Xiuli Wang

Titanium Dioxide-Based Nanomaterials for Photocatalytic Fuel Generations

Generations Yi Ma,† Xiuli Wang,† Yushuai Jia,† Xiaobo Chen,‡ Hongxian Han,*,† and Can Li*,† †State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China ‡Department of Chemistry, College of Arts and Sciences, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, Missouri 64110, United States

Trap states and carrier dynamics of TiO2 studied by photoluminescence spectroscopy under weak excitation condition

Anatase and rutile TiO(2) were investigated with photoluminescence techniques under the weak excitation condition, where trap states play a vital role in carrier dynamics. The visible emission of anatase and near-infrared (NIR) emission of rutile both exhibit extremely long lifetimes up to milliseconds. The decay processes can be well described by the power-law decay which corresponds to the trapping-detrapping effect. These results indicate that the luminescence processes in both anatase and rutile TiO(2) have a close relationship with trap states. The visible emission band was assigned to the donor-acceptor recombination. Oxygen vacancies and hydroxyl groups mainly serve as the donor and acceptor sites, respectively. The NIR luminescence is originated from the recombination of trapped electrons with free holes, while the trapped electrons were formed through two paths, direct trapping or trap-to-trap hopping. The trap states in anatase and rutile TiO(2) may largely influence the photocatalysis process of TiO(2) and determine the photocatalytic activity under stationary illumination.

Visible emission characteristics from different defects of ZnS nanocrystals

Various sized ZnS nanocrystals were prepared by treatment under H(2)S atmosphere. Resonance Raman spectra indicate that the electron-phonon coupling increases with increasing the size of ZnS. Surface and interfacial defects are formed during the treatment processes. Blue, green and orange emissions are observed for these ZnS. The blue emission (430 nm) from ZnS without treatment is attributed to surface states. ZnS sintered at 873 K displays orange luminescence (620 nm) while ZnS treated at 1173 K shows green emission (515 nm). The green luminescence is assigned to the electron transfer from sulfur vacancies to interstitial sulfur states, and the orange emission is caused by the recombination between interstitial zinc states and zinc vacancies. The lifetimes of the orange emission are much slower than that of the green luminescence and sensitively dependent on the treatment temperature. Controlling defect formation makes ZnS a potential material for photoelectrical applications.

Synthesis, Structure, and Photoluminescent Properties of Metal−Organic Coordination Polymers Assembled with Bithiophenedicarboxylic Acid

Four novel metal-organic coordination polymers with the formulas Mn(3)(btdc)(3)(DMF)(4) (1), Co(btdc)(DMF)(3) (2), Zn(btdc)(DMF)(3) (3), and Zn(btdc)(4,4-bpy)(0.5) (4), where H(2)btdc = 2,2-bithiophene-5,5-dicarboxylic acid, DMF = N,N-dimethylformamide, and 4,4-bpy = 4,4-bipyridine, have been successfully synthesized. Crystal 1 with Mn(2+) as the cation features a three-dimensional (3D) infinite framework built from trimanganese clusters, and crystals 2 and 3 with Co(2+) and Zn(2+), respectively, as the cation both have one-dimensional zigzag polymeric coordination chains. Crystal 4 synthesized using a mixture of 4,4-bpy and H(2)btdc exhibits a triply interpenetrating 3D framework built from a dizinc paddlewheel second building unit with a distorted primitive cubic single net. The results of UV/vis spectra indicate that metal binding does not disturb the detailed electronic structure of the ligand. We also demonstrate that Zn(2+) can greatly enhance the luminescence emission of the H(2)btdc ligand, and the emission intensity of crystal 4 is almost 20 times higher than that of the free H(2)btdc ligand. Steady-state and time-resolved spectroscopic measurement reveal that the more rigid environment of the btdc ligand can stabilize the highly excited long-lived states in metal-organic frameworks (MOFs), which thus greatly changes the emission properties of MOFs.

The structural transition and compaction of human telomeric G-quadruplex induced by excluded volume effect under cation-deficient conditions.

The structure polymorphism of human telomeric G-quadruplex (ht-quadruplex) is currently an important topic but remains controversy. Here, we present study of the ht-quadruplex under the cation-deficient but molecular crowding conditions by circular dichroism (CD), microchip electrophoresis (MCE) and UV-melting experiments. Our results show that with concentration increasing of poly(ethylene glycol) (PEG), the structural transition of ht-quadruplex occurs accompanied by structural compaction and enhanced stabilization, which may be caused by excluded volume effect. This work also demonstrates that ht-quadruplex can be well assembled without cations and the structure of ht-quadruplex is actually very complex in vivo.

