Preparation, Characterization, and Electrophysical Properties of

of single-source precursors in a new dimension by producing two separate, uniform products from decomposition of a single-source precursor in one step...
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J. Phys. Chem. C 2007, 111, 18538-18544

Preparation, Characterization, and Electrophysical Properties of Nanostructured BiPO4 and Bi2Se3 Derived from a Structurally Characterized, Single-Source Precursor Bi[Se2P(OiPr)2]3 Yi-Feng Lin,† Hao-Wei Chang,‡ Shih-Yuan Lu,*,† and C. W. Liu*,‡ Department of Chemical Engineering, National Tsing-Hua UniVersity, Hsinchu, Taiwan 30013, Republic of China, and Department of Chemistry, National Dong-Hwa UniVersity, Hualien, Taiwan 970, Republic of China

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ReceiVed: August 28, 2007; In Final Form: October 1, 2007

A Tris-chelated diselenophosphato complex of bismuth, Bi[Se2P(OiPr)2]3, was successfully prepared and used to obtain two separate, uniform deposits of nanostructured metal phosphate, BiPO4, and metal chalcogenide, Bi2Se3, in a one-step metal-organic chemical vapor deposition process. This work expands the applications of single-source precursors in a new dimension by producing two separate, uniform products from decomposition of a single-source precursor in one step. The inclusion of oxygen and phosphorus elements in the precursor molecule makes possible the simultaneous production of BiPO4 nanowires and Bi2Se3 nanoplates from the single-source precursor. The resulting BiPO4 nanowires and Bi2Se3 nanoplates show promising field emission properties, comparable to the more popular oxide semiconductor nanowires. The Bi2Se3 nanoplates also exhibit a superior thermoelectric property over bulk Bi2Se3.

Introduction Single-source precursors are molecules containing all constituent elements of targeted products, the decomposition of which leads to the formation of desired products often in one processing step. The use of a single-source precursor simplifies material production processes by reducing the number of involved reactant processing equipment and avoiding the use of highly toxic or moisture sensitive gases, such as H2S, AsH3, and SiH4, often necessary in multiple-source routes.1 The development of single-source precursors, particularly for use in metal-organic chemical vapor deposition (MOCVD) processes for the preparation of heterometallic oxides, metal chalcogenides, metal pnictides, transition metal silicides, and intermetallic compounds, has drawn continuing research attention since the pioneering work of the late 1970s.1 Ideally, single-product formation is desired, for most applications, from the decomposition of single-source precursors. The formation of mixed products, however, is possible and has been reported. A mixture of Y2O3 and Ba4Y2O7 together with a trace amount of BaF2 was obtained on a silica substrate from the thermal decomposition of BaY2[µ-OCH-(CF3)2]4(thd)4 run at 285-300 °C.2 Such an outcome is undesirable since it is difficult to find applications for deposits of mixture products. The formation of mixture products can, however, become desirable if these mixture products form a uniform and useful morphology, such as a core-shell structure. One such example is the densely packed and well-aligned core-shell CdS-CdO and ZnS-ZnO nanorod arrays obtained with a one-step, noncatalytic, templatefree MOCVD process by using single-source precursors Cd(O-EtXan)2 and Zn(O-EtXan)2, respectively (O-EtXan ) S2COCH2CH3).3 The decomposition of this M(O-EtXan)2 molecule led to the formation of both oxide and sulfide of the metal, with the later formed oxide shell coating on the surface of the * Corresponding authors. E-mail: (S.-Y.L.) [email protected] or (C.W.L.) [email protected]. † National Tsing-Hua University. ‡ National Dong-Hwa University.

