Infrared-emitting colloidal nanocrystals: synthesis, assembly, spectroscopy, and applications

Semiconductor nanocrystals produced by means of colloidal chemistry in a solvent medium are an attractive class of nanometer-sized building blocks from which to create complex materials with unique properties for a variety of applications. Their optical and electronic properties can be tailored easily, both by their chemical composition and particle size. While colloidal nanocrystals emitting in the infrared region have seen a burst of attention during the last decade there is clearly a paucity of review articles covering their synthesis, assembly, spectroscopic characterization, and applications. This Review comprehensively addresses these topics for II-VI, III-V, and IV-VI nanocrystals, examples being HgTe and Cd(x)Hg(1-) (x)Te, InP and InAs, and PbS, PbSe, and PbTe, respectively. Among the applications discussed here are optical amplifier media for telecommunications systems, electroluminescence devices, and noninvasive optical imaging in biology.     

Microwave-assisted synthesis of anisotropic gold nanocrystals in polymer matrix and their catalytic activities

We report a facile and fast one-pot microwave (MW) synthesis of gold nanotriangle, nanobranches and nanorod. Gold salt is reduced to gold nanoparticles by ethylene glycol/polyethylene glycol in the presence of hydroxypropyl methyl cellulose. Morphology of the nanocrystals can be tailored by varying the amount of capping agents, reducing agent and the MW irradiation time. Synthesised nanoparticles are characterised by ultraviolet–visible (UV–Vis), transmission electron microscopic and X-ray diffraction spectroscopy study. The key requirement for producing the gold triangle and gold nanorod of higher aspect ratio is the adequate MW heating time. The photocatalytic activity of gold nanocrystals is investigated by monitoring the photochemical reduction of 2,4,6-trinitrophenol (TNP) in the presence of excess NaBH₄. The reaction is first order with respect to the concentration of TNP. It has been observed that the catalytic efficiency of rod-like gold nanocrystals is higher than the other gold nanocrystals.     

Design and synthesis of polymetallic nanoparticles and their catalytic applications

High-temperature hydrogen reduction reactions enable the synthesis and processing of binary metal oxide composite nanoparticles starting from titanium, ruthenium, and silicon, while the use of a surface modifier and an organic surfactant enables the synthesis of catalytic thin films from binary semiconductor oxides. Surface characterization by XRD, SEM, TEM, AFM, Raman spectroscopy, and BET measurements indicate that the incorporation of binary oxide particles into the semiconductor materials altered the surface properties and morphology of the nanoparticles while the surface modifier and organic surfactant loading can be experimentally adjusted to obtain thin films of varying morphological characteristics.     

Shape control and applications of nanocrystals

Inorganic nanocrystals with well-defined shapes are important for understanding basic size-dependent scaling laws, and may be useful in a wide range of applications. Methods for controlling the shapes of inorganic nanocrystals are evolving rapidly. This paper will focus on how we currently control the shape of nanocrystals and this will be illustrated using CdSe and Co nanocrystals as examples for semiconductors and for metals. These materials show a more pronounced variation of fundamental properties with aspect ratio. However, to take advantage of these shape-dependent properties in possible applications, several challenges need to be overcome. Issues such as alignment, high quantum yield and photostability and precise control of three-dimensional structures need to be addressed. These challenges, as well as several potential applications, will be described briefly.     

Ultrafast synthesis of gold nanoparticles on cellulose nanocrystals via microwave irradiation and their dyes-degradation catalytic activity

This study represents a well-dispersed gold nanoparticles(AuNPs) synthesis process via cellulose nanocrystals(CNC) which acts as both reducing and supporting agent. The synthesis process was ultrafast and completed in a few seconds using microwave irradiation. The entire synthesis process was cost-effective, sustainable and eco-friendly. The synthesized(AuNPs/CNC) nanocomposite was investigated by transmission electron microscopy, selected area electron diffraction, Fourier transform infrared spectroscopy, energy dispersive X-ray analysis, X-ray diffraction, and UV–vis spectroscopy. The obtained AuNPs were well accumulated on the CNC surface and had a uniform spherical shape with an average diameter of 8 ± 5.3 nm. The diameter of AuNPs could be altered by tuning the concentration of CNC suspension. The synthesized AuNPs/CNC nanocomposite film exhibited excellent degradation properties against various organic dyes, namely, Allura red, Congo red, Rhodamine B and Amaranth. The ultrafast degradation reactions followed pseudo first order kinetics. In the catalytic degradation reaction,AuNPs/CNC was transmitting electrons from a donor(NaBH_4) to an acceptor(a dye).     

