Nanostructured TiO2 Films Research Articles

Deposition of SiO2 and TiO2 Films on Electrode Materials to Suppress Space Charge Injection

Liquid dielectrics are widely used in high-voltage transformers owing to their excellent insulation properties. Space charge injected from electrodes can damage power systems. It is believed that space charge dynamics are related to the electrodes surface closely. In this study, surface modifications of copper and stainless-steel electrodes deposited by SiO2 and TiO2 nanostructured films with radio frequency magnetron sputtering were investigated. The films were characterized. The effects of the films on the breakdown voltage and space charge dynamics characteristics of the propylene carbonate were studied. These results indicate that when a TiO2 nanostructured film was deposited on the stainless-steel electrode, the breakdown voltage of the propylene carbonate increased by 7.3%, and when SiO2 and TiO2 films were deposited on the copper electrode, the increases were 7.4% and 10.2%, respectively. Furthermore, the amount of charge injection of the stainless-steel electrode was lower than that of the copper electrode, and the nanostructured films markedly suppressed the space charge. Both the TiO2 and SiO2 nanostructured films sputtered on the electrode surface changed the barrier height of the solid–liquid interface and reduced the depth and amount of charge injection. Compared with a sputtered SiO2 film, the TiO2 film had a better space charge suppression effect.

Synthesis and Modification of TiO2 nanostructured Materials for Energy Storage

Increasing energy demand is inexorably linked to the need for efficient energy storage techniques. For practical applications, it is highly desirable to decrease the size of electrical components and to increase the storage capacities while maintaining power, stability, and charging-discharging speeds. Much focus has been directed towards the development of supercapacitors. These are often fabricated from carbonaceous or metal oxide materials with high surface areas to maximize electrode/electrolyte interactions. The use of nanostructured TiO2 electrodes has been explored for this application due to their low cost, high stability, and highly tunable morphology.Here we present the fabrication of nanostructured TiO2 films via a facile anodic process. Characterization of the films was carried out by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. Electrochemical reduction of the formed TiO2 film was further performed to increase its capacitance, which was confirmed by cyclic voltammetry and electrochemical impedance spectroscopy. The performance of the symmetric capacitor constructed with the modified TiO2 films will be presented.

Numerical modelling of anodization reaction kinetics for TiO2 nanotubular-film growth in NH4F-based electrolytes

In most reports that propose mechanisms by which nanostructured TiO2 films are synthesised, little information about chemical reaction kinetics that lead to anodic current density transients is provided. Analyses of the anodic current signal have therefore been mostly experimental, leaving previously proposed nanotube development mechanisms unverified by fundamentals of chemical reaction kinetics. Without customising an existing current/conductivity or data-fitting function, the anodic current density is simulated in this work, using a mathematical model of anodization reaction kinetics. From a system of 14 differential equations, a numerical integration algorithm is formulated and executed. The results show current transients that have a strong correlation with measurements from seven samples grown in ethylene glycol-NH4F electrolytes. Having tested the model at limits of low and high NH4F content, this technique proves that the anodic current measured during TiO2 growth is actually an integral function of the kinetics of the redox reactions of water.

Control of pore density on TiO2 nanostructure films: Effect of etchant quantity and anodic voltage

The spatial distribution of pores on a TiO2 nanotube film contribute to its specific surface area, which is a critical factor in applications such as photo-electrochemical water splitting. In this work, we show that while both anodization voltage and etchant quantity determine the TiO2 nanostructure pore density, the quantity of the etchant has a greater effect. We anodized titanium foil substrates, varying either voltage or etchant quantity. With higher voltage, the results show an increase in pore diameters which decreased nanotube density. However, with an increase in etchant quantity, the development of inter-pore voids that create tubular structures, resulted in a remarkable increase in pore density. The study shows that at low quantities of NH4F, the pore density is significantly reduced. Based on these findings, a mechanism of preparing Ti foil templates that could be used to develop spatially distributed nanotubes by multiple-step anodization in ethylene glycol-NH4F electrolytes is proposed.

