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Synthesis of tungsten nanoparticles by solvothermal decomposition of tungsten hexacarbonyl
Int. Journal of Refractory Metals & Hard Materials 27 (2009) 784–791
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Synthesis of tungsten nanoparticles by solvothermal decomposition of tungsten hexacarbonyl Prasanta Kumar Sahoo a,c,*, S.S. Kalyan Kamal a,c, M. Premkumar a, T. Jagadeesh Kumar a, B. Sreedhar b, A.K. Singh a, S.K. Srivastava c, K. Chandra Sekhar a a
Defence Metallurgical Research Laboratory (DMRL), Kanchanbagh, Hyderabad 500 058, India Indian Institute of Chemical Technology (IICT), Tarnaka, Hyderabad 500 007, India c Indian Institute of Technology Kharagpur (IIT Kharagpur), Kharagpur 721 302, India b
a r t i c l e
i n f o
Article history: Received 5 November 2008 Accepted 23 January 2009
Keywords: Thermal decomposition Tungsten nanoparticles Surfactants and agglomeration
a b s t r a c t Tungsten nanoparticles were prepared by thermal decomposition of tungsten hexacarbonyl [W(CO)6] (2 mmol) at 160 °C in presence of a mixture of (1:1) surfactants, oleic acid (6 mmol) and trioctyl phosphine oxide (TOPO) (6 mmol) under a blanket of Ar gas. The synthesized tungsten nanoparticles without surfactant are flocculated. With increase in concentration of surfactant mixture to 12 mmol each, agglomeration of several tungsten nanoparticles are observed. Characterization of surfactant coated tungsten nanoparticles were carried out using Fourier transform infrared spectroscopy (FTIR). Chemical characterizations of synthesized tungsten nanoparticles were done using X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-optical emission spectrometry (ICP-OES) and LECO gas analyzers. XPS study shows the W0 oxidation state of tungsten nanopowders. Structural characterization of synthesized tungsten nanoparticles were conducted by X-ray diffraction (XRD), which shows that the as-synthesized tungsten nanoparticles are amorphous in nature and they become body centered cubic crystalline after annealing. Particle size, shape and distribution were characterized using small-angle X-ray scattering (SAXS), environmental scanning electron microscopy (ESEM) and transmission electron microscopy (TEM). Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Recent development of uniform nanostructured materials has been intensively pursued because of their technological and fundamental scientific importance [1–6]. These nanostructured materials often exhibit exotic electrical, optical, magnetic, catalytic and chemical properties, which cannot be achieved by their bulk counterparts [7–11], for example benzene hydrogenation catalyzed by iridium and rhodium nanoparticles increases by many folds [12]. This can be attributed to the large surface to volume ratio of the nanoparticles. Gold nanoparticle solutions exhibit a range of colours with varying particle sizes. This phenomenon arises due to the surface plasmon resonance, which is not observed for the bulk metal [13–15]. Metallic nanoparticles have also been widely exploited for use in photography, catalysis, biological labeling, photonics, optoelectronics, information storage, surface-enhanced raman scattering (SERS), and the formulation of magnetic ferrofluids [16–19]. Chemical methods have been widely used to produce
* Corresponding author. Address: Defence Metallurgical Research Laboratory (DMRL), Kanchanbagh, Hyderabad 500 058, India. Tel.: +91 040 24586675; fax: +91 040 2434 0884. E-mail address: [email protected] (P.K. Sahoo). 0263-4368/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2009.01.005
nanoparticles due to their ease of operation and potential for scaling-up [20]. There are several synthetic procedures for synthesizing nanoparticles, of which the thermal decomposition method is a well-known one and is used by several investigators to prepare nanoparticles [21–24]. A general thermal decomposition method involves the dissolution of a protective agent or stabilizer in a high boiling solvent. The required metal precursor is then added to the solution and refluxed, which results in the formation of nanoparticles. The growth and stability of nanoparticles in organic solvents rely on the presence of surfactants as stabilizing agents [4]. The application of tungsten nanoparticles mainly arises from its properties as functional material [25]. In recent years, tungsten nanoparticles have gained extensive application in defence, aerospace, electronics, engine components etc.; hence the demand of tungsten nanoparticles is increasing [26–29]. Recently, these tungsten nanoparticles have been synthesized by various methods such as electrical explosion of wires (EEW) method [25,30], high energy ball milling using WO3 and Mg as starting materials [31], molten salt assisted self-propagating high-temperature synthesis (SHS) [32], plasma heating of precursor aerosol for vapor condensation [33], physical vapor deposition (PVD) [34], gas-phase combustion synthesis method [35], sealed tube synthesis employing {WCl6 + Si(SiMe3)4} [36], and chemical reduction method [37]. Size and
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Fig. 1. Sketch of the experimental set-up.
shape of the particles can be controlled by adjusting the synthetic parameters such as molar ratios of stabilizers to metal precursor, heating time and temperature [38,39]. The properties of nanostructured materials depend on their particle size and shape [40]. Therefore, this study is focused on controlling the size and shape of tungsten nanoparticles synthesized by thermal decomposition method. In this article, synthesis of tungsten nanoparticles through thermal decomposition of tungsten hexacarbonyl [W(CO)6] is discussed. It is observed that using different concentration of surfactant mixture during tungsten hexacarbonyl [W(CO)6] decomposition, results in the formation of either large or small clusters [41,42]. These nanostructured materials can be dispersed in most of the organic solvents and can be retrieved as powders by centrifugation. 2. Experimental The synthesis of nanoparticles was carried out using standard synthetic chemical procedure and commercially available reagents. Tungsten hexacarbonyl [W(CO)6] (99.9%), Diphenyl ether (99%), trioctyl phosphine oxide (TOPO) (>97%) and oleic acid (90%) were purchased form Alfa Aesar laboratories, India. Ethanol and hexane were used as received from Loba chemie, India.
