Branched WO3 nanosheet array with layered C3 N4 heterojunctions and CoOx nanoparticles as a flexible photoanode for efficient photoelectrochemical water oxidation

Pd(II) coordination molecule modified g-C3N4 for boosting photocatalytic hydrogen production

The photocatalytic H2 production activity of polymer carbon nitride (g-C3N4) is limited by the rapid recombination of photoelectron-hole pairs and slow surface reduction dynamic process. Here, a supramolecular complex (named R-TAP-Pd(II)) was fabricated via self-assembly of (R)-N-(1-phenylethyl)-4-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)benzamide (R-TAP) with Pd(II) and used to modify g-C3N4. In the R-TAP-Pd(II)@g-C3N4 composite photocatalyst, the spin polarization of R-TAP-Pd(II) can promote charge transfer and inhibit photogenerated carrier recombination, as confirmed by spectral tests and photoelectrochemical performance tests. Electrochemical tests and in situ X-ray photoelectron spectroscopy (XPS) tests proved that the Pd(II) ion in the R-TAP-Pd(II) molecule can serve as active sites to accelerate H2 production. The R-TAP-Pd(II)@g-C3N4 presented a photocatalytic H2 generation rate of 1085 μmol g-1 h-1 when exposed to visible light, which was a about 278-fold increase compared with g-C3N4. This work finds a new approach to boost the photocatalytic efficiency of g-C3N4 via supramolecular self-assembly.

Synergistic effect of dual phase cocatalysts: MoC-Mo2C quantum dots anchored on g-C3N4 for high-stability photocatalytic hydrogen evolution

The extensive examination of hexagonal molybdenum carbide (β-Mo2C) as a non-noble cocatalyst in the realm of photocatalytic H2 evolution is predominantly motivated by its exceptional capacity to adsorb H+ ions akin to Pt and its advantageous conductivity characteristics. However, the H2 evolution rate of photocatalysts modified with β-Mo2C is limited as a result of their comparatively low ability to release H through desorption. Therefore, a facile method was employed to synthesize carbon intercalated dual phase molybdenum carbide (MC@C) quantum dots (ca. 3.13 nm) containing both α-MoC and β-Mo2C decorated on g-C3N4 (gCN). The synthesis process involved a simple and efficient combination of sonication-assisted self-assembly and calcination techniques. 3-MC@C/gCN exhibited the highest efficiency in generating H2, with a rate of 4078 µmol g-1h-1 under 4 h simulated sunlight irradiation, which is 13 times higher than pristine gCN. Furthermore, from the cycle test, 3-MC@C/gCN showcased exceptional photochemical stability of 65 h, as it maintained a H2 evolution rate of 40 mmol g-1h-1. The heightened level of activity observed in the 3-MC@C/gCN system can be ascribed to the synergistic effects of MoC-Mo2C that arise due to the existence of a carbon layer. The presence of a carbon layer enhanced the transmission of photoinduced electrons, while the MoC-Mo2C@C composite served as active sites, thereby facilitating the H2 production reaction of gCN. The present study introduces a potentially paradigm-shifting concept pertaining to the exploration of novel Mo-based cocatalysts with the aim of augmenting the efficacy of photocatalytic H2 production.

Photoelectrochemical Cells with a Pyridine-Anchored Bilayer Photoanode for Water Splitting

A dye-sensitized photoanode is prepared by coassembling a Ru complex photosensitizer and a Ru water oxidation catalyst (WOC) on a TiO2 substrate, in which the WOC molecules are immobilized in a layer-by-layer fashion through metal-pyridine coordination with the aid of a bifunctional anchoring and bridging molecule containing multiple pyridine groups. Under visible-light irradiation, an anodic photocurrent of around 200 μA/cm2 has been achieved with O2 and H2 being generated at the photoanode and Pt counter electrode, respectively. The pyridine anchoring strategy provides a simple method to prepare photoelectrodes for applications in photoelectrochemical cells.

Ni(OH)2 Nanosheet as an Efficient Cocatalyst for Improved Photocatalytic Hydrogen Evolution over Cd0.9Zn0.1S Nanorods under Visible Light

Loading cocatalysts to promote spatial charge separation has been confirmed as an effective method for improving photocatalytic hydrogen production. This article reports that the synthesis of Ni(OH)2/Cd0.9Zn0.1S nanorod photocatalyst is suitable for photocatalytic H2 generation under visible light. It can be proven that the binary photocatalyst exhibits a one-dimensional nanorod morphological structure. Ni(OH)2 nanosheets occupy the top area of Cd0.9Zn0.1S nanorods. The photocatalytic H2 production rate can reach 132.93 mmol·h-1·g-1, which corresponds to an apparent quantum efficiency of up to 76.5% at a wavelength of 460 nm. In addition, the Ni(OH)2 nanosheet can aggregate the light-incited electrons of Cd0.9Zn0.1S, inhibiting the confluence of electrons and holes. The detailed analysis of its mechanism through characterization methods such as photoluminescence and electrochemical measurement shows that the significant improvement in photocatalytic performance derives from the effective spatial separation of photo-induced charge carriers. Therefore, this synthesis strategy of one-dimensional materials may bring new prospects for more efficient, stable, and sustainable photocatalysis for water splitting.

