- Email: [email protected]
CuS heterojunctions with enhanced photocatalytic performances and photoconduction
Accepted Manuscript Facile fabrication of g-C3N4/ZnS/CuS heterojunctions with enhanced photocatalytic performances and photoconduction Yanjuan Sun, Jizhou Jiang, Yuan Cao, Yi Liu, Shengli Wu, Jing Zou PII: DOI: Reference:
S0167-577X(17)31587-2 https://doi.org/10.1016/j.matlet.2017.10.111 MLBLUE 23346
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
15 August 2017 15 October 2017 26 October 2017
Please cite this article as: Y. Sun, J. Jiang, Y. Cao, Y. Liu, S. Wu, J. Zou, Facile fabrication of g-C3N4/ZnS/CuS heterojunctions with enhanced photocatalytic performances and photoconduction, Materials Letters (2017), doi: https://doi.org/10.1016/j.matlet.2017.10.111
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile
fabrication
of
g-C3N4/ZnS/CuS
heterojunctions
with
enhanced
photocatalytic performances and photoconduction Yanjuan Suna, Jizhou Jianga,b,*, Yuan Cao a, Yi Liu a, Shengli Wua, Jing Zoua,* a
School of Environmental Ecology and Bioengineering, School of Chemistry and
Environmental Engineering, Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430205, P. R. China. bDepartment of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore. * Author to whom correspondence should be addressed. E-mail: [email protected] (J. Zou); [email protected] (J. Jiang) Abstract The
construction
of
high-quality
graphitic
carbon
nitride
(g-C3N4)-based
heterojunctions remains a grand challenge. Herein, the ternary heterojunctions of g-C3N4/ZnS/CuS has been fabricated via a facile three-step process: the polymerization of melamine, the loading of ZnS nanoparticles (NPs), and further deposition of hexagonal CuS nanosheets. Compared with pure g-C3N4, there is an extended visible-light absorption over 500-800 nm for g-C3N4/ZnS/CuS, which enables the possible utilization of low-energy visible light. Moreover, ZnS NPs interface layers and hexagonal CuS nano-sheets can be both utilized as electron co-catalysts to markedly improve the separation efficiency of the photo-generated electrons and holes and decrease interface transfer resistance, resulting in highly efficient photocatalytic activity for degradation of Rhodamine B (RhB) and significantly enhanced photoconductivity in an all-solid-state device. This work can be helpful for developing other effective hetero-structured photocatalysts and photoelectrical devices.
1
1. Introduction g-C3N4 has garnered considerable attention recently owing to its unique optical, electrical, structural properties, desirable chemical stability, low cost and simple preparation [1]. However, several prominent limitations of the pristine g-C3N4, such as insufficient utilization of visible light, poor electrical conductivity and fast recombination rate of electron-hole pairs [2], remain to be addressed. A great deal of effort has been devoted to elevating their photo- and electro- catalytic activity by forming heterojunctions with other semiconductors, introducing multi-functional interface layers and changing microstructure, such as g-C3N4/rTiO2 [3], g-C3N4/Ni/NiS [4], HT-g-C3N4 [5], g-C3N4/Ag [6], which can promote charge separation, improve the dispersion and stability, increase specific surface area and accelerate surface reaction kinetics. Nevertheless, relatively low separation efficiency of electron-hole pairs, complicated and costly preparation process limited practical applications in the photoand electro- catalytic field. Hence, it is highly required to explore effective methods to enhance the catalytic performance of g-C3N4. Herein, we developed a facile technique to fabricate ternary heterojunctions by combining the processes of fabricating the micro/nano architectures, loading cocatalysts and constructing heterojunctions. The g-C3N4/ZnS/CuS ternary heterojunctions were constructed by a solvothermal method to load ZnS followed by a microwave (MW) assisted precipitation to deposit CuS, which exhibited enhanced photocatalytic activity and photoconductivity. This work would open up an avenue for g-C3N4-based heterojunctions in developing efficient photocatalysts and photoelectrical devices. 2. Experimental procedure
2
