Photocharge Separation Research Articles

Boosting bulk photocharge separation and transfer in heterojunctions for antibiotics removal: Enhanced internal electric field and interfacial electric field

The electric fields created by semiconductor heterojunctions often only separated photogenerated charges at component interface, not sufficiently inhibiting carriers’ recombination in their bulk phases. Herein, we introduced the enhanced bulk internal electric field into BixOyIz/g-C3N4 heterojunction, constructing a dual electric field structure through in-situ precipitation and calcination to boost photocharge separation and transfer at both the interface and the bulk. The interfacial electric field created by heterojunction boosted the photocharge separation, and the enhanced internal electric field resulting from iodine reduction improved the bulk charge dynamics of BixOyIz. The dual electric field endowed BixOyIz/g-C3N4 with high photocatalytic activity and broad application potential. Specifically, the photocatalytic reaction rate constants of BixOyIz/g-C3N4 calcined at 460 °C for tetracycline removal reached 0.024 min−1, which was 3.42 times higher than that of BiOI/g-C3N4. The electron paramagnetic resonance (EPR) and scavenging experiments proved that ·O2− was the predominant active species in oxidizing tetracycline. The experiments on simulated real water showed that BixOyIz/g-C3N4 has a broad range of pH applicability, strong resistance to anionic interference, and good cycling performance. This work provided new insights for inhibiting bulk phase photocharge recombination of individual components in heterojunctions.

An eco-efficient dual-function technology: Magnetically recoverable rGO-WO3/Fe3O4 ternary heterojunction catalytic system simultaneously performing malachite green photolysis and Cr(VI) reduction

This study focuses on developing a system that efficiently removes two contaminants simultaneously: malachite green (MG), an organic dye, and chromium (Cr) ions, which are used as oxidizing agents in the dyeing process. A dual-functional heterojunction photocatalyst was prepared by combining tungsten oxide (WO3, the main photocatalyst) and iron oxide (Fe3O4, a visible light absorber) on a reduced graphene oxide (rGO) support. The 10% rGO-WO3/Fe3O4 catalyst decomposes malachite green in 6 h and reduces Cr(VI) to Cr(III) in 2 h (both at 100 ppm). A mixed solution of MG and Cr(VI) required treatment for only 2 h, showing that the intermediates generated during the reaction positively synergized with each other. Here, the junction of WO3 and Fe3O4, which strongly induces a Z-scheme charge transfer, facilitates photo-charge separation and suppresses exciton recombination. The rGO extended the photocatalytic activity while continuously supplying electrons to the valence band of WO3. Finally, the rGO-WO3/Fe3O4 catalyst was completely recovered after the reaction using a magnet. Ultimately, this system will be useful for the quick, clean, and safe treatment of wastewater containing toxic heavy metals and organic pollutants.

Construction of PVDF-HKUST-1/BiVO4 hybrid electrospun flexible fiber membrane for enhanced piezo-photocatalytic reduction performance of Cr(VI)

For the practical implementation of Cr(Ⅵ) removal, effective and recyclable water treatment technology is essential. Polarized electric fields inside piezoelectric materials have proved to be a promising technique for facilitating photogenerated charge separation. In this work, Z-scheme heterojunction photocatalyst HKUST-1/BiVO4 and organic piezoelectric material PVDF were combined through electrospinning to create a novel flexible fiber PVDF-HKUST-1/BiVO4 (PVDF-HV) piezo-photocatalyst membrane. When the amount of HKUST-1/BiVO4 powder was 2.4% and the diameter of the obtained fiber film was 10 cm, the optimal reduction efficiency can be achieved by the piezoelectric photocatalytic reduction in solution with initial pH=2, and the piezo-photocatalytic efficiency of PVDF-HV reached up to 85.33% within 120 min, which was about 1.35 times and 2.19 times greater than that of single photocatalysis and piezoelectric catalysis, respectively. It successfully promoted photo charge separation based on the piezoelectric field in PVDF and the heterojunction produced between HKUST-1 and BiVO4. This research developed an efficient strategy for multi-path collection and exploitation of natural solar energy and vibration energy to boost photoactivity.

