Electron Lifetime Research Articles

Mathematical modeling, parameter identification, and electrical performance of a DSSC based on nature-inspired optimization techniques

The aim of this research is to achieve the highest efficiency for a dye-sensitized solar cell (DSSC) before the fabrication process. For DSSC efficiency improvement, six different optimization algorithms are used for the DSSC parameter extraction. The algorithms used are the genetic algorithm, grey wolf algorithm, dragonfly algorithm, moth flame algorithm, ant-lion algorithm, and whale algorithm, developed based on MATLAB coding. The physical parameters for the DSSC are the electron lifetime, electrode thickness, ideality factor, absorption coefficient, and diffusion coefficient. A comparative study is carried out among the six algorithms based on the highest efficiency and computational speed. Finally, a sensitivity analysis of environmental conditions (solar irradiance and temperature) and physical parameters is implemented and analyzed to simulate the DSSC performance for different values of these parameters. The DSSC parameters studied are short-circuit current density, open-circuit voltage, fill factor, and efficiency. The optimal electron lifetime is 100 ms, and the optimal thickness of the photoanode layer is 1 μm, reaching maximum efficiency equal to 11.79%.

First principles study of the effect of Cu/Ag/Au single doping and point defects on the magnetic and photocatalytic properties of ZnO

Experimental studies and theoretical calculations on the physical properties of ZnO with Cu/Ag/Au single doping have been reported, but reports on the magnetic and photocatalytic properties of ZnO by Zn vacancies and Cu/Ag/Au single doping and interstitial H coexistence are few. Moreover, locating the positions of the Zn vacancies is experimentally challenging. We used the GGA + U method within the framework of density functional theory to investigate the formation energy and magnetic and photocatalytic properties of Zn34MO36 (M = Cu/Ag/Au) and Zn34MHiO36 (M = Cu/Ag/Au) systems. Results show that the system is more stable under the O-rich condition. The Zn34CuO36, Zn34AgO36, and Zn34CuHiO36 systems all exhibit the magnetic properties of 2, −1 and 2 μB, and the O−-2p, Ag-4d, and Cu-3d states in the Zn34CuO36, Zn34AgO36 and Zn34CuHiO36 systems have local and itinerant electron properties. The Zn34AgHiO36 system has the longest electronic lifetime of 0.75 × 10−13 s and a longest hole lifetime of 0.70 × 10−13 s, with a relative maximum D value of 67.59, better carrier separation, and relatively high oxidation capacity. In the wavelength range of 400–800 nm, the Zn34AgHiO36 system has the strongest relative absorption spectrum intensity, the most pronounced red shift, the largest relative electric dipole moment of 109.19 × 10−30C·m and the largest relative activity. Therefore, the Zn34AgHiO36 system is the best photocatalyst for O production.

A self-powered ultraviolet photodetector with van der Waals Schottky junction based on TiO2 nanorod arrays/Au-modulated V2CTx MXene

A self-powered ultraviolet photodetector (UV PD) with van der Waals (vdW) Schottky junction based on TiO2 nanorod arrays/Au-modulated V2CTx MXene is reported. The Schottky junction enables the device to operate in self-powered mode. The dangling bond-free surface of V2CTx MXene reduces the charge recombination at the junction interface. Meanwhile, V2CTx MXene, with the work function (WF) increasing to 5.35 eV, forms a hole transport layer by contacting with Au electrode, which facilitates the carrier extraction. The electron lifetime in the device has prolonged to 8.95 μs. As a result, the responsivity and detectivity of the PD have achieved 28 mA/W and 1.2 × 1011 cm Hz1/2/W (340 nm, 65 mW/cm2, 0 V), respectively. In addition, the presence of the Au electrode prevents the vanadium from coming into contact with oxygen and oxidizing, preserving the properties of the V2CTx films. After 180 days of exposure to the atmosphere, the device performance remained at a particularly high level, indicating enhanced durability. This work points out an effective approach to modulate the properties of V2CTx to obtain the high performance and stability of the UV PD.