About 30 ml of diphenyl ether was used as solvent and a mixture of 6 mmol each of oleic acid and trioctyl phosphine oxide (TOPO) as surfactants were mixed thoroughly in a 250 ml threeneck distillation flask. High purity Ar (99%) gas was flushed into the system for 8–10 min to eliminate any traces of O2. The flask was attached with circulatory water condenser and a mechanical stirrer. A sketch of the experimental set-up has given in Fig. 1. The mixture was preheated to 130 °C for 5 min and then 2 mmol tungsten hexacarbonyl [W(CO)6] was added into the system and the mixture was refluxed for varying times ranging from 15 to 60 min. The solution turning black on the decomposition of tungsten hexacarbonyl [W(CO)6] is an indication of the formation of tungsten nanoparticles. Various experimental conditions used in the present work are shown in Tables 1 and 2. During all our experiments, argon gas was flushed into the system to avoid the oxidation. After completion of the reaction, the solution was cooled to room temperature and the tungsten nanoparticles were found to be well-dispersed. To prepare samples for analysis, ethanol (60 ml) was introduced under ambient conditions to precipitate tungsten nanoparticles. After washing several times with ethanol, the precipitated tungsten nanoparticles were separated by centrifuging the solution at 6000 rpm for 10 min and were later redispersed in hexane. Fourier transform infrared spectroscopy (FTIR) measurements were carried out using Perkin–Elmer FT-IR system, Spectrum GX in the range of 400–4000 cm 1. A few mg of the surfactant coated tungsten nanopowders were thoroughly mixed with KBr to make a pellet and the spectrum was recorded. X-ray photoelectron spectroscopic (XPS) measurements were performed using Kratos/Shimadsu AXIS 165 instrument. The Al Ka (1486.6 eV) radiation was used as X-ray source and the spectrum was acquired at a fixed analysis energy mode. Samples for elemental analysis were precipitated from their hexane dispersion by ethanol, centrifuged, washed with ethanol and dried. Analysis of as-synthesized tungsten nanoparticles were carried out using inductively coupled plasma-optical emission spectrometry (ICP-OES) model (JY ULTIMA), LECO gas analyzers of model CS-444 and TC-600, respectively. The powders were dried under inert atmosphere and the structural characterization of these tungsten nanoparticles were carried out using a PHILIPS-PW3020 diffractometer with a Cu Ka radiation source (k = 1.5418 Å). SAXS (Anton Paar-PW3830) was used to measure size, shape and distribution of the tungsten nanoparticles. The samples for environmental scanning electron microscopy (ESEM) analysis were prepared by drying a hexane dispersion of particles on a carbon tape supported on brass tub. The particles were imaged by FEI QUANTA 400 (ESEM) at an accelerating voltage of 20 kV and the elemental analysis of synthesized tungsten nano-
Table 1 Experimental conditions under different reaction times. Experiment
Tungsten hexacarbonyl (mmol)
Oleic acid (mmol)
Trioctyl phosphine oxide (mmol)
Reflux time (min)
Average particle size from SAXS (nm)
Standard deviation in particle size (nm)
1 2 3
2 2 2
6 6 6
6 6 6
15 30 60
15 25 28
0.2 0.3 0.3
Table 2 Reaction conditions with varying surfactant concentration. Experiment
Tungsten hexacarbonyl (mmol)
Oleic acid (mmol)
Trioctyl phosphine oxide (mmol)
Reflux time (min)
Average particle size from SAXS (nm)
Standard deviation in particle size (nm)
4 5 6
2 2 2
0 6 12
0 6 12
30 30 30
45 25 30
1.4 0.3 0.4
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Fig. 2. FT-IR spectra of pure oleic acid in 1300–3300 cm
1
region. Fig. 5. X-ray photoelectron spectra of as-synthesized tungsten nanoparticles capped with surfactants produced from experiment 2.
particles were conducted by using energy dispersive X-ray microanalysis (EDX). The samples for transmission electron microscopy (TEM) were prepared by drying a drop of hexane dispersion of the particles on amorphous carbon-coated copper grids. The particles were imaged using Philips, TECHNAI FE 12 microscope (TEM) at an accelerating voltage of 120 kV. 3. Results and discussion
Fig. 3. FT-IR spectra of pure trioctyl phosphine oxide (TOPO) in 1000–3200 cm region.
1
Fig. 4. FT-IR spectra of tungsten nanoparticles capped with oleic acid and trioctyl phosine oxide (TOPO) produced from experiment 2 in 800–3400 cm 1 region.
The tungsten nanoparticles were prepared under various experimental conditions-reaction time and molar ratio of surfactant to metal precursor. As trioctyl phosphine oxide (TOPO) is a high boiling point surfactant with a patulous long chain structure providing greater steric hindrance [43], it slows down the addition rate of materials to the nanoparticles during the growth, resulting in much smaller and well-dispersed nanoparticles. Oleic acid binds tightly to surface of nanoparticles. A combined effect of these two surfactant mixture were much more profound than those of individual contributions. Furthermore, the surfactants adsorbed on to the surface of tungsten nanoparticles provide a dynamic organic structure that stabilizes the tungsten nanoparticles in the solution [21].
Fig. 6. XRD of synthesized tungsten nanoparticles produced from experiment 2.
