Growth Kinetics Of Nanocrystals Research Articles

Monodispersed mesoscopic star-shaped gold particles via silver-ion-assisted multi-directional growth for highly sensitive SERS-active substrates

Surface-enhanced Raman scattering (SERS) exploits localized surface plasmon resonances in metallic nanostructures to significantly amplify Raman signals and perform ultrasensitive analyses. A critical factor for SERS-based analysis systems is the formation of numerous electromagnetic hot spots within the nanostructures, which represent regions with highly concentrated fields emerging from excited localized surface plasmons. These intense hotspot fields can amplify the Raman signal by several orders of magnitude, facilitating analyte detection at extremely low concentrations and highly sensitive molecular identification at the single-nanoparticle level. In this study, mesoscopic star-shaped gold particles (gold mesostars) were synthesized using a three-step seed-mediated growth approach coupled with the addition of silver ions. Our study confirms the successful synthesis of gold mesostars with numerous sharp tips via the multi-directional growth effect induced by the underpotential deposition of silver adatoms (AgUPD) onto the gold surfaces. The AgUPD process affects the nanocrystal growth kinetics of the noble metal and its morphological evolution, thereby leading to intricate nanostructures with high-index facets and protruding tips or branches. Mesoscopic gold particles with a distinctive star-like morphology featuring multiple sharp projections from the central core were synthesized by exploiting this phenomenon. Sharp tips of the gold mesostars facilitate intense localized electromagnetic fields, which result in strong SERS enhancements at the single-particle level. Electromagnetic fields can be further enhanced by interparticle hot spots in addition to the intraparticle local field enhancements when arranged in multilayered arrays on substrates, rendering these arrays as highly efficient SERS-active substrates with improved sensitivity. Evaluation using Raman-tagged analytes revealed a higher SERS signal intensity compared to that of individual mesostars because of interparticle hot spots enhancements. These substrates enabled analyte detection at a concentration of 10− 9 M, demonstrating their remarkable sensitivity for trace analysis applications.

A Kinetic Isotope Effect in the Formation of Lanthanide Phosphate Nanocrystals.

Mechanisms of nucleation and growth of crystals are still attracting a great deal of interest, in particular with recent advances in experimental techniques aimed at studying such phenomena. Studies of kinetic isotope effects in various reactions have been useful for elucidating reaction mechanisms, and it is believed that the same may apply for crystal formation kinetics. In this work, we present a kinetic study of the formation of europium-doped terbium phosphate nanocrystals under acidic conditions, including a strong H/D isotope effect. The nanocrystal growth process could be quantitatively followed through monitoring of the europium luminescence intensity. Hence, such lanthanide-based nanocrystals may serve as unique model systems for studying crystal nucleation and growth mechanisms. By combining the luminescence and NMR kinetics data, we conclude that the observed delayed nucleation occurs due to initial formation of pre-nucleation clusters or polymers of the lanthanide and phosphate ions, which undergo a phase transformation to crystal nuclei and further grow by cluster attachment. A scaling behavior observed on comparison of the H2O and D2O-based pre-nucleation and nanocrystal growth kinetics led us to conclude that both pre-nucleation and nanocrystal growth processes are of similar chemical nature.

Microfluidics-Assisted Synthesis of Hierarchical Cu2 O Nanocrystal as C2 -Selective CO2 Reduction Electrocatalyst.

Copper-based catalysts have attracted enormous attention due to their high selectivity for C2+ products during the electrochemical reduction of CO2 (CO2 RR). In particular, grain boundaries on the catalysts contribute to the generation of various Cu coordination environments, which have been found essential for C-C coupling. However, smooth-surfaced Cu2 O nanocrystals generally lack the ability for the surface reorganization to form multiple grain boundaries and desired Cu undercoordination sites. Flow chemistry armed with the unparalleled ability to mix reaction mixture can achieve a very high concentration of unstable reaction intermediates, which in turn are used up rapidly to lead to kinetics-driven nanocrystal growth. Herein, the synthesis of a unique hierarchical structure of Cu2 O with numerous steps (h-Cu2 O ONS) via flow chemistry-assisted modulation of nanocrystal growth kinetics is reported. The surface of h-Cu2 O ONS underwent rapid surface reconstruction under CO2 RR conditions to exhibit multiple heterointerfaces between Cu2 O and Cu phases, setting the preferable condition to facilitate C-C bond formation. Notably, the h-Cu2 O ONS obtained the increased C2 H4 Faradaic efficiency from 31.9% to 43.5% during electrocatalysis concurrent with the morphological reorganization, showing the role of the stepped surface. Also, the h-Cu2 O ONS demonstrated a 3.8-fold higher ethylene production rate as compared to the Cu2 O nanocube.