CoP-loaded BiVO4 for highly active and robust photocathodic protection of 304 stainless steel

Despite the narrow band gap and the suitable band edge position, the BiVO4 photoanode exhibits low metal anticorrosion performance due to inefficient charge separation and transfer. In this work, we introduce a CoP electrocatalyst with metal character on the BiVO4 surface by cyclic voltammetry to address this challenge, which greatly enhances the electron injection efficiency into the protected metal. The modified BiVO4 photoanode exhibits high solar water oxidation activity, resulting in a 123 mV cathodic shift on the self-corrosion potential of 304 stainless steel (304SS), enabling excellent metal protection. Additionally, the coupled composite photoanode shows high photo stability under light illumination, with only 8% decay on the open circuit potential after 51 h of photo-illumination. Our mechanism study shows that the CoP overlayer can act as the hole collector and electrocatalyst for water oxidation, and thus the BiVO4 valence band holes are effectively transferred to the CoP and subsequently to water. Such an improvement inhibits the charge recombination increasing the electron lifetime, and thus realizing the effective protection of 304SS. The present work offers a rather simple process that allows the effective deposition of CoP electrocatalyst on BiVO4 surfaces, which effectively protects metals from corrosion.

Point defect engineering of elemental phosphorus for photocatalytic hydrogen evolution

Red phosphorus (RP) has been demonstrated for photocatalytic hydrogen evolution (PHE) in recent years. Realistically, P vacancy (VP) defects are present in RP. The constructive or detrimental influences of such defects on the photocatalytic activity of RP should be revealed as the understanding shall provide a platform for further improvement. Herein, we for the first time establish a comprehensive understanding of the crucial relationships between the intrinsic VP defects and the charge dynamics together with the photocatalytic performance in RP. Two main findings are achieved: i) VP-induced deep charge trapping effect is revealed to lead to a severe loss of active electrons during the H+ reduction reaction, resulting in the inferior PHE performance of RP. ii) O doping in the VP sites is demonstrated to be an effective strategy for eliminating the detrimental VP defect states, leading to a long-lived free electron lifetime in RP for enhanced PHE performance. The point defect engineering of RP applied in this study paves a promising way in tuning the physicochemical properties of RP and other elemental-based materials for various applications.

Periodic Distributions and Ultrafast Dynamics of Hot Electrons in Plasmonic Resonators.

Understanding and managing hot electrons in metals are of fundamental and practical interest in plasmonic studies and applications. A major challenge for the development of hot electron devices requires the efficient and controllable generation of long-lived hot electrons so that they can be harnessed effectively before relaxation. Here, we report the ultrafast spatiotemporal evolution of hot electrons in plasmonic resonators. Using femtosecond-resolution interferometric imaging, we show the unique periodic distributions of hot electrons due to standing plasmonic waves. In particular, this distribution can be flexibly tuned by the size, shape, and dimension of the resonator. We also demonstrate that the hot electron lifetimes are substantially prolonged at hot spots. This appealing effect is interpreted as a result of the locally concentrated energy density at the antinodes in standing hot electron waves. These results could be useful to control the distributions and lifetimes of hot electrons in plasmonic devices for targeted optoelectronic applications.

High-temperature self-energy corrections to x-ray absorption spectra

Effects of finite-temperature quasiparticle self-energy corrections on x-ray absorption spectra are investigated within the finite-temperature quasiparticle local density $GW$ approximation up to temperatures $T$ of order the Fermi temperature. To facilitate the calculations, we parametrize the quasiparticle self-energy using low-order polynomial fits. We show that temperature-driven decrease in the electron lifetime substantially broadens the spectra in the near-edge region with increasing $T$. However, the quasiparticle shift is most strongly modified near the onset of plasmon excitations.

Towards Embedded Electrochemical Sensors for On-Site Nitrite Detection by Gold Nanoparticles Modified Screen Printed Carbon Electrodes.

