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Facile synthesis and characterization of nanostructured flower-like copper molybdate by the co-precipitation method
Journal of Crystal Growth 386 (2014) 80–87
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Facile synthesis and characterization of nanostructured flower-like copper molybdate by the co-precipitation method Zahra Shahri a, Masoud Salavati-Niasari a,b,n, Noshin Mir c, Ghazal Kianpour a a
Institute of Nano Science and Nano Technology, University of Kashan, P.O. Box. 87317-51167, Kashan, Islamic Republic of Iran Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, P.O. Box 87317-51167, Kashan, Islamic Republic of Iran c Department of Chemistry, University of Zabol, P.O. Box. 98615-538, Zabol, Iran b
art ic l e i nf o
a b s t r a c t
Article history: Received 17 July 2013 Received in revised form 7 September 2013 Accepted 19 September 2013 Communicated by K. Nakajima Available online 9 October 2013
Nanostructured flower-like copper molybdate (CuMoO4) have been successfully synthesized with remarkable morphology via the co-precipitation method process by using Cu(Sal)2 (Sal ¼ salicylidene) and (NH4)6Mo7O24 4H2O as starting materials. Influence of parameters such as temperature, reaction time, solvent and surfactant were studied to achieve the best condition. It was found that the morphology and particles size of the final products could be greatly influenced by these parameters. The as-synthesized nanostructures were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, Energy Dispersive X-ray microanalysis (EDX), and Scanning Electron Microscopy (SEM). & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A1. Nanostructures A1. X-ray diffraction B1. Inorganic compounds B1. Semiconducting materials
1. Introduction Recently, syntheses of metal molybdates (MMoO4, M¼Ca, Ba, Sr, Cd and Ni) with specific size and morphologies have attracted many interests [1] due to their versatile applications in various fields [2] such as optical fibers, humidity sensors, photoluminescence, magnetic applications, lithium batteries, and catalysts [3–8]. Different chemical synthetic methods have been employed for preparation of molybdate compounds including hydrothermal, microwave, and precipitation with different morphologies such as microtubes [4], disk-like structures [5], spirals [6], core–shells [7], and octahedral structures [8]. Moreover, other methods have been specified for synthesis of a variety of molybdate compounds such as sonochemical for NiMoO4 nanorod [9], microemulsion for SrMoO4 spindle [10], hydrothermal for Ag2MoO4 [11], FeMoO4 nanorod [12] and erythrocyte-like CaMoO4 [13]. According to the electronic structure of MMoO4 reported by Abraham et al. [14], it may possess excellent photocatalytic activity and photoluminescence properties due to its electronic versatility, reactivity, and stability [15]. Therefore, the development of a facile and effective route for synthesizing nanostructured CuMoO4 with complex morphology is of great importance to the potential studies of its physical and chemical properties. Recently, shape n Corresponding author at: Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box. 87317-51167, Islamic Republic of Iran. Tel.: þ 98 361 5912383; fax: þ 98 361 5552935. E-mail address: [email protected] (M. Salavati-Niasari).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.09.031
control of metal molybdate compounds has been studied for some transition metals such as zinc and copper [16,17]. It is evident that the hierarchical structures consisting of nanostructured building blocks have unique physical and chemical properties and potential applications in advanced functional materials due to interfacial atomic arrangement of the material which is exactly dictated by the crystal shape [18]. For instance, the reported metal molybdate nanoplates with 2D structures have shown to give a higher photocatalytic activity than that of corresponding bulk material [19]. In this work, we report synthesis of flower-like nanostructured CuMoO4 via a simple chemical method using Cu(Sal)2 and (NH4)6 Mo7O24 4H2O as the starting materials. The co-precipitation method is chosen as facile synthetic process due to its flexibility and simplicity.
2. Experimental 2.1. Materials and methods All the chemicals used in our experiments were of analytical grade, were purchased from Merck and used as received without further purification. The XRD patterns were recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-filtered Cu Kα radiation. SEM images were obtained on Philips XL-30ESEM equipped with an energy dispersive X-ray spectroscopy. Fourier transform infrared spectroscopy (FT-IR) was recorded with Shimadzu Varian 4300 spectrophotometer in KBr pellets. EDS analysis was obtained
Z. Shahri et al. / Journal of Crystal Growth 386 (2014) 80–87
81
Table 1 Reaction conditions for preparation of CuMoO4 nanostructures. Sample no.
Investigated parameter
pH
Stirring speed (rpm)
Temperature (1C)
Solvent
Surfactant
Time (h)
Temperature
5–6
750
50
Ethanol
–
1
2
5–6
750
60
Ethanol
–
1
3
5–6
750
70
Ethanol
–
1
4
5–6
750
80
Ethanol
–
1
5–6
750
60
Ethanol
–
0
5–6
750
60
Ethanol
–
2
5–6
750
60
Ethanol
PEG (0.5 cc)
1
8
5–6
750
60
Ethanol
PVP (0.1 g)
1
9
5–6
750
60
Ethanol
SDS (0.1 g)
1
10
5–6
750
60
Ethanol
CTAB (0.1 g)
1
1
5
Time
6
7
Surfactant
SEM
82
Z. Shahri et al. / Journal of Crystal Growth 386 (2014) 80–87
Table 1 (continued ) Sample no.
