- Email: [email protected]
SnS2 ternary heterojunction visible-light photocatalyst with ZnS as electron transport buffer material
Journal of Alloys and Compounds 778 (2019) 215e223
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Band structure engineering design of g-C3N4/ZnS/SnS2 ternary heterojunction visible-light photocatalyst with ZnS as electron transport buffer material Kai Dai a, 1, Jiali Lv b, c, 1, Jinfeng Zhang a, *, Changhao Liang b, **, Guangping Zhu a a
College of Physics and Electronic Information, Anhui Key Laboratory of Energetic Materials, Huaibei Normal University, Huaibei, 235000, PR China Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, 230031, PR China c Department of Materials Science and Engineering, University of Science and Technology of China, 96 Jinzhai Road, Hefei, 230026, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2018 Received in revised form 30 October 2018 Accepted 10 November 2018 Available online 14 November 2018
Semiconductor heterojunction represents a family of promising photocatalysts for visible-light photocatalysis. In this work, a novel ternary g-C3N4/ZnS/SnS2 heterostructure has been designed and synthesized by a facile one-step hydrothermal method. The obtained ternary g-C3N4/ZnS/SnS2 heterostructures exhibited high photocatalytic activity in photodegradation of organic pollutants and photocurrent response irradiated by 410 nm LED light. The results demonstrated that the formation of the heterostructures can much improve the excellent photocatalytic activity if the lattice and energy level matching among the three semiconductors be satisfied, which causes efficient separation of photoinduced carriers, resulting in the high photodegradation of methylene blue (MB). As a result, the highest apparent rate constant Kapp of g-C3N4/ZnS/SnS2 hybrid is 0.148 min1, which is 8.74, 3.22 and 37.01 times as high as that of pristine g-C3N4, SnS2 and ZnS, respectively. © 2018 Elsevier B.V. All rights reserved.
Keywords: C3N4 ZnS SnS2 Photocatalysis Photocatalytic activity Buffer layer
1. Introduction Nanostructured semiconductor with high photocatalytic degradation ability for organic pollutants has been regarded as the most promising nanomaterials for environment protection and clean energy [1e6]. Since nanostructured TiO2 was prepared, it has become the most famous photocatalysis and been widely utilized as photocatalytic hydrogen production and treatment of organic pollutants from liquid solution due to its outstanding photoactivity, stable chemical performance, low cost and nontoxic nature [7e10]. Nevertheless, TiO2 carried out photochemical reactions only in a small ultraviolet (UV) fraction with low quantum yield by reason of the wide band gap (Eg) about 3.2 eV for anatase [11,12]. With the purpose of utilizing solar energy more efficiently, large quantities of visible-light-responsive semiconductors have been desired. Since Wang and his co-workers presented the pioneering work
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (C. Liang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jallcom.2018.11.127 0925-8388/© 2018 Elsevier B.V. All rights reserved.
on exploring catalytic performance of graphitic carbon nitride (gC3N4), which exhibit extremely high photocatalytic capabilities for H2 evolution from water in 2009 [13]. This kind of metal-free semiconductor has got much attention in environment pollutants treatment fields. The element carbon and nitrogen of g-C3N4 via sp2 hydridization leads to the narrow Eg about 2.70 eV and a high chemical stability [14,15]. However, g-C3N4 often exhibits a low catalytic activity because of its narrow absorption and low quantum yield [16,17]. Recently, many studies have indicated that combining g-C3N4 with semiconductor photocatalysis to form heterojunction could extend the photoresponding range and increase the charge separation efficiency to improve the photoactivity [18,19]. For instance, Yu and his fellows showed that g-C3N4/Ag2WO4 exhibited a significantly improved catalytic activity in methyl orange photodegradation [20]. Wu and his co-workers presented that the H2 production activity of g-C3N4 was strengthened by Zn3In2S6 nanoparticles modification [21]. Similarly, Che and his fellows found that the g-C3N4/Bi3O4Cl composites show higher visible light photoactivity for treatment of antibiotic, dye and heavy metal than single g-C3N4 and Bi3O4Cl material [22]. In our previous work, we also found both g-C3N4/TiO2, g-C3N4/BiOI, g-C3N4/BiVO4 could
216
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
enhance the photocatalytic efficiency [23e25]. Although the binary coupling semiconductor exhibits improved photoactivity compared with the single semiconductor, how to get higher photocatalytic performance with band structure engineering design remains a challenge. Metal sulfides usually have photoabsorption ability in visible and near infrared regions, which enables them to be used as direct photocatalysts, such as MoS2 [26], ZnS [27,28], CdS [29,30], Cu2S [31,32], SnS2 [33,34], Bi2S3 [35,36], and so on. Among these metal sulfides, ZnS has got more attention in photodegradation field owing to its rapid photogeneration of electrons and holes and the relatively powerful reduction potential of photoexcited electrons [37]. But ZnS can only be irradiated by UV light with the wide Eg (ca. 3.4 eV) for cubic phase, which restrains its further application as an efficient photocatalyst [38]. Therefore, it is necessary to improve its practical application ability in photocatalytic field. Moreover, previous studies have verified that matching with a smaller Eg catalyst can extend the response of wide Eg photocatalyst to visible light efficiently. Zhang and his co-workers reported that CuS/ZnS composites exhibited a higher visible-light photocatalytic performance than single material [39]. Wu and his co-workers demonstrated that Bi2S3/ZnS composite exhibited better visible light photocatalytic activity than that of ZnS microparticles in degradation of Rhodamine B [40]. SnS2 is a hexagon sliced photocatalyst with a suitable Eg value [41,42], which can match well with ZnS. Besides, SnS2 is a favorable visible light semiconductor on account of its innocuity, cheapness, chemically stable performance in aqueous solution [43,44]. In contrast with single catalyst, the coupling between two different catalysts has already been proved to be effective in obtaining better catalytic activity. The enhanced catalytic performance of composites is mainly credited to the easy separation of photoinducted carriers via interfacial electrons and holes transfer [45]. The ternary coupling semiconductors for the Eg matching optimization may have perfect efficiency in degradation pollutants. Herein, we adopt ZnS as a buffer layer material to reduce the conductive band barrier height of g-C3N4 and SnS2, and a ternary gC3N4/ZnS/SnS2 composite can be prepared by the facile hydrothermal method. The catalytic performance of different catalysts has been evaluated by photodegrading methylene blue (MB) in the water. Furthermore, the visible light activity was enhanced after ZnS and SnS2 were hybridized by g-C3N4, and the stability of the ternary composites was excellent. As far as we know, it is the first report on the catalytic activities of g-C3N4/ZnS/SnS2 heterojunction up to now. Further, the synergic effect between ZnS, SnS2 and gC3N4 and the possible mechanism for photocatalytic performance were correspondingly studied.
2. Experimental
2.3. Synthesis of g-C3N4/ZnS/SnS2 heterostructures Zn(Ac)2$2H2O (1 mmol), 2 mmol SnCl4$5H2O and 8 mmol thiourea were mixed in 40 ml deionized water. Then g-C3N4 powder was added into the above mixture and stirred to form the homogeneous solution. The solution was sealed in 50 mL autoclave and heated 433 K for 12 h. The g-C3N4/ZnS/SnS2 heterostructures was obtained after filtration with deionized water and ethyl alcohol and then dried. To explore the effect of g-C3N4 on g-C3N4/ZnS/SnS2, different content on the weight percentage of g-C3N4 to ZnS/SnS2 was studied by varying the content of g-C3N4, and the sample were labeled as x%g-C3N4/ZnS/SnS2, where x is weight ratio of g-C3N4. For comparison, ZnS/SnS2 powder was also synthesized without adding g-C3N4. ZnS was prepared without adding SnCl4$5H2O and g-C3N4. SnS2 was synthesized without adding Zn(Ac)2$2H2O and gC3N4. g-C3N4/ZnS was prepared by 1 mmol Zn(Ac)2$2H2O, 0.1 g gC3N4 and 8 mmol thiourea, and g-C3N4/SnS2 was synthesized by 2 mmol SnCl4$5H2O, 0.1 g g-C3N4 and 8 mmol thiourea. All samples were synthesized under the same hydrothermal procedure. 2.4. Analytical and testing instrument Scanning electron microscopy (SEM) images and energy dispersive spectrometer (EDS) were obtained using Hitachi S5500 scanning electron microscopy. High resolution transmission electron microscopy (HRTEM) images were recorded on JEOL JEM-2010. X-ray diffraction (XRD) patterns of powder samples were measured on a Rigaku D/MAX 24000 diffractometer. X-ray photoelectron spectroscopy (XPS) of different samples was tested on a Thermo ESCALAB 250 spectrometer. UV visible (UVevis) diffuse reflectance spectroscopy (DRS) measurements were measured using a Perkin Elmer UV/VIS/NIR Spectrometer lambda 950. Photoelectrochemical measurements were tested on Shanghai Chenhua CHI-660D electrochemical system with Pt wire, saturated calomel electrode (SCE) and working electrode. 0.05 g catalyst sample, 0.01 g PEG and 0.25 mL distilled water mixed and injected onto ITO glass electrode. Then the working electrode was dried at 333 K for 2 h and calcined at 523 K for 4 h to remove PEG. 2.5. Catalytic experiment 50 W 410 nm LED light was used as light source. 0.02 g photocatalyst was totally dispersed in 30 mL MB solution (8 mg/L). The degradation efficiency (h) and apparent pseudo-first-order rate constant (kapp) were expressed as follows:
h¼ ln
C0 C 100% C0
C0 ¼ kappt C
(1)
(2)
2.1. Chemicals Zinc acetate dihydrate (Zn(Ac)2$2H2O), tin chloride pentahydrate (SnCl4$5H2O), polyethylene glycol (PEG), thiourea (CH4N2S), MB and ethyl alcohol (C2H5OH) were obtained from Sinopharm Chemical Reagent Co. Ltd. Melamine was purchased from Shanghai Chemical Reagent Co., Ltd. All chemicals were used as received.
2.2. Synthesis of g-C3N4 Polymeric g-C3N4 was synthesized by a simple pyrolysis of melamine without any templates. In a typical synthesis, 5 g melamine powder was dried and then heated to 773 K for 4 h.
