Self-assembly of zinc cadmium sulfide nanorods into nanoflowers with enhanced photocatalytic hydrogen production activity

Journal of Colloid and Interface Science 567 (2020) 357–368

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Self-assembly of zinc cadmium sulfide nanorods into nanoflowers with enhanced photocatalytic hydrogen production activity Zhiliang Jin ⇑, Yang Liu, Xuqiang Hao School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021, PR China Ningxia Key Laboratory of Solar Chemical Conversion Technology, North Minzu University, Yinchuan 750021, PR China Key Laboratory for Chemical Engineering and Technology, State Ethnic Affairs Commission, North Minzu University, Yinchuan 750021, PR China

g r a p h i c a l a b s t r a c t ZnCdS nanoflowers were prepared by adjusting the molar ratio of precursor in the absence of surfactant. ZCS nanorods self-assemble into the structure of a nanoflower, so that charges on the nanorods can transfer to each other through the center of nanoflower, which provides another effective way for charge transferring. Therefore, excellent charge separation and transferring ability is achieved over ZCS nanoflowers.

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Article history: Received 10 August 2019 Revised 7 February 2020 Accepted 8 February 2020 Available online 11 February 2020 Keywords: ZnCdS solid solution Self-assemble Nanoflower Photocatalytic hydrogen evolution

a b s t r a c t ZnCdS solid solutions have been extensively studied due to their excellent photocatalytic hydrogen evolution performance. The change of the molar ratio of precursors affects the morphology and structure of ZnCdS, with the subsequent influence on the separation of photogenerated electron–hole pairs and the hydrogen production ability. The effect of the amount of nonmetallic elements on the photocatalytic activity has been scarcely explored. In this work, the morphology of ZnCdS is regulated by varying the amount of thioacetamide as S precursor. The structure of the samples is thoroughly analyzed by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and Brunauer–Emmet t–Teller analysis. Their optical properties, photocatalytic hydrogen evolution ability, and photoelectrochemical performance are evaluated. Upon increasing the amount of thioacetamide, the crystallinity improves, the ZnCdS nanorods self-assemble into nanoflowers, and the number of defects decreases. The highest photocatalytic activity is achieved for a (Zn + Cd):S molar ratio of 1:3.5. Moreover, the photocatalyst exhibits excellent stability after six cycles. The one-dimensional nanorod structure contributes

Abbreviations: CB, Conduction band; EDS, Energy dispersive X-ray spectrometer; EIS, Electrochemical impedance spectroscopy; HER, Hydrogen evolution reaction; LSV, Linear sweep voltammetry; NHE, Normal hydrogen electrode; SCE, Saturated calomel electrode; TEM, Transmission electron microscope; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction. ⇑ Corresponding author at: School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021, PR China. E-mail address: [email protected] (Z. Jin). https://doi.org/10.1016/j.jcis.2020.02.024 0021-9797/Ó 2020 Elsevier Inc. All rights reserved.

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to the formation of a space charge region that drives the charge carriers along the nanorods. The selfassembled ZnCdS nanoflowers provide extra channels for the charge transfer, improving the separation of electron–hole pairs. Ó 2020 Elsevier Inc. All rights reserved.

1. Introduction Photocatalytic hydrogen production by splitting water over semiconductor photocatalysts is considered as a promising and challenging way to convert solar energy into chemical energy [1–3]. Photocatalysts with a high activity and visible-light sensitivity are very important for a wide range of applications [4–6]. In this regard, metal sulfides have been shown to be effective visiblelight-driven photocatalysts under the presence of sacrificial reagents [7–10]. In particular, CdS-based photocatalysts have shown a significantly high photocatalytic hydrogen generation activity [11,12]. Even so, many researchers have recently devoted themselves to the exploration of bimetallic sulfide, mainly consisting of Zn-doped CdS semiconductors. It has been reported that nanometer-sized Zn1-xCdxS powders with dual boundarydependent potential could prevent photoinduced carrier recombination [13]. However, the rapid recombination of electron–hole pairs and serious photocorrosion of these sulfide derivatives lead to poor stability of the photocatalysts. The promotion of charge separation and transfer is, therefore, one of the most effective strategies to improve the activity of sulfide catalysts [13]. In previous studies, it has been shown that ZnCdS is not a simple physical mixture of ZnS and CdS, but a kind of bimetallic solid solution [14–16]. Therefore, it has many properties superior to those of pure ZnS, bare CdS, or ZnS/CdS mixtures. The adjustable energy band structure and higher charge separation efficiency of the ZnCdS solid solution provide it with a better photocatalytic activity than that of bare ZnS and CdS [17,18]. Meanwhile, the ZnCdS solid solution has hydrogen evolution activity under acidic, neutral, and alkaline conditions [19]. In the study of ZnCdS, the construction of a heterojunction to promote electron transfer or ion doping in order to change the light absorption range and other properties is widely used with the aim of enhancing the photocatalytic activity of ZnCdS [20–25]. However, few studies have been conducted to promote photocatalyst activity through the regulation of morphology and structure. Therefore, it is of great significance to further explore the influence of morphologic and structural changes on the photocatalytic activity of ZnCdS. The microstructure of the catalyst has an important influence on the photogenerated electron–hole separation. By controlling the catalysts, effective separation of photogenerated charges and directional transport of photoelectrons can be achieved. Besides, the active sites for hydrogen production are relatively independent, and the electron–hole pair recombination can be inhibited. For example, the morphology and structure of CdS can be controlled by changing temperature [26]. Thus, it was found that the combination of small CdS nanocrystals produced at medium temperatures and CdS nanocrystals having low diameters showed greater optical activity than rod-like or linear CdS crystals produced at higher photothermal temperatures [26]. Hao et al. succeeded in regulating the morphology and structure of CdS, finally obtaining a hexagonal CdS single crystal, by controlling the molar ratio of Cd and S in CdS [27]. The type II band alignment between the exposed surfaces {0001} and {10–10} formed a small junction with continuous band curvature, which significantly promoted the separation rate of photogenerated electrons and holes. Recently, as a novel strategy, the morphology and structure of

