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Enhanced visible-light-driven photocatalytic degradation of RhB by AgIO3 /WO3 composites Qian Wen Cao a, Yi Fan Zheng b, Xu Chun Song a,∗ a b
Department of Chemistry, Fujian Normal University, Fuzhou 350007, PR China Research Center of Analysis and Measurement, Zhejiang University of Technology, Hangzhou 310014, P R China
a r t i c l e
i n f o
Article history: Received 11 June 2016 Revised 21 September 2016 Accepted 17 October 2016 Available online xxx Keywords: AgIO3 WO3 Heterojunction Photocatalytic
a b s t r a c t A novel heterojunction photocatalyst AgIO3 /WO3 was fabricated through hydrothermal and chemical precipitation methods. The AgIO3 /WO3 samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy dispersive X-ray detector, and UV–vis diffuse reflectance spectroscopy (UV-vis DRS). Moreover, the photocatalytic activities of AgIO3 /WO3 samples were estimated by the decomposition of organic dye RhB under visible light irradiation (λ >420 nm). The result reveled that AgIO3 /WO3 composite showed higher photocatalytic performance than pure AgIO3 and WO3 photocatalysts, and 50% AgIO3 /WO3 heterojunction was recorded to have the optimum rate constant. The enhancement of the photocatalytic activity could be attributed chiefly to the effective separation and migration of photogenerated electron-hole pairs at the interface of AgIO3 and WO3 . In addition, radical trap experiments confirmed that the ·OH was the primary reactive species during the photodecomposition of RhB. A possible photocatalytic mechanism of RhB decomposition over AgIO3 /WO3 heterostructures was also presented. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction During the past decades, the environmental problem is becoming more and more serious with the rapid development of the society. Since the demonstration of the photocatalysis with TiO2 under ultraviolet light irradiation by Honda and Fujishima in 1970s, semiconductor-based photocatalytic technologies have received much attention because of its practical applications in environmental purification [1,2]. However, the wide Eg (about 3.2 eV) of titanium dioxide (TiO2 ) limits its application in visible light region, which can only use no more than 5% of the solar spectrum [3]. Therefore, numerous efforts have been made to produce superb photocatalysts with high activity and stability under visible light irradiation because ∼43% of the solar energy on the earth is made up by visible light [4,5]. Among the photocatalytic materials tungsten trioxide (WO3 ) has been widely used as a visible-light response photocatalyst due to its relative narrow band-gap energy (2.4∼2.8 eV), high photoactivity, no photo-corrosion, and excellent stability [6]. A variety of WO3 nanaostructures such as nanocakes [7], nanorods [8], nanowires [9], nanorsheets [10], and mesoporous structures [11] have been prepared and their applications in pho-
∗
Corresponding author. Fax: +86 59183465376. E-mail address: [email protected] (X.C. Song).
tocatalysis have also been discussed in many fields [7–11]. Nevertheless, the reduction potential for the photoinduced electrons in the CB (conduction band) of WO3 is +0.5 V (vs. NHE), while the potential for one-electron reduction for O2 is −0.13 V (vs. NHE). Therefore, the electrons in conduction band of WO3 cannot be consumed by O2 to generate ·O2 − radicals efficiently, leading to low photocatalytic activity [8]. Moreover, the poor charge mobility of pure WO3 nanomaterial causes a high electron-hole recombination rate, which can also reduce its photocatalytic efficiency. Many strategies have been carried out to improve the photocatalytic performance of WO3 , among which forming heterostructure by WO3 and another semiconductor turned out to be a better technique. For instance, the heterojunctions of WO3 /TiO2 [6,12,13], WO3 /SiO2 [14–16], WO3 /ZnWO4 [17,18], WO3 /BiOI [19,20] and WO3 /Ag3 PO4 [21–24] all exhibited enhanced photocatalytic performance comparing to pure WO3 . Recently, metal iodates are widely studied owing to the lone pair electrons in IO3 − , which favors the formation of layered structure and make it polarized. The layered structure and polarity may play significant roles in diminishing the recombination of the photoinduced electrons (e− ) and holes (h+ ), thus enhancing the photocatalytic activity. These metal iodates such as BiIO4 [25], Bi(IO3 )3 [26], AgIO3 [27], and Y(IO3 )3 [28] all showed excellent photocatalytic performance in the dye photolysis process under ultraviolet light irradiation which suggested that metal iodates can be a promising material to develop effective
http://dx.doi.org/10.1016/j.jtice.2016.10.030 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Q.W. Cao et al., Enhanced visible-light-driven photocatalytic degradation of RhB by AgIO3 /WO3 composites, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.030
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photocatalysts. Moreover, increasing attentions have also been focused on the silver-based oxy-acid salt semiconductors due to the hybridization of O 2p orbital and Ag 4d orbital, which will help to elevate the VB (valence band), resulting in the semiconductor with higher VB and narrower band gap. For instance, AgIO3 with an orthorhombic structure [7], has been reported exhibiting good photocatalytic properties in UV region because of its wide band gap and high separation rate of photoexcited charge carriers. According to the above analysis, the heterostructure formed by AgIO3 and WO3 may exhibit good catalytic activity under UV–vis light irradiation. To the best of our knowledge, there has no report on the combination of AgIO3 with WO3 to form the heterostructured photocatalyst with increased visible light induced activity. In this work, we successfully synthesized the visible-light-active photocatalysts AgIO3 /WO3 , which possessed both fast chargeseparation rate and wide range in visible light region, thus exhibiting better photocatalytic activity than pure WO3 and AgIO3 catalysts. The WO3 precursor was synthesized via hydrothermal method, and then the AgIO3 was fabricated on WO3 through chemical precipitation. The ·OH was verified to be the major reactive species in the photodegradation process by radical trap experiments. A possible photocatalytic mechanism of the improved photocatalysis on AgIO3 /WO3 heterojunctions was deduced according to the band structures of WO3 and AgIO3 and the experimental results. 