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Fig. 7. (a) X-ray small angle scattering profile with variation of reaction time where I is the scattering intensity and q is the length of the scattering vector. (b) Pair distance distribution curve of tungsten nanoparticles with variation of reaction time. (c) Radius of gyration (Rg) vs. reaction time graph. (d) X-ray small angle scattering profile with variation of surfactant concentration. (e) Pair distance distribution curve of tungsten nanoparticles with variation of surfactant concentration. (f) Radius of gyration (Rg) vs. surfactant concentration graph.
Figs. 2 and 3 show characteristic peaks of the pure surfactants oleic acid and trioctyl phosphine oxide (TOPO), respectively. For oleic acid, the peaks at 2854 and 2926 cm 1 are due to symmetric
and asymmetric CH2 stretching modes and the peak at 3009 cm 1 is due to t(C–H) stretching mode of the C–H bond adjacent to the C@C bond. The peak at 1713 cm 1 is due to t(C@O) stretching
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mode and the weak mode at 2675 cm 1 is due to t(O–H) stretch of the dimerised acid. The spectrum of trioctyl phosphine oxide (TOPO) shows the peaks at 2854 and 2920 cm 1 which arise due to the symmetric and asymmetric CH2 stretching modes (Fig. 3). The peak at 1146 cm 1 is due to t(P@O) stretching mode. The peak at 1463 cm 1 arises from t(CH3) bending modes, present in both oleic acid and trioctyl phosphine oxide (TOPO), was present in the capped tungsten nanoparticles and attributed to both passivating ligands. The spectrum of tungsten nanoparticles (Fig. 4) produced from experiment 2 with excess surfactant shows the same characteristic peaks in the 2850–3500 cm 1 region. The peak at 1713 cm 1 is due to the t(C@O) stretch mode and indicates some fraction of the oleic acid is bonded to tungsten nanoparticles either in monodentate form or as an acid. The peak at 1512 cm 1 is due to t(–COO) vibrational mode indicates the presence of bidentate carboxylate bonding to the nanoparticles and confirms that some fraction of oleic acid is bonded to nanoparticles in bidentate form [44]. The 1146 cm 1 peak is due to the t(P@O) stretching mode which suggests that the trioctyl phosphine oxide (TOPO) was weakly coordinated to the tungsten nanoparticles surface through O atom. Tungsten nanoparticles are stabilized with oleic acid and trioctyl phosphine oxide (TOPO) [21]. The observation of both t(COO) and t(C@O) vibrational modes indicate that oleic acid acts both as a monodentate and bidentate ligand binding to tungsten via only O atom. The stretching mode of t(P@O) at 1146 cm 1 confirms that trioctyl phosphine oxide (TOPO) bonds to the tungsten nanoparticles through O atom of (P@O) group. The peaks between 3000–3100 cm 1 is due to the t(C–H) stretching modes of C–H bonds adjacent to the C@C bond. The spectrum of oleic acid shows only a single peak at 3009 cm 1 in this region. In Fig. 4, presence of several peaks in this region indicates that the C–H bonds adjacent to the C@C bond exists in several different environments. Prominent mode between 965 and 975 cm 1 can be used to differentiate between cis and trans substituted fatty acids [45]. The peak at 968 cm 1 in tungsten nanoparticles indicates the presence of trans substituted unsaturated alkyl groups. The presence of high frequency peaks due to stretching of the C–H bond adjacent to the C@C double bond suggests that there is conversion of oleic acid to elaidic acid. Three peaks were observed at 3020, 3038 and 3063 cm 1 associated with the t(C– H) stretching modes adjacent to C@C bond in elaidic acid. During the synthesis of tungsten nanoparticles there is a conversion of the alkyl chains from the oleyl form (cis-9-octadecenyl) to the elaidyl form (trans-9-octadecenyl), which is revealed by the presence of several vibrational absorption bands in the region of the olefinic C–H stretching modes [46]. The presence of elaidyl groups on tungsten nanoparticles surfaces is very important because the structure of the oleyl groups and the elaidyl groups are quite different. The elaidyl groups are expected to pack differently around the tungsten nanoparticles providing more stability. The oxidation state of tungsten in tungsten nanoparticles produced form experiment 2 was determined by X-ray photoelectron spectroscopy (XPS) (Kratos/Shimadsu AXIS 165). While the sample was being bombarded with argon ions (emission current = 10 mA, accelerating voltage = 3 kV) evolution of the XPS spectra was followed. The XPS analysis shows the presence of W, O and C in the as-synthesized nanopowders. The W4f doublet with a binding energy of 31.701 eV and 33.759 eV shows the W0 oxidation state of tungsten nanopowders (Fig. 5). Disappearance of the carbon after an erosion time of 3 min, suggests that the carbon is present only at the surface. Elemental analyses of the as-synthesized tungsten nanoparticles were performed using ICP-OES and LECO gas analyzers. It was observed that tungsten, carbon and oxygen content were in the range of 94–95%, 0.7–1% and 1.5–4%, respectively.
