Functional Materials Letters

Ternary nickel cobaltite nanostructures have found their application in many optoelectronic devices due to their excellent electronic and catalytic properties. In this review paper, we will discuss two synthetic strategies for ternary nickel cobaltite nanostructures: nickel cobaltite nanopowders and conductive substrate supported nickel cobaltite, respectively. Then selected examples utilizing ternary nickel cobaltite nanostructures as building blocks for solar cells, photodetectors and water oxidation will be highlighted. In the end, an outlook and conclusion will be given about the future research and development in this field.

Mixed reactant fuel cells (MR-FCs), are aimed at using a uniform mixture of fuel and oxygen applied to both the anode and the cathode. This allows redesign of fuel cells with a significantly simpler construction, having potentially a higher power density, better fuel utilization and be less expensive. The challenge in realizing MR-FCs is finding selective electrodes that can enhance oxygen reduction at the cathode, fuel oxidation at the anode while inhibiting the chemical reaction between the fuel and oxygen in the gas mixture. This task is in particular challenging in solid oxide fuel cells (SOFCs), as they operate at elevated temperatures, where many reactions are easily activated and selectivity is difficult to achieve. As a result no true MR-FC of the SOFC type were reported while some were found for low temperature fuel cells (FCs). The so-called single-chamber-SOFC are not true MR-FCs as they do not contain two selective electrodes, as required.

We shall discuss potential ways to search for and develop selective anodes and cathodes for SOFC type MR-FCs. We first consider material properties which should contribute to that goal. This refers to electronic properties of the bulk, band banding under adsorbed specie, point defects in the bulk and on the surface. We then proceed to show how cell design, in particular electrode structure, can contribute to selectivity. Finally operation conditions are considered and it is shown that they also can contribute to selectivity. The operation condition considered are gas mixture composition, gas mixture residence time in the hot zone, hence gas flow rate, current density and temperature. The topics discussed hold for all FC types but are crucial for the SOFC type because of the difficulty to achieve selectivity at elevated temperatures. It is suggested that a concerted effort taking advantage of all those options should allow development of a true SOFC type MR-FC.

A lignite-based activated carbon (LAC) for electrochemical capacitors (ECs) was prepared from high moisture lignite by KOH activation, and the as-prepared sample was characterized by the N₂-sorption, scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The electrochemical performances of ECs with activated carbon as electrodes in 3 M KOH aqueous solution were evaluated by constant current charge-discharge and cyclic voltammetry. The LAC exhibits a well-developed surface area of 2581 m²/g, a relative wide pore size distribution of 0.5–10 nm. The ECs with LAC as electrode materials presents a high specific capacitance of 392 F/g at a low current density of 50 mA/g, and still remains 315 F/g even at a high current density of 5 A/g. The residual specific capacitance is as high as 92.9% after 2000 cycles. Compared with the commercial activated carbon (Maxsorb: Commercial product, Kansai, Japan), the LAC based electrode materials shows superior capacitive performance in terms of specific capacitance and charge–discharge performance at the high current density.

The Cu thin film electrode grown on aluminum nitride (AlN) substrate is widely used in the thin film thermoelectric devices due to its high electrical conductivity. We have developed a new type of buffer layer by co-sputtering Ti and Cu forming Ti–Cu layer. The Ti–Cu layer was sputtered on the Ti buffered AlN substrate so that the adhesion and electrical conductivity properties of the Cu film electrode on AlN substrate could be improved. The interface between the thin films and the substrate were characterized by the scanning electron microscope (SEM). Nanoscratch tests were conducted on a nanomechanical test system to investigate the adhesion between the Cu film electrodes and AlN substrate. Meanwhile, accelerated ageing test under thermal cycling was conducted to evaluate the reliability of the thin film electrode. The results show that the adhesion and the reliability of Cu film electrode on AlN substrate have been greatly improved by employing Ti–Cu/Ti buffer layers.

Nanoscale La-doped PbTiO₃(PLT)\TiO₂ (PLTT) composite structures have been fabricated. It shows that the structure presents reliable resistive switching (RS) behavior, and importantly, has great tunability on RS characteristics such as forming/set/reset voltages and resistance ratio by adjusting the PLT layer thickness. Particularly, the set voltage can be tuned at a large range from several volts to dozens of volts. Meanwhile, the set current keeps almost the same, indicating the RS is current dominating. The space-charge-limited current (SCLC) feature indicates that the localized traps are decisive for the RS. Our result sheds light on the prospects of composite structures for designing tunable RS devices.

