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Synthesis of tungsten nanoparticles by reverse micelle method
Accepted Manuscript Synthesis of tungsten nanoparticles by reverse micelle method
Arash Hadavand Khani, A.M. Rashidi, Giti Kashi PII: DOI: Reference:
S0167-7322(16)34288-X doi: 10.1016/j.molliq.2017.06.053 MOLLIQ 7499
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
30 December 2016 9 June 2017 12 June 2017
Please cite this article as: Arash Hadavand Khani, A.M. Rashidi, Giti Kashi , Synthesis of tungsten nanoparticles by reverse micelle method, Journal of Molecular Liquids (2016), doi: 10.1016/j.molliq.2017.06.053
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Synthesis of tungsten nanoparticles by reverse micelle method
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Arash Hadavand khani*, a, A.M. Rashidi a,b, Giti Kashic
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Department of Mechanical Engineering, Razi University, p.o.box:67149, Tagh-E-Bostan Kermanshah, Islamic republic of IRAN Faculty of Engineering Razi University, p.o.box:67149, Tagh-E-Bostan Kermanshah, Islamic republic of IRAN c Faculty of Health, University of Islamic Azad University Tehran Medical Sciences Branch, Khaghani St, Shariati Ave, Tehran, IRAN
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Corresponding Author: Arash Hadavand Khani
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Department of mechanical engineering Razi university of Kermanshah Tagh-E-Bostan Kermanshah.p.o.box:67149
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Tel: +98(833)4274535-9
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Email: [email protected]
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Fax: +98(833)4283263
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ACCEPTED MANUSCRIPT Abstract Tungsten nanoparticles have been successfully synthesized via reverse micelle method in two micellar systems include nHexane-SDS (Sodium dodecyl sulfate) and Cyclohexane-Triton X-100. The inter-micellar space has been used for the reaction of sodium tungstate with two reducing agents containing Iron(II)chloride and sodium borohydride. The effects of parameters such as temperature, pH, micelle type, water to surfactant molar ratio(Kw), annealing and type of the co-surfactant on the reduction process and size of the particles were investigated. The as-synthesized and heat treated products were characterized by scanning electron microscopy(SEM), Energy dispersive X-ray(EDX) and X-ray diffraction. SEM analyze showed spherical tungsten nanoparticles with narrow size distribution and size(13nm) and both α & β structures of tungsten were observed in XRD patterns.
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Keywords: Tungsten Nanoparticles, Reverse Micelle, n-Hexane-SDS, Cyclohexane-TritoneX100
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1.Introduction
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Metallic nanoparticles are the building blocks for the future of most industries and the most widely used after ceramic nanoparticles. According to their unique properties like large surface energy, Plasmon excitation, quantum confinement, they became the key component for the electronic, optoelectronic, material and other scientific studies [1]. In recent years, a lot of attentions have been paid to the Transition metals like Zinc, Titanium, Copper and tungsten. special physical properties like high melting and boiling point, photocatalytic activity and their applications in dye and pollution removal from waste water made them the most significant metal nanostructures. Nanoparticles morphology is deeply affected by synthesis method so that it can vary from dendrite and flower to spherical, cubic or any other shape, nevertheless, the application and properties will be changed [2-9]. Considering the specific application of tungsten nanoparticles in armament, aerospace, electronics, High power batteries, coating, Microelectronic films, electrodes for gas sensors, sintering additive, Alloy for heat sinks, Vacuum contactors, vacuum lead switch, and anti-friction tools, it has special position among transition metals. Multifarious methods have been 1 offered for fabrication of WNPs encompass chemical vapor synthesis, electric wire explosion, arc discharge, Microwave plasma, sputtering, Self-propagating High-Temperature synthesis, Ball milling, Combustion, sol-gel and wet chemical based methods like thermal decomposition and co-precipitation. As a result of high melting point(3422°C) and recrystallization temperature of tungsten in traditional synthesis method which includes reducing tungsten three oxide with Hydrogen gas to metallic form, the temperature exceeds 1000⁰C and that is the cause of particle growth and agglomeration. Moreover, a lot of disadvantages to other methods like low yield, amorphous products and difficult conditions for monodisperse synthesis, make them inapplicable [10-23]. This study aims to synthesize WNPs by reverse micelle and approach the temperature below 100⁰C. Ye Gao et al synthesized WNPs by co-precipitation method, though the process is hard to control and not repeatable, besides the major part of the product is iron tungstate nanorods [24]. Reverse micelle contains a continuous oil phase, a dispersed water phase and dissolved surfactant at the oil-water interface. Dispersing water in oil phase causes to form a plethora of tiny droplets span from nanometers to micrometers in size which is so-called reverse micelle. as a result of space confinement in micelles, particle growth will be limited. controlling the parameters like water to surfactant ratio restricts the particle size and shape. surrounded space of micelle by oil phase, prevents the penetration of oxygen and impurities and therefore the oxidation possibility will be decreased. Reverse micelle method is not commercially viable for mass production, however it is important on account of narrow size distribution and proper size of as-synthesized particles so that it can be used for preparation of quantum dots [25,26]. Synthesis of WNPs by reverse micelle has received scant study. on the other hand, most of the studies focus on combined tungsten structure such as oxides and carbides. Xiong et al used reverse micro emulsion mediated method for synthesis metal tungsten nanopowder. They produced a gel by hydrolyzing tungstenisopropoxide as precursor in various micellar systems and after aging, particles were collected by precipitation and dried in cryogenic condition. Oxolation and Olation is the main reaction path and, therefore, the main product is metal oxide and reduction process with heat treatment will be required. [27]. In this study we have investigated the hydrolysis of metal salt and reduction of tungsten oxide in the micelle system without any purification.
2.Experimental 2.1 Material and experiment 1
Wolframite Nano particles
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ACCEPTED MANUSCRIPT All materials were purchased from Merck with analytical grade and used without further purification. Sodium tungstate dehydrate is the most abound tungsten precursor and as a result of low price [28], it has been selected as starting material. The phase of product determined by X-Ray powder diffraction (XRD; Philips pw1730 with CuKα(λ=1.54046⁰A). Size and size distribution analyzed by Zetasizer 3000 HAS analyzer. The powder morphology and composition of samples obtained by scanning electron microscope (FESEM; Model MIRA3 TESCAN) equipped with EDS apparatus.
