Facile One-Step Fabrication Of Phthalocyanine–Graphene .
nanomaterialsArticleFacile One-Step Fabrication eNanocomposite with Superior Catalytic PerformanceQiulin Hong and Shiliang Chen *Institute of Environmental Sciences, Qianjiang College, Hangzhou Normal University, Hangzhou 310018, China;hongql1201@163.com* Correspondence: bruceblue@zju.edu.cn; Tel.: 86-571-28861372Received: 30 July 2020; Accepted: 20 August 2020; Published: 26 August 2020 Abstract: It is generally accepted that the convenient fabrication of a metal phthalocyanine-basedheterogeneous catalyst with superior catalytic activity is crucial for its application. Herein, a noveland versatile ultrasonic-assisted biosynthesis approach (conducting ultrasonic treatment duringbiosynthesis process) was tactfully adopted for the direct immobilization of a sulfonated cobaltphthalocyanine (PcS) catalyst onto a graphene–bacterial cellulose (GBC) substrate without anymodification. The prepared phthalocyanine–graphene–bacterial–cellulose nanocomposite, PcS@GBC,was characterized by field emission scanning electron microscope (FESEM) and X-ray photoelectronspectroscopy (XPS). The catalytic activity of the PcS@GBC was evaluated based on its catalyticoxidation performance to dye solution, with H2 O2 used as an oxidant. More than a 140% increase ofdye removal percentage for the PcS@GBC heterogeneous catalyst was found compared with that ofPcS. The unique hierarchical architecture of the GBC substrate and the strong interaction betweenPcS and graphene, which were verified experimentally by ultraviolet-visible light spectroscopy(UV-vis) and Fourier transform infrared spectroscopy (FT-IR) and theoretically by density functionaltheory (DFT) calculation, were synergistically responsible for the substantial enhancement of catalyticactivity. The accelerated formation of the highly reactive hydroxyl radical (·OH) for PcS@GBC wasdirectly evidenced by the electron paramagnetic resonance (EPR) spin-trapping technique. A possiblecatalytic oxidation mechanism for the PcS@GBC–H2 O2 system was illustrated. This work provides anew 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 purificationis also promising.Keywords: graphene; bacterial cellulose; phthalocyanine; nanocomposite; synergism1. IntroductionMetal phthalocyanine complexes (MPcs) are fascinating macrocyclic compounds for manyapplications [1–3], especially in the area of bio-inspired catalysts [4–6], considering their structuralrelations to naturally occurring metal porphyrin complexes. Employing MPcs as versatile catalysts indifferent types of reactions were extensively studied [7–9], and enormous strides were made in thisfield, while the simple preparation of an MPc catalyst with excellent catalytic performance is still amajor challenge.The catalytic property of MPc is dependent on various factors; to adequately explore its catalyticperformance, the fundamental mechanism responsible for the catalytic reaction of MPc should be fullyunderstood. Firstly, MPc has a high tendency to form inactive aggregates, and the immobilization ofMPc onto appropriate support is a logical choice to offset this shortage. Secondly, the catalytic processNanomaterials 2020, 10, 1673; aterials
Nanomaterials 2020, 10, 16732 of 14of MPc is crucially dependent on the complexity of electron transfer after coordination between MPcand reactant; thus, providing a microenvironment with outstanding electron transporting property ispotentially another strategy to enhance its catalytic activity.As an important member of carbon allotropes, graphene constitutes a truly two-dimensionalplanar sheet of sp2-hybridized carbon atoms. This unique structural feature results in outstandingphysicochemical properties, including extremely large specific surface area, excellent mechanicalproperty, 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 frameworkcan be employed as an ideal support for the incorporation of various functional materials [16–18]. Basedon the hierarchical structures of both MPc and graphene, the immobilization of MPc onto graphene istheoretically and experimentally feasible [19–24]. In addition, considering that graphene possesseshigh electrical conductivity and