Nanocrystals Of Different Sizes Research Articles

Organothiol-directed synthesis and growth mechanism of Ag octahedra with tunable optical properties in aqueous solution

Shape-selective synthesis of Ag nanocrystals in the aqueous system remains a challenge, and insight into the mechanism of crystal growth is lacking. Here, we synthesized Ag octahedra (Ag Oh) with sharp corners and edges by the seed-mediated method in an aqueous solution at low temperatures (35 °C) and wide ranges of edge lengths (52–187 nm). l-cysteine directs the kinetic transformation of Ag cubes into octahedral nanocrystals of different sizes by selective deposition on the {100} plane. The thermodynamic product of the organothiol-directed synthesis was discovered to be truncated tetrahedra. The regrowth of octahedral nanocrystals in the absence of surfactant leads to corner deposition of Ag with the formation of concave Oh. Au@Ag tip-blobbed and concave Oh with tunable optical properties were synthesized using octahedral nanocrystals as a sacrificial template. Furthermore, by measuring the refractive index sensitivity and figure of merit (FOM) of Ag Oh, we have demonstrated its potential as a promising candidate for plasmonic sensors. The proposed concept of organothiol-directed deposition can be used to synthesize various nanocrystals with tunable properties as promising candidates for progress in optics, catalysis, and biomedicine applications.

Revealing the Impact of Polystyrene Functionalization of Au Octahedral Nanocrystals of Different Sizes on the Formation and Structure of Mesocrystals

AbstractThe self‐assembly of anisotropic nanocrystals (stabilized by organic capping molecules) with pre‐selected composition, size, and shape allows for the creation of nanostructured materials with unique structures and features. For such a material, the shape and packing of the individual nanoparticles play an important role. This work presents a synthesis procedure for ω‐thiol‐terminated polystyrene (PS‐SH) functionalized gold nanooctahedra of variable size (edge length 37, 46, 58, and 72 nm). The impact of polymer chain length (Mw: 11k, 22k, 43k, and 66k g⋅mol−1) on the growth of colloidal crystals (e. g. mesocrystals) and their resulting crystal structure is investigated. Small‐angle X‐ray scattering (SAXS) and scanning transmission electron microscopy (STEM) methods provide a detailed structural examination of the self‐assembled faceted mesocrystals based on octahedral gold nanoparticles of different size and surface functionalization. Three‐dimensional angular X‐ray cross‐correlation analysis (AXCCA) enables high‐precision determination of the superlattice structure and relative orientation of nanoparticles in mesocrystals. This approach allows us to perform non‐destructive characterization of mesocrystalline materials and reveals their structure with resolution down to the nanometer scale.

Optical and thermal characterization of a nanofluid of MPA-functionalized CdSe nanocrystals and Au nanorods: An energy transfer process analysis

This study investigates the optical and energy transfer properties of mercaptopropionic acid (MPA)-functionalized CdSe nanocrystals (NCs) and gold nanorods (AuNRs) into aqueous solution. CdSe NCs of different sizes exhibit size-dependent excitonic transitions, with larger sizes resulting in red shifted absorption and emission peaks. MPA surface modification induces deep trap emissions in CdSe NCs. Thermal lens measurements reveal that the addition of CdSe NCs decreases the thermal diffusivity, while the presence of AuNRs increases it. The energy transfer efficiency between CdSe NCs and AuNRs in the nanofluids was observed, with CdSe1+Au60 showing the highest energy transfer efficiency of 0.75. These findings emphasize the significance of MPA functionalization in enhancing energy transfer efficiency between NCs and AuNRs, with implications for improving the performance of photovoltaic systems.

Size-dependent miscibility controls the kinetics of anion exchange in cesium lead halide nanocrystals.

Anion exchange is a facile, post-synthetic method to tune the emission wavelength of colloidal cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals. While colloidal nanocrystals can exhibit size-dependent phase stability and chemical reactivity, the role of size in the mechanism of anion exchange in CsPbX3 nanocrystals has not been elucidated. We used single-particle fluorescence microscopy to monitor the transformation of individual CsPbBr3 nanocrystals to CsPbI3. By systematically varying the size of the nanocrystals and the concentration of substitutional iodide, we observed that smaller nanocrystals exhibit longer transition times in their fluorescence trajectories, while larger nanocrystals undergo a more abrupt transition during anion exchange. Monte Carlo simulations were used to rationalize the size-dependent reactivity, in which we varied how each exchange event affects the probability for further exchange. Greater cooperativity for simulated ion exchange leads to shorter transition times to complete the exchange. We propose that size-dependent miscibility between CsPbBr3 and CsPbI3 at the nanoscale controls the reaction kinetics. Smaller nanocrystals maintain a homogeneous composition during anion exchange. As the nanocrystal size increases, variations in the octahedral tilting patterns of the perovskite crystals lead to different structures for CsPbBr3 and CsPbI3. Thus, an iodide-rich region must first nucleate within larger CsPbBr3 nanocrystals, which is followed by rapid transformation to CsPbI3. While higher concentrations of substitutional anions can suppress this size-dependent reactivity, the inherent differences in reactivity between nanocrystals of different sizes are important to consider when scaling up this reaction for applications in solid-state lighting and biological imaging.

