Surface Engineering of Nanomaterials for Photo‐Electrochemical Water Splitting

Abstract

be regenerated through combustion or a hydrogen fuel cell. The renewable solar energy and the good availability of water on the Earth make this approach sustain able and practically feasible.The rapid expanding of nanomaterials in the past decade is the fuel for the recent development of PEC water splitting and could fundamentally changes the design of PEC devices. [1,[8][9][10][11] Nanostructured photo electrodes have several advantages over the conventional bulk films, including larger surface area, shorter carrier diffu sion length, smaller light reflection loss, tunable optical bandgap, and electronic structure. [3,10,[12][13][14] Water oxidation and reduction take place at the electrode/ electrolyte interfaces; therefore, having favorable surface properties of photo electrodes is important, in particular, for nanostructured electrodes with extremely large surface area. [15,16] A number of sur face engineering methods have been demonstrated to be effective in tuning the surface properties of nanomaterials and consequently improving their photo and electrochemical stability, charge separation/recombination effi ciency, and kinetics of surface redox reactions. Despite some excellent review articles have highlighted the recent advances of nanomaterials for PEC water splitting, [8][9][10] the surface engi neering of nanomaterials for PEC water splitting has not been reported. In this review, we start with a brief introduction of basic principles of PEC water splitting, followed by the discus sion of the advantages of nanostructured photoelectrodes, and then the comprehensive discussion of nine most efficient and widely used surface engineering methods, and finally end with a comparison for all these methods and an outlook for the future work. Basic Principles of PEC Water SplittingFigure 1 illustrates the basic structure of a conventional PEC cell and the major processes of PEC water splitting including light absorption, charge separation, and charge transfer at the electrode/electrolyte interface. In a PEC device, at least one of the two electrodes should be semiconductor material capable of harvesting sunlight. Photoelectrodes absorb light with energy equal or higher than their bandgap energies and excite electrons (e − ) from valence band (VB) to the unoccupied conduction band (CB), leaving the corresponding holes (h + ) at Photo-electrochemical water splitting represents a green and environmentally friendly method for producing solar hydrogen. Semiconductor nanomaterials with a highly accessible surface area, reduced charge migration distance, and tunable optical and electronic property are regarded as promising electrode materials to carry out this solar-to-hydrogen process. Since most of the photo-electrochemical reactions take place on the electrode surface or near-surface region, rational engineering of the surface structures, physical properties, and chemical nature of photoelectrode materials could fundamentally change their performance. Here, the recent advances in surface engineering methods, includin...

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