Facet-controlled Self-assembly of ZnO Nanocrystals by Non-Hydrolytic Aminolysis and their Photodegradation Activities

We report a method for controlling the exposed facets and hence the dimensionality and shape of ZnO nanocrystals using a non-hydrolytic aminolysis synthesis route. The effects of changes to reaction conditions on ZnO formation were investigated and possible self-assembly mechanisms proposed. The crystal facet growth and hence morphologies of the ZnO nanocrystals were controlled by varying reaction temperature and the reactant ratio. Four distinct ZnO nanocrystal types were produced (nanocones, nanobullets, nanorods and nanoplates). The relative photocatalytic activities of the exposed facets of these ZnO nanostructures were also examined, which showed the activities obviously depended on the reactivity of exposed crystal facets in the order: {1011}>>{0001}, {1010}.

Microwave synthesis and spectroscopic characterization of manganese oxalate nanocrystals

The microwave synthesis of MnC2O4·2H2O nanoparticles was performed through the thermal double decomposition of oxalic acid dihydrate (C2H2O4·2H2O) and Mn(OAc)2·4H2O solutions using a CATA-2R microwave reactor. Structural characterization was performed using X-ray diffraction (XRD), particle size and shape were analyzed using transmission electron microscopy (TEM). The chemical in the structures was investigated using electron paramagnetic resonance (EPR) as well as optical absorption spectra and near-infrared (NIR) spectroscopies. The nanocrystals produced with this method were pure and had a distorted rhombic octahedral structure.

Efficient catalysts of zeolite nanocrystals grown with a preferred orientation on nanofibers

An innovative structure — nanozeolites (as shell) grown with preferred orientation on ceramic nanofibers (as core) was proposed. The Y-zeolite nanocrystals on TiO2 nanofibers exhibited superior ability to catalyze acetalization and carboxylation reaction, achieving high conversions to desired products with selectivity of 100% under moderate conditions.

Controlled synthesis of inorganic semiconductor nanocrystals and their applications

This thesis is a comprehensive study of the synthesis of nanomaterials. It explores the synthetic methods on the control of the size, shape and phase of semiconductor nanocrystals. A number of important conclusions, including the mechanism behind crystal growth and the structure-relationship, have been drawn through the experimental and theoretical investigation. The synthesized nanocrystals have been tested for applications in gas sensing, photocatalysis and solar cells, which exhibit considerable commercialization potential.

Painting anatase (TiO2) nanocrystals on long nanofibers to prepare photocatalysts with large active surface for dye degradation and organic synthesis

Anatase TiO2 nanocrystals were painted on H-titanate nanofibers by using an aqueous solution of titanyl sulfate. The anatase nanocrystals were bonded solidly onto the titanate fibers through formation of coherent interfaces at which the oxygen atoms were shared by the nanocrystals and the fiber. This approach allowed us to create large anatase surfaces on the nanofibers, which are active in photocatalytic reactions. This method was also applied successfully to coat anatase nanocrystals on surfaces of fly ash and layered clay. The painted nanofibers exhibited a much higher catalytic activity for the photocatalytic degradation of sulforhodamine B and the selective oxidation of benzylamine to the corresponding imine (with a product selectivity >99%) under UV irradiation than both the parent H-titanate nanofibers and a commercial TiO2 powder, P25. We found that gold nanoparticles supported on H-titanate nanofibers showed no catalytic activity for the reduction of nitrobenzene to azoxybenzene, whereas the gold nanoparticles supported on the painted nanofibers and P25 could efficiently reduce nitrobenzene to azoxybenzene as the sole product under visible light irradiation. These results were different from those from the reduction on the gold nanoparticles photocatalyst on ZrO2, in which the azoxybenzene was the intermediate and converted to azobenzene quickly. Evidently, the support materials significantly affect the product selectivity of the nitrobenzene reduction. Finally, the new photocatalysts could be easily dispersed into and separated from a liquid because of their fibril morphology, which is an important advantage for practical applications.

