Structure,morphology and photocatalytic activity of Cu2O/Pt/TiO2 three-layered nanocomposite films

Novel Cu₂O/Pt/TiO₂ three-layered nanocomposite films were prepared by deposition on glass substrates using the magnetron sputtering method. Their structure, surface morphology as well as optical and photocatalytic properties were examined by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, UV–visible spectroscopy, and photoluminescence spectroscopy. As a comparison, Cu₂O/TiO₂ double-layer films were also investigated. The results show that Cu₂O/TiO₂ double-layer films have relatively smooth surfaces with agglomerated Cu₂O particle, whereas the surface layer of the Cu₂O/Pt/TiO₂ three-layered nanocomposite films was composed of fine nano-sized columnar Cu₂O and they had a rough surface morphology due to the insertion of the Pt layer. The photocatalytic activity of the three-layered films is significantly higher than that of the Cu₂O/TiO₂ double-layered composite films. Such enhancement is closely related to the presence of the Pt layer and the rough surface, which was composed of fine nano-sized Cu₂O columns; this increases the utilization of visible light as well as promotes the transfer of interfacial charge and the separation of photogenerated electron–holes.     

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.     

Effects of silver substrates on the visible light photocatalytic activities of copper-doped titanium dioxide thin films

Novel copper-doped titanium dioxide (Cu-doped TiO₂) thin films on silver (Ag) substrates with different thicknesses were prepared by sol–gel and magnetron sputtering methods. The influences of the Ag substrate thickness on the morphology and performance of the films were investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, UV–visible spectroscopy, and photocatalytic degradation testing with methylene blue aqueous solution under visible light irradiation. The results indicated that Ag substrates with an optimal thickness of 30 nm not only maintained the tiny nanocrystals but also greatly improved dispersion of the nanoparticles on the surface of the nanofilms. Furthermore, during the calcination process, part of the Ag atoms diffused from the substrates into the Cu–TiO₂ films and substituted for the Cu ions to form Ag–TiO₂. A proper Ag substrate thickness (30 nm) greatly improved the photocatalytic properties of TiO₂ with photocatalytic efficiency, reaching approximately 86% in 300 minutes under visible light irradiation. However, an excess of Ag substrate not only led to the Cu ion separating out in the form of CuO but also resulted in the agglomeration of TiO₂ particles on the surface, which were detrimental to photocatalytic activities.     

Enhancing current density of perovskite solar cells using TiO2-ZrO2 composite scaffold layer

A novel scaffold layer composed of TiO₂-ZrO₂ composite was fabricated for perovskite solar cell. Compared with pure TiO₂ nanoparticles (NPs), the relatively larger ZrO₂ NPs could increase film roughness and enhance light-scattering effect in TiO₂-ZrO₂ composite films. The device exhibited outstanding power conversion efficiency (PCE) of 11.41%. The morphology and aggregation of particles, three-dimensional roughness, as well as the ingredient and micro-structure of FTO/compact TiO₂/TiO₂-ZrO₂ was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscope (AFM), energy dispersive spectrometer (EDS), and X-ray diffraction (XRD), respectively. Moreover, the optical property of TiO₂-ZrO₂ films for visible light was characterized by UV–visible absorption spectroscopy (UV–vis), and its influence on quantum yield of the device was further demonstrated by incident photon-to-electron conversion efficiency (IPCE). Owing to the inert oxide, the short-circuit current density of perovskite solar cell using TiO₂-ZrO₂ composition as scaffold layer increased by 21% compared to the one employing pure TiO₂ mesoporous film.     

Visible light active TiO2–ZnO composite films by cerium and fluorine codoping for photocatalytic decontamination

The visible light active Ce/F codoped TiO₂–ZnO composite films with a bad gap of 1.82 eV were successfully prepared though a simple sol–gel method. Experimental results indicated that the composite films showed excellent photocatalytic performance towards photocatalytic oxidation of organic pollutants including formaldehyde, acid naphthol red (ANR) and methyl green (MG). The catalysts were characterized by photoluminescence (PL) spectra, UV–vis diffraction reflectance absorption spectra (DRS), X-ray diffraction (XRD), differential thermal analysis-thermogravimetry (DTA-TG), field emission scanning electron microscopy (FE-SEM) equipped with energy-dispersive spectroscopy (EDS), and N₂ adsorption/desorption isotherms. The DRS and PL spectra results showed that multi-modification not only induced strong visible light absorption but also reduced the recombination rate of electron–hole pairs. The DTA-TG and XRD results indicated that the crystal type of the TiO₂-based catalyst was mostly stabilized in anatase. The FE-SEM and BET surface area results revealed that the nanocrystalline Ce/F codoped TiO₂–ZnO composite samples with the larger specific surface area were composed of smaller nanoparticles compared to pure TiO₂. The mechanism of the enhanced photocatalytic activity was discussed in this study.     

