Step‐by‐Step Guide for Synthesis and Delamination of Ti3C2Tx MXene

To advance the MXene field, it is crucial to optimize each step of the synthesis process and create a detailed, systematic guide for synthesizing high‐quality MXene that can be consistently reproduced. In this study, a detailed guide is provided for an optimized synthesis of titanium carbide (Ti3C2Tx) MXene using a mixture of hydrofluoric and hydrochloric acids for the selective etching of the stoichimetric‐Ti3AlC2 MAX phase and delamination of the etched multilayered Ti3C2Tx MXene using lithium chloride at 65 °C for 1 h with argon bubbling. The effect of different synthesis variables is investigated, including the stoichiometry of the mixed powders to synthesize Ti3AlC2, pre‐etch impurity removal conditions, selective etching, storage, and drying of MXene multilayer powder, and the subsequent delamination conditions. The synthesis yield and the MXene film electrical conductivity are used as the two parameters to evaluate the MXene quality. Also the MXenes are characterized with scanning electron microscopy, x‐ray diffraction, atomic force microscopy, and ellipsometry. The Ti3C2Tx film made via the optimized method shows electrical conductivity as high as ≈21,000 S/cm with a synthesis yield of up to 38 %. A detailed protocol is also provided for the Ti3C2Tx MXene synthesis as the supporting information for this study.


Introduction
The growing palette of 2D materials has been a fast-expanding area in materials science owing to their outstanding physicochemical properties, such as large surface area, layer-dependent DOI: 10.1002/smtd.202300030 optical bandgap, carrier mobilities, mechanical flexibility, and thermal conductivity. [1,2] One of the additions to the list of 2D materials in the past decade is the family of transition metal carbides, nitrides, or carbonitrides (known as MXenes). [3,4] The general formula of MXenes is M n+1 X n T x , which denotes n+1 layers (n = 1-4) of one or more group 3-6 early transition metals (M), interleaved by n layers of carbon and/or nitrogen atoms (X), with T x denoting surface terminations (for example, -F, -O, -OH). [5] Most MXenes are typically synthesized from their corresponding three-dimensional (3D) bulk MAX phase precursors (M n+1 AX n , where "A" refers to group 11-16 atoms, such as Al, Si, or Ga). [6,7] The atomistic diversity of the MAX phases holds the key to the synthesis of novel MXenes for prospective applications. [8][9][10] A titanium carbide MXene (Ti 3 C 2 T x ) was the first synthesized MXene reported in 2011, [3] and has grown to comprise well over 70% of all studies on MXenes to date, [4] and has been explored in applications ranging from supercapacitors [11] photo/electrocatalysis [12] to urea adsorbents [13] and tribological nanogenerators. [14] This wide range of applications comes from flakes can be different from many perspectives, including the number of vacancies in the Ti and C sites, the substitution of oxygen in the carbon sites, the size of the flakes, surface termination compositions, and properties like electrical conductivity, and environmental stability. [40] The type of etchant and etching conditions (such as temperature and time) plays a vital role in controlling the kinetics of the selective etching of the MAX phase and, thus, the properties of the resulting MXene (flake size, flake surface chemistry, electrical conductivity). [23] For example, the higher concentration of HF-based etchants results in smaller flakes (∼few hundred nm) with more defects and higher content of fluoride-containing surface functional groups, whereas HF-HCl etching results in single-flakes with larger lateral sizes (≈3-50 μm) with fewer defects. [41][42][43] Beyond the selective etching parameters, the choice of intercalants in the delamination controls the interlayer spacing of the multilayer MXene and significantly affects the MXene flake sizes and quality. For instance, the intercalation of TMA + and TBA + leads to larger interlayer spacing and smaller flake sizes, [44] whereas metal cations result in smaller interlayer spacing due to their smaller ionic radius. [44] Furthermore, intercalating cation species also improves electrical conductivity by increasing the carrier density in MXenes. [39,45] Delamination processing methods, such as sonication, mechanical agitation (manual shaking or automatic shakers), and soft delamination via gentle stirring, can also control the flake size-dependent properties of MXenes. [39,44,46,47] Hence, the choice of precursor synthesis, etching, and delamination methods control the MXene flake size, defects, synthesis yield, surface functionalities (-F, -O, -Cl, or -OH), and properties (electrical conductivity, mechanical, and colloidal stability). [48] This has driven various investigations in developing the guidelines for the scalable synthesis of Ti 3 C 2 T x MXene. [23,38,39,44,47] In this study, we present a step-by-step guide on the synthesis routes of high-quality Ti 3 C 2 T x MXene known to date and provide experimental strategies to further optimize the process. We investigated the effect of Ti 3 AlC 2 MAX precursor while focusing on the optimized-Ti 3 AlC 2 MAX phase as the precursor of choice to make high-quality MXene. The optimized-Ti 3 AlC 2 MAX was made by the use of excess moles of aluminum and titanium in the mixed raw powder. We present the optimal conditions for preetch HCl processing of Ti 3 AlC 2 MAX, selective etching, storage of etched multilayered MXene, delamination parameters (temperature, reaction environment (air/argon)), and post-delamination processing by centrifugation to improve the yield and quality of resulting Ti 3 C 2 T x MXene. While focused on HF-HCl etching and delamination with LiCl, we compared the MXene quality with the commonly used synthesis method of HCl-LiF. Our focus on studying the effects of etching and delamination conditions is targeted to control the flake size, yield, and electrical conductivity of Ti 3 C 2 T x MXene. We use the bulk electrical conductivity of the MXene films as a rapid qualitative method to determine the quality of the resulting MXene. We also provide a detailed step-bystep guide for the synthesis of high-quality Ti 3 C 2 T x MXene as the Supporting Information. This detailed strategic study offers the optimal stepwise synthesis of Ti 3 C 2 T x MXene from optimized-Ti 3 AlC 2 MAX phase to single-to-few flakes MXene production to guide future development of Ti 3 C 2 T x MXene for large-scale applications. Figure 1. Schematic of Ti 3 AlC 2 MAX and Ti 3 C 2 T x MXene synthesis process. a) Ti 3 AlC 2 is synthesized via reactive pressureless sintering of elemental and carbide powders in an inert atmosphere at 1400°C. b) Ti 3 AlC 2 synthesis variables used in this study. c) After Ti 3 AlC 2 is synthesized and milled into fine powder (particle size < 71 μm), pre-etched cleaning can be used to dissolve the intermetallic impurities (TiAl 3 ), recommended when excess metals are used in the Ti 3 AlC 2 synthesis. d) When we used excess metals (reffered to as optimized-Ti 3 AlC 2 ), more than the required moles in Ti 3 AlC 2 synthesis, we dissolved the intermetallics via pre-etch cleaning in 9 M HCl at different temperatures and durations. e) Selective etching of the Al layers to synthesize Ti 3 C 2 T x MXene powders. f) The etching route used in our study. g) Delamination step to make single-flake Ti 3 C 2 T x from the etched multilayered powder. h) Our delamination variables for the synthesis of single-to-few layer Ti 3 C 2 T x MXene.

