Na Zhou ,Yan Du *,Chunyu Wang Rizhi Chen ,*

1 State Key Laboratory of Materials-Oriented Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

2 College of Environment,Nanjing Tech University,Nanjing 210009,China

Keywords:Porous carbon ZIF-8 Carbonization Oxygen Adsorption Water treatment

A B S T R A C T The state-of-the-art approaches for adjusting the structural characteristics of porous carbons are the after treatments,which are complicated and time consuming.In this work,a facile approach was developed,i.e.,controlling the initialoxygen concentration in-situ during the direct carbonization of zeolitic imidazole framework-8(ZIF-8),to adjust the pore structure and prepare hierarchically porous carbons.The introduction of oxygen can significantly affect the crystalline and pore structures of porous carbons,and promote the pore widening and the formation of mesopores.An appropriate initial oxygen concentration can notably enhance the surface area and mesopore volume of porous carbon,and then improve the adsorption capacity toward methylene blue(MB)dye from water by 3.4 times.The developed approach is more efficient at lower carbonization temperature.Moreover,the introduction of oxygen can increase the ratio ofHO--C=Ogroups on the carbon surface,leading to enhanced interaction with MB molecules and higher adsorption capacity toward MB.The as-prepared porous carbons exhibit superior adsorption capacities toward MB dye as compared to the reported ZIF-8 derived carbons.These findings would aid the development of porous carbon materials with high performance.

1.Introduction

Porous carbon materials are attracting tremendous attention owing to their high surface area,rich porosity,and good chemical and thermal stabilities.They are widely applied in wastewater treatment,catalysis and energy[1–4].So far,enormous efforts have been devoted to the development of methods for synthesizing porous carbons including template[5],activation of carbon materials[6–8]and direct pyrolysis of polymer[9].For instance,in the work of Huang and co-workers,a nitrogen and phosphorus co-doped hierarchically porous carbon(NPHPC)with higher surface area(1479 m2·g-1)was prepared through the direct pyrolysis of melamine polyphosphate[9].As an anode material for lithiumion battery,the as-fabricated NPHPC exhibited a high reversible capacity and an excellent cyclic stability.

In recent,metal–organic frameworks(MOFs)are attracting wide interest[10–14],because they can be controllably prepared by self assembly of metal ions with organic ligands in appropriate solvents,thereby tailored framework structures and multiple functions.Motivated by their designed chemical property,high and tunable porosity,and high surface area,several MOFs,such as ZIF-8,ZIF-67,MIL-88B-NH2,MIL-101 and MOF-5 have been used as the templates to synthesize porous carbons with promising applications[15–20].For example,Li et al.synthesized nanoporous carbons by one-pot pyrolysis of ZIF-8,which could be efficiently applied in the removal of ciprofloxacin from water[16].

The structural characteristics of porous carbons such as surface area,pore size and pore volume play an important role in determining their properties.To improve the structural characteristics and the corresponding properties,a lot of efforts have been made.Controlling the carbonization temperature has been proven to be an efficient approach to enhance the properties of porous carbons[21,22].It was observed that the nanoporous carbon obtained by direct carbonization of ZIF-8 at 1000°C had significantly greater adsorption capacity(186.3 mg·g-1)for the removal of methylene blue(MB)dye from water in comparison with those obtained at 600 °C(49.5 mg·g-1)and 1200 °C(36.7 mg·g-1)because of the changes in surface charge and pore size distribution[21].Many works have reported the use of after-treatment,i.e.the chemical or physical activation[23–25].And a serial of activating agents like air,steam,CO2,KOH,K2CO3and ZnCl2were attempted[26–28].In the work of Wang et al.[28],three types of ZIFs,i.e.ZIF-8,ZIF-68 and ZIF-69 were directly carbonized at 1000°C and then activated with fused KOH.It was observed that the activation with KOH sharply increased the surface area and pore volume.For instance,with respect to ZIF-68,the surface area of the activated porous carbon was almost four times as compared to the inactivated sample.On the other hand,the activation with KOH introduced some large micropores into the porous carbons.As a result,the activated porous carbon materials exhibited significantly enhanced properties in gas storage and supercapacitors.It is evident that the after-treatment is efficient to improve the structural characteristics and properties of porous carbons,but the approach is complicated and time consuming,and not environmentally friendly especially for the chemical activation.Hence,the development of a facile method for adjusting the structural characteristics of porous carbons is keeping a great challenge.

