- Email: [email protected]
Construction of octahedral BiFeWOx encapsulated in hierarchical In2S3 [email protected] heterostructure for visible-light-driven CO2 reduction
Journal of CO₂ Utilization 29 (2019) 156–162
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Construction of octahedral BiFeWOx encapsulated in hierarchical In2S3 core@shell heterostructure for visible-light-driven CO2 reduction ⁎
Yanan Wang, Yiqing Zeng, Shipeng Wan, Shule Zhang , Qin Zhong
T
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School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 photoreduction Heterojunction Core-shell structure In2S3 BiFeWOx
Novel 3D BiFeWOx@In2S3 (BFW@IS-x) core shell heterostructures were rationally designed and fabricated to be used as efficient photocatalysts for CO2 photoreduction. The constructive strategy integrated nano-octahedral BFW and hierarchical IS by growing IS ultrathin nanosheet subunits on surface of BFW. The proposed in site synthesis route resulted in a high dispersion of BFW nanocrystals and the tight interaction between BFW and IS. The obtained core-shell heterostructure boosted separation and transfer of photoinduced charge carriers, evidenced by electrochemical measurement. The hierarchical structure on the surface of BFW@IS-x catalyst enhanced light harvest and provided ample active sites for surface redox reactions. Profiting from these extraordinary chemical and physical characteristics owing to the interaction between BFW and IS, the efficiency of photo-conversion of CO2 to CH4 and CO over BFW@IS-1 heterostructures with optimal proportion was 49.9 μmol h−1 g−1 and 28.9 μmol h−1 g−1, respectively, under visible light irradiation. The CH4 generated rate was about 3.5 and 6.4 times higher than that of pure IS and BFW, and CO generated rates was 4.9 times higher than that of pure IS, respectively. The possible mechanism for the CO2 reduction over BFW@IS-1 composite was discussed.
1. Introduction The growing atmospheric concentration of CO2 has been a global environmental issue due to its inevitable effect to climate change and global warming [1,2]. Inspired by natural photosynthesis, artificial photosynthesis, which are being conducted to fulfill the solar energy conversion of the CO2 into available chemical energy (H2, CO, HCOOH, HCHO, CH3OH, CH4), is considered to be a desired and “kill two birds with one stone” approach” candidate technology to simultaneously address the energy problems of environmental issues [3,4]. In the past few years, significant progresses of CO2 conversion in semiconductor based photocatalysts have been achieved, such as TiO2 and TiO2-based materials [5,6], ZnO [7,8], SrTiO3 [9], Bi2WO6 [10,11], In2S3 [12,13], however, these photocatalysts usually have low CO2 photo-conversion efficiency. Photocatalytic process involves three steps, absorption of light, generation and separation of carriers, redox reactions at the surface. In principle, the rapid separation and transfer of charge carriers and the catalytic reactions of electrons on active sites determine photoconversion efficiency of CO2. Therefore, increasing the carrier separation efficiency is usually considered as an effective way for enhancement of photocatalytic activity, and the integration of two or three semiconductors to form hybrid nanostructures has recently attracted
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much attention to realize the purpose. Among visible light induced photocatalysts, bismuth (Bi)-based photocatalytic materials have obtained attention for owing to unique band structures [14,15]. Thereinto, Bi-based pyrochlores have been reported for owing to a defective structure, which provide the ability to handle electron/hole transfer by adjusting different elements entered into different sites, for instance, vanadate (BiVO4) [16], tungstate (Bi2WO6, BiFeWOx) [17,18], ferrites (BiFeO3) [19], titanate (Bi2Ti2O7) [20] and tellurate (BiFeTeO6) [21]. Among them, BiFeWO6 has attracted attention recently because of its unique structure, nontoxicity, chemical stability and visible light response [18]. However, the single BiFeWOx has been limited by rapid recombination of photogenerated electron-hole and poor adsorption of solar light. It is generally known that the coupling of BiFeWOx with other semiconductors is an ideal method to encourage the separation of photogenerated charge carriers. Some binary BiFeWOx-based composites have been fabricated to improve its photocatalytic efficiency [22,23]. However, related research is still very few, there is plenty of room for the development of BiFeWOxbased composites. Indium sulfides (In2S3) is an intriguing photocatalyst due to a narrow band gap (2.0–2.3 eV) and appropriate band edges for converting photon energy into chemical energy, which make it become an
Corresponding authors. E-mail addresses: [email protected] (S. Zhang), [email protected] (Q. Zhong).
https://doi.org/10.1016/j.jcou.2018.12.009 Received 4 July 2018; Received in revised form 20 November 2018; Accepted 13 December 2018 2212-9820/ © 2018 Published by Elsevier Ltd.
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obtained and denoted as BFW@IS-x, where x is the weight ratio of BFW to the In(NO3)3·xH2O.
excellent candidate to couple with BiFeWOx. In current work, we for the first time presented a simple hydrothermal method to yield hierarchical In2S3 on octahedral BiFeWOx to obtain BiFeWOx@In2S3 (BFW@IS-x) core-shell structure. As expected, compared with single BFW and IS, the BFW@IS-x displayed significantly enhanced photocatalytic performance of CO2 reduction due to facilitating the separation and high transfer efficiency of photoexcited electron-hole pairs.
