ZnO Clusters Research Articles

Unpaired Electron-Induced Wide-Range Light Absorption within Zn (or Cu) MOFs Containing Electron-Withdrawing Ligands: A Theoretical and Experimental Study.

In photocatalysis, it is of general interest to understand and design wide-range light-absorbing inorganic/organic hybrid materials with an excellent photo-induced intramolecular charge-transfer (ICT) effect. To verify the role of unpaired electrons in enhancing ICT within electron-withdrawing ligand-based metal-organic frameworks (MOFs), the molecular structure, density of states (DOS), and electronic structure of strong electron-deficient pyridine-diketopyrrolopyrrole (P-DPP)-based Zn (or Cu) MOFs were calculated in Gaussian package to validate the unpaired electron ICT. The electron spin resonance technique has detected the unpaired electrons for the coordination systems containing Zn-O or Cu-O clusters and P-DPP ligand on photoexcitation. The estimated band gaps from the DOS calculation for P-DPP-Cu and P-DPP-Zn are 1.4 and 2.4 eV, respectively, showing a good agreement with the experimental UV-vis optical spectra. The partial DOS, dipole moment, and frontier orbital analysis prove that the ICT should happen from Zn-O or Cu-O clusters to P-DPP ligands. This research may contribute to a comprehensive understanding of electron-withdrawing ligand-induced ICT within MOFs and shed light on the design of light-absorbing MOFs with excellent ICT or conductivity.

Re-Oxidation of ZnO Clusters Grown on HOPG

This article studies the chemical interaction between ZnO and highly oriented pyrolytic graphite for as grown and thermally treated samples. In-situ X-ray photoelectron spectroscopy and ex-situ Raman spectroscopy confirm that graphite is affected by these processes, becoming oxidized and defective only in the presence of ZnO clusters that become recrystallized upon thermal re-oxidation processes performed at 400 °C. By comparing these results with other identical experiments performed with ZnO clusters grown on graphene and even with CoO clusters grown on graphite, the present results show how the interaction of the ZnO clusters with graphitic substrates depend on two factors—firstly, the mode of growth and corresponding morphology, and secondly, the reactivity of the graphitic substrates, either graphene or graphite. The results presented here will help us understand the fundamental interactions in ZnO/graphitic heterostructures and to define their operating limits.

Density functional theory investigation of structure, stability, and glycerol/hydrogen adsorption on Cu, CuZn, and CuZnO clusters

AbstractExtensive density functional theory calculations are performed to analyze the structure and activity of Cu and CuZn/CuZnO clusters containing up to 10 Cu/Zn atoms. The minimum‐energy structures of CuZn and CuZnO clusters are found by doping minimum‐energy pure Cu clusters with Zn atom(s) and ZnO molecule(s), respectively, followed by energy minimization of the resultant clusters. Odd‐even alteration in properties that determine cluster stability/activity is observed with cluster size, which may be attributed to the presence/absence of unpaired electrons. The difference in behavior between Zn/ZnO doping can be interpreted in terms of charge transfer between atoms. Charge transfers from Zn to Cu in the CuZn clusters and from Cu and Zn atoms to O atom in Cu‐ZnO clusters, which implies that the Cu atom acts as an electron acceptor in the CuZn clusters but not in the CuZnO clusters. Finally, the adsorption energies of glycerol and hydrogen on CuZn/CuZnO clusters are computed in the context of the use of CuZn/CuZnO catalysts in glycerol hydrogenolysis. Glycerol adsorption is generally found to be more energetically favorable than hydrogen adsorption. Dual‐site glycerol adsorption is also observed in some of the planar clusters. Fundamental insights obtained in this study can be useful in the design of CuZn/CuZnO catalysts.