semiconductors has been regulated by changing the molar ratio of metallic and nonmetallic elements in semiconductor catalysts [27]. By using this method, researchers have explored the optimal molar ratio of elements in the semiconductor at which it achieves the optimal hydrogen production. In this work, the changes in morphology and structure of a ZnCdS solid solution was explored by fixing the amounts of Zn and Cd in ZnCdS and varying the amount of the S precursor. When the (Zn + Cd):S ratio is 1:1, nanorods and nanocrystals coexist in the catalyst structure, which is caused by incomplete growth due to an insufficient amount of S precursor. By increasing the amount of S precursor, the ZnCdS solid solutions gradually adopt the form of nanorods and agglomerate into nanoflowers. This is because the high concentration of thioacetamide provides the driving force for the growth of nanorods and the formation of nanoflowers. When the molar ratio of (Zn + Cd):S is 1:3.5 (ZCS-6), the highest photocatalytic activity and photostability are achieved. One-dimensional structures like nanorods favor the formation of a space charge region that drives the transportation of charge carriers along the nanorods [28]. In particular, the self-assembly of nanoflowers induced by nanorods leads to a more efficient electron–hole separation, thereby facilitating the progress of the hydrogen evolution reaction (HER). In recent decades, research on solid solution in the field of photocatalysis has been gradually increasing, which is due to the excellent photocatalysis effect and good controllability of solid solutions [13–15]. However, these studies mainly explored the influence of the proportion of metal elements in solid solution on the photocatalytic activity [16,20,23]. In the process of compound synthesis, the amount of nonmetallic elements has a great influence on the photocatalytic activity [27,29]. The present work provides a feasible and simple strategy for designing self-assembled ZnCdS nanoflowers with high photocatalytic activity and photostability by changing the molar ratio of precursors. Importantly, the photocatalysts are prepared by a very simple method, in which the optimum molar ratio of the precursor is obtained. The excellent hydrogen production performance of the best photocatalyst provides an important reference for the industrialization of the field of photocatalytic hydrogen production. 2. Experimental section All the chemical reagents used in the experiments were analytical reagents, and did not require purification. 2.1. Materials Zinc acetate dihydrate (Zn(OAc)2
2H2O, AR) was purchased from Tianjin damao chemical reagent factory, China, cadmium acetate dihydrate (Cd(OAc)2
2H2O, AR) was obtained from Shanghai Macklin Biochemical Co. Ltd., thioacetamide (CH3CSNH2, 98.0%) was purchased from Shanghai Aladdin Co. Ltd., and ethylenediamine (NH2CH2CH2NH2) was obtained from Yantai shuangshuang chemical Co. Ltd., China. Deionized (DI) water was used throughout this study. All of the chemicals were analytical grade and used without further treatment.

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2.2. Preparation of photocatalysts ZnCdS photocatalysts with different (Zn + Cd):S molar ratio were prepared by a one-pot hydrothermal method. Typically, 5 mmol of Zn(OAc)2
2H2O and 5 mmol of Cd(OAc)2
2H2O were dissolved into ethylenediamine (50 mL) under stirring. After 0.5 h, different molar amounts of thioacetamide (TAA; 10, 15, 20, 25, 30, 35, 40, and 45 mmol, which afforded the samples ZCS-1, ZCS-2, ZCS-3, ZCS-4, ZCS-5, ZCS-6, ZCS-7, and ZCS-8, respectively) were added to the above solution, and the resulting mixtures were stirred continuously for 1 h at room temperature. Then, the uniform solutions were transferred to an 80 mL Teflon-lined autoclave, and kept at 180 °C for 24 h. Finally, a yellow precipitate was obtained after centrifugation. The precipitate was washed several times with N, N-dimethylformamide (DMF), deionized water, and ethanol, and dried at 60 °C overnight. In addition, we prepared pure ZnS and bare CdS with the same precursor ratio of ZCS-6 under the same experimental conditions. Equimolar amounts of ZnS and CdS were dispersed in 30 mL ethanol for 30 min under ultrasonic processing, stirred for 2 h at room temperature, and finally dried at 70 °C to obtain a physical mixture of ZnS/CdS.

