Characterization and Analysis of Self-Assembly of a Highly Active Colloidal Catalyst for Water Oxidation onto Transparent Conducting Oxide Substrates

An IrO 2 colloid stabilized by citrate ions was self-assembled on an indium tin oxide (ITO) electrode when it was immersed in the colloid solution at pH 3.5. The IrO 2 colloid on the ITO surface was characterized using electrochemical, inductively coupled plasma mass spectroscopic, X-ray diffraction spectroscopic, scanning electron microscopic, and atomic force microscopic techniques. The self-assembly was promoted steeply at pH 3.5 to 4.1, although it hardly occurred at pH 5.3 to 9.7. It is considered to be caused by chemical interaction between carboxylic groups on the citrate stabilizer and hydroxyl groups of the ITO surface. The adsorption isotherm of the IrO 2 colloid onto the ITO surface was analyzed by a Langmuir adsorption isotherm to provide the maximum coverage and an adsorption equilibrium constant Γ max = 1.1 × 10 -8 mol cm -2 and K ads = 1.8 × 10 4 M -1 at 25 °C, respectively. The K ads value increased from 6.7 × 10 3 to 1.8 × 10 4 M -1 with a temperature increase from 5 to 25 °C. The temperature dependence of K ads gave Δ H ° = 36.5 kJ mol -1, Δ G ° = −24.4 kJ mol -1, and Δ S ° = 204 J mol -1 K -1 at 25 °C. The positive Δ H ° and Δ S ° values are explained by the rearrangement of solvating water molecules and counter cations surrounding the IrO 2 colloid that is involved in its assembly on the ITO surface. In electrocatalytic water oxidation, the maximum turnover frequency of the IrO 2 catalyst was 23 600 h -1 under potential static conditions at 1.3 V versus Ag/AgCl.