Point defects play important and crucial roles in the design of high performance photocatalysts. We report on the electrochemical fabrication of black Ti-Mo-Ni-O nanotubes as a promising electrode material for solar-assisted water splitting. The ternary Ti-Mo-Ni-O catalyst was annealed in hydrogen atmosphere to induce point defects in the material to enhance its conductivity, charge carriers density, and performance. The effect of annealing duration on the performance of ternary Ti-Mo-Ni-O nanotube films was investigated. The hydrogen-annealed nanotubes showed enhanced optical characteristics in the visible spectrum, which can be related to the formation of defect states upon hydrogen annealing. The 10 h-annealed sample showed an exceptionally enhanced photocurrent density of similar to 10 mA/cm(2)with a remarkable open-circuit voltage of similar to-1.0 V(Ag/AgCl)under AM 1.5G illumination. This improved photocurrent is in agreement with the obtained 75 % incident-photon-to-current-conversion-efficiency (IPCE), confirming the improved photoactivity of the hydrogen-treated mixed oxide nanotubes.

In this study, the quantification of decarburization induced during the annealing process for the fabrication of electrical steels was carried out using glow discharge optical emission spectroscopy (GD‐OES). Different calibration methods, based on external and internal standard references, were examined to optimize the quantification of carbon concentration. Accurate calibration curves for carbon at low concentration ranges were achieved by the use of carbon intensity calibrated by the internal reference, i.e. iron intensity line. This methodology was found to be beneficial for long GD‐OES measurements, providing a better correction over changes in the overall emission intensity with the sputter time. The good depth resolution obtained by the GD‐OES technique enabled the identification of specific features in the steel microstructure related to carbide coarseness. Quantitative carbon concentration profiles were obtained by GD‐OES to evaluate the decarburization effect on the microstructure of low‐carbon steels considering different initial microstructures. The effect of the spatial distribution of carbides in these microstructures on the decarburization kinetics was also studied. Through quantitative determination of carbon elemental profiles by GD‐OES, information about the morphology of the cementite in the microstructure and its development in relation to decarburization was acquired. The depth of decarburization can accurately be determined. On the basis of the global results, GD‐OES thus emerged as being a fast and reliable technique for a better understanding of decarburization kinetics. Copyright © 2015 John Wiley & Sons, Ltd.

Water splitting using sunlight is an important process for future energy supplies. TiO2 is widely used as photoanode, but has a limited light absorption range. Here, ternary Ti-Mo-Ni mixed oxide nanotube arrays were fabricated via electrochemical anodization of Ti-Mo-Ni alloy in formamide-ethylene glycol-based electrolytes, to extend the absorption range into visible light. The electrolyte composition and anodization time were found crucial in controlling the structural features of the nanotubes. By tuning these parameters, arrays of thin walled (similar to 9 nm) and similar to 8 mu M long nanotubes were obtained. In photo-electrochemical water splitting, the mixed oxides showed incident photon conversion efficiency (IPCE) up to 65% for wavelengths from 300 nm to 450 nm. This enhancement in the IPCE of the mixed oxide nanotubes, compared with pure titania, can be related to synergistic effects of Mo and Ni oxides as well as to the unique structural properties of the fabricated mixed oxide nanotubes. (C) 2015 Elsevier B.V. All rights reserved.

The electrochemistry of hydrous iridium oxide films (HIROF) is revisited. Cyclic voltammograrns of HIROFs display two reversible redox couples commonly assigned to the Ir(III)/Ir(IV) and Ir(IV)/Ir(V) transitions, respectively. However, compared to the first, the second redox couple has significantly less charge associated with it. This effect is interpreted as partial oxidation of Ir(IV) as limited by nearest neighbor repulsion of resulting Ir(V) sites. Thus, the redox process is divided into two steps: one preceding and one overlapping the oxygen evolution reaction (OER). Here, the "super-nernstian" pH dependence of the redox processes in the HIROF is used to expose how pH controls the overpotential for oxygen evolution, as evidenced by the complementary increased formation of Ir(V) oxide. A recently formulated binuclear mechanism for the OER is employed to illustrate how hydrogen bonding may suppress the OER, thus implicitly favoring Ir(V) oxide formation above the thermodynamic onset potential for the OER at low pH.

Galvanostatic electrodeposition of zinc oxide on gold electrodes was studied. Depositions were made from KCl solutions using oxygen reduction for driving the pH induced precipitation. The main focus was the influence of current density on the morphology and density of the deposit. The size, shape and density of the zinc oxide layer were shown to strongly depend on the current density. With an average rod diameter of 82nm and a rod density of 36μm−2 optimal conditions were found at −1.41mAcm−2. The experimental results are discussed within the context of zinc oxide solubility and the near surface pH. For this purpose a simple model for surface pH is considered, based on concentration profiles of reaction products near an RDE and the speciation of zinc in aqueous solution.

