Self-limiting growth of anatase TiO2: A comparison of two deposition techniques

Self-limiting deposition of titanium dioxide thin films was accomplished using pulsed plasma-enhanced chemical vapor deposition (PECVD) and plasma-enhanced atomic layer deposition (PEALD) at low temperatures (T < 200 °C) using TiCl₄ and O₂. TiCl₄ is shown to be inert with molecular oxygen at process conditions, making it a suitable precursor for these processes. The deposition kinetics were examined as a function of TiCl₄ exposure and substrate temperature. The quality of the anatase films produced by the two techniques was nominally identical. The key distinctions are found in precursor utilization and conformality. Pulsed PECVD requires 20 times less TiCl₄, while PEALD must be used to uniformly coat complex topographies.     

Hydrophilicity of anatase TiO2/Cr-doped TiO2 thin films with different band gaps

Based on the concept that the electron-hole separation effect caused by a different band-gap structure would improve its hydrophilicity, anatase-TiO₂/Cr-doped TiO₂ thin films were synthesized by DC magnetron sputtering. The optical band gaps of TiO₂ thin films decreased from 3.23 to 2.95 eV with increasing Cr-doping content. Multilayer TiO₂ thin films with different band gaps exhibited a superhydrophilicity under UV illumination. In particular, in anatase TiO₂ (3.23 eV)/4.8% Cr-doped TiO₂ (2.95 eV), the hydrophilicity, which indicated a contact angle of less than 20°, lasted for 48 h in the dark after UV illumination was discontinued. This outstanding result has rarely been reported for TiO₂ thin films, which confirmed that the prominent superhydrophilicity of anatase TiO₂/Cr-doped TiO₂/glass could be attributed to the retardation of electron-hole recombination caused by the band-gap difference.     

Dielectric polymer matrix composite films of CNT coated with anatase TiO2

This study presents the fabrication method and the dielectric property of polymer matrix composite films of carbon nanotube (CNT) coated with TiO₂. The TiO₂ was coated with sol-gel method using titanium (IV) butoxide (TNBT), HO₂ and benzyl-alcohol as the surfactant. The configuration of CNT-TiO₂ hybrid was observed with the field emission scanning electron microscope images. The coated TiO₂ was thermally treated and transformed into the anatase structure to enhance the mechanical strength and get the high insulating property. The anatase structure was proved from the diffraction angles of XRD. The CNT-TiO₂ hybrid was mixed with the epoxy resin using 3-roll-mil and casted into the films using film casting method. The structure of CNT-TiO₂ hybrid was ascertained to be maintained against the high shear stress during the mixing and casting processes. The dielectric property of the composite films was measured following IPC-TM-6550. The dielectric property at 1 GHz of the composite film of 5 wt.% CNT is about 10 and the loss tangent at 1 GHz is about 0.06.     

Fabricating highly active mixed phase TiO2 photocatalysts by reactive DC magnetron sputter deposition

Recent work points out the importance of the solid-solid interface in explaining the high photoactivity of mixed phase TiO₂ catalysts. The goal of this research is to probe the structural and functional relationships of the solid-solid interface created by the reactive DC magnetron sputtering of titanium dioxide. We show that sputter deposition provides excellent control of the phase and interface formation. We explored the effects of the process parameters of pressure, oxygen partial pressure, target power, substrate bias (RF), deposition incidence angle, and post annealing treatment on the structural and functional characteristics of the catalysts. We have successfully made pure and mixed phase TiO₂ films. These films were characterized with AFM, SEM, TEM, and XRD to determine surface morphology, phase distribution and phase content. The performance as photocatalytic surfaces was measured and compared (normalized for surface area) to mixed phase TiO₂ fabricated by other methods, including flame hydrolysis powders, and sol-gel deposited TiO₂ films. The sputtered mixed phase materials were far superior to the commercial standard (Degussa P25) and sol-gel TiO₂ as measured by the gas phase oxidation of the air pollutant acetaldehyde under UV illumination. These results demonstrate that reactive DC magnetron sputtering is a powerful tool for investigating the role of the solid-solid interface in influencing photocatalytic activity. In addition, our work illustrates the feasibility of reactive DC magnetron sputtering as a practical commercial technique for manufacturing highly active nanostructured TiO₂ photocatalysts.     