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glass surfaces (1  1.5 cm). Concomitantly, a platinum plate was used as the opposite electrode, a saturated calomel electrode (SCE, E0 = 0.24 V vs. normal hydrogen electrode, NHE, at 25 °C) was used as the reference electrode, and a 0.2 M Na2SO4 aqueous solution was used as the electrolyte. 2.4. Photocatalytic hydrogen production The photocatalytic hydrogen production process was carried out in a 62 mL adaptive reaction flask, and the light source was obtained using a perfectlight PCX-50C multichannel photochemical reaction system (5 W LED white light, k  420 nm). Typically, 10 mg of catalyst was added to 30 mL of a 0.35 M Na2S/0.25 M Na2SO3 solution and sonicated for 20 min to give a homogeneous suspension. Before the irradiation, the system was bubbled with nitrogen for 30 min to remove the air and create an anaerobic atmosphere. During the reaction, 0.5 mL of gas was taken every 0.5 h, and the evolution of H2 was analyzed by phase chromatography using a Tianmei GC7900 gas chromatograph, a TCD, and N2 as carrier. 3. Results and discussion

2.3. Characterizations 3.1. Structure and morphology analysis X-ray diffraction (XRD) patterns were obtained on an X-ray diffractometer (Rigaku RINT-2000) using Cu Ka radiation at a scan rate of 5° min1. The morphologies and microstructures of the materials were investigated using a high-resolution transmission electron microscope (TEM, FEI Tecnai TF20) equipped with an energy dispersive X-ray spectrometer (EDS). The ultraviolet–visible diffuse reflectance spectra (UV–vis DRS) of the catalyst were performed on a UV-2550 (Shimadzu) spectrometer. BaSO4 was used as a reflectance standard. The composition of the compounds and the surface states of the elements were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The Brunauer–Em mett–Teller (BET) specific surface area (SBET) of the powders were measured by nitrogen adsorption using a Micromeritics ASAP2020M nitrogen adsorption apparatus. The photoluminescence (PL) spectra were taken on a Horiba Scientific FluoroMax-4 fluorometer spectrometer at room temperature, and timeresolved fluorescence emission spectra were conducted using a Horiba Jobin Yvon Data Station operating in time-correlated single-photon counting mode. The photocurrent response, Mott– Schottky curve, electrochemical impedance spectroscopy (EIS) measurements, and linear sweep voltammetry (LSV) measurements were performed using a Princeton electrochemical workstation. The working electrodes were prepared by drop-coating homogeneous catalyst suspensions directly onto precleaned FTO

3.1.1. X-ray diffraction analysis ZnCdS solid solutions having different element proportions (Zn + Cd)/S were prepared by one-step solute thermal method. Meanwhile, ZnS/CdS samples with a mole ratio of Zn/Cd: S = 10:35 were obtained by physical mixing method. The crystal structure of a series of ZnCdS and ZnS/CdS samples was detected by XRD. As shown in Fig. 1A, all the ZCS products display similar patterns, which are close to the standard diffraction patterns of ZnS (JCPDS#1-1280) and CdS (JCPDS#75-1545). It should be noted that the diffraction peaks of the ZCS samples are between those of ZnS and CdS, which implies that ZnCdS is a kind of solid solution rather than a simple physical mixture of ZnS and CdS, as can be seen in Fig. 1B. Thus, the XRD diffraction peaks of the ZnS/CdS physically mixed sample and the ZCS-6 solid solution having the same Zn/Cd:S molar ratio are completely different. The diffraction pattern of ZnS/CdS exhibits peaks that are in good agreement with those of pure CdS and ZnS, which proves the existence of both species in the mixed sample. Besides, the three peaks that appear in the range 15°–23° in the XRD pattern of ZnS/CdS can be attributed to the S element and organic matter. As can be seen in the enlarged view in Fig. 1A corresponding to the 24°–30° area, the diffraction peaks of the ZCS samples become narrower with increasing the amount of TAA used in the hydrothermal process, which indicates

Fig. 1. XRD patterns of (A) the ZCS-X (X = 1–8) samples and the (B) ZCS-6 and ZnS/CdS physical mixture samples.

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Fig. 2. (A–D) Typical TEM images; (E–H) HRTEM images; (I–L) SAED patterns of ZCS-X (X = 1, 2, 3, 6).

