**3.3 GaN material having different doping**

water splitting, Park et al. used the plasma-assisted molecular beam epitaxy (PAMBE) technique to grow InGaN/GaN multiple quantum wells (MQWs) grown on hollow n-GaN nanowires (**Figure 4c**) [74]. The hollow and InGaN/GaN multiple quantum well structures of the nanowires allow the incident light to be refracted multiple times, increasing the absorption of light. **Figure 4d** shows the incident photon-to-current conversion efficiency value of the device, which can be found that the highest IPCE value of the device is as high as 33.3% and 415 μmol of

Nanopores, nanocones, and honeycombs are other nanostructures of GaN. **Figure 5a** shows the GaN nanopore structure [43], nanopore structure used electrochemical lateral etching and ICP etching to prepare laterally porous, vertically holes well-ordered GaN. This structure reduces the UV reflectivity. The ordered vertical holes not only help open the embedded channels to the electrolyte on both sides and reduce the migration distance of bubbles in the water splitting reaction but also help to modulate the light field. Incident light can be modulated and captured into the nanopore to enhance the absorption of light, so the saturation photocurrent was 4.5 times that of the planar structure, as shown in **Figure 5d**. Moreover, GaN with aligned nanopore structure had been fabricated by combining MOCVD using a lateral anodic etching, as shown in **Figure 5b** [75]. Laterally porous 3D hierarchical nanostructures not only provided a large contact area between the electrode and the electrolyte but also increased the absorption of light and provided a channel for the transmission of light and electrons. The device also achieved high

values of photocurrent of 0.32 mA/cm<sup>2</sup> by using etching voltages at 10 V (**Figure 5e**). Kim et al. had prepared GaN truncated nanocones [76], which was shown in **Figure 5c**. GaN truncated nanocones have concentrated incident light inside the nanostructure and enhanced the light trapping with reduced light losses from surface reflection. The relationship between current density and potential was shown in **Figure 5f**, which indicated that the photocurrent of GaN truncated

The above structures are expected, and GaN can also have nanorods [77], nanocolumns [78], nano-pyramids [79], and so on. It can be known from the above results that changing the morphology of GaN influences the efficiency of PEC water

*(a) [43], (b) [75], and (c) [76] are the structure schematics of the composite porous GaN, laterally porous GaN, and photoelectrochemical cell, respectively. (d) Relation curves between photocurrent density and voltage of the above three GaN photoelectrodes [43]. (e) The photocurrent and applied current curves under different GaN etching voltages [75]. (f) Dark (segment) and illumination (straight) conditions for photocurrent density in linear scanning voltammetry [76]. (a and d), (b and e), and (c and f) reproduced from Ref. [43, 75, 76],*

*respectively, with permission from the American Chemical Society.*

nanocones was three times higher than the planar structure.

**Figure 5.**

**70**

hydrogen gas was generated within 1 hour.

*Nanowires - Recent Progress*

Doping is a commonly used and effective method to improve the performance of materials. It mainly adjusts the energy band of the material, so that the photogenerated electrons and holes are better transported and high efficiency of PEC water splitting is obtained. Zhou and co-workers doped ZnO-GaN (GZNO) solid solution with La, as shown in **Figure 6a** [80]. La-dopant incorporation is optimized to adjust the bending of the band gap, which increases the thickness of the space charge region, thereby improving the separation of photogenerated carriers. **Figure 6c** shows the photocatalytic performance of GZNO and 3% La GZNO. It can be clearly seen that the photocatalyst doped with La produces more hydrogen and oxygen under the same conditions, which indicates that the performance of the photocatalyst is significantly improved after doping. **Figure 6b** shows the schematic of Ni-doped AlN and two-dimensional GaN monolayers [81]. By controlling the doping content of Ni, it can adjust the band bending of GaN. **Figure 6d** displays the binding strength of GaN and AlN composites with different transition metals doped. It can be found that Ni doping is the best for OER because they have small OER overpotentials.

