电沉积制备的PtNi纳米粒子用于氧还原反应:成核和生长机理

质子交换膜燃料电池(PEMFCs)电堆中阴极Pt基催化剂的高用量造成其成本居高不下,成为阻碍燃料电池汽车商业化推进的重要原因,因此开发低Pt、高活性的Pt基催化剂势在必行.Pt合金催化剂能够有效地降低Pt用量,并通过对合金颗粒的元素比例、晶面、粒径等实行精确调控,显著提升氧还原(ORR)催化活性.然而,目前常用的制备方法由于原料与制备成本高昂、过程复杂大都难以适应规模化生产需求.电化学方法通过控制施加的电流或电位控制晶体生长.在水体系中该方法已得到验证,但由于Pt化合物的热力学标准电极电位与过渡金属元素之间相差较大,且对于过渡金属来说,电负性大多小于铂,因此还原电位通常负于析氢电位,使得二者难以实现共沉积.有机体系中电位窗口比水体系大得多,Pt与电位较负的过渡金属可实现共沉积,采用小分子有机溶剂也可避免溶剂清洗问题,具有应用潜力.本文提出了一种简单的一步电沉积方法,选择易溶于水的N,N-二甲基甲酰胺(DMF)作为溶剂,将碳载体滴涂到玻碳电极上作为工作电极,通过电化学方法直接将Pt-Ni合金沉积到碳载体上,并利用物化表征与密度泛函理论(DFT)理论计算来探究共沉积机理.透射电镜表征结果表明,在不同的沉积电位下均可得到分散均匀、粒径适当的催化剂;且随着电位值降低,催化剂颗粒分散得更均匀,颗粒粒径不断减小.元素分布和晶面结果表明,铂镍元素均匀分布于颗粒中.所有样品均表现出优异的ORR性能,最高的面积比活性达到商业催化剂的6.85倍.将材料表征、电化学表征与DFT计算结合,建立起了铂镍合金生长过程的模型,并发现了有机体系中独特的成核-生长机理.将体系中的DMF换成超纯水,用同样的方法进行沉积,得到的催化剂颗粒团聚严重,说明DMF的使用能够避免颗粒团聚.在单独铂的体系中沉积发现,负载量极小,表明体系中镍前驱体的添加对于催化剂的沉积过程起到重要作用.电化学表征结果表明,在所选用的DMF有机体系中,镍的还原电位与铂的十分接近,但还原动力学更慢,趋向于先形成吸附原子后快速还原.由此可以推测,在二者合金的形成过程中,镍在碳载体表面的缓慢还原而形成的吸附原子能够成为铂还原的活性位点,从而降低了铂还原成核所需的能量,使得载体上的成核位点大大增加,这与DFT模拟结果一致.DFT建立了碳上镍的位点和铂的位点,分别在上面进行铂的还原,发现镍位点上比铂位点上更容易实现铂沉积.本文提出了铂镍共沉积的机理:在过电位(即还原能量)下,铂的还原动力学较镍稍快,于是铂先还原形成晶核,但难以达到生长的临界半径,于是单独铂体系中的沉积负载量很少.载体上还原的镍为铂还原提供了大量的活性位点,促进了铂还原,并与镍共沉积.Pt-Ni表面则进一步促进了铂的沉积和颗粒的生长.综上,本文提出了一种用于制备铂合金催化剂的有机电沉积体系,实现了单分散的碳载铂镍合金催化剂的一步制备.随后,本文将材料表征、电化学表征与DFT计算相结合,建立起了有机体系中铂镍合金成核-生长过程的机理模型.     

纳米Pd和Au催化CO2还原粒径效应的密度泛函理论计算研究

以可再生能源为能量来源,在水溶液中进行的光(电)催化CO₂还原生成高附加值化学品和燃料是解决能源危机与环境污染的有效途径之一.CO是一种简单却很重要的CO₂还原产物,它可以作为水煤气变换反应与费托合成的重要原料.具有较高CO选择性的贵金属纳米颗粒催化剂(如Au和Pd)一直受到研究者的广泛关注.一般来说,金属颗粒催化剂的催化性能与粒径大小密切相关,即所谓的粒径效应.然而在实际的理论计算研究中,由于受到计算能力的限制,催化剂模型都仅局限于简单的周期性模型或小的金属团簇模型,无法准确描述真实颗粒上复杂的反应位点的性质,导致了对催化行为的误解.因此,建立更加真实的颗粒模型对探究纳米颗粒催化剂上活性位点的性质,解释其粒径效应至关重要.本文旨在阐述Au与Pd纳米颗粒催化剂不同活性位点上CO₂还原反应与产H2副反应的竞争机制,并解释Au与Pd纳米颗粒催化剂在CO₂电还原中表现出不同粒径效应的原因.本文基于密度泛函理论,采用VASP软件,BEEF-vdW泛函进行计算.分别建立了原子数为55,147,309和561的颗粒模型和高CO^*覆盖度模型,避免了传统周期性模型的局限性,探究了金属颗粒催化剂不同反应位点上的CO选择性.结果表明,对于颗粒模型来说,(100)位点对CO的选择性优于边缘位点;但对于周期性模型来说,Au(211)对CO的选择性则优于Au(100).产生这种反差的主要原因在于Au颗粒的边缘位点对H*的吸附过强.通过对比,我们直观地展现了颗粒模型上平面位点和Edge位点与相对应的周期性模型上CO选择性的区别,突出了模型选择对揭示活性位点性质的重要性.在此基础上,通过计算理论CO法拉第效率,发现Au颗粒随着粒径的减小,CO选择性降低,与实验的趋势一致.对于Pd催化剂来说,低覆盖度模型无法正确预测活性位点的性质;而高CO覆盖度的情况下,Pd颗粒的边缘位点对COOH^*吸附能更强,这是导致边缘位点上CO选择性更高的主要原因.同样通过计算理论CO法拉第效率,发现随着粒径的减小,Pd颗粒上CO选择性升高.本文不仅成功揭示了Au与Pd颗粒催化剂上活性位点的性质,对粒径效应做出了合理解释,也强调了合理的计算模型是理论研究的基础.     