2.4. Characterization The structure, elemental composition and morphology of the assynthesized samples were analyzed by XRD patterns (a Purkinjie XD-30 system equipped with Cu Kα radiation), the transmission electron microscopy (TEM) (FEI Tecnai G2 20 microscope) and a field-emission scanning electron microscopy (FESEM, FEI Quanta 250 F) attached energy-dispersive spectroscopy (EDS). UV–vis diff ;use reflectance spectra (DRS) were conducted on a Shimadzu UV-2600 spectrometer.
2. Experimental 2.1. Preparation of BFW All the raw materials were purchased from commercial sources and used without further purification. In a typical procedure, 2.5 mmol of Bi (NO3)3·5H2O and Fe(NO3)3·9H2O was dissolved in 25 mL deionized water under magnetic stirring. Secondly, Na2WO4 solution was dissolved and added into above mixture solution. Subsequently, 100 mL certain concentrated NH3·H2O was slowly poured into the above mixture for adjusting the pH = 4. At last, the resultant solution was transferred into a 200 mL Teflon-lined autoclave and maintained at 200 °C for16 h. when it cooled to room temperature naturally, the yellow precipitates were collected and washed several times with deionized water, then dried at 60 °C in air for 12 h.
2.5. Electrochemical measurements Photocurrent measurements were carried out using an electrochemical system (CHI-770, Chenhua, China), wherein the working electrode was irradiated by a 300 W Xe lamp with a 400 nm cut-off filter. A Pt plate and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The samples were employed as working electrode: 10 mg photocatalyst was dispersed in solution containing 30 μL naphthol, 850 μL deionized water and 150 μL isopropyl alcohol. Then, the suspension was dip-coated on a FTO glass (1 cm × 2 cm) and dried at 180 ℃ for 2 h under a vacuum atmosphere. The cross-sectional view indicates that the BFW@IS-1 film has ∼240 nm thickness (Fig. S1). All measurements were measured in 0.5 M Na2SO4 solution. Prior to and during all measurements, the electrolyte was purged with nitrogen. The electrochemical impedance spectroscopy (EIS) was performed in the standard three-electrode system. EIS measurement was conducted in a frequency range of 1 Hz to 106 Hz for amplitude of 5 mV.
2.2. Preparation of IS Typically, both In(NO3)3·xH2O (mol) and 1,4-benzenedicarboxylic acid (H2BDC) were dissolved in 20 ml of DMF under stirring and followed by heating at 120 °C for 60 min A white precipitate was observed and filtrated and washed with ethanol three times. Subsequently, the obtained precipitates were dissolved in 40 ml ethanol solution containing and 200 mg thiourea was added into the ethanol solution. After stirring for 30 min. Then, the resultant mixture was sealed in a Teflon lined autoclave followed by heating at 180 °C for 6 h. The products were obtained by centrifugation and then dried at 60 °C for 24 h under vacuum
2.6. Photocatalytic performance The photocatalytic activity was evaluated by photocatalytic reduction (PCR) of CO2 at room temperature and negative atmospheric pressure. Photocatalyst (20 mg), 1 mL of triethanolamine (TEOA) and 20 mL H2O were added into a gas-closed glass reactor (80 mL in capacity). Then, the high purity CO2 gas was evacuated and refilled for three times to remove the air inside before the lamp turn on. Finally, the reactor was filled with CO2 at a pressure of 0.8 atm. A 300 W Xe arc lamp (Aulight CEL-HXF300, Beijing) was used as the light source. During the photocatalytic process, the reaction system was vigorously stirred by a magnetic stirrer. The produced gas was detected and quantified by a gas chromatography (Hope GC 9860) with both TCD and FID detectors.
2.3. Preparation of BFW@IS The BFW@IS composite photocatalysts were synthesized through a surface-functionalized hydrothermal method. In detail, a certain amount of BFW (0.05 g, 0.1 g, 0.2 g) 0.1 g of In(NO3)3·xH2O and 0.15 g of H2BDC were mixed in 15 ml of DMF and stirred for 120 min. Then, the resultant solution was placed in an oil bath at 120 °C for 60 min After the reaction, the obtained precipitate was washed with ethanol, then added into 80 ml of an ethanol solution containing 4 g of thiourea. The mixture was transferred into a Teflon-lined autoclave (100 mL in capacity) and maintained at 180 °C for 6 h. In the solvothermal process, the in-situ growth of In2S3 on BFW was achieved. After cooling to room temperature, the BFW@IS composites were
Fig. 1. XRD patterns of (a) IS, (b) BFW, and BFW@IS-x composites. 157
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Fig. 2. (a) The evolution of CH4 and CO over pure IS, BFW and BFW@IS-x photocatalysts under visible-light irradiation, (b) the recyclability of for photoreduction CO2 (evacuation every 12 h).
Fig. 3. SEM images of (a) BFW, (b) BFW@IS-1 and (c1–c6) EDX mappings of the as-synthesized BFW@IS-1.
