Morphology and Optical Properties of Highly Ordered TiO₂ Nanotubes Grown in NH₄F/o-H₃PO₄ Electrolytes in View of Light-Harvesting and Catalytic Applications

Abstract

Highly ordered titanium dioxide nanotube (TiO2 NT) arrays were grown by electrochemical anodization of titanium in a molten NH4F/o-H3PO4 electrolyte. NTs with various diameters, lengths, wall thicknesses, and intertube distances could be obtained by tuning key anodization parameters such as the applied potential, anodization time, electrolyte temperature, concentration of NH4F, and H2O content. The morphology and optical properties were characterized by scanning electron microscopy and UV–vis spectroscopy techniques. We show that all aforementioned parameters have a strong influence on the nanostructured morphology and optical characteristics (reflectivity) of the formed nanotubular layers. Their optical features were simulated numerically to support the experimental measurements. We show that the optical features of anodic TiO2 nanotube layers result from the overlay of the individual optical properties of various “structural elements”, e.g., the NT barrier layer, top opening morphology, intertube spacing, and thermally formed oxide sublayer. Our results provide tools for “a priori” design with nanoscale precision of TiO2 structures with advanced optical features for light-harvesting and catalytic applications, e.g., in sensing, photocatalytic self-cleaning processes, solar hydrogen generation, or photovoltaics.

A One‐Pot Universal Approach to Fabricate Lubricant‐Infused Slippery Surfaces on Solid Substrates

“…The PDMS grafting procedure can be applied virtually to any material. Here, we examined the following substrates: i) plain Si, and ii) Si wafer covered by 100‐nm‐thick SiO 2 layer, iii) 1‐mm‐thick standard microscope glass slides, 0.4‐mm‐thick borosilicate glass coverslips, and fluorine‐doped tin oxide (FTO) conductive glass, and quartz, iv) bare Co, v) Mo foils, vi) polished Al, and Cu, vii) vertically aligned highly ordered anodized TiO 2 nanotubular arrays (TiO 2 NTs), [ 30 ] and viii) anodized 316L grade stainless steel with sponge‐like nanoporous structure (Figure S2, Supporting Information). The WCA measured on flat and rough substrates was hydrophilic for all plain samples (Figure 1e,f).…”

“…After the second‐step anodization, the samples were extensively rinsed with a 20:80 (v/v) water–ethanol mixture and then kept in ethanol until annealed in air at 400 °C for 1 h. Ti foil was anodized according to our previously published procedure. [ 30 ] Briefly, the Ti foil was anodized in a molten o‐H 3 PO 4 electrolyte containing 3 m of NH 4 F for 120 min at an applied potential of 15 V and 100 °C. The “as‐anodized” samples were immersed in DI water for 12 h to remove residual electrolytes and finally dried under a stream of N 2 .…”

Anodic TiO2 Nanotubes: Tailoring Osteoinduction via Drug Delivery

“…Very ordered, regular, and highly-uniform TiO 2 nanotubes (initially referred to as nanostumps) can be obtained in concentrated H 3 PO 4 /NH 4 F electrolytes at high temperatures (100 °C) [ 64 , 65 ], while the intertube distance decreases by increasing the temperature in the 75–140 °C range [ 56 ]—such a nanotubular structure is shown in Figure 3 e. More interestingly, a nanotubular morphology with a two-scale organization and a distinct space in between each nanotube from the top to the bottom of the tube length can be obtained in certain organic electrolytes, such as diethylene glycol (DEG) [ 57 , 66 ], triethylene glycol [ 67 ], or dimethyl sulfoxide [ 66 , 68 ]. As shown in Figure 3 f, such nanostructures (e.g., grown in DEG-based electrolytes) are highly uniform and provide additional functionality due to the spaced morphology.…”

Optical Properties of Porous Alumina Assisted Niobia Nanostructured Films–Designing 2-D Photonic Crystals Based on Hexagonally Arranged Nanocolumns

“…Features of electrochemical behavior of valve metals (Al, Nb, Ta, Ti, etc. ), micro and nanodispersed, nanostructured oxides, including porous and tubular anodic oxides and composite materials based on them, are the objects of numerous studies [ 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 ] and review publications [ 11 , 12 , 18 , 52 , 53 , 54 , 55 , 56 , 57 , 58 ]. Particular interest in such materials is due to the fact that they have a quasi-regular structure, which is formed in the process of production as a result of self-organization processes [ 59 ].…”