Conference-ALC ’ 17-Preparation and Surface Reduction Behavior of CeO 2 Nanoparticle Layer on Al 2 O 3 ( 0001 ) Crystal Substrate

The CeO2 NP layer on Al2O3(0001) prepared by dipping method using CeO2 NPs colloid solution showed less contacted state among the primary particles even after heat treatment at 1000◦C in air. The surface reduction behavior of CeO2 NP layer on Al2O3(0001) was investigated by XPS under the Ar + sputtering in vacuum. The result indicates that Ar sputtering induced a considerable reduction of CeO2 NP layer on Al2O3(0001) and its final ratio of Ce was 79%. In comparison with CeO2 powder, the difference between their final ratio of Ce 3+


I. INTRODUCTION
Nanoparticles (NPs) materials have attracted much interest in recent years owing to novel functions with nanometer regime, including electronic, magnetic and chemical properties [1][2][3].Catalytic properties are also expected for metal and metal oxide NPs because of novel state containing active species as well as large specific area.Cerium dioxide (CeO 2 ) NPs will be useful in catalysis for its excellent properties as an oxygen storage capacity material which is used as in three-way catalysts (TWCs) [4][5][6][7][8].Recently, we have developed a fabrication process of CeO 2 NPs and its thin layer on several substrates [9,10].A simple dipping process of colloidal solution can bring higher dispersion state of NPs even after heat treatment at elevated temperature.The surface characterization is essential for understanding the catalytic properties of materials, because catalytic reactions occur at the surface of the catalyst.In particular, XPS has been widely used to study the chemical state of CeO 2 .In this study, we examine the surface reduction behavior of CeO 2 NP layer on Al 2 O 3 (0001) and characterized the state of Ce 3+/4+ by XPS under the Ar + sputtering in vacuum.

A. Synthesis of CeO2 nanoparticles
The starting CeO 2 NPs was prepared by the same method as described in previous papers [9,10].Briefly, diammonium cerium(IV) nitrate [(NH 4 ) 2 Ce(NO 3 ) 6 ] and potassium oleate (C 17 H 33 COOK) were dissolved and mixed in distilled water under stirred condition, followed by addition of ammonia aqueous solution.The solution mixture was then sealed in a Teflon-lined stainless steel autoclave (100 mL) and heated at 200 • C for 48 h, and the products were separated from the reaction solution by centrifugation [11,12].The CeO 2 precipitates were washed with distilled water three times and dried at 90 • C for 24 h in air, and then dispersed in toluene (C 6 H 5 CH 3 ), leading to stable colloid solution.

B. Preparation of CeO2 nanoparticle layer on Al2O3(0001)
Al 2 O 3 (0001) crystal plate was used as substrate for the deposition of CeO 2 NPs layer.The substrate was immersed in the CeO 2 solution for 5 s.After CeO 2 NPs were deposited, the substrate was pulled upward at a constant speed of 10 mm s −1 .Excess liquid was drained from the surface, and evaporated by drying in atmosphere.Subsequently, CeO 2 /substrate samples were heat-treated at 1000 • C for 3 h in air.As a reference, CeO 2 NPs without substrate were calcined at 400 • C for 3 h in air, followed by heat treatment at 1000 • C for 3 h in air.

C. Characterization
The crystal phase was analyzed by an X-ray diffractometer (XRD; Rigaku MiniFlex II) with Cu Kα radiation at 15 kV and 30 mA.Atomic force microscopy (AFM) studies of CeO 2 NP layers on substrates were carried out using an SPM-9700 (Shimadzu) in the noncontact mode.The AFM profiler mode was used to determine roughness and film thickness after peeling a thin film from a substrate by rubbing with a crystal.The cross-sectional microstructure of the CeO Al 2 O 3 (0001) was observed using a field emission scanning electron microscope (FE-SEM; Hitachi S-4800).The morphologies of powder and film samples were characterized using a transmission electron microscopy (TEM; JEOL JEM-2100Plus).The X-ray photoelectron spectroscopy (XPS) measurements were performed with the PHI 5000 Versa Probe equipped with monochromatic Al Kα X-rays.In order to obtain reduction behaviors of CeO 2 , the powder and film samples were subjected to 4 keV Ar + sputtering for different periods of 15-240 s.The background of XPS spectra was subtracted by the Shirley procedure and the peaks were separated to measure the content ratio of Ce 3+ /Ce 4+ after fitting by the Gaussian-Lorentzian function [13].

