ZnO纳米棒阵列到纳米管结构的自转化机制及其在相变封装材料中的应用

In the emerging field of nanoscience, tubular structures have been attracting remarkable interest due to their well-defined geometry, high specific area, and exceptional physical and chemical properties. Among them, oriented ZnO tubular arrays are regarded as promising candidates for various applications such as optoelectronics, solar cells, sensors, field emission, piezoelectrics, and catalysis. Although template-directed and selective dissolution synthesizing strategies are commonly used to prepare ZnO nanotubes, repeatability and large scale preparation are still challenging. In this study, ZnO nanotube arrays were controllably prepared by tuning the hydrothermal parameters, without the use of any additives. The mechanism underlying the self-conversion of ZnO nanorods to nanotubes was comprehensively studied based on the surface energy theory. It has been proved that the metastable top surface of the ZnO nanorods dissolves preferentially to reach a stable state during the hydrothermal growth. The specific surface energy of different crystal faces of ZnO nanorods was calculated using molecular dynamics simulation. The top surface of the ZnO nanorod, the Zn-terminated [0001] face, demonstrated much higher surface free energy than did the lateral faces, which indicated that the self-dissolution of top face (002) is energetically favorable. The self-conversion behavior of ZnO nanorod arrays with different diameters was specifically investigated by adjusting the initial precursor concentration, density of the crystal seed layers, and growth time. The dissolution-crystallization equilibrium concentration, determined by crystal surface energy, was found to be a key factor for the formation of the tubular structure. Notably, the critical equilibrium conditions for the self-conversion of ZnO nanorods to nanotubes, including zinc ion concentration and pH, have been identified by studying parameters corresponding to the dissolution-crystallization equilibrium for the metastable top surface of the ZnO nanorods. The preparation of the ZnO nanotube arrays was successfully accelerated and simplified via two-step procedure: (1) preparation of ZnO nanorod arrays and (2) self-conversion of ZnO nanorods to nanotubes. The preparation method based on the self-conversion mechanism from rods to tubes for polar oxides is simpler and more easily controllable as compared to the reported methods involving variety of additives. Because of the advantages of adaptability to a wide range of substrates, excellent conducting properties, and filling ability, the prepared ZnO nanotube array films were used in encapsulating phase-change materials. The encapsulated phase-change material exhibited excellent heat storage/release properties and heat conductivities. This indicates the potential application of precision devices for temperature control.     

Surface-plasmon-enhanced band emission of ZnO nanoflowers decorated with Au nanoparticles

A simple strategy was used to enhance band emission through the transfer of defect emission from ZnO to Au by using the energy match between the defect emission of ZnO and the surface plasmon absorbance of Au NPs through decorating the surface of ZnO nanoflowers with Au nanoparticles (Au NPs). The ZnO nanostructure, which was comprised of six nanorods that were attached on one side in a flower-like fashion, was synthesized by using a hydrothermal method. The temperature-dependent morphology and detailed growth mechanism were studied. The influence of the density of the Au NPs that were deposited onto the surface of ZnO on photoluminescence was investigated to optimize the configuration of the ZnO/Au system in terms of the maximum band emission. The sequential transfer of defect energy from ZnO to Au and electron transfer from excited Au to ZnO was proposed as a possible mechanism for the enhanced band emission.     

Surface‐Plasmon‐Enhanced Band Emission of ZnO Nanoflowers Decorated with Au Nanoparticles

A simple strategy was used to enhance band emission through the transfer of defect emission from ZnO to Au by using the energy match between the defect emission of ZnO and the surface plasmon absorbance of Au NPs through decorating the surface of ZnO nanoflowers with Au nanoparticles (Au NPs). The ZnO nanostructure, which was comprised of six nanorods that were attached on one side in a flower‐like fashion, was synthesized by using a hydrothermal method. The temperature‐dependent morphology and detailed growth mechanism were studied. The influence of the density of the Au NPs that were deposited onto the surface of ZnO on photoluminescence was investigated to optimize the configuration of the ZnO/Au system in terms of the maximum band emission. The sequential transfer of defect energy from ZnO to Au and electron transfer from excited Au to ZnO was proposed as a possible mechanism for the enhanced band emission.     

表面活性剂种类对纳米氧化锌形貌的影响

利用简单的水热法,以Zn(Ac)2.2H2O为锌源,通过调整表面活性剂的种类及碱源,制备了一系列不同形貌的纳米结构氧化锌;利用场发射扫描电子显微镜和X射线粉末衍射仪分析了产物的形貌和晶体结构,并探讨了多种表面活性剂和醋酸钠碱源对氧化锌纳米结构的影响.结果表明,以NaOH作为碱源时,在不添加任何表面活性剂的情况下,产物的形貌结构与氢氧化钠的加入量有关,当n(Zn2+)∶n(OH-)为1∶2.5、1∶10及1∶20时,分别得到片状、棒状及海胆状纳米结构的氧化锌;产物均为六方相氧化锌.     

