Efficient Z-scheme Research Articles

Hierarchical TiO2 spheroids decorated g-C3N4 nanocomposite for solar driven hydrogen production and water depollution

A novel hierarchical TiO2 spheroids embellished with g-C3N4 nanosheets has been successfully developed via thermal condensation process for efficient solar-driven hydrogen evolution and water depollution photocatalyst. The photocatalytic behaviour of the as-prepared nanocomposite is experimented in water splitting and organic pollutant degradation under solar light irradiation. The optimal ratio of TiO2 spheroids with g-C3N4 in the nanocomposite was found to be 1:10 and the resulting composite exhibits excellent photocatalytic hydrogen production of about 286 μmol h−1g−1, which is a factor of 3.4 and 2.3 times higher than that of pure TiO2 and g-C3N4, respectively. The outstanding photocatalytic performance in this composite could be ascribed as an efficient electron-hole pair's separation and interfacial contact between TiO2 spheroids with g-C3N4 nanosheets in the formed TiO2/g-C3N4 nanocomposite. This work provide new insight for constructing an efficient Z-scheme TiO2/g-C3N4 nanocomposites for solar light photocatlyst towards solar energy conversion, solar fuels and other environmental applications.

A direct Z-scheme BiOBr/TzDa COF heterojunction photocatalyst with enhanced performance on visible-light driven removal of organic dye and Cr(VI)

• New 2D/2D Z-scheme heterostructure photocatalyst BiOBr/TzDa COF (BTDC) is prepared. • RhB degradation or Cr(VI) reduction is achieved by BTDC under visible-light. • BTDC can simultaneously remove RhB/Cr(VI) mixture with synergistic effect. • BTDC has good stability and reusability in cyclic use on RhB and Cr(VI) removal. • Mechanisms for RhB degradation and Cr(VI) reduction by BTDC are proposed. Design of efficient Z-scheme heterojunction photocatalysts is important and yet challenging for purifying wastewater with solar energy. Herein, a new 2D/2D Z-scheme heterostructure photocatalyst of BiOBr/TzDa covalent organic framework (COF) composite was obtained by modification of 2D BiOBr with 2D TzDa COF via a facile hydrothermal method. The photocatalytic activity of BiOBr/TzDa COF (BTDC) under simulated visible light was studied by degradation of single Rhodamine B (RhB) (10 ppm) and reduction of single Cr(VI) (10 ppm), as well as for treating the RhB/Cr(VI) mixture. RhB could be efficaciously degraded by BTDC within a wide pH range of 2.1–8.1. Nearly complete reduction of Cr(VI) at pH 2.1 could also be achieved after irradiation for 90 min. Moreover, much higher performance of BTDC was found for simultaneous removal of RhB and Cr(VI) from a binary mixture solution than for single pollutant system due to the synergistic effect within the RhB/Cr(VI)/BTDC system. 97% of total RhB and Cr(VI) in the mixture could be removed in 20 min and 40 min, respectively. The promoted efficiency of the stable and recyclable BTDC could be mainly attributed to accelerated charge carriers separation and transport, enhanced visible light absorption as well as increased specific surface area. Photocatalytic mechanism with Z-scheme charge transfer in BTDC was suggested based on systematic studies of band structures calculation, radical quenching experiments and ESR analyses. This study may provide a strategy in constructing COF based heterojunctions for purification of wastewaters containing mixed heavy metal ions and organic pollutants.

Designing of TiO2/α-Fe2O3 coupled g-C3N4 magnetic heterostructure composite for efficient Z-scheme photo-degradation process under visible light exposures

