- Email: [email protected]
visible-light-driven photocatalytic activity
Materials Science in Semiconductor Processing 45 (2016) 51–56
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Preparation of point-line Bi2WO6@TiO2 nanowires composite photocatalysts with enhanced UV/visible-light-driven photocatalytic activity Xiaojun Sun n, Hui Zhang, Jinzhi Wei, Qi Yu, Ping Yang, Fengming Zhang n Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 November 2015 Received in revised form 15 January 2016 Accepted 20 January 2016
Bi2WO6@TiO2 nanowires composite photocatalysts (BWO-TNWS) with point-line structures have been successfully fabricated by hydrothermalsynthesis method, and characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible diffuse reflectance spectra (UV–vis DRS), photoluminescence (PL) and electrochemical impedance spectroscopy (EIS). The effects of coupling narrow-band-gap semiconductor Bi2WO6 (BWO) to photocatalytic activity for degrading Rhodamine B (RhB) and Phenol under UV–vis light irradiation were investigated. The results demonstrate that the photocatalytic activities of the prepared photocatalysts are associated with the content of Bi2WO6 (BWO). 20% BWO-TNWS (containing 20 wt% BWO) composite exhibits the highest degradation rate for RhB and Phenol up to 78% and 33%, respectively. It can be concluded that the improved photocatalytic performance of the BWO-TNWS composite is mainly ascribed to its high photoinduced charge separation rate resulting from the effective heterojunction structure of BWO and TNWS, as well as the enlarged optical response range owing to coupling narrow-band-gap semiconductor BWO. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Composite Nanowire Heterojunction Semiconductor Photocatalyticactivity
1. Introduction Since the discovery of carbon nanotubes in 1991 [1], one-dimensional (1D) nanostructured materials including nanotubes [2], nonorods [3] and nanowires [4] have been attracted significant attention due to their unique structures, excellent properties [5], and wide applications in various aspects such as light-emitting diodes [6], chemical sensors [7], solar cells [8], lithium battery [9] and photocatalysis [10]. Among the various 1D nanostructured materials, one-dimensional TiO2 is the most widely studied photocatalysts due to the high chemical and photoelectron chemical durability [11]. In particular, TNWS are a kind of excellent semiconductor materials with advantages of high electron transfer rate, short electron transport path and large specific surface area, which can greatly reduce the recombination of photoelectrons and holes, offering abundant active sites on the surface of catalyst [12,13]. However, the photocatalytic activity of the naked TiO2 is comparatively low, because TiO2 with the band gap of 3.2 eV, only can response to the ultraviolet light which just accounts for no more than 4% of the solar radiation energy [14]. Therefore, the n
Corresponding authors. E-mail address: [email protected] (F. Zhang).
http://dx.doi.org/10.1016/j.mssp.2016.01.015 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
application of TNWS is also limited to a certain extent. With the aim of obtaining enhanced photocatalytic performance, many studies on different modifications of TiO2 have been performed [15,16]. For the poor performance of metal and nonmetal doping, researchers pay their attention to the modified compound semiconductors which can effectively reduce the recombination rate of photogenerated electron-hole and broaden the range of their spectral response [17–19]. Therefore, the development of more efficient visible-light-active composite photocatalysts based on TNWS is urgent and indispensable. BWO, as the simplest member and the most studied example in Aurivillius family, is an important photosensitizer with a direct band gap of 2.8 eV, also used as visible light-responsive photocatalyst [20–25]. However, its application remains limited because of its high electron-hole recombination rate in photocatalytic process. To resolve this problem, the best way may be the fabrication of heterojunction photocatalysts by coupling of another semiconductor with appropriate band edges [26]. Up to now, extensive studies about coupling BWO with TiO2 have been reported. Huang et al. [23] synthesized anatase TiO2-modified flower-like Bi2WO6 nanostructures by a simple hydrothermal reaction followed by layer-by-layer deposition and calcination. Wang et al. [27] successfully prepared Bi2WO6/TiO2 possessed enhanced visible-light-induced activity in photocatalytic degradation of
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contaminants in aqueous/gaseous phases. Thatt Yang Tan et al. [28] demonstrated the preparation of porous Bi2WO6/TiO2 heterojunction bilayer films on glass substrates using a super hydrophilicity-assisted dip-coating process. Li et al. [29] fabricated a three-dimensional TiO2/Bi2WO6 hierarchical heterostructure by a simple and practical liquid phase method. Attributed to the coupling effects of TiO2, these composites have exhibited enhanced activities in photocatalytic degradation. As a result, photosensitization of TNWS with narrow-band-gap BWO and design of the composite photocatalysts with point-line structures are promising ways to achieve high photocatalytic activity. Inspired by the work above, we report a successful attempt at the fabrication of point-line BWO-TNWS composite photocatalysts via the facile hydrothermal synthesis method. TNWS were selected as the matrixes and soaked in the precursor solution containing Bi3 þ . Thus, the Bi3 þ ions attached to the carrier by immersion. By the introduction of WO24− ions, nano-particles BWO can grow insitu on TNWS. To evaluate the photocatalytic activity of the assynthesized composites, RhB is chosen as representative organic substance. Besides, in order to avoid the photosensitization [30], colorless organic pollutant Phenol is also chosen as model reaction to further illustrate composite catalyst present higher photocatalytic degradation efficiency, compared with un-modifided ones.
