Florian Le Formal

Imperial College London, Londinium, England, United Kingdom

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Publications (24)112.94 Total impact

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    ABSTRACT: The electrochemical behavior of α-Fe2O3 photoelectrodes prepared by spray pyrolysis with different thicknesses was examined under dark and illumination conditions. The main charge transport phenomena occurring in the PEC cell photoelectrodes were characterized by electrochemical impedance spectroscopy (EIS) operating under dark conditions. The impedance spectra were fitted to an equivalent electrical circuit model for obtaining relevant information concerning reaction kinetics and charge transfer phenomena occurring at the semiconductor/electrolyte interface. A three-electrode configuration was used to carry out the electrochemical measurements allowing a detailed study concerning the double charged layer at the semiconductor/electrolyte interface that arises under dark conditions. The model parameters determined by EIS were then related to the film thickness to assess the role of electronic conduction in the performance of the cell. Moreover, by correlating the sample thickness differences with their electrochemical impedance spectroscopy response, it was possible to discriminate the two main phenomena occurring on semiconductor/electrolyte interfaces of photoelectrochemical systems under dark conditions: the space charge layer and the electrical double layer.
    Physical Chemistry Chemical Physics 07/2014; · 3.83 Impact Factor
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    ABSTRACT: Transient absorption spectroscopy on sub-picosecond to second timescales is used to investigate photogenerated charge carrier recombination in Si-doped nanostructured hematite (α-Fe2O3) photoanodes as a function of applied bias. For unbiased hematite, this recombination exhibits a 50% decay time of ~6 ps, ~103 times faster than TiO2 under comparable conditions. Anodic bias significantly retards hematite recombination dynamics, and causes the appearance of electron trapping on ps-μs timescales. These ultra-fast recombination dynamics, their retardation by ap-plied bias and the associated electron trapping are discussed in terms of their implications for efficient water oxidation.
    Journal of the American Chemical Society 06/2014; · 10.68 Impact Factor
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    ABSTRACT: The kinetic competition between electron / hole recombination and water oxidation is a key consideration for the development of efficient photoanodes for solar driven water splitting. In this study, we employed three complementary techniques, transient absorption spectroscopy (TAS), transient photocurrent spectroscopy (TPC) and electrochemical impedance spectroscopy (EIS), to address this issue for one of the most widely studied photoanode systems: nanostructured hematite thin films. For the first time, we show a quantitative agreement between all three techniques. In particular all three methods show the presence of a recombination process on the 10 ms - 1 s time scale, with the time constant and yield of this loss process being dependent upon applied bias. From comparison of data between these techniques, we are able to assign this recombination phase to recombination of bulk hematite electrons with long-lived holes accumulated at the semiconductor / electrolyte interface. The data from all three techniques are shown to be consistent with a simple kinetic model based on competition between this, bias dependent, recombination pathway and water oxidation by these long-lived holes. Contrary to most existing models, this simple model does not require the consideration of surface states located energetically inside the band gap. These data suggest two distinct roles for the space charge layer developed at the semiconductor / electrolyte interface under anodic bias. Under modest anodic bias (just anodic of flatband), this space charge layer enables the spatial separation of initially generated electrons and holes following photon absorption, generating relatively long lived holes (milliseconds) at the semiconductor surface. However under such modest bias conditions, the energetic barrier generated by the space charge layer fields is insufficient to prevent the subsequent recombination of these holes with electrons in the semiconductor bulk on a timescale faster than water oxidation. Preventing this back electron / hole recombination requires the application of stronger anodic bias, and is a key reason why the onset potential for photocurrent generation in hematite photoanodes is typically ~ 500 mV anodic of flat band and therefore needs to be accounted for in electrode design for PEC water splitting.
    Journal of the American Chemical Society 01/2014; · 10.68 Impact Factor
  • Florian Le Formal, Kevin Sivula, Michael Grätzel
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    ABSTRACT: 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.
    The Journal of Physical Chemistry C. 12/2012; 116(51):26707–26720.
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    ABSTRACT: TiO2 nanorod arrays were prepared on top of a transparent conductive glass substrate covered with a thin TiO2 compact layer. Solid-state dye-sensitized solar cells (SSDSCs) were fabricated using these structured TiO2 films sensitized with C106 dye as a photoanode and 2,2′,7,7′-tetrakis-(N,N-dipmethoxyphenylamine) 9,9′-spirobifluorene (spiro-MeOTAD) as the organic hole-transporting material. Photovoltaic power conversion efficiency of 2.9% was obtained at full sunlight intensity. The electron lifetime as well as the electron diffusion coefficient in the device was determined by charge extraction, transient photovoltage decay, and open-circuit photovoltage decay experiments.
    The Journal of Physical Chemistry C. 01/2012; 116(5).