2.2 preparation of tungsten nanoparticles by co-precipitation
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For this purpose, we followed the work of Gao et al precisely [24]. 199ml solution FeCl2.4H2O of 0.2M prepared in a beaker at room temperature and located in a water bath and stirred simultaneously. By adding specific amount of HCl to the solution, pH adjusted to number 2. afterwards 50 ml solution of NaWO4.2H2O of 0.2 M was added to the mother solution. The addition must be done in correct sequence, otherwise the product will be different. The Reaction conducted at 80⁰C, Molar ratio X= [ II+ 2Fe /WO4 ] =4 and stabled pH at 2. Within 90 minutes of stirring, dark precipitates were separated from the mother solution and washed by deionized water repeatedly to eliminate the iron and tungstate ions. Finally, the sediment was dried at T=60⁰C for 3 hours and sent for characterization by SEM&EDX with Nu0.
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2.3 Synthesis by Reverse Micelle
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The synthesis reaction in micelle system included precipitation and reduction. The most important factors in these reactions 2 were pH and Temperature. Two types of micelles were examined included SDS -normal hexane and Cyclohexane-TritoneX100. Normal butanol has been used as co-surfactant in both micelles.
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2.3.1 First Micelle (SDS-nhexane)
WO3+2NaCl+H2O
(1)
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Na2WO4+2HCl
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The micelle includes sodium dodecyl sulfate(SDS) as an anionic surfactant and Normal Hexane as a continuous oil phase. Preparation of micelle followed by dissolving 8.01gr SDS in 100ml of n-Hexane under vigorous stirring and ultrasonic for 20 minutes. then the n-butanol added dropwise to the solution until the solution became transparent and stable. finally, emulsion 2was divided into two parts. for reaction of tungsten precursor with FeCl2 at the optimum molar ratio(X=[Fe(II)/WO4 ] =4), 1gr NaWO4.2H2O and 2.41gr FeCl2.4H2O, dissolved in 5ml pre-degassed deionized water separately then added to the micelles. HCl 37% added to theFeCl2 solution to adjust the pH, then mixed with micelle in a three neck flask and sealed. the reaction between sodium tungstate and HCl is strong and form three oxide that it can be described as equation (1), therefor the precursor micelle should have higher pH before the reaction.
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Fig 1 illustrates the sketch of the reaction apparatus used for reverse micelle. Monitoring the amounts pH during the reaction was done by locating a pH meter probe into the solution. The total volume of water in micelles was 10 ml that is designed to attain the water-to-surfactant molar ratio(Kω) to 20. Titration of metal container micelle under the flow of Argon gas to the other micelle started after temperature approached 60⁰C. black sediments were separated from oil phase within 60 minutes of reaction time and soaked in methanol for 2hr to eliminate the sodium element, then washed repeatedly with distilled water. The solution of water and dark sediments centrifuged 2min at5000rpm speed and dried at 60⁰C in atmosphere. Reactions conducted at a constant temperature, stoichiometry and at 4 different pH values. The conditions of reactions are presented in table 1. 2-
Reaction with sodium borohydride at various stoichiometry (X=[NaBH4]/[WO4 ] =10,15,20,30) was investigated. Preparation of two micelles was done under the same conditions. owing to the vigorous reaction between sodium borohydride and water at low pH values which causes to form BO2 (Eq2), NaOH added to the 5ml pre-degassed deionized water then reducing solution was prepared. -
BH4 + 2H2O
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BO2 + 4H2
(2)
Subsequently, solution mixed with the micelle in the reflux system. the reaction between sodium tungstate and sodium borohydride at PH values more than 1 causes to form sodium tungsten bronze as it is shown in Equations3&4[29], therefore the pH was reduced to 0.5 during the reduction. Reactions data with sodium borohydride at 60⁰C in this micelle are presented in table 2.
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Sodium dodecyl sulfate
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NaBO2+NaxWO3+(2-x) NaOH+(4-0.5x) H2 NaBO2+WO3-α+2NaOH+(4-α) H2
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NaBH4+Na2WO4+(3-α) H2O
(3)
Figure. 1. a sketch of reaction media
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2.3.2 Second Micelle(Cyclohexane-TritonX100)
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In comparison with other refractory and transition metals such as molybdenum, tungsten metal has a complicated reduction process. This is because the reduction is depended on temperature also the pH is effective [30]. In addition, the double bond dissociation of tungsten with oxygen is an obstacle for reduction. Based on the last studies the best particle size and size distribution determined in co-precipitation at temperature 80⁰C [24]. moreover, the boiling point of n-hexane is 68⁰C, thus cyclohexane was used instead of normal hexane. The best surfactant for mixing with cyclohexane is triton x-100. Additionally, by this micelle combination the separation process of synthesized nanoparticles will be done at lower temperature. The experiments in this micelle aims to understand the stoichiometry of reaction and investigate the effect of co-surfactant, water to surfactant molar ratio and temperature on products. Table1-reactions of FeCl2 in first type micelle No 1 2 3 4
Hexane (ml) 100 100 100 100
SDS (gr) 8.01 8.01 8.01 8.01
Kw= [SDS/H2O] 20 20 20 20
Butanol (ml) 5 5 5 5
pH * 1 2 3 8
T (⁰C) 60 60 60 60
X= [ Fe(II)/WO42-] 4 4 4 4
Table2-Reaction of NaBH4 in first micelle No
Hexane (ml)
SDS (gr)
Kw= [SDS/H2O]
Butanol (ml)
4
pH
T (⁰C)
X= [NaBH4]/[WO42-] *
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120 120 120
10.68 10.68 10.68
15 15 15
10 10 10
0.5 0.5 0.5
60 60 60
15 20 30
Table 3-Reaction of FeCl2 in second micelle Cyclohexane (ml)
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Triton X-100 (ml) 11.19
Kw= [T-X100]/[H2O]
pH
T (⁰C)
30
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20
Butanol (ml)
X= [ Fe(II)/WO42-]
60
Isoamyl Alcohol (ml) ……
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11.19
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13.43
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1
80
60
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11
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22.39
15
1
80
60
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13
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22.39
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Table4- Reaction of NaBH4 in second micelle
14 15 16
60 60 60
Triton X-100 (ml) 22.39 33.59 22.39
Kw= [TX-100]/ [H2O] * 15 10 15
3. Results and discussion
pH
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Butanol (ml)
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80 80 80
60 60 60
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X= [NaBH4]/[WO42-] * 20 30 35
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Cyclohexane (ml)
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the SEM micrograph of co-precipitation sample0(Fig2a) shows highly agglomerated particles with unsuitable mean size which is calculated 121.9nm however the morphology is nearly spherical. the SEM micrograph of as synthesized particles in the first micelle (sample 1) in Fig2b exhibits particles with more spherical morphology and ameliorated resolution. The mean size diameter for sample1 is measured 66.86nm. the differences in size result from volume confinement of micelles and web structure of capping agents such as surfactants. Methyl groups of SDS, bond with oxygen and form a web that entraps the nanoparticles and cause the coagulated structure. 3. effect of temperature
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3.effect of co-surfactant
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SEM of particles prepared in the Cyclo-TX100 micelle at 20⁰C and Kw=30 (sample8) in Fig2c indicates particles with Nano size coagulation and as a result of low resolution, the morphology cannot be determined. However, the isometry of particles does not follow a particular rule at this Kω. the SEM of particles produced in Cyclo-TX100 micelle(Fig2d) at 80⁰C and the same Kω(sample9) shows that particles have faceted morphology and they are agglomerated but have better resolution nonetheless. This could be resulting from temperature increment. The mean particles size calculated for S9 is 47.5nm. comparing Fig 2(c)&2(d), reveals that, particle size has decreased by increasing temperature. The micelle stability increases with elevating temperature and there is a decline in probability of micelle's rupture. the micellar crust became harder and prevented the water entrance. So, the smaller particles with narrow size distribution were synthesized at higher temperature and constant Kw. Based on EDS data presented in table 5 and by comparing sample 15 with 7, it was concluded that the reduction of tungsten by sodium borohydride is affected by temperature vigorously.
