Plasmon-Enhanced Photodegradation of Ionic Liquids with Ag

Oct 29, 2018 - Ionic liquids (ILs) have been a hot topic in the past decades because of their unique properties and ... In this work, Ag nanocubes/ZnO...
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Cite This: Ind. Eng. Chem. Res. 2018, 57, 15597−15605

Plasmon-Enhanced Photodegradation of Ionic Liquids with Ag Nanocubes/ZnO Microsphere Composites Weiwei Lu,*,† Qingling Huang,† Yuan Zhang,† Kaisheng Yao,† and Jianji Wang*,‡ †

Ind. Eng. Chem. Res. 2018.57:15597-15605. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/21/18. For personal use only.

School of Chemical Engineering and Pharmaceutics, Henan University of Science and Technology, Luoyang, Henan 471003, P. R. China ‡ Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, P. R. China S Supporting Information *

ABSTRACT: Ionic liquids (ILs) have been a hot topic in the past decades because of their unique properties and promising applications in various areas. However, the toxicity of ILs and their possible risk to the environment and aquatic organisms have also been confirmed. Therefore, it is imperative to seek an efficient way to remove or degrade ILs in the polluted aqueous system. In this work, Ag nanocubes/ZnO microspheres composites have been designed, prepared, and used for the photocatalytic degradation of eight commonly used imidazolium-based ILs in aqueous solution. It is found that ZnO semiconductor photocatalyst with an Ag content of 1.12 at. % can degrade 90% of all these ILs within 7 h. The structural characterization, photoluminescence analysis, and discrete dipole approximation calculations showed that the loading of Ag nanocubes onto ZnO microspheres significantly improves the performance of ZnO for the IL degradation through hot electron transfer and electrical field enhancement. In addition, the formed intermediates in the degradation process of [C4 mim]Cl have been detected by gas chromatography−mass spectroscopy. A possible degradation mechanism is proposed and compared with those previously reported in the chemical degradation.

1. INTRODUCTION Ionic liquids (ILs) refer to a class of salts that generally have melting point lower than 100 °C. They are typically constituted by large organic cations and relatively small anions. Over the past decades, ILs have been a hot topic because of their unusual properties such as broad temperature range of liquid state, high stability from thermal and chemical decomposition, wide electrochemical window, and easy tunability of structure and property.1,2 Furthermore, because ILs have negligible vapor pressure, they are often believed to be “green” in nature. However, it is gradually realized that a low vapor pressure does not necessarily lead to an environmentally friendly process. Before such a conclusion can be made, many other facts have to be considered. On one hand, different studies have been carried out to assess the toxicity of ILs in in vitro assays as well as in aquatic organisms.3−7 These studies indicate that some ILs are toxic and have possible risks to the environment and aquatic organisms. A stronger toxicity has been reported for ILs with Cl− anion than those with PF6−8, and a significant impact of the carbon number in the side chain on the toxicity of imidazole-based ILs is also observed.9,10 On the other hand, with widespread and large-scale use in industry, certain amounts of ILs will inevitably be brought © 2018 American Chemical Society

into the environment through wastewater resulting from synthesis, purification, and recycling. Because of the high chemical stability and water solubility, ILs may become a class of harmful pollutants to the environment and the aquatic organism. Therefore, it is desirable to remove or degrade ILs in the polluted aqueous system. At present, the existing methods for the degradation of ILs mainly focus on biodegradation11,12 and chemical degradation.13−16 However, it is shown from biodegradation studies that ILs, especially the commonly used imidazolium ILs, are very difficult to be degraded by microbial.11,12 Compared with biodegradation, chemical oxidation has been demonstrated to be more efficient for the degradation of ILs. For example, oxidative degradation of the ILs with imidazolium cation with different anions have been studied in H2O2 acid medium, where 99% of the tested compounds can be degraded within 72 h.13 There are also other studies on the Fenton oxidative degradation of ILs by using the created reactive peroxides.14−16 Nevertheless, the chemical oxidation degradation of ILs Received: Revised: Accepted: Published: 15597

August 14, 2018 October 29, 2018 October 29, 2018 October 29, 2018 DOI: 10.1021/acs.iecr.8b03890 Ind. Eng. Chem. Res. 2018, 57, 15597−15605

