Cr2WO6 nanoparticles prepared by hydrothermal assisted method with selective adsorption properties for methylene blue in water

Cr2WO6 nanoparticles prepared by hydrothermal assisted method with selective adsorption properties for methylene blue in water

Materials Science in Semiconductor Processing 34 (2015) 170–174 Contents lists available at ScienceDirect Materials Science in Semiconductor Process...

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Materials Science in Semiconductor Processing 34 (2015) 170–174

Contents lists available at ScienceDirect

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Short Communication

Cr2WO6 nanoparticles prepared by hydrothermal assisted method with selective adsorption properties for methylene blue in water Wenmin Zhou, Jianfeng Huang n, Jiayin Li, Zhanwei Xu, Liyun Cao, Chunyan Yao, Jing Lu School of Materials Science & Engineering, Shaanxi University of Science and Technology, Xi'an, Shaanxi 710021, China

a r t i c l e in f o

Keywords: Cr2WO6 Nanocrystallites Methylene blue Adsorption performance

abstract Cr2WO6 nanocrystallites were prepared using a facile hydrothermal assisted process. The microstructures and preferential adsorption performance for methylene blue (MB) of the prepared nanoparticles were investigated using a solid state reaction resulted in Cr2WO6 crystallites as a baseline. Their structural information were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometer, transmission electron microscopy, N2–sorption BET surface area, thermogravimetric and differential scanning calorimeters. Results show that the Cr2WO6 nanocrystallines with  50 nm in size are achieved by calcination at 650 1C for only 2 h using a hydrothermal product as precursor, while the micrometer sized Cr2WO6 crystallites synthesized by the solid state reaction at higher temperature (950 1C) for at least 20 h. For the as-prepared Cr2WO6 nanocrystallines, the adsorption performance were tested using methyl orange (MO), Rhodamine B (RhB) and methylene blue (MB). The adsorption ability was found to be in the order of MB4 4 RhB4MO owing to electrostatic interactions between the adsorbent and the dye. Results of this study provide a kind of promising alternative adsorbent for color removal from industrial wastewaters. & 2015 Elsevier Ltd. All rights reserved.

1. Introduction Tungsten oxide and chromium oxide have stimulated great interest because of their excellent performance in many fields, including environmental purification [1,2], dye-sensitized solar cells [3] and gas sensor [4,5]. Given this background, considerable researches were also addressed to their compound materials, including WO3/Cr2O3 [6,7] and chromium tungstate [8]. Especially, the exploitation of chromium tungstate has been reported in many fields like gas sensors [7], antiferromagnetic materials [9] and negative thermal expansion [10], magnetoelectric materials [11], which were found to


Corresponding author. Tel./fax: þ 86 029 86168802. E-mail address: [email protected] (J. Huang). 1369-8001/& 2015 Elsevier Ltd. All rights reserved.

widely open our minds with great expansion in the practical applications of chromium tungstate. This leads to fact that the preparation of chromium tungstate is one of the key issues at present. However, the most of the commonly used synthetic methods in the preparation of chromium tungstate powder relate to conventional solid state reactions [8,9,12], involving the disadvantages of high energy consumption, uncontrollable shape or difficulties in the economic efficiency of practical applications [13]. Therefore, it still remains a challenge to develop facile methods for the preparation of chromium tungstate powder for current research. Hydrothermal is a well-established approach for the preparation of various functional nanomaterials with desired micro/nanostructures for a variety of potential applications [14]. In particular, the formation of crystalline products through a hydrothermal

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Table 1 The characteristics of these dyes. Molecule



Molecular weight

Methyl orange(MO)



Rhodamine B(RhB)



Methylene blue (MB)



process can be produced at temperatures substantially lower than those required by solid state reaction. Therefore, in this paper, we demonstrate the synthesis of chromium tungstate nanocrystallites using a facile hydrothermal assisted method. Meanwhile, the preferential adsorption performance of Cr2WO6 nanocrystallines on methylene blue (MB) has been discussed and proposed. It is hoped that our work could provide a new insight into the preparation of tungstate materials and their selective adsorbent for wastewater treatment.

emission scanning electron microscopy (FE-SEM, Hitachi S4800, acceleration voltage: 5 kV) equipped with energy dispersive spectrometer (EDS). Transmission electron images (TEM) and high-resolution transmission electron images (HRTEM) were obtained on a JEM-3100 microscope (accelerating voltage: 300 kV). Zeta potentials of particles in water suspensions were measured by using Zeta Meter(NAMO-ZS) and the specific surface areas of the samples were measured by nitrogen adsorption method using an American Quantachrome NOVA–2200E instrument.

