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ZnIn2S4 composite photocatalyst for enhanced mineralization of chloramphenicol under visible light
Accepted Manuscript Title: Polypyrrole/ZnIn2 S4 composite photocatalyst for enhanced mineralization of chloramphenicol under visible light Authors: Bo Gao, Weiping Chen, Shaonan Dong, Jiadong Liu, Tingting Liu, Lei Wang, Mika Sillanp¨aa¨ PII: DOI: Reference:
S1010-6030(17)31109-7 http://dx.doi.org/10.1016/j.jphotochem.2017.09.018 JPC 10863
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
29-7-2017 1-9-2017 5-9-2017
Please cite this article as: Bo Gao, Weiping Chen, Shaonan Dong, Jiadong Liu, Tingting Liu, Lei Wang, Mika Sillanp¨aa¨ , Polypyrrole/ZnIn2S4 composite photocatalyst for enhanced mineralization of chloramphenicol under visible light, Journal of Photochemistry and Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.09.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polypyrrole/ZnIn2S4 composite photocatalyst for enhanced mineralization of chloramphenicol under visible light Bo Gaoa, b*, Weiping Chena, Shaonan Donga, JiadongLiua, TingtingLiua, Lei Wanga, Mika Sillanpääb, c a
Key Laboratory of Membrane Separation of Shaanxi Province, School of Environmental and
Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, PR China b
Laboratory of Green Chemistry, Faculty of Technology, Lappeenranta University of
Technology, Sammonkatu 12, FIN-50130 Mikkeli, Finland c
Department of Civil and Environmental Engneering, Florida International University, Miami,
FL-33174, USA Corresponding
author.Tel. +358 0503368529; +86 13689296473;
E-mail address: [email protected] (B. Gao).
Graphical abstract
Highlights
Mineralization of chloramphenicol is enhanced by PPy-ZnIn2S4 composite.
More reactive oxygen radicals are produced by PPy-ZnIn2S4 composite.
The coexistence of hexagonal and cubic phase in composite is advantageous to charge separation.
ABSTRACT Functional PPy-ZnIn2S4 composite was prepared via facile hydrothermal method in the presence of PPy powder. The X-ray diffraction (XRD) pattern showed single hexagonal phase in pure ZnIn2S4 and mixed phases of hexagonal and cubic in
PPy-ZnIn2S4 composite. The coexistence of hexagonal and cubic phase in PPy-ZnIn2S4 composite can be advantageous to charge separation. The absorbance within visible light range was enhanced by PPy-ZnIn2S4 composite compared with pure ZnIn2S4. The time required for complete removal of chloramphenicol by PPy-ZnIn2S4 composite (60min) was shorter than pure ZnIn2S4 (120min), indicating better deep degradation. 48.5% TOC was achieved by PPy-ZnIn2S4 after 3h photocatalytic degradation, which was two times higher than ZnIn2S4 (23.5%).
Keywords:Ternary sulfide particles; Photocatalysis; Conducting polymer; Antibiotics
1. Introduction
The ternary metallic sulfide with narrow band gap had been extensively studied as visible light photocatalyst [1, 2]. However, the electrons and holes generated by photoexcitation of single semiconductor can easily recombine as a result of the random motion of photo-generated carriers, which significantly limited the photocatalytic efficiency under visible light. The modification of ZnIn2S4 in order to prevent the recombination had become a research hotspot. Recently, effects of graphene and carbon analogs for modification of ZnIn2S4 were contrastively studied to enhance charge separation [3]. An electron transport bridge was built up in MoS2-graphene cocatalyst and resulted in effective charge transfer in ZnIn2S4 [4]. It was reported that a hybrid organic-inorganic TiO2 nanocomposite could enhance photocatalytic properties [5]. Therefore, functional modification according to the property of photocatalyst can achieve improvement in photocatalytic activity. Polypyrrole (PPy), as a commonly used conductive polymer, has an extended
π-conjugated electron system, which possesses high stability and relatively high carrier mobility [6]. It is reported that PPy could serve as effective electron donor and stable photosensitizer, and integration of PPy with semiconductor could improve the photocatalytic activity [7]. The photocatalytic activity of Bi2WO6 was enhanced by ‘in situ’ deposition of polyprrole [6]. The PPy on the surface of photocatalyst could realize the directional separation of carrier and inhibit the recombination of electrons and holes. PPy can form a layer of conductive network on the surface of CdS, which can promote the separation of carrier and it also can suppress CdS from photocorrosion, so that coupled semiconductor PPy/CdS greatly improved the photocatalytic hydrogen production [8]. Based on the photosensitization of PPy, composite semiconductor PPy/TiO2 can overcome the defect of low visible light utilization of TiO2, and thus further enhance the photocatalytic degradation of organic compounds [9, 10] and hydrogen production [11] under visible light. And the synergetic effect of adsorption and visible light photocatalysis was observed on Ag2O/TiO2/PPy composite during removal of methylene blue [12]. Also, the transportation of photogenerated charges was investigated on composite semiconductors α-Fe2O3@PPy [13] and Fe3O4@PPy [14]. Unlike above-mentioned composite semiconductors, photoelectrons were trapped by conductive PPy in AgCl/PPy photocatalyst [15] and then transferred to the adsorbed O2 to form the active oxygen species. On the other hand, holes were scavenged by Cl- to form active oxidation production, thereby, the separation of photogenerated electron-hole pairs can be realized. To sum up, PPy has flexible and tunable properties, which can serve as effective electron donor, electron transporter and holes acceptor and transporter. PPy may work differently in different composite photocatalyst, as there is always a synergetic effect between PPy and semiconductor. So PPy is expected to be an
efficient mediator for the modification of semiconductor to enhance photoactivity under visible light. The aim of the present work is to explore functional modification of ZnIn2S4 with PPy in order to enhance the photocatalytic activity of ZnIn2S4. Composite photocatalyst combining ZnIn2S4 and PPy in the present study was synthesized through a facile hydrothermal method in the presence of PPy powder. The influence of addition PPy powder in the precursor solution on morphology, crystal structure and photocatalytic activity was investigated in detail. The detection of concentration up to 28 ngl-1 of chloramphenicol in urban water supplies of Shanghai (China) [16] had received increasing concern. The antibacterial property of chloramphenicol had prevented the removal efficiencies by conventional biological treatment process [17]. The photocatalytic degradation of chloramphenicol might provide a good option. Some researches related to the photodegradation of chloramphenicol were summarized in Table 1 [18-25], indicating that the photocatalytic degradation efficiency and mineralization of chloramphenicol under visible light needed to be further improved. Therefore, the degradation and mineralization of chloramphenicol was investigated in this research.
2. Materials and Methods
2.1. Chemicals
Unless otherwise specified, the chemicals used in this experiment were of analytical purity and used without further purification. The selected target organic pollutant chloramphenicol (CHL) and methylene blue (MB) was purchased from Aladdin® and Sinopharm Chemical Reagent Co., Ltd. The methanol was
chromatographically pure. The pyrrole (Py) and ammonium persulfate (APS, (NH4)2S2O8) were used for polymerization of polypyrrole (PPy). Ethylenediamine tetraacetic acid (C10H16N2O8), isopropanol ((CH3)2CHOH) and p-benzoquinone (C6H4O2, Aladdin®) were used as trapping reagent. 5-tert-Butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO) was purchased from Dojindo Molecular Technologies. The other reagents for the preparation of ZnIn2S4 photocatalyst such as Zn(NO3)2·6H2O, CH3CSNH2 (TAA) and In(NO3)3·5H2O, were the same as our previous research [26].
2.2. Preparation of ZnIn2S4 and PPy-ZnIn2S4 photocatalyst
Two steps were involved in the preparation of PPy-ZnIn2S4 photocatalyst. First of all, PPy was synthesized via liquid-phase polymerization of pyrrole. In a typical and simple procedure, 13.07g (NH4)2S2O8 was dissolved in 1000ml deionized water and then 4ml of pyrrole was added in the above solution to start the polymerization reaction in ice water bath under magnetic stirring. After 2h, the black product PPy was collected by filtration, and then washed twice with deionized water and finally dried at 80℃. After grinding and sieving through 300 mesh sieve, the PPy powder was used for the subsequent procedure. A certain amount of prepared PPy powder was added into precursor solution of ZnIn2S4 (including 0.5mmol Zn(NO3)2·6H2O, 1.0mmol In(NO3)3·5H2O and an excessive (4mmol) CH3CSNH2) before transferring to the autoclave. Then the autoclave was sealed and kept in 80℃ water bath with constant stirring for 6h. The product was collected by suction filtration and washed twice with deionized water. Different amount of PPy in the composite photocatalyst was marked as PPy (X)-ZnIn2S4, where the X (0.5%, 1.0%, 2.0%, 4.0%, 8.0%)
represented the weight of PPy versus the weight of ZnIn2S4. The similar procedure without adding PPy powder was adopted to prepare pure ZnIn2S4.
