Elaboration and characterization of TiO2 nanoparticles incorporated in SiO2 host matrix

Nanometer-scale TiO₂ particles have been synthesized by sol-gel method. It was incorporated in a glass-based silica aerogel. The composite was characterized by various techniques such as particle size analysis, scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray diffraction (XRD), infrared spectroscopy (IR) and photoluminescence (PL). The bulk glass presents a strong luminescence at wavelengths ranging from 750 to 950 nm. This PL was attributed to various non-bridging oxygen hole centers (NBOHCs) defects resulting from thermal treatment and crystallization of TiO₂ at the interface between titania nanoparticles and silica host matrix.     

Applications of vertically oriented TiO2 micro-pillars array on the electrode of dye-sensitized solar cell

In this research, dye-sensitized solar cells based on TiO₂ micro-pillars fabricated by inductive couple plasma etcher were investigated by analyses of X-ray diffraction (XRD), scanning electron microscopy (SEM), contact angle, ultraviolet-visible absorption spectra (UV-vis), and current-voltage characteristics. X-ray diffraction patterns show that the TiO₂ anatase phase forms while sintering at 450 °C for 30 min. The SEM images reveal that the diameter and height of TiO₂ micro-pillars are about 3 and 0.8 μm, respectively. The measurements of contact angle between TiO₂ micro-pillars and deionized water (DI water) reveal that the TiO₂ micro-pillars is super-hydrophilic while annealed at 450 °C for 30 min.The absorption spectrum of TiO₂ micro-pillars is better than TiO₂ thin film and can be widely improved in visible region with N3 dye adsorbed. The results of current-voltage (I-V) characteristics analysis reveal that dye-sensitized solar cell with TiO₂ micro-pillars electrode has better I-V characteristics and efficiency than TiO₂ film electrodes. This result may be due to the annealed TiO₂ micro-pillars applied on the electrode of dye-sensitized solar cell can increase the contact area between TiO₂ and dye, resulting in the enhancement of I-V characteristics and efficiency for dye-sensitized solar cell.     

N, S-doped TiO2 anode effect on performance of dye-sensitized solar cells

The modification of non-metallic elements N and S to nanocrystalline TiO₂ anode results in the energy gap is reduced to 2.63 eV and a strong redshift to the visible region occurred in the UV–visible spectrum. Poly (3-decylthiophene) (P3DT) is synthesized. Ultraviolet–visible spectra (UV–vis) shows that the light absorption of P3DT (Poly (3-decylthiophene)) and N719 (RuL2(NCS)2:2TBA (L=2,2′-bipyridyl-4, 4′-dicarboxylic acid)) are complementary to cover the entire visible region. Solar cell based on N–S/TiO₂ is co-sensitized by P3DT and N719. The photoelectric conversion efficiency of co-sensitized solar cell increases 56.8% comparing with the single dye-sensitized solar cell.     

Preparation and characterization of cerium oxide doped TiO2 nanoparticles

A mixed oxide consisting of TiO₂ as the major phase and CeO_2−y (0<y<0.5) as the dopant phase was prepared via the sol-gel reaction of Ti(i-OC₃H₇)₃ in an aqueous solution of Ce(NO₃)₃. The resulting oxide powders with different CeO_2−y contents were all composed of nano-sized spheres. The CeO_2−y phase was identified to have retarding effect on the phase transition from anatase TiO₂ to rutile TiO₂ at calcinations temperature as high as 800 °C, which would otherwise be a thorough conversion. The CeO_2−y-TiO₂ powders could apparently shift the UV-absorption band of TiO₂ toward visible range, and there was an optimal CeO_2−y content in association with the maximum absorbance. This effect is interpreted as the existence of an n-type impurity band, due to the substitution of Ti⁴⁺ for Ce^3+/4+ at the interface between the two oxides, in the gap of TiO₂. According to X-ray photoelectron spectroscopy (XPS) investigation, the Ti element mainly existed as the chemical state of Ti⁴⁺ and the Ce oxide doping did not affect the peak position of Ti 2p. The Ce 3d spectrum of CeO_2−y-doped TiO₂ sample basically denotes a mixture of Ce^3+/4+ oxidation states giving rise to a myriad of peaks.     

