One-step electrochemical preparation of the ternary (BixSb1−x)2Te3 thin films on Au(1 1 1): Composition-dependent growth and characterization studies

This study reports on the synthesis of ternary semiconductor (Bi_xSb_1−x)₂Te₃ thin films on Au(1 1 1) using a practical electrochemical method, based on the simultaneous underpotential deposition (UPD) of Bi, Sb and Te from the same solution containing Bi³⁺, SbO⁺, and HTeO₂⁺ at a constant potential. The thin films are characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS) and reflection absorption-FTIR (RA-FTIR) to determine structural, morphological, compositional and optic properties. The ternary thin films of (Bi_xSb_1−x)₂Te₃ with various compositions (0.0 ≤ x ≤ 1.0) are highly crystalline and have a kinetically preferred orientation at (0 1 5) for hexagonal crystal structure. AFM images show uniform morphology with hexagonal-shaped crystals deposited over the entire gold substrate. The structure and composition analyses reveal that the thin films are pure phase with corresponding atomic ratios. The optical studies show that the band gap of (Bi_xSb_1−x)₂Te₃ thin films could be tuned from 0.17 eV to 0.29 eV as a function of composition.     

Amorphous LiCoO2 thin films on Li1+x+yAlxTi2−xSiyP3−yO12 prepared by radio frequency magnetron sputtering for all-solid-state Li-ion batteries

Amorphous LiCoO₂ thin films were deposited on the NASICON-type glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP), by radio frequency (RF) magnetron sputtering below 180 °C. The as-deposited LiCoO₂ thin films were characterized by X-ray diffraction, scanning electron microscopy and atomic force microscope. All-solid-state Li/PEO₁₈-Li (CF₃SO₂)₂N/LATSP/LiCoO₂/Au cells were fabricated using the amorphous film. The electrochemical performance of the cells was investigated by galvanostatic cycling, cyclic voltammetry, potentiostatic intermittent titration technique and electrochemical impedance spectroscopy. It was found that the amorphous LiCoO₂ thin film shows a promising electrochemical performance, making it a potential application in microbatteries for microelectronic devices.     

Preparation and cycling performance of Aland F co-substituted compounds Li4AlxTi5−xFyO12−y

Li₄Al_xTi_5−xF_yO_12−y compounds were prepared by a solid-state reaction method. Phase analyses demonstrated that both Al³⁺ and F⁻ ions entered the structure of spinel-type Li₄Ti₅O₁₂. Charge-discharge cycling results at a constant current density of 0.15 mA cm⁻² between the cut-off voltages of 2.5 and 0.5 V showed that the Al³⁺ and F⁻ substitutions improved the first total discharge capacity of Li₄Ti₅O₁₂. However, Al³⁺ substitution greatly increased the reversible capacity and cycling stability of Li₄Ti₅O₁₂ while F⁻ substitution decreased its reversible capacity and cycling stability slightly. The electrochemical performance of the Al³⁺-F⁻-co-substituted specimen was better than the F⁻-substituted one but worse than the Al³⁺-substituted one.     

Electrochemically deposited thermoelectric n-type Bi2Te3 thin films

Electrochemically deposited n-type BiTe alloy thin films were grown from nitric acid baths on sputtered Bi_xTe_y/SiO/Si substrates. The film compositions, which varied from 57 to 63 at.% Te were strongly dependent on the deposition conditions. Surface morphologies varied from needle-like to granular structures depending on deposited Te content. Electrical and thermoelectric properties of these electrodeposited Bi_xTe_y thin films were measured before and after annealing and compared to those of bulk Bi₂Te₃. Annealing at 250 °C in reducing H₂ atmosphere enhanced thermoelectric properties by reducing film defects. In-plane electrical resistivity was highly dependent on composition and microstructure. In-plane Hall mobility decreased with increasing carrier concentration, while the magnitude of the Seebeck coefficient increased with increasing electrical conductivity to a maximum of −188.5 μV/K. Overall, the thermoelectric properties of electrodeposited n-type BiTe thin films after annealing were comparable to those of bulk BiTe films.     

