Investigating the role of base chemistry in doping efficiency and crystallinity of F-doped SnO2 nanoparticles

Abstract

Fluorine-doped tin oxide (FTO) is a widely used transparent conductive oxide, yet its synthesis through hydrothermal methods remains challenging due to low dopant efficiency. The pH of the reaction medium, adjusted by acids or bases, plays a crucial role in determining doping efficiency by influencing the hydrolysis rate of Sn precursors and the solubility of fluorine sources. This study systematically investigated the effect of pH on doping efficiency and crystallinity in F-doped SnO₂ nanoparticles synthesised hydrothermally using two base adjusters: ammonium hydroxide (NH₄OH) and tetramethylammonium hydroxide (TMAH). The results showed that pH directly affected both crystallite size and fluorine incorporation. Fine crystalline SnO₂ nanoparticles with a tetragonal rutile phase were successfully obtained, with average crystallite sizes decreasing from 4.51 nm (pH ∼0.6, unadjusted) to 3.65 nm (NH₄OH, pH 7) and 3.45 nm (TMAH, pH 7), driven by enhanced hydrolysis and nucleation at higher pH. While decreasing proton concentration (at pH > 3) promotes Sn–F interaction, excessive OH⁻ and base-derived cations can hinder fluorine diffusion. The optimal F/Sn ratios of 0.61 (NH₄OH, pH 3) and 0.36 (TMAH, pH 2) were achieved under conditions that balance hydrolysis kinetics and cation interference. Elemental and spectroscopic analysis further confirmed that fluorine was successfully incorporated into the SnO₂ lattice via O-to-F substitution. These findings emphasise the importance of precise pH control in tailoring the structural and chemical properties of F-doped SnO₂, reinforcing its potential for future optoelectronic applications.