Herein,we developed a facile approach for the preparation of hierarchically porous carbons from carbonization of ZIF-8 by simply controlling the initial oxygen concentration in-situ during the carbonization process.ZIF-8 is very promising for many applications due to the easy synthesis and excellent stability[29–32],and hence was selected as the model system.The function of adjusting the initial oxygen concentration was investigated in detail by characterizing the microstructures of carbonized ZIF-8 samples using XRD,Raman spectroscopy,XPS,FESEM and N2sorption and testing their adsorption capacities for the removal of MB dye from water.

2.Experimental

2.1.Chemicals

Zn(NO3)2·6H2O(98%)was obtained from Shanghai Aladdin Biochemical Technology Co.,Ltd.,China.2-Methylimidazole(Hmim,99%)was provided by Sigma-Aldrich.Absolute ethanol(99.5%)was obtained from Wuxi Yasheng chemical Co.,Ltd.,China.Methanol(99.5%)was acquired by Shanghai Ling Feng Chemical Reagent Co.,Ltd.,China.Methylene blue(98.5%)was purchased from Tianjin Chemical Reagent Co.,Ltd.,China.All chemicals were applied as received.

2.2.Preparation of ZIF-8 and nanoporous carbon

ZIF-8 was synthesized in methanol as follows.After the dissolution of 2 g of Hmim in 60 g of methanol,1.81 g of Zn(NO3)2·6H2O was added under agitating.The above solution was agitated atroom temperature for10 min followed by reaction for4 h without agitating.Then,the powders were collected by centrifugation,washed with ethanol for several times,and dried at 60°C overnight.

Porous carbons were prepared through carbonization of ZIF-8 powders in a furnace under nitrogen flow.In this study,after putting ZIF-8 powders in the furnace,a certain amount of air in the furnace was removed with a vacuum pump,and then nitrogen was fed to make the pressure in the furnace reach the atmosphere pressure.Thus,a certain initial oxygen concentration in the furnace could be achieved.After that,the furnace was heated to the set temperature(5 °C·min-1),held for 5 h,and then cooled down to room temperature.The as-prepared carbon is marked as C-x-y,where x and y express the carbonization temperature and initial oxygen concentration,respectively.

2.3.Characterization

A powder X-ray diffraction(XRD,Rigaku Mini flex 600)was used to characterize the crystalline structure.The surface morphologies of the samples were obtained by a field-emission scanning electron microscope(FESEM,Hitachi S-4800).The surface areas and pore structures of the samples were analyzed by nitrogen sorption on a Micromeritics ASAP 2020 adsorption apparatus at 77 K.Each sample was degassed at 120 °C for 12 h before sorption measurement.The Barret–Joyner–Hanlenda(BJH)model was used to determine the pore size distribution.A NETZSCH STA449F3 apparatus was applied to evaluate the thermal stability of ZIF-8 crystals under nitrogen atmosphere at a heating rate of 10 °C·min-1.The Raman spectra were obtained by a Horiba LabRam HR800 spectrometer with a 514.5 nm He–Cd laser beam excitation.The element composition and chemical state of the samples were characterized by X-ray photoelectron spectroscopy(XPS)using a Thermo ESCALAB 250 spectrometer equipped with a monochromatized Al KR radiation.

2.4.Adsorption experiments

Aqueous MB dye was applied as a toxic model substance to test the adsorption capacity of ZIF-8 carbonized materials.In a typical adsorption process,10 mg of adsorbent was added into 100 ml of 100 mg·L-1aqueous MB dye solution,and then the solution containing the adsorbent was mixed well at30°C.The solution was taken atcertain intervals,separated from the adsorbent by centrifugation(6000 r·min-1,20 min),and then analyzed by an ultraviolet visible spectrophotometer(UV-5500)for determining the MB concentration.In this study,the adsorption capacity was expressed by the adsorbed amount of MB per unit weight of adsorbent,Q(mg·g-1),which was calculated as follows.

where C0is the initial MB concentration(mg·L-1),Ctis the MB concentration after adsorption for a pre-determined time(mg·L-1),V is the solution volume(L),and M is the adsorbent mass(g).

