Charge Separation In Photocatalysis Research Articles

Periodic quantum well mediated oriented charge separation in Cd0.3Zn0.7S twin crystal towards optimized photocatalytic hydrogen evolution

Interface engineering is vital for promoting charge separation in photocatalysis. Herein, a twin crystal interface in Cd0.3Zn0.7S is engineered, which leads to a variation of the electric polarization along the interface and the formation of a periodic quantum well along z axis. The periodic quantum well could effectively facilitate the oriented charge separation and significantly reduce the diffusion distance simultaneously. Density functional theory (DFT) calculations confirm that Cd0.3Zn0.7S twin crystal possesses a relative low work function and an appropriate hydrogen adsorption Gibbs free energy (ΔGH*), making each step of the cascaded hydrogen evolution reactions optimized. As a result, the resultant twin crystal exhibits an excellent visible light photocatalytic hydrogen evolution rate (13148.98 μmol·g−1·h−1), which is almost 10 and 30 times higher than those of CdS and ZnS. Importantly, it also shows a good stability because of the formation of twin crystal interface. In addition, the introduction of S vacancy defect results in narrowing the band gap and extending the photo-response to long wavelength region. Such a twin crystal interface engineering strategy provides a basic guideline for designing high-efficient photocatalysts with tunable electric polarization.

Hollow SrTiO3 photocatalyst with spatially separated OER and HER cocatalysts for photocatalytic overall water splitting

Overcoming the challenge of sluggish charge separation and the concurrent occurrence of overall water splitting (OWS) half reactions at the same catalytic sites is crucial for achieving efficient photocatalysis. This issue often leads to a high recombination rate and the back reaction of OWS. In this study, we addressed this challenge by enhancing interfacial spatial charge separation. Specifically, we designed hollow SrTiO3 spheres and spatially segregated the hydrogen and oxygen evolution sites on the outer and inner surfaces, respectively, for OWS. The spatially distributed Pt/SrTiO3/CoOx catalysts exhibited exceptional separation of photogenerated charges, resulting in 3-times higher stoichiometric OWS compared to Pt/SrTiO3. This work underscores the effectiveness of a well-tailored spatially distributed structure in promoting charge separation in photocatalysis, thereby opening avenues for the development of efficient artificial photosynthesis systems.

Visible-light responsive BiVO4 nanocompositions: Enhanced photocatalytic, electrical and optical performance through Ni and Ni/Co doping

The pronounced light sensitivity of BiVO4 has garnered significant attention, establishing it as a visible light-responsive photocatalyst. This study was focused on enhancing the optical, dielectric constant properties, and photocatalytic activity of BiVO4 compositions. By employing a modified combustion method, pure BiVO4, Ni-mono-doped (BiNi0.02V0.98VO4), and Ni/Co-bi-doped (BiCo0.02Ni0.02V0.96VO4) were synthesized. X-ray diffraction (XRD) analysis validated the formation of a monoclinic BiVO4 phase. The incorporation of Ni2+ and Co2+ ions into the BiVO4 lattice was supported by variations in the lattice parameters, volume expansion and increased average crystallite size. Morphological investigation via scanning electron microscopy (SEM-EDAX) confirmed the purity of the synthesized nanocompositions and unveiled distinctive rod-like and needle-like particle shapes in BiNi0.02Co0.02V0.96VO4, with an aspect ratio of 0.07–0.1, indicating potential benefits for efficient charge separation in photocatalysis. Optical examination highlighted redshifts in band gap energy (2.36 eV) due to both Ni and (Co, Ni) co-doping, leading to direct band gaps of 2.34 eV and 2.32 eV, respectively. High refractive index values exceeding 2.5, conducive to optical applications, were observed across all samples. Through Ni and Ni/Co doping process, a notable enhancement in conductivity emerged, particularly within the low-frequency domain. The co-doping of Ni and Co demonstrated elevated relative permittivity (εʹ) values exclusively within the low-frequency span, up to 100 Hz. In contrast, the Ni-mono-doping interestingly disclosed an enhancement in (εʹ) across all frequency ranges. Photocatalytic results demonstrated heightened degradation activity in doped samples, with the BiNi0.02Co0.02V0.96VO4, sample exhibiting the highest rate, achieving 88 % degradation efficiency in 180 min. This exceptional performance can be attributed to its visible-light-responsive band gap, increased electron-hole trapping sites, and distinct morphology.

