Reductive dissolution of Fe₃O₄ facilitated by the Au domain of an Fe₃O₄/Au hybrid nanocrystal: formation of a nanorattle structure composed of a hollow porous silica nanoshell and entrapped Au nanocrystal

Abstract

The Fe₃O₄ grain of a Fe₃O₄/Au hybrid nanocrystal encapsulated in a silica nanosphere was rapidly and exclusively dissolved through a reductive process facilitated by the attached Au grain, resulting in the formation of a nanorattle structure which has utility as a nanoreactor to template the growth of nanocrystals inside the cavity.

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Recently, several hybrid nanocrystals, such as Au/Fe₃O₄, Ag/Fe₃O₄, CdS/FePt, and γ-Fe₂O₃/metal sulfides, have been synthesized by integrating chemically different species through heterojunction, and have attracted much attention due to their novel properties and unique applicability which cannot be achieved with single component nanocrystals.¹ In this study, we observed the spontaneous generation of Fe₃O₄/Au hybrid nanocrystals during the encapsulation reaction of Fe₃O₄ and Au³⁺ complexes with silica nanospheres. Additionally, in the course of treating the hybrid nanocrystals with NaBH₄, it was revealed that the Fe₃O₄ grains are rapidly and exclusively removed from the hybrid nanocrystals through the reductive process, facilitated by the attached Au grain. This is a unique and interesting phenomenon which had not been expected with any single component nanocrystals and has never been reported, to the best of our knowledge. Along with providing fundamental knowledge, this finding also offers a simple and novel method to fabricate “rattle type” nanostructures, which have received attention recently as catalytic and biosensing materials.² The selective dissolution of Fe₃O₄ accompanied by silica etching left a Au nanocrystal inside the cavity of a hollow and porous silica nanoshell, which enabled the stabilization of the active metal core, even under severe conditions, while providing the reacting molecules with a pathway for diffusion.³ Based on the resulting nanostructure, it was envisioned that the nucleation and growth of the metal species could be guided, occurring only inside the hollow shell by the Au core, thus the nanorattle could act as a nanoreactor to spatially-confine the synthesis of nanoparticles. Herein, we report the synthesis of Fe₃O₄/Au hybrid nanocrystals in silica nanospheres, the reductive dissolution of their Fe₃O₄ grain facilitated by the attached Au, and the formation of the nanorattle structure with a porous and hollow silica nanoshell capturing a Au nanocrystal inside the cavity. We also report the successful employment of the resulting nanorattles to template Ag nanocrystal synthesis, which demonstrates their utility as a platform intermediate for a wide variety of metal core–silica shell nanoparticles (Scheme 1).

Scheme 1 The syntheses of Fe₃O₄/Au@SiO₂, Au@h-SiO₂, and Ag@SiO₂.

The encapsulation of Fe₃O₄ nanocrystals and Au complexes within silica shells was conducted in a microemulsion system containing a HAuCl₄ precursor in water droplets and 8 (±1) nm sized Fe₃O₄ nanocrystals in the external cyclohexane phase.⁴ The formation of a silica shell around the Fe₃O₄ nanocrystals was initiated by the successive addition of an NH₄OH aqueous solution and tetraethyl orthosilicate (TEOS) and proceeded for 12 h. Transmission electron microscopy (TEM) and X-ray photoelectron spectrometry (XPS) analyses of the resulting solids revealed the reduction of Au³⁺ complexes during the reaction and the growth of several tiny Au nanocrystals of 1–2 nm diameter around the Fe₃O₄ nanocrystals, generating an Fe₃O₄/Au hybrid nanocrystal in a silica nanosphere of 29 (±1) nm diameter (Fe₃O₄/Au@SiO₂) (Fig. 1, ESI†). The formation of the hybrid nanocrystal can be understood by the reduction of AuCl₄⁻ by polyoxyethylenenonylphenyl ether (Igepal CO-520, containing 50 mol% hydrophilic groups) and the preferential nucleation of Au at the Fe₃O₄ surface.⁵ A control reaction carried out in the absence of the Fe₃O₄ nanocrystal allowed the formation of a Au nanocrystal with an average size of 3.5 (±0.4) nm within a silica nanosphere (Au@SiO₂) (ESI†).

Fig. 1 TEM images and histograms showing the size distribution of the silica nanospheres, hollow cavities, and Fe₃O₄ and Au grains of (a) Fe₃O₄/Au@SiO₂ and (b) Au@h-SiO₂ synthesized from 8 nm sized Fe₃O₄ nanocrystals.

When NaBH₄ (60 mM) was added to an aqueous suspension of the Fe₃O₄/Au@SiO₂ nanospheres with stirring, the dark brown color gradually faded with the evolution of H₂ gas over 30 min and a pale purple suspension was generated (Fig. 2a). The UV-Vis spectrum change showed the reduction and disappearance of the broad absorption below 500 nm, which is associated with the Fe₃O₄ nanocrystals, and the emergence of a surface plasmon band of the Au nanocrystals at around 520 nm (ESI†). TEM and X-ray diffraction (XRD) analyses confirmed the dissolution of Fe₃O₄ from the hybrid nanocrystals during the reaction (Fig. 1b, 2b). The removal of the Fe₃O₄ grains left voids with a 14 (±2) nm diameter in the silica nanospheres, thus leading to the formation of Au@h-SiO₂ with a rattle-like nanostructure consisting of a hollow and porous silica shell with an outer diameter of 28 (±2) nm and a Au nanocrystal with an average size of 4 (±1) nm. Their nitrogen adsorption/desorption isotherm revealed the marked increase in BET surface area from 86 m² g⁻¹ to 371 m² g⁻¹ and the specification of a bimodal mesoporous system that consists of large pores with a narrow size distribution centered at 19.9 nm and small pores broadly dispersed in a 1–5 nm range (ESI†). The reduced size and the porosity of the resulting silica shell indicate the partial etching of silica during the reaction with NaBH₄, which is consistent with the recent discovery reported by Yin et al.⁶ The larger cavity size of Au@h-SiO₂, compared with the size of the removed Fe₃O₄ grain, can be understood by the subsequent etching of the cavity surface newly generated after Fe₃O₄ dissolution. When the Fe₃O₄ grain was etched from Fe₃O₄/Au@SiO₂ with an aqueous HCl solution and the resulting hollow nanosphere was subsequently treated with NaBH₄, as a control experiment, it was observed that the hollow cavity expanded during the NaBH₄ treatment to give a hollow and porous shell, which is quite similar to that observed in Au@h-SiO₂ (ESI†). The growth of a Au nanocrystal relative to that in Fe₃O₄/Au@SiO₂ is most likely due to the coalescence or ripening of Au particles within the cavity during the etching reaction. The cavity size of Au@h-SiO₂ was found to be readily controlled simply by changing the size of the sacrificial Fe₃O₄ nanocrystal.⁷ For instance, a Fe₃O₄/Au@SiO₂ nanosphere prepared by using 5 (±1) nm sized Fe₃O₄ provided a Au@h-SiO₂ nanorattle with an average cavity size of 10 (±1) nm (ESI†).

Fig. 2 (a) Photographs and (b) XRD patterns of Fe₃O₄/Au@SiO₂ (left photo, blue line) and Au@h-SiO₂ (right photo, red line).

While there are previous reports on the reductive dissolution of iron oxide occurring in several microorganisms, such a rapid and selective dissolution of the crystalline Fe₃O₄ phase during the abiotic reaction was an unexpected and unique finding.⁸ The treatment of silica nanospheres encapsulating Fe₃O₄ nanocrystals, Fe₃O₄@SiO₂, prepared without the addition of HAuCl₄, or their mixture with Au@SiO₂, with NaBH₄ did not exhibit any detectable change in the Fe₃O₄ crystals, while the silica nanospheres were rendered porous by etching, which proved that there was no dissolution of Fe₃O₄ without the attachment to Au (ESI†). Thus it can be inferred that the Au grain of the hybrid nanocrystal facilitates the electron transfer from BH₄⁻ (donor) to the adjacent Fe₃O₄ grain (acceptor) and catalyzes the reductive dissolution of Fe₃O₄. A similar electron relaying mechanism has been proposed in a number of previous reports to explain the catalytic performance of Au nanocrystals in the reduction of organic compounds by NaBH₄.^2b,3a,9 Very recently, Lucas et al. reported the partial dissolution of γ-Fe₂O₃ nanoparticles into Fe²⁺ ions during electrochemical reduction on a gold electrode, which also supports our proposed mechanism.¹⁰

When AgNO₃ (3.0 mM) was reacted with reductive N₂H₄ in an aqueous suspension containing 0.3 mg ml⁻¹ of Au@h-SiO₂ nanospheres, Ag nanocrystals were grown exclusively inside the hollow shells, creating Ag@SiO₂ nanospheres in which the Ag cores are well coated by porous silica shells (Fig. 3a).¹¹ It was found that larger Ag nanocrystals were formed as the initial concentration of AgNO₃ increased, which demonstrates the preferential nucleation of Ag at the Au surface inside the cavity followed by gradual growth (Fig. 3b, ESI†). The purified solids of Ag@SiO₂ were readily dispersed in an aqueous suspension to generate a stable colloid (Fig. 3c and d). Control reactions with hollow and porous silica nanospheres, prepared from Fe₃O₄@SiO₂, through etching with HCl followed by treatment with NaBH₄, or Au encapsulating hollow silica nanospheres, synthesized by etching Fe₃O₄/Au@SiO₂ with a HCl solution, gave rise to the growth of Ag with a relatively large size on the outer surface of the silica sphere (ESI†). These observations indicate that both the Au core and the porous shell are indispensable factors to spatially confine the synthesis of Ag nanocrystals inside the hollow cavities of the Au@h-SiO₂ nanospheres.

Fig. 3 (a) TEM, HRTEM, and SEM images and histograms showing the size distribution of the silica nanospheres and Ag nanocrystals of Ag@SiO₂. (b) TEM images and histograms showing the size distribution of Ag nanocrystals at various AgNO₃ concentrations. (c) Photographs and (d) UV-Vis absorption spectra of aqueous suspensions containing Ag@SiO₂ nanospheres (0.8 mg ml⁻¹).

In summary, we synthesized Fe₃O₄/Au hybrid nanocrystals encapsulated in silica nanospheres and found the rapid and selective dissolution of their Fe₃O₄ grains through a reductive process, facilitated by the adjacent Au, which has never been achieved with any single component nanocrystals. By employing the reductive Fe₃O₄ dissolution, we demonstrated a method to generate a nanorattle structure consisting of a hollow and porous silica nanoshell and a Au nanocrystal. We also demonstrated the utility of the nanorattles as nanoreactors to template the growth of nanocrystals inside the cavities. We believe that the results of this study will provide a novel approach for developing a variety of core–shell nanomaterials, which have many advantages in catalytic and biosensing applications.

This work was supported by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2008-314-C00192).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures and characterization, XPS of Fe₃O₄/Au@SiO₂, UV-Vis spectra and N₂ sorption isotherms of Au@h-SiO₂, and TEM images of Au@h-SiO₂ synthesized from 5 nm sized Fe₃O₄ nanocrystals, Ag@SiO₂, and adducts obtained from the control reactions. See DOI: 10.1039/b915240g

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