The (photo)electrochemical behavior of n‐GaN[0001] in various aqueous solutions was studied using rotating‐disk voltammetry, cyclic voltammetry, and electrical impedance measurements. It was found that the bandedges of the semiconductor shift over 60 mV/pH unit, indicating acid‐base equilibria at the interface. In H2SO4 and KOH solutions, the photocurrent under anodic bias is associated with the oxidation of the semiconductor according to a three‐equivalent reaction, leading to dissolution and roughening of the surface. In 1.2 M HCl solutions, n‐GaN is stabilized for anodic decomposition due to the competing oxidation of Cl− ions. In the presence of oxalic acid and citric acid, anodic photocurrent multiplication was observed. Under cathodic polarization in the dark, Fe3+ , Ce4+ , HIO3 , in acid medium and normalFefalse(CN)63− in alkaline medium are electrochemically reduced at a diffusion‐limited rate. In 1 M KOH a high reactivity for O2/H2O reduction is observed, explaining why n‐GaN can be photoetched under open‐circuit conditions in this solution. © 2000 The Electrochemical Society. All rights reserved.

A vanadium 2,6‐naphthalenedicarboxylate, VIII(OH)(O2C–C10H6–CO2)·H2O, denoted as COMOC‐3as (COMOC = Center for Ordered Materials, Organometallics and Catalysis, Ghent University), has been synthesized under hydrothermal conditions by means of both a solvothermal and a microwave synthesis procedure. The structure shows the topology of an aluminium 2,6‐naphthalenedicarboxylate, the so‐called MIL‐69 (MIL = Materials of the Institute Lavoisier). After calcination at 250 °C in air, the VIII center was oxidized to VIV with the structure of VIVO(O2C–C10H6–CO2) (COMOC‐3). The oxidation process was verified by cyclic voltammetry and EPR spectroscopy. The crystallinity was investigated by variable‐temperature XRD. The title compound is stable against air and moisture. The catalytic performance of COMOC‐3 was examined in the liquid‐phase oxidation of cyclohexene. COMOC‐3 exhibited similar catalytic performance to MIL‐47 [VO(O2C–C6H4–CO2)]. The compound is reusable and maintains its catalytic activity through several runs.

A comparative study between n-GaAs/Au contacts, formed by electrochemical deposition or by vacuum evaporation, is presented. The main parameter, the barrier height B , was determined using three methods, i.e. classical current-voltage and capacitance-voltage measurements as well as STM-based ballistic electron emission microscopy (BEEM). The latter method allowed us to determine the distribution of B over the contact area on a nanometre scale and showed that the electrochemically made contacts are inhomogeneous. The main result, confirmed by the three methods, was that B was higher for the electrochemically deposited contacts than for the evaporated ones. This higher value is attributed to O − groups, present at the interface during the electrochemical metallization, and forming interfacial dipoles with the Au, leading to an increase of the barrier.

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The photoelectrochemical behaviour of n-GaN in contact with 1 M H 2 SO 4 and with acidic solutions containing Cl À ions was studied using rotating-ring-disk voltammetry, electrochemical impedance spectroscopy and etching experiments. It was found that n-GaN is stabilized against photoanodic decomposition in the presence of Cl À ions due to the competing oxidation of Cl À to Cl 2 . The competition kinetics were interpreted on the basis of a mechanism, in which intrinsic surface states take part in the photoanodic oxidation of Cl À . The participation of such intrinsic surface states may explain the discrepancies found in the literature concerning the photoelectrochemical behaviour of n-GaN.