GaN doped with Mn [82], Mg [83], or CrO are also reported [84]. Doping is also a good method to improve the efficiency of PEC water splitting. It mainly adjusts the energy band of GaN through doping, thereby promoting the separation of photogenerated electrons and holes and effectively preventing the recombination of

#### **Figure 6.**

*(a) TEM images of 3% La GZNO [80]. (b) Schematic diagram of Ni-doped structure [81]. (c) The amount of H2 and O2 produced by overall water splitting after 8 hours of GZNO and 3% La GZNO [80]. (d) Binding strength of OH or O of different transition metal-doped GaN and AlN composites [81]. (a and c) Reproduced from Ref. [73] with permission from The Royal Society of Chemistry. (b and d) Reproduced from Ref. [74] with permission from the American Chemical Society.*

carriers. However, excessive doping will deteriorate the crystal quality of the material. So, it is important to choose the doping material and control dopant incorporation.

### **3.4 Composition of solid solution**

The solid solution is a wurtzite structure composite material composed of GaN and ZnO mixed in a certain proportion. It adjusts the doping content of ZnO to change the band gap of the solid solution and realizes PEC of water splitting under the visible light. This concept was first proposed by Maeda and co-workers [85]. And then, Ohno et al. used Rh2 *<sup>y</sup>*Cr*y*O3 nanoparticles to modify the solid solution, and the device shows outstanding stability; it has been working continuously for half a year under light irradiation, as shown in **Figure 7a** [86]. The co-catalyst is beneficial to suppress the oxidative decomposition of the solid solution, thereby making the device more stable. NiCoFeP and flux-assisted method can also be used to modify the solid solution to improve the efficiency of PEC water splitting [87, 88]. The conversion efficiency of solar energy using NiCoFeP-modified solid solution exceeds 1% at 1.23 V vs. RHE. To further improve the efficiency of PEC water splitting, solid solution nanosheets modified with Rh nanoparticles have been proposed, as shown in **Figure 7d** [89]. This shows 0.7 μmol h<sup>1</sup> g<sup>1</sup> of hydrogen production in an aqueous H2SO4 solution. The nitridation process was used to change the morphology from hexagonal 2D ZnGa2O4 nanosheets to 2D (GaN)1 x(ZnO)x nanosheets, reducing the path of carrier transportation and decreasing the recombination of electrons and holes. So, the composition of a solid solution or multiple-metal incorporation can expand the light absorption range of the device, improving the absorption of light and increasing the efficiency of PEC water splitting.

nanowire arrays on Si substrate by catalyst-free MBE, as shown in **Figure 8a** [72]. It has a photoelectric conversion efficiency of up to 27% under ultraviolet and visible light irradiation. The photoelectrode continued to work for 10 hours, and the hydrogen production was consistent with the theoretical value (**Figure 8d**), which indicates that the photoelectrode has good stability and hydrogen production ability. And the quadruple-band InGaN nanowire arrays were integrated on a nonpolar substrate, which includes In0.35Ga0.65N, In0.27Ga0.73N, In0.20Ga0.80N, and GaN and

*(a) Schematic of the InGaN/GaN core-shell structure [72]. (b) Schematic diagram of ideal light absorption structure of a multiband InGaN stack with different indium compositions [95]. (c) Schematic diagram of photocatalytic overall water splitting reaction mechanism [48]. (d) Hydrogen production in 1 mol/L HBr at 0.2 V vs. the counter electrode [72]. (e) Hydrogen and oxygen produced as a function of time under multiple experiment cycles [95]. (f) Hydrogen and oxygen evolution as a function of irradiation time under full arc (>300 nm) 300 W xenon lamp irradiation [48]. (a and d) and (c and e) Reproduced from Ref. [48, 72] with permission from the American Chemical Society. (b and f) Reproduced from Ref. [95] with permission from*

*Recent Progress in Gallium Nitride for Photoelectrochemical Water Splitting*

*DOI: http://dx.doi.org/10.5772/intechopen.92848*

(**Figure 8b**) [95]. Multiband nanowire arrays enhance light absorption to improve the performance of PEC water splitting. Moreover, the multiband nanowire array photoelectrode has good stability and high photocatalyst efficiency for overall water splitting, as shown in **Figure 8e**. To improve the efficiency of the photolysis of water, InGaN heterostructures have been proposed. Kibria and co-workers have fabricated InGaN/GaN nanowire heterostructures, in which the internal quantum efficiency is about 13% [48]. The nanowire heterostructure is shown in **Figure 8c**. The combination of GaN and InGaN expands the light absorption range of GaN from ultraviolet light to visible light, which greatly improves the light absorption range and improves

photoelectrode also exhibits extremely high stability and high hydrogen production capabilities, as shown in **Figure 8f**. Moreover, nanowire arrays [96], tunnel junction

In summary, the multiple-metal incorporation can greatly improve the efficiency of PEC water splitting of GaN. The structures and In content will greatly affect the efficiency of PEC water splitting. So, it is important to choose a suitable structure and the In content while preparing the GaN-based photoelectrode.

