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ZnS heterojunction for high performance and stable photocatalytic activity
Journal of Photochemistry & Photobiology A: Chemistry 380 (2019) 111859
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Novel method of constructing CdS/ZnS heterojunction for high performance and stable photocatalytic activity ⁎
Kejie Zhanga,b, , Ling Jinc, Yuncan Yanga, Kechun Guoa, Fang Hud,
T
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a
School of Materials Science and Engineering, Nanjing Institute of Technology, No. 1, Hongjing Road, Jiangning District, Nanjing, 211167, China Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, No. 1, Hongjingdadao, Jiangning District, Nanjing, 211167, China c National Graphene Products Quality Supervision and Inspection Center (Jiangsu), Jiangsu Province Special Equipment Safety Supervision and Inspection Institute Branch of Wuxi, 214174, China d Key Laboratory of Biomedical Functional Materials School of Science, China Pharmaceutical University, Nanjing, 211198, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: CdS/ZnS Heterojunction Photocurrent Photocatalytic performance
A series of CdS/ZnS heterojunction nanocomposites with different contents of CdS were successfully synthesized through liquid-phase thermal decomposition of a single precursor and ion adsorption method. The structures and properties of these nanocomposites were characterized by XRD, SEM, TEM, ultraviolet visible diffuse reflectance spectrum, electrochemical workstation and photodegradation test. Therefore, the influences of different contents of CdS on the photocatalytic performance of as-prepared samples were demonstrated explicitly. The CdS/ZnS (7:1) nanocomposite exhibited the highest photocatalytic efficiency when exposed to ultraviolet light for 35 min, the degradation efficiencies of rhodamine B (RhB) and methylene blue reached 96.1% and 99.6%, respectively, which is higher than other reported CdS/ZnS based photocatalysts. Moreover, with a 10-cycle test of the degradation of RhB under natural light, the degradation efficiency remained around 96%, showing the desirable photocatalytic stability of CdS/ZnS (7:1) nanocomposite. The approach provides new possibility to produce other metal sulfide nanocomposites.
1. Introduction The construction of nano-semiconductor heterojunctions is an effective method to synthesize nanocomposites with excellent photocatalytic properties [1]. On the one hand, nanoheterojunctions possess the splendid properties of nanomaterials, such as surface effect, small size effect, macroscopic quantum tunneling effect, quantum confinement effect, etc [2,3]. On the other hand, nanoheterojunctions also have the advantages of heterojunctions, for example, mobility increases, two-dimensional electron system and so on [4]. The nanoheterojunctions formed by metal chalcogenides, particularly sulfide-oxide composites [5,6] and sulfide-sulfide complexes [7–9], often have good photocatalytic properties [10–14], such as photocatalytic hydrogen generation, degradation of organic matter, reduction of CO2, etc [6,15–19]. Therefore, the study of chalcogenide nano-semiconductor heterojunctions is of great significance for the development of photocatalytic materials. CdS is a typical II–VI direct semiconductor material with suitable
conduction band position and narrow band gap (2.42 eV). It has high utilization of solar energy and is easy to extract and prepare. Therefore, it has a wide range of applications in photovoltaics [20], photocatalysis [21], bio-imaging [22], sensing technology [23] and light-emitting devices [24]. However, pure CdS is prone to photocorrosion, which results in greatly reduced catalytic activity [25]. ZnS has a band gap of 3.66 eV and is resistant to photocorrosion. It is widely used in photocatalytic hydrogen generation [26] and photocatalytic reduction of CO2 [27]. However, the band gap of ZnS is so wide that it can only react under ultraviolet light, which suggests that its application range is greatly limited [28]. When CdS and ZnS are composed of semiconductor with nanoheterojunction, CdS/ZnS nanoparticles not only make up for their own shortcomings, but also exhibit higher photocatalytic activity than pure CdS or ZnS [29]. Common preparation methods of CdS/ZnS nanoheterojunction materials include ion exchange method [6,30], ion adsorption method [31,32], solvothermal method [8,9,33–36], solution bath deposition [15,37,38], chemical vapor deposition [39,40], microwave radiation method [41], solid
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Corresponding author at: School of Materials Science and Engineering, Nanjing Institute of Technology, No. 1, Hongjingdadao, Jiangning District, Nanjing, 211167, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (K. Zhang), [email protected] (F. Hu). https://doi.org/10.1016/j.jphotochem.2019.111859 Received 23 December 2018; Received in revised form 22 April 2019; Accepted 12 May 2019 Available online 13 May 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
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drying. The CdS/ZnS (3:1, 5:1, 7:1) nanocomposites were prepared with the same method.
phase thermogenesis method [42,43] and in situ synthesis method [44,45]. Zhang et al. used cation exchange method to prepare a metalfree core ZnO/CdS/ZnS composite ball with high hydrogen production, good stability and low cost, and H2 generation rate can reach 11.37 mmol h−1 g−1 [6]. Jiang et al. prepared the CdS/ZnS core-shell material with one-step thermal synthesis and solution bath deposition, and the photocatalytic hydrogen generation rate of CdS/ZnS composites was almost unchanged within 12 h, which indicated that the CdS/ZnS composites were of good stability [15]. Xing et al. synthesized CdS/ZnS composites by solvothermal method with ethylenediamine, and the photocatalytic efficiency was 90 times that of pure ZnS [33]. Although good methods for synthesizing CdS/ZnS are endless, the method for preparing CdS/ZnS by liquid-phase thermal decomposition of a single precursor and ion adsorption has not been reported. Herein, CdS/ZnS nanocomposites were synthesized through liquidphase thermal decomposition and ion-adsorption method using Cd (Ac)2·2H2O, 2-Mercaptobenzothiazole (HMBT), Na2S·9H2O and Zn (Ac)2·2H2O as raw materials. We characterized the structure and composition of the samples by XRD, SEM, TEM and ultraviolet visible diffuse reflectance spectrum, and the photocatalytic performances of the samples were also discussed.
