Hysteresis,nucleation and growth phenomena in spin-crossover solids

The observation and the study of first-order phase transitions in cooperative spin-crossover (SCO) solids exhibiting hysteresis behaviours are of particular interest and currently constitute a burgeoning area in the field of bistable molecular materials. The understanding and the control of the transition mechanisms (nucleation and growth processes) and their dynamics within the hysteresis region appear to be a general and appealing problem from a fundamental point of view and for technological applications as well. This review reports on the recent progresses and most important findings made on the spatiotemporal dynamics of the spin transition in SCO solids, particularly through the universal nucleation and growth process. Both thermally induced and light-induced spin transitions are discussed. We open up this review to the central question of the evolution of the transition mechanisms and dynamics in SCO nano-objects, which constitute promising systems to reach ultra-fast switching, and the experimental issues inherent to such studies at the micro- and nanometric scale.     

Current transient study of the kinetics of nucleation and diffusion-controlled growth of bimetallic phases

A model to describe the kinetics of nucleation and diffusion-controlled growth of bimetallic phases has been developed, and analytical expressions have been obtained for the elucidation of nucleation kinetics through determination of the number density N ₀ of active sites and their nucleation frequency A, from experimental current transients obtained under potentiostatic control. The validity of the model has been demonstrated with the electrodeposition of Ag–Hg phases at two distinct compositions.     

Analytical expression for transient chronoamperometric current at ultramicroband electrode

Analytical solution of non-steady state chronoamperometric current at ultramicroband electrode is derived. The analytical expression for current (Padé approximant), valid for entire time domain is compared with the analytical and digital simulation data. A satisfactory agreement with the available limiting cases of analytical expressions and digital simulation data is noted. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 10, pp. 1244–1250. The text was submitted by the authors in English.     

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.     

Mechanism for nucleation and growth of electrochemical palladium deposition on an Au(111) electrode

The initial kinetics of electrochemical deposition of palladium on an Au(111) electrode in 0.1 M H₂SO₄ solution containing K₂PdCl₄ have been investigated quantitatively by cyclic voltammetry (CV) and potentiostatic current transient measurements. A cathodic current maximum was observed in the initial stage of Pd deposition process in 0.1 mM K₂PdCl₄ solution. The analysis shows that the deposition of palladium in this solution proceeds by an instantaneous nucleation and two-dimensional (2D) growth mode. When the concentration of K₂PdCl₄ became higher, a potential-dependent cathodic current ‘plateau’ after the current maximum was observed. Simple nucleation-growth models cannot fit the transients in the region of the ‘current plateau’. The present current transient results were discussed in comparison with the results obtained by in situ scanning tunneling microscopy (STM).     

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.     

Effect of growth conditions on the structure of two-dimensional latex crystals: modeling

Essential experimental features of the nucleation and growth of a 2D colloidal crystal on a solid substrate are modeled. The crystal, composed of sub-micron-sized latex spheres, is grown by the evaporation of water from the particle suspension in a circular cell. The calculation of the meniscus profile in the cell allows the prediction of the particle volume fraction in the suspension surrounding the crystal as a function of time. This quantity enters into a convective-diffusion model for the crystal growth which calculates the crystal radius as a function of time. Comparison with experimental data for 2D latex particle crystals shows predominant convective growth over a wide range of evaporation rates set by varying the humidity of the air. Microscopic parameters of the particle assembly can also be estimated such as the particle velocity, diffusivity, characteristic time constants, Peclet number, etc. The nucleation is simulated by simultaneously solving the equations of motion for the ensemble of particles trapped in a thin liquid film using the discrete-element method. These equations account for the forces which are physically important in the system: contact particle–particle friction, increased viscous resistance during the particle motion in a wetting film, long-range capillary attraction between two particles screened by the rest of particles. The final result of the simulation is a particle cluster of hexagonal packing, whose structure resembles very much the monolayer nucleus of latex particles observed experimentally. The models proposed by us could also be implemented for the aggregation of species in a variety of practical processes such as coating, texturing, crystal growth from a melt or liquid solution, or a biological array. Received: 10 May 1999 Accepted in revised form: 6 July 1999     

