Research ArticleMATERIALS SCIENCE

Metal-organic frameworks tailor the properties of aluminum nanocrystals

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Science Advances  08 Feb 2019:
Vol. 5, no. 2, eaav5340
DOI: 10.1126/sciadv.aav5340
  • Fig. 1 Synthesis of Al NC@MOF plasmonic heterostructure based on dissolution-and-growth.

    The controlled dissolution of the oxide layer through a one-pot mild hydrothermal strategy in the presence of the requisite organic linker leads to the formation of a uniform Al-based MOF shell around the Al core. In this scenario, the native oxide layer serves as a source of Al3+ for the backbone of the MOF. As the dissolution of Al3+ progresses, the surface oxide is regrown onto the Al NC, controllably etching the nanoparticle, but always maintains a surface oxide protecting the metallic Al core.

  • Fig. 2 Synthesis and characterization of Al NC@MIL-53(Al).

    (A) Schematic illustration of the structure of MIL-53(Al) grown around the Al core based on the dissolution-and-growth approach. Each aluminum center coordinates to two axial hydroxyl functionalities and four carboxylate units from linkers, sharing the coordination with the preceding and following Al centers in the chain. (B) Powder x-ray diffraction pattern of pristine Al NC, Al@MIL-53(Al) (activated for 4 hours at 150°C under vacuum), and simulated MIL-53(Al) in large-pore configuration. a.u., arbitrary units. (C) Transmission electron micrograph (TEM) image of single Al NC with the native oxide layer observable around the edge of the particle. (D) TEM image of single Al NC@MIL-53(Al). (E) Scanning electron micrograph (SEM) image of single Al NC. (F) SEM image of single Al NC@MIL-53(Al) showing a drastic change of the surface features compared with pristine Al NC. (G to J) High-angle annular dark-field scanning transmission electron micrograph (HAADF-STEM) image of (G) single Al NC@MIL-53(Al) and (H to J) surface rendering of Al NC@MIL-53(Al) in different orientations from electron tomography performed on the particle shown in (G). See movie S1. (K to N) HAADF-STEM image of a single Al NC@MIL-53(Al) particle (K) and energy-dispersive x-ray mapping of aluminum, carbon, and oxygen (L to N). Scale bars, 50 nm.

  • Fig. 3 Al NC@MIL-53(Al) with tunable surface plasmon resonance.

    (A to D) Representative TEM images of Al NC@MIL-53(Al) with nominal Al core sizes of (A) 50 nm, (B) 85 nm, (C)110 nm, and (D)150 nm and their corresponding normalized extinction spectra before and after MIL-53(Al) growth. The extinction spectrum of the pure MIL-53(Al) is provided in (A). Scale bars, 50 nm. For particles larger than 100 nm, additional peaks appearing at wavelengths <550 nm correspond to the quadrupolar plasmon resonance. Interband absorption of Al appears at nominally 820 nm. (E) Experimental and theoretical scattering spectrum of bare and MOF-coated particles, showing narrowing of the dipolar plasmon linewidth as a result of MOF growth around Al NC. SEM images of the nanoparticles used to obtain these spectra are shown on the right. Scale bar, 100 nm. (F) FWHM and (G) quality factor (peak energy divided by FWHM) for the MOF-coated Al NCs compared to pristine Al NCs.

  • Fig. 4 Controlling the kinetics of Al NC dissolution during MOF formation.

    (A) Monitoring the LSPR shift of the Al core as a function of time during the formation of the MIL-53(Al) MOF shell in the presence of various concentrations of acetic acid (HOAc) in the initial reaction mixture. The magnitude of the LSPR shift is divided into regions (i) and (ii) representing the dissolution and growth steps, respectively (see fig. S8 for the complete extinction spectra). (B) Representative TEM images of Al NC@MOF heterostructure formed in the absence of acetic acid (orange) and the presence of 5 mM (blue), 15 mM (red), and 30 mM (green) acetic acid. Addition of increasing quantities of acetic acid to the initial reaction mixture controllably increases the rate of dissolution of the oxide layer, causing a greater degree of Al core shrinkage and formation of a thicker and denser MOF shell. (C) Size distributions of pristine Al NC and Al core encapsulated in MOF grown in the presence of 30 mM acetic acid.

  • Fig. 5 Proposed mechanism of MOF [MIL-53(Al)] formation around Al NCs.

    (A to D) Schematic depicting the proposed dissolution-and-growth mechanism and corresponding TEM images (below) of nanoparticles obtained at various stages [(A) 20 min, (B) 40 min, (C) 60 min, and (D) 80 min] showing progressive MOF formation. The particle surface remains visibly unchanged during hydration of the surface oxide layer to aluminum oxyhydroxy [Al2O3 + H2O ➔ 2AlO(OH)]. The dissolution of AlO(OH) by reacting with adsorbed H+ generates an interface with a high concentration of the aluminum aqua complex ([Al(H2O)6]3+) that then quickly coordinates with the linker molecule to form mono- or oligomers of the metal complex–linker species. Localized supersaturation of the solid/liquid interface with coordinated species promotes nucleation of the MOF crystal near the same dissolution site. The onset of the nucleation process after approximately 60 min of the reaction time is consistent with the onset of the LSPR red shift observed during MOF formation (orange dotted line in Fig. 4A). The nucleation process is followed by MOF layer growth that is nearly complete at 80 min into the reaction. We note that, because of thermodynamic equilibrium, the newly generated oxide layer is continuously reestablished while constantly dissolving to supply Al3+ for MOF growth. Scale bars, 50 nm.