2.5. Material characterization X-ray powder diffraction (XRD) patterns of samples were recorded on a Rigaku Ultimate IV X-ray diffractometer using Cu Kα radiation at 40 kV and 40 mA in the 2θ range from 10° to 80° with a scan speed of 8°/min. Scanning electron microscopy (SEM) images and elemental mapping images were acquired on an FEI Nova SEM 230. Transmission electron microscopy images were taken on a JEOL 2100 microscope. The optical properties of the samples were characterized by UV–vis diffuse reflectance spectroscopy (UV-3900), ultraviolet spectrophotometer (UV-1800). The photocurrents were measured by an electrochemical workstation (CHI600E). 2.6. Photocatalytic evaluation The photocatalytic activities of the samples were evaluated by degradation of RhB (0.01 g/L) and MB (0.01 g/L) under visible and UV light irradiation at room temperature, respectively. 500 W mercury lamp was chosen to offer visible light and UV light source. The operating procedures of photodegradation can be seen in literature [45]. 0.2 g photocatalyst was suspended in 200 mL of RhB or MB solution, and then the solution was stirred in the dark room for 30 min. During the light irradiation, 5 mL solution containing the sample was taken out from the reaction suspensions every 5 min and the photocatalyst particles were removed by centrifugation. Subsequently, the solutions were measured with the UV–vis spectrophotometer at wavelength of 552 nm and 662 nm for RhB and MB respectively. Similarly, the recycling photocatalytic tests were done for ten times under natural light, and 30 min per cycle of light.
2. Experimental details 2.1. Material preparation All reagents and solvents were used as received from commercial suppliers without further purification. Cd(Ac)2·2H2O, HMBT, Na2S·9H2O, Zn(Ac)2·2H2O, oleic acid and CH3CH2OH were commercially available from Sinopharm Chemical Co., Ltd (Shanghai, China). Deionized water was used throughout the investigation. 2.2. Synthesis of Cd(MBT)2
2.7. Photoelectrochemical measurement
For fabricating the Cd(MBT)2, 6.50 mmol Cd(Ac)2·2H2O and 13.00 mmol HMBT were firstly dispersed in 80 mL and 50 mL of anhydrous ethanol under stirring separately. Faint yellow precipitate was generated by mixing the above two solutions. The faint yellow powder Cd(MBT)2 was obtained through filtering, washing and drying in sequence.
The photocurrent measurement was performed with a standard three-electrode configuration on a CHI600E electrochemical system. The working electrodes were prepared as follows: 0.001 g CdS/ZnS nanocomposite was mixed with 0.5 mL anhydrous ethanol and 10 μL PTFE by ultrasound. Then, the resulting solution was droped on the FTO glass (1 cm × 1 cm). After evaporation of the ethanol in air, the photocatalysts were attached on the FTO glass surface. A saturated calomel electrode (SCE) was used as the auxiliary and a Pt wire was used as the reference electrode. 0.5 mol/L aqueous solution of Na2SO4 was used as the electrolyte and a 300 W Xe lamp served as the light source.
2.3. Synthesis of CdS nanoparticles The CdS nanoparticles were prepared through a method of liquidphase thermal decomposition. In a typical process, 6.13 mmol Cd (MBT)2 and 150 mL oleic acid were added into a three-necked flask and heated with reflux and magnetic stirring. Cd(MBT)2 was completely dissolved and the color of the solution was orange-red at about 150 ℃. When the solution was heated to 290 ℃ approximately, orange-yellow precipitate occurred. After 10 min of isothermal reaction, the resulting suspension was centrifuged and washed by anhydrous ethanol and deionized water for several times. The CdS nanoparticles were obtained after drying under vacuum for 12 h.
3. Results and discussion 3.1. XRD The crystallization and phase identification of the as-prepared nanocomposites were firstly studied. Fig. 1 shows the XRD patterns of the pure CdS and nanocomposites with different molar ratios of CdS/ZnS (1:1), (3:1), (5:1), (7:1), respectively. From Fig. 1, all the diffraction peaks of the samples can be well indexed to hexagonal wurtzite CdS (JCPDS No. 41-1049). The six major diffraction peaks at 24.94°, 26.60°, 28.20°, 43.86°, 47.90° and 51.94° were observed, which matched well with crystal planes (100), (002), (101), (110), (103) and (112) of hexagonal wurtzite CdS [46,47], indicating that crystalline CdS materials were successfully synthesized. Compared with the pure CdS, the diffraction peak positions corresponding to the (110) and (112) crystal plane of the CdS/ZnS nanocomposite have a slight shift. Hereon the assynthesized CdS/ZnS (7:1) nanocomposite exhibits positions of diffraction peaks at 43.92° and 51.98°, shifting about 0.06° and 0.04°
2.4. Synthesis of CdS/ZnS nanocomposites The CdS/ZnS nanocomposites were prepared successfully by an effortless ion-adsorption method. 5.05 mmol CdS nanoparticles were put into 25 mL saturated solution of Na2S with constant magnetic stirring and heating. The CdS nanoparticles covered with S2− were collected through centrifuging, followed by washing with anhydrous ethanol and deionized water alternatively for several times. Then, the as-prepared nanoparticles were uniformly dispersed into deionized water. 5.05 mmol Zn(Ac)2·2H2O configured as solution was dropwise added into the resulting solution. After sufficient reaction for 5 min, the CdS/ ZnS (1:1) nanocomposites were obtained via centrifuging, washing and 2
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the absorbance of samples also increased gradually. Strong light absorption before 550 nm indicates that they have good optical absorption properties. Among the four as-prepared samples, CdS/ZnS (7:1) nanocomposite with the strongest absorption value means that it has the best optical absorption property. As shown in Fig. 3b, the band gaps (Eg) of different samples are measured from the (Ahν)2−hν curves [49], and A, h and ν correspond to absorption ratio, Planck’s constant and light frequency, respectively. The band gaps of CdS/ZnS (1:1, 3:1, 5:1, 7:1) samples are estimated to be 2.36 eV, 2.31 eV, 2.30 eV and 2.28 eV, respectively. As everyone knows, pure CdS has a bandgap in the 2.42–2.50 eV range and the bandgap of ZnS about 3.7 eV. Compared with CdS and ZnS, the samples are red shifted. With CdS content increasing in CdS/ZnS nanocomposites, the crystal defects of the samples also increase. So red shift occurs UV–vis spectrum, and band gap decreases [50] [Udayabhaskar, 2014 #103]. CdS/ZnS (7:1) has the smallest band gap, corresponding to its optical absorption property.