Zn-promoted Hβ zeolite for gas-phase catalyzed aza-heterocyclic-aromatization of acrolein dimethyl acetal and aniline to quinolines

Catalytic activities of Zn-promoted Hβ zeolite for gas-phase aza-heterocyclic-aromatization of acrolein dimethyl acetal and aniline to quinolines were investigated. the Zn/Hβ catalyst showed better selectivity to quinoline than the parent Hβ one. Characterization results demonstrated that the Zn/Hβ catalyst prepared via deposition precipitation method existed the isolated Zn2+ cations as well as the highly dispersed ZnO clusters, which not only decreased concentration of strong acid sites but also enhanced enhance aromatization process. As a result, the decrease of strong acid sites restrained the cracking of acrolein to acetaldehyde as well as the alkylation of quinoline to ethylquinoline effectively; and the Zn species of catalyst further improved aromatization process of dihydroquinoline to quinoline. Moreover, the Zn/Hβ catalyst presented relatively enhanced ability of anti-activation and excellent regenerability, owing to decrease strong acid-induced polymerization of active intermediates to form the coking. Under the optimized operating conditions, more than 51 % yield of quinoline was achieved over Zn/Hβ catalyst; which far exceeded quinoline yield (28 %) over the pure Hβ one. Besides, a plausible reaction routes in vapor-phase acrolein diethyl acetal with aniline to quinolines were suggested in this paper.

Tuning of electronic energy levels of NH3 passivated ZnO nanoclusters: A first principle study

We have investigated the optical structural and electronic properties of NH3 capped (ZnO)21 and (ZnO)34 nanoclusters of varying sizes, and their complexes by using density functional theory (DFT. B3LYP functional with LANL2DZ basis set which are implemented in the Gaussian09 package. Structural study of bond length, bond angle and bond distortion increases slightly with increase in the surface passivation. From PDOS there is merely an equivalent contribution to the surface states from ZnO and NH3 molecules except for 3 NH3 capping where contribution of ZnO is very less. Surface of nanoclusters is capped upto five NH3 molecules for both nanoclusters have been examined. The band gap of ZnO clusters decreases with the increase in particle size, illuminating quantum confinement in nanoclusters. The band gap becomes reliable with increase in the surface passivation. Additionally, the uv–vis spectra delivers blue shift of the entire absorption spectra. The amount of spectral displacement from red to blue shift happens due to increase in NH3. Also the presence of amine group on Zn atom will extensively increases its nucleophilicity.

Density Functional Theory Studies of Zn12O12 Clusters Doped with Mg/Eu and Defect Complexes

We report a density functional theory study of ZnO cluster doped with Eu and Mg along with native point defects using the generalized gradient approximation including the Hubbard parameter. The Zn atomic positions are found to be energetically more favorable doping sites than O. The Eu has a lower formation energy than Zn and O vacancies, helps in lowering the formation energy of point defects and induces spin polarization. Mg is less favorable dopant energetically and is not inducing any magnetism in the cluster. Presence of Eu and point defects along with Mg can help in sustaining spin polarization, implying that transition metal and rare earth dopant is a favorable combination to invoke desirable properties in ZnO based materials. Eu–Eu doping pair prefers ferromagnetic orientation and a spin flip is induced by Eu in the Eu–Mg configuration. Further, Eu doping increases the value of static refractive index and optical absorption in the UV region compared to the undoped ZnO cluster.

On the structural-optical correlation of ZnO nanospheres synthesized using thermal evaporation technique

Zinc oxide clusters of different sizes and geometrical shapes have been synthesized and characterized experimentally and theoretically. The ZnO cluster models have been built and presented to study the evolution of physico-chemical properties of the metal oxide semiconductor upon reduction of size. However, most of the theoretical models are lack of support from the experimental details. This article aims to correlate the geometry of the theoretical models with their optical properties. The ZnO thin films were synthesized by thermal evaporator at vacuum pressures of 5 × 10−5 Torr. Cluster models of the synthesized ZnO were built based on its crystal structure. The clusters were evaluated as realistic using geometry optimization, and harmonic frequency calculations at the level of B3LYP functional and lanl2dz basis set. We observed that the fabricated ZnO thin film was constructed by small ZnO spherical clusters, which revealed by the Field Emission Electron Microscopy (FESEM) . This observation is in good agreement with our theoretical simulations, which were established based on its crystal structure – characterized by the X-ray diffractometer.

