**2.1 Dealloying methods**

De-alloying is an effective corrosion method for the fabrication of NPG films wherein the presence of less noble metals in the gold alloy has been exploited in a way that they are chemically or electrochemically dissolved to produce monolithic metal bodies with nanoscale pore structure. Au-Ag alloys are considered ideal due to their similar atomic volumes and continuous solid solubility allowing for coherent transformation from the master alloy to the nanoporous structure, see **Table 1** [17, 18]. Chemical dealloying has been studied employing Metropolis Monte Carlo simulations wherein the simulation of the dealloying process in the first stage describes the equilibrated systems followed by the second stage of dealloying with the exclusion of interaction parameters [19]. A simple method to dealloy the precursor alloy is to immerse it in nitric acid leading to selective etching of silver forming a 3-D pattern resulting in the formation of an open, bicontinuous highly porous network of gold with tunable ligament and channel width by varying the alloy composition, electrochemical potential, or by thermal annealing after dealloying [20]. **Figure 2** depicts the outcome of NPG structures upon a change in the experimental parameters.


**Table 1.**

*Summary of the fabrication techniques used to synthesize NPG thin films.*

#### **Figure 2.**

*SEM images of NPG structures. (a) NPG membrane made by dealloying 1* μ*m thick gold leaf in nitric acid for 1 h. (b) Coarsened NPG structure generated via annealing of (a) at 400°C for 8 h. (c) Hierarchical porous membrane structure produced by performing second dealloying (of sample b) in nitric acid for 5 min and annealing at 400°C for 4 h. (d) Cross-section micrograph of sample (c). Reproduced with permission from reference [20], Copyright 2003, American Chemical Society.*

Variables such as acid concentration, etching time, and solution temperature are known to influence the size of pores and ligaments. NPG films with ultrafine pores have been produced by a pulse electrochemical dealloying carried out at a potential of 0.6 V with 50 ms on-time and 10 ms off-time in 8 M HNO3 at 23°C [21]. It has been seen that with the use of strong acid or alkali corrosion, the rearrangement is very rapid leaving little scope for porosity adjustment in such nanoporous products. Therefore, instead of the traditional corrosive acid etching, a two-step dealloying method has been reported which utilized FeCl3 to synthesize NPG wherein the pore size was easily tunable by using a surfactant like polyvinyl pyrrolidone (PVP) or replacing corrosion reaction solvent with ethylene glycol (EG) [22]. Dealloying can be further classified into three categories namely, (i) chemical dealloying, (ii) electrochemical or potentiostatic dealloying, and (iii) liquid metal dealloying. The driving force towards the selective dissolution of the active component is varied in the abovementioned categories. It is a corrosive solution such as an acid in the case of chemical dealloying, the constant potential for electrochemical dealloying and the nature of liquid metal medium and temperature are the crucial deciding factors in the emerging field of liquid metal dealloying [11]. Extensive investigations have demonstrated that nanoporous metals with 3-D bicontinuous structures fabricated by the dealloying method, has many active sites for the excitation of localized surface plasmons and can, therefore, serve as a potential substrate for practical surface-enhanced Raman scattering (SERS) application [23]. Unlike single-sized porous materials, a hierarchical (bimodal) porous structure can impart novel properties to the material wherein large pores can favor increased mass transport and small pores can impart high specific surface area [24].

#### **2.2 Self-assembly**

Self-assembly is one of the most versatile, simple, and inexpensive methods aiding the formation of porous polymer films with finely controlled topography [25]. The self-assembly strategy is a powerful approach to create functional nanomaterials due to the high selection capability of the precursor materials and inclusion of functional groups, and nanoarchitectonics. The process is usually carried

**147**

**2.3 Sputter deposition**

*Electrodeposition of Nanoporous Gold Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94604*

out under ambient and mild conditions, making the process suitable for biological materials, see **Table 1** [26]. The self-assembled nanostructured film is a rapidly emerging field of great fundamental and practical interest due to the prospective applications in the field of microelectronics, non-linear optics, catalysis, and sensor science. Controlled covalent attachment of nanoparticles to functionalized surfaces is a versatile approach for producing thin-film structures. It has been seen that self-assembly of gold nanoparticles (AuNPs) on thiol modified surfaces exhibited non-metallic optical and electronic properties [27]. The self-assembly process of the AuNPs via aggregation and coalescence leads to the porous structure directly without relying on external assistance. Zhang and coworkers successfully fabricated porous gold films from a colloidal gold solution by evaporation induced self-assembly method (EISA) [28]. A facile synthetic approach for the self-assembly of AuNPs on sulfide functionalized polydopamine surface in high-density has been reported. AuNPs were seen to self-assemble strongly on the modified surface due to the strong interaction between gold and sulfur atoms [29]. Nanostructural control is very critical in material chemistry to bring out unique physical and chemical properties. So far, many mesoporous materials with varying compositions have been reported via self-assembly of amphiphilic organic molecules and have attracted keen interest due to their wide range of potential applications in energy storage, separation, catalysis, ion exchange, sensing, and drug delivery [30]. Another useful technique to immobilize gold thin films on micro- and nanopatterns is via the use of directed self-assembly on templates that are prepared by phase- separated mixed Langmuir Blodgett (LB) films. Atomic force microscopy (AFM), Auger electron spectroscopy, and scanning Auger electron mapping of the gold thin films revealed that the immobilized layer was following the patterns of the original mixed LB films [31]. Systematic analysis of the thermodynamics and kinetics of self-assembly in thin films of supramolecular nanocomposite has been studied in detail to extract information about the interfacial area defects, chain mobility, and activation energy needed for the diffusion of materials. Co-assemblies of nanoparticles and organic moieties are promising, and the resultant material will combine the properties of both the families of building blocks. Hierarchically structured nanocomposite thin films over macroscopic distances have been created by a fast ordering process with minimal usage of solvent via a deep understanding of the kinetic pathways [32].