Residual stresses and Raman shift relation in anatase TiO2 thin film

Anatase TiO₂ film (100-1000 nm thick) grown on glass, sapphire (0001), and Si (100) substrates by pulsed dc-magnetron reactive sputtering were evaluated for stress and strain analysis using Raman spectroscopy and curvature measurement techniques. The X-ray analysis revealed that films prepared for this study were purely anatase, and the measurements indicate that the film exhibit that (101) is the preferred growth orientation of the crystallites, especially for the film thicker than 100 nm. Curvature measurements and Raman spectroscopy, with 514.5 nm excitation wavelength, phonon line shift were used for stress analysis. A comparison between Raman lineshapes and peak shifts yields information on the strain distribution as a function of film thickness. The measurements of residual stresses for crystalline anatase TiO₂ thin film showed that all thin film were under compressive stress. A correlation between Raman shifts and the measured stress from the curvature measurements was established. The behavior of the anatase film on three different substrates shows that the strain in film on glass has a higher value compared to the strain on sapphire and on silicon substrates. The dominant 144 cm^− 1E_g mode in anatase TiO₂ clearly shifts to a higher value by 0.45-5.7 cm^− 1 depending on the type of substrate and film thickness. The measurement of the full width at half maximum values of 0.59-0.80 (2θ°) for the anatase (101) peaks revealed that these values are greater than anatase powder 0.119 (2θ°) and this exhibits strong crystal anisotropy with thermal expansion.     

Preparation and antibacterial behavior of Fe-doped nanostructured TiO2 thin films

Fe³⁺-doped nanostructured TiO₂ thin films with antibacterial activity were prepared on soda–lime–silica glass slides by using sol–gel technology. Water containing Escherichia coli K-12 with TiO₂ thin films in was exposed to low intensity fluorescent light and antibacterial efficiency was evaluated with spread plate techniques. The films are porous and have anatase phase. Iron ions increased luminous energy utilization as the absorption edge of the Fe³⁺-doped film has a red shift compared to that of the pure TiO₂ film in the UV–VIS absorption spectrum. The bacterial removal efficiency reached 95% at the optimum concentration of iron ion (about 0.5% (mol)) after 120 min irradiation. The antibacterial behavior of the doped TiO₂ films was explicitly observed using scanning electron microscopy and cell wall damage was found.     

Growth and characterization of epitaxial anatase TiO2(001) on SrTiO3-buffered Si(001) using atomic layer deposition

Epitaxial anatase titanium dioxide (TiO₂) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer grown by molecular beam epitaxy (MBE) to serve as a surface template. The growth of TiO₂ was achieved using titanium isopropoxide and water as the co-reactants at a substrate temperature of 225-250 °C. To preserve the quality of the MBE-grown STO, the samples were transferred in-situ from the MBE chamber to the ALD chamber. After ALD growth, the samples were annealed in-situ at 600 °C in vacuum (10^− 7 Pa) for 1-2 h. Reflection high-energy electron diffraction was performed during the MBE growth of STO on Si(001), as well as after deposition of TiO₂ by ALD. The ALD films were shown to be highly ordered with the substrate. At least four unit cells of STO must be present to create a stable template on the Si(001) substrate for epitaxial anatase TiO₂ growth. X-ray diffraction revealed that the TiO₂ films were anatase with only the (004) reflection present at 2θ = 38.2°, indicating that the c-axis is slightly reduced from that of anatase powder (2θ = 37.9°). Anatase TiO₂ films up to 100 nm thick have been grown that remain highly ordered in the (001) direction on STO-buffered Si(001) substrates.     

