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Typical low-temperature frustrated triangular antiferromagnet CuFeO2 is attracting extensive interest due to its narrow-band-gap semiconductor properties. High-quality and impurity-free CuFeO2 epitaxial thin films would be preferable for fundamental studies on the physical and chemical properties. However, the heteroepitaxial growth of impurity-free CuFeO2 thin films has been a significant challenge due to its narrow formation window in the Cu–Fe–O system as well as the metastable nature of the Cu1+ cations. This work reports for the first time the fabrication and characterization of high-quality and impurity-free (00.1)-oriented CuFeO2 epitaxial thin films grown with relaxed interfaces on (00.1) sapphire substrates by pulsed laser deposition. Below the critical thickness of around 16 nm, the films exhibit a rhombohedral structure with relatively good crystalline quality where all Cu ions appear to be in the 1+ oxidation state, while the rocking curves display a narrow full width at half maximum of about 0.11°. Increasing the thickness, the (111)-oriented γ-Fe2O3 nanograins grow embedded in the CuFeO2 films. Here, an excess Fe3+-assisted growth mechanism is proposed to explain the iron oxide grain formation. This study provides insight into the heteroepitaxial growth of relaxed CuFeO2 thin films with high purity and crystalline quality as an ideal sample design to characterize the fundamental properties of this material in view of potential device applications. [ABSTRACT FROM AUTHOR]

Nanoscale thin films are widely implemented across a plethora of technological and scientific areas, and form the basis for many advancements that have driven human progress, owing to the high degree of functional tunability based on the chemical composition. Pulsed laser deposition is one of the multiple physical vapour deposition routes to fabricate thin films, employing laser energy to eject material from a target in the form of a plasma. A substrate, commonly a single-crystal oxide, is placed in the path of the plume and acts as a template for the arriving species from the target to coalesce and self-assemble into a thin film. This technique is tremendously useful to produce crystalline films, due to the wide range of atmospheric conditions and the extent of possible chemical complexity of the target. However, this flexibility results in a high degree of complexity, oftentimes requiring rigorous optimisation of the growth parameters to achieve high quality crystalline films with desired composition. In this tutorial review, we aim to reduce the complexity and the barrier to entry for the controlled growth of complex oxides by pulsed laser deposition. We present an overview of the fundamental and practical aspects of pulsed laser deposition, discuss the consequences of tailoring the growth parameters on the thin film properties, and describe in situ monitoring techniques that are useful in gaining a deeper understanding of the properties of the resultant films. Particular emphasis is placed on the general relationships between the growth parameters and the consequent structural, chemical and functional properties of the thin films. In the final section, we discuss the open questions within the field and possible directions to further expand the utility of pulsed laser deposition. [ABSTRACT FROM AUTHOR]

Strain engineering is the art of inducing controlled lattice distortions in a material to modify specific physicochemical properties. Strain engineering is applied not only for basic fundamental studies of physics and chemistry of solids but also for device fabrication through the development of materials with new functionalities. Thin films are one of the most important tools for strain engineering. Thin films can in fact develop large strain due to the crystalline constraints at the interface with the substrate and/or as a result of specific morphological features that can be selected by an appropriate tuning of the deposition parameters. Within this context, the in situ measurement of the substrate curvature is a powerful diagnostic tool allowing real time monitoring of the stress state of the growing film. This paper reviews a few recent applications of this technique and presents new measurements that point out the great potentials of the substrate curvature measurement in strain engineering. Our study also shows how, due to the high sensitivity of the technique, the correct interpretation of the results can be in certain cases not trivial and require complementary characterizations and an accurate knowledge of the physicochemical properties of the materials under investigation. [ABSTRACT FROM AUTHOR]