2. Experimental

2.3. Adsorption testing

2.1. Preparation

In this study, two different types of organic dyes were selected as adsorbates. Methyl orange (MO) is a typical anionic dye in the aqueous solution [15]. Methylene blue (MB) and rhodamine B (RhB) are typical cationic dyes [16]. Under experimental conditions MB and RhB are electrolytically dissociated in the aqueous solution: MB dye remains in cationic form, whereas the presence of carboxylic group in the RhB dye makes it negatively charged [17]. The characteristics of these dyes were listed in Table 1. The product adsorption properties were evaluated in a Pyrex glass vessel with 50 mL dye solution (10 mg L  1) under dark conditions. 50 mg of the HS-product or Sproduct were added with magnetic stirring in the solution, afterwards the solution was collected per 5 min under vigorous stirring. The collected solution was further centrifuged to quantify the residual concentration of the dye solution in an UV–vis spectrophotometer (Unico UV-2600).

Analytical Cr(NO3)3  9H2O and Na2WO4  2H2O were used without any purification. The Cr2WO6 nanocrystallites were prepared by a hydrothermal assisted process. In a typical synthesis, 20 mL of the Na2WO4 solution (2.5 mmol) was added into 20 mL of Cr(NO3)3 solution (5 mmol) under magnetic stirring. Then the pH value of this solution was adjusted to 5 with the addition of NaOH solution (1 M). The adjusted solution was hydrothermal-treated in a 100 mL Teflon-lined autoclave at 200 1C for 24 h. The precursor precipitates were separated and washed with deionized water and anhydrous alcohol for several times, dried at 60 1C for 3 h (H-product). To obtain well crystallized Cr2WO6, the dried precipitates were calcined in air at 650 1C for 2 h (HS-product). Fine powder of WO3 and Cr2O3 (molar ratio 1:1) were heated at 800 1C and 950 1C for 20 h as a baseline (S-product) to find their product differences in the phase and microstructure.

3. Results and discussion

2.2. Characterization

3.1. Structure and composition analyses

The product phases were characterized by a powder X-ray diffraction (XRD, Rigaku D/max-2000) with Cu Kα radiation (λ ¼0.15406 nm) at 40 kV and 40 mA in the 2θ range of 151–701. The product thermal conversion characteristics were analyzed by thermogravimetric (TG) and differential scanning calorimeters (DSC) using a NETZSCH STA 409PC thermal analyzer at the heating rate of 20 1C/min in air from room temperature to 1200 1C. The product composition and microstructure were characterized by field

Fig. 1a shows the XRD patterns of the powders prepared by different processes. All of the four peaks of H-product at about 36.1, 41.5, 54.8 and 65.11could be indexed to rhombohedral Cr2O3 (JCPDS Card No. 70-3765) structure with low peak intensities. To investigate the phase composition of Hproduct, corresponding EDS spectrum (Fig. 2f, taken from Fig. 2a) was tested. The actual Cr: W: O molar ratio is 2:1:6 in the H-product, indicating that the H-product may be a mixture of Cr2O3 crystals and amorphous WO3. The above


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Fig. 1. XRD patterns of the powders prepared by different processes; (a) TG–DSC curves of the products; (b) hydrothermal product and (c) the mixture of WO3 and Cr2O3.

Fig. 2. SEM and TEM images of the H-product (a, d), HS-product (b, e) and S-product (c). (The selected area in Fig. 2a stands for the specific area of the EDS spectrum in Fig. 2f).

results indicate that further heat treatment was needed to obtain Cr2WO6 structure. Hence, TG–DSC test was carried out to find the thermal conversion characteristics of the Hproduct, as shown in Fig. 1b. The figure exhibits a broad exothermic peak at 650 1C with no major weight loss in the corresponding TG curve. Therefore, the H-product was calcined at 650 1C to obtain the HS-product. As shown in Fig. 1a, all diffraction peaks of the HS-product could be readily indexed to a pure tetragonal Cr2WO6 (JCPDS Card No. 35-0791) structure with well crystallinity and no peak of Cr2O3 or WO3 can be found, indicating that the mixture of Cr2O3 crystals and amorphous WO3 were completely transformed to pure Cr2WO6 crystals by heat treatment at 650 1C. However, this temperature may not provide enough energy to obtain the same Cr2WO6 crystallites under conventional solid state condition. This is suggested by the TG–DSC results

