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: 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: (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 GaTe_0.97 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 GaTe_x 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 Ga₂O_3-x is manifestly smaller (more than 0.5 eV) than over GaTe_x substrates, the Ga₂O_3-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 GaTe_x substrate. In the second step, photo-generated electrons migrate from conduction bands of the GaTe_x substrate to the states with lower energies in the conduction band of the Ga₂O_x 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)