Direct Z-scheme Mechanism Research Articles

Bilayer MoSe2/HfS2 Nanocomposite as a Potential Visible-Light-Driven Z-Scheme Photocatalyst.

Visible-light-driven photocatalytic overall water splitting is deemed to be an ideal way to generate clean and renewable energy. The direct Z-scheme photocatalytic systems, which can realize the effective separation of photoinduced carriers and possess outstanding redox ability, have attracted a huge amount of interest. In this work, we have studied the photocatalytic performance of the bilayer MoSe2/HfS2 van der Waals (vdW) heterojunction following the direct Z-scheme mechanism by employing the hybrid density functional theory. Our calculated results show that the HfS2 and MoSe2 single layers in this heterojunction are used for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. The charge transfer between the two layers brought about an internal electric field pointing from the MoSe2 layer to the HfS2 slab, which can accelerate the separation of the photoinduced electron–hole pairs and support the Z-scheme electron migration near the interface. Excitingly, the optical absorption intensity of the MoSe2/HfS2 heterojunction is enhanced in the visible and infrared region. As a result, these results reveal that the MoSe2/HfS2 heterojunction is a promising direct Z-scheme photocatalyst for photocatalytic overall water splitting.

Pt–TiO2–Nb2O5 heterojunction as effective photocatalyst for the degradation of diclofenac and ketoprofen

Pt–TiO2–Nb2O5 heterojunction was synthetized and studied for the photocatalytic removal of diclofenac (DCF) and ketoprofen (KTF) under UV light irradiation. The physical-chemical properties of the prepared catalysts were analysed by different characterization techniques revealing that the lowest platinum nanoparticle size and the better metal distribution was observed in Pt–TiO2–Nb2O5 sample. The Pt–TiO2–Nb2O5 heterojunction possessed the best photocatalytic activity toward both the photodegradation and mineralization of the two selected pollutants. The optimal photocatalyst showed a DCF and KTF mineralization rate of 0.0555 and 0.0746 min−1, respectively, which were higher than those of Pt–TiO2 (0.0321 min−1 for DCF and 0.0597 min−1 for KTF). The experiments driven to analyse the effects of free radical capture showed that ·OH, ·O2− and h+ have a primary role in reactive during the photocatalytic reaction. The improved photocatalytic performances of the Pt–TiO2–Nb2O5 heterojunction could be argue by a direct Z-scheme mechanism in which the Pt0 nanoparticles could act as a bridge between TiO2 and Nb2O5, improving the electron-hole separation and, ultimately, enhancing the photocatalytic removal rate of both DCF and KTF.

Integrating photocatalytic reduction of CO2 with selective oxidation of tetrahydroisoquinoline over InP–In2O3 Z-scheme p-n junction

The development of a facile strategy to construct stable hierarchal porous heterogeneous photocatalysts remains a great challenge for efficient CO2 reduction. Additionally, hole-trapping sacrificial agents (e.g., triethanolamine, triethylamine, and methanol) are mostly necessary, which produce useless chemicals, and thus cause costs/environmental concerns. Therefore, utilizing oxidation ability of holes to develop an alternative photooxidation reaction to produce value-added chemicals, especially coupled with CO2 photoreduction, is highly desirable. Here, an in situ partial phosphating method of In2O3 is reported for synthesizing InP–In2O3 p-n junction. A highly selective photooxidation of tetrahydroisoquinoline (THIQ) into value-added dihydroisoquinoline (DHIQ) is to replace the hole driven oxidation of typical sacrificial agents. Meanwhile, the photoelectrons of InP–In2O3 p-n junction can induce the efficient photoreduction of CO2 to CO with high selectivity and stability. The evolution rates of DHIQ and CO are 2 and 3.8 times higher than those of the corresponding In2O3 n-type precursor, respectively. In situ irradiated X-ray photoelectron spectroscopy and electron spin resonance are utilized to confirm that the direct Z-scheme mechanism of InP–In2O3 p-n junction accelerate the efficient separation of photocarriers.

