Introduction In the past decades, numerous semiconducting materials [1], especially oxide semiconductors, have been investigated as potential photoelectrodes in photoelectrochemical (PEC) system with a view to efficient light-induced water splitting for solar-hydrogen conversion.Compared to other metal oxide semiconductors, a-Fe2O3 (hematite) has the advantage such as the small band gap energy of ~2.0 eV, which enables it to absorb most of the photons of solar spectrum [2]. Furthermore, its chemical stability, widespread availability and innocuity make it promising to be employed in a large scale. Unfortunately, the ultrafast recombination of the photogenerated carriers and the poor minority charge carrier mobility lead to a short hole diffusion length in a-Fe2O3, which severely limits the overall photocurrents produced by solar light. It was reported that doping with metal ions displayed positive effects on the efficiency of a-Fe2O3photoanodes, which was mostly attributed to the improved charge conductivity.ZnO, as a wide band gap semiconductor, has many attractive material properties, including non-toxic, low cost, large excitation binding energy, high electronic conductivity [3]. Accordingly, ZnO has been utilized for various applications, e.g., chemical and gas sensors, UV light emitters, metal-ion batteries, surface acoustic wave devices and photoanodes for solar energy conversion. However, either ZnO particles or nanostructured ZnO films have a wide band gap of ~3.2 eV, suggesting that only ultraviolet (UV) light could be utilized. Effective and controlled doping with metal or non-metal ions is a very common method to modify the electronic and optical properties for various wide band gap semiconductors. The doped ions can create impurity levels in the forbidden band of substrate semiconductor materials, and further narrow band gaps for efficient visible light utilization.Core/shell structured a-Fe2O3 nanorod arrays with Ag surface doping A facile solution-based method was developed to fabricate hematite nanorods coated with ultrathin overlayer of AgxFe2-xO3. The core/shell nanorod structures of a-Fe2O3/AgxFe2-xO3 were obtained by annealing solution-fabricated �-FeOOH nanorod arrays which were first ultrasonicated in Ag+ aqueous solution. The formed overlayer of AgxFe2-xO3 was estimated to be ~3 nm in HRTEM images, and evenly coated on the surface of a-Fe2O3 nanorod core. For the a-Fe2O3/AgxFe2-xO3 nanorod structure, the surface doping of Ag+ions gave rise to increased electron donor density as estimated from the Mott-Schottky results, which also led to the enhancement in PEC performance with IPCE at 400 nm increased from ~2.0% to ~7.5%. N-doped ZnO nanorod arrays with gradient band structure Solution based ZnO nanorod arrays were modified with controlled N doping by an advanced ion implantation method, and then utilized as photoanodes for PEC water splitting under visible light irradiation. An obvious N dopant concentration of 3.5 wt% was acquired by varying the implantation doses up to 1�1015 ions/cm2. N doped ZnO nanorod photoanodes displayed markedly enhanced PEC performance, a photocurrent of 150 �A/cm2was achieved under visible light illumination, which was about 2 orders of magnitude higher than that of pure ZnO. A comprehensive characterization was performed to explore how N dopant influences the physicochemical properties of ZnO nanorod arrays acting as water-splitting photoanodes. The relationship between optical properties, band structure, photoluminescence intensity and PEC performance indicated that gradient N ion doping can extend optical absorption ability, introduce internal driving force for photo-induced charge transfer and hence increase the PEC performance of ZnO nanorod arrays (Figure 2). These results proposed the potential application of ion implantation method for exploiting novel water splitting materials with improved visible light response.  Conclusions Surface doping of Ag into a-Fe2O3nanorod arrays leads to greatly enhanced photoelectrochemical water splitting performance, mainly due to the increased electron donor density. N ions doping extended the optical absorption edges to visible light region and enhanced the separation efficiency of photogenerated electron-hole pairs in ZnO. As a result, visible light driven PEC performance of N doped ZnO films was improved significantly.Acknowledgement. The author gratefully acknowledges the financial support of the National Natural Science Foundation of China (no. 51102194)References (1) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Chem. Rev. 2010  , 110, 6503.(2) Sivula, K.; Le Formal, F.; Gr�tzel, M. ChemSusChem 2011  , 4, 432.(3) Vayssieres, L. Adv. Mater. 2003  , 15, 464.

Doping to metal oxide nanorod arrays: Engineered electronic property and band structure for improved photoanodic performances

247th National Meeting (2014)

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