Fig 3a shows the SEM of sample 11 that is produced in the second micelle at 80⁰C and Kω=15. Aggregation and particle size have significantly decreased. The morphology is spherical and better than faceted shape of particles in Fig2d. Fig3b shows the SEM of sample 13 using Isoamyl Alcohol as co-surfactant instead of butanol. Any specific change was not observed in morphology but regarding to the results presented on table 6, the size distribution and particles size have decreased in 13. On comparison, the Isoamyl alcohol was more effective of the two co-surfactant and, as a result, it was selected for the next experiments. 3. effect of annealing Fig 3c shows SEM micrograph of sample 15. due to the low water to surfactant molar ratio(Kw=15), the particles are very small but have absolutely spherical morphology. The particle size spanned from 8 to 20nm and The least particle size obtained under this condition. Fig 3(d) indicates the sample 15 after annealing at 1000⁰C for 2hr under Argon flow. Most of the excess impurities like surfactants, oil phase and reducing agent were sublimated at this temperature. In spite of significant particle growth, their size did not exceed 100nm and therefore the annealing was not inappropriate factor. The particle diameter calculated 83. 37nm.
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(b)
(c)
(d)
Figure. 2. Scanning electron microscope of (a)sample0 (b)sample 1 ( SDS-nHexane) (c) sample 8 (Cyclo-TX100 at T=20⁰ C) (d) sample 9 (Cyclo-TX100 at T=80⁰ C)
(c)
(d)
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Figure. 3. Scanning electron microscope of samples (a) 11 (b) 13 (c) 15(d) 19(15 after annealing at 1000⁰ C)
FeCl2 + H2
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Two kind of EDS patterns of as-prepared powder prepared by reverse micelle are presented in Fig4a&b. patterns of samples contain iron, Oxygen and tungsten peaks and some of them have sodium and boron. The iron peaks observed in the powder produced by FeCl2. Due to the reverse reaction of Fe(II) with HCl, Iron existence is nearly unavoidable.
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Furthermore, many of Fe(II) ions react with tungstate ion and form iron tungstate. The sodium peak in samples may referred to the incomplete washing process. Based on EDS data, the tungsten reduction is depended on pH values so that the highest reduction for FeCl2 occurred at pH=1 was %50.25. The optimum stoichiometry ratio for reaction between NaBH 4 and Na2WO4.2H2O (samples 5, 6 & 7) was X=30 and the reduction amount was%65.75. no precipitation observed in X=10. The relation between pH, stoichiometry and reduction illustrated in diagram Fig 5&6.
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The crystal structure of as-synthesized powder was determined by XRD. The XRD patterns of tungsten nanoparticles obtained under different conditions (stoichiometry, reducing agent, temperature). Most of the products was amorphous but some of them had very tiny peaks. A sample of amorphous peak is shown in Fig7a. Products were annealed at 600⁰C in a tube furnace under Argon flow for 2hr to obtain their crystal structure. The weak peak observed in Fig7b (sample 1), correspond to the reflection of plane (1 1 0) of α-tungsten with bcc structure. Other peaks in the pattern match with peaks of the Ferberite (Iron tungstate). tungsten percentage in this sample was %67. Respect to the TEM presented in the synthesis via co-precipitation method [24] and due to the low reducing potential of iron chloride,
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Figure. 4. EDS patterns of powder produced by (a)FeCl2 (b) NaBH4
File1.CVS here
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Figure. 5. Plot of Relation between pH and tungsten reduction
File2.CVS here
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Figure. 6. Plot of Relation between Stoichiometry and tungsten reduction 2-
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it is inferred that the most of the Fe(II) in FeCl2.4H2O react with WO4 and leads to form iron tungstate and tungsten iron core shell even in reverse micelle method. Pattern of sample 5 is shown in Fig 7c. Owing to the low stoichiometry ratio of NaBH4 to tungstate (X=15), a small peak of tungsten observed which is belonged to the (2 0 0) plane. WO3 strong peak shows that the reducing agent, reduced tungstate ion Table. 5. Data of EDX
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Reducing Agent FeCl2 FeCl2 FeCl2 FeCl2 FeCl2 NaBH4 NaBH4 NaBH4 FeCl2 FeCl2 FeCl2 FeCl2 FeCl2 FeCl2 NaBH4 NaBH4 NaBH4
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4 4 4 4 4 15 20 30 1 1 3 4 5 6 20 30 35
T (⁰C) 80 60 60 60 60 60 60 60 20 80 80 80 80 80 80 80 80
pH 2 1 2 3 8 0.5 0.5 0.5 1 1 1 1 1 1 0.5 0.5 0.5
%Wt. W 29.35 50.25 36 34.08 8.11 5.35 30.60 65.75 17.30 31.46 45.23 52.45 35.47 42.74 68.04 90.63 76.01
Table. 6. Data of size and errors No
0 1 9 11 13 15 19
Largest size (nm) 301 138 138 115 85 18.93 127
The least size (nm) 26.3 10 13.21 24 14.5 5.93 50
Size Distribution (nm) 274.7 128 124.79 91 70.5 13 77
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Arithmetic Mean 𝑥̅ (𝑛𝑚) 121.91 66.8637 47.5779 66.016 51.4615 12.8927 83.3714
Standard Deviation 𝜎(nm) 63.8192 29.9143 28.1149 23.9847 18.0856 3.5799 18.3493
Error 𝜎𝑚 6.3819 2.9914 2.8114 2.3985 1.8086 0.3579 1.8349