2. Experimental method
2.2. Characterization of the catalysts X-ray powder diffraction (XRD) analysis was carried out on a D/Max-rB rotating anode X-ray diffractometer with monochromatized Cu-Kα radiation (λ ¼0.15406 nm) at a setting of 45 kV and 40 mA. The scanning rate was 0.02° (2θ)/s, and the scanning range was 20–75°. The general morphology of the composite photocatalysts was examined using scanning electron microscopy (SEM) on a FEI.SIRION instrument operated at 20 kV. Transmission electron microscope (TEM) was carried out on a JEOL H-7650 microscope at 100 kV. Energy dispersive X-ray spectroscopy (EDS) was used to determine elemental composition of the particles, using an Oxford 7200 IncaPentaFET-x3 Energy Dispersive X-ray Spectrometer. For the TEM observation, the catalysts were dispersed in ethanol by ultrasonic treatment and dropped on the support film with carbon coating copper grids. The UV–vis diffuse reflectance spectra (DRS) of samples were recorded over a UV–vis spectrophotometer (UV-3010) using BaSO4 as reference. Scans range was 200–800 nm. Photoluminescence (PL) spectra of the photocatalysts were detected using a fluorescence spectrophotometer RF-5301PC made in Japan. The electrochemical impedance spectra (EIS) were on the CHI760D by using three-electrode cells. The samples were fabricated into thin film electrodes via the uniformly dispersed by mixing catalyst with methanol solutions for 30 min on the FTO glass substrate. Then the working electrodes were dried at 80 °C for 48 h in air. The frequency range covers from 10000.0 kHz to 0.01 Hz with modulation amplitude of 5 mV.
2.1. Catalysts preparation 2.3. Photocatalytic activity A typical synthesis of point-line 20% BWO-TNWS by the hydrothermal synthesis method is shown in the following way: firstly, Bi(NO3)3 5H2O (0.349 g) was dissolved in EG, to which TNWS (1.0 g) was added under vigorous magnetic stirring at room temperature. After stirring for 2 h, EG was removed by vacuum distillation. Na2WO4 2H2O (0.119 g) dissolved in EG was added into the above product. After vigorous magnetic stirring for 3 h, the suspension was transferred into a 50 mL Teflon-lined autoclave with a stainless steel tank. The autoclave was heated at 160 °C for 10 h. Finally the sample was separated by centrifugation and washed with ethanol and water several times, followed by drying under vacuum at 50 °C to obtain the 20% BWO-TNWS composite. The weight contents of 10%, 15%, 20%, 25% and 30% BWO in the composite photocatalysts were synthesized and expressed as 10% BWO-TNWS, 15% BWO-TNWS, 20% BWO-TNWS, 25% BWO-TNWS and 30% BWO-TNWS, respectively. The detail synthetic route is shown in Scheme 1. For comparison, BWO nano-particles were prepared according to the above-mentioned methods without TNWS. TNWS were prepared according to the literature [31]. TiO2 sol (15 g) was added into NaOH solution (10 mol/L, 60 mL), with magnetic stirring for 2 h. Then the mixture was transferred into a 100 mL Teflon-lined autoclave with a stainless steel tank. The autoclave was heated at 180 °C for 48 h. After washed with HCl (1 mol/L), the sample was then immersed in HCl (1 mol/L) for 12 h. Finally, the sample was dried and calcined after washed with water.
Scheme 1. Synthesis path diagram of BWO-TNWS composite.
RhB and Phenol solution, acting as colored and colorless organic pollutants, respectively, are chosen as representative organic substances to evaluate the photocatalytic activity of the as-prepared BWO-TNWS composite catalyst. The photocatalytic activity of the samples was determined in a photochemical glass reactor equipped with a 500 W high-pressure xenon lamp, which was placed at about 15 cm from the reactor. For the decolorization of RhB, photocatalyst (0.1 g) was added into RhB solution (20 mg/L, 100 mL) and stirred in the dark for 1 h, maintaining the temperature at 2571 °C, in order to make the reactive system uniform and the adsorption equilibrium, then begin to illuminate. About 5 mL of the suspension continually was taken from the reaction cell at given time intervals. The catalyst in the mixture was removed by centrifugation, and the residual dyes in the solution were determined over a UV–vis spectrophotometer at the wavelength of 553 nm. The formula of removal rate is (1 C/C0) 100%¼ (1 A/A0) 100, C0, C, A0, A are concentrations and absorbance values of RhB before and after light irradiation, respectively. For photodegradation of Phenol, similar to that of RhB, the photocatalysts (0.1 g) and Phenol solution (20 mg/L, 80 mL) were used, and the Phenol concentration was measured by the colorimetric method of 4-aminoantipyrine at the wavelength of 505 nm.
