Nanoscale Characterization of Redox and Acid Properties of Keggin-Type Heteropolyacids by Scanning Tunneling Microscopy and Tunneling Spectroscopy: Effect of Heteroatom Substitution

详细信息    查看全文

• 作者：In K. Song ; Russell B. Shnitser ; Jennifer J. Cowan ; Craig L. Hill ; and Mark A. Barteau
• 刊名：Inorganic Chemistry
• 出版年：2002
• 出版时间：March 11, 2002
• 年：2002
• 卷：41
• 期：5
• 页码：1292 - 1298
• 全文大小：185K
• 年卷期：v.41,no.5(March 11, 2002)
• ISSN：1520-510X

文摘

Nanoscale characterization of acid and redox properties of Keggin-type heteropolyacids (HPAs) with differentheteroatoms, H_nMW₁₂O₄₀ (M = P, Si, B, Co), was carried out by scanning tunneling microscopy (STM) and tunnelingspectroscopy (TS) in this study. HPA samples were deposited on highly oriented pyrolytic graphite surfaces toobtain images and tunneling spectra by STM before and after pyridine adsorption. All HPA samples formed well-ordered 2-dimensional arrays on graphite before and after pyridine exposure. NDR (negative differential resistance)peaks were observed in the tunneling spectra. Those measured for fresh HPA samples appeared at less negativevoltages with increasing reduction potential of the HPAs and with increases in the electronegativity of the heteroatom,but with decreases in the overall negative charge of the heteropolyanions. These results support the conclusionthat more reducible HPA samples show NDR behavior at less negative applied voltages in their tunneling spectra.Introduction of pyridine into the HPA arrays increased the lattice constants of the 2-dimensional HPA arrays by ca.6 Å. Exposure to pyridine also shifted NDR peak voltages of H_nMW₁₂O₄₀ (M = P, Si, B, Co) samples to lessnegative values in the TS measurements. The NDR shifts of HPAs obtained before and after pyridine adsorptionwere correlated with the acid strengths of the HPAs, suggesting that tunneling spectra measured by STM couldserve to probe acid properties of HPAs. These results show how one can relate the bulk acid and redox propertiesof HPAs to surface properties of nanostructured HPA monolayers determined by STM.