that the crystallinity of the ZCS sample gradually increases. Notably, the impurity peak between 15° and 23° decreases with increasing the TAA amount, suggesting that the purity of the ZCS samples also increases. In the enlarged XRD pattern of the region between 24° and 30°, another diffraction peak can be observed at 28.8° for ZCS-7 and ZCS-8, which is due to the sulfur formed by the presence of excess thioacetamide during the reaction (JCPDS#1-478). In addition, there is no obvious deviation in the positions of the diffraction peaks, which indicates that the element composition of the ZCS samples is very close [29]. 3.1.2. Transmission electron microscopy characterization The influence of the amount of TAA added in the ZCS solid solution on the structure and morphology of the samples was studied by TEM. As shown in Fig. 2A, when the (Zn + Cd):S molar ratio is 1:1, the ZCS solid solution presents two morphological structures, i.e., sheet-like and rod-like. According to a previous study [30], both sheet-like and rod-like structures are shaped by the ZCS solid solution. Gradually, upon increasing the amount of TAA, the ZCS sample becomes a pure rod-like structure, and the nanorods agglomerate to form a nanoflower (Fig. 2B). By further increasing the amount of TAA, the ZCS solid solution shows a clearer nanostructure (Fig. 2C and Fig. 2D). Thus, the presence of high concentrations of TAA provides the driving force for the growth of nanorods and nanoflowers [29]. Furthermore, the ZCS-6 nanoflowers (diameter, 0.88 mm) are slightly smaller than the ZCS-2 (diameter, 1.07 mm) and ZCS-3 nanoflowers (diameter, 1.03 mm) (Fig. 3A and B). According to previous studies, more ZCS cores will rapidly form when the initial concentration of TAA is high, which will eventually lead to a reduction in the microcrystalline size of the ZCS samples [29].

HRTEM and SAED images provide further information on the crystal state of the samples. In Fig. 2E, nanorod structures with obvious and ordered lattice stripes can be observed in the areas circled by the yellow curve. However, the lattice stripes present in other areas are vague and disordered, which can be attributed to the incomplete growth of the sample due to an insufficient amount of S source. The lattice fringe becomes clearer as the amount of TAA increases, indicating that the crystallinity of the sample is higher, which is consistent with the XRD results. The HRTEM images of all samples show a lattice spacing of 0.33 nm, indicating the successful synthesis of the ZCS solid solution. However, the SAED images show distinct layers of annular dark fields, which is indicative of the presence of different exposed crystal surfaces. Importantly, the increase in crystallinity and the combination of nanostructures have a significant contribution to photocatalytic reactions. On the one hand, a good crystallinity provides more exposed crystal surfaces, thus increasing the number of active sites. On the other hand, one-dimensional structures such as nanorods favor the formation of space charge regions to drive the transportation of charge carriers along the nanorods. Furthermore, nanorods self-assemble into nanoflowers, providing a pathway for charge transport between nanorods, and significantly increasing the charge transfer efficiency. Moreover, the EDX spectrum of ZCS-6 (Fig. 3C) further proves the presence of Zn, Cd, and S elements in both ZCS systems. 3.1.3. X-ray photoelectron spectroscopy To determine the surface chemical composition and valence details of the as-fabricated photocatalysts, the XPS spectra of ZCS6 were measured. The XPS survey spectrum confirms that the

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Fig. 3. (A–B) Typical TEM images of ZCS-3 and ZCS-6 and (C) EDX image of ZCS-6.

Fig. 4. (A) Survey XPS spectra of ZCS-6; (B) Zn 2p, (C) Cd 3d, (D) S 2p XPS patterns of the ZCS-6 photocatalyst.

catalyst is mainly composed of Zn, Cd, S, C, and O elements (Fig. 4A), among which C and O stem from the background elements and the oxygen adsorbed on the surface, respectively. In Fig. 4C, the highresolution XPS spectra of Cd 3d for ZCS-6 shows two peaks at 404.5 and 411.3 eV, which can be assigned to Cd 3d5/2 and Cd 3d3/2 respectively [31]. The peaks of Zn 2p in ZCS-6 appearing at binding energies of 1021.0 and 1044.1 eV can be assigned to Zn 2p3/2 and Zn 2p1/2 (Fig. 4B) [32], which is attributable to the Zn2+ valence state. In Fig. 4D, the peak observed at binding energies of 161.0 and 162.2 eV can be ascribed to S 2p3/2 and S 2p1/2. The above results are consistent with those reported in the literature [33], indicating the successful preparation of the ZCS solid solution. The ratio of the elements present on the surface of the composite photocatalyst was quantitatively analyzed. Moreover, according to a previous

study on a ZCS solid solution similar to ZCS-1 [30], we compared the surface element content to prove the existence and change of surface S vacancies. As shown in Table 1, the (Zn + Cd):S atomic ratio of ZCS-1 and ZCS-6 was found to be 1:0.81 and 1:0.99, respectively. To obtain a more rigorous result, we repeated the test for ZCS-1, which afforded a result of 1:0.76. The similar results obtained in the two tests further confirm the presence of S vacancies in ZCS-1. Meanwhile, with increasing the amount of S precursor, the amount of S vacancies decreased gradually, and ZCS-6 can be considered a perfect crystal without S vacancies. 3.1.4. Brunauer–Emmett–Teller analysis The N2 sorption technique was employed to measure the specific surface areas (SBET), pore volumes, and average pore sizes of the

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Table 1 Chemical composition of ZCS-1 and ZCS-6 measured by XPS. Samples

Zn (at%)

Cd (at%)

S (at%)

(Zn + Cd):S

ZCS-1 ZCS-1 (repeated) ZCS-6

30.06 31.69 12.58

13.49 13.84 7.94

35.23 27.63 20.37

1:0.81 1:0.76 1:0.99

Fig. 5. (A) Nitrogen adsorption–desorption isotherms and (B) pore size distribution of the ZCS-X (X = 1, 3, 6, 8) samples.

