A novel van der Waals material for electrochemistry, photochemistry and gas sensing

Gallium telluride (GaTe) is a van der Waals semiconductor currently adopted for photonic and optoelectronic devices. The fast degradation of GaTe in the ambient atmosphere is allegedly assumed as disadvantageous for device applications. Recent work, carried out at the Elettra Synchrotron by an international multidisciplinary team, opens new perspectives for GaTe-based applications in catalysis, by exploiting the inevitable surface oxidation resulting from the interaction of this material with air.

When exposed to oxidative environments, GaTe transforms into Ga2O3 upon the following reaction pathway:

12GaTe + 3O2 →4Ga2Te3 + 2Ga2O3
2Ga2Te3 + 3O2 →2Ga2O3 + 6Te

 Thus, upon exposure to atmosphere, a protective, wide-energy-gap Ga2O3 skin forms over narrow-energy-gap, bulk GaTe.

Experimentally, the oxidation of the material was assessed using X-ray photoemission Spectroscopy (XPS) and X-ray Photo-Emission Electron Microscopy (XPEEM) measurements at the BACH and Nanospectroscopy beamlines. The ambient stability of defect-free and defective GaTe surfaces was monitored while exposing the sample to selective gases, i.e., oxygen and water. The stoichiometric GaTe sample resulted chemically inert towards these gases even after a 1011 L exposure (1 L=10-6 Torr·s). Indeed, the formation of surface oxides was not detected, in agreement with theoretical expectations. On the other hand, core-level spectra on O2-dosed defective GaTe surfaces displayed gallium-oxide and Ga2Te3 components.

Figure 1 by Bondino et al.

Figure 1: Bottom, (a) Ga-3d and (b) Te-3d core levels (XPS) for as-cleaved GaTe crystal surface and its aging upon exposure to air for 1 min, 10 min, 2h, 21 h, 7 days. Top: TEM image at the border of one GaTe nanosheet after 3 months in air, showing the presence of an enveloping amorphous oxide layer with a thickness of ≈3.5 nm.

Already upon 1 minute air exposure (Figure 1), changes in core-level spectra became evident, with the formation of components associated with the intercalation of ambient gases in the subsurface region. After 3 months in air, an amorphous oxide skin with thickness of 3.5 nm, as imaged by TEM, was formed. Correspondingly, the investigation of atomically thin layers of GaTe exfoliated in liquid phase by microspot-XPS spectra indicated the full oxidation of GaTe nanosheets (Figure 2).

Figure 2 by Bondino et al.

Figure 2: (a, b) XPEEM images of exfoliated GaTe nanosheets at the (a) Si-2p, (b) Te-4d core levels. (c, d) XPS spectra in the region of (c) Ga 3d and (d) Te 4d core levels.

Considering the presence of the amorphous Ga2O3 skin on the surface of GaTe (both bulk and nanosheets), the catalytic activity of GaTe-based systems on the hydrogen evolution reaction (HER) was evaluated. Whereas the pristine defect-free bulk GaTe is unsuitable for HER, due to an energy cost, which even increases in the monolayer regime, defective GaTe0.97 is instead competitive with state-of-the-art catalysts Pt. Considering the reduction of cost of raw materials by 160 times compared to Pt-based electrocatalysts, the self-assembled Ga2O3/GaTe heterostructure represents a suitable catalyst for HER in acidic media.

Moreover, the wide-band-gap oxide skin formed over narrow-band-gap GaTex upon air exposure makes the self-assembled gallium-oxide/gallium-telluride heterostructure suitable for light harvesting. As the magnitude of endothermic steps of HER over Ga2O3-x is manifestly smaller (more than 0.5 eV) than over GaTex substrates, the Ga2O3-x skin could be proposed as the chemically active part of GaTe-based photocatalytic devices. Photocatalysis on Ga2O3/GaTe interface can be modelled as a two-steps process. In the first step, a photo-induced transition of electrons occurs from valence to conduction bands of the GaTex substrate. In the second step, photo-generated electrons migrate from conduction bands of the GaTex substrate to the states with lower energies in the conduction band of the Ga2Ox skin, where these electrons participate in electrochemical reactions on chemically active sites on the surface.

Finally, the Ga2O3/GaTe interface can be also considered as a perspective material for gas sensing. The sensing properties of GaTe are enhanced by surface oxidation, which induces an increase of the area of charge redistribution after adsorption of analytes, such as water, ammonia, and nitrogen dioxide. Remarkably, Ga2O3/GaTe enables high-temperature gas sensing at operational temperature as high as 600°C, so as to enable its use for detection of gaseous species in combustion processes.

All these findings pave the way for a novel generation of (photo-) electrocatalysts and gas sensors, based on self-assembled heterostructures produced by exploiting the natural interaction of van der Waals semiconductors with air.

This research was conducted by the following research team:

Federica Bondino1, Songül Duman2, Silvia Nappini1, Gianluca D'Olimpio3, Corneliu Ghica4, Tevfik Onur Menteş5, Federico Mazzola1, Marian Cosmin Istrate4, Matteo Jugovac5, Mykhailo Vorokhta6, Sergio Santoro7, Bekir Gürbulak8, Andrea Locatelli5, Danil W. Boukhvalov9, Antonio Politano3

1Consiglio Nazionale delle Ricerche (CNR)- Istituto Officina dei Materiali (IOM), Trieste, Italy
2Basic Sciences Department, Faculty of Sciences, Erzurum Technical University, Erzurum, Turkey
3Department of Physical and Chemical Sciences, University of L’Aquila, L’Aquila (AQ), Italy
4National Institute of Materials Physics, Atomistilor, Magurele, Romania
5Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy
6Charles University, Prague, Czech Republic
7Department of Environmental Engineering (DIAM), University of Calabria,  Rende (CS) Italy
8Department of Physics, Faculty of Sciences, Atatürk University, Erzurum, Turkey
9College of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry University, Nanjing, P. R. China

Contact person: ;

Local contact person: ; (BACH); ; (Nanospectroscopy)

Reference

F. Bondino, S. Duman, S. Nappini, G. D'Olimpio, C. Ghica, T.O. Menteş, F. Mazzola, M.C. Istrate, M. Jugovac, M. Vorokhta, S. Santoro, B. Gürbulak, A. Locatelli, D.W. Boukhvalov, A. Politano, "Improving the Efficiency of Gallium Telluride for Photocatalysis, Electrocatalysis, and Chemical Sensing through Defects Engineering and Interfacing with its Native Oxide", Adv. Funct. Mater., in press (2022). DOI: 10.1002/adfm.202205923

 
Last Updated on Friday, 23 September 2022 10:19