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Ag3VO4 under visible light irriadiation
Materials Science and Engineering B 178 (2013) 45–52
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Enhanced photodegradation activity of Rhodamine B by Co3 O4 /Ag3 VO4 under visible light irriadiation Lei Zhang a , Yiming He b , Ping Ye a , Wenhua Qin a , Ying Wu a,∗ , Tinghua Wu a a b
Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, 321004, PR China Department of Material Physics, College of Mathmatics Physics and Information Enginerring, Zhejiang Normal University, Jinhua, 321004, PR China
a r t i c l e
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Article history: Received 6 June 2012 Received in revised form 12 September 2012 Accepted 11 October 2012 Available online 25 October 2012 Keywords: Photocatalytic Rhodamine B Co3 O4 /Ag3 VO4 Visible light irradiation
a b s t r a c t Co3 O4 /Ag3 VO4 composite photocatalysts were synthesized by the wetness impregnation method and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive Xray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance spectroscopy (DRS). The photoactivity testing result shows that the Co3 O4 /Ag3 VO4 composite exhibits enhanced photodegradation activity for Rhodamine B (RhB) under visible-light irradiation. The Co3 O4 content and calcination temperature have a significant impact on the photocatalytic activities of the samples. The highest efficiency was observed on the 1.0 wt.% Co3 O4 /Ag3 VO4 sample calcined at 653 K. Moreover, the photoelectrochemical performance with the method of transient photocurrent–time was investigated. Based on the results of the characterizations, the mechanism of enhanced photocatalytic activity is discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Wastewater release from industries is a serious problem for environmental purification on the goal scale. Especially the discharge of these dyes contained wastewater, which is not only causing irritation of the skin, eyes and respiratory tract but also in many cases carcinogenic to humans and animals [1,2]. In order to remove these dyes, numerous efforts have been devoted to minimize the hazardous effects [3,4]. Among many options, the development of photocatalytic processes to transform the dyes into harmless compounds is one of the most effective solutions. Furthermore, owing to the visible light accounts for the largest proportion of the solar spectrum, great interests have been focused on degradation of organic pollutants under visible-light irradiation [5]. The strategies to develop high efficient and visible-light-driven photocatalysts include the modification of TiO2 [6–12] and the development of new non-TiO2 -based environmental cleaning materials. Recently, many investigations have been undertaken on the latter strategy [13–18]. A great number of novel photocatalysts, such as Bi2 WO6 [19], DyVO4 [20], BiVO4 [21,22], CaBi6 O10 [23], PbBi2 Nb2 O9 [24] and Ag2 M2 O7 (M = Mo, W) [25], were reported. The band gap of the above materials is appropriate for absorption of visible light, and therefore, they have activity under visible-light irradiation. However, the disadvantages, such as small specific
∗ Corresponding author. Tel.: +86 0579 82283907; fax: +86 0579 82282595. E-mail addresses: [email protected] (Y. Wu), [email protected] (T. Wu). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.10.011
surface areas and long migration distances for excited electron–hole pairs, were expected to lower their photocatalytic activities. It is worthwhile to note that a monoclinic scheelite Ag3 VO4 has raised much interest due to its special band structures. The valence band (VB) of Ag3 VO4 is composed of Ag 3d and O 1s orbits, whereas the conduction band (CB) is composed of V 3d orbits. The hybridization of Ag 3d and O 2p levels makes the VB largely dispersed, which enhances the mobility of photoexcited holes and the oxidation of organic pollutants [26]. As far as we know, several methods (such as deposition method [27,28] and hydrothermal treatment [29]) have been reported for the synthesis of Ag3 VO4. However, the activity of pure Ag3 VO4 is still low and needs to be modified. Co3 O4 is a semiconductor with interesting electronic, magnetic and catalytic properties [30,31]. Recently, it was also reported that Co3 O4 could promote the photocatalytic activity of BiVO4 and Bi2 WO6 [32,33]. These results indicate that Co3 O4 might be a good dopant to promote the photocataytic activity of Ag3 VO4 . In the present work, Co3 O4 /Ag3 VO4 samples with different Co concentrations and calcination temperatures were synthesized by impregnation technique. Its photocatalytic activity was evaluated by adopting RhB dye as the model pollutant under visible-light illumination. RhB, which is a highly water-soluble, basic red dye of the xanthene class, is a well-known fluorescent water tracer. The photodegradation of RhB is important with regard to the purification of dye effluents. In addition, we also reported the investigations on the photoelectrochemical performance with the method of successive sudden photocurrent–time, which is corresponded with the
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photocatalytic activity. Based on the result of investigations, the mechanism of enhanced photocatalytic activities of Co3 O4 /Ag3 VO4 is given. 2. Experimental 2.1. Catalysts preparation Co(NO3 )2 ·6H2 O (>99.0%), AgNO3 (>99.8%), NaOH (>96.0%), V2 O5 (>99.0%), Ti(OBu)4 (>98.5%), CO(NH2 )2 (>99.0%), and RhB (>99.0%) were all purchased from Sinopharm Cemical Reagent Co. Ltd. and other China’s chemical reagent Ltd. without further purification. Deionized water was used throughout this study. Ag3 VO4 was prepared by precipitation reaction as reported by Hu and Hu [27]: NaOH and V2 O5 powders in mole ratio 6:1 were put into a beaker with 20 mL distilled water, and magnetically stirred. Subsequently, the solution of 20 mL AgNO3 (1.5 mol L−1 ) was added. Yellow precipitate was formed immediately. The precipitate was aged at room temperature for one day, and washed with deionized water. The obtained sample was dried at 353 K and then calcined at 653 K for 4 h. The composite particles Co3 O4 /Ag3 VO4 were prepared by the impregnation method from an aqueous solution of Co(NO3 )2 . Ag3 VO4 powder (1.0 g) was added to 2 mL of distilled water containing an appropriate amount of Co(NO3 )2 in a ceramic dish. Water was evaporated at 353 K. The suspension was stirred using a glass rod during the evaporation. The resulting powder was collected and calcined in air at different temperatures for 2 h. The content of loading species was calculated according to the mass percentage of Co. Co3 O4 was prepared by calcining the Co(NO3 )2 at 653 K for 4 h in the air. The N-doped TiO2 was synthesized as reported by Kobayakawa et al. [34]. 2.2. Photoreaction apparatus and procedures The experiments were carried out in a self-made photoreactor (see Fig. S1). An annular quartz tube of a diameter 5 cm was used. A 500 W Xenon lamp was used as the light source. In order to obtain the visible light, optical filter ( ≥ 420 nm, Instrument Company of Nantong, China) was placed between the reactor tube and the Xenon lamp to cut off the unneeded light at certain wavelengths. The power density of the light at the position of reactor is about 35.2 mW/cm2 . A fan was used to cool down the reactor tube to prevent the effect of thermal catalytic reaction. Owing to continuous cooling, the temperature of the reaction solution was maintained at approximately 303 K. The distance between the light source and the center of the reaction solution is 10 cm. The amount of photocatalyst used was 2.0 g L−1 , the initial concentration of RhB is 1.0 × 10−5 mol L−1 . At the start of the experiment, the reaction solution (volume, 100 mL) containing reactants and photocatalyst was put in the unsealed tube. In order to disperse the photocatalyst powder and reach the adsorption–desorption equilibrium on the photocatalyst surface, the suspensions were magnetically stirred for an hour prior to irradiation. After illumination, the samples (volume of each is about 5 mL) were withdrawn from the reaction suspension at given time intervals, centrifuged at 4000 rpm for 10 min and filtered to remove the particles. The filtrate was then analyzed using a UV–vis spectrophotometer (TU-1810) measuring the absorption of RhB in the range of 400–700 nm. 2.3. Photoelectrochemical (transient photocurrent–time) experiments The electrochemical experiments (photocurrent measurements) were carried out with an electrochemical workstation (CHI660B) in a standard three-electrode quartz cell under zero bias
Fig. 1. XRD patterns of pure Co3 O4 , Ag3 VO4 and CoOx /Ag3 VO4 composite photocatalysts with different Co concentrations.
