12:25 PM - EL03.05.03
Tuning the Electronic Properties of LaFeO3 Thin-Films Photoelectrodes via Partial Cation Replacement
David Fermin1,Xin Sun1,Devendra Tiwari2
University of Bristol1,Northumbria University2
Fe2O3 thin-films have been extensively investigated as photoanodes, with a variety of different strategies being proposed to mitigate key performance limiting factors, namely short carrier lifetimes and surface recombination kinetics.1 On the other hand, significantly less is known about the properties of perovskite ferrite absorbers, including LaFeO3,2-4 YFeO3,5 PrFeO3,6 and BiFeO3.7 These materials exhibit a wide range of intrinsic defects, which often lead to p-type conductivity. Our recent studies have shown that highly crystalline LaFeO3 nanoparticles can promote hydrogen evolution under illumination at potential as positive as 1.47 V vs RHE,8 one of the highest photovoltages reported for a single p-type absorber layer. However, the performance of these perovskites is limited to quantum yields below 1%. In this contribution, we will examine the nature of the states involved in the loss of photogenerated carriers and the effect of partial substitution by alkaline-earth metal cations (AMC).
LaFeO3 thin-film with a thickness of 95 nm were prepared by thermolysis (600 °C) of sol-gel precursors incorporating citric acid as chelating agent.9 The ratio of AMC (Mg2+, Ca2+, Ba2+ and Sr2+) to La3+ were adjusted in the range of 0 to 10% in the precursor solution, while keeping the Fe3+ concentration constant. XRD confirms the formation of single-phase cubic LaFeO3 thin films across the whole composition range. Interestingly, we observe subtle trends in lattice constant variations which are closely correlated to shifts in the binding energies of Fe 2p3/2 and O 1s measured by XPS. These trends are the result of the complex interplay between differences in ionic radii of the cations and changes in the oxidation state of Fe sites. Indeed, we establish a scaling factor between these two photoemission peaks, revealing a direct correlation between Fe oxidation state and Fe–O covalency. Electrochemical impedance spectroscopy (EIS) confirms the p-type characteristic of pristine LaFeO3 thin-films, as well as the presence of sub-bandgap electronic state (A-states) close to the valence band edge. Partial AMC replacement leads to: (i) a decrease in the density of A-states, (ii) an increase in density of majority carriers (shallow acceptor states), and (iii) a shift of the valence band edge toward more positive potentials. In addition, AMC-substituted films exhibit deeper states centered at 0.6 eV above the valence band edge (B-states). These sub-band gap states have contrasting effects on the photoelectrochemical responses towards the oxygen reduction and the hydrogen evolution reactions. These trends are rationalized in terms of the position of the sub-bandgap states, majority carrier mobility, charge transfer and recombination kinetics.
1. W. Yang, R.R. Prabhakar, J. Tan, S.D. Tilley and J. Moon, Chem. Soc. Rev., 48, 4979 (2019)
2. V. Celorrio, K. Bradley, O.J. Weber, S.R. Hall, and D.J. Fermín, ChemElectroChem, 1, 1667 (2014)
3. G.P. Wheeler and K.S. Choi, ACS Energy Lett., 2, 2378 (2017).
4. G.P. Wheeler, V.U. Baltazar, T.J. Smart, A. Radmilovic, Y. Ping, and K.-S. Choi, Chem. Mater., 31, 5890 (2019)
5. M.I. Díez-García, V. Celorrio, L. Calvillo, D. Tiwari, R. Gómez, and D.J. Fermín, Electrochim. Acta, 246, 365 (2017).
6. E. Freeman, S. Kumar, S.R. Thomas, H. Pickering, D.J. Fermín, S. Eslava, ChemElectroChem, 7, 1365 (2020)
7. D. Tiwari, D.J. Fermín, T.K. Chaudhuri, A. Ray J. Phys. Chem C, 119, 5872 (2015)
8. X. Sun, D. Tiwari and D. J. Fermín, J. Electrochem. Soc., 166, H764 (2019)
9. X. Sun, D. Tiwari and D. J. Fermín, ACS Appl. Mater. Inter., 12, 31486 (2020)