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Magnetically Separable CdS/ZnFe2O4 Composites with Highly Efficient Photocatalytic Activity and Photostability under Visible Light Lei Zou, Haoran Wang, Guoliang Yuan, and Xiong Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00243 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018

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ACS Applied Nano Materials

Magnetically Separable CdS/ZnFe2O4 Composites with Highly Efficient Photocatalytic Activity and Photostability under Visible Light Lei Zou, Haoran Wang, Guoliang Yuan*, and Xiong Wang* School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China *Corresponding author. Email: [email protected]

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ABSTRACT: Novel CdS/ZnFe2O4 composites were prepared through a two-step hydrothermal process. The homogeneous ZnFe2O4 nanoparticles are decorated on the self-assembled CdS spheres. Compared to pure CdS and blank ZnFe2O4, the photocatalytic activity and stability of the magnetically separable CdS/ZnFe2O4 composites are considerably increased. The results of photoluminescence and electrochemical impedance

spectroscopy

further

validate

that

the

performance

enhancement results from the construction of heterojunction structure, leading to high charge separation efficiency. Based on the calculation and the trapping test, a heterojunction photocatalytic mechanism is proposed. KEYWORDS: Photocatalytic, CdS/ZnFe2O4, Heterostructure, Composite, Magnetic, Mechanism

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INTRODUCTION In the past decades, novel visible-light-driven semiconductors capable of efficiently solar harvesting have attracted much attention and been regarded as the most promising materials in the photocatalytic field, especially for the application in degradation of organic contaminants and the conversion of solar energy.1-4 Cadmium sulfide (CdS), as a visible-light-induced semiconductor (Eg=2.4 eV),5-7 has extensive application prospects on degrading organic pollutants since its conduct -

band (CB) is negative than the reduction potential for O2 /·O2 .8-11 Unfortunately, bulk CdS suffers some serious problems, such as low quantum yield and photocorrosion of S2−,12, 13 which seriously limit its further utilization in practical wastewater remediation. The construction of heterojunctions is a suitable strategy to promote the carriers’ separation, stabilize the structure, and improve the photocatalytic activity of CdS.14-17 Recently, the fabrications of CdS-TiO2,18 CdS-GO,19 CdS-Bi2MoO6,20 and CdS-Bi2WO621 have been reported, which exhibited excellent photocatalytic activity and stability. Spinel-type complex oxides, such as MnFe2O4,22 ZnFe2O4,23 NiFe2O4,24 have been extensively exploited in solar transformation, photocatalysis, and H2 evolution reaction (HER) due to their unique structures. Among them, ZnFe2O4 possessing narrow bandgap (~1.9 eV vs NHE),25-27 can harvest large amount of sunlight. Moreover, some advantages like high 3



photocatalytic activity, good stability, low cost and easy synthesis，hugely increase the possibility of commercialization. For CdS/ZnFe2O4 (CdS/ZFO), small ZnFe2O4 nanoparticles (ZFO NPs) will be easy to attach onto the CdS surface, which can provide more active sites. Large amounts of interfaces between ZFO and CdS also can accelerate the charge separation. Furthermore, due to the more positive valence band (VB) of CdS, the photoinduced holes will promptly transfer from CdS to ZFO,28,

29

thus preventing CdS from photocorrosion. However, little

previous work regrading on the photocatalytic performances of CdS/ZFO hybrid system has been reported. In this work, a two-step hydrothermal route to synthesis of CdS/ZFO nanocomposites was reported. The photoactivity and photostability of as-obtained nanocomposites was estimated from the degradation of organic dye (RhB) irradiated with visible light. The main active oxygen species (AOSs) were examined by the radical trapping test and the photocatalytic mechanism was also investigated. The heterojunction energy band structure accounts for the improved photoactivity and superior stability of CdS/ZFO composites. Additionally, due to its room-temperature superparamagnetic characteristics, the composite photocatalyst can be easily magnetically separated and recycled. RESULTS AND DISCUSSION Phase and morphological analysis. The phase structure of 4


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as-synthesized product was examined by XRD. Observed from Figure 1, the characteristic reflections of pristine CdS at 2θ angles of 26.48°, 44.02°, and 52.11°, are readily assigned to the (111), (220) and (311) planes, respectively, for the cubic zinc-blende structure of CdS (JCPDS No.: 80-0019). And the distinct broadening of reflections implies a small crystallite size. As calculated from the Scherrer formula, the mean crystallite size of CdS is ~6.4 nm. Pure ZFO was crystallized in cubic spinel structure (JCPDS No.: 22-1012) with Fd-3m space group. Both the diffraction peaks of CdS and ZFO can be observed from the CdS/ZFO composites without other detectable impurities. The diffraction intensity of the spinel tends to be stronger with the increase of ZFO content.