Er3+掺杂Li2O-SrO-ZnO-Bi2O3玻璃中Er3+离子在1.53 μm处的荧光发射特性

采用传统的熔融法制备了Er~(3+)掺杂的新型铋酸盐玻璃(Li_2O-SrO-ZnO-Bi_2O_3,LSZB),并对其光谱性质进行了表征,分析了玻璃的拉曼光谱、吸收光谱、荧光光谱,利用Judd-Ofelt理论研究了其荧光特性。LSZB玻璃样品中Er~(3+)的~4I_(13/2)→~4I_(15/2)跃迁发射峰位于1.53 μm处,半高宽约为78 nm。样品中Er~(3+)的~4I_(13/2)能级寿命为2.848 ms,量子效率为99.93%,受激发射截面达到9.76×10~(-21)cm~2。以上结果显示,Er~(3+)掺杂LSZB玻璃有良好的光谱特性。     

Understanding the dehydrogenation mechanism over iron nanoparticles catalysts based on density functional theory

The conversion of chemical feedstock materials into high value-added products accompanied with dehydrogenation is of great value in the chemical industry.However,the catalytic dehydrogenation reaction is inhibited by a limited number of expensive noble metal catalysts and lacks understanding of dehydrogenation mechanism.Here,we report the use of heterogeneous non-noble metal iron nanoparticles(NPs) incorporated mesoporous nitrogen-doped carbon to investigate the dehydrogenation mechanism based on experiment observation and density functional theory(DFT) method.Fe NPs catalyst displays excellent performance in the dehydrogenation of 1,2,3,4-tetrahydroquinoline(THQ)with 100% selectivity and 100% conversion for 10-12 h at room temperature.The calculated adsorption energy implies that THQ prefers to adsorb on Fe NPs as compared with absence of Fe NPs.What is more,the energy barrier of transition state is relatively low,illustrating the dehydrogenation is feasible.This work provides an atomic scale mechanism guidance for the catalytic dehydrogenation reaction and points out the direction for the design of new catalysts.     

The Chemical Bond: When Atom Size Instead of Electronegativity Difference Determines Trend in Bond Strength

We have quantum chemically analyzed element−element bonds of archetypal H_nX−YH_n molecules (X, Y=C, N, O, F, Si, P, S, Cl, Br, I), using density functional theory. One purpose is to obtain a set of consistent homolytic bond dissociation energies (BDE) for establishing accurate trends across the periodic table. The main objective is to elucidate the underlying physical factors behind these chemical bonding trends. On one hand, we confirm that, along a period (e. g., from C−C to C−F), bonds strengthen because the electronegativity difference across the bond increases. But, down a period, our findings constitute a paradigm shift. From C−F to C−I, for example, bonds do become weaker, however, not because of the decreasing electronegativity difference. Instead, we show that the effective atom size (via steric Pauli repulsion) is the causal factor behind bond weakening in this series, and behind the weakening in orbital interactions at the equilibrium distance. We discuss the actual bonding mechanism and the importance of analyzing this mechanism as a function of the bond distance.     

受限于两平行板间的对称星形共聚物Am Bm熔体相行为的格子自洽场理论研究

采用格子自洽场理论计算研究了受限于2个平行板间的对称星形共聚物AmBm(m=1,2,3,4,5)熔体形成的层状相结构.在给定的相互作用下(χNAB不变,χ为Flory-Huggins相互作用参数,NAB=(N?1)/m为单个聚合物分子中一对AB臂的总链节数目),针对平行板间距为体相周期的情况,系统考察了共聚物链长N和单个聚合物分子中A(或B)臂数目m对受限层结构细节及层取向的影响.由计算结果,当N或NAB不变时,受限层的归一化界面宽度随m的增大而减小.受限板为中性时,垂直层结构的单链自由能比平行层结构的低.随着板对共聚物中一种嵌段的选择作用Λ的增大,体系发生垂直层到平行层的转变,该转变为一阶相变.当m不变时,N越小,上述转变出现在越大的Λ值处,体系越容易保持垂直层结构.并且N越小,层状结构周期越小.当N或NAB不变时,m越大体系越容易保持垂直层结构.总之,星形共聚物的链长越短、臂数越多时,垂直层稳定的Λ区间越大、层状结构的界面宽度越小.这些结论可以指导刻蚀应用中对体系参数的选择.     

