Low temperature TiO2 rutile phase thin film synthesis by chemical spray pyrolysis (CSP) of titanyl acetylacetonate

Rutile phase TiO₂ thin films have been synthesized using chemical spray pyrolysis of titanyl acetylacetonate TiAcAc in ethanol at 500 °C. The first part of the paper focuses on the thermal decomposition behavior of the precursor by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) coupled with differential scanning calorimetry (DSC). The second part of the paper focuses on the evolution of TiO₂ thin films and their structural transformation with substrate temperature. XRD revealed amorphous TiO₂ thin film at low substrate temperatures (<350 °C) and on high substrate temperatures anatase (3.84 g/cm³) or rutile (4.25 g/cm³) crystalline structure was obtained. The lattice constant, grain size, microstrain and the dislocation density of the film were obtained from the peak width. FTIR spectra of both anatase and rutile TiO₂ revealed stretching vibration of the Ti–O bond for tetrahedral and octahedral surroundings of the titanium atom. Scanning electron micrograph showed the compactness of the rutile film.     

Preparation of highly photocatalytically active rutile titania nanorods decorated with anatase nanoparticles produced by a titanyl-oxalato complex solution

Rutile phase titania (TiO₂) nanorods and anatase nanoparticles were successfully synthesized from a titanyl-oxalato complex solution prepared using titanium (IV) sulfate and oxalic acid by a hydrothermal process. The impact of various hydrothermal conditions on the formation, morphology, phase, and grain size of the TiO₂ nanocrystals was investigated using fourier transformation infrared spectroscopy, X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and nitrogen adsorption. The photocatalytic activities have been evaluated for the photo-decomposition of phenol under ultraviolet visible illumination. The results revealed that the TiO₂ rutile nanorods decorated with anatase nanoparticles (with ~22% anatase) prepared at 160 °C for 72 h exhibit a higher photocatalytic activity than those pure anatase nanoparticles. This behavior was closely related to the better charge carrier separation in the cases of rutile–anatase mixtures. In addition, the possible growth mechanism and phase development of the rutile nanorods and anatase nanoparticles were illustrated.     

Structural phase-dependent resistivity of intrinsic-extrinsic co-doped transparent titanium dioxide films

The authors report a method of enhancing the conductivity of TiO₂ films by controlling their structural phases. Thin films of Nb:TiO₂ (TNO) were prepared on glass and silicon substrates by RF sputtering with varying Nb content at 200 °C. It is shown that fine control over the structural phases of TiO₂ is critical for achieving low resistivity. The resistivity values of the films doped with oxygen vacancies and Nb⁺⁵ decreased from 3.8 × 10⁻¹ to 4.1 × 10⁻³ Ω cm when the weight percent of rutile in anatase-rutile phase mixture decreases from 52.8% to 32%. Furthermore, the lowest resistivity value of 2.37 × 10⁻³ Ω cm was obtained for the doped TiO₂ films having single phase anatase structure. The physical processes responsible for the diverse electrical properties are discussed and are associated with the growth conditions. Our result indicates that highly conductive doped-TiO₂ film can be obtained by controlling the anatase phase formation via the growth temperature. The obtained results can significantly contribute to the development of transparent electrodes by RF sputtering, a suitable technique for coating large area substrates.     

Room-temperature synthesis of nanocrystalline titanium dioxide via electrochemical anodization

This paper presents a study of the growth of nanoporous anatase and rutile phases of titanium dioxide (TiO₂) subjected to electrochemical anodization at room temperature without post-thermal treatment, using sulfuric acid as the electrolyte. Effects of the applied voltage on the morphological, structural, and photoelectrochemical (PEC) properties were examined. Images from field emission scanning electron microscopy reveal that pore size could be manipulated by changing the anodization voltage. In addition, X-ray diffraction (XRD) results indicate that anatase and rutile phases of TiO₂ appeared in samples subjected to minimum anodization voltages of 100 V and 150 V. The Scherrer method was used to calculate the mean crystallite size, and the interplanar d-spacing formula was used to obtain the in-plane and out-of-plane strains. XRD measurements reveal that the amount of anatase and rutile crystallinity and their mean crystallite sizes were affected significantly by the anodization voltage. Results of the PEC studies reveal that the photocurrent density and photoconversion efficiency increased with increasing anodization voltage. In addition, the synthesized nanoporous TiO₂ showed stable photoresponse where only a small decay of photocurrent density is observed in numerous on-to-off illumination cycles.     