The conversion of solar light into hydrogen by photoelectrochemical water splitting is one of the potential strategies that can allow the development of a carbon‐neutral energy cycle. Oxynitride semiconductors are promising materials for this application, although important limitations must still to be addressed. One of the most important issues is physicochemical degradation of the semiconductor, at the interface with water, where the electrochemical reactions occur. In this regard, thin films, with well‐defined and atomically flat surfaces, are invaluable tools for characterizing material properties and degradation mechanisms, while identifying strategies to mitigate detrimental effects. Thin oxynitride films may allow the use of complementary characterizations, not applicable to conventional powder samples. In particular, the study of the solid–liquid interface can benefit enormously from the use of thin films for synchrotron‐based surface‐sensitive X‐Ray scattering methods and neutron reflectometry. These investigation approaches promise to speed up the design and discovery of new materials for the production of solar fuels, while paving the way for similar applications in other research fields. This work aims at reviewing the literature contributions on oxynitride thin films for solar water splitting summarizing what is learnt so far and suggesting experimental strategies to unveil what is still not clear. [ABSTRACT FROM AUTHOR]

Oxynitrides have been shown to be promising visible light water splitting photocatalysts, but rapidly degrade under operating conditions. With a custom designed photoelectrochemical cell, we perform operando grazing incidence X-ray absorption spectroscopy measurements on the oxynitride semiconductor SrTaOxNy during photocatalytic solar water splitting. We show that the nature of the A-site (Sr) and its evolution during operation have large impacts on the overall stability and catalytic activity of the material, leading to an enriched BO2 (Ta(OH)/TaO(OH)) like surface. However, this usually beneficial effect with respect to increased surface hydrophilicity has complications for the efficiency of the photocatalytic process, as the OH and O(OH) intermediates formed are in competition between O2 generation and NOx species formation in the initial stages of operation. Operando characterisation of the evolution of the electronic structure of the photocatalyst proves to be an invaluable tool for the rational design and discovery of new and better performing materials. [ABSTRACT FROM AUTHOR]

Understanding the mechanism of the oxygen evolution reaction (OER) on perovskite materials is of great interest for the development of more active catalysts. Despite a lot of literature reports, the complexity of catalytic systems and scarce in situ and operando surface sensitive spectroscopic tools render the detection of active sites and the understanding of reaction mechanisms challenging. Here, we carried out and compared in situ and ex situ ambient pressure X-ray photoelectron spectroscopy experiments on a La0.2Sr0.8CoO3−δ perovskite OER catalyst. The experimental results show that segregated surface strontium, which is present in the as prepared sample, is leached into the electrolyte after immersion, leading to surface cobalt active site enrichment. Such a cobalt-enriched oxide surface evolves into a new phase, whose spectral feature is detected in situ and after the OER. With the help of theoretical simulations, such a species is assigned to cobalt oxyhydroxide, providing direct evidence of its formation and surface segregation during the oxygen evolution reaction. [ABSTRACT FROM AUTHOR]

Pulsed reactive crossed-beam laser ablation is an effective technique to govern the chemical activity of plasma species and background molecules during pulsed laser deposition. Instead of using a constant background pressure, a gas pulse with a reactive gas, synchronized with the laser beam, is injected into vacuum or a low background pressure near the ablated area of the target. It intercepts the initially generated plasma plume, thereby enhancing the physicochemical interactions between the gaseous environment and the plasma species. For this study, kinetic energy resolved mass-spectrometry and time-resolved plasma imaging were used to study the physicochemical processes occurring during the reactive crossed beam laser ablation of a partially 18O substituted La0.6Sr0.4MnO3 target using oxygen as gas pulse. The characteristics of the ablated plasma are compared with those observed during pulsed laser deposition in different oxygen background pressures. [ABSTRACT FROM AUTHOR]

Heteroepitaxy of complex oxide thin films is a significant challenge when a large mismatch in the lattice parameters (>8%) and difference in the crystallographic symmetry coexist between the film and substrate. Herein, the heteroepitaxial growth of a hexagonal delafossite CuFeO2 thin film with (00.1) orientation on a cubic perovskite (001) SrTiO3 substrate through translational and rotational domain matching epitaxy is reported. The rotational in‐plane domain orientation relationships are CuFeO2 [11.0]//SrTiO3 [110] and CuFeO2 [21¯.0]//SrTiO3 [110] with about 10% in‐plane lattice mismatch. The 14.8 nm‐thick (00.1) CuFeO2 thin film shows high‐crystalline quality with a full width at half maximum of rocking curve of about 0.24° and exhibits a possible indirect optical bandgap of 1.43 eV or direct optical bandgap of 1.94 eV. Herein, not only a model system demonstrating translational and rotational domain matching heteroepitaxy of complex oxides is reported, but also a way to thin‐film heterostructures integrating hexagonal delafossite with cubic perovskite materials for functional oxide devices is opened. [ABSTRACT FROM AUTHOR]