TiO₂-based nanomaterials (BNMs) photoelectrode was successfully synthesized via a facile and controllable hydrothermal procedure. The as-prepared TiO₂-BNMs photoelectrode was characterized by field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). The resulting TiO₂-BNMs samples assembled by different morphologies were obtained through changing hydrothermal temperature that were: (i) One-dimensional (relative to nanosheets) nanobelts at 140°C, (ii) two-dimensional nanosheets at 160–180°C and (iii) three-dimensional network nanowires at 200–220°C. Structural investigation of the obtained nanomaterials revealed that the content of anatase increased as increasing the apparent dimensionality of the materials. The photoelectrochemical performance of TiO₂-BNMs photoelectrode was elucidated by comparative investigation on the electron transport and electron–hole (e^-/h⁺) recombination in TiO₂-BNMs sample with typically morphology. The photoelectrochemical performance of the TiO₂-BNMs sample was obviously dependent on the morphology. Compared to the nanobelts and the network nanowires structure, the two-dimensional nanosheets displayed the more effective photogenerated electron transfer and reduced electron–hole recombination rate. Moreover, two-dimensional nanosheets had significantly enhanced photoelectrocatalytic efficiency for degradation of methyl orange (MO).

Experimental data on dielectric spectra of calcium copper titanate, CaCu₃Ti₄O₁₂ (CCTO) family functional ceramics have been studied and analyzed. It is shown that there are both non-Debye relaxation and resonance regions in their spectra. An occurrence of a retardation of complex permittivity and a relaxation of electric modulus is established. An average relaxation frequency of the electric modulus is considerably (in some cases several orders of magnitude) larger than the retardation frequency of the permittivity. A parallel connection of the capacity and complex conductivity is used to model and interpret experimental data on a negative permittivity in the infralow frequency range. Computer simulation enables us to reveal that the hopping conductivity, characteristic for disordered heterogeneous systems, is to be taken into account to describe adequately experimental data on passing the real part of the capacity (or permittivity) through zero. We have found a critical frequency at which the parallel resonance would take place.

Reduced graphene oxide (RGO) wrapped TiO₂/Ag ternary nanocomposite was synthesized via a wet chemical method. The synthesized nanocomposite was characterized using UV-Vis spectroscopy, Raman spectroscopy, photoluminescence spectroscopy, Scanning electron microscope (SEM), Energy-dispersive X-ray spectroscopy (EDX) and X-ray Diffraction (XRD). The average size of RGO wrapped TiO₂ and deposited Ag nanoparticles were 3.2 μm and 48 nm, respectively. Due to the highly facilitated electron transport by the synergistic effect between RGO sheets and Ag nanoparticles, the TiO₂/RGO/Ag nanocomposite exhibited a significant enhancement of photocatalytic activity towards degradation of methylene blue under UV light.

Ca₃LiRuO₆ single crystals were grown using the flux method, magnetic properties measurement results along different directions of needle-like single crystals are reported for the first time and show Ca₃LiRuO₆ occurs a paramagnetic (PM)-weak ferromagnetic (FM) transition at a temperature Tc = 120 K, also the magnetic moment aligns in the ab plane. Although Ca₃LiRuO₆ is characterized by one-dimensional configuration in crystallographic structure, it has three-dimensional (3D) magnetic order, the value of Tc is far below the magnitude of Curie–Weiss temperature. Θ_cw ≈ -306 K indicates complex nature of the magnetism is in this compound.

Magnetic and dielectric properties of 3Y-TZP/20 wt.% Ba_1-xSr_xFe₁₂O₁₉ (x = 0, 0.25, 0.5, 0.75) composites prepared by solid state reaction method are investigated. The magnetic properties are improved in the composites with the strontium doped barium ferrite. When x = 0.25, the saturation magnetization of the ferrite reaches the maximum. This is due to the migration of Fe³⁺ induced by the Sr²⁺ doping. The dielectric properties are also improved in the composite with the strontium doped barium ferrite. When x = 0.5, the dielectric constant and dielectric loss possess the maximum. This is caused by the lattice distortion resulting from the Sr²⁺ doping. The dielectric properties are analyzed by the universal relaxation law.

A Keggin-type vanadium-substituted tungstovanadozincic heteropoly acid H₇ZnW₁₁VO₄₀ ⋅ 8H₂O, with the transition metal as central atom, was firstly synthesized and characterized. Its proton conductivity was measured by the electrochemical impedance spectrum (EIS), and the result indicates that the H₇ZnW₁₁VO₄₀ ⋅ 8H₂O is a solid high-proton conductor with conductivity of 3.26 × 10^-3S ⋅ cm^-1 at 58°C, 50% relative humidity. Its activation energy is 29.50 kJ ⋅ mol^-1, which suggests that the mechanism of proton conduction is the Vehicle mechanism.

