**Meet the editor**

Prof. (Dr.) Kaushik Pal was born in India and received his PhD from the University of Kalyani (India). He has received many prestigious awards: Marie-Curie Experienced Researcher (Postdoctoral Fellowship) offered by the European Commission in Greece; Brain Korea National Research Foundation Visiting Scientist Fellowship in South Korea; Senior Postdoctoral Fellow at Wu-

han University, China; and Chief-Scientist & Faculty CAS Fellow by the Chinese Academy of Science. He is now working as a research professor, group leader, and independent scientist at the Department of Nanotechnology, Bharath University (BIHER), Chennai. He was invited as a visiting professor in March 2019 at University Technology Malaysia and University of Malaya, Kuala Lumpur, Malaysia. He is acting editor-in-chief of international peer-reviewed journals from publishers such as Pan Standford, Elsevier, and Springer. He has edited and contributed to significant numbers of book chapters (15) and review articles (70) and has reviewed 95 research articles. Prof. Pal has organized and has been the chairperson for around 25 national/international events/symposiums/conferences/workshops and has contributed to around 8 plenary, 25 keynote, and 30 invited lectures. He is distinguished in the worldwide nanotechnology and materials research community.

Contents

**Preface VII**

Kaushik Pal

**Conducting Films" 1**

**Conducting Films 9**

**Nanotube 31**

Guodong Liang

**Applications 53**

**Section 1 Prosperity for the Commercialisation of "Transparent**

Chapter 1 **Introductory Chapter: Transparent Conducting Films 3**

**Section 2 Novel Growth Mechanism of Nanocarbon for Transparent**

**Conducting Films Utilization 7**

Chapter 2 **Interfacial Engineering of Flexible Transparent**

Joong Tark Han and Geon-Woong Lee

**Section 3 A Facile Fabrication Criteria of Carbon Nanotube for Transparent Conducting Films Application 29**

**Section 4 Highly Conductive and Carbon Nanotube Activated**

**Transparent Thin-film 51**

Chapter 3 **Transparent Conducting Thin Film Preparation of Carbon**

Xiaogang Sun, Jie Wang, Wei Chen, Xu Li, Manyuan Cai, Long Chen,

Zhiwen Qiu, Yapan Huang, Chengcheng Wei, Hao Hu and

Chapter 4 **Carbon Nanotube-Activated Thin Film Transparent Conductor**

Iskandar Yahya, Seri Mastura Mustaza and Huda Abdullah

## Contents

**Preface VII**



**Section 5 Application of Chemically Synthesized Conductive Polymers for Biosensing 73**

## Chapter 5 **Cyclic Voltammetry and Electrical Impedance Spectroscopy of Electrodes Modified with PEDOT:PSS-Reduced Graphene Oxide Composite 75**

Nurul Izzati Ramli, Nur Alya Batrisya Ismail, Firdaus Abd-Wahab and Wan Wardatul Amani Wan Salim

Preface

nanoelectronics, and flexible electronics.

application counterparts.

its intended scope.

Transparent conducting film (TCF) is a unique class of designing process that exhibits trans‐ parency and electronic conductivity simultaneously. The novelty of research has wide‐ spread utilization in displays, photovoltaics, low-e windows, optoelectronics,

In many aspects of these applications, TCFs are used in their role as transparent contacts. However, increasingly, the demands required have extended beyond the combination of conductivity and transparency, where higher performance is needed, and now includes work function, synthesis, structural morphology, designing processes and patterning re‐ quirements, long-term stability, cost-effectiveness, and elemental abundance/green nanoma‐ terials. As these needs began to emerge over the last 5 years, they have stimulated a dramatic resurgence of research in the field leading to many new materials and processes. The purpose of this book is both to provide a snapshot of the unique and enabling work in the field and to provide indications of what might be coming over the next few decades. Over the past 5–10 years, the field has exploded to include a vastly increased number of ntype materials and a class of new p-type materials. In addition, the historically held view that crystalline materials have superior properties has been challenged by the emergence of new materials-coating TCFs that have properties as good as or better than their potential

These materials have led to the development of amorphous oxide transistors, which offer the advantage of low-temperature processing and the promise of flexible electronics on pol‐ ymer substrates. In their role as a channel material in thin film transistor structures, trans‐ parent conducting oxides with controlled carrier densities are often termed transparent oxide semiconductors since their key properties may lie in the limited to nonconductive re‐ gime. We have organized the book to capture this diversity of materials, processes, and ap‐ plications. Over the next few years, we expect these materials will become increasingly important for TCF techniques. Their inclusion in this volume at present is, however, beyond

> **Professor (Dr.) Kaushik Pal** Bharath University, India

## Preface

**Section 5 Application of Chemically Synthesized Conductive Polymers for**

Chapter 5 **Cyclic Voltammetry and Electrical Impedance Spectroscopy of**

**Electrodes Modified with PEDOT:PSS-Reduced Graphene Oxide**

Nurul Izzati Ramli, Nur Alya Batrisya Ismail, Firdaus Abd-Wahab and

**Biosensing 73**

**VI** Contents

**Composite 75**

Wan Wardatul Amani Wan Salim

Transparent conducting film (TCF) is a unique class of designing process that exhibits trans‐ parency and electronic conductivity simultaneously. The novelty of research has wide‐ spread utilization in displays, photovoltaics, low-e windows, optoelectronics, nanoelectronics, and flexible electronics.

In many aspects of these applications, TCFs are used in their role as transparent contacts. However, increasingly, the demands required have extended beyond the combination of conductivity and transparency, where higher performance is needed, and now includes work function, synthesis, structural morphology, designing processes and patterning re‐ quirements, long-term stability, cost-effectiveness, and elemental abundance/green nanoma‐ terials. As these needs began to emerge over the last 5 years, they have stimulated a dramatic resurgence of research in the field leading to many new materials and processes. The purpose of this book is both to provide a snapshot of the unique and enabling work in the field and to provide indications of what might be coming over the next few decades.

Over the past 5–10 years, the field has exploded to include a vastly increased number of ntype materials and a class of new p-type materials. In addition, the historically held view that crystalline materials have superior properties has been challenged by the emergence of new materials-coating TCFs that have properties as good as or better than their potential application counterparts.

These materials have led to the development of amorphous oxide transistors, which offer the advantage of low-temperature processing and the promise of flexible electronics on pol‐ ymer substrates. In their role as a channel material in thin film transistor structures, trans‐ parent conducting oxides with controlled carrier densities are often termed transparent oxide semiconductors since their key properties may lie in the limited to nonconductive re‐ gime. We have organized the book to capture this diversity of materials, processes, and ap‐ plications. Over the next few years, we expect these materials will become increasingly important for TCF techniques. Their inclusion in this volume at present is, however, beyond its intended scope.

> **Professor (Dr.) Kaushik Pal** Bharath University, India

**Section 1**

**Prosperity for the Commercialisation of**

**"Transparent Conducting Films"**

**Prosperity for the Commercialisation of "Transparent Conducting Films"**

**Chapter 1**

**Provisional chapter**

**Introductory Chapter: Transparent Conducting Films**

**Introductory Chapter: Transparent Conducting Films**

**1. Implementation and benefits of "Transparent Conducting Films"**

There has been an increasing demand for functional films, which combine a film substrate with various features. However, the main research goal of the transparent conducting films (TCF) and materials has been rapidly promising to scientists as well as industries. This continuing transformation is taking place at all levels: technologies, applications, developers and suppliers. Owing to their processability, stability, and high conductivity, carbon nanotubes has received significant attention from electronics-industry researchers over the past several years as an alternative to ITO. As per current trends for transparent conductive films increases, transparent electrode materials alternatives to ITO and active research and development for commercialization of such materials are being conducted. Meanwhile, transparent conductive films that have conductivity while being transparent are heavily used as essential elements for touch panels of smartphones or tablets or transparent electrodes of solar cells or other products. In particular entitled book "Transparent Conducting Films", we provide the most comprehensive and authoritative chapters are based upon years of research as we have been tracking and analyzing TCF industry since 2008. Those useful chapters are listed below in contained book;

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

DOI: 10.5772/intechopen.85577

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

• Carbon nanotube transparent conducting film

• Nanocarbon-based transparent conducting films

• Conductive polymers in biosensors

• Carbon nanotube activated thin-film transparent conductor applications

Our expert team of reviewers and editors has also been independently analyzed and peer reviewed those individual articles to flourish emerging target applications. Indeed, most of the articles, particularly concentrated on OLED lighting, wearable technology, in-mold electronics, smart windows, OPVs, DSSCs, perovskites, and touch screens. This enables us to assess

http://dx.doi.org/10.5772/intechopen.85577

Kaushik Pal

Kaushik Pal

### **Introductory Chapter: Transparent Conducting Films Introductory Chapter: Transparent Conducting Films**

DOI: 10.5772/intechopen.85577

Kaushik Pal Kaushik Pal

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.85577

## **1. Implementation and benefits of "Transparent Conducting Films"**

There has been an increasing demand for functional films, which combine a film substrate with various features. However, the main research goal of the transparent conducting films (TCF) and materials has been rapidly promising to scientists as well as industries. This continuing transformation is taking place at all levels: technologies, applications, developers and suppliers. Owing to their processability, stability, and high conductivity, carbon nanotubes has received significant attention from electronics-industry researchers over the past several years as an alternative to ITO. As per current trends for transparent conductive films increases, transparent electrode materials alternatives to ITO and active research and development for commercialization of such materials are being conducted. Meanwhile, transparent conductive films that have conductivity while being transparent are heavily used as essential elements for touch panels of smartphones or tablets or transparent electrodes of solar cells or other products. In particular entitled book "Transparent Conducting Films", we provide the most comprehensive and authoritative chapters are based upon years of research as we have been tracking and analyzing TCF industry since 2008. Those useful chapters are listed below in contained book;


Our expert team of reviewers and editors has also been independently analyzed and peer reviewed those individual articles to flourish emerging target applications. Indeed, most of the articles, particularly concentrated on OLED lighting, wearable technology, in-mold electronics, smart windows, OPVs, DSSCs, perovskites, and touch screens. This enables us to assess

the market from an application as well as technology point of view. The approach mainly the author used for fabrication is highly reproducible and creates a chemically stable configuration with a tunable tradeoff between transparency and conductive properties. In the new study, the contributors used an approach called colloidal lithography to create transparent conductive silver thin films.

**References**

[1] Rakesh AA. Transparent conducting oxide films forvarious applications: A review.

[2] Mizoguchi H, Woodward PM. Electronic structure studies of main group oxides possessing edge-sharing octahedra: Implications for the design of transparent conducting

[3] Pal K, Maiti UN, Majumder TP, Dash P, Mishra NC, Bennis N, et al. Ultraviolet visible spectroscopy of CdS nano-wires doped ferroelectric liquid crystal. Journal of Molecular

[4] Pal K, Majumder TP, Neogy C, Debnath SC. Optical, dielectric and microscopic obser-

[5] Pal K, Maiti UN, Majumder TP, Debnath SC, Ghosh S, Roy SK, et al. Switching of ferroelectric liquid crystal doped with cetyltrimethyl ammonium bromide assisted CdS

[6] Sagadevan S, Das I, Pal K, Murugasen P, Singh P. Optical and electrical smart response of chemically stabilized graphene oxide. Journal of Materials Science: Materials in

[7] Pal K, Mohan MLNM, Foley M, Ahmed W. Emerging assembly of ZnO-nanowires/graphene dispersed liquid crystal for switchable device modulation. Organic Electronics.

metal host nanowires. Journal of Molecular Structure.

Introductory Chapter: Transparent Conducting Films http://dx.doi.org/10.5772/intechopen.85577 5

Reviews on Advanced Materials Science. 2018;**53**:79-89

oxides. Chemistry of Materials. 2004;**16**(16):5233-5248

nanostructures. Nanotechnology. 2013;**24**:125702

Liquids. 2011;**164**:233-238

2012;**1016**:30-38

2018;**56**:291-304

vation of different phases TiO2

Electronics. 2017;**28**(7):5235-5243

If researchers would like to get specific knowledge on this topic from the beginning, the best advice would be to choose firstly the branch among an incomprehensible canopy of transparent conducting films and its various applied studies [1, 2]. The book aimed to show how the field is studied in different countries and what is common for all spectroscopic or microscopic investigations. The results from these experimental studies are very important outcomes of model experiments carried out on cultivating thin film techniques.

The phase, purity, stability and morphology of the composite and its constitutes have been also analyzed in those chapters. Hence, it possesses superior thermal properties and higher thermal stabilities of its layers [3, 4], qualifying it to be used in various thermo-electric devices [5] and photovoltaics. Indeed, the optical properties can be studied by utilizing optical absorption spectrum calculated optical energy band gap of the conducting film [6, 7]. The electrical parameters such as dielectric constant, tangent loss, AC conductivity as a function of frequency with fixed typical temperature also analyze.

The overall studies and investigated results in our individual chapter. Through the entire book in this year will get scope to learn more about the market trends, the key questions, latest prices, novelty of applications, e.g., transparent electrodes, flexible displays or wearable devices, OPV (organic photovoltaics) cells, light control glasses or films, organic EL lighting, transparent antennas, transparent electric wave shielding materials, and fine-tuned our analysis, insight and forecasts to reflect the latest research.

We also believe that it will be most help beginner research scholars, scientists, academicians in current understanding and advise them quite novel and non-standard approaches to find and decipher the mechanisms of transparent conducting film methodology and its application.

Finally, we would like to thank all the concern authors for their endless contributions and hard work to match and unify the "philosophy" of this book. We also thank to our colleagues from University Federal Rio de Jenerio, Brazil and Mahatma Gandhi University, Kerala, India who supported us and helped us in preparation and edition of the chapters, especially to those who raised complex questions and promoted us to answer them. We are personally very grateful to the "In-Tech" Publisher, especially Ms. Anita Condic, who assisted us in the arrangement of the book and scheduling our activities.

## **Author details**

Kaushik Pal

Address all correspondence to: kaushikphysics@gmail.com

Department of Nanotechnology, Bharath Institute of Higher Education and Research, Bharath University, Chennai, Tamil Nadu, India

## **References**

the market from an application as well as technology point of view. The approach mainly the author used for fabrication is highly reproducible and creates a chemically stable configuration with a tunable tradeoff between transparency and conductive properties. In the new study, the contributors used an approach called colloidal lithography to create transparent

If researchers would like to get specific knowledge on this topic from the beginning, the best advice would be to choose firstly the branch among an incomprehensible canopy of transparent conducting films and its various applied studies [1, 2]. The book aimed to show how the field is studied in different countries and what is common for all spectroscopic or microscopic investigations. The results from these experimental studies are very important outcomes of

The phase, purity, stability and morphology of the composite and its constitutes have been also analyzed in those chapters. Hence, it possesses superior thermal properties and higher thermal stabilities of its layers [3, 4], qualifying it to be used in various thermo-electric devices [5] and photovoltaics. Indeed, the optical properties can be studied by utilizing optical absorption spectrum calculated optical energy band gap of the conducting film [6, 7]. The electrical parameters such as dielectric constant, tangent loss, AC conductivity as a function

The overall studies and investigated results in our individual chapter. Through the entire book in this year will get scope to learn more about the market trends, the key questions, latest prices, novelty of applications, e.g., transparent electrodes, flexible displays or wearable devices, OPV (organic photovoltaics) cells, light control glasses or films, organic EL lighting, transparent antennas, transparent electric wave shielding materials, and fine-tuned our

We also believe that it will be most help beginner research scholars, scientists, academicians in current understanding and advise them quite novel and non-standard approaches to find and decipher the mechanisms of transparent conducting film methodology and its application. Finally, we would like to thank all the concern authors for their endless contributions and hard work to match and unify the "philosophy" of this book. We also thank to our colleagues from University Federal Rio de Jenerio, Brazil and Mahatma Gandhi University, Kerala, India who supported us and helped us in preparation and edition of the chapters, especially to those who raised complex questions and promoted us to answer them. We are personally very grateful to the "In-Tech" Publisher, especially Ms. Anita Condic, who assisted us in the

model experiments carried out on cultivating thin film techniques.

of frequency with fixed typical temperature also analyze.

analysis, insight and forecasts to reflect the latest research.

arrangement of the book and scheduling our activities.

Address all correspondence to: kaushikphysics@gmail.com

Bharath University, Chennai, Tamil Nadu, India

Department of Nanotechnology, Bharath Institute of Higher Education and Research,

**Author details**

Kaushik Pal

conductive silver thin films.

