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Plate-like WO3 inserting into I-deficient BiO1.2I0.6 microsphere for highly efficient photocatalytic degradation of VOCs
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Plate-like WO3 inserting into I-deficient BiO1.2 I0.6 microsphere for highly efficient photocatalytic degradation of VOCs Shaomang Wang a, Yuan Guan a,∗, Ruiheng Zeng a, Zhijun Zhang b, Liang Liu c, Zhongyu Li a,∗, Wen An a, Yang Fu a a b c
School of Environment and Safety Engineering, School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China Huaide College, Changzhou University, Jingjiang, Jiangsu 214500, China Green Intelligence Environmental School, Yangtze Normal University, Fuling, Chongqing 408100, China
a r t i c l e
i n f o
Article history: Received 22 June 2019 Revised 5 September 2019 Accepted 19 September 2019 Available online xxx Keywords: Photocatalysis Pollutant degradation VOCs I-deficient BiO1.2 I0.6 /WO3
a b s t r a c t The key to photocatalytic efficient decomposition of VOCs is to significantly improve carrier separationefficiency of the photocatalyst. Herein, we synthesized a novel photocatalytic composite material of Ideficient BiO1.2 I0.6 /WO3 by solvothermal method coupling with thermal decomposition. The PL spectrum and photoelectrochemical tests found that WO3 compounded by 40 wt% BiO1.2 I0.6 greatly promoted its separation efficiency of carriers. This was mainly caused by the potential difference of band edge between the 40 wt% BiO1.2 I0.6 and WO3 driving reverse shift of carriers in their space charge-layer. The 40 wt% BiO1.2 I0.6 /WO3 achieved 58.8% mineralization ratio in the process of degrading typical VOCs, toluene after 8 h irradiation, which was 15.9 and 2 times those of BiO1.2 I0.6 and WO3 , respectively. Gaussian calculation and ESR investigation demonstrated that toluene was decomposed by h+ and hydroxyl radical (·OH) under irradiation. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Photocatalytic degradation of VOCs is earning much attention due to the expected use of solar energy [1–5]. However, the lack of a photocatalyst with high quantum efficiency greatly inhibits its practical application. A photocatalyst with broad light absorption, high separation and injection efficiency of carriers exhibits high quantum efficiency [6]. WO3 as a visible-light catalytic material has received extensive attention because its band-gap energy is about 2.6 eV and maximum adsorption wavelength can reach 480 nm [7–11]. In addition, WO3 also exhibited high hole injection-efficiency owning to its deep valence-band energy level of 3.3 V [12,13]. Unfortunately, pure WO3 displays low photocatalytic activity due to its low separation efficiency of photo-generated carriers [14,15]. Constructing a composite of two materials with the potential difference between the conduction band (CB) and the valence band (VB) to drive reverse movement of electron and hole is an effective method to improve separation efficiency of carriers. Some WO3 composites
∗
Corresponding authors. E-mail addresses: [email protected] (Y. Guan), [email protected] (Z. Li).
such as TiO2 /WO3 [16,17], Ag3 VO4 /WO3 [18], CdS/WO3 [19,20], Ag3 PO4 /WO3 [21], ZnO/WO3 [22], Bi2 O3 /WO3 [23], g-C3 N4 /WO3 [24], and BiVO4 /WO3 [25] have been successfully prepared and exhibit enhanced photocatalytic activity. However, the current WO3 composite photocatalytic materials still have some shortcomings such as narrow light absorption or poor stability or low activity. Recently, a photocatalyst of BiOI with the absorption light up to 660 nm and good photocatalytic activity and stability receives widespread attention [26–30]. We have synthesized I-deficient BiO1.2 I0.6 in previous study, and found that the iodine vacancy obviously promoted the photocatalytic activity of BiOI [31]. The CB potential of BiO1.2 I0.6 is about 0.07 V Vs NHE, which is lower than that (∼0.74 V Vs NHE) of WO3 . The VB potential of WO3 is higher than that (∼2.61 V Vs NHE) of BiO1.2 I0.6 . Thus, WO3 compounded by BiO1.2 I0.6 should distinctly improve its separation efficiency of photo-generated carriers. Moreover, the BiO1.2 I0.6 /WO3 composite will exhibit wide light absorption and good stability. In this work, we prepared the composites of BiO1.2 I0.6 /WO3 for the first time. Their performances were investigated through photocatalytic degradation of typical VOCs, toluene. The mechanisms of the enhanced photocatalytic activity of BiO1.2 I0.6 /WO3 were discussed. The main active species for degradation of toluene were revealed.
https://doi.org/10.1016/j.jtice.2019.09.015 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: S. Wang, Y. Guan and R. Zeng et al., Plate-like WO3 inserting into I-deficient Bio1.2I0.6 microsphere for highly efficient photocatalytic degradation of VOCs, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.09. 015
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2. Experimental section 2.1. Photocatalyst preparation 2.1.1. Fabrication of I-deficient Bio1.2 I0.6 I-deficient BiO1.2 I0.6 was prepared according to our previous research [31]. The 40 mL glycol containing 1.9 g of KI was dropwisely added into the same volume of glycol containing 5.5 g of Bi(NO3 )3 ·5H2 O. The mixture was transferred into an autoclave and reacted at 160 °C for 12 h after it was magnetically stirred for 1 h. The cooled product was washed by deionized water and then dried at 80 °C to obtain BiOI. Then, 1 g of as-synthesized BiOI was placed in a muffle furnace and calcined at 400 °C for 5 h with a heat rate of 5 °C min−1 . After the sample was naturally cooled to room temperature, the BiO1.2 I0.6 was obtained. 2.1.2. Preparation of WO3 1 g of H2 WO4 was placed in the muffle furnace and calcined at 500 °C for 3 h with a heating rate of 10 °C min−1 . After the sample was naturally cooled to room temperature, the WO3 was obtained. Fig. 1. XRD patterns of BiO1.2 I0.6 , WO3 and BiO1.2 I0.6 /WO3 composites.
2.1.3. Synthesis of BiO1.2 I0.6 /WO3 composites The composites of BiO1.2 I0.6 /WO3 were prepared as follows. 0.46 g of BiOI and 0.60 g WO3 were fully ground for 0.5 h with addition of 5 mL of ethanol. The mixture was dried at 100 °C for 1 h and then uniformly ground again. Subsequently, it was calcined at 400 °C for 5 h with a heating rate of 5 °C min−1 . The 40 wt% BiO1.2 I0.6 /WO3 was obtained after naturally cooled to room temperature. The other BiO1.2 I0.6 /WO3 composites with different content of BiO1.2 I0.6 were prepared via the similar method. 2.2. Characterization The crystal structure of the samples was identified via powder X-ray diffraction (XRD) (Rigaku Ultima III, Japan) with Cu Kα radi˚ The morphology of the samples was observed ation (λ = 1.5418 A). by the scanning electron microscope (Nova Nano 230) and transmission electron microscope (JEM-200CX). X-ray photoelectron spectroscopy (XPS) test was conducted on a PHI50 0 0 Versa Probe (ULVAC-PHI, Japan) with monochromatized Al Kα X-ray radiation (1486.6 eV). Light absorption characteristics of the samples was characterized by a UV–vis diffuse reflectance spectra (UV-2550, Shimadzu). The specific surface area based on the BET equation was analyzed by N2 adsorption–desorption on an instrument (SA-3100, Beekman Coulter). The photoluminescence spectra (PL) was collected by Fluoromax-4 (HORIBA, USA) with an excitation wavelength at 400 nm. Electron spin resonance (ESR) test was conducted on an apparatus (JES-FA200, JEOL). Photoelectrochemical measurement was conducted on an electrochemical analyzer (CHI-660D, Shanghai Chenhua, China) in a standard three-electrode system. A platinum sheet and a Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. Working electrode was prepared as follows: 30 mg of the sample and 10 mg of iodine were added to 25 mL of acetone with ultrasonic shock for half an hour. Then, the sample was deposited on a fluorine-doped tin oxide conducting substrate with a fixed area of 1 cm2 by a voltage of 12 V for 3 min. Na2 SO4 (0.1 mol L−1 ) aqueous solution was used as the electrolyte. A 300 W Xe lamp was utilized as the light source.