The XRD pattern of the as-synthesized tungsten nanoparticles produced from all the experiments (1–6) exhibits only two broad peaks. Absence of sharp diffraction peaks in the spectrum indicates that the nanopowders are amorphous in nature. After annealing at 600 °C for 1 h in argon atmosphere the powders convert from a fully amorphous state to a crystalline body centered cubic structure. The annealing temperature raised to 600 °C at a heating rate 10 °C per 1 min and was then kept for 1 h. There was no great variation of XRD pattern observed for the tungsten nanopowders produced from all the experiments (1–6). Fig. 6 shows the XRD pattern of the tungsten nanoparticles produced from experiment 2. XRD results for the annealed tungsten nanoparticles reveals the presence of tungsten with the lattice parameter 3.1607 Å which is very close to the lattice parameter of tungsten reported (JCPDS card. No. 04-0806). The SAXS results of all the six samples are shown in Fig. 7. As observed in Fig. 7a that the scattering intensity is increasing with increase in reaction time, indicating that there is an increase in average particle size [47]. It is known that when slope of the plot log I (q) vs. log q is 0 (line collimation) then the scattering curves reveal that the shape of the particles are spherical [48]. The slopes of all the curves in Fig. 7a exhibit deviation from zero. Based on the scattering curves, it can be concluded that the shape of the particles are not spherical. With varying concentration of surfactant, SAXS experiment has been carried out for the tungsten nanoparticles and the results are shown in Fig. 7d. Without the addition of surfactant the intensity is highest which indicates that the particles are either agglomerated or having large size. With addition of 6 mmol each of the surfactants the intensity has decreased
Fig. 8. (a) ESEM image of the synthesized tungsten nanoparticles with a mixture of (1:1) surfactants oleic acid (6 mmol) and trioctyl phosphine oxide (TOPO) (6 mmol) and corresponding, (b) EDX spectrum.
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and further increase in surfactant mixture concentration to 12 mmol each the particle size has again increased. The slopes of all the curves in Fig. 7d also exhibit deviation from zero indicating that the particles are not spherical [48]. Pair distance distribution function (PDDF) p(r) has been calculated from the scattering data by using the programme GIFT [49]. The p(r) is the Fourier inversion of the scattering curve and defined R I (q) qr Sin qr dq], where r is the distance or as [(1/2p2) radius of the particles. Fig. 7b and e show the pair distance distribution functions as a function of r. The shape of the p(r) vs r curves also resembles that the particles are not spherical. The PDDF curves also reveal that the average particle size for experiments 1–3 (Table 1) and 4–6 ( Table 2) vary form 15 to 28 nm and 25 to 45 nm, respectively. The radius of gyration (Rg) has been plotted against the reaction time (Fig. 7c). This clearly demonstrates that with increase in reaction time the radius of gyration increases which in turn indicates the increase in particle size [50]. Radius of gyration (Rg) has also been plotted against varying surfactant concentration (Fig. 7f). Without addition of surfactant the radius of gyration (Rg) is maximum and on addition of 6 mmol of each of the surfactant the radius of gyration (Rg) is minimum. With increase in surfactant concentration to 12 mmol each the radius of gyration increases which clearly shows that the particle size increases with addition of more surfactants. Fig. 8a shows the ESEM image of the synthesized tungsten nanoparticles (experiment 2) with a mixture of (1:1) surfactants
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oleic acid (6 mmol) and trioctyl phosphine oxide (TOPO) (6 mmol). Its EDX spectrum excited by an electron beam (20 kV) shows the peaks for the elements of W, O and C (Fig. 8b). From EDX it was confirmed that there is little amount of C and O impurities. The appearance of C peak is due to the carbon tape used for sampling and surfactants present on nanoparticle surface. The O peak is due to the surfactant mixture oleic acid and trioctyl phosphine oxide (TOPO) and a partial surface oxidation of tungsten nanoparticles during sampling. There are no other impurities in as-synthesized tungsten nanoparticles. Fig. 9 shows the TEM images of the tungsten nanoparticles synthesized by thermal decomposition of tungsten hexacarbonyl [W(CO)6] at 160 °C in presence of mixture of surfactants (1:1) at different reaction times (experiments 1–3). The average size of the prepared tungsten nanoparticles was increased from 15 to 28 nm as the decomposition time increases from 15 to 60 min, respectively (Fig. 9a–c). As shown in Fig. 10, following with the different surfactant concentration, sizes of the tungsten nanoparticles obtained are also different. The tungsten nanoparticles synthesized without surfactants seem to be an agglomeration of small particles and they are flocculated with an average particle size of 45 nm (Fig. 10a). An addition of a mixture of surfactants (1:1) has resulted in nanopowders with average particle size about 25 nm as shown in Fig. 10b. With increase in surfactant amount twice up to 12 mmol each, the obtained nanoparticles looked like agglomeration of several particles with average particle size of
Fig. 9. TEM image of the synthesized tungsten nanoparticles after (a) 15 min, (b) 30 min and (c) 60 min of reaction.
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Fig. 10. TEM image of the synthesized tungsten nanoparticles (a) without surfactant, (b) with a mixture of (1:1) surfactants oleic acid (6 mmol) and trioctyl phosphine oxide (TOPO) (6 mmol) and (c) with a mixture of (1:1) surfactants oleic acid (12 mmol) and trioctyl phosphine oxide (TOPO) (12 mmol).
30 nm (Fig. 10c) [24]. Though not all the ESEM pictures are presented here in this paper, but both ESEM and TEM results are in good agreement with the SAXS results. Therefore, it can be concluded that the role of surfactant is very important in the formation of tungsten nanoparticles by thermal decomposition method. This controls both the size as well as shape of the nanoparticles. 4. Conclusion Different size and shapes of the tungsten nanoparticles were successfully synthesized by thermal decomposition with tungsten hexacarbonyl depending on the decomposition times and surfactant concentrations. The sizes of the tungsten nanoparticles varied from 15 to 28 nm with increasing the decomposition times from 15 to 60 min. The particles are agglomerated without surfactant. The presence of surfactant reduces the particle size and with further increase in surfactant concentration increases the particle size. The synthesized tungsten nanoparticles are amorphous and it converts to crystalline bcc phase after annealing in argon atmosphere.
