Lifetime Of Photoexcited Electrons Research Articles

An ultrahigh hydrogen production rate of photocatalyst Fe2S2(CO)6/MoS2/CdS integrated mimic hydrogenase and capacitor for H2 evolution with longer lifetime of photoexcited electrons

The hydrogenase mimics provide an innovative strategy for photocatalytic hydrogen production; however, the efficiency mismatch between electron production and utilization results in low hydrogen production rate. Herein, we report the integration of non-noble metal cocatalyst MoS2 decorated with hydrogenase model complex Fe2S2(CO)6 and CdS QDs to assemble Fe2S2(CO)6/MoS2/CdS QDs, which exhibited an excellent hydrogen production rate of 10,836 μmol/g/h. This rate is 127.4, 9.1, and 1.6 times higher than that of CdS QDs, Fe2S2(CO)6/CdS QDs, and MoS2/CdS QDs, respectively. In comparison to CdS or MoS2/CdS, transient photovoltage (TPV) measurements revealed that the photovoltage response of Fe2S2(CO)6/MoS2/CdS increased numerically by 3.3 times; meanwhile, Fe2S2(CO)6/MoS2/CdS significantly extended the lifetime of photo-generated electrons. The excellent performance was attributed to the distinct capacitance effect and functional active site of Fe2S2(CO)6/MoS2/CdS, which boosted electron storage and redistribution at the interface to alleviate recombination losses and facilitate hydrogen production.

Heterostructured grafting of NiFe-layered double hydroxide@TiO2 for boosting photoelectrochemical cathodic protection.

Accelerating the oxidation process at photoanode-electrolyte interfaces can prolong the lifetime of photoexcited electrons and improve the efficiency of photoelectrochemical cathodic protection (PECCP) systems without relying on hole scavengers. However, the systematic design of precisely structured heterostructures for efficient photoanodes remains challenging. Here we meticulously engineered a type-II heterostructure featuring precise spatial organization, wherein NiFe-layered double hydroxide nanosheets (NiFe-LDH NSs) were assembled onto annealed TiO2 nanorod arrays (ATNAs), demonstrating their effectiveness in achieving efficient PECCP. The interfacial electronic coupling and appropriate energy alignment between the NiFe-LDH NSs and ATNAs allowed rapid hole extraction from the ATNAs to the NiFe-LDH NSs. Furthermore, the uniform distribution of the NiFe-LDH NSs on top of ATNAs drastically reduced the overpotential of oxygen evolution reactions (OER) from 370 to 200 mV and Tafel slope from 162 to 56 mV dec-1, leading to significantly improved cathodic protection of 304 stainless steel (SS) under extended illumination and interesting post-illumination protection. In addition, with the increase of testing cycles, the as-prepared NiFe-LDH NSs@ATNAs demonstrated a progressively enhanced cathodic protection potential from 0.15 to 0.13 V vs. RHE over 50 cycles. These findings provide important guidelines for the design of future high-efficiency green metal protection through rational photoanode design.

Plasmonic Au Nanoparticle of a Au/TiO2-C3N4 Heterojunction Boosts up Photooxidation of Benzyl Alcohol Using LED Light.

Plasmonic Au nanoparticles (NPs) employing localized surface plasmon resonance excitation have exhibited superior visible light absorption for many organic transformations. In this work, we prepared a ternary composite catalyst comprising plasmonic Au NPs and a 2D/2D TiO2-C3N4 heterojunction via a photoreduction method of chloroauric acid in the presence of TiO2-C3N4. The introduction of plasmonic nanogold particles embedded onto the TiO2 surface of the TiO2-C3N4 heterojunction can significantly improve the photocatalytic performance during photooxidation of benzyl alcohol to benzaldehyde under mild conditions (1 bar air, white LED irradiation at ambient temperature). The productivity over Au/TiO2-C3N4 (0.25 mmolreactedBA gcat.-1 h-1) is found to be ∼5.6, 8.3, and 8.2-fold of these over the Au/TiO2, TiO2-C3N4, and C3N4-Au-TiO2 heterojunctions, respectively. Trapping experiments and electron spin resonance (ESR) spectroscopy confirm that the superoxide (·O2-) and hydroxyl radicals (·OH) act as the reactive oxygen species during photooxidation. Furthermore, the experimental results combined with density functional theory calculations reveal that the chemisorbed benzyl alcohol population, surface oxygen vacancies, and lifetime of photoexcited electrons and holes are largely improved by plasmonic Au NPs. This study on nanogold composites provides some hints for developing new efficient and practical photocatalysts.

