Controlled hydrodynamic conditions on the formation of iron oxide nanostructures synthesized by electrochemical anodization: Effect of the electrode rotation speed

Iron oxide nanostructures are of particular interest because they can be used as photocatalysts in water splitting due to their advantageous properties. Electrochemical anodization is one of the best techniques to synthesize nanostructures directly on the metal substrate (direct back contact). In the present study, a novel methodology consisting of the anodization of iron under hydrodynamic conditions is carried out in order to obtain mainly hematite (α-Fe2O3) nanostructures to be used as photocatalysts for photoelectrochemical water splitting applications. Different rotation speeds were studied with the aim of evaluating the obtained nanostructures and determining the most attractive operational conditions. The synthesized nanostructures were characterized by means of Raman spectroscopy, Field Emission Scanning Electron Microscopy, photoelectrochemical water splitting, stability against photocorrosion tests, Mott-Schottky analysis, Electrochemical Impedance Spectroscopy (EIS) and band gap measurements. The results showed that the highest photocurrent densities for photoelectrochemical water splitting were achieved for the nanostructure synthesized at 1000 rpm which corresponds to a nanotubular structure reaching ∼0.130 mA cm−2 at 0.54 V (vs. Ag/AgCl). This is in agreement with the EIS measurements and Mott-Schottky analysis which showed the lowest resistances and the corresponding donor density values, respectively, for the nanostructure anodized at 1000 rpm

Interfacial insight in multi-junction metal oxide photoanodes for water-splitting applications

Photoelectrochemical (PEC) properties of nanostructured hematite (Fe2O3) thin films prepared using plasma-enhanced chemical vapor deposition (PE-CVD) were investigated against the influence of processing parameters and post-synthesis heat-treatment procedures.Annealing at high temperatures (> 500 \ub0C) was found to substantially affect the micro-structure (grain growth and densification) and electronic (interdiffusion at the film/substrate interface) concomitantly manifested in an enhancement in the PEC behavior. The Sn impuritylevel in hematite films was found to increase with the annealing temperature with highest values achieved in samples heat-treated at 750 \ub0C, due to the interdiffusion and substitution of Sn(IV) species at Fe(III) sites. Sn:Fe2O3 films exhibited significantly high photocurrent density of 1.33 mAcm-2 at the water oxidation level of 1.23V vs. RHE. The diffusion of Sn ions into iron oxide lattice altered the electronic properties of hematite films duetoelectron\u2013donor behavior of the dopants that was verified by X-ray photoelectron spectroscopy and secondary ion mass spectroscopy (SIMS) analyses.Deposition of a thin overlayer of TiO2 (10 nm) on hematite films by atomic layer deposition (ALD) was found to furthe rimprove the photocurrent density to 1.8 mAcm-2 at 1.23V vs.RHE. Ab-initio calculations on the effect of substitutional Sn(IV) dopants in the Fe2O3 lattice on the electronic structure and the band alignment between hematite and theTiO2 over layer revealed that Sn-dopants led to the generation of localized Fe (II) centers augmenting then-type behavior of hematite.No effect of the Sn-dopingonthe electrostatic potential was found on a macroscopic scale.However, the charge transfer from the Sn-doping to the Fe(II) centers would cause high electric fields on the nanometer scale and might hence play an important role in the efficient separation o felectron and holes.The simulations showed that the hematite band edges are enclosed by the TiO2 band edges and therefore electron depletion at the surface\u2013liquid interface is enhanced.This might lead to reduced recombination rates near the surface and consequently to increased photocurrents, since the Fe2O3/TiO2 interface constitutes a barrier for hole transport

Vapor Phase Processing of α-Fe2O3Photoelectrodes for Water Splitting: An Insight into the Structure/Property Interplay

Harvesting radiant energy to trigger water photoelectrolysis and produce clean hydrogen is receiving increasing attention in the search of alternative energy resources. In this regard, hematite (α-Fe2O3) nanostructures with controlled nano-organization have been fabricated and investigated for use as anodes in photoelectrochemical (PEC) cells. The target systems have been grown on conductive substrates by plasma enhanced-chemical vapor deposition (PE-CVD) and subjected to eventual ex situ annealing in air to further tailor their structure and properties. A detailed multitechnique approach has enabled to elucidate the interrelations between system characteristics and the generated photocurrent. The present α-Fe2O3 systems are characterized by a high purity and hierarchical morphologies consisting of nanopyramids/organized dendrites, offering a high contact area with the electrolyte. PEC data reveal a dramatic response enhancement upon thermal treatment, related to a more efficient electron transfer. The reasons underlying such a phenomenon are elucidated and discussed by transient absorption spectroscopy (TAS) studies of photogenerated charge carrier kinetics, investigated on different time scales for the first time on PE-CVD Fe2O3 nanostructures

Electrochemical Biosensors for miRNA Detection

MicroRNAs (miRNAs) are intensely studied as candidates for diagnostic and prognostic biomarkers. They are naturally occurring small RNAs (approximately 22 nucleotides in length) that act as regulators of protein translation. Because many diseases are caused by the misregulated activity of proteins, miRNAs have been implicated in a number of diseases including a broad range of cancers, heart disease, and immunological and neurological diseases. A great deal of effort, therefore, has been devoted to developing analytical methods for miRNA analysis. The consideration when selecting existing or designing new methods for miRNA analysis includes sensitivity and multiplexing capability without PCR. In this chapter, novel electrochemical strategies for miRNA detection and quantification will be reviewed