POM Heterojunctions with Excellent Fenton Catalytic

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

FeOCl/POM Heterojunctions with Excellent Fenton Catalytic Performance via Different Mechanisms Jian Zhang,*,† Mingyu Zhan,† Lulu Zheng,† Chen Zhang,† Guodong Liu,*,† Jingquan Sha,† Shaojie Liu,‡ and Shuo Tian§ †

Department of Chemistry and Chemical Engineering, Jining University, Qufu 273100, P. R. China Department of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R. China § Animal Husbandry and Veterinary Bureau of Jinan, Jinan 250002, P. R. China ‡

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S Supporting Information *

ABSTRACT: To enhance the Fenton catalytic performance in a neutral solution under indoor sunlight, a novel FeOCl/polyoxometalate (POM) (FeOCl/POM-W and FeOCl/POM-Mo) composite was successfully synthesized for the first time, which shows significantly improved Fenton catalytic activity and stability for phenol degradation compared with FeOCl. Furthermore, the degradation constants (k) of FeOCl/POM-Mo (0.08 min−1) and FeOCl/POM-W (0.06 min−1) are a factor of 4 and 3 times greater than that of FeOCl (0.02 min−1), respectively. The enhanced catalytic activity is attributed to the formation of FeOCl/POM heterojunctions, which results in efficient separation of photoinduced electron−hole pairs and electron transfer from POM to FeOCl. Density functional theory calculations indicate a strong interface interaction of Fe−O−Mo and Fe−O−W in the FeOCl/POM heterojunctions. A Z-scheme mechanism for FeOCl/POM-Mo and a double-transfer mechanism for FeOCl/POM-W are proposed for the enhanced catalytic performance. This study sheds new light on the design and fabrication of high-performance photoFenton catalysts to overcome the environmental crisis.

1. INTRODUCTION Environmental and energy crises have become two major challenges in recent years, which hinder the sustainable development of human society. To overcome the serious environmental crisis, advanced oxidation processes have emerged as a powerful solution because they are highly efficient, environmentally friendly, and have no secondary pollution effect. Among the technologies, Fenton reactions have received considerable attention. Owing to the main shortcoming of iron sludge produced in homogeneous Fenton processes (FeII−H2O2), great efforts have been focused on heterogeneous Fenton reactions (FeIII−H2O2).1−3 However, because the reduction rate of FeIII to FeII in the heterogeneous Fenton reaction is sluggish, this is the first issue that we should address.4 Because of the major disadvantage of the strong acidic solution (pH = 3−4) used in Fenton reactions,5−12 recent developments have highlighted the photoinduced heterogeneous Fenton reaction in a neutral solution.13,14 However, the catalytic performance in the heterogeneous photo-Fenton reaction is limited within the ultraviolet and visible (UV−vis) regions. Therefore, the development of heterogeneous photo-Fenton catalysts with the accelerated reduction rate of FeIII to FeII in a neutral solution under natural light is an enormous challenge. FeOCl- and FeOCl-based composites (FeOCl/SBA-15 and FeOCl/SiO2) have shown excellent catalytic performance as © XXXX American Chemical Society

heterogeneous Fenton catalysts in an acidic solution by Han’s group.15−17 In this aspect, two kinds of FeOCl-based photoFenton catalysts, namely, two-dimensional FeOCl and FeOCl/ iron hydroxide nanosheets, have been reported by our group, which show outstanding catalytic performance in a neutral solution under visible light.18,19 However, the narrow band gap of FeOCl (1.90 eV) causes a low separation efficiency of the electron−hole pairs. Furthermore, it is very difficult to achieve enhanced catalytic performance with a single material component. One possible solution is to form a heterojunction between FeOCl and other semiconductors to increase the separation efficiency of electron−hole pairs. Polyoxometalates (POMs), well-defined metal−oxygen clusters with desirable reversibility in multielectron redox reactions, have been proven to be a new kind of photocatalyst.20−26 However, the strong adsorption of organic pollutants on the POM surface limits its photocatalytic performance.27,28 Therefore, the construction of a FeOCl/POM structure, especially a Z-scheme photocatalytic system, will be propitious to improve the catalytic performance of FeOCl and POM. On the basis of the above-mentioned consideration, we present a possible solution to these challenges by forming heterojunctions of FeOCl/POM for the first time. Our Received: August 17, 2018