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Synthesis of tungsten nanoparticles by solvothermal decomposition of tungsten hexacarbonyl Prasanta Kumar Sahoo a,c,*, S.S. Kalyan Kamal a,c, M. Premkumar a, T. Jagadeesh Kumar a, B. Sreedhar b, A.K. Singh a, S.K. Srivastava c, K. Chandra Sekhar a a
Defence Metallurgical Research Laboratory (DMRL), Kanchanbagh, Hyderabad 500 058, India Indian Institute of Chemical Technology (IICT), Tarnaka, Hyderabad 500 007, India c Indian Institute of Technology Kharagpur (IIT Kharagpur), Kharagpur 721 302, India b
a r t i c l e
i n f o
Article history: Received 5 November 2008 Accepted 23 January 2009
Keywords: Thermal decomposition Tungsten nanoparticles Surfactants and agglomeration
a b s t r a c t Tungsten nanoparticles were prepared by thermal decomposition of tungsten hexacarbonyl [W(CO)6] (2 mmol) at 160 °C in presence of a mixture of (1:1) surfactants, oleic acid (6 mmol) and trioctyl phosphine oxide (TOPO) (6 mmol) under a blanket of Ar gas. The synthesized tungsten nanoparticles without surfactant are flocculated. With increase in concentration of surfactant mixture to 12 mmol each, agglomeration of several tungsten nanoparticles are observed. Characterization of surfactant coated tungsten nanoparticles were carried out using Fourier transform infrared spectroscopy (FTIR). Chemical characterizations of synthesized tungsten nanoparticles were done using X-ray photoelectron spectroscopy (XPS), inductively coupled plasma-optical emission spectrometry (ICP-OES) and LECO gas analyzers. XPS study shows the W0 oxidation state of tungsten nanopowders. Structural characterization of synthesized tungsten nanoparticles were conducted by X-ray diffraction (XRD), which shows that the as-synthesized tungsten nanoparticles are amorphous in nature and they become body centered cubic crystalline after annealing. Particle size, shape and distribution were characterized using small-angle X-ray scattering (SAXS), environmental scanning electron microscopy (ESEM) and transmission electron microscopy (TEM). Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Recent development of uniform nanostructured materials has been intensively pursued because of their technological and fundamental scientific importance [1–6]. These nanostructured materials often exhibit exotic electrical, optical, magnetic, catalytic and chemical properties, which cannot be achieved by their bulk counterparts [7–11], for example benzene hydrogenation catalyzed by iridium and rhodium nanoparticles increases by many folds [12]. This can be attributed to the large surface to volume ratio of the nanoparticles. Gold nanoparticle solutions exhibit a range of colours with varying particle sizes. This phenomenon arises due to the surface plasmon resonance, which is not observed for the bulk metal [13–15]. Metallic nanoparticles have also been widely exploited for use in photography, catalysis, biological labeling, photonics, optoelectronics, information storage, surface-enhanced raman scattering (SERS), and the formulation of magnetic ferrofluids [16–19]. Chemical methods have been widely used to produce
* Corresponding author. Address: Defence Metallurgical Research Laboratory (DMRL), Kanchanbagh, Hyderabad 500 058, India. Tel.: +91 040 24586675; fax: +91 040 2434 0884. E-mail address: [email protected] (P.K. Sahoo). 0263-4368/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2009.01.005
nanoparticles due to their ease of operation and potential for scaling-up [20]. There are several synthetic procedures for synthesizing nanoparticles, of which the thermal decomposition method is a well-known one and is used by several investigators to prepare nanoparticles [21–24]. A general thermal decomposition method involves the dissolution of a protective agent or stabilizer in a high boiling solvent. The required metal precursor is then added to the solution and refluxed, which results in the formation of nanoparticles. The growth and stability of nanoparticles in organic solvents rely on the presence of surfactants as stabilizing agents [4]. The application of tungsten nanoparticles mainly arises from its properties as functional material [25]. In recent years, tungsten nanoparticles have gained extensive application in defence, aerospace, electronics, engine components etc.; hence the demand of tungsten nanoparticles is increasing [26–29]. Recently, these tungsten nanoparticles have been synthesized by various methods such as electrical explosion of wires (EEW) method [25,30], high energy ball milling using WO3 and Mg as starting materials [31], molten salt assisted self-propagating high-temperature synthesis (SHS) [32], plasma heating of precursor aerosol for vapor condensation [33], physical vapor deposition (PVD) [34], gas-phase combustion synthesis method [35], sealed tube synthesis employing {WCl6 + Si(SiMe3)4} [36], and chemical reduction method [37]. Size and
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Fig. 1. Sketch of the experimental set-up.
shape of the particles can be controlled by adjusting the synthetic parameters such as molar ratios of stabilizers to metal precursor, heating time and temperature [38,39]. The properties of nanostructured materials depend on their particle size and shape [40]. Therefore, this study is focused on controlling the size and shape of tungsten nanoparticles synthesized by thermal decomposition method. In this article, synthesis of tungsten nanoparticles through thermal decomposition of tungsten hexacarbonyl [W(CO)6] is discussed. It is observed that using different concentration of surfactant mixture during tungsten hexacarbonyl [W(CO)6] decomposition, results in the formation of either large or small clusters [41,42]. These nanostructured materials can be dispersed in most of the organic solvents and can be retrieved as powders by centrifugation. 2. Experimental The synthesis of nanoparticles was carried out using standard synthetic chemical procedure and commercially available reagents. Tungsten hexacarbonyl [W(CO)6] (99.9%), Diphenyl ether (99%), trioctyl phosphine oxide (TOPO) (>97%) and oleic acid (90%) were purchased form Alfa Aesar laboratories, India. Ethanol and hexane were used as received from Loba chemie, India.
