Precipitation synthesis and characterization of cobalt molybdates nanostructures

Superlattices and Microstructures 58 (2013) 120–129

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Superlattices and Microstructures

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . co m / l o c a t e / s u p e r l a t t i c e s

Precipitation synthesis and characterization ofcobalt molybdates nanostructures

0749-6036/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.spmi.2013.01.014

⇑ Corresponding author at: Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box51167, Islamic Republic of Iran. Tel.: +98 361 5912383; fax: +98 361 5552935.

E-mail address: [email protected] (M. Salavati-Niasari).

Ghazal Kianpour a, Masoud Salavati-Niasari a,b,⇑, Hamid Emadi c

a Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republic of Iranb Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, P.O. Box 87317-51167, Islamic Republicof Iranc School of Chemistry, University College of Science, University of Tehran, Tehran, Islamic Republic of Iran

a r t i c l e i n f o

Article history:Received 21 November 2012Received in revised form 13 January 2013Accepted 30 January 2013Available online 16 March 2013

Keywords:NanorodsCoMoO4

PrecipitationComplex precursorElectron microscopy

a b s t r a c t

CoMoO4 nanorods have been successfully synthesized by precipita-tion method using Co(C7H5O2)2�4H2O and (NH4)6Mo7O24�4H2O asstarting materials. The effect of some parameters including reactiontime, temperature, concentration, and surfactant were investigatedto reach optimum condition. The as-synthesized nanostructureswere characterized by X-ray diffraction (XRD), scanning electronmicroscopy (SEM), transmittance electron microscopy (TEM),photoluminescence (PL) spectroscopy, Fourier transform infrared(FT-IR) spectra, and energy dispersive X-ray microanalysis (EDX).Facile preparation and separation are important features of thisroute. This work has provided a general, simple, and effectivemethod to control the composition and morphology of CoMoO4 inaqueous solution, which revealed potential new insight intoinorganic synthesis methodology.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Metal molybdates, as two important families of inorganic materials, have a potential application invarious fields such as magnetic, catalysis [1], photoluminescence [2], and humidity sensors [3].CoMoO4 is well known as an active and selective catalyst in the oxidation of hydrocarbons. It isaccepted that these reactions proceed via the redox mechanism. First, the hydrocarbon molecule is

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oxidized by an oxygen ion from the catalyst lattice and then it is reoxidized by oxygen from the gasphase [4].

CoMoO4 may exist in several phases: the low temperature a-phase, the high temperature b-phase,the high pressure hp-phase, and the hydrate [5]. Generally, CoMoO4 can be prepared by two ways [6]:(i) the reaction between molybdenum trioxide [7] and (ii) by soft chemical, e.g. precipitation fromaqueous solutions of soluble salts of Mo and Co [8,9]. Most of the previous reported approaches to ob-tain molybdates need high temperature and harsh reaction condition, such as a solid state reaction at1000 �C [10] or the sol–gel method [11]. It has already been demonstrated that the soft chemistryroutes can be as an appropriate method for controllable synthesis of this group of materials, suchas two dimensional CdWO4 nanocrystals [12].

In current investigation, a simple and facile precipitation method to prepare the CoMoO4 nano-structures at mild temperature under atmospheric pressure by employing Co(C7H5O2)2 as a new pre-cursor was developed. The presented approach is not restricted to complicated instruments, difficultprocesses, extra additives, and harsh conditions such as high temperature and pressure. The effect ofdifferent synthetic conditions such as reaction time and concentration of precursors, different surfac-tants, and different temperatures on the morphology and size of the final products were investigated.

2. Experimental

2.1. Materials and physical measurements

All the chemical reagents used in the experiment were of analytical grade and used as receivedwithout further purification. X-ray diffraction (XRD) patterns were recorded by a Philips-X’pertpro,X-ray diffractometer using Ni-filtered Cu Ka radiation. Scanning electron microscopy (SEM) imageswere obtained on LEO-1455VP equipped with an energy dispersive X-ray spectroscopy. TEM imagewas obtained on a Philips EM208 transmission electron microscope with an accelerating voltage of200 kV. Fourier transform infrared (FT-IR) spectra were recorded on Nicolet Magna-550 spectrometerin KBr pellets. Room temperature PL was studied on a Perkin Elmer (LS 55) fluorescence spectropho-tometer that with an excitation slit width of 5 nm.

