Visible light response and heterostructure of composite [email protected]–ZnO to enhance its photocatalytic activity

Journal of Alloys and Compounds 813 (2020) 152190

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Visible light response and heterostructure of composite CdS@ZnSeZnO to enhance its photocatalytic activity Qianlong Zhou a, Li Li a, b, *, Zichan Xin a, Yan Yu b, Lixian Wang b, Wenzhi Zhang a a b

College of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar, Heilongjiang, 161006, China College of Materials Science and Engineering, Qiqihar University, Qiqihar, Heilongjiang, 161006, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 April 2019 Received in revised form 5 September 2019 Accepted 6 September 2019 Available online 6 September 2019

In this work, CdS@ZnSeZnO composites were prepared by the temperature-programmed hydrothermal method combined with the microwave-assisted hydrothermal method. The composite consisted mainly of cubic phase ZnS, hexagonal phase CdS and hexagonal phase ZnO, and presented a spherical structure with relatively uniform size and shape. There were heterostructures between CdS, ZnS and ZnO in the spherical structure. At the same time, the grain size, specific surface area, average pore diameter and pore volume of the composited material changed significantly, and its light absorption performance in the visible light region was obviously enhanced, and the photocatalytic activity was also greatly improved. Under ultraviolet light, CdS@ZnSeZnO can degrade simulated pollutants by more than 90% within 20 min. In addition, the results of hydrogen evolution from photolysis of water showed that the ternary composite CdS@ZnSeZnO had excellent hydrogen evolution capacity compared with pure CdS, ZnS and ZnO, whose hydrogen evolution amount can reach 3647 mmol g
1 in 8 h, was 298 times than that of P25. Moreover, it still maintained a high hydrogen evolution capacity after 4 cycles. © 2019 Elsevier B.V. All rights reserved.

Keywords: Microwave-assisted hydrothermal method CdS@ZnSeZnO Heterojunction Photocatalysis Hydrogen evolution

1. Introduction The rapid development of the economy has contributed to the rapid consumption of fossil energy, which has also brought about a series of environmental and energy problems. Therefore, the search for alternative energy sources that are clean, efficient, renewable, and stable has gradually become an issue, which scientists in the world are eager to solve [1e5]. Among various ongoing green earth and renewable energy projects, semiconductor photocatalysis has become one of the most promising technologies. On the one hand, semiconductor photocatalysts can effectively degrade organic pollutants by using sunlight, which can decompose pollutants in air or water into simple, harmless inorganic substances. On the other hand, the electronehole pairs generated by photoexcitation in the photocatalytic material can be used to reduce and oxidize protons and hydroxide ion in the water to generate hydrogen and oxygen [6e10]. Due to its high calorific value and cleanliness, hydrogen can be used as a raw material in various chemical industries. Therefore, this method is one of the most ideal ways to convert solar energy

* Corresponding author. College of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar, Heilongjiang, 161006, China. E-mail address: [email protected] (L. Li). https://doi.org/10.1016/j.jallcom.2019.152190 0925-8388/© 2019 Elsevier B.V. All rights reserved.

into chemical energy. Among many photocatalytic materials, nano-ZnO is a novel inorganic semiconductor photocatalytic material with stable physical and chemical properties, non-toxicity, high electron mobility, and catalytic activity. ZnO is widely used in biomedicine, sensors, solar cells and photocatalysis, especially in photocatalytic degradation of organic pollutants [11e14]. Moreover, ZnO is highly active, environmentally friendly, and low cost, which is an important direct bandgap semiconductor material with large exciton binding energy (60 meV) and high electron mobility (115e155 cm2 V
1 s
1). ZnO nanomaterials have become another important semiconductor photocatalytic material after TiO2. Meanwhile, as a multifunctional material, ZnO has a variety of morphologies, such as nanospheres, nanorods, nanotubes, nanobelts, nanowires and nanocrystals [15]. However, due to the large band gap of ZnO nanomaterials, only the ultraviolet region radiation can be used for the photocatalytic process, which greatly limits its practical application [16,17]. At present, many researchers modify ZnO to improve its photocatalytic performance, such as doping transition metal ions [18,19], surface modification of plasmon resonance [20,21] and suitable narrow bandgap semiconductor composite [22e24], etc. The use of narrow-bandgap semiconductors to form

2

Q. Zhou et al. / Journal of Alloys and Compounds 813 (2020) 152190

heterojunctions with wide-bandgap semiconductor materials is a common method used by researchers. ZnS is an ideal material for constructing heterojunctions with ZnO, whose negative conduction band position makes photogenerated electrons to have stronger reduction ability. Under proper photoexcitation, electronehole pairs can be rapidly generated. In the presence of the hole sacrificial agent Na2SeNa2SO3, the photocatalytic quantum efficiency of the light source at 313 nm can be as high as 90% without the noble metal promoter [25]. Guadalupe et al. prepared ZnOcore-ZnSshell nanoparticles in ethanol by hydrothermal method and proved that the photocatalytic H2 production could be enhanced remarkably by depositing a suitable amount of ZnS on ZnO-1D surface [26]. Zou et al. prepared C/ZnO hollow spheres by C microsphere template method and prepared C/ZnS/ZnO by further vulcanization. The results showed that both trace C and ZnS can improve the surface electron transfer efficiency of C/ZnS/ZnO hollow nanospheres. In the photocatalytic activity experiment, the composite can remove almost 81% tetracycline (TC) within 180 min [27]. However, both ZnS and ZnO have strong absorption only under ultraviolet region and almost no absorption under the visible light region, thus greatly limiting their applications. As an important semiconductor material, CdS has favorable photoelectric properties. On the one hand, it has a narrow optical band gap, which can lead to its better photocatalytic activity in the visible region. On the other hand, the conduction band (CB) of CdS is more negative than the electrode potential of Hþ/H2, and it is an favorable photocatalytic hydrogen evolution catalytic material [28e30]. However, due to the metal sulfide characteristics of CdS itself, it is easily oxidized and photo-etching occurs, which limits its practical application. But it is still a very effective method to utilize CdS to support the ZnSeZnO composite to enhance the photocatalytic activity of the composite. In this paper, the ternary composite CdS@ZnSeZnO was synthesized by the combination of the temperature-programmed hydrothermal and the microwave-assisted hydrothermal method. First, microwave radiation will affect the physical properties and photocatalytic properties of the synthesized samples. Then, improving the photocatalytic activity and light resistance of ZnO can be achieved by introducing ZnS. Certainly, the ZnSeZnO binary composite can only absorb ultraviolet light, and the introduction of CdS in the synthesis can expand the response range of the composite in the visible region and improve the light absorption capacity of the composite. In addition, the combination of CdS and ZnSeZnO can effectively construct the heterostructure between CdS, ZnS and ZnO in the composite, and establish multi-channel electron transfer, which leads to the successful separation of photogenerated carriers. Therefore, we hope that the construction of CdS and ZnSeZnO can further enhance its photocatalytic activity and photolysis water hydrogen production capacity.

