Enhanced Low Bias Performance in Textured BiVO4 Photoanodes through Halide and Phosphate Treatments

Over the last couple of years, BiVO4 has merged as a promising photoanode material owing to its earth abundance, suitable band gap (2.4eV), high absorption coefficient and a reasonable hole diffusion length (100nm)[1]. The intrinsic electron mobility has been shown to be a bottleneck[2] in achieving high photocurrents with a standalone BiVO4 photoanode. Recently photocurrents as high as 5mA/cm2 at OER have been demonstrated using a heterojunction with WO3 rods[3]. This however does not solve the intrinsic electron transport problems of BiVO4 and hence enhancing the photoresponse at relatively low biases close to the flatband potential still remains a challenge and limits its application in a high efficiency tandem device. Here we report photocurrents in excess of 3mA/cm2 at applied biases as low as 0.6V vs RHE with an onset potential of 0.19V vs RHE for sulfite oxidation.

A common route to improving the elctronic performance of BiVO4 is via addition of W or Mo as dopants. However, the addition of both W and Mo always results in an anodic shift in the onset potential, limiting the photoresponse at lower biases. In the this study, phosphate-based treatments of the fluorine-doped tin oxide are shown to eliminate the small rectifying junction existing at the back contact with BiVO4. Simultaneously, halide treatments were used to improve electron transport in undoped –BiVO4. Using results from our previous work[4], we have been able to successfully texture 300nm thin films with the previously mentioned phosphate and halide treatments to unity carrier separation at OER and over 80% at 0.6V RHE. Owing to the {004} texturing, these devices also demonstrate a record high catalytic activity for a standalone BiVO4 photoanode towards oxygen evolution. Such high carrier separation was achieved via halide treatment as well as surface texturing which reduces the density of in-gap surface defects states. UPS and XPS measurements were used to ascertain the nature and magnitude of these in-gap states. Impedance spectroscopy shows that the enhanced charge transfer on the surface of the textured photoelectrodes is the origin for the catalytic efficiency of these devices.

This study hopes to pave the way towards a rational improvement in the electronic and catalytic properties of standalone undoped- BiVO4 as well as a new approach in designing highly efficient photoanodes for oxygen evolution.

[1] Abdi F.F., Savenije T.J., May M.M., Dan B., Krol R., J.Phys.Chem.Lett., 2013,4,2752-2757

[2] Park Y., McDonald K.J., Choi K.S., Chem.Soc. Rev., 2013,422321-2337.

[3] Shi X., Choi Y., Zhang K., Kwon J., Kim D.Y., Lee J., Ho L.S., Kim J.K., Park J.H., Nat. Comm., 5, 2014, 4775.

[4] Nair V.V., Perkins C.L., Law M., Submitted.