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ZnS nanoplate with high adsorption capability and photocatalytic activity
Accepted Manuscript Facile ion-exchange synthesis of mesoporous Bi2S3/ZnS nanoplate with high adsorption capability and photocatalytic activity Dan-Ni Xiong, Gui-Fang Huang, Bing-Xin Zhou, Qian Yan, An-Lian Pan and Wei-Qing Huang PII: DOI: Reference:
S0021-9797(15)30325-8 http://dx.doi.org/10.1016/j.jcis.2015.11.015 YJCIS 20866
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
29 July 2015 5 November 2015 8 November 2015
Please cite this article as: D-N. Xiong, G-F. Huang, B-X. Zhou, Q. Yan, A.W-Q. Huang, Facile ion-exchange synthesis of mesoporous Bi2S3/ZnS nanoplate with high adsorption capability and photocatalytic activity, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.11.015
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Facile ion-exchange synthesis of mesoporous Bi2S3/ZnS nanoplate with high adsorption capability and photocatalytic activity Dan-Ni Xiong, Gui-Fang Huang, Bing-Xin Zhou, Qian Yan, An-Lian Pan and Wei-Qing Huang# Department of Applied Physics, School of Physics and Electronics, Hunan University, Changsha 410082, China
Abstract: Novel Bi2S3/ZnS nanoplates have been successfully prepared by simple reflux and cation exchange reaction between the preformed ZnS spheres and Bi(NO 3)3·5H2O. The synthesized Bi2S3/ZnS nanoplates are mesoporous structures, possess a high specific surface area of 101.30 m2/g and exhibit high adsorption capability and photocatalytic activity for methylene blue (MB) degradation under UV light irradiation. The high adsorption capability and photocatalytic activity can be ascribed to the fact that the formation of Bi2S3/ZnS nanoplates with large specific surface area provides more reactive sites and facilitates the separation of photogenerated electron–hole pairs. The possible formation mechanism of Bi2S3/ZnS nanoplates is proposed based on the time-dependent observation. Moreover, a tentative mechanism for degradation of MB over Bi2S3/ZnS has been proposed involving ·OH radical and photoinduced holes as the active species, which is confirmed by using methanol or ammonium oxalate as scavengers. This work provides a cost-effective method for large-scale synthesis of composite with controlled architectural morphology and highly promising applications in photocatalysis.
Keywords: Bi2S3/ZnS; mesoporous; photocatalytic activity; nanoplates.
.Corresponding author. E-mail address: [email protected] #.Corresponding author. E-mail address: [email protected] 1
1． Introduction Semiconductor-based photocatalysis is a promising avenue to solve the worldwide energy shortage and environmental pollution using the abundant solar light [1-4]. The prerequisite for its application is to design efficient photocatalysts for pollutant degradation and water splitting. So far, numerous photocatalysts have been reported to show activity for the photodegradation of organic pollutants or/and water splitting. Among the various semiconductor photocatalysts, TiO 2 is undoubtedly the most popular photocatalyst that have been extensively studied. However, its large band gap and the rapid recombination of photogenerated electrons and holes greatly restrict its practical applications. Thus, various modified TiO2 and TiO2-alternative photocatalysts have been designed and fabricated [5, 6]. Metal sulfides, in particular ZnS, have been extensively investigated in photocatalysis owing to their unique catalytic functions compared to those of TiO 2 [7, 8]. Moreover, many strategies, such as nonmetal and metal doping [9, 10], surface sensitization and composites formation with various other semiconductors, have been developed to improve the photocatalytic efficiency of ZnS. Especially, composite formation can effectively decrease the recombination of photogenerated electrons and holes and thus enhance the photocatalytic activity [4, 11-13]. For example, Yu et al. demonstrate that the Zn1-xCdxS solid solution synthesized using hydrothermal method exhibit high photocatalytic activity for the H 2-production under visible light illumination [14]. It is reported that CuS/ZnS porous nanosheet photocatalysts prepared by hydrothermal and cation exchange reaction show enhanced visible light photocatalytic H 2-production activity due to the interfacial charge transfer from the valence band of ZnS to CuS [15]. Bi2S3 with a narrow band gap of 1.3 eV, has received great attention in photocatalysis. However, the photocatalytic activity of pure Bi2S3 is low due to the rapid recombination of photogenerated charges [16]. It is reported that Bi2S3 can be used as a potential efficient photocatalyst through the combination with other semiconductors such as TiO 2 [17], BiOI [18], ZnO [19], Bi2O3 [20] and CdS [16] owing to the efficient separation of photogenerated carriers. More recently, Wu et al. have synthesized Bi 2S3/ZnS composite spheres by hydrothermal method at 120 ℃, and found that the nanocomposites exhibit enhanced photochemical efficiency for the
2
degradation of rhodamine B (RhB) [21]. It is generally considered that the photocatalysis reaction occurs on the surface of photocatalysts, and the photocatalytic activity of catalysts greatly depends on their morphology, microstructure, and surface properties. Therefore, great efforts have been made to obtain photocatalysts
with
controllable
morphologies
and
structures.
Two-dimensional
(2D)
nanostructures with large specific surface area can provide more reactive sites at the surface for adsorbing reactant molecules and the photocatalytic reaction, thus are highly desirable for obtaining superior catalytic performances. For example, CeO2/TiO2 nanobelt heterostructures [22] synthesized via a hydrothermal method exhibit a markedly enhanced photocatalytic activity in the degradation of organic pollutants such as methyl orange (MO) under either UV or visible light irradiation. It is reported that flower-like CdSe architectures composed of ultrathin nanosheets show much better photocatalytic H 2 evolution activity under visible light irradiation compared with well-studied CdSe quantum dots [23]. In the present strategy, we report the synthesis of Bi2S3/ZnS nanocrystals by a low cost and simple reflux and cation exchange method using the preformed ZnS nanospheres and Bi(NO3)3·5H2O as a precursor. It is interesting that 2D mesoporous Bi2S3/ZnS nanoplates with large specific surface area can be obtained. Moreover, the synthesized Bi 2S3/ZnS composites exhibit high adsorption capability and photocatalytic activity under UV light irradiation.
2.