3.Results and Discussion

3.1.Synthesis and characterization of porous carbons

Pure nano-sized ZIF-8 crystals with a specific surface area of 1561 m2·g-1and a pore volume of 0.69 m3·g-1were synthesized by a precipitation method(Fig.S1),and were directly carbonized without other additional carbon sources under N2atmosphere by controlling the initial oxygen concentration to produce porous carbons.

To investigate the feasibility of the developed method,ZIF-8 crystals were first carbonized at800°C under different initial oxygen concentrations to produce C-800 samples.XRD,Raman spectroscopy,XPS,FESEM and N2sorption were used to analyze the microstructures of the C-800 materials.

Fig.1.XRD patterns of(a)C-800-0,(b)C-800-2.1,(c)C-800-4.2,(d)C-800-6.3,(e)C-800-8.4 and(f)C-800-10.5.

Fig.2.Raman spectra of(a)C-800-0,(b)C-800-8.4 and(c)C-800-10.5.

Fig.1 shows the XRD patterns of C-800 samples.Clearly,all the C-800 samples show one broad diffraction peak at 25°irrespective of the initial oxygen concentration,which can be ascribed to the(002)diffraction of graphitic carbon[33].The broad peak suggests the presence of some amorphous porous carbon species in the materials[28].There are no any other peaks for impurities like Zn and ZnO in the XRD patterns,which should be related with the carbonization process of ZIF-8.ZnO can be produced during the carbonization of ZIF-8 and then reduced to Zn by carbon[21].Higher temperature like 800°C may promote the reduction of ZnO and vaporization of Zn,leading to lower Zn content and nonexistence of Zn or ZnO peaks.Similar work was reported by Yamauchi and co-workers[22].However,the existence of Zn in the C-800 samples can be confirmed by the following XPS characterization.The results indicate that porous carbon can be successfully synthesized by simple carbonization of ZIF-8.The results in Fig.1 highlight that the initial oxygen concentration can significantly affect the crystalline structure of porous carbons.The addition of oxygen makes the(002)diffraction peak move to right,especially at higher initial oxygen concentration,which suggests a decreased interlayer spacing d(002)[34].Moreover,with increasing initialoxygen concentration,the peak intensity enhances,indicating the higher degree of graphitization with the introduction of oxygen during carbonization.

Three typical samples,i.e.,C-800-0,C-800-8.4 and C-800-10.5,were characterized by Raman spectroscopy.Obvious D and G bands can be observed for the three samples(Fig.2).The D band at around 1350 cm-1indicates the presence of disordered carbon,while the G band at around 1600 cm-1supports the presence of crystalline graphitic carbon.The relative intensity ratio of D/G bands(ID/IG)is considered as an indication of the defects amount in the porous carbons[21].As presented in Fig.2,three samples have similar ID/IGvalues,suggesting that the initial oxygen concentration has no significant influence on the formation of defects in the ZIF-8 derived carbons.

The presence of C,N,O and Zn in C-800-0,C-800-8.4 and C-800-10.5 was confirmed by XPS spectra(Fig.S2).Similar C 1s,N 1s and Zn 2p spectra were observed for the three samples.However,there are obvious differences in the O 1s spectra(Fig.3).The high resolution O 1 s spectrum can be deconvoluted into three peaks positioned at 531.1,532.2 and 533.2 eV,corresponding to the functional groups HO--C=O,C=O and C--OH,respectively[35].Clearly,the ratio of HO--C=O in C-800-8.4 or C-800-10.5 is significantly higher than the one in C-800-0(Table 1),indicating that the introduction of oxygen during the carbonization of ZIF-8 can adjust the surface composition and increase the ratio of HO--C=O groups on the surface of porous carbon.Similar phenomenon was reported by Chen et al.[35].