Morphology‐Dependent Gas‐Sensing Properties of ZnO Nanostructures for Chlorophenol

The crystal‐plane effect of ZnO nanostructures on the toxic 2‐chlorophenol gas‐sensing properties was examined. Three kinds of single‐crystalline ZnO nanostructures including nanoawls, nanorods, and nanodisks were synthesized by using different capping agents via simple hydrothermal routes. Different crystal surfaces were expected for these ZnO nanostructures. The sensing tests results showed that ZnO nanodisks exhibited the greatest sensitivity for the detection of toxic 2‐chlorophenol. The results revealed that the sensitivity of these ZnO samples was heavily dependent on their exposed surfaces. The polar (0001) planes were most reactive and could be considered as the critical factor for the gas‐sensing performance. In addition, calculations using density functional theory were employed to simulate the gas‐sensing reaction involving surface reconstruction and charge transfer both of which result in the change of electronic conductance of ZnO.     

Microstructure control of Zn/ZnO core/shell nanoparticles and their temperature-dependent blue emissions

The microstructure of the Zn/ZnO core/shell nanoparticles synthesized by laser ablation in liquid medium can be facilely controlled. With the surfactant concentration increased over the critical micelle concentration, the nanoparticle transformed from pure ZnO to a Zn/ZnO core/shell structure. Further, with a decrease of the applied laser power, the ZnO shell thickness was monotonously reduced till 2.5 nm and the ultrafine ZnO nanocrystals embedded in the nanoshells were also reduced till 1.5 nm, which induced the increase of the disorder degree of the nanoshell lattice. The controlling mechanism was discussed according to the competition of capping protection and the oxidation reaction of laser-induced plasma. Blue photoluminescence from the ZnO nanoshells was observed. The emission band exhibited abnormal red-blue shift and narrowing with increasing temperature. Such temperature-dependent behaviors can be well described by a localization model involving an interstitial zinc defect center. These results indicate that this method provides a convenient and universal way to obtain various metal/oxide core/shell nanoparticles with controllable microstructure, and it will be beneficial to an understanding of the physical origins of the blue emission in nanostructured ZnO as well as to extending its optical and electronic applications.     

Secondary nucleation and growth of ZnO

Recently we discovered that under certain conditions new crystal growth (branch) can be induced on specific crystalline planes of the same material. This is a new phenomenon and is in sharp contrast to typical nucleation and growth in which a crystal will simply grow larger in preferred directions depending on the surface energy of the specific crystalline planes. Based on our observation, we developed a sequential nucleation and growth technique offering the power to assemble complex hierarchical crystals step-by-step. However, the key questions of when and how the secondary nucleation takes place have not been answered. Here we systematically study secondary ZnO crystal growth using organic diamine additives with a range of chain lengths and concentration. We found that ZnO branches form for a narrow diamine concentration range with a critical lower and upper critical nucleation concentration limit, which increases by about a factor of 5 for each additional carbon in the diaminoalkane chain. Our results suggest that the narrow window for secondary growth is dictated by the solubility of the ZnO crystals, where the low critical nucleation concentration is determined by slight etching of the surface to produce new nucleation sites, and the upper critical concentration is determined by the supersaturation concentration. Kinetic measurements show that the induction time and growth rate increase with increasing diamine concentration and follow classical nucleation and growth theory. Observations of branch morphological evolution reveal the mechanisms guiding the tunable crystal size and morphology.     

Morphology effects on the biofunctionalization of nanostructured ZnO

A stepwise surface functionalization methodology was applied to nanostructured ZnO films grown by metal organic chemical vapor deposition (MOCVD) having three different surface morphologies (i.e., nanorod layers (ZnO films-N), rough surface films (ZnO films-R), and planar surface films (ZnO films-P). The films were grown on glass substrates and on the sensing area of a quartz crystal microbalance (nano-QCM). 16-(2-Pyridyldithiol)-hexadecanoic acid (PDHA) was bound to ZnO films-N, -R, and -P through the carboxylic acid unit, followed by a nucleophilic displacement of the 2-pyridyldithiol moiety by single-stranded DNA capped with a thiol group (SH-ssDNA). The resulting ssDNA-functionalized films were hybridized with complementary ssDNA tagged with fluorescein (ssDNA-Fl). In a selectivity control experiment, no hybridization occurred upon treatment with non complementary DNA. The ZnO films' surface functionalization, characterized by FT-IR-ATR and fluorescence spectroscopy and detected on the nano-QCM, was successful on films-N and -R but was barely detectable on the planar surface of films-P.     