of WO3 and Cr2O3 mixture shown in Fig. 1c, indicating that 900 1C may be the proper temperature to react WO3 with Cr2O3 under solid state reaction condition. When calcining these mixtures at 800 1C and 950 1C, corresponding XRD patterns in Fig. 1a further confirm the Cr2WO6 is not able to crystallize until the heating temperature is increased to 950 1C for 20 h, presenting high activation energy is needed in the conventional solid state reaction process. Fig. 2a shows the good dispersity and uniformity of Hproduct Cr2O3 with an average size of 10 nm (inset image in Fig. 2a). This is further confirmed by the TEM and HRTEM results in Fig. 2d, in which the crystal lattice are determined to be (110) crystal plane of rhombohedral Cr2O3 (JCPDS Card No. 70-3765). Fig. 2b displays the HS-product Cr2WO6 morphology with increased particle size of 50 nm. The TEM and HRTEM images in Fig. 2e further identify the crystal lattice of (110)

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plane of tetragonal Cr2WO6 (JCPDS Card No. 35-0791) structures, which being in good accordance with the XRD results. By combining the above results together, it is revealed that using hydrothermal assisted method could successfully obtain Cr2WO6 nanocrystallites. Compared with the HS-product, the Cr2WO6 crystallites synthesized by the solid state reaction (Fig. 2c) present a morphology of microscale bipyramid-like structure. In conclusion, the hydrothermal process providing an effective method to prepare Cr2WO6 nanocrystals. 3.2. Adsorption activities of Cr2WO6 nanocrystallites prepared by different processes Dyes are the major sources for textile, leather, food, and paper industries and dumping of these wastes is a major environmental concern [17]. Different adsorbents has been reported for the selective adsorption of cationic and anionic dyes based on the surface charge of the adsorbents and their electrostatic interactions with the dye molecules [18,19]. In our research, the as-prepared Cr2WO6 nanoparticles suspended in aqueous media develop a negative zeta potential value(39 mV), which suggests the as-prepared sample with highly negatively charged surface under experiment conditions [20]. To further evaluate the potential application


of Cr2WO6 nanocrystallites in water treatment, the adsorption capacities of methyl orange (MO), methylene blue (MB) and Rhodamine B (RhB) were investigated, as is shown in Fig. 3a. The adsorption rate of the HS-product for MB, RhB and MO dye is 93%, 15% and 2%, respectively. This result suggests highly preferential adsorption of the HS-product on MB dye. In aqueous solution, MB exists in cationic form, whereas the presence of carboxylic group in the RhB dye makes it negatively charged. MO dye remains in anionic form in aqueous solution. The poor adsorption behavior of highly negatively charged HS-product towards MO and RhB may due to the mutually rejection of the same charge. The better adsorption of HS-product on MB dye is possibly because of the electrostatic interaction between the opposite charges. In contrast, it was found that the adsorption rate of the S-product on MB dye is only 5%. The better adsorption performance of the HS-product for MB may arise from its bigger specific surface area, as N2 adsorption– desorption isotherms for the samples show (Fig. 3b). The specific surface area of the HS-product and S-product is calculated to be 148.058 and 19.021 m2/g, respectively. The difference is because that the HS-product has small particle size (50 nm), whereas the S-product has larger particle size in micrometer. Usually, adsorbent with small particle size is

Fig. 3. (a) Adsorption characteristics of organic dye solution (MO, RhB and MB) on the different samples; (b) nitrogen adsorption–desorption isotherm of Cr2WO6 crystallites prepared using different methods.

Fig. 4. Schematic comparison of different synthesis processes and corresponding products.


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beneficial for the enhancement of adsorption capacity by enlarging its specific surface area. These results show that the hydrothermal condition plays a crucial role in the improvement of adsorption performance of Cr2WO6 nanocrystallites. It has been reported, typical adsorbent activated carbon and clays [21] present great adsorption performance to MB dyes. Comparing with these typical adsorbent, the adsorption activity of the prepared Cr2WO6 nanocrystallites still needs to be improved in the future research. Though, the hydrothermal assisted Cr2WO6 nanocrystallites in our research may provide good potential as adsorbent material for organic wastewater treatment applications. By concluding the above structure and property results, the schematic synthesis processes and the corresponding products are shown in Fig. 4. As illustrated the mixture of Cr2O3 nanocrystals and amorphous WO3 were obtained during hydrothermal reaction. Afterwards, the Cr2WO6 nanocrystals formed by a calcination process at 650 1C. As for the solid state reaction, difficulties were found to directly obtain pure Cr2WO6 at 800 1C for 20 h. At even higher temperature of 950 1C, Cr2WO6 microparticles were finally produced. Therefore, we found that using the hydrothermal assisted method may require much lower temperature to obtain the final Cr2WO6 nanocrystals, suggesting obvious lowered activation energy of the reaction in our approach.

