Cerium Impurities Research Articles

Potential enhancements in the photocatalytic activity of the anatase 001 surface by adding cerium impurity

AbstractThis research investigates the structural and electronic properties of titanium dioxide ($$ TiO_2 $$ T i O 2 ) surfaces with the addition of cerium (Ce) impurities using first principles calculations within the framework of density functional theory, employing the pseudopotential method and the computational package Quantum ESPRESSO. Density of states (DOS) calculations reveal that the clean surface exhibits semiconductor behavior. In contrast, the titanium dioxide surface with Ce impurities on the fifth layer generates intermediate states within the energy band gap, resulting in a reduction in the band, with an intraband energy gap of 0.7882 eV induced. Moreover, the inclusion of a Ce atom on the fourth layer of the surface results it is observed that the surface an important characteristic is the appearance of intermediate states in the band gap, which are close to the valence band with a magnetic moment of 0.91 $$ \mu \beta /cell $$ μ β / c e l l . The ntraband gap can enhance the utilization of the visible spectrum. Additionally, the Ce impurity on the $$ TiO_2 $$ T i O 2 surface could promote greater photocatalytic activity in pollutant degradation and increase the oxidative capacity of titanium dioxide.

Activation of carbon dioxide on zinc oxide surfaces doped with cerium

AbstractThe adsorption of carbon dioxide on the surfaces of zinc oxide doped with cerium was studied. For this, a theoretical study was carried out using computational simulations based on density functional theory to determine possible improvements in photocatalytic activity. for the capture and dissociation of carbon dioxide. Calculations were performed using density functional theory within the Perdew-Burke-Ernzerhof generalized gradient approximation, also the Hubbard correction together with ultrasmooth atomic pseudopotentials and a basic plane wave implemented in the Quantum-ESPRESSO package. The doping concentration level considered in this work was 6.25%, among the results, the semiconducting character of zinc oxide was evident from the density of states calculations, it was also found that by adding cerium impurities to zinc oxide, the values of the lengths and angles of the bonds vary a little, this may be due to the small difference in covalent radius that the zinc atom has with respect to the cerium atom, consequently, changes occurred in the electronic properties that consist in intermediate states in the energy bandgap located around the Fermi energy. This may suggest that the cerium-doped zinc oxide system can probably absorb visible light, which could lead to possible improvements in the photocatalytic properties of the material. Furthermore, we found that by adding carbon dioxide to the cerium-doped zinc oxide surface, the carbon dioxide is activated and this could suggest the dissociation or reduction of this contaminant.

Effect of cerium impurity on the stable growth of the 4H-SiC polytype by the physical vapour transport method

Effect of cerium impurity in the SiC source material on the 4H-SiC growth was investigated. 4H-SiC crystals were grown on 6H-SiC crystal seeds by physical vapor transport and using the open seed backside method. Cerium is the impurity used for promotion of the 4H polytype nucleation. The optimal amount of CeO2 for the growth of 4H-SiC with a good crystalline quality and without undesired 15R- and 6H-SiC inclusions was identified. The open seed backside method combined with the presence of cerium impurity in the growth chamber were more effective for the crystal growth process due to a lower degradation of the backside surface of the grown crystal.

Enhancement of Spin-Charge Conversion in Dilute Magnetic Alloys by Kondo Screening.

We derive a kinetic theory capable of dealing both with large spin-orbit coupling and Kondo screening in dilute magnetic alloys. We obtain the collision integral nonperturbatively and uncover a contribution proportional to the momentum derivative of the impurity scattering S matrix. The latter yields an important correction to the spin diffusion and spin-charge conversion coefficients, and fully captures the so-called side-jump process without resorting to the Born approximation (which fails for resonant scattering), or to otherwise heuristic derivations. We apply our kinetic theory to a quantum impurity model with strong spin-orbit, which captures the most important features of Kondo-screened Cerium impurities in alloys such as Ce_{x}La_{1-x}Cu_{6}. We find (1)a large zero-temperature spin-Hall conductivity that depends solely on the Fermi wave number and (2)a transverse spin diffusion mechanism that modifies the standard Fick's diffusion law. Our predictions can be readily verified by standard spin-transport measurements in metal alloys with Kondo impurities.

