---
title: 'Cl-Terminated MXenes: Synthesis & Properties'
url: https://www.emergentmind.com/topics/cl-terminated-mxene
type: topic
---

# Cl-Terminated MXenes: Synthesis & Properties

Searching arXiv for the cited Cl-terminated MXene papers to ground the article in current indexed records.
arxiv_search({"query":"Cl-terminated MXene Ti3C2Cl2 1901.05212 2102.10262 2006.09578 2302.07720 2201.00298", "max_results": 10, "sort_by": "relevance"})
Search results reviewed; fetching a broader query to ensure coverage of synthesis, structure, oxidation, superconductivity, and exfoliation work on Cl-terminated MXenes.
arxiv_search({"query":"\"Cl-terminated MXene\" OR Ti3C2Cl2 OR Ti2CCl2 OR Nb2CClx", "max_results": 10, "sort_by": "lastUpdatedDate"})
Cl-terminated MXene denotes a class of two-dimensional transition-metal carbides and nitrides whose exposed metal surfaces are predominantly capped by chloride ligands after removal of the A layer from a parent MAX phase. A convenient formula is $M_{n+1}X_n\mathrm{Cl}_2$, where $M$ is an early transition metal, $X$ is C or N, and “$\mathrm{Cl}_2$” denotes two chloride terminations per $M_{n+1}X_n$ unit. Experimentally established members include $\mathrm{Ti}_2\mathrm{C}\mathrm{Cl}_2$, $\mathrm{Ti}_3\mathrm{C}_2\mathrm{Cl}_2$, and $\mathrm{Nb}_2\mathrm{CCl}_x$, with the first demonstrations of fully saturated Ti-based examples emerging from Lewis acidic molten salts and later extensions using molten-salt-assisted electrochemical etching, chlorosilane vapor etching, and CdCl$_2$-based routes [1901.05212; 1901.05120; 2102.10262; 2201.00298; 2509.11380]. The topic is defined by unusually ordered halide termination layers, fluorine-free processing routes, and a surface chemistry that modifies interlayer spacing, electronic structure, thermal stability, and downstream reactivity.

## 1. Definition, compositional scope, and identification

Cl-terminated MXenes are most explicitly represented by $\mathrm{Ti}_2\mathrm{C}\mathrm{Cl}_2$ and $\mathrm{Ti}_3\mathrm{C}_2\mathrm{Cl}_2$, for which fully saturated, single-component Cl termination was established by atomic-resolution HAADF-STEM imaging, image simulations, lattice-resolved EDX mapping, EELS, XRD, and XPS [1901.05212; 1901.05120]. In Ti-based systems synthesized from Lewis acidic melts, the reported Ti:Cl ratios are very close to $2\!:\!2$ for Ti$_2$C and $3\!:\!2$ for Ti$_3$C$_2$, consistent with $\mathrm{Ti}_2\mathrm{C}\mathrm{Cl}_2$ and $\mathrm{Ti}_3\mathrm{C}_2\mathrm{Cl}_2$ [1901.05212]. In molten-salt-assisted electrochemical etching of Ti$_3$AlC$_2$, the dominant product is likewise described as Ti$_3$C$_2$Cl$_2$, with XPS giving Ti:Cl $\approx 2.94\!:\!2$ and EDS giving Ti/Cl atomic ratio $\approx 1.85$ [2102.10262]. Vapor-phase chlorosilane etching explicitly produced $\mathrm{Ti}_2\mathrm{C}\mathrm{Cl}_2$ and $\mathrm{Ti}_3\mathrm{C}_2\mathrm{Cl}_2$, while CdCl$_2$ etching produced superconducting $\mathrm{Nb}_2\mathrm{CCl}_x$, modeled in DFT as $\mathrm{Nb}_2\mathrm{CCl}_2$ with $x \approx 2$ [2509.11380; 2201.00298].

