Yexun Shi,Chang Li,Liming Shen,Ningzhong Bao

College of Chemical Engineering,State Key Laboratory of Material-Oriented Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

Keywords:Graphene oxide powder Spray drying Re-dispersibility Oriented assembly Layered structure

ABSTRACT The graphene oxide powder (GOP) obtained from the spray drying process often exhibits poor redispersibility which is considered due to the partial reduction of GO sheets.The reduction of drying temperature can effectively increase the re-dispersibility of GOP,but result in a decreased drying efficiency.Herein,we found that the re-dispersibility of GOP is strongly affected by its microstructure,which is determined by the feed concentration.With the increase of feed concentration,the GO nanosheet assembly varies from the disordered stacking to relatively oriented assembly,making the morphology of the GOP transform from ball-like (the most crumpled one) to flake-like (the least crumpled one),and the 0.8 mg﹒ml-1 is the threshold concentration for the morphology,structure,and re-dispersibility change.Once the feed concentration reaches 0.8 mg﹒ml-1,the appearance of the nematic phase in droplet ensures the relatively oriented assembly of GO sheets to form the layered structure with a low crumpling degree,which greatly improves the polar parts surface tension of the solid GOP,making the GOP easier to form hydrogen bonding with water during the redispersion process,thus stabilizing dispersion.This work provides useful information for understanding the relationships between the morphology,microstructure,and final re-dispersibility of GOPs.

1.Introduction

Graphene oxide(GO),as a precursor of graphene,is a promising two-dimensional material for the fabrication of carbon-based materials due to its processibility and low cost of manufacture[1–4].GO possesses abundant oxidation groups and extraordinarily high surface area,and thus is suitable for many applications such as separation,catalyst,energy storage,and so forth [5–15].However,as-prepared GO is usually present in the form of GO aqueous dispersion with a low concentration (generally<2 mg﹒ml-1).Such low concentration is processable for lab-scale research but it is difficult to meet manufacturers’ demands that generally require tons of highly concentrated GO slurry or GO powders.Providing re-dispersible GO powders not only saves costs on transportation and post-drying process for users but also improves the shelf life of products[16].Till now,various drying technologies for producing graphene oxide powder (GOP) have been developed such as freeze-drying,vacuum drying,and spray drying [17–19].Among them,the spray drying has been widely used in the manufacture of powders,especially in the pharmaceutical and food industries due to its low cost,effective drying,easy scale-up,and rapid solvent loss[20,21],which are all beneficial to the production of GOP on a large scale.

So far,several studies on the spray drying of GO dispersion have been reported and they intensively investigated the crumpling process of GO and interpreted the evaporation-induced assembly behavior [18,22–25].Luo et al.reported the synthesis of crumpled graphene balls under capillary compression from rapidly evaporating GO droplets [23].Ma et al.explained that the crumpling of GO nanosheets is driven by the capillary force associated with a rapid solvent loss after calculating[24].Wang et al.established a universal equation to correlate the confinement force with various operating parameters and revealed that the evaporation rate plays an important role in controlling the crumpling process [25].The spray-dried GOPs studied in these works usually possess a high crumpling degree,and would not re-disperse in water [18].Some studies have also concluded that the partial reduction of GO resulted from the high-temperature thermal drying process (normally about 200°C) makes the dispersion of GOP worse [16,18].The GOP with excellent re-dispersibility is highly desirable because it may significantly facilitate its practical application.To solve the problem of re-dispersion,Chen et al.reduced the drying temperature to 130°C by adding N,N-Dimethylformamide(DMF)in original solvent (water) and obtained a well-dispersed flower-shaped GO(fGO) [16].We have been looking for an alternative solution to increase the re-dispersibility of GOPs,without dropping the drying temperature and sacrificing the drying efficiency.

Inspired by the studies of spray drying of other materials [26],which showed that the morphology and structure of powders can affect its re-dispersibility,we suspect whether similar property can be obtained for GOP.In this case,we can solve the problem of re-dispersibility of GOP,while keep the high drying efficiency at normal temperature of 200°C.For the spray drying of pure GO aqueous solution without any additional surfactants or solvents,the surface tension of GO dispersion does not change significantly,and thus the GO concentration is the only variable that changes the slurry properties.There have been some studies about the effect of GO concentration on the final product structure.Luo et al.found that the size of the crumpled particles became larger with higher initial GO concentration during the atomization drying process[23].But Yang et al.claimed that the concentration of the GO precursor solution has a low,ignorable impact on the product structure [27].The GO property varies greatly with the GO concentrations,from dispersed disorderly at low concentration to arranged in order at high concentration [28].However,for the spray drying of GO aqueous dispersion,there is no systematic report on the influence of concentration on the internal structure and the final dispersion of the GOP before.

