nan Gurudayal

Perovskite–Hematite Tandem Cells for Efficient Overall Solar Driven Water Splitting

Photoelectrochemical water splitting half reactions on semiconducting photoelectrodes have received much attention but efficient overall water splitting driven by a single photoelectrode has remained elusive due to stringent electronic and thermodynamic property requirements. Utilizing a tandem configuration wherein the total photovoltage is generated by complementary optical absorption across different semiconducting electrodes is a possible pathway to unassisted overall light-induced water splitting. Because of the low photovoltages generated by conventional photovoltaic materials (e.g., Si, CIGS), such systems typically consist of triple junction design that increases the complexity due to optoelectrical trade-offs and are also not cost-effective. Here, we show that a single solution processed organic-inorganic halide perovskite (CH3NH3PbI3) solar cell in tandem with a Fe2O3 photoanode can achieve overall unassisted water splitting with a solar-to-hydrogen conversion efficiency of 2.4%. Systematic electro-optical studies were performed to investigate the performance of tandem device. It was found that the overall efficiency was limited by the hematites photocurrent and onset potential. To understand these limitations, we have estimated the intrinsic solar to chemical conversion efficiency of the doped and undoped Fe2O3 photoanodes. The total photopotential generated by our tandem system (1.87 V) exceeds both the thermodynamic and kinetic requirements (1.6 V), resulting in overall water splitting without the assistance of an electrical bias.

Improving the Efficiency of Hematite Nanorods for Photoelectrochemical Water Splitting by Doping with Manganese

Here, we report a significant improvement of the photoelectrochemical (PEC) properties of hematite (α-Fe2O3) to oxidize water by doping with manganese. Hematite nanorods were grown on a fluorine-treated tin oxide (FTO) substrate by a hydrothermal method in the presence on Mn. Systematic physical analyses were performed to investigate the presence of Mn in the samples. Fe2O3 nanorods with 5 mol % Mn treatment showed a photocurrent density of 1.6 mA cm(-2) (75% higher than that of pristine Fe2O3) at 1.23 V versus RHE and a plateau photocurrent density of 3.2 mA cm(-2) at 1.8 V versus RHE in a 1 M NaOH electrolyte solution (pH 13.6). We attribute the increase in the photocurrent density, and thus in the oxygen evolving capacity, to the increased donor density resulting from Mn doping of the Fe2O3 nanorods, as confirmed by Mott-Schottky measurement, as well as the suppression of electron-hole recombination and enhancement in hole transport, as detected by chronoamperometry measurements.

Iron based photoanodes for solar fuel production

In natural photosynthesis, the water splitting reaction of photosystem II is the source of the electrons/reducing equivalents for the reduction of carbon dioxide to carbohydrate while oxygen is formed as the by-product. Similarly, for artificial photosynthesis where the end product is a solar fuel such as hydrogen, a water splitting-oxygen evolving system is required to supply high energy electrons to drive the reductive reactions. Very attractive candidates for this purpose are iron based semiconductors which have band gaps corresponding to visible light and valence band energies sufficient to oxidise water. The most studied system is hematite (Fe2O3) which is highly abundant with many attributes for incorporation into photoelectrochemical (PEC) cells. We review the recent progress in manipulating hematite for this purpose through nanostructuring, doping and surface modifications. We also consider several hybrid iron-based semiconducting systems like ferrites and iron titanates as alternatives to hematite for light driven water splitting emphasizing their advantages with respect to their band levels and charge transport properties.

Core-shell hematite nanorods: a simple method to improve the charge transfer in the photoanode for photoelectrochemical water splitting.

