Valence Band Of ZnO Research Articles

Sol–gel synthesis and photocatalytic activity of ZnO–SnO2 nanocomposites

ZnO–SnO2 nanocomposites were synthesized by a facile sol–gel synthesis route and characterized through X-ray diffraction, BET specific surface area analysis, UV–vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy. The photocatalytic activity of the samples was tested using the degradation of 4-nitrophenol under UV light as model reaction. The ZnO/SnO2 molar ratio was varied in order to study its influence on the photoefficiency of the samples. The ZnO–SnO2 nanocomposites showed higher photoactivity than the pure oxides and in particular the sample with ZnO/SnO2 molar ratio equal to 1/0.05 resulted the best one among the tested powders. The high activity of the mixed samples was attributed to the presence of heterojunctions between the two oxides, which allows an improved charge separation of the photogenerated electron–hole pairs, due to the differences between the energy levels of the conduction and valence bands of ZnO and SnO2. Photovoltage measurements were performed to determine the energy band structure of the ZnO–SnO2 heterojunction.

Effects of Interface States on Photoexcited Carriers in ZnO/Zn2SnO4 Type-II Radial Heterostructure Nanowires

Type-II band alignment of heterostructure contributes to spatially separate electrons and holes leading to an increase in minority carrier lifetime, which has much more advantages in photocatalytic activities and photovoltaic device applications. Here, Zn2SnO4-sheathed ZnO radial heterostructure nanowires were constructed to investigate systematically interfacial charge separation. The lattice mismatch between ZnO and Zn2SnO4 induces interface states to exist at their heterointerface. At low pump fluence, photoexcited charges are localized within the ZnO core rather than separated due to the large interface barrier. Correspondingly, only ZnO-related bandedge ultraviolet (UV) and green emissions are dominated in photoluminescence spectra. At high pump fluence, however, impurities are ionized and electrons trapped in interface states are excited, resulting in a decrease in interface barrier, which makes photogenerated charges efficiently separated at their heterointerface by direct tunneling, and, consequently, an additional blue-violet emission, attributed to the heterointerface recombination of electrons in Zn2SnO4 conduction band (CB) and holes in ZnO valence band. Additionally, the heterointerface can separate effectively photoexcited carriers and form a photovoltaic effect. Our results provide the localization/separation condition of photogenerated charges for the type-II band alignment of core/shell heterostructure, which should be very useful for the realization of underpinned mechanism of the developed optoelectronic devices.

Effect of different type of organic compounds on the photocatalytic reduction of Cr(VI) in presence of ZnO nanoparticles

ABSTRACTEffect of different type of organic compounds (humic acid, oxalate, ethylenediaminetetraacetic acid, nitrilotriacetic acid, phenol) on the photocatalytic reduction of Cr(VI) with illuminated ZnO was studied in this work with variation of solution pH, contact time, initial Cr(VI) concentration, and type of organic compounds. Maximum removal of Cr(VI) was observed at near neutral pH because of the reduced photocatalytic activity of ZnO at exceedingly low and high pH values originated from either acidic/photochemical corrosion of the catalyst and/or surface passivation with Zn(OH)2. Removal efficiency of Cr(VI) increased in the presence of oxalate compared to that without presence of organic compound because of positive holes in the valence band of ZnO were scavenged by oxalate. However, it was decreased in the presence of the other organic compounds due to the blocking effects of electron transfer between ZnO surface and Cr(VI). The reduction pattern of Cr(VI) was better described by first-order kin...

Verification of symmetry assignment for the top valence band of ZnO by magneto-optical studies of the free A exciton state

Circularly-polarized and angular-resolved magneto-photoluminescence spectroscopy was carried out to study the free A exciton 1S state in wurtzite ZnO at 5 K. The Γ 7 symmetry of the top valence band symmetry is confirmed according to the unique selection rules of Zeeman splitting lines. The out-of-plane component B∥ of the magnetic field, which is parallel to ZnO's c-axis, leads to linear Zeeman splitting of both the dipole-allowed Γ 5 exciton state and the weakly allowed Γ 1 /Γ 2 exciton states. The in-plane field B⊥, which is perpendicular to the c-axis, increases the oscillator strength of the weak Γ 1 /Γ 2 states by forming a mixed exciton state. For the Γ 7 symmetry, the lower energy Zeeman splitting peak of the weak Γ 1 /Γ 2 can only be σ+ polarization.

