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Facile large scale synthesis of WS2 nanotubes from WO3 nanorods prepared by a hydrothermal route
Solid State Sciences 7 (2005) 67–72 www.elsevier.com/locate/ssscie
Facile large scale synthesis of WS2 nanotubes from WO3 nanorods prepared by a hydrothermal route Helen Annal Therese a , Jixue Li b , Ute Kolb b , Wolfgang Tremel a,∗ a Institut für Anorganische Chemie und Analytische Chemie der Johannes Gutenberg-Universität, Duesbergweg 10-14, 55099 Mainz, Germany b Institut für Physikalische Chemie, Welderweg 11, 55099 Mainz, Germany
Received 17 June 2004; received in revised form 27 July 2004; accepted 8 October 2004 Available online 13 December 2004
Abstract Hexagonal WO3 nanorods of 5–50 nm in diameter and 150–250 nm in length have been synthesised in gram quantities by a low temperature hydrothermal route using citric acid as a structural modifier and hexadecylamine as a templating agent. The ratio of [A]/[W] play an important role on WO3 nanorods formation. These WO3 nanorods were found highly suitable as a precursor for the synthesis of a good yield of multiwalled WS2 nanotubes by reducing them with H2 S at 840 ◦ C for 30 min. The length and the wall thickness of the WS2 nanotubes could be altered by controlled reduction of the oxide precursor. The morphology, structure and the composition of the WO3 nanorods and WS2 nanotubes were characterised by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray analysis (EDX) and by selected area electron diffraction techniques (SAED). 2004 Elsevier SAS. All rights reserved. Keywords: WO3 ; Nanorods; Hydrothermal route; WS2 ; Nanotubes
1. Introduction Following the discovery of carbon nanotubes in 1991 [1], nanostructured inorganic materials and their syntheses have attracted tremendous attention due to their superior mechanical properties, their unique electronic behaviour and their high potential in making technologically advanced nanodevices. The interesting properties of these nanomaterials arise from their enormous surface area, strength and the quantum size. Among nanomaterials, nanowires and nanotubes are good candidates for studying the phenomena such as electrical resistivity, strength, magnetic and optical properties in one dimension. During the past few years, nanotubes of various materials with graphite-like layered structures has been synthesised successfully using techniques such as arc discharge [2], laser ablation [3], electron beam irradiation [4], sonochemical [5], hydrothermal reaction [6] and * Corresponding author.
E-mail address: [email protected] (W. Tremel). 1293-2558/$ – see front matter 2004 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.10.006
iodine transport [7], etc. Among the various classes of noncarbon nanotubes, transition metal chalcogenides of the general formula MQ2 (M = W, Mo, V, Nb, Ta, Zr; Q = S, Se) are significant materials and reveal interesting electronic and optical properties [8–10]. Inorganic fullerene like MoS2 nanoparticles and MoS2 nanotubes exhibit excellent lubricating properties [11] and show high scope as tips for scanning probe microscopes [12]. Recent electrochemical studies on MoS2 nanotubes revealed that the nanotubes can store relatively large amounts of gaseous hydrogen by electrochemical storage [13]. MoS2 nanotubes of up to 5 µm length and 10–20 nm in diameter were first synthesised by Tenne and co-workers by reducing MoO3 in the atmosphere of a mixture of H2 /N2 and H2 S gas [14]. Recently, WO3 nanorods were found to be a versatile precursor in the synthesis of WS2 nanotubes apart from its utility in electrochromic devices [15], gas sensors [16], rechargeable lithium batteries [17], memory devices [18], etc. For example WS2 nanotubes were synthesised by annealing in a H2 S stream a nanostructured tungsten oxide precursor, which was produced by heating tungsten filaments
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under reduced pressure in the presence of water vapour [19, 20]. Walton and co-workers have synthesised WS2 tubes from W18 O49 rods produced by heating tungsten foil at high voltages under Ar in the presence of SiO2 [21]. Various strategies have been employed for the synthesis of WO3 nanorods as synthesising large quantities WO3 nanostructures of the required size becomes a greater task. WO3 nanorods were produced by heating the tungsten filament in an inert atmosphere either in the presence of oxides like SiO2 [22], B2 O3 [23] or in the presence of water vapour [20] or by IR irradiation of W in the presence of air [24]. Electrochemical etching followed by heating also yielded WO3 nanorods [25]. Controlled removal of the surfactants from the mesolamellar precursors resulted in WO3 nanorods in large quantity [26]. Recently, the synthesis of WO3 nanorods at a large scale [27] has been described by the oxidation of W(CO)6 with an amine oxide in a low-volatile solvent (oleylamine) at 270 ◦ C. Most of the above mentioned synthetic techniques are very tedious and require expensive experimental setup or inert atmosphere. Above all, obtaining a single phase compound of similar morphology and dimensions in considerable quantity still is a difficult task. In this article we report the synthesis of WO3 nanorods of 5–50 nm in diameter and 150–250 nm in length, in gram quantities by a simple hydrothermal method. WO3 nanorods were reduced with H2 S to obtain a very high yield of multiwalled WS2 nanotubes.
