Growth of Pt Nanoparticles on Reducible CeO₂(111) Thin Films: Effect of Nanostructures and Redox Properties of Ceria

Abstract

Pt nanoparticles grown on fully oxidized and partially reduced CeO x (111) thin films have been studied by scanning tunneling microscopy and X-ray photoelectron spectroscopy to understand the effect of redox properties and nanostructures of ceria supports on the growth of Pt. Deposition of 0.2 ML of Pt on CeO2 at 300 K produces two atomic layer high nanoparticles, while on reduced ceria films Pt favors the growth of smaller particles of one−two layer thick with a larger particle density. With the increase of Pt coverage, Pt particles on CeO2 grow in size while the Pt particle density significantly increases on the reduced ceria. Heating the surface to higher temperatures causes the Pt particle agglomeration, but Pt particles sinter less on the reduced ceria compared to those on the fully oxidized ceria. New particle structures are formed on reduced ceria as a result of heating which are suggested due to the encapsulation of Pt particles by ceria. In addition to the structural changes of the Pt particles, modifications of electronic properties of both ceria and Pt were observed upon Pt deposition as well as after heating. Our combined scanning tunneling microscopy and X-ray photoelectron spectroscopy studies suggest a complex growth behavior of Pt on ceria and a strong interaction between the Pt and the ceria support.

Controlling the charge state of supported nanoparticles in catalysis: lessons from model systems

“…34 In particular, shifts towards high binding energies are often observed for small supported nano-particles as compared to the bulk, but are not necessarily connected to the existence of a positive charge. 35,36 Another method is electron paramagnetic resonance (EPR), which changes as the spin state of a system varies and may be used when applied to model systems or powder samples. In certain cases, where the structure of a nanoparticle is known from scanning tunneling microscopy (STM) and the various states and symmetries of the system, if present, have been determined by scanning tunneling spectroscopy (STS) (i.e.…”

Oxygen Vacancies versus Fluorine atCeO2(111): A Case of Mistaken Identity?

“…More explicitly, calculations predict that the vacancy is more stable in the subsurface layer than in the surface layer [14], which would suggest that the number of surface defects should decrease with annealing time; that its diffusion barrier is small [15], which would render it highly mobile at the elevated temperatures at which the experiments were carried out; and that clustering of surface vacancies is energetically unfavourable [16], implying that they should exist primarily in isolation. Despite the doubt cast by these inconsistencies upon the interpretation of the defect structure of CeO 2 ð111Þ in terms of oxygen vacancies alone, no alternative model has been proposed, and numerous authors have subsequently used the "vacancy model" to interpret the results of their experiments, based both on STM and on other techniques [17][18][19][20].…”

Epitaxial CeO2 thin films for a mechanism study of resistive random access memory (ReRAM)

“…3c. The measured size of the Pt islands was approximately 8 nm in width and 1 nm in height; however, the reported size of Pt island prepared in a similar way was 2.2-3.5 nm in width and 0.8-0.9 nm in height, which were measured using STM [20,21]. Because of the lower lateral resolution in our AFM measurement using a PtIr-coated tip, we consider that the measured width of the Pt islands was bigger relative to their real size.…”

“…The appearance of a high Ce 4+ signal can be taken as evidence of the formation of cerium dioxide CeO 2 [16,24]. The spectra for Pt deposited CeO 2 show two more pairs of doublets indicated as f′ 1 and f′ 2 , which originated from different Ce 4f configurations in both the initial and final states associated with Ce 3+ ions [20,25]. The spectra are fitted based on the method described, e.g., in [26], and an example of the fitting for Pt/CeO 2 is shown in Fig.…”

“…The nominal thickness of Pt calculated from the attenuation of Ce 4f peak is about 0.1 ∼ 0.2 nm, which is much less than 1 ML. Pt is expected to form small islands in such conditions [20,21]. Because the tip was coated with PtIr, it is probable that there was a strong interaction between the tip and small Pt islands, and the Pt islands stuck to the tip, causing formation of lines instead of the image of Pt islands.…”