In this work,all the spray drying processes of GO dispersions were carried at 200°C,and the relationships between the feed concentration and the morphology,internal structure,and final redispersibility of GOPs were studied systematically.By varying the feed concentration,the morphology change of GOP is similar to the unfolding process of the crumpled paper ball,and 0.8 mg﹒ml-1is the threshold concentration for the crumpled GOPs being able to unfold and regain the dispersibility in water.It was found that the existence of the nematic phase in droplet makes the relatively oriented assembly of GO sheets to form layered GOP when the feed concentration reaches 0.8 mg﹒ml-1,which promotes the polar part surface tension of solid GOP and thus final re-dispersibility.Our work helps the controllable preparation of GOPs with different morphologies and provides useful information to prepare redispersible GOP on a large scale.

2.Experimental

2.1.Materials

Natural flake graphite with a size of around 6.5 μm(2000 mesh)was purchased from Aladdin Industrial Corporation.Potassium permanganate(KMnO4,99.5%,analytical grade),potassium nitrate(KNO3,99.5%,analytical grade),sulfuric acid (H2SO4,98%),and hydrogen peroxide aqueous solution(H2O2,30%)were bought from Shanghai Lingfeng Chemical Reagents Company.All chemicals were used as received without further purification.

2.2.Preparation of GOP by the spray drying process

The GO dispersions with different mass concentrations were first prepared using the modified Hummers method[29,30].Spray drying was performed using a spray-dryer BüCHI B-290 (Flawil,Switzerland) with a standard 0.7 mm nozzle.As shown in Fig.1,in the spray drying process of GO,the GO dispersion is first broken into a large number of small droplets by a nozzle under pressure,then the droplets are mixed with hot air with rapid evaporation of water to result in the formation of particles,and finally,these particles are separated from the hot exhaust air [31,32].The inlet temperature was set at 200°C,and the aspiration flow rate was maintained at 40 m3﹒h-1.For the operating parameters in the atomization process,the liquid feed rate,atomization pressure,and nozzle airflow rate were fixed at 6 ml﹒min-1,0.4 MPa,439 L﹒h-1,respectively.The feed concentration was adjusted in the range of 0.1–8.0 mg﹒ml-1(the feed concentration here refers to the concentration of GO aqueous dispersion).The dryer was running for at least 10 min with DI water before the GO dispersion was fed to ensure a steady outlet temperature.After the drying process,the GOPs were collected from both the cyclone wall and the collection vessel.

2.3.Characterization

X-ray diffraction(XRD)measurements were carried out using a D8-Advance Bruker AXS diffractometer(Cu-Kα radiation source)at 40 kV and 40 mA.Thermogravimetric analysis (TGA) was performed using an STA 499C analyzer (NETZSCH,Germany) from 25°C to 700°C at a heating rate of 10°C﹒min-1in a nitrogen atmosphere.The morphology and structure of GOP samples were observed with a field-emission scanning electron microscope(FESEM,HITACHI S-4800).Particle size measurement was performed using a dynamic light scattering instrument(S3500,Microtrac Inc.).X-ray photoelectron spectroscopy (XPS) analysis was carried on a PHI 5000 Versa Probe III X-ray photoelectron spectrometer(ULVAC-PHI Inc.,Japan).The contact angle was measured with a contact angle meter (SDC-350,Sindin,China).The specific surface areas were measured using the Brumauer-Emmett–Teller(BET) method based on the nitrogen adsorption–desorption isotherms at 77 K on a BELSORP-mini (BEL,Japan).Fourier transform infrared (FTIR) tests were carried out by using a spectrometer(RQUINOX55,Bruker,Germany).The re-dispersion of GOP in water was carried out by dissolving 25 mg GOP in 50 ml DI water(0.5 mg﹒ml-1) with the aid of 30 min sonication (40 kHz,300 W),and the similar method was used to investigate the re-dispersion of GOP in other solvents.

3.Results and Discussion

3.1.Formation path of GOP with layered structure

Fig.1.Schematic illustrations of the spray drying process of GO dispersions and the detailed view of bi-fluid nozzle.

