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Synthesis of tungsten disulphide nanoparticles by the chemical vapor condensation method
ARTICLE IN PRESS Microelectronics Journal 40 (2009) 687–691
Contents lists available at ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
Synthesis of tungsten disulphide nanoparticles by the chemical vapor condensation method E.S. Vasilyeva a,, O.V. Tolochko a, B.K. Kim b, D.W. Lee b, D.S. Kim b a b
Material Science Faculty, State Technical University, Polytechnicheskaya Street, 29, 195251 Saint-Petersburg, Russian Federation Korea Institute of Material Science 66, Sangnam-Dong, Changwon, Kyungnam 641-010, South Korea
a r t i c l e in fo
abstract
Article history: Received 28 February 2008 Received in revised form 13 November 2008 Accepted 24 November 2008 Available online 31 January 2009
Crystalline tungsten disulphide nanoparticles were successfully synthesized by the chemical vapor condensation (CVC) method. The process performed as decomposition of tungsten hexacarbonyl over sulphur vapor in inert gas flow, where WS2 nanoparticles were synthesized by direct reaction between as formed pure tungsten nanoclusters and sulphur vapor. Influence of experimental parameters on shape, size distribution, structure and phase composition of nanoparticles were evaluated by transmission and scanning electron microscopy and X-ray diffraction analysis. The produced nanoparticles have closely spherical shape with the mean size in the range 20–70 nm in diameter dependently of process parameters. Nested ‘‘onion-like’’ structure of nanoparticles was observed. The mean value of interlayer distance in the {0 0 0 1} direction, is about 0.635870.031 nm. Due to nanodimensional size, physical properties and layered structure tungsten disulphide nanoparticles have great potential as a solid lubricant material. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Tungsten disulphide Nanoparticles Fulleren-like structures Nanofabrication Tribology Solid lubricants
1. Introduction It is well known that chalcogenides of transition metals forming layered structure like disulphides, diselenides, ditellurides and fluorides of molybdenum, titanium, vanadium, chromium and others suitable as lubricants, the tungsten chalcogenides have better oxidation resistance at high temperature, quite low friction coefficient and chemical stability as in high vacuum as well in atmospheric air conditions [1]. Hence, WS2 nanopowder has potential beneficial effects as solid lubricants and as additives in the liquid lubricants. In analogy to graphite, nanoparticles of named compounds with a layered structure can form a various enclosed structures such as polyhedrical, fullerene-like (IF) and nanotubular structures [2,3]. Such structures are characterized by weak interatomic interactions like van der Waals force between their hexagonal planes. Hence these compounds can be used as solid lubricants and have great potential in high-temperature tribology. Varying interplane distances in the ‘‘c’’-direction are usually found in such structures [4], whereas the distances in the ‘‘a’’-direction are always close to the bulk material. Recently, two major methods have been used for synthesis of tungsten disulphide nanoparticles, and both require the sulfidization process of tungsten powder through the H2S gas or sulphur
Corresponding author. Tel./fax: +7 812 552 89 69.
E-mail addresses: [email protected], [email protected] (E.S. Vasilyeva). 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.11.066
vapor atmosphere. However, in one of the methods, tungsten nanoparticles form through the reduction of WO3 as in experiments of Tenne [2] or sometimes particles form from a tungsten metalorganic precursor by microwave plasma synthesis as shown in the experiments of Vollath and Szabo [5]. However, both require expensive equipment, though they provide processes involving only few operations, it is not easy to control the particle mean size. In this view, developing a technique for the synthesis of such structures is a subject of great interest. The chemical vapor condensation (CVC) process, originally developed for the synthesis of single-component metallic nanoparticles, was used to produce pure iron, cobalt, copper, tungsten nanoparticles and theirs alloys in a variety of particles mean size from 6 to 100 nm [6,7]. In the present work, we study the synthesis conditions and structure of WS2 nanoparticles by the chemical vapor condensation method. The friction and wear behavior of composite materials filled by produced WS2 nanopowder were also studied.
