Facile One-Step Fabrication of Phthalocyanine–Graphene–Bacterial–Cellulose Nanocomposite with Superior Catalytic Performance

nanomaterials

Article Facile One-Step Fabrication of Phthalocyanine–Graphene–Bacterial–Cellulose Nanocomposite with Superior Catalytic Performance

Qiulin Hong and Shiliang Chen *

Institute of Environmental Sciences, Qianjiang College, Hangzhou Normal University, Hangzhou 310018, China; [email protected] * Correspondence: [email protected]; Tel.: +86-571-28861372

Received: 30 July 2020; Accepted: 20 August 2020; Published: 26 August 2020

Abstract: It is generally accepted that the convenient fabrication of a metal phthalocyanine-based heterogeneous catalyst with superior catalytic activity is crucial for its application. Herein, a novel and versatile ultrasonic-assisted biosynthesis approach (conducting ultrasonic treatment during biosynthesis process) was tactfully adopted for the direct immobilization of a sulfonated cobalt phthalocyanine (PcS) catalyst onto a graphene–bacterial cellulose (GBC) substrate without any modification. The prepared phthalocyanine–graphene–bacterial–cellulose nanocomposite, PcS@GBC, was characterized by field emission scanning electron microscope (FESEM) and X-ray photoelectron spectroscopy (XPS). The catalytic activity of the PcS@GBC was evaluated based on its catalytic oxidation performance to dye solution, with H2O2 used as an oxidant. More than a 140% increase of dye removal percentage for the PcS@GBC heterogeneous catalyst was found compared with that of PcS. The unique hierarchical architecture of the GBC substrate and the strong interaction between PcS and graphene, which were verified experimentally by ultraviolet-visible light spectroscopy (UV-vis) and Fourier transform infrared spectroscopy (FT-IR) and theoretically by density functional theory (DFT) calculation, were synergistically responsible for the substantial enhancement of catalytic activity. The accelerated formation of the highly reactive hydroxyl radical ( OH) for PcS@GBC was · directly evidenced by the electron paramagnetic resonance (EPR) spin-trapping technique. A possible catalytic oxidation mechanism for the PcS@GBC–H2O2 system was illustrated. This work provides a new insight into the design and construction of a highly reactive metal phthalocyanine-based catalyst, and the practical application of this functional nanomaterial in the field of environmental purification is also promising.

Keywords: graphene; bacterial cellulose; phthalocyanine; nanocomposite; synergism

1. Introduction Metal phthalocyanine complexes (MPcs) are fascinating macrocyclic compounds for many applications [1–3], especially in the area of bio-inspired catalysts [4–6], considering their structural relations to naturally occurring metal porphyrin complexes. Employing MPcs as versatile catalysts in different types of reactions were extensively studied [7–9], and enormous strides were made in this field, while the simple preparation of an MPc catalyst with excellent catalytic performance is still a major challenge. The catalytic property of MPc is dependent on various factors; to adequately explore its catalytic performance, the fundamental mechanism responsible for the catalytic reaction of MPc should be fully understood. Firstly, MPc has a high tendency to form inactive aggregates, and the immobilization of MPc onto appropriate support is a logical choice to offset this shortage. Secondly, the catalytic process

Nanomaterials 2020, 10, 1673; doi:10.3390/nano10091673 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1673 2 of 14 of MPc is crucially dependent on the complexity of electron transfer after coordination between MPc and reactant; thus, providing a microenvironment with outstanding electron transporting property is potentially another strategy to enhance its catalytic activity. As an important member of carbon allotropes, graphene constitutes a truly two-dimensional planar sheet of sp2-hybridized carbon atoms. This unique structural feature results in outstanding physicochemical properties, including extremely large specific surface area, excellent mechanical property, and high electrical conductivity. The application of graphene in various fields, such as sensors, electrodes, and nanofiller, has been frequently reported [10–15]. Particularly, the graphene framework can be employed as an ideal support for the incorporation of various functional materials [16–18]. Based on the hierarchical structures of both MPc and graphene, the immobilization of MPc onto graphene is theoretically and experimentally feasible [19–24]. In addition, considering that graphene possesses high electrical conductivity and superior electron mobility, synergistically enhanced performance is reasonably expected with the combination of MPc and graphene. In contrast to individual graphene nanosheets, macroscale graphene-based architectures with three-dimensional structures may be a better choice when used as support for MPc catalyst. The 3D structures can not only improves the dispersion of graphene and reduce the stack of graphene nanosheets, but also promotes the diffusion adsorption of the reactants and improves the accessibility of reactants to the active sites, which is also important to the heterogeneous catalyst [25–27]. In our previous work, a graphene-incorporated bacterial cellulose (GBC) nanohybrid was employed as support for the covalent immobilization of tetraamino cobalt phthalocyanine (CoPc) catalyst [28], and an improved catalytic activity of CoPc was found. However, several disadvantages of this technique should be noted. Firstly, MPc was not directly immobilized onto graphene; thus, the communication efficiency between these two electroactive components was significantly reduced, which in return affects the catalytic performance of MPc. Secondly, the covalently binding method is relatively complex, and organic solvent was essential for the immobilization. Moreover, the microstructure of the BC support was inevitably damaged during the chemical treatment. These issues created the objective of the present work, in which a facile and convenient one-step ultrasonic-assisted biosynthesis approach (conducting ultrasonic treatment during biosynthesis process) [29–31] was developed for the direct immobilization of sulfonated cobalt phthalocyanine (PcS) catalyst onto the graphene–bacterial cellulose (GBC) substrate. The prepared nanocomposite, PcS@GBC, was employed as the heterogeneous catalyst for the catalytic oxidation of reactive red X-3B dye molecules. The influence of GBC substrate on the dye removal efficiency of PcS was thoroughly investigated, and the strong interaction between PcS and graphene were identified experimentally by Fourier transform infrared spectroscopy (FT-IR), ultraviolet-visible light spectroscopy (UV-vis), and electron paramagnetic resonance (EPR) technologies and theoretically by density functional theory (DFT) calculation.

