Crystal Growth Kinetics of Nanocrystalline ZnS under Surface Adsorption

The crystal growth mechanism,kinetics,and microstructure development play a fundamental role in tailoring the materials with controllable size and morphology. In this study,by introducing the strong surface adsorption of the concentrated NaOH,two-stage crystal growth kinetics of ZnS nanoparticles was observed. In the first stage,the primary particles grow into a size over a hundred times of the original volume and the growth is controlled by the crystal-lographically specific oriented attachment. The first stage data were fitted by the "multistep OA kinetic model" built based on the molecular collision and reaction. In the second stage,following the dispersal of nanoparticles,an abrupt transition from asymptotic to parabola growth kinetics occurs,which can be fitted by a standard Ostwald ripening volume diffusion model. The presence of surface adsorption causes the two-stage growth kinetics and permits an almost exclusive OA-based growth to dominate in the first stage.     

NaOH concentration effect on the oriented attachment growth kinetics of ZnS

In this work, the crystal growth kinetics of ZnS nanoparticles coarsened under 100 degrees C with NaOH concentration from 2 to 8 M was investigated, aiming to study the role of NaOH concentration on the oriented attachment growth kinetics. It reveals that 2 M NaOH is sufficient to lead to two-stage growth kinetics of ZnS nanoparticles, resulting in pure and multistep oriented attachment growth characteristics in the first stage. When the NaOH concentration increases, the rate of crystal growth by oriented attachment mechanism increases, while the time period for crystal growth at the pure oriented attachment stage was similar. We suggest that the concentration of solute is critical to enhance the oriented attachment growth rate and achieve exclusively oriented attachment growth of nanoparticles at a large size scale.     

Oriented attachment kinetics for ligand capped nanocrystals: coarsening of thiol-PbS nanoparticles

In this work, the growth kinetics of thiol-capped PbS nanoparticles was studied. Two-stage growth process was observed, which was controlled first by oriented attachment (OA) mechanism and then by the hybrid Ostwald ripening (OR) and OA mechanism. Different from the NaOH-ZnS system, where OA will occur between any two multilevel nanoparticles, an OA kinetic model only considering the attachment related to original particles was fitted well with the experimental results. Analysis reveals that this model may be a universal one to describe the OA crystal growth process of nanocrystals capped with easily destroyed ligands, such as thiol-ZnS in the previous report. The OA crystal growth characteristics determined by the surface agent were discussed and compared. We propose that with stronger surface capping, the OR growth of nanocrystals is hindered, which facilitates the size controlling via OA kinetics during nanosynthesis.     

纳米晶取向接合生长动力学研究及其在量子点发光性质调控中的应用

对晶体生长机制、动力学与微结构衍化的认识是实现纳米材料的尺寸和形貌可控制备的基础．以表面溶解沉积为特征的奥斯特瓦尔德熟化（0R）理论常用来解释传统的晶体生长过程．在该生长模式下,纳米晶体的生长呈现出小颗粒溶解而大颗粒逐渐长大的特征．在纳米材料体系,近来还发现了一种重要的新的晶体生长模式——“取向接合（OA）”机制,在该机制下,两个晶格取向一致的初级纳米颗粒可通过直接接合和结构调整,从而长成一个新的晶体．这一机制已被证实在许多纳米材料体系中广泛存在,并对所合成的纳米材料的形貌、微结构具有非常显著的影响,在构筑新型纳米结构方面具有潜在的优势．本文我们首先回顾了OA生长机制的认识历程和这一机制对材料科学的重大意义;进而,基于我们的研究工作系统介绍了OA生长动力学模型的建立与发展,进一步阐述了这一机制的微观过程及其对材料内部缺陷的特殊影响,深入地分析和讨论了表面包裹的强弱、表面作用的性质对OR机制和OA机制的抑制和调控作用;基于上述表面包裹可调控纳米材料的生长机制的认识,我们结合近期研究结果,从动力学角度分析了量子点的生长机制与其发光特性的内在关联,阐明了表面包裹调控量子点的发光性质的本质原因,为制备不同发光特征的量子点及理解其发光性质衍化规律提供了重要的理论指引．     

In situ kinetic study of zinc sulfide activation using a quartz crystal microbalance with dissipation (QCM-D)

