Tailoring WO 3 thin film properties via synthesis parameter control Frederike von der Haar, Roland Marschall Physical Chemistry III, University of Bayreuth, 95447 Bayreuth, Germany Tungsten oxide (WO 3 ) thin films have been extensively investigated for a wide range of applications, including photoelectrochemical (PEC) oxidation, photo- and electrochromic devices, and gas sensors. 1–4 Each of these applications requires specific material properties. For example, PEC applications demand a band gap in the visible light range and low overpotentials, 4 while photochromic devices benefit from fast and reversible changes in optical properties. 3 Additionally, WO 3 thin films need to possess a large active surface area and long-term stability to be suitable for different applications. However, precisely tuning multiple material properties to the needs of a given application using one synthesis approach remains a significant challenge. In this work, we demonstrate that the key material characteristics of WO 3 thin films can be easily tailored through simple variations of synthesis parameters, providing a straightforward recipe to obtain application-specific properties. Using a simple sol-gel synthesis method adapted from Hillard et al., 5 we systematically investigated the influence of calcination temperature, film thickness, and porosity on the structural, optical, and electronic properties of WO 3 thin films. As a case study, we focused on PEC water oxidation and investigated the influence of the varied material properties on the PEC performance. Careful optimisation resulted in WO 3 photoanodes exhibiting photocurrents exceeding 2 mA cm-² at 1.23 V vs. RHE along with excellent reproducibility and stability. The high-performance WO 3 photoanodes obtained in this study provide an ideal basis for further modification such as the formation of heterojunctions with materials like BiVO4. 6,7 This work establishes a practical guide for the design of WO 3 thin films via accessible synthesis parameters, paving the way for their tailored use in desired 2. Dong, C.; Zhao, R.; Yao, L.; Ran, Y.; Zhang, X.; Wang, Y., Journal of Alloys and Compounds 2020 , 820 , 153194. 3. Dong, X.; Lu, Y.; Liu, X.; Zhang, L.; Tong, Y., Journal of Photochemistry and Photobiology C: Photochemistry Reviews 2022 , 53 , 100555. 4. Wang, Y.; Tian, W.; Chen, C.; Xu, W.; Li, L., Advanced Functional Materials 2019 , 29 , 1809036. 5. Hilliard, S.; Baldinozzi, G.; Friedrich, D.; Kressman, S.; Strub, H.; Artero, V.; Laberty-Robert, C., Sustainable Energy & Fuels 2017 , 1 , 145–153. 6. Pihosh, Y.; Turkevych, I.; Mawatari, K.; Uemura, J.; Kazoe, Y.; Kosar, S.; Makita, K.; Sugaya, T.; Matsui, T.; Fujita, D.; Tosa, M.; Kondo, M.; Kitamori, T., Scientific reports 2015 , 5 , 11141. 7. Lee, M. G.; Kim, D. H.; Sohn, W.; Moon, C. W.; Park, H.; Lee, S.; Jang, H. W., Nano Energy 2016 , 28 , 250–260. applications. References 1. Ade, M.; Schumacher, L.; Marschall, R., Sustainable Energy & Fuels 2023 , 7 , 4332–4340.
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