2. Materials and Methods

2.1. Materials and Reagents Cellulose-forming bacterium Acetobacer xylinum (A. xylinum) was purchased from BeNa Culture Collection Co. Ltd. (Beijing, China). Graphene solution (0.4–0.5 wt %, with 0.4–0.5 wt % dispersant) was purchased from Aladdin Co. Ltd. (Shanghai, China). Sulfonated cobalt phthalocyanine (PcS, 98 wt %) was purchased from Energy Chemical Co., Ltd. (Shanghai, China) and was puriﬁed by a recrystallization process. Reactive red X-3B (RR) was purchased from Shanghai Chemical Reagent Factory (Shanghai, China). 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was purchased from Sigma Chemical Co. (Saint Louis, MO, USA). All other common chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China). Nanomaterials 2020, 10, 1673 3 of 14

2.2. Preparation of PcS@GBC PcS@GBC nanocomposite was prepared through direct immobilization of PcS onto the GBC substrate during the biosynthesis process; the typical preparation procedure is as follows. A mixed culture medium composed of 10.0 wt % D-glucose, 1.0 wt % yeast extract, 0.5 wt % peptone, and 1.0 wt % ethanol was sterilized at 121 ◦C in an autoclave for 30 min. To initiate the biosynthesis process, the bacterium Acetobacter xylinum was added into the mixture, and the temperature was kept constant at 30 ◦C. After cultivation for 24 h, a certain amount of PcS solution and graphene solution were separately added every 24 h, and the resulting mixture was conducted with ultrasonic treatment. After cultivation for 10 days, the sample was collected, incubated in a NaOH solution (0.10 mol/L) for 30 min, thoroughly washed with ultraﬁltration water, and subsequently stored in ultraﬁltration water for future use. The immobilized PcS amount of PcS@GBC (µmol/g) was calculated as follows: n immobilized PcS (µmol/g) = 1 (1) m0 where n1 is the mole number of PcS, which equals the mole number of the Co element and was measured by atomic absorption spectrometry (Thermo Sollar M6); m0 is the weight of the PcS@GBC nanocomposite. The graphene content of PcS@GBC (mg/g) was calculated as follows: m graphene content (%) = 2 100% (2) m0 × where m2 is the weight of graphene in PcS@GBC, which was measured from the precipitated weight of PcS@GBC after digestion; m0 is the weight of the PcS@GBC nanocomposite.

2.3. Characterization The morphologies and compositions of pure BC, GBC, and PcS@GBC nanocomposites were monitored by ﬁeld emission scanning electron microscopy (FESEM, Serion, FEI, USA) and X-ray photoelectron spectroscopy (XPS). XPS spectra of all samples were recorded on a Kratos Axis Ultra XPS system with Al (mono) Kα irradiation (hν = 1486.6 eV). The binding energy peaks of all the XPS spectra were calibrated by placing the principal C 1s binding energy peak at 284.6 eV. The functional groups of PcS, graphene, PcS–graphene mixture, and PcS–graphene nanohybrid were characterized by Fourier transform infrared spectra (FT-IR, Brucker Optics, Switzerland). Each spectrum of FT-IR was 1 taken by 32 scans at a nominal resolution of 4 cm− . The Gaussian09 program package was used to perform the density functional theory (DFT) calculation [32]. The B3LYP-D3 with a 6-31G(d) basis set was used for the geometry optimization.

2.4. Catalytic Oxidation Studies and Analysis To study the catalytic activity of the PcS@GBC nanocomposites, RR dye solution was employed as the model target and H2O2 was employed as an oxidant. The reaction was carried out in a stirred tank glass reactor and placed in a thermostatic water bath with the temperature set to 50 ◦C. The typical composition of the reaction mixture was 5 mL of RR dye solution (initial concentration 100 µmol/L) and 0.75 mg of PcS@GBC nanocomposite (immobilized PcS: 43 µmol/g, graphene content: 20.50%). The pH value of the RR solution was adjusted to the desired value by using 1 mol/L HClO4 and 1 mol/L NaOH. To initiate the catalytic process, a given volume of H2O2 was added into the above-mentioned reaction mixture. The concentration of RR solution, which is proportional to its maximum absorbance at 539 nm, was monitored by a UV-Vis absorption spectrometer UV-2450. The dye removal percentage of the solution was expressed as the value of (1 C/C ), where C is the instant concentration of RR solution, − 0 Nanomaterials 2020, 10, 1673 4 of 14

and C0 is the initial concentration of RR solution. The catalytic activity of PcS@G/BC nanocomposite was evaluated by the value of dye removal eﬃciency, which was calculated as follows:

3 100µmol/L 5 10− L (1 (C/C0) Dye removal eﬃciency (µmol/g) = × × × − (3) 7.5 10 4g × − where (1 (C/C )) is the percentage of removed RR dye after treatment. The EPR signals of radical − 0 spin-trapped by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were detected with a Bruker-A300 X-band EPR spectrometer (Bruker, Karlsruhe, Germany). To test the stability of PcS@G/BC for cyclic runs, the heterogeneous catalyst was recycled after treatment, thoroughly washed with ultrapure water, and vacuum dried at 25 ◦C for 24 h for the next use.

3. Results and Discussion

3.1. Materials Characterization PcS@GBC nanocomposite was prepared by the direct immobilization of PcS onto the GBC substrate. The macro- and microstructures of BC, GBC, and PcS@GBC were observed by digital images and FESEM, respectively. As expected, the pure BC membrane shows a white color (inset of Figure1A), and an interconnected three-dimensional (3D) network morphology was found (Figure1A), which was important for the good dispersion of graphene and the subsequent immobilization of PcS. With the incorporation of graphene, the membrane turns to dark black (inset of Figure1B), and the adsorption of graphene onto BC can be easily observed (Figure1B). After PcS was immobilized onto GBC, the resulting PcS@GBC membrane displays a deep green color (inset of Figure1C), and the morphology became much denser compared with that of GBC (Figure1C). In addition, the elemental mapping images (Figure1D–F) and the energy dispersive X-ray spectroscopy (EDS) spectrum (Figure1G) of PcS@GBC clearly show the distribution and existence of N, S, and Co elements, indicating the uniformly immobilization of PcS onto GBC and the successful preparation of the PcS@GBC nanocomposite. The chemical compositions of pure BC, the GBC nanohybrid, and the PcS@GBC nanocomposite were monitored by XPS. For BC, the characteristic peaks at 284.6 eV and 531.6 eV were ascribed to the binding energies of C 1s and O 1s, respectively (Figure2A(a)). When graphene was incorporated into BC, the decrease of O 1s peak intensity (with decreasing the O atomic ratio from 44% to 28%) and the increase of C 1s peak intensity (with increasing the C atomic ratio from 55% to 71%) were found (Figure2A(b)). For PcS@GBC, the marked increased peak at 398.6 eV was observed, which was the typical signal of N 1s (Figure2A(c)). A Co 2p 3/2 peak and Co 2p1/2 peak located within the range of 777–781 eV and 792–796 eV were detected, implying the existence of a Co element for PcS@GBC (Figure2B). In addition, the S2p peaks spinning at 166.4 eV and 161.7 eV correspond to the sulfinyl group and sulfide group, and the binding energy located at 168.2 eV can be ascribed to sulfonyl group of PcS (Figure2C). The detection of S and Co elements further verified the successful preparation of the PcS@GBC nanocomposite.