Fig.3.XPS results of(a)O 1s and O 1s spectra of(b)C-800-0,(c)C-800-8.4,(d)C-800-10.5.

Table 1O 1s XPS results of C-800 obtained with different amounts of oxygen

The C-800 samples were investigated by FESEM to identify the change in the morphology during carbonization.As shown in Fig.4,typical crystal morphology similar to that of the pristine ZIF-8(Fig.S1)is observed for each C-800 sample,indicating no obvious influence of initial oxygen concentration on the morphology of porous carbons.Similar observations were reported in the literatures[16,21,22,36,37].However,after carbonization,the particle size slightly decreases,especially at higher initial oxygen concentration,possibly because of the loss of oxygen containing groups and volatile Zn[36].The results indicate that the introduction of oxygen can promote the carbonization of ZIF-8 crystals.

Fig.5.Nitrogen sorption isotherms of(a)C-800-0,(b)C-800-8.4,(c)C-800-10.5.Solid symbols indicate gas sorption and open symbols express gas desorption.

Fig.4.FESEM images of(a)C-800-0,(b)C-800-2.1,(c)C-800-4.2,(d)C-800-6.3,(e)C-800-8.4 and(f)C-800-10.5.

Table 2Textural properties of C-800 obtained with different amounts of oxygen

The adsorption capacity of an adsorbent material strongly depends on its surface area,pore volume and pore size.Hence,some typical C-800 samples were analyzed by N2sorption.Fig.5 shows the N2adsorption–desorption isotherms of C-800-0,C-800-8.4 and C-800-10.5,respectively.And Table 2 summarizes their specific surface areas and pore volumes.It is evident,besides the microporous characters originated from ZIF-8,the isotherms show hysteresis loops,especially for the samples carbonized with the introduction of oxygen.The hysteresis loop indicates the production of mesopores during the carbonization of ZIF-8.The pore size distribution curves confirm that the introduction of oxygen can significantly promote the formation of mesopores(Fig.6).The results suggest the successful synthesis of hierarchically porous carbons.In comparison with the pristine ZIF-8(Fig.S1d),the adsorption amounts of nitrogen for the C-800 samples significantly reduce,thereby the decreased surface areas owing to part collapsing of ZIF-8 framework[21,33,38].More importantly,the surface area and pore volume of C-800 strongly depend on the initial oxygen concentration.With increasing initial oxygen concentration,both the surface area and pore volume significantly increase.For the C-800-10.5,the pore volume is even larger than that of ZIF-8.But the mesopore volume has a different trend with initial oxygen concentration(Table 2).The C-800-8.4 has the highest mesopore volume.Further increasing the initial oxygen concentration will lead to a reduction in mesopore volume.And the proportion of mesopore volume also first increases from 50%to 60%and then reduces to 49%.

Fig.6.Pore size distribution curves of(a)C-800-0,(b)C-800-8.4 and(c)C-800-10.5.

These results in Figs.5 and 6 and Table 2 highlight that the introduction of oxygen during the carbonization of ZIF-8 can notably affect the development of pore structure.In fact,oxygen activation has been considered to be a good method for preparing porous carbons because of its cheapness and higher reactivity[39].As oxygen is added during the carbonization of ZIF-8,the pore evolution has two mechanisms.One is the micropore formation,caused by the reaction between oxygen and carbon.Another is the pore widening,a result of oxygen reaction inside the opened pores.At higher oxygen concentration,the pore widening will play the key role in the pore evolution[40],resulting in the increase of mesopores(Table 2).But as the oxygen concentration is relatively high,the oxygen reaction is pronounced,leading to the disappearing of some mesopores,thereby the reduction in mesopore volume(Table 2).The pore evolution in the porous carbon will significantly affect its adsorption capacity for the removal of MB dye from water as discussed below.

To investigate the universality of the developed method,i.e.,adjusting the initial oxygen concentration during the carbonization to control the microstructure of porous carbon,ZIF-8 crystals were also carbonized at 600 °C and 1000 °C under different initial oxygen concentrations to yield C-600 and C-1000 samples,respectively.