Binary pyrene-benzene polymer/Znln2S4 S-scheme photocatalyst for enhanced hydrogen evolution and antibiotics degradation

Recently, Step-scheme (S-scheme) photocatalysts possess great prospects because of the unique charge transport route and high efficiency of charge separation in photocatalysis. Herein, binary Znln2S4/pyrene-benzene polymer (ZIS-PyP) S-scheme heterojunction was fabricated by an in-situ method. The resulting ZIS-PyP exhibits an excellent photocatalytic H2 evolution rate of 54 μmol h−1 which is 2.5 and 10.8-fold than pure ZnIn2S4 and PyP, respectively. The trapping agent experiments reveal the main active species are ·O2− and h+ in the TCH degradation reaction. The characteristic experiments and DFT calculations were used to elucidate the photocatalytic mechanism. The average hydrogen adsorption free energy of ZIS-PyP-H is determined to be −1.51 eV, smallest than that of ZIS-H and PyP-H, which indicates the most favorable for hydrogen mass transfer in ZIS-PyP. Driven by band bending, built-in electric field and Coulombic repulsion, a unique electronic transmission along S-scheme route between ZnIn2S4 and PyP was formed. Therefore, the powerful electrons left in the LUMO of PyP and strong holes in VB of ZnIn2S4 attended to reactions, ultimately achieved an optimal hydrogen production and TCH degradation efficiency in ZIS-PyP S-scheme system.

MoS2/polyaniline (PANI)/polyacrylonitrile (PAN)@BiFeO3 bilayer hollow nanofiber membrane: Photocatalytic filtration and piezoelectric effect enhancing degradation and disinfection

A novel MoS2/polyaniline (PANI)/polyacrylonitrile (PAN)@BiFeO3 bilayer hollow nanofiber membrane (PPBM-H) was successfully synthesized by coaxial electrospinning technique. In the nanofiber, BiFeO3 nanoparticles (NPs) and MoS2 nanosheets (NSs) were loaded in the middle and outer layers of the PANI/PAN composites, respectively, which constructs a type II heterojunction with spatially separated microtopography, thus significantly improving the charge separation in photocatalysis. Moreover, the hollow structure and the vast number of exposed groups on the surface of PPBM-H help to improve the mass transfer efficiency and pollutant adsorption performance in wastewater treatment. In addition, PPBM-H can generate H2O2 by in-situ activation of BiFeO3/MoS2 for photo-Fenton catalysis, enabling Fe3+ and Fe2+ recycling. Also, PPBM-H can produce piezoelectric polarisation under ultrasonic excitation, which can further enhance the efficiency of electron/hole separation and transfer, and induce the generation of active free radicals. Owing to its wonderful self-cleaning effect, the PPBM-H has good mechanical strength (2.95 Mpa), hydrophilicity (11.6°), water flux (1248 L·m−2·h−1), BSA rejection (98.8 %), and exhibits distinguished photocatalytic filtration efficiencies (99.5 % tetracycline hydrochloride (TCH) and 99.9 % methyl orange (MO) within 60 min), piezo-photocatalysis (99.2 % TCH within 2 h), disinfection performance for Escherichia coli (E. coli) (100 %, within 60 min).

Unraveling the Role of Interface in Photogenerated Charge Separation at the Anatase/Rutile Heterophase Junction

Heterophase junctions have emerged as an effective strategy for spatial charge separation in photocatalysis. However, the photogenerated charge separation pathway at the junction region remains in doubt, and how the interfacial structure affects charge separation is unclear. Herein, we fabricate well-defined TiO2 heterophase junctions between rutile and anatase and employ in situ photochemical probing reactions to track the interfacial charges distribution by means of spherical aberration correction transmission electron microscope (CS aberration TEM). The photoreduction deposition of platinum (Pt) nanoclusters and photo-oxidation deposition of metal oxide nanoparticles demonstrate that the photogenerated electrons smoothly migrate across the interfacial phase junction from rutile to anatase phases and the photogenerated holes move in an opposite direction, which provides direct evidence on the spatial charge separation at the heterophase junction. Moreover, the atomic interfacial structure of anatase/rutile TiO2 heterophase junction formed is conducive for photogenerated charges moving from one phase to the other. This study revises the photogenerated charge separation at the anatase/rutile heterophase junction and demonstrates the important role of the interfacial structure on spatial charge separation.

