WANG Qian（王 倩），SHI Penghui（時(shí)鵬輝），2，ZHU Shaobo（朱少波），LI Jiebing（李潔冰），ASIF Hussain，LI Dengxin（李登新）＊

1College of Environmental Science and Engineering，Donghua University，Shanghai 201620，China

2College of Environmental and Chemical Engineering，Shanghai University of Electronic Power，Shanghai 200090，China

Introduction

The Nobel Prize in Physics 2010was awarded to Andre Geim and Konstantin Novoselov “for ground breaking experiments regarding the two-dimensional material graphene”.The discovery of graphene caused the research upsurge in the world for its extraordinary mechanical and electronic transport properties.However，it is difficult to deposit metal or metal oxide on its surface to synthesize graphene-based hybrid materials because of its poorly soluble in water and polar organic solvents［1］.

It is well known that graphite oxide（GO）is a hydrophilic material owing to the presence of hydroxyl groups decorated on the surface of carbon sheets and the carboxyl groups located at the edges［2］.In the middle of the 20th century，there were mainly three kinds of classic methods to prepare GO：Brodie method［3］，staudenmaier method［4］and Hummers method［5］，respectively.Nevertheless，a conventionally-modified Hummers method became most popular for its properties of simpleness，security and time saving.Proportional amounts of oxidants，for instance，potassium permanganate，sodium nitrate and concentrated sulfuric acid are used in the fabrication process.Different from graphene，GO consists of a hexagonal ring based carbon network with both sp2and sp3hybridized carbon atoms.As a starting material，GO has been applied into inorganic or organic hybrid nanocomposite systems more and more due to its favorable properties［6-7］.For instance，Gao et al.［8］reported a novel graphite oxide GO-TiO2microsphere hierarchical membrane，which performed well and exhibited promising potential in clean water production field.

As the intermediate product of making graphene，GO has been widely applied in the preparation of cheap graphene and functionalized graphene［9-10］.Therefore，spectroscopy characterization and structure analysis of the different degree of oxidation of GO and associated catalyst have important theoretical and practical significances for revealing the structural transformation rule in the process of oxidation and the preparation of GO and surface modification research.

Recently，there has been a considerable amount of interest in the advanced oxidation processes（AOP）which hydroxyl radicals are the main oxidants involved in detoxification of dyeing wastewater.More lately，sulfate radical based-advanced oxidation technologies were engaged in the area of water treatment and other environmental applications.SO-4·is a major oxidizing specie with high standard redox potential（2.5-3.1V）compared with·OH radicals（1.89-2.72V）［11-12］.Furthermore，the high reactivity of sulfate radicals could remain in the range of pH 3-8［13］.Plenty of studies［14-16］have fully proven the applicability of cobalt ion as an efficient catalyst for the activation of peroxymonosulfate（PMS）to produce sulphate radicals.However，the adverse effect of dissolved cobalt in water still needs to be disposed.Therefore，it is better to activate PMS via a heterogeneous way that could deal with the problem of leached cobalt.Anipsitakis et al.first reported the hetero-PMS-act employing commercially available Co3O4particles［15］.Lately，well-dispersed nanocrystalline Co3O4particles were immobilized onto GO supporter and had played a good role in the water treatment［17］.As a potential material，the nanoparticles may be further used in dyeing wastewater treatment.Therefore，the interaction between mGO and Co3O4would be very critical.

In this study，a series of Co3O4／mildly oxidized graphite oxide （mGO）nanocatalysts were prepared and used as a mediator for the heterogeneous PMS activation.The performance of Co3O4／mGO／PMS system was investigated using acid orange 7（AO7），a textile azo-dye，as a model compound.As a result，along with the increase in hydrophilicity during the oxidation process，GO obtained with the maximum possible degree of oxidation expressed the best efficiency along with more oxygen atoms and hydroxyl groups grafted on both sides of them.The formation of Co—OH complexes at the surface of Co3O4／mGO nanoparticles and the synergistic catalytic mechanism between Co3O4and mGO were discussed.

