The preparation and characterization of nanoparticle TiO2/Ti films and their photocatalytic activity

Nanoparticle TiO₂/Ti films were prepared by a sol–gel process using Ti(OBu)₄ as raw material, the as-prepared film samples were also characterized by TG-DTA, XRD, TEM, SEM, XPS, DRS, PL, SPS and EFISPS testing techniques. TiO₂ nanoparticles experienced two processes of phase transition, i.e. amorphous to anatase and anatase to rutile at the calcining temperature range from 450 to 700 °C. TiO₂ nanoparticles calcined at 600 °C had similar composition, structure, morphology and particle size with the internationally commercial P-25 TiO₂ particles. Thus, the conclusion that 600 °C might be the most appropriate calcining temperature during the preparation process of nanoparticle TiO₂/Ti film photocatalysts could be made by considering the main factors such as the properties of TiO₂ nanoparticles, the adhesion of nanoparticle TiO₂ film to Ti substrate, the effects of calcining temperature on Ti substrate and the surface characteristics and morphology of nanoparticle TiO₂/Ti film for the practice view. The Ti element mainly existed on the nanoparticle TiO₂/Ti(3) film calcined at 600 °C as the chemical state of Ti⁴⁺, while O element mainly existed as three kinds of chemical states, i.e. crystal lattice oxygen, hydroxyl oxygen and adsorbed oxygen with increasing band energy. Its photoluminescence (PL) spectra with a peak at about 380 nm could be observed using 260 nm excitation, possibly resulting from the electron transition from the bottom of conduction band to the top of valence band. The PL peak position was nearly the same as the onset of its diffuse reflection spectra (DRS) and surface photovoltage spectroscopy (SPS), demonstrating that the effects of the quantum size on optical property were greater than that of the Coulomb and surface polarization. The PL spectra with two peaks related to the anatase and rutile, respectively, could be observed using the excited wavelength of 310 nm. Weak PL spectra could be observed using the excited wavelength of 450 nm, resulting from surface states. In addition, during the experimental process of the photocatalytic degradation phenol, the photocatalytic activity of nanoparticle TiO₂/Ti film with three layers calcined at 600 °C was the highest.     

Energy transfer induced Eu3+ photoluminescence enhancement in tellurite glass

In this work, structural, thermal and optical properties of Eu³⁺ doped TeO₂–La₂O₃–TiO₂ glass were investigated. The differential scanning calorimetry (DSC) measurements reveal an important stability factor ΔT=143.52 K, which indicates the good thermal and mechanical stabilities of tellurite glass. From the absorption spectrum, the optical band gap was found to be direct with E_g=3.23 eV. The temperature dependences of photoluminescence (PL) properties of Eu-doped and Eu–Tb codoped tellurite glass are investigated. As the temperature increases from 7 to 300 K, both the PL intensity and the PL lifetime relative to the ⁵D₂→⁷F₀ are nearly constant below 230 K and then an enhancement takes place. This anomalous feature is attributed to the thermally activated carrier transfer process from charged intrinsic defects states to Eu³⁺ energy levels.By co-doping tellurite glasses with Eu and Tb, a strong Eu³⁺ PL enhancement is shown due to excitation transfer from Tb³⁺ and intrinsic defects to Eu ions.     

Absorption,luminescence, and electron paramagnetic resonance of molybdenum ions in CdWO4

A single crystal of cadmium tungstate (CdWO₄) containing approximately 200 ppm of molybdenum was grown by the Czochralski method and then characterized in a series of optical absorption, photoluminescence (PL), photoluminescence excitation (PLE), and electron paramagnetic resonance (EPR) experiments. The Mo⁶⁺ ions substitute for W⁶⁺ ions and serve as recombination sites for electrons and holes when the crystal is exposed to ionizing radiation. A charge-transfer absorption band for the Mo⁶⁺ ions was observed near 320 nm at 10 K. The PL experiments, performed at low temperature with 325 nm excitation, showed a Mo-associated emission peaking near 680 nm. A direct correlation of the 680 nm emission and the 320 nm absorption band was established by the PLE data. When these doped CdWO₄ crystals are exposed at low temperature either to light that is near or above the band gap or to X-rays, the Mo⁶⁺ ions can trap an electron and form stable Mo⁵⁺ ions. The EPR spectrum of the Mo⁵⁺ ions was observed at temperatures near 15 K, and a complete set of parameters describing the g matrix was obtained from an angular dependence study.     