Preparation, characterization and electrical conductivity studies of NdYb1−xGdxZr2O7 (0 ≤ x ≤ 1.0) ceramics

Several compositions of NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 1.0) ceramics were prepared by pressureless-sintering method at 1973 K for 10 h in air. The relative density, microstructure and electrical conductivity of NdYb_1−xGd_xZr₂O₇ ceramics were analyzed by the Archimedes method, X-ray diffraction, scanning electron microscopy and impedance plots measurements. NdYb_1−xGd_xZr₂O₇ (0 ≤ x ≤ 0.3) ceramics have a single phase of defect fluorite-type structure, and NdYb_1−xGd_xZr₂O₇ (0.7 ≤ x ≤ 1.0) ceramics exhibit a single phase of pyrochlore-type structure; however, the NdYb_0.5Gd_0.5Zr₂O₇ composition shows mixed phases of both defect fluorite-type and pyrochlore-type structures. The measured values of the grain conductivity obey the Arrhenius relation. The grain conductivity of each composition in NdYb_1−xGd_xZr₂O₇ ceramics gradually increases with increasing temperature from 673 to 1173 K. NdYb_1−xGd_xZr₂O₇ ceramics are oxide-ion conductor in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest grain conductivity value obtained in this work is 1.79 × 10⁻² S cm⁻¹ at 1173 K for NdYb_0.3Gd_0.7Zr₂O₇ composition.     

Fabrication and characterization of Li1+x−yNb1−x−3yTix+4yO3 substrates using aqueous tape casting process

An aqueous system for tape casting Li_1+x−yNb_1−x−3yTi_x+4yO₃ (LNT) ceramics was developed using poly(vinyl alcohol) (PVA) binder, ethylene alcohol (EG) plasticizer and ammonium salt of polycarboxylate (PCA-NH₄) dispersant. The zeta potential measurement showed that the isoelectric point of the LNT particles moved slightly toward more acid region after the dispersant absorbed on the particles, while the zeta potential increased significantly. The rheological test indicated that the ceramic slurry exhibited a typical pseudoplastic behavior without thixotropy. The effect of solid loading on the properties of the green tapes was investigated. The increase in the solid loading increased the tensile strength and the green density of the tapes. TGA analysis indicated that the organic additives in the green tapes can be completely removed by heat treatment at 600 °C. SEM micrographs showed that the microstructure of the green and sintered tapes was homogeneous.     

Study on the rapid synthesis of LiNi1−xCoxO2 cathode material for lithium secondary battery

LiNi_1−xCo_xO₂ (x = 0, 0.1, 0.2) cathode materials were successfully synthesized by a rheological phase reaction method with calcination time of 0.5 h at 800 °C. All obtained powders are pure phase with α-NaFeO₂ structure (R-3m space group). The samples deliver an initial discharge capacity of 182, 199 and 189 mAh g⁻¹ (25 mA g⁻¹, 4.35-3.0 V), respectively. The reaction mechanism was also discussed, which consists of a series of defect reactions. As a result of these defect reactions, the reaction of forming LiNi_1−xCo_xO₂ takes place in high speed.     

Synthesis and electrochemical characterization of mesoporous CoxNi1−x layered double hydroxides as electrode materials for supercapacitors

A novel nanostructured mesoporous Co_xNi_1−x layered double hydroxides (Co_xNi_1−x LDHs), which both Co(OH)₂ and Ni(OH)₂ exhibit, has been successfully synthesized by a chemical co-precipitation route using polyethylene glycol as the structure-directing reagent. Structural and morphological characterizations were performed using powder X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The component and thermal stability of the sample were measured by energy dispersed X-ray spectrometry (EDS), FT-IR and thermal analyses, including thermogravimetry (TG) and differential thermal analysis (DTA). Cyclic voltammogram and galvanostatic charge-discharge testified that the Co_xNi_1−x LDH has a specific capacitance of 1809 F g⁻¹ at a current density of 1 A g⁻¹ and remains at about 90.2% of the initial value after 1000 cycles at a current density of 10 A g⁻¹. The relationship between the chemical composition and the capacitance is discussed.     

