Rapid Plasma Synthesis of Blackened Oxygen-Deficient Zinc Oxide

Zinc oxide (ZnO) has long attracted attention in a range of fields such as electronics, solar cells, photocatalysis and biological applications.¹ It has a characteristic large bandgap of 3.1-3.4 eV giving rise to absorption of light in the UV range below 400 nm. However, for applications which favour absorption over a wider energy range such as photocatalysis, solar thermal harvesting this restriction to UV is a limitation to application. Therefore, increasing the range of absorption represents a necessity to drive these technologies further. More recently it is emerging that modifying the number of defects in the Zinc material can give rise to enhanced photocatalytic activities and even absorption in the visible range.<br/>Several groups have utilised chemical methods to formulate hydrogenated ZnO and other forms of defect-rich ZnO by chemical methods.^2–4 These materials do not display the characteristic white colouration, appearing visibly black, further demonstrated by a reduction from 94% to sub-45% reflection in the visible light range.² These materials demonstrated enhanced photocatalytic properties when compared to traditional ZnO with test cases of methylene blue² or methyl orange degradation³ and hydrogen evolution.⁵ Thus far the theories presented in literature have highlighted the key role of defects in the crystal lattice, in particular oxygen deficiencies or vacancies in the colour change from the typical ZnO white to black.^2,6<br/>In this work we report a new method to produce Zn-rich ZnO complex produced using atmospheric-pressure plasma. The addition of a 2% H₂/Ar mixture at 0.1 litres per minute into the standard 1 litre per minute flow of helium gas is critical to the formation of the darkened powder. We have characterized the synthesised material, utilising transmission electron microscopy, alongside X-ray diffraction and X-ray photoelectron spectroscopy to characterise the structural information including defects and surface chemistry respectively. The material we produce is determined to contain two long-ordered and well-defined structures, a shell of Wurtzite ZnO and a core that is comprised of either O-deficient Zn or Zn in a hexagonal lattice. There is a clear and sharp change in atomic plane between these regimes. In addition, the stability of the material in several typical solvents or under thermal annealing at several hundred degrees Celsius is explored, before we test the material for several different applications, such as solar thermal and biological applications.<br/><b>References</b><br/>(1) Qi, K.; Cheng, B.; Yu, J.; Ho, W. <i>J. Alloys Compd.</i> <b>2017</b>, <i>727</i>, 792–820. https://doi.org/10.1016/j.jallcom.2017.08.142.<br/>(2) Xia, T.; Wallenmeyer, P.; Anderson, A.; Murowchick, J.; Liu, L.; Chen, X. <i>RSC Adv.</i> <b>2014</b>, <i>40</i> (78), 41654–41658. https://doi.org/10.1039/c4ra04826a.<br/>(3) Yan, C.; Yan, G.; Cheng, W.; Qiu, Y.; Wang, L. <i>Mater. Lett.</i> <b>2020</b>, <i>265</i>, 127442. https://doi.org/10.1016/j.matlet.2020.127442.<br/>(4) Badreldin, A.; Abdel-Wahab, A.; Balbuena, P. B. <i>ACS Appl. Energy Mater.</i> <b>2020</b>, <i>3</i> (11), 10590–10599. https://doi.org/10.1021/acsaem.0c01642.<br/>(5) Zhang, N.; Shan, C. X.; Tan, H. Q.; Zhao, Q.; Wang, S. P.; Sun, Z. C.; Xia, Y. De; Shen, D. Z. <i>Nanotechnology</i> <b>2016</b>, <i>27</i> (22), 1–7. https://doi.org/10.1088/0957-4484/27/22/22LT01.<br/>(6) Sharma Kanakkillam, S.; Krishnan, B.; Sepulveda Guzman, S.; Amilcar Aguilar Martinez, J.; Avellaneda Avellaneda, D.; Shaji, S. <i>Appl. Surf. Sci.</i> <b>2021</b>, <i>567</i> (April), 150858. https://doi.org/10.1016/j.apsusc.2021.150858.