title: Nickel oxide particles catalyze photochemical hydrogen evolution from water--nanoscaling promotes p-type character and minority carrier extraction.
abstract: Nickel(II) oxide (NiO) is an important wide gap p-type semiconductor used as a hole transport material for dye sensitized solar cells and as a water oxidation electrocatalyst. Here we demonstrate that nanocrystals of the material have increased p-type character and improved photocatalytic activity for hydrogen evolution from water in the presence of methanol as sacrificial electron donor. NiO nanocrystals were synthesized by hydrolysis of Ni(II) nitrate under hydrothermal conditions followed by calcination in air. The crystals have the rock salt structure type and adopt a plate-like morphology (50-90 nm × 10-15 nm). Diffuse reflectance absorbance spectra indicate a band gap of 3.45 eV, similar to bulk NiO. Photoelectrochemical measurements were performed at neutral pH with methylviologen as electron acceptor, revealing photo-onset potentials (Fermi energies) of 0.2 and 0.05 eV (NHE) for nanoscale and bulk NiO, respectively. Nano-NiO and NiO-Pt composites obtained by photodepositon of H2PtCl6 catalyze hydrogen evolution from aqueous methanol at rates of 0.8 and 4.5 μmol H2 h(-1), respectively, compared to 0.5 and 2.1 μmol H2 h(-1) for bulk-NiO and NiO-Pt (20 mg of catalyst, 300 W Xe lamp). Surface photovoltage spectroscopy of NiO and NiO-Pt films on Au substrates indicate a metal Pt-NiO junction with 30 mV photovoltage that promotes carrier separation. The increased photocatalytic and photoelectrochemical performance of nano-NiO is due to improved minority carrier extraction and increased p-type character, as deduced from Mott-Schottky plots, optical absorbance, and X-ray photoelectron spectroscopy data.
authors: †Department of Chemistry, University of California Davis, One Shields Avenue, Davis, California 95616, United States.
authors: Benjamin A
authors: BA
authors: Nail
authors: †Department of Chemistry, University of California Davis, One Shields Avenue, Davis, California 95616, United States.
authors: Jorie M
authors: JM
authors: Fields
authors: †Department of Chemistry, University of California Davis, One Shields Avenue, Davis, California 95616, United States.
authors: Jing
authors: J
authors: Zhao
authors: †Department of Chemistry, University of California Davis, One Shields Avenue, Davis, California 95616, United States.
authors: Jiarui
authors: J
authors: Wang
authors: ‡Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States.
authors: Matthew J
authors: MJ
authors: Greaney
authors: ‡Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States.
authors: Richard L
authors: RL
authors: Brutchey
authors: †Department of Chemistry, University of California Davis, One Shields Avenue, Davis, California 95616, United States.
authors: Frank E
authors: FE
authors: Osterloh
keywords: nanoscale junction; p-type metal oxide; photocatalysis; photocorrosion; surface photovoltage spectroscopy