Where: C0 is initial dy concentration and C is aqueous MB concentration at time t. 3. Result and discussion The synthetic process of g-C3N4/ZnS/SnS2 nanocomposites is shown in Fig. 1. Firstly, Zn(Ac)2$2H2O and SnCl4$5H2O were added in the deionized water to form homogeneous solution. Then, the ZnS/SnS2 heterostructures were prepared by an in-situ hydrothermal growth process after adding thiourea into the mixed aqueous solution. A portion of g-C3N4/ZnS/SnS2 was prepared through combining with g-C3N4 at 200 C. Fig. 2A, B, C and D show SEM images of as-synthesized ZnS, SnS2,
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
217
Fig. 1. The synthetic process of g-C3N4/ZnS/SnS2.
g-C3N4 and 20%g-C3N4/ZnS/SnS2, respectively. Fig. 2A displays the nanosphere morphology of ZnS, composed of nanoparticle with the diameter of about 60e80 nm. As indicated in Fig. 2B, hexagonal SnS2 nanoplate can be observed distinctly, the size is 400e600 nm and the thickness is ca. 20 nm. As indicated in Fig. 2C, it is obvious that g-C3N4 presented a thin laminar structure. Fig. 2D shows the SEM image of 20%g-C3N4/ZnS/SnS2 composite. The SnS2 nanoplate and ZnS nanoparticle were tightly connected with g-C3N4. This feature provides a channel for the efficient transmission and separation of the photoexcited electron and hole pairs among the gC3N4, ZnS and SnS2, which can improve the catalytic performance. Fig. 2E and F shows TEM image and HRTEM image of synthesized 20%g-C3N4/ZnS/SnS2, respectively. A clear fringe with an interval of 0.31 nm can be indexed to the (100) lattice plane of tetragonal SnS2 and that of 0.63 nm corresponds to the (100) crystallographic plane of ZnS, which further demonstrated that ZnS/SnS2 was deposited on g-C3N4. As indicated in Fig. 3, strong Bragg diffraction peaks at 2q ¼ 28.63 , 47.54 , 56.43 can be indexed to (111), (220), (311) planes for cubic phase of ZnS (JCPDS card No. 77-2100), and the peaks at 2q ¼ 27.84 , 31.88 , 41.42 , 49.63 , 52.24 , 54.56 , 60.44 and 67.12 were indexed to (100), (101), (102), (110), (111), (103), (201) and (202) planes for hexagonal phase of SnS2 (JCPDS No. 230677), respectively. For the pristine g-C3N4, the XRD peak at 27.32 was indexed as (002) diffraction plane (JCPDS card No. 87-1526), respectively. The g-C3N4/ZnS/SnS2 sample presented a three-phase composition: g-C3N4, ZnS and SnS2. No impurity phase is found in the g-C3N4/ZnS/SnS2 samples. Notably, the intensity of g-C3N4 peaks significantly strengthened with the increase of g-C3N4 component in g-C3N4/ZnS/SnS2 hybrids. The band structure of photocatalyst plays the key role in the photocatalytic activity. Fig. 4A displays the UVevis DRS of the gC3N4/ZnS/SnS2 hybrids, ZnS, SnS2 and g-C3N4. SnS2 and g-C3N4 present a wide absorbance in both UV and visible light ranges, while ZnS only presents absorbance in UV ranges. The absorption of
g-C3N4, ZnS and SnS2 are 663, 382 and 458 nm, respectively. When the three photocatalysts are combined together, g-C3N4/ZnS/SnS2 hybrid exhibits the absorption edges between 550 and 611 nm. This result shows that all of g-C3N4/ZnS/SnS2 hybrids can be used as visible-light-driven catalysts. The Eg energy of the semiconductors can be estimated by the following formula [46]:
.
ahv ðhv EgÞðn=2Þ hv
(3)
Where: a is the absorption coefficient. n ¼ 1 or 4 for direct-gap catalyst or indirect-gap catalyst. Therefore, on the basis of Eq. (3), plots of ðahvÞ2 versus energy (hv) for SnS2 and g-C3N4 were shown in Fig. 4B, Eg values of SnS2 and g-C3N4 are 1.66 and 2.68 eV, respectively. The plot of ðahvÞ1=2 versus energy (hv) for ZnS was shown in Fig. 4C, Eg of ZnS is 3.24 eV. To further investigate the interaction among g-C3N4, ZnS and SnS2, the chemical state and element composition for 20%g-C3N4/ ZnS/SnS2 were analyzed by XPS (Fig. 5). The fully scanned spectra show in Fig. 5A reveals that C, N, S, Zn, and Sn elements exist in the 20%g-C3N4/ZnS/SnS2 heterostructures. No impurity element is evident in the g-C3N4/ZnS/SnS2, which is in accordance with the XRD and EDS data. The C1s XPS spectra were showed in Fig. 5B, the broad and asymmetrical features suggested that the co-existence of distinguishable models. In addition, the deconvolution core level spectra at about 288.7 and 284.8 eV have been found. The peak at 284.8 eV is exclusively assigned to CeC bonding in pure carbon environment [47]. The peak at 288.7 eV is originated from carbon atoms bonded to three nitrogen atoms in the g-C3N4 lattice [48,49]. The high resolution N1s spectra in Fig. 5C shows the asymmetrical feature, indicating the co-existence of a number of distinguishable nitrogen environment, fitting with 3 results in binding energy of 398.14, 398.76 and 399.82 eV. The 2 peaks at 398.76 and 399.82 eV can be indexed as tertiary nitrogen (N-(C)3) and amino functional groups with a hydrogen atom (C-N-H) [50]. The peak centered at 398.14 eV is typically attributed to N atoms sp2-bonded to two
218
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
Fig. 2. SEM images of (A) ZnS, (B) SnS2, (C) g-C3N4, (D) 20%g-C3N4/ZnS/SnS2 and (E) TEM image and (F) HRTEM image of 20%g-C3N4/ZnS/SnS2.
Fig. 3. XRD patterns of g-C3N4, SnS2, ZnS, g-C3N4/SnS2, g-C3N4/ZnS and g-C3N4/ZnS/ SnS2 heterostructures.
carbon atoms (C]N-C), thus confirming the presence of graphitelike sp2-bonded g-C3N4 [51,52]. As indicated in Fig. 5D, the binding energies of Sn 3d 5/2 and Sn 3d 3/2 at 482.7 and 491.7 eV are in agreement with pure SnS2 [53]. Fig. 5E shows that Zn 2p 1/2 and Zn 2p 3/2 peaks of ZnS appeared at 1040.9 and 1017.7 eV respectively [54]. The high resolution S2p XPS spectra in Fig. 5F shows the peaks at 162.5 and 161.3 eV, which is assigned to S-Sn and S-Zn bond [43,55]. Fig. 6A exhibits the MB photodegradation by as-prepared gC3N4/ZnS/SnS2 hybrids under the irradiation of 410 nm LED light. The blank sample (no catalyst), individual g-C3N4, ZnS, SnS2, gC3N4/ZnS, g-C3N4/SnS2 and ZnS/SnS2 were also tested for comparison. Indeed, MB is very stable in visible light treatment when no catalysts were used. The pristine ZnS shows very low photoactivity due to the large Eg. The pure SnS2 and g-C3N4 show some visible light activity. When two components of g-C3N4, ZnS and SnS2 are composited together, the MB photodegradation is obviously improved over that of the individual components. Particularly, when g-C3N4, ZnS and SnS2 are combined together, constructing ternary C3N4/ZnS/SnS2 hybrid, the photocatalytic activity is dramatically enhanced under identical conditions. Among these ternary composite photocatalysts, 20%g-C3N4/ZnS/SnS2 hybrid can photodegrade 95% MB within 20 min, which is higher than other photocatalysts. As indicated in Fig. 6B, the linear relationship
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
219
Fig. 4. (A) UVevis DRS spectra of x%g-C3N4/ZnS/SnS2; (B) Plots of ðahvÞ2 versus energy for SnS2 and g-C3N4; (C) Plot of ðahvÞ1=2 versus energy for ZnS.
between lnC0/C and t suggests that the photocatalytic reaction is pseudo-first-order. According to Eq. (2) and Fig. 6B, the Kapp of the MB degradation is shown in Fig. 6C. The highest Kapp of g-C3N4/ZnS/ SnS2 hybrid is 0.148 min1, which is 8.74, 3.22 and 37.01 times as high as that of pristine g-C3N4, SnS2 and ZnS, respectively. Photocurrent is usually recorded to investigate the total amount of electrons and holes generation by the semiconductor. Fig. 7 shows the relationship between photocurrent and time curves of g-C3N4, ZnS, SnS2 and g-C3N4/ZnS/SnS2 with 2 on and off intermittent excitation cycles. The electrode of the different photocatalysts exhibits a rapid photocurrent response when 410 nm LED light illumination is work on on-off mode. The photocurrent suddenly decreases to its dark current state as soon as the LED light is turned off, and photocurrent increases sharply when the light is turned on. And the 20%g-C3N4/ZnS/SnS2 exhibits higher photocurrent intensity than that of ZnS, g-C3N4, SnS2 and other g-C3N4/ ZnS/SnS2 samples, which is in good accordance with the MB photodegradation. The valence band (VB) potentials of a photocatalyst can be theoretically obtained by the following equation:
EVB ¼ X Ec þ 0:5Eg
(4)
Where: EVB and X are the VB edge potential and the electronegativity of the catalyst, respectively. Ec is 4.5 eV. At the same time, the conduction band edge potential (ECB ) can be calculated by ECB ¼
EVB Eg . According Fig. 4 and eq. (4), the CB and VB of g-C3N4 are determined at 1.12 and þ 1.56 eV, respectively. The CB and VB edge potentials of SnS2 are calculated at 0.04 and þ 1.62 eV, respectively. The CB and VB edge potentials of ZnS are calculated at 0.95 and þ 2.29 eV, respectively. According to above results, the band structure of g-C3N4/ZnS/ SnS2 and possible photocatalytic degradation mechanism were displayed in Fig. 8. Both SnS2 and g-C3N4 semiconductor can be irradiated by visible light and generate electron and hole pairs. However, the photo-generated electrons and holes will recombine rapidly [56]. As indicated in Fig. 8, if electrons migration directly from g-C3N4 to SnS2, they will overcome a barrier height of 1.08 eV. Such high barrier height may hinder electrons migration [57,58]. Thus, how to design a proper buffer layer according to band structure engineering to smooth barrier height for easy carrier transport is a challenge [59]. The CB value of ZnS is 0.95 eV, which is a proper value between the CB of SnS2 and g-C3N4. Thus, the introduction of ZnS will produce g-C3N4/ZnS and ZnS/SnS2 interfaces. The barrier heights of the g-C3N4/ZnS and ZnS/SnS2 interfaces are calculated as 0.17 and 0.91 eV, respectively. As shown in Fig. 8, when g-C3N4/ZnS/SnS2 irradiated by 410 nm LED light, there is a new channel for the electron migration from g-C3N4 to SnS2 with lower barrier height, the electron of g-C3N4 CB can immediately transfer to ZnS CB and then transfer to SnS2 CB. The new channel can collect photo-generated electrons from g-C3N4 by a low barrier height of 0.17 eV and transfer them to SnS2 by another
220
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
Fig. 5. The overview (A) and the corresponding high-resolution XPS spectra (B) C1s, (C) N1s, (D) Sn3d, (E) Zn2p and (F) S2p of the as-prepared 20%g-C3N4/ZnS/SnS2.
lower barrier height of 0.91 eV. At the same time, the holes of SnS2 VB can immediately transfer to g-C3N4 VB. According to this method, the photoinduced charge carriers could be effectively separated, which can be described as follows:
H2O þ O 2 / OH þ OH /H2O2
(8)
hþþH2O/ OH þ Hþ
(9)
OH þ MB / degradation product
C3N4/ZnS/SnS2þhv / C3N4(hþþe)þSnS2(hþþe)/SnS2(e)þ ZnS(hþ)
(5)
e þ O2 / O2
(6)
2eþ2Hþ þ O2 þ 2Hþ/H2O2
(7)
(10)
As a result, the O 2 and OH were produced during the whole reaction, the organic pollutant could be mineralized due to these two strong oxidizing ions [60,61]. The photocatalytic performance of g-C3N4/ZnS/SnS2 is much better than that of ZnS and SnS2, which is attributed to that photoexcited electron-hole pairs would
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
221
Fig. 7. Photocurrentetime curves of different photocatalysts.