A time-resolved luminescence spectroscopy study of non-linear optical crystals K2Al2B2O7

We report the results of complex study of luminescence and dynamics of electronic excitations in K₂Al₂B₂O₇ (KABO) crystals obtained using low-temperature luminescence-optical vacuum ultraviolet spectroscopy with sub-nanosecond time resolution under selective photoexcitation with synchrotron radiation. The paper discusses the decay kinetics of photoluminescence (PL), the time-resolved PL emission spectra (1.2–6.2 eV), the time-resolved PL excitation spectra and the reflection spectra (3.7–21 eV) measured at 7 K. On the basis of the obtained results three absorption peaks at 4.7, 5.8 and 6.5 eV were detected and assigned to charge-transfer absorption from O^2? to Fe³⁺ ions; the intrinsic PL band at 3.28 eV was revealed and attributed to radiative annihilation of self-trapped excitons, the defect luminescence bands at 2.68 and 3.54 eV were separated; the strong PL band at 1.72 eV was revealed and attributed to a radiative transition in Fe³⁺ ion.     

Sol–gel synthesis and luminescent properties of SiO2/Zn2SiO4 and SiO2/Zn2SiO4:V composite materials

Undoped and vanadium-doped Zn₂SiO₄ particles embedded in silica host matrix were prepared by a simple solid-phase reaction after the incorporation of ZnO and ZnO:V nanoparticles, respectively, in silica monolith using the sol–gel method with supercritical drying of ethyl alcohol in two steps. After supercritical drying and annealing in the temperature range between 1423 and 1473 K in an air atmosphere, the photoluminescence (PL) measurements show a band centered at about 760 nm in the case of non-doped Zn₂SiO₄ which is attributed to energy transfer from Zn₂SiO₄ particles to NBOHs interface defects. In the case of vanadium doped Zn₂SiO₄, the PL reveals a band centered at about 540 nm attributed to the vanadium in the interfaces between Zn₂SiO₄ particles and SiO₂ host matrix. Photoluminescence excitation (PLE) measurements show different origins of the emission bands. The PLE band (～240–350 nm) may be understood as an energy transfer process from O^2? to V⁵⁺ which occurs intrinsically in the vanadyl group.     

Fabrication and luminescent properties of Al2O3:Cr3 + microspheres via a microwave solvothermal route followed by heat treatment

AlOOH:Cr^3 + powders were synthesized via a microwave solvothermal route at 433 K for 30 min and were used as the precursor and template for the preparation of γ-Al₂O₃:Cr^3 + by thermal transformation at 773 K for 2 h in air. The obtained γ-Al₂O₃ based powders were microspheres with an average diameter about 1.9 μm. Photoluminescence (PL) spectra showed that the Al₂O₃:Cr^3 + particles presented a symmetric broad R band at 696 nm without appreciable splitting when excited at 462 nm. It is shown that the 0.04 mol% of doping concentration of Cr^3 + ions in γ-Al₂O₃:Cr^3 + is optimum. According to Dexter's theory, the critical distance between Cr^3 + ions for energy transfer was determined to be 47.54 Å. Based on the corresponding PL spectrum, full width at half maximum (FWHM) of Al₂O₃:Cr^3 + (0.04 mol%) was calculated to be 3.35 nm.     

Defect related luminescence in Hg0.2Cd0.8Te nano- and micro-crystals grown by the solvothermal method

The photoluminescence (PL) properties of nano- and micro-crystalline Hg_1?xCd_xTe (x≈0.8) grown by the solvothermal method have been studied over the temperature range 10–300 K. The emission spectra of the samples excited with 514.5 nm Ar⁺ laser consist of five prominent bands around 0.56, 0.60, 0.69, 0.78 and 0.92 eV. The entire PL band in this NIR region is attributed to the luminescence from defect centers. The features like temperature independent peak energy and quite sensitive PL intensity, which has a maximum around 50 K is illustrated by the configuration coordinate model. After 50 K, the luminescence shows a thermal quenching behavior that is usually exhibited by amorphous semiconductors, indicating that the defects are related to the compositional disorder.     