3. Results and discussion 3.1. XRD analysis Fig. 1 showed the XRD patterns of the samples. The XRD patterns of sole TNWS and BWO were offered. All the peaks for the sample are readily indexed to the anatase TNWS (JCPDS card no. 89-4921) and mix orthorhombic phase of BWO (JCPDS card no. 731126). No other characteristic peaks were observed, indicting the high purity of the as-prepared samples. Moreover, the narrow and sharp peaks indicate high crystallinity. It is observed that, with
X. Sun et al. / Materials Science in Semiconductor Processing 45 (2016) 51–56
Fig. 1. XRD patterns of as-prepared samples: (a) TNWS, (b) 10% BWO-TNWS, (c) 15% BWO-TNWS, (d) 20% BWO-TNWS, (e) 25% BWO-TNWS, (f) 30% BWO-TNWS and (g) BWO.
ratio increasing of BWO, the characteristic peaks of TNWS gradually weaken, which is similar to that of the reported composit [26]. In order to illustrate the chemical stability of the composite photocatalysts, XRD patterns of as-prepared samples after degradation organic have been provided in Fig. S2. 3.2. SEM and TEM analysis To further discover the microscopic structure information, SEM and TEM analysis of the samples were carried out. From Fig. 2a and b, the typical SEM and TEM images of the pure TNWS sample, it can be seen that it possesses line structure with the diameter of about 80 nm and the length up to few micrometers. The SEM and TEM images of 20% BWO-TNWS, in Fig. 2c and d, respectively, confirm that the composite TNWS retains 1D morphology where BWO nano-particles imbedded into the 1D TiO2 nanowires closely.
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Fig. 3. UV–vis diffuse reflectance spectra of samples.
Energy dispersive X-ray spectroscopy (EDS) confirmed the presence of BWO in the samples (Fig. S1). These results confirm that Bi2WO6@TiO2 nanowires composite photocatalysts (BWO-TNWS) with point-line structures have been successfully fabricated. 3.3. UV–vis analysis The DRS of the as-prepared samples was shown in Fig. 3. Compared with pure TNWS, the absorption of BWO-TNWS within the visible light range are obviously enhanced and red-shifted. For comparison, 20% TNWS and BWO mechanical mixing was detected as shown in Fig. S2. It was generally known that the light absorption characteristics of the semiconductor depended on the energy band structure [32]. The BWO-TNWS composite catalyst exhibit an obvious adsorption in visible light range, which is ascribed to the presence of BWO with a narrow energy band gap and the suitability of the energy gap between TNWS and BWO. Therefore, it can be demonstrated that the surface-modification
Fig. 2. SEM and TEM images of the samples: (a) (b) TNWS, (c) (d) 20% BWO-TNWS.
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3.5. EIS analysis The EIS Nyquist plots of the samples were shown in Fig. 5. The semicircle in the high frequency region can be ascribed to chargetransfer resistance (Rct), showing the charge transfer through the electrode/electrolyte interface [37]. Compared with TiO2 nanoparticles, TNWS have the smaller Nyquist semicircular, which is due to providing a good channel for electronic transmission and accelerating the migration rate of electron. Furthermore, the observed arc radii of TNWS are decreased after coupling with an appropriate amount of BWO, which indicates that its charge transfer resistance becomes somewhat smaller. In short, this is because that the charge carriers behave as massless fermions between TNWS and BWO heterojunction structure, leading to unique transport properties.
Fig. 4. PL spectra of TNWS and 20% BWO-TNWS composite.
with BWO can extend the optical response to the visible range.
3.4. PL analysis PL analysis was applied to reveal the lattice defects and separation of the photoinduced electrons and holes in the semiconductors [33]. The PL spectra of the samples were recorded at room temperature as shown in Fig. 4. TNWS show a wide absorption band between 390 and 600 nm which may be related to the recombination of photogenerated electron hole, along with the luminescence of free-exciton and bound-exciton [34]. The peak at 390 nm is corresponding to the band gap of TiO2. Moreover, the bands at 420 nm and 460 nm represent the free exciton emitting and bound exciton light caused by the oxygen vacancies [35,36]. The peak around 510 nm may be associated with the defects of band levels and electronic transitions related to interface states. The positions of the emission peaks of TNWS after being coupled with BWO remain almost unchanged, suggesting that the interaction between TNWS and BWO is chemical absorption [27]. In addition, the 20% BWO-TNWS composite have a lower peak than that of TNWS, which indicates that the composite improve the separation efficiency of the photo-induced electron-hole pairs. Effective heterojunction prompts electron of conduction band of TNWS transiting to the conduction band of BWO. Furthermore, BWO-TNWS composite effectively suppresses the recombination of photogenerated electrons and holes.
