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Zinc vanadate nanorods and their visible light photocatalytic activity
Journal of Alloys and Compounds 631 (2015) 90–98
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Zinc vanadate nanorods and their visible light photocatalytic activity L.Z. Pei ⇑, N. Lin, T. Wei, H.D. Liu, H.Y. Yu Key Lab of Materials Science and Processing of Anhui Province, School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan, Anhui 243002, PR China
a r t i c l e
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Article history: Received 29 November 2014 Received in revised form 10 January 2015 Accepted 18 January 2015 Available online 24 January 2015 Keywords: Zinc vanadate nanorods Crystal growth Electron microscopy Solar light Photocatalytic activity
a b s t r a c t Zinc vanadate nanorods have been synthesized by a simple hydrothermal process using zinc acetate and sodium vanadate as the raw materials. The zinc vanadate nanorods have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), highresolution TEM (HRTEM) and solid UV–vis diffuse reflectance spectrum. XRD pattern and HRTEM image show that the zinc vanadate nanorods are composed of single crystalline monoclinic Zn2V2O7 phase. SEM and TEM observations show that the diameter and length of the zinc vanadate nanorods are 50–100 nm and about 5 lm, respectively. Sodium dodecyl sulfonate (SDS) has an essential role in the formation of zinc vanadate nanorods. The SDS-assisted nucleation and growth process have been proposed to explain the formation and growth of the zinc vanadate nanorods. Solid UV–vis diffuse reflectance spectrum shows that the zinc vanadate nanorods have a band gap of 2.76 eV. The photocatalytic activities of the zinc vanadate nanorods have been evaluated by the photocatalytic degradation of methylene blue (MB) under solar light irradiation. The MB with the concentration of 10 mg L1 can be degraded totally under the solar light irradiation for 4 h. It is suggested that the zinc vanadate nanorods exhibit promising application potential for the degradation of organic pollutants under solar light irradiation. Ó 2015 Published by Elsevier B.V.
1. Introduction Great research interest has been devoted to the removal of organic pollutes owing to their long term environmental toxicity and public health damage to humans [1–3]. Various methods, such as coagulation [4], adsorption [5], membrane filtration [6] and photocatalytic technology [7] have been used to remove organic pollutants from aqueous environment. Among these methods, photocatalytic technology is especially attractive due to its high efficiency, simplicity of design and simple operation process. However, it requires photocatalysts with narrow band gap, large specific surface area and well-defined morphology, which can remove organic pollutants under visible light. Zinc vanadate nanomaterials fit these criteria well and have been extensively considered as the potential materials in the fields of photocatalytic treatment of organic pollutants, lithium batteries, hydrogen storage, electrochemical supercapacitors and various new application fields. Double-shelled ZnV2O4 hollow nanostructures have been synthesized through a facile one-pot template-free solvothermal method using ZnCl2 and vanadium (IV) oxide bis (2,4pentanedionate) (VO(acac)2) as the raw materials, N,N-dimethyl formamide (DMF) as the solvent [8]. And the inward–outward
⇑ Corresponding author. Fax: +86 5552311570. E-mail addresses: [email protected], [email protected] (L.Z. Pei). http://dx.doi.org/10.1016/j.jallcom.2015.01.115 0925-8388/Ó 2015 Published by Elsevier B.V.
ripening formation mechanism has been proposed to explain the formation and growth of the ZnV2O4 hollow spheres. Zinc nitrate can also be used as the Zn raw material for the formation of ZnV2O4 hollow spheres [1]. The ZnV2O4 hollow spheres show a good adsorption capacity of methylene blue (MB) organic pollutant. Clewlike hollow ZnV2O4 spheres have also been synthesized by the reaction between zinc nitrate hexahydrate and ammonium metavanadate in benzyl alcohol [9]. The clewlike hollow ZnV2O4 spheres exhibited stable cycling performance to maintain a capacity of 524 mA hg1 over 50 cycles suggesting that the clewlike ZnV2O4 hollow spheres were promising for lithium ion batteries. Butt et al. [10] reported the synthesis of ZnV2O4 hierarchical nanospheres using NH4VO3 and Zn(NO3)2 as the raw materials, oxalic acid dehydrated as the chelating agent, H2O2 and HNO3 as the solvents. The ZnV2O4 hierarchical nanospheres were used as the electrochemical supercapacitor electrode showing good capacitance and retention capacity of 89% after 1000 cycles. Sun et al. [11] reported the preparation of monoclinic ZnV2O6 nanowires using Zn(NO3)2 and NH4VO3 as the raw materials showing the ZnV2O6 nanowires were promising anode electrode materials for lithium ion batteries. ZnV2O4 spinel oxide nanosheets have also been prepared in oxalic acid dehydrated using zinc acetate and ammonium metavanadate as the raw materials [12]. The ZnV2O4 nanosheets are a prospective material for hydrogen energy. Among these zinc vanadate nanomaterials, zinc vanadate nanorods have attracted tremendous interest as a special class of
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semiconductor photocatalytic materials due to narrow band gap which can absorb solar visible light. However, the preparation of zinc vanadate generally requires complex and high temperature process [13,14]. Therefore, it is essential to develop a facile and low temperature route for the synthesis of zinc vanadate nanostructures with favorable shapes owing to the affinitive relationship between the morphology and properties. To date, zinc vanadate nanorods and their photocatalytic performance have not been reported. Hydrothermal synthesis takes the advantages of facile synthesis process, low cost, good morphology control of nanoscale materials and simple apparatus showing extensive application for the synthesis of one-dimensional (1D) nanoscale materials [15–17]. MB belongs to important organic dye and has wide application in the fields of paper coloring, cottons or wools dyeing and coating for paper stock [18]. MB is a representative dye and serves as a model to evaluate the photocatalytic activities of the photocatalysts due to its strong adsorption onto the solids [19,20]. In the paper, a facile and simple hydrothermal process has been developed to synthesize zinc vanadate nanorods using zinc acetate and sodium vanadate as the raw materials, sodium dodecyl sulfonate (SDS) as the surfactant by adjusting the pH value of hydrothermal solution. The formation mechanism of the zinc vanadate nanorods has been investigated based on the morphology and structures of the zinc vanadate products synthesized from different hydrothermal conditions. The zinc vanadate nanorods have been used as the photocatalyst for the photocatalytic degradation of MB under solar light irradiation. The roles of irradiation times, content of the zinc vanadate nanorods and MB concentrations on the photocatalytic activities of the zinc vanadate nanorods have been analyzed. The good photocatalytic activities make the zinc vanadate nanorods promising potential for the application in the photocatalytic degradation of organic pollutants. 