  Gallium selenide (GaSe) is a layered compound, which has been exploited in
nonlinear optical applications and photodetectors due to its anisotropic
structure and pseudo-direct optical gap. Theoretical studies predicted that its
two-dimensional (2D) form is a potential photocatalyst for water splitting
reactions. Herein, we first report the photoelectrochemical (PEC)
characterization of GaSe nanoflakes (single-/few-layer flakes), produced via
liquid phase exfoliation, for hydrogen evolution reaction (HER) and oxygen
evolution reaction (OER) in both acidic and alkaline media. In 0.5 M H2SO4, the
GaSe photoelectrodes display the best PEC performance, i.e. a ratiometric
power-saved metric for HER ({\Phi}saved,HER) of 0.09% and a ratiometric
power-saved metric for OER ({\Phi}saved,OER) of 0.25%. When used as PEC-type
photodetectors, GaSe photoelectrodes show a responsivity of ~0.16 A W-1 upon
455 nm illumination at light intensity of 63.5 uW cm-2 and applied potential of
-0.3 V vs. reversible hydrogen electrode (RHE). The stability analysis of the
GaSe photodetectors evidences a durable operation over tens of cathodic linear
sweep voltammetry scans in 0.5 M H2SO4 for HER. Viceversa, degradation effects
have been observed in both alkaline and anodic operation due to highly
oxidizing environment and O2-induced (photo-)oxidation effects. Our results
provide new insight into PEC properties of GaSe nanoflakes for their
exploitation in photoelectrocatalysis, PEC-type photodetectors and
(bio)sensors.
