Improved photoelectrochemical performance of Nb-substituted LaTi(O,N)₃

Photoelectrochemical water-splitting is a possible path toward sustainably produced hydrogen, which is a potential solar fuel of the future. Complex tuning of material properties is necessary to further enhance efficiency and, therefore, ensure economic viability of this process. In this study, the influence of Ti-site substitution in the perovskite-related oxynitride LaTi(O,N)₃ is investigated. Oxide materials, La₂Ti_2-2xNb_2xO_7±δ without and with a substitution of up to 8% of the Ti-sites, are synthesized and, subsequently, transformed into the corresponding phase pure oxynitrides, LaTi_1-xNb_x(O,N)_3±δ. The incorporation of Nb is confirmed via x-ray diffraction and energy dispersive spectroscopy. Changes in morphology, nitrogen content, optical properties, and charge carrier density are investigated using scanning electron microscopy, gas adsorption, thermal gravimetric analysis, and UV-vis and electrochemical impedance spectroscopies. Charge compensation of the positive charge introduced by Nb⁵⁺ via additional N³⁻ incorporation is identified as a mechanism that occurs during thermal ammonolysis and results in an increased N content in substituted compounds, probably preventing charge carrier density increase. An improvement of up to 30% of the photocurrent density at 1.23 V vs RHE is achieved for x = 0.01 in comparison to unsubstituted LaTi(O,N)₃. The performance increase in this material is related to a reduction in (crystalline) defects at low substitution degrees. These results underline the necessity to tune the synthesis conditions carefully for material design.