Absorption spectra and photocurrent densities of Ag nanoparticle/TiO2 composite thin films with various amounts of Ag

To study the absorption spectra and photocurrent densities of metallic Ag nanoparticle/titania (Ag NP)/TiO₂ composite thin films, COMP-Agn, with various amounts of Ag (10 mol% ≤n ≤80 mol%) were fabricated on a quartz glass substrate at 600 °C using the molecular precursor method. Respective precursor solutions for Ag-nanoparticles and titania were prepared from Ag salt and a titanium complex. Apart from a surface plasmon resonance (SPR) peak around 400 nm, additional wide-range absorption spread in the wide vis-region at wavelengths greater than 400 nm was observed in the composite thin films. The wide-range absorption is due to a tip–tip plasmon mode, intraparticle plasmonic coupling of tip and cavity resonances (LSPR). The absorption spectral patterns could be categorized into three types, depending on the Ag NP content. The photocurrent density of the TiO₂ thin film and COMP-Agn was measured under natural potential by a conventional three-electrode method using a Ag plate as a counter electrode. The photocurrent densities of COMP-Agn were comparable to those of the three patterns categorized according to their absorption spectra. The cathodic photocurrent densities generated by COMP-Agn with Ag content greater than 45 mol% could be observed under both UV- and visible (vis-) light irradiation. The enhanced photocurrent density was observed till the Ag content was increased up to 70 mol%. Under dark conditions, cathodic current densities were 1/5–1/20 of those under vis-light irradiation probable due to chemical redox reactions that may occur to the system. On the basis of photoexcited electron transfer from Ag NPs to the TiO₂ conduction band and the electrical conductivity of COMP-Agn, the excellent response to vis-light and major factors affecting the photoresponse and photocurrent polarity were clarified by LSPR.     

Microstructure,electrical and humidity sensing properties of TiO<Subscript>2</Subscript>/polyaniline nanocomposite films prepared by sol–gel spin coating technique

High sensitive resistive type humidity sensor based titanium oxide/polyaniline (TiO₂/PANI) nanocomposite thin films prepared by a sol–gel spin coating technique on an alumina substrate. The resultant nanocomposites were characterized by using X-ray diffraction (XRD), Field emission electron microscopy, Fourier transform infrared spectroscopy (FTIR), UV–Vis absorbance and energy dispersive spectra analysis. In the XRD patterns of both pure and TiO₂/PANI composite confirms the deposition of PANI on TiO₂ and the average size of the composite particle was found to be 32 nm. Large number of nano grain surface being covered by PANI, which agrees very well with the results obtained by XRD studies. FTIR and UV–Vis spectra reveal that the PANI component undergoes an electronic structure modification as a result of the TiO₂ and PANI interaction. The room temperature resistivity was found to be for TiO₂ and TiO₂/PANI nanocomposite films 1.42?×?10⁶ and 2.56?×?10³ Ω cm respectively. The obtained TiO₂/PANI nanocomposites sensor exhibited higher humidity sensing performance such as high sensitivity, fast response (20 s) and recovery time (15 s) and high stability.     

Maximum SiO2 layer thickness by utilizing polyethylene glycol as the surfactant in synthesis of core/shell structured TiO2–SiO2 nano-composites

TiO₂–SiO₂ nano-composites with the core/shell structure have been prepared by means of a technique based on an extension of well-known Stöber process. In this way, the silica coating of TiO₂ nano-particles in the presence of various commercially available surfactants of cationic, anionic and nonionic has been conducted with the aim to increase barrier properties against UV (UV blocking) radiation, in order to optimize photo-killing ability of the TiO₂ nano-particles and decline of the high photo-catalytic property of titania. The influences of varying coating parameters such as time and temperature on the silica content of nano-composites have been studied and optimum conditions for attaining a thick layer of SiO₂ have been determined. Electro-phoretic mobility measurements indicated that the silica coating shifted the iso-electric point of titania toward that of a typical pure colloidal silica. Surface elemental composition of core/shell structured TiO₂–SiO₂ nano-composites was verified by using energy dispersive X-ray analysis. It was found that maximum silica shell thickness can be obtained in the presence of polyethylene glycol as a nonionic surfactant at 80 °C for 360 min. The photo-catalytic activities were evaluated by the degradation of an aqueous solution of methylene blue under UV light irradiation. In addition, the resultant optimum nano-composites have been characterized by FESEM, TEM, BET, FTIR and UV–Vis spectroscopy.     

