*3.3.2 Piezoelectric energy harvesting with ZTO nanostructures*

Nanogenerators are devices that can convert external stimulus into electrical energy, being highly interesting for smart and self-sustainable surfaces, as they can be used for sustainable energy sources, biomedical systems and smart sensors [91]. Due to its excellent ferroelectric and piezoelectric properties, different ZnSnO3 nanostructures (i.e., nanowires, nanoplates, nanocubes) have been widely explored for energy harvesting devices and sensitive human motion sensors, through their piezoelectric (induction of electrical charge by the applied mechanical strain) and piezoresistive (electrical resistivity change by the applied mechanical strain) effects, respectively [45, 66, 92–94]. The fcc-ZnSnO3 nanocubes have been the most popular ZTO structures for these applications. For instance, Wang *et al*. reported the nanogenerators of fcc-ZnSnO3 nanocubes mixed with polydimethylsiloxane (PDMS), reaching a maximum output of 400 V, 28 μA at a current density of <sup>7</sup> <sup>μ</sup><sup>A</sup>cm<sup>2</sup> [95]. While, Paria *et al.* mixed fcc-ZnSnO3 nanocubes with polyvinyl chloride (PVC), achieving a maximum output of ≈40 V and ≈1.4 μA, corresponding to a power density of 3.7 <sup>μ</sup><sup>W</sup>cm<sup>3</sup> (**Figure 8a**) [94].

#### **Figure 8.**

*h*<sup>þ</sup> þ *H*2*O* ! *H*<sup>þ</sup> þ *OH*\_ (17) *h*<sup>þ</sup> þ *OH*� ! *OH*\_ (18)

<sup>þ</sup> *dye* ! *degradation products* (21)

� *cb*

*Dye* <sup>þ</sup> *OH:* ! *CO*<sup>2</sup> <sup>þ</sup> *<sup>H</sup>*2*<sup>O</sup>* <sup>þ</sup> *degradation products* (28)

*Zn*2*SnO*<sup>4</sup> *<sup>h</sup>*<sup>þ</sup> <sup>þ</sup> *OH*� ! *OH:* (24)

*H*2*O*<sup>2</sup> ! 2*OH*\_ (27)

<sup>þ</sup> *<sup>O</sup>*<sup>2</sup> ! *Zn*2*SnO*<sup>4</sup> <sup>þ</sup> *<sup>O</sup>*�*:*

<sup>2</sup> þ *H*<sup>þ</sup> ! *HO*2\_ (20)

� (19)

<sup>þ</sup> *Zn*2*SnO*<sup>4</sup> *<sup>h</sup>*<sup>þ</sup> (22)

<sup>2</sup> (23)

) [81].

<sup>2</sup> (25)

*H*2*O*<sup>2</sup> (26)

*e*

*O*° �

*Zn*2*SnO*<sup>4</sup> þ *hv* ! *Zn*2*SnO*<sup>4</sup> *e*

� *cb*

behavior on methylene blue degradation under visible light (0.0156 min�<sup>1</sup>

2.5 h, with a degradation rate of 4.5 � <sup>10</sup>�<sup>2</sup> min�<sup>1</sup> [87].

**Figure 7.**

**14**

Alternatives to the conventional photocatalytic approach have also been explored, making use of the piezoelectric properties of materials such as ZnSnO3 (nanowires and nanoplates) for piezocatalysis (in the dark) [87] or for piezophotocatalysis (under illumination) [42, 87, 88]. Indeed, piezoelectricity and ferroelectricity (associated with perovskite structures) have shown to play an important role in photocatalysis, since the photogeneration of electron–hole pairs is enhanced by the dipole moment formed by the polarization electric field across polar materials [89, 90]. A schematic representation of the piezocatalytic mechanism is presented in **Figure 7b** and shows the influence of the characteristic polarization of the piezoelectric materials, which contributes to the generation of hydroxyl radicals and consequently degradation of rhodamine B. The dye degradation was achieved in

