On the Theoretical and Experimental Control of Defect Chemistry and Electrical and Photoelectrochemical Properties of Hematite Nanostructures

Facile Interfacial Reduction Suppresses Redox Chemical Expansion and Promotes the Polaronic to Ionic Transition in Mixed Conducting (Pr,Ce)O2-δ Nanoparticles

Mixed ionic/electronic conductors (MIECs) are essential components of solid-state electrochemical devices, such as solid oxide fuel/electrolysis cells. For efficient performance, MIECs are typically nanostructured, to enhance the reaction kinetics. However, the effect of nanostructuring on MIEC chemo-mechanical coupling and transport properties, which also impact cell durability and efficiency, has not yet been well understood. In this work, Pr0.2Ce0.8O2-δ (PCO20) nanopowders were prepared by coprecipitation, then sintered in a modified dilatometer at three different temperatures (600, 725, and 850 °C) for microstructure evolution, resulting in three samples with different average particle sizes (23, 30, and 53 nm). The chemical strain and electronic/ionic conductivity were then measured simultaneously on stable nanostructures in four isotherms from 550 to 400 °C with steps in pO2 (1 to 10-4 atm O2). A microcrystalline bar was prepared and measured for comparison. Particle size reduction led to a monotonically decreasing isothermal redox chemical strain, confirmed by in situ high-temperature, controlled-atmosphere XRD measurements. The corresponding conductivity measurements provided defect chemical insight into the particle size-dependent chemical expansion behavior. The significant weakening of the pO2 dependence and decreased activation energy for electrical conduction with decreasing particle size indicated a decrease in the reduction enthalpy of PCO, shifting the transition from (Pr) polaronic to ionic behavior to higher pO2. STEM-EELS measurements confirmed the majority of Pr was reduced to 3+ in the nanoparticles, while Ce remained 4+. These results demonstrate suppression of deleterious chemical expansion and tailoring of the dominant charge carrier simply through controlling the particle size, providing insights for MIEC microstructural design.

Revisiting strategies to improve the performance of hematite photoanodes for water photoelectrolysis

Photoelectrochemical (PEC) water splitting is emerging as a sustainable approach for producing green hydrogen. The design of efficient photoanodes is the key for the step forward technological application in which morphology optimization and defect engineering play central roles. In this perspective, the intricate interplay between morphology optimization, band engineering and chemical modifications is critically discussed. First, a brief introduction of the relevant aspects of semiconductors applied in PEC devices is provided. Then, a critical analysis of the influence of morphology and chemical modifications is presented, with hematite employed as a model system. Lastly, insights into future directions and outlooks for existing challenges on the development of photoelectrochemical devices for water splitting are displayed.

Self-Diffusion versus Intentional Doping: Beneficial and Damaging Impact on Hematite Photoanode Interfaces

The comprehension of side effects caused by high-temperature thermal treatments in the design of (photo)electrodes is essential to achieve efficient and cost-effective devices for solar water splitting. This investigation explores the beneficial and damaging impacts of thermal treatments in the (photo)electrode design, unraveling the impact of self-diffusion and its consequences. The industrial-friendly polymeric precursor synthesis (PPS) method, which is known for its easy technological application, was chosen as the fabrication technique for hematite photoabsorbers. For substrate evaluation, two types of conductive glass substrates, aluminum borosilicate and quartz, both coated with fluorine-doped tin oxide (ABS/FTO and QTZ/FTO, respectively), were subjected to thermal treatments following the PPS protocol. Optical and structural analyses showed no significant alterations in substrate properties, whereas X-ray photoelectron spectroscopy (XPS) revealed the migration of silicon and calcium ions from the glass component to the FTO surface. This diffusion can be further mitigated by an oxide buffer layer. To track the potential ion diffusion on the photoabsorber surface and assess its effect on the photoelectrode performance, hematite was selected as the model material and deposited onto the glass substrates. From all the ions that could possibly migrate, only Si4+ and Ca2+ originating from the glass component, as well as Sn4+ from the fluorine-doped tin oxide (FTO), were detected on the surface of the hematite photoabsorber. Interestingly, the so-called "self-diffusion" of these ions did not result in any beneficial effect on the hematite photoelectrochemical response. Instead, intentional modifications showed more substantial impacts on the photoelectrochemical efficiency compared to unintentional self-diffusion. Therefore, "self-diffusion", which can unintentionally dope the hematite, is not sufficient to significantly impact the final photocurrent. These findings emphasize the importance of understanding the true effect of thermal treatments on the photoelectrode properties to unlock their full potential in photoelectrochemical applications.

