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Enhanced photocatalytic performance of g-C3N4 with BiOCl quantum dots modification
Materials Research Bulletin 55 (2014) 212–215
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Short communication
Enhanced photocatalytic performance of g-C3N4 with BiOCl quantum dots modification Chun-zhi Zheng * , Chun-yong Zhang, Guo-hua Zhang, De-jian Zhao, Ya-zhen Wang Jiangsu University of technology, Changzhou 213001, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 January 2014 Received in revised form 14 April 2014 Accepted 18 April 2014 Available online 19 April 2014
Novel BiOCl quantum dots modified g-C3N4 photocatalyst was synthesized by a one-step chemical bath method at low temperature. The photocatalyst was characterized using X-ray diffraction, high-resolution transmission microscopy UV–visible light diffusion reflectance spectrometry, and photoluminescence spectroscopy. The results indicated that BiOCl quantum dots were dispersed on g-C3N4 to form heterojunction structures with high specific surface area. BiOCl-g-C3N4 showed much higher photocatalytic activity than pure g-C3N4 and BiOCl for rhodamine B degradation. The enhanced performance was induced by the high separation efficiency of photoinduced carriers. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Semiconductors Surfaces Nanostructures Structural materials Layered compounds
1. Introduction Semiconductor photocatalysis technique is a “green” method to completely eliminate most kinds of environmental contaminations [1]. The development of visible-light-driven (VLD) photocatalysts has been done by more and more researchers. Recently, graphitelike carbon nitride (g-C3N4) has emerged as a potential photocatalyst with excellent visible-light photocatalytic activity. g-C3N4 is recognized as the most stable allotrope of carbon nitride and have an optical band gap of 2.7 eV [2,3]. It can be synthesized from a simple precursor via a series of polycondensation reactions without any metal involvement [2–5]. Due to its high nitrogen content and facile synthesis procedure, g-C3N4 may provide more active reaction sites than other N-carbon materials. Recently, Wang et al. firstly reported that g-C3N4 can be used for hydrogen or oxygen production from water splitting under visible light irradiation [2]. But, many followed reports showed that its photocatalytic activity is very low. In order to improve its photocatalytic activity, the g-C3N4 composite photocatalysts (TaON-g-C3N4 [6], TiO2-g-C3N4 [7,8], ZnO-g-C3N4 [9,10], Bi2WO6g-C3N4 [11,12], SrTiO3-g-C3N4 [13]) were prepared and used for the photodegradation of azo dyes.
* Corresponding author. Tel.: +86 51986953266; fax: +86 51986953269. E-mail addresses: [email protected] (C.-z. Zheng), [email protected] (C.-y. Zhang), [email protected] (G.-h. Zhang), [email protected] (D.-j. Zhao), [email protected] (Y.-z. Wang). http://dx.doi.org/10.1016/j.materresbull.2014.04.037 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
Bismuth oxyhalides (BiOX) have indirect-transition band-gap and layered structure. The indirect-transition band-gap of BiOX results in that of the excited electron has to travel a certain k-space distance to be emitted to the valence band (VB) which reduces the recombination probability of the excited electron and the hole [14–16]. On the other hand, the layered structure provides a large enough space to polarize the related atoms and orbitals, and then induce the presence of internal static electric fields perpendicular to the [Bi2O2] slabs and halogen anionic slabs in BiOX [14–16]. At the end, effective separation of the photoinduced electron–hole pairs along the [0 0 1] direction can be displayed. So, BiOX