Fig. 8. Diagrammatic sketch of photocatalytic degradation mechanism.
same conditions. Fig. 9 shows photostability test of 20%g-C3N4/ZnS/ SnS2 heterostructures, the photocatalytic efficiency does not exhibit significant decrease after 20%g-C3N4/ZnS/SnS2 heterostructures were used for 3 times. This result demonstrated that 20% g-C3N4/ZnS/SnS2 heterostructures could exhibit excellent photocatalytic activity and stability. This superior stability enables recycling to be more effective in practical applications.
Fig. 6. (A) Photocatalytic degradation of MB under the irradiation of 410 nm LED light by different samples, (B) The ln(C0/C) as a function of irradiation time (t) for MB degradation, (C) The apparent pseudo-first-order rate constant kapp with different catalysts.
recombine hardly on the surface and interior of the sample [62,63]. And 20% g-C3N4 of g-C3N4/ZnS/SnS2 showed the best MB photodegradation among the different g-C3N4/ZnS/SnS2. Nevertheless, when the weight content of g-C3N4 further added exceeding its optimum value, the catalytic activity worse, that is because some gC3N4 will act as recombination center [64,65]. To provide reusable performance, the recycled photocatalytic experiment of 20%g-C3N4/ZnS/SnS2 was investigated under the
4. Conclusions In summary, ternary g-C3N4/ZnS/SnS2 heterojunctions with different content of g-C3N4 can be prepared by a simple hydrothermal method. Such a ternary g-C3N4/ZnS/SnS2 heterojunction can efficiently improve the photoactivity for degradation organic dyes by visible light as compared with binary ZnS/SnS2 heterojunction, pure SnS2, pure ZnS, g-C3N4/SnS2 or g-C3N4/ZnS. 20%gC3N4/ZnS/SnS2 heterojunctions displayed the highest photocatalytic performance with degradation of 95% MB with the visible light irradiation. The improved catalytic activities are attributed to effective charges transfer across heterojunction interface and decreased Eg value.
222
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
Fig. 9. Stability of photodegradation performance within three cycles for 20% g-C3N4/ ZnS/SnS2 heterostructures.
Acknowledgments This work was supported by the National Natural Science Foundation of China (51572103 and 51502106), the Distinguished Young Scholar of Anhui Province (1808085J14), the Foundation for Young Talents in College of Anhui Province (gxyqZD2017051), the Key Foundation of Educational Commission of Anhui Province (KJ2016SD53) and Anhui Provincial Innovation Team of Design and Application of Advanced Energetic Materials (KJ2015TD003). References [1] J.X. Low, J.G. Yu, M. Jaroniec, S. Wageh, A.A. Al-Ghamdi, Heterojunction photocatalysts, Adv. Mater. 29 (2017) 1601694. [2] Y. Xu, M. Kraft, R. Xu, Metal-free carbonaceous electrocatalysts and photocatalysts for water splitting, Chem. Soc. Rev. 45 (2016) 3039e3052. [3] J. Lv, J. Zhang, J. Liu, Z. Li, K. Dai, C. Liang, Bi SPR-promoted Z-scheme Bi2MoO6/ CdS-diethylenetriamine composite with effectively enhanced visible light photocatalytic hydrogen evolution activity and stability, ACS Sustain. Chem. Eng. 6 (2018) 696e706. [4] M. Hojamberdiev, R.M. Prasad, K. Morita, Y.F. Zhu, M.A. Schiavon, A. Gurlo, R. Riedel, Template-free synthesis of polymer-derived mesoporous SiOC/TiO2 and SiOC/N-doped TiO2 ceramic composites for application in the removal of organic dyes from contaminated water, Appl. Catal. B Environ. 115 (2012) 303e313. [5] Y. Huo, J. Zhang, K. Dai, Q. Li, J. Lv, G. Zhu, C. Liang, All-solid-state artificial Zscheme porous g-C3N4/Sn2S3-DETA heterostructure photocatalyst with enhanced performance in photocatalytic CO2 reduction, Appl. Catal. B Environ. 241 (2019) 528e538. [6] J. Liu, H. Wang, M. Antonietti, Graphitic carbon nitride “reloaded”: emerging applications beyond (photo)catalysis, Chem. Soc. Rev. 45 (2016) 2308e2326. [7] K. Dai, L. Lu, Q. Liu, G. Zhu, Q. Liu, Z. Liu, Graphene oxide capturing surfacefluorinated TiO2 nanosheets for advanced photocatalysis and the reveal of synergism reinforce mechanism, Dalton Trans. 43 (2014) 2202e2210. [8] X.F. Zhu, B. Cheng, J.G. Yu, W.K. Ho, Halogen poisoning effect of Pt-TiO2 for formaldehyde catalytic oxidation performance at room temperature, Appl. Surf. Sci. 364 (2016) 808e814. [9] H.G. Yu, P. Xiao, J. Tian, F.Z. Wang, J.G. Yu, Phenylamine-functionalized rGO/ TiO2 photocatalysts: spatially separated adsorption sites and tunable photocatalytic selectivity, ACS Appl. Mater. Interfaces 8 (2016) 29470e29477. [10] J. Zhang, J. Fu, S. Chen, J. Lv, K. Dai, 1D carbon nanofibers@TiO2 core-shell nanocomposites with enhanced photocatalytic activity toward CO2 reduction, J. Alloys Compd. 746 (2018) 168e176. [11] K. Dai, J. Lv, J. Zhang, G. Zhu, L. Geng, C. Liang, Efficient visible-light-driven splitting of water into hydrogen over surface-fluorinated anatase TiO2 nanosheets with exposed {001} facets/layered CdSediethylenetriamine nanobelts, ACS Sustain. Chem. Eng. 6 (2018) 12817e12826. [12] W.K. Wang, D.F. Xu, B. Cheng, J.G. Yu, C.J. Jiang, Hybrid carbon@TiO2 hollow spheres with enhanced photocatalytic CO2 reduction activity, J. Mater. Chem. A 5 (2017) 5020e5029. [13] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production
from water under visible light, Nat. Mater. 8 (2009) 76e80. [14] N. Tian, Y.H. Zhang, X.W. Li, K. Xiao, X. Du, F. Dong, G.I.N. Waterhouse, T.R. Zhang, H.W. Huang, Precursor-reforming protocol to 3D mesoporous gC3N4 established by ultrathin self-doped nanosheets for superior hydrogen evolution, Nano Energy 38 (2017) 72e81. [15] X.F. Wang, J.J. Cheng, H.G. Yu, J.G. Yu, A facile hydrothermal synthesis of carbon dots modified g-C3N4 for enhanced photocatalytic H2-evolution performance, Dalton Trans. 46 (2017) 6417e6424. [16] H.J. Yu, L. Shang, T. Bian, R. Shi, G.I.N. Waterhouse, Y.F. Zhao, C. Zhou, L.Z. Wu, C.H. Tung, T.R. Zhang, Nitrogen-Doped porous carbon nanosheets templated from g-C3N4 as metal-free electrocatalysts for efficient oxygen reduction reaction, Adv. Mater. 28 (2016) 5080e5086. [17] J. Low, C. Jiang, B. Cheng, S. Wageh, A.A. Al-Ghamdi, J. Yu, A review of direct Zscheme photocatalysts, Small Methods 1 (2017) 1700080. [18] J.X. Low, B. Cheng, J.G. Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Appl. Surf. Sci. 392 (2017) 658e686. [19] M. Wu, J. Zhang, C. Liu, Y. Gong, R. Wang, B. He, H. Wang, Rational design and fabrication of noble-metal-free NixP cocatalyst embedded 3D N-TiO2/g-C3N4 heterojunctions with enhanced photocatalytic hydrogen evolution, Chemcatchem 10 (2018) 3069e3077. [20] B.C. Zhu, P.F. Xia, Y. Li, W.K. Ho, J.G. Yu, Fabrication and photocatalytic activity enhanced mechanism of direct Z-scheme g-C3N4/Ag2WO4 photocatalyst, Appl. Surf. Sci. 391 (2017) 175e183. [21] Y. Wu, H. Wang, W. Tu, Y. Liu, S. Wu, Y.Z. Tan, J.W. Chew, Construction of hierarchical 2D-2D Zn3In2S6/fluorinated polymeric carbon nitride nanosheets photocatalyst for boosting photocatalytic degradation and hydrogen production performance, Appl. Catal. B Environ. 233 (2018) 58e69. [22] H. Che, G. Che, H. Dong, W. Hu, H. Hu, C. Liu, C. Li, Fabrication of Z-scheme Bi3O4Cl/g-C3N4 2D/2D heterojunctions with enhanced interfacial charge separation and photocatalytic degradation various organic pollutants activity, Appl. Surf. Sci. 455 (2018) 705e716. [23] K. Dai, L. Lu, C. Liang, Q. Liu, G. Zhu, Heterojunction of facet coupled g-C3N4/ surface-fluorinated TiO2 nanosheets for organic pollutants degradation under visible LED light irradiation, Appl. Catal. B Environ. 156e157 (2014) 331e340. [24] J. Zhang, J. Fu, Z. Wang, B. Cheng, K. Dai, W. Ho, Direct Z-scheme porous gC3N4/BiOI heterojunction for enhanced visible-light photocatalytic activity, J. Alloys Compd. 766 (2018) 841e850. [25] Z. Wang, J. Lv, J. Zhang, K. Dai, C. Liang, Facile synthesis of Z-scheme BiVO4/ porous graphite carbon nitride heterojunction for enhanced visible-lightdriven photocatalyst, Appl. Surf. Sci. 430 (2018) 595e602. [26] D. Lu, Q. Wang, K.K. Kondamareddy, A. Wang, H. Hao, Q. Wu, Efficiently visible-light-induced photoactivity of MoS2 nanoflowers/chromic oxide/protonated titanate nanoflakes edge-on ternary heterostructures for production of hydrogen, J. Alloys Compd. 761 (2018) 31e40. [27] L. Kashinath, K. Namratha, K. Byrappa, Sol-gel assisted hydrothermal synthesis and characterization of hybrid ZnS-RGO nanocomposite for efficient photodegradation of dyes, J. Alloys Compd. 695 (2017) 799e809. [28] Z. Ren, L. Li, B. Liu, X. Liu, Z. Li, X. Lei, C. Li, Y. Gong, L. Niu, L. Pan, Cr(VI) reduction in presence of ZnS/RGO photocatalyst under full solar spectrum radiation from UV/vis to near-infrared light, Catal. Today 315 (2018) 46e51. [29] J.F. Zhang, S. Wageh, A. Al-Ghamdi, J.G. Yu, New understanding on the different photocatalytic activity of wurtzite and zinc-blende CdS, Appl. Catal. B Environ. 192 (2016) 101e107. [30] Z. Li, J. Zhang, J. Lv, L. Lu, C. Liang, K. Dai, Sustainable synthesis of CeO2/CdSdiethylenetriamine composites for enhanced photocatalytic hydrogen evolution under visible light, J. Alloys Compd. 758 (2018) 162e170. [31] P. Sahatiya, A. Kadu, H. Gupta, P.T. Gomathi, S. Badhulika, Flexible, disposable cellulose-paper-based MoS2/Cu2S hybrid for wireless environmental monitoring and multifunctional sensing of chemical stimuli, ACS Appl. Mater. Interfaces 10 (2018) 9048e9059. [32] L. Yang, Y. Yao, G. Zhu, M. Ma, W. Wang, L. Wang, H. Zhang, Y. Zhang, Z. Jiao, Co doping of worm-like Cu2S: an efficient and durable heterogeneous electrocatalyst for alkaline water oxidation, J. Alloys Compd. 