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to the lower valence. Another peak belongs to the hydrocarbon compounds which means that the washing process had a failure. The same peak of WO3 was observed in Fig 7d that shows the pattern of sample 14 with X=20. But the α-tungsten with (1 1 0), (2 0 0) and (2 2 0) planes was observed too. The pattern of the sample 15 is shown in Fig 7e. the peak of WO3 eliminated in X=30 and α-W was observed. The highest tungsten percentage obtained in this sample(%Wt=90.63). by comparing patterns of Fig7c&d&e it is clear that in higher stoichiometry portion, the purity of product increased. 3 samples were annealed under argon flow at 1000⁰ C and 2hours. Fig8a shows the sample17(5 after annealing). By comparing patterns before and after annealing for sample 5, it was found that the peak of (2 0 0) is eliminated and most of the peaks belong to the tungsten trioxide. Due to the pH=1, the reaction (3) & (4) might be the reason of formation of WO3, sodium metaborate and sodium tungsten bronze. Fig8b shows x-ray pattern of sample 18 (14 after annealing). The peaks of sodium tungsten oxide(Na2W2O7), tungsten dioxide(WO2) and Disodium octaborate(Na2B8O13) were observed. The highest peak belongs to the tungsten trioxide. the x-ray pattern of sample 15 after annealing (19) is shown in Fig8c. both α and β structure of tungsten were observed. Also the peaks of sodium tungsten oxide and disodium octaborate were detected. Due to the result of SEM, EDS and XRD the best condition to produce metallic tungsten, belongs to sample15.
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Figure. 7. XRD patterns of amorphous sample(a) and four heat treated sample at 600⁰ C under argon flow include samples (b)1 (c) 5 (d) 14 (e) 15
Figure. 8. XRD patterns of three samples (a) sample 5 (b)sample 14 (c) sample15 annealed at 1000⁰ C under argon flow.
4. conclusion reverse micelle method has been developed for synthesis of tungsten nanoparticles from sodium tungstate, FeCl2.4H2O and NaBH4 as starting materials. effects of parameters such as temperature, pH, micelle type, stoichiometry, type of reducing agent
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and annealing on morphology, reduction and structure of product have been investigated. SEM analysis showed spherical particles with mean diameter 13nm and narrower size distribution in Cyclo-TX100 than the SDS-nhexane micelle. comparing SEM micrograph of second micelle (Cyclohexane-TritonX-100) at two temperature showed that increasing Temperature affects the micelle shape extremely. Regarding to the EDX data, temperature increment ameliorates the reduction of tungsten. The best pH for reducing by NaBH4 attained 0.5 and the best molar ratio achieved at [NaBH4/Na2WO4] =30. both α&β structures of tungsten were observed in x-ray diffraction patterns. XRD pattern of powder produced by FeCl2, showed that, the majority of the products are iron tungstate in the form of core and shell. consequently, sodium borohydride acts much better. EDX patterns showed pure tungsten peak in sample 15 by %90.63 purity. In comparison with the work of Xiong et al [27], as synthesized particles collection is more simple, however the mean size of particles was larger. Additionally, no more purification and reduction process is needed. Based on our knowledge it was the first time that tungsten nanoparticles were synthesized directly from sodium tungstate via this method.
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ACCEPTED MANUSCRIPT 5. Acknowledgment
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This research is conducted at nanotechnology laboratory of department of physic. The authors like to thank the department physic of Razi university of Kermanshah for facilities and instrumental supports.
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[11] Y.S. Kwon, A.A. Gromov, A.P. Ilyin, A.A. Ditts, J.S. Kim, S.H. Park, M.H. Hong, Features of passivation, oxidation and combustion of tungsten nanopowders by air, Int. J. Refract. Met. Hard Mater. 22 (2004) 235–241. http://dx.doi.org/10.1016/j.ijrmhm.2004.06.005. [12] T. Ryu, H.Y. Sohn, K.S. Hwang, Z.Z. Fang, Chemical vapor synthesis (CVS) of tungsten nanopowder in a thermal plasma reactor, Int. J. Refract. Metals Hard Mater. 27 (2009) 149–154, https://doi.org/10.1016/j.ijrmhm.2008.06.002. [13] M.Z. Kassaee, S. Soleimani-Amiri, F. Buazar, Diverse tungsten nanoparticles via arc discharge, J. Manuf. Process. 12 (2010) 85–91, https://doi.org/10.1016/j.jmapro.2010.06.001. [14] R. Sarathi, T.K. Sindhu, S.R. Chakravarthy, A. Sharma, K. V Nagesh, Generation and characterization of nano-tungsten particles formed by wire explosion process, J. Alloys Compd. 475 (2009) 658–663, https://doi.org/10.1016/j.jallcom.2008.07.092. [15] K. Murugan, S.B. Chandrasekhar, J. Joardar, Nanostructured α/β-tungsten by reduction of {WO3} under microwave plasma, Int. J. Refract. Met. Hard Mater. 29 (2011) 128–133, https:// doi.org/10.1016/j.ijrmhm.2010.09.005. [16] X.-X. Zhang, Q.-Z. Yan, C.-T. Yang, T.-N. Wang, M. Xia, C.-C. Ge, Recrystallization temperature of tungsten with different deformation degrees, Rare Met. 35 (2016) 566–570, https://10.1007/s12598-014-0315-2.
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[17] K.K. K., L. Couëdel, C. Arnas, Growth of tungsten nanoparticles in direct-current argon glow discharges, Phys. Plasmas. 20 (2013), https://doi.org/10.1063/1.4802809. [18] T.J. Petty, J.W. Bradley, Tungsten nanostructure formation in a magnetron sputtering device, J. Nucl. Mater. 453 (2014) 320–322, https://doi.org/10.1016/j.jnucmat.2014.07.023. [19] A.S. Bolokang, M.J. Phasha, K. Maweja, S. Bhero, Structural characterization of mechanically milled and annealed tungsten powder, Powder Technol. 225 (2012) 27–31, https://doi.org/10.1016/j.powtec.2012.03.028.
