Chen Gu,Wenqiang Weng,Cong Lu,Peng Tan,Yao Jiang,Qiang Zhang,Xiaoqin Liu,*,Linbing Sun,*

1 State Key Laboratory of Materials-Oriented Chemical Engineering,Jiangsu National Synergetic Innovation Center for Advanced Material (SICAM),College of Chemical Engineering,Nanjing Tech University,Nanjing 211816,China

2 Department of Chemistry,Washington State University,Pullman,Washington 99163,United States

Keywords:Ti3C2Tx MXene MOF nanoparticles Adsorption Composites

ABSTRACT MXenes have attracted increasing research enthusiasm owing to their unique physical and chemical properties.Although MXenes exhibit exciting potential in cations adsorption due to their unique surface groups,the adsorption capacity is limited by the low specific surface area and undeveloped porosity.Our work aims at enhancing the adsorption performance of a well-known MXene,Ti3C2Tx,for methylene blue(MB) by decorating tiny ZIF-8 nanoparticles in the interlayer.After the incorporation of ZIF-8,suitable interspace in the layers resulting from the distribution of tiny ZIF-8 appears.When employing in MB,the adsorption capacity of composites can reach up to 107 mg·g-1 while both ZIF-8 (3 mg·g-1) and Ti3C2Tx (9 mg·g-1) show nearly no adsorption capacity.The adsorption mechanism was explored,and the good adsorption capacity is caused by the synergistic effect of ZIF-8 and Ti3C2Tx,for neither of them is of suitable interspace or surface groups for MB adsorption.Our work might pave the way for constructing functional materials based on the introduction of nanoparticles into layered materials for various adsorption applications.

1.Introduction

The treatment of water pollutant,especially the ones contain synthetic dyes,has received extensive environmental attention[1-5].Over the past decades,various physical,chemical,and biological processes have been employed to remove the dyes in water.Low cost,simple operation,fast kinetics,and low toxicity make adsorption one of the most promising technologies in pollutant removal [6-8].Thence,developing efficient adsorbents is of great significance since the adsorption process largely depends on the adsorbents.In recent years,a typical kind of two-dimensional(2D)transition metal carbide named MXene exhibits great application prospects in the field of water treatment for the removal of pollutants (e.g.,heavy metals,synthetic dyes,and radionuclides)due to its unique physical and chemical properties [9-14].Originally,Peng and co-workers conducted experimental and computational studies on the adsorption behavior of the Pb(II) in 2D titanium carbide,and the result showed that the MXene possesses an excellent Pb(II) adsorption performance,and the adsorption behavior is associated with the hydroxyl groups in activated Ti sites[15].This research prompts MXene to become a potential candidate in adsorption.Then,Gogotsi’s group investigated the dye adsorption on MXene and found that the adsorption mechanism was similar to Pb(II)adsorption and MXene was an effective cationic dye adsorbent[16].Enhancing performance in current applications[17-22]is accounted for the most considerable fraction of all works on MXenes,which also contains developing novel etching and delaminated methods[23-26],exploring potential application fileds[27-30]and taking from the lab to commercial products[31].However,improving the adsorption performance on water pollutants by physical or chemical modification has scarcely been reported.

Metal-organic frameworks (MOFs),which are known for their high porosity,show excellent application value in the field of gas adsorption[32-35].However,the pore size of most common MOFs for gas adsorption is less than 1 nm,which restricts their applications in bulky molecule adsorption.Interestingly,some attempts on attaching tiny MOFs onto support materials to enhance performance in applications have been reported [36-38].For example,Yaghi’s group reported that different nano MOFs can be incorporated into devices after doping with graphene,which shows enhanced capacitance [39].Moreover,nano ZIF-8 has been introduced into the mesoporous tunnels of SBA-15 for trypsin immobilization.The porous composites exhibit much higher adsorption capacity than both SBA-15 and ZIF-8,which can be attributed to the suitable pore system constructed from the arrangement of nano ZIF-8 in SBA-15 [40].MXenes possess a particular layered structure with functional groups between layers.Thus metal ions should be easily incorporated through the hydroxyl groups in activated Ti sites and act as the precursors for MOFs crystallization between layers.This might lead to the formation of new MOF/MXene composites with small MOF nanoparticles due to the confined effect of interlayers,which is expected for the capture of bulky molecules like dyes.