762 (2018) 637e642. [33] X. Jia, C. Tang, R. Pan, Y. Long, C. Gu, J. Li, Thickness-dependently enhanced photodetection performance of vertically grown SnS2 nanoflakes with large size and high production, ACS Appl. Mater. Interfaces 10 (2018) 18073e18081. [34] M. Bagherzadeh, R. Kaveh, A new SnS2-BiFeO3/reduced graphene oxide photocatalyst with superior photocatalytic capability under visible light irradiation, J. Photochem. Photobiol. A 359 (2018) 11e22. [35] J. Wang, J. Jin, X. Wang, S. Yang, Y. Zhao, Y. Wu, S. Dong, J. Sun, J. Sun, Facile fabrication of novel BiVO4/Bi2S3/MoS2 n-p heterojunction with enhanced photocatalytic activities towards pollutant degradation under natural sunlight, J. Colloid Interface Sci. 505 (2017) 805e815. [36] Y. Mi, H. Li, Y. Zhang, R. Zhang, W. Hou, One-pot synthesis of belt-like Bi2S3/ BiOCl hierarchical composites with enhanced visible light photocatalytic activity, Appl. Surf. Sci. 423 (2017) 1062e1071. [37] Y.-S. Lee, C.V.V.M. Gopi, M. Venkata-Haritha, H.-J. Kim, Recombination control in high-performance quantum dot-sensitized solar cells with a novel TiO2/ ZnS/CdS/ZnS heterostructure, Dalton Trans. 45 (2016) 12914e12923. [38] X. Hao, Y. Wang, J. Zhou, Z. Cui, Y. Wang, Z. Zou, Zinc vacancy-promoted photocatalytic activity and photostability of ZnS for efficient visible-lightdriven hydrogen evolution, Appl. Catal. B Environ. 221 (2018) 302e311. [39] J. Zhang, J.G. Yu, Y.M. Zhang, Q. Li, J.R. Gong, Visible light photocatalytic H2-
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
[40] [41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49] [50]
[51]
[52]
[53]
production activity of CuS/ZnS porous nanosheets based on photoinduced interfacial charge transfer, Nano Lett. 11 (2011) 4774e4779. Y. He, Y.F. Zhu, Solvothermal synthesis of sodium and potassium tantalate perovskite nanocubes, Chem. Lett. 33 (2004) 900e901. L. Wang, G. Jin, Y. Shi, H. Zhang, H. Xie, B. Yang, H. Sun, Co-catalyst-free ZnSSnS2 porous nanosheets for clean and recyclable photocatalytic H2 generation, J. Alloys Compd. 753 (2018) 60e67. H. Liu, C. Du, H. Bai, Y. Su, D. Wei, Y. Wang, G. Liu, L. Yang, Fabrication of plateon-plate Z-scheme SnS2/Bi2MoO6 heterojunction photocatalysts with enhanced photocatalytic activity, J. Mater. Sci. 53 (2018) 10743e10757. F. Zhang, Y. Zhang, G. Zhang, Z. Yang, D.D. Dionysiou, A. Zhu, Exceptional synergistic enhancement of the photocatalytic activity of SnS2 by coupling with polyaniline and N-doped reduced graphene oxide, Appl. Catal. B Environ. 236 (2018) 53e63. E. Liu, J. Chen, Y. Ma, J. Feng, J. Jia, J. Fan, X. Hu, Fabrication of 2D SnS2/g-C3N4 heterojunction with enhanced H2 evolution during photocatalytic water splitting, J. Colloid Interface Sci. 524 (2018) 313e324. J. Lv, K. Dai, J. Zhang, L. Lu, C. Liang, L. Geng, Z. Wang, G. Yuan, G. Zhu, In situ controllable synthesis of novel surface plasmon resonance-enhanced Ag2WO4/Ag/Bi2MoO6 composite for enhanced and stable visible light photocatalyst, Appl. Surf. Sci. 391 (2017) 507e515. K. Dai, L. Lu, C. Liang, G. Zhu, Q. Liu, L. Geng, J. He, A high efficient graphiticC3N4/BiOI/graphene oxide ternary nanocomposite heterostructured photocatalyst with graphene oxide as electron transport buffer material, Dalton Trans. 44 (2015) 7903e7910. Q.L. Xu, B. Cheng, J.G. Yu, G. Liu, Making co-condensed amorphous carbon/gC3N4 composites with improved visible-light photocatalytic H2-production performance using Pt as cocatalyst, Carbon 118 (2017) 241e249. P.F. Xia, B.C. Zhu, J.G. Yu, S.W. Cao, M. Jaroniec, Ultra-thin nanosheet assemblies of graphitic carbon nitride for enhanced photocatalytic CO2 reduction, J. Mater. Chem. A 5 (2017) 3230e3238. S.W. Cao, Y. Li, B.C. Zhu, M. Jaroniec, J.G. Yu, Facet effect of Pd cocatalyst on photocatalytic CO2 reduction over g-C3N4, J. Catal. 349 (2017) 208e217. J.W. Fu, B.C. Zhu, C.J. Jiang, B. Cheng, W. You, J.G. Yu, Hierarchical porous Odoped g-C3N4 with enhanced photocatalytic CO2 reduction activity, Small 13 (2017). C.Y. Liu, Y.H. Zhang, F. Dong, A.H. Reshak, L.Q. Ye, N. Pinna, C. Zeng, T.R. Zhang, H.W. Huang, Chlorine intercalation in graphitic carbon nitride for efficient photocatalysis, Appl. Catal. B Environ. 203 (2017) 465e474. J. Zhang, J. Lv, K. Dai, Q. Liu, C. Liang, G. Zhu, Facile and green synthesis of novel porous g-C3N4/Ag3PO4 composite with enhanced visible light photocatalysis, Ceram. Int. 43 (2017) 1522e1529. X. Cheng, B. Zhang, J. Shi, J. Zhang, L. Zheng, J. Zhang, D. Shao, X. Tan, B. Han, G. Yang, Tin(IV) sulfide greatly improves the catalytic performance of UiO-66 for carbon dioxide cycloaddition, Chemcatchem 10 (2018) 2945e2948.
223
[54] J. Lv, J. Zhang, K. Dai, C. Liang, G. Zhu, Z. Wang, Z. Li, Controllable synthesis of inorganiceorganic Zn1xCdxS-DETA solid solution nanoflowers and their enhanced visible-light photocatalytic hydrogen-production performance, Dalton Trans. 46 (2017) 11335e11343. [55] X. Hao, J. Zhou, Z. Cui, Y. Wang, Y. Wang, Z. Zou, Zn-vacancy mediated electron-hole separation in ZnS/g-C3N4 heterojunction for efficient visiblelight photocatalytic hydrogen production, Appl. Catal. B Environ. 229 (2018) 41e51. [56] J. Lv, K. Dai, L. Lu, L. Geng, C. Liang, G. Zhu, Cu/Ag/Ag3PO4 ternary composite: a hybrid alloy-semiconductor heterojunction structure with visible light photocatalytic properties, J. Alloys Compd. 682 (2016) 778e784. [57] Q. Wu, J. Hou, H. Zhao, Z. Liu, X. Yue, S. Peng, H. Cao, Charge recombination control for high efficiency CdS/CdSe quantum dot co-sensitized solar cells with multi-ZnS layers, Dalton Trans. 47 (2018) 2214e2221. [58] P. Duy Phong, S. Kim, L. Anh Huy Tuan, J. Park, J. Yi, Diminished band discontinuity at the phi interface of narrow-gap a-SiGe:H solar cell by hydrogenated amorphous silicon oxide buffer layer, J. Alloys Compd. 762 (2018) 616e620. [59] S. Lie, M.I. Sandi, Y.F. Tay, W. Li, J.M.R. Tan, D.M. Bishop, O. Gunawan, L.H. Wong, Improving the charge separation and collection at the buffer/ absorber interface by double-layered Mn-substituted CZTS, Sol. Energy Mater. Sol. Cells 185 (2018) 351e358. [60] H. Wang, X. Qiu, W. Liu, D. Yang, Facile preparation of well-combined ligninbased carbon/ZnO hybrid composite with excellent photocatalytic activity, Appl. Surf. Sci. 426 (2017) 206e216. [61] H. Yu, B. Huang, H. Wang, X. Yuan, L. Jiang, Z. Wu, J. Zhang, G. Zeng, Facile construction of novel direct solid-state Z-scheme AgI/BiOBr photocatalysts for highly effective removal of ciprofloxacin under visible light exposure: mineralization efficiency and mechanisms, J. Colloid Interface Sci. 522 (2018) 82e94. [62] J. Zhang, J. Lv, K. Dai, C. Liang, Q. Liu, One-step growth of nanosheet-assembled BiOCl/BiOBr microspheres for highly efficient visible photocatalytic performance, Appl. Surf. Sci. 430 (2018) 639e646. [63] Q.C. Yuan, D. Liu, N. Zhang, W. Ye, H.X. Ju, L. Shi, R. Long, J.F. Zhu, Y.J. Xiong, Noble-metal-free janus-like structures by cation exchange for Z-scheme photocatalytic water splitting under broadband light irradiation, Angew. Chem. Int. Ed. 56 (2017) 4206e4210. [64] A. Wu, C. Tian, Y. Jiao, Q. Yan, G. Yang, H. Fu, Sequential two-step hydrothermal growth of MoS2/CdS core-shell heterojunctions for efficient visible light-driven photocatalytic H2 evolution, Appl. Catal. B Environ. 203 (2017) 955e963. [65] Y. Zou, J.-W. Shi, D. Ma, Z. Fan, C. Niu, L. Wang, Fabrication of g-C3N4/Au/CTiO2 hollow structures as visible-light-driven Z-scheme photocatalysts with enhanced photocatalytic H2 evolution, Chemcatchem 9 (2017) 3752e3761.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Band structure engineering design of g-C3N4/ZnS/SnS2 ternary heterojunction visible-light photocatalyst with ZnS as electron transport buffer material Kai Dai a, 1, Jiali Lv b, c, 1, Jinfeng Zhang a, *, Changhao Liang b, **, Guangping Zhu a a
College of Physics and Electronic Information, Anhui Key Laboratory of Energetic Materials, Huaibei Normal University, Huaibei, 235000, PR China Key Laboratory of Materials Physics, Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, 230031, PR China c Department of Materials Science and Engineering, University of Science and Technology of China, 96 Jinzhai Road, Hefei, 230026, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 August 2018 Received in revised form 30 October 2018 Accepted 10 November 2018 Available online 14 November 2018
Semiconductor heterojunction represents a family of promising photocatalysts for visible-light photocatalysis. In this work, a novel ternary g-C3N4/ZnS/SnS2 heterostructure has been designed and synthesized by a facile one-step hydrothermal method. The obtained ternary g-C3N4/ZnS/SnS2 heterostructures exhibited high photocatalytic activity in photodegradation of organic pollutants and photocurrent response irradiated by 410 nm LED light. The results demonstrated that the formation of the heterostructures can much improve the excellent photocatalytic activity if the lattice and energy level matching among the three semiconductors be satisfied, which causes efficient separation of photoinduced carriers, resulting in the high photodegradation of methylene blue (MB). As a result, the highest apparent rate constant Kapp of g-C3N4/ZnS/SnS2 hybrid is 0.148 min1, which is 8.74, 3.22 and 37.01 times as high as that of pristine g-C3N4, SnS2 and ZnS, respectively. © 2018 Elsevier B.V. All rights reserved.