Investigated parameter
pH
Stirring speed (rpm)
Temperature (1C)
Solvent
Surfactant
Time (h)
11
Solvent
5–6
750
60
EG
–
1
5–6
750
60
PG
–
1
12
SEM
Fig. 1. XRD patterns of as prepared samples: (a) Standard CuMoO4 (b) Cu(Sal)2, (c) no. 2, (d) no. 4, (e) no. 5, and (f) no.6.
on Philips EM208. Room temperature photoluminescence (PL) was studied on a Perkin Elmer (LS 55) fluorescence spectrophotometer. 2.2. Synthesis of Cu(sal)2 complex
Fig. 2. FT-IR spectra of CuMoO4 samples: (a) Cu(Sal)2, (b) no. 1, (c) no. 9 (d) no. 10, and (e) no. 11.
[Cu(Sal)2] was synthesized as follows: copper (II) acetate [Cu (CH3COO)2 2H2O] (2 mmol) was dissolved in 40 ml distilled water, a solution of salicylaldehyde (4 mmol) dissolved in the same
volume of ethanol was added dropwise to the above solution under magnetic stirring. After addition of all reagents, the mixture was refluxed for about 3 h. The obtained green crystals were
Z. Shahri et al. / Journal of Crystal Growth 386 (2014) 80–87
recrystallized from ethanol, dried at 30 1C for 6 h. The obtained product was characterized by FT-IR and XRD. 2.3. Synthesis of CuMoO4 nanostructures CuMoO4 nanoflowers were synthesized by a simple coprecipitation method. In a typical procedure, an aqueous solution of Cu(Sal)2 in the presence of different surfactants, such as polyethylene glycol (PEG 600), polyvinylpyrrolidone (PVP), sodium dodecyl sulfate (SDS) and cetyltrimethyl ammonium bromide (CTAB), was mixed with (NH4)6Mo7O24 4H2O aqueous solution and solution heated up to 60 1C for 1 h. The green product was filtered, washed with distilled water and methanol for several
Fig.3. PL spectra at room temperature of (a) no. 3 and (b) no. 11 samples.
83
times and dried in vacuum at 60 1C. The effects of temperature, reaction time, solvent, and surfactant on the morphology, the particle sizes and the phase of CuMoO4 samples were investigated and the results are listed in Table 1.
3. Results and discussions The crystal structure and phase composition of the obtained products were characterized by powder X-ray diffraction pattern and are shown in Fig. 1 Comparison of the standard XRD pattern of CuMoO4 (JCDPS no.: 31-0449) shown in Fig. 1a with the XRD spectra of the as-prepared samples reveals their high purity as well as nanocrystalline nature of the as-prepared samples due to the broadening of the diffraction peaks. Fig. 1b shows the XRD pattern of Cu(Sal)2 precursor which is completely different from the as-prepared products. The XRD patterns of samples no. 2 and no. 4 are shown in Fig. 1c and d, respectively. The crystalline phases of these products prepared in absence of surfactant in 60 and 80 1C, respectively, are well characterized. It is shown that all the peaks in Fig. 1c and d except for the ones labeled with (▲) are representative for CuMoO4 phase. The impurity peaks are originated from the small amount of precursor which slightly remained in the sample. On the other hand, making a comparison between spectra in Fig. 1c and d indicates that due to the sharper peaks in sample no. 4, the crystallinity of the products is increased with increasing the temperature. Fig. 2 shows FT-IR spectra of Cu(sal)2 precursor and CuMoO4 nanoflowers. For [Cu(Sal)2] (see Fig. 2a), two bands at 1521 and 1608 cm 1 are assigned to metal-bonded C–O stretching vibration (vCO). The peaks at 1434 and 1336 cm 1 are attributed to C–C stretching vibrations (vC–C) of salicylaldehyde. In uncoordinated salicylaldehyde, vC–O appears at 1680 and 1660 cm 1, and vC–C
Fig. 4. SEM images of samples obtained at different temperatures: (a) 50 1C (sample no. 1), (b) 60 1C (sample no. 2), (c) 70 1C (sample no. 3) and (d) 80 1C (sample no. 4).
84
Z. Shahri et al. / Journal of Crystal Growth 386 (2014) 80–87
Fig. 5. SEM images with different magnifications for time effect, (a) and (b) no. 5, (c) and (d) no. 2, (e) and (f) no. 6 (0 h, 1 h, 2 h, respectively).
appears at 1490 and 1380 cm 1. The low-shifting of these two absorption frequencies indicates chelation of the salicylaldehyde group to the metal center. A very broad peak at 3440 cm 1 is assigned to the stretching vibration of absorbed water. Absorption peaks at 500–650 cm 1 are due to Cu–O bonds. However, there are no absorption bands around this range in salicylaldehyde. Therefore the added salicylaldehyde is considered to be fully chelated to the copper ion, and no free salicylaldehyde exists in the solution [20]. Fig 2b, c, d, and e shows the FT-IR spectra of CuMoO4 nanoflowers obtained at 60 1C (sample no. 2), in presence of SDS (sample no. 9), CTAB (sample no. 10), and EG (sample no.11), respectively. Two observed peaks at 810 and 920 cm 1 in all spectra are assigned to stretching vibrations of the OMo2 (Mo… O…Mo) [9]. The peak at around 990 cm 1 is assigned to MoQO [21]. The peaks around 3430 cm–1 are assigned to the stretching vibrations of absorption water, the H–OH bending vibration appears at 1620 cm–1.
