Water Oxidation at Hematite Photoelectrodes: The Role of Surface States

The potential versus current state of water splitting with hematite

This review describes the potential of hematite as a photoanode material for photoelectrochemical (PEC) water splitting.

The pH dependence of OH radical formation in photo-electrochemical water oxidation with rutile TiO2 single crystals

The experimental results in photoelectrolysis with rutile (100) and (110) TiO₂ single crystals support a plausible reaction mechanism that the surface Ti–O–O–Ti structure is an intermediate of water oxidation process, by which mechanism the O₂ production becomes favorable in alkaline solution.

Can we judge an oxide by its cover? The case of platinum over α-Fe2O3 from first principles

Solar water splitting may be improved by reduced charge carrier mass in Fe₂O₃ covered with Pt.

Improvement of the electron collection efficiency in porous hematite using a thin iron oxide underlayer: towards efficient all-iron based photoelectrodes

By combining different iron oxide morphologies, the interfacial selectivity towards charge carriers generated in sol–gel hematite photoelectrodes is improved.

Diethylenetriamine-assisted hydrothermal synthesis of dodecahedral α-Fe2O3 nanocrystals with enhanced and stable photoelectrochemical activity

A facile diethylenetriamine-assisted protocol is developed to prepare dodecahedral α-Fe₂O₃ nanocrystals, which exhibit efficient photoelectrocatalytic water splitting activity.

Cubic In2O3 Microparticles for Efficient Photoelectrochemical Oxygen Evolution

Cubic In2O3 microparticles with exposed {001} facets as well as single morphology and size are produced on a large scale on silicon with a high yield. The morphological evolution during chemical vapor deposition is investigated and the new knowledge enables precise facet cutting. The synthesized Cubic In2O3 microparticles possess superior photoelectrocatalytic activity and excellent chemical and structural stability in oxygen evolution reaction on account of the unique surface structure and electronic band structure of the {001} facets. Our results reveal that it is feasible to promote the photolectrochemical water splitting efficiency of photoanode materials by controlling the growth on specific crystal facets. The technique and concept can be extended to other facet-specific materials in applications such as sensors, solar cells, and lithium batteries.

Nanowires for Photovoltaics and Artificial Photosynthesis

As the world's population grows and modernizes, developing inexpensive and efficient technologies for solar energy conversion is becoming increasingly important. Photovoltaics and artificial photosynthesis are two approaches for transforming solar energy into a usable form, either electricity or chemical fuels. While both technologies have been actively researched for decades, semiconductor nanowires possess unique properties that make them promising candidates for efficient photovoltaics and artificial photosynthesis. Because many optical and electronic processes occur over nanometer length scales, nanowires can offer improved capabilities to absorb light, collect photogenerated charges, and perform chemical reactions, functions that are all essential for solar energy conversion. Additionally, the increasing dexterity with which scientists synthesize, fabricate, and integrate nanoscale structures suggests that efficient devices that can take full advantage of these unique properties are not too far in the future.

Nano-sized quaternary CuGa2In3S8 as an efficient photocatalyst for solar hydrogen production

The synthesis of quaternary metal sulfide (QMS) nanocrystals is challenging because of the difficulty to control their stoichiometry and phase structure. Herein, quaternary CuGa2In3S8 photocatalysts with a primary particle size of ≈4 nm are synthesized using a facile hot-injection method by fine-tuning the sulfur source injection temperature and aging time. Characterization of the samples reveals that quaternary CuGa2In3S8 nanocrystals exhibit n-type semiconductor characteristics with a transition band gap of ≈1.8 eV. Their flatband potential is located at -0.56 V versus the standard hydrogen electrode at pH 6.0 and is shifted cathodically by 0.75 V in solutions with pH values greater than 12.0. Under optimized conditions, the 1.0 wt % Ru-loaded CuGa2In3S8 photocatalyst exhibits a photocatalytic H2 evolution response up to 700 nm and an apparent quantum efficiency of (6.9±0.5) % at 560 nm. These results indicate clearly that QMS nanocrystals have great potential as nano-photocatalysts for solar H2 production.

