Boosting the Yield of MXene 2D Sheets via a Facile Hydrothermal

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Functional Nanostructured Materials (including low-D carbon)

Boosting the Yield of MXene 2D Sheets via a Facile Hydrothermal-Assisted Intercalation Fei Han, Shaojuan Luo, Luoyuan Xie, Jiajie Zhu, Wei Wei, Xian Chen, Fuwei Liu, Wei Chen, Jinlai Zhao, Lei Dong, Kuai Yu, Xierong Zeng, Feng Rao, Lei Wang, and Yang Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22339 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Applied Materials & Interfaces

Boosting the Yield of MXene 2D Sheets via a Facile Hydrothermal-Assisted Intercalation Fei Han,†,‡ Shaojuan Luo,§ Luoyuan Xie,† Jiajie Zhu,† Wei Wei,# Xian Chen,*,† Fuwei Liu,† Wei Chen,*, || Jinlai Zhao,† Lei Dong, Kuai Yu,∮ Xierong Zeng,† Feng Rao,† Lei Wang,† and Yang Huang*,† † College of Materials Science and Engineering, Shenzhen University, Shenzhen 518055, China ‡ Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China § School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China # Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China || Institute of Medical Engineering, School of Basic Medical Sciences, Xi’an Jiaotong University, Xi’an, Shaanxi, 710061, China  Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China

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∮ College of Electronic Science and Technology, Shenzhen University, Shenzhen 518060, China KEYWORDS: Hydrothermal-assisted intercalation (HAI), Ti3C2Tx, MXene, high yield, facile

ABSTRACT: Ti3C2Tx (MXene) exhibits attractive properties in different applications. However, traditional synthesis leads to unsatisfactory yield of two-dimensional (2D) Ti3C2Tx, e.g. lower than 20%, which stems from the strong interactions of potential Ti-Ti bonds and residual Ti-Al bonds between the adjacent Ti3C2 layers, hindering the effective intercalation and delamination. Herein, we propose a facile hydrothermal-assisted intercalation (HAI) strategy to boost the yield of 2D sheets, achieving a record high value of 74%. This HAI assists the diffusion and intercalation of reagent effectively, promoting the subsequent delamination; meanwhile, antioxidant is applied to protect these Ti3C2Tx from oxidation during HAI process. Therefore, massive Ti3C2Tx 2D sheets can be easily synthesized. Thanks to the synergistic effect of high conductivity and substantial terminated functionalities, these Ti3C2Tx 2D sheets show promising application in supercapacitor, providing a high capacitance of 482 F g-1. Besides, the ultrafast carrier dynamics results of Ti3C2Tx 2D sheets clearly imply the promising application in photocatalysis, due to the relatively long bleaching relaxation time. Our work not only paves the way for the mass production of Ti3C2Tx 2D sheets, but also provides insights into their electronic and optical properties.

1. INTRODUCTION The successful delamination of single layer graphene has created an upsurge of the investigation on two-dimensional (2D) materials because of the distinctive properties compared with the bulk


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form.1-4 In 2011, a big family of 2D materials, MXenes, were discovered, including transition metal carbides and nitrides, which are with a general formula of Mn+1XnTx (n =1, 2, or 3), where M represents an early transition metal (e.g. Ti, Nb, Mo, Cr and so on), X is carbon/nitrogen and Tx denotes surface terminations.5-6 The MXenes are obtained via etching A elements from the parent MAX phases, which are with a general composition of Mn+1AXn (n = 1, 2, or 3), where A stands for elements from the group 13 and 14 of periodic table.7 To date, over 20 species of MXene family have been successfully synthesized.6, 8 Compared with most other 2D materials, MXenes exhibit high metallic conductivities (~6000-8000 S cm-1) and hydrophilic surfaces, showing promising prospects in energy storage,9-14 electromagnetic interference shielding,15 bioimaging,16 catalysis,17 water purification,18 gas separation,19-20 to name a few, which makes them a rising star of 2D materials. Among the big family of MXenes, Ti3C2Tx is the most widely studied one, because it is the first reported MXene in 2011. Normally, Ti3C2Tx 2D sheets are synthesized via a two-step method: first, Ti3AlC2 MAX is etched via aqueous hydrofluoric acid (HF), HF-containing or HFforming etchants to eliminate the metallic bond of M-A,6, 21-22 forming a multiple-layer structure attached with different functionalities (e.g. -O, -F or -OH); second, different intercalating compounds, for example, dimethyl sulfoxide (DMSO), are employed to expand the interlayer space of Ti3C2Tx MXene and consequently realize the delamination of separated 2D sheets under an external shearing force.11, 23 Currently, most of the researches about Ti3C2Tx MXene mainly focus on its applications leading to many attractive results.4, 8, 10, 16, 24 However, the low yield of Ti3C2Tx 2D sheets is a long-neglected and critical issue waiting to be resolved, which significantly impedes the application extension in the research fields that require large amount, for example, environmental


