Functional Materials Letters

Lead-free multifunctional ceramics of Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃-x mol% Dy have been prepared by an ordinary sintering method and the effects of Dy₂O₃ doping on structure, piezoelectric, ferroelectric and photoluminescent properties of the ceramics have been studied. The ceramics possess a single phase perovskite structure. The grain growth of the ceramics is prohibited and the ferroelectric–paraelectric phase transition at T_C becomes more diffusive after the addition of Dy₂O₃. Dy₂O₃ doping improves the piezoelectricity of the ceramics and the optimal piezoelectric properties d₃₃ = 335 pC/N is obtained at x = 0.5. The addition of 2 mol% Dy enhances the photoluminescent properties of the ceramics and strong emissions at ∼ 478 nm and ∼ 575 nm are observed. Our study shows that the ceramics with low Dy₂O₃ levels exhibit simultaneously the strong piezoelectricity, ferroelectricity and photoluminescence and may have a potential application in mechano-electro-optic integration and coupling device.

Spinel LiMn₂O₄ was synthesized by a modified solid state reaction. We pretreated the reactants using tartaric acid as complexing agent through a grinding process to obtain uniform distribution of metal ions at atomic level. The structures, morphologies and electrochemical properties of the products were studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and galvanostatic charge-discharge test. The results show that adding tartaric acid during the chemical pretreatment plays an important role in the formation of regular and uniform particles, which is beneficial to the electrochemical performance of LiMn₂O₄. At the current density of 100 mA g^-1, the discharge capacity is 118 mAh g^-1 after 50 cycles with the capacity retention of 97%.

Deep brain activity is conventionally recorded with surgical implantation of electrodes. During the neurosurgery, brain tissue damage and the consequent side effects to patients are inevitably incurred. In order to eliminate undesired risks, we propose that deep brain activity should be measured using the noninvasive scalp electroencephalography (EEG) technique. However, the deeper the neuronal activity is located, the noisier the corresponding scalp EEG signals are. Thus, the present study aims to evaluate whether deep brain activity could be observed from EEG recordings. In the experiment, a three-layer cylindrical head model was constructed to mimic a human head. A single dipole source (sine wave, 10 Hz, altering amplitudes) was embedded inside the model to simulate neuronal activity. When the dipole source was activated, surface potential was measured via electrodes attached on the top surface of the model and raw data were recorded for signal analysis. Results show that the dipole source activity positioned at 66 mm depth in the model, equivalent to the depth of deep brain structures, is clearly observed from surface potential recordings. Therefore, it is highly possible that deep brain activity could be observed from EEG recordings and deep brain activity could be measured using the noninvasive scalp EEG technique.

A given amount of gold is electrodeposited on the reduced graphene oxide (RGO)/glassy carbon (GC) electrodes to form Au/RGO/GC electrodes, which are carried out at different potentials. The Au/RGO/GC electrode with Au loading of 250 μg cm^-2 prepared at a constant potential of -0.30 V exhibits the best electrocatalytic activity to glucose oxidation in alkaline solutions because of homogeneous dispersion of gold nanoparticles with smaller sizes. This electrode shows long-term stability, rapid charge transfer ability, and higher current density compared to other gold electrodes reported previously.

Development of non-precious electrocatalysts with applicable electrocatalytic activity towards the oxygen reduction reaction (ORR) is important to fulfill broad-based and large-scale applications of metal/air batteries and fuel cells. Herein, nickel and cobalt molybdates with uniform nanorod morphology are synthesized using a facile one-pot hydrothermal method. The ORR activity of the prepared metal molybdate nanorods in alkaline media are investigated by using cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperomety in rotating disk electrode (RDE) techniques. The present study suggests that the prepared metal molybdate nanorods exhibit applicable electrocatalytic activities towards the ORR in alkaline media, promising the applications as non-precious cathode in fuel cells and metal–air batteries.