We have studied the activation kinetics of zinc sulfide (ZnS) using silver as an activator by a quartz crystal microbalance with dissipation (QCM-D). The zinc sulfide coating on QCM-D sensor was shown to have similar crystallographic structure, composition, and surface properties as nature sphalerite through the characterization of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and xanthate adsorption measurement using QCM-D. The activation of ZnS sensor by silver was confirmed by the mass increase in ZnS sensor coupled with subsequent xanthate adsorption during QCM-D measurement, the change of surface wettability, and the presence of Ag(2)S on the surface. Two distinct stages on the silver uptake vs. time curve were identified and fitted well by a logarithmic function for the initial stage and a parabolic law in the later stage, which agrees with the two-stage zinc-silver reaction kinetics reported previously. Argon sputtering followed by XPS measurement on the ZnS surface demonstrated the penetration of silver into the bulk ZnS after activation. The present study is the first of its kind to apply the QCM-D technique to investigate sphalerite activation, which introduces a new in situ approach to investigate surface adsorption and activation in many mineral processes and surface modifications.     

The Kinetics of Analcime Synthesis in Alkaline Solution

In this study glass has been used as a starting material in order to increase the resolution of the experiment and establish the kinetics of analcimes during hydrothermal synthesis. The experimental results show that the crystal growth curves represent two‐stage linear growth. The first stage is marked by rapid crystal growth whilst the second stage indicates much slower growth. The nuclei appear before the crystals start growing. The crystal growth rate is dependent on the experimental conditions. The nucleation rate increases as the glass dissolves and reaches a maximum when the glass is completely dissolved. Thereafter, the concentration decreases with the consumption of nutrients, and both nucleation and crystal growth stops when supersaturation occurs.     

ZnS纳米球的水热法制备及其光催化性能研究

在表面活性剂十六烷基三甲基溴化铵(CTAB)的辅助下,以乙酸锌为锌源,硫脲(NH2)2CS为硫源,使用水热法通过改变反应时间,成功制备了不同粒径的ZnS球状颗粒。利用X射线衍射(XRD)、扫描电子显微镜(SEM)、X-射线能谱,高分辨透射电子显微镜(HRTEM))、紫外可见分光光谱和光致发光谱(PL)等测试手段对样品的晶体结构、形貌、光学性质进行了分析。通过对不同粒径的ZnS纳米颗粒对亚甲基蓝的光催化降解的催化活性进行了评估。实验结果表明:在表面活性剂CTAB的作用下,随着反应时间的增加,生成的ZnS晶核生长成纳米颗粒,然后ZnS纳米颗粒将进一步发生团聚从而形成平均粒径超过500nm的ZnS纳米球,但制备的ZnS产物的晶体结构均为立方纤锌矿结构。随着ZnS粒径的增加,样品的紫外吸收峰从418nm逐渐蓝移到362nm,而PL发射峰位的峰强随着粒径的增大而增强。光催化结果显示,反应12h制备的ZnS纳米球的光催化性能最佳。     

ZnS纳米球的水热法制备及其光催化性能研究

在表面活性剂十六烷基三甲基溴化铵(CTAB)的辅助下,以乙酸锌为锌源,硫脲(NH₂)₂CS为硫源,使用水热法通过改变反应时间,成功制备了不同粒径的ZnS球状颗粒.利用X射线衍射(XRD)、扫描电子显微镜(SEM)、X射线能谱,高分辨透射电子显微镜(HRTEM))、紫外可见分光光谱和光致发光谱(PL)等测试手段对样品的晶体结构、形貌、光学性质进行了分析.通过对不同粒径的ZnS纳米颗粒对亚甲基蓝的光催化降解的催化活性进行了评估.实验结果表明:在表面活性剂CTAB的作用下,随着反应时间的增加,生成的ZnS晶核生长成纳米颗粒,然后ZnS纳米颗粒将进一步发生团聚从而形成平均粒径超过500nm的ZnS纳米球,但制备的ZnS产物的晶体结构均为立方纤锌矿结构.随着ZnS粒径的增加,样品的紫外吸收峰从418nm逐渐蓝移到362nm,而PL发射峰位的峰强随着粒径的增大而增强.光催化结果显示,反应12h制备的ZnS纳米球的光催化性能最佳.     