Mechanism insight into boosting photocatalytic purification of tetracycline hydrochloride over 2D/2D S-scheme (BiO)2CO3/BiOCl heterojunctions

The (BiO)2CO3/BiOCl heterojunction holds promise for photocatalytic purification of antibiotics, which is primarily attributed to the type-II and Z-scheme photocharge separation mechanisms. However, it lacks a report on S-scheme heterojunction for efficiently promoting photocharge separation and retaining powerful redox capability. In this study, a 2D/2D S-scheme (BiO)2CO3/BiOCl (denoted as (BC/BL)1) heterojunction was prepared through the hydrothermal synthesis method. The as-synthesized (BC/BL)1 exhibited a significantly enhanced photocatalytic activity, with a degradation rate of tetracycline hydrochloride (TC-HCl) reaching 95.3% after 60 minutes. Its kinetic constant of photodegradation surpassed that of (BiO)2CO3 and BiOCl by 20.8 and 3.8 times, respectively. A series of characterizations revealed that the improved photocatalytic activity of the (BC/BL)1 sample stemmed from the formation of a built-in electric field at the intimate interface, which strongly drove the photogenerated carriers to achieve the S-scheme separation. This study highlights the potential application prospects of photocatalyst (BC/BL)1 in antibiotic degradation.

Bismuth sulfide-impregnated cobalt oxide nanocrystals: An enhanced photocatalyst for improved hydrogen production beneath visible light

Hydrogen (H2) is the lightest and greenest fuel compared to conventional energy resources. The production of H2 could be achieved via several techniques as renewable energy sources. Due to its simplicity, the visible-light H2 evolution over nanocomposite photocatalysts is an emerging method. Following this, the surfactant-aided sol–gel way has grown Cobalt oxide (Co3O4) nanoparticles. 3.0–12.0 wt% of bismuth sulfide (Bi2S3) nanocrystals were added to Co3O4 by impregnating technique to enhance the visible-light photoactivity. The produced nanostructures were utilized to generate H2 by photocatalytic reaction in a water/glycerol medium with platinum traces cocatalyst. The impregnation of Bi2S3 at 9.0 wt% has endorsed the photogenerated H2 by ∼ 8.7 folds (∼1547 µmol g−1 h−1) than only using bare Co3O4 (∼178 µmol g−1 h−1) owing to bandgap reduction down to 1.71 eV and suppression of charge recombination. The dose control of 2.5 gL−1 has augmented the H2 generation to 3.095 mmol g−1 h−1 with a significant regeneration to reach 96.5 % after the fifth run. The improved performance of the formed 9.0 % Bi2S3/Co3O4 heterojunction is accredited to the enhanced photocharge separation between Bi2S3 and Co3O4.

Molecular surface-dependent light harvesting and photo charge separation in plant-derived carbon quantum dots for visible-light-driven OH radical generation for remediation of aromatic hydrocarbon pollutants and real wastewater

Despite the growing emphasis on eco-friendly nanomaterials as energy harvesters, scientists are actively searching for metal-free photocatalysts to be used in environmental remediation strategies. Developing renewable resource-based carbon quantum dots (CQDs) as the sole photocatalyst to harvest visible light for efficient pollutant degradation is crucial yet challenging, particularly for addressing the escalating issue of water deterioration. Moreover, the photocatalytic decomposition of H2O2 under visible light irradiation remains an arduous task. Based on this, we designed two types of CQDs, C-CQDs (carboxylic-rich) and A-CQDs (amine-rich) with distinct molecular surfaces. Owing to the higher amount of upward band bending induced by amine-rich molecular surface, A-CQDs efficiently harvest the visible light and prevent recombination kinetics resulting in prolonged lifetimes (25 ps), and augmented charge carrier density (35.7 × 1018) of photoexcited charge carriers. A-CQDs enabled rapid visible-light-driven photolysis of H2O2 (k = 0.058 min−1) and produced higher quantity of •OH radicals (0.158 μmol/sec) for the mineralization of petroleum waste, BETX (i.e. Benzene, Ethylbenzene, Toluene and Xylene) (k = 0.017–0.026 min−1) and real textile wastewater (k = 0.026 min−1). To assess comparative toxicities of both remediated and non-remediated real wastewater samples in a time and dose depended manner, Drosophila melanogaster was used as a model organism. The findings unequivocally demonstrate the potential of remediated wastewater for watering urban forestry.