2. Experimental 2.1. Synthesis of WO3 nanorods An aqueous solution containing a mixture of 1.32 g of (NH4 )10 W12 O41 ·7H2 O and 2.10 g of citric acid was heated around 120 ◦ C under constant stirring for 4–5 h until a gel was formed, which was allowed to stand overnight. 2.45 g of hexadecyl amine dissolved in ethanol was added as an additive to the gel and stirred for 10 h. The resulting mixture was transferred to a Teflon autoclave with a stainless steel protective outer body and heated at 180 ◦ C for 7 days. The product obtained was washed with ethanol, cyclohexene, water and finally with ethanol and dried at room temperature. The importance of citric acid as a structural modifier was studied by replacing citric acid with hydrochloric acid, while maintaining the pH of the reactant same as the corresponding experiment carried with citric acid. 2.2. Conversion of WO3 nanorods to WS2 nanotubes An alumina crucible containing WO3 nanorods was placed in a tubular furnace and heated up to 840 ◦ C in Ar gas flow, then switched to H2 S gas for 30 min at 840 ◦ C to allow the complete conversion of oxide nanorods to tungsten sulphide nanotubes and finally cooled to room temperature in
the presence of Ar. A heating rate of 5 ◦ C min−1 was maintained during the heating/cooling process. 2.3. Characterisation The products obtained from hydrothermal reactions after washing and drying were analysed by X-ray powder diffraction in θ/2θ reflection geometry using Siemens D8 powder diffractometer equipped with a position sensitive detector. Data was collected between 2θ = 5◦ and 60◦ (using Cu-Kα radiation), at an operation potential of 40 kV and a current of 40 mA. The morphology of the WO3 nanorods and WS2 nanotubes was characterised by high-resolution transmission electron microscopy (FEI Tecnai F30 ST operated at an extraction voltage of 300 kV, equipped with an EDXA energy dispersive X-ray spectrometer) and by selected area electron diffraction techniques (SAED). For transmission electron microscopic (TEM) studies, the sample was prepared by crushing them mechanically with a mortar and pestle followed by dispersing the powder ultrasonically in absolute ethanol and placing a drop of this suspension on to a copper grid coated with a holey carbon films.
3. Results and discussion Representative TEM images of the tungsten oxide samples obtained from two different trials of hydrothermal reactions are given in Fig. 1 (with m and w given in the caption of Fig. 1). Fig. 1a shows a part of a WO3 particle with many nonseparable rods protruding out. TEM image of these rods are shown at a slightly higher magnification in the inset of Fig. 1a. This sample consists of particles exclusively grown in a bunch-like fashion. Fig. 1b shows very well separated WO3 nanorods aggregated together due to the high surface energy owing to their nanosize. The nanorod lengths in Fig. 1b range from 150 to 250 nm and their diameters vary from 5 to 50 nm. A calculation of the particle size distribution of these samples shows that more than 85% of the tungsten oxide rods are within the range of 15–50 nm in diameter. The powder X-ray diffraction pattern of the WO3 nanorods (shown in Fig. 2) could be well indexed based on a hexagonal cell of WO3 with lattice constants a = 7.37 Å and c = 3.77 Å. The lattice parameters of the WO3 rods reported here vary slightly from the reported (ICSD code = 32,001; a = 7.298 Å, c = 3.899 Å) values for the hexagonal WO3 . The reflection half-widths indicate the presence of nanoscale WO3 rods. The particle size of the nanorods calculated from the XRD pattern using Scherrer’s formula varies between 50 and 185 nm. Fig. 3 shows experimental and simulated HRTEM images and SAED diffraction patterns of a WO3 nanorod. The lattice parameters of 0.38 and 0.63 nm correspond to the d-spacings of (001) and (100) of the WO3 hexagonal cell.