Optical properties of CuMoO4 nanocrystals were investigated by PL technique. The nature of the optical transitions of molybdates is still unclear, but by analogy with the tungstate crystals, the bands could be interpreted as the recombination of the electronhole pairs localized on [MoO42 ] group [22]. On the basis of previous reflectivity measurements [23] and the current knowledge of their electronic structure [24], these two compounds have similar crystal structures. Generally, the measured emission spectrum of MMoO4 shows the charge-transfer transitions between the O 2p orbitals and the Mo 4d orbitals within the [MoO42 ] ion complex [9]. Fig. 3 shows PL of CuMoO4 for an excitation wavelength of 300 nm (samples no. 3 and no. 11). The spectrum shows a sharp emission at 377 nm. As the crystallite size decreases, the peaks shift to higher frequencies and their intensities increase, indicating quantum confinement effect. The morphology of the resulting samples was investigated by SEM images. Fig. 4a–d shows the SEM images of samples no. 1–no. 4,
Z. Shahri et al. / Journal of Crystal Growth 386 (2014) 80–87
85
Scheme 1. Effect of reaction time.
Fig. 6. SEM image of sample prepared with different surfactants, (a) no. 7, (b) no. 8, (c) no. 9 and (d) no. 10.
respectively. All of the samples have been obtained in the absence of surfactant. The SEM images show that samples no. 1 and no. 2 have formed by assembling of small nanodisks or nanoplates which have resulted in formation of fine nanoflowers with indistinct substructures. The morphologies of samples no. 3 and no. 4 which have been prepared in higher temperatures were carefully studied. The SEM images shown in Fig. 4b–d indicate that with increasing the reaction temperature from 60 to 80 1C, the size of
the nanoplates and their agglomeration increases which leads to thickening of the plates and corresponding flower-like structures. We assume that 60 1C is the best temperature for the formation of nanostructured flowers with the most appropriate morphology, thus other reactions were conducted at this temperature. The influence of the reaction time on the sample morphologies was investigated in different reaction times of 0 h, 1 h, and 2 h. Fig. 5 shows the SEM images of the CuMoO4 samples prepared in
86
Z. Shahri et al. / Journal of Crystal Growth 386 (2014) 80–87
nanoplates as the building blocks of nanoflowers. The obtained products in presence of either EG or PG are mostly nanoparticles. EDX analysis was employed to investigate the chemical composition and purity of as-synthesized CuMoO4 structures. The EDX pattern of CuMoO4 (sample no. 3) in Fig. 8 shows that only Cu, Mo, and O elements exist in the sample. No peak of any impurity is observed indicating the high purity of the product. To investigate the effect of molybdate precursor on morphology, (NH4)6Mo7O24 4H2O was replaced with Na2MoO4, but no reaction occurred between Cu(Sal)2 and Na2MoO4, and Cu(Sal)2 remained unchanged. In other word, Na2MoO4 does not have enough affinity to react with Cu to segregate it from Sal ligand. The proposed mechanism for the synthesis of CuMoO4 could be explained as follows:
Scheme 2. Effect of SDS on morphology. .
0 h, 1 h and 2 h. The sample prepared at 0 h, include agglomerated nanoparticles which have formed plates or layers. Increasing the reaction time to 1 h, causes the plates or layers to assemble and make flower-like structures. However, increasing the reaction time to 2 h does not change the morphology. From the SEM results, the optimum time in our co-precipitation reaction conditions was considered to be 1 h. Effect of reaction time is shown in Scheme 1. It is known that the morphology and shape of the crystals can be controlled by the change of the surface energy of the nuclei through selective adsorption [8] To investigate the effect of surfactant on the morphology and size of the product, SEM images of prepared samples in presence of polyvinylpyrrolidone (PVP), polyethylene glycol 600 (PEG), sodium dodecyl sulfate (SDS), and cetyltrimethyl ammonium bromide (CTAB) are shown in Fig. 6. The SEM image (Fig. 6a) shows that nanoflowers are also formed in presence of PEG 600 because this polymer attaches to the particle surface through its active oxygen groups. This type of bonding is much weaker than that observed in PVP for absorbing on the surface of the plates and is easily separated. Therefore, though in presence of PEG 600 nanoplates assemble and form the flower structures, PVP stops the substructures to stick together and form ordered and uniform morphology (Fig. 6b). SDS, as an anionic surfactant, has different influence on the morphology. Due to its amphiphilic nature, it forms stable micelles in water in which spherical nanostructures form easily [7]. Formation of spherical morphology in micelle is illustrated in Scheme 2. The high magnification of SEM image for sample no. 9 shown in Fig. 8c reveals that surface of the microspheres is not smooth and is covered with nanosheets. CTAB as a cationic surfactant was employed to investigate the influence of cationic active groups on morphology of the products. Due to its cationic head group, CTAB easily interact with free oxygen groups on the surface of nanoplates and changes the whole plate-like appearance of the as-formed flowers. Fig. 6d shows SEM image of the CuMoO4 sample prepared in presence of CTAB. It can be clearly observed that CTAB has changed the morphology. For investigating the effects of solvent on morphology and particle size, the reaction was carried out in Propylene glycol (PG) and Ethylene glycol (EG) [9]. Fig. 7 presents SEM image of the CuMoO4 nanoparticles prepared in PG and EG at 60 1C. EG and PG solvents having two polar heads, prevent the formation of
Fig. 7. SEM images of samples prepared in different solvents (a) EG and (b) PG.