Direct Z-scheme charge separation mechanism and photocatalytic properties of (BiO)2CO3-BiOCl composites prepared in-situ

In this work, (BiO)2CO3-BiOCl heterojunctions were in-situ constructed by acid etching of (BiO)2CO3 using HCl. The formation of (BiO)2CO3-BiOCl heterojunctions was confirmed by XRD and high-resolution transmission electron microscopy (HRTEM). The separation properties of photoinduced e−/h+ pairs were investigated by surface photovoltage spectroscopy (SPS). The results show that O2− and OH were yielded in the (BiO)2CO3-BiOCl composite photocatalytic system. The photocatalytic abilities of the samples toward degradation of methyl orange (MO) were evaluated, the results exhibit that when the molar ratio of (BiO)2CO3/BiOCl is 1/8, the sample displays the highest activity. A direct Z-scheme mechanism was proposed.

Synthesis of sol–gel derived holmium aluminium garnet on exfoliated g-C3N4: a novel visible-light-driven Z-scheme photocatalyst for the degradation of sunset yellow FCF

The synthesis of harmless, cost-effective, and visible-light-driven photocatalytic materials has received significant attention to the researchers in the environmental decontamination applications. In this perception, we report the synthesis of a novel hybrid photocatalyst using holmium aluminium garnet nanostructures modified graphitic carbon nitride (HoAG/g-C3N4) for the first time. A facile sol–gel route and thermal polycondensation methods have been used for the photocatalyst preparation. Different physicochemical characterization methods were employed to confirm the structural, chemical, optical, and morphological properties of the HoAG/g-C3N4 photocatalyst. The visible light absorption enhancement of HoAG/g-C3N4 nanocomposites is due to the broad absorption of trivalent ion (Ho3+) and thus proves to be a promising candidate for the visible-light-driven photocatalysis. The HoAG5/g-C3N4 nanocomposite shows 90.66% removal of sunset yellow FCF dye within 60 min under visible-light irradiation. Also, the catalytic activity of the HoAG5/g-C3N4 nanocomposite was 1.28, and 3.13 times greater than that of pristine g-C3N4 and HoAG, respectively. The photocatalytic degradation of dye on HoAG/g-C3N4 hierarchical structure followed direct Z-scheme mechanism. Photoelectrochemical and recycling measurements show the higher photogenerated charge transfer efficiencies and stability of the HoAG/g-C3N4 nanocomposites.

Surfactant-assisted synthesis of direct Z-scheme AgBr/β-Ag2WO4 heterostructures with enhanced visible-light-driven photocatalytic activities

Compared with the conventional two-step ion exchange method used for the fabrication of AgBr/β-Ag2WO4 heterostructures, herein, a series of AgBr/β-Ag2WO4 direct Z-scheme heterostructures were efficiently fabricated via a simple surfactant-assisted one-step precipitation strategy. The phase structure, morphology, chemical composition as well as photocatalytic mechanism of the as-obtained AgBr/β-Ag2WO4 heterostructures were systematically characterized by using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Ultraviolet–Visible diffuse reflectance spectra (UV–vis DRS), X-ray photoelectron spectroscopy (XPS), Photoluminescence spectrum (PL), photoelectrochemical tests. Photocatalytic degradation of Rhodamine (RhB) under visible light was carried out as a probe to assess its photocatalytic activity. The result indicates that AgBr/β-Ag2WO4-30% exhibits the highest photocatalytic activity, which is approximately 1.7 and 135.9 times as high as that of pure AgBr and pure β-Ag2WO4, respectively. The pathway of charge carriers separation and transfer in the AgBr/β-Ag2WO4 heterostructures and the direct Z-scheme mechanism were depicted in details based on the systematic characterizations and measurements. The enhancement of the photocatalytic performances of the as-prepared AgBr/β-Ag2WO4 heterostructures are mainly attributed to the larger specific surface area and Z-scheme system formed between these two semiconductors, leading to the spatial separation of the photoinduced electron and hole. This work may provide a new sight to understand the mechanism of AgBr/β-Ag2WO4 heterostructures, which is direct Z-scheme heterojunctions rather than type-II heterojunctions.

