Shi Yuan,Yang Li,Ruosang Qiu,Yun Xia,Cordelia Selomulya,2,Xiwang Zhang,

1 Department of Chemical Engineering,Monash University,Clayton,Victoria 3800,Australia

2 School of Chemical Engineering,University of New South Wales,New South Wales 2052,Australia

Keywords:rGO membranes Graphene quantum dots Non-selective defects Water permeance NaCl rejection

ABSTRACT Reduced graphene oxide (rGO) membranes have been intensively evaluated for desalination and ionic sieving applications,benefiting from their stable and well-confined interlayer channels.However,rGO membranes generally suffer from low permeability due to the high transport resistance resulting from the narrowed two-dimensional (2D) channels.Although high permeability can be realized by reducing membrane thickness,membrane selectivity normally declines because of the formation of nonselective defects,in particular pinholes.In this study,we demonstrate that the non-selective defects in ultrathin rGO membranes can be effectively minimised by a facile posttreatment via surfacedeposition of graphene quantum dots (GQDs).The resultant GQDs/rGO membranes obtained a good trade-off between water permeance (14 L·m-2·h-1·MPa-1) and NaCl rejection (91%).This work provides new insights into the design of high quality ultrathin 2D laminar membranes for desalination,molecular/ionic sieving and other separation applications.

1.Introduction

Graphene and other two-dimensional(2D)materials have been recognized recently as promising building blocks for the fabrication of 2D laminar membranes[1–3].Among these 2D nanosheets,graphene oxide (GO) nanosheets,a derivative of graphene with abundant oxygen functional groups [4],are highly dispersible in various solvents [5],allowing feasible re-stacking into GO laminates[6–8].These laminar GO membranes theoretically have great potential in achieving superior water flux as well as high selectivity for ionic sieving owing to their unique 2D interlayer channels[9,10].However,due to the hydration of oxygen functional groups,GO nanosheets become negatively charged and GO membranes tend to swell in aqueous solutions,leading to expanded 2D nanochannels and thus a decline in selectivity [11].To stabilize GO membranes in water,several physicochemical strategies have been developed,e.g.,by utilizing physical confinement by epoxy glue [12] and external pressure regulation [13],crosslinking adjunct GO nanosheets with organic compounds [14,15] and ions[16,17] via non-covalent or covalent bonds,and reducing oxygen functionalities on GO nanosheets [18–21].Among these stabilized GO-based membranes,reduced graphene oxide (rGO) membranes provide the advantages of simplicity,stability and tuneability[22,23].

In general,GO membranes can be reduced to rGO membranes by either thermal treatment [24,25] or chemical reduction with reductants,such as hydrazine [26,27],hydroiodic acid (HI)[20,28],poly-dopamine (PDA) [22,29] and etc.After the partial removal of oxygen functional groups from GO nanosheets,rGO membranes exhibit good stability with confined interlayer channels in water.The sizes of 2D nanochannels in rGO membranes could narrow down to 0.4 nm in water,which should enable the effective rejection of small ions like Na+(hydrated diameter:0.72 nm) [30].In fact,due to the presence of inevitable defects,in particular pinholes and nanowrinkels in GO/rGO membranes,the NaCl rejection of most reported rGO membranes was much lower than theoretical value in hydraulic pressure-driven filtration systems [21,30,31].In our previous work [32],we demonstrated that nanowrinkles in rGO membranes could be minimised by a mild thermal reduction in air,thus enhancing NaCl rejection to some extent.After that,we further improved NaCl rejection by adopting potassium cations (K+) to insert into rGO laminates (KrGO) [33].The well-confined interlayer spacing by strong K–π interaction in K-rGO membranes could restrict the permeation of Na+into rGO layers,resulting in a high NaCl rejection at 91%.Unfortunately,water permeance of these rGO membranes declined with the increase of salt rejection.

Because water molecules normally suffer from high transport resistance in rGO membranes due to the tortuous layered transport passages and narrowed interlayer spacing,a trade-off between membrane flux and salt rejection has been commonly observed.Similar to polymeric membranes,membrane permeability can be enhanced by reducing the thickness of rGO membranes [20,34].Nevertheless,when the thickness of these membranes is down to 10 nm,it is inevitable to form large non-selective defects,pinholes[35,36],which could significantly lower their selectivity.Recently,Yu et al.demonstrated that the defects in one-dimensional (1D)carbon nanotubes(CNTs)membranes were minimised by depositing zero-dimensional (0D) graphene quantum dots (GQDs) onto membrane,achieving improved separation performance for gas separation and desalination [37].Therefore,GQDs could be potential fillers to seal the non-selective defects of rGO membranes.

