There is a growing interest in the conversion of water and solar energy into clean and renewable H2 fuels using earth-abundant materials due to the depletion of fossil fuel and its serious environmental impact. This critical review highlights some key factors influencing the efficiency of heterogeneous semiconductors for solar water splitting (i.e. improved charge separation and transfer, promoted optical absorption, optimized band gap position, lowered cost and toxicity, and enhanced stability and water splitting kinetics). Moreover, different engineering strategies, such as band structure engineering, micro/nano engineering, bionic engineering, co-catalyst engineering, surface/interface engineering of heterogeneous semiconductors are summarized and discussed thoroughly. The synergistic effects of the different engineering strategies, especially for the combination of co-catalyst loading and other strategies seem to be more promising for the development of highly efficient photocatalysts. A thorough understanding of electron and hole transfer thermodynamics and kinetics at the fundamental level is also important for elucidating the key efficiency-limiting step and designing highly efficient solar-to-fuel conversion systems. In this review, we provide not only a summary of the recent progress in the different engineering strategies of heterogeneous semiconductors for solar water splitting, but also some potential opportunities for designing and optimizing solar cells, photocatalysts for the reduction of CO2 and pollutant degradation, and electrocatalysts for water splitting.

Engineering heterogeneous semiconductors for solar water splitting

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

Hydrogen (H2) production via photocatalytic water splitting is one of the most promising technologies for clean solar energy conversion to emerge in recent decades. The achievement of energy production from water splitting would mean that we could use water as a fuel for future energy need. Among the various photocatalytic materials, titanium dioxide (TiO2) is the dominant and most widely studied because of its exceptional physico-chemical characteristics. Surface decoration of metal/non-metal on TiO2 nanoparticles is an outstanding technique to revamp its electronic properties and enrich the H2 production efficiency. Metal dopants play a vital role in separation of electron-hole pairs on the TiO2 surface during UV/visible/simulated solar light irradiation. In this paper, the basic principles, photocatalytic-reactor design, kinetics, key findings, and the mechanism of metal-doped TiO2 are comprehensively reviewed. We found that Langmuir-Hinshelwood kinetic model is commonly employed by the researchers to demonstrate the rate of H2 production. Copper (Cu), gold (Au) and platinum (Pt) are the most widely studied dopants for TiO2, owing to their superior work function. The metal dopants can amplify the H2 production efficiency of TiO2 through Schottky barrier formation, surface plasmon resonance (SPR), generation of gap states by interaction with TiO2 VB states. The recent advances and important consequences of 2D materials, perovskites, and other novel photocatalysts for H2 generation have also been reviewed.

Photocatalytic hydrogen production using metal doped TiO2: A review of recent advances

2D Layered Double Hydroxides for Oxygen Evolution Reaction: From Fundamental Design to Application

Self – cleaning superhydrophobic coatings: Potential industrial applications

Self-cleaning transparent superhydrophobic coatings through simple sol–gel processing of fluoroalkylsilane

Non-noble metal Cu as a cocatalyst on TiO2 nanorod for highly efficient photocatalytic hydrogen production

Cr/Ta co-doped SrTiO 3 (STO:Cr/Ta) photocatalysts with high photocatalytic activity for water splitting under visible light have been synthesized by a polymerizable complex (PC) method. UV–vis diffuse reflectance spectra (DRS) showed that the Cr/Ta co-doping extended the absorption edge of STO to visible light region at the wavelength of 540 nm. Calculations based on density functional theory (DFT) indicated that, after doping, Cr 3d and O 2p orbits formed new impurity states within the forbidden gap of STO and facilitated the excitation of photons with low energy, as the Ta was mainly used to restrain the appearance of undesired Cr 6+ . The photoelectrochemical measurements and XRD, SEM, BET analysis revealed that the excellent photocatalytic performance of photocatalyst prepared by a PC method was attributed to the high crystalline quality as well as relative large specific surface area. The photocatalytic activity of STO: (1% Cr/Ta) for H 2 evolution prepared by a PC method is 10 times higher than that of the sample prepared by a solid state reaction (SSR) method. The Pt/STO: (2% Cr/Ta) calcined at 1100 °C possessed the highest photocatalyitc activity for water splitting under visible light, thereby producing the average rate of H 2 evolution of 122.6 μmol h −1 (ca. 2.6% AYQ) measured at λ = 420 nm. Also, under a Z-scheme system, the Pt/STO: (2% Cr/Ta) exhibited AQY as high as 1.52% at λ = 420 nm for overall water splitting while Pt/WO 3 acting as an O 2 evolution photocatalyst.

Li Gao

Papers

Self-cleaning transparent superhydrophobic coatings through simple sol–gel processing of fluoroalkylsilane

Non-noble metal Cu as a cocatalyst on TiO2 nanorod for highly efficient photocatalytic hydrogen production

Polymerizable complex synthesis of SrTiO3:(Cr/Ta) photocatalysts to improve photocatalytic water splitting activity under visible light

In situ photodeposition of cobalt on CdS nanorod for promoting photocatalytic hydrogen production under visible light irradiation

Nickel phosphate materials regulated by doping cobalt for urea and methanol electro-oxidation