first formed sulfide core. The oxide shell may serve as the protective and/or passivation layer for the sulfide core. In this case, the mixture product is desirable for boosting the functional performance of the core materials. In this work, we expand the applications of single-source precursors in a new dimension by producing two separate, uniform products from decomposition of a single-source precursor. More specifically, we prepared and characterized the molecular structure of a Tris-chelated diselenophosphato complex of bismuth, Bi[Se2P(OiPr)2]3, and obtained two separate, uniform deposits of a nanostructured metal phosphate, BiPO4, and a metal chalcogenide, Bi2Se3, in a one-step MOCVD process. The inclusion of oxygen and phosphorus elements in the precursor molecule makes possible the simultaneous production of BiPO4 nanowires and Bi2Se3 nanoplates from the singlesource precursor. Both BiPO4 and Bi2Se3 are important functional materials with a wide range of applications. Bismuth phosphate finds applications in catalysis4 and ion sensing5 and can be used for separating radioactive elements, such as uranium,6a neptunium,6b and americium,6c and for improving the electric properties of phosphate glasses.7,8 Despite this wide range of applications, the development of nanostructured BiPO4 and the investigation of its functional properties in the nanoscale have not yet been seriously conducted, and to our best knowledge, there is only one report for the fabrication of BiPO4 nanorods by a sonochemical process.8b In this work, single-crystalline BiPO4 nanowires, together with single-crystalline Bi2Se3 nanoplates, were obtained from the Au-assisted MOCVD process by using Bi[Se2P(OiPr)2]3 as the precursor. The field emission behavior of the BiPO4 nanowires was measured for the first time to assess their potential as field emitters and was found to obey the classic Fowler-Nordheim (F-N) theory, indicating a quantum mechanical tunneling process for the field emitted electrons. Bismuth selenide, a V-VI semiconductor with a band gap of about 0.3 eV,9 finds applications in photosensitive devices, photoelectrochemical cells,10 and thermoelectric devices.9a Its promising thermoelectric property is of particular recent research

10.1021/jp076886b CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007

BiPO4 and Bi2Se3 Derived from Bi[Se2P(OiPr)2]3

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Figure 1. (a) Thermal ellipsoid drawing of Bi[Se2P(OiPr)2]3. Selected bond lengths (Å): Bi(1)-Se(1) 2.8873(7), Bi(1)-Se(2) 2.9194(8), Se(1)P(1) 2.1607(19), and Se(2)-P(1) 2.149(2) and angles (deg): Se(1)-Bi(1)-Se(2A) 76.07(2) and Se(2)-P(1)-Se(1B) 112.21(8). (b) Intermolecular Se-Se interactions, 3.853 Å, of Bi[Se2P(OiPr)2]3 with isopropyl groups omitted for clarity.

interest. A wide variety of Bi2Se3 nanostructures, including nanospheres,11 thin films,9a,12 nanoflakes/nanosheets,12,13 nanotubes,13 nanowires,14 and nanobelts,15 has been reported in the literature. In the present study, another 2-D nanostructure, quite similar in morphology to nanoflakes and nanosheets, of Bi2Se3 was obtained from decomposition of the present single-source precursor. We call this 2-D nanostructure a nanoplate for its smaller width/thickness ratio and flatter surface as compared

to nanoflakes and nanosheets. The field emission behavior of the Bi2Se3 nanoplates was also investigated for the first time, and the resulting low turn-on field indicated that Bi2Se3 nanoplates may be a promising candidate material for field emitting devices. In addition, the thermoelectric property of the Bi2Se3 nanoplates was measured and showed superiority over bulk Bi2Se3, complying with the expectation that the nanostructure generally promotes the thermoelectric property.16

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Figure 2. SEM images of (a) BiPO4 nanowires and (b) Bi2Se3 nanoplates. Insets are enlarged SEM images of a BiPO4 nanowire and a Bi2Se3 nanoplate, respectively. (c) XRD patterns of BiPO4 nanowires and Bi2Se3 nanoplates with corresponding standard references included for comparison.

Experimental Procedures Synthesis of Bi[Se2P(OiPr)2]3, 1. All operations were carried out under N2 atmosphere with standard Schlenk techniques. To a suspension of NH4[Se2P(OiPr)2] (0.84 g, 2.584 mmol) in 40 mL of dichloromethane was added Bi(NO3)3‚5H2O (0.418 g, 0.861 mmol), and the resulting mixture was stirred for 4 h. The yellow filtrate was collected and evaporated to dryness using a rotary evaporator under reduced pressure. The resulting solid was dissolved with hexane (20 mL) and filtered through Celite and evaporated to dryness. After evaporation of the solvent, the compound Bi[Se2P(OiPr)2]3 was afforded as an orange powder. Yield: 82% (0.798 g) 1: elemental anal. calcd for C18H42O6BiSe6P3: C, 19.13; H, 3.75%. Found: C, 19.17; H, 3.70%. 31P {1H} NMR (CDCl3): δ ) 68.5 (JP-Se ) 637 Hz). 1H NMR (CDCl ): δ ) 1.35 [d, 3J 3 H,H ) 6 Hz, 36H, CH(CH3)2], 4.88 [m, 6H,CH(CH3)2]. 77Se NMR (CDCl3, 25 °C): δ ) 193.8 (JSe-P ) 643 Hz). MALDI-TOF MS: m/z 823.18 [(M - L)+, 823.19]. Crystal Data for Bi[Se2P(OiPr)2]3. C18H42O6P3BiSe6: T ) 293 K, trigonal, space group R3h, a ) 23.1571(8) Å, c ) 12.4167(8) Å, V ) 5766.4(5) Å3, Z ) 6, Fcalcd ) 1.953 g cm-3, µ ) 10.417 mm-1, 2θmax ) 55.82°, R1 ) 0.0435, wR2 )

