Synthesis and Characterization of Noble Metal (Pd, Pt, Au, Ag

19818

J. Phys. Chem. C 2008, 112, 19818–19824

ARTICLES Synthesis and Characterization of Noble Metal (Pd, Pt, Au, Ag) Nanostructured Materials Confined in the Channels of Mesoporous SBA-15 Zhou-jun Wang, Yongbing Xie, and Chang-jun Liu* Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, P. R. China ReceiVed: June 23, 2008; ReVised Manuscript ReceiVed: October 25, 2008

Highly dispersed metal (Pd, Pt) nanoparticles and uniformly distributed metal (Au, Ag) nanowires have been synthesized in ordered mesoporous silica SBA-15 via conventional incipient wetness impregnation followed by novel glow discharge plasma reduction. N2 adsorption-desorption isotherms and the low-angle X-ray diffraction (XRD) patterns indicate that the parent ordered mesoporous structure was well-maintained during the synthesis process. The wide-angle XRD patterns and transmission electron microscope images demonstrate that spherical Pd and Pt nanoparticles as well as rodlike Au and Ag nanowires were fabricated within the channels of SBA-15. The diameters of the metal nanoparticles and the metal nanowires were effectively controlled by the mesopores of the SBA-15 host. The population of the metal nanoparticles and the length of the metal nanowires can be tuned by the metal loading amount. In particular, the novel plasma reduction at ambient temperature is green, economical, and non-time-consuming, showing great advantages over the traditional hydrogen reduction at elevated temperature. This very simple synthesis method with the use of plasma reduction will be very promising as a general technique for the preparation of metal nanostructured materials confined in the host architectures. 1. Introduction The discovery of highly ordered mesoporous silica1,2 has stimulated extensive studies on the synthesis of silica-supported nanostructured materials, which have potential applications in catalysis, gas sensors, nano electronic/optical devices, medical diagnosis, etc.3-6 Mesoporous silica frameworks with uniform pore diameters of 2.0-30.0 nm and high surface areas of 600-1000 m2/g provide an ideal scaffold for the synthesis of metal nanoparticles and nanowires. Up to now, various strategies have been developed for the synthesis of silica-supported nanostructured materials, which can be generally classified into three groups: (1) wet impregnation or ion exchange of the appropriate metal salt into the host silica, followed by the reduction procedure to fabricate metal nanoparticles and nanowires;7,8 (2) chemical vapor infiltration of the volatile metal precursors and their subsequent deposition within the silica frameworks;9 and (3) synthesis of the mesoporous materials around the preprepared metal nanocrystals or using intensive sonication to hammer the preprepared metal nanocrystals into the previously formed silica pores.10 Although these strategies have been extensively applied in the synthesis of silica-supported nanostructured materials, the present state-of-the-art status of the technologies is far from perfect, and the exploration of new preparation methods is still very urgent. In the traditional strategy, such as wetness impregnation or ion exchange, metal salts diffuse easily to the outer surface of the host silica to form large metal aggregates * Corresponding author. Phone: +86 22 27406490. Fax: +86 22 2789 0078. E-mail: [email protected].

during the reduction or thermal treatment process.11-13 To overcome this disadvantage, Chao et al. functionalized the intrachannel surface of the host silica and then fabricated highly dispersed metal nanoparticles via anion exchange.14 Shi and his co-workers developed a new in situ reduction to produce the Pd nanocoating in the channels of selectively modified mesoporous silica SBA-15.15 However, for the above-cited methods, complicated surface functionalization steps are unavoidably required. In the chemical vapor infiltration strategy, the metal precursors are always limited in terms of high volatility, and long-time vapor treatment is usually needed. In the third strategy, presynthesis of metal nanostructured materials and careful encapsulation processes are needed. Herein we present a quick, convenient, and effective method for the synthesis of silica-supported nanostructured materials via conventional incipient wetness impregnation followed by novel glow discharge plasma reduction. Glow discharge is a conventional nonthermal plasma that is characterized by high electron temperature (as high as 10 000-100 000 K) and low gas temperature (as low as room temperature).16,17 The active species induced in the glow discharge plasma can reduce the ionic metals into the metallic states effectively.18,19 The significance of this new synthesis method using plasma reduction is that it is very simple without any surface functionalization step and is energy-efficient, as well, because of the very mild working conditions. In this work, we aim at the fabrication of noble metal nanostructured materials in ordered mesoporous silica using novel glow discharge plasma reduction operated at room temperature. Highly dispersed metal (Pd, Pt) nanoparticles and

