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Enhanced sunlight-driven photocatalytic performance of Bi-doped CdMoO4 benefited from efficient separation of photogenerated charge pairs
Accepted Manuscript Enhanced sunlight-driven photocatalytic performance of Bi-doped CdMoO4 benefited from efficient separation of photogenerated charge pairs
Jiao Huang, Huanhuan Liu, Junbo Zhong, Qi Yang, Jiufu Chen, Jianzhang Li, Dongmei Ma, Ran duan PII:
S1293-2558(17)31263-3
DOI:
10.1016/j.solidstatesciences.2018.04.015
Reference:
SSSCIE 5685
To appear in:
Solid State Sciences
Received Date:
27 December 2017
Revised Date:
18 March 2018
Accepted Date:
17 April 2018
Please cite this article as: Jiao Huang, Huanhuan Liu, Junbo Zhong, Qi Yang, Jiufu Chen, Jianzhang Li, Dongmei Ma, Ran duan, Enhanced sunlight-driven photocatalytic performance of Bidoped CdMoO4 benefited from efficient separation of photogenerated charge pairs, Solid State
Sciences (2018), doi: 10.1016/j.solidstatesciences.2018.04.015
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Graphical abstract
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ACCEPTED MANUSCRIPT Enhanced sunlight-driven photocatalytic performance of Bi-doped CdMoO4 benefited from efficient separation of photogenerated charge pairs Jiao Huang1, Huanhuan Liu1, Junbo Zhong1*, Qi Yang1, Jiufu Chen1, Jianzhang Li1, Dongmei Ma1, Ran duan2 1 Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Environmental Engineering, Sichuan University of Science and Engineering, Zigong, 643000, P R China 2 Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P R China *Corresponding author (E-mail: [email protected]) Abstract In this paper, to further boost the photocatalytic performance of CdMoO4, Bi3+ was successfully doped into CdMoO4 by a facile microwave hydrothermal method. The Bi-doped CdMoO4 photocatalysts prepared were characterized by Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), electron spin-resonance (ESR) and surface photovoltage spectroscopy (SPS). The results exhibit that doping Bi3+ into CdMoO4 remarkably boosts the separation rate of photoinduced charge pairs and the specific surface area, decrease the crystal size, narrows the band gap of the CdMoO4 and induces the binding energy shift of Cd, all these advantageous factors result in the promoted photocatalytic performance of CdMoO4. Using rhodamine B (RhB) as model toxic pollutant, the photocatalytic activities of the 1
ACCEPTED MANUSCRIPT photocatalysts were evaluated under a 500 W Xe lamp irradiation. When the molar ratio of Bi/Cd is 0.2%, Bi-CdMoO4 prepared displays the best photocatalytic performance, the photocatalytic performance of the 0.2% sample is more than twice of that of the reference CdMoO4. Key words: CdMoO4, doping, microwave hydrothermal method; photocatalytic performance; separation of carriers
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ACCEPTED MANUSCRIPT 1. Introduction Photocatalytic oxidation technology has become an ideal way to solve environment pollutant because of its mild reaction conditions, low energy consumption, low cost and basic non-pollutant advantages [1,2]. To effectively degrade various toxic contaminants, tremendous catalyst systems, such as CeO2, Co3O4, ZnO, CdS, Bi2O3, (BiO)2CO3 and so on were developed [3-16]. Among these catalysts developed, metal molybdates have attracted widespread concern owing to its photocatalytic ability. CdMoO4 with scheelite-type structure is a typical metal molybdate material, the band gap of CdMoO4 is 3.25 eV [17-24], which indicates that CdMoO4 can be excited by UV light and used as a photocatalyst. In fact, it has been reported that CdMoO4 could effectively degrade RhB under UV light illumination [25]. However, it is still a challenge to prepare CdMoO4 with excellent photocatalytic performance, since the practical application of CdMoO4 has been significantly hindered by its high recombination of photoinduced charge pairs [25,26]. Therefore, it is absolutely necessary to boost the photocatalytic performance by modulating the separation of photoinduced charge pairs. As well known, among the factors which influence the photocatalytic performance of a photocatalyst, separation behaviors of photoinduced charge pairs play a predominant role [27]. To achieve high separation of photoinduced charge pairs, tremendous approaches have been executed, such as construction of heterojuctions[28,29], choice of preparation methods[30-33] and doping ions into the lattice of CdMoO4[34], these methods can greatly enhance the photocatalytic performance of CdMoO4 by accelerating the separation of photoinduced charge pairs. As a layered aurivillus-related oxide, Bi2MoO6 consists Bi2O22+ layers sandwiched between perovskite polyhedra and MoO42−slabs [35,36]. Bi2MoO6 with a narrow band gap (2.5-2.8 eV)[2] 3
ACCEPTED MANUSCRIPT displays excellent photocatalytic performance [37,38]. More importantly, both CdMoO4 and Bi2MoO6 consist MoO42−, if doping Bi3+ into lattice of CdMoO4, it is feasible to form Bi2MoO6, which will greatly influence the electronic structure of CdMoO4, consequently significantly affecting the separation of photoinduced charge pairs. Herein, a series of Bi-doped CdMoO4 photocatalysts were prepared by a facile microwave hydrothermal method. The photocatalysts prepared were studied HRTEM, XPS and SPS. The results of HRTEM confirm the presence of Bi2MoO6, the XPS results reveal that Bi3+ was successfully doped into CdMoO4. The SPS results exhibit that Bi-doped CdMoO4 displays higher separation rate of photoinduced charge pairs than the bare CdMoO4, resulting in promoted photocatalytic performance.
2. Experimental 2.1 Preparation of the samples Bi-doped CdMoO4 photocatalysts were prepared by a microwave hydrothermal process. In a typical preparation process, 6.1694 g (0.02 mol) Cd(NO3)2·4H2O and desired Bi(NO3)3·5H2O were added into 50 mL of deionized water under intensely stirring, the molar ratios of Bi/Cd were 0%, 0.1%, 0.2% and 0.3%, respectively, then desired Na2MoO4·2H2O was added into the solution mentioned above, the molar amount of MoO42-=Cd2+ +0.5 Bi3+. The mixture was sonicated for 10 min, and then was stirred for 24 h, forming a suspension system. The suspension system was transferred to a microwave hydrothermal reactor and maintained at 403 K for 2 h. After cooling down to room temperature naturally, the solid was filtered and washed with deionized water and absolute ethanol many times, and then dispersed in absolute ethanol and dried at 333K overnight.
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ACCEPTED MANUSCRIPT The samples with different molar ratios of Bi/Cd (0%, 0.1%, 0.2% and 0.3%) were marked as 0%, 0.1%, 0.2% and 0.3%, respectively. 2.2 Characterization Specific surface area parameters were measured on the SSA-4200 surface analyzer using Brunauer-Emmett-Teller (BET) method. The XRD patterns were performed on a DX-2600 using X-ray diffractometer with a radiation source of Cu Kα (λ=0.15406 nm), the X-ray tube was operated at 35 kV and 25 mA. The UV-Vis diffuse reflectance spectra were measured on a spectrometer (UH-4150) using barium sulphate as the reference. SEM images were observed on a VEGA 3 SBU scanning electron microscope, using an accelerating voltage of 15 kV. EDS profiles of the sample were conducted on a X Flash Detector 410-M energy dispersive spectrometer. HRTEM was recorded on a JEOL-2100F microscope, utilizing an accelerating voltage of 200 KV. X-ray photoelectron spectroscopy (XPS) measurements were performed on a XSAM 800 using Mg Kα at 12 kV and 12 mA. SPS spectra were recorded on a home-built apparatus, and the detailed operation procedures were given in the Ref.[39]. ESR experiments were performed on Bruker E 500, the detailed experimental procedures were described in the Ref.[40] using 5,5dimethyl-1-pyrroline-N-oxide (DMPO) method. 2.3 Photocatalytic tests In this paper, RhB (10 mgL-1, pH=6.00) was used as a model dye. The detailed photocatalytic evaluation procedures were given in the Ref.[41], the light source was a 500W Xe lamp (simulated sunlight), and the dosage of photocatalysts was 1 gL-1.