4 Transparent Conducting Films


**Section 2**

**Novel Growth Mechanism of Nanocarbon for**

**Transparent Conducting Films Utilization**

**Novel Growth Mechanism of Nanocarbon for Transparent Conducting Films Utilization**

**Chapter 2**

**Provisional chapter**

**Interfacial Engineering of Flexible Transparent**

**Interfacial Engineering of Flexible Transparent** 

DOI: 10.5772/intechopen.80259

One-dimensional (1D) carbon nanotubes (CNTs) and silver nanowires (AgNWs) have been used as replacements for brittle indium tin oxide (ITO) in the fabrication of transparent conducting films (TCFs), which can be used in opto-electronic devices such as screen panels, solar cell panels, and organic light-emitting diodes. This chapter describes a fabrication method of high-performance TCFs by solution processing of single-walled CNTs (SWCNTs) and AgNWs. Highly uniform TCFs with SWCNTs and AgNW inks were fabricated using spray deposition. Their performance was modulated by interfacial engineering such as overcoating with silane compound for densification of SWCNT networks and chemical or photothermal welding of SWCNT networks on thermoplastic substrates. Moreover, the hybridization of SWCNTs, AgNWs, and graphene oxide nanosheets is a promising approach to mitigate their drawbacks via p-type doping, electrical stabilization, or interfacial stabilization on plastic substrates. The rational control of 1D material networks can provide a good opportunity to fabricate high-performance TCFs for flexible opto-electronic devices.

**Keywords:** single-walled carbon nanotubes, silver nanowires, interfacial engineering,

One-dimensional (1D) conducting nanomaterials such as carbon nanotubes (CNTs) and metal nanowires have been studied to replace brittle indium tin oxide (ITO) films for flexible opto-electronic devices because of their flexibility and high electrical conductivity as well as solution processability [1–5]. There are growing needs for high-performance transparent conducting films (TCFs) with flexibility to realize flexible displays or solar cells. Solution processing of conducting nanomaterials for TCFs has many challenging issues in order to

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Conducting Films**

**Abstract**

**1. Introduction**

**Conducting Films**

Joong Tark Han and Geon-Woong Lee

Joong Tark Han and Geon-Woong Lee

http://dx.doi.org/10.5772/intechopen.80259

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

graphene oxide, dispersion, sheet resistance

## **Interfacial Engineering of Flexible Transparent Conducting Films Interfacial Engineering of Flexible Transparent Conducting Films**

DOI: 10.5772/intechopen.80259

Joong Tark Han and Geon-Woong Lee Joong Tark Han and Geon-Woong Lee

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.80259

## **Abstract**

One-dimensional (1D) carbon nanotubes (CNTs) and silver nanowires (AgNWs) have been used as replacements for brittle indium tin oxide (ITO) in the fabrication of transparent conducting films (TCFs), which can be used in opto-electronic devices such as screen panels, solar cell panels, and organic light-emitting diodes. This chapter describes a fabrication method of high-performance TCFs by solution processing of single-walled CNTs (SWCNTs) and AgNWs. Highly uniform TCFs with SWCNTs and AgNW inks were fabricated using spray deposition. Their performance was modulated by interfacial engineering such as overcoating with silane compound for densification of SWCNT networks and chemical or photothermal welding of SWCNT networks on thermoplastic substrates. Moreover, the hybridization of SWCNTs, AgNWs, and graphene oxide nanosheets is a promising approach to mitigate their drawbacks via p-type doping, electrical stabilization, or interfacial stabilization on plastic substrates. The rational control of 1D material networks can provide a good opportunity to fabricate high-performance TCFs for flexible opto-electronic devices.

**Keywords:** single-walled carbon nanotubes, silver nanowires, interfacial engineering, graphene oxide, dispersion, sheet resistance

## **1. Introduction**

One-dimensional (1D) conducting nanomaterials such as carbon nanotubes (CNTs) and metal nanowires have been studied to replace brittle indium tin oxide (ITO) films for flexible opto-electronic devices because of their flexibility and high electrical conductivity as well as solution processability [1–5]. There are growing needs for high-performance transparent conducting films (TCFs) with flexibility to realize flexible displays or solar cells. Solution processing of conducting nanomaterials for TCFs has many challenging issues in order to

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Figure 1.** Scheme of hybrid TCFs fabricated with 1D/1D hybrid materials and 1D/2D hybrid materials by solution processing.

achieve high performance, including the intrinsic properties of the materials, the dispersion of nanomaterials, and interfacial engineering of coating films on plastic substrates. Moreover, to mitigate the drawbacks of each conducting nanomaterial, we need a rational hybridization strategy to achieve the fabrication of high performance TCFs on plastic substrates (**Figure 1**).

shows the contact angle (CA) change with an increase in UVO irradiation time of polycarbonate substrates. The CA of the SWCNT/surfactant solution decreases from 15 to 10°. The CA of the aqueous AgNW solution decreases from 68.7 to 36.6° with an increasing UVO irradiation time, and the size of the deposited liquid droplet increases from 8 to 13 mm. The nozzle height

**Figure 3.** Contact angles and spread droplet sizes of (a) the aqueous SWCNT solution dispersed by sodium dodecylbenzene sulfonate and (c) the aqueous AgNW solution containing PVP on polycarbonate substratesby varying the UVO exposure time. The inset photoimages in (a) and (c) show the spread SWCNT and AgNW droplet sizes on the substrate by varying the UVO-irradiation time indicated by values. (b) and (d) Schematics of spreading of the SWCNT

**Figure 2.** Schematic of the automatic spray coating system with mass flow controller, injection pump, and atomizing

Interfacial Engineering of Flexible Transparent Conducting Films

http://dx.doi.org/10.5772/intechopen.80259

11

and the spraying pitch were optimized to 70 and 7 mm, respectively.

nozzle. The X- and Y-direction can be controlledautomatically by robotics [6].

(b) and AgNW (d) droplets on substrates [6].

Therefore, this chapter describes some of the research on the fabrication of high-performance TCFs based on single-walled CNTs (SWCNTs) and silver nanowires (AgNWs) over the past 8 years that addresses these and other challenges, with an emphasis on our own efforts. We begin with the realization of TCFs with high uniformity by spray deposition and then describe the interfacial engineering of TCFs on plastic substrates. Furthermore, we describe the fabrication of flexible TCFs with 1D/1D hybrid structures and 1D/2D hybrid materials with SWCNTs and AgNWs as 1D materials and graphene oxide as a 2D material. We conclude with some discussion of future directions and the remaining challenges in chemically exfoliated graphene technologies.

## **2. Fabrication of TCFs by spray coating**

Spray coating methods can be used to fabricate flexible TCFs with aqueous single-walled carbon nanotube (SWCNT) solutions or silver nanowire (AgNW) solutions on plastic substrates. As shown in **Figure 2**, thin films were deposited on the substrate by the atomization of aqueous solution using high-pressure nitrogen gas through a spray nozzle. The gas flow rate, nozzle height, and pitch should be controlled to fabricate uniform films with high opto-electrical performance. As a model system, SWCNT solution dispersed in aqueous surfactant and aqueous AgNW solution containing a small amount (0.01 wt%) of polyvinylpyrrolidone (PVP) were used to investigate the spreading behavior on surface energy-controlled substrates. To control the surface energy of the substrate, plastic substrates were irradiated with UV-ozone (UVO). The wettability of coating inks is critical for fabrication of uniform films by spraying. **Figure 3**

Interfacial Engineering of Flexible Transparent Conducting Films http://dx.doi.org/10.5772/intechopen.80259 11

**Figure 2.** Schematic of the automatic spray coating system with mass flow controller, injection pump, and atomizing nozzle. The X- and Y-direction can be controlledautomatically by robotics [6].

achieve high performance, including the intrinsic properties of the materials, the dispersion of nanomaterials, and interfacial engineering of coating films on plastic substrates. Moreover, to mitigate the drawbacks of each conducting nanomaterial, we need a rational hybridization strategy to achieve the fabrication of high performance TCFs on plastic substrates (**Figure 1**). Therefore, this chapter describes some of the research on the fabrication of high-performance TCFs based on single-walled CNTs (SWCNTs) and silver nanowires (AgNWs) over the past 8 years that addresses these and other challenges, with an emphasis on our own efforts. We begin with the realization of TCFs with high uniformity by spray deposition and then describe the interfacial engineering of TCFs on plastic substrates. Furthermore, we describe the fabrication of flexible TCFs with 1D/1D hybrid structures and 1D/2D hybrid materials with SWCNTs and AgNWs as 1D materials and graphene oxide as a 2D material. We conclude with some discussion of future directions and the remaining challenges in chemically exfoli-

**Figure 1.** Scheme of hybrid TCFs fabricated with 1D/1D hybrid materials and 1D/2D hybrid materials by solution

Spray coating methods can be used to fabricate flexible TCFs with aqueous single-walled carbon nanotube (SWCNT) solutions or silver nanowire (AgNW) solutions on plastic substrates. As shown in **Figure 2**, thin films were deposited on the substrate by the atomization of aqueous solution using high-pressure nitrogen gas through a spray nozzle. The gas flow rate, nozzle height, and pitch should be controlled to fabricate uniform films with high opto-electrical performance. As a model system, SWCNT solution dispersed in aqueous surfactant and aqueous AgNW solution containing a small amount (0.01 wt%) of polyvinylpyrrolidone (PVP) were used to investigate the spreading behavior on surface energy-controlled substrates. To control the surface energy of the substrate, plastic substrates were irradiated with UV-ozone (UVO). The wettability of coating inks is critical for fabrication of uniform films by spraying. **Figure 3**

ated graphene technologies.

processing.

10 Transparent Conducting Films

**2. Fabrication of TCFs by spray coating**

shows the contact angle (CA) change with an increase in UVO irradiation time of polycarbonate substrates. The CA of the SWCNT/surfactant solution decreases from 15 to 10°. The CA of the aqueous AgNW solution decreases from 68.7 to 36.6° with an increasing UVO irradiation time, and the size of the deposited liquid droplet increases from 8 to 13 mm. The nozzle height and the spraying pitch were optimized to 70 and 7 mm, respectively.

**Figure 3.** Contact angles and spread droplet sizes of (a) the aqueous SWCNT solution dispersed by sodium dodecylbenzene sulfonate and (c) the aqueous AgNW solution containing PVP on polycarbonate substratesby varying the UVO exposure time. The inset photoimages in (a) and (c) show the spread SWCNT and AgNW droplet sizes on the substrate by varying the UVO-irradiation time indicated by values. (b) and (d) Schematics of spreading of the SWCNT (b) and AgNW (d) droplets on substrates [6].

The conductivity, *σDC*, of the disordered nanotube films depends on the number density of the

< *RJ* >

Here, *K* is the proportionality factor that scales with the bundle length. Therefore, if we can

this, the SWCNT films were coated with silane sols by considering their surface energy. Considering the interfacial tension between the SWCNT film and silane sols, two top-coating materials such as a tetraorthosilicate (TEOS) sol with silanol groups and methyltrimethoxysilane (MTMS) sol with hydrophobic methyl groups were used. It is worth noting that top-

ally increased. This large disparity between MTMS and TEOS sols can be explained by a change in the contact resistance between the bundles. Hydrophilic TEOS sol can densify the hydrophobic SWCNT networks, while MTMS sol, having methyl groups, can penetrate the hydrophobic SWCNT networks, resulting in an increase of the contact resistance of SWCNTs. This interfacial tension effect was minimized by deposition of gold chloride solution onto the

hard-coated PET and glass. These results imply that the CTE value should be considered in order to obtain highly stable SWCNT TCFs on plastic substrates. To illustrate this phenomenon, a Raman spectroscopic study was performed, and the G+ and G− peak positions related to the strain of SWCNTs were compared. The G-band frequencies for SWCNT films on bare

, which in turn scales with the network morphology through the film

, the sheet resistance of the SWCNT films can be improved. To realize

change after heating at 130°C and cooling. Bare PET, hard-coated PET,

versus transmittance plot of SWCNT film deposited by spraying on PET substrates. (b) Wettability of

by varying spray coating times of top-coating materials (methyl trimethoxysilane (MTMS) sol, tetraethoxysilane (TEOS)

of the SWCNTs on bare PET substrates increased by 40%

after heating at 130°C and cooling, which corresponds to a

increase was suppressed in bare SWCNT films on

change of pristine and doped SWCNT films

<sup>&</sup>lt; *<sup>D</sup>* <sup>&</sup>gt;<sup>3</sup> (2)

Interfacial Engineering of Flexible Transparent Conducting Films

http://dx.doi.org/10.5772/intechopen.80259

of the film to less than 80% of the *Rs*

values of MTMS sol-coated SWCNT films gradu-

>

13

changes of

, the mean diameter, *<D>*, of the bundles, and the mean junction resistance, <*RJ*

*Vf* 2

\_\_\_\_\_\_\_\_

network junctions, *Nj*

> and *Vf*

**Figure 6** shows the *Rs*

**Figure 5.** (a) The *Rs*

sol) after baking at 80°C for 1 h [12].

SWCNT films. Interestingly, the *Rs*

PET were up-shifted by 1–2 cm−<sup>1</sup>

relative to the initial values, while the *Rs*

pristine SWCNT film and SWCNT filmdoped with gold chloride. (c) The *Rs*

*<sup>σ</sup>DC* <sup>=</sup> \_\_\_\_\_\_\_ *<sup>K</sup>*

coating with TEOS sol unexpectedly decreased the *Rs*

SWCNT film (**Figure 5b**) to make it hydrophilic, as shown in **Figure 5c**.

and glass substrates were used to illustrate the CTE mismatch effect on the *Rs*

of the as-prepared film. However, the *Rs*

fill-factor, *Vf*

reduce <*RJ*

[8–11],

**Figure 4.** The sheet resistance (*Rs* ) uniformity of (a–c) the SWCNT films and (d–f) the AgNW films on (a, d) pristine polycarbonate (PC), (b, e) UVO-irradiated PC substrates, and (c, f) after graphene oxide (HOGO) coating of the conducting films fabricated on UVO-treated substrates [6].

Another way to enhance the uniformity of the films is deposition of hydrophilic graphene oxide (GO) nanosheets onto the substrate. **Figure 4** shows the sheet resistance (*Rs* ) distribution of the SWCNT and the AgNW films spray-coated on surface energy-controlled substrates and after deposition of GO nanosheets onto the films. After UVO treatment, the *Rs* uniformity of AgNW films was dramatically improved and reached 7.2%, resulting in *T* = 98% and *Rs* = 100 Ω/sq for the highlyoxidized GO (HOGO)-coated AgNW films.

## **3. Interfacial engineering for high-performance TCFs**

## **3.1. Modulation of the sheet resistance of SWCNT-based TCFs by silane sol**

In this study, we investigated the effect of the interfacial tension between bare SWCNT network films and a top-coating of passivation materials on the *Rs* of the film. We demonstrated that the *Rs* of the SWCNT film can be affected by a thermal expansion coefficient (CTE) mismatch between the substrate and the SWCNT film.

The spray-coated SWCNT films have porous structures on a scale of tens of nanometers. The *Rs* and transmittance are related by [7].

$$T(\lambda) = \left(1 + \frac{188.5}{R\_i} \frac{\sigma\_\phi(\lambda)}{\sigma\_{\text{DC}}}\right)^{-2},\tag{1}$$

where *σDC* and *σOp* are the DC and optical conductivities, respectively.

The conductivity, *σDC*, of the disordered nanotube films depends on the number density of the network junctions, *Nj* , which in turn scales with the network morphology through the film fill-factor, *Vf* , the mean diameter, *<D>*, of the bundles, and the mean junction resistance, <*RJ* > [8–11],

$$
\sigma\_{\rm DC} = \frac{K}{} \frac{V\_{\uparrow}^2}{^3} \tag{2}
$$

Here, *K* is the proportionality factor that scales with the bundle length. Therefore, if we can reduce <*RJ* > and *Vf* , the sheet resistance of the SWCNT films can be improved. To realize this, the SWCNT films were coated with silane sols by considering their surface energy. Considering the interfacial tension between the SWCNT film and silane sols, two top-coating materials such as a tetraorthosilicate (TEOS) sol with silanol groups and methyltrimethoxysilane (MTMS) sol with hydrophobic methyl groups were used. It is worth noting that topcoating with TEOS sol unexpectedly decreased the *Rs* of the film to less than 80% of the *Rs* of the as-prepared film. However, the *Rs* values of MTMS sol-coated SWCNT films gradually increased. This large disparity between MTMS and TEOS sols can be explained by a change in the contact resistance between the bundles. Hydrophilic TEOS sol can densify the hydrophobic SWCNT networks, while MTMS sol, having methyl groups, can penetrate the hydrophobic SWCNT networks, resulting in an increase of the contact resistance of SWCNTs. This interfacial tension effect was minimized by deposition of gold chloride solution onto the SWCNT film (**Figure 5b**) to make it hydrophilic, as shown in **Figure 5c**.

