Tuning the potentials of "extra" electrons in colloidal n-type ZnO nanocrystals via Mg2+ substitution

Colloidal reduced ZnO nanocrystals are potent reductants for one-electron or multielectron redox chemistry, with reduction potentials tunable via the quantum confinement effect. Other methods for tuning the redox potentials of these unusual reagents are desired. Here, we describe synthesis and characterization of a series of colloidal Zn(1-x)Mg(x)O and Zn(0.98-x)Mg(x)Mn(0.02)O nanocrystals in which Mg(2+) substitution is used to tune the nanocrystal reduction potential. The effect of Mg(2+) doping on the band-edge potentials of ZnO was investigated using electronic absorption, photoluminescence, and magnetic circular dichroism spectroscopies. Mg(2+) incorporation widens the ZnO gap by raising the conduction-band potential and lowering the valence-band potential at a ratio of 0.68:0.32. Mg(2+) substitution is far more effective than Zn(2+) removal in raising the conduction-band potential and allows better reductants to be prepared from Zn(1-x)Mg(x)O nanocrystals than can be achieved via quantum confinement of ZnO nanocrystals. The increased conduction-band potentials of Zn(1-x)Mg(x)O nanocrystals compared to ZnO nanocrystals are confirmed by demonstration of spontaneous electron transfer from n-type Zn(1-x)Mg(x)O nanocrystals to smaller (more strongly quantum confined) ZnO nanocrystals.     

Comparison of extra electrons in colloidal n-type Al(3+)-doped and photochemically reduced ZnO nanocrystals

The "extra" electrons in colloidal n-type ZnO nanocrystals formed by aliovalent doping and photochemical reduction are compared. Whereas the two are similar spectroscopically, they show very different electron-transfer reactivities, attributable to their different charge-compensating cations (Al(3+)vs. H(+)).     

A simple solvothermal route towards the morphological control of ZnO and tuning of its optical and photocatalytic properties

ZnO nanoparticles with different morphologies were solvothermally synthesized by controlling the alkali (sodium hydroxide) concentration in an isopropanol solution. The products were characterized by means of powder X-ray diffraction, UV-visible absorption spectra, scanning electron microscopy, transmission electron microscopy, and selected area electron diffraction. The morphologies of the formed ZnO nanocrystals were dependent on the concentration of the alkali, and with increases of sodium hydroxide concentration, the ZnO nanocrystals evolved from rod to hexagonal bipyramid, and then to a flower-like nanostructure. The flower-like nanostructure resulted from the etching of the hexagonal bipyramid by the excess alkali. The photoluminescence and photocatalytic properties of the prepared ZnO were investigated. The difference of green emission among the ZnO nanocrystals indicated that a higher sodium hydroxide concentration led to a higher level of defects. The size, the surface structure and defects in the ZnO nanocrystals affected its photo-degradation characteristics.     

Synthesis and Characterization of Mn-Doped ZnO Nanocrystals

We report the synthesis and characterization of several sizes of Mn-doped ZnO nanocrystals, both in the free-standing and the capped particle forms. The sizes of these nanocrystals could be controlled by capping them with polyvinylpyrollidone under different synthesis conditions and were estimated by X-ray diffraction and transmission electron microscopy. The absorption properties of PVP-capped Mn-doped ZnO exhibit an interesting variation of the band gap with the concentration of Mn. Fluorescence emission, electron paramagnetic resonance, and X-ray absorption spectroscopy provide evidence for the presence of Mn in the interior as well as on the surface of the nanocrystals.     

Photo-physical studies of pyridine capped ZnO nanostructures

Pyridine capped ZnO nanocrystals with different sizes were synthesized at room temperature by wet chemical synthesis. Pyridine provides the control over the morphology of final product. X-ray study confirms the crystalline hexagonal structure of the capped and uncapped ZnO nanocrystals. The particle size was found to decrease with increase in capping concentration. Electron microscopy investigation reveals the uniform morphology of the product. Optical absorption studies indicate the blue shift effect for pyridine capped ZnO as compare to uncapped ZnO.     

