Iridium-based oxides: Recent advances in coloration mechanism, structural and morphological characterization

Films of iridium–tantalum oxide and iridium oxide have been prepared by sputtering and studied regarding their structure and electrochemical properties. X-ray diffraction and transmission electron microscopy showed an average grain size of 3–4 nm for both films. Point energy dispersive X-ray spectrometry showed an inhomogeneous distribution of iridium and tantalum indicating that the iridium–tantalum oxide may be a mixture of small IrO₂ and Ta₂O₅ grains, which is consistent with the determined composition IrTa_1.4O_5.6. X-ray photoelectron spectroscopy gave valuable information on the stabilization process of the as-deposited films involving an uptake of oxygen, and on a coloration mechanism only including protons.     

Promotion of electrochromism in spray-deposited molybdenum oxide-doped iridium oxide thin films

Electrochromic molybdenum oxide-doped iridium oxide thin films were prepared by using a pneumatic spray pyrolysis technique onto fluorine-doped tin oxide (FTO) coated conducting glass substrates. An aqueous solution of 0.01 M ammonium molybdate was mixed with 0.01 M iridium trichloride in different volume proportions and the resultant solution was used as a precursor for spraying. An aqueous electrolyte (0.5 N H₂SO₄) was used to study electrochromic properties of thin films using cyclic voltammetry (CV), chronoamperometry (CA) and spectrophotometry. During the potential scan the iridium oxide electrode switches between coloured and bleached state due to Ir⁺⁴–Ir⁺³ intervalency charge transfers. The optical density difference (ΔOD)_λ=630 nm and colouration efficiency was maximum for 2% molybdenum oxide-doped sample. Moreover, loss in charge density during extended cycling is less than undoped and other doped (>2%) samples.     

Effects of tungsten oxide addition on the electrochemical performance of nanoscale tantalum oxide-based electrocatalysts for proton exchange membrane (PEM) fuel cells

In the present study, the properties of non-platinum based nanoscale tantalum oxide/tungsten oxide-carbon composite catalysts were investigated for potential use in catalyzing the oxygen reduction reaction on the cathode side of a PEM fuel cell. All of the tantalum oxide-based catalysts exhibit high ORR on-set potentials, comparable with the commercial Pt/C catalyst even though oxygen reduction current was limited. The tungsten oxide doping to tantalum oxide improved catalytic performance. The performance enhancement was due to a decrease in resistance polarization with increasing tungsten content mainly due to the decrease in resistance polarization. XPS results indicate that the oxidation state of tungsten is +6 and that of the tantalum is +5, suggesting that excess oxygen is generated in the resulting oxide structure. This compositional effect seems to reduce resistance polarization by altering the surface chemistry of the tantalum oxide and enhancing the reaction steps such as surface diffusion. Maximum performance was achieved with a catalyst containing 32 mol% of tungsten oxide, reaching a mass specific current density of ∼7% that of the commercial Pt/C catalyst at 0.6 V vs. NHE and ∼35% at 0.2 V vs. NHE. In term of area-specific current density, five-fold increase in loading of the doped catalyst leads to a 4-4.5 fold increase in area specific current density at 0.6 V vs. NHE, reaching 66% that of the Pt/C catalyst at 100 rpm and 35% at 2400 rpm.     

Optical properties of electrochromic iridium oxide and iridium-tantalum oxide thin films in different colouration states

Electrochromic iridium oxide (IrOx) and iridium-tantalum oxide (IrTaOx) thin films were prepared by sputtering. Complex refractive indices were determined for samples deposited on indium-tin oxide covered glass in different colouration states, and for as-deposited samples on sapphire and Corning glass. The refractive index was found to be practically constant for both IrOx (∼1.3) and IrTaOx (∼2). The extinction coefficient was found to vary between the coloured and bleached states with ∼35% for IrOx and ∼55% for IrTaOx at 660 nm. This is believed to be a result of the removal of intraband transitions within the Ir t_2g band during bleaching.     

