Synthesis of Ti2B2Clx MBenes in molten salts from theoretical and experimental perspectives

Andrejs Petruhins; David Portehault; Dominique Thiaudi\`ere; Durga Sankar Vavilapalli; Emile Defoy; Johanna Rosen; Jonas Bj\"ork; Justinas Palisaitis; Lan Tang; Lianghao Yu

Molten ZnCl2 etches Ti2InB2 into multilayer Ti2B2Clx MBene by a direct biphasic path whose free energy is lower than every competing process.

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2026-07-14 04:48 UTC pith:QS7M4YIS

load-bearing objection Solid experimental synthesis of Cl-terminated multilayer Ti2B2Clx MBene with clean biphasic kinetics and a useful free-energy screen; modeling idealizations are secondary.

arxiv 2607.11550 v1 pith:QS7M4YIS submitted 2026-07-13 cond-mat.mtrl-sci physics.comp-ph

Synthesis of Ti2B2Clx MBenes in molten salts from theoretical and experimental perspectives

Rodrigo M. Ronchi , Emile Defoy , Andrejs Petruhins , Justinas Palisaitis , Lianghao Yu , Lan Tang , Solenn Reguer , Dominique Thiaudi\`ere

show 6 more authors

Ningjun Chen Durga Sankar Vavilapalli David Portehault Jonas Bj\"ork Per O. {\AA}. Persson Johanna Rosen

This is my paper

classification cond-mat.mtrl-sci physics.comp-ph

keywords 2D materialsMAB phaseMBenemolten saltin situ XRDDFTLi-ion batterysurface terminations

verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

Two-dimensional metal borides (MBenes) are hard to make: most etching routes leave the A-layer partly in place, oxidize the material, or collapse the sheets into a 3D boride. This paper shows that a ZnCl2 molten-salt reaction cleanly removes every indium atom from Ti2InB2 and replaces them with chlorine surface groups, yielding multilayer Ti2B2Clx (1.1 < x < 1.4). In-situ XRD records an abrupt biphasic jump from the parent MAB phase to the expanded MBene lattice with no intermediate A-substituted phase. A density-functional free-energy map ranks the four plausible outcomes and finds that only MBene formation is exergonic; A-element substitution, complete disintegration and collapse to 3D TiB are all uphill. The same map correctly predicts that CuCl2 fails to produce a clean MBene. Early battery tests give stable capacities comparable to or better than related MAX phases. Together the experiment and the free-energy ranking supply a practical route and a predictive filter for growing the still-small family of 2D borides.

Core claim

Selective molten-salt etching of the MAB phase Ti2InB2 with ZnCl2 produces multilayer Ti2B2Clx (1.1 < x < 1.4) by a direct biphasic transformation; density-functional reaction free energies show that this pathway is the only exergonic process among the four scenarios considered, while A-element substitution to Ti2ZnB2 is endergonic.

What carries the argument

Process-specific free-energy phase diagrams that constrain the product set to one of four etching scenarios (A-substitution, MBene formation, complete disintegration, 3D-boride collapse) and rank their Gibbs free energies versus salt-to-MAB ratio.

Load-bearing premise

The free-energy ranking treats the multilayer product as ideal stoichiometric Ti2B2Cl2 with fixed chlorine sites and mixes harmonic phonon energies with tabulated salt data; large errors in the van-der-Waals functional, vibrational entropy or neglected kinetic barriers could reorder the pathways.

What would settle it

An in-situ XRD or STEM experiment that captures a Zn-substituted intermediate (Ti2ZnB2) or a progressive interlayer expansion before the MBene (002) peak appears would falsify the claimed direct biphasic mechanism and the free-energy ordering that excludes substitution.

Watch this falsifier — get emailed when new claim-graph text bears on it.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Desk Editor's Note

Solid experimental synthesis of Cl-terminated multilayer Ti2B2Clx MBene with clean biphasic kinetics and a useful free-energy screen; modeling idealizations are secondary.

read the letter

This one actually delivers a clean MAB-to-MBene conversion that earlier acid and molten-salt attempts on related phases mostly failed to achieve. They etch Ti2InB2 with ZnCl2, fully remove In, put Cl terminations on the basal faces (coverage 1.1–1.4), keep the multilayer stack, and show by in-situ synchrotron XRD that the transformation is direct and biphasic—no Zn-substituted intermediate, no progressive swelling, no amorphous stage at the time resolution they have. That pathway difference from many MAX-to-MXene molten-salt routes is the real experimental novelty.

