A quantum paraelectric SrTiO3 is a material situated in close proximity to a quantum critical point (QCP) of ferroelectric transition in which the critical temperature to the ferroelectric state is ...suppressed down to 0 K. However, the understanding of the behavior of the phase transition in the vicinity of this point remains challenging. Using the concentration x of Pb in solid solution Sr1−xPbxTiO3 (PSTx) as a tuning parameter and applying the combination of Raman and dielectric spectroscopy methods, we approach the QCP in PSTx and study the interplay of classical and quantum phenomena in the region of criticality. We obtain the critical temperature of PSTx and the evolution of the temperature-dependent dynamical properties of the system as a function of x to reveal the mechanism of the transition. We show that the ferroelectric transition occurs gradually through the emergence of the polar nanoregions inside the non-polar tetragonal phase with their further expansion on cooling. We also study the ferroelastic cubic-to-tetragonal structural transition, occurring at higher temperatures, and show that its properties are almost concentration-independent and not affected by the quantum criticality.
The crystal structure and lattice dynamics of quantum paraelectric BaxSr1−xTiO3 (x = 0, 0.01, 0.02) solid solutions are studied using X‐ray diffraction (XRD), Raman and terahertz‐infrared (THz‐IR) ...spectroscopies in a temperature range of 4–300 K. XRD and Raman spectroscopy reveal the cubic‐to‐tetragonal nonpolar structural phase transition at about 100 K. At the same time, Raman spectra manifest the presence of polar modes, TO2 and TO4, normally prohibited in paraelectric phase. Emergence of these modes indicates the appearance of the polar nanoregions in a broad temperature range. The modes become more intensive at low temperatures, and temperature dependence of their intensities on cooling reveals the kink‐like change of the slope from flat to steep, indicating on activation of polar nanoregions. The transmission THz‐IR spectra show that squared frequency of the polar TO1 soft mode, responsible for ferroelectric transition, follows Cochran's behavior at high temperatures. However, at low temperatures, it doesn't vanish at extrapolated Curie temperature but saturates, demonstrating the plateau feature below 20 K. This behavior, coherent with known saturation of the dielectric constant, indicates that transition to ferroelectric phase in BaxSr1−xTiO3 is suppressed by quantum fluctuations and the system stays in quantum paraelectric state at very low temperatures.
Crystal structure and lattice dynamics of quantum paraelectric BaxSr1−xTiO3 are studied at temperatures 4–300 K. Polar nanoclusters appear in a broad temperature range and extend with increasing concentration of Ba ions. Clusters’ activation locally stabilizes the ferroelectric phase and reduces the temperature of structural transition. Quantum fluctuations suppress the ferroelectric state, conserving the paraelectric state at very low temperatures.
In the present work, the effect of grain size distribution on the diffraction profile shape is inspected via analysis of the mutual ratio of Lorentz and Gauss components in pseudo‐Voigt function ...which is used for simulating X‐ray profiles of nanoparticles. As established from the plotted dependences, the error in the average Pt nanoparticles size determination reaches 56% and the discrepancy between calculated Pt nanoparticle surface areas attains 60%. Furthermore, the determination error becomes greater with increasing the Lorentz contribution to pseudo‐Voigt function, or, in fact, with enlarging particle size distribution. The empirically found electrochemical surface area of Pt/C electrocatalyst is compared with that evaluated from XRD data using the Scherrer formula and particle size distribution data analysis.
In this work, on the basis of analysis of the diffraction profiles of X‐ray diffraction patterns and the particle size distributions, it is shown that the use of only a well‐known Scherrer formula without analyzing the particle size distributions can lead to an error in the average Pt nanoparticles size determination reaching 56%, and the discrepancy between calculated Pt nanoparticle surface areas attains 60%.
Supported PtCu/C electrocatalysts containing core–shell bimetallic PtCu nanoparticles were synthesized by sequential chemical reduction of Cu2+ and Pt(IV) in a carbon suspension, prepared on the ...basis of ethylene glycol–water solvent, and then treated at different temperatures in the range from 250 to 350 °C. The structural characterization of “as-prepared” PtCu nanoparticles and of those obtained after the thermal treatments was performed by transmission electron microscopy, X-ray diffraction, and Pt L3- and Cu K-edge extended X-ray absorption fine structure spectroscopy. The atomic cluster models of PtCu nanoparticles before and after the thermal treatment, reflecting the character of the components’ distribution, were generated. The electrochemical performance of the obtained PtCu/C electrocatalysts in oxygen reduction reaction was studied by cycling and linear voltammetry.
