Growth and Microstructural Characterization of Single Crystalline

Growth and Microstructural Characterization of Single Crystalline Nb3Te4 Nanowires Hannah K. Edwards,†,‡ Pamela A. Salyer,† Martin J. Roe,‡ Gavin S. Walker,‡ Paul D. Brown,*,‡ and Duncan H. Gregory*,†

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School of Chemistry, and School of Mechanical, Materials and Manufacturing Engineering, University of Nottingham, University Park, Nottingham, NG7 2RD, United Kingdom Received April 7, 2005;

Revised Manuscript Received May 24, 2005

ABSTRACT: Novel Nb3Te4 nanowires have been successfully synthesized through the direct reaction and annealing of the elemental powders, by means of a vapor transport mechanism. This is the first instance of the production of group 5 transition metal telluride nanowires. Synthesis involved the initial formation of NbTe2 crystallites which were subsequently annealed to generate crystalline Nb3Te4 nanowires. Characterization using powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and X-ray photoelectron spectroscopy (XPS) confirmed the wires to be single crystalline, with their longitudinal axes coincident with the crystallographic c-direction of Nb3Te4. The nanowires ranged in dimension from 50 nm to ∼1 µm in diameter and from 1 to 30 µm in length. Since the discovery of carbon nanotubes in the early 1990s, there has been a rapid increase in research into carbon and inorganic nanotubes and nanowires. The promise of a greater breadth of chemistry and functional properties offered by inorganic nanostructures over carbon nanostructures has led to the active search for inorganic compounds with the capacity to be synthesized as nanowires or nanotubes. Further, nanomaterials chemistry offers the promise of novel properties as a result of the transition from the bulk to nanoscale when effects such as quantum confinement come into play. The first inorganic nanowires were synthesized by Tenne et al. in 1992,1 and since then many compounds have been produced in the form of nanotubes and nanowires through various methods. These include elemental nanotubes such as Fe, Cu, Ni, Pb, Bi, Te, Se, and Si2-10 and boron- and silicon-based tubes (e.g., BN,2,11,12 B-C-N,11,13 SiC,14-16 and Si3N417), but the largest category of inorganic anisotropic nanostructures produced have been the transition metal chalcogenides. In the bulk state, these compounds often have layered structures that seem to be strongly associated with an ability to form nanotubes.18 Those transition metal chalcogenides synthesized to date include the disulfides and diselenides of metals from groups 4, 5 and 6; TiS2, ZrS2, HfS2, TaS2, MoS2, MoSe2, WS2, and WSe2,12,19-23 in addition to various oxides; TiO2, V2O5, VOx, MoO3, and W18O49.2,24 In this context, Nb-based nanostructures have not been exhaustively researched, with reports only of NbS2,25-29 NbSe230,31 and more recently NbSe332 nanowires. Further, only one instance of a metal telluride nanowire has been previously reported, that of MoTe2.33 In an earlier communication, we reported the discovery of the first member of a new family of A3X4 * To whom correspondence should be addressed. (D.H.G.) E-mail: [email protected]; fax: 0115 951 3563; tel: 0115 951 4594. (P.D.B.) E-mail: [email protected]; fax: 0115 951 3800; tel: 0115 951 3748. † School of Chemistry. ‡ School of Mechanical, Materials and Manufacturing Engineering.

chalcogenide nanowires, Nb3Te4.34 Here we describe, in detail, the growth and characterization of this novel single crystalline, nanostructured telluride. Nb3Te4 is an interesting departure from the dichalcogenide wires produced to date in terms of both its stoichiometry and its pseudo-one-dimensional structure. Bulk Nb3Te4 has been found to superconduct with a Tc of 1.8 K.35 It is also an interesting candidate for nanowire growth because its crystal structure contains Nb chains and hollow channels centered about a 6-fold screw axis, both parallel to the crystallographic c-axis, which are enclosed by Te atoms.36 These channels are wide enough to accept other atoms and have been exploited to synthesize intercalation compounds such as MxNb3Te4 in the bulk (where M ) In, Hg, Tl, and Cu and 0 < x e 1).37,38 Experimental Methods (a) Synthesis and Nanowire Growth. The nanowires were synthesized through a direct chemical vapor transport method. The elemental powders of niobium (Aldrich, 99.9%) and tellurium (Alpha, 99.99%) were ground together in a 3:4 molar ratio in a nitrogen-filled recirculating glovebox (O2 < 1 ppm, H2O < 5 ppm). Powdered mixtures were placed in 12-mm-diameter silica ampules, which were evacuated to ∼10-3 mbar and then sealed. The ampules were wrapped in quartz wool insulation and heated initially to 900 °C in a box furnace for between 1 h and 1 week. This initial heating, below the boiling point of Te (988.9 °C),39 served to prevent the rapid vaporization of the chalcogen. After air quenching, the ampules were subsequently annealed at 1160 °C for 2 days. (b) Characterization. Structural characterization of intermediate mixtures after heating to 900 °C and the resultant low density black powder following high temperature annealing were carried out initially by PXRD using a Philips X’Pert θ-2θ diffractometer (Cu KR radiation). Cell parameters and phase purity were evaluated from 2 h step scans over 5°-80° 2θ with step size 0.02° 2θ using DICVOL9140 for indexing and the Philips IDENTIFY routine for phase identification. The latter package allows access to the ICDD PDF database and provides a facile