Experimental Variables
Before we discuss the findings of this study, it is crucial to lay out the key variables in the MXene synthesis and the parameters we have studied here. Figure 1 presents the four main steps in Ti 3 C 2 T x MXene synthesis: the optimized-Ti 3 AlC 2 MAX phase synthesis, pre-etch HCl wash of the MAX phase, selective A-layer etching of the MAX phase, and exfoliation of multilayered MXenes to single-to-few-layer flakes via delamination. Clearly, there are many variables, which cannot be thoroughly investigated in one study. Thus, we selected routes and variables that are known to lead to high-quality Ti 3 C 2 T x MXene. To understand the effect of each step, we modified one synthesis variable at a time and kept the rest constant (marked with red background in Figure 1). We measured the MXene synthesis yield and the free-standing MXene film electrical conductivity as two quick measures to identify MXene quantity and its quality. The detailed synthesis steps can be found in the experimental section at the end of this study and in the Supporting Information.

Known Methods
Ti 3 AlC 2 MAX phases can be synthesized from a wide range of raw materials, such as pure elemental powders, that is, Ti, Al, and C, or compounds containing metals and carbon, such as TiH 2 , TiC, Al 4 C 3 , or even from economic precursors, such as TiO 2 , scrap Al and tires (as carbon source). [49][50][51][52] The type of carbon can also be altered, such as graphite, carbides, or carbon fibers. [22,53] More recently, it was shown that the use of excess metal, including more than two times the moles of aluminum (instead of one mole), and 0.25 more moles of Ti, gives the highest purity Ti 3 AlC 2 MAX with the least amount of X-site substitutional impurities. However, the use of the stoichiometric ratio of pre-cursors (1Ti:1Al:2TiC) leads to the synthesis of an oxycarbide Ti 3 Al(C,O) 2 MAX phase with oxygen substitutional impurities in the X-sites. [25] All these variables control the synthesis yield and quality of the resulting MXene. The precursor powder mixture for a MAX synthesis is usually dry mixed in a horizontal tumbler mixer, 3D tumbling mixer, and v-shape ball mill for a few hours to 24 h or more in air, followed by reactive sintering at ≈1350 -1400°C under a controlled environment. [24,54] Different high-temperature reaction sintering methods can be utilized, such as tube furnace, [24] hot press, [52] hot isostatic press, [55] selfpropagating high-temperature synthesis (SHS), [56] microwave sintering, [49] and spark plasma sintering (SPS). [57]

Our Method Variables
For optimized-Ti 3 AlC 2 MAX, we selected the excess-metal route with Ti, TiC, and Al as our raw materials in a 1.25Ti:2.2Al:2TiC ratio and tumbler milled for 18 h in a jar mill with zirconia media at 60 RPM, followed by reactive sintering in a tube furnace at 1400°C for 4 h under an argon flow. We also made Ti 3 AlC 2 MAX using the stoichiometric mixing of 1Ti:1Al:2TiC mole ratio under the same conditions (labelled as regular-Ti 3 AlC 2 ) to compare the resulting Ti 3 C 2 T x MXene. We chose these two sintering routes as they have been proven to result in high-quality Ti 3 C 2 T x MXene. [15,24] The remaining MAX synthesis parameters, such as mixing and sintering parameters (temperature, ramp rates, and dwell times), were the same as described in the experimental methods section.

Known Methods
After the Ti 3 AlC 2 MAX is synthesized, the sintered block needs to be milled into fine powder. Usually, hand or drill press milling Small Methods 2023, 7,2300030 is used at the lab scales, followed by sieving to the desired particle sizes. The effect of particle size was studied earlier via different techniques, including sieving [58] and size-selection sedimentation procedure. [59] The latter is a method for separating large MAX particles when MXene with large lateral sizes is desired. When non-stoichiometric ratios of elemental powders are used (for example, excess metal), impurities such as intermetallic or carbide phases can form. Removing impurities can improve the etching effectiveness with more control over the resulting MXene. An acid pre-wash, for example, HCl, can dissolve the intermetallic phases before the selective etching step. [24]

Our Method Variables
When the excess metal powder mixture is used to synthesize Ti 3 AlC 2 MAX (labeled as optimized-Ti 3 AlC 2 ), the sintered block has a metallic finish and is relatively hard to be milled manually. We used a drill press with TiN-coated mill bits to prepare the MAX powder and sieved the resulting powder to < 71 μm particle size. To remove the intermetallic impurities from the optimized-Ti 3 AlC 2 MAX, we treated the powders for 18 h in 9 M HCl at room temperature (RT) while stirring. We examined the effect of the pre-etch HCl wash by decreasing the time to 4 h at room temperature and increasing the temperature and time to 55°C for 18 and 72 h and studied the synthesis yield and electrical conductivity of resulting Ti 3 C 2 T x MXene. As a point of comparison, we also investigated the resulting Ti 3 C 2 T x MXene synthesized without pre-etch HCl wash of optimized-Ti 3 AlC 2 MAX and with preetch HCl wash on regular-Ti 3 AlC 2 MAX.

Known Methods
As discussed in the introduction, several etching methods have been used for the selective etching of Al from Ti 3 AlC 2 MAX to synthesize multilayer Ti 3 C 2 T x MXene. The most used method to date is HF-etching, either pure HF, mixed HF with other acids [24] or in situ HF formation via a fluoride salt such as lithium fluoride (LiF) mixed with HCl. [60] The pure HF method is a straightforward method used when multilayer powder is needed. However, it is challenging to delaminate the HF-etched MXene, and organic molecules such as TMAOH are usually required to make single-flake MXenes. The mix of HF with another acid, such as HF-HCl, is the desired method to produce high-quality MXene with subsequent alkali cations intercalation and delamination using chloride salts, such as LiCl, NaCl, and KCl. The etching temperature range is usually 24 to 55°C, and the duration is ≈24 h for Ti 3 C 2 T x . Nitrogen bubbling supplied during etching has been reported to improve the quality of Ti 3 C 2 T x MXene with higher electrical conductivity. [61]

Our Method Variables
We used HF-HCl mixed acids at 35°C for 24 h as our standard etching method in this study. We also considered longer etching times of 48 and 72 h while keeping the other variables constant. As a point of comparison, we used the HCl-LiF method at 35°C for 24 h and compared the synthesis yield and electrical conductivity of the resulting MXene. The same etching route was used for regular-Ti 3 AlC 2 MAX to synthesize Ti 3 C 2 T x MXene as a control.

Known Methods
The choice of delamination process of the multilayer powders to single-flake Ti 3 C 2 T x MXene depends on the chosen etching route. For example, if etched with only HF, usually organic molecules, such as TMAOH, can be used in the delamination. While etching routes via alkali metal halides, such as LiF, do not require subsequent delamination steps. However, when a mix of HF and HCl is used for the etching, different metal halides can be used for the delamination. [24,39] When the HCl-LiF method is used, no further intercalation is needed because Li + can directly be used as a delamination agent, and after raising the pH to neutral, single-to-few layer flake MXene can be obtained. In the molten salt etching route, intercalants such as DMSO, [62] n-butyl lithium, [63] and TBAOH [28] are needed to make single-flake MXenes. Spontaneous delamination of multilayered Ti 3 C 2 T x MXene is explored by the use of aryl diazonium salts, which leads to the grafting of aryl groups on MXene surfaces. [64] In addition, multilayered Ti 3 C 2 T x MXene can also be delaminated and intercalated using phosphorous vapor evolved from red phosphorous. [65] MXene delamination is usually conducted in aqueous solutions at temperatures ranging from room temperature to 65°C.

