Controllable Synthesis of Undoped and Doped Calcium Niobate Nanocrystals for Tailored Structural, Electronic, and Luminescent Properties

In this work, we report on the phase and particle size control of Ca₂Nb₂O₇ nanocrystals with an aim of tailoring structural, electronic, and luminescent properties. Using citric acid as the capping agent, the as-prepared nanocrystals exhibited a phase transformation from orthorhombic CaNb₂O₆ to cubic Ca₂Nb₂O₇. The samples were carefully characterized by X-ray diffraction, transmission electron microscopy, Fourier transformed infrared spectroscopy, UV–vis diffuse reflectance spectroscopy, and luminescence spectroscopy. It is found that Ca₂Nb₂O₇ showed particle sizes ranging from 14.6 to 26.9 nm by regulating the pH value under hydrothermal conditions. Contrary to the theoretical predictions of the quantum size effect, Ca₂Nb₂O₇ showed an abnormal band gap narrowing with particle size reduction, which can be well-defined as a function of lattice volumes, surface defects, and the surface dipole layer. As for the Eu³⁺-doped samples, it is shown that Eu³⁺ and K⁺ were simultaneously substituted at Ca²⁺ sites in the Ca₂Nb₂O₇ host lattice, which allows one to vary the local symmetry surrounding Eu³⁺ for an enhanced luminescence property. As the consequence of the particle size effect and variation of the local symmetry, a maximum quantum yield of 28% was observed for 19.2 nm Ca₂Nb₂O₇

Additional file 1: of Synthesis and Characterization of Modified BiOCl and Their Application in Adsorption of Low-Concentration Dyes from Aqueous Solution

Figure S1. Adsorption capacities of BiOCl and Fe/BiOCl toward MB (a) and AO (b). Figure S2. Adsorption capacities of MO, MB, RhB, and AO as a function of time in mixed dye solutions on BiOCl. Figure S3. Adsorption capacities of MO, MB, RhB, and AO as a function of time in mixed dye solutions on Fe/BiOCl. Figure S4. Freundlich isotherm for adsorption RhB on BiOCl (a) and Fe/BiOCl (b). Figure S5. Pseudo-second-order kinetics for adsorption RhB on BiOCl (a) and Fe/BiOCl (b). Table S1. Parameters based on the pseudo-second-order kinetics for adsorption RhB on BiOCl and Fe/BiOCl. Figure S6. FT-IR spectra (a) and photographs of various samples (1-RhB, 2-BiOCl, 3-Fe/BiOCl, 4-BiOCl after adsorption, 5-Fe/BiOCl after adsorption, 6-BiOCl after adsorption and photodegradation, 7-Fe/BiOCl after adsorption and photodegradation). (DOCX 669 kb

Toward the Long-Term Stability of Cobalt Benzoate Confined Highly Dispersed PtCo Alloy Supported on a Nitrogen-Doped Carbon Nanosheet/Fe₃C Nanoparticle Hybrid as a Multifunctional Catalyst for Zinc-Air Batteries

This work reports a new type of platinum-based heterostructural electrode catalyst that highly dispersed PtCo alloy nanoparticles (NPs) confined in cobalt benzoate (Co-BA) nanowires are supported on a nitrogen-doped ultra-thin carbon nanosheet/Fe3C hybrid (PtCo@Co-BA-Fe3C/NC) to show high electrochemical activity and long-term stability. One-dimensional Co-BA nanowires could alleviate the shedding and agglomeration of PtCo alloy NPs during the reaction so as to achieve satisfactory long-term durability. Moreover, the synergistic effect at the interface optimizes the surface electronic structure and prominently accelerates the electrochemical kinetics. The oxygen reduction reaction half-wave potential is 0.923 V, and the oxygen evolution reaction under the condition of 10 mA•cm–2 is 1.48 V. Higher power density (263.12 mW•cm–2), narrowed voltage gap (0.49 V), and specific capacity (808.5 mAh•g–1) for PtCo@Co-BA-Fe3C/NC in Zn-air batteries are achieved with long-term cycling measurements over 776 h, which is obviously better than the Pt/C + RuO2 catalyst. The interfacial electronic interaction of PtCo@Co-BA-Fe3C/NC is investigated, which can accelerate electron transfer from Fe to Pt. Density functional theory calculations also indicate that the interfacial potential regulates the binding energies of the intermediates to achieve the best performance

Unexpected Catalytic Performance in Silent Tantalum Oxide through Nitridation and Defect Chemistry

This work reports on the preparation of a noble-metal-free and highly active catalyst that proved to be an efficient and green reductant with renewable capacity. Nitridation of a silent Ta_1.1O_1.05 substrate led to the formation of a series of TaO_<i>x</i>N_<i>y</i> hollow nanocrystals that exhibited outstanding activity toward catalytic reduction of nitrobenzenes under ambient conditions. ESR and XPS results indicated that defective nitrogen species and oxygen vacancies at the surfaces of the TaO_<i>x</i>N_<i>y</i> nanocrystals may play synergetic roles in the reduction of nitrobenzenes. The underlying mechanism is completely different from those previously reported for metallic nanoparticles. This work may provide new possibilities for the development of novel defect-meditated catalytic systems and offer a strategy for tuning any catalysts from silent to highly reactive by carefully tailoring the chemical composition and surface defect chemistry

Particle Size and Structural Control of ZnWO₄ Nanocrystals via Sn²⁺ Doping for Tunable Optical and Visible Photocatalytic Properties

In this work, we report on the microwave-assisted hydrothermal synthesis of Sn²⁺-doped ZnWO₄ nanocrystals with controlled particle sizes and lattice structures for tunable optical and photocatalytic properties. The samples were carefully characterized by X-ray diffraction, transmission electron microscopy, inductive coupled plasma optical emission spectroscopy, UV–vis diffuse reflectance spectroscopy, and Barrett–Emmett–Teller technique. The effects of Sn²⁺ doping in ZnWO₄ lattice on the crystal structure, electronic structure, and photodegradation of methylene orange dye solution were investigated both experimentally and theoretically. It is found that part of the Sn²⁺ ions were homogeneously incorporated in the ZnWO₄ host lattice, leading to a monotonous lattice expansion, and part of Sn²⁺ ions were expelled at surface sites for decreased crystallinity and particle size reduction. By Sn²⁺ doping, ZnWO₄ nanocrystals showed a significant XPS binding energy shift of Zn 2p, W 4f, and O 1s, which is attributed to the combination of electronegativity between Sn²⁺ and Zn²⁺, lattice variation, and particle size reduction. Meanwhile, the BET surface areas were also greatly enlarged from 40.1 to ∼110 m²·g^–1. Contrary to the theoretical predictions of the quantum size effect, Sn²⁺-doped ZnWO₄ nanocrystals showed an abnormal band gap narrowing, which can be well-defined as a consequence of bulk and surface doping effects as well as lattice variations. With well-controlled particle size, crystallinity, and electronic structure via Sn²⁺ doping, the photocatalytic performance of Sn²⁺-doped ZnWO₄ nanocrystals was optimized at Sn²⁺ doping level of 0.451