Humidity sensing characteristics of Sn doped Zinc oxide based quartz crystal microbalance sensors

The electrical, optical and humidity sensor properties of nanostructured ZnO samples were investigated. The structural properties of Sn doped ZnO samples were characterized by X-ray diffraction and atomic force microscopy. It was found that the all samples have a hexagonal crystal structure. The electrical conductivity of the samples indicates that undoped and Sn doped ZnO samples exhibit the semiconducting behavior. The optical absorption method was used to determine the optical band gaps of the samples. The optical band gap and activation energy values of the ZnO samples were changed with Sn doping. The ZnO based on quartz crystal microbalance humidity sensors were prepared and sensing properties of the sensors were changed with Sn doping. The response time required to reach 70 % is about 13–16 s, while the recovery time from 70 to 30 % RH is about 13–15 s. The fast response of the sensors is due to easy diffusion of water molecules between ZnO nanopowders. The prepared sensors have a high reproducibility and sensitivity for humidity sensing applications.     

Fabrication of 3D rotor-like ZnO nanostructure from 1D ZnO nanorods and their morphology dependent photoluminescence property

A facile and eco-friendly sonochemical route to fabricate well-defined dentritic (rotor-like) ZnO nanostructures from 1D ZnO nanorods without alloying elements, templates and surfactants has been reported. Phase and structural analysis has been carried out by X-ray diffraction (XRD) and Fourier Transform Infra-Red (FTIR) spectroscopy, showed the formation of hexagonal wurtzite structure of ZnO. Scanning electron microscopic (SEM) study showed the formation of rotor-like ZnO nanostructure having a central core which is surrounded by side branches nanocones. Transmission electron microscopic (TEM) study showed that these nanocones grow along [0001] direction on the six {01–10} planes of central core ZnO nanorods. A plausible formation mechanism of rotor-like ZnO nanostructures was studied by SEM which indicates that the size and morphology of side branches can be controlled by adjusting the concentration of OH^? ions and time duration of growth. The photoluminescence (PL) spectrum of the synthesized rotor-like ZnO nanostructures exhibited a weak ultraviolet emission at 400 nm and a strong green emission at 532 nm recorded at room temperature. The influence of morphology on the origin of green emission was discussed in detail. The results suggested a positive relationship among polar plane, oxygen vacancy and green emission.     

Fe3+改性纳米ZnO光催化降解壬基酚聚氧乙烯醚

采用氨浸法制备了不同Fe3 含量的Fe3 /ZnO光催化剂,并用X射线衍射、N2吸附、X射线光电子能谱和紫外-可见漫反射光谱对纳米Fe3 /ZnO进行了表征.以壬基酚聚氧乙烯醚(NPE-10)为模型污染物,分别在紫外光和可见光下考察了纳米Fe3 /ZnO的光催化活性.结果表明,该方法能成功地将Fe掺杂到ZnO晶体上,且随着Fe3 添加量的增加,ZnO的晶粒尺寸逐渐减小,比表面积逐渐增大.与纳米ZnO样品相比,Fe3 /ZnO中Fe2p结合能减小,而Zn2p和O1s结合能增大,ZnO表面的羟基氧和吸附氧含量增加,光催化活性提高.当Fe3 的添加量大于0.5%时,Fe3 /ZnO样品的吸收光谱发生红移,在可见光区出现吸收.光催化降解结果显示,0.5?3 /ZnO样品的光催化活性最高,在紫外光和可见光照射3h后对NPE-10的降解率分别比纯ZnO提高18%和69%.     

Site specific interaction between ZnO nanoparticles and tryptophan: a first principles quantum mechanical study

First principles density functional theory calculations are performed on tryptophan-ZnO nanoparticles complex in order to study site specific interactions between tryptophan and ZnO. The calculated results find the salt bridge structure involving the -COOH group and ZnO cluster to be energetically more favorable than other interacting sites, such as indole and amine groups in tryptophan. The interaction between tryptophan and ZnO appears to be mediated by both ionic and hydrogen bonds. The calculated molecular orbital energy levels and charge distributions suggest non-radiative energy transfer from an excited state of tryptophan to states associated with ZnO, which may lead to a reduction in the emission intensity assigned to the π-π* transition of the indole functional group of tryptophan.     