Our Method Variables
In this study, we used 1 g of LiCl per gram of pre-etch HCl washed Ti 3 AlC 2 MAX, dissolved in 50 mL of deionized (DI) water, added etched Ti 3 C 2 T x MXene powder, and stirred at different temperatures and durations, to identify the best delamination conditions. In addition, a continuous inert gas flow was used during the delamination by supplying the aqueous solution with argon/nitrogen bubbling, as well as no inert gas bubbling for comparison. We also present the effect of etched MXene powder storage and drying before delamination on the yield and quality of the delaminated MXene flakes. Furthermore, to understand how the quality of Ti 3 C 2 T x MXene is affected by stirring speed during LiCl delamination, we conducted an optimized delamination study at different stirring rates. In order to improve the MXene synthesis yield, the effect of centrifugation speeds (3234 RCF vs 21913 RCF) on the delamination washes was investigated. Further, the flake size dependency on the processing of delaminated Ti 3 C 2 T x MXene was studied with respect to different centrifugation speeds (1500 RCF vs 2380 RCF).

Results and Discussion
First, we present the effect of MXene synthesis steps using the optimized-Ti 3 AlC 2 MAX phase, where only the discussed variables are changed, and other processing conditions remain constant. The optimized synthesis variables of this study are marked with red background in Figure 1. To summarize, the standard variables are pre-etch HCl washing of optimized-Ti 3 AlC 2 MAX in 9 M HCl at room temperature (24°C) for 18 h, selective etching in HF-HCl for 24 h at 35°C, and delamination using 1 M LiCl at 65°C for 1 h with argon bubbling. After displaying the optimized parameters of synthesis of Ti 3 C 2 T x MXene from optimized-Ti 3 AlC 2 MAX, we compare our results with Ti 3 C 2 T x MXene with other synthesis routes.

Effect of Pre-etch HCl Wash Conditions on the Quality of MXene
The pre-etch washing of optimized-Ti 3 AlC 2 MAX using HCl acid was carried out at four different conditions as presented in Figure 2a, (i) 4 h at room temperature (24°C), (ii) 18 at room temperature (24°C), (iii) 18 h at 55°C, and (iv) 72 h at 55°C. After the pre-etch HCl wash was complete, the samples were neutralized to pH 7 by repeated centrifugation with deionized water (≈250 mL/1 g of MAX) and the powder was vacuum-filtered using a 2 μm cellulose filter membrane. The mass of the pre-etch HCl-washed Ti 3 AlC 2 MAX powders was measured after drying in a vacuum oven at 200°C for 18 h (Figure 2b). It was observed that all four pre-etch HCl-wash conditions led to a mass loss, suggesting that HCl dissolved intermetallic impurities during the pre-etch-wash as was reported in the first study of optimized-Ti 3 AlC 2 MAX. [24] The x-ray diffraction (XRD) pattern of the optimized-Ti 3 AlC 2 MAX shows the peaks of Ti 3 AlC 2 with the presence of TiAl 3 intermetallic impurities ( Figure S1, Supporting Information). However, after pre-etch HCl wash for 18 h these impurity peaks were no longer present. When we used 4 h at room temperature for pre-etch HCl wash, the mass loss was the lowest (11.12 ± 1.08 %). The MAX powder pre-etch HCl-washed for 18 h at room temperature had a mass loss of 20.24 ± 1.5 %.
www.advancedsciencenews.com www.small-methods.com When a higher temperature (55°C) was used, the mass loss was increased to 22.42 ± 1.76 % and 25.93 ± 1.22 % for 18 and 72 h samples, respectively. The scanning electron microscopy (SEM) images and particle size distribution of the optimized-Ti 3 AlC 2 MAX and pre-etch HCl washed MAX are presented in Figure  S2 (Supporting Information). The SEM images of the pre-etch washed optimized-Ti 3 AlC 2 MAX samples show that prolonged exposure to HCl for more than 18 h results in smaller particle size (≈8.93 μm). It is also observed that after 72 h of exposure to HCl, the MAX particles show stepwise etching features on grain edges and opening of the MAX layers, which suggests non-selective etching ( Figure S3, Supporting Information).
To understand the effect of 4 h and longer pre-etch HCl wash time (> 18 h) on the resulting Ti 3 C 2 T x MXene, we etched all HCl pre-washed samples using the HF-HCl etching route at 35°C for 24 h with subsequent delamination with LiCl at 65°C with argon bubbling for 1 h. As a point of comparison, we investigated the effect of the no pre-etch HCl wash on optimized-Ti 3 AlC 2 , which was directly etched at 35°C for 24 h and delaminated using LiCl at 65°C with argon bubbling for 1 h. We first measured the yield of Ti 3 C 2 T x MXene from the four pre-etch HCl-washed methods ( Figure 2b). In this study, MXene yield is defined as the mass of the delaminated Ti 3 C 2 T x MXene divided by the mass of pre-etch HCl-washed optimized-Ti 3 AlC 2 . We further explain this method in detail in the experimental section. The yield of synthesized MXene was highest at ≈36.75 ± 2.19 % for the pre-etch HCl-washed optimized-Ti 3 AlC 2 at room temperature for 18 h as compared to HCl-washed Ti 3 AlC 2 at room temperature for 4 h (≈25.92 ± 1.81%), pre-etch HCl-washed Ti 3 AlC 2 at 55°C for 18 h (≈5.6 ± 0.6 %), and 72 h (≈2.5 ± 0.4 %).
The decrease in the yield with the increase in temperature and duration, indicates that the pre-etch HCl wash of the optimized-Ti 3 AlC 2 MAX may be partially etching and damaging the MAX particles in addition to dissolving the intermetallic phases. Previous studies agree with our findings here, as it has been shown that HCl can selectively etch Ti 3 AlC 2 via the hydrothermal method at 180°C temperature for 8-32 h. [66] As our method is not using higher-temperature hydrothermal methods, we would not necessarily expect full exfoliation of the MAX phase with HCl, which agrees with our results. The yield of MXene synthesized from optimized-Ti 3 AlC 2 MAX without a pre-etch HCl wash was ≈22.87 ± 2.29 %, which is comparable to the yield produced from 4-h pre-etch HCl acid washed MAX, as shown in Figure S4 (Supporting Information). The lower MXene synthesis yield(≈25.92 ± 1.81%) observed for the pre-etch HCl wash for 4 h compared to 18 h, is possibly due to the presence of intermetallics in the etching step, which interferes with the etching kinetics for Al removal from MAX.
Along these lines, although it is known that harsher etching conditions, such as higher concentration HF and prolonged etching, can damage the resulting MXene, [23,41,67,68] little is known about the effects of pre-etch processing of the Ti 3 AlC 2 powder. [69,70] To understand whether the pre-etch HCl wash of the Ti 3 AlC 2 powder could affect the resulting MXene, we made films from the delaminated MXene solutions (Figure 2a) via vacuumassisted filtration and measured the MXene films' electrical conductivities ( Figure 2c). The MXene films made from the 18 h pre-etch HCl-washed Ti 3 AlC 2 at room temperature exhibited a high electrical conductivity of ≈19 560 ± 1440 S/cm compared to films produced from the room temperature pre-etch HCl-washed for 4 h (≈14 360 ± 1370 S/cm), 55°C pre-etch HCl-washed for 18 h (≈10 460 ± 1340 S/cm), and 72 h (≈7450 ± 2050 S/cm). To best compare these values, we kept all the MXene synthesis steps identical, and the only variable in this section was the pre-etch HCl wash temperature and time. As a result, the observed reduction in electrical conductivity could be due to harsher pre-etch HCl wash conditions, which suggests that our reduced electrical conductivity might be a result of damage to the resulting MXene flakes and their alignment in film form.
To further understand the effect of Ti 3 AlC 2 HCl pre-wash on Ti 3 C 2 T x , we drop-casted diluted delaminated MXene solutions on a porous alumina membrane and compared the flake morphology using SEM (Figure 2d-g). The MXene flakes (Figure 2d) made from pre-etch HCl-washed MAX processed for 4 h at room temperature sample were found to be decorated with nano to micro-size particles. The subsequent free-standing film also had the same nature of contamination on its surface ( Figure  S5, Supporting Information). Similar observations were found on MXene flakes synthesized without pre-etch HCl wash of the MAX ( Figure S6, Supporting Information). In contrast, the MXene flakes made from pre-etch HCl-washed MAX processed for 18 h at room temperature had distinctive and straight edges with no observable particles or SEM-visible holes (Figure 2e). In contrast, MXene flakes derived from higher processing temperatures of pre-etch HCl-wash at 55°C for 18 h and 72 h (Figure 2f,g) had nanometer-size holes outlined in red in SEM micrographs, which may be due to HCl-induced over-etching. [41] In addition, the Ti 3 C 2 T x flakes produced using pre-etch HCl-wash at 55°C for 18 h and 72 h had more broken edges compared to the smooth flakes of the room temperature pre-etched HCl-washed sample for 18 h.
Since the etching and delamination conditions were kept identical for all Ti 3 C 2 T x MXenes, the results suggest that HCl itself, without the presence of HF and high temperature (> 100°C) [66,71] or applied potential, [72,73] can etch the Ti 3 AlC 2 particles. We investigated the effect of the pre-etch HCl wash on regular-Ti 3 AlC 2 and studied the yield and electrical conductivity of the resulting Ti 3 C 2 T x ( Figure S7, Supporting Information). The lower measured electrical conductivity (8460 ± 710 S/cm) could be attributed to the HCl damage and non-selective etching of regular-Ti 3 AlC 2 MAX during pre-etch HCl wash. Thus, HCl can lead to several defects in the final Ti 3 C 2 T x flakes, which reduces the quality of the synthesized MXenes. At the same time, further studies on the detailed mechanism of HCl interaction with Ti 3 AlC 2 are required. Based on our findings in this section, we believe that the optimized pre-etch processing conditions of the optimized-Ti 3 AlC 2 MAX are 9 M HCl at room temperature for 18 h.