Fig. 8. EDX pattern of CuMoO4 (sample no. 4).
Z. Shahri et al. / Journal of Crystal Growth 386 (2014) 80–87
Cu(C6H4(CHO)O)2-Cu2 þ þ2C6H4(CHO)O þ
(NH4)6Mo7O24 4H2O-6NH4 þ 7MoO4
2
þ8H
þ
þ
C6H4(CHO)O þH -C6H4(CHO)OH Cu2 þ þ MoO42 -CuMoO4 The pH value is an influential factor for converting [Mo7O24]6 to [MoO4]2 and eventually formation of MMoO4. According to previous reports, formation of MMoO4 occurs in pH ¼5 7. It has been observed that if pH of the solution decreases to 5, Mo will polymerize and form Mo8O264 [25]. In current experiment, we have used Cu(Sal)2 as Cu source. By using this precursor due to formation of salicylidene ion during the reaction, pH of reaction was measured to be higher than 5.5. 4. Conclusions In summary, CuMoO4 nanoflowers have been successfully synthesized from Cu(Sal)2 and (NH4)6Mo7O24 4H2O via the simple co-precipitation method, under low temperature and pressure. Different temperatures, reaction times, solvents, and surfactants were examined to investigate their effects on the morphology of final product. The results show that different kinds of surfactants, reaction times, and solvent are crucial factors for the formation of CuMoO4 nanoflowers. To the best of our knowledge, it is the first time that Cu(Sal)2 is used as Cu source for the synthesis of CuMoO4 and flowerlike morphology is obtained by the co-precipitation method. In comparison to other similar works, the presented method is facile, low-cost and employs non-toxic materials and solvent. Acknowledgment Authors are grateful to the council of the University of Kashan for providing financial support to undertake this experiment by Grant no. 159271/102. References [1] X. Jiang, J. Ma, B. Lin, Y. Ren, J. Liu, X. Zhu, J. Tao, Y. Wang, L. Xie, Hydrothermal synthesis of CdMoO4 nano-particles, Journal of the American Ceramic Society 90 (2007) 977–979. [2] A. Phuruangrat, N. Ekthammathat, T. Thongtem, S. Thongtem, Microwaveassisted synthesis and optical property of CdMoO4 nanoparticles, Journal of Physics and Chemistry of Solids 72 (2011) 176–180. [3] A. Sun, D. Wang, Z. Wu, Q. Chen, Mechanochemical synthesis of Mo–Cu nanocomposite powders, Journal of Alloys and Compounds 509 (2011) L74–L77. [4] K. Li, L. Wang, W. Liu, T. Ying, Ionic liquid-assisted sacrificial templating route to hollow CdMoO4 microtubes, Journal of the Ceramic Society of Japan 118 (2010) 253–255.
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[5] G. Xing, Y. Xu, C. Zhao, Y. Wang, Y. Li, Z. Wu, T. Liu, G. Wu, Photoluminescence properties of CdMoO4 disk and hollow microsphere-like crystals synthesized by hydrothermal conventional method, Journal of Powder Technology 213 (2011) 109–115. [6] Y. Ren, J. Ma, Y. Wang, X. Zhu, B. Lin, J. Liu, X. Jiang, J. Tao, Shape-Tailored hydrothermal synthesis of CdMoO4 crystallites on varying pH conditions, Journal of the American Ceramic Society 90 (2007) 125s1–1254. [7] W.S. Wang, L. Zhen, C.Y. Xu, W.Z. Shao, Room temperature synthesis, growth mechanism, photocatalytic and photoluminescence properties of cadmium molybdate core-shell microspheres, Crystal Growth and Design 9 (2009) 1558–1568. [8] Q. Gong, G. Li, X. Qian, H. Cao, W. Du, X. Ma, Synthesis of single crystal CdMoO4 octahedral microparticles via microemulsion-mediated route, Journal of Colloid and Interface Science 304 (2006) 408–412. [9] G. Kianpour, M. Salavati-Niasari, H. Emadi, Sonochemical synthesis and characterization of NiMoO4 nanorods, Ultrasonics Sonochemistry 20 (2013) 418–424. [10] J. Liu, J. Ma, B. Lin, Y. Ren, X. Jiang, J. Tao, X. Zhu, Room temperature synthesis and optical properties of SrMoO4 crystallites by w/o microemulsion, Journal of the European Ceramic Society 34 (2008) 1557–1560. [11] X. Cui, S.H. Yu, L. Li, L. Biao, H. Li, M. Mo, X.M. Liu, Selective synthesis and characterization of single-crystal silver molybdate/tungstate nanowires by hydrothermal process, Chemistry: A European Journal 10 (2004) 218–223. [12] Z. Zhang, Ch. Hu, M. Hashim, P. Chen, Y. Xiong, C. Zhang, Synthesis and magnetic property of FeMoO4 nanorod, Journal of Materials Science and Engineering B 176 (2011) 756–761. [13] Y.S Luo, X.J. Dai, W.D. Zhang, Y. Yang, C.Q. Sun, S.Y. Fu, Controllable synthesis and luminescent properties of novel erythrocyte-like CaMoO4 hierarchical nanostructures via a simple surfactant-free hydrothermal route, Dalton Transactions 39 (2010) 2226–2231. [14] Y. Abraham, N.A.W. Holzwarth, R.T. Willams, Electronic structure and optical properties of CdMoO4 and CdWO4, Physical Review B 62 (2000) 1733–1741. [15] L. Zhen, W.S. Wang, C.Y. Xu, W.Z. Shao, M.M. Ye, Z.L. Chen, High photocatalytic activity and photoluminescence property of hollow CdMoO4 microspheres,, Scripta Materialia 58 (2008) 461–464. [16] G. Zhang, S. Yu, Y. Yang, W. Jiang, S. Zhang, B. Huang, Synthesis, morphology and phase transition of the zinc molybdates ZnMoO4 0.8H2O/α-ZnMoO4/ ZnMoO4 by hydrothermal method, Journal of Crystal Growth 312 (2010) 1866–1874. [17] J. Xu, D. Xue, Y. Zhu, Room Temperature Synthesis of Curved Ammonium Copper Molybdate Nanoflake and Its Hierarchical Architecture, Journal of Physical Chemistry B 110 (2006) 17400–17405. [18] J. Liu, G. Zhang, Template-free synthesis and high photocatalytic activity of hierarchical Zn2GeO4 microspheres, CrystEngComm 15 (2013) 