Citrate adsorption on gold: Understanding the shaping mechanism of nanoparticles

Advanced applications of colloidal nanoparticles (NPs) become to depend on their specific shape, which is controlled by the adsorption behavior of the capping agent involved in their synthesis. To understand the way in which citric acid determines the shape of gold NPs, the adsorption behavior of citrate on gold under the synthesis conditions is here investigated from electrochemical experiments on well-defined surfaces. Gibb excesses and charge numbers for the citrate adlayers deposited on the Au (111), Au(100) and Au(110) electrodes when a potential is applied were estimated at pHs 1 and 3. From these results, FTIR spectra and DFT calculations, it is concluded that solvated citrate can become simultaneously adsorbed through three dehydrogenated carboxylic groups in bidentate configuration on Au(111), but only through two on Au(100) and Au(110). As a result of this behavior, citrate can become more strongly adsorbed on Au(111) than on the other two basal planes of gold under the synthesis conditions, which would explain why tetrahedral and octahedral colloidal gold NPs are preferentially shaped when citric acid is used as the capping agent in water. This conclusion coincides with the previously one obtained on platinum, suggesting that the mechanism here described would operate also on other metals having fcc structure.