Identification of Cl termination relies on convergent structural and spectroscopic signatures. In Ti$_3$C$_2$Cl$_2$, XRD shows the Ti$_3$AlC$_2$ (002) peak at $2\theta = 9.8^\circ$ shifting to $8.1^\circ$ after MS-E-etching, increasing the $c$-lattice parameter from $18.2$ Å to $21.9$ Å [2102.10262]. In ZnCl$_2$-derived Ti$_3$C$_2$Cl$_2$, the (0002) peak appears at $2\theta = 7.94^\circ$, with $c = 22.24$ Å [1901.05120]. XPS resolves Ti–Cl bonding directly: Ti–Cl $2p_{3/2}$ at $457.4$ eV in MS-E-etched Ti$_3$C$_2$Tx, Ti–Cl $2p_{3/2}$ at $457.8$ eV and Cl $2p_{3/2}$ at $199.82$ eV in Si-coated Ti$_2$CCl$_2$, and Ti–Cl $2p_{3/2}$ at $458.1$ eV with Cl 2p peaks at $198.6$ eV and $200.1$ eV in ZnCl$_2$-derived Ti$_3$C$_2$Cl$_2$ [2102.10262; 2509.11380; 1901.05120].

A frequent misconception is that any detected chlorine in MXene automatically proves an exclusive basal-plane termination. Atom probe tomography of wet-etched Ti$_3$C$_2T_x$ detected Cl at $2.49$ at% within the MXene region of interest, but the study states that APT “does not resolve whether Cl is bound as a surface termination (T = Cl) on basal planes or edges, intercalated between layers, or present as residual species associated with byproducts” [2305.19822]. That caveat applies to wet-chemical Ti$_3$C$_2T_x$, not to the ordered Ti$_2$CCl$_2$ and Ti$_3$C$_2$Cl$_2$ structures established by STEM and image simulation in molten-salt-derived systems.

## 2. Synthetic routes and reaction frameworks

Several distinct synthetic strategies generate Cl-terminated MXenes, but they share a common logic: selective A-layer extraction coupled to chloride delivery at exposed metal sites. In the original ZnCl$_2$-based replacement route, Al-MAX powders were mixed with ZnCl$_2$ and heated at $550\,^\circ\mathrm{C}$ for $5$ h under Ar; an Al-MAX:ZnCl$_2$ ratio of $1\!:\!1.5$ yielded Zn-MAX phases, whereas excess ZnCl$_2$ at $1\!:\!6$ enabled exfoliation to Ti$_3$C$_2$Cl$_2$ and Ti$_2$CCl$_2$ [1901.05120]. In the related Ti$_2$AlC/Ti$_3$AlC$_2$ Lewis acidic melt route, powders were mixed with ZnCl$_2$ and heated at $550\,^\circ\mathrm{C}$ for $5$ h, then cleansed with aqueous HCl and washed with deionized water, producing fully saturated Ti$_2$CCl$_2$ and Ti$_3$C$_2$Cl$_2$ [1901.05212]. Molten-salt-assisted electrochemical etching used a LiCl–KCl eutectic (1:1 wt%) at $450\,^\circ\mathrm{C}$, a typical applied cell voltage of $2.0$ V, and a total etching time of $24$ h, with the process constrained so that the anode potential remained lower than $0.91$ V vs Ag/AgCl to avoid Ti dissolution; the reported overall etching yield was $\approx 94.6\%$ [2102.10262]. Chlorosilane vapor etching used SiCl$_4$ as a Lewis acid, with a $4\!:\!5$ Al-MAX:SiCl$_4$ ratio favoring formation of Cl-terminated MXene plus amorphous Si, while CdCl$_2$ etching of Nb$_2$AlC used a $10\!:\!1$ CdCl$_2$:Nb$_2$AlC ratio, ball milling for $12$ h, then heat treatment at $300\,^\circ\mathrm{C}$ for $8$ h followed by $750\,^\circ\mathrm{C}$ for $36$ h [2509.11380; 2201.00298].