Colloidal transition-metal-doped ZnO quantum dots

Methods for introducing new magnetic, optical, electronic, photophysical, or photochemical properties to semiconductor nanocrystals are attracting intense applications-oriented interest. In this communication, we report the preparation and electronic absorption spectroscopy of colloidal ZnO DMS-QDs. Our synthetic procedure involves modification of literature methods known to yield highly crystalline and relatively monodisperse nanocrystals of pure ZnO to allow introduction of transition-metal dopants. We use ligand-field electronic absorption spectroscopy as a dopant-specific optical probe to monitor dopant incorporation during nanocrystal growth and to verify internal substitutional doping in Co2+:ZnO and Ni2+:ZnO DMS-QDs. To the best of our knowledge, these are the first free-standing oxide DMS-QDs reported. The synthesis of colloidal oxide DMS-QDs introduces a new category of magnetic semiconductor materials available for detailed physical study and application in nanotechnology.     

Enhanced Raman scattering and photocatalytic activity of Ag/ZnO heterojunction nanocrystals

In this work, we study the enhancement of Raman signals and photocatalytic activity of Ag/ZnO heterojunctions with an Ag content of 1 at.%, which were synthesized by photochemical deposition of Ag nanoparticles onto pre-synthesized ZnO nanorods. A strong interaction between Ag and ZnO nanocrystals were evidenced by XPS and UV-vis spectroscopy. The binding energy of Ag nanoparticles shifts toward lower energy compared to that of pure Ag nanoparticles, revealing that electrons transfer from Ag to the ZnO nanocrystals. The red shift of the plasmon absorption peak of Ag nanoparticles in Ag/ZnO heterojunctions further confirms the strong interaction between the two components. This strong interaction, arising from the coupling between Ag and ZnO nanocrystals, is responsible for the enhancement of Raman signals and photocatalytic activity of the Ag/ZnO heterojunctions.     

Morphology-controlled large-scale synthesis of ZnO nanocrystals from bulk ZnO

Gram-scale, teardrop-like, monodisperse ZnO nanocrystals with a well-resolved absorption onset and a strong sharp UV emission were prepared by thermal decomposition of ZnO-oleic acid complexes in hexadecylamine.     

Structural,optical, electrical,and photocatalytic properties of manganese doped zinc oxide nanocrystals

Manganese-doped and undoped ZnO nanocrystals were synthesized via wet-chemical methods. The structure, physico-chemical, electrical and optical properties of the as-prepared products were characterized by using the X-ray diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PLS) and electrochemical impedance spectroscopy (EIS) techniques. The photocatalytic activity of Mn-doped ZnO nanocrystal (mixed phases) has been examined under the visible-irradiation by using photocatalytic oxidation of rhodamine B (RhB) dye as a model reaction, and compared with that of known system such as pure ZnO nanocrystal (single-phase). The results showed that Mn doped ZnO nanocrystals bleaches RhB much faster than undoped ZnO upon its exposure to the visible light. The enhancement of the photocatalytic activity was discussed as an effect due to the Mn doping in the Mn-doped ZnO semiconductors, which shifts the optical absorption edge to the visible region and alters the electron-hole pair separation conditions. These factors are responsible for the higher photocatalytic performance of Mn/ZnO composites.     