(see Fig. S2). An aqueous solution of 0.5 M Na2 SO4 was used as the electrolyte. A saturated Ag/AgCl electrode and a platinum wire were used as the reference and counter electrode, respectively. A 500 W Xe lamp positioned aside the cell window was used as the visiblelight source. An optical filter was placed in front of the lamp to allow the passage of light with wavelength longer than 420 nm. The working electrodes were irradiated from the side with samples in order to generate photocurrent effectively. The working electrodes were prepared on indium tin oxide (ITO) glass plates. The ITO glass pieces of a size of 1.5 cm × 5 cm were sonicated in acetone and degreased in boiling NaOH (0.1 M), and then rinsed with deionized water and dried in an air stream. The electrically conductive adhesive with a size of 0.8 cm × 0.8 cm was pressed on the bottom middle of ITO glass. Then the right amount of prepared powder was closely compacted on the exposed electrically conductive adhesive hard. 2.4. Characterization The X-ray diffraction (XRD) spectra of the catalysts were obtained on a Philips X-ray diffractometer (PW3040/60) using Cu K␣ radiation. The accelerating voltage and the current were 40 kV and 40 mA, respectively. The SEM pictures were taken on a field emission scanning electron microscope (Hitachi S-4800) and the chemical composition of the photocatalysts was determined using an energy dispersive X-ray spectrometer (EDS) attached to the SEM. The binding energies of Co, V, O, and Ag were measured at room temperature using an X-ray photoelectron spectroscope (XPS, VG ESCALAB MARK II) using Mg K␣ radiation. The UV–vis absorption spectra of the samples were measured in the range of 200–800 nm on a UV–vis Spectrometer (Nicolet, evolution 500, Thermo). 3. Results and discussion 3.1. XRD analysis The XRD patterns of CoOx /Ag3 VO4 samples are shown in Fig. 1. The as-prepared Ag3 VO4 sample is in its monoclinic phase (JCPDS No. 43-0542). Besides, the additive of Ag2 O was also observed at 32.9◦ (JCPDS No. 41-1104), which is consistent with the result of Hu and Hu [27]. It is reported that Ag2 O additive is usually accompanied with Ag3 VO4 . Fortunately, the additive is considered as “an observer” and shows no effect on the photocatalytic reaction [28,35,36]. The main diffraction peaks and their intensities of the
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Fig. 2. XRD patterns of 1.0 wt.% CoOx /Ag3 VO4 photocatalyst calcined at different temperature.
CoOx /Ag3 VO4 catalysts have no obvious changes after the doping of Co, indicating the crystal structure of the samples keeps stable. The literatures have reported that Co(NO3 )2 can be transformed into Co3 O4 during the calcination process [32,33]. We also obtained pure Co3 O4 (JCPDS 78-1969) by calcining Co(NO3 )2 at 653 K. Therefore, in the case of CoOx /Ag3 VO4 composite, we believe that the Co component might disperse on the surface of Ag3 VO4 powder in the form of Co3 O4 . However, the characteristic peaks of Co3 O4 were not found in the XRD analysis, which might be attributed to the low Co content. Fig. 2 shows the XRD patterns of the 1.0 wt.% CoOx /Ag3 VO4 prepared at different temperatures. As shown in Fig. 2, only monoclinic Ag3 VO4 was observed, and no fresh phase was detected. It indicates that the calcination process (below 773 K) cannot influence the crystal forms of Ag3 VO4 . 3.2. SEM–EDS and BET analysis The morphology of pure Ag3 VO4 and 1.0 wt.% CoOx /Ag3 VO4 composite (calcined at 653 K) are shown in Fig. 3. From Fig. 3a, it can be seen that the pure Ag3 VO4 is mainly composed of irregular spherical grains (with smooth surfaces) with average size about 70 nm. The CoOx /Ag3 VO4 composite (Fig. 3b) presents a similar morphology with pure Ag3 VO4 . It is difficult to distinguish the Co oxide from the Ag3 VO4 particle. However, the EDS result (Fig. 3c) shows the Co element was detected in addition to the original signals for Ag, V and O elements. It is revealed that the Co exists in the CoOx /Ag3 VO4 samples. The atomic ratio of Ag:V:Co is 2.8:1:0.06, which is close to the theoretical value of 3:1:0.05 for the sample of 1.0 wt.% Co3 O4 /Ag3 VO4 . The BET surface area of 1.0 wt.% Co3 O4 /Ag3 VO4 composite is 0.72 m2 /g, which is larger than that of Ag3 VO4 powder (0.31 m2 /g). The enlarged BET might be attributed to the introduction of the nanoparticles CoOx on the surface of Ag3 VO4 . Yet their BET values are nearly of the same magnitude, both of which are still very low. 3.3. XPS analysis Although the EDS technique is an effective tool in elemental analysis, it cannot discriminate the chemical states of elements. Therefore, XPS measurement was performed on the 1.0 wt.% CoOx /Ag3 VO4 photocatalyst. The full XPS spectrum of 1.0 wt.% CoOx /Ag3 VO4 catalyst is shown in Fig. 4. It can be seen that the main peaks at about 284, 530, 516, 374 (368) and 796 (781) eV are assigned to the binding energies of C 1s, O 1s, V 2p, Ag 3d and Co 2p, respectively. It indicates the existence of these elements, which is consistent with the EDS result. The high-resolution XPS
Fig. 3. SEM images of pure Ag3 VO4 (a), 1.0 wt.% CoOx /Ag3 VO4 (b) and EDS spectrum of the 1.0 wt.% CoOx /Ag3 VO4 (c).
spectra of the Ag 3d, O 1s, V 2p and Co 2p regions are shown in Fig. 5. The Ag 3d3/2 and Ag 3d5/2 binding energies were found at 374.1 and 367.9 eV, which can be assigned to Ag+ in Ag3 VO4 [37]. Like Ag component, the V element was also found in its highest valence state (V5+ ) [35,36]. The O 1s peak is fitted into two peaks at 532.7 eV and 530.0 eV (Fig. 5b), which can be assigned to the lattice oxygen in the prepared sample and the adsorbed H2 O, respectively [38,39]. For Co species, two peaks at 780.9 and 796.9 eV, which can be assigned to the Co 2p3/2 and Co 2p1/2 respectively, were observed. These peaks are very close to those of Co3 O4 [39], suggesting that Co3 O4 might be present on the surfaces of the corresponding Co-loaded Ag3 VO4 catalysts.
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Fig. 4. XPS of 1.0 wt.% Co3 O4 /Ag3 VO4 composite photocatalyst.
3.4. UV–vis analysis To investigate the optical absorption properties of the catalysts, pure Co3 O4 , Ag3 VO4 , N-doped TiO2 , and Co3 O4 /Ag3 VO4 composites calcined at 653 K with different Co contents were investigated by DRS. The results are shown in Fig. 6. Both N-doped TiO2 and Ag3 VO4 can absorb the visible light. The absorption edge of the two samples is at 550 nm and 650 nm, respectively. Co3 O4 presents a higher photoabsroption ability than N-doped TiO2 or Ag3 VO4 . It has absorption in nearly all the visible-light range. This fact induces an extension of the light absorption spectrum of the composite semiconductor even at low cobalt contents. As shown in Fig. 6a, the Co3 O4 /Ag3 VO4 composite shows an obvious red shift in the absorption band compared with the pure Ag3 VO4 , and the absorption edge depends largely on the amounts of Co content. After Co3 O4 was loaded, the ability of light absorption is enhanced greatly, especially in the visible-light region. This can be attributed to the small band-gap of Co3 O4 with direct transition at 1.45 and 2.07 eV, corresponding to edges of O2− to Co3+ excitation and O2− to Co2+ charge transfer respectively. The latter is the basic optical band gap energy for interband transitions [40]. The DRS spectra of 1.0 wt.% Co3 O4 /Ag3 VO4 composites calcined at different temperatures are displayed in Fig. 6b. The sample prepared at low (573 K) calcination temperature reveals the weaker absorbance than the catalyst calcined at 653 K in the region of 500–800 nm. However, when the calcination temperature gets up to the higher one, the absorbance intensity declines sharply, which may be attributed to the melting of Ag3 VO4 [41,42]. At high temperature, the Co3 O4 particle on the surface would be covered by the melted Ag3 VO4 . Thus, no light can be absorbed by the Co3 O4 particle which results in the sharp decrease in photoabsorption performance. The optical band gap of the prepared samples can be estimated by the following formula: (Ahv)2 = (hv − Eg ) (for direct band gap material) [43,44]. By plotting (Ahv)2 versus Eg , the band gap energy of Ag3 VO4 is estimated to be 2.05 eV (Fig. 6c), which is very close to the value of the reported literature [27]. 3.5. Transient photocurrent–time performance of the catalysts To study the photocurrent response of the electrodes to visiblelight irradiation [45,46], the photocurrent–time measurements were carried out for a period of 500 s under visible-light illumination in an on-and-off cycle mode. The results are shown in Fig. 7. It can be seen clearly that both the Ag3 VO4 and 1.0 wt.% Co3 O4 /Ag3 VO4 (calcined at 653 K) electrodes demonstrated a rapid photocurrent response when the visible light illumination was on
Fig. 5. High-resolution XPS spectra of (a) Ag 3d, (b) O 1s and (c) V 2p, Co 2p.
and off. The photocurrent intensity of the 1.0 wt.% Co3 O4 /Ag3 VO4 electrode was about 3 times as high as that of the Ag3 VO4 electrode. In general, the higher generation of photocurrent might result from a better electron transfer rate between the two materials so that the photogenerated electrons are more easily to form a greater current in the external electric field. In our experiment, the photocurrent enhancement of the Co3 O4 /Ag3 VO4 photocatalyst indicated an enhanced photoinduced electrons and holes separation, which could be attributed to the synergetic effect of Co3 O4
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Fig. 7. Unbiased photocurrent changes corresponding to successive sudden switching on and switching off the visible fight of Ag3 VO4 and 1.0 wt.% Co3 O4 /Ag3 VO4 photocatalysts.