Two kinds of carbon black (CB) (i.e., CB#300 and CB#3350) were added into poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP), respectively, to improve its piezoelectricity. The results revealed that when 0.5 wt.% CB was added, the best performance of the PVDF-HFP/CB composite films was obtained. The calibrated open circuit voltage and the density of harvested power of 0.5 wt.% CB#3350 contained composite films were 204%, and 464% (AC) and 561% (DC) of those of neat PVDF-HFP films. Similarly, for 0.5 wt.% CB#300 contained films, they were 211%, and 475% (AC) and 624% (DC), respectively. The enhancement mechanisms of piezoelectricity were clarified by the observation of Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and scanning electron microscope (SEM). We found that the added CBs act as nucleate agents to promote the formation of elongated, oriented and fibrillar β-phase crystals during the fabrication process, which increase the piezoelectricity. Overdosed CBs lead to a lower crystallinity degree, resulting in the lower piezoelectricity. Compared with CB#3350, CB#300 performs slightly better, which may be ascribed to its higher specific surface area.

We have investigated the effect of ambient gases on the structural, electrical, and optical characteristics of Zn-doped indium tin oxide (ZITO) thin films intended for use as anode contacts in organic light emitting diodes (OLED) devices. These ZITO thin films are deposited by radio frequency (RF) magnetron sputtering under different ambient gases (Ar, Ar + O₂ and Ar + H₂) at 300°C. In order to investigate the influences of the oxygen and hydrogen, the flow rate of oxygen and hydrogen in argon mixing gas has been changed from 0.1 sccm to 0.5 sccm and from 0.1 sccm to 1 sccm, respectively. The intensity of the (400) peak in the ZITO thin films increases with increasing H₂ flow rate whilst the (400) peak was nearly invisible in an atmosphere of Ar + O₂. The electrical resistivity of the ZITO thin films increases with increasing O₂ flow rate, whereas the electrical resistivity decreases with increasing H₂ flow rate. The change of electrical resistivity with changes in the ambient gas composition is mainly interpreted in terms of the charge carrier mobility and the charge carrier concentration. All the films show an average transmittance of over 80% in the visible range. The optical bandgap of the ZITO films increases with increasing H₂ flow rates, whereas the optical bandgap of the ZITO films deposited in an O₂ atmosphere decreases with increasing O₂ flow rates. The current density and the luminance of the OLED devices with ZITO thin films deposited in 1 sccm of H₂ ambient gas are the highest among all the films. The optical bandgap energy of ZITO thin films plays a major role in OLED device performance, especially the current density and luminance.

The temporal evolution of domain structure and its piezoelectric behavior of ferroelectric material BaTiO₃ during the transition process from rhombohedral to tetragonal phase under an applied electric field have been studied by employing Landau–Ginzburg theory and the phase-field method. The results obtained show that, during the transformation process, the intermediate phase was monoclinic M_A phase, and several peak values of piezoelectric coefficient appeared at the stage where obvious change of domain pattern occurred. In addition, by comparing the cases of applied electric field with different frequencies, it was found that the maximum piezoelectric coefficient obtained decreased with increasing frequency value. These results are of great significance in tuning the properties of engineering domains in ferroelectrics, and could provide more fundamentals to the design of ferroelectric devices.

Nanostructured ZnO films were fabricated by chemical vapor deposition (CVD) method with different Sn source concentrations for ethanol and acetone sensing applications. The samples prepared with Sn:ZnO (weight ratio) of 0.1:1, 0.2:1 and 0.3:1 are named as S1, S2 and S3, respectively. It was found that hexagonal wurtzite structure of ZnO is maintained after Sn doping. The S1 sample shows the highest response to acetone, while the S2 sample shows the highest response to ethanol, which confines that the as-grown ZnO films with different Sn source concentrations exhibit obvious selectivity to ethanol and acetone. Working temperature can also be reduced by the increase of Sn source concentration. All the samples have very short average recovery time. Our work provides a prospective approach to obtain high performance gas sensor for ethanol and acetone.