superior electron mobility, synergistically enhanced performance isreasonably expected with the combination of MPc and graphene.In contrast to individual graphene nanosheets, macroscale graphene-based architectures withthree-dimensional structures may be a better choice when used as support for MPc catalyst. The 3Dstructures can not only improves the dispersion of graphene and reduce the stack of graphenenanosheets, but also promotes the diffusion adsorption of the reactants and improves the accessibilityof 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 wasemployed as support for the covalent immobilization of tetraamino cobalt phthalocyanine (CoPc)catalyst [28], and an improved catalytic activity of CoPc was found. However, several disadvantagesof 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 methodis relatively complex, and organic solvent was essential for the immobilization. Moreover, themicrostructure 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-stepultrasonic-assisted biosynthesis approach (conducting ultrasonic treatment during biosynthesisprocess) [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-3Bdye molecules. The influence of GBC substrate on the dye removal efficiency of PcS was thoroughlyinvestigated, and the strong interaction between PcS and graphene were identified experimentally byFourier transform infrared spectroscopy (FT-IR), ultraviolet-visible light spectroscopy (UV-vis), andelectron paramagnetic resonance (EPR) technologies and theoretically by density functional theory(DFT) calculation.2. Materials and Methods2.1. Materials and ReagentsCellulose-forming bacterium Acetobacer xylinum (A. xylinum) was purchased from BeNa CultureCollection 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 arecrystallization process. Reactive red X-3B (RR) was purchased from Shanghai Chemical ReagentFactory (Shanghai, China). 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) was purchased from SigmaChemical Co. (Saint Louis, MO, USA). All other common chemicals were of analytical grade andpurchased from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China).
Nanomaterials 2020, 10, 16733 of 142.2. Preparation of PcS@GBCPcS@GBC nanocomposite was prepared through direct immobilization of PcS onto the GBCsubstrate during the biosynthesis process; the typical preparation procedure is as follows. A mixedculture 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 biosynthesisprocess, the bacterium Acetobacter xylinum was added into the mixture, and the temperature was keptconstant at 30 C. After cultivation for 24 h, a certain amount of PcS solution and graphene solutionwere 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) for30 min, thoroughly washed with ultrafiltration water, and subsequently stored in ultrafiltration waterfor future use. The immobilized PcS amount of PcS@GBC (µmol/g) was calculated as follows:immobilized PcS (µmol/g) n1m0(1)where n1 is the mole number of PcS, which equals the mole number of the Co element andwas measured by atomic absorption spectrometry (Thermo Sollar M6); m0 is the weight of thePcS@GBC nanocomposite.The graphene content of PcS@GBC (mg/g) was calculated as follows:graphene content (%) m2 100%m0(2)where m2 is the weight of graphene in PcS@GBC, which was measured from the precipitated weight ofPcS@GBC after digestion; m0 is the weight of the PcS@GBC nanocomposite.2.3. CharacterizationThe morphologies and compositions of pure BC, GBC, and PcS@GBC nanocomposites weremonitored by field emission scanning electron microscopy (FESEM, Serion, FEI, USA) and X-rayphotoelectron spectroscopy (XPS). XPS spectra of all samples were recorded on a Kratos Axis UltraXPS system with Al (mono) Kα irradiation (hν 1486.6 eV). The binding energy peaks of all the XPSspectra were calibrated by placing the principal C 1s binding energy peak at 284.6 eV. The functionalgroups of PcS, graphene, PcS–graphene mixture, and PcS–graphene nanohybrid were characterized byFourier transform infrared spectra (FT-IR, Brucker Optics, Switzerland). Each spectrum of FT-IR wastaken by 32 scans at a nominal resolution of 4 cm 1 .