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Insight into the Growth Mechanism and Photocatalytic Behavior of Tubular Hierarchical ZnO Structures: An Integrated Experimental and Theoretical Approach

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... . The influence of an alcoholic solvent on the ZnO particle growth and morphology has been proved experimentally and theoretically [6,[8][9][10][11][12], and microwave-assisted growth has also been demonstrated [3,13,14]. A comprehensive review of different preparation strategies has recently appeared [15]. ...
... Similarly, the photocatalytic reduction of CO 2 by ZnO microspheres, microflowers and nanorods sensitively depends on the percentage of exposed [0001] facets and the size of the catalysts [30]. The EXAFS experiments revealed detectable differences in the performance of hollow tubular ZnO nanostructures for the photocatalytic degradation of organic dye pollutants [11], while XANES showed the ability to distinguish coordination changes and surface disorder of the Zn-O adsorption complexes [31]. ...
... The performance of hollow tubular ZnO nanostructures for the photocatalytic degradation of organic dye pollutants can be sensitively monitored by EXAFS experiments that reveal detectable differences [11]. Moreover, XANES has proven to differentiate the coordination changes and surface disorder of the Zn-O adsorption complexes [31]. ...
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Citation: Eckelt, F.; Rothweiler, P.; Braun, F.; Voss, L.; Šarić, A.; Vrankić, M.; Lützenkirchen-Hecht, D. In Situ Observation of ZnO Nanoparticle Formation by a Combination of Time-Resolved X-ray Absorption Spectroscopy and X-ray Diffraction. Materials 2022, 15, 8186. https:// Abstract: The formation of ZnO nanomaterials from different Zn acetylacetonate precursor solutions was studied in situ by employing simultaneous, time-resolved X-ray diffraction (XRD) and X-ray absorption spectroscopy (EXAFS) at the Zn K-edge. The precursor solutions were heated from room temperature to the desired reaction temperatures in a hermetically sealed cell dedicated to X-ray experiments. In general, the first indications for the formation of hexagonal ZnO were found for elevated temperatures of about 80 • C both by XRD and EXAFS, and the contributions increase with temperature and time. However, no reaction intermediates could be proved in addition to the Zn precursors and the formed hexagonal ZnO materials. Furthermore, the results show that the efficiency of the reaction, i.e., the conversion of the precursor material to the ZnO product, strongly depends on the solvent used and the reaction temperature. ZnO formation is accelerated by an increased temperature of 165 • C and the use of 1-octanol, with a conversion to ZnO of more than 80% after only a ca. 35 min reaction time according to a detailed analysis of the EXAFS data. For comparison, an identical concentration of Zn acetylacetonate in water or dilute alkaline NaOH solutions and a reaction temperature of around 90 • C leads to a smaller conversion of approximately 50% only, even after several hours of reaction. The particle size determined from XRD for different orientations shows a preferred orientation along the c-direction of the hexagonal crystal system, as well in accordance with scanning electron microscopy. The LaMer model explained this highly non-uniform growth of needle-like ZnO crystallites.
... . The influence of an alcoholic solvent on the ZnO particle growth and morphology has been proved experimentally and theoretically [6,[8][9][10][11][12], and microwave-assisted growth has also been demonstrated [3,13,14]. A comprehensive review of different preparation strategies has recently appeared [15]. ...
... Similarly, the photocatalytic reduction of CO 2 by ZnO microspheres, microflowers and nanorods sensitively depends on the percentage of exposed [0001] facets and the size of the catalysts [30]. The EXAFS experiments revealed detectable differences in the performance of hollow tubular ZnO nanostructures for the photocatalytic degradation of organic dye pollutants [11], while XANES showed the ability to distinguish coordination changes and surface disorder of the Zn-O adsorption complexes [31]. ...
... The performance of hollow tubular ZnO nanostructures for the photocatalytic degradation of organic dye pollutants can be sensitively monitored by EXAFS experiments that reveal detectable differences [11]. Moreover, XANES has proven to differentiate the coordination changes and surface disorder of the Zn-O adsorption complexes [31]. ...