Synthesis of colloidal Mn2+:ZnO quantum dots and high-TC ferromagnetic nanocrystalline thin films

We report the synthesis of colloidal Mn(2+)-doped ZnO (Mn(2+):ZnO) quantum dots and the preparation of room-temperature ferromagnetic nanocrystalline thin films. Mn(2+):ZnO nanocrystals were prepared by a hydrolysis and condensation reaction in DMSO under atmospheric conditions. Synthesis was monitored by electronic absorption and electron paramagnetic resonance (EPR) spectroscopies. Zn(OAc)(2) was found to strongly inhibit oxidation of Mn(2+) by O(2), allowing the synthesis of Mn(2+):ZnO to be performed aerobically. Mn(2+) ions were removed from the surfaces of as-prepared nanocrystals using dodecylamine to yield high-quality internally doped Mn(2+):ZnO colloids of nearly spherical shape and uniform diameter (6.1 +/- 0.7 nm). Simulations of the highly resolved X- and Q-band nanocrystal EPR spectra, combined with quantitative analysis of magnetic susceptibilities, confirmed that the manganese is substitutionally incorporated into the ZnO nanocrystals as Mn(2+) with very homogeneous speciation, differing from bulk Mn(2+):ZnO only in the magnitude of D-strain. Robust ferromagnetism was observed in spin-coated thin films of the nanocrystals, with 300 K saturation moments as large as 1.35 micro(B)/Mn(2+) and T(C) > 350 K. A distinct ferromagnetic resonance signal was observed in the EPR spectra of the ferromagnetic films. The occurrence of ferromagnetism in Mn(2+):ZnO and its dependence on synthetic variables are discussed in the context of these and previous theoretical and experimental results.     

Structures and optical properties of Zn1?x Ni x O nanoparticles by coprecipitation method

Nanocrystals of undoped and nickel-doped zinc oxide (Zn_1?x Ni _x O, where x?=?0.00?C0.05) were synthesized by the coprecipitation method. Crystalline size, morphology, and optical absorption of prepared samples were determined by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), and UV?Cvisible spectrometer. XRD and SEM studies revealed that Ni-doped ZnO crystallized in hexagonal wurtzite structure. Doping of ZnO with Ni²⁺ was intended to enhance the surface defects of ZnO. The incorporation of Ni²⁺ in place of Zn²⁺ provoked an increase in the size of nanocrystals as compared to undoped ZnO. Crystalline size of nanocrystals varied from 10 to 40?nm as the calcination temperature increased. Enhancement in the optical absorption of Ni-doped ZnO indicated that it can be used as an efficient photocatalyst under visible light irradiation. Optical absorption measurements indicated a red shift in the absorption band edge upon Ni doping. The band gap value of prepared undoped and Ni-doped ZnO nanoparticles decreased as annealing temperature was increased up to 800?°C.     

Luminescence and photocatalytic activity of ZnO nanocrystals: correlation between structure and property

Low-dimensional ZnO nanocrystals with controlled size, aspect ratio, and oxygen defects (e.g., type and concentration) are successfully prepared through simple solvothermal and thermal treatment methods. The structure of the as-synthesized samples is characterized by XRD, N2 physical adsorption, TEM, and IR and XPS spectra. The results show that the aspect ratio and size of the as-synthesized ZnO nanocrystals increase with increasing [OH-]/[Zn2+]; the morphology evolves from nanorod to nanoparticle with an increase in the annealing temperature; the BET surface areas of the corresponding samples decrease during these processes, respectively; and different oxygen defects, which are likely to be oxygen vacancy (Vo**) and interstitial oxygen (Oi'), are formed in our experiments accordingly. With evolution of the structure, IR absorption bands and visible photoluminescence emission peaks of the synthesized ZnO nanocrystals shift and split, which is ascribed to the change of oxygen defects. In addition, it is found that the photocatalytic activity of the synthesized ZnO nanocrystals is mainly dependent on the type and concentration of oxygen defects. The relationship of structure-property and the possible photocatalytic mechanism are discussed in detail.     