For heterogeneous processes, such as electrochemical reactions taking place at an electrode, the chemical conditions at the reaction interface can differ significantly from the bulk. Consumption of reactants leads to local depletion, whereas product formation results in excess concentration. For reactions involving protons and hydroxide ions, these changes can be described in terms of local pH. In this thesis, a method to measure near surface pH with rotating ring disc electrodes (RRDE) was investigated. In this method, the RRDE is not used in the classical generator collector mode with amperometric detection on the ring, but with the ring as a potentiometric pH sensor. Hydrous iridium oxide films (HIROF) were electrodeposited to serve as thin film pH sensors. The chemistry of the deposition medium used for preparing the HIROFs was studied in detail. The existence of crystalline nanoparticles in the film was proven by transmission electron microscopy. HIROF exhibit a pH response greater than 60 mV, up to 90 mV, depending on the amount of crystalline matter and the redox buffering in the film. Variations in the pH sensitivity were related to the average oxidation state, by measuring titration curves after potential conditioning at different potentials. The pH sensitivity followed the cyclic voltammogram (CV) of HIROF, with two maxima near the two redox couples in the CV. The films did not fully adapt to conditioning potentials more positive than the first redox couple. This phenomenon was discussed within the context of a DFT study on the binuclear mechanism for the oxygen evolution reaction. Combination of experimental observations and results from DFT calculations, revealed that the full oxidation of Ir(IV) to Ir(V) oxide is inhibited and only partial oxidation to Ir(V) is allowed before the start of oxygen evolution. After determining the pH sensing properties of HIROFs, films were deposited on the ring of RRDEs and succesfully used in in situ near surface pH measurements. The ocp of HIROF ring electrode was measured, during water and oxygen reduction on the disc. Next, the ocp values were correlated to pH, using calibration curves measured in standard buffer solutions, to obtain information on the near surface pH changes Hydrogen peroxide, a redox compound, is one of the products of O2 reduction. Redox reactions between the EIROF and such species, could hinder pH measurements. However, for low H2O2 concentrations, a suitable potential conditioning of the HIROF, was shown to subdue this influence. Thus, optimisation for systems under study is possible, facilitating in situ pH measurements even in the presence of electroactive substances in solution. Finally, the growth of ZnO rods was studied, with focus on electrodeposition on gold. Deposition is induced by changing near surface pH with a electrochemical reaction, like oxygen reduction, to induce precipation of ZnO onto the electrode. The current density, related to the local pH, was found to have a profound influence on coverage density and rod dimensions.

The highly dispersed nature of hydrous iridium oxide combined with its electrochemical properties makes it a very interesting material. Possible applications can be found in electrocatalysis, neural stimulation, electrochromic devices and pH sensors. In the present work a commonly used electrodeposition solution, based on IrCl4, oxalic acid, H2O2 and NaCO3, was studied with electrochemical methods as well as UV–vis spectroscopy. The hexachloroiridate (IV) complex was initially observed in both UV–vis and cyclic voltammetry. No oxalato complexes were detected, instead oxalate is proposed to act as a stabilising agent in nanoparticle formation. Initially hydrogen peroxide was found to reduce Ir(IV) complexes to Ir(III). However, after increasing the pH by addition of sodium carbonate it was shown to act as an oxidising agent instead. During development of the solution UV–vis showed the formation of multinuclear complexes and with aging also scattering from solid materials was observed. Transmission electron microscopy confirmed the formation of nanoparticles of iridium oxide with a diameter of ∼3nm. The role of nanoparticles and non-particulate species in the deposition process is discussed.

Many electrochemical reactions taking place in aqueous solution consume or produce protons. The pH in the diffusion layer can therefore be significantly altered during the reaction and there is a need for in situ pH measurements tracing this near surface pH. In the present paper the rotating ring disc technique was used to measure near surface pH changes during oxygen reduction, utilising hydrous iridium oxide as the pH sensing probe. Before such experiments a good understanding of the pH sensing properties of these films is required and the impact of the oxidation state of the film on the pH sensing properties was investigated as well as the influence of solution redox species. The pH sensitivity (depicted by dE/dpH) was found to depend on the average oxidation state of the film in a manner resembling the cyclic voltammetry response. In all cases the pH response is “supernernstian” with more than one proton per electron. The origin of this behaviour is discussed in the context of acid-base properties of the film and the existence of both hydrous and anhydrous oxide phases. The pH response depends also on the redox properties of the solution but can be optimised for various purposes by conditioning the film at different potentials. This was clearly illustrated by adding hydrogen peroxide, an intermediate in the oxygen reduction reaction, to the solution. It was shown that hydrous iridium oxide can be used as a reliable in situ pH sensor provided that care is taken to optimise the oxidation state of the film.