Sputter deposition is an industry-relevant, high-rate, large-scale, and wellcontrollable deposition technique used to prepare gold films and other device fabrication processes. It is a large-scale deposition method, allowing high-rate vacuum coating with nanoscale precision, see **Figure 3**. Sputter deposition is an ideal method for preparing nanometer-thick films along with precise control over the deposition rate, highly adhesive films, large area uniformity, uniform temperature, and the ability to coat a variety of substrates including the non-heat resistant [33].

Sputtering is a pollution-free ("green") technique that offers the advantage of reproducibility and low price of the final product. Nanostructured gold film adhesion and electrical contact properties are strongly influenced by interface structure, see **Table 1** [34]. Thin gold films prepared by this strategy are found to be continuous and relatively homogeneous, with distinct grain surfaces without showing the formation of islands. Films of thickness ranging between 5 and 52 nm have been prepared for the study of third-order non-linear properties where the third-order

interaction enhancement has helped to construct nanodevices to be used in metrology, sensing, imaging, and telecommunications [35]. It has been shown that pure

esu. This non-linear optical

susceptibility was found to be of the order of 10**<sup>−</sup>**<sup>9</sup>

#### *Electrodeposition of Nanoporous Gold Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94604*

*Nanofibers - Synthesis, Properties and Applications*

*reference [20], Copyright 2003, American Chemical Society.*

Variables such as acid concentration, etching time, and solution temperature are known to influence the size of pores and ligaments. NPG films with ultrafine pores have been produced by a pulse electrochemical dealloying carried out at a potential of 0.6 V with 50 ms on-time and 10 ms off-time in 8 M HNO3 at 23°C [21]. It has been seen that with the use of strong acid or alkali corrosion, the rearrangement is very rapid leaving little scope for porosity adjustment in such nanoporous products. Therefore, instead of the traditional corrosive acid etching, a two-step dealloying method has been reported which utilized FeCl3 to synthesize NPG wherein the pore size was easily tunable by using a surfactant like polyvinyl pyrrolidone (PVP) or replacing corrosion reaction solvent with ethylene glycol (EG) [22]. Dealloying can be further classified into three categories namely, (i) chemical dealloying, (ii) electrochemical or potentiostatic dealloying, and (iii) liquid metal dealloying. The driving force towards the selective dissolution of the active component is varied in the abovementioned categories. It is a corrosive solution such as an acid in the case of chemical dealloying, the constant potential for electrochemical dealloying and the nature of liquid metal medium and temperature are the crucial deciding factors in the emerging field of liquid metal dealloying [11]. Extensive investigations have demonstrated that nanoporous metals with 3-D bicontinuous structures fabricated by the dealloying method, has many active sites for the excitation of localized surface plasmons and can, therefore, serve as a potential substrate for practical surface-enhanced Raman scattering (SERS) application [23]. Unlike single-sized porous materials, a hierarchical (bimodal) porous structure can impart novel properties to the material wherein large pores can favor increased mass transport and small pores can impart high

*SEM images of NPG structures. (a) NPG membrane made by dealloying 1* μ*m thick gold leaf in nitric acid for 1 h. (b) Coarsened NPG structure generated via annealing of (a) at 400°C for 8 h. (c) Hierarchical porous membrane structure produced by performing second dealloying (of sample b) in nitric acid for 5 min and annealing at 400°C for 4 h. (d) Cross-section micrograph of sample (c). Reproduced with permission from* 

Self-assembly is one of the most versatile, simple, and inexpensive methods aiding the formation of porous polymer films with finely controlled topography [25]. The self-assembly strategy is a powerful approach to create functional nanomaterials due to the high selection capability of the precursor materials and inclusion of functional groups, and nanoarchitectonics. The process is usually carried

**146**

specific surface area [24].

**2.2 Self-assembly**

**Figure 2.**

out under ambient and mild conditions, making the process suitable for biological materials, see **Table 1** [26]. The self-assembled nanostructured film is a rapidly emerging field of great fundamental and practical interest due to the prospective applications in the field of microelectronics, non-linear optics, catalysis, and sensor science. Controlled covalent attachment of nanoparticles to functionalized surfaces is a versatile approach for producing thin-film structures. It has been seen that self-assembly of gold nanoparticles (AuNPs) on thiol modified surfaces exhibited non-metallic optical and electronic properties [27]. The self-assembly process of the AuNPs via aggregation and coalescence leads to the porous structure directly without relying on external assistance. Zhang and coworkers successfully fabricated porous gold films from a colloidal gold solution by evaporation induced self-assembly method (EISA) [28]. A facile synthetic approach for the self-assembly of AuNPs on sulfide functionalized polydopamine surface in high-density has been reported. AuNPs were seen to self-assemble strongly on the modified surface due to the strong interaction between gold and sulfur atoms [29]. Nanostructural control is very critical in material chemistry to bring out unique physical and chemical properties. So far, many mesoporous materials with varying compositions have been reported via self-assembly of amphiphilic organic molecules and have attracted keen interest due to their wide range of potential applications in energy storage, separation, catalysis, ion exchange, sensing, and drug delivery [30]. Another useful technique to immobilize gold thin films on micro- and nanopatterns is via the use of directed self-assembly on templates that are prepared by phase- separated mixed Langmuir Blodgett (LB) films. Atomic force microscopy (AFM), Auger electron spectroscopy, and scanning Auger electron mapping of the gold thin films revealed that the immobilized layer was following the patterns of the original mixed LB films [31]. Systematic analysis of the thermodynamics and kinetics of self-assembly in thin films of supramolecular nanocomposite has been studied in detail to extract information about the interfacial area defects, chain mobility, and activation energy needed for the diffusion of materials. Co-assemblies of nanoparticles and organic moieties are promising, and the resultant material will combine the properties of both the families of building blocks. Hierarchically structured nanocomposite thin films over macroscopic distances have been created by a fast ordering process with minimal usage of solvent via a deep understanding of the kinetic pathways [32].

