Charge Carrier Transfer Process Research Articles

In-situ formation of SrTiO3/Ti3C2 MXene Schottky heterojunction for efficient photocatalytic hydrogen evolution

Surface and interface engineering of composite photocatalysts are effective ways to enhance the dynamics of photo-generated charge carriers. In this work, SrTiO3/Ti3C2 MXene (STO/TC) Schottky heterojunction is constructed by in-situ growth of SrTiO3 (STO) on Ti3C2 MXene (TC) through Sr(OH)2 etching the surfaces of TC. This in-situ growth strategy not only creates the tight chemically bonded interfaces by SrTiO3 nanoparticles uniformly anchoring on the surface of two-dimensional Ti3C2 MXene nanosheets for promoting the photo-generated charge carrier separation, but also introduces surface Ti vacancies as the efficient catalytic active sites to accelerate the charge carrier transfer process for efficient hydrogen production. The photocatalytic system constructed by interface and surface engineering optimizes the photo-generated charge carrier dynamics and refines the photocatalytic hydrogen evolution performance (6.8 times higher than pristine SrTiO3) and stability. This work is expected to provide an alternative strategy to construct highly efficient photocatalysts with hydrogen evolution.

Recent advances of modification effect in Co3O4-based catalyst towards highly efficient photocatalysis

Tricobalt tetroxide (Co3O4) has been developed as a promising photocatalyst material for various applications. Several reports have been published on the self-modification of Co3O4 to achieve optimal photocatalytic performance. The pristine Co3O4 alone is inadequate for photocatalysis due to the rapid recombination process of photogenerated (PG) charge carriers. The modification of Co3O4 can be extended through the introduction of doping elements, incorporation of supporting materials, surface functionalization, metal loading, and combination with other photocatalysts. The addition of doping elements and support materials may enhance the photocatalysis process, although these modifications have a slight effect on decreasing the recombination process of PG charge carriers. On the other hand, combining Co3O4 with other semiconductors results in a different PG charge carrier mechanism, leading to a decrease in the recombination process and an increase in photocatalytic activity. Therefore, this work discusses recent modifications of Co3O4 and their effects on its photocatalytic performance. Additionally, the modification effects, such as enhanced surface area, generation of oxygen vacancies, tuning the band gap, and formation of heterojunctions, are reviewed to demonstrate the feasibility of separating PG charge carriers. Finally, the formation and mechanism of these modification effects are also reviewed based on theoretical and experimental approaches to validate their formation and the transfer process of charge carriers.

Enhanced charge carrier transfer process in nickel titanate nanostructures for environmental remediation of industrial dye.

The effective charge carrier transfer process in one-dimensional (1D) NiTiO3 nanofibers and NiTiO3 nanoparticles was demonstrated experimentally, showcasing an effective photocatalytic enhancement under visible light ambience. The rhombohedral crystal structure of NiTiO3 nanostructures was confirmed using X-ray diffractometer (XRD). The morphology and optical characteristics of the synthesized nanostructures were characterized using scanning electron microscopy (SEM) and UV-visible spectroscopy (UV-Vis). Nitrogen adsorption-desorption analysis corresponding to NiTiO3 nanofibers showcased porous structures with an average pore size of ~3.9 nm. The photoelectrochemical (PEC) measurement studies revealed an enhanced photocurrent for the NiTiO3 nanostructures, confirming enhanced charge carriers transportation in fibers than in particles due to the delocalized electrons in the conduction band, thereby hindering the photoexcited charge carrier's recombination. The photodegradation efficiency of methylene blue (MB) dye under the visible light irradiation revealed an enhancement in the rate of degradation for NiTiO3 nanofibers when compared to NiTiO3 nanoparticles.

