Growth Mechanism Of Nanocrystals Research Articles

Anisotropic Growth of Covalent Inorganic Complexes to Nanoplatelets.

Colloidal II-VI semiconductor nanoplatelets (NPLs) provide a new platform in material science due to their unique growth mode and advanced optical properties. However, in contrast to the rapid development of zinc blend structured NPLs, studies on the formation of wurtzite (WZ) NPLs have been limited to the lamellar assembly of specific magic-sized nanoclusters (MSCs). Therefore, the study of new precursors is important for enriching the synthesis strategy, improving the study of two-dimensional (2D) nanocrystal growth mechanisms, and constructing complex nanostructures. Here, we demonstrated that covalent inorganic complexes (CICs), as novel functional intermediates, can be directly used to form NPLs without involving MSCs. Using in situ absorption spectra, we demonstrated that the evolution followed a pseudo-first-order kinetics (kobs = 0.02 min-1 (t1/2 = 34.7 min)). Several types of binary WZ NPLs, including CdSe, CdS, CdTe, and ZnS, have been directly prepared based on this mechanism through the anisotropic growth of CICs. In addition, CICs can also be used to prepare Mn-doped CdSe NPLs. The present study not only affords new precursors for the synthesis of WZ NPLs but also advances our understanding of the synthesis mechanism of nanocrystals.

Chemical transformation mechanism for blue-to-green emitting CsPbBr3 nanocrystals.

Recently, metal-halide perovskites have rapidly emerged as efficient light emitters with near-unity quantum yield and size-dependent optical and electronic properties, which have attracted considerable attention from researchers. However, the ultrafast nucleation rate of ionic perovskite counterparts severely limits the in-depth exploration of the growth mechanism of colloidal nanocrystals (NCs). Herein, we used an inorganic ligand nitrosonium tetrafluoroborate (NOBF4) to trigger a slow post-synthesis transformation process, converting non-luminescent Cs4PbBr6 NCs into bright green luminescent CsPbBr3 NCs to elucidate the concrete transformation mechanism via four stages: (i) the dissociation of pristine NCs, (ii) the formation of Pb-Br intermediates, (iii) low-dimensional nanoplatelets (NPLs) and (iv) cubic CsPbBr3 NCs, corresponding to the blue-to-green emission process. The desorption and reorganization of organic ligands induced by NO+ and the involvement of BF4- in the ligand exchange process played pivotal roles in this dissolution-recrystallization of NCs. Moreover, controlled shape evolution from anisotropic NPLs to NCs was investigated through variations in the amount of NOBF4. This further validates that additives exert a decisive role in the symmetry and growth of nanostructured perovskite crystals during phase transition based on the ligand-exchange mechanism. This finding serves as a source of inspiration for the synthesis of highly luminescent CsPbBr3 NCs, providing valuable insights into the chemical mechanism in post-synthesis transformation.

Structural Diversity Design, Four Nucleation Methods Growth and Mechanism of 3D Hollow Box TiO2 Nanocrystals with a Temperature-Controlled High (001) Crystal Facets Exposure Ratio.

Three-dimensional (3D) hollow box TiO2 nanocrystals with structural diversity have been designed and grown by four nucleation methods, including the acid dissolution denucleation method with Fe2O3 as heterogeneous nucleation, the topological phase transition method, the sonic solvothermal method, and the air atmosphere sintering method with TiOF2 as homogeneous nucleation. Through full morphology analysis and structural characterization, reasonable growth mechanisms of 3D hollow box TiO2 nanocrystals were proposed, including nucleation dissolution, Oswald ripening, and hydrolysis reactions. It was found that the high energy (001) crystal facets exposure ratio was closely correlated with reaction temperature of four nucleation-methods, which even reached 92% for the first time. Under simulated sunlight irradiation, their hydrogen production performance and photocatalytic degradation efficiency on model dye molecules rhodamine B (RhB) and methylene blue (MB) were evaluated, and as-prepared hollow box TiO2 nanocrystals prepared by the sonic solvothermal method exhibited the best photocatalytic performance, with a hydrogen production rate of 93.88 μmol/g/h. Within 70 min, the photocatalytic degradation rates of RhB and MB reached 96.59 and 75.25%, respectively, which were 5.74 and 5.54 times that of P25. Their properties are closely connected with the orderly cubic and hierarchy configuration structure of hollow box TiO2 nanocrystals, which have a high exposure ratio of (001) facet controlled by reaction temperatures, thereby greatly improving the photocatalytic activity. This study provides a classic reference for improving the properties of hollow box TiO2 nanocrystals through structural diversity design and various methods of nanocrystal growth.

