Monodisperse Nanocrystals Research Articles

Structural and optical properties of solvothermal synthesized nearly monodispersed CdSe nanocrystals

Water soluble nearly monodisperse CdSe nanocrystals have been successfully synthesized via aqueous phase solvothermal route in non ionic surfactant glycolic acid ethoxylate 4-non phenyl ether (GAEPE). X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) are used to determine the phase, structural parameters such as lattice constants, strain, x-ray density and specific surface area, morphology, shape and size distribution, respectively, whereas optical properties are studied by UV-visible absorption and photoluminescence (PL) spectroscopy. All the diffraction peaks of XRD pattern indexed to wurtzite phase of hexagonal system of CdSe and crystallite sizes estimated to be 13–29 nm along some stronger and narrower peaks which is also consistent with TEM measurement while crystallinity and defects have been analyzed with selective area electron diffraction (SAED) pattern. Optical absorption spectrum shows that the as prepared sample exhibits primary and secondary absorption band centered at 2.15 eV and 1.82 eV, respectively, which is blue shifted as compared to bulk value (1.74 eV) of band gap due to quantum confinement effect. Photoluminescence spectrum shows sharp excitonic emission band centered at 583 nm which is nearer to primary band gap energy.

Seed mediated synthesis of highly mono-dispersed gold nanoparticles in the presence of hydroquinone

Gold nanoparticles (AuNPs) are being studied for several biomedical applications, including drug delivery, biomedical imaging, contrast agents and tumor targeting. The synthesis of nanoparticles with a narrow size distribution is critical for these applications. We report the synthesis of highly mono-dispersed AuNPs by a seed mediated approach, in the presence of tri-sodium citrate and hydroquinone (HQ). AuNPs with an average size of 18 nm were used for the synthesis of highly mono-dispersed nanocrystals of an average size 40 nm, 60 nm, 80 nm and ∼100 nm; but the protocol is not limited to these sizes. The colloidal gold was subjected to UV–vis absorbance spectroscopy, showing a red shift in lambda max wavelength, peaks at 518.47 nm, 526.37 nm, 535.73 nm, 546.03 nm and 556.50 nm for AuNPs seed (18 nm), 40 nm, 60 nm, 80 nm and ∼100 nm respectively. The analysis was consistent with dynamic light scattering and electron microscopy. Hydrodynamic diameters measured were 17.6 nm, 40.8 nm, 59.8 nm, 74.1 nm, and 91.4 nm (size by dynamic light scattering—volume %); with an average poly dispersity index value of 0.088, suggesting mono-dispersity in the size distribution, which was also confirmed by transmission electron microscopy analysis. The advantage of a seed mediated approach is a multi-step growth of nanoparticle size that enables us to control the number of nanoparticles in the suspension, for size ranging from 24.5 nm to 95.8 nm. In addition, the HQ-based synthesis of colloidal nanocrystals allowed control of the particle size and size distribution by tailoring either the number of seeds, amount of gold precursor or reducing agent (HQ) in the final reaction mixture.