382–389. [19] J. Yu, A. Kudo, Hydrothermal synthesis and photocatalytic property of 2-dimensional bismuth molybdate nanoplates, Chemistry Letters 34 (2005) 1528–1529. [20] M. Sabet, M. Salavati-Niasari, F. Davar, Facile one-step microwave to prepare CuInS2/CuS nanocomposite for solar cells, Micro and Nano Letters 6 (2011) 904–908. [21] D. Klissurski, R. Irodanova, M. Milanova, D. Radev, S. Vassilve, Mechanochemically assisted synthesis of Cu (II) molybdate, Chemie Inorganique 56 (2003) 39–42. [22] Y.J. Hsiao, Y.S. Chang, G.J. Chen, Y.H. Chang, Synthesis and the luminescent properties of CdNb2O6 oxides by sol–gel process, Journal of Alloys and Compounds 471 (2009) 259–262. [23] R. Grasser, E. Pitt, A. Scharmann, G. Zimmerer, Optical properties of CaWO4 and CaMoO4 crystals in the 4 to 25 eV region, Physica Status Solidi B 69 (1975) 359–368. [24] Y. Zhang, N.A.W. Holzwarth, R.T. Williams, Electronic band structures of the scheelite materials CaMoO4, CaWO4, PbMoO4, and PbWO4, Physical Review B 57 (1998) 12738–12750. [25] C.V. Krishnan, R. Munoz-Espi, Q. Li, C. Burger, B. Chu, Formation of molybdenum oxide nanostructures controlled by poly(Ethyelene oxide), Chinese Journal of Polymer Science 27 (2009) 11–22.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Facile synthesis and characterization of nanostructured flower-like copper molybdate by the co-precipitation method Zahra Shahri a, Masoud Salavati-Niasari a,b,n, Noshin Mir c, Ghazal Kianpour a a
Institute of Nano Science and Nano Technology, University of Kashan, P.O. Box. 87317-51167, Kashan, Islamic Republic of Iran Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, P.O. Box 87317-51167, Kashan, Islamic Republic of Iran c Department of Chemistry, University of Zabol, P.O. Box. 98615-538, Zabol, Iran b
art ic l e i nf o
a b s t r a c t
Article history: Received 17 July 2013 Received in revised form 7 September 2013 Accepted 19 September 2013 Communicated by K. Nakajima Available online 9 October 2013
Nanostructured flower-like copper molybdate (CuMoO4) have been successfully synthesized with remarkable morphology via the co-precipitation method process by using Cu(Sal)2 (Sal ¼ salicylidene) and (NH4)6Mo7O24 4H2O as starting materials. Influence of parameters such as temperature, reaction time, solvent and surfactant were studied to achieve the best condition. It was found that the morphology and particles size of the final products could be greatly influenced by these parameters. The as-synthesized nanostructures were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, Energy Dispersive X-ray microanalysis (EDX), and Scanning Electron Microscopy (SEM). & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Characterization A1. Nanostructures A1. X-ray diffraction B1. Inorganic compounds B1. Semiconducting materials
1. Introduction Recently, syntheses of metal molybdates (MMoO4, M¼Ca, Ba, Sr, Cd and Ni) with specific size and morphologies have attracted many interests [1] due to their versatile applications in various fields [2] such as optical fibers, humidity sensors, photoluminescence, magnetic applications, lithium batteries, and catalysts [3–8]. Different chemical synthetic methods have been employed for preparation of molybdate compounds including hydrothermal, microwave, and precipitation with different morphologies such as microtubes [4], disk-like structures [5], spirals [6], core–shells [7], and octahedral structures [8]. Moreover, other methods have been specified for synthesis of a variety of molybdate compounds such as sonochemical for NiMoO4 nanorod [9], microemulsion for SrMoO4 spindle [10], hydrothermal for Ag2MoO4 [11], FeMoO4 nanorod [12] and erythrocyte-like CaMoO4 [13]. According to the electronic structure of MMoO4 reported by Abraham et al. [14], it may possess excellent photocatalytic activity and photoluminescence properties due to its electronic versatility, reactivity, and stability [15]. Therefore, the development of a facile and effective route for synthesizing nanostructured CuMoO4 with complex morphology is of great importance to the potential studies of its physical and chemical properties. Recently, shape n Corresponding author at: Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P.O. Box. 87317-51167, Islamic Republic of Iran. Tel.: þ 98 361 5912383; fax: þ 98 361 5552935. E-mail address: [email protected] (M. Salavati-Niasari).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.09.031
control of metal molybdate compounds has been studied for some transition metals such as zinc and copper [16,17]. It is evident that the hierarchical structures consisting of nanostructured building blocks have unique physical and chemical properties and potential applications in advanced functional materials due to interfacial atomic arrangement of the material which is exactly dictated by the crystal shape [18]. For instance, the reported metal molybdate nanoplates with 2D structures have shown to give a higher photocatalytic activity than that of corresponding bulk material [19]. In this work, we report synthesis of flower-like nanostructured CuMoO4 via a simple chemical method using Cu(Sal)2 and (NH4)6 Mo7O24 4H2O as the starting materials. The co-precipitation method is chosen as facile synthetic process due to its flexibility and simplicity.
