Article pubs.acs.org/Langmuir
Highly Efficiently Delaminated Single-Layered MXene Nanosheets with Large Lateral Size Gengnan Li, Li Tan, Yumeng zhang, Binghan Wu, and Liang Li* Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *
ABSTRACT: Single layered Ti3C2(OH)2 nanosheets have been successfully fabricated by etching its Ti3AlC2 precursor with KOH in the presence of a small amount of water. The OH group replaced the Al layer within the Ti3AlC2 structure during etching, and Ti3C2(OH)2 nanosheets could be easily and efficiently achieved through a simple washing process. The delaminated single-layered nanosheets are clearly revealed by atomic force microscopy to be several micrometers in lateral size. Interestingly, the exfoliated Ti3C2(OH)2 nanosheets could be restacked to form a new layerstructured material after drying. When redispersing this restacked Ti3C2(OH)2 materials in water again, it could be re-delaminated easily only after shaking for several hours. The easy delamination and restacking properties, coupled with intrinsic metallic conductivity and hydrophilicity, make it an ideal two-dimensional building block for fabricating a wide variety of functional materials.
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INTRODUCTION Two-dimensional (2D) nanosheets have been one of the most extensively studied classes of materials in the past decade due to their exceptional properties originating from the dimensional effect. The extremely small thickness of around 1 nm produced ultimate two-dimensional anisotropy results in specific physical and chemical features different from their bulk counterpart.1 Apart from the well-established 2D materials, such as graphene,2 Ti1−xO2,3 Mn1−xO2,4 and layered double hydroxide,5 recently, a new family of layered compounds consisting of twodimensional atomic crystals, labeled as MXene (Mn+1XnTx: M = Ti, V, Nb, etc.; X = C, N; n = 1−3; Tx is the variable termination group), has been extensively studied.1,6−8 Owning to the aqueous medium used during synthesis, MXene flakes are always terminated with hydrophilic surface moieties (Tx), such as −OH, −O, and −F. The intrinsic metallic conductivity and good mechanical properties coupled with hydrophilicity make them good building blocks for the fabrication of a wide variety of functional nanostructured materials, especially for polymer composites and energy storage devices as they can be easily intercalated by organic molecules and ions.8−16 Among various kinds of MXenes, the most studied is Ti3C2, which is normally prepared by immersing Ti3AlC2 precursor in hydrofluoric acid (HF) or HF-containing etchants to extract the Al layers.1,8 The resulting Ti3C2Tx is terminated with −F and −OH/=O functional groups. Thus, the original metallic bonds between Ti3C2 layers through the Al atoms were replaced by weak bonds between surface functional groups. Ultrathin 2D Ti3C2 nanosheets could be achieved by a further sonicating process of the Ti3C2Tx stacks. However, the use of © 2017 American Chemical Society
HF-containing reagents in the etching process is dangerous and toxic, and the inevitably residual −F group on the surface of Ti3C2 nanosheet may affect its further applications. When used as electrode material, as mentioned in the literature, F− ion may be detrimental to the charge storage process.17 Besides, the HF solution not only etches the Al layers but also corrodes the Ti element within the Ti 3 C 2 structure, which has been demonstrated by such unstable titanium based materials in the solution containing fluoride groups or ions.18,19 More importantly, one of the etching byproducts, AlF3, is difficult to remove from the mixture because of its insolubility in any solvent under mild conditions. Therefore, the well-adopted multistep centrifugation protocol to separate the MXene from the mixture causes unnecessary inefficiency of the exfoliation. Only some small fragments of the delaminated MXene nanosheets could be residual after the separation process.8,20,21 Many attempts have been made so far to develop a fluoridefree synthesis strategy for preparing the MXenes.22,23 Researchers attempted to use alkaline activation treatment by immersing the MXene obtained from the HF etching process in NaOH solution to replace the −F group by hydroxy.1 However, the Al was etched only from the outer surface of the MAX phase and a small amount of F− ion could still be detected in the MXene materials.10,18 Based on these techniques, we demonstrate that potassium hydroxide in the presence of a small amount of water could be Received: April 19, 2017 Revised: August 5, 2017 Published: August 14, 2017 9000
DOI: 10.1021/acs.langmuir.7b01339 Langmuir 2017, 33, 9000−9006
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Langmuir used as etchant instead of −F-containing materials to extract the Al layers within the Ti3AlC2 structure. High-quality, large lateral-size single-layer Ti3C2 nanosheets without F− ion thus can be directly fabricated by simple liquid phase exfoliation. The resulting 2D material shows improved energy storage properties compared with that produced by HF etching method. More importantly, the strategy reported here could also be used in other MAX materials to synthesis their corresponding MXenes.