The formation path of GOP in the spray drying process has been reported before,and the overall process includes the atomization stage and the drying stage,which determine the size and morphology of final GOP,respectively [18,24,33–35].In this paper,it is found that the formation path can be divided into three distinct types,each controlled by the feed concentration.As shown in Fig.2,when the feed concentration is low (C<0.8 mg﹒ml-1),similar to the reported before,the GO sheets will accumulate at the droplet boundary first due to the high evaporation rate(compared with the diffusion rate of the GO sheets),and thus results in the formation of a shell at boundary [18,36].Due to the flexibility of the thin shell,it can easily collapse toward the droplet center to form a compact crumpled morphology with the further evaporation of water[37].The low feed concentration makes the GO sheets randomly distributed inside droplets,leading to the GOP a product of disordered stacking of sheets after drying.However,once the feed concentration reaches 0.8 mg﹒ml-1,the situation is very different.When the feed concentration is medium(0.8 ≤C<4.0 mg﹒ml-1),although the bulk concentration of the droplets has not yet formed a nematic phase.The concentration of GO sheets resulted from the fast water evaporation makes the edge first form a relatively oriented assembly,and finally the layered GOP due to the drying and extrusion.Further increasing the feed concentration (4.0

3.2.Preparation of GOP with layered structure

The feed concentration in the range of 0.1–8.0 mg﹒ml-1was used for GOP preparation to understand the structure of GOP varies with the feed concentration.Fig.3 shows the SEM images of the GOP samples prepared with different feed concentrations.The morphology evolution with the feed concentration is similar to the unfolding process of a crumpled paper ball.These GOPs all possess crumpled morphology,but the crumpling degrees make differences.At the low end of the feed concentration (Fig.3a1,a2),the GOP looks like small rough paper balls;while at the high end of the feed concentration (Fig.3h1,h2),the GOP exhibits the morphology of relatively smooth flakes with wrinkles on the surface.The 0.8 mg﹒ml-1concentration is the threshold value for the unfolding process to initialize.More crumpled GOPs (rough balllike) can be obtained when the feed concentration is less than 0.8 mg﹒ml-1,and less crumpled GOPs (smooth flake-like) can be prepared once the concentration reaches 0.8 mg﹒ml-1.This finding explains why the low concentration (0.1 mg﹒ml-1) was used to study the crumpling process of GO[18]and the feed concentration(≥0.6 mg﹒ml-1) had a low impact on the structure [27].

Accompanied by the decline of the crumpling degree,the particle size of the GOP becomes larger as the feed concentration increases.As depicted by Fig.4,the size distribution constantly shifts to the right side [Fig.4(a)],and the average particle size(D50) of GOP increases from 2.26 to 10.34 μm when the feed concentration increases from 0.1 to 8.0 mg﹒ml-1[Fig.4(b)].The crumpled morphology of GOP in the spray drying process is mainly dependent on the capillary force exerted by the evaporation of water [24].The capillary forces of different GOPs were calculated using the equation of confinement force proposed by Wang et al.[25] (Supplementary Information,discussion S1),and the results were plotted as a function of concentration,as shown in Fig.4(b).From the feed concentration of 0.1 to 1.0 mg﹒ml-1,the confinement force decreases from 248 to 96 nN.In this period,the thin and flexible GO sheets at 0.1 mg﹒ml-1can be easily crumpled to small balls when subjected to a high capillary force of 248 nN.With the decline of the capillary force,the crumpled GOPs gradually unfold,making the particle size increase,correspondingly.When the feed concentration is above 1.0 mg﹒ml-1,the feed concentration dominates the capillary force.The high capillary force not only induces the crumpling process but also creates crease patterns on the GO surface[40].However,the effects of capillary force on the morphology of the GOP is limited.It is because the prepared GOPs consist of thick stacks of GO sheets which provide higher bend strength to resist the deformation,thus exhibits the least crumpled morphology [23,41].

Fig.2.Schematic formation paths of the less crumpled GOP and more crumpled GOP from a droplet with low-and high-concentration,respectively.

Fig.3.(a-h)SEM images of GOP samples with feed concentration of 0.1(a1,a2),0.2(b1,b2),0.4(c1,c2),0.6(d1,d2),0.8(e1,e2),1.0(f1,f2),2.0(g1,g2)and 8.0 mg﹒ml-1(h1,h2).

Fig.4.(a) Particle size distribution of GOPs prepared with different feed concentrations.(b) D50 and capillary force as a function of feed concentration.