2. Experimental details 2.1. Nanoparticles synthesis Synthesis of nanoparticles by the CVC method was described previously [6–8]. The general scheme of the chemical vapor condensation method is presented in Fig. 1.
ARTICLE IN PRESS 688
E.S. Vasilyeva et al. / Microelectronics Journal 40 (2009) 687–691
In order to remove the excess of sulphur, as-prepared nanoparticles were heat-treated in the two-zone furnace in a sealed silica tube. Excess of free pure sulphur from the powder was evaporated at the temperature of 200 1C and condensed in the cold zone of the reactor at room temperature. 2.2. Characterization methods The phase and structural analysis of samples was carried out on a DRON 2.0 diffractometer with monochromatic Ka copper radiation (l ¼ 0.154051 nm). The morphological characterization and size distribution of nanoparticles were determined by means of high-resolution transmission electron microscopy (HR TEM). Specimen preparation of samples to study individual particles was done in the following way: produced nanopowders were ultrasonically dispersed in ethanol and dropped into the carbon-coated Cu-grids. The average particle size of each sample was calculated as the center of gravity of particles size distribution, which was estimated by measuring of diameters 200–300 nanoparticles from TEM microphotographs. Previously, the particle mean size was identified based on the width of diffraction peaks, using the Sherrer equation [9]. 2.3. Tribological testing
Fig. 1. Scheme of (CVC) laboratory setup designed for WS2 nanoparticles production.
First, pure tungsten nanoparticles were prepared for the evaluation of process parameters. The experiment was performed in a quartz tube passing a heated furnace; carrier gas of highpurity argon or helium was fed through a heated bubbling unit containing the metalorganic precursor of tungsten hexocarbonyl (W(CO)6) at the vaporization temperature in range 95–140 1C at a carrier gas flow rate of 500 cc/min. Concentration of precursor in the gas phase was controlled by the temperature of its evaporation. The flow of argon gas mixed with the precursor vapor passes through a heated pipe system into the high-temperature reactor where nanoparticles are formed as a result of precursor decomposition with further condensation in a water-cooled collecting chamber. In order to produce tungsten nanoparticles, the precursor decomposition temperature was estimated in the range 700–1100 1C. Using both accelerating gas and carrier gas allowed changing precursor residential time in the reaction zone and concentration of precursor in the gas phase. In order to produce nanocrystalline particles of tungsten disulphide, decomposition of tungsten hexocarbonyl was carried out in the presence of sulphur vapor with the partial pressure of S2 vapor being 0.02–0.04 atm. In such conditions we achieve about 10 times more pure sulphur in the gas phase. A specially designed two-zoned furnace (Fig. 1) allows fixing two different temperatures at two different heights of the reactor. The removable cartridge filled by crystalline sulphur was located in the first (lower) zone of the reactor at the evaporation temperature in interval from 325 up to 400 1C. The gas flow inclusive vapor of the precursor passed subsequently through both zones of the reactor. In the lower zone of the reactor, sulphur vapor was mixed with carbonyl vapor. In the second zone, mixed gas was heated up to the carbonyl pyrolysis conditions, as shown in Fig. 1. Hence, precursor decomposition and nanoparticles formation run at 800–1000 1C in the sulphur-contained atmosphere. As a result, the formation of crystalline pure tungsten disulphide nanoparticles with excess of crystalline sulphur was observed.
Tribological testing of nanocomposite samples containing tungsten disulphide nanoparticles was carried out in couple with bronze using a pin-on-disc wear tester. All tests were performed at room temperature at a load of 300 N and a sliding speed of 0.2 m/s in dry friction conditions.
3. Results and discussion Phase composition of synthesized powders depends on experimental parameters such as accelerating gas flow rate and reactor temperature. Fig. 2 shows the phase composition of products as a function of experimental parameters. The region of pure WS2 existence becomes wider with the increase in reaction
Fig. 2. Phase diagram of products of synthesis shows dependence of nanoparticles chemical and phase composition from a gas flow rate and precursor decomposition temperature. Evaporation temperature of W(CO)6. Gas flow passed through the precursor contained a unit and accelerating gas was not used.
