Ligand-Mediated Surface Restructuring of Copper Nanoclusters and Its Impact on the Reduction of 4-Nitrophenol to 4-Aminophenol

While nanocatalysis is fascinating, our understanding of the issues related to nanocatalysts, such as surface restructuring and induction time, is still in its infancy. If a nanocatalyst is made in such a way that it performs its role as a nanocatalyst and at the same time provides information about its surroundings through a suitable optical output signal, it is anticipated that several insights into the catalytic mechanisms will emerge. These insights, in turn, can facilitate the design of more effective nanocatalysts. With an aim to understand this, two types of fluorescent Cu-based nanocatalysts, tannic acid (TA-CuNCs) and cysteine-capped (Cys-CuNCs), have been synthesized, and their catalytic performance in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol has been investigated. Along with the usual analytical methods, the fluorescence property of the nanocatalysts has also been exploited to extract relevant information on the catalytic reaction. The data have also been compared with ligand-free CuNCs. Kinetic data obtained from absorption studies have revealed that the rate constants for the nanocatalyst-catalyzed reaction follow the order: kTA–CuNCs > kCuNCs > kCys–CuNCs. Quite interestingly, it has been observed that unlike TA-CuNCs, Cys-CuNCs, and ligand-free CuNCs exhibit an induction time in the catalytic reaction, indicating the crucial role of ligand-mediated surface restructuring. Furthermore, it has been found that eliminating dissolved oxygen from the reaction medium increases the rate constants and eliminates the induction time. This finding has essentially indicated the influence of dissolved oxygen on facilitating the reverse oxidation of 4-nitrosophenol (an intermediate) to 4-aminophenol. It has also been observed that the catalytic activity decreases with an increase in 4-NP concentration, suggesting a Langmuir–Hinshelwood mechanism for the event. The outcome of the present work sheds light on the role of surface ligands in nanocatalysis and presents new opportunities for designing new nanocatalysts for practical applications.