Magneto-optical properties of α-Fe2O3@ZnO nanocomposites prepared by the high energy ball-milling technique

Magnetic–fluorescent nanocomposites (NCs) with 10 wt% of α-Fe₂O₃ in ZnO have been prepared by the high energy ball-milling. The crystallite sizes of α-Fe₂O₃ and ZnO in the NCs are found to vary from 65 nm to 20 nm and 47 nm to 15 nm respectively as milling time is increased from 2 to 30 h. XRD analysis confirms presence of α-Fe₂O₃ and ZnO in pure form in all the NCs. UV–vis study of the NCs shows a continuous blue-shift of the absorption peak and a steady increase of band gap of ZnO with increasing milling duration that are assigned to decreasing particle size of ZnO in the NCs. Photoluminescence (PL) spectra of the NCs reveal three weak emission bands in the visible region at 421, 445 and 485 nm along with the strong near band edge emission at 391 nm. These weak emission bands are attributed to different defect – related energy levels e.g. Zn-vacancy, Zn interstitial and oxygen vacancy. Dc and ac magnetization measurements show presence of weakly interacting superparamagnetic (SPM) α-Fe₂O₃ particles in the NCs. ⁵⁷Fe-Mössbauer study confirms presence of SPM hematite in the sample milled for 30 h. Positron annihilation lifetime measurements indicate presence of cation vacancies in ZnO nanostructures confirming results of PL studies.     

Near-infrared photoluminescence and thermally stimulated current in Cu3Ga5Se9 layered crystals: A comparative study

Near-infrared photoluminescence (PL) and thermally stimulated current (TSC) spectra of Cu₃Ga₅Se₉ layered crystals grown by Bridgman method have been studied in the photon energy region of 1.35–1.46 eV and the temperature range of 15–115 K (PL) and 10–170 K (TSC). An infrared PL band centered at 1.42 eV was revealed at T = 15 K. Radiative transitions from shallow donor level placed at 20 meV to moderately deep acceptor level at 310 meV were suggested to be the reason of the observed PL band. TSC curve of Cu₃Ga₅Se₉ crystal exhibited one broad peak at nearly 88 K. The thermal activation energy of traps was found to be 22 meV. An energy level diagram demonstrating the transitions in the crystal band gap was plotted taking account of results of PL and TSC experiments conducted below room temperature.     

Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr³⁺ centers in submicron and nanostructured Al₂O₃ crystals of different phase compositions at temperatures of 300 and 7.5 K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of ²E, ⁴T₂, and ⁴T₁ excited states of Cr³⁺ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al₂O₃ nanoscale crystals is formed by intracenter transitions (2.5–5.5 eV region), by charge transfer band (6.9 eV) and by effective formation of impurity-bound excitons (9.0 eV region). Such impurity-bound excitons correspond to O₂p→Al₃s electron transition in surroundings of an impurity Cr³⁺ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100 nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al₂O₃ crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al₂O₃ at an excitation energy above 19 eV (more than 2E_g).     

Preparation and photoluminescence properties of the Sr1.56Ba0.4SiO4:0.04Eu2+ phosphor

The Sr_1.56Ba_0.4SiO₄:0.04Eu²⁺ phosphors were prepared via a combustion reaction and following the calcination method at low temperature. The influences of the amount of the uncommonly used SrCl₂ flux, different calcination temperatures and time on the structure and the photoluminescence (PL) properties of the phosphors were investigated. Under the excitation of 450 nm blue light, the phosphor shows the intense broad emission band from 490 nm to 650 nm, and the emission peak is centered at 553 nm. The luminescence intensity of Sr_1.56Ba_0.4SiO₄:0.04Eu²⁺ was very sensitive to the crystallinity and morphology characteristics of the phosphor. The phosphor calcined at 950 °C for 3 h in 20%H₂/80%Ar atmosphere exhibits improved PL properties due to its high crystallinity and excellent morphology characteristics. The use of the SrCl₂ flux provides a novel way to improve the crystallinity of the silicates phosphors at low preparation temperature.     

Up-conversion luminescence and optical temperature-sensing properties of Er3+-doped perovskite Na0.5Bi0.5TiO3 nanocrystals

In this work we demonstrate the preparation of Er³⁺ doped perovskite ferroelectric Na_0.5Bi_0.5TiO₃ nanocrystals and their application in temperature sensing. The samples were synthesized via a facile hydrothermal method. Upconversion emission at 528 nm and 547 nm from two thermodynamically coupled excited states of Er³⁺ were recorded in the temperature from 80 K to 480 K under the excitation of a 980 nm diode laser. The emission intensity ratio (I₅₂₈/I₅₄₇) as a function of the temperature was investigated. A sensitivity of 0.0053 K⁻¹ is observed at 400 K, suggesting they are promising candidate for nanothermometers.     

