Elsevier

Synthetic Metals

Volume 160, Issues 19–20, October 2010, Pages 2099-2103
Synthetic Metals

One-step preparation of silver nanoparticles confined in functionalized-free SBA-15 channels

https://doi.org/10.1016/j.synthmet.2010.07.037Get rights and content

Abstract

Supported Ag nanoparticles were successfully prepared by a one-step in situ procedure within channels of SBA-15 mesoporous silica. The reaction was carried out in N,N-dimethylformamide (DMF) at 25 °C with no need for silica surface functionalization or demanding time-consuming steps, and presents important advantages over the usual method. The small-angle X-ray scattering (SAXS) pattern of SBA-15 presents well-defined reflections associated with P6mm hexagonal symmetry. TEM micrographs of supported Ag nanoparticles (SBA-15/Ag) clearly show the presence of spheroidal and elongated Ag nanoparticles that are confirmed by surface plasmon resonance band in the UV–vis absorption spectrum. No evidence of Ag nanoparticle agglomeration was observed throughout the inorganic matrix. Ag nanoparticle formation within channels of SBA-15 decreases its specific surface area and total pore volume, as evidenced by nitrogen adsorption–desorption isotherms, with no damage to the silica framework. The method shows itself to be very efficient, fast and convenient for the synthesis of supported Ag nanoparticles in SBA-15 channels.

Introduction

The possibility of manipulating and controlling matter on the nanometer scale has proven to be a very efficient approach in the development of new technologies in several areas such as catalysis [1], energy conversion [2], sensors [3], electronic and optical devices [4] and medical diagnosis [5]. The great interest in nanoscale materials is clearly a result of remarkable properties presented by nanoparticles (NPs) ranging from large surface-to-volume ratios to quantum confinement effects that are not observed with their bulk counterparts [6], [7]. In the case of noble metals, metallic nanoparticles (MNPs) provide significantly more chemical activity centers in catalysis compared to bulk [8], but the NP free surface energy for this size regime often leads to particle growth or aggregation and loss of important properties achieved at nano level.

At this point, the size, shape and dispersion control of MNPs becomes a challenging task that determines the efficiency and the development of new devices and technologies. Regarding this prerequisite, porous solids with rigid frameworks and high surface area have been considered as a good alternative to support and stabilize MNPs, especially those with long-range ordered structures and tunable textural properties, e.g. mesoporous silicas [9], [10], [11]. Due to the confinement effects provided by uniform pore structures, through the manipulation of these textural properties, it should be possible to control the size of supported metal nanoparticles (SMNPs) [12], inhibiting aggregation and producing highly active centers. Several approaches have been published describing the preparation routes to SMNPs, including: (i) wet impregnation of an appropriate metal salt into a functionalized silica host, followed by a reduction procedure to form MNPs [13]; (ii) chemical vapor deposition (CVD) of volatile metal precursors within the silica framework [14]; (iii) electrochemical reduction [15] and (iv) co-precipitation methods, which involve the simultaneous precipitation of the metal and the support [16]. Some drawbacks of the methods above are mainly related to demanding time-consuming surface functionalization reactions (wet impregnation), volatility and the mass-transfer kinetics of metal precursors (CVD). Also an undesirable disordering effect on the porous structure caused by the interference of metallic precursors in the polymerization of the inorganic support (co-precipitation) is a problem to be overcome. Even though these and other strategies have been extensively applied in the synthesis of SMNPs, the development of new methodologies is still necessary in order to have quicker and more convenient methods for this purpose.

This work presents a one-step procedure to prepare supported Ag nanoparticles within the channels of SBA-15, using a very simple route without any surface functionalization and carried out under mild reaction conditions. The method is based on the work of Pastoriza-Santos and Liz-Marzán [17] who introduced the use of N,N-dimethylformamide (DMF) as a reaction medium for synthesis of silver nanoparticles in solution, where DMF acts as both solvent and reducing agent [18].

Section snippets

Synthesis of mesoporous SBA-15

In the preparation of SBA-15 [11], 4.0 g of Pluronic P123 (Mav = 5800, Aldrich) was dissolved in 150 mL of a 1.6 mol L−1 aqueous HCl solution with stirring at 40 °C. Then 8.5 g of tetraethoxysilane (TEOS, Aldrich 98%) was added. Stirring at 40 °C was continued for 8 min. The mixture was aged for 24 h at 40 °C and subsequently submitted to hydrothermal treatment in a Teflon-lined autoclave at 100 °C for 24 h. The solid product was recovered, washed with water and dried at 100 °C for 8 h. Calcination was carried

Results and discussion

The porous structure organization of the prepared SBA-15 mesoporous silica was evaluated by SAXS. The SAXS pattern, presented in Fig. 1, shows three well-resolved peaks that can be indexed as (1 0 0), (1 1 0) and (2 0 0) reflections attributed to P6mm hexagonal symmetry [9], [19]. The corresponding d spacings for (1 0 0), (1 1 0) and (2 0 0) are respectively 102, 59 and 51 Å, in accordance with previously synthesized SBA-15 materials [9], [11], [19].

TEM images of SBA-15 (Fig. 2) confirm the long-range

Conclusion

Ag nanoparticles were successfully prepared in the channels of SBA-15 mesoporous silica by a facile one-step procedure where Ag+ ions are reduced to Ag nanoparticles within the SBA-15 framework.

The UV–vis absorption spectrum suggests the presence of elongated metallic nanostructures in the well structured confined space of SBA-15 mesoporous silica, which was confirmed by TEM. Transmission electronic micrographs also show that silver nanoparticles (spheroids and ellipsoids) are highly dispersed

Acknowledgements

CMM and LPC are indebted to CNPq for Postdoctoral and PhD fellowships. FAS, IOM and YG are indebted to CNPq and FAPESP for financial supports and to Prof. C.H. Collins (IQ-UNICAMP, Brazil) for English revision. Contributions from Brazilian Synchrotron Light Laboratory (LNLS, Campinas–SP, Brazil) for SAXS and TEM (LME-LNLS) analysis are also gratefully acknowledged. This is a contribution of the National Institute of Science and Technology in Complex Functional Materials (CNPq-MCT/Fapesp).

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