The multi-technique package is tight: powder XRD, SEM-EDX, HAADF-STEM, STEM-EDX, EELS, and the in-situ run all converge on the same product identity and expanded c-axis. Lattice parameters from the DFT model match experiment, phonons are clean, and the free-energy ranking (only the MBene channel is exergonic) correctly predicts both the observed product and the absence of Ti2ZnB2. They also show the CuCl2 case goes the other way, which is a useful negative control. Yield numbers (~70 %) and the sealed-tube vs open-crucible comparison are practical details people will use.

Soft spots are real but secondary. The DFT free energies treat ideal Ti2B2Cl2 with fixed hollow Cl sites and mix harmonic phonons with tabulated salt data; that is the usual approximation and does not reverse the ranking, but it is still an idealization. Edge oxidation and sub-stoichiometric Cl are acknowledged and consistent with MXene literature. Battery data are preliminary and not the point. Citation pattern is normal for the group; the free-energy method is their prior MAX work, but the experimental observables stand independently.

This is for people who make or screen 2D borides and for anyone running molten-salt etching. Methods are detailed enough to re-implement. I would send it to referees without hesitation and would cite the synthesis and the biphasic kinetics.

Referee Report

0 major / 6 minor

Summary. The manuscript reports selective molten-salt etching of the MAB phase Ti2InB2 with ZnCl2 to produce multilayer Ti2B2Clx (1.1 < x < 1.4). Multi-technique characterization (powder XRD, SEM-EDX, HAADF-STEM, STEM-EDX, EELS) establishes complete In removal, Cl surface terminations (primarily on basal faces), an expanded c-axis (~19.1 Å), and residual TiB2 impurity. In situ synchrotron XRD shows a direct biphasic MAB-to-ml-MBene conversion without detectable Zn-substituted intermediate or progressive interlayer expansion. A DFT free-energy framework (adapted from prior MAX-phase work) ranks ml-MBene formation as the only exergonic pathway among four scenarios, consistent with experiment; CuCl2 control experiments yield partial In removal and oxidation rather than clean MBene. Initial Li-ion battery data give stable capacities comparable to related MAX phases and borides. The work combines synthesis, in situ pathway mapping, and thermodynamic ranking to support expansion of the MBene family.

Significance. If the result holds, the paper supplies a concrete, scalable route to Cl-terminated multilayer Ti2B2Tx MBene and an experimentally corroborated thermodynamic ranking that correctly predicts both successful (ZnCl2) and unsuccessful (CuCl2) outcomes. The in situ XRD evidence of a biphasic transformation without A-layer substitution is a clear mechanistic distinction from many MS-derived MXenes and is valuable for the field. The free-energy framework, already published for MAX phases, is shown to transfer to MAB phases without fitted parameters, giving a predictive tool for screening other layered borides. Battery performance is secondary but places the material in a useful comparative context. Strengths include orthogonal experimental observables that stand independently of the DFT ranking, dynamical stability of the model structure, and lattice-parameter agreement between DFT (rev-vdW-DF2) and STEM/XRD.

minor comments (6)

Abstract and Conclusions: the coverage range is written both as 1.1 < x < 1.4 and 1.1 < 𝑥 < 1.4; unify notation and ensure the same bounds appear consistently with Tables S2–S3.
Figure 1 caption and main text: the schematic shows Ti2B2Cl2 while experiment reports non-stoichiometric Cl coverage; a brief note that the schematic is idealized would avoid confusion.
Results (CuCl2 section): the statement that mass loss is ~10% versus ~50% for ZnCl2 is useful; adding the corresponding In/Ti and Cl/Ti ratios from SEM-EDX (if available) would strengthen the comparison.
Methods (Computational Details): the choice of TiB (Pnma) over a collapsed multilayer structure is justified by energy, but a short sentence on whether the collapsed structure was ever relaxed would help readers reproduce the ranking.
Figure 2b and SI: the vertical dotted lines marking completion of each process are helpful; labeling the exact salt-to-MAB ratios used in the sealed-tube experiments on the same plot would make the experimental–theory link more immediate.
Typographical: “A-element substitution into to the 3D Ti2ZnB2” (abstract) and occasional missing spaces around units (e.g., 0.5h) should be corrected in proof.

