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Fabrication of Ellipsoidal Silica Yolk-Shell Magnetic Structures with Extremely Stable Au Nanoparticles as Highly Reactive and Recoverable Catalysts Jiasheng Fang, Yiwei Zhang, Yuming Zhou, Shuo Zhao, Chao Zhang, Hongxing Zhang, Xiaoli Sheng, and Kunpeng Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03873 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 2, 2017
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Fabrication of Ellipsoidal Silica Yolk-Shell Magnetic Structures with Extremely Stable Au Nanoparticles as Highly Reactive and Recoverable Catalysts Jiasheng Fang, Yiwei Zhang*, Yuming Zhou*, Shuo Zhao, Chao Zhang, Hongxing Zhang, Xiaoli Sheng, Kunpeng Wang School of Chemistry and Chemical Engineering, Southeast University, Jiangsu Optoelectronic Functional Materials and Engineering Laboratory, Nanjing 211189, P. R. China. * Corresponding authors. E-mail:
[email protected];
[email protected]: +86 25 52090617; Fax: +86 25 52090617.
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ABSTRACT A novel strategy was reported for the fabrication of yolk-shell magnetic MFSVmS-Au nanocomposites (NCs) consisting of double-layered ellipsoidal mesoporous silica shells, numerous sub-4 nm Au nanoparticles (NPs), and magnetic Fe
central
cores.
The
hierarchical
FSVmS
NCs
with
ellipsoidal
α-Fe2O3@mSiO2/mSiO2 as yolks/shells were firstly prepared through the facile sol-gel template-assisted method, and plenty of extremely stable ultrafine Au NPs were
post
encapsulated
within
interlayer
cavities
through
the
unique
deposition-precipitation method mediated with Au(en)2Cl3 compounds. Notably, ethylenediamine ligands were used to synthesize the stable cationic complexes [Au(en)2]3+ that readily underwent the deprotonation reaction to chemically modify on negatively charged mesoporous silica under alkaline conditions. The subsequent two-stage programmed hydrogen annealing initiated the in-situ formation of Au NPs and reduction of α-Fe2O3 into magnetic Fe, where the synthesized Au NPs were highly resistant to harsh thermal sintering even at 700℃. Given its structural superiority and magnetic nature, the MFSVmS-Au was demonstrated as a highly efficient and recoverable nanocatalyst with superior activity and reusability toward the reduction of 4-nitrophenol to 4-aminophenol, and the pristine morphology still retained after six recycling tests. KEYWORDS: yolk-shell; α-Fe2O3 ellipsoid; Au(en)2Cl3; Au nanocatalyst; magnetic recovery
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1. INTRODUCTION Ultrafine metallic gold nanoparticles (Au NPs) dispersed on high-surface-area materials are greatly promising as heterogeneous catalysts for their fascinating catalytic performances in various important chemical reactions.1-3 Whereas, these noble NPs suffer from serious instability provoked by the high surface energy, and tend to agglomerate and deform during the catalysis process, resulting in distinct deterioration of their intrinsic activity and cycling lifetime.3-6 To alleviate this problem, existing strategies for the Au NPs’ stabilization to maintain their tiny size and recyclability were mainly focused on the physical separation.4,
7, 8
Through
embedding Au NPs onto support surfaces or surrounding them with porous oxide materials, the colloidal Au NPs can be protected from contacting and consequent aggregation to some extent.6, 9-11 However, due to the weak physical adsorption, these metal NPs on support surfaces may be inclined to migrate or detach to occur inevitable agglomeration under harsh processing conditions.7, 12-14 Hence, it’s highly desirable to develop an efficient approach to improve the dispersion stability of supported Au NPs, which is a critical concern in designing high-performance heterogeneous catalysts. Yolk-shell
nanocomposites
(NCs)
with
distinctive
core@void@shell
architectures have generated tremendous current researches due to their outstanding advantages of structural and functional combinations, displaying fascinating potentials in heterogeneous catalysis.15-18 More importantly, highly tailorable