Review of non-reactive and reactive wetting of liquids on surfaces

Wettability is a tendency for a liquid to spread on a solid substrate and is generally measured in terms of the angle (contact angle) between the tangent drawn at the triple point between the three phases (solid, liquid and vapour) and the substrate surface. A liquid spreading on a substrate with no reaction/absorption of the liquid by substrate material is known as non-reactive or inert wetting whereas the wetting process influenced by reaction between the spreading liquid and substrate material is known as reactive wetting. Young's equation gives the equilibrium contact angle in terms of interfacial tensions existing at the three-phase interface. The derivation of Young's equation is made under the assumptions of spreading of non-reactive liquid on an ideal (physically and chemically inert, smooth, homogeneous and rigid) solid, a condition that is rarely met in practical situations. Nevertheless Young's equation is the most fundamental starting point for understanding of the complex field of wetting. Reliable and reproducible measurements of contact angle from the experiments are important in order to analyze the wetting behaviour. Various methods have been developed over the years to evaluate wettability of a solid by a liquid. Among these, sessile drop and wetting balance techniques are versatile, popular and provide reliable data. Wetting is affected by large number of factors including liquid properties, substrate properties and system conditions. The effect of these factors on wettability is discussed. Thermodynamic treatment of wetting in inert systems is simple and based on free energy minimization where as that in reactive systems is quite complex. Surface energetics has to be considered while determining the driving force for spreading. Similar is the case of spreading kinetics. Inert systems follow definite flow pattern and in most cases a single function is sufficient to describe the whole kinetics. Theoretical models successfully describe the spreading in inert systems. However, it is difficult to determine the exact mechanism that controls the kinetics since reactive wetting is affected by a number of factors like interfacial reactions, diffusion of constituents, dissolution of the substrate, etc. The quantification of the effect of these interrelated factors on wettability would be useful to build a predictive model of wetting kinetics for reactive systems.     

Nucleation and growth mechanism for the electropolymerization of aniline in trifluoroacetic acid/lithium perchlorate/propylene carbonate medium

In the present work, the nucleation and growth mechanism for the electropolymerization of aniline in propylene carbonate medium containing 0.06 M trifluoroacetic acid and 0.05 M lithium perchlorate was investigated at different potentials on highly oriented pyrolytic graphite (HOPG) by potentiostatic current-time transients (i-t) and atomic force microscopic (AFM) measurements. The electrochemical data fitted with the theoretical curves for nucleation and growth suggest that the electropolymerization of aniline consists of progressive nucleation followed by 3D growth at an early stage and layer-by-layer growth in subsequent periods. The results obtained from transient analysis were in good agreement with the results of the AFM analysis. In our previous studies with aqueous solutions, we observed only progressive nucleation followed by a 3D growth mechanism for the electropolymerization of aniline in a higher potential range, 1.5–2.0 V vs. Ag/AgCl. Hence, the results obtained from the present work indicate that the nucleation and growth mechanism depends on the medium.     

Effects of diffusion-controlled association in kinetic model of crystal nucleation in external orienting field

Translational and rotational diffusion equation of orientable single elements in external orienting potential field is used to discuss effective rate of association and dissociation during cluster growth. First and second harmonic terms of orientation-dependent potential energy of single elements are taken into consideration. Tolerance angle accounting for orientational limitations for association is introduced.Effective rate of association of single elements is derived with first-order correction for the effects of orienting potential. Net rate of cluster growth depends on orientation in the field, and it is controlled by longrange diffusion (rotational and translational) of single elements. Influence of diffusion is higher, the narrower the tolerance angle and the faster the association.At slow association, long-range diffusion does not influence the process, the growth of a cluster is controlled by thermodynamic, orientation-dependent potential of the species involved in the process.Effective rate constants of association and dissociation, as well as effective concentration of single elements are derived as functions of orientation angle in the field.The Fokker-Planck equation is proposed for the distribution of cluster size in external orienting potential, with the effects of finite translational and rotational diffusion of single elements, for the case of non-polar clusters.     

碳纤维微晶结构的形成与生长

碳纤维的微晶结构是影响其性能的决定性因素,本论文采用广角X-射线衍射法研究了聚丙烯腈预氧纤维在碳化阶段中微晶结构的形成、生长与转变过程。研究结果表明:在1000℃以下,经预氧化反应形成的不连续多环芳香平面结构沿平行于纤维轴的方向堆积并逐渐靠近,形成类似于晶核的微晶结构生长中心,表现为类石墨片层间距的减小而晶粒增长缓慢;当温度高于1000℃时,晶粒的生长速度明显加快,形成三维有序的微晶结构;在温度高于1500℃的石墨化阶段,类石墨微晶结构进行重排,晶粒尺寸迅速增加。根据这一规律,可以通过控制晶核生成和晶粒生长速度的匹配,进行碳纤维的结构设计和调控。     