shows the UV–vis spectral changes of RhB solution recorded for 0.1 wt.% Co3 O4 /Ag3 VO4 sample calcined at 653 K as a function of irradiation time. The RhB characteristic band centered at 553 nm was decreased promptly upon light irradiation, and floated to 505 nm which might be attributed to the formation of a series of N-de-ethylated intermediates of RhB since no obvious pH value
Fig. 6. UV–vis diffuse absorption spectra of N-doped TiO2 , Co3 O4 /Ag3 VO4 series catalysts with different Co content (a), 1.0 wt.% Co3 O4 /Ag3 VO4 calcined at different temperature (b) and estimated band gap of pure Ag3 VO4 (c).
and the Ag3 VO4 semiconductor [47]. That means the charge transfer might occur between the two semiconductors, which decrease the recombination of electron–hole pairs and subsequently prolong the lifetime of charges. 3.6. Photocatalytic activities The photocatalytic degradation of RhB dye on Co3 O4 /Ag3 VO4 composites was carried out under visible-light irradiation. Fig. 8a
Fig. 8. UV–vis spectral changes of RhB solution recorded for 1.0 wt.% Co3 O4 /Ag3 VO4 sample calcined at 653 K as a function of irradiation time (a) and the photocatalytic activity of Co3 O4 /Ag3 VO4 composite with different Co3 O4 content (b).
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Fig. 9. Degradation curves of RhB over the 1.0 wt.% Co3 O4 /Ag3 VO4 composite powders calcined at different temperature.
change was observed in the solution (see Fig. S3) [28]. After 80 min of irradiation, the RhB solution was colorized completely. Other Co3 O4 /Ag3 VO4 composites were also tested in the reaction system. Based on the equation of = (1 − Ct /C0 ) × 100%, where is the photocatalytic efficiency; C0 is the concentration of reactant before illumination; Ct is the concentration of reactant after illumination time t, the photocatalytic degradation efficiency of RhB was calculated. The result is shown in Fig. 8b. The blank test was carried out to determine the contribution of photolysis of RhB. It showed that there has little influence on the results of our experiments. The Co3 O4 semiconductor shows no apparent photocatalytic activity for the RhB degradation, while poor activity was observed on P25. Considering that P25 cannot absorb the visible light, the poor photoactivity can be attributed to the photosensitization effect of RhB. Besides, it also indicates that the contribution of the dye photosensitization effect is limited. N-doped TiO2 presents good activity because of its good photoabsorption ability. Due to the same reason, higher photocatalytic activity was observed on pure Ag3 VO4 . After 80 min of visible-light irradiation, about 65% of RhB was photodegraded. The doping of Co3 O4 to Ag3 VO4 can promote the photocatalytic activity. As shown in Fig. 8b, for the Co3 O4 /Ag3 VO4 samples, the photodegradation efficiency of RhB increases initially with the increasing of Co3 O4 contents, and then decreases as its content gets up to a higher level. The 1.0 wt.% Co3 O4 /Ag3 VO4 sample presents the highest activity. Fig. 9 shows the photocatalytic activities of 1.0 wt.% Co3 O4 /Ag3 VO4 composite calcined at different temperatures under visible light irradiation. The photocatalytic degradation efficiency of RhB increases gradually with the increase of the calcination temperature up to 653 K. When the temperature is above 653 K, however, the degradation efficiency decreases rapidly. It is also found that RhB degradation over Co3 O4 /Ag3 VO4 with different Co3 O4 contents is fitted by the pseudo-first-order kinetics. The linear plot of −ln(C/C0 ) versus irradiation time t is shown in Fig. 10. It can be seen that the Co-loaded samples exhibited higher photocatalytic degradation rates than both pure Ag3 VO4 and N-doped TiO2 . For Co3 O4 /Ag3 VO4 samples, the degradation rate attained maximum value of 0.0434 min−1 , which is 2.8 and 6.8 times higher than those of pure Ag3 VO4 and N-doped TiO2 , respectively. The pseudo-first-order constant and relative coefficients are summarized in Table S1. The Co3 O4 /Ag3 VO4 composites exhibit strong photocatalytic activity. However, in view of practical application, its stability is very important. Therefore, the circulating runs in the photocatalytic degradation of RhB in the presence of optimal Co3 O4 /Ag3 VO4
Fig. 10. Kinetic fit for the degradation of RhB with pure Ag3 VO4 , N-doped TiO2 and Co3 O4 /Ag3 VO4 catalysts with different Co3 O4 contents.
composite were carried out. The result shown in Fig. 11 suggests that the photocatalytic activity does not exhibit any significant loss after four terms of recycling. The XRD pattern and UV–vis spectrum of the used sample also shows that no change was observed (see Figs. S4 and S5). It indicates that the Co3 O4 /Ag3 VO4 composite photocatalysts are stable in photocatalysis. Chemical oxygen demand (COD) measurements of the RhB solution were also performed to verify whether dye is completely degraded to carbonate. The result shows a COD removal of 70.8% during the photocatalytic process (see Fig. S6), suggesting that there exist some smaller organic molecules. The further modification is still needed to promote the complete oxidation ability of the Co3 O4 /Ag3 VO4 composite. 3.7. Discussion of mechanism Both Ag3 VO4 and Co3 O4 /Ag3 VO4 present very low specific surface areas. The adsorption experiment in the dark further shows that no correlation between the RhB adsorption and photoactivity was observed (see Fig. S7). It indicates that the enhanced photocatalytic performance of Co3 O4 /Ag3 VO4 can mainly be demonstrated by the synergism and coupling effect between the two semiconductors, which favors an effective photoexcited electron–hole
Fig. 11. Cycling runs in photocatalytic degradation of RhB in the presence of 1.0 wt.% Co3 O4 /Ag3 VO4 composite calcined at 653 K under visible light irradiation.
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Therefore, the photocatalytic activity of the sample calcined at 773 K is significantly decreased as compared to the catalyst prepared at 653 K heat-treatment. On the basis of the above analysis, it is suggested that the enhanced photocatalytic activity of Co3 O4 loaded Ag3 VO4 could be attributed to the synergetic effect of the above factors. 4. Conclusions
Fig. 12. Schematic diagram of synergistic effect of Co3 O4 /Ag3 VO4 .
separation in the two materials [48,49]. The conduction band (CB) and valence band (VB) potentials of Co3 O4 and Ag3 VO4 at the point of zero charge can be calculated based on the previous literatures [50,51]. The results are shown in Table S2. The CB edge potential of Ag3 VO4 (0.12 eV) is more negative than that of Co3 O4 (0.37 eV), hence, photoinduced electrons on the Ag3 VO4 particle surface can easily migrate to Co3 O4 via interfaces; similarly, photoinduced holes on the Co3 O4 surface can transfer to Ag3 VO4 owing to the different VB edge potentials, as shown in Fig. 12. As a result, a larger amount of electrons on the Co3 O4 surface and holes on the Ag3 VO4 surface can participate in photocatalytic reactions to directly or indirectly decompose RhB. Besides the obvious matching ability of band potentials between semiconductors described above, the photocatalytic activity of the catalysts also strongly depends on the optical properties and the crystallinity of semiconductor. The result of DRS shows that the doping of Co3 O4 can promote the photoabsorption performance. The absorption wavelength range of Co3 O4 /Ag3 VO4 catalyst is shifted to the longer wavelength, which can facilitate the absorption in the visible-light region. The larger red shift in absorption indicates that the catalyst can absorb more photons and generate more electron–hole pairs, and the photocatalytic activity is thus enhanced [38]. However, it is interesting to note that the photocatalytic activities of Co3 O4 /Ag3 VO4 catalysts enhance with the increase in Co3 O4 content up to 1.0 wt.% and then decrease. A similar result was also reported by Xu et al. [46]. It is considered that high Co3 O4 content (>1.0 wt.%) is harmful to the photocatalytic activity because of the excess coverage of active sites on the Ag3 VO4 surface by Co3 O4 particles [46]. The calcination temperature was also found to play a vital role in the photocatalytic activity. The photoactivity of the 1.0 wt.% Co3 O4 /Ag3 VO4 samples calcined at a different temperature follows the order: 653 K > 573 K > 773 K (Fig. 9). In this work, the Co(NO3 )2 precursor was gradually decomposed into cobalt oxide during calcination process, and the crystallinity of Co3 O4 was enhanced by increasing the temperature. Generally, higher crystallinity of the sample is always beneficial to the separation of electron and hole pairs as compared to the amorphous structure [32]. As a consequence, the photocatalytic activity of the Co3 O4 /Ag3 VO4 was enhanced with increasing temperature. However, at higher calcination temperatures (≥653 K), the Co3 O4 particle might be agglomerated, thus decrease photocatalytic activity of the sample. In addition, as the calcination temperature exceeds 653 K, the Co3 O4 nanoparticles are entrapped or encapsulated in the melted Ag3 VO4 [43,44].
