Article pubs.acs.org/JPCC
In Situ Co-Crystallization for Fabrication of g‑C3N4/Bi5O7I Heterojunction for Enhanced Visible-Light Photocatalysis Chengyin Liu,† Hongwei Huang,*,† Xin Du,‡ Tierui Zhang,§ Na Tian,† Yuxi Guo,† and Yihe Zhang*,†
J. Phys. Chem. C 2015.119:17156-17165. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/16/18. For personal use only.
†
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China ‡ Research Center for Bioengineering and Sensing Technology, Department of Chemistry & Biological Engineering, University of Science & Technology Beijing, Beijing 100083, P. R. China § Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijng 100190, China S Supporting Information *
ABSTRACT: We developed for the first time an in situ co-crystallization route for fabrication of a heterojunctional photocatalyst g-C3N4/Bi5O7I by adopting melamine and BiOI as coprecursors. This synthetic method enables intimate interfacial interaction with chemical bonding between g-C3N4 and Bi5O7I, which is beneficial for charge transfer at the interface. The photocatalysis properties of g-C3N4/Bi5O7I composites were studied by photodegradation of Rhodamine B (RhB) and phenol and generation of transient photocurrent with illumination of visible-light (λ > 420 nm), The results revealed that the g-C3N4/Bi5O7I composite shows enhanced photocatalytic reactivity compared to the pristine g-C3N4 and Bi5O7I samples. Investigations on the behaviors of charge carriers via electrochemical impedance spectra (EIS) and photoluminescence (PL) spectra suggests that the g-C3N4/Bi5O7I heterojunctional structure constructed of the in situ cothermolysis approach is responsible for the efficient separation and transfer of photogenerated electrons (e−) and holes (h+), thus giving rise to the higher photocatalytic activity. The present work opens a new avenue for manipulation of high-performance semiconductor heterojunction for photocatalytic and photoelectrochemical application.
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INTRODUCTION Visible-light photocatalysts have attracted worldwide attention because of their excellent capacity for environmental purification, energy production, and efficient solar-energy utilization.1−3 However, a single photocatalyst always suffers from the drawback of deficient separation and transfer of charge carriers, which counteracts the high-activity performance of photocatalysts. Many strategies are hence developed to conquer the above problems, such as doping with metal (Fe, Zn, Co)4−6 or nonmetal (N, Br, Cl)7,8 elements, noble metal (Au, Ag)9,10 deposition, fabrication of semiconductor-heterostructure photocatalysts,11 and exploitation of novel photocatalytic materials.12 Among these methods, heterostructure construction has the enormous potential to improve the charge separation and transfer efficiency and meanwhile conquer the drawbacks of the two components. Therefore, many researchers have been attracted to focus on the preparation of composite photocatalysts. In heterostructure, band structures are vitally important to determine the photoabsorption, photogenerated charge separation efficiency, and photooxidation ability of the whole system. Unlike most oxide semiconductors, Bi-based oxides are found possessing unique band structures, in which the valence band (VB) is always hybridized by Bi 6s and O 2p levels.13,14 This band configuration would greatly contribute to © 2015 American Chemical Society
the band gap narrowing and visible-light absorption of Bi-based oxides. Nevertheless, much narrower band gap with less positive VB position may lead to reduced photooxidation ability of holes (h+) generated from VB. Thus, it is of great interest and importance to develop Bi-based photocatalysts with appropriate band gap. Bi5O7I is a newly found visible-light-driven photocatalyst, which was prepared by a hydrothermal method.14 Its dispersed hybridized valence bands with moderate band gap (∼2.87 eV) endow Bi5O7I with efficient photocatalytic activity in the decomposition of rhodamine B (RhB) in water and acetaldehyde (CH3CHO) in air under visible light. However, there still exist some drawbacks in Bi5O7I, such as deficient light absorption, low transfer efficiency of photogenerated charge carriers in the local three-dimensional crystal structure of Bi5O7I, and so forth. Therefore, construction of heterojunctional structure by combining Bi5O7I with other appropriate semiconductors may be a constructive and instructive method to further improve the photocatalytic activity of Bi5O7I. Recently, research on graphite-like carbon nitride (g-C3N4) has been very active because of the charming two-dimensional Received: April 17, 2015 Revised: July 12, 2015 Published: July 13, 2015 17156
