INFORME CUARTA SESIÓN

Figure 1.13: (a) Schematic of a PEC set up under tandem illumination and (b) band edge potentials of various nitride materials against water redox potentials (green dotted lines) and band edge positions of InxGa1-xN (red-dotted lines) with increasing x from left to right (0 to1)

[40].

The development of efficient and economically viable photocatalytic materials is essential for hydrogen generation at the price of $2-3 kg-1 set by the U.S. Department of Energy to compete with conventional energy resources [8]. A wide variety of materials such as metal oxides, sulphides, nitrides and many other semiconductor materials have been investigated for photocatalytic water splitting over four decades [37, 38]. However, to date, the reported STH conversion of photocatalytic materials is low due to poor carrier mobility, inefficient absorption and insufficient redox potentials. Inefficient absorption due to large band gaps in excess of 3 eV and poor hole mobility are the main drawbacks preventing oxides from being efficient

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photoelectrodes [38, 39]. On the other hand, sulphides and oxynitrides require sacrificial reagents for hydrogen generation, although they have optimum band gaps and favourable energy band positions against water redox potentials, which is not ideal for hydrogen generation. There is a lack of single robust materials that can meet the ideal photocatalytic material requirements such as suitable band gap, high carrier mobility, stability and band edge positions for efficient photocatalytic water splitting.

The PEC tandem cell (Z scheme), consisting of photoactive- anode and cathode components with complementary energy band gaps, has been perceived to be an ideal and alternative approach for unassisted water splitting. A schematic of a PEC tandem cell for overall water splitting is shown in Figure 1.13(a). The simultaneous excitation of the photoelectrodes eliminates the basic requirement of favourable band edge potentials for a single material for overall water splitting as the reduction and oxidation reactions occur at different semiconductor surfaces. Moreover, the two semiconductors can have smaller band gaps because each needs to support only half reactions. Consequently, light absorption by the photoelectrodes can be extended into the visible region of the solar spectrum. However, for overall water splitting using PEC tandem cells to occur, the band edge potentials of the photocathode and photoanode must straddle the water reduction and oxidation potentials, respectively. Also, part of the sunlight should be transmitted from the front to the rear photoelectrode. Most importantly, the photoexcited photoanode and photocathode must provide sufficient photovoltage to oxidize and reduce water at the semiconductor/electrolyte interfaces without electrical energy from an external source. The combination of InGaN as a larger band gap front electrode and InP as a small band gap rear electrode as the photoanode and photocathode, respectively, can make an excellent PEC tandem cell to achieve outstanding STH conversion efficiency. The band gap of InGaN can be tuned by changing the In content in such a way that it can make an ideal tandem device with InP. The efficiency of a PEC tandem cell depends on the performances of the individual photoelectrodes and their half-cell conversion efficiencies. Therefore, the development of efficient GaN-based and InP photoelectrodes could enable high efficiency PEC tandem systems for overall water splitting.

The GaN-based ternary alloy InGaN is a promising material for photoelectrodes owing to its variable band gap, band edges that straddle the H2 and O2 redox potentials, larger carrier

mobility and high chemical stability [41]. The band gap of InGaN can be varied from 0.7 (InN) to 3.4 eV (GaN) by varying the In concentration, which allows its light absorption to be tuned from the UV to far IR regions [42]. Moreover, up to an In content of 50%, the valence and conduction band edges of InGaN straddle both the hydrogen and oxygen evolution reaction

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(HER and OER) potentials, which means that it can drive unassisted water splitting (Figure 1.13(b)) and also be used either as a photoanode or photocathode [22, 40, 43]. Due to its highly crystalline nature and strong ionic bonds, InGaN exhibits high carrier transport and chemical stability. On the other hand, InP possesses several attractive attributes such as a well-matched band gap of 1.35 eV to the solar spectrum, a favourably aligned conduction band for the water reduction reaction and low surface recombination velocity. The conduction band of InP lies above the water reduction potential to facilitate electron transfer for hydrogen generation, which makes it an optimum material for photocathodes. Further, the low surface recombination velocity of InP is an important characteristic for non-planar materials, specifically, nanostructured materials for photocatalytic water splitting to raise the photoconversion efficiency.

Further, nanostructures offer great potential for solar water splitting over their planar counterparts. Nanostructures possess several essential attributes towards achieving efficient water splitting such as enhanced light absorption, reduced carrier transfer lengths, large surface area to facilitate efficient charge transfer at the semiconductor/electrolyte interface and enlarged depletion area at the nanostructured surface that drives charge separation efficiently. In the case of GaN alloys, nanostructures have the potential to accommodate higher In concentrations while maintaining good quality InGaN layers given that nanostructures can be made stress-free [22, 44]. Nanostructures can be synthesized either using bottom-up or top- down approaches. Out of these two approaches, the top-down method is an ideal approach for the formation of nanostructures with controlled morphology and uniform doping as it allows for the use of high quality GaN epilayers grown using well-matured planar growth technology wherein the doping concentration can be precisely controlled. The morphology of the nanostructures can be controlled by well-established lithography techniques or random masking techniques. On the other hand, bottom-up approaches suffer from a lack of control over the morphology of nanostructures and doping of NPs [45, 46]. Therefore, a top-down approach provides the opportunity to analyse the influence of nanostructure parameters on their PEC performance.

This thesis is devoted to developing highly stable and efficient GaN-based and InP nanopillar (NP) photoelectrodes using a top-down approach for PEC water splitting applications by making use of the outstanding photocatalytic properties of GaN-based alloys and InP and the advantages of a top-down approach. We investigated the influence of carrier concentration, NP dimensions and band gap engineering of the nanostructures on the PEC performance to achieve optimum photoconversion efficiency. Several characterization

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techniques were employed to elucidate the PEC results of NP photoelectrodes. We also carried out photostability tests for these photoelectrodes.