Reduction In Interfacial Energy Research Articles

Microstructural evolution and spheroidization mechanism of powder metallurgy Ti-6Al-4 V alloys after high-temperature forging

After high-temperature forging and rapidly cooling, the microstructure evolution and spheroidization mechanism of powder metallurgy Ti-6Al-4 V alloy have been clarified. The spheroidization process is divided into two asynchronous stages. The initial stage is primarily characterized by the initial shearing and twisting of GBα grain boundaries, as well as the deformation of lamellar structure. Subsequently, the fragmentation of original GBα grain boundaries and the fracture of intra-granular α lamellae are observed. This process is accompanied by grain twisting and substructure formation, especially the division of lamellae through interface splitting and the segregation of local chemical elements. The differential accumulation of elements around the fractured and twisted grains suggests that element segregation and interfacial energy play a significant role in microstructure evolution. Therefore, the static spheroidization process largely depends on the formation and evolution of dislocation substructures, and the reduction of interfacial energy may be a key factor driving this process. These findings provide a theoretical basis for manufacturing metals with specific anisotropic properties by controlling microstructure evolution.

Wettability and microstructural evolution of copper filler in W and EUROFER brazed joints

In terms of wettability, active systems are characterized by a reduction in interfacial energy as the time at specific conditions is increased. This article aims to investigate the evolution of wettability and microstructure, which undergoes a critical transformation at temperatures and dwell times near brazing conditions due to their significant impact on resultant mechanical properties. The objective is to enhance wettability and prevent the formation of different phases that can occur rapidly within the brazing window conditions. Up to 1105 °C, complete fusion of the filler does not occur. However, once it happens, the expansion of the copper filler in EUROFER increases up to 400%, and the contact angle reduces from 100° to 10°, indicating an active wetting behavior. On the other hand, when copper is used with tungsten, an inert behavior is observed, maintaining the contact angle around 70°. Brazed joints carried out under the most promising wetting conditions demonstrated that at 1110 °C-1 min, various phenomena began to occur. This includes solid-state diffusion of copper in the EUROFER, following the austenitic grain boundaries, and partial dissolution of Fe in the copper braze. Increasing the brazing time from 2 to 5 min achieved high interfacial adhesion properties and controlled the diffusion layer and Fe-rich band formed at the W-braze interface, resulting in the best mechanical results (295 MPa).

First-principles study of Re-W interactions and their effects on the mechanical properties of γ/γ′ interface in Ni-based single-crystal alloys

The distribution of solutes and their interactions play a crucial role in determining the mechanical properties of the γ/γ′ interface in Ni-based single-crystal alloys. In this study, atomic interactions between Re and W and their alloying effects on the inter-phase cohesion of the γ/γ′ interface are investigated by first-principles calculations. Our results show that W atom exhibits a preference for partitioning into the γ phase, while the stability of the γ/γ′ interface can be enhanced due to the partitioning of W to the γ′ phase. Moreover, our results reveal that partitioned W atoms in the γ′ phase contribute to the strengthening of the γ/γ′ interface. Conversely, the dissolution of W atoms in the γ phase weakens the inter-phase cohesion. However, this detrimental effect can be mitigated by introducing of Re into the γ/γ′ interface. Partitioning of Re and W into separate phases yields minimal alterations in interaction energies, resulting in a notable enhancement of inter-phase cohesion when compared to the partitioning of Re and W within γ phase of the γ/γ′ interface. Additionally, the partitioning of solute atoms at the γ/γ′ interface leads to local lattice distortion and interfacial energy reduction, which contribute to the enhancement of inter-phase cohesion of the γ/γ′ interface. As a result, a model is proposed for interpretation of crack propagation at the γ/γ′ interface at the threshold region with the presence of tensile stress in Ni-based single-crystal alloys.

Revealing the role of liquid preordering in crystallisation of supercooled liquids

The recent discovery of non-classical crystal nucleation pathways has revealed the role of fluctuations in the liquid structural order, not considered in classical nucleation theory. On the other hand, classical crystal growth theory states that crystal growth is independent of interfacial energy, but this is questionable. Here we elucidate the role of liquid structural ordering in crystal nucleation and growth using computer simulations of supercooled liquids. We find that suppressing the crystal-like structural order in the supercooled liquid through a new order-killing strategy can reduce the crystallisation rate by several orders of magnitude. This indicates that crystal-like liquid preordering and the associated interfacial energy reduction play an essential role in nucleation and growth processes, forcing critical modifications of the classical crystal growth theory. Furthermore, we evaluate the importance of this additional factor for different types of liquids. These findings shed new light on the fundamental understanding of crystal growth kinetics.