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Fig. 1. TEM images of WO3 nanorods synthesised by hydrothermal reactions at different mole percentages m = [W]/([A] + [W] + [HDA]) (%) and ratios of w = [W]/[HAD] (%) ([W], [A] and [HAD] correspond to the number of moles of ammonium tungstate, citric acid and hexadecylamine). (a) Depicts the bunch-like morphology of the WO3 particle grown at m = 3.42 and w = 18.6, whereas (b) shows the morphology of WO3 rods obtained at grown at m = 1.8 and w = 1.6. The WO3 rods are shown at higher magnification in the inset. w values < 5 and m values 1 resulted in the formation of nanorods.
Fig. 2. Powder X-ray diffraction pattern of WO3 nanorods. All reflections are indexed based on a hexagonal WO3 cell with a = 7.37 Å, c = 3.77 Å.
These values are also in agreement with the lattice parameters obtained from the powder XRD pattern. All rods tend to grow along the ‘c’ direction. The high resolution image filtered via FFT with DM3.6 (see inset (a)) is in good agreement with the image of zone [010] (see inset (b)) simulated by multislice method [28,29] (thickness of 37 Å, defocus −855 Å, Cs = 1.2 mm) using Cerius [30]. The same holds for the dynamically calculated diffraction pattern compared with the experimental SAED pattern. Selected area energy dispersive X-ray analyses (EDX) of individual nanorods exhibit the existence of tungsten and oxygen in an atomic ratio of 1:3. Orthorhombic WO3 ·1/3H2 O needles has been synthesised by Figlarz and co-workers [31] by hydrothermal synthesis of tungstic gel at 120 ◦ C. These needles on heating at 250 ◦ C yielded hexagonal WO3 , but there was nothing mentioned about the yield of such needles. In the present report, it is important to mention that samples prepared for m 1 and w < 5 contain exclusively hexagonal WO3 nanorods. Recently, nanotubes of VOx have been synthesised [32–34] by low temperature sol–gel techniques. In both cases of VOx nanotube synthesis, hexadecyl amine used as a template gets
Fig. 3. Experimental HRTEM image of a WoO3 nanorod along b axis (top); the filtered image is shown as inset (a) the corresponding simulated image as inset (b) together with experimental SAED pattern (bottom, left-hand side) and dynamically calculated ED pattern for zone [010] (bottom, right-hand side).
intercalated into the vanadium oxide structure, resulting in larger d-spacings (∼ 3 nm). From the lattice spacings of the WO3 rods reported here one could see that the hexadecyl amine is not intercalated into the WO3 nanorods. This also helps us in availing this material as a precursor for WS2 –NT synthesis. Synthesis of WO3 nanorods was also carried out using hydrochloric acid instead of citric acid, while maintaining the pH similar to the reaction which yielded WO3 nanorods. This reaction resulted in a product containing a
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Fig. 4. A low resolution TEM image of the WS2 nanotubes (a) indicates the yield of nanotubes during the conversion process. TEM image of hollow WS2 nanotubes (b). The sheet-like appearance in (b) are due to Moire patterns. The open end of a nanotube is indicated by an arrow. HRTEM micrograph of a typical MWNT (c) along with its SAED pattern (d). The chiral angle of the nanotubes could be calculated as ∼ 10◦ based on the SAED.
