5 research outputs found
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 Ca<sub>2</sub>Nb<sub>2</sub>O<sub>7</sub> 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 CaNb<sub>2</sub>O<sub>6</sub> to cubic Ca<sub>2</sub>Nb<sub>2</sub>O<sub>7</sub>. 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 Ca<sub>2</sub>Nb<sub>2</sub>O<sub>7</sub> 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, Ca<sub>2</sub>Nb<sub>2</sub>O<sub>7</sub> 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 Eu<sup>3+</sup>-doped samples, it is shown that Eu<sup>3+</sup> and K<sup>+</sup> were simultaneously substituted at Ca<sup>2+</sup> sites in the Ca<sub>2</sub>Nb<sub>2</sub>O<sub>7</sub> host lattice, which allows one to vary the local symmetry surrounding Eu<sup>3+</sup> 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 Ca<sub>2</sub>Nb<sub>2</sub>O<sub>7</sub>
Toward the Long-Term Stability of Cobalt Benzoate Confined Highly Dispersed PtCo Alloy Supported on a Nitrogen-Doped Carbon Nanosheet/Fe<sub>3</sub>C 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
Tunable Optical and Photocatalytic Performance Promoted by Nonstoichiometric Control and Site-Selective Codoping of Trivalent Ions in NaTaO<sub>3</sub>
The present work explores a solid
state route to synthesis of trivalent ions (Eu<sup>3+</sup>, La<sup>3+</sup>, etc.) doped NaTaO<sub>3</sub> with controlled nonstoichiometric
chemistry and lattice parameters with an aim to exploring electronic
structure and photocatalytic performance. All samples were fully characterized
using X-ray diffraction (XRD), transmission electron microscopy (TEM),
atomic absorption spectrophotometry, UV–vis diffuse reflectance
spectroscopy, and photoluminescence measurement. By employing Eu<sup>3+</sup> as a model trivalent ion doped in NaTaO<sub>3</sub> lattice,
the effects of site-selective doping and nonstoichiometric chemistry
on the lattice parameters, band gap structure, photocatalytic activity
toward methylene blue solution, and photocatalytic hydrogen evolution
were systematically investigated. A nonstoichiometric Na/Ta molar
ratio led to site-selective occupation of Eu<sup>3+</sup> ions which
was changed from sole substitution to dual substitutions. Meanwhile,
the nonstoichiometric Na/Ta molar ratio and site-selective occupation
of Eu<sup>3+</sup> resulted in a monotonous lattice expansion and
local symmetry distortion. Lattice variation, doping effects, and
its relevant defect chemistry had a great impact on the ν<sub>3</sub> mode vibration of the O–Ta bond, which became asymmetric
and shifted toward higher wavenumbers. Moreover, contrary to theoretical
predictions, Eu<sup>3+</sup>-doped NaTaO<sub>3</sub> nanocrystals
showed an abnormal narrowing of the band gap energies and weak visible
light absorption with variation of Na/Ta molar ratios, which is thought
to be related to doping effects, defect chemistry, and variation of
lattice parameters. With well-defined lattice structure and defect
centers and electronic structure via nonstoichiometric control and
trivalent ions doping, the photocatalytic activity of trivalent ions-doped
NaTaO<sub>3</sub> can be well regulated and optimized
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<sub>1.1</sub>O<sub>1.05</sub> substrate led to the formation of a series of TaO<sub><i>x</i></sub>N<sub><i>y</i></sub> 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<sub><i>x</i></sub>N<sub><i>y</i></sub> 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 ZnWO<sub>4</sub> Nanocrystals via Sn<sup>2+</sup> Doping for Tunable Optical and Visible Photocatalytic Properties
In this work, we report on the microwave-assisted hydrothermal
synthesis of Sn<sup>2+</sup>-doped ZnWO<sub>4</sub> 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 Sn<sup>2+</sup> doping in ZnWO<sub>4</sub> 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 Sn<sup>2+</sup> ions
were homogeneously incorporated in the ZnWO<sub>4</sub> host lattice,
leading to a monotonous lattice expansion, and part of Sn<sup>2+</sup> ions were expelled at surface sites for decreased crystallinity
and particle size reduction. By Sn<sup>2+</sup> doping, ZnWO<sub>4</sub> 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 Sn<sup>2+</sup> and Zn<sup>2+</sup>, lattice variation, and
particle size reduction. Meanwhile, the BET surface areas were also
greatly enlarged from 40.1 to ∼110 m<sup>2</sup>·g<sup>–1</sup>. Contrary to the theoretical predictions of the quantum
size effect, Sn<sup>2+</sup>-doped ZnWO<sub>4</sub> 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 Sn<sup>2+</sup> doping, the photocatalytic performance
of Sn<sup>2+</sup>-doped ZnWO<sub>4</sub> nanocrystals was optimized
at Sn<sup>2+</sup> doping level of 0.451