Rapid charge transfer in covalent organic framework via through-bond for enhanced photocatalytic CO2 reduction

Charge transfer efficiency between discrete photosensitizers and catalytic sites is a key limiting factor in artificial photosynthesis. It is highly desirable but challenging to efficiently combine the two sections into an integration system and get insight into the kinetics and mechanisms. Here in, the photosensitizer [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) and active cobalt porphyrin (Co-Por) sites were integrated into a covalent organic framework (COF), named COF-RuBpy-Co, for efficient charge transfer and photocatalytic CO2 reduction. The catalyst COF-RuBpy-Co exhibited excellent CO2 photoreduction activity towards CO production with a rate of 547 μmol g−1h−1, which is 1.4-fold enhancement over the physical mixture of Ru(bpy)3Cl2 and COF-Bpy-Co. In situ X-ray photoelectron spectroscopy combined with theoretical calculation results revealed that COF-RuBpy-Co achieved efficient photoelectron transfer from [Ru(bpy)3]2+ to cobalt porphyrin. More importantly, transient absorption spectroscopy indicated that the covalent linking [Ru(bpy)3]2+and Co-Por units realized a faster charge transfer (44.2 ps) over the large π-conjugated system. This work provides vital insights into the charge carrier transfer process and demonstrates the potential of COFs as a platform in artificial photosynthesis.

A novel PEBAX-1657/PES-(BiFeO3@ZnS) photocatalytic membrane for integrated hybrid systems coupling CO2 separation and photoreduction

Suspended photocatalyst systems face the problems of poor light distribution, photocatalyst separation, and inappropriate transfer of reducing agent and CO2 gas to the active photocatalyst sites. To overcome these issues, Poly(ether-block-amide)−1657/Polyethersulfone (PEBAX-1657/PES) -(BiFeO3 @ZnS) photocatalytic membrane was synthesized and applied for simultaneous separation and photoreduction of CO2 to methanol as the valuable product. The two-phase membrane photoreactor was tested under both ultraviolet (UV) and visible light irradiation in the presence of water. Based on UV–visible and photoluminescence spectroscopy (PL) analyses, PEBAX-1657/PES-(BiFeO3@ZnS) exhibited wider light absorption range and lower recombination rate of photogenerated species compared with pristine PEBAX-1657/PES, respectively. Meanwhile, the separation features of the photocatalytic membrane remained almost constant even after 6 h reaction time. In addition, the schematic representation of proposed mechanism was clarified in detail based on the p-n junction photogenerated charge carrier transfer process of BiFeO3@ZnS. Finally, photocatalytic performance of the prepared photocatalytic membrane with different water flow rates and light powers were examined. The photocatalytic activity tests revealed the maximum methanol concentrations of 0.85 mmol/L and 0.56 mmol/L and the maximum methanol production yields of 5100 and 3360 µmol/gcat.h under UV and visible irradiation, respectively at the optimized operating conditions (water flow rate of 3 mL/min and light power of 250 W). Therefore, the present study provides a new insight into the effective visible-light-driven photocatalytic membranes for simultaneous separation and photocatalytic conversion of CO2 to solar fuels.

Spin-Orbit Coupling Is the Key to Promote Asynchronous Photoinduced Charge Transfer of Two-Dimensional Perovskites.

Two-dimensional (2D) perovskites are emerging as promising candidates for diverse optoelectronic applications because of low cost and excellent stability. In this work, we explore the electronic structures and interfacial properties of (4Tm)2PbI4 with both the collinear and noncollinear DFT (PBE and HSE06) methods. The results evidently manifest that explicitly considering the spin–orbit coupling (SOC) effects is necessary to attain correct band alignment of (4Tm)2PbI4 that agrees with recent experiments (Nat. Chem.2019, 11, 115131712613; Nature2020, 580, 61432350477). The subsequent time-domain noncollinear DFT-based nonadiabatic carrier dynamics simulations with the SOC effects reveal that the photoinduced electron and hole transfer processes are asymmetric and associated with different rates. The differences are mainly ascribed to considerably different nonadiabatic couplings in charge of the electron and hole transfer processes. Shortly, our current work sheds important light on the mechanism of the interfacial charge carrier transfer processes of (4Tm)2PbI4. The importance of the SOC effects on correctly aligning the band states of (4Tm)2PbI4 may be generalized to similar organic–inorganic hybrid 2D perovskites having heavy Pb atoms.

