This perspective reviews recent advances in inverse opal structures, how they have been developed, studied and applied as catalysts, catalyst support materials, as electrode materials for batteries, water splitting applications, solar-to-fuel conversion and electrochromics, and finally as photonic photocatalysts and photoelectrocatalysts. Throughout, we detail some of the salient optical characteristics that underpin recent results and form the basis for light-matter interactions that span electrochemical energy conversion systems as well as photocatalytic systems. Strategies for using 2D as well as 3D structures, ordered macroporous materials such as inverse opals are summarized and recent work on plasmonic–photonic coupling in metal nanoparticle-infiltrated wide band gap inverse opals for enhanced photoelectrochemistry are provided.
Devices composed of transparent materials, particularly those utilizing metal oxides, are of significant interest due to increased demand from industry for higher fidelity transparent thin film transistors, photovoltaics and a myriad of other optoelectronic devices and optics that require more cost-effective and simplified processing techniques for functional oxides and coatings. Here, we report a facile solution processed technique for the formation of a transparent thin film through an inter-diffusion process involving substrate dopant species at a range of low annealing temperatures compatible with processing conditions required by many state-of-the-art devices. The inter-diffusion process facilitates the movement of Si, Na and O species from the substrate into the as-deposited vanadium oxide thin film forming a composite fully transparent V0.0352O0.547Si0.4078Na0.01. Thin film X-ray diffraction and Raman scattering spectroscopy show the crystalline component of the structure to be α-NaVO3 within a glassy matrix. This optical coating exhibits high broadband transparency, exceeding 90-97% absolute transmission across the UV-to-NIR spectral range, while having low roughness and free of surface defects and pinholes. The production of transparent films for advanced optoelectronic devices, optical coatings, and low- or high-k oxides is important for planar or complex shaped optics or surfaces. It provides opportunities for doping metal oxides to ternary, quaternary or other mixed metal oxides on glass, encapsulants or other substrates that facilitate diffusional movement of dopant species.
Uniform thin films of vanadium pentoxide were dip-coated from a high-concentration vanadium oxytriisopropoxide precursor which is shown to be resistant to the dewetting processes which can form surface pinhole defects. Through appropriate withdrawal speed choice, the thin films have a smooth uniform surface morphology with a low rms roughness of <1 nm in both their amorphous and crystallized states. The structure of the thin films follows that of bulk vanadium pentoxide but in a nanostructured form. The deposition methods shown can be applied to prepare thin films upon a variety of different substrates and other alkoxide based metal oxide materials.
We report on the electrodeposition of 3D macroporous vanadium oxide inverse opals and binary inverse opals on transparent conducting oxide substrates and stainless steel and thermally oxidized stainless steel substrates. The electrodeposition follows a diffusion limited growth mode to form 3D porous crystalline V 2 O 5 after removal of a colloid photonic crystal template of selfassembled polystyrene spheres. Inverse opals were grown using spheres ranging in diameter from 0.5 μm to 6 μm, and binary inverse opals were also electrodeposited using binary mixtures of sphere sizes. We demonstrate that the ionic diffusion that leads to growth has charge-to-mass Coulombic efficiency ranging from 60-90%, depending on the voltage used. Additionally, the tortuosity in ionic diffusion through the opal to the substrate is significantly increased when large sphere diameter templates and binary opal templates are used. Analysis of the contribution of true substrate active area and the influence of template structure on ionic diffusivity confirms that inverse opal growth is dictated by the size of opal spheres, interstitial void clogging by smaller spheres in binary opals, and the conductivity of the substrate active area. which benefit from the large active surface-area to volume ratio and the ability of IOs to be fashioned with porosity that enhances the capture and waveguiding of light at certain energies at various angles of incidence, because IOs exhibit a pseudo photonic bandgap structure. 32Vanadium pentoxide (V 2 O 5 ) has been the subject of much research for over 40 years 33 and has been widely investigated as a cathode material for Li-ion batteries due to its high theoretical specific capacity, according to the following intercalation reaction: V 2 O 5 + xLi34,35 V 2 O 5 is useful for reversible Li-ion insertion and removal due to its mixed valance and layered structure. 25,31The mixed valence (V 4+ and V 5+ ) of V 2 O 5 allows for material expansion during intercalation with more electrolyte accessing the increased surface area, also helping decrease structural deformation of the material. 25,36 To date, a wide range of nanostructured V 2 O 5 mate- * Electrochemical Society Member. z E-mail: c.odwyer@ucc.ie rials (including nanowires, nanorods, etc. 34,37 ) have been examined as Li-ion cathode materials. In particular, 3D V 2 O 5 IO structures have shown extremely promising results in both and half-cell and full-cell configurations, prompting further research into mechanism of formation and control over their structure, geometry, crystallinity, size and composition/valence. 38 V 2 O 5 IOs have been formed using various routes including simple dropcast methods and more controlled electrodeposition (ED) approaches.7,25,32 ED of V 2 O 5 IOs has been achieved using a VOSO 4 aqueous solution (with subsequent heating allowing a transformation to crystalline V 2 O 5 ) producing dense material in the sphere template voids. Aside from the many alternative infiltration methods, creating binary inverse opals from binary opal t...
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