Figure 1. XRD patterns of pristine CdS, pure ZFO, and CdS/ZFO composites with various molar ratios.

5



Figure 2. SEM images of (a) pristine CdS, (b) CdS/ZFO-3:1, (c) CdS/ZFO-2:1, and (d) CdS/ZFO-1:1. (e-j) Typical electron photograph and EDX elemental mappings of CdS/ZFO-2:1. All the scale bars in Fig. 2a-d are 2 µm. The further morphological observation was conducted on SEM and TEM. The SEM images (Figure 2) display the microstructures evolving from CdS submicrometer spheres to CdS/ZFO composite microspheres. Figure 2a depicts the well-uniformed pristine CdS spheres in a size range of 150-200 nm with smooth surface. After being loaded with ZFO NPs (see Figure S1, Supporting Information), the submicrospherical structure 6


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of CdS is maintained. The surface of CdS spheres becomes rough and is dotted with ZFO NPs. When the ZFO/CdS molar ratio is relatively low (1:3), only a few ZFO NPs are decorated on the surface (Figure 2b). With gradually increasing the ratio to 1:2, the NPs adhered to CdS surface also increase (Figure 2c). As further increasing the molar ratio (1:1, Figure 2d), partial agglomeration can be observed from the sample. The representative EDX elemental mappings (Figure 2e-j) for CdS/ZFO-2:1 manifest the uniform distribution of ZFO NPs on the CdS surface.

Figure 3. TEM images of (a, b) pristine CdS and (c, d) CdS/ZFO-2:1. The insets are the corresponding ED patterns. CdS and ZFO NPs were respectively marked with red and green dashed lines for the sake of visual 7



distinguish. More morphological details can be observed from the TEM images (Figure 3). It is found that the pristine CdS particles aggregate into large clusters (Figure 3a), and a close observation reveals that CdS submicrospheres are in fact self-assembled with even much smaller NPs so as to minimize the Gibbs free energy (Figure 3b). The local high-resolution TEM image of CdS/ZFO-2:1 (marked in Figure 3c) is shown in Figure 3d. The lattice spacings of 2.06 and 2.52 Å at the interface are in agreement with the CdS (220) plane and the ZFO (311) plane, respectively, indicating that ZFO NPs are tightly attached to the CdS surface. For these two solids, both them are homogeneous in size ranging from 5-10 nm, which is in line with values calculated from the Scherrer formula. Furthermore, the SAED pattern (inset of Fig. 3d) exhibits two sets of ring patterns, which can be indexed to blended CdS and spinel ZFO. All the results validate the successful preparation of CdS/ZFO heterojunctions through the two-step hydrothermal strategy. The formation of CdS/ZFO composites can be briefly illustrated as follows (Figure 4). At first, Cd2+ cations react with thiourea to obtain CdS nuclei in the autoclave. Then the newly formed CdS nuclei gradually grow up into NPs. In the hydrothermal environment, the primary CdS NPs are self-assembled into submicrospheres with the assistance of PVP. Because of the point of zero charge (PZC) of CdS spheres is around 3.5, 8


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the spherical surface is negatively charged in the present reaction system. The Zn2+ and Fe3+ cations tend to be adsorbed onto the solid surface by electrostatic attraction. Ultimately, under the alkaline hydrothermal condition, the hydroxide precursor is converted to ZFO NPs depositing over the CdS surface.