2. Experimental section 2.1. Sample preparation 2.1.1. Synthesis of WO3 precursor WO3 was synthesized via a NaCl-assisted hydrothemal method. Typically, 2 mmol Na2 WO4 ·2H2 O (Alaadin) was firstly dissolved in 15 mL distilled water. At the same time, 4 mmol NaCl (Sinopharm Chemical Reagent Co., Ltd) was dissolved in 15 mL distilled water and subsequently it was added to the Na2 WO4 solution dropwise under continuously stirring. After 10 min strongly stirring, 5 mmol HCl (Sinopharm Chemical Reagent Co., Ltd) was added into the mixture solution under continuously stirring. After vigorous stirring for another 30 min, the obtained mixture was transferred into a 50 mL teflon-lined stainless steel autoclave for hydrothermal treatment. The hydrothermal process was carried out at 180 °C for 12 h. After cooling down to room temperature naturally, the WO3 precipitation was washed by distilled water for three times. Finally, the product was dried 60 °C. 2.1.2. Preparation of a series of AgIO3 /WO3 1 mmol WO3 (0.2318 g) was dispersed uniformly in 20 mL distilled water to form suspension A1 , and suspension A2 and A3 were prepared in the same procedure. Then 2 mmol, 1 mmol, 0.25 mmol of AgNO3 (Guangdong Chemical) were added into suspension A1 , A2 and A3 respectively and stirred vigorously to ensure it dissolved completely. Meanwhile, 2 mmol, 1 mmol, 0.25 mmol of KIO3 (Alaadin) were dissolved in 15 mL distilled water to form solution B1 , B2 and B3 respectively. Finally, the KIO3 solutions were dropped slowly into the above corresponding suspensions with intense stirring (A1 vs. B1 , A2 vs. B2 , A3 vs. B3 ). Then the AgIO3 /WO3 composites with different more ratios of AgIO3 were successfully achieved with continuous stir for another 30 min. All the processes were operated at room temperature. The products were washed with distilled water, collected and dried at 60 °C. The final obtained products were labeled as 67% AgIO3 /WO3 , 50% AgIO3 /WO3 and 20% AgIO3 /WO3 composites according to the mole ratios of AgIO3 and WO3 . For comparison, pure AgIO3 photocatalysts were also prepared. Firstly, 5 mmol of AgNO3 (0.8494 g) and 5 mmol of KIO3 (1.07 g)
were dissolved in 20 mL distilled water, respectively. Then the KIO3 solution was dropped slowly into AgNO3 under strongly stirring, and simultaneously a white precipitate was obtained. At last, the precipitate was washed, collected, and dried in oven at 60◦ C. 2.2. Characterization The crystal structural information of the as-synthesized samples were recorded by X-ray diffraction (XRD) analysis with a Xray diffractometer (Thermo ARL SCINTAG X’TRA) using Cu Ka radi˚ at room temperature and it ation (40 mA, 45 kV and λ = 1.5406 A) was carried out in a range of 2θ = 10–80°. The ultraviolet-visible absorption spectra of the as-obtained samples were collected using a Lambda 850 UV–vis spectrophotometer, in which the BaSO4 was used as reference. The morphologies of the as-prepared samples were characterized by a scanning electron microscope (Hitachi S-4700 SEM), with scanning voltage of 15 kV. The chemical composition of the as-synthesized sample was analyzed on an energy dispersive X-ray detector (Thermo Noran VANTAG-ESI). The absorption of RhB in solution was detected by an ultraviolet-visible spectrophotometer (UV759S). 2.3. Photocatalytic experiments The photocatalytic activities of the as-fabricated samples were evaluated by photodegradation of RhB under visible light irradiation. Firstly, 0.1 g of the as-prepared sample was dispersed evenly in 200 mL RhB solution (2 × 10−5 mol/L) under intensely stirring for 30 min in a dark place to ensure the adsorption-desorption equilibrium between the organic dye molecules and photocatalyst. After that, the suspension was illuminated by a 300 W Xe lamp (1900 mW/cm2 ) with a 420 nm cut off filter, which worked as the visible-light source. During the irradiation about 4 mL of the suspension was sampled at the interval of every 20 min and then the samples were centrifuged to get rid of the photocatalyst powders. The absorbance of RhB in the filtrate was analyzed using a UV759S UV–vis spectrophotometer. 3. Results and discussions The crystal structure and phase purity of the as-synthesized WO3 , AgIO3 , and AgIO3 /WO3 composites are confirmed by XRD characterization shown in Fig. 1. It can be seen that pure WO3 displays the distinctive diffraction peaks of (020), (0 02), (20 0), (120), (112), (022), (004) and (040) which are in good accordance with the crystalline phase of monoclinic WO3 (JCPDS Card No. 72-0677). On the other hand, pure AgIO3 shows characteristic diffraction peaks of (021), (040), (210), (041), (211), (230), (002), (060), (212), (232), and (271) which belong to the orthorhombic AgIO3 (JCPDS Card No. 71-1928). The XRD results indicate that the well-crystallized WO3 and AgIO3 were successfully fabricated under the experimental condition. For AgIO3 /WO3 composites, the XRD patterns exhibit diffraction peaks corresponding to AgIO3 and WO3 phases, implying that AgIO3 and WO3 are well coupled and AgIO3 /WO3 has been synthesized successfully. Moreover, it can be investigated that the relative intensity of some diffraction peaks (marked with ‘ ’ in Fig. 1), especially the peaks of (021), (041), and (211), increased faintly as the concentration of AgIO3 increases in the AgIO3 /WO3 compound. The SEM was used to investigate the morphologic structure of the fabricated samples. As can be observed in Fig. 2a, WO3 have nanoplate like structures with length and width of 10 0–80 0 nm. As for AgIO3 , irregular nanoplates of pure AgIO3 with length and width of 0.4–1.5 μm can be investigated in Fig. 2b, which is much larger than pure WO3 nanoplate. Fig. 2c–e exhibit the SEM
Please cite this article as: Q.W. Cao et al., Enhanced visible-light-driven photocatalytic degradation of RhB by AgIO3 /WO3 composites, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.030
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Fig. 1. XRD diffraction patterns of WO3, AgIO3, 67%AgIO3/WO3, 50%AgIO3/WO3, and 20%AgIO3/WO3, respectively.
Fig. 3. (a) UV–vis absorption spectra of WO3, AgIO3, 20%AgIO3/WO3, 50%AgIO3/WO3, and 67%AgIO3/WO3, respectively; and (b) plot of the (α hν )1/2 vs. hν of WO3 and AgIO3.