**Figure 6** shows the *Rs* change after heating at 130°C and cooling. Bare PET, hard-coated PET, and glass substrates were used to illustrate the CTE mismatch effect on the *Rs* changes of SWCNT films. Interestingly, the *Rs* of the SWCNTs on bare PET substrates increased by 40% relative to the initial values, while the *Rs* increase was suppressed in bare SWCNT films on hard-coated PET and glass. These results imply that the CTE value should be considered in order to obtain highly stable SWCNT TCFs on plastic substrates. To illustrate this phenomenon, a Raman spectroscopic study was performed, and the G+ and G− peak positions related to the strain of SWCNTs were compared. The G-band frequencies for SWCNT films on bare PET were up-shifted by 1–2 cm−<sup>1</sup> after heating at 130°C and cooling, which corresponds to a

Another way to enhance the uniformity of the films is deposition of hydrophilic graphene

polycarbonate (PC), (b, e) UVO-irradiated PC substrates, and (c, f) after graphene oxide (HOGO) coating of the

) uniformity of (a–c) the SWCNT films and (d–f) the AgNW films on (a, d) pristine

of the SWCNT and the AgNW films spray-coated on surface energy-controlled substrates

mity of AgNW films was dramatically improved and reached 7.2%, resulting in *T* = 98% and

In this study, we investigated the effect of the interfacial tension between bare SWCNT net-

The spray-coated SWCNT films have porous structures on a scale of tens of nanometers. The

*Rs*

of the SWCNT film can be affected by a thermal expansion coefficient (CTE) mis-

*<sup>σ</sup>Op*(*λ*) \_\_\_\_\_ *σDC* )

−2

) distribution

of the film. We demonstrated

, (1)

unifor-

oxide (GO) nanosheets onto the substrate. **Figure 4** shows the sheet resistance (*Rs*

**3.1. Modulation of the sheet resistance of SWCNT-based TCFs by silane sol**

*Rs* = 100 Ω/sq for the highlyoxidized GO (HOGO)-coated AgNW films.

**3. Interfacial engineering for high-performance TCFs**

work films and a top-coating of passivation materials on the *Rs*

where *σDC* and *σOp* are the DC and optical conductivities, respectively.

match between the substrate and the SWCNT film.

*<sup>T</sup>*(*λ*) <sup>=</sup> (<sup>1</sup> <sup>+</sup> \_\_\_\_\_ 188.5

and transmittance are related by [7].

that the *Rs*

**Figure 4.** The sheet resistance (*Rs*

12 Transparent Conducting Films

conducting films fabricated on UVO-treated substrates [6].

*Rs*

and after deposition of GO nanosheets onto the films. After UVO treatment, the *Rs*

**Figure 5.** (a) The *Rs* versus transmittance plot of SWCNT film deposited by spraying on PET substrates. (b) Wettability of pristine SWCNT film and SWCNT filmdoped with gold chloride. (c) The *Rs* change of pristine and doped SWCNT films by varying spray coating times of top-coating materials (methyl trimethoxysilane (MTMS) sol, tetraethoxysilane (TEOS) sol) after baking at 80°C for 1 h [12].

**Figure 6.** (a, b) The *Rs* changes of SWCNT films with different transmittance values, after heating at 130°C, as a function of thermal treatment time. (c, d) Scheme of thermal expansion mismatch between the SWCNT layers and bare PET or hard-coated PET after heating and cooling. (e–g) Raman spectra (G band) of SWCNT films fabricated on (e) bare PET, (f) hard-coated PET, and (g) glass after heating at 130°C for 20 min, followed by cooling [12].

compressive strain of ~0.1%. This compressive strain may cause the increase of the *Rs* of the SWCNT film on bare PET.

deposition show clearly that the electrical conductivity of the SWCNT films was enhanced

surfaces, and after deposition of solvents and dopants. (b) In-situ conductivity measurements of SWCNT films after

versus transmittance plots for pristine SWCNT films prepared from a SWNT/SDBS solution on PET

changes of bare SWCNT films in comparison with the same films treated

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**Figure 8** shows that large SWCNT bundles were welded, and small bundles were embedded on the PET substrate after spraying aromatic hydrocarbons, while spraying cyclohexane did not trigger welding. The strain induced on the SWCNT networks during network formation on the substrate may cause an initial high resistance in the SWCNT network film. Thus, solvent-induced chemical welding of the SWCNT film can release their strain. The recovery of the G band in the Raman spectra of the SWCNT films demonstrates strain relaxation via

Thermal treatment is an alternative way to produce SWCNT film-substrate welding without any chemicals. In particular, fast selective heating of CNTs on plastic substrates can provide an interesting opportunity for thermal welding [14, 15]. Microwaves irradiate the SWCNT films inside the rectangular waveguide microwave applicator, within which the microwave electric field is well defined and controlled. The microwave mode in the applicator is a

after toluene deposition, which can swell PET substrates.

with solvents: toluene (T), benzene (B), hexane (H), and cyclohexane (C) [13].

chemical welding.

**Figure 7.** (a) *Rs*

deposition of toluene and gold chloride. (c) *Rs*

## **3.2. Self-passivation of SWCNT films on plastic substrates by nanowelding**

Plastic substrates are generally used to fabricate flexible TCFs by deposition of CNTs or metal nanowires. In particular, the electrical properties of SWCNT network films are sensitive to humidity and temperature. In this context, top-coating with passivation materials or hybridization with binder materials are applicable for improving the stability of TCFs. Another way to passivate TCFs is welding or embedding in plastic substrates by chemical or thermal treatments. **Figure 7** shows the *Rs* change of the SWCNT films after deposition of solvents. To investigate the solvent effects, we used solvents with optimal polarity and affinity for the PET substrate. Moreover, the presence of electron-donating and electron-withdrawing groups in the solvent molecules can affect the electronic structure of the SWCNTs. Thus, nonpolar solvents were selected. In particular, aromatic hydrocarbon, benzene, and toluene can swell the PET substrate. Most interestingly, deposition of toluene or benzene decreased the *Rs* of the SWCNT films. After doping with gold chloride, the *Rs* and transmittance of the film were measured to be 85 Ω/sq and 90%, respectively. Moreover, I-V plots measured after solvent

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**Figure 7.** (a) *Rs* versus transmittance plots for pristine SWCNT films prepared from a SWNT/SDBS solution on PET surfaces, and after deposition of solvents and dopants. (b) In-situ conductivity measurements of SWCNT films after deposition of toluene and gold chloride. (c) *Rs* changes of bare SWCNT films in comparison with the same films treated with solvents: toluene (T), benzene (B), hexane (H), and cyclohexane (C) [13].

**Figure 6.** (a, b) The *Rs*

14 Transparent Conducting Films

SWCNT film on bare PET.

ments. **Figure 7** shows the *Rs*

changes of SWCNT films with different transmittance values, after heating at 130°C, as a function

change of the SWCNT films after deposition of solvents. To

of the

of

and transmittance of the film were

of thermal treatment time. (c, d) Scheme of thermal expansion mismatch between the SWCNT layers and bare PET or hard-coated PET after heating and cooling. (e–g) Raman spectra (G band) of SWCNT films fabricated on (e) bare PET, (f)

Plastic substrates are generally used to fabricate flexible TCFs by deposition of CNTs or metal nanowires. In particular, the electrical properties of SWCNT network films are sensitive to humidity and temperature. In this context, top-coating with passivation materials or hybridization with binder materials are applicable for improving the stability of TCFs. Another way to passivate TCFs is welding or embedding in plastic substrates by chemical or thermal treat-

investigate the solvent effects, we used solvents with optimal polarity and affinity for the PET substrate. Moreover, the presence of electron-donating and electron-withdrawing groups in the solvent molecules can affect the electronic structure of the SWCNTs. Thus, nonpolar solvents were selected. In particular, aromatic hydrocarbon, benzene, and toluene can swell the PET substrate. Most interestingly, deposition of toluene or benzene decreased the *Rs*

measured to be 85 Ω/sq and 90%, respectively. Moreover, I-V plots measured after solvent

compressive strain of ~0.1%. This compressive strain may cause the increase of the *Rs*

**3.2. Self-passivation of SWCNT films on plastic substrates by nanowelding**

hard-coated PET, and (g) glass after heating at 130°C for 20 min, followed by cooling [12].

the SWCNT films. After doping with gold chloride, the *Rs*

deposition show clearly that the electrical conductivity of the SWCNT films was enhanced after toluene deposition, which can swell PET substrates.

**Figure 8** shows that large SWCNT bundles were welded, and small bundles were embedded on the PET substrate after spraying aromatic hydrocarbons, while spraying cyclohexane did not trigger welding. The strain induced on the SWCNT networks during network formation on the substrate may cause an initial high resistance in the SWCNT network film. Thus, solvent-induced chemical welding of the SWCNT film can release their strain. The recovery of the G band in the Raman spectra of the SWCNT films demonstrates strain relaxation via chemical welding.

Thermal treatment is an alternative way to produce SWCNT film-substrate welding without any chemicals. In particular, fast selective heating of CNTs on plastic substrates can provide an interesting opportunity for thermal welding [14, 15]. Microwaves irradiate the SWCNT films inside the rectangular waveguide microwave applicator, within which the microwave electric field is well defined and controlled. The microwave mode in the applicator is a

**Figure 8.** Atomic force microscope images of (a) an as-prepared film (99% transmittance at 550 nm), and the film after spraying of (b) cyclohexane and (c) toluene. (d) Height profile of the nanotube bundles indicated by the inverted triangles in (a–c). The left and right images in (a), (b), and (c) are the height and phase images, respectively. Deformed SWCNT bundles are indicated by arrows in (a). The green dotted circles in (c) indicate embedded SWCNT bundles after deposition of toluene because of swelling of PET [13].

fundamental transverse electric (*TE10*) mode (*Ez* = 0) with a frequency of 2450 MHz, so the microwave electric field (*Ey* ) is sinusoidally distributed along the x- and z-axes and constant along the y-axis. Immediate flash Ohmic heating with an energy conversion of greater than 99% can be realized because the microwave electric field is parallel to the overall SWCNT film and can efficiently induce a fast oscillating current in the film. The amplitude of the conduction current density, *J s* , induced on the CNT film by the microwave electric field intensity, *EMW*, may be described as follows [16]:

$$J\_s = \sigma\_{\rm CNT} E\_{\rm MVV} \tag{3}$$

7 s irradiation at 40 W without heat deflection. Of interest is that the *Rs*

relative humidity, despite embedding of the nanotubes in the plastic substrates.

**3.3. CNT-induced migration of AgNW networks into plastic substrates**

water molecules. The *Rs*

**Figure 9.** (a) Measured surface temperatures and *Rs*

of irradiation, due to the occurrence of chemical welding. The Raman spectra in **Figure 9e** show the strain relaxation of the SWCNT network. The SEM image also shows clearly that the SWCNTs are welded or embedded in the plastic substrate. Importantly, the MW-irradiated SWCNT networks are protected by a self-passivation layer that protects the nanotubes from

microwave irradiation time. (b) The SWCNT film on PC heated in a conventional heating oven at 150°C. (c) The SWCNT film irradiated with microwaves. (d) Scheme of microwave-irradiated selective heating of CNTs on a plastic substrate, wherein a rapidly oscillating current induced along the CNTs is efficiently generated by the microwave electric field parallel to the SWCNT film. (e) Raman spectra of SWCNT powder and SWCNT films on PC before and after microwave irradiation for 7 s. Inset SEM image shows the microwave-nanowelded SWCNT film on the PC substrate [17].

AgNW-based TCFs are not very environmentally stable without some form of passivation. If the AgNW network can be welded onto a thermoplastic substrate, it can be self-passivated, as was accomplished with SWCNT film. However, the surface tension of AgNWs (~500 mN/m of liquid silver in air) is much different from that of the hydrophobic PC substrate (~34.2 mN/m), which prevents the AgNWs from completely embedding in the plastic substrate, as illustrated in **Figure 10**. This surface tension mismatch can be solved by deposition of SWCNTs onto

values of the SWCNT films increase by less than 10% at 80°C and 90%

changes of SWCNT films on PC substrates as a function of

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decreased after 7 s

where *σCNT* is the electric conductivity of the SWCNT film.

**Figure 9a** shows the surface temperature and *Rs* changes of the SWCNT film by varying the irradiation time. The surface temperature of the SWCNT film is dramatically increased after Interfacial Engineering of Flexible Transparent Conducting Films http://dx.doi.org/10.5772/intechopen.80259 17

**Figure 9.** (a) Measured surface temperatures and *Rs* changes of SWCNT films on PC substrates as a function of microwave irradiation time. (b) The SWCNT film on PC heated in a conventional heating oven at 150°C. (c) The SWCNT film irradiated with microwaves. (d) Scheme of microwave-irradiated selective heating of CNTs on a plastic substrate, wherein a rapidly oscillating current induced along the CNTs is efficiently generated by the microwave electric field parallel to the SWCNT film. (e) Raman spectra of SWCNT powder and SWCNT films on PC before and after microwave irradiation for 7 s. Inset SEM image shows the microwave-nanowelded SWCNT film on the PC substrate [17].

7 s irradiation at 40 W without heat deflection. Of interest is that the *Rs* decreased after 7 s of irradiation, due to the occurrence of chemical welding. The Raman spectra in **Figure 9e** show the strain relaxation of the SWCNT network. The SEM image also shows clearly that the SWCNTs are welded or embedded in the plastic substrate. Importantly, the MW-irradiated SWCNT networks are protected by a self-passivation layer that protects the nanotubes from water molecules. The *Rs* values of the SWCNT films increase by less than 10% at 80°C and 90% relative humidity, despite embedding of the nanotubes in the plastic substrates.

## **3.3. CNT-induced migration of AgNW networks into plastic substrates**

fundamental transverse electric (*TE10*) mode (*Ez* = 0) with a frequency of 2450 MHz, so the

**Figure 8.** Atomic force microscope images of (a) an as-prepared film (99% transmittance at 550 nm), and the film after spraying of (b) cyclohexane and (c) toluene. (d) Height profile of the nanotube bundles indicated by the inverted triangles in (a–c). The left and right images in (a), (b), and (c) are the height and phase images, respectively. Deformed SWCNT bundles are indicated by arrows in (a). The green dotted circles in (c) indicate embedded SWCNT bundles after

along the y-axis. Immediate flash Ohmic heating with an energy conversion of greater than 99% can be realized because the microwave electric field is parallel to the overall SWCNT film and can efficiently induce a fast oscillating current in the film. The amplitude of the conduc-

irradiation time. The surface temperature of the SWCNT film is dramatically increased after

) is sinusoidally distributed along the x- and z-axes and constant

, induced on the CNT film by the microwave electric field intensity,

*<sup>s</sup>* = *σCNT EMW* , (3)

changes of the SWCNT film by varying the

microwave electric field (*Ey*

*s*

deposition of toluene because of swelling of PET [13].

**Figure 9a** shows the surface temperature and *Rs*

where *σCNT* is the electric conductivity of the SWCNT film.

*EMW*, may be described as follows [16]:

*J*

tion current density, *J*

16 Transparent Conducting Films

AgNW-based TCFs are not very environmentally stable without some form of passivation. If the AgNW network can be welded onto a thermoplastic substrate, it can be self-passivated, as was accomplished with SWCNT film. However, the surface tension of AgNWs (~500 mN/m of liquid silver in air) is much different from that of the hydrophobic PC substrate (~34.2 mN/m), which prevents the AgNWs from completely embedding in the plastic substrate, as illustrated in **Figure 10**. This surface tension mismatch can be solved by deposition of SWCNTs onto

**Figure 10.** (a) AFM image and (b) height profile of the AgNW film after thermal treatment at 150°C for 3 h on a PC substrate. (c, d) Schematic illustration of the limited migration of AgNW networks into the plastic substrate due to a surface tension mismatch [18].

the AgNW network because of the low surface tension of CNTs (40–80 mN/m). Therefore, SWCNTs can trigger the migration of AgNWs into plastic substrates by thermal or chemical treatment. Moreover, the high thermal electrical conductivity of the SWCNT can promote the self-passivation of AgNWs by stable Joule heating of the film with an applied DC voltage. **Figure 11** shows the surface morphology of the SWCNT-overcoated AgNW film after a voltage of 20 V was applied. In stark contrast to AgNWs in AgNW film shown in **Figure 10a**, AgNWs were fully embedded in the plastic substrate by electrical heating. Atomic force microscopy (AFM) height profiles also demonstrate the embedding of the AgNW–SWCNT network in the plastic substrate. This self-passivation of AgNW networks assisted by SWCNTs with electrical heating improved the mechanical and hydrothermal stability of the film.

