OXIDATIVE COUPLING OF METHANE OVER Na_2CO_3 DOPED ZIRCONIUM DIORIDE

Na_2CO_3/ZrO_2 catalyst shows a high activity of oxidative coupling of methane. It possesses stronger electron donor ability than that of ZrO_2 catalyst. The activation of methane is supposed to relate to the O_2^- or O_2^(2-) and O^- species.

Effects of Na^+ doping on crystalline structure and electrochemical performances of Li Ni_(0.5)Mn_(1.5)O_4 cathode material

Pristine LiNi0.5Mn1.5O4and Na-doped Li0.95Na0.05Ni0.5Mn1.5O4cathode materials were synthesized by a simple solid-statemethod.The effects of Na+doping on the crystalline structure and electrochemical performance of LiNi0.5Mn1.5O4cathode materialwere systematically investigated.The samples were characterized by XRD,SEM,FT-IR,CV,EIS and galvanostatic charge/dischargetests.It is found that both pristine and Na-doped samples exhibit secondary agglomerates composed of well-defined octahedralprimary particle,but Na+doping decreases the primary particle size to certain extent.Na+doping can effectively inhibit the formationof LixNi1-xO impurity phase,enhance the Ni/Mn disordering degree,decrease the charge-transfer resistance and accelerate the lithiumion diffusion,which are conductive to the rate capability.However,the doped Na+ions tend to occupy8a Li sites,which forces equalamounts of Li+ions to occupy16d octahedral sites,making the spinel framework less stable,therefore the cycling stability is notimproved obviously after Na+doping.

Na-doping-induced modification of the Cu_(2)ZnSn(S,Se)_(4)/CdS heterojunction towards efficient solar cells

It is very important to understand why a small amount of alkali metal doping in Cu_(2)ZnSn(S,Se)_(4)(CZTSSe)solar cells can improve the conversion efficiency.In this work,Na-doped CZTSSe is prepared by a simple solution method,and then the effects on the surface properties of the absorber layer,the buffer layer growth,and the modifications of the solar cell performance induced by the Na doping are studied.The surface of the absorber layer is more Cu-depletion and less roughness due to the Na doping.In addition,the contact angle of the surface increases because of Na doping.As a consequence,the thickness of the CdS buffer layer is significantly reduced and the optical losses in the CdS buffer layer are decreased.The difference of quasi-Fermi levels(EFn-EFp) increases with a small amount of Na doping in the CZTSSe solar cell,so that open circuit voltage(VOC) increased significantly.This work offers new insights into the effects of Na doping on CZTSSe via a solution-based approach and provides a deeper understanding of the origin of the efficiency improvement of Na-doped CZTSSe thin film solar cells.

A High-Temperature β-Phase NaMnO2 Stabilized by Cu Doping and Its Na Storage Properties

The high-temperature β-phase NaMnO2 is a promising material for Na-ion batteries（NIBs） due to its high capacity and abundant resources. However, the synthesis of phase-pure -NaMnO2 is burdensome and costineffective because it needs to be sintered under oxygen atmosphere at high temperature and followed by a quenching procedure. Here we first report that the pure β phase can be stabilized by Cu-doping and easily synthesized by replacing a proportion of Mn with Cu via a simplified process including sintering in air and cooling to room temperature naturally. Based on the first-principle calculations, the band gap decreases from 0.7 eV to 0.3 eV, which indicates that the electronic conductivity can be improved by Cu-doping. The designed -NaCu（0.1）Mn（0.9）O2 is applied as cathode in NIBs, exhibiting an energy density of 419 Wh/kg and better performance in terms of rate capability and cycling stability than those in the undoped case.

Boosting High-Voltage and Ultralong-Cycling Performance of Single-Crystal LiNi_(0.5)Co_(0.2)Mn_(0.3)O_(2) Cathode Materials via Three-in-One Modification

LiNi_(0.5)Co_(0.2)Mn_(0.3)O_(2) is extensively researched as one of the most widely used commercially materials for Li-ion batteries at present.However,the poor high-voltage performance(≥4.3 V)with low reversible capacity limits its replacement for LiCoO_(2) in high-end digital field.Herein,three-in-one modification,Na-doping and Al_(2)O_(3)@Li_(3)BO_(3) dual-coating simultaneously,is explored for single-crystalline LiNi_(0.5)Co_(0.2)Mn_(0.3)O_(2)(N-NCM@AB),which exhibits excellent high-voltage performance.N-NCM@AB displays a discharge-specific capacity of 201.8 mAh g^(−1) at 0.2 C with a high upper voltage of 4.6 V and maintains 158.9 mAh g^(−1) discharge capacity at 1 C over 200 cycles with the corresponding capacity retention of 87.8%.Remarkably,the N-NCM@AB||graphite pouch-type full cell retains 81.2% of its initial capacity with high working voltage of 4.4 V over 1600 cycles.More importantly,the fundamental understandings of three-in-one modification on surface morphology,crystal structure,and phase transformation of N-NCM@AB are clearly revealed.The Na+doped into the Li–O slab can enhance the bond energy,stabilize the crystal structure,and facilitate Li+transport.Additionally,the interior surface layer of Li^(+)-ions conductor Li_(3)BO_(3) relieves the charge transfer resistance with surface coating,whereas the outer surface Al_(2)O_(3) coating layer is beneficial for reducing the active materials loss and alleviating the electrode/electrolyte parasite reaction.This three-in-one strategy provides a reference for the further research on the performance attenuation mechanism of NCM,paving a new avenue to boost the high-voltage performance of NCM cathode in Li-ion batteries.