Oxidation resistance of multi-walled carbon nanotubes coated with polycarbosilane-derived SiCxOy ceramic

Multi-walled carbon nanotubes (MWCNTs) have been successfully coated with a thin SiC_xO_y coating when polycarbosilane (PCS) was used as precursor and pyrolyzed in a coke bed. Meanwhile, effect of PCS concentration on oxidation resistance of the coated MWCNTs is studied. The results showed that the pyrolysis products of PCS were composed of amorphous SiC_xO_y as the main phase, together with β-SiC and SiO₂ as the minor phases whose amount increased a little with the increase of temperature from 1000 °C to 1500 °C. The thickness of SiC_xO_y coating on the surface of MWCNTs increased a little from 1 wt.% to 5 wt.%, but decreased dramatically with PCS concentration in the range of 10-30 wt.%. The oxidation resistance of the coated MWCNTs was greatly improved in comparison with as-received ones. The oxidation peak temperature of the coated MWCNTs reached 783.7 °C, much higher than 652.2 °C for as-received ones.     

Synthesis and characterization of MoOx-Pt/C and TiOx-Pt/C nano-catalysts for oxygen reduction

The oxygen reduction reaction (ORR) was studied at carbon supported MoO_x-Pt/C and TiO_x-Pt nanocatalysts in 0.5 mol dm⁻³ HClO₄ solution, at 25 °C. The MoO_x-Pt/C and TiO_x-Pt/C catalysts were prepared by the polyole method combined by MoO_x or TiO_x post-deposition. Home made catalysts were characterized by TEM and EDX techniques. It was found that catalyst nanoparticles were homogenously distributed over the carbon support with a mean particle size about 2.5 nm. Quite similar distribution and particle size was previously obtained for Pt/C catalyst. Results confirmed that MoO_x and TiO_x post-deposition did not lead to a significant growth of the Pt nanoparticles.The ORR kinetics was investigated by cyclic voltammetry and linear sweep voltammetry at the rotating disc electrode. These results showed the existence of two E − log j regions, usually observed with polycrystalline Pt in acid solution. It was proposed that the main path in the ORR mechanism on MoO_x-Pt/C and TiO_x-Pt/C catalysts was the direct four-electron process with the transfer of the first electron as the rate-determining step. The increase in catalytic activity for ORR on MoO_x-Pt/C and TiO_x-Pt/C catalysts, in comparison with Pt/C catalyst, was explained by synergetic effects due to the formation of the interface between the platinum and oxide materials and by spillover due to the surface diffusion of oxygen reaction intermediates.     

Effects of B-site ion (M) substitution on the ionic conductivity of (Li3xLa2/3−x)1+y/2(MyTi1−y)O3 (M=Al, Cr)

Effect on the ionic conductivity of B-site ion (M) substitution in (Li_3xLa_2/3−x)_1+y/2M_yTi_1−yO₃ (M=Al, Cr) has been investigated. It has been found that partial substitution of smaller Al³⁺ for Ti⁴⁺ is effective to enhance the ionic conductivity of Li_3xLa_2/3−xTiO₃. At 300 K, the maximum bulk conductivity of (1.58±0.01)×10⁻³ S cm⁻¹ is observed from the composition of (Li_0.39La_0.54)_1−y/2Al_yTi_1−yO₃ with y=0.02 (x=0.13), that is the highest yet reported for known perovskite solutions at room temperature. The conductivity enhancement is interpreted as being due to the substitution-induced bond-strength change rather than due to bottleneck size change for Li migration, TiO₆-octahedron tilting or A-site cation ordering.     