## **3.4. Interfacial engineering with GO for AgNW TCFs**

In terms of the applications of metal nanowire networks, interfacial engineering is an important step to improve their performance with respect to electrical conductivity, environmental stability, surface roughness, and work function modulation. In particular, interfacial engineering of AgNW film can affect the opto-electrical performance because of junction formation in the network. In this study, HOGO nanosheets were utilized for efficient thermal joining of AgNW networks on thermoplastic substrates (**Figure 12a**). **Figure 12b** shows the *Rs* changes of the AgNW network films on bare PC, GO-modified PC, and glass after heating at 150°C with increasing exposure time. The *Rs* was dramatically reduced by thermal treatment via a junction joining of the networks. Importantly, the *Rs* decrease of the AgNW film was

more efficient on GO-modified PC than on bare PC and glass. Interestingly, the changed *Rs*

on glass gradually increased, even after 30 min, due to air oxidation. This result provides an opportunity to obtain high-performance AgNW TCFs by a combination of thermal welding and junction joining of AgNW networks. SEM and AFM images in **Figure 12** show clearly that on GO nanosheets, limited embedding or welding of AgNWs was observed. This demon-

**Figure 11.** Field emission SEM images of AgNW overcoated with SWCNTs (a) before and (b) after heating under a current flow of thin film heater. AFM images of the same film (c) before heating and (d) after heating. (e) Height profile

of AgNWs on the GO-modified PC.

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AgNW films on PC was stable even after heating for 180 min, while the *Rs*

strates the more efficient reduction of *Rs*

of the SWCNT-overcoated AgNW film under a current flow [18].

of

of the AgNW film

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the AgNW network because of the low surface tension of CNTs (40–80 mN/m). Therefore, SWCNTs can trigger the migration of AgNWs into plastic substrates by thermal or chemical treatment. Moreover, the high thermal electrical conductivity of the SWCNT can promote the self-passivation of AgNWs by stable Joule heating of the film with an applied DC voltage. **Figure 11** shows the surface morphology of the SWCNT-overcoated AgNW film after a voltage of 20 V was applied. In stark contrast to AgNWs in AgNW film shown in **Figure 10a**, AgNWs were fully embedded in the plastic substrate by electrical heating. Atomic force microscopy (AFM) height profiles also demonstrate the embedding of the AgNW–SWCNT network in the plastic substrate. This self-passivation of AgNW networks assisted by SWCNTs with electrical

**Figure 10.** (a) AFM image and (b) height profile of the AgNW film after thermal treatment at 150°C for 3 h on a PC substrate. (c, d) Schematic illustration of the limited migration of AgNW networks into the plastic substrate due to a

In terms of the applications of metal nanowire networks, interfacial engineering is an important step to improve their performance with respect to electrical conductivity, environmental stability, surface roughness, and work function modulation. In particular, interfacial engineering of AgNW film can affect the opto-electrical performance because of junction formation in the network. In this study, HOGO nanosheets were utilized for efficient thermal joining of AgNW networks on thermoplastic substrates (**Figure 12a**). **Figure 12b** shows the *Rs* changes of the AgNW network films on bare PC, GO-modified PC, and glass after heating at

was dramatically reduced by thermal treatment

decrease of the AgNW film was

heating improved the mechanical and hydrothermal stability of the film.

**3.4. Interfacial engineering with GO for AgNW TCFs**

surface tension mismatch [18].

18 Transparent Conducting Films

150°C with increasing exposure time. The *Rs*

via a junction joining of the networks. Importantly, the *Rs*

**Figure 11.** Field emission SEM images of AgNW overcoated with SWCNTs (a) before and (b) after heating under a current flow of thin film heater. AFM images of the same film (c) before heating and (d) after heating. (e) Height profile of the SWCNT-overcoated AgNW film under a current flow [18].

more efficient on GO-modified PC than on bare PC and glass. Interestingly, the changed *Rs* of AgNW films on PC was stable even after heating for 180 min, while the *Rs* of the AgNW film on glass gradually increased, even after 30 min, due to air oxidation. This result provides an opportunity to obtain high-performance AgNW TCFs by a combination of thermal welding and junction joining of AgNW networks. SEM and AFM images in **Figure 12** show clearly that on GO nanosheets, limited embedding or welding of AgNWs was observed. This demonstrates the more efficient reduction of *Rs* of AgNWs on the GO-modified PC.

**Figure 12.** (a) Scheme showing AgNW film on GO-modified PC. (b) *Rs* changes of AgNW films on bare PC and GO-modified PC, and on glass after heating at 150°C by varying the exposure time. (c, d) SEM images of AgNW films on (c) bare PC and (d) GO-modified PC substrates after heating at 150°C for 1 h. (e) AFM image of AgNW networks on GO-modified PC. (f) Height profiles of embedded AgNWs and AgNWs floated on the GO nanosheet indicated in (e) as numbers [19].

## **4. High-performance TCFs by hybridization of 1D or 2D materials**

## **4.1. Graphene oxide-modified SWCNT-based TCFs**

SWCNT-based TCFs with a low haze value are suitable for highly transparent opto-electronic devices. However, for achievement of a low *Rs* value of the films, one challenge is the development of an efficient and stable dopant. In addition, their high porosity and hydrophobic surface properties are a drawback as an electrode material in opto-electronic devices. In this context, we introduced easily deformable GO nanosheets containing electron-withdrawing groups on the basal plane and edges, which can give a p-type doping effect on the SWCNT film. **Figure 13** shows that the *Rs* of the SWCNT film can be dramatically reduced by up to 40% compared to the as-prepared SWCNT film by deposition of GO solution onto the film by spraying. The efficiency of *Rs* reduction depends on the lateral sizes of the GO nanosheets. Small-sized GO nanosheets prepared by decanting the first supernatant (S1) by centrifugation were more efficient than larger GO nanosheets. As shown in **Figure 14**, the SWCNT bundles are easily wrapped with small GO nanosheets, while larger GO nanosheets can be freestanding between SWCNT networks. This means that densification of the SWCNT network is more efficient using small GO than large GO. The reduction of porosity and junction resistance of the SWCNT network can have a positive effect on the decrease of *Rs* . Moreover, the effect of p-type doping by GO is clearly shown in Raman spectra (**Figure 14c** and **d**). An upshift of 3.5 cm−<sup>1</sup> in the G+ band for the semiconducting SWCNTs by small GO nanosheets (S1) demonstrates p-type doping of the SWCNTs from the GO nanosheets via a charge transfer mechanism.

fabricated (**Figure 15**). For fabrication of the layered structure of the OPV cells, the wettability of the electrode on the upper loaded aqueous PEDOT:PSS solution is important. As shown in **Figure 15a**, the hydrophobic SWCNT film was converted to hydrophilic by deposition of hydrophilic GO nanosheets. Moreover, importantly, the work function of the SWCNT film changed from 4.7 to 5.05 eV by deposition of S1-GO nanosheets, which induces a facile hole injection from the HOMO of P3HT (5.0 eV) to the electrode. The resultant device performance with the GO-modified SWCNT anodes shows a significant enhancement in overall photovoltaic performance compared to devices fabricated on pristine SWCNT electrodes, as shown in **Figure 15d**.

changes of SWCNT films by increasing the number of spray coatings of GO solution obtained by centrifugation (the first

the SWCNT bundle. (d) The I-V measurement scheme performed on SWCNT films after deposition of the GO solution. (e) Photo image of a gold-patterned SWCNT film and I-V plots for SWCNT films by increasing the amount of deposited

versus transmittance plots of SWCNT films before and after deposition of GO nanosheets. (b) Relative *Rs*

as a function of the SWCNT film transmittance

changes of the film due to contact area change between GO and

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Under high current flow, metal NW scan be disrupted by Joule heating at the junction due to a relatively high junction resistance between metal NWs. Self-joining of NW network junctions can solve this problem via post-treatment. Another approach is to interconnect the NWs with other conducting materials or metal oxides. For more efficient processing of metal NW-based TCFs, we need to exclude additional steps, such as irradiation with light, heating at high

**4.2. Electrically stable SWCNT/AgNW hybrid TCFs**

to fourth supernatant solutions are denoted as S1 to S4). (c) Relative *Rs*

showing thickness dependence of GO deposition on *Rs*

GO solution (in the direction of the arrow) [20].

**Figure 13.** (a) *Rs*

To evaluate the opto-electrical performance of a GO-SWCNT electrode on PET, organic photovoltaic (OPV) cells with a PET/GO-SWCNT/PEDOT-PSS/active layer/LiF/Al structure were

Interfacial Engineering of Flexible Transparent Conducting Films http://dx.doi.org/10.5772/intechopen.80259 21

**Figure 13.** (a) *Rs* versus transmittance plots of SWCNT films before and after deposition of GO nanosheets. (b) Relative *Rs* changes of SWCNT films by increasing the number of spray coatings of GO solution obtained by centrifugation (the first to fourth supernatant solutions are denoted as S1 to S4). (c) Relative *Rs* as a function of the SWCNT film transmittance showing thickness dependence of GO deposition on *Rs* changes of the film due to contact area change between GO and the SWCNT bundle. (d) The I-V measurement scheme performed on SWCNT films after deposition of the GO solution. (e) Photo image of a gold-patterned SWCNT film and I-V plots for SWCNT films by increasing the amount of deposited GO solution (in the direction of the arrow) [20].

fabricated (**Figure 15**). For fabrication of the layered structure of the OPV cells, the wettability of the electrode on the upper loaded aqueous PEDOT:PSS solution is important. As shown in **Figure 15a**, the hydrophobic SWCNT film was converted to hydrophilic by deposition of hydrophilic GO nanosheets. Moreover, importantly, the work function of the SWCNT film changed from 4.7 to 5.05 eV by deposition of S1-GO nanosheets, which induces a facile hole injection from the HOMO of P3HT (5.0 eV) to the electrode. The resultant device performance with the GO-modified SWCNT anodes shows a significant enhancement in overall photovoltaic performance compared to devices fabricated on pristine SWCNT electrodes, as shown in **Figure 15d**.

## **4.2. Electrically stable SWCNT/AgNW hybrid TCFs**

**4. High-performance TCFs by hybridization of 1D or 2D materials**

SWCNT-based TCFs with a low haze value are suitable for highly transparent opto-electronic

PC, and on glass after heating at 150°C by varying the exposure time. (c, d) SEM images of AgNW films on (c) bare PC and (d) GO-modified PC substrates after heating at 150°C for 1 h. (e) AFM image of AgNW networks on GO-modified PC. (f) Height profiles of embedded AgNWs and AgNWs floated on the GO nanosheet indicated in (e) as numbers [19].

opment of an efficient and stable dopant. In addition, their high porosity and hydrophobic surface properties are a drawback as an electrode material in opto-electronic devices. In this context, we introduced easily deformable GO nanosheets containing electron-withdrawing groups on the basal plane and edges, which can give a p-type doping effect on the SWCNT

40% compared to the as-prepared SWCNT film by deposition of GO solution onto the film

Small-sized GO nanosheets prepared by decanting the first supernatant (S1) by centrifugation were more efficient than larger GO nanosheets. As shown in **Figure 14**, the SWCNT bundles are easily wrapped with small GO nanosheets, while larger GO nanosheets can be freestanding between SWCNT networks. This means that densification of the SWCNT network is more efficient using small GO than large GO. The reduction of porosity and junction resistance of the

doping by GO is clearly shown in Raman spectra (**Figure 14c** and **d**). An upshift of 3.5 cm−<sup>1</sup> in the G+ band for the semiconducting SWCNTs by small GO nanosheets (S1) demonstrates p-type doping of the SWCNTs from the GO nanosheets via a charge transfer mechanism.

To evaluate the opto-electrical performance of a GO-SWCNT electrode on PET, organic photovoltaic (OPV) cells with a PET/GO-SWCNT/PEDOT-PSS/active layer/LiF/Al structure were

value of the films, one challenge is the devel-

changes of AgNW films on bare PC and GO-modified

. Moreover, the effect of p-type

of the SWCNT film can be dramatically reduced by up to

reduction depends on the lateral sizes of the GO nanosheets.

**4.1. Graphene oxide-modified SWCNT-based TCFs**

**Figure 12.** (a) Scheme showing AgNW film on GO-modified PC. (b) *Rs*

SWCNT network can have a positive effect on the decrease of *Rs*

devices. However, for achievement of a low *Rs*

film. **Figure 13** shows that the *Rs*

20 Transparent Conducting Films

by spraying. The efficiency of *Rs*

Under high current flow, metal NW scan be disrupted by Joule heating at the junction due to a relatively high junction resistance between metal NWs. Self-joining of NW network junctions can solve this problem via post-treatment. Another approach is to interconnect the NWs with other conducting materials or metal oxides. For more efficient processing of metal NW-based TCFs, we need to exclude additional steps, such as irradiation with light, heating at high

temperatures, and the removal of surfactant molecules after the deposition of AgNWs or AgNW hybrid materials. Thus, we suggest that a small amount of SWCNTs can stabilize the AgNW networks under current flow without post-treatment. To realize this, the major challenge is the fabrication of a stable dispersion of SWCNTs in liquid medium without dispersant molecules that can be removed after deposition. To solve this issue, the SWCNTs were functionalized with quadruple hydrogen bonding (QHB) motifs of 2-ureido-4[1H]pyrimidinone (UHP) moieties through a previously reported sequential coupling reaction [21]. The AgNW/SWCNT mixture solution was easily prepared by direct mixing of the aqueous AgNW solution with a paste of SWCNTs functionalized with UHP (UHP-SWCNTs) by shaking, as shown in **Figure 16a**. The

used to fabricate transparent film heaters to investigate the effect of SWCNTs on the electrical stability of the AgNW films under current flow. Notably, the breaking up of AgNWs at junctions was observed at 9 V (**Figure 17a**), which might have been induced by rapid joule heating

stark contrast, after hybridization with SWCNTs, a new current pathway through the AgNW-SWCNT junction may be formed because of the relatively low contact resistance between the AgNW and SWCNT (R12 ≈ 10<sup>3</sup> Ω) when compared to R11, resulting in the formation of stable network films even at 15 V. Moreover, a very small work function difference between AgNW and UHP-SWCNTs, based on the Φ values of AgNW (4.1 eV) and UHP-SWCNTs (4.3 eV), can

**Figure 16.** (a) Preparation of AgNW/SWCNT solution by direct mixing of aqueous AgNW solution and UHPfunctionalized SWCNTs. (b) Optical transmission of the AgNW and AgNW/SWCNT hybrid films with *Rs* ≈ 20 ohm/sq. fabricated by spraying. Inset image shows the lighting of an LED lamp at 3 V on bendable AgNW/SWCNT hybrid film on a polycarbonate substrate. (c) Raman spectra of the QHB-SWCNT film prepared by paste and AgNW/UHP-SWCNT

hybrid films fabricated by mixture inks [22].

at the junctions because of the high junction resistance of the AgNWs (R<sup>11</sup> ≈ 10<sup>3</sup>

promote the current pathway through the AgNW-SWCNT junction (**Figure 18**).

value of ~20 Ω/sq. and *T* > 90% and was

Interfacial Engineering of Flexible Transparent Conducting Films

http://dx.doi.org/10.5772/intechopen.80259

–109 Ω). In

23

spray-coated AgNW/SWCNT hybrid film has an *Rs*

**Figure 14.** Tilted SEM images of SWCNT surfaces coated with (a) S1-GO nanosheets and (b) S4-GO nanosheets. Inset schemes show the structure of the GO-coated SWCNT networks. (c) Raman spectra of a pristine SWCNT film and films coated with S1, S2, S3, and S4 using a spray-coater 20 times. (d) Raman spectra of SWCNT films coated with S1-GO by increasing the number of coating layers from 5 to 30. Values in brackets in (c) and (d) indicate G+ band position. Scale bars in (a) and (b) are 300 nm [20].

