One of the most classic size-effects in materials is the increase in the strength and hardness as the grain size decreases. However, a practical low size limit for this so-called grain boundary strengthening has been extensively reported for both metals and ceramics. Here, it is demonstrated that this limit is not observed in fully dense nanocrystalline magnesium aluminate, where hardness increases from 17.2 GPa to 28.4 GPa (surpassing sapphire hardness) when grain sizes are refined from 188 nm to 7.1 nm, respectively. The increasing trend is proportional to the square root of the grain size, following the Hall-Petch relationship, reassuring that common weakening mechanisms described in nanocrystalline metals might not be present in ceramics. To achieve such small grain sizes in fully dense ceramics, a new processing technique is introduced, Deformable Punch Spark Plasma Sintering, DP-SPS, in which nanoparticles are sheared under high pressures (~2 GPa) during densification at moderate temperatures (720-870°C). This inhibits grain growth due to the low processing temperatures and destabilizes/eliminate isolated residual pores, known to detrimentally affect mechanical behavior of ceramics. Noticeably, the sintered material showed high transparency in the visible spectrum, being reported as one of the hardest transparent oxide material to date.

Nanoparticle stability against coarsening is one of the keys to allow better exploitation of the properties of nanoscale materials. The intrinsically high interfacial energies of nanoparticles constitute the driving force for coarsening, and therefore can serve as targets to design materials with improved thermal stability. In this study, we discuss the surface engineering of TiO2 nanocatalysts for artificial photosynthesis by exploiting the spontaneous segregation of Ba2+ ions to the interfaces of TiO2 nanocrystals. Ba2+ is a strong candidate for photoelectrocatalytic reduction of CO2 and its effects on interfacial energies lead to a remarkable increase in thermal stability. By using a systematic lixiviation method, we quantified the Ba2+ content located at both the surface and at grain boundary interfaces and combined with direct calorimetric measurements of surface energies and microstructural studies to demonstrate that Ba2+ excess quantities directly impact coarsening of TiO2 nanocatalysts by creating meta-equilibrium configurations defined by the Ba2+ content and segregation potentials at each individual interface. The results establish the fundamental framework for the design of ultrastable nanocatalysts.

The efficiency of a nanostructure applied to photo‐electrocatalysis is fundamentally governed by the capability of the nanostructure surface to sustain a reaction without the occurrence of electron trapping or a recombination of the photogenerated holes. This Minireview summarizes the latest progress in the use of cocatalysts on hematite electrodes and their application in water splitting induced by sunlight. The major drawback of hole diffusion through a solid‐liquid interface is addressed through the selection of the best cocatalyst for increasing the efficiency of an iron‐based material. Finally, the most promising modification routes and outstanding materials for enhancing the low kinetics of the oxygen evolution reaction during sunlight‐driven water oxidation reactions in hematite are provided and those materials that may have a significant impact on the overall photoelectrochemical performance are discussed.

Semiconductor-based solar water splitting is a promising strategy for the production of fuels from a clean and sustainable source, following the global trend of replacing fossil fuels. Herein, a systematic study of the application of single-phase pseudobrookite Fe2TiO5 nanoparticles as oxygen-evolving photocatalyst for water splitting, in a suspended particle system, is presented. A solvothermal route was employed for the synthesis of Fe2TiO5 nanoparticles with average diameter of (34 ± 8) nm. The obtained orange powder absorbs a broad portion of the solar spectrum (band gap of 2.1 eV) and produces 7.0 μmol of O2 within 5 h, under visible light irradiation. In order to enhance the catalytic activity of Fe2TiO5, NiO and Co3O4 cocatalyst nanoparticles were, separately, attached to the surface through the conventional impregnation and the magnetron sputtering deposition (MSD) methods. The homogeneous coverage with cocatalysts nanoparticles provided by the latter, allied to the reduced dimensions of the formed oxide nanoparticles (∼1 nm), resulted in an enhanced photoactivity of Fe2TiO5. The Co3O4-modified materials prepared through magnetron sputtering and impregnation depositions produced 58.8 and 26.2 μmol of O2 within 5 h under visible light, respectively. Similar behavior was observed for the NiO-modified nanomaterial, which generated 25.5 and 12.6 μmol of O2, respectively. These results reflect the potentiality of Fe2TiO5 nanoparticles to be employed as a particulate water splitting photocatalyst, especially when containing homogeneously distributed nanoparticles of the Co3O4 cocatalyst on the surface. Photoelectrochemical measurements further confirmed these conclusions, as Co3O4 and NiO fine deposits substantially reduce the water oxidation overpotential of a Fe2TiO5 photoanode and enhance the intensity of the anodic photocurrent due to the improved oxidation kinetics, the reduced charge recombination rates, and the lowered charge transfer resistance at the solid/liquid interface.

It has been empirically observed that decreasing grain size in polycrystalline materials leads to an increase in hardness following a linear dependence with the inverse square root of the size. This so-called Hall−Petch relationship was originally observed in metals and was attributed to the blocking effect of grain boundaries on dislocation motion. However, the Hall−Petch theory is not tenable for explaining similar phenomena in oxide ceramics, where dislocations are virtually immobile at ambient temperature. In this study, multinuclear ( 27 Al, 25 Mg) magnetic resonance spectroscopy is utilized to demonstrate that a reduction in the grain size of MgAl 2 O 4 spinel nanoceramics induces structural disorder in the form of cation site inversion in the grain boundary region, which leads to an increase in the average bond density and bond strength. The rapid increase in the volume fraction of such structurally disordered and strong grain boundary regions with decreasing grain size of the nanoceramics is responsible for the corresponding remarkable increase in the hardness.