Abstract
Abstract Photo-electrochemical (PEC) water splitting (WS) using metal oxide semiconductors is regarded as a promising approach for the renewable production of fuels and energy vectors such as hydrogen (H2). Among metal oxide semiconductors, iron oxide in the form of hematite (α-Fe2O3) is one of the most researched photo-anode materials, mainly due to its ability to absorb photons up to 600 nm combined to a set of desirable properties such as high photocorrosion resistance, environmental friendliness, large abundance and relatively low production costs. However, hematite main disadvantages are a low electrical conductivity and a high rate of charge recombination; both these shortcomings drastically limit functionality and efficiency of hematite-based photo-anodes in PEC devices. One-dimensional (1D) nanostructuring is a powerful tool to tackle such disadvantages as it provides the photoelectrode material with increased surface area along with directional charge transport properties and short charge diffusion distances to the electrolyte – these features can improve the lifetime of photo-generated charges and/or enhance the charge transfer efficiency, and can consequently lead to a superior photo-electrochemical performance. At the same time, chemical/physical modification can also compensate natural weaknesses of hematite in water photoelectolysis. The present mini-review outlines a series of most effective strategies for the fabrication of 1D hematite nanostructures as well as for their physicochemical modification, mainly by doping or co-catalyst decoration, to achieve superior PEC activity.
Highlights
Several semiconductors were explored for photoelectrochemical water splitting such as Si, Ge, SiC, CdSe and, etc. [2]; these semiconductors generally suffer from severe photocorrosion and from chemical instability in aqueous electrolytes; on the contrary, photoelectrodes based on transition metal oxides such as hematite can provide a proper band gap positioning for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) reactions together with appropriate chemical and photo-electrochemical stability
A most effective method to accelerate the hole transfer step and thereby accelerate the oxygen evolution reaction (OER) at the photo-anode surface is by the modification of hematite with an oxygen evolution catalyst (OEC)
A first reason is that compared to more classic semiconductor materials (Si, CdS, CdSe, GaAs, etc.), hematite is substantially more chemically stable againstcorrosion in a wide range ofchemical conditions, but is much cheaper, and more abundant on earth, easy to synthesize and nanostructure, and eco-friendly
Summary
Fossil fuels are the main source of global energy. The energy production by fossil fuels is typically achieved by combustion, which results in major production of CO2 and subsequent global warming. The overall reaction shows a Gibbs free energy ∆rG0 = 273.2 kJ/mol. Relevant stages in photo-electrochemical water splitting are: light absorption, separation of charge carriers, charges transport holes (hVB+) towards the photoelectrode surface and electrons (eCB−) to the back contact, and surface reactions (see Figure 1a). In view of utilizing sunlight to drive PEC water splitting, it is a strict requirement that the semiconductor absorbs a significant portion of visible light, i.e. to be able to efficiently convert sunlight to hydrogen and oxygen. [2]; these semiconductors generally suffer from severe photocorrosion and from chemical instability in aqueous electrolytes; on the contrary, photoelectrodes based on transition metal oxides such as hematite can provide a proper band gap positioning for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) reactions together with appropriate chemical and photo-electrochemical stability Several semiconductors were explored for photoelectrochemical water splitting such as Si, Ge, SiC, CdSe and, etc. [2]; these semiconductors generally suffer from severe photocorrosion and from chemical instability in aqueous electrolytes; on the contrary, photoelectrodes based on transition metal oxides such as hematite can provide a proper band gap positioning for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) reactions together with appropriate chemical and photo-electrochemical stability
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