Keywords: C3N4 ZnS SnS2 Photocatalysis Photocatalytic activity Buffer layer
1. Introduction Nanostructured semiconductor with high photocatalytic degradation ability for organic pollutants has been regarded as the most promising nanomaterials for environment protection and clean energy [1e6]. Since nanostructured TiO2 was prepared, it has become the most famous photocatalysis and been widely utilized as photocatalytic hydrogen production and treatment of organic pollutants from liquid solution due to its outstanding photoactivity, stable chemical performance, low cost and nontoxic nature [7e10]. Nevertheless, TiO2 carried out photochemical reactions only in a small ultraviolet (UV) fraction with low quantum yield by reason of the wide band gap (Eg) about 3.2 eV for anatase [11,12]. With the purpose of utilizing solar energy more efficiently, large quantities of visible-light-responsive semiconductors have been desired. Since Wang and his co-workers presented the pioneering work
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Zhang), [email protected] (C. Liang). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.jallcom.2018.11.127 0925-8388/© 2018 Elsevier B.V. All rights reserved.
on exploring catalytic performance of graphitic carbon nitride (gC3N4), which exhibit extremely high photocatalytic capabilities for H2 evolution from water in 2009 [13]. This kind of metal-free semiconductor has got much attention in environment pollutants treatment fields. The element carbon and nitrogen of g-C3N4 via sp2 hydridization leads to the narrow Eg about 2.70 eV and a high chemical stability [14,15]. However, g-C3N4 often exhibits a low catalytic activity because of its narrow absorption and low quantum yield [16,17]. Recently, many studies have indicated that combining g-C3N4 with semiconductor photocatalysis to form heterojunction could extend the photoresponding range and increase the charge separation efficiency to improve the photoactivity [18,19]. For instance, Yu and his fellows showed that g-C3N4/Ag2WO4 exhibited a significantly improved catalytic activity in methyl orange photodegradation [20]. Wu and his co-workers presented that the H2 production activity of g-C3N4 was strengthened by Zn3In2S6 nanoparticles modification [21]. Similarly, Che and his fellows found that the g-C3N4/Bi3O4Cl composites show higher visible light photoactivity for treatment of antibiotic, dye and heavy metal than single g-C3N4 and Bi3O4Cl material [22]. In our previous work, we also found both g-C3N4/TiO2, g-C3N4/BiOI, g-C3N4/BiVO4 could
216
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
enhance the photocatalytic efficiency [23e25]. Although the binary coupling semiconductor exhibits improved photoactivity compared with the single semiconductor, how to get higher photocatalytic performance with band structure engineering design remains a challenge. Metal sulfides usually have photoabsorption ability in visible and near infrared regions, which enables them to be used as direct photocatalysts, such as MoS2 [26], ZnS [27,28], CdS [29,30], Cu2S [31,32], SnS2 [33,34], Bi2S3 [35,36], and so on. Among these metal sulfides, ZnS has got more attention in photodegradation field owing to its rapid photogeneration of electrons and holes and the relatively powerful reduction potential of photoexcited electrons [37]. But ZnS can only be irradiated by UV light with the wide Eg (ca. 3.4 eV) for cubic phase, which restrains its further application as an efficient photocatalyst [38]. Therefore, it is necessary to improve its practical application ability in photocatalytic field. Moreover, previous studies have verified that matching with a smaller Eg catalyst can extend the response of wide Eg photocatalyst to visible light efficiently. Zhang and his co-workers reported that CuS/ZnS composites exhibited a higher visible-light photocatalytic performance than single material [39]. Wu and his co-workers demonstrated that Bi2S3/ZnS composite exhibited better visible light photocatalytic activity than that of ZnS microparticles in degradation of Rhodamine B [40]. SnS2 is a hexagon sliced photocatalyst with a suitable Eg value [41,42], which can match well with ZnS. Besides, SnS2 is a favorable visible light semiconductor on account of its innocuity, cheapness, chemically stable performance in aqueous solution [43,44]. In contrast with single catalyst, the coupling between two different catalysts has already been proved to be effective in obtaining better catalytic activity. The enhanced catalytic performance of composites is mainly credited to the easy separation of photoinducted carriers via interfacial electrons and holes transfer [45]. The ternary coupling semiconductors for the Eg matching optimization may have perfect efficiency in degradation pollutants. Herein, we adopt ZnS as a buffer layer material to reduce the conductive band barrier height of g-C3N4 and SnS2, and a ternary gC3N4/ZnS/SnS2 composite can be prepared by the facile hydrothermal method. The catalytic performance of different catalysts has been evaluated by photodegrading methylene blue (MB) in the water. Furthermore, the visible light activity was enhanced after ZnS and SnS2 were hybridized by g-C3N4, and the stability of the ternary composites was excellent. As far as we know, it is the first report on the catalytic activities of g-C3N4/ZnS/SnS2 heterojunction up to now. Further, the synergic effect between ZnS, SnS2 and gC3N4 and the possible mechanism for photocatalytic performance were correspondingly studied.
2. Experimental
2.3. Synthesis of g-C3N4/ZnS/SnS2 heterostructures Zn(Ac)2$2H2O (1 mmol), 2 mmol SnCl4$5H2O and 8 mmol thiourea were mixed in 40 ml deionized water. Then g-C3N4 powder was added into the above mixture and stirred to form the homogeneous solution. The solution was sealed in 50 mL autoclave and heated 433 K for 12 h. The g-C3N4/ZnS/SnS2 heterostructures was obtained after filtration with deionized water and ethyl alcohol and then dried. To explore the effect of g-C3N4 on g-C3N4/ZnS/SnS2, different content on the weight percentage of g-C3N4 to ZnS/SnS2 was studied by varying the content of g-C3N4, and the sample were labeled as x%g-C3N4/ZnS/SnS2, where x is weight ratio of g-C3N4. For comparison, ZnS/SnS2 powder was also synthesized without adding g-C3N4. ZnS was prepared without adding SnCl4$5H2O and g-C3N4. SnS2 was synthesized without adding Zn(Ac)2$2H2O and gC3N4. g-C3N4/ZnS was prepared by 1 mmol Zn(Ac)2$2H2O, 0.1 g gC3N4 and 8 mmol thiourea, and g-C3N4/SnS2 was synthesized by 2 mmol SnCl4$5H2O, 0.1 g g-C3N4 and 8 mmol thiourea. All samples were synthesized under the same hydrothermal procedure. 2.4. Analytical and testing instrument Scanning electron microscopy (SEM) images and energy dispersive spectrometer (EDS) were obtained using Hitachi S5500 scanning electron microscopy. High resolution transmission electron microscopy (HRTEM) images were recorded on JEOL JEM-2010. X-ray diffraction (XRD) patterns of powder samples were measured on a Rigaku D/MAX 24000 diffractometer. X-ray photoelectron spectroscopy (XPS) of different samples was tested on a Thermo ESCALAB 250 spectrometer. UV visible (UVevis) diffuse reflectance spectroscopy (DRS) measurements were measured using a Perkin Elmer UV/VIS/NIR Spectrometer lambda 950. Photoelectrochemical measurements were tested on Shanghai Chenhua CHI-660D electrochemical system with Pt wire, saturated calomel electrode (SCE) and working electrode. 0.05 g catalyst sample, 0.01 g PEG and 0.25 mL distilled water mixed and injected onto ITO glass electrode. Then the working electrode was dried at 333 K for 2 h and calcined at 523 K for 4 h to remove PEG. 2.5. Catalytic experiment 50 W 410 nm LED light was used as light source. 0.02 g photocatalyst was totally dispersed in 30 mL MB solution (8 mg/L). The degradation efficiency (h) and apparent pseudo-first-order rate constant (kapp) were expressed as follows:
h¼ ln
C0 C 100% C0
C0 ¼ kappt C
(1)
(2)
2.1. Chemicals Zinc acetate dihydrate (Zn(Ac)2$2H2O), tin chloride pentahydrate (SnCl4$5H2O), polyethylene glycol (PEG), thiourea (CH4N2S), MB and ethyl alcohol (C2H5OH) were obtained from Sinopharm Chemical Reagent Co. Ltd. Melamine was purchased from Shanghai Chemical Reagent Co., Ltd. All chemicals were used as received.
2.2. Synthesis of g-C3N4 Polymeric g-C3N4 was synthesized by a simple pyrolysis of melamine without any templates. In a typical synthesis, 5 g melamine powder was dried and then heated to 773 K for 4 h.
