The key factor determining the anodic deposition of vanadium oxides

This work demonstrates that anodic deposition of vanadium oxide (denoted as VO_x·nH₂O) can be considered as the chemical co-precipitation of V⁵⁺ and V⁴⁺ oxy-/hydroxyl species and the accumulation of V⁵⁺ species at the vicinity of electrode surface is the key factor for the successful anodic deposition of VO_x·nH₂O at a potential much more negative than the equilibrium potential of the oxygen evolution reaction (OER). The results of in situ UV-vis spectra show that the V⁴⁺/V⁵⁺ ratio near the electrode surface can be controlled by varying the applied potential, leading to different, three-dimensional (3D) nanostructures of VO_x·nH₂O. The accumulation of V⁵⁺ species due to V⁴⁺ oxidation at potentials ≥0.4 V (vs. Ag/AgCl) has been found to be very similar to the phenomenon by adding H₂O₂ in the deposition solution. The X-ray photoelectron spectroscopic (XPS) results show that all VO_x·nH₂O deposits can be considered as aggregates consisting of mixed V⁵⁺ and V⁴⁺ oxy-/hydroxyl species with the mean oxidation state significantly increasing with the applied electrode potential.     

Effects of complex agents on the anodic deposition and electrochemical characteristics of cobalt oxides

The anodic deposition rate of cobalt oxide from CoCl₂·6H₂O is strongly affected by the type of complex agents (acetate ion (AcO⁻), citrate ion, EDTA) added into the deposition solutions. The oxidation potential of CoCl₂·6H₂O, examined by linear sweep voltammetry (LSV), is negatively shifted from ca. 1.1 V to about 0.8, 0.5, and 0.2 V by adding AcO⁻, citrate ion, and EDTA, respectively. The deposition rate of cobalt oxide is found to depend not only on the coordinating strength between Co and ligands but also on the conversion rate of the Co-L complexes (L: ligand) into the oxy-hydroxyl-Co species after electron transfer. The textural and electrochemical characteristics of resultant Co oxides, examined by X-ray photoelectron spectroscopic (XPS), scanning electron microscopic (SEM), open-circuit potential versus time, and cyclic voltammetric analyses, are also influenced by varying the complex agents. The deposition rate is the highest when the Co oxide is deposited from the precursor solution containing AcO⁻, which also exhibits the highest specific capacitance of ca. 230 F g⁻¹ among all Co oxide deposits (as the oxide loading ≥0.05 mg cm⁻²), demonstrating its most promising applicability in the electrochemical supercapacitors.     

Anodic deposition of hydrous ruthenium oxide for supercapaciors: Effects of the AcO concentration, plating temperature, and oxide loading

Effects of the sodium acetate (NaCH₃COO, denoted as NaAcO) concentration, plating temperature, and oxide loading on the pseudocapacitive characteristics of hydrous ruthenium oxide (denoted as RuO₂·xH₂O) films anodically plated from aqueous RuCl₃·xH₂O media were systematically investigated in this work. The electrochemical behavior of RuO₂·xH₂O with annealing in air at 200 °C for 2 h is approximately independent of the NaAcO concentration and plating temperature although a negative shift in the onset and peak potentials of deposition with rising the plating temperature is found. The morphologies and adhesion of RuO₂·xH₂O deposits strongly depend on the deposition rate which is obviously influenced by varying the above two deposition variables. The specific capacitance of RuO₂·xH₂O is monotonously decreased from 760 to 505 F g⁻¹ when the oxide loading is gradually increased from 0.34 to 1.0 mg cm⁻², due to the longer pathways of both electrons and protons during the redox transitions. The XRD and Raman spectroscopic analyses reveal the extremely localized crystalline nature of as-deposited RuO₂·xH₂O. All RuO₂·xH₂O deposits show the ideal pseudocapacitive characteristics, definitely illustrating the merits of RuO₂·xH₂O prepared by anodic deposition without considering the advantages of its simplicity, one-step, reliability, low cost, and versatility for electrode preparation.     

