Enhanced Electrocatalytic Activity in GaSe and InSe Nanosheets: The Role of Surface Oxides

Abstract

Gallium selenide (GaSe) is a van der Waals semiconductor widely used for optoelectronic devices, whose performances are dictated by bulk properties, including band-gap energy. However, recent experimental observations that the exfoliation of GaSe into atomically thin layers enhances performances in electrochemistry and photocatalysis have opened new avenues for its applications in the fields of energy and catalysis. Here, it is demonstrated by surface-science experiments and density functional theory (DFT) that the oxidation of GaSe into Ga 2 O 3 , driven by Se vacancies and edge sites created in the exfoliation process, plays a pivotal role in catalytic processes. Specifically, both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are energetically unfavorable in pristine GaSe, due to energy barriers of 1.9 and 5.7-7.4 eV, respectively. On the contrary, energy barriers are reduced concurrently with surface oxidation. Especially, the Heyrovsky step (H ads + H + + e − → H 2) of HER becomes energetically favorable only in sub-stoichiometric Ga 2 O 2.97 (−0.3 eV/H +). It is also discovered that the same mechanisms occur for the case of the parental compound indium selenide (InSe), thus ensuring the validity of the model for the broad class of III-VI layered semiconductors.

Chemical reactions on surfaces for applications in catalysis, gas sensing, adsorption-assisted desalination and Li-ion batteries: opportunities and challenges for surface science

“…As evident from the analysis of X-ray photoelectron spectroscopy (XPS) data in Fig. 1b, GaSe is stable in a vacuum, while air exposure induces the emergence of spectral features related to the Ga +3 oxidation state in Ga 2 O 3 for both the bulk 87,93 and nanosheets. 81,93 Definitely, oxidation proceeds with an intermediate Ga 2 Se 3 -like configuration, with the ultimate emergence of Ga 2 O 3 for prolonged storage in air.…”

“…1b, GaSe is stable in a vacuum, while air exposure induces the emergence of spectral features related to the Ga +3 oxidation state in Ga 2 O 3 for both the bulk 87,93 and nanosheets. 81,93 Definitely, oxidation proceeds with an intermediate Ga 2 Se 3 -like configuration, with the ultimate emergence of Ga 2 O 3 for prolonged storage in air. 93 Actually, the incomplete picture of the physicochemical mechanisms ruling GaSe oxidation should be attributed to the insufficient surface sensitivity of the spectroscopic tools used so far.…”

“…81,93 Definitely, oxidation proceeds with an intermediate Ga 2 Se 3 -like configuration, with the ultimate emergence of Ga 2 O 3 for prolonged storage in air. 93 Actually, the incomplete picture of the physicochemical mechanisms ruling GaSe oxidation should be attributed to the insufficient surface sensitivity of the spectroscopic tools used so far. Indeed, the study of the oxidation state is usually performed using Raman spectroscopy, 80,82 regardless of its low surface sensitivity, in consideration of its probing depth in the 300-650 nm range.…”

A Biaxial Strain Sensor Using a Single MoS2 Grating

“…Over the past years, 2D materials have attracted tremendous research effort. Following the introduction of grapheme [12,13], the family of 2D materials has been enlarged by many new members such as double atomic thin black phosphorus [14], triple atomic thin transition metal dichalcogenides [15], quadruple atomic thin group-III metal monochalcogenides [16], and other nonlayered 2D materials [17]. Many interesting properties have been found in these materials, keeping them under the spotlight of materials science [18][19][20][21][22][23][24].…”

Optical microscopy–based thickness estimation in thin GaSe flakes

“…This observation points out that the increased transmittance upon aging must be due to the transformation of GaSe into more transparent chemical compounds, containing Ga2Se3, Ga2O3, and/or amorphous selenium (a-Se), in agreement with previous Raman measurements. [40,[53][54][55]58] The topography of the flake has also changed substantially, becoming much rougher than the pristine flake. It seems that the degradation is accompanied by the generation of 'bubbles' or 'blisters' on the surface.…”