A novel ternary g-C3N4 coupling with TiO2 and α-Fe2O3 composed magnetic nanocomposite was fabricated by facile calcination and hydrothermal process. The chemical, surface and magnetic characterizations of as-obtained g-C3N4/TiO2/α-Fe2O3 magnetic nanocomposite was explored by various analytical systems. The superior degradation efficiency of magnetic g-C3N4/TiO2/α-Fe2O3 ternary nanocomposite is about 95.7% of Rhodamine B (RhB) in 50 min under visible-light exposure. The photocatalytic upgrading performance was recognized to the synergetic interface among TiO2, g-C3N4 and α-Fe2O3, with effective separation of electron/hole (e-/h+) pair’s, increases in the redox species and great visible-light exploitation owing to less bandgap. The scavenging species presented in superoxide (.O2-) radicals and photo-holes (h+) were played a substantial part and hence establish the Z-scheme RhB dye photo-degradation mechanism. The magnetic g-C3N4/TiO2/α-Fe2O3 ternary PCs still maintain the excellent photo-degradation rate (95.7–88.4%) for the RhB dye aqueous solution after five consecutive recycles, which signifies that the great photocatalyst durability, and is more beneficial for wastewater treatment.

LaTaON2 Mesoporous Single Crystals for Efficient Photocatalytic Water Oxidation and Z-Scheme Overall Water Splitting.

LaTaON2 porous single crystals (PSCs), integrating structural coherence and porous microstructures, will warrant promising photocatalytic performance. The absence of grain boundaries in PSCs ensures rapid photocarrier transportation from bulk to the surface, thereby mitigating photocarriers' recombination. Porous microstructures not only provide ample reachable surface to host photochemical reactions but also reinforce photon-matter interactions by additional photon reflection/scattering. Here, we have synthesized LaTaON2 PSCs via a topotactic route and show significantly improved photocatalytic performance. Efficient water oxidation into O2 has been realized by LaTaON2 PSCs with an apparent quantum efficiency as high as 5.7% at 420 ± 20 nm. Stable overall water splitting into stoichiometric H2 and O2 has also been achieved in a Z-scheme setup using LaTaON2 PSCs as the O2 evolution photocatalyst. These results not only prove that PSCs facilitate photocarrier migrations, which in turn deliver exceptional photocatalytic performance, but also imply that PSCs are useful to reinvigorate conventional semiconductor photocatalysts toward efficient solar energy conversions.

Construction of Chemically Bonded Interface of Organic/Inorganic g-C3N4/LDH Heterojunction for Z-Schematic Photocatalytic H2 Generation.

The design and synthesis of a Z-schematic photocatalytic heterostructure with an intimate interface is of great significance for the migration and separation of photogenerated charge carriers, but still remains a challenge. Here, we developed an efficient Z-scheme organic/inorganic g-C3N4/LDH heterojunction by in situ growing of inorganic CoAl-LDH firmly on organic g-C3N4 nanosheet (NS). Benefiting from the two-dimensional (2D) morphology and the surface exposed pyridine-like nitrogen atoms, the g-C3N4 NS offers efficient trap sits to capture transition metal ions. As such, CoAl-LDH NS can be tightly attached onto the g-C3N4 NS, forming a strong interaction between CoAl-LDH and g-C3N4 via nitrogen–metal bonds. Moreover, the 2D/2D interface provides a high-speed channel for the interfacial charge transfer. As a result, the prepared heterojunction composite exhibits a greatly improved photocatalytic H2 evolution activity, as well as considerable stability. Under visible light irradiation of 4 h, the optimal H2 evolution rate reaches 1952.9 μmol g−1, which is 8.4 times of the bare g-C3N4 NS. The in situ construction of organic/inorganic heterojunction with a chemical-bonded interface may provide guidance for the designing of high-performance heterostructure photocatalysts.

Constructing a Z-scheme ZnIn2S4-S/CNTs/RP nanocomposite with modulated energy band alignment for enhanced photocatalytic hydrogen evolution

Energy band structures greatly determine the charge separation and transfer properties in heterojunction photocatalysts and consequently their photocatalytic activities. Herein, a well-designed Z-scheme ZnIn2S4-S/CNTs/RP (ZIS-S/CNTs/RP) nanocomposite was fabricated according to an energy band alignment steering strategy to realize superior photocatalytic H2 evolution performance. The ZIS-S/CNTs/RP nanocomposite shows an efficient photocatalytic H2 evolution rate of 1639.9 μmol g-1h−1, which is noticeably higher than that of pristine red phosphorus (RP) and CNTs/RP and ZIS-S/RP composites, as well as those of the compared heterojunctions using bulk RP or ZnIn2S4. Owing to the modification of nanosized RP and the introduction of sulfur vacancies in ZnIn2S4, a tailored energy band alignment of the heterojunction with a higher reduction potential and larger Fermi level potential difference was achieved, which resulted in significantly increased photogenerated electron-hole separation efficiency and a more efficient Z-scheme charge transfer mechanism, thus promoting the photocatalytic activity of ZIS-S/CNTs/RP. This work aims to provide a novel effective strategy for the construction of high-performance heterojunction photocatalysts by energy band engineering.