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Citation: Eckelt, F.; Rothweiler, P.; Braun, F.; Voss, L.; Šarić, A.; Vrankić, M.; Lützenkirchen-Hecht, D. In Situ Observation of ZnO Nanoparticle Formation by a Combination of Time-Resolved X-ray Absorption Spectroscopy and X-ray Diffraction. Materials 2022, 15, 8186. https:// Abstract: The formation of ZnO nanomaterials from different Zn acetylacetonate precursor solutions was studied in situ by employing simultaneous, time-resolved X-ray diffraction (XRD) and X-ray absorption spectroscopy (EXAFS) at the Zn K-edge. The precursor solutions were heated from room temperature to the desired reaction temperatures in a hermetically sealed cell dedicated to X-ray experiments. In general, the first indications for the formation of hexagonal ZnO were found for elevated temperatures of about 80 • C both by XRD and EXAFS, and the contributions increase with temperature and time. However, no reaction intermediates could be proved in addition to the Zn precursors and the formed hexagonal ZnO materials. Furthermore, the results show that the efficiency of the reaction, i.e., the conversion of the precursor material to the ZnO product, strongly depends on the solvent used and the reaction temperature. ZnO formation is accelerated by an increased temperature of 165 • C and the use of 1-octanol, with a conversion to ZnO of more than 80% after only a ca. 35 min reaction time according to a detailed analysis of the EXAFS data. For comparison, an identical concentration of Zn acetylacetonate in water or dilute alkaline NaOH solutions and a reaction temperature of around 90 • C leads to a smaller conversion of approximately 50% only, even after several hours of reaction. The particle size determined from XRD for different orientations shows a preferred orientation along the c-direction of the hexagonal crystal system, as well in accordance with scanning electron microscopy. The LaMer model explained this highly non-uniform growth of needle-like ZnO crystallites.
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ZnO nanoparticles doped with different Eu3+ percentages were synthesized in water (ZnO:Eu(x%)-W) and other solvents (methanol ZnO:Eu(x%)-M and ethanol ZnO:Eu(x%)-E). X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), optical absorption and photoluminescence (PL) spectroscopy were used for characterization of the nanoparticles. Our results showed influence of europium doping and solvents on size, particles agglomeration, light absorption and photocatalytic activity. Improvement in photocatalytical activity with addition of Eu3+ doping was detected. Particle size increased with Eu3+ doping in water samples, while it decreased in methanol. Agglomeration was more prominent in ZnO:Eu(x)-W samples.Greater amount of surface OH groups in case of ZnO:Eu(x%)-M samples was detected by PL, XPS and FTIR measurements. Influence of europium doping, as an electron trap, and surface OH groups, as a hole trap, was studied in sunlight photocatalytic degradation of cationic methylene blue (MB) and anionic methyl orange (MO). Improved photocatalytic behavior was discussed and influence of active species was further investigated using hole and hydroxyle radical scavengers. The degradation pathway of MB and MO, using high performance liquid chromatohraphy (HPLC), is also examined.
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Electron spin resonance spectroscopy (ESR), Fourier transform infrared spectroscopy (FTIR), and monitoring of pyridine (py) and boric acid trimethyl ester (BATE) adsorption for determining surface acidity and basicity, respectively, were used to carry out further characterization of mixed ZnO/TiO2 polycrystalline solids prepared by different methods. Moreover, the powders were tested in a batch photoreactor for a probe reaction, i.e., 4-nitrophenol photodegradation in aqueous medium. ESR results indicated the presence of signals attributable to Zn+ species in ZnO/TiO2 (anatase) solids, while in ZnO/TiO2 (rutile) samples the presence of zinc induced only the formation of signals probably due to Ti3+ centers. FT-IR spectra showed no significant differences of the surface hydroxylation degree of the various photocatalysts, whereas the surface acidic properties generally decrease by increasing the amount of ZnO. Coupling of ZnO and TiO2 semiconducting powders was not so beneficial, as expected on the basis of the intrinsic electronic properties, to enhance the photoreactivity for the studied reaction, although some of the powders showed photoactivities slightly higher than those of bare TiO2 and ZnO. Nevertheless, some of the samples (the mixed particles ZnO/TiO2 (rutile)) appear promising from an application point of view because no filtration was needed after the occurrence of the photoreactivity tests to separate them from the solution because they decanted easily. The mineralization of 4-nitrophenol was checked by determining the total organic carbon (TOC) and a complete photooxidation occurred in few hours in the presence of some of the samples.