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properties of the cores and shells bestows these modulated yolk-shell NCs with great versatility to be flexibly designed for specific demands or particular catalytic performances.17-20 And the application of yolk-shell nanostructures was considered as suitable supports to effectively stabilize Au NPs.4, 7 Noteworthy, the yolk-shell NCs featuring single metal core within a hollow shell can prohibit these NPs from aggregating and deactivating to some extent.4, 21-23 Undesirably, several major defects inevitably arise from low reaction rates and difficulties in pursuing fine morphologies.3, 24, 25 Alternatively, through uniformly entrapping Au NPs within both the cores and shells from modulated yolk-shell NCs, high catalyst-loading content and enhanced thermal stability can be concurrently implemented to promote their catalytic functions.24, 26-29 Until now, various synthetic approaches for yolk-shell nanostructures, such as selective etching, bottom-up, Kirkendall effect, Ostwald ripening, etc., have been developed.30,
31
With the merits of well-difined shape, adjustable structure,
monodispersed dispersibility, facile and replicable manipulations, template-assisted methods are still the most general and reliable strategies to obtain desired high-quality yolk-shell NCs.24, 26, 32-34 Resorcinol-formaldehyde (RF) polymer resins were one of the most popular carbon precursors as interlayer sacrificial templates via the simple extended Stöber method, enabling the obtained void spaces be feasible to control accordingly.29 Since gold is expensively precious and limited in supply, facile recovery of Au-based nanocatalysts become highly desirable. In this aspect, integrating iron-containing magnetic materials (e.g., Fe3O4, γ-Fe2O3 and Fe) into
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yolk-shell NCs as magnetically recoverable nanocatalysts can be a more efficient technique to realize convenient catalyst recycling.5, 18, 35, 36 Besides, a silica shell can protect the coating magnetic particles from losing their magnetic properties in harsh conditions and provide further functionalization.35 Moreover, by engineering the “shell-in-shell”
hierarchical
architecture,
a
mesoporous
outer
shell
and
silica-protected magnetic yolk structure with ultrafine active metal NPs distributed on the surfaces within the interlayer cavities can endow the yolk-shell nanocatalyst with excellent stability and dispersibility of immobilized active sites as well as rapid magnetic responsivity. Commonly, it’s known that most of yolk-shell nanostructures are spherical in shape, for the uniform nonspherical nanostructures is difficult to synthesize due to their high curvature surfaces.36, 37 Nevertheless, taking into account of industrial and commercial factors, nonspherical nanostructures are also necessary to be adapted to different catalytic reactions and operation procedures that requires catalyst structures with different shapes.18,
36
Thus, shape-controlled synthesis of such nonspherical
nanostructures with uniform sizes has become an issue of importance to expand their application in various fields. Herein, a facile synthetic route was developed for novel Au-loaded ellipsoidal yolk-shell magnetic NCs (namely MFSVmS-Au) with double hollow mesoporous silica shells. Using α-Fe2O3 ellipsoids and RF resins as cores and sacrificial templates, the ellipsoidal hierarchical nanostructures composing of α-Fe2O3@mSiO2/mSiO2 as yolks/shells were well constructed firstly. And then, encapsulation of ultrafine Au NPs
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and integration of magnetic nature were post accomplished concurrently through the unique deposition-precipitation (DP) process based on in-situ two-stage programmed hydrogen reduction. Specifically, ethylenediamine ligands were used to react with [AuCl4]- to form stable cationic complexes [Au(en)2]3+ that were ready to chemically modify on double ellipsoidal mesoporous silica shells in the DP process. Induced by subsequent two-stage hydrogen annealing, extremely thermal stable Au NPs were in-situ reduced from [Au(en)2]3+ precursors, and α-Fe2O3 ellipsoids were transformed into smaller magnetic Fe cores with inner voids for ellipsoidal double-shell hollow magnetic architectures. Toward catalyzing the reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) by NaBH4, the MFSVmS-Au was shown as a highly efficient and recoverable catalyst with superior activity and reusability.
