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 TiFxCly 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.