Enhanced photoluminescence stability of CdS nanocrystals through a zinc acetate reagent.

In this study, the role of a zinc acetate precursor in improving the luminescence stability of purple-emitting CdS nanocrystals is investigated. The oleate-capped core of CdS nanocrystals exhibits intense photodarkening under prolonged UV excitation. From the results of photoluminescence experiments, we can observe that photobleaching is responsible for the degradation of temporal stability, i.e., decline in photoluminescence intensity. Herein, we demonstrate that by adding zinc acetate to the synthesis solution, one can enhance the photoluminescence stability by the complete suppression of the bleaching processes of nanocrystals. We can distinguish between the effects caused by zinc ions and those caused by acetate ligands. Acetate ligands improve the photoluminescence stability of the core of CdS nanocrystals. However, only when zinc acetate is used, the PL stability can be conserved at high excitation power. Simultaneously, we have studied the influence of zinc cations and acetate ligands on the kinetics of nanocrystal growth. The presented results underline the importance of short surface capping ligands and zinc cations in CdS nanocrystal synthesis. This study exhibits a new advantage of exploiting zinc acetate reagents in one-pot nanocrystal synthesis.

Control of Electron Beam-Induced Au Nanocrystal Growth Kinetics through Solution Chemistry.

Measurements of solution-phase crystal growth provide mechanistic information that is helpful in designing and synthesizing nanostructures. Here, we examine the model system of individual Au nanocrystal formation within a defined liquid geometry during electron beam irradiation of gold chloride solution, where radiolytically formed hydrated electrons reduce Au ions to solid Au. By selecting conditions that favor the growth of well-faceted Au nanoprisms, we measure growth rates of individual crystals. The volume of each crystal increases linearly with irradiation time at a rate unaffected by its shape or proximity to neighboring crystals, implying a growth process that is controlled by the arrival of atoms from solution. Furthermore, growth requires a threshold dose rate, suggesting competition between reduction and oxidation processes in the solution. Above this threshold, the growth rate follows a power law with dose rate. To explain the observed dose rate dependence, we demonstrate that a reaction-diffusion model is required that explicitly accounts for the species H(+) and Cl(-). The model highlights the necessity of considering all species present when interpreting kinetic data obtained from beam-induced processes, and suggest conditions under which growth rates can be controlled with higher precision.

Polyoxometalate-mediated one-pot synthesis of Pd nanocrystals with controlled morphologies for efficient chemical and electrochemical catalysis.

Polyoxometalates (POMs), as inorganic ligands, can endow metal nanocrystals (NCs) with unique reactivities on account of their characteristic redox properties. In the present work, we present a facile POM-mediated one-pot aqueous synthesis method for the production of single-crystalline Pd NCs with controlled shapes and sizes. The POMs could function as both reducing and stabilizing agents in the formation of NCs, and thus gave a fine control over the nucleation and growth kinetics of NCs. The prepared POM-stabilized Pd NCs exhibited excellent catalytic activity and stability for electrocatalytic (formic acid oxidation) and catalytic (Suzuki coupling) reactions compared to Pd NCs prepared without the POMs. This shows that the POMs play a pivotal role in determining the catalytic performance, as well as the growth, of NCs. We envision that the present approach can offer a convenient way to develop efficient NC-based catalyst systems.

One-pot synthesis of Au@Pd core-shell nanocrystals with multiple high- and low-index facets and their high electrocatalytic performance.

