Charge Separation Mechanism Research Articles

N-doped Z-scheme heterojunctions on cobalt foam for enhanced photocatalytic degradation of levofloxacin and reduction of hexavalent chromium: Insights into charge separation and catalytic mechanisms

The transfer of photoelectrons across heterogeneous materials stands as a cornerstone process within the domain of photocatalysis. Elucidating the mechanisms underlying electron transfer at heterojunction interfaces provides profound insights that are crucial for the rational design of efficient catalysts. In this study, we employ a hierarchical synthesis approach to fabricate N-doped Z-scheme heterojunctions directly on the surface of cobalt foam (CoF). Both experimental observations and density functional theory (DFT) calculations reveal that N-doped CoS2 nanosheets exhibit asymmetric charge distribution, functioning as localized electron traps. Further analysis of differential charge density elucidates that positive charges are predominantly concentrated in proximity to N-CoS2, whereas negative charges accumulate near ZnO, indicating substantial charge exchange between these two phases. The integration of a potent built-in electric field upon illumination facilitates the splitting of photogenerated carriers in both transverse and longitudinal directions, thereby enabling their effective separation and migration. The CoF@N-CS@ZO composites demonstrated a remarkable capacity for simultaneous degradation and reduction, achieving a 93.6 % degradation of levofloxacin (LEV) and a 96.9 % reduction of hexavalent chromium (Cr(VI)) under visible light irradiation for 85 min. Furthermore, due to the active species being anchored on the surface of the CoF, which serves as a recyclable substrate, the composites could be directly recycled post-use. Notably, CoF@N-CS@ZO maintained an efficiency of over 80 % for both LEV degradation and Cr(VI) reduction even after undergoing more than ten cycles. This research elucidates the intricate interplay of the synergistic N-doped built-in electric field on the charge transfer process, offering profound insights into the targeted design of charge-modulated materials for enhanced photocatalytic performance.

Direct visualization of the charge transfer state dynamics in dilute-donor organic photovoltaic blends

The interconversion dynamics between charge transfer state charges (CTCs) and separated charges (SCs) is still an unresolved issue in the field of organic photovoltaics. Here, a transient absorption spectroscopy (TAS) study of a thermally evaporated small-molecule:fullerene system (α6T:C60) in different morphologies (dilute intermixed and phase separated) is presented. Spectral decomposition reveals two charge species with distinct absorption characteristics and different dynamics. Using time-dependent density functional theory, these species are identified as CTCs and SCs, where the spectral differences arise from broken symmetry in the charge transfer state that turns forbidden transitions into allowed ones. Based on this assignment, a kinetic model is formulated allowing the characterization of the charge generation, separation, and recombination mechanisms. We find that SCs are either formed directly from excitons within a few picoseconds or more slowly (~30–80 ps) from reversible splitting of CTCs. These findings constitute the first unambiguous observation of spectrally resolved CTCs and SCs.

Surface modification of TiO2 photoanode in dye-sensitized solar cells using reduced graphene oxide: A computational and experimental study

This study investigated the modification of TiO2 photoanodes using reduced graphene oxide (rGO) to enhance electron transport channels and prevent recombination processes, thereby improving the photovoltaic performance. Through the ultraviolet (UV)-assisted photoreduction of GO on TiO2 coated on a fluoride tin oxide substrate (FTO|TiO2), we demonstrated the successful integration of rGO. This was evidenced by the increased Csp2 content observed during X-ray photoelectron spectroscopy and reduced photogenerated electron–hole recombination observed during photoluminescence spectroscopy. The incorporation of rGO significantly improved the photocurrent density and power conversion efficiency (PCE). A 12 % increase was observed in the PCE, which reached 8.5 % when the UV irradiation time was optimized from 10 to 15 min compared with the 7.57 % in the standard cell (rGO-0 min). Electrochemical impedance spectroscopy confirmed that the optimized rGO content enhanced the electron lifetime and recombination resistance, attributable to the high conductivity and large specific surface area of rGO. DFT simulation further elucidated how improved charge separation and transport mechanisms of TiO2–rGO heterojunction. This study highlights the potential of TiO2–rGO materials as promising electrodes for improving the efficiency, capacity, and stability of dye-sensitized solar cells.

