NaBH4-mediated syntheses of colloidal gold nanocatalysts in water: are additives really needed?

Abstract

Nanomaterials are relevant for several applications in electronics, sensing, biomedicine, optics, and catalysis, among other fields. As a consequence, a wide range of colloidal syntheses have been reported in which a precursor is reduced to metallic nanoparticles (NPs). In numerous protocols, stabilizers such as surfactants are added to ensure colloidal stability and/or to achieve size control. The actual need for such added chemicals, often derived from fossil fuels, is rarely questioned. Here, we investigate the effect(s), pros and cons of using common additives, such as sodium citrate (NaCt), polyvinylpyrrolidone (PVP) or sodium dodecylsulfate (SDS), in the fast (seconds-long) room temperature synthesis of gold (Au) NPs obtained otherwise using only water and NaBH₄ in its surfactant-free version. The effects of the NaBH₄/Au molar ratio, additive/Au molar ratio and concentration of HAuCl₄ used as a precursor are jointly investigated in a parametric study of over 130 samples. The influence of the order of the addition of the chemicals is also discussed. It is found that there is no benefit in using additives for size control, stability or catalysis. Control over the NP size in the range of 3–10 nm is easily achieved without additives by controlling the NaBH₄/Au molar ratio. A benefit of PVP is that it leads to NPs smaller than 3 nm even at a relatively high concentration of HAuCl₄ up to 3–4 mM. An advantage of the additive-free approach is not only to develop simpler and more sustainable syntheses of colloidal NPs that are stable over time but also to lead, in most cases, to catalysts as active as or even more active than the NPs prepared with additives. This is exemplified by the 4-nitrophenol reduction, a model reaction for water treatment, and by the electrocatalytic ethanol oxidation reaction (EOR), a model reaction for energy conversion. Finally, the surfactant-free approach is shown to be easily scalable to 1 L of solution, e.g. by a factor of 500 compared with the parametric study performed. Overall, the results demonstrate the benefits of surfactant-free approaches to develop gold-based nanomaterials and provide guidelines to optimize their synthesis towards more sustainable nanotechnologies.