Electron Lifetime Research Articles

Construction of Ternary Bismuth-Based Heterojunction by Using (BiO)2 CO3 as Electron Bridge for Highly Efficient Degradation of Phenol.

Inspired by nature, it has been considered an effective approach to design artificial photosynthetic system by fabricating Z-scheme photocatalysts to eliminate environmental issues and alleviate the global energy crisis. However, the development of low cost, environment-friendly, and high-efficient photocatalysts by utilizing solar energy still confronts huge challenge. Herein, we constructed a Bi2 O3 /(BiO)2 CO3 /Bi2 MoO6 ternary heterojunction via a facile solvothermal method and calcination approach and used it as a photocatalyst for the degradation of phenol. The optimized Bi2 O3 /(BiO)2 CO3 /Bi2 MoO6 heterojunction delivers a considerable activity for phenol photodegradation with an impressive removal efficiency of 98.8 % and about total organic carbon (TOC) of 68 % within 180 min under visible-light irradiation. The excellent photocatalytic activity was ascribed to the formation of a Z-scheme heterojunction, more importantly, the presence of (BiO)2 CO3 as an electron bridge greatly shortens the migration distance of photogenerated electron from ECB of Bi2 O3 to EVB of Bi2 MoO6 , thus prolonging the lifetime of photogenerated electrons, which is verified by trapping experiments, electron spin-resonance spectroscopy (ESR) results, and density functional theory (DFT) calculations. This work provides a potential strategy to fabricate highly efficient Bi-based Z-scheme photocatalysts with wide application prospects in solar-to-fuel conversion and environmental protection.

Co3O4 Nanosheet/g-C3N4 Hybrid Photocatalysts for Promoted H2 Evolution

Co3O4 is an attractive semiconductor in the photocatalytic field due to its proper band gap; however, its energy level structure is mismatched for H2 evolution. Herein, the Co3O4-based photocatalysts, Co3O4 nanosheet/g-C3N4 hybrids, were successfully prepared by an ultrasonic self-assembly method for enhanced photocatalytic H2 evolution. The optimized hybrid with 20 wt % g-C3N4, 20-CNCo, displayed the best photocatalytic performances, and the average H2 evolution rate was 134.6 μmol·g–1·h–1. The enhanced photocatalytic H2 evolution activities of 20-CNCo were due to the formation of heterojunctions between Co3O4 nanosheets and g-C3N4, which can increase the optical absorption ability, promote the separation of photogenerated charges, accelerate the electron transfer, and prolong the lifetime of the excited electrons. Moreover, following the unique step-scheme (S-scheme) charge transfer mechanism, the strong redox ability of the Co3O4 nanosheet/ g-C3N4 hybrid was retained, which was beneficial to the H2 evolution. This work provides strategies for designing active catalysts for photocatalytic reactions.

Performance Improvement of Dye-Sensitized Solar Cells with Pressed TiO2 Nanoparticles Layer

A simple and low-cost fabrication method of dye-sensitized solar cells (DSSCs) was developed to improve the structure and performance of the photoanode with the pressed layer and compact TiO2 thin film using spin coating, screen printing, and mechanical compression. In this study, four different TiO2 layers were adopted to fabricate photoanodes: a mesoporous TiO2 nanoparticles (NPs) layer, a pressed TiO2 NPs layer, a mesoporous TiO2 NPs layer on the TiO2 compact thin film, and a pressed TiO2 NPs layer on the TiO2 compact thin film. The compact thin film was deposited on the fluorine-doped tin oxide (FTO) glass via spin coating, while the mesoporous TiO2 NPs layer was deposited via the screen-printing method. The pressed TiO2 NPs layer was produced by compressing the mesoporous TiO2 NPs layer with a hydraulic press machine. When using the pressed TiO2 NPs layer for the photoanode of DSSC, the power conversion efficiency of DSSC was enhanced the most. The electron lifetime for DSSC with photoanodes based on the pressed TiO2 NPs and mesoporous TiO2 NPs layers were 8.217 and 6.287 ms, respectively. The power conversion efficiency of DSSC with photoanodes based on the pressed TiO2 NPs layer was 5.4%, while that based on the mesoporous TiO2 NPs layer was 4.08%. DSSC with photoanodes based on the pressed TiO2 NPs layer showed a significant increase in the power conversion efficiency by 36.16% compared to that based on the mesoporous TiO2 NPs layer.