### **2.3 Sputter deposition**

Sputter deposition is an industry-relevant, high-rate, large-scale, and wellcontrollable deposition technique used to prepare gold films and other device fabrication processes. It is a large-scale deposition method, allowing high-rate vacuum coating with nanoscale precision, see **Figure 3**. Sputter deposition is an ideal method for preparing nanometer-thick films along with precise control over the deposition rate, highly adhesive films, large area uniformity, uniform temperature, and the ability to coat a variety of substrates including the non-heat resistant [33].

Sputtering is a pollution-free ("green") technique that offers the advantage of reproducibility and low price of the final product. Nanostructured gold film adhesion and electrical contact properties are strongly influenced by interface structure, see **Table 1** [34]. Thin gold films prepared by this strategy are found to be continuous and relatively homogeneous, with distinct grain surfaces without showing the formation of islands. Films of thickness ranging between 5 and 52 nm have been prepared for the study of third-order non-linear properties where the third-order susceptibility was found to be of the order of 10**<sup>−</sup>**<sup>9</sup> esu. This non-linear optical interaction enhancement has helped to construct nanodevices to be used in metrology, sensing, imaging, and telecommunications [35]. It has been shown that pure

**Figure 3.**

*Schematic illustration of the* in situ *grazing-incidence small-angle X-ray scattering (GISAXS) set up showing sputtering of gold atoms on spiro-OMeTAD film. A two-dimensional detector is detecting the signal produced by scattered X-rays. Reproduced with permission from reference [33], Copyright 2020, American Chemical Society.*

sputtered gold films can match the hardness of gold electrodeposits. The hardness of sputtered films and the grain size of the deposit is controlled by maintaining the temperature of the substrate during deposition [36]. It has been reported that ultrathin semi-transparent gold films have been deposited using radio frequency (RF) magnetron sputtering at room temperature over a small area (23 mm**<sup>2</sup>** ) of the porous silicon layer. Various film thicknesses were obtained by changing the sputtering time from 5 to 20 s at constant chamber pressure and argon gas flow rate [37]. Thin films have been deposited by varying the experimental conditions concerning the substrate tilt angle and background pressure. Growth regimes of thin gold films deposited via magnetron sputtering at oblique angles and low temperatures have been studied from both theoretical and experimental points of view [38]. The growth and morphology of a room temperature sputter-coated thin gold film on a soft polymeric substrate from nucleation to thin film formation has been investigated using AFM. It was observed that an initial 3-D island-type growth starts with the deposition and with increasing time the morphology evolved from hemispherical islands to partially coalesced worm-like island structures, to percolation, and finally to a rough and continuous film [39]. In a study, it was seen that after the stage of nucleation, the growth of gold clusters proceeds mainly in the lateral direction. As the discontinuous islands change into a continuous thin film, a rapid decline in the resistance of the gold layer has been observed [40]. A unique method for synthesizing porous gold films by co-deposition of Au-Cu alloy has been done via co-sputtering Au and Cu using a multi-target sputtering system at room temperature. Selective removal of Cu was done via corrosive dealloying leading to the formation of the porous gold film via the physical–chemical combination method [41]. Sputter deposited thin porous gold films find applications in the field of electrochemical biosensing for enhancing redox signals by modulating the nanopore size and film thickness. Higher detection resolution is exhibited to that obtained by conventional bulk gold electrodes [42]. The atom sputtering deposition technique has allowed scientists to study the optical and electrical properties, density, and crystalline structure of gold nanostructures sputtered on glass [40].

#### **2.4 Electrodeposition**

Electrodeposition is a simple, controllable, and cost-effective method that can control the growth process by varying deposition current density. Kinetic control over the growth process can give insights into the mechanisms in the synthesis of deposits with various sizes and shapes. The mass of gold on the electrode is controlled by the electric charge passed during the electrodeposition process, based

**149**

more negative potentials [54].

*Electrodeposition of Nanoporous Gold Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94604*

providing the diffusion control, forming AuCl2

immediately giving off Au atoms and AuCl4

the reaction, HCl depletes and AuCl2

multiple sized pores [47].

electrodes [50].

on Faraday's laws of electrolysis [43]. Deng and co-workers gave a mechanistic viewpoint to the facile method of electrodeposition in the electrochemical cell, where a process of electrodissolution-disproportion-deposition is involved. Upon the potential step, the gold substrate undergoes active electrodissolution in HCl

−

−

−

aggregate and deposit on the substrate to form NPG film [44, 45]. NPG morphology evolution has a direct relationship with the topography of the underlying substrate. A study has very well demonstrated that micropattern widths that are within the magnitude of film thickness produce tunable pores with less cracking [46]. Porous nanomaterials particularly NPG has drawn tremendous research interests mainly in the development of electrochemical sensors due to the high surface area and the trait of being electrocatalytically active. Electrosynthesis can impart precise control over the bicontinuous network of interconnected nanometric gold grains and

Conductive substrates coated with AuNPs can be exploited as biochemical sensors and electrodes owing to their excellent electrocatalytic activity. A simple, powerful, and cost-effective way for attachment of AuNPs onto a substrate has been electrodeposition with pulsating current resulting in smoother, brighter, finer, and less porous Au grains concerning the direct current [48]. A simple template-less, surfactant-less and effective electrochemical method for the preparation of AuNP arrays with an average diameter of ~14 nm onto indium tin oxide (ITO) glass has been reported. The system exhibits excellent catalytic properties due to the presence of large active sites on the surface [49]. The deposition of thin metallic films onto non-conductive surfaces by the wet processing technique of electrodeposition is another route for fabricating various unique electronic devices and systems. Amine terminated self-assembled monolayer (SAM) modified insulator surface has been employed for the lateral growth of electrodeposited gold. Controlled morphology and thickness of laterally grown metal films has paved a way to create nanogap

Electrochemical deposition methods have been classified into two groups namely, templated electrodeposition and self-templated electrodeposition.

