Monserrat Bizarro

Polymorphous silicon thin films obtained by plasma-enhanced chemical vapor deposition using dichlorosilane as silicon precursor.

Polymorphous silicon thin films (pm-Si) have been deposited from mixtures of dichlorosilane and hydrogen, using argon as the diluting gas by plasma-enhanced chemical vapor deposition. The deposition conditions were chosen to simultaneously obtain both Si nanocrystallites and an amorphous silicon matrix in the as-grown samples. High resolution transmission electron microscopy studies show the crystallinity of Si domains whose dimensions are in the interval of 2-14 nm. The surface passivation state of the silicon nanocrystals was inferred from Fourier transform infrared spectroscopy analysis. Two optical absorption edges, corresponding to the amorphous matrix and the Si nanocrystals, were observed for all the pm-Si thin films. Intense visible photoluminescence was observed for the as-grown samples. The possibility of using these thin films for the down-conversion effect in silicon solar cells is discussed.

Photocatalytic Performance of ZnO: Al Films under Different Light Sources

ZnO and Al doped ZnO films were produced by spray pyrolysis. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis spectroscopy, and photoluminescence. Their photocatalytic activity was evaluated by the decomposition of the methyl orange dye using different light sources: ultraviolet light, artificial white light, and direct sunlight. The films were also tested under darkness for comparison. The ZnO films were able to degrade the test pollutant under UV and sunlight in more than a 60% after 180 min of irradiation and a scarce degradation was obtained using white light. However, the Al doped ZnO films presented a very high degradation rate not only under UV and sunlight (100% degradation), but also under white light (90% degradation after the same irradiation time). An unexpected high degradation was also obtained in the dark, which indicates that a nonphotonic process is taking place parallel to the photocatalytic process. This can be due to the extra electrons—provided by the aluminum atoms—that migrate to the surface and produce radicals favoring the decomposition process even in the dark. The high activity achieved by the ZnO: Al films under natural conditions can be potentially applied to water treatment processes.

Sprayed Pyrolyzed ZnO Films with Nanoflake and Nanorod Morphologies and Their Photocatalytic Activity

There is an increasing interest on the application of ZnO nanorods in photocatalysis and many growth methods have been applied, in particular the spray pyrolysis technique which is attractive for large scale production. However it is interesting to know if the nanorod morphology is the best considering its photocatalytic activity, stability, and cost effectiveness compared to a nonoriented growth. In this work we present a systematic study of the effect of the precursor solution type of salt, solvent, and concentration on the morphology of sprayed ZnO films to obtain nanoflakes and nanorods without the use of surfactants or catalysts. The surface properties and structural characteristics of these types of films were investigated to elucidate which morphology is more favorable for photocatalytic applications. Wettability and photocatalytic experiments were carried out in the same conditions. After UV irradiation both morphologies became hydrophilic and achieved a dye discoloration efficiency higher than 90%; however, the nanoflake morphology provided the highest photocatalytic performance 99% dye discoloration and stability and the lowest energy consumption during the synthesis process. The surface-to-volume ratio revealed that the nanoflake morphology is more adequate for photocatalytic water treatment applications and that the thin nanorods should be preferred over the large ones.

The Effect of the Process Conditions on the Synthesis of Zirconium-Aluminum Oxide Thin Films Prepared by Ultrasonic Spray Pyrolysis

Zirconium-aluminum oxide thin films with different aluminum/zirconium ratios (Al/Zr) have been prepared by ultrasonic spray pyrolysis, using metallic acetylacetonates as source materials. The effect of variations of substrate temperature (T s ), carrier gas flow rate (F g ), and aluminum concentration in the start solution [(Al/Zr) s ] on the Al/Zr ratio, the deposition rate (R d ), and refractive index of deposited films is analyzed. The Al/Zr ratio increases when the T s and [(Al/Zr) s ] ratios increase, but it diminishes as F g is augmented. Moreover, R d grows when the deposition parameters are incremented. Refractive index acquires values around 1.85 in all cases. X-ray photoelectron spectroscopy results indicate that the Zr 3d and O Is core levels shift toward low binding energies when aluminum is incorporated. The location of line Al 2p and its symmetry indicates that aluminum atoms are completely oxidized, forming a ternary oxide with zirconium atoms. Infrared transmittance spectra show small absorption bands related with Zr-O and Al-O bonds in a ternary (ZrAlO) oxide. X-ray diffraction spectra show that all the films containing aluminum are amorphous, except that deposited at the highest T s . In all other studied cases the films are amorphous. The dielectric constant has values between 11.84 and 20.96 depending on the deposition conditions.

