Tuning diazonium modification of gold nanoparticles for fuel cell application

Products made from alcohol oxidation are very important and used in food industry, pharmaceuticals and dryers to name but a few. In this regard, precious metals such as gold operate in various shapes and sizes as effective catalysts for the oxidation of alcohols. In this work, 20 nm-sized gold nanoparticles were synthesized and modified with either 4-carboxybenzenediazonium tetrafluoroborate (AB) (single salt catalyst) or a mixture of the latter with 3,5-dimethylbenzenediazonium tetrafluoroborate (AB-DMB, double-salt catalyst). The relative concentrations of the diazonium salts were tuned in order to maintain the electrocatalytic property of the arylated gold nanoparticles in the electro-oxidation of ethanol. The physicochemical properties (size, aryl layer thickness and composition) and electrocatalytic performances of the capped gold nanoparticles (in single and double salt catalyst systems) were investigated by means of TEM, XPS, UV-visible, TGA-DTA and electrochemistry. The aryl capping layer thickness is below 1 nm when the total salt concentration was 510-5 M, without any significant loss of current compared to pure gold nanoparticles. For 1 μL of pristine or arylated Au NPs, coated on 0.07 cm² glassy carbon electrode disk, we achieved a low oxidation potential of 200 mV and high current ethanol oxidation peak intensity levelling off at ~80 μA for the single diazonium arylation. It is demonstrated that dilution of the carboxylic acid-functionalized diazonium is more important than mixing this compound with 3,5-dimethylbenzene diazonium salt at equal total concentration. The dual arylation strategy returned lower ethanol oxidation intensity likely due to hydrophic effect imparted by the 3,5-dimethylphenyl groups, thus hindring access of ethanol to the nanocatalyst surface. This work demonstrates that arylated gold nanoparticles are unique nanocatalysts for ethanol oxidation reaction, with remarkable performances. This electrocatalytic performance of the nanoparticles was maintained through the use of finely tuned aryl capping layer. The strategy opens avenues for other arylated electrocatalysts and uses thereof.