J. Phys. Chem. C 2007, 111, 5331-5336
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Anchoring of Silver Nanoparticles on Graphite and Isomorphous Lattices Rita Patakfalvi,† David Diaz,*,† Patricia Santiago-Jacinto,‡ Geonel Rodriguez-Gattorno,†,| and Roberto Sato-Berru§ Facultad de Quı´mica, Instituto de Fı´sica, y Centro de Ciencias Aplicadas y Desarrollo Tecnolo´ gico, UniVersidad Nacional Auto´ noma de Me´ xico, Coyoaca´ n, 04510, Me´ xico D.F., Me´ xico, and Departamento de Fı´sica Aplicada, Centro de InVestigaciones AVanzadas del IPN, Me´ rida 97310, Yucata´ n, Me´ xico ReceiVed: December 14, 2006; In Final Form: February 9, 2007
Recently, multiwalled N-doped carbon nanotubes were decorated with Ag nanoparticles (NPs) by a simple procedure. Now, as a continuation of our earlier work, Ag NP-graphite and Ag NP-molybdenite composites were prepared with this easy, fast, and inexpensive method. Graphite or molybdenite (MoS2) powder was suspended in N,N′-dimethylformamide (DMF) in the presence of different silver salts. As DMF is a good reducing agent of silver cations, in situ formation of Ag NPs took place in the suspension. We supposed that the Ag NP formation is possible on the surface of the supports due to the interaction between the unoccupied silver orbitals and the π-electrons of graphite or the 3p sulfur orbitals of MoS2. This is the first report about the Ag NP-MoS2 nanocomposites synthesis. A complete spectral and structural characterization of these composites was accomplished by UV-vis and Raman spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, and particle charge detector. According to these measurements, nanocomposites were formed depending on the original silver salt, support, surface charge, and stabilizing agent.
Introduction Recently, Ag nanoparticles (NPs) have been anchored successfully onto the surface of nitrogen-doped multiwalled carbon nanotubes by our group.1 Since, there are no any simple syntheses of metallic NP-graphite composites, we decided to test the previously applied method. Ag NP-MoS2 composites could be also interesting in the catalysis field; then, this synthesis pathway was also using with molybdenite as supporting material. Because of the multilayered structure of graphite, a wide variety of intercalated graphite compounds can be synthesized. One possibility to prepare transition-metal NP-graphite composites (Pd, Pt, Rh) is the reduction of the convenient metal chloride intercalated compounds.2,3 The advantage of these methods is that the particles could be formed between the graphite layers, stabilizing therefore the particles and controlling their size. Pt nanosheets were formed between graphite layers by reduction of PtCl4-graphite intercalation compounds.2 Twodimensional Pd NPs with a wide size distribution were prepared by a similar method; the two-dimensional character of the NPs was confirmed by H2 sorption experiments.3 However, the intercalation process requires some special reaction conditions: high temperature and pressure. Pd nanoparticles with a narrow size distribution were prepared on a graphite fiber surface by the impregnation method.4 But in many cases this method results in a broad particle size range.5 Other pathways for preparing metallic NPs on the graphite surface are electrochemical methods6,7 and metal evaporation.8 Controlling the deposition over potential, different size Ag nanoparticles can be synthesized.7 * To whom correspondence should be addressed. E-mail:
[email protected]. Phone and Fax: +52 55 5622-3813. † Facultad de Quı´mica, Universidad Nacional Auto ´ noma de Me´xico. ‡ Instituto de Fı´sica, Universidad Nacional Auto ´ noma de Me´xico. § Centro de Ciencias Aplicadas y Desarrollo Tecnolo ´ gico, Universidad Nacional Auto´noma de Me´xico. | Centro de Investigaciones Avanzadas del IPN.