Influence of Antisite Defects in Yttrium–Aluminum Garnet on Paramagnetic Centers of Ce3+ and Tb3+

In yttrium aluminum garnet (YAG) crystals containing terbium and cerium impurities, along with the main EPR signals of Tb3+ and Се3+ ions located in the dodecahedral sites of the YAG lattice in a regular environment, EPR lines with a lower (several percent) intensity were observed. They also belong to the paramagnetic centers of terbium and cerium, but are characterized by slightly altered parameters—the initial level splitting for non-Kramers Tb3+ ions and g-factors for Се3+ ions. It is shown that the nature of such centers and their number can be explained by the presence of YAl antisite defects, i.e. yttrium ions in the octahedral aluminum positions, in the environment of Tb3+ and Се3+.

A pulse cathodoluminescence and transient optical absorption kinetics simulation in Li6ReB3O9 (Re = Gd, Y) crystals doped with Ce-ions

A mechanism for the radioluminescence excitation involving radiation-induced mobile defects of the lithium sublattice has been revealed for double lithium orthoborates Li6YxGd1−x B3O9 (x = 0.0, 0.5) doped with a cerium impurity (0.5–1.0 at%). The pulse cathodoluminescence and transient optical absorption decay kinetics for both the single crystals and crystal-fibers have been studied using time-resolved absorption and luminescence spectroscopy techniques upon excitation with a nanosecond electron beam at T = 293 K. A numerical simulation of both processes had been performed. Based on the simulation results we explained the role of radiation defects of the lithium sublattice in increasing the radioluminescence intensity and the creation of inertial components of the luminescence decay kinetics.

Kinetics of Pulse Cathodoluminescence in Lithium–Gadolinium Orthoborate Crystals Doped with Cerium Impurity

The relaxation kinetics of electron excitations in single-crystal and crystal-fiber Li6GdB3O9 samples was studied by the time-resolved methods of pulse absorption spectroscopy and pulse cathodoluminescence under excitation with a nanosecond electron beam at 293 K. Based on the results of the numerical modeling of recombination processes, a pulse cathodoluminescence excitation mechanism was proposed.

Structural and Optical Properties of Ce3+-Doped TiO2 Nanocrystals Prepared by Sol–Gel Precursors

Ce-doped TiO2 nanoparticles (NPs) with dopant percentages of 1%, 3%, 5% and 10% were synthesized by the sol–gel method with precursors of tetratitanium isopropoxide and cerium nitrate hexahydrate Ce(NO3)3·6H2O. Based on the x-ray diffraction analysis, it was found that the size of NPs declines with an elevation of Ce impurity; for the pure state of 39 nm and the impure sample of 10%, its value declines up to 9 nm. Furthermore, with the addition of cerium impurity, the tetragonal structure of the TiO2 sample remains unchanged with slight variations in the lattice’s constants. The mean size and shape of the NPs were investigated by the transmission electron microscopy analysis, which were observed to be about 12.5 nm and spherical for NPs with 5% dopant. The results of the diffused reflection spectroscopy optical test showed that the band gap of the prepared samples decreases by 10% with increasing of cerium dopant from 2.75 eV for bare TiO2 NPs up to 2.00 eV for NPs with cerium impurity. In addition, photoluminescence analysis shows the intensity diminishes due to doping, causing a recombination of the electron–hole pair in the Ce-doped TiO2 NPs.