1 Experimental

1.1 Materials

Flake graphite（300 mesh）was supplied by Shanghai Yifan Graphite Co.， Ltd.Concentrated sulfuric acid（H2SO4，98%）， potassium permanganate （KMnO4），sodium nitrate（NaNO3），hydrogen peroxide（H2O2，30%），cobalt nitrate hexahydrate［Co（NO3）2·6H2O］，AO7（98%purity），PMS［2（KHSO5）·KHSO4·K2SO4］，and 4.5%-4.9% active oxygen were manufactured by DuPont.All chemicals used in this study are analytical grade.

1.2 Synthesis

In this experiment，three kinds of catalysts were composed.Raw material mGO were prepared with the modified Hummers method［5，18］.By changing the dosage of KMnO4（2.5，7.5，and 15 g），we obtained a series of samples of different oxidation degree which were marked as mGO-1，mGO-2，and mGO-3.To compose the catalysts，mGO（including mGO-1，mGO-2and mGO-3，200mg）was dissolved into 120mL of hexyl alcohol and sonicated for 2 h.Meanwhile，Co（NO3）2·6H2O （1 mmol）was dissolved into another 80mL of hexyl alcohol.The mixture was heated to 140 ℃under constant magnetic stirring for 12h after stirred for 2h.The resulted product was collected by centrifugation and washed with ethanol and water for several times.Finally，the sediment was dried in a vacuum oven at 60 ℃for 24 h.And the product was tagged as Co3O4／mGO（including Co3O4／mGO-1，Co3O4／mGO-2and Co3O4／mGO-3）.

1.3 Characterizations

X-ray powder diffraction （XRD） patterns were obtained on a Rigaku X-ray diffractometer （D／Max-2550PC，Japan）with a Cu-Ka radiation source operated at 40kV and 200mA in the 2θrange 5°-90°.

The nanoscale structures were observed using transmission electron microscopy（TEM，JEOL JEM-2100F）with an accelerating voltage of 200kV.

The Fourier transform infrared spectroscopy （FTIR）spectra of the materials were recorded between 4 000and 500cm-1using a Thermo Nicolet NEXUS 670 FTIR spectrometer.

Raman spectra were recorded on the Nicolet Micro-Raman System（NEXUS-670，USA）using a 1 200lines／mm grating and a 50objective lens.He：Ne green laser with 633 nm wavelength was used to excite Raman signal with the power of 17mW.

The atomic composition of Co3O4／mGO was detected by X-ray photoelectron spectroscopy （XPS）.XPS experiments were carried out on an RBD upgraded PHI-5000CESCA system （Perkin Elmer）with Al Kαradiation（hυ＝1 486.6eV）.Binding energies were calibrated by using the containment carbon（C 1s＝284.6eV）.The data analysis was carried out by using XPS Peak 4.1.

1.4 Experimental

Firstly，100mL dye wastewater（0.2mmol／L）was put into a 250 mL conical flask.Then，the pH of the solution was adjusted to neutral using sodiumm bicarbonate solution（NaHCO3，0.5mol／L）after PMS（0.2mmol／L）was added into it.Finally，0.05g catalyst was put into the mixture and the vessel was placed in a water-bathing constanttemperature vibrator controlled at 25 ℃throughout the process.At given reaction time intervals，samples were taken for analysis.

2 Results and Discussion

2.1 Degradation of AO7performances

The degradation curves of AO7 under the effect of different catalysts are shown in Fig.1， where C／Co represents the residual rate of AO7in the water and smaller value means higher removal rate.The removal rate of AO7 depends on the concentration of sulfate radicals generated，and hence it can be used to evaluate the performance of the supported cobalt catalysts as activators of PMS.In order to achieve good efficiency，the experiments were carried in neutral conditions which were adjusted with 0.5mol／L sodium bicarbonate buffer.The catalysts （Co3O4／mGO）dosage was 0.05g／L.All experiments were conducted at room temperature.And pH was constant in the whole process of reaction［19］.

