提高氧化铁光电催化分解水效率的策略进展

doi:10.1016/S1872-2067(20)63712-3

摘要/Abstract

摘要：

为了解决能源危机与环境污染问题, 发展一种可再生的清洁能源至关重要. 太阳能是一种取之不尽用之不竭的清洁能源, 而氢气是一种良好的能源载体. 利用太阳能光电催化水分解制氢, 是一项有望能够解决能源与环境问题的技术, 具有很大的应用前景. 其中, 氧化铁因为具有合适的能带位置与带隙、良好的稳定性与廉价无毒等优点, 成为一种理想的光阳极材料. 但是, 在实际的测试中, 氧化铁仅仅只能得到一个较低的光电转换效率, 这可能是因为其较短的空穴扩散距离、较低的电导率以及极度缓慢的水氧化反应动力学所致.
整个光电催化水氧化可分为三个过程, 即光吸收过程、电荷分离过程以及表面空穴注入过程. 这三个过程的效率共同决定了器件的太阳能转化效率. 鉴于此, 本文将从如何提高这三个效率的角度出发, 总结近期研究报道中提高氧化铁光电催化分解水效率的一些策略.
光吸收过程是指半导体中价带的电子在吸收具有一定能量的光子后发生跃迁, 产生空穴-电子对的过程. 其光子的损失主要来源于光的反射、透射以及半导体吸收边的限制. 提高光吸收效率的主要策略包括制备具有特定纳米结构的氧化铁电极、利用表面等离子体共振效应以及组成双光吸收系统和掺杂等.
电荷分离过程指的是受光激发产生的空穴电子对, 在内建电场的作用下发生电荷分离, 即光生空穴流向电极表面, 光生电子流向半导体内部并从外电路导出. 电荷分离效率的损失主要来源于光生载流子在迁移过程中的复合. 因此, 为了提高电荷分离效率, 常见的策略是提高载流子在电荷分离过程中的复合时间𝜏₁和减少电荷迁移到表面(空穴)或者基底(电子)的时间𝜏₂. 具体的策略包括制备特定的纳米结构(缩短体表相距离, 减少𝜏₂)、构建异质结(增强能带弯曲, 提高𝜏₁和减少𝜏₂)、掺杂(减少𝜏₂)和钝化复合中心(提高𝜏₁)等.
表面空穴注入是指到达表面的光生空穴发生水氧化反应生成氧气的过程. 除了空穴注入外, 表面还可能存在复合与逆反应过程. 因此, 为了提高表面空穴注入效率, 我们既可以提高水氧化反应动力学, 具体的手段包括引入水氧化催化剂、F掺杂和碱处理等; 也可以采用减少复合反应的策略, 具体的方法包括引入钝化层、酸处理和高温热处理等; 还可以采用减少逆反应的方法, 最常见的手段就是在基底与氧化铁层之间引入电子阻挡层. 上述三种途径都能提高表面空穴注入效率.
最后, 通过结合上述的一些策略, 目前得到的最高性能的氧化铁电极在1.23 V(相对于可逆氢电极)能够达到6 mA cm^-2的光电催化分解水电流, 但这个值依然明显低于氧化铁的理论值(12.6 mA cm^-2). 这可能是由于体相复合所致. 除此之外, 氧化铁表面的水氧化机理现在依然不清晰, 这些都是需要我们在未来解决的问题.

关键词: 氧化铁, 光吸收效率, 电荷分离效率, 表面空穴注入效率

Abstract:

Photoelectrochemical (PEC) water splitting is one of the most promising approaches toward achieving the conversion of solar energy to hydrogen. Hematite is a widely applied photoanode material in PEC water splitting because of its appropriate band structure, non-toxicity, high stability, and low cost. Nevertheless, its relatively low photochemical conversion efficiency limits its application, and enhancing its PEC water splitting efficiency remains a challenge. Consequently, increasing efforts have been rendered toward improving the performance of hematite photoanodes. The entire PEC water splitting efficiency typically includes three parts: the photon absorption efficiency, the separation efficiency of the semiconductor bulk, and the surface injection efficiency. This review briefly discusses the recent advances in studies on hematite photoanodes for water splitting, and through the enhancement of the three above-mentioned efficiencies, the corresponding strategies toward improving the PEC performance of hematite are comprehensively discussed and summarized.

