Bio-deterioration of concrete, which is very common in sewer system and waste water treatment plant, results in significant structure degradation. Normally, the process can be described by the two ...following steps: Biochemical reactions producing biogenic aggressive species (H2SO4 is one of the most significant biogenic acid in sewer pipes), and chemical reactions between biogenic aggressive species and cement hydration products which is responsible for concrete degradation. A reactive transport model is proposed to simulate the chemical degradation process of cementitious materials in contact with H2SO4 solution. The dissolution of portlandite (CH) and calcium silicate hydrates (C–S–H) and the precipitation of gypsum (p) are described by mass action law and threshold of ion activity products. To take into account the continuous decrease of the Ca/Si ratio during the dissolution of C–S–H a generalization of the mass action law is applied. A simplified damage model is introduced to characterize the degradation of concrete due to the swelling of gypsum. Some experiments reported in literature are simulated. The numerical results and the experimental observations are compared and discussed.
•A reactive transport modeling of sulfuric acid attack of concrete is developed•Swelling of gypsum induces damage of concrete•Transport, chemistry and material damage are coupled•A general approach is applied to characterize the decalcification of C-S-H•Simulation of laboratory experiments and long term prediction are conducted
Single molecules: Large enhancements of single‐molecule fluorescence up to 1100 times by using synthesized gold nanorods are reported (see picture). This high enhancement is achieved by selecting a ...dye with its adsorption and emission close to the surface plasmon resonance of the gold nanorods.
Halide perovskites possess enormous potential for various optoelectronic applications. Presently, a clear understanding of the interplay between the lattice and electronic effects is still elusive. ...Specifically, the weakly absorbing tail states and dual emission from perovskites are not satisfactorily described by existing theories based on the Urbach tail and reabsorption effect. Herein, through temperature-dependent and time-resolved spectroscopy on metal halide perovskite single crystals with organic or inorganic A-site cations, we confirm the existence of indirect tail states below the direct transition edge to arise from a dynamical Rashba splitting effect, caused by the PbBr
octahedral thermal polar distortions at elevated temperatures. This dynamic effect is distinct from the static Rashba splitting effect, caused by non-spherical A-site cations or surface induced lattice distortions. Our findings shed fresh perspectives on the electronic-lattice relations paramount for the design and optimization of emergent perovskites, revealing broad implications for light harvesting/photo-detection and light emission/lasing applications.
The electrochemical reduction of CO2 to multi-carbon products has attracted much attention because it provides an avenue to the synthesis of value-added carbon-based fuels and feedstocks using ...renewable electricity. Unfortunately, the efficiency of CO2 conversion to C2 products remains below that necessary for its implementation at scale. Modifying the local electronic structure of copper with positive valence sites has been predicted to boost conversion to C2 products. Here, we use boron to tune the ratio of Cuδ+ to Cu0 active sites and improve both stability and C2-product generation. Simulations show that the ability to tune the average oxidation state of copper enables control over CO adsorption and dimerization, and makes it possible to implement a preference for the electrosynthesis of C2 products. We report experimentally a C2 Faradaic efficiency of 79 ± 2% on boron-doped copper catalysts and further show that boron doping leads to catalysts that are stable for in excess of ~40 hours while electrochemically reducing CO2 to multi-carbon hydrocarbons.
Formamidinium-lead-iodide (FAPbI
)-based perovskites with bandgap below 1.55 eV are of interest for photovoltaics in view of their close-to-ideal bandgap. Record-performance FAPbI
-based solar cells ...have relied on fabrication via the sequential-deposition method; however, these devices exhibit unstable output under illumination due to the difficulty of incorporating cesium cations (stabilizer) in sequentially deposited films. Here we devise a perovskite seeding method that efficiently incorporates cesium and beneficially modulates perovskite crystallization. First, perovskite seed crystals are embedded in the PbI
film. The perovskite seeds serve as cesium sources and act as nuclei to facilitate crystallization during the formation of perovskite. Perovskite films with perovskite seeding growth exhibit a lowered trap density, and the resulting planar solar cells achieve stabilized efficiency of 21.5% with a high open-circuit voltage of 1.13 V and a fill factor that exceeds 80%. The Cs-containing FAPbI
-based devices show a striking improvement in operational stability and retain 60% of their initial efficiency after 140 h operation under one sun illumination.
Defect engineering in photocatalysts represents a fundamental method toward tailoring their solar-to-chemical energy conversion performance, although determining the nature and impact of subsurface ...defects remains challenging. Single-unit-cell Bi2WO6 monolayers, forming a sandwich-like structure, BiO+–WO42––BiO+, exhibit promising photocatalytic performance and are an ideal system for isolating subsurface defects. We report the single-step synthesis of Bi2WO6 monolayers rich in stable interior W vacancies and characterize their influence on the physical properties necessary for effective photocatalytic surface reactions. Defect-rich monolayers benefit from enhanced visible-light absorption and photocarrier transport, boosting the solar photocatalytic oxidation of benzylic alcohols by 140% at no cost to selectivity or stability. This work highlights the importance of subsurface defects within surface-driven photocatalytic applications and prescribes a general strategy for their isolated study via 2D compounds exhibiting symmetric surface termination.