*(a) Piezocatalysis using ZnSnO3 nanoparticles under ultrasound exposure. (b) Schematic of the piezocatalytic*

*mechanism. Reprinted with permission from [87]. Copyright (2020) American Chemical Society.*

*OH*°, *O*°

the following equations:

*Novel Nanomaterials*

� <sup>2</sup> , *HO*°2

*Zn*2*SnO*<sup>4</sup> *e*

*O*�*:*

*HO:*

� þ *O*<sup>2</sup> ! *O*2\_

Considering photocatalytic activity under visible light, Jain *et al.* [84] proposes

<sup>2</sup> <sup>þ</sup> *<sup>H</sup>*2*<sup>O</sup>* ! *OH*� <sup>þ</sup> *HO:*

Zn2SnO4 nanocrystals were used for the degradation of 50% of reactive red 141 dye in 270 min under sunlight [85]. Different ZnSnO3 structures such as nanowires and nanoplates were already used as photocatalysts for organic pollutants (for example, methylene blue and rhodamine B) [33, 40, 86]. Due to its high optical band gap (3.3– 3.9 eV) UV light is usually required to photoactivate this material. Nevertheless, fcc-ZnSnO3 nanoparticles were already reported with a very satisfactory photocatalytic

<sup>2</sup> <sup>þ</sup> *<sup>H</sup>*2*<sup>O</sup>* ! *OH:*

*Hybrid nanogenerators of: (a) a composite film based on fcc-ZnSnO3 nanocubes and PVC. Reprinted with permission from [94], copyright 2020 American Chemical Society; and (b) a composite film based on orth-ZnSnO3 nanowires and PDMS. (c) Schematic of the charge generation mechanism in the micro-structured devices of (b). Images (b) and (c) were reprinted with permission from [63], copyright 2020 American Chemical Society.*

ZnSnO3 nanoplates were also applied for nanogenerators. Guo *et al.* reported produced nanogenerators fabricated with orth-ZnSnO3 nanoplates embedded in flat films of PDMS, reaching voltage and current outputs of 20 V and 0.6 μA, respectively, under bending stress [45]. More recently in our group, orth-ZnSnO3 nanowires were mixed with PDMS to fabricate nanogenerators of micro-structured composites (**Figure 8b**) [63]. In the same work, a charge generation and displacement mechanism was proposed, as depicted in **Figure 8c**. Briefly, the microstructures induced in PDMS are suggested to improve the force delivery to the nanowires, enhancing its piezoelectric signal, while bringing also a triboelectric contribution to the nanogenerator output. This results in an output voltage, current and instantaneous power of approximately 9 V, 1 <sup>μ</sup>A and 3 <sup>μ</sup><sup>W</sup>cm<sup>2</sup> , respectively, when applying a force of only 10 N. For higher forces the devices were capable to reach outputs around 120 V and 13 μA, which was shown to be enough energy to light up LEDs and several small electronic devices [63].

typically presenting smaller response times and superior stabilities compared to binary compounds [103]. Moreover, the implementation of these nanostructures in sensors allows miniaturization of the devices, as well as cost reduction. ZnSnO3 has been reported as an excellent humidity sensor, in different nanostructure forms such as nanoparticles or even in composites of ZnSnO3 nanocubes and Ag nanowires [29, 104]. Additionally, ZnSnO3 nanoparticles were used as electrochemical biosensors for label free sub-femtomolar detection of cardiac biomarker troponin T and a composite of Zn2SnO4 nanoparticles and graphene was used for morphine and codeine detection [105, 106]. Recently, Durai *et al.*. reported ultraselective sensors, based on ZnSnO3 nanocubes modified glassy carbon electrode (GCE), for simultaneous detection of uric acid and dopamine through differential pulse voltammetry technique [107]. Zn2SnO4 and ZnSnO3 nanostructures of different shapes such as nanoparticles, nanowires and nanocubes, have also been widely explored as photoconductors [23, 108–111]. While the optical band gap of these materials is typically in the UV energy levels (hence their transparency in visible range), quantum confinement effects or even defect levels near the band edges can be explored to increase the absorption for lower energy levels. Other applications that have been explored using ZTO nanostructures are related with energy storage and conversion. Zn2SnO4 has been widely used as photoanode for dye solar cells in different nanostructure morphologies such as nanoparticles and nanowires [21, 35]. Cherian *et al.* reported the performance of nanowires and compared with nanoplates of Zn2SnO4 for Li-batteries [34]. Supercapacitors (SC) have also started to be explored using ZTO nanostructures, with Bao *et al.* having reported the use of Zn2SnO4/MnO2 core shell in carbon fibers showing a capaci-

