Ultra small gold nanoparticles–graphitic carbon nitride composite: an efficient catalyst for ultrafast reduction of 4-nitrophenol and removal of organic dyes from water

Abstract

Synthesis of supported ultra small gold particles is important for their unusual properties and catalytic applications. We report a facile method for synthesis of ultra small gold nanoparticles supported on carbon nitride sheets (Au–CN_x). The ultrasonication was used to form Au–CN_x composite from carbon nitride quantum dots and HAuCl₄ without any aid of external reducing agent. The Au–CN_x composite was well characterized by tunneling electron microscopy, selected area electron diffraction, energy dispersive X-ray spectroscopy, powder X-ray diffraction, X-ray photo electron spectroscopy methods. Electron microscopic measurements confirm that thin graphitic carbon nitride sheets provided two dimensional supports to stabilize ultra small, 1–3 nm sized gold nanoparticles (AuNPs). The Au–CN_x composite showed excellent catalytic activity towards the reduction of 4-nitrophenol in aqueous medium in presence of sodium borohydride (NaBH₄) with very fast kinetics and good stability. The Au–CN_x catalyst can be used as an absorbent for the removal of organic dye [Rhodamine B (RhB), methylene blue (MB), and methyl red (MR)] from water. It showed excellent adsorption capacity for RhB and MB due to electrostatic interaction between anionic catalyst and cationic organic dye molecules. The catalyst can easily be reused after removing adsorbed dye from the catalyst simply by changing pH of the solution. In addition, Au–CN_x composite exhibited superior photo catalytic degradation of various dyes (RhB, MB and MR) on irradiation of UV, visible light and natural sunlight. Excellent photodegradation rate constants for RhB (0.024 min⁻¹), MB (0.024 min⁻¹) and MR (0.02 min⁻¹) were observed although high concentration of dyes were used for degradation. Au–CN_x is one of the best adsorbent for RhB adsorption with maximum adsorption capacity of 400 mg g⁻¹ and catalyst for nitrophenol reduction. We hope that this Au–CN_x composite will find its application as an effective catalyst for water purification.