15 research outputs found
Fabrication of La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> Crystals Using an Alkali-Metal Molybdate Flux Growth Method and Their Nitridability To Form LaTiO<sub>2</sub>N Crystals under a High-Temperature NH<sub>3</sub> Atmosphere
Flux
growth is a promising method that allows one to control over the crystalline
phase, crystal shape, crystal size, and crystal surface through the
selection of a suitable flux. In this work, lanthanum titanate (La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub>) crystals with different morphologies
were grown using the Na<sub>2</sub>MoO<sub>4</sub>, K<sub>2</sub>MoO<sub>4</sub>, NaCl, and mixed NaCl + K<sub>2</sub>MoO<sub>4</sub> (molar
ratio = 3:7) fluxes, and their nitridability to form LaTiO<sub>2</sub>N crystals under a high-temperature NH<sub>3</sub> atmosphere was
also investigated. The effects of the solute concentration and cooling
rate on the growth of the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals were also studied. The X-ray diffraction results revealed
that the {100} plane was dominant in the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> platelet crystals grown using the alkali-metal molybdate
fluxes. When the solute concentration was increased from 1 to 20 mol
%, the average size of the crystals decreased without considerable
alteration of the overall crystal morphology. The La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals with the preferred ⟨010⟩
and ⟨001⟩ growth directions along the <i>b</i> and <i>c</i> axes were grown using the Na<sub>2</sub>MoO<sub>4</sub> and K<sub>2</sub>MoO<sub>4</sub> fluxes, respectively. Compared
to the Na<sub>2</sub>MoO<sub>4</sub> flux, the K<sub>2</sub>MoO<sub>4</sub> flux did not show a cooling-rate-dependent effect on the
growth of the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals.
It was found that conversion of the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals to the LaTiO<sub>2</sub>N crystals was strongly
dependent on the flux used to grow the precursor La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals. That is, the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals grown using the K<sub>2</sub>MoO<sub>4</sub> and NaCl fluxes were nearly completely converted into the
LaTiO<sub>2</sub>N crystals, while conversion of the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals grown using the Na<sub>2</sub>MoO<sub>4</sub> and mixed NaCl + K<sub>2</sub>MoO<sub>4</sub> fluxes
to the LaTiO<sub>2</sub>N crystals seemed to be not completed yet
even after nitridation at 950 °C for 15 h using NH<sub>3</sub> because of the larger crystal size and the presence of unintentional
impurities (sodium and molybdenum from the flux) in the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystal lattice. Nevertheless, the LaTiO<sub>2</sub>N crystals fabricated by nitriding the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals grown using the K<sub>2</sub>MoO<sub>4</sub> and NaCl fluxes should be suitable for direct solar water
splitting
Template-Assisted Size Control of Polycrystalline BaNbO<sub>2</sub>N Particles and Effects of Their Characteristics on Photocatalytic Water Oxidation Performances
The photocatalytic
water oxidation using solar irradiation is a
sustainable way to convert a natural source to energy. The perovskite-type
oxynitride BaNbO<sub>2</sub>N is a candidate photocatalyst for this
process because its long-range light absorbance of up to ca. 740 nm
leads to the high ability of energy conversion. However, it is necessary
to improve its poor performance by optimizing the crystallographic
characteristics, chemical formula, depositions of cocatalysts, and
so on. In this study, we aimed to identify the dominant factors of
the photocatalytic performance of BaNbO<sub>2</sub>N. We controlled
the particle characteristics by nitriding size-controlled Ba<sub>5</sub>Nb<sub>4</sub>O<sub>15</sub> crystals in sizes of 0.2–50 μm
as sacrificial templates. Porous BaNbO<sub>2</sub>N secondary particles
of different sizes were achieved, and they exhibited distinctive photocatalytic
performances for O<sub>2</sub> evolution with rates between 14.1 and
113.9 μmol·h<sup>–1</sup>, depending on the precursor
size and nitriding time. By correlating the performance with the basal
particle characteristics, we assume that the crystallinity and anion
deficiency are the two dominant factors that competitively affect
the photocatalytic performance of BaNbO<sub>2</sub>N
Fabrication of NIR-Vis Upconversion YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Ln (Ln = Yb, Er, Ho, and Tm) Crystal Layers by Flux Coating and Investigation of Growth Manner
High-quality upconversion YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Ln (Ln = Yb, Er, Ho, and Tm) layers