Fabrication of dual Z-scheme MIL-53(Fe)/α-Bi2O3/g-C3N4 ternary composite with enhanced visible light photocatalytic performance

Z-scheme heterojunction photocatalyst can simultaneously reduce the recombination of photo-generated electron-hole pairs and preserve its outstanding redox ability during the process of photocatalytic reaction. In this study, Z-scheme MIL-53(Fe)/α-Bi2O3/g-C3N4 photocatalyst was successfully fabricated via an accessible method and comprehensive characterized by XRD, FT-IR, SEM, TEM, XPS, N2 adsorption-desorption isotherms, UV–vis diffuse reflection spectroscopy and photoelectrochemical analysis. The as-obtained MIL-53(Fe)/α-Bi2O3/g-C3N4 composite exhibited superior photocatalytic activity for amino black 10B degradation than that of pure g-C3N4, α-Bi2O3, MIL-53(Fe) and α-Bi2O3/g-C3N4 binary composite under visible light irradiation and the photodegradation rate would achieve the maximum when the mass fraction of MIL-53(Fe) was 32% in ternary composite. The enhanced photocatalytic performance of MIL-53(Fe)/α-Bi2O3/g-C3N4 ternary composite could be mainly ascribed to improved visible light absorption range and the construction of an effective interfacial heterojunction driven by direct solid-state dual Z-scheme mechanism, which significantly facilitates the separation and transfer of photo-generated charge carriers. Eventually, the as-prepared ternary composite was confirmed to have prominent stability and reusability after consecutive 4 recycling run.

Fabrication of BiVO4/Bi12O15Cl6 heterojunction photocatalyst with enhanced photocatalytic performance under visible light

In this paper, BiVO4/Bi12O15Cl6 was prepared by combination of ion exchange process and hydrothermal method. The photocatalytic performance of the as-prepared photocatalyst was systematically investigated via the photodegradation of tetracycline (TC) under visible light irradiation. The BiVO4/Bi12O15Cl6 composites showed much higher photocatalytic efficiency than that of BiVO4 and Bi12O15Cl6. The increased photocatalytic performance is attributed to the formation of heterojunction between BiVO4 and Bi12O15Cl6, which results in the effective charges separation and visible light absorption. Significantly, hydroxy radical (·OH) and superoxide radical were confirmed to be the dominant reactive species. Additionally, a direct Z-scheme mechanism was proposed on the basis of photoelectrochemical measurement and bang gap analyses.

Evidence of direct Z-scheme g-C3N4/WS2 nanocomposite under interfacial coupling: First-principles study

Understanding is far from satisfactory on the high photocatalytic performances of g-C3N4/WS2 nanocomposites which are experimentally reported. Here, we investigate systematically for the first time the interfacial interactions and electronic properties of triazine-based g-C3N4/WS2 nanocomposites by first-principles calculations. The results confirm the reasonable existence of vdW interaction and typical direct Z-scheme mechanism based on the prominent interfacial binding energies and the energy level lineup at interface. Both energy band structures and work functions show that the conduction and valence band edges of the g-C3N4 layer are more negative than those of the WS2 layer after their intimate contact, respectively. Meanwhile, the interfacial interaction makes WS2 with negative charges and g-C3N4 with positive charges, forming a built-in electric field from g-C3N4 to WS2. Thus, photogenerated e––h+ pairs are separated to different places with prolonged lifetime and high redox potential. This work shines some lights on a direct Z-scheme mechanism for the g-C3N4/WS2 heterostructured photocatalyst.