**Figure 15.** (a) PEDOT:PSS solution drop images on a, b are SWCNT surface and on the GO-coated area (dotted area). (b) Schematic structure and (c) photo image of OPV cell. (d) Current density (J) versus voltage (V) characteristics of pristine SWCNTs and GO-modified SWCNT photovoltaic cells under 100 mW/cm2 AM 1.5G spectral illumination at various transmittance and *Rs* values [20].

temperatures, and the removal of surfactant molecules after the deposition of AgNWs or AgNW hybrid materials. Thus, we suggest that a small amount of SWCNTs can stabilize the AgNW networks under current flow without post-treatment. To realize this, the major challenge is the fabrication of a stable dispersion of SWCNTs in liquid medium without dispersant molecules that can be removed after deposition. To solve this issue, the SWCNTs were functionalized with quadruple hydrogen bonding (QHB) motifs of 2-ureido-4[1H]pyrimidinone (UHP) moieties through a previously reported sequential coupling reaction [21]. The AgNW/SWCNT mixture solution was easily prepared by direct mixing of the aqueous AgNW solution with a paste of SWCNTs functionalized with UHP (UHP-SWCNTs) by shaking, as shown in **Figure 16a**. The spray-coated AgNW/SWCNT hybrid film has an *Rs* value of ~20 Ω/sq. and *T* > 90% and was used to fabricate transparent film heaters to investigate the effect of SWCNTs on the electrical stability of the AgNW films under current flow. Notably, the breaking up of AgNWs at junctions was observed at 9 V (**Figure 17a**), which might have been induced by rapid joule heating at the junctions because of the high junction resistance of the AgNWs (R<sup>11</sup> ≈ 10<sup>3</sup> –109 Ω). In stark contrast, after hybridization with SWCNTs, a new current pathway through the AgNW-SWCNT junction may be formed because of the relatively low contact resistance between the AgNW and SWCNT (R12 ≈ 10<sup>3</sup> Ω) when compared to R11, resulting in the formation of stable network films even at 15 V. Moreover, a very small work function difference between AgNW and UHP-SWCNTs, based on the Φ values of AgNW (4.1 eV) and UHP-SWCNTs (4.3 eV), can promote the current pathway through the AgNW-SWCNT junction (**Figure 18**).

**Figure 14.** Tilted SEM images of SWCNT surfaces coated with (a) S1-GO nanosheets and (b) S4-GO nanosheets. Inset schemes show the structure of the GO-coated SWCNT networks. (c) Raman spectra of a pristine SWCNT film and films coated with S1, S2, S3, and S4 using a spray-coater 20 times. (d) Raman spectra of SWCNT films coated with S1-GO by increasing the number of coating layers from 5 to 30. Values in brackets in (c) and (d) indicate G+ band position. Scale

**Figure 15.** (a) PEDOT:PSS solution drop images on a, b are SWCNT surface and on the GO-coated area (dotted area). (b) Schematic structure and (c) photo image of OPV cell. (d) Current density (J) versus voltage (V) characteristics of pristine SWCNTs and GO-modified SWCNT photovoltaic cells under 100 mW/cm2 AM 1.5G spectral illumination at various

bars in (a) and (b) are 300 nm [20].

22 Transparent Conducting Films

transmittance and *Rs*

values [20].

**Figure 16.** (a) Preparation of AgNW/SWCNT solution by direct mixing of aqueous AgNW solution and UHPfunctionalized SWCNTs. (b) Optical transmission of the AgNW and AgNW/SWCNT hybrid films with *Rs* ≈ 20 ohm/sq. fabricated by spraying. Inset image shows the lighting of an LED lamp at 3 V on bendable AgNW/SWCNT hybrid film on a polycarbonate substrate. (c) Raman spectra of the QHB-SWCNT film prepared by paste and AgNW/UHP-SWCNT hybrid films fabricated by mixture inks [22].

**5. Summary**

We have briefly reviewed recent research progress on TCF technologies based on SWCNTs, AgNWs, and GO nanosheets via interfacial engineering and hybridization strategies. Onedimensional (1D) conducting nanomaterials such as CNTs and metal nanowires have been studied intensively because of their fascinating properties and offer tremendous potential for flexible opto-electronic applications in touch screen panels, flexible displays, solar cells, thin film heaters, signage, etc. To realize these applications, we need to develop high-performance TCFs with flexibility using a low-temperature process with scalable processing techniques on flexible plastic substrates. In this chapter, therefore, a scalable spray coating process using SWCNTs and AgNW solutions was introduced by demonstrating the wettability of the solution on surface energycontrolled substrates. One of the most important strategies for high-performance TCFs is interfacial engineering. Matching the interfacial tension between top-coating materials and the film is an important practical concept for fabrication of passivated TCFs that are environmentally stable at high humidity and temperature, as well as to improve their opto-electrical properties. Moreover, rational use of GO nanosheets and SWCNTs can improve AgNW network TCFs by welding in plastic substrates and efficient junction joining of AgNW junctions. Chemical or thermal welding of SWCNT networks is also useful for self-passivation of films on thermoplastic substrates.

Interfacial Engineering of Flexible Transparent Conducting Films

http://dx.doi.org/10.5772/intechopen.80259

25

In addition, recently developed AgNW/SWCNT hybrid TCF technologies can be commercially used to fabricate large area flexible TCFs by a roll-to-roll process because of fabrication

For large opto-electronic devices with flexibility and stretchability, there are still many challenging issues for commercial application, including newly designed anisotropic conducting

This work was supported by the Center for Advanced Soft-Electronics as Global Frontier Project (2014M3A6A5060953) funded by the Ministry of Science, ICT and Future Planning and by the Primary Research Program (18-12-N0101-18) of the Korea Electrotechnology Research Institute.

[1] Ye S, Rathmell AR, Chen Z, Stewart IE, Wiley BJ. Metal nanowire networks: The next generation of transparent conductors. Advanced Materials. 2014;**26**:6670-6687. DOI: 10.1002/

of coating solutions without additional dispersant molecules.

materials and their solution processing.

Joong Tark Han\* and Geon-Woong Lee

\*Address all correspondence to: jthan@keri.re.kr

Korea Electrotechnology Research Institute, Republic of Korea

**Acknowledgements**

**Author details**

**References**

adma.201402710

**Figure 17.** (a, b) Time-dependent temperature profiles of (a) AgNW and (b) AgNW/SWCNT hybrid films. The inset images are infrared thermal images of the film heaters. (c, d) Tilted SEM images of (c) AgNW and (d) AgNW/SWCNT hybrid films after heating at an input voltage of 9 V. (e) Schematic of AgNW/SWCNT hybrid networks showing possible current flow pathways (I, II). R1 and R2 indicate the resistivity of AgNWs and SWCNTs, respectively. R11 or R12 indicate the contact resistances between AgNWs or between AgNW and SWCNTs.

**Figure 18.** (a, b) Schematic diagram showing poor contact between AgNWs (a) and good contact between AgNW and UHP-SWCNTs. (c) Ultraviolet photoelectron spectroscopy spectra of AgNW, UHP-SWCNT, and thermally treated UHP-SWCNT films. (d) Schematic showing the reason for the current pathway through SWCNTs in terms of the work function.

## **5. Summary**

We have briefly reviewed recent research progress on TCF technologies based on SWCNTs, AgNWs, and GO nanosheets via interfacial engineering and hybridization strategies. Onedimensional (1D) conducting nanomaterials such as CNTs and metal nanowires have been studied intensively because of their fascinating properties and offer tremendous potential for flexible opto-electronic applications in touch screen panels, flexible displays, solar cells, thin film heaters, signage, etc. To realize these applications, we need to develop high-performance TCFs with flexibility using a low-temperature process with scalable processing techniques on flexible plastic substrates. In this chapter, therefore, a scalable spray coating process using SWCNTs and AgNW solutions was introduced by demonstrating the wettability of the solution on surface energycontrolled substrates. One of the most important strategies for high-performance TCFs is interfacial engineering. Matching the interfacial tension between top-coating materials and the film is an important practical concept for fabrication of passivated TCFs that are environmentally stable at high humidity and temperature, as well as to improve their opto-electrical properties. Moreover, rational use of GO nanosheets and SWCNTs can improve AgNW network TCFs by welding in plastic substrates and efficient junction joining of AgNW junctions. Chemical or thermal welding of SWCNT networks is also useful for self-passivation of films on thermoplastic substrates.

In addition, recently developed AgNW/SWCNT hybrid TCF technologies can be commercially used to fabricate large area flexible TCFs by a roll-to-roll process because of fabrication of coating solutions without additional dispersant molecules.

For large opto-electronic devices with flexibility and stretchability, there are still many challenging issues for commercial application, including newly designed anisotropic conducting materials and their solution processing.

## **Acknowledgements**

**Figure 17.** (a, b) Time-dependent temperature profiles of (a) AgNW and (b) AgNW/SWCNT hybrid films. The inset images are infrared thermal images of the film heaters. (c, d) Tilted SEM images of (c) AgNW and (d) AgNW/SWCNT hybrid films after heating at an input voltage of 9 V. (e) Schematic of AgNW/SWCNT hybrid networks showing possible current flow pathways (I, II). R1 and R2 indicate the resistivity of AgNWs and SWCNTs, respectively. R11 or R12 indicate

**Figure 18.** (a, b) Schematic diagram showing poor contact between AgNWs (a) and good contact between AgNW and UHP-SWCNTs. (c) Ultraviolet photoelectron spectroscopy spectra of AgNW, UHP-SWCNT, and thermally treated UHP-SWCNT films. (d) Schematic showing the reason for the current pathway through SWCNTs in terms of the work function.

the contact resistances between AgNWs or between AgNW and SWCNTs.

24 Transparent Conducting Films

This work was supported by the Center for Advanced Soft-Electronics as Global Frontier Project (2014M3A6A5060953) funded by the Ministry of Science, ICT and Future Planning and by the Primary Research Program (18-12-N0101-18) of the Korea Electrotechnology Research Institute.

## **Author details**

Joong Tark Han\* and Geon-Woong Lee

\*Address all correspondence to: jthan@keri.re.kr

Korea Electrotechnology Research Institute, Republic of Korea

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**Section 3**

**A Facile Fabrication Criteria of Carbon Nanotube**

**for Transparent Conducting Films Application**

**A Facile Fabrication Criteria of Carbon Nanotube for Transparent Conducting Films Application**

**Chapter 3**

**Provisional chapter**

**Transparent Conducting Thin Film Preparation of**

**Transparent Conducting Thin Film Preparation of** 

DOI: 10.5772/intechopen.79164

Transparent conducting films have a wide range of applications in the fields of flat panel displays, solar cells, and touch panels for their both good conductivity and light transmittance. Carbon nanotubes (CNTs) transparent conducting film has become a potential alternative for next-generation transparent conducting film systems owing to high conductivity, light transmittance and flexibility. The multiwalled carbon nanotubes (MWCNTs) conductive liquid was prepared by dispersing MWCNTs in alcohol through ultrasonic and high-speed shearing process with an addition of carbon nanotube alcohol dispersant (TNADIS) as the dispersant. The transparent conducting film was fabricated on polyethylene terephthalate (PET) transparent film by spin-coating process. The film was used as interlayer between the electrode and the separator to improve electrochemi-

**Keywords:** multiwalled carbon nanotubes, transparent conducting film, lithium-sulfur

Carbon nanotubes have been the hotpot of scientific research ever since their discovery. Due to their unique structure, carbon nanotubes (CNTs) have shown outstanding performance in electromagnetics, mechanics, heat and optics [1–5], which have made them attractive in lithium ion batteries, supercapacitors, composite materials and many other aspects [2–6]. At present, carbon

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

distribution, and reproduction in any medium, provided the original work is properly cited.

**Carbon Nanotube**

**Carbon Nanotube**

Guodong Liang

Guodong Liang

**Abstract**

Xiaogang Sun, Jie Wang, Wei Chen, Xu Li, Manyuan Cai, Long Chen, Zhiwen Qiu,

Xiaogang Sun, Jie Wang, Wei Chen, Xu Li, Manyuan Cai, Long Chen, Zhiwen Qiu,

Yapan Huang, Chengcheng Wei, Hao Hu and

Yapan Huang, Chengcheng Wei, Hao Hu and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

cal performance of lithium-sulfur (Li-S) batteries.

batteries, transmittance

**1. Introduction**

http://dx.doi.org/10.5772/intechopen.79164

### **Transparent Conducting Thin Film Preparation of Carbon Nanotube Transparent Conducting Thin Film Preparation of Carbon Nanotube**

DOI: 10.5772/intechopen.79164

Xiaogang Sun, Jie Wang, Wei Chen, Xu Li, Manyuan Cai, Long Chen, Zhiwen Qiu, Yapan Huang, Chengcheng Wei, Hao Hu and Guodong Liang Xiaogang Sun, Jie Wang, Wei Chen, Xu Li, Manyuan Cai, Long Chen, Zhiwen Qiu, Yapan Huang, Chengcheng Wei, Hao Hu and Guodong Liang

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.79164

## **Abstract**

Transparent conducting films have a wide range of applications in the fields of flat panel displays, solar cells, and touch panels for their both good conductivity and light transmittance. Carbon nanotubes (CNTs) transparent conducting film has become a potential alternative for next-generation transparent conducting film systems owing to high conductivity, light transmittance and flexibility. The multiwalled carbon nanotubes (MWCNTs) conductive liquid was prepared by dispersing MWCNTs in alcohol through ultrasonic and high-speed shearing process with an addition of carbon nanotube alcohol dispersant (TNADIS) as the dispersant. The transparent conducting film was fabricated on polyethylene terephthalate (PET) transparent film by spin-coating process. The film was used as interlayer between the electrode and the separator to improve electrochemical performance of lithium-sulfur (Li-S) batteries.

**Keywords:** multiwalled carbon nanotubes, transparent conducting film, lithium-sulfur batteries, transmittance

## **1. Introduction**

Carbon nanotubes have been the hotpot of scientific research ever since their discovery. Due to their unique structure, carbon nanotubes (CNTs) have shown outstanding performance in electromagnetics, mechanics, heat and optics [1–5], which have made them attractive in lithium ion batteries, supercapacitors, composite materials and many other aspects [2–6]. At present, carbon

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

nanotubes have been produced in large scale. However, the carbon nanotubes entangled with each other and shown severe agglomeration effect [7, 8]. The carbon nanotubes are nanoscale materials and the specific surface area is large, the surface energy is high, and there is a great Van der Waals force between the carbon nanotubes [9–15]. In addition, the carbon nanotubes exhibited a structure of one-dimensional tubular and the aspect ratio is relatively large, they have similar interlocking characteristics to fibers, which result in easy agglomeration of carbon nanotubes. In order to solve the technical problem and obtain a stable carbon nanotube dispersion, many dispersion methods are introduced to prepare the dispersion liquid of the carbon nanotube. Physical dispersion methods include grinding, ball milling, ultrasonic, and highspeed shear. The chemical dispersion methods include strong acid and alkali treatment and the addition of dispersant [16–18]. Each dispersion method has its own advantages but all have some drawbacks that make it difficult to produce a very stable dispersion of carbon nanotubes.

Transparent conductive films are widely used in the fields of flat panel displays, solar cells, touch panels owing to good electrical conductivity and light transmission. Currently, many transparent conductive films are studied: metal film, n-type transparent conductive oxide film, p-type transparent conductive oxide film, special film system (TiN conductive film, etc.) and multilayer film system. Carbon nanotubes (CNTs) have also become the focus of research due to their good properties in conductivity, light transmission and flexibility. Therefore, the CNTs transparent conducting films have also become the focus of research. The dispersibility of carbon nanotubes has an important influence on the quality of the film of the conductivity as well as transparency. In this chapter, MWCNTs ethanol conductive liquid was prepared by ultrasonic vibration and high-speed shearing process. The MWCNTs transparent conductive film was prepared by spin-coating.

hydrogen as carrier gas. The reaction temperature was around 1200°C. The raw MWCNTs

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The SEM image of multiwalled carbon nanotube was shown in **Figure 1(a)**, it was observed that the MWCNTs have a one-dimensional tubular structure and are not entangled with each other. This suggested that the line MWCNTs can be easily dispersed in various matrix. The MWCNTs have also excellent mechanical and physicochemical properties. The TEM (**Figure 1(b)**) image shows that MWCNTs have one-dimensional tubular structure and the

The raw multiwalled carbon nanotubes (R-MWCNTs) and graphitized multiwalled carbon nanotubes (G-MWCNTs) were, respectively, ball-milled in a ball mill for 2 h. The TNADIS dispersant was dissolved in anhydrous ethanol and its mass concentration was 0, 0.025, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, and 1%, respectively. Then, two types of MWCNTs were, respectively, added to the above solution with a concentration of 1 wt.%. The dispersion liquid of MWCNTs was prepared by ultrasonically dispersing for 30 min and high-speed shearing for 1 h. The

Raman spectroscopic analysis, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to analyze and observe the morphology and structure of MWCNTs. The optimal addition ratio of dispersant was determined by measuring the precipitation after centrifugation. In addition, the stability of the conductive liquid was observed and analyzed. The stability of the conductive liquid of MWCNTs was characterized by detect-

R-MWCHTs are marked as 1#-8# and the G-MWCNTs are marked as 9–16#.

ing the Tyndall effect after the conductive liquid was left standing for 5 months.

were treated at a high temperature of 3000°C for graphitization.

carbon atoms are arranged in a regular and orderly manner.

