Synergistic Effect of Co3O4 Nanoparticles and Graphene as Catalysts

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Synergistic Effect of Co3O4 Nanoparticles and Graphene as Catalysts for Peroxymonosulfate-Based Orange II Degradation with High Oxidant Utilization Efficiency Chengxiang Wang,† Penghui Shi,*,†,‡ Xiaodong Cai,† Qunjie Xu,*,†,‡ Xuejun Zhou,† Xiaolv Zhou,† Dong Yang,† Jinchen Fan,† Yulin Min,†,‡ Honghua Ge,† and Weifeng Yao† †

Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, Shanghai University of Electric Power, Shanghai 200090, P. R. China ‡ Shanghai Key Laboratory of Chemical Assessment and Sustainability, Shanghai 200090, P. R. China S Supporting Information *

ABSTRACT: Cobalt oxide and graphene nanocomposites (Co3O4/graphene) are fabricated as heterogeneous catalysts to accelerate sulfate radical generation in Orange II degradation. The Co3O4/graphene catalyst is characterized through X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy. Results show that the Co3O4/graphene catalysts are prepared successfully. Co3O4 or graphene solely exhibits slight catalytic activity, but their hybrid (Co3O4/graphene) efficiently degrades and removes Orange II from an aqueous solution in the presence of peroxymonosulfate (PMS). Orange II is completely removed or degraded (100%) within 7 min by using the composite catalysts; by contrast, Orange II is partially removed when Co3O4 or graphene is used alone under the same conditions. These phenomena suggest a synergistic catalytic activity of Co3O4 and graphene in the hybrid. To investigate the causes of the synergistic interactions of the Co3O4/graphene composites, we summarize previous studies and propose an electron transfer pathway between Co3O4 and graphene. We then perform density functional theory calculations to describe the specific features of the composite structures. The hybrid structure is more conductive than the individual semiconductor cobalt oxide clusters because of the hybridization between Co-4d orbital and graphene-p orbital. Fukui indices of electrophilic attack indicate that Co2+, not Co3+, is the active site. Therefore, the PMS activation processes and Orange II degradation pathways are involved in an electrochemical process. Graphene functions as a wire because of its excellent electrical conductivity during oxidation. catalyst activators.8 However, cobalt ions are toxic; these ions may also cause metal contamination or metal poisoning as a consequence of cobalt leaching and difficulty in catalyst recycling in homogeneous systems. Therefore, various cobalt precursors as heterogeneous activators loaded onto various supports have been prepared to activate PMS and to provide environmental benefits. Co3O4, a p-type semiconductor with efficient electronic and magnetic properties, is considered as the most versatile oxide among the transition metal oxides and is widely applied in various fields.9 Pure Co3O410−12 and its composites13−16 have been extensively investigated. Cobalt ions in the form of Co3O4 are a significant advancement in catalytic studies. Likewise, graphene, as a new 2D atomic layer of carbon atoms with sp2-hybridized carbon material, has been explored because of its unique 2D structure with a large surface area, flexibility, chemical stability, and superior electrical conductivity.17−20 Furthermore, graphene-based composites with metal

1. INTRODUCTION Dye wastewater not only negatively affects human life but also threatens the ecological environment. As such, various physical, chemical, and biological wastewater treatment methods have been applied. Among these methods, advanced oxidation processes (AOPs) are considered as easy, highly efficient, and environmentally friendly techniques to decompose dye wastewater. AOPs based on the generation of highly active radicals, such as •OH, •O2−, •OOH, and SO4−•, have been considered as an effective wastewater treatment to degrade organic pollutants.1−3 SO4−• yields a higher oxidative potential (2.5− 3.1 V) than other radicals; with this property, SO4−• exhibits a powerful oxidation ability to decompose refractory organic wastewater and enhance the degradation rate.4,5 SO4−• can react selectively in a wide pH range (2−9) and completely mineralize organic pollutants into final simple nontoxic products, such as H2O and CO26,7 as a result, targeted reactions produce high yields. In SO4−• production, peroxymonosulfate (PMS) can be simply and efficiently activated by transition metals, such as Cu2+, Mn2+, Fe2+, and Co2+; among transition metals, cobalt ions coupled with PMS are optimum © XXXX American Chemical Society