Electrocatalytic activity of iridium oxide nanoparticles coated on carbon for oxygen reduction as cathode catalyst in polymer electrolyte fuel cell

The iridium oxide nanoparticles supported on Vulcan XC-72 porous carbon were prepared for cathode catalyst in polymer electrolyte fuel cell (PEFC). The catalyst has been characterized by transmission electron microscopy (TEM) and in PEFC tests. The iridium oxide nanoparticles, which were uniformly dispersed on carbon surface, were 2-3 nm in diameter. With respect to the oxygen reduction reaction (ORR) activity was also studied by cyclic voltammetry (CV), revealing an onset potential of about 0.6 V vs. an Ag/AgCl electrode. The ORR catalytic activity of this catalyst was also tested in a hydrogen-oxygen single PEFC and a power density of 20 mW cm⁻² has been achieved at the current density of 68.5 mA cm⁻². This study concludes that carbon-supported iridium oxide nanoparticles have potential to be used as cathode catalyst in PEFC.     

A novel non-platinum group electrocatalyst for PEM fuel cell application

Precious-metal catalysts (predominantly Pt or Pt-based alloys supported on carbon) have traditionally been used to catalyze the electrode reactions in polymer electrolyte membrane (PEM) fuel cells. However as PEM fuel systems begin to approach commercial reality, there is an impending need to replace Pt with a lower cost alternative. The present study investigates the performance of a carbon-supported tantalum oxide material as a potential oxygen reduction reaction (ORR) catalyst for use on the cathode side of the PEM fuel cell membrane electrode assembly. Although bulk tantalum oxide tends to exhibit poor electrochemical performance due to limited electrical conductivity, it displays a high oxygen reduction potential; one that is comparable to Pt. Analysis of the Pourbaix electrochemical equilibrium database also indicates that tantalum oxide (Ta₂O₅) is chemically stable under the pH and applied potential conditions to which the cathode catalyst is typically exposed during stack operation. Nanoscale tantalum oxide catalysts were fabricated using two approaches, by reactive oxidation sputtering and by direct chemical synthesis, each carried out on a carbon support material. Nanoscale tantalum oxide particles measuring approximately 6 nm in size that were sputtered onto carbon paper exhibited a mass-specific current density as high as one-third that of Pt when measured at 0.6 V vs. NHE. However, because of the two-dimensional nature of this particle-on-paper structure, which limits the overall length of the triple-phase boundary junctions where the oxide, carbon paper, and aqueous electrolyte meet, the corresponding area-specific current density was quite low. The second synthesis approach yielded a more extended, three-dimensional structure via chemical deposition of nanoscale tantalum oxide particles on carbon powder. These catalysts exhibited a high ORR onset potential, comparable to that of Pt, and displayed a significant improvement in the area-specific current density. Overall, the highest mass-specific current density of the carbon-powder supported catalyst was ˜9% of that of Pt.     

Tantalum-tungsten oxide thermite composites prepared by sol-gel synthesis and spark plasma sintering

Energetic composite powders consisting of sol-gel derived nanostructured tungsten oxide were produced with various amounts of micrometer-scale tantalum fuel metal. Such energetic composite powders were ignition-tested and the results show that the powders are not sensitive to friction, spark and/or impact ignition. Initial consolidation experiments, using the High-Pressure Spark Plasma Sintering (HPSPS) technique, on the sol-gel derived nanostructured tungsten oxide produced samples with higher relative density than can be achieved with commercially available tungsten oxide. The sol-gel derived nanostructured tungsten oxide with immobilized tantalum fuel metal (Ta-WO₃) energetic composite was consolidated to a density of 9.17 g cm⁻³ or 93% relative density. In addition, those samples were consolidated without significant pre-reaction of the constituents, thus retaining their stored chemical energy.     

