Surface States Of ZnO Research Articles

Energy Alignment of Quantum-Confined ZnO Particles with Copper Oxides for Heterojunctions with Improved Photocatalytic Performance.

The ability to control electronic states by utilizing quantum confinement of one of the material components in heterojunctions is a promising approach to perform energy-level matching. In this work, we report the possibility to achieve optimum energy alignment in heterojunctions made from size-controlled quantum dots (Q-dots) of ZnO in combination with three copper oxides: Cu2O, Cu4O3, and CuO. Quantum confinement effects on the ZnO nanoparticles in the diameter range 2.6-7.4 nm showed that the direct optical band gap decreased from 3.99 to 3.41 eV, with a dominating shift occurring in the conduction band (CB) edge, and thus the possibility to obtain close to 0.6 eV CB edge shift by controlling the size of ZnO. The effect was utilized to align the electronic bands in the ZnO Q-dot/copper oxide heterojunctions to allow for charge transfer between the materials and to test the ability to improve the photocatalytic performance for the system, evaluated by the transformation of a dye molecule in water. The catalyst materials were investigated by X-ray diffraction, scanning electron microscopy, ultraviolet-visible (UV-vis), photoluminescence, and Raman spectroscopy. The most promising material combination was found to be the Cu2O copper oxide in combination with an energy aligned ZnO Q-dot system with approximately 7 nm diameter, showing strong synergy effects in good agreement with the energy-level analysis, outperforming the added effect of its individual components, ZnO-Q-dots and Cu2O, by about 140%. The results show that utilization of a heterojunction with controllable energy alignment can provide a drastically improved photocatalytic performance. Apart from increased photocatalytic activity, specific surface states of ZnO are quenched when the heterojunction is created. It is anticipated that the same approach can be utilized in several material combinations with the added benefit of a system with controllable overpotential and thus added specificity for the targeted reduction reaction.

Low barrier height in a ZnO nanorods/NbSe2 heterostructure prepared by van der Waals epitaxy

Two-dimensional (2D) materials as contacts for semiconductor devices have attracted much attention due to minimizing Fermi level pinning. Schottky–Mott physics has been widely employed to design 2D material-based electrodes and to elucidate their contact behavior. In this study, we revealed that charge transfer across a 2D/semiconductor heterointerface and materials characteristics besides work function should be accounted for in fabrication of electrodes based on 2D materials. Our density functional theory (DFT) calculations predicted that charge transfer between ZnO and NbSe2 lowers the barrier height at the heterojunction and that conductive surface states of ZnO provide an additional conduction channel in the ZnO/NbSe2 heterostructures. Crystalline ZnO/NbSe2 heterostructures were prepared by the hydrothermal method. Electrical characterizations of the ZnO/NbSe2 heterostructures showed Ohmic-like behavior as predicted by the DFT calculations, opposed to the prediction based on the Schottky–Mott model.

Enhanced thermal stability of electron transport layer-free perovskite solar cells via interface strain releasing

The thermal decomposition of perovskite films on ZnO surfaces is generally believed to originate from specific surface states of ZnO and the impact from the lattice mismatch between ZnO and perovskite films on this process has long been ignored. In this research, the role of lattice mismatch in the thermal degradation process of cesium-containing perovskite films on Al doped ZnO (AZO) is studied. A Ba(OH)2 buffer layer on the surface of AZO is employed to release the lattice mismatch and suppress the thermal degradation of perovskite films resulted from ZnO. Consequently, perovskite films with enhanced thermal stability and crystalline properties are obtained. Meanwhile, the Ba(OH)2 films efficiently passivate the surface trap states and reduce the vacuum level of the AZO surfaces. On this basis, electron transport layer-free perovskite solar cells yield the best efficiency of 18.18% and the thermal stability is obviously improved.

Conformation-mediated Förster resonance energy transfer (FRET) in blue-emitting polyvinylpyrrolidone (PVP)-passivated zinc oxide (ZnO) nanoparticles

Homopolymers, such as polyvinylpyrrolidone (PVP), are commonly used to passivate the surface of blue-light emitting ZnO nanoparticles during colloid nucleation and growth. However, although PVP is known to auto-fluoresce at 400nm, which is near the absorption edge of ZnO, the impact of PVP adsorption characteristics on the surface of ZnO and the surface-related photophysics of PVP-capped ZnO nanoparticles is not well understood. To investigate, we have synthesized ZnO nanoparticles in solvents containing PVP of 3 concentrations—0.5, 0.7, and 0.11gmL−1. Using time-domain NMR, we show that the adsorbed polymer conformation differs with polymer concentration—head-to-tail under low concentration (e.g., 0.05gmL−1) and looping, then train-like, with increasing concentration (e.g., 0.07gmL−1 and 0.11gmL−1, respectively). When the surface-adsorbed PVP is entrained, the surface states of ZnO are passivated and radiative emission from surface trap states is suppressed, allowing emission to be dominated by exciton transitions in the UV (ca. 310nm). Moreover, the reduced proximity between the PVP molecule and the ZnO gives rise to increased efficiency of energy transfer between the exciton emission of ZnO and the HOMO-LUMO absorption of PVP (ca. 400nm). As a result, light emission in the blue is enhanced in the PVP-capped ZnO nanoparticles. We thus show that the emission properties of ZnO can be tuned by controlling the adsorbed PVP conformation on the ZnO surface via the PVP concentration in the ZnO precipitation medium.