2.3. Photocatalytic activity
The visible light photocatalytic activity of the prepared samples was conducted by degradation of CHL and MB. The photocatalytic reaction was carried out in a 500ml beaker with constant magnetic stirring. A 100W iodine-gallium lamp (PHILIPS) with light intensity of 50mW·cm-2 was positioned horizontally above the beaker and the distance between the beaker and the lamp was 10cm. The initial concentration of MB was 10mg·l-1 and photocatalyst dosage for MB degradation was 0.1g·l-1. The initial concentration of 30mg·l-1 and 0.3g·l-1 photocatalyst dosage were applied in degradation of CHL. A 400ml reaction solution was adopted in this experiment. For the degradation of MB, the adsorption and photocatalytic experiment was separated and each experiment lasted 120min. For CHL degradation, the suspension was maintained in dark for 30min prior to irradiation and then was continuously irradiated for 180min. Samples were withdrawn at specific time intervals and then filtered through 0.22μm membrane for further analysis. After 180min degradation of CHL, the reaction solution was collected by suction filtration using 0.22μm membrane in order to evaluate the mineralization of CHL via a total organic carbon analyzer (Shimadzu, TOC-L PCN).
The degradation of MB was
evaluated by UV spectrophotometer (Unic, UV-2102C) at a wavelength of 633nm. The concentration of CHL was determined by high performance liquid chromatography, which is the same as in our reported research [26].
2.4. Determination of reactive oxygen species and photocatalytic intermediates
Ethylenediamine tetraacetic acid (EDTA) [27], isopropanol (IPA) and p-benzoquinone (BQ) [28] with concentration of 1mmol·l-1 were served as trapping reagent to capture hole (h+), hydroxyl radical (·OH) and superoxide radical (·O2-), respectively. One of the above trapping reagents was added into 400ml CHL solution and then 120mg photocatalyst (ZnIn2S4 or PPy-ZnIn2S4) was added after homogeneous mixing. The following step was the same as the photocatalytic experiment including 30min adsorption and 180min photocatalytic experiment. The electron paramagnetic resonance (EPR) signals of spin adduct formed by the reaction of reactive oxygen species and BMPO were recorded with a Bruker EMM2098 spectrometer, operating with microwave frequency of 9.85GHz and power of 6.32mW. 50μl of photocatalyst (ZnIn2S4 or PPy-ZnIn2S4) with concentration of 0.3g·l-1 and 30μl of BMPO with concentration of 80mmol·l-1 were mixed together to prepare samples for EPR analysis. The samples were analyzed immediately by EPR after illumination for 1min under 100W iodine-gallium lamp. The intermediates formed during photocatalytic degradation of CHL by PPy-ZnIn2S4 at specific time of 30min, 60min, 120min and 180min were qualitative detected using high performance liquid chromatograph/mass spectrometer (LCMS-IT-TOF, Shimadzu). The analysis method and used instruments of LCMS were the same as our reported research [26].
3. Results and discussion
3.1. Characterization of photocatalyst
X-ray photoelectron spectroscopy (XPS, Escalab250Xi, Thermo Scientific) was performed to determine the surface chemical composition and electronic state of ZnIn2S4 and PPy-ZnIn2S4 (Figure 1). From the survey, characteristic binding energy of 443.68eV and 451.08eV for In3d, 1020.58eV and 1044.18eV for Zn2p and 161.18eV for S2p were detected over pure ZnIn2S4 and PPy-ZnIn2S4 composite, and no obvious binding energy shift was observed. No obvious peak of N1s was observed, but the atomic ratio showed the existence of N1s (0.47%) in PPy(4%)-ZnIn2S4 composite. The interference peak of C1s showed up both in ZnIn2S4 and PPy-ZnIn2S4, so that it was hard to confirm the presence of C in α, β position of PPy. However, the atomic ratio showed that the content of C1s (55.09%) in PPy-ZnIn2S4 was more than that in ZnIn2S4 (50.86%), which can indirectly explain that the C in PPy was existed in the composite photocatalyst. The atomic ratio of Zn, In, S in the surface of pure ZnIn2S4 was 1: 2.5: 5, the In/S ratio was ½, but the Zinc content seemed to be a little lower than expected value (ZnIn2S4). XPS is a surface analysis and the ratio does not represent the information in the bulk. In some way, this information can indirectly suggest that there were Zn vacancies in the surface of ZnIn2S4. The atomic ratio of Zn, In, S in PPy-ZnIn2S4 composite was changed to 1: 2.8: 5.6, while the In/S ratio remained to be ½, but much lower Zinc was observed, which suggested that the existence of PPy powder during synthesis of ZnIn2S4 resulted in more Zn vacancies in the surface of composite photocatalyst. The X-ray diffraction (XRD) patterns (UltimanIV, Rigaku) of ZnIn2S4 and PPy-ZnIn2S4 in Figure 2 were obtained to determine the crystal structure. All the diffraction peaks of 2θ values at 21.5°, 27.6°, 32.4°, 47.2°, 52.4° and 55.6° corresponding to the (006), (102), (105), (110), (116) and (202) in pure ZnIn2S4 can be indexed as hexagonal phase (JCPDS. No. 01-072-0773) [29]. Some small changes
were observed from XRD diffraction peaks in PPy-ZnIn2S4 composite photocatalyst. Except some peaks of hexagonal phase, PPy-ZnIn2S4 composite photocatalyst showed additional peaks at 2θ=14.4°, 44.2°, 60.3°, 67.7° and 70.9°, which can be assigned to the (111), (511), (444), (731) and (800) crystallographic planes of cubic ZnIn2S4 (JCPDS. No. 00-048-1778) [30]. That is to say, the addition of PPy powder in the precursor solution can affect formation of crystal structure during the synthesis process and result in coexistence of hexagonal and cubic ZnIn2S4 in composite PPy-ZnIn2S4 photocatalyst. The characteristic peak of PPy at 2θ=20-30° was not observed in the composite photocatalyst, which might be attributed to the low amount of PPy. Figure 3 presented the FT-IR spectra (IRPrestige-21, Shimadzu) of ZnIn2S4 and PPy-ZnIn2S4. Two peaks at ~1396cm-1 and ~1610cm-1 for both samples probably corresponded to the adsorbed water molecules and hydroxyl groups on photocatalyst [31]. The spectrum of PPy showed characteristic bands at 1555cm-1, 1478cm-1, 1299cm-1, 1174cm-1, 1032cm-1 and 917cm-1. As expected, the spectrum of PPy-ZnIn2S4 composite clearly exhibited characteristic bands at 917cm-1, 1032 cm-1 and 1174cm-1 attributed to PPy [32], which indicated the existence of PPy in this composite. Scanning electron microscope (SEM, JSM-6510LV, JEOL, Ltd) and transmission electron microscope (TEM, JEM2100) equipped with an energy dispersive spectrometer (EDS) were applied to observe the microstructures of as-prepared samples. As shown by SEM images in Figure 4, unlike the morphology of ZnIn2S4 prepared by hydrothermal method in our reported research [33], the lamellar microsphere structure was not observed in both ZnIn2S4 and PPy-ZnIn2S4 samples. An amorphous blocky with lamellar structure was existed in these two samples, which might be attributed to the magnetic stirring during the hydrothermal reaction. Static
hydrothermal reaction was adopted in the previous research, so microsphere morphology was observed in the samples. In order to remain the PPy powder suspended, magnetic stirring was applied during hydrothermal reaction, which might result in collapse of microsphere and formation of lamellar blocky structure. The size of the amorphous blocky ranged from 3-5μm, which had the similar size with the microsphere synthesized previously [33]. The SEM spectrum of PPy showed uniform granules and these granules can be observed in PPy-ZnIn2S4 composite sample. The similar results can be obtained from TEM images in Figure 5. Except the similar amorphous sheets in ZnIn2S4 sample, some leaf-like fibers were observed in PPy-ZnIn2S4. As shown in EDS elemental mapping, three main feature elements of Zn, In and S were detected in ZnIn2S4 sample and an additional element of N was also detected in PPy-ZnIn2S4 sample. The Brunauer-Emmett-Teller (BET) surface areas were measured by nitrogen adsorption and desorption (V-Sorb2800P, GAPP, China) to evaluate the specific surface area of prepared ZnIn2S4 and PPy-ZnIn2S4. The nitrogen adsorption-desorption isotherms of ZnIn2S4 and PPy-ZnIn2S4 in Figure 6a were characteristic of a type Ⅳ isotherm with a hysteresis loop, which indicated that mesoporous structures existed in both samples. Almost no change of nitrogen adsorption-desorption isotherm for PPy-ZnIn2S4 was observed comparing with ZnIn2S4. From the isotherm, the capillary condensation appeared only when the pressure was close to saturated vapor pressure, which indicated that the hysteresis loop curve can be regarded as the H4 type [34]. It turned out that silt pores with parallel plate structure were existed in both photocatalysts, which was consistent with the lamellar structure of photocatalysts observed in SEM. Both ZnIn2S4 and PPyZnIn2S4 had similar BET specific surface areas, which were 150 m2·g-1 and 142 m2·g-1
for ZnIn2S4 and PPy-ZnIn2S4, respectively. The corresponding pore size distribution curve can be obtained from adsorption and desorption data and sometimes probably gave different results as in Figure 6b and Figure 6c. The peak at 4nm always appears, which is considered as a BJH false peak mainly caused by the connectivity of the internal pores, so that adsorption data in Figure 6b was used to determine the pore size distribution and showed an average pore size of 2-7nm. Moreover, the peak at ~100nm appeared in both Figure 6b and Figure 6c, indicating there were 100nm pores existed in the material since the structure in Figure 4 showed size ranging from 3 to 5μm. Figure 7a showed the typical UV-Vis diffuse reflectance spectra (SU 3900) of the as-synthesized pure ZnIn2S4 and PPy and PPy-ZnIn2S4 composite powder. Both had wide range absorbency from UV to visible region. It can be found that the absorbance within visible light range was enhanced by PPy-ZnIn2S4 composite compared with pure ZnIn2S4, which was in agreement with the sample color from yellow (ZnIn2S4) to dark green (PPy-ZnIn2S4). Previous research indicated that the band gap of hexagonal ZnIn2S4 was estimated based on direct-allowed transition [35, 36]. As shown in Figure 7b, the band gap of as-prepared samples based on direct-allowed transition can be estimated by plotting of (αhν)2 with respect to hν [36]. A classical extrapolation method was adopted to get the band gap by extrapolating the straight line to the horizontal axis (photon energy axis hν). The band gap (Eg) of pure ZnIn2S4 and PPy-ZnIn2S4 calculated in this way were 2.2eV and 2.0eV, respectively. According to the calculation in our recent research [37], the conduction band (CB) and valence band (VB) potential for pure ZnIn2S4 were estimated to be -0.78eV and 1.42eV, respectively. The highest occupied molecular orbital (HOMO) level of PPy (1.05eV) [38] was lower than that of ZnIn2S4 (1.42eV), which might be beneficial to
the charge separation and transfer demonstrated in Scheme 1.