Combustion synthesis and photoluminescence of Eu, Dy-doped La2Zr2O7 nanocrystals

A facile and energy saving sol-gel combustion method has been used to prepare La₂Zr₂O₇ nanocrystallines. The pyrochlore La₂Zr₂O₇ nanocrystals have been obtained at a relatively low temperature with the grain size ranging from 45 to 70 nm. Eu³⁺ and Dy³⁺ have been introduced into the La₂Zr₂O₇ crystal structure, respectively, and the intense photoluminescence was observed. The relative intensity of electric dipole transition and magnetic dipole transition is considered for luminescence emission both of Eu³⁺ and Dy³⁺. The dependence of luminescence intensity on dopant concentration and the effect of Dy³⁺ co-doping on Eu³⁺ luminescence are also discussed.     

Optical and photocatalytic properties of nanocrystalline TiO2 synthesised by solid-state chemical reaction

Nanoparticulate TiO₂ is of interest for a variety of technological applications, including optically transparent UV-filters and photocatalysts for the destruction of chemical waste. The successful use of nanoparticulate TiO₂ in such applications requires an understanding of how the synthesis conditions effect the optical and photocatalytic properties. In this study, we have investigated the effect of heat treatment temperature on the properties of nanoparticulate TiO₂ powders that were synthesised by solid-state chemical reaction of anhydrous TiOSO₄ with Na₂CO₃. It was found that the photocatalytic activity increased with the heat treatment temperature up to a maximum at 600 °C and thereafter declined. In contrast, the optical transparency decreased monotonically with the heat treatment temperature. These results indicate that solid-state chemical reaction can be used to prepare powders of nanoparticulate TiO₂ with properties that are optimised for use as either optically transparent UV-filters or photocatalysts.     

A new photoluminescence emission peak of ZnO-SiO2 nanocomposites and its energy transfer to Eu ions

This work reports a new photoluminescence (PL) emission peak at about 402 nm from amorphous ZnO nanoparticles in a silica matrix, and the energy transfer from it to Eu³⁺ ions. The amorphous ZnO-SiO₂ nanocomposites were prepared by the sol-gel method, which is verified by X-ray diffraction (XRD) profiles and FT-IR spectra. The luminescence emission spectra are fitted by four Gauss profiles, two of which at longer wavelength are due to the defects of the material and the others to amorphous ZnO nanoparticles and the Zn-O-Si interface state. With the reduction of Zn/Si ratio and diethanolamine, the relative intensities of visible emission decrease. The weak visible emission is due to the reduction of defects after calcined at high temperature. The new energy state at the Zn-O-Si interface results in strong emission at about 402 nm. When Eu³⁺ ions are co-doped, weak energy transfer from ZnO-SiO₂ nanocomposites to Eu³⁺ emission are observed in the excitation spectra.     

Visible luminescence of Al2O3 nanoparticles embedded in silica glass host matrix

This paper deals with the sol-gel elaboration and defects photoluminescence (PL) examination of Al₂O₃ nanocrystallites (size ∼30 nm) confined in glass based on silica aerogel. Aluminium oxide aerogels were synthesized using esterification reaction for hydrolysis of the precursor and supercritical conditions of ethyl alcohol for drying. The obtained nanopowder was incorporated in SiO₂ host matrix. After heating under natural atmosphere at 1150 °C for 2 h, the composite Al₂O₃/SiO₂ (AS) exhibited a strong PL bands at 400-600 and 700-900 nm in 78-300 K temperature range. PL excitation (PLE) measurements show different origins of the emission. It was suggested that OH-related radiative centres and non-bridging oxygen hole centres (NBOHCs) were responsible for the bands at 400-600 and 700-900 nm, respectively.     

