Carbon Nitride Materials for Water Splitting Photoelectrochemical Cells

Convenient rapid prototyping microphysiological niche for mimicking liver native basement membrane: Liver sinusoid on a chip

Liver is responsible for the metabolization processes of up to 90 % of compounds and toxins in the body. Therefore liver-on-a-chip systems, as an in vitro promising cell culture platform, have great importance for fundamental science and drug development. In most of the liver-on-a-chip studies, seeding cells on both sides of a porous membrane, which represents the basement membrane, fail to resemble the native characteristics of biochemical, biophysical, and mechanical properties. In this study, polycarbonate (PC) and polyethylene terephthalate (PET) membranes were coated with gelatin to address this issue by accurately mimicking the native basement membrane present in the space of Disse. Various coating methods were used, including doctor blade, gel micro-injection, electrospinning, and spin coating. Spin coating was demonstrated to be the most effective technique owing to the ability to produce thin gel thickness with desirable surface roughness for cell interactions on both sides of the membrane. HepG2 and EA.HY926 cells were seeded on the upper and bottom sides of the gelatin-coated PET membrane and cultured on-chip for 7 days. Cell viability increased from 90 % to 95 %, while apoptotic index decreased. Albumin secretion notably rose between days 1-7 and 4-7, while GST-α secretion decreased from day 1 to day 7. In conclusion, the optimized spin coating process reported here can effectively modify the membranes to better mimic the native basement membrane niche characteristics.

Supramolecular interaction of a molecular catalyst with a polymeric carbon nitride photoanode enhances photoelectrochemical activity and stability at neutral pH

Polymeric carbon nitride (CN) emerged as an alternative, metal-free photoanode material for water-splitting photoelectrochemical cells (PECs). However, the performance of CN photoanodes is limited due to the slow charge separation and water oxidation kinetics due to poor interaction with water oxidation catalysts (WOCs). Moreover, operation under benign, neutral pH conditions is rarely reported. Here, we design a porous CN photoanode connected to a highly active molecular Ru-based WOC, which also acts as an additional photo-absorber. We show that the strong interaction between the π-system of the heptazine units within the CN with the CH groups of the WOC's equatorial ligand enables a strong connection between them and an efficient electronic communication path. The optimized photoanode exhibits a photocurrent density of 180 ± 10 μA cm-2 at 1.23 V vs. the reversible hydrogen electrode (RHE) with 89% faradaic efficiency for oxygen evolution with turnover numbers (TONs) in the range of 3300 and a turnover frequency (TOF) of 0.4 s-1, low onset potential, extended incident photon to current conversion, and good stability up to 5 h. This study may lead to the integration of molecular catalysts and polymeric organic absorbers using supramolecular interactions.

Dual-mode electrochemiluminescence sensing and phone imaging assays based on bipolar electrode for kanamycin detection

Excessive use of antibiotics will enter the water environment and soil through the biological chain, and then transfer to the human body through food, resulting in drug resistance, kidney toxicity and other health problems, so it is urgent to develop highly sensitive detection methods of antibiotics. Here, we designed a dual-mode sensor platform based on closed bipolar electrode (cBPE) electroluminescence (ECL) and mobile phone imaging to detect kanamycin in seawater. The prepared CN-NV-550 displayed extremely intense ECL signal, allowing for convenient mobile phone imaging. The cBPE was combined with DNA cycle amplification technology to prevent the mutual interference between target and the luminescent material, and realized the amplification of signal. In the presence of target Kana, Co3O4 was introduced to the cBPE anode by DNA cycle amplification product, and accelerated the oxidation rate of uric acid (UA). Thus, the electroluminescence response of CN-NV-550 on cBPE cathode was much improved due to the charge balance of the cBPE, achieving both ECL detection and mobile phone imaging assay of Kana, which much improved the accuracy and efficiency of assay. The limit of detection (LOD) in this work is 0.23 pM, and LOD for mobile phone imaging is 0.39 pM. This study integrate ECL imaging visualization of CN-NV-550 and high electrocatalytic activity of Co3O4 into cBPE-ECL detection, providing a new perspective for antibiotic analysis, and has great potential for practical applications, especially in Marine environmental pollution monitoring.

Electronic Structure and Functions of Carbon Nitride in Frontier Green Catalysis

ConspectusGraphitic carbon nitride-based materials have emerged as promising photocatalysts for a variety of energy and environmental applications owing to their "earth-abundant" nature, structural versatility, tunable electronic and optical properties, and chemical stability. Optimizing carbon nitride's physicochemical properties encompasses a variety of approaches, including the regulation of inherent structural defects, morphology control, heterostructure construction, and heteroatom and metal-atom doping. These strategies are pivotal in ultimately enhancing their photocatalytic activities. Previous reviews with extensive examples have mainly focused on the synthesis, modification, and application of carbon nitride-based materials in photocatalysis. However, there has been a lack of straightforward and in-depth discussion to understand the electronic characteristics and functions of various engineered carbon nitrides as well as their precise tailoring strategies and ultimately to explain the regularity and specificity of their improved performance in targeted photocatalytic systems. In the past ten years, our group has conducted extensive investigations on carbon nitride-based materials and their application in photocatalysis. These studies demonstrate the close yet intricate relationship between the electronic structure of carbon nitride materials and their photocatalytic reactivity. Understanding the electronic structure and functions of carbon nitride, as well as different engineering strategies, is essential for the improvement of photocatalytic processes from fundamental study to practical applications.To this end, in this Account, we first delve into the nature of the electronic properties of carbon nitride, highlighting the electronic structures, including band structure, density of states, molecular orbitals, and band center, as well as its electronic functions, such as the charge distribution, internal electric field, and external electric force. Subsequently, based on recent research in our group, we present a detailed discussion of the strategic modifications of carbon nitride and the consequential impacts on the physicochemical properties, particularly the optical properties and intrinsic electronic characteristics, for enhancing the photocatalytic performance. These modifications are categorized as follows: (i) component changing, which involves intralayer and interface heterojunctions as well as homojunctions, to modulate the band-edge potentials and reactivity of photoinduced electrons and holes toward surface redox reactions; (ii) dimensional tuning, which engineers the dimensional structure of carbon nitride, to influence the electron transfer direction; (iii) defect and heteroatom modification, which introduces a symmetry break in the carbon nitride framework, to promote charge redistribution for altering the charge density and electronic structure; and (iv) anchoring of single-atom metals to facilitate orbital hybridization and charge transfer enhancement through the unique metal-N coordination configurations. Finally, we propose an appraisal of the prospects and challenges in the precise manipulation and characterization of the electronic structure and functions of carbon nitride. The integration of in situ electronic structure analysis, theoretical calculation based on machine learning, and precise mechanism study may propel its substantial development in the light-driven circular economy. We hope this Account aspires to offer novel insights and perspectives into the operational mechanisms and tailored structure of carbon nitride-based materials in photocatalysis.

