Spectroscopic study on spontaneously grown silver@ultra-thin cerium oxide nanostructures

Abstract

Ag@CeO₂ nanostructures have been recently reported to show unique catalytic properties but synthetic methods for them are limited. This study investigates microstructural characteristics of Ag@CeO₂ nanowires and nanoparticles with a spontaneously-grown ultra-thin ceria shell (∼0.5 nm), which were synthesized on CeO₂ substrate without using oxide precursors. Elemental mapping and line scanning by electron energy loss spectroscopy (EELS) suggest that the Ce in the CeO₂ substrate dissolved in the molten silver nitrate salt and was repelled to the surface of the Ag nanostructures to form continuous oxide shells during the growth of Ag single-crystals. X-ray absorption near edge structure (XANES) spectra verify that the valence of the Ce ions in the oxide layer was between Ce³⁺ and Ce⁴⁺.

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Among the common metal oxides, CeO₂, which is regarded as a wide band gap semiconductor,^1,2 has attracted much interest due to its unique properties.^3,4 To enhance functionality and performance, core–shell nanostructured materials have recently been reported and developed, due to the fact that the properties of single material are sometimes limited. The combination of a Ag core and ceria shell is of interest. Ag@CeO₂ core–shell nanostructures exhibit good catalytic ability and high chemical selectivity as well. For instance, Khan et al.⁵ investigated the photocatalytic performance of a Ag/CeO₂ nanocomposite and demonstrated that it possesses superior visible light photocatalytic activity compared with pure CeO₂. Zhang et al.⁶ successfully fabricated Ag@CeO₂ NPs through hydrothermal method and found that after annealing at 500 °C for 5 h the catalytic properties of Ag@CeO₂ NPs are superior to their single structure counterparts (either Ag or CeO₂).

Previous works^6–11 on the Ag@CeO₂ core–shell structures are listed in Table 1. As tabulated, most of them deal with the synthesis of Ag@CeO₂ nanoparticles (NPs) and only one study successfully prepared one-dimensional Ag@CeO₂. Mondragón-Galicia et al.¹¹ developed the synthesis the Ag@CeO₂ nanotubes through sol–gel method. In this method, a hollow structure is formed by reacting the Ag nanowires (NWs) with ammonium hydroxide followed by the migration of Ag atoms. During the heating process, outer-diffusion of the remnant Ag through the interface is faster than the inter-diffusion of the CeO shell, which eventually gives rise to a hollow Ag@CeO₂. With respect to Ag@CeO₂ nanowires, the relevant reports are quite rare. To have a simple yet effective approach to prepare the Ag@CeO₂ nanostructures with adjustable morphologies and dimensions would be a great challenge.

Table 1 A summary of previous works on the Ag@CeO₂ core–shell structures. NPs and NTs indicate nanoparticles and nanotubes^a

Metal@oxide	Method	Surfactant (S), precipitation (P)	Size	Core configuration	Precursor
a Igepal CO-520 (polyoxyethylene nonylphenyl ether), PVP (polyvinylpyrrolidone).
Ag@CeO2 NPs^6	Hydro-thermal	PVP (S)	Ag 18.3 nm	Single Ag	Ce(NO3)3·6H2O
			CeO2 6.2 nm
Ag@CeO2 NPs^7,8	Coprecipitation	Ammonia (P)	Ag 28 nm	Ag agglomeration	Ce(NO3)3·6H2O
			CeO2 14 nm
Ag@CeO2 NPs^9,10	Reverse micelle	Igepal CO-520 (S)	Ag 10 nm	Ag agglomeration	Ce(NO3)3·6H2O
		Cyclohexane (S)	CeO2 3 nm
Ag@CeO2 NTs^11	Sol–gel	PVP (S)	Agcore 49 nm	Ag hollow	Ce(NO3)3·6H2O
			CeO2 shell 78 nm

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This study synthesized Ag@CeO₂ nanostructures grown on CeO₂ thin film, including nanoparticles and nanowires, with the absence of oxide precursors, templates, and surfactants. Microstructural characteristics of the core–shell nanostructures were studied by TEM, electron energy loss spectroscope (EELS) and X-ray absorption near edge structure (XANES). The growth mechanisms were proposed and discussed.

CeO₂ thin film with the thickness of 720 nm was fabricated through sol gel method by dipping Si wafer substrate into the gel, for which the solution was prepared from CeCl₃O·7H₂O, citric acid, and ethanol with a volume ratio of 1 : 2.5 : 50 and stirred for 10 min before aging at room temperature (20 °C) for 2 days. It was then spun at 1000 rpm for 30 s. To achieve well crystallization, annealing of CeO₂ films was performed at 500 °C for 8 h in an oxygen atmosphere. Silver nitrate (AgNO₃) was used as the precursor to grow Ag@CeO₂ nanostructures. In order to obtain Ag@CeO₂ NPs, 15 μl of 0.05 M aqueous AgNO₃ was dropped onto the UV light-exposed (48 h) CeO₂ film, while the Ag@CeO₂ NWs was synthesized by dropping the salt solution onto non-UV-exposed CeO₂ film. Afterward, the samples were isothermally heated in air at 300 °C for 3 h using an infrared furnace, and cooled down to ambient temperature.