Template technology offers advantages for designing new types of electrode materials aided by "hard" or "soft" templates that give rise to functional materials with diverse structures and morphologies, such as, one-dimensional nanostructures, two-dimensional films, and 3-D porous frameworks. In templated electrodeposition, porous gold is deposited on a template by reducing gold near the electrode surface via fixed potential application followed by the removal of the template. The self-templated approach initializes with the generation of H2 bubbles in the solution containing a supporting electrolyte by applying a potential of at least −2 V vs. saturated calomel electrode (SCE), allowing a gold reduction in the interstitial space [51, 52]. The monolithic NPG film has been synthesized using a bottom-up synthesis from a bicontinuous microemulsion (BME) acting as a dynamic soft template, depicted in **Figure 4**. The aqueous phase of BME acted as a medium for gold electrodeposition and the intertwined structure came from the aqueous and oil phases of BME compartmentalized by surfactant and cosurfactant. The resultant film nanostructure has been controlled by adjusting the initial BME composition and by varying kinetic parameters such as deposition potential and time [53]. The role of electrodeposition potential on the growth morphologies has been seen in a study where gold deposition from sulfite electrolyte exhibits a range of geometries from vertically oriented nanowires to lenticular grains and dendrites at

and AuCl4

−

starts accumulating near the gold surface

wherein the newly formed Au atoms

. With the progress of

#### *Electrodeposition of Nanoporous Gold Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94604*

*Nanofibers - Synthesis, Properties and Applications*

**Figure 3.**

sputtered gold films can match the hardness of gold electrodeposits. The hardness of sputtered films and the grain size of the deposit is controlled by maintaining the temperature of the substrate during deposition [36]. It has been reported that ultrathin semi-transparent gold films have been deposited using radio frequency (RF) magnetron sputtering at room temperature over a small area (23 mm**<sup>2</sup>**

*Schematic illustration of the* in situ *grazing-incidence small-angle X-ray scattering (GISAXS) set up showing sputtering of gold atoms on spiro-OMeTAD film. A two-dimensional detector is detecting the signal produced by scattered X-rays. Reproduced with permission from reference [33], Copyright 2020, American Chemical Society.*

porous silicon layer. Various film thicknesses were obtained by changing the sputtering time from 5 to 20 s at constant chamber pressure and argon gas flow rate [37]. Thin films have been deposited by varying the experimental conditions concerning the substrate tilt angle and background pressure. Growth regimes of thin gold films deposited via magnetron sputtering at oblique angles and low temperatures have been studied from both theoretical and experimental points of view [38]. The growth and morphology of a room temperature sputter-coated thin gold film on a soft polymeric substrate from nucleation to thin film formation has been investigated using AFM. It was observed that an initial 3-D island-type growth starts with the deposition and with increasing time the morphology evolved from hemispherical islands to partially coalesced worm-like island structures, to percolation, and finally to a rough and continuous film [39]. In a study, it was seen that after the stage of nucleation, the growth of gold clusters proceeds mainly in the lateral direction. As the discontinuous islands change into a continuous thin film, a rapid decline in the resistance of the gold layer has been observed [40]. A unique method for synthesizing porous gold films by co-deposition of Au-Cu alloy has been done via co-sputtering Au and Cu using a multi-target sputtering system at room temperature. Selective removal of Cu was done via corrosive dealloying leading to the formation of the porous gold film via the physical–chemical combination method [41]. Sputter deposited thin porous gold films find applications in the field of electrochemical biosensing for enhancing redox signals by modulating the nanopore size and film thickness. Higher detection resolution is exhibited to that obtained by conventional bulk gold electrodes [42]. The atom sputtering deposition technique has allowed scientists to study the optical and electrical properties, density, and crystalline structure of gold nanostructures sputtered on glass [40].

Electrodeposition is a simple, controllable, and cost-effective method that can control the growth process by varying deposition current density. Kinetic control over the growth process can give insights into the mechanisms in the synthesis of deposits with various sizes and shapes. The mass of gold on the electrode is controlled by the electric charge passed during the electrodeposition process, based

) of the

**148**

**2.4 Electrodeposition**

on Faraday's laws of electrolysis [43]. Deng and co-workers gave a mechanistic viewpoint to the facile method of electrodeposition in the electrochemical cell, where a process of electrodissolution-disproportion-deposition is involved. Upon the potential step, the gold substrate undergoes active electrodissolution in HCl providing the diffusion control, forming AuCl2 − and AuCl4 − . With the progress of the reaction, HCl depletes and AuCl2 − starts accumulating near the gold surface immediately giving off Au atoms and AuCl4 − wherein the newly formed Au atoms aggregate and deposit on the substrate to form NPG film [44, 45]. NPG morphology evolution has a direct relationship with the topography of the underlying substrate. A study has very well demonstrated that micropattern widths that are within the magnitude of film thickness produce tunable pores with less cracking [46]. Porous nanomaterials particularly NPG has drawn tremendous research interests mainly in the development of electrochemical sensors due to the high surface area and the trait of being electrocatalytically active. Electrosynthesis can impart precise control over the bicontinuous network of interconnected nanometric gold grains and multiple sized pores [47].