This review mainly introduces the application of GaN in the PEC water splitting

system and summarizes the methods to improve the efficiency of PEC water

exhibits a solar-to-hydrogen efficiency of 5.2% in a relatively stable state

the efficiency of photolysis. The InGaN/GaN nanowire heterostructure

nanowire [97], have also been reported.

**4. Conclusion**

**73**

**Figure 8.**

*The Royal Society of Chemistry.*

#### **3.5 The multiple-metal incorporation**

The method of forming multiple-metal incorporation is similar to that of a solid solution. Different In content incorporation can change the band gap of GaN to widen the absorption spectrum range. Many different multiple-metal incorporations have been proposed [90–94]. AlOtaibi et al. grown InGaN/GaN core-shell

#### **Figure 7.**

*(a) Curve of total hydrogen production over time [86]. (b) The relationship between photocurrent and voltage for GaN-ZnO in different ways of treatment [87]. (c) Variation curve of the amount of hydrogen and oxygen produced with time of (GaN)1* <sup>x</sup>*(ZnO)*<sup>x</sup> *solid solutions from Zn2Ga-LDH modified with Rh2 yCryO3 nanoparticles [88]. (d) Structure conversion flowchart from 2D ZnGa2O4 to 2D (GaN)1* <sup>x</sup>*(ZnO)*<sup>x</sup> *[89]. (a–d) Reproduced from Ref. 86–89 with permission from the American Chemical Society.*

*Recent Progress in Gallium Nitride for Photoelectrochemical Water Splitting DOI: http://dx.doi.org/10.5772/intechopen.92848*

#### **Figure 8.**

carriers. However, excessive doping will deteriorate the crystal quality of the material. So, it is important to choose the doping material and control dopant incorporation.

The solid solution is a wurtzite structure composite material composed of GaN and ZnO mixed in a certain proportion. It adjusts the doping content of ZnO to change the band gap of the solid solution and realizes PEC of water splitting under the visible light. This concept was first proposed by Maeda and co-workers [85]. And then, Ohno et al. used Rh2 *<sup>y</sup>*Cr*y*O3 nanoparticles to modify the solid solution, and the device shows outstanding stability; it has been working continuously for half a year under light irradiation, as shown in **Figure 7a** [86]. The co-catalyst is beneficial to suppress the oxidative decomposition of the solid solution, thereby making the device more stable. NiCoFeP and flux-assisted method can also be used to modify the solid solution to improve the efficiency of PEC water splitting [87, 88]. The conversion efficiency of solar energy using NiCoFeP-modified solid solution exceeds 1% at 1.23 V vs. RHE. To further improve the efficiency of PEC water splitting, solid solution nanosheets modified with Rh nanoparticles have been proposed, as shown in **Figure 7d** [89]. This shows 0.7 μmol h<sup>1</sup> g<sup>1</sup> of hydrogen production in an aqueous H2SO4 solution. The nitridation process was used to change the morphology from hexagonal 2D ZnGa2O4 nanosheets to 2D (GaN)1 x(ZnO)x nanosheets, reducing the path of carrier transportation and decreasing the recombination of electrons and holes. So, the composition of a solid solution or multiple-metal incorporation can expand the light absorption range of the device, improving the absorption of light and

The method of forming multiple-metal incorporation is similar to that of a solid solution. Different In content incorporation can change the band gap of GaN to widen the absorption spectrum range. Many different multiple-metal incorporations have been proposed [90–94]. AlOtaibi et al. grown InGaN/GaN core-shell

