NAIL ET AL. VOL. 9 ’ NO. 5 ’ 5135–5142 ’ 2015 www.acsnano.org 5135 April 14, 2015 C 2015 American Chemical Society Nickel Oxide Particles Catalyze Photochemical Hydrogen Evolution from Water ; Nanoscaling Promotes P‑Type Character and Minority Carrier Extraction Benjamin A. Nail, † Jorie M. Fields, † Jing Zhao, † Jiarui Wang, † Matthew J. Greaney, ‡ Richard L. Brutchey, ‡ and Frank E. Osterloh * ,† † Department of Chemistry, University of California Davis, One Shields Avenue, Davis, California 95616, United States and ‡ Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States P hotoelectrochemical and photocata- lytic water splitting have attracted significant attention as methods for generating carbon free fuels from solar energy. 1À3 Many photocatalysts have been reported to evolve hydrogen from water in the presence of a sacrificial reagent, but stable visible-light responsive photocata- lysts for overall water splitting are still very rare. 4À6 In tandem or z-scheme photocata- lysts, water splitting is achieved by the combined photoaction of photocathode and photoanode materials that are con- nected in series. 7À10 However, visible light responsive p-type metal oxide photocatalysts and photoelectrodes are still uncommon. 11À17 In terms of its structure and composition, nickel oxide (NiO) is one of the simplest p-type metal oxides. 18 Its wide band gap precludes it from visible light absorption, but it is a popular hole transport material in dye sensitized solar cells, 19À21 and in thin film photo- voltaics. 22 It also has been used as a cocatalyst in water splitting photocatalysts. 6,23À27 Some- times the function of NiO is not clear; 28 it has been interpreted as a proton-reduction electrocatalyst, 29À32 as a water oxidation electrocatalyst, 33,34 or as a photocathode. 35,36 * Address correspondence to fosterloh@ucdavis.edu. Received for review January 20, 2015 and accepted April 14, 2015. Published online 10.1021/acsnano.5b00435 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 (50À90 nm  10À15 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 H 2 PtCl 6 catalyze hydrogen evolution from aqueous methanol at rates of 0.8 and 4.5 μmol H 2 h À1 , respectively, compared to 0.5 and 2.1 μmol H 2 h À1 for bulk-NiO and NiO-Pt (20 mg of catalyst, 300 W Xe lamp). Surface photovoltage spectroscopy of NiO and NiOÀPt 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 MottÀSchottky plots, optical absorbance, and X-ray photoelectron spectroscopy data. KEYWORDS: photocatalysis . p-type metal oxide . nanoscale junction . surface photovoltage spectroscopy . photocorrosion ARTICLE