3. Results and discussion 5
ACCEPTED MANUSCRIPT 3.1 Characterization of the samples The BET results of the samples were shown in Table 1. It can be seen from Table 1 that doping Bi3+ into the lattice of CdMoO4 can significantly increase the specific surface area of CdMoO4, which implies that Bi3+ restrains the growth of CdMoO4, which can be further verified by the results of XRD. Similar phenomena were observed when doping Ga3+ and Bi3+ into ZnO [42,43]. Combined with the results of XRD, it is apparent that the increased specific surface area of CdMoO4 arises from the small crystal size. It is widely acknowledged that relative high specific surface area can offer more active sites, which can effectively adsorb more pollutants, promoting the photcatalytic performance of CdMoO4. The XRD profiles of the samples were exhibited in Fig.1. It is distinct that all the diffraction peaks accord well with the standard tetragonal phase of CdMoO4 (JCPDS No.07-0209). No other impurity peaks were observed, indicating high purity of the photocatalysts. No diffraction peaks of Bi2MoO6 were detected, which may attribute to the relatively low content of Bi2MoO6 or Bi2MoO6 is highly dispersed in the lattice of CdMoO4. Furthermore, the full width at half maximum (FWHM) gradually increases as the loading of Bi3+ elevating. According to the Scherrer formula, the wider FWHM means smaller the crystal size of CdMoO4. The crystal sizes of the samples were displayed in Table 1. As exhibited in Table 1, the crystal sizes of the samples gradually decrease as the amount of Bi3+ elevating, which indicates that Bi3+ effectively restrains the growth of CdMoO4. The radius of Bi3+ (0.103 nm) is bigger than that of Cd2+ (0.095 nm), it is impossible for Bi3+ ions enter into the crystal cell of CdMoO4 through the interstitial mode. However, it is possible for Bi3+ to substitute Cd2+ and thus occupies the positions of Cd2+ ions in CdMoO4, greatly influencing the physicochemical properties of CdMoO4. The decreased crystal 6
ACCEPTED MANUSCRIPT size suggests that Bi was doped into CdMoO4.The result further reveals that the decreased crystal size results in the BET surface area increase, which fits well with the results of BET surface area. The UV-Vis diffuse reflectance spectra of the photocatalysts were displayed in Fig.2. As shown in Fig.2, the band gap of the samples can be further calculated using formula Eg = 1240/λ, where λ is the absorption edge [44]. The absorption edge and the corresponding band gap were shown in Table1. It is evident that Bi-doped CdMoO4 photocatalyst appear red-shift, which suggests that the band gap of Bi-doped CdMoO4 becomes narrow. It is clear that change in band gap is induced by the presence of Bi2MoO6 in the lattice of CdMoO4, since Bi2MoO6 is a photocatalyst with a narrow band gap (2.5-2.8 eV)[2]. The red-shift of the absorption edge is in good consistent with the results of SPS. The narrowed band gap of CdMoO4 can greatly increase the light response ability, boosting the sunlight-driven photoctalytic performance. SEM images of the 0% and 0.2% sample were demonstrated in Fig. 3. As shown in Fig. 3A and B, no obvious difference in shape of the samples was observed. Both samples display lumplike shape, indicating that doping Bi3+ into CdMoO4 cannot effectively alter the morphology of the sample. Due to the relative small amount of Bi, no Bi was detected for the 0.3% sample, to further confirm the existence of Bi in CdMoO4, 1.0% sample was prepared and characterized by EDS. As exhibited in Fig.3C, Cd, Mo, Bi, O elements were detected for the 1.0% sample, which further confirms the existence of Bi3+ in the photocatalysts. The presence of Bi2MoO6 in the phtocatalyst can be further supported by HRTEM. As demonstrated in Fig.3D, the spacing of 0.1529 nm was clearly observed, which can be ascribed to the (153) crystallographic planes of Bi2MoO6 (JCPDS card No. 21-0102). Furthermore, the lattice spacing of 0.2317 nm agrees well with the (105) plane of CdMoO4 (JCPDS card No. 07-0209). The results of UV-Vis DRS, EDS and HRTEM firmly 7
ACCEPTED MANUSCRIPT confirm the successful preparation of Bi3+-doped CdMoO4 and formation of Bi2MoO6. The Bi3+ in the lattice of CdMoO4 is anticipated to significantly affect the separation of photogenerated charge pairs, proven by the results of SPS. X-ray photoelectron spectroscopy (XPS) was carried out to determine the surface chemical composition of the samples. Fig.4A shows the surface survey of the 0.1% sample. As expected, XPS spectra reveal the characteristic peaks for Cd, O, Mo and Bi, which accords well with the results of EDS and HRTEM. Fig.4B shows the high-resolution XPS spectra of Cd 3d, for the 0% sample, two strong symmetrical characteristic spin-orbit splitting of Cd 3d peaks at 404.7 eV and 411.5 eV, which can be ascribed to Cd 3d3/2 and Cd 3d5/2, respectively, suggesting that Cd is Cd2+ cations [45-48]. However, for the 0.1%, 0.2% and 0.3 % samples, the binding energy of Cd 3d3/2 gradually shifts to lower binding energy, which implies that the chemical environment of Cd has been altered. If Bi3+ ions enter into the crystal cell of CdMoO4 through the interstitial mode, which will execute minor even no influence on the chemical environment of Cd. The binding energy shift of Cd 3d3/2 substantially confirms that Bi3+ substitutes part of the Cd2+ ions and thus occupies the positions of the Cd2+ ions in CdMoO4, affecting the specific surface area, crystal size, band gap, binding energy of Cd and separation rate of photoinduced charge pairs. Fig.4C displays the XPS peaks of Mo 3d3/2 and Mo 3d5/2 at 231.9 eV and 235.0 eV, respectively, corresponding to Mo6+ of CdMoO4 [49]. As shown in Fig.5, the O 1s spectra can be fitted with two peaks at 531.8 eV and 529.7eV, which can be attributed to O-H and Cd-O bonds, respectively [50]. Table 2 lists the curve-fitting results of O1s XPS spectra for the photocatalysts (Fig.5), where Ri (%) represents the ratio of the different kinds of oxygen contributions. As shown in Table 2, due to the relative low amount of Bi3+, the hydroxyl content on all the samples has no obvious difference. Therefore, 8