Herein,we demonstrated a facile strategy to seal defects in rGO membranes using GQDs fillers for achieving high water flux as well as high salt rejections.The GQDs/rGO membranes were prepared via the surface deposition of GQDs onto as-prepared GO membranes,followed by a thermal treatment in air.The resultant membranes exhibit both high water permeance and enhanced NaCl rejection,compared with original rGO membranes and GQDs-rGO membranes which were fabricated by the filtration of GQDs and GO mixture solution in one step.This work provides new insights into the design of high-performance ultrathin membranes using 2D materials as building blocks for simultaneously realizing high permeability and selectivity.

2.Materials and Methods

2.1.Materials

Graphene oxide flakes(XFnano,China)were applied to prepare 3.2 mg·L-1GO aqueous solutions with Milli-Q water (Millipore,USA).Commercial polyethersulfone (PES) membrane filters(47 mm,pore size of 30 nm,Sterlitech Corp.,USA) were utilized as the substrates of GO membranes.H2O2(30% (mass),Sigma-Aldrich,Germany),Na2CO3and NaCl (AR,Sigma-Aldrich) were used without further purification.

2.2.Fabrication of GQDs and GQDs/rGO membranes

GQDs were synthesized by the oxidation of GO flakes in H2O2and Na2CO3solutions.Typically,100 mg of GO flakes was firstly dispersed into 300 ml of H2O2,followed by adding 0.318 g of Na2-CO3.After 15 min ultra-sonication treatment,the above solution was then heated up to 70 °C in a water bath and refluxed for 24 h.Finally,the solution was purified in a dialysis bag (MWCO 14000 Da,Sigma,USA) with Milli-Q water for 3 days to obtain a transparent GQDs solution (Fig.1).

Ultrathin flat-sheet GQDs/rGO membranes were prepared as shown in Fig.2.Firstly,a 10 ml 3.2 mg·L-1GO solution was diluted with Milli-Q water to 200 ml.After that,a flat-sheet 10 nm-thick GO membrane was assembled by filtrating the diluted GO solution onto a polyethersulfone(PES)substrate(47 mm,pore size:30 nm,Sterlitech Corporation,USA).Then,the as-prepared 10 nm-thick GO membrane was dried and stabilized in an oven(air drying oven,Jinghong,China)at 80°C for 24 h.Afterward,a 1 ml of GQDs solution was diluted into a 10 ml GQDs solution,followed by filtrating the GQDs solution onto the dried 10 nm-thick GO membrane to deposit GQDs onto the GO membrane.Finally,the GQDs/GO membrane was thermally treated at 150 °C for 1.5 h to fabricate the GQDs/rGO membrane.For comparison,rGO or GQD-rGO membranes were prepared via filtrating same amount of pure GO solution or GQD and GO mixture solution on PES substrates,respectively,followed by the air-thermal treatment at 80 °C for 24 h and the thermal reduction at 150 °C for 1.5 h.

2.3.Characterization of membranes

The morphology and size of GQDs were characterized by transmission electron microscopy (TEM,Tecnai F20 G2,FEI,USA).Raman spectra were monitored with 10 nm-thick membranes using a Renishaw RM 2000 Confocal micro-Raman System (Renishaw,UK) with a laser at an excitation wavelength of 514 nm to identify the structural defects.The contact angles of membranes were measured by a contact angle measuring device (OCAH-230,Dataphysics,USA) with 2 μl dosing amount.X-ray diffraction(XRD) patterns were taken on a diffractometer (Bruker D2 Advance,Germany) with Cu source (40 kV,40 mA,λ=0.154060 n m) to determine the interlayer spacing with 500 nm-thick membranes to exclude the influence of PES substrates.Atomic force microscopy (AFM,Dimension Icon,USA) was used to characterize the surface roughness of membranes with 10 nm-thick membranes.Average surface roughness (Ra) was derived from tapping mode AFM images using NanoScope 1.5 analysis software.

2.4.Membrane performance test

A dead-end cell (Sterlitech,HP4750,USA) was applied to measure water permeance and salt rejection of membranes.Membranes were firstly compacted in water under 0.6 MPa for 1 h,and then the water permeance was obtained by filtrating water under 0.6 MPa for 2 h.A digital balance was used to monitor the mass of the permeate water (m).The water permeate was calculated by the Eq.(1) below:

where P refers to the water permeance(L·m-2·h-1·MPa-1),m is the mass of the water permeate(kg),ρ represents the density of water(1 kg·L-1),A is the effective area of rGO membranes (m2,here was constant to 3.14×10-4m2),t denotes the time interval(h)and p is the transmembrane pressure(in our experiment,p was set constant to 0.6 MPa).