Figure 4. Illustration showing the formation process for CdS/ZFO composites. UV-vis diffuse reflectance spectra (DRS). The optical absorption properties were also investigated for the products and the UV-vis DRS are shown in Figure 5. The absorption edge for pristine CdS lies on around 530 nm, responsive to visible light. Comparatively, the DRS of CdS/ZFO composites exhibit obvious red shifts and the visible light absorptions are also gradually enhanced with the increasing ZFO contents. Thereby, the deposition of ZFO NPs on CdS extends the absorption range 9



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of the composites and promotes the visible light harvesting, which are crucial

for

superior

photocatalytic

performance.30

From

the

Kubelka-Munk plots (inset of Figure 5), the bandgaps of pure ZFO and CdS are respectively determined to be 1.96 and 2.42 eV (vs. NHE), which agree with the existing reports.31-33

Figure 5. UV-vis DRS for pristine CdS, pure ZFO, and CdS/ZFO composites. The inset displays Kubelka-Munk plots of CdS and ZFO for band gap estimation. Photocatalytic performance. The photocatalytic performances were assessed through photodegradation of the typical organic dye RhB irradiated with visible light. Figure 6a shows the photocatalytic activities of pure ZFO, pristine CdS and CdS/ZFO composites. Obviously, RhB molecules are stable in the visible-light photolysis process without any catalysts. Pure ZFO and pristine CdS exhibit poor photocatalytic activities, and only 67% and 34% of RhB are removed within 2 h 10


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irradiation, respectively, which result from the rapid recombination of photoexcited charges. Once CdS particles are hybridized with ZFO, the photodegradation rate for the composites prominently increases.

Figure 6. (a, b) Time-dependent photodegradation of RhB over different photocatalysts

and

the

corresponding

degradation

kinetics.

Visible-light-driven photodegradation efficiency of CdS/ZFO-2:1 (c) at different pH and (d) in the presence of different scavengers at pH=7. Dye concentration: 5 mg L−1. Catalyst suspended: 1 g L−1. Duration: 120 min. All the CdS/ZFO samples deliver superior photocatalytic activity. Among them, CdS/ZFO-2:1 represents the best catalytic performance with a degradation rate of 93% within 120 min, which is improved by 11



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more than 4.5 and 2.6 times (k = 0.0215 min-1, Figure 6b) compared to pure CdS (k = 0.0047 min-1) and pristine ZFO (k = 0.0081 min-1), respectively.

However,

further

increasing

the

ZFO

content

(CdS/ZFO-1:1), the degradation rate reduces. It might be due to the agglomeration of excessive ZFO NPs, which would decrease the contact interface between CdS and ZFO and shield the light rays reaching the CdS surface. This result suggests that there is an optimum synergistic interaction between CdS and ZFO for the best photocatalytic performance. Moreover, the photodegradation efficiency of CdS/ZFO-2:1 in the allowed pH range (pH 3~9) was also investigated. As shown in Figure 6c, the optimal pH scope for RhB photodegradation is found to be 5~7. Since RhB exists predominately as two forms in acidic or basic system,34 in a solution with much lower pH value (pH=3), the electrostatic repulsion force between the cationic RhB molecules (acidic form) and the positively charged catalyst particles results in the decrease of dye adsorption amount, and thus weakens the photocatalytic efficiency. Meanwhile, the composite catalyst will also be partly acid-etched at lower pH. In a basic environment, RhB molecules (basic form) can attach to the solid photocatalyst surface by the carboxylic or amino group,35 which is greatly pH-dependent. The combined effects result into the gradually decline of photoactivity in the basic range. The CdS/ZFO-2:1 12


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sample can keep a high level of photoactivity in the pH range from 5 to 7. Photocatalytic mechanism of CdS/ZFO composite. To more deeply explore the dominating AOSs directly participating in the photocatalytic reaction for CdS/ZFO composites, the radical scavenging experiment was conducted. By adding various radical quenchers, the different influence of AOSs can be apperceived. As observed form Figure 6d, the existence of p-benzoquinone (BQ) and isopropyl alcohol (IPA), as the quenchers for superoxide (·O2−) and hydroxyl radicals (·OH), respectively, greatly suppresses the photocatalytic activity of CdS/ZFO-2:1. Only 22% and 38% of RhB can be removed within 120 min, respectively, indicating ·O2− and ·OH radicals as the predominant dominant AOSs in the reaction system. On the other hand, the photocatalytic activity is slightly suppressed (13.4%) by the hole quencher ammonium oxalate (AO). It means that the oxidation of dye molecules directly by the photoinduced holes occurs as a minor reaction.

