A bonding study of cyclopentene (c-C5H8) adsorption on Ni(1 1 1) surface

The adsorption of cyclopentene (c-C₅H₈) on Ni(1 1 1) was studied using DFT and semiempirical calculations. Preferred site and geometry calculations were carried out considering a Ni(1 1 1) surface and a unit cell of 64-atoms. The tetrahedral threefold hollow position was identified as the most favorable site, with a surface-molecule minimum distance of 1.83 Å. A bending structure is adopted when the molecule is adsorbed where the carbon atoms of the double bond are closer to the surface forming an angle of 160° among non-equivalents carbon atoms. The metal surface was represented by a two-dimensional slab with an overlayer of c-C₅H₈/Ni of 1/9 ratio. We also computed the density of states (DOS) and the crystal orbital overlap populations (COOP) corresponding to CC, CNi, CH, and NiNi bonds. We found that both NiNi bonds interacting with the ring, and the CC bond are weakened after adsorption, this last bond is linked significantly to the surface. The hydrogen atoms belonging to the saturated carbon atoms also participate in the adsorbate–surface bonding. The main interactions include the 4s, 3p_z and 5d_z² bands of nickel and 2p_z bands of the carbon atoms of the double bond.     

EQCM studies on Pd–Ni alloy oxidation in basic solution

Processes of electrochemical oxidation of Pd-rich Pd–Ni alloys in basic solutions were studied with the aim of electrochemical quartz crystal microbalance. Potentials of current peaks of Ni(II)/Ni(III) redox couple are independent of alloy composition. On the other hand, Ni(II)/Ni(III) redox couples formed on Pd–Ni alloys and Ni differ in respect to the structure of involved compounds and the processes of transport of the species accompanying oxidation/reduction reaction. The process of oxidation of Pd exhibits some differences between pure Pd and Pd–Ni alloys. This concerns mainly on participation of adsorbed water/OH⁻ in Pd oxidation process. In the initial stages of Pd oxidation, the source of oxygen is water/OH⁻ from the bulk of the solution. At this stage of the process, the product of Pd oxidation could be described as Pd(OH)₂ or PdOH₂O. With further progress in oxidation process, adsorbed species, water/OH⁻, start to play a decisive role. Hydrous species, i.e. Pd(OH)₂ or PdOH₂O, are also reduced in the final stages of Pd(II) reduction process. This study is dedicated to the 70th birthday of Professor Oleg Petrii.     

Development of Methane Decomposition Catalysts for CO<Subscript>x</Subscript> Free Hydrogen

Now-a-days, catalytic decomposition of methane (CDM) into hydrogen and carbon is a promising technique for production of fuel cell grade hydrogen. The Ni based catalysts seems promising particularly for the production of CO_x free H₂ by methane decomposition process. The CDM activity and longevity of the Ni based catalysts are mainly influenced by the amount of Ni and type of support material. In this paper the CDM activity results are correlated with NiO crystallite size, Ni metal surface area and acidity of the catalysts. In case of bimetallic catalysts addition of Cu to Ni catalysts lead to enhance the CDM activity at higher temperature thus resulting in the increased concentration of hydrogen in the outlet stream. Finally, some of the carbon-based catalysts are studied for methane decomposition activity at higher temperature. The surface changes over carbon catalysts with methane decomposition are studied using various characterization techniques.     

Reactivity of metal oxides: Thermal and photochemical dissolution of MO and MFe2O4 (M=Ni, Co, Zn)

Dissolution rates of NiO, CoO, ZnO, α-Fe₂O₃ and the corresponding ferrites in 0.1 mol dm⁻³ oxalic acid at pH 3.5 were measured at 70 °C. The dissolution of simple oxides proceeds through the formation of surface metal oxalate complexes, followed by the transfer of surface complexes (rate-determining step). At constant pH, oxalate concentration and temperature, the trend in the first-order rate constant for the transfer of the surface complexes (k_Me; Me=Ni, Co, Zn, Fe) parallels that of water exchange in the dissolved metal ions (k_−w). Thus, the most important factor determining the rates of dissolution of metal oxides is the lability of Me-O bonds, which is in turn defined by the electronic structure of the metal ion and its charge/radius ratio. UV (384 nm) irradiation does not increase significantly the dissolution rates of NiO, CoO and ZnO, whereas hematite is highly sensitive to UV light. For ferrites, the reactivity order is ZnFe₂O₄>CoFe₂O₄?NiFe₂O₄. Dissolution is congruent, with rates intermediate between those of the constituent oxides, Fe₂O₃ and MO (M=Co, Ni, Zn), reflecting the behavior of very thin leached layers with little Zn and Co, but appreciable amounts of Ni. The more robust Ni²⁺ labilizes less the corresponding ferrite. The correlation between log k_M and log k_−w is somewhat blurred and displaced to lower k_M values. Fe(II), either photogenerated or added as salt, enhances the rate of Fe(III) phase transfer. A simple reaction mechanism is used to interpret the data.     