display high photocatalytic activity [17–22]. Among the BiOX photocatalysts, BiOCl, with a band gap of 3.2–3.6 eV, has been used primarily as a catalyst for the oxidative cracking of hydrocarbons, as a photoluminescent material, and as a pigment in cosmetics [22]. Recently, many researchers [22–24] have reported that BiOCl is an efficient photocatalyst for the decomposition of organic pollutants under UV light. These results suggest that BiOCl exhibits higher photocatalytic activity than TiO2 under UV-light irradiation, even though its band gap is considerably larger than that of anatase TiO2. The literature contains numerous reports related to BiOCl heterojunction catalysts, such as BiOCl/Bi2O3 [25], BiOCl/NaBiO3 [26] and BiOCl/WO3 [27], which were developed to enable the utilization of BiOCl in the visible-light region. The successful development of these catalysts shows that BiOCl is a very good heterojunction component due to its suitable conduction- and valance- band levels [28]. However, the specific surface area of these heterojunction catalysts is often low and the heterojunction
C.- Zheng et al. / Materials Research Bulletin 55 (2014) 212–215
contact surface is often poor due to the use of inorganic salts as reactants. In this study, because of the suitable conduction and valance band levels of BiOCl and g-C3N4, we prepared the novel BiOCl quantum dots modified g-C3N4(BiOCl-g-C3N4) photocatalyst. The aim was to further improve the photocatalytic activity of BiOCl and g-C3N4. The results showed that the BiOCl-g-C3N4 photocatalyst had a remarkably enhanced RhB photodegradation activity under visible light irradiation. To the best of our knowledge, this is the first report on the fabrication and enhanced photocatalytic activity of g-C3N4 with BiOCl quantum dots modification. 2. Experimental 2.1. Materials Thiourea (Tu, analytical pure) was bought from Tianjin Taixing Chemical Reagent Factory. Bi(NO3)35H2O, terephthalic acid (TA), p-benzoquinone (BQ), isopropanol (IPA), triethanolamine (TEOA), nitro blue tetrazolium (NBT), cetyltrimethylammonium chloride (CTAC) and ethanol were analytical pure and from Sinopharm Chemical Reagent Co., Ltd. Rhodamine B (RhB) was analytical pure and used without further purification. 2.2. Synthesis g-C3N4 was synthesized by annealing Tu at 550 C for 3 h with the heating rate of 5 C min 1 [29–31]. The BiOCl-g-C3N4 was synthesized as follows: 0.12 g Bi(NO3)35H2O were dissolved in 50 mL deionized water (pH 3 by adding HNO3); 0.05 g g-C3N4 was suspended in 50 mL CTAC solution (8 10 3 mol/L). Then Bi(NO3) 35H2O solution were dropped in g-C3N4 suspension. The mixture was stirred and heated in water bath at 80 C for 3 h. After completion of the reaction, the suspension was filtered. The precipitate was washed by absolute ethanol and deionized water for three times and transferred to oven to dry at 80 C for 24 h. The BiOCl-g-C3N4 was defined as BiOCl-g-C3N4-50, and the BiOCl-gC3N4-10, BiOCl-g-C3N4-30 and BiOCl-g-C3N4-70 were synthesized by changing the amount of Bi(NO3)35H2O. For comparison, pure BiOCl were synthesized using the similar processes without adding g-C3N4. 2.3. Characterization X-ray diffraction (XRD) analysis was performed on a Rigaku D/ MAX 2500 X-ray diffractometer equipped with a Cu Ka radiation source. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were collected with a JEOL JEM-2100F electron microscope. UV–vis diffuse reflectance spectra (DRS) were recorded at room temperature between 200 and 800 nm using a Shimadzu UV2550 spectrophotometer with BaSO4 as a reference. The photoluminescence (PL) spectra of the samples were measured using a Hitachi F-4600 fluorescence spectrophotometer to observe the combination rate of electron–hole pairs.