Dynamic mode optimization for the deposition of homogeneous TiO2 thin film by atmospheric pressure PECVD using a microwave plasma torch

An atmospheric pressure plasma-enhanced chemical vapor deposition process using a microwave plasma torch has been used for titania thin film synthesis. A dynamic deposition mode was set up to cover a square centimeter surface with a nanostructured TiO2 film. The process parameters were studied and optimized to control the coating crystallinity and morphology and limit the formation of powder in the plasma phase. Contrary to static deposition, the substrate movement promotes a film growth by particles agglomeration in the reference conditions, leading to a cauliflower-like morphology. Then, the precursor proportion in the plasma appears to be determinant in the TiO2 film microstructure. For a precursor flow rate beyond 0.2 slpm, the titania nanoparticles formation in the gas phase is promoted and the thin film is growing by particles agglomeration, leading to a columnar cauliflower-like morphology. At a flow rate of 0.2 slpm, the growth by surface reaction is promoted and the TiO2 film is columnar, where each column is an anatase crystal. After the optimization of the substrate holder movement, it was possible to deposit this last microstructure homogeneously on a square centimeter surface.

Plasmon-polariton oscillations in three-dimensional disordered nanotubes excited by a moving charge

We systematically investigated the plasmon-polariton oscillations generated by a fast radiating charge (Cherenkov radiation) in a three-dimensional (3D) strongly disordered nanostructure. We studied the dynamic properties of an optical field in a random composition of empty single-wall nanotubes by using a 3D numerical finite-difference time domain technique. In our approach, only parameters of nanotube structures are fixed. The dynamic spectrum of internal field excitations was left to be defined as a result of numerical simulation. The patterns of total field (charge + carbon nanotubes) are determined by the interference of a moving charge field and the spectrum of surface plasmon-polaritons in disordered nanotubes. We found that the field energy losses, as a function of the charge velocity, has a clearly pronounced maximum when the characteristic frequency scale (defined by a charge velocity) is close to the frequency of the surface plasmon-polariton resonances generated in coupled nanotubes, even at a significant level of disorder. Our studies show that the shape of the resonance peak, depending on the charge velocity, is similar for carbon and TiO2 nanostructures, but, only for frequencies from the range of the surface plasmon polaritons of respective materials. The TiO2 nanostructure films for a classic cylindrical polytetrafluoroethylene cell was synthesized in our experiments too.

Colorful nanostructured TiO2 film with superhydrophobic–superhydrophilic switchable wettability and anti-fouling property

Superhydrophobic nanostructured TiO2 film was successfully fabricated on the Ti plate by anodic oxidation and lauric acid modification. By varying the applied voltage, TiO2 films with various nanostructures and colors were prepared. The water contact angles (WCA) on the modified TiO2 film increased as the applied voltage increased, and the highest WCA on the TiO2 nanovesuvianite film was about 161.23°. When the applied voltage was 5 V and 20 V, the obtained TiO2 nanopores and TiO2 nanotubes could be used to build two improved Cassie models for designing superhydrophobic surface. Moreover, the superhydrophobic TiO2 film showed good anti-fouling property, particularly it could maintain superhydrophobicity both when immersed in and taken out from the organic solvents. The superhydrophobic film could be changed into superhydrophilic under ultraviolet (UV) irradiation, and the superhydrophilic film recovered to hydrophobic or superhydrophobic gradually at 60 °C in the dark. Additionally, the superhydrophilic film showed underwater superoleophobicity, which exhibited excellent anti-fouling property by degrading the oil under UV light.

