Flavio L. Souza

Vertically oriented iron oxide films produced by hydrothermal process: effect of thermal treatment on the physical chemical properties.

Our study describes the influence of the thermal treatment on the fundamental properties of the vertical oriented iron oxide nanorods synthesized under hydrothermal condition onto a conductor substrate. X-ray diffraction and X-ray absorption near edge structure spectra were used to investigate the phase evolution from iron oxyhydroxide (β-FeOOH) to pure hematite phase. The formation of nanorods distributed along of substrate was observed by top-view SEM images and the rod growth preferentially oriented at the highly conductive (001) basal plane of hematite, perpendicular to the substrate. Light absorption capacity increases with the temperature of treatment and the electronic transitions (direct and indirect electronic transition) were estimated from this result. From the electrochemical measurement, the hematite/electrolyte interface was evaluated. These findings demonstrated that the temperature plays an important role on the hematite (structural, morphological, and catalytic) properties and that many influences must work in great harmony in order to produce a promising hematite photoanode.

Morphological and structural evolution from akaganeite to hematite of nanorods monitored by ex situ synchrotron X-ray powder diffraction

Hematite (α-Fe2O3) is one of the most abundant minerals in nature and a thermodynamically-stable phase of iron oxide. One-dimensional (1D) hematite nanostructures have attracted, due to their fundamental characteristics such as excellent chemical stability in aqueous environments and low cost, considerable interest for a series of technical applications (as sensors, catalysis, lithium ion batteries and solar energy production and storage). Our study describes the phase evolution of iron oxide nanorods, from β-FeOOH to α-Fe2O3, with increasing temperatures of thermal treatment monitored by ex situ synchrotron X-ray powder diffraction data. These data used in conjunction with the Rietveld method allowed us to infer the phase concentration, elucidate the influence of temperature on the unit cell parameters and preferred growth orientation of the nanorods. Only the use of synchrotron radiation allowed us to precisely identify and quantify the presence of a minor Fe2O3 phase, which has never before been reported for those systems. Moreover, the formation of one-dimensional nanorods was accompanied by using scanning and transmission electron microscopies (SEM and TEM). At the highest temperatures of thermal treatments the hematite nanorods exhibited changes that varied from a quasi-perfect crystal up to polycrystalline rods (>500 °C). These findings were used for discussing and explaining recent results using this kind of nanostructure synthesized under hydrothermal conditions. Changes in the preferred orientation of the crystal faces of the rods could be responsible for a low photocatalytic performance reported by this strategy.

Enhancing Hematite Photoanode Activity for Water Oxidation by Incorporation of Reduced Graphene Oxide.

Two effective methods to prepare reduced graphene oxide (rGO)/hematite nanostructured photoanodes and their photoelectrochemical characterization towards water splitting reactions are presented. First, graphene oxide (GO) is reduced to rGO using hydrazine in a basic solution containing tetrabutylammonium hydroxide (TBAOH), and then deposited over the nanostructured hematite photoanodes previously treated at 750 °C for 30 min. The second method follows the deposition of a paste containing a mixture of hematite nanoparticles and rGO sheets by the doctor-blade method, varying the rGO concentration. Since hematite suffers from low electron mobility, a low absorption coefficient, high recombination rates and slow reaction kinetics, the incorporation of rGO in the hematite can overcome such limitations due to graphenes exceptional properties. Using the first method, the rGO incorporation results in a photocurrent density increase from 0.56 to 0.82 mA cm(-2) at 1.23 VRHE. Our results indicate that the rGO incorporation in the hematite photoanodes shows a positive effect in the reduction of the electron-hole recombination rate.

Large-area plasmonic substrate of silver-coated iron oxide nanorod arrays for plasmon-enhanced spectroscopy.

One-dimensional iron oxide materials fabricated on conducting glass substrates and their unique properties make these nanostructures promising candidates for a wide range of applications. Herein, vertically oriented α-Fe2O3 nanorod arrays synthesized under hydrothermal conditions over a large area are described, as an active platform for surface-enhanced resonance Raman scattering (SERRS) and surface-enhanced fluorescence (SEF). From scanning electron microscopy images the formation of a homogeneous distribution of vertically oriented rods in a large area is confirmed. For activating the localized surface plasmon resonances, which are responsible for SERRS and SEF, a 6 nm layer of Ag is deposited onto the α-Fe2O3 nanorod arrays by physical vapor deposition to form Ag islands.

Synthesis, growth mechanism, optical properties and catalytic activity of ZnO microcrystals obtained via hydrothermal processing

In the present study, typical ZnO microcrystals exhibiting the wurtzite hexagonal crystal structure were produced successfully, characterized by a high degree of crystallinity, via hydrothermal processing at 120, 150 and 180 °C, assisted by N-cetyl-N,N,N-trimethylammonium (CTAB). The samples were characterised by XRD, Raman and infrared, FE-SEM, UV-Vis by diffuse reflectance and photoluminescence (PL). The experimental results confirm that all hydrothermally synthesised ZnO samples were crystallised into a wurtzite hexagonal structure. The ZnO crystals exhibit the morphology of hexagonal columns in the absence and double hexagonal columns in the presence of CTAB. The length and average diameter of the microstructures decrease with increasing processing temperature. It is evident that all the synthesised samples present very similar profiles and band positions in the PL emission spectra, with an emission band in the violet range at approximately 400 nm, a small peak in the UV range at approximately 380 nm, and highly superposed and intense emission bands between 440 and 750 nm (blue to red emission), with a maximum at approximately 610 nm. Furthermore, a nucleation and growth model was proposed to explain the formation of ZnO microcrystals, based on the experimental conditions, that were preferably grown in the [001] direction. In addition, the ZnO exhibited excellent performance in the photocatalytic degradation of rhodamine B (RhB) and methyl orange (MO), achieving 97% and 99% photodegradation of RhB and MO, respectively, when ZnO obtained at 120 °C, in the absence of CTAB, was used as catalyst.