samples. Fig. 5A shows the nitrogen adsorption–desorption isotherms and the corresponding aperture distribution curves of ZCS-X. It can be seen that all samples have type IV isotherms and H3-type hysteresis loops with a relative pressure (P/P0) of 0.8– 1.0, indicating the presence of mesopores in their structures. Meanwhile, the pore size distribution obtained by the Barrett–Joy ner–Halenda (BJH) method is within the range of 0–50 nm (Fig. 5B). According to the IUPAC definition, pore size structures of 0–50 nm are classified into micropores and mesopores, which is in agreement with the results obtained from the adsorption and desorption isotherms. On the basis of the BET characterization results, which were repeated twice, we obtained the ratio and error values of the ZCS-X specific surface area, pore volume, and pore size. As shown in Table 2, we found that the specific surface area of the photocatalysts gradually decreased with the increase of the amount of the S precursor, which is related to the gradual decrease of the size of the ZCS nanorods. The specific surface area of the rod-like ZCS samples is much smaller than that of the ZCS in which sheet-like and rod-like structures coexist. Moreover, the diameter of the nanorods decreases as the amount of the S precursor increases, resulting in the reduction of the specific surface area of the sample. The slight difference in specific surface area between ZCS-6 and ZCS-8 indicates that their morphology and structure are similar. Therefore, the difference in specific surface area is not the reason for the change of the photocatalytic hydrogen production rate [34]. Table 2 BET surface area (SBET), pore diameter (Dp), and pore volume (Vp) of ZCS-X (X = 1, 3, 6, 8).

a

Samples

BET specific surface area (m2g1)a

Pore size (nm)b

Pore volume (cm2 g1)b

ZCS-1 ZCS-3 ZCS-6 ZCS-8

37.31 23.03 20.17 18.79

22 16 20 18

0.19 0.08 0.09 0.08

± ± ± ±

1.78 0.10 1.12 2.37

± ± ± ±

4 1 1 4

± ± ± ±

0.04 0.00 0.00 0.02

Obtained from BET method. Total pore volume obtained from the N2 adsorption volume at a relative pressure (P/P0) of 0.99. b

3.2. Optical properties 3.2.1. Ultraviolet–visible diffuse reflectance spectra The optical properties of the ZCS-1, ZCS-3, ZCS-6, and ZCS-8 samples were investigated by ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS). As can be seen from Fig. 6A, ZCS-3, ZCS-6, and ZCS-8 have similar absorption spectra with an absorption edge of ~478 nm. In contrast, the absorption edge of ZCS-1 shows a slight red shift, which is attributable to the presence of more defects [32]. Importantly, all of the ZnCdS samples exhibit apparent absorption bands in the visible region. The corresponding band gap can be calculated by the Tauc equation (Eq. (1)) as shown in Fig. 6B.

ahv ¼ Aðhv  Eg Þ

1=2

ð1Þ

where a is the absorption coefficient, hv is the photon energy, and Eg is the band gap energy. As presented in Fig. 7, the band gap values for ZCS-X (X = 1, 3, 6, 8) are 2.54, 2.58, 2.59, and 2.60 eV, respectively. With increasing the amount of TAA, the band gap of the photocatalyst shows an increasing trend. It has been reported that the narrowing of the band gap may be caused by its defect state [29]. The defect state in the ZCS samples was evaluated by performing a semiquantitative XPS analysis. It was found that the defect state in the ZCS solid solution decreases gradually as the amount of TAA increases, which is consistent with the results summarized in Table 1. It should be noted that the reduction of defects is beneficial to the effective separation of charges. 3.2.2. Photoluminescence analysis To further prove the excellent charge transfer performance of the ZCS nanoflowers, the photoluminescence spectra (PL) of the prepared samples were determined. Fig. 8A shows the PL spectra of the ZCS samples excited at 420 nm. ZCS-1 exhibits an intrinsic emission band of about 531 nm whose high intensity stems from the high recombination rate of photogenerated electron–hole pairs [35]. The emission peak of ZCS-3 is blue-shifted (about 18 nm) compared with that of ZCS-1, which can be attributed to Stokes

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Fig. 6. (A) UV–vis DRS spectra and (B) plots of (ahv)2 versus band gap energy (hv) of ZCS-X (X = 1, 3, 6, 8).