Influence of the reactant concentrations on the synthesis of ZnO nanoparticles

We report on the synthesis of ZnO nanoparticles from Zn(CH3CO2)2 and NaOH in 2-propanol. Nucleation and growth are fast, and hence at longer times the particle size is controlled by coarsening. The coarsening kinetics are independent of the Zn(CH3CO2)2 concentration between 0.5 and 1.25 mM at a fixed [Zn(CH3CO2)2]:[NaOH] ratio of 0.625. The width of the size distribution was found to increase only slightly with aging time. In addition, at a fixed Zn(CH3CO2)2 concentration of 1 mM, the kinetics are independent of the [Zn(CH3CO2)2]:[NaOH] ratio between 0.476 and 0.625. The presence of water in the reaction mixture was found to only slightly affect the coarsening kinetics for water contents larger than about 20 mM. For lower water concentrations, the nucleation and growth of ZnO were very slow. It can be concluded that the synthesis method described provides a reliable source of ZnO nanoparticles due to its insensitivity to the reactant concentrations and the presence of water.     

Mechanism and modeling of nanorod formation from nanodots

A population balance model based on Smoluchowski aggregation kinetics is developed to explain the formation of nanorods from a colloidal suspension of spherical nanoparticles (nanodots). Our model shows that linear pearl-chain aggregates form by the oriented attachment (OA) of nanodots during the early stages of synthesis, since it occurs with a time scale smaller than the coalescence time scale of nanodots present within an aggregate. The slower coalescence step leads to the transformation of the linear pearl-chain aggregate into a smooth nanorod over a longer time scale of many hours, as observed in experiments. The attachment kinetics is modeled by a modified Brownian collision frequency, with the latter decreasing with nanorod length, leading to the experimentally observed slower growth in nanorod length at longer times. The collision frequency also includes the effects of attractive dipole-dipole and van der Waals interactions between nanodots, which are primarily responsible for OA. Our model predictions are general, and they compare favorably with available experimental data in the literature on the distribution of the aspect ratio (length to diameter) of ZnO and ZnS nanorods over different time scales.     

Early stages of ZnS growth studied by stopped-flow UV absorption spectroscopy: effects of educt concentrations on the nanoparticle formation

The growth of ZnS nanoparticles by precipitation from supersaturated aqueous solution is studied by stopped-flow UV absorption spectroscopy. The average size, size distribution, and concentration of the particles are monitored within the sub-second time regime with a resolution of 1.28 ms. Particle growth at these early stages is governed by pronounced ripening. The UV absorption data strongly suggest that growth occurs by preferential adsorption of HS- anions relative to Zn(2+) or ZnOH(+) cations. Correspondingly, the initial sulfide concentration has a much more pronounced influence on the growth kinetics than the initial zinc concentration. These findings are verified by zeta-potential measurements which confirm that the particle surfaces are negatively charged under near-neutral pH conditions.     

Synthesis and Optical Properties of Mn2+-Doped ZnS Nanoparticles in Solutions and Coatings

Mn2+-doped ZnS nanoparticles with different Mn-doping concentrations stabilized by hydroxypropyl cellulose (HPC) have been synthesized in ethanolic solutions and coatings. Their optical and structural properties have been characterized by means of UV-vis spectroscopy, luminescence spectroscopy, high resolution transmission electron microscopy (HRTEM) and small angle X-ray scattering (SAXS). Solutions and coatings exhibit a strong luminescence at 590 nm when excited with UV light showing that Mn2+ is incorporated into the ZnS nanoparticles. The highest luminescence intensity is obtained with an Mn2+ concentration of 2 mol%. HRTEM and SAXS investigations show that the particles are crystalline and are 3 ± 0.5 nm in size. Irradiation of the coatings with UV light leads to a photochemical oxidation of the particles, as shown by the decreasing absorption of the coating with irradiation time and a blue shift of the absorption maximum. Furthermore, the luminescence intensity first strongly increases and then decreases completely with UV-irradiation time. Both phenomena can be explained by the photochemical oxidation of the particles.     