The transition of electrochemical sensors from lab-based measurements to real-time analysis requires special attention to different aspects in addition to the classical development of new sensing materials. Several critical challenges need to be addressed including a reproducible fabrication procedure, stability, lifetime, and development of cost-effective sensor electronics. In this paper, we address these aspects exemplarily for a nitrite sensor. An electrochemical sensor has been developed using one-step electrodeposited (Ed) gold nanoparticles (EdAu) for the detection of nitrite in water, which shows a low limit of detection of 0.38 µM and excellent analytical capabilities in groundwater. Experimental investigations with 10 realized sensors show a very high reproducibility enabling mass production. A comprehensive investigation of the sensor drift by calendar and cyclic aging was carried out for 160 cycles to assess the stability of the electrodes. Electrochemical impedance spectroscopy (EIS) shows significant changes with increasing aging inferring the deterioration of the electrode surface. To enable on-site measurements outside the laboratory, a compact and cost-effective wireless potentiostat combining cyclic and square wave voltammetry, and EIS capabilities has been designed and validated. The implemented methodology in this study builds a basis for the development of further on-site distributed electrochemical sensor networks.

Assembly of a novel Fe2TiO5-impregnated donor-π-acceptor conjugated carbon nitride for highly efficient solar water splitting

Carbon nitride (CN) exhibits low photocatalytic performance for the energy conversion during photocatalytic water splitting (PWS) under visible light spectrum. Designing novel high-performance CN-based photocatalysts via providing advanced intrinsic characteristics to pristine CN material may further endorse the PWS process. Integrating organic molecules within the skeleton of CN is an interesting strategy to lessen the extent of charge recombination by promoting effective charge separation and transportation, thereby boosting light absorption and photocatalytic performances. Yet, limited progresses have been made along this line. We have carefully chosen and fused previously unexplored organic donor-π-acceptor-type 2,6-diaminopurine (DP) molecules within the skeleton of CN via a superficial organic molecular engineering. Moreover, the subsequent introduction of Fe2TiO5 with copolymerized CN-DP10.0 and pristine CN provide considerably more surface-active sites to form heterojunction material, synergistically extending both the optical absorption range and the lifetime of photoinduced electron. More importantly, with a typical type-II heterojunction formation, the CN and Fe2TiO5 energy levels are overlapped and compatible, thereby significantly accelerating both electron and hole transfer processes. These beneficial characteristics in turn lead to an outstanding apparent quantum yield (AQY) of 61.7% at a wavelength of 420 nm, together with highly efficient and greatly improved photocatalytic hydrogen (H2) (165.7 μmol/h) along with oxygen (O2) (6.2 μmol/h) evolution reactions in visible spectrum at 420 nm. These values are 15.7 and 11.4 times more than those of the pristine CN (9.9 μmol/h and 0.5 μmol/h). Consistent with these experimental findings, our computational simulations show that integration of Fe2TiO5 and DP molecules within the CN framework decreases the band gap from 3.25 to 2.55 eV, suggesting remarkable photocatalytic activities of Fe2TiO5/CN-DP10.0. As a result, it is anticipated that this work will offer a significant avenue for scientific investigation into the next generation of renewable energy employing improved heterostructure photocatalyst predicated on organic ligands.

Dual Charge-Transfer Channels Harmonize Carrier Separation for Efficient U(VI) Photoreduction

The low efficient transfer of photogenerated electrons to an active catalytic site is a pivotal problem for the photoreduction of highly soluble hexavalent uranium [U(VI)] to low soluble tetravalent uranium [U(IV)]. Herein, we successfully synthesized a TiO2-x/1T-MoS2/reduced graphene oxide heterojunction (T2-xTMR) with dual charge-transfer channels by exploiting the difference in Fermi levels between the heterojunction interfaces, which induced multilevel separation of photogenerated carriers. Theoretical and experimental results demonstrate that the presence of the electron buffer layer promoted the efficient migration of photogenerated electrons between the dual charge-transfer channels, which achieved effective separation of photogenerated carriers in physical/spatial dimensions and significantly extended the lifetime of photogenerated electrons. The migration of photogenerated electrons to the active catalytic site after multilevel spatial separation enabled the T2-xTMR dual co-photocatalyst to remove 97.4% of the high concentration of U(VI) from the liquid-phase system within 80 min. This work provides a practical reference for utilizing multiple co-catalysts to accomplish directed spatial separation of photogenerated carriers.

Unveiling Long-Lived Hot-Electron Dynamics via Hyperbolic Meta-antennas.

