Hybrid Nanocomposites with Tunable Alignment of the Magnetic Nanorod Filler

For many important applications, the performance of polymer–anisotropic particle nanocomposite materials strongly depends on the orientation of the nanoparticles. Using the very peculiar magnetic properties of goethite (α-FeOOH) nanorods, we produced goethite–poly­(hydroxyethyl methacrylate) nanocomposites in which the alignment direction and the level of orientation of the nanorods could easily be tuned by simply adjusting the intensity of a magnetic field applied during polymerization. Because the particle volume fraction was kept low (1–5.5 vol %), we used the orientational order induced by the field in the isotropic phase rather than the spontaneous orientational order of the nematic phase. At the strongest field values (up to 1.5 T), the particles exhibit almost perfect antinematic alignment, as measured by optical birefringence and small-angle X-ray scattering. The results of these two techniques are in remarkably good agreement, validating the use of birefringence measurements for quantifying the degree of orientational order. We also demonstrate that the ordering induced by the field in the isotropic suspension is preserved in the final material after field removal. This work illustrates the interest, for such problems, of considering the field-induced alignment of anisotropic nanoparticles in the isotropic phase, an approach that is effective at low filler content, that avoids the need of controlling the nematic texture, and that allows tuning of the orientation level of the particles at will simply by adjusting the field intensity

Engineering the Optical Response of the Titanium-MIL-125 Metal–Organic Framework through Ligand Functionalization

Herein we discuss band gap modification of MIL-125, a TiO₂/1,4-benzenedicarboxylate (<b>bdc</b>) metal–organic framework (MOF). Through a combination of synthesis and computation, we elucidated the electronic structure of MIL-125 with aminated linkers. The band gap decrease observed when the monoaminated <b>bdc-NH</b>_<b>2</b> linker was used arises from donation of the N 2p electrons to the aromatic linking unit, resulting in a red-shifted band above the valence-band edge of MIL-125. We further explored in silico MIL-125 with the diaminated linker <b>bdc-(NH</b>_<b>2</b><b>)</b>_<b>2</b> and other functional groups (−OH, −CH₃, −Cl) as alternative substitutions to control the optical response. The <b>bdc-(NH</b>_<b>2</b><b>)</b>_<b>2</b> linking unit was predicted to lower the band gap of MIL-125 to 1.28 eV, and this was confirmed through the targeted synthesis of the <b>bdc-(NH</b>_<b>2</b><b>)</b>_<b>2</b>-based MIL-125. This study illustrates the possibility of tuning the optical response of MOFs through rational functionalization of the linking unit, and the strength of combined synthetic/computational approaches for targeting functionalized hybrid materials