Photoinduced electron storage and surface plasmon modulation in Ag@TiO2 clusters

The reversible charging and discharging effects associated with photoexcitation of a TiO2 shell in a Ag@TiO2 composite are described. The photoinduced charge separation in the TiO2 shell is followed by electron injection into the silver core. Interestingly, the charging of the silver core is associated with the shift in the surface plasmon band from 460 to 430 nm. The stored electrons are discharged upon exposure of the charged Ag@/TiO2 composite to an electron acceptor. As the electrons from the silver core are discharged, the original surface plasmon absorption of the Ag core is restored.     

核壳结构p-n异质结复合光催化剂BiFeO3@TiO2的制备及其在可见光催化降解中的应用

化石燃料的迅速消耗导致环境污染与能源危机日益加剧.太阳能高效利用与转换是解决该难题的有效途径之一.在众多光催化剂中,TiO2因其高催化活性、高稳定性、低毒性以及低成本等优势而普遍受到关注,但TiO2存在着带隙过宽而无法利用可见光的缺陷,严重制约了其在光催化方向的实际应用.核壳型复合纳米材料具有较大的比表面积、较高的光吸收能力以及所吸附的污染物分子易于从吸附面扩散到光致降解面等特点,而表现出较强的有机污染物吸附性能以及良好的光催化活性.以磁体材料为核将TiO2包覆于磁体表面,可以制备具有磁分离特性的磁载光催化剂.而铁酸铋作为一种在室温下同时具有铁电性和铁磁性的钙钛矿型材料,由于其较窄的带宽(2.1 eV)在可见光催化氧化方面也受到了极大的关注.本文首先通过柠檬酸自燃烧法制备了可磁性分离的BiFeO3粉体,再以水解沉淀法将TiO2包裹在BiFeO3前驱体上形成了不同质量比(1:1,1:2,2:1)的核壳结构的BiFeO3@TiO2复合粉体,并以甲基紫为例,对其在紫外光和可见光照射下的光催化性能分别展开了研究.结果表明,BiFeO3@TiO2复合粉体的光催化性能较单独的BiFeO3或TiO2均有所提升.其中质量比为1:1的BiFeO3@TiO2复合粉体(TiO2壳层厚度为50–100 nm)展现出最强的光催化氧化活性,且在可见光下有更高的光催化效率.经表征分析,该复合粉体光催化性能高的原因可能归结于BiFeO3与TiO2两者之间形成了p-n异质结界面,有效地提高了电荷载流子的传输分离效率,同时BiFeO3较窄的禁带宽度拓展了纳米TiO2的光谱吸收范围,增强其光吸收能力.光电化学Mott-Schottky测试结果进一步证实:BiFeO3粉体在与TiO2复合之后,其电荷载流子传输与供体密度均有显著提升.自由基猝灭实验表明,在甲基紫光催化降解中起主要作用的为羟基自由基与光生电子,并结合能带理论与自建电场理论对降解机理进行了阐述.进一步研究表明,甲基紫降解效果最优条件为:复合粉体的投加量为1 g/L,甲基紫初始浓度为10 mg/L,初始pH为5.另外,质量比1:1的BiFeO3@TiO2对甲基橙和刚果红染料废水也具有较好的降解效果,表现出良好的工业应用前景.     

Catalysis with TiO2/gold nanocomposites. Effect of metal particle size on the Fermi level equilibration

Photoexcited semiconductor nanoparticles undergo charge equilibration when they are in contact with metal nanoparticles. Such a charge distribution has direct influence in dictating the energetics of the composite by shifting the Fermi level to more negative potentials. The transfer of electrons to Au nanoparticles has now been probed by exciting TiO(2) nanoparticles under steady-state and laser pulse excitation. Equilibration with the C(60)/C(60)(-) redox couple provides a means to determine the apparent Fermi level of the TiO(2)-Au composite system. The size-dependent shift in the apparent Fermi level of the TiO(2)-Au composite (20 mV for 8-nm diameter and 40 mV for 5-nm and 60 mV for 3-nm gold nanoparticles) shows the ability of Au nanoparticles to influence the energetics by improving the photoinduced charge separation. Isolation of individual charge-transfer steps from UV-excited TiO(2) --> Au --> C(60) has provided mechanistic and kinetic information on the role of metal in semiconductor-assisted photocatalysis and size-dependent catalytic activity of metal-semiconductor nanocomposites.     