As the carbonization temperature is 600°C,no ZIF-8 peaks are observed(Fig.S3),which is due to the decomposition ofZIF-8 frameworks at about 600 °C as indicated in Fig.S1b.In contrast to 800 °C(Fig.1),the peaks of ZnO appear at 600°C.A possible reason is that the reduction of ZnO to Zn by carbon may be inhibited at the lower carbonization temperature,thereby higher Zn content and the existence of ZnO peaks.Furthermore,the intensity of the ZnO peaks increases with increasing initial oxygen concentration,indicating that higher oxygen concentration is in favor in the formation of ZnO.At 600°C,the carbonized materials also almost keep the ZIF-8 morphology with smaller particle size(Fig.S4),in agreement with the C-800 samples(Fig.4).

Figs.7 and S5 present the textural characteristics of C-600-0,C-600-6.3 and C-600-21,respectively.Similarly to the C-800 samples(Figs.5 and 6),there are micropores and mesopores in the obtained C-600 materials.But the adsorption amounts of nitrogen for the C-600 samples notably reduce,thereby the decreased surface areas and pore volumes.The existence of ZnO in the carbon structure(Fig.S3)should be responsible for the observations.Clearly,the initial oxygen concentration also significantly affects the surface area and pore volume of C-600.With increasing initial oxygen concentration,the surface area and pore volume first rise and then reduce.Particularly,at relatively high initial oxygen concentration,the mesopore volume decreases.The phenomena should be related with the oxygen activation.As discussed above,the introduced oxygen during the carbonization of ZIF-8 can react with carbon,and then some new pores are formed,thereby the increase in surface area and pore volume(Table 3).When the oxygen concentration is relatively high,more oxygen can react with carbon inside the opened pores[40],leading to the disappearing of some pores including mesopores and the reduction in surface area and pore volume.

Table 3Textural properties of C-600 obtained with different amounts of oxygen

At1000°C,the carbonized ZIF-8 materials are porous carbons having typical morphology similar to ZIF-8 as presented in Figs.S6 and S7.Additionally,no ZIF-8 and any other peaks for impurities like Zn and ZnO are observed,indicating the complete carbonization of ZIF-8 under the carbonization conditions.As the carbonization temperature is higher than the boiling point of Zn(908°C),Zn will evaporate and leave with the N2flow,leading to the nonexistence of Zn impurities peaks[21].Compared to the C-800 samples(Fig.1),another peak at about44°assigned to the(100)diffraction of carbon can be observed,indicating the increased degree of graphitization at higher carbonization temperature.In addition,there are no obvious differences in the peak intensity and position for the C-1000 samples,indicating that the initial oxygen concentration has no significant influence on the crystalline structure of ZIF-8 derived porous carbons at 1000°C,possibly because of the sufficient carbonization of ZIF-8 at higher carbonization temperature.

Similarly,three typical C-1000 samples were also analyzed by N2sorption,and the results are given in Figs.8 and S8 and Table 4.In comparison with the C-600 and C-800 samples,no obvious hysteresis loops in the N2adsorption–desorption isotherms of the C-1000 samples are observed irrespective of the initial oxygen concentration,further confirming the weak influence of initial oxygen concentration at 1000°C.The C-1000 samples exhibit higher adsorption amounts of nitrogen,and then have greater surface areas and pore volumes,indicating that higher carbonization temperature is in favor of the development of pores[22].The effect of heat treatment on the surface area and pore volume of carbonized sample is in consistent with the results reported by Abbasi et al.[21],which should be related with the thermal decomposition behavior of ZIF-8.ZIF-8 organic linkers start to decompose at around 600°C(Fig.S1b)and the generated Zn species probably block the pores,resulting in lower surface area and pore volume(Tables 2 and 3).As the decomposition temperature is larger than 908°C,Zn species will evaporate and this might lead to the production of porosity within the framework,thereby higher surface area and pore volume(Table 4).Evidently,the surface area and pore volume of C-1000 are dependent on the initial oxygen concentration.Unlike the change trend as the carbonization temperature is 600 °C or 800 °C(Tables 2 and 3),in this case,the surface area and total pore volume gradually decrease with increasing initial oxygen concentration,due to the relatively high oxygen concentration and the pronounced oxygen reaction inside the opened pores at higher carbonization temperature as discussed above.Interestingly,the mesopore volume of C-1000 first increases and then reduces with increasing initial oxygen concentration(Table 4),which should be caused by the pore evolution during the carbonization of ZIF-8 with the introduction of oxygen[40].