Modification of Hematite Electronic Properties with Trimethyl Aluminum to Enhance the Efficiency of Photoelectrodes

The electronic properties of hematite were investigated by means of synchrotron radiation photoemission (SR-PES) and X-ray absorption spectroscopy (XAS). Hematite samples were exposed to trimethyl aluminum (TMA) pulses, a widely used Al-precursor for the atomic layer deposition (ALD) of Al2O3. SR-PES and XAS showed that the electronic properties of hematite were modified by the interaction with TMA. In particular, the hybridization of O 2p states with Fe 3d and Fe 4s4p changed upon TMA pulses due to electron inclusion as polarons. The change of hybridization correlates with an enhancement of the photocurrent density due to water oxidation for the hematite electrodes. Such an enhancement has been associated with an improvement in charge carrier transport. Our findings open new perspectives for the understanding and utilization of electrode modifications by very thin ALD films and show that the interactions between metal precursors and substrates seem to be important factors in defining their electronic and photoelectrocatalytic properties.

Gradient FeO(x)(PO4)(y) layer on hematite photoanodes: novel structure for efficient light-driven water oxidation

Hematite has been receiving increasing attention for its application in photoelectrochemical (PEC) water oxidation but usually exhibits poor efficiency. We fabricated a stable gradient-structured FeOx(PO4)y layer on hematite by diffusively incorporating phosphate onto the surface layer of hematite films at a low temperature. X-ray photoelectron spectroscopy depth profile and Fe K-edge grazing-incidence X-ray absorption near-edge structure and extended X-ray absorption fine structure analysis demonstrated the formation of a ∼50 nm overlayer with a gradient phosphorus distribution and structural evolution from the outer surface to the depth. The as-prepared photoanodes showed highly improved PEC water oxidation performance. Up to 8.5-fold enhancement in the photocurrent density at 1.23 V versus reversible hydrogen electrode was achieved relative to the pristine anode. This strategy is applicable for hematite photoanodes prepared by different methods and with different morphologies and structures. The improvement in the water oxidation activity is mainly attributed to the enhanced separation of photogenerated electron-hole pairs, which is derived from the increased hole diffusion length in the gradient-structured overlayer. This work develops a simple and universal method to boost the PEC water oxidation efficiency with versatile hematite photoanodes.

Morphology-tunable synthesis of ZnO nanoforest and its photoelectrochemical performance

Understanding and manipulating synthesis reactions and crystal growth mechanisms are keys to designing and constructing the morphology and functional properties of advanced materials. Herein, the morphology-controlled synthesis of three-dimensional (3D) ZnO nanoforests is reported via a facile hydrothermal route. Specifically, the respective and synergistic influence of polyethylenimine (PEI) and ammonia on tuning the architecture of ZnO nanoforests is systematically studied. An in-depth understanding of the mechanism of hydrothermal growth is vital for advancing this facile approach and incorporating special 3D nanostructures into versatile nanomanufacturing. More importantly, its unique architectural characteristics endow the willow-like ZnO nanoforest with prominent photoelectrochemical water splitting performance, including small charge transfer resistance, long photoelectron lifetime, a high photocurrent density of 0.919 mA cm(-2) at +1.2 V (vs. Ag/AgCl), and more important, a high photoconversion efficiency of 0.299% at 0.89 V (vs. RHE), which leads the realm of homogeneous ZnO nanostructures. In all, it is expected that this work will open up an unprecedented avenue to govern desirable 3D ZnO nanostructures and broaden the application potentials of 3D nanotechnology.

Enhanced Water Splitting Efficiency Through Selective Surface State Removal

Hematite (α-Fe2O3) thin film electrodes prepared by atomic layer deposition (ALD) were employed to photocatalytically oxidize water under 1 sun illumination. It was shown that annealing at 800 °C substantially improves the water oxidation efficiency of the ultrathin film hematite electrodes. The effect of high temperature treatment is shown to remove one of two surface states identified, which reduces recombination and Fermi level pinning. Further modification with Co-Pi water oxidation catalyst resulted in unprecedented photocurrent onset potential of ∼0.6 V versus reversible hydrogen electrode (RHE; slightly positive of the flat band potential).