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[20] H.H. Nersisyan, J.H. Lee, C.W. Won, A study of tungsten nanopowder formation by self-propagating high-temperature synthesis, Combust. Flame. 142 (2005) 241–248, https://doi.org/10.1016/j.combustflame.2005.03.012.
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[22] Han, Yuxian, Tai Qiu, and Tao Song. "Preparation of Ultrafine Tungsten Powder by Sol-Gel Method." Journal of materials science & technology 24.5 (2008).pp 816-818, https://doi/ 10.3321/j.issn:1005-0302.2008.05.029.
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[23] P.K. Sahoo, S.S.K. Kamal, M. Premkumar, T.J. Kumar, B. Sreedhar, A.K. Singh, S.K. Srivastava, K.C. Sekhar, Synthesis of tungsten nanoparticles by solvothermal decomposition of tungsten hexacarbonyl, Int. J. Refract. Met. Hard Mater. 27 (2009) 784–791, https://doi.org/10.1016/j.ijrmhm.2009.01.005.
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[24] Y. Gao, J. Zhao, Y. Zhu, S. Ma, X. Su, Z. Wang, Wet chemical process of rod-like tungsten nanopowders with iron (II) as reductive agent, Mater. Lett. 60 (2006) 3903–3905. doi:http:// doi.org/10.1016/j.matlet.2006.03.137. [25] H. L. Rosano and M. Clausse, Microemulsion Systems. Taylor & Francis, vol6, issue 3, PP.14-32, 1985.
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[26] D. Ganguli and M. Ganguli, Inorganic Particle Synthesis via Macro and Microemulsions: A Micrometer to Nanometer Landscape. Springer US, vol 20, (2012). pp 1-20.
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[27] L. Xiong, T. He, Synthesis and characterization of ultrafine tungsten and tungsten oxide nanoparticles by a reverse microemulsion-mediated method, Chem. Mater. 18 (2006) 2211–2218, https://doi.org/ 10.1021/cm052320t. [28] T. A. G. Duarte, “Sodium Tungstate Dihydrate,” ChemInform, vol. 45, no. 33. (2014)., https://doi.org/ 0.1002/chin.201433266.
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[29] C. Tsang, S.Y. Lai, A. Manthiram, Reduction of aqueous Na2WO4 by NaBH4 at ambient temperatures to obtain lower valent tungsten oxides, Inorg. Chem. 36 (1997) 2206–2210, https://doi.org/10.1021/ic9610039.
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[30] A. Döring, C. Schulzke, Tungsten’s redox potential is more temperature sensitive than that of molybdenum, Dalt. Trans. 39 (2010) 5623–5629. https://doi.org/ 10.1039/C000489H
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Highlights A facile method under low pressure and temperature for synthesis of tungsten nanoparticles Relatively cheap precursor Tiny and monodisperse particles with controllable morphology Stable and protected nanoparticles
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Arash Hadavand Khani, A.M. Rashidi, Giti Kashi PII: DOI: Reference:
S0167-7322(16)34288-X doi: 10.1016/j.molliq.2017.06.053 MOLLIQ 7499
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
30 December 2016 9 June 2017 12 June 2017
Please cite this article as: Arash Hadavand Khani, A.M. Rashidi, Giti Kashi , Synthesis of tungsten nanoparticles by reverse micelle method, Journal of Molecular Liquids (2016), doi: 10.1016/j.molliq.2017.06.053
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Synthesis of tungsten nanoparticles by reverse micelle method
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Arash Hadavand khani*, a, A.M. Rashidi a,b, Giti Kashic
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Department of Mechanical Engineering, Razi University, p.o.box:67149, Tagh-E-Bostan Kermanshah, Islamic republic of IRAN Faculty of Engineering Razi University, p.o.box:67149, Tagh-E-Bostan Kermanshah, Islamic republic of IRAN c Faculty of Health, University of Islamic Azad University Tehran Medical Sciences Branch, Khaghani St, Shariati Ave, Tehran, IRAN
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Corresponding Author: Arash Hadavand Khani
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Department of mechanical engineering Razi university of Kermanshah Tagh-E-Bostan Kermanshah.p.o.box:67149
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Tel: +98(833)4274535-9
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Email: [email protected]
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Fax: +98(833)4283263
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ACCEPTED MANUSCRIPT Abstract Tungsten nanoparticles have been successfully synthesized via reverse micelle method in two micellar systems include nHexane-SDS (Sodium dodecyl sulfate) and Cyclohexane-Triton X-100. The inter-micellar space has been used for the reaction of sodium tungstate with two reducing agents containing Iron(II)chloride and sodium borohydride. The effects of parameters such as temperature, pH, micelle type, water to surfactant molar ratio(Kw), annealing and type of the co-surfactant on the reduction process and size of the particles were investigated. The as-synthesized and heat treated products were characterized by scanning electron microscopy(SEM), Energy dispersive X-ray(EDX) and X-ray diffraction. SEM analyze showed spherical tungsten nanoparticles with narrow size distribution and size(13nm) and both α & β structures of tungsten were observed in XRD patterns.
