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The Reaction Mechanism for the Hydrogen Evolution Reaction on the Basal Plane Sulfur Vacancy Site of MoS2 Using Grand Canonical Potential Kinetics
摘要： We develop the grand canonical potential kinetics (GCP-K) formulation based on thermodynamics from quantum mechanics calculations to provide a fundamental basis for understanding heterogeneous electrochemical reactions. Our GCP-K formulation arises naturally from minimizing the free energy using a Legendre transform relating the net charge of the system and the applied voltage. Performing this macroscopic transformation explicitly allows us to make the connection of GCP-K to the traditional Butler-Volmer kinetics. Using this GCP-K based free energy, we show how to predict both the potential and pH dependent chemistry for a specific example, the hydrogen evolution reaction (HER) at a sulfur vacancy on the basal plane of MoS2. We find that the rate determining steps in both acidic and basic conditions are the Volmer reaction in which the second hydrogen atom is adsorbed from the solution. Using the our GCP-K formulation, we show that the stretched bond distances change continuously as a function of the applied potential. This shows that the main reason for the higher activity in basic conditions is that the transition state is closer to the product, which leads to a more favorable Tafel slope of 60mV/dec. In contrast if the transition state were closer to the reactant, where the transfer coefficient is less than 0.5 we would obtain a Tafel slope of almost 150mV/dec. Based on this detailed understanding of the reaction mechanism, we conclude that the second hydrogen at the chalcogenide vacant site is the most active towards the hydrogen evolution reaction. Using this as a descriptor, we compare to the other 2H group VI metal dichalcogenides and predict that vacancies on MoTe2 will have the best performance towards HER.
关键词： Grand Canonical Potential,Hydrogen Evolution Reaction,Molybdenum Disulfide,Electrochemistry
更新于2025-09-23 15:21:21
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Outage and BER performances of indoor relay-assisted hybrid RF/VLC systems
摘要： In this study, the authors analyse the outage performance of the indoor relay-assisted hybrid radio frequency (RF)/visible light communication (VLC) systems. They derive closed-form expressions for the outage probability of the end-to-end signal-to-noise ratio. Then they obtain the optimal approximation parameter with optimising the outage probability using the differential evolution algorithm. Moreover, they analysed the average bit error rate (BER) performance of the hybrid RF/optical system using pulse position modulation method while assuming the timing error in synchronisation. Finally, they present some numerical results utilising the newly derived exact closed-form expressions.
关键词： pulse position modulation,hybrid RF/VLC,BER,differential evolution algorithm,relay-assisted,outage probability
更新于2025-09-23 15:21:21
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Modified Nanopillar Arrays for Highly Stable and Efficient Photoelectrochemical Water Splitting
摘要： Atomically modified graphitic carbon nitride quantum dots (QDs), characterized by strongly increased reactivity and stability, are developed. These are deposited on arrays of TiO2 nanopillars used as a photoanode for the photoelectrochemical water splitting. This photoanode shows excellent stability, with 111 h of continuous work without any performance loss, which outperforms the best-reported results by a factor of 10. Remarkably, our photoanode produces hydrogen even at zero bias. The excellent performance is attributed to the enhancement of photoabsorption, as well as to the promotion of charge separation between TiO2 nanopillars and the QDs.
关键词： hydrogen evolution,quantum dots,graphitic carbon nitride,photoelectrochemistry,TiO2 nanopillars
更新于2025-09-23 15:21:21
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The MUSE <i>Hubble</i> Ultra Deep Field Survey
摘要： The physical origin of the near-ultraviolet Mg ii emission remains an underexplored domain, unlike more typical emission lines that are detected in the spectra of star-forming galaxies. We explore the nebular and physical properties of a sample of 381 galaxies between 0.70 < z < 2.34 drawn from the MUSE Hubble Ultra Deep Survey. The spectra of these galaxies show a wide variety of profiles of the Mg ii λλ2796, 2803 resonant doublet, from absorption to emission. We present a study on the main drivers for the detection of Mg ii emission in galaxy spectra. By exploiting photoionization models, we verified that the emission-line ratios observed in galaxies with Mg ii in emission are consistent with nebular emission from Hii regions. From a simultaneous analysis of MUSE spectra and ancillary Hubble Space Telescope information through spectral energy distribution fitting, we find that galaxies with Mg ii in emission have lower stellar masses, smaller sizes, bluer spectral slopes, and lower optical depth than those with absorption. This leads us to suggest that Mg ii emission is a potential tracer of physical conditions that are not merely related to those of the ionized gas. We show that these differences in Mg ii emission and absorption can be explained in terms of a higher dust and neutral gas content in the interstellar medium (ISM) of galaxies showing Mg ii in absorption, which confirms the extreme sensitivity of Mg ii to the presence of the neutral ISM. We conclude with an analogy between the Mg ii doublet and the Lyα line that lies in their resonant nature. Further investigations with current and future facilities, including the James Webb Space Telescope, are promising because the detection of Mg ii emission and its potential connection with Lyα could provide new insights into the ISM content in the early Universe.
