Faculty of Material and Manufacturing Technologies, Malek Ashtar University of Technology, Tehran, IranDepartment of Chemistry, Imam Hossein University, Tehran, IranIslamic Azad University, Varamin Pishva Branch, Varamin, Iran
r t i c l e i n f o
rticle history:eceived 30 April 2012eceived in revised form5 September 2012ccepted 27 September 2012vailable online 8 October 2012
eywords:anoparticle
a b s t r a c t
Taguchi robust design was applied to optimize experimental parameters for controllable, simple andfast synthesis of nickel tungstate nanoparticles. NiWO4 nanoparticles were synthesized by precipitationreaction involving addition of nickel ion solution to the tungstate aqueous reagent and then formation ofnickel tungstate nucleolus which are insoluble in aqueous media. Effects of various parameters such asnickel and tungstate concentrations, flow rate of reagent addition and reactor temperature on diameterof synthesized nickel tungstate nanoparticles were investigated experimentally by the aid of orthogonalarray design. The results for analysis of variance (ANOVA) showed that particle size of nickel tungstate canbe effectively tuned by controlling significant variables involving nickel and tungstate concentrations and
flow rate; while, temperature of the reactor has a no considerable effect on the size of NiWO4 particles.The ANOVA results proposed the optimum conditions for synthesis of nickel tungstate nanoparticles viathis technique. Also, under optimum condition nanoparticles of NiWO4 were prepared and their structureand chemical composition were characterized by means of EDAX, XRD, SEM, FT-IR spectroscopy, UV–visspectroscopy, and photoluminescence. Finally, catalytic activity of the nanoparticles in a cycloaddition
reaction was examined.
. Introduction
During last decades, preparation and characterization of inor-anic structures with nano-sized dimensions and morphologicalpecificity has obtained a great importance and interest in vari-us fields of science and nanotechnology. Since, nano-structuredaterials usually have low density and high specific surface fea-
ures; they are appropriate candidates for specific usages. Today,ano-structured materials play important roles in medicine, biol-gy, electronics, chemical technology and many other fields [1–3].
Tungstate compounds correspond to the relatively large biva-ent cations (MWO4 with ionic radius larger than 0.99 A, similaro the some tungstate salts of Ca, Ba, Pb, and Sr) exist in thecheelite structure form, while in their structure the tungsten atom
dopts tetrahedral coordination. However, tungstate salts of biva-ent cations with smaller radius (MWO4 with ionic radius less than.77 A; such as tangestate salts of Fe, Mn, Ni, and Mg) possesses
wolframite structure, where in their structure the tungsten atomadopts an overall six-fold coordination [4,5].
Nickel tungstate or as an important inorganic salt belongs tometal tungstate family. NiWO4 has an appropriate potential forapplication in various fields of industries such as catalysts [6] andhumidity sensors [7] due to its attractive catalytic activity and goodsensitivity to humidity. Also, NiWO4 could be used extensively inother fields such as microwave devices [5], photoanodes [8], scintil-lator materials [9], microwave applications [10] and optical fibers[11]. Meanwhile, metal tungstate compounds are used as photocat-alyst for removal of various organic pollutants especially organicdyes from water samples [12–14]. On the other hand, in orderto obtain NiWO4 material with desired properties, the key stepis how to synthesize NiWO4 powders with interested morphol-ogy, high chemical purity and good phase composition. Recently,NiWO4 nanoparticles have obtained much attention because oftheir large surface area and remarkable quantum size effect, whichcause lower sinter temperature and better photocatalytic activity[8–10].