2. Experimental details 2.1. Chemical reagent Sodium sulfite, zinc acetate, cadmium nitrate, thioacetamide (TAA), ethylenediaminetetraacetic acid, isopropanol, and p-benzoquinone were purchased from Tianjin Kaitong Chemical Reagent Co., Ltd. Ethylene glycol and absolute ethanol (C2H5OH) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Sodium sulfide purchased from New Jersey, USA. Malachite green (MG), Methyl orange (MO), Congo red (CR), Methylene blue (MB) and Rhodamine B (RhB) were all commercially available analytically pure; the water used in the experiment was secondary deionized water.

2.2. Preparation of composite material CdS@ZnSeZnO Synthesis of ZnO: 0.75 g of zinc acetate was poured into 25 mL of ethylene glycol and stirred for 30 min to dissolve it sufficiently. The reaction was carried out in a polytetrafluoroethylene reactor at 150  C for 7 h. The obtained product was washed with deionized water and ethanol and dried at 60  C for 12 h. ZnO monomer was obtained. Synthesis of Composite ZnSeZnO: 0.02 g of TAA was dissolved in 25 mL of deionized water, mixed well, and then 0.02 g of zinc acetate was dissolved in the solution. After it was completely dissolved, 0.1 g of ZnO prepared in advance was added, stirred for 30 min, and then placed in a microwave reactor at 100  C for 45 min. The obtained sample was washed three times with deionized water and ethanol and dried at 60  C for 12 h to obtain a ZnSeZnO composite. Synthesis of ternary composite CdS@ZnSeZnO: 0.01 g of TAA was dissolved in 25 mL of deionized water, mixed well, and then 0.01 g of cadmium nitrate was dissolved in the solution. After it was completely dissolved, 0.1 g of the prepared ZnSeZnO was added, stirred for 30 min, and then placed in a microwave reactor at 100  C for 45 min. The obtained sample was washed three times with deionized water and ethanol and dried at 60  C for 12 h to obtain the composite CdS@ZnSeZnO. 2.3. Characterization The X-ray diffraction (XRD) analysis of the sample used a German Bruker-AXS (D8) X-ray diffractometer (Cu target Ka radiation (l ¼ 0.15406 nm), tube voltage 60 kV, tube current 80 mA, scanning range 20 e80 ). The X-ray photoelectron spectroscopy (XPS) spectra of the sample were measured using a VG-ADES400 Xray photoelectron spectrometer using an Mg K-ADES source with a residual gas pressure of less than 10
8 Pa. The scanning electron microscope (SEM) analysis of the sample was performed using a Hitachi S-4700 scanning electron microscope with an operating voltage of 5 kV. The specific surface area and pore diameter of the sample were measured by a 3H-2000 type specific surface area and pore size analyzer of Beijing Beishide Instrument Co., Ltd., and the measurement temperature was 77 K. The transmission electron microscopy and high-resolution transmission electron microscopy (TEM and HR-TEM) of the sample were performed by Hitachi H7650 of Hitachi, Japan and JEM-2100F of JEOL of Japan (acceleration voltage of 200 kV). The UVevisible diffuse reflectance absorption spectroscopy (UVeVis/DRS) of the sample was carried out on a TU1901 ultravioletevisible dual-beam spectrophotometer (integral sphere) produced by Beijing General Analysis Co., Ltd. The photoluminescence (PL) of the sample was made by F-7000 of Hitachi, Japan. The electrochemical impedance was measured using a PEC1000 photoelectrochemical test system manufactured by Perfectlight Co., Ltd. The absorbance of the sample solution was measured by a TU-1901 ultravioletevisible double-beam spectrophotometer manufactured by Beijing P&A General Electric Co., Ltd. The hydrogen evolution experiments were carried out in the photolysis water hydrogen production system (Labsolar-III(AG), Beijing Perfectlight Technology Co. Ltd.). 2.4. Photocatalytic performance experiments The photocatalytic activity experiments were carried out by previous report [14]. The detailed experimental process is as follows: Multi-modal photocatalytic performance research: the photocatalytic activity of samples was evaluated using malachite green dye as a main model molecule under multi-mode illumination with

Q. Zhou et al. / Journal of Alloys and Compounds 813 (2020) 152190

ultraviolet, visible light, and simulated sunlight. The experimental apparatus for the multi-mode photocatalytic reaction was selfmade. In photocatalysis experiments, 125 W high-pressure mercury lamp as a UV light source (located in the main emission line 313.2 nm), 400 W xenon lamp as a visible light source (emission line located mainly 410 nm), 1000 W xenon lamp as a simulated daylight source (external type, Shanghai Bao Xun Instrument Co., Ltd., the emission spectrum closes to full spectrum) were used, and the concentration of the reaction solution was 50 mg/L. The catalysts used under the ultraviolet, visible and simulated sunlight were: 0.15 g, 0.3 g, and 0.15 g, respectively, and the samples were dispersed in 90 mL, 220 mL and 100 mL of the reaction solution, respectively. The photocatalytic reaction was as follows: the suspension was stirred for 30 min in the dark to achieve adsorptionedesorption equilibrium. The solutions were then irradiated with respective light sources. The samples were collected at certain time intervals and centrifuged to remove the residual catalyst. The absorbance values were measured by the TU-1901 dual-beam ultravioletevisible spectrophotometer at lmax of dyes. After light irradiation, the catalyst was washed with ethanol and deionized water, dried, calcined and recovered. Photocatalytic hydrogen evolution experiment: the photocatalytic performances for hydrogen evolution were performed in a closed vacuum reactor connected to the loop system. 0.1 g catalyst was dispersed in 50 mL deionized water with different sacrificial agents, after vacuum degassing, the test for photocatalytic hydrogen evolution began under stirring. The reactor was irradiated with a 300 W Xe lamp (420 nm filter to remove the ultraviolet light to test the hydrogen evolution performance of the composite under visible light) with high purity nitrogen as the carrier gas, and the output pressure was 0.4e0.6 MPa, the operating voltage was about 20 mV, and operating current was 50 mA. During the reaction, the circulating condensed water temperature was maintained at 5  C. The gas was collected after 8 h, and the hydrogen production was analyzed by an on-line gas chromatograph. The column was a 5 Å molecular sieve column and the detector was TCD. The amount of hydrogen production was calculated based on the peak area measured at different reaction times and the catalytic activity was measured by the total hydrogen production for 8 h. 3. Results and discussion 3.1. XRD analysis In order to determine the crystal structure of the ternary composite CdS@ZnSeZnO, XRD tests were carried out, and the results are shown in Fig. 1. After 7 h of hydrothermal reaction at 150  C, the pure ZnO obtained has a hexagonal phase structure, and the diffraction peaks are mainly located at 31.7, 34.4 , 36.2 , 47.5 , 56.5 , 62.8 , and 68.0 , respectively, corresponding to (100), (002), (101), (102), (110), (103), and (112) crystal faces of ZnO (JCPDS No. 36e1453). And pure ZnS is a cubic phase structure with diffraction peaks at 28.6 , 48.1, and 56.5 , corresponding to (111), (220), and (220) crystal faces of ZnS (JCPDS NO.05e0566). On the basis of this, after the ZnS and ZnO are combined by microwave radiation reaction, the diffraction peaks of the obtained composite are consistent with the diffraction peaks of pure ZnO, but a small peak is observed near 28.6 , which corresponds to characteristic diffraction peak of (111) crystal plane of ZnS. It shows that the microwave radiation does not destroy the crystal structure of ZnO, while ZnS has been combined with ZnO to some extent. After loading CdS, compared with ZnSeZnO, the crystal structure of the obtained ternary composite CdS@ZnSeZnO is not changed. Among them, the characteristic diffraction peaks of pure CdS are mainly located at

3

Fig. 1. XRD pattern of ZnS, CdS, ZnO, ZnSeZnO, and CdS@ZnSeZnO composites.