3.6. Photocatalytic performance of BWO-TNWS photocatalysts RhB and colorless organic pollutants Phenol solution were chosen as model pollutants to evaluate photocatalytic activities of the BWO-TNWS composites with different ratios of BWO, as shown in Fig. 6. Obviously, the RhB degradation rates of 10%, 15%, 20%, 25%, 30% BWO-TNWS composite catalysts are 46%, 52%, 78%, 63%, 56%, respectively. 20% BWO-TNWS shows the best degradation efficiency, which is 4-fold higher than that of free TNWS and 3-fold higher than that of BWO under the same condition. In addition, Fig. 7 showed the time-dependent UV–vis absorption spectra of RhB solution in the presence of 20% BWO-TNWS. After the irradiation for 90 min, the maximum absorption peak at λ ¼553 nm disappeared, accompanied by the color of RhB solution changing from fuchsia to light red and then colorless. This means that the chromophoric group of RhB had been destroyed. At the same time, the maximum absorption peak showed a blue-shift, which could be attributed to the step-by-step de-ethylation process [38]. In order to further study the reaction kinetics of photocatalytic degradation of RhB, the kinetics of reaction rate constant for different proportion of composite catalyst were obtained. It was found that the changes of the RhB concentration versus the reaction time over the BWO-TNWS composites followed pseudofirst-order kinetics plot by the equation of – ln(C/C0)¼kt, where k is the rate constant, C0 and C are the RhB concentrations in solution at time 0 and t, respectively. Fig. S3 showed that the largest rate constants of 20% BWO-TNWS composite catalyst 0.013 min 1. For photodegradation of Phenol, the degradation rates of 10%, 15%, 20%, 25%, 30% BWO-TNWS composite catalysts are18%, 24%, 33%, 28%, 25%, respectively. As expected, 20% BWO-TNWS possess much higher photocatalytic activities for degrading Phenol than pure TNWS and BWO. It need to be noticed that the rate constant of 20% BWO-TNWS is 2 times larger about than that of TNWS, and the modification with a proper amount of BWO corresponds to the high photoactivity (Fig. S4). Furthermore, the catalysts all reserve their structures after degradation pollution as verified by XRD in Fig. S5. Accordingly, it is concluded that the enhanced activity is attributed to the improved separation of photogenerated charges. However, when content of BWO is too high, the active sites in the surface of the material from the heterojunction structure became less, reducing the activity of the catalyst. 3.7. Proposed mechanism
Fig. 5. EIS Nyquist plots of BWO, TNP, TNWS and 20% BWO-TNWS electrodes in 0.5 M Na2SO4 aqueous solution.
The photocatalysis in both RhB and Phenol solution proved that the 20% BWO-TNWS presents the enhanced UV–vis photocatalytic activity. Based on the result of photodegradation, a schematic mechanism of the BWO-TNWS is suggested, as shown in Scheme 2. Under the UV–vis light irradiation, both TNWS and BWO are easily excited the photogenerated electron-hole pairs.
X. Sun et al. / Materials Science in Semiconductor Processing 45 (2016) 51–56
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Fig. 6. Photodegradation rates of RhB (A) and Phenol (B) on the BWO-TNWS samples with different ratios of BWO.
recombination, which in accordance with the result of the PL and EIS. Therefore, the photocatalytic activities of BWO-TNWS composite for degrading RhB and Phenol can be greatly enhanced.
4. Conclusions
Fig. 7. Temporal UV–vis absorption spectral changes during the photocatalytic degradation of RhB in aqueous solution in the presence of 20% BWO-TNWS composite.
In our work, the in-situ synthesis method has been successfully developed to fabricate photocatalyst in which BWO attached on the surface of TNWS homogeneously. Among the series of the composites with different content of TNWS, the 20% BWO-TNWS composite showed optimal photocatalytic activities for degradation of RhB and Phenol solution with the highest degradation rate of 78% and 33%, respectively. The significant enhancement of photoactivity can be ascribed to the extent of the optical response range and efficient separation of photogenerated electrons and holes. The present study provides a new strategy to employing 1D nanostructured materials as supports to fabricate point-line composite photocatalysts with promising potential applications.
Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 21501036).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.mssp.2016.01.015.
References
Scheme 2. Photogenerated charge transfer and separation processes in the BWOTNWS composite.