3. Results and discussion
3.2. Photocatalytic performance
3.1. Crystal structure
The photocatalytic activity of the BFW@IS-x heterostructure was evaluated by photocatalytic CO2 reduction performance under irradiation from a 300 W Xe-lamp. As shown in Fig. 2a, both CO and CH4 can be observed in the reaction system. Among the samples, the pure BFW and IS exhibited the moderate activity for CO2 photoreduction with a CH4 generation rate of 7.8 μmol h−1 g−1 and 14.1 μmol h−1 g−1, and CO generation rate of 0 μmol h−1 g−1 and 5.9 μmol h−1 g−1, respectively. After in-situ growing procedure, the obtained BFW@IS-x heterostructure samples exhibited substantially enhanced CO2 photoreduction performance. Strikingly, the BFW@IS-1 catalysts presented the highest CO evolution rate of 49.9 μmol h−1 g−1, which was about 3.5 times higher than that of IS, 6.4 times higher than that of BFW. In
Fig. 1 shows the XRD patterns of IS, BFW and BFW@IS-x samples. The peaks of the IS (Fig. 1a) matched well with the standard card of cubic In2S3 phase (JCPDS card 32-0456) [24]. A series of BFW@IS-x exhibited similar XRD patterns with slight distinctions in the peak density. As the amount of BFW decreased, the peaks gradually decreased because the surface of BFW was covered by IS. Due to weak peak intensity of IS, slight peaks belonging to In2S3 (marked by diamonds) were observed from the XRD pattern of BFW@IS-x. This result confirmed the coexistence of BFW and IS.
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Fig. 4. (a, b) TEM and (c) HAADF-STEM image (c1–c5) and elemental mapping of Bi, W, Fe, In and S.
heterojunctions facilitating the efficient separation and transfer of charge carriers; The CO2 photoreduction performance over BFW@IS-1 was carried out for 5 cycles to evaluate the stability. As shown in Fig. 2b, no significant loss in the CO and CH4 evolution rate was observed, reflecting the high stability of the BFW@IS-1catalyst. 3.3. Morphology, optical property and the separation and transfer of charge carriers Morphology and composition of samples were investigated by SEM, TEM, and EDS. A perfect octahedral nanostructure of pure BFW was obtained. The SEM image of pure IS sample is presented in Fig. S2a. it showed a hierarchical structure, and every wrinkling or rolling nanoflakes was cross-linked to form vast three-dimensional (3D) network channels (Fig. S2b). The HRTEM image (Fig. S2c) clearly reveals the interplanar distances was found to be 0.32 nm, corresponding to the planes (109) in cubic In2S3. As to BFW@IS-1 (Fig. 3b), it can be observed that nano-octahedral BFW was encapsulated in a large number of hierarchical nanosheets, forming a fluffy flower-like structure on the surface of the BFW, confirming the formation of BFW@IS-1 composites. In addition, the component elements and distribution were measured by EDX mapping. As shown in Fig. 3c, the Bi, Fe, W, S and In elements represented by yellow, purple, green, red and pink dots separately were detected. While, as the ration of BFW went up to 2, it can be observed that numerous BFW octahedra were exposed (Fig. S2d). As to BFW@IS-
Fig. 5. UV–vis diffuse reflectance spectra of IS, BFW and BFW@IS-x composites.
addition, the photo-reduction efficiency of CO2 to CO (28.9 μmol h−1 g−1) was 4.9 times of that for the IS. The coupling effect among IS and BFW significantly promoted the CO and CH4 evolution. The enhanced photocatalytic efficiency of BFW@IS-x composite could be attributed to (1) improved the sunlight response region; (2) the formed 159
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Fig. 6. (a) Transient photocurrent responses, (b) EIS Nyquist plots of the BFW, IS, [email protected], BFW@IS-1 and BFW@IS-2.
Fig. 7. Mott-Schottky (MS) plots of (a) BFW and (b) IS photocatalysts.
images (Fig. 4c1–c5) further confirmed the existence of Bi, Fe, W, In and S elements. The corresponding elemental mapping images showed all elements were uniformly dispersed on BFW@IS-1 heterostructure. Meanwhile, it unambiguously revealed the BFW@IS-1 heterostructure with different acreage, in which the regions of Bi, Fe and W elements were smaller than that of In and S elements, representing that Bi, Fe and W elements were distributed in the core and S and In elements were homogeneously dispersed in the shell. The detailed elemental mapping results demonstrated the spatial distribution of the BFW@IS-1 composite, confirming the core-shell structure of the obtained BFW@IS-1 heterostructure. The hierarchical core-shell heterostructure intimately formed by BFW and IS could facilitate the migration of charge carries and provide pathways to harvest light. Moreover, synergistic effects from the complementary effect between BFW and IS could maximize the asset, thereby, enhancing the photocatalytic efficiency [25,26]. In general, the photoreduction processes mainly involves three steps, light harvesting, generation and separation of photogenerated carriers and redox reactions. The UV–vis DRS spectra are measured to analyze the optical property of photocatalysts. As shown in Fig. 5, the pure IS and BFW samples could harvest the light from UV to visible light. The band gaps calculated from the Tauc plots (Fig. S3a, b) were 2.19 and 2.22 eV, respectively. After the hybridization of IS and BFW, a significant red-shift of absorption edge from UV light region to the visible light region could be found, compared with pure IS. It could be deduced the core-shell formation with wrinkled hierarchical surface provided more pathways to harvest light, resulting in a narrow the band gap. That was consistent with the results of SEM and TEM. Consequently, the narrow band gap facilitated the photoreduction performance. The separation and transfer of charge carries was an important step during the photocatalytic process. Photoelectrochemical tests were carried out to evaluate the charge transfer efficiency of the obtained
Fig. 8. Diagram for energy band levels of BFW@IS-1 composite and the possible charge separation process.
