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

Nickel phosphate-based materials with excellent durability for urea electro-oxidation

Ni(OH)2 loaded on TaON for enhancing photocatalytic water splitting activity under visible light irradiation

A novel route is developed to synthesize highly uniform N-doped porous carbon spheres (NCS) with a tailorable size ranging from 86 to 205 nm depending on their precursors of phenolic-resin-based analogues. Solid-state NMR and FTIR spectra confirm that the as-synthesized polymer spheres are composed of typical phenolic resin. After activation treatment with KOH, the NCS with a diameter of 86 nm possesses a high surface area of 1462 m2 g–1 coupled with hierarchical (micro and meso) pore structures. Upon these advantages, the prepared carbon sample endows the supercapacitor with high specific capacitance up to 247 F g–1 at 1 A g–1 with excellent rate performance and high cycling stability. Also, the specific energy density is 6.74 Wh kg–1 at a current density of 0.1 A g–1. Therefore, the as-prepared uniform N-doped porous carbon spheres represent a promising candidate for an efficient electrode material.

Synthesis of Highly Uniform N-Doped Porous Carbon Spheres Derived from Their Phenolic-Resin-Based Analogues for High Performance Supercapacitors

Cobalt phthalocyanine (Pd/CoPc) supported palladium nanoparticle composite materials are synthesized by using a facile methanol-mediated weakly-capping growth approach with anhydrous methanol as the mild reductant. Pd/CoPc nanorods have been used to modify a glassy carbon electrode to fabricate a sensitive and selective electrochemical nitrite sensor. The morphology and structure of Pd/CoPc are studied by XRD, UV-vis, FTIR, Raman, HRTEM and HAADF-STEM techniques. Transmission electron microscopy and high-angle annular dark-field imaging-scanning transmission electron microscopy images indicated that the diameters of spherical Pd particles are in the range of 4–6 nm. The electrode exhibits enhanced electro-catalytic behavior to the oxidation of nitrite compared to commercial activated carbon-supported palladium and pure cobalt phthalocyanine (CoPc) modified glassy carbon electrodes. Meanwhile, due to the unique structure of the Pd/CoPc modified electrode, the prepared nitrite biosensor displays good performance. The fabricated sensor exhibits excellent analytical performance for nitrite detection linearly over two different concentration ranges, which are from 0.2 to 50 μM and from 500 to 5000 μM with a low detection limit of 0.10 μM nitrite and a sensitivity of 0.01 μA μM−1 (S/N = 3). Moreover, the proposed electrochemical sensor also shows good reproducibility and excellent anti-interference ability against metal ions, and exhibits satisfactory recovery for detecting nitrite in cured food.

A sensitive and selective electrochemical nitrite sensor based on a glassy carbon electrode modified with cobalt phthalocyanine-supported Pd nanoparticles

Li Gao

Papers

Nickel phosphate-based materials with excellent durability for urea electro-oxidation

Ni(OH)2 loaded on TaON for enhancing photocatalytic water splitting activity under visible light irradiation

Synthesis of Highly Uniform N-Doped Porous Carbon Spheres Derived from Their Phenolic-Resin-Based Analogues for High Performance Supercapacitors

A sensitive and selective electrochemical nitrite sensor based on a glassy carbon electrode modified with cobalt phthalocyanine-supported Pd nanoparticles

Facile synthesis of ultrathin Ni(OH) 2 -Cu 2 S hexagonal nanosheets hybrid for oxygen evolution reaction