The nanostructures thus synthesized were removed from CeO₂ substrate in alcohol by ultrasonic oscillation for investigation. Phase identification was carried out using X-ray diffraction (XRD, D8 Discover X-ray diffractometer) (incidence angle of 0.3°) with graphite monochromatic Cu Kα radiation (λ = 0.15418 nm) at a scanning rate of 2° per minute from 20° to 70°. The structure and composition distribution of the Ag@CeO₂ NPs and NWs were characterized using a transmission electron microscope (TEM). Scanning transmission electron microscopy/electron energy loss spectroscopy (STEM/EELS) was done using a USTEM-NION microscope (USTEM200) and a GATAN EELS modified spectrometer. In order to limit the damage, the microscope was operated at 100 keV.

The X-ray absorption near edge structure (XANES) spectroscopy measurements at the Ce L3-edge were performed using Wigger beamline 17C at National Synchrotron Radiation Research Center (NSRRC), Taiwan, at room temperature, for which the monochromator Si (111) crystals were used. The energy resolution at the Ce L3-edge (5723 eV) was about 0.4 eV.

Fig. 1(a) and (b) show the Ag nanostructures grown on CeO₂ substrate. Ag NPs thus produced were with an average diameter of 18 ± 6 nm, while the length and diameter of Ag NWs were 7 μm and 100 ± 9 nm respectively. XRD patterns given in Fig. 1(c) depict that only the diffraction peaks of Ag could be detected. The growth mechanism of zero and one-dimensional Ag on oxide substrates has been proposed elsewhere.^12,13 The shape and dimensions of nanocrystals are closely related to the degree of excitation of the oxide substrate by UV exposure, and the amount of photo-induced defects. In brief, UV-excited oxide substrates, e.g. TiO₂ and CeO₂, can reduce metallic ions to form metallic nanostructure. The reduction of metallic ions is related to the photo-induced defects. Take TiO₂ for example, the reduction mechanism of metallic ions are proposed as follows.

Ti⁴⁺ + e⁻ → Ti³⁺ (3)

M⁺ + Ti³⁺ → M + Ti⁴⁺ (4)

where e⁻ and h⁺ are electron and hole. V_o is oxygen vacancy, and M⁺ represents metallic ion. A fully-excited oxide substrate possesses homogeneous distribution of photo-induced defects which form nanoparticles covering the whole oxide surface. In contrast, a limited excitation by black blackbody radiation during annealing process results in inhomogeneous distribution of photo-induced surface defects. This contributes to more localized nucleation sites and thus the growth of nanowires.

Fig. 1 (a) NPs formed on UV-exposed CeO₂ (the observed particles were already removed from substrate), (b) NWs grown non UV-exposed substrate, and (c) XRD patterns of the nanostructures shown in (a) and (b).

Fig. 2(a) shows the HRTEM images of an as-synthesized Ag NP, depicting a lattice spacing of 0.23 nm, which could be referred to {111} planes of Ag. Diffraction pattern obtained by Fourier transform was shown in Fig. 2(b). There is only one set of reciprocal lattices, indicating that the Ag nanoparticle is a single crystal. EELS mapping in Fig. 2(c) and elemental line scanning in Fig. 2(d) demonstrate the presence of a very thin shell layer on Ag NPs comprising Ce and O, of which the thickness was about 0.5 nm. The fact that only the lattice fringes of Ag can be observed brings us to believe that the oxide shell was amorphous or with a poor degree of crystallization. Fig. 2(d) further pinpoints the location of O and Ce, which was at the edge of the Ag signals, again verifying the core–shell structure. Elemental line scanning also shows that the oxygen signal was overlapped with Ce but much narrower and close to the outer surface. Our previous study¹⁴ suggested that this could be ascribed to the inward diffusion of oxygen and subsequent oxidation.

Fig. 2 (a) HRTEM image of Ag NPs synthesized on the CeO₂ substrates after heating at 300 °C for 3 h, (b) the corresponding electron diffraction patterns by fast Fourier transform, (c) EELS mapping and (d) EELS elemental line-scans.