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etchant. Thus, the etching process may follow the simplified reactions listed below: Ti3AlC2 + KOH + H 2O = Ti3C2 + KAlO2 + 3/2H 2 (1)
Ti3C2 + 2H 2O = Ti3C2(OH)2 + H 2
(2)
To make sure of the complete extraction of the Al layers within the Ti3AlC2 structure, Ti3AlC2 powder was mixed with KOH and a small amount of water first and ground finely into paste. Bubbles, presumed to be H2, could be observed during grinding, suggesting the reaction of chemical etching. The mixture was then transferred into the autoclave and heated at 180 °C for 24 h. The synthesis process is schematically shown in Scheme 1.
EXPERIMENTAL SECTION
Synthesis of Ti3C2Tx. Potassium hydroxide (0.35 g), Ti3AlC2 (0.1 g), and a small amount of water (0.05 mL) were mixed and ground finely into paste and transferred into a polytetrafluoroethylene-lined stainless steel autoclave. After heating at 180 °C for 24 h, the autoclave was taken out and the solid product was collected and washed with deionized water repeatedly until the pH of the filtrate reached 7. The as-obtained sample was dried in a vacuum oven at 60 °C for 12 h. The materials synthesized under different conditions were obtained by changing the amount of KOH, heating temperature, and reaction time, respectively. For comparison, the Ti3C2Tx prepared by the HF method was fabricated according to the literature.1 In a typical synthesis, the Ti3AlC2 powder (1 g) was immersed in 10 mL of 50% HF solution at room temperature for 2 h. The resulting suspension was washed several times to get rid of the residual HF. Materials Characterization. Powder X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Focus powder diffractometer with Cu Kα radiation (λ = 0.154 05 nm) at 40 kV, 40 mA. Laser Raman spectra (LRS) of samples were collected at ambient condition on a LabRAM HR-800 spectrometer. A laser beam (λ = 532 nm) was used for excitation. The morphology of the materials was observed on a Quanta 250 FEG scanning electron microscope (SEM). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) observations were carried out on a field emission JEM2100 (JEOL) electron microscope operated at 300 kV. X-ray photoelectron spectroscopy (XPS) signals were collected on a VG Micro MK II instrument using monochromatic Al Kα X-rays at 1486.6 eV operated at 200 W. The elemental binding energy calibration was referenced to the C(1s) line signal at 284.6 eV. Atomic force microscopy (AFM) images were acquired in tapping mode on an SPI4000. For the SEM, TEM, and AFM analyses, the exfoliated nanosheets were obtained directly from the paste after KOH treatment and a deionized water washing process. Electrochemical Measurement. The working electrodes were prepared by thoroughly mixing as-prepared composites, polyvinylidene fluoride (PVDF) and acetylene black (AB) in N-methyl-2-pyrrolidone (NMP) with the mass ratio of 8:1:1. The obtained slurries were then brushed onto copper foil substrate and dried in a vacuum oven at 120 °C for 12 h. Lithium foil was used as the half-cell counter electrode. The Celgard 2400 microporous polyethylene membrane was used as separator. The electrolyte was a mixture of 1 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (EC:DMC = 1:1, v/v). The cells were assembled in an argon-filled glovebox. The charge/discharge cycling was performed on a battery test instrument (CT2001A, LAND Battery Program-control Test System).
