Effect of Au clustering on ferromagnetism in Au doped TiO2 films: theory and experiments investigation
TL;DR: In this article, the magnetic properties of the TiO 2 films and Au cluster doped TiO2 films fabricated by sol-gel and sputtering methods combined experiments and first-principles calculations.
About: This article is published in Journal of Physics and Chemistry of Solids.The article was published on 2017-01-01. It has received 11 citations till now. The article focuses on the topics: Vacancy defect & Doping.
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05 Nov 2018
TL;DR: A review of the most recent works that have employed the doping, decoration, and structural modification of TiO 2 particles for applications in photocatalysis can be found in this article.
Abstract: Titanium dioxide (TiO 2 ) has been widely used as a photocatalyst in many environmental and energy applications due to its efficient photoactivity, high stability, low cost, and safety to the environment and humans. However, its large band gap energy, ca. 3.2 eV limits its absorption of solar radiation to the UV light range which accounts for only about 5% of the solar spectrum. Furthermore, the photocatalytic activity of TiO 2 is also limited by the rapid recombination of the photogenerated electron-hole pairs. When used in water treatment applications, TiO 2 has a poor affinity toward organic pollutants, especially hydrophobic organic pollutants. Several strategies have been employed to reduce its band gap energy, its electron-hole recombination rates as well as enhance its absorption of organic pollutants. In this chapter, we review some of the most recent works that have employed the doping, decoration, and structural modification of TiO 2 particles for applications in photocatalysis. Additionally, we discuss the effectiveness of these dopants and/or modifiers in enhancing TiO 2 photoactivity as well as some perspective on the future of TiO 2 photocatalysis.
100 citations
Cites background from "Effect of Au clustering on ferromag..."
...Noble metal nanoparticles such as Ag [35], Pt [34], Pd [36], Rh [37] and Au [38] have also been used to modify TiO 2 for photocatalysis and have been reported to efficiently hinder electron-hole recombination due to the resulting Schottky barrier at the metal-TiO 2 interface....
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TL;DR: The ensemble of the obtained experimental results was analyzed in order to discover the actual state of Mo in the final materials, showing the occurrence of both bulk doping and Mo surface species, with progressive segregation of MoOx species occurring only at a higher Mo content.
Abstract: TiO₂ nanoparticles containing 0.0, 1.0, 5.0, and 10.0 wt.% Mo were prepared by a reverse micelle template assisted sol⁻gel method allowing the dispersion of Mo atoms in the TiO₂ matrix. Their textural and surface properties were characterized by means of X-ray powder diffraction, micro-Raman spectroscopy, N₂ adsorption/desorption isotherms at -196 °C, energy dispersive X-ray analysis coupled to field emission scanning electron microscopy, X-ray photoelectron spectroscopy, diffuse reflectance UV⁻Vis spectroscopy, and ζ-potential measurement. The photocatalytic degradation of Rhodamine B (under visible light and low irradiance) in water was used as a test reaction as well. The ensemble of the obtained experimental results was analyzed in order to discover the actual state of Mo in the final materials, showing the occurrence of both bulk doping and Mo surface species, with progressive segregation of MoOx species occurring only at a higher Mo content.
25 citations
TL;DR: In this article, the authors calculated the system of TiO2 with different valence states and concentrations of Cu substituted for Zn as well as Cu substitution and vacancy complex defects.
Abstract: Cu doped TiO2 can help to understand the magnetic origin of TiO2-based diluted magnetic semiconductors. As such, herein, we calculated the systems of TiO2 with different valence states and concentrations of Cu substituted for Zn as well as Cu substitution and vacancy complex defects. For the substitution ratio of 1/48, ferromagnetism appears only in Cu0 substitution system. By increasing the concentration of Cu to 1/24, Cu2+ substitution system also shows a ferromagnetic ground state when the Cu substitutions are not separated by empty regions (low charge density regions) and along the (111) direction. The Cu substitution and oxygen vacancy complex defect systems are easily polarized due to carriers induced by vacancy. Carriers change the bond of Ti ions around the defect, and arouse residual magnetic moment; however, the contribution of carriers to the electronic structure had a saturation limit. Oxygen vacancy leads to the formation of non-ferromagnetic coupling between two Cu substitutions in a 2 × 2 × 2 supercell. Cu substitution and Ti vacancy composite defect give rise to the weakening of p–d hybridization, and the lattice distortion caused by defects and the strong interaction of Cu–O bond is the source of ferromagnetism. The room-temperature ferromagnetism in Cu doped monocrystalline TiO2 samples could be mediated by vacancies or carriers.