Hydrothermal synthesis of TiO2 nanosheets photoelectrocatalyst on Ti mesh for degradation of norfloxacin: Influence of pickling agents

TiO₂ nanosheets (TiO₂-NSs) photoanodes were synthesized on the surface of Ti meshes via the hydrothermal method, and then washed by HCl, HNO₃ and H₃PO₄, respectively. The TiO₂-NSs washed by different pickling agents were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electrons microscopy (SEM), and UV–vis diffuse reflectance spectra (DRS). The results showed that P and N were transferred into the lattice of TiO₂. The TiO₂-NSs samples washed by HCl and HNO₃ both had a mixture of anatase and rutile structures, while the TiO₂-NSs washed by H₃PO₄ only contained anatase phase. It could be concluded that H₃PO₄ pickling inhibited the formation of rutile phase. Moreover, TiO₂-NSs washed by HNO₃ (TiO₂-NSs-HNO₃) exhibited the strongest light absorption ability in visible region. Photoelectrochemical properties of the TiO₂-NSs photoelectrodes washed by different agents were investigated via transient photocurrent response (TPR), open-circuit potential (OCP) and electrochemical impedance spectroscopy (EIS). The photocatalytic (PC) and photoelectrocatalytic (PEC) performances were evaluated by the degradation of norfloxacin and the yield of •OH radicals in the reactors. The results showed that the TiO₂-NSs-HNO₃ photoelectrode revealed the highest photoinduced current of 1.08 mA/cm², open-circuit photovoltage of −0.22 V, PEC efficiency of 93% for the removal of norfloxacin. In addition, the enhanced PEC mechanism for TiO₂-NSs-HNO₃ was proposed. The high PEC activity of TiO₂-NSs-HNO₃ could be attributed to the introduction of N during acid-washing procedure, which could not only reduce the band gap energy but also accelerate the separation and transportation of photogenerated charge carriers.     

Fabrication of WO3 nanostructures by anodization method for visible-light driven water splitting and photodegradation of methyl orange

Visible light-responsive WO₃ nanostructures were synthesized by anodization in a NH₄F/Na₂SO₄ electrolyte solution. Applied potential and anodization time play an important role in the formation of self-organized WO₃ nanostructures, further developed upon anodization. The average pore diameter of ～80 nm with thickness of～300 nm of WO₃ nanoporous layer was successfully synthesized at 50 V for 15 min. The uniform and regular WO₃ nanoporous layer exhibited better photocurrent density of～0.18 mA/cm² at 0.7 V vs. SCE and better photodegradation of MO solution of ～50% after 5 h of visible-light illumination. The larger active surface area of WO₃ nanoporous layer played a significant role to generate more electron-hole pairs, which triggered the PEC water splitting reaction and photodegradation reaction much more effectively.     

Influence of Ti film thickness and oxidation temperature on TiO2 thin film formation via thermal oxidation of sputtered Ti film

Single-phase rutile TiO₂ films with good crystallinity were obtained by thermal oxidation of sputtered Ti films on Si and quartz substrates. The influence of the Ti film thickness on oxidation was systematically investigated. A temperature of 823 K was sufficient to fully oxidize Ti films of <0.2 μm in thickness, but 923 K was required for complete oxidation of thicker films. The crystal structure, phase, composition, and optical properties of the TiO₂ films were investigated using X-ray diffraction (XRD), Raman spectroscopy, energy-dispersive X-ray analysis (EDAX), and UV-vis-NIR spectroscopy. XRD and Raman analyses showed that the TiO₂ films are rutile phase. The bandgap of the TiO₂ films decreased with increasing thickness. A growth mechanism for TiO₂ thin films due to thermal oxidation of sputtered Ti films is proposed. Oxidation commences from the surface and proceeds inside the bulk and Ti→TiO₂ phase transformation occurs via different intermediate phases. We found that the oxidation temperature rather than the duration is the dominant factor in the growth of TiO₂ thin films.     

Preparation and characterization of TiO2 thin film by thermal oxidation of sputtered Ti film

Titanium dioxide (TiO₂) thin films were successfully prepared on quartz substrate by thermal oxidation of sputtered titanium film in air. The structure, composition, morphology and optical properties of oxidized TiO₂ films were characterized by Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy and UV-visible spectroscopy. Meanwhile, the photocatalytic activity of the films was evaluated on the basis of the degradation of methyl orange solution under UV irradiation. Ti films after oxidation present mainly in TiO₂ form with a larger amount of adsorbed O₂, and oxidation temperature has a strong impact on the crystal structure and properties of the films. A phase transformation of anatase to rutile for oxidized TiO₂ films occurred in the temperature range of 700–800 °C. The energy band gap of oxidized TiO₂ films decreased first and then increased with annealing temperature. Furthermore, TiO₂ film oxidized at 600 °C exhibited the best photocatalytic activity due to suitable crystal phase and size. These results might contribute to the synthesis of metal oxide thin films with expectant structural morphology and properties by thermal oxidation methods.     