3.4. Photocatalytic performance
Fig. 1. XRD patterns of as-obtained pure CdS and CdS/ZnS nanocomposites (1:1, 3:1, 5:1, 7:1).
As shown in Fig. 4a, the photocatalytic activity of the four samples is evaluated by the degradation of organic dyes. Before turn on the light, four CdS/ZnS samples (1:1, 3:1, 5:1, 7:1) adsorbed 65%, 67%, 89%, 92% RhB, respectively. This can be seen that the prepared samples have outstanding adsorption performance because of their large surface area. And the color of the solid obtained by centrifugation remains unchanged, indicating that the samples not only have good adsorption performance, but also can undergo photodegradation. Moreover, under ultraviolet light radiation, the degradation efficiencies of RhB of CdS/ZnS samples (1:1, 3:1, 5:1, 7:1) are 92%, 93%, 95%, 96% within 35 min, respectively. The CdS/ZnS (7:1) sample shows the best photocatalytic performance of the four samples. As displayed in Fig. 4a, at 25 min, the degradation efficiency of MB by the CdS/ZnS (7:1) sample reaches 99% under ultraviolet light radiation. In addition, MB is almost completely degraded at 35 min. By contrast with other literatures [51,52], the CdS/ZnS samples have better photocatalytic activity for the degradation of MB and RhB. And CdS/ZnS (7:1) has the best photocatalytic property among the prepared samples. The degradation profiles of dyes in the presence of different samples have been given in Fig. 4b. The photodegradation process can be described by a first order kinetic equation.
severally. The diffraction peaks at 28.20, 47.76° and 56.64° can be designated to ((111), (220) and (311) crystal planes of the zinc blende ZnS. (JCPDS No. 05-0566). It should be noted that strong CdS crystalline phases appeared in all samples and ZnS peaks were not obvious, probably due to low ZnS content or poor crystallization [15]. On account of the above analysis, CdS/ZnS nanocomposites were successfully synthesized. This conclusion can also be proved by SEM data. 3.2. SEM and TEM The morphology and internal microstructure of the samples were observed by SEM and TEM. Fig. 2 shows the FESEM images of CdS/ZnS samples with different molar ratios of CdS to ZnS, in which Fig. 2a-d stand for the FESEM of CdS/ZnS (1:1, 3:1, 5:1, 7:1), respectively. As depicted in Fig. 2a-d, the CdS/ZnS nanocomposites occur as a similar ellipse and a particle size of approximately 25 nm. The particle sizes and morphologies of the four samples are essentially unchanged. That is to say, the molar ratio of CdS has little effect on the particle size and shape of the composites. The elemental mappings of S, Cd, Zn elements (Fig. 2e-h) combined with XRD results prove that CdS/ZnS nanocomposite has been successfully synthesized. Fig. 2i-j show the TEM image of as-synthesized CdS/ZnS (7:1) nanocomposite. As can be seen from Fig. 2i, the particles of CdS/ZnS (7:1) nanocomposite present similar ellipse morphology with a particle size of 25 nm, which is consistent with the SEM observation. Meanwhile, a typical HRTEM image (Fig. 2j) of CdS/ZnS (7:1) sample exhibits clear lattice fringes, indicating that the sample has good crystallinity. What’s more, the image shows the lattice fringes with interplanar spacing of 0.31 nm and 0.36 nm of CdS/ZnS (7:1) sample, corresponding to the (111) and (100) plane of the ZnS [46] and CdS [48], respectively. The energy dispersive X-ray (EDX) results are displayed in Fig. 2k, C and Cu are from the copper mesh of the sample tray while S, Cd, and Zn are from as-synthesized CdS/ZnS (7:1) nanocomposite, which is in line with the SEM results of CdS/ZnS (7:1) nanocomposite. Furthermore, calculated by embedded data in the EDX spectrum, the content of Cd is about seven times as much as the content of Zn. Based on the results of XRD, the asprepared sample is further identified as the CdS/ZnS (7:1) structures.
－ln(C/C0)=kt In the formula, C is the concentration of organic pollutants changing with time, C0 is the initial concentration of organic pollutions, k is the first order kinetic constant, and t is the reaction time [53]. As shown in Fig. 4b, the linear correlations of the reaction kinetic models of the samples are high. Furthermore, the degradation rate of the CdS/ZnS (7:1) is the highest and it degrades MB at a rate of 0.072 min−1 under ultraviolet light irradiation. It is further proved that CdS/ZnS (7:1) has good photocatalytic activity, which is consistent with the above experimental results. For the industrial application of samples, we studied the cyclic degradation of CdS/ZnS (7:1) under natural sunlight irradiation. As displayed in Fig. 5 at the first degradation, the photodegradation efficiency of RhB is 98.6%, while at the 10th degradation; the photodegradation efficiency is 96%. In other word, after 10 cycle times, the degradation efficiency of RhB decreases slightly by 2.6%. Compared with the data of Fig. 4, the degradation efficiency of RhB under natural sunlight irradiation is better than that under ultraviolet light irradiation when CdS/ ZnS (7:1) used as the photocatalyst. The reason is that natural sunlight contains ultraviolet light, visible light and other ingredients. In addition, Fig. 3 illustrates the sample has good absorption of ultraviolet and visible light. Thus the catalytic effect of CdS/ZnS (7:1) on RhB under natural sunlight irradiation is better than that under ultraviolet light irradiation and the results are in agreement with the above analysis. In
3.3. UV–vis DRS Fig.3a shows the UV–vis DRS spectra of CdS/ZnS nanocomposites (1:1, 3:1, 5:1, 7:1) samples. All of the as-obtained samples have absorption properties in both ultraviolet and visible light regions. In addition, with the increasing of CdS content in CdS/ZnS nanocomposites, 3
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Fig. 2. SEM images (a, b, c, d) of various CdS/ZnS (1:1, 3:1, 5:1, 7:1); elemental mapping images (e, f, g, h) of the CdS/ZnS nanocomposite (7:1); TEM (i), HRTEM (j) images of the CdS/ZnS (7:1) nanocomposite; EDX (k) spectrum of the CdS/ZnS nanocomposite (7:1).
