P掺杂锐钛矿相TiO2的第一性原理计算

利用基于密度泛函理论的第一性原理对不同P掺杂形式(P替位Ti, P替位O, 间隙P)的锐钛矿相TiO₂的晶格常数、电荷布居、能带结构、分态密度和吸收光谱进行了计算. 结果表明, P替位Ti时, TiO₂体积减小, P替位O和间隙P的存在使TiO₂的体积膨胀; 替位Ti的P和间隙P均有不同程度的氧化, 而替位O的P带有负电荷. 三种P掺杂形式均导致锐钛矿相TiO₂禁带宽度的增大, 并在TiO₂禁带之内引入了掺杂局域能级. P掺杂导致TiO₂禁带宽度增大的程度依次为: 间隙P>P替位Ti>P替位O. 吸收光谱的计算结果表明, P替位Ti并不能增强TiO₂的可见光吸收能力, 但间隙P的存在大幅提高了TiO₂的可见光光吸收能力, 间隙P有可能是造成实验上P掺杂增强锐钛矿相TiO₂光催化活性的重要原因. 关键词： P掺杂 2')" href="#">锐钛矿相TiO₂ 第一性原理     

Ab Initio Simulation of Charge Transfer at the Semiconductor Quantum Dot/TiO2 Interface in Quantum Dot‐Sensitized Solar Cells

Quantum dot‐sensitized solar cells (QDSSCs) have emerged as a promising solar architecture for next‐generation solar cells. The QDSSCs exhibit a remarkably fast electron transfer from the quantum dot (QD) donor to the TiO₂ acceptor with size quantization properties of QDs that allows for the modulation of band energies to control photoresponse and photoconversion efficiency of solar cells. To understand the mechanisms that underpin this rapid charge transfer, the electronic properties of CdSe and PbSe QDs with different sizes on the TiO₂ substrate are simulated using a rigorous ab initio density functional method. This method capitalizes on localized orbital basis set, which is computationally less intensive. Quite intriguingly, a remarkable set of electron bridging states between QDs and TiO₂ occurring via the strong bonding between the conduction bands of QDs and TiO₂ is revealed. Such bridging states account for the fast adiabatic charge transfer from the QD donor to the TiO₂ acceptor, and may be a general feature for strongly coupled donor/acceptor systems. All the QDs/TiO₂ systems exhibit type II band alignments, with conduction band offsets that increase with the decrease in QD size. This facilitates the charge transfer from QDs donors to TiO₂ acceptors and explains the dependence of the increased charge transfer rate with the decreased QD size.     

Deposition dynamics and chemical properties of size-selected Ir clusters on TiO2

We report a study of Ir_n/TiO₂ samples prepared by size and energy-selected deposition of Ir_n⁺ (n=1, 2, 5, 10, 15) on rutile TiO₂(1 1 0) at room temperatures. The Ir clusters are found to be formally in the zero oxidation state, and there are no significant shifts in Ir 4f binding energy with cluster size. Over a wide range of impact energies, both Ir XPS intensity and peak position are constant, indicating constant sticking coefficient, and no impact-driven redox chemistry. Low energy ion scattering spectroscopy (ISS) suggests that the deposited Ir clusters remain largely intact, neither fragmenting nor agglomerating, and retaining 3-D structures for the larger sizes. For impact energies above 10 eV/atom, comparison of ISS and XPS data show that the Ir clusters are penetrating into the TiO₂ surface, with the extent of penetration increasing with both per atom energy and cluster size. Temperature programmed desorption (TPD) of CO is used to further characterize the deposited Ir_n. This system shows pronounced substrate-mediated adsorption (SMA) in low CO exposures, with strong dependence on cluster size. ISS and sputtering experiments indicate that CO adsorbed via SMA is bound differently than CO adsorbed in high dose experiments. In experiments with sequential C¹⁶O and C¹⁸O doses, facile C¹⁶O → C¹⁸O exchange is observed for Ir₅ and larger clusters, but not for Ir₂. The peak CO desorption temperature is found to decrease with cluster size. The cycle of CO adsorption and heating comprising a TPD experiment have a dramatic effect on the sample morphology, leading to encapsulation of Ir by a thin TiO_x layer.     

