Citation

Jianguang Xu, Jiale Zhu, Chen Gong, Zexin Guan, Dan Yang, Zhihan Shen, Wei Yao, Haijiang Wu. Achieving high yield of Ti₃C₂T_x MXene few-layer flakes with enhanced pseudocapacior performance by decreasing precursor size[J]. Chin. Chem. Lett., 2020, 31(4): 1039-1043  

Jianguang Xu^a,*, Jiale Zhu^a,b, Chen Gong^a,b, Zexin Guan^a, Dan Yang^a, Zhihan Shen^a, Wei Yao^a,*, Haijiang Wu^b,*     

MXenes, a family of two-dimensional (2D) early transition metal carbides or nitrides, have attracted great attention recently owing to their unique 2D structure and properties. The general formula of MXenes is M_n+1X_nT_x, where M is an early transition metal, X is carbon or nitrogen, and T is surface functional group such as O, OH and F. Since the first MXene Ti₃C₂T_x has been discovered in 2011 [1], MXenes show promising potentials in numerous applications such as energy storage [2-7], catalysis [8-12], electromagnetic shielding [13-15], antibacterical [16, 17], memristive device [18-20], water purification [21]. Up to now, the major strategy for preparation of MXene few-layer flakes is etching MAX powders with HF or HCl and LiF, then followed by a delamination process such as sonication or hand shaking. However, the yield of MXene few-layer flakes via this two-step process is estimated lower than 20% in most situations [22], which may retard the steps of MXenes in real applications.

To overcome the challenge of low yield, some novel strategies were employed recently to boost the yield of Ti₃C₂T_x few-layer flakes. For example, a microwave-assisted etching method has been reported to apply in the preparation of Ti₃C₂T_x MXene, yet only a relatively low sheet yield of less than 10% was reached [23]. Furthermore, by using an electrochemical etching process in a binary aqueous system, high yield (>90%) of single or bilayer Ti₃C₂T_x flakes was achieved [24]. Nevertheless, this number is the percentage of monolayer and/or bilayer flakes among all the layered flakes after centrifugal separation, which does not reflect the real yield calculated by using the weight ratio of few-layer flakes to multiple-layer Ti₃C₂T_x precursor. Two other strategies are selective lithiation-expansion-microexplosion synthesis [25] and hydrothermal alkali treatment [26], demonstrating excellent effectiveness to obtain 2D Ti₃C₂T_x sheets, but the exact yield was kept unknown. Fortunately, a successful approach was reported that by adding a hydrothermal treatment before sonication process, a high yield of 74% was achieved [27]. However, to further save the cost of preparation of the Ti₃C₂T_x few-layer flakes, seeking simple and cost effective solutions to achieve high yield of MXenes is still a challenging topic for researchers.

Herein, we proposed a simple strategy to achieve relatively high yield of Ti₃C₂T_x few-layer flakes by decreasing the precursor size. In this work, Ti₃AlC₂ powders with three different sizes (300 mesh, 400 mesh and 500 mesh) were used as precursors. When etching the smallest one (500 mesh Ti₃AlC₂ powders) with HCl and LiF at 35 ℃, we received as high as 65% Ti₃C₂T_x few-layer flakes of the received Ti₃C₂T_x MXene product. In addition, the as-received Ti₃C₂T_x few-layer flakes show better electrical conductivity, a more expanded interlayer space, more O content on the surface compared to other two products etched from bigger Ti₃AlC₂ powders, resulting in an enhanced pseudocapacitor performance.