3.2. Photocatalytic activity of ZnIn2S4 and PPy-ZnIn2S4 composite
Degradation of dye was a simple way to evaluate the photocatalytic activity of photocatalyst and MB was selected as one target organic contaminant in order to optimize the content of PPy in composite photocatalyst. From the results (Figure S1), the composite photocatalyst exhibited the best photocatalytic degradation of MB when the weight ratio of PPy versus ZnIn2S4 was 4%. So PPy(4%)-ZnIn2S4 composite photocatalyst was chosen for degradation of chloramphenicol. Besides the degradation rate, mineralization of CHL was also taken into account in the following experiment. ZnIn2S4 and PPy-ZnIn2S4 showed poor adsorption performance for the removal of CHL and no obvious removal of CHL was observed after 30min adsorption. Figure 8 showed the decrease of CHL concentration vs. light irritation time on the photocatalysts of PPy-ZnIn2S4 and ZnIn2S4 samples. It seemed that there was little difference in the first 30min, but the time required for complete removal of CHL by PPy-ZnIn2S4 composite (60min) was shorter than pure ZnIn2S4 (120min), indicating better deep degradation of CHL. Based on our previous research, the degradation of CHL started from the cleavage of C-N bonds and then some intermediates with stronger chemical bonds including C-OH, C=O, C-C and benzene ring were produced. It is even more difficult to decompose these intermediates. It is relatively easy for the preliminary degradation of CHL, so the degradation in the initial 30min was fast and the difference between two photocatalyst was small. Therefore, besides detection of CHL concentration, mineralization of CHL was another important evaluation criterion
for photocatalytic activity. To some extent, PPy-ZnIn2S4 composite photocatalyst can improve the mineralization of chloramphenicol. From the inset of Figure 8, 48.5% TOC was achieved by PPy-ZnIn2S4 after 3h degradation, which was two times higher than ZnIn2S4 (23.5%). Although the TOC removal was not high enough, it was noticeable for visible light photocatalysis, especially for degradation of persistent antibiotic chloramphenicol. The intermediates generated in photocatalytic degradation of chloramphenicol by ZnIn2S4 were detected and the degradation pathways were discussed in our published research [26]. Thus, only the degradation pathways on PPy-ZnIn2S4 were discussed in detail in the following section.
3.3. Identification of intermediates formed by PPy-ZnIn2S4 composite photocatalyst
Identification of intermediates formed during photocatalytic degradation of CHL can give evidence for intensive study of photocatalytic degradation pathways. The intermediate products resulting from photocatalytic degradation of CHL by PPy-ZnIn2S4 at 30min, 60min, 120min and 180min were analyzed. The single mass spectra of detected intermediates were shown in support information and the formation and disappearance of each intermediate at different reaction time were listed in Table S1. The identification of intermediates was performed with the information of mass-to-charge ratio (m/z) by loss or gain a proton. The molecular formulas, molecular mass, mass-to-charge ratio (m/z) of detected fifteen intermediates were summarized in Table 1. Based on these data, a possible photocatalytic degradation pathway of CHL by PPy-ZnIn2S4 including radical reaction, ring open and final mineralization was proposed in Figure 9. According to Table S1, three protonated intermediate products corresponding to m/z of 94, 115, 130 and three
deprotonated ones with m/z 138, 197, 203 were detected after 30min. Because the dissociation energies of phenyl-nitryl, C-OH, C-Cl and C-N bonds were weaker than other chemical bonds in CHL [39], the cleavage of these chemical bonds was occurred in the first place. After 60min, the characteristic peak of CHL was vanished and subsequently decomposed to smaller and simpler compounds. The ring-opening product began to appear after 120min, which would be beneficial to the mineralization of CHL. Four kinds of simpler compounds with protonated m/z of 82, 91, 109 and 156 were still existed in the photocatalytic reaction solution after 180min. Comparing with the formed intermediates for CHL degradation by ZnIn2S4 in our previous study [26], fewer and simpler intermediates were detected in PPy-ZnIn2S4 system. The ring-opening product appeared after 180min and ten kinds of intermediates existed in the solution after 180min reaction in ZnIn2S4 system. That is the reason for increased TOC removal rate of CHL degradation by PPy-ZnIn2S4.
3.4. Determination of reactive species
In order to determine the contributing reactive species, a series of radical trapping experiments were conducted in the photocatalytic system containing ZnIn2S4 and PPy-ZnIn2S4, respectively. Some trapping reagents were organic chemicals, which could have some interference on the measurement of TOC, so that only the concentration of chloramphenicol was analyzed. From Figure 10a, the photocatalytic degradation of CHL was slightly inhibited when isopropanol was added, which suggested that hydroxyl radical (·OH) gave some contribution to photocatalytic degradation of CHL in ZnIn2S4 system. While photocatalytic activity was significantly suppressed and only 73% CHL was removed after 180min photocatalytic
reaction when superoxide radical (·O2-) was trapped, which declared that superoxide radical was a dominating reactive radical in ZnIn2S4 system. For PPy-ZnIn2S4 photocatalytic system, trapping holes and hydroxyl radical had slight influence on photocatalytic activity and similar results as in ZnIn2S4 photocatalytic system were obtained after adding ·O2- trapping reagent (Figure 10b). That is to say, ·O2- remained to be the dominating active species in photocatalytic degradation of CHL by PPy-ZnIn2S4. It was found that the conduction band of ZnIn2S4 was negative enough that photogenerated electrons can reduce dissolved oxygen to produce superoxide radical ·O2- (Eo(O2/·O2-)=-0.33 V vs NHE) [40]. Thus, it is reasonable that ·O2- was detected as the main reactive species. On the other hand, hydroxyl radical (·OH) cannot be generated directly because of band position of ZnIn2S4 [40]. Our experimental results indicated that the ·OH was produced indirectly via superoxide dependent mechanism [41]. There is one phenomenon should be noticed from Figure 10 that the inhibition of photocatalytic activity in PPy-ZnIn2S4 system was diminished comparing with that in ZnIn2S4 system, indicating the formation of a higher concentration of reactive radicals (·O2- and ·OH) in PPy-ZnIn2S4 system. Because that the concentration of trapping reagent was 1mmol·l-1 in both system and there were spare reactive radicals after in-situ capture by scavengers. The direct detection of oxygen species formation by EPR would help to prove which reactive oxygen species were involved in the photocatalytic reaction. The BMPO spectra in Figure 11 with four main peaks associated with some fine structure were characteristic of BMPO/·O2and BMPO/·OH radicals [41, 42]. Previous research [43] claimed that light illumination could convert ·O2- adduct to ·OH adduct. And also our previous study [41] have proved that the BMPO/·O2- was reduced to BMPO/·OH under visible light system and the ·OH formation in ZnIn2S4 system was via superoxide radical (·O2-).
