On the Morphology and Interfaces of Nanostructured Hematite Photoanodes for Solar-Driven Water Splitting

A sustainable route to store the energy provided by the Sun, is to directly convert sunlight into molecular hydrogen using a semiconductor performing water photolysis. Hematite (α-Fe2O3) is promising for this application due to its ample abundance, chemical stability and significant light absorption (with a band gap of 2.0 - 2.2 eV). Despite these advantageous properties, several drawbacks restrain the utilization of iron oxide for photoelectrochemical water splitting. The first limitation, namely the conduction band edge lower than the water reduction potential, can be straightforwardly overcome by adding a second solar system in tandem, which can absorb a complementary part of the solar spectrum and bring the electron at an energetic level higher than the hydrogen evolution potential. The second drawback arises from the disaccord between the short charge carrier diffusion length and the large light penetration depth. It is therefore necessary to control the hematite morphology on a length scale similar to the hole transport length. To further enhance the photoelectrochemical performance, a new concept for water splitting is introduced in this thesis. The host-guest approach consists in decoupling the different tasks of the photoanode (on one side light harvesting and water oxidation center, on the other side electron conduction to the substrate) by depositing a thin layer of hematite onto a mesoporous host (WO3 in this study). This concept has been demonstrated to increase the photocurrent by ca. 20% due to enhanced quantum efficiencies at long wavelengths. This demonstration has been nonetheless limited by the iron oxide thin films overall efficiency. Thin films photoactivity is then investigated by two means: first by controlling their nucleation on a modified substrate and secondly by incorporating plasmonic nanoparticles aimed to localize absorption in the thin film. The formation of a SiOx buffer layer on the substrate prior to deposition of hematite by Fe(acac)3 spray is shown to modify the film formation mode and its physical properties. These films exhibit photoactivity from an optical thickness of 12.5 nm (as compared to 25 nm without underlayer). The study of hematite photoanodes with gold nanoparticles, embedded or deposited on its surface, establish that charge transfer from metal nanoparticles is occurring only at overlapping wavelengths between the plasmonic resonance and the semiconductor absorption. Nevertheless, photoelectrochemical performances are reduced because of high recombination rate at the metal/semiconductor interface. Finally the third limitation, i.e. the large overpotential required to observe the onset of water splitting photocurrent, is tackled in the last part. The onset potential of photocurrent is decreased by a very thin coating of Al2O3 (0.1 - 2 nm), deposited by ALD, on the nanostructured photoanode. The subsequent application of the Co2+ catalyst further reduces the overpotential and results in a record photocurrent at 0.9 VRHE of over 0.4 mA cm-2. This investigation clearly distinguishes two causes for this energy loss: surface traps and slow oxidation kinetics. The charge accumulation and the Fermi level pinning, observed at low bias potential and assigned to these surface states were further rationalized in an investigation on photocurrent and photovoltage transients

Examining architectures of photoanode-photovoltaic tandem cells for solar water splitting

Given the limitations of the materials available for photoelectrochemical water splitting, a multiphoton (tandem) approach is required to convert solar energy into hydrogen efficiently and durably. Here we investigate a promising system consisting of a hematite photoanode in combination with dye-sensitized solar cells with newly developed organic dyes, such as the squaraine dye, which permit new configurations of this tandem system. Three configurations were investigated: two side-by-side dye cells behind a semitransparent hematite photoanode, two semitransparent dye sensitized solar cells (DSCs) in front of the hematite, and a trilevel hematite/DSC/DSC architecture. Based on the current-voltage curves of state-of-the-art devices made in our laboratories, we found the trilevel tandem architecture (hematite/SQ1 dye/N749 dye) produces the highest operating current density and thus the highest expected solar-to-hydrogen efficiency (1.36% compared with 1.16% with the standard back DSC case and 0.76% for the front DSC case). Further investigation into the wavelength-dependent quantum efficiency of each component revealed that in each case photons lost as a result of scattering and reflection reduce the performance from the expected 3.3% based on the nanostructured hematite photoanodes. We further suggest avenues for the improvement of each configuration from both the DSC and the photoanode part

A Gibeon meteorite yields a high-performance water oxidation electrocatalyst

Examining the electrocatalytic performance of naturally-occurring metallic minerals is of interest for energy conversion applications given their unique atomic composition and formation history. Herein, we report the electrocatalytic function of an iron-based Gibeon meteorite for the oxygen evolution reaction (OER). After ageing under operational conditions in an alkaline electrolyte, an activity matching or possibly slightly superior to the best performing OER catalysts emerges, with stable overpotentials as low as 270 mV (for 10 mA cm(-2)) and Tafel slopes of 37 mV decade(-1). The Faradaic efficiency for the OER was unity and no deterioration in performance was detected during 1000 hours of OER operation at 500 mA cm(-2). Mechanistic studies suggest an operando surface modification involving the formation of a 3D oxy(hydroxide) layer with a metal atom composition of Co0.11Fe0.33Ni0.55, as indicated by Raman and XPS studies and trace Ir as indicated via elemental analysis. The growth of the catalyst layer was self-limiting to <200 nm after ca. 300 hours of operation as indicated through XPS depth profiling and cyclic voltammetry. The unique composition and structure of the Gibeon meteorite suggest that further investigation of Ir-Co-Ni-Fe systems or other alloys inspired by natural materials for water oxidation are of interest

