Sol-gel processing and characterization of potassium niobate nano-powders by an EDTA/citrate complexing method

The present research describes a modified sol-gel technique used to obtain nano-crystalline potassium niobate (KNbO₃) powders by using ethylene diamine tetraacetic acid (EDTA)/citrate as a complexing agent. The metal ions chemically interact with EDTA in the precursor sol. The aging treatments lead to the formation of a precursor-polymeric gel network. The effects of the amounts of citric acid and EDTA on the stability of the precursor sol are investigated. The influence of excess K on the formation of pure-phase KNbO₃ powders is also studied. The obtained gels and powders are characterized by thermogravimetric-differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicate that a stable precursor sol is formed when n(CA):n(Mⁿ⁺) = 3:1 and n(EDTA) :n(NH₄OH) = 1:3.5. The xerogel is calcined at 700–850 °C to prepare the KNbO₃ nano-powder. The smallest grain size of the sample obtained at 850 °C is about 60 nm when the K/Nb molar ratio equals 1.2.     

Synthesis of the nitrogen-doped carbon nanotube (NCNT) bouquets and their electrochemical properties

The nitrogen-doped carbon nanotube (NCNT) bouquets have been synthesized by the pyrolysis of nitrogen-containing ion-exchange resin. The material shows excellent oxygen reduction performance after being supported by the Pt nanoparticles (Pt/NCNTs) compared with commercial Pt/C (46.7 wt.% Pt, TKK, Japan) in terms of the onset potential, half-wave potential, mass activity and durability. The better ORR performance of the Pt/NCNTs indicates potential applications in PEMCs.     

钯催化溴代水杨醛与吡啶硼酸的Suzuki交叉偶联反应

通过研究不同种类钯催化剂[Pd(dppf)2Cl2,Pd(OAc)2,Pd(PPh3)4]、碱(Na2CO3,Na HCO3,K2CO3,K3PO4,Cs2CO3,Cs F)、溶剂(DME/H2O,DMF/H2O,Dioxane/H2O)及温度(70,80,100℃)对5-溴-3-叔丁基水杨醛与吡啶-4-硼酸制备5-(4-吡啶)-3-叔丁基水杨醛化合物的Suzuki偶联反应的影响,开发出一种高效催化吸或供电子基取代的芳基硼酸与吸电子基取代的溴代芳烃的偶联反应的方法,该反应在Pd(PPh3)4,K2CO3,Dioxane/H2O(V∶V=4∶1)、80℃的条件下产率达到97%,且具有分离简单、重现性好的特征;但对供电子基取代的溴代芳烃参与的反应催化效果一般.     

The First-principles Calculations of the Electronic Structures and Optical Properties of Ⅱ-Ⅲ_2-Ⅵ_4(Ⅱ = Zn,Cd; Ⅲ = In; Ⅵ = Se,Te)

The electronic structures and optical properties of II?III2?VI4(II = Zn, Cd; III = In; VI = Se, Te) compounds are studied by the density functional theory(DFT) using the Vienna ab initio simulation package(VASP). Geometrical optimization of the unit cell is in good agreement with the experimental data. Our calculations show that the valence band maximum(VBM) and conduction band minimum(CBM) are located at G resulting in a direct energy gap. The optical properties are analyzed, and the independent second harmonic generation(SHG) coefficients are determined. By an analysis of the band structure, we can get that SHG response of the system can be attributed to the transitions from the bands near the top of valence band that are derived from the Se/Te p states to the unoccupied bands contributed by the p states of In atoms.     

Understanding the Activity of Co-N4−xCx in Atomic Metal Catalysts for Oxygen Reduction Catalysis

Atomic metal catalysis (AMC) provides an effective way to enhance activity for the oxygen reduction reaction (ORR). Cobalt anchored on nitrogen-doped carbon materials have been extensively reported. The carbon-hosted Co-N₄ structure was widely considered as the active site; however, it is very rare to investigate the activity of Co partially coordinated with N, for example, Co-N_4−xC_x. Herein, the activity of Co-N_4−xC_x with tunable coordination environment is investigated as the active sites for ORR catalysis. The defect (di-vacancies) on carbon is essential for the formation of Co-N_4−xC_x. N species play two important roles in promoting the intrinsic activity of atomic metal catalyst: N coordinated with Co to manipulate the reactivity by modification of electronic distribution and N helped to trap more Co to increase the number of active sites.     