Scanning electrochemical microscopy study of ion annihilation electrogenerated chemiluminescence of rubrene and [Ru(bpy)3]2+

Scanning electrochemical microscopy (SECM) was used for the study of electrogenerated chemiluminescence (ECL) in the radical annihilation mode. The concurrent steady-state generation of radical ions in the microgap formed between a SECM probe and a transparent microsubstrate provides a distance-dependent ECL signal that can provide information about the kinetics, stability, and mechanism of the light emission process. In the present study, the ECL emission from rubrene and [Ru(bpy)(3)](2+) was used to model the system by carrying out experiments with the SECM and light-detecting apparatus inside an inert atmosphere box. We studied the influence of the distance between the two electrodes, d, and the annihilation kinetics on the ECL light emission profiles under steady-state conditions, as well as the ECL profiles when carrying out cyclic voltammetry (CV) at a fixed d. Experimental results are compared to simulated results obtained through commercial finite element method software. The light produced by annihilation of the ions was a function of d; stronger light was observed at smaller d. The distance dependence of the ECL emission allows the construction of light approach curves in a similar fashion as with the tip currents in the feedback mode of SECM. These ECL approach curves provide an additional channel to describe the reaction kinetics that lead to ECL; good agreement was found between the ECL approach curve emission profile and the simulated results for a fast, diffusion-limited second-order annihilation process (k(ann) > 10(7) M(-1) s(-1)). In the CV mode at fixed distance, the ECL emission of rubrene showed two distinct signals at different potentials when fixing the substrate to generate the radical cation and scanning the tip to generate the radical anion. The first signal (pre-emission) corresponded to an emission well before reaching the generation of the radical anion and was more intense on Au than on Pt. The second ECL signal showed the expected steady-state behavior from the second-order annihilation reaction and agreed well with the simulation. A comparison of the emission obtained with rubrene and [Ru(bpy)(3)](2+) to test the direct formation of lower energy triplets directly at the electrode showed that triplets are not the cause of the pre-emission observed. Wavelength selection experiments for the rubrene system showed that the pre-emission ECL signal also appeared slightly red-shifted with respect to the main luminophore emission; a possible explanation for this phenomenon is inverse photoemission, where the injection of highly energetic holes by the oxidized species into the negatively biased tip electrode causes emission of states in the metal that appear at a different wavelength than the singlet emission from the ECL luminophore.     

The use of radionuclides for the study of crystal structure of solids

It is well known that by the coordinated action of atoms arranged in rows and planes in the crystal lattice, the motion of charged particles such as protons, alpha particles and heavier ions can be influenced so that their range in the single crystals is considerably enhanced in low-index directions. A technique has been developed based on such enhanced penetration (channeling) of radioactive atoms (²²⁰Rn) emitted by recoil with a 100 keV energy from a²²⁴Ra point source to record channeling patterns which show the crystal structure. The radioactive recoil atoms impinging from this source on the surface of a single crystal penetrate deeper in places where their direction of impact is identical with low index crystal directions and planes. These places can be visualized by autoradiography when having first stripped a thin layer from the surface corresponding to the random range of the atoms. This technique is generally applicable in close packed crystals and gives information about the crystal structure of very thin surface layers.     

Growth and Characterization of [001] ZnO Nanorod Array on ITO Substrate with Electric Field Assisted Nucleation

This paper reports direct growth of [001] ZnO nanorod arrays on ITO substrate from aqueous solution with electric field assisted nucleation, followed with thermal annealing. X-ray diffraction analyses revealed that nanorods have wurtzite crystal structure. The diameter of ZnO nanorods was 60–300 nm and the length was up to 2.5 μm depending on the growth condition. Photoluminescence spectra showed a broad emission band spreading from 500 to 870 nm, which suggests that ZnO nanorods have a high density of oxygen interstitials. Low and nonlinear electrical conductivity of ZnO nanorod array was observed, which was ascribed to non-ohmic contact between top electrode and ZnO nanorods and the low concentration of oxygen vacancies.     

Effect of annealing on photoluminescence of blue-emitting ZnO nanoparticles by sol–gel method

In this work, we investigated the influence of annealing on the crystallinity, microstructures, and photoluminescence (PL) properties of ZnO nanoparticles prepared by sol–gel method. The annealing was carried out both in air and vacuum. X-ray powder diffraction, scanning electron microscopy, and ultraviolet–visible spectroscopy were used to characterize the crystal structures, diameter, surface morphology, and PL properties of ZnO nanoparticles. It has been found that both the as-grown and annealed ZnO nanoparticles had a hexagonal wurtzite crystal structure, and their average diameter and crystallinity increased with the anneal time and temperature. Pure blue-emitting behavior was observed in all samples. The emission intensity of ZnO nanoparticles was found to be enhanced after annealing, but it was highly dependent on the annealing conditions. Optimal annealing conditions both in air and vacuum were obtained for achieving maximum emission intensity in the ZnO nanoparticles. The dependence of PL properties of the ZnO nanoparticles on the annealing conditions was discussed.     