Hematite Surface Activation by Chemical Addition of Tin Oxide Layer

In this study, the effect of tin (Sn(4+) ) modification on the surface of hematite electrodes synthesized by an aqueous solution route at different times (2, 5, 10, 18, and 24 h) is investigated. As confirmed from X-ray diffraction results, the as-synthesized electrode exhibits an oxyhydroxide phase, which is converted into a pure hematite phase after being subjected to additional thermal treatment at 750 °C for 30 min. The tin-modified hematite electrode is prepared by depositing a solution of Sn(4+) precursor on the as-synthesized electrode, followed by thermal treatment under the same abovementioned conditions. This modification results in an enhancement of the photocurrent response for all hematite electrodes investigated and attains the highest values of around 1.62 and 2.3 mA cm(-2) at 1.23 and 1.4 V versus RHE, respectively, for electrodes obtained in short synthesis times (2 h). Contact angle measurements suggest that the deposition of Sn(4+) on the hematite electrode provides a more hydrophilic surface, which favors a chemical reaction at the interface between the electrode and electrolyte. This result generates new perspectives for understanding the deposition of Sn(4+) on the hematite electrode surface, which is in contrast with several studies previously reported; these studies state that the enhancement in photocurrent density is related to either the induction of an increased donor charge density or shift in the flat-band potential, which favors charge separation.

Facile Routes to Produce Hematite Film for Hydrogen Generation from Photoelectro-Chemical Water Splitting

In this chapter, we brief review a recent progress in chemical synthesis used to prepare very promise material to be applied as photoanode in a PEC cell. We discuss the important parameters such as; the interface solid/liquid showing the different challenge that needs to be addressed for obtains higher semiconductor photoanode performance. In addition, we discuss the impact of a variety of morphology applied in a PEC cell to split water and generate hydrogen and oxygen molecular. Finally, we have pointed out the progress of molecular oxygen evolution mechanism from water oxidation under solar light irradiation.

UV-Light Effects on Cytochrome C Modulated by the Aggregation State of Phenothiazines

The present study shows the factors that modulate the photodamage promoted by phenothiazines. Cytochrome c was irradiated with UV light for 120 min, over a pH range from 4.0 to 8.0, in the absence and in the presence of different concentrations of thioridazine (TR) and fluphenazine (FP). In the absence of phenothiazines, the maximal rate of a Soret band blue shift (nm/min) from 409 to 406 nm was obtained at pH 4.0 (0.028 nm/min). The presence of phenothiazines at the concentration range 10-25 µmol/L amplified and accelerated a cytochrome c blue shift (409 to 405 nm, at a rate = 0.041 nm/min). Above 25 µmol/L, crescent concentrations of phenothiazines contributed to cytochrome c protection with (maximal at 2500 µmol/L). Scanning electronic microscopy revealed the formation of nanostructures. The pH also influenced the effect of low phenothiazine concentrations on cytochrome c. Thus, the predominance of phenothiazine-promoted cytochrome c damage or protection depends on a balance of the following factors: the yield of photo-generated drug cation radicals, which is favored by acidic pH; the stability of the cation radicals, which is favored by the drug aggregation; and the cytochrome c structure, modulated by the pH.

New ultrasonic assisted co-precipitation for high surface area oxide based nanostructured materials

Owing to their enhanced properties as compared to bulk materials, the prospective applications for nanomaterials have experienced unprecedented growth, gaining attention from all levels of industry, from medical to electronics, chemistry, catalysis and mechanics. However, one of the greatest challenges of the nanomaterial industry lies in developing a production system that assures low cost and high production capabilities while maintaining quality standards. Here, we show a new method for the synthesis of metal oxide nanoparticles based on an aqueous precipitation method. The system makes use of ultrasonic probes and continuous precipitation chambers which allow it to operate continuously. Catalyst support materials, such as MgAl2O4 and γ-Al2O3, were synthesized showing high BET surface areas of 338.61 and 366.10 m2 g−1, hollow spherical morphologies and crystallite sizes as small as 3.2 and 2.1 nm, respectively.

Latest Advances on the Columnar Nanostructure for Solar Water Splitting

In this chapter, we briefly review a recent progress in chemical synthesis used to prepare promising and active material to be applied as photoanode in a PEC cell. A variety of morphology, crystal alignment, and bulk recombination was discussed during the light-induced water oxidation reaction evaluation. The major drawback related to the hole diffusion through the solid/liquid interface was addressed in terms of high annealing temperature combined with dopant addition. In this chapter, a critical view and depth understanding of the role of synergistic effect of these two parameters were discussed focusing on the molecular oxygen evolution mechanism from sunlight-driven water oxidation reaction.