Article
The facet-dependent photocatalytic performance of ZnO is still controversial, for the reason that ZnO samples with different high energy facets are hard to prepare. In this paper, three kinds of ZnO with different facet exposures, including {0001}, {101̅1}, and {101̅0} facets, were prepared by a scalable solvothermal method without using any crystal seeds, environmentally harmful chemicals, or severe reaction conditions. The morphologies of ZnO were gradually varied from prism to polyhedron with the increase of ethylene glycol volume fraction. Meanwhile, the predominant exposed facets changed from {101̅0} to {101̅1}. Increasing the volume fraction of ethylene glycol in the mixture solution further, the ZnO spheres formed, where the {0001} facet is predominantly exposed. The formation mechanism for different ZnO geometric structures and their facet-dependent photocatalytic performances were explored in detail, which indicates that the photocatalytic performance is dependent on the crystal facet exposures in the order {0001} > {101̅1} > {101̅0}. Efforts were made to explain the facet-dependent photocatalytic activities basing on the related X-ray photoelectron spectroscopy and atomic stacking model of various exposed surfaces.
Article
Dumbbell-shaped ZnO microstructures have been successfully synthesized by a facile hydrothermal method using only Zn(NO3)(2)center dot 6H(2)O and NH3 center dot H2O as raw materials at 150 degrees C for 10 h. The results from X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) show that the prepared ZnO samples exhibit dumbbell-shaped morphology and hexagonal wurtzite structure. The length of ZnO dumbbells is about 5-20 mu m, the diameters of the two ends and the middle part are about 1-5 mu m and 0.5-3 mu m, respectively. The dumbbell-shaped ZnO microstructures may be formed by self-assembly of ZnO nanorods with 1-5 mu m in length and 100-200 nm in diameter. The photoluminescence (PL) spectrum of dumbbell-shaped ZnO microstructures at room temperature shows three emission peaks at about 362, 384 and 485 nm.
Article
Crystalline nanoparticles often preferentially display particular facets, and varying the growth conditions can favor different facets. This study provides fundamental insights into photocatalytic and photoelectrochemical performance of three types of TiO2 nanoparticles with predominantly exposed {101}, {010}, or {001} facets. Photodegradation of methyl orange reveals that {001}-TiO2 has 1.79 and 3.22 times higher photocatalytic activity than {010}- and {101}-TiO2, respectively. This suggests that the photochemical performance is highly correlated with the surface energy and the number of under-coordinated atoms. In contrast, the photoelectrochemical performance of the faceted TiO2 nanoparticles sensitized with the commercially available MK-2 dye was highest with {010}-TiO2 which yielded an overall cell efficiency of 6.1%, compared 3.2% for {101}-TiO2 and 2.6% for {001}-TiO2 prepared under analogous conditions. Measurement of desorption kinetics and accompanying computational modeling suggest a stronger covalent interaction of the dye with the {010} and {101} facets compared with the {001} facet. Time-resolved THz spectroscopy and transient absorption spectroscopy measure faster electron injection and recombination dynamics when MK-2 is bound to {010} compared to other facets, consistent with computational results which indicate that {010} facet provides the most efficient and direct pathway for interfacial electron transfer. Our experimental and computational results establish for the first time that photoelectrochemical performance is dependent upon the binding energy of the dye as well as the crystalline structure of the facet, as oppose to surface energy alone.