ACCEPTED MANUSCRIPT the difference in photocatalytic performance is not generated by the hydroxyl content. Fig.6 exhibits the SPS responses of the photocatalysts as prepared, it can be seen from Fig.6 that all the photocatalysts display obvious response in the range of 300-375 nm, which is due to the electron transition from valence band to conduction band and UV-Vis DRS. It is interesting to notice that the response peak shifts to higher wavelength, which is due to the presence of Bi2MoO6 in the photocatalyst, proven by the results of HRTEM and XPS. As a visible light response photocatalyts, compared to CdMoO4, the SPS response peak of Bi2MoO6 located at higher wavelength. Moreover, the SPS responses of the photocatalysts gradually boosts as the loading of Bi3+ increasing, then dramatically drops once the molar ratio of Bi/Cd is beyond 0.2%. Among all these photocatalytsts, the 0.2% sample holds the strongest SPS response. Generally, the surface barrier becomes higher and the space charge region of CdMoO4 becomes narrower as the loading of Bi3+ increases, the charge pairs can be efficiently and easily separated by the large electric field. However, when the loading of Bi3+ is relative too high, the space charge region becomes very narrow; resulting in the recombination of the photogenerated charge pairs becomes easier [51]. According to the principle of SPS measurement, strong SPS response stems from high separation rate of the photoinduced charge pairs [52], therefore the 0.2% sample possesses the highest separation rate of the photoinduced charge pairs. It is commonly accepted that the separation rate of photo-generated charge carriers takes a predominant role in deciding the photocatalytic performance of photocatalyst [53], therefore it is naturally anticipated that the 0.2% sample exhibits the best photocatalytic performance, which accords well with the results of the photocatalytic evaluation. ESR experiments were performed to detect the active free radicals under the simulated 9
ACCEPTED MANUSCRIPT sunlight illumination and the results were illustrated in Fig.7. As displayed in Fig.7, DMPO-•OH and DMPO-•O2− signals were detected, which substantially supports that •OH and •O2− radicals exist in the photocatalytic system. To further investigate the role of active free radicals, scavenger experiments were carried out by adding ammonium oxalate (AO), isopropyl alcohol (IPA), benzoquinone (BQ) into the 0.2% photocatalytic reaction system, respectively. As exhibited in Fig.8A, the existence of BQ in the photocatalytic reaction system dramatically inhibits the decolorization efficiency of RhB, the decolorization efficiency of RhB is the lowest, which indicates that BQ quenches •O2−, resulting in low decolorization efficiency of RhB. The results firmly prove that •O2− takes a predominate role in decolorization of RhB and •OH performs a minor effect. To further investigate the separation efficiency of photoinduced charge pairs of the different samples, BQ was added into the different photocatalytic reaction system, the effects of BQ on the declorozation efficiency of RhB over the different photocatalysts were shown in Fig.8B. It is apparent that the decolrozation of RhB over the 0.2% sample is still the highest, which suggests that the 0.2% sample possesses the highest separation efficiency of photoinduced charge pairs to some degree, fitting well with the results of SPS. 3.2 Photocatalytic performance The decolorization of rhodamine B due to photolysis and adsorption on the different sample after 3h is less than 9%, compared with the decolorization efficiency with photocatalyst under simulated sunlight irradiation; the decolorization of RhB is due to photocatalysis. The decay of RhB over the different photocatalysts was presented in Fig.9A. It is clear that all the Bi-doped CdMoO4 photocatalsyts display higher photocatalytic performance than the bare CdMoO4. The photocatalytic activity of the sample gradually decreases as the molar ratio of Bi/Cd increasing, 10
ACCEPTED MANUSCRIPT and then the photocatalytic activity of the sample drops sharply. Among all the samples, the 0.2% samples exhibits the highest photocatalytic activity. Furthermore, it is interesting to notice that the trend of photocatalytic activity accords well with the trend of SPS, which further implies that separation rate of photo-generated charge carriers plays a predominant role in influencing the photocatalytic performance of photocatalyst. Moreover, the decay of rhodamine B on the catalysts abides by a first-order kinetic equation, and the apparent discoloration rate constants for rhodamine B over the 0%, 0.1%, 0.2% and 0.3% sample are 0.004 min-1, 0.006 min-1, 0.01min-1, and 0.005 min-1,respectively. The photocatalytic performance of the 0.2% sample is more than twice of that of the 0% sample. Doping Bi3+ into lattice of CdMoO4 can remarkably boost the photocatalytic performance of CdMoO4. Fig. 9B described the photocatalytic stability of the 0.2% sample. After recycling four times, the decolorization efficiency of RhB over the 0.2% sample is still higher than 80%, demonstrating that the 0.2% sample displays high photocatalytic stability and has a promising application prospect. Furthermore, the preparation method has the advantages of simple operation, easy availability of raw materials, this facile method can be employed to prepare other photocatalys.
4 Conclusions In summary, Bi-doped CdMoO4 photocatalysts with remarkably enhanced photocatalytic performance were successfully synthesized by a facile microwave hydrothermal method. The experimental data reveal that doping Bi3+ into lattice of CdMoO4 can remarkably boost the separation rate of photoinduce charge pairs and the specific surface area, narrow the band gap of the CdMoO4, all these advantageous factors result in dramatically promoted photocatalytic 11
ACCEPTED MANUSCRIPT performance of CdMoO4. When the molar ratio of Bi/Cd is 0.2%, Bi-doped CdMoO4 holds the best photocatalytic performance. This facile method can be employed to prepare other photocatalysts.
Acknowledgements This project was financially supported by the program of Science and Technology Department of Sichuan province (No.2015JY0081), the Project of Zigong city (No.2014HX14, No. 2014HX09) and the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No. LZJ1302, No. LZJ1301).