2. Experimental details All raw materials were used without further purification. Sodium vanadate (Na3VO4) (AR grade, purity: P99.9%) and zinc acetate (Zn(CH3COO)22H2O) (AR grade, purity: P99.9%) were purchased from Aladdin Reagent Co., Ltd. of PR China and Sinopharm Chemical Reagent Co., Ltd. of PR China, respectively. SDS (AR grade, purity: P99.9%) was purchased from Sinopharm Chemical Reagent Co., Ltd. of PR China. In a typical procedure, sodium vanadate, zinc acetate and SDS with different concentrations were dissolved in 60 mL deionized water, respectively under vigorous stirring. The mole ratio of the sodium vanadate and zinc acetate is 2:1. Hydrochloric acid and sodium hydrate were used to adjust the pH value from 2 to 12. Then, the mixture was placed in a 100 mL autoclave with a Teflon liner. The autoclave was maintained at 80–180 °C for different duration times. Subsequently the autoclave was cooled naturally in air. The obtained white precipitates were filtered, washed with deionized water for several times and dried at 60 °C in air. Finally, white zinc vanadate nanorod products were obtained. The obtained products were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED) and solid UV–vis diffuse reflectance spectrum. XRD pattern of the products was performed on a Bruker AXS D8 X-ray diffractometer equipped with a graphite monochromatized Cu Ka radiation (k = 1.5406 Å). The samples were scanned at a scan rate of 0.05°/s in 2h range of 20–80°. SEM observation was carried out using JEOL JSM-6490LV SEM with a 15-kV accelerating voltage. TEM and HRTEM samples were prepared by putting several drops of solution with zinc vanadate nanorods onto a standard copper grid with a porous carbon film after the zinc vanadate nanorods samples were dispersed into deionized water and treated for about 10 min using supersonic wave apparatus. TEM and HRTEM observations were performed using JEOL JEM-2100 TEM operating with 1.9 Å point-to-point resolution operating with a 200-kV accelerating voltage with a GATAN digital photography system. Solid UV–vis diffuse reflectance spectrum of the zinc vanadate nanorods was obtained using a UV3600 UV–vis spectrometer (Shimadzu International Co., Ltd. of Japan) and a thermo Electron Corporation with a reflectance diffuse accessory. The photocatalytic degradation of MB was used to evaluate the photocatalytic activities of the zinc vanadate nanorods. The photocatalytic tests were carried out in the OCRS-IV photocatalytic system which was purchased from Kaifeng Hongxing Technology Co., Ltd. of Henan province of PR China. MB was AR grade and purchased from Aladdin Reagent Co., Ltd. of PR China and used without further treatment. The photocatalytic tests were performed under solar light irradiation. 2.5–20 mg zinc vanadate nanorods were used in 10 mL MB solution with the con-
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centration of 10 mg L1 in a quartz glass cell. Before solar light irradiation, the zinc vanadate nanorods suspension was magnetically stirred and maintained in the dark for 30 min to ensure the adsorption and desorption equilibrium between zinc vanadate nanorods and MB. Then the mixture with zinc vanadate nanorods and MB was exposed to solar light under magnetic stirring. All photocatalytic experiments were performed at room temperature in air. The MB solution was separated from zinc vanadate nanorods by filter unit. The obtained solution was analyzed by UV756 UV–vis spectrometer (Shanghai Youke Instrument Co., Ltd. of PR China) to record the intensity of the maximum band at 665 nm in the UV–vis absorption spectra. The absorption was converted to MB concentration referring to a standard curve showing a linear behavior between the MB concentration and intensity of the absorption peak at 665 nm.
3. Results and discussion The structure of the zinc vanadate products synthesized from 180 °C for 24 h with the pH value of 5 and 1 wt.% SDS has been characterized by XRD. The XRD pattern is shown in Fig. 1. The sharp reflection can be indexed to be monoclinic Zn2V2O7 phase (JCPDS card, PDF No. 38-0251). The monoclinic Zn2V2O7 phase is obviously different from the cubic ZnV2O4 phase of the zinc vanadate microscale and nanoscale materials obtained from other methods [1,8–11]. No characteristic peaks are observed from other impurities showing high purity of the zinc vanadate products. The morphologies of the zinc vanadate products were examined using SEM, TEM and HRTEM. Fig. 2a and b shows the typical morphology and size of the zinc vanadate products with different magnifications. As shown in the SEM image of Fig. 2a, the zinc vanadate products are composed of a large amount of free-standing nanorods. The diameter and length of the zinc vanadate nanorods are in the range of 50–100 nm and about 5 lm, respectively. The magnified SEM image (Fig. 2b) indicates the smooth surface of the nanorods. The zinc vanadate nanorods have semi-circular closed tips. The morphology of the zinc vanadate nanorods is similar to that of the nanorods with different compositions synthesized from the hydrothermal process [21–23]. TEM image of the zinc vanadate nanorods (Fig. 3a) further shows the nanrod morphology. The zinc vanadate nanorods with the diameter of 50–100 nm have straight structure and smooth surface. The dot-shaped SAED pattern of the zinc vanadate nanorods (inset in the upper-left part of Fig. 3a) shows that the zinc vanadate nanorods are composed of single crystalline structure. The HRTEM image (Fig. 3b) further indicates that the zinc vanadate nanorods have clear parallel fringes showing good single crystalline structure. The fringe spacing is determined to be 0.53 nm which is same to the lattice spacing of {1 1 0} plane of monoclinic
Fig. 1. XRD pattern of the zinc vanadate nanorods obtained from 180 °C for 24 h with the pH value of 5 and 1 wt.% SDS.
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Fig. 3. Transmission electron microscopy images of the zinc vanadate nanorods. (a) TEM image, the inset in the upper-left part is the SAED pattern of the zinc vanadate nanorods. (b) HRTEM image.
Fig. 2. SEM images of the zinc vanadate nanorods with different magnifications.
Zn2V2O7 phase. The SAED pattern, clear and regular lattice fringes show that the zinc vanadate nanorods consist of good single crystalline nature of monoclinic Zn2V2O7 phase. Similar to the nanorods with different compositions [21–23], special hydrothermal growth conditions are required to prepare regular zinc vanadate nanorods. The hydrothermal experiments with the pH value of 2, 7, 9 and 12 were performed so as to investigate the effects of pH value on the morphologies and phases of the zinc vanadate products. Zinc acetate, sodium vanadate and 1 wt.% SDS were used as the raw materials. The hydrothermal temperature and duration time were 180 °C and 24 h, respectively. Fig. 4 shows the SEM image of the zinc vanadate products synthesized from the pH value of 7. Different from the zinc vanadate nanorods, the zinc vanadate products consist of rod-shaped morphology with sub-microscale size. The average length and diameter of the zinc vanadate microrods are about 2 lm and 250 nm, respectively. The results show that pH value plays an important role in the diameter of the zinc vanadate rod-shaped morphology. Fig. 5 shows the SEM images of the zinc vanadate nanorods synthesized from 180 °C for 24 h with the pH value of 2, 9 and 12, respectively. The zinc vanadate products still consist of zinc vanadate nanorods with decreasing the pH value from 5 to 2 (Fig. 5a and b). The average length and diameter of the zinc vanadate nanorods are 10 lm and 50–100 nm, respectively. However, some microrods with the diameter of about 1–2 lm exist in the zinc vanadate products besides zinc vanadate nanorods with the pH value increasing from 5 to 9 and 12, respectively (Fig. 5c– f). Especially, the length of the zinc vanadate nanorods decreases to about 1 lm when the experiment was conducted in the pH value of 12 (Fig. 5e and f). And the diameter of the zinc vanadate
Fig. 4. SEM images of the zinc vanadate microrods obtained from 180 °C for 24 h with the pH value of 7 and 1 wt.% SDS.
nanorods is similar when the experiments were carried out from different pH values. pH value has an important role in the structure of the nanorods with different compositions [24,25]. Fig. 6 shows the XRD patterns of the zinc vanadate products synthesized from different pH values demonstrating the role of the pH value on the phase change of the zinc vanadate products. The zinc vanadate nanorods obtained from different pH values are same which is composed of monoclinic Zn2V2O7 phase (Fig. 6a, b, d and f). However, the structure of the zinc vanadate microrods obtained from the pH value of 7 is very different from that of the zinc vanadate nanorods synthesized from
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Fig. 5. SEM images of the zinc vanadate products obtained from 180 °C for 24 h with different pH values and 1 wt.% SDS. (a) and (b) pH = 2, (c) and (d) pH = 9, (e) and (f) pH = 12.