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Edwards et. al.

Figure 2. SE images of (a) NbTe2 particles after 1 h at 900 °C and (b) Nb3Te4 nanowires after 2 days annealing at 1160 °C.

Figure 1. (a-c) XRD patterns of 900 °C sintered samples after 1 h, 24 h, and 1 week, showing the gradual transformation of NbTe2 to Nb3Te4. These samples did not contain nanowires. (d) XRD pattern of a sample sintered for 1 h at 900 °C followed by annealing at 1160 °C for 2 days. This sample contained nanowires. Principal reflections for Nb3Te4 are identified in panel d.

means for assigning known phases. Lattice parameters were refined by least-squares fitting of PXD data. Investigation of the morphology of samples and elemental microanalysis was initially performed using a JEOL 6400 SEM with an Oxford Instruments ISIS EDX system operated at 20 kV. Near surface composition was subsequently probed by XPS using a VG Scientific Escalab Mark II spectrometer with unmonochromated AlKR X-rays operating at an anode potential of 10 kV and a filament emission current of 20 mA. Nanoscale imaging and SAED were performed using a JEOL 2000fx TEM with an ISIS EDX system operated at 200 kV. The samples for SEM and XPS were prepared by depositing the powder onto a carbon tab with sufficient thickness to prevent the detection of the underlying carbon. Samples for TEM were prepared by sonicating the powders in acetone for 3 min and then pipetting drops of the suspension onto holey carbon film copper grids.

Results The transition of the starting material from a ground mixture of elemental powder to product was followed ex-situ by PXRD.41 Figure 1a-c shows off-set diffractograms obtained from a series of samples sintered in the furnace at 900 °C for increasing time points. After 1 h, the Nb and Te react to form NbTe2 (Figure 1a). This phase was then gradually converted over a period of 1 week to Nb3Te4 (Figure 1c). After intermediate times of 4 h, 1 day, and 3 days, both phases (NbTe2 and Nb3Te4) were identified as being present (e.g., Figure 1b). These samples sintered at 900 °C did not contain nanowires. The PXRD pattern of the material held at 900 °C for 1 h and then annealed at 1160 °C for 2 days shows that the initial NbTe2 fully converts to Nb3Te4 during the annealing process (Figure 1d). This sample was found to contain nanowires. The PXRD pattern of the annealed sample was indexed and refined by leastsquares fitting, giving the lattice parameters a )

Figure 3. (a) SEM image of a cluster of Nb3Te4 wires. As a general observation, the nanowires appear to emanate from larger particles of Nb3Te4. (b) EDX spectra acquired from a large volume of material with an inset table of elemental ratios (At %).

10.665(5) Å and c ) 3.643(2) Å, which are in good agreement with those of Nb3Te4 (a ) 10.6710 Å and c ) 3.6468 Å).35 A small number of minor peaks were observed in the powder pattern for the annealed nanowire sample that could not be assigned to either NbTe2 or Nb3Te4. These peaks could not be identified as any other known compounds (e.g., containing Nb and/or Te) that may have formed during the reaction. SEM observation clarified that the growth of nanowires occurred during the anneal stage of the synthesis at 1160 °C. The secondary electron (SE) images Figure 2a,b shows NbTe2 particles after 1 h of sintering at 900 °C and Nb3Te4 nanowires after the 1160 °C anneal treatment, respectively. Closer examination of the sample demonstrated that the wires grow out from a central core as “sea-urchin-like” clusters (Figure 3a). The wires appear to have lengths on the scale of 30 µm. Figure 3b shows a typical EDX spectrum acquired from a large volume of material, which revealed the sample to have a Nb/Te atomic ratio of 43(2):57(2), consistent with the formula Nb3Te4 and in agreement with PXRD results. Quantitative EDX could not be performed on the very fine scale individual wires due to the porosity of the sample. However, backscattered electron (BSE) imaging indicated a lack of high contrast between the wires, suggesting the composition of the sample to be relatively constant throughout. Traces of oxygen and silicon were also identified within the sample. The likely source of these low-level contaminants is material from the walls of the SiO2 reaction vessel. SEM observation detected particles other than those apparently seeding nanowires, although again their porosity prevented quantitative EDX analysis. Figure 4a presents a representative TEM image of a nanowire recorded at a low magnification to illustrate dimensions. The wires were found to be straight with diameters of 50 nm to 1 µm and lengths from 1 µm to

