O2 Evolution Rate Research Articles

New Modified Sol–Gel Method for the Preparation KNb3O8 as a Hydrogen Evolution Photocatalyst in Z-Scheme Overall Water Splitting

The layered potassium niobate, KNb3O8, is evident as a photocatalyst for hydrogen production from water splitting under UV light. We hereby reported a new modified sol–gel method for the preparation of potassium niobate photocatalyst. Photocatalytic water splitting has been improved in this work by loading Pt nanoparticles as cocatalyst. The catalysts were characterized by powder X-ray diffraction (XRD), Ultraviolet–Visible spectroscopy (UV–Vis), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Their photocatalytic activities were in (i) an aqueous MeOH, used as sacrificial hole scavengers for hydrogen production and (ii) a new Z-scheme photocatalytic water splitting system where Pt/KNb3O8 was used for H2 evolution and Pt/WO3 was used for O2 evolution. The I−/IO3− aqueous solution was used as redox couple for the photocatalytic Z-scheme over all water splitting under UV light irradiation. The photocatalysts were prepared by loading platinum on sol–gel synthesis KNb3O8 and commercial WO3 via impregnation method. We find that, this KNb3O8 belonging to a orthorhombic structure with rod-like morphology. The Pt nanoparticles were sufficiently well-dispersed on the surface of photocatalyst, and enhanced photocatalytic properties. Our results show that I− concentration significantly influenced the photocatalytic activity. The combination of Pt/KNb3O8 with Pt/WO3 achieves a high H2 evolution rate (539 μmol g−1 h−1) and O2 evolution rate (140 μmol g−1 h−1) in 2 mM NaI solution.

A Commonly Used Photosynthetic Inhibitor Fails to Block Electron Flow to Photosystem I in Intact Systems.

In plant science, 2,4-dinitrophenylether of iodonitrothymol (DNP-INT) is frequently used as an alternative to 2,5-dibromo-6-isopropyl-3-methyl-1,4-benzoquinone (DBMIB) to examine the capacity of plastoquinol and semiquinone to reduce O2. DNP-INT is considered to be an effective inhibitor of the photosynthetic electron transfer chain (PETC) through its binding at the Q0 site of Cyt-b6f. The binding and action of DNP-INT has been previously characterized spectroscopically in purified Cyt-b6f complex reconstituted with Plastocyanin, PSII membranes and plastoquinone, as well as in isolated thylakoids based on its property to block MV-mediated O2 consumption. Contrary to the conclusions obtained from these experiments, we observed clear reduction of P700+ in samples incubated with DNP-INT during our recent investigation into the sites of oxygen consumption in isolated thylakoids. Therefore, we carried out an extensive investigation of DNP-INT’s chemical efficacy in isolated thylakoids and intact leaves. This included examination of its capacity to block the PETC before PSI, and therefore its inhibition of CO2 fixation. P700 redox kinetics were measured using Dual-PAM whilst Membrane Inlet Mass Spectrometry (MIMS) was used for simultaneous determination of the rates of O2 evolution and O2 consumption in isolated thylakoids and CO2 fixation in intact leaves, using two stable isotopes of oxygen (16O2, 18O2) and CO2 (12C, 13C), respectively. Based on these investigations we confirmed that DNP-INT is unable to completely block the PETC and CO2 fixation, therefore its use may produce artifacts if applied to isolated thylakoids or intact cells, especially when determining the locations of reactive oxygen species formation in the photosynthetic apparatus.