Fig. 7. Band gap energy (Eg) of the ZCS-X samples for (A) X = 1, (B) X = 3, (C) X = 6, and (D) X = 8.

displacement caused by S vacancies in ZCS-1. Meanwhile, the emission peak position of the samples remains unchanged as the amount of TAA increases, which further proves the effective decrease of the amount of S vacancies [27]. Importantly, ZCS-6 has the lowest emission intensity, suggesting that less electron– hole recombination occurs with this catalyst. Due to the orderliness of the structure, which preserves the free-charge transfer capacity in perfect crystals, the recombination of holes and electrons is prevented. In addition, four weak peaks are observed at 482, 492, 570, and 688 nm. The emission bands at 482 and 492 nm can be attributed to the surface states of both solid solutions [36], and the peaks at 570 and 688 nm are related to the

S vacancies [37,38], which is consistent with the results of the XPS semiquantitative analysis. Moreover, ZCS-1 gives rise to the strongest emission peak, indicating that this sample has the highest amount of surface defects. Time-resolved emission and decay spectra were studied to determine the electron transfer kinetics of the ZCS solid solutions. The time-resolved photoluminescence (TRPL) spectra were fitted using a three-exponential decay model expressed by Eq. (2). As shown in Fig. 8B, ZCS-6 undergoes the slowest decay, which can be related to a good hydrogen evolution performance. The specific parameters of the excited electron lifetime and the percentage of each sample are shown in Table 3. For a better analysis of the

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Fig. 8. (A) Photoluminescence spectra and (B) time-resolved fluorescence spectra of the ZCS-X (X = 1, 3, 6, 8) samples.

Table 3 Kinetic analysis of emission decay for the ZCS-X (X = 1, 3, 6, 8) samples. Samples

s1 (ns)

s2 (ns)

s3 (ns)

ZCS-1 ZCS-3 ZCS-6 ZCS-8

5.72 5.67 5.72 5.66

0.90 0.93 0.94 0.91

152.82 155.17 154.60 153.25

(27.20%) (26.84%) (26.15%) (26.76%)

(25.25%) (23.63%) (23.64%) (23.87%)

charge separation mechanism, we calculated the average lifetime (sav) for all samples according to Eq. (3). As shown in Table 3, the average lifetime was found to be 3.03, 3.29, 3.33, and 3.19 ns for the core state emission of ZCS-1, ZCS-3, ZCS-6, and ZCS-8, respectively, among which ZCS-6 exhibits the longest average lifetime. According to the previous report [27], the extended average PL lifetime indicates that the separation efficiency of photogenerated charge carriers increases, which can significantly improve the hydrogen evolution activity.

IðtÞ ¼

X

Bi expðt=si Þ

ð2Þ

i¼1;2;3

where I is the normalized emission intensity, t is the time after the pulsed laser excitation, si are the respective decay lifetimes, and Bi is the amplitude (preexponential factor).

hsi ¼

X

i¼1;2;3

Bi s2i =

X

Bi si

ð3Þ

i¼1;2;3

where is the average lifetime, si is the respective decay times, and Bi is the amplitude (preexponential factor). 3.3. Photocatalytic hydrogen evolution activity The photocatalytic H2 production activity of the prepared ZCS solid solution was evaluated by using a mixed aqueous solution of Na2S and Na2SO3 as the sacrificial agent under visible-light irradiation (k  420 nm). As shown in Fig. 9A, ZCS-6 shows the best photocatalytic activity, with a total amount of 573.2 lmol H2 produced by 10 mg of photocatalyst after 5 h of reaction, which is 4.5 times higher than that of ZCS-1 (128.8 lmol) under the same experimental conditions. In Fig. 9B, the average hydrogen production rate of two repeated tests follows an increasing trend at first and then decreases with the increase of the amount of TAA. In particular, the corresponding H2 production rates are 2.03, 5.17, 9.79, 10.13, 10.51, 12.57, 10.70, and 8.54 mmol h1 g1 for the ZCS-1, ZCS-2, ZCS-3, ZCS-4, ZCS-5, ZCS-6, ZCS-7, and ZCS-8 samples, respectively. As expected, ZCS-6 exhibits the highest hydrogen production rate. The enhancement of the photocatalytic activity is due

(47.55%) (49.53%) (50.21%) (49.37%)

Average lifetime (ns)

x2

3.03 3.29 3.33 3.19

1.78 1.68 1.72 1.78

to the increased crystallinity of the self-assembled nanoflowers, which prevents the rapid recombination of photogenerated holes and electrons [28]. With increasing the amount of TAA, the nanorods self-assemble into nanoflowers, and the charges are transferred through the junctions of the nanorods, which provides additional channels for the charge transfer. As a result, the nanoflowers are more conductive to charge transfer, which results in higher hydrogen evolution rates. However, the hydrogen production rates of ZCS-7 and ZCS-8 are lower than that of ZCS-6, which can be ascribed to the excess of organic species and element sulfur from the hydrothermal reaction of TAA that surrounds the ZCS nanorods [29] resulting in a reduction of the photogenerated charge transport. Bare physical mixtures of ZnS, pure CdS, and ZnS/CdS were prepared following the same method, using the same molar ratio of Zn/S and Cd/S precursors as for ZCS-6. The hydrogen production rates of ZnS, CdS, the ZnS/CdS physical mixture, and the ZCS-6 solid solution are shown in Fig. 9C, in which the error bar was obtained from two repeated experiments. As can be seen, ZnS does not exhibit any hydrogen production activity under visible light, and CdS shows low hydrogen production activity (0.61 mmol h1 g1). When equimolar amounts of ZnS and CdS are physically mixed, the photocatalytic activity of the sample improves (4.17 mmol h1 g1) due to the interaction between CdS and ZnS. More importantly, the hydrogen production that results from ZCS-6 is strikingly higher (12.57 mmol h1 g1), which is enough to prove that the ZCS solid solution is not a simple physical mixture of ZnS and CdS. This is consistent with the results of the XRD analysis. The stability of the ZCS-6 photocatalyst was investigated by a hydrogen evolution cyclic experiment. As displayed in Fig. 9D, after irradiating the photocatalyst for 5 h, the catalyst was continuously subjected to a photocatalytic reaction by replacing H2 in the reaction flask with N2. It can be seen from the figure that the catalyst is gradually activated from the second cycle, thus the hydrogen production increases gradually to a maximum of 913.4 lmol (10 mg, 5 h). After six cycles of experiments, no obvious decrease in the H2 evolution rate is detected, which signifies the high stability and durability of ZCS-6. Table 4 lists a comparison of the hydrogen