Enhanced photoelectric desalination of Co3O4@NC/BiVO4 photoanode via in-situ construction of hole transport layer

The solar-driven photoelectrochemical desalination (SD-PED) technology, as a new emerging desalination technique, has been developed and attracted the increasing attention. However, practical application remains hampered by several constraints, including the rapid deterioration of photocurrent, and the long-term stability of system. In this research, MOF-derived nitrogen-doped carbon@Co3O4/BVO (Co3O4@NC/BVO) heterostructured photoanode was design for efficient and durable solar driven redox desalination. It exhibits an initial photocurrent of 2.40 mA/cm2 and a desalination rate of 69.01 μg/(cm2·min) in the zero-bias state using the light as the driving force, without consuming electrical energy. Furthermore, the solar energy consumption of the photoanode is 0.187 μmol/J. The salt removal rate fluctuates within 1.36 μg/(cm2·min) throughout five cycles without any substantial decrease. Photo-luminescence, EIS and Mott-Schottky analysis are also performed to investigate interface reaction, charge separation and transfer mechanism between photoanode and electrolyte. The analysis of the charge-transfer paths on the heterojunction interface is conducted through in situ irradiation XPS. Further analysis of the generation and separation of •OH and h+ in the Co3O4@NC/BVO photoanode using electron paramagnetic resonance (EPR) showed that Co3O4@NC as an efficient hole transfer layer can effectively promote the separation and transfer of photo-generated electrons and holes. The excellent desalination performance is attributed to the synergistic effect of electron transfer in the Co3O4@NC/BVO heterojunction and hole transport in the Co3O4@NC efficient hole transport layer. This work is significant for the development of solar redox flow desalination.

Distance-Dependent Symmetry-Breaking Charge Transfer in 9,10- Bis(phenylethynyl)anthracene Dimers.

To investigate the effect of the through-bond coupling strength on the symmetry-breaking charge separation (SB-CS) dynamics and mechanism, three 9,10-bis((4-hexylphenyl)ethynyl)anthracene dimers with varying distances, viz., a single-bond linked dimer (0-dimer), a phenylene linked dimer (1-dimer) and a para-biphenylene linked dimer (2-dimer), were synthesized and studied systematically using steady-state and transient spectroscopy. Steady-state absorption spectra revealed that the electronic coupling strength decreased gradually with the increase of the inter-chromophore distance, and the transition of S0→S1 includes the dark charge transfer (CT) excitation. fs-TA spectra demonstrated that SB-CS could be conducted in both weakly and highly polar solvents for 0-dimer, but the SB-CS dynamics has a significant difference. In weakly polar solvents, SB-CS only produces the partial CT (PCT) state, but it could generate the CT state via the PCT state in highly polar solvent. In comparison, SB-CS is only proceeded in highly polar solvents in 1-dimer and 2-dimer to produce the CT state directly. These results demonstrate that the SB-CS dynamics is strongly dependent on the inter-chromophore electronic coupling, and the relatively strong electronic coupling is crucial for the occurrence of SB-CS in weakly polar environment that is commonly presented in photoelectric devices.

Controlled growth of two-dimensional MoS2/WSe2 heterostructure solar cell by chemical vapor deposition

The development of high-quality type II semiconductor heterostructures is crucial for solar energy conversion applications. Here, we report the sequential growth of MoS2/WSe2 two-dimensional semiconductor type II heterostructure using a one-step chemical vapor deposition. The morphological, Raman, and chemical analysis revealed that the WSe2 layer is deposited on the rhombus-shaped MoS2, forming a vertical MoS2/WSe2 heterostructure. The solar cell fabricated using the grown heterostructure exhibits a photovoltaic response with a conversion efficiency of 2.5 % and an open circuit voltage of 0.22 V, respectively. The numerical simulation study unravels the mechanism of charge separation and transport in the MoS2/WSe2 heterostructure solar cell.

The Role of Thermally Activated Charge Separation in Organic Solar Cells

AbstractIn recent years, organic solar cells (OSCs) have shown high power efficiencies approaching 20%. However, the fundamental mechanisms of charge separation in these highly efficient devices have been a subject of intensive debates. Here, the charge separation efficiency (CSE) is extensively investigated across a wide range of blend systems with different energetic offsets. The findings unveil the temperature‐dependent nature of charge separation in low‐offset systems, emphasizing its significant contribution to the overall CSE. An intriguing inverse correlation between CSE and charge separation activation energy in relation to the offset is also observed. These results shed new light on the factors underlying the high CSE observed in the state‐of‐the‐art devices.