Molecular Structures of ZnO by Aggregates of Atoms and Interaction of Two Monolayers

The first-principles calculations are useful for determining electronic and structural properties for a model that simulates a material composed of atomic clusters of ZnO through the analysis of interaction energies and charge distribution. The two-dimensional structural form of ZnO aggregates shows regularly flat hexagons obtained in models of 6, 27 and 54 atoms of Zinc and Oxygen. The structure of a three-dimensional system was determined by dynamics calculations by using the interaction of a pair of monolayers consisting of 108 atoms and as a result, a cage structure was formed from a cluster of Zn54 and O54 identifying only bond atoms at the ends that promote the union of monolayers. The stable structure shows modifications of the atomic bonds in whose centers hexagonal rings prevailed and at the arrangements of the end of triangles, squares, pentagons and even rings of 10 and 11 atoms were obtained. Atomic positions and charge distribution were analyzed based on the methodology used Density Functional Theory (DFT), with the becke88-LYP exchange and correlation functional.

Molecular Structures of ZnO by Aggregates of Atoms and Interaction of Two Monolayers

The first-principles calculations are useful for determining electronic and structural properties for a model that simulates a material composed of atomic clusters of ZnO through the analysis of interaction energies and charge distribution. The two-dimensional structural form of ZnO aggregates shows regularly flat hexagons obtained in models of 6, 27 and 54 atoms of Zinc and Oxygen. The structure of a three-dimensional system was determined by dynamics calculations by using the interaction of a pair of monolayers consisting of 108 atoms and as a result, a cage structure was formed from a cluster of Zn54 and O54 identifying only bond atoms at the ends that promote the union of monolayers. The stable structure shows modifications of the atomic bonds in whose centers hexagonal rings prevailed and at the arrangements of the end of triangles, squares, pentagons and even rings of 10 and 11 atoms were obtained. Atomic positions and charge distribution were analyzed based on the methodology used Density Functional Theory (DFT), with the becke88-LYP exchange and correlation functional.

Propylene Transformation on Zn-Modified Zeolite: Is There Any Difference in the Effect of Zn2+ Cations or ZnO Species on the Reaction Occurrence?

The peculiar role of either Zn2+ cations or small ZnO clusters located inside the zeolite pores in propylene transformation to aromatic hydrocarbons on Zn2+/H-ZSM-5 and ZnO/H-BEA zeolites has been analyzed. 13C MAS NMR and FTIR spectroscopic investigation has revealed similar properties of different Zn-sites with respect to propylene oligomerization. Both Zn2+ cations and ZnO species are capable to convert propylene to higher C6–C9 olefins which represent the precursors of simple benzene-based aromatics. The mechanism of such oligomerization has been inferred to include the following stages: (a) adsorption of propylene on the Zn-sites in the form of a π-complex; (b) transformation of the π-complex of propylene to σ-allylzinc surface species at elevated temperature; (c) formation of C6+ olefins; (d) transformation of oligomeric C6+ olefins to simple aromatic molecules (benzene, toluene, and xylenes). NMR evidences that Bronsted acid sites (BAS) of both zeolite samples are not involved in the oligomerizatio...

The active site of syngas conversion into ethanol over Cu/ZnO/Al2O3 ternary catalysts in slurry bed