Band structure tuning of ZnO/CuO composites for enhanced photocatalytic activity

The toxic dye pigments, even in small quantities, can damage ecosystems. Removing organic, inorganic, and microbiological contaminants from wastewater via heterogeneous photocatalysis is a promising method. Herein, we report the band structure tuning of ZnO/CuO nanocomposites to enhance photocatalytic activity. The nanocomposites were synthesized by a chemical approach using step-wise implantation of p-type semiconductor CuO to n-type semiconductor ZnO. Various characterization techniques such as X-ray diffraction analysis (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDX) and UV spectroscopy were used to investigate the crystal structure, surface morphology, elemental composition and optical properties of the synthesized samples. As the CuO content increased from 10% to 50% in ZnO/CuO nanocomposites, the optical bandgap decreased from 3.36 to 2.14 eV. The photocatalytic activity of the samples was evaluated against the degradation of methylene blue (MB) under visible irradiation. Our study demonstrates a novel p–n junction oxide photocatalyst based on wt. 10% CuO/ZnO with superior photocatalytic activity. Effectively 66.6% increase in degradation rate was achieved for wt. 10% CuO/ZnO nanocomposite compared to pure ZnO nanoparticles.     

2,4,6-三硝基-2,4,6-三氮杂环己酮的晶体形貌预测

利用Materials Studio 5.0软件包中的Morphology模块所含的BFDH、Growth Morphology和Equilibri-um Morphology三种方法计算了2,4,6-三硝基-2,4,6-三氮杂环己酮的晶体形貌,得到了特定晶面的面积、附着能、表面能及晶面相对生长速率等参数,确定了形态学上重要的生长晶面.各晶面的表面结构分析结果表明,(101)和(111)晶面为强极性晶面,(002)、(110)和(021)晶面为极性晶面,而(020)晶面为非极性晶面.据此可以预测,在强极性的质子溶剂中,(101)和(111)晶面为形态学上重要的晶面,(002)、(110)和(021)晶面的显露面可能增加,而(020)晶面会变小或消失.在非极性溶剂中,情况则可能刚好相反.     

Determination of nitrite based on its quenching effect on anodic electrochemiluminescence of CdSe quantum dots

A novel method for electrochemiluminescent (ECL) detection of nitrite was proposed based on its quenching effect on anodic ECL emission of CdSe quantum dots (QDs). The ECL emission could be greatly enhanced by sulfite and dissolved oxygen in a neutral system and occurred at a relatively low potential in comparison with traditional anodic ECL emitter, leading to high sensitivity and good selectivity. The quenching mechanism followed an “electrochemical oxidation inhibition” process, which was completely different from those of some analytes on the ECL emission of QDs. The coincidence of photoluminescence and ECL spectra of the QDs indicated that the ECL emission resulted from the redox process of QDs core and the sulfite acted as a coreactant. The nitrite quenched ECL emission could be analyzed according to the treatment of Stern-Volmer equation with a linear range from 1 μM to 0.5 mM for detection of nitrite. This work presented a new efficient ECL methodology for quencher-related detection.     

Influence of different precursors and Mn doping concentrations on the structural,optical properties and photocatalytic activity of single-crystal manganese-doped ZnO

Mn-doped ZnO single-crystal micronuts were synthesized via hydrothermal method in an hexamethylenetetramine aqueous solution. These micronuts are of wurtzite crystal structure. The effects of Mn doping amount and precursor concentration on the structural, optical properties and photocatalytic activity have been investigated. The synthesized Mn-doped ZnO was characterized by X-ray powder diffraction, field emission scanning electron microscopy (FESEM), UV–Vis absorption and photoluminescence spectroscopy. The structural analyses based on X-ray diffraction revealed the absence of Mn-related secondary phases. According to FESEM results, the length of ZnO micronuts was in the range of 5–8 μm. The band gap energy increased on increasing Mn doping concentration. The photocatalytic activity was studied by degradation of methyl orange aqueous solution, which showed that the Mn-doped ZnO micronuts prepared in precursor concentration of 0.1 M and 4% Mn doping had the highest photocatalytic activity. The effects of crystal defect and band gap energy on photocatalytic activity of Mn-doped ZnO samples were studied in different precursors and Mn doping amounts.