Metallic NP-graphite composites have various applications as catalysts.4,5,9-11 Different types of graphite nanofibers, as support for Ni particles, were studied in the catalytic hydrogenation of croton aldehyde to crotyl alcohol.5 Enhanced activity and selectivity was observed using a graphite support in comparison with Ni/alumina catalyst. Graphite nanofiber supported Pd nanoparticles was found to be a selective catalyst for alkene hydrogenation.4 Polycrystalline silver impregnated on graphite, in low concentrations, is mainly utilized as electric contacts.12 Moreover, MoS2 is a semiconductor with layered structure. Each layer consists of three, hexagonally arranged, atomic sheets: one Mo sheet between two sulfide sheets. These sheets are connected by strong covalent bonds, whereas the layers are held together only by weak van der Waals forces. This is the reason for the low molybdenite friction, and that is why MoS2 is a common solid lubricant. MoS2 has many other important applications, as catalyst in the hydrodesulphurisation13,14 and methanation processes15,16 and as semiconductor in photocatalysis.17,18 Tenne and co-workers observed first that on the analogy of carbon fullerenes, hollow structures and nanotubes of transition metal dichalcogenides could be prepared.19,20 These new structured nanomaterials got the general inorganic fullerenelike materials (IF) acronym. Among these materials MoS2 and WS2 have the main interest due to their excellent physical and chemical properties.21-23 Many times, after addition of a transition or alkali metal to the surface of MoS2, the catalytic activity can be enhanced. This was the motivation to study the chemical and electronic properties of silver atoms on MoS2 surface.24 The deposited silver atoms remain in metallic state below 300 K and above 400 K Ag atoms diffuse into the MoS2 forming AgMoSx compounds. It is important to state that the synthesis and applications of metallic NP-MoS2 compounds are not found in the literature.
10.1021/jp068609v CCC: $37.00 © 2007 American Chemical Society Published on Web 03/20/2007
5332 J. Phys. Chem. C, Vol. 111, No. 14, 2007 Recently, N. Kotov25 has suggested that “the graphene composites could be very useful: for example, in the manufacture of fuselages for aircraft, which must combine low weight, high strength, and electrical conductivity. This last property is necessary for protection against lightning strikes while in flight. Nevertheless, the conductivities of these composites26 are still several orders of magnitude lower than those of the best examples of nanotube mats (which are made entirely of nanotubes).” Some advantages of the graphene-sheet composites are their low cost and the plentiful supply of graphite. Taking in account this point of view, we have nanocomposites made of disaggregated graphite or molybdenite and silver nanoparticles as materials with great potential in this area. We present here a very simple but successful preparation method for Ag-graphite and MoS2 nanocomposite synthesis, under mild reaction conditions. The reaction does not require high temperature, high energy, pressure, high vacuum, ball milling,27 electricity, or long reaction periods. In our case the reaction goes fast and easy. Spontaneous reduction of Ag(I) ions takes place in N,N′dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) at room temperature,28,29 and Ag NPs with narrow size distribution are formed using stabilizing agents. In this work, we assumed that the π-electrons of graphite and the 3p sulfur orbitals of MoS2 might have a strong interaction with the 5p unoccupied orbitals of the silver NPs; therefore, the formation of Ag nanoparticles on the surface is possible. Experimental Section Materials. During the synthesis the following materials and methods were used: silver nitrate (AgNO3, Aldrich, 99%), silver 2-ethylhexanoate (Ag(ethex), Strem Chemicals, 99%) as precursors, and trisodium citrate dihydrated (Aldrich, 99%) used as a stabilizing agent. The solid supports were graphite (B & A Chemicals) or molybdenite (MoS2, Aldrich, 99%). The solvent was DMF (Baker 99.9%) and was heated at 80 °C, during 40 min and bubbled simultaneously with Ar (PRAXAIR, 99.999%) before being used. Ag powder (Strem Chemicals, 99.9%) was used for Raman measurement. Preparation of Samples. Graphite or MoS2 powders were suspended in DMF and disrupted using a high-power sonicator until we got a suspension containing a very fine fraction of the particles. The graphite and MoS2 suspensions were centrifuged (at 2000 rpm, during 20 min) and washed with acetone. After drying, this fine fraction was used in the further experiments. The graphite and MoS2 supported Ag nanoparticles were prepared by 3 variants of this general method. Method 1. The necessary quantity to prepare a 10-4 M