Article

Industrial & Engineering Chemistry Research

For the preparation of the Ag nanocubes/ZnO microsphere composite photocatalysts (designated as Ag/ZnO), 0.5 g of ZnO was first dispersed in 10 mL of the solution containing the Ag nanocubes. The solution was stirred for 6 h to deposit the Ag nanocubes onto ZnO microspheres. Then, the Ag/ZnO composites were centrifuged and dried under vacuum for 12 h at 60 °C. A series of Ag/ZnO with different Ag contents were prepared with the similar procedure by changing the amounts of Ag nanocubes in the solutions. 2.3. Sample Characterization. The morphology of the samples was characterized by FE-SEM (Hitachi S-4800). XPS (PHI Quantum 2000 Scanning XPS Microprobe) was used to examine the surface property of all the samples with Al K-alpha radiation (1486.6 eV). The C 1s signal of contaminated hydrocarbon was chosen as internal reference for calibrating the absolute binding energy. Origin 7.0 software was used to fit the XPS curves. The PL spectra were determined by using a fluorescence spectrometer (JASCO FP-6200) with an excitation wavelength at 325 nm. The scattering and absorption of light and the near-field distribution of the 50 nm Ag nanocubes were calculated using discrete dipole approximation (DDA) code DDSCAT 7.3,26,27 and the number of dipoles was 32 768. The ambient medium was water, and its refractive index was 1.33. The efficiency factor of extinction (Qext), scattering (Qsca), and absorption (Qabs) was calculated on the average of 27 target orientations and two incident polarizations. 2.4. Photodegradation Experiment. The photodegradation experiment was carried out in a cylindrical cell with a quartz cap and a circulating water jacket. The irradiation light is supplied by a 300 W Xe lamp (Beijing Aulight Co., Ltd.) with light density tuning function. In each degradation experiment, stock solution of each IL at a concentration of 50 mM was prepared in purified water. Then, 2 mL of the 50 mM IL solution was added to generate final concentration of 1 mM. Typically, a aqueous solution (100 mL) contained 1 mM IL, and 0.1 g of photocatalyst was stirred without light irradiation at 25 °C for 15 min in the reactor to let IL reach adsorption equilibrium on photocatalyst surface. After the reaction was initiated by irradiation with a 300 W Xe arc lamp, 2 mL of reaction solution was taken out every 60 min. Subsequently, to quantitatively analyze the remaining IL by UV−vis, the catalyst particles have to be completely removed through centrifuge and filter. The quantitative determination of the remaining IL in the degradation process, which is based on the Beer−Lambert Law, was performed at 212 nm using a UV/ vis spectrophotometer (PERSEE TU-1900). In addition, the standard curve was established from three independent measurements. The degradation percent (D%) was calculated according to the following equation: D% = (C0 − Ct)/C0 × 100, where C0 is the starting concentration of ILs and Ct is the concentration of ILs at the photodegradation time t. The organic intermediates were analyzed by GC−MS (Agilent 6890/5793N) equipped with a HP-5 capillary column (Hewlett-Packard, 30 m × 0.25 mm × 0.25 μm).

generally involves complicated chemical process, and the degradation efficiency is not always high. In the past decades, photocatalytic degradation has been proven to be a facile and efficient technology for degrading organic pollutants, including ionic liquids.17−19 In this method, photoenergy is utilized to excite semiconductor catalysts to generate holes and the primary oxidizing species of ·OH. Then, the free radicals are able to unselectively attack and even mineralize the organic substances. However, in the degradation process, the high recombination of photocreated charge carriers always limited the performance of semiconductor photocatalysts. In recent years, with the in-depth investigation of the localized surface plasmon resonance (LSPR) property of noble metals, such as Au, Ag, and Cu, the plasmonic metal nanoparticles have shown their significant roles in plasmoninduced charge and energy transfers from the metal to semiconductor,20,21 electric field-enhanced electron−hole production within the proximity of the metal−semiconductor interface,22 the plasmon-induced increase of the temperature, and direct activation of the molecular reactant.23 Although these studies confirmed that the deposition of Au or Ag with the unique LSPR property in the visible light zone can significantly increase the efficiency of semiconductor photocatalysts, the underlying roles of plasmons require further clarification. In this work, the photodegradation of eight commonly used imidazolium-based ILs with different cations and anions by Ag/ZnO photocatalysts was performed. In doing so, threedimensional ZnO microspheres were chosen to act as a semiconductor photocatalyst. Ag nanocubes with strong SPR effect were then loaded on the ZnO microspheres surface to enhance the production and separation of the photogenerated charge carriers. The samples were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy. Then, the photocatalytic performance of the prepared Ag/ZnO samples was explored, and the organic intermediates during the degradation process were analyzed to understand degradation mechanism of the imidazolium-based ILs.