Figure 7. Schematic sketches for the photodegradation of organic dye 13



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over CdS/ZFO and the carrier migration between ZFO and CdS. Besides, the band edges of a semiconductor are vital and closely entwined with its physicochemical properties. Thereby the band edge positions at the PZC were estimated from the equation E0VB = χ − 1

Ee + Eg .36, 37 The calculated band edges at the PZC are corrected to pH 7 2

by the following formulas EVB =E0VB +0.059(PZC − 7) and ECB =EVB − Eg . The VB top (EVB) and the CB bottom (ECB) for CdS at pH 7 are determined to be 1.56 and –0.86 eV (vs. NHE), and 0.34 and –1.62 eV (vs. NHE) for ZFO (PZC = 6.3), respectively. These results confirm that CdS and ZnFe2O4 can form an overlapping band structure.38-40 In view of the above-mentioned discussion, a heterojunction photocatalytic mechanism is proposed considering the charge transfer process. The schematic illustration was shown in Figure 7. Illuminated with visible light, both the narrow-band materials are irradiated to yield e−/h+ pairs. The photogenerated CB e− of CdS will react with adsorbed oxygen to produce superoxide anion radicals (E0

-

O2 /·O2

= -0.28 eV vs NHE)

and finally form ·OH. The VB holes of ZFO cannot react with H2O to 0 yield ·OH (EH

2 O/·OH

= 2.68 eV vs NHE) because of its more negative

potential (EVB = 0.34 eV vs NHE),41 while the photoexcited holes can oxidize RhB molecules directly.22 Since the EVB of CdS is more positive, the photoinduced holes will timely transfer from CdS to ZFO. Meanwhile, the CB of ZFO is more negative, and thus the photoirradiated electrons of 14


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ZFO will immigrate to the CB of CdS. In order to verify the migration mechanism, the interface electric resistance was measured by EIS. Figure 8a shows the EIS Nyquist plots of pure ZFO, pristine CdS, and CdS/ZFO-2:1 composite at frequencies ranging from 0.1 to 105 Hz. The semicircle in high frequency range is ascribed to charge transfer resistance (Rct). It is found that CdS/ZFO-2:1 has a smaller resistance than ZFO and CdS, implying that the charge transfer between CdS/ZFO interface is easier than pristine CdS or pure ZFO.

Figure 8. (a) EIS Nyquist plots and (b) Room-temperature PL spectra of pristine CdS, pure ZFO, and CdS/ZFO-2:1 composite. The separation of photoexcited carriers was further validated by their photoluminescence

(PL)

property.

Figure

8b

exhibits

the

room-temperature PL spectra of ZFO, pristine CdS, and CdS/ZFO-2:1. Comparatively, the PL intensity for CdS/ZFO-2:1 is weakened distinctly, indicating the retarded recombination and the effective separation of the photogenerated e−/h+ pairs.42 15



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Figure 9. Magnetic hysteresis loops of ZnFe2O4 and CdS/ZFO-2:1 (magnified image in inset). Equally importantly, the CdS/ZFO heterojunction photocatalyst exhibits

superior

stability.

The

photodegradation

efficiency

of

CdS/ZFO-2:1 reduces slightly during the cycling (Figure S2), and the crystal structure is sustained throughout the photocatalytic process, which can be clearly observed from the phase analysis on the sample cycled for four runs (Figure S3). Due to the timely transfer of photoinduced h+, the CdS photocorrosion is effectively restrained, bringing about the enhanced photostability. Furthermore, the room-temperature magnetic property of the composites was also investigated by VSM (Figure 9). Because of the diamagnetic of CdS, the saturation magnetization of CdS/ZFO-2:1 composite is reduced to 5.4 emu g−1 as compared with that of ZFO (40.3 emu g−1). However, as expected, the composite still exhibits 16


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superparamagnetic characteristics. Thus it can easily be magnetically separated from the solution using an extra magnet and also be readily redispersed back into the aqueous phase due to its near zero coercivity (as shown in Figure S4).43 The facile separation and recovery of CdS/ZFO composite is very favorable for its practical applications. CONCLUSION In summary, the CdS/ZFO composites successfully prepared through the two-step hydrothermal route exhibit highly efficient visible-light-driven photocatalytic activities. Compared with pristine CdS and pure ZFO NPs, CdS/ZFO-2:1 presents the highest efficiency among all the as-prepared photocatalysts. The enhanced photoactivity and photostability is accredited to the promotion of e−/h+ separation and the efficient transfer of photogenerated carriers through the heterojunction interface. This magnetically separable heterojunction photocatalytic system will shed light on the potential applications in environmental remediation and photosynthesis. EXPERIMENTAL SECTION Preparation of CdS spheres. Briefly, 5 mmol of cadmium acetate and thiourea were added into 50 mL of ethylene glycol. Polyvinylpyrrolidone (PVP, k30, 1.0 g) were then dissolved into the above solution by vigorously stirring. About half an hour later, the transparent solution was poured in a 100 mL hydrothermal autoclave, sealed, and heated to 180 °C. 17



