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

Improved performance of transparent silver nanowire electrodes by adding carbon nanotubes

We present an easy and scalable production method for transparent electrodes based on silver nanowires and silver nanowire/single-wall carbon nanotube hybrid films. We applied dip coating on glass and flexible polyethylene terephtalate foils. The foils were treated with oxygen plasma in order to increase the hydrophilicity of the surface. After the plasma treatment, the foils could be wet-coated as easy and fast as the glass substrates. Several silver nanowire films were coated with two different carbon nanotube inks. Hereby, the carbon nanotubes are supposed to function as an electrical bridge between the silver nanowires in order to decrease the sheet resistance. We found that shorter metallic enriched carbon nanotubes contributed a stronger increase on the conductivity than longer unsorted carbon nanotubes. This result for silver/carbon nanotube hybrid films is opposed to the trends for pure carbon nanotube networks, where the length of the carbon nanotubes has a stronger effect compared to the amount of metallic carbon nanotubes

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

In-situ plasma hydrogenated TiO₂ thin films for enhanced photoelectrochemical properties

In this paper, we report the effect of in-situ plasma hydrogenation of TiO2 (iH:TiO2) thin films by the incorporation of known amount of hydrogen in the Ar plasma during rf-sputter deposition of TiO2 films. As compared to pristine TiO2 films (∼0.43 mA/cm2 at 0.23 V vs Ag/AgCl), hydrogenated TiO2 showed enhanced photoelectrochemical activity in terms of improved photocurrent density of ∼1.08 mA/cm2 (at 0.23 V vs Ag/AgCl). These results are explained in terms of reduction in band gap energy, shift in valence band maximum away from the Fermi level, improved donor density and more negative flat band potential in iH:TiO2 sample. The presence of Ti2+ states in iH:TiO2 films in addition to Ti3+ states in pristine TiO2 act as additional electronic states in the TiO2 band gap and increases the optical absorption in the visible region. This method of in-situ hydrogenation can be used as a general method for improving the properties of metal oxide thin films for photoelectrochemical and photocatalytic applications