Experimental Section
2.1 Materials Sodium Citrate (Na3C6H5O7), Zinc sulfate heptahydrate (ZnSO4·7H2O), bismuth nitrate hexahydrate (Bi(NO3)3·5H2O), ethyl alcohol, thiourea (CN2H4S) and ammonium hydroxide (NH3·H2O) are analytical grade and purchased from Shanghai Chemical Reagent Factory of China without further treatment.
2.2 Preparation of ZnS nanospheres and Bi2S3/ZnS composite ZnS nanospheres are synthesized with following procedure. 25 mmol Na 3C6H5O7 and 10 mmol ZnSO 4·7H2O are dissolved in 100 mL deionized water under vigorous stirring and marked as solution A. The pH of solution A is adjusted to about 9.0, using NH 3·H2O. At the same time, 30
3
mmol CN 2H4S is dissolved in 50 mL deionized water under vigorous stirring, marked as solution B. When solution A is heated to 80 ℃ in thermostatic water bath with stirring, B solutions is added dropwise slowly. The mixture is stirred continuously at 80 ℃ for 3 h. The as-synthesized white products are separated from the solution by centrifugation and washed with deionized water and alcohol several times. Bi2S3/ZnS nanoplates are synthesized by simple reflux and cation exchange method using dispersed ZnS solid spheres as a precursor at 120 ℃ for 3 h. Typically, the as-prepared ZnS nanospheres are dispersed in 150 mL deionized water and sonicated for 1 h. Then, the suspension is transferred into a 250 mL round-bottom flask. Appropriate Bi(NO3)3·5H2O aqueous solutions is added dropwise under vigorous stirring when the above-mentioned suspension reach 120 ℃. It can be observed that the color changes gradually from white to bistre. After the reaction for 3 h, the flask is cooled naturally. The final burnt sienna solid product is centrifuged, washed with deionized water and alcohol several times, and finally dried at 60 ℃ for 12 h. For comparison, pure Bi2S3 nanorods are obtained under the same experimental conditions with CN2H4S as the sulfur source in the absence of ZnS.
2.3 Characterization The crystal structures of the obtained samples are characterized by power X-ray diffraction (XRD, Siemens D-5000 diffractometer with Cu Kαirradiation). The morphological details and nanoparticle size of the prepared samples are probed by an S-4800 field emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images are observed using a FEI Tecai F20 electron microscope to identity the morphology, size and nanocrystal structures of the sample. The sample for TEM observation is prepared by dispersing the Bi 2S3/ZnS composites in an absolute ethanol solution under ultrasonic irradiation, and then the dispersion is dropped on a carbon-copper grid. The Brunauer-Emmett-Teller (BET) specific surface area of the products is analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus.
2.4 Measurement of photocatalytic activity The photocatalytic activity of the prepared samples is evaluated by degradation of MB 4
aqueous solution under the irradiation of 300 W UV lamps. Before irradiation, solutions suspended with photocatalysts are sonicated in the dark for 20 minutes to ensure the adsorption-desorption equilibrium of MB on the surface of photocatalysts. During the UV light irradiation, 4 mL suspension is taken out at given time interval and centrifuged for analysis by a UV–VIS spectroscopy. The characteristic absorption peak of MB at 664 nm is used to determine the degradation efficiency.
2.5. Active species trapping experiments In order to detect the active species in the photocatalytic system, 0.5 mM methanol and 0.5 mM ammonium oxalate are added as the active species scavengers of the hydroxyl radicals (•OH) and holes (h+), respectively [24, 25]. The procedure is similar to the former photocatalytic activity experiment.
3.
Results and discussion
3.1 Morphology and structure characterization of catalyst Bi2S3/ZnS nanoplates are prepared by cation exchange reaction using the as-prepared ZnS nanospheres and Bi(NO3)3 as precursors at 120 ℃ for 3 h under reflux condition. The crystal structure of the prepared samples is characterized by XRD analysis. Fig. 1 displays the XRD patterns of pure Bi2S3, ZnS and Bi2S3/ZnS nanoplates. It is clear that all the diffraction peaks of pure Bi2S3 (Fig. 1a) match perfectly with the crystalline planes of orthorhombic Bi2S3 (JCPDS No. 17-0320). The characteristic peaks are located at the 2θ values of 15.7°, 17.6°, 22.4°, 23.8°, 25.0°, 27.5°, 28.6°, 31.8°, 33.0°, 34.0°, 35.6°, 39.2°, 39.9°, 45.7°, 46.5° and 52.6°, which correspond to the (020), (120), (220), (101), (130), (021), (211), (221), (301), (311), (240), (041), (141), (002), (431) and (351) crystal planes of Bi2S3, respectively. The pattern of pristine ZnS (Fig. 1b) could be readily indexed to the pure cubic phase of sphalerite ZnS (JCPDS No. 05-0566). While the XRD patterns of Bi2S3/ZnS composites (Fig. 1c) clearly indicate the bi-phase existence of Bi2S3 and ZnS, the broadening of the diffraction peaks of Bi2S3 is caused by the small crystallite size. As is expected, there is no third phase formation except for Bi2S3 and ZnS in the XRD pattern of Bi2S3/ZnS composite. The typical morphology of the prepared samples is observed by SEM. Pure Bi2S3 (Fig. 2a) is
5
found to consist of nanorod with an average length of about 500 nm. The morphology of pure ZnS (Fig. 2b) shows spherical nanoparticles. The size of the nanospheres is uniform with an average diameter of approximately 60 nm. Whereas, Bi2S3/ZnS composites mainly remain nanoplate morphology as shown in Fig. 2c. Further observation in detail shows that thickness of the nanoplates is about 10 nm and the surface of Bi 2S3/ZnS nanoplates is rough and porous. Moreover, according to SEM observation, the diameter of the nanoplates mainly ranges from 80 nm to 90 nm as shown in the histogram of nanoparticles size distribution (Fig. 3). TEM and HRTEM are used to further observe the morphology, size and the crystallographic structure of Bi2S3/ZnS composite. Fig. 2d shows the typical TEM image of Bi 2S3/ZnS composite. It can be seen that the primary morphology is nanoplates with diameters of approximately 80–90 nm, which is in agreement with the SEM observation. The corresponding HRTEM images of the same Bi2S3/ZnS composite in Figs. 2(e, f) show clear lattice fringes, which indicate that the prepared Bi2S3/ZnS composite are well crystalline and may be used to identify the crystallographic spacing. Figs. 2(e, f) clearly show two distinct sets of lattice fringes. The interlayer spacing d = 0.312 nm matches to the (111) plane of sphalerite ZnS, while the interlayer spaces 0.504 nm is corresponding to the (220) plane of orthorhombic Bi2S3, respectively.
