Enhanced Photocatalytic Hydrogen Evolution from Transition-Metal Surface-Modified TiO2.

Abstract

This study describes the UV solution photodeposition of several earth-abundant 3d transition metals (Co, Ni, and Cu) onto the surface of nanoparticulate TiO2. Irradiated methanolic metal dichloride solutions with suspended Degussa P25-TiO2 (1–2 wt % metal to TiO2) yield visibly colored titanias, whereas the bulk TiO2 structure is unchanged; X-ray photoelectron spectroscopy confirms that metals are present on the titania surface in either reduced metal (Cu/Cu+) or metal cation states (Co2+ and Ni2+), and UV–vis diffuse reflectance spectroscopy shows new visible absorbance features. The analyzed bulk metal contents (∼0.04–0.6 at. %, highest for copper) are lower than the nominal metal solution content. Mixed-metal solution photodeposition reactions roughly parallel observations for single metals, with copper deposition being most favored. These 3d metal surface-modified titanias show significant (∼5–15×) improvement in UV photocatalytic H2 evolution versus unmodified TiO2. H2 evolution rates as high as 85 μmol/h (8500 μmol h–1 g–1) were detected for Cu-coated TiO2 using continuous monitoring of reactor headspace gases by portable mass spectrometry. Control experiments verify the necessity of the methanol sacrificial oxidant in both metal deposition and H2 evolution. In situ metal surface deposition is quickly followed by enhanced H2 evolution relative to TiO2, but at lower levels than isolated metal surface-modified titanias. The photodeposited 3d metal species on the TiO2 surface likely act to reduce electron–hole recombination by facilitating the transfer of photoinduced TiO2 conduction band electrons to protons in solution that are reduced to H2. This study demonstrates a facile method to modify photoactive TiO2 nanoparticles with inexpensive 3d transition metals to improve photocatalytic hydrogen evolution, and it shows the utility of quantitative real-time gas evolution monitoring by portable mass spectrometry.