ARTICLE IN PRESS E.S. Vasilyeva et al. / Microelectronics Journal 40 (2009) 687–691
temperature and decrease in Ar-accelerated gas flow rate. Due to laminar gas flow in the vertical-type reactor, the residence time of particles in the reactor and the time of reaction between tungsten and sulphur increased almost linearly with decrease in gas flow rate. As residential time in the reactor decreases, pure W nanoparticles could be observed. In that case, considering that sulphur excess always exist in the reaction system, it is possible to assume that there is not enough time to complete the reaction: W(s)+S2(g) ¼ WS2(s). At the same time the Gibbs energy of this reaction increases with increase in temperature. It allows supposing that kinetically reaction rate increases with increase in temperature. The typical appearance of synthesized particles is depicted in Fig. 3. Some agglomeration of nanoparticles can be observed after synthesis. Two kinds of nanoparticles found out from HR TEM microphotographs in the as-synthesized powders: crystalline tungsten disulphide with an onion-like structure (Fig. 3(a) and (c)) and nanoparticles of pure sulphur (Fig. 3(b)). Produced WS2 nanoparticles generally have a close round shape and their TEM image demonstrates the formation of the so-called onion-like
689
structure, which consists of concentric layers. Nanoparticles have mean size in the interval of 20–70 nm. The particle mean size insignificantly increases with the increase in reaction temperature and decrease in accelerated gas flow rate. Crystals of pure sulphur having been overall removed from powders after annealing at 200 1C, mean size and structure of WS2 nanoparticles after annealing at such low temperatures were found to be the same as initial WS2 nanoparticles. Typical X-ray diffraction patterns for nanoparticles synthesized at the different precursor decomposition temperatures are presented in Fig. 4. As the reaction temperature increases, there were changes in WS2 structure modification from 2H to 3R. The observed XRD patterns are in good agreement with reference data. There is good consistency in the number and positions of diffraction maxima; however, there are relevant differences in the relative intensities of particular reflections and significant broadening of experimental peaks in comparison with theoretical data for WS2 micron-sized powder. The particle’s mean size, which was estimated based on the width of diffraction peaks, was estimated to be in the interval of 10–25 nm as a function of residential time and temperature of
Fig. 3. TEM images of WS2 nanoparticles (a and c) and pure sulphur (b) nanoparticles.
ARTICLE IN PRESS 690
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may be supported by broadened diffraction maximum in the 23.5–241 locations. It has to be noticed that the detailed study of TEM images detected on samples with particle mean sizes up to 30 nm lattice spacing in the {0 0 0 1} direction is irrespective of particle size and process parameters. In synthesized samples, formation of odd-shaped particles was not observed, but edge dislocations are found in quantity (Fig. 4). This corresponds with the characterization data of similar nanostructures presented in publications of Tenne et al. [12–13] and Vollath and Szabo [4,5]. Preliminary tribological testing of the product was provided. The plasma-sprayed porous alumina coating was tested in the couple with bronze at dry friction condition. Friction coefficient was 0.2. Coating porosity was about 5–7%. In order to prepare nanocomposite materials, WS2 nanoparticles of average size of 25 nm were dispersed in mineral oil. The suspension was ultrasonically impregnated in the porous surface of the coating. After such treatment, samples were dried at 300 1C for 1 h. The friction test in couple with bronze exhibited the value of friction coefficient to be about 0.08 and slow wear rate over a wide range of the pressure–velocity (PV) parameters. Fig. 4. X-ray diffraction patterns of WS2 powders synthesized at different precursor decomposition temperatures (synthesis conditions: sulphur evaporation at 400 1C, Ar flow rate ¼ 3500 cm3/min).