The intensity of the signal was a little higher than that of ZnIn2S4, indicating more reactive oxygen radicals were generated by PPy-ZnIn2S4.
4. Conclusions A series of PPy-ZnIn2S4 composites with different amounts of PPy were prepared in this study. There was obvious difference in crystalline phase and optical property between PPy-ZnIn2S4 and pure ZnIn2S4 photocatalyst, which the cubic phase was appeared and absorbance within visible light range was enhanced in PPy-ZnIn2S4. PPy-ZnIn2S4 showed the best photoactivity when the content of PPy was 4% by weight, which shortened the reaction time for complete removal of chloramphenicol and doubled the mineralization rate. The modification of ZnIn2S4 with polypyrrole provided a good candidate for improving photocatalytic activity, especially the mineralization efficiency. The cleavage of phenyl-nitryl, C-OH, C-Cl and C-N bonds was occurred in the first place for photocatalytic degradation of chloramphenicol by PPy-ZnIn2S4.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (51708442, 51608428), Natural Science Foundation of Shaanxi province (2017JQ5019) and the science and technology foundation for talents and young researcher from Xi’an University of Architecture and Technology (Project No. RC1441, RC1440, QN1515 and QN1516).
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Figure captions Figure 1. XPS spectra of ZnIn2S4 and PPy(4%)-ZnIn2S4, including expanded spectra of In3d, Zn2p and S2p. Figure 2. XRD patterns of ZnIn2S4, PPy(4%)-ZnIn2S4 photocatalyst and pure PPy. Figure 3. FTIR spectra of ZnIn2S4, PPy(4%)-ZnIn2S4 photocatalyst and pure PPy. Figure 4. SEM images of ZnIn2S4, PPy(4%)-ZnIn2S4 photocatalyst and pure PPy. Figure 5. TEM images, HRTEM images and EDS spectra of ZnIn2S4 and PPy(4%)-ZnIn2S4. Figure 6. (a) Nitrogen adsorption-desorption isotherms of ZnIn2S4 and PPy(4%)-ZnIn2S4; (b) The pore size distribution curves related to adsorption data; (c) The pore size distribution curves related to desorption data; Figure 7. (a) UV-Vis diffuse reflectance spectra of PPy, ZnIn2S4 and PPy(4%)-ZnIn2S4; (b) Plots of (αhν)2 with respect to photon energy (hν). Figure 8. Photodegradation of chloramphenicol by ZnIn2S4 and PPy(4%)-ZnIn2S4 (Inset: TOC removal rate); Figure 9. The proposed reaction pathways of photocatalytic degradation of CHL by PPy(4%)-ZnIn2S4. Figure 10. Photodegradation of chloramphenicol in the presence of EDTA, Isopropanol and BQ, (a) In ZnIn2S4 system; (b) In PPy-ZnIn2S4 composite system. Figure 11. BMPO spin-trapping EPR spectra from ZnIn2S4 and PPy-ZnIn2S4 under the irradiation of visible-light.
Figure 1
PPy (4%)-ZnIn2S4
O1 s
Zn 2p
In3d S2p C1s
Zn2p
PPy (4%)-ZnIn2S4
ZnIn2S4 At.%: Zn2p=4.73 In3d=12.17 S2p=23.63 C1s=50.86 O1s=8.63
Counts (a. u.)
O1 s
S2p C1s
Counts (a. u.)
Zn 2p
In3d
Survey
ZnIn2S4 ZnIn2S4
0
400
800
In3d
1200
1020
PPy (4%)-ZnIn2S4
ZnIn2S4
155
450 460 Binding Energy / E (eV)
160 165 170 175 Binding Energy / E (eV)
Figure 2 * hexagonal phase # cubic phase (102)
* *
(116)
(105)
*
(202)
(006)
(110)
PPy *
ZnIn2S4
* *
JCPDS-01-072-0773
* #
*
(511)
#
*
PPy(4%)-ZnIn2S4 (444)
* #
(731) (800)
*
(111)
# #
JCPDS-00-048-1778
10
20
30
40 50 o 2 Theta / degree ( )
At.%: Zn2p=3.91 In3d=11.1 S2p=21.97 N1s=0.47 C1s=55.09 O1s=7.46
PPy (4%)-ZnIn2S4
Counts (a. u.)
Counts (a. u.)
ZnIn2S4
1060
S2p
PPy (4% )-ZnIn2S4
440
1040
60
70
80
180
Figure 3 90
80
100 1032 1478 1299
PPy
95
T(%) 60
90
PPy (4% )-ZnIn2S4
85
917 1174
50
80 1396
40 500
Figure 4
1000
1610
1500
2000 2500 3000 -1 Wavenumber (cm )
ZnIn2S4 3500
75 4000
T(%)
1555
70
Figure 5
Figure 6 Volume (cm3 g-1)
100
a
PPy(4%)-ZnIn2S4 ZnIn2S4
80
60
40 0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
b
Adsorption
-1 Pore volume (ml g )
0.016
c
0.04
Desorption
ZnIn2S4
ZnIn2S4
PPy(4%)-ZnIn2S4
PPy(4% )-ZnIn2S4 0.02
0.008
0.00
0.000 1
10 Pore size (nm)
100
1
10 Pore size (nm)
100
Figure 7 1.5
40
a
b PPy
30 2
PPy(4%)-ZnIn2S4
ah
Abs
1.0
0.5
20
PPy-ZnIn2S4
10 ZnIn2S4
0.0 200
ZnIn2S4 0
400 600 Wavelength (nm)
800
1
2
3
h (eV)
4
5
6
Figure 8
60
0.8 0.6
48.5%
40 23.5% 20 0
0.4
ZnIn2S4
PPy-ZnIn2S4
ZnIn2S4
0.2 0.0 -30
Figure 9
TOC removal (%)
Chloramphenicol (C/C0)
1.0
PPy-ZnIn2S4 0
30
60 90 120 Reaction time (min)
150
180
Figure 10 1.0
Chloramphenicol (C/C0)
1.0
PPy-ZnIn2S4
ZnIn2S4 0.8
0.8 No quencher EDTA Isopropanol BQ
0.6
0.6
0.4
0.4
0.2
0.2
b
a 0.0 -30
Figure 11
No quencher EDTA Isopropanol BQ
0
30
60 90 t (min)
120
150
0.0 180 -30
0
30
60 90 t (min)
120
150
180
PPy-ZnIn2S4 ZnIn2S4
3480
3500
3520 Field
3540
3560
Scheme 1. The proposed photocatalytic mechanism over PPy-ZnIn2S4 composite photocatalyst.
Scheme 1
Table legends Table 1. Some researches related to the photodegradation of chloramphenicol. Table 1 Dosage
Light source
Time
Initial concentration
Efficiency
TiO2
0.94g/l
300W medium pressure mercury lamp (λ≥365nm)
60min
19.97mg/l
85.97%
[18]
TiO2, ZnO, TiONa
1.0g/l
9W Osram Dulux lamp
90min
50mg/l
ZnO 90% TiO2 37% TiONa 23%
[19]
ZnO Film
----
240min
8mg/l
40%
[20]
TiO2
1.6g/l
120min
25mg/l
100%
[21]
24h
10mg/l
97%
[22]
50min
20mg/l
99%
[23]
120min
20mg/l
61%
[24]
140min
15mg/l
66.2%
[25]
Catalyst
SrFeO3−x/gC3N4 Ce(MoO4)2/ GO ATP/Cu2O/ Cu/g-C3N4 Fe3+/Er3+ co-doped Bi5O7I
0.1g/l 0.5g/l 0.08g/l 1.0g/l
18W UV lamp (315-400nm) 450W Xenon arc lamp 150W Xenon arc lamp 500W tungsten lamp 300W Xenon lamp 400W halogen lamp
Table 2
Table 2. Intermediates identified by LCMS-IT-TOF.