Novel Nanoscale Electrode Materials for Sodium-Ion Batteries

Colloidal nanocrystals can serve as ideal model electrode materials for studying the effects of downsizing on electrode performance. We will present two examples of monodisperse nanocrystals as anode and cathode materials for Li-ion and Na-ion batteries (LiBs and NiBs). Fluoroperovskites of the form NaMF3 and LiMF3 (M = Ni, Mn, Fe) are identified as perspective oxygen-free cathode materials for LiBs and NiBs. Among them, NaFeF3 would have a clear priority due to its unbeatable combination of the high natural abundance of Fe and its non-toxicity. At the same time, mixed-metal fluorides exhibit a poor electronic conductivity. Therefore, fine particles of complex fluorides that provide short conductive path for electrons are required for high ion diffusion and improving the capacity of the cathodes. We have developed colloidal synthesis of NaFeF3 nanocrystals using decomposition of heterometallic fluorinated β-diketonates NaM(hfac)3 (M = Mn, Fe, Co and Ni; hfac=hexafluoroacetylacetonate) at 200-300 °C in a medium of high-boiling apolar solvent. NaM(hfac)3 compounds are stable in moist air and can be obtained by simple synthetic procedure from cheap, commercially available starting reagents. Such crystalline NaFeF3 nanoplates are capable of fast Na and Li intercalation/deintercalation. The use of NaFeF3 cathode material in form of nanoplates allowed to increase its cyclic stability and rate capability performance. In addition, electrochemical performance of NaFeF3 nanoplates was measured in a full cell configuration with antimony anode. The charge capacity of NaFeF3 nanoparticles after 20 cycles at 20mA/g was equal to 150mAh/g. Antimony sulfide has high potential as anode material for NiBs with are high theoretical capacity (946 mA h g-1). Nanostructured Sb2S3 can improve kinetic performance of NiB due to reducing the transport length of Na ions and electrons as well as accommodating the volume changes. We have developed hot-injection colloidal synthesis of monodisperse Sb2S3 nanoparticles of various shapes and sizes via reaction SbCl3 and bis(trimethylsilyl) sulfide in hot-boiling solvent. Crystalline and amorphous nanoparticles were tested as potential anode materials of LiBs and NiBs. In comparison with bulk Sb2S3, we show that monodisperse crystalline or amorphous Sb2S3 nanoparticles have enhanced the cycling stability. 20 nm amorphous Sb2S3 nanospheres exhibited higher capacity and excellent cycle performance. In particular, they showed Lion and Na-ion capacities of 1100 and 950 mAh/g, respectively, at a current density 330 mA/g. Besides good cycling performance, nanoscale Sb2S3 showed an excellent rate capability. Metal phosphides such as FeP, CoP, NiP2, and CuP2 remain essentially unexplored as electrode materials for SIBs, despite their high theoretical charge storage capacities of 900-1300 mAh g-1. We have developed the synthesis of metal phosphide NCs and assessed their electrochemical properties as anode materials for SIBs, as well as for LIBs. We also compared the electrochemical characteristics of phosphides with their corresponding sulfides, using the environmentally benign iron compounds, FeP and FeS2, as a case study. We show that despite the appealing initial charge storage capacities of up to 1200 mAh g-1, enabled by effective nanosizing of the active electrode materials, further work toward optimization of the electrode/electrolyte pair is needed to improve the cycling performance of FeP and other phosphides.

Highly active Ag clusters stabilized on TiO2 nanocrystals for catalytic reduction of p-nitrophenol

Ag/TiO2 nanocomposites comprising of Ag clusters on TiO2 nanocrystal surfaces are of great significance in catalysts and advanced functional materials. Herein a novel method to synthesize Ag/TiO2 nanocomposites with Ag clusters under 2nm on TiO2 nanocrystal surfaces have been developed. The success of this method relies on a silver mirror reaction in toluene, which refers to the reduction of silver-dodecylamine complexes by acetaldehyde in the presence of mono-dispersed TiO2 nanocrystals. The prepared Ag/TiO2 nanocomposites have been characterized by FT-IR spectra, UV–vis absorption spectra, X-ray diffraction (XRD) analysis, ultra high resolution scanning electron microscope (Ultra-HRSEM), high resolution transmission electron microscope (HRTEM) and X-ray photoelectron spectra (XPS). Catalytic activity of Ag/TiO2 nanocomposites is evaluated for the reduction of p-nitrophenol (4-NP) into p-aminophenol (4-AP) by NaBH4. Results demonstrate that Ag/TiO2 nanocomposites have shown an outstanding catalytic activity as well as a good stability in successive reduction of 4-NP. Noticeably, TOF of Ag/TiO2-0.75 nanocomposites obtained in this work is the highest among Ag based catalysts previously reported.

One-Step Synthesis of a High Performance Pt-Fe3O4 Catalyst: Intermetallic Al13Fe4 as a Platform and Precursor

A one-step process was developed to prepare catalysts with monodispersed Pt nanocrystals on Fe3O4, based on leaching of the intermetallic compounds Al76.5−xFe23.5Ptx (at%: x = 0, 0.05, 0.5, 1) with an aqueous 10 wt% NaOH solution. In this process, the intermetallic Al13Fe4 lattice acts as a platform in which Pt atoms can be homogeneously dispersed, and also serves as a precursor for subsequent leaching. Compared with conventional preparation methods, including impregnation and coprecipitation, this one-step process eliminates the need for thermal treatments such as high temperature calcination and/or reduction, thus minimizing the sintering or crystal grain growth problems caused by these treatments. Consequently, the materials derived from Al76Fe23.5Pt0.5 exhibit high catalytic performance during CO oxidation and the CO-PROX reaction due to the monodispersed Pt nanocrystals in the porous or nanoparticle Fe3O4 matrix. A high performance nanocomposite Pt-Fe3O4 catalyst, consisting of monodispersed Pt nanocrystals in a porous Fe3O4 matrix, was prepared by a one-step process based on caustic leaching of a bulk Al13(Fe + Pt)4 intermetallic compound.