The room-temperature charge carrier mobility and excitation–emission properties of metal halide perovskites are governed by their electronic band structures and intrinsic lattice phonon scattering ...mechanisms. Establishing how charge carriers interact within this scenario will have far-reaching consequences for developing high-efficiency materials for optoelectronic applications. Herein we evaluate the charge carrier scattering properties and conduction band environment of the double perovskite Cs2AgBiBr6 via a combinatorial approach; single crystal X-ray diffraction, optical excitation and temperature-dependent emission spectroscopy, resonant and nonresonant Raman scattering, further supported by first-principles calculations. We identify deep conduction band energy levels and that scattering from longitudinal optical phononsvia the Fröhlich interactiondominates electron scattering at room temperature, manifesting within the nominally nonresonant Raman spectrum as multiphonon processes up to the fourth order. A Fröhlich coupling constant nearing 230 meV is inferred from a temperature-dependent emission line width analysis and is found to be extremely large compared to popular lead halide perovskites (between 40 and 60 meV), highlighting the fundamentally different nature of the two “single” and “double” perovskite materials branches.
Efficient wide-bandgap perovskite solar cells (PSCs) enable high-efficiency tandem photovoltaics when combined with crystalline silicon and other low-bandgap absorbers. However, wide-bandgap PSCs ...today exhibit performance far inferior to that of sub-1.6-eV bandgap PSCs due to their tendency to form a high density of deep traps. Here, we show that healing the deep traps in wide-bandgap perovskites-in effect, increasing the defect tolerance via cation engineering-enables further performance improvements in PSCs. We achieve a stabilized power conversion efficiency of 20.7% for 1.65-eV bandgap PSCs by incorporating dipolar cations, with a high open-circuit voltage of 1.22 V and a fill factor exceeding 80%. We also obtain a stabilized efficiency of 19.1% for 1.74-eV bandgap PSCs with a high open-circuit voltage of 1.25 V. From density functional theory calculations, we find that the presence and reorientation of the dipolar cation in mixed cation-halide perovskites heals the defects that introduce deep trap states.
The sensitive detection of X‐rays embodies an important research area, being motivated by a common desire to minimize the radiation doses required for detection. Among metal halide perovskites, the ...double‐perovskite Cs2AgBiBr6 system has emerged as a promising candidate for the detection of X‐rays, capable of high X‐ray stability and sensitivity (105 μC Gy−1 cm−2). Herein, the important photophysical pathways in single‐crystal Cs2AgBiBr6 are detailed at both room (RT) and liquid‐nitrogen (LN2T) temperatures, with emphasis made toward understanding the carrier dynamics that influence X‐ray sensitivity. This study draws upon several optical probes and an RT excitation model is developed which is far from optimal, being plagued by a large trap density and fast free‐carrier recombination pathways. Substantially improved operating conditions are revealed at 77 K, with a long fundamental carrier lifetime (>1.5 µs) and a marked depopulation of parasitic recombination pathways. The temperature dependence of a single‐crystal Cs2AgBiBr6 X‐ray detecting device is characterized and a strong and monotonic enhancement to the X‐ray sensitivity upon cooling is demonstrated, moving from 316 μC Gy−1 cm−2 at RT to 988 μC Gy−1 cm−2 near LN2T. It is concluded that even modest cooling—via a Peltier device—will facilitate a substantial enhancement in device performance, ultimately lowering the radiation doses required.
The sensitive detection of X‐rays using devices based on metal halide perovskite semiconductors embodies a rapidly emerging field of research. The photophysical pathways within a single‐crystal Cs2AgBiBr6 detector exhibiting high sensitivity to X‐rays are detailed. By evaluating carrier‐recombination pathways at both high and low temperatures, the dramatic enhancements to performance realized upon cooling the device are elucidated.
Organometal halide perovskites show promising features for cost-effective application in photovoltaics. The material instability remains a major obstacle to broad application because of the poorly ...understood degradation pathways. Here, we apply simultaneous luminescence and electron microscopy on perovskites for the first time, allowing us to monitor in situ morphology evolution and optical properties upon perovskite degradation. Interestingly, morphology, photoluminescence (PL), and cathodoluminescence of perovskite samples evolve differently upon degradation driven by electron beam (e-beam) or by light. A transversal electric current generated by a scanning electron beam leads to dramatic changes in PL and tunes the energy band gaps continuously alongside film thinning. In contrast, light-induced degradation results in material decomposition to scattered particles and shows little PL spectral shifts. The differences in degradation can be ascribed to different electric currents that drive ion migration. Moreover, solution-processed perovskite cuboids show heterogeneity in stability which is likely related to crystallinity and morphology. Our results reveal the essential role of ion migration in perovskite degradation and provide potential avenues to rationally enhance the stability of perovskite materials by reducing ion migration while improving morphology and crystallinity. It is worth noting that even moderate e-beam currents (86 pA) and acceleration voltages (10 kV) readily induce significant perovskite degradation and alter their optical properties. Therefore, attention has to be paid while characterizing such materials using scanning electron microscopy or transmission electron microscopy techniques.
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