Facile fabrication of ultra-light N-doped-rGO/g-C3N4 for broadband microwave absorption

"Thin thickness", "lightweight", "wide absorption bandwidth" and "strong absorption" are the new standards of contemporary science and technology for microwave absorption(MA) material. In this study, N-doped-rGO/g-C3N4 MA material was prepared for the first time by simple heat treatment, which the N atoms were doped into rGO and g-C3N4 was dispersed on the surface of N-doped-rGO, and its density is only 0.035 g/cm3. The impedance matching of the N-doped-rGO/g-C3N4 composite was well adjusted by decreasing the dielectric constant and attenuation constant due to the g-C3N4 semiconductor property and the graphite-like structure. Moreover, the distribution of g-C3N4 among N-doped-rGO sheets can produce more polarization effect and relaxation effect by increasing the lamellar spacing. Furthermore, the polarization loss of N-doped-rGO/g-C3N4 could be increased successfully by doping N atoms and g-C3N4. Ultimately, the MA property of N-doped-rGO/g-C3N4 composite was optimized significantly, with a loading of 5 wt%, the N-doped-rGO/g-C3N4 composite exhibited the RLmin of -49.59 dB and the effective absorption bandwidth could reach 4.56 GHz when the thickness was only 1.6 mm. The "thin thickness", "lightweight", "wide absorption bandwidth" and "strong absorption" of MA material are actually achieved by the N-doped-rGO/g-C3N4.

p-n heterojunction constructed by γ-Fe2O3 covering CuO with CuFe2O4 interface for visible-light-driven photoelectrochemical water oxidation

Fe₂O₃ is a promising n-type semiconductor as the photoanode of photoelectrochemical water-splitting method due to its abundance, low cost, environment-friendly, and high chemical stability. However, the recombination of photogenerated holes and electrons leads to low solar-to-hydrogen efficiency. In this work, to overcome the recombination issue, a p-type semiconductor, CuO, is introduced underneath the γ-Fe₂O₃ to synthesize γ-Fe₂O₃/CuO on the FTO substrate. Along with the formation of p-n heterojunction, CuFe₂O₄ is in situ generated at the interface of γ-Fe₂O₃ and CuO. The existence of Cu₂O in CuO and CuFe₂O₄ promotes the charge transfer from CuO to γ-Fe₂O₃ and within CuFe₂O₄, respectively, resulting in creating an internal electric field in γ-Fe₂O₃/CuO and leading to the conduction band of CuO bending up and γ-Fe₂O₃ bending down. Additionally, Cu(II) in CuFe₂O₄ contributes to fast electron capture. Consequently, the charge transfer efficiency and charge separation efficiency of photo-generated holes are promoted. Hence, γ-Fe₂O₃/CuO exhibits an enhanced photocurrent density of 13.40 mA cm^-2 (1.9 times higher than γ-Fe₂O₃). The photo corrosion resistance of CuO is dramatically increased with the protection of CuFe₂O₄, resulting in superior high chemical stability, i.e. 85% of the initial activity remains after a long-term test.

Effects of iron oxide contents on photocatalytic performance of nanocomposites based on g-C3N4

α-Fe₂O₃/Fe₃O₄/g-C₃N₄ nanocomposites were prepared in-situ by solution combustion as magnetically separable photocatalysts using ferric nitrate as oxidant, glycine as organic fuel, and g-C₃N₄. The effects of various amounts of iron oxides, on the magnetic, optical, and photocatalytic properties were explored by different characterization methods. The magnetite (Fe₃O₄) phase as ferrimagnetic material disappeared with the increase in ferric nitrate contents, leading to the decrease of magnetic properties. The bandgap energy decreased from 2.8 to 1.6 eV with the increase of the hematite (α-Fe₂O₃) phase.The photocatalytic results showed that the type and amount of iron oxides had a significant effect on the decolorization of methylene blue, rhodamine B and methyl orange dyes under visible-light irradiation. The activity of the nanocomposite sample containing 37 wt. % iron oxides was more effective than that of the pristine g-C₃N₄ sample to photodegrade the methylene blue, rhodamine B and methyl orange, respectively. Moreover, the nanocomposites exhibited a higher photocurrent density than that of the pristine g-C₃N₄, mainly due to their lower charge recombination rate.

Synergistic role of hydrogen treatment and heterojunction in H-WO3-x/TiO2-x NT/Ti foil-based photoanodes for photoelectrochemical wastewater detoxification and antibacterial activity

The process of photoelectrochemical wastewater detoxification is limited by significant charge recombination, which is difficult to suppress with efficient single-material photoanodes. We demonstrated the effectiveness of hydrogen treatment in evaluating charge separation properties in WO_3-x/TiO_2-x NT/Ti foil heterojunction photoanodes. The influence of varying hydrogen annealing (200-400 °C) on the structural and photoelectrochemical properties of WO₃/TiO₂ NS/NT heterojunction is studied systematically. Additionally, after hydrogen treatment of pristine WO₃/TiO₂ NT/Ti foil photoanodes, substoichiometric H-WO_3-x/TiO_2-x NT-300 achieved the 1.21 mA/cm² photocurrent density, which is 8.06 and 3.27 times than TiO₂ NT and WO₃/TiO₂ NT. The hydrogen-treated H-WO_3-x/TiO_2-x NT-300 electrode exhibits 3 times greater bulk efficiencies than the WO₃/TiO₂ NT electrode due to the production of oxygen vacancies at the interface. Additionally, optimum H-WO_3-x/TiO_2-x NS/NT-300 photoanode exhibited 93.8% E. coli and 99.8% BPA decomposition efficiencies. The present work shows the effectiveness of microwave-assisted H-WO_3-x/TiO_2-x NT heterojunction photoanodes for organic decomposition and antibacterial activity in a neutral environment without surface-loaded co-catalysts.