g-C3N4 was prepared by polymerization of melamine at 520 °C for 4 h. Thioacetamide (TAA) (4.2 mmol), acetoxyzinc dihydrate (1.4 mmol), diacetone (DA) (4.2 mmol) and g-C3N4 (1.2 g) were dispersed into 10 mL ethanol under ultrasound irradiation (US), respectively. After 10 min, DA solution was dropped into acetoxyzinc solution. TAA and g-C3N4 dispersions solution were successively added into the above mixed solution. After US for 15 min, the mixture was transferred into a stainless Teflon-lined autoclave and heated at 180 °C for 24 h, then cooled naturally to obtain the g-C3N4/ZnS. To prepare the g-C3N4/ZnS/CuS heterojunctions, g-C3N4/ZnS (1.4 g), cupric acetate monohydrate (1.4 mmol) and ethylenediamine (60 mmol) were introduced into 20 mL of TAA (60 mmol) aqueous solution under US. After 15 min, the suspension was transferred into a simultaneous US/MW/UV apparatus (XH-300 UL, China) at 50 oC for 5 min, 105 oC for 30 min with a MW power of 500 W. The resultant product was washed repeatedly with water and ethanol. The samples were characterized by TEM (JSM-2100, Electronics Co., Japan) equipped with an EDX spectrometer, XRD (Bruker D8 Advance TXS, Germany), Raman spectrometer (Thermo Fischer), XPS (VG Multilab 2000) spectrometer, UV-Vis diffuse reflectance spectrophotometer (DRS, Agilent Cary 5000) and fluorescence (FP-6200, Japan). Electrochemical impedance spectroscopy (EIS) and photoresponse were carried out using a CHI-660C electrochemical workstation (China) in a standard three-electrode configuration. The photocatalytic performance was evaluated by the degradation of RhB (C0=10 mg L-1) with a photocatalytic reactor (BL-GHX-V, Bilang). Visible light was provided by a 500 W xenon lamp with a UV-cutoff filter (λ≥420 nm). 3. Results and discussion
3
The TEM image of g-C3N4 (Fig. 1a) clearly illustrates it has a sheet-like morphology with a width of >2 µm. Fig. 1b reveals that the ZnS NPs are monodisperse with an average diameters of ~40 nm. Regular hexagonal nano-sheets with side lengths of ~200 nm are observed in the TEM image of CuS (Fig. 1c). From the TEM image of g-C3N4/ZnS/CuS heterojunctions (Fig. 1d), the obvious superimposed contours and morphological features indicate that ZnS NPs and CuS hexagonal nano-sheets are overlaid on the surface of g-C3N4 micro-sheets, suggesting the successful fabrication of g-C3N4/ZnS/CuS heterojunctions. The selected area electron diffraction pattern (SAED) of g-C3N4/ZnS/CuS heterojunctions exhibits clear ring patterns (Fig. 1e), indicative of the polycrystalline nature of the heterojunctions, which ascribe to the (311) and (220) lattice planes of cubic ZnS (JCPDS card No.: 05-0566) and the (102) lattice plane of hexagonal CuS (JCPDS card No.: 06-0464), respectively. EDX (Fig. 1f) and XPS (Fig. S1) analyses confirm that the g-C3N4/ZnS/CuS heterojunctions are mainly composed of C, N, Cu, S and Zn elements. The SAED, EDX and XPS also imply the successful construction of g-C3N4/ZnS/CuS heterojunctions. Fig. 2a shows XRD patterns of pure g-C3N4, ZnS, CuS and g-C3N4/ZnS/CuS heterojunctions. In the XRD pattern of the ternary heterojunctions, almost all characteristic diffraction peaks of the g-C3N4, cubic ZnS (JCPDS card No.: 05-0566) and hexagonal CuS (JCPDS card No.: 06-0464) are observed, in accordance with the SAED result. The Raman spectra of all samples are shown in Fig. 2b. Several characteristic peaks were observed at 709 and 749 cm-1 for g-C3N4, corresponding to the out of plane deformation vibrations of CN heterocycles [7]. The peaks at 256 and 341 cm-1 correspond to the longitudinal- and transverse- optic modes of ZnS [8]. S-S
4
stretching vibration peak of CuS is observed around 474 cm-1 [9]. The above characteristic peaks all appear in the Raman spectrum of the ternary heterojunctions. These results of XRD and Raman also indicate that we have successfully fabricated the ternary heterojunctions. The photoluminescence (PL) emission spectrum can be used to estimate the separation efficiency of the photo-generated electrons and holes. Fig 2c displays the PL emission spectra of different samples at excitation wavelength of 316 nm. g-C3N4 presents a strong peak at 437 nm, but the presence of ZnS causes a slight decrease of PL peak intensity. After loading CuS, the PL peak intensity further decreases drastically. This implies that there is an interaction among g-C3N4, ZnS and CuS (through N atoms), making the photo-excited electrons from g-C3N4 transfer to ZnS and CuS, thus greatly inhibiting the charge recombination. It also indicates a synergistic effect between ZnS and CuS within the