A High Yield Synthesis of Ligand-Free Iridium Oxide Nanoparticles with High Electrocatalytic Activity

Abstract

Stable blue suspensions of 2 nm diameter iridium oxide (IrO x 3 nH 2 O) nanoparticles were obtained by hydrolyzing IrCl 6 2-in base at 90°C to produce [Ir(OH) 6 ] 2-and then treating with HNO 3 at 0°C. UV-visible spectra show that acid condensation of [Ir(OH) 6 ] 2-results in quantitative conversion to stable, ligand-free IrO x 3 nH 2 O nanoparticles, which have an extinction coefficient of 630 ( 50 M -1 cm -1 at 580 nm. In contrast, alkaline hydrolysis alone converts only 30% of the sample to IrO x 3 nH 2 O at 2 mM concentration. The acidified nanoparticles are stable for at least one month at 2°C and can be used to make colloidal solutions between pH 1 and 13. At pH 7 and above, some hydrolysis to form [Ir(OH) 6 ] 2-occurs. Uniform IrO x 3 nH 2 O electrode films were grown anodically from pH 1 solutions, and were found to be highly active for water oxidation between pH 1 and 13.SECTION: Nanoparticles and Nanostructures R ecent research activity in artificial photosynthesis has intensified the search for water oxidation catalysts that can function at high turnover rates and low overpotentials. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Despite the high cost and low terrestrial abundance of iridium, hydrated iridium oxide (IrO x 3 nH 2 O) has been useful for fundamental studies of the water splitting reaction because it can be made as stable nanoparticles, and because it is highly active for water oxidation over a broad range of pH. [16][17][18][19][20][21][22][23][24][25][26] The synthesis of IrO x 3 nH 2 O colloids was first reported over 100 years ago, 27 and that synthetic method (alkaline hydrolysis of [IrCl 6 ] 2-) produces blue colloids with particle sizes in the 1-2 nm range. Recently, Murray and co-workers used this method to deposit electrode films of IrO x 3 nH 2 O, which they showed are very good electrocatalysts over a broad range of pH. 22,28 Alternative syntheses of IrO x 3 nH 2 O colloids have used stabilizing ligands with multiple carboxylate groups, such as malonate or succinate. 29-32 With these stabilizing ligands, the colloids may be incorporated into photoelectrodes and other assemblies for overall light-driven water splitting. 23,33-35 In syntheses using capping ligands, the yield of stable colloid is rarely quantitative, and some of the IrO x 3 nH 2 O precipitates as large particles. Similarly, in our hands the alkaline route to uncapped colloids gives solutions of varying color, from pale to deep blue. Another complication of current synthetic methods is that IrO x 3 nH 2 O nanoparticles are not stable under acidic conditions; for example citrate-capped IrO x 3 nH 2 O precipitates at pH < 3, 36 and ligandfree IrO x 3 nH 2 O nanoparticles synthesized by alkaline hydrolysis are unstable at neutral pH. 22 Because of these problems, we have conducted a study of the alkaline hydrolysis process, the results of which are reported here. We identify conditions for obtaining quantitative yields of catalytically active, uncapped colloids that are stable over a wide range of pH.Hydrolysis o...

Boosting water oxidation layer-by-layer

“…[15][16][17] Thus, the four-electron electrooxidation of water proceeds at potentials far in excess of the thermodynamic value, necessitating an efficient electrocatalyst. Iridium oxide nanoparticles (IrO 2 NPs) exhibit high electrocatalytic activity at moderate overpotentials (0.20-0.29 V) [18][19][20][21][22][23][24] and stability against anodic corrosion over a wide pH range, whether incorporated into electrochemical or photo-electrochemical OER systems. [25][26][27] The scarcity of Ir and the enhanced activity of nanoscale (higher surface-to-volume ratio) 28 versus bulk heterogeneous catalysts or solid IrO 2 electrodes promotes the use of IrO 2 as NPs, typically studied as colloidal solutions or films on various electrode surfaces.…”

“…The latter loss of stability may reflect decomposition of the IrO 2 NPs, with formation of less reactive species under alkaline conditions, or simply a chemical degradation of the PDDA polymer that binds the IrO 2 NPs to the FTO electrode surface. Considering the high stability of IrO 2 NPs and their low tendency to corrode in alkaline media, 20 the leaching of the IrO 2…”

Improving O2 production of WO3 photoanodes with IrO2 in acidic aqueous electrolyte

“…A second IrO 2 electrodeposition procedure was tried as well (see ESI †). 39 Drop-cast particulate IrO 2 on WO 3 was also fabricated and tested (see ESI †).…”

Periodic DFT Study of Rutile IrO2: Surface Reactivity and Catechol Adsorption

“…Indeed, although the structural [8][9][10] , electronic 7,[11][12][13][14] , conductivity 15,16 and optic [16][17][18][19] properties are well known for the bulk of iridium oxide, there is an evident lack of knowledge regarding the surface properties and the surface reactivity of this material. In particular, an atomic description of the surfaces is missing, limiting the knowledge and the interpretation of the physico-chemical phenomena taking place.…”