In conclusion, a novel coupled photocatalyst, Co3 O4 /Ag3 VO4 , was prepared by impregnation method. The synthesized Co3 O4 /Ag3 VO4 catalysts showed high photocatalytic efficiency for the degradation of RhB under visible-light irradiation, and the optimal activity was obtained on the sample calcined at 653 K with 1.0 wt.% Co content. In the range of concentrations used, pseudozero-order kinetics was most suitable to describe the change of RhB concentration with time in the initial photocatalytic removal phase. The enhanced photocatalytic activity of Co3 O4 /Ag3 VO4 was mainly ascribed to the coupling effect in the separation of photoexcited electron–hole pairs. The Co3 O4 /Ag3 VO4 photocatalyst is promising for practical application for water purification because of its high activity and stability. In addition, the success of the preparation of Co3 O4 /Ag3 VO4 composite indicates that coupling two different semiconductors with suitable band potential is an efficient way to synthesize visible-light-driven photocatalyst with high activity. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21003109, 21003108), the Applied Research Foundation of Public Welfare Technologies of Zhejiang Province (2012C37057), and the program for Zhejiang Leading Team of Science and Technology Innovation (2009R50020). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mseb. 2012.10.011. References [1] J. Rochat, P. Demenge, J.C. Rerat, Toxicological European Research. Recherche Europeenne En Toxicologie 1 (1978) 23–26. [2] R. Jain, M. Mathur, S. Sikarwar, A. Mittal, Journal of Environment Management 85 (2007) 956–964. [3] O. Hamdaoui, Desalination 271 (2011) 279–286. [4] W.L. Zhang, Y. Li, C. Wang, P.F. Wang, Desalination 266 (2011) 40–45. [5] W. Choi, Catalysis Surveys from Asia 10 (2006) 16–28. [6] M. Zhang, C.C. Chen, W.H. Ma, J.C. Zhao, Angewandte Chemie International Edition 47 (2008) 1–5. [7] G. Liu, Y.N. Zhao, C.H. Sun, F. Li, G.Q. Lu, H.M. Cheng, Angewandte Chemie International Edition 120 (2008) 4592–4596. [8] X.C. Wang, J.C. Yu, Y.L. Chen, L. Wu, X.Z. Fu, Environmental Science and Technology 40 (2006) 2369–2374. [9] D.F. Zhang, Russian Journal of Physical Chemistry A 86 (2012) 675–680. [10] D.F. Zhang, Russian Journal of Physical Chemistry A 86 (2012) 498–503. [11] D.F. Zhang, Monatshefte fur Chemie 143 (2012) 729–738. [12] D.F. Zhang, Journal of Sol–Gel Science and Technology 58 (2011) 312–318. [13] Y.N. Rajeswari, B.A. Chandra, Science of Advanced Materials 4 (2012) 673–680. [14] Y.N. Rajeswari, B.A. Chandra, Science of Advanced Materials 4 (2012) 54–60. [15] C.R. Adrian, A. Luminita, D. Anca, Journal for Nanoscience and Nanotechnology 11 (2011) 9095–9101. [16] J. Zhen, F.G. Tao, H.X. Ye, W. Min, D.Z. Li, Journal of Nanoengineering and Nanomanufacturing 2 (2012) 49–53. [17] J. Kim, C.W. Lee, W. Choi, Platinized, Environmental Science and Technology 44 (2010) 6849–6854. [18] D.F. Zhang, F.B. Zeng, Journal of Materials Science 47 (2012) 2155–2161. [19] Z. He, C. Sun, S. Yang, Y. Ding, H. He, Z. Wang, Journal of Hazardous Materials 162 (2009) 1477–1486. [20] Y.M. He, L.H. Zhao, Y.J. Wang, H. Lin, T. Li, X.T. Wu, Y. Wu, Chemical Engineering Journal 169 (2011) 50–57.
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[21] L. Zhang, D. Chen, X. Jiao, Journal of Physical Chemistry B 110 (2006) 2668–2673. [22] G. Li, D. Zhang, J. Yu, Chemistry of Materials 20 (2008) 3983–3992. [23] Y.J. Wang, Y.M. He, T.T. Li, J. Cai, M.F. Luo, L.H. Zhao, Catalysis Communications 18 (2012) 161–164. [24] H.G. Kim, D.W. Hwang, J.S. Lee, Journal of the American Chemical Society 126 (2004) 8912–8913. [25] D.W. Kim, I.S. Cho, S. Lee, S.T. Bae, S.S. Shin, G.S. Han, H.S. Jung, K.S. Hong, Photophysical, Journal of the American Ceramic Society 93 (2010) 3867– 3872. [26] R. Konta, H. Kato, H. Kobayashi, A. Kudo, Physical Chemistry Chemical Physics 5 (2003) 3061–3065. [27] X.X. Hu, C. Hu, Journal of Solid State Chemistry 180 (2007) 725–732. [28] H. Xu, H.M. Li, G.S. Sun, J.X. Xia, C.D. Wu, Z.X. Ye, Q. Zhang, Chemical Engineering Journal 160 (2010) 33–41. [29] C.M. Huang, K.W. Cheng, G.T. Pan, W.S. Chang, T.C.K. Yang, Chemical Engineering Science 65 (2010) 148–152. [30] H.A.E. Hagelin-Weaver, G.B. Hoflund, D.M. Minahan, G.N. Salaita, Applied Surface Science 235 (2004) 420–448. [31] M. Casas-Cabanas, G. Binotto, D. Larcher, A. Lecup, V. Giordani, J.M. Tarascon, Chemistry of Materials 21 (2009) 1939–1947. [32] M. Long, W.M. Cai, J. Cai, B.X. Zhou, X.Y. Chai, Y.H. Wu, Journal of Physical Chemistry B 110 (2006) 20211–20216. [33] Q. Xiao, J. Zhang, C. Xiao, X.K. Tan, Catalysis Communications 9 (2008) 1247–1253. [34] K. Kobayakawa, Y. Murakami, Y. Sato, Journal of Photochemistry and Photobiology A 170 (2005) 177–179. [35] H. Xu, H.M. Li, L. Xu, G.S. Sun, Y.G. Xu, J.Y. Chu, Industrial and Engineering Chemistry Research 48 (2009) 10771–10778.
[36] S.F. Chen, W. Zhao, W. Liu, H.Y. Zhang, X.L. Yu, Y.H. Chen, Journal of Hazardous Materials 172 (2009) 1415–1423. [37] J. Bai, Y. Li, M. Li, S. Wang, C. Zhang, Q. Yang, Applied Surface Science 254 (2008) 4520–4523. [38] B.Z. Tian, C.Z. Li, J.L. Zhang, Chemical Engineering Journal 191 (2012) 402–409. [39] A. Gulino, I. Fragala, Inorganica Chimica Acta 358 (2005) 4466–4472. [40] D. Barreca, C. Massignan, S. Daolio, M. Fabrizio, C. Piccirillo, L. Armelao, E. Tondello, Chemistry of Materials 13 (2001) 588–593. [41] E. Wenda, Journal of Thermal Analysis 30 (1985) 879–887. [42] E. Wenda, A. Bielanski, Journal of Thermal Analysis and Calorimetry 92 (2008) 921–929. [43] H. Xu, H. Li, C. Wu, J. Chu, Y. Yan, H. Shu, Z. Gu, Journal of Hazardous Materials 153 (2008) 877–884. [44] H. Xu, H. Li, C. Wu, J. Chu, Y. Yan, H. Shu, Materials Science and Engineering B 147 (2008) 52–56. [45] M. Long, W. Cai, H. Kisch, Journal of Physical Chemistry C 112 (2008) 548–554. [46] R. Panda, V. Rathor, M. Rathore, V. Shelke, N. Badera, L.S. Sharath Chandra, D. Jain, M. Gangrade, T. Shripati, V. Ganesan, Applied Surface Science 258 (2012) 5086–5093. [47] C. He, X.Z. Li, N. Graham, Y. Wang, Applied Catalysis A: General 305 (2006) 54–63. [48] Y.J. Wang, R. Shi, J. Lin, Y.F. Zhu, Energy and Environmental Science 4 (2011) 2922–2929. [49] H.J. Huang, D.Z. Li, Q. Lin, W.J. Zhang, Y. Shao, Y.B. Chen, M. Sun, X.Z. Fu, Environmental Science and Technology 43 (2009) 4164–4168. [50] Y. Xu, M.A.A. Schoonen, American Mineralogist 85 (2000) 543–556. [51] J.W. Tang, J.H. Ye, Chemical Physics Letters 410 (2005) 104–107.