10 citations
TL;DR: In this paper, microwave assisted sol-gel method and sonochemistry assisted solgel method (SC method) with varying gold load were used to synthesize Au-TiO2 nano powders.
Abstract: TiO2 is one of the most studied photocatalyst, however, in order to enhance the photocatalytic activity, several strategies for chemical or physical modifications have been reported. Among these strategies, microwave and sonochemistry assisted synthesis methods have been attracting attention due to the unique characteristics that can be achieved through it. Au–TiO2 nano powders were synthesized via microwave assisted sol–gel method (SG method) and sonochemistry assisted sol–gel method (SC method) with varying gold load, (containing 0.05, 0.1, 0.3, 0.7, 1.0, 3.0 and 5.0 wt% for SG method and 0.05, 0.1, 0.3, 0.7, 1.0 wt% for SC method). Subsequently, a calcination process was carried out at 450 °C for 3 h. Materials obtained were physicochemical analyzed by SEM, XPS, and XRD analysis. According to XRD analysis, the main crystalline phase of the materials was anatase. Average crystallite size and microstrain present in the powders were studied using the Williamson–Hall method and Debye–Scherrer equation. The crystal structure of all samples was refined by the Rietveld method, and a compression on the unit cell parameters was determined. These analyses revealed an increment in the unit cell strain when Au concentration was increased, and a decrease of the crystallinity in the powders when SG method was used. In the case of SC method samples, crystallinity and strain was found to remain constant.
9 citations
TL;DR: In this article, the physical properties of the TiO2 and Au-doped thin films fabricated by sol-gel dip-coating route are investigated and diffraction peaks of XRD spectra confirmed the formation of anatase phase.
Abstract: Physical properties of the TiO2 and Au-doped TiO2 films fabricated by sol–gel dip-coating route are investigated. The diffraction peaks of XRD spectra confirmed the formation of anatase phase. Crystallite size was found to be decreased by increasing dopant concentration from 12.02 to 10.10 nm. All the thin films annealed in air exhibit significant room-temperature ferromagnetism displaying anatase phase that can be employed in spintronic devices. The saturation magnetization increases from 6.8 to 10.35 emu/cm3 with the increase in Au concentration. The coercivity values vary between 360.82 and 478.515 Oe and remnant magnetization ranges between 0.46 and 0.71 emu/cm3. Dielectric parameters obeyed Maxwell-Wagner model and Koop’s theory and were explained by hopping mechanism. Small values of dielectric constant made them favorable for high-frequency devices. The band gaps of the undoped and Au-doped TiO2 thin films is in the range 3.5 to 3.38 eV, which are lesser than those of reported pure TiO2 (3.7 eV) that is favorable for enhancing solar cells efficiency. Au-doped TiO2 leads to an optimum antimicrobial agent. The photocatalysts having 5 wt% Au exhibit the highest photoactivities. The degradation of methylene blue under sunlight made them promising materials for water treatment.
8 citations
References
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TL;DR: A simple derivation of a simple GGA is presented, in which all parameters (other than those in LSD) are fundamental constants, and only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked.
Abstract: Generalized gradient approximations (GGA’s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. [S0031-9007(96)01479-2] PACS numbers: 71.15.Mb, 71.45.Gm Kohn-Sham density functional theory [1,2] is widely used for self-consistent-field electronic structure calculations of the ground-state properties of atoms, molecules, and solids. In this theory, only the exchange-correlation energy EXC › EX 1 EC as a functional of the electron spin densities n"srd and n#srd must be approximated. The most popular functionals have a form appropriate for slowly varying densities: the local spin density (LSD) approximation Z d 3 rn e unif
146,533 citations
TL;DR: An efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set is presented and the application of Pulay's DIIS method to the iterative diagonalization of large matrices will be discussed.
