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Template-free hydrothermal synthesis and high photocatalytic activity of ZnWO4 nanorods
Materials Science and Engineering B 177 (2012) 1126–1132
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Template-free hydrothermal synthesis and high photocatalytic activity of ZnWO4 nanorods Bin Gao a,b , Huiqing Fan a,∗ , Xiaojun Zhang b , Lixun Song b a b
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China School of Science, Xi’an Polytechnic University, Xi’an 710048, China
a r t i c l e
i n f o
Article history: Received 28 January 2012 Received in revised form 4 May 2012 Accepted 28 May 2012 Available online 13 June 2012 Keywords: Photocatalytic Hydrothermal synthesis ZnWO4 nanorod Methylene blue
a b s t r a c t ZnWO4 nanorods are successfully synthesized by a template-free hydrothermal method, and are characterized in detail by X-ray diffractometer (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). The results show that the ZnWO4 nanorods with wolframite structure are well-crystallized single crystallites. The crystallinity of the products is influenced by the pH value of initial precursor suspension. The width and length of the synthesized samples increase with hydrothermal reaction temperature. The photocatalytic efficiency of the ZnWO4 nanorods for degradation of methylene blue (MB) in aqueous solution under UV light irradiation declines greatly with increasing crystallinity. The ZnWO4 nanorods prepared at pH of 4 have the best activity in photo-degradation of MB. After six recycles, photocatalytic activity loss of the catalyst is not obvious. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Until now, metal tungstates, with high luminescence efficiency, great density, strong resistance to radiation damage, and no deliquescence, etc., have a good application prospect in many fields [1–3]. As a member of metal tungstates family, ZnWO4 crystal with a monoclinic wolframite structure has a high application potential in various fields as an X-ray, ␥-scintillator, microwave system, solid-state laser host, photocatalyst, gas and humidity sensors, acoustic and optical fibers, and magnetic material due to its unique characteristics [4]. Namely, ZnWO4 crystal with advantages of an average high refractive index, a high X-ray absorption coefficient, high chemical stability, low radiation damage, high luminescence intensity, and long afterglow to luminescence, is considered a promising light-emitting material [5–7]. Recently, there have been a number of reports concerning the improvement of luminescence properties of ZnWO4 by introducing rare earth ions [8–11]. Owing to energy transfer from the excited tungstate groups to the rare earth ions the emission intensity of ZnWO4 is improved. As an important inorganic material, ZnWO4 crystal has been synthesized by various methods, such as Czochralski [12], solid state [13], solid-state metathetic [14], sol–gel [15], aqueous solution growth [16], polymerized complex matrix [17], molten salt [18], template [19], microwave solvothermal [20], self-propagating combustion [21], hydrothermal [22], and hydrothermal followed
by annealing [23]. Most of the aforementioned synthesis methods can only obtain macro-size ZnWO4 at high temperature and harsh reaction condition. It is well known that chemical and physical properties of a solid material are closely associated with size, morphology and micro-structure of the material. Therefore, exploring the controlling synthesis of ZnWO4 nanocrystal with a special morphology is one of the most challenging issues. Dyes are important organic pollutants, and their release as wastewater in the ecosystem is a dramatic source of esthetic pollution, eutrophication, and perturbations in aquatic life [24]. Photocatalytic degradation of organic compounds for the purpose of purifying wastewater from industries and households has attracted much attention in recent years [25–27]. So far, the ZnWO4 little endeavor has been dedicated to study the nanorods as a photocatalytic activity material for degradation of organic matters. The hydrothermal synthesis is a very popular synthesis method to synthesize photocatalysts and plays a key role in tailoring the properties of nanomaterials due to its less-cost, mild temperature, and potential controllability over size and morphology [28]. In this work, we reported the synthesis of ZnWO4 nanorods by the hydrothermal method without any templates or surfactants using sodium tungstate dihydrate (Na2 WO4 • 2H2 O) and zinc dichloride (ZnCl2 ) as mainly raw materials. The photocatalytic activity of the as-prepared samples for photodegradation of methylene blue (MB) was investigated. 2. Experimental
∗ Corresponding author. Tel.: +86 29 88494463; fax: +86 29 88492642. E-mail addresses: [email protected] (B. Gao), [email protected] (H. Fan). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.05.022
All the chemical reagents used in our experiments were analytical grade without further purification and were purchased from
B. Gao et al. / Materials Science and Engineering B 177 (2012) 1126–1132