Quantum Dots, Passivation Layer and Cocatalysts for Enhanced Photoelectrochemical Hydrogen Production

Solar-driven photoelectrochemical (PEC) hydrogen production is one potential pathway to establish a carbon-neutral society. Nowadays, quantum dots (QDs)-sensitized semiconductors have emerged as promising materials for PEC hydrogen production due to their tunable bandgap by size or morphology control, displaying excellent optical and electrical properties. Nevertheless, they still suffer from anodic corrosion during long-term cycling, offering poor stability. This Review discussed advancements to improve long-term stability of QDs particularly in terms of cocatalysts and passivation layers. The working principle of PEC cells was reviewed, along with all important configurations adopted over recent years. The equations to assess PEC performance were also described. A greater emphasized was placed on QDs and incorporation of cocatalysts or passivation layers that could enhance the PEC performance by influencing the charge transfer and surface recombination processes.

Recent Advances in Self-Supported Semiconductor Heterojunction Nanoarrays as Efficient Photoanodes for Photoelectrochemical Water Splitting

Growth of semiconductor heterojunction nanoarrays directly on conductive substrates represents a promising strategy toward high-performance photoelectrodes for photoelectrochemical (PEC) water splitting. By controlling the growth conditions, heterojunction nanoarrays with different morphologies and semiconductor components can be fabricated, resulting in greatly enhanced light-absorption properties, stabilities, and PEC activities. Herein, recent progress in the development of self-supported heterostructured semiconductor nanoarrays as efficient photoanode catalysts for water oxidation is reviewed. Synthetic methods for the fabrication of heterojunction nanoarrays with specific compositions and structures are first discussed, including templating methods, wet chemical syntheses, electrochemical approaches and chemical vapor deposition (CVD) methods. Then, various heterojunction nanoarrays that have been reported in recent years based on particular core semiconductor scaffolds (e.g., TiO₂ , ZnO, WO₃ , Fe₂ O₃ , etc.) are summarized, placing strong emphasis on the synergies generated at the interface between the semiconductor components that can favorably boost PEC water oxidation. Whilst strong progress has been made in recent years to enhance the visible-light responsiveness, photon-to-O₂ conversion efficiency and stability of photoanodes based on heterojunction nanoarrays, further advancements in all these areas are needed for PEC water splitting to gain any traction alongside photovoltaic-electrochemical (PV-EC) systems as a viable and cost-effective route toward the hydrogen economy.

Self-Doped Carbon Dots Decorated TiO2 Nanorods: A Novel Synthesis Route for Enhanced Photoelectrochemical Water Splitting

Herein, we have successfully prepared self-doped carbon dots with nitrogen elements (NCD) in a simple one-pot hydrothermal carbonization method, using L-histidine as a new precursor. The effect of as-prepared carbon dots was studied for photoelectrochemical (PEC) water splitting by decorating NCDs upon TiO2 nanorods systematically by changing the loading time from 2 h to 8 h (TiO2@NCD2h, TiO2@NCD4h, TiO2@NCD6h, and TiO2@NCD8h). The successful decorating of NCDs on TiO2 was confirmed by FE-TEM and Raman spectroscopy. The TiO2@NCD4h has shown a photocurrent density of 2.51 mA.cm−2, 3.4 times higher than the pristine TiO2. Moreover, TiO2@NCD4h exhibited 12% higher applied bias photon-to-current efficiency (ABPE) than the pristine TiO2. The detailed IPCE, Mott–Schottky, and impedance (EIS) analyses have revealed the enhanced light harvesting property, free carrier concentration, charge separation, and transportation upon introduction of the NCDs on TiO2. The obtained results clearly portray the key role of NCDs in improving the PEC performance, providing a new insight into the development of highly competent TiO2 and NCDs based photoanodes for PEC water splitting.

Floating perovskite-BiVO4 devices for scalable solar fuel production

Photoelectrochemical (PEC) artificial leaves hold the potential to lower the costs of sustainable solar fuel production by integrating light harvesting and catalysis within one compact device. However, current deposition techniques limit their scalability¹, whereas fragile and heavy bulk materials can affect their transport and deployment. Here we demonstrate the fabrication of lightweight artificial leaves by employing thin, flexible substrates and carbonaceous protection layers. Lead halide perovskite photocathodes deposited onto indium tin oxide-coated polyethylene terephthalate achieved an activity of 4,266 µmol H₂ g^-1 h^-1 using a platinum catalyst, whereas photocathodes with a molecular Co catalyst for CO₂ reduction attained a high CO:H₂ selectivity of 7.2 under lower (0.1 sun) irradiation. The corresponding lightweight perovskite-BiVO₄ PEC devices showed unassisted solar-to-fuel efficiencies of 0.58% (H₂) and 0.053% (CO), respectively. Their potential for scalability is demonstrated by 100 cm² stand-alone artificial leaves, which sustained a comparable performance and stability (of approximately 24 h) to their 1.7 cm² counterparts. Bubbles formed under operation further enabled 30-100 mg cm^-2 devices to float, while lightweight reactors facilitated gas collection during outdoor testing on a river. This leaf-like PEC device bridges the gulf in weight between traditional solar fuel approaches, showcasing activities per gram comparable to those of photocatalytic suspensions and plant leaves. The presented lightweight, floating systems may enable open-water applications, thus avoiding competition with land use.