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ACCEPTED MANUSCRIPT 17790-17797. [36] X. Ding, W. Ho, J. Shang, L. Zhang, Self doping promoted photocatalytic removal of no under visible light with Bi2MoO6: indispensable role of superoxide ions, Appl. Catal. B.182 (2016) 316-325. [37] G. Tian, Y. Chen, W. Zhou, K. Pan, Y. Dong, C. Tian, H. Fu, Facile solvothermal synthesis of hierarchical flower-like Bi2MoO6 hollow spheres as high performance visible-light driven photocatalysts, J. Mater. Chem. 21 (2010) 887-892. [38] M. Shang, W. Wang, J. Ren, S. Sun, L. Zhang, Nanoscale kirkend all effect for the synthesis of Bi2MoO6 boxes via a facile solution-phase method, Nanoscale 3 (2011) 1474-1476. [39] J. Zhong, J. Li, F. Feng, Y. Lu, J. Zeng, W. Hu, Z. Tang, Improved photocatalytic performance of SiO2-TiO2 prepared with the assistance of SDBS, Mol. Catal. A: Chem. 357 (2012) 101-105. [40] M. Yang, Q. Yang, J. Zhong, S. Huang, J. Li, J. Song, Enhanced photocatalytic performance of Ag2O/BiOF composite photocatalysts originating from efficient interfacial charge separation, Appl. Surf. Sci. 416 (2017) 666-671. [41] X. Liu, J. Zhong, J. Li, S. Huang, W. Song, PEG-assisted hydrothermal synthesis of BiOCl with enhanced photocatalytic performance, Appl. Phys. A119 (2015) 1203-1208. [42] J. Zhong, J. Li, J. Zeng, X. He, W. Hu, Y. Shen, Enhanced photocatalytic performance of Ga3+-doped ZnO, Mater. Res. Bull. 47 (2012) 3893-3896. [43] J. Zhong, J. Li, Y. Lu, X. He, J. Zeng, W. Hu, Y. Shen, Fabrication of Bi3+-doped ZnO with enhanced photocatalytic performance, Appl. Surf. Sci. 258 (2012) 4929-4933. [44] W. Yang, B. Ma, W. Wang, Y. Wen, D. Zeng, B. Shan, Enhanced photosensitized activity of 17
ACCEPTED MANUSCRIPT a BiOCl-Bi2WO6 heterojunction by effective interfacial charge transfer, Phys. Chem. Chem. Phys., 15 (2013) 19387-19394. [45] Y. Ren, J. Ma, Y. Wang, X. Zhu ,B. Lin, J. Liu, X. Jiang, J, Tao, Shape-tailored hydrothermal synthesis of CdMoO4, crystallites on varying pH conditions, J. Am. Ceram. Soc. 90 (2007) 12511254. [46] X. Jiang, J. Ma, B. Lin, Y. Ren, J. Liu, X. Zhu, J. Tao, Y. Wang, L. Xie, Hydrothermal Synthesis of CdMoO4 nano-particles, J. Am. Ceram. Soc. 90 (2007) 977-979. [47] C. Papelis, X-ray photoelectron spectroscopic studies of cadmium and selenite adsorption on aluminum oxides, Environ. Sci. Technol. 29 (1995)1526-1533. [48] J. Bi, Z. Zhou, M. Chen, S. Liang, Y. He, Z. Zhang, L. Wu, Plasmonic Au/CdMoO4, photocatalyst: Influence of surface plasmon resonance for selective photocatalytic oxidation of benzylic alcohol, Appl. Surf. Sci. 349 (2015) 292-298. [49] R. Adhikari, S. Malla, G. Gyawali, T. Sekino , S. W. Lee, Synthesis, characterization and evaluation of the photocatalytic performance of Ag-CdMoO4 solar light driven plasmonic photocatalyst, Mater Res Bull. 48 (2013) 3367-3373. [50] J. Li, X. Liu, Z. Sun, L. Pan, Novel Bi2MoO6/TiO2 heterostructure microspheres for degradation of benzene series compound under visible light irradiation, J. Colloid Interface Sci. 463 (2016) 145-153. [51] Y. Cong, B. Tian, J. Zhang, Improving the thermal stability and photocatalytic activity of nanosized titanium dioxide via La3+ and N co-doping, Appl. Catal. B. Environ. 101 (2011) 376381. [52] L. Kronik, Y. Shapira, Surface photovoltage phenomena: theory, experiment and application, 18
ACCEPTED MANUSCRIPT Surf. Sci. Rep. 37(1999) 1-206. [53] X. Zheng, Q. Yang, S. Huang, J. Zhong, J. Li, R. Yang, Y. Zhang, Enhanced separation efficiency of photo-induced charge pairs and sunlight-driven photocatalytic performance of TiO2 prepared with the assistance of NH4Cl, J Sol-Gel Sci Technol. 83 (2017) 174-180.
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Caption for Figures Fig.1 XRD profiles of the samples Fig.2 UV-Vis diffuse reflectance spectra of the photocatalysts Fig.3 SEM, EDS and HRTEM of the samples; (A) 0%; (B) 0.2%; (C) 1.0%; (D) 0.2% Fig.4 (A) XPS survey spectrum for the 0.1% sample; (B) High resolution XPS spectra of the Cd 3d of the photocatalysts; (C) High resolution XPS spectra of the Mo 3d on the surface of the photocatalysts Fig.5 High resolution XPS spectra of the O1s region on the surface of photocatalysts; (A) 0%; (B) 0.1%; (C) 0.2%; (D) 0.3%. Fig.6 SPS spectra of the photocatalysts Fig.7 DMPO-•OH and DMPO-•O2− signals of the 0.2% sample under irradiation for 2 min Fig.8 (A) Photocatalytic decolorization efficiency of RhB on the 0.2% sample in the presence of different scavengers (irradiation time=3h, scavenger dosage = 0.2 mmol/L); (B) Effects of BQ on decolorization efficiency of RhB over the different photocatalyst (illumination time = 3h, BQ dosage = 0.2 mmol/L) Fig.9 (A) Decay of RhB over the different photocatalysts; (B) Decolorization efficiency of rhodamine B over the 0.2% sample, the illumination time=3h.
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Tables
Table 1 physicochemical parameter of the photocatalysts Sample
0%
0.1%
0.2%
0.3%
SBET (m2/g)
4.2
6.0
7.1
8.0
Crystal size (nm)
33.7
32.6
31.8
31.3
Absorption edge (nm)
370
375
383
385
Band gap (eV)
3.35
3.30
3.24
3.33
Table 2 Curve-fitting results of the high resolution XPS spectra for the O1s region O1s(Cd-O)
O1s (O-H)
Photocatalyst Eb /eV
Ri /%
Eb /eV
Ri /%
0%
529.8
73.7
531.8
26.3
0.1% Bi
530.1
73.0
531.7
27.0
0.2% Bi
529.8
69.7
531.9
30.3
0.3% Bi
529.9
73.2
532.2
26.8
Note: Ri (%) represents the ratio Ai/ΣAi (Ai is the area of each peak).
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0.3%
Intensity (a.u.)
30000
0.2% 20000 0.1% 10000 0% 0
30
40
50
2 Theta (degree) Fig.1 XRD profiles of the samples
22
60
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1.0 0% 0.1% 0.2% 0.3%
Absorbance (a.u.)
0.8 0.6 0.4 0.2 370
0.0 300
350
400
450
500
550
600
Wavelength (nm) Fig.2 UV-Vis diffuse reflectance spectra of the photocatalysts.
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ACCEPTED MANUSCRIPT
A
B
Intensity (a.u.)
1600
Mo
D
(C)
1200 Mo
800
Cd Bi
O
400
Cd Cd
0 0
1
2
3
Energy (keV)
4
5
Fig.3 SEM, EDS and HRTEM of the samples; (A) 0%; (B) 0.2%; (C) 1.0%; (D) 0.2%
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ACCEPTED MANUSCRIPT
O(KLL) O1s
(B) Cd 3d
(A)
Mo 3p
Cd3d
0.3%
Intensity (a.u.)
Cd4d Mo3d Mo4p C1s
Intensity (a.u.)
Cd3p
0.2%
0.1%
Bi4f
1000
800
600 400 Binding energy (eV)
404.45
200
404.50
404.60
404.70
0% 0
416
412
408
404
Binding energy (eV)
231.9
(C) Mo 3d
0.3% Bi
Intensity (a.u.)
231.8
0.2% Bi
231.9
0.1% Bi
231.9
0% 240
238
236
234
232
230
228
226
224
Binding energy (eV)
Fig.4 (A) XPS survey spectrum for the 0.1% sample; (B) High resolution XPS spectra of the Cd 3d of the photocatalysts; (C) High resolution XPS spectra of the Mo 3d on the surface of the photocatalysts
25
400
ACCEPTED MANUSCRIPT
22000
40000 (A)
(B)
35000
18000
Intensity (a.u.)
Intensity (a.u.)
20000
O1s(Cd-O) 16000
O1s(OH)
14000
30000
O1s (Cd-O) O1s (OH)
25000
12000 20000
10000 536
534
532
530
528
526
536
534
532
530
528
526
Binding energy (eV)
Binding energy (eV) 45000 (C)
32000
(D)
28000
35000 30000
Intensity (a.u.)
Intensity (a.u.)
