Effects of pH value and hydrothermal treatment on the microstructure and natural-sunlight photocatalytic performance of ZnSn(OH)6 photocatalyst

Journal of Alloys and Compounds 810 (2019) 151955

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Effects of pH value and hydrothermal treatment on the microstructure and natural-sunlight photocatalytic performance of ZnSn(OH)6 photocatalyst Shuying Dong*, Longji Xia, Fangyuan Zhang, Fengzi Li, Yuyao Wang, Lingfang Cui, Jinglan Feng, Jianhui Sun** School of Environment, Henan Normal University, Key Laboratory for Yellow River and Huai River Water Environmental and Pollution Control, Ministry of Education, Henan Key Laboratory for Environmental Pollution Control, Xinxiang, Henan, 453007, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 April 2019 Received in revised form 13 August 2019 Accepted 21 August 2019 Available online 21 August 2019

The ZnSn(OH)6 nanocubes were prepared under different pH (3e12.4) by liquid precipitation, well characterized and further used for wastewater treatment. The results indicated that the ZnSn(OH)6 nanocubes prepared at pH ¼ 11.1 possessed of uniform size distribution and excellent photocatalytic activity, the degradation efficiency to MB reaches 76.3% after 5 h natural sunlight irradiation. Subsequently, the prepared ZnSn(OH)6 nanocubes was further thermal treated (200  C, 24 h) in four different solvents, containing deionized water, stock solution, mixed solution (Vwater: Vethanol: Vglacial acetic acid ¼ 3:1:1) and ethanol, named as H-1, H-2, H-3 and H-4, respectively. H-2 and H-3 showed superior photocatalytic performance and the removal efficiency of MB improved to be almost 100%, as well as redshifted light absorption edge and the band gap energies reduced 0.9 and 0.93 eV, respectively. Moreover, free radical capture experiments showed that the hþ and $O 2 are the main active species for the 1 ZnSn(OH)6 nanocubes and H-2, respectively, while all these three $OH, $O 2 and O2 radicals for H-3. Those results suggested that both pH regulation and thermal treatment could efficiently improve the microscopic morphology, crystal structure and photocatalytic activity of the ZnSn(OH)6, which might pave the way for the artful design of other high-performance catalysts. © 2019 Elsevier B.V. All rights reserved.

Keywords: Liquid precipitation pH ZnSn(OH)6 nanocubes Thermal treatment Photocatalytic

1. Introduction Recent studies have shown that compared with traditional wastewater treatment methods, photocatalytic technology has a hopeful application prospect in wastewater treatment due to its environmental friendly and energy saving, mild reaction conditions, the potential to utilize sunlight, and without secondary pollution [1e5]. The essence of photocatalytic redox reaction is that it acts as an electron transporter. The most important process for the photocatalytic reaction is the excitation and migration of photo-generated e-hþ pairs. The excitation usually controlled by the inner electron band structure of the photocatalyst, which determined the band gap width and the excitation wavelength, as well as the possibility of the reaction [6e8]. The mobility of photogenerated e-hþ determines the catalytic activity and quantum

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Dong), [email protected] (J. Sun). https://doi.org/10.1016/j.jallcom.2019.151955 0925-8388/© 2019 Elsevier B.V. All rights reserved.

yield [9,10]. It is generally considered that those processes are closely related to the microscopic surface structure such as crystal phase structure, crystallinity, surface area and co-catalyst. Therefore, the material category and their controllable preparation are vital for their photocatalytic activity [11e13]. Compared with traditional simple oxide photocatalyst, multicomponent oxides usually have multiple valence band and better acid-based resistance, complex crystal lattice, diverse crystal phase and morphology, as well as the multiple regulatory possibility, prompting these catalysts have preferable photocatalytic activity under various light sources and reaction conditions [14,15]. Therefore, multicomponent oxides such as BiVO4 [16e19], Bi2WO6 [20e22] and ZnWO4 [16e18,23e25] are widely studied in ultraviolet or visible light photocatalytic system. To construct an efficient and stable visible-light-responsive catalytic system, we should pay much attention not only to the electronic structure, but also to the effects of material type, morphology structure, crystallinity and surface properties. Therefore, material selection is particularly important due to that it determines the degree of visible light response and overall efficiency of the semiconductor material. As a

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typical multicomponent oxide, MSnO3 (M ¼ Zn, Ca, Sr, Ba, and Ag), ZnSn(OH)6 and Zn2SnO4 are widely used as gas-sensitive materials, electronic devices, lithium ion batteries, and photocatalytic materials [26e31]. Among them, ZnSn(OH)6 has attracted extensive research interests as a promising photocatalyst, due to its inner characteristics of applicable stability, non-toxicity, easy to use and excellent optical properties [30,32]. Hence, controllable synthesis of high-performance ZnSn(OH)6 visible-light-responsive photocatalysts via a simple preparation method has far-reaching significance for exploring versatile catalytic materials in wastewater treatment, as well as promoting the industrial application of photocatalytic technology. The main objectives of this study were to investigate the effect of pH and heat treatment on the microstructure and photocatalytic performance of the synthesized ZnSn(OH)6 photocatalyst. Owing to this, the ZnSn(OH)6 nanocubes was prepared by a simple liquid phase precipitation with pH as a regulator, and further annealing treatment in different solvents. The prepared samples were well characterized by a series of characterization techniques, and the photocatalytic activity was evaluated by the degradation of organic wastewater under natural sunlight.