Fig.8.Nitrogen sorption isotherms of(a)C-1000-0,(b)C-1000-16.8,(c)C-1000-21.Solid symbols indicate gas sorption and open symbols express gas desorption.

Table 4Textural properties of C-1000 obtained with different amounts of oxygen

3.2.Adsorption study

MB dye is often is selected as a model substance to evaluate the potential application of various sorbents[20,21,38,41].Here we also used aqueous MB dye solution to test the adsorption capacities of the carbonized ZIF-8 materials.

Fig.9 shows the adsorption capacities of C-800 samples toward MB dye versus contact time.Clearly,the adsorbed quantity of MB onto each C-800 sample increases rapidly with adsorption time during the initial stage,and then increases at a slower rate and finally reaches equilibrium after about 600 min,in agreement with the previous reports[20,21].The results in Fig.9 indicate that the initial oxygen concentration during the carbonization process significantly affects the adsorption capacity of ZIF-8 derived porous carbon.With increasing initial oxygen concentration,the adsorption capacity of C-800 first significantly increases,reaches the highest value,and then reduces.The adsorption capacity of C-800-8.4 toward MB dye is 201 mg·g-1,about 2.6 times higher than that of C-800-0.The results highlight that controlling the initial oxygen concentration during the carbonization process can significantly improve the adsorption performance of ZIF-derived porous carbon.

Fig.9.Methylene blue adsorption capacity of C-800.

Fig.10.Methylene blue adsorption capacity of C-600.

Fig.11.Methylene blue adsorption capacity of C-1000.

To further verify the universality of adjusting the initial oxygen concentration,the adsorption capacities of C-600 and C-1000 samples were also investigated in aqueous MB dye solution.As seen in Figs.10 and 11,in comparison with the C-800 samples,similar adsorption curves are observed for the C-600 and C-1000 samples,but the time for adsorption equilibrium is significantly different.With respect to each C-600 sample,after only about 400 min,the adsorption toward MB dye can reach equilibrium.While for each C-1000 sample,the needed time for adsorption equilibrium is about 1200 min,significantly longer than that for the C-800 or C-600 samples.Larger adsorption capacity due to higher carbonization temperature can be responsible for the longer time for adsorption equilibrium[21].Higher carbonization temperature can increase the surface area and pore volume of porous carbon(Tables 2-4),thereby the enhanced adsorption capacity[21,38].Likewise,the adsorption capacities of porous carbons derived from ZIF-8 at 600 °C and 1000 °C strongly depend on the initial oxygen concentration.An optimal initial oxygen concentration is beneficial for the enhancement of adsorption capacity of ZIF-8 derived carbon.However,it should be noted that the influence of initial oxygen concentration at different carbonization temperatures is significantly different.The lower the carbonization temperature,the more obvious the influence of initial oxygen concentration.For example,the adsorption capacity of C-600-6.3 toward MB dye is about 3.4 times higher than that of C-600-0.While the adsorption capacity of C-1000-16.8 is only 1.1 times as compared to C-1000-0.The more obvious influence of initial oxygen concentration at lower carbonization temperature should be related with the carbonization process of ZIF-8.During the carbonization process,the Zn ions are reduced by carbon to Zn metals,and then the Zn metals will evaporate and leave at temperatures above the boiling point of Zn(908°C),leading to the formation of porous carbon[21,34].At 1000°C,almost all the Zn can leave from ZIF-8 under N2flow,and a carbon with well-developed pore structure can be formed[34],decreasing the influence of initial oxygen concentration.In contrast,at 800 °C or 600 °C,parts of Zn can be remained,and the pore structure will be not well developed[21].As a result,the introduction of oxygen will play an important role in the development of pores and the corresponding adsorption performance.These results confirm that controlling the initial oxygen concentration is a facile method for adjusting the microstructures of ZIF-derived porous carbons and improving their adsorption capacities.