DOI: 10.1021/acs.jpcc.5b03707 J. Phys. Chem. C 2015, 119, 17156−17165
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The Journal of Physical Chemistry C Scheme 1. Schematic Illustration of the Preparation of the g-C3N4/Bi5O7I Composite Structure
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EXPERIMENTAL SECTION Preparation of the Photocatalyst. All the chemicals were of analytical grade, and used as received without further purification. BiOI precursor was obtained by using the coprecipitation method. Representatively, 4 mmol of Bi(NO3)3·5H2O was added into 30 mL glycol and then the mixture was placed in an ultrasonic bath to get a uniform solution. Meanwhile, 4 mmol of KI was added to 30 mL deionized water. Then, the solution KI was added to the Bi(NO3)3 glycol solution step by step under magnetic stirring for 2 h at room temperature. The resulting red-colored solid was collected by filtration, washed, and dried at 80 °C in air. The g-C 3N4/Bi5 O7I composites were synthesized by thermolysis of melamine and BiOI precursor. The two materials with melamine weight percentages of 10%, 20%, 30%, and 40% were mixed and ground thoroughly in an agate mortar. Then, they were placed in the crucible and heated in a muffle furnace at 520 °C for 4 h (the rate of heating was set as 3 °C/min). When the furnace was finally cooled to room temperature, a series of g-C3N4/Bi5O7I composite photocatalysts were obtained. According to the melamine content (weight percentages of 10%, 20%, 30%, and 40%), we have renamed the composites 10% g-C3N4/Bi5O7I, 20% g-C3N4/Bi5O7I, 30% g-C3N4/Bi5O7I, and 40% g-C3N4/Bi5O7I, respectively. In addition, pure Bi5 O 7 I and g-C 3N 4 samples were also synthesized under the same conditions. Characterization. X-ray diffraction (XRD) data were collected on a Bruker D8 focus Advance diffractometer (Cu Ka radiation). The scanning electron microscopy (SEM) on a Hitachi S-4800 instrument was employed to observe the morphologies of the photocatalysts. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) by JEM-2100 made in Japan were used to study the microstructure of the as-prepared samples. Fourier-transform infrared (FTIR) spectra were obtained using a Bruker spectrometer in the frequency range of 4000−450 cm−1. A Cary 5000 UV−vis spectrometer made in America Varian was used to get the UV− vis diffuse reflectance spectra (DRS) of the photocatalysts. The Brunauer−Emmett−Teller (BET) specific surface areas of the samples were analyzed by nitrogen adsorption−desorption (Micromeritics ASAP 2460, USA). The photoluminescence (PL) spectra were recorded by a fluorescence spectrophotometer (Hitachi F-4600) made in Japan, equipped with a 150 W Xe lamp at 400 V. Thermal properties of g-C3N4/Bi5O7I composite were examined by differential scanning calorimetric
crystal structure and appropriate band gap of 2.7 eV, which can effectively remove pollutants in air and water15 and photocatalyze H2 evolution from water.16 However, the photocatalytic activity of pure g-C3N4 is restricted by low quantum efficiency owing to its fast charge recombination. Among various modification methods, such as morphology modulation,17 doping with metal or nonmetal species,4−8 heterojunction fabrication,11 and so forth, heterojunction fabrication is the most effective and sustainable to improve the photocatalytic activity of C3N4, which would also not import side effects that other methods produce, like undesirable thermal instability.18−20 In situ crystallization, as an effective synthesis strategy, is always used to construct semiconductor heterojunctions for establishing firm interface interaction. Nevertheless, most of the reported work on C3N4-based semiconductor heterojunctions fabricated by in situ crystallization route involves depositing or mechanical-compositing one material on the surface of C3N4 sheets,21 which could only change the surface property of the latter component without improving that of C3N4. Thus, it is highly desirable to develop an in situ co-crystallization route to synthesize C3N4-based heterojunctional