2.4. Investigation of photocatalytic performances 2.4.1. Evaluation of photocatalytic activity The photocatalytic degradation of toluene was performed in a gas sealed container with a volume of 250 mL. The as-synthesized photocatalyst (0.1 g) was uniformly dispersed on the bottom of the reaction container. Toluene gas with 50 μL of H2 O was injected into the reactor such that the initial concentration of toluene was 2800 mg m−3 . The system was kept in the dark for 1 h to ensure that the adsorption–desorption equilibrium was established. A 300 W Xenon lamp was utilized as light source. During the irradiation, 100 μL of gas withdrawn from the system every hour was analyzed through a gas chromatography (GC9790, Fuli) of hydrogen flame. The temperatures of injector and column were all 120 °C. Detector temperature was 150 °C. 2.4.2. Stability test of photocatalyst The stability measurement of a photocatalyst was carried out as follows. The reactor was emptied after the end of a round of experiment. The original gas in the reactor was replaced using fresh air. Then, the reactor was sealed again and the same concentration of toluene gas was injected to conduct the next round of degradation reaction. 3. Results and discussion 3.1. Characterization of photocatalysts As shown in Fig. 1, the tetragonal phase of BiOI (PDF#10-0445) calcined at 400 °C for 5 h did not convert to Bi2 O3 [32,33]. According to the weight loss before and after the calcination of BiOI, it is believed that BiOI was converted to I-deficient BiO1.2 I0.6 . The WO3 was consistent with its orthorhombic structure (PDF#20-1324). According to Scherrer formula (1) [34,35], average particle diameters of BiO1.2 I0.6 and WO3 were calculated to be about 20.13 and 24.83 nm.
D= 2.3. Calculation method The function of b3lyp was used to calculate the HOMO/LUMO energy level. The basis set was 6–31 g and the convergence criteria was tight. The d-polarized orbit and p-polarized orbit were added to non-hydrogen atoms and hydrogen atoms, respectively.
K×λ β × Cos θ
(1)
K and λ are equal to 0.89 and 0.15418 nm, respectively. β is the peak width at half height of the diffraction peak, and θ is the diffraction angle. Both characteristic peaks of BiO1.2 I0.6 and WO3 were observed in the composites of BiO1.2 I0.6 /WO3 . With increasing content of BiO1.2 I0.6 , the peak intensity of BiO1.2 I0.6 gradually
Please cite this article as: S. Wang, Y. Guan and R. Zeng et al., Plate-like WO3 inserting into I-deficient Bio1.2I0.6 microsphere for highly efficient photocatalytic degradation of VOCs, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.09. 015
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Fig. 2. SEM images of (a and b) BiO1.2 I0.6 , (c and d) WO3 and (e and f) 40 wt% BiO1.2 I0.6 /WO3 .
increased while that of WO3 weakened by degrees. The results of XRD display that the composites of BiO1.2 I0.6 /WO3 have been successfully prepared. Morphology and particle size of BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 are illustrated in Fig. 2. BiO1.2 I0.6 was a flower-like microsphere, which was consisted of many nano-plates. The diameter of microsphere was about 2 μm. WO3 was composed of many plate-like bulks with length from 10 nm to 350 nm. The thickness of plate was approximately 50 nm. For 40 wt% BiO1.2 I0.6 /WO3 , the microspheres of BiO1.2 I0.6 were tightly wrapped by many plate-like
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bulks of WO3 . Many plate-like WO3 inserted into nano-plate gap of BiO1.2 I0.6 microspheres. The results distinctly show that BiO1.2 I0.6 and WO3 are tightly combined. The same morphology as above, BiO1.2 I0.6 and WO3 were flower-like microsphere and plate-like bulks (Fig. 3a–b). It is clearly seen that BiO1.2 I0.6 was covered by many particles of WO3 in 40 wt% BiO1.2 I0.6 /WO3 (Fig. 3c). The high-resolution TEM lattice image depicted interfacial interaction between BiO1.2 I0.6 and WO3 (Fig. 3d). Lattice spacing of 0.37 nm belonged to BiO1.2 I0.6 (101) and that of 0.26 nm originated from WO3 (220). The chemical composition and elemental valence in the surface of as-prepared photocatalysts are depicted in Fig. 4. The binding energies of Bi, O and I in the BiO1.2 I0.6 corresponded to Bi3+ , O2− and I− , respectively. For 40 wt% BiO1.2 I0.6 /WO3 , the binding energies of Bi 4f7/2 , O 1 s and I 5d3/2 were higher than those in BiO1.2 I0.6 . It is mainly due to the fact that the electronegativity of W is greater than that of Bi resulting in the decline of outer electronic cloud density around Bi 4f7/2 , O 1s and I 5d3/2 in 40 wt% BiO1.2 I0.6 /WO3 . It causes the shielding effect between electrons and atomic nuclear to be decreased. The binding energy of I 5d5/2 in 40 wt% BiO1.2 I0.6 /WO3 was lower than that in BiO1.2 I0.6 . It might be that the decrease of outer electronic cloud density of I 5d3/2 leads to the density increase of spin and orbital angular momentum in the same direction. In addition, the binding energies of W 4f and O 1 s in WO3 corresponding to W6+ and O2− were higher than those in 40 wt% BiO1.2 I0.6 /WO3 [36]. Compared to WO3 , the density of outer electronic cloud around W 4f and O 1 s in 40 wt% BiO1.2 I0.6 /WO3 is higher because the electronegativity of Bi is smaller than that of W. It causes the shielding effect between electrons and atomic nuclear to be enhanced. The results of XPS confirm that the chemical composition and elemental valence are in accord with as-fabricated photocatalysts. BiOI is not converted to Bi2 O3 after the calcination at 400 °C for 5 h. There is a strong interaction between BiO1.2 I0.6 and WO3 in 40 wt% BiO1.2 I0.6 /WO3 . The light-absorption property of BiO1.2 I0.6 , WO3 and BiO1.2 I0.6 /WO3 composites is shown in Fig. 5. The as-synthesized samples had good property of visible-light absorption. The maximum adsorption wavelength of BiO1.2 I0.6 reached about 500 nm, and WO3 could also absorb light at the maximum wavelength of approximately 480 nm. The optical absorption of BiO1.2 I0.6 /WO3 composites was between 480 nm and 500 nm.