the photocurrent test with repetitive light on/off cycles. Fig. 6 shows the surface light current response curves of different samples. The photocurrents of five kinds of samples show the following orders: CdS/ ZnS (7:1) > CdS/ZnS (3:1) > CdS/ZnS (5:1) > CdS > CdS/ZnS (1:1). The photocurrent of CdS/ZnS (7:1) nanocomposite is about 45 times than that of pure CdS. This is because the photogenerated electrons in the conduction band (CB) of CdS were transferred to the CB of ZnS under visible light irradiation. Driven by potential energy [54], the photogenerated holes were transferred from the valence band (VB) of ZnS to the valence band (VB) of CdS [55], which can better separate the charge and prolong the lifetime of photogenerated electrons, greatly
conclusion, the CdS/ZnS (7:1) nanocomposite with excellent photocatalytic activity and stability has a good application prospect in industrialization. 3.5. Photoelectric properties The photoelectric properties of semiconductor nanomaterials mainly depend on the generation of optical electrons, the separation of electron-hole pairs and the transfer efficiency of charge vectors. Therefore, the photoelectrochemical performances of pure CdS and CdS/ZnS nanocomposites with different ratios can be demonstrated by 4
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Fig. 3. UV–vis diffuse reflectance spectra of CdS/ZnS nanocomposites (1:1, 3:1, 5:1, 7:1) (a) and corresponding Tauc’s plots of different samples (b).
Fig. 4. Photocatalytic degradation of dyes (a) and kinetic data for the degradation of dyes (b) in the presence of CdS/ZnS samples (1:1, 3:1, 5:1, 7:1).
Fig. 5. Effects of cycle times on the degradation efficiency of RhB by CdS/ZnS (7:1) under natural sunlight irradiation.
improving the catalytic activity of CdS/ZnS (7:1). However, the more the content of ZnS was, the smaller the photocurrent responses of samples were. The photocurrent of the CdS/ZnS (7:1) is about 200 times that of the CdS/ZnS (1:1). The possible reason is that the excess ZnS led to a large band gap of CdS/ZnS, which reduced the photon yield and hindered the separation of electron-hole pairs [56]. In addition, as the irradiation time is prolonged, the photocurrent response of the CdS/ ZnS (7:1) gradually decreases, which may be caused by the poor binding between CdS/ZnS and FTO substrate as we observed the drop of CdS/ZnS catalyst from FTO substrate in our test [39,56]. Electrons cannot reach the FTO substrate in time and accumulate in the sample, resulting in the recombination of redundant electron-hole pairs. Nevertheless, the photocurrent density of CdS/ZnS (7:1) has a tendency for stabilization after several on/off cycles, which indicates that once the balance of carrier generation, separation, transport and recombination is achieved, the photocurrent density of the sample will become stable [57]{Juncao Bian, 2015 #19}. In summary, the heterostructure of CdS/ZnS can effectively prevent photo-corrosion of CdS
Fig. 6. Transient photocurrent density of pure CdS and CdS/ZnS nanocomposites.
[58], and can be used in the production of hydrogen by photolysis of water and the reduction of CO2 [59,60]. 3.6. Synthesis mechanism The formation mechanism of CdS/ZnS nanoparticles is illustrated in Scheme 1. The mechanism is as follows: (1) CdS nanoparticles were generated from Cd(MBT)2 by thermal decomposition in oleic acid. (2) CdS nanoparticles were uniformly dispersed into saturated NaS2 solution, the S2− ions were absorbed onto the surfaces of the CdS nanoparticles. (3) CdS nanoparticles adsorbing S2− ions were washed and uniformly scattered in deionized water. (4) Quantitative Zn(Ac)2 solution was added to the above suspension, CdS/ZnS heterojunction nanocomposites was fabricated after full reaction. 5
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in 35 min, respectively. What’s more, after ten-cycle tests under natural light, the degradation efficiency still reached around 96% showing the wonderful photocatalytic stability of CdS/ZnS (7:1) nanocomposite, which suggested the potential of environmental purification and water pollution treatment. In this work, a simple two-step method was described to synthesize CdS/ZnS nanocomposites, which broadened the way for the preparation of other metal sulfide nanocomposites. Acknowledgments The authors are grateful to the financial supports of National Natural Science Foundation of China (Grant No. 21603101), National Natural Science Foundation of Jiangsu Province (Grant No. BK20160774), the Innovation Fund of Nanjing Institute of Technology (TB201902014, TB201902058), the Opening Project of Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology and the Outstanding Scientific and Technological Innovation Team in Colleges and Universities of Jiangsu Province.
Scheme 1. Schematic representation of the synthesis processes of CdS/ZnS nanoparticles.
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Fig. 7. Reaction mechanism of the CdS/ZnS nanoparticles for the degradation of organics.
3.7. Mechanism for photocatalytic The photocatalytic degradation mechanism of CdS/ZnS nanoparticles is shown in Fig. 7. The bandgap energies of CdS and ZnS are 2.42 [61] and 3.7 [62] eV, respectively. First, when exposed to light, both of the CdS and ZnS could be excited to produce photogenerated electrons [49] and the positively holes (h+). Second, because the ECB of CdS is more positive than that of ZnS, the photogenerated electrons on the conduction band (CB) of ZnS can transfer toward the conduction band of CdS. Meanwhile, the holes would be shifted from the valence band (VB) of ZnS to the valence band of CdS since the EVB of CdS is more negative than that of ZnS. Third, under the influence of an electric field, the photogenerated electrons could react with O2 adsorbed onto CdS/ZnS nanoparticles to form superoxide radicals (%O2−) [63]. The superoxide radicals could be combined with H+ to form H2O2 which could react with %O2− to form hydroxyl radicals (%OH) [64,65]. Fourth, the positively charged holes (h+) on CdS and ZnS could also have a reaction with H2O to generate %OH. Finally, hydroxyl radicals could react with RhB or MB to become CO2 and H2O.