棒状氢氧化镁的合成

以氯化镁和氢氧化钠为原料合成了棒状氢氧化镁粉体。考察了氢氧化钠浓度、陈化温度、陈化时间对氢氧化镁形貌的影响。采用SEM、HRTEM、XRD、TG等对所得氢氧化镁颗粒的形貌、颗粒大小、晶习及热稳定性进行了表征。结果表明,当陈化温度为90℃时,所得棒状氢氧化镁随氢氧化钠浓度的升高及陈化时间的增加,晶形更加完整;当陈化温度在120℃时,所得氢氧化镁颗粒的形貌为片状,不能得到棒状氢氧化镁。     

大长径比ZnS纳米线的制备、结构和生长机理

通过碳热辅助化学气相沉积法, 以Au作为催化剂, 在较低温度(800 益)制备了ZnS纳米线, 其尺寸均匀, 表面光滑, 直径约为40 nm, 具有很大的长径比, 是典型的单晶纤锌矿六方结构. 高分辨透射电镜和选区电子衍射分析表明, 纳米线的生长方向为[1100], 与已报道的生长方向不同. 纳米线的生长是由气-液-固(vapor-liquid-solid)机理控制的.     

Size-dependent phase transformation kinetics in nanocrystalline ZnS

Nanocrystalline ZnS was coarsened under hydrothermal conditions to investigate the effect of particle size on phase transformation kinetics. Although bulk wurtzite is metastable relative to sphalerite below 1020 degrees C at low pressure, sphalerite transforms to wurtzite at 225 degrees C in the hydrothermal experiments. This indicates that nanocrystalline wurtzite is stable at low temperature. High-resolution transmission electron microscope data indicate there are no pure wurtzite particles in the coarsened samples and that wurtzite only grows on the surface of coarsened sphalerite particles. Crystal growth of wurtzite stops when the diameter of the sphalerite-wurtzite interface reaches approximately 22 nm. We infer that crystal growth of wurtzite is kinetically controlled by the radius of the sphalerite-wurtzite interface. A new phase transformation kinetic model based on collective movement of atoms across the interface is developed to explain the experimental results.     

Synthesis of ZnO nanoparticles in 2-propanol by reaction with water

We report on the synthesis of ZnO particles from Zn(CH(3)CO(2))(2) in 2-propanol as a function of the concentration of water, in the absence of a base such as NaOH. Particles with diameters of 3-5 nm are formed depending on time, temperature, and water concentration. The nucleation and growth are slower than in the presence of NaOH, and at longer times the increase in particle size is dominated by diffusion-limited coarsening. The rate constant for coarsening increases with increasing water concentration up to 150 mM, above which the rate constant is 1.1 x 10(-4) cm(3) s(-1), independent of the water concentration. The width of the particle size distribution decreases with increasing water concentration, and at 250 mM water, the full width at half-maximum of the distribution function is essentially the same as for the synthesis of ZnO using NaOH as a reactant. The temperature dependence of coarsening is determined by the bulk solubility of the ZnO nanoparticles and yields an apparent activation energy of 1.12 eV. This is significantly larger than the activation energy of 0.35 eV for coarsening of ZnO from 1 mM Zn(CH(3)CO(2))(2) in 2-propanol with 1.6 mM NaOH.     

Atomic resolution electron microscopy of small metal clusters

Atomic resolution imaging of cluster structures has been performed with high resolution transmission electron microscopy (HRTEM). Metal particles of the sizes 1 nanometer to tens of nanometers have been surface profile imaged on different supports; like zeolites, cordierite and amorphous carbon. It is shown that organic ligands in Schmid-clusters coordinated to the metal surface are desorbed or destroyed by the electron beam. Dynamic events on the surfaces and in the bulk of small metal particles have been recorded for small crystals of Au, Pt, Rh and Pb and can be classified under three headings; The smaller the crystals are the faster rearrangements of the crystal structure; “clouds” of atoms existing outside some surfaces are involved in extensive structural rearrangements of the surface or crystal surface growth; localized atom hopping on surfaces during crystal growth and desorption also occurs.     

