钬掺杂提高TiO2纳米晶光催化活性的光谱性能机制研究

采用酸催化的溶胶-凝胶法制备了未掺杂和掺杂0.5%Ho(摩尔分数)的纳米TiO2光催化剂,以甲基橙的光催化降解为探针反应,评价了其光催化活性。运用XRD,TEM,EDS,FT-IR,UV-Vis DRS以及PL光谱表征技术考察了Ho掺杂对纳米TiO2的微晶尺寸、晶体结构、表面组成与光学性能的影响,并对改性机制作了探讨。结果表明,Ho掺杂可以抑制TiO2由锐钛矿相向金红石相的转变,阻碍TiO2晶粒增长,使TiO2的光吸收带边发生蓝移,增加表面羟基含量,促进光生载流子分离,从而使Ho掺杂TiO2样品光催化降解甲基橙的能力显著增强。     

Direct Observation of a Li‐Ionic Space‐Charge Layer Formed at an Electrode/Solid‐Electrolyte Interface

When two different materials come into contact, mobile carriers redistribute at the interface according to their potential difference. Such a charge redistribution is also expected at the interface between electrodes and solid electrolytes. The redistributed ions significantly affect the ion conduction through the interface. Thus, it is essential to determine the actual distribution of the ionic carriers and their potential to improve ion conduction. We succeeded in visualizing the ionic and potential profiles in the charge redistribution layer, or space‐charge layer (SCL), formed at the interface between a Cu electrode and Li‐conductive solid electrolyte using phase‐shifting electron holography and spatially resolved electron energy‐loss spectroscopy. These electron microscopy techniques clearly showed the Li‐ionic SCL, which dropped by 1.3 V within a distance of 10 nm from the interface. These techniques could contribute to the development of next‐generation electrochemical devices.     

梯形高聚物PTL导电机理的研究

本文采用EHMO/CO半经验计算方法,以准一维体系对梯形高聚物聚吩噻嗪(简称PTL)本征态及掺杂态电子能带进行了计算,讨论了PTL的导电机理,提出了当掺杂剂为质子酸时的荷电孤子或极化子导电模型及p-型掺杂时的极化子导电模型。     

Efficient charge transport along phenylene-vinylene molecular wires

We have studied the motion of charge carriers along isolated phenylene-vinylene (PV) chains using a combination of experimental and theoretical methods. The conductive properties of positive charges along PV chains in dilute solution were studied by using the pulse-radiolysis time-resolved microwave conductivity (TRMC) technique. This technique enables the measurement of high-frequency (tens of GHz) charge carrier mobilities along isolated PV chains without the use of electrodes. The charge carrier mobility along PV chains with finite and infinite length was studied theoretically by charge transport simulations with parameters from density functional theory (DFT) calculations. The high-frequency charge carrier mobility is found to depend strongly on the conjugation length of the PV chains and is found to increase both with increasing length of the PV chain and with increasing conjugation fraction. The experimental results are in good agreement with the calculated results. On the basis of this combined experimental and theoretical study an intrachain charge carrier mobility of a few tens of cm2/Vs is expected for an infinitely long PV chain without conjugation breaks.     

Asymmetric deformation density analysis in carbon nanotubes

Electrons and holes as charge carriers appear when a molecular system is exposed to external electric field. For nonsymmetric molecules, the distribution of charge carriers is also nonsymmetric. Although symmetric distribution of charge carriers is expected when the molecular system is symmetric, as the present work shows, they are not symmetric unexpectedly; that is, electrons and holes are not each other's mirror images. In this respect, asymmetric deformation density analysis is introduced to measure the extent of asymmetric distribution of electrons and holes as charge carriers when the symmetric system is exposed to symmetric external potential. Segments of (5,0) carbon nanotubes with different lengths are selected as symmetric systems, and a linear electric field is applied along the principal axis as symmetric potential. Results show that, at high electric fields, electrons tend to localize at the ends, while holes tend to occupy the middle area of carbon nanotube segments. While charge carriers play a vital role in molecular conductivity, asymmetric distribution of electrons and holes in symmetric systems has not yet been reported.     

Freezing of low energy excitations in charge density wave glasses

Thermally stimulated discharge current measurements were performed to study slow relaxation processes in two canonical charge density wave systems K(0.3)MoO(3) and o-TaS(3). Two relaxation processes were observed and characterized in each system, corroborating the results of dielectric spectroscopy. Our results are consistent with the scenario of the glass transition on the charge density wave superstructure level. In particular, the results directly prove the previously proposed criterion of charge density wave freezing based on the interplay of charge density wave pinning by impurities and screening by free carriers. In addition, we obtained new information on distribution of relaxation parameters, as well as on nonlinear dielectric response both below and above the threshold field for charge density wave sliding.     

