title: Size-selected TiO₂ nanocluster catalysts for efficient photoelectrochemical water splitting.
abstract: Nanoclusters (NCs) are of great interest because they provide the link between the distinct behavior of atoms and nanoparticles and that of bulk materials. Here, we report precisely controlled deposition of size-selected TiO2 NCs produced by gas-phase aggregation in a special magnetron sputtering system. Carefully optimized aggregation length and Ar gas flow are used to control the size distribution, while a quadrupole mass filter provides precise in situ size selection (from 2 to 15 nm). Transmission electron microscopy studies reveal that NCs larger than a critical size (∼8 nm) have a crystalline core with an amorphous shell, while those smaller than the critical size are all amorphous. The TiO2 NCs so produced exhibit remarkable photoelectrochemical water splitting performance in spite of a small amount of material loading. NCs of three different sizes (4, 6, and 8 nm) deposited on H-terminated Si(100) substrates are tested for the photoelectrochemical catalytic performance, and significant enhancement in photocurrent density (0.8 mA/cm(2)) with decreasing NC size is observed with a low saturation voltage of -0.22 V vs Ag/AgCl (0.78 V vs RHE). The enhanced photoconductivity could be attributed to the increase in the specific surface area and increase in the number of active (defect) sites in the amorphous NCs. The unique advantages of the present technique will be further exploited to develop applications based on tunable, size-selected NCs.
authors.affiliation: WATLab and Department of Chemistry, University of Waterloo , Waterloo, Ontario N2L3G1, Canada.
authors.forename: Saurabh
authors.initials: S
authors.lastname: Srivastava
authors.forename: Joseph Palathinkal
authors.initials: JP
authors.lastname: Thomas
authors.forename: Md Anisur
authors.initials: MA
authors.lastname: Rahman
authors.forename: Marwa
authors.initials: M
authors.lastname: Abd-Ellah
authors.forename: Mamata
authors.initials: M
authors.lastname: Mohapatra
authors.forename: Debabrata
authors.initials: D
authors.lastname: Pradhan
authors.forename: Nina F
authors.initials: NF
authors.lastname: Heinig
authors.forename: Kam Tong
authors.initials: KT
authors.lastname: Leung
keywords: TiO2 nanoclusters; magnetron sputtering; photoelectrochemical catalysts; size selection; water splitting reaction