Bimetallic nanocrystals (NCs) enclosed by high-surface energy facets have been of enormous interest due to their pronounced catalytic performance in numerous chemical and electrochemical reactions. However, it remains a significant challenge to develop a facile method to synthesize bimetallic NCs with high-surface energy facets in the form of finely tuned structures due to the difficulties in manipulating the nucleation and growth kinetics of NCs in the presence of multiple metal precursors. In the present work, a facile one-pot aqueous synthesis method is developed for the production of bimetallic Au@Pd core-shell NCs with an unusual truncated hexoctahedral (THOH) shape without pre-synthesized seeds. The THOH Au@Pd NCs are bound by multiple high- and low-index facets. The formation of this unique structure is realized through co-reduction of Au and Pd precursors under precisely controlled kinetic conditions. The prepared THOH NCs exhibit a prominent electrocatalytic performance for ethanol oxidation, which is attributed to their characteristic structural features. This study significantly expands the understanding of NC growth and will lead to fabricating novel nanomaterials with desired morphologies and functions.

Crystal growth by oriented attachment: kinetic models and control factors

Crystal growth mechanism, kinetics, and microstructure development play fundamental roles in tailoring materials with controllable sizes and morphologies. Oriented attachment (OA) involves the spontaneous self-organization of adjacent nanocrystals, resulting in crystal growth by addition of solid particles that share a common crystallographic orientation. It is common for crystal growth to occur by more than one mechanism simultaneously, such as OA and coarsening. This complexity leads to the difficulty in studying the OA growth mechanism. Here, we briefly review progress in kinetic models involving OA and the impact of this mechanism on materials science. We concentrate mostly on recent findings that relate different OA behaviors to surface chemistry and growth conditions, aiming to elucidate this crystal growth mechanism. To explore OA-limited growth, the influence of the Ostwald ripening (OR) mechanism needs to be known and quantified. The introduction of capping ligands has been reported to play multiple roles, including i) promoting or inhibiting aggregation and thus influencing OA growth, or ii) hindering OR from occurring and thus facilitating OA. A detailed survey of nanocrystal growth kinetics under the effect of surface adsorption is presented and summarized.

ChemInform Abstract: Conversion Reactions of Cadmium Chalcogenide Nanocrystal Precursors.

AbstractReview: [259 refs.]

Growth kinetics of CdSe nanocrystals synthesized in liquid paraffin via one-pot method

The cadmium selenide (CdSe) nanocrystals are synthesized in liquid paraffin via one-pot method. The growth kinetics of the nanocrystals is investigated by the measurements of their ultraviolet–visible absorption and fluorescence emission. The results show that very fast growth occurs linearly for the nanoparticles regardless of the molar ratio of Cd to Se until a reaction time of approximately 60 s, followed by essential growth rate decrease. The nanocrystals thereafter possess a two-stage kinetic feature consisting of a rapid growth within approximately 30 min and a slow growing up to 10 h. The particles size is not governed by the reaction temperature but by the molar ratio of Cd to Se. From the viewpoint of both strong fluorescence emission and saving in raw materials, a reaction temperature of 200 °C and a molar ratio of Cd to Se of 4/1 are appropriate for the synthesis of the CdSe nanocrystals.

Conversion Reactions of Cadmium Chalcogenide Nanocrystal Precursors

We survey the chemical reactions between common precursors used in the synthesis of metal chalcogenide nanocrystals and outline how they affect the mechanism and kinetics of nanocrystal growth. We emphasize syntheses of cadmium selenide and cadmium sulfide where a variety of metal and chalcogenide precursors have been explored, though this is supplemented by studies of zinc and lead chalcogenide formation where appropriate. This review is organized into three sections, highlighting kinetics, metal precursors, and chalcogenide precursors, respectively. Section I is dedicated to the role of precursor conversion as a source of monomers and the importance of the supply rate on nanocrystal nucleation and growth. Section II describes the structure and reactivity of cadmium carboxylates, phosphonates, and chalcogenolates. Section III describes the reaction chemistry of commonly employed chalcogenide precursors and the mechanisms by which they react with metal precursors.