with good adhesion, comprising densely packed, plate-shaped idiomorphic
crystals, were directly grown on stainless steel (SUS) substrates
by flux coating. The LiNO<sub>3</sub>–KNO<sub>3</sub> flux
effectively promoted crystal growth. Additionally, the near-infrared-to-visible
(NIR-vis) upconversion fluorescence properties of the YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Ln crystal layers
could be tuned by varying the type of dopant (Ln element). YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Yb,Er, YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Yb,Ho, and YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Yb,Tm crystal layers showed red,
green, and blue fluorescence, respectively, under 980 nm laser irradiation
Fabrication of NIR-Vis Upconversion YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Ln (Ln = Yb, Er, Ho, and Tm) Crystal Layers by Flux Coating and Investigation of Growth Manner
High-quality upconversion YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Ln (Ln = Yb, Er, Ho, and Tm) layers
with good adhesion, comprising densely packed, plate-shaped idiomorphic
crystals, were directly grown on stainless steel (SUS) substrates
by flux coating. The LiNO<sub>3</sub>–KNO<sub>3</sub> flux
effectively promoted crystal growth. Additionally, the near-infrared-to-visible
(NIR-vis) upconversion fluorescence properties of the YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Ln crystal layers
could be tuned by varying the type of dopant (Ln element). YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Yb,Er, YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Yb,Ho, and YO<sub>1–<i>x</i></sub>F<sub>1+2<i>x</i></sub>:Yb,Tm crystal layers showed red,
green, and blue fluorescence, respectively, under 980 nm laser irradiation
Flux-Assisted Fabrication of Vertically Aligned Layered Double Hydroxide Plates on in Situ Formed Alumina Particles
Layered double hydroxide (LDH) is
an environmentally benign anion
exchanger that can adsorb various toxic anions. In this work, we demonstrate
the fabrication of plate-like Mg–Al-type LDH crystals on in
situ formed alumina particles using a flux method at a relatively
low temperature (∼350 °C). At or below 300 °C, the
melted Al source crystallized to form AlOOH or γ-alumina particles
in KNO<sub>3</sub>–NaNO<sub>3</sub> flux. However, LDH crystals
did not form due to the inferior crystallization properties of the
Mg precursor. Increasing the holding temperature up to 350 °C
and above facilitated crystallization of the dissolved Mg and Al species
in flux to yield plate-like LDH crystals on the preformed alumina
particles. Top-surface and cross-sectional FE-SEM and EPMA analyses
revealed the vertical alignment of the crystalline LDH plates on the
surface of the alumina particles. On the other hand, solid-state reactions
did not yield these well-grown, plate-like LDH crystals. The TG-DTA
profile of the LDH precursors with flux depicted the decomposition
and crystallization events that the Al and Mg precursors undergo.
On the basis of the results from these characterization studies, we
propose a mechanism in which LDH crystals sequentially form on the
surface of the alumina particles
Environmentally Friendly Flux Growth of High-Quality, Idiomorphic Li<sub>5</sub>La<sub>3</sub>Nb<sub>2</sub>O<sub>12</sub> Crystals
High-quality, idiomorphic, single-phase Li<sub>5</sub>La<sub>3</sub>Nb<sub>2</sub>O<sub>12</sub> crystals were successfully
grown using
a LiOH flux cooling method at the relatively low temperature of 500
°C at a solute concentration of 5 mol %. The grown Li<sub>5</sub>La<sub>3</sub>Nb<sub>2</sub>O<sub>12</sub> crystals had polyhedral
shapes with well-developed, flat {211} and {110} faces. Their shapes
were relatively uniform, and the average crystal size was approximately
59.2 μm. No aggregation was observed in scanning electron microscopy
images. The high crystallinity of the Li<sub>5</sub>La<sub>3</sub>Nb<sub>2</sub>O<sub>12</sub> crystals was confirmed by transmission
electron microscopy images. Their lattice parameter was determined
from the X-ray diffraction pattern to be <i>a</i> = 1.281
nm, which is consistent with the literature value (<i>a</i> = 1.282 nm). Furthermore, the crystal phase, form, size, and crystallinity
of the flux-grown Li<sub>5</sub>La<sub>3</sub>Nb<sub>2</sub>O<sub>12</sub> crystals were obviously dependent on the growth conditions
including the solute concentration and holding temperature
Chloride Flux Growth of La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> Crystals and Subsequent Nitridation To Form LaTiO<sub>2</sub>N Crystals
Highly crystalline, platelike La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> were grown from a NaCl flux, and
LaTiO<sub>2</sub>N crystals
were obtained by subsequent nitridation under NH<sub>3</sub> flow.