Effect of Etching Duration
We next looked at the effect of HF-HCl etching duration while keeping all other variables the same. The pre-etch HCl-washed optimized-Ti 3 AlC 2 (18 h at room temperature) was selectively etched using 12 M HCl, deionized water, and 28.4 M HF in the 6:3:1 volume ratio. The etching reaction was carried out at 35°C with 400 RPM in an oil bath set up on the hot plate for (i) 24 h, (ii) 48 h, and (iii) 72 h, as presented in Figure 3a. The neutralized etched multilayered Ti 3 C 2 T x was then delaminated using LiCl at 65°C with argon bubbling for 1 h. As the first point of comparison, we measured the MXene yields. Figure 3b shows the yield of the resulting Ti 3 C 2 T x were ≈36.75 ± 2.19 %, ≈24.43 ± 2.69 %, and ≈4.47 ± 1.23 % for the 24 h, 48 h, and 72 h etch duration. The significant drop in the MXene yield with the increase in the etching time could be explained by the over-etching of Ti 3 AlC 2 particles that may occur where Ti and C atoms are attacked by acids, resulting in the dissolution of the smaller MAX particles and possibly making few-nm size MXene flakes which can be removed during the centrifugation and filtration steps. [23] The measured electrical conductivity (Figure 3c) for the resultant MXene dropped from ≈19 560 ± 1440 S/cm to ≈7640 ± 1890 S/cm to ≈4480 ± 580 S/cm with the increase in etching time of the MAX from 24 to 48 to 72 h, respectively. Moreover, the re-sultant MXene flakes with longer etching times had SEM-visible defects, such as holes (Figure 3e,f) than the MXene flakes made from the 24-h-etched MXene (Figure 3d). These holes (marked with red circles in Figure 3e,f) and broken flakes to smaller sizes could lead to higher inter-flake contact resistance leading to lower electrical conductivity. Accordingly, it is recommended to conduct selective etching of optimized-Ti 3 AlC 2 MAX at 35°C for a maximum of 24 h.

Effect of Storage Conditions on Etched MXene
A critical parameter in MXene synthesis that still has the opportunity for exploration for the study of MXenes for commercially viable and scalable manufacturing is the storage of the etched MXene powders before delamination. After the selective etching and synthesis of the multilayered MXene powder, the delamination process can be started immediately or with a delay, or the powder can be dried. In this section, we compare the effect of the storage conditions of the multilayered MXene powder before delamination on the resulting MXene. To do so, we investigated four storage conditions (Figure 4a) of the etched Ti 3 C 2 T x MXene powder before delamination (i) no storage (instant delamination), (ii) storage in DI water for 18 h at room temperature, (iii) storage in a vacuum chamber at room temperature for 18 h, and (iv) storage in a vacuum oven at 200°C for 18 h, after which the sample was delaminated using LiCl at 65°C with argon bubbling for 1 h.
The measurement of the resultant MXene yield is presented in Figure 4b, where a sharp drop can be noticed depending on the storage conditions. The MXene yield drops by ≈14% after  [21,74,75] which suggests the presence of water molecules between the flakes is critical for the successful delamination of MXene via the LiCl method. The annealed MXene clay with almost no water molecules in between the flakes could hinder the intercalation process of Li ions into the etched multilayered MXene. The lower MXene yield after storage in water for 18 h indicates that parameters other than water intercalation can also affect the delamination yield, such as degradation at the edge of the MXenes due to hydrolysis in water-based storage. [76,77] The degradation of MXene edges leads to fewer delaminated MXene single-to-few layer flakes. Our results indicate that storage of the etched MXene powder (delay in delamination) negatively affects the delamination process and subsequent MXene synthesis yield.
To further investigate the effect of multilayer MXene powder storage on MXene quality, we prepared MXene films from the single-to-few layers of Ti 3 C 2 T x MXene solutions derived from the instant and delayed delamination conditions, presented in Figure 4c. Obtained values suggest that a delay in delamination (storage of the MXene powder) can lead to MXene with lower electrical conductivity at ≈15090 ± 730 and ≈13200 ± 630 S/cm for delayed delamination of the films compared to ≈19560 ± 1440 S/cm for no delay. We also compared the Ti 3 C 2 T x singleflake morphology of delayed delamination via SEM (Figure 4d-f) with that of no-delay before delamination Ti 3 C 2 T x (Figure 2d). The flake morphology showed the least number of SEM-visible defects (for example, holes) when delamination was done immediately after the etching. Storage in water and partial hydrolysis and oxidation can be the reason for the appearance of smaller Ti 3 C 2 T x MXene flakes with more defects. As for vacuum storage, the removal of water molecules between MXene sheets of the multilayer powder reduced the efficiency of the uniform delamination and led to smaller flakes with more defects, which could be the reason for the lower electrical conductivity of the films (Figure 4c). Therefore, it is suggested to perform delamination instantly after etching optimized-Ti 3 AlC 2 MAX to achieve higher quality Ti 3 C 2 T x MXene.