Advances in Engineered Metal Oxide Thin Films by Low-Cost, Solution-Based Techniques for Green Hydrogen Production

Functional oxide materials have become crucial in the continuous development of various fields, including those for energy applications. In this aspect, the synthesis of nanomaterials for low-cost green hydrogen production represents a huge challenge that needs to be overcome to move toward the next generation of efficient systems and devices. This perspective presents a critical assessment of hydrothermal and polymeric precursor methods as potential approaches to designing photoelectrodes for future industrial implementation. The main conditions that can affect the photoanode's physical and chemical characteristics, such as morphology, particle size, defects chemistry, dimensionality, and crystal orientation, and how they influence the photoelectrochemical performance are highlighted in this report. Strategies to tune and engineer photoelectrode and an outlook for developing efficient solar-to-hydrogen conversion using an inexpensive and stable material will also be addressed.

Effect of preparation conditions and Co–Pi groups as a noble metal-free redox mediator and hole extractor to boost the photoelectrochemical water oxidation for 1D nanorod α-Fe2O3

Co–Pi groups have a double effect on the short nanorod α-Fe2O3 photoanode: accelerating the hole separation and improving the water oxidation kinetics.

A complete ab initio thermodynamic and kinetic catalogue of the defect chemistry of hematite α-Fe2O3, its cation diffusion, and sample donor dopants

This paper studies comprehensively the defect chemistry of and cation diffusion in α-Fe₂O₃. Defect formation energies and migration barriers are calculated using density functional theory with a theoretically calibrated Hubbard U correction. The established model shows a good agreement with experimental off-stoichiometry and cation diffusivities available in the literature. At any temperature, and are the predominant ionic defects in hematite at the two extremes of oxygen partial pressure (pO₂) range, reducing and oxidizing, respectively. Between these two extremes, an intrinsic electronic regime exists where small polaronic electrons and holes are the dominant charge carriers. The calculated migration barriers show that Fe ions favor the diffusion along the 〈111〉 direction in the primitive cell through an interstitial crowdion-like mechanism. Our model suggests that cation diffusion in hematite is mainly controlled by the migration of , while may contribute to cation diffusion at extremely low pO₂. Our analysis in the presence of two sample donor dopants Ti and Sn indicates that high temperature annealing at T > 1100 K is needed to prepare n-type hematite at ambient pO₂, consistently with prior experimental findings. Alternatively, annealing at lower temperatures requires much lower pO₂ to avoid compensating the donors with Fe vacancies. A synergistic comparison of our theoretical model and the experimental results on Ti-doped hematite led us to propose that free electrons and small polarons coexist and both contribute to n-type conductivity. Our validated model of defective hematite is a foundation to study hematite in applications such as corrosion and water splitting.

The role of carbon dots - derived underlayer in hematite photoanodes

Hematite is a promising candidate as photoanode for solar-driven water splitting, with a theoretically predicted maximum solar-to-hydrogen conversion efficiency of ∼16%. However, the interfacial charge transfer and recombination greatly limits its activity for photoelectrochemical water splitting. Carbon dots exhibit great potential in photoelectrochemical water splitting for solar to hydrogen conversion as photosensitisers and co-catalysts. Here we developed a novel carbon underlayer from low-cost and environmental-friendly carbon dots through a facile hydrothermal process, introduced between the fluorine-doped tin oxide conducting substrate and hematite photoanodes. This led to a remarkable enhancement in the photocurrent density. Owing to the triple functional role of carbon dots underlayer in improving the interfacial properties of FTO/hematite and providing carbon source for the overlayer as well as the change in the iron oxidation state, the bulk and interfacial charge transfer dynamics of hematite are significantly enhanced, and consequently led to a remarkable enhancement in the photocurrent density. The results revealed a substantial improvement in the charge transfer rate, yielding a charge transfer efficiency of up to 80% at 1.25 V vs. RHE. In addition, a significant enhancement in the lifetime of photogenerated electrons and an increased carrier density were observed for the hematite photoanodes modified with a carbon underlayer, confirming that the use of sustainable carbon nanomaterials is an effective strategy to boost the photoelectrochemical performance of semiconductors for energy conversion.

Engineered Sn- and Mg-doped hematite photoanodes for efficient photoelectrochemical water oxidation

A feasible and cost-effective method was developed to improve the photoelectrochemical performance of the hematite (α-Fe2O3) photoanode. Using a hydrothermal method, tin (Sn) and magnesium (Mg) (co-)doped hematite films were prepared and characterized by X-ray diffraction (XRD), X-ray photon spectroscopy (XPS), and Raman spectroscopy. The average particle size of the α-Fe2O3 film varied from 150 to 300 nm. The photocurrent density of Sn-/Mg-co-doped α-Fe2O3 reached a maximum of 1.1 mA cm-2 at 1.23 VRHE, which increased approximately 3 times compared to that of pristine α-Fe2O3. It also yielded a maximum applied bias photon-to-current efficiency (ABPE) of 0.09% at 1.08 V vs. RHE. The excellent PEC activity could be attributed to Mg co-doping relieving the lattice distortion caused by Sn doping, and improving both the charge injection efficiency and charge separation efficiency without obviously changing the carrier concentration, which was proved by electrochemical impedance spectroscopy. This promising co-doping strategy could also be extended to other candidatephotoelectrodes.