Herein,we report the fabrication of an efficient adsorbent named ZIF-8/Ti3C2Txfor the capture of a typical cationic dye methylene blue (MB) for the first time.As illustrated in Fig.1,at the very beginning,the bulk MAX was treated by HF etching to fully remove the Al layer,and the black powder called Ti3C2Txwas successfully achieved.The MXene was then treated in the Zn(NO3)2solution to make the interlayers intercalated with Zn2+,which is beneficial to the growth of ZIF-8.Finally,the ligand 2-methylimidazole was introduced,and the tiny ZIF-8 wasin situgrown between the layers of MXene.Our method makes full use of the excellent adsorption capacity MXene for Zn2+to assist the growth of ZIF-8 between interlayers.Meanwhile,the introduced ZIF-8 nanoparticles can significantly increase the layer spacing,which can be beneficial to adsorption.When it comes to MB adsorption,the ZIF-8/Ti3C2Txcomposites show a capacity up to 107 mg·g-1,which is much higher than ZIF-8(3 mg·g-1)and Ti3C2-TxMXene (9 mg·g-1).Suitable interspace in the layers resulting from the distribution of tiny ZIF-8 may produce a great influence on the MB adsorption process in interlayers of MXene.The adsorption mechanism was explored,and the good adsorption capacity is caused by the synergistic effects of ZIF-8 on Ti3C2Tx,for neither of them can do this alone.In detail,the adsorption capacity decreases along with the increased tiny ZIF-8 nanoparticles introduced into the layers.The result suggests that the distribution of tiny ZIF-8 in the interlayer is crucial in the field of adsorption and paves a feasible way on constructing functional materials based on the introduction of nanoparticles into layered materials for the development of efficient water treatment adsorbents.

2.Materials and Methods

2.1.Materials

The Ti3AlC2MAX powder was commercially procured.Hydrofluoric acid (49%,aqueous solution) and polyvinylpyrrolidone were purchased from Alfa Aesar (China) Chemical Co.,Ltd.Zinc nitrate hexahydrate and 2-methylimidazole were purchased from Aladdin Biochemical Technology Co.,Ltd.(China).Methanol(＞99.5%),methylene blue,fast green and metanil yellow were purchased from Sinopharm (China).

2.2.Synthesis of Ti3C2Tx

The Ti3AlC2MAX powder (10 g) was mixed with 49 % HF(100 ml) solution slowly,and the obtained suspension was kept by stirring at 350 r·min-1at room temperature for 72 h to remove the Al layers.After the etching,the product was washed with deionized water till the pH of the solution was close to 7.The black powder was dried under vacuum at 60 °C for 24 h.

2.3.Synthesis of ZIF-8

The synthesis of ZIF-8 is similar to the previous report [32].The solutions of Zn(NO3)2·6H2O (30 ml,0.1 mol·L-1) and 2-methylimidazole (30 ml,0.2 mol·L-1) were mixed at room temperature by stirring for 2 h.The solution changed from clarification to milky white quickly.The white ZIF-8 was obtained by centrifugation at 8000 r·min-1for 5 min and washed with methanol three times to remove the unreacted precursors.Finally,the white powder was dried under vacuum at 50 °C for 24 h.

2.4.Synthesis of ZIF-8/Ti3C2Tx

Ti3C2Txpowder (0.5 g) was dispersed in NaOH solution (20 ml,10% (mass)) by stirring at 300 r·min-1at 30 °C in a water bath for 24 h.The final product was washed with deionized water till the pH is about 7,and the sample was dried under vacuum at 50 °C for 24 h.The treated Ti3C2Tx(0.2 g)and PVP(0.2 g)were added into the above Zn(NO3)2·6H2O solution (30 ml) with stirring at 400 r·min-1at room temperature for 2 h to make the interlayer of Ti3-C2Txfull of Zn2+.After washed with methanol three times,the sample was added into 30 ml as-prepared 2-methylimidazole with stirring at 400 r·min-1at room temperature for 2 h to induce the growth of ZIF-8 on the surface of the Ti3C2Tx.The process of ZIF-8 growth is repeated for 3 times.The product was washed with methanol three times and then dried under vacuum at 50 °C for 24 h.The obtained material is denoted as ZIF-8/Ti3C2Tx(also can be named as 1-ZIF-8/Ti3C2Tx),and the content of ZIF-8 is about 3.04%).2-ZIF-8/Ti3C2Tx(4.71%) and 3-ZIF-8/Ti3C2Tx(5.93%) represent the process of ZIF-8 growth is repeated for 2 and 3 times respectively.All the content of ZIF-8 was tested by thermogravimetric (TG) (Fig.S1,in Supplementary Material).

