An approach is introduced to calculate the thermodynamic oxidation and reduction potentials of semiconductors in aqueous solution. By combining a newly developed ab initio calculation method for ...compound formation energy and band alignment with electrochemistry experimental data, this approach can be used to predict the stability of almost any compound semiconductor in aqueous solution. Thirty photocatalytic semiconductors have been studied, and a graph (a simplified Pourbaix diagram) showing their valence/conduction band edges and oxidation/reduction potentials relative to the water redox potentials is produced. On the basis of this graph, the thermodynamic stabilities and trends against the oxidative and reductive photocorrosion for compound semiconductors are analyzed, which shows the following: (i) some metal oxides can be resistant against the oxidation by the photogenerated holes when used as the n-type photoanodes; (ii) all the nonoxide semiconductors are susceptible to oxidation, but they are resistant to the reduction by the photogenerated electrons and thus can be used as the p-type photocathodes if protected from the oxidation; (iii) doping or alloying the metal oxide with less electronegative anions can decrease the band gap but also degrade the stability against oxidation.
The formation energies and charge-transition levels of intrinsic point defects in lead halide perovskite CsPbBr3 are studied from first-principles calculations. It is shown that the formation energy ...of dominant defect under Br-rich growth condition is much lower than that under moderate or Br-poor conditions. Thus avoiding the Br-rich condition can help to reduce the defect concentration. Interestingly, CsPbBr3 is found to be highly defect-tolerant in terms of its electronic structure. Most of the intrinsic defects induce shallow transition levels. Only a few defects with high formation energies can create deep transition levels. Therefore, CsPbBr3 can maintain its good electronic quality despite the presence of defects. Such defect tolerance feature can be attributed to the lacking of bonding–antibonding interaction between the conduction bands and valence bands.
As core units of organ tissues, cells of various types play their harmonious rhythms to maintain the homeostasis of the human body. It is essential to identify the characteristics of cells in human ...organs and their regulatory networks for understanding the biological mechanisms related to health and disease. However, a systematic and comprehensive single-cell transcriptional profile across multiple organs of a normal human adult is missing.
We perform single-cell transcriptomes of 84,363 cells derived from 15 tissue organs of one adult donor and generate an adult human cell atlas. The adult human cell atlas depicts 252 subtypes of cells, including major cell types such as T, B, myeloid, epithelial, and stromal cells, as well as novel COCH
fibroblasts and FibSmo cells, each of which is distinguished by multiple marker genes and transcriptional profiles. These collectively contribute to the heterogeneity of major human organs. Moreover, T cell and B cell receptor repertoire comparisons and trajectory analyses reveal direct clonal sharing of T and B cells with various developmental states among different tissues. Furthermore, novel cell markers, transcription factors, and ligand-receptor pairs are identified with potential functional regulations in maintaining the homeostasis of human cells among tissues.
The adult human cell atlas reveals the inter- and intra-organ heterogeneity of cell characteristics and provides a useful resource in uncovering key events during the development of human diseases in the context of the heterogeneity of cells and organs.
Perovskite-based solar cells have achieved high solar-energy conversion efficiencies and attracted wide attentions nowadays. Despite the rapid progress in solar-cell devices, many fundamental issues ...of the hybrid perovskites have not been fully understood. Experimentally, it is well-known that in CH3NH3PbI3 the organic molecules CH3NH3 are randomly orientated at the room temperature, but the impact of the random molecular orientation has not been investigated. Because of the dipole moment of the organic molecule, the random orientation creates a novel system with long-range potential fluctuations unlike alloys or other conventional disordered systems. Using linear scaling ab initio methods, we find that the charge densities of the conduction band minimum and the valence band maximum are localized in nanoscales due to the potential fluctuations. The charge localization causes electron–hole separation and reduces carrier recombination rates, which may contribute to the long carrier lifetime observed in experiments.
Discovery of earth-abundant electrocatalysts to replace iridium for the oxygen evolution reaction (OER) in a proton exchange membrane water electrolyzer (PEMWE) represents a critical step in reducing ...the cost for green hydrogen production. We report a nanofibrous cobalt spinel catalyst codoped with lanthanum (La) and manganese (Mn) prepared from a zeolitic imidazolate framework embedded in electrospun polymer fiber. The catalyst demonstrated a low overpotential of 353 millivolts at 10 milliamperes per square centimeter and a low degradation for OER over 360 hours in acidic electrolyte. A PEMWE containing this catalyst at the anode demonstrated a current density of 2000 milliamperes per square centimeter at 2.47 volts (Nafion 115 membrane) or 4000 milliamperes per square centimeter at 3.00 volts (Nafion 212 membrane) and low degradation in an accelerated stress test.
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•A fixed potential method is used to simulate potential effects in electrochemical reactions.•Adjustes the total system charge to satisfy the applied electrode potential.•Yield ...different rate-limiting steps and overpotential
Potential not only governs the direction of electrochemical reactions, but also shapes the electronic properties of single-atom catalysts dramatically. However, it is a challenge to simulate the effects of potential theoretically. Normally, DFT calculations are performed at a constant number of electrons, not a constant voltage. In this work, we apply a new fixed-potential method (grand canonical method) in the DFT simulation to mimic the electrochemical processes, in which the total number of the electron in the system was floated to match the ‘‘applied voltage,’, or the electrode Fermi energy at the atomic level. Here, the single-atom catalysts on two-dimension substrates for the oxygen evolution reaction process are used as examples to test the fixed-potential method. This fixed-potential method changes the rate-determining step and yields an overpotential difference of as much as 0.48 V in comparison with the conventional charge-neutral method. The quantitative error in the overpotential is not as important as the qualitative error in rate-determining steps. These errors can be avoided via the fixed-charge method with the proper charge. We believe our work advances the understanding of the effects of potential on the catalytic process in real electrochemical reactions and offers practical guidance for designing catalysts.
Auger recombination is the main non-radiative decay pathway for multi-carrier states of colloidal quantum dots, which affects performance of most of their optical and optoelectronic applications. ...Outstanding single-exciton properties of CdSe/CdS core/shell quantum dots enable us to simultaneously study the two basic types of Auger recombination channels-negative trion and positive trion channels. Though Auger rates of positive trion are regarded to be much faster than that of negative trion for II-VI quantum dots in literature, our experiments find the two rates can be inverted for certain core/shell geometries. This is confirmed by theoretical calculations as a result of geometry-dependent dielectric screening. By varying the core/shell geometry, both types of Auger rates can be independently tuned for ~ 1 order of magnitude. Experimental and theoretical findings shed new light on designing quantum dots with necessary Auger recombination characteristics for high-power light-emitting-diodes, lasers, single-molecular tracking, super-resolution microscope, and advanced quantum light sources.
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