Non-square-well potential profile and suppression of blinking in compositionally graded Cd1−xZnxSe/CdxZn1−xSe nanocrystals

Random blinking is a major problem on the way to successful applications of semiconducting nanocrystals in optoelectronics and photonics, which until recently had neither a practical solution nor a theoretical interpretation. An experimental breakthrough has recently been made by fabricating non-blinking Cd1-xZnxSe/ZnSe graded nanocrystals [Wang et al., Nature, 2009, 459, 686]. Here, we (1) report an unequivocal and detailed theoretical investigation to understand the properties (e.g., profile) of the potential-well and the distribution of Zn content with respect to the nanocrystal radius and (2) develop a strategy to find the relationship between the photoluminescence (PL) energy peaks and the potential-well due to Zn distribution in nanocrystals. It is demonstrated that the non-square-well potential can be varied in such a way that one can indeed control the PL intensity and the energy-level difference (PL energy peaks) accurately. This implies that one can either suppress the blinking altogether, or alternatively, manipulate the PL energy peaks and intensities systematically to achieve a controlled non-random intermittent luminescence. The approach developed here is based on the ionization energy approximation and as such is generic and can be applied to any non-free-electron nanocrystals.

Colloidal semiconductor nanocrystals : controlled synthesis and surface chemistry in organic media

Colloidal semiconductor nanocrystals (CS-NCs) possess compelling benefits of low-cost, large-scale solution processing, and tunable optoelectronic properties through controlled synthesis and surface chemistry engineering. These merits make them promising candidates for a variety of applications. This review focuses on the general strategies and recent developments of the controlled synthesis of CS-NCs in terms of crystalline structure, particle size, dominant exposed facet, and their surface passivation. Highlighted are the organic-media based synthesis of metal chalcogenide (including cadmium, lead, and copper chalcogenide) and metal oxide (including titanium oxide and zinc oxide) nanocrystals. Current challenges and thus future opportunities are also pointed out in this review.

Comparative behavior of CdS and CdSe quantum dots in poly(3-hexylthiophene) based nanocomposites

CdS and CdSe nanoparticles have been prepared using conducting poly(3-hexylthiophene) (P3HT) matrix with an objective to understand the effect of nanoparticles on the polymer matrix using electrochemical and spectroscopic techniques. The spectroscopic results reveal that the electronic structure of polymer is strongly influenced by the characteristics of embedded semiconducting nanoparticles. SEM and TEM images show the ordered morphology of the CdS and CdSe nanoparticles in presence of the polymer matrix. Cyclic voltammetry performed both in the presence and absence of light enables us to understand the redox changes in P3HT due to CdS and CdSe quantum dots such as the generation of free radical in the excited state and their electrochemical band gaps.

Hybrid chalcogenide nanoparticles : 2D-WS2 nanocrystals inside nested WS2 fullerenes

The MOCVD assisted formation of nested WS2 inorganic fullerenes (IF-WS2) was performed by enhancing surface diffusion with iodine, and fullerene growth was monitored by taking TEM snapshots of intermediate products. The internal structure of the core-shell nanoparticles was studied using scanning electron microscopy (SEM) after cross-cutting with a focused ion beam (FIB). Lamellar reaction intermediates were found occluded in the fullerene particles. In contrast to carbon fullerenes, layered metal chalcogenides prefer the formation of planar, plate-like structures where the dangling bonds at the edges are stabilized by excess S atoms. The effects of the reaction and annealing temperatures on the composition and morphology of the final product were investigated, and the strength of the WS2 shell was measured by intermittent contact-mode AFM. The encapsulated lamellar structures inside the hollow spheres may lead to enhanced tribological activities.

