Letter pubs.acs.org/Langmuir
Synthesis of Diazenido-Ligated Vanadium Nanoparticles Mariko Miyachi,† Yuki Yamamoto,† Yoshinori Yamanoi,† Ai Minoda,‡ Shinji Oshima,‡ Yoshihiro Kobori,*,‡ and Hiroshi Nishihara*,† †
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Hydrogen R&D Group, Hydrogen & New Energy Research Laboratory, Research & Development Division, JX Nippon Oil & Energy Corporation, 8 Chidori-cho, Naka-ku, Yokohama 231-0815, Japan
‡
S Supporting Information *
ABSTRACT: Metallic vanadium nanoparticles stabilized with 4-octylphenyldiazenido groups (particle size: 1.7 ± 0.2 nm) were synthesized via the reduction of VCl4 with superhydride (LiBHEt3) in the presence of 4octylphenyldiazonium salt in an Ar-filled glovebox. The resulting particles were characterized using TEM, elemental analysis, and XPS measurements. The unusual reaction on the surface resulted in the passivation of V−N N−Ar covalent bonds.
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INTRODUCTION The synthesis of nanometer-scale metal nanoparticles is currently an active research area as a result of the potential of these nanoparticles for applications in sensors, optical switches, and nanoelectronic devices.1,2 The metallic surfaces of early transition metals (groups IV−VI) are especially attractive for hydrogen storage materials3 and catalysts.4 Generally, earlytransition-metal chlorides are reacted with n-BuLi or K(BHEt3) to form black colloids.5−7 However, the practical use of these particles is limited because they are easily oxidized in air, and the creation of nanoparticles containing these elements is a significant synthesis challenge. Historically, aryldiazonium salts have served as reagents in the Sandmeyer reaction. The grafting of molecular layers from solutions of aryldiazonium salts has recently been investigated for the modification of surfaces, and several groups have reported the formation of aryl monolayers on metal or metal oxide particle surfaces via the reduction of diazonium salts.8−17 The reduction of the aryldiazonium cation on the metal surface eliminates N2, forming the corresponding aryl radical that appears to attack the substrate to form metal−carbon covalent bonds. The attachment of aryldiazonium radicals on metal substrates, followed by the loss of N2, has also been proposed as an alternative pathway (Scheme 1a). For example, Chen et al. reported the preparation of titanium nanoparticles stabilized with Ti−C bonds via the reduction of TiCl4 in the presence of aryldiazonium salts (Scheme 1b).18 However, little attention has been paid to vanadium nanoparticles passivated by organic surfactants.19,20 Previously, we reported the synthesis of vanadium-doped palladium nanoparticles stabilized with n-pentyl isocyanide.21 Although we tried to synthesize vanadium nanoparticles stabilized with alkyl isocyanides, isocyanides were not effective for the stabilization of the early-transition-metal surface.22 Inspired by the above-mentioned reports, we designed a new method for © 2013 American Chemical Society
the preparation of vanadium nanoparticles using the reduction of VCl4 with superhydride in the presence of aryldiazonium salt. In contrast to the results obtained with titanium, there was a significant difference in the behavior of the diazonium salts on the vanadium metal surface (Scheme 1c).
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METHODS AND MATERIALS
Materials. Vanadium(IV) chloride, superhydride, 4-octylaniline, HBF4, NaNO2, ethanol and anhydrous acetonitrile were purchased from commercial sources. These materials were used as received without further purification. THF was purchased from commercial sources and then distilled from sodium benzophenone ketyl radical and stored over 4A molecular sieves before use. MilliQ water was supplied by AUTOPURE WD500 system. Synthesis of 4-Octylphenyldiazonium Tetrafluoroborate (1·BF4). This compound was prepared using a modified method based on the literature.23 4-Octylaniline (2.08 g, 10.1 mmol) was dissolved in MilliQ water (100 mL) and ethanol (60 mL). Then, 48 wt % HBF4(aq) (4.6 mL) and NaNO2 (2.00 g, 29 mmol) were added at −5 °C. After the solution was stirred for 10 min, white precipitate was generated. 4Octylphenyldiazonium tetrafluoroborate (1·BF4) was obtained as a pale-yellow solid using filtration and was then washed with cold ethanol (0.83 g, 27% yield). 1H NMR (DMSO-d6, 400 MHz): δ 9.45 (d, J = 8.8 Hz, 2H), 8.70 (d, J = 8.8 Hz, 2H), 3.71 (t, J = 7.6 Hz, 2H), 2.15−2.12 (m, 12H), 1.73 (t, J = 6.8 Hz, 3H). IR (KBr) 2350 cm−1 (NN+). Synthesis Procedure for Obtaining 4-OctylphenyldiazenidoStabilized Vanadium Nanoparticles (Oct-C6H4-N2-V NPs). Distilled THF (130 mL), VCl4 (0.30 mL, 4 mmol), and 4-octylphenyldiazonium tetrafluoroborate (2.15 g, 9 mmol) were mixed under an Ar atmosphere. After the solution was stirred for 2 h, superhydride (LiBHEt3, 1 M THF solution, 70 mL, 70 mmol) was added over a period of 30 min. After the mixture was stirred for 2 days, the Received: January 11, 2013 Revised: March 26, 2013 Published: April 12, 2013 5099
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Scheme 1. Metal Surface−Diazonium Salt Reaction and Titanium Nanoparticle Preparationa
a
(a) Reaction between the metal surface and diazonium salt in the presence of a reducing agent. (b) Preparation of titanium nanoparticles (ref 18). (c) Preparation of vanadium nanoparticles (this work).
