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.

A new, facile, one step, single-pot, green synthesis of (200) faceted Au-AgCl cubes, at room temperature, is reported. The simultaneous addition of aqueous solutions of HAuCl4 and AgNO3, at room temperature, to an aqueous solution of bael gum results in the formation of nanocubes consisting of Au and AgCl, as evidenced by a combination of characterization techniques such as UV–visible spectroscopy, powder X-ray diffraction, XPS, SEM, and STEM. The formation of silver chloride with (200) as the preferred surface facet might arise from the controlled release of chloride ions, which in turn could arise from the slow reduction rate of HAuCl4 by bael gum. This claim is supported by the preparation of Au-AgCl cubes having no specific orientation synthesized by the successive addition of HAuCl4 followed by AgNO3, where the release of the chloride ion is not controlled. The Au-AgCl cubes exhibit selective activity toward the oxidative dimerization of ortho-phenylenediamine (o-PDA), in the absence of H2O2. The time required for this conversion is less than 30 min. This in turn enables the detection of o-PDA in a trace amount with the minimum being 2 ppm via fluorescence and 15 ppm via UV–visible spectroscopy. Control experiments suggest the specific and rapid activity shown by the (200) facet-oriented Au-AgCl cubes might arise from its smaller size and morphology.

A new, simple, green method for the synthesis of Au nanowires (average diameter 8 nm and several micrometers in length) using Au seeds prepared from bael gum (BG) is reported. The nanowires are characterized using UV−visible absorption spectroscopy, powder X-ray diffraction, transmission electron microscopy (TEM), and high-resolution-TEM. It is observed that the rate of the reduction process might be the decisive factor for the shape selectivity, as evident from the formation of nanowires at a particular concentration of seeds and NaOH. The polysaccharide present in BG is the active ingredient for the synthesis of Au nanowires, while the small molecules present in BG, when used alone, did not result in nanowire formation. The TEM images of the precursor to the Au nanowires suggested that new, nucleated particles align in a linear manner and fuse with one another, resulting in the nanowire. The linear fusion of the newly nucleated particles could be due to the lack of adequate protecting agent and the presence of Au complex adsorbed to the surface. The electrochemical activity of the nanowires for oxygen reduction reaction (ORR) is assessed and compared with that of nanotriangles and spherical nanoparticles of Au. The performance of Au nanowire is better than Au-nanomaterials (heat-treated as well as non-heat-treated), Au seeds, and clusters. The better efficiency of the nanowires when compared to that of the other reported catalysts is attributed to the presence of active (100) facets with numerous corners, edges, and surface defects.

The green and selective synthesis of triangular nanoplates (NPs) is shown to arise out of the slow rate of reduction and generation of Au(0) from HAuCl 4 . Toward this purpose, the small molecules present along with the polysaccharide in bael gum (BG) are separated; their structures are identified, and their role in the reduction of HAuCl 4 in aqueous solution as well as their possible role in shape direction are studied. The observations suggest that in all the cases studied the slow rate of reduction could be the primary reason for shape selectivity toward formation of NPs, and the role of small molecules is possibly limited to that of a reducing agent. This was further confirmed by carrying out the reduction reaction in some detail by using imperatorin oxide (one of the molecules isolated from BG) at different concentrations. At higher concentrations of imperatorin oxide, the formation of pseudospherical and rodlike particles (instead of smaller NPs) in solution further confirmed the hypothesis. The formation of pseudospherical Au nanoparticles from BG, at high concentration and ambient temperature or relatively lower concentration and high temperature, as well as the formation of NPs from purified BG at ambient temperature reinforce the hypothesis that a moderate reduction rate results in the formation of triangular Au NPs.