Biotemplated Synthesis of Single-Crystalline W18O49@C Core−Shell

J. Phys. Chem. C 2008, 112, 1913-1919

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Biotemplated Synthesis of Single-Crystalline W18O49@C Core-Shell Nanorod and Its Capacitance Properties Raman Ganesan, Ilana Perelshtein, and Aharon Gedanken* Department of Chemistry, Kanbar Laboratory for Nanomaterials, Institute of Nanotechnology and AdVanced Materials, Bar-Ilan UniVersity, Ramat-Gan, Israel 52900 ReceiVed: August 29, 2007; In Final Form: NoVember 5, 2007

Single-crystalline W18O49@C core-shell nanorods were prepared by a simple and economical method using a naturally occurring biopolymer, chitosan, as a template. The core-shell nanorods were characterized by CHN, XRD, BET, SEM, and HRTEM analysis. The HRTEM shows that each nanorod is made up of bundle of tiny nanorods with diameters of 2-3 nm. The capacitance properties were studied in 1 M H2SO4 and 1 M Na2SO4 using cyclic voltammetry. W18O49@C prepared at 973 K shows higher capacitance compared to W18O49@C prepared at 1023 K in both acid and aqueous mediums.

Introduction Nanostructured materials have received enormous interest in recent years because of their unusual properties when compared with those of bulk materials.1-5 The design and preparation of nanomaterials with tunable physical and chemical properties is still a challenge for the scientific community. Among these materials, tungsten oxide-based materials have been studied extensively in recent years as electrocatalysts in fuel cells, as electrical contacts in microelectronics, for the development of sensors and functional coatings (e.g., smart windows), and for catalytic applications such as selective oxidation of organic compounds, hydrodesulfurization of fuels, and isomerization reaction.6-12 Recently, there is a considerable interest in preparing W18O49 nanorods and nanowires. All methods reported so far in the literature for the preparation of W18O49 require very high temperatures, vacuum conditions, and costly airsensitive precursor materials and surfactants.13-19 Alternate simple methods are required to prepare W18O49 nanorods. In recent years, the direct utilization of natural materials as templates for the formation of nanoparticles has received considerable attention. These materials are environmentally friendly and also abundant in nature. Among these materials are biopolymers, which are renewable and biocompatible and can also be biodegraded easily. Chitosan is one of the biopolymers that is widely used as a catalyst for various organic reactions. It is produced from chitin by deacetylation. It can bind metal ions effectively due to the presence of amino and hydroxyl groups.20,21 Since the basic structure of chitosan is fibrous in nature, it can be used to fabricate inorganic nanowires/ rods.22 Core-shell nanosystems are one of the topological systems for materials scientists.23-27 The isolation of the core from the surroundings can be used to create objects with properties fundamentally different from those of the bare nanocrystal. By preparing semiconducting material with carbon, we can bring good capacitive materials with good conductivity of the system. Herein, we for the first time report the biotemplated synthesis of core shell W18O49@carbon nanorods with diameters of 1-2 nm using chitosan biopolymer as a template. The capacitance of these materials is tested by cyclic voltam* Corresponding author. E-mail: [email protected].

Figure 1. XRD patterns of (a) W18O49/C973 and (b) W18O49/C1023.

Figure 2. Raman spectrum of (a) W18O49/C973 and (b) W18O49/C1023.

metry. This method is very simple and ecomonically viable compared to the reported method for the preparation of W18O49. Experimental Section We synthesized W18O49/C core-shell nanorods by heating composites composed of chitosan biopolymer as a carbon precursor and ammonium metatungstate salt (AMT) as a tungsten precursor. In a typical synthesis, 1 g of AMT was dissolved in 25 mL of water and 3 g chitosan biopolymer was added to that solution. The resulting gel was dried at 393 K for 24 h. A total of 0.3 g of these materials was transferred to a swagelog reactor (3/4-in. union parts that were plugged from both sides by standard caps). The swagelog is heated to the desired temperatures (973 K (W18O49/C973) and 1023 K (W18O49/C1023)) at a heating rate of 10 °C/min (reaction under

10.1021/jp0769227 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/18/2008

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Figure 3. SEM image of (a) W18O49/C973 and (b) W18O49/C1023 nanorods.