24.8, 27.5 , 43.7, 52.8 , corresponding to (100), (002), (110), (112) crystal planes of the hexagonal phase CdS. However, in the ternary composite CdS@ZnSeZnO, the characteristic diffraction peaks of CdS are not observed, it may be caused by the following reasons: in the synthesis process, the content of CdS is very small, lower than the detection limit of X-ray diffraction, so CdS may not be detected. In addition, it is also possible that the CdS has a small particle size in the prepared composite material CdS@ZnSeZnO, resulting in no peak belonging to CdS in the XRD pattern [31]. Usually, XPS and HRTEM have higher precision than XRD. Therefore, even if the presence of CdS cannot be detected in XRD, the existence of CdS in the composite can still be confirmed by the more accurate analysis method. In the subsequent HR-TEM analysis, the existence of the crystal phase structure of ZnS and CdS can still be confirmed. At the same time, compared with pure ZnO, the 2q of the characteristic diffraction peak of ZnO in the composite ZnSeZnO is shifted. After further loading CdS, the 2q of the characteristic diffraction peak of ZnO is further shifted, which proves to some extent that ZnS and CdS affect the crystal structure of ZnO, causing lattice distortion and the 2q to shift [32]. To further understand the crystal plane growth of ZnO in pure ZnO, binary ZnSeZnO and ternary composite CdS@ZnSeZnO, the growth of (100), (101) and (110) crystal faces of ZnO was studied with the (002) crystal plane of ZnO as the standard. The results are shown in Fig. 2. After compounding with ZnS, the (110) crystal plane of ZnO in the composite ZnSeZnO is inhibited, while the growth of (100) and (101) crystal planes of ZnO is promoted. In addition, after CdS is further loaded, the growth of three crystal faces of ZnO in the ternary composite CdS@ZnSeZnO is suppressed, and the (110) crystal plane is continuously inhibited. The above results indicate that the combination of ZnO and ZnS and the loading of CdS have a certain influence on the selective growth of the crystal plane of the material. Meanwhile, the average grain size of each of the synthesized sample was calculated according to the Scherrer's formula d ¼ Kl/ (Bcosq) (see Supplementary material) [33], and the results are shown in Table 1. The grain sizes of ZnO, ZnSeZnO and CdS@ZnSeZnO composite are 27.5 nm, 25.1 nm, and 25.2 nm, respectively. Compared with pure ZnO, the grain sizes of ZnSeZnO and CdS@ZnSeZnO composites are reduced, because the combination of ZnS and ZnO, and the loading of CdS are not destroyed under microwave radiation. The crystal structure of ZnO also promotes the high purity, narrow particle size distribution and uniform morphology of the composite during crystallization. At the

4

Q. Zhou et al. / Journal of Alloys and Compounds 813 (2020) 152190

Fig. 2. Growth of each crystal plane (measured by the (002) crystal plane of ZnO).

Table 1 Grain size (d*), specific surface area (SBET), average pore diameter (D) and pore volume (Vtotal) of ZnO, ZnSeZnO and CdS@ZnSeZnO. Sample

d*/nm

SBET/(m2$g
1)

D/nm

Vtotal/(cm3$g
1)

ZnO ZnSeZnO CdS@ZnSeZnO

27.5 25.1 25.2

37.5 33.3 28.7

10.33 4.45 4.22

0.138 0.090 0.096

same time, due to the characteristics of rapid heating and uniform heating in the microwave, the sintering time is short and the temperature distribution in the crystal is uniform, which ensures the uniformity of the grain distribution, and thus the grain size of the composite material becomes smaller [34].

3.2. SEM-EDS To investigate the influence of microwave radiation on the morphology and structure of the materials, SEM analysis of different materials was carried out. The results are shown in Fig. 3. Fig. 3(a, c) is the SEM images of pure ZnO. The synthesized ZnO has a spherical structure of uniform size, and the diameter of the sphere is ca. 1.2e1.3 mm, while the surface of the sphere is relatively smooth and exhibits the accumulation of nanoparticles. Fig. 3(def) is the SEM images of the composite ZnSeZnO. After ZnS and ZnO are combined by microwave radiation, the morphology of the composite is still spherical and the sphere is well-balanced. Compared with the pure ZnO, the spherical size of ZnSeZnO remains basically the same, but the particles on the surface of the sphere become rough, indicating that ZnS and ZnO have been combined under microwave radiation, and the microwave radiation does not destroy the spherical structure of the material. It can be seen from Fig. 3(gei) that after further loading of CdS, the composite CdS@ZnSeZnO still maintains a spherical structure, but the size of the whole sphere is slightly reduced, which could be attributed to the tighter packing between the nanoparticles under the two times microwave-assisted synthesis, resulting in increased structural tightness of the composite after compounding. In order to further determine the type and content of the constituent elements in the composite CdS@ZnSeZnO, EDS test was performed. The results are shown in Fig. 4. From Fig. 4(aed), there are four elements of O, S, Zn, and Cd in the composite, which are more evenly distributed. From the EDS element distribution map, it is not difficult to find out that the distribution of each element is consistent with the morphology of the composite material, and both are spherical. From the data in Fig. 4(e), the content of the corresponding elements is 12.9%, 13.2%, 6.89%, and 5.0%, respectively, which are consistent with the results of the respective elements under the theoretical feed ratio, further proves that the composite material CdS@ZnSeZnO is successfully prepared.

