Cu掺杂SnO_2/TiO_2复合薄膜的制备及性能研究

制备Cu掺杂的纳米Sn O2/Ti O2溶胶,采用旋涂法在载玻片上镀膜,经干燥、煅烧制得Cu掺杂的Sn O2/Ti O2薄膜,通过对比实验探讨掺杂比例、条件、复合形式等对结构和性能的影响。采用XRD、SEM、EDS、UVVis等测试手段对样品进行表征,并以甲基橙为探针考察了其光催化降解性能。XRD测试结果显示薄膜的晶型为锐钛矿型,结晶度较高。SEM谱图显示薄膜表面无明显开裂,粒子分布均匀,粒径约为20 nm。EDS测试结果表明薄膜材料中含有Cu元素,谱形一致。UV-Vis吸收光谱表明Cu掺杂以及Sn O2/Ti O2的复合使得在近紫外区的光吸收比纯Ti O2明显增强。光催化实验表明Cu掺杂后使得Sn O2/Ti O2复合薄膜对甲基橙的光催化降解效率进一步提高,Sn O2/Ti O2复合薄膜的光催化活性在10%Cu掺杂时达到最高。     

Dopant induced morphology changes in ZnO nanocrystals

Zinc oxide (ZnO) nanocrystals doped with different groups of impurities, e.g., Li, Na, Cu, Pr and Mg synthesized by solid-state reaction method under similar conditions exhibit different morphology. XRD showed monophasic wurtzite structure but change in lattice parameters and Zn-O bond length indicates incorporation of dopant ion in ZnO lattice. The morphology of ZnO nanocrystals exhibited striking dependence on type of dopant ion with the shape changing from nanorods, spherical to petal like particles. Photoluminescence (PL) shows pronounced UV emission and negligible visible emission for Li, Na and Cu doped ZnO nanocrystals with peak positions coinciding with that of undoped ZnO. Whereas signature emission of Pr³⁺ ion as well as broad visible emission from Mg doped ZnO revealed the role of intra gap metastable states formed by the dopant ion in the emission process.     

Oxygen vacancy and dopant concentration dependent magnetic properties of Mn doped TiO2 nanoparticle

Mn doped TiO₂ nanoparticles are synthesized by sol–gel method. Incorporation of Mn shifts the diffraction peak of TiO₂ to lower angle. The position and width of the Raman peak and photoluminescence intensity of the doped nanoparticles varies with oxygen vacancy and Mn doping level. The electron spin resonance spectra of the Mn doped TiO₂ show peaks at g = 1.99 and 4.39, characteristic of Mn²⁺ state. Reduction in the emission intensity, on Mn doping, is owing to the increase of nonradiative oxygen vacancy centers. Mn doped TiO₂, with 2% Mn, shows ferromagnetic ordering at low applied field. Paramagnetic contribution increases as Mn loading increases to 4% and 6%. Temperature dependent magnetic measurement shows a small kink in the ZFC curve at about 40 K, characteristic of Mn₃O₄. The ferromagnetic ordering is possibly due to the interaction of the neighboring Mn²⁺ ions via oxygen vacancy (F⁺ center). Increase in Mn concentration increases the fraction of Mn₃O₄ phase and thereby increases the paramagnetic ordering.     

Size effect on the SHG properties of Cu-doped CdI2 nanocrystals

Because the optically induced second harmonic generation (SHG) is prevented by symmetry in a centrosymmetric material, one needs to form noncentrosymmetric processes in order to observe the SHG. However, one of the efficient ways to enhance the noncentrosymmetricity of a material is to dope it with an appropriate impurity and amount. We grow Cu-doped CdI₂ layered nanocrystal structures from the mixture of CdI₂ and CuI using the standard Bridgman-Stockbarger method and investigate the nano-confined effects by studying the second-order optical effect via the measurements of SHG. The second-order susceptibility for the nanocrystals is calculated and the values at liquid helium temperature range from 0.38 to 0.83 pm V⁻¹ for the thicknesses of 10-0.8 nm respectively. The size dependence demonstrates the nano-sized quantum-confined effect with a clear increase in the SHG with decreasing the thickness of the nanocrystal or crystal temperature. Since the local electron-phonon anharmonicity is described by third-order rank tensors in disordered systems, the SHG is very similar to that one introduced for the third-order optical susceptibility. It has been confirmed by observing the large photoluminescent yield of the pure crystals. The Raman scattering spectra taken for thin nanocrystals confirm the phonon modes originating from interlayer phonons crucially responsible for the observed effects. The obtained results show that the Cu-doped CdI₂ layered nanocrystals are promising materials for applications in optoelectronic nano-devices.     