g-C3N4/ZnS/CuS heterojunctions. The lowest PL peak intensity of the g-C3N4/ZnS/CuS heterojunctions indicates that it has the highest separation efficiency. In addition, a strongest visible-light absorption is observed over 500-800 nm for g-C3N4/ZnS/CuS (Fig. S2), which enables the possible utilization of low-energy visible light. Fig. 2d presents the EIS plots of different modified electrodes. Compared with pure g-C3N4, a smaller semicircle is observed for the g-C3N4/ZnS, followed by the g-C3N4/ZnS/CuS heterojunctions, suggesting a synergistic effect between ZnS and CuS within the g-C3N4/ZnS/CuS heterojunctions and the ternary heterojunctions with the lowest interface transfer resistance. This could be ascribed to the formation of the ternary heterogeneous structure, which is more conducive for higher photoelectric transfer efficiency. Photocatalytic activities of different samples are evaluated by the RhB degradation
5
under visible-light irradiation. As shown in Fig. 3a, after 90 min of, RhB removal over pure g-C3N4, ZnS and CuS are all less than 20%. After loading ZnS or CuS, the degradation ratio of binary heterojunctions both increase. The enhanced photocatalytic activity could be partly attributed to the surface modification of g-C3N4 (i.e. the formation of oxygen-containing groups) within solvothermal procedure [5,10]. The main reason may be larger separation efficiency of the photo-generated electrons and holes and smaller interface transfer resistance of binary heterojunctions. Moreover, the g-C3N4/ZnS/CuS heterojunctions show the highest photocatalytic performance with a degradation ratio of ~90%, implying that there is a synergistic effect between ZnS and CuS within the ternary heterjunctions. The degradation rate constant of the g-C3N4/ZnS/CuS heterojunctions is ~2.5-fold and ~10-fold higher than that of g-C3N4/CuS and g-C3N4/ZnS, respectively. These results imply the g-C3N4/ZnS/CuS heterojunctions have the highest photocatalytic activity. Therefore, a possible underlying photocatalytic mechanism is proposed as following: under visible-light irradiation, the electrons could be excited from VB of g-C3N4 to its CB, leaving the holes in its VB. For the ternary heterjunctions, after implantation of ZnS and CuS with a decrease electron-transfer resistance, these photo-excited electrons in the CB of g-C3N4 can rapidly transfer to ZnS layers, followed by spread to CuS, where the multistep charge-transfer utmost inhibits recombination of electron-hole pairs, thus achieving an enhanced catalytic activity. Meanwhile, ZnS and CuS can act as co-catalysts to accept the photo-generated electrons from g-C3N4, resulting in an improvement of photocatalytic activity. Additionally, an increased visible-light absorption over 500-800 nm for g-C3N4/ZnS/CuS is also beneficial to improve the photocatalytic performance.
6
Fig. 3b shows that the photocurrent of the g-C3N4/ZnS/CuS heterojunction photoconductor is profound compared to the dark current, having a rapid photoresponse time of several micro-seconds. The g-C3N4/ZnS/CuS produces the significantly highest photocurrent than binary samples and pure g-C3N4 in the same condition, suggesting the ternary heterojunctions have lowest recombination rare and most efficient charge separation. As a novel carbon-based hybrid material, this feature makes the g-C3N4/ZnS/CuS heterojunctions promising for the applications in solid-state device. 4. Conclusion In summary, we have developed a simple approach for synthesizing the g-C3N4/ZnS/CuS ternary heterojunctions. Due to the unique micro-structure, the heterojunctions could be utilized as an efficient photocatalyst capable of degrading RhB and an excellent photoconductor in an all-solid-state device. Our proposed strategy may open up new avenues for developing visible-light photo/electro-catalytic applications. Acknowledgments: This work is financially supported by National Natural Science Foundation of China (Grant. 21471122). References [1] J. Xu, M. Antonietti, J. Am. Chem. Soc. 139 (2017) 6026. [2] Z. Tong, D. Yang, Z. Li,Y. Nan, F. Ding, Y. Shen, et al., ACS Nano 11 (2017) 1103. [3] Y. Li, K. Lv, W. Ho, F. Dong, X. Wu, Y. Xia, Appl. Catal. B 202 (2017) 611. [4] J. Wen, J. Xie, H. Zhang, A. Zhang, Y. Liu, X. Chen, et al., ACS Appl. Mater. Interfaces 9 (2017) 14031. [5] X. Hao, X. Ji, Q. Zhang, Mater. Lett. 185 (2016) 29. [6] J. Jiang, RSC Adv., 6 (2016) 47368.