Optical and electrical characteristics of 17 keV X-rays exposed TiO2 films and Ag/TiO2/p-Si MOS device

This work presents the effect of varied doses of X-rays radiation on the Ag/TiO₂/p-Si MOS device. The device functionality was observed to depend strongly on the formation of an interfacial layer composed of SiO_x and TiO_y, which was confirmed by the spectroscopic ellipsometry. The XRD patterns showed that the as prepared TiO₂ films had an anatase phase and its exposure to varied doses of 17 keV X-rays resulted in the formation of minute rutile phase. In the X-rays exposed films, reduced Ti³⁺ state was not observed; however a fraction of Ti–O bonds disassociated and little oxygen vacancies were created. It was observed that the device performance was mainly influenced by the nature and composition of the interfacial layer formed at the TiO₂/Si interface. The spectroscopic ellipsometry was used to determine the refractive indices of the interfacial layer, which was 2.80 at λ=633 nm lying in between that of Si (3.87) and TiO₂ (2.11). The dc and frequency dependent electrical measurements showed that the interface defects (traps) were for both types of charge carriers. The presence of SiO_x was responsible for the creation of positive charge traps. The interface trap density and relaxation time (τ) were determined and analyzed by dc and frequency dependent (100 Hz–1 MHz) ac-electrical measurements. The appearance of peak in G/ω vs log (f) confirmed the presence of interface traps. The interface traps initially increased up to exposure of 10 kGy and then decreased at high dose due to compensation by the positive charge traps in SiO_x part of the interface layer. It was observed that large number of interface defects was active at low frequencies and reduced to a limiting value at high frequency. The values of relaxation time, τ ranged from 4.3±0.02×10⁻⁴ s at 0 V and 7.6±0.2×10⁻⁵ s at −1.0 V.     

Facile synthesis of monodispersed nanocrystalline anatase TiO2 particles with large surface area and enhanced photocatalytic activity for degradation of organic contaminant in wastewaters

Monodispersed nanocrystalline anatase TiO₂ particles have been successfully synthesized via a facile hydrothermal process, combined with encircling hexamethylenetetramine protectors. The products are characterized in detail by X-ray diffraction, Raman spectra, and transmission electron microscope. Experimental results indicate that the catalysts possess monodispersed nanocrystalline anatase structure with small particles (∼6 nm) and large surface area (286 m² g⁻¹), which are attributed to encircling hexamethylenetetramine protectors for inhibiting the undesirable grain growth. The obtained monodispersed nanocrystalline anatase TiO₂ particles exhibit better photocatalytic activity than that of Degussa P25 TiO₂ for degradation of highly toxic 2,4-dichlorophenol in wastewaters under UV irradiation, which is ascribed to the small particle size and large surface area offering more active sites, and the improved crystallinity in favor of the separation of photogenerated electron–hole pairs.     

Application of sputter-deposited amorphous and anatase TiO2 as electron-collecting layers in inverted organic photovoltaics

We demonstrate the deposition of amorphous and anatase TiO₂ on indium tin oxide (ITO) substrates via the process of sputtering, and the use of these materials as electron-collecting layers (ECLs) in inverted-type organic photovoltaics (OPVs). Anatase TiO₂ was obtained via vacuum-annealing of as-deposited amorphous TiO₂ at 300 °C. No deterioration of optical and electrical properties of ITO was observed after both sputter-deposition of TiO₂ and annealing process. The anatase TiO₂ proved to be an effective ECL when employed in inverted OPVs using bulk heterojunction photoactive layer of poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester, achieving a power conversion efficiency of 3.3% (J_SC = 9.0 mA cm^?2, V_OC = 0.62 V and FF = 0.60).     