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    ABSTRACT: Hematite photoanodes for photoelectrochemical (PEC) water splitting are often fabricated as extremely-thin films to minimize charge recombination because of the short diffusion lengths of photoexcited carriers. However, poor crystallinity caused by structural interaction with a substrate negates the potential of ultrathin hematite photoanodes. This study demonstrates that ultrathin Ga2O3 underlayers, which were deposited on conducting substrates prior to hematite layers by atomic layer deposition, served as an isomorphic (corundum-type) structural template for ultrathin hematite and improved the photocurrent onset of PECwater splitting by 0.2 V. The benefit from Ga2O3 underlayers was most pronounced when the thickness of the underlayer was approximately 2 nm. Thinner underlayers did not work effectively as a template presumably because of insufficient crystallinity of the underlayer, while thicker ones diminished the PEC performance of hematite because the underlayer prevented electron injection from hematite to a conductive substrate due to the large conduction band offset. The enhancement of PEC performance by a Ga2O3 underlayer was more significant for thinner hematite layers owing to greater margins for improving the crystallinity of ultrathin hematite. It was confirmed that a Ga2O3 underlayer was applicable to a rough conducting substrate loaded with Sb-doped SnO2nanoparticles, improving the photocurrent by a factor of 1.4. Accordingly, a Ga2O3 underlayer could push forward the development of host–guest-type nanocomposites consisting of highly-rough substrates and extremely-thin hematite absorbers.
    Faraday Discussions 01/2012; 155:223. · 3.82 Impact Factor
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    ABSTRACT: Hematite photoanodes for photoelectrochemical (PEC) water splitting are often fabricated as extremely-thin films to minimize charge recombination because of the short diffusion lengths of photoexcited carriers. However, poor crystallinity caused by structural interaction with a substrate negates the potential of ultrathin hematite photoanodes. This study demonstrates that ultrathin Ga2O3 underlayers, which were deposited on conducting substrates prior to hematite layers by atomic layer deposition, served as an isomorphic (corundum-type) structural template for ultrathin hematite and improved the photocurrent onset of PEC water splitting by 0.2 V. The benefit from Ga2O3 underlayers was most pronounced when the thickness of the underlayer was approximately 2 nm. Thinner underlayers did not work effectively as a template presumably because of insufficient crystallinity of the underlayer, while thicker ones diminished the PEC performance of hematite because the underlayer prevented electron injection from hematite to a conductive substrate due to the large conduction band offset. The enhancement of PEC performance by a Ga2O3 underlayer was more significant for thinner hematite layers owing to greater margins for improving the crystallinity of ultrathin hematite. It was confirmed that a Ga2O3 underlayer was applicable to a rough conducting substrate loaded with Sb-doped SnO2 nanoparticles, improving the photocurrent by a factor of 1.4. Accordingly, a Ga2O3 underlayer could push forward the development of host-guest-type nanocomposites consisting of highly-rough substrates and extremely-thin hematite absorbers.
    Faraday Discussions 01/2012; 155:223-32; discussion 297-308. · 3.82 Impact Factor
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    ABSTRACT: An overview of a collaborative experimental and theoretical effort toward efficient hydrogen production via photoelectrochemical splitting of water into di-hydrogen and di-oxygen is presented here. We present state-of-the-art experimental studies using hematite and TiO(2) functionalized with gold nanoparticles as photoanode materials, and theoretical studies on electro and photo-catalysis of water on a range of metal oxide semiconductor materials, including recently developed implementation of self-interaction corrected energy functionals.
    Physical Chemistry Chemical Physics 11/2011; 14(1):49-70. · 3.83 Impact Factor
  • Energy & Environmental Science 07/2011; 4(7):2512. · 11.65 Impact Factor
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    ABSTRACT: Hematite is a promising material for inexpensive solar energy conversion via water splitting but has been limited by the large overpotential (0.5-0.6 V) that must be applied to afford high water oxidation photocurrent. This has conventionally been addressed by coating it with a catalyst to increase the kinetics of the oxygen evolution reaction. However, surface recombination at trapping states is also thought to be an important factor for the overpotential, and herein we investigate a strategy to passivate trapping states using conformal overlayers applied by atomic layer deposition. While TiO2 overlayers show no beneficial effect, we find that an ultra-thin coating of Al2O3 reduces the overpotential required with state-of-the-art nano-structured photo-anodes by as much as 100 mV and increases the photocurrent by a factor of 3.5 (from 0.24 mA cm(-2) to 0.85 mA cm(-2)) at + 1.0 V vs. the reversible hydrogen electrode (RHE) under standard illumination conditions. The subsequent addition of Co2+ ions as a catalyst further decreases the overpotential and leads to a record photocurrent density at 0.9 V vs. RHE (0.42 mA cm(-2)). A detailed investigation into the effect of the Al2O3 overlayer by electrochemical impedance and photoluminescence spectroscopy reveals a significant change in the surface capacitance and radiative recombination, respectively, which distinguishes the observed overpotential reduction from a catalytic effect and confirms the passivation of surface states. Importantly, this work clearly demonstrates that two distinct loss processes are occurring on the surface of high-performance hematite and suggests a viable route to individually address them.
    Chemical Science 03/2011; 2(4):737-743. · 8.31 Impact Factor
  • Kevin Sivula, Florian Le Formal, Michael Grätzel
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    ABSTRACT: 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 (α-Fe(2)O(3)) 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.