Effect of Delamination Conditions (Temperature and Environment) Using LiCl
This section presents four delamination conditions while keeping the rest of the process the same with the previous optimized steps of HCl pre-etch washing and etching routes and then in-stantly delaminated the multilayered MXene sample to singleto-few layers of Ti 3 C 2 T x MXene using 0.47 M LiCl for 1 h. The delamination conditions were as follows: (i) room temperature without argon, (ii) room temperature with argon, (iii) 65°C without argon, and (iv) 65°C with argon (Figure 5a). After 1 h of delamination at the above conditions, the samples were washed with a total of ≈ 200-250 mL of deionized water per gram of preetch washed MAX via four rounds of centrifugation. After the fourth centrifugation and decantation, 15-20 mL of deionized water was added to the centrifuge bottle and vortexed mixed for 30 min, followed by centrifugation at 2380 RCF for 30 min. The supernatant was then collected by pipetting to separate singleto-few-layered MXene from the clay sediment. We explained all the steps in detail in the Synthesis Protocol, Supporting Information. The UV-Vis of the MXene solutions delaminated at different conditions is presented in Figure S8 (Supporting Information). The XRD analysis of the resultant free-standing MXenes films (annealed in a vacuum at 200°C for 24 h) prepared from the above-mentioned MXene solutions are presented in Figure  S9 (Supporting Information). [3,44] The Raman spectra of Ti 3 C 2 T x MXene films were collected using 785 nm laser with grating size of 1200 gr/mm and power 0.1% are presented in Figure S10 (Supporting Information). The MXene synthesis yields for the samples delaminated at room temperature without argon, room temperature with argon, 65°C without argon, and 65°C with argon were measured to be 33.58 ± 3.8 %, 30.46 ± 1.9 %, 35.1 ± 1.7 %, and 36.75 ± 2.19 %, respectively (Figure 5b). The synthesis yield was highest for the sample delaminated for 1 h at 65°C with argon, suggesting that temperature might play some role in slightly increased yield, while the argon environment can prevent edge-driven degradation of MXene flakes, supported by the SEM images showcasing the majority of broken small flakes (Figure S11, Supporting Information), from the samples prepared via delamination at room temperature and 65°C without argon. To further investigate the reasons for the slightly higher MXene yield in delamination without argon bubbling, we measured the MXene flake size distribution using dynamic light scattering (DLS) presented in Figure 5c. The colloidal suspensions of Ti 3 C 2 T x MXene (supernatant collected after 2380 RCF) were diluted with deionized water to prepare suspensions of 1 mg mL −1 concentration. The DLS results indicated that the MXene solutions delaminated at room temperature and 65°C without argon bubbling had slightly smaller flake sizes (≈2.5 μm) than MXene solutions delaminated with argon bubbling (Figure 5c and Table 1). Moreover, defective and broken flakes can be found on SEM images of Ti 3 C 2 T x delaminated at room temperature and 65°C without argon ( Figure S11, Supporting Information). We made another set of samples to investigate the smaller flake size resulting from delamination without argon bubbling and processed them at a lower centrifugal force, 1500 RCF, then measured the particle size distribution via DLS (Figure 5d). On average, the Ti 3 C 2 T x MXene solutions processed at 1500 RCF had larger flake sizes ( Table 1) than those of 2380 RCF processed solutions ( Figure S12, Supporting Information). A lower centrifugation speed of 1500 RCF led to polydisperse flake size distribution with ≈91 % larger flakes and ≈9 % smaller flakes (Figure 5d), while with 2380 RCF, a monodisperse flake size distribution was recorded ( Figure 5c). As DLS theoretically assumes spherical particles, we also measured the flake sizes of the MXene solutions using SEM and ImageJ analysis (Table 1), which aligned well with the estimated flake size using a hydrodynamic radius from DLS ( Figure S13, Supporting Information) and is in good agreement with the previous studies. [78,79] Additionally, we conducted atomic force microscopy (AFM) imaging of spin-coated MXene flakes on glass substrates from MXenes solutions delaminated at room temperature with and without argon, 65°C with and without argon, all centrifuged at 2380 RCF (Figure 6a-d). The AFM results showed more small and broken flakes (pointed by white arrows) for MXene solutions delaminated without argon bubbling at room temperature and 65°C (Figure 6a,c, respectively). On the other hand, the AFM images showed fewer small flakes for MXene solutions delaminated with argon at room temperature and 65°C. These results are consistent with the SEM and DLS measurements.
After analyzing the effects of delamination conditions on flake morphology and size distribution, we measured the electrical conductivity of MXene films prepared by vacuum-assisted filtration and after vacuum annealing at 200°C for 24 h (Figure 7a). In all cases, an increase in the electrical conductivity was observed after annealing of the Ti 3 C 2 T x films (Figure 7a), which can be explained by the removal of entrapped water molecules between Ti 3 C 2 T x MXene flakes, reducing the inter-flake distances and minimizing the inter-flake resistance [74,[80][81][82] (Figure S9 and S14, Supporting Information). Additionally, the results indicate that delamination with argon bubbling at 65°C gives the highest electrical conductivity of 19560 ± 1440 S/cm after annealing. Interestingly, comparing the samples delaminated with and without argon bubbling indicates that argon bubbling is a required step for delamination at 65°C, as the electrical conductivity of the 65°C without argon reaches only 13290 ± 2120 S/cm after annealing. However, argon bubbling has a less significant effect on MXene electrical conductivity when delaminated at room temperature, with electrical conductivities of 16850 ± 1990 S/cm and 14730 ± 2070 S/cm for with and without argon bubbling samples, respectively. Argon bubbling during the delamination can mitigate MXene degradation, which creates defects in MXene flakes, leading to lower electrical conductivity. A higher degradation rate is expected as the temperature rises to 65°C, hence, argon bubbling is more critical in this condition. Further, we also examined the possibility of replacing argon with nitrogen gas bubbling for optimized delamination conditions at 65°C. Our observations revealed that the synthesis yield and electrical conductivity for the MXene delaminated with nitrogen bubbling is comparable to the one delaminated with argon bubbling (Figure S15, Supporting  Information) and is a cheaper alternative to argon gas. We also explored the effect of stirring speed during LiCl delamination and performed experiments with optimized delamination conditions at different RPMs ( Figure S16, Supporting Information).
To eliminate the effect of inter-flake interactions, size distribution, and electrode contact resistance on measured MXenes electrical conductivity, we also measured conductivity in the visible and near-IR optical region of the spectrum (optical conductivity) of MXene thin film using spectroscopic ellipsometry (SE) as a non-contact method to compare the quality of the produced Ti 3 C 2 T x MXenes (Figure 7b). The optical conductivities were measured for two MXene flake film thicknesses, ≈1 ± 0.3 nm (single flakes with the least overlap) and ≈18 ± 2 nm (multilayer). Given the random distribution of MXenes on the frosted glass substrate used in these studies, this is an effective film thickness, representing an assemble-averaged value over a 4 mm 2 surface area of the sample (the SE beam size). A similar trend in the conductivity of various preparations was observed in these measurements for both effective film thicknesses (Figure 7b).
We note that as the thickness increases, the calculated optical conductivity drops, for example, from ≈7000 ± 3000 S/cm to ≈1100 ± 300 S/cm for MXene delaminated at 65°C with argon. This is mainly due to the indirect calculation of the optical conductivity/resistivity based on each sample's extinction coefficient and increased resistivity between stacked MXene layers in a multilayer film geometry. The error percentage of 1 ± 0.3 nm film (single flakes with the least overlap) is 30 %, compared with 11 % for the 18 ± 2 nm (multilayer) film. Consequently, the 1 ± 0.3 nm film resistivity shows larger error bars. The results for samples with the same film thickness and preparation method can be directly compared and offer a similar trend as direct conductivity measurements for these 4 different delamination methods.