Effects of light scattering TiO2 surface-modified by dual oxide coating in dye-sensitized solar cells

The surface of light scattering TiO₂ particles in the dye-sensitized solar cell (DSSC) was dual-coated with Al₂O₃ and SiO₂ nanoparticles. The surface modification of the light scattering TiO₂ particles was performed by a modified sol–gel method using the colloidal alumina and the colloidal silica as surface coating precursors. It was revealed that the dual-coated light scattering TiO₂ particles leads to an increase in short-circuit photocurrent of DSSC device, resulting in an increase in energy conversion efficiency. This seems to be due to the increase of the light scattering by a combination of the light scattering TiO₂ particles and the oxide nanoparticles such as Al₂O₃ and SiO₂.     

Co-electrospinning fabrication and photocatalytic performance of TiO2/SiO2 core/sheath nanofibers with tunable sheath thickness

In this paper, core/sheath TiO₂/SiO₂ nanofibers with tunable sheath thickness were directly fabricated via a facile co-electrospinning technique with subsequent calcination at 500 °C. The morphologies and structures of core/sheath TiO₂/SiO₂ nanofibers were characterized by TGA, FESEM, TEM, FTIR, XPS and BET. It was found that the 1D core/sheath nanofibers are made up of anatase–rutile TiO₂ core and amorphous SiO₂ sheath. The influences of SiO₂ sheath and its thickness on the photoreactivity were evaluated by observing photo-degradation of methylene blue aqueous solution under the irradiation of UV light. Compared with pure TiO₂ nanofibers, the core/sheath TiO₂/SiO₂ nanofibers performed a better catalytic performance. That was attributed to not only efficient separation of hole–electron pairs resulting from the formation of heterojunction but also larger surface area and surface silanol group which will be useful to provide higher capacity for oxygen adsorption to generate more hydroxyl radicals. And the optimized core/sheath TiO₂/SiO₂ nanofibers with a sheath thickness of 37 nm exhibited the best photocatalytic performance.     

Highly enhanced photocatalytic degradation of dye rhodamine B on the biogenic hierarchical porous TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> with 1D/3D-chain

A hybrid photocatalyst consisting of TiO₂ and nonporous SiO₂ (TiO₂/CS-RH) is prepared by loading TiO₂ sol on one-dimensional/three-dimensional chain (1D/3D-chain) which is synthesized from rice husk. The products are characterized by X-ray diffraction, N₂-adsorption–desorption analysis and scanning electron microscopy. Meanwhile, the corresponding photocatalytic activity is evaluated by measuring the photocatalytic oxidation of rhodamine B (RhB). The results reveal that TiO₂/CS-RH displays a hierarchical porous structure from micrometer to nanometer scale with high BET surface area (574.7–719.4 cm²/g). Meanwhile, the activity of TiO₂/CS-RH for the photocatalytic degradation of RhB in aqueous slurry is significantly higher than that of the unsupported TiO₂. The optimal TiO₂ loaded on the support was two times and then treated at 600 °C for 120 min to complete the conversion of RhB. In contrast, the unsupported TiO₂ photocatalyst could convert only 20% of RhB in the same irradiation time and condition.     

Do properties of bioactive glasses exhibit mixed alkali behavior?

