Rapid synthesis of tin-doped indium oxide microcrystals in supercritical water using hydrazine as reducing agent

Tin-doped indium oxide (ITO) microcrystals were successfully synthesized in supercritical water (SCW) using hydrazine (N₂H₄) as a reducing agent. Using a mixture of tin and indium hydroxides prepared at pH = 9.4 as a precursor, ITO microcrystals were synthesized at temperatures 400-450 °C under pressures 25-30 MPa. Synthesizing in SCW effectively shortened the time required to synthesize the ITO microcrystals to below 30 mins. The effect of reducing agents (ethanol, formic acid, and N₂H₄) and reaction conditions on the formation of ITO particles were investigated, and it was found that N₂H₄, which is superior to ethanol and formic acid, played a key role in the doping of the In₂O₃ structure with Sn⁴⁺ to form ITO particles with a blue color. Addition of N₂H₄ possibly depleted the oxygen in the In(Sn)OOH structure, accelerating the formation of cubic In₂O₃ and introduced Sn⁴⁺ into the structure along with the creation of oxygen vacancies. It was also found that the high temperatures and the properties of the SCW, such as ion product, strongly affected the morphology of the ITO particles and the Sn⁴⁺ doping. Based on these results, a mechanism has been proposed for the synthesis of ITO particles under SCW conditions. This study demonstrates that due to the unique properties of SCW, the synthesis of doped oxides in SCW is a plausible alternative method.