4. Conclusions
Fig. 5. TEM microphotograph of WS2 particle with a diameter of about 20 nm with dislocation in the {0 0 0 1} planes and an interlayer distance of about 0.60570.0312 (synthesis conditions: T ¼ 800 1C, Ar flow rate ¼ 3500 cm3/min).
precursor decomposition. This size is significantly smaller when compared with values identified from TEM microphotographs. The presence of location errors and non-periodity of the structure cause the widening of diffraction peaks, which may decrease the apparent particle’s mean size as seen in X-ray patterns. The interplanar distances along the c-direction evaluated from X-ray diffraction analysis data are about 0.63770.003 and the lattice spacing estimated from TEM images is about 0.635870.031 nm as shown in Fig. 5. The measured values have to be compared with the reference values of 0.618 and 0.613 nm of a stoichiometric equilibrium WS2 crystal with sizes in the micrometer range [10] for 2R and 3H structures, respectively. Some inconsistency of experimental and theoretical data might be because of the occurrence of so-called ‘‘restacked structure’’ [11]. Such structures contain foreign atoms intercalated between the S–W–S layers, which cause weakening of the bonds and modification of the crystalline atom structure. The presence of such atoms, creating a sub-structure with weak long-range order,
The possibility of producing crystalline fullerene-like WS2 nanoparticles with mean size in the range 20–70 nm by the gas phase synthesis method was shown in this study. Phase composition and particle mean size can be controlled by experimental parameters. The produced nanopowder has uniform particles size distribution and the mean size can be varied according to the application. From TEM microphotographs, only spherical onion-like nanoparticles were found, polyhedronshaped crystals are not observed in the given temperature interval, which can be considered to be an advantage. Results of TEM investigation show the existence of a number of stable edge dislocations in the structure of nanoparticles. The interplanar distance of WS2 nanoparticles in the {0 0 0 1} direction calculated from TEM and X-ray diffraction data is much higher compared with the reference data for micron powders. There are no significant variations in the interlayer distance for the observed nanoparticles of different sizes. Preliminary tribological testing of the product shows that thermo-sprayed coating impregnated with WS2 nanoparticles in couple with bronze exhibited a significantly low friction coefficient and slow wear. Hence, the presented method is quite suitable for the mass production of good-quality fullerene-like WS2 nanoparticles. References [1] A.R. Lansdown, Molybdenium Disulphide Lubrication, Elsevier, Swansea, 1999. [2] R. Tenne, Advanced in synthesis of inorganic nanotubes and fullerene-like nanoparticles, Angew. Chem. Int. 42 (2003) 5124–5132. [3] R. Tenne, L. Margulis, M. Genut, G. Hodes, Polyhedral and cylindrical structures of tungsten disulphide, Nature 360 (1992) 444–446. [4] D. Vollath, D.V. Szabo, Nanoparticles from compounds with layered structures, Acta Mater. 48 (2000) 953–967. [5] D. Vollath, D.V. Szabo, Synthesis of nanocrystalline MoS2 and WS2 in microwave plasma, Mater. Lett. 35 (1998) 236–244. [6] C.J. Choi, O.V. Tolochko, B.K. Kim, Preparation of Iron nanoparticles by chemical vapor condensation, Mater. Lett. 56 (2002) 289–294. [7] D.W. Lee, T.S. Jang, D. Kim, O.V. Tolochko, B.K. Kim, Nano-crystalline iron particles synthesized without chilling by chemical vapor condensation, Glass Phys. Chem. 31 (2005) 545–548. [8] D. Kim, E.S. Vasilyeva, A.G. Nasibulin, D.W. Lee, O.V. Tolochko, B.K. Kim, Aerosol synthesis and growth mechanism of magnetic iron nanoparticles, Mater. Sci. Forum 534–536 (2007) 9 (on-line at www.scientific.net).
ARTICLE IN PRESS E.S. Vasilyeva et al. / Microelectronics Journal 40 (2009) 687–691
[9] B.H. Kear, P.R. Strutt, Chemical processing and applications for nanostructured materials, Nanostruct. Mater. 6 (1995) 227–236. [10] H.F. McMurdie, et al., Powder Diffraction 1 (1986) 269 (JPDS #37-1492). [11] N.N. Galtsov, A.I. Prohvatilov, G.N. Dolgova, Intercalation of fullerite C60 with N2 molecules. An investigation by X-ray powder diffraction, Fiz. Nizk. Temp. 33 (2007) 1159–1165.