Conductive substrates coated with AuNPs can be exploited as biochemical sensors and electrodes owing to their excellent electrocatalytic activity. A simple, powerful, and cost-effective way for attachment of AuNPs onto a substrate has been electrodeposition with pulsating current resulting in smoother, brighter, finer, and less porous Au grains concerning the direct current [48]. A simple template-less, surfactant-less and effective electrochemical method for the preparation of AuNP arrays with an average diameter of ~14 nm onto indium tin oxide (ITO) glass has been reported. The system exhibits excellent catalytic properties due to the presence of large active sites on the surface [49]. The deposition of thin metallic films onto non-conductive surfaces by the wet processing technique of electrodeposition is another route for fabricating various unique electronic devices and systems. Amine terminated self-assembled monolayer (SAM) modified insulator surface has been employed for the lateral growth of electrodeposited gold. Controlled morphology and thickness of laterally grown metal films has paved a way to create nanogap electrodes [50].

Electrochemical deposition methods have been classified into two groups namely, templated electrodeposition and self-templated electrodeposition. Template technology offers advantages for designing new types of electrode materials aided by "hard" or "soft" templates that give rise to functional materials with diverse structures and morphologies, such as, one-dimensional nanostructures, two-dimensional films, and 3-D porous frameworks. In templated electrodeposition, porous gold is deposited on a template by reducing gold near the electrode surface via fixed potential application followed by the removal of the template. The self-templated approach initializes with the generation of H2 bubbles in the solution containing a supporting electrolyte by applying a potential of at least −2 V vs. saturated calomel electrode (SCE), allowing a gold reduction in the interstitial space [51, 52]. The monolithic NPG film has been synthesized using a bottom-up synthesis from a bicontinuous microemulsion (BME) acting as a dynamic soft template, depicted in **Figure 4**. The aqueous phase of BME acted as a medium for gold electrodeposition and the intertwined structure came from the aqueous and oil phases of BME compartmentalized by surfactant and cosurfactant. The resultant film nanostructure has been controlled by adjusting the initial BME composition and by varying kinetic parameters such as deposition potential and time [53].

The role of electrodeposition potential on the growth morphologies has been seen in a study where gold deposition from sulfite electrolyte exhibits a range of geometries from vertically oriented nanowires to lenticular grains and dendrites at more negative potentials [54].

#### **Figure 4.**

*Schematic representation of the dynamic soft templating of a nanoporous thin gold film from BME by electrodeposition. Reproduced with permission from reference [69], Copyright 2013, American Chemical Society.*

In recent years, studies have been performed to produce 3D porous materials that have the advantage of increasing the mass transport for electrolytes but also allow rapid electrochemical reactions due to considerable active surface area. A new way of producing such materials is via concurrent generation of hydrogen bubbles with simultaneous metal deposition at high cathodic current densities. The reduction of H+ giving rise to hydrogen bubbles acts as a dynamic template for metal electrodeposition [15]. The electrodeposition parameters such as time and potential are the deciding factors for the pore size and film density. The dynamic hydrogen bubble template (DHBT) method results in a gold nanoporous structure with outstanding properties, like high specific surface area, large pore volume, uniform nanostructure, good conductivity, and enhanced electrochemical activity [55]. A simple method has been proposed for the one-step electrodeposition of NPG-islands films on the surface of the glassy carbon (GC) electrode via the hydrogen bubble template approach. The 3-D structure generated via the DHBT method is more attractive as it is clean and porous in an efficient way in the absence of inorganic and organic templates and exhibited improved electrocatalytic activity for oxygen reduction and hydrogen evolution reactions (HER) [56]. Recently, selfsupported 3D metal foams of copper, tin, and silver have been reported. 3D porous noble metals such as gold have higher equilibrium potential and lower overpotential for HER and therefore, a two-step rout was taken involving the deposition of the less noble element followed by its galvanic displacement. Chung and coworkers have therefore shown a fast one-step preparation of high surface area NPG with a multimodal pore-size distribution utilizing a DHBT [12].

Another study produced porous gold incorporating nanocorrals on gold screenprinted electrodes (SPEs) utilizing hydrogen bubbles as a dynamic template. The structure produced using this one-step electrodeposition using high overpotential had a high roughness factor [57]. Moreover, distinct pore morphology can be obtained by taking advantage of electrical conductivity and morphological plasticity of NPG. Pore morphology can be tuned using the novel and versatile technique of electro-annealing on NPG thin films at low temperatures [58]. The microstructure of electrodeposited gold films and their deposition characteristics are affected by base metal ions. It has been seen that gold deposits obtained from Co and Ni-containing electrolytes are generally hard, while Pb and Tl-containing electrolytes tend to give soft gold deposits [59]. The crystallographic structure of gold films is also dependent on the current density at which electrodeposition has been carried out. It has been seen that Au electrodeposited at a current density less than 0.25 mA cm−2 from dicyanoaurate baths with or without Cu2+ or Tl+ , gave rise to the formation of hexagonal structure [60].