Enhanced electron lifetime of CdSe/CdS quantum dot (QD) sensitized solar cells using ZnSe core–shell structure with efficient regeneration of quantum Ddots

Research on development of efficient passivation materials for high performance and stable quantum dot sensitized solar cells (QDSCs) is highly important. While ZnS is one of the most widely used passivation material in QDSCs, an alternative material based on ZnSe which was deposited on CdS/CdSe/TiO2 photoanode to form a semi-core/shell structure has been found to be more efficient in terms of reducing electron recombination in QDSCs in this work. It has been found that the solar cell efficiency was improved from 1.86% for ZnSe0 (without coating) to 3.99% using 2 layers of ZnSe coating (ZnSe2) deposited by successive ionic layer adsorption and reaction (SILAR) method. The short circuit current density (Jsc) increased nearly 1-fold (from 7.25 mA/cm2 to13.4 mA/cm2), and the open circuit voltage (Voc) was enhanced by 100 mV using ZnSe2 passivation layer compared to ZnSe0. Studies on the light harvesting efficiency (ηLHE) and the absorbed photon-to-current conversion efficiency (APCE) have revealed that the ZnSe coating layer caused the enhanced ηLHE at wavelength beyond 500 nm and a significant increase of the APCE over the spectrum 400−550 nm. A nearly 100% APCE was obtained with ZnSe2, indicating the excellent charge injection and collection process in the device. The investigation on charge transport and recombination of the device has indicated that the enhanced electron collection efficiency and reduced electron recombination should be responsible for the improved Jsc and Voc of the QDSCs. The effective electron lifetime of the device with ZnSe2 was nearly 6 times higher than ZnSe0 while the electron diffusion coefficient was largely unaffected by the coating. Study on the regeneration of QDs after photoinduced excitation has indicated that the hole transport from QDs to the reduced species (S2−) in electrolyte was very efficient even when the QDs were coated with a thick ZnSe shell (three layers). For comparison, ZnS coated CdS/CdSe sensitized solar cell with optimum shell thickness was also fabricated, which generated a lower energy conversion efficiency (η = 3.43%) than the ZnSe based QDSC counterpart due to a lower Voc and FF. This study suggests that ZnSe may be a more efficient passivation layer than ZnS, which is attributed to the type II energy band alignment of the core (CdS/CdSe quantum dots) and passivation shell (ZnSe) structure, leading to more efficient electron−hole separation and slower electron recombination.

Assembly of sol-gel-grown $Li _{(x)} CoO_{2}$ nanocrystals through electromagnetic irradiation

We report the fabrication of assembled nanostructures from the pre-synthesized nanocrystals building blocks through optical means of exciton formation and dissociation. We demonstrate that Li (x) CoO2 nanocrystals assemble to an acicular architecture, upon prolonged exposure to ultraviolet-visible radiation emitted from a 125 W mercury vapor lamp, through intermediate excitation of excitons. The results obtained in the present study clearly show how nanocrystals of various materials with band gaps appropriate for excitations of excitons at given optical wavelengths can be assembled to unusual nanoarchitectures through illumination with incoherent light sources. The disappearance of exciton bands due to Li (x) CoO2 phase in the optical spectrum of the irradiated film comprising acicular structure is consistent with the proposed mechanism of exciton dissociation in the observed light-induced assembly process. The assembly process occurs through attractive Coulomb interactions between charged dots created upon exciton dissociation. Our work presents a new type of nanocrystal assembly process that is driven by light and exciton directed.

Effect of Structural Modification on the Quantum-Size Effect in II-VI Semiconducting Nanocrystals

The electronic structure of group II-VI semiconductors in the stable wurtzite form is analyzed using state-of-the-art ab initio approaches to extract a simple and chemically transparent tight-binding model. This model can be used to understand the variation in the bandgap with size, for nanoclusters of these compounds. Results complement similar information already available for same systems in the zinc blende structure. A comparison with all available experimental data on quantum size effects in group II-VI semiconductor nanoclusters establishes a remarkable agreement between theory and experiment in both structure types, thereby verifying the predictive ability of our approach. The significant dependence of the quantum size effect on the structure type suggests that the experimental bandgap change at a given size compared to the bulk bandgap, may be used to indicate the structural form of the nanoclusters, particularly in the small size limit, where broadening of diffraction features often make it difficult to unambiguously determine the structure.