2018 Vol. 31, No. 5

Content
2018, 31(5): 0-0.
We report an ultrafast spectroscopy investigation that addresses the subtle location effect in a prototypical semiconductor-MOF hybrid system with TiO2 nanoparticles being incorporated inside or supported onto Cu3(BTC)2, denoted as TiO2@Cu3(BTC)2 and TiO2/Cu3(BTC)2, respectively. By tracking in real time the interface electron dynamics in the hybrid system, we find that the interface states formed between TiO2 and Cu3(BTC)2 can act as an ef-fective relay for electron transfer, whose effciency rests on the relative location of the two components. It is such a subtle location effect that brings on difference in photocatalytic CO2 reduction using the two semiconductor-MOF hybrids. The mechanistic understanding of the involved interface electron-transfer behavior and effect opens a helpful perspective for rational design of MOF-based hybrid systems for photoelectrochemical applications.
The dissociative photoionization of cyclopentanone was investigated by means of a reflectron time-of-flight mass spectrometer (RTOF-MS) with tunable vacuum ultraviolet synchrotron radiation in the photon energy range of 9.0-15.5 eV. The photoionization efficiency (PIE) curves for molecular ion and fragment ions were measured. The ionization energy of cyclopentanone was determined to be 9.23±0.03 eV. Fragment ions from the dissociative photoionization of cyclopentanone were identified as C5H7O+,C4H5O+,C4H8+,C3H3O+,C4H6+,C2H4O+,C3H6+,C3H5+,C3H4+,C3H3+,C2H5+, C2H4+. With the aid of the ab initio calculations at the ωB97X-D/6-31+G(d,p) level of theory, the dissociative mechanisms of C5H8O+ are proposed. Ring opening and hydrogen migrations are the predominant processes in most of the fragmentation pathways of cyclopentanone.
Hydrogen evolution reaction (HER) is the major cathodic reaction which competes CO2 reduction reaction (CO2 RR) on Pt electrode. Molecular level understanding on how these two reactions interact with each other and what the key factors are of CO2 RR kinetics and selectivity will be of great help in optimizing electrolysers for CO2 reduction. In this work, we report our results of hydrogen evolution and CO2 reduction on Pt(111) and Pt film electrodes in CO2 saturated acid solution by cyclic voltammetry and infrared spectroscopy. In solution with pH>2, the major process is HER and the interfacial pH increases abruptly during HER; COad is the only adsorbed intermediate detected in CO2 reduction by infrared spectroscopy; the rate for COad formation increases with the coverage of UPD-H and reaches maximum at the onset potential for HER; the decrease of COad formation under HER is attributed to the available limited sites and the limited residence time for the reduction intermediate (Had), which is necessary for CO2 adsorption and reduction.
The structures of human telomeric DNA have received much attention due to its significant biological importance. Most studies have focused on G-quadruplex structure formedby short telomeric DNA sequence, but little is known about the structures of long singlestranded telomeric DNAs. Here, we investigated the structure of DNA with a long sequence of d[AGGG(TTAGGG)6] (G6-DNA) and the effect of a single repeat sequence d(TTAGGG) (G01-DNA) on the structure of G6-DNA using sedimentation velocity technique, polyacrylamide gel electrophoresis, circular dichroism spectroscopy, and UV melting experiments. The results suggest that the G6-DNA can form dimers in aqueous solutions and G01-DNA can form additional G-quadruplex structures by binding to G6-DNA. However, G01-DNA has no effect on the structure of DNA with a sequence of d[AGGG(TTAGGG)3] (G3-DNA). Our study provides new insights into the structure polymorphism of long human single-stranded telomeric DNA.