As a promising technology, selective conversion of syngas to ethanol over the CuZnAl catalyst has a long-standing interest in current energy research. Here, we report cooperative ZnO and Al2O3 clusters on the Cu(1 1 1) surface for selective hydrogenation of CO to ethanol using density functional theory (DFT) and microkinetic model. Our results show that the Cu-ZnO synergy facilitates CO initial activation to CHO, the cooperation of the three components is necessary for CH3 formation via the direct dissociation of the formed CH3OH that is the rate-controlling step throughout the whole process, and the aid of the Cu-Al2O3 synergy is indispensable for the initial carbon chain formation by CO insertion into CH3 intermediate with a 0.45 activation barrier. Apart from the ethanol synthesis, we propose the possibility of syngas conversion into ethylene glycol that is a typical atom economic reaction. Moreover, thermodynamic and kinetic barriers and charge analyses results identify that the carbon chain growth via CHO coupling with the assistant of Cu-ZnO synergy is facile for the formation of ethylene glycol. In order to achieve high ethanol productivity and selectivity over the CuZnAl catalyst, the aid of a metal promoter with the function of facilitating the CHO hydrogenation and/or suppressing the CHO coupling is required. This work not only clarifies the synergy of Cu, ZnO, and Al2O3 components of CuZnAl catalyst for ethanol synthesis, but also offers an alternative nonpetroleum route for the ethylene glycol synthesis.

ZnO and TiO2 clusters as catalyst in the addition and abstraction reaction of acrylic acid with the OH radical

AbstractThe present work displays the theoretical analysis on the role of metal oxide clusters as an effective catalyst in the reaction between acrylic acid and OH radical, which has an energy barrier of 12.4 kcal/mol. The formation of metal oxide cluster such as ZnO and TiO2 with varying size from monomer to hexamer is analyzed using cohesive energy, which increases with cluster size. Adsorption of acrylic acid on clusters reveals that dimer ZnO and tetramer TiO2 are good adsorbed entities. The dimer ZnO and tetramer TiO2 clusters have reduced the barrier height. However, from the thermodynamical analysis of H‐abstraction and OH addition reaction, the dimer ZnO cluster is found to be a good catalyst than a tetramer TiO2 cluster. The favorable H abstraction and OH addition reactions are feasible at the active methylene group (–CH). OH addition reactions dominate over the H abstraction reaction. Further, the presence of metal oxide clusters enhances the rate of the reaction between acrylic acid and OH radical. The kinetics of the favorable reaction with a dimer ZnO cluster has a rate constant of 7.80 × 10−11 cm3 molecule−1 s−1, which is higher than the literature report (1.75 × 10−11 cm3 molecule−1 s−1). Overall, ZnO and TiO2 metal oxide clusters can be effectively utilized as catalyst.

Experimental and density functional theory investigations of catechol sensing properties of ZnO/RGO nanocomposites

Catechol, an essential industrial feedstock is one of the major environmental pollutant which causes several health issues including skin irritation, eye damage and genetic defects. In this work, we have investigated the catechol sensing performance of ZnO/RGO composites for its quantitative detection. ZnO/RGO composite showed superior electron transfer kinetics, sensitivity (162.04 μA mM cm−2) and lower detection limit (47 nM) than that of bare ZnO nanoparticles. Further, the change in the zeta potential towards the lower negative demonstrated the strong electrostatic interaction between ZnO-RGO and catechol where it is converted as 1, 2 Benzoquinone through redox reactions. By using state of the art of Density Functional Theory simulations, we have demonstrated the interaction of catechol molecule on various ZnO clusters and RGO supported ZnO clusters. The strong bonding between p orbital of O atom of catechol and d orbital of Zn atom and its charge transfer facilitate the oxidation of catechol molecule. In the presence of RGO, the binding energy of catechol and charge transfer increases which qualitatively infer that RGO/ZnO exhibits higher sensitivity compared to bare ZnO and in consistency with the experimental observations.