Structures, chemical bonding, magnetisms of small Al-doped zirconium clusters

The geometrical and electronic properties of small Al-doped Zrn−1 and host Zrn clusters (n=2-8) are investigated with hybrid HF/DFT functional: B3LYP. For the most favorable configurations of Zrn−1Al clusters, the Al atom prefers to be located on the surface of host zirconium clusters. The isomers that correspond to low coordination number of Zr-Al bonds are found to be more stable. The doping of Al atom in Zrn−1 clusters improves the chemical activities of host clusters. The Zr₅, Zr₇, Zr₄Al and Zr₆Al clusters behave the stronger stabilities relative to their respective neighbors. The strong s-d hybridizations are presented in all bonding Zr atoms. The values of WBI together with AIM analysis suggest that the Zr-Zr interactions are stronger than those between Zr and Al atoms. The doping of Al atom results into the decrease of spin magnetic moments for host zirconium clusters. The moments are mainly derived from the 4d electrons of bonding Zr atoms.     

密度泛函理论对ZrnPd团簇结构和性质的研究

用B3LYP/LANL2DZ方法对Zr_nPd(n =1—13)团簇的平衡几何结构、能量、频率、电子性质和磁性进行了计算.研究表明,Pd原子位于表面的异构体更为稳定,其中Zr₇Pd,Zr₁₂Pd团簇稳定性高,是幻数团簇,此外,相对于Zr_nCo与Zr_nFe团簇,Zr_nPd团簇参与化学反应的能力较弱,化学稳定性更     

The electronic structures of the core and valence ion states of metal oxides

SCF-Xα SW MO calculations on metal core ion hole states and X-ray emission (XES) and X-ray photoelectron (XPS) transition states of the non- transition metal oxidic clusters MgO₆^10?, AlO₄^5? and SiO₄^4? show relative valence orbital energies to be virtually unaffected by the creation of valence orbital or metal core orbital holes. Accordingly, valence orbital energies derived from XPS and XES are directly comparable and may be correlated to generate empirical MO diagrams. In addition, charge relaxation about the metal core hole is small and valence orbital compositions are little changed in the core hole state. On the other hand, for the transition metal oxidic clusters FeO₆^10?, CrO₆^9? and TiO₆^8? relative valence orbital energies are sharply changed by a metal core orbital or crystal field orbital hole, the energy lowering of an orbital increasing with its degree of metal character. Consequently O 2p nonbonding → M 3d-O 2p antibonding (crystal field) energies are reduced, while M 3d bonding → O 2p nonbonding and M 3d-O 2p antibonding → M 4s,p-O 2p antibonding (conduction band) energies increase. Charge relaxation about the core hole is virtually complete in the transition metal oxides and substantial changes are observed in the composition of those valence orbitals with appreciable M 3d character. This change in composition is greater for e _g than for t_2g orbitals and increases as the separation of the e_g crystal field (CF) orbitals and the O 2p nonbonding orbital set decreases. Based on the hole state MO diagrams the higher energy XPS satellite in TiO₂ (at about 13 eV) is assigned to a valence → conduction band transition. The UV PES satellites at 8.2 eV in Cr₂O₃ and 9.3 eV in FeO are tentatively assigned to similar transitions to conduction band orbitals, although the closeness in energy of the crystal field and O 2p nonbonding orbitals in the valence orbital hole state prevents a definite assignment on energy criteria alone. However the calculations do clearly show that charge transfer transitions of the e_g bonding → e_g crystal field orbital type would generally occur at lower energy than is consistent with observed satellite structure.A core electron hole has little effect upon relative orbital energies and is only slightly neutralized by valence electron redistribution for MgO and SiO₂. For the transition metal oxides a core hole lowers the relative energies of M3d containing orbitals by large amounts, reducing O → M charge transfer and increasing M 3d crystal field → conduction band energies. Large and sometimes overcomplete neutralization of the core hole is observed, increasing from CrO₆^9? to FeO₆^10? to TiO₆^8?. as the O → M charge transfer energy declines.High energy XPS satellites in TiO₂ may be assigned to O 2p nonbonding → conduction band transitions while lower energy UV PES satellites in FeO and Cr₂O₃ arise from crystal field or O 2p nonbonding → conduction band excitations. Our “shake-up” assignment for FeO₆^10?, CrO₆^9? and TiO₆^8? are less than definitive because no procedure has yet been developed to calculate “shake-up” intensities resulting from transitions of the type described. However the results do allow a critical evaluation of earlier qualitative predictions of core and valence hole effects. First, we find that the comparison of hole or valence state ionic systems with equilibrium distance systems of higher nuclear and/or cation charge (e.g. the comparison of the FeO₆^10? Fe 2p core hole state to Co₃O₄) is dangerous. For example, larger MO distances in the ion states substantially reduce crystal field splittings. Second, core and CF orbital holes sharply reduce O → M charge transfer energies, giving 2e_g → 3e_g energy separations which are generally too small to match observed satellite energies. Third, highest occupied CF-conduction band energies are only about 4–5 eV in the ground states, but increase to about 7–11 eV in the core and valence hole states of the transition metal oxides studied. The energetic arguments presented thus support the idea of CF and/or O 2p nonbonding → conduction band excitations as assignments for “shake-up” satellites, at least in oxides of metals near the beginning of the transition series.     