and filtered to remove the photocatalyst. The concentrations of the pollutants were subsequently analyzed using a UV–vis spectrophotometer. The decoloration rate is reported as C/C0, where C is the pollutant’s concentration after adsorption or photocatalysis, and C0 is the pollutant’s initial concentration. The total organic carbon (TOC) content of the samples after degradation (80 min) was measured with a Shimadzu TOC-VWS analyzer. 3. Results and discussion The XRD patterns of BiOCl, g-C3N4 and various BiOCl-g-C3N4-50 samples are shown in Fig. 1. As evident from the results in Fig. 1, the diffraction peaks of BiOCl were indexed as tetragonal-phase BiOCl (cell constants: b = 0.389 nm, c = 0.737 nm; JCPDS No.06-0249). For pure g-C3N4, the strongest XRD peak at 27.5, corresponding to 0.326 nm, can be indexed as (0 0 2) diffraction plane (JCPDS 871526). It is due to the stacking distance of the conjugated aceomatic system, as in graphene oxide. Another pronounced additional peaks found at 13.1, corresponding to a distance d = 0.675 nm. This distance is only slightly below the size of one tris-s-triazine unit (ca. 0.73 nm). When BiOCl coupled with g-C3N4, it can be found that the half band width of BiOCl decrease after BiOCl coupled with g-C3N4. It indicates that the size became small after hybridization. Fig. 2a and b shows the TEM image of g-C3N4. It can be seen the wrinkle two-dimensional structure. On the basis of the theoretic and previously experimental crystal structure of g-C3N4, it can be concluded that the top surface of wrinkle two-dimensional is the (0 0 2) facets of g-C3N4. In Fig. 2c and d, it can be found that the BiOCl quantum dots lie on (0 0 2) facets of g-C3N4 and the sizes of BiOCl quantum dots are very small. On the other hand, the HRTEM images proved the size of BiOCl quantum dots are about 10 nm which in agreement with XRD results. Fig. 3 shows the UV–vis diffuse reflectance spectra of BiOCl, g-C3N4, and BiOCl-g-C3N4-50 heterojunctions in several compositions. As evident in the figure, pure BiOCl does not absorb in the visible-light range, with an absorption edge of approximately 376 nm, whereas the fundamental absorption edge increases at 453 nm in the visible-light range for pure g-C3N4. After facets coupling, the BiOCl-g-C3N4 photocatalyst showed the absorption edge at 446 nm. The photocatalytic activities of as-prepared samples were evaluated by the degradation of RhB under xenon lamp irradiation. Fig. 4a shows the photodegradation of RhB as a function of the irradiation time over BiOCl, g-C3N4, and BiOCl-g-C3N4 samples. As evident in the figure, the RhB selfphotolysis without catalyst under visible light irradiation is not observable, the photodegradation of RhB over g-C3N4 and BiOCl was 20% and 47%, respectively, after
2.4. Photocatalytic reactivity test We chose RhB as an objective pollutant to evaluate the activity of the photocatalysts. Two hundred milliliters of an aqueous solution with RhB concentration of 10 mg/L was mixed with 20 mg of photocatalyst powder in a 500 mL beaker. Prior to the photocatalytic reaction, the suspension was stirred in darkness for 30 min to reach adsorption/desorption equilibrium. The irradiation was performed with a 300 W xenon lamp. At given time intervals, 5 mL of the suspension was sampled, centrifuged
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Fig. 1. XRD patterns of g-C3N4, BiOCl and BiOCl-g-C3N4-50.
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Fig. 2. (a, b) TEM image of g-C3N4; (c, d) TEM image of BiOCl-g-C3N4-50.
60 min irradiation. Coupling of the two semiconductors resulted in an enhancement of the degradation of RhB. For BiOCl-g-C3N4-50, there were about 99% RhB, which was decomposed after 60 min irradiation, and BiOCl-g-C3N4-50 was the best one of all BiOCl-gC3N4 samples. The reason is that efficient heterojunction interface between two components can restrain the recombination of photoinduced charges effectively [32,33]. When excess BiOCl dispersed on the surface of g-C3N4, leading to an agglomerating state, the contact area of heterojunction reduced and catalytic activity would decrease [34,35]. To further confirm the photocatalytic property of BiOCl-g-C3N4, total organic carbon (TOC) degradation was also used to compare with photodecolorization (Fig. 4b). The TOC removal rate of the solution reached 81.1% at 60 min which also higher than that of g-C3N4 and BiOCl, indicating that the as-prepared BiOCl-g-C3N4-50 showed high photocatalytic efficiency in mineralizing organic pollutants. PL emission spectra have been widely used to investigate the efficiency of charge carrier trapping, migration and transfer and to understand the fate of electron–hole pairs in semiconductor particles. PL emission is known to result from the recombination of excited electrons and holes. A lower PL intensity may indicate a lower recombination rate of electron–hole pairs under irradiation [35]. Fig. 5 shows the PL spectra of g-C3N4, BiOCl-g-C3N4-50 heterojunction and mechanically mixed BiOCl with g-C3N4 samples. The main emission peak for pure g-C3N4 is centered at approximately 460 nm, which can be attributed to the band–band PL phenomenon with the energy of light approximately equal to the band gap energy of pure g-C3N4. Pure g-C3N4 exhibited the
Fig. 3. UV–vis diffuse reflectance spectra of BiOCl, g-C3N4, and BiOCl-g-C3N4-50.