Electrodeposition of bismuth telluride films from a nonaqueous solvent

The author examines Bi₂Te₃ deposition from a DMSO solution containing TeCl₄ and Bi(NO₃)₃ × 5H₂O by means of cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). Accumulated charges and related mass changes for Bi₂Te₃ deposition on working electrodes are measured in situ. The deposit composition is more dependent on Te⁴⁺ concentrations in DMSO solution than on the potential. In a DMSO solution containing 0.01 M Te⁴⁺ and 0.0075 M Bi³⁺, Bi₂Te₃ deposits were obtained in the potential range between −0.2 and −0.8 V vs. Ag/AgCl. In a DMSO solution containing 0.05 M Te⁴⁺ and 0.0375 M Bi³⁺, Te-rich deposits were formed from −0.2 to −0.8 V vs. Ag/AgCl.     

Optical characterisation of anodic sulphide films on Hg1−xCdxTe (MCT) grown by the potential step method

The growth of native sulphide films on Hg_1−xCd_xTe (MCT) by potential steps to two different electrochemical growth regions has been studied by in-situ ellipsometry, photocurrent spectroscopy and second harmonic generation (SHG) rotational anisotropy. Films grown at −0.4 V versus SCE were porous and consisted mainly of cubic close packed (ccp) CdS. The photocurrent spectrum of the film exhibited a cut-off which closely conformed to the expected bandgap of CdS. The gradual increase in SH intensity as the sulphide film thickened was due to a contribution to the overall SH signal from the CdS film itself. No evidence of any hcp species was apparent in the rotational anisotropy patterns, with the four-fold pattern expected for the vicinal MCT surface being maintained even after film growth. When the potential was stepped to −0.3 V, the film grown absorbed the ellipsometer radiation once a thickness of 86 nm was reached. Photocurrent measurements showed a tail extending into the red region of the spectrum. These phenomena have been attributed to the incorporation of ccp HgS into the sulphide film as at this potential, the electrochemical reaction of the HgTe component of MCT can occur. SHG rotational anisotropy confirmed the four-fold symmetry of the ccp surface film.     

Synthesis and tunable thermal expansion properties of Sc2−xYxW3O12 solid solutions

A new series of rare earth solid solutions Sc_2−xY_xW₃O₁₂ was successfully synthesized by the conventional solid-state method. Effects of doping ion yttrium on the crystal structure, morphology and thermal expansion property of as-prepared Sc_2−xY_xW₃O₁₂ ceramics were investigated by X-ray diffraction (XRD), thermogravimetric analysis (TG), field emission scanning electron microscope (FE-SEM) and thermal mechanical analyzer (TMA). Results indicate that the obtained Sc_2−xY_xW₃O₁₂ samples with Y doping of 0≤x≤0.5 are in the form of orthorhombic Sc₂W₃O₁₂-structure and show negative thermal expansion (NTE) from room temperature to 600 °C; while as-synthesized materials with Y doping of 1.5≤x≤2 take hygroscopic Y₂W₃O₁₂·nH₂O-structure at room temperature and exhibit NTE only after losing water molecules. It is suggested that the obvious difference in crystal structure leads to different thermal expansion behaviors in Sc_2−xY_xW₃O₁₂. Thus it is proposed that thermal expansion properties of Sc_2−xY_xW₃O₁₂ can be adjusted by the employment of Y dopant; the obtained Sc_1.5Y_0.5W₃O₁₂ ceramic shows almost zero thermal expansion and its average linear thermal expansion coefficient is −0.00683×10⁻⁶ °C⁻¹ in the 25–250 °C range.     