2. Experimental 2.1. Materials and methods All the chemicals used in our experiments were of analytical grade, were purchased from Merck and used as received without further purification. The XRD patterns were recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-filtered Cu Kα radiation. SEM images were obtained on Philips XL-30ESEM equipped with an energy dispersive X-ray spectroscopy. Fourier transform infrared spectroscopy (FT-IR) was recorded with Shimadzu Varian 4300 spectrophotometer in KBr pellets. EDS analysis was obtained
Z. Shahri et al. / Journal of Crystal Growth 386 (2014) 80–87
81
Table 1 Reaction conditions for preparation of CuMoO4 nanostructures. Sample no.
Investigated parameter
pH
Stirring speed (rpm)
Temperature (1C)
Solvent
Surfactant
Time (h)
Temperature
5–6
750
50
Ethanol
–
1
2
5–6
750
60
Ethanol
–
1
3
5–6
750
70
Ethanol
–
1
4
5–6
750
80
Ethanol
–
1
5–6
750
60
Ethanol
–
0
5–6
750
60
Ethanol
–
2
5–6
750
60
Ethanol
PEG (0.5 cc)
1
8
5–6
750
60
Ethanol
PVP (0.1 g)
1
9
5–6
750
60
Ethanol
SDS (0.1 g)
1
10
5–6
750
60
Ethanol
CTAB (0.1 g)
1
1
5
Time
6
7
Surfactant
SEM
82
Z. Shahri et al. / Journal of Crystal Growth 386 (2014) 80–87
Table 1 (continued ) Sample no.
Investigated parameter
pH
Stirring speed (rpm)
Temperature (1C)
Solvent
Surfactant
Time (h)
11
Solvent
5–6
750
60
EG
–
1
5–6
750
60
PG
–
1
12
SEM
Fig. 1. XRD patterns of as prepared samples: (a) Standard CuMoO4 (b) Cu(Sal)2, (c) no. 2, (d) no. 4, (e) no. 5, and (f) no.6.
on Philips EM208. Room temperature photoluminescence (PL) was studied on a Perkin Elmer (LS 55) fluorescence spectrophotometer. 2.2. Synthesis of Cu(sal)2 complex
Fig. 2. FT-IR spectra of CuMoO4 samples: (a) Cu(Sal)2, (b) no. 1, (c) no. 9 (d) no. 10, and (e) no. 11.
[Cu(Sal)2] was synthesized as follows: copper (II) acetate [Cu (CH3COO)2 2H2O] (2 mmol) was dissolved in 40 ml distilled water, a solution of salicylaldehyde (4 mmol) dissolved in the same
volume of ethanol was added dropwise to the above solution under magnetic stirring. After addition of all reagents, the mixture was refluxed for about 3 h. The obtained green crystals were
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recrystallized from ethanol, dried at 30 1C for 6 h. The obtained product was characterized by FT-IR and XRD. 2.3. Synthesis of CuMoO4 nanostructures CuMoO4 nanoflowers were synthesized by a simple coprecipitation method. In a typical procedure, an aqueous solution of Cu(Sal)2 in the presence of different surfactants, such as polyethylene glycol (PEG 600), polyvinylpyrrolidone (PVP), sodium dodecyl sulfate (SDS) and cetyltrimethyl ammonium bromide (CTAB), was mixed with (NH4)6Mo7O24 4H2O aqueous solution and solution heated up to 60 1C for 1 h. The green product was filtered, washed with distilled water and methanol for several
Fig.3. PL spectra at room temperature of (a) no. 3 and (b) no. 11 samples.
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times and dried in vacuum at 60 1C. The effects of temperature, reaction time, solvent, and surfactant on the morphology, the particle sizes and the phase of CuMoO4 samples were investigated and the results are listed in Table 1.