Due to the importance and various applications of nickeltungstate, until today different processes have been proposed tosynthesis nano-sized NiWO4 crystals with different morpholo-gies. These proposed methods, including co-precipitation [15],
Table 1OA9 (34) matrix for parameter optimization in synthesis of NiWO4 nanoparticles via direct precipitation reaction and mean diameter of produced nickel tungstate as response.
olymeric precursor method [16], modified citrate complex tech-ique [17], hydrothermal method [18], molten salt method [19] andpray pyrolysis [5], have own benefits and their related limitations.hus, more investigations are interested for finding other methodsn order to facile, easy to handle and more economical synthesisf NiWO4 nanoparticles; while the method should has an appro-riate potential for scaling-up and large scale production of theroduct. Therefore, the main purpose of the present research was toptimize direct precipitation procedure in order to produce NiWO4anoparticles, composition and morphological characterization ofhe product and also investigation on the catalytic activity of syn-hesized nanoparticles in a cyclo-addition reaction. To the best ofur knowledge, various data are available on synthesis of NiWO4icro and nanoparticles by various methods [5,15–19]. However,
here is no report on the synthesis NiWO4 nanoparticles via directrecipitation without any surfactant, templates or catalyst.
. Experimental
.1. Materials and procedures
Analytical-grade nickel chloride and sodium tungstate weresed as received from Merck Company. NiWO4 particles were pre-ared via direct precipitation reaction in aqueous media by additionf Ni2+ solution, at various concentrations and under different flowates, directly to the tungstate solution under vigorous stirring andarious reactor temperatures. When the mixing process was com-leted, the formed NiWO4 suspension was filtered and washed withe-ionized distilled water three times. The filtered precipitate washen washed with ethanol and dried in oven at 90 ◦C for 2 h. Inrder to form crystalline NiWO4 particles, the prepared samplesere annealed at 600 ◦C.
The effect of various variables on the particle size of productas investigated and the experimental parameters of precipitation
eaction during synthesis of NiWO4 particles were optimized usingn experimental design approach. The variables (Ni ion concen-ration, tungstate concentration, flow rate for addition of nickel ioneagent to the tungstate solution, and temperature of reactor) weretudied under three different levels by an OA9 as shown in Table 1.
.2. Characterization of prepared nickel tungstate particles
The synthesized NiWO4 particles under different experimentalonditions after annealing at 600 ◦C were characterized by scan-ing electron microscopic (SEM) and energy-dispersive analysis by-rays (EDAX). SEM images were recorded on a Philips XL30 series
nstrument using a gold film for loading the dried particles on thenstrument. Gold films were prepared via a sputter coater model
CD005 manufactured by BAL-TEC (Switzerland). X-ray powderiffraction (XRD) analysis was carried out using a Rigaku D/max500 V diffractometer equipped with a graphite monochroma-or and a Cu target. The IR spectra were recorded by an FT-IR
40.0 30 1322.5 60 119
10.0 0 99
spectrophotometer (Perkin Elmer Spectrum 100) using KBr pellettechnique. PL spectrum was recorded by photoluminescence (PL)spectrometer (Spectro Fluorescence JASCO fp-6200) using 290 nmas excitation wavelength at room temperature.
2.3. Catalytic activity of NiWO4 nanoparticles
Catalytic activity of NiWO4 nanoparticles in cycloaddition reac-tion of nitriles with sodium azide for the synthesis of 5-substituted1H-tetrazoles was investigated. In order to study the effect of cata-lyst particle size on the reaction parameters, synthesis reaction wascarried out in identical conditions using NiWO4 with two differentaverage particle sizes (sub-micron particles with average diame-ter of 208 nm and nano-sized particles with average diameter of84 nm). In this study, 0.2 mmol (0.0578 g) of nickel tungstate pow-der was added to the solution of 2-cholorobenzonitrile (2 mmol)and sodium azide (4 mmol) in DMF (4 mL) and the resulting mixturewas stirred continuously for 9 h at 100 ◦C [20]. After the completionof the reaction (while the reaction was followed by TLC), the cat-alyst was separated by centrifugation, washed with 4 mL water,and the centrifuged was treated with 6 M HCl (10 mL). After stir-ring of the acidic mixture for 10 min, the precipitated solid wasfiltered and washed with 5 mL of distilled water and then driedat 50 ◦C to provide 5-(2-chorophenyl)tetrazole. The formation ofproduct was followed by 13C NMR and appearance of the pickaround ı = 154 ppm confirmed the formation of 5-substituted 1H-tetrazoles [20].