Article
ZnO nanoparticles (NPs) were synthesized by a seeded polyol process in di(ethylene glycol) (DEG) using zinc acetate as a precursor in the presence of inorganic hydroxides (NaOH, KOH, LiOH). The precursor was transformed into ZnO in DEG without solid intermediates as shown by Fourier transform infrared spectroscopy. Both Raman and photoluminescence spectroscopies confirmed the presence of defects in the ZnO crystal structure which generated visible light emission when excited with UV light. The molar ratio of seeds/precursor affects ZnO particle growth, and at a high ratio (10/1) growth of ZnO NPs is practically prevented, giving ZnO with a narrow particle size distribution. By decreasing the precursor concentration from 1.0 to 0.01 M, ZnO particle size decreased from 50−200 nm to 20−60 nm; however, the degree of agglomeration was increased. Type of cation plays only a minor role in the ZnO NPs formation. By increasing the hydroxide/precursor molar ratio from 1/1 to 5/1, the ZnO particle size is reduced from 50−200 nm to 20−40 nm. Therefore, the hydroxide/precursor molar ratio has a significant role in the formation of ZnO because it defines the concentration of OH− ions, which is the key factor in this process.
Article
Polygonal ZnO nanorods are synthesized with a hydrothermal technique and coated with an ultrathin CdS layer. A NiO cocatalyst layer is then deposited on the surface of the CdS coated ZnO and makes up a three-component nanostructured composite photoelectrode (NiO/CdS@ZnO). This composite photoelectrode shows an improved photoelectrochemical performance under Air Mass 1.5 (AM 1.5) simulated sunlight. It generates a photocurrent density of 950 mu A/cm(2) with a comparatively low potential -0.6 V vs. Ag/AgCl in a 0.5 M Na2S and Na2SO3 solution, which is 6.78 times as much as that of the as-grown ZnO nanorods. Furthermore, it is also an efficient photoanode to split pure water, producing a photocurrent density of 530 mu A/cm(2) at 0.5 V vs. Ag/AgCl in a 1 M Na2SO4 solution. In the composite, ZnO nanorods absorb UV light with a wide band gap, 3.2 eV, while CdS, with a narrow band gap (2.4 eV), utilizes the remaining visible light, furthermore, an ordered transfer of the charge carriers occurs in the component phases. The photoluminescence (PL) experiment results prove that the direct loading of NiO on ZnO results in an inactive material. However, both the CdS/ZnO and NiO/CdS@ZnO composites are active and the charge carrier separations in them are effectively improved than in the as-grown ZnO.
Article
In this paper, Bi2WO6 was, for the first time, utilized as crystal seeds to fine control the ratio of ZnO polar facets under a mild condition. The structure and photocatalytic properties of the ZnO were investigated by powder X-ray diffraction, Raman scattering measurements, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, UV–vis diffuse reflectance spectroscopy, total organic C analyzer, Brunauer–Emmett–Teller surface areas, and electrochemical workstation. It was found that ZnO architectures were gradually changed from nanorods to nanosheets after adding a certain amount of Bi2WO6 crystal seeds, which led to the change of the ratio of ZnO polar facets. The formation mechanism for this ZnO and the effect of ZnO polar facets on the photocatalytic activity are explored in detail. We find that the nucleation and growth rates of ZnO are controlled by adjusting the quantity of the crystal seeds. Besides, photocatalytic activity of the samples tuned by Bi2WO6 crystal seeds was compared. The ratio of ZnO polar facets is the main factor influencing the photocatalytic activity. The results clearly demonstrate that the sample with a higher proportion of Zn-terminated (0001) and O-terminated (0001̅) polar facets is beneficial to enhance photocatalytic activity.