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Enhanced photodegradation activity of Rhodamine B by Co3 O4 /Ag3 VO4 under visible light irriadiation Lei Zhang a , Yiming He b , Ping Ye a , Wenhua Qin a , Ying Wu a,∗ , Tinghua Wu a a b
Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua, 321004, PR China Department of Material Physics, College of Mathmatics Physics and Information Enginerring, Zhejiang Normal University, Jinhua, 321004, PR China
a r t i c l e
i n f o
Article history: Received 6 June 2012 Received in revised form 12 September 2012 Accepted 11 October 2012 Available online 25 October 2012 Keywords: Photocatalytic Rhodamine B Co3 O4 /Ag3 VO4 Visible light irradiation
a b s t r a c t Co3 O4 /Ag3 VO4 composite photocatalysts were synthesized by the wetness impregnation method and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive Xray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance spectroscopy (DRS). The photoactivity testing result shows that the Co3 O4 /Ag3 VO4 composite exhibits enhanced photodegradation activity for Rhodamine B (RhB) under visible-light irradiation. The Co3 O4 content and calcination temperature have a significant impact on the photocatalytic activities of the samples. The highest efficiency was observed on the 1.0 wt.% Co3 O4 /Ag3 VO4 sample calcined at 653 K. Moreover, the photoelectrochemical performance with the method of transient photocurrent–time was investigated. Based on the results of the characterizations, the mechanism of enhanced photocatalytic activity is discussed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Wastewater release from industries is a serious problem for environmental purification on the goal scale. Especially the discharge of these dyes contained wastewater, which is not only causing irritation of the skin, eyes and respiratory tract but also in many cases carcinogenic to humans and animals [1,2]. In order to remove these dyes, numerous efforts have been devoted to minimize the hazardous effects [3,4]. Among many options, the development of photocatalytic processes to transform the dyes into harmless compounds is one of the most effective solutions. Furthermore, owing to the visible light accounts for the largest proportion of the solar spectrum, great interests have been focused on degradation of organic pollutants under visible-light irradiation [5]. The strategies to develop high efficient and visible-light-driven photocatalysts include the modification of TiO2 [6–12] and the development of new non-TiO2 -based environmental cleaning materials. Recently, many investigations have been undertaken on the latter strategy [13–18]. A great number of novel photocatalysts, such as Bi2 WO6 [19], DyVO4 [20], BiVO4 [21,22], CaBi6 O10 [23], PbBi2 Nb2 O9 [24] and Ag2 M2 O7 (M = Mo, W) [25], were reported. The band gap of the above materials is appropriate for absorption of visible light, and therefore, they have activity under visible-light irradiation. However, the disadvantages, such as small specific
∗ Corresponding author. Tel.: +86 0579 82283907; fax: +86 0579 82282595. E-mail addresses: [email protected] (Y. Wu), [email protected] (T. Wu). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.10.011
surface areas and long migration distances for excited electron–hole pairs, were expected to lower their photocatalytic activities. It is worthwhile to note that a monoclinic scheelite Ag3 VO4 has raised much interest due to its special band structures. The valence band (VB) of Ag3 VO4 is composed of Ag 3d and O 1s orbits, whereas the conduction band (CB) is composed of V 3d orbits. The hybridization of Ag 3d and O 2p levels makes the VB largely dispersed, which enhances the mobility of photoexcited holes and the oxidation of organic pollutants [26]. As far as we know, several methods (such as deposition method [27,28] and hydrothermal treatment [29]) have been reported for the synthesis of Ag3 VO4. However, the activity of pure Ag3 VO4 is still low and needs to be modified. Co3 O4 is a semiconductor with interesting electronic, magnetic and catalytic properties [30,31]. Recently, it was also reported that Co3 O4 could promote the photocatalytic activity of BiVO4 and Bi2 WO6 [32,33]. These results indicate that Co3 O4 might be a good dopant to promote the photocataytic activity of Ag3 VO4 . In the present work, Co3 O4 /Ag3 VO4 samples with different Co concentrations and calcination temperatures were synthesized by impregnation technique. Its photocatalytic activity was evaluated by adopting RhB dye as the model pollutant under visible-light illumination. RhB, which is a highly water-soluble, basic red dye of the xanthene class, is a well-known fluorescent water tracer. The photodegradation of RhB is important with regard to the purification of dye effluents. In addition, we also reported the investigations on the photoelectrochemical performance with the method of successive sudden photocurrent–time, which is corresponded with the
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photocatalytic activity. Based on the result of investigations, the mechanism of enhanced photocatalytic activities of Co3 O4 /Ag3 VO4 is given. 2. Experimental 2.1. Catalysts preparation Co(NO3 )2 ·6H2 O (>99.0%), AgNO3 (>99.8%), NaOH (>96.0%), V2 O5 (>99.0%), Ti(OBu)4 (>98.5%), CO(NH2 )2 (>99.0%), and RhB (>99.0%) were all purchased from Sinopharm Cemical Reagent Co. Ltd. and other China’s chemical reagent Ltd. without further purification. Deionized water was used throughout this study. Ag3 VO4 was prepared by precipitation reaction as reported by Hu and Hu [27]: NaOH and V2 O5 powders in mole ratio 6:1 were put into a beaker with 20 mL distilled water, and magnetically stirred. Subsequently, the solution of 20 mL AgNO3 (1.5 mol L−1 ) was added. Yellow precipitate was formed immediately. The precipitate was aged at room temperature for one day, and washed with deionized water. The obtained sample was dried at 353 K and then calcined at 653 K for 4 h. The composite particles Co3 O4 /Ag3 VO4 were prepared by the impregnation method from an aqueous solution of Co(NO3 )2 . Ag3 VO4 powder (1.0 g) was added to 2 mL of distilled water containing an appropriate amount of Co(NO3 )2 in a ceramic dish. Water was evaporated at 353 K. The suspension was stirred using a glass rod during the evaporation. The resulting powder was collected and calcined in air at different temperatures for 2 h. The content of loading species was calculated according to the mass percentage of Co. Co3 O4 was prepared by calcining the Co(NO3 )2 at 653 K for 4 h in the air. The N-doped TiO2 was synthesized as reported by Kobayakawa et al. [34]. 2.2. Photoreaction apparatus and procedures The experiments were carried out in a self-made photoreactor (see Fig. S1). An annular quartz tube of a diameter 5 cm was used. A 500 W Xenon lamp was used as the light source. In order to obtain the visible light, optical filter ( ≥ 420 nm, Instrument Company of Nantong, China) was placed between the reactor tube and the Xenon lamp to cut off the unneeded light at certain wavelengths. The power density of the light at the position of reactor is about 35.2 mW/cm2 . A fan was used to cool down the reactor tube to prevent the effect of thermal catalytic reaction. Owing to continuous cooling, the temperature of the reaction solution was maintained at approximately 303 K. The distance between the light source and the center of the reaction solution is 10 cm. The amount of photocatalyst used was 2.0 g L−1 , the initial concentration of RhB is 1.0 × 10−5 mol L−1 . At the start of the experiment, the reaction solution (volume, 100 mL) containing reactants and photocatalyst was put in the unsealed tube. In order to disperse the photocatalyst powder and reach the adsorption–desorption equilibrium on the photocatalyst surface, the suspensions were magnetically stirred for an hour prior to irradiation. After illumination, the samples (volume of each is about 5 mL) were withdrawn from the reaction suspension at given time intervals, centrifuged at 4000 rpm for 10 min and filtered to remove the particles. The filtrate was then analyzed using a UV–vis spectrophotometer (TU-1810) measuring the absorption of RhB in the range of 400–700 nm. 2.3. Photoelectrochemical (transient photocurrent–time) experiments The electrochemical experiments (photocurrent measurements) were carried out with an electrochemical workstation (CHI660B) in a standard three-electrode quartz cell under zero bias
Fig. 1. XRD patterns of pure Co3 O4 , Ag3 VO4 and CoOx /Ag3 VO4 composite photocatalysts with different Co concentrations.