3. Results and discussion
3.1. Nanoparticles synthesis and optimization of procedure
In this study, Taguchi robust design was used for the optimiza-tion of variables which are potentially affective in the synthesisprocedure. By the aid of fractional factorial experiments, such asTaguchi robust design, the number of experiments required to theoptimization of variables could be strongly reduced [21–24]. InTaguchi robust design, orthogonal arrays are used to assign factorsto a series of experiment combinations whose results can then beanalyzed via a common mathematical procedure; while, the maineffects of the variables and preselected interactions are indepen-dently extracted. In this systematic design, different effects canbe separated; because the experiments are arranged orthogonal[25–27].
Direct reaction of anion and cation correspond to an insolubleinorganic salt such as NiWO4 and hence formation and precipi-tation of the salt particles is a simple and common technique forsynthesis of various water insoluble inorganic powders; while the
morphology of the produced power is strongly dependent on thereaction conditions [28–30]. The control of precipitated particlemorphology during precipitation reaction is complex processes andneeds complete interpretation the multi-interactions between the
Fig. 1. SEM images of NiWO4 particles obtained under different runs of Ta
eagents and operation parameters. However, experimental inves-igation on the effect of various parameters on the particle size viatatistical optimization of the variables may be a useful and facileechnique. Thus, in this investigation, effect of various parametersuch as concentration of Ni2+ and WO4
2− in aqueous solutions, flowate for addition of Ni2+ reagent to WO4
2− solution, and tempera-ure of reactor on the particle size of produced nickel tungstateas studied. Table 1 shows the studied factors, their levels and the
verage particle size of synthesized nickel tungstate in each exper-ment. Also, Fig. 1 shows SEM images for some NiWO4 samplesroduced under various experimental conditions of this table. Ashown in this figure and considering the average particle sizes ofynthesized samples in Table 1, variation in the reaction conditionsunder various runs) changes the size of produced NiWO4 particles.n the other hand, average particle size of nickel tungstate corre-
pond to the effect of each studied parameter under any level wasomputed [31,32] and the results for all investigated parametersre presented in Fig. 2. The graphs presented in this figure deter-ine how the size of produced NiWO4 particles will vary when the
evel of the parameters varies. Fig. 2a shows the effects of Ni2+ andO4
2− solution concentrations on the size of precipitated NiWO4articles at three different levels (0.02, 0.1, and 0.5 mol/L). As coulde seen, 0.5 M is the optimum concentration for the each reagent
n order to synthesis of NiWO4 particles with smallest size. Also,ig. 2b shows effect of various flow rates (2.5, 10, and 40 mL min−1)or addition of nickel ion reagent to the reactor containing tungstateon solution on size of produced NiWO4 particles. As shown in thisgure, 40 mL min−1 is optimum flow rate for synthesis of NiWO4anoparticles. On the other hand, this figure demonstrates that0 ◦C temperature is useful for production fine particles of NiWO4.
In the next step of this study, analysis of variance (ANOVA)as carried out on the average particle sizes of product particles
btained under conditions mentioned in Table 1. The results ofNOVA for the used array in synthesis of NiWO4 are shown inable 2. As could be seen in this table, at 90% confidence inter-al, except temperature all of other studied parameters includingickel and tungstate ion concentrations and flow rate of nickel ioneagent have significant effects in tuning the size of synthesized
iWO4 particles. In this work, the interactions between the param-ters were neglected. By considering graphs presented in Fig. 2nd the results of ANOVA shown in Table 2, the optimum condi-ions for synthesis NiWO4 nanoparticles via direct precipitation
y direct precipitation method: (a) run 1, (b) run 2, (c) run 4, and (d) run 7.
reaction could be pointed out as: 0.5 mol/L as concentration ofnickel ion and tungstate reagent solutions and 40 mL min−1 as flowrate for addition of nickel reagent to the tungstate ion solution.In systematic experiment design proposed by Taguchi [25,26], theresult under the optimum condition (in our study, direct precip-itation of NiWO4 nanoparticles with smallest diameter) could beestimated via following equation:
Flow rate (mL/min) or Temprature (ºC)
Fig. 2. Average effects of each variable at various levels on the diameter of theprecipitated NiWO4 particles.