Received: October 13, 2015 Revised: December 14, 2015

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DOI: 10.1021/acs.jpcc.5b10032 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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accelerating voltage of 40 kV, a current of 200 mA, and a scanning angle of 2θ = 10−70°. The Raman spectrum was obtained using a Spex 1403 Raman spectrometer. The morphological characteristics of the samples were investigated by using a high-resolution transmission electron microscope (HRTEM, JEOL JEM22100F) operating at 300 kV. Catalytic performance was conducted using a UV−visible-NIR spectrophotometer (UV-3101PC, Shimadzu). The atomic composition of the Co3O4/graphene hybrid was tested by X-ray photoelectron spectroscopy (XPS). The X-ray photoelectron spectroscopy (XPS) analysis was performed on a ESCALAB 250 photoelectron spectrometer (Thermo VG Scientific, USA) with Al Kα (1486.6 eV) as the X-ray source. 2.4. Evaluation of Catalytic Activity. The catalytic performance was carried out in a 250 mL conical flask with a water bath on a shaking table at constant temperature. Afterward, 100 mL of 0.3 mM Orange II was transferred to a conical flask with continuous stirring and 0.5 mM Oxone was added; saturated sodium bicarbonate solution was also added to adjust pH to 7. Subsequently, 5 mg of the catalyst was poured into the solution, and this procedure initiated the reaction. The samples were obtained at regular intervals, joined immediately with the same amount of methanol as quencher,30 and filtered. The Orange II concentration in the filtrate was measured using a UV−vis spectrophotometer (Shimadzu 2550) at 486 nm; a full spectrum scan was simultaneously obtained from 200 to 700 nm. In general, commercial oxone can be used to generate PMS; in this study, 614.7 g/L oxone was required to release 2 mM PMS. All of the experiments were conducted in triplicate. For a comparison, the experiment on the catalytic activity of pure graphene and Co3O4 in the presence of PMS was conducted in the same conditions. During the recycling experiment, the catalyst was collected by centrifugation and thoroughly washed with distilled water and ethanol after each recycle. Then the catalyst was dried in a vacuum oven at 60 °C for 24 h to remove water and ethanol. 2.5. Computational Details. The DFT calculations were performed in the framework of Perdew−Burke−Ernzerh of the exchange-correlation functional with Grimme-D2 dispersion correction as implemented in the DMol3 code. Valence electrons were expanded in a DNP basis set with DFT semicore pseudopotentials. A 6 × 6 unit cell with a slab separated by a vacuum region of 30 Å was built as a graphene substrate. Brillouin zone integration was performed on an 11 × 11 × 1 Monkhorst−Pack grid. A Fermi smearing of 0.03 eV and a global orbital cutoff of 4.5 Å were employed. The convergence of the self-consistent iterations was settled for the charge density variation within 1 × 10−6. The structure was fully relaxed until the maximal force on each atom was less than 5.44 × 10−2 eV·Å−1. The adsorption energies of the Co3O4 clusters on graphene surfaces are calculated using the following equation:

oxides demonstrate high performance in degradation, energy storage, and electrochemical properties; therefore, graphene can be a promising support material.21−25 Graphene oxide (GO) or reduced graphene oxide (RGO) incorporated with cobalt oxides enhances catalytic efficiency.26−28 Wang et al.16 reported that the combination of Co3O4 nanoparticles with graphene sheets provides a catalytic activity much higher than that of pure Co3O4; the RGO play an active role in Co3O4 dispersion and phenol removal.26 Dai et al.27 demonstrated that Co3O4/rmGO (reduced mild graphene oxide) without any N species exhibits a very similar ORR onset potential to Co3O4/ N-rmGO; Dai et al.27 also proposed that active reaction sites may be Co oxide species rather than N species but are enhanced by the N doping of mGO at the graphene-containing interface. Xiao et al.28 suggested that surface Co2+ ions are the active sites of ORR, and the catalytic ability of these ions is closely related to the density of catalytically active sites. However, to the best of our knowledge, the theoretical mechanism of graphene and cobalt oxide remains inconclusive. In this study, a simple strategy is proposed to synthesize Co3O4/graphene as an efficient heterogeneous catalyst activator of PMS for sulfate radical-based degradation of Orange II. The Co3O4/graphene catalyst activator exhibits a catalytic activity higher than that of Co3O4 or graphene alone during degradation; this finding indicates that the active Co3O4 with graphene synergistically affects PMS activation and Orange II oxidation. Density functional theory (DFT) calculations are also performed to investigate the systematic relation of graphene-incorporated Co3O4.