(see Fig. S2). An aqueous solution of 0.5 M Na2 SO4 was used as the electrolyte. A saturated Ag/AgCl electrode and a platinum wire were used as the reference and counter electrode, respectively. A 500 W Xe lamp positioned aside the cell window was used as the visiblelight source. An optical filter was placed in front of the lamp to allow the passage of light with wavelength longer than 420 nm. The working electrodes were irradiated from the side with samples in order to generate photocurrent effectively. The working electrodes were prepared on indium tin oxide (ITO) glass plates. The ITO glass pieces of a size of 1.5 cm × 5 cm were sonicated in acetone and degreased in boiling NaOH (0.1 M), and then rinsed with deionized water and dried in an air stream. The electrically conductive adhesive with a size of 0.8 cm × 0.8 cm was pressed on the bottom middle of ITO glass. Then the right amount of prepared powder was closely compacted on the exposed electrically conductive adhesive hard. 2.4. Characterization The X-ray diffraction (XRD) spectra of the catalysts were obtained on a Philips X-ray diffractometer (PW3040/60) using Cu K␣ radiation. The accelerating voltage and the current were 40 kV and 40 mA, respectively. The SEM pictures were taken on a field emission scanning electron microscope (Hitachi S-4800) and the chemical composition of the photocatalysts was determined using an energy dispersive X-ray spectrometer (EDS) attached to the SEM. The binding energies of Co, V, O, and Ag were measured at room temperature using an X-ray photoelectron spectroscope (XPS, VG ESCALAB MARK II) using Mg K␣ radiation. The UV–vis absorption spectra of the samples were measured in the range of 200–800 nm on a UV–vis Spectrometer (Nicolet, evolution 500, Thermo). 3. Results and discussion 3.1. XRD analysis The XRD patterns of CoOx /Ag3 VO4 samples are shown in Fig. 1. The as-prepared Ag3 VO4 sample is in its monoclinic phase (JCPDS No. 43-0542). Besides, the additive of Ag2 O was also observed at 32.9◦ (JCPDS No. 41-1104), which is consistent with the result of Hu and Hu [27]. It is reported that Ag2 O additive is usually accompanied with Ag3 VO4 . Fortunately, the additive is considered as “an observer” and shows no effect on the photocatalytic reaction [28,35,36]. The main diffraction peaks and their intensities of the
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Fig. 2. XRD patterns of 1.0 wt.% CoOx /Ag3 VO4 photocatalyst calcined at different temperature.
CoOx /Ag3 VO4 catalysts have no obvious changes after the doping of Co, indicating the crystal structure of the samples keeps stable. The literatures have reported that Co(NO3 )2 can be transformed into Co3 O4 during the calcination process [32,33]. We also obtained pure Co3 O4 (JCPDS 78-1969) by calcining Co(NO3 )2 at 653 K. Therefore, in the case of CoOx /Ag3 VO4 composite, we believe that the Co component might disperse on the surface of Ag3 VO4 powder in the form of Co3 O4 . However, the characteristic peaks of Co3 O4 were not found in the XRD analysis, which might be attributed to the low Co content. Fig. 2 shows the XRD patterns of the 1.0 wt.% CoOx /Ag3 VO4 prepared at different temperatures. As shown in Fig. 2, only monoclinic Ag3 VO4 was observed, and no fresh phase was detected. It indicates that the calcination process (below 773 K) cannot influence the crystal forms of Ag3 VO4 . 3.2. SEM–EDS and BET analysis The morphology of pure Ag3 VO4 and 1.0 wt.% CoOx /Ag3 VO4 composite (calcined at 653 K) are shown in Fig. 3. From Fig. 3a, it can be seen that the pure Ag3 VO4 is mainly composed of irregular spherical grains (with smooth surfaces) with average size about 70 nm. The CoOx /Ag3 VO4 composite (Fig. 3b) presents a similar morphology with pure Ag3 VO4 . It is difficult to distinguish the Co oxide from the Ag3 VO4 particle. However, the EDS result (Fig. 3c) shows the Co element was detected in addition to the original signals for Ag, V and O elements. It is revealed that the Co exists in the CoOx /Ag3 VO4 samples. The atomic ratio of Ag:V:Co is 2.8:1:0.06, which is close to the theoretical value of 3:1:0.05 for the sample of 1.0 wt.% Co3 O4 /Ag3 VO4 . The BET surface area of 1.0 wt.% Co3 O4 /Ag3 VO4 composite is 0.72 m2 /g, which is larger than that of Ag3 VO4 powder (0.31 m2 /g). The enlarged BET might be attributed to the introduction of the nanoparticles CoOx on the surface of Ag3 VO4 . Yet their BET values are nearly of the same magnitude, both of which are still very low. 3.3. XPS analysis Although the EDS technique is an effective tool in elemental analysis, it cannot discriminate the chemical states of elements. Therefore, XPS measurement was performed on the 1.0 wt.% CoOx /Ag3 VO4 photocatalyst. The full XPS spectrum of 1.0 wt.% CoOx /Ag3 VO4 catalyst is shown in Fig. 4. It can be seen that the main peaks at about 284, 530, 516, 374 (368) and 796 (781) eV are assigned to the binding energies of C 1s, O 1s, V 2p, Ag 3d and Co 2p, respectively. It indicates the existence of these elements, which is consistent with the EDS result. The high-resolution XPS
Fig. 3. SEM images of pure Ag3 VO4 (a), 1.0 wt.% CoOx /Ag3 VO4 (b) and EDS spectrum of the 1.0 wt.% CoOx /Ag3 VO4 (c).
spectra of the Ag 3d, O 1s, V 2p and Co 2p regions are shown in Fig. 5. The Ag 3d3/2 and Ag 3d5/2 binding energies were found at 374.1 and 367.9 eV, which can be assigned to Ag+ in Ag3 VO4 [37]. Like Ag component, the V element was also found in its highest valence state (V5+ ) [35,36]. The O 1s peak is fitted into two peaks at 532.7 eV and 530.0 eV (Fig. 5b), which can be assigned to the lattice oxygen in the prepared sample and the adsorbed H2 O, respectively [38,39]. For Co species, two peaks at 780.9 and 796.9 eV, which can be assigned to the Co 2p3/2 and Co 2p1/2 respectively, were observed. These peaks are very close to those of Co3 O4 [39], suggesting that Co3 O4 might be present on the surfaces of the corresponding Co-loaded Ag3 VO4 catalysts.
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Fig. 4. XPS of 1.0 wt.% Co3 O4 /Ag3 VO4 composite photocatalyst.
3.4. UV–vis analysis To investigate the optical absorption properties of the catalysts, pure Co3 O4 , Ag3 VO4 , N-doped TiO2 , and Co3 O4 /Ag3 VO4 composites calcined at 653 K with different Co contents were investigated by DRS. The results are shown in Fig. 6. Both N-doped TiO2 and Ag3 VO4 can absorb the visible light. The absorption edge of the two samples is at 550 nm and 650 nm, respectively. Co3 O4 presents a higher photoabsroption ability than N-doped TiO2 or Ag3 VO4 . It has absorption in nearly all the visible-light range. This fact induces an extension of the light absorption spectrum of the composite semiconductor even at low cobalt contents. As shown in Fig. 6a, the Co3 O4 /Ag3 VO4 composite shows an obvious red shift in the absorption band compared with the pure Ag3 VO4 , and the absorption edge depends largely on the amounts of Co content. After Co3 O4 was loaded, the ability of light absorption is enhanced greatly, especially in the visible-light region. This can be attributed to the small band-gap of Co3 O4 with direct transition at 1.45 and 2.07 eV, corresponding to edges of O2− to Co3+ excitation and O2− to Co2+ charge transfer respectively. The latter is the basic optical band gap energy for interband transitions [40]. The DRS spectra of 1.0 wt.% Co3 O4 /Ag3 VO4 composites calcined at different temperatures are displayed in Fig. 6b. The sample prepared at low (573 K) calcination temperature reveals the weaker absorbance than the catalyst calcined at 653 K in the region of 500–800 nm. However, when the calcination temperature gets up to the higher one, the absorbance intensity declines sharply, which may be attributed to the melting of Ag3 VO4 [41,42]. At high temperature, the Co3 O4 particle on the surface would be covered by the melted Ag3 VO4 . Thus, no light can be absorbed by the Co3 O4 particle which results in the sharp decrease in photoabsorption performance. The optical band gap of the prepared samples can be estimated by the following formula: (Ahv)2 = (hv − Eg ) (for direct band gap material) [43,44]. By plotting (Ahv)2 versus Eg , the band gap energy of Ag3 VO4 is estimated to be 2.05 eV (Fig. 6c), which is very close to the value of the reported literature [27]. 3.5. Transient photocurrent–time performance of the catalysts To study the photocurrent response of the electrodes to visiblelight irradiation [45,46], the photocurrent–time measurements were carried out for a period of 500 s under visible-light illumination in an on-and-off cycle mode. The results are shown in Fig. 7. It can be seen clearly that both the Ag3 VO4 and 1.0 wt.% Co3 O4 /Ag3 VO4 (calcined at 653 K) electrodes demonstrated a rapid photocurrent response when the visible light illumination was on
Fig. 5. High-resolution XPS spectra of (a) Ag 3d, (b) O 1s and (c) V 2p, Co 2p.
and off. The photocurrent intensity of the 1.0 wt.% Co3 O4 /Ag3 VO4 electrode was about 3 times as high as that of the Ag3 VO4 electrode. In general, the higher generation of photocurrent might result from a better electron transfer rate between the two materials so that the photogenerated electrons are more easily to form a greater current in the external electric field. In our experiment, the photocurrent enhancement of the Co3 O4 /Ag3 VO4 photocatalyst indicated an enhanced photoinduced electrons and holes separation, which could be attributed to the synergetic effect of Co3 O4
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Fig. 7. Unbiased photocurrent changes corresponding to successive sudden switching on and switching off the visible fight of Ag3 VO4 and 1.0 wt.% Co3 O4 /Ag3 VO4 photocatalysts.
