Spray pyrolytically deposited nanoporous Ti doped hematite thin films for efficient photoelectrochemical splitting of water

Nanoporous hematite (α-Fe₂O₃) thin films doped with Ti⁴⁺ deposited by spray-pyrolysis were successfully used in photoelectrochemical splitting of water for solar hydrogen production. X-ray diffraction, field emission scanning electron microscopy, UV–visible absorption and photoelectrochemical studies have been performed on the undoped and Ti⁴⁺ doped hematite thin films. Morphology of α-Fe₂O₃ thin films was observed to be nanoporous, with increased porosity (pore size ∼12 to 20 nm) on increasing doping concentration. A significant decrease in the bandgap energy from 1.95 to 1.27 eV was found due to doping. α-Fe₂O₃ film doped with 0.02 M Ti⁴⁺ ions exhibited best solar to hydrogen conversion efficiency (photoconversion efficiency) of 1.38% at 0.5 V/SCE. Highest photocurrent densities of 0.34 mA/cm² at zero bias and 1.98 mA/cm² at 0.5 V/SCE were obtained by incorporating 0.02 M Ti⁴⁺ in α-Fe₂O₃, which are significantly larger than earlier reported values. Donor density (30.8 × 10²⁰ cm⁻³) and flatband potential (−1.01 V/SCE) obtained were also maximum for this sample. Hydrogen collected in 1 hr at Pt electrode with the best photoelectrode was 2.44 mL with 150 mW/cm² visible light source.     

Favourable band edge alignment and increased visible light absorption in β-MoO3/α-MoO3 oxide heterojunction for enhanced photoelectrochemical performance

Optimum band gap values, favourable band edge positions and stability in the electrolyte are critical parameters required for a semiconductor to have efficient photoelectrode properties. The present investigation carried out on the phase pure α & β MoO₃ thin film shows that the low bandgap β-MoO₃ possesses a mis-alignment with the water oxidation potential, while a more suitable band alignment is observed for the comparatively large bandgap α-MoO₃. Both experimental and DFT calculations show that the valence edge of the orthorhombic (α-MoO₃) phase is located at a higher energy (0.9 eV higher in VB-XPS and 1 eV higher in the DOS plots) than the monoclinic (β-MoO₃) phase, while the conduction edge value is roughly at the same energy level (?2.5 eV) in both polymorphs. Based on the above investigations, an all oxide heterojunction comprising of β-MoO₃/α-MoO₃ is found to be suitable for improved PEC performance due to favourable energy band diagram and increased visible light absorption in β-MoO₃. Significantly higher cathodic photocurrent is observed for the β-MoO₃/α-MoO₃ (1.6 mA/cm² at applied bias of ?0.3V_RHE under simulated 1 sun irradiation) as compared to the very low anodic response in β-MoO₃ (～1.0 nA/cm²) and α-MoO₃ (32 μA/cm²).     

Columnar Fe2O3 arrays via plasma-enhanced growth: Interplay of fluorine substitution and photoelectrochemical properties

A single-step plasma enhanced-chemical vapor deposition (PE-CVD) route for the synthesis of F-doped iron(III) oxide nanomaterials is presented. Growth experiments, performed from a fluorinated Fe(II) β-diketonate precursor on Indium Tin Oxide (ITO) between 200 and 400 °C, yielded columnar β-Fe₂O₃ arrays with a preferential (100) growth direction. The fluorine content in the deposits could be adjusted by the sole variation of the deposition temperature controlling, in turn, the optical absorption and energy bandgap. Photocurrent measurements and Mott–Schottky analyses, carried out in Na₂SO₄ solution under one sun illumination, evidenced a conductivity switch from n- to p-type upon increasing fluorine amount in the obtained nanomaterials. The sample photocurrent density, donor content and flatband potential support the hypothesis that a progressive substitution of oxygen by fluorine in the iron(III) oxide lattice can alter electronic structure and extend charge carrier lifetimes, making anion-doped β-Fe₂O₃ an efficient water oxidation catalyst.     

Iron doped nanostructured TiO2 for photoelectrochemical generation of hydrogen

This paper describes the photoelectrochemical studies on nanostructured iron doped titanium dioxide (TiO₂) thin films prepared by sol-gel spin coating method. Thin films were characterized by X-ray diffraction, Raman spectroscopy, spectral absorbance, atomic force microscopy and photoelectrochemical (PEC) measurements. XRD study shows that the films were polycrystalline with the photoactive anatase phase of TiO₂. Doping of Fe in TiO₂ resulted in a shift of absorption edge towards the visible region of solar spectrum. The observed bandgap energy decreased from 3.3 to 2.89 eV on increasing the doping concentration upto 0.2 at.% Fe. 0.2 at.% Fe doped TiO₂ exhibited the highest photocurrent density, ∼0.92 mA/cm² at zero external bias. Flatband potential and donor density determined from the Mott–Schottky plots were found to vary with doping concentration from −0.54 to −0.92 V/SCE and 1.7 × 10¹⁹ to 4.3 × 10¹⁹ cm⁻³, respectively.     