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.

Critical Role of the NiO–Ni(OH)2 Species Loaded Onto TiO2 Photocatalyst in the Enhancement of H2 Evolution Activity

One of the bottlenecks in photocatalysis is to obtain reasonably cheap co-catalysts beneficially contributing to efficient photophysical and photochemical processess. Herein, we prepare a co-catalyst system consisting of Ni species embedded in anatase TiO2. We focus on clarifying its role in regulating electron behavior, and hence, photocatalytic activity. Based on the transmission electron microscopy, X-ray absorption near-edge structure, and X-ray absorption fine structure, the surface-loaded Ni species are found to exist as crystalline NiO, together with a small fraction of amorphous Ni(OH)2. Using a combination of transient microwave conductivity and static infrared absorption spectroscopy, we show that electron transfer to the Ni species instead of electron trapping by structural defects, such as oxygen vacancies or reduced titanium cations, is key mechanism responsible for the prolonged lifetime of photoexcited electrons. The prolonged electron lifetime due to an efficient electron extractionby the Ni species are behind the increased H2 evolution activity. We demonstrate that the NiO-Ni(OH)2 co-catalyst system, which works as a H2 evolution site, is auspicious to make photoexcited electrons abundantly available on the surface by localizing them, and thus, readily accessible for the desired surface reactions.

Enhanced Electron-Hole Separation in Phosphorus-Coordinated Co Atom on g-C3N4 toward Photocatalytic Overall Water Splitting.

Revealing the decoration mode of g-C3N4 and understanding the physical mechanism of overall water splitting is important for the further improvement of the photocatalytic activity of g-C3N4-based materials. With core level shift and molecular dynamics simulations based on first-principles calculations, Co1(PHx)3 anchored on the triazine of g-C3N4 is determined as a stable single-atom catalyst with high efficiency for photocatalytic overall water splitting. The separated spin-polarized charge density distribution of valence-band maximum and conduction-band minimum states is beneficial for the long lifetime of photoexcited electrons and holes. An anchored Co single atom site is the active site for oxygen evolution reaction, and nitrogen atoms act as active sites for hydrogen evolution reaction. This new decoration mode of g-C3N4 opens a possible way to functionalize g-C3N4 on both triazine and void sites to realize the separation of OER and hydrogenation reaction by water splitting.

Biocidal activity of citrus limetta peel extract mediated green synthesized silver quantum dots against MCF-7 cancer cells and pathogenic bacteria

Monodispersed silver quantum dots (Ag QDs) of sub 5 nm range are prepared through a facile and size-controllable green synthesis route using sweet lime (Citrus limetta) peel (SLP) extract. Powder X-ray diffraction studies confirm the formation of face-centered-cubic structured Ag QDs which is corroborated by selected area electron diffraction. The Ag QDs show an absorption maximum at ~415 nm due to characteristic surface plasmon resonance. The Ag QDs prepared at 80 °C exhibit high photoluminescence quenching, indicating low recombination rate and high lifetime of photoexcited electrons. The examination of SLP extract by Fourier-transform infrared spectroscopy, LC-MS and NMR spectroscopy reveals the presence of citrate and carbohydrate biomolecules that could be responsible for the formation of small and stable Ag QDs. The cytotoxic studies indicate that the synthesized Ag QDs are able to effect 71% cancer cells death at 100 µg/mL concentration. Moreover, Ag QDs exhibit minimum inhibitory concentration (MIC) value of 50 μg/mL against all the selected pathogenic bacteria. Present findings suggest that Ag QDs obtained by SLP extract mediated green synthesis can be employed as a low-cost and eco-friendly biocidal agent.