We report a simple method to produce a stable and repeatable photoanode for water splitting with a core-shell hematite (α-Fe2O3) nanorods system by combining spray pyrolysis and hydrothermal synthesis. Impedance spectroscopy revealed passivation of the surface states by the shell layer, which results in an increase of the charge injection through the hematite conduction band. In pristine hematite more holes are accumulated on the surface and the charge transfer to the electrolyte occurs through surface states, whereas in the core-shell hematite photoanode the majority of hole transfer process occurs through the valence band. As a result the photoactivity of the core-shell nanorods, 1.2 mA cm(-2), at 1.23 V vs RHE, is twice that of pristine hematite nanorods. The alteration of the interface energetics is supported by TEM, showing that the crystallinity of the surface has been improved by the deposition of the shell.

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.

Immobilization of dye pollutants on iron hydroxide coated substrates: kinetics, efficiency and the adsorption mechanism

Research on adsorbent materials has recently revolved around nanoparticles due to their enhanced surface area. However, they pose a major problem in terms of their removal and persistency in treated water, which is hazardous for consumption. This issue can be alleviated by immobilizing the pollutant on a rigid substrate. Using coated FeOOH porous thin films, we demonstrated a high adsorption capacity of 144 mg g−1 for Congo red that proved the viability of such an approach. The coated film therefore achieved unprecedented ease in separating the pollutant through immobilization. In addition, our film was grown using an environmentally friendly method at room temperature, making it highly attractive and scalable. Finally, we also examined the kinetics and adsorption mechanism of Congo red on FeOOH. We found that it is governed by a surface limited chemisorption reaction, through a unidentate complex bonding of Fe with the sulfonic group of the dye. We discussed the implication of such a mechanism by showing how the structure of our coated film plays a key role in affecting the adsorption capacity, and the theoretical limit of FeOOH adsorption.

Atomically Altered Hematite for Highly Efficient Perovskite Tandem Water-Splitting Devices

Photoelectrochemical (PEC) cells are attractive for storing solar energy in chemical bonds through cleaving of water into oxygen and hydrogen. Although hematite (α-Fe2 O3 ) is a promising photoanode material owing to its chemical stability, suitable band gap, low cost, and environmental friendliness, its performance is limited by short carrier lifetimes, poor conductivity, and sluggish kinetics leading to low (solar-to-hydrogen) STH efficiency. Herein, we combine solution-based hydrothermal growth and a post-growth surface exposure through atomic layer deposition (ALD) to show a dramatic enhancement of the efficiency for water photolysis. These modified photoanodes show a high photocurrent of 3.12u2005mAu2009cm-2 at 1.23u2005V versus RHE, (>5 times higher than Fe2 O3 ) and a plateau photocurrent of 4.5u2005mAu2009cm-2 at 1.5u2005V versus RHE. We demonstrate that these photoanodes in tandem with a CH3 NH3 PbI3 perovskite solar cell achieves overall unassisted water splitting with an STH conversion efficiency of 3.4u2009%, constituting a new benchmark for hematite-based tandem systems.

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.

Revealing the Influence of Doping and Surface Treatment on the Surface Carrier Dynamics in Hematite Nanorod Photoanodes

Photoelectrochemical (PEC) water oxidation is considered to be the rate-limiting step of the two half-reactions in light-driven water splitting. Consequently, considerable effort has focused on improving the performance of photoanodes for water oxidation. While these efforts have met with some success, the mechanisms responsible for improvements resulting from photoanode modifications are often difficult to determine. This is mainly caused by the entanglement of numerous properties that influence the PEC performance, particularly processes that occur at the photoanode/electrolyte interface. In this study, we set out to elucidate the effects on the surface carrier dynamics of hematite photoanodes of introducing manganese (Mn) into hematite nanorods and of creating a core-shell structure. Intensity-modulated photocurrent spectroscopy (IMPS) measurements reveal that the introduction of Mn into hematite not only increases the rate constant for hole transfer but also reduces the rate constant for surface recombination. In contrast, the core-shell architecture evidently passivates the surface states where recombination occurs; no change is observed for the charge transfer rate constant, whereas the surface recombination rate constant is suppressed by ∼1 order of magnitude.