Nano-crystalline Ce-doped TiO2 powders: sol-gel synthesis and optoelectronic properties

Nanoparticles (NPs) of pure titanium dioxide (TiO2) and impure titanium dioxide (Ce-doped TiO2) were made with impurity percentages of 1%, 2%, 3%, 4%, and 6% by the sol-gel technique, with a new precursor of tetratitaniumisopropoxide (TTIP) and cerium chloride (CeCl3), acetic acid as a catalyst, and ethylene glycol as stabilizer in an ethanol solvent. The morphological, structural, and optical properties of NPs were examined. An XRD analysis showed that the TiO2 NPs had a tetragonal structure as the sample structures did not change as a result of the insertion cerium impurity into the TiO2 lattice. The structural results have also shown that the size of NPs is reduced by an increase in Ce impurities as the pure size of 38 nm reduces to 21.4 nm because of 6% impurity. A FESEM analysis showed that an increase in the impurity percentage reduces NPs uniformity because of its reducing size. The apparent shape and mean diameter of NPs obtained by TEM analysis was 25.2 nm for with 3% impurities in a quasi-spherical form. The UV-DRS optical analysis results showed that the band gap increases due to an increase in the Ce impurity percentage from 2.75 eV for pure TiO2 NPs to 3.09 eV for NPs with 6% impurities. In addition, a photoluminescence analysis showed that the intensity increased following an increase in impurities because of a band-gap increase.

Structural relaxation effects on the lowest $$4f{-}5d$$ 4 f - 5 d transition of $$\hbox {Ce}^{3+}$$ Ce 3 + in garnets

The role of structural relaxations on the energy of the lowest $$4f{-}5d$$ transition of $$\hbox {Ce}^{3+}$$ in garnets is studied by means of ab initio calculations. This study completes previous studies on the roles of the interactions of the Cerium impurity with its first neighbors and with the rest of the solid hosts, before the relaxations take place. Periodic boundary conditions density functional theory calculations (DFT) and second-order perturbation theory spin–orbit coupling embedded-cluster wave function theory calculations (WFT) have been performed in the garnets $$\hbox {Y}_{3}\hbox {Al}_{5}\hbox {O}_{12}$$ , $$\hbox {Lu}_{3}\hbox {Al}_{5}\hbox {O}_{12}$$ , $$\hbox {Y}_{3}\hbox {Ga}_{5}\hbox {O}_{12}$$ , $$\hbox {Lu}_{3}\hbox {Ga}_{5}\hbox {O}_{12}$$ , and $$\hbox {Ca}_{3}\hbox {Sc}_{2}\hbox {Si}_{3}\hbox {O}_{12}$$ doped with $$\hbox {Ce}^{3+}$$ . The local relaxation effects on the $$4f{-}5d$$ transition are similar in the WFT and DFT calculations. They produce a blue shift in Al and Ga garnets in which Ce substitutes for smaller Y and Lu cations, which is found to be basically due to the local expansions around the impurity, with only minor contributions from angular relaxations. Atomic relaxations of more distant neighbors enhance the blue shift. Although the embedding effects of the undistorted garnets are known to make the differences between the $$4f{-}5d$$ transition in Al and Ga garnets, we find that the structural relaxations are responsible for the small differences between the $$4f{-}5d$$ transition in $$\hbox {Y}_{3}\hbox {Al}_{5}\hbox {O}_{12}$$ : $$\hbox {Ce}^{3+}$$ and $$\hbox {Lu}_{3}\hbox {Al}_{5}\hbox {O}_{12}$$ : $$\hbox {Ce}^{3+}$$ , and in $$\hbox {Y}_{3}\hbox {Ga}_{5}\hbox {O}_{12}$$ : $$\hbox {Ce}^{3+}$$ and $$\hbox {Lu}_{3}\hbox {Ga}_{5}\hbox{O}_{12}$$ : $$\hbox {Ce}^{3+}$$ .

Crystal fields and Kondo effect: Specific heat for Cerium compounds

The thermodynamic Bethe ansatz equations for the N=6 Coqblin-Schrieffer model with crystal fields have been solved numerically. The realistic case of three Kramers doublets with arbitrary splittings has been studied for the first time. The specific heat has been calculated for representative combinations of the ionic energy splittings providing ample material for comparison with experimental results for Cerium impurities and compounds.