Since the CB level of TNWS is by 0.78 eV higher than that of BWO, while the VB level of BWO is by 0.28 eV lower than that of TNWS, the photogenerated electron of TNWS would transfer to CB of BWO and the holes on the VB of BWO migrate to TNWS [39]. As a result, the charge transfer would suppress the electron-hole pair
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Preparation of point-line Bi2WO6@TiO2 nanowires composite photocatalysts with enhanced UV/visible-light-driven photocatalytic activity Xiaojun Sun n, Hui Zhang, Jinzhi Wei, Qi Yu, Ping Yang, Fengming Zhang n Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 30 November 2015 Received in revised form 15 January 2016 Accepted 20 January 2016
Bi2WO6@TiO2 nanowires composite photocatalysts (BWO-TNWS) with point-line structures have been successfully fabricated by hydrothermalsynthesis method, and characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible diffuse reflectance spectra (UV–vis DRS), photoluminescence (PL) and electrochemical impedance spectroscopy (EIS). The effects of coupling narrow-band-gap semiconductor Bi2WO6 (BWO) to photocatalytic activity for degrading Rhodamine B (RhB) and Phenol under UV–vis light irradiation were investigated. The results demonstrate that the photocatalytic activities of the prepared photocatalysts are associated with the content of Bi2WO6 (BWO). 20% BWO-TNWS (containing 20 wt% BWO) composite exhibits the highest degradation rate for RhB and Phenol up to 78% and 33%, respectively. It can be concluded that the improved photocatalytic performance of the BWO-TNWS composite is mainly ascribed to its high photoinduced charge separation rate resulting from the effective heterojunction structure of BWO and TNWS, as well as the enlarged optical response range owing to coupling narrow-band-gap semiconductor BWO. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Composite Nanowire Heterojunction Semiconductor Photocatalyticactivity
1. Introduction Since the discovery of carbon nanotubes in 1991 [1], one-dimensional (1D) nanostructured materials including nanotubes [2], nonorods [3] and nanowires [4] have been attracted significant attention due to their unique structures, excellent properties [5], and wide applications in various aspects such as light-emitting diodes [6], chemical sensors [7], solar cells [8], lithium battery [9] and photocatalysis [10]. Among the various 1D nanostructured materials, one-dimensional TiO2 is the most widely studied photocatalysts due to the high chemical and photoelectron chemical durability [11]. In particular, TNWS are a kind of excellent semiconductor materials with advantages of high electron transfer rate, short electron transport path and large specific surface area, which can greatly reduce the recombination of photoelectrons and holes, offering abundant active sites on the surface of catalyst [12,13]. However, the photocatalytic activity of the naked TiO2 is comparatively low, because TiO2 with the band gap of 3.2 eV, only can response to the ultraviolet light which just accounts for no more than 4% of the solar radiation energy [14]. Therefore, the n
Corresponding authors. E-mail address: [email protected] (F. Zhang).
http://dx.doi.org/10.1016/j.mssp.2016.01.015 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
application of TNWS is also limited to a certain extent. With the aim of obtaining enhanced photocatalytic performance, many studies on different modifications of TiO2 have been performed [15,16]. For the poor performance of metal and nonmetal doping, researchers pay their attention to the modified compound semiconductors which can effectively reduce the recombination rate of photogenerated electron-hole and broaden the range of their spectral response [17–19]. Therefore, the development of more efficient visible-light-active composite photocatalysts based on TNWS is urgent and indispensable. BWO, as the simplest member and the most studied example in Aurivillius family, is an important photosensitizer with a direct band gap of 2.8 eV, also used as visible light-responsive photocatalyst [20–25]. However, its application remains limited because of its high electron-hole recombination rate in photocatalytic process. To resolve this problem, the best way may be the fabrication of heterojunction photocatalysts by coupling of another semiconductor with appropriate band edges [26]. Up to now, extensive studies about coupling BWO with TiO2 have been reported. Huang et al. [23] synthesized anatase TiO2-modified flower-like Bi2WO6 nanostructures by a simple hydrothermal reaction followed by layer-by-layer deposition and calcination. Wang et al. [27] successfully prepared Bi2WO6/TiO2 possessed enhanced visible-light-induced activity in photocatalytic degradation of
52
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contaminants in aqueous/gaseous phases. Thatt Yang Tan et al. [28] demonstrated the preparation of porous Bi2WO6/TiO2 heterojunction bilayer films on glass substrates using a super hydrophilicity-assisted dip-coating process. Li et al. [29] fabricated a three-dimensional TiO2/Bi2WO6 hierarchical heterostructure by a simple and practical liquid phase method. Attributed to the coupling effects of TiO2, these composites have exhibited enhanced activities in photocatalytic degradation. As a result, photosensitization of TNWS with narrow-band-gap BWO and design of the composite photocatalysts with point-line structures are promising ways to achieve high photocatalytic activity. Inspired by the work above, we report a successful attempt at the fabrication of point-line BWO-TNWS composite photocatalysts via the facile hydrothermal synthesis method. TNWS were selected as the matrixes and soaked in the precursor solution containing Bi3 þ . Thus, the Bi3 þ ions attached to the carrier by immersion. By the introduction of WO24− ions, nano-particles BWO can grow insitu on TNWS. To evaluate the photocatalytic activity of the assynthesized composites, RhB is chosen as representative organic substance. Besides, in order to avoid the photosensitization [30], colorless organic pollutant Phenol is also chosen as model reaction to further illustrate composite catalyst present higher photocatalytic degradation efficiency, compared with un-modifided ones.