Fig. 2. SEM images of (a) WO3, (b) AgIO3, (c) 20%AgIO3/WO3, (d) 50%AgIO3/WO3, and (e) 67%AgIO3/WO3, respectively; and (f) EDS mapping for the AgIO3/WO3 composite.
images of 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 , respectively, which can indicate the adherence of WO3 on the surface of AgIO3 . Besides, there is an obviously decreasing amount of WO3 on the surfaces of AgIO3 with increasing concentration of AgIO3 and the sizes of these two kinds of nanomaterials have no obvious changes. Furthermore, the composites purely composed of AgIO3 and WO3 can be further confirmed by the energy dispersive
X-ray (EDS) spectrum. Fig. 2e displays the EDS image of the obtained 50%AgIO3 /WO3 compound from a selected area. It can be observed clearly that the major peaks refer to oxygen, wolfram, silver, and idiot, respectively. Moreover, no other element or impurity can be found in the prepared sample, demonstrating that the AgIO3 /WO3 sample is simply consist of AgIO3 and WO3 . The synergetic effect of WO3 and AgIO3 on the light absorption properties was also investigated, which may play a significant role in determining the catalytic property, especially for the photodecomposition of contaminants. The UV–vis diffuse reflectance spectroscopy (DRS) is used to characterize the optical property of the as-prepared samples, as shown in Fig. 3. It depicts that pure WO3 has an absorption ranging from ultraviolet to visible light (λ ࣚ 480 nm), on the other side, the absorption band edge of AgIO3 is estimated to 340 nm, showing that the AgIO3 only has narrow optical absorption in the UV light region. From Fig. 3a, it can be seen that the three AgIO3 /WO3 heterojunctions have similar DRS spectra in UV–vis light region and they display mixed properties of pure AgIO3 and WO3 . The result manifests that the hybrid material coupled by AgIO3 and WO3 is able to work as a better photocatalyst than pure AgIO3 and WO3 throughout the ultraviolet and visible light region. It is known that the absorption edge relates closely to the bandgap of photocatalyst, which is essential to de-
Please cite this article as: Q.W. Cao et al., Enhanced visible-light-driven photocatalytic degradation of RhB by AgIO3 /WO3 composites, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.030
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termine the photoabsorption region of the sample. On the basis of the theory of light absorption for semiconductors and the DRS results, it is proverbial to calculate the band gap energy through the following formula [1,11]: n/2
A(hv − Eg)
= ahv
(1)
Where A, h, Eg, v and a are constant, Plank constant, optical band gap energy, optical frequency, and absorption coefficient determined by scattering and reflectance spectra on the basis of Kubelka–Munk theory, respectively. And in this equation, n is a coefficient which is determined by the type of optical transition considered (n =1 is for direct transition and n = 4 is for indirect transition). Just as shown in Fig. 3b, the plot of (ahv)1/2 vs. hv is depicted. Making out a tangent, the bandgap energy can be acquired from the intercept to the X axis. The estimated Eg values of WO3 and AgIO3 are 2.46 eV and 3.38 eV, respectively, which is consistent with other values that have been reported in the literatures for WO3 and AgIO3 [3,11]. To study the potential application of AgIO3 /WO3 in the decomposition of some toxic contaminants, the photocatalytic efficiency of WO3 , AgIO3 , 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 photocatalysts are assessed, employing the photodegradation of RhB under the irradiation of visible light. Prior to illumination, the reaction system was stirred in a dark place for 30 min to ensure the adsorption-desorption equilibrium. In this system, the RhB adsorption capability of WO3 , AgIO3 , 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 were also evaluated, and the results were 11.2%, 11.2%, 9.5%, 9.1%, and 15.5%, respectively. Fig. 4a exhibits the plot of C/C0 versus the irradiation time of WO3 , AgIO3 , 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 , respectively. For pure WO3 and AgIO3 , the RhB degradation rate is 14.7% and 19.6% in 140 min, respectively. While as for the heterojunctions formed by WO3 and AgIO3 , they all show better photocatalystic activity than that of pure WO3 or AgIO3 . The RhB degradation rates of 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 are 77%, 100% and 82.7%, respectively, suggesting that the 50%AgIO3 /WO3 compound is the best photocatalyst among the as-fabricated samples, which can degrade the RhB completely in 140 min. As is known that the interface of the heterojunctions can greatly affect the photocatalytic activity of the hybrid semiconductors. In this system with more content of AgIO3 , the contact surfaces between AgIO3 and WO3 may increase. Therefore, it can be ratiocinated that the increased contact surfaces may associate with the enhanced photocatalytic activity. However, when the AgIO3 content increased excessively, the heterojunction interface may not increase linearly with the AgIO3 ratio. In the AgIO3 /WO3 heterostructures, there must exist an optimum ratio of AgIO3 and WO3 which can induce a bigger contact area between two different semiconductors. Fig. 4b shows the temporal absorption spectral changes of RhB solution over the as-prepared 50%AgIO3 /WO3 sample under visible light irradiation. It can be observed that the primary absorption band of RhB locates at λ = 553 nm, and the starting red solution fades to colorless and transparent when illuminated for 140 min, suggesting that RhB is decomposed completely. At the same time, a spectral blue-shift from 553 to 501 nm can be noted in Fig. 4b, which can be ascribed to the de-ethylation processes in the RhB dye photodegradation [29]. In order to investigate the photocatalytic capability of AgIO3 , WO3 , 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 , and to comprehend their chemical kinetics of the photodegrading RhB processes, a pseudo-first-order model has been suggested. As shown in Fig. 5, ln(C/C0 ) is observed having a linear function against the irradiation time, suggesting the photocatalytic reactions are in good accordance with the pseudo-first-order reaction, and the model can be expressed the following equation:
ln (C/C0 ) = kt
(2)
Fig. 4. (a) The plot of C/C0 vs. the irradiation time of WO3, AgIO3 and AgIO3/WO3, respectively; (b) Spectral of the photodegredation of RhB over 50 % AgIO3/WO3 photocatalyst under visible light irradiation.
Fig. 5. The plot of ln(C0/C) versus the irradiation time of AgIO3, WO3, 20%AgIO3/WO3, 50%AgIO3/WO3, and 67%AgIO3/WO3, respectively.
Please cite this article as: Q.W. Cao et al., Enhanced visible-light-driven photocatalytic degradation of RhB by AgIO3 /WO3 composites, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.030
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Fig. 6. Repetitive testing of the RhB photodegradation process on 50%AgIO3/WO3 heterjunction under simulated solar light irradiation.