impurities were separated using a membrane filter under an Ar atmosphere. Anhydrous acetonitrile (250 mL) was added to the solution, and this was followed by separation using centrifugation (4000 min−1, 15 min ×2), which yielded the vanadium nanoparticles (0.37 g, 47% based on the weight of vanadium). Characterization. Field-Emission Transmission Electron Microscopy (FE-TEM). TEM images were recorded at 200 kV using a Hitachi HF-2000 instrument equipped with an AMT-CCD camera. Vanadium nanoparticle samples were prepared for TEM at room temperature by depositing THF-dispersed particles on a carbon film supported by a copper grid. The size distribution of the nanoparticles was obtained by manually measuring the particle diameters from the TEM images. X-ray Photoemission Spectroscopy (XPS). XPS spectra were recorded using a PHI-5000 versaprobe spectrometer (Ulvac-phi, Inc.) and an Al Kα anode. Samples were mounted on an Al foil by placing a volume of the particle dispersion in THF on the film, and the sample was then dried. We used gold on mica electrode attached to the sample folder and the Au 4f peak at 83.80 eV was used as a calibration peak for all of the spectra.
Figure 1. (a) Representative TEM image of a vanadium nanoparticle. Scale bar: 20 nm. (b) Core size distribution.
had a spherical structure and relatively uniform sizes. A statistical analysis was performed by measuring 100 particles. The average diameter was 1.7 ± 0.2 nm, where the error gives the standard deviation (Figure 1b). The standard deviation was less than 15% of the average size, and these nanoparticles were considered to be monodisperse. Elemental analysis of the purified vanadium nanoparticle powder was carried out to evaluate the structure of the particle surface. The results of the elemental analysis were C, 56.30%; H, 7.92%; and N, 9.86% associated with the presence of azo bridges. The vanadium content in the particles was estimated to be 25.92%. Consequently, from these results and the TEM results, the composition of the obtained vanadium nanoparticles was estimated to be 180V123 (C, 56.87%; H, 7.16%; N, 9.48%; V, 26.49%). Ar−NN+ was partially decomposed to Ar−NH−NH2 or Ar−NH2 under the reductive conditions although they were obtained in low yields.24 These compounds can also be combined on the vanadium nanoparticle surface. For these reasons, the experimental value of the hydrogen ratio was slightly high. The total number of vanadium atoms in the nanoparticles was calculated using the following equation: N = R3/r3, with N being the total number of vanadium atoms, R being the radius of a vanadium nanoparticle (0.85 nm), and r being the radius of a vanadium atom (0.171 nm). The surface atoms were estimated to represent ca. 70% of the total number of atoms in the case of Au nanoparticles (ca. 1.5 nm).25,26 This result indicated an average of ca. 0.65 ligand molecule per
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RESULTS AND DISCUSSION The nanoparticles were synthesized in a size-controlled fashion using a one-phase method. First, stabilizing agent 4octylphenyldiazonium tetrafluoroborate (1·BF4) was prepared using a modified method based on the literature.23 We then performed the reduction of VCl4 using an excess amount of superhydride in the presence of 4-octylphenyldiazonium salt. The nanoparticle synthesis was carried out inside the Ar-filled glovebox. As the reducing reagent was added, the V(IV) ions were reduced to V(0). The sample was stirred for 2 days to ensure the formation of the nanoparticles. The vanadium particles were purified by performing two cycles of centrifugation/redispersion in THF to remove the excess unbound molecules, which were obtained as a black powder. The formation of the nanoparticles was confirmed, and their surface chemistry was analyzed using FE-TEM, elemental analysis, and XPS. The surface morphology of the particles was investigated using TEM. Figure 1a shows a representative TEM image of the vanadium nanoparticles prepared using this method. They 5100
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Figure 2. Representative curve-fitting analysis for high-resolution XPS of vanadium nanoparticles. (a) V 2p region. (b) N 1s region.