The electrochemical potential and the concentration of HAuCl4 have been modified to create a variety of morphologies in the final structure. Nanopyramidal, nano rod-like, and spherical gold nanostructures were fabricated on polycrystalline gold substrates via one-step, non-templated electrochemical overpotential

**151**

*Electrodeposition of Nanoporous Gold Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94604*

deposition (OPD) [61]. Moreover, the chemical nature of the organic ions present in the organic electrolytes has a huge role to play on the morphology of the deposits produced. The organic species adsorb onto the electrodeposits due to high surface energy and finally influence the shape of the growing grains and the roughness of gold electrodeposits [62]. Recently, a study described the electrodeposition of gold nanostructures at the interface of a Pickering emulsion. The controlled electrodeposition of AuNPs on the surface of an emulsion droplet gave rise to intricate structures with fine control over the locus or duration of nanoparticle growth. Decamethylferrocene present in the emulsion droplet acted as a heterogeneous electron transfer agent for the reduction of aqueous phase Au (III) resulting in its deposition as nanoparticles [63]. In one of the studies, it was found that SERS of the electrodeposited gold was correlated to the roughness and the size of surface nanostructures where these two parameters were largely controlled by the applied potential during deposition [64]. Over the last decade, researchers have focused on improving methods for fabricating reproducible substrates for surface-enhanced resonance Raman scattering (SERRS). Amongst these, deposited gold films have been the most heavily researched, and therefore, Bartlett and his group fabricated ordered-spherical-cavity gold films using colloidal templated electrodeposition

method for the first time. The net enhancements are found to be ~109

gold from potassium dicyanoaurate solution [66].

for electrodeposition [68].

over normal Raman [65]. Nanostructured gold films for use in localized surface plasmon resonance (LSPR) spectroscopy have been prepared from flat gold film substrate made by stripping off epoxy coated glass slides off the gold-sputtered silicon wafers and then subsequent two-step chronoamperometry to electrodeposit

Solid thin film deposition on the soft ionic liquid (IL) substrate was used for L-arginine detection. A gold monolayer was deposited on the surface of the IL substrate using the conventional electro-co-deposition (CECD) technique using an electrochemical workstation with three electrodes system in 0.5 M NaCl and 3 mM KAuCl4 electrolyte solutions [67]. A new study described the bottom-up approach of lithographically patterned nanowire electrodeposition (LPNE) for synthesizing noble metal nanowires on glass or oxidized silicon surfaces. The process of LPNE starts with the preparation of a nickel nanoband electrode which acts as the surface

LPNE has been combined with colloidal lithography to create a novel low-cost method for the lithographically patterned electrodeposition of metallic nanoring close-packed arrays over large areas. By altering the width and radius of the nanoring during the fabrication, near-infrared (NIR) plasmonic resonances could be tuned from 3500 to 8000 cm−1 with potential applications in the fabrication of plasmonic antennae, plasmonic semiconductors, and negative-index metamaterials [69]. Direct electrodeposition of porous gold nanowire arrays has been developed utilizing a one-step electrodeposition methodology utilizing nanochannel alumina templates. Current density during deposition is the deciding factor for the microstructure of gold nanowires and the resulting structure has shown excellent electro-

It has been known that the surface roughness and the structure of gold deposits can be influenced by periodically reversed or pulsed current and in some electronic applications pulse plated gold deposits are considered superior to DC plated deposits. Electrodeposition of gold using pulse plating produces a dense fine-grained structure with half the resistivity of normal DC plated films [71]. The pulse potentiostatic method (PPSM) offers numerous advantages in terms of controllability, particle size, stronger adhesion, and uniform film morphology. PPSM has been used to deposit gold nanoflowers (AuNFs) onto a polymer film. Through this simple and rapid method, a new organic–inorganic hybrid film has been fabricated with

chemical biosensing ability towards the detection of glucose [70].

for SERRS

#### *Electrodeposition of Nanoporous Gold Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94604*

*Nanofibers - Synthesis, Properties and Applications*

The reduction of H+

**Figure 4.**

In recent years, studies have been performed to produce 3D porous materials that have the advantage of increasing the mass transport for electrolytes but also allow rapid electrochemical reactions due to considerable active surface area. A new way of producing such materials is via concurrent generation of hydrogen bubbles with simultaneous metal deposition at high cathodic current densities.

*Schematic representation of the dynamic soft templating of a nanoporous thin gold film from BME by electrodeposition. Reproduced with permission from reference [69], Copyright 2013, American Chemical Society.*

metal electrodeposition [15]. The electrodeposition parameters such as time and potential are the deciding factors for the pore size and film density. The dynamic hydrogen bubble template (DHBT) method results in a gold nanoporous structure with outstanding properties, like high specific surface area, large pore volume, uniform nanostructure, good conductivity, and enhanced electrochemical activity [55]. A simple method has been proposed for the one-step electrodeposition of NPG-islands films on the surface of the glassy carbon (GC) electrode via the hydrogen bubble template approach. The 3-D structure generated via the DHBT method is more attractive as it is clean and porous in an efficient way in the absence of inorganic and organic templates and exhibited improved electrocatalytic activity for oxygen reduction and hydrogen evolution reactions (HER) [56]. Recently, selfsupported 3D metal foams of copper, tin, and silver have been reported. 3D porous noble metals such as gold have higher equilibrium potential and lower overpotential for HER and therefore, a two-step rout was taken involving the deposition of the less noble element followed by its galvanic displacement. Chung and coworkers have therefore shown a fast one-step preparation of high surface area NPG with a

Another study produced porous gold incorporating nanocorrals on gold screenprinted electrodes (SPEs) utilizing hydrogen bubbles as a dynamic template. The structure produced using this one-step electrodeposition using high overpotential had a high roughness factor [57]. Moreover, distinct pore morphology can be obtained by taking advantage of electrical conductivity and morphological plasticity of NPG. Pore morphology can be tuned using the novel and versatile technique of electro-annealing on NPG thin films at low temperatures [58]. The microstructure of electrodeposited gold films and their deposition characteristics are affected by base metal ions. It has been seen that gold deposits obtained from Co and Ni-containing electrolytes are generally hard, while Pb and Tl-containing electrolytes tend to give soft gold deposits [59]. The crystallographic structure of gold films is also dependent on the current density at which electrodeposition has been carried out. It has been seen that Au electrodeposited at a current density less

than 0.25 mA cm−2 from dicyanoaurate baths with or without Cu2+ or Tl+

The electrochemical potential and the concentration of HAuCl4 have been modified to create a variety of morphologies in the final structure. Nanopyramidal, nano rod-like, and spherical gold nanostructures were fabricated on polycrystalline gold substrates via one-step, non-templated electrochemical overpotential