Constructing a Z-scheme heterojunction of oxygen-deficient WO3-x and g-C3N4 for superior photocatalytic evolution of H2

Semiconductor-based photocatalytic water splitting enables the conversion of abundant solar energy to green and renewable hydrogen energy. Graphitic carbon nitride (g-C3N4) is synthesized using a straightforward method, demonstrating stable physicochemical properties and possessing an optimal bandgap, thus positioning it as a promising photocatalyst in the realm of environmental sustainability. Oxygen vacancies are extensively employed to modulate light absorption and surface properties of metal-oxide semiconductors. In this study, g-C3N4 nanosheets were coupled with oxygen-deficient tungsten trioxide (WO3-x) to form heterojunction photocatalysts (X-WOCN). Comprehensive material characterization results demonstrated that the constructed heterojunction extended the visible light absorption range, improved photogenerated electron-hole separation efficiency, and thus augmented photocatalytic activity. Notably, the optimum hydrogen evolution rate of 6 %-WOCN was enhanced by 5.4-fold compared to that of g-C3N4. Furthermore, we propose a Z-scheme heterojunction charge separation mechanism mediated by oxygen defects and support this mechanism through detection of surface-active substances •O2− and •OH. This study offers novel propositions into the function of oxygen defects in facilitating charge separation within Z-scheme heterojunction.

Design and Preparation of Heterostructured Cu2O/TiO2 Materials for Photocatalytic Applications.

The extensive use of fossil fuels has sped up the global development of the world economy and is accompanied by significant problems, such as energy shortages and environmental pollution. Solar energy, an inexhaustible and clean energy resource, has emerged as a promising sustainable alternative. Light irradiation can be transformed into electrical/chemical energy, which can be used to remove pollutants or transform contaminants into high-value-added chemicals through photocatalytic reactions. Therefore, photocatalysis is a promising strategy to overcome the increasing energy and environmental problems. As is well-known, photocatalysts are key components of photocatalytic systems. Among the widely investigated photocatalysts, titanium dioxide (TiO2) has attracted great attention owing to its excellent light-driven redox capability and photochemical stability. However, its poor solar light response and rapid recombination of electron-hole pairs limit its photocatalytic applications. Therefore, strategies to enhance the photocatalytic activity of TiO2 by narrowing its bandgap and inhibiting the recombination of charges have been widely accepted. Constructing heterojunctions with other components, including cuprous oxide (Cu2O), has especially narrowed the bandgap, providing a promising means of solving the present challenges. This paper reviews the advances in research on heterostructured Cu2O/TiO2 photocatalysts, such as their synthesis methods, mechanisms for the enhancement of photocatalytic performance, and their applications in hydrogen production, CO2 reduction, selective synthesis, and the degradation of pollutants. The mechanism of charge separation and transfer through the Cu2O/TiO2 heterojunctions and the inherent factors that lead to the enhancement of photocatalytic performance are extensively discussed. Additionally, the current challenges in and future perspectives on the use of heterostructured Cu2O/TiO2 photocatalysts are also highlighted.

Single-Particle Fluorescence Spectroscopy for Elucidating Charge Transfer and Catalytic Mechanisms on Nanophotocatalysts.

Photocatalysis is a cost-effective approach to producing renewable energy. A thorough comprehension of carrier separation at the micronano level is crucial for enhancing the photochemical conversion capabilities of photocatalysts. However, the heterogeneity of photocatalyst nanoparticles and complex charge migration processes limit the profound understanding of photocatalytic reaction mechanisms. By establishing the precise interrelationship between microscopic properties and photophysical behaviors of photocatalysts, single-particle fluorescence spectroscopy can elucidate the carrier separation and catalytic mechanism of the photocatalysts in situ, which provides perspectives for improving the photocatalytic efficiency. This Review primarily focuses on the basic principles and advantages of single-particle fluorescence spectroscopy and its progress in the study of plasmonic and semiconductor photocatalysis, especially emphasizing its importance in understanding the charge separation and photocatalytic reaction mechanism, which offers scientific guidance for designing efficient photocatalytic systems. Finally, we summarize and forecast the future development prospects of single-particle fluorescence spectroscopy technology, especially the insights into its technological upgrading.