Textural and pseudocapacitive characteristics of sol-gel derived RuO2·xH2O: Hydrothermal annealing vs. annealing in air

Hydrous ruthenium dioxide (RuO₂·xH₂O) prepared in a modified sol-gel process was subjected to annealing in air and water at various temperatures for supercapacitor applications. The textural and pseudocapacitive characteristics of RuO₂·xH₂O annealed in air and water were systematically compared to show the benefits of annealing in water (denoted as hydrothermal annealing). An important concept that hydrothermal annealing effectively restricts condensation of hydroxyl groups within nanoparticles, inhibits crystal growth, and maintains high water content of RuO₂·xH₂O is demonstrated in this work. The unique textural characteristics of hydrothermally annealed RuO₂·xH₂O are attributable to the high-pressured, water-enriched surroundings which restrain coalescence of RuO₂·xH₂O nanocrystallites. The crystalline, hydrous nature of hydrothermally annealed RuO₂·xH₂O favors the utilization of active species in addition to a merit of minor dependence of specific capacitance on the scan rate of CV for pseudocapacitors. As a result, RuO₂·xH₂O with hydrothermal annealing at 225 °C for 24 h exhibits 16 wt.% water, an average particle size of about 7 nm, and specific capacitance of ca. 390 F g⁻¹.     

Effects of substrates on the capacitive performance of RuOx·nH2O and activated carbon-RuOx electrodes for supercapacitors

The electrochemical energy storage and delivery on the electrodes composed of hydrous ruthenium oxide (RuO_x·nH₂O) or activated carbon-hydrous ruthenium oxide (AC-RuO_x) composites are found to strongly depend on the substrate employed. The contact resistance at the active material-graphite interface is much lower than that at the active material-stainless steel (SS) mesh interface. Thin films of gold plus RuO_x·nH₂O deposited on SS meshes (RuO_x/Au/SS) are found to greatly improve the poor contact between SS meshes and electrode materials. The maximum specific capacitance (C_S,RuOx) of RuO_x·nH₂O, 1580 F g⁻¹ (measured at 1 mV s⁻¹), very close to the theoretic value, was obtained from an AC-RuO_x/RuO_x/Au/SS electrode with 10 wt.% sol-gel-derived RuO_x·nH₂O annealed in air at 200 °C for 2 h. The highly electrochemical reversibility, high-power characteristics, good stability, and improved frequency response of this AC-RuO_x/RuO_x/Au/SS electrode demonstrate its promising application potential in supercapacitors. The ultrahigh specific capacitance of RuO_x·nH₂O probably results from the uniform size distribution of RuO_x·nH₂O nanoparticles, ranged from 1.5 to 3 nm which is clearly observed from the high-resolution transmission electron microscopy (HRTEM).     

Tailoring and structure of PtRu nanoparticles supported on functionalized carbon for DMFC applications: New evidence of the hydrous ruthenium oxide phase

The influence of the structure and morphology of PtRu nanoparticles supported on functionalized carbon black has been investigated for CO and methanol electrooxidation in a half-cell and in a DMFC single cell. Carbon black was treated with HNO₃ to obtain an oxidized surface (Vulcan-N), and PtRu nanoparticles supported on Vulcan-N were prepared via impregnation, Bönnemann's method and the sulfito-complex route. Temperature programmed reduction (TPR) measurements evidence the presence of RuO₂·xH₂O phase in the catalyst obtained by the sulfito-complex route. This phase was stabilized by metal–support interaction, whereas alloy characteristics were estimated for PtRu catalyst obtained by impregnation and Bönnemann's method. The nature of the precursor–support interaction, induced by the nature of the functional groups on the carbon surface, affects the structure of the electrocatalyst and subsequent behavior in electroactivity. When synthesized through Bönnemann's method, the surface oxygen-containing groups of the support seem to be unable to stabilize the anhydrous precursors of platinum and ruthenium, yielding crystalline RuO₂. Methanol electrooxidation performance was clearly different in the three catalysts, whereas only a few negligible differences were observed in CO oxidation. The superior performance in DMFC of the catalysts obtained by the sulfito-complex route accounts for both the presence of RuO₂·xH₂O species and the functionalization of carbon black.     