Nano-treating promoted solute dissolution for novel high strength Al-Cu-Mg alloys

High-strength Al-Cu-Mg alloys are normally designed with a low Mg content because of their tendency towards solidification cracking and a limited solubility of Cu when the Mg content exceeds 2 wt.%. Nano-treating is an emerging metallurgical method to improve the manufacturability and properties of alloys by adding a small volume fraction of ceramic nanoparticles. In this study, approximately 1.0 vol.% of TiC nanoparticles was added to cast Al-4.6Cu-xMg-0.3Mn alloys with a high Mg content ranging from 1.5 wt.% to 4.0 wt.%. Nanoparticles not only eliminated shrinkage porosities during casting but also significantly improved the dissolution of secondary phases during solution treatment of the alloys. Moreover, it was discovered that TiC nanoparticles promoted an unusual solute dissolution behavior to break the thermodynamic solubility limit of Mg in the Al-4.6Cu-xMg-0.3Mn system to achieve high mechanical performance. Theoretical analysis and TEM suggest that the promoted solute dissolution was driven by an overall reduction in interfacial energy between nanoparticles and the S phase during solutionization. The density of primary S’ precipitates was significantly increased by nano-treating, as an unusually high supersaturation was achieved in the nano-treated samples.

Chromian spinel neomineralisations and the microstructure of plastically deformed ophiolitic peridotites (Kraka massifs, Southern Urals, Russia)

Results of a microstructural study of spinel peridotite samples obtained from the Kraka massif in the Southern Urals, involving findings of Cr-spinel neomineralisations within intensive ductile deformed silicates (olivine and orthopyroxene), are presented. The new-formed Cr-spinel grains show different stages of syn-deformation growth as evidenced by investigations combining petrography, decorated dislocation structure analysis, scanning electron microscopy and electron backscatter diffraction (EBSD). Initial precipitations appearing as rods or lamellae are observed to form around structural defects of host silicate grains (olivine and orthopyroxene) by means of impurity segregation or heterogeneous nucleation in the most distorted lattice regions (i.e., in the predominant recrystallisation zones). Syn-deformational crystal growth leads to a complication and coarsening of the grain morphology by coalescence due to a reduction in grain boundary (interfacial) energy. While in the process of growing, the Cr-spinel grains capture fragments of silicate matrix in the solid-state process. The final stage of Cr-spinel growth involves a change in morphology resulting in their characteristic crystallographic forms (spheroidisation). The presence of euhedral Cr-spinel grains, typical for ophiolitic dunite bodies, is a result of interfacial energy reduction in areas of grain boundaries of the hardest phase. The general trend of the observed stages relates closely with the localisation of deformation zones in the upwelling upper mantle (diapir), which are composed by the weakest phase of olivine (dunites). The concentration of the weakest olivine phase in the mobile zones, which is energetically beneficial, explains why dunite bodies having euhedral chromite grains comprise the dynamic equilibrium rocks in the plastic flow localisation zones in upper mantle diapirs. Conversely, assemblages having pyroxene phases, which are stronger and larger in size compared to olivine, are not stable in these zones.

Li2ZnTi3O8@α-Fe2O3 composite anode material for Li-ion batteries

Li2ZnTi3O8@α-Fe2O3 composites have been successfully prepared by a facile hydrothermal process. Li2ZnTi3O8/α-Fe2O3 composites show similar irregular spherical morphologies like Li2ZnTi3O8 and relatively smaller particle sizes than pristine Li2ZnTi3O8. Among all Li2ZnTi3O8/α-Fe2O3 composites, Li2ZnTi3O8/α-Fe2O3 composite (5 wt%) exhibits the best electrochemical properties. Li2ZnTi3O8/α-Fe2O3 composite (5 wt%) delivers a reversible charge capacity of 184.8 mAh g−1 even at 1000 mA g−1 after 500 cycles, while pristine Li2ZnTi3O8 only delivers a reversible charge capacity of 110.7 mAh g−1. The strong covalent bonds between Li2ZnTi3O8 and α-Fe2O3 will be formed, which is beneficial for the reduction of interfacial energy and thus helpful for the stabilization of the composite. Because of the special synergistic effect of the multi-phase interface, Li2ZnTi3O8/α-Fe2O3 composites not only possess the advantages of single components but also show novel and attractive performances, such as the enhanced ionic conductivity, reduced interfacial charge transfer impedance, improved migration rate of lithium ions, and the enhancement of the rate performance and reversible capacity. The as-prepared Li2ZnTi3O8/α-Fe2O3 composites reveal important potentials as anode materials for next-generation rechargeable Li-ion batteries, and this work also offers an effective strategy to design high performance lithium storage materials for advanced lithium-ion batteries.