Conventional plasmonic nanoantennas enable scattering and absorption bands at the same wavelength region, making their utilization to full potential impossible for both features simultaneously. Here, we take advantage of spectrally separated scattering and absorption resonance bands in hyperbolic meta-antennas (HMA) to enhance the hot-electron generation and prolong the relaxation dynamics of hot carriers. First, we show that HMA enables extending plasmon-modulated photoluminescence spectrum toward longer wavelengths due to its particular scattering spectrum, in comparison to the corresponding nanodisk antennas (NDA). Then, we demonstrate that the tunable absorption band of HMA controls and modifies the lifetime of the plasmon-induced hot electrons with enhanced excitation efficiency in the near-infrared region and also broadens the utilization of the visible/NIR spectrum in comparison to NDA. Thus, the rational heterostructures designed by plasmonic and adsorbate/dielectric layers with such dynamics can be a platform for optimization and engineering the utilization of plasmon-induced hot carriers.

Impact of organic amine cations on photoluminescence and magnetic properties in Dion-Jacobson hybrid manganese halide perovskites

A strenuous effort has been made to design multifunctional lead-free organic–inorganic hybrid (OIH) halide compounds, which are envisioned as next generation solar cell materials. However, it is challenging to design OIH halides that can exhibit both long-range magnetic ordering and high photoluminescence quantum yield (PLQY) since the dimensionality of the compounds has a contrasting effect on them. In this article, we have shown an approach to enhance PLQY in two-dimensional (2D) Heisenberg antiferromagnets by increasing the alkylene chain length of [H3N–(CH2)m–NH3]MnCl4 (m = 2, 3, and 4) compounds. All these compounds exhibit 2D layers of corner-sharing MnCl6 octahedra where the organic cations are intercalated between them. These compounds exhibit long-range antiferromagnetic ordering confirmed by the DC magnetic susceptibility and heat capacity measurements. The Néel temperature (TN) decreases with increasing the length of spacer cations due to a decrease in interlayer exchange interactions; however, interestingly, the lifetime of photoexcited electrons and PLQY enhances from 24 to 56 µs and 8% to 23%, respectively. Furthermore, the temperature-dependent photoluminescence measurements provide insight into thermal quenching and exciton binding energy. We believe this study can help to design new OIH halides with long-range magnetic ordering and high PLQY.

Engineering high-coordinated cerium single-atom sites on carbon nitride nanosheets for efficient photocatalytic amine oxidation and water splitting into hydrogen

Developing highly-active rare-earth single atom photocatalysts have attracted extensive attention due to their excellent catalytic properties. Herein, we prepared a single-atom Ce-SA-C3N4 catalyst composed of atomically dispersed rare-earth cerium (Ce) on C3N4 nanosheets by the pyrolysis of cerium-incorporated layered precursor. The atomic distribution and high-coordinated environment of Ce sites were disclosed by aberration-corrected scanning transmission electron microscopy, electron energy-loss spectra, X-ray absorption spectroscopy, and theoretical calculations. In Ce-SA-C3N4, Ce single atoms are coordinated by four N atoms and six O atoms (Ce-N4/O6). The cooperation of single-atom Ce-N4/O6 active sites in C3N4 nanosheets tunes the electronic structure and the surface trap states, resulting in accelerated charge transfer/separation and extended lifetime of photoinduced electrons. Meanwhile, the high-coordinated Ce-N4/O6 active sites could promote the production of superoxide radicals (•O2−) and C = N bond, thus, the optimized single-atom Ce-SA-C3N4 photocatalyst exhibits highly efficient photocatalytic oxidation of amines under visible light irradiation. Furthermore, the fabricated single-atom Ce-SA-C3N4 photocatalysts are applied to split water into hydrogen, producing the maximum hydrogen yield of 33.5 mmol h−1 g−1. The apparent quantum efficiency for hydrogen evolution achieves 32.6% at 420 nm. This study provides a guideline for rationally designing efficient high-coordinated rare-earth single-atom active sites for efficient solar energy conversion and utilization.