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[12] Y. Feldman, E. Wasserman, D.J. Srolovitz, R. Tenne, High-rate, gas-phase growth of MoS2 nested inorganic fullerenes and nanotubes, Science 267 (1995) 222. [13] L. Margulis, G. Saltra, R. Tenne, M. Tallanker, Nested fullerene-like structures, Nature 365 (1993) 113–114.
Contents lists available at ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
Synthesis of tungsten disulphide nanoparticles by the chemical vapor condensation method E.S. Vasilyeva a,, O.V. Tolochko a, B.K. Kim b, D.W. Lee b, D.S. Kim b a b
Material Science Faculty, State Technical University, Polytechnicheskaya Street, 29, 195251 Saint-Petersburg, Russian Federation Korea Institute of Material Science 66, Sangnam-Dong, Changwon, Kyungnam 641-010, South Korea
a r t i c l e in fo
abstract
Article history: Received 28 February 2008 Received in revised form 13 November 2008 Accepted 24 November 2008 Available online 31 January 2009
Crystalline tungsten disulphide nanoparticles were successfully synthesized by the chemical vapor condensation (CVC) method. The process performed as decomposition of tungsten hexacarbonyl over sulphur vapor in inert gas flow, where WS2 nanoparticles were synthesized by direct reaction between as formed pure tungsten nanoclusters and sulphur vapor. Influence of experimental parameters on shape, size distribution, structure and phase composition of nanoparticles were evaluated by transmission and scanning electron microscopy and X-ray diffraction analysis. The produced nanoparticles have closely spherical shape with the mean size in the range 20–70 nm in diameter dependently of process parameters. Nested ‘‘onion-like’’ structure of nanoparticles was observed. The mean value of interlayer distance in the {0 0 0 1} direction, is about 0.635870.031 nm. Due to nanodimensional size, physical properties and layered structure tungsten disulphide nanoparticles have great potential as a solid lubricant material. & 2008 Elsevier Ltd. All rights reserved.
Keywords: Tungsten disulphide Nanoparticles Fulleren-like structures Nanofabrication Tribology Solid lubricants
1. Introduction It is well known that chalcogenides of transition metals forming layered structure like disulphides, diselenides, ditellurides and fluorides of molybdenum, titanium, vanadium, chromium and others suitable as lubricants, the tungsten chalcogenides have better oxidation resistance at high temperature, quite low friction coefficient and chemical stability as in high vacuum as well in atmospheric air conditions [1]. Hence, WS2 nanopowder has potential beneficial effects as solid lubricants and as additives in the liquid lubricants. In analogy to graphite, nanoparticles of named compounds with a layered structure can form a various enclosed structures such as polyhedrical, fullerene-like (IF) and nanotubular structures [2,3]. Such structures are characterized by weak interatomic interactions like van der Waals force between their hexagonal planes. Hence these compounds can be used as solid lubricants and have great potential in high-temperature tribology. Varying interplane distances in the ‘‘c’’-direction are usually found in such structures [4], whereas the distances in the ‘‘a’’-direction are always close to the bulk material. Recently, two major methods have been used for synthesis of tungsten disulphide nanoparticles, and both require the sulfidization process of tungsten powder through the H2S gas or sulphur
Corresponding author. Tel./fax: +7 812 552 89 69.
E-mail addresses: [email protected], [email protected] (E.S. Vasilyeva). 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.11.066
vapor atmosphere. However, in one of the methods, tungsten nanoparticles form through the reduction of WO3 as in experiments of Tenne [2] or sometimes particles form from a tungsten metalorganic precursor by microwave plasma synthesis as shown in the experiments of Vollath and Szabo [5]. However, both require expensive equipment, though they provide processes involving only few operations, it is not easy to control the particle mean size. In this view, developing a technique for the synthesis of such structures is a subject of great interest. The chemical vapor condensation (CVC) process, originally developed for the synthesis of single-component metallic nanoparticles, was used to produce pure iron, cobalt, copper, tungsten nanoparticles and theirs alloys in a variety of particles mean size from 6 to 100 nm [6,7]. In the present work, we study the synthesis conditions and structure of WS2 nanoparticles by the chemical vapor condensation method. The friction and wear behavior of composite materials filled by produced WS2 nanopowder were also studied.