Photoluminescence characterization of beryllium-implanted 6H–silicon carbide

Beryllium has been implanted into both n- and p-type 6H–SiC with post-implantation annealing at 1600 °C. Photoluminescence (PL) measurements have been performed, and PL lines at 420.5, 431 nm, and a broad band at around 505 nm have been observed. The line at 420.5 nm is attributed to an intrinsic defect D_II-center induced by beryllium implantation. The effects of excitation intensity and temperature during the PL experiments are investigated. Based on the excitation laser dependence PL result, the new doublet lines at around 431 nm are thought to be associated with beryllium related bound excitons. The broad band corresponding to the green luminescence at room temperature has been attributed to the recombination of free carriers to beryllium bound levels.     

Blue emission in Eu2+ activated MgXAl10O17 (X = Sr,Ca) phosphors

Here we reported photoluminescence properties of Eu²⁺ activated in novel and existing MgXAl₁₀O₁₇ (X = Sr, Ca) phosphor which has been prepared by combustion synthesis at 550 °C under UV and near UV excitation wavelength. The PL emission properties of MgSrAl₁₀O₁₇:Eu²⁺ were monitored at 254 nm and 354 nm respectively keeping emission wavelength at 469 nm. Whereas novel MgCaAl₁₀O₁₇:Eu²⁺ exhibit emission band at 452 nm keeping excitation at 378 nm. These blue emission corresponds to 4f⁶5d¹ → 4f⁷ transition of Eu²⁺ ions. Further phosphor was analyzed by XRD for the confirmation of desired phase and purity.     

Synthesis and luminescence of La2BaZnO5 phosphors

A modified synthesis of La₂BaZnO₅ phosphors activated with rare earths Eu³⁺, Tb³⁺, Pr³⁺ and Sm³⁺, and ns² ion Bi³⁺ is reported. RE₂BaZnO₅ compounds are conventionally prepared by two step solid state reaction. In the first step, carbonates or similar precursors are intimately mixed and heated at 900 °C to decompose the precursors to oxides. To eliminate the unwanted phases like BaRE₂O₄, the resulting powders are reheated at 1100 °C for long time. We prepared La₂BaZnO₅ phosphors activated with various activators by replacing the first step by combustion synthesis. Results on photoluminescence are presented. PL results on Eu³⁺ and Tb³⁺ are in good agreement with the literature reports. PL emission from Sm³⁺, Pr³⁺ and Bi³⁺ had not been reported earlier. Excitation spectrum of Eu³⁺ is dominated by a charge transfer band around 318 nm, while for the other rare earths a band at 240 nm is always present. This is attributed to the host absorption.     

Spectral structure of barium–phosphate–silicate phosphor Ba10(PO4)4(SiO4)2:EuM+

The new apatite–silicate phosphor doped with Eu ions in Ba₁₀(PO₄)₄(SiO₄)₂ matrix was synthesized through solid-state reaction. It was found that the as-synthesized phosphor displayed apparent mixture of band and line emission peaks giving rise to pseudo white light. The narrow emission bands peaking at 410 nm can be assigned to the 4f⁶5d→4f⁷(⁸S_7/2) transition of Eu²⁺ ions, and the other band at 507 nm is ascribed to anomalous fluorescent emission. One group of line emission peaking at 595 nm and 613 m were due to the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transition of Eu³⁺ ions. The occurrence of photostimulated luminescence and discrete emission lines in violet (410 nm), green (507 nm) and red (595 nm and 613 nm) colors indicate that this material has potential application in fields of white-light-emitting.     

Time-resolved spectroscopy in LiCaAlF6 doped with Cr3+ as a function of pressure and temperature: Excited-state crossover and phase-transition effects

This work investigates phase transition (PT) and excited-state-crossover (ESCO) effects on the photoluminescence (PL) properties of LiCaAlF₆: Cr³⁺. The structural requirements for changing the Cr³⁺ PL behavior from a broad-band emission at 1.59 eV (781 nm) at ambient conditions, to ruby-like narrow-line emission at 1.87 eV (663 nm) are analyzed in the 0–35 GPa range. We report a PL study on LiCaAlF₆: Cr³⁺ by means of time-resolved emission as a function of pressure and temperature. In particular we focus on the PL variations occurring around the pressure-induced trigonal-to-monoclinic first-order PT in LiCaAlF₆ at 7 GPa.     