Z-scheme junction Bi2O2(NO3)(OH)/g-C3N4 for promoting CO2 photoreduction

Photocatalytic CO2 reduction is believed to be an emerging strategy to address fossil fuels consumption and global warming problems. However, the rapid recombination of photogenerated carriers restricts the application of photocatalytic technology severely. Constructing heterojunction structure is one of the most effective pathways to facilitate the charge separation and photocatalytic performance. In this work, a series of direct Z-scheme heterojunction Bi2O2(NO3)(OH)/g-C3N4 (BON/CN) was developed by an electrostatic self-assembly method. The optimal BON/CN-2 photocatalyst exhibits a conspicuous photocatalytic CO2 reduction activity with a high CO evolution rate of 14.84 µmol g-1h−1 under the stimulated solar light irradiation, which is about 15 and 3.5 times higher than that of pure BON and g-C3N4, respectively. The significantly promoted photocatalytic performance is attributed to the tightly-contact interface and formation of Z-scheme between BON and g-C3N4, which favor for the separation and transfer of photogenerated electrons and holes, as confirmed by high resolution transmission electron microscope (HRTEM), electron spin resonance (ESR), photoelectrochemical measurement and photoluminescence (PL) results. This work may provide a new reference for developing rational tactics to fabricate efficient Z-scheme heterojunction photocatalysts for CO2 conversion.

Efficient hydrogen generation of vector Z-scheme CaTiO3/Cu/TiO2 photocatalyst assisted by cocatalyst Cu nanoparticles

Herein, the CaTiO3/Cu/TiO2 all-solid-state Z-scheme heterojunction is successfully designed via Cu nanoparticles situating at the interface between CaTiO3 and TiO2 with a new synthesis route. Interestingly, TiO2 nanosheets are generated in-situ on the surface of CaTiO3 in the second step hydrothermal reaction. The lifetimes of photoexcited carriers, photoluminescence emission spectra and transient photocurrent response tests have confirmed that the efficient Z-scheme charge transmission path of the CaTiO3/Cu/TiO2 is beneficial to facilitate the separation of photogenerated carriers and reduce their recombination efficiency. As expected, the hydrogen generation rate of CaTiO3/Cu/TiO2 is increased to 23.550 mmol g-1h−1 with the appropriate amount of copper loading, which is about 981 times and 93 times higher than that of pristine CaTiO3 (0.024 mmol g-1h−1) and CaTiO3/TiO2 (0.253 mmol g-1h−1), respectively. Furthermore, the CaTiO3/Cu/TiO2 sample shows good stability in cycle experiments. Particularly, experimental results show that the non-noble metal Cu nanoparticles can be an effective electron mediator. And these merits strongly demonstrate that the CaTiO3/Cu/TiO2 composites have potential application in photocatalytic field. This study can provide fundamental guidance for designing rationally efficient non-noble metal vector Z-scheme system photocatalysts with outstanding photocatalytic H2 generation performance.

Electron transfer via a carbon channel for efficient Z-scheme solar hydrogen production

Z-scheme photocatalysis provides a promising solution to photocatalytic solar water splitting, yet restricted by inferior interfacial charge transfer. Here, we demonstrate a Z-scheme composite photocatalyst made of Fe2O3, a carbon layer, and g-C3N4 that can achieve efficient hydrogen generation from solar water decomposition. The success relies on in-situ preparation of core-shell Fe2O3@C structure at the surface of g-C3N4. Carbon as an intermediate layer thus acts as a bridge that significantly accelerates the migration of photogenerated electrons from Fe2O3 conduction band to g-C3N4 valence band. As a result, the highest rate of H2 generation reaches 5.26 mmol h−1g−1. This activity is approximately 33-time greater than that achieved over pristine g-C3N4 and about 4-time larger than that obtained over a Fe2O3/g-C3N4 heterojunction without internal carbon layer. This work explicates the potential insight of the composite and paves a promising way to engineer the charge transfer behavior.