Abstract: We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order ${\mathit{N}}_{\mathrm{atoms}}^{3}$ operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ``metric'' and a special ``preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order ${\mathit{N}}_{\mathrm{atoms}}^{2}$ scaling is found for systems containing up to 1000 electrons. If we take into account that the number of k points can be decreased linearly with the system size, the overall scaling can approach ${\mathit{N}}_{\mathrm{atoms}}$. We have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable. \textcopyright{} 1996 The American Physical Society.
81,985 citations
TL;DR: In this paper, the formal relationship between US Vanderbilt-type pseudopotentials and Blochl's projector augmented wave (PAW) method is derived and the Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional.
Abstract: The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Bl\"ochl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules $({\mathrm{H}}_{2}{,\mathrm{}\mathrm{H}}_{2}{\mathrm{O},\mathrm{}\mathrm{Li}}_{2}{,\mathrm{}\mathrm{N}}_{2}{,\mathrm{}\mathrm{F}}_{2}{,\mathrm{}\mathrm{BF}}_{3}{,\mathrm{}\mathrm{SiF}}_{4})$ and several bulk systems (diamond, Si, V, Li, Ca, ${\mathrm{CaF}}_{2},$ Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.
57,691 citations
TL;DR: In this article, the presence of metal ion dopants in the TiO_2 crystalline matrix significantly influences photoreactivity, charge carrier recombination rates, and interfacial electron-transfer rates.
Abstract: A systematic study of metal ion doping in quantum (Q)-sized (2-4 nm) TiO_2 colloids is performed by measuring their photoreactivities and the transient charge carrier recombination dynamics. The presence of metal ion dopants in the TiO_2 crystalline matrix significantly influences photoreactivity, charge carrier recombination rates, and interfacial electron-transfer rates. The photoreactivities of 21 metal ion-doped colloids are quantified in terms of both the conduction band electron reduction of an electron acceptor (CCl_4 dechlorination) and the valence band hole oxidation of an electron donor (CHCl_3 degradation). Doping with Fe^(3+), Mo^(5+), Ru^(3+), Os^(3+), Re^(5+), V^(4+), and Rh^(3+) at 0.1-0.5 at.% significantly increases the photoreactivity for both oxidation and reduction while Co^(3+) and Al^(3+) doping decreases the photoreactivity. The transient absorption signals upon laser flash photolysis (λ_(ex) = 355 nm) at λ = 600 nm are extended up to 50 ms for Fe^(3+)-, V^(4+)-, Mo^(5+)-, and Ru^(3+)-doped TiO_2 while the undoped Q-sized TiO_2 shows a complete "blue electron" signal decay within 200 μs. Co^(3+)- and Al^(3+)-doped TiO_2 are characterized by rapid signal decays with a complete loss of absorption signals within 5 μs. The quantum yields obtained during CW photolyses are quantitatively correlated with the measured transient absorption signals of the charge carriers. Photoreactivities
are shown to increase with the relative concentration of trapped charge carriers. The photoreactivity of doped TiO_2 appears to be a complex function of the dopant concentration, the energy level of dopants within the TiO_2 lattice, their d electronic configuration, the distribution of dopants, the electron donor concentration, and the light intensity.
3,508 citations
TL;DR: It is proposed thatferromagnetic exchange here, and in dilute ferromagnetic nitrides, is mediated by shallow donor electrons that form bound magnetic polarons, which overlap to create a spin-split impurity band.
Abstract: Dilute ferromagnetic oxides having Curie temperatures far in excess of 300 K and exceptionally large ordered moments per transition-metal cation challenge our understanding of magnetism in solids. These materials are high-k dielectrics with degenerate or thermally activated n-type semiconductivity. Conventional super-exchange or double-exchange interactions cannot produce long-range magnetic order at concentrations of magnetic cations of a few percent. We propose that ferromagnetic exchange here, and in dilute ferromagnetic nitrides, is mediated by shallow donor electrons that form bound magnetic polarons, which overlap to create a spin-split impurity band. The Curie temperature in the mean-field approximation varies as (xdelta)(1/2) where x and delta are the concentrations of magnetic cations and donors, respectively. High Curie temperatures arise only when empty minority-spin or majority-spin d states lie at the Fermi level in the impurity band. The magnetic phase diagram includes regions of semiconducting and metallic ferromagnetism, cluster paramagnetism, spin glass and canted antiferromagnetism.
2,743 citations