10.1021/jp805538j CCC: $40.75  2008 American Chemical Society Published on Web 11/19/2008

Noble Metals in Channels of Mesoporous SBA-15

J. Phys. Chem. C, Vol. 112, No. 50, 2008 19819

Figure 1. N2 adsorption-desorption isotherms of SBA-15 and 2.0 wt % metal/SBA-15 samples: (a) SBA-15, (b) Pd/SBA-15, (c) Pt/SBA15, (d) Au/SBA-15, and (e) Ag/SBA-15.

TABLE 1: Pore Structure Parameters of SBA-15 and Metal/SBA-15 Samples Derived from the N2 Adsorption-Desorption Isotherms and Low-Angle XRD Patternsa sample

SBET (m2/g)b

VBJH (cm3/g)c

DBJH (nm)d

d100 (nm)e

a0 (nm)f

t (nm)g

SBA-15 P-SBA-15h Pd/SBA-15 Pt/SBA-15 Au/SBA-15 Ag/SBA-15

600 597 526 521 507 500

0.90 0.89 0.82 0.82 0.83 0.82

7.7 7.6 7.7 7.7 7.8 7.6

9.2 9.5 9.6 9.6 9.8 9.6

10.6 11.0 11.1 11.1 11.3 11.1

2.9 3.4 3.4 3.4 3.5 3.5

a The metal amount is 2.0 wt %. b BET specific surface area. BJH pore volume. d BJH average pore diameter. e Periodicity of host SBA-15 derived from low-angle XRD. f The unit cell parameter, a0 ) 2d100/3. g The pore wall thickness, t ) a0 - DBJH. h plasma treated SBA-15 sample. c

uniformly distributed metal (Au, Ag) nanowires have been successfully confined in the channels of mesoporous SBA-15. To the best of our knowledge, this is the first report on the use of room temperature plasma reduction for the synthesis of silicasupported nanostructured materials. 2. Experimental 2.1. Synthesis. The SBA-15 silica was synthesized in the laboratory of Prof. D. Y. Zhao (Department of Chemistry, Fudan University, P. R. China).20 To remove physically absorbed water on the surface, it was calcined in air at 500 °C for 3 h before use. The preparation of the metal (Pd, Pt, Au, or Ag)/SBA-15 composite includes only two steps: incipient wetness impregnation and argon glow discharge plasma reduction. The calcined SBA-15 powder was impregnated with an aqueous solution of the metal precursor (PdCl2, H2PtCl6, HAuCl4, or AgNO3) overnight, and the resultant sample was then reduced with glow discharge plasma for 45 min. The glow discharge plasma setup and plasma reduction protocol have been previously described in detail.21 Briefly, the sample (about 0.5 g), loaded on a quartz boat, was placed in the glow discharge cell, which was a quartz tube (i.d. 35 mm) with two stainless steel electrodes (o.d. 30 mm). When the pressure was adjusted to 100-200 Pa, the glow discharge plasma was generated by applying 900 V to the electrodes with a 100 Hz square wave using a high-voltage amplifier (Trek, 20/20B). The signal input for the high-voltage amplifier was supplied by a function/arbitrary waveform genera-

Figure 2. (a) The low-angle XRD patterns for SBA-15 silica before and after the metal loading. (b) The wide-angle XRD patterns of the 2.0 wt % metal (Pd, Pt, Au, Ag)/SBA composite.

Figure 3. The low-angle XRD patterns for SBA-15 samples before and after plasma treatment. P-SBA-15 denotes the plasma-treated SBA15 sample.

tor (Hewlett-Packard, model 33120A). The current was 4.5 mA. Ultrahigh pure grade argon (>99.999%) was used as the plasma-

19820 J. Phys. Chem. C, Vol. 112, No. 50, 2008

Wang et al.