The photocatalytic degradation constants of AgIO3 , WO3 , 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 are calculated to be 0.0 014, 0.0 011, 0.0108, 0.0178, and 0.0129 min−1 , respectively. The result shows that the 50%AgIO3 /WO3 composite has the highest degradation rate, implying the heterostructure photocatalyst with the optimum content percentage of AgIO3 show a significantly enhanced photocatalytic activity. To study the stability of the 50%AgIO3 /WO3 composite, repetitive experiments were carried out to evaluate the photodegradation of RhB over the 50%AgIO3 /WO3 heterojunction photocatalyst under visible-light irradiation (λ > 420 nm). The recycled photocatalyst was separated and retrieved from the suspension by centrifugation after each run. The results are displayed in Fig. 6, it can be discovered that although there is deactivation of catalyst in every cycling runs, the photocatalytic activity of 50%AgIO3 /WO3 is still retained nearly 80% of its original photocatalytic activity in the third experimental run, suggesting that the 50%AgIO3 /WO3 heterojunction is stable during the photodegradation of RhB under visible-light irradiation. It is well known that several reactive species will generate in the photocatalytic oxidation (PCO) process, which can be ascribed to the photogenerated electrons and holes motivated from the photocatalyst seminconductors, and these reactive species will directly determine the performances of catalyst in PCO [30,31]. To ascertain the photocatalysis mechanism of RhB degradation in detail, it is crucial to understand the functions of the active species in the RhB photodegradation processes. Therefore, several radical scavengers are applied in the reaction system, benzoquinone (BQ, 2 mg), ammonium oxalate (2.13 mg) and isopropanol (IPA, 15 mL) were added into 200 mL of RhB solution companied with 50%AgIO3 /WO3 and illuminated by visible light, respectively. Among the test, benzoquinone acts as superoxide radicals (·O2 − ) scavenger, ammonium oxalate works as holes (h+ ) scavenger, and isopropanol is hydroxyl radicals (·OH) scavenger. As can be seen from Fig. 7 the photocatalytic activity is greatly deactivated when isopropanol is added, with only 16.8% of RhB being decomposed in 140 min. While 75.5% and 95.7% of RhB have been photodegraded within the same irradiation time with ammonium oxalate and BQ added, respectively. It can be summarized that ·OH may be the major active oxygen species in the photocatalytic degradation process. For further investigation of the effect of 50%AgIO3 /WO3 catalyst, The photoluminescence (PL) analysis is usually applied to investigate the diffusion and recombination processes of photoin-
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Fig. 7. Effects of radical scavengers in the degradation process of RhB over 50%AgIO3/WO3.
Fig. 8. The PL spectra of as-prepared photocatalysts.
duced electron-hole pairs in semiconductors, which is important in photocatalytic reactions [17,32]. In general, the enhanced photocatalytic activity is related to the lower recombination rate of photogenerated electron-hole pairs, leading to lower PL intensity. Fig. 8 shows the PL spectra of pure WO3 , 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 using a 300 nm excitation wavelength. It can be seen that all the photocatalysts exhibit similar PL spectra and show a strong emission peak at about 475 nm, which can be ascribed to band-band PL phenomenon with energy of light. Moreover, comparing to pure WO3 , the emission intensity of 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 decrease significantly and the lowest intensity was observed on 50%AgIO3 /WO3 heterojunction, indicating that the recombination rate of photoinduced charge carriers on 50%AgIO3 /WO3 composites is the lowest. The results correlate well with the best activity of 50%AgIO3 /WO3 compound among all the photocatalysts. As is well-known, the separation efficiency of the photoinduced charge carriers can affect the photocatalytic properties. To comprehend the photocatalytic mechanism involved in the photodecomposion of the fabricated 50%AgIO3 /WO3 composite the edge potentials of CB and VB are necessary to be computed due to their close relationships with the PCO of organic molecules. The CB potential of WO3 is 0.09 eV (vs. NHE, pH = 7) [8], and the VB
Please cite this article as: Q.W. Cao et al., Enhanced visible-light-driven photocatalytic degradation of RhB by AgIO3 /WO3 composites, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.030
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alyst is a promising material in applications and worth researching in future.
Acknowledgment This work is financially supported by the National Nature Science Foundation of China (No. 21273034). References
Fig. 9. Schematic illustration of the photocatalysis process involved in the 50%AgIO3/WO3 composite.
potential of WO3 is 2.55 eV. The ECB and EVB of AgIO3 are calculated to be 0.45 eV and 3.83 eV, respectively [29]. The markedly enhanced photocatalytic activities of AgIO3 /WO3 composites are prevailingly attributed to the effective separation of the photoinduced electron-hole pairs. Based on the band-gap structure of AgIO3 /WO3 heterjunction and the results of free radical trapping experiments, the photocatalytic mechanism of AgIO3 /WO3 heterjunction is proposed. As shown in Fig. 9, the redox potentials of conduction band and valence band of AgIO3 are observed to be more positive than those of WO3 and there is an interactive structure formed by AgIO3 and WO3 , which is favorable for the separation of light-induced charge carriers. Under visible-light irradiation, electron-hole pairs can only be generated on the surface of WO3 . The photogenerated electrons on the CB of WO3 can efficiently migrate to the CB of AgIO3 . The redox potential of O2 /·O2 − is −0.046 eV [33], more negative than the CB edge potential of AgIO3 (0.45 eV), which makes it impossible to yield ·O2 − radicals via the reduction of O2 by CB electrons of AgIO3 . Meanwhile, the photoinduced holes on the VB of WO3 will react with the reagent RhB. For the WO3 , the VB edge potential is 2.55 eV, and the potential of OH– /·OH couple is 1.99 eV [32]. The VB edge potential of WO3 is more positive than that of OH– /·OH couple, which indicate that the photoinduced holes would react with adsorbed H2 O to form ·OH. The ·OH radicals can then effectively decompose RhB into H2 O, CO2 and other intermediates, which is consistent with the results of radical scavengers experiments. As a consequence, the photoexcited electron and hole separate efficiently on the interface of the AgIO3 /WO3 photocatalyst, which distinctively enhanced the photocatalytic activity of the catalyst. 4. Conclusion In summary, a series of AgIO3 /WO3 heterostructure photocatalysts with different content of AgIO3 have been fabricated through facile hydrothermal and chemical precipitation method. The asprepared AgIO3 /WO3 heterojunction photocatalysts displayed much higher photocatalytic efficiency for the photodecomposition of RhB compared to pure AgIO3 and WO3 photocatalysts under visible light irradiation. It can be noted that 50%AgIO3 /WO3 compound exhibits the highest photocatalytic activity, with all the RhB degraded completely in 140 min. The enhanced photocatalytic activity can be mainly attributed to the high separation rate of photogenerated electron-hole pairs and efficient migration during photocatalytic degradation process. Additionally, ·OH is considered to be the main reactive species in the photodegradation course, and a probable photocatalytic mechanism is also proposed according to the experimental results. This study indicates that AgIO3 /WO3 cat-
