Growth kinetics of multimetal oxide nanomaterials, the key for structural and property tailoring, is extremely difficult to be determined due to the crystal cracking and the quick migration of metal ions in lattice originated from the big strains after long-term calcinations at very high temperature. Herein, spinel NiFe2O4 was taken as a model multimetal oxide for a case study. NiFe2O4 nanoparticles were synthesized to show grain sizes ranging from 4.3 to 780 nm by a sol–gel autocombustion method. Through a systematic sample characterization via XRD, SEM, and TEM, the grain growth mechanism for NiFe2O4 nanoparticles is uncovered to follow an Ostwald ripening model and grain boundary diffusion model, totally different from those mechanism previously reported for simple oxide nanoparticles, such as LaMer mechanism, Ostwald ripening, oriented attachment mechanism, and their combination. With this unique model, the grain growth kinetics of NiFe2O4 nanoparticles below 700 °C was determined to quantitatively show an equation, D5 = (1.37 × 1015)t exp(−150.75/RT). With the grain growth ongoing, Fe3+ ions of surface/interface might migrate to the octahedral site of NiFe2O4 lattice in the Ostwald ripening process, while more Fe3+ ions at the surface/interface of NiFe2O4 nanoparticles would occupy the tetrahedral sites during the grain-boundary diffusion process. Further, the cell volume showed a minimum at D = 120.9 nm, which was interpreted in terms of the balance between strong repulsive interaction of the parallel defect dipoles at surfaces and the lattice strain due to the migration of Ni2+ and Fe3+ from interface to the bulk. Eventually, as particle size increases, the room-temperature saturation magnetization, Ms, increased continuously, while other magnetic parameters (e.g., coercivity Hc, remanent Mr, and paramagnetic susceptibility χp) showed crossovers. These observations were interpreted in terms of the cation occupancy and lattice variations. The findings reported in this work provide an unprecedented understanding of growth kinetics of spinel NiFe2O4 NPs and may pave an elegant route to synthesize multimetal oxide nanomaterials with specific structure and magnetic properties.

Controlling the growth of nanocrystals is one of the most challenged issues in current catalytic field, which helps to further understand the size and morphology related behaviors for catalytic applications. In this work, we investigated the plane growth kinetics of Mg(OH)2 for catalytic application in preferential CO oxidation. Nanoflakes were synthesized through hydrothermal method. The morphology and structure of nanoflakes were characterized by TEM, SEM, and XRD. By varying the reaction temperature and time, Mg(OH)2 nanoflakes underwent an anisotropic growth. Benefited from the Ostwald ripening process, the thickness of nanoflake corresponding to the (110) plane of Mg(OH)2 was tuned from 7.6 nm to 24.0 nm, while the diameter of (001) plane increased from 18.2 nm to 30.2 nm. The grain growth kinetics for the thickness was well described in terms of an equation, D5 = 7.65 + 6.9 × 108exp(−28.14/RT). After depositing Pt nanoparticles onto these Mg(OH)2 nanoflakes, an excellent catalytic performance was achieved for preferential CO oxidation in H2-rich streams with a wide temperature window from 140 °C to 240 °C for complete CO conversion due to the interaction between Pt and hydroxyl groups. The findings reported here would be helpful in discovering novel catalysts for application of proton exchange membrane fuel cells.

•High doping concentration of Tb3 + suppresses Ce3 +-Sm3 + metal-metal charge transfer quenching in Sr3Y(PO4)3 host.•The energy transfer mechanisms and efficiencies were uncovered through a systematic investigation.•The phosphor showed a superior thermal stability up to 493K, at which photoluminescence intensity still remained at 85.3%.In this work, we report on a comprehensive study about the bridge role of Tb3 + in broadband excited Sr3Y(PO4)3:Ce3 +, Tb3 +, Sm3 + phosphors. With Tb3 + acting as an energy transfer bridge, Ce3 + → (Tb3 +)n → Sm3 + energy transfer process was utilized to circumvent Ce3 +-Sm3 + metal-metal charge transfer (MMCT) quenching in Sr3Y(PO4)3 host. Sm3 + was efficiently sensitized by the broad absorption band (4f1 → 5d1 transition) of Ce3 + in near-ultraviolet spectral region. The color tones of Sr3Y(PO4)3:Ce3 +, yTb3 +, zSm3 + phosphors were tuned from blue through green and finally to orange with increasing Tb3 +/Sm3 + doping concentration. Furthermore, energy transfer mechanisms from Ce3 + to Tb3 + (dipole-dipole mechanism) and Tb3 + to Sm3 + (exchange interaction) as well as the corresponding energy transfer efficiencies (higher than 90%) are systematically investigated. Sr3Y(PO4)3:0.02Ce3 +, 0.90Tb3 +, 0.02Sm3 + phosphor shows a thermal stability up to 493 K, superior to that in analogous reports. The quantum efficiency of Sr3Y(PO4)3:0.02Ce3 +, 0.90Tb3 +, 0.02Sm3 + phosphor with 315 nm excitation was calculated to be 60%.High doping concentration of Tb3 + suppresses Ce3 +-Sm3 + metal-metal charge transfer quenching in Sr3Y(PO4)3:Ce3 +/Tb3 +/Sm3 + phosphors, which show a superior thermal stability up to 493K.Download high-res image (202KB)Download full-size image

