New Findings for the Much‐Promised Hematite Photoanodes with Gradient Doping and Overlayer Elaboration

Abstract

Herein, it is demonstrated that gradient Ti doping coupled with an overlayer of NiFeOx on hematite can markedly improve the photoelectrochemical (PEC) water‐splitting efficiency of hematite‐based photoanodes, which are prized from sustainability considerations but have met daunting challenges. First, the gradient Ti doping of hematite has effectively lowered the onset potential while maintaining the high efficiency of photo‐generated charge separation and transmission. Second, the NiFeOx layer not only substantially reduces the surface trap states, but also significantly enhances the oxygen evolution kinetics of hematite‐based photoanodes as an oxygen evolution catalyst, resulting in a further improvement of the onset potential. Consequently, with the TiO2 layer and a double electrode stack design, a remarkable photocurrent density of 4.49 mA cm−2 is achieved at 1.23 V versus reversible hydrogen electrode (RHE) for NiFeOx/(Grad Ti)‐Fe2O3/TiO2 photoanode without any hole scavenger, delivering a high applied bias photo‐to‐current efficiency of up to 0.58% at 1 V versus RHE. This multipronged attack for improving PEC water‐splitting efficiency revitalizes the great promise of hematite photoanodes and sheds light on the design and development of the next‐generation photoelectrodes.

Review on BiVO4-Based Photoanodes for Photoelectrochemical Water Oxidation: The Main Influencing Factors

“…Another formation of interfacial electric field is a homojunction, which involved the use of two same semiconductor materials to form the interface . Abdi et al designed a BiVO 4 /W-BiVO 4 homojunction to solve the poor charge carrier separation, which achieved a high 80% carrier separation efficiency .…”

“…Another formation of interfacial electric field is a homojunction, which involved the use of two same semiconductor materials to form the interface. 15 Abdi et al designed a BiVO 4 / W-BiVO 4 homojunction to solve the poor charge carrier separation, which achieved a high 80% carrier separation efficiency. 51 Soon afterward, a Zn:BiVO 4 /Mo:BiVO 4 homojunction was reported to facilitate the charge carrier transport and surface charge transfer process.…”

“…Due to the depletion of petroleum resources and supplies, the development of renewable clean energy has led to the advancement of new technologies such as energy conversion technology including water electrolysis. − Hydrogen, as a clean energy, is considered as an effective substitute to replace conventional fossil fuels. − Photoelectrochemical (PEC) water splitting remains a promising strategy for environmentally friendly H 2 generation. − The first PEC cell was reported in 1972 for water splitting, possessing one photoelectrode and responsible for light harvesting . PEC devices use solar energy to produce chemical energy to reconstruct hydrogen molecules. − The PEC water splitting efficiency is mainly determined by the nature of photoelectrode materials.…”

How titanium and iron are integrated into hematite to enhance the photoelectrochemical water oxidation: a review

“…3,[7][8][9][10] In particular, Ti-based modifications have been demonstrated to be one of the most effective approaches, with outstanding improvements in the PEC performance of hematite extensively reported. [11][12][13][14][15] Commonly, the beneficial effects of Ti-based modifications on the charge dynamics of a hematite photoanode can be classified into four categories: (i) increase in the donor density for higher charge mobility through Ti-doping, (ii) promotion of the charge separation via constructing Fe 2 TiO 5 /Fe 2 O 3 heterojunctions, (iii) passivation of the surface states by coating a TiO 2 overlayer, and (iv) suppression of the recombination of holes and back electrons via introduction of a Ti-containing underlayer. [11][12][13][14][15] Although diverse functional roles of Ti in hematite have been disclosed, a precise identification of the underlying causes to enhance the PEC performance with Ti-based modifications remains very difficult, as they could be strongly influenced by the fabrication conditions.…”

“…[11][12][13][14][15] Commonly, the beneficial effects of Ti-based modifications on the charge dynamics of a hematite photoanode can be classified into four categories: (i) increase in the donor density for higher charge mobility through Ti-doping, (ii) promotion of the charge separation via constructing Fe 2 TiO 5 /Fe 2 O 3 heterojunctions, (iii) passivation of the surface states by coating a TiO 2 overlayer, and (iv) suppression of the recombination of holes and back electrons via introduction of a Ti-containing underlayer. [11][12][13][14][15] Although diverse functional roles of Ti in hematite have been disclosed, a precise identification of the underlying causes to enhance the PEC performance with Ti-based modifications remains very difficult, as they could be strongly influenced by the fabrication conditions. For example, in 2011, Le Formal et al grew an ultrathin TiO 2 layer on the surface of hematite by atomic layer deposition (ALD), but no positive effect was reported upon using it as a passivation layer.…”