Heteroatom‐ and Bonded Z‐Scheme Channels‐Modulated Ultrafast Carrier Dynamics and Exciton Dissociation in Covalent Triazine Frameworks for Efficient Photocatalytic Hydrogen Evolution

AbstractCovalent triazine frameworks (CTF) offer a tunable platform for photocatalytic H2 generation due to their diverse structures, low costs, and precisely tunable electronic structures. However, high exciton binding energy and short lifetimes of photogenerated carriers restrict their application in photocatalytic hydrogen evolution. Herein, a novel phosphorus‐incorporated CTF is introduced to construct a chemically bonded PCTF/WO3 (PCTFW) heterostructure with a precise interface electron transfer channel. The phosphorus incorporation is found to dominantly reduce the exciton binding energy and promote the dissociation of singlet and triplet excitons into free charge carriers due to the regulation of electronic structures. High‐quality interfacial WN bonds improve the interfacial transfer of photogenerated electrons, thus prolonging the lifetime of photogenerated electrons. Femtosecond transient absorption spectroscopy characterizations and DFT calculations further confirm both phosphorus incorporation and Z‐scheme heterojunctions can synergistically boost the in‐built electric field and accelerate the migration and separation of photogenerated electrons. The optimized photocatalytic H2‐evolution rate of resultant PCTFW is 134.84 µmol h−1 (67.42 mmol h−1g−1), with an apparent quantum efficiency of 37.63% at 420 nm, surpassing many reported CTF‐based photocatalysts so far. This work highlights the significance of atom‐level interfacial exciton dissociation, and charge transfer and separation in improving photocatalysis.

Free Electron Dynamics and Intrinsic Mobility in Ga2O3 Revealed by Transient Terahertz Conductivity

Here, we investigate the photoconductivity of gallium oxide thin films at different temperatures using time-resolved terahertz spectroscopy. The photogenerated electrons in the conduction band show a monoexponential decay, implying a first-order electron depopulation mechanism. The electron lifetime increases with rising temperature, and this trend coincides with the temperature dependence of the electron mobility rather than diffusion coefficient, suggesting that electron-hole recombination is determined by directional electron drift instead of random diffusion. The electron mobilities extracted from the transient terahertz conductivity are substantially greater than the previously reported Hall mobilities over a wide temperature range, and this is probably because the electron drift in response to the terahertz field is immune from scattering with macroscopic defects. Thus, the mobilities measured here may represent the intrinsic limit of the electron mobility in gallium oxide crystals. Our finding suggests that the current Hall mobility in this wide bandgap semiconductor is still far below the limit, and the long-range electron transport can be further increased by improving the crystalline quality.

Enhanced photo conversion efficiency of Nb2O5/TiO2 bilayer photoanode for Dye-Sensitized Solar Cells

This work is focused to improve the photovoltaic parameters in DSSC by introducing a bilayer photoanode. Nb2O5/TiO2 bilayer photoanodes were prepared by screen printing method. The microstructural, crystalline nature, and morphological properties were analysed. Charge transfer processes in Nb2O5/TiO2 bilayer photoanode and TiO2 photoanode were analysed. The electron behavior in the device was investigated by electrochemical impedance spectroscopy (EIS) measurement. It was inferred that charge transfer resistance significantly lowered in the Nb2O5/TiO2 bilayer photoanode. In addition, the effective electron lifetime was significantly improved, and the recombination rate was reduced. DSSCs devices were fabricated using Nb2O5/TiO2 bilayer and TiO2 unilayers as photoanode, N719 dye, iodide-based electrolytes and platinum counter electrode. Highest efficiency of 5.4% was achieved in the bilayer photoanode system for an optimum thickness of Nb2O5.