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 AnalysisTo study the catalytic activity of the PcS@GBC nanocomposites, RR dye solution was employed asthe model target and H2 O2 was employed as an oxidant. The reaction was carried out in a stirred tankglass reactor and placed in a thermostatic water bath with the temperature set to 50 C. The typicalcomposition of the reaction mixture was 5 mL of RR dye solution (initial concentration 100 µmol/L) and0.75 mg of PcS@GBC nanocomposite (immobilized PcS: 43 µmol/g, graphene content: 20.50%). The pHvalue 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 H2 O2 was added into the above-mentioned reactionmixture. 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 thesolution was expressed as the value of (1 C/C0 ), where C is the instant concentration of RR solution,
Nanomaterials 2020, 10, 16734 of 14and C0 is the initial concentration of RR solution. The catalytic activity of PcS@G/BC nanocompositewas evaluated by the value of dye removal efficiency, which was calculated as follows:Dye removal efficiency (µmol/g) 100µmol/L 5 10 3 L (1 (C/C0 )7.5 10 4 g(3)where (1 (C/C0 )) is the percentage of removed RR dye after treatment. The EPR signals of radicalspin-trapped by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) were detected with a Bruker-A300 X-bandEPR spectrometer (Bruker, Karlsruhe, Germany).To test the stability of PcS@G/BC for cyclic runs, the heterogeneous catalyst was recycled aftertreatment, thoroughly washed with ultrapure water, and vacuum dried at 25 C for 24 h for thenext use.3. Results and Discussion3.1. Materials CharacterizationPcS@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 andFESEM, respectively. As expected, the pure BC membrane shows a white color (inset of Figure 1A),and an interconnected three-dimensional (3D) network morphology was found (Figure 1A), which wasimportant for the good dispersion of graphene and the subsequent immobilization of PcS. With theincorporation of graphene, the membrane turns to dark black (inset of Figure 1B), and the adsorptionof graphene onto BC can be easily observed (Figure 1B). After PcS was immobilized onto GBC,the resulting PcS@GBC membrane displays a deep green color (inset of Figure 1C), and the morphologybecame much denser compared with that of GBC (Figure 1C). In addition, the elemental mappingimages (Figure 1D–F) and the energy dispersive X-ray spectroscopy (EDS) spectrum (Figure 1G) ofPcS@GBC clearly show the distribution and existence of N, S, and Co elements, indicating the uniformlyimmobilization 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 nanocompositewere monitored by XPS. For BC, the characteristic peaks at 284.6 eV and 531.6 eV were ascribed tothe binding energies of C 1s and O 1s, respectively (Figure 2A(a)). When graphene was incorporatedinto 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%) werefound (Figure 2A(b)). For PcS@GBC, the marked increased peak at 398.6 eV was observed, which wasthe typical signal of N 1s (Figure 2A(c)). A Co 2p3/2 peak and Co 2p1/2 peak located within the rangeof 777–781 eV and 792–796 eV were detected, implying the existence of a Co element for PcS@GBC(Figure 2B). In addition, the S2p peaks spinning at 166.4 eV and 161.7 eV correspond to the sulfinylgroup and sulfide group, and the binding energy located at 168.2 eV can be ascribed to sulfonyl groupof PcS (Figure 2C). The detection of S and Co elements further verified the successful preparation ofthe PcS@GBC nanocomposite.3.2. Study of Interaction between PcS and GrapheneThe interaction between PcS and graphene is of vital importance to the successful preparation of thePcS@GBC nanocomposite and the subsequent catalytic performance; therefore, the detailed interactionprocess between these two nanocomponents is urged to be thoroughly understood. Figure 3A shows thedigital images of the color changes of the PcS solution after the addition of graphene and subsequentlythe ultrasonic treatment. The PcS solution exhibited a color of brilliant blue, and the color turnedto green when graphene was added. Interestingly, the color of the PcS–graphene mixture has adramatic change when ultrasonic treatment was carried out; a yellow-colored solution was formedwith ultrasonication for 4 h.