Fig. 6. XRD patterns of the zinc vanadate products synthesized from 180 °C for 24 h with different pH values and 1 wt.% SDS. (a) pH = 2, (b) pH = 5, (c) pH = 7, (d) pH = 9, (e) pH = 12.
acidic or alkaline solution. The XRD diffraction peaks contribute to the monoclinic Zn3V2O8 phase (JCPDS card, PDF No. 34-0378) (Fig. 6c). Therefore, the morphology of the zinc vanadate is closely relative to the structure of the zinc vanadate. The phase difference
of the zinc vanadate may result in the morphology difference of the zinc vanadate. Hydrogen ions or hydroxyl ions may induce the phase change from monoclinic Zn3V2O8 phase (zinc vanadate microrods) to monoclinic Zn2V2O7 phase (zinc vanadate nanorods). To understand the formation process of the zinc vanadate nanorods, the morphological evolution of the zinc vanadate products as a function of different synthetic periods has been analyzed. SDS is an anionic surfactant which can promote the formation and growth of the nanorods with different compositions [26,27]. Fig. 7 shows the SEM images of the zinc vanadate products obtained from SDS with different concentrations to understand the role of the SDS on the formation of the zinc vanadate nanorods. Only irregular particles with sub-microscale size exist in the zinc vanadate products when the experiment was performed without SDS (Fig. 7a and b). The result shows that the SDS has an essential role in the formation of zinc vanadate nanorods. Zinc vanadate nanorods are formed with adding the SDS. The diameter and length of the zinc vanadate nanorods are about 30–200 nm and 5 lm, respectively when SDS concentration is 0.1 wt.% (Fig. 7c and d). The diameter of the nanorods decreases obviously with increasing the SDS concentration to 3 wt.% (Fig. 7e and f). The diameter is about 30– 100 nm. And the length of the nanorods maintains similar. Fig. 8 shows the SEM images of the zinc vanadate products obtained from 180 °C by adjusting the duration time of 0.5, 6 and 12 h, respectively. The pH value is 5 and SDS concentration is
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Fig. 7. SEM images of the zinc vanadate products obtained from 180 °C for 24 h, pH = 5 using SDS with different concentrations. (a) and (b) Without SDS, (c) and (d) SDS 0.1 wt.%, (e) and (f) SDS 3 wt.%.
1 wt.%. At the initial reaction stage, a large amount of irregular particles with the size of smaller than 2 lm exist in the zinc vanadate products besides some nanorods when the duration time is 0.5 h (Fig. 8a). High magnification SEM image shows that the length and diameter of the zinc vanadate nanorods are about 1 lm and 30–100 nm, respectively (Fig. 8b). The amount of the zinc vanadate particles with microscale size decreases obviously with adding 0.1 wt.% SDS (Fig. 8c and d). When the SDS concentration increases to 1 wt.% and 3 wt.%, zinc vanadate particles disappear and the products are totally composed of zinc vanadate nanorods (Fig. 8e and f). Fig. 9 shows the SEM images of the zinc vanadate products obtained from different hydrothermal temperatures for 24 h. A large amount of zinc vanadate particles with the size of about 200 nm are observed from the products obtained from 80 °C for 24 h (Fig. 9a and b) besides some nanorods with the length of about 1 lm in the products. With the hydrothermal temperature increasing 120 °C, the amount of the zinc vanadate particles decreases obviously and the length of the zinc vanadate nanorods increases to 2 lm (Fig. 9c and d). Some nanorods with the length of smaller than 500 nm are also observed from the zinc vanadate products which may originate from the nanoparticles. Therefore, the zinc vanadate particles are considered as the nuclei for the formation of the zinc vanadate nanorods. Therefore, the hydrothermal temperature and duration time have important roles in the formation and growth of the zinc vanadate nanorods.
Generally, the growth process of the low dimensional nanomaterials can be explained by the ‘‘Ostwald ripening’’ process. However, it is considered that SDS has an essential role in the formation of the zinc vanadate nanorods according to the above analysis. As an anionic surfactant, SDS acts not only as the reaction moderator, but also provides a conductive environment for the nucleation and growth of the zinc vanadate nanorods. In addition, the hydrothermal conditions, such as hydrothermal temperature, duration time and pH value have important roles in the formation and growth of the zinc vanadate nanorods. Therefore, the formation process of the zinc vanadate nanorods has been proposed as a SDS-assisted nucleation and growth process based on the analysis of hydrothermal conditions on the formation of the zinc vanadate nandorods. A possible formation process schematic of the zinc vanadate nanorods is shown in Fig. 10. At the initial reaction stage, zinc acetate and sodium vanadate react to form zinc vanadate. Zinc vanadate precipitates from the hydrothermal solution forming zinc vanadate nuclei. After the formation of the zinc vanadate nuclei, the growth of zinc vanadate nuclei will be kinetically controlled by different hydrothermal parameters, such as hydrothermal temperature, duration time and surfactant in the hydrothermal system [28]. However, SDS plays an essential role in the formation of the zinc vanadate nanorods. The anionic surfactant SDS form SDS micelles in the hydrothermal solution. The SDS micelles control the surface free energy to affect the crystal growth direction and direct the
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Fig. 8. SEM images of the zinc vanadate products obtained from 180 °C for different duration times, pH = 5 and 1 wt.% SDS as the surfactant. (a) and (b) 0.5 h, (c) and (d) 6 h, (e) and (f) 12 h.
nucleation and growth of the zinc vanadate nanorods [29]. The selective adsorption of the SDS at the respective counter ions on the crystal faces of the zinc vanadate directs the formation of the zinc vanadate nanorods [30]. Under the hydrothermal conditions of low hydrothermal temperature (80 °C) and short duration time (0.5 h), short zinc vanadate nanorods form from the zinc vanadate nuclei through the SDS-assisted Ostwald ripening nucleation process. Long zinc vanadate nanorods are obtained by the further growth of short zinc vanadate nanorods with increasing the hydrothermal temperature and duration time. Fig. 11 shows the solid UV–vis diffuse reflectance spectrum of the zinc vanadate nanorods. The band gap (Eg) can be determined by the UV–vis diffuse reflectance spectrum. The band gap is calculated from the reflectance to be 2.76 eV using the Kubelka–Munk function [31]. Zhou et al. [32] reported that the band gap of the 3D chiral organic–inorganic hybrid zinc vanadate was 3.10 eV. Compared with the hybrid zinc vanadate with the band gap of 3.10 eV, the zinc vanadate nanorods exhibit a noticeable redshift of the adsorption edge and a significant enhancement of light absorption in the visible light region. The zinc vanadate nanorods show higher absorption ability in the visible light region. The absorption edge of the zinc vanadate nanorods is about 450 nm showing that the zinc vanadate nanorods have strong light absorption ability in visible light region. Therefore, zinc vanadate nanorods are strong absorbers of photons in the solar light due to the narrow band gap.
The photocatalytic activities of the zinc vanadate nanorods have been evaluated by the photocatalytic degradation of MB under solar light irradiation. For comparison, the photocatalytic activities of MB were also performed using zinc vanadate nanorods without visible light, visible light without zinc vanadate nanorods, respectively. The UV–vis absorption spectra of MB solution after different irradiation times using solar light and zinc vanadate nanorods are shown in Fig. 12a. The initial MB concentration and concent of the zinc vanadate nanorods are 10 mg L1 and 10 mg/10 mL MB solution, respectively. The characteristic absorption peak of MB is located at 665 nm. The intensity of the absorption peak at 665 nm decreases obviously with the increase of the solar light irradiation time. MB concentration decreases to 0 mg L1 after solar light irradiating for 4 h. The color of MB solution changes from deep blue1 to colorless. Fig. 12b indicates the MB concentration ratio after different irradiation times under different experimental conditions. The photocatalytic activities of the zinc vanadate nanorods for the treatment of MB in the dark conditions have also been performed. The curve between C/Co ratio and irradiation time (Fig. 12b) shows that the zinc vanadate nanorods have no photocatalytic activities for MB without visible light irradiation. In order to understand the role of the solar light on the photocatalytic activity for MB, MB degradation ratio was also analyzed under solar light 1 For interpretation of color in Fig. 12, the reader is referred to the web version of this article.