Visible light active photocatalysis on block copolymer induced strings of ZnO nanoparticles doped with carbon

We report a facile route for the fabrication of a unique configuration of carbon-doped strings of ZnO nanoparticles obtained by subjecting solvent vapor annealing followed by direct carbonization to reconstructed block copolymer inverse micelle arrays. The dual role of a carbon (C) moiety has been exploited in significantly enhancing the catalytic performance of C–ZnO nanohybrids under the illumination of visible light. The first role is to create a new energy level above the valence band of ZnO and the other is to act as a channel by which recombination of the excitons is effectively suppressed. The enhanced visible light active photocatalytic efficiency and recyclability of C–ZnO was demonstrated in terms of degradation of a target dye (p-nitrophenol) molecule. Photoelectrochemical water splitting was also performed using C–ZnO on ITO as the working electrode. A significant enhancement in the photocurrent density was obtained for C–ZnO compared with bare ZnO. A plausible mechanism to support the enhanced visible light activity of C–ZnO was proposed.

Enhanced photocatalytic activity of ZnO–graphene nanocomposites prepared by microwave synthesis

This work reports a simple one-step synthesis of ZnO nanopyramids supported on reduced graphene oxide (RGO) nanosheets using microwave irradiation (MWI) of zinc acetate and GO in the presence of a mixture of oleic acid and oleylamine. The rapid decomposition of zinc acetate by MWI in the presence of the mixture of oleic acid and oleylamine results in the formation of hexagonal ZnO nanopyramids. GO has a high affinity for absorbing MWI, which results in a high local heating effect around the GO nanosheets and facilitates the reduction of GO by the oleylamine. The RGO nanosheets act as heterogeneous surface sites for the nucleation and growth of the ZnO nanopyramids. Using ligand exchange, the ZnO–RGO nanocomposites can be dispersed in an aqueous medium, thus allowing their use as photocatalysts for the degradation of the malachite green dye in water. The ZnO–RGO nanocomposites show enhanced photocatalytic activity for the degradation of the dye over the unsupported ZnO nanopyramids. The enhanced activity is attributed to efficient charge transfer of the photogenerated electrons in the conduction band of ZnO to graphene. This enhances the oxidative pathway of the holes generated in the valence band of ZnO which can effectively lead to the degradation and mineralization of the malachite green. The ZnO nanopyramids supported on RGO could have improved performance in other photocatalytic reactions and also in solar energy conversion.

CO2 adsorption on polar surfaces of ZnO

Physical and chemical adsorption of CO2 on ZnO surfaces were studied by means of two different implementations of periodic density functional theory. Adsorption energies were computed and compared to values in the literature. In particular, it was found that the calculated equilibrium structure and internuclear distances are in agreement with previous work. CO2 adsorption was analyzed by inspection of the density of states and electron localization function. Valence bands, band gap and final states of adsorbed CO2 were investigated and the effect of atomic displacements analyzed. The partial density of states (PDOS) of chemical adsorption of CO2 on the ZnO(0001) surface show that the p orbitals of CO2 were mixed with the ZnO valence band state appearing at the top of the valence band and in regions of low-energy conduction band.

Ag3PO4/ZnO: An efficient visible-light-sensitized composite with its application in photocatalytic degradation of Rhodamine B

The efficient visible-light-sensitized Ag3PO4/ZnO composites with various weight percents of Ag3PO4 were prepared by a facile ball milling method. The photocatalysts were characterized by XRD, DRS, SEM, EDS, XPS, and BET specific area. The OH radicals produced during the photocatalytic reaction was detected by the TA–PL technique. The photocatalytic property of Ag3PO4/ZnO was evaluated by photocatalytic degradation of Rhodamine B under visible light irradiation. Significantly, the results revealed that the photocatalytic activity of the composites was much higher than that of pure Ag3PO4 and ZnO. The rate constant of RhB degradation over Ag3PO4(3.0wt.%)/ZnO is 3 times that of single-phase Ag3PO4. The optimal percentage of Ag3PO4 in the composite is 3.0wt.%. It is proposed that the OH radicals produced in the valence band of ZnO play the leading role in the photocatalytic degradation of Rhodamine B by Ag3PO4/ZnO systems under visible light irradiation.