Transient Absorption Spectrum Analysis for Photothermal Catalysis Perovskite Materials

To gain insight into photocatalytic behavior, transient absorption spectroscopy (TAS) was used to study LaCoxMn1−xO3, LaMnxNi1−xO3 and LaNixCo1−xO3 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) on a microsecond time scale. The results show that the electron lifetime is key to determining the photocatalytic reduction of CO2. This is the first time that the photogenerated electron lifetime in perovskite has been proposed to express the performance of the photocatalytic reduction of CO2 with H2O into CH4. In all cases, the decay curve can be well explained by two consecutive first-order kinetics, indicating that the electron exists within two major populations: one with a short lifetime and the other one with a long lifetime. The long-lived electrons are the rate-limiting species for the photocatalytic reaction and are related to the activity of the photocatalytic reduction of CO2 with H2O to produce CH4. For different photocatalysts, we find that the longer the electron decay lifetime is, the stronger the electron de-trapping ability is, and the electrons perform more activity. In this paper, TAS can not only detect the micro-dynamics process of carriers, but it is also demonstrated to be an easy and effective method for screening the most active catalyst in various catalysts for the photocatalytic reduction of CO2 with H2O accurately and quickly.

TiO2 passivation layers with laser derived p-n heterojunctions enable boosted photoelectrochemical performance of α-Fe2O3 photoanodes

Addressing the carrier loss at the semiconductor-liquid junction of metal oxides is of significance for developing high-performance photoelectrodes for viable solar hydrogen generation, while one of the critical challenges remains in pursuing the top passivation layer with pronounced carrier extraction capability. In the present work, we propose and experimentally verify that artificial engineering of the passivation layer via laser embedding of p-n heterointerfaces leads to significantly improved carrier transfer. By virtue of the advantage of generating nanocrystals in any desired solvents via the technology of laser synthesis and processing of colloids (LSPC), p-type CdTe nanocrystals are successfully embedded in a thin layer of TiO2, which is adopted as an efficient passivation layer to address the excessive surface defects and the slow carrier transfer at α-Fe2O3 photoanodes. The embedded p-n heterointerfaces not only provide an effective built-in electric field that accelerates carrier transport in TiO2 via boosting polaron hopping but also activate hole transfer by shifting up the valence band in TiO2. Such vivid embedding is demonstrated to generate a 12-fold increase for the lifetime of photogenerated electrons, and a 3.9-fold increase for the ηinj at 1.23 VRHE, leading to a 4.4-fold increase in the photocurrent density at 1.23 VRHE. We thus believe the present work provides a universal alternative for regulating the chemical/physical features of semiconductor-liquid junctions at metal oxides.

Room Temperature Operation of a Quantum Ratchet Intermediate Band Solar Cell

An intermediate band solar cell which generates photocurrent due to sequential absorption of two sub‐bandgap photons at temperatures up to 300 K was realized. The intermediate band is comprised of a “quantum ratchet” semiconductor nanostructure which is designed to store the photoelectron in a long‐lived state that allows for efficient optical re‐absorption; it incorporates a potential barrier designed to both increase the confinement of electrons and reduce thermal escape. This leads to extended electron lifetimes in the ratchet state and room temperature operation. At low temperature (≈12 K), electrons exhibit a ratchet state lifetime of τRB > 100 s, representing an increase by a factor of ≈107 over a device without the barrier.

Stabilized Brookite Nanotube Array Films Grown on Transparent Substrates for Photoelectrochemical Water Splitting

Photoelectrochemical water splitting is a promising route to realizing a clean energy economy. Herein, we report on the ability to selectively fabricate optimized Ni-doped brookite nanotube array films on fluorine-doped tin oxide (FTO) substrate using ultrarapid radio frequency sputtering followed by electrochemical anodization and thermal annealing under ambient air. Ni doping promotes photocatalytic performance by introducing impurity bands within the TiO2 bandgap. Further, annealing at various temperatures helps to tune the photoactivity of the fabricated photoelectrodes, which is in agreement with the nature of the trap states. The nanotubes enjoy high crystallinity and enhanced photoresponse with a noticeable bandgap of 2.7 eV. The Mott–Schottky analysis reveals the variation in the charge carriers density of the fabricated nanotubes. Most importantly, the electrochemical impedance spectroscopy (EIS) analysis unravels the involved kinetics and helps to determine the charge transfer properties of the annealed nanotubular films under open-circuit voltage (OCV) and with applied potential under dark and illumination conditions. EIS analysis reveals decreased charge transfer resistance under illumination, in agreement with the obtained photocurrent, electron lifetime, and density of trap states.

