Gradient FeO(x)(PO4)(y) layer on hematite photoanodes: novel structure for efficient light-driven water oxidation.

Hematite has been receiving increasing attention for its application in photoelectrochemical (PEC) water oxidation but usually exhibits poor efficiency. We fabricated a stable gradient-structured FeOx(PO4)y layer on hematite by diffusively incorporating phosphate onto the surface layer of hematite films at a low temperature. X-ray photoelectron spectroscopy depth profile and Fe K-edge grazing-incidence X-ray absorption near-edge structure and extended X-ray absorption fine structure analysis demonstrated the formation of a ∼50 nm overlayer with a gradient phosphorus distribution and structural evolution from the outer surface to the depth. The as-prepared photoanodes showed highly improved PEC water oxidation performance. Up to 8.5-fold enhancement in the photocurrent density at 1.23 V versus reversible hydrogen electrode was achieved relative to the pristine anode. This strategy is applicable for hematite photoanodes prepared by different methods and with different morphologies and structures. The improvement in the water oxidation activity is mainly attributed to the enhanced separation of photogenerated electron-hole pairs, which is derived from the increased hole diffusion length in the gradient-structured overlayer. This work develops a simple and universal method to boost the PEC water oxidation efficiency with versatile hematite photoanodes.

[1]  K. Wandelt.  Photoemission studies of adsorbed oxygen and oxide layers , 1982 .

[2]  L. Castro,et al.  The Spin-Polarized Electronic Structure of LiFePO4 and FePO4 Evidenced by in-Lab XPS , 2010 .

[3]  Alexander J. Cowan,et al.  Dynamics of photogenerated holes in surface modified α-Fe2O3 photoanodes for solar water splitting , 2012, Proceedings of the National Academy of Sciences.

[4]  J. Sullivan,et al.  A study of the core level electrons in iron and its three oxides by means of X-ray photoelectron spectroscopy , 1983 .

[5]  D. Klug,et al.  The role of cobalt phosphate in enhancing the photocatalytic activity of α-Fe2O3 toward water oxidation. , 2011, Journal of the American Chemical Society.

[6]  M. Takano,et al.  Large-scale synthesis of single-crystalline iron oxide magnetic nanorings. , 2008, Journal of the American Chemical Society.

[7]  Michael Grätzel,et al.  Translucent thin film Fe2O3 photoanodes for efficient water splitting by sunlight: nanostructure-directing effect of Si-doping. , 2006, Journal of the American Chemical Society.

[8]  C. Fan,et al.  Template-free synthesis of hematite photoanodes with nanostructured ATO conductive underlayer for PEC water splitting. , 2014, ACS applied materials & interfaces.

[9]  G. Sawatzky,et al.  In situ XPS analysis of various iron oxide films grown by NO2-assisted molecular-beam epitaxy , 1999 .

[10]  Ryan L. Spray,et al.  Photoactivity of Transparent Nanocrystalline Fe2O3 Electrodes Prepared via Anodic Electrodeposition , 2009 .

[11]  Juan Bisquert,et al.  Photoelectrochemical and impedance spectroscopic investigation of water oxidation with "Co-Pi"-coated hematite electrodes. , 2012, Journal of the American Chemical Society.

[12]  Yichuan Ling,et al.  Sn-doped hematite nanostructures for photoelectrochemical water splitting. , 2011, Nano letters.

[13]  Jun Lin,et al.  Lattice defect-enhanced hydrogen production in nanostructured hematite-based photoelectrochemical device. , 2012, ACS applied materials & interfaces.

[14]  Xianluo Hu,et al.  Continuous Aspect‐Ratio Tuning and Fine Shape Control of Monodisperse α‐Fe2O3 Nanocrystals by a Programmed Microwave–Hydrothermal Method , 2008 .

[15]  R. Toth,et al.  Formation of an electron hole doped film in the α-Fe2O3 photoanode upon electrochemical oxidation. , 2013, Physical chemistry chemical physics : PCCP.

[16]  W. Stickle,et al.  Handbook of X-Ray Photoelectron Spectroscopy , 1992 .

[17]  Juan Bisquert,et al.  Water oxidation at hematite photoelectrodes: the role of surface states. , 2012, Journal of the American Chemical Society.

[18]  Yichuan Ling,et al.  Facile synthesis of highly photoactive α-Fe₂O₃-based films for water oxidation. , 2011, Nano letters.

[19]  Sean C. Smith,et al.  Understanding the enhancement in photoelectrochemical properties of photocatalytically prepared TiO2-reduced graphene oxide composite , 2011 .

[20]  M. Seah,et al.  Auger and x-ray photoelectron spectroscopy , 1990 .

[21]  Michael Grätzel,et al.  Solar water splitting: progress using hematite (α-Fe(2) O(3) ) photoelectrodes. , 2011, ChemSusChem.

[22]  Jianwei Sun,et al.  Solar water oxidation by composite catalyst/alpha-Fe(2)O(3) photoanodes. , 2009, Journal of the American Chemical Society.

[23]  Pingyun Feng,et al.  A three-dimensional branched cobalt-doped α-Fe2O3 nanorod/MgFe2O4 heterojunction array as a flexible photoanode for efficient photoelectrochemical water oxidation. , 2013, Angewandte Chemie.

[24]  Chunhua Yan,et al.  Single-crystalline iron oxide nanotubes. , 2005, Angewandte Chemie.

[25]  D. Gamelin,et al.  Photoelectrochemical water oxidation by cobalt catalyst ("Co-Pi")/alpha-Fe(2)O(3) composite photoanodes: oxygen evolution and resolution of a kinetic bottleneck. , 2010, Journal of the American Chemical Society.

[26]  Xianluo Hu,et al.  α‐Fe2O3 Nanorings Prepared by a Microwave‐Assisted Hydrothermal Process and Their Sensing Properties , 2007 .

[27]  J. Kitchin,et al.  Spectroscopic Characterization of Mixed Fe–Ni Oxide Electrocatalysts for the Oxygen Evolution Reaction in Alkaline Electrolytes , 2012 .

[28]  M. Grätzel,et al.  Photo-assisted electrodeposition of cobalt–phosphate (Co–Pi) catalyst on hematite photoanodes for solar water oxidation , 2011 .

[29]  Michael Grätzel,et al.  Light-induced water splitting with hematite: improved nanostructure and iridium oxide catalysis. , 2010, Angewandte Chemie.

[30]  Michael Grätzel,et al.  Passivating surface states on water splitting hematite photoanodes with alumina overlayers , 2011 .