Measurement method for carrier concentration in TiO2 via the Mott-Schottky approach

In direct contrast to the way in which silicon is precisely doped for integrated circuit applications in order to optimize device performance, there is little nuanced understanding of the correlation between TiO₂ doping level, charge carrier concentration, and the operation of TiO₂-based photocatalysts, dye-sensitized solar cells, and sensors. The present work outlines a rigorous methodology for the determination of free carrier concentration for doped metal oxide semiconductors such as TiO₂ that are not amenable to standard metrology methods. Undoped, Cr-, Mn-, and Nb-doped polycrystalline anatase TiO₂ are synthesized via atomic layer deposition (ALD) using Ti(OCH(CH₃)₂)₄, H₂O, Cr(C₅H₇O₂)₃, Mn(DPM)₃ (DPM = 2,2,6,6-tetramethyl-3, 5-heptanedionato), and Nb(OCH₂CH₃)₅ as the source materials for Ti, O, Cr, Mn, and Nb, respectively. Chemical composition and crystallinity are investigated and a thorough “device-like” characterization of TiO₂ Schottky diodes is carried out to justify the subsequent extraction of carrier concentration values from capacitance-voltage (C-V) measurements using the Mott-Schottky approach. The influence of factors such as substrate type, contact metal type, and surface and interface preparation are examined. Measurements of donor carrier concentration are obtained for undoped, Cr-, Mn-, and Nb-doped TiO₂ synthesized by ALD. Possible causes for the obtained carrier concentrations are discussed.     

Preparation and photocatalytic study of fibrous K0.3Ti4O7.3(OH)1.7–anatase TiO2 nanocomposite photocatalyst

Nanocomposite of K_0.3Ti₄O_7.3(OH)_1.7 fiber and anatase TiO₂ nanoparticle was prepared by hydrothermal treatment of the K_0.3Ti₄O_7.3(OH)_1.7 which was synthesized by calcination of K₂CO₃ and TiO₂ at 1250 °C followed by refluxing in nitric acid. Effects of hydrothermal treatment conditions such as temperature and time on morphology, phase composition and crystal structure of the nanocomposites were extensively studied. Photocatalytic activities of the catalysts prepared at various hydrothermal conditions were evaluated by means of methylene blue decomposition under blacklight irradiation.     

Deoxidisation and phase analysis of plasma sprayed TiO2 by X-ray Rietveld method

It is fundamentally important to determine the deoxidisation and phase compositions of plasma sprayed TiO₂ coatings containing anatase. In the present study, plasma sprayed porous TiO₂ coatings containing anatase were prepared using anatase powder and both Ar-He-H₂ and Ar-He-N₂ plasma gases. The deoxidisation of TiO₂ and phase compositions of the starting powder and the prepared coatings were examined using X-ray Rietveld method by refining their crystalline parameters and scale factors. The refined oxygen occupancies showed that there were about 0.08 and 0.1 formula units of oxygen deficiencies for the rutile and anatase phases of the coatings, respectively. Such degrees of deoxidisation and the other crystalline parameters appeared independent of the plasma spraying process parameters. With considerations of the presence of organic adhesive in the starting powder and the formation of titanium ethoxide in the coatings, the degrees of deoxidisation estimated by the X-ray Rietveld method were slightly higher than those quantified by the thermogravimetry curves. The phases of the coatings determined from the refined scale factors were mainly composed of rutile with 10.0% to 22.5% anatase by weight, and the latter content increased with decreasing the intensity of the plasma jet.     

Effect of SnO2 on the photocatalytical properties of TiO2 films

TiO₂/SnO₂ thin films with different tin atomic percentages were successfully prepared on glass substrates by the spray pyrolysis method from an alcoholic solution of TiO[C₅H₇O₂]₂ with different concentrations of SnCl₄. The TiO₂/SnO₂ thin films prepared at 450 °C presented the anatase phase in polycrystalline configuration from %Sn = 0 in the starting solution up to %Sn = 20, at higher tin content the films present an amorphous configuration. The resulting thin films have a homogeneous surface structure with some porosity. The photocatalytical properties of the films were evaluated with the degradation of methylene blue. The products of the degradation reaction were identified by ¹H nuclear magnetic resonance and the film properties were studied by atomic force microscopy, scanning electron microscopy, UV–Vis spectroscopy, and X-ray diffraction.     