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Fig. 9. (A) Time-dependent photocatalytic H2 evolution and (B) average hydrogen production rates of the ZCS-X samples; (C) hydrogen production rates of pure ZnS, bare CdS, and ZnS/CdS physical mixtures; (D) stability test for H2 evolution over ZCS-6 under visible-light irradiation (k  420 nm).

Table 4 Comparison of the activity of the photocatalyst prepared in this work with that of related photocatalysts previously reported. Photocatalyst

Light source

Sacrificial reagent

Activity (lmol g1h1)

Ref.

ZnCdS Graphene-ZnxCd1-xS ZnxCd1-xS/Ni ZnxCd1-xS/few-layer phosphorene Cd0.9Zn0.1S CoP/Zn0.5Cd0.5S ZnxCd1xS-P Zncds Cd0.5Zn0.5S/C a-Fe2O3/ZnCdS

5 W LED (k  420 nm) 150WXe lamp (k  420 nm) 5 W LED (k  420 nm) 300 W Xe lamp (k  420 nm) 300 W Xe lamp (k  420 nm) Full spectrum 30  3 W LED 300 W Xe lamp (k  420 nm) 300 W Xe lamp (k  420 nm) 550 W Xe lamp (k  420 nm)

Na2S/Na2SO3 Na2S/Na2SO3 Na2S/Na2SO3 Lactic acid Na2S/Na2SO3 Ascorbic acid Na2S/Na2SO3 Na2S/Na2SO3 Na2S/Na2SO3 Na2S/Na2SO3

11,470 1060 11,993 9326 8040 12175.8 419 11,420 2018 5368

This work [14] [19] [21] [28] [41] [43] [44] [45] [46]

production activity of ZCS-6 and other catalysts previously reported, which demonstrates the excellent hydrogen production activity of our photocatalyst. 3.4. Photoelectrochemical performance of different photocatalysts The process of charge separation and migration in ZCS-X was further studied by photoelectrochemical (PEC) measurements. As shown in Fig. 10A, ZCS-6 shows the smallest resistance [47], indicating that this photocatalyst exhibits the fastest electron transfer rate [19,48]. The rapid electron migration is beneficial to the improvement of the photocatalytic activity [39]. Fig. 10B shows transient photocurrent response curves, which are measured by several on–off cycles of intermittent simulated solar irradiation at 0.4 V (k  420 nm) in a 0.2 M Na2SO4 aqueous solution. A substantial increase of the photocurrent density over ZCS-6 is observed. The results show that more photogenerated electrons are transferred in ZCS-6 during the photocatalytic process, which

leads to more effective electron–hole separation, thus promoting the photocatalytic activity [40]. The electrochemical H2 evolution activities of the ZCS samples on the electrodes were also measured by LSV method. It can be clearly seen from the current density curves that ZCS-6 has the highest cathode current. Importantly, the production of photocatalytic H2 is highly dependent on the overpotential of the HER reaction [41]. According to Fig. 10C, ZCS-6 shows a lower overpotential (0.44 V) than that of ZCS-1 (0.57 V), ZCS-3 (0.47 V), and ZCS-8 (0.50 V), clearly confirming that ZCS-6 is a better catalyst for HER [42]. Among all the samples, ZCS-6 shows the lowest overpotential, which facilitates the rapid transfer of electrons. The above results are consistent with the kinetic results of the photocatalytic hydrogen evolution (as shown in Section 3.3). To explain the mechanism governing the effect of the amount of S precursor on the photocatalytic activity of the ZCS solid solution, the energy level alignment of ZCS was studied. The Mott–Schottky (MS) plots of all the ZCS samples were determined in a 0.2 M Na2-

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Fig. 10. (A) Nyquist plots of electrochemical impedance spectroscopy (EIS); (B) transient photocurrent response; (C) LSV curve; (D) Mott–Schottky plots for the ZCS-X (X = 1, 3, 6, 8) samples.