2. EXPERIMENTAL SECTION 2.1. Materials. Flake graphite (300 mesh, 99.99%) was obtained from Shanghai Yifan Graphite Co., Ltd., China. PMS, available as a triple salt of sulfate commercially known as oxone (2KHSO5·KHSO4·K2SO4, 4.5%−4.9% active oxygen; Dupont) was purchased from Shanghai Ansin Chemical Co., Ltd. Other reagents, such as Co(NO3)2·6H2O, P2O5, K2S2O8, H2SO4 (98%), KMnO4, H2O2 (30%), methanol, and other chemicals, were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All of the chemicals were used as received without further purification. All of the solutions were prepared in 18.2 MΩ·cm Milli-Q water produced using a Milli-Q Biocel water system. 2.2. Preparation of Catalysts. Graphite oxide (GO) was obtained in accordance with the improved Hummers method by oxidizing flake graphite with strong oxidants, namely, P2O5, K2S2O8, and KMnO4, under acidic conditions.29 Co(NO3)2· 6H2O (0.29 g), NH4F (0.12 g), and urea (0.30 g) were dissolved in deionized water (10 mL). Dispersed GO (5 g/L) was sonicated for 30 min and dropped into the resulting solution; the mass ratio of GO and Co(NO3)2·6H2O was maintained at 1:5. Afterward, the solution was stirred for 1 h and then transferred to a Teflon-lined autoclave (25 mL) at 120 °C for 5 h. The precipitate was collected through centrifugation, washed thoroughly with water and ethanol to remove any impurities, and dried at 60 °C for 12 h. The solid was heated to 300 °C for 3 h at a rate of 1 °C/min in a N2 atmosphere. Pure Co3O4 and pure graphene were synthesized in the same manner but in the absence of Co(NO3)2·6H2O or GO, respectively. 2.3. Characterization Technique. X-ray diffraction (XRD) was performed using a Philips X’ Pert Pro PW 3050/ 60 powder diffractometer with Cu Kα radiation at an

Eads = ECo3O4/graphene − ECo3O4 − Egraphene

(1)

where ECo3O4, Egraphene, and ECo3O4/graphene represent the total energies of the Co3O4 clusters, graphene, and the total adsorption system, respectively. The charge density difference (Δρ) is calculated as follows: Δρ = ρCo3O4/graphene (r ) − ρCo3O4 (r ) − ρgraphene (r ) B

(2)

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Figure 1. (a) XRD spectra of Co3O4, graphene, and Co3O4/graphene. (b) Raman spectra of Co3O4/graphene.

Figure 2. TEM images of Co3O4/graphene with low (a) and high (b) magnification.

where ρCo3O4/graphene(r), ρCo3O4(r), and ρgraphene(r) are the total density difference of the system, the isolated Co3O4 clusters, or graphene, respectively. Fukui indices for nucleophilic or electrophilic attack are calculated based on Milliken and Hirshfeld algorithm and defined as the following formulas: fk + = qk (N + 1) − qk (N )