2. EXPERIMENTAL SECTION 2.1. Materials.
Chloroauric acid
(HAuCl4·3H2O),
ethylenediamine
(en),
cetyltrimethyl ammonium bromide (CTAB), polyvinylpyrrolidone (PVP K-30) and 4-nitrophenol were purchased from Sigma-Aldrich. Anhydrous FeCl3, NaH2PO4, tetraethylorthosilicate (TEOS), resorcinol, formaldehyde, ammonia (28 wt%) and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. Deionized water was used in all experiments. All chemicals were analytical grade and used as received without further purification. 2.2. Preparation of FS Ellipsoids. α-Fe2O3 ellipsoids were firstly prepared via hydrothermally incubating the homogeneous solution containing FeCl3 (0.02 M) and NaH2PO4 (0.45 mM) at 105℃ for 48 h. For SiO2 coating, the obtained α-Fe2O3
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ellipsoids were dispersed ultrasonically into the mixture containing PVP (0.45 g, K-30), ammonia (2 mL, 28%), deionized water (5 mL) and ethanol (50 mL), followed by slow addition of TEOS (0.3 mL) in ethanol (20 mL) under stirring. After reacting for 9 h, the resultant α-Fe2O3@SiO2 (denoted as FS) ellipsoids were collected and washed with ethanol. 2.3. Preparation of FSR NCs. Typically, the previous FS NCs were redispersed ultrasonically into a mixed solution of ethanol (20 mL) and deionized water (10 mL), followed by adding a resorcinol-formaldehyde (RF) precursor (0.4 g, 1:1 of weight ratio) under stirring. Then, ammonia (0.5 mL, 28%) was injected into the above dispersion to proceed the polymerization for 2 h, producing a layer of RF polymer resins on FS ellipsoids. Afterward, the resultant FS@RF (denoted as FSR) NCs were collected and washed several times with water-alcohol. 2.4. Preparation of Yolk-Shell FSVmS NCs. The prepared FSR NCs were mixed ultrasonically with ethanol (55 mL), deionized water (50 mL), ammonia (1.0 mL) and CTAB (0.2 g) for 30 min to form a homogeneous dispersion, followed by dropwise adding ethanol (20 mL) containing TEOS (0.15 mL) under stirring. After continuing for another 6 h, the multilayer core-shell FSR@mSiO2 (denoted as FSRS) precipitates were collected, washed, dried, and then air-calcined at 550℃ for 3 h to give the yolk-shell FS@Void@mSiO2 (denoted as FSVmS) NCs. 2.5. Preparation of Magnetic Au-Loaded Nanocatalysts. Au NPs were in-situ synthesized using the DP method mediated with Au(en)2Cl3 compounds, along with the reduction of α-Fe2O3 into magnetic Fe through the two-stage programmed
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hydrogen annealing process. Au(en)2Cl3 compounds were firstly prepared by reacting ethylenediamine with HAuCl4, followed by ethanol rinsing to acquire these precipitates and vacuum drying to yield the pure pale-yellow powders. For chemical modification, FSVmS NCs were impregnated ultrasonically into the freshly aqueous Au(en)2Cl3 solution (3 mg mL-1, pH=8.0~10.0) and stayed overnight under stirring to obtain the intermediates (denoted as FSVmS-AE). After rinsing and drying, the FSVmS-AE was annealed at 150℃ for 1 h and then at elevated 525℃ for 2 h under ultrapure hydrogen (99.99%) flowing, giving the final magnetic products (denoted as MFSVmS-Au). 2.6. Characterization. Transmission electron microscopy (TEM) equipped with selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS) was conducted on a JEM-1230 microscope. Scanning electron microscopy (SEM) micrographs were acquired using a JEOL JSM-5600L electron microscope. Powder X-ray diffraction (XRD) patterns were taken on a Bruker D8 Advance Diffractometer. Fourier transform infrared (FTIR) spectra were collected on a BRUKERALPHA FTIR spectrometer. The thermogravimetric analysis (TGA) was performed under air flowing using a Rigaku ThermoPlus TG8120 analyzer. Nitrogen sorption was measured on an ASAP 2020 device (Micrometrics USA). UV-vis spectra were recorded with a Shimadzu UV 3600 spectrometer. Magnetization was analyzed using a vibrating sample magnetometer (VSM, Lake Shore, USA). 2.7. Catalytic Reduction of 4-NP. Catalytic activity for the reduction of 4-NP by NaBH4 was conducted at ambient temperatures as follows. Briefly, deionized water (2
