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.

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.

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 TiO 2 overlayers show no beneficial effect, we find that an ultra-thin coating of Al 2 O 3 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 À2 to 0.85 mA cm À2 ) at +1.0 V vs. the reversible hydrogen electrode (RHE) under standard illumination conditions. The subsequent addition of Co 2+ 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 À2 ). A detailed investigation into the effect of the Al 2 O 3 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.

Water oxidation is a key chemical reaction, central to both biological photosynthesis and artificial solar fuel synthesis strategies. Despite recent progress on the structure of the natural catalytic site, and on inorganic catalyst function, determining the mechanistic details of this multiredox reaction remains a significant challenge. We report herein a rate law analysis of the order of water oxidation as a function of surface hole density on a hematite photoanode employing photoinduced absorption spectroscopy. Our study reveals a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. This study thus provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.

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 to 1 s time scale, with the time scale 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 field is insufficient to prevent the subsequent recombination of these holes with electrons in the semiconductor bulk on a time scale 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.

We use transient absorption spectroscopy and photoelectrochemical methods to study the dynamics of photogenerated holes in BiVO4 for solar water oxidation. The back electron/hole recombination is found to be slow and therefore competes with water oxidation, limiting water oxidation efficiency.

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.

A promising route to increase the performance of hematite (α‐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 1.1 V 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.