Photo-assisted self-chargeable aqueous Zn-ion energy storage device

The ever-growing demand for portable electronic devices in various applications emphasizes the necessity for continuous power sources, particularly in situations where recharging is not readily available. Although photo-assisted aqueous Zn-ion energy storage devices show promise, their slow charging rates and limited sunlight hours impede their practicality. In this study, we present a new self-charging energy storage device by investigating chemical processes for air-based recharging in photo-assisted Zn-ion technology, utilizing VO2/WO3 as a cathode. This research marks the first utilization of WO3 as a charge-separating layer alongside VO2 in photo-assisted energy storage devices. Under consistent light exposure, the electrode demonstrates a 170% increase in capacity, confirming the effectiveness of the photo-assisted charge separation mechanism. Moreover, the air-based charging achieves an Open Circuit Potential (OCP) of 1 V, reaching 0.9 V OCP within 140 s. Incorporating this technology into a double cathode-based device successfully powers small LED devices, showcasing the practicality of this approach. These discoveries pave the way for the advancement of self-rechargeable photo-assisted energy storage devices for real-world usage.

Exploring the formation of S-scheme heterojunctions in CuFe2O4/Bi2MoO6 porous cubes for photocatalytic removal of tetracycline

Exploring highly active and noble metal-free photocatalysts for the efficient photocatalytic degradation of antibiotics is of great significance. The CuFe2O4 nanostructured cube is synthesized by using a self-sacrificial template strategy, and two-dimensional Bi2MoO6 nanosheets are grown on the surface of CuFe2O4 through a solvothermal method to construct CuFe2O4/Bi2MoO6 S-scheme heterostructure. The CuFe2O4/Bi2MoO6 composite material exhibits eminent photocatalytic activity for the degradation of TC under simulated sunlight, with the optimal ratio CFO/BMO-2 achieving a degradation rate of 98.54 % within 30 min (initial concentration of TC: 50 mg L−1). In addition, CFO/BMO-2 also has excellent magnetic recyclability and cycling ability. Rice cultivation tests demonstrate that the biotoxicity of TC is effectively removed. Density functional theory calculations and in-situ irradiated X-ray spectroscopy deeply reveal the mechanisms of charge separation and transfer of S-scheme heterojunctions in CuFe2O4/Bi2MoO6. The construction of S-scheme heterojunctions promotes the separation of photo-induced charge carriers and improves photocatalytic degradation performance by retaining holes and electrons with strong redox ability. The work provides a practicable method for constructing efficient spinel photocatalysts to drive wastewater treatment reactions, and the constructed magnetically recyclable photocatalysts have certain practical application potential.

Mechanism of Carrier Formation in P3HT-C60-PCBM Solar Cells.

Solar cells convert light energy directly into electricity using semiconductor materials. The ternary system, composed of poly(3-hexylthiophene) (P3HT), fullerene (C60), and phenyl-C61-butyric-acid-methyl-ester (PCBM), expressed as P3HT-C60-PCBM, is one of the most efficient organic solar cells. In the present study, the structures and electronic states of P3HT-C60-PCBM have been investigated by means of the density functional theory (DFT) method to shed light on the mechanism of charge separation in semiconductor materials. The thiophene hexamer was used as a model of P3HT. Five geometrical conformers were obtained as the C60-PCBM binary complexes. In the ternary system, P3HT wrapped around C60 in the stable structure of P3HT-C60-PCBM. The intermolecular distances for P3HT-(C60-PCBM) and (P3HT-C60)-PCBM were 3.255 and 2.885 Å, respectively. The binding energies of P3HT + (C60-PCBM) and (P3HT-C60) + PCBM were 27.2 and 19.1 kcal/mol, respectively. The charge transfer bands were found at the low-lying excited states of P3HT-C60-PCBM. These bands strongly correlated with the carrier separation and electron transfer in solar cells. The electronic states at the ground and excited states of P3HT-C60-PCBM were discussed on the basis of the calculated results.

