Investigating the charge separation and transport in iron based photoanodes for solar-driven water splitting

Solar fuel production using Photoelectrochemical Cell has developed after Fujishima and Honda demonstrated in 1972, for the first time, solar water splitting using Titanium Dioxide (TiO2) as photoanode. After that, efforts have been made to employ cheap, abundant and non-toxic semiconducting materials as photoanodes and to improve overall efficiency and performance of PEC devices. In the last decade, hematite (α-Fe2O3) based PEC devices have attracted many scientists in exploiting its advantages like good visible light absorption and stability but the efficiency achieved (only 1-2%) is still far from the theoretical limit of Solar-to-Hydrogen (STH) efficiency (15%). This is due to the limitations of hematite like small minority carrier diffusion length, poor charge transport properties, slow water oxidation kinetics etc. To counter this, hematite nanostructures with elemental doping are used in PEC devices and are thermally activated through sintering at high temperature to achieve significant photocurrent. But high temperature annealing also deforms the substrate morphology which in turn degrades the conductivity and hence trade-offs with overall performance of PEC device. Here, we have demonstrated annealing profile which proves optimal in tackling bulk recombination and simultaneously keeping intact the crystallinity of Fe2O3 nanorods and conductivity of FTO substrates. Through measurements performed under frontside and backside illumination, we have shown electron transport drives the photocurrent under backside illumination for the optimized annealing profile. To inhibit the surface recombination probability, we employed surface passivation technique by coating hematite nanorods with a very thin Atomic Layer Deposition (ALD) grown TiOx overlayer which resulted in manifold enhancement in its performance. Owing to inability of Fe2O3 to reduce water due to its lower conduction band level than the water reduction potential, iron based photoanodes based on Fe-Ti-O systems were explored. Crystalline nanoporous single phase Fe2TiO5 (without secondary phases) thin films are obtained and characterized for the first time. This material is stable in water and has a bandgap suitable for visible light absorption (2.1eV). Using XPS, the work function was calculated to lay around 4.77 eV with respect to vacuum level and the conduction band is estimated to lie or higher than water reduction level based on XPS data. Hall measurement showed a mobility of around 6 cm-2V-1s-1 which demonstrate good charge transport properties of Fe2TiO5 thin films. To enhance charge separation and transport in pristine Fe2O3 nanorods, it was coupled with Fe2TiO5 thin films to form a Fe2O3/Fe2TiO5 heterojunction, which according to their band level positions share a type II band alignment. Synthesized using low-cost hydrothermal technique, these heterojunction films show high photocurrent, reaching 1.4 mA/cm2, as compared to pristine Fe2O3 and Fe2TiO5 films. Characterization techniques like impedance spectroscopy and PEC characterization with hole scavenger revealed the charge dynamics of the system. It is proposed that surface mediated water oxidation was enhanced for the heterojunction films which allowed firstly the trapping of holes in the surface states and secondly facile transfer of these holes into the electrolyte for water oxidation. In sum, the favorable band alignment and the improvement in catalytic activity due to integration of Fe2TiO5 film with Fe2O3 nanorods was beneficial in providing low onset voltages and higher photocurrent density for heterojunction based PEC devices.DOCTOR OF PHILOSOPHY (MSE

Dataset for 'Zn doped Fe2TiO5 photoanodes grown by aerosol-assited chemical vapor deposition'

