A novel two-step strategy to fabricate phase-pure SnS photoelectrode with tunable conductivity: formation mechanism and photoelectrochemical properties

Tin monosulfide (SnS), as a narrow band gap semiconductor for visible-light harvesting, nevertheless the easy formation of secondary phases such as Sn2S3 and SnS2 severely restricts its photoelectrochemical properties. Herein, we proposed a novel two-step strategy to fabricate phase-pure SnS photoelectrode with tunable conductivity on Ti foil substrate and carefully investigated the formation mechanism and photoelectrochemical properties. The tunable conductivity is determined by Na2SO4 pretreatment before annealing, which is supported by the EDS, XPS, and EPR characterizations. Na+ adsorbed to the edge of the precursor SnS2 nanosheets forming a dangling bond adsorption will protect S2- against reacting with the trace oxygen in the CVD system within a certain temperature range (< 525 ℃), thereby reducing the generation of S vacancies to adjust the S/Sn ratio and further regulating the conductivity type. Moreover, the anodic photocurrent density of SnS thin films was about 0.32 mA/cm2 at 1.23 V vs. RHE with the separation and injection efficiency of 1.22 % and 72.78 % and a maximum cathodic photocurrent density can reach approximately -0.36 mA/cm2 at 0 V vs. RHE with the separation and injection efficiency 1.15 % and 5.44 % respectively. The method shown in this work provides an effective approach to control the electrical conductivity of SnS thin films with considerable photocurrent response for phase-pure SnS.

Annealing effect of absorber layer on SnS/CdS heterojunction band alignments

The effect of annealing on physical properties of a SnS thin film and also on SnS/CdS heterojunction band alignment was studied. Vacuum annealing has greatly improved the crystalline quality of SnS and average grain size of 1.6 µm was achieved. Sulfur-rich secondary phases observed on the surface of as-grown SnS thin film were eliminated after vacuum annealing, resulting in a decrease of the resistivity and an increase of the carrier concentration of the film. A maximum hole mobility of 17 cm2V-1s-1 was obtained for SnS thin films annealed at 400 ֯C. A transition of SnS/CdS heterojunction from “spike” type to “cliff” type was observed when the vacuum annealed SnS thin film was post-air-annealed at 200 and 250 ֯C. The band alignment of SnS/CdS heterojunction could be adjustable between “spike” type to “cliff” type via vacuum annealing followed by post-air-annealing.

Vapor Phase Synthesis of SnS Facilitated by Ligand-Driven “Launch Vehicle” Effect in Tin Precursors

Extraordinary low-temperature vapor-phase synthesis of SnS thin films from single molecular precursors is attractive over conventional high-temperature solid-state methods. Molecular-level processing of functional materials is accompanied by several intrinsic advantages such as precise control over stoichiometry, phase selective synthesis, and uniform substrate coverage. We report here on the synthesis of a new heteroleptic molecular precursor containing (i) a thiolate ligand forming a direct Sn-S bond, and (ii) a chelating O^N^N-donor ligand introducing a “launch vehicle”-effect into the synthesized compound, thus remarkably increasing its volatility. The newly synthesized tin compound [Sn(SBut)(tfb-dmeda)] 1 was characterized by single-crystal X-ray diffraction analysis that verified the desired Sn:S ratio in the molecule, which was demonstrated in the direct conversion of the molecular complex into SnS thin films. The multi-nuclei (1H, 13C, 19F, and 119Sn) and variable-temperature 1D and 2D NMR studies indicate retention of the overall solid-state structure of 1 in the solution and suggest the presence of a dynamic conformational equilibrium. The fragmentation behavior of 1 was analyzed by mass spectrometry and compared with those of homoleptic tin tertiary butyl thiolates [Sn(SBut)2] and [Sn(SBut)4]. The precursor 1 was then used to deposit SnS thin films on different substrates (FTO, Mo-coated soda-lime glass) by CVD and film growth rates at different temperatures (300–450 °C) and times (15–60 min), film thickness, crystalline quality, and surface morphology were investigated.