Preparation, characterization and activity evaluation of p–n junction photocatalyst p-ZnO/n-TiO2

Zinc interstitials and their complexes with oxygen vacancies, harbouring a lone pair of electrons, have been discussed as suspects for an n-type character and the ensuing conductivity of nominally undoped ZnO. ZnO nanorods, with different Zn concentrations, have been studied using photoluminescence and photoluminescence excitation spectroscopy to evidence the presence of defect states, their correlation with excess Zn and their viability as donors. It is shown that each sample may emit intense violet-blue, green-orange and red emissions, as decided by the excitation energy and presence of excess Zn. Intensity variation of these individual emissions vis-a-vis their excitation energies, in the range 4.0–2.7 eV, indicates the presence of defect states not only within the band gap but crucially within the conduction band itself, but only in a Zn rich environment. The study provides the experimental signature of the theoretical predictions of the latter states and demonstrates why Zn interstitials may be responsible for the n-type doping and conductivity of ZnO. The origin of the emissions, their interdependence and tunability are further discussed.

https://pubs.rsc.org/en/Content/ArticlePDF/2015/RA/C5RA00355E

Zn interstitials and O vacancies responsible for n-type ZnO: what do the emission spectra reveal?

By combining results from positron annihilation and photoluminescence spectroscopy with data from Hall effect measurements, the characteristic deep level emission centered at ! 1.75 eV and exhibiting an activation energy of thermal quenching of 11.5 meV is associated with the zinc vacancy. Further, a strong indication that oxygen interstitials act as a dominating acceptor is derived from the analysis of charge carrier losses induced by electron irradiation with variable energy below and above the threshold for Zn-atom displacement. We also demonstrate that the commonly observed green emission is related to an extrinsic acceptorlike impurity, which may be readily passivated by oxygen vacancies.

/pdf/zinc-vacancy-and-oxygen-interstitial-in-zno-revealed-by-4ju1ke3fp6.pdf

Zinc vacancy and oxygen interstitial in ZnO revealed by sequential annealing and electron irradiation

We investigate the optical properties of four different samples of ZnO nanocrystals, with a particle size average varying from 70 up to 380nm. The photoluminescence (PL) of all samples shows at low temperature an emission band around 3.31eV, which is several orders of magnitude stronger compared to the PL of bulk ZnO at this energy. This band shows a clear dependence on the surface to volume ratio of the nanocrystals and is therefore assigned to surface states. Temperature dependent measurements reveal that this band plays a major role up to room temperature for all examined ZnO powders. Additionally, intensity dependent measurements display that the origin of this emission band can be assigned to bound exciton complexes (BECs). Compared to the well known shallow BECs the measured lifetimes of these relatively strong bound excitons states are much longer.

Surface-state related luminescence in ZnO nanocrystals

Wurtzite ZnO has many potential applications in optoelectronic devices, and the hydrogenated ZnO exhibits excellent photoelectronic properties compared to undoped ZnO; however, the structure of H-related defects is still unclear. In this article, the effects of hydrogen-plasma treatment and subsequent annealing on the electrical and optical properties of ZnO films were investigated by a combination of Hall measurement, Raman scattering, and photoluminescence. It is found that two types of hydrogen-related defects, namely, the interstitial hydrogen located at the bond-centered (H-BC) and the hydrogen trapped at a O vacancy (H-O), are responsible for the n-type background conductivity of ZnO films. Besides introducing two hydrogen-related donor states, the incorporated hydrogen passivates defects at grain boundaries. With increasing annealing temperatures, the unstable H-BC atoms gradually diffuse out of the ZnO films and part of them are converted into H-O, which gives rise to two anomalous Raman peaks at 275 and 510 cm(-1). These results help to clarify the relationship between the hydrogen-related defects in ZnO described in various studies and the free carriers that are produced by the introduction of hydrogen.

Effects of hydrogen plasma treatment on the electrical and optical properties of ZnO films: identification of hydrogen donors in ZnO.

We report on the preparation and characterization of N-doped ZnO powders. EPR measurements revealed the presence of N-acceptors similar to those found in single crystals (Aiso = 1.225 mT, Aaniso = 0.864 mT, g‖ = 1.9953, g⊥ = 1.9633). The photoluminescence of the samples shows donor to shallow acceptor recombinations. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Incorporation of nitrogen acceptors in ZnO powder

ZnO powder heat treated in NH3 atmosphere was investigated by electron paramagnetic resonance, photoluminescence, and Raman spectroscopy. We find that the treatment creates Zn interstitials Zni and complexes of Zn interstitials and nitrogen atoms substituting oxygen Zni-NO . A correlation between the Zni and an exciton at 3.366 eV I3 can be stated by a comparison with the PL results; furthermore, the Zni-NO complex seems to be related to a recombination at 3.193 eV.

/pdf/zn-interstitial-related-donors-in-ammonia-treated-zno-5fm0m2amrj.pdf

Zn interstitial related donors in ammonia-treated ZnO powders

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Incorporation of nitrogen acceptors in ZnO powder

Zn interstitial related donors in ammonia-treated ZnO powders