mixture of longer rods of various thickness and unevenly shaped crystalline particles. Similarly synthesis carried out for m < 1 and w < 5 resulted in a mixed product with more of highly crystalline WO3 particles and a few bunch-like WO3 particles. In our study, the role of citric acid as a structural modifier and the mechanism involved in the growth of WO3 is not clear. But from the above study we could infer that the amount of citric acid plays an important role in the formation of WO3 nanorods. One could speculate that at higher m values, the 3 carboxylate group of each citric acid could bind to more than one WO6 octahedron and helping in the olation of WO6 in a directed manner while hindering the oxolation in all directions due to steric effects. This could lead to the formation of shorter and thinner rods. On the other hand, at lower m values, two or more oxygens of the WO6 octahedra could be contributed from a single citric acid molecule,
hence hindering both the olation and oxolation to a large extent. During hydrothermal reaction at 180 ◦ C, when citric acid decomposes the hydrophobic hexadecyl amine template could be helping in preserving the rod like structure of the tungsten oxides. For w values > 5 the surface coverage of the growing WO3 nanorods is not sufficient to limit the particle growth and WO3 bunches and rods are obtained. When citric acid is fully replaced by hydrochloric acid condensation of WO6 takes place rather very quickly resulting in a mixture of longer and thicker rods and crystals. The nanorods were converted to WS2 nanotubes by heating the WO3 nanorods in Ar gas up to 840 ◦ C and then treating them in hydrogen disulphide atmosphere for 30 min. TEM images of WS2 nanotubes obtained after H2 S reduction (Fig. 4a) show the high yield of WS2 nanotubes. However a manifold increase in the diameter and the length of the WS2 tubes compared to the WO3 starting material was
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Fig. 5. The various defects formed in a nanotube during the reduction process (a). HRTEM micrograph of a typical open ended WS2 nanotube encapsulated by a WS2 mantle (b). The shorter nanotubes obtained when the nanorods were reduced for a duration of 10 min under H2 S are shown in (c) and (d).
observed. The nanotube thicknesses range broadly from 20 to 200 nm and their length varies approximately from 1 to 8 µm. A large fraction of the nanotubes has open ends (Fig. 4b). Studies on these nanotubes by HRTEM combined with EDX analyses reveal the complete conversion of oxide rods to sulphide tubes during the reduction process which allows the synthesis of large amounts of multiwalled nanotubes (MWNTs). A HRTEM image of one such representative MWNT is shown in Fig. 4c. The interlayer spacing of 0.65 nm between the tubular walls is consistent with the (002) d-spacing of 2H–WS2 lattice. The helicity of the nanotube (Fig. 4c) could be calculated as ∼ 10◦ based on the selected area diffraction (SAED) pattern (Fig. 4c) of the multiwalled WS2 nanotube [35]. The synthesis of WS2 nanotubes by reduction of WOx nanorods has been reported earlier, where tungsten disulphide tubes were produced by heating WOx particles first in flowing H2 /N2 mixture and then in flowing H2 S gas.
In consistency with WS2 tubes reported by Tenne and coworkers [20] the WS2 nanotubes reported in this contribution also contain both open (see the tubes marked by an arrow in Fig. 4b) and closed ends (Fig. 5b), and exhibit plenty of defects. One such nanotube with defects is shown in Fig. 5a. A mechanism for the growth of nanotubes from oxide whiskers and rods has been proposed previously [19, 20]. According to this mechanism the growth of WS2 layer starts by encapsulating the WOx particle anisotropically in the initial phase of the reaction with the H2 /N2 and H2 S flow. During the course of the reaction this embryonic WS2 layer starts growing inward as well as slowly converting the oxide, which is continuously growing on the other end of the particles by the condensation of WOx from the vapour state. A similar mechanism is plausible in the present nanotube synthesis, where the role of the reducing H2 /N2 gas has been replaced by the pretreatment of the oxide with Ar gas. A TEM analysis of the oxide rods after the pretreatment with
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Ar shows the formation of an intermediate tungsten oxide with many defects while the appearance of the rods was retained. This could be related to a higher surface activity of the oxide nanorods. We have also observed many interesting structures such as open multiwalled WS2 tubes coated with a WS2 mantle (Fig. 5b). It appears that the outer tubes have grown on top of the inner MWNT. This could be explained by the formation of WS2 in the vapour phase followed by the growth of WS2 layers on the preexisting WS2 nanotubes. When the reduction period of WO3 nanorods was decreased to 10 min we obtained MWNTs with a length of 30 nm (Fig. 5c and 5d) with 3–5 tube walls. Some of these particles also can be described as a slightly elongated fullerene structured WS2 .
4. Conclusions In conclusion, we have synthesised WO3 nanorods in large quantities by a low temperature sol–gel route. These nanorods were then converted by reduction with H2 S to get WS2 nanotubes in large quantities. This simple and inexpensive approach might be extended to the synthesis of other MS2 nanotubes. It was also possible to control the wall thickness and the length of the nanotubes by selecting appropriate reaction times for the reduction.
Acknowledgement We are grateful to the Federal Ministry for Research and Technology (BMBF) for the support of this research within the program “Multifunctional Materials and Miniaturized Devices” at the University of Mainz and the Deutsche Forschungsgmeinschaft (DFG, SFB 625).