, gave rise

multimodal pore-size distribution utilizing a DHBT [12].

to the formation of hexagonal structure [60].

giving rise to hydrogen bubbles acts as a dynamic template for

**150**

deposition (OPD) [61]. Moreover, the chemical nature of the organic ions present in the organic electrolytes has a huge role to play on the morphology of the deposits produced. The organic species adsorb onto the electrodeposits due to high surface energy and finally influence the shape of the growing grains and the roughness of gold electrodeposits [62]. Recently, a study described the electrodeposition of gold nanostructures at the interface of a Pickering emulsion. The controlled electrodeposition of AuNPs on the surface of an emulsion droplet gave rise to intricate structures with fine control over the locus or duration of nanoparticle growth. Decamethylferrocene present in the emulsion droplet acted as a heterogeneous electron transfer agent for the reduction of aqueous phase Au (III) resulting in its deposition as nanoparticles [63]. In one of the studies, it was found that SERS of the electrodeposited gold was correlated to the roughness and the size of surface nanostructures where these two parameters were largely controlled by the applied potential during deposition [64]. Over the last decade, researchers have focused on improving methods for fabricating reproducible substrates for surface-enhanced resonance Raman scattering (SERRS). Amongst these, deposited gold films have been the most heavily researched, and therefore, Bartlett and his group fabricated ordered-spherical-cavity gold films using colloidal templated electrodeposition method for the first time. The net enhancements are found to be ~109 for SERRS over normal Raman [65]. Nanostructured gold films for use in localized surface plasmon resonance (LSPR) spectroscopy have been prepared from flat gold film substrate made by stripping off epoxy coated glass slides off the gold-sputtered silicon wafers and then subsequent two-step chronoamperometry to electrodeposit gold from potassium dicyanoaurate solution [66].

Solid thin film deposition on the soft ionic liquid (IL) substrate was used for L-arginine detection. A gold monolayer was deposited on the surface of the IL substrate using the conventional electro-co-deposition (CECD) technique using an electrochemical workstation with three electrodes system in 0.5 M NaCl and 3 mM KAuCl4 electrolyte solutions [67]. A new study described the bottom-up approach of lithographically patterned nanowire electrodeposition (LPNE) for synthesizing noble metal nanowires on glass or oxidized silicon surfaces. The process of LPNE starts with the preparation of a nickel nanoband electrode which acts as the surface for electrodeposition [68].

LPNE has been combined with colloidal lithography to create a novel low-cost method for the lithographically patterned electrodeposition of metallic nanoring close-packed arrays over large areas. By altering the width and radius of the nanoring during the fabrication, near-infrared (NIR) plasmonic resonances could be tuned from 3500 to 8000 cm−1 with potential applications in the fabrication of plasmonic antennae, plasmonic semiconductors, and negative-index metamaterials [69]. Direct electrodeposition of porous gold nanowire arrays has been developed utilizing a one-step electrodeposition methodology utilizing nanochannel alumina templates. Current density during deposition is the deciding factor for the microstructure of gold nanowires and the resulting structure has shown excellent electrochemical biosensing ability towards the detection of glucose [70].

It has been known that the surface roughness and the structure of gold deposits can be influenced by periodically reversed or pulsed current and in some electronic applications pulse plated gold deposits are considered superior to DC plated deposits. Electrodeposition of gold using pulse plating produces a dense fine-grained structure with half the resistivity of normal DC plated films [71]. The pulse potentiostatic method (PPSM) offers numerous advantages in terms of controllability, particle size, stronger adhesion, and uniform film morphology. PPSM has been used to deposit gold nanoflowers (AuNFs) onto a polymer film. Through this simple and rapid method, a new organic–inorganic hybrid film has been fabricated with

superior electroactivity, electrochemical, and interfacial characteristics suggesting the suitability of such electrodes for sensors, electrocatalysis, and diode applications [72]. Epitaxial, ultrathin, semitransparent, and catalytic gold films were electrodeposited on n-type silicon (n-Si) to protect the substrate from photo-passivation, see **Figure 5**. In addition to being a good redox catalyst, the ultrathin gold layers serve to induce band bending in n-Si thereby making it a diode of an ideal quality factor due to minimal electron–hole recombination [73].

Fabrication methods have been used in combination in the past to generate a promising eco-friendly biosensor platform with advantages coming from both the techniques. One such study has been done to fabricate a glucose biosensor wherein AuNPs and glucose oxidase multilayer films were generated via electrodeposition and self-assembly respectively [74]. As an eco-friendly alternative, deep eutectic solvents (DESs) are a new class of green and sustainable solvents extensively used for electrodepositions due to their intrinsic properties of good solubility, nonflammability, low toxicities, and suitable electrochemical windows. Recently, an electrochemical method has been developed to fabricate NPG electrodes by alloying and dealloying the precursor alloy in ZnCl2-urea deep eutectic solvent [75]. The wide acceptance of DES in metal/alloy electrodeposition processes has been seen in recent times due to their ability to act as stabilizers and reducing agents in metal nanoparticle synthesis. Moreover, the nucleation rates are enhanced leading to a

#### **Figure 5.**

*Illustration showing the formation of an ultrathin epitaxial gold layer electrodeposited on n-Si (111) to form a Schottky junction in a regenerative photoelectrochemical cell. Reproduced with permission from reference [73], Copyright 2018, American Chemical Society.*

**153**

*Electrodeposition of Nanoporous Gold Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94604*

tion to form thin nanoporous films.