Fig.1 Degradation curves of AO7

Fig.2 XRD patterns of graphite and mGO

As shown in Fig.1，the adsorption abilities of PMS，Co3O4and mGO-3are weaker than that of Co3O4／mGO composites.Catalytic properties of Co3O4／mGO compounds are Co3O4／mGO-3＞Co3O4／mGO-2 ＞Co3O4／mGO-1，respectively.The cobalt catalyst supports on mGO-3exhibited a much higher catalytic efficiency than the other two materials.It is remarkable that AO7 with a same starting concentration can be nearly degraded in 6min in a typical run when Co3O4／mGO-3is used as the catalyst.That was to say the catalytic properties of the compounds increased with the improving oxidation degree.Therefore，we presume that high oxidation degree of mGO supporter with appropriate loading of Co3O4can activate PMS efficiently during the reaction.And the effect resulting from these factors is conjunct.Furthermore，the formation of the surface Co—OH complex played an import role in the whole process［18］.

2.2 Catalyst characterization

Graphite raw material shows obvious diffraction peak（002）at 26.5°which reveals that it has good degree of crystallinity.Compared with graphite materials，mGO samples show wide peak（001）at around 10°and increases interlayer spacings which accesses to the formation of large numbers of oxygen-containing groups.In addition，tests show that the electrical resistivity of mGO samples increases gradually（from 0.04to 118.6 Ω·m）with the increasing dosage of oxidant.And the results indicate the oxidation dgree of GO increases progressively as well.These groups help mGO with strong hydrophilism.As can be seen from Fig.2，with the increasing dosage of oxidant，diffraction peaks（002）of mGO samples turn to weaker and wider.And diffraction peak（001）is shaped.Combining previous studies［20-21］，the phenomenon could be explained that the process of preparation mGO at low temperature stage was a mild oxidation of the insertion process.The main effects of oxidant were to oxide layer edge or defect of flake graphite so as to help the polar sulfuric acid molecules and hydrogen sulfate ions smoothly insert into the graphite structure layer，and continue the following formation of oxygencontaining groups.

The spectra of Co3O4／mGO-1 catalyst show distinct peaks at 31.2°，36.7°，44.2°，55.6°，59.2°，65.7°and 77.4°.The results are consistent with the diffraction peaks of（220），（311），（400），（422），（511），（440）and（533）of Co3O4，which proves that the formation and existence of cobalt oxide crystal［22-24］.For Co3O4／mGO-2and Co3O4／mGO-3， with the increasing oxidation degree， the diffraction signal of Co3O4is weak and nearly disappeared.Meanwhile，diffraction peak（001）of mGO samples turn to weaker and wider.However，wide peaks at 2θ＝20°-27°can be seen.This indicates mGO layers pile up disorderly and it also proves cobalt oxide is successly loaded on both sides of mGO layers［25］.

Fig.3 XRD spectra of catalysts

Fig.4 FTIR spectra of catalysts

The chemical structure of the catalysts was investigated by FTIR spectrum.There are a lot of carboxyl，hydroxyl and epoxy groups growing on mGO surface.As shown in Fig.4，Co3O4／mGO samples have several obvious diffraction peaks.The peak at 3 437cm-1attributes to O—H stretching and bending vibration［26］.Peaks at 1 630and 2 926cm-1turn to weaker on account of the short absorpti onsofCC and C—H of graphite with the increasing oxidation degree［27］.Peak at 874cm-1is due to the deformation and vibration of N—H.It is important to see that peak at 1 439 cm-1decreases gradually with the increased oxidation degree of graphite and finally disappears.This is because with the increased dosage of oxidant，the carbon atoms and C—C bond which in the form of sp2hybridization in the structures gradually reduces［28-29］.In addition，compared with Co3O4／mGO-3，Co3O4／mGO-1 and Co3O4／mGO-2show strong absorptions at 654cm-1because of the large amount of Co3O4.And the result is in line with that of XRD.In conclusion，although the amount of oxygen-containing groups and multi-model active sites in the structure of graphite increase with the enhanced oxidation degree，the group species are alike.

The spectra of graphite and Co3O4／mGO are shown in Fig.5.Raman spectroscopy is a widely used tool for the characterization of carbon products，especially considering the fact that conjugated and C—C bonds lead to high Raman intensities.The spectrum of GO exhibits two regular peaks，corresponding to the D-band line（1 350cm-1）and the Gband line（1 580cm-1）［24］which assign to E2gand A1gspecies of the infinite crystal，respectively［23］.A D-band line is observed in the center of graphite raw material and proves the existence of a significant number of defects.Obviously，the D-band tends to sharper and stronger.We can see ID／IGincreased gradually with the increased dosage of oxidant during graphite amorphization.Therefore，it is unassailable that oxidation degree of graphite is growing.In addition，combined with XRD and FTIR analyses，we conclude that with the increasing oxidation degree，the size of the in-plane sp2domain reduces and part of the carbon atoms transforms to sp3hybridization which lead to the formation of its disordered structure degree.Strong peak at 2 717 cm-1which corresponds to 2D-band line is allowed in graphite crystal.Diffraction peak of 2D-band line turns to weaker and nearly disappears along with the growing oxidation degree of graphite.