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2016) 1–7
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Enhanced visible-light-driven photocatalytic degradation of RhB by AgIO3 /WO3 composites Qian Wen Cao a, Yi Fan Zheng b, Xu Chun Song a,∗ a b
Department of Chemistry, Fujian Normal University, Fuzhou 350007, PR China Research Center of Analysis and Measurement, Zhejiang University of Technology, Hangzhou 310014, P R China
a r t i c l e
i n f o
Article history: Received 11 June 2016 Revised 21 September 2016 Accepted 17 October 2016 Available online xxx Keywords: AgIO3 WO3 Heterojunction Photocatalytic
a b s t r a c t A novel heterojunction photocatalyst AgIO3 /WO3 was fabricated through hydrothermal and chemical precipitation methods. The AgIO3 /WO3 samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Energy dispersive X-ray detector, and UV–vis diffuse reflectance spectroscopy (UV-vis DRS). Moreover, the photocatalytic activities of AgIO3 /WO3 samples were estimated by the decomposition of organic dye RhB under visible light irradiation (λ >420 nm). The result reveled that AgIO3 /WO3 composite showed higher photocatalytic performance than pure AgIO3 and WO3 photocatalysts, and 50% AgIO3 /WO3 heterojunction was recorded to have the optimum rate constant. The enhancement of the photocatalytic activity could be attributed chiefly to the effective separation and migration of photogenerated electron-hole pairs at the interface of AgIO3 and WO3 . In addition, radical trap experiments confirmed that the ·OH was the primary reactive species during the photodecomposition of RhB. A possible photocatalytic mechanism of RhB decomposition over AgIO3 /WO3 heterostructures was also presented. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction During the past decades, the environmental problem is becoming more and more serious with the rapid development of the society. Since the demonstration of the photocatalysis with TiO2 under ultraviolet light irradiation by Honda and Fujishima in 1970s, semiconductor-based photocatalytic technologies have received much attention because of its practical applications in environmental purification [1,2]. However, the wide Eg (about 3.2 eV) of titanium dioxide (TiO2 ) limits its application in visible light region, which can only use no more than 5% of the solar spectrum [3]. Therefore, numerous efforts have been made to produce superb photocatalysts with high activity and stability under visible light irradiation because ∼43% of the solar energy on the earth is made up by visible light [4,5]. Among the photocatalytic materials tungsten trioxide (WO3 ) has been widely used as a visible-light response photocatalyst due to its relative narrow band-gap energy (2.4∼2.8 eV), high photoactivity, no photo-corrosion, and excellent stability [6]. A variety of WO3 nanaostructures such as nanocakes [7], nanorods [8], nanowires [9], nanorsheets [10], and mesoporous structures [11] have been prepared and their applications in pho-
∗
Corresponding author. Fax: +86 59183465376. E-mail address: [email protected] (X.C. Song).
tocatalysis have also been discussed in many fields [7–11]. Nevertheless, the reduction potential for the photoinduced electrons in the CB (conduction band) of WO3 is +0.5 V (vs. NHE), while the potential for one-electron reduction for O2 is −0.13 V (vs. NHE). Therefore, the electrons in conduction band of WO3 cannot be consumed by O2 to generate ·O2 − radicals efficiently, leading to low photocatalytic activity [8]. Moreover, the poor charge mobility of pure WO3 nanomaterial causes a high electron-hole recombination rate, which can also reduce its photocatalytic efficiency. Many strategies have been carried out to improve the photocatalytic performance of WO3 , among which forming heterostructure by WO3 and another semiconductor turned out to be a better technique. For instance, the heterojunctions of WO3 /TiO2 [6,12,13], WO3 /SiO2 [14–16], WO3 /ZnWO4 [17,18], WO3 /BiOI [19,20] and WO3 /Ag3 PO4 [21–24] all exhibited enhanced photocatalytic performance comparing to pure WO3 . Recently, metal iodates are widely studied owing to the lone pair electrons in IO3 − , which favors the formation of layered structure and make it polarized. The layered structure and polarity may play significant roles in diminishing the recombination of the photoinduced electrons (e− ) and holes (h+ ), thus enhancing the photocatalytic activity. These metal iodates such as BiIO4 [25], Bi(IO3 )3 [26], AgIO3 [27], and Y(IO3 )3 [28] all showed excellent photocatalytic performance in the dye photolysis process under ultraviolet light irradiation which suggested that metal iodates can be a promising material to develop effective
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Please cite this article as: Q.W. Cao et al., Enhanced visible-light-driven photocatalytic degradation of RhB by AgIO3 /WO3 composites, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.030
