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Conversion of non-van der Waals solids to 2D transition-metal chalcogenides

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Although two-dimensional (2D) atomic layers, such as transition-metal chalcogenides, have been widely synthesized using techniques such as exfoliation1–3 and vapour-phase growth4,5, it is still challenging to obtain phase-controlled 2D structures6–8. Here we demonstrate an effective synthesis strategy via the progressive transformation of non-van der Waals (non-vdW) solids to 2D vdW transition-metal chalcogenide layers with identified 2H (trigonal prismatic)/1T (octahedral) phases. The transformation, achieved by exposing non-vdW solids to chalcogen vapours, can be controlled using the enthalpies and vapour pressures of the reaction products. Heteroatom-substituted (such as yttrium and phosphorus) transition-metal chalcogenides can also be synthesized in this way, thus enabling a generic synthesis approach to engineering phase-selected 2D transition-metal chalcogenide structures with good stability at high temperatures (up to 1,373 kelvin) and achieving high-throughput production of monolayers. We anticipate that these 2D transition-metal chalcogenides will have broad applications for electronics, catalysis and energy storage. A synthetic approach is described, for efficiently converting non-van der Waals solids into two-dimensional van der Waals transition-metal chalcogenide layers with specific phases, enabling the high-throughput production of monolayers.
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492 | Nature | Vol 577 | 23 January 2020
Conversion of non-van der Waals solids to 2D
transition-metal chalcogenides
Zhiguo Du1, Shubin Yang1*, Songmei Li1, Jun Lou2, Shuqing Zhang3, Shuai Wang1, Bin Li1,
Yongji Gong1, Li Song4, Xiaolong Zou3 & Pulickel M. Ajayan2*
Although two-dimensional (2D) atomic layers, such as transition-metal
chalcogenides, have been widely synthesized using techniques such as exfoliation1–3
and vapour-phase growth4,5, it is still challenging to obtain phase-controlled 2D
structures6–8. Here we demonstrate an eective synthesis strategy via the progressive
transformation of non-van der Waals (non-vdW) solids to 2D vdW transition-metal
chalcogenide layers with identied 2H (trigonal prismatic)/1T (octahedral) phases.
The transformation, achieved by exposing non-vdW solids to chalcogen vapours, can
be controlled using the enthalpies and vapour pressures of the reaction products.
Heteroatom-substituted (such as yttrium and phosphorus) transition-metal
chalcogenides can also be synthesized in this way, thus enabling a generic synthesis
approach to engineering phase-selected 2D transition-metal chalcogenide structures
with good stability at high temperatures (up to 1,373 kelvin) and achieving high-
throughput production of monolayers. We anticipate that these 2D transition-metal
chalcogenides will have broad applications for electronics, catalysis and energy
Two-dimensional (2D) atomic-layer crystals have demonstrated many
unique physical and chemical properties as well as broad applications
in electronics
, sensors
, catalysts
and batteries
. Generally, 2D struc-
tures such as graphene, boron nitride and transition-metal sulfides can
be produced via a top-down approach, that is, by directly exfoliating
the vdW counterparts through mechanical
, liquid-phase
and electro-
chemical procedures
. In this manner, various vdW materials—such
as metal oxides12, hydroxides13 and topological insulators14—can also
be synthesized, enriching the 2D family of materials. In these 2D vdW
nanocrystals, the elemental compositions, stoichiometric ratios and
structural phases are usually inherited from their parent bulk counter-
parts, although 2D nanocrystals with phase-specific structures such
as 1T and 2H phases are difficult to synthesize selectively
. Here we
demonstrate an efficient topological conversion of non-vdW solids
such as transition-metal carbides and nitrides under chalcogen vapours
to 2D vdW transition-metal chalcogenide layers with identified 2H/1T
phases, good stability at high temperatures (<1,373K) and achieving
high-throughput production of monolayers. We anticipate that the
resultant transition-metal chalcogenide layers with favourable fea-
tures would have broad applications for electronics, energy storage
and conversions.
In the past decade, some unusual 2D nanocrystals have emerged from
non-vdW solids such as haematite
or bulk layered transition-metal
carbides and nitrides16, namely MAX phases, greatly increasing the num-
ber of 2D material compositions accessible. In particular, the non-vdW
MAX phases—where M represents a transition-metal element, A usually
represents an element from groups 13–16 of the periodic table and X
is carbon or nitrogen—have predominantly mixed covalent or ionic
M–X bonds and metallic M–A bonds
. Because the M–A bonds are
more chemically active than the M–X bonds, A species in MAX phases
can be extracted using highly reactive solvents (hydrogen fluoride and
strong bases)
, allowing few-layer-thick 2D transition-metal carbides,
carbonitrides and nitrides—called MXenes—to be created. These 2D
nanocrystals are usually terminated with defects and surface termina-
tions of -OH, -O, -F or -Cl
. Owing to the very close atomic packing
and strong chemical bonds in non-vdW solids, it remains a challenge
to convert them to 2D nanocrystals with abundant exposed surfaces
and identified phases.