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Keywords: Tungsten Nanoparticles, Reverse Micelle, n-Hexane-SDS, Cyclohexane-TritoneX100
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1.Introduction
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Metallic nanoparticles are the building blocks for the future of most industries and the most widely used after ceramic nanoparticles. According to their unique properties like large surface energy, Plasmon excitation, quantum confinement, they became the key component for the electronic, optoelectronic, material and other scientific studies [1]. In recent years, a lot of attentions have been paid to the Transition metals like Zinc, Titanium, Copper and tungsten. special physical properties like high melting and boiling point, photocatalytic activity and their applications in dye and pollution removal from waste water made them the most significant metal nanostructures. Nanoparticles morphology is deeply affected by synthesis method so that it can vary from dendrite and flower to spherical, cubic or any other shape, nevertheless, the application and properties will be changed [2-9]. Considering the specific application of tungsten nanoparticles in armament, aerospace, electronics, High power batteries, coating, Microelectronic films, electrodes for gas sensors, sintering additive, Alloy for heat sinks, Vacuum contactors, vacuum lead switch, and anti-friction tools, it has special position among transition metals. Multifarious methods have been 1 offered for fabrication of WNPs encompass chemical vapor synthesis, electric wire explosion, arc discharge, Microwave plasma, sputtering, Self-propagating High-Temperature synthesis, Ball milling, Combustion, sol-gel and wet chemical based methods like thermal decomposition and co-precipitation. As a result of high melting point(3422°C) and recrystallization temperature of tungsten in traditional synthesis method which includes reducing tungsten three oxide with Hydrogen gas to metallic form, the temperature exceeds 1000⁰C and that is the cause of particle growth and agglomeration. Moreover, a lot of disadvantages to other methods like low yield, amorphous products and difficult conditions for monodisperse synthesis, make them inapplicable [10-23]. This study aims to synthesize WNPs by reverse micelle and approach the temperature below 100⁰C. Ye Gao et al synthesized WNPs by co-precipitation method, though the process is hard to control and not repeatable, besides the major part of the product is iron tungstate nanorods [24]. Reverse micelle contains a continuous oil phase, a dispersed water phase and dissolved surfactant at the oil-water interface. Dispersing water in oil phase causes to form a plethora of tiny droplets span from nanometers to micrometers in size which is so-called reverse micelle. as a result of space confinement in micelles, particle growth will be limited. controlling the parameters like water to surfactant ratio restricts the particle size and shape. surrounded space of micelle by oil phase, prevents the penetration of oxygen and impurities and therefore the oxidation possibility will be decreased. Reverse micelle method is not commercially viable for mass production, however it is important on account of narrow size distribution and proper size of as-synthesized particles so that it can be used for preparation of quantum dots [25,26]. Synthesis of WNPs by reverse micelle has received scant study. on the other hand, most of the studies focus on combined tungsten structure such as oxides and carbides. Xiong et al used reverse micro emulsion mediated method for synthesis metal tungsten nanopowder. They produced a gel by hydrolyzing tungstenisopropoxide as precursor in various micellar systems and after aging, particles were collected by precipitation and dried in cryogenic condition. Oxolation and Olation is the main reaction path and, therefore, the main product is metal oxide and reduction process with heat treatment will be required. [27]. In this study we have investigated the hydrolysis of metal salt and reduction of tungsten oxide in the micelle system without any purification.
2.Experimental 2.1 Material and experiment 1
Wolframite Nano particles
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ACCEPTED MANUSCRIPT All materials were purchased from Merck with analytical grade and used without further purification. Sodium tungstate dehydrate is the most abound tungsten precursor and as a result of low price [28], it has been selected as starting material. The phase of product determined by X-Ray powder diffraction (XRD; Philips pw1730 with CuKα(λ=1.54046⁰A). Size and size distribution analyzed by Zetasizer 3000 HAS analyzer. The powder morphology and composition of samples obtained by scanning electron microscope (FESEM; Model MIRA3 TESCAN) equipped with EDS apparatus.
2.2 preparation of tungsten nanoparticles by co-precipitation
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For this purpose, we followed the work of Gao et al precisely [24]. 199ml solution FeCl2.4H2O of 0.2M prepared in a beaker at room temperature and located in a water bath and stirred simultaneously. By adding specific amount of HCl to the solution, pH adjusted to number 2. afterwards 50 ml solution of NaWO4.2H2O of 0.2 M was added to the mother solution. The addition must be done in correct sequence, otherwise the product will be different. The Reaction conducted at 80⁰C, Molar ratio X= [ II+ 2Fe /WO4 ] =4 and stabled pH at 2. Within 90 minutes of stirring, dark precipitates were separated from the mother solution and washed by deionized water repeatedly to eliminate the iron and tungstate ions. Finally, the sediment was dried at T=60⁰C for 3 hours and sent for characterization by SEM&EDX with Nu0.
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2.3 Synthesis by Reverse Micelle
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The synthesis reaction in micelle system included precipitation and reduction. The most important factors in these reactions 2 were pH and Temperature. Two types of micelles were examined included SDS -normal hexane and Cyclohexane-TritoneX100. Normal butanol has been used as co-surfactant in both micelles.
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2.3.1 First Micelle (SDS-nhexane)
WO3+2NaCl+H2O
(1)
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Na2WO4+2HCl
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The micelle includes sodium dodecyl sulfate(SDS) as an anionic surfactant and Normal Hexane as a continuous oil phase. Preparation of micelle followed by dissolving 8.01gr SDS in 100ml of n-Hexane under vigorous stirring and ultrasonic for 20 minutes. then the n-butanol added dropwise to the solution until the solution became transparent and stable. finally, emulsion 2was divided into two parts. for reaction of tungsten precursor with FeCl2 at the optimum molar ratio(X=[Fe(II)/WO4 ] =4), 1gr NaWO4.2H2O and 2.41gr FeCl2.4H2O, dissolved in 5ml pre-degassed deionized water separately then added to the micelles. HCl 37% added to theFeCl2 solution to adjust the pH, then mixed with micelle in a three neck flask and sealed. the reaction between sodium tungstate and HCl is strong and form three oxide that it can be described as equation (1), therefor the precursor micelle should have higher pH before the reaction.
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Fig 1 illustrates the sketch of the reaction apparatus used for reverse micelle. Monitoring the amounts pH during the reaction was done by locating a pH meter probe into the solution. The total volume of water in micelles was 10 ml that is designed to attain the water-to-surfactant molar ratio(Kω) to 20. Titration of metal container micelle under the flow of Argon gas to the other micelle started after temperature approached 60⁰C. black sediments were separated from oil phase within 60 minutes of reaction time and soaked in methanol for 2hr to eliminate the sodium element, then washed repeatedly with distilled water. The solution of water and dark sediments centrifuged 2min at5000rpm speed and dried at 60⁰C in atmosphere. Reactions conducted at a constant temperature, stoichiometry and at 4 different pH values. The conditions of reactions are presented in table 1. 2-
Reaction with sodium borohydride at various stoichiometry (X=[NaBH4]/[WO4 ] =10,15,20,30) was investigated. Preparation of two micelles was done under the same conditions. owing to the vigorous reaction between sodium borohydride and water at low pH values which causes to form BO2 (Eq2), NaOH added to the 5ml pre-degassed deionized water then reducing solution was prepared. -
BH4 + 2H2O
-
BO2 + 4H2
(2)
Subsequently, solution mixed with the micelle in the reflux system. the reaction between sodium tungstate and sodium borohydride at PH values more than 1 causes to form sodium tungsten bronze as it is shown in Equations3&4[29], therefore the pH was reduced to 0.5 during the reduction. Reactions data with sodium borohydride at 60⁰C in this micelle are presented in table 2.
