Submitted 27/07/2

Revised 20/08/2

Accepted 31/08/21

 

Introduction

Nowadays, due to rapid growth in population, fast-growing demands for non-renewable, high-pollution energy worldwide have triggered two mainstream issues: one is population and its unbalanced resource allocation; another is environmental problems [1]. In order to address these energy and environmental issues, it’s urgent to find and develop the earth-abundant, sustainable and cleaning energy source to replace the traditional fuel energy [2]. Among all of the cleaning energy, solar energy is the richest one on Earth despite its discontinuous spatio-temporal distribution and relatively low energy flow density. Besides, hydrogen energy is always considered as one of the idealist resources due to its sustainability, high energy density and non-toxicity [2][3]. Thus, it’s of great value to explore a suitable way of solar-hydrogen energy conversion to give more access to efficient energy storage and utilization. Compared with the other common industrial H2 production pathways, solar-driven water splitting has been a constantly appealing research focus because it is a milder and more efficient method [4]. Since the pioneer work of Fujishima et. al.. [5] on H2 and O2 generation via TiO2-based photocatalytic water splitting shown the boosted activity, more various semiconductors have been developed as heterogeneous photocatalysts to reduce the non-spontaneity in the photo-excited water splitting process. The solar-driven water photolysis mainly contains three steps (Fig. 1)[6][7]: (1) light harvesting: when the proton energy surpasses the band gap, the photocatalyst absorbs the light and generates the photo-induced electron-hole (e--h+) pairs; (2) charge separation and transport: the pairs are separated from each other and migrate to the surface of the catalyst, where the electrons are excited from the valence band (VB) to the conduction band (CB) with the holes left in the VB; (3) surface redox reaction: the electrons in the CB drives the H2 production in the reduction reaction with the VB holes for O2 generation in oxidation. Theoretically, for Step 1, the light absorption ability depends primarily on the band gap of the semiconductor materials; for Step 2 and 3, the corresponding efficiency can be accurately regulated via the coupling effect between catalysts and cocatalysts [8]. Theoretical Physics Letters, 06(09): 09.-13. CC. 4 INTERNATIONAL DISTRIBUTION Page237 However, at present, numbers of photocatalysts possess insufficient photocatalytic activity for H2/O2 generation because of the following problems: narrow spectral scope, inefficient electron-hole separation, poor surface reaction, and relatively high overpotential [6, 8, 9]. Accordingly, versatile strategies have been implemented to improve photo-excited charge separation and migration for enhanced quantum efficiency, like heteroatom doping, heterojunction construction, morphology modulation, and cocatalysts loaded on the semiconductors [8, 9]. Among the proposed strategies, cocatalysts play an indispensable role in photocatalytic activity and stability. Specifically, cocatalysts can harvest charge carriers for boosted electron-hole separation, expose the abundant active sites for optimized surface redox reactions, lower the overpotential for strengthened photocatalytic activity, and inhibit the light corrosion for stabler photocatalysts. Herein, we focus on the roles and mechanism of cocatalysts in the photocatalytic water splitting system based on different categories of the existing photocatalysts. We also highlight the difference between a single cocatalyst for H2 or O2 evolution and dual cocatalysts system. Moreover, from the nanoscience and photochemistry perspectives, we also discuss the optimal design of a novel, more efficient and stabler cocatalyst for tuning the surface reaction and charge separation processes.

 

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