Based on density functional theory calculations, the full hydrolysis of per NH3BH3 molecule to produce three hydrogen molecules on single Pt atoms supported on oxidized graphene (Pt1/Gr-O) is investigated. It is suggested that the first hydrogen molecule is produced by the combination of two hydrogen atoms from two successive B-H bonds breaking. Then one H2O molecule attacks the left ?BHNH3 group (? represents adsorbed state) to form ?BH(H2O)NH3 and the elongated O-H bond is easily broken to produce ?BH(OH)NH3. The second H2O molecule attacks ?BH(OH)NH3 to form ?BH(OH)(H2O)NH3 and the breaking of O-H bond pointing to the plane of Pt1/Gr-O results in the desorption of BH(OH)2NH3. The second hydrogen molecule is produced from two hydrogen atoms coming from two H2O molecules and Pt1/Gr-O is recovered after the releasing of hydrogen molecule. The third hydrogen molecule is generated by the further hydrolysis of BH(OH)2NH3 in water solution. The rate-limiting step of the whole process is the combination of one H2O molecule and ?BHNH3 with an energy barrier of 16.1 kcal/mol. Thus, Pt1/Gr-O is suggested to be a promising catalyst for hydrolysis of NH3BH3 at room temperature.
Manipulating the chemical reactivity of graphene toward oxygen reduced reduction (ORR) is of particular interest for both fundamental research and practical application in fuel cell. Deposing graphene on selected substrate provides a structure-intact strategy to enhance its chemical reactivity due to substrate-induced charge and interface effect. Here, we report the graphene deposited on one-dimensional electride Y5Si3 as an effective ORR catalyst in acidic media. Thermodynamic calculations suggest that depositing graphene on electride materials can facilitate the protonation of O2, which is the rate-determining step based on the four-electron reaction pathway and thus promote the ORR activity. Further electronic calculations reveal that low work function (3.5 eV), superior electrical conductivity and slight charge transfer from substrate to graphene result in the enhanced ORR performance of graphene. These findings shed light on the rational design of ORR catalysts based on graphitic materials and emphasize the critical role of substrates for energy-related electrochemical reactions.
Phenomenon of localized surface plasmon excitation at nanostructured materials has attracted much attention in recent decades for their wide applications in single molecule detection, surface-enhanced Raman spectroscopy and nano-plasmonics. In addition to the excitation by external light field, an electron beam can also induce the local surface plasmon excitation. Nowadays, electron energy loss spectroscopy (EELS) technique has been increasingly employed in experiment to investigate the surface excitation characteristics of metallic nanoparticles. However, a present theoretical analysis tool for electromagnetic analysis based on the discrete dipole approximation (DDA) method can only treat the case of excitation by light field. In this work we extend the DDA method for the calculation of EELS spectrum for arbitary nanostructured materials. We have simulated EELS spectra for different incident locations of an electron beam on a single silver nanoparticle, the simulated results agree with an experimental measurement very well. The present method then provides a computation tool for study of the local surface plasmon excitation of metallic nanoparticles induced by an electron beam.
Internal reformation of low steam methane fuel is highly bene cial for improving the energy effciency and reducing the system complexity and cost of solid oxide fuel cells (SOFCs). However, anode coking for the Ni-based anode should be prevented before the technology becomes a reality. A multi-physics fully coupled model is employed to simulate the operations of SOFCs fueled by low steam methane. The multi-physics model produces I-V relations that are in excellent agreement with the experimental results. The multi-physics model and the experimental non-coking current density deduced kinetic carbon activity criterion are used to examine the effect of operating parameters and the anode diffusion barrier layer on the propensity of carbon deposition. The interplays among the fuel utilization ratio, current generation, thickness of the barrier layer and the cell operating voltage are revealed. It is demonstrated that a barrier layer of 400 μm thickness is an optimal and safe anode design to achieve high power density and non-coking operations. The anode structure design can be very useful for the development of high e ciency and low cost SOFC technology.
Intense investigations have been attracted to the development of materials which can reconfigure into 3D structures in response to external stimuli. Herein we report on the design and self-folding behaviors of hydrogels composed of poly(ethylene glycol) methyl ether methacry-late (OEGMA) and 2-(2-methoxyethoxy) ethyl methacrylate (MEO2MA). Upon immersion into a variety of solvents at room temperature, the resulting P(MEO2MA-co-OEGMA) hydrogel sheets self-fold into 3D morphologies, and the observed transformation in shape is reversible. We further show that composition of the gel, gaseous environment, and preparation procedure play important roles in the self-folding behavior of the resulting hydrogels. This work provides a facile approach for fabricating self-folding hydrogels.
