Encapsulating modification of titania/titanium nanotube composites for electrochemical and photochemical applications

Highly-ordered titania (TiO₂) nanotube array with a desired pore diameter and tube length has been fabricated through an anodic oxidation in fluoride-containing electrolyte. The photoelectroactive nanocomposites of nickel oxide-titania/titanium (NiO-TiO2/Ti) and polyoxophosphotungstate-titania/titanium (POW-TiO2/Ti) have been well developed by encapsulating modification of TiO2 nanotubes with electroactive NiO or photoactive POW. The morphological characteristics and microstructure of POW-TiO2/Ti and NiO-TiO2/Ti have been fully examined by field-emission scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, Fourier transform IR spectroscopy and UV-visible spectroscopy. The photoelectrochemical properties are also investigated through polarization photocurrent response and cyclic voltammetry measurement. Regarding NiO-TiO2/Ti nanotube composite, NiO can be feasibly incorporated into TiO2 nanotubes and grow from inner wall to top surface with an open pore mouth. The loading amount of NiO can be enhanced by an electrodeposition-oxidation synthesis rather than a hydrothermal reaction synthesis. Redox supercapacitor application is achieved for the capacitive energy storage. When NiO-TiO₂/Ti composite is tailored from particulate-dispersing to flower-like structure, both redox supercapacitance and energy density are increased, meanwhile keeping high charge-discharge stability due to its highly accessible reaction sites of three-dimensional NiO on TiO₂ nanotubes. Regarding POW-TiO₂/Ti nanotube composite, POW can be well introduced into TiO2 nanotube by a poly(diallyldimethylammonium)-induced hydrothermal synthesis and POW aggregates are accordingly formed on tubule mouth with a uniform filling structure. Photocatalysis and photoelectrocatalysis applications have been also achieved for the degradation of an organic pollutant. The encapsulating configuration of POW-TiO₂/Ti nanotube composite contributes a better chemical stability of POW. The coupling structure between POW and TiO2 promotes the interfacial electron transfer and electron-hole pair separation, which consequently leads to its higher photoactivity than TiO2/Ti nanotubes alone. Higher photodegradation reaction efficiency can be achieved in photoelectrocatalysis rather than photocatalysis of POW-TiO₂/Ti nanocomposite.