2. Experimental details 2.1. Nanoparticles synthesis Synthesis of nanoparticles by the CVC method was described previously [6–8]. The general scheme of the chemical vapor condensation method is presented in Fig. 1.
ARTICLE IN PRESS 688
E.S. Vasilyeva et al. / Microelectronics Journal 40 (2009) 687–691
In order to remove the excess of sulphur, as-prepared nanoparticles were heat-treated in the two-zone furnace in a sealed silica tube. Excess of free pure sulphur from the powder was evaporated at the temperature of 200 1C and condensed in the cold zone of the reactor at room temperature. 2.2. Characterization methods The phase and structural analysis of samples was carried out on a DRON 2.0 diffractometer with monochromatic Ka copper radiation (l ¼ 0.154051 nm). The morphological characterization and size distribution of nanoparticles were determined by means of high-resolution transmission electron microscopy (HR TEM). Specimen preparation of samples to study individual particles was done in the following way: produced nanopowders were ultrasonically dispersed in ethanol and dropped into the carbon-coated Cu-grids. The average particle size of each sample was calculated as the center of gravity of particles size distribution, which was estimated by measuring of diameters 200–300 nanoparticles from TEM microphotographs. Previously, the particle mean size was identified based on the width of diffraction peaks, using the Sherrer equation [9]. 2.3. Tribological testing
Fig. 1. Scheme of (CVC) laboratory setup designed for WS2 nanoparticles production.
First, pure tungsten nanoparticles were prepared for the evaluation of process parameters. The experiment was performed in a quartz tube passing a heated furnace; carrier gas of highpurity argon or helium was fed through a heated bubbling unit containing the metalorganic precursor of tungsten hexocarbonyl (W(CO)6) at the vaporization temperature in range 95–140 1C at a carrier gas flow rate of 500 cc/min. Concentration of precursor in the gas phase was controlled by the temperature of its evaporation. The flow of argon gas mixed with the precursor vapor passes through a heated pipe system into the high-temperature reactor where nanoparticles are formed as a result of precursor decomposition with further condensation in a water-cooled collecting chamber. In order to produce tungsten nanoparticles, the precursor decomposition temperature was estimated in the range 700–1100 1C. Using both accelerating gas and carrier gas allowed changing precursor residential time in the reaction zone and concentration of precursor in the gas phase. In order to produce nanocrystalline particles of tungsten disulphide, decomposition of tungsten hexocarbonyl was carried out in the presence of sulphur vapor with the partial pressure of S2 vapor being 0.02–0.04 atm. In such conditions we achieve about 10 times more pure sulphur in the gas phase. A specially designed two-zoned furnace (Fig. 1) allows fixing two different temperatures at two different heights of the reactor. The removable cartridge filled by crystalline sulphur was located in the first (lower) zone of the reactor at the evaporation temperature in interval from 325 up to 400 1C. The gas flow inclusive vapor of the precursor passed subsequently through both zones of the reactor. In the lower zone of the reactor, sulphur vapor was mixed with carbonyl vapor. In the second zone, mixed gas was heated up to the carbonyl pyrolysis conditions, as shown in Fig. 1. Hence, precursor decomposition and nanoparticles formation run at 800–1000 1C in the sulphur-contained atmosphere. As a result, the formation of crystalline pure tungsten disulphide nanoparticles with excess of crystalline sulphur was observed.
Tribological testing of nanocomposite samples containing tungsten disulphide nanoparticles was carried out in couple with bronze using a pin-on-disc wear tester. All tests were performed at room temperature at a load of 300 N and a sliding speed of 0.2 m/s in dry friction conditions.
3. Results and discussion Phase composition of synthesized powders depends on experimental parameters such as accelerating gas flow rate and reactor temperature. Fig. 2 shows the phase composition of products as a function of experimental parameters. The region of pure WS2 existence becomes wider with the increase in reaction
Fig. 2. Phase diagram of products of synthesis shows dependence of nanoparticles chemical and phase composition from a gas flow rate and precursor decomposition temperature. Evaporation temperature of W(CO)6. Gas flow passed through the precursor contained a unit and accelerating gas was not used.