All‐solid‐state lithium ion batteries (LIB) are currently the most promising technology for next generation electrochemical energy storage. Many efforts have been devoted in the past years to improve performance and safety of these devices. Nevertheless, issues regarding chemical and mechanical stability of the different components still hinder substantial improvements. Pulsed laser deposition (PLD) has proved to be an outstanding technique for the deposition of thin films of materials of interest for the fabrication of LIB. Thanks to its versatility and possible fine tuning of the thin film properties, PLD promises to be a very powerful tool for the fabrication of model systems which would allow to study in detail material properties and mechanisms contributing to LIB degradation. Nevertheless, PLD presents difficulties in the deposition of LIB components, mainly due to the presence of elements with large difference of atomic mass in their chemical composition. In this review, we report the main challenges and solution strategies used for the deposition through PLD of complex oxides thin films for LIB. [ABSTRACT FROM AUTHOR]

Photocharge extraction in the visible-light energy range is essential to sustain efficient photoelectrochemical processes driven by solar irradiation of photoanode materials. Recent literature has shown that crystallographic orientation can have a significant impact on the extraction of photocharges, though the difficulty of discriminating bulk and surface orientation effects on charge extraction often complicates any understanding as to the role each orientation may play. Our catalyst-decorated thin-film model system allows us to cleanly separate charge extraction limiting effects of bulk vs. surface orientation. As a result, our study reveals an anisotropic visible-light induced photocharge extraction dependence on the bulk crystallographic orientation in LaTiO2N (LTON) photoanodes. In particular, the visible-light induced photocharge extraction is 30% higher for (011) films than (001) oriented thin films. Computational analysis of the LTON band diagram suggests that this is due to a higher potential energy and photocharge mobility along the (011) direction. [ABSTRACT FROM AUTHOR]

This work reveals that photoanodes based on TiNb2O7 (TNO) powder show remarkable water‐oxidation properties including nearly ideal charge‐transfer and charge‐injection efficiencies. Furthermore, using a simplified photoanode construction and carefully surveying the structural and morphological characteristics of oriented and polycrystalline thin films and powder‐based samples revealed that the water‐splitting kinetics of TNO is negligibly effected by surface morphology; instead, internal grain boundaries likely play a driving role. The current powder‐based TNO photoanodes exhibit ideal water‐oxidation kinetics and oxidize water at minimal applied biases under illumination; consequently, TNO exhibits an early onset photocurrent voltage (0.4 V vs. RHE) that rivals that of other state‐of‐the‐art photoanode materials. [ABSTRACT FROM AUTHOR]

Photoelectrochemical (PEC) water splitting is recognized as a sustainable strategy for hydrogen generation due to its abundant hydrogen source, utilization of inexhaustible solar energy, high‐purity product, and environment‐friendly process. To actualize a practical PEC water splitting, it is paramount to develop efficient, stable, safe, and low‐cost photoelectrode materials. Recently, graphitic carbon nitride (g‐C3N4) has aroused a great interest in the new generation photoelectrode materials because of its unique features, such as suitable band structure for water splitting, a certain range of visible light absorption, nontoxicity, and good stability. Some inherent defects of g‐C3N4, however, seriously impair further improvement on PEC performance, including low electronic conductivity, high recombination rate of photogenerated charges, and limited visible light absorption at long wavelength range. Construction of g‐C3N4‐based nanosized heteroarrays as photoelectrodes has been regarded as a promising strategy to circumvent these inherent limitations and achieve the high‐performance PEC water splitting due to the accelerated exciton separation and the reduced combination of photogenerated electrons/holes. Herein, we summarize in detail the latest progress of g‐C3N4‐based nanosized heteroarrays in PEC water‐splitting photoelectrodes. Firstly, the unique advantages of this type of photoelectrodes, including the highly ordered nanoarray architectures and the heterojunctions, are highlighted. Then, different g‐C3N4‐based nanosized heteroarrays are comprehensively discussed, in terms of their fabrication methods, PEC capacities, and mechanisms, etc. To conclude, the key challenges and possible solutions for future development on g‐C3N4‐based nanosized heteroarray photoelectrodes are discussed. [ABSTRACT FROM AUTHOR]

