Charles Y. Cummings

Surface State Trapping and Mobility Revealed by Junction Electrochemistry of Nano-Cr2O3

Hydrous chromium oxide nanoparticles (~15 nm diameter) are assembled from a colloidal solution onto tin-doped indium oxide (ITO) substrates by layer-by-layer electrostatic deposition with aqueous carboxymethyl-cellulose sodium salt binder. Calcination produces purely inorganic mesoporous films (average thickness increase per layer of 1 nm) of chromia Cr2O3. When immersed in aqueous carbonate buffer at pH 10 and investigated by cyclic voltammetry, a chemically reversible oxidation is observed because of a conductive layer at the chromia surface (formed during initial potential cycling). This is attributed to a surface CrIII/IV process. At more positive potentials higher oxidation states are accessible before film dissolution. The effects of film thickness and pH on voltammetric responses are studied. X-Ray photoelectron spectroscopy (XPS) evidence for higher chromium oxidation states is obtained. ITO junction experiments are employed to reveal surface conduction by CrIII/IV and CrIV/V ‘mobile surface states’ and an estimate is obtained for the apparent CrIII/IV charge surface diffusion coefficient Dapp = 10–13 m2 s–1. The junction experiment distinguishes mobile surface redox sites from energetically distinct deeper-sitting ‘trapped states’.

Redox Reactivity of Methylene Blue Bound in Pores of UMCM-1 Metal-Organic Frameworks

Redox processes were studied in the pores of a crystalline UMCM-1 metal organic framework (MOF) material. Methylene blue was employed as an absorbed redox active dye component. From the change in coloration during dye adsorption, it was concluded that an essentially irreversible adsorption process with high pore loading of the resulting MOF structure occurred. The adsorbed methylene blue remained redox active in the MOF pores and there was no evidence of significant losses during extended redox cycling. Due to the size of the pores, the reactivity of the pore-bound methylene blue is closely related to that expected for methylene blue in an aqueous solution. A study of the effect of solution pH on the voltammetric responses revealed an interesting gradual change in electrical pore conductivity form conducting poorly under acidic conditions to conducting very well under alkaline conditions. This is interpreted in terms of charge transport via single-electron hopping conduction in pores. An estimate of the apparent charge diffusion coefficient at pH 7, Dapp = 1.4 × 10−15 m2s−2, is obtained. Potential applications of the new family of redox active hybrid MOF materials are indicated.