  • Fig. 6 Enhancing gas uptake and photocatalytic activity of Al NCs through MOF shell layer growth.

    (A) N2 adsorption isotherm for Al NC@MIL-53(Al) and bare Al NCs measured at 77 K. STP, standard temperature and pressure. (B) Low-pressure CO2 adsorption isotherm for Al NC@MIL-53(Al) and bare Al NC measured at 273 K. The solid and open symbols represent adsorption and desorption, respectively. (C) Photocatalytic reactivity of AlNC@MIL-53(Al) compared to pristine Al NC for the hydrogen-deuterium (HD) exchange reaction. (D) Photocatalytic reactivity of AlNC@MIL-53(Al) compared to pristine Al NC and pure MIL-53(Al) for the rWGS reaction under 300-mW white-light illumination. CO formation under illumination was detected when CO2 and H2 were both present. Illumination of the photocatalyst in an inert He atmosphere did not produce any measurable product, verifying that CO formation was not due to the decomposition of any organic contamination that may be present. GC, gas chromatography.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/2/eaav5340/DC1

    Fig. S1. Crystal structure of the MIL-53(Al) MOF formed around Al NC.

    Fig. S2. Vibrational spectroscopy of the MIL-53(Al) framework surrounding Al NC core.

    Fig. S3. TGA analysis of Al NC@MIL-53(Al).

    Fig. S4. Transmission electron microscopy characterizations of pristine Al NCs and Al NC@MIL-53(Al).

    Fig. S5. Scanning electron microscopy characterizations of pristine Al NCs and Al NC@MIL-53(Al).

    Fig. S6. Attempt for the synthesis of MIL-53(Al) shell around Al NCs following previously established synthetic strategy.

    Fig. S7. Influence of the initial pH of the solution on formation of MIL-53(Al) around Al NCs.

    Fig. S8. Time-dependent UV-Vis extinction spectrum of the reaction mixture during MIL-53(Al) shell formation around Al NCs.

    Fig. S9. The role of organic linker on establishing MOF shell during hydrothermal dissolution of Al NC.

    Fig. S10. Influence of sodium acetate on MOF formation progress.

    Fig. S11. Pore size distribution of MIL-53(Al) shell layer around Al NC core.

    Fig. S12. Spectrum of the light source used for photocatalysis and optical characterization of Al NC@MIL-53(Al) on γ-Al2O3 support.

    Fig. S13. Product selectivity for thermally driven rWGS.

    Fig. S14. Applying the dissolution-and-growth approach to Al NCs in a solution of 1,4-NDC.

    Fig. S15. Applying the dissolution-and-growth approach to Al NCs in a solution of H3BTC.

    Fig. S16. Enhanced stability of Al NC in water through rational MOF coating.

    Fig. S17. Coupling catalytically active TM nanoparticle islands to the Al@MOF hybrid for future photocatalytic applications.

    Movie S1. 3D reconstruction of Al NC@MIL-53(Al) particle morphology using electron tomography.

    References (4345)

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Crystal structure of the MIL-53(Al) MOF formed around Al NC.
    • Fig. S2. Vibrational spectroscopy of the MIL-53(Al) framework surrounding Al NC core.
    • Fig. S3. TGA analysis of Al NC@MIL-53(Al).
    • Fig. S4. Transmission electron microscopy characterizations of pristine Al NCs and Al NC@MIL-53(Al).
    • Fig. S5. Scanning electron microscopy characterizations of pristine Al NCs and Al NC@MIL-53(Al).
    • Fig. S6. Attempt for the synthesis of MIL-53(Al) shell around Al NCs following previously established synthetic strategy.
    • Fig. S7. Influence of the initial pH of the solution on formation of MIL-53(Al) around Al NCs.
    • Fig. S8. Time-dependent UV-Vis extinction spectrum of the reaction mixture during MIL-53(Al) shell formation around Al NCs.
    • Fig. S9. The role of organic linker on establishing MOF shell during hydrothermal dissolution of Al NC.
    • Fig. S10. Influence of sodium acetate on MOF formation progress.
    • Fig. S11. Pore size distribution of MIL-53(Al) shell layer around Al NC core.
    • Fig. S12. Spectrum of the light source used for photocatalysis and optical characterization of Al NC@MIL-53(Al) on γ-Al2O3 support.
    • Fig. S13. Product selectivity for thermally driven rWGS.
    • Fig. S14. Applying the dissolution-and-growth approach to Al NCs in a solution of 1,4-NDC.
    • Fig. S15. Applying the dissolution-and-growth approach to Al NCs in a solution of H3BTC.
    • Fig. S16. Enhanced stability of Al NC in water through rational MOF coating.
    • Fig. S17. Coupling catalytically active TM nanoparticle islands to the Al@MOF hybrid for future photocatalytic applications.
    • References (4345)

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.avi format). 3D reconstruction of Al NC@MIL-53(Al) particle morphology using electron tomography.

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