Hierarchical Gold Flower with Sharp Tips from Controlled Galvanic Replacement Reaction for High Surface Enhanced Raman Scattering Activity

Highly branched Au flowers (AuFs) with sharp tips have been synthesized in high yield by controlling the kinetics of nanocrystal growth via galvanic replacement reaction. Controlled galvanic replacement takes place on appropriately chosen commercially available polystyrene bead supported Cu nanoparticles with HAuCl4. Such a reaction is thermodynamically driven through the exploitation of a Cu(II)/Cu(0) redox couple during nanocrystal formation. The hierarchical morphology of the resulting naked Au flowers (AuFs) depends on the galvanic exchange rate, which increases with increasing HAuCl4 concentration but over a specific concentration range. The formation of AuF is carefully studied, and a spontaneous assembly mechanism is proposed. The time-course experimental results show that the influence of electrostatic field force (EFF) of the charged resin beads is held responsible for prickly tipped AuF formation. Then the fabrication of AuF morphology becomes a large-scale free-standing synthetic protocol for a chemically stable substrate for surface-enhanced Raman scattering (SERS) studies down to the single molecular level. The SERS results guarantee that the as-prepared naked AuF is an excellent and stable SERS substrate. The prickly tips of the gold bearing enhanced field with a large number of embedded “hot spots” hidden within the oriented petals and absence of surfactant or capping agent invites a probe molecule to show the enhancement even for a concentration of 4-mercaptopyridine (4-MPy) down to 10–12 mol dm–3. The electric field distribution around the hot spot has been estimated from 3D-FDTD simulation studies. The selective enhancements of SER bands of the 4-MPy molecule have been unveiled from the view of Herzberg–Teller (HT) charge transfer (CT) mechanism.

The effect of electron dose on beam induced silver nanocrystal growth kinetics and morphology

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Cover Picture: Topographically controlled growth of silver nanoparticle clusters (Phys. Status Solidi RRL 5/2012)

It has been well known that a nanometer- or micrometer-scale surface roughness affects the heterogeneous nucleation and controls the growth kinetics of nanocrystals. In their Letter on pp. 202–204, Wi, Shrestha and Nagao demonstrate that the preferred nucleation of silver nanoparticles inside lithographically defined silicon nanowells allows to locate the silver nanoclusters in predetermined positions, and thus to form ordered arrays of surface enhanced Raman scattering (SERS)-active sensors. Highly enhanced surface plasmon fields from the Ag nanoparticle clusters were successfully confirmed by three-dimensional electromagnetic simulation and by confocal measurements of increased Raman signals. The proposed approach, which enables to combine a top–down nanoscale templating and a bottom–up nanoparticles synthesis, can be utilized for the growth and assembly of functional nanomaterials in a broad range of materials, array designs, and applications.

Growth kinetics of platinum nanocrystals prepared by two different methods: Role of the surface

In order to examine the applicability of the diffusion-limited Ostwald ripening model to the growth kinetics of nanocrystals, platinum nanocrystals prepared by two different methods have been investigated by a combined use of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). One of the methods of synthesis involved the reduction of chloroplatinic acid by sodium citrate while in the other method reduction was carried out in the presence of polyvinylpyrrolidone (PVP) as a capping agent. The growth of platinum nanocrystals prepared by citrate reduction in the absence of any capping agent follows a Ostwald ripening growth with a D 3 dependence. In the presence of PVP, the growth of platinum nanocrystals does not completely follow the Ostwald ripening model, making it necessary to include a surface reaction term in the growth equation. Thus, the growth of platinum nanocrystals in the presence of PVP has contributions both from diffusion and surface reaction, exhibiting a D 3 + D 2 type behavior.

Controlling the Growth Kinetics of Nanocrystals via Galvanic Replacement: Synthesis of Au Tetrapods and Star-Shaped Decahedra

By controlling the kinetics of nanocrystal growth via a galvanic replacement process, branched Au nanocrystals, including Au tetrapods and star-shaped decahedra, have been synthesized in high yield. Such control is achieved by the establishment of a Pt(II)/Pt redox couple during nanocrystal formation, which provides locally high concentrations of Au adatoms via galvanic replacement. These remarkable structures have been extensively characterized by microscopy techniques, which illustrate that the internal twin features of the nanocrystals provide a break in the symmetry of the face centered cubic lattice of the Au particles and account for the specific branching patterns observed. Significantly, this study validates a new class of redox couples for use in nanosyntheses and will enable the preparation of other nanocrystals as kinetic products.