Photocatalytic mechanisms of modified titania under visible light

Heterogeneous photocatalysis with titania under visible light has increasingly been a focus for research. Metal or non-metal doping, surface sensitization, semiconductor coupling, precious metal deposition and increasing crystal defects have been used to enhance the photocatalytic activity of titania under visible light. Based on the research results of different modification methods in recent years, some mechanisms from the excitation, bulk diffusion and surface transfer of photoinduced charge carriers, such as band gap modification, changing the excitation path, promoting the separation of photogenerated charge carrier, improving the surface adsorption and reaction, and synergistic effects, for photocatalysis under visible light are discussed and the development trend in this field is predicted.     

Photoelectrochemical investigation of ultrathin film iron oxide solar cells prepared by atomic layer deposition

Atomic layer deposition was used to grow conformal thin films of hematite with controlled thickness on transparent conductive oxide substrates. The hematite films were incorporated as photoelectrodes in regenerative photoelectrochemical cells employing an aqueous [Fe(CN)(6)](3-/4-) electrolyte. Steady state current density versus applied potential measurements under monochromatic and simulated solar illumination were used to probe the photoelectrochemical properties of the hematite electrodes as a function of film thickness. Combining the photoelectrochemical results with careful optical measurements allowed us to determine an optimal thickness for a hematite electrode of ～20 nm. Mott-Schottky analysis of differential capacitance measurements indicated a depletion region of ～17 nm. Thus, only charge carriers generated in the depletion region were found to contribute to the photocurrent.     

Nanostructured CuO/SrTiO3 bilayered thin films for photoelectrochemical water splitting

Bilayered thin films of CuO/SrTiO₃ with varying thickness of CuO were deposited by sol–gel spin-coating technique on indium tin oxide substrate and used as photoelectrode in the photoelectrochemical cell for water splitting reaction. Maximum photocurrent density of 1.85 mA/cm² at ?0.9 V/saturated calomel electrode was exhibited by 590-nm-thick CuO/SrTiO₃ bilayered photoelectrode, which is approximately eight times higher than that for CuO and 30 times higher than that for SrTiO₃. The bilayered system offered increased photocurrent density and enhanced photoconversion efficiency, attributed to improved conductivity, which ameliorate separation of the photo-generated carriers at the CuO/SrTiO₃ interface and higher value of flatband potential. Details about synthesis and various characterisations involving X-ray diffraction and scanning electron microscopy have been discussed. An energy band diagram has been proposed to dwell upon the mechanism of charge carrier transfer across the interface.     

光电催化二氧化碳还原研究进展

CO_2是最常见的化合物,作为潜在的碳一资源,可用于制备多种高附加值的化学品,如一氧化碳、甲烷、甲醇、甲酸等。传统的热催化转化CO_2方法能耗高,反应条件苛刻。因此,如何在温和条件下高效地将CO_2转化成高附加值的化学品,一直以来是催化领域的研究热点和难点之一。光催化技术反应条件温和、绿色环保。然而,纯光催化反应普遍存在太阳能利用效率有限,光生载流子分离效率低等问题。针对上述问题,在光催化的基础上引入电催化,可以提高载流子的分离效率,在较低的过电位下,实现多电子、质子向CO_2转移,从而提高催化反应效率。总之,光电催化技术可以结合光催化和电催化的优势,提高CO_2催化还原反应效率,为清洁、绿色利用CO_2提供了一种新方法。本文依据光电催化CO_2还原反应基本过程,从光吸收、载流子分离和界面反应等三个角度综述了光电催化反应的基本强化策略,并对未来可能的研究方向进行了展望。     

N掺杂TiO_2纳米粒子表面光生电荷特性与光催化活性

以尿素为氮源,采用水热法制备了不同N掺杂量的TiO2(N-TiO2)光催化剂.利用X射线衍射(XRD),紫外-可见漫反射光谱(UV-Vis DRS),X射线光电子能谱(XPS)及荧光(PL)光谱等技术对其进行了系统的表征.以罗丹明B(RhB)和甲基橙(MO)溶液的脱色降解为模型反应,分别考察了N-TiO2光催化剂在紫外和可见光区的光催化活性.利用表面光伏(SPV)和瞬态光伏(TPV)技术研究了N-TiO2纳米粒子表面光生电荷的产生和传输机制,并探讨了光生电荷与光催化活性之间的关系.结果显示,随着N含量的增大,TiO2表面光伏响应阈值红移,可见光部分光电压响应强度逐渐增强,瞬态光伏响应达到最大值的时间亦有着不同程度的延迟.这表明适量的N掺杂能够提高TiO2纳米粒子中光生载流子的分离效率,相应地延长载流子的传输时间,增加光生电荷的寿命,从而促进其光催化活性;而过量的N掺杂则增加了TiO2纳米粒子中光生载流子的复合中心,抑制其光催化活性.     