Encapsulating N‑doped graphite carbon in MoO2 as a novel cocatalyst for boosting photocatalytic hydrogen evolution

Generally, it is important to ameliorate the co-catalyst used in photocatalytic hydrogen evolution reactions (PHERs) to achieve efficient transfer and separation of photogenerated carriers, decrease the surface reaction energy barrier, and hence improve the photocatalytic activity. In this study, N-doped graphite carbon (GC) was introduced in situ to MoO₂ to ensure the presence of well-dispersed active sites, lower the overpotential of hydrogen evolution, and further achieve high conductivity. Then, the MoO₂/GC composite obtained was used as a co-catalyst of ZnIn₂S₄ (ZIS) in a PHER, resulting in a great improvement in the photocatalytic activity. Given the metallicity and large work function of MoO₂/GC, a Schottky interface can form between MoO₂/GC and ZIS, which accelerates the transmission of photogenerated electrons. As a result, the separation efficiency of photogenerated carriers improves, whereas the surface overpotential of PHERs clearly decreases for ZIS. This study proposes a new idea for exploiting efficient co-catalysts and promotes the wide and heavy use of carbon materials in the field of solar energy conversion.

Facile Preparation of Hydrogen-Bonded Organic Framework/Cu2O Heterostructure Films via Electrophoretic Deposition for Efficient CO2 Photoreduction

Photocatalytic CO₂ reduction is one of the most cost-effective and environmentally friendly techniques of converting CO₂ into high-value compounds and/or fuels. However, the performance of most current photocatalytic CO₂ reduction catalysts is less than satisfactory for practical applications. Here, we synthesized a heterogeneous structure by integrating Cu₂O and a porphyrin hydrogen-bonded organic framework (PFC-45), which was then fabricated into a thin-film catalyst on carbolic paper (CP) using a facile electrophoretic deposition technology. With improved electron-hole separation efficiency and visible-light-harvesting ability, this film (PFC-45/Cu₂O@CP) significantly enhanced CO₂-to-CO photoreduction, exceeding 2.4 and 3.2 times that of PFC-45@CP and PFC-45/Cu₂O particles, respectively. Remarkably, PFC-45/Cu₂O@CP also exhibited high selectivity (99%) and outstanding activity (11.81 μmol g^-1 h^-1) for photocatalytic CO₂ reduction in pure water without any sacrificial agent. This work demonstrates a new strategy to design photocatalysts for efficient CO₂ reduction.

Effect of a Cocatalyst on a Photoanode in Water Splitting: A Study of Scanning Electrochemical Microscopy

With a proper band gap of ∼2.4 eV for solar light absorption and suitable valence band edge position for oxygen evolution, scheelite-monoclinic bismuth vanadate (BiVO₄) has become one of the most attractive photocatalysts for efficient visible-light-driven photoelectrochemical (PEC) water splitting. Several studies have indicated that surface modification of BiVO₄ with a cocatalyst such as NiFe layered double hydroxide (LDH) can significantly increase the PEC water splitting performance of the catalyst. Herein, we experimentally investigated the charge transfer dynamics and charge carrier recombination processes by scanning electrochemical microscopy (SECM) with the feedback mode on the surface of BiVO₄ and BiVO₄/NiFe-LDH as model samples. The ratio of rate constants for photogenerated hole (k_h+⁰) to electron (k_e-⁰) via the photocatalyst of BiVO₄/NiFe-LDH reacting with the redox couple is found to be five times larger than that of BiVO₄ under illumination. In this case, the ratio of the rate constants k_h+⁰/k_e-⁰ stands for the interfacial charge recombination process. This implies the cocatalyst NiFe-LDH suppresses the electron back transfer greatly and finally reduces the surface recombination. Control experiments with cocatalysts CoPi and RuO_x onto BiVO₄ further verify this conclusion. Therefore, the SECM characterization allows us to make an overall analysis on the function of cocatalysts in the PEC water splitting system.

Nanoarray Structures for Artificial Photosynthesis

Conversion and storage of solar energy into fuels and chemicals by artificial photosynthesis has been considered as one of the promising methods to address the global energy crisis. However, it is still far from the practical applications on a large scale. Nanoarray structures that combine the advantages of nanosize and array alignment have demonstrated great potential to improve solar energy conversion efficiency, stability, and selectivity. This article provides a comprehensive review on the utilization of nanoarray structures in artificial photosynthesis of renewable fuels and high value-added chemicals. First, basic principles of solar energy conversion and superiorities of using nanoarray structures in this field are described. Recent research progress on nanoarray structures in both abiotic and abiotic-biotic hybrid systems is then outlined, highlighting contributions to light absorption, charge transport and transfer, and catalytic reactions (including kinetics and selectivity). Finally, conclusions and outlooks on future research directions of nanoarray structures for artificial photosynthesis are presented.