Selective CO2 Photoconversion on Low-Coordinate TiO2

The selective conversion of CO2 into desired hydrocarbon products remains a challenge in photocatalysis. In this issue of Chem, Feng and colleagues report a strong-magnetic-field-assisted strategy for forming active low-coordinate (100) facets on TiO2 nanotubes, enabling selective photoconversion of CO2 into the C2 product C2H5OH under visible light.

Simultaneous photodegradation of dyes by NiS/CuS-CdS composites in visible light region

NiS/CuS-CdS (NiS/CuxCd1-xS) composites were prepared via a one-pot hydrothermal route for the simultaneous photocatalytic removal of methyl orange (MO) and methylene blue (MB) in visible light region. The photocatalytic performances of obtained composites were affected by the Cu/Cd molar ratio, NiS content, MO/MB concentration ratio, pH values, and inorganic ions. Among these samples, the optimal 2-NiS/Cu0.8Cd0.2S with the NiS content of 3.45 wt% and Cu/Cd molar ratio of 4:1 exhibits the best photocatalytic activity for the simultaneous removal of MO and MB. Compared with 2-NiS/Cu0.8Cd0.2S alone for MO or MB, the obtained samples exhibits the enhanced removal efficiency of MO while the reduced removal efficiency of MB in mixture solution. The enhanced photocatalytic activity is ascribed to the efficient transfer and separation of charge carriers at the ternary interface of NiS-CuS-CdS heterojunction, in which NiS serves as the electron trapper for prolonging lifetime of photoexcited electrons. The O2− and OH radicals play the vital roles in the enhanced photocatalytic activity of 2-NiS/Cu0.8Cd0.2S for the simultaneous removal of MO and MB.

Efficient infrared light induced CO2 reduction with nearly 100% CO selectivity enabled by metallic CoN porous atomic layers

Conductors hold promise for applications in infrared-light CO2 reduction to fuels. However, their extremely high carrier densities unfortunately result in strong electron–hole recombination. Herein, an ultrathin conductor with porous structure is designed to prolong the lifetime of photoexcited electrons. Taking the synthetic metallic CoN porous atomic layers as an example, synchrotron-radiation photoelectron spectroscopy and UV-vis-NIR spectroscopy uncover they could realize simultaneous CO2 reduction and H2O oxidation under infrared-light irradiation. Ultrafast transient absorption spectroscopy first unveils the infrared-light excited electrons undergo sequential intraband relaxation and interband recombination processes, where the 9- and 1.6-fold increased time in these two processes verifies the Na2S solution dramatically prolongs the electron lifetime. The metallic CoN porous atomic layers exhibit infrared-light induced CO2 reduction with nearly 100% CO selectivity, while the CO evolution rate is increased by 50 times upon adding Na2S solution. This study affords possibilities for achieving high-efficiency infrared-light driven CO2 reduction performance.

Infrared Response in a Polymeric Carbon Nitride Via an Upconversion Mechanism

We propose an upconversion process in polymeric carbon nitride (C3N4). This phenomenon in our O-doped porous nanosheet carbon nitride (MCN1-4) results in hydrogen generation from water splitting under infrared light irradiation by two (multi)-photon excitation (λ≥800 nm, 22.84 μmol h-1g-1). In addition to exhibiting upconversion, MCN1-4 has a σ*C-O orbital localized on the triazine matrix ring resulting from O doping, a porous ultra-thin nanosheet morphology, and an elemental distribution gradient from the inside to the surface of the material. Synergy induces extension of the lifetime of photoexcited electrons, which increases the photocatalytic activity under both infrared and visible light. Under visible light (λ≥420 nm) irradiation, MCN1-4 exhibits a solar-to-hydrogen energy conversion efficiency of 0.44% and an apparent quantum efficiency of 23.02% at 420 nm.