Nacitrate solution was dissolved in a suspension consisting of 0.002 g of graphite or MoS2 in 50 mL of DMF. After the complete dissolution, an adequate aliquot of silver 2-ethylhexanoate solution (10-4 M) was added to the suspension under vigorous stirring. The suspension was heated at 60 °C during 20 min. After a few minutes, the yellowish suspension color indicates the Ag NPs formation. The stirring was continued for 4 days. During this time the suspension was protected from light to avoid the photochemical synthesis of Ag NPs. The suspensions were centrifuged (at 2000 rpm), washed with acetone, and dried. The samples were denoted as AgG1 (graphite support) and AgM1 (MoS2 support). Method 2. AgNO3 solution (final concentration 10-4 M) was added to 0.002 g graphite or MoS2/50 mL of DMF suspension under vigorous stirring. The reaction conditions (temperature,
Patakfalvi et al. heating time, etc.) were the same as in Method 1. The samples were denoted as AgG2 and AgM2, respectively. Method 3. The same procedure as in Method 1 was followed but using AgNO3. The resulting samples were labeled as AgG3 and AgM3. Characterization. The Braun-Sonic-U and Cole-Parmer ultrasonic equipments were used to prepare the graphite and MoS2 suspensions. UV-visible absorption spectra were obtained by means of an Ocean Optics CHEM2000 fiberoptic spectrophotometer before centrifuging the samples. High-resolution transmission electron micrographs (HRTEM) were obtained using a JEM FASTEM 2010 instrument, operating at 200 kV. Fast Fourier transform (FFT) of HRTEM micrograph images was done by means of DigitalMicrograph GATAN v-3.7.0 software. X-ray diffraction (XRD) measurements were taken on a Siemens D5000 equipment using Cu KR radiation (20 mA, 40 kV, λ ) 1.5418 Å). The electric potentials on the particles surfaces were measured in a Mu¨tek PCD 03 Particle Charge Detector. An Almega XR Dispersive Raman spectrometer equipped with an Olympus microscope (BX51) was used to obtain Raman spectra. An Olympus ×10 objective (N.A. ) 0.25) was used both for focusing the laser on the sample, with a spot size ∼3 µm, and collecting the scattered light in an 180° backscattering configuration. The scattered light was detected by a charge coupled device (CCD) detector, thermoelectrically cooling to -50 °C. The spectrometer used a grating (675 lines/mm) to resolve the scattered radiation and a notch filter to block the Rayleigh light. The pinhole of the monochromator was set at 25 µm. Raman spectra were accumulated over 25 s with a resolution of ∼4 cm-1. The excitation source was 532 nm radiation from a Nd:YVO4 laser (frequency-doubled) and the incident power on the sample was of ∼10 mW. XRD, HRTEM, and Raman spectroscopy measurements were carried out for all the resulting samples after centrifuging and removing the free Ag NPs. Results and Discussion During the synthesis of Ag-graphite nanocomposites the evolution of the UV-visible absorption spectra of the suspensions was monitored and the formation of Ag NPs was confirmed by the appearing of the characteristic plasmon absorption band of the Ag NPs (Figure 1a). When Na-citrate was used as stabilizer at AgG1 sample, the Ag plasmon band was narrow with a maximum centered at 412 nm. However, the corresponding spectrum of the AgG3 sample has also a shoulder at higher wavelength (493 nm). It was found for Au NPs a second absorption band at higher wavelengths corresponding to the formation of aggregated particles30 or a second population of Ag particles with a larger size, while without any stabilizer (AgG2) the plasmon band was very broad with low intensity. After the synthesis, the supernatants of the suspensions were also analyzed with UV-vis spectroscopy and Ag NPs were present. This is an indication that NPs were formed on the graphite surface and in the solvent. Since in the case of AgG2 the plasmon intensity was very low, we propose that more Ag NPs are deposited on the support surface. (The absence of Ag(I) ions in the supernatant was confirmed, all of the Ag(I) ions were reduced). XRD patterns confirm this assumption (Figure 1b). The Ag(111) reflection at 38.1° indicates the existence of metallic silver in these composites. We took for granted that the AgG1 sample has the lowest silver content because this sample had the lowest Ag (111) reflection intensity. Comparing the surface charge of the particles in the reaction suspensions,
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Figure 1. (a) UV-vis absorption spectra of the different Ag-graphite suspensions in DMF after 4 days. For the starting Ag (I) concentrations, see the experimental section. (b) XRD patterns of graphite and the different Ag-graphite composites.
Figure 2. HRTEM micrographs of the different Ag NP-graphite composites with the FFT of graphite and Ag nanoparticles.
