Revisiting the Chemical Stability of Germanium Selenide (GeSe) and the Origin of its Photocatalytic Efficiency

Abstract

Recently, germanium selenide (GeSe) has emerged as a promising van der Waals semiconductor for photovoltaics, solar light harvesting, and water photoelectrolysis cells. Contrary to previous reports claiming perfect ambient stability based on experiments with techniques without surface sensitivity, here, by means of surface-science investigations and density functional theory, it is demonstrated that actually both: i) the surface of bulk crystals; and ii) atomically thin flakes of GeSe are prone to oxidation, with the formation of self-assembled germanium-oxide skin with sub-nanometric thickness. Surface oxidation leads to the decrease of the bandgap of stoichiometric GeSe and GeSe 1−x , while bandgap energy increases upon surface oxidation of Ge 1−x Se. Remarkably, the formation of a surface oxide skin on GeSe crystals plays a key role in the physicochemical mechanisms ruling photoelectrocatalysis: the underlying van der Waals semiconductor provides electron-hole pairs, while the germanium-oxide skin formed upon oxidation affords the active sites for catalytic reactions. The self-assembled germanium-oxide/germanium-selenide heterostructure with different bandgaps enables the activation of photocatalytic processes by absorption of light of different wavelengths, with inherently superior activity. Finally, it is discovered that, depending on the specific solvent-GeSe interaction, the liquid phase exfoliation of bulk crystals can induce the formation of Se nanowires.

III–VI and IV–VI van der Waals Semiconductors InSe, GaSe and GeSe: A Suitable Platform for Efficient Electrochemical Water Splitting, Photocatalysis and Chemical Sensing

“…Upon exposure of water vapor, GeSe x is formed, due to the Se‐depleted phase and the Ge 2 Se 3 /GeSeO x , [82] while GeSeO x represents an early stage of oxidation. The exposure of the pristine GeSe to molecular oxygen evidences higher reactivity in comparison to the water, consistently with the theoretical predictions [68] …”

“…Differently from GaSe, an oxidative process was observed in the as‐cleaved GeSe after exposure towards O 2 and H 2 O. After O 2 dosage, the Ge‐3d core level exhibit five new components, [68] correlated to different oxidation states. The thickness of the mixed phase of different Ge oxides after oxygen exposure was estimated to be ≈0.4 nm.…”

“…The energetics for the decomposition of Ge x Se to GeO 2 and Se demonstrates the favorability of this process (Table 2). Accordingly, in oxidative environments GeO 2 skin is formed over GeSe, while in the case of Se‐rich GeSe samples, additional nanocrystalline Se‐based structures could be formed [68] . Regarding the GeSe‐based monolayers, the decomposition of oxygen is an exothermic process.…”

“…Secondarly, the GeSeO surface layer is transformed into a combination of GeO, Ge 2 O 3 , and Ge 2 Se 3 structures. The third step involves the formation of GeO 2 , SeO 2 , and Se nanostructures [68] . Note that intercalacation of water is unfeasible, owing to a differential enthalpy of +108.36 kJ/H 2 O.…”

“…Once GeSe is exposed to air, the oxidation process starts in a few minutes, with the formation of an oxide skin of 0.3 nm [68] . For a longer exposure (30 hours), GeO 2 starts to form and after 40 days the total conversion the intermediate oxide in GeO 2 is completed, with a partial hydroxylation in GeO 2‐x (OH) y [68,90] . Correspondingly, in Se‐3d spectra a component related to SeO x sub‐oxide species emerges [91] .…”