Interest in the development of state‐of‐the‐art methods for synthesis of Pt nanoparticles is dictated by both unique properties of platinum itself and the wide range of platinum nanoparticles ...applications. Platinum acetylacetonate and platinum nanoparticles produced by the thermal decomposition of Pt(acac)2 are studied via in situ X‐ray diffraction and Raman spectroscopy, thermal gravimetry, scanning (SEM), and transmission electronic microscopy (TEM). The experiments are made at heating rates of 1 and 5 K min−1. A comparative analysis of SEM, TEM, and XRD data highlights the formation of coarse Pt particles with sizes of ≈60–160 nm, which are composed of nanoparticles with dimensions of 1.9 and 4.1 nm for 1 K min−1‐ and 5 K min−1‐heating rates, respectively. The desirable average size of nanoparticles upon their synthesis can thus be achieved by a simple tuning of the heating rate of acetylacetate thermal decomposition. The present results open up the possibility to use a trivial non‐isothermal thermal decomposition method in the synthesis of both supported and unsupported nanoparticles of predetermined size for other chemical elements such as Pd, Ni, Co, etc. from their acetylacetonates.
Platinum nanoparticles forming through the thermal decomposition of Pt(acac)2 are studied. Desirable average size of nanoparticles during their synthesis can be achieved through a simple tuning of the decomposition heating rate. The obtained results open up the possibility to use a presented method for the synthesis of Pt, Pd, Ni, Co nanoparticles of a predetermined size using their acetylacetonates.
Polarized Raman spectra of the epitaxial Ba0.4Sr0.6TiO3 film grown on MgO substrate with thickness of 600 nm were studied in a broad temperature range of 80–600 K and the phase transition from ...paraelectric to tetragonal ferroelectric phase was observed at about 500 K. This critical temperature is substantially higher than that calculated from the lattice‐mismatch strain theory. In addition, the predicted from the theory low‐temperature orthorhombic and rhombohedral phases were not observed. We ascribe the strengthening of the tetragonal ferroelectric phase to the effect of the interface‐induced build‐in electric field resulted from the interface‐accumulated charges.
Effects of thermal post treatment and of thickness of the initial gold film coated the silicate glass surface, on localized surface plasmon resonance (SPR) of gold nanoparticles formed in glass by UV ...laser irradiation were studied to develop effective technique for production of gold nanoparticles and their arrays with required and stable characteristics of SPR. Optical spectra of the obtained Au/glass samples showed that SPR of gold nanoparticles formed by irradiation of glasses sputter coated with gold films of thickness 6–10 nm depends of T-treatment, while irradiation of glass coated with gold film ∼70 nm leads to SPR stable of temperature. The origin of such dependences was studied by the structural analysis of gold nanoparticles in Au/glass samples before and after thermal treatment using XRD, EXAFS methods, TEM-EDX images and direct calculations of optical spectra. It was revealed that the changes or stability of SPR parameters under thermal post-treatment of Au/glass samples depend respectively of the presence or absence of tin atoms in the near-surface region (shell) of the gold particles. Thus, if tin atoms are presented in the shell of gold particles (as in “as prepared” Au/glass samples initially coated with gold films of 6–10 nm) then the leave of tin atoms from the particle's volume during the heating leads to partial dissolution of the particles shell and consequently, to decreasing of the particles size, which in turn leads to the observed changes in SPR. The most probable explanation of the presence of tin atoms in the shell of gold particles formed by irradiation of samples initially coated with gold films of thickness 6–10 nm and their absence in the case of sample with film ∼70 nm was proposed, basing on the difference in the effects of the first laser pulse actions on these films on the tin-bath side of the glass samples. The sizes of coherent scattering region in gold nanoparticles before and after thermal treatment of Au/glass samples were determined and the dependence of these sizes upon the thickness of the gold film coating the sample's surface before laser irradiation was revealed.