Fig. 3. TEM images of (a) BiO1.2 I0.6 , (b) WO3 and (c and d) 40 wt% BiO1.2 I0.6 /WO3 .
Please cite this article as: S. Wang, Y. Guan and R. Zeng et al., Plate-like WO3 inserting into I-deficient Bio1.2I0.6 microsphere for highly efficient photocatalytic degradation of VOCs, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.09. 015
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Fig. 4. XPS core-level spectra of (a) Bi 4f, (b) O 1s, (c) I 5d and (d) W 4f.
Fig. 5. UV–vis absorption spectra of BiO1.2 I0.6 , WO3 and BiO1.2 I0.6 /WO3 composites.
Please cite this article as: S. Wang, Y. Guan and R. Zeng et al., Plate-like WO3 inserting into I-deficient Bio1.2I0.6 microsphere for highly efficient photocatalytic degradation of VOCs, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.09. 015
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Fig. 6. (a) Concentration of toluene over BiO1.2 I0.6 , WO3 and BiO1.2 I0.6 /WO3 composites with increasing time of irradiation. (b) Mineralization ratio of toluene over (1) BiO1.2 I0.6 , (2) WO3 and (3–7) 10–50 wt% BiO1.2 I0.6 /WO3 after irradiation for 8 h.
Fig. 7. (a) Schematic illustration of photo-induced carrier separation in 40 wt% BiO1.2 I0.6 /WO3 . (b) PL spectra of BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 .
3.2. Photocatalytic performances of photocatalysts The photocatalytic activity of as-synthesized samples were evaluated by degrading toluene. As shown in Fig. 6a, the concentration of toluene over BiO1.2 I0.6 , WO3 and BiO1.2 I0.6 /WO3 composites gradually degreased with increasing reaction time. The degradation ratio of toluene over BiO1.2 I0.6 and WO3 were about 82.3% and 73.8% under irradiation of 8 h, respectively. The photocatalytic activity of BiO1.2 I0.6 /WO3 composites was significantly higher than that of BiO1.2 I0.6 and WO3 . At the same irradiation time, the degradation ratio of toluene over 10–50 wt% BiO1.2 I0.6 /WO3 were 96.2%, 95.5%, 96.4%, 99% and 96.9%, respectively. The 40 wt% BiO1.2 I0.6 /WO3 exhibited the best photocatalytic activity. The degradation rate of toluene over 40 wt% BiO1.2 I0.6 /WO3 were 1.2 and 1.4 times those of BiO1.2 I0.6 and WO3 , respectively. Moreover, the degradation rate of toluene over 40 wt% BiO1.2 I0.6 /WO3 was higher than previous research results (Table S1). It is necessary for toxic VOCs to be effectively mineralized before discharge. The CO2 concentration was detected in every system before and after the reaction (Fig. 6b). The mineralization rate of toluene over BiO1.2 I0.6 , WO3 and 10–50 wt% BiO1.2 I0.6 /WO3 were 3.7%, 29.4%, 36.7%, 7.4%, 22%, 58.8% and 22%. Although the degradation rate of toluene over BiO1.2 I0.6 was higher than that over WO3 , the mineralization ratio was on the contrary. It is mainly due to the fact that WO3 has the deeper valence-band
energy level resulting in stronger oxidation ability. The 40 wt% BiO1.2 I0.6 /WO3 achieved the highest mineralization rate of toluene (58.8%). Degradation rate of toluene over 40 wt% BiO1.2 I0.6 /WO3 was 1.7 times mineralization rate of toluene. It might be that 40 wt% BiO1.2 I0.6 /WO3 with low specific surface area is difficult to adsorb small molecular intermediates (Fig. S3). The results of toluene degradation over 40 wt% BiO1.2 I0.6 /WO3 by cycling three times show that the degradation and mineral rates of toluene did not distinctly decrease (Fig. S1). The diffraction peaks of 40 wt% BiO1.2 I0.6 /WO3 after 3 runs of use did not change (Fig. S2). These indicate that the 40 wt% BiO1.2 I0.6 /WO3 has good stability. 3.3. Mechanisms of the enhanced photocatalytic activity For heterogeneous photocatalytic reaction, a photocatalyst with distinctly large specific surface area generally exhibits high photocatalytic activity. The specific surface area of BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 calculated by adsorption and desorption isotherm (Fig. S3) was 16.2, 11.4 and 14.7 cm2 g−1 . The specific surface area of WO3 was smaller than those of 40 wt% BiO1.2 I0.6 /WO3 and BiO1.2 I0.6 . Thence, the photocatalytic activity of WO3 is lower than 40 wt% BiO1.2 I0.6 /WO3 and BiO1.2 I0.6 . Although the specific surface area of 40 wt% BiO1.2 I0.6 /WO3 was
Please cite this article as: S. Wang, Y. Guan and R. Zeng et al., Plate-like WO3 inserting into I-deficient Bio1.2I0.6 microsphere for highly efficient photocatalytic degradation of VOCs, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.09. 015
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Fig. 8. (a) Current density of BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 under dark and irradiation. (b) Nyquist impedance of BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 .
Fig. 9. (a) The HOMO and LUMO energy level of toluene. ESR spectra of 40 wt% BiO1.2 I0.6 /WO3 using DMPO as (b) hydroxyl radical scavenger in aqueous solution and (c) superoxide radical scavenger in methanol solution respectively.
Please cite this article as: S. Wang, Y. Guan and R. Zeng et al., Plate-like WO3 inserting into I-deficient Bio1.2I0.6 microsphere for highly efficient photocatalytic degradation of VOCs, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.09. 015
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smaller than that of BiO1.2 I0.6 , the photocatalytic activity of 40 wt% BiO1.2 I0.6 /WO3 was obviously higher than BiO1.2 I0.6 . This is basically caused by separation-efficiency difference of photo-generated carriers. In generally, a photocatalyst with high separation efficiency of photo-generated electron–hole pairs demonstrates excellent photocatalytic activity. Because of the potential difference of BiO1.2 I0.6 (ECB = 0.07 and EVB = 2.61 V) and WO3 (ECB = 0.74 and EVB = 3.3 V) in 40 wt% BiO1.2 I0.6 /WO3 , photo-generated electrons from the CB of BiO1.2 I0.6 will jump to the CB of WO3 , and photo-induced holes from the VB of WO3 will migrate to the VB of BiO1.2 I0.6 (Fig. 7a). This distinctly promotes separation of photo-generated electronhole pairs in 40 wt% BiO1.2 I0.6 /WO3 , thereby greatly enhancing its photocatalytic activity. Separation efficiency of carriers in BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 was characterized by photoluminescence spectroscopy. A photocatalyst with low intensity of fluorescent peak exhibits high separation efficiency under light excitation. PL spectra of BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 is shown in Fig. 7b. WO3 emitted a strong fluorescent peak at about 487 nm. The fluorescent intensity of BiO1.2 I0.6 at approximately 485 nm was slightly lower than that of WO3 . Compared to WO3 and BiO1.2 I0.6 , the 40 wt% BiO1.2 I0.6 /WO3 emitted a fluorescent peak with significantly decreased intensity. The PL results clearly display that the separation efficiency of electron–hole pairs is 40 wt% BiO1.2 I0.6 /WO3 > BiO1.2 I0.6 > WO3 . Thus, the 40 wt% BiO1.2 I0.6 /WO3 demonstrates distinctly enhanced photocatalytic activity. Photocurrent and electrochemical impedance were utilized to further analyze separation efficiency of carriers in BiO1.2 I0.6, WO3 and 40 wt% BiO1.2 I0.6 /WO3 . A photocatalyst with high photocurrent and low impedance corresponds to high separation efficiency of carriers. The photocurrent sequence was 40 wt% BiO1.2 I0.6 /WO3 > BiO1.2 I0.6 > WO3 (Fig. 8a), while the impedance in the order was 40 wt% BiO1.2 I0.6 /WO3 < BiO1.2 I0.6 < WO3 (Fig. 8b and Fig. S4). The results confirm that the separation efficiency of carriers is 40 wt% BiO1.2 I0.6 /WO3 > BiO1.2 I0.6 > WO3 , which is in line with PL results.