Evolution of High-Index Facets Pt Nanocrystals Induced by N-Defective Sites in the Integrated Electrode for Enhanced Methanol Oxidation.

Highly efficient and durable Pt electrocatalysts are the key to boost the performance of fuel cells. The high-index facets (HIF) Pt nanocrystals are regarded as excellent catalytic activity and stability catalysts. However, nucleation, growth and evolution of high-index facets Pt nanocrystals induced by defective sites is still a challenge. In this work, tetrahexahedron (THH) and hexactahedron (HOH) Pt nanocrystals are synthesized, which are loaded on the nitrogen-doped reduced graphene oxide (N-rGO) support of the integrated electrodes by the square wave pulse method. Experimental investigations and density functional theory (DFT) calculations are conducted to analyze the growth and evolution mechanism of HIF Pt nanocrystals on the graphene-derived carbon supports. It shows that the H adsorption on the N-rGO/CFP support can induce evolution of Pt nanocrystals. Moreover, the N-defective sites on the surface of N-rGO can lead to a slower growth of Pt nanocrystals than that on the surface of reduced graphene oxide (rGO). Pt/N-rGO/CFP (20min) shows the highest specific activity in methanol oxidation, which is 1.5 times higher than that of commercial Pt/C. This research paves the way on the design and synthesis of HIF Pt nanocrystal using graphene-derived carbon materials as substrates in the future.

Crystallization heat treatments for the fabrication of volume Bragg gratings based on photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a functional optical material with broad applications in holographic optics. In this work, we examine the effect of crystallization heat treatment on the growth and distribution of sodium fluoride (NaF) nano-crystals in PTR glass using DSC, XRD, SEM, and TEM. The optical properties of the crystallized PTR glass are analyzed using a spectrophotometer. The results indicate that the NaF nano-crystals precipitated in PTR glass matrix are in the form of polycrystalline, and the growth of which is highly dependent on temperature. The growth of NaF nano-crystals saturates during the crystalline phase transition, resulting in decreased distribution uniformity. Higher crystallization temperature and longer crystallization time lead to an increase in absorption and scattering losses in the visible band. However, the crystallized PTR glass exhibits favorable absorption and scattering loss characteristics in the near-infrared band. By adjusting the crystallization treatments within the temperature range of 530 °C–570 °C for 10–120 min, refractive index modulation (RIM) values of the transmitting volume Bragg gratings (TVBG) ranging from 50 to 350 ppm can be achieved, which allows for maximum relative diffraction efficiencies exceeding 90% at 532 nm and 1053 nm using different crystallization processes. This research enhances the understanding of NaF nano-crystal growth mechanisms and provides insights into the application of holographic optical elements based on PTR glass in both the near-infrared and visible bands.

High-Yield Gold Nanohydrangeas on Three-Dimensional Carbon Nanotube Foams for Surface-Enhanced Raman Scattering Sensors.

Recently, carbon nanomaterial-supported plasmonic nanocrystals used as high-performance surface-enhanced Raman scattering (SERS) substrates have attracted increasing attention due to their ultra-high sensitivity of detection. However, most of the work focuses on the design of 2-D planar substrates with traditional plasmonic structures, such as nanoparticles, nanorods, nanowires, and so forth. Here, we report a novel strategy for the preparation of high-yield Au nanohydrangeas on three-dimensional porous polydopamine (PDA)/polyvinyl alcohol (PVA)/carbon nanotube (CNT) foams. The structures and growth mechanisms of these specific Au nanocrystals are systematically investigated. PDA plays the role of both a reducing agent as well as an anchoring site for Au nanohydrangeas' growth. We also show that the ratio of surfactant KBr to the gold precursor (HAuCl4) is key to obtain these structures in a manner of high production. Moreover, the substrate of the CNT foam-Au nanohydrangea hybrid can be employed as SERS sensors and can detect the analytes down to 10-9 M.

Diffusion growth mechanism of penta-twinned Ag nanocrystals from decahedral seeds.

Crystals with penta-twinned structures can be produced from diverse fcc metals, but the mechanisms that control the final product shapes are still not well understood. By using the theory of absorbing Markov chains to account for the growth of penta-twinned decahedral seeds via atom deposition and surface diffusion, we predicted the formation of various types of products: decahedra, nanorods, and nanowires. We showed that the type of product depends on the morphology of the seed and that small differences between various seed morphologies can lead to significantly different products. For the case of uncapped decahedra seeds, we compared predictions from our model to nanowire morphologies obtained in two different experiments and obtained favorable agreement. Possible extensions of our model are indicated.

A reactor for time-resolved X-ray studies of nucleation and growth during solvothermal synthesis.