SnO2表面卤化提高钙钛矿太阳能电池光伏性能

钙钛矿太阳能电池(PSCs)成为近几年来迅速发展的新型太阳能电池,其中将SnO₂纳米粒子层用作电子传输层(ETL)的钙钛矿太阳能电池器件得到了广泛的关注。SnO₂有着更低的制备温度,使其具备应用于柔性器件的潜力,但与钙钛矿层能级不匹配等问题限制着其发展。而在界面处加入钝化层,尤其是表面卤化的方法或可解决这一问题。本文综合研究了SnO₂表面卤化对钙钛矿太阳能电池光伏性能的影响,选用四丁基氯化铵(TBAC)、四丁基溴化铵(TBAB)和四丁基碘化铵(TBAI)三种钝化材料对SnO₂表面进行钝化处理,并对钝化材料溶液进行了浓度梯度研究。通过材料形貌、结构和光学性能表征以及电池器件性能测试分析等方法,证明了SnO₂表面卤化可提高钙钛矿层的质量和PSCs光伏性能,并从器件内部电荷传输动力学等角度解释了器件性能改善的原因。为进一步说明其性能改善的机理,采用基于密度泛函理论(DFT)的第一性原理计算方法对材料表面性质进行了深入研究,从能量、结构、电荷密度、态密度、功函数等角度解释了表面卤化提高SnO₂/钙钛矿界面处电子传输特性的原因。实验和理论计算均表明TBAC对于SnO₂具有较好的钝化效果,并随着溶液浓度的提升钝化作用越明显。SnO₂表面卤化作用的深入研究不仅对提高电池器件性能具有实际意义,还能够帮助理解太阳能电池界面现象,为界面改性提供新的研究思路。     

Simultaneous studies of pressure effect on charge transport and photophysical properties in organic semiconductors: A theoretical investigation

High-mobility and strong luminescent materials are essential as an important component of organic photodiodes, having received extensive attention in the field of organic optoelectronics. Beyond the conventional chemical synthesis of new molecules, pressure technology, as a flexible and efficient method, can tune the electronic and optical properties reversibly. However, the mechanism in organic materials has not been systematically revealed. Here, we theoretically predicted the pressure-depended luminescence and charge transport properties of high-performance organic optoelectronic semiconductors, 2,6-diphenylanthracene (DPA), by first-principle and multi-scale theoretical calculation methods. The dispersion-corrected density functional theory (DFT-D) and hybrid quantum mechanics/molecular mechanics (QM/MM) method were used to get the electronic structures and vibration properties under pressure. Furthermore, the charge transport and luminescence properties were calculated with the quantum tunneling method and thermal vibration correlation function. We found that the pressure could significantly improve the charge transport performance of the DPA single crystal. When the applied pressure increased to 1.86 GPa, the hole mobility could be doubled. At the same time, due to the weak exciton coupling effect and the rigid flat structure, there is neither fluorescence quenching nor obvious emission enhancement phenomenon. The DPA single crystal possesses a slightly higher fluorescence quantum yield ～ 0.47 under pressure. Our work systematically explored the pressure-dependence photoelectric properties and explained the inside mechanism. Also, we proposed that the external pressure would be an effective way to improve the photoelectric performance of organic semiconductors.     

On the predictivity of the non‐renormalizable quantum field theories

Following a Four Dimensional Renormalization approach to ultraviolet divergences (FDR), we extend the concept of predictivity to non‐renormalizable quantum field theories at arbitrarily large perturbative orders. The idea of topological renormalization is introduced, which keeps a finite value for the parameters of the theory by trading the usual order‐by‐order renormalization procedure for an order‐by‐order redefinition of the perturbative vacuum. One additional measurement is then sufficient to systematically compute quantum corrections at any loop order, with no need of absorbing ultraviolet infinities in the Lagrangian.     

Fe掺杂ZnTe团簇结构和磁性质

本文采用第一性原理密度泛函理论系统的研究了Fe原子单掺杂和双掺杂(ZnTe)12团簇的结构和磁性质。我们考虑了替代掺杂和间隙掺杂。不管是单掺杂还是双掺杂,间隙掺杂团簇都是最稳定结构。团簇磁矩主要来自Fe-3d态的贡献,4s 和4p 态也贡献了一小部分磁矩。由于轨道杂化,相邻的Zn和Te原子上也产生少量自旋。最重要的是,我们指出间隙双掺杂团簇是铁磁耦合,在纳米量子器件有潜在的应用价值。