p-Type Cu−Ti−O Nanotube Arrays and Their Use in Self-Biased Heterojunction Photoelectrochemical Diodes for Hydrogen Generation

Abstract

Copper and titanium remain relatively plentiful in the earth’s crust; hence, their use for large-scale solar energy conversion technologies is of significant interest. We describe fabrication of vertically oriented p-type Cu−Ti−O nanotube array films by anodization of copper rich (60% to 74%) Ti metal films cosputtered onto fluorine doped tin oxide (FTO) coated glass. Cu−Ti−O nanotube array films 1 μm thick exhibit external quantum efficiencies up to 11%, with a spectral photoresponse indicating that the complete visible spectrum, 380 to 885 nm, contributes significantly to the photocurrent generation. Water-splitting photoelectrochemical pn-junction diodes are fabricated using p-type Cu−Ti−O nanotube array films in combination with n-type TiO₂ nanotube array films. With the glass substrates oriented back-to-back, light is incident upon the UV absorbing n-TiO₂ side, with the visible light passing to the p-Cu−Ti−O side. In a manner analogous to photosynthesis, photocatalytic reactions are powered only by the incident light to generate fuel with oxygen evolved from the n-TiO₂ side of the diode and hydrogen from the p-Cu−Ti−O side. To date, we find under global AM 1.5 illumination that such photocorrosion-stable diodes generate a photocurrent of approximately 0.25 mA/cm², at a photoconversion efficiency of 0.30%.