Method for Surface Characterization Using Solid-State Nuclear Magnetic Resonance Spectroscopy Demonstrated on Nanocrystalline ZnO:Al

Nanoscale zinc-oxide doped with aluminum ZnO:Al is studied by different techniques targeting surface changes induced by the conditions at which ZnO:Al is used as support material in the catalysis of methanol. While it is well established that a variety of 1H and 27Al resonances can be found by solid-state NMR for this material, it was not clear yet which signals are related to species located close to the surface of the material and which to species located in the bulk. To this end, a method is suggested that makes use of a paramagnetically impregnated material to suppress NMR signals close to the particle surface in the blind sphere around the paramagnetic metal atoms. It is shown that it is important to use conditions that guarantee a stable reference system relative to which it can be established whether the coating procedure is conserving the original structure or not. This method, called paramagnetically assisted surface peak assignment, helped to assign the 1H and 27Al NMR peaks to the bulk and the surface layer defined by the blind sphere of the paramagnetic atoms. The assignment results are further corroborated by the results from heteronuclear 27Al{1H} dipolar dephasing experiments, which indicate that the hydrogen atoms are preferentially located in the surface layer and not in the particle core.

Activation of Molecular O2 on CoFe2 O4 (001) Surfaces: An Embedded Cluster Study

Dioxygen activation pathways on the (001) surfaces of cobalt ferrite, CoFe₂O₄, were investigated computationally using density functional theory and the hybrid Perdew‐Burke‐Ernzerhof exchange‐correlation functional (PBE0) within the periodic electrostatic embedded cluster model. We considered two terminations: the A‐layer exposing Fe²⁺ and Co²⁺ metal sites in tetrahedral and octahedral positions, respectively, and the B‐layer exposing octahedrally coordinated Co³⁺. On the A‐layer, molecular oxygen is chemisorbed as a superoxide on the Fe monocenter or bridging a Fe−Co cation pair, whereas on the B‐layer it is adsorbed at the most stable anionic vacancy. Activation is promoted by transfer of electrons provided by the d metal centers onto the adsorbed oxygen. The subsequent dissociation of dioxygen into monoatomic species and surface reoxidation have been identified as the most critical steps that may limit the rate of the oxidation processes. Of the reactive metal‐O species, [Fe^III−O]²⁺ is thermodynamically most stable, while the oxygen of the Co−O species may easily migrate across the A‐layer with barriers smaller than the associative desorption.

Mechanism of creation and destruction of oxygen interstitial atoms by nonpolar zinc oxide(101[combining macron]0) surfaces

Oxygen vacancies (VO) influence many properties of ZnO in semiconductor devices, yet synthesis methods leave behind variable and unpredictable VO concentrations. Oxygen interstitials (Oi) move far more rapidly, so post-synthesis introduction of Oi to control the VO concentration would be desirable. Free surfaces offer such an introduction mechanism if they are free of poisoning foreign adsorbates. Here, isotopic exchange experiments between nonpolar ZnO(101[combining macron]0) and O2 gas, together with mesoscale modeling and first-principles calculations, point to an activation barrier for injection only 0.1-0.2 eV higher than for bulk site hopping. The modest barrier for hopping in turn enables diffusion lengths of tens to hundreds of nanometers only slightly above room temperature, which should facilitate defect engineering under very modest conditions. In addition, low hopping barriers coupled with statistical considerations lead to important qualitative manifestations in diffusion via an interstitialcy mechanism that does not occur for vacancies.

A quantum chemical study of hydrogen adsorption on carbon-supported palladium clusters

A key step for achieving better insight into catalytic hydrogenation reactions is to understand in detail the process of hydrogen adsorption on the catalyst. The present article focuses on hydrogen adsorption on carbon-supported palladium clusters, which are nowadays one of the most common catalysts in industrial applications. Density functional theory is applied to study Pd₆ and Pd₂₁ clusters to reveal the influence of the carbon support material on the properties of the catalyst as well as on the mechanisms and energetics of the hydrogen adsorption. In general, a stepwise hydrogen adsorption process is observed consisting of molecular adsorption followed by dissociative chemisorption. The carbon support material does not noticeably affect the reaction mechanisms, but has a large influence on energy barriers and preferential adsorption sites. Our comparison of Pd₆ and Pd₂₁ systems reveals that small clusters, such as Pd₆, are able to model some but not all important properties of palladium nanoparticles and, therefore, it is essential to also study larger cluster sizes.