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remediation. One inaccurate estimate value has indicated that the yield of Ti3C2Tx 2D sheets is lower than 20% via using the traditional intercalation and delamination two-step synthesis.24 Actually, some efforts have been made to improve the yield of Ti3C2Tx 2D sheets by applying other novel strategies. But the results are far from satisfactory. For example, a microwaveassisted approach has been reported to realize the scalable synthesis of various 2D sheets, including black phosphorus (BP), transition metal dichalcogenides (TMDs), hexagonal boron nitride (h-BN), and graphitic carbon nitride (g-C3N4).25 Yet Ti3C2Tx MXene had relatively low sheet yields based on this novel microwave-assisted strategy, only reaching a value less than 10%,25 which is even lower than the traditional two-step method. In addition, through an electrochemical etching method in a binary aqueous system, single or bilayer Ti3C2Tx sheets with high yield (>90 %) was achieved.26 However, this number is the percentage of monolayer and/or bilayer sheets among all the layered flakes after centrifugal separation rather than the yield calculated by using the weight ratio of thin flakes to multiple-layer Ti3C2Tx precursor. As a matter of fact, there are no reports about the exact yield of Ti3C2Tx 2D sheets synthesized by various methods.27 More importantly, the leading cause of low yield in Ti3C2Tx 2D sheets synthesis is still unclear so far. Thus, it is a challenging topic to realize the high yield of Ti3C2Tx 2D sheets, which needs to present an effective solution based on the profound understanding of MXene’s structure. Herein, we propose a hydrothermal-assisted intercalation (HAI) strategy to boost the yield of Ti3C2Tx 2D sheets, achieving a record high value of 74% with the optimum technics (140 ℃, 24 h). The strong interaction between adjacent Ti3C2 layers is the reason for a low yield of MXene sheets, since based on the theoretical calculation we find out that potential Ti-Ti bond and residual Ti-Al bond at the interface are with high binding energy of 1.07 eV and 2.26 eV,


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respectively. Thus, intercalation reagent can’t effectively intercalate into the interlayer by its free diffusion, resulting in a low yield. Through the facile HAI, we could assist the diffusion and intercalation of reagent, tetramethylammonium hydroxide (TMAOH), effectively, and thus promoting the subsequent delamination; besides, an antioxidant, ascorbic acid (AA), was used to protect the Ti3C2 layers from oxidation. Therefore, we could synthesize a large amount of Ti3C2Tx 2D sheets with a thickness of ~1.7 nm. Owing to the high conductivity and substantial terminated functionalities, these 2D sheets have a promising application in supercapacitor, which could deliver a high capacitance of 482 F g-1 at 1 A g-1. Besides, the ultrafast carrier dynamics of Ti3C2Tx 2D sheets, investigated for the first time, indicate its promising application in photocatalysis, because of their relatively long bleaching relaxation time. Predictably, our work will inspire the effective intercalation and delamination of other MXenes. 2. RESULTS AND DISCUSSION 2.1 Effective solution for the high yield of Ti3C2Tx 2D sheets. In a typical synthesis process, multiple-layer MXene particles are obtained via the etching of Aelement atomic layer in a MAX phase, namely the aluminum layer between adjacent Ti3C2 layers as for Ti3AlC2 (Figure 1A). When etching process is finished, surface of Ti3C2 layers will be terminated with a variety of functionalities, and these layers are held together via hydrogen and/or van der Waals bonds, forming a special stacked multiple-layer structure that is unlike most other 2D materials. In such an ideal situation, various polar organic molecules, for example, DMSO and isopropylamine, can be easily intercalated into these multiple-layer Ti3C2Tx and thus expand the interlayer space between adjacent Ti3C2 layers. Afterwards, the intercalated Ti3C2Tx (I-Ti3C2Tx) can be delaminated into separated 2D sheets with desired sizes through a