Steam reforming of simulated pre-reformed naphtha in a PdAu membrane reactor

Process intensification in a membrane reactor is an efficient and compact way to produce hydrogen. A methane-rich gas mixture that simulated the composition of pre-reformed naphtha (PRN; with a steam-to-carbon ratio of 2.7) was reformed at temperatures of 550 °C–625 °C and pressures up to 40 barg. The reactor contained commercial steam reforming catalyst and a 14.8 cm long, 2.6 μm thick Pd-1.8Au (wt. %) membrane on a porous alumina support. Methane conversions approaching 90% were obtained in the membrane reactor at a gas-hourly space velocity of 676 h^?1, compared to ≤30% conversion at the same conditions in conventional reactor mode (CM) without withdrawing hydrogen through the membrane. The results were compared to steam methane reforming (SMR) in the membrane reactor at similar conditions. The nitrogen leak through the membrane increased slowly during the testing, because of both pinhole formation and some leakage through the end seals.     

Plasma-chemical reduction of iron oxide photoanodes for efficient solar hydrogen production

We demonstrate the effect of hydrogen plasma treatment on hematite films as a simple and effective strategy for modifying the existing substrate to improve significantly the band edge positions and photoelectrochemical (PEC) performance. Plasma treated hematite films were consist of mixed phases (Fe₃O₄:α-Fe₂O₃) which was confirmed by XPS and Raman analysis, treated films also showed higher absorption cross-section and were found to be a promising photoelectrode material. The treated samples showed enhance photocurrent densities with maximum of 3.5 mA/cm² at 1.8 V/RHE and the photocurrent onset potentials were shifted from 1.68 V_RHE (untreated) to 1.28 V_RHE (treated). Hydrogen plasma treatment under non-equilibrium conditions induced a valence dynamics among Fe centers in the sub-surface region that was sustained by the incorporation of hydrogen in the hematite lattice as supported by the density functional theory calculations.     

Photoelectrochemical properties of hematite films grown by plasma enhanced chemical vapor deposition

Nanostructured α-Fe₂O₃ thin films were grown by plasma-enhanced chemical vapor deposition (PE-CVD) using iron pentacarbonyl (Fe(CO)₅) as precursor. Influence of the plasma parameters on photoelectrochemical (PEC) properties of the resulting hematite thin films toward solar oxidation of water was investigated under one sun illumination in a basic (1 M NaOH) electrolyte. PEC data analyzed in conjunction with the data obtained by scanning electron microscopy, X-ray diffraction and Mott–Schottky analysis showed 100 W plasma power to be an optimal RF-power value for achieving a high photocurrent density of ∼1098 μA/cm² at 0.9 V/SCE external applied potential. The donor density, flat band potential, grain size and porosity of the films were observed to be highly affected by RF-power, which in turn resulted in enhanced photoresponse.     

Nanostructured hematite for photoelectrochemical generation of hydrogen

With the advent of nanotechnology there has been a resurge of interest in α$\alpha$-Fe₂O₃, as suitable candidate for photoelectrochemical (PEC) splitting of water to generate hydrogen. This paper describes the enhanced PEC behaviour of nanostructured α$\alpha$-Fe₂O₃ thin films modified by various techniques. Nanostructured thin film/pellets of α$\alpha$-Fe₂O₃ prepared by various techniques and various dopants were investigated for their photoelectrochemical response. Thin films prepared by spray pyrolysis having particle size of 20–30 nm exhibited better photoresponse as compared to the films prepared by sol–gel methods, which further improved on doping with Zn. These films were further modified by (i) depositing Zn dots on the surface of α$\alpha$-Fe₂O₃ films using thermal evaporation method and (ii) irradiating it with 170 MeV Au¹³⁺${\mathrm{Au}}^{13+}$ ions. When used as electrode in photoelectrochemical cell, a significant increase in the photoresponse of these modified films were observed, details of which are discussed.     

Experimental studies of a hybrid adsorbent-membrane reactor (HAMR) system for hydrogen production

An experimental study of the performance of a novel reactor system—termed the hybrid adsorbent-membrane reactor (HAMR)—is described. In the HAMR the reaction and membrane separation steps are coupled with adsorption. It was shown previously by our group for esterification reactions that this system results in significantly improved performance. The focus in this paper is on the use of the HAMR for hydrogen production. We present experimental investigations of the HAMR for the water-gas-shift (WGS) reaction using layered double hydroxides as adsorbents for CO₂ and nanoporous H₂-selective carbon molecular sieve membranes. The reactor characteristics are investigated for a range of temperatures and pressures relevant to the WGS application, and are compared with the predictions of a mathematical model previously developed by our group.