| Route | Representative products | Key conditions/features |
|---|---|---|
| Lewis acidic ZnCl$_2$ melt | Ti$_3$C$_2$Cl$_2$, Ti$_2$CCl$_2$ | Al-MAX:ZnCl$_2$ = $1\!:\!6$, $550\,^\circ\mathrm{C}$, $5$ h, Ar [1901.05120] |
| Lewis acidic melt from Ti$_2$AlC/Ti$_3$AlC$_2$ | Ti$_2$CCl$_2$, Ti$_3$C$_2$Cl$_2$ | ZnCl$_2$, $550\,^\circ\mathrm{C}$, $5$ h; post-HCl wash [1901.05212] |
| MS-E-etching in LiCl–KCl | Ti$_3$C$_2$Cl$_2$ | LiCl–KCl (1:1 wt%), $450\,^\circ\mathrm{C}$, $2.0$ V, $24$ h, yield $\approx 94.6\%$ [2102.10262] |
| SiCl$_4$ vapor etching | Ti$_2$CCl$_2$, Ti$_3$C$_2$Cl$_2$ | Al-MAX:SiCl$_4$ = $4\!:\!5$ favors MXene + Si(s) [2509.11380] |
| CdCl$_2$ molten salts | Nb$_2$CCl$_x$ | $300\,^\circ\mathrm{C}$/8 h, then $750\,^\circ\mathrm{C}$/36 h, Ar with 5% H$_2$ [2201.00298] |

The reaction chemistry is explicit in several cases. For chlorosilane etching, the generic relation is
$$
M_{n+1}AX_n + \mathrm{SiCl}_4 (g) \rightarrow M_{n+1}X_n\mathrm{Cl}_2 + A\mathrm{Cl}_3 (g) + \mathrm{Si} (s),
$$
with Ti-containing examples
$$
4\,\mathrm{Ti}_2\mathrm{AlC} + 5\,\mathrm{SiCl}_4 (g) \rightarrow 4\,\mathrm{Ti}_2\mathrm{C}\mathrm{Cl}_2 + 4\,\mathrm{AlCl}_3 (g) + 5\,\mathrm{Si} (s),
$$
and
$$
4\,\mathrm{Ti}_3\mathrm{AlC}_2 + 5\,\mathrm{SiCl}_4 (g) \rightarrow 4\,\mathrm{Ti}_3\mathrm{C}_2\mathrm{Cl}_2 + 4\,\mathrm{AlCl}_3 (g) + 5\,\mathrm{Si} (s).
$$
A redox-potential model was proposed to rationalize why Ti-containing MAX phases can yield Cl-terminated MXenes under SiCl$_4$, whereas Nb/Ta/Cr/V systems favor Si-substituted MAX phases under comparable conditions [2509.11380].

For MS-E-etching, the reported overall scheme is
$$
\mathrm{Ti}_3\mathrm{AlC}_2 + (3+x)\,\mathrm{LiCl}\,(\mathrm{KCl}) \rightarrow \mathrm{Ti}_3\mathrm{C}_2\mathrm{Cl}_x + (3+x)\,\mathrm{Li}\,(\mathrm{K}) + \mathrm{AlCl}_3(l),
$$
with AlCl$_3$ volatility providing the thermochemical driving force for Al removal and Cl$^-$ supplying the termination chemistry [2102.10262]. In ZnCl$_2$ melts, the exfoliation step was written as
$$
\mathrm{Ti}_3\mathrm{ZnC}_2 + \mathrm{ZnCl}_2 \to \mathrm{Ti}_3\mathrm{C}_2\mathrm{Cl}_2 + 2\,\mathrm{Zn},
$$
which the authors described as conceptually analogous to HF etching but mediated by Zn$^{2+}$ extraction and Cl$^-$ termination rather than H$^+$ extraction and F$^-$ termination [1901.05120].

