Simple size-controlled synthesis of Au nanoparticles and their size-dependent catalytic activity

Abstract

The controlled preparation of Au nanoparticles (NPs) in the size range of 6 to 22 nm is explored in this study. The Au NPs were prepared by the reduction of tetrachloroauric acid using maltose in the presence of nonionic surfactant Tween 80 at various concentrations to control the size of the resulting Au NPs. With increasing concentration of Tween 80 a decrease in the size of produced Au NPs was observed, along with a significant decrease in their size distribution. The size-dependent catalytic activity of the synthesized Au NPs was tested in the reduction of 4-nitrophenol with sodium borohydride, resulting in increasing catalytic activity with decreasing size of the prepared nanoparticles. Eley-Rideal catalytic mechanism emerges as the more probable, in contrary to the Langmuir-Hinshelwood mechanism reported for other noble metal nanocatalysts.

Figure 1

Size distribution of Au NPs obtained by DLS (weighted by intensity) at various concentrations of Tween 80 in the reaction mixture.

Figure 2

UV-Vis spectra of colloid dispersions containing gold nanoparticles prepared at various concentrations of Tween 80 in the reaction mixture.

Figure 3

TEM images and particle size distribution histograms of gold nanoparticles prepared via reduction of tetrachloroauric acid by maltose at alkali pH at various concentrations of Tween 80: 10 mmol/L (a), 1 mmol/L (b), 0.1 mmol/L (c), 0.01 mmol/L (d), and without addition of Tween 80 (e).

Figure 4

The kinetic curves (based on UV-Vis absorption maxima of emerging gold colloids) of Au(III) reduction by maltose in alkaline pH observed for different concentration of Tween 80 in the reaction mixture.

Figure 5

Time-dependent absorption spectra of the solution of 4-nitrophenol during its reduction by sodium borohydride to 4-aminophenol catalysed by Au NPs prepared without Tween 80, recorded in 2 minute intervals. (See Supplementary Fig. S2–S4 online for the spectra recorded in the remaining cases of Au catalytic nanoparticles).

Figure 6

Kinetic curves and apparent rate constants of 4-nitrophenol reduction by sodium borohydride heterogeneously catalysed by various sized gold nanoparticles and without gold nanoparticles (blank).

Figure 7

Dependence of the reaction rate of 4-nitrophenol reduction to 4-aminophenol by sodium borohydride catalysed by various sized Au NPs on their total surface at constant amount of gold in the reaction mixture (10 μmol/L). Experimental data are evaluated by linear (full line) and by quadratic function (dashed line).