Internal electric field modulation by copper vacancy concentration of cuprous sulfide nanosheets for enhanced selective CO2 photoreduction

Although the internal electric field (IEF) of photocatalysts is acknowledged as a potent driving force for photocharge separation, modulating the IEF intensity to achieve enhanced photocatalytic performances remains a challenge. Herein, cuprous sulfide nanosheets with different Cu vacancy concentration were employed to study IEF modulation and corresponding direct charge transfer. Among the samples, Cu1.8S nanosheets possessed intensified IEF intensity compared with those of Cu2S and Cu1.95S nanosheets, suggesting that an enhanced IEF intensity could be achieved by introducing more Cu vacancies. This intensified IEF of Cu1.8S nanosheets induced numerous photogenerated electrons to migrate to its surface, and the dissociative electrons were then captured by Cu vacancies, resulting in efficient charge separation spatially. In addition, the Cu vacancies on Cu1.8S nanosheets accumulated electrons as active sites to lower the energy barrier of rate-determining step of CO2 photoreduction, leading to the selective conversion of CO2 to CO. Herein, the manipulation of IEF intensity through Cu vacancy concentration regulation of cuprous sulfide photocatalysts for efficient charge separation has been discussed, providing a scientific strategy to rationally improve photocatalytic performances for solar energy conversion.

A multicomponent triformylphoroglucinol-based covalent organic framework for overall hydrogen peroxide photosynthesis.

A multicomponent covalent organic framework (COF-Tfp-BpyDaaq) integrating bipyridine with diaminoanthraquinone through a triformylphoroglucinol linkage was synthesized for the first time as a photocatalyst for overall H2O2 photosynthesis. It exhibits enhanced photo-charge separation and H2O2 production rate over its two-component counterparts, demonstrating the pivotal role of multicomponent synthesis in designing efficient photocatalysts.

A graphitic carbon nitride photocatalyst with a benzene-ring-modified isotype heterojunction for visible-light-driven hydrogen production

Benzene rings modified g-C3N4 with an iso-type heterojunction structure (UTPh) promotes significantly higher visible-light driven photocatalytic H2 production due to the efficient photo-charge separation and transfer, and donor-acceptor mechanism.

Mechanistic insight into near-infrared light-driven Cu2O/WO2 Ohmic contact photothermal catalysts for high-efficiency antibiotic wastewater purification.

Near-infrared (NIR) light-induced photothermal effect is beneficial for accelerating catalytic processes; thus, it is imperative to develop novel photothermal catalysts for promoting practical application. Herein, we synthesized NIR-responsive Cu2O/WO2 Ohmic contact photothermal catalysts through a facile ethylene glycol-assisted liquid-phase reduction method. In this photothermal catalyst, a new-type NIR-responsive Cu2O semiconductor is integrated with an NIR-responsive WO2 semimetal component to form an Ohmic contact, which is more beneficial for simultaneously promoting photocharge separation and enhancing NIR light absorption for a high-efficiency photothermal effect. As expected, the Cu2O/WO2 composite displays higher NIR light-driven photothermal catalytic performance for tetracycline removal from wastewater. Various characterization methods and density functional theory calculations were performed to obtain in-depth mechanistic insights into the NIR light-driven Cu2O/WO2 Ohmic contact photothermal catalysts. Hopefully, this research could provide a useful guideline for researchers focusing on the photothermal engineering of new composite photocatalysts.