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photocatalysts. Moreover, increasing attentions have also been focused on the silver-based oxy-acid salt semiconductors due to the hybridization of O 2p orbital and Ag 4d orbital, which will help to elevate the VB (valence band), resulting in the semiconductor with higher VB and narrower band gap. For instance, AgIO3 with an orthorhombic structure [7], has been reported exhibiting good photocatalytic properties in UV region because of its wide band gap and high separation rate of photoexcited charge carriers. According to the above analysis, the heterostructure formed by AgIO3 and WO3 may exhibit good catalytic activity under UV–vis light irradiation. To the best of our knowledge, there has no report on the combination of AgIO3 with WO3 to form the heterostructured photocatalyst with increased visible light induced activity. In this work, we successfully synthesized the visible-light-active photocatalysts AgIO3 /WO3 , which possessed both fast chargeseparation rate and wide range in visible light region, thus exhibiting better photocatalytic activity than pure WO3 and AgIO3 catalysts. The WO3 precursor was synthesized via hydrothermal method, and then the AgIO3 was fabricated on WO3 through chemical precipitation. The ·OH was verified to be the major reactive species in the photodegradation process by radical trap experiments. A possible photocatalytic mechanism of the improved photocatalysis on AgIO3 /WO3 heterojunctions was deduced according to the band structures of WO3 and AgIO3 and the experimental results. 2. Experimental section 2.1. Sample preparation 2.1.1. Synthesis of WO3 precursor WO3 was synthesized via a NaCl-assisted hydrothemal method. Typically, 2 mmol Na2 WO4 ·2H2 O (Alaadin) was firstly dissolved in 15 mL distilled water. At the same time, 4 mmol NaCl (Sinopharm Chemical Reagent Co., Ltd) was dissolved in 15 mL distilled water and subsequently it was added to the Na2 WO4 solution dropwise under continuously stirring. After 10 min strongly stirring, 5 mmol HCl (Sinopharm Chemical Reagent Co., Ltd) was added into the mixture solution under continuously stirring. After vigorous stirring for another 30 min, the obtained mixture was transferred into a 50 mL teflon-lined stainless steel autoclave for hydrothermal treatment. The hydrothermal process was carried out at 180 °C for 12 h. After cooling down to room temperature naturally, the WO3 precipitation was washed by distilled water for three times. Finally, the product was dried 60 °C. 2.1.2. Preparation of a series of AgIO3 /WO3 1 mmol WO3 (0.2318 g) was dispersed uniformly in 20 mL distilled water to form suspension A1 , and suspension A2 and A3 were prepared in the same procedure. Then 2 mmol, 1 mmol, 0.25 mmol of AgNO3 (Guangdong Chemical) were added into suspension A1 , A2 and A3 respectively and stirred vigorously to ensure it dissolved completely. Meanwhile, 2 mmol, 1 mmol, 0.25 mmol of KIO3 (Alaadin) were dissolved in 15 mL distilled water to form solution B1 , B2 and B3 respectively. Finally, the KIO3 solutions were dropped slowly into the above corresponding suspensions with intense stirring (A1 vs. B1 , A2 vs. B2 , A3 vs. B3 ). Then the AgIO3 /WO3 composites with different more ratios of AgIO3 were successfully achieved with continuous stir for another 30 min. All the processes were operated at room temperature. The products were washed with distilled water, collected and dried at 60 °C. The final obtained products were labeled as 67% AgIO3 /WO3 , 50% AgIO3 /WO3 and 20% AgIO3 /WO3 composites according to the mole ratios of AgIO3 and WO3 . For comparison, pure AgIO3 photocatalysts were also prepared. Firstly, 5 mmol of AgNO3 (0.8494 g) and 5 mmol of KIO3 (1.07 g)
were dissolved in 20 mL distilled water, respectively. Then the KIO3 solution was dropped slowly into AgNO3 under strongly stirring, and simultaneously a white precipitate was obtained. At last, the precipitate was washed, collected, and dried in oven at 60◦ C. 2.2. Characterization The crystal structural information of the as-synthesized samples were recorded by X-ray diffraction (XRD) analysis with a Xray diffractometer (Thermo ARL SCINTAG X’TRA) using Cu Ka radi˚ at room temperature and it ation (40 mA, 45 kV and λ = 1.5406 A) was carried out in a range of 2θ = 10–80°. The ultraviolet-visible absorption spectra of the as-obtained samples were collected using a Lambda 850 UV–vis spectrophotometer, in which the BaSO4 was used as reference. The morphologies of the as-prepared samples were characterized by a scanning electron microscope (Hitachi S-4700 SEM), with scanning voltage of 15 kV. The chemical composition of the as-synthesized sample was analyzed on an energy dispersive X-ray detector (Thermo Noran VANTAG-ESI). The absorption of RhB in solution was detected by an ultraviolet-visible spectrophotometer (UV759S). 2.3. Photocatalytic experiments The photocatalytic activities of the as-fabricated samples were evaluated by photodegradation of RhB under visible light irradiation. Firstly, 0.1 g of the as-prepared sample was dispersed evenly in 200 mL RhB solution (2 × 10−5 mol/L) under intensely stirring for 30 min in a dark place to ensure the adsorption-desorption equilibrium between the organic dye molecules and photocatalyst. After that, the suspension was illuminated by a 300 W Xe lamp (1900 mW/cm2 ) with a 420 nm cut off filter, which worked as the visible-light source. During the irradiation about 4 mL of the suspension was sampled at the interval of every 20 min and then the samples were centrifuged to get rid of the photocatalyst powders. The absorbance of RhB in the filtrate was analyzed using a UV759S UV–vis spectrophotometer. 3. Results and discussions The crystal structure and phase purity of the as-synthesized WO3 , AgIO3 , and AgIO3 /WO3 composites are confirmed by XRD characterization shown in Fig. 1. It can be seen that pure WO3 displays the distinctive diffraction peaks of (020), (0 02), (20 0), (120), (112), (022), (004) and (040) which are in good accordance with the crystalline phase of monoclinic WO3 (JCPDS Card No. 72-0677). On the other hand, pure AgIO3 shows characteristic diffraction peaks of (021), (040), (210), (041), (211), (230), (002), (060), (212), (232), and (271) which belong to the orthorhombic AgIO3 (JCPDS Card No. 71-1928). The XRD results indicate that the well-crystallized WO3 and AgIO3 were successfully fabricated under the experimental condition. For AgIO3 /WO3 composites, the XRD patterns exhibit diffraction peaks corresponding to AgIO3 and WO3 phases, implying that AgIO3 and WO3 are well coupled and AgIO3 /WO3 has been synthesized successfully. Moreover, it can be investigated that the relative intensity of some diffraction peaks (marked with ‘ ’ in Fig. 1), especially the peaks of (021), (041), and (211), increased faintly as the concentration of AgIO3 increases in the AgIO3 /WO3 compound. The SEM was used to investigate the morphologic structure of the fabricated samples. As can be observed in Fig. 2a, WO3 have nanoplate like structures with length and width of 10 0–80 0 nm. As for AgIO3 , irregular nanoplates of pure AgIO3 with length and width of 0.4–1.5 μm can be investigated in Fig. 2b, which is much larger than pure WO3 nanoplate. Fig. 2c–e exhibit the SEM
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Fig. 1. XRD diffraction patterns of WO3, AgIO3, 67%AgIO3/WO3, 50%AgIO3/WO3, and 20%AgIO3/WO3, respectively.
Fig. 3. (a) UV–vis absorption spectra of WO3, AgIO3, 20%AgIO3/WO3, 50%AgIO3/WO3, and 67%AgIO3/WO3, respectively; and (b) plot of the (α hν )1/2 vs. hν of WO3 and AgIO3.