In situ studies of oxide nucleation,growth, and transformation using slow electrons

Surface processes such as metal oxidation and metal oxide growth invariably influence the physical and chemical properties of materials and determine their interaction with their surroundings and hence their functionality in many technical applications. On a fundamental level, these processes are found to be governed by a complex interplay of thermodynamic variables and kinetic constraints, resulting in a rich variety of material-specific phenomena. In this review article, we discuss recent results and insights on transition metal oxidation and rare-earth oxide growth acquired by low-energy electron microscopy and related techniques. We demonstrate that the use of in situ surface sensitive methods is a prerequisite to gaining a deeper understanding of the underlying concepts and the mechanisms responsible for the emerging oxide structure and morphology. Furthermore, examples will be provided on how structural and chemical modifications of the oxide films and nanostructures can be followed in real-time and analyzed in terms of local reactivity and cooperative effects relevant for heterogeneous model catalysis.     

Electrochemical nucleation of lead and copper on indium-tin oxide electrodes

The nucleation process of Pb and Cu onto indium-tin oxide electrodes was studied by single potentiostatic steps using 0.1×10^–4 mol L^–1 Cu(NO₃)₂ and 2×10^–2 mol L^–1 Pb(NO₃)₂ solutions in aqueous 1.0 mol L^–1 NaNO₃. The current, I_max, and the time, t_max, corresponding to the maximum, were evaluated by 3D nucleation with a diffusion-controlled growth model for instantaneous and progressive nucleation. The non-dimensional plots of (I/I_max)² vs. t/t_max showed that Cu and Pb nucleation closely followed the response predicted for instantaneous nucleation. Other parameters, such as I vs. t^1/2 and I²_maxt_max, as well as nuclei micrographs, were analyzed, corroborating the results obtained.     

Nucleation,growth, and fractal dimensions of icosahedral and polyicosahedral clusters

In this paper, the concept offractal geometry was applied to the growth of icosahedral and polyicosahedral clusters. Fractal dimensions were calculated for high-frequency icosahedral casters, vertex-sharing polyicosahedral clusters, and linked polyicosahedral clusters. These cluster growth pathways were compared with the fractal growth mechanisms of colloidal particles. Close similarities were found despite the tremendous differences in particle sizes and the forces governing their nucleation and growth processes.Dedicated to Prof. Lawrence F. Dahl on his 65th birthday; based in part on a lecture presented by BKT at the Dahl symposium held at the University of Wisconsin, Madison, Wisconsin on September 17, 1994.     

The kinetics of homogeneous and two-step nucleation during protein crystal growth from solution

Many experimental reports for the kinetics of crystal nucleation and growth, from an isothermal solution, point to a sigmoidal-like behavior for the process. Here we consider three different nucleation models from the literature and show that all lead to sigmoidal or sigmoidal-like behavior for the kinetics of nucleation. A two-step nucleation process is known to occur within certain supersaturated protein solutions, and it is demonstrated in this report how the sigmoidal law yields kinetic information for the two-step and homogeneous nucleation modes. We propose here that two-step solute-rich associates form in the solution around seed nuclei that are already present at or near the point in time when the solution is prepared. Using this hypothesis, we are able to model the time-dependent volume of the two-step phase per unit volume of solution and show that this compares well with reported experimental data. A kinetic model is given for the proposed process, which leads to a sigmoidal rate law. Additionally, a relation between the initial and final nuclei densities and the induction time is derived. As a result of this study, the conclusion is that two-step activity increases with increasing initial supersaturation or increasing salt concentration.     

聚苯胺的成核及生长机理

本文通过恒电位阶跃法研究了聚苯胺在不同介质中的成核与膜的生长过程动力学。结果表明, 在硫酸介质中, 成核过程为扩散控制下的三维连续成核, 得到疏松、多孔的膜; 而在高氯酸介质中, 成核则是电化学动力学控制下的二维成核过程。在高电位时(E>1.02V, vs, SCE)为二维连续成核过程, 而在较低的电位时, 主要表现为二维瞬时成核, 膜层呈网状且致密。     

The Kinetics of Crystal Growth

The fundamentals of the theory of crystal growth comprise the two concepts of growth via two-dimensional nuclei and growth with the aid of dislocations. Measurements of the relationship between rate of growth and supersaturation are particularly suitable for testing theoretical predictions. Experimental results are compared with predictions for crystal growth from the vapor phase, from solution, and from the melt. Depending on the experimental conditions, examples of both types of growth are met irrespective of the nature of the mother phase.