0.1066 for 21 290 data points (3065 independent), 104 parameters. Max/min: 1.409/-1.952 eÅ-3. Single-crystal X-ray diffraction analysis was performed on a Bruker APEX II CCD diffractometer at T ) 293 K with Mo KR radiation (λ ) 0.71073 Å). The data were collected using the 2θ-ω scan technique. Absorption corrections were applied by using the multiscan program SADABS. The structure was solved by the use of direct methods, and the refinement was performed by least-squares methods on F2 with the SHELXL-97 package, incorporated in SHELXTL/PC V5.10. Formation of BiPO4 Nanowires and Bi2Se3 Nanoplates. The CVD reaction was conducted in a three-zone hot wall furnace under reduced pressure. The silicon plates coated with the gold catalyst, placed downstream of the reactor, were used as the substrates for the growth of BiPO4 nanowires and Bi2Se3 nanoplates. The single-source precursor, Bi[Se2P(OiPr)2]3, placed upstream was heated to 300 °C for the generation of vapors. The furnace temperature was set at 700 °C, and the BiPO4 nanowires and Bi2Se3 nanoplates were collected at locations with temperatures of 450 and 350 °C, respectively (about 3 cm apart). Deposition was run for 4 h at a carrier gas (N2) flow rate of 200 sccm and system pressure of 30 Torr.

BiPO4 and Bi2Se3 Derived from Bi[Se2P(OiPr)2]3

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Figure 3. (a) TEM and (b) lattice-resolved TEM images of a BiPO4 nanowire. Inset of panel a is the corresponding SAED pattern. (c) EDX spectra of the BiPO4 nanowire obtained from the tip and stem regions.

Characterization of BiPO4 Nanowires and Bi2Se3 Nanoplates. The morphology and dimensions of the as-deposited nanowires and nanoplates were examined with a field emission scanning electron microscope (Hitachi, S-4700). The crystallographic structures of the nanowires and nanoplates were investigated with an X-ray diffractometer (MAC Sience MXP18), a transmission electron microscope (JEOL JEM-2010, operated at 200 kV), and a high-resolution transmission electron microscope (JEOL JEM-400EX, operated at 400 kV). The elemental analyses of the nanowires were conducted with an energy dispersive spectrometer, an accessory of the transmission electron microscope (JEM-2010). The field emission measurements of the nanowire arrays were conducted in a vacuum chamber maintained at a pressure of 10-6 Torr. A copper electrode probe was taken as the anode with an area of 7.1 × 10-3 cm2. The anode was placed at a distance of 100 and 250 µm from the tips of the BiPO4 and Bi2Se3 samples, respectively, during the measurement. The Seebeck coefficient was measured with a standard dc technique. A temperature gradient across the sample was established by a 10 Ω resistor, and the temperatures at the cold and hot ends were measured with a calibrated carbon-glass thermometer. The Seebeck voltage was measured between the two signal leads connected by a pair of copper wires.