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Fig. 9. SEM images of the zinc vanadate products obtained from different hydrothermal temperatures for 24 h, pH = 5 using 1 wt.% SDS as the surfactant. (a) and (b) 80 °C, (c) and (d) 120 °C.
Fig. 10. The formation process schematic of the zinc vanadate nanorods.
without any photocatalysts. The MB degradation ratio is about 27.36% with the solar light irradiation time reaching 4 h. Therefore, the photocatalytic activities for the MB degradation are limited by solar light irradiation without any photocatalysts. The result is similar to that reported by Guo et al. using bismuth vanadate as the photocatalysts for MB degradation [33]. The MB can be degraded totally under the solar light irradiation for 4 h. Therefore, the zinc vanadate nanorods have good photocatalytic activities for the degradation of MB under solar light. The photocatalytic activities are believed to be associated with highly reactive species of peroxide (O2) and hydroxyl radical (⁄OH) generated by electrons and holes on the surface with water [34]. The photocatalytic activities are known to be dependent on the band gap, surface area, morphology and can be improved by the excitation wavelength to lower energy range and increasing the amount of the surface adsorbed reactant species [35]. The zinc vanadate nanorods have narrow band gap and good visible light absorption ability. When the zinc vanadate nanorods absorb photons with sufficient energy which is higher than 2.76 eV, the photogenerated electrons are promoted from the valence band to the conduction band. The electrons in the conduction band reduce the molecular oxygen while the positively charged holes in the valence band oxidize water. The activation energy necessary to overcome the band gap using the zinc vanadate nanorods is low owing to the low band gap showing high photocatalytic efficiency under solar light irradiation for practical application. The photocatalytic reaction is followed by the diffusion of the charge carriers to the surface of the zinc vanadate nanorods. The photocatalytic reactions can be listed as follows [36]: þ
Zinc vanadate nanorods þ hm ! Zinc vanadate nanorods eCB þ hVB
ð1Þ þ
Fig. 11. Solid UV–vis diffuse reflectance spectrum of the zinc vanadate nanorods.
H2 O þ h ! OH þ Hþ
ð2Þ
O2 þ e ! O2
ð3Þ
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Fig. 12. (a) Absorption spectra of MB solution after different irradiation times using solar light in 10 mL MB solution. (b) MB concentration ratio after different irradiation times treated by zinc vanadate nanorods in 10 mL MB solution. Zinc vanadate nanorods, 10 mg; MB, 10 mg L1.
The electrons in the conduction band and holes in valence band are originated from Eq. (1). Eq. (2) shows the oxidation process of water by the holes. The photocatalytic reaction process of MB is shown as follows:
MB þ O2 ! HCl þ H2 SO4 þ HNO3 þ CO2 þ H2 O
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Fig. 13. Absorption spectra of MB solution (a) and MB concentration ratio (b) treated using zinc vanadate nanorods with different contents in 10 mL MB solution. Irradiation time, 4 h; MB, 10 mg L1.
degradation ratio decreases from 87.84% to 85.51% with the MB concentration increasing from 15 to 20 mg L1. All active sites are entirely exposed and used at lower content of the zinc vanadate nanorods. Only a part of active sites are exposed and occupied by MB at higher content of the zinc vanadate nanorods. The diffusion driving force of MB adsorbed by the zinc vanadate nanorods increases which originates from the larger MB concentration gradi-
ð4Þ
The role of the content of the zinc vanadate nanorods on the photocatalytic degradation ratio of MB has been analyzed. The solar light irradiation time and MB concentration are 4 h and 10 mgL1, respectively. Fig. 13 shows the absorption spectra of MB solution and MB concentration ratio using the zinc vanadate nanorods with different contents. MB degradation ratio decreases to 77.71% with the content of the zinc vanadate nanorods decreasing from 50 wt.% (10 mg/10 mL MB solution) to 20 wt.% (2.5 mg/ 10 mL MB solution). The results indicate that the content of the zinc vanadate nanorods plays an important effect in the photocatalytic activities for the degradation of MB. Zinc vanadate nanorods belong to oxide semiconductor. The photocatalytic degradation process of the zinc vanadate nanorods is the direct absorption process of a photon by the energy band gap. The photocatalytic process can be improved by the combination of the zinc vanadate nanorods and MB. Fig. 14 shows the MB concentration ratio treated using the zinc vanadate nanorods in 10 mL MB solution with the MB concentration from 2.5 to 20 mg L1. The MB solution with the concentration of less than 10 mg L1 can be degraded totally. However, the MB
Fig. 14. The photocatalytic activity of the zinc vanadate nanorods in 10 mL MB solution with different concentrations. Zinc vanadate nanorods, 10 mg; irradiation time, 4 h.
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ent [37,38]. At the same content of the zinc vanadate nanorods, higher MB concentration requires higher equilibrium adsorption capacity resulting in the decrease of the MB degradation ratio. 4. Conclusions In summary, single crystalline zinc vanadate nanorods with monoclinic Zn2V2O7 phase have been synthesized via the simple hydrothermal process using SDS as the surfactant by adjusting the pH value. The zinc vanadate nanorods have the diameter and length of 50–100 nm and 5 lm, respectively. The pH value, hydrothermal temperature, duration time and SDS concentration have important roles in the structure, morphology and size of the zinc vanadate nanorods. pH value of the hydrothermal solution induces the phase change of the zinc vanadate products from monoclinic Zn3V2O8 phase (zinc vanadate microrods) to monoclinic Zn2V2O7 phase (zinc vanadate nanorods). The zinc vanadate nanorods have been explained according to the SDS-assisted nucleation and growth process based on the roles of the hydrothermal conditions in formation of the zinc vanadate nanorods. Solid UV– vis diffuse reflectance spectrum shows that the zinc vanadate nanorods have a band gap of 2.76 eV exhibiting that the zinc vanadate nanorods have strong light absorption ability in visible light region. The zinc vanadate nanorods are good visible light photocatalysts for the photocatalytic degradation of MB under solar light irradiation. The MB with the concentration of 10 mg L1 can be degraded totally after solar light irradiation for 4 h over 10 mg zinc vanadate nanorods in 10 mL MB solution. The zinc vanadate nanorods have promising application potential in the field of visible light photocatalysts. Acknowledgments This work was supported by the Innovative Research Foundation of Postgraduate of Anhui University of Technology (2014077) and Natural Science Foundation of Anhui Province (1208085QE98). References [1] F. Duan, W.F. Dong, D.J. Shi, M.Q. Chen, Appl. Surf. Sci. 258 (2011) 189. [2] Y. Gu, Z.D. Xu, L. Guo, Y.Q. Wan, CrystEngComm 16 (2014) 10997. [3] M. Wang, Y.F. Tang, T.M. Sun, G.Q. Jiang, Y.J. Shi, CrystEngComm 16 (2014) 11035. [4] B.Y. Shi, G.H. Li, D.S. Wang, C.H. Feng, H.X. Tang, J. Hazard. Mater. 143 (2007) 567.