Thermodynamics of CO2 adsorption on cellulose‐derived biochar prepared in ionic liquid

AbstractThis work focused on thermodynamic analysis of carbon dioxide (CO2) adsorption on a promising biochar as a CO2 adsorbent. The biochar was prepared by catalytic pyrolysis of cellulose material in ionic liquid at moderate temperature. The adsorption characteristics, such as adsorption capacity, interfacial potential, Gibbs free energy change, enthalpy change, entropy change, and internal energy change, influenced by adsorption temperature and gas pressure, were systematically investigated. The results indicated that CO2 adsorption on cellulose‐derived biochar was a spontaneous, physical, exothermic, and entropic decrement process, accompanied by adsorption capacity of 5.2 mmol/g and interfacial potential of −18.2 J/g at 273 K and 100 kPa. The process could be well described by adsorption potential theory. Then a quasi‐Gaussian distribution of site energy was verified for CO2 adsorption. The interfacial potential was found to be a monotropic function of the amount of CO2 adsorbed, and the latter was actually a differential of the former via adsorption potential. The positive temperature effect and negative pressure effect on negative Gibbs free energy change indicated that reducing adsorption temperature and increasing gas pressure were beneficial to CO2 uptake, accompanied by the increase of adsorption capacity and the reduction of interfacial energy, entropy, enthalpy, and internal energy. The strongest temperature effects on entropy change, enthalpy change, and internal energy change existed at given pressure or temperature. The pressure effect was stronger and more sensitive to pressure at lower adsorption pressure. More interestingly, the peak pressure or peak temperature with the strongest pressure effect possibly existed during CO2 adsorption.

Coupled segregation mechanisms of Sc, Zr and Mn at [formula omitted] interfaces enhances the strength and thermal stability of Al-Cu alloys

The refinement and thermal stability of intermediate theta-prime (θ′) precipitates are critical in the development of new high strength 2xxx series aluminium-copper (Al-Cu) alloys for high temperature applications. In this work, we use trace additions of Sc, Zr and Mn in an Al-6.5 wt.% Cu alloy to refine and stabilise the θ′ precipitates. The formation of Al3(Sc, Zr) core/shell dispersoids significantly refine the θ′ precipitates by acting as preferential nucleation sites during artificial ageing. Adding Mn results in a significant increase of hardness during ageing at 190 °C. Hardness is maintained during thermal exposure at 280 °C for up to 24 h. Transmission electron microscopy (TEM) reveals that the addition of Mn leads to a finer and denser distribution of θ′ precipitates, and greatly slows the growth and coarsening of the θ′ precipitates at elevated temperatures. Differential scanning calorimetry (DSC) shows that this can be attributed to an enhanced nucleation and improved coarsening resistance of the θ′ precipitates in the presence of Mn. Atom probe tomography (APT) reveals that the enhanced age-hardening kinetics and thermal stability arise from the independent segregation mechanisms of Mn, Sc and Zr at the semi-coherent and coherent interfaces of the θ′ precipitates. The segregation is quantified by calculating the Gibbsian interfacial excess and corresponding reduction in interfacial energy. These calculations reveal that while Sc and Zr play a significant role in the refinement of the θ′ precipitates, Mn not only refines the θ′ precipitates, but also greatly enhances their coarsening resistance and corresponding alloy's thermal stability.

Nanoscale segregation mechanism of cation dopant at the matrix/oxide interface in oxide dispersion-strengthened alloys