Fig.5.(a) XRD patterns of GOPs with different feed concentrations.(b) The boundary concentrations vary with feed concentrations.

From the SEM images of GOPs,only the differences in crumpling degree can be observed.To further investigate whether the internal structure changes with feed concentration,XRD measurements were performed.Fig.5(a) shows the XRD patterns of different GOPs.When the feed concentration is 0.1 mg﹒ml-1,the XRD pattern does not contain the diffraction peak at～10°which is the fingerprint of the lamellar microstructure of GOP [42].This indicates that the GO nanosheets are assembled in a disordered manner so that the most crumpled GOP exhibits isotropic property and no diffraction peaks exist.As the feed concentration increases,the characteristic diffraction peak at～10° appears,indicating the improved ordered structure of GO sheets.When the feed concentration reaches 0.8 mg﹒ml-1,the strong (002) diffraction peaks appear no matter the feed concentration is high or low.The high peak intensity indicates the existence of the relatively oriented assembly of GO sheets,promoting the formation of GOP with a layered structure similar to graphite oxide.The XRD results reveal that the feed concentration indeed affects the assembly of GO sheets and thus the morphology and structure of GOP.

To further explain why the completely different assembly forms with lower or higher feed concentration than the threshold concentration of 0.8 mg﹒ml-1,we further calculated the GO concentration in the droplet.In fact,during the drying process,the evaporation of water induces the accumulation of GO sheets on the surface and results in the formation of a shell at the liquid–gas interface.The concentration at the boundary will cause the boundary concentration(Cboundary)to be much higher than the bulk concentration (Cbulk) of the droplet,so the boundary concentrations were quantitatively calculated (Supplementary Information,Discussion S2)[43].According to the DLVO theory,as the feed concentration increases,the Van der Waals forces between the GO sheets gradually increase[38].The combined effect of electrostatic repulsion and van der Waals forces make the existence of the nematic phase (4.0 mg﹒ml-1),and the GO sheets tend to be arranged in long-range orientational order [43,44].As shown in Fig.5(b),with the feed concentration increases from 0.1 to 0.6 mg﹒ml-1,the boundary concentration increases from 0.15 to 2.48 mg﹒ml-1,correspondingly,which is still too low for the nematic phase formation.However,once the feed concentration reaches 0.8 mg﹒ml-1,the boundary concentration will increase up to 4.15 mg﹒ml-1,at which characteristic nematic phase schlieren textures have been observed before[43].Further increase the feed concentration to more than 4 mg﹒ml-1,the bulk concentration of the droplets has reached the concentration formed by the nematic phase.Under these conditions,the relatively oriented assembly of GO sheets makes the GOP thick enough to resist deformation,resulting in the formation of layered GOP with a low crumpling degree (smooth flake-like).These results are consistent with the SEM images and XRD patterns,indicating that the threshold concentration of 0.8 mg﹒ml-1exists.Only when the feed concentration reaches 0.8 mg﹒ml-1,the layered GOP with a low crumpling degree can be formed due to the relatively oriented assembly of the GO sheets.However,even at the highest feed concentration(8.0 mg﹒ml-1),the boundary concentration is only 342.92 mg﹒ml-1,which is much lower than 1200 mg﹒ml-1,where the film forms at the gas–liquid interface [43].The thin film is a product of tight stacking of GO sheets,so this result shows that the layered structure of GOPs obtained by the spray drying process is far less dense than thin film.Unlike the drying of a GO film,the absence of the substrate makes GO droplet not compress in one direction toward the substrate during the drying process,resulting in a relatively loose and oriented structure.This result can be confirmed by the BET data.From Fig.S1,the least crumpled GOP possesses the specific surface area of 45.77 m2﹒g-1,much larger than the GO film(2.78 m2﹒g-1),indicating the loose and relatively oriented assembly of GO sheets inside the GOP [23].

3.3.Re-dispersibility of different GOPs

Fig.6.Digital images of DI dispersions of PGO and GOPs obtained with feed concentrations ranging from 0.1 to 8.0 mg﹒ml-1 (a) right after 30 min sonication and (b) after standing for 24 h.(c) SEM images of GOP after redispersion with feed concentrations of 0.1 mg﹒ml-1.(d)Schematic process of water molecules entering the GOP during the redispersion.(e) SEM images of GOP after redispersion with feed concentrations of 8.0 mg﹒ml-1.

