Metal oxide-to-MIL-53(Al) conversion of ALD-alumina coated SiC foamsThe conversion of Al2O3 layers deposited by ALD on pristine SiC foams (Fig. 1a) into MIL-53(Al) resulted in the formation of large, rectangular crystals as observed by SEM (Fig. 1b,c). Indeed, the surface of SiC foam is uniformly covered with MOF crystals forming a unidirectional urchin-like pattern. We note an average MIL-53(Al) crystal size of 2.5 ± 0.8 µm × 0.5 ± 0.2 µm. Moreover, the crystal length of MIL-53(Al) is rather constant, around ⁓ 2.0 µm (Fig. 1d). Thus, the upwards growth of MIL-53(Al) crystals in the presence of MWs (i.e. in the direction of their length) appears to be homogeneous on SiC support. The nature of the MOF material was confirmed by Raman spectroscopy due to the observed characteristic vibration bands of MIL-53(Al) in the range 800–1800 cm−1 (Figure S2). To the author’s knowledge, such large MOF crystals presenting this type of urchin-like morphology, well-ordered and anchored to the surface of a ceramic support have never been reported in the literature.Fig. 1SEM images of pristine SiC (a) foams; MIL-53(Al) urchin-like crystals grown on SiC foam (b,c) and its cross-section (d) by MW-assisted hydrothermal conversion of a thin ALD-alumina layer.EDX mapping (Figure S3) is evidencing the uniform distribution of MOF crystals on the surface of SiC foam. Moreover, the latter showed that carbon and oxygen atoms present in the MIL-53(Al) framework were solely located on the ceramic foams’ surface, confirming virtually zero growth of MOF crystals in the bulk of the supporting material.Compared to the ordered urchin structure of MIL-53(Al) formed by alumina conversion under MW irradiation (i.e., MIL-53@SiC), conventional heating generates entangled and disorganized nanorods (average diameter ~ 0.20 µm) when applying the same synthesis temperature and ligand concentration (Fig. 2). In addition, a significant difference in support weight increase was noticed between the two samples after MOF growth: 6.5 wt% and 9.1 wt% for MIL-53@SiC-CONV and MIL-53@SiC respectively (Table 1). Thus, as expected, higher conversion rates of ALD alumina to MIL-53(Al) were therefore achieved via MW heating. However, the morphological effects associated to the heating method do not seem to impact on the specific surface area and pore volume of the MOF. Indeed, the SBET values determined for MIL-53@SiC and MIL-53@SiC-CONV are very similar (80 and 77 m2/g, respectively—Table 1).Fig. 2SEM images of MIL-53@SiC-CONV synthesized via the hydrothermal conversion under conventional heating of a thin ALD-alumina top layer.Table 1 Textural characteristics measured for MIL-53(Al) grown on SiC foams and extrapolated to the MIL-53(Al) material- Values of SBET and pore volume (Vp) were attributed to the sole contribution of the MOF material (An error of ~ 10% is estimated on the SBET, Vp).Hence based on these observations one would question which concomitant phenomena are occurring in the presence of MWs and are inducing the particular observed morphology of MIL-53(Al) and its epitaxial growth. Indeed, as mentioned in the beginning SiC is an excellent thermal dissipater under MW irradiation14,22. Additionally to this property, SiC is benefitting from its native silica layer rich in hydroxyl groups necessary for the germination process of MOF(Al) on the SiC support23. Herein a synergistic effect of these two characteristics needs to be considered since it could favor the creation of “anchoring sites” for MIL-53(Al) nuclei, leading to a uniform distribution of MIL-53(Al) crystals on SiC surface.Besides, since the SiC-foam surface is expected to be heated more homogeneously during the reaction under MW irradiation due to high dielectric loss constant (Table S1), it enables a better control of crystal growth. Yet, it does not explain the formation of epitaxial grown urchins.Therefore, one can question the role of ALD-Al2O3. The thin ALD-Al2O3 layer is amorphous, rich in hydroxyl groups and very reactive. In the work of Kim et al.24 it was demonstrated that the epitaxial growth of MIL-53(Al) on metal particles seemed to depend strongly on the competition between coordination and dissolution kinetics. The later was favored at synthesis temperatures exceeding 200 °C, and subsequently induced the growth of 1D oriented MOF(Al) crystals. Since ALD-alumina is strongly reactive the dissolution of Al3+ at 220 °C might have favored the epitaxial growth of urchins. Interestingly, comparative experiments carried out on γ-alumina Accu®spheres (St-Gobain) led to the same epitaxial growth of urchin-like MIL-53(Al) crystals (Figure S4). The amorphous nature of the ALD-alumina layer as well as the SiC support are therefore not responsible for both the formation of urchin-like crystals and epitaxial growth.Yanai et al.25 demonstrated the feasibility of aligning ZIF-8 crystals under an imposed electric field. Similarly, when MW-heating is applied, samples are also exposed to the electric component of MW irradiation. Indeed, during the oxide-to-MOF conversion, the coordinated aluminum-terephthalic acid (Al-TA) complexes and water molecules are sensitive to the orientation of the electric field. Given their high dielectric constant (ε’) (Table S1)13 and dipolar characteristics, they orient themselves based on the applied electric field (Fig. 3a). Laybourn et al.13 have demonstrated that the dielectric properties of Al3+ and water are strongly