Xi’an Chemical Reagent Co. Ltd. ZnWO4 nanorods were synthesized by a hydrothermal course. A typically experimental process is described as follows: 2 mmol (0.66 g) of Na2 WO4 • 2H2 O was dissolved in 30 ml de-ionized water in a glass beaker and stirred with a glass rod until the formation of saturated solution. 20 ml of aqueous solution of ZnCl2 with 0.1 mol dm−3 concentration was added drop-wise into the above-mentioned saturated solution under violent magnetic stirring, and then the solution immediately became white and opaque. The solution gradually became yellow transparent with stirring, subsequently became white suspension by sequentially adding to ZnCl2 solution. Finally, the white suspension no longer turned into transparent solution. Afterwards, the pH value of the white suspension was adjusted by the addition of either dilute hydrochloric acid or sodium hydroxide solution. After stirring for 1 h, the white pH-adjusted precursor suspension was transferred into a 50 ml Teflon-lined stainless steel autoclave and 80% filled to its capacity and then sealed, followed by the heating of the Teflon-lined stainless steel autoclave to 180 ◦ C in a stove with a rate of 5 ◦ C min−1 . The autoclave was maintained at 180 ◦ C for 12 h until the reaction was complete, and then cooled down to room temperature naturally. White precipitations were formed at the bottom of the autoclave. The products were centrifuged, filtered out, and rinsed with deionized water and absolute alcohol several times, finally dried at 80 ◦ C for 10 h in air, and light yellow powders were obtained. The ZnWO4 nanorods were finally synthesized. The crystal structures of the obtained products were detected by X-ray powder diffraction (XRD; D/Max-2400, Rigaku, Tokyo, Japan) with Cu K␣ radiation at = 0.15418 nm. The morphologies and microstructures of the synthesized products were characterized using a high-resolution transmission electron microscopy (HRTEM; JEM-3000F, JEOL, Tokyo, Japan) operating at 200 kV equipped with selected area electron diffraction (SAED). The photoluminescence (PL) spectra were measured at room temperature on a steady state fluorescence and phosphorescence lifetime spectrometer (FLSP 920, Edinburgh Instruments Ltd., Livingston, UK) with an excitation wavelength of 320 nm. The excitation source was steady-state Xe-arc lamp with an output power of 450 W. The preparation procedure of the samples for PL measurement is as follows: the synthetic ZnWO4 nanorods powder was added to the beaker filled with ethanol and dispersed ultrasonically for 30 min, configured as a suspension with concentration of 600 mg/l. Photocatalytic activity of the ZnWO4 nanorods was evaluated by decomposition of methylene blue (MB) under UV light irradiation. Ultraviolet source was a Hg 10 W lamp ( = 253.7 nm; CEL-LUV 254). The photoactivity experiments were performed at ambient temperature. Experimental details were as follows: 80 mg of the prepared ZnWO4 nanorods was dispersed into a 200 ml aqueous solution of MB with concentration of 15 mg dm−3 in a beaker. Prior to irradiation, the solution was magnetically stirred in the dark for ca. 30 min to ensure the adsorption equilibrium of the working solution. Then initial absorbance A0 of the solution was measured. The solution was placed below an ultraviolet lamp (10 W) with a wavelength of 253.7 nm for 2 h Analytical samples (10 ml) were drawn from the reaction solution every 10 min, and then catalyst was removed from the samples by centrifugation. The change of absorbance measured using a 721-type spectrophotometer at wavelength 662 nm was applied to identify the concentration of MB in the irradiated solution. The percentage of degradation is reported as A/A0 in which A is the maximum peak of the absorption spectra of MB for each irradiated time interval at 662 nm, and A0 is the absorbance of the initial concentration when adsorption/desorption equilibrium was achieved. To test its photocatalytic lifetime, the ZnWO4 nanorods were recycled and reused six times in the decomposition of MB under the same conditions. After each photocatalytic reaction, the aqueous solution was centrifuged to reuse the ZnWO4 nanorods dried at 100 ◦ C for next test.
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Fig. 1. XRD patterns of the ZnWO4 nanorods synthesized at different pH values.
3. Results and discussion X-ray powder diffraction analyses were employed to determine the crystalline structure and the phase composition of the assynthesized ZnWO4 nanorods, and X-ray radiation was produced from the Cu target ( = 0.15418 nm). Fig. 1 shows the XRD patterns of the ZnWO4 nanorods prepared at 180 ◦ C for 12 h by the hydrothermal process, where the pH values of the white precursor suspensions are 3, 4, 7, 9 and 10 respectively. It is showed that all the diffraction peaks in the three patterns for samples synthesized at pH of 4, 7 and 10 can be indexed to a pure monoclinic phase of crystallized ZnWO4 with wolframite structure (C2h point symmetry and P2/c space group), with calculated cell parameters of a = 0.4688 nm, b = 0.5721 nm and c = 0.4936 nm, which are well consistent with the values for standard monoclinic phase ZnWO4 (JCPDS, No. 15-0774, a = 0.4693 nm, b = 0.5721 nm and c = 0.4928 nm). It is interesting to note that other phase characteristic peaks are not found in the patterns of XRD, indicating that the products are pure phases. The sharp diffraction peaks indicate that well-crystallized ZnWO4 can be obtained under current synthetic conditions. XRD analyses also show that the intensity of XRD diffraction peaks of the ZnWO4 nanorods synthesized at pH of 7 reaches a maximum value, suggesting that this preparation condition is the optimal circumstance for crystalline of the synthesized ZnWO4 nanorods. The relatively broad peaks probably arise from the small size of the synthesized ZnWO4 nanorods. It can be seen that from Fig. 1 when the pH value of white pH-adjusted precursor suspension increased to 10, the XRD analysis shows that the products are a mixture of ZnWO4 and ZnO nanocrystals (JCPDS, No. 36-1451); at pH of 3, the product is WO3 nanocrystal (JCPDS, No. 72-1465). From the above results, it is indicated that pure monoclinic phase of crystallized ZnWO4 with wolframite structure is obtained only within the pH ranging from 4 to 9. Recently, Song et al. have reported that the initial pH value was an important factor in the synthesis of ZnWO4 nanorods [29] and Chen et al. accounted that the low pH value favors the formation of polytungstate species and Zn2+ ion, while the high one favors
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Fig. 2. TEM images of the ZnWO4 nanorods prepared at pH of (a) 4, (b) 7 and (c) 9 respectively.