Novel Method of Preparation of Gold‐Nanoparticle‐Doped TiO2 and SiO2 Plasmonic Thin Films: Optical Characterization and Comparison with Maxwell–Garnett Modeling

SiO₂ and TiO₂ thin films with gold nanoparticles (NPs) are of particular interest as photovoltaic materials. A novel method for the preparation of spin‐coated SiO₂–Au and TiO₂–Au nanocomposites is presented. This fast and inexpensive method, which includes three separate stages, is based on the in situ synthesis of both the metal‐oxide matrix and the Au NPs during a baking process at relatively low temperature. It allows the formation of nanocomposite thin films with a higher concentration of Au NPs than other methods. High‐resolution transmission electron microscopy studies revealed a homogeneous distribution of NPs over the film volume along with their narrow size distribution. The optical manifestation of localized surface plasmon resonance was studied in more detail for TiO₂‐based Au‐doped nanocomposite films deposited on glass (in absorption and transmittance) and silicon (in specular reflectance). Maxwell–Garnett effective‐medium theory applied to such metal‐doped nanocomposite films describes the peculiarities of the experimental spectra, including modification of the antireflective properties of bare TiO₂ films deposited on silicon by varying the concentration of metal NPs. The antireflective capabilities of the film are increased after a wet etching process.     

Synthesis of Pd co-doped nano-TiO2–SO42– and its synergetic effect on the solar photodegradation of Reactive Red 120 dye

Pd co-doped TiO₂–SO₄^2– photocatalyst was synthesized by sol–gel, photodeposition methods and characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) and BET surface area measurements. Elements present in the catalyst are shown by X-ray photoelectron spectroscopy. X-ray diffraction analysis reveals that the photocatalyst has an anatase structure. Sulfation by sulfuric acid reduces agglomeration, inhibits the phase transformation and enhances the stability of TiO₂. The solar photocatalytic activity of the Pd–TiO₂–SO₄^2– is higher than that of the TiO₂–P25, Pd–TiO_2, bare TiO₂ and TiO₂–SO₄^2– at pH 7 for the mineralization of Reactive Red 120. The modification of palladium shows higher adsorption with synergistic effect and also enhances the separation of photogenerated electron–hole pairs, leading to higher photodegradation efficiency. The effects of operational parameters such as the amount of photocatalyst, initial pH on photo mineralization have been analyzed. A dual mechanism of degradation by Pd–TiO₂–SO₄^2– is proposed to explain its higher activity in solar light. The mineralization of Reactive Red 120 has also been confirmed by CV and COD measurements. The catalyst is found to be stable and reusable. Based on the degradation intermediates identified by GC–MS analysis, a reaction pathway is proposed.     

Multistep hydrothermal route for nanocoral architecture of anatase TiO2: synthesis and characterization of dye‐sensitized solar cell performance

Multistep hydrothermal (MSH) process is employed for growth of TiO₂ nanocorals onto the conducting fluorine‐doped tin oxide‐coated glass substrates. The surface morphological features and physical properties of TiO₂ films were investigated by field emission scanning electron microscopy, high resolution transmission electron microscopy, X‐ray diffraction, Fourier transform infrared spectroscopy, Fourier transform Raman spectroscopy, room temperature photoluminescence spectroscopy and X‐ray photoelectron spectroscopy. The surface morphology revealed the formation of TiO₂ corals having nanosized (30–40 nm) polyps. The photoelectrochemical properties of the TiO₂ nanocoral electrodes were investigated in 0.1 M NaOH electrolyte under ultraviolet illumination. The results presented in this study highlight two major findings: (i) tuning the photoelectrochemical response and photoconversion efficiency via controlled thickness of TiO₂ nanocorals by MSH route and (ii) the substantial increase in short‐circuit photocurrent (J_sc) because of the improved charge transport through TiO₂ nanocorals prepared via MSH process. Copyright © 2012 John Wiley & Sons, Ltd.     

