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

Abstract

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.

Full text

492 | Nature | Vol 577 | 23 January 2020
Article
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
storage.
Two-dimensional (2D) atomic-layer crystals have demonstrated many
unique physical and chemical properties as well as broad applications
in electronics
2
, sensors
5
, catalysts
9
and batteries
10,11
. 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
3
, liquid-phase
1
and electro-
chemical procedures
2
. 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
6,7
. 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
15
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
17,18
. 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)
16,19
, 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
20–22
. 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
y
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)
23,24
, germanium chalcogenides (GeS, GeSe)
https://doi.org/10.1038/s41586-019-1904-x
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: yangshubin@buaa.edu.cn; ajayan@rice.edu
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