Circularity Check

1 steps flagged

No significant circularity: experimental product identity and biphasic pathway stand independently of the authors' prior free-energy framework, which is used only for corroborative ranking.

specific steps

self citation load bearing [RESULTS AND DISCUSSION, paragraph introducing free-energy framework; METHODS Computational Details]
"a rational framework for this pathway could be obtained by employing a recently developed theoretical methodology 23, where ml-Ti2B2Cl2 exhibited the lowest (negative) reaction free energy among possible pathways, while the intermediate compound (Ti2ZnB2) had positive reaction free energy. ... To assess the thermodynamics of this reaction, we used a recently developed methodology applied to MAX phases and MXenes 23, which maps different competing reactions by constraining the product term in Eq. {1} to a set of possible compounds that are specific to each etching scenario."

The free-energy ranking that 'corroborates' experiment is taken from the authors' own prior MAX-phase paper (ref. 23). However, the citation is not load-bearing for the synthesis claim: product identity and biphasic pathway are independently measured. The self-citation supplies a ranking tool, not a uniqueness theorem that forces the experimental outcome, so circularity is minor.

full rationale

The paper's strongest claims (complete In removal, Cl termination with 1.1 < x < 1.4, expanded interlayer spacing, and direct biphasic MAB-to-ml-MBene transformation without intermediate) are established by multi-technique experimental data (ex situ XRD, SEM-EDX, HAADF-STEM, EELS, in situ synchrotron XRD). The DFT process-specific free-energy ranking (Fig. 2b) is imported from the authors' prior MAX-phase methodology (ref. 23) and applied to MAB phases; it correctly predicts that ml-MBene formation is the only exergonic pathway and that A-element substitution is endergonic, matching the experimental absence of Ti2ZnB2. This is ordinary self-citation of a computational tool, not a load-bearing uniqueness theorem or a fitted parameter re-labeled as prediction. No equation reduces by construction to its own inputs, no parameter is fitted to the target observables and then 'predicted,' and the experimental observables do not presuppose the free-energy ranking. The modeling idealizations (stoichiometric Ti2B2Cl2, harmonic phonons, tabulated salt data) affect quantitative accuracy of the ranking but do not create circularity. Score 1 reflects only the minor, non-load-bearing self-citation of the framework.

Axiom & Free-Parameter Ledger

2 free parameters · 4 axioms · 0 invented entities

Central claim rests on standard solid-state and DFT practice plus one previously published free-energy mapping protocol. No new particles or forces are postulated; the main modeling choices (stoichiometric Cl coverage, harmonic free energies, tabulated salt data) are domain-standard but still load-bearing for the thermodynamic ranking.

free parameters (2)

salt-to-MAB molar ratio and temperature schedule
Experimentally optimized (4:1, 600 °C, 5 h) rather than predicted a priori; the free-energy diagrams are evaluated at the experimental temperature but the precise ratio that completes conversion is empirical.
Cl surface coverage x in experimental formula
Measured range 1.1 < x < 1.4 from EDX/EELS; DFT calculations use ideal x = 2 for free-energy ranking.

axioms (4)

domain assumption Reaction free energies can be ranked by minimizing linear combinations of formation free energies under process-specific product constraints (the four scenarios of Fig. 2a).
Imported from the authors’ prior MAX-phase methodology (ref. 23); assumes equilibrium thermodynamics dominate over kinetics.
domain assumption Harmonic phonon free energies plus tabulated salt free energies give sufficiently accurate Gibbs energies at 900 K.
Standard approximation stated in Methods; anharmonic or liquid-structure effects are neglected.
domain assumption rev-vdW-DF2 functional correctly captures interlayer van der Waals binding in the multilayer MBene.
Chosen after lattice-parameter agreement with experiment; other vdW functionals could shift absolute energies.
ad hoc to paper Cl atoms occupy hollow sites above B and layers stack A-B-A.
Adopted from prior MBene/MXene literature and relaxed; used for all free-energy and phonon calculations.