The influence of grain size and catalytic doping on magnesium hydride nanocrystals for hydrogen storage

Magnesium hydride (MgH2) nanocrystals of different sizes were constructed by a facile hydrogenated ball milling method. X-ray diffraction (XRD) analysis showes that the size of MgH2 nanocrystals decreased gradually with the increasing milling time. The as-prepared MgH2 nanocrystals started to release and take up hydrogen from 281 to 119 °C, respectively. By doping the prepared Nb2O5@MOF into MgH2 nanocrystals, the initial hydrogen desorption and absorption temperatures were further reduced to 228 and 25 °C. Microstructure analysis revealed that the uniform distribution of Nb2O5 on MgH2 nanocrystals contributed to the greatly enhanced de/hydrogenation kinetics of modified MgH2 system.

Size dependent photocatalytic activities of rod-shape SnO2 nanocrystals

Various rod-shape SnO2 nanocrystals of different sizes and shapes were synthesized to study the desirable morphology for photocatalytic applications. An optimal apparent photocatalytic rate constant of 0.0809/min−1 was achieved for smaller SnO2 nanorods with diameter of ∼2.9 nm and with length of ∼9.3 nm, which is attributed to suitable nanostructures and high crystallinity. Photocatalytic activities as a function of growth and evolution of SnO2 nanorods are revealed.

Investigation into the crystal structure-dielectric property correlation in barium titanate nanocrystals of different sizes.

For high capacitance multilayer ceramic capacitors, high dielectric constant and lead-free ceramic nanoparticles are highly desired. However, as the particle size decreases to a few tens of nanometers, their dielectric constant significantly decreases, and the underlying mechanism has yet to be fully elucidated. Herein, we report a systematic investigation into the crystal structure-dielectric property relationship of combustion-made BaTiO3 (BTO) nanocrystals. When the nanocrystal size was 100 nm and below, a metastable paraelectric cubic phase was found in the as-received BTO (denoted as arBTO) nanocrystals based on an X-ray diffraction (XRD) study. A stable ferroelectric tetragonal phase was present when the nanocrystal size was above 200 nm. Notably, the cubic arBTO (particle size ≤100 nm) exhibited tetragonal fluctuations as revealed by Raman spectroscopy, whereas the tetragonal arBTO (particle size ≥200 nm) contained ∼10% cubic fraction according to the Rietveld fitting of the XRD profiles. Thermal annealing of the multi-grain tetragonal arBTO at 950 °C yielded single crystals of annealed BTO (denoted as anBTO), whose dielectric constants were higher than those of arBTO. However, the single crystalline anBTO prevented the formation of 90° domains; therefore, they exhibited a low dielectric constant of ∼300. Although X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy could not identify the exact structural defects, our study revealed that surface and bulk defects formed during synthesis affect the final crystal structures and thus the dielectric properties of BTO nanocrystals with different sizes. The understanding obtained from this study will help us design high dielectric constant perovskite nanocrystals for next-generation multilayer ceramic capacitor applications.

Systematic synthesis of different-sized AgInS2/GaS x nanocrystals for emitting the strong and narrow excitonic luminescence

This paper presents for the first time the systematic synthesis of AgInS2 (AIS) nanocrystals (NCs) with different sizes of 2.6–6.8 nm just by controlling only the reaction temperature. The synthesis of AIS core NCs was carried out in 2 steps: (i) synthesis of Ag2S NCs and then (ii) partial exchange of Ag+ with In3+ in the template Ag2S NCs. For step (i), Ag2S NCs of different sizes were synthesized by reaction of the Ag and S precursors at different temperatures of 30 °C to 130 °C, for the same reaction time of 30 min. For step (ii), AIS NCs were created by the exchange of Ag+ with In3+ at 120 °C for 60 min. Finally, GaS x was shelled on AIS core NCs to produce the AgInS2/GaS x core/shell structures. The synthesized AIS/GaS x NCs demonstrate the clear excitonic absorptions and strong, narrow excitonic luminescence peaking at 530–606 nm depending on the size of AIS core NCs.