0.5 (Fig. S2e), the single BFW were covered with numerous petals and its surface presented large aggregated particles. Different from the BFW@IS-2 and [email protected], BFW@IS-1 composite exhibited loose and uniform stacking petals, which contributed to multiple scattering of the light, increasing light absorption and light utilization rate. And an appropriate proportion of BFW and IS caused a tight interfacial interaction and optimal synergistic effect to facilitate the migration and separation of charge carriers, which would improve the photocatalytic efficiency. The BFW octahedra wrapped by hierarchical IS structure were further analyzed using TEM techniques. Fig. 4a and b show the TEM images of as-prepared BFW@IS-1, demonstrating that the BFW was wrapped by wrinkled IS architectures to form uniform core-shell hierarchical nano-octahedra, which was well agreement with the results of SEM. The high-angle annular dark-field scanning TEM (HAADF-STEM) 160
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Province (BY2016004-09), Jiangsu Province Scientific and Technological Achievements into a Special Fund Project (BA2015062, BA2016055 and BA2017095), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions, Industry-Academia Cooperation Project of Datang Pro-environment (DNEPT_CZ_179_16). A Project by the Priority Academic Program Development of Jiangsu Higher Education Institutions, Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_0446).
photocatalysts. The transient photocurrent response displayed in Fig. 6a exhibited that pure IS and BFW showed the smaller photocurrent response. While, all BFW@IS-x composites showed enhanced photocurrent density due to the formation of binary core-shell structure, demonstrating more effective separation of photo-induced electron and hole [27]. Among the BFW@IS-x composites, the BFW@IS-1 composite exhibited the highest photocurrent intensity, deducing the most effective separation and transfer of charge carries owing to uniform and tight core-shell heterojunctions. Electrochemical impedance spectroscopy (EIS) were carried out further to certify the high transfer efficiency of photogenerated charge carries (Fig. 6b). As is known, a smaller arc radius usually represents a lower electron transfer resistance. It can be clearly observed from EIS Nyquist plots that the arc radius of the BFW@IS-1 heterojunction was the smallest compared with other samples, demonstrating a rapid transfer of electrons. The results of transient photocurrent response and EIS were in accordance with their photocatalytic CO and CH4 production rates, it suggested that the optimal heterojunctions contributed to the interfacial separation and transfer efficiency of photogenerated charge carriers in BFW@IS-1 composite.
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3.4. Photocatalytic mechanism The reduction reactions are closely associated with the location of conduction band (CB), which could be further evaluated by MottSchottky (MS) measurements by using the impedance-potential method [28,29]. The Mott-Schottky plots of the pure IS and BFW shown in Fig. 7 displayed positive slopes, respectively, suggesting they were ntype semiconductors. On the basis of the intercept of the plots on the X axis, the values of the flat band potentials (Vfb) of BFW and IS were estimated to be about −0.22 and −0.60 V vs Ag/AgCl (equivalent to -0.02 and -0.40 eV vs. NHE), respectively. As the Vfb was more 0.2 V positive than CB potential (ECB) for a n-type semiconductor, the CB of BFW and IS of were -0.22 and -0.60 V vs NHE, respectively. The EVB was determined through using Eq. (1) EVB = ECB + Eg
(1)
the EVB of the BFW and IS were 2.00 and 1.59 eV vs. NHE, respectively. According to above values, the energy band positions of the BFW and IS were drawn in Fig. 8. Both the VB and CB of BFW are more negative than that of IS, photogenerated electrons and holes would migrate to BFW and IS, respectively, which achieved spatially effective separation of photogenerated charge carriers, resulting in facilitating the photoreduction efficiency. 4. Conclusions Hierarchical BFW@IS-x nanotoctahedral heterojunctions, as superior photocatalysts for CO2 reduction were synthesized through a facile in-situ hydrothermal method. The BFW@IS-x composites exhibited remarkable photocatalytic activity of CO2 reduction under visible light irradiation, especially BFW@IS-1. The enhanced photocatalytic performance for deoxygenative CO2 reduction of BFW@IS-1 was ascribed mainly to the increased the light-harvest,and the formation of heterojunctions, which promoted effective separation and migration of photoinduced electrons and hole. This work may contribution to developing the novel core-shell heterostructured photocatalysts for the applications of environmental remediation. Acknowledgements This work was financially supported by the Key Project of Chinese National Programs for Research and Development (2016YFC0203800), the National Natural Science Foundation of China (51578288), Industry-Academia Cooperation Innovation Fund Projects of Jiangsu 161
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Construction of octahedral BiFeWOx encapsulated in hierarchical In2S3 core@shell heterostructure for visible-light-driven CO2 reduction ⁎