NH3在Ni/Pt(111)和Ni/WC(001)表面上分解的第一性原理研究

采用密度泛函理论和slab模型,研究NH₃在Ni单原子层覆盖的Pt（111）和WC（001）表面上的物理与化学行为,计算了Ni单原子覆盖表面的电子结构以及NH₃的吸附与分解.表面覆盖的单原子层中,Ni原子的性质与Ni（111）面上的Ni原子明显不同.与Ni（111）相比,Ni/Pt（111）和Ni/WC（001）表面上Ni原子dz²轨道上的电子更多地转移到了其它位置,该轨道上电荷密度降低有利于NH₃吸附.在Ni/Pt（111）和Ni/WC（001）面上NH₃吸附能均大于Ni（111）,NH₃分子第一个N-H键断裂的活化能则明显比Ni（111）面上低,有利于NH₃的分解,吸附能增大使NH₃在Ni/Pt（111）和Ni/WC（001）面上更倾向于分解,而不是脱附.N₂分子的生成是NH₃分解的速控步骤,该反应能垒较高,说明N₂分子只有在较高温度下才能生成.WC与Pt性质相似,但Ni/Pt（111）和Ni/WC（001）的电子结构还是有差异的,与Ni（111）表面相比,NH₃在Ni/Pt（111）表面上分解速控步骤的能垒降低,而在Ni/WC（001）上却升高.要获得活性好且便宜的催化剂,需要对Ni/WC（001）表面做进一步改进,降低N₂分子生成步骤的活化能.     

GaAs纳米线阵列太阳能电池的设计与优化

为了得到纳米线阵列太阳能电池的最优转换效率,通过仿真计算对GaAs轴向pin结纳米线阵列进行了结构优化.首先利用三维有限时域差分法分析了GaAs纳米线阵列的光吸收特性,并对其直径、密度等结构参量进行优化,优化后的GaAs纳米线阵列的光吸收率可达87.4%.在此基础上,利用Sentaurus软件包中的电学仿真模块分析了电池的电学性能,并根据光生载流子在纳米线中的分布,对轴向pin结结构进行优化,最终优化过的太阳能电池功率转换效率可达到17.6%.分析结果表明,通过钝化处理以降低GaAs纳米线的表面复合速率,可显著提升电池的功率转换效率,而通过减小纳米线顶端高掺杂区域的体积,可减少载流子复合损耗,从而提高电池效率.该研究可为制作高性能的纳米线太阳能电池提供参考.     

Fabrication and optical properties of type‐II InP/InAs nanowire/quantum‐dot heterostructures

The growth and optical properties of InAs quantum dots on a pure zinc blende InP nanowire are investigated. The quantum dots are formed in Stranski–Krastanov mode and exhibit pure zinc blende crystal structure. A substantial blueshift of the dots peak with a cube‐root dependence on the excitation power is observed, suggesting a type‐II band alignment. The peak position of dots initially red‐shifts and then blue‐shifts with increasing temperature, which is attributed to the carrier redistribution among the quantum dots. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Piezoelectric effect in GaAs nanowires: experiment and theory

The anomalous piezoelectric effect in GaAs nanowires was detected (the piezoelectric strain coefficient up to d₃₃ ≈ 26 pC/N) that is 12 times larger than the theoretically estimated value. This result is explained by the dominant content of the phase with the wurtzite‐type crystal structure in GaAs nanowires and an increased pressing force by the contact layer on the nanowire. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

The origin of yellow band emission and cathodoluminescence of Au-catalyzed wurtzite GaN nanowires

GaN nanowires with large yield are directly synthesized by simply ammoniating the gallium oxide powders in the presence of ammonia gas at 1000 °C, under the assistance of Au nanocatalysts. The microstructure and crystallinity of as-synthesized GaN nanowires are well studied by using high-resolution transmission electron microscope (HRTEM) and some structural defects such as stacking faults are found in the GaN nano-crystal. Cathodoluminescence measurement shows that a strong near-band-edge (NBE) emission band centered at 384 nm and a broad yellow band in the range of 500–800 nm are observed. Finally, the growth mechanism and possible optical emission process of GaN nanowires are discussed.     

不同制备条件对聚偏氟乙烯纳米线生长的影响

为提高铁电聚合物聚偏氟乙烯(PVDF)的压电性能,利用纳米限域效应,采用模板直接浸润法,将多孔氧化铝(AAO)模板在不同浓度的PVDF的DMF溶液中自然浸润,并添加聚乙烯吡咯烷酮(PVP)作为表面修饰剂,制备了一维材料PVDF纳米线。分别研究了浸润温度、溶液浓度及表面修饰剂等因素对PVDF纳米线生长过程的影响。通过FTIR、SEM、XRD等对样品的形貌结构及性能进行了表征,进一步讨论了AAO模板中PVDF纳米线的生长机制。结果表明:模板法生长的PVDF纳米线形貌主要受溶液浓度的影响,并且当浓度为0.10 g/m L时形貌较优,其平均长度为50μm,平均直径为180 nm,与AAO模板孔径尺寸相当;表面修饰剂PVP可在一定程度上防止纳米线团聚并且优化其尺寸均一性;AAO模板中生长的PVDF纳米线由于纳米限域效应优先向β晶相结晶,并且在生长过程中PVDF并未参与任何化学反应。