    ChemSusChem 03/2011; 4(4):432-49. · 7.48 Impact Factor
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    ABSTRACT: The electronic structure of hydrated SrTiO3 (001) surfaces is investigated using density-functional models. It is shown that adsorbed water molecules give rise to unoccupied electron states similar in localization and shape to wet-electron states recently reported for other oxide-water interfaces, and believed to serve as a preferred path for transfer of conduction electrons to the surface water molecules. Additionally, we found that chemisorbed water and hydrogen have donor levels in the band gap, and that chemisorbed hydrogen is oxidized and released in the presence of free holes. These gap states can serve as surface recombination centers in photoelectrochemical cells.
    Applied Physics Letters 01/2011; 98(1):012106-012106-3. · 3.79 Impact Factor
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    ABSTRACT: An experimental study of the influence of gold nanoparticles on α-Fe(2)O(3) 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.
    Nano Letters 01/2011; 11(1):35-43. · 13.03 Impact Factor
  • Chem. Sci. 01/2011; 2(4):737-743.
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    ABSTRACT: Sustainable hydrogen production through photoelectrochemical water splitting using hematite (alpha-Fe(2)O(3)) is a promising approach for the chemical storage of solar energy, but is complicated by the material's nonoptimal optoelectronic properties. Nanostructuring approaches have been shown to increase the performance of hematite, but the ideal nanostructure giving high efficiencies for all absorbed light wavelengths remains elusive. Here, we report for the first time mesoporous hematite photoelectodes prepared by a solution-based colloidal method which yield water-splitting photocurrents of 0.56 mA cm(-2) under standard conditions (AM 1.5G 100 mW cm(-2), 1.23 V vs reversible hydrogen electrode, RHE) and over 1.0 mA cm(-2) before the dark current onset (1.55 V vs RHE). The sintering temperature is found to increase the average particle size, and have a drastic effect on the photoactivity. X-ray photoelectron spectroscopy and magnetic measurements using a SQUID magnetometer link this effect to the diffusion and incorporation of dopant atoms from the transparent conducting substrate. In addition, examining the optical properties of the films reveals a considerable change in the absorption coefficient and onset properties, critical aspects for hematite as a solar energy converter, as a function of the sintering temperature. A detailed investigation into hematite's crystal structure using powder X-ray diffraction with Rietveld refinement to account for these effects correlates an increase in a C(3v)-type crystal lattice distortion to the improved optical properties.
    Journal of the American Chemical Society 06/2010; 132(21):7436-44. · 10.68 Impact Factor
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    ABSTRACT: Multi-walled carbon nanotubes functionalized by carboxylic groups, exhibit better affinity towards TiO2 (P90, Degussa) as compared to that of pristine nanotubes. Also the electrochemical performance of TiO2 is improved by nanotube networking, but the Li-storage capacity of TiO2 is unchanged. Whereas the composite of TiO2 with non-functionalized nanotubes demonstrates simple superposition of the behavior of pure components, the composite with functionalized nanotubes shows unique faradaic pseudocapacitance which is specific for this composite only. The surface functionalization of nanotubes enhances charge storage capacity and reversibility of a composite with LiMnPO4 (olivine), but mediates also the electrolyte breakdown at potentials >4.2 V. Whereas the electrochemical activation of LiMnPO4 (olivine) by functionalized nanotubes is quite modest, excellent performance was found for LiFePO4 (olivine) in composite materials containing only 2 wt% of functionalized nanotubes. (C) 2010 Elsevier B.V. All rights reserved.
    01/2010;
  • Florian Le Formal, Michael Graetzel, Kevin Sivula
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    ABSTRACT: 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.
    Advanced Functional Materials 01/2010; · 9.77 Impact Factor
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    ABSTRACT: A ruthenium sensitizer (coded C101, NaRu (4,4'-bis(5-hexylthiophen-2-yl)-2,2'-bipyridine) (4-carboxylic acid-4'-caboxylate-2,2'-bipyridine) (NCS)(2)) containing a hexylthiophene-conjugated bipyridyl group as an ancillary ligand is presented for use in solid-state dye-sensitized solar cells (SSDSCs). The high molar. extinction coefficient of this dye is advantageous compared to the widely used Z907 dye, (NaRu (4-carboxylic acid-4'-carboxylate) (4,4'-dinonyl-2,2'-bipyridine) (NCS)(2)). In combination with an organic hole-transporting material (spiro-MeOTAD, 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine) 9, 9'-spirobifluorene), the C101 sensitizer exhibits an excellent power-conversion efficiency of 4.5% under AM 1.5 solar (100 mW cm(-2)) irradiation in a SSDSC. From electronic-absorption, transient-photovoltage-decay, and impedance measurements it is inferred that extending the pi-conjugation of spectator ligands induces an enhanced light harvesting and retards the charge recombination, thus favoring the photovoltaic performance of a SSDSC.,
    Advanced Functional Materials 01/2010; · 9.77 Impact Factor
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    ABSTRACT: 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 parts.
    Journal of Materials Research 01/2010; 25(1):17-24. · 1.82 Impact Factor
  • Kevin Sivula, Florian Le Formal, Michael Grätzel
    Chemistry of Materials - CHEM MATER. 01/2009; 21(13):2862-2867.