The salt rejection was determined by filtrating 1 g·L-1salt solution under 0.6 MPa with a magnetic stir bar stirring at 300 r·min-1on the near surface of membranes after filtrating the water.The concentration of salt in the feed (cf,g·L-1) and permeate (cp,g·L-1) was measured by a conductivity meter.The salt rejection rate was calculated by the Eq.(2) below:

The salt permeation rate(Psalt,g·m-2·h-1)was calculated by the Eq.(3) below:

P′(L·m-2·h-1·MPa-1) is the water permeance when the feed are 1 g·L-1salt solutions.ρ′is the density of permeate which could approximate to the density of water,ρ.p refers the transmembrane pressure (in our experiment,p was set constant to 0.6 MPa).

Fig.1.Schematic illustration of the fabrication process of GQDs solutions.

Fig.2.Schematic illustration of the fabrication process of GQDs/rGO membranes.

3.Results and Discussion

3.1.Synthesis of graphene oxide quantum dots

GQDs were synthesized through the oxidation of GO flakes using H2O2and Na2CO3mixture solutions.As illustrated in Eqs.(4)–(7),H2O2could react withto generate various reactive oxygen species (ROSs),e.g.,·OH andWhen heating up GO dispersion with H2O2and Na2CO3,these ROSs could oxidize sp2carbon networks and break the aromatic rings of GO nanosheets.Hence,GO nanosheets could be gradually transferred to nano-sized GQDs after the oxidation [39].As shown in the TEM images of GQDs(Fig.3(a)and(b)),as-prepared GQDs are uniformly distributed and the lattices of the GQDs are observed in the high-resolution TEM image.As shown in Fig.3(c),the size of these GQDs is in the range of 2 to 5.5 nm with an average size of 3.8 nm.Such sub-10-nm GQDs tend to interact strongly with graphenebased nanosheets through the π–π stacking interaction [40,41],enabling the coverage of the nano-sized defects on graphenebased membranes.

Fig.3.(a) and (b) TEM images of GQDs in different magnifications.(c) Size distribution of GQDs in (a).

3.2.Structural characteristics of GQDs/rGO membranes

The structures of rGO and GQDs-rGO and GQDs/rGO membranes were firstly explored by Raman characterization.Fig.4(a)shows two peaks at Raman shift of 1350 and 1600 cm-1for each membrane,which represent the sp3(D peak)and sp2(G peak)carbon,respectively.The intensity ratio of D peak to G peak (ID/IG)could indicate the order of laminar structure in graphene-based membranes [42].The ID/IGof these rGO membranes were compared as shown in Fig.4(b).After incorporating GQDs into rGO membranes,the ID/IGratio declines significantly from 0.72 to as small as 0.64,demonstrating that GQDs could cover the structural defects on the graphitic regions on rGO nanosheets[43].It is worth noting that the surface coated GQDs/rGO membranes exhibit smaller ID/IGthan the mixed GQDs-rGO membranes,indicating that the surface coating with GQDs could form ultrathin rGO membranes with more ordered laminar structure,which is favourable for achieving high selectivity.

Fig.4.(a) Raman spectra and (b) ID/IG of 10 nm-thick rGO,GQDs-rGO and GQDs/rGO membranes.

In addition to Raman characterization,their 2D nanochannels were further measured by XRD characterization.In Fig.5(a),there is no obvious(0 0 2)peak of these rGO membranes,since the peak of PES at 2θ of 18° is too strong and cover characteristic peaks of these rGO membranes.In general,the (0 0 2) peak of GO membranes is stronger before the reduction.Hence,we further investigate the XRD results of these membranes before the reduction to reflect the trend in their interlayer nanostructures.As shown in Fig.5(b),the (0 0 2) peak of GO shift left from 11.9° to 11.62°and 11.79° after the incorporation of GQDs into the laminates and onto the membrane surface,indicating the interlayer spacing of 0.74,0.76 and 0.75 nm of GO,GQDs-GO and GQDs/GO membranes(Fig.5(c)),respectively.GQDs/GO membranes share similar interlayer channels with GO,while GQDs-GO exhibit a larger 2D channel size.The full width at half maximum (FWHM) values of these GO membranes were then analysed to evaluate the order of their laminar structures.Fig.5(c) shows that GQDs-GO exhibit the largest FWHM of 0.853,while GO and GQDs/GO experience smaller FWHM at around 0.83,demonstrating the relatively disordered 2D nanostructures in GQD-GO.Therefore,it is reasonable to predict that,after the reduction,the surface-deposited GQODs/rGO could show similar interlayer structures with pure rGO membranes.In contrast,GQDs-rGO membranes will experience larger interlayer spacing as well as disordered laminar structures after the insertion of GQDs into rGO laminates,which could result to the low selectivity.