Effect of Centrifugation Speed on Improving the Yield of MXene
To improve the yield of Ti 3 C 2 T x MXene, we used higher centrifugal forces to collect more MXene flakes. We observed the effect of centrifugation speed (i) 3234 RCF with a swing rotor and (ii) 21913 RCF with a fixed angle centrifugation on the MXene yield. Upon increasing the centrifugation force (speed) during the de-lamination washes, the yield of MXenes synthesized at different delamination conditions was increased ( Figure S17, Supporting Information). The highest yield for the Ti 3 C 2 T x MXene from the optimized-Ti 3 AlC 2 was ≈45 % via delamination at room temperature without argon bubbling and a 37 % yield for the 65°C with argon bubbling. The use of high-force centrifugation, even at ≈22000 RCF, did not increase the MXene yield beyond a few percent. This finding suggests that ≈2380 RCF is enough to synthesize high-quality Ti 3 C 2 T x MXene.

Comparison of Optimized Ti 3 C 2 T x MXene Synthesis with the State-of-the-Art Routes
In the last step of this study, we compared our results with the results of the state-of-the-art synthesis routes of Ti 3 C 2 T x . However, to eliminate synthesis and measurement variables from one study to another, we synthesized Ti 3 C 2 T x MXene using the common synthesis routes and compared the MXene film's electrical conductivities with the optimized synthesis route discussed in this study. For comparison, we synthesized Ti 3 C 2 T x MXene using (i) our optimized method here (HF-HCl etched and delaminated using LiCl for 1 h in an argon environment at 65°C) but starting from regular-Ti 3 AlC 2 MAX phase as the precursor instead of optimized-Ti 3 AlC 2 , (ii) typical HCl-LiF etching and delamination route with optimized-Ti 3 AlC 2 MAX phase as the precursor, (iii) optimized-Ti 3 AlC 2 MAX phase, HCl pre-washed and etched with HF-HCl but delaminated using LiCl for 18 h at room temperature as previously reported, [39] and (iv) our optimized method which uses optimized-Ti 3 AlC 2 with pre-etch HCl wash, etched with HF-HCl, and delaminated in LiCl for 1 h at 65°C with argon bubbling (Figure 1, red highlighted parameters). Free-standing films of these MXenes were prepared by vacuum-assisted filtration of the colloidal MXene solutions vacuum annealed at 200°C for 24 h, after which the films' electrical conductivities were measured (Figure 8).
As observed, the optimized method of synthesizing Ti 3 C 2 T x MXene using optimized-Ti 3 AlC 2 MAX phase followed by preetch HCl wash, etching with HF-HCl, and delamination using LiCl for 1 h in an argon environment at 65°C exhibited the highest electrical conductivity of ≈19560 ± 1440 S/cm. The electrical conductivity (≈11830 ± 870 S/cm) of the Ti 3 C 2 T x MXene synthesized from the regular-Ti 3 AlC 2 MAX phase was 39.5 % lower, which can be due to more defects and oxygen content present in the carbon sublattice of MXene. [25] While delaminating multilayered MXene synthesized for 18 h with LiCl, we observed a 38.1 % drop in electrical conductivity (≈12100 ± 900 S/cm) compared to the value obtained from our optimized protocol. The decrease in electrical conductivity is attributed to prolonged exposure of MXene to water, which damages MXene flakes and promotes edge-driven oxidation. However, this value was still higher than previously reported in the literature due to the use of optimized-Ti 3 AlC 2 MAX as the starting precursor and varied processing parameters (centrifugation speed and time, vortex duration). [39] Ti 3 C 2 T x MXene synthesized using HCl-LiF etching and delamination of optimized-Ti 3 AlC 2 MAX showed a 60 % drop in electrical conductivity (∼ 7900 ± 620 S/cm) due to the presence of more defective and small flakes. [