Cr2O3/C@TiO2核壳型复合材料的设计合成及其光催化产氢性能

随着社会经济的快速发展,能源危机和环境污染问题成为世界各国关注的焦点.通过光催化剂将太阳能用于污染物降解、分解水产氢、CO2还原及有机物合成等领域,是解决上述问题的理想途径.过渡金属氧化物TiO2因其稳定性高、催化活性好、制备简单等优点,被认为是最理想的光催化材料.然而,TiO2带隙较宽、光响应范围窄、光量子效率低等缺点限制了其实际应用.将碳或Cr2O3与TiO2结合形成复合结构已被证明可以有效提升其光催化性能.另一方面,金属离子的掺杂可以有效提高氧化钛的可见光响应.本文利用具有高比表面积的金属有机骨架材料MIL-101(Cr)纳米材料作为模板、镉源和碳源,首先在MIL-101(Cr)表面可控生长TiO2纳米颗粒,获得MIL-101(Cr)@TiO2复合结构;然后在氮气保护下碳化形成Cr2O3/C@TiO2核壳型复合材料.碳化后,制备的复合材料具有模板的八面体形貌和高比表面积,MIL-101(Cr)中的Cr元素一部分会形成Cr2O3,一部分会掺杂到TiO2中,使得TiO2的吸收边红移.此外,Cr2O3/C@TiO2中的C有利于光的吸收和载流子的分离.这种独特的纳米结构赋予Cr2O3/C@TiO2复合材料优异的光催化性能.在300 W氙灯照射下,该复合材料光解水产氢的速率为446μmol h?1 g?1,约为纯TiO2的4倍.在可见光照射下,Cr2O3/C@TiO2分解水产氢的速率为25.5μmol h?1 g?1.将获得的粉体催化剂制备成光电极发现,Cr2O3/C@TiO2在全幅光照射下的光电流密度在0.4 V(vs.Ag/AgCl)下达到2.3 mA/cm2,约为纯TiO2的3.5倍.Cr2O3/C@TiO2光催化产氢活性的提高一方面是由于Cr掺杂到TiO2中使得其具有可见光响应,另一方面MIL-101碳化获得的Cr2O3/C有效促进了光生载流子的分离.     

Cr2O3/C@TiO2核壳型复合材料的设计合成及其光催化产氢性能

随着社会经济的快速发展,能源危机和环境污染问题成为世界各国关注的焦点.通过光催化剂将太阳能用于污染物降解、分解水产氢、CO2还原及有机物合成等领域,是解决上述问题的理想途径.过渡金属氧化物TiO2因其稳定性高、催化活性好、制备简单等优点,被认为是最理想的光催化材料.然而,TiO2带隙较宽、光响应范围窄、光量子效率低等缺点限制了其实际应用.将碳或Cr2O3与TiO2结合形成复合结构已被证明可以有效提升其光催化性能.另一方面,金属离子的掺杂可以有效提高氧化钛的可见光响应.本文利用具有高比表面积的金属有机骨架材料MIL-101(Cr)纳米材料作为模板、镉源和碳源,首先在MIL-101(Cr)表面可控生长TiO2纳米颗粒,获得MIL-101(Cr)@TiO2复合结构;然后在氮气保护下碳化形成Cr2O3/C@TiO2核壳型复合材料.碳化后,制备的复合材料具有模板的八面体形貌和高比表面积,MIL-101(Cr)中的Cr元素一部分会形成Cr2O3,一部分会掺杂到TiO2中,使得TiO2的吸收边红移.此外,Cr2O3/C@TiO2中的C有利于光的吸收和载流子的分离.这种独特的纳米结构赋予Cr2O3/C@TiO2复合材料优异的光催化性能.在300 W氙灯照射下,该复合材料光解水产氢的速率为446μmol h?1 g?1,约为纯TiO2的4倍.在可见光照射下,Cr2O3/C@TiO2分解水产氢的速率为25.5μmol h?1 g?1.将获得的粉体催化剂制备成光电极发现,Cr2O3/C@TiO2在全幅光照射下的光电流密度在0.4 V(vs.Ag/AgCl)下达到2.3 mA/cm2,约为纯TiO2的3.5倍.Cr2O3/C@TiO2光催化产氢活性的提高一方面是由于Cr掺杂到TiO2中使得其具有可见光响应,另一方面MIL-101碳化获得的Cr2O3/C有效促进了光生载流子的分离.     