Thermally driven isotropic crystallinity breaking of nanocrystals: Insight into the assembly of EuS nanoclusters and nanorods with oleate ligands

Clusters of EuS nanocrystals formed through the thermal assembly of 2.5 nm nanocrystal monomers by varying the annealing temperature from 300 to 340 °C. Below 310 °C, oleate ligands stabilized on the surface of the EuS nanocrystals, giving rise to their low solubility in triethanolamine while facilitating monomer–monomer oriented attachment into short-chain structures. Above 320 °C, the oleate ligands thermally detached from the surface and were replaced by oleyamine. This reaction mechanism was a multilevel oriented attachment, based on calculations of the nanocrystal growth kinetics, whose evolution gave rise to the formation of EuS nanorods at 340 °C.

Progress of nanocrystalline growth kinetics based on oriented attachment

The crystal growth mechanism, kinetics, and microstructure development play a fundamental role in tailoring the materials with controllable sizes and morphologies. The classical crystal growth kinetics-Ostwald ripening (OR) theory is usually used to explain the diffusion-controlled crystal growth process, in which larger particles grow at the expense of smaller particles. In nanoscale systems, another significant mechanism named "oriented attachment (OA)" was found, where nanoparticles with common crystallographic orientations directly combine together to form larger ones. Comparing with the classical atom/molecular-mediated crystallization pathway, the OA mechanism shows its specific characteristics and roles in the process of nanocrystal growth. In recent years, the OA mechanism has been widely reported in preparing low-dimension nanostructural materials and reveals remarkable effects on directing and mediating the self-assembly of nanocrystals. Currently, the interests are more focused on the investigation of its role rather than the comprehensive insight of the mechanism and kinetics. The inner complicacy of crystal growth and the occurrence of coexisting mechanisms lead to the difficulty and lack of understanding this growth process by the OA mechanism.In this context, we review the progress of the OA mechanism and its impact on materials science, and especially highlight the OA-based growth kinetics aiming to achieve a further understanding of this crystal growth route. To explore the OA-limited growth, the influence of the OR mechanism needs to be eliminated. The introduction of strong surface adsorption was reported as the effective solution to hinder OR from occurring and facilitate the exclusive OA growth stage. A detailed survey of the nanocrystal growth kinetics under the effect of surface adsorption was presented and summarized. Moreover, the development of OA kinetic models was systematically generalized, in which the "molecular-like" kinetic models were built to take the OA nanocrystal growth behavior as the collision and reaction between molecules. The development of OA growth kinetics can provide a sufficient understanding of crystal growth, and the awareness of underlying factors in the growth will offer promising guidance on how to control the size distribution and shape development of nanostructural materials.

Growth Kinetics of Monodisperse Cu−In−S Nanocrystals Using a Dialkyl Disulfide Sulfur Source

Well-defined and monodisperse wurtzite Cu−In−S nanocrystals were synthesized via a solution-based method using di-tert-butyl disulfide as the sulfur source. Reaction control (i.e., coordinating solvent, capping ligand, and sulfur source) proved critical for providing a kinetic pathway to the metastable wurtzite phase. The crystal phase was confirmed by powder X-ray diffraction and selected area electron diffraction. Quantitative nanocrystal growth kinetics were studied on a Cu−In−S system for the first time, clearly showing that the nanocrystals pass through a size focusing event before undergoing steady-state Ostwald ripening at later stages of growth. The size-focused 6.9 nm Cu−In−S nanocrystals have an optical band gap of Eg = 1.47 eV and an electrochemical band gap of Eg = 1.84 eV.

Observation of Nucleation and Growth of CdS Nanocrystals in a Two-Phase System

The nucleation and growth kinetics of CdS nanocrystals in a two-phase synthesis system have been investigated. It was found that the nucleation process is quite lengthy and overlapped with the growth process; nevertheless, as formed nanocrystals show extremely narrow size distribution owing to the unique heterogeneous reacting environment and Ostwald ripening growth. The nucleation and growth kinetics of the nanocrystals were also influenced strongly by the monomer concentration, capping agent concentration, and solvent polarity. It was also found that a high monomer concentration, a low capping agent concentration, and low solvent polarity lead to a higher maximum nucleus concentration and nanocrystal concentration, while high polarity solvents are favorable for the formation of nanocrystals with narrower size distribution and higher photoluminescence quantum yield.