ARTICLE IN PRESS E.S. Vasilyeva et al. / Microelectronics Journal 40 (2009) 687–691
temperature and decrease in Ar-accelerated gas flow rate. Due to laminar gas flow in the vertical-type reactor, the residence time of particles in the reactor and the time of reaction between tungsten and sulphur increased almost linearly with decrease in gas flow rate. As residential time in the reactor decreases, pure W nanoparticles could be observed. In that case, considering that sulphur excess always exist in the reaction system, it is possible to assume that there is not enough time to complete the reaction: W(s)+S2(g) ¼ WS2(s). At the same time the Gibbs energy of this reaction increases with increase in temperature. It allows supposing that kinetically reaction rate increases with increase in temperature. The typical appearance of synthesized particles is depicted in Fig. 3. Some agglomeration of nanoparticles can be observed after synthesis. Two kinds of nanoparticles found out from HR TEM microphotographs in the as-synthesized powders: crystalline tungsten disulphide with an onion-like structure (Fig. 3(a) and (c)) and nanoparticles of pure sulphur (Fig. 3(b)). Produced WS2 nanoparticles generally have a close round shape and their TEM image demonstrates the formation of the so-called onion-like
689
structure, which consists of concentric layers. Nanoparticles have mean size in the interval of 20–70 nm. The particle mean size insignificantly increases with the increase in reaction temperature and decrease in accelerated gas flow rate. Crystals of pure sulphur having been overall removed from powders after annealing at 200 1C, mean size and structure of WS2 nanoparticles after annealing at such low temperatures were found to be the same as initial WS2 nanoparticles. Typical X-ray diffraction patterns for nanoparticles synthesized at the different precursor decomposition temperatures are presented in Fig. 4. As the reaction temperature increases, there were changes in WS2 structure modification from 2H to 3R. The observed XRD patterns are in good agreement with reference data. There is good consistency in the number and positions of diffraction maxima; however, there are relevant differences in the relative intensities of particular reflections and significant broadening of experimental peaks in comparison with theoretical data for WS2 micron-sized powder. The particle’s mean size, which was estimated based on the width of diffraction peaks, was estimated to be in the interval of 10–25 nm as a function of residential time and temperature of
Fig. 3. TEM images of WS2 nanoparticles (a and c) and pure sulphur (b) nanoparticles.
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may be supported by broadened diffraction maximum in the 23.5–241 locations. It has to be noticed that the detailed study of TEM images detected on samples with particle mean sizes up to 30 nm lattice spacing in the {0 0 0 1} direction is irrespective of particle size and process parameters. In synthesized samples, formation of odd-shaped particles was not observed, but edge dislocations are found in quantity (Fig. 4). This corresponds with the characterization data of similar nanostructures presented in publications of Tenne et al. [12–13] and Vollath and Szabo [4,5]. Preliminary tribological testing of the product was provided. The plasma-sprayed porous alumina coating was tested in the couple with bronze at dry friction condition. Friction coefficient was 0.2. Coating porosity was about 5–7%. In order to prepare nanocomposite materials, WS2 nanoparticles of average size of 25 nm were dispersed in mineral oil. The suspension was ultrasonically impregnated in the porous surface of the coating. After such treatment, samples were dried at 300 1C for 1 h. The friction test in couple with bronze exhibited the value of friction coefficient to be about 0.08 and slow wear rate over a wide range of the pressure–velocity (PV) parameters. Fig. 4. X-ray diffraction patterns of WS2 powders synthesized at different precursor decomposition temperatures (synthesis conditions: sulphur evaporation at 400 1C, Ar flow rate ¼ 3500 cm3/min).