3.3. TEM and HR-TEM To further demonstrate the morphology and the composition of CdS@ZnSeZnO composite, TEM and HR-TEM measurement were performed, and the results are shown in Figs. 5 and 6. Fig. 5 shows TEM images of CdS@ZnSeZnO at different magnifications. It can be more intuitively shown that CdS@ZnSeZnO composite has a spherical structure. Meanwhile, it can be observed in Fig. 6(a) that the intermediate portion of the obtained composite material is a ZnO microsphere, and the outer surface of the microsphere is covered with ZnS and CdS. It can be observed from the mark in Fig. 6(b) that the heterojunction is formed between the square region ZnS and the circular region ZnO. Meanwhile, the hexagonal region (CdS) is also in close contact with the circular region (ZnO). In addition, in order to more accurately prove that the synthesized sample is CdS@ZnSeZnO, the HR-TEM analysis and Gatan Digital Micrograph technology were carried out. Fig. 6(c, f) shows the HR-TEM analysis images of the square region ZnS. In Fig. 6(c, f), the lattice spacing is 0.31 nm, 0.27 nm, 0.19 nm, corresponding to (111), (220), (220) crystal faces of the cubic phase ZnS [35,36]. Fig. 6 (d, g) are the HR-TEM analysis image of the hexagonal region CdS. In Fig. 6 (d, g), the lattice spacing is 0.32 nm and 0.35 nm, which corresponds to (101), (100) crystal plane of the hexagonal phase CdS [37,38]. Fig. 6(e, h) is the HR-TEM analysis image of ZnO in a circular region. In Fig. 6(e, h), the lattice spacing is 0.26 nm, which corresponds to (002) crystal plane of the hexagonal phase ZnO [39]. The above results show that there are ZnO hexagonal phase, CdS hexagonal phase and cubic phase ZnS in the synthesized ternary composite CdS@ZnSeZnO. Simultaneously, a close connection is formed between ZnS and ZnO, thereby forming the heterojunction, which is advantageous for photogenerated carrier separation, and improving the separation efficiency of photogenerated electronehole pairs. In addition, after further loading CdS, on the one hand, since the CdS conduction band potential is more negative than ZnO, the recombination of photogenerated carriers inside the ZnO microspheres can be effectively suppressed, and the lifetime of the photogenerated carriers of the composite material is further improved. On the other hand, the photogenerated electrons on the ZnO conduction band and the photogenerated holes on the CdS valence band can be recombined, thereby promoting the accelerated separation of the photogenerated electronehole pairs on the CdS, further improving the photocatalytic activity of the composite. 3.4. XPS analysis To determine the chemical form of each element on the surface of the composite, XPS analysis of the composite CdS@ZnSeZnO was performed. The results are shown in Fig. 7. From Fig. 7(a), the sample contains four elements of Zn, Cd, S, and O. Fig. 7(b) shows the XPS spectrum of Cd at Cd 3d with an electron binding energy of 405.1 eV, 411.8 eV, respectively, wherein the split energy between

Q. Zhou et al. / Journal of Alloys and Compounds 813 (2020) 152190

5

Fig. 3. SEM images of different samples: (aec) ZnO; (def) ZnSeZnO; (gei) CdS@ZnSeZnO.

Cd 3d3/2 and Cd 3d5/2 is 6.7 eV, indicating that Cd exists as Cd2þ [40]. Fig. 7(c) shows the characteristic peaks of Zn in Zn 2p3/2 and Zn 2p1/2, where the electron binding energy region is located at 1021.4 eV and 1044.4 eV, respectively, corresponding to Zn2þ ion in ZnO [41], while 1022.5 eV and 1045.5 eV belong to Zn2þ ion in ZnS [42]. Fig. 7(d) is the XPS spectrum of O at O1s in composite CdS@ZnSeZnO with electron binding energy regions of 530.3 eV, 531.3 eV, and 532.8 eV, respectively, corresponding to lattice oxygen (ZneO) and characteristic peak of adsorbed oxygen (H2O or O2) [43,44]. Fig. 7(d) is the XPS spectrum of S in S 2p of the ternary composite CdS@ZnSeZnO with an electron binding energy region of 161.7 eV and 162.7 eV, respectively, attributed to the S2
ion in CdeS bond and ZneS bond [45,46]. The above results can further prove the existence of each substance in the ternary composite CdS@ZnSeZnO. 3.5. UVevis/DRS analysis In order to examine the optical absorption properties of samples, UVevis/DRS spectral analysis was performed on the prepared composite, and the results are shown in Fig. 8. Among them, the absorption spectrum of different catalysts is shown in Fig. 8(a), while Fig. 8(b) is the KubelkaeMunk energy plot of different samples. From Fig. 8(a), ZnO, ZnS and ZnSeZnO samples have strong absorption in the ultraviolet region and almost no absorption in the visible region. Since CdS has absorption in the visible region, after loading CdS, compared with pure ZnO and ZnS, the

absorption band edge of the ternary composite CdS@ZnSeZnO has a significant redshift, and redshift is about 90 nm. It indicates that CdS is successfully loaded on the composite ZnSeZnO to a certain extent, so that it has a certain light response in the visible region, which can enhance the absorption capacity of the composite. According to Fig. 8(b) and KubelkaeMunk formula, the band gap energy of the prepared sample was calculated [47]:

ahn ¼ A(hn-Eg)n/2 Wherein a represents an absorption coefficient; n represents an optical frequency, and A is a proportional constant. It can be concluded from the above formula that the band gap values of ZnS, ZnO, CdS, ZnSeZnO, ZnSeZnO, and CdS@ZnSeZnO are 3.34, 3.20, 2.27, 3.23, and 2.67 eV, respectively. Compared with pure ZnS, the band gap value of ZnSeZnO composite is significantly reduced, which is mainly due to the heterojunction formation between ZnS and ZnO, thus effectively reducing the band gap value of the composite. After loading CdS, the band gap value is further reduced. The above results show that CdS is successfully supported on ZnSeZnO, which further reduces the forbidden bandwidth of the composite and can effectively improve its photocatalytic activity. 3.6. N2 adsorptionedesorption analysis To explore the surface physicochemical property of CdS@ZnSeZnO composite, a series of composite materials were

6

Q. Zhou et al. / Journal of Alloys and Compounds 813 (2020) 152190

Fig. 4. EDS spectrum of composite CdS@ZnSeZnO: (a, g) O; (b, h) S; (c, i) Zn; (d, j) Cd; (f) SEM; (e) EDS spectrum of composite CdS@ZnSeZnO (illustrated as element type and content analysis).

Fig. 5. TEM images of ternary composite CdS@ZnSeZnO at different magnifications.

subjected to N2 adsorptionedesorption measurement. The results are shown in Fig. 9. The N2 adsorptionedesorption isotherms of three photocatalytic materials all exhibit type IV and have an H3 type hysteresis loop. The type of this isotherm and its hysteresis loop are caused by the condensation of the capillary and the aggregation of the particles in the structure, indicating that the synthesized composite has a mesoporous structure. While there is no adsorption limit at high P/P0 in the three samples, indicating the

presence of pores has plate-like particles or slit shapes. The BET specific surface areas of three photocatalytic materials are shown in Table 1, in which the pure ZnO has the largest specific surface area (37.5 m2 g
1). After microwave irradiation, the specific surface area of the obtained photocatalytic material ZnOeZnS is reduced (33.3 m2 g
1), attributed to the accumulation of nanoparticles inside the spherical structure during the composite process of ZnS and ZnO, which make it more compact, and cause some of the slitshaped holes to be blocked. This is also the main reason why the BET specific surface area and pore volume of the obtained photocatalytic material CdS@ZnSeZnO are further reduced (28.7 m2 g
1) after microwave radiation again. Meanwhile, it can also be confirmed by the BJH pore size distribution curve shown in the inset of Fig. 9. As can be seen from the data in Table 1, the average pore diameters of ZnO, ZnOeZnS, and CdS@ZnSeZnO are 10.33 nm, 4.45 nm, 4.22 nm, respectively. Obviously, in the process of sample synthesis, the microwave radiation effect can lead to a decrease in the pore size of the material, while the two times microwave effects can lead to further tightening of the material and smaller pore size.