Ag3PO4 modified phosphorus and sulphur co-doped graphitic carbon nitride as a direct Z-scheme photocatalyst for 2, 4-dimethyl phenol degradation

Very recently, hybrid photocatalysts are gaining importance due to their unique and enhanced photocatalytic activity. In precedent study, we have successfully prepared Ag3PO4 (AP) and P and S co-doped g-C3N4 (PSGCN) based AP/PSGCN photocatalyst via facile deposition-precipitation method. The P and S co-doped g-C3N4 was prepared via thermal poly-condensation using hexachlorotriphosphazene (HCCP) and thiourea as precursors. The photocatalysts were characterized by X-ray diffraction pattern (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectra (FTIR), X- ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) and ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS). The thickness of AP/PSGCN was less than 9.0 nm. The zeta potential and Tyndall effect experiments validated the formation of well dispersed suspension of AP/PSGCN in water. The co-doping resulted in lowering of optical band gap of g-C3N4. The Tyndall effect experiments ascertained the formation of well dispersed suspension of AP/PSGCN. The photoluminescence and electrochemical impedance analysis confirmed reduction in recombination of photogenerated electron and hole pairs. The photodegradation of 2,4-dimethyl phenol (DMP) followed pseudo first order kinetics. The enhanced photocatalysis was due to direct Z-scheme mechanism. Hydroxyl and superoxide radicals were the two main reactive species during DMP degradation. The COD, HPLC and LCMS investigations ascertained mineralization of DMP. AP/PSGCN displayed high stability and recycle efficiency significant for ten catalytic cycles.

Excellent visible light photocatalytic H2 evolution activity of novel noble-metal-free Ni12P5/CdS composite

Novel noble-metal-free Ni12P5/CdS composite was designed and synthesized by a hydrothermal method. The photocatalytic H2 generation activity of the as-prepared Ni12P5/CdS is 1.93 fold of that of pure CdS under visible light (λ > 420 nm) irradiation. The enhanced activity of Ni12P5/CdS is ascribed to the improved separation efficiency of photocarriers over a direct Z-scheme mechanism. It provides a potential strategy to improve the photocatalytic H2 evolution activity of CdS greatly by loading noble-metal-free Ni12P5 as electron co-catalyst via direct Z-scheme mechanism.

Direct Z-scheme ZnO/CdS hierarchical photocatalyst for enhanced photocatalytic H2-production activity

Herein, ZnO/CdS hierarchical composite was prepared through a hydrothermal and chemical bath deposition (CBD) process. Its photocatalytic H2-production performance was tested. Mass ratio of CdS acted a pivotal part in light absorption and photocatalytic properties. Noticeably, promoted photocatalytic H2-production activity of 4134 μmol g−1 h-1 was achieved by the sample with optimal CdS content. Significantly, the photoluminescence (PL) detection of hydroxyl radicals, as well as the in-situ XPS measurements was selected to verify the direct Z-scheme charge migration mechanism. This mechanism endowed the composite with strong capability for hydrogen evolution and elucidated the improved photocatalytic performance. The improvement of photocatalytic activity was due to hierarchical structure, extended visible light response and direct Z-scheme mechanism. This work will give an innovative vision in constructing direct Z-scheme photocatalytic system with great photocatalytic H2-production activity.

All-solid-state artificial Z-scheme porous g-C3N4/Sn2S3-DETA heterostructure photocatalyst with enhanced performance in photocatalytic CO2 reduction

Nowadays, the increasing CO2 emissions have attracted widespread attentions and it is necessary to reduce CO2 emissions to solve the global warming problem. So photocatalytic reduction of CO2 into chemical fuels is a promising strategy. Here, a Z-scheme porous g-C3N4/Sn2S3-diethylenetriamine (Pg-C3N4/Sn2S3-DETA) composite without an electron intermediary is designed. Photocatalytic performance of the as-fabricated samples is investigated on the basis of photocatalytic CO2 reduction (PCR) to form CH4 and CH3OH. We find that the Z-scheme heterostructure photocatalysts show a higher PCR performance than Pg-C3N4 and Sn2S3-DETA. An optimized Pg-C3N4/Sn2S3-DETA heterostructure sample displays high CH4 production rate of 4.84 μmol h−1 g−1 and CH3OH-production rate of 1.35 μmol h−1 g−1 with 5% Pg-C3N4 content. The superior PCR performance could be ascribed to the special structure of a direct Z-scheme Pg-C3N4/Sn2S3-DETA photocatalyst, which is beneficial to efficient separation of electron-hole pairs. Density functional theory (DFT) calculation further confirms the presence of direct Z-scheme mechanism. This Z-scheme heterostructure photocatalyst with superior performance may inaugurate the perspective on a new promising hierarchy of materials on CO2 photoreduction.