Here we demonstrate an efficient strategy that enables us to convert
a family of non-vdW bulk solids such as MAX phases to 2D transition-
metal chalcogenides with well-defined phases. As depicted in Fig.1a,
under chalcogen-containing vapours (H
Z, where Z represents sulfur,
selenium or tellurium and y is 0 or 2) at high temperatures, non-vdW
MAX phases and transition-metal borides, silicides and carbides (Sup-
plementary Fig.1) have high activities. In particular, the active M–A
bonds in MAX phases react easily with chalcogen-containing gases,
resulting in products of AZ and MZ compositions. Such reactions must
produce an AZ intermediate product at high vapour pressure, which
would allow rapid evaporation rates, thus boosting the continuous
reaction into the bulk of the reactant material. Thermodynamically,
if the reaction temperature were high enough, all the post-transition-
metal A (Si, Al, Sn, Ge) species in MAX phases could be transformed to
metal chalcogenide gases (Supplementary Figs.2–5), which facilitates
the conversion of MAX phases to 2D nanostructures. As an example,
based on temperature–vapour pressure relationships (Fig.1b and
Supplementary Fig.6)
, germanium chalcogenides (GeS, GeSe)
Received: 2 March 2019
Accepted: 25 November 2019
Published online: 22 January 2020
1Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, School of Materials Science and Engineering, Beihang University, Beijing, China. 2Department of
Materials Science and NanoEngineering, Rice University, Houston, TX, USA. 3Shenzhen Geim Graphene Center and Low-Dimensional Materials and Devices Laboratory (LDMD), Tsinghua-
Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen, China. 4National Synchrotron Radiation Laboratory, CAS Center for Excellence in Nanoscience, University of Science and
Technology of China, Hefei, China. *e-mail:;
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... 5,6 Results of theoretical modeling, and synthesis of powders and nanocrystals, demonstrate the interest and potential of isomorphous TMD alloys. [5][6][7] The work here addresses a need for progress in thin film processing. ...
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... MXene-derived functional materials, including oxide, 96 ferroelectric crystals, 114 2D MOF crystals and films, 116 and chalcogenides, 135 have recently been demonstrated. Some of these MXene-derived materials can play unique roles in integrated MXetronic devices. ...
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Investigations of two-dimensional transition-metal chalcogenides (TMCs) have recently revealed interesting physical phenomena, including the quantum spin Hall effect1,2, valley polarization3,4 and two-dimensional superconductivity 5 , suggesting potential applications for functional devices6-10. However, of the numerous compounds available, only a handful, such as Mo- and W-based TMCs, have been synthesized, typically via sulfurization11-15, selenization16,17 and tellurization 18 of metals and metal compounds. Many TMCs are difficult to produce because of the high melting points of their metal and metal oxide precursors. Molten-salt-assisted methods have been used to produce ceramic powders at relatively low temperature 19 and this approach 20 was recently employed to facilitate the growth of monolayer WS2 and WSe2. Here we demonstrate that molten-salt-assisted chemical vapour deposition can be broadly applied for the synthesis of a wide variety of two-dimensional (atomically thin) TMCs. We synthesized 47 compounds, including 32 binary compounds (based on the transition metals Ti, Zr, Hf, V, Nb, Ta, Mo, W, Re, Pt, Pd and Fe), 13 alloys (including 11 ternary, one quaternary and one quinary), and two heterostructured compounds. We elaborate how the salt decreases the melting point of the reactants and facilitates the formation of intermediate products, increasing the overall reaction rate. Most of the synthesized materials in our library are useful, as supported by evidence of superconductivity in our monolayer NbSe2 and MoTe2 samples21,22 and of high mobilities in MoS2 and ReS2. Although the quality of some of the materials still requires development, our work opens up opportunities for studying the properties and potential application of a wide variety of two-dimensional TMCs.
The Mn+1AXn, or MAX, phases are nanolayered, hexagonal, machinable, early transition-metal carbides and nitrides, where n = 1, 2, or 3, M is an early transition metal, A is an A-group element (mostly groups 13 and 14), and X is C and/or N. These phases are characterized by a unique combination of both metallic and ceramic properties. The fact that these phases are precursors for MXenes and the dramatic increase in interest in the latter for a large host of applications render the former even more valuable. Herein we describe the structure of most, if not all, MAX phases known to date. This review covers ~155 MAX compositions. Currently, 16 A elements and 14 M elements have been incorporated in these phases. The recent discovery of both quaternary in-and out-of-plane ordered MAX phases opens the door to the discovery of many more. The chemical diversity of the MAX phases holds the key to eventually optimizing properties for prospective applications. Since many of the newer quaternary (and higher) phases have yet to be characterized, much work remains to be done.