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combination of sonication and centrifugation methods. Apparently, the yield of Ti3C2Tx 2D sheets should achieve a satisfactory level as expected, if these intercalating compounds are so easily insert into the interspace of adjacent layers. However, the yield of Ti3C2Tx 2D sheets is actually claimed to be lower than 20%, meanwhile the exact value is still unknow, because most studies rarely indicate their yields.24, 27 Obviously, there should be some obstacles that make the intercalation reagents less effective and cause great difficulties in a high yield of Ti3C2Tx 2D sheets after delamination, one critical issue waiting to be uncovered and addressed. Density Functional Theory (DFT) should be a helpful method to figure out the possible reasons for the less effective intercalation, since it can simulate the interactions between the adjacent Ti3C2 layers from the theoretical level.28-29 As shown in Figure 1B, it is inferred that there might be another three possible interlayer interactions between adjacent Ti3C2 layers besides of hydrogen and/van der Waals bonds that are structurally stable, and they are X-X (X = O, F, and OH), Ti-Ti and/or Ti-Al bonds, respectively. As for X-X bonds, the surface of Ti3C2 layers will be terminated with different functionalities (-O, -F, or -OH) after etching, and these layers could interact with each other via these functionalities, forming a binding energy of 0.26 eV. As for Ti-Ti bonds, with a binding energy of 1.07 eV, their existence can be attributed to a spontaneous process during etching, in which the dissociation of aluminum layer will generate considerable amount of heat in a local space leading to such interactions; whereas Ti-Al bonds, with a binding energy of 2.26 eV, are due to the residual aluminum that is not fully etched by the corrosive etchant. Noticeably, binding energy of Ti-Ti and Ti-Al is much higher than that of hydrogen and/or van der Waals bonds with a typical binding energy of 0.1 eV/atom. As a result, intercalation reagent can’t break through such high energy barrier and realize interlayer space expansion by means of its free diffusion, which explains why even with high power


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ultrasonication, the yield of separated Ti3C2Tx 2D sheets is still relatively low, as most of the multiple-layer Ti3C2Tx particles are not effectively intercalated. In the light of thermodynamic theory exp (-E / kT), where E and T refer to the energy barrier and temperature, the opportunity of intercalation reagent to break through energy barrier is positively associated with its initial temperature. Consequently, intercalation reagent should be with higher initial temperature to achieve enough energy to break through the strong Ti-Ti and/or Ti-Al bonds and subsequently expand the adjacent layers of Ti3C2 as expected. Among the feasible schemes, heating the system is generally considered to be a preferred method, because of its convenience and effectiveness for liquid intercalation reagent, for example, DMSO. When mixture of multiple-layer Ti3C2Tx and intercalation reagent is heated with a higher temperature, the diffusion energy of reagent should be increased, making them easily break through the barriers of high-energy bonds. Besides, choosing an appropriate intercalation reagent is important to realize the selective attack of the potential bonding, especially for the Ti-Al bonds, which is with a binding energy of 2.26 eV. The alkali etchant can be a promising candidate, because the dissociative OH- from the etchant can react with the remaining Al atoms to form Al(OH)4– and thus break the potential Ti-Al bonds. Such selective attack should be more effective under a higher temperature owing to the better diffusion. However, with an increased intercalation temperature, there will be an unavoidable problem we have to deal with: possible oxidation of Ti3C2 layers under the coexistence of high temperature and oxygen.30-31 As a matter of fact, antioxidants can be a good solution, since animals and plants take advantages of complex antioxidant systems to balance the oxidative state all the time. Based on the above ideas, we propose a facile HAI strategy to remarkably boost the yield of Ti3C2Tx 2D sheets as schematically illustrated in Figure 1C. The multiple-layer Ti3C2Tx powders


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are dispersed in deionized water containing the intercalation agent of TMAOH and the antioxidant of AA. Then, this mixture solution is transferred to a Teflon-lined stainless-steel autoclave and heated to certain temperature for several hours. When HAI is finished, the highly expanded I-Ti3C2Tx is collected and washed; after subsequent sonication and centrifugation, a large amount of delaminated Ti3C2Tx 2D sheets can be obtained. Herein, it is noted that TMAOH is employed as the intercalation reagent rather than other organic molecule (e.g. DMSO) because of several reasons: first, TMAOH is widely used as an effective etchant for Al that can sufficiently react with the remaining Al between adjacent Ti3C2 layers as assisted by the HAI; second, the spatial structure of TMAOH is relatively large enough to weaken the interaction between adjacent Ti3C2 sheets; third, after the reaction between Al with TMAOH, the formed Al(OH)4– can serve as a titanium-mediation moiety on the surface of Ti3C2 to further expand the interlayer space.24 Thus, TMAOH can effectively intercalate in the Ti3C2Tx interlayers with the help of high temperature and facilitate the subsequent delamination; meanwhile, the excessive dose of AA, a water-soluble antioxidant, will effectively protect these I-Ti3C2Tx from oxidation during the HAI, ensuring a high yield of Ti3C2Tx 2D sheets (Figure 1C). After the experience of complete synthesis routine as we have proposed (Figure 1), macroscopic and microscopic appearances of the MAX and MXene will change tremendously. For example, the grayish and dense Ti3AlC2 powder changes into dark purple/black and fluffy multiple-layer Ti3C2Tx after the etching step, whereas the volume of multiple-layer Ti3C2Tx powders will become highly expanded after the intercalation process (Figure S1). Moreover, the scanning electron microscopy (SEM) images demonstrate that Ti3AlC2, multiple-layer Ti3C2Tx, I-Ti3C2Tx and Ti3C2Tx 2D sheets present very different morphologies (Figure 2): Ti3AlC2 MAX phase particle exhibits a closely compact structure (Figure 2A); after etching, the original