2.2. Preparation of CoMoO4 nanostructures

In a typical procedure, 0.048 mmol of (NH4)6Mo7O24�4H2O was dissolved in 30 ml of distilled waterand then was added dropwise to 30 ml solution containing 0.3 mmol of Co(C6H5O2)2 under magneticstirring. This solution was stirred till a homogeneous solution obtained. The final solution was heatedat 70 �C for 30 min under stirring. During these processes no sediment was observed. Then the tem-perature was increased to 90 �C and kept constant at 90 �C for 1 h which led to formation of a violetprecipitate. Finally, the precipitates were separated and dried under vacuum at 60 �C for further char-acterization. To reach optimum condition for synthesis, the same process was repeated which eachdifferent condition is presented in Table 1 with more details. To investigate the surfactant role, appro-priate amount of the surfactant was added after mixing precursors.

3. Results and discussion

The crystal structure and purity of the products were characterized by powder X-ray diffractionpattern. Fig. 1 shows the XRD pattern of CoMoO4 prepared at 90 �C for 1 h. The diffraction peaksare in good agreement with the literature data (JCPDS No. 15-0439), and with those reported previ-ously in ref [13]. Besides, several weak diffraction peaks were attributed to CoMoO6�0.9H2O. The smallcrystallite size and poor crystallization are evidenced by the broad and weak diffraction peaks.

Chemical composition of the as prepared products was confirmed by EDX. As can be observedin Fig. 2, Co and Mo signals are detected in the collected peaks of the products. No impurities weredetected in the EDX survey.

Table 1Reaction conditions for preparation of NiMoO4 nanostructures.

SampleNo.

Temperature(�C)

Time(min)

pH Amount of precursor(mmol)

Surfactant SEM

1 50 60 5.5 0.3 –

2 70 60 5.5 0.3 –

3 90 60 5.5 0.3 –

4 100 60 5.5 0.3 –

5 90 120 5.5 0.3 –

6 90 240 5.5 0.3 –

7 90 60 6.5 0.3 –

8 90 60 5.5 0.15 –

9 90 60 5.5 0.6 –

10 90 60 5.5 0.3 PEG-600

11 90 60 5.5 0.3 SDS

12 90 60 5.5 0.3 CTAB

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Fig. 1. XRD pattern of as prepared samples.

Fig. 2. EDX pattern of the as-prepared samples.

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Fig. 3a and b display the FT-IR spectra of the samples No. 3 (with no surfactant) and No. 10 (withpoly ethylene glycol (PEG) (surfactant), respectively. It is obvious that there are similarities betweentwo spectra and on the other hand no vibration peak related to PEG chemical bonds is observed inFig. 3b. Thus, it can be concluded that no or small amount of surfactant was absorbed on the surfaceof samples. The bands at around 2920 and 2850 cm�1 which is observed in both spectra are assignedto the antisymmetric and symmetric C–H stretching vibrations of hydrocarbon moiety remained fromprecursor on the surface of nanostructures. Two peaks at 820 and 870 cm�1 are assigned to stretchingvibrations of the Mo–O–Mo. The vibration band at 963 cm�1 is attributed to activation of the m1 vibra-tion of the distorted MoO4 tetrahedral presented in CoMoO4. The absorption peak at 743 cm�1 is as-signed to m3 vibration of the same group. The low frequency peak at 444 cm�1 is superposition of m4

and m5 of MoO and m3 of CoO6 building groups of CoMoO4 [14–19]. These spectra are in good agree-ment with those reported previously in Ref. [20], and Fig. 3c is related to Co(C7H6O2)2 as the precursor.The observed peaks between 1000 and 1400 cm�1 in Fig. 3c are attributable to the symmetric andasymmetric stretching of the ACH2 groups, terminal ACH3 and @CH of the precursor.