Fabrication of Z-Scheme WO3/KNbO3 Photocatalyst with Enhanced Separation of Charge Carriers

Z-Scheme photocatalysts as a research focus perform strong redox capability and high photocatalytic performance. WO₃/KNbO₃ photocatalysts were fabricated by ball milling method, and performed higher photocatalytic activity in liquid degradation(rhodamine B, methylene blue and bisphenol A), compared with WO₃ or KNbO₃ monomer. This is due to that Z-scheme heterojunction is formed between WO₃ and KNbO₃, and the holes photo-excited in valence band of KNbO₃ are quickly combined with the electrons in conduction band of WO₃. The electrons accumulated in conduction band of KNbO₃ show high reducibility, thereby reducing O₂ to ^·O₂^-, and the holes in valence band of WO₃ show high oxidative to oxidize H₂O to ^·OH, respectively. Furthermore, it is proved by means of electron spin resonance(ESR) spectra, terephthalic acid photoluminescence probing technique(TA-PL), and UV-Vis absorption spectra of nitroblue tetrazolium. This work indicates that the fabrication of Z-scheme structure can improve the photocatalytic activity by efficiently separating the photogenerated electrons and holes in the photocatalytic reaction system, which is helpful to deeply understand the migration mechanism of photoexcited carrier(band-band transfer and Z-scheme transfer) in heterojunction photocatalysts.     

Water Oxidation with Cobalt-Loaded Linear Conjugated Polymer Photocatalysts

The first examples of linear conjugated organic polymer photocatalysts that produce oxygen from water after loading with cobalt and in the presence of an electron scavenger are reported. The oxygen evolution rates, which are higher than for related organic materials, can be rationalized by a combination of the thermodynamic driving force for water oxidation, the light absorption of the polymer, and the aqueous dispersibility of the relatively hydrophilic polymer particles. We also used transient absorption spectroscopy to study the best performing system and we found that fast oxidative quenching of the exciton occurs (picoseconds) in the presence of an electron scavenger, minimizing recombination.     

LaOCl-Coupled Polymeric Carbon Nitride for Overall Water Splitting through a One-Photon Excitation Pathway

It is a challenge to explore photocatalytic materials for sunlight-driven water splitting owing to the limited choice of a single semiconductor with suitable band energy levels but with a minimized band gap for light harvesting. Here, we report a one-photon excitation pathway by coupling polymeric carbon nitride (PCN) semiconductor with LaOCl to achieve overall water splitting. This artificial photosynthesis composite catalyzes the decomposition of H₂O into H₂ and O₂, with evolution rates of 22.3 and 10.7 μmol h⁻¹, respectively. The high photocatalytic performance of PCN/LaOCl can be ascribed to the simultaneously accomplished reduction and oxidation of water on LaOCl and PCN domains, respectively, as well as the fast charge separation and migration induced by the interfacial electric field related to LaOCl modification. This study provides new insights on the development of composite photocatalysts for pure water splitting based on polymer-based materials via charge modulation.     

Heteroatom Dopants Promote Two-Electron O2 Reduction for Photocatalytic Production of H2O2 on Polymeric Carbon Nitride

Polymeric carbon nitride modified with selected heteroatom dopants was prepared and used as a model photocatalyst to identify and understand the key mechanisms required for efficient photoproduction of H₂O₂ via selective oxygen reduction reaction (ORR). The photochemical production of H₂O₂ was achieved at a millimolar level per hour under visible-light irradiation along with 100 % apparent quantum yield (in 360–450 nm region) and 96 % selectivity in an electrochemical system (0.1 V vs. RHE). Spectroscopic analysis in spatiotemporal resolution and theoretical calculations revealed that the synergistic association of alkali and sulfur dopants in the polymeric matrix promoted the interlayer charge separation and polarization of trapped electrons for preferable oxygen capture and reduction in ORR kinetics. This work highlights the key features that are responsible for controlling the photocatalytic activity and selectivity toward the two-electron ORR, which should be the basis of further development of solar H₂O₂ production.     

Asymmetric Acceptor–Donor–Acceptor Polymers with Fast Charge Carrier Transfer for Solar Hydrogen Production

Construction of local donor–acceptor architecture is one of the valid means for facilitating the intramolecular charge transfer in organic semiconductors. To further accelerate the interface charge transfer, a ternary acceptor–donor–acceptor (A₁-D-A₂) molecular junction is established via gradient nitrogen substituting into the polymer skeleton. Accordingly, the exciton splitting and interface charge transfer could be promptly liberated because of the strong attracting ability of the two different electron acceptors. Both DFT calculations and photoluminescence spectra elucidate the swift charge transfer at the donor-acceptor interface. Consequently, the optimum polymer, N₃-CP, undergoes a remarkable photocatalytic property in terms of hydrogen production with AQY_{405 nm}=26.6 % by the rational design of asymmetric molecular junctions on organic semiconductors.