Conclusion
In summary, we have reported the stepwise optimized guidelines for the lab-scale synthesis of Ti 3 C 2 T x MXene (step-by-step protocol can be found in Supporting Information), which holds the potential to be scaled to manufacturing levels. We presented the optimal conditions for pre-etch processing of optimized-Ti 3 AlC 2 MAX, selective etching, storage of etched multi-layered MXene, delamination parameters (temperature, reaction environment (air/argon)), and post-delamination processing by centrifugation to improve the yield and quality of resulting Ti 3 C 2 T x MXene. We showed that the pre-etch HCl washing of optimized-Ti 3 AlC 2 for 18 h at room temperature was optimal for removing intermetallic impurities in the MAX phase. We optimized the selective etching conditions for optimized-Ti 3 AlC 2 MAX and observed that 35°C for 24 h was optimal for the complete etching of MAX. We demonstrated the importance of delaying the delamination step after etching and suggested instant delamination of the multilayered MXene for high yield and quality of resultant Ti 3 C 2 T x MXene. Furthermore, we examined the delamination conditions and showed the effect of temperature and reaction environment, which control the flake size, yield, and electrical conductivity of Ti 3 C 2 T x MXene. Our study suggests performing LiCl delamination at 65°C for 1 h under argon, or N 2 bubbling is optimal to yield single-to-few layered large Ti 3 C 2 T x MXene flakes with the highest electrical conductivity (≈19560 ± 1440 S/cm). Thus, using optimized-Ti 3 AlC 2 precursor MAX, with HCl pre-wash for 18 h at room temperature, HF-HCl etching for 24 h at 35°C, and immediate delamination with LiCl at 65°C with argon or nitrogen bubbling for 1 h is the method of choice for achieving high-quality Ti 3 C 2 T x flakes with high electrical conductivity and a minimum number of defects. While this method has ≈38 % yield with our current setup, regular-Ti 3 AlC 2 gives up to 80 % yield with lower MXene electrical conductivity. This detailed study established a better understanding of various experimental conditions and provided an optimal stepwise synthesis of high-quality Ti 3 C 2 T x MXene from the optimized-Ti 3 AlC 2 MAX phase, which is crucial to expand the future development of Ti 3 C 2 T x for largescale applications.
Methods-Synthesis of Ti 3 AlC 2 MAX: The Ti 3 AlC 2 MAX phase was synthesized using a modified MAX phase synthesis protocol with excess metal (aluminum and titanium). [24] The chemical powders of titanium carbide, titanium, and aluminum were mixed in a 2:1.25:2.2 molar ratio. The powders were then jar-milled in 250 mL Nalgene high-density polyethylene bottle (≈30 g batch) with yttria-stabilized zirconia balls for 18 h in 2:1 ball-topowder mass ratio without any shear at 60 RPM. Based on the total mass of the MAX, the size of the bottles can be varied. For instance, we used smaller bottles for smaller batches. The precursor powder was not sieved after jar-milling and then packed into an alumina crucible. The top of the crucible was covered with graphite foil and placed in a high-temperature tube furnace (Carbolite Gero, 1700°C model) for reactive pressureless sintering. Initially, the furnace was purged with argon (99.999 % purity) for 20 min at room temperature. After purging, the ball-milled precursor powders were heated to 1400°C at a 3.5°C min −1 ramp rate and held for 4 h under a constant argon flow. After 4 h, the furnace was cooled to room temperature at a 10°C min −1 rate. Then the alumina crucible containing sintered bock of optimized-Ti 3 AlC 2 MAX was removed and drilled to a fine powder using a TiN-coated drill bit in a drill press setup. For regular-Ti 3 AlC 2 MAX phase synthesis, the precursor powders of titanium carbide, titanium, and aluminum were mixed in a 2:1:1 molar ratio and followed similar steps for optimized-Ti 3 AlC 2 .
Methods-Synthesis of Ti 3 C 2 T x MXene: Effect of Pre-Etch HCl Wash Conditions: The optimized-Ti 3 AlC 2 MAX powder was sieved using a 71 μm sieve before Ti 3 C 2 T x MXene synthesis. The intermetallic impurities formed during the optimized-Ti 3 AlC 2 MAX synthesis were dissolved by an acid wash before proceeding with the selective etching of MAX. For 1 g of sieved optimized-Ti 3 AlC 2 MAX powder, 7.5 mL of 12 M HCl is added to 2.5 mL deionized water in a flat-bottomed glass bottle with a stir bar. The reaction bottle was kept in an ice bath, and the powder was added slowly at a rate of ≈0.5 g min to reduce the vigorous gas evolution during the HCl wash and kept stirring at 400 RPM. The effect of reaction time and temperature on removing impurities from optimized-Ti 3 AlC 2 MAX powder was for (i) 4 h at room temperature, (ii) 18 h at room temperature, (iii) 18 h at 55°C, and (iv) 72 h at 55°C. After adding MAX powder, it was recommended to monitor the reaction closely for at least 15-20 min to ensure no sudden gas evolution and vigorous changes. The reaction vessel should not be capped or closed during the reaction time, reducing the potential risk of gaseous pressure build-up. The acid-washed optimized-Ti 3 AlC 2 MAX was neutralized to pH 7 by repeated centrifugation in an Eppendorf centrifuge at 3234 RCF for 5 min with ≈200-250 mL deionized water per 1 g of MAX. Then the neutralized optimized-Ti 3 AlC 2 MAX solution was vacuum filtered using a 2 μm filter membrane. The vacuum filtration of the neutralized optimized-Ti 3 AlC 2 MAX solution takes ≈15-30 min for 1 g of MAX.
Methods-Synthesis of Ti 3 C 2 T x MXene: Effect of Etching Conditions: For 1 g of pre-etch HCl-washed optimized-Ti 3 AlC 2 MAX powder, 30 mL of etchant was used. The etchant solution was a mixture of 12 M HCl, deionized water, and 28.4 M HF in the 6:3:1 volume ratio. To prepare the etchant mixture, the subsequent addition of HCl was recommended to deionized water first and then adding HF, wearing all necessary protective equipment. The etchant was taken mixed in a 125 mL Nalgene highdensity polyethylene bottle with a magnetic stir bar. The MAX powder was then added slowly at a rate of ≈0.5 g min −1 . The effect of reaction time on the selective etching of the Al layer from optimized-Ti 3 AlC 2 MAX powder was investigated. The etching reaction was carried out at 35°C with 400 RPM in an oil bath on a hot plate for (i) 24 h, (ii) 48 h, and (iii) 72 h. The reaction vessel was capped with a condenser column on the top to release evolved gases during the reaction and prevent liquid evaporation. An in-house condenser column was utilized for our etching reactions using disposable plastic pipettes, as shown in Synthesis Protocol, Supporting Information. The etched multilayered Ti 3 C 2 T x MXene was washed to neutral pH with ≈200-250 mL per 1 g of precursor MAX with deionized water by repeated centrifugation in an Eppendorf centrifuge at 3234 RCF for 5 min. Then the neutralized etched multi-layered Ti 3 C 2 T x MXene solution was vacuum-filtered using a 0.8 μm filter membrane, which took ≈30 min. For immediate delamination, the neutralized etched multilayered Ti 3 C 2 T x MXene was directly used without vacuum filtration.
Methods-Synthesis of Ti 3 C 2 T x MXene: Effect of Delayed Delamination on Etched MXene: The etched multilayered Ti 3 C 2 T x MXene was delaminated to single-to-few layers of Ti 3 C 2 T x MXene using 0.47 M LiCl as an intercalant. The effect of deionized water, entrapped in between the layers of etched MXene, which affect the yield of delaminated MXene, was investigated. The experimental study focused on instant versus delayed delamination of etched multilayered Ti 3 C 2 T x MXene. To perform delayed delamination, the etched filtered Ti 3 C 2 T x MXene was stored for 18 h in (i) deionized water, (ii) a vacuum oven (Across International, AT09e) at room temperature, and (iii) annealed in a vacuum oven at 200°C.