Optical properties of CoMoO4 nanostructures (sample No. 3) were investigated by using photolu-minescence (PL) spectroscopy. The PL spectroscopy was done by ultrasonically dispersing of CoMoO4

nanostructures in absolute ethanol. The nature of the optical transitions of molybdates is still unclear,but by analogy with the tungstate crystals, the bands can be interpreted as the radiative recombina-tion of the electron–hole pairs localized at the [MoO2�

4 ] group [21] on the basis of previous reflectivity

Fig. 3. FTIR spectra of CoMoO4 samples: (a) CoMoO4 without further purification and (b) CoMoO4 obtained by poly ethyleneglycol as the surfactant, Co(C7H6O2)2 as the precursor.

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measurements [22] and the current knowledge of their electronic structure [23,24]. These compoundshave similar crystal structure and the tetrahedral [MoO2�

4 ] oxyanion complex is argued to be the prin-cipal constitutive element, which defines the optical properties in the ultraviolet energy region. Thetop of the valence band of the lowest unoccupied states are composed of the 4d Mo states split intwo sets of bands with e (primarily Mo 4d) and t (primarily O 2pp) symmetry (1) (Scheme 1) [25].Fig. 4 shows a typical photoluminescence (PL) spectrum of CoMoO4 nanostructures. A sharp peak at417 nm was observed and band gap of sample was calculated about 2.97 eV.

Fig. 4. The room-temperature photoluminescence spectrum of the nanorods CoMoO4.

Fig. 5. Typical SEM images of CoMoO4 products prepared at (a and b) 90 �C for 1 h (sample No. 3).

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SEM technique was employed to depict the genuine size and shape of the as-prepared CoMoO4

sample. Fig. 5 shows typical SEM images taken at different magnifications for the product preparedat 90 �C for 1 h (sample No. 3). These images clearly reveal that the product consists of a large numbersof uniform discrete nanorods. The micrograph shows that the nanorods are well segregated. The nano-rod sizes are 50–100 nm in diameters and 1–3 lm in length.

In order to investigate the effect of the experiment conditions on the products, various experimentswere carried out at different condition by variation of reaction parameters. It was found that the reac-tion temperature had a remarkable influence on the formation of products. When the reaction temper-ature was set below 50 �C, no reaction occurred even after 2 h and only pink transparent solution was

Fig. 6. Typical SEM images of CoMoO4 products prepared at: (a) 50 �C (sample No. 1); (b) 70 �C (sample No. 2); (c and d) 100 �C(sample No. 4) for 1 h.

Scheme 1. Schematic diagram of the crystal-field splitting and hybridization of the molecular orbitals of a tetrahedral [MoO2�4 ]

complex. The numbers in parentheses indicate the degeneracy of the [MoO2�4 ] complex, with the ‘⁄’ indicating anti-bonding

(unoccupied) states.

Fig. 7. Typical SEM images of CoMoO4 products prepared at 90 �C for: (a) 2 h (sample No. 5) and (b) 4 h (sample No. 6).

Scheme 2. Schematic illustration of the formation process of the CoMoO4 products (from left to right: 50 �C, 70 �C, 90 �C).

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obtained. As the solution temperature increase up to 50 �C, nanoparticle and nanorods with smallaspect ratio (length/diameter) were formed (sample No. 1) (Fig. 6a). By increasing the temperatureto 70 (sample No. 2) and 100 �C (sample No. 4), nanorods with the increased length of 1–2 lm andhigher aspect ratios were obtained (Fig. 6b–d). It can be concluded that by increasing the temperatureto 90 �C (Fig. 5) uniform nanorods with regular shapes are obtained. However, with increasing thetemperature to 100 �C long nanorods with irregular shapes are formed.

Fig. 8. Typical TEM image of CoMoO4 product prepared at 90 �C for 1 h.

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Moreover, the effect of reaction time on the morphology of the products was investigated. The sam-ple No. 3 was chosen as the best morphology and with the fixed temperature at 90 �C, the time wasvaried. Fig. 7a and b exhibit SEM images of the samples prepared at 90 �C for 2 h and 4 h, respectively.When the reaction time was increased to 2 h (sample No. 5) (Fig. 7a) and 4 h (sample No. 6) (Fig. 7b),nanorods with large sizes, small aspect ratios and irregular morphologies was obtained.