Synthesis, characterization and catalytic activity of CdO nanocrystals

In this paper, we report the synthesis of nanocrystalline cadmium oxide (CdO) and its characterization by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Its catalytic activity was investigated on the thermal decomposition of 1,2,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX), ammonium perchlorate (AP), hydroxyl terminated polybutadiene (HTPB) and composite solid propellants (CSPs) using thermogravimetric analysis (TG), simultaneous thermogravimerty and differential scanning calorimetry (TG-DSC) and ignition delay measurements. Kinetics of thermal decomposition of AP + CdO has also been investigated using model free (isoconversional) and model-fitting approaches which have been applied to data for isothermal TG decomposition. All these studies show enhancement in the rate of decomposition of AP, HTPB and CSPs but no effect on HMX. The burning rate of CSPs has also been found to be increased with CdO nanocrystals.     

Syntheses and applications of Mn-doped II-VI semiconductor nanocrystals

Luminescent Mn-doped II-VI semiconductor nanocrystals have been intensively investigated over the last ten years. Several semiconductor host materials such as ZnS, CdS, and ZnSe have been used for Mn-doped nanocrystals with different synthetic routes and surface passivation. Beyond studies of their fundamental properties including photoluminescence and size, these luminescent nanocrystals have now been tested for practical applications such as electroluminescent displays and biological labeling agents (biomarkers). Here, we first review ZnS:Mn, CdS:Mn/ZnS core/shell, and ZnSe:Mn nanocrystal systems in terms of their synthetic chemistries and photoluminescent properties. Second, based on ZnS:Mn and CdS:Mn/ZnS core/shell nanocrystals as electroluminescent components, direct current electroluminescent devices having a hybrid organic/inorganic multilayer structure are reviewed. Highly luminescent and photostable CdS:Mn/ZnS nanocrystals can further be used as the luminescent biomarkers and some preliminary results are also discussed here.     

Fungal xylanases‐mediated synthesis of silver nanoparticles for catalytic and biomedical applications

Green synthesis of nanoparticles has fuelled the use of biomaterials to synthesise a variety of metallic nanoparticles. The current study investigates the use of xylanases of Aspergillus niger L3 (NEA) and Trichoderma longibrachiatum L2 (TEA) to synthesise silver nanoparticles (AgNPs). Characterisation of AgNPs was carried out using UV–Vis spectroscopy, Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy, while their effectiveness as antimicrobial, antioxidant, catalytic, anticoagulant, and thrombolytic agents were determined. The colloidal AgNPs was brownish with surface plasmon resonance at 402.5 and 410 nm for NEA‐AgNPs and TEA‐AgNPs, respectively; while FTIR indicated that protein molecules were responsible for the capping and stabilisation of the nanoparticles. The spherical nanoparticles had size of 15.21–77.49 nm. The nanoparticles significantly inhibited the growth of tested bacteria (63.20–88.10%) and fungi (82.20–86.10%), and also scavenged DPPH (37.48–79.42%) and hydrogen peroxide (20.50–96.50%). In addition, the AgNPs degraded malachite green (78.97%) and methylene blue (25.30%). Furthermore, the AgNPs displayed excellent anticoagulant and thrombolytic activities using human blood. This study has demonstrated the potential of xylanases to synthesise AgNPs which is to the best of our knowledge the first record of such. The present study underscores the relevance of xylanases in nanobiotechnology.Inspec keywords: visible spectra, catalysis, ultraviolet spectra, silver, microorganisms, antibacterial activity, transmission electron microscopy, surface plasmon resonance, nanoparticles, nanofabrication, colloids, blood, Fourier transform infrared spectra, particle sizeOther keywords: Ag, fungal xylanases‐mediated synthesis, silver nanoparticles, catalytic applications, biomedical applications, green synthesis, metallic nanoparticles, Trichoderma longibrachiatum L2, transmission electron microscopy, antimicrobial agents, antioxidant agents, catalytic agents, thrombolytic agents, surface plasmon resonance, spherical nanoparticles, FTIR spectra, anticoagulant agents, colloidal nanoparticles, biomaterials, Aspergillus niger L3, UV‐vis spectroscopy, Fourier transform infrared spectroscopy, protein molecules, DPPH, hydrogen peroxide, malachite green, methylene blue, human blood, nanobiotechnology     