In-situ construction of direct Z-scheme Bi2WO6/g-C3N4 composites with remarkably promoted solar-driven photocatalytic activity

As metal-free visible light-driven semiconductor, graphitic carbon nitride (g-C3N4) has attracted increasing attention owing to its excellent properties. However, the photocatalytic performance of the bare g-C3N4 has been significantly hampered by its inherent drawbacks. Herein, to further boost the photocatalytic performance of g-C3N4, Bi2WO6 was in-situ successfully anchored onto g-C3N4 via a routine hydrothermal method. The as-prepared Bi2WO6/g-C3N4 heterojuctions display highly efficient photocatalytic activity toward decay of rhodamine B (RhB) and phenol aqueous solution under simulated sunlight irradiation. When the molar ratio of Bi:g-C3N4 was 4%, the Bi2WO6/g-C3N4 sample prepared exhibits highest photocatalytic activity. The highest photocatalytic performance of the 4% sample can be definitely attributed to the highest separation rate of the photogenerated electron-hole pairs, supported by the results of surface photovoltage spectroscopy (SPS). The cycling experiments show that Bi2WO6/g-C3N4 composites display excellent stability. The photoinduced electron-hole separation and transfer of the Bi2WO6/g-C3N4 composites obey a direct Z-scheme mechanism based on the matched energy band, the analysis of main active species and the enhanced separation rate of photogenerated electron-hole pairs.

Enhanced photocatalytic hydrogen evolution using a novel in situ heterojunction yttrium-doped Bi4NbO8Cl@Nb2O5

The novel in situ Z-scheme heterostructure materials Y-doped Bi4NbO8Cl@Nb2O5 (Bi4-xYxNbO8Cl, x = 0, 1, 1.33, 2, 2.67, 3) have been synthesized successfully via a solid-state method. The as-prepared samples were characterized by XRD, Raman spectrum, SEM, EDS, element mapping, HRTEM, XPS and UV–vis spectrum to explore the structures, morphologies and optical properties. Photocatalytic activities were evaluated for hydrogen generation using the Pt as the co-catalysts. HRTEM results indicated the Pt particles were deposited on the surface of the Bi4NbO8Cl. Photocatalytic activities were evaluated by hydrogen generation. While photocatalytic results showed that BiY3NbO8Cl composites exhibited the best performance of hydrogen production under the full-range irradiation (λ > 300 nm) while the Y-doped Bi4NbO8Cl@Nb2O5 with Y:Bi molar ratio 1:1 obtained the highest efficiency with ultraviolet light eliminated. The H2 production was 1.35 mmol and 0.9 mmol in 8 h, respectively. Furthermore, a direct Z-scheme mechanism with enhanced hydrogen evolution competent for accelerating the separation of photogenerated carries has been presented and proved by electrochemical impedance spectroscopy (EIS). Finally, considering the conclusions of the electron spin-resonance spectroscopy (EPR), ·OH radicals served as an active species played an important role in the hydrogen production. Mechanisms about the action of the ·OH radicals were also proposed.