Where: C0 is initial dy concentration and C is aqueous MB concentration at time t. 3. Result and discussion The synthetic process of g-C3N4/ZnS/SnS2 nanocomposites is shown in Fig. 1. Firstly, Zn(Ac)2$2H2O and SnCl4$5H2O were added in the deionized water to form homogeneous solution. Then, the ZnS/SnS2 heterostructures were prepared by an in-situ hydrothermal growth process after adding thiourea into the mixed aqueous solution. A portion of g-C3N4/ZnS/SnS2 was prepared through combining with g-C3N4 at 200 C. Fig. 2A, B, C and D show SEM images of as-synthesized ZnS, SnS2,
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
217
Fig. 1. The synthetic process of g-C3N4/ZnS/SnS2.
g-C3N4 and 20%g-C3N4/ZnS/SnS2, respectively. Fig. 2A displays the nanosphere morphology of ZnS, composed of nanoparticle with the diameter of about 60e80 nm. As indicated in Fig. 2B, hexagonal SnS2 nanoplate can be observed distinctly, the size is 400e600 nm and the thickness is ca. 20 nm. As indicated in Fig. 2C, it is obvious that g-C3N4 presented a thin laminar structure. Fig. 2D shows the SEM image of 20%g-C3N4/ZnS/SnS2 composite. The SnS2 nanoplate and ZnS nanoparticle were tightly connected with g-C3N4. This feature provides a channel for the efficient transmission and separation of the photoexcited electron and hole pairs among the gC3N4, ZnS and SnS2, which can improve the catalytic performance. Fig. 2E and F shows TEM image and HRTEM image of synthesized 20%g-C3N4/ZnS/SnS2, respectively. A clear fringe with an interval of 0.31 nm can be indexed to the (100) lattice plane of tetragonal SnS2 and that of 0.63 nm corresponds to the (100) crystallographic plane of ZnS, which further demonstrated that ZnS/SnS2 was deposited on g-C3N4. As indicated in Fig. 3, strong Bragg diffraction peaks at 2q ¼ 28.63 , 47.54 , 56.43 can be indexed to (111), (220), (311) planes for cubic phase of ZnS (JCPDS card No. 77-2100), and the peaks at 2q ¼ 27.84 , 31.88 , 41.42 , 49.63 , 52.24 , 54.56 , 60.44 and 67.12 were indexed to (100), (101), (102), (110), (111), (103), (201) and (202) planes for hexagonal phase of SnS2 (JCPDS No. 230677), respectively. For the pristine g-C3N4, the XRD peak at 27.32 was indexed as (002) diffraction plane (JCPDS card No. 87-1526), respectively. The g-C3N4/ZnS/SnS2 sample presented a three-phase composition: g-C3N4, ZnS and SnS2. No impurity phase is found in the g-C3N4/ZnS/SnS2 samples. Notably, the intensity of g-C3N4 peaks significantly strengthened with the increase of g-C3N4 component in g-C3N4/ZnS/SnS2 hybrids. The band structure of photocatalyst plays the key role in the photocatalytic activity. Fig. 4A displays the UVevis DRS of the gC3N4/ZnS/SnS2 hybrids, ZnS, SnS2 and g-C3N4. SnS2 and g-C3N4 present a wide absorbance in both UV and visible light ranges, while ZnS only presents absorbance in UV ranges. The absorption of
g-C3N4, ZnS and SnS2 are 663, 382 and 458 nm, respectively. When the three photocatalysts are combined together, g-C3N4/ZnS/SnS2 hybrid exhibits the absorption edges between 550 and 611 nm. This result shows that all of g-C3N4/ZnS/SnS2 hybrids can be used as visible-light-driven catalysts. The Eg energy of the semiconductors can be estimated by the following formula [46]:
.
ahv ðhv EgÞðn=2Þ hv
(3)
Where: a is the absorption coefficient. n ¼ 1 or 4 for direct-gap catalyst or indirect-gap catalyst. Therefore, on the basis of Eq. (3), plots of ðahvÞ2 versus energy (hv) for SnS2 and g-C3N4 were shown in Fig. 4B, Eg values of SnS2 and g-C3N4 are 1.66 and 2.68 eV, respectively. The plot of ðahvÞ1=2 versus energy (hv) for ZnS was shown in Fig. 4C, Eg of ZnS is 3.24 eV. To further investigate the interaction among g-C3N4, ZnS and SnS2, the chemical state and element composition for 20%g-C3N4/ ZnS/SnS2 were analyzed by XPS (Fig. 5). The fully scanned spectra show in Fig. 5A reveals that C, N, S, Zn, and Sn elements exist in the 20%g-C3N4/ZnS/SnS2 heterostructures. No impurity element is evident in the g-C3N4/ZnS/SnS2, which is in accordance with the XRD and EDS data. The C1s XPS spectra were showed in Fig. 5B, the broad and asymmetrical features suggested that the co-existence of distinguishable models. In addition, the deconvolution core level spectra at about 288.7 and 284.8 eV have been found. The peak at 284.8 eV is exclusively assigned to CeC bonding in pure carbon environment [47]. The peak at 288.7 eV is originated from carbon atoms bonded to three nitrogen atoms in the g-C3N4 lattice [48,49]. The high resolution N1s spectra in Fig. 5C shows the asymmetrical feature, indicating the co-existence of a number of distinguishable nitrogen environment, fitting with 3 results in binding energy of 398.14, 398.76 and 399.82 eV. The 2 peaks at 398.76 and 399.82 eV can be indexed as tertiary nitrogen (N-(C)3) and amino functional groups with a hydrogen atom (C-N-H) [50]. The peak centered at 398.14 eV is typically attributed to N atoms sp2-bonded to two
218
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
Fig. 2. SEM images of (A) ZnS, (B) SnS2, (C) g-C3N4, (D) 20%g-C3N4/ZnS/SnS2 and (E) TEM image and (F) HRTEM image of 20%g-C3N4/ZnS/SnS2.
Fig. 3. XRD patterns of g-C3N4, SnS2, ZnS, g-C3N4/SnS2, g-C3N4/ZnS and g-C3N4/ZnS/ SnS2 heterostructures.
carbon atoms (C]N-C), thus confirming the presence of graphitelike sp2-bonded g-C3N4 [51,52]. As indicated in Fig. 5D, the binding energies of Sn 3d 5/2 and Sn 3d 3/2 at 482.7 and 491.7 eV are in agreement with pure SnS2 [53]. Fig. 5E shows that Zn 2p 1/2 and Zn 2p 3/2 peaks of ZnS appeared at 1040.9 and 1017.7 eV respectively [54]. The high resolution S2p XPS spectra in Fig. 5F shows the peaks at 162.5 and 161.3 eV, which is assigned to S-Sn and S-Zn bond [43,55]. Fig. 6A exhibits the MB photodegradation by as-prepared gC3N4/ZnS/SnS2 hybrids under the irradiation of 410 nm LED light. The blank sample (no catalyst), individual g-C3N4, ZnS, SnS2, gC3N4/ZnS, g-C3N4/SnS2 and ZnS/SnS2 were also tested for comparison. Indeed, MB is very stable in visible light treatment when no catalysts were used. The pristine ZnS shows very low photoactivity due to the large Eg. The pure SnS2 and g-C3N4 show some visible light activity. When two components of g-C3N4, ZnS and SnS2 are composited together, the MB photodegradation is obviously improved over that of the individual components. Particularly, when g-C3N4, ZnS and SnS2 are combined together, constructing ternary C3N4/ZnS/SnS2 hybrid, the photocatalytic activity is dramatically enhanced under identical conditions. Among these ternary composite photocatalysts, 20%g-C3N4/ZnS/SnS2 hybrid can photodegrade 95% MB within 20 min, which is higher than other photocatalysts. As indicated in Fig. 6B, the linear relationship
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
219
Fig. 4. (A) UVevis DRS spectra of x%g-C3N4/ZnS/SnS2; (B) Plots of ðahvÞ2 versus energy for SnS2 and g-C3N4; (C) Plot of ðahvÞ1=2 versus energy for ZnS.
between lnC0/C and t suggests that the photocatalytic reaction is pseudo-first-order. According to Eq. (2) and Fig. 6B, the Kapp of the MB degradation is shown in Fig. 6C. The highest Kapp of g-C3N4/ZnS/ SnS2 hybrid is 0.148 min1, which is 8.74, 3.22 and 37.01 times as high as that of pristine g-C3N4, SnS2 and ZnS, respectively. Photocurrent is usually recorded to investigate the total amount of electrons and holes generation by the semiconductor. Fig. 7 shows the relationship between photocurrent and time curves of g-C3N4, ZnS, SnS2 and g-C3N4/ZnS/SnS2 with 2 on and off intermittent excitation cycles. The electrode of the different photocatalysts exhibits a rapid photocurrent response when 410 nm LED light illumination is work on on-off mode. The photocurrent suddenly decreases to its dark current state as soon as the LED light is turned off, and photocurrent increases sharply when the light is turned on. And the 20%g-C3N4/ZnS/SnS2 exhibits higher photocurrent intensity than that of ZnS, g-C3N4, SnS2 and other g-C3N4/ ZnS/SnS2 samples, which is in good accordance with the MB photodegradation. The valence band (VB) potentials of a photocatalyst can be theoretically obtained by the following equation:
EVB ¼ X Ec þ 0:5Eg
(4)
Where: EVB and X are the VB edge potential and the electronegativity of the catalyst, respectively. Ec is 4.5 eV. At the same time, the conduction band edge potential (ECB ) can be calculated by ECB ¼
EVB Eg . According Fig. 4 and eq. (4), the CB and VB of g-C3N4 are determined at 1.12 and þ 1.56 eV, respectively. The CB and VB edge potentials of SnS2 are calculated at 0.04 and þ 1.62 eV, respectively. The CB and VB edge potentials of ZnS are calculated at 0.95 and þ 2.29 eV, respectively. According to above results, the band structure of g-C3N4/ZnS/ SnS2 and possible photocatalytic degradation mechanism were displayed in Fig. 8. Both SnS2 and g-C3N4 semiconductor can be irradiated by visible light and generate electron and hole pairs. However, the photo-generated electrons and holes will recombine rapidly [56]. As indicated in Fig. 8, if electrons migration directly from g-C3N4 to SnS2, they will overcome a barrier height of 1.08 eV. Such high barrier height may hinder electrons migration [57,58]. Thus, how to design a proper buffer layer according to band structure engineering to smooth barrier height for easy carrier transport is a challenge [59]. The CB value of ZnS is 0.95 eV, which is a proper value between the CB of SnS2 and g-C3N4. Thus, the introduction of ZnS will produce g-C3N4/ZnS and ZnS/SnS2 interfaces. The barrier heights of the g-C3N4/ZnS and ZnS/SnS2 interfaces are calculated as 0.17 and 0.91 eV, respectively. As shown in Fig. 8, when g-C3N4/ZnS/SnS2 irradiated by 410 nm LED light, there is a new channel for the electron migration from g-C3N4 to SnS2 with lower barrier height, the electron of g-C3N4 CB can immediately transfer to ZnS CB and then transfer to SnS2 CB. The new channel can collect photo-generated electrons from g-C3N4 by a low barrier height of 0.17 eV and transfer them to SnS2 by another
220
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
Fig. 5. The overview (A) and the corresponding high-resolution XPS spectra (B) C1s, (C) N1s, (D) Sn3d, (E) Zn2p and (F) S2p of the as-prepared 20%g-C3N4/ZnS/SnS2.
lower barrier height of 0.91 eV. At the same time, the holes of SnS2 VB can immediately transfer to g-C3N4 VB. According to this method, the photoinduced charge carriers could be effectively separated, which can be described as follows:
H2O þ O 2 / OH þ OH /H2O2
(8)
hþþH2O/ OH þ Hþ
(9)
OH þ MB / degradation product
C3N4/ZnS/SnS2þhv / C3N4(hþþe)þSnS2(hþþe)/SnS2(e)þ ZnS(hþ)
(5)
e þ O2 / O2
(6)
2eþ2Hþ þ O2 þ 2Hþ/H2O2
(7)
(10)
As a result, the O 2 and OH were produced during the whole reaction, the organic pollutant could be mineralized due to these two strong oxidizing ions [60,61]. The photocatalytic performance of g-C3N4/ZnS/SnS2 is much better than that of ZnS and SnS2, which is attributed to that photoexcited electron-hole pairs would
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
221
Fig. 7. Photocurrentetime curves of different photocatalysts.