Characterization of Nb3Te4 Nanowires

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Figure 4. TEM images and an SAED pattern of the Nb3Te4 nanowires. (a and b) TEM images of single nanowires. (c and d) HREM image and corresponding single-crystal SAED pattern of the (1 0 1 h 0) lattice fringes of an Nb3Te4 nanowire. The ringed spots have been indexed. (e) High magnification TEM image of rounded nanowire ends. The amorphous surface layer found on some nanowires is indicated by an arrow.

several micrometers. Many of the nanowires exhibited a bundled structure as if composed of many smaller filaments (Figure 4b), although SAED confirmed the wires to be single crystalline, suggesting a cooperative growth mechanism. Indexing of nanowire SAED patterns was consistent with their composition being hexagonal Nb3Te4 with the space group P63/m.35 The nanowires were found to fracture quite easily during sonication, suggesting they are mechanically quite weak, but when intact a majority were found to have rounded ends (Figure 4e). TEM observation also detected the presence of a thin surface layer of what appears to be amorphous material covering the ends and sides of some wires. HREM and SAED indexing revealed that the Nb3Te4 nanowires grow with the [0001] direction parallel to the wire long axis. The orientation of the Nb3Te4 planes within the wire has been further confirmed by measuring lattice fringes. Figure 4, panels c and d show a high magnification TEM image of planes within a nanowire and the corresponding SAED pattern of the [1 2h 1 0] projection, respectively. The plane spacings measure ∼9.24 Å and so correspond to the (1 0 1h 0) plane of Nb3Te4 (9.241 Å). This crystal structure orientation leads to Nb-Nb chains and open channels running along the long axis of the Nb3Te4 wire. Figure 5 illustrates the crystal structure of Nb3Te4. Figure 6a-c shows XPS survey scans and highresolution spectra of the fibrous sample annealed for 2 days, providing detail on the near surface chemistry of the nanowires. Nb and Te peak areas (Figure 6b,c) were compared to give a percent concentration of each element within an analyzed area of ∼1 cm2. The Nb/Te elemental ratio was found to be 0.76(1), which closely matches the expected ratio of 0.75 for the compound Nb3Te4. The high-resolution spectra also provide binding energy information. The Nb and Te peaks were compared with possible values for candidate Nb and Te compounds.42 Peak positions of 202.94 and 572.68 eV were found to closely match the Nb and Te binding

Figure 5. The crystal structure of Nb3Te4. Nb and Te atoms are indicated by red and blue spheres, respectively. Note (a) the Nb-Nb zigzag chains and (b) the open channels, both running parallel to the c-axis. At 2.86 Å, the intrachain NbNb distance is shorter than the interchain Nb-Nb distance of 3.854 Å.

Figure 6. (a) XPS survey scan, (b and c) high-resolution spectra of Nb and Te regions, and (d) table of detected and reference binding energies.

energies corresponding to Nb3Te4, of 202.8 and 572.6 eV, respectively. Figure 6d contains the tabulated binding energy data for the detected and reference peaks of Nb and Te. In addition, oxygen was also identified within the survey scans and oxide peaks were identified within the Nb and Te high-resolution spectra. The Nb oxide peak