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compact particle exhibits an accordion-like structure (Figure 2B), indicating that most of the Al layers in Ti3AlC2 has been eliminated; after intercalation, the I-Ti3C2Tx is further expanded, and as a result, a certain amount of thick Ti3C2Tx layers with a relatively large lateral size are spontaneously delaminated due to a weaken interlayer interaction as shown in Figure 2C; after ultrasonication, most of the I-Ti3C2Tx are delaminated into Ti3C2Tx 2D sheets, which are with different sizes, irregular shapes and sharp edges (Figure 2D). In addition, different synthesis stages will bring about significant structural changes. After etching, the (002) peak of Ti3AlC2 at 9.50° broadens and shifts to a lower angle of 8.84° (Figure 3A), which is ascribed to the removal of Al and the introduction of terminations, such as -F, -O and -OH.8 After intercalation, the (002) peak shifts further to a much lower angle of 5.84°, indicating an expanded interlayer space between adjacent Ti3C2 layers, attributed to the formation of Al(OH)4– and the intercalated TMA+ cation. This explains why some of the Ti3C2Tx layer would delaminate spontaneously after intercalation, since the interlayer interaction decreased apparently. The corresponding Fourier transform infrared (FTIR) spectra and Raman shifts also reveal similar characters, namely, the presence of functional groups, suggesting the successful intercalation via our HAI (Figure S2A, B). The X-ray photoelectron spectroscopy (XPS) a helpful characterization technique was employed to further study the structural variation of the samples. Compared to the original MAX powder (Figure S2C), the Ti2p spectra of multiple-layer Ti3C2Tx and I-Ti3C2Tx show very different patterns. The disappearance of Ti−Al bond and enhancement of Ti−C bond indicate the Al atoms are substituted by OH− and F− mediation, thus resulting in the occurrence of Ti−F stretching at 464.8 eV (Figure 3B, E), suggesting that most Al are etched by HF.33-36 In addition, I-Ti3C2Tx sample exhibits an additional signal for Ti−N at 455.0 eV (Figure 3E) and N signal at approximately 400 eV in the


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full spectra (Figure 3H), confirming the intercalation of TMA+ ions after HAI.24, 37 Meanwhile, the Ti–N bond (N–I bond) peak at 396.0 eV suggests that TMA+ ions anchor on the Ti3C2Tx structure via N atoms (Figure S2H).38-39 O1s core level spectra are displayed in Figures S2D and Figure 3C, F. The spectrum of MAX is fitted by five deconvoluted Gaussian peaks corresponding to the Ti−O, C−Ti−Ox, C−Ti-(OH)x, Al−O and O−C−(OH)x bonds that are located at 529.4, 530.2, 531.5, 532.2 and 532.8 eV, respectively (Figure S2D).33, 41-42 Deconvolution of O1s of multiple-layer Ti3C2Tx and I-Ti3C2Tx show five peaks corresponding to Ti−O, C−Ti−Ox, C−Ti-(OH)x, O−C−(OH)x and H2O species (Figure 3C, F), and the disappearance of Al−O peak indicates the successful etching of Al atoms that is in accordance with the Ti2p core level. As for C1s spectra, beyond the strong C–C peak at 284.6 eV, additional four deconvoluted Gaussian peaks can be recognized: C–Ti–Tx, C–O and O＝C–O, confirming the presence of oxygenated functional groups (Figure S2E–G).42 The distinct enhancement of C–Ti–Tx peaks intensity in multiple-layer Ti3C2Tx and I-Ti3C2Tx indicates that the oxygenated nature of surface termination on C species are replaced by –F, –OH or –N, attributed to the intense reactions of etching and HAI. The high resolution F1s region indicate that both of multiple-layer Ti3C2Tx and I-Ti3C2Tx contain F− terminated Ti and AlFx (Figure 3D and G).41 Actually, Al atoms can react with TMAOH and hydrolyze to form Al(OH)4– (withdraw from AlFx). Owing to its negative charge, Al(OH)4− is readily interacted with the outmost Ti atoms of Ti3C2 layer through O–Ti bonds, resulting in an expanded structure of adjacent Ti3C2 sheets terminated by TMA+ (Figure 1C),24 which meets with the low-angle shifting in the XRD. The sharp decrease of Al/Ti molecular ratio (from about 1/3 to 1/11, calculated basing on the integrated area of Al–F to Ti–F bonds in Figure 3D and G) elucidates the further extraction of residual Al atom from I-Ti3C2Tx that results in