Article
The layout and the characteristics of the hard X-ray beamline BL10 at the superconducting asymmetric wiggler at the 1.5 GeV Dortmund Electron Accelerator DELTA are described. This beamline is equipped with a Si(111) channel-cut monochromator and is dedicated to X-ray studies in the spectral range from ∼4 keV to ∼16 keV photon energy. There are two different endstations available. While X-ray absorption studies in different detection modes (transmission, fluorescence, reflectivity) can be performed on a designated table, a six-axis kappa diffractometer is installed for X-ray scattering and reflectivity experiments. Different detector set-ups are integrated into the beamline control software, i.e. gas-filled ionization chambers, different photodiodes, as well as a Pilatus 2D-detector are permanently available. The performance of the beamline is illustrated by high-quality X-ray absorption spectra from several reference compounds. First applications include temperature-dependent EXAFS experiments from liquid-nitrogen temperature in a bath cryostat up to ∼660 K by using a dedicated furnace. Besides transmission measurements, fluorescence detection for dilute sample systems as well as surface-sensitive reflection-mode experiments are presented.
Article
Formation of hollow ZnO particles by simple hydrolysis of zinc acetylacetonate [Zn(acac)2] at 90 °C was monitored. Isolated precipitates were characterized with XRD, FT-IR and FE-SEM. Different amounts of Zn(acac)2 underwent the hydrolysis in 1 × 10−3 M NaOH for 24 h. The conditions for the formation of hollow ZnO particles were determined and the process was explained by the aggregation mechanism.
Article
ZnS cristallise dans P6 3 mc avec a=8227 et c=6,2607 A, Z=2; affinement jusqu'a R=0,017. ZnO cristallise dans P6 3 mc avec a=3,2501 et c=5,2071 A, Z=2 affinement jusau'a R=0,012. Les atomes Zn occupent la position speciale 2(b) avec les coordonnees 1/3, 2/3, Os et O occupent egalement la position speciale 2(b) avec les coordonnees 1/3, 2/3, u. Pour ZnS, u=0,3748 et pour ZnO, u=0,3817. Dans les deux cas, u et c/a se conforment a la correlation comme entre ces parametres
Article
Electron spin resonance spectroscopy (ESR), Fourier transform infrared spectroscopy (FTIR), and monitoring of pyridine (py) and boric acid trimethyl ester (BATE) adsorption for determining surface acidity and basicity, respectively, were used to carry out further characterization of mixed ZnO/TiO2 polycrystalline solids prepared by different methods. Moreover, the powders were tested in a batch photoreactor for a probe reaction, i.e., 4-nitrophenol photodegradation in aqueous medium. ESR results indicated the presence of signals attributable to Zn+ species in ZnO/TiO2 (anatase) solids, while in ZnO/TiO2 (rutile) samples the presence of zinc induced only the formation of signals probably due to Ti3+ centers. FT-IR spectra showed no significant differences of the surface hydroxylation degree of the various photocatalysts, whereas the surface acidic properties generally decrease by increasing the amount of ZnO. Coupling of ZnO and TiO2 semiconducting powders was not so beneficial, as expected on the basis of the intrinsic electronic properties, to enhance the photoreactivity for the studied reaction, although some of the powders showed photoactivities slightly higher than those of bare TiO2 and ZnO. Nevertheless, some of the samples (the mixed particles ZnO/TiO2 (rutile)) appear promising from an application point of view because no filtration was needed after the occurrence of the photoreactivity tests to separate them from the solution because they decanted easily. The mineralization of 4-nitrophenol was checked by determining the total organic carbon (TOC) and a complete photooxidation occurred in few hours in the presence of some of the samples.
Article
A structure refinement method is described which does not use integrated neutron powder intensities, single or overlapping, but employs directly the profile intensities obtained from step-scanning measurements of the powder diagram. Nuclear as well as magnetic structures can be refined, the latter only when their magnetic unit cell is equal to, or a multiple of, the nuclear cell. The least-squares refinement procedure allows, with a simple code, the introduction of linear or quadratic constraints between the parameters.