Mechanistic investigation of photocatalytic degradation of Bisphenol-A using MIL-88A(Fe)/MoS2 Z-scheme heterojunction composite assisted peroxymonosulfate activation

In this study, a green synthesis method was employed to synthesize MoS2 incorporated MIL-88A(Fe) composite photocatalyst, MIL-88A(Fe)/MoS2 (MSMIL). Its catalytic property was utilized to degrade Bisphenol-A (BPA) in aqueous solution, using peroxymonosulfate (PMS) as oxidant under UV irradiation (intensity: 48 W). Doping of 20 wt% MoS2 was found to produce heterojunction with optimal activity, increased surface area (15.8 m2/g for MIL-88A(Fe) to 78.2 m2/g for MSMIL20) and abundant number of active sites. Insitu formation of intimate heterojunction, coupled with hierarchical porosity, conductive network, and abundant exposed active sites results efficient charge carrier migration to the catalyst surface and accelerates PMS activation to yield highly active radicals. X-ray photoelectron spectroscopy indicates that surface bound Fe3+/Fe2+ and Mo4+/Mo6+ redox interconversions play significant roles in surface adsorbed PMS activation. Scavenging experiments established that both radicalOH·,SO4·-,O2·-and non-radical O21,h+ based degradation pathways were responsible for BPA degradation. The excellent photocatalytic activity was due to the formation of suitable heterojunction which resists e-/h+ recombination, along with formation of an efficient Z-scheme heterojunction. LCMS/MS analysis was employed to determine the intermittent intermediates during BPA degradation and plausible pathways were designed. The apparent degradation rate constant of MSMIL(20)/PMS system was 0.083 min−1 which was 16.6 times and 25.4 times that of PMS alone and MSMIL(20) alone, in presence of UV irradiation. This work elicits critical insights towards synthesizing highly efficient and stable MOF based heterojunction photocatalyst and corresponding mechanistic elucidation of the catalytic activity towards degradative removal of refractory contaminants.

Cobalt oxide/copper bismuth oxide/samarium vanadate (Co3O4/CuBi2O4/SmVO4) dual Z-scheme heterostructured photocatalyst with high charge-transfer efficiency: Enhanced carbamazepine degradation under visible light irradiation

Herein, a dual Z-scheme heterojunction photocatalyst consisting of Co3O4, CuBi2O4, and SmVO4 for carbamazepine (CBZ) degradation was synthesised and characterised by XRD, FTIR, UV–Vis DRS, XPS, FE-SEM, and TEM. The reduction in electron-hole recombination was evaluated by PL, LSV, and EIS analysis. The heterojunction, Co3O4/CuBi2O4/SmVO4 (CCBSV) showed enhanced photocatalytic activity of 76.1% ± 3.81 CBZ degradation under visible light irradiation, ascribed to the improved interfacial contact, visible light capturing ability, and enhanced electron-hole separation and transportation through the formation of Z-scheme heterojunction. The formation of dual Z-scheme was confirmed by active radical experiments and XPS analysis that helped to prose the mechanism of degradation. The catalyst showed sustained stability after 4 cycles of reuse. Ultra-high-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UHPLC-QTOF-MS) was employed to identify the degradation by-products of CBZ, and a possible mechanistic degradation pathway was proposed. This study provided an insight into the development of efficient dual Z-scheme heterojunction photocatalyst for remediation of CBZ which can be extended to other organic pollutants.