4. Conclusion In summary, CdS/ZnS nanocomposites were successfully synthesized through the combination of one-step liquid-phase thermal decomposition and ion adsorption method. The effects of different CdS contents on the photocatalytic properties of CdS/ZnS nanocomposites were investigated by multiple characterizations and tests. Notably, the photocurrent of CdS/ZnS (7:1) reached the highest and its photocatalytic property was excellent among the prepared samples. The degradation efficiencies of RhB and MB can reach 96.1% and 99.6% only 6
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Novel method of constructing CdS/ZnS heterojunction for high performance and stable photocatalytic activity ⁎
Kejie Zhanga,b, , Ling Jinc, Yuncan Yanga, Kechun Guoa, Fang Hud,
T
⁎⁎
a
School of Materials Science and Engineering, Nanjing Institute of Technology, No. 1, Hongjing Road, Jiangning District, Nanjing, 211167, China Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, No. 1, Hongjingdadao, Jiangning District, Nanjing, 211167, China c National Graphene Products Quality Supervision and Inspection Center (Jiangsu), Jiangsu Province Special Equipment Safety Supervision and Inspection Institute Branch of Wuxi, 214174, China d Key Laboratory of Biomedical Functional Materials School of Science, China Pharmaceutical University, Nanjing, 211198, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: CdS/ZnS Heterojunction Photocurrent Photocatalytic performance
A series of CdS/ZnS heterojunction nanocomposites with different contents of CdS were successfully synthesized through liquid-phase thermal decomposition of a single precursor and ion adsorption method. The structures and properties of these nanocomposites were characterized by XRD, SEM, TEM, ultraviolet visible diffuse reflectance spectrum, electrochemical workstation and photodegradation test. Therefore, the influences of different contents of CdS on the photocatalytic performance of as-prepared samples were demonstrated explicitly. The CdS/ZnS (7:1) nanocomposite exhibited the highest photocatalytic efficiency when exposed to ultraviolet light for 35 min, the degradation efficiencies of rhodamine B (RhB) and methylene blue reached 96.1% and 99.6%, respectively, which is higher than other reported CdS/ZnS based photocatalysts. Moreover, with a 10-cycle test of the degradation of RhB under natural light, the degradation efficiency remained around 96%, showing the desirable photocatalytic stability of CdS/ZnS (7:1) nanocomposite. The approach provides new possibility to produce other metal sulfide nanocomposites.
1. Introduction The construction of nano-semiconductor heterojunctions is an effective method to synthesize nanocomposites with excellent photocatalytic properties [1]. On the one hand, nanoheterojunctions possess the splendid properties of nanomaterials, such as surface effect, small size effect, macroscopic quantum tunneling effect, quantum confinement effect, etc [2,3]. On the other hand, nanoheterojunctions also have the advantages of heterojunctions, for example, mobility increases, two-dimensional electron system and so on [4]. The nanoheterojunctions formed by metal chalcogenides, particularly sulfide-oxide composites [5,6] and sulfide-sulfide complexes [7–9], often have good photocatalytic properties [10–14], such as photocatalytic hydrogen generation, degradation of organic matter, reduction of CO2, etc [6,15–19]. Therefore, the study of chalcogenide nano-semiconductor heterojunctions is of great significance for the development of photocatalytic materials. CdS is a typical II–VI direct semiconductor material with suitable
conduction band position and narrow band gap (2.42 eV). It has high utilization of solar energy and is easy to extract and prepare. Therefore, it has a wide range of applications in photovoltaics [20], photocatalysis [21], bio-imaging [22], sensing technology [23] and light-emitting devices [24]. However, pure CdS is prone to photocorrosion, which results in greatly reduced catalytic activity [25]. ZnS has a band gap of 3.66 eV and is resistant to photocorrosion. It is widely used in photocatalytic hydrogen generation [26] and photocatalytic reduction of CO2 [27]. However, the band gap of ZnS is so wide that it can only react under ultraviolet light, which suggests that its application range is greatly limited [28]. When CdS and ZnS are composed of semiconductor with nanoheterojunction, CdS/ZnS nanoparticles not only make up for their own shortcomings, but also exhibit higher photocatalytic activity than pure CdS or ZnS [29]. Common preparation methods of CdS/ZnS nanoheterojunction materials include ion exchange method [6,30], ion adsorption method [31,32], solvothermal method [8,9,33–36], solution bath deposition [15,37,38], chemical vapor deposition [39,40], microwave radiation method [41], solid
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Corresponding author at: School of Materials Science and Engineering, Nanjing Institute of Technology, No. 1, Hongjingdadao, Jiangning District, Nanjing, 211167, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (K. Zhang), [email protected] (F. Hu). https://doi.org/10.1016/j.jphotochem.2019.111859 Received 23 December 2018; Received in revised form 22 April 2019; Accepted 12 May 2019 Available online 13 May 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
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drying. The CdS/ZnS (3:1, 5:1, 7:1) nanocomposites were prepared with the same method.
phase thermogenesis method [42,43] and in situ synthesis method [44,45]. Zhang et al. used cation exchange method to prepare a metalfree core ZnO/CdS/ZnS composite ball with high hydrogen production, good stability and low cost, and H2 generation rate can reach 11.37 mmol h−1 g−1 [6]. Jiang et al. prepared the CdS/ZnS core-shell material with one-step thermal synthesis and solution bath deposition, and the photocatalytic hydrogen generation rate of CdS/ZnS composites was almost unchanged within 12 h, which indicated that the CdS/ZnS composites were of good stability [15]. Xing et al. synthesized CdS/ZnS composites by solvothermal method with ethylenediamine, and the photocatalytic efficiency was 90 times that of pure ZnS [33]. Although good methods for synthesizing CdS/ZnS are endless, the method for preparing CdS/ZnS by liquid-phase thermal decomposition of a single precursor and ion adsorption has not been reported. Herein, CdS/ZnS nanocomposites were synthesized through liquidphase thermal decomposition and ion-adsorption method using Cd (Ac)2·2H2O, 2-Mercaptobenzothiazole (HMBT), Na2S·9H2O and Zn (Ac)2·2H2O as raw materials. We characterized the structure and composition of the samples by XRD, SEM, TEM and ultraviolet visible diffuse reflectance spectrum, and the photocatalytic performances of the samples were also discussed.