2. Experimental method
2.2. Characterization of the catalysts X-ray powder diffraction (XRD) analysis was carried out on a D/Max-rB rotating anode X-ray diffractometer with monochromatized Cu-Kα radiation (λ ¼0.15406 nm) at a setting of 45 kV and 40 mA. The scanning rate was 0.02° (2θ)/s, and the scanning range was 20–75°. The general morphology of the composite photocatalysts was examined using scanning electron microscopy (SEM) on a FEI.SIRION instrument operated at 20 kV. Transmission electron microscope (TEM) was carried out on a JEOL H-7650 microscope at 100 kV. Energy dispersive X-ray spectroscopy (EDS) was used to determine elemental composition of the particles, using an Oxford 7200 IncaPentaFET-x3 Energy Dispersive X-ray Spectrometer. For the TEM observation, the catalysts were dispersed in ethanol by ultrasonic treatment and dropped on the support film with carbon coating copper grids. The UV–vis diffuse reflectance spectra (DRS) of samples were recorded over a UV–vis spectrophotometer (UV-3010) using BaSO4 as reference. Scans range was 200–800 nm. Photoluminescence (PL) spectra of the photocatalysts were detected using a fluorescence spectrophotometer RF-5301PC made in Japan. The electrochemical impedance spectra (EIS) were on the CHI760D by using three-electrode cells. The samples were fabricated into thin film electrodes via the uniformly dispersed by mixing catalyst with methanol solutions for 30 min on the FTO glass substrate. Then the working electrodes were dried at 80 °C for 48 h in air. The frequency range covers from 10000.0 kHz to 0.01 Hz with modulation amplitude of 5 mV.
2.1. Catalysts preparation 2.3. Photocatalytic activity A typical synthesis of point-line 20% BWO-TNWS by the hydrothermal synthesis method is shown in the following way: firstly, Bi(NO3)3 5H2O (0.349 g) was dissolved in EG, to which TNWS (1.0 g) was added under vigorous magnetic stirring at room temperature. After stirring for 2 h, EG was removed by vacuum distillation. Na2WO4 2H2O (0.119 g) dissolved in EG was added into the above product. After vigorous magnetic stirring for 3 h, the suspension was transferred into a 50 mL Teflon-lined autoclave with a stainless steel tank. The autoclave was heated at 160 °C for 10 h. Finally the sample was separated by centrifugation and washed with ethanol and water several times, followed by drying under vacuum at 50 °C to obtain the 20% BWO-TNWS composite. The weight contents of 10%, 15%, 20%, 25% and 30% BWO in the composite photocatalysts were synthesized and expressed as 10% BWO-TNWS, 15% BWO-TNWS, 20% BWO-TNWS, 25% BWO-TNWS and 30% BWO-TNWS, respectively. The detail synthetic route is shown in Scheme 1. For comparison, BWO nano-particles were prepared according to the above-mentioned methods without TNWS. TNWS were prepared according to the literature [31]. TiO2 sol (15 g) was added into NaOH solution (10 mol/L, 60 mL), with magnetic stirring for 2 h. Then the mixture was transferred into a 100 mL Teflon-lined autoclave with a stainless steel tank. The autoclave was heated at 180 °C for 48 h. After washed with HCl (1 mol/L), the sample was then immersed in HCl (1 mol/L) for 12 h. Finally, the sample was dried and calcined after washed with water.
Scheme 1. Synthesis path diagram of BWO-TNWS composite.
RhB and Phenol solution, acting as colored and colorless organic pollutants, respectively, are chosen as representative organic substances to evaluate the photocatalytic activity of the as-prepared BWO-TNWS composite catalyst. The photocatalytic activity of the samples was determined in a photochemical glass reactor equipped with a 500 W high-pressure xenon lamp, which was placed at about 15 cm from the reactor. For the decolorization of RhB, photocatalyst (0.1 g) was added into RhB solution (20 mg/L, 100 mL) and stirred in the dark for 1 h, maintaining the temperature at 2571 °C, in order to make the reactive system uniform and the adsorption equilibrium, then begin to illuminate. About 5 mL of the suspension continually was taken from the reaction cell at given time intervals. The catalyst in the mixture was removed by centrifugation, and the residual dyes in the solution were determined over a UV–vis spectrophotometer at the wavelength of 553 nm. The formula of removal rate is (1 C/C0) 100%¼ (1 A/A0) 100, C0, C, A0, A are concentrations and absorbance values of RhB before and after light irradiation, respectively. For photodegradation of Phenol, similar to that of RhB, the photocatalysts (0.1 g) and Phenol solution (20 mg/L, 80 mL) were used, and the Phenol concentration was measured by the colorimetric method of 4-aminoantipyrine at the wavelength of 505 nm.