Yanan Wang, Yiqing Zeng, Shipeng Wan, Shule Zhang , Qin Zhong
T
⁎
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 photoreduction Heterojunction Core-shell structure In2S3 BiFeWOx
Novel 3D BiFeWOx@In2S3 (BFW@IS-x) core shell heterostructures were rationally designed and fabricated to be used as efficient photocatalysts for CO2 photoreduction. The constructive strategy integrated nano-octahedral BFW and hierarchical IS by growing IS ultrathin nanosheet subunits on surface of BFW. The proposed in site synthesis route resulted in a high dispersion of BFW nanocrystals and the tight interaction between BFW and IS. The obtained core-shell heterostructure boosted separation and transfer of photoinduced charge carriers, evidenced by electrochemical measurement. The hierarchical structure on the surface of BFW@IS-x catalyst enhanced light harvest and provided ample active sites for surface redox reactions. Profiting from these extraordinary chemical and physical characteristics owing to the interaction between BFW and IS, the efficiency of photo-conversion of CO2 to CH4 and CO over BFW@IS-1 heterostructures with optimal proportion was 49.9 μmol h−1 g−1 and 28.9 μmol h−1 g−1, respectively, under visible light irradiation. The CH4 generated rate was about 3.5 and 6.4 times higher than that of pure IS and BFW, and CO generated rates was 4.9 times higher than that of pure IS, respectively. The possible mechanism for the CO2 reduction over BFW@IS-1 composite was discussed.
1. Introduction The growing atmospheric concentration of CO2 has been a global environmental issue due to its inevitable effect to climate change and global warming [1,2]. Inspired by natural photosynthesis, artificial photosynthesis, which are being conducted to fulfill the solar energy conversion of the CO2 into available chemical energy (H2, CO, HCOOH, HCHO, CH3OH, CH4), is considered to be a desired and “kill two birds with one stone” approach” candidate technology to simultaneously address the energy problems of environmental issues [3,4]. In the past few years, significant progresses of CO2 conversion in semiconductor based photocatalysts have been achieved, such as TiO2 and TiO2-based materials [5,6], ZnO [7,8], SrTiO3 [9], Bi2WO6 [10,11], In2S3 [12,13], however, these photocatalysts usually have low CO2 photo-conversion efficiency. Photocatalytic process involves three steps, absorption of light, generation and separation of carriers, redox reactions at the surface. In principle, the rapid separation and transfer of charge carriers and the catalytic reactions of electrons on active sites determine photoconversion efficiency of CO2. Therefore, increasing the carrier separation efficiency is usually considered as an effective way for enhancement of photocatalytic activity, and the integration of two or three semiconductors to form hybrid nanostructures has recently attracted
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much attention to realize the purpose. Among visible light induced photocatalysts, bismuth (Bi)-based photocatalytic materials have obtained attention for owing to unique band structures [14,15]. Thereinto, Bi-based pyrochlores have been reported for owing to a defective structure, which provide the ability to handle electron/hole transfer by adjusting different elements entered into different sites, for instance, vanadate (BiVO4) [16], tungstate (Bi2WO6, BiFeWOx) [17,18], ferrites (BiFeO3) [19], titanate (Bi2Ti2O7) [20] and tellurate (BiFeTeO6) [21]. Among them, BiFeWO6 has attracted attention recently because of its unique structure, nontoxicity, chemical stability and visible light response [18]. However, the single BiFeWOx has been limited by rapid recombination of photogenerated electron-hole and poor adsorption of solar light. It is generally known that the coupling of BiFeWOx with other semiconductors is an ideal method to encourage the separation of photogenerated charge carriers. Some binary BiFeWOx-based composites have been fabricated to improve its photocatalytic efficiency [22,23]. However, related research is still very few, there is plenty of room for the development of BiFeWOxbased composites. Indium sulfides (In2S3) is an intriguing photocatalyst due to a narrow band gap (2.0–2.3 eV) and appropriate band edges for converting photon energy into chemical energy, which make it become an
Corresponding authors. E-mail addresses: [email protected] (S. Zhang), [email protected] (Q. Zhong).
https://doi.org/10.1016/j.jcou.2018.12.009 Received 4 July 2018; Received in revised form 20 November 2018; Accepted 13 December 2018 2212-9820/ © 2018 Published by Elsevier Ltd.
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obtained and denoted as BFW@IS-x, where x is the weight ratio of BFW to the In(NO3)3·xH2O.
excellent candidate to couple with BiFeWOx. In current work, we for the first time presented a simple hydrothermal method to yield hierarchical In2S3 on octahedral BiFeWOx to obtain BiFeWOx@In2S3 (BFW@IS-x) core-shell structure. As expected, compared with single BFW and IS, the BFW@IS-x displayed significantly enhanced photocatalytic performance of CO2 reduction due to facilitating the separation and high transfer efficiency of photoexcited electron-hole pairs.