Novel S-scheme 2D/2D BiOBr/g-C3N4 heterojunctions with enhanced photocatalytic activity

The design and construction of heterojunction photocatalysts, which possess a staggered energy band structure and appropriate interfacial contact, is an effective way to achieve outstanding photocatalytic performance. In this study, 2D/2D BiOBr/g-C3N4 heterojunctions were successfully obtained by a convenient in situ self-assembly route. Under simulated sunlight irradiation, 99% of RhB (10 mg·L−1, 100 mL) was efficiently degraded by 1.5-BiOBr/g-C3N4 within 30 min, which is better than the performance of both BiOBr and g-C3N4, and it has superior stability. In addition, the composite also exhibits enhanced photocatalytic activity for H2 production. The enhanced activity can be attributed to the intimate interface contact, the larger surface area, and the highly efficient separation of photoinduced electron–hole pairs. Based on the experimental results, a novel S-scheme model was proposed to illuminate the transfer process of charge carriers. This study presents a simple way to develop novel step-scheme photocatalysts for environmental and related applications.

Termination dependence and electric field modification of band alignment in a CNT/CH3NH3PbI3 heterojunction.

Carbon nanotube (CNT) and perovskite composite materials possessing the combined advantages of CNTs and perovskites have drawn substantial attention due to their promising applications in photovoltaic and optoelectronic devices. Understanding the band alignment of heterojunctions is crucial for further performance improvement. Here, we systematically investigated the interfacial electronic structure and optical absorption of a semiconducting CNT/CH3NH3PbI3 heterojunction via density functional theory calculations. It was found that the CNT/PbI2-terminated CH3NH3PbI3 (001) surface heterojunction is a type-I band alignment, while the CNT/CH3NH3I-terminated CH3NH3PbI3 (001) surface heterojunction is a type-II band alignment, suggesting the different charge carrier transfer processes as well as termination dependence of band alignment in the CNT/CH3NH3PbI3 heterojunction. Further investigation indicated that applying electric fields can modify the band alignment type in the CNT/CH3NH3PbI3 heterojunction. Our results provide the first insight into the interfacial electronic structure of the CNT/CH3NH3PbI3 heterojunction, which may give a new route for designing optoelectronic devices.

Photoluminescence Quenching and Photo-Induced Charge Transfer Processes in Poly(3-octylthiophene) Polymer Based Hybrid Nano-composites by Ion Irradiation for Possible Optoelectronic Applications

Structural and spectroscopy studies have been carried out on conducting polymer poly(3-octylthiophene) (P3OT) and its copper-doped ZnO (Cu-ZnO/P3OT) hybrid nanocomposites (HNCs) under swift heavy ion (SHI) irradiation at different electronic energy depositions. The photoluminescence (PL) spectra of irradiated films exhibit a significant decrease in the intensity of emissions at higher ion fluences which is ascribed to the trapping of a photo-induced electron-hole by irradiation-induced free radicals and extrinsic non-radiative trap centers leading to the quenching effects. The generation of such free radicals and non-radiative recombination centers occurred through a chemical transformation of polymers in terms of polymer chain disordering, chain scission, chain aggregation, and bond breaking by ion irradiation depending upon the electronic energy depositions and ion fluences. The structural, vibrational, morphological, and optical properties of the irradiated P3OT and Cu-ZnO/P3OT HNCs films have been studied. Interestingly, the glancing-angle x-ray diffraction patterns of irradiated films reveal that the polymer and HNC films retain their chemical structures after high electronic deposition at lower ion fluences which leads to insignificant degradation of polymer and HNCs. However, a relative change in the intensity of characteristic peaks of polymer and ZnO was observed at higher ion fluences and is attributed to the disordering of polymer chains by high electronic depositions. Fourier transform infrared spectroscopy (FTIR) measurements also show similar observation, attributed to a decrease in the intensity of a few methyl and octyl functional groups of P3OT and HNCs. Further, optical study has shown a significant modification in the process of inter-chain and interfacial charge transfer. Finally, from these concurrent effects, PL quenching and photo-induced charge carrier transfer processes are understood by developing a schematic charge transfer diagram.

