Ultrathin ZrO₂ thickness control on TiO₂/ZrO₂ core/shell nanoparticles using ZrO₂ atomic layer deposition and etching

Atomic layer deposition (ALD) and atomic layer etching (ALE) techniques were used to control the ZrO₂ shell thickness on TiO₂/ZrO₂ core/shell nanoparticles. ALD and ALE were performed at 200 °C while the nanoparticles were agitated using a rotary reactor. To increase the ZrO₂ shell thickness, ZrO₂ ALD films were deposited using sequential exposures of tetrakis(dimethylamino) zirconium and H₂O. Ex situ analysis using transmission electron microscopy (TEM) observed the growth of the ZrO₂ shells. The ZrO₂ ALD led to more spherical ZrO₂ shells on the crystalline and irregular TiO₂ cores. The ZrO₂ ALD on the nanoparticles had a growth rate of 0.9 ± 0.1 Å/cycle. Tunable ZrO₂ coatings were observed with thicknesses ranging from 5.9 to 27.1 nm after 240 ZrO₂ ALD cycles. To demonstrate the decrease in the ZrO₂ shell thickness, the ZrO₂ film was then etched using sequential hydrogen fluoride (HF) and TiCl₄ exposures. Quadrupole mass spectrometry experiments performed in a separate reactor identified the volatile products during ZrO₂ ALE. H₂O was monitored during HF exposures, and ZrCl₄ etch products and TiF_xCl_y ligand-exchange products were observed during TiCl₄ exposures. Ex situ TEM studies revealed that the ZrO₂ shells remained spherical during ZrO₂ ALE. The ZrO₂ ALE on the nanoparticles had an etch rate of 6.5 ± 0.2 Å/cycle. Tunable ZrO₂ coatings were produced from 27.1 down to 7.6 nm using 30 ZrO₂ ALE cycles. This study demonstrated that ZrO₂ ALD and ZrO₂ ALE can control the thickness of ZrO₂ shells on TiO₂/ZrO₂ core/shell nanoparticles without inducing nanoparticle agglomeration.