## 4 Experimental Section
_Hydrothermal Synthesis of V-Doped Co\({}_{3}\)O\({}_{4}\) and V-Doped MoO\({}_{3}\)_  
For a typical hydrothermal synthesis of V-doped Co\({}_{3}\)O\({}_{4}\) and V-doped MoO\({}_{3}\), adapting procedures reported in the literature [41; 73], a given amount of metal precursor are dissolved in deionized water (10 mL). The amounts of cobalt acetate tetrahydrate ([PERSON], Milano, Italy, pur. \(>\)99%) and ammonium petamolybdate tetrahydrate (Alfa Aesar, Kandel, Germany, pur. 99%) are respectively 0.3736 g and 0.58 g. At 0.15 M, solutions of cobalt acetate are added 0.0018, 0.0053, 0.0088, 0.0175, 0.0351 g of ammonium metavanadate (pur. 99%), with a molar ratio to the cobalt precursor of 0.01, 0.03. 0.05, 0.1 and 0.2, respectively, to have in the mixtures' different atomic ratios between the vanadium and cobalt precursor (1%, 3%, 5%, 10%, 20% at). In a similar way, to the 0.05 M solutions of ammonium petamolybdate 0.0746, 0.2237, 0.3728, 0.6984, 1.3968 g of ammonium metavanadate are added, with a molar ratio to the molybdenum precursor of 0.0746, 0.2237, 0.3728, 0.6984 and 1.3968, respectively, to achieve different atomic ratios between the vanadium and molybdenum precursor (1%, 3%, 5%, 10%, 20% at). At the cobalt precursor solution, 0.135 g of oxalic acid (pur. \(>\)99%) with a molar ratio of 1:1 with respect to cobalt acetate are added, followed by stirring for 30 min. The molybdenum precursor solution is instead stirred for 20 min, then 275 \(\upmu\)L of HCl 37% v/v are added (0.005 molar ratio to ammonium molybdate), and next the solution is stirred for other 20 min. Each solution is poured in a 23 mL A255 AC PTFE cup (Parr Instrument Company, IL, USA). **Supplementary Materials:** The following are available online at [[https://www.mdpi.com/2076](https://www.mdpi.com/2076)]([https://www.mdpi.com/2076](https://www.mdpi.com/2076)) -3417/11/5/2016/s1, Figure S1: Diffractograms of (a) Co\({}_{3}\)O\({}_{4}\)-related compounds and (b) MoO\({}_{3}\)-doped samples. Figure S2. Reflectance spectra of (a) Co\({}_{3}\)O\({}_{4}\)-related compounds and (b) MoO\({}_{3}\)-doped samples used for Tauc plot analysis. Figure S3. Diffractograms and corresponding Pawley refinements of (a) Co\({}_{3}\)O\({}_{4}\) (Rwp = 9.27 %), (b) MoO\({}_{3}\) (Rwp = 9.41 %) and (c) MoO\({}_{3}\); V 20% (at.) (Rwp = 9.47 %) with unit cell parameters of: a = 3.9617 \(\pm\) 0.0004 A, b = 13.8660 \(\pm\) 0.0013 A, c = 3.6934 \(\pm\) 0.0003 A. **Author Contributions:** Conceptualization, S.G., T.G. and F.L. ; methodology, S.G., T.G. and F.L. ; validation, S.G., T.G. and F.L. ; formal analysis, T.G. and F.L. ; investigation, M.C., P.D.F., F.B., F.T., F.D., M.S., M.P., S.G., T.G. and F.L. ; resources, M.S., M.P., F.T., S.G., T.G. ; data curation, T.G. and F.L. ; writing--original draft preparation, M.C., P.D.F., T.G. and F.L. ; writing-review and editing P.D.F., M.C., F.T., F.B., S.G., T.G. and F.L. ; visualization, F.L. and T.G. ; supervision, S.G., T.G. and F.L. ; project administration, S.G., T.G. and F.L. ; funding acquisition, S.G., T.G. and F.L. All authors have read and agreed to the published version of the manuscript. **Funding:** P.D.F., F.L. and S.G. gratefully acknowledge the Interdepartmental Centre Giorgio Levi Cases for Energy Economics and Technology of the University of Padova for financial support within the project AMON-RA. S.G. thanks the Department of Excellence Project \"NEXUS\" for providing the P-XRD equipment and laboratory spaces. T.G. and M.C. thank the European Commission for the H2020 FET-PROACTIVE-EIC-07-2020 project LIGHT-CAP (project number 101017821) and the Verband der Chemischen Industrie e.V. for the \"Fonds der Chemischen Industrie\". F.T. and F.B. gratefully acknowledge for funding the PhD course in Molecular Sciences and the PhD Course in Science and Engineering of Materials and Nanostructures, University of Padova, Italy, respectively.