**3. MWCNTs transparent conducting film**

*3.1.1. Preparation of MWCNTs conductive liquid*

**Figure 1.** (a) SEM and (b) TEM images of MWCNT.

**3.1. Experiment**

*3.1.2. Performance testing*

The high theoretical capacity of 1675 mAh/g and high energy density of 2600 Wh/kg, lithium− sulfur (Li-S) batteries have become the most promising alternatives for next-generation electrochemical energy storage systems [19–22]. In addition, abundant resources, low cost and ecofriendliness of sulfur make Li-S batteries have higher commercial value [23–25]. However, the actual capacity of the current lithium-sulfur battery is greatly lower than the theoretical capacity and the cycle life is poor, which seriously hampered the practical application of lithium-sulfur battery [26]. The main reason is that the diffusion and dissolution of intermediate lithium polysulfides during cycling (Li<sup>2</sup> Sn, 4 ≤ n ≤ 8) which led to notorious shuttle effects. This resulted in high self-discharge, active material loss and low Coulombic efficiency [27–31]. In this work, we report multiwalled carbon nanotubes paper (MWCNTsP) as a current collector, MWCNTs transparent conductive film was used as the interlayer between the positive electrode and the separator [32, 33]. The new structure of Li-S battery retarded the dissolution and dispersion of lithium polysulfides (LPSs). The Li-S batteries with MWCNTs transparent conductive film showed high discharge capacity, excellent cycle stability and high sulfur loading.

## **2. Fabrication and characterization of MWCNTs**

MWCNTs were synthesized by chemical vapor deposition with benzene being used as carbon feedstock, ferrocene as a catalyst precursor, thiophene as growth promotion agent, and

**Figure 1.** (a) SEM and (b) TEM images of MWCNT.

hydrogen as carrier gas. The reaction temperature was around 1200°C. The raw MWCNTs were treated at a high temperature of 3000°C for graphitization.

The SEM image of multiwalled carbon nanotube was shown in **Figure 1(a)**, it was observed that the MWCNTs have a one-dimensional tubular structure and are not entangled with each other. This suggested that the line MWCNTs can be easily dispersed in various matrix. The MWCNTs have also excellent mechanical and physicochemical properties. The TEM (**Figure 1(b)**) image shows that MWCNTs have one-dimensional tubular structure and the carbon atoms are arranged in a regular and orderly manner.

## **3. MWCNTs transparent conducting film**

## **3.1. Experiment**

nanotubes have been produced in large scale. However, the carbon nanotubes entangled with each other and shown severe agglomeration effect [7, 8]. The carbon nanotubes are nanoscale materials and the specific surface area is large, the surface energy is high, and there is a great Van der Waals force between the carbon nanotubes [9–15]. In addition, the carbon nanotubes exhibited a structure of one-dimensional tubular and the aspect ratio is relatively large, they have similar interlocking characteristics to fibers, which result in easy agglomeration of carbon nanotubes. In order to solve the technical problem and obtain a stable carbon nanotube dispersion, many dispersion methods are introduced to prepare the dispersion liquid of the carbon nanotube. Physical dispersion methods include grinding, ball milling, ultrasonic, and highspeed shear. The chemical dispersion methods include strong acid and alkali treatment and the addition of dispersant [16–18]. Each dispersion method has its own advantages but all have some drawbacks that make it difficult to produce a very stable dispersion of carbon nanotubes.

Transparent conductive films are widely used in the fields of flat panel displays, solar cells, touch panels owing to good electrical conductivity and light transmission. Currently, many transparent conductive films are studied: metal film, n-type transparent conductive oxide film, p-type transparent conductive oxide film, special film system (TiN conductive film, etc.) and multilayer film system. Carbon nanotubes (CNTs) have also become the focus of research due to their good properties in conductivity, light transmission and flexibility. Therefore, the CNTs transparent conducting films have also become the focus of research. The dispersibility of carbon nanotubes has an important influence on the quality of the film of the conductivity as well as transparency. In this chapter, MWCNTs ethanol conductive liquid was prepared by ultrasonic vibration and high-speed shearing process. The MWCNTs transparent conductive

The high theoretical capacity of 1675 mAh/g and high energy density of 2600 Wh/kg, lithium− sulfur (Li-S) batteries have become the most promising alternatives for next-generation electrochemical energy storage systems [19–22]. In addition, abundant resources, low cost and ecofriendliness of sulfur make Li-S batteries have higher commercial value [23–25]. However, the actual capacity of the current lithium-sulfur battery is greatly lower than the theoretical capacity and the cycle life is poor, which seriously hampered the practical application of lithium-sulfur battery [26]. The main reason is that the diffusion and dissolution of inter-

effects. This resulted in high self-discharge, active material loss and low Coulombic efficiency [27–31]. In this work, we report multiwalled carbon nanotubes paper (MWCNTsP) as a current collector, MWCNTs transparent conductive film was used as the interlayer between the positive electrode and the separator [32, 33]. The new structure of Li-S battery retarded the dissolution and dispersion of lithium polysulfides (LPSs). The Li-S batteries with MWCNTs transparent conductive film showed high discharge capacity, excellent cycle stability and

MWCNTs were synthesized by chemical vapor deposition with benzene being used as carbon feedstock, ferrocene as a catalyst precursor, thiophene as growth promotion agent, and

Sn, 4 ≤ n ≤ 8) which led to notorious shuttle

film was prepared by spin-coating.

32 Transparent Conducting Films

high sulfur loading.

mediate lithium polysulfides during cycling (Li<sup>2</sup>

**2. Fabrication and characterization of MWCNTs**

## *3.1.1. Preparation of MWCNTs conductive liquid*

The raw multiwalled carbon nanotubes (R-MWCNTs) and graphitized multiwalled carbon nanotubes (G-MWCNTs) were, respectively, ball-milled in a ball mill for 2 h. The TNADIS dispersant was dissolved in anhydrous ethanol and its mass concentration was 0, 0.025, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, and 1%, respectively. Then, two types of MWCNTs were, respectively, added to the above solution with a concentration of 1 wt.%. The dispersion liquid of MWCNTs was prepared by ultrasonically dispersing for 30 min and high-speed shearing for 1 h. The R-MWCHTs are marked as 1#-8# and the G-MWCNTs are marked as 9–16#.

## *3.1.2. Performance testing*

Raman spectroscopic analysis, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to analyze and observe the morphology and structure of MWCNTs. The optimal addition ratio of dispersant was determined by measuring the precipitation after centrifugation. In addition, the stability of the conductive liquid was observed and analyzed. The stability of the conductive liquid of MWCNTs was characterized by detecting the Tyndall effect after the conductive liquid was left standing for 5 months.

## **3.2. Results and discussion**

## *3.2.1. Raman spectroscopy and TEM, SEM analysis*

The Raman spectra of MWCNTs was shown in **Figure 2**, it can be used to analyze the crystallinity of MWCNTs. The relative intensity of D and G peaks (IG/ID) can reflect the degree of crystallization of MWCNTs samples. G-MWCNTs have a much higher IG/ID of 4.16 than R-MWCNTs (IG/ID = 0.67). The G peak at 1585 cm−1 is called the tangential stretching mode of the MWCNTs, which is a reflection of the degree of order, and similar peaks are observed in the graphite. The D peak at 1334 cm−1 is a reflection of the defect and disorder degree in the MWCNTs, and the D peak of the MWCNTs originates from the structural defects of the carbon nanotubes. Both the D peak after graphitization and the 2D peak at 2656 cm−1 were observed due to the double resonance process of the two resonant electron states of the MWCNTs.

The TEM image of raw MWCNTs was shown in **Figure 3(a)**, and it was observed that the arrangement of carbon atoms on the surface of R-MWCNTs is disordered. It indicated there are a lot of amorphous carbons and defects on surface of R-MWCNTs. After MWCNTs were graphitized at 3000°C, the carbon atoms shown a regular and orderly manner (**Figure 3(b)**).The G-MWCNTs have a high degree of crystallinity which is consistent with the results obtained by Raman spectroscopy. **Figure 2(c)** and **(d)** shows the SEM images before and after graphitization of MWCNTs, respectively. It can be seen that the MWCNTs used in the experiment are linear whisker carbon nanotubes. However, G-MWCNTs and R-MWCNTs are agglomerated before being dispersed due to van der Waals forces. From the SEM images, it can be seen that the graphitized carbon nanotubes have less impurities and higher purity.

## *3.2.2. Effect of dispersant TNADIS content on dispersion of MWCNTs*

The precipitation mass versus dispersant content was exhibited (**Figure 4**), which depicts a linear reduce with increasing dispersant content until the minimum precipitation mass is achieved. Hereafter the sediment increased with increasing dispersant. The minimum

precipitation quality is around 0.0943 g with a dispersant of 0.05 wt.%. Van der Waals attraction between MWCNTs causes MWCNTs flocculation resulting in the formation of the aggregates and precipitation. The centrifugation treatment accelerated the sediment. The dispersant

**Figure 4.** The precipitation mass of MWCNTs conducting liquids after centrifuged 10 min.

**Figure 3.** The TEM of R-MWCNTs (a), G-MWCNTs (b), the SEM of R-MWCNTs (c), and G-MWCNTs (d).

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**Figure 2.** The Raman spectra of graphitization and raw MWCNTs.

**3.2. Results and discussion**

34 Transparent Conducting Films

and higher purity.

*3.2.1. Raman spectroscopy and TEM, SEM analysis*

The Raman spectra of MWCNTs was shown in **Figure 2**, it can be used to analyze the crystallinity of MWCNTs. The relative intensity of D and G peaks (IG/ID) can reflect the degree of crystallization of MWCNTs samples. G-MWCNTs have a much higher IG/ID of 4.16 than R-MWCNTs (IG/ID = 0.67). The G peak at 1585 cm−1 is called the tangential stretching mode of the MWCNTs, which is a reflection of the degree of order, and similar peaks are observed in the graphite. The D peak at 1334 cm−1 is a reflection of the defect and disorder degree in the MWCNTs, and the D peak of the MWCNTs originates from the structural defects of the carbon nanotubes. Both the D peak after graphitization and the 2D peak at 2656 cm−1 were observed due to the double resonance process of the two resonant electron states of the MWCNTs.

The TEM image of raw MWCNTs was shown in **Figure 3(a)**, and it was observed that the arrangement of carbon atoms on the surface of R-MWCNTs is disordered. It indicated there are a lot of amorphous carbons and defects on surface of R-MWCNTs. After MWCNTs were graphitized at 3000°C, the carbon atoms shown a regular and orderly manner (**Figure 3(b)**).The G-MWCNTs have a high degree of crystallinity which is consistent with the results obtained by Raman spectroscopy. **Figure 2(c)** and **(d)** shows the SEM images before and after graphitization of MWCNTs, respectively. It can be seen that the MWCNTs used in the experiment are linear whisker carbon nanotubes. However, G-MWCNTs and R-MWCNTs are agglomerated before being dispersed due to van der Waals forces. From the SEM images, it can be seen that the graphitized carbon nanotubes have less impurities

The precipitation mass versus dispersant content was exhibited (**Figure 4**), which depicts a linear reduce with increasing dispersant content until the minimum precipitation mass is achieved. Hereafter the sediment increased with increasing dispersant. The minimum

*3.2.2. Effect of dispersant TNADIS content on dispersion of MWCNTs*

**Figure 2.** The Raman spectra of graphitization and raw MWCNTs.

**Figure 3.** The TEM of R-MWCNTs (a), G-MWCNTs (b), the SEM of R-MWCNTs (c), and G-MWCNTs (d).

**Figure 4.** The precipitation mass of MWCNTs conducting liquids after centrifuged 10 min.

precipitation quality is around 0.0943 g with a dispersant of 0.05 wt.%. Van der Waals attraction between MWCNTs causes MWCNTs flocculation resulting in the formation of the aggregates and precipitation. The centrifugation treatment accelerated the sediment. The dispersant can disperse the MWCNTs aggregates and form stable dispersion liquid of ethanol. When the dispersant content exceeded a critical value, the micelles of the dispersant are formed and resulted in the aggregation of MWCNTs and more precipitates.

In addition, the precipitation produced by G-MWCNTs conducting liquid after centrifugation is higher than that of R-MWCNTs conductive liquid as shown in **Figure 4**. This is owing to that R-MWCNTs absorbed the dispersant molecules on the surface which checked the aggregation of R-MWCNTs. However, graphitization eliminated the surface defects of G-MWCNTs, which reduces the adsorption of dispersant molecules on the surface. Therefore, the conductive liquid of G-MWCNTs is easier to precipitate than R-MWCNTs after centrifugation treatment.

## *3.2.3. Observation of MWCNTs conductive liquid*

After 5 days of standing of R-MWCNTs dispersion, only the 1# shown obvious delamination and sediment phenomenon and other samples maintained unchanging (**Figure 5(a)**). **Figure 5(b)** shows the conductive liquid of G-MWCNTs. The 9#(0% dispersant), 13#(0.4%), 14#(0.6%), 15#(0.8%) and 16#(0.1%) demonstrated obvious delamination and sediment. The 10#, 11# and 12# hold unchanging. The results showed that the stability of conductive liquid of R-MWCNTs was better than that of G-MWCNTs. This is attributed that R-MWCNTs have a lot of surface defects and absorbed much functional groups as OH. In addition, the stability of dispersion with different dispersant TNADIS mass fractions was analyzed (9#, 13#, 14#, 15#, and 16#). The results demonstrated the aggregation and sediment were enhanced with increasing dispersant which surpass the critical concentration.

MWCNTs conductive liquid (3#) with 0.05 wt.% TNADIS showed excellent stability and no obvious sedimentation was observed after resting at room temperature for 5 months. When a beam of light passes through the colloid, a bright path in the colloid can be observed from the direction of the incident light. This phenomenon is called the Tyndall effect. The Tyndall effect of the conductive liquid of R-MWCNTs was examined before and after standing for 5 months. The optical path can be seen in the conductive liquid (**Figure 6**). It is shown that the conductive liquid of R-MWCNTs has a good stability and still remained colloidal properties after standing for 5 months.

*3.2.4. Preparation and properties of MWCNTs transparent conductive film*

**Figure 7.** The R-MWCNTs (a, b, c) and G-MWCNTs (d, e, f) transparent conducting film.

The preparation method of MWCNTs transparent conductive films has high requirements on the dispersion properties of MWCNTs. The dispersion properties have a great influence on the film quality, electrical conductivity and transparency [34–41]. The 3#R-MWCNTs

**Figure 6.** The Tyndall effect of R-MWCNTs conductive liquid before (a) and after (b) resting for 5 months.

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**Figure 5.** The raw (a) and graphitization (b) MWCNTs conducting liquid of stewing 5 days.

**Figure 6.** The Tyndall effect of R-MWCNTs conductive liquid before (a) and after (b) resting for 5 months.

can disperse the MWCNTs aggregates and form stable dispersion liquid of ethanol. When the dispersant content exceeded a critical value, the micelles of the dispersant are formed and

In addition, the precipitation produced by G-MWCNTs conducting liquid after centrifugation is higher than that of R-MWCNTs conductive liquid as shown in **Figure 4**. This is owing to that R-MWCNTs absorbed the dispersant molecules on the surface which checked the aggregation of R-MWCNTs. However, graphitization eliminated the surface defects of G-MWCNTs, which reduces the adsorption of dispersant molecules on the surface. Therefore, the conductive liquid of G-MWCNTs is easier to precipitate than R-MWCNTs after centrifuga-

After 5 days of standing of R-MWCNTs dispersion, only the 1# shown obvious delamination and sediment phenomenon and other samples maintained unchanging (**Figure 5(a)**). **Figure 5(b)** shows the conductive liquid of G-MWCNTs. The 9#(0% dispersant), 13#(0.4%), 14#(0.6%), 15#(0.8%) and 16#(0.1%) demonstrated obvious delamination and sediment. The 10#, 11# and 12# hold unchanging. The results showed that the stability of conductive liquid of R-MWCNTs was better than that of G-MWCNTs. This is attributed that R-MWCNTs have a lot of surface defects and absorbed much functional groups as OH. In addition, the stability of dispersion with different dispersant TNADIS mass fractions was analyzed (9#, 13#, 14#, 15#, and 16#). The results demonstrated the aggregation and sediment were enhanced with

MWCNTs conductive liquid (3#) with 0.05 wt.% TNADIS showed excellent stability and no obvious sedimentation was observed after resting at room temperature for 5 months. When a beam of light passes through the colloid, a bright path in the colloid can be observed from the direction of the incident light. This phenomenon is called the Tyndall effect. The Tyndall effect of the conductive liquid of R-MWCNTs was examined before and after standing for 5 months. The optical path can be seen in the conductive liquid (**Figure 6**). It is shown that the conductive liquid of R-MWCNTs has a good stability and still remained colloidal properties after standing

resulted in the aggregation of MWCNTs and more precipitates.

increasing dispersant which surpass the critical concentration.