3.2 BET specific surface area and porosity To further study the porous structure of the synthesized Bi 2S3/ZnS nanoplates, nitrogen adsorption–desorption isotherms are measured to determine the specific surface area and the pore size. Fig. 4 shows the nitrogen adsorption–desorption isotherms of Bi2S3/ZnS nanoplates. It can be observed that the isotherms show a linear absorption in the low range of relative pressure (P/P0<0.7), while the curves exhibit a small hysteresis loop in the high relative pressure range (P/P0>0.7). The hysteresis observed in the isotherms suggests the existence of abundant mesoporous structures in the architectures according to IUPAC classification [26, 27]. The quantitative calculation reveals that the synthesized Bi2S3/ZnS nanoplate possesses a high specific surface area of 101.30 m2/g, which is much higher than the reported pure ZnS (31.09 m2/g) [28] and Bi2S3 (20.30 m2/g) [29]. The large surface area may be attributed to the presence of mesoporosity in the nanoplates, which is beneficial for the applications as photocatalyst for the adsorption of the pollutants and would provide efficient transport pathways to reactant molecules 6
and products, and thus enhance the adsorption capability and the photocatalytic activity.
3.3 Formation mechanism As 2D Bi2S3/ZnS nanoplate morphology has rarely been studied before, studying the formation mechanism of Bi2S3/ZnS nanoplates would be of particular interest. To substantially understand the growth mechanisms of Bi2S3/ZnS nanoplates, we study the time-dependent evolutions of the morphology by SEM, as shown in Fig. 5. The typical SEM image of the sample collected at a reaction time of 10 min is presented in Fig. 5a. Fig. 5a shows that the samples display plate-like morphology interspersed with some nanoparticles. It is worth noting that the size of spheres (about 30 nm) is much smaller than that of the pristine ZnS spheres as shown in Fig. 2b, suggesting the decomposition of the original ZnS nanospheres and the formation of Bi2S3 on the ZnS surface to form composite structure. The morphology information in Fig. 5a implies that the transformation of ZnS to Bi2S3 via the cation exchange reaction proceeds as Bi(NO3)3·5H2O is added into the ZnS suspension solution. It can be verified by the observed color change from white to bistre during the experiment. As the reaction time proceeds to 30 min, it is notable that the nanoplates are basically formed as reflected in Fig. 5b. Based on the above observations, a possible growth process for Bi2S3/ZnS nanoplate can be reasonably inferred to be the decomposition and regrowth mechanism as proposed in Scheme 1. After the ZnS nanospheres are dispersed in deionized water, there is a small amount of S2− in the solution due to the dynamic equilibrium and local decomposition of ZnS nanospheres (equation (1)). As Bi (NO3)3·5H2O is added in the suspensions, Bi2S3 will produce according to the chemical (equation (2)) due to the small solubility constants (Ksp) of Bi 2S3 (1.82×10−99). As shown in Fig. 5a, the smaller ZnS spheres are uniformly distributed in Bi2S3 crystallites and form composite structure, indicating that the nucleation and growth process of Bi 2S3 is initialized preferentially on the rough surface of the dissolving ZnS spheres and the formed Bi2S3 crystallites gradually cover the surface of ZnS spheres since it can provide high-energy nucleation sites for the growth of Bi2S3. As the reaction proceeds, the concentration of S2− in the solution around the ZnS nanospheres is higher than that in the bulk owing to the continous decomposition of ZnS nanospheres and the concentration polarization. Therefore, Bi2S3 crystallites basically grow around the ZnS nanospheres, and form a nanoplate structure with a size of about 80-90 nm as 7
shown in Fig. 5b through the nucleation–aggregation deposition process as the reaction time increases to 30 minutes. As the reaction time further increases, the continous decomposition of ZnS and nucleation–aggregation deposition process of Bi2S3 lead to the formation of Bi2S3/ZnS nanoplate as shown in Fig. 2c.
ZnS ⇌ Zn2++S2-
(1)
2Bi3++3S2- ⇌ Bi2S3
(2)
3.4 Photocatalytic behavior The time-dependent absorption spectrum of MB solution under UV light illumination in the presence of Bi2S3/ZnS nanoplates is shown in Fig. 6a. It can be found that the characteristic absorption peaks corresponding to MB decrease greatly as the adsorption-desorption equilibrium of MB on the surface of photocatalysts reaches after 20 min ultrasonic treatment, indicating that Bi2S3/ZnS nanoplates show high adsorption capacities for MB molecules. The characteristic absorption peaks of MB further diminish as the exposure time increases due to the degradation of MB. To evaluate the photocatalytic performance quantitatively, the degradation efficiency is determined by the following function:
C0 Ct A 100% (1 t ) 100% C0 A0 Where C0 is the initial concentration of MB and Ct is the concentration after degradation. At and A0 are the corresponding absorbance values. For comparison, the photocatalytic performances of pure ZnS and Bi2S3 are also evaluated. The photocatalytic activity of Bi 2S3/ZnS nanoplates, pure ZnS and pure Bi 2S3 are presented in Fig. 6b. We can see clearly from Fig. 6b that all of the samples show adsorption abilities to MB during the dark, that is, the absorption amount of Bi2S3/ZnS nanoplates may reach about 46.5 % of MB, which is much higher than that of pure ZnS (6.4 %) and pure Bi2S3 (12.0 %). The enhanced adsorption capacity for Bi2S3/ZnS nanoplates can be attributed to the modified surface with larger specific surface area as verified by the BET measurement. After 20 min UV light irradiation, the remaining MB molecules are about 12.3 % for Bi2S3/ZnS nanoplates, which is much lower than those for pure ZnS (25.1 %) and pure Bi 2S3 (40.4 %) sample. The results suggest that Bi2S3/ZnS is 8
an excellent composite photocatalyst for pollutant degradation.