关键词： ultraviolet: ISM,ISM: lines and bands,ultraviolet: galaxies,galaxies: evolution,galaxies: ISM
更新于2025-09-23 15:21:21
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A new method to identify subclasses among AGB stars using <i>Gaia</i> and 2MASS photometry
摘要： Aims. We explore the wealth of high-quality photometric data provided by data release 2 (DR2) of the Gaia mission for long-period variables (LPVs) in the Large Magellanic Cloud (LMC). Our goal is to identify stars of various types and masses along the asymptotic giant branch. Methods. For this endeavour, we developed a new multi-band approach combining Wesenheit functions WRP,BP?RP and WKs,J?Ks in the Gaia BP, RP, and 2MASS J, Ks spectral ranges, respectively, and use a new diagram, (WRP,BP?RP ? WKs,J?Ks ) versus Ks, to distinguish between di?erent kinds of stars in our sample of LPVs. We used stellar population synthesis models to validate our approach. Results. We demonstrate the ability of the new diagram to discriminate between O- and C-rich objects, and to identify low-mass, intermediate-mass, and massive O-rich red giants, as well as extreme C-rich stars. Stellar evolution and population synthesis models guide the interpretation of the results, highlighting the diagnostic power of the new tool to discriminate between stellar initial masses, chemical properties, and evolutionary stages.
关键词： stars: evolution,Magellanic Clouds,stars: AGB and post-AGB,stars: variables: general
更新于2025-09-23 15:21:21
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Theoretical gravity and limb-darkening coefficients for the MOST satellite photometric system
摘要： Aims. We present new calculations of limb and gravity-darkening coe?cients to be used as input in many ?elds of stellar physics such as synthetic light curves of double-lined eclipsing binaries and planetary transits, studies of stellar diameters or line pro?les in rotating stars. Methods. We compute the limb-darkening coe?cients speci?cally for the photometric system of the satellite MOST (Microvariability and Oscillations in STars). All computations were performed by adopting the least-square method, but for completeness we also performed calculations for the linear and bi-parametric approaches by adopting the ?ux conservation method. The passband gravity-darkening coe?cients y(λ) were computed by adopting a more general di?erential equation, which also takes the e?ects of convection into account. Results. We used two stellar atmosphere models: ATLAS (plane-parallel) and PHOENIX (spherical and quasi-spherical). We adopted six laws to describe the speci?c intensity distribution: linear, quadratic, square root, logarithmic, exponential, and a more general one with four terms. The covered ranges of Te?, log g, metallicities, and microturbulent velocities are (1500–50 000 K, 0–5.5, ?5.0–+1.0, 0–8 km s?1), respectively.
关键词： binaries: close,binaries: eclipsing,stars: evolution,stars: atmospheres,planetary systems
更新于2025-09-23 15:21:21
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The GALEX Ultraviolet Virgo Cluster Survey (GUViCS)
摘要： We study the role of the environment on galaxy evolution using a sample of 868 galaxies in the Virgo cluster and in its surrounding regions that are selected from the GALEX Ultraviolet Virgo Cluster Survey (GUViCS) with the purpose of understanding the origin of the red sequence in dense environments. The sample spans a wide range in morphological types (from dwarf ellipticals to Im and BCD) and stellar masses (107 (cid:2) Mstar (cid:2) 1011.5 M(cid:3)). We collected multifrequency data covering the whole electromagnetic spectrum for most of the galaxies, including UV, optical, mid- and far-infrared imaging data, as well as optical and HI spectroscopic data. We ?rst identify the di?erent dynamical substructures that compose the Virgo cluster, and we calculate the local density of galaxies using di?erent methods. We then study the distribution of galaxies belonging to the red sequence, the green valley, and the blue cloud within the di?erent cluster substructures or as a function of galaxy density. Our analysis indicates that all the most massive galaxies (Mstar (cid:3) 1011 M(cid:3)) are slow rotators and are the dominant galaxies of the di?erent cluster substructures, which are generally associated with a di?use X-ray emission. They are probably the result of major merging events that occurred at early epochs, as also indicated by their very old stellar populations. Slow rotators of lower stellar mass (108.5 (cid:2) Mstar (cid:2) 1011 M(cid:3)) are also preferentially located within the di?erent high-density substructures of the cluster. Their position in the velocity space indicates that they are virialised within the cluster; thus, they are Virgo members since its formation. They have been shaped by gravitational perturbations occurring within the infalling groups that later form the cluster (pre-processing). On the contrary, low-mass star-forming systems are extremely rare in the inner regions of the Virgo cluster A, where the density of the intergalactic medium is at its maximum. Our ram pressure stripping models consistently indicate that these star-forming systems can be rapidly deprived of their interstellar medium during their interaction with the intergalactic medium. The lack of gas quenches their star-formation activity transforming them into quiescent dwarf ellipticals. This mild transformation does not perturb the kinematic properties of these galaxies, which still have rotation curves typical of star-forming systems.