Visible-light-driven Z-scheme Zn3In2S6/AgBr photocatalyst for boosting simultaneous Cr (VI) reduction and metronidazole oxidation: Kinetics, degradation pathways and mechanism

It is urgently needed to develop high-performance materials that can synchronously remove heavy metals and organic pollutants. Herein, the visible-light responsive Zn3In2S6/AgBr composites were prepared for concurrent removals of metronidazole (MNZ) and Cr (VI). In the Cr (VI)-MNZ coexisting system, the removals of MNZ and Cr (VI) using the optimized Zn3In2S6/AgBr-15 photocatalyst reached 98.2% and 94.8% within 2 h, respectively; higher than those using counterparts. The radical species trapping and electron spin resonance (ESR) results demonstrated that ·OH was the most dominated species for MNZ oxidation, and photo-generated electrons were responsible for Cr (VI) reduction. Besides, slight competition for ·O2ˉ during the simultaneous MNZ degradation and Cr (VI) reduction occurred. Energy band structure analysis, ESR and the outstanding photocatalytic performance for MNZ and Cr (VI) removals demonstrated that the Zn3In2S6/AgBr-15 was a Z-scheme photocatalyst, which promoted photo-induced carrier′s separation. Possible MNZ degradation pathways and mechanism over the Z-scheme Zn3In2S6/AgBr were also proposed based on the identified intermediates. This study could inspire new ideas for design of efficient Z-scheme photocatalysts for wastewater treatment.

Constructing Heterogeneous Direct Z-Scheme Photocatalysts Based on Metal-Organic Cages and Graphitic-C3N4 for High-Efficiency Photocatalytic Water Splitting.

The development of artificial devices that mimic the highly efficient and ingenious photosystems in nature is worthy of in-depth study. A metal-organic cage (MOC) Pd2(M-4)4(BF4)4, denoted as MOC-Q1, integrating four organic photosensitized ligands M-4 and two Pd2+ catalytic centers is designed for a photochemical molecular device (PMD). MOC-Q1 is successfully immobilized on graphitic carbon nitride (g-C3N4) by hydrogen bonds to obtain a robust heterogeneous direct Z-scheme g-C3N4/MOC-Q1 photocatalyst for H2 generation under visible light. The optimized g-C3N4/MOC-Q1 (2 wt %) system shows high hydrogen evolution activity (4495 μmol g-1 h-1 based on the catalyst mass) and exhibits stable performances for 25 h (a turnover number of 19,268 based on MOC-Q1), significantly outperforming pure MOC-Q1, g-C3N4, and comparsion materials Pd/g-C3N4/M-4, which is the highest one of all reported heterogeneous MOC-based photocatalysts under visible irradiation. This enhancement can be ascribed to the synergistic effects of high-efficient electron transfer, extended visible-light response region, and good protective environment for MOC-Q1 arising from an efficient direct Z-scheme heterostructure of g-C3N4/MOC-Q1. This rationally designed and synthesized MOC/g-C3N4-based heterogeneous PMD is expected to have great potential in photocatalytic water splitting.

Construction of direct Z-scheme photocatalyst by the interfacial interaction of WO3 and SiC to enhance the redox activity of electrons and holes

The direct Z-scheme heterojunction structure benefits separation and migration of photoinduced carriers while maintaining original redox ability of each component. Nowadays, most Z-scheme structures are fabricated by g-C3N4 with other narrow band photocatalysts due to its low conduction band (CB). In this paper, SiC, another kind of photoelectric semiconductor with low CB, was employed to prepare direct Z-scheme photocatalyst with 2D WO3 by simple water oxidation precipitation method. The component and interface band structure of Z-scheme heterojunction WO3/SiC (WS) were verified by XPS, KPFM, Mott-Schottky method. The photodegradation efficiency and rate constant values of WS-1 for degrading RhB enhanced 2.5 and 5.3 times respectively compared with pristine WO3. Radical capture experiments and ESR tests affirmed that WS-1 photocatalyst produced •OH and •O2－active species, which further confirmed the photogenerated carriers were transmitted through the Z-scheme mode in principle. Band structure investigation showed that the direct Z-scheme structure assembled by WO3 with high valence band (VB) and SiC with low CB could maintain the high photocatalytic activity of active species. Therefore, this study offers a feasible method for construction of a novel and efficient direct Z-scheme photocatalyst.