Modeling with Hybrid Density Functional Theory the Electronic Band Alignment at the Zinc Oxide–Anatase Interface

The band alignment at semiconductor interfaces can be theoretically computed using periodic slab models together with hybrid functional density functional theory methods in which Hartree–Fock exchange mixing coefficients are properly chosen (as justified by their relationship with the dielectric constant), and the calculated electrostatic potential inside each slab is used as reference for the band-edge energies. This principle is applied here to the interface between wurtzite-type ZnO and anatase-type TiO2, two oxides with nearly identical band gap widths. According to the results, in a composite of both materials the conduction and valence bands of ZnO will lie ca. 0.3 eV lower in energy than those of anatase, influencing the way in which photogenerated electrons and holes will be routed in photocatalytic or photovoltaic systems which include interfaces between these two oxides. The performance improvement observed in dye-sensitized solar cells based on a nanostructured ZnO electrode when the surface of the latter is covered by a thin TiO2 layer is thus justified. The system may serve as an example for the estimation of the band alignment in other semiconductor junctions of interest in different kinds of devices.

Competition between ferromagnetic and anti-ferromagnetic couplings in Co doped ZnO with vacancies and Ga co-dopants

Spin-polarized first-principles electronic structure and total energy calculations have been performed to better understand the magnetic properties of Co doped ZnO (ZnO:Co) with vacancies and Ga co-dopants. The paramagnetic state of ZnO:Co, in which Co ions lose their magnetic moments, has been found to be unstable. The total energy results show that acceptor-like Zn vacancies and donor-like Ga co-dopants render the anti-ferromagnetic (AFM) and ferromagnetic (FM) states to be more favorable, respectively. With O vacancies, ZnO:Co has been found to be in the weak FM state. These magnetic properties can be understood by the calculated O- and Zn-vacancies and Ga co-dopant induced changes of the electronic structure, which suggest that AFM and FM Co-Co couplings are mediated by O 2p-Co majority (↑)-spin 3d hybridized states in the valence band of ZnO and O-vacancy-derived p states or Ga sp states in the ZnO band gap, respectively. For ZnO:Co with Zn vacancies (Ga co-dopants) the AFM (FM) coupling outweighs the FM (AFM) coupling and results in the AFM (FM) state, while for ZnO:Co with O vacancies, both the FM and AFM couplings are enhanced by similar degrees and result in the weak FM state. This study reveals a competition between FM and AFM couplings in ZnO:Co with vacancies and Ga co-dopants, the detailed balancing between which determines the magnetic properties of these materials.

Physical Chemistry of the Mn/ZnO (0001̅) Interface Probed by Hard X-ray Photoelectron Spectroscopy

The oxidation of a thin Mn film grown on a ZnO (0001̅) surface and the subsequent diffusion of Mn into the oxide single crystal are investigated in situ by using high-energy X-ray photoelectron spectroscopy (HAXPES). Using hard X-rays allows one not only to investigate the chemistry at the heterojunction but also to describe in detail the thermal diffusion process and the electron energy band alignment at the Mn/ZnO interface. Charge transfer occurs between the metallic Mn film and the ZnO surface which causes the ZnO valence band to bend downward in the interfacial region. Annealing at 630 K leads to the formation of a thin two-dimensional MnO film which induces an upward bending of ZnO bands. Upon annealing at 800 K, Mn diffuses into the substrate crystal. A Mn concentration profile is derived, and a diffusion coefficient of 1.4 × 10–19 m2/s is experimentally determined.