2.5.Materials characterization

Fig.1.Schematic illustration of the synthesis of ZIF-8/Ti3C2Tx.

X-ray diffraction(XRD)patterns of the materials were recorded on a D8 Advance diffractometer (Bruker,Germany) with Cu Kα radiation in the 2θ range from 2° to 80° at 40 kV and 40 mA.Scanning electronic microscopy (SEM) images were recorded on a S4800 electron microscope (Hitachi,Japan) operating at 20 kV.Fourier transform infrared (IR) spectra were recorded on a Nexus 470 spectrometer (Nicolet,USA) with KBr wafer.Transmission electron microscopy (TEM) was performed on a JEM-200CX electron microscope(JEOL,Japan)and a Tecnai 12 electron microscope(Philips,Netherlands).X-ray photoelectron spectroscopy (XPS)spectrum was recorded on PHI 5000 VersaProbe (Ulvac,Japan).TG analysis was performed in an air flow from room temperature to 800 °C on a Thermo balance (STA-490C,NETZSCH,Germany).The N2adsorption-desorption isotherms were measured using an ASAP2020 apparatus(Micromeritics,USA)at-196°C.The samples were degassed at 150°C for 2 h before analysis.UV-visible absorption (UV/Vis) spectrum was performed on Lambda 950 (Perkin Elmer,USA) with a scan wavelength range from 400 to 1000 nm.Elemental analysis was recorded on Vario EL elemental analyzer(Elementar,Germany),and all samples were taken about 2 mg.

2.6.Adsorption test

15 mg as-synthesized adsorbents and 200 ml dye solution were mixed under stirring.About 3 ml of the above solution was transferred into glass cuvette at 0,5,10,15,20,30,45,60,90,120,150,180,240,300,360,420,480,540 and 600 min for further testing the UV/Vis absorption spectrum.After each measurement,the solution was poured back.

3.Results and Discussion

As shown in Fig.2(a) and (b),the Ti3C2Txpresents a layered structure just like an accordion,indicating the successful etching of the Al layer in MAX.Fig.2(c)-(f) depict the surface morphology of the ZIF-8/Ti3C2Tx.The ZIF-8 nanoparticles cover the entire surface of the Ti3C2Tx,and as for the side face,the same case is also existing.All of the results stress the ZIF-8/Ti3C2Txcomposites are synthesized,and the two-dimensional layered structure of Ti3C2Txkeeps well in the process of hybridization.The arrangement of tiny ZIF-8 on Ti3C2Txdensely packed.Compared to Fig.2(b) and (f),porosity in interlayer can be observed clearly after the incorporation of tiny ZIF-8,indicating the interspace emerges.Fig.2(g)shows the TEM and EDX-mapping images of ZIF-8/Ti3C2Tx.From the images,both intrinsic elements of Ti and F from Ti3C2Txas well as intrinsic elements of Zn and N from ZIF-8 distribute uniformly on the hybrid material,which also suggests the successful introduction of ZIF-8 in whole bulk Ti3C2Txand the completion is consistent with the SEM and TEM images.

Fig.2.SEM images of (a) Ti3C2Tx,(c,e) ZIF-8/Ti3C2Tx,TEM images of (b) Ti3C2Tx,(d,f) ZIF-8/Ti3C2Tx,and EDX-mapping images of (g) ZIF-8/Ti3C2Tx.

