title: Efficient chemical and visible-light-driven water oxidation using nickel complexes and salts as precatalysts.
abstract: Chemical and visible-light-driven water oxidation catalyzed by a number of Ni complexes and salts have been investigated at pH 7-9 in borate buffer. For chemical oxidation, [Ru(bpy)3](3+) (bpy = 2,2'-bipyridine) was used as the oxidant, with turnover numbers (TONs) >65 and a maximum turnover frequency (TOFmax) >0.9 s(-1). Notably, simple Ni salts such as Ni(NO3 )2 are more active than Ni complexes that bear multidentate N-donor ligands. The Ni complexes and salts are also active catalysts for visible-light-driven water oxidation that uses [Ru(bpy)3](2+) as the photosensitizer and S2 O8 (2-) as the sacrificial oxidant; a TON>1200 was obtained at pH 8.5 by using Ni(NO3)2 as the catalyst. Dynamic light scattering measurements revealed the formation of nanoparticles in chemical and visible-light-driven water oxidation by the Ni catalysts. These nanoparticles aggregated during water oxidation to form submicron particles that were isolated and shown to be partially reduced β-NiOOH by various techniques, which include SEM, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, XRD, and IR spectroscopy. These results suggest that the Ni complexes and salts act as precatalysts that decompose under oxidative conditions to form an active nickel oxide catalyst. The nature of this active oxide catalyst is discussed.
authors.affiliation: Department of Biology and Chemistry, City University of Hong Kong and Institute of Molecular Functional Materials, Tat Chee Avenue, Kowloon Tong, Hong Kong (China), Fax: (+852) 34420522.
authors.forename: Gui
authors.initials: G
authors.lastname: Chen
authors.forename: Lingjing
authors.initials: L
authors.lastname: Chen
authors.forename: Siu-Mui
authors.initials: SM
authors.lastname: Ng
authors.forename: Tai-Chu
authors.initials: TC
authors.lastname: Lau
mesh_terms: D002384:Catalysis; D056831:Coordination Complexes / Q000737:chemistry* / Q000528:radiation effects; D008027:Light; D053768:Metal Nanoparticles / Q000737:chemistry; D009532:Nickel / Q000737:chemistry* / Q000528:radiation effects; D009942:Organometallic Compounds / Q000737:chemistry / Q000528:radiation effects; D010084:Oxidation-Reduction; D012428:Ruthenium / Q000737:chemistry / Q000528:radiation effects; D012492:Salts / Q000737:chemistry; D017670:Sodium Compounds / Q000737:chemistry / Q000528:radiation effects; D013431:Sulfates / Q000737:chemistry / Q000528:radiation effects; D014867:Water / Q000737:chemistry*
chemical_list: D056831:Coordination Complexes; D009942:Organometallic Compounds; D012492:Salts; D017670:Sodium Compounds; D013431:Sulfates; D014867:Water; D009532:Nickel; D012428:Ruthenium; C024625:sodium persulfate
keywords: nanoparticles; nickel; oxidation; photocatalysis; water splitting