3.4. Main active species for toluene degradation It is well-known that hydroxyl radical (·OH) and superoxide radical (·O2 − ) are primary active species besides photo-generated electron and hole in the process of photocatalytic degradation [37–39]. The results of Gaussian calculation demonstrated that the HOMO and LUMO energy level of toluene were 1.9 V (Vs NHE) and −4.6 V (Vs NHE), respectively (Fig. 9a). The HOMO energy level of toluene was less than the VB potentials of BiO1.2 I0.6 (2.61 V Vs NHE) and WO3 (3.3 V Vs NHE), as well as oxidation potential of ·OH (2.8 Vs NHE), but greater than oxidation potential of ·O2 − (1.75 Vs NHE). Its energy level of the LUMO was smaller than the CB potential of BiO1.2 I0.6 (0.07 V Vs NHE) and WO3 (0.74 V Vs NHE). This indicates that toluene should be decomposed by ·OH and h+ . Radical generated over 40 wt% BiO1.2 I0.6 was detected using DMPO as a radical scavenger by ESR technique under dark and light irradiation (Fig. 9b–c). No peaks were observed under the dark. Four peaks with the signal intensity of 1:2:2:1 appeared in H2 O solution under irradiation for 5 min, which is a typical ESR signal of ·OH. No signal of ·O2 − was found under either irradiation or dark for 5 min. ESR results demonstrate that main radical for the photocatalytic degradation of toluene over 40 wt% BiO1.2 I0.6 is ·OH not ·O2 − . This further confirms that the toluene is mainly degraded by ·OH and h+ . According to the detected active species, the probable mechanism of the photocatalytic degradation of toluene can be summarized as follows. [40,41]
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40 wt% BiO1.2 I0.6 /WO3 + hV → 40 wt% BiO1.2 I0.6 /WO3 ∗ + e− + h+ h+ + H2 O → ·OH[40, 41] h+ + ·OH + toluene → degradation products 4. Conclusions A novel photocatalyst of 40 wt% BiO1.2 I0.6 /WO3 was successfully prepared. The 40 wt% BiO1.2 I0.6 /WO3 exhibited high separation efficiency of photo-induced carriers. Moreover, it also absorbed broad spectrum photons with good stability for reuse. Although this photocatalyst still needs further improvement in the ability to deeply decompose toluene, this work opens up a promising strategy for developing high performance photocatalysts for efficient purification of VOCs. Acknowledgments This work was supported primarily by the National Natural Science Foundation of China (21876015), the Natural Science Foundation of Jiangsu Province (BK20161277 and BK20190934), the Natural Science Foundation of Jiangsu Education Department (16KJB610 0 02), the Postdoctoral Science Foundation of China (2017M611784). Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2019.09.015. References [1] Liu H, Ma Y, Chen J, Wen M, Li G, An T. Highly efficient visible-light-driven photocatalytic degradation of VOCs by CO2 -assisted synthesized mesoporous carbon confined mixed-phase TiO2 nanocomposites derived from MOFs. Appl Catal B-Environ 2019;250:337–46. [2] Sansotera M, Kheyli SGM, Baggioli A, Bianchi CL, Pedeferri MP, Diamanti MV, Navarrini W. Absorption and photocatalytic degradation of VOCs by perfluorinated ionomeric coating with TiO2 nanopowders for air purification. Chem Eng J 2019;361:885–96. [3] Li JA, Li XY, Zeng LB, Fan SY, Zhang MM, Sun WB, Chen X, Tade MO, Liu SM. Functionalized nitrogen-doped carbon dot-modified yolk-shell ZnFe2 O4 nanospheres with highly efficient light harvesting and superior catalytic activity. Nanoscale 2019;11:3877–87. [4] Nie L, Duan B, Lu A, Zhang LN. Pd/TiO2 @ carbon microspheres derived from chitin for highly efficient photocatalytic degradation of volatile organic compounds. ACS Sustain Chem Eng 2019;7:1658–66. [5] Zhang X, Wang L, Zhou X, Ni Z, Xia S. Investigation into the enhancement of property and the difference of mechanism on visible light degradation of gaseous toluene catalyzed by ZnAl layered double hydroxides before and after Au support. ACS Sustain Chem Eng 2018;6:13395–407. [6] Wang SL, Zhu Y, Luo X, Huang Y, Chai J, Wong TI, Xu GQ. 2D WC/WO3 heterogeneous hybrid for photocatalytic decomposition of organic compounds with vis-nir light. Adv Funct Mater 2018;28:1705357–64. [7] Zhou H, Wen Z, Liu J, Ke J, Duan X, Wang S. Z-scheme plasmonic Ag decorated WO3 /Bi2 WO6 hybrids for enhanced photocatalytic abatement of chlorinated-VOCs under solar light irradiation. Appl Catal B-Environ 2019;242:76–84. [8] Katsumata K, Motoyoshi R, Matsushita N, Okada K. Preparation of graphitic carbon nitride (g-C3 N4 )/WO3 composites and enhanced visible-light-driven photodegradation of acetaldehyde gas. J Hazard Mater 2013;260:475–82. [9] Zhang L, Qin M, Yu W, Zhang Q, Xie H, Sun Z, Shao Q, Guo X, Hao L, Zheng Y, Guo Z. Heterostructured TiO2 /WO3 nanocomposites for photocatalytic degradation of toluene under visible light. J Electrochem Soc 2017;164:1086–90. [10] Liu Y, Xie C, Li H, Chen H, Liao Y, Zeng D. Low bias photoelectrocatalytic (PEC) performance for organic vapour degradation using TiO2 /WO3 nanocomposite. Appl Catal B-Environ 2011;102:157–62. [11] Sun SM, Watanabe M, Wu J, An Q, Ishihara T. Ultrathin WO3 center dot 0·33H2 O nanotubes for CO2 photoreduction to acetate with high selectivity. J Am Chem Soc 2018;140:6474–82. [12] Wang TY, Quan W, Jiang DL, Chen LL, Li D, Meng S, Chen M. Synthesis of redox-mediator-free direct Z-scheme Agi/WO3 nanocomposite photocatalysts for the degradation of tetracycline with enhanced photocatalytic activity. Chem Eng J 2016;300:280–90.
