Two-Dimensional Metal Dichalcogenides and Oxides for Hydrogen
Evolution: A Computational Screening Approach
Mohnish Pandey,† Aleksandra Vojvodic,‡ Kristian S. Thygesen,†,§ and Karsten W. Jacobsen*,†
†Center for Atomic-Scale Materials Design, Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby,
Denmark
‡SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park,
California 94025, United States
§Center for Nanostructured Graphene (CNG), Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby,
Denmark
*S Supporting Information
ABSTRACT: We explore the possibilities of hydrogen evolution by basal planes of
2D metal dichalcogenides and oxides in the 2H and 1T class of structures using the
hydrogen binding energy as a computational activity descriptor. For some groups of
systems like the Ti, Zr, and Hf dichalcogenides the hydrogen bonding to the 2H
structure is stronger than that to the 1T structure, while for the Cr, Mo, and W
dichalcogenides the behavior is opposite. This is rationalized by investigating shifts in
the chalcogenide p levels comparing the two structures. We find that usually for a
given material only at most one of the two phases will be active for the hydrogen
evolution reaction; however, in most cases the two phases are very close in formation
energy, opening up the possibility for stabilizing the active phase. The study points to
many new possible 2D HER materials beyond the few that are already known.
Hydrogen holds a crucial place in many chemical syntheses
and in energy production;1,2 however, an economical
process for hydrogen production has not been fully realized yet.
One of the main challenges lies in finding a cheap catalyst that
can evolve hydrogen efficiently. Platinum, which is known to be
one of the best catalysts for hydrogen evolution, is prohibitively
expensive, thus precluding it to be used on large scales. Several
other metals, metal surface alloys and metal oxides, have been
studied for the same reaction, but unfortunately most of these
are not both efficient and cheap at the same time.3−5 Only
recently a few and interesting candidates have been identified
for hydrogen evolution reaction (HER), for example, Ni2P.6,7
Recent promising experiments on 2D metal sulfides have
opened up a new class of materials that could contain
promising candidates for HER.8−12 The 2D nature of these
materials gives additional flexibility of nanostructuring and
manipulating the structures, which is otherwise challenging in
the 3D bulk form. For example, MoS2 exists in both 2H and 1T
phases in monolayer form, whereas the 1T phase is
thermodynamically unfavorable in the bulk.13 Despite the fact
that the 2H-MoS2 is one of the most studied 2D sulfides for
HER, it has active sites on the edges only,14,15 and the limited
activity is ascribed to the inability of the 2H-MoS2 basal plane
to adsorb hydrogen.10 The above limitation has been overcome
by contemporary experiments on 1T-MoS2 and WS2, in which
the entire sheet has been found to be catalytically active for
HER.8−10 The unusual difference between the 2H and 1T
phases thus expands the material space to more structures that
might be relevant for the given application.
In the present work, we explore the HER activity of the basal
planes of 100 dichalcogenides and oxides (MX2) in both the
2H and 1T class of structures using the free energy of hydrogen
adsorption as a descriptor for the activity of the material.3,16
Rather than assuming the existence of perfectly symmetrical 2H
and 1T structures, we carefully look for deviations of the atomic
structure from the perfectly symmetric 2H and 1T phases and
choose the structure with minimum energy. (We continue
using the terminology 2H and 1T for distorted structures as
well to avoid cluttering of notations.) We choose ‘M’ from a set
of 25 elements (shown in the ordinate of Figure 2) and ‘X’
from a set of 4 (shown in the abscissa of Figure 2) elements
(chalcogens and oxygen). We find a significant difference in the
hydrogen adsorption energy of the 2H and 1T phases of a given
compound; on the other hand, the 2H and 1T phases show
similar thermodynamic stability, thus making it possible to
stabilize the structure showing activity toward HER despite the
fact that it is not the most stable structure. To find a correlation
between the adsorption energies and the nature of metal atoms,
we group the compounds based on the position of the metal
atoms in the periodic table. For the groups showing apparent
Received: February 17, 2015
Accepted: April 10, 2015
Published: April 10, 2015
Letter
pubs.acs.org/JPCL
© 2015 American Chemical Society 1577 DOI: 10.1021/acs.jpclett.5b00353
J. Phys. Chem. Lett. 2015, 6, 1577−1585