Electron transfer between colloidal ZnO nanocrystals

Colloidal ZnO nanocrystals capped with dodecylamine and dissolved in toluene can be charged photochemically to give stable solutions in which electrons are present in the conduction bands of the nanocrystals. These conduction-band electrons are readily monitored by EPR spectroscopy, with g* values that correlate with the nanocrystal sizes. Mixing a solution of charged small nanocrystals (e(-)(CB):ZnO-S) with a solution of uncharged large nanocrystals (ZnO-L) caused changes in the EPR spectrum indicative of quantitative electron transfer from small to large nanocrystals. EPR spectra of the reverse reaction, e(-)(CB):ZnO-L + ZnO-S, showed that electrons do not transfer from large to small nanocrystals. Stopped-flow kinetics studies monitoring the change in the UV band-edge absorption showed that reactions of 50 μM nanocrystals were complete within the 5 ms mixing time of the instrument. Similar results were obtained for the reaction of charged nanocrystals with methyl viologen (MV(2+)). These and related results indicate that the electron-transfer reactions of these colloidal nanocrystals are quantitative and very rapid, despite the presence of ~1.5 nm long dodecylamine capping ligands. These soluble ZnO nanocrystals are thus well-defined redox reagents suitable for studies of electron transfer involving semiconductor nanostructures.     

Synthesis and Photocatalytic Properties of Coupled Nanocrystalline ZnO/ZnS in Nafion Membrane

The coupled nanocrystalline ZnO/ZnS was fabricated and immobilized in Nafion membrane by using sodium sulfide (Na2S) as the single anion precursor. The molar ratio of ZnO to ZnS can be controlled by simply adjusting the reaction time. The as-prepared ZnO/ZnS-Nafion samples were characterized by various methods, including optical absorption, X-ray diffraction and high-resolution transmission electron microscopy. These coupled ZnO/ZnS nanocrystals embedded in Nafion membrane displayed excellent photocatalytic activities for their efficient charge separation properties. A mechanism of ZnO/ZnS nanoparticle fabrication in Nafion was deduced from the solubility difference, and the photocatalytic mechanism of coupled ZnO/ZnS was discussed as well.     

Photoelectrochemical property of the BiOBr-BiOI/ZnO heterostructures with tunable bandgap

Layered BiOBr-BiOI composites with tunable bandgap were deposited onto ZnO nanowire arrays by spin-coating. The prepared BiOBr-BiOI/ZnO heterostructures were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV-visible absorption, and photoelectrochemical response. The bandgap of the BiOBr-BiOI can be tuned by varying the ratio of BiOBr/BiOI. The BiOBr-BiOI composites were proved to be n-type semiconductors, which serve as sensitizers in the BiOBr-BiOI/ZnO heterostructures. BiOBr-BiOI/ZnO heterostructures show much higher visible light photoelectrochemical activity than ZnO nanowire arrays because of the visible light absorption of BiOBr-BiOI and the formation of heterojunction between BiOBr-BiOI and ZnO, which reduces the recombination of photogenerated electrons and holes. In addition, the bandgap of BiOBr-BiOI directly affects the photoelectrochemical performance of the BiOBr-BiOI/ZnO heterojunctions. The smaller bandgap of the BiOBr-BiOI is, the more visible light is absorbed and the higher photoelectrochemical performance of the BiOBr-BiOI/ZnO heterojunctions achieves. The BiOBr-BiOI/ZnO heterostructures can be developed for application in water splitting and other optoelectrical devices.     

The Effect of CdSe and InP Quantum Dots on the Interaction of ZnO with NO<Subscript>2</Subscript> under Visible Light Irradiation

Nanocomposites based on nanocrystalline ZnO and CdSe and InP nanocrystals (quantum dots) have been synthesized by chemical precipitation and high-temperature colloidal synthesis. The microstructure parameters of the oxide matrix and the size of the CdSe and InP nanocrystals have been determined. A correlation was established between the spectral dependence of the photoconductivity of nanocomposites and the optical absorption spectra of quantum dots. The influence of CdSe and InP quantum dots on the interaction of ZnO with NO₂ under visible light irradiation has been studied. It has been shown that the synthesized nanocomposites can be used to detect NO₂ under illumination with green light without additional thermal heating.     