Lithium-ion insertion kinetics of Na-doped Li2TiSiO5 as anode materials for lithium-ion batteries

Li2TiSiO5 receives much interest recently in lithium-ion battery anodes because of its attractive Liinsertion/extraction potential at 0.28 V(vs. Li+/Li), which bridges the potential gap between graphite and Li4 Ti5 O12. However, Li2TiSiO5 suffers from the low intrinsic electronic conductivity and sluggish Liion transfer kinetics. In this work, we report lithium-ion insertion kinetics of Li2TiSiO5 by Na doping,achieving high-rate capability. Rietveld refinement of X-ray diffraction results reveals that Na doping can enlarge the space of Li slabs, thus reducing the Li-ion transfer barrier and enhancing the Li-ion diffusion kinetics. According to first-principles calculations, Na doping can tune the band structure of Li2TiSiO5 from indirect to direct band, leading to improved electronic conductivity and electrochemical performance. In particular, the Na-doped Li2TiSiO5(Li1.95 Na(0.05)TiSiO5) electrode exhibits outstanding rate capability with a high capacity of 101 m A h g^(-1) at 5 A g^(-1) and superior cyclability with a reversible capacity of 137 m A h g^(-1) under 0.5 A g^(-1) over 150 cycles.

Investigation of the growth and optical properties of a Co^2＋-doped Na5Lu9F(32) single crystal

A 0.1 mol.% CoF2-doped Na5Lu9F（32）single crystal with high quality in the size of -φ10 mm×100 mm was grown by the Bridgman method. Three peaks located at 504, 544, and 688 nm and a broad band in the range of 1200–1600 nm centered at 1472 nm were observed in the absorption spectra. The absorption peak position suggests cobalt ions in the divalent state in the grown crystal. Moreover, the cobalt ions are confirmed to locate in the distorted cubic crystal structure. Upon excitation of 500 nm light, a sharp emission peak at 747 nm ascribed to the ^2T2（H1） →^4A2（F） transition was observed for the crystal. The Co^2＋-doped Na5Lu9F（32）crystal shows a potentially promising material for the application of a passively Q-switched laser operating in the near-infrared range.

Room-temperature electroluminescence of p-Zn_xMg_(1-x)O:Na/n-ZnO p-n junction light emitting diode

p-ZnxMg1-xO：Na/n-ZnO p-n junction light emitting diode （LED） was produced on n-ZnO （0001） single-crystal substrate using pulsed laser deposition. The realization of band gap engineering was achieved by the incor-poration of Mg in ZnO layers and was confirmed by photoluminescence spectrum. The p-type ZnxMg1-xO：Na film with low resistance was obtained at 500 ℃ and in which, Na has taken effect evidenced by Hall and X-ray photo-electron spectroscopy measurements. The current-voltage curve of LED showed a rectifying behavior and obvious electroluminescence was realized by feeding a direct current up to 40 mA. Furthermore, its structural and electric characters are discussed as well.

One-step supramolecular preorganization constructed crinkly graphitic carbon nitride nanosheets with enhanced photocatalytic activity

Here we report a new one-step thermal polycondensation process to form crinkly graphitic carbon nitride nanosheets(CGCNNs)via supramolecular preorganization,using a mixture of urea and melamine as the starting material.Systematical studies reveal that the newly developed CGCNNs significantly strengthen the optical absorption,widen the bandgap,and increase the Hall mobility and carrier density compared to that of its bulk counterpart,regardless of the similar chemical composition and structure.As a result,the photocatalytic hydrogen production rate is improved by 7 times.Moreover,Na doping of CGCNNs can further promote its photocatalytic activity,leading to an excellent photocatalytic hydrogen production rate of 250.9μmol h^(–1),which is approximately 10.5 times higher than its bulk counterpart.Moreover,an impressive apparent quantum efficiency of 19.12%is achieved at 420 nm.This study provides a facile strategy for the design of efficient low-cost carbon-nitride-based photocatalysts for solar fuel production.

Adjusting Na doping via wet-chemical synthesis to enhance thermoelectric properties of polycrystalline SnS

Tin sulfide (SnS) has analogous structural features to tin selenide (SnSe), but contains more abundant resources as compared with SnSe. SnS has elicited attention as a potential eco-friendly therm oelectric (TE) material. However, the intrinsic carrier concentration of SnS is very low, thereby hindering the performance improvement of the material. This study proposes that the TE properties of polycrystalline Nadoped SnS (synthesized through an improved chemical coprecipitation) can be significantly enhanced. The maximum power factor (PF) of 362 μW m^-1K^-2 at 873 K was achieved, presenting a state-of-the-art value for the polycrystalline SnS. Considering the merits of the improved electrical properties and lower thermal conductivity of SnS, the highest ZT was up to 0.52 at 873 K even without intentional chemical doping. This study offers an effective approach for improving the PF to achieve high ZT in SnS. Hence, we expect that this new perspective can be extended to other dopants and broaden the scope of synthesis technology.