SrTiO3 NPs/g-C3N4 NSs Coupled Si NWs based Hybrid Photocathode for Visible Light Driven Photoelectrochemical Water Reduction

We report a novel class of silicon nanowire-based hybrid heterostructures consisting of perovskite SrTiO₃ nanoparticle-coupled g-C₃N₄ nanosheets as a promising photocathode for the solar water reduction process. The SrTiO₃ nanoparticle-coupled g-C₃N₄ nanosheets as the interfacial layer with Si NWs provides excellent visible light photocatalytic activity for the proton reduction processes. The hybrid heterojunction photocathode for the water reduction reaction at neutral pH exhibits the highest photocurrent density of 28 mA cm–², along with a photon-to-current efficiency of 5.4% under visible light irradiation. As a result, the hybrid photocathode shows an unprecedented enhancement in the low photocurrent onset potential (606 mV vs RHE) and hydrogen evolved rate of 18 μmole cm–² by increasing the interfacial charge carrier separation and the migration process at the photocathode/electrolyte interface. Moreover, the electrochemical impedance spectroscopy studies suggest that the hybrid photocathode holds an impressive charge transfer resistance reduction with an extended lifetime of photoexcited electrons, owing to the well-aligned photocathode/electrolyte interface. The improved photoelectrochemical performances can be ascribed to the high in-built electrostatic field in the interface, and the photocathode is secure from electrolytes by wrapping in an external hybrid sheath. The proposed hybrid Si NWs heterostructure photocathode is expected to provide new opportunities for a very low cost solar energy utilization platform toward a solar-assisted water reduction process.

Investigations of Interfacial Electric Field on Reduced‐Graphene‐Oxide‐Supported Molybdenum Oxide @ Silver Phosphate Ternary Hybrid Composite: Highly Efficient Visible‐Light‐Driven Photocatalyst

AbstractReduced graphene oxide supported molybdenum oxide @ silver phosphate (RGO‐MoO3@Ag3PO4) ternary hybrid composite was prepared by simple mixing of RGO supported MoO3 nanorods into Ag3PO4 nanosphere. Upon visible light exposure, the photocatalytic degradation of MB revealed that the 30 wt.% RGO‐MoO3@Ag3PO4 photocatalyst led to 2.9 times higher activities than pure Ag3PO4, 6.2 and 8.7 times compared to RGO‐MoO3 and pure MoO3, respectively. Degradation of MB using 30 wt.% RGO‐MoO3@Ag3PO4 is 2.9 and 15 times higher compared to RhB and MO respectively. The downward band bending of Ag3PO4 at the interface of RGO‐MoO3@Ag3PO4 was investigated based on the electric field formed. The formed electric field at the interface of RGO‐MoO3@Ag3PO4 is found to be 50 mV which avoids the recombination rate of electron‐hole pairs at the interfaces. This formed electric field promotes the higher separation efficiency of electrons‐holes pairs and longer life time of photoexcited electrons in the RGO‐MoO3@Ag3PO4 resulting higher efficiency and stability. RGO act as an electron acceptor and transporter during charge transfer process between MoO3 and Ag3PO4.

Spin depolarization dynamics of WSe2 bilayer**Project supported by the National Natural Science Foundation of China (Grant No. 11474276) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDPB0603).

In this work, the spin dynamics of a centrosymmetric WSe2 bilayer has been investigated by the two-color time-resolved Kerr rotation together with helicity-resolved transient reflectance techniques. Two depolarization processes associated with the direct transition are discovered at a low temperature of 10 K, with the characteristic decaying time of ∼3.8 ps and ∼20 ps, respectively. The short decay time of ∼3.8 ps is suggested to be the exciton spin lifetime of the WSe2 bilayer, which is limited by the short exciton lifetime of the WSe2 bilayer and the rapid intervalley electron–hole exchange interaction between K+ and K− valley in the same layer as that of monolayer. The long decay time of ∼20 ps is suggested to be the spin lifetime of photo-excited electrons, whose spin relaxation is governed by the rapid intervalley scattering from the K valley to the global minimum Σ valley and the subsequent interlayer charge transfer in WSe2 bilayer. Our experimental results prove the existence of the spin-polarized excitons and carriers even in centrosymmetric transition metal dichalcogenides (TMDCs) bilayers, suggesting their potential valleytronic and spintronic device applications.