The application of computer curve fitting techniques to regnault-pfaundler calculations of calorimetric data

A curve fitting programme, using a cubic-spline technique, is described for fitting the results from reaction calorimetry. The coefficients from the technique are used to determine the mean resistance and other correction terms in a Regnault-Pfaundler analysis; the results agree with hand-worked results to better the 0.5%. The difficulties of using polynomial techniques are discussed and the importance is noted of computer plotting of results in checking the behaviour of the techniques.     

Evaluating the charge of nanofiltration membranes

In this study, several methods were used to determine the charge of commercially available nanofiltration membranes, and were compared. First the ion-exchange capacity was determined by titration, this method is able to distinguish between positively and negatively charged functional groups on the membrane. Secondly, measurements of the streaming potential gave a value for the charge density at the exterior membrane surface; the effect of the pH of the solution on the membrane charge was studied. Finally, measurements of the membrane potential allowed to evaluate the total membrane charge density.The results of the three methods were used to compare the membrane charge of nanofiltration membranes mainly in a qualitative way. It is shown that measurements of the membrane potential are preferred for the evaluation of the membrane charge.     

Dependence of charge transfer reorganization energy on carrier localisation in organic molecular crystals

Taking the organic molecular material dithiophene-tetrathiafulvalene (DT-TTF) as an example of a high mobility organic molecular material, we use density functional calculations to calculate the dependency of the reorganization energy associated with charge carrier transport on: (i) the geometric and electronic responsiveness of the local molecular crystal environment, and, (ii) the local spatial extent of the charge carrier. We find that in our most realistic extended models the charge transfer reorganization energy is strongly dependent on carrier localization. In particular, whereas highly localized carriers are found to be highly susceptible to their charge transfer efficiency being affected by changes in the local crystal environment, more delocalized carriers are better able to maintain their low reorganization energies. Considering that maintaining a relatively small charge transfer reorganization energy magnitude is an important factor in achieving high carrier mobilities, we suggest that those materials better able to sustain carriers with short-range thermally resistant intermolecular delocalisation should be sought for device applications.     

Association behavior of poly(methyl methacrylate-block-methacrylic acid) in aqueous medium

The atom transfer radical polymerization technique was used to synthesize the poly(methyl methacrylate-block-methacrylic acid) (P(MAA-b-MMA)) copolymer in order to study the aggregation behavior in aqueous solution over the course of neutralization. Combinations of static and dynamic light scattering (SLS, DLS) and potentiometric titration techniques were used to investigate the size and shape of the micelle at various degrees of neutralization (alpha). By comparing the effect of different polymer chain length with similar MMA/MAA ratio on the aggregation behavior during neutralization, we found relatively strong entanglement of long MMA polymer chains. The comparison between the different MMA/MAA ratios showed that longer MMA chains produced more entanglements. Conductometric titration was used to determine the counterion condensation phenomenon during the course of neutralization. At a critical micellar charge density observed at alpha approximately 0.4, Na+ ions are condensed on the polymer chains. The amount of condensed Na+ was evaluated by the conductivity change, yielding the condensation ratio when the polymer was completely neutralized.     

Topological analysis of the electronic charge density in the ethene protonation reaction catalyzed by acidic zeolite

In the present work, the distribution of the electronic charge density in the ethene protonation reaction by a zeolite acid site is studied within the framework of the density functional theory and the atoms in molecules (AIM) theory. The key electronic effects such as topological distribution of the charge density involved in the reaction are presented and discussed. The results are obtained at B3LYP/6-31G(**) level theory. Attention is focused on topological parameters such as electron density, its Laplacian, kinetic energy density, potential energy density, and electronic energy density at the bond critical points (BCP) in all bonds involved in the interaction zone, in the reactants, pi-complex, transition state, and alkoxy product. In addition, the topological atomic properties are determined on the selected atoms in the course of the reaction (average electron population, N(Omega), atomic net charge, q(Omega), atomic energy, E(Omega), atomic volume, v(Omega), and first moment of the atomic charge distribution, M(Omega)) and their changes are analyzed exhaustively. The topological study clearly shows that the ethene interaction with the acid site of the zeolite cluster, T5-OH, in the ethene adsorbed, is dominated by a strong O-H...pi interaction with some degree of covalence. AIM analysis based on DFT calculation for the transition state (TS) shows that the hydrogen atom from the acid site in the zeolitic fragment is connected to the carbon atom by a covalent bond with some contribution of electrostatic interaction and to the oxygen atom by closed shell interaction with some contribution of covalent character. The C-O bond formed in the alkoxy product can be defined as a weaker shared interaction. Our results show that in the transition state, the dominant interactions are partially electrostatic and partially covalent in nature, in which the covalent contribution increases as the concentration and accumulation of the charge density along the bond path between the nuclei linked increases.     