Stability of the Bulk Phase of Layered ZnO

Recently a novel phase of ZnO has been synthesized which is analogous to α-boron nitride, although more three dimensional, and consists of planar hexagonal sheets of ZnO. Examining the dynamic stability of the structure, we find unstable phonon modes over a considerable part of the Brillouin zone. Local-density approximation (LDA) and generalized gradient approximation level calculations have usually been able to predict the structural stability of s-p bonded systems. The failure in the present case is a surprise and is traced to the self-interaction error which incorrectly locates the localized Zn d states in the valence band of ZnO. Correcting for this with a Hubbard-like U on the Zn d states, the optimized structure is predicted to be stable. This highlights the fact that the large bond length contraction that one finds in going from sp(3)- to sp(2)-type bonding results in an increased necessity to correct for self-interaction errors.

Nitrogen is a deep acceptor in ZnO

Zinc oxide is a promising material for blue and UV solid-state lighting devices, among other applications. Nitrogen has been regarded as a potential p-type dopant for ZnO. However, recent calculations [Lyons, Janotti, and Van de Walle, Appl. Phys. Lett. 95, 252105 (2009)] indicate that nitrogen is a deep acceptor. This paper presents experimental evidence that nitrogen is, in fact, a deep acceptor and therefore cannot produce p-type ZnO. A broad photoluminescence (PL) emission band near 1.7 eV, with an excitation onset of ∼2.2 eV, was observed, in agreement with the deep-acceptor model of the nitrogen defect. The deep-acceptor behavior can be explained by the low energy of the ZnO valence band relative to the vacuum level.

The luminescence of ZnO film grown on GaAs interlayer by PA-MOCVD

Abstract ZnO film was firstly prepared by PA-MOCVD method on the substrate pre-coated with GaAs interlayer. Hall measurement found that the GaAs interlayer had important effects on the electrical behavior of the ZnO film. It could make the ZnO film convert to p-type conductivity. The XPS results confirmed that the acceptor was arsenic. And the acceptor level was 130 meV above the ZnO valence band maximum. Low-temperature PL measurement was introduced to investigate the optical properties of both as-grown n-type and arsenic doped p-type ZnO films. Then, based on this technology, ZnO homojunction light emitting device (LED) was fabricated with arsenic doped p-type ZnO and unintentionally doped n-type ZnO on GaAs/p+-Si substrate. Its current–voltage (I–V) character showed a typical rectification behavior, which was different from the n-ZnO/p+-Si structure. The UV–visible (385–580 nm) electroluminescence was detected under relatively low current injection condition from the n-ZnO/p-ZnO/p+-Si LED.

Band structure engineering of ZnO1−xSex alloys

ZnO 1 − x Se x alloys with Se substitutional composition x<0.12 were synthesized using pulsed laser deposition. Incorporation of small concentrations of Se results in a greater than 1 eV red shift in the ZnO optical absorption edge which is quantitatively explained in the framework of the band anticrossing model. The Se defect level is found to be located at 0.9 eV above the ZnO valence band and the band anticrossing coupling constant is determined to be 1.2 eV. These parameters allow prediction of the composition dependence of the band gap as well as the conduction and the valence band offsets in the full composition range of ZnO1−xSex alloys.

INFLUENCE OF pH VALUE ON THE STRUCTURES AND OPTICAL PROPERTIES OFZnONANOCRYSTALLINE SYNTHESIZED WITH A SOLUTION PROCESS

Three types of hexagonal wurtzite ZnO nanostructures namely nanorods, nanodisks and nanoflowers were prepared in three different pH value solutions. The results indicated that the concentration of H+ions in the solution would greatly affect the growth behaviors of ZnO crystal. In addition to a broad ultraviolet emission found on the photoluminescence spectra, two weaker peaks corresponding to the blue emission of ZnO were observed at 2.75 and 2.68 eV, which probably originate from the electron transition from the shallow donor level of interstitial Zn to the valence band of ZnO .

