2019 Vol. 32, No. 6

Content
Content
2019, 32(6): i-i.
Chinese abstract
Chinese Abstracts
2019, 32(6): ii-iii.
Article
In this work, we investigated the energy transfer (EnT) and electron transfer (ET) processes as well as their relationship in the carbon quantum dots/rhodamine B (CQDs/RhB) including o-CQDs/RhB and m-CQDs/RhB systems by using photoluminescence spectroscopy in combination with steady-state and transient absorption spectroscopy. We found that the ET process is negligible in the o-CQDs/RhB system with an EnT efficiency as high as 73.2%, while it becomes pronounced in the m-CQDs/RhB system whose EnT efficiency is lower than 33.5%. Such an interplay of EnT and ET processes revealed in the prototypical composite system consisting of carbon quantum dots and dye molecules would provide helpful insights for applications of relevance to exciton quenching.
The kinetics of formic acid oxidation (FAO) on Pd(111) in 0.1 mol/L H2SO4+0.1 mol/L HCOOH with and without addition of Na2SO4 is studied using cyclic voltammetry and potential step method, which is compared with that in 0.1 mol/L HClO4. It is found that adsorbed sulfate has significant inhibition effect on FAO kinetics. After addition of 0.05 mol/L or 0.1 mol/L Na2SO4, FAO current in the negative-going scan is found to be significantly smaller than that at the same potential in the positive-going scan. We speculate that at potentials positive of the phase transition potential for the (SO4ad*)m+[(H2O)n-H3O+] or (SO4ad*)m+[Na+(H2O)n-H3O+] adlayer, the adlayer structure probably becomes denser and more stable with the increase of potential or with the addition of Na2SO4. The formation of connected adlayer network greatly enhance the stability of the adlayer, and the insertion of positive-charged H+ or Na+ into the adlayer network further reduces the electrostatic repulsion between partially charged sulfates. As a result, the destruction/desorption of compact sulfate adlayer becomes more difficult, which leaves much less free sites on the surface for FAO, and thus FAO kinetics at higher potentials and in the subsequent negative-going potential scan is significantly inhibited.
The formation and qualification of redox sites in transition metal oxides are always the active fields related to electronics, catalysis, sensors, and energy-storage units. In the present study, the temperature dependence of thermal reduction of MoO3 was surveyed at the range of 350 ℃ to 750 ℃. Upon reduction, the formed redox species characterized by EPR spectroscopy are the MoV ion and superoxide anion radical (O2-) when the reduction was induced at the optimal temperature of 300-350 ℃. When heating-up from 350 ℃, the EPR signals started to decline in amplitude. The signals in the range of 400-450 ℃ decreased to half of that at 350 ℃, and then to zero at ~600 ℃. Further treatment at even higher temperature or prolonged heating time at 500 ℃ caused more reduction and more free electrons were released to the MoO3 bulk, which results in a delocalized means similar to the anti-ferromagnetic coupling. These data herein are helpful to prepare and study the metal-oxide catalysts.
High quality LiLuF4 single crystals doped with various Pr3+ ions were synthesized by a vertical Bridgman method in completely sealed platinum crucibles. The excitation spectra spans from 420 nm to 500 nm. The prepared single crystals exhibit a blue band at 480 nm (3P03H4), a green band at 522 nm (3P13H5), and a red band at 605 nm (1D23H4) when excited at 446 nm; their corresponding average lifetimes are 38.5 μs, 37.3 μs, and 36.8 μs, respectively, which are much longer than those in oxide single crystals. The effects of excitation wavelength and doping concentration on emission intensities and chromaticity coordinates are investigated. The optimal Pr3+ concentration is confirmed to be 0.5%. The temperature dependent emission shows that the emission intensity constantly decreases with the increase of temperature from 298 K to 443 K due to the enhancement of non-radiative quenching at high temperature. The 3P03H4 transition is the most vulnerable to temperature, followed by the 3P13H5 transition and 1D23H4 transition.
