Green, selective, seedless and one-pot synthesis of triangular Au nanoplates of controlled size using bael gum and mechanistic study

A seedless, one pot, green and selective synthesis of triangular gold nanoplates (NPs) by the reduction of chloroauric acid using Aegle marmelos gum (bael fruit derived gum; BG), with control over size is reported. The size of the NPs is found to be dependent critically on the concentration of the BG with triangular, triangular with truncation at the apex and hexagonal plates being the predominant shapes over a wide concentration range, at room temperature. At very high concentration of BG and at room temperature, the shape of the particles is spherical. A fine-tuning of the temperature, at a particular concentration of BG, results in the formation of perfect triangular NPs. Increasing the temperature of reduction to 90 °C and especially above a particular BG concentration, Au nanoparticles that are elongated spheres are obtained as the major product. The possible mechanism for the Au NPs formation is investigated using techniques such as HR-TEM, SEM, XRD and UV-visible spectroscopy. The formation of single and polycrystalline Au seeds and fused particles along with multiple twinning suggest that a particular well-faceted seed might not be the prerequisite for the formation of NPs but rather a critical rate of formation of Au(0). A schematic diagram is also presented representing the concentration and temperature regime over which anisotropic and especially triangular NPs can be synthesized with bael gum.