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better intercalation and delamination. Thus, with the HAI strategy, a large amount of Ti3C2Tx 2D sheets can be readily synthesized via a simple ultrasonication (Figure 1C and Figure 2D). 2.2 Boosting the yield of Ti3C2Tx 2D sheets by optimizing HAI. Our proposed HAI strategy indeed can produce a certain amount of Ti3C2Tx 2D sheets. Based on our calculation results and inferences, it is believed that this strategy has a great potential to boost the yield of Ti3C2Tx 2D sheets. Thus, a series of parallel experiments with different conditions were carried out to optimize this HAI. As aforementioned, the dose of antioxidant, AA, is excessive to protect the multiple-layer Ti3C2Tx from oxidation with the co-existing of high temperature and oxygen. As shown in Figure S3A and B, the I-Ti3C2Tx samples synthesized with our HAI strategy at 140 ℃ for 24 h with and without AA show different appearances. Obvious oxidation occurs in the I-Ti3C2Tx sample without AA, as partial of the black sediment has turned white (Figure S3A); in contrast, the I-Ti3C2Tx sediment with AA’s protection maintains its original black color after the hydrothermal process (Figure S3B). Actually, TiO2 is easily generated on Ti3C2Tx under high temperature and aerobic condition, which is the origin of oxidation.30-31 This is verified by the corresponding SEM images, because numerous small particles are grown on the I-Ti3C2Tx and multiple-layer Ti3C2Tx samples after hydrothermal process without the protection of AA (Figure S3C, D). However, the particles will preserve their smooth surface and sharp edges while adding the AA (Figure S3E). As discussed previously, TMAOH is indispensable for the intercalation of Ti3C2Tx that will expand the interlayer space significantly with the assistance of a hydrothermal process. There will be huge differences between the multiple-layer Ti3C2Tx samples treated with and without TMAOH. As shown in Figure S3C, there are no obvious multiple-layer Ti3C2Tx remained after the TMAOH assisted HAI, since most of the Ti3C2Tx layers are delaminated spontaneously because of a weaken


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interaction resulted from an expanded interlayer space; while the particle maintains its original multiple-layer structure without the addition of TMAOH, because no intercalation occurs during the hydrothermal process (Figure S3D, E). It was found out that 2 wt% TMAOH would be enough for the effective intercalation of Ti3C2Tx via our proposed HAI strategy. Even with a higher concentration of TMAOH, for example, 10 wt%, the yield of Ti3C2Tx 2D sheets is almost unchanged; whereas the addition of TMAOH is lower than 2 wt% (e.g. 0.5 wt% and 1 wt%), the yield of Ti3C2Tx 2D sheets will decrease due to a lack of intercalation reagent. In fact, the temperature of HAI strategy is the key factor to achieve a high yield of Ti3C2Tx 2D sheets according to our theoretical analysis and the relative inferences. Accordingly, a series of temperatures (room temperature, 60 ℃, 100 ℃, 140 ℃ and 180 ℃) were applied to the HAI strategy to systematically investigate the thermal effects on the yield of Ti3C2Tx 2D sheets. As shown in Figure 4A, the freshly synthesized Ti3C2Tx 2D sheets can disperse in water homogeneously and form a dark black solution, which can be stored for months without obvious precipitation (Figure S4), indicating great stability. The excellent dispersity and hydrophilicity of Ti3C2Tx 2D sheets are further convinced by the typical Tyndall scattering effect, as the green laser can pass through the colloidal solution (Figure 4A). Because water, a solvent with high polarity, is a good candidate for the dispersion of Ti3C2Tx 2D sheets,43 we could qualitatively compare the yields of Ti3C2Tx 2D sheets of different hydrothermal temperatures through comparing their absorbance spectra. The absorbance spectra of Ti3C2Tx 2D sheet solutions obtained by different hydrothermal temperatures are shown in Figure 4B. We can detect two shoulder peaks in the UV range of 200 nm to 400 nm and a broad peak in the near-infrared range of 700 nm to 1000 nm in all the samples, which means the Ti3C2Tx 2D sheets yielded by different hydrothermal temperature exhibit very similar structural and optical properties. It is