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position of 206.84 eV fell between the reference values of NbO (204.7 eV) and Nb2O5 (207.5 eV), suggesting either an intermediate oxide such as Nb4O9 or a ternary niobium-tellurate. Similarly, the Te oxide peak of 576.26 eV fell between TeO2 (575.7 eV) and TeO3 (576.6 eV), pointing to the presence of Te2O5, Te4O9, or a ternary oxide, such as Nb2Te3O11. According to a paper published by Garbassi et al.43 on the XPS analysis of ternary niobium tellurates, both Nb2Te3O11 and Nb2Te4O13 have the same Nb and Te binding energies of 207.0 and 576.1 eV, respectively. These binding energies correspond very well to the nanowire results, indicating that the amorphous film is likely to be one of these ternary oxides. The complementary oxygen peak at 532.28 eV was too broad to distinguish between the candidate Nb oxide and Te oxide peak positions overlapping at 529-531 and 530 eV, respectively. Given the combined X-ray and electron diffraction results which confirm the wires to be Nb3Te4, it is considered that the wires have an Nb3Te4 core with an amorphous outer surface oxide layer. Discussion The structural characterization techniques of powder X-ray and electron diffraction, combined with the EDX and XPS techniques for chemical microanalysis, have confirmed the fabrication of Nb3Te4 nanowires with a thin oxide outer layer, by means of the synthesis technique described. The nanowires were found to be single crystalline with dimensions varying from 50 nm to 1 µm in diameter and 1-30 µm in length. Although a precise growth mechanism for these wires has not yet been established, from our investigation of samples sintered at 900 °C and then reheated to 1160 °C, it is evident that the annealing process is fundamental to the growth of the wires. The initial reaction of the elemental powders forms randomly orientated NbTe2 crystallites, which then become converted to Nb3Te4 wires and particles at 1160 °C. It is noted that Te has a melting point of 449.5 °C;39 therefore, elemental Te would tend to be molten during the initial sinter reaction with Nb and formation of the NbTe2 crystals. It is considered that a vapor transport process has subsequently occurred under the elevated annealing temperature and reduced pressure, with the resultant formation of the Nb3Te4 nanowires from the gas phase growing outward from an initial core of material (also transformed from NbTe2 to Nb3Te4). The suggestion of vapor phase mass transport is reinforced by the SE image of Figure 3a, which shows that the wires grow from a central point giving their characteristic seaurchin-like form. The SE observations also indicate the formation of hollowed core particles consistent with mass transportation from the inside out. The transformation from NbTe2 to Nb3Te4 should proceed via Te loss, but it is worth noting that no Te-containing phases other than Nb3Te4 are detected by PXD in the products. Indexing of the NbTe2 phase reveals contraction along c (i.e., between Nb-Te layers) relative to the reported parameters,44 suggesting the intial telluride formed is Nb1+xTe2 by analogy to Nb1+xS2 and, for example, V1+xTe2.45,46 This ditelluride phase is a subject of our further investigations. Structural investigation confirmed that the Nb3Te4 c-axis is parallel to the wire long axis. This is considered

Edwards et. al.

to be the growth direction. It is noted that Nb3Te4 has a large a and a small c lattice parameter of 10.671 and 3.6468 Å, respectively, and that growth direction of inorganic nanowires tends to be associated with the shortest lattice parameter in the compound. This makes these Nb3Te4 wires distinct from most other layered transition metal chalcogenide wires that have shorter a than c lattice parameters and so grow with their [001] direction perpendicular to the wire long axis.1,18 With reference to Figure 5, the wire long axis is also the direction of the shortest Nb-Nb bond length, with Nb chains running along the c-axis. This crystallographic orientation relationship is particularly interesting when considering the electrical and magnetic properties of Nb3Te4. The intrachain Nb-Nb bond lengths are 2.973 Å, approaching that of the Nb-Nb metallic bond distance of 2.86 Å, whereas the interchain Nb-Nb bond length is much larger at 3.854 Å.36 Ishihara et al.47 found a maximum in the electrical conduction in the [0001] direction through magnetoresistance experiments. It is suggested that the Nb chains are capable of carrying charge linearly, anisotropically decoupled by the intervening Te atoms. The disparate intra- and interchain Nb-Nb bond lengths accentuate the anisotropic conduction in Nb3Te4. Consequently, it is considered that a wire growth direction with a longitudinal axis parallel to the Nb3Te4 c-axis potentially offers some very attractive properties, such as high electrical conduction and possible superconductivity coupled with the potential for directional intercalation. TEM and XPS investigations further indicate the presence of a surface oxide. The layer is a few nanometers thick and of constant thickness across the top and sides of some of the wires. Images recorded at high magnification show that the original crystal planes of the Nb3Te4 appear to have been replaced with what is most probably a stabilizing amorphous film, indicating that oxidation occurred after wire growth. A similar amorphous layer was detected on the surface of some NbS2 nanowires.26 In this case, it was suspected that a nonstoichiometric, amorphous NbS2 phase had formed on the surface of the wires, which may gradually be converted to oxide. Nb3Te4 is isostructural with both Nb3Se4 and Nb3S4, but to date only NbSe2,30,31 NbS2,25-27 and NbSe332 have been synthesized as nanowires or nanotubes. Both the NbSe2 and NbSe3 nanowire syntheses are similar to methods employed to produce the telluride nanowires described here in that the elemental powders were sealed in silica ampules and heated to elevated temperature. However, the two groups working with NbS2 have found that the use of iodine as a transport medium is essential for the growth of nanowires. The 2H-NbSe2 and 3R-NbS2 nanowires have been found to be single crystals with their respective (002) and (003) crystal planes orientated perpendicularly to the long axis of the wires. The NbSe3 nanowires and ribbons very recently discovered by Hor et al.32 bear the closest resemblance among known nanostructures to the Nb3Te4 nanowires presented here. Both chalcogenides display a chargedensity-wave state at low temperature in the bulk and possess a pseudo-one-dimensional structure of Nb-Nb chains. In the case of NbSe3, the long axis of the