Yellow emitting Sm₃₊-doped NaBa_1-xBO₃(0.01 ≤ x ≤ 0.13) phosphors were synthesized by conventional solid state reaction method. The phase structure and luminescence properties of the as-prepared phosphors were investigated. These phosphors can be effectively excited by 403 nm near-ultraviolet light and feature a satisfactory yellow performance. The emission peaks are observed at 560 nm, 603 nm and 650 nm, originating from the transitions of ⁴G_5∕2→⁶H_5∕2, ⁴G_5∕2→⁶H_7∕2 and ⁴G_5∕2→⁶H_9∕2 respectively. Investigation of Sm³⁺ concentration-dependent emission spectra indicates that the NaBa_0.95BO₃:0.05Sm³⁺ phosphor shows the strongest yellow emission intensity and exhibits the CIE value of x = 0.4760 and y = 0.5090. Through the theoretical calculation, a high color purity of 96.0% was determined for the NaBaBO₃:Sm³⁺ phosphor, and the color purity of NaBaBO₃:Sm³⁺ is much higher than that of the commercial yellow phosphor YAG:Ce³⁺. This work is highly relevant to the development of a new type of potential down-conversion (DC) yellow phosphor for near ultraviolet white or yellow light-emitting diodes (LEDs).

Titanium dioxide (TiO₂) nanoparticles were prepared by resistive heating Ti filament under an oxygen atmosphere. The obtained nanoparticles were confirmed to be a tetragonal crystal TiO₂ and the particle size increased with increasing the oxygen pressure. The NO₂ gas sensing properties of the TiO₂ nanoparticles were investigated. All the sensors made of TiO₂ nanoparticles with different particle sizes exhibited the maximum sensitivities to 1 ppm NO₂ at a relative low operating temperature of 150°C. Comparing with the large particle size of TiO₂ nanoparticles, the ones with the smallest particle size exhibited the highest sensitivity and the best response and recovery characteristic to various NO₂ gas concentrations.

We demonstrated a biomimetic green synthesis of bimetallic Au–Ag nanoparticles (NPs) on graphene nanosheets (GNs). The spherical protein, ferritin (Fr), was bound onto GNs and served as the template for the synthesis of GN/Au–Ag nanohybrids. The created GN/Au–Ag nanohybrids were further utilized to fabricate a non-enzymatic amperometric biosensor for the sensitive detection of hydrogen peroxide (H₂O₂), and this biosensor displayed high performances to determine H₂O₂ with a detection limit of 20.0 × 10^-6 M and a linear detection range from 2.0 μM to 7.0 mM.

In this study, poly(ether block amide) was used as coating material to develop a novel composite polymer membrane for CO₂ capture. Polyacrylonitrile (PAN) ultrafiltration membrane was applied as substrate. Between them, a gutter layer prepared from cross-linked polydimethylsiloxane (PDMS) blending with amino silicone was introduced to improve separation performance of the composite membrane. The separation properties of resultant triple layer composite membrane was characterized using pure CO₂ and N₂ gases. It was found that the crosslinking degree of the gutter layer had great influence on membrane performance, which could be significantly improved with the help of amino-PDMS gutter layer compared with that of the membrane having Pebax coating directly onto PAN substrate. Using 2 wt.% Pebax coating solution and properly cross-linked gutter layer, the designed Pebax/amino-PDMS/PAN composite membrane showed CO₂ permeance of 147 GPU and CO₂/N₂ selectivity of 62.

Novel deep red phosphor Li₂MgGeO₄:Mn⁴⁺ is synthesized by high temperature solid state reaction method in air. The strongest PL band peaking at ~ 671 nm in the range of 600–750 nm is due to the ²E → ⁴A₂ transition of Mn⁴⁺ ion and the PLE spectra shows broad band peaking at ~ 323 within the range 220–550 nm owing to the ⁴A₂ → ⁴T₁ transitions of Mn⁴⁺ ion. The optimum Mn⁴⁺ doping concentration is about 0.4 mol.%. The luminous mechanism is explained by Tanabe–Sugano diagram of Mn⁴⁺ ion. The results indicate that red phosphor Li₂MgGeO₄:Mn⁴⁺ is a beneficial phosphor for use in white light-emitting-diodes (LEDs).