2.4. Characterization The structure, elemental composition and morphology of the assynthesized samples were analyzed by XRD patterns (a Purkinjie XD-30 system equipped with Cu Kα radiation), the transmission electron microscopy (TEM) (FEI Tecnai G2 20 microscope) and a field-emission scanning electron microscopy (FESEM, FEI Quanta 250 F) attached energy-dispersive spectroscopy (EDS). UV–vis diff ;use reflectance spectra (DRS) were conducted on a Shimadzu UV-2600 spectrometer.
2. Experimental 2.1. Preparation of BFW All the raw materials were purchased from commercial sources and used without further purification. In a typical procedure, 2.5 mmol of Bi (NO3)3·5H2O and Fe(NO3)3·9H2O was dissolved in 25 mL deionized water under magnetic stirring. Secondly, Na2WO4 solution was dissolved and added into above mixture solution. Subsequently, 100 mL certain concentrated NH3·H2O was slowly poured into the above mixture for adjusting the pH = 4. At last, the resultant solution was transferred into a 200 mL Teflon-lined autoclave and maintained at 200 °C for16 h. when it cooled to room temperature naturally, the yellow precipitates were collected and washed several times with deionized water, then dried at 60 °C in air for 12 h.
2.5. Electrochemical measurements Photocurrent measurements were carried out using an electrochemical system (CHI-770, Chenhua, China), wherein the working electrode was irradiated by a 300 W Xe lamp with a 400 nm cut-off filter. A Pt plate and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. The samples were employed as working electrode: 10 mg photocatalyst was dispersed in solution containing 30 μL naphthol, 850 μL deionized water and 150 μL isopropyl alcohol. Then, the suspension was dip-coated on a FTO glass (1 cm × 2 cm) and dried at 180 ℃ for 2 h under a vacuum atmosphere. The cross-sectional view indicates that the BFW@IS-1 film has ∼240 nm thickness (Fig. S1). All measurements were measured in 0.5 M Na2SO4 solution. Prior to and during all measurements, the electrolyte was purged with nitrogen. The electrochemical impedance spectroscopy (EIS) was performed in the standard three-electrode system. EIS measurement was conducted in a frequency range of 1 Hz to 106 Hz for amplitude of 5 mV.
2.2. Preparation of IS Typically, both In(NO3)3·xH2O (mol) and 1,4-benzenedicarboxylic acid (H2BDC) were dissolved in 20 ml of DMF under stirring and followed by heating at 120 °C for 60 min A white precipitate was observed and filtrated and washed with ethanol three times. Subsequently, the obtained precipitates were dissolved in 40 ml ethanol solution containing and 200 mg thiourea was added into the ethanol solution. After stirring for 30 min. Then, the resultant mixture was sealed in a Teflon lined autoclave followed by heating at 180 °C for 6 h. The products were obtained by centrifugation and then dried at 60 °C for 24 h under vacuum
2.6. Photocatalytic performance The photocatalytic activity was evaluated by photocatalytic reduction (PCR) of CO2 at room temperature and negative atmospheric pressure. Photocatalyst (20 mg), 1 mL of triethanolamine (TEOA) and 20 mL H2O were added into a gas-closed glass reactor (80 mL in capacity). Then, the high purity CO2 gas was evacuated and refilled for three times to remove the air inside before the lamp turn on. Finally, the reactor was filled with CO2 at a pressure of 0.8 atm. A 300 W Xe arc lamp (Aulight CEL-HXF300, Beijing) was used as the light source. During the photocatalytic process, the reaction system was vigorously stirred by a magnetic stirrer. The produced gas was detected and quantified by a gas chromatography (Hope GC 9860) with both TCD and FID detectors.
2.3. Preparation of BFW@IS The BFW@IS composite photocatalysts were synthesized through a surface-functionalized hydrothermal method. In detail, a certain amount of BFW (0.05 g, 0.1 g, 0.2 g) 0.1 g of In(NO3)3·xH2O and 0.15 g of H2BDC were mixed in 15 ml of DMF and stirred for 120 min. Then, the resultant solution was placed in an oil bath at 120 °C for 60 min After the reaction, the obtained precipitate was washed with ethanol, then added into 80 ml of an ethanol solution containing 4 g of thiourea. The mixture was transferred into a Teflon-lined autoclave (100 mL in capacity) and maintained at 180 °C for 6 h. In the solvothermal process, the in-situ growth of In2S3 on BFW was achieved. After cooling to room temperature, the BFW@IS composites were
Fig. 1. XRD patterns of (a) IS, (b) BFW, and BFW@IS-x composites. 157
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Fig. 2. (a) The evolution of CH4 and CO over pure IS, BFW and BFW@IS-x photocatalysts under visible-light irradiation, (b) the recyclability of for photoreduction CO2 (evacuation every 12 h).
Fig. 3. SEM images of (a) BFW, (b) BFW@IS-1 and (c1–c6) EDX mappings of the as-synthesized BFW@IS-1.