Cross-linked branching nanohybrid polymer electrolyte with monodispersed TiO2 nanoparticles for high performance lithium-ion batteries

Nanohybrid polymer electrolytes (NHPE) with ceramic particles have attracted significant attention owing to their improvement in electrochemical performance. However, particle aggregation and weak nanoparticle/polymer matrix interaction restrict their further application in lithium-ion batteries (LIBs). We demonstrate a facile in-situ polymerization/crystallization method to synthesize a homogeneous TiO2-grafted NHPE with a cross-linked branching structure, comprised of ion-conducting poly(ethylene glycol) methyl ether methacrylate (PEGMEM) and non-polar stearyl methacrylate (SMA). This technique is different from existing methods of blending functionalized ceramic particles into the polymer matrix. Highly monodispersed TiO2 nanocrystals enhance the effective interfacial interactions between particles and polymer matrix, which suppress the crystallization of ethylene oxide (EO) groups and facilitate forming continuously interconnected ion-conducting channels. Moreover, an increased dissociation degree of Li salt can also be achieved. The TiO2-grafted NHPE exhibits superior electrochemical properties with an ionic conductivity of 1.1 × 10−4 S cm−1 at 30 °C, a high lithium ion transference number and excellent interfacial compatibility with the lithium electrode. In particular, a lithium-ion battery based on TiO2-grafted NHPE demonstrates good C-rate performance, as well as excellent cycling stability with an initial discharge capacity of 153.5 mAh g−1 and a capacity retention of 96% after 300 cycles at 1 C (80 °C).

Improved Near-Infrared Up-Conversion Emission of Tm3+ Sensitized by Yb3+ and Ho3+ in LuF3 Nanocrystals.

In the present work, mono-disperse and uniform orthorhombic lutetium fluoride (LuF3) nanocrystals with an average size of about 35 nm have been successfully synthesized by a simple ionothermal method without any template. The infrared (IR) to visible up-conversion (UC) photoluminescence of LuF3 doped with Yb3+, Tm3+, and Ho3+ under 980 nm excitation was systemically studied. The intensity of near infrared (NIR) to visible up-conversion emission of Tm3+ was improved efficiently by adding Yb3+ and Ho3+ in LuF3, especially for the broad NIR emission band located at 812 nm. Meanwhile, compared to the Yb3+ and Tm3+ co-doped LuF3, the ratio of red to green emission in the Yb3+, TmS+, and HoS+ co-doped LuF3 changed greatly, and a bright yellowish-green emission was observed under 980 nm laser excitation. It shows that Yb3+, Tm3+ and Ho3+ co-doped LuF3 nanocrystals provided a potential application in vitro and in vivo bio-imaging, color displays and optical storage.

Photodetectors: Inorganic UV–Visible–SWIR Broadband Photodetector Based on Monodisperse PbS Nanocrystals (Small 10/2016)

Synthesis of highly mono-dispersed large PbS nanocrystals (NCs) with low particle size dispersion is demonstrated by F. So and co-workers on page 1328, by controlling the sulfur concentration below the nucleation threshold during multiple injections of the sulfur precursor. These NCs are used to fabricate multi-spectral photodetectors and high detectivity values in the UV-Visible-SWIR (short-wave-IR) wavelength range from 350 nm to 2000 nm are demonstrated.