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Zinc vanadate nanorods and their visible light photocatalytic activity L.Z. Pei ⇑, N. Lin, T. Wei, H.D. Liu, H.Y. Yu Key Lab of Materials Science and Processing of Anhui Province, School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan, Anhui 243002, PR China
a r t i c l e
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Article history: Received 29 November 2014 Received in revised form 10 January 2015 Accepted 18 January 2015 Available online 24 January 2015 Keywords: Zinc vanadate nanorods Crystal growth Electron microscopy Solar light Photocatalytic activity
a b s t r a c t Zinc vanadate nanorods have been synthesized by a simple hydrothermal process using zinc acetate and sodium vanadate as the raw materials. The zinc vanadate nanorods have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), highresolution TEM (HRTEM) and solid UV–vis diffuse reflectance spectrum. XRD pattern and HRTEM image show that the zinc vanadate nanorods are composed of single crystalline monoclinic Zn2V2O7 phase. SEM and TEM observations show that the diameter and length of the zinc vanadate nanorods are 50–100 nm and about 5 lm, respectively. Sodium dodecyl sulfonate (SDS) has an essential role in the formation of zinc vanadate nanorods. The SDS-assisted nucleation and growth process have been proposed to explain the formation and growth of the zinc vanadate nanorods. Solid UV–vis diffuse reflectance spectrum shows that the zinc vanadate nanorods have a band gap of 2.76 eV. The photocatalytic activities of the zinc vanadate nanorods have been evaluated by the photocatalytic degradation of methylene blue (MB) under solar light irradiation. The MB with the concentration of 10 mg L1 can be degraded totally under the solar light irradiation for 4 h. It is suggested that the zinc vanadate nanorods exhibit promising application potential for the degradation of organic pollutants under solar light irradiation. Ó 2015 Published by Elsevier B.V.
1. Introduction Great research interest has been devoted to the removal of organic pollutes owing to their long term environmental toxicity and public health damage to humans [1–3]. Various methods, such as coagulation [4], adsorption [5], membrane filtration [6] and photocatalytic technology [7] have been used to remove organic pollutants from aqueous environment. Among these methods, photocatalytic technology is especially attractive due to its high efficiency, simplicity of design and simple operation process. However, it requires photocatalysts with narrow band gap, large specific surface area and well-defined morphology, which can remove organic pollutants under visible light. Zinc vanadate nanomaterials fit these criteria well and have been extensively considered as the potential materials in the fields of photocatalytic treatment of organic pollutants, lithium batteries, hydrogen storage, electrochemical supercapacitors and various new application fields. Double-shelled ZnV2O4 hollow nanostructures have been synthesized through a facile one-pot template-free solvothermal method using ZnCl2 and vanadium (IV) oxide bis (2,4pentanedionate) (VO(acac)2) as the raw materials, N,N-dimethyl formamide (DMF) as the solvent [8]. And the inward–outward
⇑ Corresponding author. Fax: +86 5552311570. E-mail addresses: [email protected], [email protected] (L.Z. Pei). http://dx.doi.org/10.1016/j.jallcom.2015.01.115 0925-8388/Ó 2015 Published by Elsevier B.V.
ripening formation mechanism has been proposed to explain the formation and growth of the ZnV2O4 hollow spheres. Zinc nitrate can also be used as the Zn raw material for the formation of ZnV2O4 hollow spheres [1]. The ZnV2O4 hollow spheres show a good adsorption capacity of methylene blue (MB) organic pollutant. Clewlike hollow ZnV2O4 spheres have also been synthesized by the reaction between zinc nitrate hexahydrate and ammonium metavanadate in benzyl alcohol [9]. The clewlike hollow ZnV2O4 spheres exhibited stable cycling performance to maintain a capacity of 524 mA hg1 over 50 cycles suggesting that the clewlike ZnV2O4 hollow spheres were promising for lithium ion batteries. Butt et al. [10] reported the synthesis of ZnV2O4 hierarchical nanospheres using NH4VO3 and Zn(NO3)2 as the raw materials, oxalic acid dehydrated as the chelating agent, H2O2 and HNO3 as the solvents. The ZnV2O4 hierarchical nanospheres were used as the electrochemical supercapacitor electrode showing good capacitance and retention capacity of 89% after 1000 cycles. Sun et al. [11] reported the preparation of monoclinic ZnV2O6 nanowires using Zn(NO3)2 and NH4VO3 as the raw materials showing the ZnV2O6 nanowires were promising anode electrode materials for lithium ion batteries. ZnV2O4 spinel oxide nanosheets have also been prepared in oxalic acid dehydrated using zinc acetate and ammonium metavanadate as the raw materials [12]. The ZnV2O4 nanosheets are a prospective material for hydrogen energy. Among these zinc vanadate nanomaterials, zinc vanadate nanorods have attracted tremendous interest as a special class of
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semiconductor photocatalytic materials due to narrow band gap which can absorb solar visible light. However, the preparation of zinc vanadate generally requires complex and high temperature process [13,14]. Therefore, it is essential to develop a facile and low temperature route for the synthesis of zinc vanadate nanostructures with favorable shapes owing to the affinitive relationship between the morphology and properties. To date, zinc vanadate nanorods and their photocatalytic performance have not been reported. Hydrothermal synthesis takes the advantages of facile synthesis process, low cost, good morphology control of nanoscale materials and simple apparatus showing extensive application for the synthesis of one-dimensional (1D) nanoscale materials [15–17]. MB belongs to important organic dye and has wide application in the fields of paper coloring, cottons or wools dyeing and coating for paper stock [18]. MB is a representative dye and serves as a model to evaluate the photocatalytic activities of the photocatalysts due to its strong adsorption onto the solids [19,20]. In the paper, a facile and simple hydrothermal process has been developed to synthesize zinc vanadate nanorods using zinc acetate and sodium vanadate as the raw materials, sodium dodecyl sulfonate (SDS) as the surfactant by adjusting the pH value of hydrothermal solution. The formation mechanism of the zinc vanadate nanorods has been investigated based on the morphology and structures of the zinc vanadate products synthesized from different hydrothermal conditions. The zinc vanadate nanorods have been used as the photocatalyst for the photocatalytic degradation of MB under solar light irradiation. The roles of irradiation times, content of the zinc vanadate nanorods and MB concentrations on the photocatalytic activities of the zinc vanadate nanorods have been analyzed. The good photocatalytic activities make the zinc vanadate nanorods promising potential for the application in the photocatalytic degradation of organic pollutants. 