the monotungstate ion WO4 2− and Zn(OH)2 [30], which are consistent with our results. When pH was lower than 4.0 or over 10.0, the formative polytungstate or Zn(OH)2 with high concentration prevents the formation of the crystalline ZnWO4 . The XRD patterns also show that the intensity of diffraction peaks of (1 1 1) is much stronger than the rest, which mean that the synthesized ZnWO4 nanocrystals exhibit a significant anisotropy and preferred growth in the crystal formation process. The morphology and size of the ZnWO4 nanorods were examined by a transmission electron microscopy (TEM). Fig. 2 shows the TEM images of the ZnWO4 nanorods prepared at 180 ◦ C and pH of 4, 7 and 9 respectively for 12 h by the hydrothermal process. The TEM images indicate that the products obtained at the three different pH values are all composed of large-scale nanorods and almost no difference, which suggests that almost 100% yield of ZnWO4 nanorods can be obtained under a wide pH range by the simple hydrothermal method. The synthesized nanorods exhibit a narrow distribution of width ranging from 70 to 80 nm and length of 900–1100 nm. The
nanorods are straight and have smooth round tips. From the above results, it can be seen that the morphology, dimension, and size of the samples are irrespective of the pH value of the precursor solution. Fig. 3 shows the TEM images of the ZnWO4 nanorods obtained at pH of 7 under different hydrothermal treating temperatures for 12 h. The products prepared at 120 ◦ C display a rodlike morphology with length of 300–400 nm and width of 30–50 nm. The nanorods exhibit nonuniform sizes (Fig. 3a). The samples obtained at 150 ◦ C are composed of large-scale nanorods. The diameter of the nanorods is about 40–60 nm and the length is approximately 500–700 nm, and the dimension of the nanorods has a narrower distribution compared with that synthesized at 120 ◦ C. The width and length of samples synthesized at the two reaction temperatures are both smaller than that at 180 ◦ C, and size-uniform of the nanorods is poorer compared with that of the latter (Fig. 2). These results suggest that the width and length of the prepared ZnWO4 nanorods increase with reaction temperature. The
B. Gao et al. / Materials Science and Engineering B 177 (2012) 1126–1132
Fig. 3. TEM images of the ZnWO4 samples obtained under different hydrothermal treating temperatures: (a) 120 ◦ C and (b) 150 ◦ C.
nanorods obtained at higher temperatures display a better uniformity. As previously reported [30], nanorods with low aspect ratio were transformed into well-crystalline nanorods with high aspect ratio when hydrothermal temperature was increased. Our results are consistent with this report, that is, short rod-like ZnWO4 nanostructures formed at low temperature might anisotropically grow along orientation to give long well-crystalline nanorods with increasing reaction temperature. A low magnification TEM image of a randomly chosen single ZnWO4 nanorod synthesized at 180 ◦ C and pH of 7 for 12 h is shown in Fig. 4a. The inset is its corresponding selected area electron diffraction (SAED) pattern recorded with incident electron beam perpendicular to the growth axis. We can see from the low magnification TEM image that the synthesized ZnWO4 nanorod with the width of about 85 nm and length of approximately 900 nm is straight and uniform and has smooth round tips. Fig. 4b displays a lattice fringes image of the individual ZnWO4 nanorod by
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high resolution transmission electron microscopy (HRTEM). The lattice spacing of 0.49 nm and 0.57 nm in the HRTEM image is corresponded to (0 0 1) and (0 1 0) planes of the ZnWO4 monoclinic cell respectively. It was evidenced that the lattice with an interplanar spacing of 0.49 nm is vertical to preferred growth direction, which is in agreement with the distance between the (0 0 1) crystal planes [31]. It could be inferred that the ZnWO4 nanorods grow anisotropically along the [0 0 1] direction. The SAED pattern (inset in Fig. 4a) shows clear diffraction spots, indicating that the nanorod synthesized by the present procedure is a single-crystal ZnWO4 . The SAED pattern can be attributed to the [1 0 0] zone axis diffraction of the ZnWO4 nanorod, namely, the top/bottom surfaces are (1 0 0) crystal faces and the side surfaces are (0 0 1)/(0 1 0) planes (Fig. 4b), from which suggest that the ZnWO4 nanorod with single crystal structure grow in the [0 0 1] direction. This is well consistent with the results of HRTEM and XRD analyses. We employed SAED to characterize different nanorods, as well as different parts of the same nanorod. All SAED patterns indicate almost that preferential growth of ZnWO4 nanorods is in the [0 0 1] direction and the ZnWO4 nanorods are well-crystallized single crystallites. Fig. 4c and d displays a lattice fringe image of a part of an individual ZnWO4 nanorod synthesized at pH value of 4 and 9 respectively. It can be seen from the images that the nanorods have the same lattice structure as that obtained at pH 7, indicating that the products synthesized at the two pH are ZnWO4 nanorods. The images also show that the nanorod synthesized at pH 4 has more structural defects within the detection limit than that obtained at pH 9, indicating that the sample has poorer crystalline quality than the latter. Moreover, the structural defect amount of the ZnWO4 nanorods obtained at the two pH vaules is larger than that of nanorod prepared at pH of 7 (Fig. 4b). Therefore, the pH value in the final prepared opaque suspension has a prominent influence on crystallity of the ZnWO4 nanorods. PL spectra were measured at room temperature on a steady state fluorescence and phosphorescence lifetime spectrometer with an excitation wavelength of 320 nm. A steady-state Xe-arc lamp with an output power of 450 W was employed as excitation source. Fig. 5 shows the room temperature PL spectra of the ZnWO4 nanorods prepared at 180 ◦ C and different pH values. It can be seen that the luminescence spectra of the ZnWO4 nanorods exhibit broad blue-green emission bands in the range of 400–550 nm peaking at 465 nm with a shoulder at 496 nm. The PL spectra also demonstrate that the excitonic PL intensity of the ZnWO4 nanorods prepared at different pH values is variable. The intensity of the PL emission of the ZnWO4 nanorods prepared at pH of 4 is maximum, and is minimum at pH 7. Comparing the results with TEM and XRD characteristics, we can see the intensity of the PL emission is remarkably associated with the crystalline of the ZnWO4 nanorods and increases with the crystallinity decrease. It is known that excitonic PL of semiconductor nanoparticles mainly results from surface oxygen vacancies and defects [32]. The emission spectrum of the metal tungstates might be ascribed to the charge-transfer transitions within the [WO4 ] clusters. During the excitation process at room temperature, the electrons situated at lower intermediary energy levels (oxygen 2p states) absorb the photon energies at 320 nm. As consequence of this phenomenon, the energetic electrons are promoted to higher intermediary energy levels (tungsten 5d states) located near the conduction band. When the electrons fall back to lower energy states again via radiative return processes, the energies arising from this electronic transition are converted in photons. Thus, the crystallinity of semiconductor (ZnWO4 nanorods) is poorer, the amount of surface oxygen vacancy and defect is higher, the absorption of the excitation light is stronger, and the intensity of excitonic PL spectrum is higher. The photocatalytic activity of the ZnWO4 nanorods prepared at 180 ◦ C and different pH values was evaluated by photodegradation
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Fig. 4. TEM images of a randomly chosen single ZnWO4 nanorod synthesized at 180 ◦ C and different pH values for 12 h: (a) a low magnification TEM image of the single ZnWO4 nanorod synthesized at pH of 7. The inset is its corresponding SAED pattern, which reveals the [0 0 1] preferred growth direction of the ZnWO4 nanorods. (b), (c) and (d) HRTEM image of the individual ZnWO4 nanorod synthesized at pH value of 7, 4 and 9, respectively.