2. EXPERIMENTAL SECTION 2.1. Materials. Sodium alginate was purchased from Acros. Zinc acetate dehydrate (≥99.5%), sodium sulfide (≥98%), ethylene glycol (≥98%), and silver nitrate (≥99.5%) were all supplied by Beijing Chem. Co. Ltd. (Beijing, China). 1-Butyl3-methylimidazolium chloride ([C4 mim]Cl, ≥99.5%), 1-butyl3-methylimidazolium bromide ([C4 mim]Br, ≥99.5%), 1butyl-3-methylimidazolium tetrafluoroborate ([C4 mim]BF4, ≥99.5%), 1-butyl-3-methylimidazolium trifluoromethansulfonate ([C4 mim]TfO, ≥99.5%), 1-ethyl-3-methylimidazolium chloride ([C2 mim]Cl, ≥99.5%), 1-hexyl-3-methylimidazolium chloride ([C6 mim]Cl, ≥99.5%), 1-hydroxyethyl-3-methylimidazolium chloride ([HOC2 mim]Cl, ≥99.5%), and 1-aminoethyl-3-methylimidazolium chloride ([H 2 NC 2 mim]Cl, ≥99.5%) were all supplied by Chengjie Chem. Co. Ltd. (Shanghai, China). 2.2. Preparation of ZnO and Ag/ZnO Photocatalysts. The 3D ZnO hollow microspheres were prepared by the procedures described in our previous paper.24 Polyol method reported by Xia and co-workers25 was used to prepare Ag nanocubes. The initially prepared Ag nanocubes were washed by using acetone and ethanol and then stored in water for future usage.

3. RESULTS AND DISCUSSION 3.1. Characterization of ZnO and Ag/ZnO. Shapes of the ZnO and Ag/ZnO samples were observed by SEM, as shown in Figure 1. The low-magnification image (Figure 1a) clearly indicates that the as-obtained ZnO product has the morphology of quasi-sphere and the diameter of 3−5 μm. The medium-scale magnification images in Figure 1c present an 15598

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3d5/2 (368.2 eV) and Ag 3d3/2 (374.2 eV),29 the BE value of Ag in Ag/ZnO remarkably moves to the lower value, which resulted from the decrease of electron density of Ag. This result is in accordance with previous studies and stemmed from the electrons transferring from Ag to ZnO due to fact that the work function of ZnO is higher than that of Ag.30−32 In Figure 2c, the position of the Zn 2p3/2 and Zn 2p1/2 for the Ag/ZnO is at about 1021.4 and 1044.5 eV, respectively. Both values are almost the same as those of pure ZnO, which indicates the chemical state of Zn element is Zn2+ on the surface. For O 1s (Figure 2d), two peaks (530.1 and 531.4 eV) can be identified, thereby reflecting two different chemical states of O species. The peak at 530.1 eV is related to the lattice oxygen of ZnO, while the peak at 531.4 eV can be attributed to the oxygen from surface hydroxyl (OH).30 Since the active ·OH are mainly produced through the entrapment of holes by surface hydroxyl, the quantitative calculation of surface hydroxyl from O 1s XPS data was further performed. It is shown that the content of OH was about 30% on ZnO surface no matter what percent of Ag are loaded. These results are different from those of our previous study,24 in which the surface hydroxyl contents of ZnO can be modified by the Ag deposition. This difference may be due to the use of different preparation methods in the present and previous studies. Unlike the one-pot method adopted to form composite of Ag and ZnO used in our previous work,24 Ag nanocubes and ZnO microspheres are separately prepared, and then these two parts are combined together to form Ag/ZnO composite in the present work. This means that the surface hydroxyl content of ZnO was already fixed during the ZnO formation, such that the subsequent deposition of the Ag nanocubes minimally influenced the surface hydroxyl content. 3.2. Photodegradation Performance. In this work, first, a commonly used imidazole-based IL, [C4 mim]Cl, was representatively selected to assess the degradation efficiency of the Ag/ZnO composite photocatalyst with different Ag nanocube loadings. According to the results in Figure 3, the photodegradation performance of Ag/ZnO did not monotonously increase with the Ag content. Relatively lower Ag contents ( 2.01 at. % Ag/ZnO > 4.58 at. % Ag/ZnO. Based on these results, the optimal Ag content was calculated to be approximately 1.12 at. %. No matter what percent of Ag nanocubes are loaded, all the degradation efficiency of Ag/ ZnO composite is much higher than that of pure ZnO. The difference in photodegradation efficiency of Ag/ZnO with different Ag loadings may be related to the distribution status of Ag nanocubes on the ZnO microsphere, which has been provided by the SEM characterization (Figure S1). It can be seen from Figure S1a that for 0.65 at. % Ag/ZnO, Ag nanocubes sparsely anchor on ZnO microsphere surface, and most part of ZnO is not covered by Ag nanocubes. This insufficient contact between Ag and ZnO means that only excited ZnO play a role in photodegradation process, and the plasmon-enhanced effect of Ag naonocubes on ZnO semiconductor cannot be fully utilized in this composite at such a low content of Ag. When Ag content increases to 1.12 at. %, Ag nanocubes are evenly distributed on ZnO microspheres