To further test whether the GOP with layered structure is conducive to the redispersion,the dispersion tests of different GOPs were carried out,as shown in Fig.6.Fig.6(a) shows the digital images of GOP dispersions right after the ultrasonic treatment.The feed concentration of 0.8 mg﹒ml-1is a turning point.For the GOPs obtained with low feed concentrations (0.1,0.2,0.4,and 0.6 mg﹒ml-1),their DI dispersions are turbid,through which the black line drawn on the backside of the glass vial is hardly visible.After the feed concentration reaches 0.8 mg﹒ml-1,the appearance of brown-colored supernatant reflects the presence of redispersion and the DI dispersion becomes clearer as the feed concentration increases.The clarity of the brown-colored dispersion with the least crumpled GOP (8.0 mg﹒ml-1) is similar to that of pristine GO (PGO) dispersion.After standing at room temperature for 24 h [Fig.6(b)],the dispersibilities of these GOP samples appear two states.The GOPs obtained with lower feed concentrations(0.1,0.2,0.4,and 0.6 mg﹒ml-1) were all settled at the bottom of glass vials,which means that the crumpled particles without layered structures are easier to precipitate,and they still retain their original crumpled paper ball shape after sitting in water for 24 h[Fig.6(c)].It is noteworthy that the heavily crumpled structure obtained with 0.1 mg﹒ml-1could not unfold even after 24 h of sonication(Fig.S2a).It is because the disordered structure usually hinders the water movement,and the water molecules can only infiltrate from the outer pores step by step[Fig.6(d)].On the contrary,the less crumpled GOPs with layered structure obtained from higher concentrations (≥0.8 mg﹒ml-1) can stably disperse in water;meanwhile,the amount of precipitation settled at the vial bottom gradually decreases with the increase of feed concentration.Fig.6(e)shows the sheet morphology in GOP dispersion with a feed concentration of 8.0 mg﹒ml-1.It is found that the layered GOP with a low crumpling degree has been unfolded and exfoliated,and the size is significantly reduced compared to the stacked GOP.It is mainly due to the reason that the layered GOP with relatively loose and oriented structure ensures the direct enter of water molecules to the inner layer through different pores or interlayer galleries,making the movement of water molecules faster,thus conducive to the redispersion of GOPs.In addition to water,the layered GOPs with low crumpling degrees are also redispersible in N,N-Dimethylformamide (DMF),N-Methyl pyrrolidone (NMP),and ethylene glycol (EG) after 30 min sonication(Fig.S2b).Compared with the conventional drying process such as freeze-drying,oven drying,and vacuum drying,the spray drying method is the most time-saving and can ensure the good redispersibility of GOP at the same time (Table S3).

3.4.Comparisons of GOPs with different structures

Due to the distinct dispersibilities of GOPs with different structures,the physicochemical properties of different GOPs were further compared.The prior study reported that the GO can be partially reduced in the thermal drying process,which may be responsible for the poor re-dispersion of GOP[18].So the chemical compositions of GOPs with different structures were first tested to verify whether it is the determining factor for the final redispersion.Fig.7(a)displays the broad scan XPS spectra of GOPs.The elemental analysis illustrates that C/O atomic ratio of the least crumpled GOPs with layered structure (1.99) is similar to that of the most crumpled one with disordered structure (2.02),which suggests that the chemical compositions of GOPs with different structures are the same,indicating that they are all rich in oxygen-containing functional groups after spray drying.Since the two types of GOPs have the same C/O atomic ratio,but different re-dispersibilities,the XPS result implies that the partial reduction of GO cannot explain the poor re-dispersibility of the crumpled GOP with disordered structure.It is noteworthy that the C/O atomic ratio of our soluble least crumpled GOPs with layered structure (1.99) obtained at 200°C is lower than the ratio of the reported soluble fGO (2.17) obtained at 130°C [16],implying that the normal drying temperature does not result in a significant reduction degree of GO in our spray drying process.The chemical composition of GOP is also verified by thermogravimetric analysis(TGA)and Fourier transform infrared(FTIR)data.As shown in Fig.7(b),the TGA curves of the GOPs and the PGO show the same mass loss of 10% between the room temperature and 125°C due to the removal of interlayer moistures [45].The major weight loss of GOP between 150°C and 220°C is attributed to the decomposition of labile oxygen functional groups such as hydroxyl and epoxide groups [46].The difference in weight loss between the spraydried GOPs and PGO in the range of 150°C to 220°C is less than 1%,which suggests that almost all the oxygen-containing functional groups were retained during the thermal drying process.The FTIR spectrum also reveals that the presence of CO(1720 cm-1),C—C (1620 cm-1),and C—OH (1403 cm-1) in all GOP samples(Fig.S3)[47,48],and no obvious differences between peak intensities can be observed,indicating the similar functional group composition.Although the functional group is retained,the poor re-dispersibility of our most crumpled GOP implies that its disordered structure plays a more significant influence on the final redispersion.