pith-pipeline@v1.1.0-grok45 · 20978 in / 2720 out tokens · 31184 ms · 2026-07-14T04:48:56.811220+00:00 · methodology

0 comments

read the original abstract

The unique properties and application possibilities of two-dimensional (2D) materials motivates the exploration of different nanolaminated compounds. Here, by using a molten salt approach, we selectively etch Ti2InB2 with ZnCl2 to produce a multilayer (ml) Ti2B2Clx MBene. Scanning transmission electron microscopy, in combination with energy dispersive X-ray, and electron energy loss spectroscopies show that In atoms are completely removed from the precursor upon etching, being replaced by chlorine surface terminations with a coverage 1.1 < x < 1.4. Further, in situ X-ray diffraction indicates a direct biphasic transformation from Ti2InB2 to ml-MBene, with no signs of intermediate phase formation. A computational framework based on density functional theory further corroborates these experimental observations by showing a negative reaction free energy for the formation of ml-MBene, favourable over all competing processes. In addition, A-element substitution into to the 3D Ti2ZnB2 phase is predicted to be endergonic, consistent with the absence of experimental evidence for its formation. Initial Li-ion battery performance evaluation showed a stable discharge capacity similar or better than MAX phases and other borides. Altogether, the theoretical framework combined with materials synthesis and characterization provides a general approach for 2D materials development, for further expansion of the family of 2D materials.

discussion (0)

Reference graph

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[1] [1]

Novoselov, K. S. et al. Electric Field Effect in Atomically Thin Carbon Films. Science (80- . ). 306, 666–669 (2004)

2004

[2] [2]

C., Girit, Ç

Pacilé, D., Meyer, J. C., Girit, Ç. Ö. & Zettl, A. The two-dimensional phase of boron nitride: Few-atomic-layer sheets and suspended membranes. Appl. Phys. Lett. 92, 1–4 (2008)

2008

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Chhowalla, M. et al. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nat. Chem. 5, 263–275 (2013)

2013

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Novoselov, K. S. et al. Two-dimensional atomic crystals. Proc. Natl. Acad. Sci. U. S. A. 102, 10451–10453 (2005)

2005

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Naguib, M. et al. Two-dimensional nanocrystals produced by exfoliation of Ti 3AlC2. Advanced Materials vol. 23 4248–4253 (2011)

2011

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Yang, T., Lin, H., Zheng, X., Loh, K. P. & Jia, B. Tailoring pores in graphene -based materials: from generation to applications. J. Mater. Chem. A 5, 16537–16558 (2017)

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VahidMohammadi, A., Rosen, J. & Gogotsi, Y. The world of two-dimensional carbides and nitrides (MXenes). Science 372, eabf1581 (2021)

2021

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S., Mishchenko, A., Carvalho, A

Novoselov, K. S., Mishchenko, A., Carvalho, A. & Castro Neto, A. H. 2D materials and van der Waals heterostructures. Science (80-. ). 353, aac9439 (2016)

2016

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Zhou, J. et al. Boridene: Two-dimensional Mo4/3B2-x with ordered metal vacancies obtained by chemical exfoliation. Science (80-. ). 373, 801–805 (2021)

2021

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Shen, Q. et al. Engineering Oxygen-Terminated Hexagonal MBene for Accelerated Lithium Migration and Exceptional Lithium-Ion Battery Performance. Adv. Sci. e13674, 1–12 (2025)

2025

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Helmer, P. et al. Investigation of 2D Boridene from First Principles and Experiments. Adv. Funct. Mater. 2109060, (2022)

2022

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Bury, D. et al. Wet-Chemical Etching and Delamination of MoAlB into MBene and Its Outstanding Photocatalytic Performance. Adv. Funct. Mater. 33, 1–11 (2023)

2023

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Zhang, H. et al. Phase pure and well crystalline Cr2AlB2: A key precursor for two- dimensional CrB. J. Mater. Sci. Technol. 35, 1593–1600 (2019)

2019

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