Synthesis and electrochemical performance of novel loose structured submicro/micro-sized NiSnx alloy composites for lithium batteries

An effective method of carbothermal reduction was employed to prepare spherical microcrystal NiSn_x alloy powders from oxides of Sn and Ni used as anode materials for Li-ion battery. According to XRD, SEM and TEM analysis, the synthesized spherical NiSn_x powders show a loose submicro/micro-sized structure and a multi-phase composition. The prepared NiSn_x alloy composite electrode exhibits a stable discharge capacity of electrode is ca. 380 mAh g⁻¹ at constant current density of 50 mA g⁻¹, and can be retained at 350 mAh g⁻¹ after 25 cycles. Moreover, NiSn_x alloys exhibit excellent high rate performance, i.e. stable discharge capacities of 300-310 mAh g⁻¹ and the coulombic efficiencies of 97.5-99.5% have been obtained at the current density of 500 mA g⁻¹. The loose submicro-sized particle structural characteristic and the Ni addition in Sn matrix should be responsible for the improvement of cycling stability of NiSn_x electrode. The carbothermal reduction method is simple, low-cost and mass-productive, which should be viable to other alloy composite materials system of rechargeable lithium ion batteries.     

Influence of A-site Ba substitution on microwave dielectric properties of (BaxMg1−x)(A0.05Ti0.95)O3 (A=Zr,Sn) ceramics

The microwave dielectric properties of (Ba_xMg_1−x)(A_0.05Ti_0.95)TiO₃ (A=Zr, Sn) ceramics were investigated with regard to substitution of Ba for Mg of A-site. The microwave dielectric properties were correlated with the Ba content. With an increase in Ba content from 0.01 to 0.1, the dielectric constant and the τ_f value increased, but the Q×f value decreased. The sintered (Ba_xMg_1−x)(Zr_0.05Ti_0.95)TiO₃ (called B_xMZT) ceramics had a permittivity in the range of 19.1−20.6, quality factor from 180,000 to 25,000 GHz, and variation in temperature coefficient of resonant frequency from −35 to −39 ppm/°C with increasing composition x. For sintered (Ba_xMg_1−x)(Sn_0.05Ti_0.95)TiO₃ (called B_xMST) ceramics, the dielectric constant increased from 19 to 20.5, Q×f value increased from 120,000 to 37,000 (GHz), and the τ_f value increased from −50 to −3.3 ppm/°C as the x increased from 0.01 to 0.1. When A=Sn and x=0.1, (Ba_0.1Mg_0.9)(Sn_0.05Ti_0.95)TiO₃ ceramics exhibited dielectric constant of 20.5, Q×f value of 37,000 (GHz), and a near-zero τ_f value of −3.3 ppm/°C sintered at 1210 °C for 4 h.     

Electrochemical and thermal studies of Li[NixLi(1/3−2x/3)Mn(2/3−x/3)]O2 (x = 1/12, 1/4, 5/12, and 1/2)

Samples of the layered cathode materials, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (x = 1/12, 1/4, 5/12, and 1/2), were synthesized at 900 °C. Electrodes of these samples were charged in Li-ion coin cells to remove lithium. The charged electrode materials were rinsed to remove the electrolyte salt and then added, along with EC/DEC solvent or 1 M LiPF₆ EC/DEC, to stainless steel accelerating rate calorimetry (ARC) sample holders that were then welded closed. The reactivity of the samples with electrolyte was probed at two states of charge. First, for samples charged to near 4.45 V and second, for samples charged to 4.8 V, corresponding to removal of all mobile lithium from the samples and also concomitant release of oxygen in a plateau near 4.5 V. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples with x = 1/4, 5/12 and 1/2 charged to 4.45 V do not react appreciably till 190 °C in EC/DEC. Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples charged to 4.8 V versus Li, across the oxygen release plateau, start to significantly react with EC/DEC at about 130 °C. However, their high reactivity is similar to that of Li_0.5CoO₂ (4.2 V) with 1 μm particle size. Therefore, Li[Ni_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ samples showing specific capacity of up to 225 mAh/g may be acceptable for replacing LiCoO₂ (145 mAh/g to 4.2 V) from a safety point of view, if their particle size is increased.     