Carbon nanoscrolls: synthesis and applications

Carbon nanoscrolls (CNSs), as an emerging family member of carbon nanomaterials, are a spirally wrapped 2D graphene sheet with a 1D tubular structure resembling that of multi-walled carbon nanotube. Due to its unique topological structure, CNSs not only share the remarkable mechanical, electronic properties and thermal conductivity exhibited by carbon nanotubes and graphene but also are expected to exhibit novel features. So they have attracted much attention from material scientists, chemists and physicists. Here, we review the research advances of preparative strategies of 1D CNSs with arc-discharge, CVD, self-scrolling, freeze-drying, cold quenching, functional groups/nanoparticles modification, mechanical ball milling, ultrasound-assisted and Langmuir–Blodgett methods, and potential applications in lithium ion/sulfur bateries, fuel Cells, supercapacitors, hydrogen storage, sensors, oscillators, photocatalytic materials and the other applications. We believe that CNSs will become another bright star after CNTs and graphene in the foreseeable future.     

Recent advances in germanium nanocrystals: Synthesis,optical properties and applications

Germanium nanocrystals (Ge NCs) have recently attracted renewed scientific interest as environmentally friendlier alternatives to classical II–VI and IV–VI QDs containing toxic elements such as Hg, Cd and Pb. Importantly, Ge NCs are nontoxic, biocompatible, and electrochemically stable. An essential requirement is the ability to prepare Ge NCs with narrow size distributions and well characterized surface chemistry, as these define many of their photophysical properties. However, a thorough discussion on these criteria has not been achieved to date. Here, size, surface control, and mechanisms for light emission in Ge NCs are discussed and their exciting recent applications are highlighted. The beneficial properties of Ge NCs suggest that this material can improve the performance of numerous devices like solar cells, photodetectors, and lithium ion batteries.     

Review of cellulose nanocrystals patents: preparation, composites and general applications

This review attempts to visualize the actual impact of nanocellulose-based materials in different areas. A detailed search in recent patent databases on nanocellulose showed the importance of this material, as well as relevant topics concerning its technological preparations to obtain versatile new composites materials, and the applications of nanocellulose in different domains. At the present moment, the most common techniques for nanocellulose preparation were found to be acid and enzymatic procedures, oxidation, electrospinning, high pressure homogenization, and steam explosion processes. Concerning nanocellulose composites, several aspects were found in recent patents ranging from simple to complex structures with different properties. As unique materials, nanocellulose can be used in different areas of expertise, such as in biomedical and technical applications. This review is a useful tool for researchers to provide an update on nanocellulose patents in an expanding and interesting field of nanotechnology.     