40000 O1s (Cd-O) O1s (OH)
25000
O1s(Cd-O) 24000
O1s(OH)
20000 16000
20000 536
534 532 530 Binding energy (eV)
528
526
536
534
532
530
528
526
Binding energy (eV)
Fig.5 High resolution XPS spectra of the O1s region on the surface of photocatalysts; (A) 0%; (B) 0.1%; (C) 0.2%; (D) 0.3%.
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0.24 0.2%
Photovoltage (mV)
0.18 0.1% 0.12 0.3%
0.06
0% 0.00 300
320
340
360
Wavelength (nm) Fig.6 SPS spectra of the photocatalysts
27
380
400
ACCEPTED MANUSCRIPT
400000
DMPO-O2
Intensity (a.u.)
300000
200000
DMPO-OH
100000
0 3340
3360
3380
3400
3420
3440
Magnetic field (G)
Fig.7 DMPO-•OH and DMPO-•O2− signals of the 0.2% sample under irradiation for 2 min
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ACCEPTED MANUSCRIPT
100
85.8
84.1 73.7
60 40
31.2
20 0
(B) 75
Blank
IPA
AO
Decolorization (%)
Decolorization (%)
80
90
(A)
60
30 15 0
BQ
Xe lamp, irradiation time :3h Xe lamp+BQ,irradiation time :3h
45
0%
0.1%
0.2%
0.3%
Photocatalyst
Scavenger
Fig.8 (A) Photocatalytic decolorization efficiency of RhB on the 0.2% sample in the presence of different scavengers (irradiation time=3h, scavenger dosage = 0.2 mmol/L); (B) Effects of BQ on decolorization efficiency of RhB over the different photocatalyst (illumination time = 3h, BQ dosage = 0.2 mmol/L)
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ACCEPTED MANUSCRIPT
100
(A)
1.0
85.8
84.9
83.7
84.4
1
2
3
4
(B)
C/C0
0.6 0% 0.4
0.3% 0.1%
0.2
Decolorization (%)
80 0.8
0.2% 0
40
80
120
160
60 40 20 0
200
Cycle times
Reaction time (min)
Fig.9 (A) Decay of RhB over the different photocatalysts; (B) Decolorization efficiency of rhodamine B over the 0.2% sample, the illumination time=3h.
30
ACCEPTED MANUSCRIPT Research Highlights
Bi3+ was successfully doped into CdMoO4.
>
The band gap of Bi-doped CdMoO4 narrows.
>
The photo-induced charge separation rate has been greatly enhanced.
>
The sunlight driven-photocatalytic activity has been greatly boosted.
>
Jiao Huang, Huanhuan Liu, Junbo Zhong, Qi Yang, Jiufu Chen, Jianzhang Li, Dongmei Ma, Ran duan PII:
S1293-2558(17)31263-3
DOI:
10.1016/j.solidstatesciences.2018.04.015
Reference:
SSSCIE 5685
To appear in:
Solid State Sciences
Received Date:
27 December 2017
Revised Date:
18 March 2018
Accepted Date:
17 April 2018
Please cite this article as: Jiao Huang, Huanhuan Liu, Junbo Zhong, Qi Yang, Jiufu Chen, Jianzhang Li, Dongmei Ma, Ran duan, Enhanced sunlight-driven photocatalytic performance of Bidoped CdMoO4 benefited from efficient separation of photogenerated charge pairs, Solid State
Sciences (2018), doi: 10.1016/j.solidstatesciences.2018.04.015
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Graphical abstract
0.24 0.2%
1.0 0.8
0.1% 0.12
C/C0
Photovoltage (mV)
0.18
0.3%
0.6 0% 0.4
0.06
0.3% 0.1%
0.2
0%
0.2%
0.00 300
320
340
360
Wavelength (nm)
380
400
0
40
80
120
Reaction time (min)
160
200
ACCEPTED MANUSCRIPT Enhanced sunlight-driven photocatalytic performance of Bi-doped CdMoO4 benefited from efficient separation of photogenerated charge pairs Jiao Huang1, Huanhuan Liu1, Junbo Zhong1*, Qi Yang1, Jiufu Chen1, Jianzhang Li1, Dongmei Ma1, Ran duan2 1 Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, College of Chemistry and Environmental Engineering, Sichuan University of Science and Engineering, Zigong, 643000, P R China 2 Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P R China *Corresponding author (E-mail: [email protected]) Abstract In this paper, to further boost the photocatalytic performance of CdMoO4, Bi3+ was successfully doped into CdMoO4 by a facile microwave hydrothermal method. The Bi-doped CdMoO4 photocatalysts prepared were characterized by Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), electron spin-resonance (ESR) and surface photovoltage spectroscopy (SPS). The results exhibit that doping Bi3+ into CdMoO4 remarkably boosts the separation rate of photoinduced charge pairs and the specific surface area, decrease the crystal size, narrows the band gap of the CdMoO4 and induces the binding energy shift of Cd, all these advantageous factors result in the promoted photocatalytic performance of CdMoO4. Using rhodamine B (RhB) as model toxic pollutant, the photocatalytic activities of the 1
ACCEPTED MANUSCRIPT photocatalysts were evaluated under a 500 W Xe lamp irradiation. When the molar ratio of Bi/Cd is 0.2%, Bi-CdMoO4 prepared displays the best photocatalytic performance, the photocatalytic performance of the 0.2% sample is more than twice of that of the reference CdMoO4. Key words: CdMoO4, doping, microwave hydrothermal method; photocatalytic performance; separation of carriers
2
ACCEPTED MANUSCRIPT 1. Introduction Photocatalytic oxidation technology has become an ideal way to solve environment pollutant because of its mild reaction conditions, low energy consumption, low cost and basic non-pollutant advantages [1,2]. To effectively degrade various toxic contaminants, tremendous catalyst systems, such as CeO2, Co3O4, ZnO, CdS, Bi2O3, (BiO)2CO3 and so on were developed [3-16]. Among these catalysts developed, metal molybdates have attracted widespread concern owing to its photocatalytic ability. CdMoO4 with scheelite-type structure is a typical metal molybdate material, the band gap of CdMoO4 is 3.25 eV [17-24], which indicates that CdMoO4 can be excited by UV light and used as a photocatalyst. In fact, it has been reported that CdMoO4 could effectively degrade RhB under UV light illumination [25]. However, it is still a challenge to prepare CdMoO4 with excellent photocatalytic performance, since the practical application of CdMoO4 has been significantly hindered by its high recombination of photoinduced charge pairs [25,26]. Therefore, it is absolutely necessary to boost the photocatalytic performance by modulating the separation of photoinduced charge pairs. As well known, among the factors which influence the photocatalytic performance of a photocatalyst, separation behaviors of photoinduced charge pairs play a predominant role [27]. To achieve high separation of photoinduced charge pairs, tremendous approaches have been executed, such as construction of heterojuctions[28,29], choice of preparation methods[30-33] and doping ions into the lattice of CdMoO4[34], these methods can greatly enhance the photocatalytic performance of CdMoO4 by accelerating the separation of photoinduced charge pairs. As a layered aurivillus-related oxide, Bi2MoO6 consists Bi2O22+ layers sandwiched between perovskite polyhedra and MoO42−slabs [35,36]. Bi2MoO6 with a narrow band gap (2.5-2.8 eV)[2] 3
ACCEPTED MANUSCRIPT displays excellent photocatalytic performance [37,38]. More importantly, both CdMoO4 and Bi2MoO6 consist MoO42−, if doping Bi3+ into lattice of CdMoO4, it is feasible to form Bi2MoO6, which will greatly influence the electronic structure of CdMoO4, consequently significantly affecting the separation of photoinduced charge pairs. Herein, a series of Bi-doped CdMoO4 photocatalysts were prepared by a facile microwave hydrothermal method. The photocatalysts prepared were studied HRTEM, XPS and SPS. The results of HRTEM confirm the presence of Bi2MoO6, the XPS results reveal that Bi3+ was successfully doped into CdMoO4. The SPS results exhibit that Bi-doped CdMoO4 displays higher separation rate of photoinduced charge pairs than the bare CdMoO4, resulting in promoted photocatalytic performance.