3. Results and discussion
3.2. Photocatalytic performance
3.1. Crystal structure
The photocatalytic activity of the BFW@IS-x heterostructure was evaluated by photocatalytic CO2 reduction performance under irradiation from a 300 W Xe-lamp. As shown in Fig. 2a, both CO and CH4 can be observed in the reaction system. Among the samples, the pure BFW and IS exhibited the moderate activity for CO2 photoreduction with a CH4 generation rate of 7.8 μmol h−1 g−1 and 14.1 μmol h−1 g−1, and CO generation rate of 0 μmol h−1 g−1 and 5.9 μmol h−1 g−1, respectively. After in-situ growing procedure, the obtained BFW@IS-x heterostructure samples exhibited substantially enhanced CO2 photoreduction performance. Strikingly, the BFW@IS-1 catalysts presented the highest CO evolution rate of 49.9 μmol h−1 g−1, which was about 3.5 times higher than that of IS, 6.4 times higher than that of BFW. In
Fig. 1 shows the XRD patterns of IS, BFW and BFW@IS-x samples. The peaks of the IS (Fig. 1a) matched well with the standard card of cubic In2S3 phase (JCPDS card 32-0456) [24]. A series of BFW@IS-x exhibited similar XRD patterns with slight distinctions in the peak density. As the amount of BFW decreased, the peaks gradually decreased because the surface of BFW was covered by IS. Due to weak peak intensity of IS, slight peaks belonging to In2S3 (marked by diamonds) were observed from the XRD pattern of BFW@IS-x. This result confirmed the coexistence of BFW and IS.
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Fig. 4. (a, b) TEM and (c) HAADF-STEM image (c1–c5) and elemental mapping of Bi, W, Fe, In and S.
heterojunctions facilitating the efficient separation and transfer of charge carriers; The CO2 photoreduction performance over BFW@IS-1 was carried out for 5 cycles to evaluate the stability. As shown in Fig. 2b, no significant loss in the CO and CH4 evolution rate was observed, reflecting the high stability of the BFW@IS-1catalyst. 3.3. Morphology, optical property and the separation and transfer of charge carriers Morphology and composition of samples were investigated by SEM, TEM, and EDS. A perfect octahedral nanostructure of pure BFW was obtained. The SEM image of pure IS sample is presented in Fig. S2a. it showed a hierarchical structure, and every wrinkling or rolling nanoflakes was cross-linked to form vast three-dimensional (3D) network channels (Fig. S2b). The HRTEM image (Fig. S2c) clearly reveals the interplanar distances was found to be 0.32 nm, corresponding to the planes (109) in cubic In2S3. As to BFW@IS-1 (Fig. 3b), it can be observed that nano-octahedral BFW was encapsulated in a large number of hierarchical nanosheets, forming a fluffy flower-like structure on the surface of the BFW, confirming the formation of BFW@IS-1 composites. In addition, the component elements and distribution were measured by EDX mapping. As shown in Fig. 3c, the Bi, Fe, W, S and In elements represented by yellow, purple, green, red and pink dots separately were detected. While, as the ration of BFW went up to 2, it can be observed that numerous BFW octahedra were exposed (Fig. S2d). As to BFW@IS-
Fig. 5. UV–vis diffuse reflectance spectra of IS, BFW and BFW@IS-x composites.
addition, the photo-reduction efficiency of CO2 to CO (28.9 μmol h−1 g−1) was 4.9 times of that for the IS. The coupling effect among IS and BFW significantly promoted the CO and CH4 evolution. The enhanced photocatalytic efficiency of BFW@IS-x composite could be attributed to (1) improved the sunlight response region; (2) the formed 159
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Fig. 6. (a) Transient photocurrent responses, (b) EIS Nyquist plots of the BFW, IS, [email protected], BFW@IS-1 and BFW@IS-2.
Fig. 7. Mott-Schottky (MS) plots of (a) BFW and (b) IS photocatalysts.
images (Fig. 4c1–c5) further confirmed the existence of Bi, Fe, W, In and S elements. The corresponding elemental mapping images showed all elements were uniformly dispersed on BFW@IS-1 heterostructure. Meanwhile, it unambiguously revealed the BFW@IS-1 heterostructure with different acreage, in which the regions of Bi, Fe and W elements were smaller than that of In and S elements, representing that Bi, Fe and W elements were distributed in the core and S and In elements were homogeneously dispersed in the shell. The detailed elemental mapping results demonstrated the spatial distribution of the BFW@IS-1 composite, confirming the core-shell structure of the obtained BFW@IS-1 heterostructure. The hierarchical core-shell heterostructure intimately formed by BFW and IS could facilitate the migration of charge carries and provide pathways to harvest light. Moreover, synergistic effects from the complementary effect between BFW and IS could maximize the asset, thereby, enhancing the photocatalytic efficiency [25,26]. In general, the photoreduction processes mainly involves three steps, light harvesting, generation and separation of photogenerated carriers and redox reactions. The UV–vis DRS spectra are measured to analyze the optical property of photocatalysts. As shown in Fig. 5, the pure IS and BFW samples could harvest the light from UV to visible light. The band gaps calculated from the Tauc plots (Fig. S3a, b) were 2.19 and 2.22 eV, respectively. After the hybridization of IS and BFW, a significant red-shift of absorption edge from UV light region to the visible light region could be found, compared with pure IS. It could be deduced the core-shell formation with wrinkled hierarchical surface provided more pathways to harvest light, resulting in a narrow the band gap. That was consistent with the results of SEM and TEM. Consequently, the narrow band gap facilitated the photoreduction performance. The separation and transfer of charge carries was an important step during the photocatalytic process. Photoelectrochemical tests were carried out to evaluate the charge transfer efficiency of the obtained
Fig. 8. Diagram for energy band levels of BFW@IS-1 composite and the possible charge separation process.