**Figure 5.** The raw (a) and graphitization (b) MWCNTs conducting liquid of stewing 5 days.

*3.2.3. Observation of MWCNTs conductive liquid*

tion treatment.

36 Transparent Conducting Films

for 5 months.

**Figure 7.** The R-MWCNTs (a, b, c) and G-MWCNTs (d, e, f) transparent conducting film.

## *3.2.4. Preparation and properties of MWCNTs transparent conductive film*

The preparation method of MWCNTs transparent conductive films has high requirements on the dispersion properties of MWCNTs. The dispersion properties have a great influence on the film quality, electrical conductivity and transparency [34–41]. The 3#R-MWCNTs


conductive liquid and the 11#G-MWCNTs conductive liquid were applied onto the PET transparent film by spin-coating. The content of MWCNTs on the transparent film was controlled by controlling the number of spin-coating. After being spin-coated for once, twice, and thrice, respectively, the films were dried in vacuum drying oven. The square resistance of the dried MWCNTs transparent conductive film was measured with an ST-2258C multi-

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**Figure 7** shows spin-coated MWCNTs transparent conductive film. The transmittance of the

Compared with the resistance and transmittance of the R-MWCNTs and G-MWCNTs transparent conductive films, it was found that the transmittance of conductive films with the same number of spin-coating is almost the same. But, the conductivity of G-MWCNTs transparent conductive films was significantly better than that of R-MWCNTs as shown in **Table 1**. The main reason was that the G-MWCNTs obtained very high crystallinity and purity after high temperatures treatment of 3000°C. This resulted in higher electrical conductivity of G-MWCNTs transparent conductive films. In addition, as the number of spincoating increases, the light transmittance of films gradually decreases and the conductivity

Compared with the SEM image (**Figure 8**) of the conductive films, it was found that as the spin-on time increases, the MWCNTs gradually formed a continuous and dense mesh. The electronic transmission path was constructed, and the electrical conductivity was improved. After the spin-coating of G-MWCNTs conductive liquid for three times, the square resistance

The MWCNT powder was dispersed in distilled water by sonication for 1 h and followed by high-speed shearing for 1 h with an addition of sodium dodecyl sulfate (SDS) as a surfactant. The cellulose fibers were prepared by smashing recycled papers in distilled water by highspeed shearing for 1 h. The MWCNT dispersion liquid and the cellulose fibers were mixed by high-shear emulsifier to form suspension for 2 h. The MWCNTs paper (MWCNTsP) was obtained by vacuum filtration through the suspension liquid of cellulose and MWCNTs. The

Sulfur, MWCNTs and carbon black (CB) were mixed by balling for 1 h at 200 r/min. The slurry of sulfur was prepared by balling process with N-methyl-2-pyrrolidone (NMP) as solution

MWCNTsP was rolled and tailored as current collector to host sulfur for cathodes.

**4. MWCNTs transparent conducting film as interlayer for Li-S** 

film gradually decreases with increasing the number of spin-coating.

function digital four-probe tester.

of the conductive film was 0.34 kΩ/sq.

*4.1.1. Preparation of MWCNT paper collector*

*4.1.2. Preparation of sulfur electrodes*

increased.

**batteries**

**4.1. Experiment**

**Table 1.** The square resistant and transmittance of MWCNTs transparent conducting film.

**Figure 8.** The SEM MWCNTs transparent conducting film. (a, b, c) R-MWCNTs spin-coating for once, twice, and thrice. (d, e, f) G-MWCNTs spin-coating for once, twice, and thrice.

conductive liquid and the 11#G-MWCNTs conductive liquid were applied onto the PET transparent film by spin-coating. The content of MWCNTs on the transparent film was controlled by controlling the number of spin-coating. After being spin-coated for once, twice, and thrice, respectively, the films were dried in vacuum drying oven. The square resistance of the dried MWCNTs transparent conductive film was measured with an ST-2258C multifunction digital four-probe tester.

**Figure 7** shows spin-coated MWCNTs transparent conductive film. The transmittance of the film gradually decreases with increasing the number of spin-coating.

Compared with the resistance and transmittance of the R-MWCNTs and G-MWCNTs transparent conductive films, it was found that the transmittance of conductive films with the same number of spin-coating is almost the same. But, the conductivity of G-MWCNTs transparent conductive films was significantly better than that of R-MWCNTs as shown in **Table 1**. The main reason was that the G-MWCNTs obtained very high crystallinity and purity after high temperatures treatment of 3000°C. This resulted in higher electrical conductivity of G-MWCNTs transparent conductive films. In addition, as the number of spincoating increases, the light transmittance of films gradually decreases and the conductivity increased.

Compared with the SEM image (**Figure 8**) of the conductive films, it was found that as the spin-on time increases, the MWCNTs gradually formed a continuous and dense mesh. The electronic transmission path was constructed, and the electrical conductivity was improved. After the spin-coating of G-MWCNTs conductive liquid for three times, the square resistance of the conductive film was 0.34 kΩ/sq.

## **4. MWCNTs transparent conducting film as interlayer for Li-S batteries**

## **4.1. Experiment**

**Figure 8.** The SEM MWCNTs transparent conducting film. (a, b, c) R-MWCNTs spin-coating for once, twice, and thrice.

**The number of spin-coating One Twice Three** R-MWCNTs Square resistance kΩ/sq 103.3 10.6 3.7

G-MWCNTs Square resistance kΩ/sq 53.6 2.8 0.34

**Table 1.** The square resistant and transmittance of MWCNTs transparent conducting film.

38 Transparent Conducting Films

Transmittance (%) 68.3 57.9 52.8

Transmittance (%) 68.9 58.1 53.3

(d, e, f) G-MWCNTs spin-coating for once, twice, and thrice.

## *4.1.1. Preparation of MWCNT paper collector*

The MWCNT powder was dispersed in distilled water by sonication for 1 h and followed by high-speed shearing for 1 h with an addition of sodium dodecyl sulfate (SDS) as a surfactant. The cellulose fibers were prepared by smashing recycled papers in distilled water by highspeed shearing for 1 h. The MWCNT dispersion liquid and the cellulose fibers were mixed by high-shear emulsifier to form suspension for 2 h. The MWCNTs paper (MWCNTsP) was obtained by vacuum filtration through the suspension liquid of cellulose and MWCNTs. The MWCNTsP was rolled and tailored as current collector to host sulfur for cathodes.

## *4.1.2. Preparation of sulfur electrodes*

Sulfur, MWCNTs and carbon black (CB) were mixed by balling for 1 h at 200 r/min. The slurry of sulfur was prepared by balling process with N-methyl-2-pyrrolidone (NMP) as solution and PVDF as binder. The ratio of S∶MWCNT∶CB∶PVDF = 60∶15∶15∶10. The blend slurry was coated on to porous MWCNTsP. Then the sulfur electrodes (S-MWCNTsP) were dried at 60°C under vacuum for 12 h.

dispersed evenly and well-connected, which should improve the conductivity of the electrodes and trapped more active material in its micropores toward the cathode side than that toward the anode side. The excellent electrolyte immersion and active material encapsulation also confirm the intimate connection between the insulating active material and the conduc-

It can be indicated from **Figure 10** that the TCF@S-MWCNTsP electrode had a sulfur content of around 14 wt.% according to the main weight loss at T1 interval, which was attributed to sulfur sublimation. The weight loss at T2 interval which was attributed to carbonization of paper fibers. The weight of the electrode is 26 mg and is the average mass of the pole piece, and this can be calculated as the areal mass loading of sulfur in the

Cyclic voltammetry (CV) plots of the TCF@S-MWCNTsP electrode for the initial three cycles are shown in **Figure 11(a)**, recorded at a slow scan rate of 0.1 mV/s between 1.6 and 2.8 V. In the first electrode scan, two characteristic reduction peaks at 2.29 and 1.99 V can be observed,

two oxidation peaks are observed at 2.43 and 2.47 V, which owing to the conversion of

cycle CV curve is highly similar to the second-cycle curve, thus it can be expected that the TCF@S-MWCNTsP electrode will show favorable cycling stability and high reversibility

S2 /Li<sup>2</sup> ) to long-chain PSs (Li<sup>2</sup>

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S, respectively. In the anodic sweep,

Sn,4 ≤ n ≤ 8)

[24, 42, 43].The third-

tive matrix.

cathode is 2.4 mg/cm<sup>2</sup>

*4.2.2. GTG of the TCF@S-MWCNTsP electrode*

.

corresponding to the reduction of elemental sulfur (S<sup>8</sup>

short-chain to long-chain PSs, and the subsequent oxidization to S<sup>8</sup>

and the subsequent formation of short-chain Li<sup>2</sup>

**Figure 10.** TG curve of the TCF@S-MWCNTsP electrode.

*4.2.3. Cyclic voltammetric characteristics*

The conductive liquid of MWCNTs was prepared by ultrasonication and high-shear process with NMP as solution and PVDF as binder. The ratio of MWCNTs∶CB∶PVDF = 60∶30∶10. Subsequently followed by overlaying the conductive liquid onto S-MWCNTsP electrodes, the obtained electrode with MWCNTs transparent conducting film (TCF@S-MWCNTsP).

## *4.1.3. Assembling of cell and electrochemical measurements*

The tailored S-MWCNTsP and TCF@S-MWCNTsP electrodes were, respectively, used as working electrodes. Lithium foil was used as the counter electrode and Celgard 2300 was used as the separator. The solution of 1.0 M LiTFSI in DOL:DME (1,1, vol.) with 1.0%LiNO3 was utilized as the electrolyte. CR2025 coin-type cells were assembled in an Ar filled glove box (MBRAUN LABSTAR, Germany). The electrochemical characterization of the cells was measured by a cell tester (CT-4008-5V5mA-164). The galvanostatic charge-discharge current density was set at 0.2 to 5C. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) within a potential window of 1.6–2.8 V by an electrochemical workstation (CHI 660B) were measured.

## **4.2. Results and discussion**

## *4.2.1. The SEM of MWCNTsP and TCF@S-MWCNTsP electrode*

Top surface SEM of the MWCNTsP in **Figure 9(a)** displays its porous matrix and the coalescing fiber network. The MWCNTsP demonstrated homogenous incorporation of MWCNTs in the cellulose fiber network. Porous structure can effectively improve the carrying capacity of sulfur, and adsorbed PSs. The superior electrolyte absorbability and hierarchical open channel of MWCNTsP can store more sulfur and contributes to stabilize electrochemical reactions and anchored PSs within the MWCNTsP effectively and suppressing shuttle effects. The SEM image of TCF@S-MWCNTsP electrode in **Figure 9(b)** also shows that MWCNTs and S were

**Figure 9.** The SEM image of MWCNTsP (a) and TCF@S-MWCNTsP electrode (b).

dispersed evenly and well-connected, which should improve the conductivity of the electrodes and trapped more active material in its micropores toward the cathode side than that toward the anode side. The excellent electrolyte immersion and active material encapsulation also confirm the intimate connection between the insulating active material and the conductive matrix.

## *4.2.2. GTG of the TCF@S-MWCNTsP electrode*

It can be indicated from **Figure 10** that the TCF@S-MWCNTsP electrode had a sulfur content of around 14 wt.% according to the main weight loss at T1 interval, which was attributed to sulfur sublimation. The weight loss at T2 interval which was attributed to carbonization of paper fibers. The weight of the electrode is 26 mg and is the average mass of the pole piece, and this can be calculated as the areal mass loading of sulfur in the cathode is 2.4 mg/cm<sup>2</sup> .

## *4.2.3. Cyclic voltammetric characteristics*

Cyclic voltammetry (CV) plots of the TCF@S-MWCNTsP electrode for the initial three cycles are shown in **Figure 11(a)**, recorded at a slow scan rate of 0.1 mV/s between 1.6 and 2.8 V. In the first electrode scan, two characteristic reduction peaks at 2.29 and 1.99 V can be observed, corresponding to the reduction of elemental sulfur (S<sup>8</sup> ) to long-chain PSs (Li<sup>2</sup> Sn,4 ≤ n ≤ 8) and the subsequent formation of short-chain Li<sup>2</sup> S2 /Li<sup>2</sup> S, respectively. In the anodic sweep, two oxidation peaks are observed at 2.43 and 2.47 V, which owing to the conversion of short-chain to long-chain PSs, and the subsequent oxidization to S<sup>8</sup> [24, 42, 43].The thirdcycle CV curve is highly similar to the second-cycle curve, thus it can be expected that the TCF@S-MWCNTsP electrode will show favorable cycling stability and high reversibility

**Figure 10.** TG curve of the TCF@S-MWCNTsP electrode.

**Figure 9.** The SEM image of MWCNTsP (a) and TCF@S-MWCNTsP electrode (b).

and PVDF as binder. The ratio of S∶MWCNT∶CB∶PVDF = 60∶15∶15∶10. The blend slurry was coated on to porous MWCNTsP. Then the sulfur electrodes (S-MWCNTsP) were dried at 60°C

The conductive liquid of MWCNTs was prepared by ultrasonication and high-shear process with NMP as solution and PVDF as binder. The ratio of MWCNTs∶CB∶PVDF = 60∶30∶10. Subsequently followed by overlaying the conductive liquid onto S-MWCNTsP electrodes, the

The tailored S-MWCNTsP and TCF@S-MWCNTsP electrodes were, respectively, used as working electrodes. Lithium foil was used as the counter electrode and Celgard 2300 was used as the separator. The solution of 1.0 M LiTFSI in DOL:DME (1,1, vol.) with 1.0%LiNO3 was utilized as the electrolyte. CR2025 coin-type cells were assembled in an Ar filled glove box (MBRAUN LABSTAR, Germany). The electrochemical characterization of the cells was measured by a cell tester (CT-4008-5V5mA-164). The galvanostatic charge-discharge current density was set at 0.2 to 5C. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) within a potential window of 1.6–2.8 V by an electrochemical workstation (CHI 660B) were measured.

Top surface SEM of the MWCNTsP in **Figure 9(a)** displays its porous matrix and the coalescing fiber network. The MWCNTsP demonstrated homogenous incorporation of MWCNTs in the cellulose fiber network. Porous structure can effectively improve the carrying capacity of sulfur, and adsorbed PSs. The superior electrolyte absorbability and hierarchical open channel of MWCNTsP can store more sulfur and contributes to stabilize electrochemical reactions and anchored PSs within the MWCNTsP effectively and suppressing shuttle effects. The SEM image of TCF@S-MWCNTsP electrode in **Figure 9(b)** also shows that MWCNTs and S were

obtained electrode with MWCNTs transparent conducting film (TCF@S-MWCNTsP).

*4.1.3. Assembling of cell and electrochemical measurements*

*4.2.1. The SEM of MWCNTsP and TCF@S-MWCNTsP electrode*

under vacuum for 12 h.

40 Transparent Conducting Films

**4.2. Results and discussion**

**Figure 11.** CV curves of Li-S batteries. (a) The first three cycles of CV profiles for TCF@S-MWCNTsP electrode. (b) The second cycle of CV profiles for TCF@S-MWCNTsP and S-MWCNTsP electrodes.

[44]. **Figure 11(b)** shows the second cycle of the CV plots for the two cathodes. There is a voltage shift between TCF@S-MWCNTs and S-MWCNTs electrodes other than the shape difference. This proves that using MWCNTsP as a current collector is beneficial to improve the electrochemical performance of a Li-S batteries. In addition, the TCF@S-MWCNTs electrode shows a higher voltage value at the reduction peaks and the oxidation peaks have a lower voltage value. This shows that the TCF@S-MWCNTs electrode has a higher discharge platform, which is conducive to improving the specific capacity and suppressing the shuttle effect. Meanwhile, the sharp reduction and oxidation peaks are also clear evidence of high reactivity of sulfur in the TCF@S-MWCNTs electrode. These results suggested that the electrode with MWCNTs transparent conducting film can inhibit effectively the shuttle effect and anchored PSs.