Spatiotemporal imaging of charge transfer in photocatalyst particles.

The water-splitting reaction using photocatalyst particles is a promising route for solar fuel production1-4. Photo-induced charge transfer from a photocatalyst to catalytic surface sites is key in ensuring photocatalytic efficiency5; however, it is challenging to understand this process, which spans a wide spatiotemporal range from nanometres to micrometres and from femtoseconds to seconds6-8. Although the steady-state charge distribution on single photocatalyst particles has been mapped by microscopic techniques9-11, and the charge transfer dynamics in photocatalyst aggregations have been revealed by time-resolved spectroscopy12,13, spatiotemporally evolving charge transfer processes in single photocatalyst particles cannot be tracked, and their exact mechanism is unknown. Here we perform spatiotemporally resolved surface photovoltage measurements on cuprous oxide photocatalyst particles to map holistic charge transfer processes on the femtosecond to second timescale at the single-particle level. We find that photogenerated electrons are transferred to the catalytic surface quasi-ballistically through inter-facet hot electron transfer on a subpicosecond timescale, whereas photogenerated holes are transferred to a spatially separated surface and stabilized through selective trapping on a microsecond timescale. We demonstrate that these ultrafast-hot-electron-transfer and anisotropic-trapping regimes, which challenge the classical perception of a drift-diffusion model, contribute to the efficient charge separation in photocatalysis and improve photocatalytic performance. We anticipate that our findings will be used to illustrate the universality of other photoelectronic devices and facilitate the rational design of photocatalysts.

Fast Interlayer Charge Separation and Transmission in ZnIn2S4/CNTs/ZnS Heterojunctions for Efficient Photocatalytic Hydrogen Evolution

AbstractThe construction of close contact heterojunction structures is crucial to modify photocatalytic performance. Here a series of heterojunction ZnIn2S4/CNTs/ZnS nanocomposites for photocatalytic hydrogen evolution were synthesized by a simple thermal reflux approach. The ideal ZnIn2S4/CNTs/ZnS sample has a photocatalytic hydrogen evolution rate of 0.9363 mmol ⋅ g−1 ⋅ h−1 with favorable cycle stability, which is about 16.9 and 5.5 times that of pure ZnIn2S4 and binary ZnIn2S4/ZnS. The tight interfacial contact between ZnIn2S4, ZnS, and CNTs raises photogenerated charge carrier separation efficiency. CNTs have a high electrical conductivity, which allows them to receive and transport photogenerated electrons effectively. Therefore, the recombination of photon‐generated charges may be efficaciously restrained, and the accepted electrons can be rapidly transported to reactive sites for production of hydrogen. Simultaneously, density functional theory (DFT) simulations show that introducing CNTs can enhance electronic characteristics and add additional charge carriers. This study shows that well‐designed CNTs‐based heterojunction structure can effectively promote charge separation in photocatalysis.

In-situ-formed red phosphorus nanosheet on bulk red phosphorus for boosting charge separation in photocatalysis：The role of multiple interfacial effects

Red phosphorus (RP) has recently attracted much attention in photocatalysis for its low cost, environmental benign and optical properties. However, its photocatalytic activity is still restricted since the low-charge carrier mobility and the high water-solubility. Herein, RP-S-12 h (Combination of RP nanosheet (NS) and bulk RP structure) was prepared by hydrothermal method. Results showed that the multiple reactive active sites, the energy level matching, the emergence of intermediate state and the internal unequal potential for RP-S-12 h ensured higher charge density and efficient carrier transport, improving the photocatalytic activity and stability, emphasizing the role of multiple interfacial effects. The optimum hydrogen evolution rate was 0.33 mmol g−1 h−1, and the quantum efficiency at 420 nm is 0.45%. In addition, RP-S-12 h could completely degrade 50 ppm MO in 12 min. This study provides a novel insight into the interface design of metal-free photocatalyst for environmental remediation.