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Plate-like WO3 inserting into I-deficient BiO1.2 I0.6 microsphere for highly efficient photocatalytic degradation of VOCs Shaomang Wang a, Yuan Guan a,∗, Ruiheng Zeng a, Zhijun Zhang b, Liang Liu c, Zhongyu Li a,∗, Wen An a, Yang Fu a a b c
School of Environment and Safety Engineering, School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China Huaide College, Changzhou University, Jingjiang, Jiangsu 214500, China Green Intelligence Environmental School, Yangtze Normal University, Fuling, Chongqing 408100, China
a r t i c l e
i n f o
Article history: Received 22 June 2019 Revised 5 September 2019 Accepted 19 September 2019 Available online xxx Keywords: Photocatalysis Pollutant degradation VOCs I-deficient BiO1.2 I0.6 /WO3
a b s t r a c t The key to photocatalytic efficient decomposition of VOCs is to significantly improve carrier separationefficiency of the photocatalyst. Herein, we synthesized a novel photocatalytic composite material of Ideficient BiO1.2 I0.6 /WO3 by solvothermal method coupling with thermal decomposition. The PL spectrum and photoelectrochemical tests found that WO3 compounded by 40 wt% BiO1.2 I0.6 greatly promoted its separation efficiency of carriers. This was mainly caused by the potential difference of band edge between the 40 wt% BiO1.2 I0.6 and WO3 driving reverse shift of carriers in their space charge-layer. The 40 wt% BiO1.2 I0.6 /WO3 achieved 58.8% mineralization ratio in the process of degrading typical VOCs, toluene after 8 h irradiation, which was 15.9 and 2 times those of BiO1.2 I0.6 and WO3 , respectively. Gaussian calculation and ESR investigation demonstrated that toluene was decomposed by h+ and hydroxyl radical (·OH) under irradiation. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Photocatalytic degradation of VOCs is earning much attention due to the expected use of solar energy [1–5]. However, the lack of a photocatalyst with high quantum efficiency greatly inhibits its practical application. A photocatalyst with broad light absorption, high separation and injection efficiency of carriers exhibits high quantum efficiency [6]. WO3 as a visible-light catalytic material has received extensive attention because its band-gap energy is about 2.6 eV and maximum adsorption wavelength can reach 480 nm [7–11]. In addition, WO3 also exhibited high hole injection-efficiency owning to its deep valence-band energy level of 3.3 V [12,13]. Unfortunately, pure WO3 displays low photocatalytic activity due to its low separation efficiency of photo-generated carriers [14,15]. Constructing a composite of two materials with the potential difference between the conduction band (CB) and the valence band (VB) to drive reverse movement of electron and hole is an effective method to improve separation efficiency of carriers. Some WO3 composites
∗
Corresponding authors. E-mail addresses: [email protected] (Y. Guan), [email protected] (Z. Li).
such as TiO2 /WO3 [16,17], Ag3 VO4 /WO3 [18], CdS/WO3 [19,20], Ag3 PO4 /WO3 [21], ZnO/WO3 [22], Bi2 O3 /WO3 [23], g-C3 N4 /WO3 [24], and BiVO4 /WO3 [25] have been successfully prepared and exhibit enhanced photocatalytic activity. However, the current WO3 composite photocatalytic materials still have some shortcomings such as narrow light absorption or poor stability or low activity. Recently, a photocatalyst of BiOI with the absorption light up to 660 nm and good photocatalytic activity and stability receives widespread attention [26–30]. We have synthesized I-deficient BiO1.2 I0.6 in previous study, and found that the iodine vacancy obviously promoted the photocatalytic activity of BiOI [31]. The CB potential of BiO1.2 I0.6 is about 0.07 V Vs NHE, which is lower than that (∼0.74 V Vs NHE) of WO3 . The VB potential of WO3 is higher than that (∼2.61 V Vs NHE) of BiO1.2 I0.6 . Thus, WO3 compounded by BiO1.2 I0.6 should distinctly improve its separation efficiency of photo-generated carriers. Moreover, the BiO1.2 I0.6 /WO3 composite will exhibit wide light absorption and good stability. In this work, we prepared the composites of BiO1.2 I0.6 /WO3 for the first time. Their performances were investigated through photocatalytic degradation of typical VOCs, toluene. The mechanisms of the enhanced photocatalytic activity of BiO1.2 I0.6 /WO3 were discussed. The main active species for degradation of toluene were revealed.
https://doi.org/10.1016/j.jtice.2019.09.015 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: S. Wang, Y. Guan and R. Zeng et al., Plate-like WO3 inserting into I-deficient Bio1.2I0.6 microsphere for highly efficient photocatalytic degradation of VOCs, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.09. 015
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2. Experimental section 2.1. Photocatalyst preparation 2.1.1. Fabrication of I-deficient Bio1.2 I0.6 I-deficient BiO1.2 I0.6 was prepared according to our previous research [31]. The 40 mL glycol containing 1.9 g of KI was dropwisely added into the same volume of glycol containing 5.5 g of Bi(NO3 )3 ·5H2 O. The mixture was transferred into an autoclave and reacted at 160 °C for 12 h after it was magnetically stirred for 1 h. The cooled product was washed by deionized water and then dried at 80 °C to obtain BiOI. Then, 1 g of as-synthesized BiOI was placed in a muffle furnace and calcined at 400 °C for 5 h with a heat rate of 5 °C min−1 . After the sample was naturally cooled to room temperature, the BiO1.2 I0.6 was obtained. 2.1.2. Preparation of WO3 1 g of H2 WO4 was placed in the muffle furnace and calcined at 500 °C for 3 h with a heating rate of 10 °C min−1 . After the sample was naturally cooled to room temperature, the WO3 was obtained. Fig. 1. XRD patterns of BiO1.2 I0.6 , WO3 and BiO1.2 I0.6 /WO3 composites.