highest intensity among these samples, which implies that it has the fastest recombination rate of electrons and holes. After the BiOCl was introduced, the intensity of the PL emission decreased. The positions of the PL emissions of mechanically mixed sample and BiOCl-g-C3N4 heterojunctions are similar to that of pure gC3N4. The emission intensity of BiOCl-g-C3N4 heterojunctions significantly decreased compared with that of the mechanically mixed sample, which indicates that the recombination rate of photogenerated charge carriers is lower in BiOCl-g-C3N4 heterojunctions. The PL results confirm the importance of the heterojunctions in hindering the recombination of electrons and holes
Fig. 4. (a) The PCD percentage of RhB under Xe lamp irradiation; (b) TOC removal at 60 min of RhB photocatalytic degradation.
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Fig. 5. Photoluminescence spectra of BiOCl, g-C3N4, and BiOCl-g-C3N4-50.
and explain the reason of increasing photocatalytic performance of g-C3N4 with BiOCl quantum dot modification. 4. Conclusions In summary, BiOCl quantum dot modified g-C3N4 photocatalyst was synthesized by a one-step chemical bath method at low temperature. XRD, HRTEM, DRS and PL results revealed that the the BiOCl quantum dots lied on (0 0 2) facets of g-C3N4 and the sizes of BiOCl nanoparticles are very small with about 10 nm. The modification result in efficiency separation of charge carrier and enhance the photocatalytic activity for RhB degradation. References [1] C. Chen, W. Ma, J. Zhao, Chemical Society Reviews 39 (2010) 4206–4219. [2] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nature Materials 8 (2009) 76–80. [3] A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J.O. Muller, R. Schloglb, J.M. Carlsson, Journal of Materials Chemisty 18 (2008) 4893–4908. [4] G. Liu, P. Niu, C. Sun, S.C. Smith, Z. Chen, G.Q. Lu, H.M. Cheng, Journal of the American Chemical Society 132 (2010) 11642–11648. [5] X.H. Li, X. Wang, M. Antonietti, Chemical Science 3 (2012) 2170–2174. [6] S.C. Yan, S.B. Lv, Z.S. Li, Z.G. Zou, Dalton Transactions 39 (2010) 1488–1491. [7] H. Yan, H. Yang, Journal of Alloys and Compounds 509 (2011) L26–L29.
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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Short communication
Enhanced photocatalytic performance of g-C3N4 with BiOCl quantum dots modification Chun-zhi Zheng * , Chun-yong Zhang, Guo-hua Zhang, De-jian Zhao, Ya-zhen Wang Jiangsu University of technology, Changzhou 213001, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 January 2014 Received in revised form 14 April 2014 Accepted 18 April 2014 Available online 19 April 2014
Novel BiOCl quantum dots modified g-C3N4 photocatalyst was synthesized by a one-step chemical bath method at low temperature. The photocatalyst was characterized using X-ray diffraction, high-resolution transmission microscopy UV–visible light diffusion reflectance spectrometry, and photoluminescence spectroscopy. The results indicated that BiOCl quantum dots were dispersed on g-C3N4 to form heterojunction structures with high specific surface area. BiOCl-g-C3N4 showed much higher photocatalytic activity than pure g-C3N4 and BiOCl for rhodamine B degradation. The enhanced performance was induced by the high separation efficiency of photoinduced carriers. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Semiconductors Surfaces Nanostructures Structural materials Layered compounds
1. Introduction Semiconductor photocatalysis technique is a “green” method to completely eliminate most kinds of environmental contaminations [1]. The development of visible-light-driven (VLD) photocatalysts has been done by more and more researchers. Recently, graphitelike carbon nitride (g-C3N4) has emerged as a potential photocatalyst with excellent visible-light photocatalytic activity. g-C3N4 is recognized as the most stable allotrope of carbon nitride and have an optical band gap of 2.7 eV [2,3]. It can be synthesized from a simple precursor via a series of polycondensation reactions without any metal involvement [2–5]. Due to its high nitrogen content and facile synthesis procedure, g-C3N4 may provide more active reaction sites than other N-carbon materials. Recently, Wang et al. firstly reported that g-C3N4 can be used for hydrogen or oxygen production from water splitting under visible light irradiation [2]. But, many followed reports showed that its photocatalytic activity is very low. In order to improve its photocatalytic activity, the g-C3N4 composite photocatalysts (TaON-g-C3N4 [6], TiO2-g-C3N4 [7,8], ZnO-g-C3N4 [9,10], Bi2WO6g-C3N4 [11,12], SrTiO3-g-C3N4 [13]) were prepared and used for the photodegradation of azo dyes.