3.7. PL and EIS analysis In order to study the migration, transfer, and recombination of photogenerated electronehole pairs in photocatalytic materials, the PL and EIS analysis were carried out. Generally, lower PL intensities indicate lower recombination rates of photogenerated electronehole pairs, resulting in the higher photocatalytic activity

Q. Zhou et al. / Journal of Alloys and Compounds 813 (2020) 152190

7

Fig. 6. (a, b) HR-TEM images of CdS@ZnSeZnO; (c, f) HR-TEM analysis images of ZnS in CdS@ZnSeZnO; (d, g) HR-TEM analysis images of CdS in CdS@ZnSeZnO (e, h) HR-TEM analysis images of ZnO in CdS@ZnSeZnO.

of the semiconductor composite. As can be seen from Fig. 10(a), CdS@ZnSeZnO has the lowest luminescence intensity, indicating the recombination rate with the lowest electronehole pairs. At the same time, the intensity of PL is ZnO > ZnSeZnO > CdS@ZnSeZnO, which means that the introduction of ZnS and CdS can effectively reduce the recombination rate of electronehole pairs in the composite. At the same time, the smaller arc radius is related to the lower charge transfer resistance. EIS Nyquist plots (Fig. 10(b)) further confirmed that the composite CdS@ZnSeZnO has the lowest charge transfer resistance. The above characterization results show that the construction of ternary composite CdS@ZnSeZnO can effectively reduce the recombination of electronehole pairs and further prolong the lifetime of photogenerated carriers, thus effectively improving its photocatalytic activity.

3.8. Photocatalytic performance study In order to study the photocatalytic properties of composites, a series of multi-mode photocatalytic experiments were carried out under ultraviolet light, visible light and simulated sunlight. The results are shown in Fig. 11 and Fig. S1. The experimental results of the degradation of different dyes by the ternary composite

CdS@ZnSeZnO are shown in Fig. 11(a). Under the same experimental conditions, the degradation rate of CdS@ZnSeZnO photocatalytic degradation of MG can reach 95% within 30 min. Obviously, the ternary composite has excellent degradation effect on the malachite green, therefore, MG is selected as the model molecule. In addition, as can be seen from Fig. 11(a), the composite material has the certain degradation ability to other dyes under ultraviolet light irradiation, indicating that the composite material has a certain degradation effect on different types of dyes. In Fig. 11(b), the photocatalytic activity of CdS@ZnSeZnO is significantly higher than that of other samples under UV irradiation, and its catalytic activity is significantly enhanced, which is consistent with the previous UVevisible diffuse reflectance absorption spectroscopy. It can be seen from Fig. 11(c) and (d) that CdS@ZnSeZnO exhibits the best excellent degradation activity under visible light and simulated sunlight, which can greatly increase the photocatalytic degradation efficiency of the dye. Moreover, CdS@ZnSeZnO has high photocatalytic activity under multi-mode photocatalytic condition, which is related to its strong visible light absorption and the heterostructure existing in the composite. Meanwhile, based on the above data, a photocatalytic degradation kinetic curve was drawn. It is apparent from Fig. S1 that -lnCt/C0 is substantially linear with the reaction time t, which indicates that

8

Q. Zhou et al. / Journal of Alloys and Compounds 813 (2020) 152190

Fig. 7. XPS spectra of composite CdS@ZnSeZnO: (a) XPS full spectrum of CdS@ZnSeZnO; (b) Cd 3d; (c) Zn 3p; (d) O 1s; (e) S 2p.

the degradation of the dye molecule follows the quasi-first-order reaction kinetics. To investigate the reason for the high activity of the composite, a capture experiment was carried out in this paper. The specific results are shown in Fig. 11(e). Under the same experimental conditions, the superoxide radical (O$e 2 ) trapping agent benzoquinone (BQ), hydroxyl radical ($OH) trapping agent isopropanol (IPA), hole (hþ) capturing agent disodium edetate (EDTA-2Na) were added for UV photocatalytic capture experiments. It can be seen from Fig. 11(e) that the degradation of MG is significantly reduced after

the addition of different capture agents under ultraviolet light irradiation. Among them, after the addition of superoxide radical (O$e 2 ) capture agent benzoquinone (BQ), the degradation of malachite green is obviously inhibited, which indicates that the main active species of CdS@ZnSeZnO composite is O$e 2 in photocatalytic reaction, while the holes (hþ) and hydroxyl radicals ($OH) play a certain auxiliary role. In addition, in order to further study the hydrogen-discharging ability of composite materials, the experiments of different catalysts for photohydrolysis of water were carried out. The results of

Q. Zhou et al. / Journal of Alloys and Compounds 813 (2020) 152190

9

Fig. 8. UVevis/DRS absorption spectra (a) and KubelkaeMunk energy curve plots (b) of different samples.

Fig. 9. N2 adsorptionedesorption measurement results of different materials (illustrated as pore size distribution curve): (a) ZnO; (b) ZnOeZnS; (c) CdS@ZnSeZnO.

hydrogen evolution experiments of different catalysts in 1 M Na2S and Na2SO3 system are shown in Fig. 12(a). Compared with the hydrogen evolution of pure ZnO and pure CdS after 8 h of 300 W xenon lamp irradiation, the hydrogen evolution of ZnSeZnO is greatly improved, which can be attributed to the formation of an effective heterojunction between ZnS and ZnO. After CdS loading, the hydrogen evolution amount reaches 3647 mmol g
1, which indicates that the composite material CdS@ZnSeZnO has more excellent photohydrolysis water hydrogen evolution performance. In view of the above results, we believe that although the hydrogen evolution efficiency of ZnO is extremely low, while combined with CdS and ZnS, the hydrogen evolution of the composite material is

greatly improved, indicating that the presence of CdS and ZnS compensates for the disadvantage that the ZnO conduction band is not sufficiently negative (ECB ¼
0.26 eV vs NHE) too close to the hydrogen evolution potential (0.0 eV vs NHE), which improves the hydrogen evolution capacity of the composite to some extent. In addition, in order to investigate the stability of hydrogen evolution by photolysis of the ternary composite CdS@ZnSeZnO, a cycle experiment was carried out, and the results are shown in Fig. 12(b). After four cycles, the ternary composite CdS@ZnSeZnO still has a high hydrogen evolution capacity, which could still reach 3425 mmol g
1, indicating that the prepared photocatalyst has excellent stability. Meanwhile, in order to further confirm the

10

Q. Zhou et al. / Journal of Alloys and Compounds 813 (2020) 152190

Fig. 10. (a) PL spectra (excitation: 325 nm) and (b) EIS Nyquist plots of different samples.

Fig. 11. (a) Experimental results of degradation of different dyes by ternary composite CdS@ZnSeZnO under ultraviolet light; (b) Experimental results of degradation of malachite green by different catalysts under ultraviolet light (t ¼ 30 min); (c) Different experimental results of degrading malachite green under visible light (t ¼ 180 min); (d) Experimental results of degrading malachite green by different catalyst under simulated sunlight (t ¼ 180 min); (e) Ternary composite CdS@ZnSeZnO ultraviolet light capture experimental results. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 12. (a) Experimental results of hydrogen production by different catalysts for photolysis; (b) Hydrogen cycle experiment of ternary composite CdS@ZnSeZnO photolysis.

hydrogen evolution performance of the composite under visible light irradiation, hydrogen evolution experiments under visible light were carried out (Fig. S2). Under visible light irradiation, ZnO,

ZnS, CdS and ZnSeZnO rarely have hydrogen evolution performance, but the hydrogen evolution of the composite can reach 801.2 mmol g
1, and also exhibits favorable stability in the cycling

Q. Zhou et al. / Journal of Alloys and Compounds 813 (2020) 152190

experiment, further confirming that the ternary composite has an excellent visible light response and hydrogen evolution performance.