Temperature-dependent thermal behavior of impurity hydrogen trapped in vacancy-type defects in single crystal ZnO

Interacting nature between impurity hydrogen atoms and vacancy-type defects in single crystal ZnO was investigated by means of positron annihilation lifetime spectroscopy. In order to clarify the observation of their thermal behavior, the sample was implanted with ¹H⁺ using an electrostatic accelerator. After the implantation, the positron lifetime became shorter, which suggests that the hydrogen atoms were captured by zinc vacancies (V_Zn) to form vacancy-hydrogen complexes (V_Zn + nH). The complexes decompose by heat treatment: most of the hydrogen atoms gradually dissociate from V_Zn + nH in the temperature range 393-773 K. It was also suggested that large vacancy clusters were formed by the agglomeration of smaller clusters during the process of stepwise isochronal annealings at temperatures from 773 to 1073 K, and their decomposition took place at 1173-1373 K. Temperature-dependent thermal behaviors of hydrogen atoms and vacancy-type defects in ZnO are discussed.

Light-Induced Peroxide Formation in ZnO: Origin of Persistent Photoconductivity

The persistent photoconductivity (PPC) in ZnO has been a critical problem in opto-electrical devices employing ZnO such as ultraviolet sensors and thin film transistors for the transparent display. While the metastable state of oxygen vacancy (V_O) is widely accepted as the microscopic origin of PPC, recent experiments on the influence of temperature and oxygen environments are at variance with the V_O model. In this study, using the density-functional theory calculations, we propose a novel mechanism of PPC that involves the hydrogen-zinc vacancy defect complex (2H-V_Zn). We show that a substantial amount of 2H-V_Zn can exist during the growth process due to its low formation energy. The light absorption of 2H-V_Zn leads to the metastable state that is characterized by the formation of (peroxide) around the defect, leaving the free carriers in the conduction band. Furthermore, we estimate the lifetime of photo-electrons to be ~20 secs, which is similar to the experimental observation. Our model also explains the experimental results showing that PPC is enhanced (suppressed) in oxygen-rich (low-temperature) conditions. By revealing a convincing origin of PPC in ZnO, we expect that the present work will pave the way for optimizing optoelectronic properties of ZnO.

Surface Structure Modification of ZnO and the Impact on Electronic Properties

Zinc oxide (ZnO) is a widely utilized, versatile material implemented in a diverse range of technological applications, particularly in optoelectronic devices, where its inherent transparency, tunable electronic properties, and accessible nanostructures can be combined to confer superior device properties. ZnO is a complex material with a rich and intricate defect chemistry, and its properties can be extremely sensitive to processing methods and conditions; consequently, surface modification of ZnO using both inorganic and organic species has been explored to control and regulate its surface properties, particularly at heterointerfaces in electronic devices. Here, the properties of ZnO are described in detail, particularly its surface chemistry, along with the role of defects in governing its electronic properties, and methods employed to modulate the behavior of as-grown ZnO. An outline is also given on how the native and modified oxide interact with molecular materials. To illustrate the diverse range of surface modification methods and their subsequent influence on electronic properties, a comprehensive review of the modification of ZnO surfaces at molecular interfaces in hybrid photovoltaic (hPV) and organic photovoltaic (OPV) devices is presented. This is a case study rather than a progress report, aiming to highlight the progress made toward controlling and altering the surface properties of ZnO, and to bring attention to the ways in which this may be achieved by using various interfacial modifiers (IMs).

A successive photocurrent transient study to probe the sub-band gap electron and hole traps in ZnO nanorods

Probing of the sub-band gap electron and hole traps in ZnO nanorods has been carried out using a simple technique of successive photocurrent transients.