*DOI: http://dx.doi.org/10.5772/intechopen.94294*

*Hydrothermal Synthesis of Zinc Tin Oxide Nanostructures for Photocatalysis, Energy Harvesting…*

Expanding LAE to IoT and smart surface concepts requires an increasing number of objects to have embedded electronics, sensors and connectivity, driving a demand for compact, smart, multifunctional and self-sustainable technology with low associated costs. While nanomaterials are thought to be able to meet these requirements, playing an important role in the future technological world, low cost and sustainable technologies are demanded. For this, both low cost fabrication methods and sustainable materials must be considered. This chapter shows the versatility of the hydrothermal method to control the growth and morphology of zinc tin oxide (ZTO) nanostructures, and the variety of shapes that can be produced for each of the different ZTO phases. Compared to other preparation methods, especially vapor phase methods, hydrothermal synthesis reveals a large set of advantages from both research and industrial viewpoints. First, while the multitude of parameters to control requires an in-depth understanding of their role in the final products, it also brings enormous flexibility to tune the synthesis process for the desired results. Also, it can be performed at low temperature (< 200°C), which is compatible with a wide range of substrates for direct growth, while assuring lower costs. This links perfectly with the demonstrated upscaling capability of hydrothermal synthesis which is a crucial aspect for industrial

Furthermore, a summary of exciting results that have been reported regarding

application in devices of these ZTO nanostructures over the past few years is presented. The multifunctionality of this material system is highlighted by its successful implementation in energy harvesters, photocatalysis, electronic devices,

tance of 621.6 F<sup>g</sup> <sup>1</sup> [112].

**4. Conclusions**

implementation.

sensors, and others.

**17**

#### *3.3.3 Electronic applications*

Electronic applications are always a relevant drive for materials. Multicomponent semiconductor nanostructures as ZTO are particularly interesting for these applications, with wide band gap semiconductors allowing for high-power and high-frequency operations [50]. Field-effect transistors (FETs) are the key elements enabling today's electronics, being 1D nanostructures particularly interesting in this regard, given the easiness of confining migratory direction of charge carriers through its length, i.e., between source and drain electrodes. Indeed, 1D nanostructures have already proven great usefulness for the upcoming generations of semiconductors in FETs [96]. While several reports already demonstrated ZTO as a candidate for replacement of IGZO in thin film technologies [13], similarly, ZTO is also one of the most promising multicomponent metal oxides for transistors with nanostructures [62]. Demonstrations of discrete Zn2SnO4 nanotransistors have already been made using nanotransfer molding of ZTO inks followed by annealing at 500°C, or by simple pick-and-place approach of drop-casted ZTO nanowires prepared by CVD above 700°C and by thermal evaporation at 1000°C [39, 97, 98]. While the achievement of on/off ratio ≈10<sup>6</sup> and field-effect mobility ≈20 cm<sup>2</sup> /Vs is a good demonstration of the ZTO's potential, transistors using ZTO nanostructures synthesized by solution processes have not been reported yet. Furthermore, these nanostructures have also been used for the resistive switch layer in the emerging type of memory devices known as memristors. Reports show ZTO as the active material in memristors in the form of both Zn2SnO4 nanowires and ZnSnO3 nanocubes, being the latter especially relevant for this application due to its ferroelectric properties. Properties such as high off/on ratios (>10<sup>5</sup> ), long retention times (>5 months) and fast response speeds (<20 ns) are obtained for these devices [99, 100].