III. RESULTS AND DISCUSSION
The XRD patterns of the CeO 2 layers deposited on Al 2 O 3 (0001) were assigned to CeO 2 on the substrates, and no solid-state reactions between them were observed after heat treatment at 1000 • C for 3 h in air. Figure 1 shows the AFM images of the surface of the CeO 2 NPs layer on Al 2 O 3 (0001), heated at 1000 • C. The surface morphology of CeO 2 NP layers become a granular microstructure with roughness (a root mean square: R rms ) of 3.2 nm.The film thickness was approximately 90 nm.
Figure 2 shows the TEM images of (a) CeO 2 powder and (b) CeO 2 NP layer on Al 2 O 3 (0001), respectively.In CeO 2 powder the formation of agglomerated larger grains was observed by sintering (and grain growth) among the starting nanoparticles with ca.50 nm in average diameter.On the other hand, CeO 2 NP layer after peeling a thin film form a coated substrate showed less contacted state among the primary particles even after heat treatment at 1000 • C in air.The particle size was approximately 20-40 nm which was consistent with the surface image of film on substrate in AFM (Fig. 1).
Before Ar + sputtering (0 s), the amount of Ce 3+ versus total Ce (Ce 3+ and Ce 4+ ) was 27% in the CeO 2 powder, and that of Ce 3+ was 28% in the CeO 2 NPs on Al 2 O 3 (0001).Therefore, in both the CeO 2 powder and the CeO 2 NP layer, Ce cation is mainly in the +4 oxidation state on the surface.The intensity of Ce 3d photoelectron peak due to Ce 4+ decreased as Ar + sput- Figure 4 shows the relative amount of Ce 3+ as a function of the Ar + sputtering time for both CeO 2 samples.The Ce 3+ fraction in the CeO 2 NP layer on Al 2 O 3 (0001) increased much faster than that in the CeO 2 powder sample at the initial stage as a function of sputtering time, then finally reached an equilibrium fraction (180 s).During the periods from 0 s to 180 s, the increasing fraction of Ce 3+ was larger for the CeO 2 NP layer on Al 2 O 3 (0001) (from 28% to 79%) than that of the CeO 2 powder (from 27% to 68%).
If the surface of polycrystalline CeO 2 NPs is exposed to vacuum in the XPS apparatus, the reduction of CeO 2 will be expected by the same manner as H 2 -reduction in the temperature programed reduction experi-ment which is widely used in catalytic research.Especially, since the fast reduction rate bring the better redox catalysis of CeO 2 -based catalyst, for example, in application of the oxygen storage capacity.In a previous paper, we examined the reduction behavior using the TPR technique, and concluded that CeO 2 on Al 2 O 3 substrate lead to fast reduction rate due to the stabilization of CeO 2 NPs on substrate even after heat treatment at 1000 • C [9].The TEM images (Fig. 2) suggest the open-structured and nano-polycrystalline morphology of CeO 2 when it is on the substrate.Since the surface oxygen is active in CeO 2 , the TPR result is reasonable regarding with catalytic properties of nanocrystalline CeO 2 .
Although the reduction of CeO 2 was accelerated by Ar + during sputtering in this experiment, the final ratio of Ce 3+ was 79% for the CeO 2 NP layer on Al 2 O 3 (0001) and 68% for CeO 2 powder, which difference was small.Thus, our experiment showed that the reduction was slightly influenced by the difference in reducing ability of each CeO 2 , suggesting the existence of more important factor for reduction behavior.The Ar + sputtering induces the reduction of Ce 4+ to Ce 3+ in CeO 2 by partially chemical reaction (like H 2 -reduction), however maybe major cause will be physical effect.When Ar + ions are bombarded on the solids (e.g., CeO 2 ), its energy is gradually decreased by interactions with atoms in solid, and Ar + ions penetrate to a certain depth depending on its energy [20].During the penetrating to the solid, Ar + ions repeat collision and scattering and induce collision cascade, leading to production of thermal energy and surface atoms sputtering as a result of development of collision cascade up to solid surface.The depth profile of the implanted ions is predicted as a Gaussian distribution having the maximum concentration [21].The maximum of the implanted ions moves to the surface with increasing dose and the profiles become constant, because of increased density at the solid surface, resulting in blocking the Ar + ions penetration into the solid.After that, Ar + ions recoil and induce the sputtering at the solid surface, then, a preferential loss of the lighter component (i.e., oxygen in CeO 2 ), so-called preferential sputtering, is happened [22].This is the physical effect which mainly causes the reduction process of CeO 2 .The reduction of metal oxides in XPS depth profile measurement with Ar + sputtering has been reported by several researchers [21,23].Mitchell et al. have reported preliminary experimental results of the reduction in XPS measurement with Ar + sputtering [22].In the removal process during ion irradiation (depth profile), the quantity of material is represented by a function of the voltage and mass of Ar ion.On the other hand, at the initial stage of sputtering time, while Ar + ions are penetrating to the solid, kinetic energy of Ar + ions transfers thermal energy in solid lattice.In general, the reduction reaction of metal oxide can be represented by the Ellingham diagram, which is known as oxygen potential diagram and associates the stability of metal oxide with its equilibrium oxygen partial pressure.CeO 2 can be easily reduced to nonstoichiometric CeO 2−x (0 ≤ x ≤ 0.5) at elevated temperatures and under reduced partial pressures of oxygen, and the equilibrium reaction is described as CeO 2−x + 1/2O 2 = CeO 2 .Therefore, the relation of equilibrium composition of CeO 2−x vs T and P O2 has been discussed [24].The energy transfer of irradiated Ar + (e.g., with 4 keV in present experiment) to thermal energy in solid lattice should induces the reduction of Ce 4+ to Ce 3+ in vacuum, depending on elevated temperatures.Such chemical effect in CeO According to Fig. 4, the final ratio of Ce 3+ was 79% for the CeO 2 NP layer on Al 2 O 3 (0001) and 68% for CeO 2 powder, which difference was 11%.We cannot precisely estimate the local heat-area in present porous powder and thin-layer/substrate samples.However, the difference in reduction ratio will be explained by such chemical effect which is not only physical preferential-sputtering during Ar + ions irradiation.Thus, the slightly better reducingcapacity of CeO 2 /Al 2 O 3 system is explained with the geometrical morphology in the substrate-stabilized and dispersed CeO 2 NPs.