Synthesis of highly monodispersed PtCuNi nanocrystals with high electro-catalytic activities towards oxygen reduction reaction

Pt-based ternary alloy PtCuNi nanocrystals were synthesized through a highly straightforward and adaptable hydrothermal approach. By using appropriate combination of capping agents, the size of the nanocrystals can be controlled between 7–9nm with narrow size distribution, and their composition can be tuned in broad ranges. The composition and crystal structure of the nanocrystals were fully characterized by transmission electron microscope (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectrum (EDX) and X-ray photoelectron spectroscopic (XPS). The nanocrystals show single-crystalline nature without twinning. The proposed growth mechanism involves forming Cu-rich interior at the early stage followed by Pt-rich surface. The electro-catalysis measurements towards oxygen reduction reaction indicate that PtCuNi nanocrystals, when loaded on high surface area carbon black, exhibit enhanced activity compared with binary PtCu nanocrystals. Among the four catalyst samples with different compositions, Pt23Cu51Ni26/C catalyst exhibit the highest specific activity at 2.1mA/cm2 and mass activity at 0.9A/mgPt, which shows at least 2-time enhancement home-made PtCu/C catalyst. The enhancement possibly originates from the Pt-rich surface structure and the synergistic interaction between Cu and Ni atoms. This work demonstrates the unique advantage of using Pt-based ternary alloy for reduction of oxygen, and the potential of using them as cathode catalysts for proton exchange membrane fuel cell.

Enhanced upconversion in Dy3+, Yb3+ co-doped Gd2O3 monodisperse nanocrystals

Dy3+/Yb3+ co-doped Gd2O3 monodispersed nanocrystals have been synthesized using the sodium dodecyl benzene sulfonate (SDBS) as the surfactant. The SDBS surfactant plays a critical role in the forming of monodisperse Gd2O3 structures with sphere-like morphologies. The growth mechanism of the nanocrystals was proposed in detail. The samples annealed at 400 °C for 4 h exhibit good crystallinity and narrow size distribution with average size of 6–9 nm. Green and red emission of Dy3+ related to 4G11/2–6H11/2 and 4G11/2–6H9/2 transition was observed under the laser excitation at 980 nm. An involvement of three photons has been observed for these emissions. The energy distribution of upconversion spectra depended on doping concentration and the intensity changed with the increasing of Dy3+ concentration. The mechanisms of upconversion by multi-photon absorption and energy transfer processes transitions were interpreted and explained.

Surface Atomic Structure and Growth Mechanism of Monodisperse {1 0 0}-Faceted Strontium Titanate Zirconate Nanocubes

The highly sensitive and selective properties of monodisperse faceted nanocrystals inherently stem from the atomic and electronic structures on the faceted surfaces. For elemental nanocrystals, the atomic structure on the surfaces is merely determined by the geometric shape itself. However, for compound materials such as alloys and complex oxides, atomic details on the faceted surfaces need to be studied on the atomic level. Here, we demonstrate that the surface atomic structure of faceted nanocrystals of complex oxides, {1 0 0}-faceted strontium titanate zirconate nanocubes, can be unambiguously resolved by aberration-corrected scanning transmission electron microscopy. The resolved surface atomic details reveal a layerwise growth process of the nanocubes, thereby allowing an in-depth understanding of the growth mechanism.

Band-Structure-Engineered Materials Synthesis

Various aspects of preparing band-structure-engineered materials involving nanostructured building blocks are examined in this review. In continuation with enjoying great success in controlling the charge carrier transport propertiesof crystalline semiconductors, it was found better to choose a periodic structure comprising of monodisperse nanocrystals, where the adjustment of the wave function overlaps between nearest neighbors provided sufficient delocalization of the electron states necessary for charge carrier transport with adequate mobility. Of course, for maximum advantage from such a situation, it was necessary to employ monodisperse nanostructured building blocks with uniform inter-dot coupling, which turned out to be rather difficult to achieve in practice. For instance, nanocrystals size variations within 5% are currently considered monodisperse. For achieving higher degree of monodispersity, it is necessary to understand the phenomenon of nucleation and particle formation in more detail by examining the existing models and introducing more refinements, if possible. In case of adjusting inter-dot spacing, molecular linkers are adequately appropriate for this purpose. Moreover, the solution processing of nanostructured species possesses the unique feature of self-assembly, which is currently being explored for hierarchical superstructures ranging from nm to cm scales in dimension. This kind of material growth, being more close to biomimetic processes, when combined with conventional lithography and patterning, would certainly offer opportunities to use these hierarchical superstructures in numerous unprecedented applications.