which was confirmed by the appearance of two broad peaks at 516 and 524 eV associated with vanadium oxide.
vanadium atom and showed that the nanoparticle surfaces were densely covered with diazenido groups. Finally, XPS measurements were carried out to obtain further evidence of the vanadium nanoparticles. XPS provides valuable information on the oxidation state of metals and the ligands adsorbed on their surface. The main peaks observed in the survey scans of the sample were V 2p and N 1s peaks centered at ca. 513 and 398 eV, respectively. 27−29 The V 2p photoelectron peaks between 512 and 518 eV were assigned to metallic vanadium (V(0)), V−N species, and vanadium(III) oxide on the basis of the binding energies (Figure 2a). The binding energy of V(0) is between 512 and 513 eV according to the literature.30,31 The peak at 512.5 eV can be attributed to V(0), which is derived from the surface atoms or inner atoms of the nanoparticles. The large peak at approximately 514.0 eV was in reasonable agreement with the binding energy for V−N species given in the literature.27 The result is in good agreement with that of elemental analysis, as suggested by the V−N bond formation on the particle surface. The peak at around 515.7 eV was ascribed to oxidized vanadium(III) species, which is close to the peak of V(III) in the previous report.32 It is likely that partial oxidation occurred as a result of the difficulty in maintaining a complete inert atmosphere during the synthesis or measurements. The broad peak in the N 1s region (395−400 eV) can be decomposed into three peaks (Figure 2b). They were interpreted as the presence of azo groups (NN) (398.1 eV), V−N species (396.6 eV), and amino groups directly bound to the vanadium surface (V−NH2−Ar or V−NH2− NH−Ar) (395.5 eV).26 Initially, the N 1s peak of NN and V−N should be observed in the XPS in view of the structure of the protecting group. Because the binding energy of NN should be higher than that of V−N according to Mesnage’s study,33 the peak at 398.1 eV can be attributed to NN. Although the binding energy of the peak is lower than that of the NN peak in Mesnage’s report, Kudo et al. reported that the binding energy of NN was 394.8 eV.34 In consideration of this difference in binding energy, we concluded that the peak at 398.1 eV should be derived from the NN bond. Next, the peak at 396.6 eV can be attributed to V−N in the protecting group. Although this peak position is 0.6 eV lower than that of V−N according to Romand’s report,35 Ti−N, Cr−N, and other M−N bonds show the peak position at around 396.5 eV.36,37 We concluded that this peak can be derived from V−N in the protecting group. Finally, the peak at 395.5 eV is assigned to the V−NH2 or V−NH2−NH−Ar structure on the surface in light of the results of elemental analysis. This result showed that the particle surface was partially covered with amino groups, which were generated by the decomposition of azo groups.24 There were no signals at 403.8 and 405.1 eV, consistent with the presence of unreacted diazonium cations.38 Exposing the vanadium nanoparticles to air resulted in surface oxidation,
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CONCLUSIONS
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ASSOCIATED CONTENT
We observed the unusual reactivity of aryldiazonium salt on vanadium surfaces in the presence of a reducing reagent. The vanadium surface had a V−NN−Ar structure, as revealed by elemental analysis and XPS; this was contrary to expectations based on previously reported studies on Ti or late-transitionmetal surfaces. This provides strong evidence for the presence of diazo groups on the surface. Koval’chuk et al. reported that no reaction was observed when a vanadium surface contacted a diazonium salt solution.39 Because the chemical reduction of diazonium cations on metal surfaces proceeds through absorption on the substrate, the differences in coverage can be ascribed to differences in the absorption energy on the substrate surface. Although the mechanism responsible for the unusual behavior described here remains unclear, we believe that the strong V−N bond served as a driving force and the V− C bond energy was lower than the vanadium lattice energy.
* Supporting Information S
TEM images and survey XPS of vanadium nanoparticles. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +81-3-5841-4346. Fax: +81-3-5841-8063. E-mail:
[email protected];
[email protected]. co.jp. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We thank Ms. Kimiyo Saeki and Dr. Aiko Kamitsubo of the Elemental Analysis Center of the University of Tokyo for performing elemental analysis. We also thank Ms. Naomi Miyazawa for her technical assistance and the Research Hub Advanced Nano Characterization (School of Engineering, The University of Tokyo) for the X-ray photoelectron spectroscopy measurements. This work was financially supported by Grantin-Aids for Scientific Research on Innovative Areas “Coordination Programming” (area 2107, no. 21108002) and the Global COE Program for “Chemistry Innovation through the Cooperation of Science and Engineering” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. 5101
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