Earth-Abundant Co-Catalysts to Replace Noble Metals in Photocatalysis

Climate change due to rising CO2 concentrations in the atmosphere require the transformation from a fossil-fuel based energy economy towards a sustainable, emission-free one centred around renewable energy sources. Photocatalysis offers the possibility to directly obtain H2 as a green fuel from water under irradiation of a photocatalyst with a suitable band gap for light absorption. Up to date, photocatalytic hydrogen evolution (HER) can, however, still not compete with traditional hydrogen production processes, such as steam reforming, due to low efficiencies [1]. Co-catalysts can be added to a photocatalytic system to improve the separation of excited charge carriers, provide additional active sites, lower the overpotential of a reaction, and thus increase the overall activity. Many common co-catalysts are noble-metal-based, though [2]. Therefore, the addition of a noble co-catalyst increases the material costs. This is also a common problem in electrochemistry, where platinum is used as standard material for the hydrogen evolution reaction. In recent years, sulphides and phosphides, such as MoS2, have emerged as earth-abundant alternatives and subsequently been employed as co-catalysts in photocatalysis as well [3]. Nickel and iron based materials are especially interesting for the hydrogen evolution reaction, since both metals are common motifs in hydrogenase enzymes [4].<br/>We herein present binary and ternary sulphides and oxides as novel, earth-abundant co-catalysts on TiO2, with a special focus on Ni2FeS4. All of the presented materials can be obtained as nanoparticles in a very fast microwave-assisted solvothermal reaction at temperatures not exceeding 200 °C. The synthesis time can be decreased down to 1 min, without a significant loss of product crystallinity and no impedimental effects on the activity. The addition of the transition metal sulphides to P25 TiO2 could significantly boost the activity for photocatalytic hydrogen evolution under 1 sun solar light irradiation, as well as under UV irradiation. A combination of control experiments, material characterisation and the realisation of astonishingly low transition metal loadings of 0.5 wt.% (&lt; 0.3 wt.% of metals) underline the role of Ni2FeS4 and other sulphides as efficient co-catalysts that show promise for replacing noble metal co-catalysts.[5]<br/><br/>References<br/>[1] L. Lin, T. Hisatomi, S. Chen, T. Takata, K. Domen, Trends in Chemistry 2 (2020) 813.<br/>[2] J. Yang, D. Wang, H. Han, C. Li, Acc. Chem. Res. 46 (2013) 1900.<br/>[3] K. Chang, X. Hai, J. Ye, Adv. Energy Mater. 6 (2016) 1502555.<br/>[4] M. Isegawa, T. Matsumoto, S. Ogo, Dalton Trans. 51 (2022) 312.<br/>[5] J. Zander, R. Marschall, J. Mater. Chem. A 11 (2023) 17066.