CoS_2-碳(C_(60),石墨,碳纳米管)/TiO_2复合光敏剂光降解有机染料(英文)

CoS2,CoS 2-C60 /TiO2,CoS2-CNT/TiO2,and CoS2-Graphene/TiO2 were prepared.The TiO2 products had the anatase phase structure and interesting surface compositions.X-ray diffraction patterns of the CoS2-carbon/TiO2 composites showed a single and clear anatase phase and the CoS2 structure.Scanning electron microscopy characterization of the texture on the CoS 2-carbon/TiO2 composites showed a homogenous composition.Energy-dispersive X-ray spectra for elemental identification showed the presence of C and Ti with strong Co and S peaks from the CoS2-carbon/TiO2 composites.The composites obtained were also characterized by transmission electron microscopy and UV-Vis spectroscopy.CoS2-carbon/TiO2 composites showed excellent photocatalytic activity for the degradation of methylene blue under visible light irradiation.This was attributed to both photocatalysis on the TiO2 support and charge transfer by the carbon nanomaterial,and the introduction of CoS2 to enhance transfer of photogenerated electrons.     

Effective shell charge of electrons on a sphere

By ignoring the radial motions of the electrons in the valence shell of an atom one formally obtains the problem of electrons constrained to move on a sphere. This sphere will be attracted by the core as if it had an effective charge equal to the number of electrons on it minus a certain quantity resulting from the mutual repulsion of the electrons. This “effective shell charge” is a very simple, but still precise and quantitative concept which provides a good understanding of many empirical facts about atoms and ions, most notably Hund's rules. Implications for negative ions are discussed and chemical bonding is touched briefly. A qualitative difference in physical behavior for small and large sphere radii is pointed out.     

Quantum dot solar cells. harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO2 films

By using bifunctional surface modifiers (SH-R-COOH), CdSe quantum dots (QDs) have been assembled onto mesoscopic TiO(2) films. Upon visible light excitation, CdSe QDs inject electrons into TiO(2) nanocrystallites. Femtosecond transient absorption as well as emission quenching experiments confirm the injection from the excited state of CdSe QDs into TiO(2) nanoparticles. Electron transfer from the thermally relaxed s-state occurs over a wide range of rate constant values between 7.3 x 10(9) and 1.95 x 10(11) s(-1). The injected charge carriers in a CdSe-modified TiO(2) film can be collected at a conducting electrode to generate a photocurrent. The TiO(2)-CdSe composite, when employed as a photoanode in a photoelectrochemical cell, exhibits a photon-to-charge carrier generation efficiency of 12%. Significant loss of electrons occurs due to scattering as well as charge recombination at TiO(2)/CdSe interfaces and internal TiO(2) grain boundaries.     