Growth of SiC by PVT method with different sources for doping by a cerium impurity, CeO2 or CeSi2

SiC crystals grown by a Physical Vapor Transport (PVT) method in the presence of varying Ce impurity contents (from 0.1wt% up to 2.5wt%) added to SiC source material are investigated. The presence of the cerium vapor in the growth atmosphere is confirmed by X-ray photoelectron spectroscopy measurements. Different electrical properties of SiC crystals are obtained while use was made of non-oxide (CeSi2) or oxide (CeO2) sources of Ce impurity. The impact of cerium on low concentrations of boron atoms in bulk SiC is reported. Presence of Ce in the vapor stabilizes the growth of the 4H-SiC polytype.

Spectroscopic properties of cerium doped YVO4 crystals and analysis on valence state of cerium ion

High-quality cerium doped YVO4 crystals are grown by the Czochralski method in a furnace heated by medium frequency induction. Cerium carbonate and ceria of 2 at% are respectively doped into the different crystals. The X-ray diffraction spectra show that all samples present YVO4 crystallographic phase. The absorption spectra, excitation spectra and photoluminescence spectra of the samples are measured. It is found that the spectroscopic properties of the samples doped with the different cerium impurities are similar. The two kinds of the samples each radiate a wideband blue light centered at 440 nm when excited by 325 nm light. This blue light emission is due to the 5d → 4f intrinsic emission of Ce3+ ions. In addition, the two kinds of the samples each radiate a wideband red light centered at 620 nm when excited by 460 nm light. The X-ray photoelectron spectroscopy (XPS) show that the O(1s) peak splits into two peaks at 529.8 eV and 531.8 eV, which indicate that there are two types of coordinate oxygen ions in the crystal. One kind of coordinate oxygen ion is normal coordinated oxygen ion (O2-) and another kind of coordinate oxygen ion is a hole (O-). According to recorded 5 peaks of Ce(3d) XPS, it is deduced that the Ce3+ ions and Ce4+ ions coexist in each of these samples. It is considered that the wideband red light emission of Ce:YVO4 crystal is due to the charge transfer transition of the Ce4+-O2- ion pairs, and Ce:YVO4 powders may become a novel red phosphor used for white LED.

The Blue Luminescence of CeCl<sub>3</sub> Doped Aluminum Oxide Thin Film

Aluminum oxide film doped with Ce3+ has been deposited by the medium frequency reactive magnetron sputtering technique. The photoluminescence emission from these films show peaks at range of 374-405 nm. The relative intensity of these peaks is strongly dependent on the amount of Ce incorporated in the films. The presence of Ce3+ as well as the stoichiometry of these films has been determined by energy dispersive x-ray spectroscope (EDS) measurements. It is proposed that the light emission observed generated by luminescent center associated with cerium chloride molecular rather than to atomic cerium impurities. The reason for a dominance of the lower energy transition as the amount of Ce3+ in the oxide films is increased is that the energy difference of 4f1and 5d1 decreases, with the increase of the Ce3+ concentration. These luminescent films are potentially good candidates for photonics applications.

Visible-light-driven boron/ferrum/cerium/titania photocatalyst

A boron/ferrum/cerium/titania photocatalyst with visible-light-induced performance was prepared by the conventional sol–gel method, in which tetrabutyl titanate (Ti(O- nC 4H 9) 4) was used as the precursor and boric acid (H 3BO 3), ferric nitrate enneahydrate (Fe(NO 3) 3·9H 2O), and cerium nitrate hexahydrate (Ce(NO 3) 3·6H 2O) were the sources of boron, ferrum, and cerium, respectively. The microstructure and optical property of the photocatalyst were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), N 2 adsorption–desorption isotherm, and UV–vis diffusive reflectance spectroscopy (DRS). It was found that the as-prepared photocatalyst composed of anatase and that the presence of impurities could retard phase transformation of TiO 2 from anatase to rutile at elevated temperatures and could prohibit growth of polycrystalline, which favored forming photocatalyst with large surface area. Degradation of 2,4-dichlorophenol (DCP) under visible light illumination was used to evaluate the photocatalytic activity. Comparative investigations showed that photocatalytic performance of the boron/ferrum/cerium/titania photocatalyst was the highest among the all test samples. It was testified that boron doping led to the band gap narrow and the response to visible light, and that ferrum and cerium impurities, which presented in the forms of FeO/Fe 2O 3 and Ce 2O 3/CeO 2, dispersed on the surface of TiO 2, suppressed the electron and hole recombination, and resulted in the enhancement of catalytic performance. The results exhibited that the synergistic effects of boron, ferrum and cerium played an important role in the band gap narrow and the increase of photoactivity.