Results and Discussion The single-crystal X-ray diffraction data revealed that Bi[Se2P(OiPr)2]3 was crystallized in a trigonal space group R3h where the unit cell was composed of six molecules. Six Se atoms from three diselenophosphate, Se2P(OR)2–, (dsep) ligands were found to be coordinated to bismuth with a distorted octahedral geometry. The two Se atoms of each dsep ligand have unequal distances, 2.8873(7), and 2.9194(8) Å, from the central bismuth, and the bite angles of Se-Bi-Se and Se-P-Se within the chelated four-membered ring are 76.07(2) and 112.21(8)°, respectively. It is isostructural with Bi(Se2CNBu2)312a and Bi[N(SePR2)2]3 (R ) iPr,17a Ph17b). These three molecules all display a stereochemically nonactive electron lone pair around the BiIII center. The thermal ellipsoid drawing of Bi[Se2P(OiPr)2]3 is shown in Figure 1a with selected bond lengths and bond angles given in the caption. Interestingly, several secondary Se‚‚‚Se bonding interactions were observed between the selenium atoms in the dsep ligands of the neighboring molecules to form a loosely bound dimer. They are represented by dotted lines in Figure 1b, and the distance, 3.853 Å, is less than the sum of van der Waals radii (4.0 Å, Se, Se).18 BiPO4 nanowires and Bi2Se3 nanoplates were prepared in an MOCVD process. The CVD reaction was conducted in a threezone hot wall furnace under reduced pressure. The silicon plates coated with a gold catalyst, placed downstream of the reactor,

18542 J. Phys. Chem. C, Vol. 111, No. 50, 2007 were used as the substrates for the growth of BiPO4 nanowires and Bi2Se3 nanoplates. The single-source precursor, Bi[Se2P(OiPr)2]3, placed upstream, was heated to 300 °C for the generation of vapors. The furnace temperature was set at 700 °C, and the BiPO4 nanowires and Bi2Se3 nanoplates were collected at locations with temperatures of 450 and 350 °C, respectively (about 3 cm apart). The collection location of the BiPO4 nanowires was closer to the reactor center than that of the Bi2Se3 nanoplates. Deposition was run for 4 h at a carrier gas (N2) flow rate of 200 sccm and system pressure of 30 Torr. It is apparent from Figure 2a,b that the deposit morphology collected at the two different locations is completely different, one nanowire and the other nanoplate. The morphological purities of the two nanostructures were high with a large scale morphological uniformity observed on the collecting substrates, which were 3 cm × 1 cm in size. The insets of Figure 2a,b more clearly show the shape and dimension of the deposit, with the nanowire having a diameter of 40-60 nm and the nanoplate a thickness of 40-50 nm. In addition, a nanoparticle situates at the tip of the nanowire. Figure 2c shows the XRD patterns of the BiPO4 nanowires and Bi2Se3 nanoplates. Also included in Figure 2c for comparison are the XRD patterns of the reference monoclinic BiPO4 crystals with a space group P21/n (No. 14) (JCPDS file no. 15--0767) and hexagonal Bi2Se3 crystals with a space group R3hm (No. 166) (JCPDS file no. 33--0214). The agreement is quite good for Bi2Se3 nanoplates. As for the BiPO4 nanowires, the diffraction peaks of (011) and (022) in the nanowire growth direction of [011] (to be discussed in the HRTEM analysis) stand out. The detailed crystalline structures of the BiPO4 nanowires and Bi2Se3 nanoplates were further examined with TEM and HRTEM. The BiPO4 nanowires were 40-60 nm in diameter and 1 µm in length as shown in the TEM image of Figure 3a. The inset of Figure 3a shows the selected area electron diffraction (SAED) pattern of the BiPO4 nanowires. The zone axis of the SAED pattern is in the [0-11] direction, and the dot pattern suggests the single-crystalline structure of the BiPO4 nanowires with the dots indexed to the reflections of monoclinic BiPO4 crystals, consistent with the corresponding XRD pattern shown in Figure 2c. From the HRTEM image shown in Figure 3b, the lattice spacing is determined to be 0.47 nm, which is in good agreement with the d-spacing of the (011) planes of the monoclinic BiPO4. The axis of the BiPO4 nanowires is parallel to the [011] direction, indicating that the BiPO4 nanowires grow along the [011] direction. Figure 3c displays the EDX spectra of a single BiPO4 nanowire obtained from the tip and stem regions, respectively. The copper and carbon signals observed in both spectra come from the holey carbon film supported copper grid used for holding the samples. For the tip region, only the gold signal was detected, while the Bi, P, and O signals were detected from the stem region with an atomic ratio of about 1:1:4 for Bi/P/O, implying a gold nanoparticle headed BiPO4 nanowire. The presence of the gold tip reveals the vaporliquid-solid (VLS) mechanism for the growth of BiPO4 nanowires.19 As for the Bi2Se3 nanoplates, Figure 4a shows the TEM image of a Bi2Se3 nanoplate. The image was further enlarged at the marked region to show the lattice fringes, from which the lattice spacing was determined to be 0.21 nm, in good agreement with the d-spacing of the (11-20) planes of the hexagonal Bi2Se3. The dot pattern of SAED shown in Figure 4b suggests the single-crystalline nature of the Bi2Se3 nanoplates and can be indexed to the reflections of the hexagonal Bi2Se3 crystal, consistent with the corresponding XRD pattern shown in Figure 2c. Note that the zone axis of the SAED pattern is in

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Figure 4. (a) TEM images of a Bi2Se3 nanoplate. (b) Corresponding SAED pattern of the Bi2Se3 nanoplate. (c) Corresponding latticeresolved TEM image of the marked region in panel a.

