Strong thickness dependence in thin film photocatalytic heterojunctions: the ZnO-Bi2O3 case study.

Semiconductor heterojunctions are an effective way to achieve efficient photocatalysts, as they can provide an adequate redox potential with visible light excitation. Several works have reported synergistic effects with nanoparticle semiconductor materials. The question is still open for thin film heterojunctions formed by stacked layers, as photocatalysis is considered a surface phenomenon. To investigate if the internal layer really affects or modifies the photocatalytic properties of the external material, we analyze the thin film heterojunction with ZnO and Bi2O3 semiconductors deposited by spray pyrolysis in two configurations: substrate/ZnO/Bi2O3 and substrate/Bi2O3/ZnO. Microstructural analysis was performed to verify the formation of the physical junction of the materials and discard new ternary phases. The photocatalytic activity was analyzed as a function of the thickness of the layers under blue light irradiation. We determined the conduction and valence bands positions, the carrier concentrations, mobilities, Fermi levels, etc. that allowed us to distinguish two reaction mechanisms depending on the configuration. There is a strong compromise between the order and thickness of the layers with the photocatalytic activity. The internal electric field produced in the interface defines the route of the photogenerated charges, and therefore the photocatalytic response. Thus, well-designed thin film heterojunctions can indeed improve the photocatalytic activity of the surface layer.

Synthesis of BiOI/Mordenite Composites for Photocatalytic Treatment of Organic Pollutants Present in Agro-Industrial Wastewater.

Recently, bismuth oxyiodide (BiOI) is an attractive semiconductor to use in heterogeneous photocatalysis processes. Unfortunately, BiOI individually shows limited photocatalytic efficiency, instability, and a quick recombination of electron/holes. Considering the practical application of this semiconductor, some studies show that synthetic zeolites provide good support for this photocatalyst. This support material permits a better photocatalytic efficiency because it prevents the quick recombination of photogenerated pairs. However, the optimal conditions (time and temperature) to obtain composites (BiOI/ synthetic zeolite) with high photocatalytic efficiency using a coprecipitation-solvothermal growth method have not yet been reported. In this study, a response surface methodology (RSM) based on a central composite design (CCD) was applied to optimize the synthesis conditions of BiOI/mordenite composites. For this purpose, eleven BiOI/mordenite composites were synthesized using a combined coprecipitation-solvothermal method under different time and temperature conditions. The photocatalytic activities of the synthesized composites were evaluated after 20 min of photocatalytic oxidation of caffeic acid, a typical organic pollutant found in agro-industrial wastewater. Moreover, BiOI/mordenite composites with the highest and lowest photocatalytic activity were physically and chemically characterized using nitrogen adsorption isotherms, scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and diffuse reflectance spectroscopy (DRS). The optimal synthesis conditions prove to be 187 °C and 9 h. In addition, the changes applied to the experimental conditions led to surface property modifications that influenced the photocatalytic degradation efficiency of the BiOI/mordenite composite toward caffeic acid photodegradation.

Synergistic photocatalytic effect of BiOBr-BiOI heterojunctions due to appropriate layer stacking.

The increasing interest in acquiring efficient visible-light active photocatalytic materials has led to the formation of heterojunctions with different combinations of semiconductors. Despite the fact that increasingly more complex structures are proposed, there are still many unclear factors affecting their performance and limiting their prompt implementation. In this work, we used the spray pyrolysis technique to deposit individual visible light-active BiOBr and BiOI films and formed the heterojunctions BiOBr-BiOI and BiOI-BiOBr to determine the effect of the stacking order of semiconductors. These materials were widely characterized; their structural, optical, (photo)electrochemical, and photocatalytic properties were evaluated, revealing that the configuration BiOI-BiOBr boosted the photocatalytic indigo carmine dye removal under simulated sunlight irradiation, but the opposite layout quenched it. The high efficiency is attributed to a better use of the incident radiation and the effective migration of the photogenerated carriers. BiOBr - with a wider band gap and a less negative conduction band with respect to BiOI - provides its good attributes to the heterostructure, such as high stability and low recombination rates, when it is at the surface. We demonstrated that in thin-film heterostructures, the order in which the layers are stacked becomes decisive for the photocatalytic performance and that the energy band gap and the relative band positions of both semiconductors are the principal features that govern the photocatalytic mechanism. These findings provide a key to designing more efficient photocatalysts without several unsuccessful trials.