5D0→7F1 and 5D0→7F2 transition in europium doped halosulphates for mercury-free lamps

Polycrystalline Na₃SO₄F:Eu and NaMgSO₄F:Eu halosulphate phosphors prepared by a wet chemical method have been studied for its photoluminescence (PL) and thermoluminescence (TL) characteristics. Two well resolved peaks are observed at 593 nm and 614 nm, which are assigned to due to ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions of Eu³⁺ ions. TL is observed at temperatures between 100 °C and 300 °C. In this paper, we report PL emission spectra of Eu³⁺ and TL glow curves, which are more sensitive than the standard TLD-CaSO₄:Dy. The presented phosphors are applicable for the mercury free lamps and solid state lighting devices.     

Optical properties,luminescence quenching mechanism and radiation hardness of Eu-doped GaN red powder phosphor

We report on the luminescence quenching mechanism of Eu-doped GaN powder phosphor produced with a low-cost, high yield rapid-ammonothermal method. We have studied as-synthesized and acid rinsed Eu-doped GaN powders with the Eu concentration of ～0.5 at.%. The Eu-doped GaN photoluminescence (PL) was investigated with 325 nm excitation wavelength at hydrostatic pressures up to 7.7 GPa in temperature range between 12 K and 300 K. The room temperature integrated Eu³⁺ ion PL intensity from acid rinsed material is a few times stronger than from the as-synthesized material. The temperature dependent PL studies revealed that the thermal quenching of the dominant Eu³⁺ ion transition (⁵D₀ → ⁷F₂) at 622 nm is stronger in the chemically modified phosphor indicating more efficient coupling between the Eu³⁺ ion and passivated GaN powder grains. Furthermore, it was found that thermal quenching of Eu³⁺ ion emission intensity can be completely suppressed in studied materials by applied pressure. This is due to stronger localization of bound exciton on Eu³⁺ ion trap induced by hydrostatic pressure. Furthermore, the effect of 2 MeV oxygen irradiation on the PL properties has been investigated for highly efficient Eu-doped GaN phosphor embedded in KBr–GaN:Eu³⁺ composite. Fairly good radiation damage resistance was obtained for 1.7 × 10¹² to 5 × 10¹³ cm^?2 oxygen fluence. Preliminary data indicate that Eu-doped GaN powder phosphor can be considered for devices in a radiation environment.     

Methylene bromide chemistry and photochemistry on rutile TiO2(110)

The chemistry and photochemistry of methylene bromide (CD₂Br₂) on the rutile TiO₂(110) surface was probed using temperature programmed desorption (TPD). CD₂Br₂ desorbed in three desorption states at 145, 160 and 250 K tentatively assigned to desorption from the multilayer, from an η¹-CD₂Br₂ species and a bridging η²-CD₂Br₂ species, respectively. The latter two TPD states presumably involve binding of CD₂Br₂ molecules to the surface through Br coordination at five-coordinate Ti⁴⁺ surface sites. The 160 and 250 K TPD states saturated at coverages of 1.0 and 0.33 ML, respectively, where 1 ML is equivalent to the surface Ti⁴⁺ site density (5.2 × 10¹⁴ cm^? 2). No thermal decomposition of CD₂Br₂ was observed on either the clean surface or with preadsorbed O₂. UV irradiation of CD₂Br₂ on TiO₂(110) resulted in predominately photodesorption, with trace amounts of photodecomposition evidenced in TPD. The rate of CD₂Br₂ photodesorption from TiO₂(110) occurred with a low cross section (~ 2 × 10^? 21 cm²) similar to that expected from direct optical excitation of CD₂Br₂. This observation suggests that charge carriers generated in TiO₂(110) were no more effective in activating adsorbed CD₂Br₂ molecules than would be expected through direct molecular excitation. These findings suggest that photocatalytic destruction of halocarbons such as CD₂Br₂ on TiO₂ may preferentially occur though indirect processes (such as OH radical attack) as opposed to direct electron transfer processes involving charge carriers generated in TiO₂ by bandgap excitation.