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Sodium dodecyl sulfate
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NaBO2+NaxWO3+(2-x) NaOH+(4-0.5x) H2 NaBO2+WO3-α+2NaOH+(4-α) H2
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NaBH4+Na2WO4+(3-α) H2O
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Figure. 1. a sketch of reaction media
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2.3.2 Second Micelle(Cyclohexane-TritonX100)
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In comparison with other refractory and transition metals such as molybdenum, tungsten metal has a complicated reduction process. This is because the reduction is depended on temperature also the pH is effective [30]. In addition, the double bond dissociation of tungsten with oxygen is an obstacle for reduction. Based on the last studies the best particle size and size distribution determined in co-precipitation at temperature 80⁰C [24]. moreover, the boiling point of n-hexane is 68⁰C, thus cyclohexane was used instead of normal hexane. The best surfactant for mixing with cyclohexane is triton x-100. Additionally, by this micelle combination the separation process of synthesized nanoparticles will be done at lower temperature. The experiments in this micelle aims to understand the stoichiometry of reaction and investigate the effect of co-surfactant, water to surfactant molar ratio and temperature on products. Table1-reactions of FeCl2 in first type micelle No 1 2 3 4
Hexane (ml) 100 100 100 100
SDS (gr) 8.01 8.01 8.01 8.01
Kw= [SDS/H2O] 20 20 20 20
Butanol (ml) 5 5 5 5
pH * 1 2 3 8
T (⁰C) 60 60 60 60
X= [ Fe(II)/WO42-] 4 4 4 4
Table2-Reaction of NaBH4 in first micelle No
Hexane (ml)
SDS (gr)
Kw= [SDS/H2O]
Butanol (ml)
4
pH
T (⁰C)
X= [NaBH4]/[WO42-] *
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0.5 0.5 0.5
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15 20 30
Table 3-Reaction of FeCl2 in second micelle Cyclohexane (ml)
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Triton X-100 (ml) 11.19
Kw= [T-X100]/[H2O]
pH
T (⁰C)
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Butanol (ml)
X= [ Fe(II)/WO42-]
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Isoamyl Alcohol (ml) ……
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80
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11
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22.39
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80
60
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Table4- Reaction of NaBH4 in second micelle
14 15 16
60 60 60
Triton X-100 (ml) 22.39 33.59 22.39
Kw= [TX-100]/ [H2O] * 15 10 15
3. Results and discussion
pH
T (⁰C)
Butanol (ml)
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60 60 60
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Cyclohexane (ml)
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the SEM micrograph of co-precipitation sample0(Fig2a) shows highly agglomerated particles with unsuitable mean size which is calculated 121.9nm however the morphology is nearly spherical. the SEM micrograph of as synthesized particles in the first micelle (sample 1) in Fig2b exhibits particles with more spherical morphology and ameliorated resolution. The mean size diameter for sample1 is measured 66.86nm. the differences in size result from volume confinement of micelles and web structure of capping agents such as surfactants. Methyl groups of SDS, bond with oxygen and form a web that entraps the nanoparticles and cause the coagulated structure. 3. effect of temperature
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3.effect of co-surfactant
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SEM of particles prepared in the Cyclo-TX100 micelle at 20⁰C and Kw=30 (sample8) in Fig2c indicates particles with Nano size coagulation and as a result of low resolution, the morphology cannot be determined. However, the isometry of particles does not follow a particular rule at this Kω. the SEM of particles produced in Cyclo-TX100 micelle(Fig2d) at 80⁰C and the same Kω(sample9) shows that particles have faceted morphology and they are agglomerated but have better resolution nonetheless. This could be resulting from temperature increment. The mean particles size calculated for S9 is 47.5nm. comparing Fig 2(c)&2(d), reveals that, particle size has decreased by increasing temperature. The micelle stability increases with elevating temperature and there is a decline in probability of micelle's rupture. the micellar crust became harder and prevented the water entrance. So, the smaller particles with narrow size distribution were synthesized at higher temperature and constant Kw. Based on EDS data presented in table 5 and by comparing sample 15 with 7, it was concluded that the reduction of tungsten by sodium borohydride is affected by temperature vigorously.
Fig 3a shows the SEM of sample 11 that is produced in the second micelle at 80⁰C and Kω=15. Aggregation and particle size have significantly decreased. The morphology is spherical and better than faceted shape of particles in Fig2d. Fig3b shows the SEM of sample 13 using Isoamyl Alcohol as co-surfactant instead of butanol. Any specific change was not observed in morphology but regarding to the results presented on table 6, the size distribution and particles size have decreased in 13. On comparison, the Isoamyl alcohol was more effective of the two co-surfactant and, as a result, it was selected for the next experiments. 3. effect of annealing Fig 3c shows SEM micrograph of sample 15. due to the low water to surfactant molar ratio(Kw=15), the particles are very small but have absolutely spherical morphology. The particle size spanned from 8 to 20nm and The least particle size obtained under this condition. Fig 3(d) indicates the sample 15 after annealing at 1000⁰C for 2hr under Argon flow. Most of the excess impurities like surfactants, oil phase and reducing agent were sublimated at this temperature. In spite of significant particle growth, their size did not exceed 100nm and therefore the annealing was not inappropriate factor. The particle diameter calculated 83. 37nm.
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(c)
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Figure. 2. Scanning electron microscope of (a)sample0 (b)sample 1 ( SDS-nHexane) (c) sample 8 (Cyclo-TX100 at T=20⁰ C) (d) sample 9 (Cyclo-TX100 at T=80⁰ C)
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Figure. 3. Scanning electron microscope of samples (a) 11 (b) 13 (c) 15(d) 19(15 after annealing at 1000⁰ C)
FeCl2 + H2
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Two kind of EDS patterns of as-prepared powder prepared by reverse micelle are presented in Fig4a&b. patterns of samples contain iron, Oxygen and tungsten peaks and some of them have sodium and boron. The iron peaks observed in the powder produced by FeCl2. Due to the reverse reaction of Fe(II) with HCl, Iron existence is nearly unavoidable.
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Furthermore, many of Fe(II) ions react with tungstate ion and form iron tungstate. The sodium peak in samples may referred to the incomplete washing process. Based on EDS data, the tungsten reduction is depended on pH values so that the highest reduction for FeCl2 occurred at pH=1 was %50.25. The optimum stoichiometry ratio for reaction between NaBH 4 and Na2WO4.2H2O (samples 5, 6 & 7) was X=30 and the reduction amount was%65.75. no precipitation observed in X=10. The relation between pH, stoichiometry and reduction illustrated in diagram Fig 5&6.