The current study has demonstrated that cation dopant segregation at matrix/oxide interface opened up a new route to refine and disperse secondary oxide particles in oxide dispersion-strengthened (ODS) alloys. Thus, a unified theory that explains the physical origins of this interfacial segregation phenomenon is needed for designing ODS alloys with excellent oxide dispersity and ensuing high performance. Here, taking W–Y2O3 system for example, we firstly assess the possible driving forces for cation dopant interfacial segregation based on the experimental observation from Sc3+-, La3+-, Ti4+-, Zr4+- and Hf4+-doped W–Y2O3 alloys. It was suggested that elastic energy, oxygen chemical potential gradient and interfacial energy reduction are three main driving forces for the cation dopant segregation at W/Y2O3 interface. Then, an analytical model was developed in this work to quantitatively calculate the contributions of these three factors to the total segregation energy. Finally, the coupled results are further validated with the density functional theory (DFT)-calculated total segregation energy, and the good consistency confirms again the underlying mechanism behind cation dopant segregation phenomenon in W-based ODS alloy. On this basis, it can be predicted that a chemically expanded lattice and a large oxygen affinity will promote dopant interfacial segregation and enable the microstructure of ODS alloys to be tailored desirably. More importantly, the results and analytical model in our work can provide theoretical guidance for choosing proper cation dopant for other ODS alloys and then enhancing their strength and ductility simultaneously. Besides, the high-temperature instability of secondary oxide particles under extreme working environment also can be solved easily using this method.

Evolution of Thin-Liquid Films Surrounding Bubbles in Microfluidics and Their Impact on the Pressure Drop and Fluid Movement.

The evolution of thin-liquid films in a microchannel is one of the most critical and intricate phenomena to understand two-phase movement, evaporation, micromixing, heat transfer, chemical synthesis, biological processes, and efficient energy devices. In this paper, we demonstrate experimentally the effect of a liquid film on the removal of an initially dry and lodged bubble in laser-etched poly(methyl methacrylate) microfluidic networks and discuss the evolution of the liquid film in accordance with the bubble superficial velocity and the effect of liquid properties and branch angle on the evolution of the liquid film and the pressure drop. During the removal of a dry bubble, four stages have been observed in the bubble velocity profile and they directly relate to the evolution of the liquid film. The correlation of maximum bubble velocity has been derived as a function of bubble length, fluid viscosity, surface tension, geometry of the cross-sectional area, and dimensions of the microchannel and agrees with the experimental results. The bubble moving distance required for the full deposition of a continuous and stable thin-liquid film is affected by the liquid viscosity and network branch angle. The liquid with a higher viscosity will increase the pressure drop for removing dry bubbles from microfluidic networks, while this effect will be hampered by increasing the microfluidic network complexity. The deposition of the thin-liquid film surrounding bubbles significantly decreases the pressure drop required to remove bubbles from microfluidics. Compared with deionized water, the glycerol solution is prone to acting as the lubricating liquid due to its strong H-bond interaction with the channel wall and the reduction in interfacial energy of the gas-water interface.

Growth behavior and strengthening mechanism of Cu-rich particles in sanicro 25 austenitic heat-resistant steel after aging at 973 K

Precipitation and growth behaviors of Cu-rich particles in Sanicro 25 austenitic heat-resistant steel after aging at 973 K were systematically investigated by using transmission electron microscope (TEM) and three-dimensional atom probe (3DAP), and the strengthening mechanism of Cu-rich particles was discussed. The results show that many fine and well-dispersed Cu-rich particles precipitate at the early stage of aging at 973 K. This is because Cu atoms are supersaturated in the matrix after solid solution treatment and Cu-rich particles are coherent with austenitic matrix. The growth of Cu-rich particles during aging at 973 K follows diffusion-controlling LSW theory. The reduction of interfacial energy due to the segregation of Mn is estimated to be 8.1, 8.4, 14.7 and 16.7 mJ/m2 after aging for 10, 50, 100 and 1000 h, respectively. The segregation of Mn at interfaces of Cu-rich particle/matrix retards the growth of Cu-rich particles by reducing the interfacial energy, and the retarding effect increases with increasing aging time. The strengthening mechanism of Cu-rich particles in Sanicro 25 transforms from the combination of coherency strain and stacking-fault strengthening to Orowan strengthening when the average radius is about 13 nm. Cu-rich particles play an important role in affecting the strength and hardness of Sanicro 25 during aging. Increasing the content of Cu properly is beneficial to improve the strength of Sanicro 25 from the viewpoint of increasing the volume fraction of Cu-rich particles. Also, the addition of Mn is useful to improve the strength of Sanicro 25 since it can reduce the interfacial energy of Cu-rich particle/matrix.