4. Conclusion In summary, tetragonal Cr2WO6 nanocrystallites were obtained by using a facile hydrothermal assisted process. It reveals that the hydrothermal process may greatly reduce the product reaction temperature. Preferential adsorption performance of the Cr2WO6 nanocrystallites on cationic dye further presents a good application potential in the removal of particular dye from wastewater effluents. Besides, a facile approach to synthesis Cr2WO6 was developed in our work. This method may be applicable in large scale preparation of other tungstate materials that need high temperature in conventional solid state reactions.

Acknowledgments This work was financially supported by the National Key Technology R&D Program (No. 2013BAF09B02), Innovation Team Assistance Foundation of Shaanxi Province (2013KCT06), National Natural Science Foundation of China (51472152), Scientific Research Foundation (BJ14-16), and Graduate Innovation Fund of Shaanxi University of Science and Technology. References [1] L. Li, X.H. Liu, Q.F. Han, X.X. Yao, X. Wang, J. Mater. Chem. A 1 (2013) 1246–1253. [2] C.W. Lai, S. Sreekantan, Mater. Sci. Semicond. Process. 16 (2013) 303–310. [3] H.W. Zhou, Y.T. Shi, L. Wang, H. Zhang, C.Y. Zhao, A. Hagfeldt, T.L. Ma, Chem. Commun. 49 (2013) 7626–7628. [4] S.L. Bai, K.W. Zhang, R.X. Luo, D.Q. Li, A.F. Chen, C.C. Liu, Mater. Lett. 111 (2013) 32–34. [5] V. Balouria, A. Kumar, A. Singh, S. Samanta, A.K. Debnath, A. Mahajan, R.K. Bedi, D.K. Aswal, S.K. Gupta, J.V. Yakhmi, Sens. Actuators B: Chem. 157 (2011) 466–472. [6] Q. Diao, C.G. Yin, Y.W. Liu, J.G. Li, X. Gong, X.S. Liang, S.Q. Yang, H. Chen, G.Y. Lu, Sens. Actuators B: Chem. 180 (2013) 90–95. [7] C. Cantalin, J. Eur. Ceram. Soc. 24 (2004) 1421–1424. [8] G. Bayer, J. Am. Soc. 9 (1960) 495–496. [9] N.S. Saleh, J. Phys. C: Solid State Phys. 17 (1984) 3087–3090. [10] W. Kunnmann, S.L. Placa, L.M. Corliss, J.M. Hastings, J. Phys. Chem. Solids 29 (1968) 1359–1364. [11] Y. Fang, L.Y. Wang, Y.Q. Song, T. Tang, D.H. Wang, Y.W. Du, Appl. Phys. Lett. 104 (2014) 132908–132912. [12] K.T. Jacob, J. Mater. Sci. 15 (1980) 2167–2174. [13] L. Zhen, W.S. Wang, C.Y. Xu, W.Z. Shao, L.C. Qin, Mater. Lett. 62 (2008) 1740–1742. [14] L.K. Pan, X.J. Liu, Z. Sun, C.Q. Sun, J. Mater. Chem. A 1 (2013) 8299–8326. [15] M. Arshadi, F.S. Vahid, J.W.L. Salvacion, M. Soleymanzadeh, Appl. Surf. Sci. 280 (2013) 726–736. [16] M.A. Lazar, W.A. Daoud, RSC Adv. 2 (2012) 447–452. [17] T.S. Natarajan, H.C. Bajaj, R.J. Tayade, J. colloid interface sci. 433 (2014) 104–114. [18] Y.L. Na, Y.I. Kim, D.W. Cho, D. Pradhan, Y. Sohn, Mater. Sci. Semicond. Process. 27 (2014) 181–190. [19] G. Moussavi, R. Khosravi, Chem. Eng. Res. Des. 89 (2011) 2182–2189. [20] C.H. Lin, C.H. Gung, J.J. Sun, S.Y. Suen, J. Membr. Sci. 471 (2014) 285–298. [21] M. Rafatullah, O. Sulaiman, R. Hashim, A. Ahmad, J. hazard. Mater. 177 (2010) 70–80.