3. RESULTS AND DISCUSSION 3.1. Characterizations of Catalysts. The structure of the as-synthesized samples was characterized through powder XRD. The XRD patterns of pure Co3O4, graphene, and Co3O4/graphene hybrids are shown in Figure 1a. The diffraction peaks of Co3O4 at 19.3°, 31.5°, 37.0°, 44.6°, 59.6°, and 65.4° correspond to (111), (220), (311), (400), (511), and (440) reflections, respectively; these peaks also match well with the characteristic peaks of the Co3O4 cubic phase (JCPDS no. 42-1467). This finding confirms the crystalline nature and phase purity of Co3O4 nanoparticles. The sharp diffraction peaks of graphene at 26.5° (002) and 43.6° (100) are typical for highly purified graphene.35 Compared with that of pure Co3O4, the signal intensity of Co3O4 in Co3O4/graphene slightly decreases because of the low percentage content and low crystallinity of graphene. Furthermore, the typical (002) diffraction peak of graphene fades compared with that of the prepared pure graphene; this result suggests that the face-toface stacking likely disappears and less graphene agglomeration occurs; this phenomenon is possibly attributed to the interconnection between Co3O4 and graphene.36 Moreover, this phenomenon confirms the presence of graphene in the Co3O4/graphene composite. The peaks of Raman shift (Figure 1b) at 192, 468, 516, 608, and 677 cm−1 are ascribed to F12g, Eg, F22g, F32g, and A1g Raman-active modes of the Co3O4 cubic phase, respectively.37 A small low-intensity D peak (1360 cm−1) is the dominant sp2 Raman signature of defects associated with the breathing modes of carbon hexagons coming from the transverse optical (TO) phonons near the k-point.38 The strong G peak (1580 cm−1) caused by a double-degenerated in-plane sp2 C−C stretching mode that corresponds to the E2g phonon at Brillouin zone center39 is the main Raman signature of sp2 carbon-based materials. The 2D band (2728 cm−1) originates from the

(for nucleophilic attack) (3)

fk − = qk (N ) − qk (N − 1)

(for electrophilic attack) (4)

where qk is partial charge of an atom in the anionic or cationic system with N + 1 electrons and N − 1 electrons, respectively.31 Clusters can be used as good model systems to simulate real surface reactions and to reveal surface reaction mechanisms because their isolation and properties are localized via theoretical techniques. In our study, cobalt oxide clusters were used in accordance with the method described by Bernstein et al.32 The geometric mode structures were then optimized to determine the most stable configurations. 2.6. Electrochemical Measurement. The electrochemical measurements were performed using a CHI 660E Electrochemical Analyzer in a standard three-electrode cell. The powder microelectrodes (PMEs) were prepared as working electrodes and the specific prepared methods according to Locatelli et al.33 and Qiao et al.34 The schematic diagram of the PMEs is shown in Figure S1 in Supporting Information. A saturated calomel electrode (SCE) and a large Pt foil were used as the reference electrode and the counter electrode, respectively. Electrochemical impedance spectroscopy (EIS) impedance was performed in a fresh 0.3 mM Orange II neutral solution adjusted with certain amount of saturated sodium bicarbonate and oxone. C

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The Journal of Physical Chemistry C double resonant scattering of two TO phonons near the k-point of Brillouin zone.40 These properties are the classical features of graphene. The intensity ratio of the D band to the G band (ID/ IG ratio) is an indication of graphene disorder and defects.41 The ID/IG ratio (0.12) indicates that the prepared graphene is of good quality with less defects. These results provide strong evidence supporting the existence of both graphene and Co3O4 in the prepared composites. TEM was performed to further investigate the morphological characteristics of the samples. The typical TEM images (Figure 2) show the morphological characteristics of the synthesized Co3O4/graphene. In Figure 2a, the Co3O4 particles (3−5 nm in size) are uniformly distributed on the graphene surface. A lattice structure is observed in the high-magnification TEM image of the area near the Co3O4 particles on graphene (Figure 2b). The lattice fringes are arranged with a spacing of 0.244 and 0.290 nm; this finding is consistent with the theoretical spacing of the (311) and (220) planes of Co3O4 crystals, respectively. The images confirm that the catalyst preparation applied in this study produces large graphene sheets homogeneously decorated with well-dispersed Co3O4 nanoparticles. The chemical composition of the synthesized Co3O4/ graphene material was determined with the X-ray photoelectron spectroscopy (XPS), as shown in Figure 3a. The