Numerical modelling of the potential inside the cylindrical shaped nanocrystallite metal oxide semiconductors and its effect on gas sensor response

The surface of metal oxide semiconductors exhibits great gas sensing response when the size of the grains becomes much smaller, i.e. in the range of the Debye length. In the presented work, the gas sensing properties of nanocrystalline cylindrical shaped metal oxide semiconductors have been studied. The effect of crystal size and surface states on gas sensor response has been investigated. The electric potential inside the grains has been calculated by solving the Poisson’s equation coupled with electroneutrality condition. The relationship between the electric potential and the size of the nanograins has been established as a function of available surface states, surface temperature and occupied surface states. It has been found that the critical value of available surface state density increases with an increase in the size of nanograins. Total carrier concentration inside the grain volume has been simulated as a function of available surface state density and grain size. Eventually, the sensitivity of the gas sensor has been simulated and compared for different sizes of nanocrystals. From the simulated studies, it is clear that the smaller cylindrical-shaped nanostructured grains could result in better gas sensing performances. Presented model has universal application for all metal oxides (both n-type and p-type) based semiconductors and can be extended for both oxidising and reducing gases with essential modifications.

Relationship of the electric potential inside the nano crystallite with the chemisorbed gas molecules at metal oxide semiconductor surface and its effect on gas sensor response

The metal oxide semiconductors surface exhibit great gas sensing response when the size of the grains becomes much smaller i.e. in the range of debye length. In the presented study, the effect of crystal size and surface states has been studied on gas sensing properties for cylindrical shaped nanostructures of metal oxide semiconductors. To calculate the grain potential Poisson’s equation coupled with electroneutrality condition in cylindrical shaped nanostructures has been solved. The shape of the potential profile inside the nanograins has been calculated as a function of available surface states, surface temperature and occupied surface states. Eventually, the sensitivity of the gas sensor has been simulated and compared for different sizes of nanocrystals.

Unravelling three-dimensional adsorption geometries of PbSe nanocrystal monolayers at a liquid-air interface

The adsorption, self-organization and oriented attachment of PbSe nanocrystals (NCs) at liquid-air interfaces has led to remarkable nanocrystal superlattices with atomic order and a superimposed nanoscale geometry. Earlier studies examined the NC self-organization at the suspension/air interface with time-resolved in-situ X-ray scattering. Upon continuous evaporation of the solvent, the NC interfacial layer will finally contact the (ethylene glycol) liquid substrate on which the suspension was casted. In order to obtain structural information on the NC organization at this stage of the process, we examined the ethylene glycol/NC interface in detail for PbSe NCs of different sizes, combining in-situ grazing-incidence small-and-wide-angle X-ray scattering (GISAXS/GIWAXS), X-ray reflectivity (XRR) and analytical calculations of the adsorption geometry of these NCs. Here, we observe in-situ three characteristic adsorption geometries varying with the NC size. Based on the experimental evidence and simulations, we reveal fully three-dimensional arrangements of PbSe nanocrystals at the ethylene glycol-air interface with and without the presence of rest amounts of toluene.

Engineering of colloidal nanocrystal thin films and their optoelectronic properties: A simple and effective route

This work investigates the effect of heat treatments on CdSe/CdS core–shell nanocrystals assemblies with different sizes and capping ligands. Nanostructured thin films, consisting of an array of CdSe quantum dots embedded in a continuous matrix of CdS, have been obtained by mild thermal treatments of CdSe/CdS core–shell nanocrystals deposited on glass substrates from colloidal solution. These materials are regarded as promising photon upconverters, or absorbers for high-efficiency photovoltaic devices. The treatments were conducted in the range 300–550 °C for up to 150 min. The nanoscale evolution during the heat treatment was investigated by monitoring optical properties of the films. Initially, the well-defined photoluminescence band, corresponding to the first quantum dots exciton state, rapidly shifts towards lower energy, indicating delocalization of the exciton and suggesting that nanocrystal CdS shells are merging by sintering mechanisms, thus forming a continuous matrix. Control over this phase of the heat treatment leads to the desired nanoscale morphology and properties. The kinetics of sintering has been characterized, and the procedure has been systemically applied to CdSe/CdS nanocrystals of different sizes and ligand types. This research provides novel values for the activation energy of the sintering process and strongly support the general applicability of such process to control and optimize optoelectronic properties of nanostructured thin films for applications like light-emitting diodes, tunable emitters and photoconductors among others.