Synthesis and characterization of TiO2@C core-shell nanowires and nanowalls via chemical vapor deposition for potential large-scale production

TiO(2) nanowires and nanowalls core structures covered with carbon shell were selectively synthesized by a simple chemical vapor deposition (CVD) method using commercial titanium powder as the starting material. Morphology and structure of the products were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The core shell structure is composed of single crystalline rutile titanium dioxide wrapped by amorphous carbon shell. By adjusting the growth temperature, morphology of the products can be controlled from one-dimensional nanowires to two-dimensional nanowalls. While TiO(2)@C nanowires were a preferred structure at higher temperature, TiO(2)@C nanowalls dominated the final product at lower temperature. A growth mechanism was proposed based on the initial growth state of these nanostructures, in which solid-state diffusion of the elements involved in the reaction was assumed to play an essential role. The obtained TiO(2)@C core shell structures may find potential applications in various nanoscale realms such as optoelectronic, electronic and electrochemical nanodevices and the simple synthesis procedure promises large scale production and commercialization of the titanium oxide@carbon nanostructures.     

Probing the surface adsorption and photocatalytic degradation of catechols on TiO2 by solid-state NMR spectroscopy

The local structure of the TiO2 surface modified with electron-donating bidentate ligands, such as catechols, has been investigated by solid-state NMR spectroscopy. The adsorption and degradation processes of catechols at the TiO2 surface were observed. The photocatalytic degradation mechanism of catechols at the TiO2 surface was interpreted in terms of the interfacial charge recombination reaction with conduction band electrons.     

Enhanced diffraction efficiency in a photorefractive liquid crystal cell with poly(9-vinylcarbazole)-infiltrated mesoporous TiO2 layers

The photorefractive effect of a layer-structured liquid crystal cell was significantly enhanced when a C60-doped poly(9-vinylcarbazole) (PVK)/TiO2 nanocomposite was used in two photoconductive layers. The C60-doped PVK/TiO2 nanocomposite film was prepared by infiltrating C60-doped PVK into a highly ordered mesoporous TiO2 layer. The addition of the TiO2 layer to the C60-doped PVK layer increased the first-order Raman-Nath diffraction efficiency from 24% to 42.9%. This enhancement of diffraction efficiency is attributed to a blocking effect of charge recombination in the composite layer. The electron transfer from the PVK layer into the TiO2 layer would decrease the recombination of photogenerated charges in the PVK layer, while charges in the PVK layer could participate in the formation of a space-charge field.     

Charge Separation and Catalytic Activity of Fe3O4@Ag “Nanospheres”

Nanospheres of Ag‐coated Fe₃O₄ were successfully synthesized and characterized. Photocatalytic properties of Fe₃O₄@Ag composites have been investigated using steady‐state studies and laser pulse excitations. Accumulation of the electrons in the Ag shell was detected from the shift in the surface plasmon band from 430 to 405 nm, which was discharged when an electron acceptor such as O₂, Thionine (TH) or C₆₀ was introduced into the system. Charge equilibration with redox couple such as C₆₀^●–/C₆₀ indicated the ability of these core–shell structures to carry out photocatalytic reduction reactions. As well, outer Ag layer could boost charge separation in magnetic core through dual effects of Schottky junction and localized surface plasmonic resonance (LSPR)‐powered band gap breaking effect under sunlight irradiation; resulted in higher photocatalytic degradation of diphenylamine (DPA). The maximum photocatalytic degradation rate was achieved at optimum amount of Ag‐NP loading to products. Adsorption studies confirmed that degradation of DPA dominantly occurred in solution. Moderately renewability of the nanocatalysts under sunlight was due to oxidation and dissolution of the outer Ag layer.     

Electron transport in silver-semiconductor nanocomposite films exhibiting multicolor photochromism

In the multicolor photochromism of TiO2 nanoporous films loaded with photocatalytically deposited Ag nanoparticles, visible light-induced electron transfer from Ag to oxygen molecules plays an essential role. Here we examined the effect of TiO2 on the electron transfer. We found that not only photocatalytically deposited Ag, but also electrodeposited Ag and commercially available Ag nanoparticles in a nanoporous TiO2 film exhibit the multicolor photochromism. The electrodeposited Ag exhibits the multicolor photochromism also in a nanoporous ZnO film, but not in nanoporous indium-tin oxide (ITO) and SiO2 matrices. Photoelectrochemical measurements for the Ag-TiO2 nanocomposite elucidated that some of the photo-excited electrons on Ag are transferred to oxygen molecules via TiO2 and non-excited Ag. Thus, an n-type semiconductor plays an important role in the charge separation between the excited electrons and Ag+. Non-excited Ag on TiO2 also plays an important role in the charge separation and/or catalysis of oxygen reduction. Replacement of the non-excited Ag with Pt accelerated the electron transport from the photo-excited Ag to oxygen molecules and the photochromic behavior.     