Optical studies of TiO2–lead phthalocyanine nanocomposite thin films prepared by electron beam evaporation

The nanocomposite thin films of titanium dioxide (TiO₂)–lead phthalocyanine (PbPc) have been prepared on glass substrates by the electron beam evaporation technique. The optical properties of TiO₂/PbPc nanocomposite thin films have been investigated using a spectrophotometric measurement of the absorbance and transmittance at normal incident of light in the wavelength region 300–800 nm. Surface morphology of thin films has been characterized using field emission scanning electron microscopy (FESEM). The UV–vis analysis has been performed to determine the type of electronic transition and the optical energy band gap. The analysis of the spectral behavior of the absorption coefficient in the intrinsic absorption region reveals that the absorption mechanism is due to direct transition. Moreover, by studying the absorption coefficient spectra just below the fundamental absorption edge, the width of band tails of localized states (Urbach energy), steepness parameter and width of the defect states have been evaluated. The obtained results of this novel nanocomposite (TiO₂/PbPc) support the desirable features for the optoelectronic devices.     

Synthesis of TiO2 thin films with highly efficient surfaces using a sol–gel technique

Three different strong acid catalysts were used in a simple sol–gel synthesis to produce TiO₂ thin films with increased homogeneity and enhanced photocatalytic activity on their mesoporous surfaces. Various techniques were used to characterize the samples, including UV–visible spectrophotometry, X-ray diffraction, micro-Raman spectrometry, photobleaching, scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The band gaps varied from 3.73 to 3.75 eV and the transmittance was >80%. An anatase phase was obtained in all the samples and the crystal size varied from 20 to 45 nm as a function of the annealing temperature. The increase in the efficiency of the surface of the TiO₂ thin films was evaluated by photodegradation of methylene blue in water. The results showed that the acid catalysts used in the synthesis had an important effect on the morphology and photocatalytic activity of the thin films, resulting in more efficient surfaces. Synthesis with hydrofluoric acid produced thin films with a homogenous mesoporous structure and improved the photodegradation of the methylene blue dye to 92% in 2.5 h.     

A green approach to the fabrication of titania–graphene nanocomposites: Insights relevant to efficient photodegradation of Acid Orange 7 dye under solar irradiation

Environment friendly and efficient strategy for the preparation of titanium dioxide (TiO₂)–graphene (GR) based hybrid nanocomposite has been demonstrated by simple chemical approach for the photodegradation of Acid Orange 7 (AO7) dye under solar irradiation. The resultant nanocomposite structure and composition has been characterized by Ultraviolet Diffusive Reflectance Spectroscopy (UV-DRS), Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), Raman spectroscopy and X-ray diffraction (XRD) studies. The incorporation of TiO₂ nanoparticles on the surface of GR was confirmed by High Resolution Transmission Electron Microscopy (HRTEM) and Field Emission Scanning Electron Microscopy (FESEM) studies. Electrochemical Impedance spectroscopy (EIS) and Cyclicvoltammetry (CV) studies revealed that the incorporation of GR with TiO₂ nanoparticles significantly enhanced the redox property and electrical conductivity. During photocatalysis, the TiO₂–GR nanocomposites have high photocatalytic activity compared with that of TiO₂ towards AO7 dye degradation under solar light irradiation. The enhanced photocatalytic activity might be attributed to the role GR played as an electron acceptor and transporter in the composite film, which effectively suppressed the charge recombination and promoted the charge transfer within the composite.     

Visible light assisted photocatalytic activity of TiO2–metal vanadate (M=Sr,Ag and Cd) nanocomposites

TiO₂–metal vanadate nanocomposites (TiO₂–MV) were synthesized by the precipitation method and successfully characterized using UV–visible diffuse reflectance spectroscopy (UV–vis-DRS), powder X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) techniques. The photocatalytic activity of TiO₂–MV was investigated for the degradation of fast green (FG) dye under visible light irradiation. The photocatalytic activity of TiO₂–silver vanadate [TiO₂–Ag₃VO₄] was found to be much higher than that of TiO₂–cadmium vanadate [TiO₂–CdV₂O₆], TiO₂–strontium vanadate [TiO₂–Sr₃(VO₄)₂] and TiO₂. The effect of operational parameters such as pH, photocatalyst concentration and initial dye concentration on the photodegradation of FG was examined in detail. The mineralization of FG was confirmed by chemical oxygen demand (COD) and total organic carbon (TOC) measurements. Moreover, TiO₂–Ag₃VO₄ was found to be a reusable photocatalyst.     

Capacitance-voltage analysis of a high-k dielectric on silicon

Device characteristics of TiO₂ gate dielectrics deposited by a sol-gel method and DC sputtering method on a P-type silicon wafer are reported. Metal-oxide-semiconductor capacitors with Al as the top electrode were fabricated to study the electrical properties of TiO₂ films. The films were physically characterized by using X-ray diffraction, a capacitor voltage measurement, scanning electron microscopy, and by spectroscopy ellipsometry. The XRD and DST-TG indicate the presence of an anatase TiO₂ phase in the film. Films deposited at higher temperatures showed better crystallinity. The dielectric constant calculated using the capacitance voltage measurement was found to be 18 and 73 for sputtering and sol-gel samples respectively. The refractive indices of the films were found to be 2.16 for sputtering and 2.42 for sol-gel samples.     