The effect of substituting K₂O for Na₂O on the physical and chemical properties of 15 glasses in the system Na₂O–K₂O–CaO–P₂O₅–SiO₂ was studied for three series: low (52 mol% SiO₂), medium (60 mol% SiO₂) and high (66 mol% SiO₂) silica. The SiO₂ content expressed as weight-% varied from 46 to 64 wt%, thus suggesting that the compositions were either bioactive or biocompatible. The crystallization tendency and sintering behavior were studied using differential thermal analysis and hot stage microscopy. Formation of silica- and hydroxy-apatite-rich layers were studied for glass plates immersed in static simulated body fluid. The release of inorganic ions into Tris buffer solution was analyzed using inductively coupled plasma optical emission spectrometer in dynamic and static conditions. Substitution of K₂O for Na₂O suggested mixed alkali effect (MAE) for the thermal properties with a minimum value around 25% substitution. With increased share of K₂O in total alkali oxides, the hot working window markedly expanded in each series. Silica and hydroxyapatite layers were seen only on the low silica glasses, while a thin silica-rich layer formed on the other glasses. In each series, greater dissolution of alkali and alkali earth ions was seen from K-rich glasses. Clear MAE and preferential ion dissolution were recorded for medium and high silica series, while for low silica glasses, the initial MAE dissolution trends become rapidly covered by other simultaneous surface reactions. MAE enables designing especially low silica bioactive glasses for improved hot working properties and medium and high silica glasses for controlled dissolution.     

Nanostructured mesoporous Au/TiO2 for photocatalytic degradation of a textile dye: the effect of size similarity of the deposited Au with that of TiO2 pores

Mesoporous Au/TiO₂ nanocomposites with different Au particle size (7.3–11.8 nm) were synthesized via deposition–precipitation method. The synthesized nanocomposites have been characterized by XRD, TEM, XPS, DLS, ICP-OES, N₂ sorpometry, and UV–Vis spectroscopy. Au/TiO₂ showed higher quantum yield and greater photocatalytic efficiency compared to pure TiO₂ under both ultraviolet and sunlight illumination. The increase of the photocatalytic efficiency of TiO₂ upon deposition with gold nanoparticles, Au NPs, is due to the interface electron transfer from Au nanoparticles to TiO₂ under visible light illumination and from TiO₂ to Au nanoparticles under UV illumination. For the first time, the effect of Au particle sizes when it is very similar to the interparticles pores of TiO₂ has been investigated. The highest reaction rate (5.7 × 10^?2 min^?1) and degradation efficiency of Safranin-O (SO) dye (97 %) were observed when the deposited gold nanoparticles are the smallest among the studied samples (sAu/TiO₂). In spite of blocking a high percentage of the TiO₂ pores, the sAu/TiO₂ sample demonstrated a complete degradation of SO dye in 50 min which is more efficient than any other reported catalysts in the literature.     

Oxygen-rich TiO2 decorated with C-Dots: Highly efficient visible-light-responsive photocatalysts in degradations of different contaminants

Providing novel photocatalysts with high photocatalytic efficiency is of great significance. In the present work, hydrogen peroxide and carbon dots (C-Dots) were utilized to enhance the photocatalytic performance of TiO₂ under visible light. The fabricated TiO₂-peroxo/C-Dots photocatalysts were analyzed by XRD, HRTEM, SEM, EDX, BET, FT-IR, XPS, PL, UV–Vis DRS, EIS, and photocurrent density. Photocatalytic abilities of the nanocomposites were evaluated by photocatalytic removal of RhB, MO, MB, fuchsine, and Cr (VI) upon visible-light illumination. The results demonstrated that the binary nanocomposites exhibited remarkably enhanced photocatalytic activity compared with the TiO₂ and TiO₂-peroxo photocatalysts. The best photocatalytic performance was obtained using 0.75?mL of C-Dots, which was approximately 79.2, 17.1, 71.4, and 40.5 times higher than the pure TiO₂ for degradations of RhB, MO, MB, and fuchsine, respectively. Furthermore, the TiO₂-peroxo/C-Dots nanocomposites exhibited high stability in consecutive photocatalytic processes. Based on the results, the TiO₂-peroxo/C-Dots photocatalyst is expected to become a promising photocatalyst for practical applications in water purification.     