obviously sharp peaks are centered at the survey region (0− 1100 eV) corresponding to the characteristic peaks of Co 2p, O 1s, N 1s, and C 1s,42 respectively, confirming the existence of cobalt, oxygen, nitrogen, and carbon elements in the sample. In the high-resolution Co 2p XPS spectrum (Figure 3b), two major peaks with binding energies at 779.7 and 794.9 eV are fitted to Co 2p3/2 and Co 2p1/2,43 respectively, which demonstrates the presence of Co3O4 in the composites. The C 1s XPS spectrum of Co3O4/graphene is shown in Figure 3c, which can be deconvoluted into three peaks that correspond to carbon atoms in different functional groups: carbon in sp2 C−C at 284.6 eV, carbon in C−O/C−N at 286.3 eV due to calcination in the N2 atmosphere, and carbon in C−O−Co at 288.9 eV. Two peaks in the O 1s XPS spectra of the Co3O4/ graphene hybrids in Figure 3d are distinguished at 529.85 and 531.55 eV, in accordance with the Co−O−C bond and Co− O−Co bond,44 separately. The result is consistent with the XRD and Raman which illustrates the existence of both Co3O4 and graphene in the as-prepared composite. 3.2. Orange II Degradation Performance. The Orange II degradation curves of Co3O4, graphene, and Co3O4/graphene versus time and UV−vis spectra of Orange II are shown in Figure 4. As shown in Figure 4a, curves A, B, and C represent the Orange II adsorption only in the presence of Co3O4, graphene, or Co3O4/graphene, respectively. The efficiency of Orange II degradation decreases; approximately 10% of Orange II is achieved and a plateau is reached. Curve D shows the degradation activity of PMS without a catalyst. This finding indicates that degradation occurs very slowly and about a half of Orange II is degraded in the first 35 min. Curves E, F, and G reveal the catalytic activities of graphene, Co3O4, and Co3O4/ graphene, respectively. Although the catalytic activity of graphene is low in the presence of PMS, the catalytic activity of Co3O4 satisfactorily improves. The Orange II degradation removal at 100% is achieved within 20 min using Co3O4 as the catalysts. Co3O4/graphene exhibits a catalytic activity higher than that of graphene and Co3O4; 100% Orange II degradation removal is achieved within 7 min. The degradation efficiency is significantly improved in terms of the catalytic activity when Co3O4/graphene hybrid is used. This finding can be attributed to the remarkable synergistic effect of the heterogeneous surface between Co3O4 and graphene sheets. The temporal evolution of UV−vis spectra of the Orange II solution using Co3O4/graphene as catalyst is shown in Figure 4b. As the reaction time proceeded, the two characteristic peaks at 403

Figure 3. XPS spectra of Co3O4/graphene: (a) survey scan; (b) Co 2p region; (c) C 1s region; (d) O 1s region.

Figure 4. (a) Curves of Orange II degradation under different conditions: (A) [Co3O4/graphene] = 0.05 g/L, (B) [graphene] = 0.05 g/L, (C) [Co3O4] = 0.05 g/L, (D) [PMS] = 2 mM, (E) [graphene] = 0.05 g/L and [PMS] = 2 mM, (F) [Co3O4] = 0.05 g/L and [PMS] = 2 mM, (G) [Co3O4/graphene] = 0.05 g/L and [PMS] = 2 mM; (b) UV−vis spectra of Orange II over Co3O4/graphene. D

DOI: 10.1021/acs.jpcc.5b10032 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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based on the generation of sulfate radicals through the cobalt-mediated decomposition of PMS is initiated by the following reactions:47−51

and 485 nm, which was ascribed to the NN group of azo form and the transition n−σ (the hydroxyl group toward the nitrogen bridge of hydrazone form),45,46 decreased simultaneously, suggesting that the two tautomeric forms react at the same time. Finally, the two characteristic absorption peaks disappeared. 3.3. Stability of Co3O4/Graphene in Multiple Runs. We performed three cycling tests to evaluate the stability of the Co3O4/graphene catalyst under the same reaction conditions. The results are shown in Figure 5. Compared with the activity

Co2 + + HSO5− → Co3 + + SO4 − · + OH− SO4 − · + Orange II → (...many steps...) → CO2 + H 2O