2.2. Characterizations The crystal phase of the synthesized ZnSn(OH)6 was analyzed by X-ray diffraction (XRD) using a Bruker-D8-AXS diffraction system (Bruker Co., Germany), which equipped with a Cu Ka radiation (l ¼ 1.5406 Å) in the 2q range of 10e80 . Fourier transform infrared spectroscopy (FT-IR) was recorded on a Spectrum 400 system in the range of 400e4000 cm1 using KBr as a reference. Structure and morphology of the sample were inspected by using a JSM-6390LV scanning electron microscopy (SEM) and JEM-2100 transmission electron microscopy (TEM). The photoluminescence spectrum (PL) was recorded by a fluorescence spectrophotometer (FP-6500, Japan) equipped with a xenon lamp at room temperature with an excitation wavelength of 325 nm. The UVeVis diffuse reflectance spectra (DRS) of the samples were measured by a UVeVis spectrophotometer (Lambda17, PerkinElmer) at the wavelength range of 200e800 nm. X-ray photoelectron spectroscopy (XPS) spectra was performed on an Escalab-250Xi photoelectron spectrometer with Mg Ka X-ray source to describe the superficial elemental composition of the samples. 2.3. Photocatalytic activity evaluation

2. Experimental 2.1. Synthesis of ZnSn(OH)6 nanocubes Reagents purchased from Sinopharm Chemical Reagent Co., Ltd. are analytical grade and used without further purification. The water used in the experiment was deionized pure. Synthesis of ZnSn(OH)6 nanocubes by liquid precipitation method: In a typical ZnSn(OH)6 nanocubes synthesis procedure as shown in Scheme 1, firstly, 0.8178 g ZnCl2 and 2.1036 g SnCl4$5H2O were dissolved in 60 mL H2O and 30 mL CH3CH2OH, respectively. After complete dissolution, mixed the two solution under continues stirring (450 rpm) and adjusted pH to certain value (3e12.4) with NaOH (2 M) solution. The product collected after filtration and dried in an oven at 60  C overnight to get the final white sample. Subsequent hydrothermal treatment: After a series of degradation reactions, superior photocatalyst prepared under specific pH (11.1) was selected for subsequent heat treatment study. Therefore, four parallel trials were conducted. As mentioned above, after adjusting the mixed solution to the corresponding pH, one was transferred directly to a Teflon-lined stainless steel autoclave, while other three were centrifuged, washed with water and then transferred to Teflon-lined stainless steel autoclave containing equal volume of four different solvents, deionized water, containing stock solution, mixed solution (Vwater: Vethanol: Vglacial acetic acid ¼ 3:1:1) and ethanol, respectively. Lastly, they were heated at 200  C for 24 h, and the obtained samples were named as heat treatment-1 (H-1), heat treatment-2 (H-2), heat treatment-3 (H-3) and heat treatment-4 (H-4), respectively.

The photocatalytic activities of the prepared ZnSn(OH)6 nanocubes were evaluated by the degradation of methylene blue (MB) and ciprofloxacin hydrochloride (CIP) under natural sunlight irradiation in a 250 mL photochemical batch reactor. In order to make sure the constant illumination of the sunlight, all of the photocatalytic reactions were purposely executed between 9.00 a.m. and 3.00 p.m. (the reaction time was 5 h) on those sunny days during the summer 2018. The illumination intensity of the natural sunlight was measured by using a digital Lux meter (UA1010B, Shenzhen UYIGAO E&T Co., China), where the average time-dependent illuminations of the natural sunlight were about 60000e80000 Lux. Taking the MB degradation process as an example, 0.1 g of sample added into 200 mL of a 10 mg/L MB aqueous solution (pH ¼ 6.4) and stirred in the dark for 40 min to achieve an adsorption-desorption equilibrium before the irradiation. The dark state adsorption rates of different ZnSn(OH)6 samples on MB wastewater are shown in Fig. S1a, b and c. The results indicate that the ability of the catalyst to adsorb dye molecules has little effect on the degradation efficiency of the catalyst under natural sunlight. Took 5 mL of the sample at regular intervals after lighting and immediately centrifuged to remove particles for further analysis. The concentration of MB was analyzed by recording the variations of the absorption at maximum absorption wavelength and the degradation efficiency was determined according to the following equation [33]:

  Co  Ct  100% Degradation efficiency % ¼ Ct where C0 was the absorbance of MB or CIP after 40 min adsorptiondesorption equilibrium in the dark and Ct was the absorbance of MB

Scheme 1. Schematic diagram of ZnSn(OH)6 prepared under pH control and its heat treatment process in different solvents.