3. Results and discussions The crystal structure and phase composition of the obtained products were characterized by powder X-ray diffraction pattern and are shown in Fig. 1 Comparison of the standard XRD pattern of CuMoO4 (JCDPS no.: 31-0449) shown in Fig. 1a with the XRD spectra of the as-prepared samples reveals their high purity as well as nanocrystalline nature of the as-prepared samples due to the broadening of the diffraction peaks. Fig. 1b shows the XRD pattern of Cu(Sal)2 precursor which is completely different from the as-prepared products. The XRD patterns of samples no. 2 and no. 4 are shown in Fig. 1c and d, respectively. The crystalline phases of these products prepared in absence of surfactant in 60 and 80 1C, respectively, are well characterized. It is shown that all the peaks in Fig. 1c and d except for the ones labeled with (▲) are representative for CuMoO4 phase. The impurity peaks are originated from the small amount of precursor which slightly remained in the sample. On the other hand, making a comparison between spectra in Fig. 1c and d indicates that due to the sharper peaks in sample no. 4, the crystallinity of the products is increased with increasing the temperature. Fig. 2 shows FT-IR spectra of Cu(sal)2 precursor and CuMoO4 nanoflowers. For [Cu(Sal)2] (see Fig. 2a), two bands at 1521 and 1608 cm 1 are assigned to metal-bonded C–O stretching vibration (vCO). The peaks at 1434 and 1336 cm 1 are attributed to C–C stretching vibrations (vC–C) of salicylaldehyde. In uncoordinated salicylaldehyde, vC–O appears at 1680 and 1660 cm 1, and vC–C
Fig. 4. SEM images of samples obtained at different temperatures: (a) 50 1C (sample no. 1), (b) 60 1C (sample no. 2), (c) 70 1C (sample no. 3) and (d) 80 1C (sample no. 4).
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Fig. 5. SEM images with different magnifications for time effect, (a) and (b) no. 5, (c) and (d) no. 2, (e) and (f) no. 6 (0 h, 1 h, 2 h, respectively).
appears at 1490 and 1380 cm 1. The low-shifting of these two absorption frequencies indicates chelation of the salicylaldehyde group to the metal center. A very broad peak at 3440 cm 1 is assigned to the stretching vibration of absorbed water. Absorption peaks at 500–650 cm 1 are due to Cu–O bonds. However, there are no absorption bands around this range in salicylaldehyde. Therefore the added salicylaldehyde is considered to be fully chelated to the copper ion, and no free salicylaldehyde exists in the solution [20]. Fig 2b, c, d, and e shows the FT-IR spectra of CuMoO4 nanoflowers obtained at 60 1C (sample no. 2), in presence of SDS (sample no. 9), CTAB (sample no. 10), and EG (sample no.11), respectively. Two observed peaks at 810 and 920 cm 1 in all spectra are assigned to stretching vibrations of the OMo2 (Mo… O…Mo) [9]. The peak at around 990 cm 1 is assigned to MoQO [21]. The peaks around 3430 cm–1 are assigned to the stretching vibrations of absorption water, the H–OH bending vibration appears at 1620 cm–1.
Optical properties of CuMoO4 nanocrystals were investigated by PL technique. The nature of the optical transitions of molybdates is still unclear, but by analogy with the tungstate crystals, the bands could be interpreted as the recombination of the electronhole pairs localized on [MoO42 ] group [22]. On the basis of previous reflectivity measurements [23] and the current knowledge of their electronic structure [24], these two compounds have similar crystal structures. Generally, the measured emission spectrum of MMoO4 shows the charge-transfer transitions between the O 2p orbitals and the Mo 4d orbitals within the [MoO42 ] ion complex [9]. Fig. 3 shows PL of CuMoO4 for an excitation wavelength of 300 nm (samples no. 3 and no. 11). The spectrum shows a sharp emission at 377 nm. As the crystallite size decreases, the peaks shift to higher frequencies and their intensities increase, indicating quantum confinement effect. The morphology of the resulting samples was investigated by SEM images. Fig. 4a–d shows the SEM images of samples no. 1–no. 4,
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Scheme 1. Effect of reaction time.
Fig. 6. SEM image of sample prepared with different surfactants, (a) no. 7, (b) no. 8, (c) no. 9 and (d) no. 10.
respectively. All of the samples have been obtained in the absence of surfactant. The SEM images show that samples no. 1 and no. 2 have formed by assembling of small nanodisks or nanoplates which have resulted in formation of fine nanoflowers with indistinct substructures. The morphologies of samples no. 3 and no. 4 which have been prepared in higher temperatures were carefully studied. The SEM images shown in Fig. 4b–d indicate that with increasing the reaction temperature from 60 to 80 1C, the size of
the nanoplates and their agglomeration increases which leads to thickening of the plates and corresponding flower-like structures. We assume that 60 1C is the best temperature for the formation of nanostructured flowers with the most appropriate morphology, thus other reactions were conducted at this temperature. The influence of the reaction time on the sample morphologies was investigated in different reaction times of 0 h, 1 h, and 2 h. Fig. 5 shows the SEM images of the CuMoO4 samples prepared in
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nanoplates as the building blocks of nanoflowers. The obtained products in presence of either EG or PG are mostly nanoparticles. EDX analysis was employed to investigate the chemical composition and purity of as-synthesized CuMoO4 structures. The EDX pattern of CuMoO4 (sample no. 3) in Fig. 8 shows that only Cu, Mo, and O elements exist in the sample. No peak of any impurity is observed indicating the high purity of the product. To investigate the effect of molybdate precursor on morphology, (NH4)6Mo7O24 4H2O was replaced with Na2MoO4, but no reaction occurred between Cu(Sal)2 and Na2MoO4, and Cu(Sal)2 remained unchanged. In other word, Na2MoO4 does not have enough affinity to react with Cu to segregate it from Sal ligand. The proposed mechanism for the synthesis of CuMoO4 could be explained as follows:
Scheme 2. Effect of SDS on morphology. .