Characterization of Nb3Te4 Nanowires

nanowires and nanoribbons is coincident with the crystallographic b-axis, and hence as with Nb3Te4, Nb-Nb chains propagate along the length of the wire. Although this contrasts with Nb3Te4 in that in the latter compound it is the c-axis that is aligned parallel to the wire long axis, the structural basis is consistent for both one-dimensional structures. For NbSe3 with unit cell dimensions of a ) 10.009 Å, b ) 3.4805 Å, and c ) 13.629 Å,48 the b lattice parameter is the shortest. With regard to the possible formation mechanisms for transition metal chalcogenide nanowires, regardless of the transport effects of iodine, it has been proposed by Wu et al.25 that carbon may exert a “catalytic promotive” role during the growth of related NbS2 nanowires. The study of Nb3Te4 nanowires presented here demonstrates that such niobium chalcogenide wires can be produced in the absence of carbon or iodine. It has also been suggested by Qui et al.49 that based on the scarcity of literature on the subject, the preparation of telluride nanowires may be more difficult than that of the other chalcogen analogues. From our production of the first group 5 telluride nanowires, it appears that in fact Nb3Te4 nanowires are relatively straightforward to synthesize. Growth of novel single crystalline Nb3Te4 nanowires has been achieved by means of a chemical vapor route by the heating of Nb and Te elemental powders to produce NbTe2 crystals, followed by a high-temperature annealing stage to promote conversion to Nb3Te4 wires. The wires exhibited diameters ranging from 50 nm to 1µm and lengths from 1 to 30 µm. The Nb3Te4 nanowires grew radially from a central core. The wires propagate along the [0001] axis (coincident with the main wire axis). Nb-Nb chains and open channels hence also run parallel to the principal wire axis. Some surface oxidation of the wires was also identified attributed to postsynthesis surface reaction after exposure to the atmosphere. Acknowledgment. We would like to acknowledge the EPSRC and the University of Nottingham IDTC in Nanoscience and Nanotechnology for funding and the use of the EPSRC’s Chemical Database service at Daresbury. References (1) Tenne, R. Nature 1992, 360, 444. (2) Remskar, M. Adv. Mater. 2004, 16, 1497. (3) Zhang, X. Y.; Dai, J. Y.; Zheng, R. K.; Zhang, X. X.; Wang, N. Nanotechnology 2004, 15, 1166. (4) Mayers, B.; Xia, Y. N. Adv. Mater. 2002, 14, 279. (5) Yu, H.; Gibbons, P. C.; Buhro, W. E. J. Mater. Chem. 2004, 14, 595. (6) Shen, G.; Chen, D.; Tang, K.; Qian, Y. Mater. Res. Bull. 2004, 39, 2077. (7) Wei, G.; Deng, Y.; Lin, Y. H.; Nan, C. W. Chem. Phys. Lett. 2003, 372, 590. (8) An, C. H.; Tang, K. B.; Liu, X. M.; Qian, Y. T. Eur. J. Inorg. Chem. 2003, 2003, 3250. (9) Gautam, U. K.; Nath, M.; Rao, C. N. R. J. Mater. Chem. 2003, 13, 2845. (10) Cheng, B.; Samulski, E. T. Chem. Commun. 2003, 16, 2024. (11) Ma, R. Z.; Bando, Y.; Sato, T.; Kurashima, K. Chem. Mater. 2001, 13, 2965. (12) Lee, R. S.; Gavillet, J.; de la Chapelle, M. L.; Loiseau, A.; Cochon, J. L.; Pigache, D.; Thibault, J.; Willaime, F. Phys. Rev. B 2001, 64, 121405-1.

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