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Facile large scale synthesis of WS2 nanotubes from WO3 nanorods prepared by a hydrothermal route Helen Annal Therese a , Jixue Li b , Ute Kolb b , Wolfgang Tremel a,∗ a Institut für Anorganische Chemie und Analytische Chemie der Johannes Gutenberg-Universität, Duesbergweg 10-14, 55099 Mainz, Germany b Institut für Physikalische Chemie, Welderweg 11, 55099 Mainz, Germany
Received 17 June 2004; received in revised form 27 July 2004; accepted 8 October 2004 Available online 13 December 2004
Abstract Hexagonal WO3 nanorods of 5–50 nm in diameter and 150–250 nm in length have been synthesised in gram quantities by a low temperature hydrothermal route using citric acid as a structural modifier and hexadecylamine as a templating agent. The ratio of [A]/[W] play an important role on WO3 nanorods formation. These WO3 nanorods were found highly suitable as a precursor for the synthesis of a good yield of multiwalled WS2 nanotubes by reducing them with H2 S at 840 ◦ C for 30 min. The length and the wall thickness of the WS2 nanotubes could be altered by controlled reduction of the oxide precursor. The morphology, structure and the composition of the WO3 nanorods and WS2 nanotubes were characterised by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), energy dispersive X-ray analysis (EDX) and by selected area electron diffraction techniques (SAED). 2004 Elsevier SAS. All rights reserved. Keywords: WO3 ; Nanorods; Hydrothermal route; WS2 ; Nanotubes
1. Introduction Following the discovery of carbon nanotubes in 1991 [1], nanostructured inorganic materials and their syntheses have attracted tremendous attention due to their superior mechanical properties, their unique electronic behaviour and their high potential in making technologically advanced nanodevices. The interesting properties of these nanomaterials arise from their enormous surface area, strength and the quantum size. Among nanomaterials, nanowires and nanotubes are good candidates for studying the phenomena such as electrical resistivity, strength, magnetic and optical properties in one dimension. During the past few years, nanotubes of various materials with graphite-like layered structures has been synthesised successfully using techniques such as arc discharge [2], laser ablation [3], electron beam irradiation [4], sonochemical [5], hydrothermal reaction [6] and * Corresponding author.
E-mail address: [email protected] (W. Tremel). 1293-2558/$ – see front matter 2004 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.10.006
iodine transport [7], etc. Among the various classes of noncarbon nanotubes, transition metal chalcogenides of the general formula MQ2 (M = W, Mo, V, Nb, Ta, Zr; Q = S, Se) are significant materials and reveal interesting electronic and optical properties [8–10]. Inorganic fullerene like MoS2 nanoparticles and MoS2 nanotubes exhibit excellent lubricating properties [11] and show high scope as tips for scanning probe microscopes [12]. Recent electrochemical studies on MoS2 nanotubes revealed that the nanotubes can store relatively large amounts of gaseous hydrogen by electrochemical storage [13]. MoS2 nanotubes of up to 5 µm length and 10–20 nm in diameter were first synthesised by Tenne and co-workers by reducing MoO3 in the atmosphere of a mixture of H2 /N2 and H2 S gas [14]. Recently, WO3 nanorods were found to be a versatile precursor in the synthesis of WS2 nanotubes apart from its utility in electrochromic devices [15], gas sensors [16], rechargeable lithium batteries [17], memory devices [18], etc. For example WS2 nanotubes were synthesised by annealing in a H2 S stream a nanostructured tungsten oxide precursor, which was produced by heating tungsten filaments
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under reduced pressure in the presence of water vapour [19, 20]. Walton and co-workers have synthesised WS2 tubes from W18 O49 rods produced by heating tungsten foil at high voltages under Ar in the presence of SiO2 [21]. Various strategies have been employed for the synthesis of WO3 nanorods as synthesising large quantities WO3 nanostructures of the required size becomes a greater task. WO3 nanorods were produced by heating the tungsten filament in an inert atmosphere either in the presence of oxides like SiO2 [22], B2 O3 [23] or in the presence of water vapour [20] or by IR irradiation of W in the presence of air [24]. Electrochemical etching followed by heating also yielded WO3 nanorods [25]. Controlled removal of the surfactants from the mesolamellar precursors resulted in WO3 nanorods in large quantity [26]. Recently, the synthesis of WO3 nanorods at a large scale [27] has been described by the oxidation of W(CO)6 with an amine oxide in a low-volatile solvent (oleylamine) at 270 ◦ C. Most of the above mentioned synthetic techniques are very tedious and require expensive experimental setup or inert atmosphere. Above all, obtaining a single phase compound of similar morphology and dimensions in considerable quantity still is a difficult task. In this article we report the synthesis of WO3 nanorods of 5–50 nm in diameter and 150–250 nm in length, in gram quantities by a simple hydrothermal method. WO3 nanorods were reduced with H2 S to obtain a very high yield of multiwalled WS2 nanotubes.