0.5 (Fig. S2e), the single BFW were covered with numerous petals and its surface presented large aggregated particles. Different from the BFW@IS-2 and [email protected], BFW@IS-1 composite exhibited loose and uniform stacking petals, which contributed to multiple scattering of the light, increasing light absorption and light utilization rate. And an appropriate proportion of BFW and IS caused a tight interfacial interaction and optimal synergistic effect to facilitate the migration and separation of charge carriers, which would improve the photocatalytic efficiency. The BFW octahedra wrapped by hierarchical IS structure were further analyzed using TEM techniques. Fig. 4a and b show the TEM images of as-prepared BFW@IS-1, demonstrating that the BFW was wrapped by wrinkled IS architectures to form uniform core-shell hierarchical nano-octahedra, which was well agreement with the results of SEM. The high-angle annular dark-field scanning TEM (HAADF-STEM) 160
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Province (BY2016004-09), Jiangsu Province Scientific and Technological Achievements into a Special Fund Project (BA2015062, BA2016055 and BA2017095), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions, Industry-Academia Cooperation Project of Datang Pro-environment (DNEPT_CZ_179_16). A Project by the Priority Academic Program Development of Jiangsu Higher Education Institutions, Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_0446).
photocatalysts. The transient photocurrent response displayed in Fig. 6a exhibited that pure IS and BFW showed the smaller photocurrent response. While, all BFW@IS-x composites showed enhanced photocurrent density due to the formation of binary core-shell structure, demonstrating more effective separation of photo-induced electron and hole [27]. Among the BFW@IS-x composites, the BFW@IS-1 composite exhibited the highest photocurrent intensity, deducing the most effective separation and transfer of charge carries owing to uniform and tight core-shell heterojunctions. Electrochemical impedance spectroscopy (EIS) were carried out further to certify the high transfer efficiency of photogenerated charge carries (Fig. 6b). As is known, a smaller arc radius usually represents a lower electron transfer resistance. It can be clearly observed from EIS Nyquist plots that the arc radius of the BFW@IS-1 heterojunction was the smallest compared with other samples, demonstrating a rapid transfer of electrons. The results of transient photocurrent response and EIS were in accordance with their photocatalytic CO and CH4 production rates, it suggested that the optimal heterojunctions contributed to the interfacial separation and transfer efficiency of photogenerated charge carriers in BFW@IS-1 composite.
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3.4. Photocatalytic mechanism The reduction reactions are closely associated with the location of conduction band (CB), which could be further evaluated by MottSchottky (MS) measurements by using the impedance-potential method [28,29]. The Mott-Schottky plots of the pure IS and BFW shown in Fig. 7 displayed positive slopes, respectively, suggesting they were ntype semiconductors. On the basis of the intercept of the plots on the X axis, the values of the flat band potentials (Vfb) of BFW and IS were estimated to be about −0.22 and −0.60 V vs Ag/AgCl (equivalent to -0.02 and -0.40 eV vs. NHE), respectively. As the Vfb was more 0.2 V positive than CB potential (ECB) for a n-type semiconductor, the CB of BFW and IS of were -0.22 and -0.60 V vs NHE, respectively. The EVB was determined through using Eq. (1) EVB = ECB + Eg
(1)
the EVB of the BFW and IS were 2.00 and 1.59 eV vs. NHE, respectively. According to above values, the energy band positions of the BFW and IS were drawn in Fig. 8. Both the VB and CB of BFW are more negative than that of IS, photogenerated electrons and holes would migrate to BFW and IS, respectively, which achieved spatially effective separation of photogenerated charge carriers, resulting in facilitating the photoreduction efficiency. 4. Conclusions Hierarchical BFW@IS-x nanotoctahedral heterojunctions, as superior photocatalysts for CO2 reduction were synthesized through a facile in-situ hydrothermal method. The BFW@IS-x composites exhibited remarkable photocatalytic activity of CO2 reduction under visible light irradiation, especially BFW@IS-1. The enhanced photocatalytic performance for deoxygenative CO2 reduction of BFW@IS-1 was ascribed mainly to the increased the light-harvest,and the formation of heterojunctions, which promoted effective separation and migration of photoinduced electrons and hole. This work may contribution to developing the novel core-shell heterostructured photocatalysts for the applications of environmental remediation. Acknowledgements This work was financially supported by the Key Project of Chinese National Programs for Research and Development (2016YFC0203800), the National Natural Science Foundation of China (51578288), Industry-Academia Cooperation Innovation Fund Projects of Jiangsu 161
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