Fig. 9. Typical SEM images of CoMoO4 products prepared at: (a and b) different amount of precursor (sample No. 8 and sampleNo. 9 respectively); (c) pH 6.5(sample No. 7) and (d) salt precursor.

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The formation mechanism of nanorods is proposed as follows (Scheme 2): the growing process ofcrystal includes two steps; an initial nucleating stages and growth stage. At the beginning, the mixedtransparent solution became supersaturated solution when temperature is increased to 50 �C, the nu-clei created and then many of them grew up into particles and nanorods with small length (Fig. 6a),which by temperature rise to 100 �C, the length and thickness of nanorod was increased (Fig. 6c andd).

For further investigating of crystal structure and growth process of CoMoO4 nanostructures, TEMwas used to examine the product structure. Fig. 8 shows that the sample is made up of many nanorods(sample No. 3), in agreement with the SEM images (Fig. 5). The TEM micrograph depicts nanorods withthe length of several hundred nm to 2–3 lm and the diameters of 50–100 nm. From TEM image it canbe observed that nanorods are bundles of small nanobelts which have been assembled together andformed the nanorods. (Circle marked area in Fig. 9).

With decreasing reactant concentrations to half (sample No. 8) (Fig. 9a), it was observed that nano-rods have the same structures to sample No. 3. With increasing the concentration to two-fold, nano-rods with large diameter formed (sample No. 9) (Fig. 9b).

To investigate the pH effect on product morphology, more amount of ammonia was added to reac-tion medium to achieve pH = 6.5 (sample No. 7). Fig. 9c is related to as-prepared samples obtainedafter addition of ammonia which are composed of nanorods with small length. It can be concludedthat an increase in pH of solution changes the reaction rate which finally leads to an increase in nucle-ation rate. Therefore, due to the high nucleation rate, a large number of nanorods do not find thechance to growth giving rise to formation of long length nanorods, in which nanorods were agglom-erated and fused to each other.

In current experiment, we have used cobalt salicylaldehyde as a cobalt precursor, while the previ-ous works [10,11,13,20] for synthesis of CoMoO4, generally used cobalt salts. To investigate the effectof complex precursor on the morphology of the prepared samples, the same reaction was conducted

Fig. 10. Typical SEM images of CoMoO4 products prepared at surfactant effect: (a) PEG-600 (sample No. 10); (b) SDS (sampleNo. 11), (c and d) CTAB (sample No.12).

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by using cobalt acetate as the precursor (Fig. 9d). As can be seen, by using the complex precursor theproducts are rod-like with cubic cross-sections that have more regular morphology than that of thesalt precursor.

The choice of metal precursor is a key step in the preparation of nanometal oxides. Many differentprecursors have been used for simple preparation of these nanoparticles. The applied precursor usedin this work have a great steric hindrance which raises the need of using a surfactant. In order to deter-mine the effect of precursor structure in the presence of surfactants on products morphology, threedifferent surfactant were added to the reaction. In the experiment, sodium dodecyle sulfate (SDS)was used as an anionic (sample No. 11) (Fig. 10a), cetyl trimethylammonium bromide (CTAB) as a cat-ionic (sample No. 12) (Fig. 10c and d) and poly ethylene glycol (PEG-600) as a neutral surfactant (sam-ple No. 10) (Fig. 10b). The results showed that using any type of surfactant along with the inorganicprecursor not only is not beneficial to obtain a regular morphology, but also give rise to inhomoge-neous products. Thus, with using the inorganic precursor there is no need to use any other surfactant.

4. Conclusion

In summary, the regular and homogeneous CoMoO4 nanorods have been successfully preparedthrough a simple and facile precipitation method. This study demonstrates that the complex precursoris an excellent choice for synthesis of CoMoO4 without any surfactants. Indeed using of surfactant ledto irregular morphology and increase time reaction. The effect of some parameter, such as concentra-tion of initial precursors, time of reaction, pH, and surfactant were also investigated.

Acknowledgement

Supporting of this investigation by University of KASHAN is gratefully acknowledged.

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