Collision-induced dissociation studies on gas-phase titanium oxide cluster cations

Titanium oxide cluster cations $\mathrm{Ti}_{x}\mathrm{O}_{y}^{+}$ are produced in a molecular beam by combining laser ablation of titanium with the supersonic expansion of oxygen into vacuum. The size distribution of the clusters produced is analyzed by time-of-flight reflectron mass spectrometry. The stable clusters appearing in the mass spectrum can be described by the general formula $(\mathrm{TiO})_{m}(\mathrm{TiO}_{2})_{n}(\mathrm{O}_{2})_{k}^{+}$ (with m,n=0,1,2,?? and k=0,1). Additionally, collision-induced dissociation studies of mass selected clusters colliding with Kr atoms in a gas cell have been performed. The results show that the clusters lose neutral O₂, TiO and/or (TiO₂) _n units, and the remaining charged fragments are those with the lowest ionization potentials. From these results the fragmentation cross section of the selected clusters is obtained.     

Geometries, stabilities, and electronic properties of gold-magnesium (AunMg) bimetallic clusters

The geometrical structures, relative stabilities, and electronic properties of bimetallic Au_nMg (n=1-8) clusters have been systematically investigated by means of first-principle density functional theory. The results show that the ground-state isomers have planar structures for n=1-7. Here, the calculated fragmentation energies, the second-order difference of energies, the highest occupied-lowest unoccupied molecular orbital energy gaps, and the hardness exhibit a pronounced odd-even alternation, manifesting that the clusters, especially Au₂Mg, with even-number gold atoms have a higher relative stability. On the basis of natural population analysis, the charge transfer and magnetic moment are also discussed.     

Electronic properties of anatase TiO2 doped by lanthanides: A DFT+U study

The effects of mono-doping of 4f lanthanides with and without oxygen vacancy defect on the electronic structures of anatase TiO₂ have been studied by first-principles calculations with DFT+U (DFT with Hubbard U correction) to treat the strong correlation of Ti 3d electrons and lanthanides 4f electrons. Our results revealed that dopant Ce is easy to incorporate into the TiO₂ host by substituting Ti due to its lower substitutional energy (∼−2.0 eV), but the band gap of the system almost keeps intact after doping. The Ce 4f states are located at the bottom of conduction band, which mainly originates from Ti 3d states. The magnetic moment of doped Ce disappears due to electron transfer from Ce to the nearest O atoms. For Pr and Gd doping, their substitutional energies are similar and close to zero, indicating that both of them may also incorporate into the TiO₂ host. For Pr doping, some 4f spin-down states are located next to the bottom of the conduction band and narrow the band gap of the doping system. However, for Gd doping, the 4f states are located in deep valence band and there is no intermediate band in the band gap. The magnetic moment of dopant Gd is close to the value of isolated Gd atom (∼7 μB), indicating no overlapping between Gd 4f with other orbitals. For Eu, it is hard to incorporate into the TiO₂ host due to its very higher substitutional energy. The results also indicated that oxygen vacancy defect may enhance the adsorption of the visible light in Ln-doped TiO₂ system.     

