Spontaneous water dissociation on cleaved GaN(1-100) surface
Shih-Yu Wu1*, Laing-Wei Lan1, Pei-Yang Cai2, Yun-Wen Chen3, Yu-Ling Lai4, Ming-Wei Lin4, Yao-Jane Hsu4, Wei-I Lee5, Jer-Lai Kuo3, Meng-Fan Luo2, Chien-Cheng Kuo1
1Department of Physics, National Sun Yat-sen University, Kaohsiung, Taiwan
2Department of Physics and Center for Nano Science and Technology, National Central University, Jhongli, Taiwan
3Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan
4National Synchrotron Radiation Research Center, Hsinchu, Taiwan
5Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
* presenting author:Shih-Yu Wu, email:wu.shihyu@gmail.com
Semiconductor as a photocatalyst to dissociate water molecules has been used to generate clean and renewable hydrogen source. Among of different candidates, GaN is of interest because of its excellent carrier-photon conversion capability for generating electron-hole pairs which stimulate the photocatalytic process. Especially for the nonpolar GaN(1-100) surface, calculations suggest that water molecules are spontaneously dissociated on the surface [1,2]. Recent experiments have also demonstrated a high efficient and recyclable photocatalyst by utilizing the nonpolar GaN(1-100) surface [3,4,5]. However, comparing to the comprehensive calculations and experiments, the experimental evidence for the spontaneous dissociation of water molecules on the nonpolar GaN(1-100) surface is still lacking. The main reason is the difficulty for preparing an atomically clean and ordered nonpolar GaN(1-100) surface. Here, the adsorption of the water molecules on the cleaved nonpolar GaN(1-100) surface is studied. By using synchrotron-based X-ray photoemission spectroscopy (SR-XPS), the adsorption process is clarified by monitoring the reaction in real-time with time frame ~1 min. The results show that the water molecules are spontaneously dissociated on the nonpolar surface at room temperature. The reaction rate is almost the same even the temperature is as low as 130 K, which indicates a much lower dissociation energy than the thermal energy at room temperature. The atomic model is further studied by comparing with the calculations. Our results provide a clear evidence for the dissociation of water molecules on the nonpolar GaN(1-100) surface in microscopic point of view.


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Keywords: water dissociation, GaN, X-ray photoemission spectroscopy, nonpolar surface