Microstructural characteristics of bijel-templated porous materials

The geometric arrangement of pore features is a critical aspect of three-dimensional (3D) materials design for a diverse set of applications. Transport properties such as diffusivity and conductivity are intimately linked to this pore arrangement, thus, significant effort has been invested into designing processes and systems that offer predictable final morphologies. Minimal surface structures comprising bicontinuous, symmetric phases are predicted to provide optimal transport properties. The bicontinuous interfacially jammed emulsion gel (bijel) is a class of soft matter that forms by kinetically arresting spinodal decomposition phase separation. Driven by the reduction of interfacial energy between phases, bijel morphology develops in a dynamically self-similar fashion with a near-minimal surface (spinodal) interface. Here, bijels were used as template structures to generate carbon and polymer scaffolds, and morphological characterization of distinctive features was carried out on 3D reconstructions of micro-computed tomography (µCT) data. Specific emphasis is placed on the characterization of size distribution, interfacial curvature, continuity, tortuosity, and self-similarity exhibited by pore networks within these structures. Microstructural attributes are compared to three additional porous media to demonstrate bijel-derived materials as near-minimal surface structures with high transport potential.

Mechanisms for stabilizing θ′(Al2Cu) precipitates at elevated temperatures investigated with phase field modeling

While most Al–Cu and Al–Si–Cu alloys strengthened by the metastable θ′ phase exhibit extensive microstructural degradation above 200 °C, recent experimental work has demonstrated that θ′ precipitates can be stabilized to 350 °C by microalloying additions of Mn and Zr, resulting in improved mechanical properties at elevated temperatures. The present work utilizes phase field modeling to study the relationship between microalloying solute elements and the coarsening resistance of θ′. Simulations are designed to parse out the relative influence of various stabilization mechanisms on microstructural evolution of θ′ precipitates at elevated temperatures. Specifically, a ternary alloying element is added to a virtual microstructure to study the operation and effectiveness of stabilization mechanisms including solute drag, diffusion barriers, interfacial energy reduction, and lattice strain modification. Simulation results are compared with atom probe tomography observations. The simulations rationalize experimental observations of microstructural evolution and solute segregation in Al–Cu–Mn–Zr alloys, and reveal the interlinked thermodynamic and kinetic mechanisms that determine the elevated temperature stability of θ′ precipitates.

Experimental investigation of turbulent forced heat transfer of Fe3O4 ethylene glycol – Water nanofluid with highly disaggregated particles

This work aims to investigate turbulent heat transfer of Fe3O4 ethylene glycol – water nanofluid with highly disaggregated particles in a circular pipe. The surfaces of Fe3O4 nanoparticles were modified with citric acid. Thus, the particles can be highly disaggregated by rising up the pH of the suspension. Eventually, the size of particle aggregate was reduced to be 2.4 times the individual particle. The convective heat transfer coefficients of the nanofluid (0.23 vol%) and the base liquid were compared at constant Re number. To ensure the validity of the results, thermal physical properties of both fluids were experimentally determined including density, heat capacity, viscosity and thermal conductivity. Highly disaggregated particles did not benefit the heat transfer of the nanofluid. The convective heat transfer coefficient of the nanofluid was lower than the base liquid by 7%. Furthermore, their physical thermal properties were almost the same. To explain this deteriorated heat transfer, we suggested considering another particle migration driven by interfacial energy reduction, which should move the particles toward the interface between the pipe wall and the fluid.

Synergistic effect of Cu and Ni on the formation of reversed austenite in super martensitic stainless steel

ABSTRACTThe interplay of Cu and Ni on the formation of reversed austenite in super martensitic stainless steel (SMSS) was discussed in this study. Ni concentration in reversed austenite increases from 9.93 to 11.96 wt-% as Cu content increases from 1.5 to 3 wt-%, while Cu concentration increases from 1.43 to 1.89 wt-% as Ni increases from 5 to 6.5 wt-%. The enrichment of Ni and Cu in reversed austenite is much higher than that in martensite matrix. Reversed austenite formed in the dispersion area of ε-Cu inherits the twinning structure of ε-Cu and the crystal orientation relationship with the matrix during tempering. So Cu helps increase the enrichment of Ni, which in turn promotes Cu enrichment and induces the precipitation of ε-Cu due to the reduction of interfacial energy for reversed austenite formation. Our work reveals the synergism of Cu and Ni on the formation of reversed austenite.