A

DOI: 10.1021/acs.inorgchem.8b02329 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

(generalized gradient approximation with the Perdew, Burke, and Ernzerhof functional).34,35 The self-consistent convergence accuracy, the convergence criterion, and maximum displacement were set at 1 × 10−5 eV, 3 × 10−2 eV/Å, and 1 × 10−3 Å, respectively. In addition, the plane-wave cutoff energy was 500 eV. The Mulliken bond population and charge density were calculated to study the surface state of FeOCl/POM.

assumption is confirmed by a series of catalytic activities and active species-trapping measurements. As predicted, the FeOCl/POM heterojunctions show excellent indoor-sunlightdriven photo-Fenton catalytic performance in a neutral solution. The bigger surface area and surface potential of FeOCl/POM than that of FeOCl contribute to increased catalytic activity. Two different mechanisms are proposed for the enhanced catalytic activity, namely, the Z-scheme mechanism for FeOCl/POM-Mo and the double-transfer mechanism for FeOCl/POM-W, resulting in an efficient separation of photoinduced electron−hole pairs and a rapid electron transfer from POM to FeOCl.

3. RESULTS AND DISCUSSION 3.1. Structure Characterization of FeOCl/POMs. To determine the crystallographic structure of the prepared sample, XRD measurements were carried out (Figure 1a),

2. EXPERIMENTAL SECTION 2.1. Preparation. All the chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. We used a facile one-step calcination method to prepare the FeOCl/POM heterojunction. In a typical process, 0.018 g of POMMo (phosphomolybdic acid, H3[P(Mo3O10)4]) was added to 2 mL of FeCl3 solution (0.25 g/mL) with continuous stirring for 30 min at room temperature. Then, the solution was dried at 40 °C and then placed in an alumina crucible, which was calcined at 230 °C for 1 h in a N2 atmosphere. The product was washed with acetone three times to remove the unreacted ferric chloride and then dried at 90 °C in a vacuum oven for 12 h. In addition, the preparation of FeOCl/POMW was the same as the above method except for POM-W (phosphotungstic acid, H3O40PW12). 2.2. Characterization. X-ray diffraction (XRD) technique was carried out to measure the phase structure of the prepared samples on a Rigaku D/Max 2200-PC diffractometer. Transmission electron microscopy (TEM) (JEM-100CXII) and scanning electron microscopy (SEM) (field emission-SEM ZEISS system) were adopted to descript the morphology. A N2 adsorption technique at 77 K (QUADRASORB SI) was used to measure the surface area. Energydispersive X-ray spectrometry (EDXS) was performed at 15 kV. Fourier transform infrared (FTIR) spectra (Nicolet 5DX-FTIR spectrometer), a UV−vis spectrophotometer (LAMBDA-35, PerkinElmer), and a fluorescence spectrophotometer (Cary Eclipse) were adopted to characterize the structure. The composite structures were characterized by X-ray photoelectron spectra (PerkinElmer PHI-5300 ESCA). The content of elements was studied by inductively coupled plasma-mass spectrometry (Nu AttoM). An F-4600 fluorescence spectrophotometer (Hitachi) was used to study the photoluminescence (PL) spectra. The surface potential was measured by a zetapotential meter (Delsa Nano C). Electrochemical workstation (CHI760E) was used to study the electrochemical impedance spectroscopy (EIS) spectra, Mott−Schottky plots, and photocurrent. Modified fluorine-doped tin oxide, Pt wire, and saturated calomel electrode (SCE) were used as the working electrode, counter electrode, and reference electrode, respectively. 2.3. Catalysis Measurements. The test method for Fenton catalytic degradation of phenol by FeOCl/POM was the same as in the reported literatures.18,19,29 In a typical experiment, a 100 mL suspension of FeOCl/POM (0.05 g) and phenol (1 mg) was stirred in darkness for 60 min. Then, 0.5 mL of H2O2 aqueous solution (30 wt %) was added in the suspension under sunlight irradiation. In the catalytic process, 5 mL of the mixture was taken out every 10 min to record the UV−vis absorption spectra. 2.4. Active Species Measurements. Benzoquinone (BQ, 1 mM) and iso-propyl alcohol (IPA, 1 mM) were used to investigate superoxide radical (•O2−) and hydroxyl radicals (•OH) in the photoFenton catalytic process.30,31 According to the literature,32,33 a fluorescence technique was used to study the decomposition of H2O2 on FeOCl/POM by adding terephthalic acid in the reaction system. 2.5. Calculation Methods. To study the catalytic mechanism, density functional theory (DFT) calculations were adopted in the CASTEP program by using the exchange−correlation function