Primary growth of binary nanoparticle superlattices with distinct systems contingent on synergy: softness and crystalline anisotropy

Binary nanoparticle superlattices (BNSLs), which are studied for developing multicomponent materials and multifunctional nanodevices, are distinguished by their outstanding synergetic properties. Owing to complex factors that might influence the formation of BNSLs, it is necessary to comprehensively explore the self-assembly of nanocrystals. A variety of BNSL structural configurations, such as NaZn13, Cu3Au, NaCl, AlB2, CaCu5-type structures, were obtained by cocrystallizing Au–Ni and Ag–Ni nanocrystals of different sizes. Specifically, at a similar effective size ratio, the supercrystal structures formed by the Au–Ni and Ag–Ni systems coated with different organic ligands are not essentially the same. We attribute this diversity to the different softness of effective entities and facet-dependent interdigitation between the ligands tethered on the nanoparticle surface. Inherent crystallinity directed formation of the final BNSL structures is likely driven by anisotropic or isotropic cocrystallization. The synergistic effect of the coating agent and crystallinity directed assembly process is reasonably believed to be the important factor affecting the growth of BNSLs, which provides a new idea for the study of kinetics during the self-assembly of supercrystals.

Surfactant-Induced Anion Exchange and Morphological Evolution for Composition-Controlled Caesium Lead Halide Perovskites with Tunable Optical Properties.

Environmentally stable lead halide perovskite nanostructures with engineered composition and morphology are attractive because of their exotic optical properties. Here, we report the synthesis of monodispersed (∼20 nm) CsPbI3 cubic perovskite nanocrystals (NCs) using edible olive oil as a solvent as well as a chelating reagent. Thereafter, bromide anion exchange reaction using the cetyl trimethyl ammonium bromide surfactant in hexane is carried out at relatively lower temperatures to synthesize caesium lead halide perovskites with variable halide compositions and tunable band gaps. Interestingly, because of the formation of micelles, continuous morphology evolution varying from NCs of different sizes to nanowires (NWs) and nanosheets is observed. The anion exchange temperature has a distinct effect on the morphology of the CsPbBr3 nanostructure and the anion exchange reaction rate. Finally, an easy solution-processed photoconductive device is demonstrated using as-grown CsPbBr3 NWs, indicating its potential for optoelectronic applications.

Precise synthesis of monodisperse PdAg nanoparticles for size-dependent electrocatalytic oxidation reactions

The fabrication of nanomaterials with tunable shape and size has attracted much attention in chemical and physical fields. It is particularly significant to study the effect of catalyst particle size on catalytic performance in the field of catalysis. At present, there are relatively few studies in this field, the main challenge lies in the difficulty of synthesizing catalysts with uniform size. If the particle size of a certain size is not uniform, it is meaningless to compare the catalytic performance with that of other catalysts for size effect. Herein, we successfully synthesized the monodisperse PdAg nanoparticles with three different sizes by changing the type of reducing agents. Among the three catalysts, the PdAg-S NPs (PdAg-Small nanoparticles) with an average size of 3.1 nm exhibited superior electrocatalytic performances of ethylene glycol and glycerol oxidation reactions, which were obviously better than others size PdAg NPs catalysts and commercial Pd/C catalyst. This work revealed the size effect of PdAg nanoparticles in the alcohols electrooxidation and provided a simple method in the design of different sizes of nanocrystals.

The influence of grain size on the magnetic properties of Fe3O4 nanocrystals synthesized by solvothermal method

The peculiar magnetic properties of superparamagnetic iron oxide nanoparticles (SIONs) have spurred great interest in their potential application in enhancing medical images and drug delivery. Currently hindering the application of nanomagnetic particles in these fields is due to SIONs nonuniform size distribution, agglomeration, non-crystallinity, and low magnetic moment. In this paper, magnetic nanocrystals of different sizes and morphologies were prepared by the solvothermal method, which utilized an n-hexane-surfactant system with Fe(acac)3 and iron-powder precursors. The crystalline structure and particle size of grown SIONs are characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). HRTEM showed that the SIONs were approximately 5.6–6.8 nm in size with no agglomeration and a uniform distribution. Within the reaction time, HRTEM also confirmed that the shape and nanocrystals of SIONs had changed from an irregular crystal shape to triangles and quadrilaterals, and finally to hexagonal nanoparticles. According to the test results of the magnetic properties test and the literature, the material that has been synthesized is a superparamagnetic material. And the maximum saturation magnetization was 62.65 emu/g, which is reported among the largest magnetic properties of SIONs at a size of 6.8 nm.

Chemistry of Shape-Controlled Iron Oxide Nanocrystal Formation.