SO4−• from the PMS/Co3O4/graphene system plays a significant role in dye wastewater degradation; Co2+-to-HSO5− electron transfer is a key process in radical generation in the heterogeneous PMS activation. The rate of free radical reaction is quite fast; the SO4−• production is considered as a ratedetermining step, and Co2+ is used as an active site in our work. Therefore, we hypothesize that the cobalt oxide/graphene composite accelerates the electron transfer from Co2+ to HSO5− and that graphene promotes this phenomenon based on specific factors. The surface interaction of the Co3O4/graphene composite was subjected to DFT calculations to test our hypothesis. The geometrically optimized structures of the Co3O4 clusters are displayed in Figure 6a, as described in a previous report.32 Several different adsorption sites on graphene are probed; the results reveal that the Co3O4 clusters are the most stable optimized configurations on graphene (Figure 6a and 6b). In the system, the adsorption energy of −0.88 eV is thermodynamically feasible, and this parameter confirms that the geometric structures are reasonable. The total charge density and the difference charge density are calculated to investigate the electronic properties of the Co3O4/graphene system. The total charge density is shown in Figure 6c and 6d. In the graphene plane, an electron cloud is mainly distributed on the center of six-membered ring, and the electron density of C atom decreases. These results help position the C atoms on the graphene surface. Figure 6d shows the electron density difference plots to investigate electron transfer. The isosurface value of the electron density difference plots is 0.03 e/Å3, where the accumulation and depletion of electrons

Figure 5. Orange II degradation in continuous runs by using the recycled Co3O4/graphene catalyst at neutral pH: [Orange II] = 0.3 mM, [Co3O4/graphene] = 0.05g/L, and [PMS] = 2 mM.

of the initial catalyst, the activity of Co3O4/graphene decreases slightly. However, the Orange II degradation after the third run occurs in 9 min, and this phenomenon is similar to that in the first run. This finding indicates that the catalyst exhibits excellent long-term stability. 3.4. Geometric and Electronic Structure of the Graphene−Co3O4 System. The Orange II degradation

Figure 6. Geometrically optimized structures of the Co3O4 clusters (a) and the Co3O4 cluster/graphene composite (b), the total charge density (c), and difference charge density (d). E

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The Journal of Physical Chemistry C are represented in blue and yellow, respectively. The electrons coming from the C atom adjacent to the Co3+ atom accumulate. This finding indicates that Co3+ obtains charge from graphene. The Co3O4 clusters and the total density of states (DOS) were calculated to determine the electronic conductivity of the structure. Figure 7 illustrates the band gap of 0.422 eV of the

Figure 8. Electronic structures of Co3O4 clusters/graphene. Fermi level is set to 0.

Figure 7. Calculated total densities of states of the Co3O4 clusters and the Co3O4 clusters/graphene composites.

bulk Co3O4 clusters; this finding is consistent with the reported 0.34 eV when the PBE function is used.52 The bulk Co3O4 clusters yield a wide band gap, which is a semiconductor behavior; this finding indicates that the clusters are not conducive to electron transport. However, the total DOS reveals a continuous plot at Fermi level (EF), which is a conductor behavior; therefore, the Co3O4/graphene composites are conducive to electron transport. In other words, the band gap of the Co3O4 clusters decreases from 0.422 eV to 0 upon adsorption on graphene. This result demonstrates that the semiconductor cobalt oxide clusters are converted into a form more conductive than that of the semiconducting cobalt oxide clusters; as a result, the electron transfer occurs more easily from the valence band maximum (VBM) to the conduction band minimum (CBM). This result helps accelerate the following processes: electron transfer from Co3+ to HSO5−, rate-determining step, SO4−• production, and Orange II degradation. The electronic DOS plots of the 3s, 3p, and 4d orbitals of Co3O4 and the 2s and 2p orbitals of the neighboring C atoms are illustrated in Figure 8 to determine whether the interaction between graphene and Co3O4 improves the conductivity performance of hybrids. The overlapping shaded region indicates that the stability of the Co3O4/graphene composite is mainly attributed to the hybridization between Co-4d orbital and graphene-p orbital. This result suggests that the decrease in the Co3O4 band gap from 0.422 eV to 0 should be attributed to the enhancement of the superior charge transport properties rather than to Co3O4 or graphene alone because of the interaction of the hybridization between Co-4d orbital and graphene-p orbital. To prove the transport ability of electrons, EIS measurements were carried out and the Nyquist plots of Co3O4/ graphene and Co3O4 electrodes are shown in Figure 9. A typical

Figure 9. Nyquist plots of Co3O4 and Co3O4/graphene powder microelectrodes.