Ultrafast electron dynamics in Au/Fe/MgO(001) analyzed by Au- and Fe-selective pumping in time-resolved two-photon photoemission spectroscopy: Separation of excitations in adjacent metallic layers

The transport of optically excited, hot electrons in heterostructures is analyzed by femtosecond, time-resolved two-photon photoelectron emission spectroscopy (2PPE) for epitaxial Au/Fe/MgO(001). We compare the temporal evolution of the 2PPE intensity upon optically pumping Fe or Au, while the probing occurs on the Au surface. In the case of Fe-side pumping, assuming independent relaxation in the Fe and Au layers, we determine the hot electron relaxation times in these individual layers by an analysis of the Au layer thickness dependence of the observed, effective electron lifetimes in the heterostructure. We show in addition that such a systematic analysis fails for the case of Au-side pumping due to the spatially distributed optical excitation density, which varies with the Au layer thickness. This work extends a previous study [Beyazit et al., Phys. Rev. Lett. 125, 076803 (2020)] by new data leading to reduced error bars in the determined lifetimes and by a non-linear term in the Au-thickness dependent data analysis which contributes for similar Fe and Au film thicknesses.

Promoted photocatalytic hydrogen evolution via double-electron migration in Ag@g-C3N4 heterojunction

The unsaturated edge Ag introduced on the surface of photocatalysts plays an important role in boosting photo-excited electrons for the photoreduction H2O reaction. However, a moderate and tractable strategy to efficiently expose edge Ag remains an enormous challenge. For this purpose, a core skeleton with ‘Ag conductive nanosphere array’ inside and a two-dimensional sheet structure with g-C3N4 layer outside are formed. The introduction of Ag nanospheres into g-C3N4 not only enlarges the distance between g-C3N4 nanolayers, but also increases the exposure of Ag nanospheres. When Ag intimately contacts with g-C3N4, the electrons of g-C3N4 voluntarily flow to the Ag. Consequently, a built-in electric field (IEF) has been formed at interface of Ag@g-C3N4 heterojunction, which prevents the continuous flow of electrons from g-C3N4 to Ag. Under irradiation, the e− accumulated in Ag tend to recombine with the h+ in the valence band of g-C3N4 which is driven by Coulomb interaction and IEF. Ag nanospheres are fabricated as a co-catalyst to decorate the g-C3N4 nanolayer, which hinder the conglomeration of g-C3N4 nanolayers. Moreover, Ag@g-C3N4 heterojunction provides polyunsaturated edge Ag as active sites, inducing prolonged lifetime of photogenerated electrons and formed the unique charge transfer channels. In addition, abundant nitrogen vacancies are formed, which strengthens the chemisorption of H2O. As a result, supreme Ag@g-C3N4 realizes a high H2 evolution of 312.5 μmol and preserves a good sustainability. This paper emphasizes the importance of unique electron transfer pathway and chemisorption of water for photoreduction H2O.

The Effect of Acetylene Carbon Black (ACB) Loaded on Polyacrylonitrile (PAN) Nanofiber Membrane Electrolyte for DSSC Applications

Nanofiber membranes are starting to be used as an electrolyte storage medium because of their high porosity, which causes ionic conductivity, producing high energy. The ability of nanofiber membranes to absorb electrolytes proves their stability when used for a long time. In this study, the loading of acetylene carbon black (ACB) on polyacrylonitrile (PAN) is made by the electrospun method, which in turn is applied as an electrolyte medium in DSSC. Materials characterization was carried out through FTIR to determine the functional groups formed and SEM to observe morphology and diameter distribution. Furthermore, for DSSC performance, efficiency and EIS tests were carried out. The optimum nanofiber membrane was shown by esPACB1, with the highest efficiency reaching 1.92% with a porosity of 73.43%, nanofiber diameter of 172.9 ± 2.2 nm, an absorbance of 1850, and an electron lifetime of 0.003 ms.