2. Experimental 2.1 Preparation of the samples Bi-doped CdMoO4 photocatalysts were prepared by a microwave hydrothermal process. In a typical preparation process, 6.1694 g (0.02 mol) Cd(NO3)2·4H2O and desired Bi(NO3)3·5H2O were added into 50 mL of deionized water under intensely stirring, the molar ratios of Bi/Cd were 0%, 0.1%, 0.2% and 0.3%, respectively, then desired Na2MoO4·2H2O was added into the solution mentioned above, the molar amount of MoO42-=Cd2+ +0.5 Bi3+. The mixture was sonicated for 10 min, and then was stirred for 24 h, forming a suspension system. The suspension system was transferred to a microwave hydrothermal reactor and maintained at 403 K for 2 h. After cooling down to room temperature naturally, the solid was filtered and washed with deionized water and absolute ethanol many times, and then dispersed in absolute ethanol and dried at 333K overnight.
4
ACCEPTED MANUSCRIPT The samples with different molar ratios of Bi/Cd (0%, 0.1%, 0.2% and 0.3%) were marked as 0%, 0.1%, 0.2% and 0.3%, respectively. 2.2 Characterization Specific surface area parameters were measured on the SSA-4200 surface analyzer using Brunauer-Emmett-Teller (BET) method. The XRD patterns were performed on a DX-2600 using X-ray diffractometer with a radiation source of Cu Kα (λ=0.15406 nm), the X-ray tube was operated at 35 kV and 25 mA. The UV-Vis diffuse reflectance spectra were measured on a spectrometer (UH-4150) using barium sulphate as the reference. SEM images were observed on a VEGA 3 SBU scanning electron microscope, using an accelerating voltage of 15 kV. EDS profiles of the sample were conducted on a X Flash Detector 410-M energy dispersive spectrometer. HRTEM was recorded on a JEOL-2100F microscope, utilizing an accelerating voltage of 200 KV. X-ray photoelectron spectroscopy (XPS) measurements were performed on a XSAM 800 using Mg Kα at 12 kV and 12 mA. SPS spectra were recorded on a home-built apparatus, and the detailed operation procedures were given in the Ref.[39]. ESR experiments were performed on Bruker E 500, the detailed experimental procedures were described in the Ref.[40] using 5,5dimethyl-1-pyrroline-N-oxide (DMPO) method. 2.3 Photocatalytic tests In this paper, RhB (10 mgL-1, pH=6.00) was used as a model dye. The detailed photocatalytic evaluation procedures were given in the Ref.[41], the light source was a 500W Xe lamp (simulated sunlight), and the dosage of photocatalysts was 1 gL-1.
3. Results and discussion 5
ACCEPTED MANUSCRIPT 3.1 Characterization of the samples The BET results of the samples were shown in Table 1. It can be seen from Table 1 that doping Bi3+ into the lattice of CdMoO4 can significantly increase the specific surface area of CdMoO4, which implies that Bi3+ restrains the growth of CdMoO4, which can be further verified by the results of XRD. Similar phenomena were observed when doping Ga3+ and Bi3+ into ZnO [42,43]. Combined with the results of XRD, it is apparent that the increased specific surface area of CdMoO4 arises from the small crystal size. It is widely acknowledged that relative high specific surface area can offer more active sites, which can effectively adsorb more pollutants, promoting the photcatalytic performance of CdMoO4. The XRD profiles of the samples were exhibited in Fig.1. It is distinct that all the diffraction peaks accord well with the standard tetragonal phase of CdMoO4 (JCPDS No.07-0209). No other impurity peaks were observed, indicating high purity of the photocatalysts. No diffraction peaks of Bi2MoO6 were detected, which may attribute to the relatively low content of Bi2MoO6 or Bi2MoO6 is highly dispersed in the lattice of CdMoO4. Furthermore, the full width at half maximum (FWHM) gradually increases as the loading of Bi3+ elevating. According to the Scherrer formula, the wider FWHM means smaller the crystal size of CdMoO4. The crystal sizes of the samples were displayed in Table 1. As exhibited in Table 1, the crystal sizes of the samples gradually decrease as the amount of Bi3+ elevating, which indicates that Bi3+ effectively restrains the growth of CdMoO4. The radius of Bi3+ (0.103 nm) is bigger than that of Cd2+ (0.095 nm), it is impossible for Bi3+ ions enter into the crystal cell of CdMoO4 through the interstitial mode. However, it is possible for Bi3+ to substitute Cd2+ and thus occupies the positions of Cd2+ ions in CdMoO4, greatly influencing the physicochemical properties of CdMoO4. The decreased crystal 6
ACCEPTED MANUSCRIPT size suggests that Bi was doped into CdMoO4.The result further reveals that the decreased crystal size results in the BET surface area increase, which fits well with the results of BET surface area. The UV-Vis diffuse reflectance spectra of the photocatalysts were displayed in Fig.2. As shown in Fig.2, the band gap of the samples can be further calculated using formula Eg = 1240/λ, where λ is the absorption edge [44]. The absorption edge and the corresponding band gap were shown in Table1. It is evident that Bi-doped CdMoO4 photocatalyst appear red-shift, which suggests that the band gap of Bi-doped CdMoO4 becomes narrow. It is clear that change in band gap is induced by the presence of Bi2MoO6 in the lattice of CdMoO4, since Bi2MoO6 is a photocatalyst with a narrow band gap (2.5-2.8 eV)[2]. The red-shift of the absorption edge is in good consistent with the results of SPS. The narrowed band gap of CdMoO4 can greatly increase the light response ability, boosting the sunlight-driven photoctalytic performance. SEM images of the 0% and 0.2% sample were demonstrated in Fig. 3. As shown in Fig. 3A and B, no obvious difference in shape of the samples was observed. Both samples display lumplike shape, indicating that doping Bi3+ into CdMoO4 cannot effectively alter the morphology of the sample. Due to the relative small amount of Bi, no Bi was detected for the 0.3% sample, to further confirm the existence of Bi in CdMoO4, 1.0% sample was prepared and characterized by EDS. As exhibited in Fig.3C, Cd, Mo, Bi, O elements were detected for the 1.0% sample, which further confirms the existence of Bi3+ in the photocatalysts. The presence of Bi2MoO6 in the phtocatalyst can be further supported by HRTEM. As demonstrated in Fig.3D, the spacing of 0.1529 nm was clearly observed, which can be ascribed to the (153) crystallographic planes of Bi2MoO6 (JCPDS card No. 21-0102). Furthermore, the lattice spacing of 0.2317 nm agrees well with the (105) plane of CdMoO4 (JCPDS card No. 07-0209). The results of UV-Vis DRS, EDS and HRTEM firmly 7