Properties of fluorine-doped tin oxide films prepared by an improved sol-gel process

Fluorine-doped tin oxide (FTO) films were prepared by an improved sol-gel process, in which FTO films were deposited on glass substrates using evaporation method, with the precursors prepared by the conventional sol-gel method. The coating and sintering processes were combined in the evaporation method, with the advantage of reduced probability of films cracking and simplified preparation process. The effects of F-doping contents and structure of films on properties of films were analyzed. The results showed the performance index (Φ_TC=3.535×10⁻³ Ω⁻¹ cm) of the film was maximum with surface resistance (R_sh) of 14.7 Ω cm⁻¹, average transmittance (T) of 74.4% when F/Sn=14 mol%, the reaction temperature of the sol was 50 °C, and the evaporation temperature was 600 °C in muffle furnace, and the film has densification pyramid morphology and SnO_2−xF_x polycrystalline structure with tetragonal rutile phase. Compared with the commercial FTO films (Φ_TC=3.9×10⁻³ Ω⁻¹ cm, R_sh=27.4 Ω cm⁻¹, T=80%) produced by chemical vapor deposition (CVD) method, the Φ_TC value of FTO films prepared by an improved sol-gel process is close to them, the electrical properties are higher, and the optical properties are lower.     

Sol–gel fabrication and enhanced optical properties,photocatalysis, and surface wettability of nanostructured titanium dioxide films

TiO₂ photocatalytic film, annealed at temperatures of 500 °C and 700 °C, was prepared on SiO₂ pre-coated glass via sol–gel technique for photocatalytic purposes and effects of catalyst-type on its properties were investigated by an X-ray diffractometer (XRD), Scanning Electron Microscope, UV–vis spectrophotometer, and contact angle measurements. The XRD results showed that present phases depend upon catalyst used in the solution and phase transformation behaves in a temperature-dependent manner. For the layers derived from sols containing acidic catalysts, the anatase structure dominated and exhibited much better photocatalytic activity. The results indicated that the sample derived from sol comprises H₂SO₄ as catalyst, and exhibits anatase grains with the lowest size. This could be the reason for its better photocatalytic activity. Finally, samples derived from sol containing acidic catalysts showed superhydrophilicity and superior cleaning ability.     

Performance analysis of dye solar cell with additional TiO2 layer under different light Intensities

Deployment of dye solar cells (DSCs) for building integration application would require a highly efficient solar cell that work well in diffused light. In order to improve the efficiency of dye solar cell, an additional layer of ultrathin anatase titanium dioxide (TiO₂) has been deposited for strengthening the adhesion of the porous TiO₂-based photo electrode to the conductive transparent substrate, which can lead to an enhancement in electron transportation. Fabricated cells of 1 cm² area were tested under different light intensities (100, 33 and 10 mW cm⁻²) and characterized by scanning electron microscopy (SEM), Raman spectroscopy and electrochemical impedance spectroscopy (EIS). Analysis showed an increment in overall quantum conversion efficiency (η), as high as 35% compared to the standard cell without the additional layer of TiO₂. EIS analysis has proven that the additional ultrathin anatase layer has improved the collection efficiency (Φ_COLL) as the result of the enhancement in both electron transport and lifetime within the porous TiO₂ film which translated into better conversion efficiency of DSCs.     

A study of ultrasonic spray pyrolysis deposited rutile-TiO2-based metal-semiconductor-metal ultraviolet photodetector

This work uses ultrasonic spray pyrolysis deposition to grow the TiO₂ film on a Si substrate. The TiO₂ film was annealed at 800 °C for 2 h to form rutile phase. X-ray diffraction, Raman spectrum, X-ray photoelectron spectroscopy were used to characterized rutile phase TiO₂. The optical characteristics like refractive index, extinction coefficient and absorption coefficient were measured. The rutile TiO₂-based metal-semiconductor-metal ultraviolet photodetector was fabricated and investigated, including current-voltage characteristic, photoresponsivity, external quantum efficiency, response time, noise equivalent power, and detectivity.     

Performance improvement of pentacene-doped P3HT:PCBM inverted polymer solar cells with AZO nanorod array passivated using photoelectrochemical technique

The pentacene-doped P3HT:PCBM inverted polymer solar cells (IPSCs) with Al-doped zinc oxide (AZO) nanorod array were fabricated. The AZO nanorod array could enhance the carrier collection and carrier extraction capability. The AZO nanorod array formed by the laser interference photolithography method and the wet etching process sequentially was used as the carrier collection and carrier transportation layers. The defects on the sidewall surface of the AZO nanorods were passivated by using the photoelectrochemical (PEC) method. It was demonstrated that the better performance of the IPSCs was obtained by PEC treatment. Compared with the IPSCs without PEC treatment, the short current density and power conversion efficiency of the IPSCs with PEC treatment for 60 s increased from 14.56 mA/cm² to 15.85 mA/cm² and 5.45% to 6.13%, respectively.     