Figure 5. TG analysis curve of the single-source precursor, Bi[Se2P(OiPr)2]3.

the [0001] direction (the c-axis), indicating that the top and bottom surfaces of the nanoplate are the (0001) planes, perpendicular to the c-axis of the crystal. In this work, the silicon substrates were coated with a 1 nm thick gold film to serve as the catalyst for possible induction of anisotropic growth of materials to form 1-D nanostructures through the VLS mechanism. In a typical VLS process, the metal catalysts act as the energetically favored sites for the absorption of product vapors to form the alloy phase of the metal and product. Once the product concentration becomes oversaturated, the product material precipitates out from either the vaporalloy or alloy-substrate interface to form 1-D nanostructures. For the present work, the melting point of BiPO4 is much lower than that of Bi2Se3. Consequently, it is much easier for BiPO4 to form a liquid alloy with the Au catalyst for nanowire growth. Bi2Se3, on the other hand, with a melting point of above 700 °C, did not proceed with the VLS growth mechanism at

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Figure 6. J-E curves of (a) BiPO4 nanowires and (b) Bi2Se3 nanoplates. Insets are the corresponding F-N plots. (c) Plot of electrical conductivity (open circles) and resistivity (solid circles) vs temperature for Bi2Se3 nanoplate samples. (d) Curves of Seebeck coefficient (solid circles) and power factor (S2σ, open circles) vs temperature for Bi2Se3 nanoplate samples.

the collection temperature of 350 °C. To confirm that Au did not play any role in the growth of Bi2Se3 nanoplates, we ran the MOCVD process without Au sputtered on the silicon substrate and found that Bi2Se3 nanoplates were again obtained. To further understand why two different products of different morphologies can be produced from one precursor, we performed TG analyses and positive MASS measurements for the single-source precursor, Bi[Se2P(OiPr)2]3. The TG analysis (Figure 5) showed clearly that the precursor went through a two-step decomposition, one sharp partial decomposition at around 200 °C and one gradual further decomposition after 200 °C. This phenomenon implies that different fragment species were released at different temperatures, leading to more than one product from the decomposition of the present precursor. From the positive MASS spectrum (data not shown), we identified the existence of the fragment species of {BiPOiPr}+7 (m/z ) 43), implying that the bonds of Bi-P did form during the decomposition of the precursor, in addition to the originally existing Bi-Se bonds, for the eventual formation of BiPO4. The possible reasons for the formation of 2-D nanostructures of Bi2Se3 and its structurally identical partner Bi2Te3 have been discussed in detail in the literature.13,20 Briefly speaking, the intrinsically anisotropic, layered crystalline structure of Bi2Se3 plays an important role in favoring the crystal growth along the direction perpendicular to the c-axis of hexagonal Bi2Se3 for the formation of 2-D nanostructures, instead of along the c-axis for the formation of 1-D nanostructures. Furthermore, the precursor fragments produced from the thermal decomposition of the single-source precursor may contribute to further favor the formation of 2-D nanostructures by capping onto the (0001) planes of the growing crystal to retard the growth rate along the c-axis.20 The field emission properties of the as-grown BiPO4 nanowires and Bi2Se3 nanoplates were measured to assess their potential as field emitters. Figure 6a,b shows the plots of field

