José R. Ramos-Barrado

ZnO nanorod/CdS nanocrystal core/shell-type heterostructures for solar cell applications

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.

Unraveling the conduction mechanism of Al-doped ZnO films by valence band soft x-ray photoemission spectroscopy

We report on the correlation between the electrical behavior and valence band spectra of undoped and Al-doped ZnO films, obtained by using x-ray photoelectron spectroscopy. Although Al-doping can induce a conductivity increase of two orders of magnitude, we show that the gap persists and there is no semiconductor–metal transition upon doping. For the 3% Al-doped ZnO film, we measure a reduction in the band gap of ∼150meV with respect to the undoped and the 1% doped films. Our results suggest that the band conduction mechanism proposed for undoped ZnO at room temperature still dominates the conduction process in doped films.

Differences in n-type doping efficiency between Al- and Ga-ZnO films

A careful and wide comparison between Al and Ga as substitutional dopants in the ZnO wurtzite structure is presented. Both cations behave as n-type dopants and their inclusion improves the optical and electrical properties of the ZnO matrix, making it more transparent in the visible range and rising up its electrical conductivity. However, the same dopant/Zn ratio leads to a very different doping efficiency when comparing Al and Ga, being the Ga cation a more effective dopant of the ZnO film. The measured differences between Al- and Ga-doped films are explained with the hypothesis that different quantities of these dopant cations are able to enter substitutionally in the ZnO matrix. Ga cations seem to behave as perfect substitutional dopants, while Al cation might occupy either substitutional or interstitial sites. Moreover, the subsequent charge balance after doping appear to be related with the formation of different intrinsic defects that depends on the dopant cation. The knowledge of the doped-ZnO films microstructure is a crucial step to optimize the deposition of transparent conducting electrodes for solar cells, displays, and other photoelectronic devices.

Single-Crystalline Silicon Nanowire Array-Based Photoelectrochemical Cells

Single-crystalline n-type silicon nanowire (n-SiNW) arrays have been synthesized by electroless metal deposition on a silicon wafer chip in an ionic Ag/HF solution through selective etching. The photoelectrochemical properties of the SiNW arrays were studied (under 40 mW cm -2 white illumination conditions) in an aqueous electrolyte [with Fe(CN) 4- 6 /Fe(CN) 3- 6 redox couple] and compared to those of its untreated bare polished Si sample counterpart (n-Si). The open-circuit photovoltage was lower, on average, for SiNWs array photoelectrodes, while short-circuit current density, fill factor, and overall energy conversion efficiency of the SiNWs array photoelectrodes were generally superior to those of the n-Si junction devices.

Synthesis, Characterization, and Electrochemical Properties of Nanocrystalline Silver Thin Films Obtained by Spray Pyrolysis

ler. The substrate was moved forward and backward at a fixed frequency by an electronically controlled step motor. An aqueous solution of 0.05 M Ag(CH3COO) was used as precursor. The solution was pumped into the airstream in the spray nozzle at a rate of 50 mL h 21 with a syringe pump for a preset time of 20 min. An airstream of 25 L min 21 measured at 1.25 bar was used to atomize the solution. Circular disks 0.4 mm thick and of 7.5 mm diam of commercial 304 stainless steel were used as substrates. They were kept at temperatures over the range 225-300°C.

Electrodeposited Nanostructured α-Fe2O3 Photoanodes for Solar Water Splitting: Effect of Surface Co-Modification on Photoelectrochemical Performance

Photoelectrochemical response of electrodeposited hematite (a-Fe2O3) thin films, whose surface has been modified by a catalytic cobalt layer, has been studied. Structural and morphological characteristics of the resulting nanostructured hematite films have been studied by X-ray diffraction and scanning electron microscopy techniques. Surface characterization of the Co-modified hematite films has been carried out by X-ray photoelectron spectroscopy in order to study the chemical nature of Co onto these films. The hematite films exhibited an n-type behavior, and a donor carrier concentration, ND ¼3 � 10 17 and 10 18 cm � 3 for the un-modified and Co-modified ones, respectively. The photoelectrochemical performance of the un-modified and Co-modified electrochemically grown hematite thin films has been explored using a liquid-junction in a photoelectrochemical system in a 0.1 M NaOH electrolytic solution, under monochromatic illumination of wavelengths over the range 350‐600 nm. Significant performance gaining can be seen upon Co-modification throughout the illumination wavelengths. Cobalt modification enhances the IPCE by a factor of 1.1‐2.0 (370‐450 nm range), and by a factor of 2.0 (at 350 nm). With an IPCE of 46% at 370 nm at 1.43 V vs. RHE, this electrode is much more efficient in water photo-oxidation than the best published Fe2O3 single-crystal specimen.