2.5. Material characterization X-ray powder diffraction (XRD) patterns of samples were recorded on a Rigaku Ultimate IV X-ray diffractometer using Cu Kα radiation at 40 kV and 40 mA in the 2θ range from 10° to 80° with a scan speed of 8°/min. Scanning electron microscopy (SEM) images and elemental mapping images were acquired on an FEI Nova SEM 230. Transmission electron microscopy images were taken on a JEOL 2100 microscope. The optical properties of the samples were characterized by UV–vis diffuse reflectance spectroscopy (UV-3900), ultraviolet spectrophotometer (UV-1800). The photocurrents were measured by an electrochemical workstation (CHI600E). 2.6. Photocatalytic evaluation The photocatalytic activities of the samples were evaluated by degradation of RhB (0.01 g/L) and MB (0.01 g/L) under visible and UV light irradiation at room temperature, respectively. 500 W mercury lamp was chosen to offer visible light and UV light source. The operating procedures of photodegradation can be seen in literature [45]. 0.2 g photocatalyst was suspended in 200 mL of RhB or MB solution, and then the solution was stirred in the dark room for 30 min. During the light irradiation, 5 mL solution containing the sample was taken out from the reaction suspensions every 5 min and the photocatalyst particles were removed by centrifugation. Subsequently, the solutions were measured with the UV–vis spectrophotometer at wavelength of 552 nm and 662 nm for RhB and MB respectively. Similarly, the recycling photocatalytic tests were done for ten times under natural light, and 30 min per cycle of light.
2. Experimental details 2.1. Material preparation All reagents and solvents were used as received from commercial suppliers without further purification. Cd(Ac)2·2H2O, HMBT, Na2S·9H2O, Zn(Ac)2·2H2O, oleic acid and CH3CH2OH were commercially available from Sinopharm Chemical Co., Ltd (Shanghai, China). Deionized water was used throughout the investigation. 2.2. Synthesis of Cd(MBT)2
2.7. Photoelectrochemical measurement
For fabricating the Cd(MBT)2, 6.50 mmol Cd(Ac)2·2H2O and 13.00 mmol HMBT were firstly dispersed in 80 mL and 50 mL of anhydrous ethanol under stirring separately. Faint yellow precipitate was generated by mixing the above two solutions. The faint yellow powder Cd(MBT)2 was obtained through filtering, washing and drying in sequence.
The photocurrent measurement was performed with a standard three-electrode configuration on a CHI600E electrochemical system. The working electrodes were prepared as follows: 0.001 g CdS/ZnS nanocomposite was mixed with 0.5 mL anhydrous ethanol and 10 μL PTFE by ultrasound. Then, the resulting solution was droped on the FTO glass (1 cm × 1 cm). After evaporation of the ethanol in air, the photocatalysts were attached on the FTO glass surface. A saturated calomel electrode (SCE) was used as the auxiliary and a Pt wire was used as the reference electrode. 0.5 mol/L aqueous solution of Na2SO4 was used as the electrolyte and a 300 W Xe lamp served as the light source.
2.3. Synthesis of CdS nanoparticles The CdS nanoparticles were prepared through a method of liquidphase thermal decomposition. In a typical process, 6.13 mmol Cd (MBT)2 and 150 mL oleic acid were added into a three-necked flask and heated with reflux and magnetic stirring. Cd(MBT)2 was completely dissolved and the color of the solution was orange-red at about 150 ℃. When the solution was heated to 290 ℃ approximately, orange-yellow precipitate occurred. After 10 min of isothermal reaction, the resulting suspension was centrifuged and washed by anhydrous ethanol and deionized water for several times. The CdS nanoparticles were obtained after drying under vacuum for 12 h.
3. Results and discussion 3.1. XRD The crystallization and phase identification of the as-prepared nanocomposites were firstly studied. Fig. 1 shows the XRD patterns of the pure CdS and nanocomposites with different molar ratios of CdS/ZnS (1:1), (3:1), (5:1), (7:1), respectively. From Fig. 1, all the diffraction peaks of the samples can be well indexed to hexagonal wurtzite CdS (JCPDS No. 41-1049). The six major diffraction peaks at 24.94°, 26.60°, 28.20°, 43.86°, 47.90° and 51.94° were observed, which matched well with crystal planes (100), (002), (101), (110), (103) and (112) of hexagonal wurtzite CdS [46,47], indicating that crystalline CdS materials were successfully synthesized. Compared with the pure CdS, the diffraction peak positions corresponding to the (110) and (112) crystal plane of the CdS/ZnS nanocomposite have a slight shift. Hereon the assynthesized CdS/ZnS (7:1) nanocomposite exhibits positions of diffraction peaks at 43.92° and 51.98°, shifting about 0.06° and 0.04°
2.4. Synthesis of CdS/ZnS nanocomposites The CdS/ZnS nanocomposites were prepared successfully by an effortless ion-adsorption method. 5.05 mmol CdS nanoparticles were put into 25 mL saturated solution of Na2S with constant magnetic stirring and heating. The CdS nanoparticles covered with S2− were collected through centrifuging, followed by washing with anhydrous ethanol and deionized water alternatively for several times. Then, the as-prepared nanoparticles were uniformly dispersed into deionized water. 5.05 mmol Zn(Ac)2·2H2O configured as solution was dropwise added into the resulting solution. After sufficient reaction for 5 min, the CdS/ ZnS (1:1) nanocomposites were obtained via centrifuging, washing and 2
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the absorbance of samples also increased gradually. Strong light absorption before 550 nm indicates that they have good optical absorption properties. Among the four as-prepared samples, CdS/ZnS (7:1) nanocomposite with the strongest absorption value means that it has the best optical absorption property. As shown in Fig. 3b, the band gaps (Eg) of different samples are measured from the (Ahν)2−hν curves [49], and A, h and ν correspond to absorption ratio, Planck’s constant and light frequency, respectively. The band gaps of CdS/ZnS (1:1, 3:1, 5:1, 7:1) samples are estimated to be 2.36 eV, 2.31 eV, 2.30 eV and 2.28 eV, respectively. As everyone knows, pure CdS has a bandgap in the 2.42–2.50 eV range and the bandgap of ZnS about 3.7 eV. Compared with CdS and ZnS, the samples are red shifted. With CdS content increasing in CdS/ZnS nanocomposites, the crystal defects of the samples also increase. So red shift occurs UV–vis spectrum, and band gap decreases [50] [Udayabhaskar, 2014 #103]. CdS/ZnS (7:1) has the smallest band gap, corresponding to its optical absorption property.