Synergistic effects of dopant (Ti or Sn) and oxygen vacancy on the electronic properties of hematite: a DFT investigation

Hematite has been widely studied as one of the most promising photoanodes in the photoelectrochemical decomposition of water. At present, a prevailing strategy of coupling dopant (Ti or Sn) with oxygen vacancies has been proposed by experiment, and effectively improves the photocatalytic activity. In order to clarify the intrinsic reasons for the improvement of the photochemical activity, density functional theory is adopted to calculate the formation mechanism and electronic properties of hematite with doping ions and oxygen vacancies. The result shows that the doped atom is beneficial to the formation of oxygen vacancies in hematite, thus forming a stable structure containing doping ions and oxygen vacancies. Due to the synergistic effects of dopant and oxygen vacancies, the bandgap of hematite decreases, and donor levels are introduced into the bandgap, which lead to the increase of carrier concentration. In the system with doped Ti and oxygen vacancies, donor levels are introduced at 1.47 eV and 1.73 eV below the bottom of the conduction band, respectively. For the case containing Sn and oxygen vacancies, the donor level is introduced at 1.75 eV from the conduction band minimum. Our results elaborate the reasons for the enhancement of carrier densities in terms of electronic structure, and provide some guidance for the future modification of photocatalysts.

Hematite has been widely studied as one of the most promising photoanodes in the photoelectrochemical decomposition of water.

Tuning metal oxide defect chemistry by thermochemical quenching

The low-temperature defect chemistry of monoclinic and tetragonal ZrO₂ and hematite Fe₂O₃ is studied in the non-equilibrium state of thermochemical quenching; that is, rapid cooling starting from a certain high temperature and oxygen chemical potential. This non-equilibrium state is of great interest because many metal oxides are used at low temperatures below their growth temperatures. This paper addresses the importance of considering this non-equilibrium state rather than applying equilibrium thermodynamics as commonly used when studying point defects from first principles. Based on point defect formation energies calculated previously using density functional theory, we compare the type of dominant defects at equilibrium to those at a quenched state originating from a certain initial growth temperature and oxygen partial pressure. The comparison is facilitated by casting the dominant defects in a dominance diagram on the temperature - oxygen partial pressure plane. We consider two scenarios to model the quenched state. In the first, only electronic defects equilibrate whereas all ionic defects are frozen. Whereas, in the second, electronic defects and interstitials are allowed to equilibrate under the assumption of mobile interstitials at low temperatures. We find that new ionic charge compensation modes can appear on the dominance diagram after quenching. Additionally, purely ionic charge compensation modes consisting of vacancies and/or interstitials expand in the dominance diagram at the expense of purely electronic compensation modes. For the ZrO₂ phases, we argue that scenario 2 is more realistic and leads to difficulty in achieving n-type doping by thermochemical quenching. For Fe₂O₃, and regardless of the quenching scenario, iron vacancies occupy a wider zone of domination, which limits the performance of this oxide as a water splitting photoanode. Our study shows that by controlling the growth thermochemical conditions, it is possible to tune the Fermi level of oxides over a considerable range within the band gap by quenching. This provides an extra tool to tune the electric conductivity of metal oxides beyond traditional extrinsic doping. This work indicates that non-equilibrium thermodynamic analysis is necessary to understand and control defect chemistry at low temperatures.

FeO-Based Hierarchical Structures on FTO Substrates and Their Photocurrent

As one of the most promising photoanode materials for photoelectrochemical (PEC) water oxidation, earth-abundant hematite has been severely restricted by its poor electrical conductivity, poor charge separation, and sluggish oxygen evolution reaction kinetics. FeO has an ability to produce hydrogen, while its preparation needs high temperature to reduce Fe3+ to Fe2+ by using H2 or CO gases. Here, Fe2O3- and FeO-based nanorods (NRs) on fluorine-doped tin oxide (FTO) substrate have been prepared, where the latter was obtained by doping Sn4+ ions in FeOOH to reduce Fe3+ ions to Fe2+. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements indicate that the dominant content of Fe element on the surface of Sn-doped Fe2O3 and Sn-FeOOH samples is Fe2+. FeO-based NRs have a Fe3O2/FeO heterostructure with some SnO2 nanoparticles distributed on their surface. These prepared samples were used as PEC photoanodes under a visible-light irradiation. The results showed that the modified FeO-based NRs have a photocurrent density of 0.2 mA cm-2 at 1.23 V vs reference hydrogen electrode (RHE) using Hg/HgO electrode as the reference electrode. Furthermore, they also have a better photocatalytic hydrogen evolution activity with a rate of 2.3 μmol h-1 cm-1. The improved photocurrent and photocatalytic activity can be ascribed to the Sn-dopant, as the introduction of Sn4+ not only leads to the formation of the Fe3O2/FeO heterostructure but also increases the carrier concentration. Fe3O2/FeO heterostructure with SnO2 nanoparticles on its surface has a good band energy alignment, which is beneficial to the PEC water oxidation and reduction.