Theoretical Insight of Excited States at Large Scale Organic/Organic Interface for Designing Novel Organic Electronic Devices

Revealing electronically excited states across organic/organic interfaces is important to speculate the charge photo-generation mechanisms in organic devices. As organic solar cells are large and unorganized systems so it is very tough to access it using conventional quantum chemical approaches. To unearth the charge separation mechanism a massive scale of excited states should be considered. For this purpose in this study, we have applied recently developed fragment molecular orbital (FMO) method that can effectively consider large-scale molecular aggregates. In our model, we have treated for different configurations of pentacene/C60 bilayer hetero-junction structures. Here, we have also discussed the charge delocalization effect, structural deformations. The measured energy dynamics low-lying interfacial CT states are in well match with experimental reports. J. of Sci. and Tech. Res. 5(1): 169-178, 2023

A novel approach of polychiral single walled carbon nanotubes functionalization for heterojunction solar cell

This study introduces an innovative method for functionalizing polychiral single-walled carbon nanotubes (f-SWCNTs) by depositing thin films onto n-type crystalline silicon (n-Si), thereby creating efficient heterojunction solar cells. The approach capitalizes on the effective charge separation facilitated by the built-in potential resulting from the functionalization of polychiral SWCNTs, leading to enhanced electron transfer from f-SWCNTs to Si driven by functional groups. The resulting f-SWCNT/Si heterojunction device demonstrates remarkable performance, attributed to the high optical absorption and improved charge separation and transfer mechanisms of the f-SWCNT film. Moreover, by adjusting the concentration of f-SWCNTs, the film properties and overall device performance have been customized, achieving a notable power conversion efficiency of 12 %. These discoveries serve as a foundation for the advancement of highly desirable, efficient, cost-effective, and broadband high-performance f-SWCNT/Si heterojunction solar cells

Construction of polarized 2D/2D g-C3N4/BiOCl nanosheet dual piezoelectric Z-scheme heterojunction and its efficient charge separation mechanism

A multi-electric field synergistic strategy based on heterojunction construction and polarization engineering enhances carrier separation and migration, achieving excellent piezoelectric photocatalytic performance of g-C3N4 based materials. Here, through the in-situ growth of BiOCl nanosheets (BOCs) on g-C3N4 nanosheets (GCNs), a 2D/2D Z-scheme GCNs/BOCs heterojunction was successfully fabricated, and its outstanding piezo-photocatalytic property was demonstrated. Compared to individual BOCs and GCNs, the high degradation rate (97.64 %) of rhodamine B (RhB) can be achieved over the optimal composite (GCNs/BOCs-50) after 50 min irradiation. Inspiringly, the polarized GCNs/BOCs-50 composite degraded RhB almost completely within 30 min under the simultaneous light irradiation and ultrasound, which reduced the photocatalytic time by 40 %, and its k reached 0.1410 min−1 (12.59 and 2.48 times that of BOCs and GCNs). This work is of great significance for designing a novel heterojunction photocatalysts using polarized ferroelectric materials.

Enhancement of visible light response of TiO2 photocatalyst by 3D-deposited Ag nanowires and its charge separation mechanism

In 3D-deposited AgNW/TiO2, which is prepared by spray-applying titanium dioxide suspension to deposited silver nanowire sheets, the synergistic effects of increased crossing points of AgNWs to enhance localized surface plasmon resonance excitation and longer-lived electrons in the conduction band of TiO2 generated by plasmon-induced charge transfer have successfully resulted in photocatalytic activity in the visible light range. We have developed photocatalytic sheets in which TiO2 particles are uniformly attached to 3D-deposited AgNWs. Regarding the prepared sheet, it was confirmed that TiO2 was indeed well adhered to the AgNWs, and electron transfer was efficient at the interface. This sheet solves the problem that the response wavelength range of the photocatalytic reaction using TiO2 is only in the ultraviolet region and exhibits sufficient photocatalytic effect in the visible light region. Transient absorption spectroscopy measurements in the diffuse reflectance configuration confirmed that the electrons of AgNWs actually move into the conduction band of TiO2 under visible light and that the interaction is independent of the excitation light intensity, thereby extending the lifetime of the electrons.