Synthesis of vertical WO3 nanoarrays with different morphologies using the same protocol for enhanced photocatalytic and photoelectrocatalytic performances

Tungsten trioxide (WO3) nanoarrays with different morphologies were successfully synthesized by a hydrothermal method on an FTO substrate. Various nanostructures of WO3 including nanoflakes, nanoplates, nanoflowers and nanorods were obtained by adjusting only the acidity of the precursor solution. XRD patterns confirmed that the as-prepared orthorhombic WO3·0.33H2O transformed to the monoclinic WO3 phase under annealing at 500 °C. UV-Vis absorbance spectroscopy indicated that the absorption edge of WO3 nanoflowers exhibited a slight red-shift compared to other morphologies of WO3. The obtained WO3 nanoflower arrays exhibit the highest photocurrent density and photocatalytic degradation activity towards methylene blue. Finally, the mechanism of the photocatalytic degradation of methylene blue by WO3 is discussed.

Photo-driven Oxygen Vacancies Extends Charge Carrier Lifetime for Efficient Solar Water Splitting

A photocharge/discharge strategy is proposed to initiate the WO₃ photoelectrode and suppress the main charge recombination, which remarkably improves the photoelectrochemical (PEC) performance. The photocharged WO₃ surrounded by a 8-10 nm overlayer and oxygen vacancies could be operated more than 25 cycles with 50 h durability without significant decay on PEC activity. A photocharged WO₃ /CuO photoanode exhibits an outstanding photocurrent of 3.2 mA cm^-2 at 1.23 V_RHE with a low onset potential of 0.6 V_RHE , which is one of the best performances of p-n heterojunction structure. Using nonadiabatic molecular dynamics combined with time-domain DFT, we clarify the prolonged charge carrier lifetime of photocharged WO₃ , as well as how electronic systems of photocharged WO₃ /CuO semiconductors enable the effective photoinduced electrons transfer from WO₃ into CuO. This work provides a feasible route to address excessive defects existed in photoelectrodes without causing extra recombination.

Tubular g-C3N4/carbon framework for high-efficiency photocatalytic degradation of methylene blue

The preparation of high-efficiency, pollution-free photocatalysts for water treatment has always been one of the research hotspots. In this paper, a carbon framework formed from waste grapefruit peel is used as the carrier. A simple one-step chemical vapor deposition (CVD) method allows tubular g-C₃N₄ to grow on the carbon framework. Tubular g-C₃N₄ increases the specific surface area of bulk g-C₃N₄ and enhances the absorption of visible light. At the same time, the carbon framework can effectively promote the separation and transfer of charges. The dual effects of static adsorption and photodegradation enable the g-C₃N₄/carbon (CNC) framework to quickly remove about 98% of methylene blue within 180 min. The recyclability indicates that the tubular g-C₃N₄ can stably exist on the carbon framework during the photodegradation process. In the dynamic photocatalytic test driven by gravity, roughly 77.65% of the methylene blue was degraded by the CNC framework. Our work provides an attractive strategy for constructing a composite carbon framework photocatalyst based on the tubular g-C₃N₄ structure and improving the photocatalytic performance.

Tubular g-C₃N₄ grown on a carbon framework increased the surface area of bulk g-C₃N₄, enhanced the absorption of visible light and promoted the photocatalytic performance.

Engineering Nanostructure-Interface of Photoanode Materials Toward Photoelectrochemical Water Oxidation

Photoelectrochemical (PEC) water oxidation based on semiconductor materials plays an important role in the production of clean fuel and value-added chemicals. Nanostructure-interface engineering has proven to be an effective way to construct highly efficient PEC water oxidation photoanodes with good light capture, carrier transport, and water oxidation kinetics. However, from theoretical and application perspectives, the relationship between the nanostructure and interface of photoanode materials and their PEC performance remains unclear. In this review, the PEC water oxidation reaction mechanism and evaluation criteria are briefly presented. The theoretical basis and research status of the nanostructure-interface engineering on constructing high-performance PEC water oxidation photoanodes are summarized and discussed. Finally, the current challenges and the future opportunities of nanostructure-interface engineering for the PEC reactions are pointed out.

The edge-epitaxial growth of yellow g-C3N4 on red g-C3N4 nanosheets with superior photocatalytic activities

g-C₃N₄ has been used as a photocatalyst to overcome the issues of environmental crises and energy shortages. Here, red g-C₃N₄ nanosheets (E_g: ∼ 1.89 eV) were used as seeds for the edge-epitaxial growth of yellow g-C₃N₄ (E_g: ∼ 2.59 eV) to form type II heterostructures. The heterostructures revealed superior photocatalytic activity for enhanced H₂ (3996 μmol g^-1 h^-1), CO (3.8 μmol g^-1 h^-1), and CH₄ (1.8 μmol g^-1 h^-1) evolution.