Figure 1. (a) XRD patterns of FeOCl, FeOCl/POM-W, and FeOCl/ POM-Mo, (b) TEM image of FeOCl/POM-Mo, and corresponding EDXS elemental mapping images of (c) total, (d) Fe, (e) O, (f) Cl, (g) P, and (h) Mo elements.

which show that the diffraction peaks of FeOCl agree with the pure phase of FeOCl (orthorhombic lattice, JCPDS, no. 241005). The main diffraction peaks of FeOCl/POM can also be assigned to the FeOCl phase. The results from the TEM and SEM images of FeOCl/POM reveal that both FeOCl/POM-W and FeOCl/POM-Mo are plates piled with nanosheets (Figures 1b and S1a,b). The smaller thickness and weaker crystallinity of FeOCl/POM compared with FeOCl indicate that POM prevents the crystal growth of FeOCl (Figures S2 and 1a), suggesting the intimate contact between FeOCl and POM. Furthermore, the EDXS measurements of FeOCl/ POMs show that all the essential elements including Fe, O, Cl, P, W, and Mo are detected. The corresponding EDXS images of FeOCl/POM reveal the uniform distribution of each element in the FeOCl/POM photocatalyst (Figures 1c−h and S1c−h). The above results indicate that the heterojunction provides enough contact between FeOCl and POMs, which facilitates the charge carrier transfer. XPS measurements were carried out to verify the chemical state of FeOCl/POM (Figure 2). The peaks of binding energy centered at 711.44 and 725.32 eV correspond to Fe 2p3/2 and B


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cm−1 is attributed to the Fe−O vibration (Figure 3a,b). All the IR absorption bands at 810 cm−1 (Mo−Oc−Mo), 850 cm−1 (Mo−Ob−Mo), 980 cm−1 (Mo−O), and 1080 cm−1 (P−O) can be ascribed to the characteristic bands of POM-Mo (Figure 3a).37,38 The IR absorption bands of POM-W lie at 820, 880, 1010, and 1100 cm−1, which are attributed to the W−Oc−W, W−Ob−W, W−O, and P−O vibration (Figure 3b).39,40 To further study the interaction between FeOCl and POMs, the FTIR spectra of pure POMs and the complexes (FeCl3/POM-Mo and FeCl3/POM-W) before calcination were also recorded (Figure S3b,c), which confirm the retention of POM structures in the synthesis of FeOCl/POM. Moreover, the peak shift of POMs in FeOCl/POMs compared with pure POMs indicates the interaction between POMs and FeOCl. Therefore, the FTIR spectra of FeOCl/POMs further suggest the successful combination of FeOCl and POMs. Furthermore, we recorded the UV−vis diffusive reflectance spectra (Figure 3c), which show that the optical absorptions of FeOCl/POMs are much better than that of FeOCl.23−27 Moreover, FeOCl/ POM-Mo shows better optical absorption than FeOCl/POMW.41,42 As is well-known, the number of active sites in the catalyst and the adsorption amount of organic pollutants on the catalyst depend on its surface area. Therefore, the surface areas of FeOCl/POMs were measured (Figure 4). The results show that the surface areas of FeOCl/POM-W and FeOCl/POMMo are 15.6 and 16.0 m2/g (Figure 4b,c), respectively, which are approximately a factor of 6 greater than that of FeOCl (2.5 m2/g, Figure 4a). The increased surface areas of FeOCl/POMs indicate that the composite structure can provide more active sites and higher adsorption capacity for organic pollutants than FeOCl, which will improve the catalytic activity of FeOCl/ POMs. On the basis of the above structure analysis, the successful combination of POM and FeOCl provides a large contact area between them and is beneficial to the separation of electron− hole pairs. Therefore, it is expected that FeOCl/POMs would boast the photo-Fenton catalytic activity in a neutral solution under sunlight. 3.2. Enhanced Photo-Fenton Catalytic Performance. To verify the above hypothesis, a natural solution under indoor sunlight for phenol degradation was adopted in our experiments. The photo-Fenton catalytic activities of FeOCl, FeOCl/POM-Mo, and FeOCl/POM-W were first studied (Figure 5a). The degradation rates reach 51% over FeOCl, 94% over FeOCl/POM-W, and 100% over FeOCl/POM-Mo after 40 min (Figure 5a), which suggests that POMs effectively

Figure 2. XPS spectrums of (a) Fe 2p and (b) Mo 3d in FeOCl/ POM-Mo, (c) Fe 2p, and (d) W 4f in FeOCl/POM-W.