关键词： galaxies: interactions,galaxies: star formation,galaxies: clusters: general,galaxies: clusters: individual: Virgo,galaxies: ISM,galaxies: evolution
更新于2025-09-23 15:21:21
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Total infrared luminosity estimation from local galaxies in AKARI all sky survey
摘要： We aim to use the a new and improved version of AKARI all sky survey catalogue of far-infrared sources to recalibrate the formula to derive the total infrared luminosity. We cross-match the faint source catalogue (FSC) of IRAS with the new AKARI-FIS and obtained a sample of 2430 objects. Then we calculate the total infrared (TIR) luminosity LTIR from the formula previously estimated from IRAS data and compare it with total infrared luminosity from AKARI FIS bands to obtain new coe?cients for the general relation to convert FIR luminosity from AKARI bands to the TIR luminosity.
关键词： infrared: galaxies,galaxies: star formation,galaxies: evolution
更新于2025-09-23 15:21:21
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The [CII] 158 <i>μ</i> m line emission in high-redshift galaxies
摘要： Gas is a crucial component of galaxies, providing the fuel to form stars, and it is impossible to understand the evolution of galaxies without knowing their gas properties. The [CII] fine structure transition at 158 μm is the dominant cooling line of cool interstellar gas, and is the brightest of emission lines from star forming galaxies from FIR through metre wavelengths, almost unaffected by attenuation. With the advent of ALMA and NOEMA, capable of detecting [CII]-line emission in high-redshift galaxies, there has been a growing interest in using the [CII] line as a probe of the physical conditions of the gas in galaxies, and as a star formation rate (SFR) indicator at z ≥ 4. In this paper, we have used a semi-analytical model of galaxy evolution (G.A.S.) combined with the photoionisation code CLOUDY to predict the [CII] luminosity of a large number of galaxies (25 000 at z (cid:39) 5) at 4 ≤ z ≤ 8. We assumed that the [CII]-line emission originates from photo-dominated regions. At such high redshift, the CMB represents a strong background and we discuss its effects on the luminosity of the [CII] line. We studied the L[CII]–SFR and L[CII]–Zg relations and show that they do not strongly evolve with redshift from z = 4 and to z = 8. Galaxies with higher [CII] luminosities tend to have higher metallicities and higher SFRs but the correlations are very broad, with a scatter of about 0.5 and 0.8 dex for L[CII]–SFR and L[CII]–Zg, respectively. Our model reproduces the L[CII]–SFR relations observed in high-redshift star-forming galaxies, with [CII] luminosities lower than expected from local L[CII]–SFR relations. Accordingly, the local observed L[CII]–SFR relation does not apply at high-z (z (cid:38) 5), even when CMB effects are ignored. Our model naturally produces the [CII] deficit (i.e. the decrease of L[CII]/LIR with LIR), which appears to be strongly correlated with the intensity of the radiation field in our simulated galaxies. We then predict the [CII] luminosity function, and show that it has a power law form in the range of L[CII] probed by the model (1 × 107–2 × 109 L(cid:12) at z = 6) with a slope α = ?1. The slope is not evolving from z = 4 to z = 8 but the number density of [CII]-emitters decreases by a factor of 20×. We discuss our predictions in the context of current observational estimates on both the differential and cumulative luminosity functions.
关键词： galaxies: high-redshift,galaxies: ISM,galaxies: evolution
更新于2025-09-23 15:21:21
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Effect of laser parameters on microstructure and phase evolution of Ti-Si-C composites prepared by selective laser melting
摘要： In this paper, blended Ti/SiC powder was processed by selective laser melting (SLM) method to prepare a novel kind of Ti-Si-C composite. Microstructure, elements distribution and phase evolution of Ti-Si-C composites fabricated by SLM were studied. SEM, EDS, TEM and XRD were performed to evaluate the microstructure and phase evolution during the SLM process. The results showed that processing parameters significantly affected the microstructure and element distribution of the samples. It was found that higher laser energy density could promote the densification process and hence suppress balling phenomenon of Ti-rich phases. At the micro-level, the microstructure could be classified into the dense area and the porous area, which was closely related to the redistribution of elements. The phases obtained by the SLM process were composed by TiCx, TiSi2, Ti5Si3, and Ti5Si4, which appeared in different microstructures. The results indicated that composites with different phases and microstructure could be obtained by merely modifying laser processing parameters, which could give experimental guidance for the fabricating of gradient Ti-Si-C composites by selective laser melting without additional apparatus on powder feeding.
关键词： Microstructure evolution,Phase evolution,Ti-Si-C composite,Selective laser melting
更新于2025-09-23 15:21:01