Theoretical Characterization of Electronic Transitions in Co2+- and Mn2+-Doped ZnO Nanocrystals

Linear response time-dependent hybrid density functional theory has been applied for the first time to describe optical transitions characteristic of Co2+- and Mn2+-doped ZnO quantum dots (QDs) with sizes up to 300 atoms (∼1.8 nm diam) and to investigate QD size effects on the absorption spectra. Particular attention is given to charge-transfer (CT or “photoionization”) excited states. For both dopants, CT transitions are calculated to appear at sub-band-gap energies and extend into the ZnO excitonic region. CT transitions involving excitation of dopant d electrons to the ZnO conduction band occur lowest in energy, and additional CT transitions corresponding to promotion of ZnO valence band electrons to the dopant d orbitals are found at higher energies, consistent with experimental results. The CT energies are found to depend on the QD diameter. Analysis of excited-state electron and hole density distributions shows that, for both CT types, the electron and hole are localized to some extent around the impurity ion, which results in “heavier” photogenerated carriers than predicted from simple effective mass considerations. In addition to CT transitions, the Co2+-doped ZnO QDs also exhibit characteristic d−d excitations whose experimental energies are reproduced well and do not depend on the size of the QD.

Valence band offset of the ZnO/AlN heterojunction determined by x-ray photoemission spectroscopy

The valence band offset of ZnO/AlN heterojunctions is determined by high resolution x-ray photoemission spectroscopy. The valence band of ZnO is found to be 0.43±0.17 eV below that of AlN. Together with the resulting conduction band offset of 3.29±0.20 eV, this indicates that a type-II (staggered) band line up exists at the ZnO/AlN heterojunction. Using the III-nitride band offsets and the transitivity rule, the valence band offsets for ZnO/GaN and ZnO/InN heterojunctions are derived as 1.37 and 1.95 eV, respectively, significantly higher than the previously determined values.

Biaxial stress-dependent optical band gap, crystalline, and electronic structure in wurtzite ZnO: Experimental and ab initio study

The relationship between band gap and biaxial stress in wurtzite ZnO thin films has been investigated by side-inclination x-ray diffraction technique and optical absorbance spectrum as well as ab initio calculation. The experimental result shows that differing from other semiconductor thin films with hexagonal structure, such as GaN, the band gap of ZnO thin films increases with the increase in biaxial tensile stress. For explaining the difference, ab initio calculation is performed to simulate the relationship between band gap and biaxial stress of wurtzite ZnO and GaN. The calculated result indicates that the band gap of ZnO increases under biaxial tensile stress but GaN is opposite, supporting our experimental result. The band offset calculation shows that the conduction-band minimum (CBM) and the valence-band maximum (VBM) of ZnO and GaN offset to low energy under biaxial tensile stress. The VBM offset of ZnO is larger than the CBM, responsible for the increase in band gap. The VBM offset of GaN is smaller than the CBM, responsible for the decrease in band gap. The difference between ZnO and GaN is attributed to the strong p-d coupling in valence band of ZnO, but strong p-p coupling in valence band of GaN.

Investigation of pure and Co2+-doped ZnO quantum dot electronic structures using the density functional theory: choosing the right functional

The electronic structures of pure and Co2+-doped ZnO quantum dots (QDs) with sizes up to 300 atoms were investigated using three different density functional theory approximations: local spin density approximation (LSDA), gradient-corrected Perdew–Burke–Ernzerhof (PBE) and the hybrid PBE1 functionals with LANL2DZ pseudo-potential and associated basis set. Qualitative agreement among the three methods is found for the pure ZnO nanostructures, but only the hybrid functional reproduces the correct bandgap energies quantitatively. For Co2+-doped ZnO QDs, both LSDA and PBE incorrectly model interactions between Co2+ d levels and the valence band of ZnO, which will strongly impair predictions of dopant–carrier magnetic exchange interactions based on such computational results. Experimental observations are reproduced well in calculations at the hybrid PBE1 level of theory, making this the method of choice for exploring the magnetism of transition metal ions in ZnO QDs computationally. The qualitative features of the Co2+ 3d levels do not change appreciably with changes in cluster size over the range examined, leading to size-dependent dopant-band edge energy differences. The results presented here thus provide an experimentally calibrated framework for future ab initio descriptions of dopant–carrier and dopant–dopant magnetic exchange interactions in diluted magnetic semiconductors (DMS) nanocrystals.