Formation of Micro- and Nanostructured TiO2 Films by Anodic Oxidation for Enhanced Photocatalytic Activities

This paper presents growth of micro- and nanostructured TiO2 layers on Ti6Al4 V by thermal and anodic spark oxidation for photocatalysis. Effects of anodization parameters on surface morphology and photocatalytic behavior were investigated. The photocatalysts divided into two groups consisting of four samples were prepared for growth of micro- and nanostructured TiO2 layer on Ti6Al4 V. The microstructured surfaces were obtained in 1 M H2SO4 electrolyte in 5 min at the anodization voltage in the range of 100–400 V, whilst the nanostructured TiO2 layers were prepared in 1% HF solution in the range of 10–40 V in equal time. Surface morphology, phase structure, band gap and surface roughness values of the micro- and nanostructured TiO2 photocatalysts were analyzed by scanning electron microscope, X-ray diffraction, UV–Vis diffuse reflectance spectroscopy and surface profilometer, respectively. Photocatalytic performances of TiO2 photocatalysts obtained with different anodization parameters were investigated via methylene blue aqueous solution. It was observed that the anodization voltage has a great effect on photocatalytic performances due to the formation of different crystallinity and morphology. Among the samples, the layer prepared at 100 V exhibited the best photocatalytic activity with 91.15% degradation efficiency and 95.5 × 10−4 min−1 kinetic rate constant. Moreover, the further investigation determining active species were carried out using ammonium oxalate, benzoquinone, and isopropyl alcohol. In this process, the photocatalytic mechanism revealed that the superoxide radicals $$(^{ \cdot } {\text{O}}_{ 2}^{ - } )$$ play a very important role during the photodegradation process. The TiO2 layer on Ti6Al4 V anodized at 100 V also showed high activity and stability, suggesting its potential for wastewater treatment.

Preparation of nanostructured TiO2 films with high catalytic activity and their 3D spatial distribution of anatase and rutile phases

In this study, a facile, cheap, scalable, and environmentally friendly method has been developed to prepare nanostructured TiO2 coatings with a high catalytic activity. In addition, a protocol to characterize the prepared films in order to obtain consistent results between their texture, their structure, their microstructure, and their catalytic behavior has been established. For that, two types of TiO2 coatings have been fabricated by applying once TiO2 aqueous suspensions, prepared from commercial nanopowders, by immersion followed by an adequate thermal treatment. The first type of films is constituted by coatings with anatase as single phase, and the second one by films with a rutile molar fraction of XR = 0.07 ± 0.02, determined from grazing incidence X-ray diffraction (GI-XRD) data. High spatial resolution chemical imaging, obtained by confocal Raman microscopy (CRM), of the surface and of the cross section for the films showed a good distribution of rutile without any segregation of this phase. Furthermore, high resolution electron microscopy (HREM) confirmed the presence of anatase/rutile biphasic domains in those areas in which both phases were previously found together by CRM. In addition, the catalytic data fit to a first-order kinetic model with high catalytic reaction constants (k′), especially when rutile is present. Furthermore, consistent results have been found in this study between the textural, structural, and microstructural studied features with the catalytic properties for the prepared TiO2 films.

Photocatalytic Degradation of Azithromycin by Nanostructured TiO₂ Film: Kinetics, Degradation Products, and Toxicity.

In this paper, nanostructured TiO2 film was prepared by the by sol-gel process and dip-coating technique with titanium tetraisopropoxide as a precursor. After heat treatment at 550 °C, the deposited film was characterized by means of micro-Raman spectroscopy and atomic force microscopy (AFM). It was found that the TiO2 film consisted of only the TiO2 anatase phase and showed a granular microstructure. Photocatalytic degradation of azithromycin by using sol-gel nanostructured TiO2 film was studied to define the most effective degradation process for potential use in wastewater treatment. Different factors were evaluated during photocatalysis, such as pH (3, 7, and 10), water matrix (ultrapure water and synthetic municipal waste water effluent), influence of another pharmaceutically active compound (sulfamethoxazole, one of the most often detected pharmaceutic compounds in waste waters), and radiation sources (low pressure ultraviolet (UV) mercury lamps with a UV-A and UV-C range; a light-emitting diode (LED) lamp with a radiation peak at 365 nm). The most effective degradation process was achieved with the UV-C irradiation source in matrices at pH 10. The water matrix had little effect on the photocatalytic degradation rates of azithromycin. The presence of sulfamethoxazole in the water matrix decreased the degradation rate of azithromycin, however, only in matrices with a pH level adjusted to 10. During the experiments, five azithromycin degradation products were identified and none of them showed toxic properties, suggesting effective removal of azithromycin. LED 365 nm as the irradiation source was not as effective as the UV-C lamp. Nevertheless, considering the cost, energy efficiency, and environmental aspects of the irradiation source, the LED lamp could be a “real-life” alternative.