2. Experimental details All raw materials were used without further purification. Sodium vanadate (Na3VO4) (AR grade, purity: P99.9%) and zinc acetate (Zn(CH3COO)22H2O) (AR grade, purity: P99.9%) were purchased from Aladdin Reagent Co., Ltd. of PR China and Sinopharm Chemical Reagent Co., Ltd. of PR China, respectively. SDS (AR grade, purity: P99.9%) was purchased from Sinopharm Chemical Reagent Co., Ltd. of PR China. In a typical procedure, sodium vanadate, zinc acetate and SDS with different concentrations were dissolved in 60 mL deionized water, respectively under vigorous stirring. The mole ratio of the sodium vanadate and zinc acetate is 2:1. Hydrochloric acid and sodium hydrate were used to adjust the pH value from 2 to 12. Then, the mixture was placed in a 100 mL autoclave with a Teflon liner. The autoclave was maintained at 80–180 °C for different duration times. Subsequently the autoclave was cooled naturally in air. The obtained white precipitates were filtered, washed with deionized water for several times and dried at 60 °C in air. Finally, white zinc vanadate nanorod products were obtained. The obtained products were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected area electron diffraction (SAED) and solid UV–vis diffuse reflectance spectrum. XRD pattern of the products was performed on a Bruker AXS D8 X-ray diffractometer equipped with a graphite monochromatized Cu Ka radiation (k = 1.5406 Å). The samples were scanned at a scan rate of 0.05°/s in 2h range of 20–80°. SEM observation was carried out using JEOL JSM-6490LV SEM with a 15-kV accelerating voltage. TEM and HRTEM samples were prepared by putting several drops of solution with zinc vanadate nanorods onto a standard copper grid with a porous carbon film after the zinc vanadate nanorods samples were dispersed into deionized water and treated for about 10 min using supersonic wave apparatus. TEM and HRTEM observations were performed using JEOL JEM-2100 TEM operating with 1.9 Å point-to-point resolution operating with a 200-kV accelerating voltage with a GATAN digital photography system. Solid UV–vis diffuse reflectance spectrum of the zinc vanadate nanorods was obtained using a UV3600 UV–vis spectrometer (Shimadzu International Co., Ltd. of Japan) and a thermo Electron Corporation with a reflectance diffuse accessory. The photocatalytic degradation of MB was used to evaluate the photocatalytic activities of the zinc vanadate nanorods. The photocatalytic tests were carried out in the OCRS-IV photocatalytic system which was purchased from Kaifeng Hongxing Technology Co., Ltd. of Henan province of PR China. MB was AR grade and purchased from Aladdin Reagent Co., Ltd. of PR China and used without further treatment. The photocatalytic tests were performed under solar light irradiation. 2.5–20 mg zinc vanadate nanorods were used in 10 mL MB solution with the con-
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centration of 10 mg L1 in a quartz glass cell. Before solar light irradiation, the zinc vanadate nanorods suspension was magnetically stirred and maintained in the dark for 30 min to ensure the adsorption and desorption equilibrium between zinc vanadate nanorods and MB. Then the mixture with zinc vanadate nanorods and MB was exposed to solar light under magnetic stirring. All photocatalytic experiments were performed at room temperature in air. The MB solution was separated from zinc vanadate nanorods by filter unit. The obtained solution was analyzed by UV756 UV–vis spectrometer (Shanghai Youke Instrument Co., Ltd. of PR China) to record the intensity of the maximum band at 665 nm in the UV–vis absorption spectra. The absorption was converted to MB concentration referring to a standard curve showing a linear behavior between the MB concentration and intensity of the absorption peak at 665 nm.
3. Results and discussion The structure of the zinc vanadate products synthesized from 180 °C for 24 h with the pH value of 5 and 1 wt.% SDS has been characterized by XRD. The XRD pattern is shown in Fig. 1. The sharp reflection can be indexed to be monoclinic Zn2V2O7 phase (JCPDS card, PDF No. 38-0251). The monoclinic Zn2V2O7 phase is obviously different from the cubic ZnV2O4 phase of the zinc vanadate microscale and nanoscale materials obtained from other methods [1,8–11]. No characteristic peaks are observed from other impurities showing high purity of the zinc vanadate products. The morphologies of the zinc vanadate products were examined using SEM, TEM and HRTEM. Fig. 2a and b shows the typical morphology and size of the zinc vanadate products with different magnifications. As shown in the SEM image of Fig. 2a, the zinc vanadate products are composed of a large amount of free-standing nanorods. The diameter and length of the zinc vanadate nanorods are in the range of 50–100 nm and about 5 lm, respectively. The magnified SEM image (Fig. 2b) indicates the smooth surface of the nanorods. The zinc vanadate nanorods have semi-circular closed tips. The morphology of the zinc vanadate nanorods is similar to that of the nanorods with different compositions synthesized from the hydrothermal process [21–23]. TEM image of the zinc vanadate nanorods (Fig. 3a) further shows the nanrod morphology. The zinc vanadate nanorods with the diameter of 50–100 nm have straight structure and smooth surface. The dot-shaped SAED pattern of the zinc vanadate nanorods (inset in the upper-left part of Fig. 3a) shows that the zinc vanadate nanorods are composed of single crystalline structure. The HRTEM image (Fig. 3b) further indicates that the zinc vanadate nanorods have clear parallel fringes showing good single crystalline structure. The fringe spacing is determined to be 0.53 nm which is same to the lattice spacing of {1 1 0} plane of monoclinic
Fig. 1. XRD pattern of the zinc vanadate nanorods obtained from 180 °C for 24 h with the pH value of 5 and 1 wt.% SDS.
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Fig. 3. Transmission electron microscopy images of the zinc vanadate nanorods. (a) TEM image, the inset in the upper-left part is the SAED pattern of the zinc vanadate nanorods. (b) HRTEM image.
Fig. 2. SEM images of the zinc vanadate nanorods with different magnifications.
Zn2V2O7 phase. The SAED pattern, clear and regular lattice fringes show that the zinc vanadate nanorods consist of good single crystalline nature of monoclinic Zn2V2O7 phase. Similar to the nanorods with different compositions [21–23], special hydrothermal growth conditions are required to prepare regular zinc vanadate nanorods. The hydrothermal experiments with the pH value of 2, 7, 9 and 12 were performed so as to investigate the effects of pH value on the morphologies and phases of the zinc vanadate products. Zinc acetate, sodium vanadate and 1 wt.% SDS were used as the raw materials. The hydrothermal temperature and duration time were 180 °C and 24 h, respectively. Fig. 4 shows the SEM image of the zinc vanadate products synthesized from the pH value of 7. Different from the zinc vanadate nanorods, the zinc vanadate products consist of rod-shaped morphology with sub-microscale size. The average length and diameter of the zinc vanadate microrods are about 2 lm and 250 nm, respectively. The results show that pH value plays an important role in the diameter of the zinc vanadate rod-shaped morphology. Fig. 5 shows the SEM images of the zinc vanadate nanorods synthesized from 180 °C for 24 h with the pH value of 2, 9 and 12, respectively. The zinc vanadate products still consist of zinc vanadate nanorods with decreasing the pH value from 5 to 2 (Fig. 5a and b). The average length and diameter of the zinc vanadate nanorods are 10 lm and 50–100 nm, respectively. However, some microrods with the diameter of about 1–2 lm exist in the zinc vanadate products besides zinc vanadate nanorods with the pH value increasing from 5 to 9 and 12, respectively (Fig. 5c– f). Especially, the length of the zinc vanadate nanorods decreases to about 1 lm when the experiment was conducted in the pH value of 12 (Fig. 5e and f). And the diameter of the zinc vanadate
Fig. 4. SEM images of the zinc vanadate microrods obtained from 180 °C for 24 h with the pH value of 7 and 1 wt.% SDS.
nanorods is similar when the experiments were carried out from different pH values. pH value has an important role in the structure of the nanorods with different compositions [24,25]. Fig. 6 shows the XRD patterns of the zinc vanadate products synthesized from different pH values demonstrating the role of the pH value on the phase change of the zinc vanadate products. The zinc vanadate nanorods obtained from different pH values are same which is composed of monoclinic Zn2V2O7 phase (Fig. 6a, b, d and f). However, the structure of the zinc vanadate microrods obtained from the pH value of 7 is very different from that of the zinc vanadate nanorods synthesized from
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Fig. 5. SEM images of the zinc vanadate products obtained from 180 °C for 24 h with different pH values and 1 wt.% SDS. (a) and (b) pH = 2, (c) and (d) pH = 9, (e) and (f) pH = 12.
