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Tailoring the Surface Chemical Reactivity of Transition‐Metal Dichalcogenide PtTe2 Crystals

Recently, the PtX2 (X=S, Se, Te) class of transition-metal dichalcogenides has emerged as one of the most promising among layered materials “beyond graphene” for the presence of high room-temperature electron mobility and, moreover, bulk type-II Dirac fermions, arising from a tilted Dirac cone.
Information on the ambient stability of PtTe2 is a crucial step in order to evaluate the feasibility of its exploitation in technology. Moreover, the possibility to tune surface chemical reactivity by appropriate surface modification is an essential step for its employment for diverse applications, especially in catalysis.
By means of experiments with several surface-science spectroscopies and density functional theory, an international team of researchers from Italy, Republic of Korea, and Taiwan (coordinated by Graphene Labs of Istituto Italiano di Tecnologia) has investigated the reactivity of the PtTe2 surface toward most common ambient gases (oxygen and water), under the framework of the European Graphene Flagship-Core1 project.
To assess the surface chemical reactivity of PtTe2, X-ray photoelectron spectroscopy (XPS) carried out at the APE-HE beamline has been combined with high-resolution electron energy loss (HREELS) experiments and with density functional theory.
From the analysis of Te 3d core-level spectra in XPS and from the featureless vibrational spectrum in HREELS, it has been demonstrated that as-cleaved defect-free PtTe2 surface is inert toward most common ambient gases (oxygen and water).
In the evaluation of the ambient stability of PtTe2, the possible influence of Te vacancies on surface chemical reactivity deserves particular attention. As a matter of fact, Te vacancies may appear on non-stoichiometric samples during the growth process. To check the influence of Te vacancies on ambient stability of PtTe2, Te vacancies have been intentionally introduced in stoichiometric PtTe2 samples by Ar-ion sputtering. After exposing to O2 the PtTe2 surface defected by ion sputtering, with a Pt:Te ratio of 39:61, spectral features related to Te(IV) species appear, arising from the formation of Te=O bonds in a tellurium-oxide phase. The Te(IV) components are the most intense lines in the Te 3d XPS spectra for the case of air-exposed defected samples (see Figure 1). Concerning reactivity to water, it adsorbs molecularly even at room temperature on defected PtTe2. These findings also imply that the presence of Te vacancies is able to jeopardize the ambient stability of uncapped PtTe2-based devices, with a subsequent necessity to reduce the amount of Te vacancies for a successful technological exploitation of PtTe2.
The surface chemical reactivity has been also modified by surface carbon doping, which represents a powerful tool for tuning the physicochemical properties of the surfaces of layered materials hosting Dirac fermions, already for trace amounts. Moreover, carbon atoms are the most common residual impurities incorporated in the bulk of single crystals. The catalytic activity is strongly enhanced by surface doping of carbon atoms, which transform the unreactive pristine PtTe2 into a selective catalyst. In particular, hydroxyl groups, essential step for water splitting and the water gas shift reaction, are stably adsorbed in PtTe2 surfaces modified by the joint presence of Te vacancies and substitutional carbon atoms. Therefore, C-doped PtTe2 can be proposed as a suitable catalyst for water-gas shift and water splitting.
These findings open new perspectives for the use of functionalized Dirac materials in catalysis.

Figure 1 XPS spectra of Te-3d core levels acquired for: defected PtTe2 (green curve), the same surface exposed to 106 L of O2 (black curve) and air-exposed defected PtTe2 (yellow curve). The photon energy is 745 eV. 

 

This research was conducted by the following research team:

Antonio Politano1, Vittorio Pellegrini1, Gennaro Chiarello2, Chia-Nung Kuo3, Chin Shan Lue3, Raju Edla4, Piero Torelli4, Danil W. Boukhvalov5
 

Istituto Italiano di Tecnologia, Graphene Labs, Genova, Italy
Department of Physics, University of Calabria, Rende (CS), Italy
Department of Physics, National Cheng Kung University, Tainan, Taiwan
CNR-IOM, Trieste, Italy
Department of Chemistry, Hanyang University, Seoul, Republic of Korea


Contact persons:

Antonio Politano, e-mail: antonio.politano@iit.it
Piero Torelli, e-mail: piero.torelli@elettra.eu
 

Reference

Antonio Politano, Gennaro Chiarello, Chia-Nung Kuo, Chin Shan Lue, Raju Edla, Piero Torelli, Vittorio Pellegrini, Danil W. Boukhvalov, "Tailoring the Surface Chemical Reactivity of Transition‐Metal Dichalcogenide PtTe2 Crystals", Advanced Functional Materials (2018); doi:10.1002/adfm.201706504

 
Last Updated on Monday, 26 March 2018 15:37