Anomalous eutectics in intermediately and highly undercooled Ni-29.8at.%Si eutectic alloy

The transition from regular eutectics to anomalous eutectics is a well-known non-equilibrium phenomenon for undercooled eutectic alloys, the mechanism of which has been studied extensively but is still controversial. In this work, a Ni-29.8at.%Si eutectic alloy, as an ideal modeling system whose eutectic products are two stoichiometric intermetallic compounds, whose rapid solidification does not suffer any phase-selection and whose transformation path does not include any solid-state phase transformation, was undercooled by a melt fluxing technique and observed in-situ by a high-speed camera. A common transition from uncoupled eutectics to anomalous eutectics and a unique transition from uncoupled coarse lamellar-eutectics to anomalous eutectics were found at intermediate and high undercooling, respectively. The formation of coarse lamellar-eutectics is highly related to the abrupt increase of growth velocity at ΔT ≈ 149 K, which is accompanied by a transition from dual-recalescence to single-recalescence. An electron back-scattering diffraction analysis shows that epitaxial growth of the second δ-Ni2Si phase on the primary γ-Ni31Si12 phase follows a particular eutectic orientation relationship. Anomalous eutectics were concluded to be formed by uncoupled eutectic growth with the reduction of interfacial energy but not chemical superheating as the driving force.

The influence of Sc solute partitioning on ductile fracture of Sc-microalloyed Al-Cu alloys

Al-XCu (X = 1.0, 1.5, and 2.5 wt%) alloys with and without 0.3 wt% Sc addition were prepared respectively. The effects of composition and heat treatment processes on the microstructural evolution were systematically investigated by using transmission electron microscope and atom probe tomography (APT). Both Al3Sc dispersoids and θ′-Al2Cu precipitates coexisted after artificial aging, and a strong Sc segregation at θ′-Al2Cu/matrix interfaces was detected and quantified through APT examinations. A Sc solute partitioning was demonstrated between in the Al3Sc dispersoids and at the θ′-Al2Cu/matrix interfaces, mediated by the Cu content. Increasing the Cu content, both the size and volume fraction of the Al3Sc decreased after solid solution treatment. As a result, the interfacial Sc segregation was accordingly intensified during subsequent aging treatment. A parameter of reduction in interfacial energy (Δγ), derived from APT analyses, was proposed to characterize the degree of interfacial Sc segregation. The coupling effect of Al3Sc dispersoids and θ′-Al2Cu precipitates on ductile fracture was discussed, where a micromechanical model was developed to describe a quantitative relationship between the fracture strain with Δγ as well as parameters of the Al3Sc dispersoids.

Building Materials from Colloidal Nanocrystal Assemblies: Molecular Control of Solid/Solid Interfaces in Nanostructured Tetragonal ZrO2

We here describe a bottom-up approach to control the composition of solid/solid interfaces in nanostructured materials, and we test its effectiveness on tetragonal ZrO2, an inorganic phase of great technological significance. Colloidal nanocrystals capped with trioctylphosphine oxide (TOPO) or oleic acid (OA) are deposited, and the organic fraction of the ligands is selectively etched with O2 plasma. The interfaces in the resulting all-inorganic colloidal nanocrystal assemblies are either nearly bare (for OA-capped nanocrystals) or terminated with phosphate groups (for TOPO-capped nanocrystals) resulting from the reaction of phosphine oxide groups with plasma species. The chemical modification of the interfaces has extensive effects on the thermodynamics and kinetics of the material. Different growth kinetics indicate different rate limiting processes of growth (surface diffusion for the phosphate-terminated surfaces and dissolution for the “bare” surfaces). Phosphate termination led to a higher activation energy of growth, and a 3-fold reduction in interfacial energy, and facilitated significantly the conversion of the tetragonal phase into the monoclinic phase. Films devoid of residual ligands persisted in the tetragonal phase at temperatures as high as 900 °C for 24 h.

Ligand-Exchange Dynamics on Gold Nanocrystals: Direct Monitoring of Nanoscale Polyvinylpyrrolidone-Thiol Domain Surface Morphology.

We report direct high-resolution monitoring of an evolving mixed nanodomain surface morphology during thiol adsorption on polyvinylpyrrolidone (PVP)-stabilized single crystal gold nanocrystals. The thiol adsorption and replacement dynamics are much more complex than a simple complete substitution of the initial polymer ligand. We observed that during ligand exchange with linear thiol, the nanocrystal surface evolved from an initial 1 nm uniform PVP coating into a remarkably stable network of globular PVP domains 20-100 nm in size and ∼4 nm in height surrounded by thiol self-assembled monolayers. The final stability of such a mixed thiol-PVP surface morphology can possibly be attributed to the interfacial energy reduction from partially solvophilic surfaces and the entropic gain from mixed ligand surface layers. The ligand-exchange dynamics and the unusual equilibrium morphology revealed here provide important insights into both displacement dynamics of surface-bound molecules and the nanoscale peculiarities of surface functionalization of colloidal metal substrates.