of methylene blue (MB) under UV light irradiation. The photoactivity reaction experiments were performed at room temperature. Ultraviolet source was a 10 W Hg lamp ( = 253.7 nm). Fig. 6 shows the photocatalytic degradation rates of methylene blue (MB) under UV light irradiation on ZnWO4 nanorods synthesized at pH of 4, 7 and 9 respectively. It can be seen that the sample obtained at pH of 4 has the highest photocatalytic activity, and the conversion of MB can be nearly up to 100% after 60 min irradiation. The black experiment without catalyst of ZnWO4 nanorods shows that MB is almost not decomposed under the same irradiation time, indicating that the ZnWO4 nanorods photocatalyst leads to the photodegradation of MB. It is also seen from Fig. 6 that the pH value of white precursor suspension has a significant effect on the photocatalytic activity of the as-prepared ZnWO4 nanorods. Photocatalytic activity of the sample synthesized at pH of 7 is the lowest. The product prepared at higher pH value (pH = 9) or lower pH value (pH = 4) exhibits higher
ability to decompose MB compared with the one prepared at pH of 7. To combine the results with XRD analyses, where the intensity of XRD diffraction peaks of the ZnWO4 nanorods synthesized at pH of 7 is maximum, we can acquire that the crystalline structure is a vital factor that affects the photocatalytic activity of the material in the present work. It is well known that the photocatalytic activity of photocatalyst mainly results from the photo-induced electrons and holes [33]. The produced holes can react with H2 O or OH– adsorbed on the active material (photocatalyst) surface to generate hydroxyl groups (• OH), and the electrons deacidize O2 into • O − , which reacts with H O to create hydroxyl radicals (• OH). The 2 2 hydroxyl radical is very strong oxidizing agents, which can react with most organic matters to produce CO2 , H2 O and other products [34]. Equations are as follows: e− + O2 → • O2 −
B. Gao et al. / Materials Science and Engineering B 177 (2012) 1126–1132
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Fig. 5. PL spectra of the ZnWO4 nanorods obtained at different pH values at 320 nm excitation.
•O − 2
+ H2 O → • OH + OH−
h+ + OH− → • OH • OH
+ MB → degradationproduct
As is known to all, lattice defects may act as traps for electrons and holes. The crystallinity of catalyst is poorer, the amount of surface oxygen vacancy and defect is higher, and the trap is much. During the process of photocatalytic reaction of degradation of MB with ZnWO4 nanorods, oxygen vacancies and defects can act as photo-generated holes and photo-generated electrons traps, so that the recombination of the photo-induced electrons and holes can be effectively inhibited. Therefore, the photocatalytic activity of the ZnWO4 nanorods increases with decrease of photocatalysis crystallinity [35]. One explanation for the rapid increase of the photocatalytic activity of the ZnWO4 nanorods obtained at pH value departing from 7 might be that the abundant lattice defects rapidly increase due to poorer crystallinity of samples and act as
Fig. 7. Stability for photocatalytic activity of the ZnWO4 nanorods synthesized at pH of 4.
photo-generated holes and photo-generated electrons traps. On the other hand, the sample prepared at pH value of 7 exhibits the lowest activity due to its highest crystallinity. The stability of photocatalytic activity of the ZnWO4 nanorods synthesized at pH of 4 was investigated due to its importance in application. Fig. 7 displays lifetime for photo-degradation of MB by the sample. After six recycles for the photo-degradation of MB, the ZnWO4 nanorods catalyst did not exhibit any obvious loss of photocatalytic activity, indicating that the ZnWO4 nanorods are not photodeteriorative during the photocatalytic decomposition of the pollutant. 4. Conclusions Using sodium tungstate dihydrate (Na2 WO4 • 2H2 O) and zinc dichloride (ZnCl2 ) as mainly raw materials, ZnWO4 nanorods with single crystal structure are successfully synthesized by a templatefree hydrothermal method. The well-crystallized ZnWO4 nanorods have a wolframite structure. The crystallinity of the products is influenced by the pH value of initial precursor suspension. The aspect ratio and size uniform of the ZnWO4 nanorods increase with reaction temperature. The photocatalytic efficiency of the ZnWO4 nanorods for degradation of methylene blue (MB) in aqueous solution under UV light irradiation declines greatly with increase of the catalyst crystallinity. The ZnWO4 nanorods prepared at pH of 4 have the best activity in the photo-degradation of MB. The ZnWO4 nanorods exhibit excellent stability of photocatalytic activity. After six recycles, photocatalytic activity loss of the catalyst is not obvious. Acknowledgments This work was supported by the National Nature Science Foundation (51172187), the SPDRF and 111 Program (B08040) of MOE of China. References
Fig. 6. Photocatalytic degradation efficiency of MB on ZnWO4 nanorods synthesized under different pH values.