Influence of Ag Photodeposition Conditions over SERS Intensity of Ag/ZnO Microspheres for Nanomolar Detection of Methylene Blue.

Surface enhanced Raman spectroscopy (SERS) is considered a versatile and multifunctional technique with the ability to detect molecules of different species at very low molar concentration. In this work, hierarchical ZnO microspheres (ZnO MSs) and Ag/ZnO MSs were fabricated and decorated by hydrothermal and photodeposition methods, respectively. For Ag deposition, precursor molar concentration (1.9 and 9.8 mM) and UV irradiation time (5, 15, and 30 min) were evaluated by SEM, TEM, X-ray diffraction and Raman spectroscopy. X-ray diffraction showed a peak at 37.9° corresponding to the (111) plane of Ag, whose intensity increases as precursor concentration and UV irradiation time increases. SEM images confirmed the formation of ZnO MSs (from 2.5 to 4.5 µm) building by radially aligned two-dimensional ZnO nanosheets with thicknesses below 30 nm. The Raman spectra of Ag/ZnO MSs exhibited a vibration mode at 486 cm-1 which can be directly associated to Ag deposition on ZnO MSs surface. The performance of SERS substrate was evaluated using rhodamine 6G. The SERS substrate grown at 9.8 mM during 30 min showed the best SERS activity and the ability to detect methylene blue at 10-9 M.

ZnO Nanowires/N719 Dye With Different Aspect Ratio as a Possible Photoelectrode for Dye-Sensitized Solar Cells.

The vapor-liquid-solid (VLS) process was applied to fabricate zinc oxide nanowires (ZnO NWs) with a different aspect ratio (AR), morphological, and optical properties. The ZnO NWs were grown on a system that contains a quartz substrate with transparent conductive oxide (TCO) thin film followed by an Al-doped ZnO (AZO) seed layer; both films were grown by magnetron sputtering at room temperature. It was found that the ZnO NWs presented high crystalline quality and vertical orientation from different structural and morphological characterizations. Also, NWs showed a good density distribution of 69 NWs/µm2 with a different AR that offers their capability to be used as possible photoelectrode (anode) in potential future device applications. The samples optical properties were studied using various techniques such as photoluminescence (PL), absorption, and transmittance before and after sensitization with N719 dye. The results demonstrated that NW with 30 nm diameter had the best characteristics as feasible photoelectrode (anode) (high absorption, minimum recombination, high crystallinity). Also, the present samples optical properties were found to be improved due to the existence of N719 dye and Au nanoparticles on the tip of NWs. NWs grown in this work can be used in different photonic and optoelectronic applications.

The bismuth oxyhalide family: thin film synthesis and periodic properties.

Bismuth oxyhalides (BiOX, where X = F, Cl, Br, I) are interesting materials due to their layered structure, which can be useful for different applications. In this work, we present the synthesis of the complete BiOX family in the thin film form. The tetragonal phase Bi2O3 film deposited onto a glass substrate was transformed into BiOF, BiOCl or BiOBr by a simple immersion at ambient temperature in a halide (X = F, Cl, Br) containing solution. For these films, a residual phase from the oxide was present and for BiOF another phase (tentatively identified as Bi7O5F11) was present too. For the BiOI film synthesis, an iodine and bismuth containing solution was sprayed onto the glass substrate heated at 275 °C and a pure phase was obtained. Microstructural and morphological characterization was performed by X-ray diffraction and scanning electron microscopy, while the chemical environment was studied by X-ray photoelectron spectroscopy. Optical and photocatalytic properties were also obtained. The physical and chemical characteristics of the BiOX films follow a correlation with the atomic radius of the halogen atom incorporated into the corresponding lattice. All the BiOX films showed a photocatalytic response for the photodiscoloration of indigo carmine dye under simulated sunlight irradiation in an alkaline medium. The photocatalytic reactions occurred via 2 proton-electron transfer from the oxide or oxyhalide to the adsorbed IC dye, favoring its reduction to the corresponding leuco IC form.

Gold as an intruder in ZnO nanowires.

