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The scarcity of fuels, high pollution levels, climate change, and other major environmental issues are critical challenges that modern societies are facing, mostly originating from fossil fuels-based economies. These challenges can be addressed by developing green, eco-friendly, inexpensive energy sources and energy storage devices. Electrochemical energy storage materials possess high capacitance and superior power density. To engineer highly efficient next-generation electrochemical energy storage devices, the mechanisms of electrochemical reactions and redox behavior must be probed in operational environments. They can be studied by investigating atomic and electronic structures using in situ x-ray absorption spectroscopy (XAS) analysis. Such a technique has attracted substantial research and development interest in the field of energy science for over a decade. The mechanisms of charge/discharge, carrier transport, and ion intercalation/deintercalation can be elucidated. Supercapacitors generally store energy by two specific mechanisms—pseudocapacitance and electrochemical double-layer capacitance. In situ XAS is a powerful tool for probing and understanding these mechanisms. In this Review, both soft and hard x rays are used for the in situ XAS analysis of various representative electrochemical energy storage systems. This Review also showcases some of the highly efficient energy and power density candidates. Furthermore, the importance of synchrotron-based x-ray spectroscopy characterization techniques is enlightened. The impact of the electronic structure, local atomic structure, and electronically active elements/sites of the typical electrochemical energy storage candidates in operational conditions is elucidated. Regarding electrochemical energy storage mechanisms in their respective working environments, the unknown valence states and reversible/irreversible nature of elements, local hybridization, delocalized d-electrons spin states, participation of coordination shells, disorder, and faradaic/non-faradaic behavior are thoroughly discussed. Finally, the future direction of in situ XAS analysis combined with spatial chemical mapping from operando scanning transmission x-ray microscopy and other emerging characterization techniques is presented and discussed. [ABSTRACT FROM AUTHOR]

The observation of a spontaneous electron enrichment at the apexes of bismuth vanadate (BiVO4) nanopyramidal arrays is achieved by performing synchrotron-based angular dependent X-ray absorption spectroscopy. In accordance with density function theory calculations, the formation of (112) facets enriched apexes is proposed. Such intrinsically electron-enriched (112) facets at the apexes produce a potential facilitating the diffusion of photoexcited holes, which not only mitigates the recombination with the photoexcited electrons, but also provide an active reaction site for the photoassisted selective growth of cobalt phosphate (CoPi), a well known and highly active co-catalyst for water oxidation at the apex of BiVO4 nanopyramids. Benefiting from the element-resolved properties of synchrotron-based X-ray spectroscopy, 3d electrons on the vanadium site are directly resolved by resonant inelastic X-ray scattering measurements. Due to this unique morphology, the charge at the apex is expected to induce a strong interaction with CoPi, as a result of the metal-to-ligand charge transfer spectroscopy feature observed from the cobalt site. Substantial evidences are collected by means of comprehensive soft X-ray spectroscopy techniques with high spectral resolution revealing the atomic-scale origin and the nature of the synergy as well as the strong correlation between its unique structural properties and the electronic structure of this novel highly-ordered hybrid photocatalyst and its significantly improved photoelectrochemical performance.

Photoelectrodes nanoscale interface design has become a key factor to enhancing their photoelectrochemical performance for water splitting by reducing the photogenerated charge recombination, thus ensuring their efficient separation, transport, and collection. In this work, hematite (α‐Fe2O3) photoanodes were prepared from a simple and scalable methodology capable of synergistically mitigating the charge loss and recombination at all interfaces (i.e., fluorine‐doped tin oxide/hematite, hematite/hematite, and hematite/electrolyte) and achieving overall efficiency of ∼50% for the water oxidation reaction compared to pristine photoelectrodes. The external quantum efficiency at 1.23 V versus reversible hydrogen electrode of pristine hematite was enhanced 6.7 times with the modifications of the three interfaces (Al2O3/NbH/NiFeOx). Electrochemical impedance spectroscopy and intensity‐modulated photocurrent spectroscopies were applied to probe and monitor the photogenerated charge carrier dynamics revealing a substantial improvement in charge separation and collection at the back‐contact interface as well as a partial mitigation of the surface states at the hematite–electrolyte interface. [ABSTRACT FROM AUTHOR]

Halide perovskites have shown high potential for developing self‐driven filterless narrowband photodetectors with the advantages of excellent color recognition, low cost, and simple design. Among the various perovskites, all‐inorganic perovskites are most promising for optoelectronic devices due to their unmatched thermal stability, but they are often more difficult to be fabricated into a phase‐pure form with large grain sizes. Herein, for the first time, a self‐driven all‐inorganic perovskite narrowband photodetector (PNPD) fabricated by a scalable postripening aerosol–liquid–solid deposition method in ambient conditions is reported. Significantly, simply applying a short thermal stress to a pristine film with heterogeneous phases and small grain sizes triggers Ostwald ripening and leads to more than tenfold enlarged grains of phase‐pure CsPbBr3 with considerably improved optoelectronic properties. The self‐driven CsPbBr3‐based all‐inorganic PNPD shows a narrowband response, with a peak responsivity of 106.6 mA W−1, specific detectivity of 2.2 × 1012 Jones at 535 nm, and a narrow full width at half maximum of only 15 nm, placing it among the best of its kind. The PNPD is also fast (trise/tfall = 3.9/3.21 ms) and stable (above 10 000 on/off switches). This work opens the path of tapping all‐inorganic perovskites for next‐generation optoelectronics with high performance and reliability. [ABSTRACT FROM AUTHOR]

A report is presented on the inexpensive fabrication of a large three-dimensional and highly oriented porous microrod array of n-type ZnO semiconductor. This semiconductor has a unique designed architecture consisting of perpendicularly oriented single-crystalline hexagonal rods.