Influence of Adsorption of para‐Aminothiophenol on Trap States and Recombination Kinetics at Plasmonic Ag‐TiO2 Heterostructure Interfaces

AbstractThe variation of trap states and the involving kinetics on the Ag‐TiO2 interfaces after the adsorption of reactant molecules were explored with the help of the combination of spectroscopic, electrochemical and photoelectrochemical methods by using para‐aminothiophenol (PATP) as a probe molecule. The results of photoluminescence spectroscopy and chronoamperometry revealed the distribution of trap states of Ag‐TiO2 heterostructures and the influence of PATP's adsorption. Kinetic measurements of open‐circuit photovoltage showed that the adsorption of PATP affects not only the electron lifetime (τn) but also the recombination mechanism of charge‐carrier on Ag‐TiO2 heterostructures. The analysis of the obtained effective recombination orders indicated that the charge‐carrier recombination on the Ag‐TiO2 electrode followed second‐order kinetics, while the one of the Ag‐TiO2/PATP electrode evolved from first‐order kinetics at first (at higher Voc) to second‐order kinetics (at lower Voc).

LSPR-enhanced carbon-coated In2O3/W18O49 S-scheme heterojunction for efficient CO2 photoreduction

The special defect structure and localized surface plasmon resonance (LSPR) effect offer W18O49 extraordinary potential and research value in photocatalysis. The LSPR effect optimizes the design of W18O49-sensitized photocatalytic composites and broadens the light-response range of W18O49. However, the high-energy “hot electrons” generated by W18O49 under the LSPR effect exhibit an extremely short lifetime and cannot be fully utilized. Therefore, the high electron conductivity of carbon can be used to increase the rate of hot-electron transfer, thereby extending the lifetime of hot electrons. In this study, a heterojunction photocatalyst was formed by growing a high-absorbance one-dimensional nanowire W18O49 on the surface of carbon-coated porous In2O3 nanorods (C-In2O3) derived from In-MOF. The C-In2O3/W18O49composites exhibited optical responses in both the visible and near-infrared regions. The carbon coatings acted as transport channels to accelerate the transfer of carriers and hot electrons, and the activity of photocatalytic CO2 reduction (PCR) was significantly enhanced. The 40%C-In2O3/W18O49 composites had the highest CO yield in the photocatalytic reactions, which was 2.99 and 2.84 times greater than that of pure C-In2O3 and W18O49, respectively. The internal electronic transfer in the S-scheme heterojunction and LSPR-induced hot electrons injected into C-In2O3 achieved dual-path electron transfer for PCR.

Benzimidazole-based covalent organic framework embedding single-atom Pt sites for visible-light-driven photocatalytic hydrogen evolution

Visible-light-driven photocatalytic hydrogen evolution reaction (HER) over a semiconductor provides an effective avenue to produce renewable clean energy and alleviate energy and environmental crises. However, the HER efficiency is still limited by the sluggish electron transfer process. Herein, a highly active covalent organic framework (COF) was constructed from the unusual benzimidazole monomer in a microwave-assisted solvothermal pathway. With single-atom Pt sites as cocatalyst, the catalyst exhibited an HER rate up to 115 mmol g–1 h–1 and a high turnover frequency of 4475.1 h–1 under visible-light irradiation. The above performance relied on the combination of benzimidazole moieties and COF framework, which, on the one hand, stabilized photogenerated electrons to prolong the electron lifetime, and on the other hand provided a strong host-guest interaction that resulted in the creation of single-atom Pt sites and the acceleration of the electron-transfer to the active sites for proton reduction. This work demonstrates the perspective of electron stabilization and interfacial charge transfer avenue construction in the HER process, which can be reached by a molecular-level design of COF-based organic semiconductors by using structural and functional diverse asymmetric building blocks.

Femtosecond Autocorrelation of Localized Surface Plasmons.

Plasmon electronic dephasing lifetime is one of the most important characteristics of localized surface plasmons, which is crucial both for understanding the related photophysics and for their applications in photonic and optoelectronic devices. This lifetime is generally shorter than 100 fs and measured using the femtosecond pump-probe technique, which requires femtosecond laser amplifiers delivering pulses with a duration even as short as 10 fs. This implies a large-scale laser system with complicated pulse compression schemes, introducing high-cost and technological challenges. Meanwhile, the strong optical pulse from an amplifier induces more thermal-related effects, disturbing the precise resolution of the pure electronic dephasing lifetime. In this work, we use a simple autocorrelator design and integrate it with the sample of plasmonic nanostructures, where a femtosecond laser oscillator supplies the incident pulses for autocorrelation measurements. Thus, the measured autocorrelation trace carries the optical modulation on the incident pulses. The dephasing lifetime can be thus determined by a comparison between the theoretical fittings to the autocorrelation traces with and without the plasmonic modulation. The measured timescale for the autocorrelation modulation is an indirect determination of the plasmonic dephasing lifetime. This supplies a simple, rapid, and low-cost method for quantitative characterization of the ultrafast optical response of localized surface plasmons.