Sm3+-doped La2O3–Al2O3–SiO2-glasses: structure, fluorescence and thermal expansion

This paper reports on the effect of the chemical composition on the glass structure, the coefficients of thermal expansion and the fluorescence properties of Sm³⁺-doped La₂O₃–Al₂O₃–SiO₂-glasses. The silica concentration was varied between 50 and 70 mol% and the La₂O₃:Al₂O₃ ratio between 50:50 and 30:70. The glass formation and the densities are evaluated and FTIR reflectance spectra, coefficients of thermal expansion and fluorescence lifetimes are determined. It is shown that high SiO₂ concentrations and low La₂O₃:Al₂O₃ ratios result in relatively high fluorescence lifetime (2.19 ms, ⁴G_5/2) and low coefficients of thermal expansion (4.6 × 10^?6/K). The coefficients of thermal expansion and the fluorescence lifetimes show a linear dependency on the ratio LaO_3/2/(AlO_3/2 + SiO₂).     

Effect of substrate type, dopant and thermal treatment on physicochemical properties of TiO2–SnO2 sol–gel films

Thin nanocrystalline TiO₂–SnO₂ films (0–50 mol% SnO₂) were prepared on quartz and stainless steel substrates by sol–gel coating method. The obtained films were investigated by XRD, Raman spectroscopy and XPS. The size of the nanocrystallites was determined by XRD–LB measurements. We ascertained that the increase of treatment temperature and concentration of SnO₂ in the films favour the crystallization of rutile phase. The substrate type influences more substantially the phase composition of the TiO₂–SnO₂ films. It was established that a penetration of elements took place from the substrate into the films. TiO₂ films deposited on quartz substrate include a Si which stabilizes anatase phase up to 600 °C. The films which are deposited on stainless steel substrate and treated at 700 °C show the presence of significant quantity of rutile phase. This phenomenon could be explained by the combined effect of Sn dopant as well as Fe and Cr, which also are penetrated in the films from the steel substrate. The titania films doped up to 10 mol% SnO₂ on stainless steel possess only 12–17 nm anatase crystallites, whereas the TiO₂–(10–50 mol%) SnO₂ films contain very fine grain rutile phase (4 nm).     

A novel route for the preparation of CoCr<Subscript>2</Subscript>O<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> nanocomposite starting from Co(II)–Cr(III) carboxylate complex combinations

This study reports the preparation and characterization of the spinel CoCr₂O₄. In order to obtain 20% CoCr₂O₄/80% SiO₂ and 50% CoCr₂O₄/50% SiO₂ (mol%) nanocomposites, we have used a versatile pathway based on the thermal decomposition of some particular precursors, Co(II) and Co(III) carboxylate-type complex combinations, inside the SiO₂ matrix. The ligands of these coordination compounds result in the redox reaction between Co(II) and Cr(III) nitrates and 1,3-propylene glycol by heating at 150 °C of the gels (tetraethylorthosilicate–metal nitrates–1,3-propylene glycol). The as-obtained precursors, embedded in silica gels, were characterized by FT-IR spectrometry and thermal analysis. Both precursors decompose up to 350 °C, leading to the corresponding metal oxides inside the silica matrix. X-ray diffraction of the powders annealed at different temperatures has evidenced the formation of CoCr₂O₄ starting with 400 °C for 20% CoCr₂O₄/SiO₂ and 300 °C for 50% CoCr₂O₄/SiO₂. This behaviour can be explained by the fact that, by thermal decomposition of the chromium carboxylates, a nonstoichiometric chromium oxide Cr₂O_3+x is formed. At ~ 400 °C, Cr₂O_3+x turns to α-Cr₂O₃, which interacts with CoO leading to cobalt chromite nuclei inside the pores of the silica matrix. CoCr₂O₄ has been obtained as nanocrystallites homogenously dispersed within the silica matrix as resulted from XRD, TEM and EDX mapping, with mean particle size in the range 5–20 nm.     

