Research Article


2018, 11(5): 2412–2423


Elucidation of thermally induced internal porosity in zinc oxide nanorods

Albertus D. Handoko1,§ (*), Laura-Lynn Liew1,2,§, Ming Lin1, Gopinathan Sankar3, Yonghua Du4, Haibin Su2, Zhili Dong2, and Gregory K. L. Goh1,2 (*)

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1 Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore 138634, Singapore
2 School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
3 Department of Chemistry, University College London, London WC1H 0AJ, UK
4 Institute of Chemical and Engineering Sciences, Agency for Science, Technology and Research (A*STAR), Singapore 627833, Singapore
§ Albertus D. Handoko and Laura-Lynn Liew contributed equally to this work.

Keywords: zinc oxide, solution growth, hydrothermal, porosity, photocatalysis, photocurrent
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ABSTRACT In situ electron microscopy, tomography, photoluminescence, and X-ray absorption spectroscopy were utilized to monitor and explain the formation and growth of internal pores within ZnO nanorods. Careful examination using electron microscopy and tomography indicate that nanosized internal pores start appearing within the individual solution-grown ZnO nanorods upon exposure to 200 °C. The pore volume growth rate is proportional to the heat treatment time, indicating that the process is diffusion controlled, akin to a reverse Ostwald ripening-like process. A manageable pore growth rate of 1.4–4.4 nm3·min−1 was observed at 540 °C, suggesting that the effective control over internal porosity can be achieved by carefully controlling the heat-treatment profile. Mechanistic studies using X-ray absorption spectroscopy indicated that the pore formation is linked to the significant reduction of the number of zinc vacancies after heat treatment. An optimum condition exists where most of the native surface defects are removed, while the bulk defects are contained within the internal pores. It is also demonstrated that the internal porosity can be exploited to improve the visible light absorption of ZnO. A combination of the lower defect density and improved light absorption of the heat-treated ZnO films thus lead to an increase in the photoelectrochemical response of more than 20× compared to that of the as-grown ZnO.
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Elucidation of thermally induced internal porosity in zinc oxide nanorods. Nano Res. 2018, 11(5): 2412–2423

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