Hydrothermal synthesis of well-dispersed ultrafine N-doped TiO2 nanoparticles with enhanced photocatalytic activity under visible light

Ultrafine nitrogen-doped TiO₂ nanoparticles with narrow particle size distribution, good dispersion, and high surface area were synthesized in the presence of urea and PEG-4000 via a hydrothermal procedure. TEM observation, N₂ adsorption, XRD, UV-vis spectroscopy, the Raman spectroscopy and XPS analysis were conducted to characterize the synthesized TiO₂ particles. The synthesized TiO₂ particles were a mixture of 49.5% anatase and 50.5% rutile with a size of around 5 nm. The photocatalytic activities were tested in the degradation of an aqueous solution of a reactive Brilliant Blue KN-R under both UV and visible light. The synthesized TiO₂ particles showed much higher photocatalytic activity than a commercial P25 TiO₂ powder under both UV and visible light irradiations. The high performance is associated to N doping, the reduced particle size, good dispersion, high surface area, and a quantum size effect.     

Red colored transparent PMMA-SiO2 hybrid films

Red colored transparent organic-inorganic hybrids films of a cross-linked polymethyl-methacrylate (PMMA) and silica were prepared via the sol-gel route using tetraethoxy-silane (TEOS) as precursor and a commercial organic red dye. 3-(Trimethoxysilyl) propyl methacrylate (TMSPM) was used to make compatible the organic and inorganic components of the precursor solution mixture. Four type of colored hybrid films were deposited using precursor solutions with the reactants molar ratio 1:0.5:1 for TEOS: TMSPM:MMA, respectively, and four different weight contents of the organic red dye. The hybrid films were studied by Fourier transform infrared spectroscopy, atomic force microscopy and optical transmission and reflection spectroscopy. The hardness of the films was determined from a pencil hardness test referred to ASTM Standard D 3363-92. The results showed that the colored hybrid films have a thickness of about 2 μm and consist of a homogeneous cross-linked organic-inorganic matrix with embedded dye molecules very well dispersed. The hardness of the hybrid films was enhanced with respect to that of the pure organic component. The AFM measurements showed very flat and smooth film surfaces with rms average roughness about 0.3 nm. The optical properties of the hybrid films including their color properties were determined from optical transmission and reflection spectroscopy. We found that the intensity of the color in the hybrid films and the corresponding color coordinates depend on the amount of red dye in the hybrid films. Photodegradation studies were performed by monitoring the optical density of the films as a function of the exposure time to illumination for several values of illumination intensity.     

EPR study of nitrogen-doped mesoporous TiO2 powders

Mesoporous titanium dioxide powders were synthesized by the sol-gel route either directly or using the P123 templating agent and nitrogen doping from thiourea. X-ray diffraction studies show that doping and structuring give rise to rutile growth at a relatively low temperature of 400 °C compared to the transition point of pure TiO₂, which is close to 900 °C. Systematic EPR experiments were performed in order to monitor the involved paramagnetic centres with respect to calcination treatments and also to probe the nitrogen doping efficiency in the host media. Several paramagnetic species were found in these mesostructured samples mainly with oxygen radicals for calcinations at 400 °C, while nitrogen centres appear for calcination temperature as high as 500 °C. The effect of material microstructure on the effective incorporation of nitrogen in the TiO₂ lattice is also examined and compared in representative samples.     

Sol-gel synthesis of iron(III) oxyhydroxide nanostructured monoliths using Fe(NO3)3·9H2O/CH3CH2OH/NH4OH ternary system

Iron(III) oxyhydroxide xerogels were prepared through sol-gel technology, using iron(III) nitrate nonahydrate as precursor, ethanol as solvent and ammonium hydroxide as gelation agent. This base is used for propylene oxide substitution, which was the gelation agent in previous works. Synthesis of a gel using NH₄OH as a gelation agent is an innovative result with this type of precursor, since with metal salts the addition of a strong base commonly results in precipitation of the solid. The gel synthesis was achieved by controlling the base addition time. The dried material has a residual amount of organic impurities, in contrast with the significant amount detected in xerogels prepared using propylene oxide. The iron phase prevailing in the produced xerogels can be defined as γ-FeO(OH) (lepidocrocite), according to FTIR and Mössbauer analyses. The xerogels are formed by large clusters of well connected nanocrystallites of this phase. XRD revealed a crystalline phase retained inside the iron oxyhydroxide amorphous structure, which corresponds to NH₄NO₃ and results from the combination of NO₃⁻ and NH₄⁺ ions in solution. The produced xerogel has a promising composition to be an oxidizing composite for the energetic materials area.     