Fe 2p1/2, respectively, suggesting that the Fe element in FeOCl presents as the FeIII state (Figure S3a). The peaks of Fe 2p3/2 and Fe 2p1/2 shift to 710.85 and 724.47 eV in FeOCl/POMMo (Figure 2a) and then decrease to 710.54 and 724.03 eV in FeOCl/POM-W (Figure 2c), suggesting the partial reduction of FeIII in the FeOCl/POM heterojunctions. POM, serving as a reducing agent, can transfer electrons to FeOCl, which is beneficial not only for the separation of electron−hole pairs but also for electron accumulation in FeOCl. The binding energies of Fe 2p in POM-W are smaller than those in POMMo, owing to the stronger reducing ability of POM-W than that of POM-Mo.36 Figure 2b indicates that the XPS peaks of Mo 3d5/2 and Mo 3d3/2 in FeOCl/POM-Mo appear at 232.32 and 235.45 eV, respectively, which are characteristic of the Mo6+ state.37,38 The W 4f XPS spectrum of FeOCl/POM-W demonstrates two peaks of binding energies at 35.68 eV for W 4f7/2 and 37.81 eV for W 4f5/2 (Figure 2d), which reveal that the W element in FeOCl/POM-W presents as the W6+ state.39,40 The XPS results further prove the successful composite of FeOCl and POMs. To present the detailed structure of FeOCl/POM, FTIR spectra were also recorded (Figure 3a,b). The peak at 530

Figure 3. FTIR spectrum of (a) FeOCl/POM-Mo and (b) FeOCl/POM-W, and (c) UV−vis diffusive reflectance spectra of FeOCl and FeOCl/ POMs. C


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Figure 4. N2 adsorption−desorption isotherms of (a) FeOCl, (b) FeOCl/POM-W, and (c) FeOCl/POM-Mo.

verify this point, we further measured the catalytic activities of POMs, which show that POMs are inactive on phenol degradation.27,28 Therefore, excessive POMs in FeOCl/ POMs will decrease the catalytic activity. In addition, cycling degradation experiments were also carried out to study the catalytic stability of FeOCl/POMs (Figure S5b,c), which show that there is no decrease in the catalytic activity of FeOCl/POMs after four cycling runs, suggesting the excellent stability of FeOCl/POMs. To further study the structural stability, we also measured the phase structures and morphologies of FeOCl/POMs after the catalytic reaction (Figures S5d and S6). The morphologies of FeOCl/POMs are well preserved (Figure S6), and the XRD patterns remain unchanged (Figure S5d). In addition, the FTIR spectrum and EDXS measurement of FeOCl/POMs after the catalytic reaction were also measured as seen in Figure S7. The FTIR spectrum of FeOCl/POMs after the catalytic reaction indicates that the catalyst still maintains the composite structure of FeOCl and POMs. The EDXS measurements of FeOCl/POMs in Figure S5e,f show that all the essential elements (Fe, O, Cl, P, Mo, and W) are detected. In conclusion, the structure of FeOCl/POM after the photoFenton reaction is not destroyed, indicating the excellent structure stability. Simultaneously, the pH was 5.2 in the FeOCl system after the catalytic reaction, whereas the pH values were 6.1 and 6.3 in FeOCl/POM-W and FeOCl/POMMo systems, respectively. Compared with most of the Fenton catalysts used in strong acidic solutions (pH = 3 or 4), FeOCl/ POMs show excellent catalytic performance in a near-neutral solution. To further certify the effective protection of POMs for FeOCl, we further performed Fe leaching. The leaching amounts of Fe ions in FeOCl/POM-W and FeOCl/POM-Mo are 1.82 and 1.68 ppm, respectively, whereas it is 3.01 ppm in FeOCl. The leaching amounts of Fe ions in FeOCl/POMs are small and will not cause secondary pollution to water. Moreover, FeOCl/POM-Mo shows stronger protection than FeOCl/POM-W. Thus, we can draw the conclusion that POMs do improve the structural stability of FeOCl. 3.3. Mechanism for Enhanced Catalytic Performance. To study the effect of surface area on the catalysis, the catalytic activities of FeOCl, FeOCl/POM-W, and FeOCl/POM-Mo with the same total surface area were also measured (Figure 6a). To ensure the same total surface area, the required weights of FeOCl, FeOCl/POM-W, and FeOCl/POM-Mo are 0.32, 0.051 and 0.05 g, respectively. The improved catalytic activity of FeOCl (from 51 to 69%, Figures 4a and 6a) indicates that the surface area does contribute to the enhanced catalytic activity. Because the surface areas of FeOCl/POM-Mo and