Hydrothermal synthesis and electrochemical capacitance of RuO2·xH2O loaded on benzenesulfonic functionalized MWCNTs

Amorphous RuO₂·xH₂O was well coated on the benzenesulfonic functionalized multi-wall carbon nanotubes (f-MWCNTs) successfully via hydrothermal method. The decorated benzenesulfonic groups served as a bifunctional role both for solubilizing and dispersing MWCNTs into aqueous solution and for tethering Ru³⁺ precursor to facilitate the following uniform chemical deposition of RuO₂·xH₂O. The electrochemical performance of RuO₂/f-MWCNTs and utilization of RuO₂·xH₂O were evidenced by cyclic voltammetry and galvanostatic charge/discharge tests. The specific capacitance of 1143 Fg⁻¹ for RuO₂·xH₂O was obtained from RuO₂/f-MWCNTs with 32 wt.% RuO₂·xH₂O, which was much higher than that of just 798 Fg⁻¹ for the RuO₂/p-MWCNTs. Even though the RuO₂·xH₂O loading increases to 45 wt.%, the utilization of RuO₂·xH₂O still possesses as high as 844.4 Fg⁻¹, indicating a good energy capacity in the case of high loading.     

Grass blade-like microparticle MnPO4·H2O prepared by a simple precipitation at room temperature

Grass blade-like microparticle MnPO₄·H₂O was synthesized by a simple precipitation at room temperature using a mixture of manganese sulphate monohydrate, phosphoric acid and water at pH = 7. The thermogravimetric study indicates that the synthesized compound is stable below 500 °C and its final decomposed product is Mn₂P₂O₇. The pure monoclinic phases of the synthesized MnPO₄·H₂O and its final decomposed product Mn₂P₂O₇ are verified by XRD data. FTIR spectra indicate the presences of the PO₄³⁻ ion and water molecules in the MnPO₄·H₂O structure and the P₂O₇⁴⁻ ion in the Mn₂P₂O₇ structure. The thermal stability, crystallite size, and grass blade-like microparticle of MnPO₄·H₂O in this work are different from previous reports, which may be caused by the starting reagents and reaction condition for the precipitation.     

Photoluminescence emission of nanoporous anodic aluminum oxide films prepared in phosphoric acid

The photoluminescence emission of nanoporous anodic aluminum oxide films formed in phosphoric acid is studied in order to explore their defect-based subband electronic structure. Different excitation wavelengths are used to identify most of the details of the subband states. The films are produced under different anodizing conditions to optimize their emission in the visible range. Scanning electron microscopy investigations confirm pore formation in the produced layers. Gaussian analysis of the emission data indicates that subband states change with anodizing parameters, and various point defects can be formed both in the bulk and on the surface of these nanoporous layers during anodizing.     

Morphological and structural studies of nanoporous nickel oxide films fabricated by anodic electrochemical deposition techniques

Porous nickel oxide films are directly deposited onto conducting indium tin oxide coated glass substrates by cyclic voltammetric (CV), galvanostatic, and potentiostatic strategies in a plating bath of sodium acetate, nickel sulfate, and sodium sulfate. By tuning the deposition parameters, it is possible to prepare nickel oxide films with various morphologies and structures. Film formation relies on the oxidation of dissolved Ni²⁺ to Ni³⁺, which further reacts with the available hydroxide ions from a slightly alkaline electrolyte to form insoluble nickel oxide/hydroxide deposits on the substrate. A compact film with particularly small pores is obtained by CV deposition in a potential range of 0.7-1.1 V. A galvanostatically deposited film is structurally denser near the surface of the substrate, and becomes less dense further away from the surface. Interestingly, a potentiostatically deposited film has pores distributed uniformly throughout the entire film. Therefore, for obtaining a uniform film with suitable pore size for electrolyte penetration, potentiostatic deposition technique is suggested. In addition, except for CV deposition, the deposited films resemble closely to cubic NiO when the annealing temperature exceeds 200 °C.     