Fig. 6. XRD patterns of the zinc vanadate products synthesized from 180 °C for 24 h with different pH values and 1 wt.% SDS. (a) pH = 2, (b) pH = 5, (c) pH = 7, (d) pH = 9, (e) pH = 12.
acidic or alkaline solution. The XRD diffraction peaks contribute to the monoclinic Zn3V2O8 phase (JCPDS card, PDF No. 34-0378) (Fig. 6c). Therefore, the morphology of the zinc vanadate is closely relative to the structure of the zinc vanadate. The phase difference
of the zinc vanadate may result in the morphology difference of the zinc vanadate. Hydrogen ions or hydroxyl ions may induce the phase change from monoclinic Zn3V2O8 phase (zinc vanadate microrods) to monoclinic Zn2V2O7 phase (zinc vanadate nanorods). To understand the formation process of the zinc vanadate nanorods, the morphological evolution of the zinc vanadate products as a function of different synthetic periods has been analyzed. SDS is an anionic surfactant which can promote the formation and growth of the nanorods with different compositions [26,27]. Fig. 7 shows the SEM images of the zinc vanadate products obtained from SDS with different concentrations to understand the role of the SDS on the formation of the zinc vanadate nanorods. Only irregular particles with sub-microscale size exist in the zinc vanadate products when the experiment was performed without SDS (Fig. 7a and b). The result shows that the SDS has an essential role in the formation of zinc vanadate nanorods. Zinc vanadate nanorods are formed with adding the SDS. The diameter and length of the zinc vanadate nanorods are about 30–200 nm and 5 lm, respectively when SDS concentration is 0.1 wt.% (Fig. 7c and d). The diameter of the nanorods decreases obviously with increasing the SDS concentration to 3 wt.% (Fig. 7e and f). The diameter is about 30– 100 nm. And the length of the nanorods maintains similar. Fig. 8 shows the SEM images of the zinc vanadate products obtained from 180 °C by adjusting the duration time of 0.5, 6 and 12 h, respectively. The pH value is 5 and SDS concentration is
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Fig. 7. SEM images of the zinc vanadate products obtained from 180 °C for 24 h, pH = 5 using SDS with different concentrations. (a) and (b) Without SDS, (c) and (d) SDS 0.1 wt.%, (e) and (f) SDS 3 wt.%.
1 wt.%. At the initial reaction stage, a large amount of irregular particles with the size of smaller than 2 lm exist in the zinc vanadate products besides some nanorods when the duration time is 0.5 h (Fig. 8a). High magnification SEM image shows that the length and diameter of the zinc vanadate nanorods are about 1 lm and 30–100 nm, respectively (Fig. 8b). The amount of the zinc vanadate particles with microscale size decreases obviously with adding 0.1 wt.% SDS (Fig. 8c and d). When the SDS concentration increases to 1 wt.% and 3 wt.%, zinc vanadate particles disappear and the products are totally composed of zinc vanadate nanorods (Fig. 8e and f). Fig. 9 shows the SEM images of the zinc vanadate products obtained from different hydrothermal temperatures for 24 h. A large amount of zinc vanadate particles with the size of about 200 nm are observed from the products obtained from 80 °C for 24 h (Fig. 9a and b) besides some nanorods with the length of about 1 lm in the products. With the hydrothermal temperature increasing 120 °C, the amount of the zinc vanadate particles decreases obviously and the length of the zinc vanadate nanorods increases to 2 lm (Fig. 9c and d). Some nanorods with the length of smaller than 500 nm are also observed from the zinc vanadate products which may originate from the nanoparticles. Therefore, the zinc vanadate particles are considered as the nuclei for the formation of the zinc vanadate nanorods. Therefore, the hydrothermal temperature and duration time have important roles in the formation and growth of the zinc vanadate nanorods.
Generally, the growth process of the low dimensional nanomaterials can be explained by the ‘‘Ostwald ripening’’ process. However, it is considered that SDS has an essential role in the formation of the zinc vanadate nanorods according to the above analysis. As an anionic surfactant, SDS acts not only as the reaction moderator, but also provides a conductive environment for the nucleation and growth of the zinc vanadate nanorods. In addition, the hydrothermal conditions, such as hydrothermal temperature, duration time and pH value have important roles in the formation and growth of the zinc vanadate nanorods. Therefore, the formation process of the zinc vanadate nanorods has been proposed as a SDS-assisted nucleation and growth process based on the analysis of hydrothermal conditions on the formation of the zinc vanadate nandorods. A possible formation process schematic of the zinc vanadate nanorods is shown in Fig. 10. At the initial reaction stage, zinc acetate and sodium vanadate react to form zinc vanadate. Zinc vanadate precipitates from the hydrothermal solution forming zinc vanadate nuclei. After the formation of the zinc vanadate nuclei, the growth of zinc vanadate nuclei will be kinetically controlled by different hydrothermal parameters, such as hydrothermal temperature, duration time and surfactant in the hydrothermal system [28]. However, SDS plays an essential role in the formation of the zinc vanadate nanorods. The anionic surfactant SDS form SDS micelles in the hydrothermal solution. The SDS micelles control the surface free energy to affect the crystal growth direction and direct the
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Fig. 8. SEM images of the zinc vanadate products obtained from 180 °C for different duration times, pH = 5 and 1 wt.% SDS as the surfactant. (a) and (b) 0.5 h, (c) and (d) 6 h, (e) and (f) 12 h.
nucleation and growth of the zinc vanadate nanorods [29]. The selective adsorption of the SDS at the respective counter ions on the crystal faces of the zinc vanadate directs the formation of the zinc vanadate nanorods [30]. Under the hydrothermal conditions of low hydrothermal temperature (80 °C) and short duration time (0.5 h), short zinc vanadate nanorods form from the zinc vanadate nuclei through the SDS-assisted Ostwald ripening nucleation process. Long zinc vanadate nanorods are obtained by the further growth of short zinc vanadate nanorods with increasing the hydrothermal temperature and duration time. Fig. 11 shows the solid UV–vis diffuse reflectance spectrum of the zinc vanadate nanorods. The band gap (Eg) can be determined by the UV–vis diffuse reflectance spectrum. The band gap is calculated from the reflectance to be 2.76 eV using the Kubelka–Munk function [31]. Zhou et al. [32] reported that the band gap of the 3D chiral organic–inorganic hybrid zinc vanadate was 3.10 eV. Compared with the hybrid zinc vanadate with the band gap of 3.10 eV, the zinc vanadate nanorods exhibit a noticeable redshift of the adsorption edge and a significant enhancement of light absorption in the visible light region. The zinc vanadate nanorods show higher absorption ability in the visible light region. The absorption edge of the zinc vanadate nanorods is about 450 nm showing that the zinc vanadate nanorods have strong light absorption ability in visible light region. Therefore, zinc vanadate nanorods are strong absorbers of photons in the solar light due to the narrow band gap.
The photocatalytic activities of the zinc vanadate nanorods have been evaluated by the photocatalytic degradation of MB under solar light irradiation. For comparison, the photocatalytic activities of MB were also performed using zinc vanadate nanorods without visible light, visible light without zinc vanadate nanorods, respectively. The UV–vis absorption spectra of MB solution after different irradiation times using solar light and zinc vanadate nanorods are shown in Fig. 12a. The initial MB concentration and concent of the zinc vanadate nanorods are 10 mg L1 and 10 mg/10 mL MB solution, respectively. The characteristic absorption peak of MB is located at 665 nm. The intensity of the absorption peak at 665 nm decreases obviously with the increase of the solar light irradiation time. MB concentration decreases to 0 mg L1 after solar light irradiating for 4 h. The color of MB solution changes from deep blue1 to colorless. Fig. 12b indicates the MB concentration ratio after different irradiation times under different experimental conditions. The photocatalytic activities of the zinc vanadate nanorods for the treatment of MB in the dark conditions have also been performed. The curve between C/Co ratio and irradiation time (Fig. 12b) shows that the zinc vanadate nanorods have no photocatalytic activities for MB without visible light irradiation. In order to understand the role of the solar light on the photocatalytic activity for MB, MB degradation ratio was also analyzed under solar light 1 For interpretation of color in Fig. 12, the reader is referred to the web version of this article.