shows the UV–vis spectral changes of RhB solution recorded for 0.1 wt.% Co3 O4 /Ag3 VO4 sample calcined at 653 K as a function of irradiation time. The RhB characteristic band centered at 553 nm was decreased promptly upon light irradiation, and floated to 505 nm which might be attributed to the formation of a series of N-de-ethylated intermediates of RhB since no obvious pH value
Fig. 6. UV–vis diffuse absorption spectra of N-doped TiO2 , Co3 O4 /Ag3 VO4 series catalysts with different Co content (a), 1.0 wt.% Co3 O4 /Ag3 VO4 calcined at different temperature (b) and estimated band gap of pure Ag3 VO4 (c).
and the Ag3 VO4 semiconductor [47]. That means the charge transfer might occur between the two semiconductors, which decrease the recombination of electron–hole pairs and subsequently prolong the lifetime of charges. 3.6. Photocatalytic activities The photocatalytic degradation of RhB dye on Co3 O4 /Ag3 VO4 composites was carried out under visible-light irradiation. Fig. 8a
Fig. 8. UV–vis spectral changes of RhB solution recorded for 1.0 wt.% Co3 O4 /Ag3 VO4 sample calcined at 653 K as a function of irradiation time (a) and the photocatalytic activity of Co3 O4 /Ag3 VO4 composite with different Co3 O4 content (b).
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Fig. 9. Degradation curves of RhB over the 1.0 wt.% Co3 O4 /Ag3 VO4 composite powders calcined at different temperature.
change was observed in the solution (see Fig. S3) [28]. After 80 min of irradiation, the RhB solution was colorized completely. Other Co3 O4 /Ag3 VO4 composites were also tested in the reaction system. Based on the equation of = (1 − Ct /C0 ) × 100%, where is the photocatalytic efficiency; C0 is the concentration of reactant before illumination; Ct is the concentration of reactant after illumination time t, the photocatalytic degradation efficiency of RhB was calculated. The result is shown in Fig. 8b. The blank test was carried out to determine the contribution of photolysis of RhB. It showed that there has little influence on the results of our experiments. The Co3 O4 semiconductor shows no apparent photocatalytic activity for the RhB degradation, while poor activity was observed on P25. Considering that P25 cannot absorb the visible light, the poor photoactivity can be attributed to the photosensitization effect of RhB. Besides, it also indicates that the contribution of the dye photosensitization effect is limited. N-doped TiO2 presents good activity because of its good photoabsorption ability. Due to the same reason, higher photocatalytic activity was observed on pure Ag3 VO4 . After 80 min of visible-light irradiation, about 65% of RhB was photodegraded. The doping of Co3 O4 to Ag3 VO4 can promote the photocatalytic activity. As shown in Fig. 8b, for the Co3 O4 /Ag3 VO4 samples, the photodegradation efficiency of RhB increases initially with the increasing of Co3 O4 contents, and then decreases as its content gets up to a higher level. The 1.0 wt.% Co3 O4 /Ag3 VO4 sample presents the highest activity. Fig. 9 shows the photocatalytic activities of 1.0 wt.% Co3 O4 /Ag3 VO4 composite calcined at different temperatures under visible light irradiation. The photocatalytic degradation efficiency of RhB increases gradually with the increase of the calcination temperature up to 653 K. When the temperature is above 653 K, however, the degradation efficiency decreases rapidly. It is also found that RhB degradation over Co3 O4 /Ag3 VO4 with different Co3 O4 contents is fitted by the pseudo-first-order kinetics. The linear plot of −ln(C/C0 ) versus irradiation time t is shown in Fig. 10. It can be seen that the Co-loaded samples exhibited higher photocatalytic degradation rates than both pure Ag3 VO4 and N-doped TiO2 . For Co3 O4 /Ag3 VO4 samples, the degradation rate attained maximum value of 0.0434 min−1 , which is 2.8 and 6.8 times higher than those of pure Ag3 VO4 and N-doped TiO2 , respectively. The pseudo-first-order constant and relative coefficients are summarized in Table S1. The Co3 O4 /Ag3 VO4 composites exhibit strong photocatalytic activity. However, in view of practical application, its stability is very important. Therefore, the circulating runs in the photocatalytic degradation of RhB in the presence of optimal Co3 O4 /Ag3 VO4
Fig. 10. Kinetic fit for the degradation of RhB with pure Ag3 VO4 , N-doped TiO2 and Co3 O4 /Ag3 VO4 catalysts with different Co3 O4 contents.
composite were carried out. The result shown in Fig. 11 suggests that the photocatalytic activity does not exhibit any significant loss after four terms of recycling. The XRD pattern and UV–vis spectrum of the used sample also shows that no change was observed (see Figs. S4 and S5). It indicates that the Co3 O4 /Ag3 VO4 composite photocatalysts are stable in photocatalysis. Chemical oxygen demand (COD) measurements of the RhB solution were also performed to verify whether dye is completely degraded to carbonate. The result shows a COD removal of 70.8% during the photocatalytic process (see Fig. S6), suggesting that there exist some smaller organic molecules. The further modification is still needed to promote the complete oxidation ability of the Co3 O4 /Ag3 VO4 composite. 3.7. Discussion of mechanism Both Ag3 VO4 and Co3 O4 /Ag3 VO4 present very low specific surface areas. The adsorption experiment in the dark further shows that no correlation between the RhB adsorption and photoactivity was observed (see Fig. S7). It indicates that the enhanced photocatalytic performance of Co3 O4 /Ag3 VO4 can mainly be demonstrated by the synergism and coupling effect between the two semiconductors, which favors an effective photoexcited electron–hole
Fig. 11. Cycling runs in photocatalytic degradation of RhB in the presence of 1.0 wt.% Co3 O4 /Ag3 VO4 composite calcined at 653 K under visible light irradiation.
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Therefore, the photocatalytic activity of the sample calcined at 773 K is significantly decreased as compared to the catalyst prepared at 653 K heat-treatment. On the basis of the above analysis, it is suggested that the enhanced photocatalytic activity of Co3 O4 loaded Ag3 VO4 could be attributed to the synergetic effect of the above factors. 4. Conclusions
Fig. 12. Schematic diagram of synergistic effect of Co3 O4 /Ag3 VO4 .
separation in the two materials [48,49]. The conduction band (CB) and valence band (VB) potentials of Co3 O4 and Ag3 VO4 at the point of zero charge can be calculated based on the previous literatures [50,51]. The results are shown in Table S2. The CB edge potential of Ag3 VO4 (0.12 eV) is more negative than that of Co3 O4 (0.37 eV), hence, photoinduced electrons on the Ag3 VO4 particle surface can easily migrate to Co3 O4 via interfaces; similarly, photoinduced holes on the Co3 O4 surface can transfer to Ag3 VO4 owing to the different VB edge potentials, as shown in Fig. 12. As a result, a larger amount of electrons on the Co3 O4 surface and holes on the Ag3 VO4 surface can participate in photocatalytic reactions to directly or indirectly decompose RhB. Besides the obvious matching ability of band potentials between semiconductors described above, the photocatalytic activity of the catalysts also strongly depends on the optical properties and the crystallinity of semiconductor. The result of DRS shows that the doping of Co3 O4 can promote the photoabsorption performance. The absorption wavelength range of Co3 O4 /Ag3 VO4 catalyst is shifted to the longer wavelength, which can facilitate the absorption in the visible-light region. The larger red shift in absorption indicates that the catalyst can absorb more photons and generate more electron–hole pairs, and the photocatalytic activity is thus enhanced [38]. However, it is interesting to note that the photocatalytic activities of Co3 O4 /Ag3 VO4 catalysts enhance with the increase in Co3 O4 content up to 1.0 wt.% and then decrease. A similar result was also reported by Xu et al. [46]. It is considered that high Co3 O4 content (>1.0 wt.%) is harmful to the photocatalytic activity because of the excess coverage of active sites on the Ag3 VO4 surface by Co3 O4 particles [46]. The calcination temperature was also found to play a vital role in the photocatalytic activity. The photoactivity of the 1.0 wt.% Co3 O4 /Ag3 VO4 samples calcined at a different temperature follows the order: 653 K > 573 K > 773 K (Fig. 9). In this work, the Co(NO3 )2 precursor was gradually decomposed into cobalt oxide during calcination process, and the crystallinity of Co3 O4 was enhanced by increasing the temperature. Generally, higher crystallinity of the sample is always beneficial to the separation of electron and hole pairs as compared to the amorphous structure [32]. As a consequence, the photocatalytic activity of the Co3 O4 /Ag3 VO4 was enhanced with increasing temperature. However, at higher calcination temperatures (≥653 K), the Co3 O4 particle might be agglomerated, thus decrease photocatalytic activity of the sample. In addition, as the calcination temperature exceeds 653 K, the Co3 O4 nanoparticles are entrapped or encapsulated in the melted Ag3 VO4 [43,44].