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noteworthy that the absorbance first increases with an increased temperature and achieves the maximum value by 140 ℃ sample, then interestingly the absorbance decreases with an even higher intercalation temperature. Because absorbance is normally positively correlation with the solution concentration, it can infer that 140 ℃ should be the optimum temperature for our HAI strategy, which can yield the largest amount of Ti3C2Tx 2D sheets comparing with other temperatures. In regards to the lower yield at 180 ℃, we assume that the antioxidant might not be so effective under such a high temperature, and as a result, some of the multiple-layer Ti3C2Tx might oxidize leading to a lower yield of Ti3C2Tx 2D sheets. In fact, with a higher temperature, interlayer space between adjacent Ti3C2 layers should be larger owing to the better intercalation. This is confirmed by the corresponding XRD spectra of intercalated Ti3C2Tx (Figure 4C, D): the (002) peak slightly shifts from 5.90° to 5.76° with an increased temperature, indicating the interlayer space between adjacent layers of I-Ti3C2Tx expands as the temperature increases. In order to further confirm their morphology characteristics (e.g. thickness and sizes), the obtained Ti3C2Tx 2D sheets were studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM images provide a convincing evidence for the successful synthesis of Ti3C2Tx 2D sheets via our HAI strategy, since dozens of sheets distribute randomly on the substrate with irregular shapes and sharp edges (Figure 5A, B). The thickness of these 2D sheets is estimated to be ~1.7 nm in terms of the height profile (Figure 5C), indicating that the colloidal solution indeed consists of bilayer Ti3C2Tx nanosheets. The TEM images reveal a 2D structure of the delaminated Ti3C2Tx as well, which are with lateral dimensions in the range of 300 nm to 1 μm (Figure 5D, E). The 2D sheets are so thin that we can observe their obvious stacking, since the electron beam can pass through the samples easily. Besides, crumpled or scrolled sheets are observed in the detected area, suggesting that the Ti3C2Tx 2D sheets are


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highly flexible. Meanwhile, as shown in the high-angle annular dark-field imaging (HAADF, Figure 5G), these Ti3C2Tx 2D sheets are varied in sizes and shapes, but with similar sharp edges and corners, consistent with the AFM results. Energy-dispersive X-ray spectroscopy (EDS) mapping of Ti and Al is presented in Figure 5H and I, corresponding to a selected area in the HADDF image. The distribution of Ti in elemental mapping matches well with the shape of selected area, whereas only a tiny amount of Al is detected, indicating that these 2D sheets are indeed Ti3C2Tx and the residual Al is further eliminated by HAI. In addition, the Ti3C2Tx 2D sheets synthesized under different temperatures (room temperature, 60 ℃, 100 ℃ and 180 ℃) exhibit similar morphologies to that synthesized at 140 ℃, as shown in Figure S5. As predicted by the absorbance spectra, 140 ℃ should be the optimum temperature for our HAI strategy to achieve the highest yield of Ti3C2Tx 2D sheets. However, exact value of the yield still awaits to be ascertained. Through gravimetric method, we can calculate the exact yield of these 2D sheets via the HAI strategy. As shown in Figure 5F, a remarkably high yield of 74% is achieved at the optimum temperature of 140 ℃ for 24 h. To the best of our knowledge, this is the highest yield of Ti3C2Tx 2D sheets, which is significantly higher than the estimated number of 20% as claimed by other research.24, 27 The yields of other hydrothermal temperatures, namely, room temperature, 60 ℃, 100 ℃ and 180 ℃, are 11%, 33%, 47% and 64%, respectively (Figure 5F), which is in line with the variation of absorbance spectra. Besides temperature, duration time of the HAI also affects the yield of Ti3C2Tx 2D sheets. When the duration time is decreased to 12 h, the yield decreases to 42%, due to the less effective intercalation. However, when it is increased to 36 h, the yield doesn’t increase as expected, on the contrary, it decreases to 63%. It is assumed that longer duration will affect the stability of antioxidant, a tricky case similar to the higher temperature intercalation, resulting in the slight oxidation of Ti3C2Tx layer and thus, the