ACCEPTED MANUSCRIPT confirm the successful preparation of Bi3+-doped CdMoO4 and formation of Bi2MoO6. The Bi3+ in the lattice of CdMoO4 is anticipated to significantly affect the separation of photogenerated charge pairs, proven by the results of SPS. X-ray photoelectron spectroscopy (XPS) was carried out to determine the surface chemical composition of the samples. Fig.4A shows the surface survey of the 0.1% sample. As expected, XPS spectra reveal the characteristic peaks for Cd, O, Mo and Bi, which accords well with the results of EDS and HRTEM. Fig.4B shows the high-resolution XPS spectra of Cd 3d, for the 0% sample, two strong symmetrical characteristic spin-orbit splitting of Cd 3d peaks at 404.7 eV and 411.5 eV, which can be ascribed to Cd 3d3/2 and Cd 3d5/2, respectively, suggesting that Cd is Cd2+ cations [45-48]. However, for the 0.1%, 0.2% and 0.3 % samples, the binding energy of Cd 3d3/2 gradually shifts to lower binding energy, which implies that the chemical environment of Cd has been altered. If Bi3+ ions enter into the crystal cell of CdMoO4 through the interstitial mode, which will execute minor even no influence on the chemical environment of Cd. The binding energy shift of Cd 3d3/2 substantially confirms that Bi3+ substitutes part of the Cd2+ ions and thus occupies the positions of the Cd2+ ions in CdMoO4, affecting the specific surface area, crystal size, band gap, binding energy of Cd and separation rate of photoinduced charge pairs. Fig.4C displays the XPS peaks of Mo 3d3/2 and Mo 3d5/2 at 231.9 eV and 235.0 eV, respectively, corresponding to Mo6+ of CdMoO4 [49]. As shown in Fig.5, the O 1s spectra can be fitted with two peaks at 531.8 eV and 529.7eV, which can be attributed to O-H and Cd-O bonds, respectively [50]. Table 2 lists the curve-fitting results of O1s XPS spectra for the photocatalysts (Fig.5), where Ri (%) represents the ratio of the different kinds of oxygen contributions. As shown in Table 2, due to the relative low amount of Bi3+, the hydroxyl content on all the samples has no obvious difference. Therefore, 8
ACCEPTED MANUSCRIPT the difference in photocatalytic performance is not generated by the hydroxyl content. Fig.6 exhibits the SPS responses of the photocatalysts as prepared, it can be seen from Fig.6 that all the photocatalysts display obvious response in the range of 300-375 nm, which is due to the electron transition from valence band to conduction band and UV-Vis DRS. It is interesting to notice that the response peak shifts to higher wavelength, which is due to the presence of Bi2MoO6 in the photocatalyst, proven by the results of HRTEM and XPS. As a visible light response photocatalyts, compared to CdMoO4, the SPS response peak of Bi2MoO6 located at higher wavelength. Moreover, the SPS responses of the photocatalysts gradually boosts as the loading of Bi3+ increasing, then dramatically drops once the molar ratio of Bi/Cd is beyond 0.2%. Among all these photocatalytsts, the 0.2% sample holds the strongest SPS response. Generally, the surface barrier becomes higher and the space charge region of CdMoO4 becomes narrower as the loading of Bi3+ increases, the charge pairs can be efficiently and easily separated by the large electric field. However, when the loading of Bi3+ is relative too high, the space charge region becomes very narrow; resulting in the recombination of the photogenerated charge pairs becomes easier [51]. According to the principle of SPS measurement, strong SPS response stems from high separation rate of the photoinduced charge pairs [52], therefore the 0.2% sample possesses the highest separation rate of the photoinduced charge pairs. It is commonly accepted that the separation rate of photo-generated charge carriers takes a predominant role in deciding the photocatalytic performance of photocatalyst [53], therefore it is naturally anticipated that the 0.2% sample exhibits the best photocatalytic performance, which accords well with the results of the photocatalytic evaluation. ESR experiments were performed to detect the active free radicals under the simulated 9
ACCEPTED MANUSCRIPT sunlight illumination and the results were illustrated in Fig.7. As displayed in Fig.7, DMPO-•OH and DMPO-•O2− signals were detected, which substantially supports that •OH and •O2− radicals exist in the photocatalytic system. To further investigate the role of active free radicals, scavenger experiments were carried out by adding ammonium oxalate (AO), isopropyl alcohol (IPA), benzoquinone (BQ) into the 0.2% photocatalytic reaction system, respectively. As exhibited in Fig.8A, the existence of BQ in the photocatalytic reaction system dramatically inhibits the decolorization efficiency of RhB, the decolorization efficiency of RhB is the lowest, which indicates that BQ quenches •O2−, resulting in low decolorization efficiency of RhB. The results firmly prove that •O2− takes a predominate role in decolorization of RhB and •OH performs a minor effect. To further investigate the separation efficiency of photoinduced charge pairs of the different samples, BQ was added into the different photocatalytic reaction system, the effects of BQ on the declorozation efficiency of RhB over the different photocatalysts were shown in Fig.8B. It is apparent that the decolrozation of RhB over the 0.2% sample is still the highest, which suggests that the 0.2% sample possesses the highest separation efficiency of photoinduced charge pairs to some degree, fitting well with the results of SPS. 3.2 Photocatalytic performance The decolorization of rhodamine B due to photolysis and adsorption on the different sample after 3h is less than 9%, compared with the decolorization efficiency with photocatalyst under simulated sunlight irradiation; the decolorization of RhB is due to photocatalysis. The decay of RhB over the different photocatalysts was presented in Fig.9A. It is clear that all the Bi-doped CdMoO4 photocatalsyts display higher photocatalytic performance than the bare CdMoO4. The photocatalytic activity of the sample gradually decreases as the molar ratio of Bi/Cd increasing, 10
ACCEPTED MANUSCRIPT and then the photocatalytic activity of the sample drops sharply. Among all the samples, the 0.2% samples exhibits the highest photocatalytic activity. Furthermore, it is interesting to notice that the trend of photocatalytic activity accords well with the trend of SPS, which further implies that separation rate of photo-generated charge carriers plays a predominant role in influencing the photocatalytic performance of photocatalyst. Moreover, the decay of rhodamine B on the catalysts abides by a first-order kinetic equation, and the apparent discoloration rate constants for rhodamine B over the 0%, 0.1%, 0.2% and 0.3% sample are 0.004 min-1, 0.006 min-1, 0.01min-1, and 0.005 min-1,respectively. The photocatalytic performance of the 0.2% sample is more than twice of that of the 0% sample. Doping Bi3+ into lattice of CdMoO4 can remarkably boost the photocatalytic performance of CdMoO4. Fig. 9B described the photocatalytic stability of the 0.2% sample. After recycling four times, the decolorization efficiency of RhB over the 0.2% sample is still higher than 80%, demonstrating that the 0.2% sample displays high photocatalytic stability and has a promising application prospect. Furthermore, the preparation method has the advantages of simple operation, easy availability of raw materials, this facile method can be employed to prepare other photocatalys.
4 Conclusions In summary, Bi-doped CdMoO4 photocatalysts with remarkably enhanced photocatalytic performance were successfully synthesized by a facile microwave hydrothermal method. The experimental data reveal that doping Bi3+ into lattice of CdMoO4 can remarkably boost the separation rate of photoinduce charge pairs and the specific surface area, narrow the band gap of the CdMoO4, all these advantageous factors result in dramatically promoted photocatalytic 11
ACCEPTED MANUSCRIPT performance of CdMoO4. When the molar ratio of Bi/Cd is 0.2%, Bi-doped CdMoO4 holds the best photocatalytic performance. This facile method can be employed to prepare other photocatalysts.