For MXene,intercalation is of great importance for the introduction of functional molecules or ions [41,42].To explore the change of layer spacing,XRD is an effective strategy.As shown in Fig.S2,the peak at 39°almost disappears,indicating the successful etching of Al layer.The characteristic peaks of pure ZIF-8 cannot be observed in the sample ZIF-8/Ti3C2Tx(Fig.3(a)).The result suggests the tiny size of ZIF-8 particles[34].The most prominent difference between ZIF-8/Ti3C2Txand Ti3C2Txis that the peak corresponding to (002) moves in the direction to low angle,which illustrates the expansion of layer spacing.The detailed information derived from the XRD patterns according to Bragg’s law is listed in Table 1.An increase of 0.683 nm for the layer spacing emerges after the ZIF-8 growing,and the increasing percentage is 34% based on the original interlayer spacing of the Ti3C2Tx,which origins from the presence of ZIF-8 nanoparticles between layers.

Table 1Structural parameters of Ti3C2Tx and ZIF-8/Ti3C2Tx derived from XRD results

The IR spectra of Ti3C2Txhas no distinct characteristic peak,which is consistent with the previous report (Fig.S3) [25].Compared to the spectra of ZIF-8 and ZIF-8/Ti3C2Tx,the peaks of ZIF-8/Ti3C2Txat 2916,1557,and 1134 cm-1belong to the absorption peaks of C-H,C=N,and C-N bonds of ZIF-8,respectively.The surface chemical environment of all as-prepared samples was measured by the XPS (Fig.3(b)).For pure ZIF-8 and Ti3C2Tx,the results are in accordance with the previous report [19,34].Moreover,the peaks attributed to the feature elements of Ti3C2TxMXene(Ti and F) and ZIF-8 (Zn and N) can be observed clearly in the hybrid material,indicating the ZIF-8/Ti3C2Txhybrid material wasfabricated successfully.In order to confirm the existence of ZIF-8,further analysis of the Zn in both ZIF-8 and ZIF-8/Ti3C2Txwas carried out.The peaks attributed to Zn in the hybrid material are similar to that of pure ZIF-8,indicating the tiny particles on the surface of bulk Ti3C2Txare indeed ZIF-8 (Fig.S4).

Moreover,it’s worth considering the content of the ZIF-8 loaded in the ZIF-8/Ti3C2Txhybrid material.Table S1 offers the elemental composition of N,C,and H.Because PVP was used in the synthesis process,the N content in the hybrid material is obviously higher than that comes from the loaded ZIF-8.TG was chosen to estimate the content of loaded ZIF-8.The TG curves are presented in Fig.3(c).The curves of pure ZIF-8 and Ti3C2Txin the N2atmosphere in accordance with the results in the literature and the curve of ZIF-8/Ti3C2Txexhibits most weight loss between 400 and 550°C,which belongs to the decomposition of the loaded ZIF-8.

Fig.3.(a) XRD patterns of Ti3C2Tx,alk-Ti3C2Tx and ZIF-8/Ti3C2Tx.(b) XPS spectra of Ti3C2Tx,ZIF-8 and ZIF-8/Ti3C2Tx.(c) TG curves of Ti3C2Tx,ZIF-8 and ZIF-8/Ti3C2Tx.(d) N2 adsorption-desorption isotherms of Ti3C2Tx,and ZIF-8/Ti3C2Tx.

The composites may have some effects on the pore and specific surface area of the materials,which play an important role in adsorption.As shown in Fig.3(d),although the transformation on morphological and structural after HF etching,from threedimensional bulk to accordion-like structural,N2adsorption capacity of Ti3C2Txis very limited.From the N2adsorption-desorption isotherms of ZIF-8/Ti3C2Tx,a significant rise can be observed in the low-pressure region,implying the hybrid material possesses micropores.The generation of micropores endows ZIF-8/Ti3C2Txwith complex pore structure,and developed porosity much different from original Ti3C2Txappears.However,the N2adsorption of ZIF-8/Ti3C2Txis of little increase compared with Ti3C2Tx,which results from the low content of the ZIF-8.In the traditional adsorption field,the interlayer of Ti3C2Txis of sufficient size for various molecules or ions in and out.After the growth of tiny ZIF-8,the environment in the interlayer changes a lot,and the generated space may provide ZIF-8/Ti3C2Txsome unique performance in the field of adsorption.

Fig.4.Adsorption curves of MB on Ti3C2Tx,ZIF-8,and ZIF-8/Ti3C2Tx.