Code P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15
Molecular formula C11H11NO3 C9H11NO4 C6H5NO3 C2H2O2Cl2 C2H2OCl2 C2H4NOCl C11H14NO3Cl C9H10O C10H9NO2 C7H6O2 C9H9NO C6H4O2 C6H5NO4 C5H6O C2H2O4
Molecular mass 205 197 139 129 114 94 243 134 175 122 147 108 155 82 90
m/z Positive ion ([M+H]+) —— —— —— 130.07 115.12 94.05 —— 135.00 —— 122.55 —— 109.05 156.04 81.52 90.97
Negative ion ([M-H]-) 203.97 197.96 138.02 —— —— —— 242.96 —— 174.00 —— 146.99 —— —— —— ——
S1010-6030(17)31109-7 http://dx.doi.org/10.1016/j.jphotochem.2017.09.018 JPC 10863
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
29-7-2017 1-9-2017 5-9-2017
Please cite this article as: Bo Gao, Weiping Chen, Shaonan Dong, Jiadong Liu, Tingting Liu, Lei Wang, Mika Sillanp¨aa¨ , Polypyrrole/ZnIn2S4 composite photocatalyst for enhanced mineralization of chloramphenicol under visible light, Journal of Photochemistry and Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.09.018 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Polypyrrole/ZnIn2S4 composite photocatalyst for enhanced mineralization of chloramphenicol under visible light Bo Gaoa, b*, Weiping Chena, Shaonan Donga, JiadongLiua, TingtingLiua, Lei Wanga, Mika Sillanpääb, c a
Key Laboratory of Membrane Separation of Shaanxi Province, School of Environmental and
Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, PR China b
Laboratory of Green Chemistry, Faculty of Technology, Lappeenranta University of
Technology, Sammonkatu 12, FIN-50130 Mikkeli, Finland c
Department of Civil and Environmental Engneering, Florida International University, Miami,
FL-33174, USA Corresponding
author.Tel. +358 0503368529; +86 13689296473;
E-mail address: [email protected] (B. Gao).
Graphical abstract
Highlights
Mineralization of chloramphenicol is enhanced by PPy-ZnIn2S4 composite.
More reactive oxygen radicals are produced by PPy-ZnIn2S4 composite.
The coexistence of hexagonal and cubic phase in composite is advantageous to charge separation.
ABSTRACT Functional PPy-ZnIn2S4 composite was prepared via facile hydrothermal method in the presence of PPy powder. The X-ray diffraction (XRD) pattern showed single hexagonal phase in pure ZnIn2S4 and mixed phases of hexagonal and cubic in
PPy-ZnIn2S4 composite. The coexistence of hexagonal and cubic phase in PPy-ZnIn2S4 composite can be advantageous to charge separation. The absorbance within visible light range was enhanced by PPy-ZnIn2S4 composite compared with pure ZnIn2S4. The time required for complete removal of chloramphenicol by PPy-ZnIn2S4 composite (60min) was shorter than pure ZnIn2S4 (120min), indicating better deep degradation. 48.5% TOC was achieved by PPy-ZnIn2S4 after 3h photocatalytic degradation, which was two times higher than ZnIn2S4 (23.5%).
Keywords:Ternary sulfide particles; Photocatalysis; Conducting polymer; Antibiotics
1. Introduction
The ternary metallic sulfide with narrow band gap had been extensively studied as visible light photocatalyst [1, 2]. However, the electrons and holes generated by photoexcitation of single semiconductor can easily recombine as a result of the random motion of photo-generated carriers, which significantly limited the photocatalytic efficiency under visible light. The modification of ZnIn2S4 in order to prevent the recombination had become a research hotspot. Recently, effects of graphene and carbon analogs for modification of ZnIn2S4 were contrastively studied to enhance charge separation [3]. An electron transport bridge was built up in MoS2-graphene cocatalyst and resulted in effective charge transfer in ZnIn2S4 [4]. It was reported that a hybrid organic-inorganic TiO2 nanocomposite could enhance photocatalytic properties [5]. Therefore, functional modification according to the property of photocatalyst can achieve improvement in photocatalytic activity. Polypyrrole (PPy), as a commonly used conductive polymer, has an extended
π-conjugated electron system, which possesses high stability and relatively high carrier mobility [6]. It is reported that PPy could serve as effective electron donor and stable photosensitizer, and integration of PPy with semiconductor could improve the photocatalytic activity [7]. The photocatalytic activity of Bi2WO6 was enhanced by ‘in situ’ deposition of polyprrole [6]. The PPy on the surface of photocatalyst could realize the directional separation of carrier and inhibit the recombination of electrons and holes. PPy can form a layer of conductive network on the surface of CdS, which can promote the separation of carrier and it also can suppress CdS from photocorrosion, so that coupled semiconductor PPy/CdS greatly improved the photocatalytic hydrogen production [8]. Based on the photosensitization of PPy, composite semiconductor PPy/TiO2 can overcome the defect of low visible light utilization of TiO2, and thus further enhance the photocatalytic degradation of organic compounds [9, 10] and hydrogen production [11] under visible light. And the synergetic effect of adsorption and visible light photocatalysis was observed on Ag2O/TiO2/PPy composite during removal of methylene blue [12]. Also, the transportation of photogenerated charges was investigated on composite semiconductors α-Fe2O3@PPy [13] and Fe3O4@PPy [14]. Unlike above-mentioned composite semiconductors, photoelectrons were trapped by conductive PPy in AgCl/PPy photocatalyst [15] and then transferred to the adsorbed O2 to form the active oxygen species. On the other hand, holes were scavenged by Cl- to form active oxidation production, thereby, the separation of photogenerated electron-hole pairs can be realized. To sum up, PPy has flexible and tunable properties, which can serve as effective electron donor, electron transporter and holes acceptor and transporter. PPy may work differently in different composite photocatalyst, as there is always a synergetic effect between PPy and semiconductor. So PPy is expected to be an
efficient mediator for the modification of semiconductor to enhance photoactivity under visible light. The aim of the present work is to explore functional modification of ZnIn2S4 with PPy in order to enhance the photocatalytic activity of ZnIn2S4. Composite photocatalyst combining ZnIn2S4 and PPy in the present study was synthesized through a facile hydrothermal method in the presence of PPy powder. The influence of addition PPy powder in the precursor solution on morphology, crystal structure and photocatalytic activity was investigated in detail. The detection of concentration up to 28 ngl-1 of chloramphenicol in urban water supplies of Shanghai (China) [16] had received increasing concern. The antibacterial property of chloramphenicol had prevented the removal efficiencies by conventional biological treatment process [17]. The photocatalytic degradation of chloramphenicol might provide a good option. Some researches related to the photodegradation of chloramphenicol were summarized in Table 1 [18-25], indicating that the photocatalytic degradation efficiency and mineralization of chloramphenicol under visible light needed to be further improved. Therefore, the degradation and mineralization of chloramphenicol was investigated in this research.
2. Materials and Methods
2.1. Chemicals
Unless otherwise specified, the chemicals used in this experiment were of analytical purity and used without further purification. The selected target organic pollutant chloramphenicol (CHL) and methylene blue (MB) was purchased from Aladdin® and Sinopharm Chemical Reagent Co., Ltd. The methanol was
chromatographically pure. The pyrrole (Py) and ammonium persulfate (APS, (NH4)2S2O8) were used for polymerization of polypyrrole (PPy). Ethylenediamine tetraacetic acid (C10H16N2O8), isopropanol ((CH3)2CHOH) and p-benzoquinone (C6H4O2, Aladdin®) were used as trapping reagent. 5-tert-Butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO) was purchased from Dojindo Molecular Technologies. The other reagents for the preparation of ZnIn2S4 photocatalyst such as Zn(NO3)2·6H2O, CH3CSNH2 (TAA) and In(NO3)3·5H2O, were the same as our previous research [26].
2.2. Preparation of ZnIn2S4 and PPy-ZnIn2S4 photocatalyst
Two steps were involved in the preparation of PPy-ZnIn2S4 photocatalyst. First of all, PPy was synthesized via liquid-phase polymerization of pyrrole. In a typical and simple procedure, 13.07g (NH4)2S2O8 was dissolved in 1000ml deionized water and then 4ml of pyrrole was added in the above solution to start the polymerization reaction in ice water bath under magnetic stirring. After 2h, the black product PPy was collected by filtration, and then washed twice with deionized water and finally dried at 80℃. After grinding and sieving through 300 mesh sieve, the PPy powder was used for the subsequent procedure. A certain amount of prepared PPy powder was added into precursor solution of ZnIn2S4 (including 0.5mmol Zn(NO3)2·6H2O, 1.0mmol In(NO3)3·5H2O and an excessive (4mmol) CH3CSNH2) before transferring to the autoclave. Then the autoclave was sealed and kept in 80℃ water bath with constant stirring for 6h. The product was collected by suction filtration and washed twice with deionized water. Different amount of PPy in the composite photocatalyst was marked as PPy (X)-ZnIn2S4, where the X (0.5%, 1.0%, 2.0%, 4.0%, 8.0%)
represented the weight of PPy versus the weight of ZnIn2S4. The similar procedure without adding PPy powder was adopted to prepare pure ZnIn2S4.