Figure 1. SEM images of ZnO and Ag/ZnO samples: (a), (c), and (e) in the left are images of ZnO at different magnification levels; (b), (d), and (f) in the right are images of the Ag/ZnO composites at the same magnification levels as that in the left panel.

individual microsphere, wherein the surface of the ZnO microsphere is not smooth and has some pores. A closer observation of Figure 1e reveals that the shell of the microsphere is formed by many polygonal ZnO nanorods. This can be confirmed by a broken fragment (inset of Figure 1e), which shows that the ZnO nanorods are 1−2 μm and are aligned in orientation. The right panel of Figure 1 (e.g., (b), (d), and (f)) shows the FESEM images of ZnO that have been deposited with Ag nanocubes. A comparison between the corresponding images in the left panel and those on the right clearly show that the Ag nanocubes with side length of 40−50 nm are successfully loaded on ZnO microspheres, though a few Ag nanocube aggregates have been observed. The Ag content on the surface of ZnO and its chemical bonding state in the 3D Ag/ZnO composites are analyzed by XPS. The Ag content on the ZnO surface was calculated by using the sensitivity factor of each atom.28 The results show that the molar contents of Ag on the four Ag/ZnO samples are 0.58%, 1.12%, 2.01%, and 4.85%. To further analyze the surface state of the Ag/ZnO hollow microsphere, a representative XPS curve for 1.12 at. % sample is shown in Figure 2. The value of C 1s (284.8 eV) was used to calibrate the binding energies (BE) for these XPS peaks in spectra. All the signals in the scan survey spectra of Figure 2a can be assigned to Ag, Zn, O, and C element, and no any signals of other element appeared. The appearance of the C peak can be significantly attributed to the adventitious hydrocarbon contaminant that commonly exists in XPS analyses. The high-resolution spectra for Ag, Zn, and O are shown in Figure 2b, c, and d, respectively. As observed in Figure 2b, the BE of Ag 3d5/2 and Ag 3d3/2 for Ag/ZnO are at 367.1 and 373.2 eV. As compared to the standard BE of Ag 15599

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Figure 2. Representative XPS spectrum of ZnO and 1.12 at. % Ag/ZnO sample: (a) full survey spectra, (b) Ag 3d, (c) Zn 2p, and (d) O 1s.

