Nanoscale characterization of acid properties of heteropolyacids by scanning tunneling microscopy and tunneling spectroscopy Original Research Article

Nanoscale characterization of the acid properties of H3PMo12−xWxO40 (x = 0, 3, 6, 9, 12) was carried out by scanning tunneling microscopy (STM) and tunneling spectroscopy. Pyridine binding with the acid sites of heteropolyacids (HPAs) was reflected in both the STM images and FT-IR spectra of these materials. All the HPAs investigated formed well-ordered monolayer arrays on highly oriented pyrolytic graphite (HOPG) before and after pyridine adsorption. Exposure to pyridine increased the lattice constants of the two-dimensional HPA arrays by ca. 6 Å. Exposure to pyridine also shifted the negative differential resistance (NDR) peak voltages of HPAs to less negative values in the tunneling spectroscopy measurements. The NDR shifts of HPAs obtained before and after pyridine adsorption were correlated with acid properties of HPAs for the first time, suggesting that tunneling spectra measured by STM could serve to fingerprint acid properties of HPAs.