## 3. Atomic structure, termination registry, and interlayer geometry

Atomic-resolution microscopy and first-principles calculations show that Cl occupies ordered surface sites rather than a random adlayer. In Ti$_2$CCl$_2$ and Ti$_3$C$_2$Cl$_2$, plan-view STEM images display honeycomb Ti arrangements, and contrast analysis indicates that Cl preferentially occupies fcc hollow sites, not the hcp hollow or atop sites [1901.05212]. HAADF-STEM image simulations with Cl on fcc sites reproduce the experimental intensity distributions for both Ti$_2$CCl$_2$ and Ti$_3$C$_2$Cl$_2$, and cross-sectional imaging from both $[11\bar{2}0]$ and $[1\bar{1}00]$ orientations confirms fcc-site occupancy [1901.05212]. DFT independently found that fcc hollow sites are favored over hcp or mixed configurations for both Ti$_2$C and Ti$_3$C$_2$ [1901.05212].

A defining structural feature of Cl termination is its large vertical offset from the outer Ti layer. Experimentally, the vertical separation between the Cl termination layer and the nearest Ti layer is $\sim 1.71$ Å for Ti$_2$C and $\sim 1.84$ Å for Ti$_3$C$_2$, corresponding to Ti–Cl bond lengths of $\sim 2.45$ Å and $\sim 2.55$ Å when $a \approx 3.05$ Å is used [1901.05212]. DFT gives Ti–Cl vertical separation $1.6766$ Å for Ti$_2$C and $1.7022$ Å for Ti$_3$C$_2$, with Ti–Cl bond length $\approx 2.51$ Å for both [1901.05212]. Prior work on pristine O/F-terminated Ti$_3$C$_2$ gives Ti–O/F $\approx 2.11$ Å and termination–Ti layer separation $\approx 1.23$ Å, so Cl sits significantly farther from the Ti surface [1901.05212]. This suggests increased local surface corrugation and, in multilayer stacks, a larger average interlayer spacing that can benefit ion accessibility.

The crystallographic response is consistent across synthesis routes. In MS-E-etched Ti$_3$C$_2$Cl$_2$, the (002) peak shift from $9.8^\circ$ to $8.1^\circ$ corresponds to $c \approx 21.9$ Å [2102.10262]. In ZnCl$_2$-derived Ti$_3$C$_2$Cl$_2$, the (0002) peak at $2\theta = 7.94^\circ$ corresponds to $c = 22.24$ Å, close to the DFT value of $22.34$ Å [1901.05120]. In the SiCl$_4$-derived Ti$_2$CCl$_2$ composite, the (002) reflection appears at $2\theta \approx 10.2^\circ$, giving $d_{002} \approx 8.7$ Å when Cu K$\alpha$ radiation is used [2509.11380]. Cl-terminated sheets are also reported to be laterally translated relative to each other such that fcc sites of adjacent layers do not stack directly atop one another, unlike common O-terminated multilayers [1901.05212].

Spectroscopy supports the structural model. In situ EELS from Ti$_3$C$_2$Cl$_2$ shows the Cl-$L_{2,3}$ edge at $\sim 200$ eV and Ti-$L_{2,3}$ at $\sim 450$ eV [1901.05212]. In MS-E-etched Ti$_3$C$_2$Cl$_2$, $^{13}$C NMR shifts from $566$ ppm in Ti$_3$AlC$_2$ to $350$ ppm in Ti$_3$C$_2$Tx [2102.10262]. XPS shows no Al in the SiCl$_4$-derived Ti$_2$CCl$_2$ composite and no F 1s signal above the detection limit in Nb$_2$CCl$_x$ [2509.11380; 2201.00298].