Understanding the nucleation and growth mechanisms of nanocrystals under hydro- and solvothermal conditions is key to tailoring functional nanomaterials. High-energy and high-flux synchrotron radiation is ideal for characterization by powder X-ray diffraction and X-ray total scattering in real time. Different versions of batch-type cell reactors have been employed in this work, exploiting the robustness of polyimide-coated fused quartz tubes with an inner diameter of 0.7 mm, as they can withstand pressures up to 250 bar and temperatures up to 723 K for several hours. Reported here are recent developments of the in situ setups available for general users on the P21.1 beamline at PETRA III and the DanMAX beamline at MAX IV to study nucleation and growth phenomena in solvothermal synthesis. It is shown that data suitable for both reciprocal-space Rietveld refinement and direct-space pair distribution function refinement can be obtained on a timescale of 4 ms.

Copper assisted symmetry and size control of gold nanobars

Gold nanocuboids provide a simple nanoparticle system for investigating nanocrystal growth mechanisms. Symmetry breaking in the presence of copper additive is caused by rapid deposition on {111} facets coupled with slow surface diffusion of Au on Cu.

How a Facet of a Nanocrystal Is Formed: The Concept of the Symmetry Based Kinematic Theory.

Conventional nanocrystal (NC) growth mechanisms have overwhelmingly focused on the final exposed facets to explain shape evolution. However, how the final facets are formed from the initial nuclei or seeds, has not been specifically interrogated. In this concept paper, we would like to concentrate on this specific topic, and introduce the symmetry based kinematic theory (SBKT) to explain the formation and evolution of crystal facets. It is a crystallographic theory based on the classical crystal growth concepts developed to illustrate the shape evolution during the NC growth. The most important principles connecting the basic NC growth processes and morphology evolution are the preferential growth directions and the properties of kinematic waves. On the contrary, the final facets are just indications of how the crystal growth terminates, and their formation and evolution rely on the NC growth processes: surface nucleation and layer advancement. Accordingly, the SBKT could even be applied to situations where non-faceted NCs such as spheres are formed.

Identifying the crystal orientation of epitaxially grown MoO2 nanoflakes on c-sapphire

Identifying the crystallographic orientations and understanding the growth mechanism of MoO2 nanocrystals are important for device applications. Here, we investigate the effect of precursor concentrations in chemical vapor deposition on the orientation of MoO2 nanoflakes with respect to the substrate surface. X-ray diffraction, optical microscopy, (tilted) scanning electron microscopy, high resolution transmission electron microscopy, selected area electron diffraction, and angle-resolved polarized Raman spectroscopy were used to determine the crystal orientations of the nanoflakes and their epitaxial relationship with c-sapphire substrates: lying-down MoO2 nanoflakes for MoO2(010)∥sapphire(0001) and MoO2 <001>//sapphire<11¯00>, and standing-up ones for MoO2(100)∥sapphire(0001) and MoO2 <001>∥sapphire<11¯00>. Tentative atomic epitaxial models are proposed to explain the orientation and alignment. The investigations also demonstrated that angle-resolved polarized Raman spectroscopy is a quick and nondestructive method to identify the crystallographic orientations of nanoflakes.

Revealing the Aqueous Sequential Growth Mechanism between Au and Ag Nanocrystals of Segmented Ag-Au-Ag Heterojunction Nanorods via Redox Reaction Kinetics

Although the growth mechanisms (e.g., seed-induced growth and capping agent orientation) of bimetal nanocrystals (e.g., core–shell, alloy, segmented, and branched) from artificial experimental speculation and theoretical calculation have been widely accepted, precisely revealing their growth mechanisms is still tremendously challenging. In this work, we utilized redox reaction kinetics for the first time to successfully reveal the aqueous sequential growth mechanism between Au and Ag nanocrystals of segmented Ag-Au-Ag heterojunction nanorods (HJNRs) in a one-step and high-temperature aqueous system. Herein, electrode potentials of different electrical pairs (e.g., Ag+/Ag and AuCl4–/Au) at 200 °C could be calculated through using the Helgeson–Kirkham–Flowers state and other equations, from which whether Au and Ag nanocrystals grew successively and formed segmented Ag-Au-Ag HJNRs could be correctly assessed. The redox reaction kinetics mechanism can also explain well the aqueous-phase growth mechanisms of other bimetal nanocrystals and paves a promising avenue for the design and synthesis of other one-dimensional segmented metal nanostructures.