Microscopic theory of the influence of dipole superparamagnetics (type <beta-CD<FeSO_4>>) on current flow in semiconductor layered structures (type GaSe, InSe)

A statistical approach to description of the charge carrier transfer processes in hybrid nanostructures taking into account electromagnetic fields is proposed using the method of the nonequilibrium statistical operator Zubarev. Generalized transfer equations are obtained, which describe non-Markov processes of charge transfer in the system taking into account magnetic and polarization processes under the influence of external and induced internal electromagnetic fields. Weakly nonequilibrium charge transfer processes in nanostructures are considered, and a nonequilibrium statistical operator is obtained, by means of which the magneto-diffusion transfer equations for electrons in layered nanostructures are obtained. A generalized Cattaneo-type diffusion equation in time fractional derivatives is obtained for electrons with a characteristic relaxation time and a generalized model is proposed that takes into account the complexity of relaxation electro-magnetic diffusion processes for electrons in layered nanostructures.

Tuning charge transfer process of MoS2 photoanode for enhanced photoelectrochemical conversion of ammonia in water into gaseous nitrogen

Charge transfer efficiency of semiconductor photoanode in photoelectrochemical (PEC) system is one of crucial factor determining the overall PEC performance. Previously, we have developed a MoS2-based PEC system that is able to specifically convert ammonia nitrogen in aqueous solution into gaseous nitrogen (e.g. N2) utilizing oxysulfur radicals. Herein, in order to solve the slow charge transfer issue of reported MoS2-based PEC oxidation system, we further improve the PEC conversion efficiency by tuning composition of MoS2/WS2 nanosheets heterojunction photoanode. The hybrid MoS2/WS2(1:1) electrode displays the highest PEC performance and ammonia conversion efficiency compared to the individual MoS2 or WS2 constituent under visible light irradiation, which can be mainly ascribed to the reduced electrode resistance and improved charge transfer efficiency according to the physiochemical, electrochemical and spectroscopic analysis. The results prove that both interfacial and interior charge carrier transfer processes of photoanode are crucial and tailorable in order to efficiently generate reactive radical species for ammonia conversion.

CdS nanocrystallites sensitized ZnO nanorods with plasmon enhanced photoelectrochemical performance

A novel architecture of CdS/ZnO nanorods with plasmonic silver (Ag) nanoparticles deposited at the interface of ZnO nanorods and CdS nanocrystallites, was designed as a photoanode for solar hydrogen generation, with photocurrent density achieving 4.7 mA/cm2 at 1.6 V (vs. RHE), which is 8 and 1.7 times as high as those of pure ZnO and CdS/ZnO nanorod films, respectively. Additionally, with optical absorption onset extended to ∼660 nm, CdS/Ag/ZnO nanorod film exhibits significantly increased incident photo-to-current efficiency (IPCE) in the whole optical absorption region, reaching 23.1% and 9.8% at 400 nm and 500 nm, respectively. The PEC enhancement can be attributed to the one-dimensional ZnO nanorod structure maintained for superior charge transfer, and the extended visible-light absorption of CdS nanocrystallites. Moreover, the incorporated plasmonic Ag nanoparticles could further promote the interfacial charge carrier transfer process and enhance the optical absorption ability, due to its excellent plasmon resonance effect.

Interfacial oxygen vacancy layer of a Z-scheme BCN–TiO2 heterostructure accelerating charge carrier transfer for visible light photocatalytic H2 evolution

Interfacial oxygen vacancy layer of BCN–TiO2 heterostructures as an effective interfacial mediator can promotes the direct Z-scheme charge carrier transfer process and visible light photocatalytic activity for H2 evolution.

Light-Enhanced Carbon Dioxide Activation and Conversion by Effective Plasmonic Coupling Effect of Pt and Au Nanoparticles.