Bifunctional photocatalysis of TiO2/Cu2O composite under visible light: Ti in organic pollutant degradation and water splitting

Photocatalytic experiment results under visible light demonstrate that both TiO₂ and Cu₂O have low activity for brilliant red X-3B degradation and neither can produce H₂ from water splitting. In comparison, TiO₂/Cu₂O composite can do the both efficiently. Further investigation shows that the formation of Ti³⁺ under visible light has great contribution. The mechanism of photocatalytic reaction is proposed based on energy band theory and experimental results. The photogenerated electrons from Cu₂O were captured by Ti⁴⁺ ions in TiO₂ and Ti⁴⁺ ions were further reduced to Ti³⁺ ions. Thus, the photogenerated electrons were stored in Ti³⁺ ions as the form of energy. These electrons trapped in Ti³⁺ can be released if a suitable electron acceptor is present. So, the electrons can be transferred to the interface between the composite and solution to participate in photocatalytic reaction. XPS spectra of TiO₂/Cu₂O composite before and after visible light irradiation were carried out and provided evidence for the presence of Ti³⁺. The image of high-resolution transmission electron microscopy demonstrates that TiO₂ combines with Cu₂O tightly. So, the photogenerated electrons can be transferred from Cu₂O to TiO₂.     

Optical properties of sol-gel fabricated Ni/SiO2 glass nanocomposites

Nickel nanoparticles were grown in silica glass by annealing of the sol-gel prepared silicate matrices doped with nickel nitrate. TEM characterization of Ni/SiO₂ glass proves the formation of isolated spherical nickel nanoparticles with mean sizes 6.7 and 20 nm depending on annealing conditions. The absorption and photoluminescence spectra of Ni/SiO₂ glasses were measured. In the absorption spectra, we observed the band related to the surface plasmon resonance (SPR) in Ni nanoparticles. The broadening of SPR was observed with decrease of Ni nanoparticle size. The width of the surface plasmon band decreases 1.5 times at the lowering of temperature from 293 to 2 K because of strong electron-phonon interaction. The spectra proved the creation of nickel oxide NiO clusters and Ni²⁺ ions in silica glass as well.     

Crystalline-structure-dependent green and red upconversion emissions of Er-Yb-Li codoped TiO2

Crystalline structures and infrared-to-visible upconversion luminescence spectra have been investigated in 1 mol% Er³⁺, 10 mol% Yb³⁺ and 0-20 mol% Li⁺ codoped TiO₂ [1Er10Yb(0-20)Li:TiO₂] nanocrystals. The crystalline structures of 1Er10Yb(0-20)Li:TiO₂ were divided into three parts by the addition of Yb³⁺ and Li⁺. Both green and red upconversion emissions were observed from the ²H_11/2/⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ in Er³⁺-Yb³⁺-Li⁺ codoped TiO₂, respectively. The green and red upconversion emissions of 1Er:TiO₂ were enhanced significantly by Yb³⁺ and Li⁺ codoping, in which the intensities of green and red emissions and the intensity ratio of green to red emissions (I_green/I_red) were highly dependent on the crystalline structures. The significant enhanced upconversion emissions resulted from the energy migration between Er³⁺ and Yb³⁺ as well as the distortion of crystal field symmetry of Er³⁺ caused by the dissolving of Li⁺ at lower Li⁺ codoping concentration and the phase transformation at higher Li⁺ concentration. It is concluded that codoping with ions of smaller ionic radius like Li⁺ can efficiently improve the upconversion emissions of Er³⁺ or other rare-earth ions doped luminsecence materials.     

