finement towards Janus Iron Oxide Nanocube - American Chemical

to reductive-annealing at varying Tann (300 °C-700 °C), resulting the reversion of the crystalline phase to superparamagnetic Fe3O4 at Tann above 30...
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Monofacet-Selective Cavitation within Solid-State SilicaNanoconfinement towards Janus Iron Oxide Nanocube Sunyi Lee, Nitee Kumari, Ki-Wan Jeon, Amit Kumar, Sumit Kumar, Jung Hun Koo, Jihwan Lee, Yoon-Kyoung Cho, and In Su Lee J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09869 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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Monofacet-Selective Cavitation within Solid-State Silica-Nanoconfinement towards Janus Iron Oxide Nanocube Sunyi Lee,†,§ Nitee Kumari,†,§ Ki-Wan Jeon,† Amit Kumar,† Sumit Kumar,‡ Jung Hun Koo,† Jihwan Lee,† Yoon-Kyoung Cho,‡ In Su Lee*† †

National Creative Research Initiative Center for Nanospace-confined Chemical Reactions (NCCR) and Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang 37673 (South Korea) ‡ Center for Soft and Living Matter, Institute for Basic Science (IBS) and Department of Biomedical Engineering, School of Life Sciences Ulsan National Institute of Science and Technology (UNIST) Ulsan, 44919 (South Korea). Supporting Information Placeholder ABSTRACT: Here, a highly selective solid-state nanocrystal conversion strategy is developed towards concave iron oxide (Fe3O4) nanocube with an open-mouthed cavity engraved exclusively on a single face. The strategy is based on a novel heat-induced nanospace-confined domino-type migration of Fe2+ ions from the SiO2Fe3O4 interface towards the surrounding silica shell and concomitant self-limiting nanoscale phase-transition to the Fe-silicate form. Equipped with the chemically unique cavity, the produced Janustype concave iron oxide nanocube was further functionalized with controllable density of catalytic Pt-nanocrystals exclusively on concave sites and utilized as highly diffusive catalytic Janus nanoswimmer for the efficient degradation of pollutant-dyes in water.

Unlike the facile synthesis of nanocrystals (NCs) having convex or flat surfaces enclosed by energy-minimized low-index facets, NCs with reactive concave surfaces, having high-index facets with densely populated low-coordinated atoms within the negative curvature, are rarely synthesized. The demand of such concave surface-NCs (CS-NCs) stems from their diverse applications in catalysis, plasmonics, medicine and their post-synthetic engineering to create programmable compositions and geometries, integrating multi-functionalities in single nanostructure.1 The reported strategies towards CS-NCs either employ the etching of specific set of polygonal facets on the pre-synthesized metal NC2 or the overgrowth of atoms derived from metal-precursors at specific sites.3 These methods generate negative curvatures indiscriminately around the chemically equivalent sites of 3-dimensional NC (such as nanocube), resulting symmetric structure of CS-NCs; however, the controllable selective fabrication of single concave-cavity, resulting a Janus structure has rarely been realized.4 Recently, iron oxide (Fe3O4)-nanocubes with flat or concave surfaces have received tremendous attention for synthesis5 and assembly,6 owing to their unique multi-faceted shape and application-oriented magnetic and catalytic properties; however, facet-selective asymmetric shape-modulation and post-synthetic functionalization of these NCs have never been attempted. For water remediation and environmental applications, dynamic mixing of catalyst in fluids is important for improving the catalytic activity, since quiescent conditions lead to catalytic inefficiency

Scheme 1. Monofacet-Selective Cavitation of Iron Oxide Nanocubes towards Janus structures

and long operation hours; therefore, coupling of catalyst with autonomous motion has emerged as effective strategy.7a,12 Here, we report a novel and unexpected thermal conversion chemistry of Fe3O4-nanocube confined within a silica-shell (Fe3O4@SiO2) into an asymmetric concave iron oxide nanocube (conc-Fe3O4@SiO2) with exclusive single-facet-selectivity via a heat-induced dimensionally confined domino-type diffusion of Fe2+ ions, from one facet of the cubic Fe3O4 toward surrounding SiO2 shell (Scheme 1). Owing to the unique nanoscale architecture and distinguished chemical nature of concave cavity in the transformed iron oxide nanocube (conc-Fe3O4), catalytic Pt-grains were exclusively and controllably functionalized within the cavity, resulting Janus Pt@conc-Fe3O4 which can act as catalytically self-propelled highly diffusive nanoswimmers for the efficient removal of pollutant-dyes from water.7 First, oleate capped-iron oxide (OA-Fe3O4) nanocube-precursors with an average edge length of 21 ± 1 nm were prepared following reported protocol, and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) (Figure S1a,b).5b Next, the encapsulation of OA-Fe3O4 nanocubes by silica shell to synthesize OAFe3O4@SiO2 was accomplished by a modified reverse-microemulsion method. TEM images of the OA-Fe3O4@SiO2 revealed the

