Photocatalytic Water Splitting-The Untamed Dream: A Review of Recent Advances

Abstract

Photocatalytic water splitting using sunlight is a promising technology capable of providing high energy yield without pollutant byproducts. Herein, we review various aspects of this technology including chemical reactions, physiochemical conditions and photocatalyst types such as metal oxides, sulfides, nitrides, nanocomposites, and doped materials followed by recent advances in computational modeling of photoactive materials. As the best-known catalyst for photocatalytic hydrogen and oxygen evolution, TiO₂ is discussed in a separate section, along with its challenges such as the wide band gap, large overpotential for hydrogen evolution, and rapid recombination of produced electron-hole pairs. Various approaches are addressed to overcome these shortcomings, such as doping with different elements, heterojunction catalysts, noble metal deposition, and surface modification. Development of a photocatalytic corrosion resistant, visible light absorbing, defect-tuned material with small particle size is the key to complete the sunlight to hydrogen cycle efficiently. Computational studies have opened new avenues to understand and predict the electronic density of states and band structure of advanced materials and could pave the way for the rational design of efficient photocatalysts for water splitting. Future directions are focused on developing innovative junction architectures, novel synthesis methods and optimizing the existing active materials to enhance charge transfer, visible light absorption, reducing the gas evolution overpotential and maintaining chemical and physical stability.

Keywords: hydrogen; metal oxides; nanomaterials; nanotechnology; photocatalysis; photocatalysts; semiconductors; solar fuels; water splitting.

Figure 1

Schematic representation of photochemical water splitting. Figure adapted from reference [90] of Currao work.

Figure 2

Schematic representation of photoelectrochemical water splitting, Figure adapted from reference [90] of Currao work.

Figure 3

Band structure illustration of various semiconductors with respect of the redox potentials of water splitting. Figure adapted from reference [142] of Ong et al. work.

Figure 4

Schematic band gap diagram of TiO₂. Figure adapted from references [4,149] of Moniz et al. and Miao et al. works respectively.

Figure 5

Schematic band gap alignment of S-doped, Fe-doped, and V-doped TiO₂. Figure adapted from reference [158] of Babu et al. work.

Figure 6

Schematic band gap alignment of TiO₂/BiVO₄ heterojunction. Figure adapted from references [4,161] of Moniz et al. and Resasco et al. works respectively.

Figure 7

Schematic illustration of heterojunction between Au nanoparticles and TiO₂ semiconductors. Pathway I shows the extraction of photo-generated electron from TiO₂ conduction band to Au NPs. Pathway II shows the coupling of exciton of TiO₂ and surface plasmon of Au. Figure adapted from references [150,168] Chen et al. and Dutta et al works respectively.