One-dimensional TiO2 nanomaterials: preparation and catalytic applications

This work reports on the syntheses of one-dimensional (1D) H2Ti3O7 materials (nanotubes, nanowires and their mixtures) by autoclaving anatase titania (Raw-TiO2) in NaOH-containing ethanol-water solutions, followed by washing with acid solution. The synthesized nanosized materials were characterized using XRD, TEM/HRTEM, BET and TG techniques. The autoclaving temperature (120-180 degrees C) and ethanol-to-water ratio (V(EtOH)/V(H2O) = 0/60 approximately 30/30) were shown to be critical to the morphology of H2Ti3O7 product. The obtained H2Ti3O7 nanostructures were calcined at 400-900 degrees C to prepare 1D-TiO2 nanomaterials. H2Ti3O7 nanotubes were converted to anatase nanorods while H2Ti3O7 nanowires to TiO2(B) nanowires after the calcination at 400 degrees C. The calcination at higher temperatures led to gradual decomposition of the wires to rods and phase transformation from TiO2(B) to anatase then to rutile. Photocatalytic degradation of methyl orange was conducted to compare the photocatalytic activity of these 1D materials. These 1D materials were used as new support to prepare Au/TiO2 catalysts for CO oxidation at 0 degrees C and 1,3-butadiene hydrogenation at 120 degrees C. For the CO oxidation reaction, Au particles supported on anatase nanorods derived from the H2Ti3O7 nanotubes (Au/W-180-400) were 1.6 times active that in Au/P25-TiO2, 4 times that in Au/Raw-TiO2, and 8 times that on TiO2(B) nanowires derived from the H2Ti3O7 nanotubes (Au/M-180-400). For the hydrogenation of 1,3-butadiene, however, the activity of Au particles in Au/M-180-400 was 3 times higher than those in Au/W-180-400 but similar to those in Au/P25-TiO2. These results demonstrate that the potential of 1D-TiO2 nanomaterials in catalysis is versatile.     

Water-soluble Ln-doped calcium fluoride nanocrystals: Controlled synthesis and luminescence properties

Water-soluble CaF₂ nanocrystals doped with different lanthanide ions (Eu³⁺, Tb³⁺) have been synthesized via a novel method using methanol as solvent. These nanocrystals can well disperse in water, forming a stable and transparent colloidal solution. The colloids of the Eu³⁺ and Tb³⁺ doped nanocrystals display intense red and green luminescence under ultraviolet excitation, respectively. The phase structures, morphology, surface structure and luminescence properties of CaF₂:Ln³⁺ nanocrystals were explored in detail. All of the results showed that these nanocrystals own powerful potentials as Down-conversion fluorescent label materials.     

Luminescent CdTe and CdSe semiconductor nanocrystals: preparation, optical properties and applications

The novel optical and electrical properties of luminescent semiconductor nanocrystals are appealing for ultrasensitive multiplexing and multicolor applications in a variety of fields, such as biotechnology, nanoscale electronics, and opto-electronics. Luminescent CdSe and CdTe nanocrystals are archetypes for this dynamic research area and have gained interest from diverse research communities. In this review, we first describe the advances in preparation of size- and shape-controlled CdSe and CdTe semiconductor nanocrystals with the organometallic approach. This article gives particular focus to water soluble nanocrystals due to the increasing interest of using semiconductor nanocrystals for biological applications. Post-synthetic methods to obtain water solubility, the direct synthesis routes in aqueous medium, and the strategies to improve the photoluminescence efficiency in both organic and aqueous phase are discussed. The shape evolution in aqueous medium via self-organization of preformed nanoparticles is a versatile and powerful method for production of nanocrystals with different geometries, and some recent advances in this field are presented with a qualitative discussion on the mechanism. Some examples of CdSe and CdTe nanocrystals that have been applied successfully to problems in biosensing and bioimaging are introduced, which may profoundly impact biological and biomedical research. Finally we present the research on the use of luminescent semiconductor nanocrystals for construction of light emitting diodes, solar cells, and chemical sensors, which demonstrate that they are promising building blocks for next generation electronics.     