## 4. Electronic structure, bonding strength, and thermal behavior

Cl-terminated Ti-based MXenes are metallic. DFT calculations on Ti$_2$CCl$_2$ and Ti$_3$C$_2$Cl$_2$ show finite density of states at the Fermi level, with the Fermi level dominated by Ti $d$-orbitals and strong hybridization between Ti $d$ and C/Cl $p$ states between $-2$ and $-7$ eV [1901.05212]. Ti$_3$C$_2$Cl$_2$ has more Ti and C bands, and a small gap present around $-2$ eV in Ti$_2$CCl$_2$ is closed in Ti$_3$C$_2$Cl$_2$ [1901.05212]. Formation energies quantify strong binding:
$$
E_{\mathrm{form}} = E(\mathrm{Ti}_{n+1}\mathrm{C}_nT_2) - E(\mathrm{Ti}_{n+1}\mathrm{C}_n) - E(T_2),
$$
with $E_{\mathrm{form}} = -6.901$ eV per formula unit for Ti$_2$CCl$_2$ and $-6.694$ eV per formula unit for Ti$_3$C$_2$Cl$_2$ [1901.05212]. For comparison, Ti$_3$C$_2$O$_2$ was reported as $-9.656$ eV/f.u. and Ti$_3$C$_2$N$_2$ as $-2.596$ eV/f.u., so Cl binds more strongly than N but less strongly than O [1901.05212].

Thermal robustness is one of the most distinctive features of Cl termination. In situ STEM/EELS heating experiments on Ti$_3$C$_2$Cl$_2$ showed that Cl terminations remain largely intact up to $\sim 750\,^\circ\mathrm{C}$ [1901.05212]. Above $\sim 750\,^\circ\mathrm{C}$, Cl gradually desorbs, most markedly at thin multilayer edges, consistent with diffusion-limited desorption; at $800\,^\circ\mathrm{C}$ only residual Cl remains, the lamination persists, and nanoscale voids appear, interpreted as microstructural damage due to local gas evolution as Cl leaves [1901.05212]. The reported stability hierarchy is O $>$ Cl $\approx$ F [1901.05212]. A separate molten-salt reduction study further showed that Ti$_3$C$_2$Cl$_x$ annealed in eutectic LiCl–KCl at $500\,^\circ\mathrm{C}$ under inert atmosphere without Na exhibited no structural or compositional change, supporting the conclusion that CdCl$_2$-derived Cl terminations are stable at $500\,^\circ\mathrm{C}$ in LiCl–KCl without a reductant [2507.05741].

Cl also acts as an electronically consequential surface group. In Ti$_3$C$_2$Cl$_x$, the authors of the Na-mediated reduction study state that “−Cl terminations, with high electronegativity ($\chi_{\mathrm{Cl}} \approx 3.16$ vs. $\chi_{\mathrm{Ti}} \approx 1.54$), extract electron density from outer Ti sites, lowering near-Fermi electron density and increasing work function” [2507.05741]. In that work, the work function decreased from $4.25$ eV for Ti$_3$C$_2$Cl$_x$ to $3.79$ eV after Na-mediated dechlorination, with carrier concentration increasing by $392.49\%$, mobility by $162.85\%$, and conductivity by $1196\%$ under the optimal reduction condition [2507.05741]. These values do not characterize Cl-terminated MXene itself as a poor conductor in every context; rather, they isolate the electronic role of Cl as an electron-withdrawing terminal in that specific Ti$_3$C$_2$Cl$_x$ system.

## 5. Functional properties and application-specific behavior

The large Cl–Ti separation and preserved metallicity make Cl-terminated MXenes relevant to electrochemical transport, but the performance outcome depends strongly on the device mechanism. For Ti-based systems, the unusually large Ti–Cl vertical separation “suggests enhanced interlayer spacing in multilayers and improved ion accessibility,” and theory predicts “a very large voltage window (3.5–4 V) for exclusively Cl-terminated Ti$_2$C” [1901.05212]. In MS-E-etched Ti$_3$C$_2$Cl$_2$, Cl is the direct intermediate in one-pot conversion to O- and S-containing terminations: after etching, a pellet of Li$_2$O or Li$_2$S is added directly into the same molten LiCl–KCl bath, and XPS shows disappearance of Ti–Cl peaks with emergence of Ti–O or Ti–S/Ti–O components [2102.10262]. The O-terminated product exhibited capacitance of $225$ and $205$ F/g at $1$ and $10$ A/g, with retention above $100\%$ after $10{,}000$ cycles at $10$ A/g [2102.10262]. Here, Cl termination functions as a controllable precursor state rather than the final electrochemically optimized state.