Synthesis and interlayer modification of RbLaTa2O7

A layered semiconductor, lanthanum tantalum oxide was prepared by solid reaction at high temperature, and the processes for the modification of the interlayers by protonation, intercalation and pillaring were investigated. n-Butylamine could easily be intercalated into the interlayers of HLaTa₂O₇ to significantly enhance the interlayer distance, which facilitated the exchange of cation with n-butylamine. Finally, CdO pillars in the interlayer of lanthanum tantalum oxide were formed via calcinating at 500°C in air.     

Optical properties of dense zirconium and tantalum diborides for solar thermal absorbers

Ultra-high temperature ceramics (UHTCs) are interesting materials for a large variety of applications under extreme conditions. This paper reports on the production and extensive characterization of highly dense, pure zirconium and tantalum diborides, with particular interest to their potential utilization in the thermal solar energy field. Monolithic bulk samples are produced by Spark Plasma Sintering starting from elemental reactants or using metal diboride powders previously synthesized by Self-propagating High-temperature Synthesis (SHS). Microstructural and optical properties of products obtained by the two processing methods have been comparatively evaluated. We found that pure diborides show a good spectral selectivity, which is an appealing characteristic for solar absorber applications. No, or very small, differences in the optical properties have been evidenced when the two investigated processes adopted for the fabrication of dense TaB₂ and ZrB₂, respectively, are compared.     

Polypyrrole assisted synthesis of nanosized iridium oxide for oxygen evolution reaction in acidic medium

The application of electrochemical water splitting process in acidic medium is restricted by the lack of highly efficient and stable oxygen evolution reaction (OER) electrocatalyst. In this work, we report a facile soft template method to synthesize nanosized iridium oxide for electrocatalytic OER in acidic medium. The fabrication process involves thermal treatment of iridium complex and polypyrrole in air. The compositions and structures of the resulting catalytic materials are significantly influenced by the annealing temperature. The nanostructured iridium oxide synthesized at optimal 450 °C exhibits low overpotential (291.3 ± 6 mV) to reach 10 mA/cm² current density towards OER, which is better than the commercial iridium oxide. Further investigation indicates that nanosized iridium oxide synthesized at 450 °C has high electrochemical active surface area to expose abundant accessible active sites, which can accelerate the OER rate. This method can also provide new guidance to prepare other metal oxide nanoparticles for various applications.     

Preparation and characterisation of SOFC anodic materials based on Ce–Cu

Ce–Cu mixed oxide precursors with varing Ce:Cu atomic ratio have been prepared by freeze-drying and microemulsion coprecipitation methods. Nanostructured particles having different properties have been obtained. Physicochemical properties have been studied with X-ray diffraction, UV–vis spectroscopy, nitrogen adsorption–desorption, mercury intrusion porosimetry, ICP-AES, conductivity measurement and thermal expansion coefficient. All samples show fluorite structure with slight copper surface enrichment for samples having high copper content. Microemulsion method allows the introduction of a large quantity of copper into the cerium oxide structure, obtaining a nanostructured mixed oxide of high surface area. On the other hand, freeze-drying samples does not show evidence of copper incorporation to the lattice of cerium oxide. All materials have a thermal expansion coefficient similar to other components of SOFC.     

N/C doped nano-size IrO2 catalyst of high activity and stability in proton exchange membrane water electrolysis

It is highly desirable to synthesize and deploy low-cost and highly efficient catalysts for the oxygen evolution reaction (OER) to catalyze water splitting. We show that N/C doped amorphous iridium oxide combines the benefits of nano-size (approximately 2 nm), which results in exposure to large active surface areas and features of oxygen defects, which make for an electronic structure suitable for the OER. Systematic studies indicate that the OER activity of the iridium oxide catalyst is accelerated by the effect of the structure and chemical state of the iridium element. Remarkably, the N/C doped amorphous iridium oxide catalyst shows a lower cell voltage of 1.774 V at 1.5 A cm⁻², compared with IrO₂ (1.847 V at 1.5 A cm⁻²), and it can maintain such a high current density for over 200 h without noticeable performance deterioration. This work provides a promising method for the improving OER electrocatalysts and the construction of an efficient and stable PEM water cracking system.     