* Corresponding author. Tel.: +86 51986953266; fax: +86 51986953269. E-mail addresses: [email protected] (C.-z. Zheng), [email protected] (C.-y. Zhang), [email protected] (G.-h. Zhang), [email protected] (D.-j. Zhao), [email protected] (Y.-z. Wang). http://dx.doi.org/10.1016/j.materresbull.2014.04.037 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
Bismuth oxyhalides (BiOX) have indirect-transition band-gap and layered structure. The indirect-transition band-gap of BiOX results in that of the excited electron has to travel a certain k-space distance to be emitted to the valence band (VB) which reduces the recombination probability of the excited electron and the hole [14–16]. On the other hand, the layered structure provides a large enough space to polarize the related atoms and orbitals, and then induce the presence of internal static electric fields perpendicular to the [Bi2O2] slabs and halogen anionic slabs in BiOX [14–16]. At the end, effective separation of the photoinduced electron–hole pairs along the [0 0 1] direction can be displayed. So, BiOX display high photocatalytic activity [17–22]. Among the BiOX photocatalysts, BiOCl, with a band gap of 3.2–3.6 eV, has been used primarily as a catalyst for the oxidative cracking of hydrocarbons, as a photoluminescent material, and as a pigment in cosmetics [22]. Recently, many researchers [22–24] have reported that BiOCl is an efficient photocatalyst for the decomposition of organic pollutants under UV light. These results suggest that BiOCl exhibits higher photocatalytic activity than TiO2 under UV-light irradiation, even though its band gap is considerably larger than that of anatase TiO2. The literature contains numerous reports related to BiOCl heterojunction catalysts, such as BiOCl/Bi2O3 [25], BiOCl/NaBiO3 [26] and BiOCl/WO3 [27], which were developed to enable the utilization of BiOCl in the visible-light region. The successful development of these catalysts shows that BiOCl is a very good heterojunction component due to its suitable conduction- and valance- band levels [28]. However, the specific surface area of these heterojunction catalysts is often low and the heterojunction
C.- Zheng et al. / Materials Research Bulletin 55 (2014) 212–215
contact surface is often poor due to the use of inorganic salts as reactants. In this study, because of the suitable conduction and valance band levels of BiOCl and g-C3N4, we prepared the novel BiOCl quantum dots modified g-C3N4(BiOCl-g-C3N4) photocatalyst. The aim was to further improve the photocatalytic activity of BiOCl and g-C3N4. The results showed that the BiOCl-g-C3N4 photocatalyst had a remarkably enhanced RhB photodegradation activity under visible light irradiation. To the best of our knowledge, this is the first report on the fabrication and enhanced photocatalytic activity of g-C3N4 with BiOCl quantum dots modification. 2. Experimental 2.1. Materials Thiourea (Tu, analytical pure) was bought from Tianjin Taixing Chemical Reagent Factory. Bi(NO3)35H2O, terephthalic acid (TA), p-benzoquinone (BQ), isopropanol (IPA), triethanolamine (TEOA), nitro blue tetrazolium (NBT), cetyltrimethylammonium chloride (CTAC) and ethanol were analytical pure and from Sinopharm Chemical Reagent Co., Ltd. Rhodamine B (RhB) was analytical pure and used without further purification. 