On the Importance of Surface Reactions on Semiconductor Photocatalysts for Solar Water Splitting
Xiao-Xia CHANG*(),Jin-Long GONG
Collaborative Innovation Center of Chemical Science and Engineering, Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China
One of the most appealing ways to resolve the worldwide energy crisis and environmental pollution is by converting solar energy into storable chemical energy as hydrogen through solar water splitting. The redox reactions of photogenerated charge carriers occurring on the surface of photocatalysts during the process of solar water splitting are particularly complex. Owing to the high reaction overpotentials and sluggish desorption kinetics of gas products, surface reaction is the rate-determining step in the solar water splitting process. Therefore, a great deal of attention has been focused on this specific research area. The recent advances and prospects for future directions regarding the importance of surface reactions for solar water splitting are presented. The main strategies to enhance the surface water splitting reaction kinetics are summarized. The roles and classifications of surface cocatalysts, as well as the effects of passivating the surface states and coating surface protective layers, are discussed by integrating the principles of photocatalysis. Prospects for the future development of surface reaction research are also proposed.
Received: 01 September 2015
Published: 19 October 2015
Xiao-Xia CHANG,Jin-Long GONG. On the Importance of Surface Reactions on Semiconductor Photocatalysts for Solar Water Splitting. Acta Phys. -Chim. Sin., 2016, 32(1): 2-13.
Fig 1 Schematic illustration of different steps during photocatalytic water splitting
Fig 2 (a) Schematic reaction mechanism of overall water splitting on Rh/Cr2O3 loaded (Ga1–xZnx)(N1–xOx) composite15; (b) schematic photocatalytic H2 evolution and back-reaction processes on Pt and PtO loaded TiO2, respectively16; (c) schematic diagram of charge separation and H2 generation on TiO2/graphene/MoS219; (d) comparison of the photocatalytic H2 production activity by different photocatalysts19
Fig 3 (a) Energy diagram of Co-Pi/W:BiVO4 photoanode; (b) current–potential curves of W:BiVO4 photoanode before and after the deposition of Co-Pi23; (c) photocurrent curves and (d) stability of Ta3N5 based photoanodes with different cocatalysts for solar water oxidation24
Fig 4 (a) Separated Pt and IrO2 on the core/shell Ta3N5 photocatalyst as effective charge collectors28; (b) SEM image ofBiVO4 with Pt and MnOx deposited on {010} and {110} facets. The inset shows the schematic illustration of the structure29; (c) photocatalytic water oxidation activity of BiVO4 with cocatalysts deposited by different methods29
Fig 5 (a) HRTEM image and SAED patterns (inset) of rutile TiO2 nanowires (NWs) with 150 cycles of a TiO2 thin film using ALD (atomic layer deposition) method32; (b) photocurrent curves for 1.8 μm long TiO2 nanowire electrodes coated with various ALD cycles of TiO232; (c) band diagram and (d) photocurrent curves for water oxidation of α-Fe2O3 electrodes annealed at 500 and 800 ℃33
Fig 6 (a) Stability and (b) photoconversion efficiencies of planar and nanopillar p-InP electrodes protected by ALD-grown TiO2 and sputter-deposited Ru cocatalyst34; (c) model and stability of np+-Si photoanode protected by ALD-grown TiO2(44 nm)36; (d) model and stability of ZnO nanorod electrodes before and after protected by ALD-grown Ta2O537
RH was increased by 39 times; quantum efficiency = 9.7% (365 nm)
19
OEC
β-Ge3N4
RuO2(1%(w))
impregnation
450 W Hg lamp; aqueous solution
non-oxide photocatalyst for overall water splitting
21
BiVO4
Co-Pi
photoelectrodeposition
AM 1.5G; 0.1 mol·L–1 KPi (pH 8)
cathodic shift (440 mV) of onset potential
23
BiVO4
NiOOH/FeOOH
photoelectrodeposition
AM 1.5G; 0.5 mol·L–1 phosphate buffer (pH 7)
photocurrent of 2.73 mA·cm–2at 0.6 V (vs RHE)
25
HEC/OEC
Ta3N5
Pt/CoOx(IrOx)
impregnation
visible light; 25% (volume fraction) methanol/water for H2 evolution; 0.02 mol·L–1 AgNO3 for O2 evolution
RH = 171.9; RO = 12250
28
BiVO4
Pt/MnOx
photodeposition
visible light; 0.02 mol·L–1 NaIO3
RO = ~650
29
passivating surface states
α-Fe2O3
Al2O3
ALD
AM 1.5G; 1 mol·L–1 NaOH (pH 13.6)
cathodic shift (100 mV) of onset potential; Photocurrent was increased by 3.5 times at 1.0 V (vs RHE)
31
rutile TiO2
epitaxial rutile TiO2
ALD
AM 1.5G; 1 mol·L–1 NaOH(pH 13.6)
photocurrent was increased by 1.5 times at 1.5 V (vs RHE)
32
α-Fe2O3
annealing at 800 ℃; Co-Pi
calcination; Photoelectrodeposition
AM 1.5G; 1 mol·L–1 KOH
photocurrent onset potential of ~0.6 V (vs RHE)
33
coating protective layers
p-InP
Ru/TiO2
ALD
AM 1.5G; 1 mol·L–1 HClO4 (pH 0.5) (vs SCE)
high stability; energy conversion efficiency of 14%
34
np+-Si
Ni/TiO2
ALD
1.25 Sun; 0.93 V (vs SCE); 1 mol·L–1 KOH
high stability during continuous OER operation of 100 h
36
ZnO
Ta2O5
ALD
AM 1.5G; 1.23 V (vs RHE); 0.1 mol·L–1 KOH
high stability for OER operation of 5 h
37
Table 1Examples of enhancing surface reaction for improved photocatalytic water splitting activity
Fig 7 (a) Photocurrent curves of BiVO4 photoanodes loading with different cocatalysts for water oxidation25; (b) photocurrent curves of BiVO4 photoanodes loading with CoOx cocatalysts and NiO layer for water oxidation41; (c) band diagram and mechanism of charge separation for p-n Co3O4/BiVO4 heterojunction (pH = 7)42; (d) photocurrent curves of BiVO4 and Co3O4/BiVO4 photoanodes for water oxidation42
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