Layer-structured O3 type cathode materials Na1-xCr1-xTixO2(x=0, 0.03, 0.05) are fabricated by a thermo-polymerization method. The structures and morphologies are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) respectively. It has been found that the appropriate Ti doping effectively leads to the formation of uniform morphology. As a cathode, the x=0.03 sample delivers a quite high discharge capacity of 110 mAh/g at 32 C in the voltage range from 2.0 V to 3.6 V (vs. Na/Na+) and with a capacity retention of 96% after 100 cycles at 0.2 C. The Na//Na0.97Cr0.97Ti0.03O2 cell exhibits very high coulombic e ciency (above 96%). All these results suggest that Na0.97Cr0.97Ti0.03O2 is very promising for high-rate sodium ion batteries.
A novel water-soluble luminescent complex consisting of Eu(ally-dbm)3-2Tppo and poly(N-isopropyl acrylamide) (PNIPAM) is synthesized through a series of chemical reactions. The structure of the complex is characterized by TGA, GPC, HNMR, and the thermal-responsive fluorescence of the complex in aqueous solution is investigated. It is found that PNIPAM collapse above the lower critical solution temperature causes the coordination bond breaking, leading to weakening of the fluorescence from Eu3+ and enhancing of the fluorescence from the ligands. When temperature decreases, the fluorescence from Eu3+ is found to boost up and the fluorescence from ligands weakens accordingly. It is deduced from this phenomenon that the ligands re-coordinate with europium ions again along with the temperature decreasing, which is further con rmed by IR measurements. This thermal-responsive fluorescence is of reversibility, which can be used as molecular probes for biological imaging and collapse studying of PNIPAM.
Graphene oxide (GO) is a kind of water soluble two-dimensional materials containing a large amount of oxygen-containing groups which infuse GO with water solubility, biocompatibility and functionality, etc. But GO can be easily reduced by losing oxygen-containing groups under some circumstances such as irradiation of γ-ray or ultraviolet (UV). In this work, we found that acetone can signi cantly slow down the reduction process of GO under the irradiation of either γ-ray or UV, which was supported by analysis results with UV-visible (UV-Vis) absorption spectra, X-ray photoelectron spectroscopy, etc. Acetone can capture and remove strongly reducible hydrated electrons generated under γ-irradiation. GO reduction by UV also involves electron transfer process which can be affected by the presence of acetone. Hence, acetone can be used to stabilize, adjust the radiation reduction process of GO. This would be interesting not only in radiation and radiation protection, but also in understanding the redox properties of GO.
Oxygen evolution reaction is one of the key processes in the promising renewable energy technique of electrocatalytic water splitting. Developing high effcient oxygen evolution reaction (OER) catalysts requires determination of the optimal values of the descriptor parameters. Using spinel CoFe2O4 as the model catalyst, this work demonstrates that irradiation with pulsed UV laser can control the quantity of surface oxygen vacancy and thus modify the OER activity, in a volcano-shape evolution trend. This strategy sheds light on quantita-tively investigation of the relationship between surface cation valence, anion vacancy, and physicochemical properties of transition-metal-based compounds.
Photocatalytic reduction of CO2 into various types of fuels has attracted great interest, and serves as a potential solution to addressing current global warming and energy challenges. In this work, Ag-Cu nanoparticles are densely supported on N-doped TiO2 nanowire through a straightforward nanofabrication approach. The range of light absorption by N-doped TiO2 can be tuned to match the plasmonic band of Ag nanoparticles, which allows synergizing a resonant energy transfer process with the Schottky junction. Meanwhile, Cu nanoparticles can provide active sites for the reduction of CO2 molecules. Remarkably, the performance of photocatalytic CO2 reduction is improved to produce CH4at a rate of 720 μmol·g-1·h-1 under full-spectrum irradiation.