Plasmon excitation facilitating generation of electrons and reactive oxygen species for broad spectrum photocatalytic activity

The coupling of semiconductor and plasmonic nanoparticles (NPs) has attracted significant attention due to improved light harvesting and charge separation for photocatalysis. However, the enhancement effect of surface plasmon resonance (SPR) has not been fully utilized, since it is related to a variety of factors, and the dominant factors remain unclear. It is realized that the spatial arrangement of semiconductor and plasmonic NPs may be of critically importance. To prove this concept, Au/TiO2 plasmonic nanostructures were fabricated as model system by distributing Au nanorods (AuNRs) uniformly on TiO2 nanosheets through mercaptopropionic acid assisted method. AuNRs/TiO2 possesses tunable SPR induced light absorption centered at 520 nm and 795 nm, ranging from visible to NIR that depends on the shape of AuNPs. The sample exhibits simultaneously enhanced photocatalytic activity toward reduction of H2O to produce H2 and oxidation of 3,3′,5,5′-tetramethylbenzidine when exposed to visible or NIR light. The production and reactive behaviors of electrons and ROS including ·OH, ⋅O2– and 1O2 from irradiated Au/TiO2 were detected by electron spin resonance, and correlated with SPR enhanced photocatalytic activity. These results unveil critical factor in SPR enhanced photocatalysis and suggest a highly preferred form of the hybrid nanostructures for their photocatalytic applications.

First-principles calculations of 0D/2D GQDs-MoS2 mixed van der Waals heterojunctions for photocatalysis: a transition from type I to type II.

The fabrication of type II heterojunctions is an efficient strategy to facilitate charge separation in photocatalysis. Here, mixed dimensional 0D/2D van der Waals (vdW) heterostructures (graphene quantum dots (GQDs)-MoS2) for generating hydrogen from water splitting are investigated based on density functional theory (DFT). The electronic and photocatalytic properties of three heterostructures, namely, C6H6-MoS2, C24H12-MoS2 and C32H14-MoS2 are estimated by analyzing the density of states, charge density difference, work function, Bader charge, absorption spectra and band alignment. The results indicated that the built-in electric fields from GQDs to MoS2 boost charge separation. Meanwhile, all the GQDs-MoS2 exhibit strong absorption in the visible light region. Surprisingly, the transition of heterojunctions from type I to type II is realized by tuning the size of GQDs. In particular, C32H14-MoS2 with enhanced visible-light absorption and an appropriate band edge position, as a type II heterostructure, may be a promising photocatalyst for generating hydrogen from water splitting. Thus, in this work a novel type II 0D/2D nanocomposite as a photocatalyst is constructed that provides a strategy to regulate the type of heterostructure from the perspective of theoretical calculation.

Piezo-Photocatalysis over Metal-Organic Frameworks: Promoting Photocatalytic Activity by Piezoelectric Effect.

The built-in electric field can be generated in the piezoelectric materials under mechanical stress. The resulting piezoelectric effect is beneficial to charge separation in photocatalysis. Meanwhile, the mechanical stress usually gives rise to accelerated mass transfer and enhanced catalytic activity. Unfortunately, it remains a challenge to differentiate the contribution of these two factors to catalytic performance. Herein, for the first time, isostructural metal-organic frameworks (MOFs), i.e., UiO-66-NH2 (Zr) and UiO-66-NH2 (Hf), are adopted for piezo-photocatalysis. Both MOFs, featuring the same structures except for diverse Zr/Hf-oxo clusters, possess distinctly different piezoelectric properties. Strikingly, UiO-66-NH2 (Hf) exhibits ≈2.2 times of activity compared with that of UiO-66-NH2 (Zr) under simultaneous light and ultrasonic irradiation, though both MOFs display similar activity in the photocatalytic H2 production without ultrasonic irradiation. Given their similar pore features and mass transfer behaviors, the activity difference is unambiguously assignable to the piezoelectric effect. As a result, the contributions of the piezoelectric effect to the piezo-photocatalysis can be clearly distinguished owing to the stronger piezoelectric property of UiO-66-NH2 (Hf).