3.4. Photocatalytic performance
Fig. 1. XRD patterns of as-obtained pure CdS and CdS/ZnS nanocomposites (1:1, 3:1, 5:1, 7:1).
As shown in Fig. 4a, the photocatalytic activity of the four samples is evaluated by the degradation of organic dyes. Before turn on the light, four CdS/ZnS samples (1:1, 3:1, 5:1, 7:1) adsorbed 65%, 67%, 89%, 92% RhB, respectively. This can be seen that the prepared samples have outstanding adsorption performance because of their large surface area. And the color of the solid obtained by centrifugation remains unchanged, indicating that the samples not only have good adsorption performance, but also can undergo photodegradation. Moreover, under ultraviolet light radiation, the degradation efficiencies of RhB of CdS/ZnS samples (1:1, 3:1, 5:1, 7:1) are 92%, 93%, 95%, 96% within 35 min, respectively. The CdS/ZnS (7:1) sample shows the best photocatalytic performance of the four samples. As displayed in Fig. 4a, at 25 min, the degradation efficiency of MB by the CdS/ZnS (7:1) sample reaches 99% under ultraviolet light radiation. In addition, MB is almost completely degraded at 35 min. By contrast with other literatures [51,52], the CdS/ZnS samples have better photocatalytic activity for the degradation of MB and RhB. And CdS/ZnS (7:1) has the best photocatalytic property among the prepared samples. The degradation profiles of dyes in the presence of different samples have been given in Fig. 4b. The photodegradation process can be described by a first order kinetic equation.
severally. The diffraction peaks at 28.20, 47.76° and 56.64° can be designated to ((111), (220) and (311) crystal planes of the zinc blende ZnS. (JCPDS No. 05-0566). It should be noted that strong CdS crystalline phases appeared in all samples and ZnS peaks were not obvious, probably due to low ZnS content or poor crystallization [15]. On account of the above analysis, CdS/ZnS nanocomposites were successfully synthesized. This conclusion can also be proved by SEM data. 3.2. SEM and TEM The morphology and internal microstructure of the samples were observed by SEM and TEM. Fig. 2 shows the FESEM images of CdS/ZnS samples with different molar ratios of CdS to ZnS, in which Fig. 2a-d stand for the FESEM of CdS/ZnS (1:1, 3:1, 5:1, 7:1), respectively. As depicted in Fig. 2a-d, the CdS/ZnS nanocomposites occur as a similar ellipse and a particle size of approximately 25 nm. The particle sizes and morphologies of the four samples are essentially unchanged. That is to say, the molar ratio of CdS has little effect on the particle size and shape of the composites. The elemental mappings of S, Cd, Zn elements (Fig. 2e-h) combined with XRD results prove that CdS/ZnS nanocomposite has been successfully synthesized. Fig. 2i-j show the TEM image of as-synthesized CdS/ZnS (7:1) nanocomposite. As can be seen from Fig. 2i, the particles of CdS/ZnS (7:1) nanocomposite present similar ellipse morphology with a particle size of 25 nm, which is consistent with the SEM observation. Meanwhile, a typical HRTEM image (Fig. 2j) of CdS/ZnS (7:1) sample exhibits clear lattice fringes, indicating that the sample has good crystallinity. What’s more, the image shows the lattice fringes with interplanar spacing of 0.31 nm and 0.36 nm of CdS/ZnS (7:1) sample, corresponding to the (111) and (100) plane of the ZnS [46] and CdS [48], respectively. The energy dispersive X-ray (EDX) results are displayed in Fig. 2k, C and Cu are from the copper mesh of the sample tray while S, Cd, and Zn are from as-synthesized CdS/ZnS (7:1) nanocomposite, which is in line with the SEM results of CdS/ZnS (7:1) nanocomposite. Furthermore, calculated by embedded data in the EDX spectrum, the content of Cd is about seven times as much as the content of Zn. Based on the results of XRD, the asprepared sample is further identified as the CdS/ZnS (7:1) structures.
－ln(C/C0)=kt In the formula, C is the concentration of organic pollutants changing with time, C0 is the initial concentration of organic pollutions, k is the first order kinetic constant, and t is the reaction time [53]. As shown in Fig. 4b, the linear correlations of the reaction kinetic models of the samples are high. Furthermore, the degradation rate of the CdS/ZnS (7:1) is the highest and it degrades MB at a rate of 0.072 min−1 under ultraviolet light irradiation. It is further proved that CdS/ZnS (7:1) has good photocatalytic activity, which is consistent with the above experimental results. For the industrial application of samples, we studied the cyclic degradation of CdS/ZnS (7:1) under natural sunlight irradiation. As displayed in Fig. 5 at the first degradation, the photodegradation efficiency of RhB is 98.6%, while at the 10th degradation; the photodegradation efficiency is 96%. In other word, after 10 cycle times, the degradation efficiency of RhB decreases slightly by 2.6%. Compared with the data of Fig. 4, the degradation efficiency of RhB under natural sunlight irradiation is better than that under ultraviolet light irradiation when CdS/ ZnS (7:1) used as the photocatalyst. The reason is that natural sunlight contains ultraviolet light, visible light and other ingredients. In addition, Fig. 3 illustrates the sample has good absorption of ultraviolet and visible light. Thus the catalytic effect of CdS/ZnS (7:1) on RhB under natural sunlight irradiation is better than that under ultraviolet light irradiation and the results are in agreement with the above analysis. In
3.3. UV–vis DRS Fig.3a shows the UV–vis DRS spectra of CdS/ZnS nanocomposites (1:1, 3:1, 5:1, 7:1) samples. All of the as-obtained samples have absorption properties in both ultraviolet and visible light regions. In addition, with the increasing of CdS content in CdS/ZnS nanocomposites, 3
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Fig. 2. SEM images (a, b, c, d) of various CdS/ZnS (1:1, 3:1, 5:1, 7:1); elemental mapping images (e, f, g, h) of the CdS/ZnS nanocomposite (7:1); TEM (i), HRTEM (j) images of the CdS/ZnS (7:1) nanocomposite; EDX (k) spectrum of the CdS/ZnS nanocomposite (7:1).