Transforming ZTO or other nanostructures into well-established LAE semiconductor materials, while highly desirable from the performance and functionality point of view, will still require significant advances in reliable techniques for alignment and density control in transparent (and flexible) substrates [101].

#### *3.3.4 Other applications*

Besides the applications briefly presented above, ZTO nanostructures have also been widely used in sensing applications, with gas sensors being the most popular [102]. Their small crystallite size, high surface-to-volume ratios and surface reactivity result in enhanced sensitivities/selectivity, with multicomponent materials

*Hydrothermal Synthesis of Zinc Tin Oxide Nanostructures for Photocatalysis, Energy Harvesting… DOI: http://dx.doi.org/10.5772/intechopen.94294*

typically presenting smaller response times and superior stabilities compared to binary compounds [103]. Moreover, the implementation of these nanostructures in sensors allows miniaturization of the devices, as well as cost reduction. ZnSnO3 has been reported as an excellent humidity sensor, in different nanostructure forms such as nanoparticles or even in composites of ZnSnO3 nanocubes and Ag nanowires [29, 104]. Additionally, ZnSnO3 nanoparticles were used as electrochemical biosensors for label free sub-femtomolar detection of cardiac biomarker troponin T and a composite of Zn2SnO4 nanoparticles and graphene was used for morphine and codeine detection [105, 106]. Recently, Durai *et al.*. reported ultraselective sensors, based on ZnSnO3 nanocubes modified glassy carbon electrode (GCE), for simultaneous detection of uric acid and dopamine through differential pulse voltammetry technique [107]. Zn2SnO4 and ZnSnO3 nanostructures of different shapes such as nanoparticles, nanowires and nanocubes, have also been widely explored as photoconductors [23, 108–111]. While the optical band gap of these materials is typically in the UV energy levels (hence their transparency in visible range), quantum confinement effects or even defect levels near the band edges can be explored to increase the absorption for lower energy levels. Other applications that have been explored using ZTO nanostructures are related with energy storage and conversion. Zn2SnO4 has been widely used as photoanode for dye solar cells in different nanostructure morphologies such as nanoparticles and nanowires [21, 35]. Cherian *et al.* reported the performance of nanowires and compared with nanoplates of Zn2SnO4 for Li-batteries [34]. Supercapacitors (SC) have also started to be explored using ZTO nanostructures, with Bao *et al.* having reported the use of Zn2SnO4/MnO2 core shell in carbon fibers showing a capacitance of 621.6 F<sup>g</sup> <sup>1</sup> [112].

### **4. Conclusions**

ZnSnO3 nanoplates were also applied for nanogenerators. Guo *et al.* reported produced nanogenerators fabricated with orth-ZnSnO3 nanoplates embedded in flat films of PDMS, reaching voltage and current outputs of 20 V and 0.6 μA, respectively, under bending stress [45]. More recently in our group, orth-ZnSnO3

nanowires were mixed with PDMS to fabricate nanogenerators of micro-structured composites (**Figure 8b**) [63]. In the same work, a charge generation and displacement mechanism was proposed, as depicted in **Figure 8c**. Briefly, the microstructures induced in PDMS are suggested to improve the force delivery to the nanowires, enhancing its piezoelectric signal, while bringing also a triboelectric contribution to the nanogenerator output. This results in an output voltage, current

when applying a force of only 10 N. For higher forces the devices were capable to reach outputs around 120 V and 13 μA, which was shown to be enough energy to

