Synthesis and Characterization of Hematite Nanotube Arrays for Photocatalysis

Abstract

The synthesis and characterization of hematite nanotube arrays for use as photocatalysts are described in this paper. Novel morphologies, including multilayered and wave-like nanotube arrays, were synthesized via electrochemical anodization of iron foils in electrolytic solutions containing ammonium fluoride, ethylene glycol and water. The nanotube formation mechanism was investigated by tracking the current response data during anodization.The results indicate that there are four distinct stages associated with the evolution of the nanotube arrays: an ohmic response stage, an oxide film formation stage, a chemical dissolution/pitting stage, and a steady-state growth stage. Morphological, optical, and photoelectrochemical properties of the resulting materials were characterized using X-ray diffraction, scanning electron microscopy, UV-Vis diffuse reflectance spectroscopy, linear sweep voltammetry, electrochemical impedance spectroscopy, and incident photon to current efficiency (IPCE). The IPCE for the wave-like nanotube arrays at 350 nm was ~3 times that for the single layer nanotube arrays and ~12 times that for the multilayer nanotube arrays. The IPCE also increased with electrochemical surface area and played a key role with regard to the photocatalytic performance.

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

“…Therefore, it can be pointed out that there are no differences among the samples which means that the indirect band gap is not affected by the hydrodynamic conditions. Both values of direct and indirect band gap for hematite nanostructures were in agreement with the literature[9,60,66,67]. In general terms, smaller band gap values indicate that a wider solar spectrum fraction will be absorbed generating more electron-hole pairs[9], but for photoelectrochemical water splitting an energy close to ~2 eV is necessary for the charge carriers to have enough energy to split water[9,[68][69][70].…”

Iron oxide nanostructures for photoelectrochemical applications: Effect of applied potential during Fe anodization

“…Figure 1 shows the current density vs. time registers during electrochemical anodization of iron at the different applied potentials. All the curves indicated the typical tendency of the formation of iron oxide nanostructures with the tree typical stages: (1) formation of a compact oxide layer, (2) tiny pits in the compact layer due to the fluoride ions in the electrolyte and the applied potential that leads to nanoporous structures, and (3) dissolution and cation-cation repulsion; this formation continues until reaching equilibrium between formation of oxide layer and its chemical dissolution, leading to nanotubular structures [34,38].…”

“…Since iron oxide (in particular in its hematite form) is one of the most promising materials due to its properties, several studies are focused on obtaining iron oxide nanostructures with simple and low cost methods such as electrochemical anodization. Until the moment, these iron oxide nanostructures achieve current density values in the order of a few mA • cm -2 [38,[51][52][53][54]. Then, the main research objective is to optimize the parameters of the processes to achieve higher efficiencies.…”

Functional principle of the synergistic effect of co-loaded Co-Pi and FeOOH on Fe2O3 photoanodes for photoelectrochemical water oxidation

“…This is mostly attributed to the low mobility of carriers (<0.1 cm 2 V −1 s −1 ) [7] and the sluggish oxygen evolution reaction (OER) kinetics, which are affected by the surface states [8,9]. The PEC performance of Fe2O3 could be improved by using nanostructured Fe2O3 [10,11] and foreign element doping [12]. However, the onset potential of doped nanostructured Fe2O3 photoanodes could still be as high as ~0.9 V [13,14], which is much higher than that of other photoanode materials, such as BiVO4 [15] and TiO2 [16], and represents a major drawback.…”