Toward efficient solar water splitting over hematite photoelectrodes
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matite has been considered as one of the most promising materials for solar water splitting, although its photoelectrochemical performance is still not very high and limited by its intrinsic properties. In the past few years, sizable advances in the development of hematite photoelectrodes for enhanced water splitting activities have been achieved by a variety of rational modification strategies, including nanostructure design for efficient charge collection, metal ion doping for promoted charge carrier transfer, heterojunctions for efficient charge separation, and surface and/or interface modification for retarded charge recombination and enhanced light absorption. In this article, research work and milestone achievement actually focused on hematite photoelectrodes for water splitting is reviewed in detail. A review on this topic by answering the key question, “how to modify or design hematite photoelectrode to improve its conductivity, enhance charge separation as well as catalyze surface water oxidation,” in authors’ view, can be potentially helpful to enable hematite for further efficient solar energy conversion, which will be very inspiring and important to this field.
I. INTRODUCTION
Efficient solar energy conversion for hydrogen production via photoelectrochemical (PEC) water splitting has been considered as one of the most promising approaches to solving the world energy crisis. The PEC process utilizes solar energy to excite a semiconductor to generate electrons and holes, which transfer to cathodes and anodes for water splitting into hydrogen and oxygen. Although the concept is simple, the main challenge is to search for a semiconductor material as a photoelectrode to absorb enough solar energy to drive this water splitting reaction. As a ideal candidate photoelectrode, the main requirements include: (i) excellent light or solar absorption, (ii) fast charge carrier transport, (iii) suitable band structure for water reduction and/or oxidation, (iv) abundant and low cost, and (v) good chemical and photochemical stability. Since the pioneer report on photoinduced water splitting using TiO2 photoanodes,1 significant advances in PEC water splitting by semiconductor photoelectrodes have been achieved in the past few decades.2–6 Despite these advances and numerous photoelectrodes developed, there is still no single semiconductor that satisfies all the criteria for its use in the PEC water splitting system by meeting the required performance benchmarks for efficiency, durability, and cost. A. Why hematite?
Compared to other semiconductors, hematite, the “rust,” has many potential advantages for PEC water splitting, with
a favorable combination of good chemical stability, visiblelight absorption, nontoxicity, abundance, and low cost. With a band energy gap of 2.1 eV and valence band edge substantially lower than the water oxidation potential, hematite turns to be a promising photoanode material for PEC water splitting, and can theoretically produce water oxidation photocurrent densities as high as 12.6 mA/cm2 under AM 1.5G
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