Formation of hydroxyapatite nanorods and anatase TiO2 on CaTiO3 powder using hydrothermal treatment

Hydroxyapatite (HAp) nanorods and anatase TiO₂ (ATO) were fabricated on the surface of CaTiO₃ (CTO) by hydrothermally treating a CTO powder with a β-glycerophosphoric acid disodium salt pentahydrate (β-GP)-containing solution. The aspects of HAp and ATO formation were examined by changing the pH of the hydrothermal solutions to 5, 7, 9, 11 and 13. There were no products formed on the CTO powder by hydrothermal treatment in strong alkaline environments of pH 13. HAp nanorods and ATO were observed on the CTO surface at pH 11-7. Their contents increased with decreasing pH of the hydrothermal solution. In an acidic solution (pH 5), the amount of CTO decreased drastically due to dissolution with the concomitant formation of tricalcium phosphate.     

Anodic TiO2 from stirred Na2SO4/NaF electrolytes: Effect of applied voltage and stirring

TiO₂ was anodically formed on titanium from electrolyte containing 1M Na₂SO₄ and 0.5 wt.% NaF. Oxidation was carried out for 1 h at potentiostatic 20, 30 and 40 V from unstirred as well as stirred baths. At 20 V amorphous and nanotubular oxide is produced irrespective of the stirring conditions. Stirring increased the tubular length compared to the unstirred condition at 20 V. Oxidations at 30 and 40 V produced flat, crystalline anatase under both the bath conditions. A nanotubular network may be formed in the first 5 min and then anodic oxidation may occur laterally, perpendicular to the long-axis of the nanotubes.     

Optical and structural properties of nanocrystalline anatase (TiO2) thin films prepared by non-aqueous sol-gel dip-coating

Anatase (TiO₂) thin films were grown by non-aqueous sol-gel dip-coating using titanium (IV) n-butoxide as precursor and 1-butanol as solvent. High withdrawal speed of 4.7 mm/s in dip-coating resulted in defect free films of 100 nm average film thickness after subsequent heat treatments. According to scanning electron microscope and X-ray diffraction measurements, the films consisted of nanocrystalline anatase with 30 nm mean crystallite size. Refractive index n(λ) and extinction coefficient k(λ) were determined over the wavelength range from 200 to 1650 nm. The optical band gap of the film material was approximately 3.2 eV. The results showed very similar optical characteristics to those that are accomplished with chemically more reactive aqueous sol-gel processes. Furthermore, it was found that in addition to porosity, coordination number of Ti atoms to nearest oxygen neighbors is likely to have a significant role in explaining differences of optical properties between bulk anatase and thin film materials of the present work.     

Investigations on the self-organized growth of TiO2 nanotube arrays by anodic oxidization

The formation mechanism of a thin film of self organized TiO₂ nanotube arrays prepared by anodic oxidization of a pure titanium sheet in electrolyte solutions containing potassium fluoride and sulfate was investigated through near-real time monitoring the anode mass, the current density, and the surface topography during the anodization. Energy dispersive X-ray spectrometry was used to monitor the surface composition change during the anodization. The titanium surface was first electrochemically oxidized to form a layer of dense oxide under which nanotubes were originated. With the protection of the oxide layer, long nanotubes could be formed in electrolyte solutions with relatively high pH. The surface composition analysis indicates that the nanotubes were not totally oxidized to TiO₂. However, no other elements but Ti and O were found in the oxide film. This work reveals a way to fabricate long nanotubes with defined sizes.     