Figure 5. (a) Catalytic activity and (b) corresponding kinetic plots over FeOCl, FeOCl/POM-W, and FeOCl/POM-Mo.

enhance the catalytic activity of FeOCl. To study the effect of sunlight on the photo-Fenton catalytic activity, the degradation activities over FeOCl, FeOCl/POM-Mo, and FeOCl/POM-W in darkness were also measured (Figure S4), which indicate that the degradation rates over FeOCl, FeOCl/POM-Mo, and FeOCl/POM-W are 32, 84, and 89%, respectively. The degradation activity under darkness is lower than that under sunlight, suggesting that sunlight promotes the Fenton catalytic activity. The corresponding kinetic plots of FeOCl, FeOCl/ POM-Mo, and FeOCl/POM-W are depicted in Figure 5b, which show a good linearity, confirming the role of the firstorder kinetic model in the photo-Fenton catalytic reaction. The degradation constants (k) of FeOCl/POM-Mo (0.08 min−1) and FeOCl/POM-W (0.06 min−1) are a factor of 4 times and 3 times greater than that of FeOCl (0.02 min−1) (Table 1), respectively. Furthermore, the catalytic activity of FeOCl/POM-Mo is higher than that of FeOCl/POM-W. Table 1. Rate Constants (k) and Regression Coefficients (R2) of Photo-Fenton Catalytic Activities over FeOCl, FeOCl/POM-W, and FeOCl/POM-Mo photocatalyst

k (min−1)

R2

FeOCl FeOCl/POM-W FeOCl/POM-Mo

0.02 0.06 0.08

0.99 0.98 0.96

To optimize the conditions, the catalytic activities of FeOCl/POMs with different contents of POMs were measured (Figure S5a), which show that the optimized contents of POM-Mo and POM-W in FeOCl/POMs are 6.9 and 7.3 wt %, respectively. Hence, FeOCl plays a major role in the photoFenton catalysis, whereas POMs play a secondary role. To D