2. Experimental 2.1. Synthesis of WO3 nanorods An aqueous solution containing a mixture of 1.32 g of (NH4 )10 W12 O41 ·7H2 O and 2.10 g of citric acid was heated around 120 ◦ C under constant stirring for 4–5 h until a gel was formed, which was allowed to stand overnight. 2.45 g of hexadecyl amine dissolved in ethanol was added as an additive to the gel and stirred for 10 h. The resulting mixture was transferred to a Teflon autoclave with a stainless steel protective outer body and heated at 180 ◦ C for 7 days. The product obtained was washed with ethanol, cyclohexene, water and finally with ethanol and dried at room temperature. The importance of citric acid as a structural modifier was studied by replacing citric acid with hydrochloric acid, while maintaining the pH of the reactant same as the corresponding experiment carried with citric acid. 2.2. Conversion of WO3 nanorods to WS2 nanotubes An alumina crucible containing WO3 nanorods was placed in a tubular furnace and heated up to 840 ◦ C in Ar gas flow, then switched to H2 S gas for 30 min at 840 ◦ C to allow the complete conversion of oxide nanorods to tungsten sulphide nanotubes and finally cooled to room temperature in
the presence of Ar. A heating rate of 5 ◦ C min−1 was maintained during the heating/cooling process. 2.3. Characterisation The products obtained from hydrothermal reactions after washing and drying were analysed by X-ray powder diffraction in θ/2θ reflection geometry using Siemens D8 powder diffractometer equipped with a position sensitive detector. Data was collected between 2θ = 5◦ and 60◦ (using Cu-Kα radiation), at an operation potential of 40 kV and a current of 40 mA. The morphology of the WO3 nanorods and WS2 nanotubes was characterised by high-resolution transmission electron microscopy (FEI Tecnai F30 ST operated at an extraction voltage of 300 kV, equipped with an EDXA energy dispersive X-ray spectrometer) and by selected area electron diffraction techniques (SAED). For transmission electron microscopic (TEM) studies, the sample was prepared by crushing them mechanically with a mortar and pestle followed by dispersing the powder ultrasonically in absolute ethanol and placing a drop of this suspension on to a copper grid coated with a holey carbon films.
3. Results and discussion Representative TEM images of the tungsten oxide samples obtained from two different trials of hydrothermal reactions are given in Fig. 1 (with m and w given in the caption of Fig. 1). Fig. 1a shows a part of a WO3 particle with many nonseparable rods protruding out. TEM image of these rods are shown at a slightly higher magnification in the inset of Fig. 1a. This sample consists of particles exclusively grown in a bunch-like fashion. Fig. 1b shows very well separated WO3 nanorods aggregated together due to the high surface energy owing to their nanosize. The nanorod lengths in Fig. 1b range from 150 to 250 nm and their diameters vary from 5 to 50 nm. A calculation of the particle size distribution of these samples shows that more than 85% of the tungsten oxide rods are within the range of 15–50 nm in diameter. The powder X-ray diffraction pattern of the WO3 nanorods (shown in Fig. 2) could be well indexed based on a hexagonal cell of WO3 with lattice constants a = 7.37 Å and c = 3.77 Å. The lattice parameters of the WO3 rods reported here vary slightly from the reported (ICSD code = 32,001; a = 7.298 Å, c = 3.899 Å) values for the hexagonal WO3 . The reflection half-widths indicate the presence of nanoscale WO3 rods. The particle size of the nanorods calculated from the XRD pattern using Scherrer’s formula varies between 50 and 185 nm. Fig. 3 shows experimental and simulated HRTEM images and SAED diffraction patterns of a WO3 nanorod. The lattice parameters of 0.38 and 0.63 nm correspond to the d-spacings of (001) and (100) of the WO3 hexagonal cell.