Effect of Water Addition in the Spray Solution on the Synthesis of Zirconium–Aluminum Oxide Films Prepared by the Pyrosol Process

Zirconium-aluminum oxide films were deposited by the pyrosol process using metallic acetylacetonates. The effect of addition of water, in volumes from 20 to 1000 μL per 30 mL of the starting solution on the synthesis of the films deposited is analyzed. X-ray diffraction spectra show that the films are of amorphous phase. The deposition rate decreases while the aluminum incorporated in the films increases as the water volume is augmented. These behaviors are related to the action that the water molecules have on the reactants, promoting the decomposition of the reactants even at relatively low temperatures, resulting in carbon-free and transparent films. Refractive index acquires values around 1.85. Fourier transform infrared spectra show absorption bands related with vibration of Zr-O and Al-O bonds forming a ternary oxide. The energy gap determined has values of the order of 5.85 eV, which is between the values of zirconium oxide and aluminum oxide. Current-voltage characteristics in metal-oxide-metal structures show that the effect of water is to diminish the leakage current density, although its magnitude is large. From capacitance measurements, the dielectric constant has values of the order of 23.8, which is higher than that calculated for films prepared by the same process without the addition of water. The dielectric loss factor calculated from the capacitance-voltage characteristics has high values, in agreement with the large leakage current densities observed.

Physicochemical Characterization of Photocatalytic Materials

A wide range of analytical techniques has been employed to obtain the physical–chemical properties of photocatalytic semiconductors prepared as powders or thin films. The photocatalytic activity of a semiconductor material depends on both surface and structural properties. For bulk materials, the composition, crystalline structure, and electronic properties of the materials are intrinsically correlated; however, surface characteristic such as surface area, particle size distribution, and porosity can be independently modified. On the other hand, for nanomaterials, the size becomes an important feature affecting the physical properties, such as the optical band gap. Moreover, the particle size is of primary importance in heterogeneous photocatalysis, because it is directly related to the efficiency of a catalyst through the enhancement of its specific surface area.

Photoreduction of 4-Nitrophenol in the presence of carboxylic acid using CdS nanofibers

The photocatalytic reduction of 4-Nitrophenol was assisted with blue light irradiation using CdS semiconductor nanofiber. The kinetic study of the blue-photoreduction of 4-Nitrophenol to 4-Aminophenol was performed by using Na2SO3 as “hole scavenger” in the presence of middle carboxylic acids (acetic, oxalic and citric acid). The effect of photocatalyst load, the carboxylic acid concentration and of the pH solution (acid, neutral and alkaline media) were investigated. The maximum 4-Nitrophenol photoreduction rate was reached in neutral media and it is related to the enhanced “hole scavenging” effect caused by the SO32− or HSO32− ions in the presence of carboxylic acid. Electrochemical characterization of CdS nanofiber was made in the presence of 4-NP and citric acid. The 4-NP photoreduction reaction follows a zero order reaction and a possible mechanism to explain the fast electron transfer process is discussed as a consequence of the carboxylic acid adsorbed on the CdS surface.

Structural, morphological and optical properties of sol-gel prepared Cu doped BiVO4 powders

Cu-doped BiVO4 nanoparticles were synthesized by sol-gel technique. Doping effects of the host BiVO4 materials were characterized for structural, vibrational, morphological and optical investigations. It was deduced from XRD and micro Raman analysis, that Cu-doped BiVO4 showed lattice doping of Cu in the host crystal sites. Cu doped BiVO4 show 50 - 100 nm range sized particles as compared to pristine sample, which shows larger particle size in the range of 400-500 nm. Broad optical absorption in the range of 500 -800 nm is seen for Cu doped BiVO4 as compared to pristine form with slight band gap reduction as observed from DRS analysis. The calculated energy band gaps are 2.49 and 2.54 eV for 2 at.% Cu doped and pure BiVO4 respectively.