Properties of In2O3 films obtained by thermal oxidation of sprayed In2S3

In₂S₃ thin films were grown by the chemical spray pyrolysis (CSP) method using indium chloride and thiourea as precursors at a molar ratio of S:In=2.5. The deposition was carried out at 350 °C on quartz substrates. The film thickness is about 1 µm. The films were then annealed for 2 h at 550, 600, 650 and 700 °C in oxygen flow. This process allows the transformation of nanocrystal In₂O₃ from In₂S₃ and the reaction is complete at 600 °C. X-ray diffraction spectra show that In₂O₃ films are polycrystalline with a cubic phase and preferentially oriented towards (222). The film grain size increases from 19 to 25 nm and RMS values increase from 9 to 30 nm. In₂O₃ films exhibit transparency over 70–85% in the visible and infrared regions due to the thickness and crystalline properties of the films. The optical band gap is found to vary in the range 3.87–3.95 eV for direct transitions. Hall effect measurements at room temperature show that resistivity is decreased from 117 to 27 Ω cm. A carrier concentration of 1×10¹⁶ cm^?3 and mobility of about 117 cm² V^?1 s^?1 are obtained at 700 °C.     

High dielectric constant TiO2 film grown on polysilicon by liquid phase deposition

In this study, titanium dioxide (TiO₂) films were grown on polycrystalline silicon by liquid phase deposition (LPD) with ammonium hexafluoro-titanate and boric acid as sources. The film structure is amorphous as examined by X-ray diffraction (XRD). A uniform composition of LPD-TiO₂ was observed by SIMS examination. The leakage current density of an Al/LPD-TiO₂/poly-Si/p-type Si metal–oxide–semiconductor (MOS) structure is 1.9 A/cm² at the negative electric field of 0.7 MV/cm. The dielectric constant is 29.5 after O₂ annealing at 450 °C. The leakage current densities can be improved effectively with a thermal oxidized SiO₂ added at the interface of LPD-TiO₂/poly-Si. The leakage current density can reach 3.1×10⁻⁴ A/cm² at the negative electric field of 0.7 MV/cm and the dielectric constant is 9.8.     

Efficient dye-sensitized solar cells based on CNTs and Zr-doped TiO2 nanoparticles

The light scattering, harvesting and adsorption effects in dye-sensitized solar cells (DSSCs) are studied by preparation of coated carbon nanotubes (CNTs) with TiO₂ and Zr-doped TiO₂ nanoparticles in the forms of mono- and double-layer cells. X-ray diffraction (XRD) analysis reveals that the phase composition of Zr-doped TiO₂ electrode is a mixture of anatase and rutile phases with major rutile content, whereas it is the same mixture with major anatase content for coated CNTs with TiO₂. Furthermore, the average crystallite size of Zr-doped TiO₂ electrode is slightly decreased with Zr introduction. Field emission scanning electron microscope (FE-SEM) images show that the porosity of Zr-doped TiO₂ electrodes is higher than that of undoped electrode, enhancing dye adsorption. UV–visible spectroscopy analysis reveals that the absorption onset of Zr-doped TiO₂ electrodes is slightly shifted to longer wavelength (the red-shift) in comparison with that of undoped TiO₂ electrode. Moreover, the band gap energy of TiO₂ nanoparticles is decreased by Zr introduction, enhancing light absorption. It is found that electron injection of monolayer TiO₂ electrode is improved by introduction of 0.025 mol% Zr, resulted in enhancement of its power conversion efficiency (PCE) up to 6.81% compared with 6.17% for pure TiO₂ electrode. Moreover, electron transport and light scattering are enhanced by incorporation of 0.025 wt% coated CNTs with TiO₂ in the over-layer of double layer electrode. Therefore, double layer solar cell composed of 0.025 mol% Zr-doped TiO₂ nanoparticles as the under-layer and mixtures of these nanoparticles and 0.025 wt% coated CNTs with TiO₂ as the over-layer shows the highest PCE of 8.19%.     

Effect of different ethanol/water solvent ratios on the morphology of SnO2 nanocrystals and their electrochemical properties

We report on the effect of different ethanol/water solvent ratios on the morphology of SnO₂ nanocrystals prepared by the conventional hydrothermal method and their electrochemical properties. The nanocrystals were structurally and morphologically characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), surface area measurements, and transmission electron microscopy. The XRD patterns indicate that the sphere-like SnO₂ microcrystals have a rutile-type tetragonal structure and FESEM images show that the microspheres have a diameter of 2–5 μm. We found that the ethanol/water volume ratio plays an important role in formation of the final product. Electrochemical tests revealed that the SnO₂ microspheres had a high initial capacity of 1546 mAh g^?1 at a current density of 100 mA g^?1 and retained a reversible capacity of 439 mAh g^?1 after 30 discharge cycles.