Fig.5.XRD patterns of 500 nm-thick(a)rGO,GQDs-rGO and GQDs/rGO membranes;(b)GO,GQDs-GO and GQDs/GO membranes.(c)Interlayer spacing and full width at half maximum of (0 0 2) peak of GO,GQDs-GO and GQDs/GO membranes.

In principle,rGO membranes should become hydrophobic after the reduction of GO.As shown in Fig.6(a),water droplet,in fact,spreads quickly on the surface of rGO and GQDs-rGO membranes with the dramatic decline in their water contact angles from around 70° to 20° in30 s,indicating the good hydrophilicity of these two rGO membranes.This abnormal trend in hydrophilicity of rGO membranes might be due to the presence of defects,e.g.,pinholes and nanowrinkles,which allow fast water transport in rGO laminates.In contrast,the water contact angle of GQDs/rGO membranes only declined slightly and remained at around 55°,benefiting from their defect-free surface after the deposition of GQDs onto rGO membranes.These results demonstrate the defect-free structure of our GQDs/rGO membranes after the surface-deposition of GQDs,which is desirable for high selective rGO membranes.

Fig.6.(a) Water contact angles of 10 nm-thick rGO,GQDs-rGO and GQDs/rGO membranes vs.time.Optical images of water contact angle at the time of 30 s:(b)rGO,(c) GQDs-rGO and (d) GQDs/rGO membranes.

The surface structure of these rGO membranes were further directly analysed by AFM characterizations.Since these rGO membranes are only around 10 nm in thickness,it is difficult to prepare freestanding rGO membranes on silica wafer.Fig.7 shows the surface height information of rGO,GQDs-rGO and GQDs/rGO membranes on PES substrates.Although the surface roughness of these membranes is quite similar,the roughest surface is observed in GQDs-rGO membranes,indicating the most disordered layered structure due to the incorporation of GQDs into rGO laminates.In contrast,the surface deposited GQDs/rGO membranes exhibit similar surface structure to the pristine rGO membranes with average surface roughness of 9.72 nm.This result is in accordance with XRD and water contact angle results,demonstrating the disordered structure in GQDs-rGO membranes and the coverage of defects in GQDs/rGO membranes.Therefore,as-prepared GQDs/rGO membranes have great potential to achieve high permeability as well as high selectivity.

3.3.Membrane performance

The water permeance and salt rejection of these rGO membranes were evaluated in a dead-end cell under 0.6 MPa with Milli-Q water and 1 g·L-1NaCl solution,respectively.Similar to our previous rGO membranes,the 10 nm-thick pure rGO membranes exhibit water permeance of 16 L·m-2·h-1·MPa-1with a low NaCl rejection of 75% due to the presence of defects (Fig.8(a)).Although GQDs-rGO membranes experience higher water permeance of around 31 L·m-2·h-1·MPa-1,their NaCl rejection rate drops down to 65%,which is a result of the disordered laminar structure after the incorporation of GQDs into rGO layers.In contrast,the surface deposited GQDs-rGO membranes achieve a high NaCl rejection of 91% without obviously sacrificing the water permeance(14 L·m-2·h-1·MPa-1).As shown in Fig.8(b),NaCl permeates 2.5 and 7 times faster in pure rGO and GQDs-rGO membranes than that does in GQDs/rGO membranes,further demonstrating that high permeability and high selectivity can be realized in our ultrathin and less defective GQDs/rGO membranes.Comparing with reported GO-based membranes,the GQDs/rGO membranes exhibit the state-of-the-art NaCl rejection with a high water permeance (Fig.8(c)).

Fig.8.(a)Water permeance and NaCl rejections and (b)NaCl permeation rate of 10 nm-thick rGO,GQDs-rGO and GQDs/rGO membranes,respectively.(c) The summary of NaCl rejection vs.water permeance of reported GO-based laminar membranes investigated in hydraulic pressure-driven separation system [6,19,21,24,25,30,31,44–54].