Figure 3. TEM micrograph and silver particle size distribution of AgG2 and AgG3 samples.
the results are as expected. The graphite surface has relatively small negative charge in DMF (-285 mV streaming potential was measured for pure DMF and -341 mV for the graphite suspension). When this potential is applied to a AgNO3 and graphite suspension with no stabilizer, Ag(I) ions bind to the graphite sheets, thus the reduction process takes place on the surface (AgG2). In this case, due to the absence of stabilizer species is possible to find large particles. However, if Na-citrate and 2-ethylhexanoate ions are present in the solution, Ag(I) ions and the forming Ag NPs have more affinity to the carboxyl group. Na-citrate is a very good stabilizer of the Ag NPs, but these NPs will have a more negative charge (due to the presence of the two kinds of carboxylate anions). The majority of these highly negatively charged NPs could not bind to the graphite surface; therefore, the particle formation is more favorable in the solvent phase. HRTEM images agree with the previous inferences and XRD patterns confirm that AgG2 sample has
higher silver content, in comparison with the other two samples. There are small Ag NPs with 3.6 ( 1.3 nm average diameters (Figure 2) in the case of the AgG1 sample. But in this image the metallic particles are not on the graphite surface. We can see only discrete Ag NPs or silver free graphite nanosheets. The HRTEM images of AgG2 samples are different (Figure 2). Without any stabilizer the average diameter of the Ag NPs is larger and the size distribution is wider (10.1 ( 5.5 nm, see Figure 3). Additionally, the Ag NPs are located on the graphite surface as we can see on the image. There are smaller NPs (7.6 ( 2.8 nm) and also on the graphite surface in the AgG3 sample (Figure 3). But according to the X-ray diffractograms (Figure 1b) the amount of silver in the AgG3 sample is smaller than in AgG2, but larger than in AgG1. The reason is that in AgG3 only the citrate molecules stabilize the Ag NPs giving less negative charge in comparison with AgG1 (see Table S1 of the Supporting Information). The Ag NPs could form in the solvent
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Figure 4. Raman spectra of (a) graphite and the Ag NP-graphite composites and (b) MoS2 and the Ag NP-molybdenite composites.
Figure 5. (a) UV-vis absorption spectra of the different Ag-molybdenite suspensions in DMF. (b) XRD patterns of molybdenite and the different Ag-molybdenite composites.
phase, but due to the interaction between the unoccupied 5p orbitals of silver and the graphite π electrons, Ag NPs are found on the graphite surface also. Raman spectroscopy was used to decide if any charge-transfer interaction takes place between the metallic NPs and the graphite support.31 The well-crystallized, pure graphite has a Raman band in the first-order region at ∼1580 cm-1, this is the so-called G band.32,33 Our original graphite sample shows this band at 1570 cm-1 (Figure 4). If the well ordered structure is disrupted, a second band appears at ∼1350 cm-1 (defect or D band).32,33 In the case of AgG1, the position of the G band is not changing (1568 cm-1), but we can see the appearance of the D band at 1322 cm-1. However, its intensity is very low and the band is very broad. We assume that the formation of the Ag NPs is decreasing the ordering of the graphite structure. AgG2 shows the D band at 1341 cm-1 and the G band at 1570 cm-1. The intensity of the D band is higher, almost half of the G band. The two bands overlap. The higher intensity of the D band is indicative of a higher effect of the formed Ag NPs on the lamellar structure. While the G bands of graphite and AgG1 are very symmetric, AgG2 shows an asymmetric band with a shoulder around 1600 cm-1. The shifting of the D band to higher wave numbers suggests a charge-transfer process,31 that is, the presence of a chemical interaction between the Ag NPs and the graphite surface. These results agree with the HRTEM images; AgG2 is a nanocomposite. The Raman spectrum of AgG3 also has a symmetric G band at 1566 cm-1 and a well-separated D band of less intensity at 1346 cm-1. The reason of this might be again the effect of the Ag NPs.
The same composite preparation procedure was performed with the MoS2 support. The UV-vis spectra of the suspensions show similar results as those for graphite (Figure 5a). In the presence of stabilizing agents (AgM1 and AgM3), the characteristic silver plasmon band is centered at 415 nm with high intensity (Amax ) 1.43 (AgM1) and 1.03 (AgM3)). The plasmon of AgM1 is more symmetric and narrower, indicating the presence of smaller NPs. The corresponding AgM2 plasmon band has very low intensity, Amax ) 0.16. In this case, the formation of a Ag mirror was observed. The supernatants of the AgMn suspensions show the formation of Ag NPs in every case. XRD patterns were used as criterion to decide if Ag NPs were also formed on the support or not (Figure 5b). The Ag reflections can be observed only on the AgM2 pattern and silver was not detected in the other two samples. However, the presence of Ag NPs was confirmed by HRTEM in all cases (Figure 6). It is possible that the concentration of the NPs was below the detectable limit of XRD; 3.9 ( 0.7 nm was the average Ag NPs diameter for the AgM1 sample (Figure 7). In agreement to the HRTEM micrographs Ag NP-MoS2 nanocomposites were observed in all cases. The surface particle charges were also checked for these samples. The MoS2/DMF suspension has also a negative streaming potential (-360 mV), but we should take into account the high affinity of the Ag atoms toward sulfur atoms. This property favors the formation of Ag NPs on the MoS2 surface. The Ag NP-MoS2 composites were also studied by Raman spectroscopy (Figure 4b). The characteristic bands of MoS234 are located at 381 and 407 cm-1. However, in the case of Ag NP-molybdenite composites, the samples showed two new
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Figure 6. HRTEM micrographs of the different Ag NP-molybdenite composites with the FFT of MoS2 and Ag nanoparticles.