Room temperature magnetism of Fe doped CdS nanocrystals

Undoped and Fe doped CdS nanocrystals with Fe content of 2–5 at% of average crystallite size 1.2–2 nm have been obtained using chemical co-precipitation method with 2-mercaptoethonal as capping agent at 80 °C. X-ray diffraction (XRD) results showed that the undoped CdS nanocrystals were in mixed phase of cubic and hexagonal, where as the doped CdS nanocrystals were in hexagonal phase. Room-temperature ferromagnetism has been observed in Fe-doped CdS nanocrystals. Magnetic studies indicated diamagnetism in undoped, ferromagnetism in lightly doped (2 and 3 at%) and paramagnetism in samples of higher Fe content (4 and 5 at%). The substitutional incorporation of Fe³⁺ ion in Cd²⁺ sites was reflected in structural and electron paramagnetic resonance (EPR) measurements. Isolated as well as interacting Fe³⁺ ions are observed in EPR.     

Non-magnetic impurity induced magnetism in rutile TiO2:K compounds

Recent ab initio studies have theoretically predicted room temperature ferromagnetism in several oxide materials of the type AO(2) in which the cation A(4+) is substituted by a non-magnetic element of the 1 A column. Our purpose is to address experimentally the possibility of magnetism in Ti(1-x)K(x)O(2) compounds. The samples have been synthesized via the solid state route method at equilibrium. Our study has shown that Ti(1-x)K(x)O(2) is thermodynamically unstable and leads to a phase separation, in contradiction with the hypothesis of ab initio calculations. In particular, the crystalline TiO(2) grains appear to be surrounded by K-based phase. The oxidization state of the Ti ion is found to be in Ti(4+) as confirmed from the x-ray photoelectron spectra measurement. Nevertheless, K:TiO(2) compounds exhibit weak paramagnetism with the highest magnetic moment of ~0.5 μ(B) K(-1) but no long-range ferromagnetic order. The observed moment in these compounds remains much smaller than the predicted moment of 3 μ(B) by ab initio calculation. The apparent contradictions between our experiments and first-principles studies are discussed.     

First-principles study on magnetism and electronic structure of V-doped rutile TiO2

We have investigated the magnetism and the electronic structure of V-doped rutile TiO₂ using the first-principles full potential linearized augmented plane-wave (FP-LAPW) method. Total energy calculations reveal that V-doped rutile TiO₂ has a stable ferromagnetic ground state. Meanwhile, the electronic structure analysis indicates that V-doped rutile TiO₂ is a half-metal within the local density approximation (LDA) while a semiconductor within the LDA + U (Hubbard coefficient). The calculated magnetic moment in V-doped rutile TiO₂ mainly arises from the V atom with a little contribution from the nearest-neighboring O atoms due to the hybridization between the V 3d states and the nearest-neighboring O 2p states.     

Hydrothermal synthesis of flower-like TiO2 nanocrystals/graphene oxide nanocomposites

A facile hydrothermal method has been developed to be capable of decorating graphene oxide (GO) with flower-like TiO₂ nanocrystals without using any bridging species. The flower-like TiO₂ nanocrystals were uniformly self-assembled on the surface of GO nanosheets. The photocatalytic activity experiment indicated that the prepared TiO₂/GO nanocomposites exhibited a higher photocatalytic activity for the photocatalytic degradation of rhodamine B (RB) aqueous solution under the UV illumination, this methodology made the synthesis of TiO₂/GO nanocomposites possible and may be further extended to prepare more complicated nanocomposites based on GO for technological applications.     

Temperature dependent optical properties of Si nanocrystals embedded in SiO2 matrix

The optical properties of Si nanocrystals (nc-Si) with different sizes (2～5 nm) embedded in a SiO₂ matrix synthesized by the SiO _x /SiO₂ superlattice approach were studied in a temperature range from room temperature to 600 K by spectroscopic ellipsometry. The Maxwell–Garnett effective medium approximation and Lorentz oscillator model were employed to extract the dielectric function of nc-Si. The results show that the temperature dependence of optical properties of nc-Si is sensitive to its size. An empirical expression of Varshni approximation was obtained to characterize the band-gap energy change of nc-Si at different temperatures.     

