Electronic Tuning of Zinc Oxide by Direct Fabrication of Chromium (Cr) incorporated photoanodes for Visible-light driven Water Splitting Applications

Abstract

Herein, we report the synthesis of Cr incorporated ZnO sheets arrays microstructures and construction of photoelectrode through a direct aerosol assisted chemical vapour deposition (AAcVD) method. The as-prepared Cr incorporated ZnO microstructures were characterized by transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, powdered X-ray spectroscopy, X-ray photoelectron spectroscopy and UV-Vis diffused reflectance spectroscopy. The Cr incorporation in ZnO red shifted the optical band gap of as-prepared photoanodes. The 15% Cr incorporation in ZnO has shown enhanced PEC performance. The AACVD method provides an efficient in situ incorporation approach for the manipulation of morphological aspects, phase purity, and band structure of photoelectrodes for an enhanced pec performance. Solar energy utilization has become the topic of research interest worldwide in order to cope with environmental issues and energy crisis 1,2. Hydrogen generation as a clean fuel from solar energy through photoelectrochemical (PEC) water splitting is regarded as one of the most efficient strategy 3-5. Since, Fujishima and Honda has demonstrated the use of TiO 2 for PEC water splitting 6 , a variety of transition metals based semiconductors, such as metal oxides 7-13 , metal nitrides, and sulfides/selenides 14-16 , have been extensively investigated as photoanodes for PEC cells. Among various semiconductors, ZnO has been widely explored as photoanodes due to its non-toxic, abundant, thermal and chemical stability and suitable optical band edges 17-23. But, wide optical band gap of plain ZnO limits its large scale practical utility as it is only active in ultraviolet (UV) region of light. To widen its absorption window from ultra violet (UV) to visible region, among various strategies, the incorporation of other metal into plain ZnO thin films to modulate band gap is an effected strategy. ZnO thin films manifest interesting electrical and optical features upon incorporation of metal atoms. Various transition metals like Co 24 , Fe 25 , Mn 26 , Cu 27 and Ga 28 have been extensively investigated in terms of their incorporation into ZnO material. Among these, Cr has attracted significant attention 29-31 because the incorporated Cr ions can significantly improve the optical characteristics of ZnO thin films by narrowing band gap energy. Meanwhile, the ionic radius of Cr 3+ is quite close to the ionic radius of Zn 2+ , which reveals that Cr 3+ can eagerly enter into the lattice and can replace Zn 2+ in ZnO lattice 32. The incorporated Cr atoms behave like donor species and provide free electrons. Conversely, this increases the concentration of free carriers, hence decreasing the electrical resistivity of the material. Another way to improve PEC water-splitting performance by reducing electron recombination is to design new complex structural architecture of photoelectrode. Researchers are trying their best to explore new photoelectrode architectures 33. Complex arrays such as 3D architectures are...

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