Effect of TiO2 mixtures on the optical, structural and electrochromic properties of Nb2O5 thin films

Metal oxide films are important for various optical devices and especially for solar energy materials. TiO₂-mixed Nb₂O₅ thin films have been produced by sol–gel dip-coating method. Several parameters such as heat treatment, thickness, and mixture percentages are studied for the effect of the optical, structural and electrochromic properties of the materials. Optical parameters of the films were calculated through transmission and reflection measurement by a refractive index, extinction coefficient and thickness analyzer. Structural, electrochromic and surface analyses of the films were done by X-ray diffractometer, potentiostat/galvanostat and atomic force microscope systems.     

Structure and optical properties of electrochromic copper oxide films prepared by reactive and conventional evaporation techniques

Copper oxide films (Cu_xO) are deposited by thermal evaporation techniques using copper oxide (CuO) or copper (Cu) as starting material. By varying the deposition parameters, two main types of Cu_xO film exhibiting different optical properties form. These are reddish gray and colorless films. The samples are characterised optically and morphologically. X-ray diffraction spectra reveal that evaporated Cu_xO films are amorphous. Fourier-transform infrared spectra of the samples were studied to evaluate chemical identification. The refractive index, the extinction coefficient and the thickness of the films are evluated from transmittance characteristics in the ultraviolet, visible and near-infrared regions. The refractive indices of the samples are between 2.9 and 3.1. The values determined for the optical constants are in aggreement with the results found in the literature. We report for the first time that Cu_xO films show reversible optical switching from the colored to bleached state. Optical transmittance measurements of the copper oxide film relative to indium tin oxide coated glass varied during coloring from spectral transmittance T_s = 85−40%     

Optical properties of sol–gel dip-coated Ta2O5 films for electrochromic applications

Amorphous Ta₂O₅ films were prepared by sol–gel dip process on different substrates. The dip-coating technique was used to prepare amorphous Ta₂O₅ films by hydrolysis and condensation of tantalum ethoxide, Ta(OC₂H₅)₅, precursor. Stable coating solutions were prepared using acetic acid as a chelating ligand and catalyzer. Single layer and multi-layered Ta₂O₅ films were fabricated at a dipping rate of 107 mm/min. The microstructure, stoichiometry and optical properties of these films were investigated as a function of the film thickness. Room temperature CV measurements clearly revealed a protonic conductor behavior for Ta₂O₅ films. Optical properties such as refractive index, extinction coefficient and optical band gap value of the Ta₂O₅ films were calculated from optical transmittance measurements. It was found that the refractive index and extinction coefficient values were affected by the thickness of the coatings. The refractive index at a wavelength of 550 nm increased from 1.70 to 1.72 with increasing film thickness. The optical band gap value (3.75±0.12 eV) of the coating was unaffected by the film thickness. These results indicate that sol–gel-deposited Ta₂O₅ films have a promising application as proton conductors in electrochromic devices.     

Pulsed electrodeposition of CdTe thin films: effect of pulse parameters over structure, stoichiometry and optical absorption

CdTe thin films were electrochemically deposited using unipolar current pulses of high magnitude between 2.5 and 30 mA/cm² in an aqueous solution. Parametric study of the effect of periodic current pulse magnitude, average current and ON and OFF duration was undertaken to understand the effect of pulse variables on CdTe film properties. Increasing pulse deposition current modifies crystalline growth phase from single cubic to mixed cubic and hexagonal growth phases. In addition to the modification in CdTe growth phases, there is an increasing tendency of the oxide formation particularly CdTeO₃. Increase in pulse current density or average current yields Cd rich CdTe films. The optical absorption coefficient decreases with the decrease in pulse current density, whereas an increase is observed as the OFF time decreases. The optical energy gap is found to increase with OFF time. A systematic study on the effect of pulse variables over the structure, compositional and optical properties of CdTe film is described.     