By contrast, in aqueous Zn-ion batteries, Ti$_3$C$_2$Cl$_2$ showed no distinct redox peaks and a quasi-linear charge/discharge profile, with discharge capacity $46.5$ mAh g$^{-1}$ at $0.5$ A g$^{-1}$ [2006.09578]. Ti$_3$C$_2$(OF) behaved similarly, while Br- and I-containing MXenes exhibited distinct discharge platforms and higher capacities, including $97.6$ mAh g$^{-1}$ for Ti$_3$C$_2$Br$_2$ and $135$ mAh g$^{-1}$ for Ti$_3$C$_2$I$_2$ [2006.09578]. The paper attributes the lower conversion activity of Cl to stronger Ti–Cl bonding ($E_f = -6.694$ eV) and smaller interlayer spacing ($c = 20.90$ Å) relative to Br and I [2006.09578]. This corrects another common overgeneralization: Cl termination expands the MXene property space, but it is not the universally best terminal for every electrochemical duty cycle.

Cl termination also enables functionalities outside conventional energy storage. Nb$_2$CCl$_x$ is superconducting, with Meissner onset and zero-resistivity-derived $T_c \approx 5.2$ K, $H_{c1}(0) \approx 0.283$ T, $H_{c2}(0) \approx 3.3$–$3.6$ T, coherence length $\xi(0) \approx 96.08$ Å, penetration depth $\lambda(0) \approx 231.36$ Å, and Ginzburg–Landau parameter $\kappa_{GL} \approx 2.41$, establishing Nb$_2$CCl$_x$ as a type-II superconductor [2201.00298]. DFT gave electron–phonon coupling constant $\lambda = 0.63$ and $T_c \approx 5.3$ K for $\mu^\ast = 0.10$, while the F-terminated analogue was dynamically unstable with imaginary phonons at the M point [2201.00298].

Cl-terminated Ti$_3$C$_2$ can also serve as a chemically addressable precursor for topological reconstruction. A “simple topological reaction between chlorine-terminated MXenes and selected metals” produced metal-bonded atomic layers; in the Al case, interlayer spacing shrank from $11.47$ Å in pristine Ti$_3$C$_2$Cl$_x$ to $9.45$ Å in Al–Ti$_3$C$_2$Cl$_x$, sheet resistance dropped from $4.0\times10^4$ $\Omega/\square$ to $9.3$ $\Omega/\square$, and the total EMI shielding effectiveness reached $39$ dB at a thickness of $3.1$ $\mu$m [2302.07720]. Here Cl acts as a removable handle: Al–Cl bond energy ($507\pm1.0$ kJ/mol) exceeds Ti–Cl ($405.4\pm10.5$ kJ/mol), AlCl$_3$ sublimates at $180\,^\circ\mathrm{C}$, and residual Al bridges adjacent Ti$_3$C$_2$ slabs [2302.07720].

Processing-specific properties have also been quantified for few-layer Cl-terminated Ti$_3$C$_2$. Gaseous scissor-mediated electrochemical exfoliation yielded few-layer Ti$_3$C$_2$Cl$_2$ nanoflakes with ultrahigh yield of $\sim 93\%$, average thickness $\sim 2.15$ nm, work function $\phi \approx 4.06$ eV, water contact angle $\sim 68^\circ$, and stable colloids for $\geq 4$ weeks [2410.10214]. In the same study, Cl-terminated MXene lubricants improved tribovoltaic nanogenerator output from a dry short-circuit current of $\approx 44$ nA to the $\mu$A range, though Ti$_3$C$_2$Br$_2$ outperformed Ti$_3$C$_2$Cl$_2$ because of lower work function and greater hydrophobicity [2410.10214].