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The crystal structure of as-synthesized powder was determined by XRD. The XRD patterns of tungsten nanoparticles obtained under different conditions (stoichiometry, reducing agent, temperature). Most of the products was amorphous but some of them had very tiny peaks. A sample of amorphous peak is shown in Fig7a. Products were annealed at 600⁰C in a tube furnace under Argon flow for 2hr to obtain their crystal structure. The weak peak observed in Fig7b (sample 1), correspond to the reflection of plane (1 1 0) of α-tungsten with bcc structure. Other peaks in the pattern match with peaks of the Ferberite (Iron tungstate). tungsten percentage in this sample was %67. Respect to the TEM presented in the synthesis via co-precipitation method [24] and due to the low reducing potential of iron chloride,
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Figure. 4. EDS patterns of powder produced by (a)FeCl2 (b) NaBH4
File1.CVS here
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Figure. 5. Plot of Relation between pH and tungsten reduction
File2.CVS here
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Figure. 6. Plot of Relation between Stoichiometry and tungsten reduction 2-
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it is inferred that the most of the Fe(II) in FeCl2.4H2O react with WO4 and leads to form iron tungstate and tungsten iron core shell even in reverse micelle method. Pattern of sample 5 is shown in Fig 7c. Owing to the low stoichiometry ratio of NaBH4 to tungstate (X=15), a small peak of tungsten observed which is belonged to the (2 0 0) plane. WO3 strong peak shows that the reducing agent, reduced tungstate ion Table. 5. Data of EDX
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0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Reducing Agent FeCl2 FeCl2 FeCl2 FeCl2 FeCl2 NaBH4 NaBH4 NaBH4 FeCl2 FeCl2 FeCl2 FeCl2 FeCl2 FeCl2 NaBH4 NaBH4 NaBH4
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4 4 4 4 4 15 20 30 1 1 3 4 5 6 20 30 35
T (⁰C) 80 60 60 60 60 60 60 60 20 80 80 80 80 80 80 80 80
pH 2 1 2 3 8 0.5 0.5 0.5 1 1 1 1 1 1 0.5 0.5 0.5
%Wt. W 29.35 50.25 36 34.08 8.11 5.35 30.60 65.75 17.30 31.46 45.23 52.45 35.47 42.74 68.04 90.63 76.01
Table. 6. Data of size and errors No
0 1 9 11 13 15 19
Largest size (nm) 301 138 138 115 85 18.93 127
The least size (nm) 26.3 10 13.21 24 14.5 5.93 50
Size Distribution (nm) 274.7 128 124.79 91 70.5 13 77
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Arithmetic Mean 𝑥̅ (𝑛𝑚) 121.91 66.8637 47.5779 66.016 51.4615 12.8927 83.3714
Standard Deviation 𝜎(nm) 63.8192 29.9143 28.1149 23.9847 18.0856 3.5799 18.3493
Error 𝜎𝑚 6.3819 2.9914 2.8114 2.3985 1.8086 0.3579 1.8349
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to the lower valence. Another peak belongs to the hydrocarbon compounds which means that the washing process had a failure. The same peak of WO3 was observed in Fig 7d that shows the pattern of sample 14 with X=20. But the α-tungsten with (1 1 0), (2 0 0) and (2 2 0) planes was observed too. The pattern of the sample 15 is shown in Fig 7e. the peak of WO3 eliminated in X=30 and α-W was observed. The highest tungsten percentage obtained in this sample(%Wt=90.63). by comparing patterns of Fig7c&d&e it is clear that in higher stoichiometry portion, the purity of product increased. 3 samples were annealed under argon flow at 1000⁰ C and 2hours. Fig8a shows the sample17(5 after annealing). By comparing patterns before and after annealing for sample 5, it was found that the peak of (2 0 0) is eliminated and most of the peaks belong to the tungsten trioxide. Due to the pH=1, the reaction (3) & (4) might be the reason of formation of WO3, sodium metaborate and sodium tungsten bronze. Fig8b shows x-ray pattern of sample 18 (14 after annealing). The peaks of sodium tungsten oxide(Na2W2O7), tungsten dioxide(WO2) and Disodium octaborate(Na2B8O13) were observed. The highest peak belongs to the tungsten trioxide. the x-ray pattern of sample 15 after annealing (19) is shown in Fig8c. both α and β structure of tungsten were observed. Also the peaks of sodium tungsten oxide and disodium octaborate were detected. Due to the result of SEM, EDS and XRD the best condition to produce metallic tungsten, belongs to sample15.
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Figure. 7. XRD patterns of amorphous sample(a) and four heat treated sample at 600⁰ C under argon flow include samples (b)1 (c) 5 (d) 14 (e) 15
Figure. 8. XRD patterns of three samples (a) sample 5 (b)sample 14 (c) sample15 annealed at 1000⁰ C under argon flow.
4. conclusion reverse micelle method has been developed for synthesis of tungsten nanoparticles from sodium tungstate, FeCl2.4H2O and NaBH4 as starting materials. effects of parameters such as temperature, pH, micelle type, stoichiometry, type of reducing agent
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and annealing on morphology, reduction and structure of product have been investigated. SEM analysis showed spherical particles with mean diameter 13nm and narrower size distribution in Cyclo-TX100 than the SDS-nhexane micelle. comparing SEM micrograph of second micelle (Cyclohexane-TritonX-100) at two temperature showed that increasing Temperature affects the micelle shape extremely. Regarding to the EDX data, temperature increment ameliorates the reduction of tungsten. The best pH for reducing by NaBH4 attained 0.5 and the best molar ratio achieved at [NaBH4/Na2WO4] =30. both α&β structures of tungsten were observed in x-ray diffraction patterns. XRD pattern of powder produced by FeCl2, showed that, the majority of the products are iron tungstate in the form of core and shell. consequently, sodium borohydride acts much better. EDX patterns showed pure tungsten peak in sample 15 by %90.63 purity. In comparison with the work of Xiong et al [27], as synthesized particles collection is more simple, however the mean size of particles was larger. Additionally, no more purification and reduction process is needed. Based on our knowledge it was the first time that tungsten nanoparticles were synthesized directly from sodium tungstate via this method.
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ACCEPTED MANUSCRIPT 5. Acknowledgment
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This research is conducted at nanotechnology laboratory of department of physic. The authors like to thank the department physic of Razi university of Kermanshah for facilities and instrumental supports.
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Highlights A facile method under low pressure and temperature for synthesis of tungsten nanoparticles Relatively cheap precursor Tiny and monodisperse particles with controllable morphology Stable and protected nanoparticles
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