Fig.7.(a)Broad scan XPS spectra of the least crumpled(8.0 mg﹒ml-1)and the most crumpled(0.1 mg﹒ml-1)GOPs.(b)TGA curves of GOPs and PGO.(c)Water and ethylene glycol contact angles of GOPs with different feed concentrations.(d) Calculated surface tensions of GOPs with different feed concentrations.

The re-dispersibility mainly depends on the interaction between the water and different GOPs during the wetting process.To further evaluate the wettability of GOP with different structures,the contact angle tests (CA) were carried out.Fig.7(c) displays the water CA and ethylene glycol (EG) CA between different GOPs.With the feed concentration increases,the water CA and EG CA all decreases,correspondingly,and the EG CA is usually smaller than water CA at the same feed concentration.The measured CA can be used to determine the surface tension of the solid GOP (σs) (Supplementary Information,Discussion S3) [49].The calculated surface tension of the GOP is mainly composed of two parts,the dispersive parts and polar parts,which are controlled by van der Waals force and hydrogen bonding,respectively.As shown in Fig.7(d),as the feed concentration increased from 0.1 to 8.0 mg﹒ml-1,the total surface tension of GOP increased from 29.6 mN﹒m-1to 54.1 mN﹒m-1.When the feed concentration is less than 0.8 mg﹒ml-1,the surface tension of the GOP is mainly controlled by the dispersive parts.Due to van der Waals force,the GOPs are more likely to attract each other and sedimentation,which adversely affects the redispersion of GOP.Once the feed concentration reaches the threshold concentration (0.8 mg﹒ml-1),polar parts dominate.The enhanced effect of hydrogen bonding facilitates the formation of more hydrogen bonds between the GOP and the solvent,which is more conducive to the stable dispersion of the GOP.At the same time,compared to low feed concentration,the enhancement of polar parts greatly increases the total surface tension of GOP,making it closer to the surface tension of water (72.8 mN﹒m-1).This indicates that the GOP prepared at high feed concentration is more easily compatible with water,which contributes to the unfolding and exfoliation in the subsequent redispersion process,and thus the final stable dispersion.This result reveals that the GOPs with different structures do possess very different wettability,the layered GOP with low crumpling degree helps to increase surface tension by enhancing the polar parts,making it more compatible with water,and improving the dispersion of the final powder.At the same time,the difference in surface tension caused by the difference in the GOP structure leads to the final difference in re-dispersibility,which also makes the threshold concentration of re-dispersibility and the threshold concentration of morphology structure unified.

4.Conclusions

We report on the preparation of GOP with excellent redispersibility via the spray drying process.The experimental results have shown that the good re-dispersibility of GOPs is closely related to the layered structure formed by the relatively oriented assembly of GO sheets,which is controlled by the feed concentration.And the 0.8 mg﹒ml-1is found to be the threshold concentration for the morphology,structure,and re-dispersibility change.Once the feed concentration reaches 0.8 mg﹒ml-1(0.8 mg﹒ml-1≤C<4.0 mg﹒ml-1),the concentrated boundary concentration just increases up to the concentration for nematic phase formation,which contributes to the relatively oriented assembly of GO sheets,and thus the formation of layered GOP.Further increasing the feed concentration(4.0 mg﹒ml-1≤C ≤8.0 mg﹒ml-1)allows the main body of the droplet exhibit nematic phase,and thus more easily led to the generation of layered GOP with a low crumpling degree.Due to the layered structure,the polar part surface tension of solid GOP greatly improves,which promotes the final redispersibility by enhancing the hydrogen bonding effects with water.The XPS and TGA results of the GOPs with different structures indicate that all the oxygen-containing functional groups are retained during the thermal drying process.

Acknowledgements

This research was supported by the National Key R&D Program of China (2019YFD1101200,2019YFD1101204),Natural Science Foundation of China (51772150),and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) Jiangsu Provincial Key Research and Development Program (BE2018008-1).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.08.023.