the photocurrent test with repetitive light on/off cycles. Fig. 6 shows the surface light current response curves of different samples. The photocurrents of five kinds of samples show the following orders: CdS/ ZnS (7:1) > CdS/ZnS (3:1) > CdS/ZnS (5:1) > CdS > CdS/ZnS (1:1). The photocurrent of CdS/ZnS (7:1) nanocomposite is about 45 times than that of pure CdS. This is because the photogenerated electrons in the conduction band (CB) of CdS were transferred to the CB of ZnS under visible light irradiation. Driven by potential energy [54], the photogenerated holes were transferred from the valence band (VB) of ZnS to the valence band (VB) of CdS [55], which can better separate the charge and prolong the lifetime of photogenerated electrons, greatly
conclusion, the CdS/ZnS (7:1) nanocomposite with excellent photocatalytic activity and stability has a good application prospect in industrialization. 3.5. Photoelectric properties The photoelectric properties of semiconductor nanomaterials mainly depend on the generation of optical electrons, the separation of electron-hole pairs and the transfer efficiency of charge vectors. Therefore, the photoelectrochemical performances of pure CdS and CdS/ZnS nanocomposites with different ratios can be demonstrated by 4
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Fig. 3. UV–vis diffuse reflectance spectra of CdS/ZnS nanocomposites (1:1, 3:1, 5:1, 7:1) (a) and corresponding Tauc’s plots of different samples (b).
Fig. 4. Photocatalytic degradation of dyes (a) and kinetic data for the degradation of dyes (b) in the presence of CdS/ZnS samples (1:1, 3:1, 5:1, 7:1).
Fig. 5. Effects of cycle times on the degradation efficiency of RhB by CdS/ZnS (7:1) under natural sunlight irradiation.
improving the catalytic activity of CdS/ZnS (7:1). However, the more the content of ZnS was, the smaller the photocurrent responses of samples were. The photocurrent of the CdS/ZnS (7:1) is about 200 times that of the CdS/ZnS (1:1). The possible reason is that the excess ZnS led to a large band gap of CdS/ZnS, which reduced the photon yield and hindered the separation of electron-hole pairs [56]. In addition, as the irradiation time is prolonged, the photocurrent response of the CdS/ ZnS (7:1) gradually decreases, which may be caused by the poor binding between CdS/ZnS and FTO substrate as we observed the drop of CdS/ZnS catalyst from FTO substrate in our test [39,56]. Electrons cannot reach the FTO substrate in time and accumulate in the sample, resulting in the recombination of redundant electron-hole pairs. Nevertheless, the photocurrent density of CdS/ZnS (7:1) has a tendency for stabilization after several on/off cycles, which indicates that once the balance of carrier generation, separation, transport and recombination is achieved, the photocurrent density of the sample will become stable [57]{Juncao Bian, 2015 #19}. In summary, the heterostructure of CdS/ZnS can effectively prevent photo-corrosion of CdS
Fig. 6. Transient photocurrent density of pure CdS and CdS/ZnS nanocomposites.
[58], and can be used in the production of hydrogen by photolysis of water and the reduction of CO2 [59,60]. 3.6. Synthesis mechanism The formation mechanism of CdS/ZnS nanoparticles is illustrated in Scheme 1. The mechanism is as follows: (1) CdS nanoparticles were generated from Cd(MBT)2 by thermal decomposition in oleic acid. (2) CdS nanoparticles were uniformly dispersed into saturated NaS2 solution, the S2− ions were absorbed onto the surfaces of the CdS nanoparticles. (3) CdS nanoparticles adsorbing S2− ions were washed and uniformly scattered in deionized water. (4) Quantitative Zn(Ac)2 solution was added to the above suspension, CdS/ZnS heterojunction nanocomposites was fabricated after full reaction. 5
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in 35 min, respectively. What’s more, after ten-cycle tests under natural light, the degradation efficiency still reached around 96% showing the wonderful photocatalytic stability of CdS/ZnS (7:1) nanocomposite, which suggested the potential of environmental purification and water pollution treatment. In this work, a simple two-step method was described to synthesize CdS/ZnS nanocomposites, which broadened the way for the preparation of other metal sulfide nanocomposites. Acknowledgments The authors are grateful to the financial supports of National Natural Science Foundation of China (Grant No. 21603101), National Natural Science Foundation of Jiangsu Province (Grant No. BK20160774), the Innovation Fund of Nanjing Institute of Technology (TB201902014, TB201902058), the Opening Project of Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology and the Outstanding Scientific and Technological Innovation Team in Colleges and Universities of Jiangsu Province.
Scheme 1. Schematic representation of the synthesis processes of CdS/ZnS nanoparticles.
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Fig. 7. Reaction mechanism of the CdS/ZnS nanoparticles for the degradation of organics.
3.7. Mechanism for photocatalytic The photocatalytic degradation mechanism of CdS/ZnS nanoparticles is shown in Fig. 7. The bandgap energies of CdS and ZnS are 2.42 [61] and 3.7 [62] eV, respectively. First, when exposed to light, both of the CdS and ZnS could be excited to produce photogenerated electrons [49] and the positively holes (h+). Second, because the ECB of CdS is more positive than that of ZnS, the photogenerated electrons on the conduction band (CB) of ZnS can transfer toward the conduction band of CdS. Meanwhile, the holes would be shifted from the valence band (VB) of ZnS to the valence band of CdS since the EVB of CdS is more negative than that of ZnS. Third, under the influence of an electric field, the photogenerated electrons could react with O2 adsorbed onto CdS/ZnS nanoparticles to form superoxide radicals (%O2−) [63]. The superoxide radicals could be combined with H+ to form H2O2 which could react with %O2− to form hydroxyl radicals (%OH) [64,65]. Fourth, the positively charged holes (h+) on CdS and ZnS could also have a reaction with H2O to generate %OH. Finally, hydroxyl radicals could react with RhB or MB to become CO2 and H2O.
4. Conclusion In summary, CdS/ZnS nanocomposites were successfully synthesized through the combination of one-step liquid-phase thermal decomposition and ion adsorption method. The effects of different CdS contents on the photocatalytic properties of CdS/ZnS nanocomposites were investigated by multiple characterizations and tests. Notably, the photocurrent of CdS/ZnS (7:1) reached the highest and its photocatalytic property was excellent among the prepared samples. The degradation efficiencies of RhB and MB can reach 96.1% and 99.6% only 6
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