Table 2ANOVA results for synthesis NiWO4 particles via precipitation procedure using OA9 (34) matrix while diameters of synthesized NiWO4 nanoparticles (nm) are as responses.
appeared at 876 and 820 cm arise from vibration of the WO2entity present in the W2O8 groups. The absorption bands at 705and 637 cm−1 are typical of a two-oxygen bridge (W2O2) and cor-respond to the asymmetric stretching of the same units. Also, the
Error E – –
a The critical value was at 90% confidence level; pooled error results from poolin
In this equation T/N is average diameter of NiWO4 particlesbtained by experiments performed according to Table 1; while,is the grand total of particle size for all experiments, N is totalumber of performed experiments, Yopt is particle size of productnder optimum condition, Cx, Cy, and Fz are optimum concentra-ion for reagents (Ni2+, WO4
2−) and flow rate of reagent additiono the reactor, respectively. Meanwhile, in this systematic design,he confidence interval (C.I.) for the result (particle size of NiWO4)nder optimum condition could be determined as follow [25,26]:
.I. = ±√
F˛(f1, f2)Ve
ne
In this equation, Ve corresponds to the variance of error term,˛(f1,f2) is computed value of F at degree of freedom (DOF) of f1nd f2 and desired confidence level (in our study, ˛ = 90%). f1 = DOFf mean (which always equals 1), f2 = DOF of the error term, ne ishe effective number of replications, which could be obtained byollowing equation:
e = Number of trialsDOF of mean (always 1) + DOF of all factors used in estimatio
Based on the calculations, the estimated diameter of NiWO4 par-icles under optimum conditions is 84 ± 15 nm. In the next part ofhis investigation, the proposed optimum conditions by ANOVAere applied for the synthesis of NiWO4 nanoparticles. Fig. 3resents the SEM image of the synthesized nickel tungstate sam-le under these optimum conditions. The prepared particles underptimum conditions have 88 nm average diameters; which are ingreement with the predicted range by ANOVA data for diameterf NiWO4 nanoparticles under the optimum condition of synthesis.he NiWO4 nanoparticles prepared under these conditions were
sed for composition and structure characterization studies suchs EDAX, XRD, FT-IR, photoluminescence and investigation on theiratalytic activity.
ig. 3. SEM images for synthesized NiWO4 nanoparticles under optimum condition.
– 2 69 – 3.0
nificant effect.
3.2. Characterization of nickel tungstate nanoparticles
NiWO4 nanoparticles precipitated under optimum conditionwere investigated by EDAX and X-ray powder diffraction (XRD)analysis for evaluating the composition and purity. Fig. 4 shows typ-ical EDAX pattern for synthesized NiWO4 nanoparticles. The peaksof the EDAX pattern confirm that the product is highly pure, andthe average atomic percentage ratio of Ni and W is about 41.9:58.1.This elemental analysis confirms the presence of corresponding ele-ments in their stochiometric percentage. Also, Fig. 5 shows the XRDpattern for precipitated nickel tungstate nanoparticles under opti-mum conditions. Relatively strong intensive and sharp diffractionpeaks for the produced nickel tungstate were observed. The diffrac-tion peaks indexed in the XRD pattern of sample were in agreementof monoclinic nickel tungstate with the wolframite structure fromJCPDS: 15-0755.
