Fabrication of MXene transparent conductive films via transfer process

In this study, we fabricate MXene transparent conductive films via a transfer process. The results show that the transferred transparent conductive films using titanium carbide MXene have ca. 100 times higher electrical conductivity than conventional spray-coated samples. Transparency and electrical conductivity are easily controlled by changing the amount of MXene materials. The scanning electron microscopy observations reveal that the transferred films have a smooth and uniform MXene flake network. The closer interlayer distance of the MXene flakes when compared to the spray-coated sample, which enables superior electrical conductivity, is confirmed by the X-ray diffraction measurement.

M Xenes, which are emerging two-dimensional (2D) nanomaterials represented by the general formula M n +1 X n T x (M: early transition metals; X: C and/or N, T: a terminal functional group, such as -F, −OH, and =O), have attracted attention in many fields [ Fig. 1(a)]. [1][2][3][4][5] Adding to the optical transparency and mechanical flexibility derived from the layered structure, their electronic band structures are drastically varied from semiconducting to metallic characters depending on the M, X, and T atom combinations. 6,7) MXenes also possess facile solution processability because they are dispersible in aqueous and polar organic solvents without surfactants [ Fig. 1(b)] due to the presence of the surface hydroxy groups (-OH), which are different from other 2D nanomaterials, such as graphene, hexagonal boron nitride, and transition metal dichalcogenides. Therefore, various applications, including batteries, [8][9][10] hydrogen evolution catalysts, 11,12) supercapacitors, [13][14][15] and flexible sensors, [16][17][18] have been demonstrated in recent years.
The titanium carbide MXenes (e.g. Ti 2 CT x and Ti 3 C 2 T x ) used in transparent conductive film applications have particularly been well studied due to their excellent electrical conductivity. [19][20][21][22][23][24][25][26][27][28] These transparent conductive films are generally fabricated via spin casting [19][20][21][22][23] or spray coating methods, [24][25][26][27] but both have limitations. Spin casting is only applicable to hydrophilic substrates because of its solvent polarity. It also requires time-consuming optimization and significant material waste to avoid uneven coating. Although the spray coating method is applicable to any kind of substrate, the electrical conductivity of spray-coated films is several hundreds of times lower than that of spin-casted ones. Thus, a concise and versatile method must be developed to realize MXene transparent conductive films exhibiting a sufficiently practical electrical conductivity.
This study describes the transfer method for fabricating Ti 3 C 2 T x MXene transparent conductive films. We recently established the exfoliation of Al-residual multilayer MXene using tetramethylammonium bases to obtain an aqueous Ti 3 C 2 T x dispersion. 27) A Ti 3 C 2 T x sheet prepared on a membrane filter showed a sheet resistance of ca. 0.2 kΩ·sq −1 , which is a superior conductivity compared to that of the spray-coated transparent conductive films (i.e. ∼10 kΩ·sq −1 ). Therefore, we decided to investigate the transfer of Ti 3 C 2 T x MXene sheets from the filter to the desired substrates. Accordingly, we conducted microscopic observations and spectroscopic characterizations of the fabricated films to clarify the differences in the fabrication process.
To begin our investigation on the Ti 3 C 2 T x MXene film transfer, we first examined a membrane dissolution process used to fabricate carbon nanotube (CNT) transparent conductive films 29,30) and CNT thin film transistors. 31,32) During the dissolution process of a nitrocellulose-based membrane filter, most of the Ti 3 C 2 T x flakes were leached out from the substrate to the solvent. Ishizaki and Kurihara recently reported an alternative transfer method for the CNT films instead of dissolving the membrane filter. 33) Referring to this, we tested the wetted transfer method here [ Fig. 2(a)]. A Ti 3 C 2 T x MXene sheet was prepared through the suction filtration of diluted dispersions in 50 ml DI water on a hydrophilic polytetrafluoroethylene (PTFE) membrane filter (H010A047A, ADVANTEC). The filtrated sheet on a membrane filter was immersed into DI water. The upper Ti 3 C 2 T x side of the filter was then affixed to a polyethylene naphthalate (PEN) substrate (Teonex® Q65FA, DuPont Teijin Films). The Ti 3 C 2 T x sheet was transferred onto the substrate after drying over the wetted film composite by heating at 110°C and peeling off the PTFE membrane filter. Some of the Ti 3 C 2 T x flakes remained on the PTFE filter, and the resultant film showed a mottled pattern, unsuitable for transparent conductive film applications [ Fig. 2(b)]. As regards extensive screening, the use of a mixed cellulose ester (MCE) membrane filter (VMWP04700, MF-Millipore) provided uniform Ti 3 C 2 T x MXene films. After the heating treatment until the solvent drying, a composited MCE filter was spontaneously peeled off, and the filtrated Ti 3 C 2 T x MXene sheet was completely transferred onto the PEN substrate. The developed transfer method enabled the fabrication of MXene transparent conductive films on various substrates, including silicon, glass, and stretchable polymers [ Fig. 2(c)].