2.3. Photocatalytic activity
The visible light photocatalytic activity of the prepared samples was conducted by degradation of CHL and MB. The photocatalytic reaction was carried out in a 500ml beaker with constant magnetic stirring. A 100W iodine-gallium lamp (PHILIPS) with light intensity of 50mW·cm-2 was positioned horizontally above the beaker and the distance between the beaker and the lamp was 10cm. The initial concentration of MB was 10mg·l-1 and photocatalyst dosage for MB degradation was 0.1g·l-1. The initial concentration of 30mg·l-1 and 0.3g·l-1 photocatalyst dosage were applied in degradation of CHL. A 400ml reaction solution was adopted in this experiment. For the degradation of MB, the adsorption and photocatalytic experiment was separated and each experiment lasted 120min. For CHL degradation, the suspension was maintained in dark for 30min prior to irradiation and then was continuously irradiated for 180min. Samples were withdrawn at specific time intervals and then filtered through 0.22μm membrane for further analysis. After 180min degradation of CHL, the reaction solution was collected by suction filtration using 0.22μm membrane in order to evaluate the mineralization of CHL via a total organic carbon analyzer (Shimadzu, TOC-L PCN).
The degradation of MB was
evaluated by UV spectrophotometer (Unic, UV-2102C) at a wavelength of 633nm. The concentration of CHL was determined by high performance liquid chromatography, which is the same as in our reported research [26].
2.4. Determination of reactive oxygen species and photocatalytic intermediates
Ethylenediamine tetraacetic acid (EDTA) [27], isopropanol (IPA) and p-benzoquinone (BQ) [28] with concentration of 1mmol·l-1 were served as trapping reagent to capture hole (h+), hydroxyl radical (·OH) and superoxide radical (·O2-), respectively. One of the above trapping reagents was added into 400ml CHL solution and then 120mg photocatalyst (ZnIn2S4 or PPy-ZnIn2S4) was added after homogeneous mixing. The following step was the same as the photocatalytic experiment including 30min adsorption and 180min photocatalytic experiment. The electron paramagnetic resonance (EPR) signals of spin adduct formed by the reaction of reactive oxygen species and BMPO were recorded with a Bruker EMM2098 spectrometer, operating with microwave frequency of 9.85GHz and power of 6.32mW. 50μl of photocatalyst (ZnIn2S4 or PPy-ZnIn2S4) with concentration of 0.3g·l-1 and 30μl of BMPO with concentration of 80mmol·l-1 were mixed together to prepare samples for EPR analysis. The samples were analyzed immediately by EPR after illumination for 1min under 100W iodine-gallium lamp. The intermediates formed during photocatalytic degradation of CHL by PPy-ZnIn2S4 at specific time of 30min, 60min, 120min and 180min were qualitative detected using high performance liquid chromatograph/mass spectrometer (LCMS-IT-TOF, Shimadzu). The analysis method and used instruments of LCMS were the same as our reported research [26].
3. Results and discussion
3.1. Characterization of photocatalyst
X-ray photoelectron spectroscopy (XPS, Escalab250Xi, Thermo Scientific) was performed to determine the surface chemical composition and electronic state of ZnIn2S4 and PPy-ZnIn2S4 (Figure 1). From the survey, characteristic binding energy of 443.68eV and 451.08eV for In3d, 1020.58eV and 1044.18eV for Zn2p and 161.18eV for S2p were detected over pure ZnIn2S4 and PPy-ZnIn2S4 composite, and no obvious binding energy shift was observed. No obvious peak of N1s was observed, but the atomic ratio showed the existence of N1s (0.47%) in PPy(4%)-ZnIn2S4 composite. The interference peak of C1s showed up both in ZnIn2S4 and PPy-ZnIn2S4, so that it was hard to confirm the presence of C in α, β position of PPy. However, the atomic ratio showed that the content of C1s (55.09%) in PPy-ZnIn2S4 was more than that in ZnIn2S4 (50.86%), which can indirectly explain that the C in PPy was existed in the composite photocatalyst. The atomic ratio of Zn, In, S in the surface of pure ZnIn2S4 was 1: 2.5: 5, the In/S ratio was ½, but the Zinc content seemed to be a little lower than expected value (ZnIn2S4). XPS is a surface analysis and the ratio does not represent the information in the bulk. In some way, this information can indirectly suggest that there were Zn vacancies in the surface of ZnIn2S4. The atomic ratio of Zn, In, S in PPy-ZnIn2S4 composite was changed to 1: 2.8: 5.6, while the In/S ratio remained to be ½, but much lower Zinc was observed, which suggested that the existence of PPy powder during synthesis of ZnIn2S4 resulted in more Zn vacancies in the surface of composite photocatalyst. The X-ray diffraction (XRD) patterns (UltimanIV, Rigaku) of ZnIn2S4 and PPy-ZnIn2S4 in Figure 2 were obtained to determine the crystal structure. All the diffraction peaks of 2θ values at 21.5°, 27.6°, 32.4°, 47.2°, 52.4° and 55.6° corresponding to the (006), (102), (105), (110), (116) and (202) in pure ZnIn2S4 can be indexed as hexagonal phase (JCPDS. No. 01-072-0773) [29]. Some small changes
were observed from XRD diffraction peaks in PPy-ZnIn2S4 composite photocatalyst. Except some peaks of hexagonal phase, PPy-ZnIn2S4 composite photocatalyst showed additional peaks at 2θ=14.4°, 44.2°, 60.3°, 67.7° and 70.9°, which can be assigned to the (111), (511), (444), (731) and (800) crystallographic planes of cubic ZnIn2S4 (JCPDS. No. 00-048-1778) [30]. That is to say, the addition of PPy powder in the precursor solution can affect formation of crystal structure during the synthesis process and result in coexistence of hexagonal and cubic ZnIn2S4 in composite PPy-ZnIn2S4 photocatalyst. The characteristic peak of PPy at 2θ=20-30° was not observed in the composite photocatalyst, which might be attributed to the low amount of PPy. Figure 3 presented the FT-IR spectra (IRPrestige-21, Shimadzu) of ZnIn2S4 and PPy-ZnIn2S4. Two peaks at ~1396cm-1 and ~1610cm-1 for both samples probably corresponded to the adsorbed water molecules and hydroxyl groups on photocatalyst [31]. The spectrum of PPy showed characteristic bands at 1555cm-1, 1478cm-1, 1299cm-1, 1174cm-1, 1032cm-1 and 917cm-1. As expected, the spectrum of PPy-ZnIn2S4 composite clearly exhibited characteristic bands at 917cm-1, 1032 cm-1 and 1174cm-1 attributed to PPy [32], which indicated the existence of PPy in this composite. Scanning electron microscope (SEM, JSM-6510LV, JEOL, Ltd) and transmission electron microscope (TEM, JEM2100) equipped with an energy dispersive spectrometer (EDS) were applied to observe the microstructures of as-prepared samples. As shown by SEM images in Figure 4, unlike the morphology of ZnIn2S4 prepared by hydrothermal method in our reported research [33], the lamellar microsphere structure was not observed in both ZnIn2S4 and PPy-ZnIn2S4 samples. An amorphous blocky with lamellar structure was existed in these two samples, which might be attributed to the magnetic stirring during the hydrothermal reaction. Static
hydrothermal reaction was adopted in the previous research, so microsphere morphology was observed in the samples. In order to remain the PPy powder suspended, magnetic stirring was applied during hydrothermal reaction, which might result in collapse of microsphere and formation of lamellar blocky structure. The size of the amorphous blocky ranged from 3-5μm, which had the similar size with the microsphere synthesized previously [33]. The SEM spectrum of PPy showed uniform granules and these granules can be observed in PPy-ZnIn2S4 composite sample. The similar results can be obtained from TEM images in Figure 5. Except the similar amorphous sheets in ZnIn2S4 sample, some leaf-like fibers were observed in PPy-ZnIn2S4. As shown in EDS elemental mapping, three main feature elements of Zn, In and S were detected in ZnIn2S4 sample and an additional element of N was also detected in PPy-ZnIn2S4 sample. The Brunauer-Emmett-Teller (BET) surface areas were measured by nitrogen adsorption and desorption (V-Sorb2800P, GAPP, China) to evaluate the specific surface area of prepared ZnIn2S4 and PPy-ZnIn2S4. The nitrogen adsorption-desorption isotherms of ZnIn2S4 and PPy-ZnIn2S4 in Figure 6a were characteristic of a type Ⅳ isotherm with a hysteresis loop, which indicated that mesoporous structures existed in both samples. Almost no change of nitrogen adsorption-desorption isotherm for PPy-ZnIn2S4 was observed comparing with ZnIn2S4. From the isotherm, the capillary condensation appeared only when the pressure was close to saturated vapor pressure, which indicated that the hysteresis loop curve can be regarded as the H4 type [34]. It turned out that silt pores with parallel plate structure were existed in both photocatalysts, which was consistent with the lamellar structure of photocatalysts observed in SEM. Both ZnIn2S4 and PPyZnIn2S4 had similar BET specific surface areas, which were 150 m2·g-1 and 142 m2·g-1
for ZnIn2S4 and PPy-ZnIn2S4, respectively. The corresponding pore size distribution curve can be obtained from adsorption and desorption data and sometimes probably gave different results as in Figure 6b and Figure 6c. The peak at 4nm always appears, which is considered as a BJH false peak mainly caused by the connectivity of the internal pores, so that adsorption data in Figure 6b was used to determine the pore size distribution and showed an average pore size of 2-7nm. Moreover, the peak at ~100nm appeared in both Figure 6b and Figure 6c, indicating there were 100nm pores existed in the material since the structure in Figure 4 showed size ranging from 3 to 5μm. Figure 7a showed the typical UV-Vis diffuse reflectance spectra (SU 3900) of the as-synthesized pure ZnIn2S4 and PPy and PPy-ZnIn2S4 composite powder. Both had wide range absorbency from UV to visible region. It can be found that the absorbance within visible light range was enhanced by PPy-ZnIn2S4 composite compared with pure ZnIn2S4, which was in agreement with the sample color from yellow (ZnIn2S4) to dark green (PPy-ZnIn2S4). Previous research indicated that the band gap of hexagonal ZnIn2S4 was estimated based on direct-allowed transition [35, 36]. As shown in Figure 7b, the band gap of as-prepared samples based on direct-allowed transition can be estimated by plotting of (αhν)2 with respect to hν [36]. A classical extrapolation method was adopted to get the band gap by extrapolating the straight line to the horizontal axis (photon energy axis hν). The band gap (Eg) of pure ZnIn2S4 and PPy-ZnIn2S4 calculated in this way were 2.2eV and 2.0eV, respectively. According to the calculation in our recent research [37], the conduction band (CB) and valence band (VB) potential for pure ZnIn2S4 were estimated to be -0.78eV and 1.42eV, respectively. The highest occupied molecular orbital (HOMO) level of PPy (1.05eV) [38] was lower than that of ZnIn2S4 (1.42eV), which might be beneficial to
the charge separation and transfer demonstrated in Scheme 1.