Synthesis and Photocatalytic Properties of Coupled Nanocrystalline ZnO/ZnS in Nation Membrane

T The coupled nanocrystalline ZnO/ZnS was fabricated and immobilized in Nafion membrane by using sodium sulfide (Na_2S) as the single anion precursor. The molar ratio of ZnO to ZnS can be controlled by simply adjusting the reaction time. The as-prepared ZnO/ZnS-Nafion samples were characterized by various methods, including optical absorption, X-ray diffraction and high-resolution transmission electron microscopy. These coupled ZnO/ZnS nanocrystals embedded in Nafion membrane displayed excellent photocatalytic activities for their efficient charge separation properties. A mechanism of ZnO/ZnS nanoparticle fabrication in Nation was deduced from the solubility difference, and the photocatalytic mechanism of coupled ZnO/ZnS was discussed as well.     

Synthesis of temperature and pH-responsive crosslinked micelles from polypeptide-based graft copolymer

We report a method for synthesizing small-diameter ZnO nanorods at room temperature (20 °C), under normal atmospheric pressure (1 atm), and using a relatively short reaction time (1 h) by adding gallium salts to the reaction solution. The ZnO nanorods were, on average, 92 nm in length and 9 nm in diameter and were single crystalline in nature. Quantitative analyses revealed that gallium atoms were not incorporated into the synthesized nanocrystals. On the basis of the experimental results, we propose a mechanism for the formation of small-diameter ZnO nanorods in the presence of gallium ions. The optical properties were probed by UV-Vis diffuse reflectance spectroscopy. The absorption band of the small-diameter ZnO nanorods was blue-shifted relative to the absorption band of the ~230 nm diameter ZnO nanorods (control samples). Control experiments demonstrated that the absence of metal ion-containing precipitants (except ZnO) at room temperature is essential, and that the ZnO nanorod diameter distributions were narrow for the stirred reaction solution and broad when prepared without stirring.     

Infrared absorptive properties of Al-doped ZnO divided powder

Al-doped ZnO powders were synthesised by a Pechini process in order to obtain visible non-absorbent and near-Infrared absorbent particles. Firstly, it has been shown that synthesis under argon combined with the lowest synthesis temperatures (700 °C) allows getting the optimal properties for pure ZnO compounds due to creation of n-type defects segregated on oxide grain surface (Zn/O ratio superior to 1). Nevertheless, the near-Infrared absorption properties of the pure ZnO compounds remain low. The Al³⁺ doping of ZnO compounds was then investigated. The Al solubility limit inside ZnO doped compounds decreases drastically with the grain size, i.e. with the synthesis temperature. Then, the Al cations distribution varies inside ZnO grains, Al³⁺ segregation at the grain surfaces taking place for high synthesis temperatures. The optimal optical properties (high near-Infrared absorption) are reached combining Al-doping and adequate synthesis conditions: annealing under argon at low temperatures. In these conditions, the highest extrinsic (via Al doping) and intrinsic n-types defects rates are indeed reached.     

Study of the Growth of Capped ZnO Nanocrystals: A Route to Rational Synthesis

We report the study of complex and unexpected dependencies of nanocrystal size as well as nanocrystal‐size distribution on various reaction parameters in the synthesis of ZnO nanocrystals using poly(vinyl pyrollidone) (PVP) as a capping agent. This method establishes a qualitatively different growth mechanism to the anticipated Ostwald ripening behavior. The study of size‐distribution kinetics and an understanding of the observed non‐monotonic behaviors provides a route to rational synthesis. We used a simple, but accurate, approach to estimate the size‐distribution function of nanocrystals from the UV‐absorption spectrum. Our results demonstrate the accuracy and generality of this approach, and we also illustrate its application to various semiconducting nanocrystals, such as ZnO, ZnS, and CdSe, over a wide size range (1.8–5.3 nm).