Many chemical reactions are pH dependent and for electrochemical reactions taking place at an electrode surface, changes in the near surface pH can be decisive for their outcome. Near surface pH changes have successfully been utilised for formation of oxide films and to control the shape and morphology of the deposit. The mechanistic insight into such processes is hampered by the difficulty to measure the local pH in situ. In the present paper the rotating ring disc electrode (RRDE) configuration is used to measure the near surface pH for two model reactions, hydrogen evolution and oxygen reduction. Iridium oxide is electrodeposited on the titanium ring and used as the pH sensing material. At low current densities and therefore low hydroxide ion concentrations, the pH response is rather slow, limiting the applicability of potentiodynamic sweep experiments under such conditions. At higher current densities a linear relationship between the logarithm of the current and pH is found. Tracking of small pH changes can be made by step experiments where the response is measured as a function of time and steady state conditions can be assured. Key issues for successful use of the RRDE configuration with iridium oxide as the pH sensing material are pre-conditioning of the ring electrode to obtain well defined redox properties of the film and choice of ring substrate onto which the iridium oxide is deposited. (C) 2012 Elsevier B.V. All rights reserved.

A wet chemical route was used to prepare the oxygen deficient codoped perovskite oxide BaZr0.5In0.25Yb0.25O3-delta. Analysis of X-ray powder diffraction data showed that the sample belongs to the cubic crystal system with space group Pm (3) over barm. Dynamic thermogravimetric (TG) analysis confirmed complete filling of oxygen vacancies (V ''(O)) by protonic defects (OH'(O)) during the hydration process. The proton conductivity was investigated by impedance spectroscopy. The bulk and total conductivities of prehydrated BaZr0.5In0.25Yb0.25O3-delta were found to be 8.5 x 10(-4) and 2.2 x 10(-5) S cm(-1), respectively, at 300 degrees C. The total conductivity in the codoped perovskite oxide was higher compared to that of the respective single doped perovskite oxides with the same doping level. The bulk and grain-boundary mobility and diffusion coefficients of protons were calculated at 200 degrees C using impedance and TG data to obtain the conductivity and proton concentration, respectively. The high bulk diffusivity (2.3 x 10(-7) cm(2) s(-1)) was obtained which indicates that the protons are more free to move in the heavily doped matrix compared to the lightly doped systems where trapping of protons occurs. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3494119] All rights reserved.

BaZr0.9In0.05M0.05O3−δ (M = Ga3+ or Yb3+) has been prepared by solid-state synthesis route. Rietveld analysis of neutron powder diffraction data on as-prepared samples showed that both samples crystallise in the cubic space group Pm3¯m. Scanning electron microscopy analysis showed that the grains in the BaZr0.9In0.05Ga0.05O3−δ sample were larger than the BaZr0.9In0.05Yb0.05O3−δ sample. Dynamic thermogravimetric analysis indicates that the proton concentration was higher in pre-hydrated BaZr0.9In0.05Ga0.05O3−δ sample (40% of theoretical) than that of BaZr0.9In0.05Yb0.05O3−δ sample (28% of theoretical). Proton conductivity was studied on pre-hydrated samples (under both dry and wet Ar atmospheres). The bulk proton conductivities of the heating cycle of pre-hydrated BaZr0.9In0.05M0.05O3−δ (M = Ga3+ or Yb3+) samples were 2.1 × 10−5 S cm−1 and 1.9 × 10−4 S cm−1 at 350°C, respectively. The effect of co-doping on proton conductivity was investigated and the results are compared with single doped systems e.g. BaZr0.9M0.1O3−δ (M = Ga3+, In3+ or Yb3+) samples.

Spark-plasma sintering method was used to prepare dense proton conducting perovskite oxide BaZr0.5In0.5O3-delta. Analysis of X-ray powder diffraction data showed that the sample adopt the cubic crystal structure having the space group Pm3m. Thermogravimetric analysis of prehydrated samples showed significant mass losses beyond 300 degrees C due to loss of protons as water vapor. Scanning electron microscope images show that the grain size of the spark-plasma sintered dense sample was smaller than that of solid-state sintered porous sample. The highest total proton conductivity (2 X 10(-3) S cm(-1) at 450 degrees C) was found for dense spark-plasma sintered sample under wet H-2 than the samples prepared by other routes. (C) 2010 The Electrochemical Society. [D01: 10.1149/1.3482016] All rights reserved.