IV. CONCLUSION
The CeO 2 NP layer on Al 2 O 3 (0001) was prepared by dipping method using CeO 2 NPs colloid solution.It showed less contacted state among the primary particles even after heat treatment at 1000 • C in air, in comparison with CeO 2 powder prepared as a reference.The surface reduction behavior of CeO 2 NP layer on Al 2 O 3 (0001) and CeO 2 powder were investigated by XPS under the Ar + sputtering in vacuum.The results indicate that Ar + sputtering induced a considerable reduction of both CeO 2 samples.Although the reduction of CeO 2 was accelerated by Ar + during sputtering, the final ratio of Ce 3+ was 79% for the CeO 2 NP layer on Al 2 O 3 (0001) and 68% for CeO 2 powder, which difference was small.Therefore, it was indicated that the Ar + sputtering induced the reduction of Ce 4+ to Ce 3+ in CeO 2 by mainly physical effect, however the chemical effect attributed to the local thermal energy induced by Ar + ions bonbardment varies by sample geometry.CeO 2 /Al 2 O 3 system with the substrate-stabilized and dispersed CeO 2 NPs showed slightly better reducing capacity of Ce 4+ to Ce 3+ .
2 powders or layered CeO 2 NPs is attributed to difference in efficiency during thermal energy transfer from irradiated Ar + ions around CeO 2 aggregates.Because both powdered and layered CeO 2 NPs are porous and have low density, irradiated Ar + ions with 4 keV might pass through the CeO 2 NPs with approximately 50 nm and 20-40 nm in diameter, respectively.The passed through Ar + ions will implant into CeO 2 NPs or Al 2 O 3 substrate located far away spatially surface CeO 2 NPs.In the CeO 2 NP layer on Al 2 O 3 (0001), the thermal energy effectively induces by Ar + ions bombardment in high-density Al 2 O 3 (0001) substrate which is laying just under CeO 2 layer with approximately 90 nm in thickness.This is compared with CeO 2 powder itself with porous structure, in which Ar + ions can penetrate with long distance and heat effect is more reduced than that layer CeO 2 on Al 2 O 3 substrate.