Nanolaminated materials are important because of their exceptional properties and wide range of applications. Here, we demonstrate a general approach to synthesize a series of Zn-based MAX phases and Cl-terminated MXenes originating from the replacement reaction between the MAX phase and the late transition metal halides. The approach is a top-down route that enables the late transitional element atom (Zn in the present case) to occupy the A site in the pre-existing MAX phase structure. Using this replacement reaction between Zn element from molten ZnCl2 and Al element in MAX phase precursors (Ti3AlC2, Ti2AlC, Ti2AlN, and V2AlC), novel MAX phases Ti3ZnC2, Ti2ZnC, Ti2ZnN, and V2ZnC were synthesized. When employing excess ZnCl2, Cl terminated MXenes (such as Ti3C2Cl2 and Ti2CCl2) were derived by a subsequent exfoliation of Ti3ZnC2 and Ti2ZnC due to the strong Lewis acidity of molten ZnCl2. These results indicate that A-site element replacement in traditional MAX phases by late transition metal halides opens the door to explore MAX phases that are not thermodynamically stable at high temperature and would be difficult to synthesize through the commonly employed powder metallurgy approach. In addition, this is the first time that exclusively Cl-terminated MXenes were obtained, and the etching effect of Lewis acid in molten salts provides a green and viable route to prepare MXenes through an HF-free chemical approach.
The development of stable and efficient hydrogen evolution reaction (HER) catalysts is essential for the production of hydrogen as a clean energy resource. A combination of experiment and theory demonstrates that the normally inert basal planes of 2D layers of MoS2 can be made highly catalytically active for the HER when alloyed with rhenium (Re). The presence of Re at the ≈50% level converts the material to a stable distorted tetragonal (DT) structure that shows enhanced HER activity as compared to most of the MoS2‐based catalysts reported in the literature. More importantly, this new alloy catalyst shows much better stability over time and cycling than lithiated 1T‐MoS2. Density functional theory calculations find that the role of Re is only to stabilize the DT structure, while catalysis occurs primarily in local Mo‐rich DT configurations, where the HER catalytic activity is very close to that in Pt. The study provides a new strategy to improve the overall HER performance of MoS2‐based materials via chemical doping. Re‐doped MoS2 atomic layers in the distorted tetragonal structure show excellent activity and stability for electrocatalytic hydrogen production. Atomic‐level scanning transmission electron microscopy combined with density functional theory calculations reveal active local Mo‐rich structures and explain the best performance in Re0.55Mo0.45S2. The study provides a new catalyst design strategy through chemical doping.
The transformation from semiconducting to metallic phase, accompanied by a structural transition in 2D transition metal dichalcogenides has attracted the attention of the researchers world-wide. We describe the unconventional structural transformation of fluorinated WS2 (FWS2) into the 1T phase. The energy difference between the two phases debug this transition, as fluorination enhances the stability of 1T FWS2 and makes it energetically favorable at higher F concentration. Investigation of the electronic and optical nature of FWS2 is supplemented by possible band structures and bandgap calculations. Magnetic centers in the 1T phase appear in FWS2 possibly due to the introduction of defect sites. A direct consequence of the phase transition and associated increase in interlayer spacing is a change in friction behavior. Friction Force Microscopy is used to determine this effect of functionalization accompanied phase transformation.
Structural design on the atomic level can provide novel chemistries of hybrid MAX phases and their MXenes. Herein, density functional theory is used to predict phase stability of quaternary i‐MAX phases with in‐plane chemical order and a general chemistry (W2/3M²1/3)2AC, where M² = Sc, Y (W), and A = Al, Si, Ga, Ge, In, and Sn. Of over 18 compositions probed, only two—with a monoclinic C2/c structure—are predicted to be stable: (W2/3Sc1/3)2AlC and (W2/3Y1/3)2AlC and indeed found to exist. Selectively etching the Al and Sc/Y atoms from these 3D laminates results in W1.33C‐based MXene sheets with ordered metal divacancies. Using electrochemical experiments, this MXene is shown to be a new, promising catalyst for the hydrogen evolution reaction. The addition of yet one more element, W, to the stable of M elements known to form MAX phases, and the synthesis of a pure W‐based MXene establishes that the etching of i‐MAX phases is a fruitful path for creating new MXene chemistries that has hitherto been not possible, a fact that perforce increases the potential of tuning MXene properties for myriad applications.