S. Dong et al. / Journal of Alloys and Compounds 810 (2019) 151955

or CIP at certain reaction time t (min). The mineralization of MB and CIP was measured by the decrease of total organic carbon (TOC) of the reaction solution, where the TOC was measured by a TOC analyzer (VarioTOC, Elementar Analysensysteme GmbH, Germany).

3. Results and discussion 3.1. Effect of pH The photocatalytic activity of the ZnSn(OH)6 prepared at different pH was evaluated by degradation of 10 mg/L MB under natural sunlight irradiation. Fig. S3a shows the photocatalytic degradation activity of a series of ZnSn(OH)6 photocatalysts synthesized by crude pH adjustment, where the ZnSn(OH)6 prepared at pH ¼ 11.1 exhibits superior photocatalytic performance. Therefore, the photocatalytic activity of ZnSn(OH)6 synthesized by finetuned pH with pH ¼ 11.1 as the center was further studied, and the results are shown in Fig. S3b. From which we can see that after 5 h natural sunlight irradiation, the ZnSn(OH)6 photocatalyst prepared at pH ¼ 11.1 still shows the best photocatalytic activity, and the degradation efficiency to MB reaches 76.3%. The photocatalytic activity of ZnSn(OH)6 prepared at different pH was further evaluated by degrading 10 mg/L ciprofloxacin hydrochloride (CIP) under natural sunlight, and then the optimal pH of the preparation was determined. As shown in Fig. 1a and b, ZnSn(OH)6 prepared at pH ¼ 11.1 also exhibits the most excellent photocatalytic performance for CIP degradation, which can degrade 64.67% CIP after 3 h natural sunlight irradiation. Those phenomena coincide with the results from Figs. S2a and b, therefore, ZnSn(OH)6 prepared at pH ¼ 11.1 was chose as the optimal photocatalyst for subsequent study. To verify whether the MB is mineralized or only decolorized, TOC removal efficiency was further conducted to investigate the mineralization efficiency. After 5 h reaction, the TOC removal efficiency of MB and CIP are about 60.31% and 35.17%, respectively. The results show that both MB and CIP molecule can only be partly mineralized by ZnSn(OH)6 photocatalyst under sunlight irradiation, and MB molecular was more easily destructed than that of CIP. Moreover, series of characterization techniques were carried out to investigate the effect of pH regulation on its microstructure and morphology. XRD was employed to probe the crystal phases of prepared ZnSn(OH)6 samples, and the results were displayed in Fig. 2. As shown in Fig. 2a, typical XRD patterns of ZnSn(OH)6 synthesized by

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crude pH adjustment were coincide with the cubic phase of ZnSn(OH)6 (JSPDS: 73e2384) when the pH greater than or equal to 7. However, when the preparation pH lowers than 5, the obtained ZnSn(OH)6 sample presented low crystallinity or even amorphous form. Table S2 shows the fine structure parameters of the XRD results of the prepared ZnSn(OH)6. The values of R-factors as determined by authors are less than 10%, this confirms the goodness of refining. As well as the crystallite size of ZnSn(OH)6 obtained by fitting can correspond well with the SEM results. Fig. 2b shows the diffraction peaks of ZnSn(OH)6 synthesized by finetuned pH with pH ¼ 11.1 as the center, which indicates that the pattern did not change obviously the pH increasing from 10 to 12.4 except the peak intensity. Those results are consistent with the photocatalytic activity. The typical SEM images of the prepared ZnSn(OH)6 samples are shown in Figs. 3 and 4. From the images of Fig. 3a-d, it can be seen that the morphology of the synthesized sample gradually changes from massive to stacked cubes as the pH is increased from 3 to 9. While the pH further increases to 11.1, the obtained ZnSn(OH)6 sample presents a nanocubes with uniform size distribution, as shown in Fig. 3e. After a close-up view of Fig. 4aed, we can see that the ZnSn(OH)6 sample prepared lower than 11.1 possess a rough surface, which may be due to the incomplete reaction at the insufficient alkalinity environment. As shown in Fig. 4eeg, the ZnSn(OH)6 sample aggregates again when the pH further increases to 12.4. Figs. 3f and Fig. 4jei displays the EDS spectra and element mapping of ZnSn(OH)6 nanocubes prepared at pH ¼ 11.1, indicating that there are only Sn, Zn and O elements in the ZnSn(OH)6 nanocubes and dispersed uniformly. The TEM images of ZnSn(OH)6 nanocubes prepared at pH ¼ 11.1 are shown in Fig. 4h and i, from which detailed morphological features of the prepared samples can be obtained. It can be seen from these images that the ZnSn(OH)6 sample prepared under the optimal conditions exhibited a regular square, which consistent with the results of SEM characterization. FTIR is an effective tool for characterizing chemical group in the sample. FTIR spectra of ZnSn(OH)6 nanocubes prepared under different pH conditions are shown in Fig. 5a and b. The band observed at 538 cm1 belongs to vibrations of M  O or M-O-M groups for ZnSn(OH)6. The bands at 774, 851, 1180, and 2335 cm1 maybe originate from bending vibration of M  OH or MeOHeM groups for ZnSn(OH)6, while peak situated at 3220 cm1 indicates the presence of OeH stretching vibration in ZnSn(OH)6 [34]. It is worth noting that the sample prepared at the alkaline environment

Fig. 1. The degradation efficiency of CIP under natural sunlight irradiation in the presence of ZnSn(OH)6 nanocubes prepared at different pH (a) and (b).