ZnO/Cu2O heterostructure nanopillar arrays: synthesis, structural and optical properties

Vertically aligned ZnO/Cu2O heterostructure nanopillar arrays consisting of a ZnO core and a Cu2O shell were fabricated by a two-step electrochemical deposition method. Morphological, structural and optical properties of the nanopillar heterojunctions were investigated. The surface of the single-crystalline ZnO nanopillars was coated uniformly, conformally and densely over the entire nanopillar length by numerous Cu2O nanocrystals (25?35?nm mean diameter), constituting a conformal shell layer 90?nm thick, integrating these two materials into an electronically intimate composite. The optical properties can be interpreted, by appropriate fittings of each feature, as being due to the properties of the bare ZnO nanopillar array plus the increased absorption of Cu2O. This study demonstrates that electrodeposition is a suitable and accessible technique for large-scale fabrication of nanopillar heterostructures and to achieve conformal coverage of nanostructured samples.

XPS analysis of an electrochemically modified electrode surface of natural enargite

A natural enargite (Cu 3 AsS 4 ) electrode has been studied by cyclic voltammetry (CV) and XPS. Cyclic voltammetry was carried out in a preparation chamber filled with Ar gas and coupled to the spectrometer in order to analyse by XPS, quasi-in situ, the electrochemically modified electrode surface. By CV, different potentials of oxidation and reduction were applied to the electrode in a disodium tetraborate decahydrate electrolyte solution of pH 9.2 at room temperature. Cycling in the anodic direction, XPS analysis has shown that at +500 mV the electrode surface is oxidized to CuO, CuSO 4 and As 2 O x (x = 3; 5) and the corresponding hydroxides and polysulphide are formed, whereas at lower oxidation potentials the Cu 2p and As 3d photoelectron peaks do not alter. However, with increasing oxidation potential up to +200 mV a new contribution in the S 2p signal and Cu loss indicate the formation of non-stoichiometric (metal-deficient) enargite and polysulphides in the electrode surface. Cycling then from +500 mV in the cathodic direction until -800 mV, we observe with increasing reduction potential a reverse process in the S 2p signal and Cu concentration. Furthermore, even at +200 mV the reduction from Cu(II) to Cu(I) occurs and no sulphate species are observed any more. These observations indicate that the copper concentration at the electrode surface is a sweep-direction-independent function of applied potential and that the oxide species formed at the electrode surface are only stable by application of high oxidation potentials. For comparison, the surfaces of the fractured mineral and the original polished electrode surface were also studied.

Voltammetry and XPS analysis of a chalcopyrite CuFeS2 electrode

Abstract A chalcopyrite CuFeS 2 electrode obtained from the “El Teniente” mine (Chile) has been studied by cyclic voltammetry (CV) in a borax alkaline solution at pH 9.2 and by X-ray photoelectron spectroscopy (XPS) before and after the electrochemical treatment at oxidation and reduction potentials. The voltagram shows two peaks in the positive sweep direction which can be assigned to an oxidation process at the electrode surface that involve the formation of Fe 2 O 3 and CuO giving rise to an irregular surface layer of the electrode with islands of different chemical composition. The XPS results confirm the formation of Fe 2 O 3 and CuO at the electrode surface and their stability against decomposition at reduction potential. In addition, the formation of FEOOH, CuFeO and CuS species at the electrode surface can be deduced by XPS as well as a covering of the whole electrode surface by species due to carbon–oxygen groups.

High-energy, efficient and transparent electrode for lithium batteries

In this work, we describe for the first time a high-energy and transparent thin film electrode made from nanosized LiFeO2 grown on an indium tin oxide (ITO) substrate and in situ mixed with submicronic grains of Ag homogeneously distributed to improve the conductivity. An accurate description of its texture and structure was accomplished by combining various techniques such as XPS, SEM, TEM, EDX and AFM. The electrode has the ability to deliver capacity values above 160 Ah kg−1 upon extensive cycling with capacity retention near to 98%, an average voltage of 3.0 V vs. Li+/Li and a specific energy close to 410 W h kg−1 as a result.