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Fig. 1. TEM images of WO3 nanorods synthesised by hydrothermal reactions at different mole percentages m = [W]/([A] + [W] + [HDA]) (%) and ratios of w = [W]/[HAD] (%) ([W], [A] and [HAD] correspond to the number of moles of ammonium tungstate, citric acid and hexadecylamine). (a) Depicts the bunch-like morphology of the WO3 particle grown at m = 3.42 and w = 18.6, whereas (b) shows the morphology of WO3 rods obtained at grown at m = 1.8 and w = 1.6. The WO3 rods are shown at higher magnification in the inset. w values < 5 and m values 1 resulted in the formation of nanorods.
Fig. 2. Powder X-ray diffraction pattern of WO3 nanorods. All reflections are indexed based on a hexagonal WO3 cell with a = 7.37 Å, c = 3.77 Å.
These values are also in agreement with the lattice parameters obtained from the powder XRD pattern. All rods tend to grow along the ‘c’ direction. The high resolution image filtered via FFT with DM3.6 (see inset (a)) is in good agreement with the image of zone [010] (see inset (b)) simulated by multislice method [28,29] (thickness of 37 Å, defocus −855 Å, Cs = 1.2 mm) using Cerius [30]. The same holds for the dynamically calculated diffraction pattern compared with the experimental SAED pattern. Selected area energy dispersive X-ray analyses (EDX) of individual nanorods exhibit the existence of tungsten and oxygen in an atomic ratio of 1:3. Orthorhombic WO3 ·1/3H2 O needles has been synthesised by Figlarz and co-workers [31] by hydrothermal synthesis of tungstic gel at 120 ◦ C. These needles on heating at 250 ◦ C yielded hexagonal WO3 , but there was nothing mentioned about the yield of such needles. In the present report, it is important to mention that samples prepared for m 1 and w < 5 contain exclusively hexagonal WO3 nanorods. Recently, nanotubes of VOx have been synthesised [32–34] by low temperature sol–gel techniques. In both cases of VOx nanotube synthesis, hexadecyl amine used as a template gets
Fig. 3. Experimental HRTEM image of a WoO3 nanorod along b axis (top); the filtered image is shown as inset (a) the corresponding simulated image as inset (b) together with experimental SAED pattern (bottom, left-hand side) and dynamically calculated ED pattern for zone [010] (bottom, right-hand side).
intercalated into the vanadium oxide structure, resulting in larger d-spacings (∼ 3 nm). From the lattice spacings of the WO3 rods reported here one could see that the hexadecyl amine is not intercalated into the WO3 nanorods. This also helps us in availing this material as a precursor for WS2 –NT synthesis. Synthesis of WO3 nanorods was also carried out using hydrochloric acid instead of citric acid, while maintaining the pH similar to the reaction which yielded WO3 nanorods. This reaction resulted in a product containing a
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Fig. 4. A low resolution TEM image of the WS2 nanotubes (a) indicates the yield of nanotubes during the conversion process. TEM image of hollow WS2 nanotubes (b). The sheet-like appearance in (b) are due to Moire patterns. The open end of a nanotube is indicated by an arrow. HRTEM micrograph of a typical MWNT (c) along with its SAED pattern (d). The chiral angle of the nanotubes could be calculated as ∼ 10◦ based on the SAED.