(Figure S1b). This appropriate distribution status can guarantee both Ag naocubes and ZnO microsphere to be excited by the light, and the plasmonic Ag nanocubes can play their unique role to enhance the degradation efficiency of ZnO semiconductor. However, when Ag content further increases to 2.01 and 4.58 at. %, the SEM images (Figure S1c and Figure S1d) show that Ag nanocubes cover more and more surface of ZnO and even some aggregates of Ag nanocubes begin to occur. Therefore, the higher loading of Ag nanocubes than the optimum may block a large part of light to reach and then excite ZnO. As a result, although the plasmonic Ag nanocubes are fully excited by the light, the degradation efficiency of ZnO with overloading of Ag begins to decrease. So, for the prepared Ag naocubes/ZnO microsphere composite photocatalysts in this work, the optimal Ag content is 1.12 at. %. Furthermore, other than [C4 mim]Cl, photodegradation of seven additional commonly used ILs also was chosen to test the degradation performance of the prepared Ag/ZnO photocatalysts. Among these additional ILs, [C4 mim]Br, [C4 mim]BF4, and [C4 mim]TfO have the same cation but different anions with [C4 mim]Cl. On the other hand, [C2

mim]Cl, [C6 mim]Cl, [HOC2 mim], and [H2NC2 mim]Cl have the same anion but different cations with [C4 mim]Cl, and these four ILs can be further categorized into two groups. The first group includes [C2 mim]Cl and [C6 mim]Cl, which have different carbon number in the alkyl chain of imidazolium cation. The second group, including [HOC2 mim]Cl and [H2NC2 mim]Cl, is commonly used functionalized ILs. The experimental photodegradation results of these ILs with different anions and cations by using 1.12 at. % Ag/ZnO composites are show in Figure S2. It was shown from Figure S2 that, no matter what anions and cations these studied imidazolium-based ILs have, the degradation results of these ILs show similar profiles. These degradation tests confirmed the high efficiency of the prepared Ag nanocubes/ZnO microsphere photocatalysts. The enhancement effect of Ag nanocubes can be understood by the unique SPR property of the Ag nanocubes, which can be seen from the proposed mechanism for hot electrons transfer from Ag to ZnO under light irradiation (Figure 4). The SPR refer to the resonant oscillation of electrons in material as stimulated by the incident light.33 During this course, the 15600

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V vs SHE).33 Therefore, the transfer of plasmonic electrons from the Ag nanocubes to ZnO overcomes this barrier for photocatalytic aerobic oxidation. The PL characterization has been commonly used to study the electronic structure as well as the optical and photochemical properties of semiconductor composite. Furthermore, the influence of component on the charge carrier status, such as the trapping, immigration, and transfer, can be analyzed from PL spectra. So, the PL spectra of the samples (Figure 5)

Figure 3. Degradation percent of the [C4 mim]Cl over ZnO and Ag/ ZnO with various Ag loading as a function of the reaction time. The error bar means ±1 standard error calculated from three independent experiments.

Figure 5. PL spectra of ZnO with various surface Ag loadings.

were determined to verify the electron transferring from plasmonic Ag nanocubes to semiconductor ZnO. It can be seen that the PL intensities of Ag/ZnO composites were all higher than that of ZnO. This result indicates the presence of more accumulated electrons at the surface of Ag/ZnO than that of pure ZnO because only a few active species (i.e., oxygen, water molecules, and the organic substance) exist in the solid PL analysis in the degradation solution. Therefore, the photogenerated electrons and holes have no facile way to transfer from the semiconductor to the adjacent surroundings to participate in the reaction and separate from each other. As a result, these accumulated electrons will relax by recombining with the holes to give the PL signals, such that the presence of more electrons generates a higher probability of recombination with the holes to produce a higher PL intensity. On this basis, the plasmonic electrons transferring from Ag to ZnO account for the stronger PL intensity of Ag/ZnO than that of ZnO. Figure 5 also shows that the 1.12 at. % Ag/ZnO gives the highest PL intensity. With the decreasing of Ag content (0.65 at. %), the PL intensity is only higher than that of pure ZnO, but lower than those of three other Ag/ZnO samples. This phenomenon can be easily understood because the lower Ag content means less plasmonic electrons produced and transferred to ZnO, and so the lower PL intensity. Alternatively, when the Ag content (2.01 and 4.58 at. %) is higher than 1.12 at. %, the PL intensity begins to decrease, though PL intensity of both are higher than that of 0.65 at. % Ag/ZnO and pure ZnO. Referenced to the SEM image in Figure S2, the decrease of PL intensity at higher Ag content can be ascribed to the fact that overloading of Ag nanocubes blocks the light to excited ZnO and then less holes and electrons are produced in ZnO. Therefore, the PL signal produced by recombination of holes and electrons decreases at higher Ag content.