photocatalysts for producing more strong and intimate interfacial interaction. It may drastically promote the separation and transfer of photogenerated charge carriers and further our understanding on the relationship between photocatalytic activity and microstructure/interfacial properties. In this work, we report the construction of the heterostructural photocatalyst g-C3N4/Bi5O7I via an in situ cothermolysis crystallization route by using melamine and BiOI as precusors. The synchronous thermolysis of g-C3N4 and Bi5O7I results in the successful synthesis of g-C3N4/Bi5O7I composites (Scheme 1). Their crystal phase, microstructure and optical properties are investigated by a series of techniques. The gC3N4/Bi5O7I composites exhibit much higher photodegradation activity of RhB and phenol and photocurrent response under visible-light (λ > 420 nm) illumination. This enhancement in photocatalytic and photoelectrochemical properties should be ascribed to the fabrication of g-C3N4/Bi5O7I heterojunction, which significantly promotes the separation and transfer of photogenerated charge carriers. The charge transfer behavior along with the photodegradation mechanism over the g-C3N4/Bi5O7I composites were investigated in detail. 17157
DOI: 10.1021/acs.jpcc.5b03707 J. Phys. Chem. C 2015, 119, 17156−17165
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Figure 1. (a) XRD patterns of BiOI, Bi5O7I, g-C3N4, and g-C3N4/Bi5O7I composites. Crystal structures of (b) g-C3N4, (c) BiOI, and (d) Bi5O7I.
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RESULTS AND DISCUSSION XRD Analysis. The phase purity of g-C3N4, Bi5O7I and the g-C3N4/Bi5O7I composites were studied by XRD. As shown in Figure 1a, the diffraction peaks of samples can be well indexed into the BiOI (JCPDS 10−0445), g-C3N4 (JCPDS 87−1526), and Bi5O7I (JCPDS 40−0548), indicating that the BiOI precursor and as-prepared g-C3N4 and Bi5O7I are all single phase and well crystallized. The pristine g-C3N4 sample exhibits two main diffraction peaks. The strongest peak at 27.4° is assigned to its (002) crystal plane, and the other one found at 13.09° corresponds to (100) diffraction plane of the gC3N4.22,23 By comparing the diffraction patterns of BiOI obtained at room temperature and that calcinated at 520 °C, we can find that BiOI was completely transformed into Bi5O7I at 520 °C. Due to the low crystallinity of g-C3N4 and overlap between the main peaks of Bi5O7I and g-C3N4 at 27.4°, the gC3N4 phase in g-C3N4/Bi5O7I composites may not well be identified by XRD. The successful compositing of g-C3N4/ Bi5O7I is confirmed by the following FTIR, SEM, and TEM analysis. Figure 1b−d displays the crystal structures of g-C3N4, BiOI, and Bi5O7I, respectively. They all exhibit layered configuration. BiOI crystallizes in the space group P4/nmm with cell parameters a = b = 3.99 Å and c = 9.14 Å. The infinite [Bi2O2]2+ slabs stretch along the a−b plane with the I− ions interleaved in the space between anionic layers.24 For Bi5O7I, it possesses an orthorhombic crystal structure with space-group Ibca and cell parameters a = 16.26 Å, b = 5.35 Å, and c = 11.50 Å. In its structure, the [Bi5O7]+ slices stack in an ordered manner with each other along the c-axis to form the threedimensional (3D) crystal structure, and I− ions fill in the tunnels. In contrast to the case of BiOI, the alleviatived layered structure of Bi5O7I may indicate that its crystallization products are more likely 3D crystals instead of 2D flakes.14
(DSC) analysis on the Labsys TG-DTA16 (SETARAM) thermal analyzer (the DSC was calibrated with Al2O3). Photocatalytic Activity Experiment. Photocatalytic decomposition of RhB and phenol in aqueous solutions under visible-light (500 W xenon lamp, λ > 420 nm) illumination was chosen to test the photocatalytic activities of the as-prepared samples. 