Site Identification of Mixed Arrays of Keggin-type Heteropolyacids by Scanning Tunneling Microscopy and Tunneling Spectroscopy

Abstract

Scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) studies on the mixed arrays of Keggin-type heteropolyacids (HPAs) deposited on a highly oriented pyrolytic graphite (HOPG) surface were carried out to test the ability of these techniques to distinguish structurally similar molecules. An equimolar aqueous mixture of HPAs was deposited on a graphite surface for STM measurements. The mixture of Keggin-type HPAs, H3PW12O40 and H3PMo12O40, with identical net anionic charges and identical charge-compensating cations (protons), formed ordered two-dimensional arrays on a graphite surface similar to those formed by their pure components. The periodicities of the arrays were in good agreement with the shape and dimension of the pure Keggin HPAs. It was evident from the STM image of the mixed array that some features appeared brighter than others; the ratio of bright to dim features was almost unity as designed, and their distribution within the ordered array on the surface appeared to be random. TS measurements revealed that the bright corrugations were [PW12O40]3- anions and that the less bright ones were [PMo12O40]3- anions. It is successfully demonstrated that HPAs with indistinguishable geometric structures and sizes can be distinguished based on differences in their electronic properties using STM operated in air.