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mL), 4-NP (40 µL, 0.02 M) and freshly aqueous NaBH4 (1.0 mL, 0.12 M) were mixed into a quartz cuvette, followed by adding catalyst dispersion (0.15 mL, 0.2 mg mL-1). The reaction progress was monitored by UV-vis spectrophotometry to determine the concentration of 4-NP. To evaluate the catalytic stability and reusability, the amounts of 4-NP and catalyst were increased to 20 times of the above system respectively. Prior to next cycle, the reusable catalyst was magnetically separated, rinsed and dried for regeneration. 3. RESULTS AND DISCUSSION 3.1. Morphology, Structure and Formation Process. The design of MFSVmS-Au (Scheme 1) combines the sol-gel template-assisted process for well-defined yolk-shell FSVmS and the DP process mediated with Au(en)2Cl3 as gold precursors for the synthesis of extremely stable Au NPs. Induced by hydrogen reduction with two-stage programmed temperatures from 150 to 525℃, numerous ultrafine Au NPs were in-situ formed, and α-Fe2O3 ellipsoids were converted into magnetic Fe particles with partial voids penetrated into the yolks, yielding the black Au-loaded MFSVmS-Au with double ellipsoidal mesoporous silica hollow shells and excellent magnetic recoverability. The shapes and sizes of different samples in this procedure were explicitly illustrated by TEM observation. The α-Fe2O3 ellipsoids obtained by hydrothermal synthesis were monodispersed in size with high aspect ratio (Figure 1a), and used as morphology-directed templates to fabricate magnetic nanocapsules below. Assisted with coupling stabilizer PVP, a uniform silica coating ( ~ 14 nm in thick) was formed
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on α-Fe2O3 ellipsoids by hydrolysis and condensation of TEOS to give FS NCs (Figure 1b). Through the polymerization of RF precursors in the alkaline ethanol-water solution, a RF resin layer of ~ 45 nm was uniformly generated on FS ellipsoids to produce sandwich-like FSR NCs (Figure 1c). Noticeably, upon coating RF resins, the well-defined FSR NCs remained distinct ellipsoidal shape with smoother surfaces, due to the electrostatic interaction between RF precursors and NH4+ ions located on FS’s surface and thereby the preferential adsorption of polymer resins.38 Besides, the thickness of RF resin layers can be easily tailored by varying the concentration of RF precursors (Figure S1). The smooth RF layer surface enabled the advantages of abundant phenol hydroxyl groups for good dispersity in polar solvents (water and ethanol) and further modification. With assembly of CTAB and TEOS in the sol-gel reaction, a mesostructured silica composite layer of ~ 28 nm was further evenly deposited on FSR seeds for the core-shell multilayer FSRS NCs (Figure 1d). Afterward, the obtained FSRS NCs underwent the thermal calcination to burn out all the hybrid organics (RF resins, CTAB and PVP) for the yolk-shell FSVmS NCs with interlayer voids and mesoporous shells (Figure 1e). TEM imaging revealed that FSVmS possessed the distinct “ellipsoid-in-ellipsoid” configuration comprising a movable ellipsoid within an ellipsoidal shell. The distance between the yolk and shell was ~ 42 nm, well consistent with the thickness of RF resins. And magnified TEM (Figure S2a) confirmed the ordered mesopore channels in outer silica shells (~ 25 nm) that would allow the rapid mass transfer. SEM imaging (Figure 2c) also validated the ellipsoidal
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yolk-shell structure and interlayer voids can be clearly discerned. Besides, TG/DSC analysis (Figure S3) revealed the major weight loss from 200 to 500 ℃ with several DSC peaks at 330, 405 and 500℃, indicative of the exothermic nature resulting from the drastic organic combustion. Particularly, the unique Au-loaded method mediated with Au(en)2Cl3 compounds was adopted to synthesize a high content of ultrafine Au NPs within these well-modulated yolk-shell NCs, accompanied by in-situ reducing α-Fe2O3 ellipsoids into magnetic Fe particles to hybridize these nanocatalysts with strong magnetic nature. In the DP process, ethylenediamine ligands were used to react with Au(Ⅲ) by displacing the chloride ligands in [AuCl4]-, forming stable cationic complexes [Au(en)2]3+ that can chemically immobilize on double ellipsoidal mesoporous silica shells with negatively charged surfaces through the deprotonation reaction under alkaline conditions.39 The general mechanism was depicted as equ 1, where (Si-O-) was the surface anionic species on silica and interacted with singly charged cations M+ through the cation-exchange reaction prior to grafting cationic gold species onto silica surfaces.40, 41 The high positive charge density of Au (III) would strengthen the acidity of ethylenediamine ligands and enabled the protons inclined to amine groups be easily deprotonated under alkaline conditions, forming the deprotonated gold cationic complexes [Au(en)(d-en)]2+ (d-en represented deprotonated ethylenediamine ligands).42 Accordingly, the concentration of [Au(en)(d-en)]2+ increased by raising pH and became the dominant species at pH≥8.39 From equ 2, the [Au(en)(d-en)]2+ held positive charge of 2, disclosing that only one outer chloride remained neutrality when