50 mg of the photocatalyst was ultraphonically suspended in a 50 mL of RhB (0.01 mM) aqueous solution in a quartz tube. Then, the suspensions were magnetically stirred for 30 min in the dark to obtain absorption−desorption equilibrium. Under visible-light illumination, 2.5 mL of liquid was held over a certain period time and centrifuged to obtain the supernatant. The concentrations of RhB and phenol were analyzed by measuring the absorbance of supernatant at 554 and 270 nm on a Cary 5000 UV−vis spectrophotometer. Active Species Trapping Experiment. The active species generated in the photocatalytic reaction, involving superoxide radicals (•O2−), hydroxyl radicals (•OH), and holes (h+), were analyzed in another RhB photodegradation experiment by the addition of 1 mM benzoquinone (BQ) and 1 mM isopropanol (BPA) and 1 mM ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) as scavengers, respectively. Photoelectrochemical Measurement. The electrochemical impedance spectra (EIS) and photocurrent response were measured in a standard three-electrode system, and the electrochemical station (CHI-660B, China) is set at 0.0 V with light intensity of 1 mW/cm2 and Na2SO4 (0.1 M) as the electrolyte solution. Saturated calomel electrodes (SCE) were used as the reference electrode, and platinum wires were used as counter electrode. The working electrodes were g-C3N4, Bi5O7I, and 30% g-C3N4/Bi5O7I films coated on ITO. 17158
DOI: 10.1021/acs.jpcc.5b03707 J. Phys. Chem. C 2015, 119, 17156−17165
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The Journal of Physical Chemistry C Thermal Behavior. Figure 2 displays the TG curves of BiOI. It can be easily observed that two stages of weight loss
chemical bonds with strong interfacial connections are formed between g-C3N4 and Bi5O7I rather than simply physical attachment. These differences of FTIR spectra demonstrated that g-C3N4/Bi5O7I composites with chemical bonding are successfully constructed by our in situ co-thermolysis preparation method, which is conducive to charge transfer. SEM and TEM Analysis. The morphology of the pure gC3N4, Bi5O7I, and the g-C3N4/Bi5O7I composites in varying proportions is investigated by SEM. As shown in Figure 4a, the BiOI presents uniform 3D microsphere structure. After thermal decomposition, the BiOI microspheres transform into Bi5O7I agglomerated nanorods (Figure 4b). The pure g-C3N4 shows microscale sheets with stacked layers and smooth surface, as depicted in Figure 4f. By controlling the relative content of the melamine and BiOI precursor, the as-prepared composites containing g-C3N4 and Bi5O7I exhibit different morphologies and microstructures (Figure 4c−e). With increasing proportion of melamine precursor, we can see that the amount of Bi5O7I nanorods assembled on the surface of g-C3N4 correspondingly decreases, and some Bi5O7I nanorods start to evenly disperse with further elevation of the melamine precursor content (Figure 4d). These results demonstrate that the g-C3N4/Bi5O7I composites were obtained and the microstructures of g-C3N4/ Bi5O7I composites can be tuned, which may have large effect on the photocatalytic activity. Figure 4g shows the schematic illustration of the transformation and fabrication process of the BiOI precursor, Bi5O7I, and g-C3N4/Bi5O7I composites along with g-C3N4. In order to better understand the interfacial interaction between Bi5O7I and g-C3N4, the composites of gC3N4/Bi5O7I, pristine g-C3N4, and Bi5O7I were further analyzed by TEM and HRTEM. Figure 5a,b displays the TEM image of 30% g-C3N4/Bi5O7I composite, in which the closely composited Bi5O7I and g-C3N4 are seen. Figure 5b,c also confirms the nanorod structure of Bi5O7I and the layered structure of g-C3N4, respectively. It further confirms the heterojunctional structure of g-C3N4/Bi5O7I composites. From the fast Fourier transform (FFT) pattern and HRTEM image of Bi5O7I single crystal (Figure 5d,e), we can observe that the lattice fringe with the interval of 0.317 nm corresponds to the (312) lattice plane of pure Bi5O7I, which is consistent with the strongest peak of Bi5O7I in the XRD pattern. UV−vis Diffuse Reflectance Analysis. The optical absorption of all samples is investigated by UV−vis diffuse reflectance spectra (DRS). As shown in Figure 6, Bi5O7I can absorb light with an absorption edge at about 430 nm, consistent with the reported value.20 Comparatively, the pure gC3N4 sample exhibits a longer absorption edge, which is located at about 460 nm. For the g-C3N4/Bi5O7I composites, the optical absorption edge is closely connected to different proportions of g-C3N4 and Bi5O7I. With increasing g-C3N4 content, the absorption edge of g-C3N4/Bi5O7I composites is gradually extended to longer wavelength or elevated in visible light region. This result demonstrates that the photoabsorption of Bi5O7I can be enhanced by g-C3N4. For the semiconductor, the absorption of light is determined by band gap, which can be obtained by the following formula:
Figure 2. TG curve of BiOI.