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After hydrothermal treatment for 4 h, the powders were sampled by centrifugation, and rinsed with distilled water and absolute alcohol several times. Preparation of CdS/ZnFe2O4. The typical synthetic route to CdS/ZFO composites was described as follows: 1 mmol (144 mg) of the obtained CdS spheres were dispersed into a 50 mL aqueous solution containing stoichiometric amounts of zinc and ferric nitrates under ultrasonication for 0.5 h. The pH value of the mixture was maintained at 13 by dripping NaOH solution (1.0 M). After 0.5 h for stirring, the resulting solution was poured into a 100 mL Teflon-liner. Then the liner was sealed in an autoclave and heated to 100 °C for 6 h. Afterwards, the supernatant was decanted and centrifuged to obtain the final products. According to the components, the resulting composites were named as CdS/ZFO-3:1,

CdS/ZFO-2:1

and

CdS/ZFO-1:1,

respectively,

corresponding to the different molar ratios of CdS to ZnFe2O4. Characterization of samples. The crystalline structures of the obtained samples were analyzed by XRD (Bruker-AXS D8 Advance) with Cu Kα (1.54178 Å). The morphological observation and the elemental mappings were carried out on transmission electron microscope (TEM, JEOL 2100) and a field emission scanning electron microscope (FESEM, Quant 250FEG). The optical properties were evaluated from their UV-vis diffuse reflectance spectra (DRS) determined on an 18


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UV-2450 spectrophotometer (Shimadzu) with BaSO4 as reflectance standard. The electrochemical impedance spectroscopies (EIS) were performed on a CHI660E workstation (China) in a three-electrode system with 0.5 M KCl electrolyte. The photoluminescence (PL) spectra were measured at room temperature on an Agilent G9800A spectrophotometer (excitation at 350 nm). The room-temperature magnetic properties were measured by a 7400-S vibrating sample magnetometer (VSM, Lakeshore). Photocatalytic tests. The photocatalytic tests were conducted on photochemical reactor equipped by a 500 W Xe lamp with a cut-off filter (>420 nm) as described elsewhere.44 Briefly, the catalyst powders (20 mg) were added in RhB aqueous solution (5 mg L-1, 20 mL) in a quartz tube and kept in the dark for 1 h with stirring before illumination. After the reaction, 3 mL of suspensions were decanted from the tube and centrifuged to remove the catalyst particles. The supernatant was analyzed with a spectrophotometer for quantification ASSOCIATED CONTENT Supporting Information TEM image and size distribution of ZFO NPs, cycling runs for photodegradation of RhB, XRD patterns of CdS/ZFO-2:1 before/after the photocatalysis, and magnetic separation photograph. ACKNOWLEDGMENTS 19



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This research was supported by the NSFC (21001064) and the NSF of Jiangsu Province (BK2010487). REFERENCES (1) Li, Y.; Wang, P. F.; Huang, C. P.; Yao, W. F. Synthesis and Photocatalytic Activity of Ultrafine Ag3PO4 Nanoparticles on Oxygen Vacated TiO2, Appl. Catal. B: Environl., 2017, 205, 489−497. (2) Guo, S. N.; Zhu, Y.; Yan, Y. Y.; Min, Y. L.; Fan, J. C.; Xu, Q. J. Holey Structured Graphitic Carbon Nitride Thin Sheets with Edge Oxygen Doping via Photo-Fenton Reaction with Enhanced Photocatalytic Activity, Appl. Catal. B: Environ., 2016, 185, 315−321. (3) Wu, Q.; Wang, P. F.; Niu, F. T.; Huang, C. P.; Li, Y.; Yao, W. F. A Novel Molecular Sieve Supporting Material for Enhancing Activity and Stability of Ag3PO4 Photocatalyst, Appl. Surf. Sci., 2016, 378, 552−563. (4) Wei, H. H.; Yang, Z. J.; Min, Y. L.; Fan, J. C.; Xu, Q. J. Light Auxiliary Hydrogen-evolution Catalyst Based on

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ZnFe2O4 ... - ACS Publications

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