title: Scanning tunneling microscopy and tunneling spectroscopy of nano-structured H6P2MoxW(18-x)O62 (x = 0, 3, 9, 15, 18) Wells-Dawson heteropolyacids.
abstract: Scanning tunneling microscopy (STM) and tunneling spectroscopy studies of nano-structured H6P2MoxW(18-x)O62 (x = 0, 3, 9, 15, 18) Wells-Dawson heteropolyacids (HPAs) were carried out to examine redox properties of the HPAs. STM images of H6P2MoxW(18-x)O62 HPAs clearly showed self-assembled and well-ordered 2-dimensional arrays on graphite surface. Tunneling spectroscopy measurements revealed that all H6P2MoxW(18-x)O62 HPAs exhibited a negative differential resistance (NDR) behavior in their tunneling spectra. NDR peak voltage of H6P2MoxW(18-x)O62 HPAs appeared at less negative applied voltage with increasing molybdenum substitution. Reduction potential of H6P2MoxW(18-x)O62 HPAs measured by an electrochemical method increased and absorption edge energy determined by UV-visible spectroscopy shifted to lower value with increasing molybdenum substitution. In other words, NDR peak voltage of H6P2MoxW(18-x)O62 HPAs appeared at less negative applied voltage with increasing reduction potential and with decreasing absorption edge energy of the HPAs; more reducible H6P2MoxW(18-x)O62 HPAs showed NDR behavior at less negative applied voltage. These results indicate that NDR peak voltage of nano-structured HPAs measured by STM could be utilized as a correlating parameter for the redox properties of bulk HPAs.
authors: School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea.
authors: Jung Ho
authors: JH
authors: Choi
authors: Dong Ryul
authors: DR
authors: Park
authors: Sunyoung
authors: S
authors: Park
authors: In Kyu
authors: IK
authors: Song
mesh_terms: D000838:Anions / Q000737:chemistry; D002384:Catalysis; D016252:Microscopy, Scanning Tunneling; D008958:Models, Molecular; D008982:Molybdenum / Q000737:chemistry*; D049329:Nanostructures / Q000737:chemistry* / Q000648:ultrastructure; D010084:Oxidation-Reduction; D010756:Phosphoric Acids / Q000737:chemistry*; D010772:Phosphotungstic Acid / Q000737:chemistry*; D013057:Spectrum Analysis
chemical_list: D000838:Anions; D010756:Phosphoric Acids; D010772:Phosphotungstic Acid; D008982:Molybdenum; C003125:phosphomolybdic acid