Article
This study reports on the formation of unique oriented ZnO structures (face oriented hexagonal discs, 3D-trapezoids, rings, doughnuts, and hemispheres) with tunable percentage of exposed polar facets synthesized via a simple hydrothermal route in aqueous base environment. The significance of the synthetic strategy is the generation of exotic structures without using any templates / structure directing agents and successful realization of morphologies with increased polar to nonpolar facet ratio. Detailed investigation reveals that the size and shape of ZnO microstructures can be conveniently tailored by systematically exercising control on the choice of precursor (zinc source), concentration of reactants, use of trizma as a base (pH control) which is also seen as structure directing agent. The trizma base with an appreciably short pH range effectively controls the rate of hydrolysis, regulates nucleation and restrains rapid growth leading to these well defined ZnO structures. Photocatalytic degradation of methylene blue as a model system was used to showcase the morphology dependent enhanced photoactivity under UV-light. The enhanced photocatalytic performance could be attributed to the higher fraction of exposed (0001) and (000ī) ZnO polar facets present.
Article
Ab initio effective core potentials (ECP’s) have been generated to replace the Coulomb, exchange, and core‐orthogonality effects of the chemically inert core electron in the transition metal atoms Sc to Hg. For the second and third transition series relative ECP’s have been generated which also incorporate the mass–velocity and Darwin relativistic effects into the potential. The ab initio ECP’s should facilitate valence electron calculations on molecules containing transition‐metal atoms with accuracies approaching all‐electron calculations at a fraction of the computational cost. Analytic fits to the potentials are presented for use in multicenter integral evaluation. Gaussian orbital valence basis sets are developed for the (3d,4s,4p), (4d,5s,5p), and (5d,6s,6p) orbitals of the first, second, and third transition series atoms, respectively. All‐electron and valence‐electron atomic excitation energies are also compared for the low‐lying states of Sc–Hg, and the valence‐electron calculations are found to reproduce the all‐electron excitation energies (typically within a few tenths of an eV).
Article
The growth of single-crystal tubular ZnO whiskers was achieved via a process of first reduction and following oxidation of ZnS powders. The products were characterized using x-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and photoluminescence spectroscopy. The whiskers are tubular single crystals with the [001] growth crystallographic direction, and most have outer diameters of ∼ 400 nm, lengths of up to 15 μm, and wall thickness range of 100–150 nm. Room-temperature photoluminescence spectrum of the whiskers reveals a strong and sharp UV emission band at 381 nm and a weak and broad green emission band at 583 nm. Possible growth mechanism of the ZnO whiskers was briefly discussed. © 2003 American Institute of Physics.
Article
We report on first-principles density functional calculations about the adsorption of water molecules on the nonpolar ZnO(101̅0) surface, as a function of the molecular coverage. Our results allow us to unravel the reaction mechanisms that drive the partial dissociation of water molecules at saturation coverage: Although not a favored event, concurrent adsorption/dissociation may occur as a compromise between steric repulsion and covalent and hydrogen bond formation with both the substrate and the impinging molecules. The scenario is altered by the presence of defects. We discuss the role of the most common point and extended defects at the outermost layer: these systems exhibit remarkably different electronic properties leading to peculiar and unexpected characteristics for the wet defective surface. Enhanced reactivity of edged nanostructures is predicted, while the catalytic role of oxygen vacancies is questioned. The effects of metallicity induced by hydrogen adsorption on the interaction with water are finally analyzed. We suggest also experimental probes to identify the various adsorption geometries and fully characterize the water vapor/oxide interface.