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Template-free hydrothermal synthesis and high photocatalytic activity of ZnWO4 nanorods Bin Gao a,b , Huiqing Fan a,∗ , Xiaojun Zhang b , Lixun Song b a b
State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China School of Science, Xi’an Polytechnic University, Xi’an 710048, China
a r t i c l e
i n f o
Article history: Received 28 January 2012 Received in revised form 4 May 2012 Accepted 28 May 2012 Available online 13 June 2012 Keywords: Photocatalytic Hydrothermal synthesis ZnWO4 nanorod Methylene blue
a b s t r a c t ZnWO4 nanorods are successfully synthesized by a template-free hydrothermal method, and are characterized in detail by X-ray diffractometer (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). The results show that the ZnWO4 nanorods with wolframite structure are well-crystallized single crystallites. The crystallinity of the products is influenced by the pH value of initial precursor suspension. The width and length of the synthesized samples increase with hydrothermal reaction temperature. The photocatalytic efficiency of the ZnWO4 nanorods for degradation of methylene blue (MB) in aqueous solution under UV light irradiation declines greatly with increasing crystallinity. The ZnWO4 nanorods prepared at pH of 4 have the best activity in photo-degradation of MB. After six recycles, photocatalytic activity loss of the catalyst is not obvious. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Until now, metal tungstates, with high luminescence efficiency, great density, strong resistance to radiation damage, and no deliquescence, etc., have a good application prospect in many fields [1–3]. As a member of metal tungstates family, ZnWO4 crystal with a monoclinic wolframite structure has a high application potential in various fields as an X-ray, ␥-scintillator, microwave system, solid-state laser host, photocatalyst, gas and humidity sensors, acoustic and optical fibers, and magnetic material due to its unique characteristics [4]. Namely, ZnWO4 crystal with advantages of an average high refractive index, a high X-ray absorption coefficient, high chemical stability, low radiation damage, high luminescence intensity, and long afterglow to luminescence, is considered a promising light-emitting material [5–7]. Recently, there have been a number of reports concerning the improvement of luminescence properties of ZnWO4 by introducing rare earth ions [8–11]. Owing to energy transfer from the excited tungstate groups to the rare earth ions the emission intensity of ZnWO4 is improved. As an important inorganic material, ZnWO4 crystal has been synthesized by various methods, such as Czochralski [12], solid state [13], solid-state metathetic [14], sol–gel [15], aqueous solution growth [16], polymerized complex matrix [17], molten salt [18], template [19], microwave solvothermal [20], self-propagating combustion [21], hydrothermal [22], and hydrothermal followed
by annealing [23]. Most of the aforementioned synthesis methods can only obtain macro-size ZnWO4 at high temperature and harsh reaction condition. It is well known that chemical and physical properties of a solid material are closely associated with size, morphology and micro-structure of the material. Therefore, exploring the controlling synthesis of ZnWO4 nanocrystal with a special morphology is one of the most challenging issues. Dyes are important organic pollutants, and their release as wastewater in the ecosystem is a dramatic source of esthetic pollution, eutrophication, and perturbations in aquatic life [24]. Photocatalytic degradation of organic compounds for the purpose of purifying wastewater from industries and households has attracted much attention in recent years [25–27]. So far, the ZnWO4 little endeavor has been dedicated to study the nanorods as a photocatalytic activity material for degradation of organic matters. The hydrothermal synthesis is a very popular synthesis method to synthesize photocatalysts and plays a key role in tailoring the properties of nanomaterials due to its less-cost, mild temperature, and potential controllability over size and morphology [28]. In this work, we reported the synthesis of ZnWO4 nanorods by the hydrothermal method without any templates or surfactants using sodium tungstate dihydrate (Na2 WO4 • 2H2 O) and zinc dichloride (ZnCl2 ) as mainly raw materials. The photocatalytic activity of the as-prepared samples for photodegradation of methylene blue (MB) was investigated. 2. Experimental
∗ Corresponding author. Tel.: +86 29 88494463; fax: +86 29 88492642. E-mail addresses: [email protected] (B. Gao), [email protected] (H. Fan). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.05.022
All the chemical reagents used in our experiments were analytical grade without further purification and were purchased from
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Xi’an Chemical Reagent Co. Ltd. ZnWO4 nanorods were synthesized by a hydrothermal course. A typically experimental process is described as follows: 2 mmol (0.66 g) of Na2 WO4 • 2H2 O was dissolved in 30 ml de-ionized water in a glass beaker and stirred with a glass rod until the formation of saturated solution. 