Electrocatalytic oxidation and reduction of H2O2 on vertically aligned Co3O4 nanowalls electrode: Toward H2O2 detection

Single crystal and vertically aligned cobalt oxide (Co₃O₄) nanowalls were synthesized by directly heating Co foil on a hot-plate under ambient conditions. The vertically aligned Co₃O₄ nanowalls grown on the plate show excellent mechanical property and were facilely attached to the surface of a glassy carbon (GC) electrode using conductive silver paint. The prepared Co₃O₄ nanowalls electrode was then applied to study the electrocatalytic oxidation and reduction of hydrogen peroxide (H₂O₂) in 0.01 M pH 7.4 phosphate buffer medium. Upon the addition of H₂O₂, the Co₃O₄ nanowalls electrode exhibits significant oxidation and reduction of H₂O₂ starting around +0.25 V (vs. Ag/AgCl), while no obvious redox activity is observed at a bare GC electrode over most of the potential range. The superior electrocatalytic response to H₂O₂ is mainly attributed to the large surface area, minimized diffusion resistance, high surface energy, and enhanced electron transfer of the as-synthesized Co₃O₄ nanowalls. The same Co₃O₄ nanowalls electrode was also applied for the amperometric detection of H₂O₂ and showed a fast response and high sensitivity at applied potentials of +0.8 V and −0.2 V (vs. Ag/AgCl), respectively. The results also demonstrate that Co₃O₄ nanowalls have great potential in sensor and biosensor applications.     

Solvent-effect on the interconversion among one-, two- and four-electron reduction waves for the 18-molybdodiphosphate complex, [(P2O7)Mo18O54]4−

For the [(P₂O₇)Mo₁₈O₅₄]^4? complex, the presence of small cations such as H⁺, Li⁺ and Na⁺ caused one-electron waves to be converted into four- and two-electron waves in a complex manner. With the addition of a trace amount of H⁺, a four-electron reduction wave was obtained in solvents of weaker basicity like acetone, acetonitrile and propylene carbonate (PC); the relative permittivity did not affect the appearance of the four-electron wave. On the other hand, two-electron waves were obtained in solvents of stronger basicity like N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), and N-methylpyrrolidinone (NMP). With the addition of Li⁺ or Na⁺, the one-electron waves were converted into two-electron waves only in acetone, indicating that the conversion can occur in solvents of both weak basicity and low relative permittivity.     

Preparation of perovskite-type composite oxide LaNi0.5Ti0.5O3–NiFe2O4 and its application in glucose biosensor

A novel nanostructured perovskite-type composite oxide LaNi_0.5Ti_0.5O₃–NiFe₂O₄ (LNT–NFO) was synthesized by sol–gel method and characterized by X-ray diffractometer (XRD) and transmission electron microscopy (TEM). Amperometric glucose biosensors based on the carbon paste electrode (CPE) were constructed by immobilizing glucose oxidase (GOD) with LNT–NFO and the experimental conditions such as the amount of GOD, pH value, and applied potential were investigated. Under the optimum conditions, the electrochemical performances of the LNT–NFO with different ratios of LNT-NFO (10:1, 20:1, 30:1) modified CPE have been researched on the oxidation of glucose. The results show that LNT–NFO (20:1) can immobilize GOD more effectively. The biosensor based on LNT–NFO/CS/GOD modified CPE exhibits good reproducibility, stability and selectivity in glucose determination with linear signal-to-glucose concentration range of 0.5–10 mM and a detection limit (S/N = 3) of 0.04 mM.     

Voltammetric studies of the interaction of the lithium cation with reduced forms of the Dawson [S2Mo18O62]4− polyoxometalate anion

Multiple electron transfer steps observed in the reduction of [S₂Mo₁₈O₆₂]^4?, when LiClO₄ is used as electrolyte, are accounted for by assuming that Li⁺ acts as a moderately strong Lewis acid. For example, in the 95:5 CH₃CN+H₂O solvent mixture (0.1 M NBu₄ClO₄), the voltammetric behavior obtained on addition of Li⁺ can be simulated according a reaction scheme involving an extensive series of reversible potentials and equilibrium constants. Precipitation of highly reduced polyoxometalate salts occurring at the electrode surface complicates the voltammetry at very negative potentials when 0.1 M LiClO₄ is used as the electrolyte.     