Atom‐Resolved Investigation on Dynamic Nucleation and Growth of Platinum Nanocrystals (Small Methods 6/2022)

Inside Front Cover In article number 2200171, Sun, Wang, and coworkers focused on the nucleation and growth mechanism of nanocrystals. The dynamic transformation of a solid-phase model platinum precursor was investigated at atomic resolution. Those findings shall provide a deeper understanding of the solid-phase nucleation mechanism. The in-situ method can be applied to study the nucleation process of other systems.

Peculiarities of nucleation and growth of InGaN nanowires on SiC/Si substrates by HVPE

The growth of InGaN layers on hybrid SiC/Si substrates with orientations (100), (110), and (111) by the HVPE method was studied at temperatures that wittingly exceed the temperature of InN decomposition onto nitrogen atoms and metallic In (1000oC). On substrates with orientations (110) and (111), the formation of InGaN whisker nanocrystals was observed. The shape and growth mechanisms of nanocrystals were investigated. It is shown that nanocrystals nucleate on the (111) surface only inside V-defects formed at the points where screw dislocations exit onto the surface. On the (110) surface, nanocrystals are formed only on pedestals that arise during the film growth. An explanation is given for the difference in the growth mechanisms of nanocrystals on substrates of different orientations. Keywords: InGaN, heterostructures, SiC on Si, silicon, whisker nanocrystals, nanostructures, atomic substitution method

Особенности зарождения и роста нитевидных нанокристаллов InGaN на подложках SiC/Si методом хлорид-гидридной эпитаксии

The growth of InGaN layers on hybrid SiC/Si substrates with orientations (100), (110), and (111) by the HVPE method was studied at temperatures that wittingly exceed the temperature of InN decomposition onto nitrogen atoms and metallic In (1000C). On substrates with orientations (110) and (111), the formation of InGaN nanocrystals was observed. The shape and growth mechanisms of nanocrystals were investigated. It is shown that nanocrystals nucleate on the (111) surface only inside V-defects formed at the points where screw dislocations exit onto the surface. On the (110) surface, nanocrystals are formed only on pedestals that arise during the film growth. An explanation is given for the difference in the growth mechanisms of nanocrystals on substrates of different orientations.

Multi-step atomic mechanism of platinum nanocrystals nucleation and growth revealed by in-situ liquid cell STEM

The understanding of crystal growth mechanisms has broadened substantially. One significant advancement is based in the conception that the interaction between particles plays an important role in the growth of nanomaterials. This is in contrast to the classical model, which neglects this process. Direct imaging of such processes at atomic-level in liquid-phase is essential for establishing new theoretical models that encompass the full complexity of realistic scenarios and eventually allow for tailoring nanoparticle growth. Here, we investigate at atomic-scale the exact growth mechanisms of platinum nanocrystals from single atom to final crystals by in-situ liquid phase scanning transmission electron microscopy. We show that, after nucleation, the nanocrystals grow via two main stages: atomic attachment in the first stage, where the particles initially grow by attachment of the atoms until depletion of the surrounding zone. Thereafter, follows the second stage of growth, which is based on particle attachment by different atomic pathways to finally form mature nanoparticles. The atomic mechanisms underlying these growth pathways are distinctly different and have different driving forces and kinetics as evidenced by our experimental observations.

Dopants in the Design of Noble Metal Nanoparticle Electrocatalysts and their Effect on Surface Energy and Coordination Chemistry at the Nanocrystal Surface

AbstractCommercialization of energy conversion technologies, such as water electrolysis or fuel cells, has become a common prioritized goal in academia and industry due to the accelerated environmental crisis caused by the rapid increase in demand for fossil fuel‐based energy resources. However, these state‐of‐the‐art technologies require the use of expensive noble metal‐based electrocatalysts, which significantly undermine the feasibility of their commercial success. The introduction of less expensive elements into the noble metal‐based electrocatalysts, namely, doping, has become common in nanocatalysis research because of the lower catalyst preparation costs. Interestingly, recent studies have revealed additional roles of dopants in noble metal catalysts; doping can enhance the catalytic activity and selectivity by modulating the band structure, optimizing the surface energy of the catalysts, controlling the binding strength of adsorbates, and thus affecting the reaction kinetics. Dopants can also intervene in the nanocrystal growth mechanism, leading to shape‐ and phase‐controlled nanocrystals. This review discusses the great versatility of dopants in nanoparticle‐based catalysis in all aspects, from nanocrystal growth to nanocatalyst performance. The future challenges and outlook for dopants in nanocrystals and their applications are further discussed.