An efficient Z-scheme CdS/g-C3N4 nano catalyst in methyl orange photodegradation: Focus on the scavenging agent and mechanism

The work focused on estimating the band gap energy (Eg) for various electronic transitions in the CdS/g-C3N4 coupled sample and the potential positions of the valence and conduction bands. Based on the data obtained and the effects of the scavenging agents, suitable energy diagrams were constructed for illustrating the photodegradation mechanism pathway. The coupled catalyst showed a red shift in Eg-value concerning the g-C3N4 alone (Eg (eV): CdS: 1.88, g-C3N4: 2.83, CdS/g-C3N4: 2.17). The pHpzc values of 5.1, 6.2, and 5.4 were estimated for CdS, g-C3N4, and CdS/g-C3N4, respectively. The highest methyl orange MO degradation was achieved at acidic pH 3–5. In this pH range, the zwitterion MO species is the predominant MO species in media which the catalyst can adsorb via hydrogen bonding by the MO sulfonic group. The photogenerated e- ≈ photogenerated h+ > superoxide radicals > hydroxyl radicals trend was obtained for the importance of the reactive species in MO photodegradation. In the proposed system, MO degradation can begin by both reduction and oxidation processes, and both are more efficient when the direct Z-scheme mechanism is considered. >85% of MO molecules can be degraded at conditions of catalyst dose: 0.5 g/L, pH: 3, CMO: 2 ppm, and irradiation time: 90 min. No considerable hydroxyl radicals can be produced by the system based on the potential positions of the species (E (eV): CdS-VB: 1.93, g-C3N4-VB: 1.63, OH /OH–: 1.9). The direct Z-scheme pathway can prove the H2-production and O2-evolution by the proposed system.

LaNiO3/g-C3N4 nanocomposite: An efficient Z-scheme photocatalyst for wastewater treatment using direct sunlight

The major findings in this report are (i) development of nanocomposite photocatalyst working through Z-scheme charge transfer pathway across the heterojunction, (ii) utilization of direct sunlight as the photo-source, and (iii) prospect of ligand-hole in photocatalysis through enhanced sub-band gap absorption. The photocatalysts, namely LaNiO3, g-C3N4 and LaNiO3/g-C3N4 nanocomposites were synthesized via facile route and were characterized for their structure, morphology, microstructure, texture, elemental mapping and surface oxidation states by using several physicochemical techniques. The photocatalytic performance of the nanocomposite was tested through the degradation of hazardous azo dye pollutants, namely reactive black 5 and methylene blue as well as the colorless antibiotic-pollutant tetracycline hydrochloride in aqueous solution in presence of natural sunlight with excellent recycling activity. The 10%LaNiO3/g-C3N4 nanocomposite sample shows the best catalytic activity, degrading respectively 94%, 98.6% and 88.1% of reactive black 5, methylene blue and tetracycline hydrochloride in 60, 180 and 120 min. The photocatalytic activity of the nanocomposite phase is several times superior to that of the pure phases. The improvements of photocatalytic activity of g-C3N4 in the nanocomposite have been rationalized through the construction of direct Z-scheme heterojunction and suppression of electron–hole pair recombination efficiency. The enhanced photo-absorption of the nanocomposite can possibly be related to sub-bandgap absorption, which is associated to the midgap state originating from ligand-hole formation or defects in the structure. The photodegradation process is mediated through the formation of super oxide radical (˙O2¯) and hole (h+) as the main responsible species.

A novel vacancy-strengthened Z-scheme g-C3N4/Bp/MoS2 composite for super-efficient visible-light photocatalytic degradation of ciprofloxacin

An efficient ternary Z-scheme visible-light-driven photocatalyst g-C3N4/Bp/MoS2 (CBM) was fabricated by a hydrothermal-calcination method. The morphology of modified g-C3N4 was controlled by addition trace amount of black phosphorus (Bp) and different content of MoS2. The results showed that optimal sample (CBM2) obtained the hollow porous structure that could dramatically inhibit the agglomeration and increase the specific surface area. Especially, the reasonable ternary Z-scheme structure could not only strengthen the visible-light harvest capacity, but also accelerate the photogenerated carriers separation and migration. In addition, the introduction of N vacancies via preparation could further inhibit the recombination of photoinduced electron-hole pairs. CBM possessed the super-high degradation efficiency (over 99%) of CIP within 60 min under visible-light irradiation. The apparent rate constant of the optimal sample increased by a factor of 6.0, 3.8 and 4.0 in comparison to that of CN, CN/Bp and CN/MoS2, respectively. In particular, the experiments of radical scavenging and electron spin resonance signal trapping revealed that photoexcited holes and superoxide radical were identified as the dominant reactive species in the reaction systems. The main intermediates were determined by LC-MS/MS and the transfer mechanism of photogenerated carriers on Z-scheme structure was further proposed.