The size of the band gap and the energy position of the band edges make several oxynitride semiconductors promising candidates for efficient hydrogen and oxygen production under solar light illumination. Intense research efforts dedicated to oxynitride materials have unveiled the majority of their most important properties. However, two crucial aspects have received much less attention: One is the critical issue of compositional/structural surface modifications that occur during operation and how these affect photoelectrochemical performance. The second concerns the relation between electrochemical response and the crystallographic surface orientation of the oxynitride semiconductor. These are indeed topics of fundamental importance, since it is exactly at the surface where the visible-light-driven electrochemical reaction takes place. In contrast to conventional powder samples, thin films represent the best model system for these investigations. This study reviews current state-of-the-art oxynitride thin film fabrication and characterization, before focusing on LaTiO2N, selected as a representative photocatalyst. An investigation of the initial physicochemical evolution of the surface is reported. Then, it is shown that after stabilization the absorbed photon-to-current conversion efficiency of epitaxial thin films can differ by about 50% for different crystallographic surface orientations, and be up to 5 times larger than for polycrystalline samples. [ABSTRACT FROM AUTHOR]

The nitrogen substitution into the oxygen sites of several oxide materials leads to a reduction of the band gap to the visible-light energy range, which makes these oxynitride semiconductors potential photocatalysts for efficient solar water splitting. Oxynitrides typically show a different crystal structure compared to the pristine oxide material. As the band gap is correlated to both the chemical composition and the crystal structure, it is not trivial to distinguish which modifications of the electronic structure induced by the nitrogen substitution are related to compositional and/or structural effects. Here, X-ray emission and absorption spectroscopy are used to investigate the electronic structures of orthorhombic perovskite LaTiO xN y thin films in comparison with films of the pristine oxide LaTiO x with similar orthorhombic structure and cationic oxidation state. Experiment and theory show the expected upward shift in energy of the valence band maximum that reduces the band gap as a consequence of the nitrogen incorporation. This study also shows that the conduction band minimum, typically considered almost unaffected by nitrogen substitution, undergoes a significant downward shift in energy. For a rational design of oxynitride photocatalysts, the observed changes of both the unoccupied and occupied electronic states have to be taken into account to justify the total band-gap narrowing induced by the nitrogen incorporation. [ABSTRACT FROM AUTHOR]

We performed a comparative study on the photoelectrochemical performance of LaTaON2 loaded with NiOx, Ni0.7Fe0.3Ox, CoOx and IrOx as cocatalysts. Ni-based oxides lead to the highest improvement on the photoelectrochemical performance, while CoOx and IrOx also enhance the performance though to a lower extent, but they simultaneously introduce more pseudocapacitive current thus resulting in an inefficient utilization of the photo-generated holes. Repetitive voltage cycling between 1.0 VRHE and 1.6 VRHE transforms the NiOx and Ni0.7Fe0.3Ox into oxyhydroxides known to possess higher catalytic activities. However, these oxyhydroxides lead to lower photoelectrochemical performance compared to the as-loaded oxides, most probably due to the decay of the passivation centers at the photoelectrode–cocatalyst interface. High catalytic activities cannot be achieved without sufficient passivation of surface recombination states. Despite that the photoelectrochemical performance of LaTaON2 can be improved by cocatalysts, the maximum achievable photocurrent density is still not comparable to that reported for other oxynitride compounds. Our study suggests that poor electronic conductivity or severe bulk recombination of the photo-generated electron–hole pairs are the main limiting factors for the photon-to-current conversion efficiency in LaTaON2 photoanodes. [ABSTRACT FROM AUTHOR]

