Synthesis and characterization of band gap-reduced ZnO:N and ZnO:(Al,N) films for photoelectrochemical water splitting
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Kwang-Soon Ahn Energy & Environment Laboratory, Samsung Advanced Institute of Technology, Yongin-si, Gyeonggi-do 446-712, Republic of Korea
Todd Deutsch and Heli Wang National Renewable Energy Laboratory, Golden, Colorado 80401
Nuggehalli Ravindra New Jersey Institute of Technology, Newark, New Jersey 07102
Yanfa Yan,b) John Turner,c) and Mowafak Al-Jassim National Renewable Energy Laboratory, Golden, Colorado 80401 (Received 13 May 2009; accepted 7 August 2009)
ZnO thin films with significantly reduced band gaps were synthesized by doping N and codoping Al and N at 100 C. All the films were synthesized by radiofrequency magnetron sputtering on F-doped tin-oxide-coated glass. We found that codoped ZnO: (Al,N) thin films exhibited significantly enhanced crystallinity compared with ZnO doped solely with N, ZnO:N, at the same growth conditions. Furthermore, annealed ZnO:(Al,N) thin films exhibited enhanced N incorporation over ZnO:N films. As a result, ZnO:(Al,N) films exhibited better photocurrents than ZnO:N films grown with pure N doping, suggesting that charge-compensated donor–acceptor codoping could be a potential method for band gap reduction of wide-band gap oxide materials to improve their photoelectrochemical performance. I. INTRODUCTION
Transition-metal oxides are potential candidates for photoelectrochemical (PEC) H2 production from water.1,2 However, to date, only TiO2 has received extensive attention.3–6 ZnO has similar band gap and band-edge positions compared with TiO2,1 but ZnO has a direct band gap and higher electron mobility than TiO2.7 Thus, the PEC property of ZnO also needs to be studied.8 Like TiO2, the band gap of ZnO (3.3 eV) is too large to effectively use visible light.3 Therefore, it is critical to reduce the band gap of ZnO. To date, impurity incorporation has been the main method of reducing the band gap of TiO2. It has been reported that N, C, and S doping can successfully narrow the band gap of TiO2 and push the photoresponse into the long-wavelength region.3–6 Although band gap reduction Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] c) This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http:// www.mrs.org/jmr_policy DOI: 10.1557/JMR.2010.0017 J. Mater. Res., Vol. 25, No. 1, Jan 2010
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of TiO2 has been extensively studied, very limited research exists on band gap narrowing of ZnO by impurity incorporation. Significant amounts of N can be incorporated into ZnO and WO3 only at low temperatures.9,10 However, films grown at low temperature usually exhibit poor crystallinity, which is extremely detrimental to PEC performance. This dilemma hinders the PEC performance of N-incorporated ZnO and WO3 films. A possible cause for the inferior crystallinity may be uncompensated charged N atoms. This problem could be overcome by charge-compensated d
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