Web Release Date: February 15,
Characterization and Analysis of Self-Assembly of a Highly Active Colloidal Catalyst for Water Oxidation onto Transparent Conducting Oxide Substrates
Faculty of Education and Human Sciences, Center for Transdisciplinary Research, Niigata University, 8050 Ikarashi-2, Niigata 950-2181, Japan
Received: October 9, 2007
In Final Form: November 22, 2007
Abstract:
An IrO2 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 IrO2 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 IrO2 colloid onto the ITO surface was analyzed by a Langmuir adsorption isotherm to provide
the maximum coverage and an adsorption equilibrium constant 
max = 1.1 × 10-8 mol cm-2 and Kads = 1.8
× 104 M-1 at 25 
C, respectively. The Kads value increased from 6.7 × 103 to 1.8 × 104 M-1 with a temperature
increase from 5 to 25 C. The temperature dependence of Kads gave 
H = 36.5 kJ mol-1, G = -24.4 kJ
mol-1, and S = 204 J mol-1 K-1 at 25 C. The positive H and S values are explained by the
rearrangement of solvating water molecules and counter cations surrounding the IrO2 colloid that is involved
in its assembly on the ITO surface. In electrocatalytic water oxidation, the maximum turnover frequency of
the IrO2 catalyst was 23 600 h-1 under potential static conditions at 1.3 V versus Ag/AgCl.
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