Synthesis, characterization and photocatalytic performance of W6+ impregnated g-C3N4 for the removal of chlorophenol derivatives in natural sunlight exposure

The surface of the g-C₃N₄ was altered by impregnating W⁶⁺ ions that transformed to homogeneously coated oxide layer by a calcination process. An enhanced absorption and the suppressed de-excitation in the emission spectra, with the increasing W⁶⁺ loading, exposed the supporting role of the coated layer in extending the spectral response as well as the prolonged life span of excitons. The same was further supported by electrochemical impedance spectroscopy (EIS). The XRD and XPS analysis revealed the coated layer as highly crystalline pure phase monoclinic WO₃ with the majority of impregnated tungsten ions in 6+ oxidation state respectively, whereas the FESEM and HRTEM analysis substantiated the uniformity of the coated layer with the interlayer spacing of the 0.369 nm. Additionally, the probable formation of individual WO₃ nanoparticles or clusters was ruled out. The as-synthesized impregnated photocatalysts, in comparison to pure g-C₃N_4, were subjected to natural sunlight exposure for the photocatalytic removal of chlorophenol derivatives (2-CP, 3-CP, 4-CP, 2,3-DCP, 2,4-DCP, 2,4,6-TCP and PCP) that revealed the 5 wt% coating as the optimum level for significant removal. The progress of the photocatalytic process was monitored by periodic HPLC analysis whereas ion chromatography (IC) was used for the estimation of released ions. The mineralization capability of the as-synthesized W⁶⁺ coated catalysts was measured by the time scale TOC measurements. As the formation of intermediates was indicated in HPLC analysis, selected samples were subjected to GC-MS analysis for the identification of the nature of intermediates. The variable degree of removal of chlorophenol derivatives signified the role of the position and orientation of Cl group. The kinetics of the removal process was evaluated with the calculation of rate constants. The results extracted from the analytical tools and the associated band edge potentials were correlated to speculate the probable mechanism as well as the identification of major reactive oxygen species (ROS) involved in the removal process.

α-Fe2O3/Ag/CdS ternary heterojunction photoanode for efficient solar water oxidation

By taking full advantage of the α-Fe2O3/Ag/CdS ternary heterojunction in charge separation and transfer, light harvesting and electrocatalytic water oxidation, obviously improved water oxidation performance was achieved on the photoanode.

Nanocarbon-Enhanced 2D Photoelectrodes: A New Paradigm in Photoelectrochemical Water Splitting

Solar-driven photoelectrochemical (PEC) water splitting systems are highly promising for converting solar energy into clean and sustainable chemical energy. In such PEC systems, an integrated photoelectrode incorporates a light harvester for absorbing solar energy, an interlayer for transporting photogenerated charge carriers, and a co-catalyst for triggering redox reactions. Thus, understanding the correlations between the intrinsic structural properties and functions of the photoelectrodes is crucial. Here we critically examine various 2D layered photoanodes/photocathodes, including graphitic carbon nitrides, transition metal dichalcogenides, layered double hydroxides, layered bismuth oxyhalide nanosheets, and MXenes, combined with advanced nanocarbons (carbon dots, carbon nanotubes, graphene, and graphdiyne) as co-catalysts to assemble integrated photoelectrodes for oxygen evolution/hydrogen evolution reactions. The fundamental principles of PEC water splitting and physicochemical properties of photoelectrodes and the associated catalytic reactions are analyzed. Elaborate strategies for the assembly of 2D photoelectrodes with nanocarbons to enhance the PEC performances are introduced. The mechanisms of interplay of 2D photoelectrodes and nanocarbon co-catalysts are further discussed. The challenges and opportunities in the field are identified to guide future research for maximizing the conversion efficiency of PEC water splitting.

Biuret-A Crucial Reaction Intermediate for Understanding Urea Pyrolysis To Form Carbon Nitrides: Crystal-Structure Elucidation and In Situ Diffractometric, Vibrational and Thermal Characterisation

The crystal structure of biuret was elucidated by means of XRD analysis of single crystals grown through slow evaporation from a solution in ethanol. It crystallises in its own structure type in space group C2/c (a=15.4135(8) Å, b=6.6042(3) Å, c=9.3055(4) Å, Z=8). Biuret decomposition was studied in situ by means of temperature-programmed powder XRD and FTIR spectroscopy, to identify a co-crystalline biuret-cyanuric acid phase as a previously unrecognised reaction intermediate. Extensive thermogravimetric studies of varying crucible geometry, heating rate and initial sample mass reveal that the concentration of reactive gases at the interface to the condensed sample residues is a crucial parameter for the prevailing decomposition pathway. Taking these findings into consideration, a study on the optimisation of carbon nitride synthesis from urea on the gram scale, with standard solid-state laboratory techniques, is presented. Finally, a serendipitously encountered self-coating of the crucible inner walls by graphite during repeated synthetic cycles, which prove to be highly beneficial for the obtained yields, is reported.

An Efficient and Stable MoS2 /Zn0.5 Cd0.5 S Nanocatalyst for Photocatalytic Hydrogen Evolution

Photocatalytic hydrogen evolution by water splitting is highly important for the application of hydrogen energy and the replacement of fossil fuel by solar energy, which needs the development of efficient catalysts with long-term catalytic stability under light irradiation in aqueous solution. Herein, Zn_0.5 Cd_0.5 S solid solution was synthesized by a metal-organic framework-templated strategy and then loaded with MoS₂ by a hydrothermal method to fabricate a MoS₂ /Zn_0.5 Cd_0.5 S heterojunction for photocatalytic hydrogen evolution. The composition of MoS₂ /Zn_0.5 Cd_0.5 S was fine-tuned to obtain the optimized 5 wt % MoS₂ /Zn_0.5 Cd_0.5 S heterojunction, which showed a superior hydrogen evolution rate of 23.80 mmol h^-1  g^-1 and steady photocatalytic stability over 25 h. The photocatalytic performance is due to the appropriate composition and the formation of an intimate interface between MoS₂ and Zn_0.5 Cd_0.5 S, which endows the photocatalyst with high light-harvesting ability and effective separation of photogenerated carriers.