3.5 Photocatalytic degradation mechanism of MB over Bi 2S3/ZnS nanocomposite On the basis of above experimental observations, a tentative mechanism can be proposed for the degradation of MB over Bi2S3/ZnS photocatalyst. As is well known, the adsorption of the pollutants on the catalyst's surface is an essential process for photocatalytic oxidation. The high adsorption capacities of Bi2S3/ZnS composites as shown in Fig. 6 is exciting for the large adsorption and active removal of MB. In the case of pure Bi2S3, the photocatalytic activity is largely restricted by the recombination of the photoinduced electrons and holes because of its narrow band gap. While the photoinduced electrons and holes may transfer effectively when Bi2S3/ZnS composites are used as the photocatalysts under UV light irradiation. The photoinduced electrons on the Bi 2S3 surface would easily transfer to ZnS since the reformed conduction band (CB) edge potential of Bi2S3 (−1.479 eV) is more negative than that of ZnS (−0.945 eV) [21]. The electrons transferring on the CB edge of ZnS can be subsequently trapped by O2 molecules adsorbed on the surface of the composite to form H2O2. H2O2 reacts with electrons in succession to form active ·OH, which are crucial oxidizing species responsible for the MB degradation. Meanwhile, the photoinduced holes tend to keep in the valence band (VB) of Bi2S3. Since the VB position of Bi 2S3 (1.48 eV) [30] is much more negative than the potential of the H2O/·OH (2.7 eV) couple, the photogenerated holes on the VB of Bi 2S3 cannot react with absorbed water molecules to form ·OH radicals. Therefore, the photoinduced electrons form reactive ·OH radicals, which are responsible for the subsequent degradation of pollutants, while the photoinduced holes can directly oxide MB molecules without forming ·OH radicals, and finally produce CO2, H2O, etc. This is similar to Houas et al.’s charge separation mechanism [31]. The formation of oxidative intermediate species, such as photoinduced holes and ·OH radicals, under photoreaction conditions and their role in the MB degradation process are investigated indirectly, with the addition of appropriate scavengers of these species. The comparison between the original photocatalytic degradation of MB over MB–Bi2S3/ZnS dispersions with those obtained after addition of scavengers in the initial solution, under otherwise identical conditions, is presented in Fig. 7. 9
It is noticeable that the photocatalytic activity of Bi 2S3/ZnS nanoplates is largely suppressed by the addition of methanol or ammonium oxalate as presented in Fig. 7. The presence of methanol and ammonium oxalate, which act as the active species scavengers of the •OH and holes, respectively, greatly suppresses the photodegradation of MB, indicating that the •OH and photoinduced holes serves as the main active oxidative species for the photodegradation of MB over Bi2S3/ZnS under UV light irradiation. Generally, the photocatalytic activity of the photocatalyst is mainly attributed to its ability for adsorbing target pollutant, and the separation of the photoinduced electrons and holes in the catalyst. Herein, the synthesized 2D mesoporous Bi2S3/ZnS nanoplates with large specific surface area can provide more surface-active sites for adsorbing target pollutant. Moreover, the synthesized Bi2S3/ZnS composites exhibit high adsorption capability and photocatalytic activity.
4. Conclusion In conclusion, we have demonstrated a new and facile reflux method to successfully synthesize 2D Bi2S3/ZnS nanoplate composite using ZnS nanospheres and Bi(NO3)3·5H2O as starting reactant. The synthesized Bi2S3/ZnS nanoplates are mesoporous structures with a high specific surface area of 101.30 m2/g. Moreover, the as-prepared Bi2S3/ZnS nanoplates show high adsorption capability and photocatalytic activity in the degradation of MB in aqueous solution. The possible growth process for Bi 2S3/ZnS nanoplates can be inferred to be the decomposition and regrowth mechanism based on the time-dependent observation. A tentative mechanism for degradation of MB over Bi2S3/ZnS is proposed involving ·OH radical and photoinduced holes as the active oxidative species. This work provides a cost-effective method for large-scale synthesis of composite with controlled architectural morphology. Furthermore, the inherent characteristics of Bi2S3/ZnS nanoplates may open up the prospect to understand and modify other semiconductor materials, which may have promising for environment and energy applications.
Acknowledgments This work is supported by the Changsha Science and Technology Plan Projects, China (Grant No. K1403067-11) and Environmental Protection Science and Technology Project of Hunan Province.
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12
20
** * ** * * * # * 30
40
50
(242)
(152) (721)
# ZnS
(a)
(311)
(220)
*# * ** **
(351)
(211)
(301)(221) (311) (240) (041) (141) (241) (002) (431) (251)
(130)
(111)
(021)
(220) (101)
(020) (120)
Intensity(a.u.)
* Bi2S3
(b)
#* 60
(c)
* 70
2Theta(degree) Fig. 1. XRD patterns of Bi2S3 (a), ZnS (b) and Bi2S3/ZnS nanocomposite (c).
13
Fig. 2. SEM images of as-prepared photocatalysts: Bi2S3 (a), ZnS (b), Bi2S3/ZnS nanocomposite (c), and TEM image (d), HRTEM image of Bi2S3/ZnS nanocomposite (e, f).
14
Freqency 50
60
70
80 90 100 Diameter(nm)
110
120
Fig. 3. Size distribution of Bi2S3/ZnS nanoparticle.
15
3
Volume adsorbed(cm /g,STP)
80
60
40 Adsorption Desorption
20 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure(P/P0)
Fig. 4. Nitrogen adsorption–desorption isotherms and the corresponding pore size distributions (inset) of Bi2S3/ZnS nanocomposite.
16
(b)
(a)
Fig. 5. SEM images of reaction time depended ZnS / Bi2S3 nanocomposite: 10min (a) 30min (b).
17
Fig. 6. (a) Absorption spectra of MB aqueous solution in the presence Bi2S3/ZnS nanocomposite under UV light irradiation; (b) Photodegradation efficiencies (Ct/C0) of MB as a function of irradiation time by different photocatalysts under UV light irradiation.
18
1.0
A-Bi2S3/ZnS M-Bi2S3/ZnS Bi2S3/ZnS
Ct/C0
0.8 0.6 0.4 0.2 -20
dark light on
0
5
10 Time(min)
15
20
Fig. 7. Photocatalytic degradation of MB over Bi2S3/ZnS photocatalysts alone and with the addition of methanol (M-Bi2S3/ZnS) or ammonium oxalate (A-Bi2S3/ZnS).
19
Scheme 1. Schematic illustration of morphology evolution of Bi2S3/ZnS nanoplates: (a) ZnS nanospheres decomposite and release S2- ; (b) Bi2S3 crystallites basically grow around the ZnS nanospheres and (c) form a nanoplate.