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indicate that the •OH radical is the reactive oxygen species in FeOCl and FeOCl/POMs. On the other hand, it also reveals that the production of •OH radicals over FeOCl-POMs is larger than that over FeOCl. Meanwhile, FeOCl/POM-Mo produces more •OH radicals than FeOCl/POM-W because of the higher suppressed degradation degree of FeOCl/POMMo. To prove this viewpoint, the decomposition of H2O2 was measured (Figure S8). The higher decomposition rate of H2O2 reveals the higher production of •OH radicals. The results reveal that the H2O2 conversion rates over FeOCl/POMs are obviously higher than that over FeOCl, suggesting the contribution of POMs on increased production of •OH radicals. Meanwhile, it also indicates the rapid electron transfer from POMs to FeOCl to accelerate the reduction of FeIII to FeII. In addition, the higher H2O2 conversion rate over FeOCl/ POM-Mo than that over FeOCl/POM-W suggests the more efficient reduction of FeIII to FeII. In contrast to IPA, the introduction of BQ has a small effect on the photo-Fenton catalytic activity of FeOCl (from 51 to 47%), indicating that the •O2 − radicals have smaller contribution than the •OH radicals (Figure 7a,b). However, the suppressed catalytic activities of FeOCl-POMs (82% for FeOCl/POM-Mo and 84% for FeOCl/POM-W) are larger than that of FeOCl, which indicates that the amount of •O2− radicals over FeOCl-POMs increases compared with FeOCl. According to the literature,48,49 the amount of •O2− radicals can reflect the reduction of FeIII to FeII. The •O2− radicals in the Fenton reaction mainly come from two aspects: the reduction of O2 and the reaction of FeIII with H2O2. To study the source of the •O2− radicals, the band edge positions of the valence band (VB) and conduction band (CB) potentials of FeOCl, POM-W, and POM-Mo (Figure S9) were analyzed, which show that the direct optical band gaps (Eg) of FeOCl, POM-W, and POM-Mo are 1.85, 2.98, and 2.34 eV, respectively (Figures 3b and S8a,b).37,50,51 The flat band potentials (Efb) of FeOCl, POM-W, and POM-Mo are calculated to be −0.54, −0.62, and −0.05 eV (vs SCE, pH = 7) (Figure S10a−c), respectively.50 Moreover, the CB potentials of FeOCl, POM-W, and POM-Mo are calculated to be 0.11, 0.03, and 0.60 eV [vs normal hydrogen electrode (NHE), pH = 0], respectively, which are all more positive than the potential of O2/•O2− (−0.046 V vs NHE). Furthermore, to study the effect of dissolved oxygen on the reduction of FeIII to FeII, a trapping experiment of •O2− radicals was performed under deaerated conditions (using N2 sparging), which was compared to that under air conditions. It reveals that the degradation rate hardly changed over FeOCl and FeOCl/ POMs. Therefore, the generated •O2− radicals are from the reaction of FeIII with H2O2. In addition, the corresponding VB potentials of FeOCl, POM-W, and POM-Mo are 1.96, 3.01, and 2.94 eV (vs NHE, pH = 0), respectively. On the one hand, the bigger suppressed degradation degree over FeOCl/POMs than FeOCl suggests the accelerated reduction of FeIII to FeII. On the other hand, the separation efficiency of electron−hole pairs over FeOCl/POM-Mo is higher than that over FeOCl/ POM-W. The contribution of POMs in FeOCl/POMs lies on the rapid photogenerated electron transfer from POMs to FeOCl to promote the reduction of FeIII to FeII. On the basis of the above analysis, the •OH radical is the primary reactive oxygen species, whereas the •O2− radical plays a secondary role in the photo-Fenton reaction. Therefore, the oxidation mechanism of FeOCl/POMs proceeds through the attack of •OH radical to target organic compounds. POMs

Figure 6. (a) Catalytic activities with the same total surface area and (b) adsorption of phenol over FeOCl, FeOCl/POM-Mo, and FeOCl/ POM-W under dark conditions.

FeOCl/POM-W are very close, the catalytic activity over FeOCl/POM-W exhibits unobvious improvement. Another contribution of POMs is to increase the adsorption capacity for organic pollutants. Because both the life and transport distance of active species in the Fenton reaction are short,43,44 the adsorption of organic pollutants on the catalyst surface can effectively decrease the transport distance and increase the utilization rate of the active species.45 The adsorption of phenol on the FeOCl surface under dark circumstances is weak (9%), whereas that on FeOCl/POMs is much stronger than that on FeOCl (Figure 6b). According to the zeta potential measurements, the zeta potential of FeOCl is −30.46 mV, which however increases to −54.72 mV for FeOCl/POM-W and −54.13 mV for FeOCl/POM-Mo. The increased surface electronegativity of FeOCl/POMs can effectively enhance the adsorption for phenol. In addition, the increased electronegativity of FeOCl/POMs can also increase the repulsion between nanoparticles to promote the dispersion of the suspension, which reduces the agglomeration of nanoparticles and increases the adsorption of pollutants. Because the generated reactive oxygen species are mainly • OH and •O2− in the heterogeneous Fenton reactions,2,3 BQ and IPA were applied as scavengers for the •O2− and •OH radicals (Figure 7). Using scavenger as a probe molecule to

Figure 7. Trapping experiments of active species (a) •OH radical and (b) •O2 radical over FeOCl, FeOCl/POM-W, and FeOCl/POM-Mo under indoor sunlight irradiation.

detect the oxidative species in the catalytic system has been testified to be an effective method.46,47 If the radical plays an important role in the catalysis, the degradation rate will decrease greatly in the presence of the corresponding scavenger. The suppressed degradation degrees by the •OH and •O2− radicals can reflect their content in the catalytic process. The catalytic activity of FeOCl is suppressed to 45 from 51% by the introduction of IPA, which is lower than that of FeOCl/POMs (Figure 7a). On the one hand, the results E


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Figure 8. (a) PL spectra, (b) electrochemical impedance spectra (Nyquist plot), and (c) photocurrents of FeOCl, FeOCl/POM-W, and FeOCl/ POM-Mo.