Figure 4. TEM results for the 2.0 wt % metal/SBA-15 samples: (a) typical TEM image of Pd/SBA-15 composite, (b) typical TEM image of Pt/SBA-15 composite, (c) typical TEM image of Au/SBA-15 composite, (d) typical TEM image of Ag/SBA-15 composite, (e) high-resolution TEM image of Pd nanoparticle confined in SBA-15, and (f) high-resolution TEM image of Au nanowire confined in SBA-15.

forming gas. The bulk temperature of the plasma was measured by infrared imaging (Ircon, model 100PHT), indicating that the plasma reduction was conducted at ambient temperature. The metal amount is 2.0 wt % unless otherwise mentioned.

2.2. Characterizations. N2 adsorption-desorption isotherms were measured at 77 K on a Micromeritics Tristar 3000 analyzer. Before the measurements, the samples were outgassed at 300 °C for 6 h. The Brunauer-Emmett-Teller (BET) method was



Figure 6. The wide-angle XRD patterns of Pd/SBA-15 composites with different Pd loadings.

Halenda (BJH) model. X-ray diffraction (XRD) patterns were collected on a Rigaku D/MAX-2500 V/PC using Cu KR radiation (λ ) 0.154056 nm) at 40 kV and 200 mA. The lowangle XRD patterns were collected at a scanning speed of 1°/min over a 2θ range of 0.5-6°, whereas the wide-angle XRD patterns were collected at a scanning speed of 4°/min over a 2θ range of 10-90°. Transmission electron microscope (TEM) images were recorded on a Philips Tecnai G2 F20 system operated at 200 kV. The powder samples were sonicated in ethanol for 10 min and dropped and dried on the carbon-coated copper grids. X-ray photoelectron spectroscopy (XPS) was performed on a Perkin-Elmer PHI-1600 spectrometer with Mg KR (1253.6 eV) radiation. The binding energy was calibrated from the C1s peak (284.6 eV) of the surface adventitious carbon. 3. Results and Discussion

Figure 5. Typical TEM images of Pd/SBA-15 composites with different Pd loadings: (a) 0.5 wt % Pd/SBA-15; (b) 1.0 wt % Pd/SBA15; (c) 5.0 wt % Pd/SBA-15.

employed to calculate the specific surface areas. The pore volume and pore size distributions were derived from the adsorption branches of the isotherms using the Barrett-Joyner-

During plasma reduction, the samples showed a distinct change in color. Pd/SBA-15 powders turned from earth yellow to light gray, whereas Pt/SBA-15 powders changed from pale yellow to dark gray, Au/SBA-15 powders from pastel yellow to purple brown, and Ag/SBA-15 powders from pure white to gray green. These significant color changes presented the direct evidence for the formation of metal nanostructured materials in SBA-15. The metal/SBA-15 composites, together with the calcined SBA-15 silica, were characterized by N2 adsorption-desorption measurements. Figure 1 showed that the isotherms for SBA-15 and metal/SBA-15 were of type IV and exhibited a hysteresis loop of H1 type according to the IUPAC classification,22 typical for materials with pores of constant cross section, which indicated that the parent mesoporous structure was wellmaintained during the synthesis process. The pore structure parameters of SBA-15 and metal/SBA-15 samples were summarized in Table 1. The BET surface areas and pore volumes decreased slightly after synthesis, which suggested the incorporation of metal nanoparticles or nanowires into the pore channels of SBA-15.23,24 The influence of the loading of the metal was also investigated using low-angle XRD (Figure 2a). The synthesized metal/SBA15 samples displayed one major peak at about 0.92° together with two additional peaks, characteristic of the well-ordered hexagonal (p6mm) structure in SBA-15. By carefully comparing the patterns in Figure 2a, we can observe that, after synthesis, all peaks were

19822 J. Phys. Chem. C, Vol. 112, No. 50, 2008

Figure 7. Typical TEM images of Au/SBA-15 composites with different Au loadings: (a) 0.5 wt % Au/SBA-15; scale bar, 20 nm; (b) 1.0 wt % Au/SBA-15; scale bar, 50 nm; and (c) 5.0 wt % Au/SBA-15; scale bar, 100 nm.

shifted slightly to lower angles, corresponding to a slight increase of d100, as listed in Table 1. This shift suggested that the frameworks