Sol-Gel Spin Coating Synthesis of TiO<sub>2</sub> Nanostructure and Its Optical Characterization

This work focuses on the sol-gel spin coating technique of TiO2 nanostructure synthesis and its characterization. Though various methods have been used to fabricate TiO2 nanostructure, much effort has not been exerted to achieve better photoresponsive and narrowly dispersed TiO2 nanostructure using the sol-gel spin coating method. Therefore, it is imperative to realize the synthesis of TiO2 nanostructures, and investigate their properties. In this work, TiO2 is synthesized by sol-gel spin coating technique using titanium tetraisopropoxide, isopropanol, acetic acid and deionized water as starting materials and deposited on borosilicate glass substrates. The effects of annealing temperatures (300˚C, 400˚C and 500˚C) on the structural and optical properties of the films were investigated by different techniques: Scanning Electron Microscopy (SEM), optical microscopy and UV-visible spectrophotometry. The optical characterization showed the direct band gap at 3.7 eV, 3.6 eV and 3.4 eV for 300˚C, 400˚C and 500˚C, respectively, and the optical transmittance and reflectance spectra showed a greater performance at 500˚C. The grain sizes obtained from SEM annealed at 300˚C, 400˚C and 500˚C are found to be about 6.0 nm, 5.0 nm and 4.0 nm respectively. The grain size of TiO2 nanostructure films decreased with increasing annealing temperatures. The results clearly indicated that the sol-gel spin coating synthesis of TiO2 nanostructure and post-thermal treatment at 500˚C cooled naturally at room temperature result in better photoresponsive and narrowly dispersed TiO2 nanostructure films with higher photoresponsive and good optical properties.

Surface modification of TiO2 nanostructured films by inserting Mg2+ ions applied to dye sensitized solar cells

Nanostructured electrodes were prepared based on titanium dioxide (TiO2) modified by incorporating small amounts of magnesium oxide (MgO) to improve their efficiency in dye sensitized solar cells. J-V curve measurements show efficiencies increased from 2.32 to 2.55 % with the addition of MgCl2. On the other hand, the incident photon-to-current efficiency (IPCE) measures obtained show an improvement with the addition of magnesium oxide. This is in agreement with absorbance results and short circuit current values. Values from cyclic voltammetry curves show that surface modification caused energy levels to increase below the TiO2 conduction band by incorporating magnesium oxide. Electrochemical impedance spectroscopy confirmed the presence of these additional energy levels and gave evidence of increased recombination in the TiO2 film and decreasing electron lifetime from 15.95 to 6.82 ms.

Influence of nanostructured TiO2 film thickness in dye-sensitized solar cells using naturally extracted dye from Thunbergia erecta flowers as a photosensitizer

