Preparation of electrocatalysts using a thiol-amine solution processing method.

Solution processing has emerged as a promising option for the preparation of functional materials with methods that tend to be cheap, less energy intensive, and allow for high throughput and large area deposition. A recently developed method, whereby a binary thiol-amine solvent mixture is used to dissolve bulk oxides, chalcogenides, or elemental materials to yield molecular precursor inks for the deposition of phase pure chalcogenide materials, has recently garnered attention for applications in optoelectronics, thermoelectrics, and electrocatalysis. Presented here is a summary of reports on the application of this "alkahest" method for the deposition of electrocatalytic materials. Ink formulations drawing on the vast library of bulk precursor solutes have already resulted in a large collection of synthesized functional materials, with room still to explore new, or less studied, materials. Additionally, as is seen for the electrocatalysts, engineering of inks and deposition procedures can lead to higher performance through optimized morphology and form factors.

Room Temperature Dissolution of Bulk Elemental Ni and Se for Solution Deposition of a NiSe2 HER Electrocatalyst.

With hydrogen fuel becoming a more viable alternative to fossil fuels comes the need for inexpensive, low-energy hydrogen production. Here, a low-temperature direct solution-processing method is presented for the deposition of earth-abundant pyrite-type NiSe2 as an efficient hydrogen evolution reaction (HER) catalyst. Thin films of phase-pure NiSe2 are deposited from a precursor ink prepared by room-temperature dissolution of bulk elemental Ni and Se in a binary thiolamine solvent mixture. The nanostructured NiSe2 thin films demonstrate high HER catalytic activity with 100% Faradaic efficiency.

Solution processing of chalcogenide materials using thiol-amine "alkahest" solvent systems.

Macroelectronics is a major focus in electronics research and is driven by large area applications such as flat panel displays and thin film solar cells. Innovations for these technologies, such as flexible substrates and mass production, will require efficient and affordable semiconductor processing. Low-temperature solution processing offers mild deposition methods, inexpensive processing equipment, and the possibility of high-throughput processing. In recent years, the discovery that binary "alkahest" mixtures of ethylenediamine and short chain thiols possess the ability to dissolve bulk inorganic materials to yield molecular inks has lead to the wide study of such systems and the straightforward recovery of phase pure crystalline chalcogenide thin films upon solution processing and mild annealing of the inks. In this review, we recount the work that has been done toward elucidating the scope of this method for the solution processing of inorganic materials for use in applications such as photovoltaic devices, electrocatalysts, photodetectors, thermoelectrics, and nanocrystal ligand exchange. We also take stock of the wide range of bulk materials that can be used as soluble precursors, and discuss the work that has been done to reveal the nature of the dissolved species. This method has provided a vast toolbox of over 65 bulk precursors, which can be utilized to develop new routes to functional chalcogenide materials. Future studies in this area should work toward a better understanding of the mechanisms involved in the dissolution and recovery of bulk materials, as well as broadening the scope of soluble precursors and recoverable functional materials for innovative applications.

Dissolution of Sn, SnO, and SnS in a Thiol-Amine Solvent Mixture: Insights into the Identity of the Molecular Solutes for Solution-Processed SnS.

Binary solvent mixtures of alkanethiols and 1,2-ethylenediamine have the ability to readily dissolve metals, metal chalcogenides, and metal oxides under ambient conditions to enable the facile solution processing of semiconductor inks; however, there is little information regarding the chemical identity of the resulting solutes. Herein, we examine the molecular solute formed after dissolution of Sn, SnO, and SnS in a binary solvent mixture comprised of 1,2-ethanedithiol (EDT) and 1,2-ethylenediamine (en). Using a combination of solution (119)Sn NMR and Raman spectroscopies, bis(1,2-ethanedithiolate)tin(II) was identified as the likely molecular solute present after the dissolution of Sn, SnO, and SnS in EDT-en, despite the different bulk material compositions and oxidation states (Sn(0) and Sn(2+)). All three semiconductor inks can be converted to phase-pure, orthorhombic SnS after a mild annealing step (∼350 °C). This highlights the ability of the EDT-en solvent mixture to dissolve and convert a variety of low-cost precursors to SnS semiconductor material.

Solution-Phase Conversion of Bulk Metal Oxides to Metal Chalcogenides Using a Simple Thiol-Amine Solvent Mixture.

A thiol-amine solvent mixture is used to dissolve ten inexpensive bulk oxides (Cu2O, ZnO, GeO2, As2O3, Ag2O, CdO, SnO, Sb2O3, PbO, and Bi2O3) under ambient conditions. Dissolved oxides can be converted to the corresponding sulfides using the thiol as the sulfur source, while selenides and tellurides can be accessed upon mixing with a stoichiometric amount of dissolved selenium or tellurium. The practicality of this method is illustrated by solution depositing Sb2Se3 thin films from compound inks of dissolved Sb2O3 and selenium that give high photoelectrochemical current response. The direct band gap of the resulting material can be tuned from 1.2-1.6 eV by modulating the ink formulation to give compositionally controlled Sb2Se(3-x)S(x) alloys.