Effective control of solution self-assembly of P3HT/zinc salt complex for in situ template synthesis of P3HT/ZnO nanohybrids

This study explored the detailed solution self-assembly of a P3HT/zinc salt complex system as well as the following in situ growth mechanism of ZnO nanocrystals within self-assembled nanotemplates. Through controlling the solvent selectivity and aging conditions, the geometric shape of the complex system could be directly tailored in solution, forming nanoscopic spheres or highly elongated nanofibrils. These solution self-assembled nanostructures could be efficaciously transferred onto substrates to fabricate nanostructured hybrid films through drop casting. In contrast to traditional P3HT systems with only form Ⅰ crystalline structure, nanostructured P3HT complex thin films revealed a metastable crystalline phase with overlapping alkyl side chains. The formation of this interdigitated crystalline feature could be attributed to the solvent evaporation‐induced crystallization of tightly assembled complex chains existing in solution self-assembled nanostructured domains. The nanostructured P3HT/zinc salt complex assemblies were further used as nanoreactors for the in situ growth of ZnO. As complex samples were thermally treated, a phase transformation of P3HT from the metastable crystalline phase to form Ⅰ structure as well as the formation of ZnO nanocrystals were clearly observed. Both nanospherical and nanofibrillar structured P3HT/ZnO hybrids exhibited lower photoluminescence intensity than did a P3HT/ZnO blend sample. In particular, the nanofibrillar structured hybrid provided an optimized donor/acceptor structure for exciton dissociation as well as interpenetrating nanochannels for charge transport, resulting in the most effective photoluminescence quenching effect among measured samples. We believe that our study can serve as a model system and provide guidelines for efficiently controlling the hierarchical structures of π-conjugated polymer/inorganic nanocrystal hybrids through the in situ synthesis, offering benefits for advanced photoelectronic applications.

On the Reaction Pathways and Growth Mechanisms of LiNbO3 Nanocrystals from the Non-Aqueous Solvothermal Alkoxide Route.

Phase-pure, highly crystalline sub-50 nm LiNbO3 nanocrystals were prepared from a non-aqueous solvothermal process for 72 h at 230 °C and a commercial precursor solution of mixed lithium niobium ethoxide in its parent alcohol. A systematic variation of the reaction medium composition with the addition of different amounts of co-solvent including butanol, 1,3-propanediol, 1,4-butanediol, and 1,5-pentanediol resulted in the formation of nanocrystals of adjustable mean size and shape anisotropy, as demonstrated from XRD measurements and TEM imaging. Colloidal stability of ethanol- and water-based suspensions was evaluated from dynamic light scattering (DLS)/zeta potential studies and correlated with FTIR data. Thanks to the evolution in the nanocrystal size and shape distribution we observed, as well as to the available literature on the alkoxide chemistry, the reaction pathways and growth mechanisms were finally discussed with a special attention on the monomer formation rate, leading to the nucleation step. The polar, non-perovskite crystalline structure of LiNbO3 was also evidenced to play a major role in the nanocrystal shape anisotropy.

Insights into Growth Kinetics of Colloidal Gold Nanoparticles: In Situ SAXS and UV–Vis Evaluation

Understanding the formation process and kinetics is of vital importance for preparing monodisperse colloidal nanocrystals with controllable size and morphology in a predictive way. However, in-depth understanding of nucleation and growth mechanisms is impeded due to the lack of reliable and complete in situ experimental data from molecular precursors to colloidal nanocrystals. Herein we used in situ UV–vis and synchrotron-based time-resolved in situ small-angle X-ray scattering to monitor the reduction kinetics of gold salt and the fast nucleation and growth kinetics of gold nanoparticles synthesized in stopped flow microfluidics through the reduction of a gold precursor by a morpholine–borane complex in the presence of oleylamine or the combination of oleylamine and oleic acid ligands. Our method enables obtaining detailed information on the evolution of size, size distribution, the number of particles, and monomer concentration by probing the original reaction solution over time. Through quantitative analysis of in situ SAXS and UV–vis data, complex growth trajectories involving the coalescence of small particles, intraparticle growth within coalesced nanoparticles, and surface-reaction limited focusing of size-distribution events could be identified in the synthesis of monodisperse Au NPs for both cases. When an oleic acid ligand is present in the system, it can cause the dissolution of larger particles, followed by the growth of small ones at the expense of larger ones at the initial growth stage before coalescence event. The polydispersity of 20% is found to be a quantitative indicator for the transition of the intraparticle growth event to the focusing of a size distribution mechanism for both cases. In the later focusing process, the polydispersity can be significantly narrowed from 15% to 9%, while the particle size keeps almost constant. By quantitative analysis of the growth process, we can conclude that nucleation and the growth process are dependent on ligand–NP binding affinity. We believe that our findings can further advance the understanding of the growth kinetics and mechanisms of oleylamine-capped metal nanocrystals.