FT-IR spectroscopy was also used to investigate formation ofNiWO4 by precipitation reaction. The FT-IR spectra of the syn-thesized NiWO4 before and after annealing at 600 ◦C are shownin Fig. 6. As could be seen in Fig. 6a, the sample before anneal-ing shows three wide absorption peaks at about 839, 1627 and3300 cm−1. This spectrum confirms the presence of moisture inthe sample. However, six absorption bands (471, 536, 637, 705,820, and 876 cm−1) were observed in the spectrum of the annealedwolframite sample in Fig. 6b. It has been reported that the mainabsorption bands of wolframite type structure AWO4 (A = Fe, Ni,Mn, Cd, Mg, Zn) appeared in the range of 450–1000 cm−1 [33].In comparison with previous investigation [34], these peaks areattributed to the vibrational bands of the NiWO4. The bands
−1
Fig. 4. EDAX spectrum for synthesized NiWO4 nanoparticles via direct precipitationmethod under optimum condition.
red by precipitation method under optimum condition.
ovtt
apUdtstntriont
t2tasTwWPmsegPepig
uT(dpwt
Table 3Effect of NiWO4 catalyst particle size on cyclo-addition reaction time.
Particle size of NiWO4
catalyst (nm)Reactiontime (h)
Reactionyield (%)
84 9 ∼80
Fig. 5. XRD pattern of NiWO4 nanoparticles prepa
bserved absorptions in the range below 500 cm−1 are due to theibrations of the NiO6 polyhedra. Therefore, the observed peaks inhe spectrum for the annealed sample (Fig. 6b) confirm the forma-ion of crystalline NiWO4 product.
UV–vis spectrophotometry was used to characterize thebsorption properties of the obtained nano-particles sample byrecipitation under the optimum conditions. Fig. 7 shows theV–vis absorption spectrum of the nickel tungstate nanoparticlesispersed in distilled water. This spectrum shows that main absorp-ion for the sample occurred in the region of 250–350 nm. Thispectrum confirmed a small crystal size, which could be attributedo the strong quantum confinement of the excitonic transition forano-structures [19]. However, the absorption of the dispersed par-icles continued until higher wavelengths and our result shows aed-shift relative to the previously reported data [35]. This shifts due to the difference in particle size of our sample and previ-us study [36,19]. Meanwhile, UV–vis result reveals that NiWO4ano-particles have good light absorption properties not only inhe ultraviolet region but also in the visible light wavelengths [19].
Fig. 8 shows photoluminescence (PL) spectrum for the syn-hesized NiWO4 nanoparticles under optimum conditions. Using90 nm as excitation wavelength, the PL spectra shows an elec-ronic transition within wolframite anion molecular complexssociated with the intrinsic emission. The emissions for synthe-ized nano-particles were at wavelength range of 300–500 nm.he observed pattern was similar to other wolframite compounds;hich in their PL spectrum, the emission bands arise from theO6
6− complex along with some defects in the crystal structure.revious studies [37,38] proposed that blue and green emissionsay be due to the intrinsic WO6
6− complex with a double emis-ion from one and the same center (3T1u-1A1g); while, the yellowmission is come from recombination of e–h pairs localized at oxy-en atom deficient tungstate ions. Meanwhile, this pattern for theL spectrum of the sample [39] may be concern to the self-trappedxcision in tungstenite crystals with strong electron–phonon cou-ling for the blue band, and the transitions of T2u-T2g and T1g-T2g
n the WO66− complex could be responsible for the observation of
reen and yellow bands.TG/DSC graphs for nickel tungstate nanoparticles prepared
nder optimum conditions (before annealing) are shown in Fig. 9.G curve for the nickel tungstate sample reveals a mass loss�m1 = 11%) in temperature range of 100–180 ◦C due to the dehy-
ration of surface-adsorbed moisture and gases. The dehydrationrocess is continued above 180 ◦C until about 350 ◦C with a smalleight loss (�m2 = 4%), due to the releasing the trapped water in
he crystalline structure of sample. Above 350 ◦C, nickel tungstate
208 15 ∼80Without catalyst – –
nanoparticles were thermally stable and no mass loss was hap-pened in temperature range of 350–700 ◦C. DSC curve of the nickeltungstate sample shows a small and wide endothermic peak intemperature range of 100–150 ◦C corresponds to the dehydra-tion of the sample. Meanwhile, DSC curve of nickel tungstatenanoparticles shows a sharp exothermic peak without any massloss of the sample at 528 ◦C. This peak is corresponds to thephase transition of the compound from amorphous phase tomonoclinic structure [4]. These characterization studies on oursynthesized NiWO4 nanoparticles via direct precipitation, underoptimum conditions, indicate that characteristics and propertiesof this nanoparticle are similar to the nanoparticles obtained viaother methods [4,8,19]; while, our proposed method is simple,cost effective and no needs to usage any template, surfactant, orcatalyst.