In Situ Photodeposited Construction of Pt–CdS/g-C3N4–MnOx Composite Photocatalyst for Efficient Visible-Light-Driven Overall Water Splitting

For converting the renewable solar energy to hydrogen (H2) energy by photocatalytic (PC) overall water splitting (OWS), visible-light-driven photocatalysts are especially desired. Herein, a model CdS/g-C3N4 photocatalyst with a type II heterojunction is first demonstrated via a facile coupling of g-C3N4 nanosheets and CdS nanorods. After being combined with in situ photodeposited 3 wt % Pt and 4 wt % MnOx dual cocatalysts simultaneously, the optimal visible-light-driven (λ > 400 nm) composite photocatalyst of Pt-CdS/g-C3N4-MnOx gives a H2 generation rate of 9.244 μmol h-1 (924.4 μmol h-1 g-1) and a O2 evolution rate of 4.6 μmol h-1 (460 μmol h-1 g-1) in pure water, which is over 420 times higher than that of pure CdS nanorods loaded with 0.5 wt % Pt. The apparent quantum efficiency (AQE) reaches about 3.389% (at 400 nm) and 1.745% (at 420 nm), respectively. The combination of a type II heterojunction and simultaneous in situ photodeposition of the dual cocatalysts results in a dramatically improved PC efficiency and a long-term stability of the CdS/g-C3N4 visible-light-driven photocatalyst for OWS.

Easy Synthesis of BiVO4 for Photocatalytic Overall Water Splitting.

Developing a photocatalyst system to generate hydrogen from water is a topic of great interest for fundamental and practical importance. In this study, we develop a new Z-scheme photocatalytic system for overall water splitting that consists of Rh/K4Nb6O8 for H2 evolution, Pt/BiVO4 for O2 evolution, and I–/IO3– for an electron mediator under UV light irradiation. The oxygen evolution photocatalyst BiVO4 was prepared by the microwave-assisted hydrothermal method. The method is fast and simple, as compared to conventional hydrothermal synthesis. The catalysts were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and UV–visible spectroscopy. The photocatalytic water splitting is investigated in (i) aqueous AgNO3 as sacrificial electron scavengers and (ii) a Z-scheme photocatalytic water splitting system. The BiVO4 photocatalysts prepared by the microwave-assisted hydrothermal method not only showed a very high oxygen evolution rate (2622 μmol g–1 h–1) of water splitting reaction in an aqueous AgNO3 solution but also achieved a high H2 evolution rate (340 μmol g–1 h–1) and O2 evolution rate (172 μmol g–1 h–1) in a Z-scheme overall water splitting system.

Rotifers release a lipid-soluble agent that inhibits photosynthetic electron transport in Chlorella sp

Rotifer contamination, as a severe constraint in mass cultures, impedes industrial-scale cultivation of microalgae. Once contamination occurred, the rotifers released something that inhibited microalgal cell growth in addition to algal reduction due to predation. With the aim of further identifying the type and effects of the inhibitor released from rotifers, Chlorella sp. was cultured in fresh modified F/2 medium and in media containing various proportions of boiled rotifer culture-medium filtrate (RCF), as well as in media containing water-soluble or lipid-soluble fractions isolated from RCF. Then, the growth and photosynthesis of Chlorella sp. were measured. The rotifer inhibition of Chlorella growth could not be eliminated by boiling the RCF, indicating that the inhibitor was not likely a protein. The growth and photosynthesis of Chlorella sp. was not inhibited by the water-soluble RCF fraction, but they were significantly inhibited by the lipid-soluble RCF fraction, in a dose-dependent manner. Further, the lipid-soluble fraction decreased energy conservation and photosynthetic electron transport, which induced a severe decrease in PSII activity and a decrease in the net photosynthetic O2 evolution rate. Based on these physiological responses of Chlorella cells, the lipid-soluble fraction rather than the protein or water-soluble fractions was determined to contain the responsible inhibitor, suggesting the direction of further studies.

Strong-Base-Assisted Synthesis of a Crystalline Covalent Triazine Framework with High Hydrophilicity via Benzylamine Monomer for Photocatalytic Water Splitting.