Photoactive Tungsten-Oxide Nanomaterials for Water-Splitting

This review focuses on tungsten oxide (WO3) and its nanocomposites as photoactive nanomaterials for photoelectrochemical cell (PEC) applications since it possesses exceptional properties such as photostability, high electron mobility (~12 cm2 V-1 s-1) and a long hole-diffusion length (~150 nm). Although WO3 has demonstrated oxygen-evolution capability in PEC, further increase of its PEC efficiency is limited by high recombination rate of photogenerated electron/hole carriers and slow charge transfer at the liquid-solid interface. To further increase the PEC efficiency of the WO3 photocatalyst, designing WO3 nanocomposites via surface-interface engineering and doping would be a great strategy to enhance the PEC performance via improving charge separation. This review starts with the basic principle of water-splitting and physical chemistry properties of WO3, that extends to various strategies to produce binary/ternary nanocomposites for PEC, particulate photocatalysts, Z-schemes and tandem-cell applications. The effect of PEC crystalline structure and nanomorphologies on efficiency are included. For both binary and ternary WO3 nanocomposite systems, the PEC performance under different conditions-including synthesis approaches, various electrolytes, morphologies and applied bias-are summarized. At the end of the review, a conclusion and outlook section concluded the WO3 photocatalyst-based system with an overview of WO3 and their nanocomposites for photocatalytic applications and provided the readers with potential research directions.

A nanocomposite consisting of cuprous oxide supported on graphitic carbon nitride nanosheets for non-enzymatic electrochemical sensing of 8-hydroxy-2'-deoxyguanosine

Graphitic carbon nitrides supported cuprous oxide architecture is reported as an efficient electrode material for supercapacitors, especially due to its high charge-transfer conductivity of the electrochemical devices. Herein, we present an electrochemical sensor to specifically detect 8-hydroxy-2'-deoxyguanosine (8-HDG) oxidative stress biomarker using graphitic carbon nitrides that decorate a cuprous oxide cubes modified electrode. The fabricated electrochemical sensor was characterized and proved by electrochemical methods, EDX, FESEM, and amperometry (i-t). In the presence of 8-hydroxy-2'-deoxyguanosine (8-HDG), the effective interaction between graphitic carbon nitrides and 8-HDG favors the accumulation on the Cu₂O/g-C₃N₄/GCE, which increases the electrocatalytic property and amperometric response. The proposed electrochemical sensor exhibits a wide linear range for 8-HDG in 0.1 M phosphate buffer (pH 7.0) from 25 nM to 0.91 mM, and the limit of detection (LOD) is 4.5 nM. The stability of the Cu₂O/g-C₃N₄/GCE is improved when stored at 4 °C. The repeatability and reproducibility of this electrochemical sensor is good and the sensor retains its  current response for 8-HDG detection also after long time storage. The modified sensor proved high selectivity and sensitivity for 8-HDG, which made it possible to determine 8-HDG in biological samples. Furthermore, the Cu₂O/g-C₃N₄/GCE offered a favorable electron transfer between the Cu₂O/g-C₃N₄ and the electrode interface compared to Cu₂O/GCE, g-C₃N₄/GCE, and unmodified GCE. Graphical abstract Electrochemical detection of oxidative stress marker based on Cu₂O@g-C₃N₄ materials modified electrode.

Sonochemical synthesis of graphitic carbon nitrides-wrapped bimetal oxide nanoparticles hybrid materials and their electrocatalytic activity for xanthine electro-oxidation

A novel network-like magnetic nanoparticle was fabricated on a graphitic carbon nitride through a facile sonochemical route at frequency 20 kHz and power 70 W. To enhance the electrocatalytic activity of the modified materials, the graphitic carbon nitrides (g-C₃N₄) was prepared from melamine. Monitoring of xanthine concentration level in biological fluids is more important for clinical diagnosis and medical applications. As modified CuFe₂O₄/g-C₃N₄ nanocomposite exhibits better electrochemical activity towards the oxidation of xanthine with higher anodic current compared to other modified and unmodified electrode for the detection of xanthine with larger linear range (0.03-695 µM) and lower limit of detection (13.2 nM). To compare with these methods, the electrochemical techniques may be an alternative high sensitive method due to their simplicity and rapid detection time. In addition, the practical feasibility of the sensor was inspected with biological samples, reveals the acceptable recovery of the sensor in real samples.

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 % MnO_x dual cocatalysts simultaneously, the optimal visible-light-driven (λ > 400 nm) composite photocatalyst of Pt-CdS/g-C₃N₄-MnO_x 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.