About 30 ml of diphenyl ether was used as solvent and a mixture of 6 mmol each of oleic acid and trioctyl phosphine oxide (TOPO) as surfactants were mixed thoroughly in a 250 ml threeneck distillation flask. High purity Ar (99%) gas was flushed into the system for 8–10 min to eliminate any traces of O2. The flask was attached with circulatory water condenser and a mechanical stirrer. A sketch of the experimental set-up has given in Fig. 1. The mixture was preheated to 130 °C for 5 min and then 2 mmol tungsten hexacarbonyl [W(CO)6] was added into the system and the mixture was refluxed for varying times ranging from 15 to 60 min. The solution turning black on the decomposition of tungsten hexacarbonyl [W(CO)6] is an indication of the formation of tungsten nanoparticles. Various experimental conditions used in the present work are shown in Tables 1 and 2. During all our experiments, argon gas was flushed into the system to avoid the oxidation. After completion of the reaction, the solution was cooled to room temperature and the tungsten nanoparticles were found to be well-dispersed. To prepare samples for analysis, ethanol (60 ml) was introduced under ambient conditions to precipitate tungsten nanoparticles. After washing several times with ethanol, the precipitated tungsten nanoparticles were separated by centrifuging the solution at 6000 rpm for 10 min and were later redispersed in hexane. Fourier transform infrared spectroscopy (FTIR) measurements were carried out using Perkin–Elmer FT-IR system, Spectrum GX in the range of 400–4000 cm 1. A few mg of the surfactant coated tungsten nanopowders were thoroughly mixed with KBr to make a pellet and the spectrum was recorded. X-ray photoelectron spectroscopic (XPS) measurements were performed using Kratos/Shimadsu AXIS 165 instrument. The Al Ka (1486.6 eV) radiation was used as X-ray source and the spectrum was acquired at a fixed analysis energy mode. Samples for elemental analysis were precipitated from their hexane dispersion by ethanol, centrifuged, washed with ethanol and dried. Analysis of as-synthesized tungsten nanoparticles were carried out using inductively coupled plasma-optical emission spectrometry (ICP-OES) model (JY ULTIMA), LECO gas analyzers of model CS-444 and TC-600, respectively. The powders were dried under inert atmosphere and the structural characterization of these tungsten nanoparticles were carried out using a PHILIPS-PW3020 diffractometer with a Cu Ka radiation source (k = 1.5418 Å). SAXS (Anton Paar-PW3830) was used to measure size, shape and distribution of the tungsten nanoparticles. The samples for environmental scanning electron microscopy (ESEM) analysis were prepared by drying a hexane dispersion of particles on a carbon tape supported on brass tub. The particles were imaged by FEI QUANTA 400 (ESEM) at an accelerating voltage of 20 kV and the elemental analysis of synthesized tungsten nano-
Table 1 Experimental conditions under different reaction times. Experiment
Tungsten hexacarbonyl (mmol)
Oleic acid (mmol)
Trioctyl phosphine oxide (mmol)
Reflux time (min)
Average particle size from SAXS (nm)
Standard deviation in particle size (nm)
1 2 3
2 2 2
6 6 6
6 6 6
15 30 60
15 25 28
0.2 0.3 0.3
Table 2 Reaction conditions with varying surfactant concentration. Experiment
Tungsten hexacarbonyl (mmol)
Oleic acid (mmol)
Trioctyl phosphine oxide (mmol)
Reflux time (min)
Average particle size from SAXS (nm)
Standard deviation in particle size (nm)
4 5 6
2 2 2
0 6 12
0 6 12
30 30 30
45 25 30
1.4 0.3 0.4
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Fig. 2. FT-IR spectra of pure oleic acid in 1300–3300 cm
1
region. Fig. 5. X-ray photoelectron spectra of as-synthesized tungsten nanoparticles capped with surfactants produced from experiment 2.
particles were conducted by using energy dispersive X-ray microanalysis (EDX). The samples for transmission electron microscopy (TEM) were prepared by drying a drop of hexane dispersion of the particles on amorphous carbon-coated copper grids. The particles were imaged using Philips, TECHNAI FE 12 microscope (TEM) at an accelerating voltage of 120 kV. 3. Results and discussion
Fig. 3. FT-IR spectra of pure trioctyl phosphine oxide (TOPO) in 1000–3200 cm region.
1
Fig. 4. FT-IR spectra of tungsten nanoparticles capped with oleic acid and trioctyl phosine oxide (TOPO) produced from experiment 2 in 800–3400 cm 1 region.
The tungsten nanoparticles were prepared under various experimental conditions-reaction time and molar ratio of surfactant to metal precursor. As trioctyl phosphine oxide (TOPO) is a high boiling point surfactant with a patulous long chain structure providing greater steric hindrance [43], it slows down the addition rate of materials to the nanoparticles during the growth, resulting in much smaller and well-dispersed nanoparticles. Oleic acid binds tightly to surface of nanoparticles. A combined effect of these two surfactant mixture were much more profound than those of individual contributions. Furthermore, the surfactants adsorbed on to the surface of tungsten nanoparticles provide a dynamic organic structure that stabilizes the tungsten nanoparticles in the solution [21].
Fig. 6. XRD of synthesized tungsten nanoparticles produced from experiment 2.
