Silver nanoparticle catalysed redox reaction: An electron relay effect

doi:10.1016/j.matchemphys.2005.08.011

Materials Chemistry and Physics

Volume 97, Issues 2–3, 10 June 2006, Pages 283-287

https://doi.org/10.1016/j.matchemphys.2005.08.011 Get rights and content

Abstract

A silver cluster shows efficient catalytic activity in a redox reaction because the cluster acts as the electron relay centre behaving alternatively as an acceptor and as a donor of electrons. An effective transfer of electrons is possible when the redox potential of the cluster is intermediate between the electron donor and electron acceptor system.

Introduction

Recently, extensive attention has been paid to the neglected dimension of a metal, the clustering of only a few atoms or molecules. New methods have been developed in physics and chemistry for the synthesis and characterization of their crucial role in number of processes such as, phase transitions, catalysis, surface phenomenon, imaging, etc.

The electron transfer step is important in many homogeneous and heterogeneous reactions [1], [2]. In this step, there can be a large redox potential difference between the donor and acceptor, which may restrict the passage of electrons. An effective catalyst with an intermediate redox potential value between the donor and acceptor partner helps the electron transfer and acts as an electron relay system. Metal ions and metal particles are well-known examples of this type of redox catalyst. It has also been established that when metal particles become small, their electrode potential value differs from that of the bulk metals [3], [4].

Solid matrix [5], [6], [7], polymers [8], [9], ligands [10] and surfactants [11] often stabilize catalyst particles. In fact, polymer and surfactants are recommended as a viable alternative for particle stabilization. While they are loosely packed, the dynamic structure around the catalyst surface hardly affects the accessibility of the particles from the reacting species.

In this study, colloidal silver has been used as a catalyst in the electron transfer reaction because the solution remains optically transparent so allowing easy monitoring of the catalytic process and analysis of the step-by-step sequence towards the formation of the particles.

Two representative redox type of reactions have been considered in order to follow the catalytic activity of the silver colloid. While the reduction of phenosafranin and eosin is thermodynamically favourable, it is kinetically difficult. Silver nanoparticles provide an alternative path for the reaction since they require a lower energy of activation. The plasmon resonance band of colloidal silver varies from 400 to 450 nm depending on the size of the particles. In contrast, the plasmon resonance band for phenosafranine and eosin are 530 and 528 nm, respectively. It is therefore easy to monitor the redox reaction by UV–vis spectrophotometer because the dyes have different colours when in their oxidised and reduced form (Scheme 1). Since the plasmon resonance bands of the dyes are in a completely separate visible wavelength region from that of the silver, it is straightforward to observe spectrophotometrically the reduction process of the dyes and the evolution of the silver particles.

Section snippets

Experimental

Methoxy polyethylene glycol (MPEG), a water soluble polymer, was purchased form Union Carbide. Analytical grade silver nitrate, phenosafranine and eosin were purchased from Aldrich. Ultra-pure water (>17 MΩ cm) was used to prepare the solution of MPEG and silver nitrate. A stock solution of MPEG (0.5 g of MPEG dissolved in 1000 ml of water) was used, while AgNO₃ was utilised at a concentration of 10⁻² mol dm⁻³.

Photochemical reactions were carried out in well-stoppered 1 cm quartz cuvette. The cuvette,

Method used for the synthesis of silver nanocluster

In a typical reaction, 2 ml of MPEG solution was added to 60 μl of silver nitrate (10⁻² mol dm⁻³). The solution was then homogeneously mixed in a quartz cuvette and purged with nitrogen gas for 15 min in order to remove the dissolved oxygen. The cuvette was placed at an angle of 35° to the horizontal under UV-irradiation directed from above. The progress of the reaction was monitored by using a UV–vis spectrophotometer.

The colour of the solution started to change from colourless to yellow after the

Reaction route for particle formation

Energy deposition through out the solution ensures the homogeneous distribution of the photolytic radicals formed by the excitation and ionisation of the solvent [13]. However, direct photolysis of water, in the presence of UV-irradiation, is as follows: $H_{2} O \overset{h υ}{⟶} H + OH$ Methoxy polyethylene glycol, CH₃O(CH₂CH₂O)_n single bond H, that is, CH₃O(CH₂CH₂O)_n−1CH₂CH₂OH, where n indicates the average number of oxyethylene group, is a scavenger of the H and OH radicals so yielding CH₂CHOH in the following manner: $C H_{2} C H_{2} OH \overset{OH}{⟶} C$

Conclusions

The aim of this study was to investigate the thermodynamic viability and catalytic effect of nanoparticles towards a redox type reaction. The dynamics of cluster formation and the simultaneous reduction of the dyes were monitored by UV-spectrophotometry. These measurements indicated that once nanoparticle clusters reach their final size with a suitable redox potential, these metal clusters behave as excellent catalysts owing to their electronic properties. A cluster can behave as an efficient

Acknowledgments

One of us (KM) thanks the National Research Foundation and the University of the Witwatersrand for the funding of a postdoctoral research fellowship.

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Silver nanoparticle catalysed redox reaction: An electron relay effect

Abstract

Introduction

Section snippets

Experimental

Method used for the synthesis of silver nanocluster

Reaction route for particle formation

Conclusions

Acknowledgments

Curr. Opin. Colloid Interface Sci.

Appl. Catal. A

J. Mol. Catal. A

Radiat. Phys. Chem.

Comprehensive Co-ordination Chemistry

J. Phys. Chem.

Appl. Catal. A