In situ x-ray photoelectron spectroscopic and density-functional studies of Si atoms adsorbed on a C60 film

The interaction between C(60) and Si atoms has been investigated for Si atoms adsorbed on a C(60) film using in situ x-ray photoelectron spectroscopy (XPS) and density-functional (DFT) calculations. Analysis of the Si 2p core peak identified three kinds of Si atoms adsorbed on the film: silicon suboxides (SiO(x)), bulk Si crystal, and silicon atoms bound to C(60). Based on the atomic percent ratio of silicon to carbon, we estimated that there was approximately one Si atom bound to each C(60) molecule. The Si 2p peak due to the Si-C(60) interaction demonstrated that a charge transfer from the Si atom to the C(60) molecule takes place at room temperature, which is much lower than the temperature of 670 K at which the charge transfer was observed for C(60) adsorbed on Si(001) and (111) clean surfaces [Sakamoto et al., Phys. Rev. B 60, 2579 (1999)]. The number of electrons transferred between the C(60) molecule and Si atom was estimated to be 0.59 based on XPS results, which is in good agreement with the DFT result of 0.63 for a C(60)Si with C(2v) symmetry used as a model cluster. Furthermore, the shift in binding energy of both the Si 2p and C 1s core peaks before and after Si-atom deposition was experimentally obtained to be +2.0 and -0.4 eV, respectively. The C(60)Si model cluster provides the shift of +2.13 eV for the Si 2p core peak and of -0.28 eV for the C 1s core peak, which are well corresponding to those experimental results. The covalency of the Si-C(60) interaction was also discussed in terms of Mulliken overlap population between them.     

类蛋结构的可磁分离光催化剂纳米球的制备及催化性能

通过反胶束和静电自组装方法制备了一种类蛋结构的可磁分离光催化剂纳米材料SiO₂@NiFe₂O₄@TiO₂(TSN), 这种光催化剂对甲基橙废水有较好的降解效果, 并显示出了超顺磁性, 通过外加磁场方便地实现催化剂在水中的分离与回收. 该光催化剂纳米球的X射线衍射, TEM和FTIR结果表明, 铁酸镍纳米粒子被包裹在SiO₂内, 形成SiO₂@NiFe₂O₄(SN)纳米球载体, 纳米TiO₂颗粒组装在SN表面, 形成TiO₂光催化壳层. 利用甲基橙的降解考察了光催化剂纳米球的活性, 结果表明, 在NiFe₂O₄和TiO₂之间包覆一层无定形的SiO₂可以显著提高光催化剂纳米球TSN的催化活性.     

Spatial electron distribution and its origin in the nanoporous TiO2 network of a dye solar cell

Dye solar cells have been investigated by charge carrier extraction under short and open circuit conditions and an illumination intensity equivalent to 1 sun (AM 1.5). Under short circuit conditions, a surprisingly high amount of charge carriers stored in the nanoporous TiO2 network has been observed. A theoretical model was developed to describe the charge transport in the nanoporous TiO2 network of a dye solar cell, and the spatial distribution of the electron concentration was calculated. These results were compared with the experimental data of charge carriers stored in the TiO2 network under short and open circuit conditions. We were able to conclude that under short circuit conditions, the electrochemical potential of the electrons in the region far from the electrode is up to 550-570 meV higher than that of the electrons at the front electrode. This internal voltage is the driving force across the nanoporous TiO2 film under short circuit conditions.     