Article
The hydroxylation structural features of the first adsorption layer and its connection to proton transfer reactivity have been studied for the ZnO-liquid water interface at room temperature. Molecular dynamics simulations employing the ReaxFF forcefield were performed for water on seven ZnO surfaces with varying step concentrations. At higher water coverage a higher level of hydroxylation was found, in agreement with previous experimental results. We have also calculated the free energy barrier for transferring a proton to the surface, showing that stepped surfaces stabilize the hydroxylated state and decrease the water dissociation barrier. On highly stepped surfaces the barrier is only 2 kJ/mol or smaller. Outside the first adsorption layer no dissociation events were found during almost 100 ns of simulation time; this indicates that these reactions are much more likely if catalyzed by the metal oxide surface. Also, when exposed to a vacuum, the less stepped surfaces stabilize adsorption beyond monolayer coverage.
Article
We present a new hybrid meta exchange-correlation functional, called M05-2X, for thermochemistry, thermochemical kinetics, and noncovalent interactions. We also provide a full discussion of the new M05 functional, previously presented in a short communication. The M05 functional was parametrized including both metals and nonmetals, whereas M05-2X is a high-nonlocality functional with double the amount of nonlocal exchange (2X) that is parametrized only for nonmetals. In particular, M05 was parametrized against 35 data values, and M05-2X is parametrized against 34 data values. Both functionals, along with 28 other functionals, have been comparatively assessed against 234 data values:  the MGAE109/3 main-group atomization energy database, the IP13/3 ionization potential database, the EA13/3 electron affinity database, the HTBH38/4 database of barrier height for hydrogen-transfer reactions, five noncovalent databases, two databases involving metal−metal and metal−ligand bond energies, a dipole moment database, a database of four alkyl bond dissociation energies of alkanes and ethers, and three total energies of one-electron systems. We also tested the new functionals and 12 others for eight hydrogen-bonding and stacking interaction energies in nucleobase pairs, and we tested M05 and M05-2X and 19 other functionals for the geometry, dipole moment, and binding energy of HCN−BF3, which has recently been shown to be a very difficult case for density functional theory. We tested eight functionals for four more alkyl bond dissociation energies, and we tested 12 functionals for several additional bond energies with varying amounts of multireference character. On the basis of all the results for 256 data values in 18 databases in the present study, we recommend M05-2X, M05, PW6B95, PWB6K, and MPWB1K for general-purpose applications in thermochemistry, kinetics, and noncovalent interactions involving nonmetals and we recommend M05 for studies involving both metallic and nonmetallic elements. The M05 functional, essentially uniquely among the functionals with broad applicability to chemistry, also performs well not only for main-group thermochemistry and radical reaction barrier heights but also for transition-metal−transition-metal interactions. The M05-2X functional has the best performance for thermochemical kinetics, noncovalent interactions (especially weak interaction, hydrogen bonding, π···π stacking, and interactions energies of nucleobases), and alkyl bond dissociation energies and the best composite results for energetics, excluding metals.
Article
The ability to generate homogeneous particulate thin films of highly oriented and highly porous microparticles of a post transition metal oxide onto polycrystalline and single-crystalline substrates, at low cost, by a template-free, aqueous low-temperature coating process is demonstrated by the fabrication of a large three-dimensional array of perpendicularly oriented hexagonal microtubes of crystalline zincite ZnO from an aqueous solution of zinc nitrate and methenamine.
Article
We introduce a new software package for analysis of XAS data. This package is based on the IFEFFIT library of analytical and numerical algorithms and uses the perl/Tk graphics toolkit. The two main components are ATHENA, a program for XAS data processing, and ARTEMIS, a program for EXAFS data analysis using theoretical standards from FEFF. These programs provide high quality analytical capabilities in a manner that is accessible to novices yet powerful enough to meet the demands of an expert. The programs run on all major platforms and are freely available under the terms of a free software license.