Photocatalytic reduction of carbon dioxide (CO2) is attractive for the production of valuable fuels and mitigating the influence of greenhouse gas emission. However, the extreme inertness of CO2 and the sluggish kinetics of photoexcited charge carrier transfer process greatly limit the conversion efficiency of CO2 photoreduction. Herein, we report that the plasmonic coupling effect of Pt and Au nanoparticles (NPs) profoundly enhances the efficiency of CO2 reduction through dry reforming of methane reaction assisted by light illumination, reducing activation energies for CO2 reduction ∼30% below thermal activation energies and achieving a reaction rate 2.4 times higher than that of the thermocatalytic reaction. UV-visible (vis) absorption spectra and wavelength-dependent performances show that not only UV but also visible light play important roles in promoting CO2 reduction due to effective localized surface plasmon resonance (LSPR) coupling between Pt and Au NPs. Finite-difference time-domain simulations and in situ diffuse reflectance infrared Fourier transform spectroscopy further reveal that effective coupling LSPR effect generates strong local electric fields and excites high concentration of hot electrons to activate the reactants and intermediate species, reduce the activation energies, and increase the reaction rate. This work provides a new pathway toward the efficient plasmon-enhanced chemical reactions via reducing the activation energies by utilizing solar energy.

Mixed–phase α/β–AgAl0.4Ga0.6O2 solid solution with heterojunction for enhanced visible–light photocatalytic activity

Mixed–phase α/β–AgAl0.4Ga0.6O2 solid solution with heterojunction was fabricated through phase transformation. The samples with high crystallinity showed the mixed morphology of near–spherical shape of β phase and the platelet of α phase. The UV–vis absorption edges ranged from 460 to 520 nm by controlling the ratio of two phases. It was found that the mixed–phase α/β–AgAl0.4Ga0.6O2 with the weight percentage of 36% for α–AgAl0.4Ga0.6O2 showed the highest photocatalytic activity. This might be attributed to the absorption behavior for efficiently utilizing visible light and the phase heterojunction for promoting the separation of photogenerated carriers. The survey of photocatalytic mechanism indicated that reactive h+ and •O2− were the oxygen active species. In addition, a possible transfer process of charge carriers in photocatalysis was proposed. This work demonstrates that the design of mixed–phase heterojunction is an efficient approach to enhance photocatalytic activity.

Surface Natrotantite Phase Induced Efficient Charge Carrier Separation and Highly Active Surface of TaON for Superior Enhanced Photocatalytic Performance

Physicochemical properties of semiconductors are highly sensitive to the interface from other heterophase, which provides great potential to enhance the solar energy conversion performance through interfacial modification. Surface natrotantite (Na2Ta4O11)‐modified TaON photocatalyst is successfully synthesized and it is demonstrated that, although Na2Ta4O11 itself is not involved in the charge carrier transfer process, a small amount of the natrotantite phase (less than 0.1%) on TaON surface has a great effect on charge carrier separation behavior, surface reactivity, and photocatalytic performance of TaON. The photocurrent and corresponding photocatalytic performance toward water oxidation of modified TaON with optimal amount of Na2Ta4O11 under visible light are about five and seven times that of pristine TaON. In particular, the water oxidation performance on Na2Ta4O11‐modified TaON is comparable to that loaded with noble metal cocatalyst platinum (Pt), which demonstrates a facial method to modify the solar energy convention materials.