We performed high-level ab initio calculations on electronic structure of NaK. The potential energy curves (PECs) of 10 Λ-S states correlated with the three lowest dissociation limits have been calculated. On the basis of the calculated PECs, the spectroscopic constants of the bound??-S states are obtained, which are in good agreement with experimental results. The maximum vibrational quantum numbers of the singlet ground state X1Σ+ and the triplet ground state a3Σ+ have been analyzed with the semiclassical scattering theory. Transition properties including transition dipole moments, Franck-Condon factors, and radiative lifetimes have been investigated. The research results indicate that such calculations can provide fairly reliable estimation of parameters for the ultracold alkali diatomic molecular experiment.
The energetic pathways of adsorption and activation of carbon dioxide (CO2) on low-lying compact (TiO2)n clusters are systematically investigated by using electronic structure calculations based on density-functional theory (DFT). Our calculated results show that CO2 is adsorbed preferably on the bridge O atom of the clusters, forming a "chemisorption" carbonate complex, while the CO is adsorbed preferably to the Ti atom of terminal Ti-O. The computed carbonate vibrational frequency values are in good agreement with the results obtained experimentally, which suggests that CO2 in the complex is distorted slightly from its undeviating linear configuration. In addition, the analyses of electronic parameters, electronic density, ionization potential, HOMO-LUMO gap, and density of states (DOS) confirm the charge transfer and interaction between CO2 and the cluster. From the predicted energy profiles, CO2 can be easily adsorbed and activated, while the activation of CO2 on (TiO2)n clusters are structure-dependent and energetically more favorable than that on the bulk TiO2. Overall, this study critically highlights how the small (TiO2)n clusters can influence the CO2 adsorption and activation which are the critical steps for CO2 reduction the surface of a catalyst and subsequent conversion into industrially relevant chemicals and fuels.
The thermodynamic stability and lithiated/delithiated potentials of LiFe$_x$Mn$_{1-x}$PO$_4$ were studied with density functional theorical calculations. The results show that the formation free energy of the LiFe$_x$Mn$_{1-x}$PO$_4$ solid solution is slightly higher than that of the phase-separated mixture of LiFePO$_4$ and LiMnPO$_4$, and the two forms may co-exist in the actual LiFe$_x$Mn$_{1-x}$PO$_4$ materials. The calculation manifests that the lithiated/delithiated potentials of LiFe$_x$Mn$_{1-x}$PO$_4$ solid solutions vary via the Mn/Fe ratio and the spatial arrangements of the transition metal ions, and the result is used to explain the shape of capacity-voltage curves. Experimentally, we have synthesized the LiFe$_x$Mn$_{1-x}$PO$_4$ materials by solid-phase reaction method. The existence of the LiFe$_x$Mn$_{1-x}$PO$_4$ solid solution is thought to be responsible for the appearance of additional capacity-voltage plateau observed in the experiment.
The binding energy and generalized stacking-fault energy (GSFE) are two critical interface properties of two dimensional layered materials, and it is still unclear how neighboring layers affect the interface energy of adjacent layers. Here, we investigate the effect of neighboring layers by comparing the differences of binding energy and GSFE between trilayer heterostructures (graphene/graphene/graphene, graphene/graphene/boron nitride, boron nitride/graphene/boron nitride) and bilayer heterostructures (graphene/graphene, graphene/boron nitride) using density functional theory. The binding energy of the adjacent layers changes from -2.3% to 22.55% due to the effect of neighboring layer, with a very small change of the interlayer distance. Neighboring layers also make a change from -2% to 10% change the GSFE, depending on the property of the interface between adjacent layers. In addition, a new simple expression is proven to describe the GSFE landscape of graphene-like structure with high accuracy.
Iterative multireference configuration interaction (IMRCI) is proposed. It is exploited to compute the electronic energies of H$_2$O and CH$_2$ (singlet and triplet states) at equilibrium and non-equilibrium geometries. The potential energy curves of H$_2$O, CH$_2$ (singlet and triplet states) and N$_2$ have also been calculated with IMRCI as well as the Møller Plesset perturbation theory (MP2, MP3, and MP4), the coupled cluster method with single and double substitutions (CCSD), and CCSD with perturbative triples correction (CCSD(T)). These calculations demonstrate that IMRCI results are independent of the initial guess of configuration functions in the reference space and converge quickly to the results of the full configuration interaction. The IMRCI errors relative to the full configuration interaction results are at the order of magnitude of 10$^{-5}$ hartree within just 2-4 iterations. Further, IMRCI provides an efficient way to find on the potential energy surface the leading electron configurations which, as correct reference states, will be very helpful for the single-reference and multireference theoretical models to obtain accurate results.
