6 research outputs found
Colossal Dielectric Behavior of Ga+Nb Co-Doped Rutile TiO<sub>2</sub>
Stimulated by the excellent colossal
permittivity (CP) behavior achieved in In+Nb co-doped rutile TiO<sub>2</sub>, in this work we investigate the CP behavior of Ga and Nb
co-doped rutile TiO<sub>2</sub>, i.e., (Ga<sub>0.5</sub>Nb<sub>0.5</sub>)<sub><i>x</i></sub>Ti<sub>1ā<i>x</i></sub>O<sub>2</sub>, where Ga<sup>3+</sup> is from the same group as In<sup>3+</sup> but with a much smaller ionic radius. Colossal permittivity
of up to 10<sup>4</sup>ā10<sup>5</sup> with an acceptably low
dielectric loss (tan Ī“ = 0.05ā0.1) over broad frequency/temperature
ranges is obtained at <i>x</i> = 0.5% after systematic synthesis
optimizations. Systematic structural, defect, and dielectric characterizations
suggest that multiple polarization mechanisms exist in this system:
defect dipoles at low temperature (ā¼10ā40 K), polaronlike
electron hopping/transport at higher temperatures, and a surface barrier
layer capacitor effect. Together these mechanisms contribute to the
overall dielectric properties, especially apparent observed CP. We
believe that this work provides comprehensive guidance for the design
of new CP materials
Superstructure of Mullite-type KAl<sub>9</sub>O<sub>14</sub>
Large whiskers of a new KAl<sub>9</sub>O<sub>14</sub> polymorph
with mullite-type structure were synthesized. The chemical composition
of the crystals was confirmed by energy-dispersive X-ray spectroscopy,
and the structure was determined using single-crystal X-ray diffraction.
Nanosized twin domains and one-dimensional diffuse scattering were
observed utilizing transmission electron microscopy. The compound
crystallizes in space group <i>P</i>2<sub>1</sub>/<i>n</i> (<i>a</i> = 8.1880(8), <i>b</i> =
7.6760(7), <i>c</i> = 8.7944(9) Ć
, Ī² = 110.570(8)Ā°, <i>V</i> = 517.50(9) Ć
<sup>3</sup>, <i>Z</i> = 2).
Crystals of KAl<sub>9</sub>O<sub>14</sub> exhibit a mullite-type structure
with linear edge-sharing AlO<sub>6</sub> octahedral chains connected
with groups of two AlO<sub>4</sub> tetrahedra and one AlO<sub>5</sub> trigonal bipyramid. Additionally, disproportionation of KAl<sub>9</sub>O<sub>14</sub> into K Ī²-alumina and corundum was observed
using in situ high-temperature optical microscopy and Raman spectroscopy
Chessboard/Diamond Nanostructures and the <i>A</i>āsite Deficient, Li<sub>1/2ā3<i>x</i></sub> Nd<sub>1/2+<i>x</i></sub>TiO<sub>3</sub>, Defect Perovskite Solid Solution
The crystal chemical origin of nanoscale chessboard/diamond
ordering
in perovskite-related solid solutions of composition Li<sub>0.5ā3<i>x</i></sub>Nd<sub>0.5+<i>x</i></sub>TiO<sub>3</sub> (LNT, <i>x</i> ā¼ 0.02ā0.12) is investigated.
Experimental and simulated scanning transmission electron microscopy
(STEM) images are found to be consistent with the compositional modulation
model proposed by previous authors. However, these earlier models
do not satisfactorily explain other features observed in high-resolution
STEM and TEM images, such as the two-dimensional {100} lattice fringes
with the same periodicity, ā2<i>a</i><sub>p</sub> Ć ā2<i>a</i><sub>p</sub>, as the local LNT
unit cell viewed along the [001] direction (where <i>a</i><sub>p</sub> is the parent perovskite unit cell parameter). Based
on bond valence sum calculations, we propose a new set of crystal
structures for LNT in which Li ions are primarily bonded to only two
O ions, and order one-dimensionally with ā2<i>a</i><sub>p</sub> periodicity. Bright-field STEM image simulations performed
for this new model reproduced the experimentally observed ā2<i>a</i><sub>p</sub> lattice fringes, thus strongly suggesting
that the finer features of the high-resolution (S)ĀTEM images are the
result of Li ion ordering and associated local structural relaxation.