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Figure 1. (a) TEM and HRTEM (inset) images of OA-Fe3O4@SiO2 after air annealing at 500 °C. (b) TEM and HRTEM (inset) images of Fe3O4@SiO2, annealed at Ar + 4% H2 at 400 °C (c) Dark Field STEM-EDS images of conc-Fe3O4@SiO2: elemental mapping of i) Fe atoms, ii) Si atoms, iii) EDX line profiles showing Fe distribution, iv) STEM image. (d) TEM and HRTEM (inset) image of concFe3O4. (e) SEM and SEM-EDX (inset) line profile images of conc-Fe3O4. (f) A voxel projection from a tomography reconstructions of a tilt series, BF-TEM images from various projections. (g) XRD patterns of the Fe3O4@SiO2 NPs before and after the annealing at various conditions. nanoparticle (NP) size to be 41 (±1) nm, with a smooth silica shell of thickness 10 (±1) nm (Figure S1c). Preservation of Fe3O4-crystalline phase after the silica-encapsulation was validated by XRD and HRTEM analysis (Figure 1g). In a preliminary study, to investigate thermal NC-transformation behavior of the Fe3O4-nanocubes within a spatially confined silica-environment,8 OA-Fe3O4@SiO2 was annealed under reductive conditions (flow of Ar + 4% H2) at temperatures (Tann) 400 °C, resulting an unexpected generation of a concave surface-cavity selectively on one facet of some of the Fe3O4-nanocubes (25% yield) (Figure S1e). Further, OAFe3O4@SiO2 was pre-treated under air-annealing at 500 °C to avoid interference from oleate moieties. The TEM of the airannealed product at 500 °C revealed the morphology and size of the NPs were preserved (Figure 1a), however, in XRD analysis (Figure 1g), the three dominant Bragg peaks at 30°, 35.6° and 43° were slightly shifted to the higher angles, implying the formation of oxidized -Fe2O3 phase. The -Fe2O3@SiO2 was then subjected to reductive-annealing at varying Tann (300 °C-700 °C), resulting the reversion of the crystalline phase to superparamagnetic Fe3O4 at Tann above 300 °C, as confirmed by XRD and HRTEM analyses (Figure 1b, 1g). Gratifyingly, annealing at Tann = 400 °C resulted in exclusive and high yielding (100%) formation of concFe3O4@SiO2 with the emmergence of an open-mouthed cavity [dconc = 11 (±1) nm] in Fe3O4-core. Elemental mapping and line profiling with scanning transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy (STEM-EDS) (Figure 1c), of single particle of conc-Fe3O4@SiO2 revealed that Fe atoms were homogeneously distributed exclusively in the bright zcontrast region excluding the concave. The accurate visualization of the three-dimensional (3D) morphology of concave structure was reconstructed using energy-filtered-TEM-tomography on conc-Fe3O4 isolated from conc-Fe3O4@SiO2 NP by complete removal of silica-shell.9 From the BF-TEM projection images at different tilting angles (−72◦ to +68◦) and their reconstruction, it was