A novel approach for tailoring strained Pt model electrocatalysts using pulsed laser deposition is presented. The physical properties of Pt films grown on single-crystalline (100)- and (111)-oriented strontium titanate depend significantly on the various deposition parameters (e.g., fluence, temperature, background atmosphere, postannealing, and number of process steps), as revealed by scanning electron microscopy and X-ray studies. By properly selecting the deposition conditions, thin, epitaxial, and strained (100)- and (111)-oriented Pt films suitable as model electrodes can be prepared. Cyclic voltammetry measurements indicate that the strain in the Pt films changes significantly the Pt surface properties, such as the hydrogen underpotential deposition (Hupd) features. CO oxidation experiments demonstrate that the strain affects strongly also the position of the CO oxidation peak shifting it to more positive potentials. This might be attributed to a modification of the adsorption behavior of oxygenated species on the strained interface. This research clearly highlights the potential of using as-produced Pt film as model catalysts to study the effect of strain on electrocatalytic reactions while minimizing possible side effects connected to the common preparation methods for strained Pt catalysts, such as alloy formation, alloy leeching, or conductive support contributions. [ABSTRACT FROM AUTHOR]

Developing novel materials for the conversion of solar to chemical energy is becoming an increasingly important endeavour. Perovskite compounds based on bandgap tunable oxynitrides represent an exciting class of novel photoactive materials. To date, literature mostly focuses on the characterization of oxynitride powder samples which have undeniable technological interest but do not allow the investigation of fundamental properties such as the role of the crystalline quality and/or the surface crystallographic orientation toward photo-catalytic activity. The challenge of growing high quality oxynitride thin films arises from the availability of a suitable substrate, owing to strict material and processing requirements: effective lattice matching, sufficiently high conductivities, stability under high temperatures and in strongly reducing environments. Here, we have established the foundations of a model system incorporating a TiN-buffer layer which enables fundamental investigations into crystallographic surface orientation and crystalline quality of the photocatalyst against photo(electro)chemical performance to be effectively performed. Furthermore, we find that TiN as current collector enables control over the nitrogen content of oxynitride thin films produced by a modified pulsed laser deposition method and allows the growth of highly ordered LaTiO 3− x N x thin films. [ABSTRACT FROM AUTHOR]

The increasing world population and the need to improve quality of life for a large percentage of human beings are the driving forces for the search for sustainable energy production systems, alternative to fossil fuel combustion. Among the various types of alternative energy production technologies, solid oxide fuel cells (SOFCs) operating at intermediate temperatures (400–700 °C) show the advantage of possible use both for stationary and mobile energy production. To reach the goal of reducing the SOFC operating temperature, proton-conducting oxides are gaining wide interest as electrolyte materials. This critical reviewprovides a broad overview of the most recent progresses obtained tailoring the properties of proton-conducting oxides for fuel cell applications, analyzing and comparing the different strategies proposed to match high-proton conductivity with good chemical stability (170 references). [ABSTRACT FROM AUTHOR]

Reducing the operating temperature in the 500-750 °C range is needed for widespread use of solid oxide fuel cells (SOFCs). Proton-conducting oxides are gaining wide interest as electrolyte materials for this aim. We report the fabrication of BaZr0.8Y0.2O3−δ (BZY) proton-conducting electrolyte thin films by pulsed laser deposition on different single-crystalline substrates. Highly textured, epitaxially oriented BZY films were obtained on (100)-oriented MgO substrates, showing the largest proton conductivity ever reported for BZY samples, being 0.11 S cm−1 at 500 °C. The excellent crystalline quality of BZY films allowed for the first time the experimental measurement of the large BZY bulk conductivity above 300 °C, expected in the absence of blocking grain boundaries. The measured proton conductivity is also significantly larger than the conductivity values of oxygen-ion conductors in the same temperature range, opening new potential for the development of miniaturized SOFCs for portable power supply. [ABSTRACT FROM AUTHOR]

Rapidly growing attention is being directed to the investigation of ionic conductivity in oxide film heterostructures. The main reason for this interest arises from interfacial phenomena in these heterostructures and their applications. Recent results revealed that heterophase interfaces have faster ionic conduction pathways than the bulk or homophase interfaces. This finding can open attractive opportunities in the field of micro-ionic devices. The influence of the interfaces on the conduction properties of heterostructures is becoming increasingly important with the miniaturization of solid-state devices, which leads to an enhanced interface density at the expense of the bulk. This review aims to describe the main evidence of interfacial phenomena in ion-conducting film heterostructures, highlighting the fundamental and technological relevance and offering guidelines to understanding the interface conduction mechanisms in these structures. [ABSTRACT FROM AUTHOR]