Highly-porous CuFe2O4@ZnFe2O4 yolk-shell heterostructure for photocatalytic reduction of nitrobenzene

Developing novel photocatalytic materials and underlying the charge separation mechanisms are keys for achieving excellent performances. However, currently reported photocatalysts suffer low charge separation efficiency and low stability during irradiation. In this work, highly-porous CuFe2O4@ZnFe2O4 yolk-shell heterostructure was prepared by a facile hydrothermal method. The characterization of transmission electron microscopy (TEM) indicates that the as prepared composite consists a core particle located within a well-defined shell with a clear gap between the two, forming a unique yolk-shell structure. The optimal sulfur-doped CuFe2O4@ZnFe2O4 yolk-shell heterostructure presents a photocurrent of 35.42 μA·cm−2, which is five times higher than undoped sample of 7.06 μA·cm−2. Photoelectrochemical method are used to analyze the behavior of photoinduced charges to reveal the structure-property relationship. The built-in electric field of heterojunctions effectively improves the separation of photoinduced charges, and the surface sulfur ions have an enrichment effect on photoinduced electrons. The yolk-shell heterostructure exhibits superior activity for photocatalytic reduction of nitrobenzene showing a selectivity of 99% and yield of 60.6%.

Two-dimensional investigation of characteristic parameters and their gradients for the self-generated electric and magnetic fields of laser-induced zirconium plasma

Two-dimensional diagnosis of laser-induced zirconium (Zr) plasma has been experimentally performed using the time-of-flight method by employing Faraday cups in addition to electric and magnetic probes. The characteristic parameters of laser-induced Zr plasma have been evaluated as a function of different laser irradiances ranging from 4.5 to 11.7 GW cm−2 at different axial positions of 1–4 cm with a fixed radial distance of 2 cm. A well-supporting correlation between the plume parameters and the laser-plasma-produced spontaneous electric and magnetic (E and B) fields was established. The measurements of the characteristic parameters and spontaneously induced fields were observed to have an increasing trend with the increasing laser irradiance. However, when increasing the spatial distance in both the axial and radial directions, the plasma parameters (electron/ion number density, temperature and kinetic energy) did not show either continuously increasing or decreasing trends due to various kinetic and dynamic processes during the spatial evolution of the plume. However, the E and B fields were observed to be always diffusing away from the target. The radial component of electron number densities remained higher than the axial number density component, whereas the axial ion number density at all laser irradiances and axial distances remained higher than the radial ion number density. The higher axial self-generated electric field (SGEF) values than radial SGEF values are correlated with the effective charge-separation mechanism of electrons and ions. The generation of a self-generated magnetic field is observed dominantly in the radial direction at increasing laser irradiance as compared to the axial one due to the deflection of fast-moving electrons and the persistence of two-electron temperature on the radial axis.

Surface photovoltage microscopy for mapping charge separation on photocatalyst particles.

Solar-driven photocatalytic reactions offer a promising route to clean and sustainable energy, and the spatial separation of photogenerated charges on the photocatalyst surface is the key to determining photocatalytic efficiency. However, probing the charge-separation properties of photocatalysts is a formidable challenge because of the spatially heterogeneous microstructures, complicated charge-separation mechanisms and lack of sensitivity for detecting the low density of separated photogenerated charges. Recently, we developed surface photovoltage microscopy (SPVM) with high spatial and energy resolution that enables the direct mapping of surface-charge distributions and quantitative assessment of the charge-separation properties of photocatalysts at the nanoscale, potentially providing unprecedented insights into photocatalytic charge-separation processes. Here, this protocol presents detailed procedures that enable researchers to construct the SPVM instruments by integrating Kelvin probe force microscopy with an illumination system and the modulated surface photovoltage (SPV) approach. It then describes in detail how to perform SPVM measurements on actual photocatalyst particles, including sample preparation, tuning of the microscope, adjustment of the illuminated light path, acquisition of SPVM images and measurements of spatially resolved modulated SPV signals. Moreover, the protocol also includes sophisticated data analysis that can guide non-experts in understanding the microscopic charge-separation mechanisms. The measurements are ordinarily performed on photocatalysts with a conducting substrate in gases or vacuum and can be completed in 15 h.