autogenic pressure at elevated temperature (RAPET)) for 3 h and gradually cooled to room temperature. Structural Characterization. The elemental analysis of the sample was carried out by an Eager C, H, N, S analyzer. The X-ray diffraction measurements (XRD) were carried out with a Bruker AXSD Advance Powder X-ray diffractometer with a Cu KR (λ ) 1.5418 Å) radiation source. The BrunauerEmmett-Teller (BET) surface area measurements were performed by a Micromeritics (Gemini 2375) surface area analyzer. The nitrogen adsorption was measured at 77 K after degassing the samples by heating at 393 K for 1 h. The particle morphology was studied with transmission electron microscopy on a JEOL-2010 HRTEM instrument with an accelerating voltage of 200 kV. An Olympus BX41 (Jobin Yvon Horiba) Raman spectrometer was employed, using the 620-nm line of an Ar laser as the excitation source to analyze the nature of tungsten and carbon present in the products. Electrochemical Characterization. The working electrodes for the electrochemical measurements were fabricated by dispersing the catalysts in 1 mL of distilled water. The dispersion was ultrasonicated for 20 min and 25 µL of 5 wt % Nafion

added to that dispersion and again ultrasonicated for 20 min. A known amount of suspension was added on to the glassy carbon, and solvent was slowly evaporated. Pt foil and Ag/AgCl/3 M NaCl were used as counter and reference electrode, respectively. A solution of 1 M Na2SO4 and 1 M H2SO4 was used for electrochemical studies using Princeton Applied Research (PAR) voltammetry. Results and Discussion XRD, BET, and CHN Analysis. The typical XRD pattern of the blue colored W18O49/C core-shell nanorods is shown in Figure 1. The reflections of nanorods corresponds to W18O49 (JCPDS No. 00-005-0392). As temperature increases, crystallinity also increases. There are no XRD peaks corresponding to tungsten trioxide or metallic tungsten. Since the graphitic most intense peak 2θ ) 26° is found in the region of tungsten oxide, we had to rely on Raman measurements in its identification. The BET surface area of W18O49/C973 is 128 m2/g higher than that of W18O49/C1023 (87 m2/g). The C, H, and N analysis shows that both W18O49/C973 and W18O49/C1023 contain 38 wt % carbon.

Single-Crystalline W18O49@C Core-Shell Nanorod

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Figure 4. HRTEM image of (a) W18O49/C973, (b-d) W18O49/C1023, (e) the lattice fringes of core W18O49/C1023, and (f) the selected electron diffraction pattern of W18O49/C1023.

Raman Spectroscopy. The nature of the W18O49/C core shell nanorods was also studied using Raman spectroscopy. Two characteristic bands of carbonaceous materials were detected at 1327 cm-1 (D-band) and 1601 cm-1 (G-band) for W18O49/ C973 (Figure 2a). The intensity ratio of the D- and the G-bands is usually ID/IG ) 1.05 for the product. The relatively high intensity of the D-peak proves that the coating comprises disordered carbon. The G-band that is assigned to the graphitic carbon is about 50% of the carbonaceous material. We suggest that the nongraphitic nature of the carbon coating is due to the execution of the RAPET reaction at a temperature of 700 °C, which is too low for the formation of fully ordered carbonaceous product. The Raman spectra of W18O49 appear in the region of

100-1000 cm-1.28 The Raman spectrum in the low-wavenumber region includes two main peaks at 240 and 320 cm-1. These bands belong to the O-W-O bending modes. The Raman bands in the high-wavenumber region include two peaks at 727 and 798 cm-1, and they belong to the W-O stretching modes. When the temperature is increased to 1023 K (Figure 2b), a notable difference is observed. The intensity of carbon bands decreases while the intensity of the W-O bands increases. The intensity ratio of the D- and the G-bands is ID/IG ) 1.02. The increase in intensity of W-O bands shows that W18O49 treated at 1023 K is more crystalline compared to W18O49 treated at 973 K.