2.1.3. Synthesis of BiO1.2 I0.6 /WO3 composites The composites of BiO1.2 I0.6 /WO3 were prepared as follows. 0.46 g of BiOI and 0.60 g WO3 were fully ground for 0.5 h with addition of 5 mL of ethanol. The mixture was dried at 100 °C for 1 h and then uniformly ground again. Subsequently, it was calcined at 400 °C for 5 h with a heating rate of 5 °C min−1 . The 40 wt% BiO1.2 I0.6 /WO3 was obtained after naturally cooled to room temperature. The other BiO1.2 I0.6 /WO3 composites with different content of BiO1.2 I0.6 were prepared via the similar method. 2.2. Characterization The crystal structure of the samples was identified via powder X-ray diffraction (XRD) (Rigaku Ultima III, Japan) with Cu Kα radi˚ The morphology of the samples was observed ation (λ = 1.5418 A). by the scanning electron microscope (Nova Nano 230) and transmission electron microscope (JEM-200CX). X-ray photoelectron spectroscopy (XPS) test was conducted on a PHI50 0 0 Versa Probe (ULVAC-PHI, Japan) with monochromatized Al Kα X-ray radiation (1486.6 eV). Light absorption characteristics of the samples was characterized by a UV–vis diffuse reflectance spectra (UV-2550, Shimadzu). The specific surface area based on the BET equation was analyzed by N2 adsorption–desorption on an instrument (SA-3100, Beekman Coulter). The photoluminescence spectra (PL) was collected by Fluoromax-4 (HORIBA, USA) with an excitation wavelength at 400 nm. Electron spin resonance (ESR) test was conducted on an apparatus (JES-FA200, JEOL). Photoelectrochemical measurement was conducted on an electrochemical analyzer (CHI-660D, Shanghai Chenhua, China) in a standard three-electrode system. A platinum sheet and a Ag/AgCl electrode were used as counter electrode and reference electrode, respectively. Working electrode was prepared as follows: 30 mg of the sample and 10 mg of iodine were added to 25 mL of acetone with ultrasonic shock for half an hour. Then, the sample was deposited on a fluorine-doped tin oxide conducting substrate with a fixed area of 1 cm2 by a voltage of 12 V for 3 min. Na2 SO4 (0.1 mol L−1 ) aqueous solution was used as the electrolyte. A 300 W Xe lamp was utilized as the light source.
2.4. Investigation of photocatalytic performances 2.4.1. Evaluation of photocatalytic activity The photocatalytic degradation of toluene was performed in a gas sealed container with a volume of 250 mL. The as-synthesized photocatalyst (0.1 g) was uniformly dispersed on the bottom of the reaction container. Toluene gas with 50 μL of H2 O was injected into the reactor such that the initial concentration of toluene was 2800 mg m−3 . The system was kept in the dark for 1 h to ensure that the adsorption–desorption equilibrium was established. A 300 W Xenon lamp was utilized as light source. During the irradiation, 100 μL of gas withdrawn from the system every hour was analyzed through a gas chromatography (GC9790, Fuli) of hydrogen flame. The temperatures of injector and column were all 120 °C. Detector temperature was 150 °C. 2.4.2. Stability test of photocatalyst The stability measurement of a photocatalyst was carried out as follows. The reactor was emptied after the end of a round of experiment. The original gas in the reactor was replaced using fresh air. Then, the reactor was sealed again and the same concentration of toluene gas was injected to conduct the next round of degradation reaction. 3. Results and discussion 3.1. Characterization of photocatalysts As shown in Fig. 1, the tetragonal phase of BiOI (PDF#10-0445) calcined at 400 °C for 5 h did not convert to Bi2 O3 [32,33]. According to the weight loss before and after the calcination of BiOI, it is believed that BiOI was converted to I-deficient BiO1.2 I0.6 . The WO3 was consistent with its orthorhombic structure (PDF#20-1324). According to Scherrer formula (1) [34,35], average particle diameters of BiO1.2 I0.6 and WO3 were calculated to be about 20.13 and 24.83 nm.
D= 2.3. Calculation method The function of b3lyp was used to calculate the HOMO/LUMO energy level. The basis set was 6–31 g and the convergence criteria was tight. The d-polarized orbit and p-polarized orbit were added to non-hydrogen atoms and hydrogen atoms, respectively.
K×λ β × Cos θ
(1)
K and λ are equal to 0.89 and 0.15418 nm, respectively. β is the peak width at half height of the diffraction peak, and θ is the diffraction angle. Both characteristic peaks of BiO1.2 I0.6 and WO3 were observed in the composites of BiO1.2 I0.6 /WO3 . With increasing content of BiO1.2 I0.6 , the peak intensity of BiO1.2 I0.6 gradually
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Fig. 2. SEM images of (a and b) BiO1.2 I0.6 , (c and d) WO3 and (e and f) 40 wt% BiO1.2 I0.6 /WO3 .
increased while that of WO3 weakened by degrees. The results of XRD display that the composites of BiO1.2 I0.6 /WO3 have been successfully prepared. Morphology and particle size of BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 are illustrated in Fig. 2. BiO1.2 I0.6 was a flower-like microsphere, which was consisted of many nano-plates. The diameter of microsphere was about 2 μm. WO3 was composed of many plate-like bulks with length from 10 nm to 350 nm. The thickness of plate was approximately 50 nm. For 40 wt% BiO1.2 I0.6 /WO3 , the microspheres of BiO1.2 I0.6 were tightly wrapped by many plate-like
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bulks of WO3 . Many plate-like WO3 inserted into nano-plate gap of BiO1.2 I0.6 microspheres. The results distinctly show that BiO1.2 I0.6 and WO3 are tightly combined. The same morphology as above, BiO1.2 I0.6 and WO3 were flower-like microsphere and plate-like bulks (Fig. 3a–b). It is clearly seen that BiO1.2 I0.6 was covered by many particles of WO3 in 40 wt% BiO1.2 I0.6 /WO3 (Fig. 3c). The high-resolution TEM lattice image depicted interfacial interaction between BiO1.2 I0.6 and WO3 (Fig. 3d). Lattice spacing of 0.37 nm belonged to BiO1.2 I0.6 (101) and that of 0.26 nm originated from WO3 (220). The chemical composition and elemental valence in the surface of as-prepared photocatalysts are depicted in Fig. 4. The binding energies of Bi, O and I in the BiO1.2 I0.6 corresponded to Bi3+ , O2− and I− , respectively. For 40 wt% BiO1.2 I0.6 /WO3 , the binding energies of Bi 4f7/2 , O 1 s and I 5d3/2 were higher than those in BiO1.2 I0.6 . It is mainly due to the fact that the electronegativity of W is greater than that of Bi resulting in the decline of outer electronic cloud density around Bi 4f7/2 , O 1s and I 5d3/2 in 40 wt% BiO1.2 I0.6 /WO3 . It causes the shielding effect between electrons and atomic nuclear to be decreased. The binding energy of I 5d5/2 in 40 wt% BiO1.2 I0.6 /WO3 was lower than that in BiO1.2 I0.6 . It might be that the decrease of outer electronic cloud density of I 5d3/2 leads to the density increase of spin and orbital angular momentum in the same direction. In addition, the binding energies of W 4f and O 1 s in WO3 corresponding to W6+ and O2− were higher than those in 40 wt% BiO1.2 I0.6 /WO3 [36]. Compared to WO3 , the density of outer electronic cloud around W 4f and O 1 s in 40 wt% BiO1.2 I0.6 /WO3 is higher because the electronegativity of Bi is smaller than that of W. It causes the shielding effect between electrons and atomic nuclear to be enhanced. The results of XPS confirm that the chemical composition and elemental valence are in accord with as-fabricated photocatalysts. BiOI is not converted to Bi2 O3 after the calcination at 400 °C for 5 h. There is a strong interaction between BiO1.2 I0.6 and WO3 in 40 wt% BiO1.2 I0.6 /WO3 . The light-absorption property of BiO1.2 I0.6 , WO3 and BiO1.2 I0.6 /WO3 composites is shown in Fig. 5. The as-synthesized samples had good property of visible-light absorption. The maximum adsorption wavelength of BiO1.2 I0.6 reached about 500 nm, and WO3 could also absorb light at the maximum wavelength of approximately 480 nm. The optical absorption of BiO1.2 I0.6 /WO3 composites was between 480 nm and 500 nm.