semicircle and a linear portion are visible, which can be ascribed to the charge-transfer resistance and mass transfer process resistance in the high-medium frequency region and the low frequency range, respectively.53 The radius of the Co3O4/ graphene is dramatically smaller than that of the bulk Co3O4. This indicates that the charge transfer resistance of the Co3O4/ graphene composite is significantly decreased compared to the bulk Co3O4, suggesting dramatic enhancement of the electron transfer in the Co3O4/graphene composite. At low frequency, the slope of the Co3O4/graphene composite was close to zero compared with the linear portion. This reveals that electron transfer encounters almost no diffusion resistance and without hesitation promotes the electron transfer rate. A large improvement in the overall conductivity results, and the catalytic action is promoted much more efficiently and more easily than with bulk Co3O4 without graphene. Moreover, the electrochemical test results are in accord with our simulation and can be powerful evidence to support our hypotheses. The Fukui indices are widely used as an important concept in explaining and predicting chemical species reactivity, and it has a closely relationship with the frontier molecular orbital theory.54,55 Fukui indices reflect the ability of different areas to participate in electrophilic reaction by the degree changes of electron density, which are due to attack of the system by an electrophilic reagent and electron removal from the original location. Fukui indices of the electrophilic attack are calculated using the Mulliken and Hirshfeld algorithm according to formula 3 to determine the active site. The results are shown in Table 1. Usually, the higher the value of Fukui index, the higher F

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4. CONCLUSIONS Although Co3O4 or graphene oxide exhibits a slight catalytic activity in Orange II degradation, their hybrid materials induce highly efficient catalytic activities in Orange II degradation based on sulfate radicals. To identify the synergistic effect, we performed DFT calculations and found that the band gap of the Co3O4 clusters decreases to 0 after these clusters are incorporated into graphene. Graphene functions as a wire connecting the cathode to the anode; graphene also accelerates electron transfer and SO4 −• production. Furthermore, graphene enhances catalytic activities.

Table 1. Fukui Indices for Electrophilic Attack (−) atom

state

Mulliken

Hirshfeld

Co 1 O1 Co 2 Co 3 O2 O3 O4

+2 −2 +2 +3 −2 −2 −2

0.094 0.066 0.044 0.002 0.022 0.022 0.025

0.099 0.052 0.054 0.013 0.019 0.019 0.020

■

the reactivity for corresponding attacks.56 Co2+ loses electrons more easily than other cobalt ions or oxygen ions. This finding indicates that Co2+ is the active site; furthermore, the theoretical result is consistent with the actual findings.25,44−46 We propose that the PMS activation processes and pathways of Orange II degradation are involved in the electrochemical process (Figure 10). In the anodic reaction, the electron

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10032. The specific preparation methods of the PMEs, the effect of operating factors, and the effect of the compositions of Co3O4/graphene (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: +86 21 35303544. Fax: +86 21 35303545. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study was supported by National Natural Science Foundation of China (21507081 and 21271010), Shanghai Municipal Natural Science Foundation (15ZR1417800), Science and Technology Commission of Shanghai Municipality (14DZ2261000, 14DZ2261100), “Dawn” Program of Shanghai Education Commission (11SG52), and Shanghai Key Project for Fundamental Research (13JC1402800).

Figure 10. Mechanism of SO4−• production on Co3O4/graphene.

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transfer pathway occurs in the following mechanism: Co2+ loses an electron to HSO5−; Co2+ is then transformed into Co3+; as a result, SO4−• is produced. The band gap of Co3O4 clusters decreases to 0 after graphene is added; this finding indicates that the semiconductor cobalt oxide clusters of hybrid are converted into a form more conductive than that of the semiconductor cobalt oxide clusters of either graphene or Co3O4. As a result, the poor electron transport of semiconductor cobalt oxide clusters is improved by incorporating graphene. This phenomenon accelerates the electron transfer from the VBM to the CBM because of the hybridization between the graphene-p orbital and cobalt ion-4d orbital. Electrons must be added to supplement the lost electron of Co2+ from the cobalt oxide clusters in the cathodic pathway. Graphene functions as a wire that connects the cathode to the anode; electron transfer occurs via graphene to compensate for the lost electron of Co2+. Graphene exhibits an electrical conductivity more efficient than that of cobalt oxide, and the connection of the cathode and the anode via graphene accelerates the electron transfer in the cathodic pathway. Therefore, the rates of the electron transfer from Co2+ to HSO5− between Co3O4/graphene and PMS are higher and faster than those in Co3O4 alone. These rates greatly enhance catalytic activity.

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