Enhanced oxygen binding through surface mediated ionic bonds

Palladium clusters deposited on TiO₂ supports constitute important oxidation catalysts. Addition of O₂ in presence of deposited palladium clusters results in dissociated O atoms whose binding strength controls the activity of a catalytic oxidation reaction. Here we demonstrate how the presence of even remote Pd sites can more than double the energy required to remove surface O atoms from a surface. This unusual increase in binding is shown to originate in a Coulomb interaction between Pd^δ+ and O^δ? where the otherwise semiconducting support remains almost neutral acting to mediate a charge transfer from Pd to O sites. The O atoms supported on lattice Ti sites are further shown to form composite TiO motifs that can exchange sites with Pd atoms with a minimal energy, opening the pathway to Ti migration. This behavior is proposed to be linked to their identical valence pool containing d-states.     

Structural, electronic and magnetic properties of ConRh (n=1-8) clusters from density functional calculations

The geometries, stabilities, electronic and magnetic properties of Co_nRh (n=1-8) clusters have been investigated systematically within the framework of the generalized gradient approximation density-functional theory. The results indicate that the most stable structures of Co_nRh (n=1-8) clusters are all similar to those of corresponding Co_n+1 clusters. Maximum peaks of second-order energy difference are found at n=2, 4 and 7, indicating that these clusters possess relatively higher stability than their respective neighbors. The magnetism of the ground state of alloy clusters all displays ferromagnetic coupling except for Co₃Rh. In addition, the doped Rh atom exhibits an important influence on the magnetism of alloy clusters, e.g., compared with corresponding pure Co_n clusters, the local moment of Co atom is noticeably enhanced in Co_nRh alloy clusters at n=1, 2, 5, 6, 7 and 8, while reduced at n=3 and 4. Further analysis based on the average bond length, the charge transfer and the spin polarization has been made to clarify the different magnetic responses to Rh doping.     

小型二氧化钛团簇(TiO2)n (n=2-8)的低能异构体研究

钛和氧之间存在多种成键方式,但迄今为止,二氧化钛团簇均只有少数几种异构体被报道. 与广泛使用的全局优化方法不同,本工作通过优化大量的随机初始结构,获得(TiO₂)_n (n=2-8)团簇稳定异构体. 首先利用PM6半经验方法对高达一万个以上的初始结构进行初步的优化筛选,并对筛选出的结构进行进一步的DFT计算以获得二氧化钛团簇的稳定异构体. 利用这种策略,发现了大量未经报道的稳定异构体,并提出了(TiO₂)₅和(TiO₂)₈新的最稳定异构体. 这些结构中包括含3个末端氧原子的异构体、含5配位氧原子和6配位钛原子的异构体等未经报道的新结构类型. 与丰富成键特征相对应,发现异构体数目随团簇尺寸的增大而急剧增加,对于(TiO₂)₇和(TiO₂)₈,能量在30 kcal/mol以内的异构体都在50个以上. 该工作发现了大量的二氧化钛小型团簇异构体,并凸显了其多样的结构特征,增进了对二氧化钛纳米团簇的结构、成键的理解,并为进一步的理论模拟、力场优化等提供了理论基础.     