Performance and Mechanism of Photoelectrocatalytic Activity of MoSx/WO3 Heterostructures Obtained by Reactive Pulsed Laser Deposition for Water Splitting

This work studies the factors that affect the efficiency of the photoelectrochemical hydrogen evolution reaction (HER) using MoS_x/WO₃ nano-heterostructures obtained by reactive pulsed laser deposition (RPLD) on glass substrates covered with fluorinated tin oxide (FTO). Another focus of the research is the potential of MoS_x nanofilms as a precursor for MoO_z(S) nanofilms, which enhance the efficiency of the photo-activated oxygen evolution reaction (OER) using the MoO_z(S)/WO₃/FTO heterostructures. The nanocrystalline WO₃ film was created by laser ablation of a W target in dry air at a substrate temperature of 420 °C. Amorphous MoS_x nanofilms (2 ≤ x ≤ 12) were obtained by laser ablation of an Mo target in H₂S gas of varied pressure at room temperature of the substrate. Studies of the energy band structures showed that for all MoS_x/WO₃/FTO samples, photo-activated HER in an acid solution proceeded through the Z-scheme. The highest photoelectrochemical HER efficiency (a photocurrent density ~1 mA/cm² at a potential of ~0 V under Xe lamp illumination (~100 mW/cm²)) was found for porous MoS_4.5 films containing the highest concentration of catalytically active sites attributed to S ligands. During the anodic posttreatment of porous MoS_x nanofilms, MoO_z(S) films with a narrow energy band gap were formed. The highest OER efficiency (a photocurrent density ~5.3 mA/cm² at 1.6 V) was detected for MoO_z(S)/WO₃/FTO photoanodes that were prepared by posttreatment of the MoS_x~3.2 precursor. The MoO_z(S) film contributed to the effective photogeneration of electron–hole pairs that was followed by the transport of photoelectrons from MoO_z(S) into the WO₃ film and the effective participation of holes possessing strong oxidation ability in the OER on the surface of the MoO_z(S) film.

Nitrogen vacancy mediated exciton dissociation in carbon nitride nanosheets: Enhanced hydroxyl radicals generation for efficient photocatalytic degradation of organic pollutants

Polymeric materials are promising candidates as photocatalysts for environmental purification, however their catalytic performance are still unsatisfactory mainly due to the strong Coulomb interactions between electron and hole that leads to fast charge recombination. Herein, taking graphitic carbon nitride as an example, we verify that installing carbon nitride nanosheets with nitrogen vacancy could break the intrinsic electronic state distribution, forming energy disordered interfaces around the vacancies with the energy difference as large as 0.35 eV. Such a large energy difference is found energetic enough to overcome the strong Coulomb interactions between electron and hole for hot electron and hole generation, as a result showing high electron-hole separation efficiency. Benefited from these advantages, the as prepared material shows remarkable photocatalytic performance toward organic pollutants degradation. The improved catalytic performance is originated from the promoted exciton dissociation that leads to ultra high hydroxyl radical generation. This study offers a new understanding of the excitonic effects for designing advanced polymeric photocatalyst for energy and environment related applications.

Functional carbon nitride materials for water oxidation: from heteroatom doping to interface engineering

Polymeric carbon nitrides (PCNs) are promising photocatalysts and electrocatalysts for water oxidation, as they are environmentally benign materials with an adjustable structure and facilely synthesized from inexpensive and abundant starting materials. In this minireview, we examine the state-of-the-art strategies for tailoring PCNs for efficient photocatalytic, electrocatalytic, and photoelectrochemical water oxidation, including heteroatom doping and interface engineering from band structure alignment (e.g., by coupling inorganic or organic semiconductors) to hybridization with nanoscale cocatalysts (e.g., nanosheets, nanoarrays, nanoparticles, and quantum dots) and sub-nanoscale cocatalysts (e.g., metallic molecular clusters and single-atom catalysts). Through establishing the structure-activity correlations, we aim to present a clear roadmap for providing insights into the design strategies, structure modification, and the improved catalytic performances of PCN-based materials in different catalytic water oxidation processes. For future guidance, we also propose some outlooks on the perspective and challenges of PCNs towards a better application in catalytic water oxidation.

Enhanced photoelectrochemical performance of NaNbO3 nanofiber photoanodes coupled with visible light active g-C3N4 nanosheets for water splitting

Sodium niobate nanofibers (NaNbO₃-NF) have been synthesized using a hydrothermal technique and further coupled with visible light responsive graphitic carbon nitride (g-C₃N₄) nanosheets in different concentration ratios of 2:1 (2-CN), 4:1 (4-CN) and 8:1 (8-CN). A significant improvement in the photoelectrochemical (PEC) performance of the g-C₃N₄/NaNbO₃-NF (4-CN) nanostructured photoanode compared to the bare NaNbO₃ photoanode is observed. A current density of 12.55 mA cm^-2 at 1 V with respect to the Ag/AgCl reference electrode is achieved for the g-C₃N₄/NaNbO₃-NF (4-CN) photoanode, which is ∼3 times higher than the NaNbO₃-NF photoanode. Also, compared to NaNbO₃-NF, the g-C₃N₄/NaNbO₃-NF (4-CN) nanocomposite photoanode showed ∼3 times improvement in the incident photon-to-current conversion efficiency. The improvement in the PEC performance of visible light active g-C₃N₄/NaNbO₃-NF (4-CN) nanocomposite is attributed to the improved photoresponse of NaNbO₃-NF due to the coupling of g-C₃N₄ and the formation of a type-II heterojunction between them leading to the enhanced separation of the photogenerated charge carriers. A possible reaction mechanism for the improved PEC water splitting performance has been proposed for the g-C₃N₄/NaNbO₃-NF (4-CN) photoanode.