Figure 4. Proposed mechanism shows the plasmon-induced hot electrons transferring from Ag nanocubes to ZnO semiconductor.

plasmonic electrons and holes can be produced through interband or intraband excitation.33 When the energy of plasmonic electrons produced by the SPR of Ag exceeds the Schottky barrier between Ag and ZnO, these electrons will inject into the conduction band of ZnO, which is known as the plasmon-induced hot electron injection.34 Subsequently, these electrons plus the ones produced in the semiconductor of ZnO immigrates to the surface of the ZnO microsphere and are then captured by the adsorbed oxygen molecules.35 In fact, the previous report stated33 that the limitation in photocatalytic degradation process does not come from the oxidation of organic pollutes by the photogenerated holes because the holes generally have the enough oxidation power. On the contrary, the limitation is mainly from the oxygen reduction, partially because there are not continuous and sufficient electrons to accomplish the multielectron reaction of O reduction (O2 + 2H+ + 2e− = H2O2 (aq), +0.682 V vs SHE; O2 + 4H+ + 4e− = 2H2O, +1.23 15601

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Figure 6. (a) SEM image of Ag nanocubes; (b) UV−vis spectra of Ag nanocubes; (c) calculated efficiency factor of extinction (Qext), scattering (Qsca), and absorption (Qabs) for the 50 nm Ag nanocubes; (d) simulated electric field distribution with DDA method (near-field calculation is for a volume extending 0.1× the lateral length extended the cubic target in the ±x, ±y, and ±z directions).

Table 1. Detected Intermediates in Degrading [C4 mim]Cl with Ag/ZnO Photocatalyst

Figure 6a gives the SEM image of Ag nanocubes. The curve in Figure 6b defines their UV−vis extinction spectra in aqueous solution. In addition, the calculated efficiency factor of extinction, scattering, and absorption by the discrete dipole approximation method at different wavelengths is shown in Figure 6b. The recorded curve in Figure 6b is in good accordance with that calculated in Figure 6c. More importantly, according to Figure 6c, the scattering efficiency factor (Qsca) is much higher than the absorption efficiency factor (Qabs). This means that the scattering of incident photon is dominant when the light interacts with the prepared 50 nm Ag nanocubes. This strong scattering effect significantly enhanced the electric field in the proximity of the Ag

Furthermore, it is found that the variation tendency of PL intensity (Figure 5) is well consistent with that of the degradation performance (Figure 3) of Ag/ZnO with various Ag loadings, which confirms the unique plasmonic effect of Ag nanocubes on the enhancement of photodegradation efficiency of ZnO. In addition to hot electron injection effect, the influences of intensely enhanced surface electric field induced by plasmonic metal on accelerating the formation rate of charge carriers in semiconductor are also experimentally confirmed.36−38 Therefore, the LSPR spectra of Ag nanocubes were further recorded, and the plasmon-induced electric field distribution was also simulated. 15602

DOI: 10.1021/acs.iecr.8b03890 Ind. Eng. Chem. Res. 2018, 57, 15597−15605

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between the carbon atom of the side alkyl and the nitrogen atom is substantially longer than that of the C−C and C−N bonds in the imidazolium ring. This may indicate the lesser stability of the alkyl C−N bond as compared to the C−C and C−N bonds in the backbone ring. Therefore, the cleavage of the side alkyl C−N bond will favorably take place to give the intermediate compounds of (b), (c), and (d). In addition, Xray spectroscopy data45−47 showed that the bond length of N3−C4 and N1−C5 in the 1-butyl-3-methylimidazolium cation was significantly longer than those of N1−C2 and C2−N3. Thus, the N3−C4 or/and N1−C5 bond will then preferentially break to form urea (g) and oxalic acid (h).

nanocube, especially around the sharp corner of the cube (Figure 6d). In general, the light absorbance of semiconductor increases monotonously with the intensity of electric field.21 Therefore, this near-field enhancement induced by the Ag nanocubes that closely anchored on the ZnO surface increased the optical excitation and thus the formation rate of the charge carriers. In conclusion, it is the plasmon-induced hot electron injection and near-field enhancement of Ag nanocubes that account for the enhanced photodegradation efficiency of ZnO. 3.3. Degradation Mechanism of ILs through Products and Intermediates Detection. To better understand the photodegradation mechanism of the ILs, identification of the intermediate and products in the course of the degradation was conducted by GC−MS. As a representative, the obtained main degradation products of [C4 mim]Cl are listed in Table 1. In addition, a proposed degradation mechanism was presented in Scheme 1.