Enhanced H2 evolution of TiO2 with efficient multiple electrons transfer modified by tiny CuOx–NiO bimetallic oxides

CuOx–NiO bimetallic oxides modified TiO2 catalysts were prepared via a precipitation-photoreduction approach for photocatalytic H2 production. 0.9%Cu0.1%Ni/TiO2 exhibited the highest H2 evolution rate (about 55.4 μmol/g/min) under UV–visible light irradiation (350–780 nm), which is higher than the sum of CuOx/TiO2 and NiO/TiO2 catalyst. Cu species exhibited more reducing valence state including Cu0 when deposited on NiO/TiO2 compared with deposited on TiO2. The polyvalent Cu species lead to enhanced photocurrent and reduced transfer resistance confirmed by photoelectron chemical analysis. CuOx–NiO modification of the TiO2 surface is beneficial for the photoexcited electron transfer, which suppresses electrons recombination. The lifetime of photoexcited electrons was enhanced greatly from 1.16 (TiO2) to 5.15 ns (CuOx–NiO/TiO2). A multiple electrons transfer mechanism for polyvalent bimetallic oxides co-modified TiO2 was proposed based on careful analysis. This work demonstrated a polyvalent bimetallic oxides modified TiO2 system with efficient photocatalytic H2 production which can provide some insightful understanding for bimetallic metal oxides co-catalyst design and H2 evolution mechanism.

Constructing Cd0.5Zn0.5S@ZIF-8 nanocomposites through self-assembly strategy to enhance Cr(VI) photocatalytic reduction

A novel and highly efficient photocatalyst of Cd0.5Zn0.5S@ZIF-8 nanocomposite has been developed by a facile self-assembly strategy. This is the first report on the application of CdxZn1−xS and metal-organic framework (MOF) nanocomposite as photocatalysts for the reduction of Cr(VI). The resulting Cd0.5Zn0.5S@ZIF-8 exhibited higher photocatalytic activity than that of pristine Cd0.5Zn0.5S and ZIF-8. Particularly, the CZS@Z60 composite with 60 wt% of ZIF-8 exhibited a photocatalytic activity that is about 1.6 times as high as that of Cd0.5Zn0.5S. The dominant reason for the improved photocatalytic reduction potential is proved to be the newly-formed interfacial SZn bonds that firmly connect Cd0.5Zn0.5S and ZIF-8 and substantially improve the separation efficiency of photo-excited electrons and holes. The newly-formed chemical bonds are confirmed by XPS analyses, and the prolonged lifetime of photo-excited electrons is evidenced by the electrochemical measurement of photocurrent, which shows that the photocurrent on Cd0.5Zn0.5S@ZIF-8 is much higher than that of Cd0.5Zn0.5S and ZIF-8. This study clearly demonstrates that the MOF-based composite nanomaterials hold great promises for applications in the field of environmental remediation and for design of novel photocatalytic materials.

Solid‐Solution Coordination Polymers as Precursors for ZnxCd1–xS/C Composite Nanowires

The development of synthetic routes to nanomaterials has been motivated by their potential applications in electronics, optics, and catalytic reactions. Solution‐based synthesis has relied on organic stabilizers to regulate the crystal growth. Despite the beneficial effect on crystal growth, organic stabilizers attached to the resulting particles reduce the accessibility of reactants and inhibit charge transport, resulting in a lower catalytic activity than expected. In this work we have demonstrated that coordination polymers (CPs) can be used as precursors for photocatalysts without stabilizers. The pyrolysis of CP nanowires of [Cd(l‐cysteinate)]n gave CdS/C nanowires (CdS/C‐NWs) that exhibit photocatalytic activity for hydrogen generation superior to CdS nanowires with organic stabilizers (CdS/stb‐NWs). Transient absorption spectroscopy showed that CdS/C‐NWs provide an active surfaces to facilitate catalytic reactions associated with electron migration. Moreover, Zn‐doped CdS nanowires (ZnxCd1‐xS/C‐NWs) obtained by the pyrolysis of CPs exhibited higher photocatalytic activity than nondoped CdS/C‐NWs. The improvement in photocatalytic activity is attributable to the prolonged lifetime of photoexcited electrons. The pyrolysis of CPs provides a facile means to design nanocomposites that realize efficient catalytic reactions associated with facile charge transfer.