Methods to synthesize crystalline covalent triazine frameworks (CTFs) are limited and little attention has been paid to development of hydrophilic CTFs and photocatalytic overall water splitting. A route to synthesize crystalline and hydrophilic CTF-HUST-A1 with a benzylamine-functionalized monomer is presented. The base reagent used plays an important role in the enhancement of crystallinity and hydrophilicity. CTF-HUST-A1 exhibits good crystallinity, excellent hydrophilicity, and excellent photocatalytic activity in sacrificial photocatalytic hydrogen evolution (hydrogen evolution rate up to 9200 μmol g-1 h-1 ). Photocatalytic overall water splitting is achieved by depositing dual co-catalysts in CTF-HUST-A1, with H2 evolution and O2 evolution rates of 25.4 μmol g-1 h-1 and 12.9 μmol g-1 h-1 in pure water without using sacrificial agent.

Strong‐Base‐Assisted Synthesis of a Crystalline Covalent Triazine Framework with High Hydrophilicity via Benzylamine Monomer for Photocatalytic Water Splitting

AbstractMethods to synthesize crystalline covalent triazine frameworks (CTFs) are limited and little attention has been paid to development of hydrophilic CTFs and photocatalytic overall water splitting. A route to synthesize crystalline and hydrophilic CTF‐HUST‐A1 with a benzylamine‐functionalized monomer is presented. The base reagent used plays an important role in the enhancement of crystallinity and hydrophilicity. CTF‐HUST‐A1 exhibits good crystallinity, excellent hydrophilicity, and excellent photocatalytic activity in sacrificial photocatalytic hydrogen evolution (hydrogen evolution rate up to 9200 μmol g−1 h−1). Photocatalytic overall water splitting is achieved by depositing dual co‐catalysts in CTF‐HUST‐A1, with H2 evolution and O2 evolution rates of 25.4 μmol g−1 h−1 and 12.9 μmol g−1 h−1 in pure water without using sacrificial agent.

Transcriptomic analysis of hydrogen photoproduction in Chlorella pyrenoidosa under nitrogen deprivation

H2 photoproduction by green algae is a very attractive way to produce clean and renewable energy. In our previous study, Chlorella pyrenoidosa strain IOAC707S was found to produce hydrogen under nitrogen deprivation in seawater medium; however the mechanism is not entirely clear. In this work, it was found that under nitrogen deprivation, the efficiency of photosystem II photochemical activity and the photosynthetic O2 evolution rate of C. pyrenoidosa strain IOAC707S decreased, while the respiratory rate increased, which were favorable for the establishment of anoxia and H2 photoproduction. Transcriptomic analysis indicated that photosynthetic activity did not simply decline but underwent a complex adjustment to prepare for H2 photoproduction. The activities of glycolysis and the pentose phosphate pathway were up-regulated, suggesting more electron transport from endogenous substrate to H2 photoproduction. The decrease of the tricarboxylic acid cycle activity resulted in decreased levels of CO2, which would limit Calvin cycle activity, and subsequently lead to the accumulation of NADPH and a reduction of the photosynthetic electron transport chain. Under these conditions, the induced hydrogenase served as an electron valve, and H2 was produced.

Immobilization of Metal-Organic Framework MIL-100(Fe) on the Surface of BiVO4: A New Platform for Enhanced Visible-Light-Driven Water Oxidation.

The development of new dual functional photocatalysts is highly desirable for conversion and storage of solar energy. Herein, we first constructed hierarchical structure MIL-100(Fe)@BiVO4 in situ growing MIL-100(Fe) nanoparticles (NPs) on the surface of decahedron BiVO4 under mild hydrothermal conditions. The as-synthesized hybrid nanostructure is unambiguously determined using a series of characterization methods. These results demonstrate that the ultra-tiny MOF MIL-100(Fe) particles are immobilized on the surface of decahedron BiVO4 and the composite exhibits a strong interaction between BiVO4 and MIL-100(Fe). This hybrid material MIL-100(Fe)@BiVO4 is employed as a photocatalyst for water oxidation reaction and demonstrates higher O2 production activity in comparison with bare BiVO4. The best performance obtained at the optimal mass percentage of MIL-100(Fe) (8.0 wt %) reaches 333.3 μmol h-1 g-1 of the O2 evolution rate irradiated with visible light, which is almost 4.3 times higher than bare BiVO4 (77.3 μmol h-1 g-1). The enhanced water oxidation performance is due to the more efficient interfacial electron-hole transfer between MIL-100(Fe) and BiVO4, which is verified by the results of various photo-electrochemical characterizations. Moreover, the as-prepared composite MIL-100(Fe)@BiVO4 also displays excellent stability for visible-light-driven water oxidation. This study affords a rational strategy for the controllable construction by loading metal-organic frameworks on a semiconductor surface, which is a good reference for other artificial photosystems.