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Graphical abstract
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S0021-9797(15)30325-8 http://dx.doi.org/10.1016/j.jcis.2015.11.015 YJCIS 20866
To appear in:
Journal of Colloid and Interface Science
Received Date: Revised Date: Accepted Date:
29 July 2015 5 November 2015 8 November 2015
Please cite this article as: D-N. Xiong, G-F. Huang, B-X. Zhou, Q. Yan, A.W-Q. Huang, Facile ion-exchange synthesis of mesoporous Bi2S3/ZnS nanoplate with high adsorption capability and photocatalytic activity, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.11.015
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 ion-exchange synthesis of mesoporous Bi2S3/ZnS nanoplate with high adsorption capability and photocatalytic activity Dan-Ni Xiong, Gui-Fang Huang, Bing-Xin Zhou, Qian Yan, An-Lian Pan and Wei-Qing Huang# Department of Applied Physics, School of Physics and Electronics, Hunan University, Changsha 410082, China
Abstract: Novel Bi2S3/ZnS nanoplates have been successfully prepared by simple reflux and cation exchange reaction between the preformed ZnS spheres and Bi(NO 3)3·5H2O. The synthesized Bi2S3/ZnS nanoplates are mesoporous structures, possess a high specific surface area of 101.30 m2/g and exhibit high adsorption capability and photocatalytic activity for methylene blue (MB) degradation under UV light irradiation. The high adsorption capability and photocatalytic activity can be ascribed to the fact that the formation of Bi2S3/ZnS nanoplates with large specific surface area provides more reactive sites and facilitates the separation of photogenerated electron–hole pairs. The possible formation mechanism of Bi2S3/ZnS nanoplates is proposed based on the time-dependent observation. Moreover, a tentative mechanism for degradation of MB over Bi2S3/ZnS has been proposed involving ·OH radical and photoinduced holes as the active species, which is confirmed by using methanol or ammonium oxalate as scavengers. This work provides a cost-effective method for large-scale synthesis of composite with controlled architectural morphology and highly promising applications in photocatalysis.
Keywords: Bi2S3/ZnS; mesoporous; photocatalytic activity; nanoplates.
.Corresponding author. E-mail address: [email protected] #.Corresponding author. E-mail address: [email protected] 1
1． Introduction Semiconductor-based photocatalysis is a promising avenue to solve the worldwide energy shortage and environmental pollution using the abundant solar light [1-4]. The prerequisite for its application is to design efficient photocatalysts for pollutant degradation and water splitting. So far, numerous photocatalysts have been reported to show activity for the photodegradation of organic pollutants or/and water splitting. Among the various semiconductor photocatalysts, TiO 2 is undoubtedly the most popular photocatalyst that have been extensively studied. However, its large band gap and the rapid recombination of photogenerated electrons and holes greatly restrict its practical applications. Thus, various modified TiO2 and TiO2-alternative photocatalysts have been designed and fabricated [5, 6]. Metal sulfides, in particular ZnS, have been extensively investigated in photocatalysis owing to their unique catalytic functions compared to those of TiO 2 [7, 8]. Moreover, many strategies, such as nonmetal and metal doping [9, 10], surface sensitization and composites formation with various other semiconductors, have been developed to improve the photocatalytic efficiency of ZnS. Especially, composite formation can effectively decrease the recombination of photogenerated electrons and holes and thus enhance the photocatalytic activity [4, 11-13]. For example, Yu et al. demonstrate that the Zn1-xCdxS solid solution synthesized using hydrothermal method exhibit high photocatalytic activity for the H 2-production under visible light illumination [14]. It is reported that CuS/ZnS porous nanosheet photocatalysts prepared by hydrothermal and cation exchange reaction show enhanced visible light photocatalytic H 2-production activity due to the interfacial charge transfer from the valence band of ZnS to CuS [15]. Bi2S3 with a narrow band gap of 1.3 eV, has received great attention in photocatalysis. However, the photocatalytic activity of pure Bi2S3 is low due to the rapid recombination of photogenerated charges [16]. It is reported that Bi2S3 can be used as a potential efficient photocatalyst through the combination with other semiconductors such as TiO 2 [17], BiOI [18], ZnO [19], Bi2O3 [20] and CdS [16] owing to the efficient separation of photogenerated carriers. More recently, Wu et al. have synthesized Bi 2S3/ZnS composite spheres by hydrothermal method at 120 ℃, and found that the nanocomposites exhibit enhanced photochemical efficiency for the
2
degradation of rhodamine B (RhB) [21]. It is generally considered that the photocatalysis reaction occurs on the surface of photocatalysts, and the photocatalytic activity of catalysts greatly depends on their morphology, microstructure, and surface properties. Therefore, great efforts have been made to obtain photocatalysts
with
controllable
morphologies
and
structures.
Two-dimensional
(2D)
nanostructures with large specific surface area can provide more reactive sites at the surface for adsorbing reactant molecules and the photocatalytic reaction, thus are highly desirable for obtaining superior catalytic performances. For example, CeO2/TiO2 nanobelt heterostructures [22] synthesized via a hydrothermal method exhibit a markedly enhanced photocatalytic activity in the degradation of organic pollutants such as methyl orange (MO) under either UV or visible light irradiation. It is reported that flower-like CdSe architectures composed of ultrathin nanosheets show much better photocatalytic H 2 evolution activity under visible light irradiation compared with well-studied CdSe quantum dots [23]. In the present strategy, we report the synthesis of Bi2S3/ZnS nanocrystals by a low cost and simple reflux and cation exchange method using the preformed ZnS nanospheres and Bi(NO3)3·5H2O as a precursor. It is interesting that 2D mesoporous Bi2S3/ZnS nanoplates with large specific surface area can be obtained. Moreover, the synthesized Bi 2S3/ZnS composites exhibit high adsorption capability and photocatalytic activity under UV light irradiation.
2.
Experimental Section
2.1 Materials Sodium Citrate (Na3C6H5O7), Zinc sulfate heptahydrate (ZnSO4·7H2O), bismuth nitrate hexahydrate (Bi(NO3)3·5H2O), ethyl alcohol, thiourea (CN2H4S) and ammonium hydroxide (NH3·H2O) are analytical grade and purchased from Shanghai Chemical Reagent Factory of China without further treatment.