3.9. Possible photocatalytic reaction mechanism In order to investigate the migration pathway of photogenerated carriers of ternary composite CdS@ZnSeZnO, the position of the energy band is estimated by equations (1) and (2), [48]. The results are shown in Table 2. ECB ¼ c-EC-0.5 Eg

(1)

EVB ¼ ECB þ Eg

(2)

Wherein, c refers to the absolute electronegativity of the semiconductor oxide, which is expressed as the geometric mean of the absolute electronegativity of the atoms constituting the compound; EC is the potential energy of the free electron at the standard hydrogen electrode (4.5 eV). ECB and EVB are the energy band positions of the semiconductor conduction band and the valence band, respectively. Based on the above estimation results and the results of the capture experiments, the possible photocatalytic mechanism of the ternary composite CdS@ZnSeZnO is presumed, as shown in Fig. 13. First, both ZnS and ZnO can be excited and produce photogenerated electron
hole pairs under light irradiation. In the case of pure ZnO, photogenerated electrons and holes in the valence band and the conduction band can be recombined internally and on the surface during the transfer process. After being combined with ZnS, due to their band matching, a heterostructure can be formed between ZnS and ZnO, thereby effectively promoting the separation of photogenerated electrons and holes. Since EVB and ECB of ZnO are higher than ZnS, photogenerated holes can be rapidly transferred from the valence band of ZnO to the valence band of ZnS, and the

Table 2 Absolute electronegativity (c), band gap energy (Eg), conduction band (CB) and valence band (VB) band position E of ZnO, ZnS, and CdS. Semiconductor

c/eV

Eg/eV

ECB/eV

EVB/eV

ZnO ZnS CdS

5.79 5.26 5.19

3.20 3.34 2.27


0.31
0.91
0.44

2.89 2.43 1.83

11

photogenerated electrons on the ZnS conduction band are easily transferred to the conduction band of ZnO. Secondly, after further loading CdS, the photogenerated carriers in the composite have two different migration modes: direct transfer mechanism and Zscheme charge transfer process. On the one hand, due to the significant potential gradient of the conduction bands of each component in the composite, photogenerated electrons can be transferred from the conduction band of ZnS to the conduction band of CdS, and further transferred to the conduction band of ZnO, effectively extending the transfer path of photogenerated electrons; On the other hand, Xie et al. have shown that oxygen vacancies at the interface of heterostructures between CdS and ZnO are effective interfacial mediators, which can promote the direct Zscheme charge transfer process in CdS and ZnO [28]. Therefore, the photogenerated electrons transferred to the ZnO conduction band recombine with the photogenerated holes on the CdS valence band, thereby promoting the accelerated separation of the photogenerated electron
hole pairs on the CdS, further increasing the lifetime of the photogenerated carriers. During the direct transfer, electrons on the surface of CdS can react with O2 molecules and form O$2 , while the valence band potential of CdS does not oxidize OH
to $OH. However, during Z-scheme charge transfer, the valence band potential of ZnS is sufficient to oxidize OH
to $OH, so þ three active species of O$2 , $OH, and h are detected in the ultraviolet light capture experiment. Subsequently, organic pollutants are mineralized into CO2 and H2O. Therefore, the multi-mode transfer process of photogenerated charges leads to excellent photocatalytic degradation activity of the ternary composite CdS@ZnSeZnO. Under visible light irradiation, since ZnS and ZnO have no visible light response, only CdS is excited in the composite to generate photogenerated carriers. As shown in Fig. 14, in this case, the photogenerated electrons generated by CdS can be transferred from the conduction band of CdS to the conduction band of ZnO by the potential gradient, effectively extending the lifetime of the photogenerated electrons. Then, the superoxide radicals enable the composite to have visible light photocatalytic activity. As confirmed by photocatalytic activity studies, the activity of the composite CdS@ZnSeZnO under visible light is significantly improved compared with other photocatalytic materials. On the other hand, the photocatalytic activity increases as the accumulation of electrons and holes increases in the conduction band and the valence band of the material, respectively. In addition, when the material exhibits a lower charge transfer resistance at the

Fig. 13. Possible photocatalytic reaction mechanism of ternary composite CdS@ZnSeZnO under UV light irradiation: (a) direct transfer mechanism, (b) Z-scheme charge transfer process.

12

Q. Zhou et al. / Journal of Alloys and Compounds 813 (2020) 152190

Acknowledgments This study are supported by the National Natural Science Foundation of China (21376126, 21776144), The Fundamental Research Funds in Heilongjiang Provincial Universities, China (135209105), Government of Heilongjiang Province Postdoctoral (LBH-Z11108), Postdoctoral Researchers in Heilongjiang Province of China Research Initiation Grant Project (LBH-Q13172), College Students' Innovative Entrepreneurial Training Program Funded Projects of Qiqihar University (201910232028). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152190. Fig. 14. Possible photocatalytic reaction mechanism CdS@ZnSeZnO under visible light irradiation.

of

ternary

composite

catalyst surface/solution reaction interface, its hydrogen evolution performance is improved. ZnO is the material having the lowest charge transfer resistance, however, the position of its conduction band with respect to the hydrogen reduction potential cannot ~ a-Pe rez et al. research showed that restore Hþ to H2 (
0.31 eV). Pin when ZnS and ZnO semiconductors were in contact, a new energy state was generated at the interface, and the Efb of the composite was replaced by a more negative value, which was beneficial to the reduction process from Hþ to H2 [49]. The composite ZnSeZnO is a new material that is chemically bonded through a heterojunction, which has different physicochemical properties from pure ZnO and ZnS photocatalysts and thus has improved photocatalytic hydrogen evolution activity after the combination of them. After loading CdS, the multi-mode transfer process between the ZnO and CdS heterojunctions further promotes electron separation, and the position of the CdS conduction band relative to the hydrogen reduction potential is
0.41 eV, which can be reduced Hþ to H2, thereby further improve the photocatalytic hydrogen evolution performance of composite materials. 4. Conclusions In this work, the CdS@ZnSeZnO composite photocatalyst material was successfully prepared by a simple temperatureprogrammed hydrothermal method combined with the microwave hydrothermal assisted method, which consists of cubic phase ZnS and hexagonal phase CdS and ZnO. After compounding ZnS, the (110) crystal plane of ZnO is inhibited, and the (100) and (101) crystal planes of ZnO are preferentially grown. After further compounding CdS, the growth of the three crystal faces of ZnO in the composite CdS@ZnSeZnO was suppressed, and the (110) crystal plane was continuously inhibited. Moreover, the light absorption performance of CdS@ZnSeZnO composite in the visible light region was obviously enhanced. In addition, the synergistic effect of the type II heterojunction formed between ZnS and ZnO and the Zscheme charge transfer process between ZnO and CdS in CdS@ZnSeZnO results in the prepared CdS@ZnSeZnO not only having favorable photodegradation activity but also having the highest hydrogen evolution performance and excellent hydrogen evolution stability in aqueous Na2S and Na2SO3 solution. Conflicts of interest The authors declare no competing financial interest.