Next, the electrical conductivity of the transparent conductive Ti 3 C 2 T x MXene films was evaluated. The fabricated transparent conductive film was dried at 30°C for 16 h under vacuum conditions (<5 Pa) prior to the measurement. The transparency was determined by the absorption spectra recorded on an ultraviolet-visible spectrophotometer (V-770, JASCO). The sheet resistance (R s , Ω sq −1 ) was measured using the four-probe method with a Loresta-AX resistivity meter (MCP-TP06P, Mitsubishi Chemical Analytech). We easily changed the film transparency by tuning the amounts of the MXene dispersion during the filtration process. Figure 3 (red solid circles) depicts the transmittance at the 600 nm wavelength versus the average   sheet resistance of the fabricated transparent conductive films. For comparison, we also prepared spray-coated transparent conductive Ti 3 C 2 T x MXene films (Fig. 3, blue sold squares). R s at 87% transmittance was 4.7 × 10 3 Ω sq −1 , which was ∼100 times lower than the sheet resistance of the spray-coated films (3.5 × 10 5 Ω sq −1 at 82% transmittance), including those in the previous reports (0.3-1.6 × 10 6 Ω sq −1 at ca. 90% transmittance. [24][25][26] A smaller deviation was confirmed as an advantage of this transfer method. The transferred Ti 3 C 2 T x MXene films exhibited a higher electrical conductivity compared to the transparent films using graphene dispersion in the aqueous sodium cholate solution (2.7 × 10 5 Ω sq −1 at 92% transmittance) 34) or chemically reduced graphene oxide (1.1 × 10 4 Ω sq −1 at 87% transmittance). 35) They were also comparable to those of the CNT transparent electrodes (3.8 × 10 3 Ω sq −1 at 87% transmittance). 30) Scanning electron microscopy (SEM) measurements of the transparent conductive films were conducted using ETHOS NX5000 (Hitachi, Ltd.) to reveal the origin of the conductivity difference between the transfer process and spray coating. Despite the smooth and uniform surface of the transferred films in Fig. 4(a), the spray-coated films obtained a fish scale-like shape [ Fig. 4(b)]. Figures 4(c) and 4(d) illustrate magnified SEM images of the transferred and spraycoated films, respectively. Many Ti 3 C 2 T x flake aggregates were confirmed in Fig. 4(d), implying the increase of the contact resistance in the electric conductive pathway. Some aggregates laying on the top surface did not contribute to the network formation.
For further structural information, the X-ray diffraction (XRD) measurement was conducted using SmartLab (Rigaku) through Cu Kα radiation [ Fig. 5(a)]. We also fabricated the Ti 3 C 2 T x film on a Si(100) substrate through the transfer process to omit the peaks from the PEN substrate at a lower angle region. The (002) peaks of Ti 3 C 2 T x on the transferred (2θ = 7.00°) and spray-coated (2θ = 5.54°) films indicated that the corresponding average interlayer distances between the flakes were 2.70 and 3.19 nm, respectively. The difference in the interlayer distances represented that the filtrate conditions provided a closer contact of the Ti 3 C 2 T x flakes with each other, consequently enabling the interconnect resistance reduction between the flakes. Compared to the peak from the spray-coated film, a broad band was observed in the transferred film spectrum. The green solid line in Fig. 5(a) depicts the diffraction pattern of the Ti 3 C 2 T x powder. The sharp peak at 6.12°meant that the Ti 3 C 2 T x flakes were highly stacked, and the interlayer distance was 2.89 nm at a powder state. Judging from the spectra shapes, the spray-coated sample morphology was similar to that of the powder sample and consistent with the SEM microscopic observations. Figure 5(b) illustrates a schematic image of the morphology of the transferred and spray-coated Ti 3 C 2 T x MXene films. The suction filtration flow forced the Ti 3 C 2 T x flakes to randomly stack with a slight tilt. The transferred Ti 3 C 2 T x sheet maintained the close interlayer distance and smooth surface network, enabling superior macroscale electric conductivity. In contrast, multilayer Ti 3 C 2 T x aggregates were generated when the condensed droplet on the surface was dried. It was difficult for them to provide a sufficient conductive pathway.
In conclusion, we successfully fabricated Ti 3 C 2 T x MXene transparent conductive films via the transfer process. The Ti 3 C 2 T x sheet prepared on the MCE membrane filter was directly transferred to the desired substrate through a simple drying method without the membrane dissolve technique. The transferred transparent conductive films showed ∼100 times lower sheet resistance than the conventional spraycoated samples. The film transparency and electrical conductivity were easily controlled by changing the amount of the MXene material. The SEM microscopic observations and XRD characterizations revealed that the transferred film has a smooth and uniform MXene flake network and a closer layer distance than the spray-coated sample, enabling superior electrical conductivity.