The yields of few-layer Ti₃C₂T_x flakes etched from Ti₃AlC₂ powders with different sizes at different conditions are showed in Table 1. Ti₃C₂ 300-35, Ti₃C₂ 400-35 and Ti₃C₂ 500-35 are named as the products etched from 300 mesh, 400 mesh and 500 mesh Ti₃AlC₂ powders at 35 ℃ respectively. The detailed experimental is shown in the supplementary information. As expected, the size of starting Ti₃AlC₂ powders exhibited important effect on the final yield. A remarkably high yield of 65% is achieved when using 500 mesh Ti₃AlC₂ as raw material, which is much higher than the yield of using 300 mesh Ti₃AlC₂ as raw material. The highest yield (65%) of this work is very close to the yield (74%) of hydrothermalassisted intercalation method [27], and much higher than the 20% yield claimed by other research [22, 28]. Particularly, this strategy is simple, cost effective and suitable for mass production. For example, the 500 mesh Ti₃AlC₂ MAX powders is convenient to obtain through a facile ball milling process. Table 1 also shows the electrical conductivity of different Ti₃C₂T_x films. All the films exhibit excellent electrical conductivity as other reported Ti₃C₂T_x MXene films, particularly in which Ti₃C₂ 500-35 film has the best value of 3174 S/cm.

Table 1

Table 1 The yield and properties of the materials under study.

Table 1 The yield and properties of the materials under study.

Interestingly, the size of starting Ti₃AlC₂ powders also show obvious effect to the composition and structure of the received few-layer Ti₃C₂T_x flakes. As shown in Fig. 1, all the etched samples exhibited similar XRD spectra to those of delaminlated Ti₃C₂ MXenes reported in literature [11, 18, 29], indicating the thoroughly removal of Al and the establishment of surface functional groups, such as –O, –OH and –F. It can be clearly seen in Fig. 1b that the characteristic (002) peak of Ti₃C₂T_x shifts from 7.12° to a lower anger of 6.56° with the decreasing of the Ti₃AlC₂ precursors size, suggesting the interlayer space expanded from 12.40 Ǻ to 13.14 Ǻ. This expansion probably origins from the decreased Ti₃C₂T_x flakes [30], which have more surface functional groups or more intercalated water molecules between adjacent layers.

Figure 1

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Fig. 1. XRD patterns of different few-layer Ti3C2Tx films.

Except the interlayer space between Ti₃C₂T_x flakes, the flake size also has important effect on the electrochemical performance of Ti₃C₂T_x MXene [30, 31]. Thus the flakes etched from the Ti₃AlC₂ precursors with different sizes have been investigated by SEM, shown in Figs. 2a–c. Calculated at least a hundred flakes in SEM images of each sample, the average sizes of Ti₃C₂T_x flakes of Ti₃C₂ 300-35, Ti₃C₂ 400-35 and Ti₃C₂ 500-35 are 805 nm, 506 nm and 361 nm, respectively (Table 1). In addition, The Ti₃C₂T_x flakes of Ti₃C₂ 500-35 in TEM image (Fig. 2d) show a transparent morphology, indicating most of the received flakes are singlelayer or few-layer.

Figure 2

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Fig. 2. SEM micrographs of different few-layer Ti3C2Tx flakes: (a) Ti3C2 300-35; (b) Ti3C2 400-35; (c) Ti3C2 500-35 (Scale bar =2 μm). (d) TEM micrograph of Ti3C2Tx flakes of Ti3C2 500-35 (Scale bar =0.5 μm).