Fig. 3(a) reveals that the TEM image of a Ag NW and the selected-area electron diffraction pattern, which confirm that the Ag NWs were single-crystalline and grew towards [11̄0]. Inserted EDS spectrum verifies the purity of Ag NWs. The EELS mapping (Fig. 3(b)), as well as the elemental line-scanning (Fig. 3(c)), shows that the NWs also exhibited a metal@oxide core–shell structural feature. The ultra-thin spontaneously-grown shell which enriched with Ce and O is shown to be continuous, compact and well attached to the NW surface. The overlapping of Ce and O signals and the deviation in the distribution are identical to those observed in NPs.

Fig. 3 (a) TEM image of Ag NWs grown on the CeO₂ film after heating at 300 °C for 3 h (insets: SADP and EDS spectra) (the Cu signal comes from the Cu grids), (b) EELS mapping with zone axis [220], and (c) EELS elemental line-scans.

Fig. 4 presents the normalized XANES spectra of the Ce L₃-edge for the Ag@CeO₂ NPs and NWs. The comparison with the spectra obtained from CeF₃ and CeO₂ NPs as the standards are also made. Apparently, the peak at low energy side (around 5730 eV) for both Ag@CeO₂ NPs and NWs differs from the standards. In order to estimate the charge state of Ce, all the spectra are subtracted by an arctangent function to exclude the electronic transition to continuum states, and fitted with five Gaussian functions.^15–17 In this respect, the deconvoluted components of a representative example for standard CeO₂ NPs were illustrated.^18–20 Component A arises from a core excited Ce⁴⁺ final state with the configuration 2p⁴f⁰5d*, where 2p denotes a hole in the 2p shell and 5d* refers to the excited electron in the 5d state. The split of component A into two sub peak A1 and A2 is caused by the crystal-field splitting. Component B is attributed mainly to a 2p⁴f¹5d* state, where the asterisk stands for a hole in the anion ligand orbital. Component C was assigned as Ce³⁺, which is transition from the initial electron configuration 2p⁴f¹5d⁰ to the final configuration 2p⁴f¹5d*. As for the feature D, it is due to the dipole-forbidden 2p-to-4f transition. The concentration of Ce³⁺ can be expressed as the ratio of I_c to I_total, where I_c refers to the intensity of component C and I_total is the summation of intensities of A, B, C, and D.¹⁷ As to Ag@CeO₂ NPs and NWs, the calculated ratio of I_c/I_total was 17 and 20%, respectively, which is much greater than that of reference sample, CeO₂ (Ce³⁺ is ∼8%). Accordingly, it can be inferred that the charge state of Ce in Ag@CeO₂ NPs and NWs is between Ce³⁺ and Ce⁴⁺.

Fig. 4 XANES results of Ce L-edge in oxide layer for Ag@CeO₂ NPs and NWs. The theoretical fit of XANES spectra of the CeO₂ reference material is included (CeO₂ standard: CAS number 1306-38-3, ALDRICH cerium(IV) oxide, nanopowders, <25 nm in particle size; CeF₃ standard: CAS number 7758-88-5, ALDRICH cerium(III) fluoride, anhydrous powders, 99.99% trace metals basis).

Unlike the formation of CeO₂ shell on Ag nanoparticles thorough an additive process, e.g. localized surface plasmon resonance photochemical induced interface reaction,²¹ the growth of thin oxide shell occurs simultaneously with the crystallization of Ag nanostructures in our case. It can be deduced that the segregation of Ce at the Ag surface may take place through the following route. As illustrated in Fig. 5, Ce from CeO₂ film dissolved into molten AgNO₃ salt during isothermal heating at 300 °C (the melting point of AgNO₃ is 212 °C). After that, dissolved Ce in molten salt was repelled to the surface of NPs or NWs during the crystal growth of Ag and then got oxidized. To clarify this, the solubility of Ce in molten AgNO₃ salt heated at 300 °C for 3 h was experimentally obtained by measuring the Ce and Ag concentrations dissolved in nitric acid aqueous solution using inductively coupled plasma-mass spectrometry. The measured Ce/Ag value (1.6 wt%), which is closely to the one estimated from the ratio of ceria shell thickness to the Ag wire radius (1.9 wt%), supports our speculation.

Fig. 5 Schematic illustration of proposed mechanism of for the spontaneous formation of thin-oxide shell on Ag nanostructure.

In summary, the microstructural features of Ag@CeO₂ core–shell nanostructures by a one-step synthesis method were investigated in this study. Spontaneously-grown CeO₂ shell is found to cover the whole surface of nanoparticles or nanowires of Ag. Experimental results suggest that the CeO₂ shell is formed by repelling and oxidation of dissolved Ce from the substrate during Ag single-crystal growth. The XAS spectra suggest that the valence of Ce ions in this spontaneous oxide layer is in between Ce³⁺ and Ce⁴⁺.

Acknowledgements

This work was supported primarily by Ministry of Science and Technology of R.O.C. through contracts No. MOST 103-2221-E-005-018 and MOST 104-2119-M-006-012, from which the authors are grateful.

Notes and references

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