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Fig. 2. (a) and (b) are XRD spectra of ZnSn(OH)6 prepared under different pH conditions.

Fig. 3. SEM images of ZnSn(OH)6 prepared at pH ¼ 3 (a), pH ¼ 5 (b), pH ¼ 7 (c), pH ¼ 9 (d) and pH ¼ 11.1 (e), respectively; Corresponding EDS spectra of ZnSn(OH)6 prepared at pH ¼ 11.1 (f).

exhibit similar vibration peak, which coincide with the results of XRD and SEM. Fig. 6 shows the PL spectra of ZnSn(OH)6 nanocubes prepared under different pH conditions. All of the samples showed emission peaks in the visible region with wide band gaps and possess radiative defect states. The fluorescence spectra of all samples display a near-band-edge excitonic emission ranging and defect-related green emission centered at 475 nm and 530 nm, respectively. Defect-related green emission peaks appearing at 530 nm are known to be associated with zinc interstitials and oxygen vacancies. Oxygen vacancies act as luminescent centers and can form high defect levels in the bandgap region, capturing electrons from the valence band to facilitate luminescence [35,36]. As is clear from Fig. 6a, the PL intensities of the ZnSn(OH)6 samples prepared under different pH adjustments were different. Therefore, we can speculate that ZnSn(OH)6 samples prepared under different pH conditions show different photocatalytic activities maybe due to the

diverse induced oxygen vacancies [37]. 3.2. Effect of heat treatment and solvent Fig. 7 shows the SEM images of the ZnSn(OH)6 sample after hydrothermal treatment in different solvents. As can be seen from Fig. 7a and d, H-1 and H-4 still present uniformly distributed ZnSn(OH)6 nanocubes with some amorphous particles covered the surface. Fig. 7b shows the typical structure of H-2, from which we can observe that it has changed from a nanocubes to a needle-like structure (insert in yellow circle) after heat treatment. Meanwhile, the H-3 exhibits an irregular block structure, as shown in Fig. 7c. In view of this, we can conclude that heat treatment has a significant effect on the morphology of ZnSn(OH)6 sample, as well as potential effect on their photocatalytic activity. In order to study the thermostability, the ZnSn(OH)6 nanocubes prepared under optimal conditions was investigated by TG-DTA

S. Dong et al. / Journal of Alloys and Compounds 810 (2019) 151955

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Fig. 4. SEM images of ZnSn(OH)6 prepared at pH ¼ 10.0 (a), pH ¼ 10.4 (b), pH ¼ 10.8 (c), pH ¼ 11.1 (d), pH ¼ 11.6 (e), pH ¼ 12.0 (f) and pH ¼ 12.4 (g), respectively; The HRTEM images (h, i) and elemental mapping (jei) of the prepared ZnSn(OH)6 prepared at pH ¼ 11.1.

Fig. 5. (a) and (b) are FTIR spectra of ZnSn(OH)6 nanocubes prepared at different pH conditions.

under air atmosphere. The detection temperature range was 20e1000  C and the heating rate was 10  C/min. As shown in Fig. 8a, the TG-DSC curve clearly shows a rapid weight loss between 206.2  C and 267.4  C, which can be related to the thermal decomposition of ZnSn(OH)6 to ZnSnO3 or Zn2SnO4 [37]. The small weight loss at 912  C is attributed to further phase transitions from the perovskite ZnSnO3 phase to the mixture of Zn2SnO4 and SnO2. In order to study the effect of heat treatment on photocatalytic activity, the photocatalytic performance of ZnSn(OH)6 sample after thermal treatment was evaluated by the degradation of 10 mg/L MB and 10 mg/L CIP, the results are shown in Fig. S2c and Fig. 8b. By comparing the degradation rate of the ZnSn(OH)6 sample after hydrothermal treatment in different solvents, we can see that the activity of the sample after the hydrothermal treatment is improved. Among them, Heat treatment-2 (H-2) and Heat treatment-3 (H-3) showed superior photocatalytic activity, and the

