Defective ZnO Research Articles

Methanol synthesis on ZnO from molecular dynamics

AbstractThis paper reviews our efforts to simulate methanol synthesis from CO and H2 on defective ZnO surfaces using advanced molecular dynamics techniques. This apparently simple chemical reaction occurring on a seemingly well‐defined surface appears to be astonishingly complex. First of all, the preferred oxidation state of F centers at the polar oxygen terminated surface is found to be dictated by the chemical composition and the thermodynamic properties of the gas phase in contact with ${\rm ZnO}(000\overline {1} )$. Secondly, reaction intermediates and pathways along the catalytic cycle taking place at or close to these defects are found to depend in a sensitive way on their oxidation state. Thirdly, it is seen that the gas phase close to the catalytic surface might be transiently involved in some of the reaction steps in a non‐trivial manner. Last but not least, the scenario is found to be greatly enriched upon involving copper clusters on polar ZnO surfaces in view of utmost strong metal‐support interactions (SMSIs), which are directly related to the polar nature of ${\rm ZnO}(000\overline {1} )$. Taken together, an unexpectedly rich picture is unveiled by the molecular dynamics approach to computational heterogeneous catalysis when applied to methanol synthesis on bare ZnO.

Functionalized IRMOF-3: An Efficient Heterogeneous Catalyst for the Cycloaddition of Allyl Glycidyl Ether and CO<SUB>2</SUB>

Functionalized metal organic framework F-IRMOF-3 having quaternary ammonium group was prepared by a fast precipitation and solvothermal method. They showed good catalytic performance for the cycloaddition of allyl glycidyl ether (AGE) and carbon dioxide without any solvent. F-IRMOF-3 with a larger alkyl chain structure and more nucleophilic anion exhibited better reactivity for the cycloaddition reaction. The introduction of a defective ZnO to F-IRMOF-3 by the fast precipitation method becomes more advantageous than the conventional solvothermal method for the cycloaddition reaction due to acid-base bifunctional active sites. The F-IRMOF-3 can be easily recovered and reused without any considerable loss of initial activity.

Resistive switching of Au/ZnO/Au resistive memory: an in situ observation of conductive bridge formation.

A special chip for direct and real-time observation of resistive changes, including set and reset processes based on Au/ZnO/Au system inside a transmission electron microscope (TEM), was designed. A clear conducting bridge associated with the migration of Au nanoparticles (NPs) inside a defective ZnO film from anode to cathode could be clearly observed by taking a series of TEM images, enabling a dynamic observation of switching behaviors. A discontinuous region (broken region) nearby the cathode after reset process was observed, which limits the flow of current, thus a high resistance state, while it will be reconnected to switch the device from high to low resistance states through the migration of Au NPs after set process. Interestingly, the formed morphology of the conducting bridge, which is different from the typical formation of a conducting bridge, was observed. The difference can be attributed to the different diffusivities of cations transported inside the dielectric layer, thereby significantly influencing the morphology of the conducting path. The current TEM technique is quite unique and informative, which can be used to elucidate the dynamic processes in other devices in the future.

Synthesis and characterization of ZnGa2O4 particles prepared by solid state reaction

We employed solid state reaction technique to synthesize ZnGa2O4 particles, produced in steps of mixing/milling the ingredients in H2O following thermal treating under 1200°C. We compare spinel and partially inverse spinel structure in ZnGa2O4 particles using Rietveld refinement. Crystal structure of ZnGa2O4 particles was identified with two structural phases; normal spinel structure and partially inverse spinel structure using Rietveld refinement. It is found that the partially inverse spinel structures occupy nearly 13% and the rest is normal spinel structure. The obtained X-ray diffraction data show that lattice constant and the position of Oxygen atoms remain almost constant in both structures. The characterization of the particles was also improved using X-ray photoelectron spectroscopy and Fourier transforms infrared spectroscopy measurements. The optical analyses were done with UV–visible spectroscopy. The band gap, calculated from climate point of UV–visible data, was found as 4.6±0.1eV. Despite no unexpected compound (such as ZnO and Ga2O3) in the structure, the optical analyses were shown defective ZnO structure in ZnGa2O4.

Theoretical Study of Syngas Hydrogenation to Methanol on the Polar Zn-Terminated ZnO(0001) Surface

Methanol synthesis from syngas (CO/CO2/H2) hydrogenation on the perfect Zn-terminated polar ZnO(0001) surface has been investigated using periodic density functional theory calculations. Our results show that direct CO2 hydrogenation to methanol is unlikely because, in the presence of surface atomic H and O, the highly stable formate (HCOO) and carbonate (CO3) readily produced from CO2 with low barriers of 0.11 and 0.09 eV will eventually accumulate and block the active sites of the ZnO(0001) surface. In contrast, methanol synthesis from CO hydrogenation is thermodynamically and kinetically feasible on the perfect ZnO(0001) surface. CO can be consecutively hydrogenated into formyl (HCO), formaldehyde (H2CO), and methoxy (H3CO) intermediates, leading to the final formation of methanol (H3COH). The reaction route via hydroxymethyl (H2COH) intermediate, a previously proposed species on the defective O-terminated ZnO(0001) surface, is kinetically inhibited on the perfect ZnO(0001) surface. The rate-determini...