The Transient Photocurrent and Photovoltage Behavior of a Hematite Photoanode under Working Conditions and the Influence of Surface Treatments

Hematite (alpha-Fe2O3) is widely recognized as a promising candidate for the production of solar fuels via water splitting, but its intrinsic optoelectronic properties have limited its performance to date. In particular, the large electrochemical overpotential required to drive the water oxidation is known as a major drawback. This overpotential (0.4 - 0.6 V anodic of the flat band potential) has been attributed to poor oxygen evolution reaction (OER) catalysis and to charge trapping in surface states but is still not fully understood. In the present study, we quantitatively investigate the photocurrent and photovoltage transient behavior of alpha-Fe2O3 photoanodes prepared by atmospheric pressure chemical vapor deposition, under light bias, in a standard electrolyte, and one containing a sacrificial agent. The accumulation of positive charges occurring in water at low bias potential is found to be maximum when the photocurrent onsets. The transient photocurrent behavior of a standard photoanode is compared to photoanodes modified by either a catalytic or surface passivating overlayer. Surface modification shows a reduction and a cathodic shift of the charge accumulation, following the observed change in photocurrent onset. By applying an electrochemical model, the values of the space charge width (5-10 nm) and of the hole diffusion length (0.5-1.5 nm) are extracted from photocurrent transients' amplitudes with the sacrificial agent. Characterization of the photovoltage transients also suggests the presence of surface states causing Fermi level pinning at small applied potential. The transient photovoltage and the use of both overlayers on the same electrode enable differentiation of the two overlayers' effects and a simplified model is proposed to explain the roles of each overlayer and their synergetic effects. This investigation demonstrates a new method to characterize water splitting photoelectrodes-especially the charge accumulation occurring at the semiconductor/electrolyte interface during operation. It finally confirms the requirements of nanostructuring and surface control with catalytic and trap passivation layers to improve iron oxide's performance for water photolysis

Controlling Photoactivity in Ultrathin Hematite Films for Solar Water-Splitting

A promising route to increase the performance of hematite (alpha-Fe2O3) photoelectrodes for solar hydrogen production through water-splitting is to use an extremely thin layer of this visible light absorber on a nanostructured scaffold. However, the typically poor performance of ultrathin (ca. 20 nm) films of hematite has been the limiting factor in implementing this approach. Here, the surprising effect of a substrate pretreatment using tetraethoxysilicate (TEOS) is reported; it results in drastic improvements in the photoperformance of 12.5 nm thick films of hematite. These films exhibit a water oxidation photocurrent onset potential at 1.1V versus the reversible hydrogen electrode (vs. RHE) and a plateau current of 0.63 mA cm(-2) at 1.5 V vs. RHE under standard illumination conditions, representing the highest reported performance for ultrathin hematite films. In contrast, almost no photoactivity is observed for the photoanode with the same amount of hematite on an untreated substrate. A detailed study of the effects of the TEOS treatment shows that a monolayer of SiOx is formed, which acts to change the hematite nucleation and growth mechanism, increases its crystallinity, reduces the concentration of carrier trapping states of the ultrathin films, and suggests its further application to quantum-dot and extremely-thin-absorber (ETA)-type solar cells

Artificial Photosynthesis with Semiconductor-Liquid Junctions

Given the urgent need to develop a sustainable, carbon neutral energy storage system on a global scale, intense efforts are currently underway to advance the field of artificial photosynthesis: i.e. solar fuel engineering. In this review we give an overview of the field of artificial photosynthesis using a semiconductor electrolyte interface employed in a photoelectrochemical device or as a heterogeneous photocatalyst. First we present a basic description of the operation principles of a semiconductor liquid junction based device. The role of nanotechnology in the recent advances in the field is highlighted and common material systems under current study are briefly reviewed. The importance of the material surfaces are further scrutinized by presenting recent advances in interfacial engineering. Technical challenges and an outlook towards industrialization of the technology are given