removal efficiency of MB and CIP wastewater was almost 100%. The crystal phase change of the ZnSn(OH)6 particle after hydrothermal treatment was further demonstrated by the XRD characterization, and the results are shown in Fig. 8cef. By comparison with the ZnSn(OH)6, it can be found that the crystal phase of the H-1 sample hydrothermal in stock solution slightly changed, while other three samples changed significantly both the crystal phase and crystallinity. After hydrothermal treatment, some diffraction peak of Zn2SnO4 (JSPDS: 74e2184) appeared in the H-1 sample, while H-2 sample also became the mixed crystalline phase of ZnSnO3 (JSPDS: 28e1486) and Zn2SnO4 (JSPDS: 74e2184). Meanwhile, the weak and widened diffraction peaks in the XRD pattern revealed that the synthesized H-3 sample with lower crystallinity, which coincided with type of perovskite ZnSnO3 (JSPDS: 28e1486). Further, two types of diffraction peaks of ZnSnO3 (JSPDS: 52e1381) and ZnSn(OH)6 (JSPDS: 73e2384) appeared in

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Fig. 6. (a) and (b) Photoluminescence spectra of ZnSn(OH)6 prepared under different pH conditions.

Fig. 7. SEM images of ZnSn(OH)6 nanocubes after heat treatment in different solvents: (a) Heat treatment-1 (H-1); (b) Heat treatment-2 (H-2); (c) Heat treatment-3 (H-3) and (d) Heat treatment-4 (H-4).

the H-4 sample. These results indicate that the improvement in photocatalytic activity of ZnSn(OH)6 by heat treatment in different solvents may be related to the change of crystal form. The FT-IR spectra of ZnSn(OH)6 nanocubes after further hydrothermal treatment in different solvents are shown in Fig. 9a. After hydrothermal treatment, the characteristic peaks of H-1 sample are almost unchanged compared with that of pH 11.1 except the peak intensity. In contrast, the precursor at the peaks of the M  OH or MeOHeM groups in H-2 and H-3 samples almost diminished, while those characteristic peaks are feebleness in H-4 sample. The emerging peak at 3425 cm1 in the H-2, H-3 and H-4 samples is attributed to the OeH stretching vibration of adsorbed water molecules on the surface of the samples [35]. Fig. 9b shows the photoluminescence spectra of the heat-treated sample, which is similar to the PL spectra of the ZnSn(OH)6 samples prepared under different pH conditions in Fig. 6. They all show emission peaks in

the visible region with near-band edge exciton emission and defect-related green emission centered at 475 nm and 545 nm, respectively. As mentioned above, the oxygen vacancies and/or Zn interstitials associated with green emission defects displayed at 545 nm, oxygen vacancies act as luminescence centers and can form defect levels that are highly located in the band gap region, trapping electrons from the valence band and contribute to luminescence [36]. This result suggesting that further hydrothermal treatment could not improve the surface defects of the prepared photocatalyst. 3.3. Discussion on the mechanism of improving photocatalytic activity The composition and surface chemical states of the as-prepared ZnSn(OH)6 samples were characterized by XPS. Fig. 10a depicts the

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Fig. 8. (a) TG-DSC curve of ZnSn(OH)6 prepared at pH ¼ 11.1; (b) Degradation efficiency of CIP-contained wastewater under natural sunlight irradiation in the presence of ZnSn(OH)6 nanocubes after heat treatment in different solvents and (cef) the corresponding XRD spectra.

full scan spectra of ZnSn(OH)6 prepared under different conditions, which demonstrates the presence of Sn, O and Zn elements and consistent with the EDS results. As shown in Fig. 10b, the Zn 2p3/2 peak and the Zn 2p1/2 peak were approximately present at 1022.6 eV and 1045.2 eV, respectively [38,39]. The peaks of H-2 and H-3 both move toward lower energy than that of ZnSn(OH)6 before hydrothermal treatment. Similarly, it can be seen from Fig. 10c that the peaks centered at 487.2 eV and 495.7 eV attribute to Sn 3d5/2 and Sn 3d3/2 of Sn4þ in ZnSn(OH)6 also move toward lower energy region [40,41]. As shown in Fig. 10d, the high-resolution O 1s spectra centered at 531.8 eV can be assigned to the metal-oxygen bonds [42]. The photoelectron peaks of the lattices O 1s, Sn 3d and Zn 2p of H-2 and H-3 show a larger negative shift than that of ZnSn(OH)6 before hydrothermal treatment, indicating that H-2 and H-3 have a thicker space charge layer and thus have higher photocatalytic activity [43]. In addition, the valence band XPS spectra of

the as-prepared samples are shown in Fig. 10e. The top of valence band relative to the surface Fermi level can be determined by linear fitting to the intersection of the leading edge of the valence band photoemission and the background. Therefore, the EVB of ZnSn(OH)6 prepared at pH ¼ 11.1, H-2, and H-3 samples can be measured to be 3.18, 2.31, and 2.52 eV per NHE, respectively. To understand the significant enhancement photocatalytic activity of the sample after heat treatment compared with the ZnSn(OH)6 synthesized by pH adjustment, we further characterized these samples using time-resolved photoluminescence (TRPL) spectroscopy, which is capable of reflecting the transfer of photogenerated charge carriers. As shown in Fig. 10f, the TRPL data further indicates that the lifetime of the ZnSn(OH)6 prepared at pH ¼ 11.1 is much shorter than that of H-2 and H-3, which suggesting that the H-2 and H-3 have a faster charge transfer rate and potential to improve photocatalytic activity. More importantly, the

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Fig. 9. (a) FT-IR and (b) PL spectra of ZnSn(OH)6 samples after hydrothermal treatment in different solvents.