Band gap tuning and nonlinear optical characterization of TiO2–SiO2 nanocomposites with respect to composition and phase

We report on the linear and nonlinear optical studies on TiO₂–SiO₂ nanocomposites with varying percentage ratio. It is found that optical band gap of the material varies with respect to the amount of the SiO₂ in the composite. Nonlinear optical characterization of these samples was studied by using open as well as closed aperture Z-scan technique using an Nd:YAG laser (532 nm, 7 ns, 10 Hz). The nanocomposites showed enhanced nonlinear optical properties than pure TiO₂ and this can be attributed to the surface states and weak dielectric confinement of TiO₂ nanoparticles by SiO₂ matrix. The nanocomposites were thermally treated and similar studies were performed. The anatase form of TiO₂ in the nanocomposites showed superior properties relative to the amorphous and rutile phase of the composite. The involved mechanism is explained by taking into account the dominant role played by the excitons in the TiO₂ nanoparticles.     

Synthesis,characterization and photocatalytic behavior of TiO2–SiO2 mesoporous composites in hydrogen generation from water splitting

TiO₂–SiO₂ mesoporous composite photocatalysts with different proportions (in wt%) of TiO₂ and SiO₂ (TiO₂–SiO₂ = 20:80, 40:60, 60:40, 80:20 and 100:0) were prepared by loading TiO₂ on as-synthesized Si–MCM-41 using sol–gel method. The physicochemical properties of composites were investigated by powder X-ray diffraction, N₂ adsorption–desorption measurements, transmission electron microscopy and UV–Vis diffuse reflectance spectroscopy. It is revealed that the titanium species are dispersed as TiO₂ having interaction with the surface of the support. Even at high TiO₂ loading, the mesostructural feature of MCM-41 was found to be intact without pore blockages. The change in morphology of TiO₂ particle was observed with increase in TiO₂ loading which may be due to different environment for the growth of TiO₂. The photocatalytic evaluation of composites was carried out in production of hydrogen by water splitting. Among the prepared samples, mesoporous composite containing 60 % TiO₂ (MTi60) has shown the best results (0.08805 mmol of H₂/h/g of TiO₂) compared to other composite photocatalysts. The catalytic performance of this sample was further enhanced (~8 times) after loading 1 % Pt in water splitting (0.70161 mmol of H₂/h/g of TiO₂). 1 % Pt loaded on pure TiO₂ (MTi100) showed hydrogen evolution of the magnitude 0.26 mmol of H₂/h/g of TiO₂. TiO₂–SiO₂ mesoporous composite photocatalyst showed much higher activity (~1.9 times) than amorphous silica-embedded titania catalyst having same composition.     

Optical and electrical properties of E-Beam deposited TiO<Subscript>2</Subscript>/Si thin films

In this paper, optical and electrical properties of E-Beam deposited TiO₂/Si thin films have been studied and investigated extensively. The films were deposited on p-type (100) silicon wafer by using electron beam evaporation technique. The thickness of the thin films was measured by a spectroscopic reflectometer, which is about 216 nm. The fabricated titanium oxide (TiO₂) thin films were annealed at 800 °C for 1 h under N₂ ambient. X-ray diffraction measurements were performed to study the structure and phase identification of the fabricated TiO₂ thin films. For the optical properties, reflection, transmittance, refractive index and absorption coefficient were obtained and analyzed. The photocurrent and dark current of the fabricated films were measured by I–V measurements. The measurement of the current–voltage (I–V) characteristics possesses good ohmic contact. The electrical characterizations of the films were performed in the range of the low frequencies (50 and 100 kHz) and high frequencies (750 kHz and 1 MHz) by the capacitance–voltage and conductance–voltage measurements at room temperature. The capacitance of the fabricated TiO₂ MOS capacitor at both high and low frequencies increases with the decrease in frequencies. The obtained conductance curves (peaks) increase with the decreasing in the frequencies. This can be due to the interface state density, series resistance and interfacial dielectric of the fabricated MOS capacitors. The variation in the characteristics of the fabricated film shows that TiO₂ is a promising candidate to be used in the optoelectronic and future UV detector applications as a switch, such as an optical amplifier, emitter, and UV light detectors.     

