The stabilization of crystal phases and nanostructured morphologies is an essential topic in application‐driven design of mesoporous materials. Many applications, e.g. catalysis, require high temperature and humidity. Typical metal oxides transform under such conditions from a metastable, low crystalline material into a thermodynamically more favorable form, i.e. from ferrihydrite into hematite in the case of iron oxide. The harsh conditions induce also a growth of the crystallites constituting pore walls, which results in sintering and finally collapse of the porous network. Herein, a new method to stabilize mesoporous templated metal oxides against sintering and pore collapse is reported. The method employs atomic layer deposition (ALD) to coat the internal mesopore surface with thin layers of either alumina or silica. The authors demonstrate that silica exerts a very strong influence: It shifts hematite formation from 400 to 600 °C and sintering of hematite from 600 to 900 °C. Differences between the stabilization via alumina and silica are rationalized by a different interaction strength between the ALD material and the ferrihydrite film. The presented approach allows to stabilize mesoporous thin films that require a high crystallization temperature, with submonolayer quantity of an ALD material, and to apply mesoporous materials for high temperature applications.

The oxides, hydroxides, and oxo-hydroxides of iron belong to the most abundant materials on earth. They also feature a wide range of practical applications. In many environments, they can undergo facile phase transformations and crystallization processes. Water appears to play a critical role in many of these processes. Despite numerous attempts, the role of water has not been fully revealed yet. We present a new approach to study the influence of water in the crystallization and phase transformations of iron oxides. The approach employs model-type iron oxide films that comprise a defined homogeneous nanostructure. The films are exposed to air containing different amounts of water reaching up to pressures of 10 bar. Ex situ analysis via scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, and X-ray diffraction is combined with operando near-ambient pressure X-ray photoelectron spectroscopy to follow water-induced changes in hematite and ferrihydrite. Water proves to be critical for the nucleation of hematite domains in ferrihydrite, the resulting crystallite orientation, and the underlying crystallization mechanism.

The nature of the small iron-oxo oligomers in iron-(iii) aqueous solutions has a determining effect on the chemical processes that govern the formation of nanoparticles in aqueous phase. Here we report on a liquid-jet photoelectron-spectroscopy experiment for the investigation of the electronic structure of the occurring iron-oxo oligomers in FeCl aqueous solutions. The only iron species in the as-prepared 0.75 M solution are Fe monomers. Addition of NaOH initiates Fe hydrolysis which is followed by the formation of iron-oxo oligomers. At small enough NaOH concentrations, corresponding to approximately [OH]/[Fe] = 0.2-0.25 ratio, the iron oligomers can be stabilized for several hours without engaging in further aggregation. Here, we apply a combination of non-resonant as well as iron 2p and oxygen 1s resonant photoelectron spectroscopy from a liquid microjet to detect the electronic structure of the occurring species. Specifically, the oxygen 1s partial electron yield X-ray absorption (PEY-XA) spectra are found to exhibit a peak well below the onset of liquid water and OH (aq) absorption. The iron 2p absorption gives rise to signal centered between the main absorption bands typical for aqueous Fe. Absorption bands in both PEY-XA spectra are found to correlate with an enhanced photoelectron peak near 20 eV binding energy, which demonstrates the sensitivity of resonant photoelectron (RPE) spectroscopy to mixing between iron and ligand orbitals. These various signals from the iron-oxo oligomers exhibit maximum intensity at [OH]/[Fe] = 0.25 ratio. For the same ratio, we observe changes in the pH as well as in complementary Raman spectra, which can be assigned to the transition from monomeric to oligomeric species. At approximately [OH]/[Fe] = 0.3 we begin to observe particles larger than 1 nm in radius, detected by small-angle X-ray scattering.

nanoparticles or nanoporous materials, can enhance the material's performance due to increased surface area, higher sorption capacity, [8] shorter diffusion paths, [9] as well as altered magnetic [10] and electronic properties. [9] Biomedical and biological applications exploit in particular the magnetic properties inherent to nanoparticles of maghemite and magnetite, which combine favorable properties like nontoxicity, biocompatibility, and biodegradability. [11] Whereas both maghemite and magnetite are ferrimagnetic (permanent magnets) as bulk materials, they become superparamagnetic when particle sizes decrease to below about 30 nm. [10,12] Hence, small particles of maghemite and magnetite lose their permanent magnetization already at temperatures significantly lower than the material's Curie temperature and behave paramagnetic. Nanoparticles of maghemite and magnetite can be used as magnetic resonance imaging contrast agents, [11] in biomagnetic separation, [13] hyperthermia treatment, [14] as well as magnetic drug targeting, [15] often serving as magnetic solid support for immobilized enzymes, antibodies, proteins, and oligonucleotides coated onto the particles' surface. [11] Their loading capacity and release characteristics can be vastly improved when particles with well-defined pore structure are used as support with the desired molecules being transported within the pores. [16] Synthesis and applications of such porous carrier particles have been elegantly demonstrated by, e.g., Brinker and co-workers for silica microspheres that featured a micelletemplated mesopore structure [17][18][19] derived via evaporationinduced self-assembly. [20] However, producing micelle-templated mesoporous structures also from superparamagnetic iron oxides, i.e., maghemite and magnetite, has failed so far. [21,22] The synthesis of superparamagnetic iron oxides with template-controlled mesopore structure succeeded only via hard templating. [8,23,24] Jiao et al. [25] impregnated porous silica templates (KIT-6) with iron nitrate solutions and calcined the material at 600 °C, followed by NaOH leaching for template removal, which resulted in mesoporous hematite (α-Fe 2 O 3 ). A subsequent reduction at 350 °C in H 2 /Ar transformed the structure into magnetite, whereas a further oxidation at 150 °C yielded maghemite. [23] Tuysuz et al. used SBA-15 and KIT-6 as silica templates and obtained mesoporous ferrihydrite when calcining at 200 to 250 °C. [26] A further reduction at 320 °C yielded a cubic Maghemite and magnetite show superparamagnetic behavior when synthesized in a nanostructured form. The material's inducible magnetization enables applications ranging from contrast enhancing agents for magnetic resonance imaging to drug delivery systems, magnetic hyperthermia, and separation. Superparamagnetic iron oxides with templated porosity have been synthesized so far only in the form of hard-templated powders, where silicon retained from the template severely degrades the material's magnetic properties. Here, for the first time, the...