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

Gold (Au) nanoparticles (NPs) are relevant for multiple applications in electronics, sensing, bio-medicine, optics, catalysis and many more. As a consequence, a range of colloidal syntheses have been reported where a gold precursor is reduced to metallic Au NPs. In numerous protocols, stabilizers such as surfactants are added to ensure colloidal stability and/or 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), polyvinlypyrrolidone (PVP) or sodium dodecylsulfate (SDS), in the fast (seconds) room temperature synthesis of Au NPs obtained otherwise by using only water and NaBH4 in its surfactant-free version. The effect of the NaBH4/Au molar ratio, the Additive/Au molar ratio and the concentration of HAuCl4 used as precursor are jointly investigated in parametric study over 130 samples. The influence of the order of addition of the chemicals is also discussed. It is found that there is no benefit to use the additives considering size control, stability but also catalysis. A fine control over the NP size in the range 3-10 nm is easily achieved without additives by controlling the NaBH4/Au molar ratio. A benefit of PVP is to lead to NPs smaller than 3 nm even at 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 stale over time, but also to lead in most cases to more catalysts as or even more active compared to NPs prepared with additives. This is exemplified with the 4-nitrophenol reduction for a model reaction for water treatment and the ethanol electrocatalytic oxidation (EOR) as 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 x500 compared to the parametric study performed. Overall, the results stress the benefits of surfactant-free approaches to develop gold-based nanomaterials and provide guidelines to optimize their synthesis towards more sustainable nanotechnologies.