Efficient piezo-photocatalysis of Bi2O2(OH)NO3/BiOI heterojunction: Collaboration of piezoelectric polarization and interface electric field

Effective removal of recalcitrant pollutants is crucial for environmental protection. Tetracycline hydrochloride, as a representative recalcitrant organic pollutant, poses a potential threat to environmental integrity when present in aquatic ecosystems. Coupling piezoelectric polarization, capable of considerably promoting the photocharge separation, acts as a very prevalent and attractive strategy to improve the photocatalytic activity. In this study, an efficient piezo-photocatalytic system was developed in Bi2O2(OH)NO3/BiOI heterojunction by collaborating with interfacial electric field and vibration-induced piezoelectric polarization. The construction of type II heterojunction allows an interfacial electric field to propel the photogenerated electrons from BiOI to Bi2O2(OH)(NO3) under visible light irradiation. The introduction of vibration-induced piezoelectric polarization field strengthens the interfacial electric field, which further promote the separation and migration of photogenerated charges. Profiting from the above advantages, Bi2O2(OH)(NO3)/BiOI heterojunction exhibited a remarkable piezo-photocatalytic performance, achieving an over 80% degradation efficiency of tetracycline hydrochloride within 10 min, which far surpasses that under individual photocatalysis or ultrasound-induced piezocatalysis and also exceeds most of the previously reported piezo-photocatalysts. The combination of electron paramagnetic resonance, piezoresponse force microscopy, COMSOL simulation, and photoelectrochemical tests are conducted to probe the synergistically-catalytic mechanism. This work not only elucidates the comprehensive impact of interfacial electric field and piezoelectric polarization on charge separation, but also highlights the tremendous potential of piezo-photocatalysis for environmental remediation.

Structure-phase transformation of bismuth oxide to BiOCl/Bi24O31Cl10 shoulder-by-shoulder heterojunctions for efficient photocatalytic removal of antibiotic

Developing heterojunction photocatalyst with well-matched interfaces and multiple charge transfer paths is vital to boost carrier separation efficiency for photocatalytic antibiotics removal, but still remains a great challenge. In present work, a new strategy of chloride anion intercalation in Bi2O3 via one-pot hydrothermal process is proposed. The as-prepared Ta-BiOCl/Bi24O31Cl10 (TBB) heterojunctions are featured with Ta-Bi24O31Cl10 and Ta-BiOCl lined shoulder-by-shouleder via semi-coherent interfaces. In this TBB heterojunctions, the well-matched semi-coherent interfaces and shoulder-by-shoulder structures provide fast electron transfer and multiple transfer paths, respectively, leading to enhanced visible light response and improved photogenerated charge separation. Meanwhile, a type-II heterojunction for photocharge separation has been obtained, in which photogenerated electrons are drove from the CB (conduction band) of Ta-Bi24O31Cl10 to the both of bilateral empty CB of Ta-BiOCl and gathered on the CB of Ta-BiOCl, while the photogenerated holes are left on the VB (valence band) of Ta-Bi24O31Cl10, effectively hindering the recombination of photogenerated electron-hole pairs. Furthermore, the separated electrons can effectively activate dissolved oxygen for the generation of reactive oxygen species (·O2−). Such TBB heterojunctions exhibit remarkably superior photocatalytic degradation activity for tetracycline hydrochloride (TCH) solution to Bi2O3, Ta-BiOCl and Ta-Bi24O31Cl10. This work not only proposes a Ta-BiOCl/Bi24O31Cl10 shoulder-by-shoulder micro-ribbon architectures with semi-coherent interfaces and successive type-II heterojunction for highly efficient photocatalytic activity, but offers a new insight into the design of highly efficient heterojunction through phase-structure synergistic transformation strategy.

Divalent Heterometal Doped Titanium-Oxide Cluster Polymers: Structures, Photoresponse, and Photocatalysis.

Five cluster polymers based on heterometal-doped titanium-oxide cluster (TOC) monomers are reported. The monomers feature Ti10-oxide cluster cores and are connected to the divalent closed-shell heterometal anchors by salicylate ligands. The Sr2+, Ba2+, and Pb2+ dopants cause the monomers to bind head-to-head and generate linear chains, while the Ca2+ and Cd2+ lead to head-to-tail connections and zigzag chains. The cluster polymers are responsive to visible-light up to 565 nm and photo-catalytically active in both H2 evolution and CO2/epoxide cycloaddition reactions. The photo-absorption, photo-charge separation, and photocatalytic properties of the cluster polymers are dependent on the heterometal dopants in order Cd > Pb > Ba > Sr > Ca. Heterometals serve as the catalytic sites in the cluster polymers, which depending on the contribution of the pCB bottom, facilitate photo-charge separation and interfacial charge transfer, further enhancing catalytic activity. The tunable compositions and topologies of the cluster polymers shown herein may inspire the design and synthesis of more multidimensional functional metal-oxide cluster materials for a variety of applications in the future.