Scheme 1. Schematic Illustration of the Etching and Exfoliation Process for Ti3AlC2
XRD technology was used in the experiment to monitor the structural change of the Ti3AlC2 during the etching and exfoliation process (Figure 1). Figure 1b depicts the XRD pattern of the Ti3AlC2 treated with KOH and a small amount of H2O (mixture of all the products after etching). It gives clear evidence for the successful extraction of Al layer from the Ti3AlC2 structure. Compared with the initial Ti3AlC2 precursor (Figure 1a), the mixture presents only four diffraction peaks. The first three could be assigned as the (012), (020), and (021)
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RESULTS AND DISCUSSION The structure of Ti3AlC2 precursor is composed of individual Ti3C2 sheets “glued” with pure Al layer. Extraction of the Al atoms from the Ti3AlC2 structure could break the metallic bonding between neighboring Ti3C2 layers and result in delaminated Ti3C2 nanosheets. However, the exposed Ti surface within such exfoliated 2D Ti3C2 nanosheets is not stable in air and should be satisfied by suitable ligands to lower its surface energy.1 Under the proposed strategy condition, the most probable ligands should be −OH, as the mixture of potassium hydroxide and a small amount of water was used as
Figure 1. XRD patterns of Ti3AlC2 precursor (a), the mixture after etching (b), exfoliated nanosheets (c), restacked layered materials (d), and re-delaminated nanosheets of the restacked layered materials. 9001
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Figure 2. SEM images of Ti3AlC2 (A), after treatment with KOH + H2O for 6 h (B), and exfoliated nanosheets (C, D).
drying process. The thickness of a single-layer Ti3C2(OH)2 nanosheet, 0.95 nm, thus could be calculated from the abovementioned XRD result. More importantly, when redispersing this restacked Ti3C2(OH)2 material in water again, it could be re-delaminated easily only after shaking for several hours, as shown in Figure 1e. The easy delamination and restacking properties, coupled with intrinsic metallic conductivity and hydrophilicity, make it an ideal 2D building block for fabricating a wide variety of functional materials. SEM analysis for the etched sample at different stages clearly expresses a from outside-to-inside etching process, as illustrated in Scheme 1. When treated with KOH and a small amount of water for 6 h, the material shows a fanning out of the basal planes but still basically keeps the particle outline of the precursor (Figure 2A,B). Subsequent extraction of the residual Al atom would break the connection between neighboring Ti3C2 layers and result in the total delamination. The particle shape originating from the Ti3AlC2 precursor was thus destroyed, and randomly stacked nanosheets combined with some scattered nanosheets could be observed in the image, as shown in Figure 2C,D. The transparent appearance with a lateral size of several micrometers clearly indicates their atomicscale thickness. X-ray energy dispersive spectroscopy (EDS; Figure S4) shows that the exfoliated nanosheet is only composed of Ti, C, O, and K. The absence of the signal for Al element implies the replacement of Al layers by an oxygencontaining group (OH) during the etching and exfoliation process. Further TEM analysis shows a quite faint but uniform
diffraction peaks for the residual potassium hydroxide hydrate, respectively. The last one is exactly originated from the (444) diffraction peaks of KAlO2, the etching product of Al during the extraction process. The presence of KAlO2 and absence of Ticontaining byproduct in the XRD pattern confirm the selective etching of Al layer within the Ti3AlC2 structure. In addition, no visible peaks for Ti3AlC2 precursor could be observed in the pattern representing the successful extraction of Al layer and the loss of its original layered structure after the etching process. It should be noted that the etching byproduct and residual all could be dissolved in water except the MXene. This is quite different from that etched by −F-containing reagents.1 Therefore, MXene could be directly obtained by simply washing with water to remove KAlO2 and residual KOH. To explore the state of the as-synthesized MXene sample, XRD analysis was recorded directly after washing (without drying) and the result is shown in Figure 1c. Only a broadly scattered halo within the 2θ range of 20−40° suggests that the resulting MXene sheets are not in parallel to induce interference of the X-rays, implying the total exfoliation. Interestingly, two diffraction peaks gradually emerged at about 9.6 and 38.5°, respectively, during the subsequent drying process (Figure 1d), which agrees well with the diffraction peaks for the simulated XRD patterns of layered structured Ti3C2(OH)2.1 The presence of −OH group after treatment was also confirmed by XPS analysis as discussed below. It clearly indicates that the exfoliated Ti3C2(OH)2 nanosheets could be restacked together to form layer-structured material during the 9002