2.2 Preparation of ZnS nanospheres and Bi2S3/ZnS composite ZnS nanospheres are synthesized with following procedure. 25 mmol Na 3C6H5O7 and 10 mmol ZnSO 4·7H2O are dissolved in 100 mL deionized water under vigorous stirring and marked as solution A. The pH of solution A is adjusted to about 9.0, using NH 3·H2O. At the same time, 30
3
mmol CN 2H4S is dissolved in 50 mL deionized water under vigorous stirring, marked as solution B. When solution A is heated to 80 ℃ in thermostatic water bath with stirring, B solutions is added dropwise slowly. The mixture is stirred continuously at 80 ℃ for 3 h. The as-synthesized white products are separated from the solution by centrifugation and washed with deionized water and alcohol several times. Bi2S3/ZnS nanoplates are synthesized by simple reflux and cation exchange method using dispersed ZnS solid spheres as a precursor at 120 ℃ for 3 h. Typically, the as-prepared ZnS nanospheres are dispersed in 150 mL deionized water and sonicated for 1 h. Then, the suspension is transferred into a 250 mL round-bottom flask. Appropriate Bi(NO3)3·5H2O aqueous solutions is added dropwise under vigorous stirring when the above-mentioned suspension reach 120 ℃. It can be observed that the color changes gradually from white to bistre. After the reaction for 3 h, the flask is cooled naturally. The final burnt sienna solid product is centrifuged, washed with deionized water and alcohol several times, and finally dried at 60 ℃ for 12 h. For comparison, pure Bi2S3 nanorods are obtained under the same experimental conditions with CN2H4S as the sulfur source in the absence of ZnS.
2.3 Characterization The crystal structures of the obtained samples are characterized by power X-ray diffraction (XRD, Siemens D-5000 diffractometer with Cu Kαirradiation). The morphological details and nanoparticle size of the prepared samples are probed by an S-4800 field emission scanning electron microscopy (FESEM). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images are observed using a FEI Tecai F20 electron microscope to identity the morphology, size and nanocrystal structures of the sample. The sample for TEM observation is prepared by dispersing the Bi 2S3/ZnS composites in an absolute ethanol solution under ultrasonic irradiation, and then the dispersion is dropped on a carbon-copper grid. The Brunauer-Emmett-Teller (BET) specific surface area of the products is analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus.
2.4 Measurement of photocatalytic activity The photocatalytic activity of the prepared samples is evaluated by degradation of MB 4
aqueous solution under the irradiation of 300 W UV lamps. Before irradiation, solutions suspended with photocatalysts are sonicated in the dark for 20 minutes to ensure the adsorption-desorption equilibrium of MB on the surface of photocatalysts. During the UV light irradiation, 4 mL suspension is taken out at given time interval and centrifuged for analysis by a UV–VIS spectroscopy. The characteristic absorption peak of MB at 664 nm is used to determine the degradation efficiency.
2.5. Active species trapping experiments In order to detect the active species in the photocatalytic system, 0.5 mM methanol and 0.5 mM ammonium oxalate are added as the active species scavengers of the hydroxyl radicals (•OH) and holes (h+), respectively [24, 25]. The procedure is similar to the former photocatalytic activity experiment.
3.
Results and discussion
3.1 Morphology and structure characterization of catalyst Bi2S3/ZnS nanoplates are prepared by cation exchange reaction using the as-prepared ZnS nanospheres and Bi(NO3)3 as precursors at 120 ℃ for 3 h under reflux condition. The crystal structure of the prepared samples is characterized by XRD analysis. Fig. 1 displays the XRD patterns of pure Bi2S3, ZnS and Bi2S3/ZnS nanoplates. It is clear that all the diffraction peaks of pure Bi2S3 (Fig. 1a) match perfectly with the crystalline planes of orthorhombic Bi2S3 (JCPDS No. 17-0320). The characteristic peaks are located at the 2θ values of 15.7°, 17.6°, 22.4°, 23.8°, 25.0°, 27.5°, 28.6°, 31.8°, 33.0°, 34.0°, 35.6°, 39.2°, 39.9°, 45.7°, 46.5° and 52.6°, which correspond to the (020), (120), (220), (101), (130), (021), (211), (221), (301), (311), (240), (041), (141), (002), (431) and (351) crystal planes of Bi2S3, respectively. The pattern of pristine ZnS (Fig. 1b) could be readily indexed to the pure cubic phase of sphalerite ZnS (JCPDS No. 05-0566). While the XRD patterns of Bi2S3/ZnS composites (Fig. 1c) clearly indicate the bi-phase existence of Bi2S3 and ZnS, the broadening of the diffraction peaks of Bi2S3 is caused by the small crystallite size. As is expected, there is no third phase formation except for Bi2S3 and ZnS in the XRD pattern of Bi2S3/ZnS composite. The typical morphology of the prepared samples is observed by SEM. Pure Bi2S3 (Fig. 2a) is
5
found to consist of nanorod with an average length of about 500 nm. The morphology of pure ZnS (Fig. 2b) shows spherical nanoparticles. The size of the nanospheres is uniform with an average diameter of approximately 60 nm. Whereas, Bi2S3/ZnS composites mainly remain nanoplate morphology as shown in Fig. 2c. Further observation in detail shows that thickness of the nanoplates is about 10 nm and the surface of Bi 2S3/ZnS nanoplates is rough and porous. Moreover, according to SEM observation, the diameter of the nanoplates mainly ranges from 80 nm to 90 nm as shown in the histogram of nanoparticles size distribution (Fig. 3). TEM and HRTEM are used to further observe the morphology, size and the crystallographic structure of Bi2S3/ZnS composite. Fig. 2d shows the typical TEM image of Bi 2S3/ZnS composite. It can be seen that the primary morphology is nanoplates with diameters of approximately 80–90 nm, which is in agreement with the SEM observation. The corresponding HRTEM images of the same Bi2S3/ZnS composite in Figs. 2(e, f) show clear lattice fringes, which indicate that the prepared Bi2S3/ZnS composite are well crystalline and may be used to identify the crystallographic spacing. Figs. 2(e, f) clearly show two distinct sets of lattice fringes. The interlayer spacing d = 0.312 nm matches to the (111) plane of sphalerite ZnS, while the interlayer spaces 0.504 nm is corresponding to the (220) plane of orthorhombic Bi2S3, respectively.
3.2 BET specific surface area and porosity To further study the porous structure of the synthesized Bi 2S3/ZnS nanoplates, nitrogen adsorption–desorption isotherms are measured to determine the specific surface area and the pore size. Fig. 4 shows the nitrogen adsorption–desorption isotherms of Bi2S3/ZnS nanoplates. It can be observed that the isotherms show a linear absorption in the low range of relative pressure (P/P0<0.7), while the curves exhibit a small hysteresis loop in the high relative pressure range (P/P0>0.7). The hysteresis observed in the isotherms suggests the existence of abundant mesoporous structures in the architectures according to IUPAC classification [26, 27]. The quantitative calculation reveals that the synthesized Bi2S3/ZnS nanoplate possesses a high specific surface area of 101.30 m2/g, which is much higher than the reported pure ZnS (31.09 m2/g) [28] and Bi2S3 (20.30 m2/g) [29]. The large surface area may be attributed to the presence of mesoporosity in the nanoplates, which is beneficial for the applications as photocatalyst for the adsorption of the pollutants and would provide efficient transport pathways to reactant molecules 6
and products, and thus enhance the adsorption capability and the photocatalytic activity.
3.3 Formation mechanism As 2D Bi2S3/ZnS nanoplate morphology has rarely been studied before, studying the formation mechanism of Bi2S3/ZnS nanoplates would be of particular interest. To substantially understand the growth mechanisms of Bi2S3/ZnS nanoplates, we study the time-dependent evolutions of the morphology by SEM, as shown in Fig. 5. The typical SEM image of the sample collected at a reaction time of 10 min is presented in Fig. 5a. Fig. 5a shows that the samples display plate-like morphology interspersed with some nanoparticles. It is worth noting that the size of spheres (about 30 nm) is much smaller than that of the pristine ZnS spheres as shown in Fig. 2b, suggesting the decomposition of the original ZnS nanospheres and the formation of Bi2S3 on the ZnS surface to form composite structure. The morphology information in Fig. 5a implies that the transformation of ZnS to Bi2S3 via the cation exchange reaction proceeds as Bi(NO3)3·5H2O is added into the ZnS suspension solution. It can be verified by the observed color change from white to bistre during the experiment. As the reaction time proceeds to 30 min, it is notable that the nanoplates are basically formed as reflected in Fig. 5b. Based on the above observations, a possible growth process for Bi2S3/ZnS nanoplate can be reasonably inferred to be the decomposition and regrowth mechanism as proposed in Scheme 1. After the ZnS nanospheres are dispersed in deionized water, there is a small amount of S2− in the solution due to the dynamic equilibrium and local decomposition of ZnS nanospheres (equation (1)). As Bi (NO3)3·5H2O is added in the suspensions, Bi2S3 will produce according to the chemical (equation (2)) due to the small solubility constants (Ksp) of Bi 2S3 (1.82×10−99). As shown in Fig. 5a, the smaller ZnS spheres are uniformly distributed in Bi2S3 crystallites and form composite structure, indicating that the nucleation and growth process of Bi 2S3 is initialized preferentially on the rough surface of the dissolving ZnS spheres and the formed Bi2S3 crystallites gradually cover the surface of ZnS spheres since it can provide high-energy nucleation sites for the growth of Bi2S3. As the reaction proceeds, the concentration of S2− in the solution around the ZnS nanospheres is higher than that in the bulk owing to the continous decomposition of ZnS nanospheres and the concentration polarization. Therefore, Bi2S3 crystallites basically grow around the ZnS nanospheres, and form a nanoplate structure with a size of about 80-90 nm as 7
shown in Fig. 5b through the nucleation–aggregation deposition process as the reaction time increases to 30 minutes. As the reaction time further increases, the continous decomposition of ZnS and nucleation–aggregation deposition process of Bi2S3 lead to the formation of Bi2S3/ZnS nanoplate as shown in Fig. 2c.
ZnS ⇌ Zn2++S2-
(1)
2Bi3++3S2- ⇌ Bi2S3
(2)
3.4 Photocatalytic behavior The time-dependent absorption spectrum of MB solution under UV light illumination in the presence of Bi2S3/ZnS nanoplates is shown in Fig. 6a. It can be found that the characteristic absorption peaks corresponding to MB decrease greatly as the adsorption-desorption equilibrium of MB on the surface of photocatalysts reaches after 20 min ultrasonic treatment, indicating that Bi2S3/ZnS nanoplates show high adsorption capacities for MB molecules. The characteristic absorption peaks of MB further diminish as the exposure time increases due to the degradation of MB. To evaluate the photocatalytic performance quantitatively, the degradation efficiency is determined by the following function:
C0 Ct A 100% (1 t ) 100% C0 A0 Where C0 is the initial concentration of MB and Ct is the concentration after degradation. At and A0 are the corresponding absorbance values. For comparison, the photocatalytic performances of pure ZnS and Bi2S3 are also evaluated. The photocatalytic activity of Bi 2S3/ZnS nanoplates, pure ZnS and pure Bi 2S3 are presented in Fig. 6b. We can see clearly from Fig. 6b that all of the samples show adsorption abilities to MB during the dark, that is, the absorption amount of Bi2S3/ZnS nanoplates may reach about 46.5 % of MB, which is much higher than that of pure ZnS (6.4 %) and pure Bi2S3 (12.0 %). The enhanced adsorption capacity for Bi2S3/ZnS nanoplates can be attributed to the modified surface with larger specific surface area as verified by the BET measurement. After 20 min UV light irradiation, the remaining MB molecules are about 12.3 % for Bi2S3/ZnS nanoplates, which is much lower than those for pure ZnS (25.1 %) and pure Bi 2S3 (40.4 %) sample. The results suggest that Bi2S3/ZnS is 8
an excellent composite photocatalyst for pollutant degradation.