Persistence time of charge carriers in defect states of molecular semiconductors

Charge carriers in organic crystals are often trapped in point defects. The persistence time of the charge in these defect states is evaluated by computing the escape rate from this state using non-adiabatic rate theory. Two cases are considered (i) the hopping between separate identical defect states and (ii) the hopping between a defect state and the bulk (delocalized) states. We show that only the second process is likely to happen with realistic defect concentrations and highlight that the inclusion of an effective quantum mode of vibration is essential for accurate computation of the rate. The computed persistence time as a function of the trap energy indicates that trap states shallower than ～0.3 eV cannot be effectively investigated with some slow spectroscopic techniques such as THz spectroscopy or EPR commonly used to study the nature of excess charge in semiconductors.     

Role of Nickel Nanoparticles in High‐Performance TiO2/Ni/Carbon Nanohybrid Lithium/Sodium‐Ion Battery Anodes

Super‐small sized TiO₂ nanoparticles are in situ co‐composited with carbon and nickel nanoparticles in a facile scalable way, using difunctional methacrylate monomers as solvent and carbon source. Good control over crystallinity, morphology, and dispersion of the nanohybrid is achieved because of the thermosetting nature of the resin polymer. The effects of the nickel nanoparticle on the composition, crystallographic phase, structure, morphology, tap density, specific surface area, and electrochemical performance as both lithium‐ion and sodium‐ion battery anodes are systematically investigated. It is found that the incorporation of the in situ formed nickel nanoparticles with certain content effectively enhances the electrochemical performance including reversible capacities, cyclic stability and rate performance as both lithium‐ion and sodium‐ion battery anodes. The experimental I‐V profiles at different temperatures and theoretical calculations reveal that the charge carriers are accumulated in the amorphous carbon regions, which act as scattering centers to the carriers and lower the carrier mobility for the composite. With increasing nickel content, the mobility of the charge carriers is significantly increased, while the number of the charge carriers maintains almost constant. The nickel nanoparticles provide extra pathways for the accumulated charge, leading to reduced scatterings among the charge carriers and enhanced charge‐carrier transportation.     

New method to measure packing densities of self-assembled thiolipid monolayers

For a monolayer of 2,3-di-phytanyl-sn-glycerol-1-tetraethylene glycol-D,L-a-lipoic acid ester lipid (DPTL) self-assembled (SAM) at a gold electrode surface we propose a new method to determine the charge number per adsorbed molecule and the packing density (area per molecule) in the monolayer. The method relies on chronocoulometry to measure the charge density at the SAM covered gold electrode surface. Two series of measurements have to be performed. In the first series, charge densities are measured for a monolayer transferred from the air-solution to the metal-solution interface using the Langmuir-Blodgett (LB) technique. This series of measurements allows one to determine charge numbers per adsorbed DPTL molecule. The second series is performed using a gold electrode covered with a self-assembled monolayer. The charge densities obtained in this series are then used to calculate the packing density with the help of charge numbers per adsorbed DPTL determined in the first series. The area per adsorbed molecule determined by the new method was compared to the area per molecule determined by the popular reductive desorption method. The molecular area determined with the new method is about 20% larger than the area calculated from the van der Waals model, which is a physically reasonable result. In contrast, the popular reductive desorption method gives an area per molecule 20% lower than the minimum estimated based on a van der Waals model. This is a physically unreasonable result. It is also shown that the charge numbers per adsorbed molecule depend on the electrode potential and may assume values smaller than the number of electrons participating in the reductive desorption step. An explanation of the origin of the "partial charge numbers" is provided. We recommend the new method be used in future studies of thiol adsorption at metal surfaces.     

瞬态光电导谱研究Pt的负载对TiO2光生载流子寿命的影响

采用瞬态光电导谱研究了TiO₂的光电导衰减曲线,通过计算机拟合得到光生载流子的寿命,并考察了Pt的负载量对TiO₂光生载流子有效寿命的影响.结果表明,增大Pt的负载量,可延缓光电导的衰减趋势,从而延长TiO₂光生载流子的有效寿命,有效地降低了光生载流子的复合率.