3.3. Catalytic activity of NiWO4 nanoparticles
Tetrazoles are a class of heterocyclic compounds which have awide range of pharmaceutical and industrial applications [20,40].The most common route to synthesis 5-substituted 1H-tetrazoles isaddition of azide ion to the nitriles. Meanwhile, tungstate salts areefficient heterogeneous catalysts in the synthesis of 5-substituted1H-tetrazoles [20,41]. Therefore, in this study, catalytic activity ofNiWO4 nanoparticles in cyclo-addition reaction of nitriles withsodium azide in production of 5-substituted 1H-tetrazoles wasinvestigated. The results show that nickel tungstate nanoparticlescould catalyze the [2 + 3] cyclo-addition reaction of nitriles withsodium azide to form 5-substited 1H tetrazoles in DMF media. Onthe other hand, our finding showed that the catalyst particle sizehas main effect on the reaction rate.
Fig. 6. FT-IR spectrum for NiWO4 nanoparticles prepared via precipitation method under optimum condition of synthesis (a) before and (b) after annealing in 600 ◦C.
ucnmen
As could be seen in Table 3, reaction was completed after 9 h bysing nickel tungstate nanoparticles prepared under the optimumonditions (with average diameters of 84 nm); while, reaction was
ot completed before 15 h in the presence of nickel tungstate sub-icron particles prepared under conditions of run 1 (with diam-
ters of 208 nm). Also, in the absence of catalyst the reaction wasot completed even after 48 h. Our finding showed that presence
of the nickel tungstate catalyst is essential for the formation ofthe product. On the other hand, particle size of catalyst has animportant effect on its catalytic activity and hence on the rate of
synthesis reaction. The yield of reaction (about 80%) was identical inboth experiments in the presence of catalyst; however, nanosizednickel tungstate catalyst considerably decreases the required reac-tion time rather than sub-micron sized nickel tungstate particles.
S.M. Pourmortazavi et al. / Applied Surf
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
200 300 400 500 600 700
Wavelength (nm)
Inte
nsit
y
Fig. 7. UV–vis spectrum for dispersed NiWO4 nanoparticles in distilled water.
Fig. 8. PL spectrum (excitation wavelength of 290 nm) for NiWO4 nanoparticlesprepared under optimum condition of synthesis.
Fp
4
cpaobNnaot
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
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ig. 9. TG/DSC curves obtained for thermal behavior of NiWO4 nanoparticles; Sam-le Mass 3.0 mg; Heating rate 10 ◦C min−1; Nitrogen atmosphere.
. Conclusion
Nanoparticles of NiWO4 with wolframite structure were suc-essfully prepared via fast, simple and cost effective directrecipitation synthesis under optimum condition without usingny surfactant, template, or catalyst. The composition and structuref the produced nickel tungstate nanoparticles were characterizedy SEM, EDAX, XRD and FT-IR and the results confirm formation ofiWO4 nanoparticles. Also, photoluminescence emission of these
anoparticles shows the intrinsic peaks at 409–420 nm. Finally, cat-lytic activity of NiWO4 nanoparticles in cyclo-addition reactionf nitriles with sodium azide in production of 5-substituted 1H-etrazoles was investigated and our finding showed the potential
[
ace Science 263 (2012) 745–752 751
of these nanoparticles for efficient reduction of reaction time incomparison with sub-micron particles.
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