Characterization of white light emitting diodes based ZnO nano structures grown on p-Si

In this paper ZnO nanorods and nanodots (with and without a SiO2 buffer layer) were grown on p-Si, forming p-n heterojunctions. The nanorods devices showed no electroluminescence (EL) emission but a rectifying behavior with a breakdown voltage around -4V. The nanodot devices showed EL emission under forward bias conditions. The buffer layer increased both the stability and efficiency of the devices. With the buffer layer EL emission was also observed under reverse bias

Hydrous iridium oxide for in-situ pH sensing: electrodeposition, properties and applications.

For heterogeneous processes, such as electrochemical reactions taking place at an elec- trode, 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, depend- ing 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

Influence of oxidation state on the pH dependence of hydrous iridium oxide films

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. (C) 2012 Elsevier Ltd. All rights reserved

In situ pH measurements with hydrous iridium oxide in a rotating ring disc configuration

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

Deposition of ZnO rods by electrochemically induced Hydrolysis

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 82 nm and a rod density of 36 μm−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. Forthis 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 aqueoussolution

Encapsulation of emulsion droplets by organo–silica shells

Surfactant-stabilized emulsion droplets were used as templates for the synthesis of hollow colloidal particles. Monodisperse silicone oil droplets were prepared by hydrolysis and polymerization of dimethyldiethoxysiloxane monomer, in the presence of surfactant: sodium dodecyl sulphate (SDS, anionic) or Triton X-100 (non-ionic). A sharp decrease in the average droplet radius with increasing surfactant concentration was found, with a linear dependence of the droplet radius on the logarithm of the surfactant concentration. The surfactant-stabilized oil droplets were then encapsulated with a solid shell using tetraethoxysilane, and hollow particles were obtained by exchange of the liquid core. The size and polydispersity of the oil droplets and the thickness of the shell were determined using static light scattering, and hollow particles were characterized by electron microscopy. Details on the composition of the shell material were obtained from energy-dispersive X-ray analysis. In the case of sodium dodecyl sulphate, the resulting shells were relatively thin and rough, while when Triton X-100 was used, smooth shells were obtained which could be varied in thickness from very thick (≈150 nm) to very thin shells (≈17 nm). Finally, hexane droplets were encapsulated using the same procedure, showing that our method can in principle be extended to a wide range of emulsions

Revisiting the Redox Properties of Hydrous Iridium Oxide Films in the Context of Oxygen Evolution

The electrochemistry of hydrous iridium oxide films (HIROF) is revisited. Cyclic voltammograms 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

Proton Conductivity in Mixed B-Site Doped Perovskite Oxide BaZr[sub 0.5]In[sub 0.25]Yb[sub 0.25]O[sub 3 - delta]

A wet chemical route was used to prepare the oxygen deficient codoped perovskite oxide BaZr0.5In0.25Yb0.25O3−. Analysis of X-ray powder diffraction data showed that the sample belongs to the cubic crystal system with space group Pmm. Dynamic thermogravimetric (TG) analysis confirmed complete filling of oxygen vacancies (V) by protonic defects (OH) during the hydration process. The proton conductivity was investigated by impedance spectroscopy. The bulk and total conductivities of prehydrated BaZr0.5In0.25Yb0.25O3− were found to be 8.5×10−4 and 2.2×10−5 S cm−1, respectively, at 300°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°C using impedance and TG data to obtain the conductivity and proton concentration, respectively. The high bulk diffusivity (2.3×10−7 cm2 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

Influence of electrolyte composition on the formation of mixed oxide nanotube arrays for solar fuel production

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 (∼9 nm) and ∼8 μm long nanotubes were obtained. In photoelectrochemical 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.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Electrochemical Fabrication of Ternary Black Ti‐Mo‐Ni Oxide Nanotube Arrays for Enhanced Photoelectrochemical Water Oxidation

© 2020 Wiley-VCH GmbH 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 ∼10 mA/cm2 with a remarkable open-circuit voltage of ∼−1.0 VAg/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