Fig. 10. XPS spectra (a) survey spectra, (b) Zn 2p, (c) Sn 3d and (d) O 1s, (e) valence band map and (f) luminescence decay curves of ZnSn(OH)6 prepared under different conditions.

hole storage effect of H-2 and H-3 samples prolongs the fluorescence lifetime of ZnSn(OH)6 after pH adjustment. As shown in Fig. 9b and Table 1, the average PL lifetime of ZnSn(OH)6 was extended from 985 ns without heat treatment to 1573 and 1398 ns after heat treatment. Fig. 11a shows the UVevis diffuse reflectance spectra. It can be observed that ZnSn(OH)6 samples prepared before and after hydrothermal treatment exhibit distinctly different absorption intensity in the UV region, but a similar absorption curve in the visible light region. The ZnSn(OH)6 sample prepared at pH ¼ 11.1 shows a significantly lower absorption ability in the deep ultraviolet region, while both the increased absorption intensity and redshifted absorption edges can be observed after the hydrothermal treatment. The above phenomenon implies that the ZnSn(OH)6 sample after hydrothermal treatment may be more effective in

improving the light-trapping ability of ZnSn(OH)6. In addition, the band gap energy (Eg) of the samples could be obtained according to the following Tauc's equation [44]: Ahn ¼ C(hn-Eg)n/2 Where C denotes a constant, A, n, Eg and h are absorption coefficient, incident light frequency, band gap energy and Planck constant, respectively. The constant n is 1 and 4 for direct band-gap semiconductor and indirect band-gap semiconductor, respectively. The band gap energies of the ZnSn(OH)6 samples can be estimated from a plot depicting (Ahv)1/2 versus hn, as exhibited in Fig. 11b. Therefore, Eg of pH ¼ 11.1, H-2 and H-3 could be calculated to be 4.4, 3.5 and 3.47 eV, respectively. In order to determine the effect of specific surface area and pore

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Table 1 Summary of the photoluminescence decay time (t) and their relative amplitude (f) of the ZnSn(OH)6, H-2 and H-3 samples obtained from the time-resolved photoluminescence spectroscopy (Fig. 10f) by tri-exponential decays. Samples

pH ¼ 11.1 H-2 H-3 1

Decay time (ns)

Relative amplitude (%)

t1

t2

t3

A1

A2

A3

68.56 86.09 209.96

911.07 830.79 1292.75

2527.44 2418.76 1713.40

39.16 39.02 20.01

49.38 44.98 56.42

11.46 16.01 23.57

Average lifetime (, ns)1

c2

985.80 1573.26 1398.74

1.297 1.300 1.206

The average lifetime was calculated using the equation: ¼ (A1t21 þ A2t22 þ A3t23)/(A1t1 þ A2t2 þ A3t3).

c2: the goodness of fit parameter.

Fig. 11. UVevis diffusion reflectance spectra (a) and Tauc's plots (b) of ZnSn(OH)6 before and after hydrothermal treatment.

volume on the photocatalytic process, the N2 adsorptiondesorption isotherm was analyzed (Fig. S3). It can be seen that all the three samples exhibit distinct hysteresis loop belong to type IV (a) isotherms and meso-porous material, where pH 11.1 (ZnSn(OH)6 prepared at pH ¼ 11.1) and H-2 sample both exhibit type H3 hysteresis loop, while H-3 sample exhibits type H2 (a) hysteresis loop with very steep desorption branch [45]. Table S1 collects information based on BET surface area, pore volume and pore size of the three samples. It can be seen that after hydrothermal treatment, the BET specific surface area of the H-2 and H-3 increased from 11.083 m2/g to 57.353 and 220.98 m2/g, respectively, compared with that of the ZnSn(OH)6. The increase in surface area suggests that more active sites maybe participate in the photocatalytic reaction, thus enhancing photocatalytic activity. Electrochemical characterization is often used as a powerful tool for analyzing carrier transport of samples. Fig. 12a shows the electrochemical impedance spectroscopy (EIS) Nyquist plots of the electrode prepared under different conditions. It is apparent that the Nyquist arc radius of theH-3 and H-2 are smaller than that of the ZnSn(OH)6 sample prepared at pH ¼ 11.1, which indicating that hydrothermal treatment can enhance the interface electron transport rate of the ZnSn(OH)6 sample. In addition, Fig. 12b shows the CV curve of the electrode prepared under different conditions in 10 mM K3Fe(CN)6 and 0.1 M KCl solution at scan rate of 40 mV S1. Obviously, all the samples show a reversible voltammogram with potential range of 0.8 to 0.8V. It's worth noting that the redox current of the ZnSn(OH)6 electrode is much lower than that of the H-2, H-3 sample, which implying that the H-2 and H-3 electrodes foster a faster electron transfer. The photocurrent responses of these three samples with 20 s simulated visible light on/off cycles are shown in Fig. 12c. It can be observed that the photocurrent intensities exhibited by H-3 and H-2 are superior to that of