ZnO supported on high-silica HZSM-5 as efficient catalysts for direct dehydrogenation of propane to propylene

Zn/HZSM-5 catalysts are well known in the conversion of propane to aromatics. Here we report that the HZSM-5 zeolite with high SiO2/Al2O3 ratio can be used to support ZnO catalysts for the reaction of direct dehydrogenation of propane to propylene (PDH). The 10%Zn/250HZSM-5 (SiO2/Al2O3 ratio is of 250) catalyst is much more selective and stable in PDH compared with that with lower SiO2/Al2O3 ratios, still presenting a high propane conversion of about 43% with a robust propylene selectivity of 93% after reaction for 10 h. As a result, the catalytic performance and reaction stability of the ZnO catalysts increase as rationally elevating the SiO2/Al2O3 ratio of HZSM-5 support. Comprehensive techniques including N2 sorption, XRD, SEM, TEM, DR UV–vis, XPS, C3H6-TPD, NH3-TPD and TGA jointly evidence that the high-silica HZSM-5 zeolite, with good textural properties, well-defined structure and well-adjusted acidity, is helpful for dispersing Zn species and reducing side reactions. Moreover, it is proposed that the small ZnO clusters directly connected with the framework of HZSM-5 zeolite are active phases, which are massively produced upon the zeolite support with fine structure condition and sparse Al sites. The ZnO catalysts supported on high-silica HZSM-5 is low-cost, toxicity-free and reusable, is thus of great promising to be used in industrial propylene production.

ZnO nanoparticles as a sensitive platform for detection of nitration in tyrosine and tryptophan: A DFT study

Regarding the significance of protein tyrosine and tryptophan nitration in the onset and progress of various chronic disorders, this study addressed ZnO nanoparticles (NPs) interaction with tyrosine and tryptophan and their nitrated counterparts. (ZnO)12 cage-like cluster was selected to model ZnO NPs, where the density functional theory was employed to determine the binding energy, active site and electronic structure of the bioconjugated systems. The results indicated the tendency of pristine amino acids to interact with (ZnO)12 through their aromatic ring; while the nitro group is the preferred site of nitrated amino acids. Comparison of electronic structure of (ZnO)12-Tyr/Trp with Tyr/Trp clearly explain the fluorescence quenching of tryptophan and tyrosine upon interaction with ZnO NPs. Emergence of new states above the Fermi level of (ZnO)12-nTyr/nTrp can result in unique response of ZnO nanoparticles to nitrated amino acids introducing small clusters of ZnO as promising candidates for detection of nitrosative stress-induced post-translation modifications leading to early diagnosis of different chronic disorders.

An experimental and theoretical study of the effect of Ce doping in ZnO/CNT composite thin film with enhanced visible light photo-catalysis

Ce-doped ZnO/CNT composite thin film was fabricated successfully on soda-lime-silica glass substrate by sol-gel drop coating method. The structure and morphology of nanocrystalline Ce-doped ZnO/CNT thin film were characterized by X-ray Diffraction (XRD), X-ray photo-electron spectroscopy (XPS), Field Emission Scanning Electron Microscope (FESEM) and UV–Visible spectroscopy. The photocatalytic activity of Ce-doped ZnO/CNT thin film was evaluated by photocatalytic degradation of methylene blue (MB) in aqueous solution as a model pollutant under visible light irradiation. The synthesized Ce-doped ZnO/CNT composite thin film showed 76.71% photocatalytic efficiency whereas bare ZnO thin film showed that of only 25.30%. It has been reported that improved photocatalytic efficiency of composite is due to the synergistic effect of Ce doping and insertion of CNTs into ZnO matrix. The experimental photodegradation data were well fitted to first-order kinetics. The photocatalytic activity of the prepared thin film can be regenerated, which implies that the photocatalytic degradation process could be operated at a relatively low cost. The results suggest that Ce-doped ZnO/CNT composite thin film prepared by sol-gel drop coating method can be developed as an economically feasible and environmentally friendly method to degrade dye containing wastewater using visible light. Furthermore, atomic models for Ce doping in ZnO cluster was used to investigate the effect of doping on electronic structure of ZnO through density functional theory calculations. The computational study suggested a significant narrowing of the band gap and change of the maximum absorption bands towards higher wavelength. These all support the experimental results.

Porous ZnO Ultrathin Nanosheets with High Specific Surface Areas and Abundant Oxygen Vacancies for Acetylacetone Gas Sensing.

