Solution‐Processed GaSe Nanoflake‐Based Films for Photoelectrochemical Water Splitting and Photoelectrochemical‐Type Photodetectors

Abstract

Gallium selenide (GaSe) is a layered compound, which has been exploited in nonlinear optical applications and photodetectors due to its anisotropic structure and pseudodirect optical gap. Theoretical studies predict that its 2D form is a potential photocatalyst for water splitting reactions. Herein, 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 is reported. In 0.5 m H2SO4, the GaSe photoelectrodes display the best PEC performance, corresponding to a ratiometric power‐saved metric for HER (Φsaved,HER) of 0.09% and a ratiometric power‐saved metric for OER (Φsaved,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 a light intensity of 63.5 µW cm−2 and applied potential of −0.3 V versus reversible hydrogen electrode (RHE). Stability tests of GaSe photodetectors demonstrated a durable operation over tens of cathodic linear sweep voltammetry scans in 0.5 m H2SO4 for HER. In contrast, degradation of photoelectrodes occurred in both alkaline and anodic operation due to the highly oxidizing environment and O2‐induced (photo)oxidation effects. The results provide new insight into the PEC properties of GaSe nanoflakes for their exploitation in photoelectrocatalysis, PEC‐type photodetectors, and (bio)sensors.

Hybrid Organic/Inorganic Photocathodes Based on WS2 Flakes as Hole Transporting Layer Material

“…However, the degradation was ascribed to the catalyst detachments, whereas no chemical analysis of CuI after the HER operation was reported. [ 55 ] More recently, 2D materials such as graphene oxide (GO), reduced graphene oxide (RGO), and transition metal dichalcogenides (TMDs) have been demonstrated as efficient CTLs and buffer layers in organic and perovskite solar cells, [ 75–82 ] as well as in other types of photoelectrodes, [ 83,84 ] including those for PEC cells. [ 70,71,85 ] In particular, GO‐ and RGO‐based HTLs and p‐doped MoS 2 have boosted the PEC performances of rr‐P3HT:PCBM photocathodes up to, respectively, J 0vsRHE = 6.01 mA cm −2 , V op = 0.6 V, [ 85 ] and J 0vsRHE = 1.21 mA cm −2 ; V op = 0.56 V. [ 71 ] Among 2D TMDs, WS 2 represents one of the most promising candidates for HTLs, thanks to high hole mobility ( ≈ 50 cm 2 V ‐1 s −1 ) and low effective hole reduced mass ( ≈ 0.46).…”

Liquid‐Phase Exfoliated Gallium Selenide for Light‐Driven Thin‐Film Transistors

“…The concentration of the obtained dispersion was 0.13 g L −1 , as estimated by the Beer–Lambert law [ 121 ] using an extinction coefficient α ≈ 113 L g –1 m –1 at 455 nm. [ 87 ] In order to confirm the production of few layer ε‐GaSe flakes with no defects, the effectiveness of the exfoliation of the layered crystal was evaluated by morphological, chemical and structural analyses. Figure a,b report representative transmission electron microscopy (TEM) and atomic force microscopy (AFM) images of the exfoliated ε‐GaSe flakes, respectively.…”

“…Raman spectroscopy measurements, shown in Figure 2g, were performed in order to demonstrate the crystalline integrity of the material. ε‐gallium GaSe selenide displays two A′‐type modes, one E′‐type mode and one A″‐type mode [ 87,123–125 ] in the bulk samples. The exfoliated ε‐GaSe spectrum shows the reduction of the E′ and the strengthening of the A′, and this can be explained by the reduction of the interlayer forces as confirmed also by previous theoretical and experimental studies.…”

“…[ 79 ] Moreover, due to low vapor pressure ( P v ≈ 0.05 kPa) [ 80 ] and high boiling point ( T b ≈ 200 °C), [ 81 ] the aforementioned solvents leave contaminants [ 82–84 ] in the deposited layers of the exfoliated materials, affecting the electronic properties of the devices. [ 72,85 ] Hence, LPE of 2D materials in low boiling point solvents, such as acetone, [ 86 ] alcohols, [ 57,72,85,87–94 ] and water/alcohol mixtures [ 57,72,82,88,95 ] has been investigated as a strategy to reduce the use of toxic chemicals, minimizing the contamination by solvent residuals in films composed of such flakes. The dispersions produced by LPE contain exfoliated flakes that differ in lateral size and thickness.…”

“…Spray coating is a forefront technique in electronics, [ 100–102 ] being an industrially compatible process that is already applied, for example, to the implementation of perovskite‐based solar cells, [ 103,104 ] InSe‐based photodetectors, [ 85 ] carbon nanotube‐based gas and temperature sensors [ 105,106 ] and can be further optimized to produce highly uniform ε‐GaSe films. ε‐Gallium selenide has been recently exfoliated through LPE by using isopropyl alcohol (IPA), [ 87 ] allowing the effective deposition of a percolative network of flakes and the implementation of a photoelectrochemical (PEC) photodetector. Despite the ease of fabrication, PEC photodetectors, according to the results of previous works reported extensively in literature, [ 107–114 ] are generally outperformed by solid‐state photodetectors (SSP) based on Schottky junctions, in terms of photoresponsivity and response speed.…”