4. Conclusions
Fig. 5. TEM microphotograph of WS2 particle with a diameter of about 20 nm with dislocation in the {0 0 0 1} planes and an interlayer distance of about 0.60570.0312 (synthesis conditions: T ¼ 800 1C, Ar flow rate ¼ 3500 cm3/min).
precursor decomposition. This size is significantly smaller when compared with values identified from TEM microphotographs. The presence of location errors and non-periodity of the structure cause the widening of diffraction peaks, which may decrease the apparent particle’s mean size as seen in X-ray patterns. The interplanar distances along the c-direction evaluated from X-ray diffraction analysis data are about 0.63770.003 and the lattice spacing estimated from TEM images is about 0.635870.031 nm as shown in Fig. 5. The measured values have to be compared with the reference values of 0.618 and 0.613 nm of a stoichiometric equilibrium WS2 crystal with sizes in the micrometer range [10] for 2R and 3H structures, respectively. Some inconsistency of experimental and theoretical data might be because of the occurrence of so-called ‘‘restacked structure’’ [11]. Such structures contain foreign atoms intercalated between the S–W–S layers, which cause weakening of the bonds and modification of the crystalline atom structure. The presence of such atoms, creating a sub-structure with weak long-range order,
The possibility of producing crystalline fullerene-like WS2 nanoparticles with mean size in the range 20–70 nm by the gas phase synthesis method was shown in this study. Phase composition and particle mean size can be controlled by experimental parameters. The produced nanopowder has uniform particles size distribution and the mean size can be varied according to the application. From TEM microphotographs, only spherical onion-like nanoparticles were found, polyhedronshaped crystals are not observed in the given temperature interval, which can be considered to be an advantage. Results of TEM investigation show the existence of a number of stable edge dislocations in the structure of nanoparticles. The interplanar distance of WS2 nanoparticles in the {0 0 0 1} direction calculated from TEM and X-ray diffraction data is much higher compared with the reference data for micron powders. There are no significant variations in the interlayer distance for the observed nanoparticles of different sizes. Preliminary tribological testing of the product shows that thermo-sprayed coating impregnated with WS2 nanoparticles in couple with bronze exhibited a significantly low friction coefficient and slow wear. Hence, the presented method is quite suitable for the mass production of good-quality fullerene-like WS2 nanoparticles. References [1] A.R. Lansdown, Molybdenium Disulphide Lubrication, Elsevier, Swansea, 1999. [2] R. Tenne, Advanced in synthesis of inorganic nanotubes and fullerene-like nanoparticles, Angew. Chem. Int. 42 (2003) 5124–5132. [3] R. Tenne, L. Margulis, M. Genut, G. Hodes, Polyhedral and cylindrical structures of tungsten disulphide, Nature 360 (1992) 444–446. [4] D. Vollath, D.V. Szabo, Nanoparticles from compounds with layered structures, Acta Mater. 48 (2000) 953–967. [5] D. Vollath, D.V. Szabo, Synthesis of nanocrystalline MoS2 and WS2 in microwave plasma, Mater. Lett. 35 (1998) 236–244. [6] C.J. Choi, O.V. Tolochko, B.K. Kim, Preparation of Iron nanoparticles by chemical vapor condensation, Mater. Lett. 56 (2002) 289–294. [7] D.W. Lee, T.S. Jang, D. Kim, O.V. Tolochko, B.K. Kim, Nano-crystalline iron particles synthesized without chilling by chemical vapor condensation, Glass Phys. Chem. 31 (2005) 545–548. [8] D. Kim, E.S. Vasilyeva, A.G. Nasibulin, D.W. Lee, O.V. Tolochko, B.K. Kim, Aerosol synthesis and growth mechanism of magnetic iron nanoparticles, Mater. Sci. Forum 534–536 (2007) 9 (on-line at www.scientific.net).
ARTICLE IN PRESS E.S. Vasilyeva et al. / Microelectronics Journal 40 (2009) 687–691
[9] B.H. Kear, P.R. Strutt, Chemical processing and applications for nanostructured materials, Nanostruct. Mater. 6 (1995) 227–236. [10] H.F. McMurdie, et al., Powder Diffraction 1 (1986) 269 (JPDS #37-1492). [11] N.N. Galtsov, A.I. Prohvatilov, G.N. Dolgova, Intercalation of fullerite C60 with N2 molecules. An investigation by X-ray powder diffraction, Fiz. Nizk. Temp. 33 (2007) 1159–1165.
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