In this new model, the LNT chessboard supercell then results from
the ordered combinations of two sublattices: the Li ion sublattice
and its translational variants on the one hand, and the Nd<sub>0.5</sub>TiO<sub>3</sub> sublattice and its oxygen octahedral tilt twin variants
on the other. Dielectric measurements indicate the presence of long-lived
polar clusters that are easily activated under an applied electric
field. This suggests that these clusters consist of spatially correlated
Li ions
Design Synthesis of Nitrogen-Doped TiO<sub>2</sub>@Carbon Nanosheets toward Selective Nitroaromatics Reduction under Mild Conditions
The development of
a facile, low-cost, and eco-friendly approach
to the synthesis of aromatic amines remains a great scientific challenge.
TiO<sub>2</sub>, as a low-cost and earth abundant metal oxide, is
usually not active for thermo-catalyzed nitro reduction. Herein, we
report a composite nanosheet catalyst, composed of nitrogen-doped
TiO<sub>2</sub> and carbon (N-TiO<sub>2</sub>@C), which exhibits highly
efficient, thermo-catalytic performance for selective nitroaromatic
reduction at room temperature. The N-TiO<sub>2</sub>@C nanosheet catalyst
is synthesized via a facile approach where C<sub>3</sub>N<sub>4</sub> nanosheets are utilized not only as a structure-directing agent
to control the shape, size, and crystal phase of TiO<sub>2</sub> but
also as a source of nitrogen for doping into both TiO<sub>2</sub> and
carbon nanosheets. Furthermore, the origin of the superior performance
of the N-TiO<sub>2</sub>@C nanosheet composite catalyst, along with
a possible nitroaromatic reduction mechanism, has also been explored
Above-Band Gap Photoinduced Stabilization of Engineered Ferroelectric Domains
The effect of above-band
gap photons on the domains of the BiFeO<sub>3</sub> (BFO) thin film
was investigated via piezoresponse force
microscopy and Kelvin probe force microscopy. It is found that under
above-band gap illumination, the relaxation time of the polarization
state was significantly extended, while the effective polarizing voltage
for the pristine domains was reduced. We propose that this photoinduced
domain stabilization can be attributed to the interaction between
photogenerated surface charges and domains. Importantly, a similar
phenomenon is observed in other ferroelectric (FE) materials with
an internal electric field once they are illuminated by above-band
gap light, indicating that this photoinduced stabilization is potentially
universal rather than specific to BFO. Thus, this study will not only
contribute to the knowledge of photovoltaic (PV) phenomena but also
provide a new route to promote the stability of PV and FE materials
Bimetallic Ions Codoped Nanocrystals: Doping Mechanism, Defect Formation, and Associated Structural Transition
Ionic
codoping offers a powerful approach for modifying material
properties by extending the selection of potential dopant ions. However,
it has been a major challenge to introduce certain ions that have
hitherto proved difficult to use as dopants (called ādifficult-dopantsā)
into crystal structures at high concentrations, especially through
wet chemical synthesis. Furthermore, the lack of a fundamental understanding
of how codopants are incorporated into host materials, which types
of defect structures they form in the equilibrium state, and what
roles they play in material performance, has seriously hindered the
rational design and development of promising codoped materials. Here
we take In<sup>3+</sup> (difficult-dopants) and Nb<sup>5+</sup> (easy-dopants)
codoped anatase TiO<sub>2</sub> nanocrystals as an example and investigate
the doping mechanism of these two different types of metal ions, the
defect formation, and their associated impacts on high-pressure induced
structural transition behaviors. It is experimentally demonstrated
that the dual mechanisms of nucleation and diffusion doping are responsible
for the synergic incorporation of these two dopants and theoretically
evidenced that the defect structures created by the introduced In<sup>3+</sup>, Nb<sup>5+</sup> codopants, their resultant Ti<sup>3+</sup>, and oxygen vacancies are locally composed of both defect clusters
and equivalent defect pairs. These formed local defect structures
then act as nucleation centers of baddeleyite- and Ī±-PbO<sub>2</sub>-like metastable polymorphic phases and induce the abnormal
trans-regime structural transition of codoped anatase TiO<sub>2</sub> nanocrystals under high pressure. This work thus suggests an effective
strategy to design and synthesize codoped nanocrystals with highly
concentrated difficult-dopants. It also unveils the significance of
local defect structures on material properties