3.5 Photocatalytic degradation mechanism of MB over Bi 2S3/ZnS nanocomposite On the basis of above experimental observations, a tentative mechanism can be proposed for the degradation of MB over Bi2S3/ZnS photocatalyst. As is well known, the adsorption of the pollutants on the catalyst's surface is an essential process for photocatalytic oxidation. The high adsorption capacities of Bi2S3/ZnS composites as shown in Fig. 6 is exciting for the large adsorption and active removal of MB. In the case of pure Bi2S3, the photocatalytic activity is largely restricted by the recombination of the photoinduced electrons and holes because of its narrow band gap. While the photoinduced electrons and holes may transfer effectively when Bi2S3/ZnS composites are used as the photocatalysts under UV light irradiation. The photoinduced electrons on the Bi 2S3 surface would easily transfer to ZnS since the reformed conduction band (CB) edge potential of Bi2S3 (−1.479 eV) is more negative than that of ZnS (−0.945 eV) [21]. The electrons transferring on the CB edge of ZnS can be subsequently trapped by O2 molecules adsorbed on the surface of the composite to form H2O2. H2O2 reacts with electrons in succession to form active ·OH, which are crucial oxidizing species responsible for the MB degradation. Meanwhile, the photoinduced holes tend to keep in the valence band (VB) of Bi2S3. Since the VB position of Bi 2S3 (1.48 eV) [30] is much more negative than the potential of the H2O/·OH (2.7 eV) couple, the photogenerated holes on the VB of Bi 2S3 cannot react with absorbed water molecules to form ·OH radicals. Therefore, the photoinduced electrons form reactive ·OH radicals, which are responsible for the subsequent degradation of pollutants, while the photoinduced holes can directly oxide MB molecules without forming ·OH radicals, and finally produce CO2, H2O, etc. This is similar to Houas et al.’s charge separation mechanism [31]. The formation of oxidative intermediate species, such as photoinduced holes and ·OH radicals, under photoreaction conditions and their role in the MB degradation process are investigated indirectly, with the addition of appropriate scavengers of these species. The comparison between the original photocatalytic degradation of MB over MB–Bi2S3/ZnS dispersions with those obtained after addition of scavengers in the initial solution, under otherwise identical conditions, is presented in Fig. 7. 9
It is noticeable that the photocatalytic activity of Bi 2S3/ZnS nanoplates is largely suppressed by the addition of methanol or ammonium oxalate as presented in Fig. 7. The presence of methanol and ammonium oxalate, which act as the active species scavengers of the •OH and holes, respectively, greatly suppresses the photodegradation of MB, indicating that the •OH and photoinduced holes serves as the main active oxidative species for the photodegradation of MB over Bi2S3/ZnS under UV light irradiation. Generally, the photocatalytic activity of the photocatalyst is mainly attributed to its ability for adsorbing target pollutant, and the separation of the photoinduced electrons and holes in the catalyst. Herein, the synthesized 2D mesoporous Bi2S3/ZnS nanoplates with large specific surface area can provide more surface-active sites for adsorbing target pollutant. Moreover, the synthesized Bi2S3/ZnS composites exhibit high adsorption capability and photocatalytic activity.
4. Conclusion In conclusion, we have demonstrated a new and facile reflux method to successfully synthesize 2D Bi2S3/ZnS nanoplate composite using ZnS nanospheres and Bi(NO3)3·5H2O as starting reactant. The synthesized Bi2S3/ZnS nanoplates are mesoporous structures with a high specific surface area of 101.30 m2/g. Moreover, the as-prepared Bi2S3/ZnS nanoplates show high adsorption capability and photocatalytic activity in the degradation of MB in aqueous solution. The possible growth process for Bi 2S3/ZnS nanoplates can be inferred to be the decomposition and regrowth mechanism based on the time-dependent observation. A tentative mechanism for degradation of MB over Bi2S3/ZnS is proposed involving ·OH radical and photoinduced holes as the active oxidative species. This work provides a cost-effective method for large-scale synthesis of composite with controlled architectural morphology. Furthermore, the inherent characteristics of Bi2S3/ZnS nanoplates may open up the prospect to understand and modify other semiconductor materials, which may have promising for environment and energy applications.
Acknowledgments This work is supported by the Changsha Science and Technology Plan Projects, China (Grant No. K1403067-11) and Environmental Protection Science and Technology Project of Hunan Province.
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12
20
** * ** * * * # * 30
40
50
(242)
(152) (721)
# ZnS
(a)
(311)
(220)
*# * ** **
(351)
(211)
(301)(221) (311) (240) (041) (141) (241) (002) (431) (251)
(130)
(111)
(021)
(220) (101)
(020) (120)
Intensity(a.u.)
* Bi2S3
(b)
#* 60
(c)
* 70
2Theta(degree) Fig. 1. XRD patterns of Bi2S3 (a), ZnS (b) and Bi2S3/ZnS nanocomposite (c).
13
Fig. 2. SEM images of as-prepared photocatalysts: Bi2S3 (a), ZnS (b), Bi2S3/ZnS nanocomposite (c), and TEM image (d), HRTEM image of Bi2S3/ZnS nanocomposite (e, f).
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Freqency 50
60
70
80 90 100 Diameter(nm)
110
120
Fig. 3. Size distribution of Bi2S3/ZnS nanoparticle.
15
3
Volume adsorbed(cm /g,STP)
80
60
40 Adsorption Desorption
20 0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure(P/P0)
Fig. 4. Nitrogen adsorption–desorption isotherms and the corresponding pore size distributions (inset) of Bi2S3/ZnS nanocomposite.
16
(b)
(a)
Fig. 5. SEM images of reaction time depended ZnS / Bi2S3 nanocomposite: 10min (a) 30min (b).
17
Fig. 6. (a) Absorption spectra of MB aqueous solution in the presence Bi2S3/ZnS nanocomposite under UV light irradiation; (b) Photodegradation efficiencies (Ct/C0) of MB as a function of irradiation time by different photocatalysts under UV light irradiation.
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1.0
A-Bi2S3/ZnS M-Bi2S3/ZnS Bi2S3/ZnS
Ct/C0
0.8 0.6 0.4 0.2 -20
dark light on
0
5
10 Time(min)
15
20
Fig. 7. Photocatalytic degradation of MB over Bi2S3/ZnS photocatalysts alone and with the addition of methanol (M-Bi2S3/ZnS) or ammonium oxalate (A-Bi2S3/ZnS).
19
Scheme 1. Schematic illustration of morphology evolution of Bi2S3/ZnS nanoplates: (a) ZnS nanospheres decomposite and release S2- ; (b) Bi2S3 crystallites basically grow around the ZnS nanospheres and (c) form a nanoplate.
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Graphical abstract
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