Fabrication of Planar Perovskite Solar Cells Using Ternary Metal Oxide Nanocomposite as Hole-Transporting Material

This work uses a hole-transporting copper cobaltite/copper oxide nanocomposite to fabricate carbon-based MAPbI3 PSCs. The copper cobaltite/copper oxide HTM-based PSC results show the highest power conversion efficiency (PCE = 7.32%) compared with an HTM-free device. The highest photocurrent density (Jsc = 15.17 mA/cm2), open-circuit voltage (Voc = 0.82 V), and fill factor (FF = 0.59) are achieved for the PSC fabricated with hydrothermally synthesized copper cobaltite/copper oxide nanocomposites. Electrochemical impedance spectroscopy is used to analyze the charge transfer resistance (Rs) and the capacitive behavior of copper cobaltite/copper oxide nanocomposite. The maximum electron lifetime of 35.16 μs is witnessed for the PSCs fabricated with 3 mg mL−1 of copper cobaltite/copper oxide (H1). The efficiency of the copper cobaltite/copper oxide-based PSC remains unchanged, showing no further perovskite layer degradation.

Ultrathin TiO2 Blocking Layers via Atomic Layer Deposition toward High-Performance Dye-Sensitized Photo-Electrosynthesis Cells

The collection of solar energy in chemical bonds via dye-sensitized photoelectrosynthesis cells (DSPECs) is a reliable solution. Herein, atomic layer deposition (ALD) introduced ultrathin blocking layers (BLs) between a mesoporous TiO2 membrane and fluorine-doped tin oxide (FTO), and much improved photoelectrochemical water oxidation performance was well documented. Samples with different BL thicknesses deposited on FTO were obtained by ALD. In the photoanode, polypyridyl Ru(II) complexes were used as photosensitizers, and Ru(bda)-type was used as a catalyst during water oxidation. Under one sun irradiation, the BL (i) increased the photocurrent density; (ii) slowed down the open-circuit voltage decay (OCVD) by electrochemical measurement; (iii) increased the photo-generated electron lifetime roughly from 1 s to more than 100 s; and (iv) enhanced the water oxidation efficiency from 25% to 85% with 0.4 V of applied voltage bias. All this pointed out that the ALD technique-prepared layers could greatly hinder the photogenerated electron–hole pair recombination in the TiO2-based photoanode. This study offers critical backing for the building of molecular films by the ALD technique to split water effectively.

Heterogeneous activation of persulfate using delafossite AgFeO2/α-MnO2 for efficient degradation of tartrazine under visible light

For the first time, delafossite AgFeO2/α-MnO2 nanocomposites were successfully synthesized and used as heterogeneous persulfate (PS) activators for degradation of water pollutants. The potential of several systems, such as AgFeO2/α-MnO2/light, AgFeO2/α-MnO2/PS/light, etc., as well as reaction conditions, stability, and radicals involved, were all assessed for degradation of tartrazine (TZ). The exceptional activity of optimal AgFeO2/α-MnO2 (AFM50) nanocomposite was attained because of improved redox cycling between Ag0/Ag+, Fe2+/Fe3+, Mn3+/Mn4+ cycles, enhanced electron transfer, reduced e−/h+ pair recombination, long electron lifetime, high charge transfer and mobility, and electron density, which facilitated efficient PS activation and subsequent generation of various reactive oxidizing species. Interestingly, AFM50 showed exceptional degradation efficiency (67.4%) under dark conditions. The AFM50/PS/Vis system also demonstrated excellent stability and activity for degrading several dyes (rhodamine B (97.8%), methyl orange (94.5%), methylene blue (100%), and congo red (97.7%)). Based on quenching experiments, active radicals (•OH, SO4•-, O2•-), and non-radical (h+, 1O2), all contributed towards efficient degradation of TZ. Following that, a reasonable PS activation mechanism was proposed. This study propounds insightful synergistic activation of PS using delafossite AgFeO2/α-MnO2 nanocomposite under visible light and dark conditions, as well as its potential application in wastewater treatment plants.