Visible-light-driven selective oxidation of alcohols using a dye-sensitized TiO2-polyoxometalate catalyst

This study demonstrated a dye-sensitized TiO2-polyoxometalate system (denoted TiO2-(PW12-TH)8, where PW12=PW12O403− and TH=thionine) for selective oxidation of alcohols under visible light. The results showed that various substituted alcohols were transformed into their corresponding aldehydes with high selectivity. Due to more efficient electrons transfer, large surface area, and enhanced visible light absorption, the photocatalytic activity of TiO2-(PW12-TH)8 was superior to any other dye-sensitized system reported to date. The response in the photocurrent-time curves over several on/off cycles of intermittent irradiation showed good reproducibility. The photocurrent response of TiO2-(PW12-TH)8 was much higher than that of TiO2/TH (physical mixture of TiO2 and TH), SiO2-(PW12-TH)8 or P25-(PW12-TH)8 (P25=Degussa P25), attributed to the more efficient transfer and longer lifetime of photoexcited electrons. This photocatalytic process confirmed that efficient electron transfer during photocatalytic oxidation plays a vital role in determining the reaction conversion and obtaining good selectivity. Electron paramagnetic resonance (EPR) spectra and radical scavenging experiments proved that superoxide radicals and electrons were the main reactive species in the proposed system, the absence of hydroxyl radicals and holes is demonstrated to be the key of high reaction selectivity.

Enhancement of Dye-Sensitized Solar Cells Efficiency Using Mixed-Phase TiO2 Nanoparticles as Photoanode.

Dye-sensitized solar cell (DSSC) is a potential candidate to replace conventional silicon-based solar cells because of high efficiency, cheap cost, and lower energy consumption in comparison with silicon chip manufacture. In this report, mixed-phase (anatase and rutile nanoparticles) TiO2 photoanode was synthesized to investigate material characteristics, carriers transport, and photovoltaic performance for future DSSC application. Field-emission scanning electron microscope (SEM), X-ray diffraction (XRD), photoluminescence (PL), and UV-visible spectroscopy were used to characterize mixed TiO2 particles. Subsequently, various mixed-phase TiO2 anodes in DSSC devices were measured by electrical impedance spectra (EIS) and energy efficiency conversion. The overall energy conversion efficiency of DSSC chip was improved as a result of the increase of rutile phase of TiO2 (14%) in anatase matrix. Synergistic effects including TiO2 crystallization, reduction of defect density level in energy band, longer lifetime of photoexcited electrons, and lower resistance of electron pathway all contributed to high efficiency of light energy conversion.

Enhanced Photoexcited Carrier Separation in Oxygen-Doped ZnIn2 S4 Nanosheets for Hydrogen Evolution.

Limited by the relatively sluggish charge-carrier separation in semiconductors, the photocatalytic performance is still far below what is expected. Herein, a model of ZnIn2 S4 (ZIS) nanosheets with oxygen doping is put forward to obtain in-depth understanding of the role that doping atoms play in photocatalysis. It shows enhanced photocatalytic activity compared with pristine ZIS. The electron dynamics analyzed by ultrafast transient absorption spectroscopy reveals that the average recovery lifetime of photoexcited electrons is increased by 1.53 times upon oxygen incorporation into the ZIS crystals, indicating enhanced separation of photoexcited carriers in oxygen-doped ZIS nanosheets. As expected, the oxygen-doped ZIS nanosheets show a remarkably improved photocatalytic activity with a hydrogen evolution rate of up to 2120 μmol h(-1) g(-1) under visible-light irradiation, which is 4.5 times higher than that of the pristine ZIS nanosheets.