## 6. Stability in real environments, ambiguities, and open questions

The strongest evidence for Cl robustness concerns vacuum and inert-environment thermal stability, not comprehensive environmental stability. In situ STEM/EELS demonstrated stability up to $\sim 750\,^\circ\mathrm{C}$ under the electron beam and microscope vacuum [1901.05212], and CdCl$_2$-derived Ti$_3$C$_2$Cl$_x$ remained unchanged at $500\,^\circ\mathrm{C}$ in LiCl–KCl without Na [2507.05741]. However, several studies explicitly note that ambient and aqueous stability remain unresolved. The Ti-based STEM/DFT study states that “stability of Cl terminations in air, water, and electrolytes, and their resistance to hydrolysis or halide exchange, remain to be characterized,” while the chlorosilane study states that systematic studies under humidity, thermal cycling, solvents, or electrochemical cycling were not reported [1901.05212; 2509.11380].

Wet-chemical systems illustrate why this distinction matters. Atom probe tomography of HCl–LiF-etched Ti$_3$C$_2T_x$ found Cl at $2.49$ at% in as-synthesized MXene and $2.58$ at% in oxidized TiO$_2$ nanowires, with the authors emphasizing that halogens and alkalis are “inevitable” in wet synthesis and remain incorporated during oxidation [2305.19822]. Oxidation produced TiO$_2$ nanowires enriched in Li and Na, while Al decreased from $0.17$ at% to $54 \pm 30$ appm [2305.19822]. This shows that chlorine can persist through degradation pathways, but it does not by itself define a pure Cl-terminated MXene state.

Scalability and compositional generality are likewise only partly resolved. ZnCl$_2$ and LiCl–KCl routes clearly produce Ti$_2$CCl$_2$ and Ti$_3$C$_2$Cl$_2$, and MS-E-etching was also extended to Ti$_3$SiC$_2$ in supporting information [1901.05120; 2102.10262]. Chlorosilane etching established a redox-potential framework in which Ti-based MAX phases can be driven to Cl-terminated MXenes, whereas MAX phases with $M = \mathrm{Nb}, \mathrm{Ta}, \mathrm{Cr}, \mathrm{V}$ favor Si substitution under comparable conditions [2509.11380]. This suggests that Cl-termination is not controlled solely by chloride availability; the accessible MXene chemistry is also gated by the relative positions of $E(\mathrm{Si}^{4+}/\mathrm{Si})$, $E(M^{3+}/M^{2+})$, and $E(\mathrm{Al}^{3+}/\mathrm{Al})$.

Several mechanistic gaps are explicitly identified across the literature. No NEB diffusion/desorption barriers or Bader charges were reported in the Ti$_2$CCl$_2$/Ti$_3$C$_2$Cl$_2$ study [1901.05212]. Work functions were not reported there either [1901.05212]. The chlorosilane work did not quantify conductivity, long-term stability, colloid dispersion, or ease of delamination [2509.11380]. The metal-bonded Ti$_3$C$_2$Cl$_x$ study did not report DFT or XPS core-level analysis for the bonded interfaces [2302.07720]. A plausible implication is that Cl-terminated MXenes are already structurally well established, but the predictive structure–property map linking termination density, work function, intercalation energetics, ambient stability, and reaction selectivity remains incomplete.

In aggregate, Cl-terminated MXenes constitute a distinct termination-defined branch of MXene chemistry: fluorine-free in their canonical syntheses, frequently highly ordered, metallic in Ti-based cases, thermally robust up to about $750\,^\circ\mathrm{C}$ in vacuum, and sufficiently versatile to support termination exchange, superconductivity, metal-bonded heterostructures, few-layer colloids, and composite formation with in-situ amorphous Si [1901.05212; 2102.10262; 2201.00298; 2302.07720; 2410.10214; 2509.11380]. Their central scientific significance lies less in a single benchmark property than in the demonstration that chloride can function as a stable, ordered, and synthetically addressable terminal, thereby expanding the accessible MXene termination space beyond the mixed F/O/OH surfaces typical of aqueous HF-derived processing.

Source: https://www.emergentmind.com/topics/cl-terminated-mxene