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

High Resolution Image Download MS PowerPoint Slide This study describes the UV solution photodeposition of several earth-abundant 3d transition metals (Co, Ni, and Cu) onto the surface of nanoparticulate TiO 2 . Irradiated methanolic metal dichloride solutions with suspended Degussa P25-TiO 2 (1–2 wt % metal to TiO 2 ) yield visibly colored titanias, whereas the bulk TiO 2 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 (Co 2+ and Ni 2+ ), 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 H 2 evolution versus unmodified TiO 2 . H 2 evolution rates as high as 85 μmol/h (8500 μmol h –1 g –1 ) were detected for Cu-coated TiO 2 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 H 2 evolution. In situ metal surface deposition is quickly followed by enhanced H 2 evolution relative to TiO 2, but at lower levels than isolated metal surface-modified titanias. The photodeposited 3d metal species on the TiO 2 surface likely act to reduce electron–hole recombination by facilitating the transfer of photoinduced TiO 2 conduction band electrons to protons in solution that are reduced to H 2 . This study demonstrates a facile method to modify photoactive TiO 2 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.