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[Au(en)(d-en)Cl]+ species were adsorbed on a (Si-O-) surface site, and can be displaced when further interacting with a second (Si-O-) surface site.39,
43
As
concentrations of both [Au(en)(d-en)]2+ and surface (Si-O-) increased with pH, more highly active supported Au NPs were loaded via the DP process based on [Au(en)(d-en)]2+ that was proportional to (Si-O-) surface sites. Thus, alkaline conditions were the key prerequisite for the preparation of silica-supported Au catalysts with [Au(en)2]3+. [Au(en)2]3+ + 2Cl- + M+-OSi≡ → M+(aq) + [Au(en)2Cl2]+-OSi≡ (1) [Au(en)(d-en)]2+ + Cl- + M+-OSi≡ → M+(aq) + [Au(en)(d-en)Cl]+-OSi≡ (2) Magnified TEM imaging (Figure 2a) captured a great deal of gold clusters (~ 0.66 nm) densely distributed in mesoporous silica of FSVmS-AE, which may be attributed to reducible decomposition evoked by the electron beam under the microscope vacuum chamber and appeared less aggressive than common treatments at higher temperatures to induce agglomeration of gold species. EDS analysis (Figure 2b) identified 4.3% wt of N element arising from ethylenediamine ligands, and TG/DCS curves (Figure S3) showed that the weight proportion of ethylenediamine ligands accounted for 3.2% wt with an exothermic DSC peak at 175℃ resulting from the ethylenediamine combustion, validating the successful immobilization of [Au(en)2]3+ species in FSVmS. Induced by two-stage programmed hydrogen annealing, the [Au(en)2]3+ precursors were firstly reduced into ultrafine Au NPs at 150 ℃ for the FSVmS-Au, and the α-Fe2O3 was then transformed into magnetic Fe at elevated 525℃ for the MFSVmS-Au. TEM imaging (Figure 1f) presented that
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plenty of tiny Au NPs with mean size of 2.61 nm were well distributed in monodispersed FSVmS at the first stage. The lattice fringes of most Au NPs embedding into silica can be apparently identified (Figure S4a), and the SEAD pattern indicated the presence of crystalline Au and α-Fe2O3 (Figure S4b) sufficiently certifying that the gold precursors were nearly converted into metallic gold NPs in FSVmS-Au after hydrogen annealing at 150℃. When switching to 525℃ for the second stage, the supported Au NPs retained unaffected but highly stabilized with small grain size of 3.17 nm, and the central Fe cores were formed to manifest magnetic nature, giving the pod-like magnetic MFSVmS-Au with double ellipsoidal hollow shells (Figure 1g). The lattice fringes on Au surface from HRTEM showed the interplanar spacing of 0.24 and 0.21 nm indexing as the (111) and (200) planes of crystalline Au (Figure 1h), and the SEAD pattern confirmed the crystalline characteristics of Au and Fe. Despite harsh annealing for integration of the high Au-loaded content (11.10 wt%, Figure S5) and strong magnetization, the MFSVmS-Au in SEM imaging (Figure 2d) maintained the robust yolk-shell morphology with excellent dispersibility. Intriguingly, from magnified TEM (Figure S2b), the Fe cores were coated with ultrathin SiO2 layers (~5 nm) inside the inner SiO2 shells, which enabled the protected Fe cores be more stable in the nanostructures of MFSVmS-Au, even being exposed to air. UV-vis spectra (Fig. 3a) of both the FSVmS-Au and FSVmS displayed a broad visible absorption band (400 to 550 nm) arising from the narrow band gap of α-Fe2O3 (~2.2 eV),44 which also shielded the Au peak absorption in FSVmS-Au. Due to the
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reduction of α-Fe2O3 into Fe, the strong absorption band of α-Fe2O3 was almost vanished and a peak absorption at ~523 nm corresponding to the excitation of surface plasmon mode of crystalline Au was discernible for MFSVmS-Au, validating the ultrafine grain size and good dispersion of supported Au NPs.29 Moreover, there also existed an absorption band centered at ~740 nm for MFSVmS-Au, which was probably correlated to the d-d band characteristic for iron species in silica matrix.45 In addition, the magnetization curve indicated that MFSVmS-Au owned typical ferromagnetic behavior with 62.9 emu g-1 of saturation magnetization (Figure 3b). By placing a magnet beside the vial, these NCs were quickly attracted and the black dispersion became transparent within a few seconds, stating their excellent magnetic responsivity for convenient separation. N2 absorption-desorption isotherms of both the FSVmS and MFSVmS-Au were of type IV (Figure 4), revealing the mesoporous characteristics. Besides, dual hysteresis loops in isotherms indicated the bimodal pore-size distributions, coinciding with their pore-size distribution curves calculated with the BJH method. The first hysteresis loop (0.3