In this study, porous ZnO ultrathin nanosheets with abundant surface oxygen vacancies were prepared by a hydrothermal technique followed by an annealing method using graphene oxide (GO) as a template. The high specific surface area of GO with ultrathin thickness provided an important template for the ZnO ultrathin nanosheets. The as-prepared porous ZnO ultrathin nanosheets exhibited superior acetylacetone sensing performance. The sensor response of the porous ZnO ultrathin nanosheets was 191.1 for 100 ppm acetylacetone, which was approximately 4 times higher than that of ZnO clusters (prepared without GO template) at 340 °C. The porous ZnO ultrathin nanosheets also exhibited excellent selectivity and operational stability. The excellent gas sensing performance of the porous ZnO ultrathin nanosheets was due to their high specific surface area (130.5 m2/g) and abundant surface oxygen vacancy.

Synthesis of Zn-doped TiO2 nano-particles using metal Ti and Zn as raw materials and application in quantum dot sensitized solar cells

Zn-doping in TiO2 gives an n-type doped material and can tailor the optical band gap and Fermi level of TiO2. In the present work, using metal Ti and Zn as raw materials, Zn-doped TiO2 nano-particles are synthesized via chemical co-precipitation and calcination process. The effects of Zn-doping on the morphology, crystal structure, optical behavior of TiO2 films, and the performance of CdS/CdSe sensitized cells are systemically investigated, we focus on improving the power conversion efficiency of the cells based on Zn-doped TiO2 photoanodes. The results show that the solar cell assembled with the Zn0.1Ti0.9O2 film exhibits a short circuit current density of 12.32 mAcm−2, open-circuit voltage of 0.604 V, and a power conversion efficiency of 3.03%. Its power conversion efficiency is 1.5-fold higher than that of the cell assembled with the pure TiO2 film, which is ascribed to reducing electron recombination and enhancing electron transport in the TiO2 film by the optimal Zn doping. Further increasing the amount of Zn doping in the TiO2 film, the performance of the cell decreases, which is attributed to introducing a large number of ZnO clusters and producing more surface defects in the TiO2 film that act as electron–hole recombination centers.

Influence of the Crystal Structure of the Nucleus on the Morphology of t-ZnO Tetrapods

A new approach to simulating the morphology of hierarchical nanocrystals has been proposed on the example of ZnO nanotetrapods. Within this approach, twinning is considered as a point of symmetric growth bifurcation under unstable conditions. The electronic structure has been calculated for elementary ZnO clusters of different symmetries by the molecular dynamics method to reveal the processes of symmetry transformations occurring under nonequilibrium conditions during the forming of the tetrapod nucleus.

CH<sub>2</sub>O Adsorption on M (M = Li, Mg and Al) Atom Deposited ZnO Nano-Cage: DFT Study

The structural and electronic properties of Li, Mg and Al deposited ZnO nanocages and their effects on the adsorption of formaldehyde molecule have been investigated using the density functional theory (DFT) computations. To understand the behavior of the adsorbed CH2O molecule on the ZnO nanocage, results of DFT calculations of the M-deposited nanocages (M=Li, Mg and Al), as well as complex systems consisting of the adsorbed CH2O molecule on M-deposited ZnO nanocage were reported. The results presented include adsorption energies, bond lengths, electronic configurations, density of states and molecular orbitals. It was found that, the most energetically stable adsorption configurations of CH2O molecule on the bare ZnO leads to 12% dilation in C=O bond length of CH2O and 14% decrease in HOMO-LUMO gap of ZnO cluster. The most energetically stable adsorption configurations of CH2O molecule on Li, Mg and Al-deposited ZnO lead to 4%, 4% and 11% dilation in C=O bond length of CH2O and-0.66, -45 and , +66% change in HOMO-LUMO gap of ZnO nanocages, respectively. The interaction between CH2O with bare ZnO and M-deposited ZnO nanocages is attributed to charge transfer mechanism. These results may be meaningful for CH2O degradation and detection.