Photoelectrode Materials

Photoelectrode materials are crucial to water splitting. Thus, chemists and scientists continually seek the best photo-active materials that exhibit the following: a small semiconductor bandgap that covers a large fraction of the sunlight spectrum, a conduction and valence band energy that straddles water oxidization and reduction potentials, a stable property in an aqueous environment, high conversion efficiency for water splitting, and low cost. Unfortunately, no single material has yet been found that satisfies all of these requirements, though combinatorial methods have been used to quickly search for and optimize ideal materials.^[5,11] Transition metal oxides, including TiO2 and ZnO, exhibit good stability but only absorb a small fraction of incident sunlight because they have a large bandgap. Given that most sunlight is visible light, the use of visible light to

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irradiate a photoelectrode and generate hydrogen (oxygen) seems more practical than the use of UV light. Hence, research on effective visible-light photoelectrodes for water splitting using new materials for both anodic/cathodic processes should focus on the goal of realizing an efficient photoelectrochemical cell that can simultaneously drive both hydrogen generation and water oxidation reaction under visible-light radiation.

The combination of either a photoanode and a photocathode or a photoelectrode and a photovoltaic device has also been investigated to construct a tandem cell that facilitates overall water splitting associated with multi-photon absorption events.^[13] A solar energy conversion efficiency of over 12% has been achieved using III–V semiconductor materials, but the cost and stability of such materials remain major disadvantages.^[14] Although the efficiency of photoelectrochemical cells for solar energy conversion is still low, research on water splitting remains popular in solving problems associated with energy and the environment.

A wide range of semiconductor materials has been developed for photoelectrodes used for water-splitting reaction. Table 1-1 presents several representative photo-active materials. The medium of photoelectrochemical measurement has a significant effect on the stability of materials in terms of photocorrosion and photodecomposition.^[15] For example, the photoelectrode made of TiO2 can be operated under neutral or basic conditions, and ZnO can be measured under neutral conditions. The properties of the medium strongly affect the performance of the photoelectrode. For instance, the pH value of the electrolyte clearly affects the redox potential of the water and the position of the CB/valence band of the semiconductor materials.

TiO2 was the first reported photoelectrode for water splitting under UV

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irradiation.^[8] It was adopted to produce hydrogen and/or oxygen from an aqueous solution. Since then, various studies have focused on reducing the probability of recombination, increasing the surface area, and enhancing the carrier mobility of TiO2 to improve the photoconversion efficiency of the photoelectrochemical cell.^[16,17] In this regard, nanoarchitecture has been regarded as a candidate for fulfilling these demands. A mesoporous structure has been adopted to reduce the probability of charge-carrier recombination because such a structure is composed of smaller particle sizes. One-dimensional (1D) nanostructures (such as nanowires and nanotubes) can provide a high surface area and rapid diffusion in a single direction, yielding a low recombination of electron–hole pairs. Mor et al. ^[18]

formed TiO2 nanotube arrays by anodizing a Ti sheet and measured an efficiency of 4% under UV irradiation (λ = 320 nm–400 nm). Subsequently, Paulose et al. ^[19]

prepared TiO2 nanotubes with a length of 45 μm using an anodizing method and achieved a photocurrent of 26 mA/cm² and a conversion efficiency of 16.25%

under UV irradiation (λ = 320 nm–400 nm). Hartmann et al. ^[20] reported that mesoporous TiO2 films prepared using the sol-gel method were approximately 10 times as efficient in water-splitting reaction than their counterparts obtained from crystalline TiO2 nanoparticles because of their thick, continuous pore walls and lower recombination rate.

ZnO may become an alternative to TiO2 for fabricating photoelectrodes because of its various advantages, including higher carrier mobility, greater chemical and thermal stability, and lack of toxicity. The CB of ZnO is also at a more negative position than that of TiO2, such that the photogenerated electrons are more able to reduce protons, an ability that corresponds to greater solar energy conversion efficiency. In addition, the morphology of ZnO can be easily controlled by adjusting synthesis conditions. Variously shaped ZnO nanostructures have been

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demonstrated.^[21] Ahn et al. ^[22] produced a ZnO nanocoral and measured a photocurrent of 0.25 mA/cm² under a tungsten halogen lamp (λ = 350 nm–750 nm). Qiu et al. ^[23] prepared ZnO nanotetrapods by using a multi-step growth process, and measured a photocurrent of 0.12 mA/cm² and a conversion efficiency of 0.045% under irradiation from a solar simulator (λ = 350 nm–800 nm). ZnO was easy to produce into many nanostructure, for example nanorods, nanocorals or nanoflowers. The one-dimesional ZnO nanorod has many advantage for using in photoelectrode material as list in follow: 1. The one-dimensional nanorods nanostructures exhibit the high surface area which could increase the the contact interface between ZnO and electrolytes. 2. The one-dimensional nanorods provide an electron transportation route for the major carrier (electron in ZnO) and could shorten the diffusion length of the minor carrier simultaneously which inhibit the recombination possibility of the photoexcited electron hole pairs. 3. The one-dimesional nanorods also improved the light absorption owing to the increase of the light diffucsion length. 4. The one-dimensional ZnO nanorods could lower the overpotential due to increase the reaction sites.