Methods-Synthesis of Ti 3 C 2 T x MXene: Effect of Delamination Conditions (Temperature and Environment): The delamination reaction with LiCl was carried out in a 125 mL Nalgene high high-density polyethylene bottle with a magnetic stir bar for 1 g batch of the optimized-Ti 3 AlC 2 MAX powder. The reaction vessel was set up in an oil bath on the hot plate. For every 1 g of pre-etch HCl washed optimized-Ti 3 AlC 2 MAX powder, 50 mL of 0.47 M LiCl solution was used. The effects of delamination parameters such as temperature and reaction environment (air/argon) on the quality of MXene flakes were investigated. The detailed conditions of the delamination were (i) room temperature without argon (ii) room temperature with argon (iii) 65°C without argon, and (iv) 65°C with argon. The delamination reactions in the mentioned conditions were carried out for 1 h. An inlet hole was made in the cap of the reaction vessel to allow constant argon bubbling during delamination (Synthesis Protocol, Supporting Information).
The lithium intercalated Ti 3 C 2 T x MXene was washed with ≈200-250 mL deionized water for every 1 g of pre-etch washed optimized-Ti 3 AlC 2 MAX powder by repeated centrifugation and decantation at 3234 RCF for 5, 10, 15, and 20 min in an Eppendorf centrifuge. MXene clay was re-dispersed in deionized water and vortexed for 30 min, and then Ti 3 C 2 T x MXene colloidal suspension was processed by centrifugation at (i) 1500 RCF and (ii) 2380 RCF for 30 min to isolate single-to-few MXene layers. The supernatant containing single-to-few layered MXene was collected and redispersed in deionized water to obtain 1 mg mL −1 solution, which was vacuum filtered using a 0.25 μm filter membrane to prepare MXene freestanding films. The vacuum filtration of 15 mL of MXene colloidal solution with 1 mg mL −1 concentration took ≈6-8 h.
Methods-Synthesis of Ti 3 C 2 T x MXene: Effect of Centrifugation Speed on Improving the Yield of MXene: The washing with deionized water after the delamination process was carried out at a different centrifugal force to avoid loss of MXene flakes and improve the yield of Ti 3 C 2 T x MXene. The effect of centrifugation speed at (i) 3234 RCF in an Eppendorf centrifuge and (ii) 21913 RCF in the Thermo Scientific centrifuge was investigated. The yield was calculated using the following equation:  Characterization Methods-Flake Size Measurements Using DLS: Dynamic light scattering (DLS) was used to investigate the flake size distribution of Ti 3 C 2 T x MXene. 1 mg mL −1 of Ti 3 C 2 T x MXene solution was pipetted into a polystyrene cuvette and loaded into Zetasizer Nano ZS, Malvern Instruments. DLS average was taken over a total of three measurements from each sample. The flake size was measured using the following equation [79] : Flake size = 0.07 ± 0.03 × Diameter (1.5 ±0.15) hydrodyamic (2) Characterization Methods-Morphology Studies Using SEM: Fieldemission scanning electron microscopy (FESEM) was performed using a JEOL JSM-7800f FESEM with a lower electron detector at an acceleration voltage of 15 kV to study the flake size and surface morphology. The solution concentration was maintained at < 0.1 mg mL −1 and loaded on an anodic disc, followed by vacuum drying for 2 h. The samples were gold sputtered to reduce the charging and improve the sharpness of SEM images. Image J software was used to calculate the average flake size from the FESEM images.
Characterization Methods-AFM Measurements: MXene films were spun-cast from 1 mg mL −1 solution on frosted glass substrates (Laurell WS 400BZ-6NPP/Lite spin coater). Before deposition, all substrates were cleaned with acetone, ethanol, and deionized water. After drying with nitrogen gas, the substrates were exposed to oxygen plasma for 10 min. All colloidal solutions were bath-sonicated for 2 min before spun-casting. Typically, films were made on 4 cm 2 substrates using 100 mL solution via a two-step spun casting at 500 RPM for 60 s, followed by 2000 RPM for 10 s. The resulting films were vacuum dried at 200°C for 24 h before characterization. AFM imaging was performed in tapping mode using a Bruker Icon AFM (Bruker, Santa Barbara, CA) instrument. TAP300AL-G-50 (Budget Sensors) tips were utilized with a tapping frequency of 300 kHz.
In these experiments, a multi-incidence angle (55°, 60°, 65°, 70°, 75°) measurement mode was adopted to measure the ellipsometric spectra. The measured and best-fitted spectroscopic angles Psi and Delta as a function of wavelength are shown in Figure S9 (Supporting Information). The film thickness and MXene optical resistivity were deduced from modeling the ellipsometric results using the CompleteEase (6.69 version) software package. For modeling, first a Kramers-Kronig constrained B-spline model was adopted to embody the dielectric dispersion of Ti 3 C 2 T x films, initiated with a value for the film thickness as measured by AFM. Then, a generalized-oscillator (Gen-Osc) model combining two Harmonic and one Drude oscillators ( Figure S18, Supporting Information) was used to parameterize the estimated dielectric functions obtained by the B-spline fitting. The model was then used to fit the multi-angle data at all wavelengths, and the deduced thickness was consistent with the AFM. The resulting parameters from the Drude oscillator were used to evaluate the conductivity and resistivity based on the dielectric constant in the visible and near-IR optical regions of the spectrum, thus named optical conductivity and optical resistivity here. The uniqueness of all fitted parameters and the results from different models were compared to avoid overfitting, and when necessary fewer fitting parameters were used. The details of the SE modeling for Ti 3 C 2 T x and other MXenes can be found in our previous work.
Characterization Methods-Electrical Conductivity Using the 4-Probe Station: For measuring the electrical conductivity, Ti 3 C 2 T x MXene films (weight ≈15 mg)were prepared by vacuum-assisted filtration of single-tofew layered MXene colloidal suspension processed at 2380 RCF using a 0.25 μm filter membrane. Then the Ti 3 C 2 T x MXene films were annealed in a vacuum oven at 200°C for 24 h. A four-point probe with 1 mm separation connected to a 2400 Keithley Source meter was employed to measure the film resistance. Using the thickness from SEM images and sheet resistance from the four-point probe, the electrical conductivity of the film was calculated using the following equation: Electrical conductivity = 1 4.532 × resistance (Ω) × thickness (cm) Characterization Methods-XRD, UV-Visible Spectra, and Raman Spectra: The phase purity of Ti 3 C 2 T x MXene and composition were analyzed using a Bruker D8 X-ray diffractometer (XRD) with a Cu K ( = 1.5406 Å) emitter and a VANTEC 500 detector. The Ti 3 C 2 T x MXene films were made via vacuum filtration of single-to-few-layer solution, which were then annealed in a vacuum oven at 200°C for 18 h to remove the water entrapped between the layers. The samples were then mounted on Kapton tapes and scanned from 5°to 80°with a step size of 5°and a time per step of 30 s. The traditional XRD plots were obtained by merging and integrating the XRD 2 data in DIFFRAC.SUITE EVA software. The absorption spectra of Ti 3 C 2 T x MXene were collected using a Varian-Cary 50 Scan UV−vis spectrophotometer with 1 cm quartz cuvettes over a range of 200−1100 nm. The concentration of the Ti 3 C 2 T x MXene was maintained at < 0.1 mg mL −1 . All spectra were recorded in deionized water, also used as a background for all measurements. The Raman spectra of Ti 3 C 2 T x MXene films were collected using XploRA PLUS Raman microscope from Horiba scientific. For Raman spectra, 785 nm laser was used with grating size of 1200 gr mm −1 and power 0.1 %.
Statistics: In the presented study, each set of experiments shown in Figures 2, 3, 4, 5, 7, and 8 were performed five times to ensure the reproducibility of the results. The average yield was calculated from the five sets of experiments, and the standard deviation was determined. The standard deviation was then plotted as the error bars on the presented graphs. For the electrical conductivity measurements, the sheet resistance and thick-ness of the film were measured randomly from five different points during each experiment. The average values of the sheet resistance and thickness were calculated for each experimental set, and from this, the average electrical conductivity was obtained. The five average values of electrical conductivity obtained from the experiments were then used to calculate the standard deviation and error bars.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.