The analogs of ionic liquids known as deep eutectic solvents (DESs) are made up of a hydrogen bond donor and hydrogen bond acceptors. The hydrogen bond donor and acceptor combine to give a eutectic mixture whose melting point is lower than that of the individual components. DESs are biodegradable, non-toxic, and affordable. Their ability to efficiently dissolve transition metal oxides insoluble in aqueous solvents has made them attractive replacements for corrosive inorganic acids in the solution synthesis of materials. Semiconducting ternary oxides and sulfides have been explored as transducers, ferroelectrics, photocatalysts, photovoltaics, and thermoelectric materials. Their properties can be tuned by varying their synthesis method. Solution syntheses have been explored in the synthesis of these materials because they promote easy mixing of the precursor, ensuring the proper distribution of the dopant. Here, we investigate the DES-assisted synthesis of undoped and aluminum-doped SrTiO3, AgBiS2, and Cu3BiS3. The eutectic mixture of choline chloride and malonic acid is utilized for the DES-assisted synthesis of the oxides. Since TiO2 is not soluble in DES, a DES-soluble, non-volatile titanium precursor was utilized in this synthesis. The effect of two heating profiles on properties such as crystallite size, unit cell size, morphology, aluminum-doping, bandgap, and photocatalytic properties of the synthesized oxides are investigated. The eutectic mixture of choline chloride and thiourea was chosen as the DES for the synthesis of the sulfides, where thiourea is both the hydrogen bond donor and the sulfur source. The morphologies, optimal synthesis condition, and the thermal stabilities of the sulfides were investigated. The thermoelectric properties, and the in-situ high-temperature synchrotron powder X-ray diffraction data, which shows the polymorphic transition of Cu3BiS3 with changing temperature will be discussed.

Environmentally friendly synthesis of functional oxides and sulfides

ACS Fall 2021

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