The role of P2O5, TiO2 and ZrO2 as nucleating agents on microstructure and crystallization behaviour of lithium disilicate-based glass

The aim of this study was to investigate the effects of different nucleating agents (P₂O₅, TiO₂ and ZrO₂) on the crystallization behaviour and the properties of a parent glass with composition 23.7 Li₂O–2.63 K₂O–2.63 Al₂O₃–71.78 SiO₂ (mol%) and SiO₂/Li₂O molar ratio far beyond that of stoichiometric lithium disilicate (LD, Li₂Si₂O₅). The scanning electron microscopy examination of the as-cast non-annealed glasses revealed the occurrence of liquid–liquid phase separation for all the compositions. P₂O₅ revealed to be effective in promoting bulk crystallization of LD, while TiO₂ and ZrO₂ led to surface crystallization. Moreover, ZrO₂ enhances the glass polymerization and shifts T _p to higher temperatures, hindering crystallization. At 900 °C, TiO₂-containing glasses feature LD and lithium metasilicate (LMS, Li₂SiO₃), while P₂O₅- and ZrO₂-containing ones present monophasic LD and LMS glass–ceramics, respectively.     

Modified titania and titanium-containing composites as fillers exhibiting an electrorheological effect

SiO₂/TiO₂ nanocomposites and individual titania (S = 30–500 m²/g) have been prepared through cohydrolysis of organomineral silicon and titanium derivatives in the presence of templates. We demonstrate that heterovalent substitution of chromium or cerium for titanium increases the thermal stability of TiO₂ in terms of its crystallite size and specific surface area. After drying at 120°C, fillers based on SiO₂/TiO₂ composites (coprecipitated and core-shell SiO₂/TiO₂ particles) exhibit a significant electrorheological response due to appreciable hydrate and hydroxyl layers on the surface of the particles. Structural modification of titania also allows one to increase the electrorheological effect through an increase in specific surface and defect formation.     

Characterization and catalytic activity studies of sol–gel Co–SiO2 nanocomposites

Silica embedded with transition metals exhibits adequate properties for applications in catalysis, sensors and optics. Cobalt–silica (Co–SiO₂) nanocomposites were prepared by the sol–gel method and thermally treated at 700, 900, 1100 and 1250 °C. Characterization of the samples was performed by XRD and BET nitrogen adsorption. The performance of the nanocomposites was investigated by catalysis reactions of oxidation. These catalysts were found to be recyclable showing a catalytic activity even after a third recovering. The results indicate that thermal treatment of sol–gel nanocomposites at temperatures higher than 900 °C is essential for the preparation of active heterogeneous catalysts.     

Preparation and characterization of phosphate-modified mesoporous TiO<Subscript>2</Subscript> incorporated in a silica matrix and their photocatalytic properties in the photodegradation of Congo red

This study describes the development of mesostructured TiO₂ photocatalysts modified with PO₄^3- to improve its specific surface area and reduce the recombination rate of the electron—hole pairs. The mesoporous photocatalyst was successfully incorporated into a high specific surface area silica matrix by the hydrolysis reaction of tetraethyl orthosilicate (TEOS). Pluronic 123 and phosphoric acid were used as the directing agent for the structure of the mesoporous TiO₂ and as a source of phosphorus, respectively. TiO₂, P/TiO₂, TiO₂-SiO₂ and P/TiO₂-SiO₂ materials were characterized by BET, XRD, TEM-EDS, FTIR and UV-vis DRS measurements. The photoactivity of TiO₂-SiO₂ nanocomposites containing 15 wt.% photocatalyst/silica was evaluated in the degradation reaction of anionic dyes with UV radiation. The proposed nanomaterials showed high potential for applications in the remediation of wastewater, being able to reuse in several cycles of reaction, maintaining its photoactivity and stability. The separation and recovery time of the material is reduced between cycles since no centrifugation or filtration processes are required after the photooxidation reaction.