Hydrophilicity on carbon-doped TiO2 thin films under visible light

Carbon-doped TiO₂ thin films in the anatase phase with dopant concentrations of 1.1, 0.9, and 0.7 mol% were fabricated by a radio-frequency magnetron sputtering method. Dopant carbons were located at the oxygen sites. Carbon substitution caused the absorbance edge and/or the shoulder of TiO₂ to shift to a higher wavelength region. Carbon-doped TiO₂ thin films underwent a hydrophilic conversion when irradiating with visible light (400–530 nm). The hydrophilic property under visible light was inferior to that under ultraviolet light, which is explained by considering that the visible light sensitivity originates from the localized C 2p formed in the band-gap.     

On the initial growth of atomic layer deposited TiO2 films on silicon and copper surfaces

Atomic layer deposition (ALD) of TiO₂ using tetrakis(diethylamino)titanium precursor and H₂O was studied on silicon and copper surfaces in order to examine differences in nucleation. Both surfaces were patterned on the same substrate to assure identical deposition conditions. Spectral ellipsometry, X-ray photoelectron spectroscopy and surface profilometry were used to probe nucleation phenomena, growth rates, and surface morphology on both surfaces. The TiO₂ deposition on copper was found to exhibit a significant induction period of about 20-25 ALD cycles with no observable TiO₂ during the first 10-15 cycles on the copper side; in contrast, no such inhibited growth was observed in the TiO₂ deposition on silicon. This result opens up potential for selective ALD of TiO₂ films on silicon-based substrates patterned with a metal without the use of a mask, a self-assembled monolayer or soft lithography which is impractical for some nanoscale semiconductor fabrication processes. After film nucleation, the TiO₂ growth rate on both surfaces was found to be 0.10 nm/cycle.     

Hydrothermal growth of ferromagnetic Fe-doped TiO2 films

Polycrystalline Fe-doped TiO₂ anatase films were deposited on (001) oriented SrTiO₃ single crystal substrates at temperatures less than 200 °C from acidic aqueous solutions of titanyl sulfate and iron (III) nitrate. Epitaxial anatase TiO₂ films were obtained when the films were annealed in air at 900 °C. Room-temperature ferromagnetic Fe-doped TiO₂ thin films were obtained after repeated deposition and annealing steps. The observed saturation magnetization (>0.28μ_B/Fe) could not be attributed to the presence of secondary phase magnetic iron oxides or iron clusters.     

Silicon releasing sol-gel TiO2-SiO230/70 films

TiO₂-SiO₂30/70 mol% sol-gel films were prepared on glass slides by a dip-coating method. The sols were prepared by a two-step hydrolysis process, where either the Si- or Ti-sol was prehydrolyzed before mixing the sols together. The acidity of the prehydrolyzed sol was varied. The surface structure and composition of formed films were characterized by contact angle meter, atomic force microscope and X-ray photoelectron spectroscopy. The silicon dissolution was performed in tris(hydroxymethyl)aminomethane buffer. The films were able to release silicon and the surface chemistry of films was dependent on the preparation method. The prehydrolysis of Si-sol and aging of mixed sol increased surface silica concentration of the formed films. The surface morphology was dependent on the preparation method so that through Si-sol prehydrolysis it was possible to make rougher nanosurfaces with needle-like particles.     

Synthesis of pure anatase TiO2 nanocrystals in SiO2 host and the determination of crystal planes by ImageJ

Anatase TiO₂ nanocrystals in the TiO₂- SiO₂ matrix were prepared by the ultra hydroylsis sol-gel route. The samples were heat treated at 350 °C and 500 °C. The structural analyses of the samples were carried out using X-ray diffraction technique, Raman spectroscopy and Transmission electron microscopy. The size of the nanocrystals from the XRD spectra (8.3 nm) and TEM (5-8 nm) is well in agreement. The spacing for the crystal planes was also determined using the ImageJ program. The Raman peaks further confirmed the formation of only the anatase phase within the matrix.