On the Origin of the Magnetism of Mn–Zn–O Systems: Structural, Electronic, and Magnetic Study of Exotic MnO2−δ/ZnO Thin Films

The structural, electronic, and magnetic characterization of sputtered MnO2−δ/ZnO films showing room temperature ferromagnetism has been carried out by a number of techniques. To elucidate the origin of the ferromagnetic order, a novel approach by studying XAS and XMCD signals at both Mn and Zn K edges in combination with the macroscopic magnetization measurements has been performed. The analysis of the XAS profile at the Mn edge indicates the coexistence of Mn4+/Mn3+ with a slightly distorted MnO2−δ environment. The XMCD signal at the Zn K edge indicates the existence of an intrinsic magnetic moment in Zn. The Zn edge dichroic intensity is, however, 1 order of magnitude lower than in the case of Mn, being the magnetic moment of Mn, 2.1–2.4 μB per Mn cation participating in the ferromagnetic phase, the main contribution to the total magnetization. The ferromagnetic phase has been assigned to high-density regions of Mn cations with a MnO2−δ distorted environment, mainly allocated at the nonstoichiometric and highly defective ZnO grain boundaries, with a sufficient extension to overcome the thermal energy over room temperature. The ferromagnetic coupling has been related to modifications of the Mn–O–Mn bonds, distances, and angles, promoted by neighboring ZnO and forming stable magnetic configurations that lead to new forms of exchange interaction: ferromagnetic superexchange or a double-exchange mechanism between the Mn4+ and Mn3+. Highly defective regions and structural distortions, together with complex Mn oxidation states, appear to be key features in the magnetic properties of Mn–Zn–O.

Magnetic effects of defect pair formation in ZnO

In order to gain insight into the so-calledd0-magnetic properties of defective ZnO we have carried out first principles calculations onvarious types of defects formed by intrinsic defects and doped atoms as well as pairs ofthem. The doped atoms include N and H. In agreement with previous works we find severalpossibilities to create magnetic defects especially by hole formation. Our results also showthat two defects which are in the vicinity of each other and that are magnetic whenisolated, in general become non-magnetic if one of them is acceptor-like and the otherone donor-like. Furthermore, we have investigated the magnetic interaction ofdifferent defect pairs via total energy calculations, the results of which show inall cases the stability of ferromagnetic configurations. In order to reproduce theexperimentally found localization of the magnetic hole states we have investigated theeffect of applying correlation corrections on the p orbitals containing these holes.

Synthesis and Characterization of Ba Defective ZnO Nano-Particle

ZnO-BaO particles (4-10) nm have been synthesized by applying 40 KHz ultrasonic wave simultaneously with the co-precipitation process of Zn and Ba oxalate at 50 0C. DTA curves of the co-precipitate were carried out and the decomposition temperature was determined. Mixed oxide obtained from thermal decomposition of Zn2 Ba (C2O4)2.2H2O in air has been investigated by TEM, XRD, IR and UV-Vis measurements. The results revealed that the obtained particle consists of a core of highly deformed Ba contaminated ZnO nano-crystals. The ZnO crystalline core is coated by Ba rich amorphous layer which prevents its growth. Decreasing concentration of ZnCl2 in the precipitation solution from 0.1M to .05 M leads to decrease the size of the particle from (10-20) nm to(4-10) nm, in average, increases UV cut-off, the cut-off tail and rounded of the shape of the particle.

Increase in Upturn Power Dissipation of Surge Suppressors Due to Highly Defective Nanostructure of Zinc Oxide

Nanodefects are probable root causes for the observed high power dissipation of ZnO‐based surge suppression devices (SSDs). In this work, nanodefects are introduced by overgrinding ZnO for 100 hours via wet milling. Using FESEM, ZnO nanostructures are found to contain fine cracks, chipped‐off surfaces and nanofragments. For the defective ZnO, the zinc relative atomic % (EDS analysis) is observed to be much larger accompanied by higher oxygen vacancy concentration as revealed by PL green emission. Average particle size drops from 0.24 µm to 0.19 µm and specific surface area increases from 4.72 m2/g to 5.67 m2/g. Fabricated SSDs with defective ZnO exhibits higher power dissipation and bigger grain resistivity. A model is proposed to provide a correlation between nanodefects and power dissipation.

The structure of higher defective ZnO[formula omitted

The surface structure of clean and defective ZnO(101̄0) have been studied using ab initio plane wave density functional theory. Two models of surface defects have been examined; an oxygen vacancy within the surface Zn–O dimer and an oxygen vacancy in the second layer of the material. Previous models of CO oxidation on this surface have invoked the formation of isolated second layer defects. In the current study we show that the formation of such a defect costs 1.01 eV more energy than the formation of a vacancy within the dimer bond and hence models of catalytic activity that invoke the formation of isolated second layer defects need to be re-examined.