Improving fast-timing time-walk calibration standards: Lifetime measurement of the 2[formula omitted] state in 152Gd

A 152Eu source was measured for 28 days using an experimental setup consisting of four LaBr3(Ce) detectors, connected to a CAEN V1730 digitizer, implementing online interpolation constant fraction discrimination for picosecond-precise timestamp determination. Using the definition of the time-walk curve, the lifetime of the 21+ (344 keV) state in 152Gd was re-measured, resulting in τ(21+)=46.9(3) ps. Compared to the previously adopted lifetime the uncertainty is reduced by an order of magnitude. This improved lifetime is of significant importance for electronic fast-timing lifetime measurements, and the impact on the systematic correction procedure and lifetime measurements in the low picosecond regime are discussed.

D-Band Hole Dynamics in Gold Nanoparticles Measured with Time-Resolved Emission Upconversion Microscopy.

The performance of photocatalysts and photovoltaic devices can be enhanced by energetic charge carriers produced from plasmon decay, and the lifetime of these energetic carriers greatly affects overall efficiencies. Although hot electron lifetimes in plasmonic gold nanoparticles have been investigated, hot hole lifetimes have not been as thoroughly studied in plasmonic systems. Here, we demonstrate time-resolved emission upconversion microscopy and use it to resolve the lifetime and energy-dependent cooling of d-band holes formed in gold nanoparticles by plasmon excitation and by following plasmon decay into interband and then intraband electron-hole pairs.

Transport and quantum lifetimes of electrons in modulation doped Al0.3Ga0.7As/In0.15Ga0.85As double quantum well structure

The enhancement of transport and quantum lifetimes of electrons in barrier delta-doped Al0.3Ga0.7As/In0.15Ga0.85As pseudomorphic Double-Quantum-Well structure is analysed as a function of applied electric field and well width. We consider screened ionised-impurity, interface-roughness, and alloy-disorder scattering potentials and show that the quantum lifetime is dominated by ionised-impurity scattering while the transport lifetime is decided by most of the scattering mechanisms. In the absence of electric field, there is double subband occupancy, and therefore, the transport as well as quantum lifetimes are influenced by the intersubband effects. Application of electric field perpendicular to the interface of the quantum well structure enhances the transport lifetime whereas reduces quantum lifetime non-monotonically resulting in a rapid enhancement of their ratio through ionised impurity scattering. Further, as the well width increases, the effect of ionised-impurity scattering decreases leading to the enhancement of both the transport and quantum lifetimes as well as their ratio. In fact, for well width of 150 Å, an improvement in the ratio of transport and quantum lifetimes can be achieved from 9.8 to 90 by increasing the electric field from 0 to 7.6 kV/cm.

Mechanism for Long Photocurrent Time Constants in α-Ga2O3 UV Photodetectors

Deep centers and their influence on photocurrent spectra and transients were studied for interdigitated photoresistors on α-Ga2O3 undoped semi-insulating films grown by Halide Vapor Phase Epitaxy (HVPE) on sapphire. Characterization involving current-voltage measurements in the dark and with monochromatic illumination with photons with energies from 1.35 eV to 4.9 eV, Thermally Stimulated Current (TSC), Photoinduced Current Transients Spectroscopy (PICTS) showed the Fermi level in the dark was pinned at Ec−0.8 eV, with other prominent centers being deep acceptors with optical thresholds near 2.3 eV and 4.9 eV and deep traps with levels at Ec−0.5 eV, Ec−0.6 eV. Measurements of photocurrent transients produced by illumination with photon energies 2.3 eV and 4.9 eV and Electron Beam Induced Current (EBIC) imaging point to the high sensitivity and external quantum efficiency values being due to hole trapping enhancing the lifetime of electrons and inherently linked with the long photocurrent transients. The photocurrent transients are stretched exponents, indicating the strong contribution of the presence of centers with barriers for electron capture and/or of potential fluctuations.