On the Enhanced Reducibility and Charge Transport Properties of Phosphorus-Doped BiVO4 as Photocatalysts: A Computational Study

Phosphorus (P)-doped BiVO4 has been proposed as a promising photoanode for water splitting as it exhibits significant improvement of photocurrent density and photocatalytic O2 evolution rate. Previ...

Catalytic water oxidation by a single site [Ru(Fc-tpy)(bpy)OH2]2+ complex and it’s mechanistic study

The mononuclear complex [Ru(Fc-tpy)(bpy)Cl]PF6 (Ru-Cl) and its corresponding aqua [Ru(Fc-tpy)(bpy)OH2](PF6)2 (Ru-OH2) complex (Fc-tpy = 4′-(2-ferrocenyl)-2,2′:6′2″-terpyridine) have been synthesized and characterized by 1H NMR, UV–Vis spectroscopy, Mass Spectrometry and the structure of the complex (Ru-Cl) was confirmed by Single Crystal X-Ray Crystallographic studies. Under strong acidic conditions the Fe(II) centre of ferrocene moiety (Fc) undergoes protonation to generate Fe(IV)hydrido species Fe(IV)H (FcH+). The Ru-OH2 complex shows one pKa value at 10.0 due to deprotonation of the aqua ligand. The resting potential of Ru-OH2 complex in pH 1.62 phosphate buffer indicates that the Fe center in ferrocene is in FeIII state, and the Ru center is in RuIII state. This complex acts as an efficient water oxidation catalyst at pH = 1.0 in triflic acid using ceric ammonium nitrate (CAN) as a sacrificial electron acceptor. The complex exhibits a turnover number (TON) of 12, whereas parent [Ru(tpy)(bpy)(OH2)]2+ complex exhibits a TON of 10 under the similar conditions. The presence of ferrocenyl group coupled with terpyridine has the potential to act as an electron donor, and can stabilize the higher oxidation state of ruthenium complex required towards chemical water oxidation. The rate of evolution of O2Ru-OH2 is much higher as compared to parent [Ru(tpy)(bpy)(OH2)]2+ complex with 230 mV of lower onset potential. The rate of O2 evolution is 1st order with respect to the catalyst as well as oxidant (CAN) concentration. The species distribution path involves [Fc+-RuII-OH2]3+, [Fc+-RuV=O]4+, [Fc+-RuVI=O]5+, [Fc+-RuIV-O-OH]4+, [Fc+-RuV-O-O]4+, [Fc+-RuIII-OH2]4+and [Fc+-RuIII-OH]3+. The rate of O⋯O bond formation (kO⋯O), interaction of [RuIV-O-OH]2+ with Ce4+ and O2 evolution (kO2) indicates that the interaction step is the rate-determining step, where k = 6.0 × 10−4 M−1 s−1.

Algal density mediates the photosynthetic responses of a marine macroalga Ulva conglobata (Chlorophyta) to temperature and pH changes