Facile synthesis of Sm3+/Eu3+ codoped CeO2 ultrafine nanocrystals and oxygen vacancy site dependent photoluminescence

Ultrafine nanocrystals of Sm3+/Eu3+ codoped CeO2 have been prepared successfully by a simple and facile oxalate decomposition method. The structure, phase and size of the nanocrystals were characterized by X-ray diffraction and transmission electron microscope which confirmed that monodispersed nanocrystals of size below 5nm are formed by this novel technique. Spectroscopic investigations of the synthesized phosphors are carried out with the help of photoluminescence excitation, emission and decay analysis. The studies revealed that oxygen vacancies introduced in the crystal lattice for charge compensation strongly influences the luminescence properties of the phosphor material. By investigating the luminescence characteristics of the material with different doping concentrations of Sm3+ and Eu3+, the position of the oxygen vacancies with respect to the dopant ion is determined. Also it can be revealed that symmetry of Eu3+ ions are perturbed by the oxygen vacancies at nearest neighbor site which produce strong electric dipole transition emissions in the luminescence spectra. In the case of Sm3+ ions oxygen vacancies are repelled to the next nearest neighbor site and hence the cubic symmetry of the ions is preserved. In addition, occurrence of non radiative energy transfer from Sm3+ to Eu3+ in the phosphor material is confirmed by the close examination of the luminescence spectra. Chromaticity coordinates evidenced that color purity of the synthesized nanophosphors is marginally increased by the proper addition of Sm3+.

Microfluidic synthesis of monodispersed CdSe quantum dots nanocrystals by using mixed fatty amines as ligands

We developed a novel microfluidic method to synthesize full-color emitting CdSe semiconductor nanocrystals with good monodispersity and less amount of Cd by using mixed amines of oleylamine (OLA) and dodecyl amine (DDA) as ligands. By the control of mixed DDA and OLA, the stage of nucleation and growth were separated, leading to narrow size distribution of CdSe semiconductor nanocrystals. The full width at half-maximum (FWHM) of fluorescence spectrum of CdSe nanocrystals could be reduced to 27.6nm as optimizing the ratio of DDA to OLA and reaction temperature. Besides, through mixed amines as ligands, CdSe semiconductor nanocrystals which emitting fluorescence ranged from 530nm to 615nm could be synthesized by using less amount of Cd. So an environmental friendly method to synthesize full-color emitting CdSe semiconductor nanocrystals would be possible.

Oriented Metallic Nano-Objects on Crystalline Surfaces by Solution Epitaxial Growth.

Chemical methods offer the possibility to synthesize a large panel of nanostructures of various materials with promising properties. One of the main limitations to a mass market development of nanostructure based devices is the integration at a moderate cost of nano-objects into smart architectures. Here we develop a general approach by adapting the seed-mediated solution phase synthesis of nanocrystals in order to directly grow them on crystalline thin films. Using a Co precursor, single-crystalline Co nanowires are directly grown on metallic films and present different spatial orientations depending on the crystalline symmetry of the film used as a 2D seed for Co nucleation. Using films exposing 6-fold symmetry surfaces such as Pt(111), Au(111), and Co(0001), the Co heterogeneous nucleation and epitaxial growth leads to vertical nanowires self-organized in dense and large scale arrays. On the other hand, using films presenting 4-fold symmetry surfaces such as Pt(001) and Cu(001), the Co growth leads to slanted wires in discrete directions. The generality of the concept is demonstrated with the use of a Fe precursor which results in Fe nanostructures on metallic films with different growth orientations which depend on the 6-fold/4-fold symmetry of the film. This approach of solution epitaxial growth combines the advantages of chemistry in solution in producing shape-controlled and monodisperse metallic nanocrystals, and of seeded growth on an ad hoc metallic film that efficiently controls orientation through epitaxy. It opens attractive opportunities for the integration of nanocrystals in planar devices.

