Surfactant-free NaBH4-mediated synthesis of gold nanoparticles in water at room temperature: fine size control for active nanocatalysts

Size control over nanomaterials obtained by simple and cost-efficient methods is a key achievement to make of the most the unique size-dependent properties of nanoparticles (NPs). For instance, size-controlled gold (Au) NPs are in high demand for electronic, optical, catalytic or biomedical applications. In several colloidal syntheses, size control is achieved by controlling pH, temperature and/or the ratio of various chemicals. Unfortunately, in most cases, energy-demanding and/or several high purity chemicals are required to achieve a fine and reproducible size control. Here, the unique features of a room temperature synthesis of gold nanoparticles simply obtained using only NaBH4 and HAuCl4 in water, without the need for any other chemicals, are exploited. Size control is achieved by tuning the NaBH4/Au molar ratio, where larger ratios lead to larger nanoparticles, and/or by controlling the purity of the water used for the synthesis where the use of an increasing amount of lower purity water leads to larger nanoparticles. Leveraging on the NaBH4/Au molar ratio and/or water purity leads to a fine size control in the range 5-20 nm. Colloidal dispersions stable for months are easily obtained. The optimization performed in 2 mL scales up easily by a factor x50 in volume. The prepared nanoparticles are readily active nanocatalysts suitable for size-effects study for instance for water treatment and energy conversion with the examples of 4-nitrophenol reduction and the ethanol oxidation reduction.