Simultaneous electrochemical and quartz crystal microbalance studies of non-conducting microcrystalline particles of trans-Cr(CO)2(dpe)2 and trans-[Cr(CO)2(dpe)2]+ (dpe = Ph2PCH2CH2PPh2) attached to gold electrodes

Simultaneous cyclic voltammetric double potential step and electrochemical quartz crystal microbalance (EQCM) experiments on water insoluble trans-Cr(CO)₂(dpe)₂ and trans-[Cr(CO)₂(dpe)₂]X (dpe = Ph₂PCH₂CH₂PPh₂, X = Cl^?, Br^? and I^?), attached as an array of microcrystals, have been employed to probe mechanistic aspects of the redox chemistry of the [trans-Cr(CO)₂(dpe)₂]^+/0 process at the electrode |solid| solvent (electrolyte) interface in a variety of aqueous electrolytes. EQCM experiments show that the oxidation of solid trans-Cr(CO)₂(dpe)₂ involves the slow incorporation of non-solvated anions from the electrolyte solution into the solid. Interestingly, on the reverse scan of cyclic voltammetric experiments, EQCM data reveal that some but not all the anions are rapidly expelled from the crystal lattice. Double potential step experiments with the neutral chromium compound confirm that the oxidation reaction is a relatively slow process. The conclusion reached from all experiments is that the reduction process predominantly expels the anions that are relatively close to the solid|solution interface. EQCM investigations of trans-[Cr(CO)₂(dpe)₂]X compounds in electrolytes containing a different anion to that in the compound show that the anion originally in the salt is rapidly replaced by the anion in the aqueous electrolyte at open circuit potential, presumably via a rapid ion exchange process. The anion from the electrolyte is then expelled and incorporated into the solid during the reduction and oxidation steps respectively.     

不同浓度的过氧化氢对银汞合金表面性能的影响

目的研究不同浓度过氧化氢对银汞合金表面性能的影响。方法采用表面轮廓仪和扫描电镜对0 wt%、3.6 wt%、10 wt%和30 wt%四种浓度过氧化氢浸泡112 h后的牙科银汞合金的表面粗糙度、表面形貌进行比较,并运用光电子能谱仪分析0%和30%两种浓度过氧化氢浸泡后银汞合金的表面腐蚀产物。结果过氧化氢浸泡112 h后,不同银汞合金表面粗糙度的Ra和Rz的大小顺序均为0 wt%<3.6 wt%<10 wt%<30 wt%;且表面均有不同程度的腐蚀,主要表现为分布不均匀、孔径大小不一的点蚀,其中以30wt%过氧化氢浸泡组的腐蚀最明显;此外,0 wt%和30 wt%过氧化氢浸泡后,银汞合金表面分别检测到Cu、Sn、Hg、Ag、O元素,表面氧化物主要为SnO₂,与0 wt%过氧化氢相比,30 wt%过氧化氢浸泡后,银汞合金表面Ag、Hg、O元素含量明显下降,而Sn、Cu元素含量则升高。结论随着过氧化氢浓度的升高,银汞合金表面粗糙度增加,表面形貌有不同程度腐蚀,表面腐蚀产物成分有所改变,氧化程度减弱。     

Voltammetric lability of multiligand complexes: the case of ML2

The voltammetric lability of a complex system, where a metal ion M and a ligand L form the species ML and ML₂, is examined. Together with the rigorous numerical simulation of the problem, two limiting cases are analysed for the overall process ML₂ → M: (i) the most common case for aqueous complexes, where ML → M is the kinetically limiting step and (ii) the case where ML₂ → ML is limiting. In both cases, analytical expressions for the lability criteria are provided which show good agreement with the results obtained from the rigorous numerical simulation of the problem.     

Analysis of cyclic voltammograms of electrochromic a-WO3 films from voltage-dependent equilibrium capacitance measurements

The main features of cyclic voltammograms (CVs) of thin electrochromic films based on a-WO₃ were investigated. First, the chemical capacitance is defined in terms of the electrochemical potential variation with the insertion level, x, and is measured under galvanostatic (chronopotentiometry) quasi-equilibrium conditions. An equilibrium capacitance increasing rapidly with respect to insertion level or negative potential is observed, respectively C_ch∝x^a at x>10^?3(a≈0.74) and C_ch∝V^α(α≈3). Simulation methods used to generate the observed CVs are described in detail. Major CV peaks can be simply understood as charging and discharging of the variable capacitor, in conjunction with a distortion of the voltage scale due to a series transport process. Therefore a simple RC equivalent circuit allows us to explain the principal CVs characteristics of lithium intercalation and deintercalation in amorphous films.     