In general, the ZnO exhibit higher carrier mobility, greater chemical and thermal stability and lack of toxicity than TiO2. The higher conduction band level could generate more energytic electron than TiO2. In this thesis, we chosen the ZnO nanorods as our major photoelectrode material.

Although both TiO2 and ZnO have high performance in the UV region, their wide bandgap nature cannot capture solar light in the visible region, in which photons of sunlight are most abundant and thus result in low energy utilization.

Therefore, materials with a narrower bandgap are required.

With respect to narrow-bandgap materials, BiVO4 [24,25], WO3 [26,27], and Fe2O3 [27-30] are very attractive narrow-bandgap materials.

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BiVO4 thin films comprised of ordered arrays of pyramidal-shaped nanowires have been successfully fabricated through seed-mediated growth in an aqueous BiVO4 suspension, and a photocurrent of 0.4 mA/cm² was measured under AM 1.5 irradiation (λ = 400 nm–700 nm; 100 mW/cm²).^[31] Tungsten trioxide (WO3) and hematite (α-Fe2O3) have bandgaps of 2.2 and 2.6 eV respectively, which make them effective semiconductor oxides that serve as oxygen-evolving anodes to absorb photons from the blue regions of the solar spectrum. Cristino et al. ^[32]

prepared mesoporous WO3 films using an anodization method and measured a photocurrent of 9 mA/cm² under AM 1.5 irradiation (λ = 300 nm–700 nm; 370 mW/cm²). Kim et al. ^[33] prepared a WO3 photoelectrode with a mesoporous nanostructure by using polyethyleneglycol as a surfactant, and the photoelectrochemical properties of the photoelectrode revealed a photocurrent of 3.7 mA/cm² under AM 1.5 irradiation (λ = 300 nm–700 nm). Su et al. ^[34]

successfully prepared vertically aligned WO3 nanoflaskers by using a solvothermal technique and was able to fabricate nanowires and nanoflakes by adjusting the composition of the solution. A photocurrent of 1.43 mA/cm² and an incident-photon-to-current-conversion efficiency (IPCE) as high as 60% at λ = 400 nm were measured.

α-Fe2O3 , which has a small bandgap, low cost, and high chemical stability, is one of the most promising metal oxide semiconductor materials for the splitting of water using solar energy. Mohapatra et al. ^[35] reported an anodization method for growing Fe2O3 nanotubes of an ultrathin wall with a thickness of 5 nm to 7 nm and Fe foil with a length of 3 μm to 4 μm. A photocurrent density of 1.41 mA/cm² was obtained using hematite nanotube arrays, and a maximum solar-to-hydrogen conversion efficiency of 0.84% was reached under AM 1.5 conditions. Brillet et al. ^[36] elucidated a solution-based strategy and fabricated a porous electrode by

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encapsulation with an SiO2 confinement scaffold, yielding a high water oxidation photocurrent of 2.34 mA/cm² under AM 1.5 illumination. This high photocurrent density was attributed to the activation of nanostructured hematite photoanodes with control over the a wide range of particle sizes in the porous film. Generally, hematite has a relatively poor photoelectrochemical performance, which is attributable to its short charge carrier diffusion length and the slow kinetics of oxidation of water by the valence band holes.^[37]

A photoelectrode made of single-component photo-active materials cannot deliver the required performance in terms of either photocurrent or conversion efficiency to meet the demands of daily life. The limitations on their performance include low absorbance in the visible region, poor charge-carrier transportation, poor collection of photogenerated electrons, and limited chemical stability in an electrolyte under illumination.

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Table 1-1. Semiconductor materials of photoelectrode for photoelectrochemical water splitting.

Materials Structure Bandgap Electrolyte Conditions Performance ref.

TiO2 (nanotube) -- -- 1 M KOH I = 100 mW/cm^{2 ,} 320-400 nm, 50 W metal hydride lamp j = 13 mA/cm² at 0 V vs Ag/AgCl, 6.8% 18

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