Thermal and chemical stability of Cu–Zn–Cr-LDHs prepared by accelerated carbonation

Layered double hydroxides Cu_xZn_6 − xCr₂(OH)₁₆(CO₃)·4H₂O with different molar ratios of Cu/Zn/Cr were synthesized by accelerated carbonation. The products were characterized by XRD, SEM, FT-IR and TG-DTG-DSC-MS. The chemical stability was tested by the modified Toxicity Characteristic Leaching Procedure (TCLP). The results showed that the products were the mixture of Cu_xZn_6 − xCr₂(OH)₁₆(CO₃)·4H₂O and (CuZn)₂(CO₃)(OH)₂, with similar thermal behavior. All products were chemically stable with reduced leaching at pH > 6 (Cu²⁺, Zn²⁺) or > 5 (Cr³⁺).     

On the contribution of electrostriction to charge-induced stresses in anodic oxide films

Electrostriction has been known for long as a major source of internal stress developing in oxide films during anodising. However, in many studies, the contribution of electrostriction is estimated using an oversimplified equation, leading to a systematic underestimation. In this work, a modified theory is first presented for linking the in-plane electrostriction stress to the applied electric field. The corrected equation explicitly takes into account the dielectric properties of the deformed material through the appropriate electrostriction parameters of the oxide film. A new experimental procedure is then described for measuring electrostriction stresses in situ during anodising, and applied to ultrathin (<100 nm) TiO₂ films. Oxide films were first grown potentiodynamically on one side of cantilevered electrodes. The electric field in the oxide film was then systematically changed by repeatedly cycling up to the forming voltage. At the same time, the resulting in-plane electrostriction stress was determined from high-resolution in situ curvature measurements. For the TiO₂ films considered, compressive in-plane electrostriction stresses up to −240 MPa were measured. This is an order of magnitude higher than previous predictions which neglected the contribution of the electrostriction parameters. Moreover, the measured stress values, their field-dependence and the derived electrostriction parameters are shown to be in agreement with the modified theory.     

Synthesis of an industrially important zinc borate, 2ZnO·3B2O3·3H2O, by a rheological phase reaction method

2ZnO·3B₂O₃·3H₂O is an industrially important zinc borate. Herein, 2ZnO·3B₂O₃·3H₂O has been prepared via a rheological phase reaction method using zinc oxide and boric acid as starting materials. This route is facile and acceptable for green chemical synthesis, producing no pollution and giving a yield of near 100% of theoretical value. And in this method, the complete conversion of the starting materials can be achieved in the presence of only 0.04 mL water (one drop of water). The products have been characterized by X-ray powder diffraction (XRD), thermogravimetry (TG) and differential thermal analysis (DTA), scanning electron microscopy (SEM) and particle size distribution. The effects of experimental conditions on the products were investigated. The main factors that affect the formation of zinc borate are water volume, sealing state, reaction time and temperature.     

Soft template synthesis of mesoporous Co3O4/RuO2·xH2O composites for electrochemical capacitors

Co₃O₄/RuO₂·xH₂O composites with various Ru content (molar content of Ru = 5%, 10%, 20%, 50%) were synthesized by one-step co-precipitation method. The precursors were prepared via adjusting pH of the mixed aqueous solutions of Co(NO₃)₂·6H₂O and RuCl₃·0.5H₂O by using Pluronic123 as a soft template. For the composite with molar ratio of Co:Ru = 1:1 annealed at 200 °C, Brunauer-Emmet-Teller (BET) results indicated that the composite showed mesoporous structure, and the specific surface area of the composite was as high as 107 m² g⁻¹. The electrochemical performances of these composites were measured in 1 M KOH electrolyte. Compared with the composite prepared without template, the composite with P123 exhibited a higher specific capacitance. When the molar content of Ru was rising, the specific capacitance of the composites increased significantly. It was also observed that the crystalline structures as well as the electrochemical activities were strongly dependent on the annealing temperature. A capacitance of 642 F/g was obtained for the composite (Co:Ru = 1:1) annealed at 150 °C. Meanwhile, the composites also exhibited good cycle stability. Besides, the morphologies and textural characteristic of the samples were also investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM).     