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SCHEME 1: Fabrication of W18O49@C Nanorods

SEM and TEM Analysis. The structure of W18O49/C was further studied by SEM and HRTEM measurements (Figure 3 and Figure 4). The HRSEM image of W18O49/C973 (Figure 3a) and W18O49/C1023 (Figure 3b) shows the morphology of nanorods with a diameter of 100-200 nm, but each nanorod consists of very small bundles of nanorods, which is not clearly visible in the SEM. In order to look at the nature of the nanorod, the electron mapping (Figure S1, Supporting Information) was done in a direction perpendicular to that of the W18O49/C973 and W18O49/C1023 nanorods. The signal which was observed from the shell of the nanorod identified only carbon. When the beam moved from the shell to the core of the nanorod, the carbon signal decreased and the tungsten and oxygen signals increased. The tungsten signal decreased at the other side of the nanorod (see Figure S1 in the Supporting Information). There was no tungsten signal at the shell of the nanorod. Since the carbon signal was originating from both shell and carbon tape,

which is used for SEM measurements, we have mapped only the tungsten signal. This shows that each rod is a core-shell structure in which the core is made up of W18O49 and the shell is made up of a carbon layer. The HRTEM image further provides insight into the structure of the W18O49 core-shell nanorods. Each nanorod is made up of bundle of very thin nanorods with diameters of 1-2 nm (Figure 4a-c). The lengths of the rods is 3-4 µm. The shell of the rod is made up of a carbon layer with a thickness of 7 nm (Figure 4c). The carbon layer contains both amorphous and crystalline carbon phases. The lattice fringes observed for the crystalline phase at the shell of W18O49/C1023 is around 0.34 nm, which is matching with (002) planes of the graphite-2H layer (Figure 4d). The lattice spacing observed at the core of the W18O49/C1023 nanorod is 0.38 nm, which is in excellent agreement with the reported lattice constant of (010) planes of monoclinicW18O49 (Figure 4e). The corresponding selected area electron diffraction (SAED)

Single-Crystalline W18O49@C Core-Shell Nanorod

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Figure 5. Cyclic voltammogram of (a) CNT (commercial), (b) W18O49/C1023, (c) RuO2 (commercial), and (d) W18O49/C973 in 1 M Na2SO4 at a scan rate of 5 mV/s at 298 K.

Figure 6. Cyclic voltammogram of W18O49/C973 (a) 5 mV/s, (b) 10 mV/s, (c) 20 mV/s, (d) 30 mV/s, (e) 40 mV/s, and (f) 50 mV/s in 1 M Na2SO4. Inset: variation of capacitance with scan rate.

pattern is demonstrated in Figure 3h, featuring a single crystal of monoclinic W18O49 nanorods (Figure 4f). The presence of streaking lines in the SAED pattern reveals that numerous stacking faults had been formed in the direction normal to the (010) direction. The EDX analysis at the center of the rod (see Figure S2 in the Supporting Information) shows that each nanorod is made up of W, O, and C (see Figure S2 in the Supporting Information). To our best knowledge, this is the first time that W18O49@C core-shell nanorods with a diameter of 1-2 nm were prepared by a simple biotemplated approach. Chitosan is a polysaccharide which contains large number of amine and hydroxyl functional groups. The presence of lone pair electron in the amine functional group is responsible for the coordination bond with tungsten. It is well-known in the literature that tungsten forms coordination complexes with nitrogen and hydroxyl ligands.29-31 When chitosan is added to

the solution of AMT, metatungstate anions form a coordination complex with amine and hydroxyl functional groups. However, there may be interaction due to hydrogen bonding between the oxygen bonded to the tungsten and the amine/hydroxyl groups or electrostatic interaction as well as simple physical adsorption. Chitosan can restrict an irregular arrangement of tungstate anions by imposing a steric constraint around them. This leads to the formation of a 1D array of metal oxide clusters (Scheme 1). Pyrolysis of this composite at 973 K leads to the formation of a core-shell nanorod structure. Electrochemical Capacitive Properties. We also studied the capacitive nature of W18O49/C973 and W18O49/C1023 in 1 M Na2SO4 solution at 25 °C cycled under a potential in the range from 0 to 0.8 V with a scan rate of 5 mV/s (Figure 5). The CV curve W18O49/C973 (Figure 5d) is rectangular (slightly deviating from the rectangular shape), which is a characteristic of the

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Figure 7. Cyclic voltammogram of (a) CNT (commercial), (b) W18O49/C1023, (c) W18O49/C973, and (d) RuO2 (commercial) in 1 M H2SO4 at a scan rate of 5 mV/s at 298 K.