The TEM analysis indicated that the flux-grown platelike La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> crystals are single-crystalline growing
along the <i>a</i> axis. The shapes and sizes of the LaTiO<sub>2</sub>N crystals were almost unchanged from the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> precursor. In addition, LaTiO<sub>2</sub>N crystals
remained single-crystalline with a porous nanostructure. The optical
absorption edges of the La<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub> and
LaTiO<sub>2</sub>N crystals were approximately 320 and 600 nm
Low-Temperature Flux Growth and Upconversion Fluorescence of the Idiomorphic Hexagonal-System NaYF<sub>4</sub> and NaYF<sub>4</sub>:Ln (Ln = Yb, Er, Tm) Crystals
Idiomorphic NaYF<sub>4</sub> and NaYF<sub>4</sub>:Ln (Ln = Yb, Er, Tm) crystals with upconversion fluorescence were successfully grown by the NaNO<sub>3</sub> flux cooling method at a relatively low holding temperature. The grown NaYF<sub>4</sub> and NaYF<sub>4</sub>:Ln crystals had a hexagonal prismatic form, and their well-developed surfaces were relatively flat. TEM images indicated that the NaYF<sub>4</sub> crystals were of good crystallinity. Their size and shape were relatively uniform, and they were poorly aggregated. The crystal phase, form, and size depended on the growth temperature and the solute concentration. In contrast, the addition of dopant ions (Yb<sup>3+</sup>, Er<sup>3+</sup>, and Tm<sup>3+</sup>) did not affect the shape, morphology, or crystallinity of the flux-grown NaYF<sub>4</sub>:Ln crystals. Additionally, the upconversion fluorescence properties of NaYF<sub>4</sub>:Ln crystals were also dependent on the type and mixture ratio (i.e., starting composition) of the dopants. The green, orange, and blue upconversion fluorescences of NaYF<sub>4</sub>:10%Yb,1%Er, NaYF<sub>4</sub>:50%Yb,1%Er, and NaYF<sub>4</sub>:10%Yb,1%Tm crystals, respectively, were observed under 980 nm laser irradiation via two- or three-photon upconversion processes
NH<sub>3</sub>‑Assisted Flux Growth of Cube-like BaTaO<sub>2</sub>N Submicron Crystals in a Completely Ionized Nonaqueous High-Temperature Solution and Their Water Splitting Activity
As the 600 nm-class photocatalyst,
BaTaO<sub>2</sub>N is one of
the promising candidates of the perovskite-type oxynitride family
for photocatalytic water splitting under visible light. The oxynitrides
are routinely synthesized by nitriding corresponding oxide precursors
under a high-temperature NH<sub>3</sub> atmosphere, causing an increase
in the defect density and a decrease in photocatalytic activity. To
improve the photocatalytic activity by reducing the defect density
and improving the crystallinity, we here demonstrate an NH<sub>3</sub>-assisted KCl flux growth approach for the direct synthesis of BaTaO<sub>2</sub>N crystals. The effects of various fluxes, solute concentration,
and reaction time and temperature on the phase evolution and morphology
transformation of the BaTaO<sub>2</sub>N crystals were systematically
investigated. By changing the solute concentration from 10 to 50 mol
%, it was found that phase-pure BaTaO<sub>2</sub>N crystals could
only be grown with the solute concentrations of ≥10 mol % using
the KCl flux, and the solute concentration of 10 mol % was solely
favorable to directly grow cube-like BaTaO<sub>2</sub>N crystals with
an average size of about 125 nm and exposed {100} and {110} faces
at 950 °C for 10 h. The time- and temperature-dependent experiments
were also performed to postulate the direct growth mechanisms of cube-like
BaTaO<sub>2</sub>N submicron crystals. The BaTaO<sub>2</sub>N crystals
modified with Pt and CoO<sub><i>x</i></sub> nanoparticles
showed a reasonable H<sub>2</sub> and O<sub>2</sub> evolution, respectively,
due to a lower defect density and higher crystallinity achieved by
an NH<sub>3</sub>-assisted KCl flux method
High-Quality Ultralong Hydroxyapatite Nanowhiskers Grown Directly on Titanium Surfaces by Novel Low-Temperature Flux Coating Method
Idiomorphic, one-dimensional (1-D), and high-quality
hydroxyapatite
(HAp) nanocrystals were successfully, directly, and densely grown
on a Ti substrate at the relatively low temperature of 300 °C
using a KNO<sub>3</sub>–LiNO<sub>3</sub> flux coating method.
The grown HAp crystals have a 1-D shape with a very high aspect ratio
(much larger than 100) and an average size of 3250 × 25 nm (length
× width). The ultralong 1-D crystals grown at 300 °C were
identified as highly crystalline HAp by their X-ray diffraction (XRD)
patterns, which clearly displayed the four characteristic lines of
HAp between 31.5° and 34.5°. Additionally, high-resolution
transmission electron microscopy (HRTEM) images demonstrated that
these ultralong whiskers were high-quality because point and line
defects were not observed. From the energy-dispersive X-ray spectroscopy
(EDS) analysis, major components were homogeneously distributed in
the HAp whiskers. In addition, the effects of holding temperature
and starting composition on the forms and average sizes of the grown
HAp whiskers were investigated