Capped semiconductor colloids: Synthesis and photochemistry of CdS capped SnO2 nanocrystallites

Cds-capped SnO₂ (CdS@SnO₂) and SiO₂ (CdS@SiO₂) colloids of 50–80 ? in diameter have been prepared in aqueous medium. Significant quenching of CdS emission is observed in the CdS@SnO₂ system as the electrons are injected from the excited CdS shell into the SnO₂ core. Photoinduced charge separation in this system also results in transient bleaching in the 450 nm region. Picosecond laser flash photolysis studies of composite semiconductor nanoclusters that highlight the effect of the metal oxide core on the photophysical properties of the outer CdS shell are described. Contribution No. 3961 from the Notre Dame Radiation Laboratory, USA     

Quantum mechanical charge field molecular dynamics simulation of the TiO2+ ion in aqueous solution

Structural and dynamical properties of the TiO(2+) ion in aqueous solution have been investigated by using the new ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) formalism, which does not require any other potential functions except those for solvent-solvent interactions. Both first and second hydration shell have been treated at Hartree-Fock (HF) quantum mechanical level. A Ti-O bond distance of 1.5 A was observed for the [Ti=O](2+) ion. The first hydration shell of the ion shows a varying coordination number ranging from 5 to 7, five being the dominant one and representing one axial and four equatorial water molecules directly coordinated to Ti, which are located at 2.3 A and 2.1 A, respectively. The flexibility in the coordination number reflects the fast exchange processes, which occur only at the oxo atom, where water ligands are weakly bound through hydrogen bonds. Considering the first shell hydration, the composition of the TiO(2+) hydrate can be characterized as [(H(2)O)(0.7)(H(2)O)(4) (eq)(H(2)O)(ax)](2+). The second shell consists in average of 12 water molecules located at a mean distance of 4.4 A. Several other structural parameters such as radial and angular distribution functions and coordination number distributions were analyzed to fully characterize the hydration structure of the TiO(2+) ion in aqueous solution. For the dynamics of the TiO(2+) ion, different sets of dynamical parameters such as Ti=O, Ti-O(eq), and Ti-O(ax) stretching frequencies and ligands' mean residence times were evaluated. During the simulation time of 15 ps, 3 water exchange processes in the first shell were observed at the oxo atom, corresponding to a mean residence time of 3.6 ps. The ligands' mean residence time for the second shell was determined as 3.5 ps.     

Synthesis of multifunctional metal- and metal oxide core@mesoporous silica shell structures by using a wet chemical approach

We demonstrate a facile wet chemical approach for fabricating spherical metal/metal-oxide core@mesoporous silica shell hybrid nanoparticles with different core and shell thicknesses. Vertically aligned mesoporous silica (mSiO(2) ) shells were fabricated over the pre-synthesized spherical SiO(2) nanoparticles through a three-step strategy: 1)?synthesis of core materials, 2)?covering the core with an organic-inorganic composite layer, and 3)?removing the organic template through calcinations in air. The mechanisms of hybrid structure formation are proposed. The multifunctional nature of the hybrid structures could be induced by incorporating guest ions/molecules, such as Ag, Mn, and TiO(2) , into the pores of an mSiO(2) shell. Mn and TiO(2) cluster- incorporated composite structures have been tested to be antioxidizing agents and effective photocatalysts through electron spin resonance, radical scavenging tests, and the photocatalytic degradation of rhodamine B. The possibility of incorporating several hetero-element guest clusters in these mesoporous composite particles makes them highly attractive for multifunctional applications.     

Ultrasoft pseudopotentials for lanthanide solvation complexes: core or valence character of the 4f electrons

The 4f electrons of lanthanides, because of their strong localization in the region around the nucleus, are traditionally included in a pseudopotential core. This approximation is scrutinized by optimizing the structures and calculating the interaction energies of Gd(3+)(H(2)O) and Gd(3+)(NH(3)) microsolvation complexes within plane wave Perdew-Burke-Ernzerhof calculations using ultrasoft pseudopotentials where the 4f electrons are included either in the core or in the valence space. Upon comparison to quantum chemical MP2 and CCSD(T) reference calculations it is found that the explicit treatment of the 4f electrons in the valence shell yields quite accurate results including the required small spin polarization due to ligand charge transfer with only modest computational overhead.