Low carrier recombination in polysiloxane gel electrolyte for high-performance DSSC

Previous research on quasi-solid-state DSSC (QSS-DSSC) that utilized polysiloxane-based polymer gel electrolytes (PGE) showed that the functional performance of the cells was highly affected by electrolyte infiltration into the TiO2 nano-porous layers. This study evaluated the efficiency enhancement in siloxane-based cells by introducing a TiCl4 pre-treatment process twice. We compared the impedance spectrum of PGE-based DSSC without (type-1 PGE) and with (type-2 PGE) with the addition of propylene carbonate, measured under dark and light illumination. The impedance spectra of both cells showed different characteristics at different condition measurements, especially in the high-frequency region. Unlike the type-2 PGE-based DSSC, the type-1 PGE-based DSSC did not show the transmission line characteristic, which indicated less charge carrier diffusion inside the TiO2 nano-porous layer. Under light illumination, the interfacial charge transfer between electrons inside TiO2 layers with the electrolyte (Rct), and the electron lifetime inside TiO2 layers before it is recombined (τ r), became smaller for type-2 PGE-based DSSC and larger for type-1 PGE-based DSSC. This indicated that the recombination rate increased as the PGE became more vicious. This result supports the photovoltaic characteristics that yield current density and efficiency values of 16 mA cm−2 and 5.37% for type-2 PGE-based DSSC, 13.4 mA cm−2, and 4.72% for type-1 PGE-based DSSC. The challenge for further improvement in DSSC that employs PGE is to elevate the wetting capability of the gel inside the TiO2 layer without additional solvent since additional solvent eventually can reduce ionic concentration and consequently increase the Rct value as shown in the analysis of the impedance spectrum of TiO2 layer without dye.

Precisely modulate interfacial Bi-O bridge bond in Co-TCPP/Bi3O4Br to trigger long-lasting charge separation for boosting CO2 photoreduction

Insufficient charge separation and feeble CO2 activation limit the CO2 photoreduction efficiency. It is highly desirable to consciously construct organic–inorganic hybrid composites to simultaneously accelerate charge separation and provide favorable active sites. Herein, a defect-induced interfacial Bi-O bridge bond is constructed by grafting terminal O of cobalt porphyrin (Co-TCPP) with Bi3O4Br. Systematic investigations reveal that the Bi-O bridge bond as the charge migration bridge accelerates the extraction and transfer of electron from the external [Bi3O4] layers to Co-TCPP, and the millisecond separation lifetime of electrons on Co-TCPP can be achieved. Co atoms as the active sites optimized the CO2 adsorption and activation, thus promoting the formation of COOH*. As a result, the CO2 photoreduction rate of 0.5% Co-TCPP/Bi3O4Br reaches 71.3 μmol g-1h-1 in pure water, 2.53-fold of that on the pristine Bi3O4Br. This work provides atomistic insights and strategies for the construction of new organic–inorganic hybrid materials for artificial photosynthesis and CO2 photoreduction.

Boosting piezo-photocatalytic activity of BiVO4/BiFeO3 heterojunctions through built-in polarization field tailoring carrier transfer performances

The greatest challenge that limits the application of piezo-photocatalytic materials is the low separation efficiency of the generated electron-hole pairs, resulting in poor catalytic activity. Here, the semiconductor n-p heterojunctions BiVO4/BiFeO3 (BVO/BFO) were designed to enhance its piezo-photocatalytic processes. Under the excitation of piezo-vibration and the irradiation of visible light, the BVO/BFO heterojunctions exhibited ultra-high and stable piezo-photocatalytic performance with the degradation rate of Rhodamine B (RhB) solution up to 98 %, and its k value was 6.12 times than that of photocatalysis and 4.36 times than that of piezoelectric catalysis. Thanks to the n-type BVO nanoparticles with good crystallinity were uniformly distributed on the surface of the p-type piezoelectric material BFO, the built-in polarization field was formed and be advantageous to improve the carrier transport performances. A large electron diffusion coefficient (27.44 × 103 cm2·s−1), effective diffusion length (14.49 cm), and long electron lifetime (7.66 × 10-3 s) were achieved in the BVO/BFO heterojunctions, which played important roles to boost the piezo-photocatalytic activity. The preparation of BVO/BFO heterojunctions and their remarkable photo-piezoelectric properties provides a theoretical and practical reference for the development of efficient piezo-photocatalysis to apply in environmental remediation.