Enhanced electrocatalytic activity in GaSe and InSe nanosheets: the role of surface oxides

Gallium selenide (GaSe) and its parental compound indium selenide (InSe) are attracting considerable attention in recent years, in consideration of their band-gap energy in the visible, which makes them particularly suitable for photodetection. However, GaSe and InSe have also potential applications in nonlinear optics, solar cells and catalysis. In particular, their use in catalysis is strictly connected with surface chemical reactions, which could be monitored by surface-science techniques. 
An international team of researchers from Italy, Czech Republic, Turkey, Russia and China, by means of experiments at the BACH and Nanospectroscopy beamlines of Elettra synchrotron, has clarified that the surface transformation of GaSe and InSe into Ga2Oand In2Oin ambient conditions plays a pivotal rolein (photo)catalysis with GaSe and InSe nanosheets.
As-exfoliated stoichiometric GaSe single crystal assumes an amorphous Ga2Oskin upon 5 min air exposure, reaching a thickness of 1.8 ±0.2 nm after prolonged storage in air (see X-ray photoelectron spectroscopy of Ga-3d and Se-3d core levels in Fig. 1a, b). The formation of an amorphous oxide skin was proved by low-energy electron microscopy (LEEM), I-V curves and microspot low-energy electron diffraction (µ-LEED) images of the GaSe surface before and after 40 days in air (see LEEM in Fig. 1c, d). The presence of Se vacancies, as well as exfoliation in nanosheets naturally exhibiting Ga-edge sites, accelerates the oxidation process by ~10times. Remarkably, calculated values of free energies for hydrogen evolution reaction (HER) evidence unsuitability of both bulk GaSe and InSe for this reaction, with energy barriers as high as 1.9 and 1.5 eV/H+, respectively. 


Figure 1. (a) Ga-3d and (b) Se-3d core-level spectra collected from as-cleaved GaSe exposed to air for 2 hours, 1 day and 1 year. The photon energy is 830 eV and each spectrum is normalized to the corresponding maximum height. Panels (c) and (d) show LEEM images of as-cleaved GaSe and the same surface after 40 days in air (at kinetic energies of 5.0 and 0.6 eV, respectively).


The presence of Se vacancies in both GaSe and InSe significantly decreases the energy cost of the process (1.5 and 0.7 eV/H+, respectively), but the magnitude is still significantly larger than that of the Pt(111) surface (0.1 eV/H+), usually taken as standard reference. In the case of free-standing monolayers (dashed lines in Fig. 2), the values of the energy cost of HER are even larger, i.e., 2.2 and 1.8 eV/H+for GaSe and InSe, respectively. Therefore, the common picture that liquid-phase exfoliation of GaSe and InSe favors the improvement of HER performance on the basis of the prominence of edges behaving as active sites should be revised. On the contrary, calculated values for Ga2Oand the sub-stoichiometric oxide Ga2O2.97 are 1.1 and -0.3 eV/H+. Therefore, for Ga2O2.97 the Heyrovsky step (Hads+ H++ e-→ H2) of HER is exothermic. 
Accordingly, contrary to the common view, exfoliation in nanosheets does not improve HER kinetics in spite of the higher surface-to-volume ratio, but rather enhances the oxidation rate, which plays an unexpectedly beneficial role.
Recently, GaSe has been proposed for photochemical water splitting. However, it is evident that the role of GaSe in photochemical water splitting is just to generate electron-hole pairs via light harvesting, while Ga2O3skin represents the only chemically active part of the photoelectrode.
These results elucidate the role of surface oxides in electrocatalysis and photocatalysis using nanosheets of III-VI layered semiconductors. These findings pave the way for a novel generation of efficient and cost-effective (photo-) electrocatalysts, based on self-assembled heterostructures formed by taking advantage of the natural interaction of sub-stoichiometric van der Waals semiconductors with air.

Figure 1. Free energy diagram for HER in acidic environment over the surface of bulk and monolayers of GaSe, GaSe0.94, InSe, and InSe0.94 and over the surface of Ga2O3 and Ga2O2.97. Results for monolayers are shown by dashed lines. Note that the line related to the free energy for GaSe0.94 is overlapped with that associated to InSe0.94



This research was conducted by the following research team:

Silvia Nappini1, Federica Bondino1, Danil W. Boukhvalov2, Gianluca D’Olimpio3, Luca Lozzi3, Luca Ottaviano3, Antonio Politano3, Mykhailo Vorokhta4,Francesca Genuzio5, Tevfik Onur Menteş,Andrea Locatelli5, Valentina Paolucci6, Bekir Gürbulak7, Songül Duman8


1Consiglio Nazionale delle Ricerche (CNR)- Istituto Officina dei Materiali (IOM), Laboratorio TASC, Trieste, Italy
College of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry University, Nanjing, P. R. China; Theoretical Physics and Applied Mathematics Department, Ural Federal University, Ekaterinburg, Russia
Department of Physical and Chemical Sciences, University of L’Aquila, L’Aquila (AQ), Italy
Charles University, Prague, Czechia

Elettra-Sincrotrone S.C.p.A, Trieste, Italy

Department of Industrial and Information Engineering and Economics, University of L’Aquila, L’Aquila, Italy
Department of Physics, Faculty of Sciences, Atatürk University, Erzurum, Turkey
Basic Sciences Department, Faculty of Sciences, Erzurum Technical University, Erzurum, Turkey

Contact persons:

Silvia Nappini, e-mail:
Federica Bondino, email: bondino@iom.cnr.it
Antonio Politano, e-mail: antonio.politano@univaq.it


G. D'Olimpio, S. Nappini, M. Vorokhta, L. Lozzi, F. Genuzio, T.O. Menteş, V. Paolucci, B. Gürbulak, S. Duman, L. Ottaviano, A. Locatelli, F. Bondino, D.W. Boukhvalov, A. Politano, "Enhanced electrocatalytic activity in GaSe and InSe nanosheets: the role of surface oxides", Adv. Funct. Mater., 30 (2020) 2005466. DOI:10.1002/adfm.202005466


Last Updated on Friday, 02 April 2021 08:39