Fabrication of ordered arrays of Pd nanoclusters and PdO nanoparticles by using self-assembled block copolymers as templates

Palladium (Pd) and palladium oxide (PdO) nanoparticles (NPs) synthesis has gained considerable interest in the last few decades due to their numerous potential utilizations [1-3]. Systems based on Pd show high activity and selectivity in numerous catalytic processes [2, 3], like the catalytic formation of C-C bonds, one of the most useful transformations in organic synthesis [2]. Because sustainable development involves the utilization of reusable catalysts, the search for new catalytic systems to replace existing homogeneous ones is one important issue [4]. In this context, the immobilization of NPs on a solid support is an interesting alternative since the catalyst can be recycled by simple filtration and the atom/ion leaching from the particles can be minimized. Also, many electronic devices, such as resistors, use compositions containing palladium. All these applications may be more suitable with NPs in a patterned form and, since the majority of unique NPs properties are highly microstructure-dependent, to form Pd and PdO arrays on solid supports with a precise control of nanoparticles dimensions and spacing is a an important issue. Here, we report a simple method to fabricate arrays of palladium (Pd) nanoclusters and palladium oxide (PdO) nanoparticles with tunable dimensions and lateral spacing by using polystyrene-block-poly(ethylene oxide) copolymers (PS-b-PEO) as both stabilizers for Pd NPs and templates controlling the distribution of them. The cylindrical self-assembled nanostructure formed from PS-b-PEO has been used as host for selectively sequestering a nanoparticles precursor (palladium(II) acetate) in cylindrical domains of PEO. Then, Pd NPs have been obtained by reduction of the precursor in presence of the BCPs. In this way, nanocomposite thin films of well-defined morphology based on PS-b-PEO matrices and Pd NPs have been obtained. Furthermore, PdO NPs of improved stability against aggregation have been obtained on a solid and conductive support (silicon wafer) by treating the Pd containing films in air at elevated temperatures. We were able to modulate the dimensions of Pd nanoclusters and PdO NPs and their inter-distance by using as template two different PS-b-PEO copolymers having different molecular weights of both PS and PEO blocks.