7
[7] J. Jiang, L. Ou-yang, L. Zhu, A. Zheng, J. Zou, X. Yi, et al., Carbon 80 (2014) 213. [8] W. G. Nilsen, Phys. Rev. 182 (1969) 838. [9] P. Jin, Z. Yao, M. Zhang, Y. Li, H. Xing, J. Raman Spectrosc. 41 (2010) 222. [10] X. Wu, F. Chen, X. Wang, H. Yu, Appl. Surf. Sci. 427 (2018) 645. Fig. 1 TEM images of g-C3N4 (a), ZnS (b), CuS (c) and g-C3N4/ZnS/CuS (d), SAED pattern (e) and EDX spectrum (f) of g-C3N4/ZnS/CuS, respectively. Fig. 2 XRD patterns (a), Raman spectra (b), PL spectra (c) and EIS plots (d) of different samples, respectively. Fig. 3 Photocatalytic performances (a) and photoresponse (b) of different samples.
1.
2.
3. 8
Highlights ►The ternary heterojunctions of g-C3N4/ZnS/CuS has been fabricated for the first time. ►It can improve the separation efficiency of the photo-generated electrons and holes. ►The heterojunctions showed a decrease interface transfer resistance over pure g-C3 N4. ►The heterojunctions show an efficient photocatalytic activity for degradation of RhB. ►It also exhibited an enhanced photoconductivity in an all-solid-state device.
9
Graphical Abstract
10
S0167-577X(17)31587-2 https://doi.org/10.1016/j.matlet.2017.10.111 MLBLUE 23346
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
15 August 2017 15 October 2017 26 October 2017
Please cite this article as: Y. Sun, J. Jiang, Y. Cao, Y. Liu, S. Wu, J. Zou, Facile fabrication of g-C3N4/ZnS/CuS heterojunctions with enhanced photocatalytic performances and photoconduction, Materials Letters (2017), doi: https://doi.org/10.1016/j.matlet.2017.10.111
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Facile
fabrication
of
g-C3N4/ZnS/CuS
heterojunctions
with
enhanced
photocatalytic performances and photoconduction Yanjuan Suna, Jizhou Jianga,b,*, Yuan Cao a, Yi Liu a, Shengli Wua, Jing Zoua,* a
School of Environmental Ecology and Bioengineering, School of Chemistry and
Environmental Engineering, Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430205, P. R. China. bDepartment of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore. * Author to whom correspondence should be addressed. E-mail: [email protected] (J. Zou); [email protected] (J. Jiang) Abstract The
construction
of
high-quality
graphitic
carbon
nitride
(g-C3N4)-based
heterojunctions remains a grand challenge. Herein, the ternary heterojunctions of g-C3N4/ZnS/CuS has been fabricated via a facile three-step process: the polymerization of melamine, the loading of ZnS nanoparticles (NPs), and further deposition of hexagonal CuS nanosheets. Compared with pure g-C3N4, there is an extended visible-light absorption over 500-800 nm for g-C3N4/ZnS/CuS, which enables the possible utilization of low-energy visible light. Moreover, ZnS NPs interface layers and hexagonal CuS nano-sheets can be both utilized as electron co-catalysts to markedly improve the separation efficiency of the photo-generated electrons and holes and decrease interface transfer resistance, resulting in highly efficient photocatalytic activity for degradation of Rhodamine B (RhB) and significantly enhanced photoconductivity in an all-solid-state device. This work can be helpful for developing other effective hetero-structured photocatalysts and photoelectrical devices.