Fig. 3. TEM images of (a) BiO1.2 I0.6 , (b) WO3 and (c and d) 40 wt% BiO1.2 I0.6 /WO3 .
Please cite this article as: S. Wang, Y. Guan and R. Zeng et al., Plate-like WO3 inserting into I-deficient Bio1.2I0.6 microsphere for highly efficient photocatalytic degradation of VOCs, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.09. 015
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Fig. 4. XPS core-level spectra of (a) Bi 4f, (b) O 1s, (c) I 5d and (d) W 4f.
Fig. 5. UV–vis absorption spectra of BiO1.2 I0.6 , WO3 and BiO1.2 I0.6 /WO3 composites.
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Fig. 6. (a) Concentration of toluene over BiO1.2 I0.6 , WO3 and BiO1.2 I0.6 /WO3 composites with increasing time of irradiation. (b) Mineralization ratio of toluene over (1) BiO1.2 I0.6 , (2) WO3 and (3–7) 10–50 wt% BiO1.2 I0.6 /WO3 after irradiation for 8 h.
Fig. 7. (a) Schematic illustration of photo-induced carrier separation in 40 wt% BiO1.2 I0.6 /WO3 . (b) PL spectra of BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 .
3.2. Photocatalytic performances of photocatalysts The photocatalytic activity of as-synthesized samples were evaluated by degrading toluene. As shown in Fig. 6a, the concentration of toluene over BiO1.2 I0.6 , WO3 and BiO1.2 I0.6 /WO3 composites gradually degreased with increasing reaction time. The degradation ratio of toluene over BiO1.2 I0.6 and WO3 were about 82.3% and 73.8% under irradiation of 8 h, respectively. The photocatalytic activity of BiO1.2 I0.6 /WO3 composites was significantly higher than that of BiO1.2 I0.6 and WO3 . At the same irradiation time, the degradation ratio of toluene over 10–50 wt% BiO1.2 I0.6 /WO3 were 96.2%, 95.5%, 96.4%, 99% and 96.9%, respectively. The 40 wt% BiO1.2 I0.6 /WO3 exhibited the best photocatalytic activity. The degradation rate of toluene over 40 wt% BiO1.2 I0.6 /WO3 were 1.2 and 1.4 times those of BiO1.2 I0.6 and WO3 , respectively. Moreover, the degradation rate of toluene over 40 wt% BiO1.2 I0.6 /WO3 was higher than previous research results (Table S1). It is necessary for toxic VOCs to be effectively mineralized before discharge. The CO2 concentration was detected in every system before and after the reaction (Fig. 6b). The mineralization rate of toluene over BiO1.2 I0.6 , WO3 and 10–50 wt% BiO1.2 I0.6 /WO3 were 3.7%, 29.4%, 36.7%, 7.4%, 22%, 58.8% and 22%. Although the degradation rate of toluene over BiO1.2 I0.6 was higher than that over WO3 , the mineralization ratio was on the contrary. It is mainly due to the fact that WO3 has the deeper valence-band
energy level resulting in stronger oxidation ability. The 40 wt% BiO1.2 I0.6 /WO3 achieved the highest mineralization rate of toluene (58.8%). Degradation rate of toluene over 40 wt% BiO1.2 I0.6 /WO3 was 1.7 times mineralization rate of toluene. It might be that 40 wt% BiO1.2 I0.6 /WO3 with low specific surface area is difficult to adsorb small molecular intermediates (Fig. S3). The results of toluene degradation over 40 wt% BiO1.2 I0.6 /WO3 by cycling three times show that the degradation and mineral rates of toluene did not distinctly decrease (Fig. S1). The diffraction peaks of 40 wt% BiO1.2 I0.6 /WO3 after 3 runs of use did not change (Fig. S2). These indicate that the 40 wt% BiO1.2 I0.6 /WO3 has good stability. 3.3. Mechanisms of the enhanced photocatalytic activity For heterogeneous photocatalytic reaction, a photocatalyst with distinctly large specific surface area generally exhibits high photocatalytic activity. The specific surface area of BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 calculated by adsorption and desorption isotherm (Fig. S3) was 16.2, 11.4 and 14.7 cm2 g−1 . The specific surface area of WO3 was smaller than those of 40 wt% BiO1.2 I0.6 /WO3 and BiO1.2 I0.6 . Thence, the photocatalytic activity of WO3 is lower than 40 wt% BiO1.2 I0.6 /WO3 and BiO1.2 I0.6 . Although the specific surface area of 40 wt% BiO1.2 I0.6 /WO3 was
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Fig. 8. (a) Current density of BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 under dark and irradiation. (b) Nyquist impedance of BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 .
Fig. 9. (a) The HOMO and LUMO energy level of toluene. ESR spectra of 40 wt% BiO1.2 I0.6 /WO3 using DMPO as (b) hydroxyl radical scavenger in aqueous solution and (c) superoxide radical scavenger in methanol solution respectively.
Please cite this article as: S. Wang, Y. Guan and R. Zeng et al., Plate-like WO3 inserting into I-deficient Bio1.2I0.6 microsphere for highly efficient photocatalytic degradation of VOCs, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.09. 015
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smaller than that of BiO1.2 I0.6 , the photocatalytic activity of 40 wt% BiO1.2 I0.6 /WO3 was obviously higher than BiO1.2 I0.6 . This is basically caused by separation-efficiency difference of photo-generated carriers. In generally, a photocatalyst with high separation efficiency of photo-generated electron–hole pairs demonstrates excellent photocatalytic activity. Because of the potential difference of BiO1.2 I0.6 (ECB = 0.07 and EVB = 2.61 V) and WO3 (ECB = 0.74 and EVB = 3.3 V) in 40 wt% BiO1.2 I0.6 /WO3 , photo-generated electrons from the CB of BiO1.2 I0.6 will jump to the CB of WO3 , and photo-induced holes from the VB of WO3 will migrate to the VB of BiO1.2 I0.6 (Fig. 7a). This distinctly promotes separation of photo-generated electronhole pairs in 40 wt% BiO1.2 I0.6 /WO3 , thereby greatly enhancing its photocatalytic activity. Separation efficiency of carriers in BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 was characterized by photoluminescence spectroscopy. A photocatalyst with low intensity of fluorescent peak exhibits high separation efficiency under light excitation. PL spectra of BiO1.2 I0.6 , WO3 and 40 wt% BiO1.2 I0.6 /WO3 is shown in Fig. 7b. WO3 emitted a strong fluorescent peak at about 487 nm. The fluorescent intensity of BiO1.2 I0.6 at approximately 485 nm was slightly lower than that of WO3 . Compared to WO3 and BiO1.2 I0.6 , the 40 wt% BiO1.2 I0.6 /WO3 emitted a fluorescent peak with significantly decreased intensity. The PL results clearly display that the separation efficiency of electron–hole pairs is 40 wt% BiO1.2 I0.6 /WO3 > BiO1.2 I0.6 > WO3 . Thus, the 40 wt% BiO1.2 I0.6 /WO3 demonstrates distinctly enhanced photocatalytic activity. Photocurrent and electrochemical impedance were utilized to further analyze separation efficiency of carriers in BiO1.2 I0.6, WO3 and 40 wt% BiO1.2 I0.6 /WO3 . A photocatalyst with high photocurrent and low impedance corresponds to high separation efficiency of carriers. The photocurrent sequence was 40 wt% BiO1.2 I0.6 /WO3 > BiO1.2 I0.6 > WO3 (Fig. 8a), while the impedance in the order was 40 wt% BiO1.2 I0.6 /WO3 < BiO1.2 I0.6 < WO3 (Fig. 8b and Fig. S4). The results confirm that the separation efficiency of carriers is 40 wt% BiO1.2 I0.6 /WO3 > BiO1.2 I0.6 > WO3 , which is in line with PL results.
