Noble metal NPs and nanoalloys by sonochemistry directly processed on nanocarbon and TiN substrates from aqueous solutions.

The sonochemical processing of nanomaterials in a solution is well established and has been advantageously used for a variety of nanomaterials and morphologies thereof. In general, high energy and high frequency ultrasound is applied to a solution containing the ionic species of the elements to be reduced as well as a certain amount of reducing chemicals. For further applications such as catalysis washing, filtering, dispersion and mounting on or mixing in a substrate are necessary. A sonochemical processing of nanomaterials directly on a substrate could make all these steps obsolete. Herein we show that noble metal and nanoalloy nanoparticles (NPs) can directly be processed on nanocarbon and titanium nitride surfaces using a simple ultrasound laboratory cleaner in aqueous solutions that are free from any reducing chemicals. The process is demonstrated on Au-NPs and nanoalloys of AuPd and PdPt which form a dense distribution on the substrate surface. To illustrate the catalytic activity of the NPs, the electrocatalytic performance of one AuPd-nanoalloy is demonstrated. The results are discussed in terms of reduction phenomena occurring at the interface between the ultrasonic cavitation and the substrate. We think that these reduction phenomena are mediated by the formation of reducing radicals at the substrate surface that are in turn driven by OH radicals from water sonolysis. Electrochemical current measurement at 0 V seem to support the existence of reducing currents during measurements under chopped ultrasound in an aqueous solution of HAuCl in comparison to measurements in water.