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observed, the corresponding surface topography of a threedimensional (3D) image shows that the nanosheet is almost flat (Figure S6). The thickness of the nanosheets was measured at steps between a nanosheet and substrate surface. The data collected from over 10 different spots yield an average value of 1.2 ± 0.5 nm, which corresponds well to the single layer. According to the above XRD result and density functional theory (DFT) calculation, the theoretical layer thickness of Ti3C2(OH)2 nanosheets is about 0.95 nm.1 The relatively large deviation indicates a twisted characteristic of the MXene nanosheet, which may also be attributed to the absorption of H2O molecular and K ions. In any case, the thickness observed obviously indicates that the sheets are unilamellar. X-ray photoelectron spectroscopy (XPS) analysis was performed to study the changes of the surface component of the material. Figure 5A depicts the XPS survey spectra of the samples before (precursor) and after etching by the mixture of KOH and a small amount of water (exfoliated nanosheets). The almost unchanged Ti and C characteristic peaks combined with the increased oxygen content and the absence of Al species clearly indicate the extraction of the Al element and the formation of Ti3C2(OH)2 nanosheets. Detailed information could be found from the high-resolution XPS spectra of titanium, aluminum, and oxygen. The high-resolution XPS spectra in the Ti 2p region (Figure 5B) could be deconvoluted into five components. The peaks at 460.3, 455.5, 464.1, and 458.4 eV could be assigned as Ti−C 2p1/2, Ti−C 2p3/2, Ti−O 2p1/2, and Ti−O 2p3/2, respectively. The other peak at 453.5 eV exactly comes from the Ti−Al bond according to the literature.20,24 The absence of the binding energy peak at 453.5 eV proves that the Al atoms within the precursor were extracted during the etching process. This result could be further confirmed by the almost disappeared Al 2p peaks for Ti3C2(OH)2 nanosheets (Figure 5C). The XPS spectra of O 1s are presented in Figure 5D. The dominant peak at about 530.0 eV is attributed to the oxygen from Ti−O bands, while the other shoulder peak centered at 531.5 eV is associated with the surface absorbed oxygen species.25,26 The ratio of Ti−O oxygen species increases significantly after the etching and washing process, corresponding well with the formation of Ti3C2(OH)2 nanosheets. Besides, the main peaks for C 1s (Figure 5E) are consistently well with the carbide and C−C species. The slight change of C species clearly indicates that the Ti3C2 structure has not been damaged during the etching process.
contrast, which is compatible with the ultrathin thickness of the crystallites (Figure 3). Some overlapped wrinkled nanosheets
Figure 3. TEM image of exfoliated 2D nanosheets.
and crumpled edge also could be found in the TEM image, suggesting the intrinsic flexible nature of the MXene nanosheets. Besides, the SAED (selected area electron diffraction) pattern diffraction spots with typical hexagonal symmetry are observed, which indicates that the as-prepared 2D Ti3C2 nanosheets retain a hexagonal crystal structure as in Ti3AlC2 (Figure S5). All the above results exactly confirm the successful extraction of the Al layer from Ti3AlC2 precursor and the exfoliated properties of the Ti3C2(OH)2 nanosheets. The AFM image gives direct and clear evidence for the successful extraction of Al layer from Ti3AlC2 precursor and the delamination of single-layered Ti3C2(OH)2 nanosheets. Figure 4 depicts a 2D image of exfoliated nanosheets and corresponding colloidal solution. A clear Tyndall light scattering can be observed, indicating the presence of exfoliated Ti3C2(OH)2 nanosheet crystallites in the water. The AFM image presents a sheetlike object of about several micrometers in lateral size. Although fragments were also occasionally
Figure 4. AFM images of exfoliated Ti3C2(OH)2 nanosheets (A) and Tyndall effect of Ti3C2(OH)2 nanosheets dispersed in H2O (B). 9003
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Figure 5. XPS survey spectra (A) and high-resolution XPS spectra of Ti 2p (B), Al 2p (C), O 1s (D), and C 1s (E) for Ti3AlC2 (a) and exfoliated 2D nanosheets (b).
preserved during the etching process and the delamination of the Ti3AlC2 precursor. After etching away the Al layer from the Ti3AlC2 structure, the resulting 2D Ti3C2(OH)2 nanomaterial becomes one of the promising candidates for effective and efficient storing of energy due to its unique layered structure and high electronic conductivity.15−17 The electrochemical performance of assynthesized Ti3C2(OH)2 electrode in a nonaqueous Li electrolyte was thus tested. As shown in Figure 7A, the first
The as-synthesized nanosheets and its colloidal solution were further characterized by Raman spectroscopy. For comparison, the Raman spectrum of Ti3AlC2 precursor is also presented, as shown in Figure 6. The main three Raman bands for Ti3AlC2
Figure 6. Raman spectra of Ti3AlC2 (a), colloidal solution of Ti3C2(OH)2 (b), and exfoliated 2D nanosheets (c).