ZnSn(OH)6, where H-3 exhibits the maximum photocurrent intensity indicating a higher electron-hole separation efficiency. It is widely believed that Mott-Schottky curves can be utilized to explore the semiconductor type and flat band potential of electrode materials. As shown in Fig. 12d, the slopes of the samples prepared under different conditions were all positive, indicating that the prepared photocatalyst exhibited characteristics of an n-type semiconductor. It is well known that the flat band potential of semiconductor materials is approximately equal to the Fermi level. Therefore, it is estimated that the ECB of ZnSn(OH)6, H-2 and H-3 is 1.22 eV, 1.19 eV and 0.95 eV, respectively. Compared with ZnSn(OH)6, the Fermi level of H-2 and H-3 has a negative change, indicating that the donor has higher density and better photocatalytic activity. In order to explore the photocatalytic mechanism for the photocatalytic oxidation of MB, we carried out trapping experiments of reactive species in designed photocatalytic process. We chose benzoquinone (BQ), isopropanol (IPA), ammonium oxalate (AO) þ 1 and NaN3 to detect the active species such as $O 2 , $OH, h and O2, respectively, where the capture dose for the capture experiment was 0.01 M. As shown in Fig. 13a, BQ, NaN3 and IPA quenchers have 1 little influence on the MB degradation, indicating that $O 2 , O2 and $OH radicals do not play major roles in the photocatalytic process. In contrast, the photocatalytic oxidation activity of ZnSn(OH)6 can be greatly suppressed by the addition of hþ scavenger (AO), suggesting that the hþ scavenger is the main active species. However, when the BQ added into the H-2 system (Fig. 13b), the degradation rate of MB is remarkably decreased, indicating that the $O 2 is the main active species, while $OH, hþ and 1O2 have little influence on the MB degradation. For H-3 system, the degradation efficiencies of MB are notably suppressed with the addition of $OH (IPA), $O 2 (BQ) and 1O2 (NaN3) scavengers, implying that all these three $OH, $O 2

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Fig. 12. (a) EIS curve in 0.1 M Na2SO4 solution under visible light irradiation; (b) CV curve in 10 mM K3Fe(CN)6 and 0.1 M KCl solution at scan rate of 40 mV S1; (c) Photocurrent response and (d) Mott-Schottky (MS) plots of the ZnSn(OH)6 electrode.

Fig. 13. Photocatalytic degradation of MB solution over (a) ZnSn(OH)6 and (b) H-2 and (c) H-3 with and without the quenchers under natural sunlight irradiation.

and 1O2 radicals play major roles in the photocatalytic process. Therefore, hydrothermal treatment not only changes the microstructure and morphology, but also the photocatalytic degradation process. 4. Conclusions In this study, ZnSn(OH)6 nanocubes were successfully prepared by liquid precipitation method using pH as a regulator. Then the ZnSn(OH)6 samples prepared under optimal conditions (pH ¼ 11.1) were further heat treated in different solvents and detailed characterized by various techniques. The results show that the ZnSn(OH)6 nanocubes prepared at pH ¼ 11.1 has a uniform size

structure and excellent photocatalytic activity, which could degrade 76.3% MB after 5 h natural sunlight irradiation. For the heat-treated samples, H-2 and H-3 showed superior photocatalytic performance and could completely decolorize the MB in identical condition. The improved photocatalytic performance of H-2 and H3 may be attributed to that the surface possesses more activity sites, better light absorption capacity and higher electron-hole pair separation efficiency. The radical capture experiment showed that the hþ and $O 2 are the main active species for the ZnSn(OH)6 nanocubes and H-2, respectively, while all these three $OH, $O 2 and 1 O2 radicals for H-3. These results indicate that the microscopic structure and photocatalytic activity of ZnSn(OH)6 can be significantly changed by pH regulation and further heat treatment in