occurred during the thermolysis process of BiOI. The continuous weight loss in the temperature interval 350−520 °C is about 27%, which corresponds well to the transformation from BiOI to Bi5O7I. With further heating, the weight of Bi5O7I almost remains unchanged until 610 °C, indicating that Bi5O7I can stably exist between 520 and 610 °C. Then, the sample continues to lose weight from 610 to 780 °C with a ratio of 9.16%. It is consistent with the weight loss of total release of I element in Bi5O7I, resulting in the final product Bi2O3. FTIR Analysis. The chemical bonding of the pristine gC 3N 4 , Bi 5 O7 I, and the g-C 3N 4 /Bi5 O 7I composites are investigated by FTIR. As shown in Figure 3, the valent
Figure 3. FTIR spectra of Bi5O7I, g-C3N4, and g-C3N4/Bi5O7I composites.
symmetrical A2u-type vibrations of the Bi−O bond can be observed at 505 cm −1 in Bi 5 O 7 I and g-C 3 N 4 /Bi 5 O 7 I composites.24,25 The peaks at 808 and 1637 cm−1 are assigned to the mode of the triazine units and CN stretching mode, respectively.26 This indicates that the characteristic peaks of gC3N4 exist in all the g-C3N4/Bi5O7I composites. Significantly, two new peaks at 1049 and 1085 cm−1, which are not seen in pristine g-C3N4 and Bi5O7I samples, appear in the g-C3N4/ Bi5O7I composites. Thus, we can conclude that some new
αhν = A(hν − Eg )n /2
(1)
where Eg, α, h, and ν are the band gap, optical absorption coefficient, Plank constant, and photonic frequency, respectively, and A is a proportionality constant.27 In this equation, when the transition type of semiconductor is direct transition, 17159
DOI: 10.1021/acs.jpcc.5b03707 J. Phys. Chem. C 2015, 119, 17156−17165
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Figure 4. SEM images of as-prepared (a) BiOI, (b) Bi5O7I, (c) 20% g-C3N4/Bi5O7I, (d) 30% g-C3N4/Bi5O7I, (e) 40% g-C3N4/Bi5O7I, and (f) gC3N4. (g) Schematic illustration of transformation and fabrication process of the BiOI precursor and Bi5O7I, g-C3N4/Bi5O7I composites along with g-C3N4.
Figure 5. (a) TEM images of 30% g-C3N4/Bi5O7I, (b) C3N4, and (c) Bi5O7I. (d) FFT (fast Fourier transform) pattern and (e) HRTEM image of 30% g-C3N4/Bi5O7I sample.
