A New Approach to Single-Step Fabrication of TiO x -CeO x Nanoparticles

Abstract

Mixed metal oxide (MMO) nanoparticles (NPs) are hybrids consisting of two or more nanoscale metal oxides. Advantages of MMO NPs over single metal oxides include improved catalytic activity, enhanced electrical and magnetic properties, and increased thermal stability due to the synergy of the different oxide components. This study presents a novel fabrication route for TiO₂-CeO₂ NPs enriched with oxygen vacancies using a Haberland-type gas aggregation cluster source. The NPs, deposited from different segmented Ti/Ce targets under varying O₂ addition, were examined with respect to final composition, morphology, and Ti, Ce surface oxidation states. Particle formation mechanisms are proposed for the observed morphologies. Additionally, available O₂ during deposition and its impact on the formation of defective sites were investigated. Defective sites in TiO₂-CeO₂ NPs were analyzed using transfer to X-ray photoelectron spectroscopy and transmission electron microscopy without contact to ambient oxygen. The incorporation of Ce to the target exhibits synergistic effects on the synthesis process. Segmented Ti/Ce targets enable the deposition of a broad range of mixed oxide NPs with diverse compositions and morphologies at considerably enhanced deposition rates, which is vital for practical applications. The presented fabrication approach is expected to be applicable for a broad variety of MMO NPs.

Keywords: TiO2‐CeO2 nanoparticles; defect formation; deposition rate; gas aggregation cluster source; mixed metal oxides.

Figure 1

Schematic representation of the GAS setup with a segmented Ti‐Ce metallic target.

Figure 2

NPs deposited from targets with different sizes of Ce inlet targets at 50–70 W magnetron power, 30–50% duty cycle, 30 sccm Ar flow, and 0.015 sccm O₂ flow. In the first row, sketches of the targets with a) 1”, b) 0.5”, and c) 0.25” Ce inlet targets are shown. Below, BF TEM images of the resulting particles are presented. SAED patterns of the respective samples are shown as an inset with references for CeO₂ ((111), (002), (022), and (113) reflections from ICSD 621 710)^{[ 86 ]} marked in yellow and TiO ((111), (002), and (022) reflections from ICSD 77 692)^{[ 87 ]} marked in green.

Figure 3

NPs deposited at 50 W, 30 sccm Ar, and 0.015 sccm O₂ flow from a Ti target with a 0.25″ Ce inlet target. a) ADF STEM image with corresponding b) overlay EDX map, c) Ti elemental EDX map, and d) Ce elemental EDX map.

Figure 4

XPS core‐level spectrum of a sample produced with 0.015 sccm external O₂ with respective fitting functions and background: a) Ce 3d with indicated binding energy position at 916.9 eV, b) Ti 2p, and c) O 1s.

Figure 5

a) BF TEM image of particles deposited without the addition of oxygen. b) SAED pattern acquired from an area with several hundreds of particles. c) Rotational average of the SAED from (b) with references for CeO₂ (ICSD 621 710),^{[ 86 ]} Ti (ICSD 168 830),^{[ 92 ]} and rutile phase TiO₂ (ICSD 33 842).^{[ 93 ]} d,e) show ADFSTEM images of particles with core‐shell, multicore, and Janus‐type morphology. The related overlay EDX maps are given in f,g). The Ce elemental map is colored in green, Ti in red, and O in blue. The NPs were deposited from the segmented target with 0.25″ Ce inlet.

Figure 6

XPS core‐level spectrum of a sample produced without external O₂ with respective fitting functions and background: a) Ce 3d with indicated binding energy position at 916.9 eV, b) Ti 2p, and c) O 1s.

Figure 7

a) HRTEM micrograph of a single Janus‐type NP that was transferred to the TEM without contact to ambient oxygen. b) FFT of the NP depicted in (a). The reflections corresponding to cubic TiO and CeO₂ with zone axis [001] are highlighted in red and green, respectively.

Figure 8

XPS core‐level spectra recorded directly after the vacuum transfer of the NP sample produced without external oxygen: a) Ti 2p, b) Ce 3d, and corresponding spectra recorded after two months of sample exposure to the atmosphere: c) Ti 2p and d) Ce 3d. All spectra include respective fitting functions and backgrounds. The Ce 3d spectra include indication lines at the binding energy position of 916.9 eV.

Figure 9

Schematic illustration of the formation mechanism of TiO _x ‐CeO _x nanoparticles without external O₂ supply.

Figure 10

BF TEM images of particles synthesized from the segmented target with 0.25″ Ce inlet at different Ar flow rates deposited directly on the TEM grid. The amount of larger particles reduces with increasing Ar flow rates.