Figure 8. Cyclic voltammogram of W18O49/C973 (a) 5 mV/s, (b) 10 mV/s, (c) 20 mV/s, (d) 30 mV/s, (e) 40 mV/s, and (f) 50 mV/s in 1 M H2SO4. Inset: variation of capacitance with scan rate.

electrical double layer capacitor behavior. The electrochemical capacitive charging causes electrical energy to be stored in the electric double layer between the electrolyte and W18O49/C973. The specific capacitance of W18O49/C973 measured at a CV scan rate of 5 mV/s in 1 M Na2SO4 solution is 43 F/g. To understand the capacitive behavior of W18O49/C973, the scan rate was varied at 5, 10, 20, 30, 40, and 50 mV/s; the results are presented in Figure 6. All the CV curves show an almost rectangular shape and exhibit a supercapacitive behavior. The mean specific capacitances of W18O49/C973 with scan rates of 5, 10, 20, 30, 40, and 50 mV/s are 43, 31, 21, 18, 16, and 14 F/g, respectively (inset of Figure 6). We compared the W18O49/ C973 electrode with that of W18O49/C1023 (Figure 5b). The specific capacitance value of W18O49/C973 (43 F/g) is 2.5 times higher than the W18O49/C1023 (17 F/g). Similarly, the capacitance properties were also studied in 1 M H2SO4 for all these materials (Figure 7). The capacitance of W18O49/C973 is 78 F/g. The capacitance of W18O49/C973 also decreased when the scan rate was increased (Figure 8). The capacitance values are 1.8 times higher than that of W18O49/ C1023 (45 F/g). W18O49/C973 has a higher BET surface area

compared to that of W18O49/C1023. This leads to a higher capacitance behavior. Finally, the capacitance properties of these catalysts are compared with those of commercially available RuO2 (Aldrich) and carbon nanotube (CNT) (MER Corp.) in 1 M Na2SO4 and 1 M H2SO4 (Figures 5 and 7). The capacitance values of RuO2 and carbon nanotube in 1 M Na2SO4 are 28 and 1.4 F/g. W18O49/ C973 shows 1.5 and 31 times higher capacitance than RuO2 and CNT. But RuO2 shows a higher capacitance value (126 F/g) compared to W18O49/C973 (78 F/g) in 1 M H2SO4. The capacitance of W18O49/C973 can be improved by suitably adjusting the preparation conditions and increasing the surface area of the materials. Conclusions W18O49@C core shell nanorods were prepared by using chitosan biopolymer as a template. The SEM and HRTEM show that each nanorod comprises a core-shell structure in which the core is made up of tungsten oxide and shell is made up of a carbon layer. The capacitance of these materials was studied

Single-Crystalline W18O49@C Core-Shell Nanorod by cyclic voltammetry, and W18O49 prepared at 973 K shows pseudocapacitance behavior and has higher capacitance than W18O49 prepared at 1023 K. This study shows a new and simple route for the preparation of core-shell nanostructures. Acknowledgment. We acknowledge the financial help of the Israeli Ministry of Science through an infrastructure and strategic grant. Supporting Information Available: The line map and EDS analysis of W18O49@C. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Burda, C.; Chen, X.; Narayanan, R.; Elsayed, M. A. Chem. ReV. 2005, 105, 1025. (2) Sayari, A.; Wang, W. H. J. Am. Chem. Soc. 2005, 127, 12194. (3) Kim, H. J.; Lee, H. C.; Rhee, C. H.; Chung, S. H.; Lee K. H.; Lee, J. S. J. Am. Chem. Soc. 2003, 125, 13354. (4) Lee, H. C.; Kim, H. J.; Chung, S. H.; Lee, K. H.; Lee, H. C.; Lee, J. S. J. Am. Chem. Soc. 2003, 125, 2882. (5) Sadasivan, S.; Sukhorukov, G. B. J. Colloid Interface Sci. 2006, 304, 437. (6) Rajesh, B.; Thampi, K. R.; Bonard, J. M.; Xanthopoulos, N.; Mathieu, H. J.; Viswanathan, B. J. Phys. Chem. B 2003, 107, 2701. (7) Raghuveer, V.; Viswanathan, B. J. Power Sources 2005, 144, 1. (8) Dhote, A. M.; Ogale, S. B. Appl. Phys. Lett. 1994, 64, 2809. (9) Solarska, R.; Alexander, B. D.; Augustynski, J. J. Solid State Electrochem. 2004, 8, 748. (10) Yang, X. L.; Dai, W. L.; Gao, R.; Chen, H.; Li, H.; Cao, Y.; Fan, K. J. Mol. Catal., A 2005, 241, 205. (11) Murali Dhar, G.; Srinivas, B. N.; Rana, M. S.; Kumar, M.; Maity, S. K. Catal. Today 2003, 86, 45.

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