ECB = E VB − Eg
the n value is 1. Otherwise, the n value is 4 for indirect transitions. The g-C3N4 and Bi5O7I are both ascribed to indirect transitions, so the n value is given as 4.27,28 Based on the plots of (αhν)1/2 versus hv, band gaps (Eg) of g-C3N4 and Bi5O7I are estimated to be 2.70 and 2.87 eV, respectively. The results are consistent with the literature reported by Sun et al.20 Furthermore, the valence band (VB) and conduction band (CB) positions can be obtained based on the following formula:27 E VB = X − Ee + 0.5Eg
(3)
where EVB is the VB potential and ECB is the CB potential. Eg is the band gap, Ee (∼4.5 eV) is the energy of free electrons on the hydrogen scale, and X is the electronegativity of the semiconductor. For g-C3N4, the X is calculated to be 4.67 eV. The ECB and EVB are estimated to be −1.13 and 1.57 eV, respectively. The ECB and EVB values of Bi5O7I are calculated to be 0.28 and 3.15 eV, respectively. Photocatalytic Activities. The photocatalysis activities of the pristine g-C3N4, Bi5O7I, and the g-C3N4/Bi5O7I composites
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DOI: 10.1021/acs.jpcc.5b03707 J. Phys. Chem. C 2015, 119, 17156−17165
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photodegradation in organic pollutants generally adheres to the pseudo-first-order equation.29,30 ln(C0/C) = kappt
(4)
where C is the instantaneous concentration of RhB solution at time t (mol/L), C0 is the initial RhB concentration (mol/L), and kapp is the apparent pseudo-first-order rate constant (h−1). As shown in Figure 7b, the highest apparent rate constant obtained for 30% g-C3N4/Bi5O7I composite is 1.12 h−1, which is 15.3 and 2.9 times higher than that of g-C3N4 and Bi5O7I. The experimental results demonstrated that the photochemical property of Bi5O7I can be improved by combining g-C3N4 to fabricate effective heterojunctional structure, though g-C3N4 displays weaker photocatalytic activity than Bi5O7I. Because of the band potential difference and interfacial interaction between g-C3N4 and Bi5O7I, g-C3N4 herein serves as an excellent platform for promoting the separation and transfer of electron− hole pairs. As shown in Figure 7a and b, the photodegradation efficiency gradually increases with increasing content of g-C3N4, when the content of g-C3N4 is lower than 30%. Nevertheless, excessive g-C3N4 would be detrimental to the photocatalytic activity due to the relatively poor photocatalytic activity of gC3N4 (as seen for 40% g-C3N4/Bi5O7I). Therefore, there exists an optimal content of g-C3N4 where the optimal photocatalytic activity is obtained (30% g-C3N4/Bi5O7I). As shown in Figure 7c, the maximum absorbance shifts from 553 to 529 nm after 4 h visible-light irradiation, because of the appearance of Ndemethylation and de-ethylation in the processes. Figure 7d shows the photodegradation curves of phenol over Bi5O7I, g-
Figure 6. UV−vis diffuse reflectance spectra of Bi5O7I, g-C3N4, and gC3N4/Bi5O7I composites.
were measured by disintegrating RhB and phenol under illumination of visible light (λ > 420 nm). As exhibited in Figure 7a, the direct photolysis of RhB can be neglected based on the result of blank experiment, and g-C3N4 shows weaker photocatalytic activity than Bi5O7I. Almost all the g-C3N4/ Bi5O7I composites with different proportions exhibit higher photocatalytic activity than the pristine g-C3N4 and Bi5O7I, indicating that the charge transfer was improved in g-C3N4/ Bi5O7I composites. The 30% g-C3N4/Bi5O7I composite exhibits the highest photocatalytic activity, with the RhB degradation efficiency of 90% after visible-light irradiation for 2 h. The
Figure 7. (a) Photocatalytic degradation curves of RhB and (b) apparent rate constants for the photodegradation of RhB over Bi5O7I, g-C3N4, and gC3N4/Bi5O7I composites under the irradiation of visible light (λ > 420 nm). (c) UV−visible spectra of RhB at different irradiation time in the presence of 30% g-C3N4/Bi5O7I composite. (d) Photodegradation curves of phenol over Bi5O7I, g-C3N4, and 30% g-C3N4/Bi5O7I composite under visible light (λ > 420 nm). 17161
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The Journal of Physical Chemistry C C3N4 and 30% g-C3N4/Bi5O7I composite. The composite of 30% g-C3N4/Bi5O7I also displays much higher photocatalytic activity than pristine Bi5O7I and g-C3N4. This result excludes the dye sensitization effect and further confirms the enhanced photocatalytic activity of g-C3N4/Bi5O7I composites. In order to illuminate the photocatalytic mechanism of 30% g-C3N4/Bi5O7I composite, the main active species generated during the decomposition process of RhB were inspected by the trapping experiments. As molecular detectors, isopropanol (IPA), benzoquinone (BQ), and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) were applied to quench hydroxyl radicals (•OH), superoxide radicals (•O2−), and holes (h+), respectively.31,32 Figure 8 shows the effect of those
Figure 9. Comparison of transient photocurrent responses of pure Bi5O7I, g-C3N4, and g-C3N4/Bi5O7I composites (10%, 20%, 30%, and 40%) with light on/off cycles under visible light irradiation. (λ > 420 nm, [Na2SO4] = 0.1 M).