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less effective intercalation. Despite of the lower yield, Ti3C2Tx 2D sheets synthesized by different duration time (12h and 36 h) exhibit similar morphologies (Figure S6). Furthermore, this facile HAI strategy can be a very universal method to achieve high yield of Ti3C2Tx 2D sheets regardless of the manufacturer. Another Ti3AlC2 MAX powder obtained from a different company is used to synthesize Ti3C2Tx 2D sheets with a high yield according to our proposed strategy. The SEM and TEM images of MAX and MXene as shown in Figure S7 exhibit similar morphologies as previous results, and the yield of Ti3C2Tx 2D sheets is 72%, a value closed to the 74% achieved by the original powder. Besides, we have succeeded in improving the yield of other types of MXene 2D sheets. For example, we can achieve a high yield of ~ 50% for Nb2C 2D sheets at present (Figure S8). As is known, large lateral size 2D sheets are desirable in many practical applications. As for the current HAI, we could obtain 2D sheets with a lateral size < 1μm. Thus, the further optimization of HAI is needed to realize the high yield and the large lateral size at the same time, which is a challenging topic we are still working on. 2.3 Properties of Ti3C2Tx 2D sheets obtained by HAI. Though a high yield of Ti3C2Tx 2D sheets is achieved by our HAI, we are still interested in their properties: will hydrothermal treatment affect the conductivity and/or defects in these MXene sheets? Conductivity is usually a good predictor of the material quality. As for our Ti3C2Tx 2D sheets, they show a relatively high conductivity of 405 S cm-1, which is lower than previous reports26. It is reasonable since they will be easily terminated with substantial functionalities after intercalation and delamination, especially during the hydrothermal treatment. Thanks to a synergistic effect of abundant functionalities and high conductivity, these Ti3C2Tx 2D sheets show a great potential in supercapacitor. Cyclic voltammogram and galvanostatic charge-discharge exhibit quasi-rectangular and symmetric triangular curves under various scan


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rates and current densities (Figure 6A, B), respectively, indicating the fast and reversible electrochemical reactions. Noticeably, a high capacitance of 482 F g-1 is achieved at 1 A g-1, which is higher than or comparable with previously reported MXene or MXene-based hybrid materials.44-49 When the current density increases to 10 A g-1, the capacitance is 333 F g-1 (Figure 6C), suggesting a good rate capability. The electrochemical impedance spectroscopy shows a semicircle in high frequency together with a linear part in low frequency. The charge transfer resistance, diameter of semicircle in high frequency, is 6.5 Ω (Figure 6D), which should be a reason for the high capacitance and good rate capability. In addition, the 2D sheets remain over 98% of the original capacitance after 2000 cycles of charging/discharging (Figure 6E), which is owing to the outstanding structural stability. Thus, it is inferred that there are a few structural defects in the 2D sheets obtained by our HAI, or else the capacitance should deteriorate obviously after repeated charging/discharging. In addition to its electronic properties, optical properties of Ti3C2Tx 2D sheet are very attractive, owing to its promising application in ultrafast lasers, plasmonics and photothermal therapy.50 Herein, we further study the optical properties of our Ti3C2Tx 2D sheets. The photoluminescence of Ti3C2Tx 2D sheets was firstly investigated, however, no obvious emission was detected. Combined with their UV-Visual absorbance spectra, we can infer that the broad peak in the near-infrared range of 700 nm to 1000 nm could be attributed to surface plasmon resonance. Besides, the ultrafast carrier dynamics of Ti3C2Tx 2D sheets are investigated via the pump probe spectroscopy

51-52

for the first time. Figure 7 shows the resulted excitation fluence

dependent decay profiles of Ti3C2Tx 2D sheets under excitation of 800 nm fs laser with probe laser of 600 nm and 700 nm, respectively. These decay curves were fitted by a biexponential function S(t) = A1exp(−t/τ1) + A2exp(−t/τ2) + A0, where τ represents the decay times and A


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represents the relative weights of the decay components at t = 0. For probe laser at 600 nm, the time constant τ1 is in tens of picoseconds and τ2 is in hundreds of nanoseconds, while for probe laser at 700 nm, the time constant τ1 is in tens of picoseconds and τ2 is in a few of nanoseconds. The fast decay components τ1 could be ascribed to the defect trapping and the slow decay components τ2 to the electron-hole recombination. As a result, it can be inferred that there are a few structural defects in the 2D sheets obtained by our HAI, which is in accordance with the result of their relative low conductivity. Even though the defects are unavoidable due to the general two-step synthesis of invasive etching and delamination for MXenes, these Ti3C2Tx 2D sheets can still have promising applications in photocatalysis. Because the relatively long bleaching relaxation time in the ultrafast dynamics study indicates that they would have comparably large number of electron-hole pairs with enough activity and lifetime to react with target reagent in the photocatalysis process. 3. CONCLUSION In summary, we have successfully developed a facile HAI strategy to boost the yield of Ti3C2Tx 2D sheets, achieving a record high yield of 74%. The strong interlayer interaction between adjacent Ti3C2 layers would cause great difficulties in the high yield of 2D sheets, because the binding energy of potential Ti-Ti bonds and residual Ti-Al bonds can reach 1.07 eV and 2.26 eV, respectively. Thus, intercalation reagent can’t break through such strong interactions by free diffusion, resulting in a low yield. Through the HAI, we can make the diffusion and intercalation of TMAOH more effectively, bringing about a higher yield of 2D sheets after delamination. Meanwhile, AA is applied to protect the Ti3C2 layers from oxidation during HAI. Therefore, we could synthesize a large amount of Ti3C2Tx 2D sheets with a thickness of ~1.7 nm. Benefitting from the high conductivity and substantial functionalities, the Ti3C2Tx 2D sheets showed