2.2. Synthesis g-C3N4 was synthesized by annealing Tu at 550 C for 3 h with the heating rate of 5 C min 1 [29–31]. The BiOCl-g-C3N4 was synthesized as follows: 0.12 g Bi(NO3)35H2O were dissolved in 50 mL deionized water (pH 3 by adding HNO3); 0.05 g g-C3N4 was suspended in 50 mL CTAC solution (8 10 3 mol/L). Then Bi(NO3) 35H2O solution were dropped in g-C3N4 suspension. The mixture was stirred and heated in water bath at 80 C for 3 h. After completion of the reaction, the suspension was filtered. The precipitate was washed by absolute ethanol and deionized water for three times and transferred to oven to dry at 80 C for 24 h. The BiOCl-g-C3N4 was defined as BiOCl-g-C3N4-50, and the BiOCl-gC3N4-10, BiOCl-g-C3N4-30 and BiOCl-g-C3N4-70 were synthesized by changing the amount of Bi(NO3)35H2O. For comparison, pure BiOCl were synthesized using the similar processes without adding g-C3N4. 2.3. Characterization X-ray diffraction (XRD) analysis was performed on a Rigaku D/ MAX 2500 X-ray diffractometer equipped with a Cu Ka radiation source. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were collected with a JEOL JEM-2100F electron microscope. UV–vis diffuse reflectance spectra (DRS) were recorded at room temperature between 200 and 800 nm using a Shimadzu UV2550 spectrophotometer with BaSO4 as a reference. The photoluminescence (PL) spectra of the samples were measured using a Hitachi F-4600 fluorescence spectrophotometer to observe the combination rate of electron–hole pairs.
and filtered to remove the photocatalyst. The concentrations of the pollutants were subsequently analyzed using a UV–vis spectrophotometer. The decoloration rate is reported as C/C0, where C is the pollutant’s concentration after adsorption or photocatalysis, and C0 is the pollutant’s initial concentration. The total organic carbon (TOC) content of the samples after degradation (80 min) was measured with a Shimadzu TOC-VWS analyzer. 3. Results and discussion The XRD patterns of BiOCl, g-C3N4 and various BiOCl-g-C3N4-50 samples are shown in Fig. 1. As evident from the results in Fig. 1, the diffraction peaks of BiOCl were indexed as tetragonal-phase BiOCl (cell constants: b = 0.389 nm, c = 0.737 nm; JCPDS No.06-0249). For pure g-C3N4, the strongest XRD peak at 27.5, corresponding to 0.326 nm, can be indexed as (0 0 2) diffraction plane (JCPDS 871526). It is due to the stacking distance of the conjugated aceomatic system, as in graphene oxide. Another pronounced additional peaks found at 13.1, corresponding to a distance d = 0.675 nm. This distance is only slightly below the size of one tris-s-triazine unit (ca. 0.73 nm). When BiOCl coupled with g-C3N4, it can be found that the half band width of BiOCl decrease after BiOCl coupled with g-C3N4. It indicates that the size became small after hybridization. Fig. 2a and b shows the TEM image of g-C3N4. It can be seen the wrinkle two-dimensional structure. On the basis of the theoretic and previously experimental crystal structure of g-C3N4, it can be concluded that the top surface of wrinkle two-dimensional is the (0 0 2) facets of g-C3N4. In Fig. 2c and d, it can be found that the BiOCl quantum dots lie on (0 0 2) facets of g-C3N4 and the sizes of BiOCl quantum dots are very small. On the other hand, the HRTEM images proved the size of BiOCl quantum dots are about 10 nm which in agreement with XRD results. Fig. 3 shows the UV–vis diffuse reflectance spectra of BiOCl, g-C3N4, and BiOCl-g-C3N4-50 heterojunctions in several compositions. As evident in the figure, pure BiOCl does not absorb in the visible-light range, with an absorption edge of approximately 376 nm, whereas the fundamental absorption edge increases at 453 nm in the visible-light range for pure g-C3N4. After facets coupling, the BiOCl-g-C3N4 photocatalyst showed the absorption edge at 446 nm. The photocatalytic activities of as-prepared samples were evaluated by the degradation of RhB under xenon lamp irradiation. Fig. 4a shows the photodegradation of RhB as a function of the irradiation time over BiOCl, g-C3N4, and BiOCl-g-C3N4 samples. As evident in the figure, the RhB selfphotolysis without catalyst under visible light irradiation is not observable, the photodegradation of RhB over g-C3N4 and BiOCl was 20% and 47%, respectively, after