4. CONCLUSION In summary, we reported a promising and greener strategy to degrade ILs in water using plasmon-enhanced photocatalysts composed of Ag nanocubes and 3D ZnO hollow microspheres. Based on the structural and spectral characterizations and photodegradation performance, the enhanced effect of the Ag nanocubes could be mainly ascribed to two aspects: (i) the transferring of plasmon-induced electrons from Ag nanocubes to ZnO facilitated the reduction of oxygen by the CB electrons and thus overcame the limitation step of the whole degradation process; (ii) the SPR of Ag nanocubes significantly enhanced the electric field at the Ag and ZnO interface region, thereby accelerating the production rate of the photogenerated charge carriers and enhancing the photodegradation performance of ZnO. In addition, based on the intermediates detected by GC−MS technology, the proposed degradation pathway indicated that the side alkyl chain was initially cleaved off, after which the imidazole ring was subsequently attacked and opened.

Scheme 1. Proposed Photodegradation Pathway of [C4 mim]Cl in Water

At first, 1-butyl-3-methyl imidazolium (a) cation was mainly converted to 1-butylimidazole (b) by cleaving off the side methyl group. 1-Butylimidazole was then degraded into imidazole (c) and n-butylaldehyde (d) by breaking the side butyl chain. In the next step, the H atoms at the 2, 4, and 5 position of the imidazolium ring are targeted by the OH radical, thereby generating 2,4-imidazolinedione (e) by the oxidation of the C2 and C4 positions. 2,4,5-Trioxoimidazolidine (f) is also followed by the oxidation of C5. Subsequently, the unstable 2,4,5-trioxoimidazolidine (f) is further oxidized to form urea (g) and oxalic acid (h) when the bonds of N3−C4 and N1−C5 in the ring are broken. The above proposed degradation pathway of the IL is different from those in previous literatures.13,14,39,40 For example, in ultrasonic irradiation assisted H2O2/CH3COOH system,13 ultrasound-assisted Fe/C advanced oxidation system,14 and electrochemical oxidization on boron-doped diamond,39 1-butyl-3-methyl-2,4,5-trioxoimidazolidine was detected as one of the primary degradation intermediates. This means that in these degradation methods, the 1-alkyl-3methylimidazolium cations were first oxidized to form 1-alkyl3-methyl-2,4,5-trioxoimidazolidine based compound without cleaving the side alkyl, and then the imidazolium ring began to be opened and further to be degraded. However, biodegradation41 and electrocatalytic degradation42 of [C8 mim]Cl indicated that 1-carboxyethyl-3-methylimidazolium was first formed by attacking the octyl side chain through β-oxidation. In our study, the IL was first inclined to cleave off the methyl and butyl side chains to generate imidazole. The imidazole ring was then further oxidized, after which the ring opened to proceed with further degradation processes. Notably, previous studies43,44 have reported that the length of the C−N bond

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b03890.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: 3325805. *E-mail: 3325805.

[email protected]. Telephone: +86-373Fax: +86-373-3326445. [email protected]. Telephone: +86-373Fax: +86-373-3326445.

ORCID

Jianji Wang: 0000-0003-2417-4630 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (grant numbers 21673067, 21673068, U1504213) and the National Key Research and Development Program of China (2017YFA0403101).

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REFERENCES

(1) Hallett, J. P.; Welton, T. Room-temperature ionic liquids: Solvents for synthesis and catalysis. 2. Chem. Rev. 2011, 111, 3508− 3576. 15603

DOI: 10.1021/acs.iecr.8b03890 Ind. Eng. Chem. Res. 2018, 57, 15597−15605

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DOI: 10.1021/acs.iecr.8b03890 Ind. Eng. Chem. Res. 2018, 57, 15597−15605

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DOI: 10.1021/acs.iecr.8b03890 Ind. Eng. Chem. Res. 2018, 57, 15597−15605