In-situ loading of (BiO)2CO3 on g-C3N4 with promoted solar-driven photocatalytic performance originated from a direct Z-scheme mechanism

The photocatalytic performance of g-C3N4 has been significantly hindered by its inherent drawbacks; therefore it is highly desirable to ameliorate the photocatalytic performance of g-C3N4. Construction of effective direct Z-scheme photocatalysts is facile approach to boost the separation and transfer of charge pairs, remarkably promoting the photocatalytic performance of the catalysts. Herein, (BiO)2CO3/g-C3N4 heterojunctions with improved solar-driven photocatalytic performance were in-situ constructed by loading (BiO)2CO3 onto the surface of g-C3N4 via a hydrothermal route. The (BiO)2CO3/g-C3N4 heterojunctions were optimized by adjusting the molar ratio between two components. The resulting heterojunctions display much higher photocatalytic activities for rhodamine B (RhB) degradation compared to the pristine g-C3N4 and the 1.5% sample exhibits the highest photocatalytic activity. Based on the surface photovoltage spectroscopy (SPS), the enhanced photocatalytic performance of (BiO)2CO3/g-C3N4 catalysts can be definitely assigned to the significantly promoted charge transfer and separation. The results confirm that the photoinduced charge separation of (BiO)2CO3/g-C3N4 composites obeys a direct Z-scheme mechanism, proven by the results of electron spin-resonance (ESR) and band edge potential.

High efficiency visible-light-driven Fe2O3-xSx/S-doped g-C3N4 heterojunction photocatalysts: Direct Z-scheme mechanism

Several nanoporous Fe2O3-xSx/S-doped g-C3N4 (CNS) Z-scheme hybrid heterojuctions have been successfully synthesized by one-pot in situ growth of the Fe2O3-xSx particles on the surface of CNS. The characterization results show that S-doping in the g-C3N4 backbone can greatly enhance the charge mobility and visible light harvesting capability. In addition, porous morphology of hybrid composite provides available open pores for guest molecules and also improves light absorbing property due to existence of multiple scattering effects. More importantly, the Fe2O3-xSx nanoparticles formed intimate heterojunction with CNS and developed the efficient charge transfer by extending interfacial interactions occurred at the interfaces of both components. It has been found that the Fe2O3-xSx/CNS composites have an enhanced photocatalytic activity under visible light irradiation compared with isolated Fe2O3 and CNS components toward the photocatalytic degradation of methylene blue (MB). The optimal loaded Fe2O3-xSx value obtained is equal to 6.6wt% that provided 82% MB photodegradation after 150min with a reaction rate constant of 0.0092min−1 which was faster than those of the pure Fe2O3 (0.0016min−1) and CNS (0.0044min−1) under the optimized operating variables acquired by the response surface methodology. The specific surface area and the pore volume of Fe2O3(6.6)/CNS hybrid are 33.5m2/g and 0.195cm3/g, which are nearly 3.8 and 7.5 times greater compared with those of the CNS, respectively. The TEM image of Fe2O3(6.6)/CNS nanocomposite exhibits a nanoporous morphology with abundant uniform pore sizes of around 25nm. Using the Mott-Schottky plot, the conduction and valence bands of the CNS are measured (at pH=7) equal to −1.07 and 1.48V versus normal hydrogen electrode (NHE), respectively. Trapping tests prove that OH and O2− radicals are major active species in the photocatalytic reaction. It has been established that formation of the Z-scheme Fe2O3(6.6)/CNS heterojunction between CNS and Fe2O3 directly produces OH as well as O2− radicals which is consistent with the results obtained from trapping experiments.

Construction of carbon nitride and MoS2 quantum dot 2D/0D hybrid photocatalyst: Direct Z-scheme mechanism for improved photocatalytic activity