Fig. 2. SEM images of (a) WO3, (b) AgIO3, (c) 20%AgIO3/WO3, (d) 50%AgIO3/WO3, and (e) 67%AgIO3/WO3, respectively; and (f) EDS mapping for the AgIO3/WO3 composite.
images of 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 , respectively, which can indicate the adherence of WO3 on the surface of AgIO3 . Besides, there is an obviously decreasing amount of WO3 on the surfaces of AgIO3 with increasing concentration of AgIO3 and the sizes of these two kinds of nanomaterials have no obvious changes. Furthermore, the composites purely composed of AgIO3 and WO3 can be further confirmed by the energy dispersive
X-ray (EDS) spectrum. Fig. 2e displays the EDS image of the obtained 50%AgIO3 /WO3 compound from a selected area. It can be observed clearly that the major peaks refer to oxygen, wolfram, silver, and idiot, respectively. Moreover, no other element or impurity can be found in the prepared sample, demonstrating that the AgIO3 /WO3 sample is simply consist of AgIO3 and WO3 . The synergetic effect of WO3 and AgIO3 on the light absorption properties was also investigated, which may play a significant role in determining the catalytic property, especially for the photodecomposition of contaminants. The UV–vis diffuse reflectance spectroscopy (DRS) is used to characterize the optical property of the as-prepared samples, as shown in Fig. 3. It depicts that pure WO3 has an absorption ranging from ultraviolet to visible light (λ ࣚ 480 nm), on the other side, the absorption band edge of AgIO3 is estimated to 340 nm, showing that the AgIO3 only has narrow optical absorption in the UV light region. From Fig. 3a, it can be seen that the three AgIO3 /WO3 heterojunctions have similar DRS spectra in UV–vis light region and they display mixed properties of pure AgIO3 and WO3 . The result manifests that the hybrid material coupled by AgIO3 and WO3 is able to work as a better photocatalyst than pure AgIO3 and WO3 throughout the ultraviolet and visible light region. It is known that the absorption edge relates closely to the bandgap of photocatalyst, which is essential to de-
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termine the photoabsorption region of the sample. On the basis of the theory of light absorption for semiconductors and the DRS results, it is proverbial to calculate the band gap energy through the following formula [1,11]: n/2
A(hv − Eg)
= ahv
(1)
Where A, h, Eg, v and a are constant, Plank constant, optical band gap energy, optical frequency, and absorption coefficient determined by scattering and reflectance spectra on the basis of Kubelka–Munk theory, respectively. And in this equation, n is a coefficient which is determined by the type of optical transition considered (n =1 is for direct transition and n = 4 is for indirect transition). Just as shown in Fig. 3b, the plot of (ahv)1/2 vs. hv is depicted. Making out a tangent, the bandgap energy can be acquired from the intercept to the X axis. The estimated Eg values of WO3 and AgIO3 are 2.46 eV and 3.38 eV, respectively, which is consistent with other values that have been reported in the literatures for WO3 and AgIO3 [3,11]. To study the potential application of AgIO3 /WO3 in the decomposition of some toxic contaminants, the photocatalytic efficiency of WO3 , AgIO3 , 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 photocatalysts are assessed, employing the photodegradation of RhB under the irradiation of visible light. Prior to illumination, the reaction system was stirred in a dark place for 30 min to ensure the adsorption-desorption equilibrium. In this system, the RhB adsorption capability of WO3 , AgIO3 , 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 were also evaluated, and the results were 11.2%, 11.2%, 9.5%, 9.1%, and 15.5%, respectively. Fig. 4a exhibits the plot of C/C0 versus the irradiation time of WO3 , AgIO3 , 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 , respectively. For pure WO3 and AgIO3 , the RhB degradation rate is 14.7% and 19.6% in 140 min, respectively. While as for the heterojunctions formed by WO3 and AgIO3 , they all show better photocatalystic activity than that of pure WO3 or AgIO3 . The RhB degradation rates of 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 are 77%, 100% and 82.7%, respectively, suggesting that the 50%AgIO3 /WO3 compound is the best photocatalyst among the as-fabricated samples, which can degrade the RhB completely in 140 min. As is known that the interface of the heterojunctions can greatly affect the photocatalytic activity of the hybrid semiconductors. In this system with more content of AgIO3 , the contact surfaces between AgIO3 and WO3 may increase. Therefore, it can be ratiocinated that the increased contact surfaces may associate with the enhanced photocatalytic activity. However, when the AgIO3 content increased excessively, the heterojunction interface may not increase linearly with the AgIO3 ratio. In the AgIO3 /WO3 heterostructures, there must exist an optimum ratio of AgIO3 and WO3 which can induce a bigger contact area between two different semiconductors. Fig. 4b shows the temporal absorption spectral changes of RhB solution over the as-prepared 50%AgIO3 /WO3 sample under visible light irradiation. It can be observed that the primary absorption band of RhB locates at λ = 553 nm, and the starting red solution fades to colorless and transparent when illuminated for 140 min, suggesting that RhB is decomposed completely. At the same time, a spectral blue-shift from 553 to 501 nm can be noted in Fig. 4b, which can be ascribed to the de-ethylation processes in the RhB dye photodegradation [29]. In order to investigate the photocatalytic capability of AgIO3 , WO3 , 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 , and to comprehend their chemical kinetics of the photodegrading RhB processes, a pseudo-first-order model has been suggested. As shown in Fig. 5, ln(C/C0 ) is observed having a linear function against the irradiation time, suggesting the photocatalytic reactions are in good accordance with the pseudo-first-order reaction, and the model can be expressed the following equation:
ln (C/C0 ) = kt
(2)
Fig. 4. (a) The plot of C/C0 vs. the irradiation time of WO3, AgIO3 and AgIO3/WO3, respectively; (b) Spectral of the photodegredation of RhB over 50 % AgIO3/WO3 photocatalyst under visible light irradiation.
Fig. 5. The plot of ln(C0/C) versus the irradiation time of AgIO3, WO3, 20%AgIO3/WO3, 50%AgIO3/WO3, and 67%AgIO3/WO3, respectively.
Please cite this article as: Q.W. Cao et al., Enhanced visible-light-driven photocatalytic degradation of RhB by AgIO3 /WO3 composites, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.030
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Fig. 6. Repetitive testing of the RhB photodegradation process on 50%AgIO3/WO3 heterjunction under simulated solar light irradiation.