Several techniques for obtaining ZnO nanowires (ZnO NWs) have been reported in the literature. In particular, vapour-liquid-solid (VLS) with Au as a catalyst is widely used. During this process, Au impurities in the ZnO NWs can be incorporated accidentally, and for this reason we named these impurities as intruders. It is thought that these intruders may produce interesting alterations in the electronic characteristics of nanowires. In the experiment, it is not easy to detect either Au atoms in these nanowires, or the modification that intruders produce in different electrical, optical and other properties. For this reason, in this density functional theory investigation, the effect of Au intruders on ZnO NWs is analysed. Au extended (thread) and point defects (atoms replacing Zn or O, or Au interstitials) are used to simulate the presence of gold atoms. Optimised geometries, band-gaps and density of states indicate that the presence of small amounts of Au drastically modifies the electronic states of ZnO NWs. The results reported here clearly indicate that small amounts of Au have a strong impact on the electronic properties of ZnO NWs, introducing states in the band edges that may promote transitions in the visible spectral region. The presence of Au as an intruder in ZnO NWs enhances the potential use of this system for photonic and photovoltaic applications.

Spray deposited ß-Bi2O3 nanostructured films with visible photocatalytic activity for solar water treatment.

Bismuth oxide thin films were obtained by the spray pyrolysis method using bismuth acetate as the precursor salt. The films were characterized by X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), UV-vis diffuse reflectance, X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM). The XRD patterns indicated that the pure ß phase is obtained at 450 °C and was also confirmed by FTIR. This phase presents a nanoplate morphology which is adequate for the photocatalytic reactions. Moreover, the band gap value was 2.6 eV indicating a good capacity of visible light absorption. The photocatalytic degradation of the Methyl Orange (MO) dye was pH dependent, an acid solution being easier to degrade. However, the Bi2O3 films were easily converted into BiOCl when they were in contact with a solution containing HCl. In order to preserve the ß-Bi2O3 phase, the Acid Blue 113 dye with its natural pH of 8 was used to evaluate the stability of the photocatalytic activity after five degradation cycles. The photoactivity was practically stable indicating a good performance of the material. This encouraged us to test the films in a continuous flow solar reactor prototype for the degradation of the dye solution using sunlight radiation exclusively. The good performance of the ß-Bi2O3 films indicates that they can be used for sustainable water treatment applications.

Fullerene thin films functionalized by 1,5-diaminonaphthalene: preparation and properties.

The gas-phase treatment with 1,5-diaminonaphthalene (DAN) is proposed as an efficient way of chemical functionalization of fullerene C60 thin films in order to modify their electronic properties; a temperature of 190 degrees C and reaction time of 4 h were found to be optimal reaction conditions. Two amino groups of DAN add on two neighboring C60 cages, thus producing cross-links in the fullerene phase. The resulting oligomeric and/or polymeric products exhibit a lower solubility in toluene as compared to pristine C60 films. The functionalized films exhibit lower surface roughness, as found by atomic force microscopy. Raman spectra keep, with some decrease in intensity, the most important features of C60 upon functionalization. The infrared band intensities corresponding to pristine fullerene decrease even stronger, where a number of new absorption bands appear not only due to DAN moieties, but also due to the covalently derivatized C60 cages. The diamine molecules are able to penetrate throughout the entire fullerene phase to provide an efficient and uniform functionalization. As a result, the DAN-functionalized films exhibit higher conductivity as compared to that of pristine films not only along the surface layer, but also through the entire phase of C60, by one and four orders of magnitude, respectively.

Raman scattering by confined optical phonons in Si and Ge nanostructures.

A microscopic theory of the Raman scattering based on the local bond-polarizability model is presented and applied to the analysis of phonon confinement in porous silicon and porous germanium, as well as nanowire structures. Within the linear response approximation, the Raman shift intensity is calculated by means of the displacement-displacement Green's function and the Born model, including central and non-central interatomic forces. For the porous case, the supercell method is used and ordered pores are produced by removing columns of Si or Ge atoms from their crystalline structures. This microscopic theory predicts a remarkable shift of the highest-frequency of first-order Raman peaks towards lower energies, in comparison with the crystalline case. This shift is discussed within the quantum confinement framework and quantitatively compared with the experimental results obtained from porous silicon samples, which were produced by anodizing p--type (001)-oriented crystalline Si wafers in a hydrofluoric acid bath.