3. Results and discussion 3.1. XRD analysis Fig. 1 showed the XRD patterns of the samples. The XRD patterns of sole TNWS and BWO were offered. All the peaks for the sample are readily indexed to the anatase TNWS (JCPDS card no. 89-4921) and mix orthorhombic phase of BWO (JCPDS card no. 731126). No other characteristic peaks were observed, indicting the high purity of the as-prepared samples. Moreover, the narrow and sharp peaks indicate high crystallinity. It is observed that, with
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Fig. 1. XRD patterns of as-prepared samples: (a) TNWS, (b) 10% BWO-TNWS, (c) 15% BWO-TNWS, (d) 20% BWO-TNWS, (e) 25% BWO-TNWS, (f) 30% BWO-TNWS and (g) BWO.
ratio increasing of BWO, the characteristic peaks of TNWS gradually weaken, which is similar to that of the reported composit [26]. In order to illustrate the chemical stability of the composite photocatalysts, XRD patterns of as-prepared samples after degradation organic have been provided in Fig. S2. 3.2. SEM and TEM analysis To further discover the microscopic structure information, SEM and TEM analysis of the samples were carried out. From Fig. 2a and b, the typical SEM and TEM images of the pure TNWS sample, it can be seen that it possesses line structure with the diameter of about 80 nm and the length up to few micrometers. The SEM and TEM images of 20% BWO-TNWS, in Fig. 2c and d, respectively, confirm that the composite TNWS retains 1D morphology where BWO nano-particles imbedded into the 1D TiO2 nanowires closely.
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Fig. 3. UV–vis diffuse reflectance spectra of samples.
Energy dispersive X-ray spectroscopy (EDS) confirmed the presence of BWO in the samples (Fig. S1). These results confirm that Bi2WO6@TiO2 nanowires composite photocatalysts (BWO-TNWS) with point-line structures have been successfully fabricated. 3.3. UV–vis analysis The DRS of the as-prepared samples was shown in Fig. 3. Compared with pure TNWS, the absorption of BWO-TNWS within the visible light range are obviously enhanced and red-shifted. For comparison, 20% TNWS and BWO mechanical mixing was detected as shown in Fig. S2. It was generally known that the light absorption characteristics of the semiconductor depended on the energy band structure [32]. The BWO-TNWS composite catalyst exhibit an obvious adsorption in visible light range, which is ascribed to the presence of BWO with a narrow energy band gap and the suitability of the energy gap between TNWS and BWO. Therefore, it can be demonstrated that the surface-modification
Fig. 2. SEM and TEM images of the samples: (a) (b) TNWS, (c) (d) 20% BWO-TNWS.
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3.5. EIS analysis The EIS Nyquist plots of the samples were shown in Fig. 5. The semicircle in the high frequency region can be ascribed to chargetransfer resistance (Rct), showing the charge transfer through the electrode/electrolyte interface [37]. Compared with TiO2 nanoparticles, TNWS have the smaller Nyquist semicircular, which is due to providing a good channel for electronic transmission and accelerating the migration rate of electron. Furthermore, the observed arc radii of TNWS are decreased after coupling with an appropriate amount of BWO, which indicates that its charge transfer resistance becomes somewhat smaller. In short, this is because that the charge carriers behave as massless fermions between TNWS and BWO heterojunction structure, leading to unique transport properties.
Fig. 4. PL spectra of TNWS and 20% BWO-TNWS composite.
with BWO can extend the optical response to the visible range.
3.4. PL analysis PL analysis was applied to reveal the lattice defects and separation of the photoinduced electrons and holes in the semiconductors [33]. The PL spectra of the samples were recorded at room temperature as shown in Fig. 4. TNWS show a wide absorption band between 390 and 600 nm which may be related to the recombination of photogenerated electron hole, along with the luminescence of free-exciton and bound-exciton [34]. The peak at 390 nm is corresponding to the band gap of TiO2. Moreover, the bands at 420 nm and 460 nm represent the free exciton emitting and bound exciton light caused by the oxygen vacancies [35,36]. The peak around 510 nm may be associated with the defects of band levels and electronic transitions related to interface states. The positions of the emission peaks of TNWS after being coupled with BWO remain almost unchanged, suggesting that the interaction between TNWS and BWO is chemical absorption [27]. In addition, the 20% BWO-TNWS composite have a lower peak than that of TNWS, which indicates that the composite improve the separation efficiency of the photo-induced electron-hole pairs. Effective heterojunction prompts electron of conduction band of TNWS transiting to the conduction band of BWO. Furthermore, BWO-TNWS composite effectively suppresses the recombination of photogenerated electrons and holes.