The photocatalytic degradation constants of AgIO3 , WO3 , 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 are calculated to be 0.0 014, 0.0 011, 0.0108, 0.0178, and 0.0129 min−1 , respectively. The result shows that the 50%AgIO3 /WO3 composite has the highest degradation rate, implying the heterostructure photocatalyst with the optimum content percentage of AgIO3 show a significantly enhanced photocatalytic activity. To study the stability of the 50%AgIO3 /WO3 composite, repetitive experiments were carried out to evaluate the photodegradation of RhB over the 50%AgIO3 /WO3 heterojunction photocatalyst under visible-light irradiation (λ > 420 nm). The recycled photocatalyst was separated and retrieved from the suspension by centrifugation after each run. The results are displayed in Fig. 6, it can be discovered that although there is deactivation of catalyst in every cycling runs, the photocatalytic activity of 50%AgIO3 /WO3 is still retained nearly 80% of its original photocatalytic activity in the third experimental run, suggesting that the 50%AgIO3 /WO3 heterojunction is stable during the photodegradation of RhB under visible-light irradiation. It is well known that several reactive species will generate in the photocatalytic oxidation (PCO) process, which can be ascribed to the photogenerated electrons and holes motivated from the photocatalyst seminconductors, and these reactive species will directly determine the performances of catalyst in PCO [30,31]. To ascertain the photocatalysis mechanism of RhB degradation in detail, it is crucial to understand the functions of the active species in the RhB photodegradation processes. Therefore, several radical scavengers are applied in the reaction system, benzoquinone (BQ, 2 mg), ammonium oxalate (2.13 mg) and isopropanol (IPA, 15 mL) were added into 200 mL of RhB solution companied with 50%AgIO3 /WO3 and illuminated by visible light, respectively. Among the test, benzoquinone acts as superoxide radicals (·O2 − ) scavenger, ammonium oxalate works as holes (h+ ) scavenger, and isopropanol is hydroxyl radicals (·OH) scavenger. As can be seen from Fig. 7 the photocatalytic activity is greatly deactivated when isopropanol is added, with only 16.8% of RhB being decomposed in 140 min. While 75.5% and 95.7% of RhB have been photodegraded within the same irradiation time with ammonium oxalate and BQ added, respectively. It can be summarized that ·OH may be the major active oxygen species in the photocatalytic degradation process. For further investigation of the effect of 50%AgIO3 /WO3 catalyst, The photoluminescence (PL) analysis is usually applied to investigate the diffusion and recombination processes of photoin-
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Fig. 7. Effects of radical scavengers in the degradation process of RhB over 50%AgIO3/WO3.
Fig. 8. The PL spectra of as-prepared photocatalysts.
duced electron-hole pairs in semiconductors, which is important in photocatalytic reactions [17,32]. In general, the enhanced photocatalytic activity is related to the lower recombination rate of photogenerated electron-hole pairs, leading to lower PL intensity. Fig. 8 shows the PL spectra of pure WO3 , 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 using a 300 nm excitation wavelength. It can be seen that all the photocatalysts exhibit similar PL spectra and show a strong emission peak at about 475 nm, which can be ascribed to band-band PL phenomenon with energy of light. Moreover, comparing to pure WO3 , the emission intensity of 20%AgIO3 /WO3 , 50%AgIO3 /WO3 , and 67%AgIO3 /WO3 decrease significantly and the lowest intensity was observed on 50%AgIO3 /WO3 heterojunction, indicating that the recombination rate of photoinduced charge carriers on 50%AgIO3 /WO3 composites is the lowest. The results correlate well with the best activity of 50%AgIO3 /WO3 compound among all the photocatalysts. As is well-known, the separation efficiency of the photoinduced charge carriers can affect the photocatalytic properties. To comprehend the photocatalytic mechanism involved in the photodecomposion of the fabricated 50%AgIO3 /WO3 composite the edge potentials of CB and VB are necessary to be computed due to their close relationships with the PCO of organic molecules. The CB potential of WO3 is 0.09 eV (vs. NHE, pH = 7) [8], and the VB
Please cite this article as: Q.W. Cao et al., Enhanced visible-light-driven photocatalytic degradation of RhB by AgIO3 /WO3 composites, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.030
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alyst is a promising material in applications and worth researching in future.
Acknowledgment This work is financially supported by the National Nature Science Foundation of China (No. 21273034). References
Fig. 9. Schematic illustration of the photocatalysis process involved in the 50%AgIO3/WO3 composite.
potential of WO3 is 2.55 eV. The ECB and EVB of AgIO3 are calculated to be 0.45 eV and 3.83 eV, respectively [29]. The markedly enhanced photocatalytic activities of AgIO3 /WO3 composites are prevailingly attributed to the effective separation of the photoinduced electron-hole pairs. Based on the band-gap structure of AgIO3 /WO3 heterjunction and the results of free radical trapping experiments, the photocatalytic mechanism of AgIO3 /WO3 heterjunction is proposed. As shown in Fig. 9, the redox potentials of conduction band and valence band of AgIO3 are observed to be more positive than those of WO3 and there is an interactive structure formed by AgIO3 and WO3 , which is favorable for the separation of light-induced charge carriers. Under visible-light irradiation, electron-hole pairs can only be generated on the surface of WO3 . The photogenerated electrons on the CB of WO3 can efficiently migrate to the CB of AgIO3 . The redox potential of O2 /·O2 − is −0.046 eV [33], more negative than the CB edge potential of AgIO3 (0.45 eV), which makes it impossible to yield ·O2 − radicals via the reduction of O2 by CB electrons of AgIO3 . Meanwhile, the photoinduced holes on the VB of WO3 will react with the reagent RhB. For the WO3 , the VB edge potential is 2.55 eV, and the potential of OH– /·OH couple is 1.99 eV [32]. The VB edge potential of WO3 is more positive than that of OH– /·OH couple, which indicate that the photoinduced holes would react with adsorbed H2 O to form ·OH. The ·OH radicals can then effectively decompose RhB into H2 O, CO2 and other intermediates, which is consistent with the results of radical scavengers experiments. As a consequence, the photoexcited electron and hole separate efficiently on the interface of the AgIO3 /WO3 photocatalyst, which distinctively enhanced the photocatalytic activity of the catalyst. 4. Conclusion In summary, a series of AgIO3 /WO3 heterostructure photocatalysts with different content of AgIO3 have been fabricated through facile hydrothermal and chemical precipitation method. The asprepared AgIO3 /WO3 heterojunction photocatalysts displayed much higher photocatalytic efficiency for the photodecomposition of RhB compared to pure AgIO3 and WO3 photocatalysts under visible light irradiation. It can be noted that 50%AgIO3 /WO3 compound exhibits the highest photocatalytic activity, with all the RhB degraded completely in 140 min. The enhanced photocatalytic activity can be mainly attributed to the high separation rate of photogenerated electron-hole pairs and efficient migration during photocatalytic degradation process. Additionally, ·OH is considered to be the main reactive species in the photodegradation course, and a probable photocatalytic mechanism is also proposed according to the experimental results. This study indicates that AgIO3 /WO3 cat-
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Please cite this article as: Q.W. Cao et al., Enhanced visible-light-driven photocatalytic degradation of RhB by AgIO3 /WO3 composites, Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.10.030