In conclusion, a novel coupled photocatalyst, Co3 O4 /Ag3 VO4 , was prepared by impregnation method. The synthesized Co3 O4 /Ag3 VO4 catalysts showed high photocatalytic efficiency for the degradation of RhB under visible-light irradiation, and the optimal activity was obtained on the sample calcined at 653 K with 1.0 wt.% Co content. In the range of concentrations used, pseudozero-order kinetics was most suitable to describe the change of RhB concentration with time in the initial photocatalytic removal phase. The enhanced photocatalytic activity of Co3 O4 /Ag3 VO4 was mainly ascribed to the coupling effect in the separation of photoexcited electron–hole pairs. The Co3 O4 /Ag3 VO4 photocatalyst is promising for practical application for water purification because of its high activity and stability. In addition, the success of the preparation of Co3 O4 /Ag3 VO4 composite indicates that coupling two different semiconductors with suitable band potential is an efficient way to synthesize visible-light-driven photocatalyst with high activity. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21003109, 21003108), the Applied Research Foundation of Public Welfare Technologies of Zhejiang Province (2012C37057), and the program for Zhejiang Leading Team of Science and Technology Innovation (2009R50020). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mseb. 2012.10.011. References [1] J. Rochat, P. Demenge, J.C. Rerat, Toxicological European Research. Recherche Europeenne En Toxicologie 1 (1978) 23–26. [2] R. Jain, M. Mathur, S. Sikarwar, A. Mittal, Journal of Environment Management 85 (2007) 956–964. [3] O. Hamdaoui, Desalination 271 (2011) 279–286. [4] W.L. Zhang, Y. Li, C. Wang, P.F. Wang, Desalination 266 (2011) 40–45. [5] W. Choi, Catalysis Surveys from Asia 10 (2006) 16–28. [6] M. Zhang, C.C. Chen, W.H. Ma, J.C. Zhao, Angewandte Chemie International Edition 47 (2008) 1–5. [7] G. Liu, Y.N. Zhao, C.H. Sun, F. Li, G.Q. Lu, H.M. Cheng, Angewandte Chemie International Edition 120 (2008) 4592–4596. [8] X.C. Wang, J.C. Yu, Y.L. Chen, L. Wu, X.Z. Fu, Environmental Science and Technology 40 (2006) 2369–2374. [9] D.F. Zhang, Russian Journal of Physical Chemistry A 86 (2012) 675–680. [10] D.F. Zhang, Russian Journal of Physical Chemistry A 86 (2012) 498–503. [11] D.F. Zhang, Monatshefte fur Chemie 143 (2012) 729–738. [12] D.F. Zhang, Journal of Sol–Gel Science and Technology 58 (2011) 312–318. [13] Y.N. Rajeswari, B.A. Chandra, Science of Advanced Materials 4 (2012) 673–680. [14] Y.N. Rajeswari, B.A. Chandra, Science of Advanced Materials 4 (2012) 54–60. [15] C.R. Adrian, A. Luminita, D. Anca, Journal for Nanoscience and Nanotechnology 11 (2011) 9095–9101. [16] J. Zhen, F.G. Tao, H.X. Ye, W. Min, D.Z. Li, Journal of Nanoengineering and Nanomanufacturing 2 (2012) 49–53. [17] J. Kim, C.W. Lee, W. Choi, Platinized, Environmental Science and Technology 44 (2010) 6849–6854. [18] D.F. Zhang, F.B. Zeng, Journal of Materials Science 47 (2012) 2155–2161. [19] Z. He, C. Sun, S. Yang, Y. Ding, H. He, Z. Wang, Journal of Hazardous Materials 162 (2009) 1477–1486. [20] Y.M. He, L.H. Zhao, Y.J. Wang, H. Lin, T. Li, X.T. Wu, Y. Wu, Chemical Engineering Journal 169 (2011) 50–57.
52
L. Zhang et al. / Materials Science and Engineering B 178 (2013) 45–52
[21] L. Zhang, D. Chen, X. Jiao, Journal of Physical Chemistry B 110 (2006) 2668–2673. [22] G. Li, D. Zhang, J. Yu, Chemistry of Materials 20 (2008) 3983–3992. [23] Y.J. Wang, Y.M. He, T.T. Li, J. Cai, M.F. Luo, L.H. Zhao, Catalysis Communications 18 (2012) 161–164. [24] H.G. Kim, D.W. Hwang, J.S. Lee, Journal of the American Chemical Society 126 (2004) 8912–8913. [25] D.W. Kim, I.S. Cho, S. Lee, S.T. Bae, S.S. Shin, G.S. Han, H.S. Jung, K.S. Hong, Photophysical, Journal of the American Ceramic Society 93 (2010) 3867– 3872. [26] R. Konta, H. Kato, H. Kobayashi, A. Kudo, Physical Chemistry Chemical Physics 5 (2003) 3061–3065. [27] X.X. Hu, C. Hu, Journal of Solid State Chemistry 180 (2007) 725–732. [28] H. Xu, H.M. Li, G.S. Sun, J.X. Xia, C.D. Wu, Z.X. Ye, Q. Zhang, Chemical Engineering Journal 160 (2010) 33–41. [29] C.M. Huang, K.W. Cheng, G.T. Pan, W.S. Chang, T.C.K. Yang, Chemical Engineering Science 65 (2010) 148–152. [30] H.A.E. Hagelin-Weaver, G.B. Hoflund, D.M. Minahan, G.N. Salaita, Applied Surface Science 235 (2004) 420–448. [31] M. Casas-Cabanas, G. Binotto, D. Larcher, A. Lecup, V. Giordani, J.M. Tarascon, Chemistry of Materials 21 (2009) 1939–1947. [32] M. Long, W.M. Cai, J. Cai, B.X. Zhou, X.Y. Chai, Y.H. Wu, Journal of Physical Chemistry B 110 (2006) 20211–20216. [33] Q. Xiao, J. Zhang, C. Xiao, X.K. Tan, Catalysis Communications 9 (2008) 1247–1253. [34] K. Kobayakawa, Y. Murakami, Y. Sato, Journal of Photochemistry and Photobiology A 170 (2005) 177–179. [35] H. Xu, H.M. Li, L. Xu, G.S. Sun, Y.G. Xu, J.Y. Chu, Industrial and Engineering Chemistry Research 48 (2009) 10771–10778.
[36] S.F. Chen, W. Zhao, W. Liu, H.Y. Zhang, X.L. Yu, Y.H. Chen, Journal of Hazardous Materials 172 (2009) 1415–1423. [37] J. Bai, Y. Li, M. Li, S. Wang, C. Zhang, Q. Yang, Applied Surface Science 254 (2008) 4520–4523. [38] B.Z. Tian, C.Z. Li, J.L. Zhang, Chemical Engineering Journal 191 (2012) 402–409. [39] A. Gulino, I. Fragala, Inorganica Chimica Acta 358 (2005) 4466–4472. [40] D. Barreca, C. Massignan, S. Daolio, M. Fabrizio, C. Piccirillo, L. Armelao, E. Tondello, Chemistry of Materials 13 (2001) 588–593. [41] E. Wenda, Journal of Thermal Analysis 30 (1985) 879–887. [42] E. Wenda, A. Bielanski, Journal of Thermal Analysis and Calorimetry 92 (2008) 921–929. [43] H. Xu, H. Li, C. Wu, J. Chu, Y. Yan, H. Shu, Z. Gu, Journal of Hazardous Materials 153 (2008) 877–884. [44] H. Xu, H. Li, C. Wu, J. Chu, Y. Yan, H. Shu, Materials Science and Engineering B 147 (2008) 52–56. [45] M. Long, W. Cai, H. Kisch, Journal of Physical Chemistry C 112 (2008) 548–554. [46] R. Panda, V. Rathor, M. Rathore, V. Shelke, N. Badera, L.S. Sharath Chandra, D. Jain, M. Gangrade, T. Shripati, V. Ganesan, Applied Surface Science 258 (2012) 5086–5093. [47] C. He, X.Z. Li, N. Graham, Y. Wang, Applied Catalysis A: General 305 (2006) 54–63. [48] Y.J. Wang, R. Shi, J. Lin, Y.F. Zhu, Energy and Environmental Science 4 (2011) 2922–2929. [49] H.J. Huang, D.Z. Li, Q. Lin, W.J. Zhang, Y. Shao, Y.B. Chen, M. Sun, X.Z. Fu, Environmental Science and Technology 43 (2009) 4164–4168. [50] Y. Xu, M.A.A. Schoonen, American Mineralogist 85 (2000) 543–556. [51] J.W. Tang, J.H. Ye, Chemical Physics Letters 410 (2005) 104–107.