carbon contamination is also higher, so the graphite-like bands will be more intense. These results concur with the XRD and UV-visible spectroscopy measurements. Conclusion
Figure 7. TEM micrograph and silver particle size distribution of AgM1 sample.
Figure 8. Raman spectra of a glass slide, silver film prepared from AgNO3 in DMF and commercial Ag powder.
bands at 1358 and 1580 cm-1, which correspond to a graphitic structure (D and G peaks). Surface enhanced Raman spectroscopy (SERS) of different organic molecules adsorbed on Ag surface often shows Raman bands near 1350 and 1550 cm-1. It has been suggested that the origin of these bands is a graphitic contamination on the Ag surface.35-37 It should be noted that extremely low contaminants contents could be detected over silver cluster surfaces as result of SERS phenomena.38 We detected this graphite layer on Ag thin film even in a commercial silver powder sample (Figure 8). Accordingly, we propose that the presence of the D and G graphite bands of the Raman spectra of MoS2 supported samples is a proof of the Ag content. Moreover, we observed a slight change in the Raman bands of MoS2 because of the Ag NPs. The ratio of the intensities of graphite and MoS2 bands is the highest for AgM2 and the lowest for AgM1. Our opinion is that, if the Ag content is higher, the
In this study, Ag NPs were prepared on the surface of graphite and MoS2 with an innovating, simple synthesis method. The reaction was followed by UV-vis spectroscopy detecting the characteristic plasmon absorption band of the Ag NPs. XRD measurements confirmed the presence of Ag NPs in the graphite samples; however, the silver concentration in some Ag/MoS2 sample was below the detection limit. The calculated reflections of FFT images obtained from HRTEM micrographs agreed to the Ag, graphite, and MoS2 structure, accordingly the composite preparation was successful. The initial silver salt and the stabilization agent had an effect on the NPs size. Smaller Ag NPs with narrow size distribution were formed using Ag(ethex) and Na-citrate. However, a larger amount of the Ag NPs formed on the support surface using only AgNO3 as precursor salt, because of the small negative charge of the graphite and molybdenite surface in DMF. In conjunction with these and the HRTEM results, Raman spectra showed that depending on the silver salt nanocomposites could be prepared. To the best of our knowledge, this is the first literature report related to novel Ag NPs-molybdenite nanocomposites synthesis. Study of the electric and catalytic properties of the prepared composites is in progress. Acknowledgment. R.P. thanks to DGAPA UNAM for the postdoctoral fellowship. Authors thank Cecilia Salcedo-Luna for her technical assistance and the USAI facilities of the UNAM-Department of Chemistry. We also thank L. Rendon for HRTEM observation assistance. The authors are also thankful to the Central Microscopy facilities of the Institute of Physics, UNAM. D.D. wants to thank to CONACyT E43662 and DGAPA UNAM IN110405 for financial support. Supporting Information Available: Description of the operation of Mu¨tek PCD 03. Syntheses of support free Ag NPs. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Zamudio, A.; Elı´as, A. L.; Rodrı´guez-Manzo, J. A.; Lo´pez-Urı´as, F.; Rodrı´guez-Gattorno, G.; Lupo, F.; Ru¨hle, M.; Smith, D. J.; Terrones, H.; Dı´az, D.; Terrones, M. Small 2006, 3, 346. (2) Shirai, M.; Igeta, K.; Arai, M. Chem. Commun. 2000, 623. (3) Mastalir, A Ä .; Kira´ly, Z.; Walter, J.; Notheisz, F.; Barto´k, M. J. Mol. Catal. 2001, 175, 205.
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