evident that concave structure is constrained selectively to single facet of nanocubes (Figure 1f and SI Movie1). Annealing the Fe2O3@SiO2 at Tann = 500 °C and 600 °C led to the formation of a side product (c.a. 20% yield) of hollow silica-NP (h-SiO2-FeO), with interior cavity [dhol = 20 (±3) nm] and an iron oxide nanosphere [diameter (d) = 19 (±4) nm] on the outer shell-surface, along with conc-Fe3O4@SiO2 [dconc = 12 (±2) nm]. The XRD and X-ray photoelectron spectroscopy (XPS) (Figure S3) on the 600 °C-annealed adduct revealed the new dominant Bragg peaks at 36° and 42° and significantly increased area of the Fe2p peak for Fe2+ at binding energy (B.E.) = 709.7 eV, respectively, which implied the generation of the crystalline FeO-phase in the side product (hSiO2-FeO). As the annealing temperature was raised to 700 °C, the entire core@shell and h-SiO2-FeO were deformed and coalesced, composed of crystalline Fe2+-silicate phase (Fe2SiO4) leaving no Fe3O4 phase (from XRD). The result of the temperature-dependent study indicates that the conc-Fe3O4@SiO2 is an intermediate product during the conversion of Fe3O4-nanocube to the reduced FeO nanosphere. The evolution of concave structure in concFe3O4@SiO2 was therefore deduced to be an early step of releasing Fe2+-ions from the Fe3O4-NC to the surrounding silica, forming amorphous Fe2+-silicate phase that supplies Fe2+-ions for FeO-NCgrowth at the external shell-surface.10a Additionally, the deconvolution analysis of high-resolution XPS spectra of Si2p also evidenced the emergence of peaks for Fe2+-silicate phase (B.E. = 709.7 eV) at 400 °C, which gradually grew with increasing the Tann (Figure S3). The time-course TEM-studies (0 h – 48 h) during the conversion of Fe3O4@SiO2 to conc-Fe3O4@SiO2 at 400 °C revealed that concave structures in Fe3O4 [dconc = 10 (±2) nm] started to emerge within a short period of 20 min, without any observable intermediates also the overall structure of the cavity was nearly maintained up to 24 h, presumably because of the saturation of the surrounding

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Figure 2. Pt@conc-Fe3O4 synthesis and characterization: (a) TEM and HRTEM (inset) images. (b) STEM image (c) STEM-EDS elemental mapping images (d) Time courseTEM images for the synthesis of Pt@conc-Fe3O4. silicate shell with Fe2+ ions (Figure S4, S5). Whereas after 48 h, segregation of the FeO-phase started from the silicate-shell, via sequential transferring of Fe2+ ions from the Fe3O4-core to the growing FeO NC (at outer shell surface), leaving a hollow silica-shell and forming h-SiO2-FeO. When conc-Fe3O4@SiO2 was treated under air at 500 °C for 10 h — as evidenced by TEM, the concave cavity was refilled with a lattice-disconnected Fe3O4-crystalline phase, forming a separate small spherical domain, and conc-Fe3O4core reverted back to the nanocube structure close to the initial Fe3O4@SiO2 (Figure S6). Upon subjecting conc-Fe3O4@SiO2 to the thermal oxidation/reduction cycles by repeatedly switching the gaseous environments (Air/H2+Ar), the NPs underwent a reversible change between asymmetric concave and nanocube structures — after each oxidation steps, TEM revealed the identical double-domain crystalline structure, confirming the shuttle-wise movement of Fe-ions at the fixed position of iron oxide/silica-interface. As a comparative thermal transformation study of Fe3O4-nanocube confined within a hollow silica-shell (Fe3O4@h-SiO2) where Fe3O4 was minimally connected to the silica shell: no asymmetric Fe3O4 concave structure was noticed at Tann = 400-700 ºC under reductive environment (Figure S8), indicating that the tight chemical attachment of silica shell with Fe3O4 is critical for heat-induced migration of Fe2+ ions to form the concave structure. When spherical Fe3O4 NC [5 (±1) nm] encapsulated with silica, were reductively annealed at Tann = 300-700 °C, concave structure was absent, rather resulting to only hollow silica-NP (h-SiO2), presumably due to the isotropic morphology (Figure S7). When MnO-nanocubes encapsulated in silica (MnO@SiO2) was reductively annealed, unlike Fe3O4-nanonocubes, internal hollowing of the MnO-nanocube started appearing at 500 °C10 — annealing at 600 ºC generated a hollow interior cavity and at 700 ºC, the hollow nanostructures coalesced (Figure S9). Plausibly, under reductive annealing conditions, at very early stage, Fe2+-ions begin to migrate towards interfacial silica and captured by the silicate anionic sites to form thermodynamically stable Fe-silicate phase, creating nanoscopic defect