High temperature proton conductor (HTPC) oxides are attracting extensive attention as electrolyte materials alternative to oxygen-ion conductors for use in solid oxide fuel cells (SOFCs) operating at intermediate temperatures (400-700 °C). The need to lower the operating temperature is dictated by cost reduction for SOFC pervasive use. The major stake for the deployment of this technology is the availability of electrodes able to limit polarization losses at the reduced operation temperature. This review aims to comprehensively describe the state-of-the-art anode and cathode materials that have so far been tested with HTPC oxide electrolytes, offering guidelines and possible strategies to speed up the development of protonic SOFCs. [ABSTRACT FROM AUTHOR]

Abstract: Y-doped barium zirconate (BZY) is a widely investigated proton conducting electrolyte for intermediate temperature solid oxide fuel cell applications. Yttrium is generally recognized as the most suitable dopant into the barium zirconate structure to achieve proper proton conductivity. The influence of Y concentration on the BZY electrical properties is here investigated. Single phase BaZr1 − x Y x O3−δ powders were obtained using a wet chemistry process in a wide compositional range (0.2≤ x ≤0.5). Inductively coupled plasma (ICP) and X-ray diffraction (XRD) analysis suggested complete Y solubility into the BZY powders for all the tested compositions. Proton concentration as a function of Y content was evaluated in the 200–800°C temperature range using thermogravimetric analysis performed under a water partial pressure of ∼0.03atm. The larger the Y content, the larger the proton concentration. By means of electrochemical impedance spectroscopy, the electrical conductivity of BZY sintered pellets was measured under humidified 10% H2/Ar atmosphere. Despite the increase in defect concentration, the conductivity decreased with increasing the Y concentration, due to both hindered mobility and reduced charge carrier concentration. [Copyright &y& Elsevier]

Mixed ionic-electronic conductors (MIECs) such as the (La,Sr)(Co,Fe)O3-d perovskite family are well described in their charge transport through their high temperature applications, i.e., as solid-oxide fuel cell electrodes (600-1000 °C). In this study, the current-voltage (I-V) profiles of these well-known MIEC perovskites are studied between room temperature and 150 °C under bias of ±15 V for potential applications in resistance random access memories. The impact of the metal-oxide interface on the I-V characteristics ranging from ohmic to non-linear hysteretic is discussed for metals of varying work functions and redox potentials, as well as changes in metal electrode distances and areas. [ABSTRACT FROM AUTHOR]

The fabrication and characterization of photoanodes based on black-Si (b-Si) are presented using a photoelectrochemical cell in NaOH solution. B-Si was fabricated by maskless dry plasma etching and was conformally coated by tens-of-nm of TiO2 using atomic layer deposition (ALD) with a top layer of CoO x cocatalyst deposited by pulsed laser deposition (PLD). Low reflectivity R < 5 % of b-Si over the entire visible and near-IR ( λ < 2   μ m) spectral range was favorable for the better absorption of light, while an increased surface area facilitated larger current densities. The photoelectrochemical performance of the heterostructured b-Si photoanode is discussed in terms of the n-n junction between b-Si and TiO2. [ABSTRACT FROM AUTHOR]

Highly textured thin films with small grain boundary regions can be used as model systems to directly measure the bulk conductivity of oxygen ion conducting oxides. Ionic conducting thin films and epitaxial heterostructures are also widely used to probe the effect of strain on the oxygen ion migration in oxide materials. For the purpose of these investigations a good lattice matching between the film and the substrate is required to promote the ordered film growth. Moreover, the substrate should be a good electrical insulator at high temperature to allow a reliable electrical characterization of the deposited film. Here we report the fabrication of an epitaxial heterostructure made with a double buffer layer of BaZrO3 and SrTiO3 grown on MgO substrates that fulfills both requirements. Based on such template platform, highly ordered (001) epitaxially oriented thin films of 15% Sm-doped CeO2 and 8 mol% Y2O3 stabilized ZrO2 are grown. Bulk conductivities as well as activation energies are measured for both materials, confirming the success of the approach. The reported insulating template platform promises potential application also for the electrical characterization of other novel electrolyte materials that still need a thorough understanding of their ionic conductivity. [ABSTRACT FROM AUTHOR]