1
1. Introduction g-C3N4 has garnered considerable attention recently owing to its unique optical, electrical, structural properties, desirable chemical stability, low cost and simple preparation [1]. However, several prominent limitations of the pristine g-C3N4, such as insufficient utilization of visible light, poor electrical conductivity and fast recombination rate of electron-hole pairs [2], remain to be addressed. A great deal of effort has been devoted to elevating their photo- and electro- catalytic activity by forming heterojunctions with other semiconductors, introducing multi-functional interface layers and changing microstructure, such as g-C3N4/rTiO2 [3], g-C3N4/Ni/NiS [4], HT-g-C3N4 [5], g-C3N4/Ag [6], which can promote charge separation, improve the dispersion and stability, increase specific surface area and accelerate surface reaction kinetics. Nevertheless, relatively low separation efficiency of electron-hole pairs, complicated and costly preparation process limited practical applications in the photoand electro- catalytic field. Hence, it is highly required to explore effective methods to enhance the catalytic performance of g-C3N4. Herein, we developed a facile technique to fabricate ternary heterojunctions by combining the processes of fabricating the micro/nano architectures, loading cocatalysts and constructing heterojunctions. The g-C3N4/ZnS/CuS ternary heterojunctions were constructed by a solvothermal method to load ZnS followed by a microwave (MW) assisted precipitation to deposit CuS, which exhibited enhanced photocatalytic activity and photoconductivity. This work would open up an avenue for g-C3N4-based heterojunctions in developing efficient photocatalysts and photoelectrical devices. 2. Experimental procedure
2
g-C3N4 was prepared by polymerization of melamine at 520 °C for 4 h. Thioacetamide (TAA) (4.2 mmol), acetoxyzinc dihydrate (1.4 mmol), diacetone (DA) (4.2 mmol) and g-C3N4 (1.2 g) were dispersed into 10 mL ethanol under ultrasound irradiation (US), respectively. After 10 min, DA solution was dropped into acetoxyzinc solution. TAA and g-C3N4 dispersions solution were successively added into the above mixed solution. After US for 15 min, the mixture was transferred into a stainless Teflon-lined autoclave and heated at 180 °C for 24 h, then cooled naturally to obtain the g-C3N4/ZnS. To prepare the g-C3N4/ZnS/CuS heterojunctions, g-C3N4/ZnS (1.4 g), cupric acetate monohydrate (1.4 mmol) and ethylenediamine (60 mmol) were introduced into 20 mL of TAA (60 mmol) aqueous solution under US. After 15 min, the suspension was transferred into a simultaneous US/MW/UV apparatus (XH-300 UL, China) at 50 oC for 5 min, 105 oC for 30 min with a MW power of 500 W. The resultant product was washed repeatedly with water and ethanol. The samples were characterized by TEM (JSM-2100, Electronics Co., Japan) equipped with an EDX spectrometer, XRD (Bruker D8 Advance TXS, Germany), Raman spectrometer (Thermo Fischer), XPS (VG Multilab 2000) spectrometer, UV-Vis diffuse reflectance spectrophotometer (DRS, Agilent Cary 5000) and fluorescence (FP-6200, Japan). Electrochemical impedance spectroscopy (EIS) and photoresponse were carried out using a CHI-660C electrochemical workstation (China) in a standard three-electrode configuration. The photocatalytic performance was evaluated by the degradation of RhB (C0=10 mg L-1) with a photocatalytic reactor (BL-GHX-V, Bilang). Visible light was provided by a 500 W xenon lamp with a UV-cutoff filter (λ≥420 nm). 3. Results and discussion
3
The TEM image of g-C3N4 (Fig. 1a) clearly illustrates it has a sheet-like morphology with a width of >2 µm. Fig. 1b reveals that the ZnS NPs are monodisperse with an average diameters of ~40 nm. Regular hexagonal nano-sheets with side lengths of ~200 nm are observed in the TEM image of CuS (Fig. 1c). From the TEM image of g-C3N4/ZnS/CuS heterojunctions (Fig. 1d), the obvious superimposed contours and morphological features indicate that ZnS NPs and CuS hexagonal nano-sheets are overlaid on the surface of g-C3N4 micro-sheets, suggesting the successful fabrication of g-C3N4/ZnS/CuS heterojunctions. The selected area electron diffraction pattern (SAED) of g-C3N4/ZnS/CuS heterojunctions exhibits clear ring patterns (Fig. 1e), indicative of the polycrystalline nature of the heterojunctions, which ascribe to the (311) and (220) lattice planes of cubic ZnS (JCPDS card No.: 05-0566) and the (102) lattice plane of hexagonal CuS (JCPDS card No.: 06-0464), respectively. EDX (Fig. 1f) and XPS (Fig. S1) analyses confirm that the g-C3N4/ZnS/CuS heterojunctions are