Mechanism insight into the enhanced photocatalytic purification of antibiotic through encapsulated architectures coupling of crystalline Cu2O/amorphous TiFe layer double hydroxide

Amorphous materials are one of the important candidates for improving heterogeneous photocatalysts because of their unique electronic structures and abundant catalytic sites originating from disorder atomic arrangements. However, there is still much room for the development of new crystalline/amorphous heterogeneous composites for photocatalytic application. Hence efficient synthetic strategies for preparing new crystalline/amorphous heterojunctions are highly desired. Herein, we have realized the deep optimization of photocatalytic activity by fabricating crystalline/amorphous Cu2O/Ti-Fe layer double hydroxide (LDH) heterojunctions. Thanks to the typical Z-scheme mechanism originating from the crystalline/amorphous interfaces, the photocharge separation and catalytic active sites obviously enhance compared to single Cu2O and LDH counterparts. As expected, the photocatalytic removal of tetracycline (TC) of the as-prepared Cu2O/Ti-Fe LDH was over 5.2 and 2.2 times those of the pristine Cu2O nanospheres and Ti-Fe LDH nanosheets. This work illustrates the origin of crystalline Cu2O nanospheres encapsulated in amorphous Ti-Fe layer double hydroxide nanosheets for enhanced photocatalytic activity driven by visible light, and provide a general Cu2O-templated solution-phase synthetic method for the synthesis of novel double-metal layer double hydroxide amorphous nanostructures.

In-situ fabrication of MoO3 hexagonal flowers decorated with Co3O4 microrods with enhanced photocatalytic activity and stability under visible light irradiation

In the recent years, semiconductor photocatalyst has become an environmentally friendly solution for the oxidative elimination of emerging wastewater contaminants under visible light irradiance. Though, the large bandgap energy (Eg) and fast recombination of charge carriers are still problems for the definite application. Here, excellent photocatalytic activity of MoO3 based photocatalyst was explored by decorating Co3O4 microrods with MoO3 hexagonal flowers with lower concentrations (1.0, 3.0, 5.0, 7.0 wt%) of Co3O4 to form novel p-n heterojunctions of Co3O4/MoO3 via hydrothermal synthesis. The as-synthesized 3% Co3O4/MoO3 nanocomposite revealed enormously improved visible light photocatalytic activity for the degradation of rhodamine B (RhB) and alizarin yellow (AY) dyes as compared to other samples due to the development of the p-n junction between p-Co3O4 and n-MoO3. The improved 3.0 wt% Co3O4-loaded MoO3 had a total removal of 91% RhB and 67% AY after 120 min of visible-light exposure with high recyclability and stability after five cycles. The developed p-n heterojunction between 3.0 wt% Co3O4 and MoO3 decreases Eg up to 2.16 eV, shows broader visible light absorption and improved photocharge separation as compared to pristine MoO3. The utilization of this novel heterojunction photocatalyst for the degradation of both dyes is the focus of this research. Furthermore, the trapping experiments and enhanced photocatalytic mechanism was proposed also.