Evolution of the local structure and electrochemical properties of spinel LiNixMn2−xO4 (0 ≤ x ≤ 0.5)

A series of Ni substituted spinel LiNi_xMn_2−xO₄ (0 ≤ x ≤ 0.5) have been synthesized to study the evolution of the local structure and their electrochemical properties. X-ray diffraction showed a few Ni cations moved to the 8a sites in heavily substituted LiNi_xMn_2−xO₄ (x ≥ 0.3). X-ray photoelectron spectroscopy confirmed Ni²⁺ cations were partially oxidized to Ni³⁺. The local structures of LiNi_xMn_2−xO₄ were studied by analyzing the and A_1g Raman bands. The most compact [Mn(Ni)O₆] octahedron with the highest bond energy of Mn(Ni)O was found for LiNi_0.2Mn_1.8O₄, which showed a Mn(Ni)O average bond length of 1.790 Å, and a force constant of 2.966 N cm⁻¹. Electrolyte decomposition during the electrochemical charging processes increased with Ni substitution. The discharge capacities at the 4.1 and 4.7 V plateaus obeyed the linear relationships with respect to the Ni substitution with the slopes of −1.9 and +1.9, which were smaller than the theoretical values of −2 and +2, respectively. The smaller slopes could be attributed to the electrochemical hysteresis and the presence of Ni³⁺ in the materials.     

Quantitative study of pH change during LaNi5−xAlx (x = 0, 0.3) discharge process by SECM

Oxygen reduction reaction (ORR) on Pt microelectrode was used for developing a micro pH sensor for scanning electrochemical microscopy (SECM) study in this work. When the potential of Pt microelectrode was held constant in ORR region, the ORR current (cathodic current) increased with decreasing solution pH and vice versa. The response time of the ORR current to pH changes was measured to be ca. 30 ms which implies that the pH response is fast enough for monitoring the temporal pH changes. Furthermore, a fine linear relationship was found to exist between the half wave potential of ORR (E_1/2) and the solution pH value, and the slope is −46 mV/pH. The Pt micro pH sensor was located 1 μm above the LaNi_5−xAl_x (x = 0, 0.3) substrate electrode surface in pH = 9 KOH solution to perform the tip-substrate voltammetry of SECM. In tip voltammogram, the ORR tip current qualitatively reflects the transit solution pH changes during LaNi_5−xAl_x discharge reaction. Also, the minimum values of the solution pH near LaNi₅ and LaNi_4.7Al_0.3 surface during the discharge reaction were quantitatively detected; they were 7.17 and 7.57, respectively. The result indicates that Al partial substitution for Ni degrades the maximum discharge ability of the alloy and decreases the hydrogen diffusion coefficient in alloy bulk.     

Ternary CdxZn1−xSe deposited on Ag (1 1 1) by ECALE: Electrochemical and EXAFS characterization

This paper reports the characterization of ternary II-VI semiconductor nanocrystals, deposited by the electrochemical atomic layer epitaxy (ECALE) technique.In particular, morphological and structural properties of the ternary compounds of formula Cd_xZn_1−xSe deposited on Ag (1 1 1) have been characterized as a function of composition. The number of the attainable x values is limited by the necessity of using well-defined ZnSe/CdSe deposition sequences. However, the quantitative analysis carried out on the basis of both electrochemical and extended X-ray absorption fine structure (EXAFS) experiments indicates that the ECALE method is a successful way of controlling the composition of Cd_xZn_1−xSe. In addition, the electrochemical measurements show that the amount of deposition is minimum in correspondence to the compound with x = 0.5, thus corroborating the hypothesis of a higher degree of disorder suggested both by morphological and structural investigation. The morphology was studied by atomic force microscopy (AFM). The structure of the films is estimated by EXAFS which is a powerful technique for the analysis of the local structure around chosen atoms.