3.4.Mass transport mechanism

In general,the defects on ultrathin rGO membranes can be prohibited by stacking more rGO layers.Hence,thicker rGO membranes prepared via one-step filtration and two-step filtration were also adopted for comparison(Fig.9).25 nm-thick rGO membranes were fabricated by filtrating 200 ml 0.4 mg·L-1GO solution followed by the stabilization and reduction processes.Although the water and NaCl selectivity of 25 nm-thick rGO membranes is slightly improved with the NaCl rejection at around 80%,their water permeance experiences a dramatic decline to around 11 L·m-2·h-1·MPa-1due to the increase of membrane thickness.In addition,their NaCl rejection cannot be further improved as the presence of non-selective nanowrinkles as demonstrated in our previous work [33].In contrast,GQDs/rGO exhibit much higher NaCl rejection with higher water permeance by depositing a small amount of GQDs on ultrathin rGO membranes.These results indicate that the enhanced performance of GQDs/rGO might be more on the formation of other channels at GQDs/rGO interphases apart from the increase of their thickness to fill the defects of rGO membranes.

Fig.9.Water permeance and NaCl rejection of 25 nm-thick rGO,1+10 nm-thick rGO and 10 nm-thick GQDs/rGO membranes.

To further evaluate our hypothesis,10 ml 0.32 mg·L-1GO solutions were adopted to replace GQDs solutions in the preparation of GQDs/rGO membranes for the deposition of a single layer (around 1 nm)of rGO on 10 nm-thick rGO membranes(1+10 nm rGO).The resultant 1+10 nm rGO membranes only achieve similar low NaCl rejection as the pristine 10 nm-thick rGO membranes (Fig.9),might because the micrometer-scale rGO nanosheets are too large to insert into defects to form new selective pathways.In comparison,when depositing 0D GQDs on rGO membranes,the nanosized carbon dots could move into pinholes and nanowrinkles of rGO membranes,forming selective channels for water and NaCl separation,thus achieving effective NaCl rejection.

Fig.10.Schematic illustration of water and ions transport in rGO,GQDs-rGO and GQDs/rGO membranes.

As described above,the enhanced water and NaCl selectivity of GQDs/rGO membranes are due to the coverage of defects as well as the formation of selective channels at 0D/2D homogeneous carbon interphases.As illustrated in Fig.10,ultrathin rGO membranes are not able to fully retard the NaCl permeation owing to the nonselective defects through membranes.While for GQDs-rGO membranes,their laminar structures are expanded and become less ordered after the incorporation of GQDs into rGO laminates,leading to the higher water permeance but the lower NaCl rejection.In contrast,the surface-deposition of GQDs could effectively cover the defects on rGO membranes and form selective passages between GQDs and rGO networks.Since 0D/2D homogeneous layer is ultrathin (less than 1 nm),GQDs/rGO membranes show as low water transport resistance as pristine rGO membranes.Therefore,our ultrathin and less-defective 0D/2D homogeneous GQDs/rGO membranes achieve enhanced NaCl rejection as well as high water permeance.

4.Conclusions

In summary,we have demonstrated a novel method to prepare ultrathin rGO membranes with minimised defects for effective water and salt separation through the surface deposition of 0D GQDs on the 2D laminates.Benefiting from the nano-sized characteristic and carbon nature of GQD,these 0D carbon dots cover the defects with an ultrathin GQDs layer and form selective channels between 0D/2D homogeneous carbon interphases via π–π stacking interaction.As a result,the resultant GQDs/rGO membranes exhibit as high as 91%NaCl rejection without compromising the water permeance in a hydraulic pressure-driven separation system.This study provides new insights into the understanding of mass transport in GO-based membranes,which advances the design of ultrathin and defect-free 2D laminar membranes for precise molecular and ionic sieving applications.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This project is supported by the Australian Government Department of Industry,Innovation,and Science through the Australia–China Science and Research Fund (ACSRF48154),and is conducted as part of the research program at the Australia-China Joint Research Centre in Future Dairy Manufacturing (http://acjrc.eng.-monash.edu/),in collaboration with the Australia Research Council Research Hub for Energy-efficient Separation (IH 170100009).The authors thank Monash Centre for Electron Microscopy for the membrane characterizations.S.Yuan sincerely acknowledge Monash University,Australia–China Joint Research Centre in Future Dairy Manufacturing,and the Monash Centre for Atomically Thin Film Materials for his scholarship.This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF).