A seed-engineering approach toward a hollow nanoreactor suitable for the confined synthesis of less-noble Ni-based nanocrystals

Abstract

A hollow nanoreactor suitable for the cultivation of Ni-nanocrystals was developed through a distinct seed-engineering stratagem, which involved the assembly of a catalytically active Au/Pd-heterojunction-nanocrystal inside the hollow silica nanoshell. The resulting hollow nanoreactor demonstrated a targeted performance in the cavity-confined growth of the catalytic Ni nanocrystal.

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Yolk@shell nanoparticles, which carry a functional core inside a hollow and porous nanoshell, are attractive candidates for nanoreactors that allow spatially confined reactions for selectively internalized molecules.¹ Most research efforts to date have focused on developing highly durable and recyclable nanocatalyst systems that efficiently and selectively catalyze the transformation of organic molecules.^2,3 Very recently, a new angle of approach has been suggested by a few researchers to utilize hollow nanoreactors for synthesizing and decorating nanoparticles within the confines of the protected interior space.⁴ For instance, we recently devised the nanoreactor approach for templating the synthesis of noble metal nanocrystals, such as Pt, Ag, Au and their alloys, through exploitation of the seed-mediated growth inside the cavity of Au-seed containing hollow silica nanospheres, Au@h-SiO₂.⁵ The hollow nanoreactor-based approach has enabled the production of ligand-free noble-metal nanocrystals with well-controlled morphologies from a highly concentrated suspension. In this context, the current study was performed with the aim of further extending the utility of the hollow nanoreactors toward the production of less confinable non-noble metal nanocrystals beyond the noble metals. Unlike noble metals that can be easily restricted to grow on Au-seeds using mild reducing agents, the growth of less reducible non-noble metals is more complicated as it requires severe reaction conditions, which results in its homogenous nucleation in the bulk solution. In particular, this study focuses on devising a hollow nanoreactor suitable for the cultivation of Ni-nanocrystals, which plays important roles as versatile and representative non-noble metal catalysts in many applications.⁶ Herein, we report our strategy for engineering a seeding core toward the Ni-growing nanoreactor, which includes an assembly of a catalytically active Au/Pd-heterojunction-nanocrystal inside the hollow silica nanoshell. The nanoreactor-synthesized Ni-nanocrystals exhibited an active and recyclable performance in catalyzing hydrogen generation from NH₃BH₃ and the chemoselective reduction of nitroarenes in aqueous solutions at room temperature. Moreover, we demonstrate the usefulness of these hollow nanoreactors in the synthesis and post-modification of Ni-based hybrid nanocrystals.

Firstly, the employability of the previously developed Au@h-SiO₂ was investigated in growing Ni nanocrystals. NiCl₂ in a suspension of Au@h-SiO₂ was treated with a range of mild reducing agents, including ascorbic acid, H₂O₂, and hydrazine. Although these reducing agents had previously allowed the Au-seed-mediated growth of Au,^5a Pt,^5b and Ag,^5c respectively, it did not lead to the reduction of Ni²⁺. An alternative trial with a stronger reducing agent, NaBH₄, caused Ni deposition on the Au seed. However, this process also formed undesirably large Ni particles on the outer surface of the Au@h-SiO₂ nanoreactor (Fig. S1, ESI†). This preliminary study suggested that a more powerful seed needs to be engineered for restricting the Ni growth to the interior of the hollow nanoreactor. Therefore, a strategy was planned to introduce Pt, Pd, or Ni, which is known to catalyze the reductive decomposition of Ni²⁺–hydrazine complex,⁷ as a seeding material by incorporating Pt²⁺, Pd²⁺, or Ni²⁺ ions into the reductive dissolution process of the Fe₃O₄, which had been adopted previously for hollowing the (Fe₃O₄/Au)@SiO₂ into Au@h-SiO₂.^5c It was hypothesized that in situ reduced Pt, Ni, or Pd would be deposited on the Au-nanocrystal inside the cavity during the hollowing process (Scheme 1).

Scheme 1 The synthetic strategy of the Ni-growing (Au/Pd)@h-SiO₂ nanoreactor.

The incorporation of Ni²⁺, Pt²⁺, and Pd²⁺ ions was carried out by adding aliquots of Ni(NO₃)₂, Na₂PtCl₄, and Na₂PdCl₄ solutions, respectively, into the microemulsion suspension which had previously been used to prepare (Fe₃O₄/Au)@SiO₂ (for the detailed procedure, refer to the experimental section in ESI†). The transmission electron microscopy (TEM) images showed that all reactions afforded similar core@shell structures, which contain an embedded hybrid nanocrystal, composed of a Fe₃O₄ nanocrystal and surrounding satellite nanocrystals, at the core of the silica nanosphere (Fig. 1a–c). The control reactions without tetraethyl orthosilicate (TEOS) revealed that a comparable amount of Pt and Au were present around the Fe₃O₄ nanoparticle in isolated nanoparticles. Conversely, only a small amount of Ni and Pd were detected around the Fe₃O₄ nanoparticle (Fig. S2, ESI†). Additional control experiments, excluding both HAuCl₄ and TEOS, showed a sparse population of Pt element on the isolated nanoparticles (Fig. S3, ESI†). From these observations, it can be deduced that, in the resultant core@shell nanospheres, the Ni²⁺ and Pd²⁺ ions were dispersed throughout the silica shell, generating (Fe₃O₄/Au)@(SiO₂/Ni²⁺) and (Fe₃O₄/Au)@(SiO₂/Pd²⁺) nanospheres, respectively. In contrast, the Pt²⁺ ions were co-included as the components of satellite nanocrystals together with Au surrounding the Fe₃O₄ nanocrystal, resulting in the formation of a (Fe₃O₄/AuPt²⁺)@SiO₂ nanosphere. When the resultant nanospheres were exposed to a solution of NaBH₄, which is expected to reductively dissolve the Fe₃O₄ and simultaneously reduce the metal ions, they were found to undergo different reactions depending on the incorporated ions, Pt²⁺, Ni²⁺, and Pd²⁺. In the case of (Fe₃O₄/AuPt²⁺)@SiO₂, it was found that the Fe₃O₄ core was not subject to the expected reductive dissolution. This was presumably due to the reduced catalytic activity of the attached Au nanocrystal, which alloyed with Pt. The NaBH₄ treatment therefore afforded an unexpected nanostructure, (Fe₃O₄/AuPt)@h-SiO₂, in which a satellite-type nanoparticle, consisting of a 9(±1) nm-sized Fe₃O₄ core and surrounding Au/Pt bimetallic nanocrystals of 2.1(±0.5) nm size, is placed inside the 19(±2) nm-sized internal cavity, which was developed through silica etching (Fig. 1e). The interior cavity of the (Fe₃O₄/AuPt)@h-SiO₂, which was partially occupied by the undissolved Fe₃O₄ core, was regarded as unsuitable for metal growth. In comparison, when (Fe₃O₄/Au)@(SiO₂/Ni²⁺) was immersed in a NaBH₄ solution, the Fe₃O₄ core dissolved rapidly due to the reductive dissolution process, facilitated by the attached Au-nanocrystals, while the Ni²⁺ ions slip away from the silica through in situ generated small pores. Therefore, regardless of the content of the initially incorporated Ni²⁺ ions in the silica shell, the reaction yielded a nanorattle-like structure containing a spherical Au core, Au@h-SiO₂, which was found to be almost identical to the one previously prepared from (Fe₃O₄/Au)@SiO₂ (Fig. 1d and Fig. S4, ESI†). When Fe₃O₄@SiO₂/M²⁺ (M = Ni, Pd, Pt) nanospheres, which were prepared without containing Au nanocrystals, were treated with NaBH₄ solution for the purpose of control, the removal of the Fe₃O₄ core was not observed from any nanosphere, which indicates little effect of metal ions other than the Au nanoparticle on the reductive dissolution process (Fig. S5, ESI†).

Fig. 1 TEM and HRTEM (insets) images of (a) (Fe₃O₄/Au)@(SiO₂/Ni²⁺), (b) (Fe₃O₄/AuPt²⁺)@SiO₂ and (c) (Fe₃O₄/Au)@(SiO₂/Pd²⁺) core@shell nanospheres and (d) Au@h-SiO₂ (e) (Fe₃O₄/AuPt)@h-SiO₂ (insets: EDX elementary maps, Au (red), Pt (green)) and (f) (Au/Pd)@h-SiO₂ yolk@shell nanospheres, which were produced by treating the core@shell nanospehres with NaBH₄.

The NaBH₄ treatment of the (Fe₃O₄/Au)@(SiO₂/Pd²⁺) also generated a similar nanorattle-like structure. This consisted of a 21(±2) nm-sized cavity and an encaged tiny core nanocrystal which appeared ellipsoidal in TEM images rather than spherical (Fig. 1f). Further investigation using high resolution-TEM (HR-TEM), STEM, and energy-dispersive X-ray spectroscopy (EDX) identified the ellipsoidal core to be an Au/Pd-heterodimer-nanocrystal with a dumbbell-like structure that includes spherical Au and Pd grains of a similar size (ca. 2.5 nm) joined by a small interfacial area (Fig. 2d). The above-identified structure of the (Au/Pd)@h-SiO₂ represents the successful introduction of the reduced Pd into the cavity of the hollow silica nanoshell, as outlined in the initial synthetic strategy. Time-course TEM and HR-TEM studies revealed that the Fe₃O₄ at the core of the (Fe₃O₄/Au)@(SiO₂/Pd²⁺) was dissolved very early in the reaction (within 10 min), leaving a spherical Au-nanocrystal in the newly formed interior cavity (Fig. 2a). Whilst the cavity expanded gradually over the reaction period, from 12(±1) nm at 10 min to 21(±2) nm at 1 h, the Pd, as seen in the STEM images, continuously and gradually migrated inward toward the Au-seed. This nucleated a Pd grain on the Au seed at 20 min; the grain eventually proliferated to a size of 4.9(±0.8) nm (Fig. 2 and Fig. S6, ESI†). It can be deduced that the reduced Pd was gradually released inward from the hollowed silica, which was continuously bored from within, and deposited on the pre-existing Au nanocrystal in the internal cavity. When the preparation of (Au/Pd)@h-SiO₂ was attempted using an increased content of silica-incorporated Pd²⁺ ions, the Pd grain of the resulting Au/Pd-core-nanocrystal was found to increase in size, while maintaining the size of the Au grain (Fig. S7, ESI†).

Fig. 2 TEM (upper) and HRTEM (lower) images of samples isolated during the reaction of the (Fe₃O₄/Au)@(SiO₂/Pd²⁺) with NaBH₄ at (a) 10, (b) 20, (c) 30, and (d) 60 min, respectively. Insets: EDX elementary maps for showing the distribution of Au (red) and Pd (yellow) elements.

The nanorattle-like structure of the (Au/Pd)@h-SiO₂, bearing a catalytically active Pd grain as part of the internal core, signifies the formation of the targeted nanoreactor framework which guides the reduction of the Ni²⁺–hydrazine complex to occur inside the cavity, thus spatially confining the synthesis of Ni nanocrystals. A pre-mixed aliquot of NiCl₂ and hydrazine was added to an aqueous suspension containing the (Au/Pd)@h-SiO₂ in a capped vessel with constant stirring at 45 °C.^7a The color of the suspension slowly changed from violet to light brown during the initial 20 min of the reaction, and then instantly changed to black with effervescence. The TEM and STEM images of the isolated black powder after a reaction time of 1.5 h revealed the confined growth of a 23(±3) nm Ni nanocrystal exclusively on the inside of the hollow cavity, creating a Ni@SiO₂ core@shell structure in which a reduced Ni grown on the Au/Pd core was compacted within the porous silica nanoshell (Fig. 3a). HRTEM and XRD analyses confirmed the formation of a polycrystalline Ni nanoparticle with a face-centered-cubic (fcc) crystalline phase (Fig. 3b and c). Field dependent magnetization measurements show Ni@SiO₂ exhibited superparamagnetic and ferromagnetic behaviors at 300 K and 5 K, respectively (Fig. S8, ESI†). The time-course TEM images showed that 24% of the hollow nanoreactors were filled by the Ni growth at 20 min, and as the reaction proceeded, the number of (Au/Pd)@h-SiO₂ that transformed into Ni@SiO₂ increased, reaching a conversion yield of 94% at 1.5 h (Fig. S9, ESI†). These observations suggest that the nucleation of the Ni was initiated on the surface of the Pd grain, which catalyzed the decomposition of the Ni²⁺−hydrazine complexes, and the subsequent crystal growth was significantly boosted on the Ni nuclei.

Fig. 3 (a) TEM, (b) HRTEM and STEM-HAADF (inset) images and (c) XRD pattern of the Ni@SiO₂. The red line in (c) shows the position of the reflections corresponding to the fcc Ni phase (JCPDS Card No. 01-1260).

To prove the effectiveness of the synthesized Ni@SiO₂ in catalysis, we examined its catalytic performance in the hydrolytic dehydrogenation of NH₃BH₃ (ammonia borane, AB), which is required in high-performance hydrogen storage and generation systems.⁸Fig. 4a displays time-course curves of the hydrogen released of AB (2 M, 2 mL) during hydrolysis at room temperature by the Ni@SiO₂ catalyst. The experiment was conducted with various Ni/AB molar ratios from 0.064 to 0.032, which generated stoichiometric amounts of hydrogen (67 mL, 3 mmol) in less than 15 min. The reaction rate of each curve was almost linearly dependent on the Ni/AB molar ratio, resulting in consistent turnover frequency (TOF) values among the curves. The average TOF of the Ni@SiO₂ catalyst (8.0 min⁻¹), calculated on the basis of the total moles of Ni, is somewhat lower than the reported highest value (30.7 min⁻¹) for 6.5 nm-sized surfactant-free Ni nanoparticles.^8b However, when defined as the moles of hydrogen generated per unit surface area of Ni nanocrystals, the TOF of the Ni@SiO₂ catalyst (4.6 × 10⁻¹⁸ mol min⁻¹ nm⁻²) is considered to be comparable to the highest reported value (5.02 × 10⁻¹⁸ mol min⁻¹ nm⁻²), which implies high catalytic activity and well-exposedness of the surfactant-free surface of the Ni nanocrystal grown inside the porous silica nanoshell. Reuse of the magnetically recovered Ni@SiO₂ did not result in a significant decline in the catalytic activity over five consecutive runs (Fig. 4b). The usefulness of the Ni@SiO₂ catalyst was investigated further in the industrially important catalytic reduction of nitroarenes, which was recently carried out by using a Ni nanocatalyst in an aqueous solution at room temperature and using environment-friendly hydrazine as a reducing agent (Table S1, ESI†).⁹ A trial reaction of p-nitroaniline (0.1 M) and hydrazine (4 M) with 15 mol% of Ni@SiO₂ was carried out in water at room temperature. The product, p-diaminobenzene, was afforded in a quantitative yield within 30 min, and this showed a higher TOF (13 h⁻¹) than that of the reported ca. 10 nm-sized Ni nanoparticles (10 h⁻¹). The Ni@SiO₂ catalysts were well dispersed in the stirred reaction suspension, and after the reaction was completed, they could be readily retrieved using a small magnet (Fig. S10, ESI†). The reusability tests showed that the magnetically recovered catalyst retained most of its initial activity in terms of conversion yield and selectivity over three consecutive reactions. The same reaction performed using various functionalized nitroarenes also led to the complete conversion of substrates in less than 4 h. More importantly, while catalyzing the transformation of the nitro groups to the amino groups with a high TOF, the Ni@SiO₂ did not much affect other functional groups (halogen, ether, carbonyl-, or alkenyl-groups) and predominantly afforded the corresponding substituted anilines with a selectivity higher than 83%. The remarkable chemoselectivity of the Ni@SiO₂ toward the reduction of nitro groups may be attributed to the mild reaction conditions, at room temperature and using a using weak reducing agent, which is allowed by the surfactant-free catalytic surface of the Ni nanocrystals protected by the porous silica shell.

Fig. 4 Plots of volume of hydrogen generated vs. time for catalytic hydrolysis of AB (a) at various concentrations of the Ni@SiO₂ catalyst and (b) during the recycling experiment.

In addition to the synthesis of the Ni nanocrystals, the hollow nanoreactor engineered in this study was also applicable to the fabrication of alloy nanocrystals.¹⁰ Therefore, an additional injection of a mixture of CoCl₂ and hydrazine into the Ni-growing suspension containing the (Au/Pd)@h-SiO₂ and Ni²⁺–hydrazine complex led to the co-reduction of Co²⁺ together with Ni²⁺. This led to the consequent growth of a Ni/Co alloy nanocrystal inside the cavity, producing a (Ni/Co)@SiO₂ nanosphere with a core@shell-type structure (Fig. S11, ESI†). The control reaction with Co²⁺– or Fe²⁺–hydrazine complexes and the (Au/Pd)@h-SiO₂ suspension did not generate any reduced Co or Fe species, implying that the pre-grown Ni surface is responsible for reducing the Co²⁺ complex and forming the Ni/Co alloy. Meanwhile, the successive addition of CuCl₂ solution and hydrazine to the (Au/Pd)@h-SiO₂ suspension led to the homogeneous nucleation and the generation of large Cu particles at the outside solution of the hollow nonreactor. The annealing process of the synthesized (Ni/Co)@SiO₂ at temperatures as high as 700 °C led to an alteration in the morphology and crystallinity of the Ni/Co nanocrystal coated by the thermally stable silica shells, hence preserving their nanocrystalline nature (Fig. S11, ESI†). This allowed the post-synthetic modification of the hollow nanoreactor-synthesized nanocrystals.

In summary, a hollow nanoreactor suitable for templating the synthesis of less reducible and, therefore, less-confinable Ni nanocrystals was developed through a seed-engineering strategy which involved introducing a catalytically active material on the seed entrapped within a hollow silica nanoshell. The Au/Pd-heterojunction-nanocrystal-bearing nanoreactor demonstrated targeted reduction of Ni²⁺–hydrazine complexes and consequent growth of the Ni nanocrystals within the confines of its cavity. The usefulness of the nanoreactor-synthesized Ni nanocrystals was verified by catalyzing the hydrogen generation of NH₃BH₃ and chemoselective reduction of nitroarenes in the aqueous phase and at room temperature. Moreover, we demonstrated the extendable utility of the hollow nanoreactors in the synthesis and post-modification of alloy nanocrystals.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2011-0017377).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: More experimental data including TEM images. See DOI: 10.1039/c4cc07306a

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