**3. Characterization techniques for NPG thin films**

decrease in the particle size in the presence of DES as it has relatively low surface tension [76]. Remarkable optical behavior has been observed when the bulk material is transformed into nanostructured surfaces. A simple two-step electrochemical process has been developed wherein electrodeposition and anodization is used in conjunction to generate black gold surfaces that can absorb more than 93% of the incident light over the entire visible spectrum due to the canopy of dendritic nanostructures within a nanoscale roughness with potential applications in photovoltaic solar cells [77]. **Figure 6** summarizes the general methods used for electrodeposi-

The physical and morphological properties of nanoporous thin films are strongly

linked with material's porosity (defined as the ratio of void volume to the total volume of the film). Hence, many efforts have been devoted to developing a reliable self-consistent quantitative characterization of their porosity [78]. The physical characteristics of thin films have been characterized by a combination of X-ray photoelectron spectroscopy (XPS), grazing-incidence small-angle X-ray scattering (GISAXS) along with adsorption isotherm surface area measurements. Porosity and internal feature sizes range from a few to tens of nanometers [79]. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) experiments have the potential to find the electrochemically active surface area of the NPG film. Studies have shown that using CV and EIS it was shown that NPG films have 4–8.5 times more accessible surface area than thermally evaporated gold (EG) films [80]. Potential step (PS) chronoamperometry has also been combined with surface plasmon resonance (SPR) for probing electrochemical deposition, conformational changes linked with redox-initiated film reorganization, and the quantification of electrodeposited thin film thickness [81]. A well-recognized approach to determine the specific surface area of nanoporous materials is the Brunauer, Emmett, and Teller (BET) method which is based on physical adsorption of gas molecules to determine the specific surface area [82]. Many groups have extensively used scanning electron microscopy (SEM) for rapidly exploring pore size and ligament size due to their ease of use and applicability to varying types of samples. Elastic modulus is an important mechanical property to calculate residual stress in free-standing beams. This in turn determines film stiffness and therefore, sensor performance [83]. SEM and AFM techniques have been used by many researchers to characterize the surface morphology, size, and shape of the pores as well as the surface roughness of the porous gold films [84]. Maaroof and coworkers have measured the optical properties of porous gold film using the techniques of spectrophotometry and ellipsometry. The spectral response was delivered using a homogeneous Lorentz-Drude (L-D) model and showed that the optical properties of NPG films are dependent on void occupancies [44]. Nanoporous metals have significant geometric complexity in the form of random bicontinuous structures possessing bubbles within ligaments, regions of very high negative, positive, and saddlepoint curvature, and multiple facets. Erlebacher introduced methods to geometrically quantify the structure of nanoporous metals using large-scale kinetic Monte Carlo simulations using mesh-smoothing algorithms [85]. Atom probe tomography (APT) analysis of NPG material, produced by dealloying can give huge information regarding the compositional variation within the structure of nanoscale ligaments. 3-D analysis of materials at the sub-nanometer scale is possible by careful preparation of samples through a reproducible process for complete pore filling through electrodeposition of copper into finely sized pores. Compositional profiling and

#### **Figure 6.**

*A general overview of the techniques used for electrodeposition to form thin nanoporous films.*

*Electrodeposition of Nanoporous Gold Thin Films DOI: http://dx.doi.org/10.5772/intechopen.94604*

*Nanofibers - Synthesis, Properties and Applications*

due to minimal electron–hole recombination [73].

superior electroactivity, electrochemical, and interfacial characteristics suggesting the suitability of such electrodes for sensors, electrocatalysis, and diode applications [72]. Epitaxial, ultrathin, semitransparent, and catalytic gold films were electrodeposited on n-type silicon (n-Si) to protect the substrate from photo-passivation, see **Figure 5**. In addition to being a good redox catalyst, the ultrathin gold layers serve to induce band bending in n-Si thereby making it a diode of an ideal quality factor

Fabrication methods have been used in combination in the past to generate a promising eco-friendly biosensor platform with advantages coming from both the techniques. One such study has been done to fabricate a glucose biosensor wherein AuNPs and glucose oxidase multilayer films were generated via electrodeposition and self-assembly respectively [74]. As an eco-friendly alternative, deep eutectic solvents (DESs) are a new class of green and sustainable solvents extensively used for electrodepositions due to their intrinsic properties of good solubility, nonflammability, low toxicities, and suitable electrochemical windows. Recently, an electrochemical method has been developed to fabricate NPG electrodes by alloying and dealloying the precursor alloy in ZnCl2-urea deep eutectic solvent [75]. The wide acceptance of DES in metal/alloy electrodeposition processes has been seen in recent times due to their ability to act as stabilizers and reducing agents in metal nanoparticle synthesis. Moreover, the nucleation rates are enhanced leading to a

*Illustration showing the formation of an ultrathin epitaxial gold layer electrodeposited on n-Si (111) to form a Schottky junction in a regenerative photoelectrochemical cell. Reproduced with permission from reference [73],* 

*A general overview of the techniques used for electrodeposition to form thin nanoporous films.*

**152**

**Figure 6.**

**Figure 5.**

*Copyright 2018, American Chemical Society.*

decrease in the particle size in the presence of DES as it has relatively low surface tension [76]. Remarkable optical behavior has been observed when the bulk material is transformed into nanostructured surfaces. A simple two-step electrochemical process has been developed wherein electrodeposition and anodization is used in conjunction to generate black gold surfaces that can absorb more than 93% of the incident light over the entire visible spectrum due to the canopy of dendritic nanostructures within a nanoscale roughness with potential applications in photovoltaic solar cells [77]. **Figure 6** summarizes the general methods used for electrodeposition to form thin nanoporous films.