Anodic oxidation of free nitric oxide at gold electrodes modified by a film of trans-[Ru(III)(NH3)4(SO4)4pic]+ and molybdenum oxide

Gold surfaces were modified with an electrochemically deposited layer of non-stoichiometric molybdenum oxides. At these surfaces, trans-[Ru(III)(NH₃)₄(SO₄)4pic]⁺ complex was incorporated in a controlled way by cycling the potential consecutively in the range +0.50 to ?0.25 V at pH 2.6. Very reproducible voltammetric curves corresponding to the electrochemical process of the ruthenium complex were obtained, confirming the immobilisation of the material into the molybdenum oxide film. The anodic oxidation of nitric oxide (NO) at pH 7.4 was investigated at the modified electrode containing the molybdenum oxide+trans-[Ru(III)(NH₃)₄(SO₄)4pic]⁺ complex and an enhancement in the current response was observed compared with the signal at a bare electrode. The rate for NO electrochemical oxidation was dependent on the amount of catalytic ruthenium sites dispersed into the molybdenum oxide film. A linear relationship between current signals measured by square wave voltammetry and NO concentration was obtained in the 0–10 μM range. The applicability of the modified electrode as a sensor for real-time NO monitoring was also demonstrated.     

Synthesis and redox properties of LixNi2(MoO4)3: a new 3-V class positive electrode material for rechargeable lithium batteries

A new framework type Li_xNi₂(MoO₄)₃ [0?x?4] polyanion compound was synthesized via a glycine-nitrate soft-combustion process at low temperature. The annealed powders were characterized by XRD to confirm the phase formation of the stoichiometric product, Ni₂(MoO₄)₃ in its non-lithiated state. The morphology of the annealed product was found to be composed of soft agglomerates embedded by ultrafine spherical grains. Electrochemical redox properties of the synthesized product were confirmed by employing the new material as a cathode in lithium-containing test cells in an aprotic electrolyte environment (1 M LiPF₆ in EC + DMC). Slow scan cyclic voltammetry (SSCV) confirmed the redox behavior corresponding to the reduction/oxidation of the transition metals, Ni and Mo between the potential window of 3.5–1.5 V. The lithium insertion/extraction process was confirmed by galvanostatic measurements on the test cells and they exhibited well discernible discharge/charge profiles with a reversible capacity of 170 mAh/g over the potential window of 3.5–1.5 V after the first charge/discharge cycle. Nevertheless, the discharge capacity was found to deteriorate slowly upon repeated cycling, which might presumably be due to disproportion reaction of the host structure beyond the extent of insertion of two lithium ions.     

牙科修复用BiOIO3/g-C3N4复合纳米光催化剂的制备与表征

目的 研究不同剂量NaOH条件对制备BiOIO3/g-C3N4复合纳米光催化剂的结构形貌及其可吸收光谱的影响,为研制适用于口腔修复纳米光催化功能性材料提供依据.方法 水热法制备BiOIO3/g-C3N4,通过XRD(X射线衍射仪)、SEM(扫描电镜图)和UV-vis(紫外-可见光吸收光谱)对其形貌、结构、分子组成和理化...     