References [1] M. Jafari, D. Armaghan, S.M. Seyed Mahmoudi, A. Chitsaz, Thermoeconomic analysis of a standalone solar hydrogen system with hybrid energy storage, Int. J. Hydrogen Energy 44 (2019) 19614e19627. [2] I. Staffell, D. Scamman, A. Velazquez Abad, P. Balcombe, P.E. Dodds, P. Ekins, N. Shah, K.R. Ward, The role of hydrogen and fuel cells in the global energy system, Energy Environ. Sci. 12 (2019) 463e491. [3] M.Z. Ghori, S. Veziroglu, B. Henkel, A. Vahl, O. Polonskyi, T. Strunskus, F. Faupel, O.C. Aktas, A comparative study of photocatalysis on highly active columnar TiO2 nanostructures in-air and in-solution, Sol. Energy Mater. Sol. Cells 178 (2018) 170e178. [4] A. Prakash, M. Dan, S. Yu, S. Wei, Y. Li, F. Wang, Y. Zhou, In2S3/CuS nanosheet composite: an excellent visible light photocatalyst for H2 production from H2S, Sol. Energy Mater. Sol. Cells 180 (2018) 205e212. [5] S. Wang, B. Zhu, M. Liu, L. Zhang, J. Yu, M. Zhou, Direct Z-scheme ZnO/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity, Appl. Catal. B Environ. 243 (2019) 19e26. lez, C. Terashima, A. Fujishima, Applications of photo[6] V. Rodríguez-Gonza catalytic titanium dioxide-based nanomaterials in sustainable agriculture, J. Photochem. Photobiol. C Photochem. Rev. 40 (2019) 49e67. [7] Z. Wang, C. Li, K. Domen, Recent developments in heterogeneous photocatalysts for solar-driven overall water splitting, Chem. Soc. Rev. 48 (2019) 2109e2125. [8] E. Farahi, N. Memarian, Nanostructured nickel phosphide as an efficient photocatalyst: effect of phase on physical properties and dye degradation, Chem. Phys. Lett. 730 (2019) 478e484. [9] L. Wang, X. Zhang, X. Yu, F. Gao, Z. Shen, X. Zhang, S. Ge, J. Liu, Z. Gu, C. Chen, An all-organic semiconductor C3N4/PDINH heterostructure with advanced antibacterial photocatalytic therapy activity, Adv. Mater. 31 (2019) 1901965. [10] K.-H. Choi, J. Min, S.-Y. Park, B.J. Park, J.-S. Jung, Enhanced photocatalytic degradation of tri-chlorophenol by Fe3O4@TiO2@Au photocatalyst under visible-light, Ceram. Int. 45 (2019) 9477e9482. [11] M. Baradaran, F.E. Ghodsi, C. Bittencourt, E. Llobet, The role of Al concentration on improving the photocatalytic performance of nanostructured ZnO/ZnO:Al/ ZnO multilayer thin films, J. Alloy. Comp. 788 (2019) 289e301. [12] Q.T.H. Ta, E. Cho, A. Sreedhar, J.-S. Noh, Mixed-dimensional, three-level hierarchical nanostructures of silver and zinc oxide for fast photocatalytic degradation of multiple dyes, J. Catal. 371 (2019) 1e9. [13] I. Kitsou, P. Panagopoulos, T. Maggos, A. Tsetsekou, ZnO-coated SiO2 nanocatalyst preparation and its photocatalytic activity over nitric oxides as an alternative material to pure ZnO, Appl. Surf. Sci. 473 (2019) 40e48. [14] Q. Zhou, L. Li, J. Wang, X. Zhang, S. Zhou, Synergistic effect of ZnO QDs and Sn4þ ions to control anatase-rutile phase of three-dimensional ordered hollow sphere TiO2 with enhanced photodegradation and hydrogen evolution, Appl. Surf. Sci. 481 (2019) 1185e1195. [15] B. Boro, B. Gogoi, B.M. Rajbongshi, A. Ramchiary, Nano-structured TiO2/ZnO nanocomposite for dye-sensitized solar cells application: a review, Renew. Sustain. Energy Rev. 81 (2018) 2264e2270. € [16] N. Güy, M. Ozacar, Ag/Ag2CrO4 nanoparticles modified on ZnO nanorods as an efficient plasmonic photocatalyst under visible light, J. Photochem. Photobiol. A Chem. 370 (2019) 1e11. [17] Y. Zhang, J. Zhou, X. Chen, Q. Feng, W. Cai, MOF-derived C-doped ZnO composites for enhanced photocatalytic performance under visible light, J. Alloy. Comp. 777 (2019) 109e118. [18] N.T. Hanh, N. Le Minh Tri, D. Van Thuan, M.H. Thanh Tung, T.-D. Pham, T.D. Minh, H.T. Trang, M.T. Binh, M.V. Nguyen, Monocrotophos pesticide effectively removed by novel visible light driven Cu doped ZnO photocatalyst, J. Photochem. Photobiol. A Chem. 382 (2019) 111923. [19] H. Muthukumar, S.N. Mohammed, N. Chandrasekaran, A.D. Sekar, A. Pugazhendhi, M. Matheswaran, Effect of iron doped Zinc oxide nanoparticles coating in the anode on current generation in microbial electrochemical cells, Int. J. Hydrogen Energy 44 (2019) 2407e2416.