0 h, 1 h and 2 h. The sample prepared at 0 h, include agglomerated nanoparticles which have formed plates or layers. Increasing the reaction time to 1 h, causes the plates or layers to assemble and make flower-like structures. However, increasing the reaction time to 2 h does not change the morphology. From the SEM results, the optimum time in our co-precipitation reaction conditions was considered to be 1 h. Effect of reaction time is shown in Scheme 1. It is known that the morphology and shape of the crystals can be controlled by the change of the surface energy of the nuclei through selective adsorption [8] To investigate the effect of surfactant on the morphology and size of the product, SEM images of prepared samples in presence of polyvinylpyrrolidone (PVP), polyethylene glycol 600 (PEG), sodium dodecyl sulfate (SDS), and cetyltrimethyl ammonium bromide (CTAB) are shown in Fig. 6. The SEM image (Fig. 6a) shows that nanoflowers are also formed in presence of PEG 600 because this polymer attaches to the particle surface through its active oxygen groups. This type of bonding is much weaker than that observed in PVP for absorbing on the surface of the plates and is easily separated. Therefore, though in presence of PEG 600 nanoplates assemble and form the flower structures, PVP stops the substructures to stick together and form ordered and uniform morphology (Fig. 6b). SDS, as an anionic surfactant, has different influence on the morphology. Due to its amphiphilic nature, it forms stable micelles in water in which spherical nanostructures form easily [7]. Formation of spherical morphology in micelle is illustrated in Scheme 2. The high magnification of SEM image for sample no. 9 shown in Fig. 8c reveals that surface of the microspheres is not smooth and is covered with nanosheets. CTAB as a cationic surfactant was employed to investigate the influence of cationic active groups on morphology of the products. Due to its cationic head group, CTAB easily interact with free oxygen groups on the surface of nanoplates and changes the whole plate-like appearance of the as-formed flowers. Fig. 6d shows SEM image of the CuMoO4 sample prepared in presence of CTAB. It can be clearly observed that CTAB has changed the morphology. For investigating the effects of solvent on morphology and particle size, the reaction was carried out in Propylene glycol (PG) and Ethylene glycol (EG) [9]. Fig. 7 presents SEM image of the CuMoO4 nanoparticles prepared in PG and EG at 60 1C. EG and PG solvents having two polar heads, prevent the formation of
Fig. 7. SEM images of samples prepared in different solvents (a) EG and (b) PG.
Fig. 8. EDX pattern of CuMoO4 (sample no. 4).
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Cu(C6H4(CHO)O)2-Cu2 þ þ2C6H4(CHO)O þ
(NH4)6Mo7O24 4H2O-6NH4 þ 7MoO4
2
þ8H
þ
þ
C6H4(CHO)O þH -C6H4(CHO)OH Cu2 þ þ MoO42 -CuMoO4 The pH value is an influential factor for converting [Mo7O24]6 to [MoO4]2 and eventually formation of MMoO4. According to previous reports, formation of MMoO4 occurs in pH ¼5 7. It has been observed that if pH of the solution decreases to 5, Mo will polymerize and form Mo8O264 [25]. In current experiment, we have used Cu(Sal)2 as Cu source. By using this precursor due to formation of salicylidene ion during the reaction, pH of reaction was measured to be higher than 5.5. 4. Conclusions In summary, CuMoO4 nanoflowers have been successfully synthesized from Cu(Sal)2 and (NH4)6Mo7O24 4H2O via the simple co-precipitation method, under low temperature and pressure. Different temperatures, reaction times, solvents, and surfactants were examined to investigate their effects on the morphology of final product. The results show that different kinds of surfactants, reaction times, and solvent are crucial factors for the formation of CuMoO4 nanoflowers. To the best of our knowledge, it is the first time that Cu(Sal)2 is used as Cu source for the synthesis of CuMoO4 and flowerlike morphology is obtained by the co-precipitation method. In comparison to other similar works, the presented method is facile, low-cost and employs non-toxic materials and solvent. Acknowledgment Authors are grateful to the council of the University of Kashan for providing financial support to undertake this experiment by Grant no. 159271/102. References [1] X. Jiang, J. Ma, B. Lin, Y. Ren, J. Liu, X. Zhu, J. Tao, Y. Wang, L. Xie, Hydrothermal synthesis of CdMoO4 nano-particles, Journal of the American Ceramic Society 90 (2007) 977–979. [2] A. Phuruangrat, N. Ekthammathat, T. Thongtem, S. Thongtem, Microwaveassisted synthesis and optical property of CdMoO4 nanoparticles, Journal of Physics and Chemistry of Solids 72 (2011) 176–180. [3] A. Sun, D. Wang, Z. Wu, Q. Chen, Mechanochemical synthesis of Mo–Cu nanocomposite powders, Journal of Alloys and Compounds 509 (2011) L74–L77. [4] K. Li, L. Wang, W. Liu, T. Ying, Ionic liquid-assisted sacrificial templating route to hollow CdMoO4 microtubes, Journal of the Ceramic Society of Japan 118 (2010) 253–255.
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