mainly composed of C, N, Cu, S and Zn elements. The SAED, EDX and XPS also imply the successful construction of g-C3N4/ZnS/CuS heterojunctions. Fig. 2a shows XRD patterns of pure g-C3N4, ZnS, CuS and g-C3N4/ZnS/CuS heterojunctions. In the XRD pattern of the ternary heterojunctions, almost all characteristic diffraction peaks of the g-C3N4, cubic ZnS (JCPDS card No.: 05-0566) and hexagonal CuS (JCPDS card No.: 06-0464) are observed, in accordance with the SAED result. The Raman spectra of all samples are shown in Fig. 2b. Several characteristic peaks were observed at 709 and 749 cm-1 for g-C3N4, corresponding to the out of plane deformation vibrations of CN heterocycles [7]. The peaks at 256 and 341 cm-1 correspond to the longitudinal- and transverse- optic modes of ZnS [8]. S-S
4
stretching vibration peak of CuS is observed around 474 cm-1 [9]. The above characteristic peaks all appear in the Raman spectrum of the ternary heterojunctions. These results of XRD and Raman also indicate that we have successfully fabricated the ternary heterojunctions. The photoluminescence (PL) emission spectrum can be used to estimate the separation efficiency of the photo-generated electrons and holes. Fig 2c displays the PL emission spectra of different samples at excitation wavelength of 316 nm. g-C3N4 presents a strong peak at 437 nm, but the presence of ZnS causes a slight decrease of PL peak intensity. After loading CuS, the PL peak intensity further decreases drastically. This implies that there is an interaction among g-C3N4, ZnS and CuS (through N atoms), making the photo-excited electrons from g-C3N4 transfer to ZnS and CuS, thus greatly inhibiting the charge recombination. It also indicates a synergistic effect between ZnS and CuS within the g-C3N4/ZnS/CuS heterojunctions. The lowest PL peak intensity of the g-C3N4/ZnS/CuS heterojunctions indicates that it has the highest separation efficiency. In addition, a strongest visible-light absorption is observed over 500-800 nm for g-C3N4/ZnS/CuS (Fig. S2), which enables the possible utilization of low-energy visible light. Fig. 2d presents the EIS plots of different modified electrodes. Compared with pure g-C3N4, a smaller semicircle is observed for the g-C3N4/ZnS, followed by the g-C3N4/ZnS/CuS heterojunctions, suggesting a synergistic effect between ZnS and CuS within the g-C3N4/ZnS/CuS heterojunctions and the ternary heterojunctions with the lowest interface transfer resistance. This could be ascribed to the formation of the ternary heterogeneous structure, which is more conducive for higher photoelectric transfer efficiency. Photocatalytic activities of different samples are evaluated by the RhB degradation
5
under visible-light irradiation. As shown in Fig. 3a, after 90 min of, RhB removal over pure g-C3N4, ZnS and CuS are all less than 20%. After loading ZnS or CuS, the degradation ratio of binary heterojunctions both increase. The enhanced photocatalytic activity could be partly attributed to the surface modification of g-C3N4 (i.e. the formation of oxygen-containing groups) within solvothermal procedure [5,10]. The main reason may be larger separation efficiency of the photo-generated electrons and holes and smaller interface transfer resistance of binary heterojunctions. Moreover, the g-C3N4/ZnS/CuS heterojunctions show the highest photocatalytic performance with a degradation ratio of ~90%, implying that there is a synergistic effect between ZnS and CuS within the ternary heterjunctions. The degradation rate constant of the g-C3N4/ZnS/CuS heterojunctions is ~2.5-fold and ~10-fold higher than that of g-C3N4/CuS and g-C3N4/ZnS, respectively. These results imply the g-C3N4/ZnS/CuS heterojunctions have the highest photocatalytic activity. Therefore, a possible underlying photocatalytic mechanism is proposed as following: under visible-light irradiation, the electrons could be excited from VB of g-C3N4 to its CB, leaving the holes in its VB. For the ternary heterjunctions, after implantation of ZnS and CuS with a decrease electron-transfer resistance, these photo-excited electrons in the CB of g-C3N4 can rapidly transfer to ZnS layers, followed by spread to CuS, where the multistep charge-transfer utmost inhibits recombination of electron-hole pairs, thus achieving an enhanced catalytic activity. Meanwhile, ZnS and CuS can act as co-catalysts to accept the photo-generated electrons from g-C3N4, resulting in an improvement of photocatalytic activity. Additionally, an increased visible-light absorption over 500-800 nm for g-C3N4/ZnS/CuS is also beneficial to improve the photocatalytic performance.