Article
Hexagonal Al-doped zinc oxide (ZnO) powders with a nominal composition of Zn1−x Alx O (0⩽x⩽0.028) were synthesized by the co-precipitation method. The contents of the Al element in the samples were measured by the inductively coupled plasma-optical emission spectroscopy (ICP-OES) technique. The structures of the Zn1−x Alx O (0⩽x⩽0.028) compounds calcined at 1000 and 1200°C have been determined using the Rietveld full-profile analysis method. Rietveld refinements of the diffraction data indicated that the addition of Al initially has a considerably positive effect on the decreasing of the lattice parameters a and c of Zn1−x Alx O, but the effect becomes very slight and even negative with the further increase of the Al content. The solid solubility limit of Al in ZnO (mole fraction y) is 2.2l%, resulting in Zn0.978Al0.22O. It seems that when the Al content is excessive, Al prefers to form a ZnAl2O4 compound with ZnO, but not to incorporate into the ZnO lattice to occupy the Zn2+ cites. Two phases, [ZnO] (or Al-doped ZnO) and [ZnAl2O4], are obviously segregated in Zn1−x Alx O while the value of x is larger than 0.024. The UV-Vis absorption spectra show that the Al-doped ZnO exhibits a red-shift in the absorption edge without reduced transmission compared with pure ZnO, which also confirms that Al ions enter the ZnO lattice and form a Zn1−x Alx O solid solution.
Article
The utilization of ZnO nanorods as a wide gap semiconductor and its use for its catalytic, electrical, optoelectronic, and photoelectronic properties is studied. ZnO has large excitation binding energy that allows the lasing action in room temperatures also and ZnO with oxygen vacancies exhibits green emissions. ZnO is non toxic in nature and has advantage of low cost and high reactivities, thus it can be used as photocatalytic degradation materials of environmental pollutants. The hydrothermal method is used for the preparation of ZnO rods and the microstructures of holes of diameters of 5 nm was observed on the surface of the nanorods. Oriented attachment of the nanoparticles led to formation of defects in the nanorods. The UV-Vis absorption spectra of ZnO rods at room temperature shows a well defined exciton bands at 381 nm. The study provides new approaches to change the optical absorption properties of ZnO nanorods and improve the visible-light photocatalysis.
Article
The ultimate device miniaturization would be to use individual molecules as functional devices. Single-wall carbon nanotubes (SWNTs) are promising candidates for achieving this: depending on their diameter and chirality, they are either one-dimensional metals or semiconductors. Single-electron transistors employing metallic nanotubes and field-effect transistors employing semiconducting nanotubes have been demonstrated. Intramolecular devices have also been proposed which should display a range of other device functions. For example, by introducing a pentagon and a heptagon into the hexagonal carbon lattice, two tube segments with different atomic and electronic structures can be seamlessly fused together to create intramolecular metal-metal, metal-semiconductor, or semiconductor- semiconductor junctions. Here we report electrical transport measurements on SWNTs with intramolecular junctions. We find that a metal-semiconductor junction behaves like a rectifying diode with nonlinear transport characteristics that are strongly asymmetric with respect to bias polarity. In the case of a metal-metal junction, the conductance appears to be strongly suppressed and it displays a power-law dependence on temperatures and applied voltage, consistent with tunnelling between the ends of two Luttinger liquids. Our results emphasize the need to consider screening and electron interactions when designing and modelling molecular devices. Realization of carbon-based molecular electronics will require future efforts in the controlled production of these intramolecular nanotube junctions.
Article
Shape-controlled ZnO nanostructures were synthesized through a facile soft-chemical approach by varying the concentration of OH− ions. X-ray diffraction and Raman spectra reveal the formation of highly crystalline single-phase hexagonal wurtzite nanostructure. It has been observed that the concentration of OH− ions plays an important role in controlling the shape of ZnO nanostructures. TEM micrographs indicate that well-spherical particles of size about 8 nm were formed at lower concentration of OH− ions whereas the higher concentration of OH− ions favor the formation of nanorods of length 30–40 nm. The optical studies confirmed that the band gap and near band edge emission of ZnO nanostructures are strongly dependent on the shape of particles. Furthermore, the decrease in the intensity of green emission as shape of particles changes from sphere to rod indicates the suppressing of defect density, which in turn influences the photocatalytic activity and ferromagnetic-like behavior of ZnO nanostructures.