Photocatalysis is a promising pathway to direct utilization of solar energy for chemical fuel production. A major challenge is development of low cost, stable active materials that utilize a large fraction of the solar spectrum. BiVO4 is a promising candidate material that acts as oxidizing species in a Z-scheme water splitting reaction, capable of utilizing visible light (band gap 2.4 eV).
Photocatalytic performance of BiVO4 is partially limited by poor photoexcited charge transport from bulk to the surface. Pure BiVO4 is expected to be n-type due to intrinsic defects and enhanced performance was previously demonstrated by extrinsic n-type doping (by W, Mo additives).
In this work we demonstrate enhancement of photocatalytic water splitting activity of BiVO4 by doping with Zr, which may act as either n or p-type dopant. BiVO4 powder was synthesized via a reaction of NH4VO4 and Bi(NO3)3 in an acidic solution, with precipitate annealed at 500°C. Zr doped BiVO4 was produced by incorporating 1% molar fraction of Zr during synthesis. Water splitting reaction was conducted in 10mM AgNO3 aqueous solution and 2x increase of photocatalytically produced oxygen was observed vs. pure BiVO4.
Incorporation mechanism of Zr into BiVO4 lattice was studied via XRD and Raman scattering measurements. XRD measurement of pure and Zr-doped samples indicated the monoclinic phase BiVO4. Observation of diffraction peaks due to (1,0,2), (0,1,2),(1,1,1) and (1,-1,1) planes suggest presence of Bi-vacancies or Bi-site substitutions. However, the XRD signal of these planes is found to be comparable in pure and Zr-doped samples, indicating that the defects are likely intrinsic. Raman scattering peaks indicative of monoclinic BiVO4 are observed at 826, 365, and 333 cm-1 for pure BiVO4 samples. Zr-doped samples display qualitatively unchanged Raman spectrum with a shift of VO4 symmetric stretching mode from 826 cm-1 to 822cm-1. The shift is interpreted as Zr substitution on V site.
Steady state photoluminescence study reveals strong sub-gap emission at wavelengths between 700 and 900nm in pure and Zr-doped samples. Based on previous DFT modeling work these emission lines are assigned to V on Bi-site substitutions and Bi on V site substitutions, further supporting conclusion that Bi site defects observed in XRD data are intrinsic. Time resolved photoluminescence measurements show decrease of photoexcited carrier lifetime in Zr-doped samples, indicating change in bulk electrical properties due to Zr incorporation.
In summary, we report observation of improved water-splitting photocatalytic activity in BiVO4 via Zr additive incorporation. XRD, Raman, and photoluminescence measurements indicate incorporation of Zr into bulk BiVO4 lattice while retaining the monoclinic phase.