6
Fig. 3b shows that the photocurrent of the g-C3N4/ZnS/CuS heterojunction photoconductor is profound compared to the dark current, having a rapid photoresponse time of several micro-seconds. The g-C3N4/ZnS/CuS produces the significantly highest photocurrent than binary samples and pure g-C3N4 in the same condition, suggesting the ternary heterojunctions have lowest recombination rare and most efficient charge separation. As a novel carbon-based hybrid material, this feature makes the g-C3N4/ZnS/CuS heterojunctions promising for the applications in solid-state device. 4. Conclusion In summary, we have developed a simple approach for synthesizing the g-C3N4/ZnS/CuS ternary heterojunctions. Due to the unique micro-structure, the heterojunctions could be utilized as an efficient photocatalyst capable of degrading RhB and an excellent photoconductor in an all-solid-state device. Our proposed strategy may open up new avenues for developing visible-light photo/electro-catalytic applications. Acknowledgments: This work is financially supported by National Natural Science Foundation of China (Grant. 21471122). References [1] J. Xu, M. Antonietti, J. Am. Chem. Soc. 139 (2017) 6026. [2] Z. Tong, D. Yang, Z. Li,Y. Nan, F. Ding, Y. Shen, et al., ACS Nano 11 (2017) 1103. [3] Y. Li, K. Lv, W. Ho, F. Dong, X. Wu, Y. Xia, Appl. Catal. B 202 (2017) 611. [4] J. Wen, J. Xie, H. Zhang, A. Zhang, Y. Liu, X. Chen, et al., ACS Appl. Mater. Interfaces 9 (2017) 14031. [5] X. Hao, X. Ji, Q. Zhang, Mater. Lett. 185 (2016) 29. [6] J. Jiang, RSC Adv., 6 (2016) 47368.
7
[7] J. Jiang, L. Ou-yang, L. Zhu, A. Zheng, J. Zou, X. Yi, et al., Carbon 80 (2014) 213. [8] W. G. Nilsen, Phys. Rev. 182 (1969) 838. [9] P. Jin, Z. Yao, M. Zhang, Y. Li, H. Xing, J. Raman Spectrosc. 41 (2010) 222. [10] X. Wu, F. Chen, X. Wang, H. Yu, Appl. Surf. Sci. 427 (2018) 645. Fig. 1 TEM images of g-C3N4 (a), ZnS (b), CuS (c) and g-C3N4/ZnS/CuS (d), SAED pattern (e) and EDX spectrum (f) of g-C3N4/ZnS/CuS, respectively. Fig. 2 XRD patterns (a), Raman spectra (b), PL spectra (c) and EIS plots (d) of different samples, respectively. Fig. 3 Photocatalytic performances (a) and photoresponse (b) of different samples.
1.
2.
3. 8
Highlights ►The ternary heterojunctions of g-C3N4/ZnS/CuS has been fabricated for the first time. ►It can improve the separation efficiency of the photo-generated electrons and holes. ►The heterojunctions showed a decrease interface transfer resistance over pure g-C3 N4. ►The heterojunctions show an efficient photocatalytic activity for degradation of RhB. ►It also exhibited an enhanced photoconductivity in an all-solid-state device.
9
Graphical Abstract
10