3.2. Photocatalytic activity of ZnIn2S4 and PPy-ZnIn2S4 composite
Degradation of dye was a simple way to evaluate the photocatalytic activity of photocatalyst and MB was selected as one target organic contaminant in order to optimize the content of PPy in composite photocatalyst. From the results (Figure S1), the composite photocatalyst exhibited the best photocatalytic degradation of MB when the weight ratio of PPy versus ZnIn2S4 was 4%. So PPy(4%)-ZnIn2S4 composite photocatalyst was chosen for degradation of chloramphenicol. Besides the degradation rate, mineralization of CHL was also taken into account in the following experiment. ZnIn2S4 and PPy-ZnIn2S4 showed poor adsorption performance for the removal of CHL and no obvious removal of CHL was observed after 30min adsorption. Figure 8 showed the decrease of CHL concentration vs. light irritation time on the photocatalysts of PPy-ZnIn2S4 and ZnIn2S4 samples. It seemed that there was little difference in the first 30min, but the time required for complete removal of CHL by PPy-ZnIn2S4 composite (60min) was shorter than pure ZnIn2S4 (120min), indicating better deep degradation of CHL. Based on our previous research, the degradation of CHL started from the cleavage of C-N bonds and then some intermediates with stronger chemical bonds including C-OH, C=O, C-C and benzene ring were produced. It is even more difficult to decompose these intermediates. It is relatively easy for the preliminary degradation of CHL, so the degradation in the initial 30min was fast and the difference between two photocatalyst was small. Therefore, besides detection of CHL concentration, mineralization of CHL was another important evaluation criterion
for photocatalytic activity. To some extent, PPy-ZnIn2S4 composite photocatalyst can improve the mineralization of chloramphenicol. From the inset of Figure 8, 48.5% TOC was achieved by PPy-ZnIn2S4 after 3h degradation, which was two times higher than ZnIn2S4 (23.5%). Although the TOC removal was not high enough, it was noticeable for visible light photocatalysis, especially for degradation of persistent antibiotic chloramphenicol. The intermediates generated in photocatalytic degradation of chloramphenicol by ZnIn2S4 were detected and the degradation pathways were discussed in our published research [26]. Thus, only the degradation pathways on PPy-ZnIn2S4 were discussed in detail in the following section.
3.3. Identification of intermediates formed by PPy-ZnIn2S4 composite photocatalyst
Identification of intermediates formed during photocatalytic degradation of CHL can give evidence for intensive study of photocatalytic degradation pathways. The intermediate products resulting from photocatalytic degradation of CHL by PPy-ZnIn2S4 at 30min, 60min, 120min and 180min were analyzed. The single mass spectra of detected intermediates were shown in support information and the formation and disappearance of each intermediate at different reaction time were listed in Table S1. The identification of intermediates was performed with the information of mass-to-charge ratio (m/z) by loss or gain a proton. The molecular formulas, molecular mass, mass-to-charge ratio (m/z) of detected fifteen intermediates were summarized in Table 1. Based on these data, a possible photocatalytic degradation pathway of CHL by PPy-ZnIn2S4 including radical reaction, ring open and final mineralization was proposed in Figure 9. According to Table S1, three protonated intermediate products corresponding to m/z of 94, 115, 130 and three
deprotonated ones with m/z 138, 197, 203 were detected after 30min. Because the dissociation energies of phenyl-nitryl, C-OH, C-Cl and C-N bonds were weaker than other chemical bonds in CHL [39], the cleavage of these chemical bonds was occurred in the first place. After 60min, the characteristic peak of CHL was vanished and subsequently decomposed to smaller and simpler compounds. The ring-opening product began to appear after 120min, which would be beneficial to the mineralization of CHL. Four kinds of simpler compounds with protonated m/z of 82, 91, 109 and 156 were still existed in the photocatalytic reaction solution after 180min. Comparing with the formed intermediates for CHL degradation by ZnIn2S4 in our previous study [26], fewer and simpler intermediates were detected in PPy-ZnIn2S4 system. The ring-opening product appeared after 180min and ten kinds of intermediates existed in the solution after 180min reaction in ZnIn2S4 system. That is the reason for increased TOC removal rate of CHL degradation by PPy-ZnIn2S4.
3.4. Determination of reactive species
In order to determine the contributing reactive species, a series of radical trapping experiments were conducted in the photocatalytic system containing ZnIn2S4 and PPy-ZnIn2S4, respectively. Some trapping reagents were organic chemicals, which could have some interference on the measurement of TOC, so that only the concentration of chloramphenicol was analyzed. From Figure 10a, the photocatalytic degradation of CHL was slightly inhibited when isopropanol was added, which suggested that hydroxyl radical (·OH) gave some contribution to photocatalytic degradation of CHL in ZnIn2S4 system. While photocatalytic activity was significantly suppressed and only 73% CHL was removed after 180min photocatalytic
reaction when superoxide radical (·O2-) was trapped, which declared that superoxide radical was a dominating reactive radical in ZnIn2S4 system. For PPy-ZnIn2S4 photocatalytic system, trapping holes and hydroxyl radical had slight influence on photocatalytic activity and similar results as in ZnIn2S4 photocatalytic system were obtained after adding ·O2- trapping reagent (Figure 10b). That is to say, ·O2- remained to be the dominating active species in photocatalytic degradation of CHL by PPy-ZnIn2S4. It was found that the conduction band of ZnIn2S4 was negative enough that photogenerated electrons can reduce dissolved oxygen to produce superoxide radical ·O2- (Eo(O2/·O2-)=-0.33 V vs NHE) [40]. Thus, it is reasonable that ·O2- was detected as the main reactive species. On the other hand, hydroxyl radical (·OH) cannot be generated directly because of band position of ZnIn2S4 [40]. Our experimental results indicated that the ·OH was produced indirectly via superoxide dependent mechanism [41]. There is one phenomenon should be noticed from Figure 10 that the inhibition of photocatalytic activity in PPy-ZnIn2S4 system was diminished comparing with that in ZnIn2S4 system, indicating the formation of a higher concentration of reactive radicals (·O2- and ·OH) in PPy-ZnIn2S4 system. Because that the concentration of trapping reagent was 1mmol·l-1 in both system and there were spare reactive radicals after in-situ capture by scavengers. The direct detection of oxygen species formation by EPR would help to prove which reactive oxygen species were involved in the photocatalytic reaction. The BMPO spectra in Figure 11 with four main peaks associated with some fine structure were characteristic of BMPO/·O2and BMPO/·OH radicals [41, 42]. Previous research [43] claimed that light illumination could convert ·O2- adduct to ·OH adduct. And also our previous study [41] have proved that the BMPO/·O2- was reduced to BMPO/·OH under visible light system and the ·OH formation in ZnIn2S4 system was via superoxide radical (·O2-).