Acknowledgements This project was financially supported by the program of Science and Technology Department of Sichuan province (No.2015JY0081), the Project of Zigong city (No.2014HX14, No. 2014HX09) and the Opening Project of Key Laboratory of Green Catalysis of Sichuan Institutes of High Education (No. LZJ1302, No. LZJ1301).
References
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Caption for Figures Fig.1 XRD profiles of the samples Fig.2 UV-Vis diffuse reflectance spectra of the photocatalysts Fig.3 SEM, EDS and HRTEM of the samples; (A) 0%; (B) 0.2%; (C) 1.0%; (D) 0.2% Fig.4 (A) XPS survey spectrum for the 0.1% sample; (B) High resolution XPS spectra of the Cd 3d of the photocatalysts; (C) High resolution XPS spectra of the Mo 3d on the surface of the photocatalysts Fig.5 High resolution XPS spectra of the O1s region on the surface of photocatalysts; (A) 0%; (B) 0.1%; (C) 0.2%; (D) 0.3%. Fig.6 SPS spectra of the photocatalysts Fig.7 DMPO-•OH and DMPO-•O2− signals of the 0.2% sample under irradiation for 2 min Fig.8 (A) Photocatalytic decolorization efficiency of RhB on the 0.2% sample in the presence of different scavengers (irradiation time=3h, scavenger dosage = 0.2 mmol/L); (B) Effects of BQ on decolorization efficiency of RhB over the different photocatalyst (illumination time = 3h, BQ dosage = 0.2 mmol/L) Fig.9 (A) Decay of RhB over the different photocatalysts; (B) Decolorization efficiency of rhodamine B over the 0.2% sample, the illumination time=3h.
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Tables
Table 1 physicochemical parameter of the photocatalysts Sample
0%
0.1%
0.2%
0.3%
SBET (m2/g)
4.2
6.0
7.1
8.0
Crystal size (nm)
33.7
32.6
31.8
31.3
Absorption edge (nm)
370
375
383
385
Band gap (eV)
3.35
3.30
3.24
3.33
Table 2 Curve-fitting results of the high resolution XPS spectra for the O1s region O1s(Cd-O)
O1s (O-H)
Photocatalyst Eb /eV
Ri /%
Eb /eV
Ri /%
0%
529.8
73.7
531.8
26.3
0.1% Bi
530.1
73.0
531.7
27.0
0.2% Bi
529.8
69.7
531.9
30.3
0.3% Bi
529.9
73.2
532.2
26.8
Note: Ri (%) represents the ratio Ai/ΣAi (Ai is the area of each peak).
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0.3%
Intensity (a.u.)
30000
0.2% 20000 0.1% 10000 0% 0
30
40
50
2 Theta (degree) Fig.1 XRD profiles of the samples
22
60
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1.0 0% 0.1% 0.2% 0.3%
Absorbance (a.u.)
0.8 0.6 0.4 0.2 370
0.0 300
350
400
450
500
550
600
Wavelength (nm) Fig.2 UV-Vis diffuse reflectance spectra of the photocatalysts.
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A
B
Intensity (a.u.)
1600
Mo
D
(C)
1200 Mo
800
Cd Bi
O
400
Cd Cd
0 0
1
2
3
Energy (keV)
4
5
Fig.3 SEM, EDS and HRTEM of the samples; (A) 0%; (B) 0.2%; (C) 1.0%; (D) 0.2%
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O(KLL) O1s
(B) Cd 3d
(A)
Mo 3p
Cd3d
0.3%
Intensity (a.u.)
Cd4d Mo3d Mo4p C1s
Intensity (a.u.)
Cd3p
0.2%
0.1%
Bi4f
1000
800
600 400 Binding energy (eV)
404.45
200
404.50
404.60
404.70
0% 0
416
412
408
404
Binding energy (eV)
231.9
(C) Mo 3d
0.3% Bi
Intensity (a.u.)
231.8
0.2% Bi
231.9
0.1% Bi
231.9
0% 240
238
236
234
232
230
228
226
224
Binding energy (eV)
Fig.4 (A) XPS survey spectrum for the 0.1% sample; (B) High resolution XPS spectra of the Cd 3d of the photocatalysts; (C) High resolution XPS spectra of the Mo 3d on the surface of the photocatalysts
25
400
ACCEPTED MANUSCRIPT
22000
40000 (A)
(B)
35000
18000
Intensity (a.u.)
Intensity (a.u.)
20000
O1s(Cd-O) 16000
O1s(OH)
14000
30000
O1s (Cd-O) O1s (OH)
25000
12000 20000
10000 536
534
532
530
528
526
536
534
532
530
528
526
Binding energy (eV)
Binding energy (eV) 45000 (C)
32000
(D)
28000
35000 30000
Intensity (a.u.)
Intensity (a.u.)
40000 O1s (Cd-O) O1s (OH)
25000
O1s(Cd-O) 24000
O1s(OH)
20000 16000
20000 536
534 532 530 Binding energy (eV)
528
526
536
534
532
530
528
526
Binding energy (eV)
Fig.5 High resolution XPS spectra of the O1s region on the surface of photocatalysts; (A) 0%; (B) 0.1%; (C) 0.2%; (D) 0.3%.
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0.24 0.2%
Photovoltage (mV)
0.18 0.1% 0.12 0.3%
0.06
0% 0.00 300
320
340
360
Wavelength (nm) Fig.6 SPS spectra of the photocatalysts
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380
400
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400000
DMPO-O2
Intensity (a.u.)
300000
200000
DMPO-OH
100000
0 3340
3360
3380
3400
3420
3440
Magnetic field (G)
Fig.7 DMPO-•OH and DMPO-•O2− signals of the 0.2% sample under irradiation for 2 min
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100
85.8
84.1 73.7
60 40
31.2
20 0
(B) 75
Blank
IPA
AO
Decolorization (%)
Decolorization (%)
80
90
(A)
60
30 15 0
BQ
Xe lamp, irradiation time :3h Xe lamp+BQ,irradiation time :3h
45
0%
0.1%
0.2%
0.3%
Photocatalyst
Scavenger
Fig.8 (A) Photocatalytic decolorization efficiency of RhB on the 0.2% sample in the presence of different scavengers (irradiation time=3h, scavenger dosage = 0.2 mmol/L); (B) Effects of BQ on decolorization efficiency of RhB over the different photocatalyst (illumination time = 3h, BQ dosage = 0.2 mmol/L)
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100
(A)
1.0
85.8
84.9
83.7
84.4
1
2
3
4
(B)
C/C0
0.6 0% 0.4
0.3% 0.1%
0.2
Decolorization (%)
80 0.8
0.2% 0
40
80
120
160
60 40 20 0
200
Cycle times
Reaction time (min)
Fig.9 (A) Decay of RhB over the different photocatalysts; (B) Decolorization efficiency of rhodamine B over the 0.2% sample, the illumination time=3h.
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ACCEPTED MANUSCRIPT Research Highlights
Bi3+ was successfully doped into CdMoO4.
>
The band gap of Bi-doped CdMoO4 narrows.
>
The photo-induced charge separation rate has been greatly enhanced.
>
The sunlight driven-photocatalytic activity has been greatly boosted.
>