To explore the adsorption performance of ZIF-8/Ti3C2Tx,we choose MB,a typical cationic dye,as the adsorbate.The adsorption performance is described in Fig.4,as for ZIF-8 and Ti3C2Tx,both of which show nearly no adsorption capacity on MB even after 10 h.Interestingly,the ZIF-8/Ti3C2Txexhibits enhanced adsorption capacity up to 107 mg·g-1.The adsorption cycles of as-prepared material were presented in Fig.S5 for further evaluating adsorption capacity of the as-prepared composites.In order to figure out the mechanism of excellent adsorption performance,two different kinds of anionic dyes(fast green and metanil yellow)were tried (Fig.S6).Obviously,ZIF-8/Ti3C2Txshows nearly no adsorption capacity on them,confirming that charge interaction plays a considerable role in the adsorption process.Then,to investigate the adsorption performance of the prepared material,0.5-ZIF-8/Ti3C2Txwas used for MB removal (Fig.S7).Furthermore,the ZIF-8 growth process was repeated to change the porosity in the interlayer,and the MB adsorption capacity was tested(Fig.S8).The capacity of growing twice is lower than the as-prepared ZIF-8/Ti3C2Tx,but higher than growing for three times.However,all of the samples decorated with tiny ZIF-8 remove more MB than the original Ti3C2Tx.The higher content of ZIF-8 in both 2-ZIF-8/Ti3C2Txand 3-ZIF-8/Ti3C2Txcan be clearly found out from Figs.S1 and S9,which suggests that more tiny ZIF-8 has been introduced on the surface of Ti3C2Tx.Therefore,the low-angle XRD of 2-ZIF-8/Ti3C2Txand 3-ZIF-8/Ti3C2Txare presented in Fig.S10 for figuring out the distribution of the additional generated ZIF-8.The peaks corresponding to (002) of them move in a direction to a much lower angle compared to ZIF-8/Ti3C2Txafter the growth,proving that more ZIF-8 particles were piled up into the MXene interlayer.Based on the above results,the possible adsorption mechanism of MB on all samples is shown in Fig.5.For original Ti3C2Tx,the poor adsorption capacity is caused by the lack of oxygen groups on the surface and unsuitable interspace inside.As for ZIF-8/Ti3C2Tx,the environment in the interlayer changes a lot after the growth of tiny ZIF-8.The suitable interspace generated by the tiny ZIF-8 arraying in the interlayer of Ti3C2Txalso plays an important role in the MB capture.When growing twice times,the additional increased ZIF-8 changes the interlayer space once again,and the excessive ZIF-8 shows a negative impact on the adsorption capacity of MB.More adsorption sites are obscured in 3-ZIF-8/Ti3C2Tx,the adsorption capacity of which further decreasing.The result suggests the distribution of tiny ZIF-8 in the interlayer is crucial in the field of adsorption.

4.Conclusions

Fig.5.Possible adsorption mechanism of MB on (a,b) Ti3C2Tx,(c,d) ZIF-8/Ti3C2Tx,and (e,f) 2 or 3-ZIF-8/Ti3C2Tx.

In summary,we designed an effective adsorbent ZIF-8/Ti3C2Txfor cationic pollutants removal.The ZIF-8/Ti3C2Txwas synthesized by the in situ growth of ZIF-8 on Ti3C2Txand the ZIF-8 nanoparticles distribute uniformly in bulk Ti3C2Tx.Due to the cation capture capacity of Ti3C2Txand suitable interspace resulting from ZIF-8 arrays in the interlayer of Ti3C2Tx,the obtained adsorbent exhibits much better adsorption capacity (107 mg·g-1) on MB than Ti3C2Txand ZIF-8.The adsorption mechanism was explored,and the good adsorption capacity is caused by the synergistic effects of ZIF-8 on Ti3C2Tx,for neither of them can do this alone.Our work might pave the way for constructing functional materials based on the introduction of nanoparticles into layered materials in the field of water treatment as well as other extended adsorption fields.

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

We acknowledge the financial support of this work by the National Natural Science Foundation of China (21878149,22078155,and 21808110),the project funded by China Postdoctoral Science Foundation (2020M681567),Postgraduate Research&Practice Innovation Program of Jiangsu Province (SJCX20_0358)and the Natural Science Foundation of Jiangsu Province(BK20180709).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.06.004.