3.2. Study of Interaction between PcS and Graphene The interaction between PcS and graphene is of vital importance to the successful preparation of the PcS@GBC nanocomposite and the subsequent catalytic performance; therefore, the detailed interaction process between these two nanocomponents is urged to be thoroughly understood. Figure3A shows the digital images of the color changes of the PcS solution after the addition of graphene and subsequently the ultrasonic treatment. The PcS solution exhibited a color of brilliant blue, and the color turned to green when graphene was added. Interestingly, the color of the PcS–graphene mixture has a dramatic change when ultrasonic treatment was carried out; a yellow-colored solution was formed with ultrasonication for 4 h. Nanomaterials 2020, 10, x FOR PEER REVIEW 4 of 14 where (1−(C/C0)) is the percentage of removed RR dye after treatment. The EPR signals of radical spin-trapped by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were detected with a Bruker-A300 X-band EPR spectrometer (Bruker, Karlsruhe, Germany). To test the stability of PcS@G/BC for cyclic runs, the heterogeneous catalyst was recycled after treatment, thoroughly washed with ultrapure water, and vacuum dried at 25 °C for 24 h for the next use.

3. Results and Discussion

N,Nanomaterials S, and Co2020 elements,, 10, 1673 indicating the uniformly immobilization of PcS onto GBC and the successful5 of 14 preparation of the PcS@GBC nanocomposite.

Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 14

The chemical compositions of pure BC, the GBC nanohybrid, and the PcS@GBC nanocomposite were monitored by XPS. For BC, the characteristic peaks at 284.6 eV and 531.6 eV were ascribed to the binding energies of C 1s and O 1s, respectively (Figure 2A(a)). When graphene was incorporated into BC, the decrease of O 1s peak intensity (with decreasing the O atomic ratio from 44% to 28%) and the increase of C 1s peak intensity (with increasing the C atomic ratio from 55% to 71%) were found (Figure 2A(b)). For PcS@GBC, the marked increased peak at 398.6 eV was observed, which was the typical signal of N 1s (Figure 2A(c)). A Co 2p3/2 peak and Co 2p1/2 peak located within the range of 777–781 eV and 792–796 eV were detected, implying the existence of a Co element for PcS@GBCFigureFigure (Figure1. 1. FieldField 2B).emission emission In addition, scanning scanning the electron S2p electron peaks microscope spinning microscope (FESEM) at (FESEM)166.4 and eV optical and and 161.7 optical images eVimages correspond(inset) of (inset) (A) to the sulfinylbacterialof (Agroup) bacterial cellulose and sulfide cellulose (BC); group, (BC);(B) graphene–bacterial (andB) graphene–bacterial the binding energycellulose cellulose located (GBC), (GBC), at and168.2 and (CeV ()C )canthe the beprepared prepared ascribed to sulfonylphthalocyanine–graphene–bacterial–cellulosephthalocyanine–graphene–bacterial–cellulose group of PcS (Figure 2C). The detection na nanocomposite nocompositeof S and Co elements(PcS@GBC); (PcS@GBC); further the the elemental elemental verified themapping mapping successful preparationimagesimages of of of N, N, the S, S, and andPcS@GBC Co Co elements elements nanocomposite. ( (DD,–EF,F)) and and EDS EDS spectrum spectrum ( G(G)) of of PcS@GBC. PcS@GBC.

FigureFigure 2.2. ((AA)) XPSXPS spectraspectra ofof a:a: BC,BC, b:b: GBC,GBC, andand c:c: PcS@GBC; the details of the Co regionregion ((B)) andand SS regionregion ((CC)) ofof [email protected]@GBC.

3.2. Study of Interaction Between PcS and Graphene The interaction between PcS and graphene is of vital importance to the successful preparation of the PcS@GBC nanocomposite and the subsequent catalytic performance; therefore, the detailed interaction process between these two nanocomponents is urged to be thoroughly understood. Figure 3A shows the digital images of the color changes of the PcS solution after the addition of graphene and subsequently the ultrasonic treatment. The PcS solution exhibited a color of brilliant blue, and the color turned to green when graphene was added. Interestingly, the color of the PcS– graphene mixture has a dramatic change when ultrasonic treatment was carried out; a yellow-colored solution was formed with ultrasonication for 4 h.

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Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 14

FigureFigure 3.3. ((A)) Color changes of of sulfonated sulfonated cobalt cobalt phthaloc phthalocyanineyanine (PcS) (PcS) solution solution with with the the addition addition of ofgraphene graphene and and the the ultrasonic ultrasonic treatment; treatment; (B (B) )EEffectﬀect of of ultrasonic ultrasonic time on thethe UV-visUV-vis absorptionabsorption spectrumspectrum of of the the PcS–graphene PcS–graphene nanohybrid; nanohybrid; (C ()EC)ﬀ Effectect of timeof time on on the the UV-vis UV-vis absorption absorption spectrum spectrum of the of PcS–graphenethe PcS–graphene mixture. mixture.

UV-visUV-vis absorption absorption spectroscopy spectroscopy was was employed employed to furtherto further understand understand the interactionthe interaction between between PcS andPcS grapheneand graphene (Figure (Figure3B). The 3B). PcS The solution PcS showedsolution a showed strong Q-band a strong characteristic Q-band characteristic peak centered peak at 670centered nm, which at 670 was nm, the which result was of the theπ –resultπ* transition of the π of–π mobile* transition electrons of mobile of PcS electrons from the of ground PcS from state the to theground first excitedstate to state the (s0first exciteds1) [33,34 state]. An (s0 additional→s1)[33,34]. weak An vibrational additional satellite weak vibrational band centered satellite at 605 band nm → wascentered the result at 605 of nm intermolecular was the result aggregations of intermolecul betweenar aggregations the PcS units. between The Soret the PcS band units. characteristic The Soret peakband in characteristic the ultraviolet peak light in regionthe ultraviolet was also light observed, region whichwas also can observed, be attributed which to thecan transitionbe attributed from to thethe groundtransition state from to the the ground second state excited to the state second (s0 exciteds2). When state graphene (s0→s2). When was added, graphene the was decreased added, → intensitythe decreased of the Q-bandintensity peak of withthe Q-band ultrasonic peak time with was observed,ultrasonic indicatingtime was theobserved, existence indicating of the strong the interactionexistence of between the strong PcS andinteraction graphene. between Furthermore, PcS and the graphene. red shift ofFurthermore, the Q-band ofthe the red PcS–graphene shift of the nanoconjugateQ-band of the wasPcS–graphene also noticed nano (Figureconjugate S1), which was also also noticed indicated (Figur the changee S1), which of the also physicochemical indicated the propertychange of of the PcS physicochemical by graphene and property the electronic of PcS interaction by graphene between and the the electronic two nanocomponents. interaction between the twoFor comparison,nanocomponents. when no ultrasound was applied, an approximately 10% decrease of absorption strengthFor of comparison, the Q-band ofwhen the PcS–graphene no ultrasound mixture was was applied, found withinan approximately 1 h (Figure3C). 10% While decrease a further of increaseabsorption of time strength has little of the eff ectQ-band on the of absorption the PcS–graphene intensity mixture of the mixture, was found the spectrumwithin 1 h keeps (Figure almost 3C). intact,While evena further prolonging increase the of mixture time has time little to 192 effect h. Inon addition, the absorption the observation intensity of of the the shoulder mixture, peak the centeredspectrum at 605keeps nm suggestedalmost intact, that someeven of prolonging the PcS molecules the mixture remain intime the aggregationto 192 h. In state. addition, This result the impliesobservation that theof ultrasonicthe shoulder treatment peak centered was effective at 605 and nm indispensable suggested that for some the good of the dispersion PcS molecules of PcS moleculesremain in ontothe aggregation the graphene state. nanosheet. This result implies that the ultrasonic treatment was effective and indispensableFourier transform for the good infrared dispersion (FT-IR) of spectroscopyPcS molecules is onto an etheffective graphene technique nanosheet. to characterize the functionalFourier groups transform of samples. infrared Figure(FT-IR)4 spectroscopyshows the FT-IR is an absorptioneffective technique spectra ofto PcS,characterize graphene, the thefunctional PcS–graphene groups mixtureof samples. (product Figure without 4 shows ultrasonication), the FT-IR absorption and the spectra PcS–graphene of PcS, graphene, nanohybrid the 1 (productPcS–graphene with ultrasonication). mixture (product For purewithout PcS (Figure ultrasonication),4a), the characteristic and the peak PcS–graphene located at 915 nanohybrid cm − was associated(product with with ultrasonication). metal–ligand vibration, For pure which PcS (Figure revealed 4a), the the coordination characteristic between peak thelocated 3d unoccupied at 915 cm−1 orbitalwas associated of Co and thewith four meta surroundingl–ligand vibration, nitrogen atomswhich in revealed the pyrrole the rings coordination [34–36]. The between characteristic the 3d 1 1 1 peaksunoccupied at 622cm orbital− , 1326of Co cm and− ,the and four 1514 surrounding cm− represents nitrogen the atoms skeleton in the structure pyrrole rings vibration [34–36]. of PcS, The characteristic peaks at 622cm−1, 1326 cm−1, and 1514 cm−1 represents the skeleton structure vibration of PcS, the C=C stretching vibration of the aromatic nucleus, and the C–C strectching vibration of

Nanomaterials 2020, 10, 1673 7 of 14

Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 14 theNanomaterials C=C stretching 2020, 10, vibrationx FOR PEER ofREVIEW the aromatic nucleus, and the C–C strectching vibration of pyrrole.7 of 14 The characteristic peaks at 1719 cm 1 and 1028 cm 1 were attributed to the C=N stretching vibration pyrrole.pyrrole. The The characteristic characteristic peaks peaks at −at 1719 1719 cm cm−1− 1and and− 1028 1028 cm cm−1− 1were were attributed attributed to to the the C=N C=N stretching stretching and S=O stretching vibration of PcS, respectively. For graphene, the absorption peaks that appeared vibrationvibration and and S=O S=O stretching stretching vibration vibration of of PcS, PcS, resp respectively.ectively. For For graphene, graphene, the the absorption absorption peaks peaks that that at 1580 cm 1 and 1210 cm 1 were attributed to the skeletal vibration of graphene nanosheets appearedappeared at at− 1580 1580 cm cm−1− 1and and 1210 1210− cm cm−1− 1were were attributed attributed to to the the skeletal skeletal vibration vibration of of graphene graphene nanosheets nanosheets (Figure4b) [37,38] . All these peaks can be found for the PcS–graphene mixture (Figure4c). In contrast, (Figure(Figure 4b) 4b) [37,38]. [37,38]. All All these these peaks peaks can can be be found found for for the the PcS–graphene PcS–graphene mixture mixture (Figure (Figure 4c). 4c). In In an obvious blue shift of the characteristic peaks of C=N (1725 cm 1), S=O (1073 cm 1), and cobalt–ligand contrast,contrast, an an obvious obvious blue blue shift shift of of the the characteristic characteristic peaks peaks of of C=N− C=N (1725 (1725 cm cm−1−),1), −S=O S=O (1073 (1073 cm cm−1−),1), and and (960 cm 1) was found for the PcS–graphene nanohybrid (Figure4d), which provides further evidence cobalt–ligandcobalt–ligand− (960 (960 cm cm−1−)1 )was was found found for for the the PcS–graphene PcS–graphene nanohybrid nanohybrid (Figure (Figure 4d), 4d), which which provides provides of the strong interaction between PcS and graphene and the formation of a PcS–graphene nanohybrid furtherfurther evidence evidence of of the the strong strong interaction interaction betw betweeneen PcS PcS and and graphene graphene and and the the formation formation of of a aPcS– PcS– with the ultrasonic treatment. graphenegraphene nanohybrid nanohybrid with with the the ultrasonic ultrasonic treatment. treatment.

FigureFigureFigure 4. 4. Fourier FourierFourier transform transform infrared infrared spectroscopy spectroscopy spectroscopy (FT-IR) (FT-IR) (FT-IR) spectra spectra spectra of of ( of a():a ( ):aPcS, ):PcS, PcS, ( b(b): ): (graphene,b graphene,): graphene, ( c():c ):PcS– PcS– (c): PcS–graphenegraphenegraphene mixture, mixture, mixture, and and and( d(d): ):PcS–graphene ( dPcS–graphene): PcS–graphene nanohybrid. nanohybrid. nanohybrid.

BasedBasedBased onon on the the the above-mentioned above-mentioned above-mentioned results results results of UV-vis of of UV-v UV-v andis FT-IRis and and spectroscopy FT-IR FT-IR spectroscopy spectroscopy measurements, measurements, measurements, a schematic a a diagramschematicschematic of diagram thediagram interaction of of the the process interact interact betweenionion process process PcS andbetween between graphene PcS PcS isand and shown graphene graphene in Figure is is 5shown .shown Without in in Figure externalFigure 5. 5. forces,WithoutWithout the external external PcS molecules forces, forces, the were the PcS PcS randomly molecules molecules adsorbed were were randomly randomly onto the surfaceadsorbed adsorbed of graphene,onto onto the the surface surface and the of of interaction graphene, graphene, betweenandand the the theseinteraction interaction two nanocomponents between between thes these e wastwo two relativelynanocomponents nanocomponents weak. When was was ultrasonicrelati relativelyvely treatmentweak. weak. When When was conductedultrasonic ultrasonic ontreatmenttreatment the PcS–graphene was was conducted conducted mixture, on on the thethe PcS–graphene likelihoodPcS–graphene of themixture, mixture, optimum the the contactlikelihood likelihood between of of the the PcS optimum optimum and graphene contact contact wasbetweenbetween signiﬁcantly PcS PcS and and increased.graphene graphene was Thewas significantly structuralsignificantly re-arrangement increase increased.d. The The of structural structural PcS results re-arrangement re-arrangement in the stronger of of conjugatedPcS PcS results results electronicinin the the stronger stronger interaction conjugated conjugated between electronic electronic PcS and interaction interaction graphene. between between PcS PcS and and graphene. graphene.

FigureFigure 5. 5. Schematic Schematic diagram diagram of of the the interaction interaction proces process sof of PcS PcS and and graphene graphene with with ultrasonic ultrasonic Figure 5. Schematic diagram of the interaction process of PcS and graphene with ultrasonic treatment. treatment.treatment. Density functional theory (DFT) computation, an eﬀective tool to study the electronic structure of Density functional theory (DFT) computation, an effective tool to study the electronic structure nanomaterials,Density functional was adopted theory to fully (DFT) optimize computation, the structures an effective of PcS, tool graphene, to study andthe electronic the PcS–graphene structure of nanomaterials, was adopted to fully optimize the structures of PcS, graphene, and the PcS– nanohybrid.of nanomaterials, As shown was in adopted Figure6A, to the fully geometry optimize of the the fully structures optimized of PcSPcS, has graphene, a planar and macrocyclic the PcS– graphene nanohybrid. As shown in Figure 6A, the geometry of the fully optimized PcS has a planar conﬁguration,graphene nanohybrid. and the calculated As shown distance in Figure between 6A, the the geometry Co atom of and the its fully surrounding optimized N PcS atoms has (dCo–N) a planar macrocyclicmacrocyclic configuration, configuration, and and the the calculated calculated dist distanceance between between the the Co Co atom atom and and its its surrounding surrounding N N atomsatoms (dCo–N) (dCo–N) was was 1.922 1.922 Å Å (Table (Table S1). S1). The The geometry geometry of of the the fully fully optimized optimized graphene graphene is is also also planar, planar, asas expected expected (Figure (Figure 6B), 6B), while while the the bond bond distance distance (dC–C) (dC–C) was was calculated calculated to to be be 1.420 1.420 Å, Å, which which was was consistentconsistent with with the the reference reference value value [39,40]. [39,40]. For For th the ePcS–graphene PcS–graphene system, system, the the basal basal plane plane adsorption adsorption

Nanomaterials 2020, 10, 1673 8 of 14 was 1.922 Å (Table S1). The geometry of the fully optimized graphene is also planar, as expected (Figure6B), while the bond distance (dC–C) was calculated to be 1.420 Å, which was consistent with the reference value [39,40]. For the PcS–graphene system, the basal plane adsorption of PcS onto Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 14 the planar graphene was observed, and the change of the calculated dCo–N for PcS (Table S1) as well asof thePcS slightonto the deviation planar graphene of the geometrywas observed, of graphene and the change were of also the noticed calculated (Figure dCo–N6C), for implyingPcS the existence(Table of S1) an as interaction well as the slight between deviation PcS andof the graphene. geometry of The graphene extended, were also delocalized, noticed (Figure and π6C),-conjugated electronimplying system theand existence the unique of an interaction planar 18 betweenπ-electron PcS and aromatic graphene. structure The extended, of the delocalized, PcS skeleton and facilitates π-conjugated electron system and the unique planar 18 π-electron aromatic structure of the PcS this speciﬁc interaction. skeleton facilitates this specific interaction.

FigureFigure 6. Fully 6. Fully optimized optimizedgeometries geometries of of (A (A) PcS,) PcS, (B) (graphene,B) graphene, and (C and) PcS–graphene (C) PcS–graphene nanohybrid. nanohybrid. The purple,The purple, blue, blue, yellow, yellow, red, red, black, black, and and white white balls balls represent represent Co, Co, N, N, S, O,S, C,O, andC, and H atoms,H atoms, respectively. respectively. 3.3. Catalytic Oxidation Performance of PcS@GBC 3.3. Catalytic Oxidation Performance of PcS@GBC To evaluateTo evaluate its catalyticits catalytic performance, performance, the the pr preparedepared PcS@GBC PcS@GBC was employed was employed as a heterogeneous as a heterogeneous catalystcatalyst for thefor the decoloration decoloration of of RRRR solution,solution, with with H2 HO2 Oas2 anas oxidant. an oxidant. No obvious No obvious change of change dye of dye concentrationconcentration was was found found with with additionaddition of of H H2O22O, suggesting2, suggesting that RR that dye RR molecules dye molecules can hardly can be hardly be oxidizedoxidized by Hby2 OH22Oalone2 alone (Figure (Figure7 7A(a)).A(a)). When When PcS@GBC PcS@GBC was waspresent, present, a gradual a gradual decrease decrease of dye of dye concentrationconcentration was was observed, observed, which which can canbe be attributedattributed to to the the good good adsorption adsorption capacity capacity of PcS@GBC; of PcS@GBC; the the dye removal efficiency was ca. 190 μmol/g with 90 min of adsorption. Meanwhile, a further dye removal efficiency was ca. 190 µmol/g with 90 min of adsorption. Meanwhile, a further increase increase of adsorption time has little effect on the dye removal efficiency, which indicated that the of adsorptionadsorption time has reached has little a dynamic effect on equilibrium the dye removal(Figure 7A(b)). efficiency, In contrast, which when indicated both PcS@GBC that the and adsorption has reachedH2O2 were a dynamic present, equilibriuma sharp decrease (Figure of dye7A(b)). concentration In contrast, was when found, both and PcS@GBC the dye removal and H 2O2 were present,efficiency a sharp was decrease as high ofas dye600 concentrationμmol/g with only was 50 found,min of andreaction. the dyeThe removaldye removal effi ciencyefficiency was as high as 600reachedµmol/ g660 with μmol/g only when 50 min the reaction of reaction. time Thewas prolonged dye removal to 120 effi min,ciency i.e., reachedmore than 660 99%µ molof dye/g when the reactionmolecules time was were prolonged effectively to removed 120 min, (Figure i.e., more 7A(c)). than These 99% results of dye indicated molecules that were the effPcS@GBCectively removed heterogeneous catalyst has excellent catalytic activity, and dye molecules can be efficiently (Figure7A(c)). These results indicated that the PcS@GBC heterogeneous catalyst has excellent catalytic decolorized with the PcS@GBC–H2O2 reaction system. activity, and dye molecules can be efficiently decolorized with the PcS@GBC–H2O2 reaction system. To better understand the role of every component of the PcS@GBC (PcS, graphene, and BC) during the catalytic oxidation, the adsorption and catalytic oxidation behavior of different components were briefly studied (Figure7B). The dye concentration decreased by less than 10% with pure BC, and the same result was found for BC+H2O2 (Figure7B(a)), suggesting that BC has a slight adsorption capacity to dye molecules. When GBC was employed, the dye concentration decreased by more than 25%, which can be ascribed to its good affinity to organic dye molecules (Figure7B(b)). Only 4% of dye molecules can be absorbed by PcS within 90 min, while a ca. 40% decrease of dye concentration can be reached with PcS+H2O2 (Figure7B(c)), which shows that the PcS–H 2O2 system is able to catalyze the oxidation of dye molecules. Due to the formation of inactive aggregate in solution, the catalytic activity of PcS was inhibited to some extent. When PcS was immobilized onto graphene, the resulting PcS@G nanohybrid can absorb ca. 25% of dye molecules. With H2O2 added as an oxidant, the PcS@G–H2O2 Nanomaterials 2020, 10, 1673 9 of 14 reaction system shows great decoloration capacity: ca. 75% of dye molecules can be catalytic oxidized within 90 min (Figure7B(d)), which was much higher than that of PcS +H2O2. The PcS@BC can absorb 13% of dye molecules, and ca. 50% of dye molecules can be catalytic oxidized with the PcS@BC+H2O2 (Figure7B(e)), which was higher than PcS +H2O2 but lower than PcS@G+H2O2. Interestingly, the catalytic oxidation efficiency of PcS@GBC+H2O2 was significantly improved when compared with others (Figure7B(f)). With the presence of H 2O2, more than 95% of dye molecules were catalytically oxidized by PcS@GBC, which was ca. 140% higher than that of the PcS–H2O2 system. Several reasons were responsible for the substantially improved catalytic activity of PcS. Firstly, the very large surface-to-volume ratio and the distinct 3D nanofibrous network architecture of BC promotes the good dispersion of graphene, which in return improved the immobilization of PcS, and the aggregation of PcS molecules was greatly prevented; thus, the catalytic activity of PcS was accordingly enhanced. Secondly, the electron transfer efficiency plays an important role for the enhancement of catalytic activity of MPc [28,41,42]. Due to its superior electron mobility, the incorporated graphene can facilitate the electron transfer efficiency of the catalytic reaction, which also improved the catalytic activity of PcS@GBC. Therefore, the conclusion is that BC and graphene have a synergistic effect on enhancing the catalytic activity of PcS. Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 14

Figure 7. (A): Dye removal efficiency of a: H2O2 (10 mM), b: PcS@GBC (0.75 mg), and c: PcS@GBC Figure 7. (A): Dye removal efficiency of a: H2O2 (10 mM), b: PcS@GBC (0.75 mg), and c: PcS@GBC (0.75 mg) and H2O2 (10 mM). (B): Comparison chart of dye concentration change by adsorption and (0.75 mg) and H2O2 (10 mM). (B): Comparison chart of dye concentration change by adsorption and catalytic oxidation with a: BC, b: GBC, c: PcS, d: PcS@G, e: PcS@BC, and f: PcS@GBC. Reaction catalytic oxidation with a: BC, b: GBC, c: PcS, d: PcS@G, e: PcS@BC, and f: PcS@GBC. Reaction condition: initial dye concentration = 100 μM, pH = 2, T = 50 °C, reaction time = 90 min; the PcS condition: initial dye concentration = 100 µM, pH = 2, T = 50 C, reaction time = 90 min; the PcS content for c–f was kept the same. ◦ content for c–f was kept the same. To better understand the role of every component of the PcS@GBC (PcS, graphene, and BC) Figureduring8 thedisplays catalytic the oxidation, fully optimized the adsorption geometries and of catalytic PcS–H 2oxidationO2 in the behavior absence (Figureof different8A) and presencecomponents of graphene were (Figure briefly8 studiedB), respectively. (Figure 7B). The Th Co–Oe dye distance concentration (dCo–O) decreased between by PcS less and than H 10%2O2 was calculatedwith pure to be BC, 2.226 and Å the (Table same S2). result For was comparison, found for BC+H the dCo–O2O2 (Figure value 7B(a)), was 2.256suggesting Å when that graphene BC has a was present,slight which adsorption suggests capacity that the to existencedye molecules. of graphene When obviouslyGBC was employed, influences the the dye electronic concentration interaction betweendecreased PcS and by Hmore2O2 than. Furthermore, 25%, which thecan O–Obe ascribed bond distanceto its good (dO–O) affinity of to H organic2O2 was dye calculated molecules to be 1.455(Figure Å when 7B(b)). it was Only chemisorbed 4% of dye onto molecules PcS, and can the be valueabsorbed was by 1.456 PcS Åwithin when 90 graphene min, while was a alsoca. 40% present; decrease of dye concentration can be reached with PcS+H2O2 (Figure 7B(c)), which shows that the comparing with that of pristine H2O2 (1.467 Å), the obvious change of dO–O revealed that the H2O2 PcS–H2O2 system is able to catalyze the oxidation of dye molecules. Due to the formation of inactive oxidant was effectively activated by PcS. Overall, the presence of graphene can change the electronic aggregate in solution, the catalytic activity of PcS was inhibited to some extent. When PcS was interaction between PcS and H O , further influencing the catalytic activity of PcS and the resulting immobilized onto graphene,2 the2 resulting PcS@G nanohybrid can absorb ca. 25% of dye molecules. catalytic oxidation efficiency of dye molecules. With H2O2 added as an oxidant, the PcS@G–H2O2 reaction system shows great decoloration capacity: ca. 75% of dye molecules can be catalytic oxidized within 90 min (Figure 7B(d)), which was much higher than that of PcS+H2O2. The PcS@BC can absorb 13% of dye molecules, and ca. 50% of dye molecules can be catalytic oxidized with the PcS@BC+H2O2 (Figure 7B(e)), which was higher than PcS+H2O2 but lower than PcS@G+H2O2. Interestingly, the catalytic oxidation efficiency of PcS@GBC+H2O2 was significantly improved when compared with others (Figure 7B(f)). With the presence of H2O2, more than 95% of dye molecules were catalytically oxidized by PcS@GBC, which was ca. 140% higher than that of the PcS–H2O2 system. Several reasons were responsible for the substantially improved catalytic activity of PcS. Firstly, the very large surface-to-volume ratio and the distinct 3D nanofibrous network architecture of BC promotes the good dispersion of graphene, which in return improved the immobilization of PcS, and the aggregation of PcS molecules was greatly prevented; thus, the catalytic activity of PcS was accordingly enhanced. Secondly, the electron transfer efficiency plays an important role for the enhancement of catalytic activity of MPc [28,41,42]. Due to its superior electron mobility, the incorporated graphene can facilitate the electron transfer efficiency of the catalytic reaction, which also improved the catalytic activity of PcS@GBC. Therefore, the conclusion is that BC and graphene have a synergistic effect on enhancing the catalytic activity of PcS. Figure 8 displays the fully optimized geometries of PcS–H2O2 in the absence (Figure 8A) and presence of graphene (Figure 8B), respectively. The Co–O distance (dCo–O) between PcS and H2O2 was calculated to be 2.226 Å (Table S2). For comparison, the dCo–O value was 2.256 Å when graphene was present, which suggests that the existence of graphene obviously influences the electronic interaction between PcS and H2O2. Furthermore, the O–O bond distance (dO–O) of H2O2 was calculated to be 1.455 Å when it was chemisorbed onto PcS, and the value was 1.456 Å when

Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 14

graphene was also present; comparing with that of pristine H2O2 (1.467 Å), the obvious change of dO–O revealed that the H2O2 oxidant was effectively activated by PcS. Overall, the presence of Nanomaterialsgraphene 2020can , 10change, 1673 the electronic interaction between PcS and H2O2, further influencing10 of the 14 catalytic activity of PcS and the resulting catalytic oxidation efficiency of dye molecules.

Figure 8. Fully optimized geometries of PcS–H2O2 in the absence (A) and presence (B) of graphene. Figure 8. Fully optimized geometries of PcS–H2O2 in the absence (A) and presence (B) of graphene. The purple, blue, yellow, red, black, and white balls represent Co, N, S, O, C, and H atoms, respectively. The purple, blue, yellow, red, black, and white balls represent Co, N, S, O, C, and H atoms, respectively. To gain insight into the catalytic oxidation mechanism of the PcS@GBC–H2O2 reaction system, the EPR spin-trapping technique was used to detect the reactive radicals formed during the reaction. To gain insight into the catalytic oxidation mechanism of the PcS@GBC–H2O2 reaction system, As shown in Figure9A(a), no obvious EPR signal was detected with only PcS@BC. A similar spectrum the EPR spin-trapping technique was used to detect the reactive radicals formed during the reaction. was observed for the PcS@GBC nanocomposite (Figure9A(b)), which was in accordance with the As shown in Figure 9A(a), no obvious EPR signal was detected with only PcS@BC. A similar analysis of Figure7, i.e., the decoloration for PcS@GBC alone was purely a physical adsorption spectrum was observed for the PcS@GBC nanocomposite (Figure 9A(b)), which was in accordance process. The characteristic four-line spectrum with a peak intensity of 1:2:2:1 was easily observed with the analysis of Figure 7, i.e., the decoloration for PcS@GBC alone was purely a physical for the PcS@BC–H2O2 reaction system (Figure9A(c)), which was the well-known spectrum of the adsorption process. The characteristic four-line spectrum with a peak intensity of 1:2:2:1 was easily DMPO– OH adduct, indicating the formation of a highly reactive hydroxyl radical during the catalytic observed· for the PcS@BC–H2O2 reaction system (Figure 9A(c)), which was the well-known spectrum oxidation. A similar DMPO– OH spectrum can also be found for the PcS@GBC–H2O2 reaction system of the DMPO–⋅OH adduct, indicating· the formation of a highly reactive hydroxyl radical during the (Figure9A(d)). Moreover, the intensity of the signal was much higher than that of PcS–H 2O2, implying catalytic oxidation. A similar DMPO–⋅OH spectrum can also be found for the PcS@GBC–H2O2 that the GBC substrate is beneficial for the formation of OH. GBC can not only allow the good reaction system (Figure 9A(d)). Moreover, the intensity of· the signal was much higher than that of dispersion of the PcS catalyst, but also promote the electron transfer efficiency of PcS during the PcS–H2O2, implying that the GBC substrate is beneficial for the formation of ⋅OH. GBC can not only reaction; thus, more OH can be produced during the reaction, and subsequently, the catalytic activity allow the good dispersion· of the PcS catalyst, but also promote the electron transfer efficiency of PcS of PcS was significantly improved. during the reaction; thus, more ⋅OH can be produced during the reaction, and subsequently, the For practical application, the stability and the recycling performance of the heterogeneous catalyst catalytic activity of PcS was significantly improved. should be carefully considered. The catalytic activity of the PcS@GBC–H2O2 reaction system in cyclic utilization was performed, and the result was presented in Figure9B. The dye removal e fficiency has no noticeable decline after five times of repetitive use, and more than 90% of dye molecules can be decolorized within 90 min. These results showed that the PcS@GBC is a promising recyclable catalyst for the consecutive catalytic oxidation of organic pollutants. Based on the results presented above, a schematic illustration of synergistic effect of BC and graphene for the enhancement of the catalytic activity of PcS was shown in Figure 10. The super high surface-to-volume ratio and the 3D nanofibrous network architecture of BC guaranteed the good dispersion of graphene, which in return improved the immobilization of PcS molecules and prevented the formation of inactive aggregates. Besides serving as a template to immobilize the PcS catalyst, the distinct π-conjugated PcS–graphene electron donor–acceptor greatly enhanced the electron Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 14

Nanomaterials 2020, 10, 1673 11 of 14

transfer eﬃciency of the reaction system. H2O2, which was chemisorbed on the PcS, was more easily dissociatedFigure 9. to (A form): EPR the spectra highly of dye reactive solutionOH, with and existence therefore, of a: PcS@BC, the catalytic b: PcS@GBC, activity c: PcS@BC–H of PcS@GBC2O2, was · substantiallyand d: PcS@GBC–H enhanced. 2O2. (B): Cycling experiments of catalytic oxidation of dye solution (initial Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 14 concentration = 100 μM, pH = 2, T = 50 °C, reaction time = 90 min) with PcS@GBC (0.75 mg) and H2O2 (10 mM).

For practical application, the stability and the recycling performance of the heterogeneous catalyst should be carefully considered. The catalytic activity of the PcS@GBC–H2O2 reaction system in cyclic utilization was performed, and the result was presented in Figure 9B. The dye removal efficiency has no noticeable decline after five times of repetitive use, and more than 90% of dye molecules can be decolorized within 90 min. These results showed that the PcS@GBC is a promising recyclable catalyst for the consecutive catalytic oxidation of organic pollutants. Based on the results presented above, a schematic illustration of synergistic effect of BC and graphene for the enhancement of the catalytic activity of PcS was shown in Figure 10. The super high surface-to-volume ratio and the 3D nanofibrous network architecture of BC guaranteed the good dispersion of graphene, which in return improved the immobilization of PcS molecules and prevented the formation of inactive aggregates. Besides serving as a template to immobilize the PcS catalyst, the distinct π-conjugated PcS–graphene electron donor–acceptor greatly enhanced the electronFigure transfer 10. Illustration efficiency of synergistic of the reaction enhancement system. ofH 2 catalyticcaOtalytic2, which activity was ofchemisorbed PcSPcS by BC and on graphene.the PcS, was more easily dissociated to form the highly reactive ⋅OH, and therefore, the catalytic activity of 4. ConclusionsPcS@GBC was substantially enhanced. In thethe presentpresent study,study, aa greengreen andand versatileversatile ultrasonic-assistedultrasonic-assisted biosynthesisbiosynthesis approachapproach waswas presented for for a a one-step one-step fabrication fabrication of ofhighly highly reac reactivetive PcS@GBC PcS@GBC heterogeneous heterogeneous catalyst catalyst by the by direct the directimmobilization immobilization of PcSof onto PcS the onto GBC the GBCsubstrate. substrate. The interaction The interaction between between PcS and PcS graphene and graphene was wasverified verified experimentally experimentally by FT-IR by FT-IR and UV-vis and UV-vis spectr spectroscopyoscopy and theoretically and theoretically by DFT by DFTcalculation. calculation. The The PcS@GBC heterogeneous catalyst together with H2O2 can efficiently catalyze the oxidation of dye solution; more than 99% of dye molecules were effectively removed within 120 min. The GBC substrate can promote the dispersion of PcS, facilitate the electron transfer between PcS and H2O2, and subsequently accelerate the formation of hydroxyl radicals; the dye removal efficiency of PcS@GBC was as high as 660 µmol/g. The results of this work may provide a new standpoint for the design and construction of a heterogeneous MPc-based catalyst, and the practical application of this highly reactive functional nanomaterial in the field of environmental purification was also promising. Figure 10. Illustration of synergistic enhancement of catalytic activity of PcS by BC and graphene. Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/10/9/1673/s1, Figure S1: The red shift phenomenon of UV-vis absorption spectrum of the PcS–graphene nanohybrid during the4. ultrasonication Conclusions treatment, Table S1: The calculated average distances of C–C for graphene and the average In the present study, a green and versatile ultrasonic-assisted biosynthesis approach was presented for a one-step fabrication of highly reactive PcS@GBC heterogeneous catalyst by the direct immobilization of PcS onto the GBC substrate. The interaction between PcS and graphene was verified experimentally by FT-IR and UV-vis spectroscopy and theoretically by DFT calculation. The

Nanomaterials 2020, 10, 1673 12 of 14

distances of Co–N for PcS, Table S2: The calculated distances of Co–O between PcS and H2O2 and the O–O bond distance of H2O2 in the PcS–H2O2 system and graphene–PcS–H2O2 system. Author Contributions: S.C. conceived the study, designed the experiments, analyzed the data, and wrote the paper. Q.H. performed the experiments. All authors have read and agreed to the published version of the manuscript. Funding: The authors are grateful to the financial support from the National Natural Science Foundation of China (Grant No.51803044) and Zhejiang Provincial Natural Science Foundation of China (Grant No. LQ15E030005). Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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