Efficient Combination of Carbon Quantum Dots and BiVO4 for Significantly Enhanced Photocatalytic Activities

The development of highly efficient and stable photocatalysts is of critical importance for the removal of environmental pollutants, such as paraben preservatives. In this work, carbon quantum dots (CQDs) were used to modify bismuth vanadate (BiVO4) through a hydrothermal reaction. Regarding the as-formed CQDs/BiVO4 composite, TEM, XPS, and Raman spectra analysis demonstrated the strong interaction between CQDs and BiVO4, possibly leading to the elevated energy level of the composite. As compared to pristine BiVO4, CQDs/BiVO4 showed an increase in light harvesting, and significantly enhanced visible-light activities in degrading the typical paraben pollutant—benzyl paraben (BzP)—where the maximum 85.4% of BzP was degraded in 150 min. After four cycle reactions, the optimum sample 0.6%CQDs/BiVO4 still degraded 78.2% of BzP, indicating the good stability and reusability of the composite. The notably higher photocurrent and smaller arc in Nyquist plot were measured by CQDs/BiVO4, unveiling the improved photocharge separation and lowered interfacial charge transfer resistance by CQDs modification. Meanwhile, due to the promoted energy level, CQDs/BiVO4 practically produced •O2− species and thereby contributed to the BzP degradation, while they had no ability to produce •OH. This was contrary to the BiVO4 system, where •OH and h+ played the dominant roles.

P–n junction formation between CoPi and α-Fe2O3 layers enhanced photo-charge separation and catalytic efficiencies for efficient visible-light-driven water oxidation

A high performance for PEC water oxidation results from the formation of a p–n junction at the interface between CoPi and α-Fe2O3, which is first observed among the hitherto-reported α-Fe2O3/CoPi-based electrodes.

Breaking through the interfacial energy barrier limitations of type-I heterojunctions via ferroelectric polarization engineering: a case study of Bi5Ti3FeO15/BiOCl

The band structure of a heterojunction significantly affects its photocharge separation efficiency.

Surface-Enriched Boron-Doped TiO2 Nanoparticles as Photocatalysts for Propene Oxidation.

A series of nanostructured boron-TiO2 photocatalysts (B-X-TiO2-T) were prepared by sol–gel synthesis using titanium tetraisopropoxide and boric acid. The effects of the synthesis variables, boric acid amount (X) and crystallization temperature (T), on structural and electronic properties and on the photocatalytic performance for propene oxidation, are studied. This reaction accounts for the remediation of pollution caused by volatile organic compounds, and it is carried out at low concentrations, a case in which efficient removal techniques are difficult and costly to implement. The presence of boric acid during the TiO2 synthesis hinders the development of rutile without affecting the textural properties. X-ray photoelectron spectroscopy analysis reveals the interstitial incorporation of boron into the surface lattice of the TiO2 nanostructure, while segregation of B2O3 occurs in samples with high boron loading, also confirmed by X-ray diffraction. The best-performing photocatalysts are those with the lowest boron loading. Their high activity, outperforming the equivalent sample without boron, can be attributed to a high anatase and surface hydroxyl group content and efficient photo-charge separation (photoelectrochemical characterization, PEC), which can explain the suppression of visible photoluminescence (PL). Crystallization at 450 °C renders the most active sample, likely due to the development of a pure anatase structure with a large surface boron enrichment. A shift in the wavelength-dependent activity profile (PEC data) and the lowest electron–hole recombination rate (PL data) are also observed for this sample.

Purposefully designing Co-S-codoping in hierarchical BiOCl architectures and elucidating the mechanism for enhanced visible-light-driven photocatalytic activity

Simultaneously doping metal and nonmetal species into BiOCl photocatalysts is an efficient way to tailor the valence band and conduction band for enhanced both solar light harvesting and photocharge separation. However, the investigation on metal-nonmetal codoping of BiOCl photocatalysts is still less so far. Herein, the codoping of Co and S species into hierarchical BiOCl architectures (denoted as BOC-Co-S) is realized by a simple one-pot hydrothermal method. The as-prepared BOC-Co-S samples demonstrated remarkably high-efficient visible-light-driven photocatalytic performances toward the degradation of Rhodamine B and tetracycline compared with the Co-doped BiOCl, S-doped BiOCl and pure BiOCl samples. The improved photodegradation activities of BOC-Co-S samples can be ascribed to the increased solar light harvesting, efficient charge separation and sufficient activity sites derived from Co-S codoping. This study might provide a promising strategy to further explore simple and economic organic-free routes for the development of high-active bismuth oxyhalides-based photocatalysts.