Uniform Bimetallic Nanocrystals by High-Temperature Seed-Mediated Colloidal Synthesis and Their Catalytic Properties for Semiconducting Nanowire Growth

A general procedure to prepare uniform gold-based bimetallic nanocrystals (NCs) is reported. The method relies on a seed-mediated approach in which deposition and in-situ alloying of a second metal (Ag, Pt, Hg, Sn, Cd) onto monodisperse Au seeds are performed at relatively high temperatures, giving access to bimetallic NCs of tunable compositions and properties. The position of the plasmon resonance in the original Au NCs is tunable over a wide range (∼300–520 nm) of the electromagnetic spectrum. We demonstrate the catalytic properties of these monodisperse NCs for growing single-crystalline semiconductor nanowires of uniform, small diameter (∼15–30 nm) via a vapor–liquid–solid (VLS) mechanism at low temperatures. This seeded-mediated approach is not restricted to Au but can be extended to several other combinations, making this procedure a straightforward method to prepare highly monodisperse and controllable multimetallic nanocrystals for optical and catalytic applications.

Reduction of Nano-Cu2O: Crystallite Size Dependent and the Effect of Nano-Ceria Support

Copper(I) oxide (Cu2O) is an effective catalyst in the CO oxidation reaction. While high surface to volume ratio in nanoparticles will increase their catalytic efficiency, it posts a stability problem. Here we study the stability of nano-cuprite against reduction as a function of its crystallite size and upon interaction with a nano-ceria support. A systematic analysis of isothermal reduction of a series size of monodispersed Cu2O nanocrystals (±7%) with time-resolved X-ray diffraction (TR-XRD) provides the time-resolved phase fraction of Cu2O and the time when reduction product of Cu (fcc) first appears. The initial phase fraction of nano-Cu2O is less than one with the balance attributed to an amorphous CuO shell. Since no peaks of crystalline CuO (monoclinic) were observed, a core–shell structure with an amorphous CuO shell is proposed. From the analysis, Cu2+ content in corresponding to shell increases from 0 to 33% as Cu2O decreases to 8 nm from the bulk. Based on the reduction profiles, a time size reduction (TSR) diagram is constructed for the observed Cu2O phase behavior during reduction. The incorporation onto a nano-CeO2 support (7 nm) significantly stabilizes our nano-Cu2O in a reducing atmosphere. The oxygen supply propensity in terms of oxygen nonstoichiometry of CeO2–y is shown to be lower when a larger crystallite size CeO2 (20 nm) support is used. The larger oxygen capacity in smaller nano-CeO2 support is analyzed and explained by the “Madelung model” with size-dependent bulk modulus of nano-ceria.

Separation of Silver Nanocrystals for Surface-enhanced Raman Scattering Using Density Gradient Centrifugation

One-dimensional silver nanocrystals (AgNCs) have been prepared by a polyol process using sodium hydroxide and nitric acid at a constant silver source concentration. Results indicate that the acidity-basicity plays an important role in silver-nanocrystal formation. Different morphologies of AgNCs were synthesized by changing the NaOH or HNO 3 amount. We demonstrate that nearly monodisperse silver nanocrystals can be separated from polydisperse samples using density gradient centrifugation separation (DGCS). We also demonstrate that the separated AgNCs can be used as substrates for surface-enhanced Raman scattering (SERS) spectroscopy. The separation approach provides a method of improving the nanocrystal quality produced by large-scale synthetic methods.

Narrowing the size distribution of CdTe nanocrystals using digestive ripening

Digestive ripening of polydispersed colloidal CdTe nanocrystals is performed which results in monodispersed nanocrystals (NCs) as studied by optical spectroscopy. Optimization of ligand and refluxing time is carried out. Monodispersed NCs are obtained using mercaptopropionic acid (MPA) as a digestive ripening agent at a refluxing time of 1–2 h. Digestive ripening of CdTe NCs, which are less polydispersed, is also executed and it leads to more monodispersed NCs. In all the cases, there is a shift of maximum emission wavelength of CdTe NCs after digestive ripening that may be due to Ostwald ripening along with digestive ripening.

Aluminum nanocrystals.

We demonstrate the facile synthesis of high purity aluminum nanocrystals over a range of controlled sizes from 70 to 220 nm diameter with size control achieved through a simple modification of solvent ratios in the reaction solution. The monodisperse, icosahedral, and trigonal bipyramidal nanocrystals are air-stable for weeks, due to the formation of a 2-4 nm thick passivating oxide layer on their surfaces. We show that the nanocrystals support size-dependent ultraviolet and visible plasmon modes, providing a far more sustainable alternative to gold and silver nanoparticles currently in widespread use.