Flame synthesis of zinc oxide nanocrystals

Distinctive zinc oxide (ZnO) nanocrystals were synthesized on the surface of Zn probes using a counter-flow flame medium formed by methane/acetylene and oxygen-enriched air streams. The source material, a zinc wire with a purity of ～99.99% and diameter of 1 mm, was introduced through a sleeve into the oxygen rich region of the flame. The position of the probe/sleeve was varied within the flame medium resulting in growth variation of ZnO nanocrystals on the surface of the probe. The shape and structural parameters of the grown crystals strongly depend on the flame position. Structural variations of the synthesized crystals include single-crystalline ZnO nanorods and microprisms (ZMPs) (the ZMPs have less than a few micrometers in length and several hundred nanometers in cross section) with a large number of facets and complex axial symmetry with a nanorod protruding from their tips. The protruding rods are less than 100 nm in diameter and lengths are less than 1 μm. The protruding nanorods can be elongated several times by increasing the residence time of the probe/sleeve inside the oxygen-rich flame or by varying the flame position. At different flame heights, nanorods having higher length-to-diameter aspect-ratio can be synthesized. A lattice spacing of ～0.26 nm was measured for the synthesized nanorods, which can be closely correlated with the (0 0 2) interplanar spacing of hexagonal ZnO (Wurtzite) cells. The synthesized nanostructures were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HR-TEM), X-ray energy dispersive spectroscopy (EDS), and selected area electron diffraction pattern (SAED). The growth mechanism of the ZnO nanostructures is discussed.     

CuAgSe nanocrystals: colloidal synthesis,characterization and their thermoelectric performance

CuAgSe is a promising thermoelectric (TE) material for its superior carrier mobility and ultralow lattice thermal conductivity. Herein, we present a scalable colloidal method to prepare monodisperse CuAgSe nanocrystals with high yield. The collected powder sample was washed by a sulfur-free reagent of NaNH₂ to remove the surface organic ligands (CuAgSe-W) and then annealed (CuAgSe-W-A). Both kinds of ligand-free samples were then hot pressed into dense pellets to measure the TE property. The results revealed that the crystal structure of both samples changed from low-temperature β-phase to high-temperature α-phase at around 465 K. Sample CuAgSe-W shows interesting temperature-dependent transition from N-type to P-type, which could be potentially used as thermal control transistor. Sample CuAgSe-W-A does not display this transition state but it exhibits potential for intermediate temperature TE applications with a figure-of-merit zT reaching 0.68 at 566 K.     

Hydrothermal synthesis of HgTe rod-shaped nanocrystals

HgTe nanorods composed of crystalline particles with the diameter of 100-300 nm and length of up to 2-3 μm have been prepared by a hydrothermal method, and characterized by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy (TEM) and energy-dispersive X-ray analysis (EDX). It was found that ammonia played a key role in the formation of HgTe nanorods.     

Bimetallic catalysts for continuous catalytic wet air oxidation of phenol.

Catalytic wet oxidation has proved to be effective at eliminating hazardous organic compounds, such as phenol, from waste waters. However, the lack of active long-life oxidation catalysts which can perform in aqueous phase is its main drawback. This study explores the ability of bimetallic supported catalysts to oxidize aqueous phenol solutions using air as oxidant. Combinations of 2% of CoO, Fe2O3, MnO or ZnO with 10% CuO were supported on gamma-alumina by pore filling, calcined and later tested. The oxidation was carried out in a packed bed reactor operating in trickle flow regime at 140 degrees C and 900 kPa of oxygen partial pressure. Lifetime tests were conducted for 8 days. The pH of the feed solution was also varied. The results show that all the catalysts tested undergo severe deactivation during the first 2 days of operation. Later, the catalysts present steady activity until the end of the test. The highest residual phenol conversion was obtained for the ZnO-CuO, which was significantly higher than that obtained with the 10% CuO catalyst used as reference. The catalyst deactivation is related to the dissolution of the metal oxides from the catalyst surface due to the acidic reaction conditions. Generally, the performance of the catalysts was better when the pH of the feed solution was increased.     

Facile synthesis of Pt multipods nanocrystals

A facile and efficient seeded growth approach was used to fabricate single-crystal Pt multipods nanocrystals, which were intensively characterized by TEM, ED, HRTEM, XRD, EDX, and XPS. The size and shape of Pt multipod nanocrystals can be easily controlled by varying the ratio of Pt seeds to H2PtCl6. The catalytic performance of these nanocrystals as heterogeneous catalysts was examined using the hydrogenation of cyclohexene as a model reaction in a biphasic system. These Pt nanocrystals should have potential applications as catalysts in organic synthesis, electronics, sensors, and other devices.