•Crystalline to amorphous transformation of tantalum-containing oxides was achieved.•Transformation mechanism was followed by nucleation-dissolution-recrystallization.•Am-TaOx shows a photocatalytic property superior to the crystalline counterparts.Crystalline tantalum-containing oxides are usually taken as the advanced photocatalysts for water splitting. How about the amorphous counterparts? In this work, a transformation of crystalline Na2Ta2O6 (CNa2Ta2O6) to amorphous TaOx (Am-TaOx) was achieved by a facial hydrothermal method. We proposed a transformation mechanism based on nucleation-dissolution -recrystallization and further intensified the influence of base concentration on the composition, crystallinity, and morphology (CCM) as confirmed by XRD, TEM, EDS. N2-physisorption, Raman, IR, and XPS analysis. It is found that when comparing to the crystalline counterparts, amorphous samples possessed higher surface area, abundant surface hydration layers and H+ adsorption, showing an unassisted photocatalytic water splitting with a rate of 70 ± 7 μmol g−1 h−1, much larger than that of 15 ± 1 μmol g−1h−1 of CNa2Ta2O6, 11 ± 1 μmol g−1h−1 of crystalline Ta2O5 (CTa2O5), 30±2 μmol g−1h−1 of mixture with crystalline Ta2O5 and amorphous NaxTayOz (CTa2O5/Am-NaxTayOz), and even 4.6 × 10−4 μmol g−1h−1 for commercial TiO2. This observation is beneficial from the short diffusion paths of amorphous state for charge carriers, amount of catalytic sites, and stronger reducing ability. These findings develop a novel and efficient pathway towards synthesizing the different CCM of tantalum-containing compounds under hydrothermal conditions and could open opportunities for further investigating the photocatalytic property of tantalum-containing materials.Download high-res image (167KB)Download full-size image

Brookite-TiO2 is a promising next-generation semiconductor material for solar energy conversion, but it suffers from difficulty in achieving high quality and phase purity due to its metastable characteristics. Long-chain fatty acid modification or surfactant assisted methods could orient the growth of brookite; however, purifying the products is complicated and the surface reactivity is invariably undermined. Herein, we demonstrate the design and tuneable synthesis of brookite nanostructures with geometric features of quasi-octahedral (QO), ellipsoid-tipped (ET) and wedge-tipped (WT) nanorods that are exposed primarily with {210} facet via water-soluble titanium precursors. When tested as a photocatalyst for hydrogen evolution from water or for the degradation of organic pollutants, QO brookite nanocrystals exhibited the highest catalytic activity compared to ET and WT nanorod counterparts. This observation could be due to the redox facets that form a “surface-heterojunction” and promote the separation of photogenerated carriers. The precursor-directed method reported here may usher in a new phase for the synthesis of novel metastable nanocrystals with specific facet exposure that are highly useful for applications in energy conversion and environment protection.

Brookite-TiO2 is a promising next-generation semiconductor material for solar energy conversion, but it suffers from difficulty in achieving high quality and phase purity due to its metastable characteristics. Long-chain fatty acid modification or surfactant assisted methods could orient the growth of brookite; however, purifying the products is complicated and the surface reactivity is invariably undermined. Herein, we demonstrate the design and tuneable synthesis of brookite nanostructures with geometric features of quasi-octahedral (QO), ellipsoid-tipped (ET) and wedge-tipped (WT) nanorods that are exposed primarily with {210} facet via water-soluble titanium precursors. When tested as a photocatalyst for hydrogen evolution from water or for the degradation of organic pollutants, QO brookite nanocrystals exhibited the highest catalytic activity compared to ET and WT nanorod counterparts. This observation could be due to the redox facets that form a “surface-heterojunction” and promote the separation of photogenerated carriers. The precursor-directed method reported here may usher in a new phase for the synthesis of novel metastable nanocrystals with specific facet exposure that are highly useful for applications in energy conversion and environment protection.