Multicomponent semiconductor nanostructures as ZTO are particularly interesting for these applications, with wide band gap semiconductors allowing for high-power and high-frequency operations [50]. Field-effect transistors (FETs) are the key elements enabling today's electronics, being 1D nanostructures particularly interesting in this regard, given the easiness of confining migratory direction of charge carriers through its length, i.e., between source and drain electrodes. Indeed, 1D nanostructures have already proven great usefulness for the upcoming generations of semiconductors in FETs [96]. While several reports already demonstrated ZTO as a candidate for replacement of IGZO in thin film technologies [13], similarly, ZTO is also one of the most promising multicomponent metal oxides for transistors with nanostructures [62]. Demonstrations of discrete Zn2SnO4 nanotransistors have already been made using nanotransfer molding of ZTO inks followed by annealing at 500°C, or by simple pick-and-place approach of drop-casted ZTO nanowires prepared by CVD above 700°C and by thermal evaporation at 1000°C [39, 97, 98]. While the achievement of on/off ratio ≈10<sup>6</sup> and field-effect mobility ≈20 cm<sup>2</sup>

a good demonstration of the ZTO's potential, transistors using ZTO nanostructures synthesized by solution processes have not been reported yet. Furthermore, these nanostructures have also been used for the resistive switch layer in the emerging type of memory devices known as memristors. Reports show ZTO as the active material in memristors in the form of both Zn2SnO4 nanowires and ZnSnO3 nanocubes, being the latter especially relevant for this application due to its ferro-

(>5 months) and fast response speeds (<20 ns) are obtained for these devices

Transforming ZTO or other nanostructures into well-established LAE semiconductor materials, while highly desirable from the performance and functionality point of view, will still require significant advances in reliable techniques for alignment and density control in transparent (and flexible) substrates [101].

Besides the applications briefly presented above, ZTO nanostructures have also been widely used in sensing applications, with gas sensors being the most popular [102]. Their small crystallite size, high surface-to-volume ratios and surface reactivity result in enhanced sensitivities/selectivity, with multicomponent materials

electric properties. Properties such as high off/on ratios (>10<sup>5</sup>

, respectively,

/Vs is

), long retention times

and instantaneous power of approximately 9 V, 1 <sup>μ</sup>A and 3 <sup>μ</sup><sup>W</sup>cm<sup>2</sup>

Electronic applications are always a relevant drive for materials.

light up LEDs and several small electronic devices [63].

*3.3.3 Electronic applications*

*Novel Nanomaterials*

[99, 100].

**16**

*3.3.4 Other applications*

Expanding LAE to IoT and smart surface concepts requires an increasing number of objects to have embedded electronics, sensors and connectivity, driving a demand for compact, smart, multifunctional and self-sustainable technology with low associated costs. While nanomaterials are thought to be able to meet these requirements, playing an important role in the future technological world, low cost and sustainable technologies are demanded. For this, both low cost fabrication methods and sustainable materials must be considered. This chapter shows the versatility of the hydrothermal method to control the growth and morphology of zinc tin oxide (ZTO) nanostructures, and the variety of shapes that can be produced for each of the different ZTO phases. Compared to other preparation methods, especially vapor phase methods, hydrothermal synthesis reveals a large set of advantages from both research and industrial viewpoints. First, while the multitude of parameters to control requires an in-depth understanding of their role in the final products, it also brings enormous flexibility to tune the synthesis process for the desired results. Also, it can be performed at low temperature (< 200°C), which is compatible with a wide range of substrates for direct growth, while assuring lower costs. This links perfectly with the demonstrated upscaling capability of hydrothermal synthesis which is a crucial aspect for industrial implementation.

Furthermore, a summary of exciting results that have been reported regarding application in devices of these ZTO nanostructures over the past few years is presented. The multifunctionality of this material system is highlighted by its successful implementation in energy harvesters, photocatalysis, electronic devices, sensors, and others.