The intensity of the signal was a little higher than that of ZnIn2S4, indicating more reactive oxygen radicals were generated by PPy-ZnIn2S4.
4. Conclusions A series of PPy-ZnIn2S4 composites with different amounts of PPy were prepared in this study. There was obvious difference in crystalline phase and optical property between PPy-ZnIn2S4 and pure ZnIn2S4 photocatalyst, which the cubic phase was appeared and absorbance within visible light range was enhanced in PPy-ZnIn2S4. PPy-ZnIn2S4 showed the best photoactivity when the content of PPy was 4% by weight, which shortened the reaction time for complete removal of chloramphenicol and doubled the mineralization rate. The modification of ZnIn2S4 with polypyrrole provided a good candidate for improving photocatalytic activity, especially the mineralization efficiency. The cleavage of phenyl-nitryl, C-OH, C-Cl and C-N bonds was occurred in the first place for photocatalytic degradation of chloramphenicol by PPy-ZnIn2S4.
Acknowledgments
This study was supported by the National Natural Science Foundation of China (51708442, 51608428), Natural Science Foundation of Shaanxi province (2017JQ5019) and the science and technology foundation for talents and young researcher from Xi’an University of Architecture and Technology (Project No. RC1441, RC1440, QN1515 and QN1516).
References
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Figure captions Figure 1. XPS spectra of ZnIn2S4 and PPy(4%)-ZnIn2S4, including expanded spectra of In3d, Zn2p and S2p. Figure 2. XRD patterns of ZnIn2S4, PPy(4%)-ZnIn2S4 photocatalyst and pure PPy. Figure 3. FTIR spectra of ZnIn2S4, PPy(4%)-ZnIn2S4 photocatalyst and pure PPy. Figure 4. SEM images of ZnIn2S4, PPy(4%)-ZnIn2S4 photocatalyst and pure PPy. Figure 5. TEM images, HRTEM images and EDS spectra of ZnIn2S4 and PPy(4%)-ZnIn2S4. Figure 6. (a) Nitrogen adsorption-desorption isotherms of ZnIn2S4 and PPy(4%)-ZnIn2S4; (b) The pore size distribution curves related to adsorption data; (c) The pore size distribution curves related to desorption data; Figure 7. (a) UV-Vis diffuse reflectance spectra of PPy, ZnIn2S4 and PPy(4%)-ZnIn2S4; (b) Plots of (αhν)2 with respect to photon energy (hν). Figure 8. Photodegradation of chloramphenicol by ZnIn2S4 and PPy(4%)-ZnIn2S4 (Inset: TOC removal rate); Figure 9. The proposed reaction pathways of photocatalytic degradation of CHL by PPy(4%)-ZnIn2S4. Figure 10. Photodegradation of chloramphenicol in the presence of EDTA, Isopropanol and BQ, (a) In ZnIn2S4 system; (b) In PPy-ZnIn2S4 composite system. Figure 11. BMPO spin-trapping EPR spectra from ZnIn2S4 and PPy-ZnIn2S4 under the irradiation of visible-light.
Figure 1
PPy (4%)-ZnIn2S4
O1 s
Zn 2p
In3d S2p C1s
Zn2p
PPy (4%)-ZnIn2S4
ZnIn2S4 At.%: Zn2p=4.73 In3d=12.17 S2p=23.63 C1s=50.86 O1s=8.63
Counts (a. u.)
O1 s
S2p C1s
Counts (a. u.)
Zn 2p
In3d
Survey
ZnIn2S4 ZnIn2S4
0
400
800
In3d
1200
1020
PPy (4%)-ZnIn2S4
ZnIn2S4
155
450 460 Binding Energy / E (eV)
160 165 170 175 Binding Energy / E (eV)
Figure 2 * hexagonal phase # cubic phase (102)
* *
(116)
(105)
*
(202)
(006)
(110)
PPy *
ZnIn2S4
* *
JCPDS-01-072-0773
* #
*
(511)
#
*
PPy(4%)-ZnIn2S4 (444)
* #
(731) (800)
*
(111)
# #
JCPDS-00-048-1778
10
20
30
40 50 o 2 Theta / degree ( )
At.%: Zn2p=3.91 In3d=11.1 S2p=21.97 N1s=0.47 C1s=55.09 O1s=7.46
PPy (4%)-ZnIn2S4
Counts (a. u.)
Counts (a. u.)
ZnIn2S4
1060
S2p
PPy (4% )-ZnIn2S4
440
1040
60
70
80
180
Figure 3 90
80
100 1032 1478 1299
PPy
95
T(%) 60
90
PPy (4% )-ZnIn2S4
85
917 1174
50
80 1396
40 500
Figure 4
1000
1610
1500
2000 2500 3000 -1 Wavenumber (cm )
ZnIn2S4 3500
75 4000
T(%)
1555
70
Figure 5
Figure 6 Volume (cm3 g-1)
100
a
PPy(4%)-ZnIn2S4 ZnIn2S4
80
60
40 0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
b
Adsorption
-1 Pore volume (ml g )
0.016
c
0.04
Desorption
ZnIn2S4
ZnIn2S4
PPy(4%)-ZnIn2S4
PPy(4% )-ZnIn2S4 0.02
0.008
0.00
0.000 1
10 Pore size (nm)
100
1
10 Pore size (nm)
100
Figure 7 1.5
40
a
b PPy
30 2
PPy(4%)-ZnIn2S4
ah
Abs
1.0
0.5
20
PPy-ZnIn2S4
10 ZnIn2S4
0.0 200
ZnIn2S4 0
400 600 Wavelength (nm)
800
1
2
3
h (eV)
4
5
6
Figure 8
60
0.8 0.6
48.5%
40 23.5% 20 0
0.4
ZnIn2S4
PPy-ZnIn2S4
ZnIn2S4
0.2 0.0 -30
Figure 9
TOC removal (%)
Chloramphenicol (C/C0)
1.0
PPy-ZnIn2S4 0
30
60 90 120 Reaction time (min)
150
180
Figure 10 1.0
Chloramphenicol (C/C0)
1.0
PPy-ZnIn2S4
ZnIn2S4 0.8
0.8 No quencher EDTA Isopropanol BQ
0.6
0.6
0.4
0.4
0.2
0.2
b
a 0.0 -30
Figure 11
No quencher EDTA Isopropanol BQ
0
30
60 90 t (min)
120
150
0.0 180 -30
0
30
60 90 t (min)
120
150
180
PPy-ZnIn2S4 ZnIn2S4
3480
3500
3520 Field
3540
3560
Scheme 1. The proposed photocatalytic mechanism over PPy-ZnIn2S4 composite photocatalyst.
Scheme 1
Table legends Table 1. Some researches related to the photodegradation of chloramphenicol. Table 1 Dosage
Light source
Time
Initial concentration
Efficiency
TiO2
0.94g/l
300W medium pressure mercury lamp (λ≥365nm)
60min
19.97mg/l
85.97%
[18]
TiO2, ZnO, TiONa
1.0g/l
9W Osram Dulux lamp
90min
50mg/l
ZnO 90% TiO2 37% TiONa 23%
[19]
ZnO Film
----
240min
8mg/l
40%
[20]
TiO2
1.6g/l
120min
25mg/l
100%
[21]
24h
10mg/l
97%
[22]
50min
20mg/l
99%
[23]
120min
20mg/l
61%
[24]
140min
15mg/l
66.2%
[25]
Catalyst
SrFeO3−x/gC3N4 Ce(MoO4)2/ GO ATP/Cu2O/ Cu/g-C3N4 Fe3+/Er3+ co-doped Bi5O7I
0.1g/l 0.5g/l 0.08g/l 1.0g/l
18W UV lamp (315-400nm) 450W Xenon arc lamp 150W Xenon arc lamp 500W tungsten lamp 300W Xenon lamp 400W halogen lamp
Table 2
Table 2. Intermediates identified by LCMS-IT-TOF.
Code P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12 P13 P14 P15
Molecular formula C11H11NO3 C9H11NO4 C6H5NO3 C2H2O2Cl2 C2H2OCl2 C2H4NOCl C11H14NO3Cl C9H10O C10H9NO2 C7H6O2 C9H9NO C6H4O2 C6H5NO4 C5H6O C2H2O4
Molecular mass 205 197 139 129 114 94 243 134 175 122 147 108 155 82 90
m/z Positive ion ([M+H]+) —— —— —— 130.07 115.12 94.05 —— 135.00 —— 122.55 —— 109.05 156.04 81.52 90.97
Negative ion ([M-H]-) 203.97 197.96 138.02 —— —— —— 242.96 —— 174.00 —— 146.99 —— —— —— ——