2.4. Photocatalytic reactivity test We chose RhB as an objective pollutant to evaluate the activity of the photocatalysts. Two hundred milliliters of an aqueous solution with RhB concentration of 10 mg/L was mixed with 20 mg of photocatalyst powder in a 500 mL beaker. Prior to the photocatalytic reaction, the suspension was stirred in darkness for 30 min to reach adsorption/desorption equilibrium. The irradiation was performed with a 300 W xenon lamp. At given time intervals, 5 mL of the suspension was sampled, centrifuged
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Fig. 1. XRD patterns of g-C3N4, BiOCl and BiOCl-g-C3N4-50.
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Fig. 2. (a, b) TEM image of g-C3N4; (c, d) TEM image of BiOCl-g-C3N4-50.
60 min irradiation. Coupling of the two semiconductors resulted in an enhancement of the degradation of RhB. For BiOCl-g-C3N4-50, there were about 99% RhB, which was decomposed after 60 min irradiation, and BiOCl-g-C3N4-50 was the best one of all BiOCl-gC3N4 samples. The reason is that efficient heterojunction interface between two components can restrain the recombination of photoinduced charges effectively [32,33]. When excess BiOCl dispersed on the surface of g-C3N4, leading to an agglomerating state, the contact area of heterojunction reduced and catalytic activity would decrease [34,35]. To further confirm the photocatalytic property of BiOCl-g-C3N4, total organic carbon (TOC) degradation was also used to compare with photodecolorization (Fig. 4b). The TOC removal rate of the solution reached 81.1% at 60 min which also higher than that of g-C3N4 and BiOCl, indicating that the as-prepared BiOCl-g-C3N4-50 showed high photocatalytic efficiency in mineralizing organic pollutants. PL emission spectra have been widely used to investigate the efficiency of charge carrier trapping, migration and transfer and to understand the fate of electron–hole pairs in semiconductor particles. PL emission is known to result from the recombination of excited electrons and holes. A lower PL intensity may indicate a lower recombination rate of electron–hole pairs under irradiation [35]. Fig. 5 shows the PL spectra of g-C3N4, BiOCl-g-C3N4-50 heterojunction and mechanically mixed BiOCl with g-C3N4 samples. The main emission peak for pure g-C3N4 is centered at approximately 460 nm, which can be attributed to the band–band PL phenomenon with the energy of light approximately equal to the band gap energy of pure g-C3N4. Pure g-C3N4 exhibited the
Fig. 3. UV–vis diffuse reflectance spectra of BiOCl, g-C3N4, and BiOCl-g-C3N4-50.
highest intensity among these samples, which implies that it has the fastest recombination rate of electrons and holes. After the BiOCl was introduced, the intensity of the PL emission decreased. The positions of the PL emissions of mechanically mixed sample and BiOCl-g-C3N4 heterojunctions are similar to that of pure gC3N4. The emission intensity of BiOCl-g-C3N4 heterojunctions significantly decreased compared with that of the mechanically mixed sample, which indicates that the recombination rate of photogenerated charge carriers is lower in BiOCl-g-C3N4 heterojunctions. The PL results confirm the importance of the heterojunctions in hindering the recombination of electrons and holes
Fig. 4. (a) The PCD percentage of RhB under Xe lamp irradiation; (b) TOC removal at 60 min of RhB photocatalytic degradation.
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Fig. 5. Photoluminescence spectra of BiOCl, g-C3N4, and BiOCl-g-C3N4-50.
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