lifetime of the injected electrons (τ) of the photocatalysts that determined via the following expression:
τ = 1/2πf
(5)
Herein, f indicates the inverse minimum frequency. According to eq 5, the electron lifetimes are expected to be 34.7 and 39.4 μs for Bi5O7I and 30% g-C3N4/Bi5O7I, respectively. The increased lifetime of injected electrons confirmed the fast transfer and low recombination of charge carriers in 30% gC3N4/Bi5O7I. These results corroborate that the separation and transfer efficiency of photoinduced holes and electrons can be improved by forming the g-C3N4/Bi5O7I heterojuction. Separation efficiency of holes and electrons can also be elucidated by photoluminescence (PL). In the photocatalytic process, the recombination rate of photogenerated charges can be analyzed by comparing the intensity of fluorescence. Low PL intensity is always closely related to low recombination rate and high photocatalytic activity.33,36 The PL excitation (PLE) wavelength determines the PL emission intensity. In order to clearly compare the PL emission intensity of different samples, generally the optimal excitation wavelength is used to obtain the strongest emission intensity. The PLE spectra (Figure S1) of Bi5O7I and g-C3N4/Bi5O7I composites monitored at 420 nm exhibit a very broad spectral region and the highest excitation peaks are located at about 251 nm. So the wavelength of 251 nm is selected to excite the samples. The PL spectra of g-C3N4/ Bi5O7I composites and pristine Bi5O7I excited under 251 nm were displayed in Figure 11. The series of samples exhibit similar curves with emission peak at around 420 nm. From the spectra it can be noted that the emission intensity of pristine Bi5O7I is higher than that of all the g-C3N4/Bi5O7I composites. Among the samples, the 30% g-C3N4/Bi5O7I sample displays the lowest intensity, indicating that it possesses the lowest recombination rate of holes and electrons. This should be closely related to its heterojunction structure. The PL result is also in accordance with observations from above RhB degradation and photoelectrochemical experiments. According to the above results, photocatalytic mechanism involving charge transfer process of g-C3N4/Bi5O7I heterojuction irradiated by visible light is illustrated in Figure 12. Based on the above band structure analysis, g-C3N4 (ECB = −1.13 eV, EVB = 1.57 eV) and Bi5O7I (ECB = 0.28 eV, EVB =
Figure 8. Photocatalytic degradation of RhB over the 30% g-C3N4/ Bi5O7I photocatalyst alone and with the addition of EDTA-2Na, BQ, or IPA.