This dataset includes a range of experimental data collected. In particular, raw data from X-Ray diffraction patterns (XRD), UV- Vis spectroscopy, X-Ray Photoelectron Spectroscopy (XPS), linear sweep voltammetries, photcurrent times curves, IPCE and electrochemical impedance spectroscopy (EIS). Specifics of the methology employed for data collection is described in detail in the experimental part of the paper. Raw data has been labeled following the same methodology as in the paper.Physical Characterisation X- ray diffraction (XRD) patterns were collected from 10 to 60° (2θ) using a Bruker D8 diffractometer with Cu Kα (0.154 nm) radiation. Raman spectroscopy was performed on a Renishaw inVia system using a 532 nm diode-pumped solid-state laser (DPSS) manufactured by Cobolt. A 50x long distance objective was used to focus the laser beam onto the sample. UV-Vis measurements were carried out in a Lambda 950 spectrometer (Perkin Elmer) with an integrating sphere (150 mm InGaAS). The samples were mounted in the center. Diffuse-reflectance UV-Vis measurements were performed in an Agilent Cary 100 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was performed with a monochromatic Al Kα X-ray-source (1486.74 eV, Specs Focus 500 monochromator). C 1s was used for internal charge correction. Ultraviolet photoelectron spectroscopy (UPS) was carried out with a He I source (E = 21.218 eV) in the same chamber. A hemispherical analyzer (Specs Phoibos 100) was used for both XPS and UPS measurements. The base pressure of the system was ∼10−9 mbar. (Photo)electrochemical characterisation: Photoelectrochemical (PEC) performance of photoanodes was measured under simulated solar light using a WACOM Super Solar Simulator (Model WXS-505-5H, AM 1.5, Class AAA) and an EG&G Princeton Applied Research Potentiostat/Galvanostat (Model 273A). PEC cells were prepared using a three electrode configuration with Pt as the counter electrode, a silver chloride reference electrode (Ag/AgCl-reference electrode, XR300, Radiometer Analytical, EAg/AgCl=0.197 VRHE) and the as-prepared photoanodes as the working electrode. Illumination was directed towards the back of the photoanode (Glass-FTO-sample). 1 M NaOH (pH=13.6) was used as electrolyte. Incident photon-to-current efficiency (IPCE) measurements were performed using an Xe lamp (LOT, LSH302), an Acton Research monochromator (Spectra Pro 2155) and an electronic shutter (Uniblitz LS6). The intensity of the monochromated light was measured by a calibrated photodiode (PD300R-UV, Ophir) just after a clean FTO-glass substrate placed at the working electrode position, in the absence of PEC cell quartz window or PEC cell electrolyte. PEC impedance spectroscopy (PEIS) was carried out under simulated sunlight (AM 1.5G, 100 mW cm-2) using a CompactStat. Potentiostat (Ivium technologies). Measurements were performed in a frequency range from 105 to 0.1 Hz, with an AC voltage amplitude of 10 mV at a potential range of 0.6 to 1.2 VRHE with 0.05V steps, in 1M NaOH. EIS measurements in the dark were also measured to obtain Mott-Schottky plots. These measurements were performed at a fixed frequency of 100 and 1000 Hz.Data for the different technqiues are presented in different documents. Data has been labeled as follows: #_Fe2TiO5, #_Fe2TiO5_Zn, #_Fe2O3 and #_Fe2O3_Zn, where # corresponds to the nomenclature of x and y axis of the plot

Zn-Doped Fe₂TiO₅Pseudobrookite-Based Photoanodes Grown by Aerosol-Assisted Chemical Vapor Deposition

Water splitting in photoelectrochemical cells is a promising technology to produce solar hydrogen. Fe2TiO5 pseudobrookite with a bandgap of around 2 eV absorbs the predominant visible range of the solar spectrum and is emerging as a promising photoanode for such cells. Herein, we present Fe2TiO5 pseudobrookite-based films prepared by aerosol-assisted chemical vapor deposition and the positive impact of Zn2+ doping in their formation and performance. Undoped and Zn2+-doped Fe2TiO5 pseudobrookite-based photoanodes were characterized by techniques such as XRD, XPS, UPS, and Mott-Schottky analysis. We find that the Zn2+ ions are preferentially incorporated in the pseudobrookite phase over a present secondary hematite (α-Fe2O3) phase. The Zn2+ doping modifies the electronic properties of the films, increases their charge carrier concentration, and upshifts their Fermi level, significantly improving their anodic photocurrent response by a factor of three. In addition, charge transfer efficiency calculations reveal that Zn2+ doping improves both charge separation and injection efficiencies, overall demonstrating a promising approach for the design of enhanced pseudobrookite-based photoanodes.</p

Bandgap engineering of ternary sulfide nanocrystals by solution proton alloying for efficient photocatalytic H-2 evolution

Bandgap engineering is an important strategy for tailoring the optical and electronic properties of semiconductor nanocrystals. This work describes the first solution proton alloying process for tuning the bandgap energy of ternary sulfide nanocrystals at room temperature. The proposed strategy circumvents the use of toxic heavy metal ions, while maintaining the size and morphology of the nanocrystals, through a seamless tuning of the bandgap over a wide range. It was shown that proton alloying exhibited different effects on the bandgap energies of ternary sulfide nanocrystals and this could be explained by Density-Of-States (DOS) calculations. Using this approach, enhanced optoelectronic properties of ternary sulfide semiconductor nanocrystals were achieved and proton alloyed ZnIn2S4 showed eight times higher photocatalytic H-2 evolution rate than that of the untreated ones due to increased carrier density and decreased charge transfer resistance. (C) 2016 Elsevier Ltd. All rights reserved.</p