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promising application in supercapacitor, providing a high capacitance of 482 F g-1. Besides, the ultrafast carrier dynamics results of Ti3C2Tx 2D sheets, as investigated for the first time, indicates its promising application in photocatalysis, because of their relatively long bleaching relaxation time. Our work not only provides an effective strategy to boost the yield of Ti3C2Tx 2D sheets, but also provides insights into their electronic and optical properties.


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Figure 1. Schematic demonstration and illustration. (A) Etching process of multiple-layer Ti3C2Tx. (B) Potential bonds between the adjacent Ti3C2 layers. (C) The designed HAI strategy to synthesize Ti3C2Tx 2D sheets.


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Figure 2. SEM images of MAX and MXene after different treatment stages. SEM images of Ti3AlC2 particles: (A) before and (B) after etching. SEM images of (C) intercalated Ti3C2Tx and (D) Ti3C2Tx 2D sheets. Inset is the high resolution of selected area.


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Figure 3. Structural characterization. (A) X-ray diffraction (XRD) spectra of Ti3AlC2 (black), Ti3C2Tx (blue) and intercalated Ti3C2Tx (red). XPS spectra of (B – D) multiple-layer Ti3C2Tx, (E – G) I-Ti3C2Tx and (H) full survey.


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Figure 4. (A) Digital pictures of the freshly synthesized Ti3C2Tx 2D sheets solution (left) and a 30 times diluted solution showing Tyndall scattering effect (right). (B) UV-vis absorbance spectra of Ti3C2Tx 2D sheet solutions synthesized at various temperatures. Notes: the samples were diluted 100 times. Inset is the digital picture of 40 times diluted samples. (C) XRD spectra and (D) high resolution of (002) peaks of I-Ti3C2Tx synthesized at various temperatures.


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Figure 5. Ti3C2Tx 2D sheets synthesized via the optimized HAI strategy, 140 ℃ for 24 h: (A, B) AFM images, (C) height profile, (D, E) TEM images, (F) comparison of the yields at various temperatures, (G) HAADF image, and EDS mapping of (H) Ti and (I) Al.


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Figure 6. (A) CV and (B) CD curves of Ti3C2Tx 2D sheets electrodes under various scan rates and current densities. (C) Rate capability. (E) EIS. (D) Capacitance retention after 2000 charging/discharging cycles.


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Figure 7. Excitation fluence dependent decay profiles of Ti3C2Tx 2D sheets (A) at 600 nm and (B) at 700 nm under excitation at 800 nm. The solid lines are the fitting results. The bleaching relaxation times of the Ti3C2Tx 2D sheets (C) at 600 nm and (D) at 700 nm as a function of excitation fluence. ASSOCIATED CONTENT The following files are available free of charge. Experimental methods, digital pictures, XPS spectra, SEM images, UV-vis absorbance, TEM images, AFM images, height profiles, and corresponding discussions


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AUTHOR INFORMATION Corresponding Author *Email: [email protected] (X.C.) *E-mail: [email protected] (W.C.) *E-mail: [email protected] (Y.H.) Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of Shenzhen University (Grant No. 2017004

&

2018004),

the

Science

and

Technology

Foundation

of

Shenzhen

(JCYJ20170818142354137 & JCYJ20170818144438033), the Natural Science Foundation of Guangdong Province (Grant Nos. 2018A030310420 & 2016A030310048), and the National Natural Science Foundation of China (Grant Nos. 51802198 & 61704112). The authors would like to thank Dr. ZHU Minshen in the Leibniz-Institut für Festkörper- und Werkstoffforschung for his help on writing this article. REFERENCES (1) Zheng, Y.; Liu, J.; Liang, J.; Jaroniec, M.; Qiao, S. Z. Graphitic Carbon Nitride Materials: Controllable Synthesis and Applications in Fuel Cells and Photocatalysis. Energy Environ. Sci. 2012, 5, 6717-6731.


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Boosting the Yield of MXene 2D Sheets via a Facile Hydrothermal

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