*(a) Curve of total hydrogen production over time [86]. (b) The relationship between photocurrent and voltage for GaN-ZnO in different ways of treatment [87]. (c) Variation curve of the amount of hydrogen and oxygen produced with time of (GaN)1* <sup>x</sup>*(ZnO)*<sup>x</sup> *solid solutions from Zn2Ga-LDH modified with Rh2 yCryO3 nanoparticles [88]. (d) Structure conversion flowchart from 2D ZnGa2O4 to 2D (GaN)1* <sup>x</sup>*(ZnO)*<sup>x</sup> *[89].*

*(a–d) Reproduced from Ref. 86–89 with permission from the American Chemical Society.*

**3.4 Composition of solid solution**

*Nanowires - Recent Progress*

increasing the efficiency of PEC water splitting.

**3.5 The multiple-metal incorporation**

**Figure 7.**

**72**

*(a) Schematic of the InGaN/GaN core-shell structure [72]. (b) Schematic diagram of ideal light absorption structure of a multiband InGaN stack with different indium compositions [95]. (c) Schematic diagram of photocatalytic overall water splitting reaction mechanism [48]. (d) Hydrogen production in 1 mol/L HBr at 0.2 V vs. the counter electrode [72]. (e) Hydrogen and oxygen produced as a function of time under multiple experiment cycles [95]. (f) Hydrogen and oxygen evolution as a function of irradiation time under full arc (>300 nm) 300 W xenon lamp irradiation [48]. (a and d) and (c and e) Reproduced from Ref. [48, 72] with permission from the American Chemical Society. (b and f) Reproduced from Ref. [95] with permission from The Royal Society of Chemistry.*

nanowire arrays on Si substrate by catalyst-free MBE, as shown in **Figure 8a** [72]. It has a photoelectric conversion efficiency of up to 27% under ultraviolet and visible light irradiation. The photoelectrode continued to work for 10 hours, and the hydrogen production was consistent with the theoretical value (**Figure 8d**), which indicates that the photoelectrode has good stability and hydrogen production ability. And the quadruple-band InGaN nanowire arrays were integrated on a nonpolar substrate, which includes In0.35Ga0.65N, In0.27Ga0.73N, In0.20Ga0.80N, and GaN and exhibits a solar-to-hydrogen efficiency of 5.2% in a relatively stable state (**Figure 8b**) [95]. Multiband nanowire arrays enhance light absorption to improve the performance of PEC water splitting. Moreover, the multiband nanowire array photoelectrode has good stability and high photocatalyst efficiency for overall water splitting, as shown in **Figure 8e**. To improve the efficiency of the photolysis of water, InGaN heterostructures have been proposed. Kibria and co-workers have fabricated InGaN/GaN nanowire heterostructures, in which the internal quantum efficiency is about 13% [48]. The nanowire heterostructure is shown in **Figure 8c**. The combination of GaN and InGaN expands the light absorption range of GaN from ultraviolet light to visible light, which greatly improves the light absorption range and improves the efficiency of photolysis. The InGaN/GaN nanowire heterostructure photoelectrode also exhibits extremely high stability and high hydrogen production capabilities, as shown in **Figure 8f**. Moreover, nanowire arrays [96], tunnel junction nanowire [97], have also been reported.

In summary, the multiple-metal incorporation can greatly improve the efficiency of PEC water splitting of GaN. The structures and In content will greatly affect the efficiency of PEC water splitting. So, it is important to choose a suitable structure and the In content while preparing the GaN-based photoelectrode.

#### **4. Conclusion**

This review mainly introduces the application of GaN in the PEC water splitting system and summarizes the methods to improve the efficiency of PEC water

splitting. The methods to enhance efficiency are mainly carried out in the following four aspects, such as morphology, doping, surface modification, and composition of solid solution or multiple-metal incorporation. Up to now, GaN has made great progress in the application of PEC water splitting; the solar-to-hydrogen efficiency of 12.6% has already been obtained without any external bias [98], better than CoP catalyst electrodes (6.7%) reported recently [99], but it still not as excellent as TiO2 (18.5%) [100]. And its properties need to be further optimized to improve the absorption efficiency of visible light, increase the carrier migration speed, and facilitate carrier transport. The follow-up works are suggested from the following aspects:

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