First principles study of the electron structures of CaCu3Mn4O12 and CaCu3Ti4O12

The electronic structures of CaCu₃Mn₄O₁₂ and CaCu₃Ti₄O₁₂ are investigated from HF SCF LCAO calculation. In CaCu₃Mn₄O_12, the band and the density of states show a spin asymmetric ferrimagnetic character with a small energy gap. The Mn spin is anti-aligned with the Cu spin, and the total spin moment is 9 μ_B. Our calculation correctly reproduces the observed antiferromagnetic insulating character of CaCu₃Ti₄O₁₂. The gap in the band structure, which is 2.15 eV, reasonably agrees with the experimental value 1.5 eV. The electron density populations at different planes show clearly that the electron density has symmetric character. A tilted Mn(Ti) orbital implies a typical tilted three-dimensional network of MnO₆ (TiO₆) octahedra due to doping of the Jahn–Teller ion Cu. There is no covalency between Ca, Cu and Mn(Ti) atoms. In contrast, there are stronger bonds and somewhat likely covalency between Cu and O atoms, and also between Mn(Ti) and O atoms.     

Interface effects and the Auger parameter in titanium oxide thin films deposited on metals and in sandwich structures

Very thin films of TiO₂ and Ti₂O₃ were deposited by evaporation on Ag, on silver oxidized by an oxygen plasma and on Pt. Depending on the coverage, there were changes in the values of the binding energy (BE) and the Auger parameter (α′) of O and Ti. These shifts occur in the opposite direction with respect to that previously found for TiO₂ supported on insulators. Among others, reasons for these shifts are the different relaxation energy of photoholes and the occurrence of charge transfer processes at the metal oxide/metal interface. UV-visible absorption spectra of thin films of TiO₂Ag composites have shown a narrowing in the gap energy (E_g) of the oxide in respect to bulk titania. This observation shows the existence of a correlation between E_g and α′ when TiO₂ is supported on a metal surface. The Auger parameter of O and Ti is also sensitive to the intercalation of TiO₂ within a “sandwich” structure of SiO₂ and Ag, and the values found for the spectroscopic parameters are intermediate between those of TiO₂ supported on Ag and SiO₂. This result opens up the possibility of modulating the electronic properties of thin layers of TiO₂ by interaction with other materials.     

Electrodeposition of CdS quantum dots and their optoelectronic characterization by photoelectrochemical and scanning probe spectroscopies

CdS quantum dots (QDs) have been electrodeposited onto textured gold substrates from a nonaqueous electrolyte containing Cd(ClO₄)₂and elemental S. The initial deposit consisted of very small (about 3 nm) nanocrystals of CdS which were partially oriented with the Au substrate. With increasing deposit thickness, the crystal size increased and the degree of orientation decreased. Photocurrent spectroscopy and I–V spectroscopy, using a conducting scanning force microscope tip, were used to measure the CdS bandgap variations due to size quantization. The latter method also revealed room temperature conductivity peaks assigned to Coulomb charging of the QDs and evidence for charge tunneling into higher discrete energy levels.     

Electronic energy alignment at the PbSe quantum dots/ZnO(101̅0) interface

A number of solar energy conversion strategies depend on exciton dissociation across interfaces between semiconductor quantum dots (QDs) and other electron or hole conducting materials. A critical factor governing exciton dissociation and charge transfer in these systems is the alignment of electronic energy levels across the interface. We probe interfacial electronic energy alignment in a model system, sub-monolayer films of PbSe QDs adsorbed on single crystal ZnO(101?0) surfaces using ultraviolet photoemission spectroscopy. We establish electronic energy alignment as a function of quantum dot size and surface chemistry. We find that replacing insulating oleic-acid capping molecules on the QDs by the short hydrazine or ethanedithiol molecules results in pinning of the valence band maximum (VBM) of QDs to ZnO substrate states, independent of QD size. This is in contrast to similar measurements on TiO₂(110) where the alignment of the PbSe QD VBM to that of the TiO₂ substrate depends on QD size. We interpret these findings as indicative of strong electronic coupling of QDs with the ZnO surface but less with the TiO₂ surface. Based on the measured energy alignment, we predict that electron injection from the 1s_e level in photo-excited PbSe QDs to ZnO can occur with small QDs (diameter ? = 3.4 nm), but energetically unfavorably for larger dots (? = 6.7 nm). In the latter, hot electrons above the 1s_e level are necessary for interfacial electron injection.     