Fig. 8. Diagrammatic sketch of photocatalytic degradation mechanism.
same conditions. Fig. 9 shows photostability test of 20%g-C3N4/ZnS/ SnS2 heterostructures, the photocatalytic efficiency does not exhibit significant decrease after 20%g-C3N4/ZnS/SnS2 heterostructures were used for 3 times. This result demonstrated that 20% g-C3N4/ZnS/SnS2 heterostructures could exhibit excellent photocatalytic activity and stability. This superior stability enables recycling to be more effective in practical applications.
Fig. 6. (A) Photocatalytic degradation of MB under the irradiation of 410 nm LED light by different samples, (B) The ln(C0/C) as a function of irradiation time (t) for MB degradation, (C) The apparent pseudo-first-order rate constant kapp with different catalysts.
recombine hardly on the surface and interior of the sample [62,63]. And 20% g-C3N4 of g-C3N4/ZnS/SnS2 showed the best MB photodegradation among the different g-C3N4/ZnS/SnS2. Nevertheless, when the weight content of g-C3N4 further added exceeding its optimum value, the catalytic activity worse, that is because some gC3N4 will act as recombination center [64,65]. To provide reusable performance, the recycled photocatalytic experiment of 20%g-C3N4/ZnS/SnS2 was investigated under the
4. Conclusions In summary, ternary g-C3N4/ZnS/SnS2 heterojunctions with different content of g-C3N4 can be prepared by a simple hydrothermal method. Such a ternary g-C3N4/ZnS/SnS2 heterojunction can efficiently improve the photoactivity for degradation organic dyes by visible light as compared with binary ZnS/SnS2 heterojunction, pure SnS2, pure ZnS, g-C3N4/SnS2 or g-C3N4/ZnS. 20%gC3N4/ZnS/SnS2 heterojunctions displayed the highest photocatalytic performance with degradation of 95% MB with the visible light irradiation. The improved catalytic activities are attributed to effective charges transfer across heterojunction interface and decreased Eg value.
222
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
Fig. 9. Stability of photodegradation performance within three cycles for 20% g-C3N4/ ZnS/SnS2 heterostructures.
Acknowledgments This work was supported by the National Natural Science Foundation of China (51572103 and 51502106), the Distinguished Young Scholar of Anhui Province (1808085J14), the Foundation for Young Talents in College of Anhui Province (gxyqZD2017051), the Key Foundation of Educational Commission of Anhui Province (KJ2016SD53) and Anhui Provincial Innovation Team of Design and Application of Advanced Energetic Materials (KJ2015TD003). References [1] J.X. Low, J.G. Yu, M. Jaroniec, S. Wageh, A.A. Al-Ghamdi, Heterojunction photocatalysts, Adv. Mater. 29 (2017) 1601694. [2] Y. Xu, M. Kraft, R. Xu, Metal-free carbonaceous electrocatalysts and photocatalysts for water splitting, Chem. Soc. Rev. 45 (2016) 3039e3052. [3] J. Lv, J. Zhang, J. Liu, Z. Li, K. Dai, C. Liang, Bi SPR-promoted Z-scheme Bi2MoO6/ CdS-diethylenetriamine composite with effectively enhanced visible light photocatalytic hydrogen evolution activity and stability, ACS Sustain. Chem. Eng. 6 (2018) 696e706. [4] M. Hojamberdiev, R.M. Prasad, K. Morita, Y.F. Zhu, M.A. Schiavon, A. Gurlo, R. Riedel, Template-free synthesis of polymer-derived mesoporous SiOC/TiO2 and SiOC/N-doped TiO2 ceramic composites for application in the removal of organic dyes from contaminated water, Appl. Catal. B Environ. 115 (2012) 303e313. [5] Y. Huo, J. Zhang, K. Dai, Q. Li, J. Lv, G. Zhu, C. Liang, All-solid-state artificial Zscheme porous g-C3N4/Sn2S3-DETA heterostructure photocatalyst with enhanced performance in photocatalytic CO2 reduction, Appl. Catal. B Environ. 241 (2019) 528e538. [6] J. Liu, H. Wang, M. Antonietti, Graphitic carbon nitride “reloaded”: emerging applications beyond (photo)catalysis, Chem. Soc. Rev. 45 (2016) 2308e2326. [7] K. Dai, L. Lu, Q. Liu, G. Zhu, Q. Liu, Z. Liu, Graphene oxide capturing surfacefluorinated TiO2 nanosheets for advanced photocatalysis and the reveal of synergism reinforce mechanism, Dalton Trans. 43 (2014) 2202e2210. [8] X.F. Zhu, B. Cheng, J.G. Yu, W.K. Ho, Halogen poisoning effect of Pt-TiO2 for formaldehyde catalytic oxidation performance at room temperature, Appl. Surf. Sci. 364 (2016) 808e814. [9] H.G. Yu, P. Xiao, J. Tian, F.Z. Wang, J.G. Yu, Phenylamine-functionalized rGO/ TiO2 photocatalysts: spatially separated adsorption sites and tunable photocatalytic selectivity, ACS Appl. Mater. Interfaces 8 (2016) 29470e29477. [10] J. Zhang, J. Fu, S. Chen, J. Lv, K. Dai, 1D carbon nanofibers@TiO2 core-shell nanocomposites with enhanced photocatalytic activity toward CO2 reduction, J. Alloys Compd. 746 (2018) 168e176. [11] K. Dai, J. Lv, J. Zhang, G. Zhu, L. Geng, C. Liang, Efficient visible-light-driven splitting of water into hydrogen over surface-fluorinated anatase TiO2 nanosheets with exposed {001} facets/layered CdSediethylenetriamine nanobelts, ACS Sustain. Chem. Eng. 6 (2018) 12817e12826. [12] W.K. Wang, D.F. Xu, B. Cheng, J.G. Yu, C.J. Jiang, Hybrid carbon@TiO2 hollow spheres with enhanced photocatalytic CO2 reduction activity, J. Mater. Chem. A 5 (2017) 5020e5029. [13] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production
from water under visible light, Nat. Mater. 8 (2009) 76e80. [14] N. Tian, Y.H. Zhang, X.W. Li, K. Xiao, X. Du, F. Dong, G.I.N. Waterhouse, T.R. Zhang, H.W. Huang, Precursor-reforming protocol to 3D mesoporous gC3N4 established by ultrathin self-doped nanosheets for superior hydrogen evolution, Nano Energy 38 (2017) 72e81. [15] X.F. Wang, J.J. Cheng, H.G. Yu, J.G. Yu, A facile hydrothermal synthesis of carbon dots modified g-C3N4 for enhanced photocatalytic H2-evolution performance, Dalton Trans. 46 (2017) 6417e6424. [16] H.J. Yu, L. Shang, T. Bian, R. Shi, G.I.N. Waterhouse, Y.F. Zhao, C. Zhou, L.Z. Wu, C.H. Tung, T.R. Zhang, Nitrogen-Doped porous carbon nanosheets templated from g-C3N4 as metal-free electrocatalysts for efficient oxygen reduction reaction, Adv. Mater. 28 (2016) 5080e5086. [17] J. Low, C. Jiang, B. Cheng, S. Wageh, A.A. Al-Ghamdi, J. Yu, A review of direct Zscheme photocatalysts, Small Methods 1 (2017) 1700080. [18] J.X. Low, B. Cheng, J.G. Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Appl. Surf. Sci. 392 (2017) 658e686. [19] M. Wu, J. Zhang, C. Liu, Y. Gong, R. Wang, B. He, H. Wang, Rational design and fabrication of noble-metal-free NixP cocatalyst embedded 3D N-TiO2/g-C3N4 heterojunctions with enhanced photocatalytic hydrogen evolution, Chemcatchem 10 (2018) 3069e3077. [20] B.C. Zhu, P.F. Xia, Y. Li, W.K. Ho, J.G. Yu, Fabrication and photocatalytic activity enhanced mechanism of direct Z-scheme g-C3N4/Ag2WO4 photocatalyst, Appl. Surf. Sci. 391 (2017) 175e183. [21] Y. Wu, H. Wang, W. Tu, Y. Liu, S. Wu, Y.Z. Tan, J.W. Chew, Construction of hierarchical 2D-2D Zn3In2S6/fluorinated polymeric carbon nitride nanosheets photocatalyst for boosting photocatalytic degradation and hydrogen production performance, Appl. Catal. B Environ. 233 (2018) 58e69. [22] H. Che, G. Che, H. Dong, W. Hu, H. Hu, C. Liu, C. Li, Fabrication of Z-scheme Bi3O4Cl/g-C3N4 2D/2D heterojunctions with enhanced interfacial charge separation and photocatalytic degradation various organic pollutants activity, Appl. Surf. Sci. 455 (2018) 705e716. [23] K. Dai, L. Lu, C. Liang, Q. Liu, G. Zhu, Heterojunction of facet coupled g-C3N4/ surface-fluorinated TiO2 nanosheets for organic pollutants degradation under visible LED light irradiation, Appl. Catal. B Environ. 156e157 (2014) 331e340. [24] J. Zhang, J. Fu, Z. Wang, B. Cheng, K. Dai, W. Ho, Direct Z-scheme porous gC3N4/BiOI heterojunction for enhanced visible-light photocatalytic activity, J. Alloys Compd. 766 (2018) 841e850. [25] Z. Wang, J. Lv, J. Zhang, K. Dai, C. Liang, Facile synthesis of Z-scheme BiVO4/ porous graphite carbon nitride heterojunction for enhanced visible-lightdriven photocatalyst, Appl. Surf. Sci. 430 (2018) 595e602. [26] D. Lu, Q. Wang, K.K. Kondamareddy, A. Wang, H. Hao, Q. Wu, Efficiently visible-light-induced photoactivity of MoS2 nanoflowers/chromic oxide/protonated titanate nanoflakes edge-on ternary heterostructures for production of hydrogen, J. Alloys Compd. 761 (2018) 31e40. [27] L. Kashinath, K. Namratha, K. Byrappa, Sol-gel assisted hydrothermal synthesis and characterization of hybrid ZnS-RGO nanocomposite for efficient photodegradation of dyes, J. Alloys Compd. 