Dye-sensitized solar cells (DSSCs) have been considered as an alternative energy resource in recent years, due to low-cost fabrication and non-toxic compared to silicon-based and thin film solar cells. Herein, the natural dye containing anthocyanins were extracted from Thunbergia erecta natural flower petals by simple extraction techniques and used as photosensitizers in the DSSC. The extracts showed the UV–Vis absorptions in the 400–800 nm range with broad maxima in visible region around 537 nm. Fourier transform infrared (FTIR) spectrum was obtained after the dye coating on a semi-conductive layer, to identify the presence of anthocyanin according to the functional groups present in the dye molecules. To precise and progressive optimization of the TiO2 photoanodes film thicknesses were prepared by spin coating technique and characterized by atomic force microscopy, field emission scanning electron microscopy and J-V characteristics. The photovoltaic performance studies were carried out to understand the effect of the TiO2 multilayer photoanodes and the interaction with the dye molecules on the cells efficiency. Photovoltaic parameters like short circuit current (JSC), open circuit voltage (VOC) and fill factor (FF) were evaluated for fabricated cells. The optimized film thickness of the TiO2 photoanode is ∼5.5 μm with an efficiency of 0.37% under AM 1.5G illumination of sunlight. The VOC of DSSCs gradually decreases as the thickness increases of the TiO2 thin film and the highest conversion efficiency while it has the maximum short-circuit current density.

Oxygen detection using nanostructured TiO2 thin films obtained by the molecular layering method

Using the molecular layering method (ML), nanostructured TiO2 films have been synthesized that are promising for detecting oxygen over a wide range of concentrations at low operating temperatures of 150–300 °C. It has been shown that an anatase thin film is formed; the particle size is 34.9 ± 3.1 nm. It has been established that it enables the highly responsive detection of oxygen (RO2/RAr up to 26.6 at a temperature of 200 °C and O2 content of 10%). The influence of operating temperature on the response values, and response and recovery time, have been studied. It has been shown that response magnitude slightly depends on humidity, but at 100% humidity response time and signal recovery time increase significantly. A sufficiently high degree of selectivity has been found when detecting oxygen, in comparison with other analyte gases (CO, H2, CH4 and CO2).

Shape-dependent photogenerated cathodic protection by hierarchically nanostructured TiO2 films

TiO2 films with different nanostructures were prepared by means of hydrothermal technique by adjusting the concentration of reactant components and reaction time. The flower-like, rod-like and sphere-like of the TiO2 films were observed by scanning electron microscopy. The shape-dependent photogenerated cathodic protection activity of such films were investigated. The flower-like film presents the largest drop of open circuit potential and the maximum magnitude of the photocurrent once irradiated by UV light. The 304 stainless steel coupled with it has the most negative potential Ecorr in comparison with other nanostructure films as well. The electrochemical impedance spectroscopy analysis of 304 stainless steel also shows the minimum charge transfer resistance when coupled with flower-like nanostructure films. To understand the effect of nanostructure on the properties, possible schematic diagrams of UV incident light passage in the three flower-like, rod-like and sphere-like nanostructures films are proposed to explain the absorption difference. The flower-like structure with a large amount of voids is beneficial to enhance light scattering and trapping for efficient UV light harvesting.

Manipulation of surface plasmon resonance of sputtered gold-nanoparticles on TiO2 nanostructured films for enhanced photoelectrochemical water splitting efficiency

Plasmonic solar water splitting is a promising way to realize the persistent supply of green energy in a sustainable manner. Here we report a largely enhanced performance of photoelectrochemical photoanodes by introducing Au nanoparticles (NPs) decoration onto the hydrothermal process grown TiO2 nanowires (NWs). Through a simple one-step sputtering process, dense and homogeneously separated Au NPs were prepared on TiO2 NWs. Compared to pristine TiO2 electrode, 10 nm Au decorated TiO2 electrode achieved >3 and 4 times photocurrent density enhancement measured under AM 1.5G and visible light illumination, respectively. The enhancement mechanism induced by localized surface plasmonic resonance effect is analyzed by the relative band and Fermi energy of corresponding materials.

