Thiol‐Amine‐Based Solution Processing of Cu₂S Thin Films for Photoelectrochemical Water Splitting

Abstract

Cu2S is a promising solar energy conversion material owing to its good optical properties, elemental earth abundance, and low cost. However, simple and cheap methods to prepare phase‐pure and photo‐active Cu2S thin films are lacking. This study concerns the development of a cost‐effective and high‐throughput method that consists of dissolving high‐purity commercial Cu2S powder in a thiol‐amine solvent mixture followed by spin coating and low‐temperature annealing to obtain phase‐pure crystalline low chalcocite Cu2S thin films. After coupling with a CdS buffer layer, a TiO2 protective layer and a RuOx hydrogen evolution catalyst, the champion Cu2S photocathode gives a photocurrent density of 2.5 mA cm−2 at −0.3 V vs. reversible hydrogen electrode (VRHE), an onset potential of 0.42 VRHE, and high stability over 12 h in pH 7 buffer solution under AM1.5 G simulated sunlight illumination (100 mW cm−2). This is the first thiol‐amine‐based ink deposition strategy to prepare phase‐pure Cu2S thin films achieving decent photoelectrochemical performance, which will facilitate its future scalable application for solar‐driven hydrogen fuel production.

Solution-Processed Cu2S Nanostructures for Solar Hydrogen Production

“…The PEC performance of both Cu 2 S nanoplate and nanoparticle thin films were tested in a pH 7 electrolyte with FTO/Au/Cu 2 S/CdS/TiO 2 /RuO x device configurations, where 50 nm CdS was deposited on top of Cu 2 S thin films by CBD as the n-type junction layer, 100 nm TiO 2 was deposited onto the CdS layer by ALD as the protective layer, and 40 nm of photoelectrodeposited RuO x served as the hydrogen evolution reaction catalyst to facilitate the charge transfer from the electrode to the electrolyte ( Figure 5 a,b). 24 Cu 2 S thin films prepared by a similar thiol–amine-based solution-processing method were considered as the non-nanostructured Cu 2 S ref ( 14 ). Figure 5 c shows the current density response upon on–off simulated solar irradiation, and the onset potential could be determined by cyclic voltammetry (CV) scans in Figure S10 .…”

“…Figure c shows the current density response upon on–off simulated solar irradiation, and the onset potential could be determined by cyclic voltammetry (CV) scans in Figure S10. The reference photocathode based on the non-nanostructured Cu 2 S thin film gave a photocurrent density of 2.5 mA cm –2 at −0.2 V RHE with an onset potential of 0.42 V RHE , where the thickness of the Cu 2 S layer was 220 nm . Even though the thickness of the nanoplate Cu 2 S layer in the photocathode is around 100 nm, the photocathode based on the nanoplate Cu 2 S thin film gave a photocurrent density of 3.0 mA cm –2 at −0.2 V RHE with a similar onset potential of 0.43 V RHE .…”

“…Cu 2 S is one promising p-type semiconductor for solar energy conversion. The advantages of this material are high elemental earth abundance, nontoxicity, and suitable optical properties including an indirect band gap of 1.2 eV, a direct band gap of 1.8 eV, and a high absorption coefficient of over 10 4 cm –1 . , However, the presence of multiple secondary phases (monoclinic low chalcocite Cu 2 S stable below 105 °C, hexagonal high chalcocite Cu 2 S stable between 105 and 425 °C, cubic γ-chalcocite Cu 2 S stable at temperatures higher than 425 °C, monoclinic djurleite Cu 1.96 S, trigonal digenite Cu 1.8 S, orthorhombic anilite Cu 7 S 4 , and hexagonal covellite CuS) and the short minority carrier diffusion length have posed an obstacle for its practical applications. , In our recent study, we reported that phase-pure Cu 2 S thin films can be obtained by thiol–amine-based solution processing for the first time . Thiol–amine mixtures can be strong solvents, enabling the dissolution of more than 65 bulk inorganic materials, including ZnS which is insoluble in hydrazine, to form molecular inks. − Generally, upon a simple mild-temperature annealing step, the solid-state crystalline phase can be recovered.…”

“… 12 , 13 In our recent study, we reported that phase-pure Cu 2 S thin films can be obtained by thiol–amine-based solution processing for the first time. 14 Thiol–amine mixtures can be strong solvents, enabling the dissolution of more than 65 bulk inorganic materials, including ZnS which is insoluble in hydrazine, to form molecular inks. 15 − 19 Generally, upon a simple mild-temperature annealing step, the solid-state crystalline phase can be recovered.…”

A durable VO2 transition layer and defect inactivation in BiVO4via spontaneous valence-charge control

“…[1][2][3] PEC water splitting entails the use of an abundant source material (water), generates very pure H 2 , and only involves low levels of CO 2 emissions; thus, it is a signicantly attractive hydrogen-production strategy. 4,5 The core components of a PEC cell are a photoanode, which drives the oxygen evolution reaction [OER; +1.23 V vs. the reversible hydrogen electrode (RHE)], and a photocathode, which drives the hydrogen evolution reaction (HER; 0 V RHE ). 6,7 These semiconductor photoelectrodes must be designed for sufficient light absorption, effective charge separation, long-term stability, and high surface reactivity.…”