Surface Modifications of (ZnSe)0.5(CuGa2.5Se4.25)0.5 to Promote Photocatalytic Z-Scheme Overall Water Splitting

Charge separation is crucial for an efficient artificial photosynthetic process, especially for narrow-bandgap metal sulfides/selenides. The present study demonstrates the application of a p-n junction to particulate metal selenides to enhance photocatalytic Z-scheme overall water splitting (OWS). The constructed p-n junction of CdS-(ZnSe)0.5(CuGa2.5Se4.25)0.5 significantly boosted charge separation. A thin TiO2 coating layer also was introduced to inhibit photocorrosion of CdS and suppress the backward reaction of water formation from hydrogen and oxygen. By employing Pt-loaded TiO2/CdS-(ZnSe)0.5(CuGa2.5Se4.25)0.5 as a hydrogen evolution photocatalyst (HEP), we assembled a Z-scheme OWS system, together with BiVO4:Mo and Au as an oxygen evolution photocatalyst and electron mediator, respectively. An apparent quantum yield of 1.5% at 420 nm was achieved, which is by far the highest among reported particulate photocatalytic Z-scheme OWS systems with metal sulfides/selenides as HEPs. The present work demonstrates that a well-tailored p-n junction structure is effective for promoting charge separation in photocatalysis and opens new pathways for the development of efficient artificial photosynthesis systems involving narrow bandgap photocatalysts.

Nano Anatase TiO2 Quasi-Core-Shell Homophase Junction Induced by a Ti3+ Concentration Difference for Highly Efficient Hydrogen Evolution.

The fabrication of heterojunctions or homojunctions between semiconductors is a controllable strategy to facilitate charge separation in photocatalysis. The homophase junctions exhibit atomic-level contact for the fast-speed charge transfer via inducing the built-in electric fields. Herein, a new concept of TiO2 quasi-core-shell homophase junction induced by a Ti3+ concentration difference for remarkably enhancing photocatalytic activity is proposed. Nano anatase TiO2 quasi-core-shell homophase junctions are constructed between the interior with high Ti3+ concentration (quasi-core) and the surface with no detected Ti3+ (quasi-shell). Diverse Ti3+ concentration differences are obtained via regulating the mass ratio of the Ti source. The nano anatase TiO2 quasi-core-shell homophase junctions exhibit improved photocatalytic hydrogen evolution compared with commercial anatase nanoparticles. To be specific, the maximum hydrogen evolution rate of 50.02 mmol/h/g is 25.4 times superior to that of commercial anatase nanoparticles under solar illumination. Besides, the photocatalytic activity remains stable (H2 evolution rate of 49.21 mmol/h/g, activity loss of <2%) after five cycles of catalytic test. The promoted photocatalytic activities are ascribed to the constitution of a built-in electrical field between the quasi-shell and quasi-core induced by the band bending, which accelerates the spatial charge separation and suppresses the recombination of carriers. Moreover, the atomic-level contact at the homophase junction interface provides smooth channels for carrier transfer, resulting in more effective separation and transfer of photogenerated electrons and holes. The synthesis of nano anatase TiO2 quasi-core-shell homophase junctions provides new insights into the efficient separation and transfer of photogenerated carriers for photocatalytic applications.

Nanoassembly of perovskite-based photocatalysts in a nanoconfined system for photocatalytic H2 production under visible light

This study reports the polymer-inorganic nanofibers directed nanoassembly of SrTiO3 and its crystal growth behavior in a polymeric nanoconfined system with two synthesis variables, including precursor concentrations and calcination temperatures. The synthesis variables were also optimized to get nanoassembled SrTiO3 in terms of photocatalytic H2 evolution. Using the optimized nanoassembled SrTiO3 as a platform, CdS/SrTiO3 and CdS/Au/SrTiO3 heterostructures were fabricated to investigate the nanofiber directed SrTiO3 nanoassembly effect on the photoactivity of these heterostructures under visible light. The nanoassembled SrTiO3-based heterostructures demonstrate an increased photocatalytic H2 evolution rate compared to their respective unassembled heterostructures due to the advantageous interparticle nanoassembly which facilitates faster electron transport and better photoexcited charge separation. Photocatalyst heterostructure nanoassembly is one potential method of enhancing charge separation in photocatalysis.