Spectroscopic properties of Pr:KY(MoO4)2 crystal as a visible laser gain medium

A Pr³⁺-doped KY(MoO₄)₂ single crystal was grown by the Czochralski method. The polarized absorption and fluorescence spectra of the Pr³⁺:KY(MoO₄)₂ crystal were measured at room temperature. The stimulated emission cross-sections for the transitions from the ³P₀ multiplet were estimated from the fluorescence spectra. The fluorescence lifetime of the ³P₀ multiplet was estimated from the fluorescence decay curve at room temperature. The analysis of spectral properties shows that the Pr³⁺:KY(MoO₄)₂ crystal is a promising gain medium for visible lasers.     

Optical spectroscopy of Yb/Er codoped NaY(WO4)2 crystal

Erbium and ytterbium codoped double tungstates NaY(WO₄)₂ crystals were prepared by using Czochralski (CZ) pulling method. The absorption spectra in the region 290-2000 nm have been recorded at room temperature. The Judd-Ofelt theory was applied to the measured values of absorption line strengths to evaluate the spontaneous emission probabilities and stimulated emission cross sections of Er³⁺ ions in NaY(WO₄)₂ crystals. Intensive green and red lights were measured when the sample were pumped by a 974 nm laser diode (LD), especially, the intensities of green upconversion luminescence are very strong. The mechanism of energy transfer from Yb³⁺ to Er³⁺ ions was analyzed. Energy transfer and nonradiative relaxation played an important role in the upconversion process. Photoexcited luminescence experiments are also fulfilled to help analyzing the transit processes of the energy levels.     

The effect of the Bi source on optical properties of Bi2Te3 nanostructures

nanostructures were synthesized by using different Bi sources via a simple solvothermal process, in which and BiCl₃ were used as the Bi sources. Optical properties of nanostructures prepared with and BiCl₃ as the Bi sources were investigated by micro-Raman spectroscopy. The Raman scattering spectrum of hexagonal nanoplates prepared by using as the Bi source shows that the infrared (IR) active mode A_1u, which must be odd parity and is Raman forbidden for bulk crystal due to its inversion symmetry, is greatly activated and shows up clearly in the Raman scattering spectrum. We attribute the appearance of the infrared active A_1u mode in the Raman spectrum to crystal symmetry breaking of hexagonal nanoplates. However, the Raman scattering spectrum of nanostructures with irregular shape prepared by using as the Bi source only exhibits the two characteristic Raman modes of crystals. Micro-Raman measurements on nanostructures with different morphologies offer us a potential way to tailor optical properties of nanostructures by controlling the morphologies of the nanostructures, which is very important for practical applications of nanostructures in thermoelectric devices.     

Polyol-assisted synthesis of TiO2 nanoparticles in a semi-aqueous solvent

TiO₂ (anatase and rutile) nanoparticles with an average crystallite size of 20-40 nm have been prepared at room temperature by polyol-mediated synthesis technique in a semi-aqueous solvent medium using titanium iso-propoxide as the titanium source, acetone as the oil phase and ethylene glycol as the stabilizer. Phase and microstructure of the resultant materials have been characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Photocatalytic degradation of acetaldehyde using TiO₂ nanoparticles was investigated by gas-chromatography technique.     

Photoluminescence spectra of rare earth doped CaGa2S4 single crystals

Six kind CaGa₂S₄ single crystals doped with different rare earth (RE) elements are grown by the horizontal Bridgman method, and their photoluminescence (PL) spectra are measured in the temperature range from 10 to 300 K. The PL spectra of Ce or Eu doped crystals have broad line shapes due to the phonon assisted 4f-5d transitions. On the other hand, those of Pr³⁺, Tb³⁺, Er³⁺ or Tm³⁺ doped samples show narrow ones owing to the 4f-4f transitions. The assignments of the electronic levels are made in reference to the reported data of RE 4f multiplets observed in same materials.