The blue luminescence of cerium doped aluminum oxide thin film

Amorphous aluminum oxide thin films doped with cerium have been deposited by middle frequency reactive magnetron sputtering. There exist Ce3+ ions in the Al2O3:Ce thin films as shown by XPS measurement. The photoluminescence emission from these films show peaks around 374 nm which are associated with 5d to 4f transitions of Ce3+ ions. The intensity of these peaks is strongly dependent on the amount of cerium incorporated in the films. The presence of cerium as well as the stoichiometry of these films have been determined by energy dispersive X-ray spectroscopy (EDS) measurements. It is proposed that the light emission observed is generated by luminescence centers associated with trivalent ionic cerium impurities. The crystalline structure of the sample was analysed by X-ray diffraction (XRD). Auger electron spectroscopy has been used to estimate the stoichiometry of the films. This luminescence feature is advantageous for display techniques which require a purer blue emission.

Temperature dependence of the magnetic hyperfine field at cerium impurity in Co

Perturbed gamma-gamma angular correlation (PAC) technique was used to measure the magnetic hyperfine field (B hf ) at Ce impurity in Co using 140La→140Ce probe. The radioactive 140La produced by neutron irradiation of lanthanum metal with thermal neutrons was introduced in Co by arc melting in argon atmosphere. The present measurements cover the temperature range from 4.2–1300 K. Two pure magnetic interactions were observed at impurity sites, corresponding to a ferromagnetic ordering of Co moments in hcp and fcc phases. The temperature dependence of B hf for both phases, however, shows a sharp deviation from an expected standard Brillouin-like behavior for the host magnetization. The results are discussed in terms of a simple molecular-field model where the localized moment at impurity ions as well as the conduction electron contributions to the hyperfine field are taken into account.

Spectral Kinetics of Ce[sup 3+] Ions in Double-Fluoride Crystals with a Scheelite Structure

The influence of the cation composition on the spectral kinetics of Ce3+ ions in double-fluoride crystals with a scheelite structure is studied. The importance of the photodynamic processes induced in these crystals by the exciting radiation is demonstrated. The difference in luminescence quantum efficiency between Ce3+ ions in LiYF4 and LiLuF4 crystals is found to be due to the different lifetimes of color centers produced in the samples by the exciting radiation and to the different efficiency of the free-carrier recombination at cerium impurity centers. It is shown that Yb3+ ions can increase the carrier recombination rate in the crystals.

Investigation of the second harmonic generation in Ce-doped glasses in the Pb(PO3)2-Ba(PO3)2 system

The influence of cerium impurities on the efficiency and the rates of formation and decay of the photoinduced second harmonic generation in lead-containing glasses based on barium metaphosphate is experimentally investigated. It is shown that introduction of 1.5 mol % of cerium dioxide into the initial glass leads to a substantial gain in the generation efficiency and an increase of at least two orders of magnitude in the dark decay time of the induced quadratic susceptibility.

Optically detected magnetic resonance via the magnetic circular dichroism of absorption of cerium impurities in bulk paramagnetic terbium gallium garnet

The paramagnetic resonance of dilute ions in the bulk paramagnetic host is separated from the host response by means of optically detected magnetic resonance (ODMR) using the magnetic circular dichroism (MCD) of the absorption. This result shows that it is possible in principle to extend by means of the ODMR of the absorption the regime of paramagnetic resonance detection of impurity ions to magnetic hosts, which strongly interact with the microwave field and, hence, make the impurity signals unobservable by means of conventional EPR. The observed ODMR signals are attributed to and ions by correlation with the optical bands of these species. By means of ODMR via the Faraday effect it is shown that this effect is due to transitions between Tb levels. The identification of the observed magnetic resonance structures is confirmed by calculations based on known information on and in diamagnetic hosts. The role of molecular fields is discussed.