Dynamic Insight into the Interaction between Porphyrin and G-quadruplex DNAs: Time-Resolved Fluorescence Anisotropy Study

Understanding the nature of the interaction between small molecules and G-quadruplex DNA is crucial for the development of novel anticancer drugs. In this paper, we present the first data on time-resolved fluorescence anisotropy study on the interaction between a water-soluble cationic porphyrin H(2)TMPyP4 and four distinct G-quadruplex DNAs, that is, AG(3)(T(2)AG(3))(3), thrombin-binding aptamer (TBA), (G(4)T(4)G(4))2, and (TG(4)T)4. The anisotropy decay curves show the monoexponential for free H(2)TMPyP4 and the biexponential upon binding to the excess amount of G-quadruplex DNAs. The biexponential anisotropy decay can be well interpreted using a wobbling-in-the-cone model. The orientational diffusion of the bound H(2)TMPyP4 is initially restricted to a limited cone angle within the G-quadruplex DNAs, and then an overall orientational relaxation of the G-quadruplex DNA-H(2)TMPyP4 complexes occurs in a longer time scale. It was found that the dynamics of the restricted internal rotation of bound H(2)TMPyP4 strongly depends on the ending structures of the G-quadruplex DNAs. According to the order parameter (Q) calculated from the wobbling-in-the-cone model, we deduce that the degree of restriction around the bound H(2)TMPyP4 follows the order of TBA > (TG(4)T)4 > AG(3)(T(2)AG(3))(3) > (G(4)T(4)G(4))2. Especially, based on the maximum order parameter (Q) of bound H(2)TMPyP4 within TBA, a new sandwich-type binding mode for TBA-H(2)TMPyP4 complex was proposed in which both terminal G-quartet and T*T base pair stack on the porphyrin ring through pi-pi interaction. This study thus provides a new insight into the interaction between G-quadruplex DNAs and H(2)TMPyP4.

Formation and stabilization of G-quadruplex in nanosized water pools

We demonstrate here that G-quadruplex structure can form and exhibits strong stability in nanosized water pools, providing new insight into investigating G-quadruplexes in the cellular environment.

Control of Nanomorphology in All-Polymer Solar Cells via Assembling Nanoaggregation in a Mixed Solution

The formation of interconnected phase-separated domains on sub-20 nm length scale is a key requirement for all-polymer solar cells (all-PSCs) with high efficiency. Herein, we report the application of crystalline poly(3-hexylthiophene) (P3HT) nanowires via an O-dichlorobenzene/hexane mixed solution blended with poly{(9,9-dioctylfluorenyl-2,7-diyl)-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2,2″-diyl} (F8TBT) for the first time. The nanomorphology of P3HT:F8TBT all-PSCs can be controlled by P3HT nanowires. The improved film morphology leads to enhanced light absorption, exciton dissociation, and charge transport in all-PSCs, as confirmed by ultraviolet-visible absorption spectra, X-ray diffraction, transmission electron microscopy, atomic force microscopy, and time-resolved photoluminescence spectra. The P3HT nanowire:F8TBT all-PSCs could achieve a power conversion efficiency of 1.87% and a Voc of 1.35 V, both of which are the highest values for P3HT:F8TBT all-PSCs. This work demonstrates that the semiconductor nanowires fabricated by the mixed solvents method is an efficient solution process approach to controlling the nanomorphology of all-PSCs.

Photoluminescence Spectroscopic Studies on TiO2 Photocatalyst

Photoluminescence is a powerful technique in the study of semiconductor photocatalysts. This chapter deals with the application of photoluminescence techniques to the study of TiO2 in relation to its photocatalytic performance. The assignment of the visible and the near-infrared luminescence characteristics of TiO2 are discussed. The influence of the adsorbed molecules, such as H2O, O2, H2, unsaturated hydrocarbons and Pt loaded on TiO2, on the photoluminescence characteristics of TiO2 is also discussed. The relationship between the photoluminescence features of TiO2 and the photo-assisted reaction of water and methanol mixture is also summarized.

Dual Extraction of Photogenerated Electrons and Holes from a Ferroelectric Sr0.5Ba0.5Nb2O6 Semiconductor.

The separation of photogenerated charges is a critical factor in photocatalysis. Recently, anomalous photovoltaic (APV) field effects (Voc ∼ 10(3) V/cm) in ferroelectrics, with their strong driving force for charge separation, have attracted much attention in photocatalysis and photoelectrocatalysis. However, it is still unknown whether photogenerated electrons and holes can be simultaneously extracted by the strong driving force toward the surface of ferroelectrics and can become available for surface reactions. This issue becomes critically important in photocatalysis because the surface reaction utilizes both the electrons and holes that reach the surface. In this work, a model lateral symmetric structure, metal/Sr0.5Ba0.5Nb2O6/metal (metal = Ag or Pt), as an electrode was fabricated. The dual extractions of photogenerated electrons and holes on the two opposite metal electrodes were achieved, as revealed by photovoltaic and ferroelectrical hysteresis measurements and photoassisted Kelvin probe force microscopy (KPFM). It was found that the high Schottky barriers of the two opposite Sr0.5Ba0.5Nb2O6-Pt electrodes are key factors that alter the two space charge regions (SCRs) by a poling effect. The resulting built-in electrical fields with parallel directions near both electrodes significantly enhance the charge separation ability. Our model unravels the driving force of charge separation in ferroelectric semiconductors, thus demonstrating the potential for highly efficient charge separation in photocatalysis.