Graphite-like carbon nitride (g-C3N4)-based compounds have attracted considerable attention because of their excellent photocatalytic performance. In this work, a novel direct Z-scheme system constructed from two-dimensional (2D) g-C3N4 nanoplates and zero-dimensional (0D) MoS2 quantum dots (QDs) was prepared through the combination of a hydrothermal process and microemulsion preparation. The morphologies, structures, and optical properties of the as-prepared photocatalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy, and UV-vis diffuse reflectance spectroscopy. In addition, the photocatalytic performances of the prepared 2D/0D hybrid composites were evaluated based on the photodegradation of rhodamine B under visible-light irradiation. The results demonstrated that the introduction of MoS2 QDs to g-C3N4 greatly enhanced the photocatalytic efficiency. For the optimum 7% MoS2 QD/g-C3N4 photocatalyst, the degradation rate constant was 8.8 times greater than that of pure g-C3N4 under visible-light irradiation. Photocurrent and electrochemical impedance spectroscopy results further demonstrated that the MoS2 QD/g-C3N4 composites exhibited higher photocurrent density and lower chargetransfer resistance than those of the pure g-C3N4 or MoS2 QDs. Active species trapping, terephthalic acid photoluminescence, and nitro blue tetrazolium transformation experiments were performed to investigate the evolution of reactive oxygen species, including hydroxyl radicals and superoxide radicals. The possible enhanced photocatalytic mechanism was attributed to a direct Z-scheme system, which not only can increase the separation efficiency of photogenerated electron-hole pairs but also possesses excellent oxidation and reduction ability for high photocatalytic performances. This work provides an effective synthesis approach and insight to help develop other C3N4-based direct Z-scheme photocatalytic systems for environmental purification and energy conversion.

Hydrothermal synthesized novel nanoporous g-C3N4/MnTiO3 heterojunction with direct Z-scheme mechanism

Novel nanoporous g-C3N4/MnTiO3 heterojunctions were synthesized by hydrothermal method using nanoporous g-C3N4 and nano-MnTiO3 as the raw materials. The effects of g-C3N4 content on phase, microstructure and photocatalytic activity for dye degradation were studied. Hydrothermal synthesized nanoporous g-C3N4/MnTiO3 (abbreviated as HS-CN/MT) exhibited facets coupling, well-structured heterojunctions with intimate interface, which displayed enhanced absorption intensity of visible light, as well as improved separation and migration efficiency of photogenerated carriers. The adsorption rate of HS-CN/MT for methylene blue gradually increased, while its photocatalytic degradation rate initially increased and subsequently decreased with increasing g-C3N4 content. HS-CN/MT with 65 wt% g-C3N4 presented the highest degradation rate constant of 0.0411 min−1, which was up to 14.5, 2.4, and 3.9 times as high as that of bulk g-C3N4, nanoporous g-C3N4 and mechanical mixed nanoporous g-C3N4/MnTiO3. EIS spectra revealed that HS-65CN/MT exhibited the highest separation efficiency of hole-electron pairs. Active specie capture experiments proved that OH and O2¯ were the main active species, which indicated the HS-CN/MT with significantly enhanced photocatalytic performance exhibited a direct Z-scheme heterojunction mechanism, because it retained higher oxidation and reduction potentials while preventing the electron-hole pairs recombination. These were essentially attributed to the intimate contact of nanoporous g-C3N4/MnTiO3 heterojunctions, which resulted from the synergistic effects of nanoporous adsorption and hydrothermal interaction.

Construction of Z-scheme heterostructure with enhanced photocatalytic H2 evolution for g-C3N4 nanosheets via loading porous silicon

In this work, Z-scheme heterostructure were constructed over the visible light response g-C3N4 photocatalysts by loading Porous silicon (PSi) to enhance the photocatalytic H2 evolution performance. The synthesized Z-scheme g-C3N4/PSi composites with a PSi loading content of 2.50 wt% achieves the highest photocatalytic H2 evolution rate at 870.4 µmol h−1 g−1, which is about 2 times as high as the pure g-C3N4 with H2 evolution rate of 427.2 µmol h−1 g−1. Various techniques including XRD, SEM, TEM, FTIR, XPS, UPS, PL and electrochemical method were employed to demonstrate the successful construction of g-C3N4/PSi composites and to investigate the origin of the enhanced potocatalytic activity. The formed heterostructure between g-C3N4 nanosheets and PSi were verified to be the dominant reason for the enhancement of photocatalytic activity, resulting from the separation promotion of photogenerated charge carriers in a direct Z-scheme mechanism. This study presented a promising Z-scheme g-C3N4/PSi photocatalysts with promising H2 evolution performance, which might drive the progress of solar energy conversion technologies.