Dopant size dependent variable range hopping conduction in polyaniline nanorods

The present work investigates the electrical transport and dielectric relaxation of polyaniline (PAni) nanorods doped with organic camphorsulfonic acid (CSA) and inorganic hydrochloric acid (HCl) synthesized by interfacial polymerization technique. High resolution transmission electron micrographs (HRTEM) depict that initially spherical nuclei directionally grow into nanorods and CSA doped PAni produces more uniform and aligned structures. The electrical transport studies reveal that the CSA doped nanorods follow 1D Mott variable-range hopping (VRH), whereas the HCl doped nanorods exhibit 2D VRH conduction mechanism. The value of interchain charge transfer integral is found to be higher for smaller size HCl doped PAni than that for larger size CSA doped PAni. The resistivity measurements exhibit semiconducting behavior for both organic and inorganic dopants and the resistivity of the CSA doped nanorods is found to be smaller than that of the HCl doped nanorods. The dielectric relaxation studies suggest Debye type relaxation with a single relaxation peak for both the dopants and the relaxation time of the carriers of the CSA doped PAni nanorods is smaller than that of the HCl doped nanorods.     

Shape and size dependent thermophysical properties of nanocrystals

A theoretical model based on thermodynamic variables is employed in the present work to study the thermophysical properties of nanomaterials of different shapes and sizes. The model proposed by Qi and Wang [19] is applied to determine the cohesive energy of nanomaterial. The number of atoms on the surface to the total number of atoms in nanosolid is considered in terms of shape factor (α) and size of nanocrystal. The variation of cohesive energy?(E_cn?), melting temperature?(T_mN), Debye temperature (θ_DN), Specific heat capacity (C_pN), and Energy band gap (E_gN?) is studied for spherical, regular tetrahedral, regular hexahedral and regular octahedral nanocrystals. The cohesive energy, melting temperature and Debye temperature are found to decrease as the grain size is reduced. However, the energy band gap and specific heat capacity are found to increase with decrease of grain size of nanomaterial. The results achieved in the present study are compared with the available experimental and also with those calculated from other theoretical models. The consistency between the present calculated results and the results reported earlier confirms the validity of the present model theory to explain the shape and size dependence of thermophysical properties of nanomaterials.     

掺杂Mn2+的浓度对CdS纳米颗粒光致发光的影响

采用反胶束法,合成了硅土包裹的掺有不同浓度的Mn2 的CdS纳米颗粒.高分辨电镜表明这些颗粒的直径小于5 nm.仅仅改变Mn2 的掺杂浓度,研究了这些颗粒的光致发光谱和光致发光激发谱,结果表明:Mn2 浓度的大小对掺杂CdS纳米颗粒的发光产生了重要的影响.通过电子顺磁共振谱的测量和分析揭露了Mn2 浓度影响这些掺杂颗粒发光效率的原因.     

Morphology dependent dispersion of third-order optical nonlinear susceptibility in TiO2

Nanostructured TiO₂ in different shapes such as nanosheet, nanowire, and nanoflower, as well as nanosphere have been prepared and characterized. Nonlinear optical (NLO) properties were measured at 532 nm using open and closed aperture Z-scan technique with respect to the shape. NLO behaviour of the nanostructured TiO₂ is found to be shape dependent in suspensions at the investigated wavelengths and is accounted in terms of effective surface-to-volume ratio of the structures. The values of different optical constants viz, β,n ₂,χ ⁽³⁾ for different morphological structures were evaluated and compared. The optical limiting thresholds of the samples were also investigated and it is found to decrease with increase in the net light matter interaction area of the sample.     

2-D magnetism

Influence of the spatial dimensionality on magnetic ordering phenomena has been discussed. Recent work on ideal 2 dimensional magnets like Rb₂FeF₄, Rb₂MnF₄, K₂NiF₄, Rb₂Mn_xMg_1-x and K₂CuF₄, K₂Cu_xZn_{1 -x}F₄, MnSt₂ possessing planar antiferro- and ferromagnetism, respectively, has been reviewed with a view to doing planar anisotropy calculations based on the reportedly common orthorhombic structures. Existence of easy axes, easy cones in the plane and first order magnetisation processes (FOMP) in the plane and out of plane in the presence of applied magnetic field can be predicted. The results have been presented in the graphical forms as well.     