Table 5 Band gap energy, valence band edge potential, and conduction band edge potential of ZCS-X (X = 1, 3, 6, 8). Samples

Band energy Eg (eV)

Valence band EVB (eV)

Conduction band ECB (eV)

ZCS-1 ZCS-3 ZCS-6 ZCS-8

2.54 2.58 2.59 2.60

2.10 2.12 2.13 2.14

0.44 0.46 0.46 0.40

1

 
E  Efb  kT=e

ð5Þ

According to the UV–vis band gap (Eg) data, the valence band (EVB) position of ZCS-X (X = 1, 3, 6, 8) can be calculated using Eq. (6) as follows:

ECB ¼ EVB  Eg

ð6Þ

The more negative CB position of ZCS-6 indicates that it has the highest reduction potential, which is favorable for the effective separation of photogenerated charges.

SO4 aqueous solution using Eq. (4). From the plots, the flat band potential can be calculated as the intersection point of the tangent line of the MS curves and the x axis. According to Fig. 10D, all samples show a positive slope, which manifests their characteristics of n-type semiconductor. The flat band potentials of the samples were estimated to be 0.48, 0.50, 0.50, and 0.44 V vs. SCE for ZCS-1, ZCS-3, ZCS-6, and ZCS-8 respectively. As previously reported [27], the conduction band (CB) potential (ECB) of n-type semiconductors is ca. 0.1–0.2 V more negative than their flat band potential. Furthermore, the final conduction potential was converted to the potential corresponding to the hydrogen electrode (ENHE), and the results calculated using Eq. (5) are shown in Table 5. The ECB for ZCS-1, ZCS-3, ZCS-6, and ZCS-8 was determined to be 0.44, 0.46, 0.46 and 0.40 V vs. the NHE.

1=C 2 ¼ 2ðND eee0 Þ

ENHE ¼ ESCE þ 0:24 V

ð4Þ

where C is the capacitance of the space charge region, ND is the charge carrier concentration, e is the electron charge, e is the dielectric constant, e0 is the vacuum permittivity, E is the electrode applied potential, Efb is the flat band potential, k is the Boltzmann constant, and T is the absolute temperature.

3.5. Proposed photocatalytic hydrogen evolution mechanism On the basis of the above results, we propose a plausible photocatalytic hydrogen evolution mechanism as shown in Scheme 1. Initially, the ZnCdS semiconductor is excited under visible-light irradiation to produce carriers. The electrons react with H+ from water to produce H2, and the holes left in the valence band are consumed by the sacrificial reagent (Na2S/Na2SO3 solution). The nanorods self-assemble into nanoflowers, in which the nanorods are tightly attached to each other through one of their ends, while the other ends are well dispersed in all directions. The excited charges migrate in different directions of the ZCS nanoflowers, taking their centers as the charge transfer path to promote the charge transfer in all directions. The microstructure of the nanoflowers provides another channel for the charge transfer, effectively promoting the charge transfer ability, which results in the efficient inhibition of the electron–hole recombination. Therefore, the photostability and the photocatalytic activity of the visible-light-responsive ZCS-6 for hydrogen evolution is significantly improved.

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Scheme 1. Mechanism for photogenerated charge separation and redox reaction over the ZnCdS semiconductor.

4. Conclusions In summary, ZnCdS nanoflowers have been prepared by adjusting the molar ratio of precursors in the absence of a surfactant. Upon increasing the amount of TAA, the defects disappear gradually, the crystallinity of the catalyst improves, and its size decreases, which is beneficial to the charge transfer. When the (Zn + Cd):S ratio is 1:3.5, the resulting ZnCdS nanoflowers exhibit the highest hydrogen production activity, which is 4.5 times that obtained for a (Zn + Cd):S ratio of 1:1. The initial photoactivity is maintained after 30 h of continuous illumination in a strong alkaline sacrificial agent. ZnCdS nanorods self-assemble into nanoflowers, and the charges on one end of the nanorods transfer to the other end through the center of the nanoflowers, which provides another effective pathway for the charge transfer. Upon light irradiation, this remarkable charge transfer phenomenon effectively inhibits the recombination of electron–hole pairs, and improves the electron ability to produce H2. In addition, the ZnCdS photocatalysts having a nanoflower structure exhibit relatively low impedance, high photocurrent response, and low overpotential, which render them with excellent photocatalytic activity. Compared with previous studies on ZnCdS [14–16,18], this work adopts a fairly new strategy to study the effect of the molar amount of the S precursor on the photocatalytic performance of ZnCdS. Furthermore, in the absence of active agent and template, the self-assembly of ZnCdS into nanoflowers greatly promotes the charge transfer efficiency, thus increasing the hydrogen production ability [49]. We believe that this work provides a feasible and simple strategy for designing self-assembled ZnCdS nanoflowers with high photocatalytic activity and photostability by changing the molar ratio of precursors. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was financially supported by the Open Project of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University (2019-KF-36), the Chinese National Natural Science Foundation (21862002 and

41663012), the new technology and system for clean energy catalytic production, Major scientific project of North Minzu University (ZDZX201803) and the Ningxia low-grade resource high value utilization and environmental chemical integration technology innovation team project, North Minzu University. Moreover, special thanks for the revision aid from Prof.Qingjie Guo (State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University), Dr. Yupeng Zhang and Dr Yanbing Li (North Minzu University).

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