Compatibility of machined Ce-TZP/Al2O3 nanocomposite and a veneering ceramic

OBJECTIVES: Due to its high fracture toughness ceria-stabilized ZrO(2)/Al(2)O(3) nanocomposite (Ce-TZP/A) could be superior to Y-TZP in clinical use. However, the compatibility to veneering ceramics is not yet investigated and therefore subject of this study. METHODS: Fracture strength and crazing resistance of veneered Ce-TZP/A frameworks (Nanozir, Matsushita Electric Works) were investigated in comparison to sintered and hipped Y-TZP (Hint-ELs). Frameworks were machined (HiCut, Hint-ELs), sintered where applicable (hiTherm, Hint-ELs), and veneered in a standardized procedure (Cerabien ZR, Noritake). Fracture strength of the crowns was assessed in a shear test (n=10). Biaxial flexural strength of the core materials was measured according to ISO 6872 (n=10). To assess the thermal compatibility between framework and veneer the coefficient of thermal expansion (CTE) of all materials was determined (n=3) and a crazing test carried out (n=12). RESULTS: Fracture strength was equal for crowns with Ce-TZP/A (866.6+/-132.1N) and sintered Y-TZP (904.5+/-168.2N), while crowns with hipped Y-TZP were significantly stronger (1380.6+/-253.2N) (Student's t-test, p<0.05). These values correlated with the biaxial flexural strength of the three core materials (Ce-TZP/A: 1238.0+/-200.0MPa, sintered Y-TZP: 1181.8+/-232.5MPa and hipped Y-TZP: 1521.8+/-163.8MPa). Failure of both hipped and sintered Y-TZP crowns occurred by chipping of the veneer or total fracture, while the Ce-TZP/A crowns predominantly failed by chipping of the veneer. In the crazing test both sintered and hipped Y-TZP performed slightly better than Ce-TZP/A. CTE's were as follows: Ce-TZP/A: 10.3microm/mK, sintered Y-TZP: 10.7microm/mK, hipped Y-TZP: 10.9microm/mK, and Cerabien ZR: 9.9microm/mK. SIGNIFICANCE: The CTE of the veneering ceramic has to be adjusted to Ce-TZP/A frameworks.     

Electrochemical response of α-H4SiW12O40 on Ag and Au electrodes

Electrochemical measurements are presented for acidic solutions containing α-H₄SiW₁₂O₄₀ adsorbed on Ag(111) and Au(111) electrodes. These measurements show that this molecule passivates the Ag surface towards solution redox events. This passivation is unique to Ag, as it is not observed on Au or carbon electrodes. Depassivation can be accomplished by moving the potential of the Ag electrode into the hydrogen evolution region. These results are discussed in terms of formation of a reduced AgSiW₁₂O₄₀ complex that is stable only at negative potentials.     

X-ray diffraction and electrochemical impedance spectroscopy study of zinc coated LiNi0.5Mn1.5O4 electrodes

Coating and doping with zinc have been used to modify LiNi_0.5Mn_1.5O₄ powders prepared at 700 and 800 °C. As previously found in the literature, surface treatment with zinc leads to a net improvement of the electrochemical performance of the 5-V electrode in lithium cells. X-ray diffraction reveals the occurrence of surface ZnO, which disappears after heating at 500 °C. This indicates that ZnO forms a coating, although zinc can also be incorporated in the LiNi_0.5Mn_1.5O₄ framework. The electrochemical oxidation of both bare LiNi_0.5Mn_1.5O₄ and zinc treated products has been followed by ex situ X-ray diffraction of charged electrodes. For both materials a decrease in the cubic unit cell parameter and the formation of two cubic phases during lithium extraction has been found. Lattice contraction is significantly larger in the sample modified with zinc. EIS measurements give direct evidence of the better stabilization of the electrode surface in the coated material.     

Photoelectrochemical properties of layered niobate (K4Nb6O17) films prepared by electrophoretic deposition

Layered niobate oxide (K₄Nb₆O₁₇·3H₂O) films with high orientation and strong adhesion were fabricated by electrophoretic deposition. The deposited films displayed the behavior of a typical n-type semiconductor in K₂SO₄ solution. When the films were irradiated with UV light, an anodic photocurrent due to the oxidative reaction of the water between the NbO₆ layers could be observed. Consequently, the deposited film was converted by photoelectrochemical oxidation from a hydrated form (K₄Nb₆O₁₇·3H₂O) into an anhydrous form (K₄Nb₆O₁₇).     

Formation of a nickel hydroxide monolayer on Au through a self-assembled monolayer of 5,5′-dithiobis(2-nitrobenzoic acid): voltammetric,SERS and XPS investigations of the modified electrodes

The formation of self-assembled monolayers (SAM) of 5,5′-dithiobis (2-nitrobenzoic acid), DNBA on gold has enabled further derivatization of the electrode surface with functional moieties anchored to the surface bound molecules. A SAM of DNBA was formed on the Au surface. Nickel ions tethered to the SAM-covered Au surface, were subsequently derivatized electrochemically to yield nickel hydroxide overlayers, thereby showing the possibility of preparing ultra-thin films of metal oxides through solution chemistry. The nickel hydroxide surface coverage obtained on bare and SAM-covered electrodes was estimated from voltammetric peaks and it varied from one monolayer to about 300 monolayers. The formation of a monolayer of nickel hydroxide has been achieved for the first time by electrochemical modification. Further, the modified electrodes were subjected to SERS and XPS studies to understand their surface characteristics. Modified electrodes provide a catalytic pathway involving nickel hydroxide for the electro-oxidation of glucose in alkaline solutions.     