## *4.2.4. Constant current charge and discharge*

**Figure 12(a)** shows the galvanostatic discharge–charge voltage profiles of S-MWCNTsP electrode at current rate 0.2 C (1 C = 1675 mAh/g) in the potential range from 1.6 to 2.8 V. It can be seen that there are two discharge plateaus for different rate at 2.3 and 2.1 V, respectively. But, only 2.1 V plateau exhibited a longer flat range which was ascribed to conformation of short-chain sulfides of Li<sup>2</sup> S2 and Li<sup>2</sup> S. After 20 cycles, the discharge capacity also faded to 968 mAh/g from initial 1282 mAh/g. Another distinct characteristic is that Coulombic efficiency faded to 90.0% from initial 98.5%. It was considered the low Coulombic efficiency after 20 cycles was resulted from dissolution of long-chain PSs in electrolyte and subsequent migration and deposition on lithium anode. **Figure 12(b)** shows the galvanostatic discharge-charge voltage profiles of TCF@S-MWCNTsP electrode at current rate 0.2 C. The galvanostatic charge-discharge curves displayed a similar profile as S-MWCNTsP electrode. But, both the voltage plateaus of 2.3 and 2.1 V all exhibited longer flat range than ones of S-MWCNTsP electrode, indicating an excellent potential stability. The Coulombic efficiency and discharge capacity after 20 cycles reached 94.8% and 1028 mAh/g, respectively. This indicates that electrode with MWCNTs transparent conducting film can increase discharge capacity and Coulombic efficiency, inhibit shuttle effect.

Then the electrochemical performance of a Li-S cell was tested by galvanostatic discharge

**Figure 12.** Electrochemical performance of Li-S batteries. Galvanostatic charge-discharge profiles of the (a) S-MWCNTsP and (b) TCF@S-MWCNTsP electrodes at 0.2 C. (c) The rate performance of TCF@S-MWCNTsP and S-MWCNTsP electrodes. (d) Galvanostatic charge-discharge profiles of the TCF@S-MWCNTsP electrode at various rates. (e) Long

loading of sulfur in the cathode is controlled to be approximately 2.4 mg/cm<sup>2</sup>

) at different current rates, and the areal mass

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. **Figure 12(c)**

and charge from 1.6 to 2.8 V (versus Li/Li<sup>+</sup>

cycling performance of TCF@S-MWCNTsP electrode at 0.5 C.

[44]. **Figure 11(b)** shows the second cycle of the CV plots for the two cathodes. There is a voltage shift between TCF@S-MWCNTs and S-MWCNTs electrodes other than the shape difference. This proves that using MWCNTsP as a current collector is beneficial to improve the electrochemical performance of a Li-S batteries. In addition, the TCF@S-MWCNTs electrode shows a higher voltage value at the reduction peaks and the oxidation peaks have a lower voltage value. This shows that the TCF@S-MWCNTs electrode has a higher discharge platform, which is conducive to improving the specific capacity and suppressing the shuttle effect. Meanwhile, the sharp reduction and oxidation peaks are also clear evidence of high reactivity of sulfur in the TCF@S-MWCNTs electrode. These results suggested that the electrode with MWCNTs transparent conducting film can inhibit effectively the shuttle effect

**Figure 11.** CV curves of Li-S batteries. (a) The first three cycles of CV profiles for TCF@S-MWCNTsP electrode. (b) The

**Figure 12(a)** shows the galvanostatic discharge–charge voltage profiles of S-MWCNTsP electrode at current rate 0.2 C (1 C = 1675 mAh/g) in the potential range from 1.6 to 2.8 V. It can be seen that there are two discharge plateaus for different rate at 2.3 and 2.1 V, respectively. But, only 2.1 V plateau exhibited a longer flat range which was

discharge capacity also faded to 968 mAh/g from initial 1282 mAh/g. Another distinct characteristic is that Coulombic efficiency faded to 90.0% from initial 98.5%. It was considered the low Coulombic efficiency after 20 cycles was resulted from dissolution of long-chain PSs in electrolyte and subsequent migration and deposition on lithium anode. **Figure 12(b)** shows the galvanostatic discharge-charge voltage profiles of TCF@S-MWCNTsP electrode at current rate 0.2 C. The galvanostatic charge-discharge curves displayed a similar profile as S-MWCNTsP electrode. But, both the voltage plateaus of 2.3 and 2.1 V all exhibited longer flat range than ones of S-MWCNTsP electrode, indicating an excellent potential stability. The Coulombic efficiency and discharge capacity after 20 cycles reached 94.8% and 1028 mAh/g, respectively. This indicates that electrode with MWCNTs transparent conducting film can increase discharge capacity and Coulombic

S2

and Li<sup>2</sup>

S. After 20 cycles, the

and anchored PSs.

42 Transparent Conducting Films

*4.2.4. Constant current charge and discharge*

efficiency, inhibit shuttle effect.

ascribed to conformation of short-chain sulfides of Li<sup>2</sup>

second cycle of CV profiles for TCF@S-MWCNTsP and S-MWCNTsP electrodes.

**Figure 12.** Electrochemical performance of Li-S batteries. Galvanostatic charge-discharge profiles of the (a) S-MWCNTsP and (b) TCF@S-MWCNTsP electrodes at 0.2 C. (c) The rate performance of TCF@S-MWCNTsP and S-MWCNTsP electrodes. (d) Galvanostatic charge-discharge profiles of the TCF@S-MWCNTsP electrode at various rates. (e) Long cycling performance of TCF@S-MWCNTsP electrode at 0.5 C.

Then the electrochemical performance of a Li-S cell was tested by galvanostatic discharge and charge from 1.6 to 2.8 V (versus Li/Li<sup>+</sup> ) at different current rates, and the areal mass loading of sulfur in the cathode is controlled to be approximately 2.4 mg/cm<sup>2</sup> . **Figure 12(c)**

exhibits the rate performance of the two cathodes ranging from 0.2 to 5 C. Compared with the S-MWCNTsP and TCF@S-MWCNTsP cells, the TCF@S-MWCNTsP cell delivers a much higher initial capacity of 1352 mAh/g at the rate of 0.2 C, followed by a subsequent slow decrease to 960, 902, and 782 mAh/g at rates of 0.5, 1, and 2 C, respectively. In addition, at higher rates of 3 and 5 C, a reversible capacity of 584 and 513 mAh/g can still be achieved. When suddenly switching back to the initial starting rate of 0.2 C, the original capacity was recovered, indicating the excellent reversible capacity of the TCF@S-MWCNTsP cell at various rates. These results indicate that the electrode with MWCNTs transparent conducting film is conducive to immobilizing sulfur and alleviating the dissolution of polysulfides. The charge-discharge curves of the TCF@S-MWCNTsP electrode at various current rates (0.2–5 C) are illustrated in **Figure 12(d)**. All the discharge curves exhibit two typical plateaus, which are well consistent with the CV results. The charge-discharge voltage plateaus remain stable during the prolonged cycles, indicating an excellent potential stability. Additionally, the charge-discharge curves of the TCF@S-MWCNTsP electrode show also high Coulombic efficiency.

Long-term cycling stability with high-capacity retention is crucial for the practical application of Li − S batteries. **Figure 12(e)** shows the cycling performance of the TCF@S-MWCNTsP electrode at 0.5 C for 300 cycles. The electrode with MWCNTs transparent conducting film delivers a high initial reversible capacity of 960 mAh/g, and the capacity remains at 730 mAh/g after 300 cycles with stabilized coulombic efficiency above 94.2%, corresponding to a capacity retention of 76% and slow capacity decay rate of 0.08% per cycle. Additionally, the TCF@S-MWCNTsP electrode had the high Coulombic efficiency over 300 cycles, proving that the electrode with MWCNTs transparent conducting film can effectively suppress the notorious shuttle effect and improve the cycling stability.

MWCNTs transparent conducting film could effectively reuse the dissolved active materials

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The surface of R-MWCNTs has irregular arrangement of carbon atoms and much defects, which are more favorable for the adsorption of dispersant molecules. The dispersion effect and stability of R-MWCNTs are better than those of G-MWCNTs conductive fluids. With the increase of the dispersant TNADIS mass fraction, the dispersion effect of MWCNTs is getting better and better. When added to 0.05 wt.% dispersant, the dispersion effect of MWCNTs is best, and then the dispersive effect becomes worse as the dispersant TNADIS increases. With the increase of the quality of the dispersant TNADIS, the density of the MWCNTs precipitates becomes larger and larger, and the excess dispersant makes the MWCNTs agglomerate more and more closely. The conductivity of the conductive liquid with 0.05% dispersant was very high. After standing for 5 months, there was no obvious precipitation and the Tyndall effect of the colloid was still significantly maintained. Transparent conductive films prepared from conductive liquids of G-MWCNTs have better conductivity. After the three layers of spin-coating, the electrical conductivity of G-MWCNTs film surpassed three times than R-MWCNTs. In summary, the designed electrode with MWCNTs transparent conducting film (TCF@S-MWCNTsP) showed significantly enhanced improvements in capacity retention and longterm cycle stability in Li − S batteries. The synergistic effect of them contributed to good rate performance, high capacity and excellent Coulombic efficiency. The areal sulfur mass loading

mAh/g can be delivered at 0.2 C with a Coulombic efficiency of 100%. Notably, the TCF@S-MWCNTsP electrode displayed a long cycling performance with only 0.08% capacity decay per cycle over 300 cycles at 0.5 C. The improved performance is ascribed to the trapping capability of the 3D configuration electrode to reutilize the dissolved polysulfides and the reduction of charge transfer impedance of the electrode. We believe that this attempt gives

new insights on the cathode design for achieving high performance Li-S batteries.

, a high discharge specific capacity of 1352

and mitigate surface aggregation, thus providing better performance.

**Figure 13.** Electrochemical impedance spectroscopy of the two electrodes (a) before and (b) after cycles.

of electrode is controlled to be above 2.4 mg/cm<sup>2</sup>

**5. Conclusions**

## *4.2.5. Electrochemical impedance spectroscopy*

The role of the electrode with MWCNTs transparent conducting film in Li-S batteries was further probed by electrochemical impedance spectroscopy (EIS). Nyquist plots of the two cells impedance before cycles are shown in **Figure 13(a)**. In the high-frequency region, the intercept of the impedance curve on the x-axis corresponds to the electrolyte resistance (Re). In the middle-frequency region, the semicircle arises from the charge transfer resistance (Rct), which represents the charge-transfer process at the interface between the electrolyte and electrode. In the low-frequency region, an inclined line denotes the Warburg resistance (Wo), which is related with mass transfer processes [45, 46]. The TCF@S-MWCNTsP electrode has the lower Rct value, indicating a low resistance caused by the entrapment of the dissolved PSs and both good electrolyte infiltration and charge transport. After 90 cycles, the Rct of S-MWCNTsP and TCF@S-MWCNTsP electrodes all demonstrates a decline as shown in **Figure 13(b)**. This can be ascribed to good electrolyte infiltration and the dramatic improvement of electronic and ionic conductivity due to the unique porous conductive interlinked structure of MWCNTsP. But the Re of both electrodes all demonstrates a rise. The increase of Re is caused by the dissolution and diffusion of PSs into the electrolyte. The lower Re value of the TCF@S-MWCNTsP electrode can be attributed to the use of the MWCNTs transparent conducting film framework can anchor PSs effectively and suppressing shuttle effects. Simultaneously, the electrode with

**Figure 13.** Electrochemical impedance spectroscopy of the two electrodes (a) before and (b) after cycles.

MWCNTs transparent conducting film could effectively reuse the dissolved active materials and mitigate surface aggregation, thus providing better performance.

## **5. Conclusions**

exhibits the rate performance of the two cathodes ranging from 0.2 to 5 C. Compared with the S-MWCNTsP and TCF@S-MWCNTsP cells, the TCF@S-MWCNTsP cell delivers a much higher initial capacity of 1352 mAh/g at the rate of 0.2 C, followed by a subsequent slow decrease to 960, 902, and 782 mAh/g at rates of 0.5, 1, and 2 C, respectively. In addition, at higher rates of 3 and 5 C, a reversible capacity of 584 and 513 mAh/g can still be achieved. When suddenly switching back to the initial starting rate of 0.2 C, the original capacity was recovered, indicating the excellent reversible capacity of the TCF@S-MWCNTsP cell at various rates. These results indicate that the electrode with MWCNTs transparent conducting film is conducive to immobilizing sulfur and alleviating the dissolution of polysulfides. The charge-discharge curves of the TCF@S-MWCNTsP electrode at various current rates (0.2–5 C) are illustrated in **Figure 12(d)**. All the discharge curves exhibit two typical plateaus, which are well consistent with the CV results. The charge-discharge voltage plateaus remain stable during the prolonged cycles, indicating an excellent potential stability. Additionally, the charge-discharge curves of the TCF@S-MWCNTsP electrode show also high Coulombic

Long-term cycling stability with high-capacity retention is crucial for the practical application of Li − S batteries. **Figure 12(e)** shows the cycling performance of the TCF@S-MWCNTsP electrode at 0.5 C for 300 cycles. The electrode with MWCNTs transparent conducting film delivers a high initial reversible capacity of 960 mAh/g, and the capacity remains at 730 mAh/g after 300 cycles with stabilized coulombic efficiency above 94.2%, corresponding to a capacity retention of 76% and slow capacity decay rate of 0.08% per cycle. Additionally, the TCF@S-MWCNTsP electrode had the high Coulombic efficiency over 300 cycles, proving that the electrode with MWCNTs transparent conducting film can effectively suppress the notorious

The role of the electrode with MWCNTs transparent conducting film in Li-S batteries was further probed by electrochemical impedance spectroscopy (EIS). Nyquist plots of the two cells impedance before cycles are shown in **Figure 13(a)**. In the high-frequency region, the intercept of the impedance curve on the x-axis corresponds to the electrolyte resistance (Re). In the middle-frequency region, the semicircle arises from the charge transfer resistance (Rct), which represents the charge-transfer process at the interface between the electrolyte and electrode. In the low-frequency region, an inclined line denotes the Warburg resistance (Wo), which is related with mass transfer processes [45, 46]. The TCF@S-MWCNTsP electrode has the lower Rct value, indicating a low resistance caused by the entrapment of the dissolved PSs and both good electrolyte infiltration and charge transport. After 90 cycles, the Rct of S-MWCNTsP and TCF@S-MWCNTsP electrodes all demonstrates a decline as shown in **Figure 13(b)**. This can be ascribed to good electrolyte infiltration and the dramatic improvement of electronic and ionic conductivity due to the unique porous conductive interlinked structure of MWCNTsP. But the Re of both electrodes all demonstrates a rise. The increase of Re is caused by the dissolution and diffusion of PSs into the electrolyte. The lower Re value of the TCF@S-MWCNTsP electrode can be attributed to the use of the MWCNTs transparent conducting film framework can anchor PSs effectively and suppressing shuttle effects. Simultaneously, the electrode with

efficiency.

44 Transparent Conducting Films

shuttle effect and improve the cycling stability.

*4.2.5. Electrochemical impedance spectroscopy*

The surface of R-MWCNTs has irregular arrangement of carbon atoms and much defects, which are more favorable for the adsorption of dispersant molecules. The dispersion effect and stability of R-MWCNTs are better than those of G-MWCNTs conductive fluids. With the increase of the dispersant TNADIS mass fraction, the dispersion effect of MWCNTs is getting better and better. When added to 0.05 wt.% dispersant, the dispersion effect of MWCNTs is best, and then the dispersive effect becomes worse as the dispersant TNADIS increases. With the increase of the quality of the dispersant TNADIS, the density of the MWCNTs precipitates becomes larger and larger, and the excess dispersant makes the MWCNTs agglomerate more and more closely. The conductivity of the conductive liquid with 0.05% dispersant was very high. After standing for 5 months, there was no obvious precipitation and the Tyndall effect of the colloid was still significantly maintained. Transparent conductive films prepared from conductive liquids of G-MWCNTs have better conductivity. After the three layers of spin-coating, the electrical conductivity of G-MWCNTs film surpassed three times than R-MWCNTs.

In summary, the designed electrode with MWCNTs transparent conducting film (TCF@S-MWCNTsP) showed significantly enhanced improvements in capacity retention and longterm cycle stability in Li − S batteries. The synergistic effect of them contributed to good rate performance, high capacity and excellent Coulombic efficiency. The areal sulfur mass loading of electrode is controlled to be above 2.4 mg/cm<sup>2</sup> , a high discharge specific capacity of 1352 mAh/g can be delivered at 0.2 C with a Coulombic efficiency of 100%. Notably, the TCF@S-MWCNTsP electrode displayed a long cycling performance with only 0.08% capacity decay per cycle over 300 cycles at 0.5 C. The improved performance is ascribed to the trapping capability of the 3D configuration electrode to reutilize the dissolved polysulfides and the reduction of charge transfer impedance of the electrode. We believe that this attempt gives new insights on the cathode design for achieving high performance Li-S batteries.

## **Author details**

Xiaogang Sun\*, Jie Wang, Wei Chen, Xu Li, Manyuan Cai, Long Chen, Zhiwen Qiu, Yapan Huang, Chengcheng Wei, Hao Hu and Guodong Liang

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Nanchang University, Nanchang, China

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**Section 4**

**Highly Conductive and Carbon Nanotube**

**Activated Transparent Thin-film**
