Hierarchical growth of SnO 2 nanostructured films on FTO substrates: structural defects induced by Sn(II) self-doping and their effects on optical and photoelectrochemical properties† Hongkang Wang, ab Sergii Kalytchuk, bc Haihua Yang, bc Lifang He, d Chenyan Hu, c Wey Yang Teoh c and Andrey L. Rogach * b Direct hydrothermal growth of Sn(II)-doped SnO 2 films on fluorine-doped tin oxide (FTO) substrates results in the formation of upstanding SnO 2 nanosheet arrays covered by hierarchical SnO 2 nanoflowers. The n- type semiconductor films show extended photoresponse in the visible spectrum arising from the coexistence of Sn(II) dopant ions and oxygen vacancies in these hierarchical SnO 2 nanostructures, which leads to a narrowed bandgap. Photoluminescence spectroscopy revealed that the emission in the UV, blue and red spectral ranges is related to the evolution of Sn(II) dopants and oxygen vacancies with annealing temperature, whereas oxygen vacancies are mostly responsible for visible emission. The Sn(II)- doped SnO 2 films show higher photocurrent when sensitized with narrow bandgap CdS nanoparticles, serving as efficient electron acceptors. 1. Introduction Chemically robust, transparent, and efficient semiconductor oxide electrodes are essential components in solar harvesting devices, especially those for photovoltaic solar cells and pho- toelectrochemical hydrogen production, where light penetra- tion to reach the coated active layer is necessary. 1–5 For such purposes, tin dioxide (SnO 2 ) in the form of a uorine-doped tin oxide (FTO) lm on glass is one of the most widely used base substrate for electrode design. 6 This is largely due to its high electron mobility (100–200 cm 2 V 1 s 1 ), which is of several magnitudes higher than that of anatase TiO 2 (0.1–1 cm 2 V 1 s 1 ) or porous TiO 2 (10 2 cm 2 V 1 s 1 ). 7–9 Besides, SnO 2 lms in a more general sense are versatile oxide semiconductor mate- rials that nd a wide range of applications from lithium-ion batteries, gas sensing to phosphor materials. 6,10–13 Extensive efforts are ongoing in designing of SnO 2 arrays with controlled morphologies on various transparent and opaque substrates to enable their applications in high perfor- mance devices. 2,14–26 For example, polycrystalline SnO 2 nano- tube arrays can be prepared on opaque substrates such as Si wafer, stainless steel and Cu foils, using preformed ZnO nano- rod arrays as sacricial templates. 17,22 Similarly, template-free techniques have also been explored by the wet chemical synthesis of SnO 2 nanowall arrays 18,21 and electrospinning of SnO 2 nanowires, 16 both on transparent FTO substrates. Despite some success, the general fabrication of well-dened SnO 2 nanostructures on arbitrary substrates remains a great chal- lenge. The intimate interfacial contact is oen necessary for robust processing and more importantly, in allowing efficient interfacial carrier charge transport from the active layer to the conductive support substrate. Among the criteria for solar harvesting applications, the SnO 2 substrate ought to maintain transparency, be an efficient acceptor material, possess a high surface area and extended absorbance. With an intrinsic bandgap of 3.6 eV (equivalent to an absorption threshold of 345 nm), pristine SnO 2 can only be activated by a small fraction of the UV component in the solar spectrum. As shown by us and others, narrowing of the SnO 2 bandgap can be achieved through cationic doping, 27,28 whereas in the case of Sn 2+ doping, the accompanying oxygen vacancies can result in the shiing of the Fermi energy level. 29 Interest- ingly, the Sn 2+ -doped SnO 2 shows enhanced interfacial charge transfer and overall NO 2 gas sensing when formulated as paste and deposited onto an interdigitated alumina substrate. 29,30 In this paper, we showcase the direct growth of Sn 2+ -doped SnO 2 nanostructures on FTO during the hydrothermal a Center of Nanomaterials for Renewable Energy (CNRE), State Key Lab of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi'an Jiaotong University, Xi'an, China b Department of Physics and Materials Science & Centre for Functional Photonics (CFP), City University of Hong Kong, Kowloon, Hong Kong S.A.R., China. E-mail: andrey. rogach@cityu.edu.hk; Fax: +852 3442-0538 c Clean Energy and Nanotechnology (CLEAN) Laboratory, School of Energy and Environment, City University of Hong Kong, Kowloon, Hong Kong S.A.R., China d Department of Applied Chemistry, Anhui Agricultural University, Hefei, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr00672k Cite this: Nanoscale, 2014, 6, 6084 Received 6th February 2014 Accepted 22nd March 2014 DOI: 10.1039/c4nr00672k www.rsc.org/nanoscale 6084 | Nanoscale, 2014, 6, 6084–6091 This journal is © The Royal Society of Chemistry 2014 Nanoscale PAPER Published on 30 April 2014. Downloaded by Universitaetsbibliothek Kassel on 10/06/2014 08:57:49. View Article Online View Journal | View Issue