The development of Bi2WO6-based materials has become one of research hotspots due to the increasing demands on high-efficient photocatalyst responding to visible light. In this work, the effect of high energy radiation (γ-ray) on the structure and the photocatalytic activity of Bi2WO6 nanocrystals was first studied. No morphological change of Bi2WO6 nanocrystals was observed by SEM under γ-ray radiation. However, the XRD spectra of the irradiated Bi2WO6 nanocrystals showed the characteristic 2θ of (113) plane shifts slightly from 28.37° to 28.45° with the increase of the absorbed dose, confirming the change in the crystal structure of Bi2WO6. The XPS results proved the crystal structure change was originated from the generation of oxygen vacancy defects under high-dose radiation. The photocatalytic activity of Bi2WO6 on the decomposition of methylene blue (MB) in water under visible light increases gradually with the increase of absorbed dose. Moreover, the improved photocatalytic performance of the irradiated Bi2WO6 nanocrystals remained after three cycles of photocatalysis, indicating a good stability of the created oxygen vacancy defects. This work gives a new simple way to improve photocatalytic performance of Bi2WO6 through creating oxygen vacancy defects in the crystal structure by γ-ray radiation.
Distinguished from commonly used Fe2O3 and Fe3O4, a three-dimensional multilevel macromicro-mesoporous structure of FeC2O4/graphene composite has been prepared as binderfree electrode for supercapacitors. The as-prepared materials are composed of macroporous graphene and microporous/mesoporous ferrous oxalate. Generally, the decomposition voltage of water is 1.23 V and the practical voltage window limit is about 2 V for asymmetric supercapacitors in aqueous electrolytes. When FeC2O4/rGO hydrogel was used as the negative electrode and a pure rGO hydrogel was used as the positive electrode, the asymmetrical supercapacitor voltage window raised to 1.7 V in KOH (1.0 mol/L) electrolyte and reached up to 2.5 V in a neutral aqueous Na2SO4 (1.0 mol/L) electrolyte. Correspondingly it also exhibits a high performance with an energy density of 59.7 Wh/kg. By means of combining a metal oxide that owns micro-mesoporous structure with graphene, this work provides a new opportunity for preparing high-voltage aqueous asymmetric supercapacitors without addition of conductive agent and binder.
Gd doped hollow nanoscale coordination polymers with multimodal imaging capabilities were synthesized by solvothermal method and further coated by silica layer. The in vitro tests demonstrated uncoated and silica-coated nanoprobes exhibit longitudinal relaxivities (r1) of 7.38 and 13.57 (mmol/L)-1·s-1, and transverse relaxivities (r2) of 180.6 and 304.8 (mmol/L)-1·s-1, showing fairly good dual T1&T2 contrast effects, and it also emits excellent multicolor fluorescence under laser beams of various wavelengths. With the combination of magnetic resonance imaging (MRI) (both T1 and T2) and fluorescence optical imaging (FOI), the nanoprobes could correlate preoperative diagnosis with intraoperative pathology. Furthermore, it also exhibits high drug loading capacity of 1166 mg/g and encapsulation efficiency of 83.29%, which makes it a potential platform as drug carriers. The MTT assay demonstrates the moderate toxicity of the NPs, and after the silica coating process, not only the MRI contrast effects but also the biocompatibility have been enhanced. The versatility of the highly integrated systems can make up for the limitations of each imaging modality and exhibit great potentials for cancer theranostics.
Lignin only accounts for about 6% of total mass in tobacco stem, but it influences the harmful substances in the side stream smoke of cigarette in a signi cant way. Traditional researches focus only on the determination of lignin content. In the present work, we investigate four typical imidazolium-based ionic liquids for efficient extraction of lignin under mild conditions and 1-ethyl-3-methylimidazolium diethylphosphate ([Emim][DEP]) shows the best results. The pretreatment of stem using water at 80 °C for 30 min can not only remove most of the sugars but also loose the micro bers. The extractive rate of lignin reaches 85.38% at 150 °C for 4 h and the purity of lignin is 90.21%.
We present a homebuilt scanning tunneling microscope (STM) which employs an inner-wall polished sapphire guiding tube as a rail for the scanner to form a short tip-sample mechanical loop. The scanner is mounted on a square rod which is housed in the guiding tube and held by a spring strip. The sti sapphire guiding tube allows the STM body to be made in a simple, compact and rigid form. Also the material of sapphire improves the thermal stability of the STM for its good thermal conductivity. To demonstrate the performance of the STM, high quality atomic-resolution STM images of high oriented pyrolytic graphite were given.