Key words: Hematite, Photon absorption efficiency, Charge separation efficiency, Surface injection efficiency

周定华, 范科. 提高氧化铁光电催化分解水效率的策略进展[J]. 催化学报, 2021, 42(6): 904-919.

Dinghua Zhou, Ke Fan. Recent strategies to enhance the efficiency of hematite photoanodes in photoelectrochemical water splitting[J]. Chinese Journal of Catalysis, 2021, 42(6): 904-919.

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图/表 23

Fig. 1. Schematic of the complete PEC water oxidation process.

Fig. 2. Main strategies for enhancing PEC performance of hematite for water splitting.

Fig. 3. (a-c) SEM images of some 3D ordered nanophotonic structures of hematite: (a), (b), and (c) adapted with permission from Refs. [10,11], and [13]. Copyright (2016) and (2014) Royal Society of, Chemistry, and (2014) American Chemical Society, respectively. UV-vis optical absorption spectra of the NSP substrate-based device with: (d) different pitches and (e) different heights in a 1000 nm pitch. (f) J-V curves of the Ti-doped hematite photoelectrodes based on the three different NSP arrays. Adapted with permission from Ref. [13]. Copyright (2014) American Chemical Society.

Fig. 4. (a) Schematic of hematite/Ag/CoPi; UV-Vis spectra (b), and IPCE (c) for hematite photoelectrodes. Adapted with permission from Ref. [21]. Copyright (2016) Wiley.

Fig. 5. Illuminated situation of the hematite surface, G represents the generation of electron-hole pairs by photoexcitation.

Table 1 Reports on photon absorption efficiency enhancement.

Sample	Electrolyte	Photocurrent density (mA cm^-2, at 1.23 V vs. RHE)	Ref.
3D branched Fe₂O₃ nanorod	1 M NaOH	0.61	[10]
3D nanocone Ti-Fe₂O₃ array	1 M NaOH	2.24	[11]
3D Ti-Fe₂O₃nanospikes	1 M NaOH	2.42	[13]
Au-Fe₂O₃	1 M NaOH	~1.00	[19]
Ag-Fe₂O₃	1 M NaOH	3.20	[21]
Pd-Fe₂O₃	0.1 M Na₂S and 0.1 M Na₂SO₃	N/A	[22]
BiVO₄-Fe₂O₃	1 M NaOH	~0.50	[31]
WO₃-Fe₂O₃	0.1 M NaOH	0.70	[35]
WO₃-Fe₂O₃	0.5 M NaSO₄	1.66	[29]
Si-Fe₂O₃ F-doped Fe₂O₃	1 M NaOH 1 M KOH	~0.90 2.52	[26] [38]

Fig. 6. Morphological images (a-c) and PEC performance curves (d-f) of some special nanostructure hematite electrodes. Adapted with permission from Ref. [44,45,50]. Copyright (2018) American Chemical Society, (2017) and (2020) Wiley.

Fig. 7. (a) J-V curves of different photoanodes based on hematite. (b) Charge separation efficient in bulk; (c-e) the band bending schemes in hematite with different P concentrations. (f) Gradient P concentration; EF is the Fermi level. Adapted with permission from Ref. [51]. Copyright (2017) Royal Society of Chemistry.

Fig. 8. Types of heterojunctions. (a) type I; (b) type II; (c) type III. (the band bending is not shown).

Fig. 9. Mott-Schottky plots (a) and UPS spectra for hematite (b) and Fe2O3@FeNbO4 nanorods (c), and (d) corresponding band alignment (d). Adapted with permission from Ref. [66]. Copyright (2018) American Chemical Society.

Table 2 Summary of studies on enhancing the separation efficiency of bulk semiconductors.

Samples	Electrolyte	Photocurrent density (mA cm^-2, at 1.23 V vs RHE)	Ref.
Ultrafine Ti-Fe₂O₃ nanowire array	1 M NaOH	0.90	[44]
TiO₂/Ti: Fe₂O₃ BNR	1 M KOH	2.50	[45]
Ti-modified mesoporous Fe₂O₃	1 M NaOH	4.30	[50]
Sn-doped Fe₂O₃ P-doped Fe₂O₃	1 M NaOH 1 M NaOH	1.83 2.70	[54] [57]
Grad P-doped Fe₂O₃	1 M KOH	1.48	[51]
Ti, Mg co-doped Fe₂O₃	1 M NaOH	1.08	[58]
Ti/Au/Fe₂O₃	1 M NaOH	0.51	[60]
Mg-doped Fe₂O₃/ P-doped Fe₂O₃	1 M KOH	2.40	[62]
U₃O₈/Fe₂O₃	1 M NaOH	2.43	[65]
Fe₂O₃@FeNbO₄ Fe₂O₃/TiO₂	1 M NaOH 1 M KOH	2.24 2.90	[66] [70]