Growing of macroalgae increases their biomass densities in natural habitats. To explore how the altered algal density impacts their photosynthetic responses to changes of environmental factors, we compared the photosynthesis versus irradiance characteristics of a marine green macroalga Ulva conglobata under low [2.0 g fresh weight (FW) L−1], medium (6.0 g FW L−1) and high biomass densities (12.0 g FW L−1), and under a matrix of temperatures (20, 25, 30 and 35 °C) and pH levels (7.8, 8.2 and 8.6). Increased algal densities decreased the photosynthetic O2 evolution rate among all combined temperature and pH treatments, in parallel with the decrease of light-utilizing efficiency (α, the initial slope) and maximum photosynthetic rate (Pmax) and the increase of light saturation point (EK). Rising temperature interacted with lowered pH to increase the α under low but not under high algal densities. Rising temperature increased the Pmax and decreased the EK under low algal density, but not under high density. Lowered pH promoted the Pmax and EK under all three algal densities. The increased temperature enhanced the dark respiration (Rd) and light compensation point (EC), while the altered pH showed a limited effect. Moreover, the increased algal density reduced the Rd, and had a limited effect on the EC. In addition, our results indicate that changing algal densities caused the complex photophysiological changes in responses to the temperature and pH changes, and these complex responses resolved into a close relation between Rd and Pmax across the matrix of temperatures and pH levels.

The Development of Crassulacean Acid Metabolism (CAM) Photosynthesis in Cotyledons of the C4 Species, Portulaca grandiflora (Portulacaceae).

Portulaca grandiflora simultaneously utilizes both the C4 and Crassulacean acid metabolism (CAM) photosynthetic pathways. Our goal was to determine whether CAM developed and was functional simultaneously with the C4 pathway in cotyledons of P. grandiflora. We studied during development whether CAM would be induced with water stress by monitoring the enzyme activity, leaf structure, JO2 (rate of O2 evolution calculated by fluorescence analysis), and the changes in titratable acidity of 10 and 25 days old cotyledons. In the 10 days old cotyledons, C4 and CAM anatomy were evident within the leaf tissue. The cotyledons showed high titratable acid levels but a small CAM induction. In the 25 days old cotyledons, there was a significant acid fluctuation under 7 days of water stress. The overall enzyme activity was reduced in the 10 days old plants, while in the 25 days old plants CAM activity increased under water-stressed conditions. In addition to CAM, the research showed the presence of glycine decarboxylase in the CAM tissue. Thus, it appears both pathways develop simultaneously in the cotyledons but the CAM pathway, due to anatomical constraints, may be slower to develop than the C4 pathway. Cotyledons showed the ancestral Atriplicoid leaf anatomy, which leads to the question: Could a CAM cell be the precursor to the C4 pathway? Further study of this may lead to understanding into the evolution of C4 photosynthesis in the Portulaca.

Thickness-dependent carrier separation in Bi2Fe4O9 nanoplates with enhanced photocatalytic water oxidation

Herein, Bi2Fe4O9 nanoplates (BFO NPs) with controllable thickness are successfully synthesized via a facile hydrothermal method with varied content of sodium dodecyl benzene sulfonate (SDBS). The thickness along the [001] direction of the BFO NPs can be controlled by varying the SDBS content, and the decreased thickness plays multiple roles in improving the photocatalytic water oxidation performance: (1) increasing the distortion and asymmetries of the FeO4 tetrahedral and FeO6 octahedral units, and enhancing the built-in electric field in the BFO NPs accordingly, to facilitate the efficient separation of photogenerated electron-hole pairs; (2) reducing the migration distance of holes to the surface of the both oxygen- and bismuth-enriched {001} facets; (3) promoting the exposure of {001} facets of the BFO NPs which facilitates hole accumulation and provides more active sites to accelerate surface oxidation reaction. As a result, the as-prepared BFO-2 sample with optimized thickness exhibits the best photocatalytic water oxidation performance under visible-light irradiation, achieving the O2 evolution rate of 461.08 µmol g−1 h−1, about 7.87 times higher than BFO NPs prepared without using SDBS. The built-in electric field for BFO and its role in photocatalytic water oxidation process are reported for the first time. This work provides new insights into improving intrinsic activity of individual semiconductor photocatalyst by morphology and structure modulation and charge behavior regulation.