695 (2017) 799e809. [28] Z. Ren, L. Li, B. Liu, X. Liu, Z. Li, X. Lei, C. Li, Y. Gong, L. Niu, L. Pan, Cr(VI) reduction in presence of ZnS/RGO photocatalyst under full solar spectrum radiation from UV/vis to near-infrared light, Catal. Today 315 (2018) 46e51. [29] J.F. Zhang, S. Wageh, A. Al-Ghamdi, J.G. Yu, New understanding on the different photocatalytic activity of wurtzite and zinc-blende CdS, Appl. Catal. B Environ. 192 (2016) 101e107. [30] Z. Li, J. Zhang, J. Lv, L. Lu, C. Liang, K. Dai, Sustainable synthesis of CeO2/CdSdiethylenetriamine composites for enhanced photocatalytic hydrogen evolution under visible light, J. Alloys Compd. 758 (2018) 162e170. [31] P. Sahatiya, A. Kadu, H. Gupta, P.T. Gomathi, S. Badhulika, Flexible, disposable cellulose-paper-based MoS2/Cu2S hybrid for wireless environmental monitoring and multifunctional sensing of chemical stimuli, ACS Appl. Mater. Interfaces 10 (2018) 9048e9059. [32] L. Yang, Y. Yao, G. Zhu, M. Ma, W. Wang, L. Wang, H. Zhang, Y. Zhang, Z. Jiao, Co doping of worm-like Cu2S: an efficient and durable heterogeneous electrocatalyst for alkaline water oxidation, J. Alloys Compd. 762 (2018) 637e642. [33] X. Jia, C. Tang, R. Pan, Y. Long, C. Gu, J. Li, Thickness-dependently enhanced photodetection performance of vertically grown SnS2 nanoflakes with large size and high production, ACS Appl. Mater. Interfaces 10 (2018) 18073e18081. [34] M. Bagherzadeh, R. Kaveh, A new SnS2-BiFeO3/reduced graphene oxide photocatalyst with superior photocatalytic capability under visible light irradiation, J. Photochem. Photobiol. A 359 (2018) 11e22. [35] J. Wang, J. Jin, X. Wang, S. Yang, Y. Zhao, Y. Wu, S. Dong, J. Sun, J. Sun, Facile fabrication of novel BiVO4/Bi2S3/MoS2 n-p heterojunction with enhanced photocatalytic activities towards pollutant degradation under natural sunlight, J. Colloid Interface Sci. 505 (2017) 805e815. [36] Y. Mi, H. Li, Y. Zhang, R. Zhang, W. Hou, One-pot synthesis of belt-like Bi2S3/ BiOCl hierarchical composites with enhanced visible light photocatalytic activity, Appl. Surf. Sci. 423 (2017) 1062e1071. [37] Y.-S. Lee, C.V.V.M. Gopi, M. Venkata-Haritha, H.-J. Kim, Recombination control in high-performance quantum dot-sensitized solar cells with a novel TiO2/ ZnS/CdS/ZnS heterostructure, Dalton Trans. 45 (2016) 12914e12923. [38] X. Hao, Y. Wang, J. Zhou, Z. Cui, Y. Wang, Z. Zou, Zinc vacancy-promoted photocatalytic activity and photostability of ZnS for efficient visible-lightdriven hydrogen evolution, Appl. Catal. B Environ. 221 (2018) 302e311. [39] J. Zhang, J.G. Yu, Y.M. Zhang, Q. Li, J.R. Gong, Visible light photocatalytic H2-
K. Dai et al. / Journal of Alloys and Compounds 778 (2019) 215e223
[40] [41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49] [50]
[51]
[52]
[53]
production activity of CuS/ZnS porous nanosheets based on photoinduced interfacial charge transfer, Nano Lett. 11 (2011) 4774e4779. Y. He, Y.F. Zhu, Solvothermal synthesis of sodium and potassium tantalate perovskite nanocubes, Chem. Lett. 33 (2004) 900e901. L. Wang, G. Jin, Y. Shi, H. Zhang, H. Xie, B. Yang, H. Sun, Co-catalyst-free ZnSSnS2 porous nanosheets for clean and recyclable photocatalytic H2 generation, J. Alloys Compd. 753 (2018) 60e67. H. Liu, C. Du, H. Bai, Y. Su, D. Wei, Y. Wang, G. Liu, L. Yang, Fabrication of plateon-plate Z-scheme SnS2/Bi2MoO6 heterojunction photocatalysts with enhanced photocatalytic activity, J. Mater. Sci. 53 (2018) 10743e10757. F. Zhang, Y. Zhang, G. Zhang, Z. Yang, D.D. Dionysiou, A. Zhu, Exceptional synergistic enhancement of the photocatalytic activity of SnS2 by coupling with polyaniline and N-doped reduced graphene oxide, Appl. Catal. B Environ. 236 (2018) 53e63. E. Liu, J. Chen, Y. Ma, J. Feng, J. Jia, J. Fan, X. Hu, Fabrication of 2D SnS2/g-C3N4 heterojunction with enhanced H2 evolution during photocatalytic water splitting, J. Colloid Interface Sci. 524 (2018) 313e324. J. Lv, K. Dai, J. Zhang, L. Lu, C. Liang, L. Geng, Z. Wang, G. Yuan, G. Zhu, In situ controllable synthesis of novel surface plasmon resonance-enhanced Ag2WO4/Ag/Bi2MoO6 composite for enhanced and stable visible light photocatalyst, Appl. Surf. Sci. 391 (2017) 507e515. K. Dai, L. Lu, C. Liang, G. Zhu, Q. Liu, L. Geng, J. He, A high efficient graphiticC3N4/BiOI/graphene oxide ternary nanocomposite heterostructured photocatalyst with graphene oxide as electron transport buffer material, Dalton Trans. 44 (2015) 7903e7910. Q.L. Xu, B. Cheng, J.G. Yu, G. Liu, Making co-condensed amorphous carbon/gC3N4 composites with improved visible-light photocatalytic H2-production performance using Pt as cocatalyst, Carbon 118 (2017) 241e249. P.F. Xia, B.C. Zhu, J.G. Yu, S.W. Cao, M. Jaroniec, Ultra-thin nanosheet assemblies of graphitic carbon nitride for enhanced photocatalytic CO2 reduction, J. Mater. Chem. A 5 (2017) 3230e3238. S.W. Cao, Y. Li, B.C. Zhu, M. Jaroniec, J.G. Yu, Facet effect of Pd cocatalyst on photocatalytic CO2 reduction over g-C3N4, J. Catal. 349 (2017) 208e217. J.W. Fu, B.C. Zhu, C.J. Jiang, B. Cheng, W. You, J.G. Yu, Hierarchical porous Odoped g-C3N4 with enhanced photocatalytic CO2 reduction activity, Small 13 (2017). C.Y. Liu, Y.H. Zhang, F. Dong, A.H. Reshak, L.Q. Ye, N. Pinna, C. Zeng, T.R. Zhang, H.W. Huang, Chlorine intercalation in graphitic carbon nitride for efficient photocatalysis, Appl. Catal. B Environ. 203 (2017) 465e474. J. Zhang, J. Lv, K. Dai, Q. Liu, C. Liang, G. Zhu, Facile and green synthesis of novel porous g-C3N4/Ag3PO4 composite with enhanced visible light photocatalysis, Ceram. Int. 43 (2017) 1522e1529. X. Cheng, B. Zhang, J. Shi, J. Zhang, L. Zheng, J. Zhang, D. Shao, X. Tan, B. Han, G. Yang, Tin(IV) sulfide greatly improves the catalytic performance of UiO-66 for carbon dioxide cycloaddition, Chemcatchem 10 (2018) 2945e2948.
223
[54] J. Lv, J. Zhang, K. Dai, C. Liang, G. Zhu, Z. Wang, Z. Li, Controllable synthesis of inorganiceorganic Zn1xCdxS-DETA solid solution nanoflowers and their enhanced visible-light photocatalytic hydrogen-production performance, Dalton Trans. 46 (2017) 11335e11343. [55] X. Hao, J. Zhou, Z. Cui, Y. Wang, Y. Wang, Z. Zou, Zn-vacancy mediated electron-hole separation in ZnS/g-C3N4 heterojunction for efficient visiblelight photocatalytic hydrogen production, Appl. Catal. B Environ. 229 (2018) 41e51. [56] J. Lv, K. Dai, L. Lu, L. Geng, C. Liang, G. Zhu, Cu/Ag/Ag3PO4 ternary composite: a hybrid alloy-semiconductor heterojunction structure with visible light photocatalytic properties, J. Alloys Compd. 682 (2016) 778e784. [57] Q. Wu, J. Hou, H. Zhao, Z. Liu, X. Yue, S. Peng, H. Cao, Charge recombination control for high efficiency CdS/CdSe quantum dot co-sensitized solar cells with multi-ZnS layers, Dalton Trans. 47 (2018) 2214e2221. [58] P. Duy Phong, S. Kim, L. Anh Huy Tuan, J. Park, J. Yi, Diminished band discontinuity at the phi interface of narrow-gap a-SiGe:H solar cell by hydrogenated amorphous silicon oxide buffer layer, J. Alloys Compd. 762 (2018) 616e620. [59] S. Lie, M.I. Sandi, Y.F. Tay, W. Li, J.M.R. Tan, D.M. Bishop, O. Gunawan, L.H. Wong, Improving the charge separation and collection at the buffer/ absorber interface by double-layered Mn-substituted CZTS, Sol. Energy Mater. Sol. Cells 185 (2018) 351e358. [60] H. Wang, X. Qiu, W. Liu, D. Yang, Facile preparation of well-combined ligninbased carbon/ZnO hybrid composite with excellent photocatalytic activity, Appl. Surf. Sci. 426 (2017) 206e216. [61] H. Yu, B. Huang, H. Wang, X. Yuan, L. Jiang, Z. Wu, J. Zhang, G. Zeng, Facile construction of novel direct solid-state Z-scheme AgI/BiOBr photocatalysts for highly effective removal of ciprofloxacin under visible light exposure: mineralization efficiency and mechanisms, J. Colloid Interface Sci. 522 (2018) 82e94. [62] J. Zhang, J. Lv, K. Dai, C. Liang, Q. Liu, One-step growth of nanosheet-assembled BiOCl/BiOBr microspheres for highly efficient visible photocatalytic performance, Appl. Surf. Sci. 430 (2018) 639e646. [63] Q.C. Yuan, D. Liu, N. Zhang, W. Ye, H.X. Ju, L. Shi, R. Long, J.F. Zhu, Y.J. Xiong, Noble-metal-free janus-like structures by cation exchange for Z-scheme photocatalytic water splitting under broadband light irradiation, Angew. Chem. Int. Ed. 56 (2017) 4206e4210. [64] A. Wu, C. Tian, Y. Jiao, Q. Yan, G. Yang, H. Fu, Sequential two-step hydrothermal growth of MoS2/CdS core-shell heterojunctions for efficient visible light-driven photocatalytic H2 evolution, Appl. Catal. B Environ. 203 (2017) 955e963. [65] Y. Zou, J.-W. Shi, D. Ma, Z. Fan, C. Niu, L. Wang, Fabrication of g-C3N4/Au/CTiO2 hollow structures as visible-light-driven Z-scheme photocatalysts with enhanced photocatalytic H2 evolution, Chemcatchem 9 (2017) 3752e3761.