Herein, we demonstrate that meticulous and in-depth analysis of the reaction mechanisms of nanoparticle formation is rewarded by full control of the size, shape, and crystal structure of superparamagnetic iron oxide nanocrystals during synthesis. Starting from two iron sources, iron(II) and iron(III) carbonate, a strict separation of oleate formation from the generation of reactive pyrolysis products and concomitant nucleation of iron oxide nanoparticles was achieved. This protocol enabled us to analyze each step of nanoparticle formation independently in depth. The progress of the entire reaction was monitored via matrix-assisted laser desorption ionization time-of-flight mass spectrometry and gas chromatography, thus providing insight into the formation of various iron oleate species prior to nucleation. Interestingly, due to the intrinsic strongly reductive pyrolysis conditions of the oleate intermediates and redox process in early stages of the synthesis, pristine iron oxide nuclei were composed exclusively from wüstite irrespective of the oxidation state of the iron source. Controlling the reaction conditions provided a very broad range of size- and shape-defined monodispersed iron oxide nanoparticles. Curiously, after nucleation, star-shaped nanocrystals were obtained that underwent metamorphism toward cubic-shaped particles. Electron energy loss spectroscopy tomography revealed ex post oxidation of the primary wustite nanocrystal, providing a full 3D image of Fe2+ and Fe3+ distribution within. Overall, we developed a highly flexible synthesis, yielding multi-gram amounts of well-defined iron oxide nanocrystals of different sizes and morphologies.

Microstructural, optical, magnetic and photocatalytic properties of Mn doped ZnO nanocrystals of different sizes

The present work explores the differences in the various properties of Mn doped ZnO nanocrystals when the sizes of the doped nanocrystals are significantly different. Mn doped ZnO nanocrystals of sizes ≈20 nm and ≈5 nm were synthesized by the chemical co-precipitation method. The targeted Mn doping concentrations were 2%, 6% and 10% (atomic), however, the observed doping concentrations were low (≈0.5%, ≈1% and ≈2%). The microstructural, morphological, optical and magnetic properties as well as the photocatalytic activities of the nanocrystals were investigated. The variations of the lattice strain and lattice volume with increasing Mn concentrations were found to be correlated. The un-doped and Mn doped ZnO nanocrystals were found to be ferromagnetic at room temperature. Variation of band gap with increasing Mn concentrations was observed. Significant/insignificant quenching of the photoluminescence emissions was observed for sizes ≈20 nm/≈ 5 nm after doping. The saturation magnetization was found to depend on the Mn concentrations as well as on the nanocrystal sizes. Agglomerated Mn2+ resulted in a decrease in saturation magnetization of the ≈2% Mn doped nanocrystals of sizes ≈5 nm. Correlation between the photoluminescence properties and photocatalytic activities was observed.

Effect of hierarchical ZSM-5 zeolite crystal size on diffusion and catalytic performance of n-heptane cracking

Hierarchical ZSM-5 zeolite aggregates with different sizes of nanocrystals were synthesized using different amounts of the mesoporogen 3-aminopropyltriethoxysilane. The effect of the crystal size on the catalytic cracking of n-heptane was investigated and the Thiele modulus and effectiveness factor were used to determine the reaction rate-limiting step. The crystal size affected the textual properties of the catalysts but not the acidic properties of the catalysts. The reaction rate was first order with respect to the n-heptane concentration. Cracking over hierarchical zeolites with nanocrystal sizes larger than about 50 nm took place under transition-limiting conditions, whereas the reaction over hierarchical zeolites with nanocrystal sizes of 15 or 30 nm proceeded under reaction control conditions. Hierarchical ZSM-5 zeolite aggregates with smaller nanocrystals had better selectivity for light olefins which can be ascribed to the shorter diffusion path lengths and lower diffusion resistance in these catalysts. Furthermore, these catalysts had lower coking levels which can be attributed to the substantial number of mesopores which prevent the formation of coke precursors. Open image in new window

Nanocrystals for improved dermal drug delivery

Nanocrystals are composed of 100% active and possess an increased aqueous solubility and dissolution velocity when compared to larger sized materials. Nanocrystals can be used to improve the bioavailability of poorly soluble actives not only for oral, but also for topical application. In this study nanocrystals of different sizes were produced and the influence of size on dermal penetration was investigated. The influence of different excipients and vehicles on the penetration efficacy upon dermal application was also investigated. Results confirm that dermal penetration of poorly soluble actives increases with decreasing size of the nanocrystals. Unexpectedly, it was observed that many classical penetration enhancers failed to promote the penetration of actives from nanocrystals. Also hydrogels were found to be non-suitable vehicles for the formulation of nanocrystals. As most suitable vehicles for nanocrystals oleogels and creams were identified.