3.4. Main active species for toluene degradation It is well-known that hydroxyl radical (·OH) and superoxide radical (·O2 − ) are primary active species besides photo-generated electron and hole in the process of photocatalytic degradation [37–39]. The results of Gaussian calculation demonstrated that the HOMO and LUMO energy level of toluene were 1.9 V (Vs NHE) and −4.6 V (Vs NHE), respectively (Fig. 9a). The HOMO energy level of toluene was less than the VB potentials of BiO1.2 I0.6 (2.61 V Vs NHE) and WO3 (3.3 V Vs NHE), as well as oxidation potential of ·OH (2.8 Vs NHE), but greater than oxidation potential of ·O2 − (1.75 Vs NHE). Its energy level of the LUMO was smaller than the CB potential of BiO1.2 I0.6 (0.07 V Vs NHE) and WO3 (0.74 V Vs NHE). This indicates that toluene should be decomposed by ·OH and h+ . Radical generated over 40 wt% BiO1.2 I0.6 was detected using DMPO as a radical scavenger by ESR technique under dark and light irradiation (Fig. 9b–c). No peaks were observed under the dark. Four peaks with the signal intensity of 1:2:2:1 appeared in H2 O solution under irradiation for 5 min, which is a typical ESR signal of ·OH. No signal of ·O2 − was found under either irradiation or dark for 5 min. ESR results demonstrate that main radical for the photocatalytic degradation of toluene over 40 wt% BiO1.2 I0.6 is ·OH not ·O2 − . This further confirms that the toluene is mainly degraded by ·OH and h+ . According to the detected active species, the probable mechanism of the photocatalytic degradation of toluene can be summarized as follows. [40,41]
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40 wt% BiO1.2 I0.6 /WO3 + hV → 40 wt% BiO1.2 I0.6 /WO3 ∗ + e− + h+ h+ + H2 O → ·OH[40, 41] h+ + ·OH + toluene → degradation products 4. Conclusions A novel photocatalyst of 40 wt% BiO1.2 I0.6 /WO3 was successfully prepared. The 40 wt% BiO1.2 I0.6 /WO3 exhibited high separation efficiency of photo-induced carriers. Moreover, it also absorbed broad spectrum photons with good stability for reuse. Although this photocatalyst still needs further improvement in the ability to deeply decompose toluene, this work opens up a promising strategy for developing high performance photocatalysts for efficient purification of VOCs. Acknowledgments This work was supported primarily by the National Natural Science Foundation of China (21876015), the Natural Science Foundation of Jiangsu Province (BK20161277 and BK20190934), the Natural Science Foundation of Jiangsu Education Department (16KJB610 0 02), the Postdoctoral Science Foundation of China (2017M611784). Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2019.09.015. References [1] Liu H, Ma Y, Chen J, Wen M, Li G, An T. Highly efficient visible-light-driven photocatalytic degradation of VOCs by CO2 -assisted synthesized mesoporous carbon confined mixed-phase TiO2 nanocomposites derived from MOFs. Appl Catal B-Environ 2019;250:337–46. [2] Sansotera M, Kheyli SGM, Baggioli A, Bianchi CL, Pedeferri MP, Diamanti MV, Navarrini W. Absorption and photocatalytic degradation of VOCs by perfluorinated ionomeric coating with TiO2 nanopowders for air purification. Chem Eng J 2019;361:885–96. [3] Li JA, Li XY, Zeng LB, Fan SY, Zhang MM, Sun WB, Chen X, Tade MO, Liu SM. Functionalized nitrogen-doped carbon dot-modified yolk-shell ZnFe2 O4 nanospheres with highly efficient light harvesting and superior catalytic activity. Nanoscale 2019;11:3877–87. [4] Nie L, Duan B, Lu A, Zhang LN. Pd/TiO2 @ carbon microspheres derived from chitin for highly efficient photocatalytic degradation of volatile organic compounds. ACS Sustain Chem Eng 2019;7:1658–66. [5] Zhang X, Wang L, Zhou X, Ni Z, Xia S. Investigation into the enhancement of property and the difference of mechanism on visible light degradation of gaseous toluene catalyzed by ZnAl layered double hydroxides before and after Au support. ACS Sustain Chem Eng 2018;6:13395–407. [6] Wang SL, Zhu Y, Luo X, Huang Y, Chai J, Wong TI, Xu GQ. 2D WC/WO3 heterogeneous hybrid for photocatalytic decomposition of organic compounds with vis-nir light. Adv Funct Mater 2018;28:1705357–64. [7] Zhou H, Wen Z, Liu J, Ke J, Duan X, Wang S. Z-scheme plasmonic Ag decorated WO3 /Bi2 WO6 hybrids for enhanced photocatalytic abatement of chlorinated-VOCs under solar light irradiation. Appl Catal B-Environ 2019;242:76–84. [8] Katsumata K, Motoyoshi R, Matsushita N, Okada K. Preparation of graphitic carbon nitride (g-C3 N4 )/WO3 composites and enhanced visible-light-driven photodegradation of acetaldehyde gas. J Hazard Mater 2013;260:475–82. [9] Zhang L, Qin M, Yu W, Zhang Q, Xie H, Sun Z, Shao Q, Guo X, Hao L, Zheng Y, Guo Z. Heterostructured TiO2 /WO3 nanocomposites for photocatalytic degradation of toluene under visible light. J Electrochem Soc 2017;164:1086–90. [10] Liu Y, Xie C, Li H, Chen H, Liao Y, Zeng D. Low bias photoelectrocatalytic (PEC) performance for organic vapour degradation using TiO2 /WO3 nanocomposite. Appl Catal B-Environ 2011;102:157–62. [11] Sun SM, Watanabe M, Wu J, An Q, Ishihara T. Ultrathin WO3 center dot 0·33H2 O nanotubes for CO2 photoreduction to acetate with high selectivity. J Am Chem Soc 2018;140:6474–82. [12] Wang TY, Quan W, Jiang DL, Chen LL, Li D, Meng S, Chen M. Synthesis of redox-mediator-free direct Z-scheme Agi/WO3 nanocomposite photocatalysts for the degradation of tetracycline with enhanced photocatalytic activity. Chem Eng J 2016;300:280–90.
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Please cite this article as: S. Wang, Y. Guan and R. Zeng et al., Plate-like WO3 inserting into I-deficient Bio1.2I0.6 microsphere for highly efficient photocatalytic degradation of VOCs, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.09. 015