Fig. 10. Processes that may exist at the semiconductor-solution interface. G represents the generation of electron-hole pairs; Et is trap energy level in the bulk semiconductor; and Ess is the surface state energy level.

Fig. 11. Band edge pinning diagrams (a) and Fermi level pinning (b) for a photoanode system. Adapted with permission from Ref. [73]. Copyright (2015) American Chemical Society.

Fig. 12. (a) J-V curves for PEC water oxidation based on different hematite electrodes under illumination (1 Sun, AM 1.5) from 1.0 M NaOH solution; (b) OCP in the dark and under illumination for hematite electrodes. Adapted with permission from Ref. [74]. Copyright (2015) American Chemical Society.

Fig. 13. J-V curves under H2O and H2O2 oxidation conditions for hematite electrodes annealed at 500 °C (a) and 800 °C (b); CV curves scanned at 1 V s-1 under dark conditions for the electrodes annealed at 500 °C (c) and 800 °C (d). Adapted with permission from Ref. [78]. Copyright (2014) American Chemical Society.

Fig. 14. J-V curves of Fe2O3 electrodes under illumination in different pH electrolytes (1 Sun, AM 1.5): at pH 13.6 (a) and pH 7 (b) for the bare Fe2O3 (c) and Pi-Fe2O3 (d) electrodes. Adapted with permission from Ref. [86]. Copyright (2014) Wiley.

Fig. 15. Proposed RDS for water oxidation on hematite. (a) Stepwise electron/proton transfer pathway; (b) CPET transfer pathway. Adapted with permission from Ref. [90]. Copyright (2016) Royal Society of Chemistry.

Fig. 16. (a) J-V scans under illumination in a 0.5 M NaClO4 solution at different pH levels; (b) KIE values calculated from the steady photocurrent ratio in H2O and D2O at various electrolyte pH levels. Adapted with permission from Ref. [91]. Copyright (2016) American Chemical Society.

Fig. 17. WNA (a) and I2M (b) mechanisms on hematite surfaces. Adapted with permission from Ref. [88]. Copyright (2018) American Chemical Society.

Fig. 18. Summary of IMPS data for three different photoelectrodes; (a) ktr; (b) kre. Adapted with permission from Ref. [93]. Copyright (2016) Royal Society of Chemistry.

Fig. 19. Back reaction in the FTO-solution interface. (a) without a blocking layer; (b) with a blocking layer. R represents the reactant (OH- or H2O).

Fig. 20. (a) J-V curves of the Ti4+ doped Fe2O3 before and after surface corrosion in 1 M NaOH; (b) Reduction dark current before and after surface corrosion in 1 M NaOH, FTO substrate as a reference, and different gas bubbling (N2 or O2) processes. Adapted with permission from Ref. [106]. Copyright (2014) Royal Society of Chemistry.

Table 3 Reports on surface injection efficiency enhancement.

Samples	Electrolyte	Photocurrent density (mA cm^-2, at 1.23 V vs RHE)	Ref.
TiO₂-modified Fe₂O₃	1 M NaOH	1.20	[74]
Annealing at 800 °C Fe₂O₃	1 M KOH	~0.78	[78]
Acid treatment Fe₂O₃	1 M NaOH	~1.30	[83]
Pi-Fe₂O₃	1 M NaOH	1.31	[86]
	0.1 M KPi	1.26
F-doped Fe₂O₃	1 M KOH	2.52	[38]
Alkali treatment Fe₂O₃	1 M NaOH	0.63	[92]
Fe₂O₃/FeOOH	1 M NaOH	1.21	[98]
Fe₂O₃/RuOEC	1 M KOH	~2.20	[103]
Nb₂O₅ underlayer of hematite	1 M NaOH	~0.1	[104]
Acid corrosion Ti-Fe₂O₃	1 M NaOH	~1.7	[106]

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