Highly efficient photocatalytic hydrogen evolution and simultaneous formaldehyde degradation over Z-scheme ZnIn2S4-NiO/BiVO4 hierarchical heterojunction under visible light irradiation

In this work, we successfully constructed a highly efficient Z-scheme ZnIn2S4-NiO/BiVO4 photocatalytic system and reported a breakthrough study that has uncovered the photocatalytic reaction mechanism and pathway of hydrogen (H2) evolution and simultaneous formaldehyde (HCHO) degradation over the as-prepared Z-scheme photocatalytic system. By exploring the effects of oxygen (O2) and pH on the photocatalytic activity of the Z-scheme photocatalytic system, it was found that O2 and alkalinity are necessary for photocatalytic HCHO degradation and H2 production. O2 can boost the oxidation conversion of HCHO into formic acid (HCOOH) and restrain the HCHO polymerization reaction, therefore accelerate the formation of H2 in the reaction system. With the increase of pH value from 4 to 13, the reducibility of HCHO and the electronegativity of catalyst increase, which makes the activity of catalyst system increase. The results provide a new route for the optimization of reaction conditions for simultaneous photocatalytic H2 production and HCHO degradation. Under the optimal conditions (80% of O2, 13 of pH and 1.5 mol·L−1 of HCHO initial concentration), the best H2 production activity and HCHO degradation activity are 2724.89 μmol·h−1 and 17.00 mmol·h−1, respectively. On this basis, the possible pathway and mechanism of photocatalytic H2 production and simultaneous HCHO degradation were proposed, which shed lights on elucidating the reaction path and underlying mechanism for photocatalytic H2 production and pollutant degradation.

Rational design of a novel LaFeO3/g-C3N4/BiFeO3 double Z-scheme structure: Photocatalytic performance for antibiotic degradation and mechanistic insight

A novel ternary symmetric double Z-scheme LaFeO3/g-C3N4/BiFeO3 (LCB) heterojunction nanocomposite was rationally obtained using a wet chemical process. The ternary system was confirmed by structural analysis and surface and morphological studies. This double Z-scheme photocatalyst showed an outstanding photocatalytic performance for ciprofloxacin (CIP) degradation compared to g-C3N4, LaFeO3, BiFeO3, and their binary nanocomposites. The enhanced photocatalytic activities were primarily derived from the improved light absorption capability, effective spatial separation, and prolonged charge carriers lifetime in the double Z-scheme system. In particular, the scavenger tests and electron spin resonance spectra demonstrated that •O2− and •OH are the primary oxidative radical species in the photocatalytic systems and confirmed the formation of the double Z-scheme structure. The possible degradation pathways of CIP were proposed based on ultra-performance liquid chromatography tandem-mass spectrometry analysis. This study may open a new insight into the design and synthesis of highly efficient double Z-scheme photocatalyst for environmental decontamination.

Highly efficient photosynthesis of hydrogen peroxide in ambient conditions

Photosynthesis of hydrogen peroxide (H2O2) in ambient conditions remains neither cost effective nor environmentally friendly enough because of the rapid charge recombination. Here, a photocatalytic rate of as high as 114 μmol⋅g-1⋅h-1 for the production of H2O2 in pure water and open air is achieved by using a Z-scheme heterojunction, which outperforms almost all reported photocatalysts under the same conditions. An extensive study at the atomic level demonstrates that Z-scheme electron transfer is realized by improving the photoresponse of the oxidation semiconductor under visible light, when the difference between the Fermi levels of the two constituent semiconductors is not sufficiently large. Moreover, it is verified that a type II electron transfer pathway can be converted to the desired Z-scheme pathway by tuning the excitation wavelengths. This study demonstrates a feasible strategy for developing efficient Z-scheme photocatalysts by regulating photoresponses.