The influence of Al(III) supersaturation and NaOH concentration on the rate of crystallization of Al(OH)3 precursor particles from sodium aluminate solutions

The growth kinetics of colloidal Al(III)-containing particles (diameter<1000 nm), nucleated in optically clear, supersaturated sodium aluminate solutions as a precursor to Al(OH)(3) crystals, has been studied using dynamic light scattering. Two series of solutions were examined at 22 degrees C to determine the influence of Al(III) supersaturation and NaOH concentration on the initial particle growth behavior. One solution series consisted of solutions with constant Al(III) absolute supersaturation (DeltaC) of 1.48 M and [NaOH] range 1.83-4.00 M ([NaOH]/[Al(III)]=1.13-2.15) and Al(III) relative supersaturation (sigma)=3.86-10.36. The other solution series had a constant sigma of 7.55 and [NaOH] range of 1.50-4.27 M ([NaOH]/[Al(III)]=1.18-1.54) and DeltaC=0.86-3.19. The correlation between the initial particle growth rates and supersaturation (DeltaC or sigma) revealed marked anomalies over the entire supersaturation range studied. The growth rate remained substantially constant in the DeltaC range 0.86-2.55 M (for the constant sigma solution series), before increasing sharply upon a further increase of DeltaC beyond 2.55 M. The variation of the growth rate with sigma in the range 3.86-9.00 (for the constant DeltaC solution series) was remarkably weak, contrary to expectation. At higher sigma (>9.00), however, a marked increase in growth rate with increasing sigma was displayed. At constant DeltaC or sigma, the growth rate showed a strong variation with NaOH concentration, indicating that Na(+) and OH(-) species play a pivotal role in the Al(OH) precursor particles (nuclei) growth process. Furthermore, the kinetics of growth displayed by these nanosized particles are an order of magnitude slower than those observed for macroscopic gibbsite (gamma-Al(OH)(3)) crystals at similar supersaturations and temperature. The difference may be rationalized in terms of particle size and Al(OH)(3) dimorphic phase dependent solubility effects. An empirically adequate growth kinetics modeling was achieved when the growth rates were correlated with the Al(III) supersaturation (DeltaC or sigma) and the excess (free) NaOH concentration, rather than the former alone, as is commonly the case. A critical [NaOH]/[Al(III)] molar ratio of 1.27-1.35, below which the particle growth rate increased markedly and above which the rate was significantly reduced, was observed. This behavior is believed to be linked to solution speciation change that occurs at certain Al(III) and NaOH compositions.     

The growth kinetics of colloidal ZnO nanoparticles in alcohols

We have studied the synthesis of ZnO nanostructures over a wide range of parameters to determine the kinetics of the nanocrystals growth. The initial rapid nucleation and growth is kinetically controlled, the subsequent ZnO nanocrystals growth is thermodynamically controlled through the diffusion limited Ostwald coarsening. The ZnO coarsening rates increased with number of alcohol’s alkyl group carbons and temperature increase, pointing to importance of the solvent viscosity, dielectric constants, surface energy and the bulk solubility. The results are consistent with the Lifshitz–Slyozov–Wagner model. For all alcohols, in the NaOH induced reaction, a lower activation energy was observed compared to the aqueous reaction. A lower ZnO solubility, obtained by the water synthesis could be responsible for these observations. Our results point to the importance of the reactant selection in controlling the kinetics of the nanostructure formation, their size and the nature of the surface defects responsible for their luminescence.     

Controlling oriented aggregation using increasing reagent concentrations and trihalo acetic acid surfactants

Zinc oxide particle growth from homogenous solutions prepared with isopropyl alcohol was monitored using in situ UV–vis spectroscopy, and results show that the rate of ZnO particle growth and the final ZnO nanoparticle size depend strongly upon the concentrations of precursors and the identity of surfactants used. In addition, particle size versus time data was fit using the coarsening model and the simultaneous oriented aggregation and coarsening model in order to evaluate the effect of changing synthetic variables on the mechanism of nanoparticle growth. In general, an increase in growth by oriented aggregation with increasing precursor concentrations was observed, a result that was consistent with results from high-resolution transmission electron microscopy (HRTEM) characterization. The increase in precursor concentrations resulted in an increase in the number concentration of ZnO nanoparticles, which resulted in a higher probability of particle–particle interactions and hence increased growth by oriented aggregation. Additionally, particle growth in solutions of trifluoro-, trichloro-, and tribromoacetate surfactants was studied, and growth by oriented aggregation followed the trend expected based on the number concentration of zinc oxide particles. Growth with trifluoroacetate was an exception, with growth by oriented aggregation substantially suppressed.