20 ml of aqueous solution of ZnCl2 with 0.1 mol dm−3 concentration was added drop-wise into the above-mentioned saturated solution under violent magnetic stirring, and then the solution immediately became white and opaque. The solution gradually became yellow transparent with stirring, subsequently became white suspension by sequentially adding to ZnCl2 solution. Finally, the white suspension no longer turned into transparent solution. Afterwards, the pH value of the white suspension was adjusted by the addition of either dilute hydrochloric acid or sodium hydroxide solution. After stirring for 1 h, the white pH-adjusted precursor suspension was transferred into a 50 ml Teflon-lined stainless steel autoclave and 80% filled to its capacity and then sealed, followed by the heating of the Teflon-lined stainless steel autoclave to 180 ◦ C in a stove with a rate of 5 ◦ C min−1 . The autoclave was maintained at 180 ◦ C for 12 h until the reaction was complete, and then cooled down to room temperature naturally. White precipitations were formed at the bottom of the autoclave. The products were centrifuged, filtered out, and rinsed with deionized water and absolute alcohol several times, finally dried at 80 ◦ C for 10 h in air, and light yellow powders were obtained. The ZnWO4 nanorods were finally synthesized. The crystal structures of the obtained products were detected by X-ray powder diffraction (XRD; D/Max-2400, Rigaku, Tokyo, Japan) with Cu K␣ radiation at = 0.15418 nm. The morphologies and microstructures of the synthesized products were characterized using a high-resolution transmission electron microscopy (HRTEM; JEM-3000F, JEOL, Tokyo, Japan) operating at 200 kV equipped with selected area electron diffraction (SAED). The photoluminescence (PL) spectra were measured at room temperature on a steady state fluorescence and phosphorescence lifetime spectrometer (FLSP 920, Edinburgh Instruments Ltd., Livingston, UK) with an excitation wavelength of 320 nm. The excitation source was steady-state Xe-arc lamp with an output power of 450 W. The preparation procedure of the samples for PL measurement is as follows: the synthetic ZnWO4 nanorods powder was added to the beaker filled with ethanol and dispersed ultrasonically for 30 min, configured as a suspension with concentration of 600 mg/l. Photocatalytic activity of the ZnWO4 nanorods was evaluated by decomposition of methylene blue (MB) under UV light irradiation. Ultraviolet source was a Hg 10 W lamp ( = 253.7 nm; CEL-LUV 254). The photoactivity experiments were performed at ambient temperature. Experimental details were as follows: 80 mg of the prepared ZnWO4 nanorods was dispersed into a 200 ml aqueous solution of MB with concentration of 15 mg dm−3 in a beaker. Prior to irradiation, the solution was magnetically stirred in the dark for ca. 30 min to ensure the adsorption equilibrium of the working solution. Then initial absorbance A0 of the solution was measured. The solution was placed below an ultraviolet lamp (10 W) with a wavelength of 253.7 nm for 2 h Analytical samples (10 ml) were drawn from the reaction solution every 10 min, and then catalyst was removed from the samples by centrifugation. The change of absorbance measured using a 721-type spectrophotometer at wavelength 662 nm was applied to identify the concentration of MB in the irradiated solution. The percentage of degradation is reported as A/A0 in which A is the maximum peak of the absorption spectra of MB for each irradiated time interval at 662 nm, and A0 is the absorbance of the initial concentration when adsorption/desorption equilibrium was achieved. To test its photocatalytic lifetime, the ZnWO4 nanorods were recycled and reused six times in the decomposition of MB under the same conditions. After each photocatalytic reaction, the aqueous solution was centrifuged to reuse the ZnWO4 nanorods dried at 100 ◦ C for next test.
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Fig. 1. XRD patterns of the ZnWO4 nanorods synthesized at different pH values.
3. Results and discussion X-ray powder diffraction analyses were employed to determine the crystalline structure and the phase composition of the assynthesized ZnWO4 nanorods, and X-ray radiation was produced from the Cu target ( = 0.15418 nm). Fig. 1 shows the XRD patterns of the ZnWO4 nanorods prepared at 180 ◦ C for 12 h by the hydrothermal process, where the pH values of the white precursor suspensions are 3, 4, 7, 9 and 10 respectively. It is showed that all the diffraction peaks in the three patterns for samples synthesized at pH of 4, 7 and 10 can be indexed to a pure monoclinic phase of crystallized ZnWO4 with wolframite structure (C2h point symmetry and P2/c space group), with calculated cell parameters of a = 0.4688 nm, b = 0.5721 nm and c = 0.4936 nm, which are well consistent with the values for standard monoclinic phase ZnWO4 (JCPDS, No. 15-0774, a = 0.4693 nm, b = 0.5721 nm and c = 0.4928 nm). It is interesting to note that other phase characteristic peaks are not found in the patterns of XRD, indicating that the products are pure phases. The sharp diffraction peaks indicate that well-crystallized ZnWO4 can be obtained under current synthetic conditions. XRD analyses also show that the intensity of XRD diffraction peaks of the ZnWO4 nanorods synthesized at pH of 7 reaches a maximum value, suggesting that this preparation condition is the optimal circumstance for crystalline of the synthesized ZnWO4 nanorods. The relatively broad peaks probably arise from the small size of the synthesized ZnWO4 nanorods. It can be seen that from Fig. 1 when the pH value of white pH-adjusted precursor suspension increased to 10, the XRD analysis shows that the products are a mixture of ZnWO4 and ZnO nanocrystals (JCPDS, No. 36-1451); at pH of 3, the product is WO3 nanocrystal (JCPDS, No. 72-1465). From the above results, it is indicated that pure monoclinic phase of crystallized ZnWO4 with wolframite structure is obtained only within the pH ranging from 4 to 9. Recently, Song et al. have reported that the initial pH value was an important factor in the synthesis of ZnWO4 nanorods [29] and Chen et al. accounted that the low pH value favors the formation of polytungstate species and Zn2+ ion, while the high one favors
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Fig. 2. TEM images of the ZnWO4 nanorods prepared at pH of (a) 4, (b) 7 and (c) 9 respectively.