To further analyze the interface between FeOCl and POMs, DFT calculations were carried out (Figure 9), and the results

promote the separation of photogenerated electron−hole pairs and the reduction of FeIII to FeII,41 which subsequently reacts with H2O2 to generate •OH radicals on the FeOCl/POMs surface. To further verify the viewpoint that FeOCl/POMs have higher charge separation efficiency than FeOCl in the photoFenton catalysis, we further carried out PL spectroscopy measurements (Figure 8a), which shows that the PL emission intensities of FeOCl/POMs are obviously weaker than that of FeOCl, indicating the higher separation efficiency of FeOCl/ POMs.51 Therefore, the interfacial contact between FeOCl and POMs effectively increases the separation efficiency. Moreover, the weaker PL emission intensity of FeOCl/POM-Mo than that of FeOCl/POM-W indicates a higher separation efficiency of FeOCl/POM-Mo, which is in accordance with the results in the trapping experiments of active species (Figure 7). Furthermore, EIS was also measured (Figure 8b), which reveals that the arc radius of FeOCl is much larger than that of FeOCl/POMs, suggesting that FeOCl/POMs exhibit a faster charge transfer and a more efficient separation of the photogenerated charge carriers than FeOCl.52 In addition, the interfacial charge transfer and separation of the photogenerated charge carriers over FeOCl/POM-Mo is more efficient than those over FeOCl/POM-W (Figure 8b). Combining the PL spectra and EIS results, the combination of FeOCl and POMs effectively improves the charge transfer and separation of the photogenerated electron−hole pair. To further investigate the electronic interaction between FeOCl and POMs, the photocurrent transient response measurements for FeOCl, FeOCl/POM-Mo, and FeOCl/POM-W were employed. Figure 8c illustrates the current−time (I−t) characteristics in three different electrodes upon several light ON/OFF cycles, which show that the photocurrent value rapidly decreases as soon as the light turns off and comes back to a constant value when the light is on again. Compared with FeOCl, the combination of POMs and FeOCl effectively increases the photocurrent value, indicating an increase in the photoinduced carrier transport rate and the photoinduced electron−hole pair separation.53,54 FeOCl/POM-Mo displays the best photoswitching performance among the three samples. Therefore, the remarkably increased photocurrent density of FeOCl/POM-Mo can be ascribed to the fastest electron transport from the VB to the CB and the highest separation efficiency of the photogenerated electron−hole pairs among the three samples. In addition, these results are good in accord with the photocatalytic activity measurements of the asprepared samples (Figures 5 and 6).

Figure 9. Total charge densities of (a) FeOCl/POM-Mo and (b) FeOCl/POM-W interface. The isosurface level is 1.5.

indicate that the interfaces are connected by Mo−O−Fe and W−O−Fe.55−58 The bond lengths of Fe−O at the interface of FeOCl/POMs (1.78 Å in FeOCl/POM-Mo and 1.77 Å in FeOCl/POM-W) are smaller than that of FeOCl (2.03 Å, Figure S11), which indicates the stronger interaction of Fe−O in FeOCl/POMs than that in FeOCl, suggesting enhanced structural stability at the interface between FeOCl and POMs (Figure 9a,b).59,60 To understand the charge redistribution at the interface, the Mulliken bond population was calculated, which shows that the electropositivity of Fe in FeOCl/POMs (1.04 in FeOCl/POM-Mo and 1.19 in FeOCl/POM-W) is larger than that of Fe in FeOCl (0.75, Figure S10), suggesting a strong interfacial interaction.61,62 In addition, the increased electropositivity of Fe in FeOCl/POM-W compared with that of Fe in FeOCl/POM-Mo reveals the stronger reduction in POM-W than in POM-Mo, which is consistent with the XPS results (Figure 2). Furthermore, the total charge densities in FeOCl/POMs demonstrate that the charges accumulate at the interface of Fe−O−Mo and Fe−O−W.63,64 On the basis of the above discussions, a synergistic mechanism for the enhanced photo-Fenton catalytic performance of FeOCl/POMs is illustrated (Figure 10). Both FeOCl and POMs can yield photogenerated electron and hole pairs under sunlight. The CB potential of POM-W (0.03 eV) is more negative than that of FeOCl (0.11 eV), which leads to the rapid electron transfer from POM-W to FeOCl, resulting in the electron accumulation in the CB of FeOCl. This behavior F