The effect of doping Mg on the structure and electrochemical properties of Li3V2(PO4)3 cathode materials for lithium-ion batteries

The Mg-doped Li₃V_2−xMg_x(PO₄)₃ (x = 0.00, 0.01, 0.02, 0.05, 0.10, 0.20, 0.30, 0.33, 0.50, 1.00 and 1.33) compounds have been prepared by a sol–gel method in reducing atmosphere (70%Ar + 30%H₂) using citric acid as a chelating agent and a carbon source coated on the samples. The Mg-doped effects on the structural and electrochemical performance of Li₃V₂(PO₄)₃ are investigated by X-ray diffraction, galvanostatic, charge/discharge and four-point probe measurement method. The Li₃V_2−xMg_x(PO₄)₃ solid solution phase can exist stable in the composition range between x = 0.00 and 0.27. The simple improve mechanism of the electrochemical performance for Mg-doped Li₃V_2−xMg_x(PO₄)₃ system is discussed too. In the Mg-doped Li₃V_2−xMg_x(PO₄)₃ system, at a lower charge/discharge rate (0.1C), the cycle performance has no much improvement with the increasing Mg doping content. However, at higher rates, there has an excited improvement in both cycle performance and rate capability due to the increase of electrical conductivity (more than one order of magnitude). At 5C charge/discharge rate, for the Li₃V_1.95Mg_0.05(PO₄)₃ sample, the discharge capacities for the 1st and 100th cycle were 138.9 and 123.3 mAh g⁻¹. The discharge capacity retention reached to 89% (more than 51% for undoped Li₃V₂(PO₄)₃ system). More important is that, except for the first 15 cycles, the discharge capacities kept almost a constant. Based on the excellent electrochemical performance, Li₃V_1.95Mg_0.05(PO₄)₃ will be a promising cathode material for rechargeable lithium-ion batteries.     

Effect of TiF4, ZrF4, HfF4 and AmF on erosion and erosion/abrasion of enamel and dentin in situ

Objective

This in situ study aimed to analyse the impact of different tetrafluorides (TiF₄, ZrF₄ and HfF₄) and AmF on erosion and erosion plus abrasion of enamel and dentin.

Design

Ten volunteers took part in this crossover and double-blind study performed in 8 phases of each 3 days. In each phase, 2 bovine enamel and 2 dentin specimens were fixed in intraoral appliances. One enamel and one dentin sample were pretreated once with TiF₄, ZrF₄, HfF₄ or AmF (all 0.5 M F) for 60 s, while the other samples remained unfluoridated and served as control. Then, all samples were subjected to either erosion only (4 times/day, 90 s) or to erosion and abrasion (2 times/day, 30 s/sample). Toothbrushing abrasion was performed 90 min after the first and last erosion with an electrical toothbrush and fluoridated toothpaste at 1.2 N. After 3 days, enamel and dentin loss was assessed by profilometry (μm) and analysed by repeated measures ANOVA and paired t-test (p < 0.05).

Results

All fluoride solutions reduced enamel and dentin loss significantly compared to the controls. Generally, eroded samples showed less wear than eroded and abraded samples. The protective potential of the fluorides was not significantly different and was only slightly, but mostly not significantly, decreased by abrasion. The protective effect of the fluoride solutions was similar in enamel and dentin.

Conclusion

Tetrafluorides and AmF are able to reduce erosion and erosion plus abrasion in situ and are almost equally effective.     

Electrochemistry of Ni oxidation in molten Li2CO3+Na2CO3 eutectic

The concept of mixed potential was used to explain phenomena observed during Ni oxidation in molten carbonates. The effect of acidity of (Li+Na)₂CO₃ melt on the open-circuit potential characteristic of the electrode consisting of thin layer of Ni on Au was examined. These studies and scanning electron microscopy confirmed that spontaneous oxidation of Ni to NiO in basic molten carbonates proceeds through formation of an intermediate product.