Q. Zhou et al. / Journal of Alloys and Compounds 813 (2020) 152190 [20] J.J. Murcia Mesa, L.G. Arias Bolivar, H.A.R. Sarmiento, E.G.A. Martinez, C.J. Paez, M.A. Lara, J.A.N. Santos, M. Del Carmen Hidalgo Lopez, Urban wastewater treatment by using Ag/ZnO and Pt/TiO2 photocatalysts, Environ. Sci. Pollut. Res. 26 (2019) 4171e4179. [21] S. Seong, I.-S. Park, Y.C. Jung, T. Lee, S.Y. Kim, J.S. Park, J.-H. Ko, J. Ahn, Synthesis of Ag-ZnO core-shell nanoparticles with enhanced photocatalytic activity through atomic layer deposition, Mater. Des. 177 (2019) 107831. [22] G. Zhang, D. Chen, N. Li, Q. Xu, H. Li, J. He, J. Lu, Fabrication of Bi2MoO6/ZnO hierarchical heterostructures with enhanced visible-light photocatalytic activity, Appl. Catal. B Environ. 250 (2019) 313e324. [23] F. Hu, L. Tao, H. Ye, X. Li, X. Chen, ZnO/WSe2 vdW heterostructure for photocatalytic water splitting, J. Mater. Chem. C 7 (2019) 7104e7113. [24] D. Zhao, T. Wu, Y. Zhou, Dual II heterojunctions metallic phase MoS2/ZnS/ZnO ternary composite with superior photocatalytic performance for removing contaminants, Chem. Eur J. 25 (2019) 9710e9720. [25] X. Li, J. Yu, J. Low, Y. Fang, J. Xiao, X. Chen, Engineering heterogeneous semiconductors for solar water splitting, J. Mater. Chem. 3 (2015) 2485e2534. ndez-García, [26] G. Mendoza-Dami an, A. Hern andez-Gordillo, M.E. Ferna ~ a, F.J. Tzompantzi-Morales, R. Pe rez-Herna ndez, Influence of P. Acevedo-Pen ZnS wurtziteesphalerite junctions on ZnOCore-ZnSShell-1D photocatalysts for H2 production, Int. J. Hydrogen Energy 44 (2019) 10528e10540. [27] Z. Zou, X. Yang, P. Zhang, Y. Zhang, X. Yan, R. Zhou, D. Liu, L. Xu, J. Gui, Trace carbon-hybridized ZnS/ZnO hollow nanospheres with multi-enhanced visible-light photocatalytic performance, J. Alloy. Comp. 775 (2019) 481e489. [28] Y.P. Xie, Y. Yang, G. Wang, G. Liu, Oxygen vacancies promoted interfacial charge carrier transfer of CdS/ZnO heterostructure for photocatalytic hydrogen generation, J. Colloid Interface Sci. 503 (2017) 198e204. [29] R. Shi, Y. Cao, Y. Bao, Y. Zhao, G.I.N. Waterhouse, Z. Fang, L.Z. Wu, C.H. Tung, Y. Yin, T. Zhang, Self-assembled Au/CdSe nanocrystal clusters for plasmonmediated photocatalytic hydrogen evolution, Adv. Mater. 29 (2017) 1700803. [30] L. Zhang, H. Zhang, B. Wang, X. Huang, F. Gao, Y. Zhao, S. Weng, P. Liu, Construction of a dual-channel mode for wide spectrum-driven photocatalytic H2 production, J. Mater. Chem. 7 (2019) 1076e1082. [31] F. Teng, Z. Tian, G. Xiong, Z. Xu, Preparation of CdSeSiO2 core-shell particles and hollow SiO2 spheres ranging from nanometers to microns in the nonionic reverse microemulsions, Catal. Today 93e95 (2004) 651e657. [32] S. Singh, N. Khare, CdS/ZnO core/shell nano-heterostructure coupled with reduced graphene oxide towards enhanced photocatalytic activity and photostability, Chem. Phys. Lett. 634 (2015) 140e145. [33] Q. Zhou, L. Li, Y. Xin, D. Liu, X. Zhang, Three-dimensionally ordered macroporous Sn4þ-doped TiO2 with anataseerutile mixed phase via Pt loading by photoreduction method: enhanced photodegradation and hydrogen production performance, New J. Chem. 42 (2018) 15190e15199. [34] S.A.A.K. Sharma, R. Krishnamurthy, Microwave glazing of aluminaetitania ceramic composite coatings, Mater. Lett. 50 (2001) 295e301. [35] J. Hu, Y. Bando, Z. Liu, J. Zhan, D. Golberg, T. Sekiguchi, Synthesis of crystalline silicon tubular nanostructures with ZnS nanowires as removable templates, Angew Chem. Int. Ed. Engl. 43 (2004) 63e66.

13

[36] L. Wang, H. Wei, Y. Fan, X. Liu, J. Zhan, Synthesis, Optical properties, and photocatalytic activity of one-dimensional CdS@ZnS core-shell nanocomposites, Nanoscale Res. Lett. 4 (2009) 558e564. [37] H. Zhang, Xiangyang Ma, Xu Jin, Junjie Niu, Sha Jian, Deren Yang, Directional CdS nanowires fabricated by chemical bath deposition, J. Cryst. Growth 246 (2002) 108e112. [38] X. Zhao, S. Zhou, L.P. Jiang, W. Hou, Q. Shen, J.J. Zhu, Graphene-CdS nanocomposites: facile one-step synthesis and enhanced photoelectrochemical cytosensing, Chem. Eur J. 18 (2012) 4974e4981. [39] J.J. Wu, H.I. Wen, C.H. Tseng, S.C. Liu, Well-aligned zno nanorods via hydrogen treatment of ZnO films, Adv. Funct. Mater. 14 (2004) 806e810. [40] G. Yang, W. Yan, Q. Zhang, S. Shen, S. Ding, One-dimensional CdS/ZnO core/ shell nanofibers via single-spinneret electrospinning: tunable morphology and efficient photocatalytic hydrogen production, Nanoscale 5 (2013) 12432e12439. [41] M. Rakibuddin, R. Ananthakrishnan, Novel nano coordination polymer based synthesis of porous ZnO hexagonal nanodisk for higher gas sorption and photocatalytic activities, Appl. Surf. Sci. 362 (2016) 265e273. [42] Z. Mei, M. Zhang, J. Schneider, W. Wang, N. Zhang, Y. Su, B. Chen, S. Wang, A.L. Rogach, F. Pan, Hexagonal Zn1
xCdxS (0.2x1) solid solution photocatalysts for H2 generation from water, Catal. Sci. Technol. 7 (2017) 982e987. [43] S. Yi, X. Yue, D. Xu, Z. Liu, F. Zhao, D. Wang, Y. Lin, Study on photogenerated charge transfer properties and enhanced visible-light photocatalytic activity of p-type Bi2O3/n-type ZnO heterojunctions, New J. Chem. 39 (2015) 2917e2924. [44] Y.C. Wu Chiaching, Investigation of the properties of nanostructured Li-doped NiO films using the modified spray pyrolysis method, Nanoscale Res. Lett. 8 (2013) 33. [45] Q. Wang, J. Li, Y. Bai, J. Lian, H. Huang, Z. Li, Z. Lei, W. Shangguan, Photochemical preparation of Cd/CdS photocatalysts and their efficient photocatalytic hydrogen production under visible light irradiation, Green Chem. 16 (2014) 2728e2735. [46] K.S. Ranjith, A. Senthamizhan, B. Balusamy, T. Uyar, Nanograined surface shell wall controlled ZnOeZnS coreeshell nanofibers and their shell wall thickness dependent visible photocatalytic properties, Catal. Sci. Technol. 7 (2017) 1167e1180. [47] X. Chen, L. Li, W. Zhang, Y. Li, Q. Song, J. Zhang, D. Liu, Multi-pathway photoelectron migration in globular flower-like In2O3/AgBr/Bi2WO6 synthesized by microwave-assisted method with enhanced photocatalytic activity, J. Mol. Catal. A Chem. 414 (2016) 27e36. [48] S. Singh, R. Sharma, Bi2O3/Ni-Bi2O3 system obtained via Ni-doping for enhanced PEC and photocatalytic activity supported by DFT and experimental study, Sol. Energy Mater. Sol. Cells 186 (2018) 208e216. ~ a-Pe rez, O. Aguilar-Martínez, P. Acevedo-Pen ~ a, C.E. Santolalla-Vargas, [49] Y. Pin mez, F. Tzompantzi, Novel ZnS-ZnO S. Oros-Ruíz, F. Galindo-Hern andez, R. Go composite synthesized by the solvothermal method through the partial sulfidation of ZnO for H2 production without sacrificial agent, Appl. Catal. B Environ. 230 (2018) 125e134.