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nism of FeOCl/POM-W are proposed for the enhanced photoFenton catalytic performance. The study has made great strides toward enhancing our understanding of the novel photoFenton catalyst in the applications of environmental crisis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02329. TEM, SEM, EDXS, and elemental mapping images of FeOCl/POMs; XPS of FeOCl; catalytic activity under darkness; cycling degradation experiments; XRD patterns and TEM after reaction; UV−vis diffusive reflectance spectra; Mott−Schottky plots; and total charge densities of FeOCl (PDF)

Figure 10. Schematic illustration of the mechanism for the photoFenton catalytic performance of FeOCl/POMs.

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effectively accelerates the reduction of FeIII to FeII, which subsequently reacts with H2O2 to generate •OH radicals. The VB of POM-W (3.01 eV) is more positive than the •OH/H2O potential (ca. 2.27 V), which is beneficial for driving the oxidation process of H2O to form •OH radicals. In addition, the h+ in the VB of POM-W can also directly oxidize the degradation of phenol. Therefore, the enhanced photo-Fenton catalytic performance of FeOCl/POM-W is attributed to the double-transfer mechanism. If the double-transfer mechanism is adopted for FeOCl/POM-Mo, the photogenerated electrons in the CB of FeOCl could quickly transfer to POM-Mo because of the more negative CB of FeOCl (0.11 eV) than that of POM-Mo (0.60 eV), which leads to the inadequacy of electrons in the CB of FeOCl. However, both productions of • OH radicals and •O2− radicals over FeOCl/POM-Mo are larger than that over FeOCl/POM-W (Figures 7 and S7), which contradicts the hypothesis of the double-transfer mechanism. Therefore, a Z-scheme mechanism for FeOCl/ POM-Mo is adopted under our experimental conditions. As both the CB and VB positions of POM-Mo are lower than that of FeOCl, the electrons in the CB of POM-Mo tend to transfer and recombine with the holes in the VB of FeOCl. As such, the holes left behind in the VB of POM-Mo (2.94 eV) can directly oxidize the degradation of phenol. Hence, the contributions of POM-W and POM-Mo for the enhanced photo-Fenton catalytic performance of FeOCl/POMs are mainly manifested by the accumulation of electrons in FeOCl and the reduced recombination of electron−hole pairs in FeOCl. Because the CB potentials of FeOCl and POM-W are close, the separation efficiency of the electron−hole pairs of FeOCl/POM-W are lower than that of FeOCl/POM-Mo, which has been proven by the PL spectra and EIS spectra (Figure 8). Therefore, FeOCl/POM-Mo shows higher photo-Fenton catalytic activity than FeOCl/POM-W.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.Z.). *E-mail: [email protected] (G.L.). ORCID

Jian Zhang: 0000-0003-2411-3037 Jingquan Sha: 0000-0002-5925-9565 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Natural Science Foundation of Shandong Province (ZR2016BB21), the Science and Technology plan project of Shandong Colleges and Universities (J17KB063), and Talent Team Culturing Plan for Leading Disciplines of University in Shandong Province.

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4. CONCLUSIONS In summary, using a facile one-step calcination method, the novel FeOCl/POMs composite was successfully synthesized. The contributions of POMs in FeOCl/POMs are mainly reflected in increased optical absorption, surface area, and surface potential. On the basis of the above-mentioned aspects, FeOCl/POMs show greatly enhanced photocatalytic activities and structure stability for phenol degradation than FeOCl. The DFT calculations reveal the strong interface interaction by Fe− O−Mo and Fe−O−W in FeOCl/POMs. The Z-scheme mechanism of FeOCl/POM-Mo and double-transfer mechaG


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POM Heterojunctions with Excellent Fenton Catalytic

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