Hydrothermal Grown Nanoporous Iron Based Titanate, Fe₂TiO₅ for Light Driven Water Splitting

We report the synthesis of iron based titanate (Fe₂TiO₅) thin films using a simple low cost hydrothermal technique. We show that this Fe₂TiO₅ works well as a photoanode for the photoelectrochemical splitting of water due to favorable band energetic. Further characterization of thin films including band positions with respect to water redox levels has been investigated. We conclude that Fe₂TiO₅ is a promising material comparable to hematite for constructing PEC cells

Improved charge separation in WO3/CuWO4 composite photoanodes for photoelectrochemical water oxidation

Porous tungsten oxide/copper tungstate (WO3/CuWO4) composite thin films were fabricated via a facile in situ conversion method, with a polymer templating strategy. Copper nitrate (Cu(NO3)2) solution with the copolymer surfactant Pluronic®F-127 (Sigma-Aldrich, St. Louis, MO, USA, generic name, poloxamer 407) was loaded onto WO3 substrates by programmed dip coating, followed by heat treatment in air at 550 °C. The Cu2+ reacted with the WO3 substrate to form the CuWO4 compound. The composite WO3/CuWO4 thin films demonstrated improved photoelectrochemical (PEC) performance over WO3 and CuWO4 single phase photoanodes. The factors of light absorption and charge separation efficiency of the composite and two single phase films were investigated to understand the reasons for the PEC enhancement of WO3/CuWO4 composite thin films. The photocurrent was generated from water splitting as confirmed by hydrogen and oxygen gas evolution, and Faradic efficiency was calculated based on the amount of H2 produced. This work provides a low-cost and controllable method to prepare WO3-metal tungstate composite thin films, and also helps to deepen the understanding of charge transfer in WO3/CuWO4 heterojunction.NRF (Natl Research Foundation, S’pore)Published versio

Scale-up of BiVO4 photoanode for water splitting in a photoelectrochemical cell : issues and challenges

The monoclinic scheelite‐type BiVO4 is recognized as one of the promising candidate materials for a photoanode because of its 9.1 % theoretical efficiency for half‐cell solar‐to‐hydrogen conversion. Although significant research efforts have been devoted to improving the performance of the photoelectrochemical cell (PEC) of this material, they have mainly been in small anode areas with only a handful of studies on scaled‐up sizes. Herein, a facile metal–organic decomposition synthesis method was used to produce scaled‐up Mo‐doped BiVO4 photoanodes. Multiple modifications were explored and incorporated to enhance the performance of the photoanode. A large‐area (5 cm×5 cm) photoanode was successfully prepared with all modifications. The resulting photoanode gave rise to an initial photocurrent density of 2.2 mA cm−2 at 1.23 V versus reversible hydrogen electrode, under AM 1.5G illumination in a PEC, which remained at 79 % of this value after 1 h of operation. A deleterious effect of the increased anode surface area on the photocurrent density was observed, which we termed the “areal effect”. Understanding the reasons for the areal effect is indispensable for the development of large‐scale PEC devices for water splitting.NRF (Natl Research Foundation, S’pore)Accepted versio

Revealing the Role of TiO₂ Surface Treatment of Hematite Nanorods Photoanodes for Solar Water Splitting

Ultrathin TiO₂ is deposited on conventional hydrothermal grown hematite nanorod arrays by atomic layer deposition (ALD). Significant photoelectrochemical water oxidation performance improvement is observed when the ALD TiO₂-treated samples are annealed at 650 °C or higher temperatures. The electrochemical impedance spectroscopy (EIS) study shows a surface trap-mediated charge transfer process exists at the hematite–electrolyte interface. Thus, one possible reason for the improvement could be the increased surface states at the hematite surface, which leads to better charge separation, less electron–hole recombination, and hence, greater improvement of photocurrent. Our Raman study shows the increase in surface defects on the ALD TiO₂-coated hematite sample after being annealed at 650 °C or higher temperatures. A photocurrent of 1.9 mA cm^–2 at 1.23 V (vs RHE) with a maximum of 2.5 mA cm^–2 at 1.8 V (vs RHE) in 1 M NaOH under AM 1.5 simulated solar illumination is achieved in optimized deposition and annealing conditions