Byline: Lionel Vayssieres Crystal phase transformations in aqueous solutions generally operate through a dissolution-recrystallisation process to fulfil surface energy minimisation requirements. Indeed, when a solid offers several allotropic crystal phases and polymorphs, it is usually the one with the highest solubility and consequently the lowest stability (i.e., the crystallographic metastable phase), which precipitates first. This is understood by considering the nucleation kinetics of the solids. At a given supersaturation ratio, the germ size is as small (and the nucleation rate as high) as the interfacial tension of the system is low. Thus, given that the solubility is inversely proportional to the interfacial tension, the precipitation of the most soluble phase and consequently the less stable thermodynamically, is therefore kinetically promoted. Due to its solubility and metastability, this particular phase is more sensitive to secondary growth and ageing, which leads to crystallographically more stable phases, essentially by heteronucleation. This is typically the case of metal oxides such as aluminium hydroxide (boehmite to bayerite or gibbsite), titanium dioxide (anatase or brookite to rutile), or iron oxyhydroxides (goethite, akaganeite, or lepidocrocite to hematite). Secondary growth and ageing processes are delicate to control and the phase transformations appear within a few hours to a few days in solution, whence the resulting undesired mixing of allotropic phases and polymorphs. However, by careful consideration of the precipitation conditions such phenomena may be avoided when nanosystems are precipitated at low interfacial tension. Thus, the stabilisation of oxide, hydroxide and oxyhydroxide metastable crystal structures can be achieved at thermodynamically stable conditions. As an attempt to assess such behaviour, experimental studies involving iron (akaganeite, β-FeOOH) and aluminium (boehmite, γ-AlOOH) oxyhydroxides were conducted. The results show that it is possible to significantly slow down and even entirely suppress crystal phase transformation and thus stabilise crystalline metastable phases in aqueous solutions.

Byline: Lionel Vayssieres Divided and ultra-divided systems such as colloidal and nanoparticle dispersions are generally unstable with regard to the size and number of their constituents because the solid-solution interfacial tension, acting as a driving force, leads to a reduction of the surface area to minimise the dispersion free enthalpy. Such phenomenon known as surface energy minimisation induces an increase in average particle size as a result of the decrease of the surface area at constant volume. For such a reason such dispersions are usually considered thermodynamically unstable. However, they can be thermodynamically stabilised if, by adsorption, the interfacial tension of the system becomes very low. This phenomenon, well known for microemulsions, is for the first time quantitatively modelled and demonstrated for transition metal oxide nanoparticles. When the pH of precipitation is sufficiently far from the point of zero charge and the ionic strength sufficiently high, the ripening of nanoparticles is avoided and their size can be monitored over one order of magnitude by tailoring solution pH and ionic strength. A model based on Gibbs adsorption equation leads to an analytical expression of the water-oxide interfacial tension as a function of the pH and the ionic strength of the dispersion/precipitation medium. The stability condition, defined by a 'zero' interfacial tension, corresponds to the chemical and electrostatic saturation of the water-oxide interface. In such a condition, the density of charged surface groups reaches its maximum, the interfacial tension its minimum and further adsorption forces the surface area to expand and consequently, the size of nanoparticles to decrease. An excellent agreement was found between the model prediction and the experimental results obtained from the aqueous precipitation of magnetite (Fe3O4) nanoparticles in basic medium. A general control of the metal oxide nanoparticle size when precipitated far from their point-of-zero-charge is thus expected.

Byline: Lionel Vayssieres A novel growth concept and a low-temperature aqueous chemical growth (ACG) thin film processing technique are presented that aim to contribute to the rational fabrication of smart and functional metal oxide particulate thin films and coatings. Such purpose-built materials are modelled, designed, and engineered to match the physical, chemical, and structural requirements of their applications. The concept involves a thermodynamic model monitoring the nucleation, growth, and ageing processes via the chemical and electrostatic control of the interfacial free energy of the system. It enables the control of the size of nano-, meso-, and micro-crystallites, their surface morphology, their orientations onto substrates as well as their crystal structure. The templateless and surfactant-free aqueous synthesis method allows generation, at large-scale, low-cost, and mild temperatures, of advanced metal oxides thin films with designed complexity. It features two- and three-dimensional (patterned) arrays and nanocomposites of transition metal oxides consisting of various isotropic and anisotropic building-blocks with advanced architectures. Such materials are designed to develop a new generation of metal oxide-based composites for optical, optoelectronic, magnetic, and sensor devices.

Fabrication of large two-dimensional (2-D) arrays is done without any applied external field. This is useful for unique nonreciprocal optical effects observed in alpha-Cr(sub 2)O(sub 3).