Cubic Co-Co prussian blue MOF-based transition metal phosphide as an efficient catalyst for visible light-driven water oxidation

Designing and developing an efficient, economic and stable catalyst for photocatalytic water oxidation is a forefront topic of sustainable energy research. Herein, we rationally designed a CoP/NC (CoP nanoparticles embedded in nitrogen and carbon matrices) nanocube catalysts via a pyrolysis phosphidation strategy derived from Co-Co/PB (Co-Co prussian blue) MOF-based precursor, which was employed as a robust heterogeneous catalyst for photocatalytic water oxidation for the first time. The TEM and SEM tests exhibit that Co-Co/PB contributes to well-defined nanocube architecture features after phosphidation. Furthermore, the photocatalytic performance of CoP/NC nanocube catalyst was examined in photocatalytic water oxidation system using [Ru(bpy)3]2+ as photosensitizer and S2O82− as sacrificial electron acceptor. The result shows CoP/NC has a modest high O2 evolution rate of 901.5 mmol h−1 g−1, O2 yield of 36.1%, TOF of 34.5 × 10−3 molO2 molmetal−1 s−1, and quantum yield of 61.5%. Noteworthily, an amorphous CoOx layer on CoP/NC surface was observed by HRTEM test after light reaction. The amorphous CoOx layer and CoP form a core-shell structure (CoP@CoOx), which facilitates the electron transport effectively. Additionally, the CoP/NC nanocube catalyst shows good electrocatalytic oxygen evolution performance, which has a higher current density and smaller Tafel slope than CoS2/NC and Co3O4/NC derived from the same precursor of Co-Co/PB with CoP/NC. This strategy of using prussian blue analogue as template to create metal phosphide will provide a platform for exploring highly efficient and stable catalysts for energy conversion reactions.

Unraveling the Mechanism of Catalytic Water Oxidation via de Novo Synthesis of Reactive Intermediate.

Artificial photosynthesis could promise abundant, carbon-neutral energy, but implementation is currently limited by a lack of control over the multi-electron catalysis of water oxidation. Discoveries of the most active catalysts still rely heavily on serendipity. [Ru(tpy)(bpy)(H2O)]2+ (1; bpy = 2,2'-bipyridine, tpy = 2,2';6',2″-terpyridine) is representative of the largest known class of water oxidation catalysts. We undertook an extensive spectroscopic analysis of the prototypical 1 water oxidation catalyst and its fastest known analog [Ru(EtO-tpy)(bpy)(H2O)]2+ (2), capable of 10 times faster water oxidation, to investigate the mechanism of action and factors controlling catalytic activity. EPR and resonance Raman spectroscopy did not detect the proposed [RuV═O] intermediate in 1 and 2 but indicated the possible formation of N-oxides. A lag phase was observed prior to O2 evolution, suggesting catalyst modification before the onset of catalysis. The reactive intermediate [Ru(tpy)(bpy-NO)(H2O)]2+ (1-NO; bpy-NO = 2,2'-bipyridine-N-oxide) proposed by combined spectroscopic and DFT analysis was de novo synthesized and demonstrated 100-fold greater catalytic activity than 1. Thus, in situ transient formation of small amounts of the Ru complex with N-oxide ligands can significantly activate single-site Ru-based catalysts. Furthermore, the rate of O2 evolution was found to correlate with the redox potential of the ligand. This observation might assist with rational design of new catalysts.

Overall pure water splitting using one-dimensional P-doped twinned Zn0.5Cd0.5S1-x nanorods via synergetic combination of long-range ordered homojunctions and interstitial S vacancies with prolonged carrier lifetime

Efficient spatial charge separation has been a pivotal element to improve the performance of solar water splitting. Pseudobinary ZnxCd1-xS with nano-twin structures is deemed to be one of the best pristine sulfide photocatalysts for H2 evolution, attributed to the homojunctions between parallel zinc blende/wurtzite (ZB/WZ) segments which can promote effective charge separation. One step further is to introduce heteroatom doping into the electronic structure of semiconductor to form midgap impurity level that can serves as an electron trapping site. In this contribution, we report the fabrication of one-dimensional P-doped twinned Zn0.5Cd0.5S1-x nanorods (NRs) with long-range ordered homojunction incorporated with defect-induced S vacancies. The P-doped twinned Zn0.5Cd0.5S1-x NRs demonstrated visible-light-driven pure water splitting without any sacrificial reagent and co-catalyst loading, resulting in H2 and O2 evolution rates of 0.97 and 0.42 μmol h−1, respectively. The apparent quantum yield (AQY) of H2 production was measured to be 0.15% under monochromatic 420 nm.