Versatile Nanocomposite Coatings with Tunable Cell Adhesion and Bactericidity

TiO₂‐Ag nanocomposites are known for their bactericidal effect during exposure to appropriate UV radiation. While involving hazardous radiation, and limited to accessible areas, the bactericidity of these coatings is not persistent in the absence of UV light, which impedes their commercial application. Herein it is shown that TiO₂‐Ag nanocomposites can be made highly bactericidal without the need of irradiation. Beyond this, bactericidity can even be mitigated in the presence of pre‐irradiated coatings. Biocompatibility and cell adhesion are also negligibly small for the as‐processed, non‐irradiated coatings, and become fairly high when the coatings are irradiated prior to testing. This opens the possibility to pattern the coatings into areas with high and low cell adhesion properties. Indeed by irradiating the coating through a mechanical mask it is shown that fibroblast cell adherence is sharply confined to the irradiated area. These properties are achieved using TiO₂‐Ag thin films with high silver loadings of 50 wt%. The films are processed on stainless steel substrates using solution deposition. Microstructural characterization by means of X‐ray diffraction, Raman, and X‐ray photoelectron spectroscopy, high‐resolution scanning electron microscopy, and atomic force microscopy show a highly amorphous TiO₂‐Ag_xO nanocomposite matrix with scattered silver nanoparticles. UV irradiation of the films results in the precipitation of a high density of silver nanoparticles at the film surface. Bactericidal properties of the films are tested on α‐haemolyzing streptococci and in‐vitro biocompatibility is assessed on primary human fibroblast cultures. The results mentioned above as to the tunable bactericidity and biocompatibility of the TiO₂‐Ag coatings developed herein, are amenable to silver ion release, to catalytic effects of silver nanoparticles, and to specific wettabilities of the surfaces.     

Low‐temperature flexible Ti/TiO2 photoanode for dye‐sensitized solar cells with binder‐free TiO2 paste

An energy‐economical dye‐sensitized solar cell (DSSC) with highly flexible Ti/TiO₂ photoanode was developed through a low‐temperature process, using a binder‐free TiO₂ paste. Ti foils, coated with the binder‐free TiO₂ films were annealed at various temperature. Scanning electron microscopic (SEM) images of the films show uniform, mesoporous and crack‐free surface morphologies as well as interpenetrated TiO₂ network. DSSCs with binder‐free TiO₂ films annealed at 450, 350, 250 and 120°C show solar‐to‐electricity conversion efficiencies (η) of 4.33, 4.34, 3.72 and 3.40%, respectively, which are comparable to the efficiency of 4.56% obtained by using a paste with binder and annealing it at 450°C; this observation demonstrates the benefits of a binder‐free TiO₂ paste for the fabrication of energy‐fugal DSSCs. On the other hand, when organic binder was used in the TiO₂ paste for film preparation, a drastic deterioration in the cell performance with decreasing annealing temperature is noticed. Laser‐induced photo‐voltage transient technique is used to estimate the electron lifetime in various Ti/TiO₂ films. Electrochemical impedance spectroscopic (EIS) analysis shows that the lower the annealing temperature of the TiO₂ coated Ti foil, the larger the charge transfer resistance at the TiO₂/dye/electrolyte interface (R_ct2). Copyright © 2011 John Wiley & Sons, Ltd.     

Enhanced photoelectrocatalytic activity of Cr-doped TiO2 nanotubes modified with polyaniline

A facile method for preparation of Cr-doped TiO₂ nanotubes (Cr–TiO₂ NTs) modified with polyaniline (PANI) was developed. The obtained materials were analyzed with scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy. The photoelectrochemical property of PANI/Cr–TiO₂ nanotubes was studied by voltammetry, photocurrent and electrochemical impedance spectroscopy (EIS). Using PANI/Cr–TiO₂ NTs as photoanode, the removal of p-nitrophenol (PNP) by photoelectrocatalytic oxidation technique was investigated. Compared with Cr–TiO₂ NTs, PANI/Cr–TiO₂ NTs showed an increased efficiency in the photoelectrocatalytic degradation of PNP. Moreover, photoelectrocatalysis was more efficient for PNP degradation than electrochemical oxidation, direct photolysis, and photocatalysis. The influences of applied bias potential, initial concentration of PNP and solution pH on the photoelectrocatalytic degradation of PNP were investigated. Under optimized conditions, almost all PNP could be degraded on PANI/Cr–TiO₂ NTs after 2-h photoelectrocatalytic treatment.     