transiently which act as highly reactive site for successive migration of Fe2+ ions, evolving to the single cavity on the same facet where initial Fe2+-migration started. Further, the cavitation process is limited by the maximum amount of Fe2+-ions which can be accommodated by the surrounding silica-shell. Next, silica-free concFe3O4 NCs were isolated from conc-Fe3O4@SiO2 by basic hydrolysis and subsequently, functionalized with Pt-NCs by treating with Na2PtCl4 and ascorbic acid. The time-course TEM (Figure 2d, S10) of the reaction revealed that the Pt NCs were gradually deposited specifically in the concave-region of the conc-Fe3O4 over 1.5h, resulting Janus Pt@conc-Fe3O4 nanostructure with Pt-NCs of 3.0 (±1) nm size (Figure 2a, Figure S11, Table S1). The high diffusion of Pt@conc-Fe3O4 was derived from the catalytic degradation of aqueous H2O2 by Pt NCs, where release of O2 from one end of catalyst would enable the self-propulsion of [email protected] Pt@conc-Fe3O4 was first modified with phosphate-polyethylene glycol for high colloidal stability (Figure S12, Table S1 designated as PEG-Pt@conc-Fe3O4) and subjected to the measurement of diffusion coefficient (D) as the function of H2O2-concentration using nanoparticle tracking analyzer (NTA). Upon increasing the concentration of H2O2 as fuel, D of PEG-Pt@conc-Fe3O4 increased linearly, whereas D of PEG-conc-Fe3O4 was unaffected (Figure 3b, SI Movie-2, -3). Finally, PEG-Pt@conc-Fe3O4 nanoswimmers were employed as catalyst for H2O2-mediated degradation of rhodamine 6G (Rh6G) (Figure 3c-d).12 Upon using PEG-Pt@conc-Fe3O4 (1 mg/mL), >99% of Rh6G was degraded within 30 min; whereas the control catalysts PEG-Fe3O4 nanocubes, PEG-conc-Fe3O4 and a physical mixture of PEG-conc-Fe3O4 and Pt NCs resulted incomplete reactions with 5, 15, 25 % conversions, respectively. The activity of PEG-conc-Fe3O4 was found to be slightly higher than the PEG-Fe3O4 nanocubes, possibly due to the more reactive Fesites inside the cavity of conc-Fe3O4. Notably, the physical mixture of non-propelling Pt NCs and conc-Fe3O4 resulted only inferior performance (30% conversion). PEG-Pt@conc-Fe3O4 shows synergistic effect due to fast self-propulsion-mediated intermixing highly reactive interfacial low-coordination Fe3O4-Pt sites inside cavity. Additionally, symmetrical particles having Pt deposited on all the faces of Fe3O4 nanocubes (Pt@Fe3O4) (SI) showed inferior catalysis compared to the PEG-Pt@conc-Fe3O4 due to the missing asymmetric structure and concave surface (Figure S17). To rule out the possibility of adsorptive removal of Rh6G during the catalytic degradation, when PEG-Pt@conc-Fe3O4 was treated with Rh6G solution in the absence of H2O2 for 160 min, only 6% Rh6G was removed (Figure S14). The recyclability of PEG-Pt@conc-Fe3O4 was tested by magnetically separating the catalyst after each cycle, affording up to ~ 90% degradation of Rh6G even after 10 runs (Figure S15). To check the possibility of degradation of Rh6G by visible light without any catalyst, control experiments showed no change in concentration of Rh6G even after 30 min (Figure S16). In conclusion, a highly efficient and selective strategy, for installing single surface-cavity on 3-dimensional Fe3O4 nanocubes, is developed with the aid of a novel nanospace-confined facet-selective reductive metal cation migration and interfacial phase-transition process. The produced conc-Fe3O4 can be further catalytically functionalized and converted to the autonomously diffusive Janus catalytic nanoswimmers as highly efficient catalysts for removal of organic pollutants from water. The present study paves for the future development of new high-temperature chemical reactions among range of metal- and metal oxide-nanotemplates and interfacial confiners for the site-selective nanoscale engraving and surface engineering for generating multidimensional NCs and their hybrids which can exhibit unique physicochemical properties and applications.

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Figure 3. (a) Schematic representation of the degradation of Rh 6G by Pt@conc-Fe3O4. (b) Average D (nm2/s) of PEG-Pt@conc-Fe3O4 and PEG-conc-Fe3O4 after adding various concentration of H2O2, (c) Quantitative comparison of Rh6G degradation by PEG-Pt@concFe3O4 and control particles (d) Effect of H2O2 concentration on degradation Rh 6G by PEG-Pt@conc-Fe3O4. Error bar in each case is based on the results from three independent experiments

ASSOCIATED CONTENT 3.

Supporting Information Additional TEM images, XRD patterns, XPS spectra, M-H curve, DLS, NTAVideos. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author 4.

[email protected].

Author Contributions §Lee,

S. and Kumari, N. contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (Grant NRF-2016R1A3B1907559) (I.S.L.).

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