Micro-EIS of anodic thin oxide films on titanium for capacitor applications

The present work focuses on the investigation of the effect of the different crystallographic orientation of titanium grains on the formation of anodic oxide films and consequently their dielectric and semiconductive properties. By using a microcapillary cell the formation process and the electrochemical impedance spectroscopy (EIS) can be performed at high lateral resolution on variously orientated single grains of polycrystalline titanium. The oxide films were potentiodynamically formed by cyclovoltammetry. EIS measurements immediately followed by the oxide formation were used for a detailed investigation of the film properties, in particular, the relative permittivity ?_r and the donor concentration N_D. In contrast to the most publications it was found that under the chosen conditions the crystallographic orientation of titanium substrate has no significant influence on the oxide thickness d, the relative permittivity ?_r or on the donor concentration N_D of the oxide films. The relative permittivity ?_r is approximately 50. The donor concentration depends on the film thickness and amounts to approximately 3 × 10¹⁸ cm⁻³ in minimum.     

A short solid-state synthesis leading to titanate compounds with porous structure and nanosheet morphology

A novel method was developed for the synthesis of titanate nanosheets with high surface area. A solid-state mixture of NaOH and TiO₂ was reacted at 600 °C for several minutes. The aqueous dispersion of the resulting melt was aged at room temperature for periods up to 14 days. After hydrochloric acid treatment and washing procedures, the reaction product was characterized by X-ray diffraction, Raman spectroscopy, N₂-sorption and small-angle X-ray scattering measurements. The titanate compound not subjected to the aging process was amorphous and possessed a microporous framework, while the aged samples displayed nanosheet morphology and high specific surface area (396-509 m² g⁻¹). It was revealed that the very short heat treatment is of crucial importance for the titania-titanate phase transformation, while the aging process is needed for the morphological evolution of the titanate samples. The effects of the aging time on the structure and the morphology are discussed.     

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.     

Rapid deposition of YBCO films by laser CVD and effect of lattice mismatch on their epitaxial growth and critical temperature

a-Axis- and c-axis-oriented YBa₂Cu₃O_7–δ (YBCO) films were grown on (100) SrTiO₃ substrate by laser chemical vapour deposition (laser CVD). The effect of lattice mismatch between films and substrates on in-plane and out-of-plane crystallinity and critical temperature (T_C) was investigated. The preferred orientation changed from a-axis to c-axis as the deposition temperature increased from 928 to 1049 K. The c-axis-oriented YBCO showed a minimum of full width at half maximum of 0.5° for the ω-scan and 1.0° for the φ-scan. A smaller mismatch between YBCO films and substrates led a higher crystallinity for in-plane and out-of-plane epitaxial growths. A high T_C of 90 K was obtained for the c-axis-oriented YBCO films. The deposition rate of the YBCO films was 58–101 μm h⁻¹, approximately 60–1000 times higher than that of conventional CVD.     

Impact of cobalt substitution for manganese on the structural and electrochemical properties of LiNi0.5Mn0.5O2

A series of LiNi_0.5Mn_0.5−xCo_xO₂ (0 ≤ x ≤ 0.5) compounds was prepared by a solid state reaction, and their structure, surface state and electrochemical characteristics were also investigated by XRD, XPS, EIS and charge-discharge cycling. The non-equivalent substitution of cobalt for manganese induced an increase in the average valence of nickel, thereby shrinking in the lattice volume. Moreover, Co non-equivalent substitution could not only reduce the impurity content but also significantly decreased the charge transfer resistance, thereby improving the rate capabilities.     

Effects of NH3·H2O pretreatment on the fabrication of uniform titania nanocoating in an aqueous solution

An investigation is reported of the TiO₂ nanocoating on the ZnS-based phosphor via a sol-gel method in an aqueous solution using titanium diethanolamine complex as the precursor. The pretreatment of ZnS phosphors with NH₃·H₂O is important to the coating process. When NH₃·H₂O was added into the ZnS phosphor suspension, OH⁻ promoted the hydrolysis of Zn²⁺ on the surface of ZnS phosphors, which resulted in the formation of Zn-OH. Zn-OH reacted with the hydrolysis product of titanium diethanolamine complex, thus the titania coatings were obtained. The coating morphology was strongly dependant on the pH of the pretreated ZnS phosphor suspension and the weight ratio of ZnS to TiO₂.