Display omitted
•SPR of Au NPs formed by UV laser irradiation of ∼10 nm Au films depends of T-treatment.•SPR of Au NPs formed by UV laser irradiation of 70 nm film is stable upon T-treatment.•Differences in SPR of Au NPs after T-treatment are explained by XRD, EXAFS, Optics.•Significance of Au NPs shell composition for stability upon T-treatment is revealed.•Size of coherent scattering region (CSR) in Au NPs increases after T-treatment.
The crystal structure and lattice dynamics of quantum paraelectric Ba
x
Sr
1−
x
TiO
3
(
x
= 0, 0.01, 0.02) solid solutions are studied using X‐ray diffraction (XRD), Raman and terahertz‐infrared ...(THz‐IR) spectroscopies in a temperature range of 4–300 K. XRD and Raman spectroscopy reveal the cubic‐to‐tetragonal nonpolar structural phase transition at about 100 K. At the same time, Raman spectra manifest the presence of polar modes, TO
2
and TO
4
, normally prohibited in paraelectric phase. Emergence of these modes indicates the appearance of the polar nanoregions in a broad temperature range. The modes become more intensive at low temperatures, and temperature dependence of their intensities on cooling reveals the kink‐like change of the slope from flat to steep, indicating on activation of polar nanoregions. The transmission THz‐IR spectra show that squared frequency of the polar TO
1
soft mode, responsible for ferroelectric transition, follows Cochran's behavior at high temperatures. However, at low temperatures, it doesn't vanish at extrapolated Curie temperature but saturates, demonstrating the plateau feature below 20 K. This behavior, coherent with known saturation of the dielectric constant, indicates that transition to ferroelectric phase in Ba
x
Sr
1−
x
TiO
3
is suppressed by quantum fluctuations and the system stays in quantum paraelectric state at very low temperatures.
Producing green hydrogen from abundant natural resources such as seawater and human urine is a viable solution for energy and industrial applications. However, the field of electrocatalysis faces ...challenges in developing cost-effective metal catalysts for efficient electrolysis of seawater and urea sewage. Here, we reported bimetallic NiFe nanoparticles encapsulated in a biomass derived N-doped graphitic carbon core shell structure (NiFe@BNGC-700) as a stable and effective catalyst for seawater and human urine electrolysis. NiFe@BNGC-700 exhibits remarkable electrocatalytic performance, with a low overpotential requiring 300 mV for OER and 297 mV for HER in alkaline 1.0 M KOH + seawater and 1.36 V for UOR in alkaline 1.0 M KOH + human urine at 50 mA cm−2 current density, respectively. The active bifunctional NiFe@BNGC-700 electrocatalyst symmetry pair facilitates the construction of a seawater electrolyser, achieving a cell voltage of 1.74 V at 50 mA cm−2 current density. Furthermore, the human urine-mediated electrolyser employing NiFe@BNGC-700//NiFe@BNGC-700 achieves a cell voltage of 1.64 V at the same current density, maintaining stability over 12 h with negligible potential deviation. Continual evolution of hydrogen and oxygen is feasible in a solar seawater a cell assembled with NiFe@BNGC-700 electrocatalyst, delivering at cell voltage of 1.74 V, enabling sustained and effective green hydrogen delivery on a large scale. This study proposes a sustainable strategy for fabricating electrodes consisting of non-precious metals, thereby permitting the economical and extensive generation of green hydrogen on a large scale.
Utilizing natural resources such as seawater and urine sewage, bimetallic NiFe nanoparticles encapsulated in biomass derived N-doped graphitic carbon core shell structure serve as efficient electrolysis. These catalysts achieve low overpotential of 300 mV for OER, 297 for HER, in seawater, and 1.36 V for UOR in human urine, sustaining for stable electrolysis with a cell voltage of 1.74 V for seawater electrolysis and 1.64 V for human urine electrolysis at 50 mA cm−2. Solar based energy devices promise large-scale, long-lasting green hydrogen production by enhancing the fabrication of non-precious metal electrodes for wide use. Display omitted
The polar phonon modes in BaFe
12
O
19
single crystal are studied in the temperature range from 6 to 300 K by polarized infrared spectroscopy. The phonon spectrum of the crystal is strongly ...anharmonic and unstable with respect to long-wavelength fluctuations of the dielectric permittivity along the hexagonal axis. Our results suggests that in BaFe
12
O
19
hexaferrite symmetry lowering to the polar phase with the space group
P
6
3
mc
can be expected.
PDF