Interface‐Engineered Amorphous TiO2‐Based Resistive Memory Devices

Crossbar‐type bipolar resistive memory devices based on low‐temperature amorphous TiO₂ (a‐TiO₂) thin films are very promising devices for flexible nonvolatile memory applications. However, stable bipolar resistive switching from amorphous TiO₂ thin films has only been achieved for Al metal electrodes that can have severe problems like electromigration and breakdown in real applications and can be a limiting factor for novel applications like transparent electronics. Here, amorphous TiO₂‐based resistive random access memory devices are presented that universally work for any configuration of metal electrodes via engineering the top and bottom interface domains. Both by inserting an ultrathin metal layer in the top interface region and by incorporating a thin blocking layer in the bottom interface, more enhanced resistance switching and superior endurance performance can be realized. Using high‐resolution transmission electron microscopy, point energy dispersive spectroscopy, and energy‐filtering transmission electron microscopy, it is demonstrated that the stable bipolar resistive switching in metal/a‐TiO₂/metal RRAM devices is attributed to both interface domains: the top interface domain with mobile oxygen ions and the bottom interface domain for its protection against an electrical breakdown.     

Photocatalytic degradation of erythromycin under visible light by zinc phthalocyanine-modified titania nanoparticles

Zinc phthalocyanine modified TiO₂ nanoparticles (Znpc–TiO₂) were prepared by the chemical impregnation method to improve the photocatalytic activity of TiO₂ under visible light. The prepared nanoparticles were characterized by UV–vis diffuse reflectance spectroscopy (UV–vis-DRS), X-ray powder diffraction, scanning electron microscopy transmission electron microscopy and Brunauer-Emmett-Teller. surface area analysis techniques. The photocatalytic activity of Znpc–TiO₂ was investigated for the degradation of erythromycin. The results revealed that UV–vis absorption edge of Znpc–TiO₂ is slightly shifted towards visible region and it has a higher surface area than that of TiO₂. The photocatalytic activity of Znpc–TiO₂ was superior (74.21%) than that of TiO₂ (31.57%). Besides the photocatalyst (Znpc–TiO₂) is stable and may be reused for several times.     

Physical and Electrical Properties of Dy2O3 and Dy2TiO5 Metal Oxide–High-κ Oxide–Silicon-Type Nonvolatile Memory Devices

In this paper, we compare the physical and electrical properties of metal oxide–high-κ oxide–silicon (MOHOS)-type devices using Dy₂O₃ and Dy₂TiO₅ films as charge-trapping layers. X-ray diffraction and x-ray photoelectron spectroscopy revealed the structural and compositional features of these films after annealing at various temperatures. MOHOS memory devices incorporating the Dy₂TiO₅ trapping layer that had been annealed at 800°C exhibited a larger memory window of ~2.91 V (measured at a sweep voltage range of ±9 V), higher flatband voltage shift of 2 V (programming voltage at 9 V for 0.1 s), and smaller charge loss of ~10% (measured at room temperature after 10⁴ s), relative to those of the systems that had been subjected to other annealing conditions. This result suggests that the Dy₂TiO₅ film featuring a higher dielectric constant as well as a thinner silicate layer provides a higher probability of charge carrier trapping and deeper electron trapping levels. In addition, the centroid of trapped charge in the Dy₂TiO₅ layer was extracted by the constant current stress method and compared with that of the Dy₂O₃ layer.     

Effects of annealing temperature on the photoluminescence of RF sputtered Barium titanate thin films

BaTiO₃ thin films were deposited onto quartz substrates by an RF magnetron sputtering method. The films deposited at room temperature and annealed at 773–1173 K were characterized using X-ray diffraction (XRD)Scanning electron microscopy (SEM), UV–vis spectroscopy and Photoluminescence spectroscopy (PL). X-ray diffraction studies revealed that the film is amorphous in nature at 773 K and that the crystallinity increases with increase in annealing temperature. The average crystallite size of the films increased from 13–18 nm and the optical band gap decreased in the range of 4.33–3.43 eV, with increase in annealing temperature. The films exhibited good adherence to the substrates and the SEM images showed smooth surface morphology. Energy dispersive X-ray (EDX) analysis confirmed the presence of barium, titanium and oxygen in the film. The red-shifts of excitonic UV emission peaks were observed in all samples which can be attributed to the stress produced due to lattice distortions. The visible PL emission intensity showed appreciable enhancement with post-deposition annealing.