Solar Water Splitting: Progress Using Hematite (alpha-Fe2O3) Photoelectrodes

Photoelectrochemical (PEC) cells offer the ability to convert electromagnetic energy from our largest renewable source, the Sun, to stored chemical energy through the splitting of water into molecular oxygen and hydrogen. Hematite (alpha-Fe2O3) has emerged as a promising photo-electrode material due to its significant light absorption, chemical stability in aqueous environments, and ample abundance. However, its performance as a water-oxidizing photoanode has been crucially limited by poor optoelectronic properties that lead to both low light harvesting efficiencies and a large requisite overpotential for photoassisted water oxidation. Recently, the application of nanostructuring techniques and advanced interfacial engineering has afforded landmark improvements in the performance of hematite photoanodes. In this review, new insights into the basic material properties, the attractive aspects, and the challenges in using hematite for photoelectrochemical (PEC) water splitting are first examined. Next, recent progress enhancing the photocurrent by precise morphology control and reducing the overpotential with surface treatments are critically detailed and compared. The latest efforts using advanced characterization techniques, particularly electrochemical impedance spectroscopy, are finally presented. These methods help to define the obstacles that remain to be surmounted in order to fully exploit the potential of this promising material for solar energy conversion

Hematite Photoanodes for Solar Water Splitting: A Detailed Spectroelectrochemical Analysis on the pH-Dependent Performance

The effect of the electrolyte pH on the performance of metal oxide photoanodes for solar water oxidation has not been fully resolved, and contrasting views have been presented in recent reports. Herein, a comprehensive set of spectroelectrochemical techniques (impedance spectroscopy, intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS), and operando UV-vis spectroscopy) are deployed to clearly uncover the role of pH on the performance of hematite photoanodes. Our results reveal that, despite the presence of high-valent iron-oxo active sites over a wide pH range (7-13.6), the observed performance improvement with increasing pH is mainly driven by the reduction of surface accumulated charges, in the form of reactive intermediate species, that alleviates Fermi level pinning (FLP). Interestingly, IMPS data provides compelling evidence that the mitigation of FLP originates from changes in the reaction mechanism which boost the rate of charge transfer reducing, in turn, the surface charging. Additionally, we present a phenomenological analysis of the IMVS response which brings to light the additional impact of the electrolyte pH on the surface-related recombination dynamics. Our work identifies the pH-dependent kinetics of water oxidation as the key step governing the performance, defining not only the efficiency of charge transfer across the interface but also the degree of FLP that determines both the photocurrent magnitude and onset potential

Influence of Plasmonic Au Nanoparticles on the Photoactivity of Fe2O3 Electrodes for Water Splitting

An experimental study of the influence of gold nanoparticles on a alpha-Fe2O3 photoanodes for photoelectrochemical water splitting is described. A relative enhancement in the water splitting efficiency at photon frequencies corresponding to the plasmon resonance in gold was observed. This relative enhancement was observed only for electrode geometries with metal particles that were localized at the semiconductor-electrolyte interface consistent with the observation that minority carrier transport to the electrolyte is the most significant impediment to achieving high efficiencies in this system

The Transient Photocurrent and Photovoltage Behavior of a Hematite Photoanode under Working Conditions and the Influence of Surface Treatments

Hematite (α-Fe₂O₃) is widely recognized as a promising candidate for the production of solar fuels via water splitting, but its intrinsic optoelectronic properties have limited its performance to date. In particular, the large electrochemical overpotential required to drive the water oxidation is known as a major drawback. This overpotential (0.4 – 0.6 V anodic of the flat band potential) has been attributed to poor oxygen evolution reaction (OER) catalysis and to charge trapping in surface states but is still not fully understood. In the present study, we quantitatively investigate the photocurrent and photovoltage transient behavior of α-Fe₂O₃ photoanodes prepared by atmospheric pressure chemical vapor deposition, under light bias, in a standard electrolyte, and one containing a sacrificial agent. The accumulation of positive charges occurring in water at low bias potential is found to be maximum when the photocurrent onsets. The transient photocurrent behavior of a standard photoanode is compared to photoanodes modified by either a catalytic or surface passivating overlayer. Surface modification shows a reduction and a cathodic shift of the charge accumulation, following the observed change in photocurrent onset. By applying an electrochemical model, the values of the space charge width (5–10 nm) and of the hole diffusion length (0.5–1.5 nm) are extracted from photocurrent transients’ amplitudes with the sacrificial agent. Characterization of the photovoltage transients also suggests the presence of surface states causing Fermi level pinning at small applied potential. The transient photovoltage and the use of both overlayers on the same electrode enable differentiation of the two overlayers’ effects and a simplified model is proposed to explain the roles of each overlayer and their synergetic effects. This investigation demonstrates a new method to characterize water splitting photoelectrodesespecially the charge accumulation occurring at the semiconductor/electrolyte interface during operation. It finally confirms the requirements of nanostructuring and surface control with catalytic and trap passivation layers to improve iron oxide’s performance for water photolysis