Wang et al. of silica host were slightly enlarged during the synthesis process. To investigate whether this effect is caused by plasma reduction, we compared the low-angle XRD patterns for SBA-15 samples before and after plasma treatment. A similar shift can be clearly observed from Figure 3, which indicates that there may be a structural transformation during plasma treatment, enlarging the frameworks of the silica host by 05-0.6 nm. The wide-angle XRD patterns for metal/SBA-15 samples are shown in Figure 2b. From the patterns of Pd/SBA-15, four Pd diffraction peaks can be seen at 2θ ) 40.12°, 46.45°, 68.20°, and 81.69°, assigned to (111), (200), (220), and (311) reflections of the cubic Pd lattice, respectively (JCPDS card, File No.46-1043), as well as a very broad signal at 2θ ) 22.47° for amorphous silica. According to the Scherrer equation, the average particle size of Pd is calculated to be 8.5 nm, which is close to the pore diameter of the host SBA-15 (dav ) 7.8 nm). Similar results can be obtained from the patterns of Pt/SBA15; however, the patterns of Au/SBA-15 and Ag/SBA-15 exhibit much different characters. The diffraction peaks of Au/SBA15 and Ag/SBA-15 are much more intense and narrower than those of Pd/SBA-15 and Pt/SBA-15, which suggests different metal nanostructures in the channels of the SBA-15 host. This difference can further be confirmed by the TEM images. TEM analysis results of metal/SBA-15 composites are shown in Figure 4. The representative TEM images reveal that (i) the SBA-15 silica materials have a well-ordered mesoporous channel structure, which supports the aforementioned N2 sorption and low-angle XRD results; (ii) spherical Pd and Pt nanoparticles were highly dispersed in the interior of the SBA15 channels (Figure 4a, b). The average Pd and Pt nanoparticle sizes were determined to be 8.2 and 7.8 nm, respectively, with a narrow distribution (i.e., monodispersed) (it is surmised that the pore channels can effectively control the scale of particles formed inside the host); (iii) well-distributed metal (Au, Ag) nanowires were fabricated in the channels of SBA-15 (Figure 4c, d). The metal nanowires appear as dark, rodlike objects between the walls of SBA-15. The average diameter of each nanowire is 8.1 nm, which is in agreement with the pore diameter of the SBA-15 host. This agreement indicates that the silica host can control the wire diameter effectively. The lengths of the wires range from 50 to a few hundred nanometers, which is expected to be tuned by the metal loading amount. To directly observe the atomic structures of metal nanoparticles and nanowires within the pore channels of SBA-15, highresolution TEM analysis of Pd nanoparticles and Au nanowires, respectively, was performed. In Figure 4e, the lattice fringes with d ) 0.224 nm are clearly visible, which can be attributed to the (111) planes of Pd. In Figure 4f, it can be observed that the Au nanowire fabricated in SBA-15 consists of many small nanocrystals. Clear lattice fringes with d ) 0.235 and 0.204 nm can be observed, which are attributed to the (111) and (200) family of Au planes, respectively. We also investigated the metal loading effect for this new synthesis method by varying the Pd and Au loading from 0.5 to 5.0 wt %. The representative TEM images of Pd/SBA-15 composites (Figure 5) demonstrate that spherical Pd nanoparticles were uniformly dispersed in the channels of the SBA-15 host, with average particle sizes of 8.2, 8.3, and 8.3 nm for 0.5, 1.0, and 5.0 wt % samples, respectively. It well-documented that the metal nanoparticle size could be effectively controlled with SBA-15 as the host, which was also confirmed by the wideangle XRD results based on the Scherrer equation (Figure 6). For Au/SBA-15 composites, TEM images (Figure 7) show that Au nanowires grew along the silica pore channels. The diameter



Figure 8. XPS spectra for the plasma-reduced metal/SBA-15 composites: (a) Pd/SBA-15, (b) Pt/SBA-15, (c) Au/SBA-15, and (d) Ag/SBA-15.

