Combinatorial search for new solar water splitting photoanode materials in the thin-film system Fe-Ti-W-O

In order to identify new solar water splitting photoanodes, Fe-Ti-W-O materials libraries were fabricated by combinatorial reactive co-sputtering and investigated by high-throughput characterization methods to elucidate compositional, thickness, and structural properties. In addition, photoelectrochemical measurements such as potentiodynamic photocurrent determination and open circuit potential measurements were performed using an automated scanning droplet cell. In the thin-film library, a quaternary photoactive region Fe${}_{30-49}$Ti${}_{29-55}$W${}_{13-22}$O${}_x$ was identified as a hit composition region, comprising binary and ternary phases. The identified region shows a distinct surface morphology with larger grains (∼200 nm) being embedded into a matrix of smaller grains (∼80–100 nm). A maximum photocurrent density of 117 μA/cm${}^2$ at a bias potential of 1.45 V vs. RHE in NaClO${}_4$ as an electrolyte under standard solar simulating conditions was recorded. Additional samples with compositions from the hit region were fabricated by reactive co-sputtering and spin coating followed by annealing. Synchrotron X-ray diffraction of sputtered Fe${}_{32}$Ti${}_{52}$W${}_{16}$O${}_x$ thin-films, annealed in air (600 °C, 700 °C, 800 °C) revealed the presence of the phases FeTiO${}_3$ and Ti${}_{0.54}$W${}_{0.46}$O${}_2$. The composition Fe${}_{48}$Ti${}_{30}$W${}_{22}$O${}_x$ from the hit region was fabricated by spin coating and subsequent annealing for a detailed investigation of its structure and photoactivity. After annealing the spin-coated sample at 650 °C for 6 h, X-ray diffraction results showed a dominant pattern with narrow diffraction lines belonging to a distorted FeWO${}_4$ (ferberite) phase along with broad diffraction lines addressed as Fe${}_2$TiO${}_5$ and in a small fraction also, Fe${}_{1.7}$Ti${}_{0.23}$O${}_3$. In hematite, Fe can be substituted by Ti, therefore we suggest that in the newfound ferberite-type phase, Ti partially substitutes for Fe leading to a small lattice distortion and a doubling of the monoclinic unit cell. In addition, Na from the substrate stabilizes the new phase: its tentative chemical formula is NaxFe${}_{0.33}$Ti${}_{0.67}$W${}_2$O${}_8$. A maximum photocurrent density of around 0.43 mA/cm${}^2$ at 1.45 V vs. RHE in 1M NaOH (pH ∼ 13.6) as an electrolyte was measured. Different aspects of the dependence of annealing and precursor solution concentration on phase transformation and photoactivity are discussed.