25th anniversary article: semiconductor nanowires--synthesis, characterization, and applications

Semiconductor nanowires (NWs) have been studied extensively for over two decades for their novel electronic, photonic, thermal, electrochemical and mechanical properties. This comprehensive review article summarizes major advances in the synthesis, characterization, and application of these materials in the past decade. Developments in the understanding of the fundamental principles of "bottom-up" growth mechanisms are presented, with an emphasis on rational control of the morphology, stoichiometry, and crystal structure of the materials. This is followed by a discussion of the application of nanowires in i) electronic, ii) sensor, iii) photonic, iv) thermoelectric, v) photovoltaic, vi) photoelectrochemical, vii) battery, viii) mechanical, and ix) biological applications. Throughout the discussion, a detailed explanation of the unique properties associated with the one-dimensional nanowire geometry will be presented, and the benefits of these properties for the various applications will be highlighted. The review concludes with a brief perspective on future research directions, and remaining barriers which must be overcome for the successful commercial application of these technologies.

Low-temperature activation of hematite nanowires for photoelectrochemical water oxidation

Hematite (α-Fe2 O3 ) nanostructures have been extensively studied as photoanode materials for photoelectrochemical (PEC) water oxidation. However, the photoactivity of pristine hematite nanostructures is fairly low and typically requires thermal activation at temperature of 650 °C or above. Here, we report a new method for enhancing the photocurrent of hematite nanowires at a substantially lower temperature of 350 °C by means of a two-step annealing process (activation process). Hydrothermally grown β-FeOOH nanowires were first annealed in a pure N2 environment at 350 °C to form magnetite, followed by partial oxidation in air to convert magnetite to hematite. During this process, Fe(2+) sites (oxygen vacancies) were intentionally created to increase the donor density and therefore the electrical conductivity of hematite. The oxygen-deficient hematite nanowire photoanode created at low temperature (350 °C) show considerably enhanced photoactivity compared to pristine hematite sample that prepared by thermal annealing of β-FeOOH nanowires at 550 °C in air. Moreover, this low-temperature annealing method can be coupled with an element doping method to further increase the photoactivity of hematite nanowire. Sn-doped hematite nanowires prepared by the same low-temperature annealing method show at least three fold enhanced photocurrent compared to the undoped sample. Significantly, the highest temperature in the entire annealing process was 350 °C, which is the lowest activation temperature ever reported for hematite nanowire photoanodes.

Influence of surface states on the evaluation of the flat band potential of TiO(2)

Flat band potential (Vfb) is one of the most important physical parameters to study and understand semiconductor materials. However, the influence of surface states on the evaluating Vfb of titanium oxide (TiO2) and other semiconductor materials through a Mott-Schottky plot is ignored. Our study indicated that the influence of surface states should be introduced into the corresponding equivalent circuit even when the kinetic process did not occur. Ignoring the influence of surface states would lead to an underestimation of the space charge capacitance. Our paper would be beneficial for accurate determination of Vfb of semiconductor materials. We anticipate that this preliminary study will open new perspectives in understanding the semiconductor-electrolyte interface.

Back electron-hole recombination in hematite photoanodes for water splitting

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.

Template-free synthesis of hematite photoanodes with nanostructured ATO conductive underlayer for PEC water splitting

Hematite is a promising semiconductor candidate for PEC water splitting. However, hematite is far well short of the theoretical value of solar-to-fuel conversion efficiency because of the fast recombination of photogenerated carriers. To address this limitation, a facile template-free preparation of hematite photoanode with nanostructured ATO (antimony-doped tin oxide) conductive underlayer served as a scaffold to transport photogenerated electron was developed to decrease the recombination opportunities of the carriers. Furthermore, the constructed ATO scaffold could also increase the light absorption of hematite and the number of the carriers, resulting in better PEC performance of hematite.

Effect of metal doping, doped structure, and annealing under argon on the properties of 30 nm thick ultrathin hematite photoanodes

The doping of the whole hematite layer with W (9.4%) and the additional doping of the bottom half of the W-doped hematite layer with Sn (8.6%), and the subsequent annealing under argon at 600 °C give rise to a large increase in current density by ∼8 times at 1.23 V vs. RHE, under 1 sun.

Constraints to the flat band potential of hematite photo-electrodes

Reasons are discussed for the widely disparate, reported flat band potentials for semiconducting hematite (photo-) electrodes in aqueous solutions.

