The effect of anatase crystal orientation on the photoelectrochemical performance of anodic TiO2 nanotubes

Comparison of the heat-treatment effect on carrier dynamics in TiO2 thin films deposited by different methods

Polycrystalline titanium dioxide thin films are routinely used in a broad range of applications where charge carrier lifetime is essential for their performance but the effects of the fabrication method are rarely considered. Here we compare three popular deposition methods, atomic layer deposition (ALD), ion beam sputtering (IBS), and spray pyrolysis deposition (SPD). In all three cases, 30 nm thin films of TiO₂ are prepared, and the as-deposited films show no defined crystal structure and can be classified as amorphous films. Heat treatment (HT) of the films converts all of them to polycrystalline anatase TiO₂ as revealed by XRD measurements. A photophysical study was carried out by pico- to nano-second transient absorption pump-probe spectroscopy in transmittance and reflectance modes which allows taking into account the effects due to the photoinduced refractive index changes. This study shows that the HT increases the lifetime of the photo-carriers gradually to a nanosecond time domain (approx. 4 ns) as compared to a few picoseconds of the as-deposited samples. The photo-carrier dynamics of the samples become very similar after heat-treatment, though the topographical features and texture of the films observed with AFM and XRD are quite different. The measured transient absorption spectra of the samples also indicate that the photo-carrier relaxation pathway involves electron and hole trap states with the longest-lived being the hole traps. To evaluate the photoactivity of thin films, methylene blue (MB) photodegradation was tested for all the as-deposited and HT samples and the results showed a 20% higher degradation rate for the IBS HT sample due to the more textured surface.

Preferentially oriented TiO2 nanotube arrays on non-native substrates and their improved performance as electron transporting layer in halide perovskite solar cells

Anodically formed TiO₂ nanotube arrays (TNTAs) constitute an optoelectronic platform that is being studied for use as a photoanode in photoelectrocatalytic cells, as an electron transport layer (ETL) in solar cells and photodetectors, and as an active layer for chemiresistive and microwave sensors. For optimal transport of charge carriers in these one-dimensional polycrystalline ordered structures, it is desirable to introduce a preferential texture with the grains constituting the nanotube walls aligned along the transport direction. Through x-ray diffraction analysis, we demonstrate that choosing the right water content in the anodization electrolyte and the use of a post-anodization zinc ion treatment can introduce a preferential texture in sub-micron length transparent TNTAs formed on non-native substrates. The incorporation of 1.5 atom% of Zn in TiO₂ nanotubes prior to annealing, was found to consistently result in the strongest preferential orientation along the [001] direction. [001] oriented TNTAs exhibited a responsivity of 523 A W^-1 at a bias of 2 V for 365 nm photons, which is among the highest reported performance values for ultraviolet photodetection using titania nanotubes. Furthermore, the textured nanotubes without a Zn²⁺ treatment showed a significantly enhanced performance in halide perovskite solar cells that used TNTAs as the ETL.

Impact of the anodization time on the photocatalytic activity of TiO2 nanotubes

Titanium oxide nanotubes (TNTs) were anodically grown in ethylene glycol electrolyte. The influence of the anodization time on their physicochemical and photoelectrochemical properties was evaluated. Concomitant with the anodization time, the NT length, fluorine content, and capacitance of the space charge region increased, affecting the opto-electronic properties (bandgap, bathochromic shift, band-edge position) and surface hydrophilicity of TiO₂ NTs. These properties are at the origin of the photocatalytic activity (PCA), as proved with the photooxidation of methylene blue.

Active {001} Facet Exposed TiO2 Nanotubes Photocatalyst Filter for Volatile Organic Compounds Removal: From Material Development to Commercial Indoor Air Cleaner Application

TiO₂ nanotubes (TNT) have a highly ordered open structure that promotes the diffusion of dioxygen and substrates onto active sites and exhibit high durability against deactivation during the photocatalytic air purification. Herein, we synthesized {001} facet-exposed TiO₂ nanotubes (001-TNT) using a new and simple method that can be easily scaled up, and tested them for the photocatalytic removal of volatile organic compounds (VOCs) in both a laboratory reactor and a commercial air cleaner. While the surface of TNT is mainly composed of {101} facet anatase, 001-TNT's outer surface was preferentially aligned with {001} facet anatase. The photocatalytic degradation activity of toluene on 001-TNT was at least twice as high as that of TNT. While the TNT experienced a gradual deactivation during successive cycles of photocatalytic degradation of toluene, the 001-TNT did not exhibit any sign of catalyst deactivation under the same test conditions. Under visible light irradiation, the 001-TNT showed degradation activity for acetaldehyde and formaldehyde, while the TNT did not exhibit any degradation activity for them. The 001-TNT filter was successfully scaled up and installed on a commercial air cleaner. The air cleaner equipped with the 001-TNT filters achieved an average VOCs removal efficiency of 72% (in 30 min of operation) in a 8-m³ test chamber, which satisfied the air cleaner standards protocol (Korea) to be the first photocatalytic air cleaner that passed this protocol.

Tailoring morphology, structure and photoluminescence properties of anodic TiO2nanotubes

TiO2nanotube (TNT) structures were grown perpendicular to fluorine-doped tin-oxide-coated glass substrates by anodic oxidation of titanium films. The morphology, crystal structure and optical properties of the TNTs were shown to be dependent on the thickness of the titanium film, which acts as an electrode in electrochemical anodization. Field emission scanning electron microscopy measurements revealed that an increase in titanium thickness from 1.5 to 2.7 µm caused a considerable increase in both inner diameter and tube length, which in turn increases the porosity and the physical surface of the TNTs per unit area. Grazing-incidence small-angle scattering was used to infer the statistical lateral ordering of the TNTs over macroscopic length scales. X-ray diffraction data show an increase in the texture coefficient for the (004) plane as well as theI004/I101intensity ratio with titanium film thickness. All these factors lead to a significant improvement in the photoluminescence intensity from titania nanotubes, which is about five times more than from titania nanoporous materials under similar circumstances.

Influence of the anatase/rutile ratio on the charge transport properties of TiO2-NTs arrays studied by dual wavelength opto-electrochemical impedance spectroscopy

Dual wavelength IMPS allows the determination of the charge transport properties of multiphasic photo-electrodes.

A novel cost effective fabrication technique for highly preferential oriented TiO2 nanotubes

Single crystal like TiO(2) nanotubes with preferential orientation along the [001] direction, parallel to the growth direction of nanotubes, that offer ease of charge transport much higher than reported so far, are fabricated using a cost effective two step technique. The success of this method to grow the nanotubes with the anomalous intense [001] preferential orientation is attributed to the zinc assisted minimization of the (001) surface energy. The single crystal like TiO(2) nanotubes show superior performance as supercapacitor electrodes compared to the normal polycrystalline titanium dioxide nanotubes.