To study the surface functional groups on Ti₃C₂T_x flakes, X-ray photoelectron spectroscopy (XPS) was applied to compare the different samples (Fig. 3). High resolution C 1s and Ti 2p spectra of all three samples are very similar, showing that the three samples have similar surface structure. However, compared to Ti₃C₂ 300-35 flakes, the O 1s spectra of Ti₃C₂ 400-35 flakes and Ti₃C₂ 500-35 flakes exhibit some obvious changes. The enhancement of the C–Ti–O, C–Ti–(OH)_x and C–Ti–(OH)_x·H₂O bonds, which are located at 530.6, 532.1 and 533.1 eV, respectively [27, 32, 33], indicates the increase of –OH and –O terminations. This increase is also proved by the spectra of Ti 2p, which show an significant enhancement of Ti³⁺ peak at 457.2 eV in Ti₃C₂ 500-35 flakes. In addition, the element contents of different samples measured by XPS are shown in Fig. 3d. The O content of Ti₃C₂ 300-35, Ti₃C₂ 400-35 and Ti₃C₂ 500-35 flakes is 14.84%, 17.37% and 18.88%, respectively. Meanwhile, the F content of Ti₃C₂ 300-35, Ti₃C₂ 400-35 and Ti₃C₂ 500-3 flakes is 12.42%, 13.55% and 9.40%, respectively, showing an inverse trend compared to O contents. Note although the F content of Ti₃C₂ 400-35 is higher than that of Ti₃C₂ 300-35, the O/F ratio of Ti₃C₂ 400-35 is still increased compared to Ti₃C₂ 300-35, suggesting –O or –OH groups occupied more surface area of Ti₃C₂T_x flakes in Ti₃C₂ 400-35. In addition, if considering the Ti contents of the three samples, the (O + F)/Ti ratio of Ti₃C₂ 300-35, Ti₃C₂ 400-35 and Ti₃C₂ 500-35 flakes is 1.025, 1.149 and 1.178, respectively, indicating there are more surface functional groups on smaller flakes. Combining the results of element contents and XPS spectra, we can conclude that by decreasing the Ti₃AlC₂ precursor size, the surface –F functional groups can be removed or substituted by –O/–OH functional groups, resulting in more O contents in smaller Ti₃C₂T_x flakes. This change may be attributed to the choice of free energy of Ti₃C₂T_x. For the 2D Ti₃C₂T_x flakes, the surface functional groups contribute to their free energy via three interactions: adsorption site, in-plane distribution pattern and surface-surface correlation, and the adsorption site is the most important one [34]. However, although –O terminated Ti₃C₂T_x is the most energetically favorable [34], the surface functional groups on Ti₃C₂T_x flakes in most situations are the mixture of –O, –OH and F because they strongly prefer mixing to segregating [35] due to the in-plane distribution pattern. Thus when Ti₃C₂T_x flakes become smaller and thinner, –O termination will probably increase because there are less in-plane distribution pattern and surface-surface correlation interactions. In addition, -OH termination may also increase because it tends to transfer to –O termination [34].

Figure 3

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Fig. 3. High resolution C 1s (a), O 1s (b) and Ti 2p (c) XPS spectra of Ti3C2 300-35, Ti3C2 400-35 and Ti3C2 500-35. (d) Element distribution of the samples measured by XPS.

Finally, cyclic voltametry (CV) and galvanostatic chargedischarge (GCD) experiments in a three-electrode system in 3 mol/L H₂SO₄ were performed to examine the electrochemical performance of these samples. The CV curves and capacitances of the Ti₃C₂ 300-35, Ti₃C₂ 400-35 and Ti₃C₂ 500-35 film electrode are shown in Fig. 4. It has been proved previously that the pseudocapacitance of Ti₃C₂T_x is mainly attributed to the protonation of oxygen functional groups on its surface, leading to the variation in the oxidation state of the surface titanium atoms [6, 36, 37]. For the three investigated electrodes, at a low scan rate of 2 mV/s, a pair of anodic and cathodic peaks near -0.35 V with small peak separations were detected, indicating the protonation and deprotonation process on the surface of Ti₃C₂T_x flakes. Particularly, benefiting from the excellent electrical conductivity and more interlayer water molecules, the CV curves of Ti₃C₂ 500-35 film electrode exhibit a smallest peak separation among all three electrodes (Fig. S1). Moreover, this electrode shows the highest gravimetric capacitance of 364 F/g at a 2 mV/s scan rate (Fig. 4d). However, the capacitance of the electrodes markedly decreases as the flake sizes become bigger and the lowest capacitance of 296 F/g was obtained for an electrode prepared by Ti₃C₂ 300-35 MXene flakes with the biggest average size of 805 nm. The capacitance (357 F/g) of Ti₃C₂ 400-35 at a 2 mV/s scan rate is very close to that of Ti₃C₂ 500-35, while it drops quicker than Ti₃C₂ 500-35 as the scan rate increased. In addition, Ti₃C₂ 500-35 electrode shows a stable cyclic performance, demonstrating an 88% capacitance retention after 10, 000 cycles Fig. S2 in Supporting information.