Electron magnetic resonance in heterogeneous photocatalysis research

The contribution of electron magnetic resonance techniques, and in particular of CW-EPR, to the experimental research on photocatalytic phenomena is illustrated in this paper with selected examples. In the first part of the paper the role of EPR in unravelling the nature and the features of extrinsic point defects in semiconducting oxides is epitomized using the important example of the photoactive nitrogen center in various semiconducting oxides. In the second part we describe how EPR can monitor the processes that follow the initial photoinduced charge separation in photocatalysis, namely the stabilisation, migration and surface reactivity of electrons and holes. Finally, we will discuss how the role of EPR in photocatalysis is not limited to monitor phenomena occurring in the solid or at its surface but it can be extended to the investigation of the liquid phase by employing the spin trapping techniques to monitor the nature and the concentration of the reactive free radicals formed along the photocatalytic process.

Homophase Junction for Promoting Spatial Charge Separation in Photocatalytic Water Splitting

The widespread heterojunction or p–n junction strategies fabricated between different semiconductors are generally used to promote the spatial charge separation in photocatalysis and solar cells, w...

Boosting photocatalytic water splitting by tuning built-in electric field at phase junction

Constructing a built-in electric field at the interface of semiconductors has been demonstrated to provide the driving force for spatial charge separation in photocatalysis.

Charge separation via asymmetric illumination in photocatalytic Cu2O particles

Solar-driven photocatalytic reactions provide a potential route to sustainable fuels. These processes rely on the effective separation of photogenerated charges, and therefore understanding and exploring the driving force for charge separation is key to improving the photocatalytic performance. Here, using surface photovoltage microscopy, we demonstrate that the photogenerated charges can be separated effectively in a high-symmetry Cu2O photocatalyst particle by asymmetric light irradiation. The holes and electrons are transferred to the illuminated and shadow regions, respectively, of a single photocatalytic particle. Quantitative results show that the intrinsic difference between electron and hole mobilities enables a diffusion-controlled charge separation process, which is stronger than that caused by conventional built-in electric fields (40 mV versus 10 mV). Based on the findings, we assemble spatially separated redox co-catalysts on a single photocatalytic particle and, in doing so, enhance the performance for a model photocatalytic reaction by 300%. These findings highlight the driving force caused by charge mobility differences and the use of asymmetric light illumination for charge separation in photocatalysis. Photocatalysts use light to drive chemical reactions; the effective spatial separation of photogenerated charges is key to their performance in solar energy conversion. Here, using surface photovoltage microscopy, the authors show that charges can be separated in photocatalytic particles by asymmetric light irradiation.

Determining the Charge‐Transfer Direction in a p–n Heterojunction BiOCl/g‐C3N4 Photocatalyst by Ultrafast Spectroscopy

AbstractType‐II heterostructures composed of p‐type BiOCl and n‐type g‐C3N4 have received much attention because of their high efficiencies of charge separation in photocatalysis. However, some ambiguity exists regarding the transfer direction of photogenerated charge carriers in the BiOCl(p)/g‐C3N4(n) system. Although it seems to have not posed an issue in terms of organic photodegradation applications, misinterpretation of the charge‐transfer direction and hence misjudgment of the redox sites in the hybrid system would be a serious problem for other photocatalytic reactions, especially for half reactions, in which the determination of the redox sites is crucial. To solve this problem, we carefully interrogated this topical heterojunction system formed between p‐type BiOCl and n‐type g‐C3N4 by means of ultrafast transient absorption spectroscopy. Our experimental results reveal that the photogenerated electrons are transferred, on a time scale of a few picoseconds, from the conduction band of BiOCl to that of g‐C3N4 and not in the opposite direction. This work enables us to eliminate the ambiguity in using two opposing mechanistic scenarios to describe the p–n heterojunction based photocatalysis mechanism, thereby providing instructive information for the rational design of photocatalytic nanosystems.