Synthesis of bipyridine-based covalent organic frameworks for visible-light-driven photocatalytic water oxidation

Covalent organic frameworks (COFs) with band gap engineering characters are attractive organic semiconductors. Although several COFs are being utilized for photocatalytic H2 production and CO2 reduction, few of them can realize the challenging water oxidation under visible light. Herein, we present the visible-light-driven photocatalytic water oxidation on bipyridine-based COFs (Bp-COF), which was synthesized through the Schiff base condensation reaction. Bp-COF displays impressive visible-light-driven water oxidation activity with O2 evolution rate of 152 μmol g−1 h−1 after coordinating with Co2+. Furthermore, the Bp-COF could also enable photocatalytic H2 production under visible light. The unique photocatalytic performance of BpCo-COF for both water oxidation and proton reduction could be attributed to its excellent light harvesting property, appropriate energy band structure, high porosity and wettability as well as coordinated cobalt for photocatalytic water splitting. This work demonstrates the potential of COFs as semiconductors for photocatalytic solar fuels conversion.

Facile synthesis of 2D/2D Co3(PO4)2/g-C3N4 heterojunction for highly photocatalytic overall water splitting under visible light

The photocatalytic overall water splitting is an ideal pathway to generate hydrogen gas for sustainable energy production. Herein, the 2D/2D Co3(PO4)2/g-C3N4 heterojunctions were successfully synthesized for overall water splitting from pure water by a simple direct precipitation route under the effect of coulomb electrostatic interaction. The introduction of Co3(PO4)2 nanosheets broadens the light absorption range of Co3(PO4)2/g-C3N4 heterojunction, and in addition, the unusual 2D/2D heterostructure can provide more contact areas to promote the interfacial charge transfer between g-C3N4 nanosheets and Co3(PO4)2 nanosheets, which considerably enhances the photogenerated charge separation. Among the Co3(PO4)2/g-C3N4 heterojunctions, 35% Co3(PO4)2/g-C3N4 exhibits the optimal H2 and O2 evolution rates which are 375.6 and 177.4 μmol g−1 h−1, respectively. Moreover, the apparent quantum efficiency of the 35% Co3(PO4)2/g-C3N4 reaches up to 1.32% at 420 nm. Furthermore, the 2D/2D Co3(PO4)2/g-C3N4 composite possesses the prominent stability and recyclability, testifying a potential application for the conversion of the sustainable energy.

Metal-free broad-spectrum PTCDA/g-C3N4 Z-scheme photocatalysts for enhanced photocatalytic water oxidation

Exploiting highly-efficient, broad-spectrum responsive and noble-metal-free water oxidation photocatalyst is a major challenge in the field of photocatalysis. Herein, we tackle this challenge by synthesizing Z-scheme PTCDA/g-C3N4 photocatalysts, which can absorb light with wavelength short than 600 nm, for visible light photocatalytic O2 production from water. Owing to the evident conduction and valance band energy level difference between PTCDA and g-C3N4, the PTCDA/g-C3N4 photocatalysts exhibit efficient charge carrier separation through a Z-scheme mechanism, as demonstrated by the transient photocurrent responses and time-resolved fluorescence decay spectra analysis. Benefitting from the synergetic effect of the broad visible light absorption capacity and efficient charge separation, the optimized PTCDA/g-C3N4 photocatalyst shows the highest O2 evolution rate of 847 μmol·h−1·g−1 under visible light irradiation in the presence of Co3O4 as a cocatalyst and AgNO3 as the sacrificial reagent. An apparent quantum yield of 4.5% was achieved under monochromatic light irradiation at 420 nm.