RESUMEN
The interaction of graphene with metal oxides is essential for understanding and controlling new devices' fabrication based on these materials. The growth of metal oxides on graphene/substrate systems constitutes a challenging task due to the graphene surface's hydrophobic nature. In general, different pre-treatments should be performed before deposition to ensure a homogenous growth depending on the deposition technique, the metal oxide, and the surface's specific nature. Among these factors, the initial state and interaction of graphene with its substrate is the most important. Therefore, it is imperative to study the initial local state of graphene and relate it to the early stages of metal oxides' growth characteristics. Taking as initial samples graphene grown by chemical vapor deposition on polycrystalline Cu sheets and then exposed to ambient conditions, this article presents a local study of the inhomogeneities of this air-exposed graphene and how they influence on the subsequent ZnO growth. Firstly, by spatially correlating Raman and x-ray photoemission spectroscopies at the micro and nanoscales, it is shown how chemical species present in air intercalate inhomogeneously between Graphene and Cu. The reason for this is precisely the polycrystalline nature of the Cu support. Moreover, these local inhomogeneities also affect the oxidation level of the uppermost layer of Cu and, consequently, the electronic coupling between graphene and the metallic substrate. In second place, through the same characterization techniques, it is shown how the initial state of graphene/Cu sheets influences the local inhomogeneities of the ZnO deposit during the early stages of growth in terms of both, stoichiometry and morphology. Finally, as a proof of concept, it is shown how altering the initial chemical state and interaction of Graphene with Cu can be used to control the properties of the ZnO deposits.
RESUMEN
ZnO/CdS core/shell nanorod arrays were fabricated by a two-step method. Single-crystalline ZnO nanorod arrays were first electrochemically grown on SnO(2):F (FTO) glass substrates. Then, CdS nanocrystals were deposited onto the ZnO nanorods, using the successive ion layer adsorption and reaction (SILAR) technique, to form core/shell nanocable architectures. Structural, morphological and optical properties of the nanorod heterojunctions were investigated. The results indicate that CdS single-crystalline domains with a mean diameter of about 7 nm are uniformly and conformally covered on the surface of the single-crystalline ZnO nanorods. ZnO absorption with a bandgap energy value of 3.30 ± 0.02 eV is present in all optical transmittance spectra. Another absorption edge close to 500 nm corresponding to CdS with bandgap energy values between 2.43 and 2.59 eV is observed. The dispersion in this value may originate in quantum confinement inside the nanocrystalline material. The appearance of both edges corresponds with the separation of ZnO and CdS phases and reveals the absorption increase due to CdS sensitizer. The photovoltaic performance of the resulting ZnO/CdS core/shell nanorod arrays has been investigated as solar cell photoanodes in a photoelectrochemical cell under white illumination. In comparison with bare ZnO nanorod arrays, a 13-fold enhancement in photoactivity was observed using the ZnO/CdS coaxial heterostructures.
RESUMEN
Electrodic surfaces of natural chalcopyrite and natural pyrite minerals (El Teniente mine, Chile) have been studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy including microanalysis (SEM/EDX). For comparison, fractured and polished mineral surfaces were also studied by XPS. In both electrodes, the formation of Fe(III) species containing oxygen were detected and Cu(II) species containing oxygen were additionally detected for chalcopyrite at advanced oxidation states. The presence of Cu(II) species containing oxygen was not detected by XPS for the initial oxidation states of the chalcopyrite. For pyrite, the present results do not allow confirmation of the presence of polysulfurs such as have been previously proposed. In both minerals, the measurements of SEM and EDX show relevant alterations in the respective surfaces when different potential values were applied. The chalcopyrite surface shows the formation of protrusions with a high concentration of oxygen. The pyrite surface shows a layer of modified material with high oxygen content. The modifications detected by XPS, SEM, and EDX allowed the explanation of the complexity of the equivalent circuit used to simulate the experimental EIS data. At high oxidation states, both minerals showed a pseudoinductive loop in the equivalent circuit, which was due to the active electrodissolution of the minerals which takes place through a surface film previously formed.