Photoelectrochemical hydrogen production on α-Fe2O3 (0001): insights from theory and experiments

The photoelectrochemical (PEC) decomposition of organic compounds in wastewater is investigated by using quantum chemical (DFT) methods to evaluate alternatives to water splitting for the production of renewable and sustainable hydrogen. Methanol is used as a model organic species for the theoretical evaluations of electrolysis on the surface of the widely available semiconductor hematite, α-Fe2 O3 , a widely studied photocatalyst. Three different α-Fe2 O3 surface terminations were investigated, including the predominant surface found in aqueous electrolytes, (OH)3 R. The PEC oxidation of methanol is energetically downhill, producing CO2 and protons. The protons are reduced to hydrogen on the cathode. Experimental PEC measurements were also performed for several polyalcoholic compounds, glycerol, erythritol, and xylitol, on α-Fe2 O3 as the photocatalyst and showed high incident-photon-to-current-efficiencies (IPCE) that were much greater than those of water splitting. Interestingly, high IPCEs were observed for hydrogen production from polyalcohols in the absence of any applied bias, which was not thought to be possible on hematite. These results support the potential application of PEC for hydrogen production by using widely available hematite for the PEC oxidation of selected components of organic wastewater present in large quantities from anthropogenic and industrial sources.

Electrochemical properties of an AgInS2 photoanode prepared using ultrasonic-assisted chemical bath deposition

Electrochemical impedance analysis revealed the hole-transfer step taking place directly from the valence band of AgInS₂ photoanode to the electrolyte.

Identifying champion nanostructures for solar water-splitting

Charge transport in nanoparticle-based materials underlies many emerging energy-conversion technologies, yet assessing the impact of nanometre-scale structure on charge transport across micrometre-scale distances remains a challenge. Here we develop an approach for correlating the spatial distribution of crystalline and current-carrying domains in entire nanoparticle aggregates. We apply this approach to nanoparticle-based α-Fe₂O₃ electrodes that are of interest in solar-to-hydrogen energy conversion. In correlating structure and charge transport with nanometre resolution across micrometre-scale distances, we have identified the existence of champion nanoparticle aggregates that are most responsible for the high photoelectrochemical activity of the present electrodes. Indeed, when electrodes are fabricated with a high proportion of these champion nanostructures, the electrodes achieve the highest photocurrent of any metal oxide photoanode for photoelectrochemical water-splitting under 100 mW cm(-2) air mass 1.5 global sunlight.

Enhanced hematite water electrolysis using a 3D antimony-doped tin oxide electrode

We present herein an example of nanocrystalline antimony-doped tin oxide (nc-ATO) disordered macroporous "inverse opal" 3D electrodes as efficient charge-collecting support structures for the electrolysis of water using a hematite surface catalyst. The 3D macroporous structures were created via templating of polystyrene spheres, followed by infiltration of the desired precursor solution and annealing at high temperature. Using cyclic voltammetry and electrochemical impedance spectroscopy, it was determined that the use of this 3D transparent conducting oxide with a hematite surface catalyst allowed for a 7-fold increase in active surface area for water splitting with respect to its 2D planar counterpart. This ratio of surface areas was evaluated based on the presence of oxidized trap states on the hematite surface, as determined from the equivalent circuit analysis of the Nyquist plots. Furthermore, the presence of nc-ATO 2D and 3D "underlayer" structures with hematite deposited on top resulted in decreased charge transfer resistances and an increase in the number of available active surface sites at the semiconductor-liquid junction when compared to hematite films lacking any nc-ATO substructures. Finally, absorption, transmission, and reflectance spectra of all of the tested films were measured, suggesting the feasibility of using 3D disordered structures in photoelectrochemical reactions, due to the high absorption of photons by the surface catalyst material and trapping of light within the structure.

Metal Oxide Photoelectrodes for Solar Fuel Production, Surface Traps, and Catalysis

The photoelectrochemical reduction of water or CO2 is a promising route to sustainable solar fuels but hinges on the identification of a stable photoanode for water oxidation. Semiconductor oxides like Fe2O3 and BiVO4 have been gaining significant attention as promising materials. However, they exhibit a major drawback of a large required overpotential for solar water oxidation. In this Perspective, recent efforts to characterize and reduce the overpotential are critically examined. The accumulation of photogenerated holes at the semiconductor-liquid interface, recently observed with multiple techniques, is rationalized with surface state models. Transient absorption spectroscopy and electrochemical impedance spectroscopy suggest that surface treatments designed to either passivate surface traps or increase reaction rates (as catalysts) actually perform identically. This calls into question the definition of a catalyst when coupled to a semiconductor photoelectrode. In contrast, results from transient photocurrent spectroscopy suggest that two separate loss mechanisms are indeed occurring and can be addressed separately.