emission current density (J) versus electric field (E) for the BiPO4 nanowires and Bi2Se3 nanoplates, respectively. The turnon fields, defined as the field required to achieve a current density of 0.1 µA/cm2, were determined to be 7.4 and 2.4 V/µm for the BiPO4 nanowires and Bi2Se3 nanoplates, respectively. The insets of Figure 6a,b display the corresponding F-N plots for the field emission data of the BiPO4 nanowires and Bi2Se3 nanoplates, respectively. The linearity of the two F-N plots indicates that the field emission of the two samples is a quantum mechanical tunneling process and obeys the F-N law.21 Here, to our best knowledge, BiPO4 nanowires are the first example of metal phosphate to be reported for field emission properties. The turn-on field of the BiPO4 nanowires is somewhat higher than some of the more popular oxide semiconductor nanowires, such as ZnO,22 WO2,23 and In2O3.24 The lack of vertical alignment of the present BiPO4 nanowires, however, should have significantly suppressed the nanowire’s field emission ability. On the other hand, despite the unfavorable morphology of the plate structure of as-prepared Bi2Se3, these nanoplates still show a rather low turn-on field (2.4 V/µm), as compared to those of some popular semiconductor materials.22-25 This makes the asgrown Bi2Se3 nanoplates a promising candidate material for field emitting devices. The plot of electrical resistivity and conductivity versus temperature for the as-grown Bi2Se3 nanoplates is shown in Figure 6c. The two curves are quite linear, and the electrical resistivity decreases with increasing temperature, indicating the semiconductor nature of the Bi2Se3 nanoplates. The thermoelectric performance of the Bi2Se3 nanoplates was also investigated. Figure 6d shows the plot of the Seebeck coefficient (S) and power factor (S2σ) versus temperature from 230 to 300 K for the Bi2Se3 nanoplate samples. The negative values of the Seebeck coefficient indicate the n-type nature of the Bi2Se3 nanoplates. The thermoelectric figure of merit (ZT), a measure of the thermoelectric efficiency of the material, is related to the Seebeck coefficient by ZT ) S2T/Fκ, where F is the electrical

18544 J. Phys. Chem. C, Vol. 111, No. 50, 2007 resistivity and κ is the thermal conductivity of the materials. Evidently, the larger the the magnitude of S, the larger ZT the material acquires, and the better thermoelectric performance the material exhibits. The magnitude of S for the Bi2Se3 nanoplates at room temperature, 84 µV K-1, is larger than that for bulk Bi2Se3, 59 µV K-1,26 demonstrating the benefit in promoting the thermoelectric performance of a material with the introduction of nanostructure.16 For nanostructured materials, there will be present an increased amount of grain/phase boundaries and subsequent phonon scattering, resulting in decreases in thermal conductivity as compared to corresponding bulk materials.16 From the definition of the figure of merit, the value of ZT increases with decreasing thermal conductivity. Therefore, nanostructured materials are expected to perform better in thermoelectrics than corresponding bulk materials. Conclusion In conclusion, we successfully demonstrated the feasibility of producing two separate, uniform products from the thermal decomposition of a single-source precursor. This work adds a new dimension to the applications of single-source precursors. The inclusion of oxygen and phosphorus elements in the precursor molecule makes possible the simultaneous production of BiPO4 nanowires and Bi2Se3 nanoplates from the singlesource precursor, Bi[Se2P(OiPr)2]3. The resulting BiPO4 nanowires and Bi2Se3 nanoplates show promising field emission properties, comparable to the more popular oxide semiconductor nanowires. Bi2Se3 nanoplates also exhibit a superior thermoelectric property over bulk Bi2Se3. Acknowledgment. The authors are grateful to Profs. J.-J. Lin, T.-Y. Tseng, and Y.-J. Hsu of the National Chiao-Tung University of Taiwan for assistance in thermoelectric measurements, field emission measurements, and helpful discussions on TEM characterizations, respectively. This work was financially supported by the National Science Council of the Republic of China (Taiwan) under Grants NSC-95-2221-E-007-194 and NSC-95-2119-M-259-001. References and Notes (1) Gleizes, A. N. Chem. Vapor Deposition 2000, 6, 155. (2) Labrize, F.; Hubert-Pfalzgraf, L. G.; Daran, J.-C.; Halut, S.; Tobaly, P. Polyhedron 1996, 15, 2707. (3) Lin, Y.-F.; Hsu, Y.-J.; Lu, S.-Y. Nanotechnology 2006, 17, 4773. (4) (a) Chang, T.-S.; Lee, D.-K.; Cho, D.-H.; Yun, S.-S. React. Kinet. Catal. Lett. 2003, 78, 35. (b) Abadzhjieva, N.; Tzokov, P.; Uzunov, I.; Minkov, V.; Klissurski, D.; Rives, V. React. Kinet. Catal. Lett. 1994, 53, 413.

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