Fig.5 Raman spectra of catalysts

XPS spectra of supported cobalt catalysts are obtained to determine the compositions of the catalysts and reveal the nature of carbon and oxygen bonds.The peaks at 286.2，533.6，and 783.9eV are attributed to the characteristic peaks of C 1s，O 1sand Co 2p，respectively（shown in Fig.6（a），Co3O4／mGO-1）.The spectrum of Co3O4／mGO-3is similar to the Co3O4／mGO-1.The O 1score level spectra collected on Co3O4／mGO are shown in Fig.6（b）.All O 1s spectra of Co3O4／mGO are clearly asymmetric，which indicate the existence of different oxygen species at the surface of the supporter.For Fig.6（b），the main peak at about 532.5 eV corresponds to surface hydroxyl groups（Co—OH）that is ubiquitous in air-exposed cobalt oxide materials［30-31］while the peak （Co3O4／mGO-1）at lower binding energy of 530.7eV is identified to lattice oxygen species from Co3O4［32］.However，the peak is vanished from the spectrum of Co3O4／mGO-3due to the small amount of Co3O4.This phenomenon is in keeping with those of XRD and FTIR.In addition，a resolved peak at around 533.5eV is attributed to the adsorbed oxygen species such as C—O bonds and surface bound water［33］.Figure 6（c）shows Co 2p XPS spectra of Co3O4／mGO-1 and Co3O4／mGO-3.Two main peaks at 781.7eV（Co 2p3／2）and 797.1eV（Co 2p1／2）are observed.Compared with the main peaks of Co3O4／mGO-3，the peaks of Co3O4／mGO-1 are sharper and stronger.A spin-orbit splitting of 15.4eV is also considered.The Co 2p spectrum is well consistent with the XPS spectrum of Co3O4［34-35］.And the result is in line with the characterizations ahead.

Fig.6 XPS spectra of Co3O4／mGO-1and Co3O4／mGO-3：（a）full XPS spectra；（b）O 1sXPS spectra；（c）Co 2p XPS spectra

Fig.7 TEM characterizations of Co3O4／mGO-3and Co3 O4／mGO-1：low magnification image of Co3 O4／mGO-3（a）and Co3O4／mGO-1（b）；a close view of Co3O4／mGO-3（c）and Co3O4／mGO-1（d）

The catalysts were further characterized by TEM.Figures 7（a）and（b）show typical low magnification TEM images of Co3O4／mGO-3 and Co3O4／mGO-1.Co3O4nanoparticles are embedded in the mGO nanosheets，which indicates that the Co3O4nanoparticles are firmly anchored onto the supporter and they appear to be a strong interaction between Co3O4and mGO［36］.Obviously，Co3O4particles are dispersed on mGO homogeneously.It may be due to the mGO nanosheets in the composite which is helpful to suppress the aggregation and hindering the growth of nanoparticles to a certain extent［37］.In addition，we can see directly that the quantities of Co3O4loaded on the two kinds of catalysts are different.It is also reasonable to suggest that the homogeneous hybridization between Co3O4nanoparticles and mGO nanosheets is beneficial for the formation of Co—OH complexes and for achieving high rate performances.