the monotungstate ion WO4 2− and Zn(OH)2 [30], which are consistent with our results. When pH was lower than 4.0 or over 10.0, the formative polytungstate or Zn(OH)2 with high concentration prevents the formation of the crystalline ZnWO4 . The XRD patterns also show that the intensity of diffraction peaks of (1 1 1) is much stronger than the rest, which mean that the synthesized ZnWO4 nanocrystals exhibit a significant anisotropy and preferred growth in the crystal formation process. The morphology and size of the ZnWO4 nanorods were examined by a transmission electron microscopy (TEM). Fig. 2 shows the TEM images of the ZnWO4 nanorods prepared at 180 ◦ C and pH of 4, 7 and 9 respectively for 12 h by the hydrothermal process. The TEM images indicate that the products obtained at the three different pH values are all composed of large-scale nanorods and almost no difference, which suggests that almost 100% yield of ZnWO4 nanorods can be obtained under a wide pH range by the simple hydrothermal method. The synthesized nanorods exhibit a narrow distribution of width ranging from 70 to 80 nm and length of 900–1100 nm. The
nanorods are straight and have smooth round tips. From the above results, it can be seen that the morphology, dimension, and size of the samples are irrespective of the pH value of the precursor solution. Fig. 3 shows the TEM images of the ZnWO4 nanorods obtained at pH of 7 under different hydrothermal treating temperatures for 12 h. The products prepared at 120 ◦ C display a rodlike morphology with length of 300–400 nm and width of 30–50 nm. The nanorods exhibit nonuniform sizes (Fig. 3a). The samples obtained at 150 ◦ C are composed of large-scale nanorods. The diameter of the nanorods is about 40–60 nm and the length is approximately 500–700 nm, and the dimension of the nanorods has a narrower distribution compared with that synthesized at 120 ◦ C. The width and length of samples synthesized at the two reaction temperatures are both smaller than that at 180 ◦ C, and size-uniform of the nanorods is poorer compared with that of the latter (Fig. 2). These results suggest that the width and length of the prepared ZnWO4 nanorods increase with reaction temperature. The
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Fig. 3. TEM images of the ZnWO4 samples obtained under different hydrothermal treating temperatures: (a) 120 ◦ C and (b) 150 ◦ C.
nanorods obtained at higher temperatures display a better uniformity. As previously reported [30], nanorods with low aspect ratio were transformed into well-crystalline nanorods with high aspect ratio when hydrothermal temperature was increased. Our results are consistent with this report, that is, short rod-like ZnWO4 nanostructures formed at low temperature might anisotropically grow along orientation to give long well-crystalline nanorods with increasing reaction temperature. A low magnification TEM image of a randomly chosen single ZnWO4 nanorod synthesized at 180 ◦ C and pH of 7 for 12 h is shown in Fig. 4a. The inset is its corresponding selected area electron diffraction (SAED) pattern recorded with incident electron beam perpendicular to the growth axis. We can see from the low magnification TEM image that the synthesized ZnWO4 nanorod with the width of about 85 nm and length of approximately 900 nm is straight and uniform and has smooth round tips. Fig. 4b displays a lattice fringes image of the individual ZnWO4 nanorod by
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high resolution transmission electron microscopy (HRTEM). The lattice spacing of 0.49 nm and 0.57 nm in the HRTEM image is corresponded to (0 0 1) and (0 1 0) planes of the ZnWO4 monoclinic cell respectively. It was evidenced that the lattice with an interplanar spacing of 0.49 nm is vertical to preferred growth direction, which is in agreement with the distance between the (0 0 1) crystal planes [31]. It could be inferred that the ZnWO4 nanorods grow anisotropically along the [0 0 1] direction. The SAED pattern (inset in Fig. 4a) shows clear diffraction spots, indicating that the nanorod synthesized by the present procedure is a single-crystal ZnWO4 . The SAED pattern can be attributed to the [1 0 0] zone axis diffraction of the ZnWO4 nanorod, namely, the top/bottom surfaces are (1 0 0) crystal faces and the side surfaces are (0 0 1)/(0 1 0) planes (Fig. 4b), from which suggest that the ZnWO4 nanorod with single crystal structure grow in the [0 0 1] direction. This is well consistent with the results of HRTEM and XRD analyses. We employed SAED to characterize different nanorods, as well as different parts of the same nanorod. All SAED patterns indicate almost that preferential growth of ZnWO4 nanorods is in the [0 0 1] direction and the ZnWO4 nanorods are well-crystallized single crystallites. Fig. 4c and d displays a lattice fringe image of a part of an individual ZnWO4 nanorod synthesized at pH value of 4 and 9 respectively. It can be seen from the images that the nanorods have the same lattice structure as that obtained at pH 7, indicating that the products synthesized at the two pH are ZnWO4 nanorods. The images also show that the nanorod synthesized at pH 4 has more structural defects within the detection limit than that obtained at pH 9, indicating that the sample has poorer crystalline quality than the latter. Moreover, the structural defect amount of the ZnWO4 nanorods obtained at the two pH vaules is larger than that of nanorod prepared at pH of 7 (Fig. 4b). Therefore, the pH value in the final prepared opaque suspension has a prominent influence on crystallity of the ZnWO4 nanorods. PL spectra were measured at room temperature on a steady state fluorescence and phosphorescence lifetime spectrometer with an excitation wavelength of 320 nm. A steady-state Xe-arc lamp with an output power of 450 W was employed as excitation source. Fig. 5 shows the room temperature PL spectra of the ZnWO4 nanorods prepared at 180 ◦ C and different pH values. It can be seen that the luminescence spectra of the ZnWO4 nanorods exhibit broad blue-green emission bands in the range of 400–550 nm peaking at 465 nm with a shoulder at 496 nm. The PL spectra also demonstrate that the excitonic PL intensity of the ZnWO4 nanorods prepared at different pH values is variable. The intensity of the PL emission of the ZnWO4 nanorods prepared at pH of 4 is maximum, and is minimum at pH 7. Comparing the results with TEM and XRD characteristics, we can see the intensity of the PL emission is remarkably associated with the crystalline of the ZnWO4 nanorods and increases with the crystallinity decrease. It is known that excitonic PL of semiconductor nanoparticles mainly results from surface oxygen vacancies and defects [32]. The emission spectrum of the metal tungstates might be ascribed to the charge-transfer transitions within the [WO4 ] clusters. During the excitation process at room temperature, the electrons situated at lower intermediary energy levels (oxygen 2p states) absorb the photon energies at 320 nm. As consequence of this phenomenon, the energetic electrons are promoted to higher intermediary energy levels (tungsten 5d states) located near the conduction band. When the electrons fall back to lower energy states again via radiative return processes, the energies arising from this electronic transition are converted in photons. Thus, the crystallinity of semiconductor (ZnWO4 nanorods) is poorer, the amount of surface oxygen vacancy and defect is higher, the absorption of the excitation light is stronger, and the intensity of excitonic PL spectrum is higher. The photocatalytic activity of the ZnWO4 nanorods prepared at 180 ◦ C and different pH values was evaluated by photodegradation
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Fig. 4. TEM images of a randomly chosen single ZnWO4 nanorod synthesized at 180 ◦ C and different pH values for 12 h: (a) a low magnification TEM image of the single ZnWO4 nanorod synthesized at pH of 7. The inset is its corresponding SAED pattern, which reveals the [0 0 1] preferred growth direction of the ZnWO4 nanorods. (b), (c) and (d) HRTEM image of the individual ZnWO4 nanorod synthesized at pH value of 7, 4 and 9, respectively.