precursor at about 250, 400, and 600 cm−1 represent the vibration modes for nonstoichiometric titanium carbon, while the two broad peaks between 1200 and 1800 cm−1 are characteristic for the D and G modes of graphitic carbon (Figure 6a).27−29 Interestingly, the Raman band centered at 600 cm−1 obviously shifted toward a higher wavenumber first after the etching and washing process (Figure 6b) and then reduced again after the sequential centrifuge and drying treatment (Figure 6c). However, the ultimate wavenumber is still somewhat larger than that for Ti3AlC2 precursor. It has been reported that the decrease in layer thickness could result in a red shift of the Raman band due to slight hardening of the bonds.24,29 Thus, this phenomenon corresponds well with the above-mentioned delamination and restacking process. The still existing D and G bands combined with slightly red-shifted Ti− C band clearly indicate that the main Ti3C2 sheet structure is
Figure 7. Electrochemical performance (A) and cycle curves (B) of Ti3C2(OH)2 electrode at different specific currents.
discharge capacities for the cell reach 250, 44, and 35 mAh g−1 at 20, 50, and 100 mA g−1, respectively, which are higher than the values for that fabricated with the HF method (55, 40, and 31 mAh g−1, respectively) (Figure S7A). This result suggests that the as-synthesized MXene sample has larger Li ion extraction/insertion and electronic diffusivity. The cycling behavior of the cell between the cutoff voltage of 0.5−3.0 V versus Li/Li+ at various specific currents was further carried out, as shown in Figure 7B. The cell was progressively charged and discharged in a series of stages at the charge/discharge rate range of 20−100 mA g−1. As in previous work, a more or less 9004
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Langmuir linear drop in capacity with cycling is observed. This may be attributed to the formation of a solid electrolyte interphase/ interface (Supporting Information) during the discharge process. The capacity profile also becomes steeper with increasing charging rate. The sample shows reversible capacities of 44, 36, and 30 mAh g−1 at 20, 50, and 100 mA g−1 with good cycle stability after 50 cycles, respectively. After the formation process, the reversible capacity of our material remains approximately 55 mAh g−1. When the current is reduced back to 20 mA g−1 again, the discharge capacity is recovered closely to 41 mAh g−1. It should be noted that the cell shows somewhat larger capacity especially under lower current condition, compared to that fabricated with the HF method (Figure S8B), suggesting a good rate performance and stability of MXene−KOH sample. The KOH treatment is an efficient and green strategy to fabricate fluoride-free MXene with higher electrochemical performance.
ACKNOWLEDGMENTS
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REFERENCES
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CONCLUSIONS The treatment of Ti3AlC2 with KOH in the presence of a small amount of water could promote the extraction of Al layer within the Ti3AlC2 structure and result in total delamination. The Al atoms were replaced by OH groups during the etching process, and ultrathin 2D nanosheets of Ti3C2(OH)2 with a theoretical thickness of 0.95 nm could be achieved by a simple washing process. The delaminated nanosheets are revealed by atomic force microscopy to have a large lateral size and a thickness of ∼1.5 nm, which provides direct evidence for the total exfoliated Ti3C2(OH)2 single sheets. Importantly, the exfoliated Ti3C2(OH)2 nanosheets could be restacked together to form a new layer-structured material during the drying process. When redispersing this restacked Ti3C2(OH)2 material in water again, it could be re-delaminated easily only after shaking for several hours. The easy delamination and restacking properties, coupled with intrinsic metallic conductivity and hydrophilicity, make it an ideal 2D building block for fabricating a wide variety of functional materials. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01339. XRD patterns of Ti3AlC2 after etching with different amounts of KOH, at different temperatures, and for different times; elemental compositions of Ti3AlC2 precursor and Ti3C2(OH)2; ratios of elements from EDS; TEM image and corresponding SAED pattern of Ti3C2(OH)2; 3D image of AFM for exfoliated 2D nanosheets; SEM image of multilayer sample cross section; electrochemical performance and cycle curves at different specific currents forTi3C2Tx electrode prepared with HF method (PDF)
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This study was supported by the National Basic Research Program of China, 2013CB933201.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Liang Li: 0000-0002-7435-4640 Notes
The authors declare no competing financial interest. 9005
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