2.3 Catalytic activities

Similar to Co3O4／mGO，mCo3O4／GO were composed by using different dosage of Co（NO3）2·6H2O and the same dosage of GO（the dosage of GO was the same as mGO-3）.The products were labeled as Co3O4-1／GO， Co3O4-2／GO，Co3O4-3／GO，Co3O4-4／GO，and Co3O4-5／GO （the Co percentage in the compounds followed the order Co3O4-5／GO＞Co3O4-4／GO ＞Co3O4-3／GO ＞Co3O4-2／GO ＞Co3O4-1／GO）.In Fig.8，the degradation efficiency of AO7in water by using Co3O4／GO composites as catalysts follows the order Co3O4-3／GO ＞Co3O4-2／GO ＞Co3O4-1／GO ＞Co3O4-4／GO ＞Co3O4-5／GO.Further increasing the Co3O4loading results in a significant decrease of degradation activity.Co3O4-3／GO has the best degradation of AO7，which indicates that this concentration produces the best catalysis.According to the experimental result，although bare Co3O4or GO has a low catalytic activity，their hybrid（Co3O4／GO）exhibits an unexpectedly high catalytic activity in the degradation of AO7 in water by advanced oxidation technology based on sulfate radicals.In addition，the nanocomposite shows different catalytic activities with different Co3O4loadings.This phenomenon shows the synergistic catalysis exists between Co3O4and GO.

Fig.8 Degradation curves of AO7

2.4 Stability of the catalyst

Four recycling runs of the catalyst were conducted，and the catalyst（Co3O4／mGO-3）was recycled under the same reaction conditions.After every run of reaction，the catalyst was collected，washed thoroughly，and dried in a vacuum oven at 60℃before the next round.As shown in Fig.9，the regenerated catalyst exhibits good performance and stability.The activity of the catalyst dropped slightly compared with the fresh catalyst.The concentration of the dissolved cobalt ions（0.04 mg／L）from Co3O4was almost the same as the fresh catalyst detected through the inductive coupled plasma emission spectrometer （ICP）.After four runs，the degradation of AO7occurred within 40min.Therefore，the Co3O4／mGO-3has good catalytic performance，slight ion leaching，and an excellent long-term stability.

Fig.9 Degradation efficiency of AO7 with the recycled Co3O4／mGO-3

2.5 Catalytic mechanism analysis

GO is a hydrophilic material owing to the presence of hydroxyl groups decorated on the surface of carbon sheets and the carboxyl groups located at the edges.

Based on Fig.1，although bare GO or pure Co3O4or PMS alone exhibits a low catalytic activity，their hybrid（Co3O4／GO）exhibits an unexpectedly high catalytic activity in the degradation of AO7in water by advanced oxidation technology.However，higher Co3O4content in the catalyst does not automatically result in a higher catalytic activity.The highest catalytic activity is observed when the Co3O4loading is about 1 mmol in the catalyst（Co3O4-3／GO）.As shown in Fig.8，the catalytic activity first increases and then decreases with increasing Co3O4loading，which indicates a proportional relation between the Co3O4content of the catalyst and the production of a catalytic active site.

As seen in the XPS spectra and TEM images（Figs.6-7），although the amount of Co3O4crystal particles on the surface of mGO-3is lower than that of mGO-1，the catalytic activity of Co3O4／mGO-3is higher.Research［38］showed that metal loading affected the degree to which the carbonsupport surface was covered，which leaded to a decrease in specific surface area and activity.Thus，if the relative amount of Co3O4is higher than the optimal quantity，the GO surface is predominantly covered by Co3O4so that the area of the exposed GO surface available for H2O dissociation becomes limited.

XRD and FTIR analyses indicate the formation of large amount of oxygen-containing groups on GO layers.Aside from the main peak at 530.7eV that corresponds to the lattice oxygen species from Co3O4，a shoulder at a higher binding energy of 532.5eV is attributed to Co—OH on the surface from the O 1spattern（Fig.6）.

There are two reasons for the formation of Co—OH complexes.One is that the hydroxyl groups are ubiquitous in air-exposed cobalt oxide materials.The other is that Co species attach to hydroxyl groups decorated on the surface of GO directly.The relevant equations are as follows：［19，39-40］

Therefore，based on reaction （1），the generation of Co—OH complexes should be the initial step to activate PMS in the heterogeneous system.

3 Conclusions

We treated graphite with different amount of KMnO4and obtained mGO which were decorated with—COOH and—OH groups.Experiments show that higher degree of oxidation of the graphite with proper cobalt loaded on leads to a better performance.According to the charactrization and synergistic catalytic mechanism，the generation of Co—OH complexes are found to be the initial step to activate PMS in the heterogeneous system of Co3O4／mGO hybrid.

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