of methylene blue (MB) under UV light irradiation. The photoactivity reaction experiments were performed at room temperature. Ultraviolet source was a 10 W Hg lamp ( = 253.7 nm). Fig. 6 shows the photocatalytic degradation rates of methylene blue (MB) under UV light irradiation on ZnWO4 nanorods synthesized at pH of 4, 7 and 9 respectively. It can be seen that the sample obtained at pH of 4 has the highest photocatalytic activity, and the conversion of MB can be nearly up to 100% after 60 min irradiation. The black experiment without catalyst of ZnWO4 nanorods shows that MB is almost not decomposed under the same irradiation time, indicating that the ZnWO4 nanorods photocatalyst leads to the photodegradation of MB. It is also seen from Fig. 6 that the pH value of white precursor suspension has a significant effect on the photocatalytic activity of the as-prepared ZnWO4 nanorods. Photocatalytic activity of the sample synthesized at pH of 7 is the lowest. The product prepared at higher pH value (pH = 9) or lower pH value (pH = 4) exhibits higher
ability to decompose MB compared with the one prepared at pH of 7. To combine the results with XRD analyses, where the intensity of XRD diffraction peaks of the ZnWO4 nanorods synthesized at pH of 7 is maximum, we can acquire that the crystalline structure is a vital factor that affects the photocatalytic activity of the material in the present work. It is well known that the photocatalytic activity of photocatalyst mainly results from the photo-induced electrons and holes [33]. The produced holes can react with H2 O or OH– adsorbed on the active material (photocatalyst) surface to generate hydroxyl groups (• OH), and the electrons deacidize O2 into • O − , which reacts with H O to create hydroxyl radicals (• OH). The 2 2 hydroxyl radical is very strong oxidizing agents, which can react with most organic matters to produce CO2 , H2 O and other products [34]. Equations are as follows: e− + O2 → • O2 −
B. Gao et al. / Materials Science and Engineering B 177 (2012) 1126–1132
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Fig. 5. PL spectra of the ZnWO4 nanorods obtained at different pH values at 320 nm excitation.
•O − 2
+ H2 O → • OH + OH−
h+ + OH− → • OH • OH
+ MB → degradationproduct
As is known to all, lattice defects may act as traps for electrons and holes. The crystallinity of catalyst is poorer, the amount of surface oxygen vacancy and defect is higher, and the trap is much. During the process of photocatalytic reaction of degradation of MB with ZnWO4 nanorods, oxygen vacancies and defects can act as photo-generated holes and photo-generated electrons traps, so that the recombination of the photo-induced electrons and holes can be effectively inhibited. Therefore, the photocatalytic activity of the ZnWO4 nanorods increases with decrease of photocatalysis crystallinity [35]. One explanation for the rapid increase of the photocatalytic activity of the ZnWO4 nanorods obtained at pH value departing from 7 might be that the abundant lattice defects rapidly increase due to poorer crystallinity of samples and act as
Fig. 7. Stability for photocatalytic activity of the ZnWO4 nanorods synthesized at pH of 4.
photo-generated holes and photo-generated electrons traps. On the other hand, the sample prepared at pH value of 7 exhibits the lowest activity due to its highest crystallinity. The stability of photocatalytic activity of the ZnWO4 nanorods synthesized at pH of 4 was investigated due to its importance in application. Fig. 7 displays lifetime for photo-degradation of MB by the sample. After six recycles for the photo-degradation of MB, the ZnWO4 nanorods catalyst did not exhibit any obvious loss of photocatalytic activity, indicating that the ZnWO4 nanorods are not photodeteriorative during the photocatalytic decomposition of the pollutant. 4. Conclusions Using sodium tungstate dihydrate (Na2 WO4 • 2H2 O) and zinc dichloride (ZnCl2 ) as mainly raw materials, ZnWO4 nanorods with single crystal structure are successfully synthesized by a templatefree hydrothermal method. The well-crystallized ZnWO4 nanorods have a wolframite structure. The crystallinity of the products is influenced by the pH value of initial precursor suspension. The aspect ratio and size uniform of the ZnWO4 nanorods increase with reaction temperature. The photocatalytic efficiency of the ZnWO4 nanorods for degradation of methylene blue (MB) in aqueous solution under UV light irradiation declines greatly with increase of the catalyst crystallinity. The ZnWO4 nanorods prepared at pH of 4 have the best activity in the photo-degradation of MB. The ZnWO4 nanorods exhibit excellent stability of photocatalytic activity. After six recycles, photocatalytic activity loss of the catalyst is not obvious. Acknowledgments This work was supported by the National Nature Science Foundation (51172187), the SPDRF and 111 Program (B08040) of MOE of China. References
Fig. 6. Photocatalytic degradation efficiency of MB on ZnWO4 nanorods synthesized under different pH values.
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