Zinc oxide has a large energy gap and thus it has potential application in the field of solar cells by tuning the absorption of sunlight. In order to enhance its absorption of sunlight, dark color zinc oxides have been prepared by traditional hydrothermal method directly using a zinc foil as both source and substrate. We found that we could tune the optical properties of ZnO samples by changing the temperature. In particular, increasing temperature could significantly reduce the reflectivity of solar energy in the visible range. We speculate that the phenomenon is relevant to the sharp cone morphology of the ZnO nanorods grown on the surface of Zn foils, which furthermore enhance refraction and reflection of light in the nanorods. The capacity to improve the light absorption of ZnO may have a bright application in raising the efficiency of solar cells.
Direct Z-scheme CdO-CdS 1-dimensional nanorod arrays were constructed through a facile and simple hydrothermal process. The structure, morphology, photoelectrochemical properties and H$_2$ evolution activity of this catalyst were investigated systematically. The morphology of the obtained nanorod is a regular hexagonal prism with 100-200 nm in diameter. The calcination temperature and time were optimized carefully to achieve the highest photoelectrochemical performance. The as-fabricated hybrid system achieved a photocurrent density up to 6.5 mA/cm$^{2}$ and H$_{2}$ evolution rate of 240 μmol$\cdot$cm$^{-2}$$\cdot$h$^{-1}$ at 0 V vs. Ag/AgCl, which is about 2-fold higher than that of the bare CdS nanorod arrays. The PEC performance exceeds those previously reported similar systems. A direct Z-scheme photocatalytic mechanism was proposed based on the structure and photoelectrochemical performance characterization results, which can well explain the high separation efficiency of photoinduced carriers and the excellent redox ability.
A bifunctional Co modified Fe$_3$O$_4$-Mn catalyst was prepared for Fischer-Tropsch synthesis (FTS). The influence of Co loading on the synergistic effect of Fe-Co as well as FTS performance over Fe$_1$Co$_x$Mn$_1$ catalysts was studied. Incorporation of Co species into the Fe$_3$O$_4$-Mn catalyst promoted the reduction of iron oxides, increasing iron active sites during FTS. Moreover, the adding of Co species enhanced the electron transfer from Fe to Co metal, which strengthened the synergistic effect of Fe-Co, improving the catalytic performance. The Fe$_1$Co$_x$Mn$_1$ catalyst with higher Co loading promoted further the hydrogenation ability, favoring the shifting of the product distribution towards shorter hydrocarbons. Under optimized conditions of 280 ℃, 2.0 MPa and 3000 h$^{-1}$, the highest yield of liquid fuels was obtained for the Fe$_1$Co$_1$Mn$_1$ catalyst.
Transition-metal oxides have attracted much attention due to its abundant crystalline phases and intriguing physical properties. However, some of these compounds are difficult to be fabricated directly in film form due to the ease of valence variation of transition-metal elements. In this work, we reveal the reversible structural transition between SrVO$_3$ and Sr$_2$V$_2$O$_7$ films via thermal treatment in oxygen atmosphere or in vacuum. Based on this, Sr$_2$V$_2$O$_7$ epitaxial films are successfully synthesized and studied. Property characterizations show that the semitransparent and metallic SrVO$_3$ could reversibly switch into transparent and insulating Sr$_2$V$_2$O$_7$, implying potential applications in controllable electronic and optical devices.