mixture of longer rods of various thickness and unevenly shaped crystalline particles. Similarly synthesis carried out for m < 1 and w < 5 resulted in a mixed product with more of highly crystalline WO3 particles and a few bunch-like WO3 particles. In our study, the role of citric acid as a structural modifier and the mechanism involved in the growth of WO3 is not clear. But from the above study we could infer that the amount of citric acid plays an important role in the formation of WO3 nanorods. One could speculate that at higher m values, the 3 carboxylate group of each citric acid could bind to more than one WO6 octahedron and helping in the olation of WO6 in a directed manner while hindering the oxolation in all directions due to steric effects. This could lead to the formation of shorter and thinner rods. On the other hand, at lower m values, two or more oxygens of the WO6 octahedra could be contributed from a single citric acid molecule,
hence hindering both the olation and oxolation to a large extent. During hydrothermal reaction at 180 ◦ C, when citric acid decomposes the hydrophobic hexadecyl amine template could be helping in preserving the rod like structure of the tungsten oxides. For w values > 5 the surface coverage of the growing WO3 nanorods is not sufficient to limit the particle growth and WO3 bunches and rods are obtained. When citric acid is fully replaced by hydrochloric acid condensation of WO6 takes place rather very quickly resulting in a mixture of longer and thicker rods and crystals. The nanorods were converted to WS2 nanotubes by heating the WO3 nanorods in Ar gas up to 840 ◦ C and then treating them in hydrogen disulphide atmosphere for 30 min. TEM images of WS2 nanotubes obtained after H2 S reduction (Fig. 4a) show the high yield of WS2 nanotubes. However a manifold increase in the diameter and the length of the WS2 tubes compared to the WO3 starting material was
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Fig. 5. The various defects formed in a nanotube during the reduction process (a). HRTEM micrograph of a typical open ended WS2 nanotube encapsulated by a WS2 mantle (b). The shorter nanotubes obtained when the nanorods were reduced for a duration of 10 min under H2 S are shown in (c) and (d).
observed. The nanotube thicknesses range broadly from 20 to 200 nm and their length varies approximately from 1 to 8 µm. A large fraction of the nanotubes has open ends (Fig. 4b). Studies on these nanotubes by HRTEM combined with EDX analyses reveal the complete conversion of oxide rods to sulphide tubes during the reduction process which allows the synthesis of large amounts of multiwalled nanotubes (MWNTs). A HRTEM image of one such representative MWNT is shown in Fig. 4c. The interlayer spacing of 0.65 nm between the tubular walls is consistent with the (002) d-spacing of 2H–WS2 lattice. The helicity of the nanotube (Fig. 4c) could be calculated as ∼ 10◦ based on the selected area diffraction (SAED) pattern (Fig. 4c) of the multiwalled WS2 nanotube [35]. The synthesis of WS2 nanotubes by reduction of WOx nanorods has been reported earlier, where tungsten disulphide tubes were produced by heating WOx particles first in flowing H2 /N2 mixture and then in flowing H2 S gas.
In consistency with WS2 tubes reported by Tenne and coworkers [20] the WS2 nanotubes reported in this contribution also contain both open (see the tubes marked by an arrow in Fig. 4b) and closed ends (Fig. 5b), and exhibit plenty of defects. One such nanotube with defects is shown in Fig. 5a. A mechanism for the growth of nanotubes from oxide whiskers and rods has been proposed previously [19, 20]. According to this mechanism the growth of WS2 layer starts by encapsulating the WOx particle anisotropically in the initial phase of the reaction with the H2 /N2 and H2 S flow. During the course of the reaction this embryonic WS2 layer starts growing inward as well as slowly converting the oxide, which is continuously growing on the other end of the particles by the condensation of WOx from the vapour state. A similar mechanism is plausible in the present nanotube synthesis, where the role of the reducing H2 /N2 gas has been replaced by the pretreatment of the oxide with Ar gas. A TEM analysis of the oxide rods after the pretreatment with
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Ar shows the formation of an intermediate tungsten oxide with many defects while the appearance of the rods was retained. This could be related to a higher surface activity of the oxide nanorods. We have also observed many interesting structures such as open multiwalled WS2 tubes coated with a WS2 mantle (Fig. 5b). It appears that the outer tubes have grown on top of the inner MWNT. This could be explained by the formation of WS2 in the vapour phase followed by the growth of WS2 layers on the preexisting WS2 nanotubes. When the reduction period of WO3 nanorods was decreased to 10 min we obtained MWNTs with a length of 30 nm (Fig. 5c and 5d) with 3–5 tube walls. Some of these particles also can be described as a slightly elongated fullerene structured WS2 .
4. Conclusions In conclusion, we have synthesised WO3 nanorods in large quantities by a low temperature sol–gel route. These nanorods were then converted by reduction with H2 S to get WS2 nanotubes in large quantities. This simple and inexpensive approach might be extended to the synthesis of other MS2 nanotubes. It was also possible to control the wall thickness and the length of the nanotubes by selecting appropriate reaction times for the reduction.
Acknowledgement We are grateful to the Federal Ministry for Research and Technology (BMBF) for the support of this research within the program “Multifunctional Materials and Miniaturized Devices” at the University of Mainz and the Deutsche Forschungsgmeinschaft (DFG, SFB 625).
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