Figure 4

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Fig. 4. Electrochemical performance of different few-layer Ti3C2Tx film electrodes in three-electrode system. Cyclic voltammetry profiles of (a) Ti3C2 300-35, (b) Ti3C2 400-35, (c) Ti3C2 500-35. (d) Rate performance of different few-layer Ti3C2Tx film electrodes.

Note that the capacitor performance of our samples exhibit different results with flakes size variation compared to previous reports [30, 31], which show a down trend of the capacitance with the decrease with flakes lateral size (≤ 1 μm) at a low scan rate of 2 mV/s because the Ti₃C₂T_x film assembled by bigger flakes has better electrical conductivity. More interfacial contact resistance and higher number of defects introduced by sonication in smaller flakes are ascribed to the loss of conductivity [31, 38]. However, in our experiments, the Ti₃C₂ 500-35 film with smallest lateral size among all the samples has best electrical conductivity of 3174 S/cm. First of all, these samples have roughly the same number of defects because they went through the same sonication process. Furthermore, surface functional groups on Ti₃C₂T_x flakes play an important role on the electrical conductivity [39-41], and the increase of -O terminations and decrease -F terminations may benefit to the value of conductivity [39]. On the basis of XPS results, Ti₃C₂ 500-35 has the highest number of O content and lowest number of F content, and these O content exist in Ti₃C₂T_x flakes as –O and –OH terminations revealed by O 1s and Ti 2p spectra of Ti₃C₂T_x samples (Fig. 3), which may benefit to the enhancement of electrical conductivity. On the other hand, it has been proved that the pseudocapacitance of 2D Ti₃C₂T_x sheets is associated with the change of Ti oxidation state, which change is accompanied by the protonation of -O groups [5, 6, 36, 37, 42], thus Ti₃C₂ 500-35 film electrode has highest capacitance among all the received samples, confirmed by the sharper peaks of Ti₃C₂ 500-35 close to -0.35 V compared to those of Ti₃C₂ 300-35 and Ti₃C₂ 400-35 [36]. In addition, benefiting from the better ion accessibility [30] and more high-mobility water molecules between Ti₃C₂T_x interlayers of smaller flakes, which have been proved can fasten the diffusion of hydrogen ions between MXene interlayers [43], Ti₃C₂ 500-35 film electrode also deliver better rate performance than other two samples.

In summary, we have demonstrated that the precursor size of 2D Ti₃C₂T_x sheets play a very important role to their yield, structure and electrochemical performance. By using the small 500 mesh Ti₃AlC₂ powders as precursor, a high yield of 65% Ti₃C₂T_x MXene few-layer flakes was achieved through etching Ti₃AlC₂ with HCl and LiF. Furthermore, the small Ti₃C₂T_x flakes has better electrical conductivity, more O content and interlayer water molecules compared to the products etched from 300 mesh and 400 mesh Ti₃AlC₂ powders, thus delivering higher capacitance and better rate performance than the other two Ti₃C₂T_x flakes when using as capacitor electrodes. A high capacitance of 364 F/g and an 88% capacitance retention after 10, 000 cycles were achieved as a result. Our results presented here are applicable to the mass production of 2D MXenes materials and make them promising cost effective electrode materials in real energy storage related applications.

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.

J. Xu thanks the National Natural Science Foundation of China (No. 21671167) and the Joint Open Fund of Jiangsu Collaborative Innovation Center for Ecological Building Material and Environmental Protection Equipments and Key Laboratory for Advanced Technology in Environmental Protection of Jiangsu Province (No. JH201847). W. Yao thanks the National Natural Science Foundation of China (No. 51602277).

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.02.050.