Modifications of bismuth molybdates through selective additions of Zn2+: an efficient photocatalyst for solar-driven water splitting applications

Abstract

The improvement of the water oxidation capability of bismuth molybdate (BM) with selective metal addition under illumination was examined. BM and its Zn-modified (different atomic %) photoanodes were developed over conducting glass substrate through the cost-effective drop-cast method. The maximum photocurrent of ~ 240 µA/cm² at an applied potential of 1.3 V vs Ag/AgCl was recorded for the 2% Zn modified sample in 0.1 M Na₂SO₄ solution (PBS, pH 7) under 100 mW/cm² illuminations. Surface characterizations like scanning electron microscopy, X-ray diffraction, energy dispersive X-ray analysis, and optical analysis such as UV–vis absorbance, photoluminescence, FT-IR, and Raman spectroscopic analyses were performed to determine the physicochemical properties of the semiconductor. The pure bismuth molybdate shows an optical band gap of ~ 2.78 eV, which decreases for the Zn-modified sample, and a minimum of 2.55 eV is detected for the optimized sample. The XRD analysis also reveals that Zn addition into the bismuth molybdate matrix decreases crystallite size with variation in proportions of the constituent metal oxides. The stability of the semiconductors regarding the PEC water oxidation reaction indicates promising results even under continuous illumination for 1 h. The Mott-Schottky study reveals the n-type nature of the semiconductors, whereas the Nyquist analysis indicates minimum charge transfer resistance for the 2% Zn-BMO sample. The PEC action spectra for the optimized photoanode indicate a maximum of 34% incident photon to current conversion efficiency with corresponding 38% absorbed photon to current conversion efficiency, which is more than three times than that of the pure bismuth molybdate.