Nanostructured TiO2 Films with a Mixed Phase for Perovskite Solar Cells

A series of thin films made with TiO2 nanoparticles with a varied anatase/rutile phase ratio is prepared on conducting glass substrates using a spin-coating method. The structure, morphology, and optical properties of TiO2 nanopowders and thin films fabricated are studied using powder X-ray diffraction, scanning electron microscopy, and optical spectroscopy. The TiO2 nanostructured films created are used as photoelectrodes for the fabrication of perovskite solar cells (PSCs). The photovoltaic characteristics of PSCs under AM1.5 light illumination (1000 W/m2) under ambient conditions are studied. It is shown that the best efficiency of solar-to-electrical energy conversion, namely, 9.3%, is obtained for the PSC with a photoelectrode based on a TiO2 film with an anatase/rutile mixed phase ratio of 86/14%.

Enhanced visible-light photoelectrochemical and photoelectrocatalytic activity of nano-TiO2/polyimide/Ni foam photoanode

We report a novel method for fabrication of a nano-TiO2/polyimide (PI)/Ni foam photoanode. Characterization results indicated that porous nanostructured TiO2 films were successfully immobilized and dispersed into the indivisible PI/Ni foam substrate. The prepared photoanode exhibited intense visible-light absorption from 400 to 700 nm, high photoinduced current of 175 μA/cm2, and photoelectrocatalytic (PEC) efficiency of 98.8 % for degradation of methylene blue (MB) within 180 min of simulated solar light irradiation. Moreover, the TiO2/PI/Ni foam photoanode showed obvious visible-light PEC performance, mainly attributed to formation of TiO2–PI charge-transfer (CT) complexes and high separation efficiency of photoinduced charge carriers by the applied bias potential. This study provides a new perspective for preparation of cheap, high-performance visible-light photoelectrocatalytic films.

Spontaneous Synthesis of Highly Crystalline TiO2 Compact/Mesoporous Stacked Films by a Low-Temperature Steam-Annealing Method for Efficient Perovskite Solar Cells.

Highly crystalline TiO2 nanostructured films were synthesized by a simple steam treatment of a TiCl4 precursor film under a saturated water vapor atmosphere at 125 °C, here referred to as the steam-annealing method. In a single TiO2 film preparation step, a bilayer structure comprising a compact bottom layer and a mesoporous surface layer was formed. The mesoporous layer was occupied by bipyramidal nanoparticles, with a composite phase of anatase and brookite crystals. Despite the low-temperature treatment process, the crystallinity of the TiO2 film was high, comparable with that of the TiO2 film sintered at 500 °C. The compact double-layered TiO2 film was applied to perovskite solar cells (PSCs) as an electron-collecting layer. The PSC exhibited a maximum power conversion efficiency (PCE) of 18.9% with an open-circuit voltage ( VOC) of 1.15 V. The PCE and VOC were higher than those of PSCs using a TiO2 film formed by 500 °C sintering.

Enhanced UV Flexible Photodetectors and Photocatalysts Based on TiO2 Nanoplatforms

In this study, titanium dioxide (TiO2) nanostructured films were synthesized under microwave irradiation through low temperature synthesis (80 °C) and integrated in ultraviolet (UV) photodetectors and as photocatalysts. Bacterial nanocellulose (BNC), tracing paper, and polyester film were tested as substrates, since they are inexpensive, flexible, recyclable, lightweight, and when associated to low temperature synthesis and absence of a seed layer, they become suitable for several low-cost applications. The nanostructured TiO2 films and substrates were structurally characterized by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, X-ray diffraction, and Raman spectroscopy. The optical properties of all materials were investigated. The TiO2 nanostructured films were implemented as a photoactive layer of UV photodetectors and demonstrated significant increase of conductance upon exposed to UV irradiation. The photodetection behaviour of each material was investigated by in-situ Kelvin probe force microscopy experiments, in which the contact potential difference varied under dark or UV irradiation conditions, demonstrating higher shift for the BNC-based UV photodetector. Photocatalytic activity of the films was assessed from rhodamine B degradation under solar radiation, and BNC based devices revealed to be the best photocatalyst. The structural characteristics of the TiO2 films and substrates were correlated to the differences in the UV photodetection and photocatalytic performances.