immobilized onto GBC, the resulting PcS@GBC membrane displays a deep green color (inset ofFigure 1C), and the morphology became much denser compared with that of GBC (Figure 1C). Inaddition, the elemental mapping images (Figure 1D–F) and the energy dispersive X-rayspectroscopy (EDS) spectrum (Figure 1G) of PcS@GBC clearly show the distribution and existence ofN,S, and Coelements,Nanomaterials2020,10, 1673 indicating the uniformly immobilization of PcS onto GBC and the successful5 of 14preparation of the PcS@GBC nanocomposite.Nanomaterials 2020, 10, x FOR PEER REVIEW5 of 14The chemical compositions of pure BC, the GBC nanohybrid, and the PcS@GBC nanocompositewere monitored by XPS. For BC, the characteristic peaks at 284.6 eV and 531.6 eV were ascribed tothe binding energies of C 1s and O 1s, respectively (Figure 2A(a)). When graphene was incorporatedinto 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%) werefound (Figure 2A(b)). For PcS@GBC, the marked increased peak at 398.6 eV was observed, whichwas the typical signal of N 1s (Figure 2A(c)). A Co 2p3/2 peak and Co 2p1/2 peak located within therange of 777–781 eV and 792–796 eV were detected, implying the existence of a Co element forPcS@GBC2B).In addition,theS2ppeaksspinningat 1. mages(inset) of(A)to scopeand 161.7optical(inset)sulfinylgroupand sulfideandgraphene–bacterialthe binding energylocatedat and168.2eVascribed tobacterialcellulose(BC);group,(B) graphene–bacterialcellulose(GBC),thepreparedof (C)canthe bepreparedsulfonylgroup of PcS (Figure 2C). The detectionof S and Co elementsfurtherverified mposite(PcS@GBC);the tionPcS@GBCnanocomposite.images ofofofN,N,theS, andandCo elementselements(D–F) andFigure 2.2. (A)(A) XPSXPS spectraspectra ofof a:a: BC,BC, b:b: GBC,GBC, andand c:c: PcS@GBC;PcS@GBC; thethe CoCo regionregion (B)(B) andand SSFigurethe detailsdetails ofof theregion(C)ofPcS@GBC.region (C) of PcS@GBC.3.2. Study of Interaction Between PcS and GrapheneThe interaction between PcS and graphene is of vital importance to the successful preparationof the PcS@GBC nanocomposite and the subsequent catalytic performance; therefore, the detailedinteraction 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 ofgraphene and subsequently the ultrasonic treatment. The PcS solution exhibited a color of brilliantblue, and the color turned to green when graphene was added. Interestingly, the color of the PcS–
Nanomaterials 2020, 10, 1673Nanomaterials 2020, 10, x FOR PEER REVIEW6 of 146 of 14Figure3.3. (A) Color changesFigurechanges ofof sulfonatedsulfonated nictimeontheUV-visabsorptionof graphene and the ultrasonic treatment; (B) Effect of ultrasonicthe UV-vis aphenenanohybrid;Effectof C)(C)Effectof timeon onthe theUV-visabsorptionspectrumof thethe wasemployedto furtherunderstandthe ctroscopyspectroscopywasemployedto furtherunderstandthe tPcS and graphene (Figure 3B). The PcS solution showed a strong Q-band characteristic peak670nm, whichthewhichresultwasof thetransitionof mobileelectronsof PcSelectronsfrom the ofgroundstate thetocenteredat 670wasnm,theπ–π*resultof the π–π*transitionof mobilePcS fromthefirst excited[33,34].An(s0 s1)[33,34].additional weakband centeredat 605bandnmgroundstate tostatethe (s0 s1)first excitedstateAnvibrationaladditionalsatelliteweak vibrationalsatellitewasthe resultofnmintermolecularaggregationsbetweenthe PcS units.The Soretbandcharacteristiccenteredat 605was the resultof intermolecularaggregationsbetweenthe PcSunits.The Soretpeakthe ultravioletlightwas alsolightobserved,canobserved,be attributedto thefrombandincharacteristicpeakin regionthe ultravioletregionwhichwas alsowhichcan transitionbe attributedtothestateto thethe groundsecond stateexcitedstate(s0 s2).Whenwas added,the wasdecreasedthe groundtransitionfromto thesecondexcitedstategraphene(s0 s2). Whengrapheneadded,intensityof the Q-bandpeakultrasonictimewithwas observed,existenceindicatingof the strongthe decreasedintensityof withthe Q-bandpeakultrasonic indicatingtime wastheobserve
Facile One-Step Fabrication of . 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]. .
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