Dopant distribution, oxygen stoichiometry and magnetism of nanoscale Sn0.99Co0.01O2

In recent work, we have shown that chemically synthesized Sn_1−xCo_xO₂ nanoscale powders with x≤0.01 are ferromagnetic at room temperature when prepared by annealing the reaction precipitate in the narrow temperature window of 350-600 ^°C. Combined high resolution x-ray photoelectron spectroscopy (on as-prepared and Ar⁺ ion sputtered samples), x-ray diffraction and magnetometry measurements showed that the Co distribution is more uniform throughout the individual Sn_0.99Co_0.01O₂ particles when prepared at lower annealing temperatures of 350-600 ^°C and this uniform dopant distribution is essential to produce stable high temperature ferromagnetism. However, surface segregation of the dopant atoms in samples annealed at >600 ^°C destroys the room-temperature ferromagnetic behavior and reduces the Curie temperature to <300 K.     

Quantum chemical calculations of the structural influence on electronic properties in TiO2 nanocrystals

ABSTRACT

Quantum chemical calculations for two TiO₂ nanoparticle cluster models (rutile–(TiO₂)_n with n = 20, and anatase–(TiO₂)_n with n = 92), selected to represent different nanoparticle size regimes, are used to elucidate structural influences on the electronic properties. Structural and electronic properties were obtained using a variety of computational methods and structure optimisation schemes, including a comparison of results for several different density functional theory functionals, as well as complementary Hartree–Fock and semi-empirical calculations. The results demonstrate a strong dependence of electronic properties, such as the optical band gap of importance for photoelectrochemical and photocatalytic applications, on the structure of the nanocrystal. From a methodological point of view, the calculations also provide useful information of broader significance about the viability of different computational schemes to efficiently obtain reliable computational results for intrinsically nanostructured materials.     

Photoluminescence of Cu-doped and Cu-doped ZnS nanocrystallites

ZnS:Cu⁺ and ZnS:Cu²⁺ nanocrystallites have been obtained by chemical precipitation from homogeneous solutions of zinc, copper salt compounds, with S²⁻ as precipitating anion formed by decomposition of thioacetamide. X-ray diffraction (XRD) analysis shows that average diameter of particles is about 2.0-2.5 nm. The nanoparticles can be doped with copper during synthesis without altering XRD pattern. However, the emission spectrum of ZnS nanocrystallites doped with Cu⁺ and Cu²⁺ consists of two emission peaks. One is at 450 nm and the other is at 530 nm. The absorptive spectrum of the doped sample is different from that of un-doped ZnS nanoparticles. Because the emission process of the Cu⁺ luminescence center in ZnS nanocrystallites is remarkably different from that of the Cu²⁺ luminescence center, the emission spectra of Cu⁺-doped samples are different from those of Cu²⁺-doped samples.     

Density functional theory studies on the structural and physical properties of Cu-doped anatase TiO2(101) surface

Structure and physical properties of anatase TiO₂ (101) surface doped with copper have been studied by using density functional theory. Results show that Cu@Ti and Cu@O systems behave as p and n type semiconductors, respectively. Anatase TiO₂ (101) surface exhibits a blue shift in optical absorption spectra compared with pure TiO₂ bulk materials. Enhanced photocatalytic activity at wavelength around 400 nm could be contributed by the change in electronic structure.     

Preparation of ultrafine TiO2 nanocrystals via pulsed-laser ablation of titanium metal in surfactant solution

This work reports the attempts to perform pulsed-laser ablation in a liquid medium for synthesizing oxide semiconductor nanocrystalline particles. Ultrafine TiO₂ nanoparticles were synthesized by pulsed-laser ablation of a titanium target immersed in an aqueous solution of surfactant sodium dodecyl sulfate (SDS) as well as in deionized water. The surfactant concentration dependence of TiO₂ nanocrystal formation was systematically characterized by X-ray diffraction, optical transmission spectroscopy, and transmission electron microscopy. The maximum amount of ultrafine anatase nanocrystalline particles (with mean size of 3 nm in diameter) was obtained in an aqueous solution of 0.01 M SDS. A probable formation process was proposed based on laser-induced reactive quenching at plasma-liquid interfaces and surfactant-mediated growth in a liquid solution. PACS 52.38.Mf; 52.50.Jm; 82.70.Uv; 81.07.Bc; 78.40.Fy