In this paper, we report a series of homogeneous polymeric composites with enhanced dielectric properties and thermal conductivity. The composites were constituted of polyvinylidene fluorides (PVDFs) matrix and CaCu₃Ti₄O₁₂ (CCTO) monolithic or CCTO/β-SiC hybrid fillers, and prepared by simple melt blending and hot moulding technique. The influence of different types of fillers and their composition on the dielectric response and thermal conductivity of the obtained composites was studied. Results show that hybrid loading is preferred and a reasonable combination of thermal conductivity (0.80 W⋅m^-1⋅K^-1), dielectric constant (∼50) and dielectric loss (∼0.07) at 10³ Hz was achieved in the PVDF composite containing 40 vol.% CCTO and 10 vol.% β-SiC. The strong dipolar and interfacial polarization derived from the fillers are responsible for the enhancement of the dielectric constant, while the formation of thermally conductive networks/chains by β-SiC whiskers contributes to the improved thermal conductivity.

(Nd_0.7Yb_0.3)₂Zr₂O₇ and ((Nd_0.7Yb_0.3)_1-xSm_x)₂Zr₂O₇ (0 < x ≤ 0.25) ceramics have been synthesized by pressureless sintering by tailoring the chemical compositions. Microstructure and electrical conductivity of ((Nd_0.7Yb_0.3)_1-xSm_x)₂Zr₂O₇ were investigated by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and AC impedance spectroscopy. (Nd_0.7Yb_0.3)₂Zr₂O₇ ceramic exhibits a mixed crystal structure of defect fluorite and pyrochlore. After doping with Sm³⁺ cations, the structure changes from a mixed type of (Nd_0.7Yb_0.3)₂Zr₂O₇ to a single pyrochlore type of ((Nd_0.7Yb_0.3)_1-xSm_x)₂Zr₂O₇, as the addition of Sm³⁺ reduces the difference in ionic radius between Nd³⁺ and Yb³⁺. However, ((Nd_0.7Yb_0.3)_1-xSm_x)₂Zr₂O₇ ceramics contain the localized short-range disorder despite the structural order overall in the pyrochlore. The measured total conductivities of ((Nd_0.7Yb_0.3)_1-xSm_x)₂Zr₂O₇ obey the Arrhenius relation. Doping of Sm³⁺ enhances the electrical conductivity of (Nd_0.7Yb_0.3)₂Zr₂O₇ ceramic significantly, which is closely related to the variations in the concentration of oxygen vacancies at 48f sites, relatively low activation energy and high pre-exponential factor caused by the long-range order and short-range disorder.

Many organic/inorganic nanocomposites have been fabricated into fibrous materials using electrospinning techniques, because electrospinning processes have many attractive advantages and the ability to produce relatively large-scale continuous films. In this study, the polyvinyl alcohol (PVA)/silk/titanium dioxide (TiO₂) nanocomposite self-cleaning textiles were successfully produced using electrospinning technique. After optimizing electrospinning conditions, we successfully obtained the PVA/silk/TiO₂ nanocomposite fibers with average diameter of ∼220 nm and TiO₂ concentration can be as high as 18.0 wt.%. For the case of the PVA/silk/TiO₂ nanocomposite textile, the color of brilliant green coated on the textile surface changed from the initial green color to colorless after ultraviolet (UV) irradiation. Because of its worthy photocatalytic performance, the developed PVA/silk/TiO₂ nanocomposite materials in this study will be beneficial for the design and fabrication of multifunctional fibers and textiles.