Cobalt-based nanomaterials have been intensively explored as one of the most promising noble-metal-free oxygen evolution reaction (OER) electrocatalysts. However, most of their performances are still inferior to state-of-the-art precious metals especially for Ru and Ir. Herein, we apply a continuous ion exchange method and further hydrothermal treatment to synthesize the flake-like Ag-CoSO$_4$ nanohybrids beginning from Co-BTC (BTC: benzene-1, 3, 5-tricarboxylic acid) metal-organic frameworks precursor. The catalyst exhibits superior OER performance under the alkaline electrolyte solution (a low overpotential of 282 mV at 10 mA/cm$^{2}$ in 1 mol/L KOH), which is even better than RuO$_2$ due to the improved conductivity and rapid electrons transfer process via introducing small amount of Ag. The existence of Ag in the hybrids is beneficial for increasing the Co(Ⅳ) concentration, thus promoting the $^*$OOH intermediate formation process. Besides, due to the very low requirement of Ag content (lower than 1 atom%), the cost of the catalyst is also limited. This work provides a new insight for designing of inexpensive OER catalysts with high performance and low cost.
Strong near-field scattering enhancement (NFSE) of polyhedral oligomeric silsesquioxanes (POSS) nanoparticles (NPs) aggregates is found through physical simulation. An aggregation of $N$, $N'$-di-[3-(isobutyl polyhedral oligomeric silsesquioxanes) propyl] perylene diimide (DPP) which possesses POSS as scatteres experimentally performs strong NFSE, which confirms the physical simulation results. Moreover, coherent random laser is triggered from the DPP aggregates in carbon disulfide. It is the NFSE of POSS NPs connected to both ends of DPP through covalent bonds and the NFSE of their aggregation thanks to DPP's aggregation that is responsible for the coherent random laser. So, this work develops a method to improve weak scattering of system through construction of molecules, and opens a road to a variety of novel interdisciplinary investigations, involving molecular designing for disordered photonics.
A protocol for selectively oxidizing aldehyde over hydroxymethyl group is developed, using biomass starch protected gold nanoparticles (NPs) as catalyst. The Au NPs show high selectivity that aldehyde is oxidized into carboxylic acid while alcoholic hydroxyl group stays intact in selective oxidation of 4-(hydroxymethyl)-benzaldehyde. The heterogeneous catalysis system is composed of soluble catalysts and insoluble substrate. The gold catalyst is prepared, preserved and applied for catalytic oxidation all in water. After reaction conditions are optimized, H$_2$O$_2$ is found to be the best oxidizing agent with complete conversion. Besides, the gold catalyst displays good versitility for aldehyde derivatives. After reaction completes, organic components are extracted by organic solvent and gold NPs in water are separated and recycled.
Formation of volatile nickel carbonyls with CO in catalytic reaction is one of the mechanisms of catalyst deactivation. CO is one of the most popular probe molecules to study the surface properties in model catalysis. Under ultra-high vacuum (UHV) conditions, the problem of nickel carbonyl impurity almost does not exist in the case that a high purity of CO is used directly. While in the near ambient pressure (NAP) range, nickel carbonyl is easily found on the surface by passing through the Ni containing tubes. Here, the NAP techniques such as NAP-X-ray photoelectron spectroscopy and NAP-scanning tunneling microscopy are used to study the adsorption of nickel carbonyl contaminated CO gas on Cu(111) surface in UHV and NAP conditions. By controlling the pressure of contaminated CO, the Ni-Cu bimetallic catalyst can form on Cu(111) surface. Furthermore, we investigate the process of CO adsorption and dissociation on the formed Ni-Cu bi-metal surface, and several high-pressure phases of CO structures are reported. This work contributes to understanding the interaction of nickel carbonyl with Cu(111) at room temperature, and reminds the consideration of CO molecules contaminated by nickel carbonyl especially in the NAP range study.
An Nd:YAG single pulse nanosecond laser of 532 nm wavelength with an 8 ns pulse width was projected on the soil samples collected from the campus of Bengbu College under 1 standard atmospheric pressure. Laser-induced breakdown spectroscopy at different sample temperatures was achieved. The intensity and signal-to-noise ratio (SNR) changes of different characteristic spectral lines could be analyzed when the sample temperature changes. The evolution of plasma electron temperature and electron density with the sample temperature was analyzed through Boltzmann oblique line method and Stark broadening method. The cause of the radiation enhancement of laser-induced metal plasma was discussed. Experimental results demonstrated that the spectral intensity, SNR, the electron temperature and electron density of plasma are positively related to the sample temperature, and reach saturation at 100 ℃.