Chemical and size effects of nanocomposites of silver and polyvinyl pyrrolidone determined by X-ray photoemission spectroscopy

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Abstract

We have taken a systematic approach for interpreting X-ray photoemission spectroscopy data for passivated silver nanoparticles, and for the first time we have been able to determine the effects of nanoparticle size and of the nanoparticle stabilizers on the observed binding energies of Ag core electrons. The binding energy of Ag 3d5/2 core electrons was found to increase with decreasing nanoparticle size.

Introduction

Metal nanoparticles have recently received considerable attention because of their unique chemical and physical properties, which differ greatly from those of bulk materials, as well as because of their technological applications [1], [2]. Polymers have been found to be effective stabilizers of colloidal metal nanoparticles. In particular, polyvinyl pyrrolidone (PVP) is commonly used in the preparation of silver nanoparticles [3], [4], [5], [6], and also in recent syntheses of various silver nanomaterials, including nanowires, nanocubes, and fractal structures; the different shapes of these nanomaterials are obtained by varying the experimental conditions [7], [8], [9], [10], [11], [12]. Although this control of the shape of silver nanomaterials is very interesting, the role of PVP in this process is yet to be elucidated [10]. An understanding of the interaction between silver and PVP is a prerequisite for elucidating the mechanism of nanomaterial shape control. Some researchers have investigated the interaction between silver nanoparticles and PVP by FTIR and X-ray photoemission spectroscopy (XPS) [4], [13], [14], but our understanding of this interaction is still insufficient.

Although XPS is an excellent tool for the investigation of materials’ electronic structures and chemical bonding, its application to the study of colloidal metal nanoparticles has so far been limited, partly because of the charging effect, which shifts the measured binding energy, especially when the conductivity of the sample is poor. Indeed, the binding energies of the Ag 3d5/2 core electrons obtained in previous studies of silver nanoparticles have not been consistent. For example, in the case of silver clusters on graphite and Si substrates [15], [16], [17], [18], [19], [20], the binding energy of the Ag 3d5/2 core electrons was found to be 368.6 eV, which is higher than the binding energy of Ag metal, 368.3 eV. In contrast, however, the binding energy for passivated silver nanoparticles was found to be 367.5 eV, i.e., shifted to a lower value [4], [21].

The measured shifts in the binding energies of nanostructured materials can be attributed to three major effects: charging [19], size [15], [16], [17], [18], [19], [20], and chemical (bonding) effects [4], [21]. The charging effect results from the creation of ions and thus a build-up of charge at the exposed area, which in samples with poor conductivity is only slowly dissipated by the transfer of electrons from the substrate. The size effect is due to the change in electronic structure that results from changes in the boundary conditions with changes in size of the nanoparticles. In general, the band gap and binding energy of a material increase as the size of the material decreases. The chemical effect on the binding energy is due to the adsorption of polymer or organic molecules onto the nanoparticles. In this study, we have taken a systematic approach to the interpretation of the XPS spectra of silver nanoparticles, which are complicated by the three effects discussed above. Firstly, we eliminate the charging effect by careful control of the XPS data acquisition time and by monitoring the secondary cut-off tail; this effect is only due to the low conductivity of insulators and so is not relevant to a phenomenon occurring in nanosized metal. We then separate the contributions of the size and chemical effects from the measured binding energies of the Ag core electrons. We found that both contributions substantially affect the XPS spectra of silver nanoparticles. To our knowledge, this is the first time that the binding energy shift for passivated silver nanoparticles is successfully separated into chemical and size effects.

Section snippets

Experimental

Silver nanoparticles were prepared from an aqueous solution of AgNO3, isopropanol, and polyvinyl pyrrolidone (PVP; MW 55 000) by γ-irradiation (in the field of a 60Co γ-ray source with 30 kGy doses) [5]. To investigate the relationship between the size of the nanoparticles and the binding energies of their Ag 3d5/2 core electrons, two samples of silver nanoparticles were prepared at different concentrations of AgNO3 and PVP, 0.01 M AgNO3 with 1 wt% PVP and 0.5 M AgNO3 with 3 wt% PVP. The

Results and discussion

Fig. 1a and b show TEM images of the samples of silver nanoparticles, prepared from 0.01 and 0.5 M AgNO3 solutions, respectively. The average sizes of the as-prepared (final) silver nanoparticles in the samples are 12.1 ± 1.6 and 19.6 ± 2.6 nm, respectively. We have previously concluded that silver nanoparticles prepared by γ-irradiation with PVP as a stabilizer consist of smaller nanoparticles (primary nanoparticles) [5]. The existence of the smaller nanoparticles was confirmed from TEM images

Acknowledgements

This study was supported by the Ministry of Education (BK Project). H.J.S. acknowledges the support from Korean Ministry of Science and Technology (MOST) under project No. M1-021202000303-B15-02-001-00.

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