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Fig. 7. (a) X-ray small angle scattering profile with variation of reaction time where I is the scattering intensity and q is the length of the scattering vector. (b) Pair distance distribution curve of tungsten nanoparticles with variation of reaction time. (c) Radius of gyration (Rg) vs. reaction time graph. (d) X-ray small angle scattering profile with variation of surfactant concentration. (e) Pair distance distribution curve of tungsten nanoparticles with variation of surfactant concentration. (f) Radius of gyration (Rg) vs. surfactant concentration graph.
Figs. 2 and 3 show characteristic peaks of the pure surfactants oleic acid and trioctyl phosphine oxide (TOPO), respectively. For oleic acid, the peaks at 2854 and 2926 cm 1 are due to symmetric
and asymmetric CH2 stretching modes and the peak at 3009 cm 1 is due to t(C–H) stretching mode of the C–H bond adjacent to the C@C bond. The peak at 1713 cm 1 is due to t(C@O) stretching
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mode and the weak mode at 2675 cm 1 is due to t(O–H) stretch of the dimerised acid. The spectrum of trioctyl phosphine oxide (TOPO) shows the peaks at 2854 and 2920 cm 1 which arise due to the symmetric and asymmetric CH2 stretching modes (Fig. 3). The peak at 1146 cm 1 is due to t(P@O) stretching mode. The peak at 1463 cm 1 arises from t(CH3) bending modes, present in both oleic acid and trioctyl phosphine oxide (TOPO), was present in the capped tungsten nanoparticles and attributed to both passivating ligands. The spectrum of tungsten nanoparticles (Fig. 4) produced from experiment 2 with excess surfactant shows the same characteristic peaks in the 2850–3500 cm 1 region. The peak at 1713 cm 1 is due to the t(C@O) stretch mode and indicates some fraction of the oleic acid is bonded to tungsten nanoparticles either in monodentate form or as an acid. The peak at 1512 cm 1 is due to t(–COO) vibrational mode indicates the presence of bidentate carboxylate bonding to the nanoparticles and confirms that some fraction of oleic acid is bonded to nanoparticles in bidentate form [44]. The 1146 cm 1 peak is due to the t(P@O) stretching mode which suggests that the trioctyl phosphine oxide (TOPO) was weakly coordinated to the tungsten nanoparticles surface through O atom. Tungsten nanoparticles are stabilized with oleic acid and trioctyl phosphine oxide (TOPO) [21]. The observation of both t(COO) and t(C@O) vibrational modes indicate that oleic acid acts both as a monodentate and bidentate ligand binding to tungsten via only O atom. The stretching mode of t(P@O) at 1146 cm 1 confirms that trioctyl phosphine oxide (TOPO) bonds to the tungsten nanoparticles through O atom of (P@O) group. The peaks between 3000–3100 cm 1 is due to the t(C–H) stretching modes of C–H bonds adjacent to the C@C bond. The spectrum of oleic acid shows only a single peak at 3009 cm 1 in this region. In Fig. 4, presence of several peaks in this region indicates that the C–H bonds adjacent to the C@C bond exists in several different environments. Prominent mode between 965 and 975 cm 1 can be used to differentiate between cis and trans substituted fatty acids [45]. The peak at 968 cm 1 in tungsten nanoparticles indicates the presence of trans substituted unsaturated alkyl groups. The presence of high frequency peaks due to stretching of the C–H bond adjacent to the C@C double bond suggests that there is conversion of oleic acid to elaidic acid. Three peaks were observed at 3020, 3038 and 3063 cm 1 associated with the t(C– H) stretching modes adjacent to C@C bond in elaidic acid. During the synthesis of tungsten nanoparticles there is a conversion of the alkyl chains from the oleyl form (cis-9-octadecenyl) to the elaidyl form (trans-9-octadecenyl), which is revealed by the presence of several vibrational absorption bands in the region of the olefinic C–H stretching modes [46]. The presence of elaidyl groups on tungsten nanoparticles surfaces is very important because the structure of the oleyl groups and the elaidyl groups are quite different. The elaidyl groups are expected to pack differently around the tungsten nanoparticles providing more stability. The oxidation state of tungsten in tungsten nanoparticles produced form experiment 2 was determined by X-ray photoelectron spectroscopy (XPS) (Kratos/Shimadsu AXIS 165). While the sample was being bombarded with argon ions (emission current = 10 mA, accelerating voltage = 3 kV) evolution of the XPS spectra was followed. The XPS analysis shows the presence of W, O and C in the as-synthesized nanopowders. The W4f doublet with a binding energy of 31.701 eV and 33.759 eV shows the W0 oxidation state of tungsten nanopowders (Fig. 5). Disappearance of the carbon after an erosion time of 3 min, suggests that the carbon is present only at the surface. Elemental analyses of the as-synthesized tungsten nanoparticles were performed using ICP-OES and LECO gas analyzers. It was observed that tungsten, carbon and oxygen content were in the range of 94–95%, 0.7–1% and 1.5–4%, respectively.