S. Dong et al. / Journal of Alloys and Compounds 810 (2019) 151955

different solvents, which might pave the way for the rapid preparation of high performance photocatalyst. Acknowledgments This work was supported by the NSFC (Grants No. U1604137, 51808199 and 21677047), the China Postdoctoral Science Foundation (Grant No. 2018M630825 and 2019T120624), Plan for University Scientific Innovation Talent of Henan Province (19HASTIT046), the Innovation Scientists and Technicians Troop Construction Projects of Henan Province, PR China. The authors also appreciate for the support from the Fund for Excellent Young Scholars (20180545) of Henan Normal University for the PhD. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.151955. References [1] H.G. Yu, W.Y. Chen, X.F. Wang, Y. Xu, J.G. Yu, Enhanced photocatalytic activity and photoinduced stability of Ag-based photocatalysts: the synergistic action of amorphous-Ti(IV) and Fe(III) cocatalysts, Appl. Catal. B Environ. 187 (2016) 163e170. [2] H.G. Yu, L.L. Xu, P. Wang, X.F. Wang, J.G. Yu, Enhanced photoinduced stability and photocatalytic activity of AgBr photocatalyst by surface modification of Fe(III) cocatalyst, Appl. Catal. B Environ. 144 (2014) 75e82. [3] L.M. Song, Y.M. Li, S.N. Zhang, S.J. Zhang, Synthesis and characterization of Bi3þ-doped Ag/AgCl and enhanced photocatalytic properties, J. Phys. Chem. C 118 (2014) 29777e29787. [4] D.F. Xu, B. Cheng, S.W. Cao, J.G. Yu, Enhanced photocatalytic activity and stability of Z-scheme Ag2CrO4-GO composite photocatalysts for organic pollutant degradation, Appl. Catal. B Environ. 164 (2015) 380e388. [5] C.Y. Teh, T.Y. Wu, J.C. Juan, An application of ultrasound technology in synthesis of titania-based photocatalyst for degrading pollutant, Chem. Eng. J. 317 (2017) 586e612. [6] S.T. Huang, J.B. Zhong, J.Z. Li, J.F. Chen, Z. Xiang, W. Hu, M.J. Li, Z-scheme TiO2/ g-C3N4 composites with improved solar-driven photocatalytic performance deriving from remarkably efficient separation of photo-generated charge pairs, Mater. Res. Bull. 84 (2016) 65e70. [7] S.G. Meng, X.F. Ning, S.S. Chang, X.L. Fu, X.J. Ye, S.F. Chen, Simultaneous dehydrogenation and hydrogenolysis of aromatic alcohols in one reaction system via visible-light-driven heterogeneous photocatalysis, J. Catal. 357 (2018) 247e256. [8] S.G. Meng, X.J. Ye, J.H. Zhang, X.L. Fu, S.F. Chen, Effective use of photogenerated electrons and holes in a system: photocatalytic selective oxidation of aromatic alcohols to aldehydes and hydrogen production, J. Catal. 367 (2018) 159e170. [9] C.S. Guo, J.A. Xu, Y. He, Y.A. Zhang, Y.Q. Wang, Photodegradation of rhodamine B and methyl orange over one-dimensional TiO2 catalysts under simulated solar irradiation, Appl. Surf. Sci. 257 (2011) 3798e3803. [10] S. Dong, L. Cui, C. Liu, F. Zhang, K. Li, L. Xia, X. Su, J. Feng, Y. Zhu, J. Sun, Fabrication of 3D ultra-light graphene aerogel/Bi2WO6 composite with excellent photocatalytic performance: a promising photocatalysts for water purification, J. Taiwan. Inst. Chem. E. 97 (2019) 288e296. [11] C.C. Chen, H.J. Fan, J.L. Jan, Degradation pathways and efficiencies of acid blue 1 by photocatalytic reaction with ZnO nanopowder, J. Phys. Chem. C 112 (2008) 11962e11972. [12] S.Y. Dong, X.H. Ding, T. Guo, X.P. Yue, X. Han, J.H. Sun, Self-assembled hollow sphere shaped Bi2WO6/RGO composites for efficient sunlight-driven photocatalytic degradation of organic pollutants, Chem. Eng. J. 316 (2017) 778e789. [13] S. Dong, Y. Pi, Q. Li, L. Hu, Y. Li, X. Han, J. Wang, J. Sun, Solar photocatalytic degradation of sulfanilamide by BiOCl/reduced graphene oxide nanocomposites: mechanism and degradation pathways, J. Alloy. Comp. 663 (2016) 1e9. [14] M.H. Habibi, M. Mardani, Synthesis and characterization of bi-component ZnSnO3/Zn2SnO4 (perovskite/spinel) nano-composites for photocatalytic degradation of Intracron Blue: structural, opto-electronic and morphology study, J. Mol. Liq. 238 (2017) 397e401. [15] S. Dong, L. Xia, T. Guo, F. Zhang, L. Cui, X. Su, D. Wang, W. Guo, J. Sun, Controlled synthesis of flexible graphene aerogels macroscopic monolith as versatile agents for wastewater treatment, Appl. Surf. Sci. 445 (2018) 30e38. [16] F. Chen, Q. Yang, X.M. Li, G.M. Zeng, D.B. Wang, C.G. Niu, J.W. Zhao, H.X. An, T. Xie, Y.C. Deng, Hierarchical assembly of graphene-bridged Ag3PO4/Ag/BiVO4 (040) Z-scheme photocatalyst: an efficient, sustainable and heterogeneous catalyst with enhanced visible-light photoactivity towards tetracycline degradation under visible light irradiation, Appl. Catal. B Environ. 200 (2017) 330e342.

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