scavengers on degradation of RhB in photocatalytic process. The decomposition rate was largely suppressed after adding EDTA-2Na and BQ; the inhibition efficiencies are about 98% and 88%, respectively. It reveals that h+ and •O2− play important roles in the photocatalytic reaction. The degradation of RhB has almost no change with addition of IPA, indicating that the •OH has little effect in the photocatalytic process. The transfer and generation of the photoexcited charge carriers are investigated by transient photocurrent responses with or without visible-light illumination. The stronger photocurrent intensity often discloses the higher separation efficiency of holes and electrons.12 Figure 9 illustrates the transient photocurrent response curves of the pristine Bi5O7I, g-C3N4, and g-C3N4/Bi5O7I composites. Obviously, the 30% gC3N4/Bi5O7I composite exhibits the strongest photocurrent response, which is 21.6 and 1.23 times that of the pristine gC3N4 and Bi5O7I in intensity, respectively. It indicates that the more efficient separation efficiency of electrons and holes occurred in g-C3N4/Bi5O7I heterojunction, thus rendering the photocatalytic activity highly enhanced. Charge transfer efficiency at the interface of semiconductor photoelectrode can be indicated by electrochemical impedance spectra (EIS). Figure 10a shows the EIS Nyquist plots of Bi5O7I, g-C3N4, and g-C3N4/Bi5O7I composites. The radius of semicircle of the 30% g-C3N4/Bi5O7I composite is obviously smaller than that of the other samples, manifesting that the transfer resistance of charge carriers on the surface of 30% gC3N4/Bi5O7I is lower.34,35 Figure 10b shows the Bode-phase spectra of the Bi5O7I and 30% g-C3N4/Bi5O7I electrodes. The 17162
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Figure 10. (a) EIS Nyquist plots of Bi5O7I, g-C3N4, and g-C3N4/Bi5O7I composites (10%, 20%, 30%, and 40%). (b) Bode phase of the Bi5O7I and 30% g-C3N4/Bi5O7I electrodes.
negative than that of Bi5O7I, the electrons will easily go to the CB of the Bi5O7I. Thus, the photogenerated charge carriers can be effectively separated and the CB of Bi5O7I can also provide a large platform to reduce the O2 molecules producing the active •O2− radicals, and then induce the RhB degradation. Moreover, the holes of Bi5O7I migrate to the VB of g-C3N4 and react directly with RhB. The photodegradation process via h+ and •O2− matches well with the consequences of the active species experiments. As a result, the g-C3N4/Bi5O7I composites show much stronger ability to separate and transfer photogenerated electrons and holes than the single g-C3N4 and Bi5O7I, responsible for their higher photocatalytic activity.
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CONCLUSION In summary, a facile in situ co-thermolysis synthetic route was developed to prepare the g-C3N4/Bi5O7I heterojunction photocatalyst using melamine and BiOI as precusors. The asobtained g-C3N4/Bi5O7I composite shows much higher activity for photodegradation of RhB and phenol under visible light (λ > 420 nm) than the pristine g-C3N4 and Bi5O7I. Consistent with the photodegradation reactivity, the g-C3N4/Bi5O7I also displays enhanced transient photocurrent response. This enhancement in photocatalytic and photoelectrochemical properties is attributed to heterojunction structure of gC 3 N 4 /Bi 5 O 7 I fabricated by the in situ co-thermolysis preparation approach, which provides strong chemical interaction between the interfaces of two phases, thus facilitating the separation and transfer of photogenerated charge carriers as confirmed by the electrochemical impedance spectra (EIS) and photoluminescence (PL) spectra. This work may give us new hints for in situ co-crystallization for fabrication of semiconductor heterojunction with high performance.
Figure 11. Photoluminescence spectra of the g-C3N4/Bi5O7I composites and pristine Bi5O7I excited by the wavelength of 251 nm.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 12. Schematic diagrams of charge transfer mechanism of gC3N4/Bi5O7I heterojunction under visible light irradiation.
Room temperature PLE spectra. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03707.
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3.15 eV) possess matchable overlapping band energy levels for constructing an effective heterojunction structure, which can enable g-C3N4/Bi5O7I efficient separation and transfer of photoproduced electrons and holes. As shown in Figure 8, the electrons of g-C3N4 can be irradiated to the CB and holes remained in its VB. As the CB potential of g-C3N4 is more
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Corresponding Authors
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[email protected]. Tel: 86-10-82333433. *E-mail:
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DOI: 10.1021/acs.jpcc.5b03707 J. Phys. Chem. C 2015, 119, 17156−17165
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The Journal of Physical Chemistry C Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (Grant No. 51302251), the Fundamental Research Funds for the Central Universities (No. 2652013052 and No. 2652015296).
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DOI: 10.1021/acs.jpcc.5b03707 J. Phys. Chem. C 2015, 119, 17156−17165