A casting solution was prepared by dispersing titanium dioxide (TiO₂) nanoparticles in the sodium alginate and acrylamide aqueous solution. The casting solution was spread on a glass plate by a glass rod enlaced with brass wires to control the thickness of the sticky solution. Then polyacrylamide/calcium alginate/TiO₂ (PAM/CA/T) composite film was obtained after UV irradiation and cross-linking by CaCl₂. The PAM/CA/T film was characterized by scanning electron microscope and transmission electron microscope. The PAM/CA/T film had good strength and toughness. And they did not rupture after swelling in 5 wt.% NaCl solution and still had good mechanical properties. The adsorption properties of the PAM/CA/T film were investigated by using different dyes as the adsorbates. The photocatalytic degradation properties of these dyes on the PAM/CA/T films were also researched. The results indicated that there was no difference in the adsorption efficiency of PAM/CA film and PAM/CA/T-30 film. The adsorption rates of all the dyes were fast. The pre-adsorption of dyes had little effect on the catalytic degradation of dyes on PAM/CA/T film. The PAM/CA/T hydrogel film provided a suitable carrier for TiO₂ in the photocatalytic degradation of dyes and the degradation efficiency of PAM/CA/T-30 film for methyl orange reached 80.76%. The PAM/CA/T film had good reusability and could degrade dyes in NaCl solution.

Nitrogen-doped diamond-like carbon (N-DLC) films were synthesized by plasma immersion ion implantation and deposition (PIII-D) at room temperature (RT). During the process of deposition, N₂ flow was changed from 0 to 10 sccm. Lifshitz–van der Waals/acid–base (LW-AB) approach was employed to study the surface electronic state of the films, X-ray photoelectron spectroscopic (XPS) and valence band spectra (VBS) were used to study chemical bonds and electron structure information inside the films. Bandgap of the films were calculated by the data from ultraviolet spectrophotometer. The results showed that synthesized films were n-type semiconductors and doping of nitrogen element will affect the accepted–electron capability of the film. The change tendency of the bandgap coincides with that of the ratio of acidic to alkaline component of the polar acid-alkali surface energy. There was much sp hybrid electronic state existed in the films, which mainly sp²C=C/C=N and sp³C–C/C–N bonds.

Through comprehensive density functional calculations, the crystallographic, magnetic and electronic properties of Na_xCoO₂ (x = 1, 0.875, 0.75, 0.625 and 0.50) were investigated. We found that all Na ions in NaCoO₂ and Na_0.875CoO₂ share the basal coordinates with O ions. However, as x decreases, some of Na ions move within the basal plane in order to reduce the in-plane Na-Na electrostatic repulsion. Magnetically, there was strong tendency for type A antiferromagnetism in the Na_0.75CoO₂ system, while all other Na deficient systems had a weaker ferromagnetic (FM) tendency. The results on magnetism were in excellent agreement with the experiments.

Solid polymer electrolytes (SPEs) based on cellulose acetate (CA) doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt are prepared by solution cast technique. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy of the polymer salt complexes are recorded in the frequency range between 400 cm^-1 and 4000 cm^-1. The shifting of carbonyl band (C=O) at 1737 cm^-1 to a lower wavenumber confirms the occurrence of complexation between the polymer and the salt. The electrochemical impedance spectroscopy (EIS) analysis discovered that the film with 25 wt.% of salt shows the highest ionic conductivity at room temperature. The change in real dielectric permittivity (ε_r) as a function of frequency at different salt concentrations which exhibits a dispersive behavior at low frequencies and decays at higher frequencies, shows the electrode polarization and space charge effect. The real modulus formalism (M_r) analysis shows that the polymer electrolytes in this work are ionic conductors.

Graphene/hexagonal boron nitride (G/h-BN) heterostructure has attracted tremendous research efforts owing to its great potential for applications in nanoscale electronic devices. In such hybrid materials, tilt grain boundaries (GBs) between graphene and h-BN grains may have unique physical properties, which have not been well understood. Here we have conducted non-equilibrium molecular dynamics simulations to study the energetic and thermal properties of tilt GBs in G/h-BN heterostructures. The effect of misorientation angles of tilt GBs on both GB energy and interfacial thermal conductance are investigated.