Effect of oxygen content and charge on the structure,stability and optoelectronic properties of yttrium oxide clusters

The electronic and geometrical structures of neutral and charged YOn (n=2–12) clusters have been investigated using density functional theory (DFT) with generalized gradient approximation. The oxygen atom in YOn has been found to be in oxo, peroxo and in superoxo forms. The geometrical structures and topologies of small size anionic clusters resemble that of neutral clusters. Yttrium showed higher coordination number than scandium. Computed results reveal the existence of YO₁₀ cluster to have a penta-peroxo oxygen with a homoleptic Y(η² –O₂)₅ geometrical configuration. The HOMO–LUMO gaps decrease with increasing n due to the increase in 2p orbital population of oxygen atoms. It has been shown that in these clusters bonding are predominantly ionic in nature and anions are thermodynamically more stable, due to the charge delocalization between the metal atom and oxygen ligands. YO₁₀⁺ and YO₁₂⁺ were found to be highly exothermic to release one and two oxygen molecules, while YO₁₁⁺ dissociates though the ozonide dissociation channel. Computed absorption spectra of small clusters are mainly contributed by yttrium metal d and s valence orbitals. The absorbance spectra, shifts towards lower energy with cluster size increase, while charge has no substantial effect on the absorption spectrum.     

密度泛函理论研究Hg与Auqn(n=1—6, q=0,+1,-1) 团簇的相互作用

采用密度泛函理论中的广义梯度近似对Hg与小团簇Au ^q_n (n=1—6, q=0, +1, -1)的相互作用进行了系统研究. 结果表明,除Au₅^+,-团簇外,前线分子轨道理论可以成功预测大部分Au _n Hg ^q 复合物的最低能量结构. Au_n团簇对Hg的吸附受团簇尺寸大小和团簇所携带电荷的影 关键词： 密度泛函理论 汞 金团簇 吸附能     

Charge transfer between alkali cluster ions and atoms in the 1 to 10 keV collisional energy range

The cross-sections for collisional charge transfer between singly charged free clusters M _n ⁺ (M = Li, Na; n=1...50) and atomic targets A (cesium, potassium) have been measured as a function of collisional relative velocity in laboratory energy range 1–10 keV. For each cluster size, the experimental values of the charge transfer cross-section are fitted with an universal parametric curve with two independent parameters and v_m, the maximum cross-section and the corresponding velocity. For small size clusters (), the characteristic parameters show strong variations with the number of atoms in the cluster. Abrupt dips observed for n=10 and n=22 are attributed to electronic properties. Charge transfer patterns observed for various collisional systems present similarities, which appear more sensitive to cluster quantum size effects than to collision energy defects. In their whole, the and v_m parameters show differences in both their size evolution and their absolute values discussed in term of projectile and target electronic structures. Received 13 April 2000 and Received in final form 29 June 2000     

Effects of increasing number of rings on the ion sensing ability of CdSe quantum dots: a theoretical study

A computational study on the structural and electronic properties of a special class of artificial atoms, known as quantum dots, has been carried out. These are semiconductors with unique optical and electronic properties and have been widely used in various applications, such as bio-sensing, bio-imaging, and so on. We have considered quantum dots belonging to II–VI types of semiconductors, due to their wide band gap, possession of large exciton binding energies and unique optical and electronic properties. We have studied their applications as chemical ion sensors by beginning with the study of the ion sensing ability of (CdSe) _n (n?=?3, 6, 9 which are in the size range of ~?0.24, 0.49, 0.74 nm, respectively) quantum dots for cations of the zinc triad, namely Zn²⁺, Cd²⁺, Hg²⁺, and various anions of biological and environmental importance, and studied the effect of increasing number of rings on their ion sensing ability. The various structural, electronic, and optical properties, their interaction energies, and charge transfer on interaction with metal ions and anions have been calculated and reported. Our studies indicate that the CdSe quantum dots can be employed as sensors for both divalent cations and anions, but they can sense cations better than anions.