Recent advances in electrochromics for smart windows applications

Electrochromic smart windows are able to vary their throughput of radiant energy by low-voltage electrical pulses. This function is caused by reversible shuttling of electrons and charge balancing ions between an electrochromic thin film and a transparent counter electrode. The ion transport takes place via a solid electrolyte. Charge transport is evoked by a voltage applied between transparent electrical conductors surrounding the electrochromic film/electrolyte/counter electrode stack. This review summarizes recent progress concerning: (i) calculated optical properties of crystalline WO₃, (ii) electrochromic properties of heavily disordered W oxide and oxyfluoride films produced by reactive magnetron bias sputtering, (iii) novel transparent reactively sputter-deposited Zr–Ce oxide counter electrodes and (iv) a new proton-conducting antimonic-acid-based polymer electrolyte. Special in depth presentations are given on elastic light scattering from W-oxide-based films and of electronic band structure effects affecting opto–chronopotentiometry data in Zr–Ce oxide. The review also contains some new device data for an electrochromic smart window capable of very high optical transmittance.     

Photoelectrochemical water splitting with 600 keV N2+ ion irradiated BiVO4 and BiVO4/Au photoanodes

Low energy N²⁺ ion beam with 600 keV energy has been used to irradiate BiVO₄ and Au nanoparticles loaded BiVO₄ (BiVO₄/Au) thin films deposited over fluorine doped tin oxide substrates via spray pyrolysis technique. Ion irradiation results in tailoring the optical, electrical, and morphological properties of the thin films and thence also responsible for changes in electrochemical properties. The scanning electron microscope images reveal the evolution of Au nanoparticles after irradiation at 2 × 10¹⁵ fluence to a nanourchins type of morphology. In consequence of morphological changes, the signature of surface plasmon resonance peak exhibited by Au nanoparticles in BiVO₄/Au shows improvement. An increase of approximately 92% in photocurrent density in comparison to pristine BiVO₄ has been found after irradiation in BiVO₄/Au photoanode at 2 × 10¹⁵ ions/cm² fluence. Moreover, irradiation also aids in improving photoelectrochemical response of BiVO₄ photoanodes without Au nanoparticles. The enhancement can be attributed to the notable changes in onset potential, charge separation, charge transfer resistance and optical properties.     

Synthesis of cobalt–silicon oxide thin films

Silicon–cobalt oxide thin films were prepared by the dipping sol–gel process. Samples with different number of dipping–annealing cycles were prepared. Some data regarding the precursor sol are given from small angle X-ray scattering characterization. Composition, structure, surface morphology and optical properties are obtained from X-ray diffraction, reflectance, transmittance, FTIR, scanning electron microscopy and EDX spectroscopy measurements. The silicon–cobalt oxide thin films prepared in this work are mostly amorphous. They have a high absorption coefficient in the visible and infrared regions. A refractive index from 2.15 to 1.79 (at 1200 nm wavelength), and a band gap between 3.73 and 3.68 eV with increasing film thickness were measured in the films. Sol–gel prepared Si–Co oxide thin films could be well suited for use in photothermal solar collectors.     

Studies on anodic oxide coating with low absorptance and high emittance on aluminum alloy 2024

Anodization of AA 2024 in sulfuric acid bath containing glycerol, lactic acid and ammonium metavenadate has been studied to develop white anodic oxide coating. Investigation on the influence of various operating parameters — coating thickness, current density and ammonium metavenadate concentration on the optical properties was carried out to optimize the process. Infrared, atomic absorption spectroscopic techniques and scanning electron micrograph were used to characterize the coating. The obtained oxide coating provides a ratio of solar absorptance (α) to infrared emittance (), as low as 0.2. The optical properties and hardness values measured under optimum experimental conditions support its use as a thermal control coating.