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Fig. 9. SEM images of the zinc vanadate products obtained from different hydrothermal temperatures for 24 h, pH = 5 using 1 wt.% SDS as the surfactant. (a) and (b) 80 °C, (c) and (d) 120 °C.
Fig. 10. The formation process schematic of the zinc vanadate nanorods.
without any photocatalysts. The MB degradation ratio is about 27.36% with the solar light irradiation time reaching 4 h. Therefore, the photocatalytic activities for the MB degradation are limited by solar light irradiation without any photocatalysts. The result is similar to that reported by Guo et al. using bismuth vanadate as the photocatalysts for MB degradation [33]. The MB can be degraded totally under the solar light irradiation for 4 h. Therefore, the zinc vanadate nanorods have good photocatalytic activities for the degradation of MB under solar light. The photocatalytic activities are believed to be associated with highly reactive species of peroxide (O2) and hydroxyl radical (⁄OH) generated by electrons and holes on the surface with water [34]. The photocatalytic activities are known to be dependent on the band gap, surface area, morphology and can be improved by the excitation wavelength to lower energy range and increasing the amount of the surface adsorbed reactant species [35]. The zinc vanadate nanorods have narrow band gap and good visible light absorption ability. When the zinc vanadate nanorods absorb photons with sufficient energy which is higher than 2.76 eV, the photogenerated electrons are promoted from the valence band to the conduction band. The electrons in the conduction band reduce the molecular oxygen while the positively charged holes in the valence band oxidize water. The activation energy necessary to overcome the band gap using the zinc vanadate nanorods is low owing to the low band gap showing high photocatalytic efficiency under solar light irradiation for practical application. The photocatalytic reaction is followed by the diffusion of the charge carriers to the surface of the zinc vanadate nanorods. The photocatalytic reactions can be listed as follows [36]: þ
Zinc vanadate nanorods þ hm ! Zinc vanadate nanorods eCB þ hVB
ð1Þ þ
Fig. 11. Solid UV–vis diffuse reflectance spectrum of the zinc vanadate nanorods.
H2 O þ h ! OH þ Hþ
ð2Þ
O2 þ e ! O2
ð3Þ
L.Z. Pei et al. / Journal of Alloys and Compounds 631 (2015) 90–98
Fig. 12. (a) Absorption spectra of MB solution after different irradiation times using solar light in 10 mL MB solution. (b) MB concentration ratio after different irradiation times treated by zinc vanadate nanorods in 10 mL MB solution. Zinc vanadate nanorods, 10 mg; MB, 10 mg L1.
The electrons in the conduction band and holes in valence band are originated from Eq. (1). Eq. (2) shows the oxidation process of water by the holes. The photocatalytic reaction process of MB is shown as follows:
MB þ O2 ! HCl þ H2 SO4 þ HNO3 þ CO2 þ H2 O
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Fig. 13. Absorption spectra of MB solution (a) and MB concentration ratio (b) treated using zinc vanadate nanorods with different contents in 10 mL MB solution. Irradiation time, 4 h; MB, 10 mg L1.
degradation ratio decreases from 87.84% to 85.51% with the MB concentration increasing from 15 to 20 mg L1. All active sites are entirely exposed and used at lower content of the zinc vanadate nanorods. Only a part of active sites are exposed and occupied by MB at higher content of the zinc vanadate nanorods. The diffusion driving force of MB adsorbed by the zinc vanadate nanorods increases which originates from the larger MB concentration gradi-
ð4Þ
The role of the content of the zinc vanadate nanorods on the photocatalytic degradation ratio of MB has been analyzed. The solar light irradiation time and MB concentration are 4 h and 10 mgL1, respectively. Fig. 13 shows the absorption spectra of MB solution and MB concentration ratio using the zinc vanadate nanorods with different contents. MB degradation ratio decreases to 77.71% with the content of the zinc vanadate nanorods decreasing from 50 wt.% (10 mg/10 mL MB solution) to 20 wt.% (2.5 mg/ 10 mL MB solution). The results indicate that the content of the zinc vanadate nanorods plays an important effect in the photocatalytic activities for the degradation of MB. Zinc vanadate nanorods belong to oxide semiconductor. The photocatalytic degradation process of the zinc vanadate nanorods is the direct absorption process of a photon by the energy band gap. The photocatalytic process can be improved by the combination of the zinc vanadate nanorods and MB. Fig. 14 shows the MB concentration ratio treated using the zinc vanadate nanorods in 10 mL MB solution with the MB concentration from 2.5 to 20 mg L1. The MB solution with the concentration of less than 10 mg L1 can be degraded totally. However, the MB
Fig. 14. The photocatalytic activity of the zinc vanadate nanorods in 10 mL MB solution with different concentrations. Zinc vanadate nanorods, 10 mg; irradiation time, 4 h.
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ent [37,38]. At the same content of the zinc vanadate nanorods, higher MB concentration requires higher equilibrium adsorption capacity resulting in the decrease of the MB degradation ratio. 4. Conclusions In summary, single crystalline zinc vanadate nanorods with monoclinic Zn2V2O7 phase have been synthesized via the simple hydrothermal process using SDS as the surfactant by adjusting the pH value. The zinc vanadate nanorods have the diameter and length of 50–100 nm and 5 lm, respectively. The pH value, hydrothermal temperature, duration time and SDS concentration have important roles in the structure, morphology and size of the zinc vanadate nanorods. pH value of the hydrothermal solution induces the phase change of the zinc vanadate products from monoclinic Zn3V2O8 phase (zinc vanadate microrods) to monoclinic Zn2V2O7 phase (zinc vanadate nanorods). The zinc vanadate nanorods have been explained according to the SDS-assisted nucleation and growth process based on the roles of the hydrothermal conditions in formation of the zinc vanadate nanorods. Solid UV– vis diffuse reflectance spectrum shows that the zinc vanadate nanorods have a band gap of 2.76 eV exhibiting that the zinc vanadate nanorods have strong light absorption ability in visible light region. The zinc vanadate nanorods are good visible light photocatalysts for the photocatalytic degradation of MB under solar light irradiation. The MB with the concentration of 10 mg L1 can be degraded totally after solar light irradiation for 4 h over 10 mg zinc vanadate nanorods in 10 mL MB solution. The zinc vanadate nanorods have promising application potential in the field of visible light photocatalysts. Acknowledgments This work was supported by the Innovative Research Foundation of Postgraduate of Anhui University of Technology (2014077) and Natural Science Foundation of Anhui Province (1208085QE98). References [1] F. Duan, W.F. Dong, D.J. Shi, M.Q. Chen, Appl. Surf. Sci. 258 (2011) 189. [2] Y. Gu, Z.D. Xu, L. Guo, Y.Q. Wan, CrystEngComm 16 (2014) 10997. [3] M. Wang, Y.F. Tang, T.M. Sun, G.Q. Jiang, Y.J. Shi, CrystEngComm 16 (2014) 11035. [4] B.Y. Shi, G.H. Li, D.S. Wang, C.H. Feng, H.X. Tang, J. Hazard. Mater. 143 (2007) 567.
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