Preparation and characterization of ZnO/SiO2/Ag nanoparticles as highly sensitive substrates for surface-enhanced Raman scattering

Metal/semiconductor nanostructure materials have been used as surface-enhanced Raman scattering (SERS) active substrates and have drawn much attention due to their widespread applications in both optical and electronics fields. Here, we report a simple approach to prepare a large-scale ZnO/SiO2/Ag nanoparticles (NPs) by the reduction of [Ag(NH3)2]+ with the reducing agent SnCl2·H2O in aqueous solution on the surface of activated ZnO/SiO2 microspheres. The thin SiO2 layer inserted between the ZnO and Ag has been demonstrated to be able to passivate the surface states of ZnO and improve the dispersion of Ag on the surface of NPs, thus enhanced the scattering intensity. The fabricated hybrids are found to be effective SERS substrates because of their homogeneous morphology and the ability to create a strong polarization induced local electromagnetic field by the metal–semiconductor heterojunction.

Surface potential measurement of As‐doped homojunction ZnO nanorods by Kelvin probe force microscopy

In this study, we demonstrate the electronic properties of As‐doped homojunction ZnO nanorods by Kelvin probe force microscopy (KPFM). The self‐assembled undoped/As‐doped homojunction ZnO nanorods were grown on Si (111) substrates by using vapor phase transport. Individual nanorods were transferred onto Au films grown on Si substrates. The morphology and surface potentials of the ZnO nanorods were simultaneously measured by KPFM. For the homojunction nanorods with ~200 nm in diameter, the KPFM images show obviously doping transition across the junction region, indicating local doping types. Also, the surface potential difference across the junction was measured to be ~85 mV, which could be the result from the work function difference between undoped and As‐doped region. It is in good agreement with the work function difference of ~95 meV between the As‐doped p‐type and intrinsic n‐type nanorods in the same measurement condition. The work function of the doped ZnO nanorods measured by KPFM is discussed in terms of surface band bending induced by surface states of ZnO. Copyright © 2011 John Wiley & Sons, Ltd.

Electronic structures and surface states of ZnO finite well structures

Electronic band structures and surface states were investigated for ZnO finite wells or slabs grown along ⟨0 0 0 1⟩ and ⟨1 − 1 0 0⟩ directions using tight-binding representation. The dangling bonds on two end-surfaces caused surface bands for different direction grown slabs, of which the wavefunctions tend to localize at the end-surfaces. The increasing splitting of the degenerate surface bands at the Γ point was observed to decrease with the thickness of the nonpolar [1 − 1 0 0] slab. And the quantum confinement effect is distinctively enhanced by the extra electron field induced in the ⟨0 0 0 1⟩ grown finite well with the polar end-surfaces.

Photoluminescence and micro-Raman scattering in ZnO nanoparticles: The influence of acetate adsorption

Abstract Ultraviolet (UV) emitting ZnO nanoparticles have been prepared by direct solvent evaporation of an alcohol solution containing KOH and zinc acetate. The particle size of ZnO becomes stable at an average diameter of 4–5 nm. Upon drying, the ZnO nanoparticle powder shows strong UV emission, suggesting that the surface states of ZnO have been eliminated with acetate molecules adsorption. Raman spectra show the presence of Zno E 2 ( 2 ) optical phonon mode, which is red-shifted when compared to bulk ZnO. The CO stretching mode in the Raman spectra is red-shifted to 1401 cm −1 , indicating strong binding of acetate on the ZnO surfaces.

Time-Resolved Radio Frequency Conductivity (TRRFC) Studies of Charge-Carrier Dynamics in Aqueous Semiconductor Suspensions

The time-resolved radio-frequency conductivity (TRRFC) method provides a useful tool for in situ measurements of charge carrier dynamics in aqueous suspensions of semiconductor particles. In this report, the effects of pH on surface states of ZnO and the effects of hole scavengers (2-propanol) are examined. The experimental results are interpreted in terms of surface mediated recombination processes, in which holes are trapped in a fast process by surficial sites on the ZnO. Recombination rates appear to be governed by the reaction rate of electrons with surface-trap sites. At higher pH (i.e., pH 12), electrostatic repulsion due to a negatively charged ZnO surface leads to slower surface recombination rates compared to lower pH (Le., pH 7) conditions. Addition of a hole scavenger significantly decreases the absolute charge-carrier concentration as detected by TRRFC. This decrease is attributed to the loss of trapped surface holes or surface-bound OH due to oxidation of the hole scavenger. When 2-propanol is used as a solvent, the holes react in a fast step with the solvent at the semiconductor interface within the time resolution of the experiment. The observed TRRFC signal is then due to electrons which are thought to be predominantly transferred to dissolved oxygen (O_2) leading to the formation of hydroperoxyl radicals (HO_2) and subsequently hydrogen peroxide (H_2O_2).