affected by the concentration of Al-substrate and applied temperature. In short, when the concentration of Al(III)-salt is low (i.e. around 5 mM) the dielectric properties of Al3+ are dominated by those of water, but at higher Al(III)-salt concentrations (i.e. 500 mM) the effect is opposite. If the total amount of ALD-Al2O3 (i.e. 4 wt% determined by EDX) was accessible for MOF conversion, the reaction solution would have a maximum alumina concentration of 10 mM—a value closer to the range in which dielectric properties of water are the main parameter governing the reaction mechanism. As a result, it seems plausible that the water molecules are aligned with the applied electric field under MW irradiation and likely do orient also the Al-TA complexes. Yet, at the solid/liquid interface, a gradient of concentration does exist. Herein, it is most likely that many point areas are present on the surface, generating hotspots due to the increased dielectric loss of Al-ions at higher concentrations. Those hotspots could potentially promote coordination and nucleation on the surface of supported alumina layers, forming so-called “anchor points”.Fig. 3(a) Orientation of Al-TA complexes and water molecules in an electric field. (b) Schematic representation of the proposed mechanism for the formation of urchin-like MIL-53(Al) crystals on a ceramic support coated with ALD-alumina, by reaction with the TA ligand in water and under MW irradiation. (The electric field is present at all stages).In addition, by simply forming a metal–ligand complex with Al3+ (Table S1), ε’ increases from 2.53 to 7013. As a results, Al-TA complexes in solution and when grafted to the anchor sites on Al2O3 coating, are oscillating with the electric field. Hence, we hypothesize that dissolved Al-TA complexes or pre-organized nuclei are assembled a long the direction of the electric field, inducing the formation of bundled fibers with a rectangular cross-section. At this stage it is expected that MW irradiation induces an abundance of closely spaced anchor points and an excess of Al3+ ions or Al-TA complexes in the aqueous solution. This rapid accumulation of species could lead to steric effects at the anchor points, resulting in the formation of urchin-like structures. Contrary, without these steric effects, parallelly, ordered fibers would have been formed on the substrates. The proposed mechanism is illustrated schematically in Fig. 3b.Potential applications of the designed materialsAs mentioned in the introduction, the as-synthetized supported materials could be used in many fields such as adsorption, filtration or catalysis. In all cases, good adhesion of the MOF to the support and high mechanical stability of the system are crucial. Sonication tests carried out in ethanol with MIL-53@SiC clearly validated the stability of the sample and the excellent adhesion of crystals to the support in the tested sonication conditions. Indeed, SEM observations confirmed the preservation of urchin-like crystals on SiC foams (Figure S7). Furthermore, EDX analysis proved that the aluminum loss was very low (⁓0.4 wt%) even during extended sonication treatments (Table S3), therefore confirming the system’s stability and the good adhesion of MOF crystals to the SiC substrate.Attractively, the proposed synthesis approach can be applied to other ceramic supports such as ZrO2 (Figure S8). It is however important to note that a much smaller quantity of MIL-53(Al) crystals was grown on the ZrO2 support which is quasi-inert to MW irradiation (ε’’⁓ 0.02 and 28 for ZrO2 and SiC, respectively). Therefore, a weight increase of only 1.1 wt% was measured for the ZrO2 support while it reached 9.1 wt% for SiC (Table S4).The presented synthesis method can also be be exploited for the preparation of original catalyst carriers with high specific surface areas, by calcining the final ceramic supported MOF material. The concept is applicable to a wide range of ceramic supports, with special interest for MW-absorptive substrates such as SiC-based ceramics. SiC offers attractive physicochemical properties (chemical inertness, thermal conductivity, mechanical stability, magnetic, semiconductor, optical and electronic properties)26, but the low specific surface area limits its use as catalytic carrier. In order to overcome this issue, MIL-53(Al) urchin-like crystals synthetized in this work could be further advantageously transformed into hierarchical nano-structured porous alumina via calcination. As demonstrated in Fig. 4, Table S4 and Figure S9, calcination of MIL-53@SiC at 1000 °C leads to the formation of a major θ-Al2O3 phase with a specific surface area of ⁓227 m2/g. Obtaining remarkably high SBET values at high calcination temperatures opens the possibility to advantageously exploit these materials for the dispersion of active species (e.g. metal nanoparticles, transition metal dichalcogenides, etc.), yielding novel heterogeneous catalysts. At the same time, the high thermal and mechanical stability of the as-synthetized θ-alumina and its SiC support endows the composite material with robustness relevant for a variety of multiphase reactions conducted under harsh conditions (i.e. high pressure and temperature). Finally, preserving the urchin-like structure could strongly improve the contact of the active surface with reactants.Fig. 4SEM images of Al2O3 fibers obtained on SiC support after calcination of MIL-53@SiC at 1000 °C in air.