The XRD pattern of the as-synthesized tungsten nanoparticles produced from all the experiments (1–6) exhibits only two broad peaks. Absence of sharp diffraction peaks in the spectrum indicates that the nanopowders are amorphous in nature. After annealing at 600 °C for 1 h in argon atmosphere the powders convert from a fully amorphous state to a crystalline body centered cubic structure. The annealing temperature raised to 600 °C at a heating rate 10 °C per 1 min and was then kept for 1 h. There was no great variation of XRD pattern observed for the tungsten nanopowders produced from all the experiments (1–6). Fig. 6 shows the XRD pattern of the tungsten nanoparticles produced from experiment 2. XRD results for the annealed tungsten nanoparticles reveals the presence of tungsten with the lattice parameter 3.1607 Å which is very close to the lattice parameter of tungsten reported (JCPDS card. No. 04-0806). The SAXS results of all the six samples are shown in Fig. 7. As observed in Fig. 7a that the scattering intensity is increasing with increase in reaction time, indicating that there is an increase in average particle size [47]. It is known that when slope of the plot log I (q) vs. log q is 0 (line collimation) then the scattering curves reveal that the shape of the particles are spherical [48]. The slopes of all the curves in Fig. 7a exhibit deviation from zero. Based on the scattering curves, it can be concluded that the shape of the particles are not spherical. With varying concentration of surfactant, SAXS experiment has been carried out for the tungsten nanoparticles and the results are shown in Fig. 7d. Without the addition of surfactant the intensity is highest which indicates that the particles are either agglomerated or having large size. With addition of 6 mmol each of the surfactants the intensity has decreased
Fig. 8. (a) ESEM image of the synthesized tungsten nanoparticles with a mixture of (1:1) surfactants oleic acid (6 mmol) and trioctyl phosphine oxide (TOPO) (6 mmol) and corresponding, (b) EDX spectrum.
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and further increase in surfactant mixture concentration to 12 mmol each the particle size has again increased. The slopes of all the curves in Fig. 7d also exhibit deviation from zero indicating that the particles are not spherical [48]. Pair distance distribution function (PDDF) p(r) has been calculated from the scattering data by using the programme GIFT [49]. The p(r) is the Fourier inversion of the scattering curve and defined R I (q) qr Sin qr dq], where r is the distance or as [(1/2p2) radius of the particles. Fig. 7b and e show the pair distance distribution functions as a function of r. The shape of the p(r) vs r curves also resembles that the particles are not spherical. The PDDF curves also reveal that the average particle size for experiments 1–3 (Table 1) and 4–6 ( Table 2) vary form 15 to 28 nm and 25 to 45 nm, respectively. The radius of gyration (Rg) has been plotted against the reaction time (Fig. 7c). This clearly demonstrates that with increase in reaction time the radius of gyration increases which in turn indicates the increase in particle size [50]. Radius of gyration (Rg) has also been plotted against varying surfactant concentration (Fig. 7f). Without addition of surfactant the radius of gyration (Rg) is maximum and on addition of 6 mmol of each of the surfactant the radius of gyration (Rg) is minimum. With increase in surfactant concentration to 12 mmol each the radius of gyration increases which clearly shows that the particle size increases with addition of more surfactants. Fig. 8a shows the ESEM image of the synthesized tungsten nanoparticles (experiment 2) with a mixture of (1:1) surfactants
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oleic acid (6 mmol) and trioctyl phosphine oxide (TOPO) (6 mmol). Its EDX spectrum excited by an electron beam (20 kV) shows the peaks for the elements of W, O and C (Fig. 8b). From EDX it was confirmed that there is little amount of C and O impurities. The appearance of C peak is due to the carbon tape used for sampling and surfactants present on nanoparticle surface. The O peak is due to the surfactant mixture oleic acid and trioctyl phosphine oxide (TOPO) and a partial surface oxidation of tungsten nanoparticles during sampling. There are no other impurities in as-synthesized tungsten nanoparticles. Fig. 9 shows the TEM images of the tungsten nanoparticles synthesized by thermal decomposition of tungsten hexacarbonyl [W(CO)6] at 160 °C in presence of mixture of surfactants (1:1) at different reaction times (experiments 1–3). The average size of the prepared tungsten nanoparticles was increased from 15 to 28 nm as the decomposition time increases from 15 to 60 min, respectively (Fig. 9a–c). As shown in Fig. 10, following with the different surfactant concentration, sizes of the tungsten nanoparticles obtained are also different. The tungsten nanoparticles synthesized without surfactants seem to be an agglomeration of small particles and they are flocculated with an average particle size of 45 nm (Fig. 10a). An addition of a mixture of surfactants (1:1) has resulted in nanopowders with average particle size about 25 nm as shown in Fig. 10b. With increase in surfactant amount twice up to 12 mmol each, the obtained nanoparticles looked like agglomeration of several particles with average particle size of
Fig. 9. TEM image of the synthesized tungsten nanoparticles after (a) 15 min, (b) 30 min and (c) 60 min of reaction.
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Fig. 10. TEM image of the synthesized tungsten nanoparticles (a) without surfactant, (b) with a mixture of (1:1) surfactants oleic acid (6 mmol) and trioctyl phosphine oxide (TOPO) (6 mmol) and (c) with a mixture of (1:1) surfactants oleic acid (12 mmol) and trioctyl phosphine oxide (TOPO) (12 mmol).
30 nm (Fig. 10c) [24]. Though not all the ESEM pictures are presented here in this paper, but both ESEM and TEM results are in good agreement with the SAXS results. Therefore, it can be concluded that the role of surfactant is very important in the formation of tungsten nanoparticles by thermal decomposition method. This controls both the size as well as shape of the nanoparticles. 4. Conclusion Different size and shapes of the tungsten nanoparticles were successfully synthesized by thermal decomposition with tungsten hexacarbonyl depending on the decomposition times and surfactant concentrations. The sizes of the tungsten nanoparticles varied from 15 to 28 nm with increasing the decomposition times from 15 to 60 min. The particles are agglomerated without surfactant. The presence of surfactant reduces the particle size and with further increase in surfactant concentration increases the particle size. The synthesized tungsten nanoparticles are amorphous and it converts to crystalline bcc phase after annealing in argon atmosphere.
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