3.6. Photocatalytic performance of BWO-TNWS photocatalysts RhB and colorless organic pollutants Phenol solution were chosen as model pollutants to evaluate photocatalytic activities of the BWO-TNWS composites with different ratios of BWO, as shown in Fig. 6. Obviously, the RhB degradation rates of 10%, 15%, 20%, 25%, 30% BWO-TNWS composite catalysts are 46%, 52%, 78%, 63%, 56%, respectively. 20% BWO-TNWS shows the best degradation efficiency, which is 4-fold higher than that of free TNWS and 3-fold higher than that of BWO under the same condition. In addition, Fig. 7 showed the time-dependent UV–vis absorption spectra of RhB solution in the presence of 20% BWO-TNWS. After the irradiation for 90 min, the maximum absorption peak at λ ¼553 nm disappeared, accompanied by the color of RhB solution changing from fuchsia to light red and then colorless. This means that the chromophoric group of RhB had been destroyed. At the same time, the maximum absorption peak showed a blue-shift, which could be attributed to the step-by-step de-ethylation process [38]. In order to further study the reaction kinetics of photocatalytic degradation of RhB, the kinetics of reaction rate constant for different proportion of composite catalyst were obtained. It was found that the changes of the RhB concentration versus the reaction time over the BWO-TNWS composites followed pseudofirst-order kinetics plot by the equation of – ln(C/C0)¼kt, where k is the rate constant, C0 and C are the RhB concentrations in solution at time 0 and t, respectively. Fig. S3 showed that the largest rate constants of 20% BWO-TNWS composite catalyst 0.013 min 1. For photodegradation of Phenol, the degradation rates of 10%, 15%, 20%, 25%, 30% BWO-TNWS composite catalysts are18%, 24%, 33%, 28%, 25%, respectively. As expected, 20% BWO-TNWS possess much higher photocatalytic activities for degrading Phenol than pure TNWS and BWO. It need to be noticed that the rate constant of 20% BWO-TNWS is 2 times larger about than that of TNWS, and the modification with a proper amount of BWO corresponds to the high photoactivity (Fig. S4). Furthermore, the catalysts all reserve their structures after degradation pollution as verified by XRD in Fig. S5. Accordingly, it is concluded that the enhanced activity is attributed to the improved separation of photogenerated charges. However, when content of BWO is too high, the active sites in the surface of the material from the heterojunction structure became less, reducing the activity of the catalyst. 3.7. Proposed mechanism
Fig. 5. EIS Nyquist plots of BWO, TNP, TNWS and 20% BWO-TNWS electrodes in 0.5 M Na2SO4 aqueous solution.
The photocatalysis in both RhB and Phenol solution proved that the 20% BWO-TNWS presents the enhanced UV–vis photocatalytic activity. Based on the result of photodegradation, a schematic mechanism of the BWO-TNWS is suggested, as shown in Scheme 2. Under the UV–vis light irradiation, both TNWS and BWO are easily excited the photogenerated electron-hole pairs.
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Fig. 6. Photodegradation rates of RhB (A) and Phenol (B) on the BWO-TNWS samples with different ratios of BWO.
recombination, which in accordance with the result of the PL and EIS. Therefore, the photocatalytic activities of BWO-TNWS composite for degrading RhB and Phenol can be greatly enhanced.
4. Conclusions
Fig. 7. Temporal UV–vis absorption spectral changes during the photocatalytic degradation of RhB in aqueous solution in the presence of 20% BWO-TNWS composite.
In our work, the in-situ synthesis method has been successfully developed to fabricate photocatalyst in which BWO attached on the surface of TNWS homogeneously. Among the series of the composites with different content of TNWS, the 20% BWO-TNWS composite showed optimal photocatalytic activities for degradation of RhB and Phenol solution with the highest degradation rate of 78% and 33%, respectively. The significant enhancement of photoactivity can be ascribed to the extent of the optical response range and efficient separation of photogenerated electrons and holes. The present study provides a new strategy to employing 1D nanostructured materials as supports to fabricate point-line composite photocatalysts with promising potential applications.
Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 21501036).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.mssp.2016.01.015.
References
Scheme 2. Photogenerated charge transfer and separation processes in the BWOTNWS composite.
Since the CB level of TNWS is by 0.78 eV higher than that of BWO, while the VB level of BWO is by 0.28 eV lower than that of TNWS, the photogenerated electron of TNWS would transfer to CB of BWO and the holes on the VB of BWO migrate to TNWS [39]. As a result, the charge transfer would suppress the electron-hole pair
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