Received: November 8, 2007

In Final Form: December 19, 2007

Abstract:

Bulk Cu₂O or cuprite is the only stable copper(I) compound present in plentiful amount in earth's crust. It is a challenging job to take bulk Cu₂O to a nanoregime and to stabilize it in solution. No wonder that Cu₂O in its nanoregime would act as a photocatalyst. We report a new synthetic protocol for the first time to obtain monodispersed, stable, exclusively cubic Cu₂O nanoparticles in surfactant-free condition and its catalytic action for methylene blue (MB)-hydrazine reaction in aqueous medium. The blue color of the dye, MB, faded away upon the addition of hydrazine, producing colorless leuco methylene blue (LMB) indicating the progress of the redox reaction. The rate of this redox reaction has been found to be enhanced in the presence of the nanocatalyst, Cu₂O. The success of the reaction demonstrates a simple 'clock reaction'. An oscillation between a blue MB color and colorless solution due to formation of LMB is observed on periodic shaking. This oscillation continues for over 15 cycles. Studies on the effect of bulk Cu₂O and nanoparticles of CuO and Cu(0) have not been successful for demonstration of the 'clock reaction'. Thus, the importance of Cu₂O nanoparticles in the clock reaction is established beyond doubt. The Cu₂O nanoparticles were characterized by different physical methods. TEM studies authenticate the cube shaped monodispersed particles. The electrochemical studies indicate that nano-Cu₂O shows a couple of redox peaks which correspond to the redox Cu(II)/Cu(I) system. Kinetic studies authenticate a first-order reaction mechanism. Further, quantum chemical calculations reveal that the nanoparticles reduce the activation energy by ~17 kcal/mol, thereby making the reaction 2.4 × 10⁷ times faster compared to the gas phase.

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