TABLE 2: Data and Assignments of XPS Spectra for the 2 wt % Metal/SBA-15 Samples sample

transition peak

BE (eV)

assignment

ref

Pd/SBA-15 Pt/SBA-15 Au/SBA-15 Ag/SBA-15

Pd 3d5/2 Pt 4f7/2 Au 4f7/2 Ag 3d5/2

335.2 71.3 83.7 368.0

Pd Pt Au Ag

26 27 28 29

of the nanowires was effectively controlled by the channels of the SBA-15 host, whereas the length of the nanowires could be tuned by the metal contents. The nanowire length increased from 25 to several hundred nm as the Au loadings increased from 0.5 to 5.0 wt %. These results indicate that (i) the diameter of the metal nanoparticles and the metal nanowires can be tailored according to the pore diameter of the SBA-15 host (the pore diameter of the SBA-15 materials can be easily tuned by altering the temperature of the postsynthesis hydrothermal restructuring procedures25) and (ii) the population of the metal nanoparticles and the length of the metal nanowires can be tuned by changing the metal loading amount. The chemical state of the metal in metal/SBA-15 has been examined by the XPS test. For all the metal species, two symmetrical peaks are observed, and no other peaks can be deconvoluted (Figure 8). The data and the assignments of XPS spectra for the metal/SBA-15 composites are listed in Table 2.26-29 These results definitely indicate that the plasma reduction can reduce the ionic metals completely into the metallic states.

The plasma reduction is the key step for this new synthesis method. We propose that the mechanism of plasma reduction may be related to two processes. One is a direct process, in which the high-energy electrons generated by the plasma can directly reduce the metal ions through a recombination route. Another is an indirect process. At room temperature, the silica frameworks contain moisture, as reported for the MCM-48.30 The high-energy species generated by the plasma may induce the dissociation and ionization of the water molecules to afford active species, such as hydrated electrons eaq- and H• radicals, which are very strong reducing agents31 and can easily reduce the metal ions to the metallic nanoparticles or nanowires. The different morphologies of the nanostructured metal materials (nanoparticles for Pd and Pt, nanowires for Au and Ag) may be attributed to the intrinsic properties of the different metals, such as surface energy, redox potential, etc. 32,33 Further investigation is being conducted for a better understanding. Plasma reduction at ambient temperature provides a convenient and effective process for the synthesis of silica-supported nanostructured materials. From the calculation of energy consumption, the energy required for the plasma reduction is around 38% of that required for the hydrogen reduction at elevated temperature. Moreover, because plasma reduction does not require expensive hydrogen or other hazardous chemical reducing agents, the material cost of the plasma reduction is very low (around 1% of that required for hydrogen reduction

19824 J. Phys. Chem. C, Vol. 112, No. 50, 2008 at elevated temperature). In addition, plasma reduction at ambient temperature is simple, green, and non-time-consuming. 4. Conclusion We first employ the novel grow discharge plasma reduction for the synthesis of noble metal (Pd, Pt, Au, Ag) nanostructured materials in mesoporous silica SBA-15. Highly dispersed metal (Pd, Pt) nanoparticles and uniformly distributed metal (Au, Ag) nanowires can be conveniently and effectively synthesized with this new method. Spherical Pd and Pt nanoparticles as well as rodlike Au and Ag nanowires were fabricated within the channels of SBA-15. The diameters of the metal nanoparticles and the metal nanowires can be tailored according to the pore diameter of the SBA-15 host. The population of the metal nanoparticles and the length of the metal nanowires can be tuned by varying the metal loading amount. Importantly, the novel plasma reduction operated at ambient temperature is much more energy-efficient and economically effective than the traditional hydrogen reduction at elevated temperature. Initial results indicate that the plasma-reduced Pd/SBA-15 sample is very suitable for Suzuki coupling reactions. The significance of this new synthesis method using plasma reduction is that it is very simple without any surface functionalization and energy-efficient due to a mild working condition. It is expected that this quick, convenient, and effective method will be very promising as a general technique for the synthesis of metal nanoparticles or nanowires confined in the host architectures, which will be both fundamentally and technologically useful for catalytic, optical, electronic, magnetic, and energy-storage applications. Acknowledgment. Support from the National Natural Science Foundation of China (#20490203) and the instruments supplied by ABB Switzerland are greatly appreciated. Supporting Information Available: The details of energy consumption and material cost evaluation. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548.

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Synthesis and Characterization of Noble Metal (Pd, Pt, Au, Ag

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