Electrochemical impedance spectroscopy (EIS): An efficiency method to monitor resin curing processes

Electrochemical impedance spectroscopy (EIS) is widely used to characterize the charge carrier transfer and charge storage process. Charge carrier transfer process may be affected by the media viscosity. Here, EIS was used to monitor the curing processes of epoxy/amine blends for the first attempt, and a new equation was proposed to character the curing process and estimate the curing degree of resins, while, resins, curing agents and the curing processes have significant effect on product properties. Monitoring curing processes of certain resin/curing agent system is very helpful to design an appropriate formulation. The epoxy/amine blends, the model system, were prepared with Diglycidyl ether of bisphenol-A as epoxy resin and different ratio of phenalkamine modified with cardanol as curing agent. The curing processes of epoxy/amine were also investigated by differential scanning calorimetry (DSC), which was employed as a comparative method to verify the EIS results. A good agreement is obtained between the two methods, especially under higher curing temperature condition, which demonstrates the great promise to monitor the curing processes using EIS. The EIS results were also modeled to equivalent electrical circuits (EEC) by ZSimpleWin software for the further analysis. These results might provide some insights on optimizing the epoxy/amine ratio and the performance of cured epoxy resin and monitoring some process related to viscosity change.

Facile composition-controlled preparation and photocatalytic application of BiOCl/Bi2O2CO3 nanosheets

The BiOCl, BiOCl/Bi2O2CO3 composites and Bi2O2CO3 were successfully fabricated by a facile composition-controlled preparation technology at room temperature for the first time. The X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) adsorption isotherm, UV–vis diffuse reflectance spectra (UV–vis DRS) and first-principles methods were employed to characterize the phase structures, morphologies, surface areas, optical properties, and photocatalytic mechanism of as-prepared samples. The BiOCl/Bi2O2CO3 composites exhibited the higher photocatalytic activity than individual BiOCl and Bi2O2CO3 for the degradation of MO under simulated sunlight irradiation. The COD removal efficiency of MO solution over BiOCl/Bi2O2CO3 composite achieved 95% after 8h reaction time. In addition, the formation mechanism and excellent photocatalytic activity of BiOCl/Bi2O2CO3 composite have been investigated and discussed in detail. The enhanced photocatalytic performance of BiOCl/Bi2O2CO3 composites is closely related to the suitable conduction band (CB) interaction and efficient separation of photo-induced electron-hole pairs by the synergistic effect of BiOCl and Bi2O2CO3 under the simulated sunlight irradiation. Combined with the theoretical and experimental findings, the photocatalytic mechanism of BiOCl/Bi2O2CO3 composite and the charge carrier transfer process between Bi2O2CO3 and BiOCl semiconductors have been proposed and investigated.

Layered WO3/TiO2 nanostructures with enhanced photocurrent densities

Layered WO3/TiO2 nanostructures, fabricated by magnetron sputtering, demonstrate significantly enhanced photocurrent densities compared to individual TiO2 and WO3 layers. First, a large quantity of compositions having different microstructures and thicknesses were fabricated by a combinatorial approach: diverse WO3 microstructures were obtained by adjusting sputtering pressures and depositing the films in form of wedges; later layers of TiO2 nanocolumns were fabricated thereon by the oblique angle deposition. The obtained photocurrent densities of individual WO3 and TiO2 films show thickness and microstructure dependence. Among individual WO3 layers, porous films exhibit increased photocurrent densities as compared to the dense layer. TiO2 nanocolumns show length-dependent characteristics, where the photocurrent increases with increasing film thickness. However, by combining a WO3-wedge type layer with a layer of TiO2 nanocolumns, PEC properties strikingly improve, by about two orders of magnitude as compared to individual WO3 layers. The highest photocurrent that is measured in the combinatorial library of porous WO3/TiO2 films is as high as 0.11 mA/cm2. Efficient charge-separation and charge carrier transfer processes increase the photoconversion efficiency for such films.

Electrical Conductivity and Seebeck Effect inNi0.80Fe2.20O4

Electrical conductivity and Seebeck effect have been measured as functions of temperature on single-crystal samples of ${\mathrm{Ni}}_{0.80}$${\mathrm{Fe}}_{2.20}$${\mathrm{O}}_{4}$. A thermal hysteresis has been observed in the conductivity but not in the Seebeck effect. This suggests that the hysteresis involves the charge carrier transfer process but not the production of carriers. The activation energy associated with the transfer process is estimated to be 0.10 ev in the high-temperature state and 0.06 ev in the low-temperature state.