Combined with the previous characterization results,we can conclude that the introduction of oxygen during the carbonization of ZIF-8 can significantly affect the development of pore structure,and then influence the surface area and pore volume as well as the adsorption capacity.For example,at 600°C,the addition of oxygen makes the surface area and pore volume of ZIF-8 carbonized material first increase and then reduce(Table 3).Correspondingly,the adsorption capacity toward MB dye also first rises and then decreases(Fig.10),indicating that higher surface area and pore volume can enhance the adsorption capacity of porous carbon[21].More interestingly,the mesopore volume plays an important role in determining the adsorption capacity of ZIF-8 carbonized material toward MB dye.For instance,the surface area and pore volume of C-800-8.4 are all lower in comparison with C-800-10.5,but the former has a higher adsorption capacity toward MB dye(Fig.9),in consistence with the greater mesopore volume(Table 2).The observation should be related with the microstructures of porous carbon and MB.The MB molecules have a relatively larger dimension(1.43 nm×0.61 nm×0.4 nm),and cannot easily diffuse into the smaller pores[21,38].Thus,a porous carbon with larger mesopore volume is beneficial for the diffusion of MB molecules,thereby higher adsorption capacity[41].

The adsorption of MB on an adsorbent is considered to be a process of MB interaction with the various functional sites of the adsorbent[35].Therefore,besides the textural properties,other factors including the surface properties and elemental composition of porous carbons also can affect their adsorption performance[21,35].The results in Figs.2 and 3 indicate that the introduction of oxygen during the carbonization of ZIF-8 almost has no influence on the formation of defects in the produced carbons,but significantly affects the surface composition.Higher ratio of HO--C=O groups on the surface of C-800-8.4 or C-800-10.5 can be achieved(Table 1),which can increase the interaction with the MB molecules[35],thereby higher adsorption capacity toward MB(Fig.9).Moreover,the adsorption capacities of C-800-0,C-800-8.4 and C-800-10.5 toward MB dye can match well with the ratio of HO--C=O groups on the carbon surface(Fig.9 and Table 1).The results suggest that the HO--C=O functional groups also plays an important role in determining the adsorption capacity of ZIF-8 derived porous carbon toward MB dye.

In comparison with the ZIF-8 derived carbons reported in the literatures[21,35],the developed porous carbons in this study exhibit superior adsorption capacities toward MB dye from water.The as-synthesized C-1000-16.8 sample has an adsorption capacity of 418 mg·g-1,about 2.2 times higher than that obtained at 1000°C in Wang group[21].Chen et al.prepared a porous carbon by direct carbonization of ZIF-8 in the presence of water steam at 800°C[35],and found that its adsorption capacity toward MB dye was 99 mg·g-1,significantly lower than the value of 201 mg·g-1for the C-800-8.4 sample.The superior adsorption capacity of the developed porous carbon should be related with its especially hierarchical pore structure and surface composition as discussed above.The results further declare that our developed method is an efficient approach for enhancing the adsorption capacity of porous carbon.

4.Conclusions

Hierarchically porous carbons were fabricated by direct carbonization of ZIF-8,and the influence of the oxygen introduction in-situ during the carbonization on the microstructures of carbonized ZIF-8 materials and their adsorption capacities toward MB dye from water were investigated.The oxygen addition can significantly affect the pore structure and surface composition of porous carbon,and then influence the adsorption capacity.And the influence is more obvious at lower carbonization temperature.The mesopore volume and ratio of HO--C=O groups are key factors that determine the adsorption capacity toward MB dye.The developed hierarchically porous carbons exhibit superior adsorption capacities for the removal of MB dye from water,and may have potential application in the wastewater treatment.

Supplementary Material

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.cjche.2018.05.014.