NC Meets CN: Porous Photoanodes with Polymeric Carbon Nitride/ZnSe Nanocrystal Heterojunctions for Photoelectrochemical Applications

The utilization of photoelectrochemical cells (PEC) for converting solar energy into fuels (e.g., hydrogen) is a promising method for sustainable energy generation. We demonstrate a strategy to enhance the performance of PEC devices by integrating surface-functionalized zinc selenide (ZnSe) semiconductor nanocrystals (NCs) into porous polymeric carbon nitride (CN) matrices to form a uniformly distributed blend of NCs within the CN layer via electrophoretic deposition (EPD). The achieved type II heterojunction at the CN/NC interface exhibits intimate contact between the NCs and the CN backbone since it does not contain insulating binders. This configuration promotes efficient charge separation and suppresses carrier recombination. The reported CN/NC composite structure serves as a photoanode, demonstrating a photocurrent density of 160 ± 8 μA cm-2 at 1.23 V vs a reversible hydrogen electrode (RHE), 75% higher compared with a CN-based photoelectrode, for approximately 12 h. Spectral and photoelectrochemical analyses reveal extended photoresponse, reduced charge recombination, and successful charge transfer at the formed heterojunction; these properties result in enhanced PEC oxygen production activity with a Faradaic efficiency of 87%. The methodology allows the integration of high-quality colloidal NCs within porous CN-based photoelectrodes and provides numerous knobs for tuning the functionality of the composite systems, thus showing promise for achieving enhanced solar fuel production using PEC.

Single-Molecule Discrimination of Saccharides Using Carbon Nitride Nanopores

Structural complexity brings a huge challenge to the analysis of sugar chains. As a single-molecule sensor, nanopores have the potential to provide fingerprint information on saccharides. Traditionally, direct single-molecule saccharide detection with nanopores is hampered by their small size and weak affinity. Here, a carbon nitride nanopore device is developed to discern two types of trisaccharide molecules (LeApN and SLeCpN) with minor structural differences. The resolution of LeApN and SLeCpN in the mixture reaches 0.98, which has never been achieved in solid-state nanopores so far. Monosaccharide (GlcNAcpN) and disaccharide (LacNAcpN) can also be discriminated using this system, indicating that the versatile carbon nitride nanopores possess a monosaccharide-level resolution. This study demonstrates that the carbon nitride nanopores have the potential for conducting structure analysis on single-molecule saccharides.

Frontiers in Photoelectrochemical Catalysis: A Focus on Valuable Product Synthesis

Photoelectrochemical (PEC) catalysis provides the most promising avenue for producing value-added chemicals and consumables from renewable precursors. Over the last decades, PEC catalysis, including reduction of renewable feedstock, oxidation of organics, and activation and functionalization of C─C and C─H bonds, are extensively investigated, opening new opportunities for employing the technology in upgrading readily available resources. However, several challenges still remain unsolved, hindering the commercialization of the process. This review offers an overview of PEC catalysis targeted at the synthesis of high-value chemicals from sustainable precursors. First, the fundamentals of evaluating PEC reactions in the context of value-added product synthesis at both anode and cathode are recalled. Then, the common photoelectrode fabrication methods that have been employed to produce thin-film photoelectrodes are highlighted. Next, the advancements are systematically reviewed and discussed in the PEC conversion of various feedstocks to produce highly valued chemicals. Finally, the challenges and prospects in the field are presented. This review aims at facilitating further development of PEC technology for upgrading several renewable precursors to value-added products and other pharmaceuticals.

Isotype heterojunction: tuning the heptazine/triazine phase of crystalline nitrogen-rich C3N5 towards multifunctional photocatalytic applications

Photocatalytic technology has been well studied as a means to achieve sustainable energy generation through water splitting or chemical synthesis. Recently, a low C/N atomic ratio carbon nitride allotrope, C3N5, has been found to be highly prospective due to its excellent electronic properties and ample N-active sites compared to g-C3N4. Tangentially, crystalline g-C3N4 has also been a prospective candidate due to its improved electron transport and extended π-conjugated system. For the first time, our group successfully employed a one-step molten salt calcination method to prepare novel N-rich crystalline C3N5 and elucidate the effect of calcination temperature on the heptazine/triazine phase. Calcination temperatures of 500 °C (CC3N5-500) and 550 °C (CC3N5-550) lead to crystalline carbon nitride with both heptazine and triazine phases, forming an intimate isotype heterojunction for robust interfacial charge separation. An excellent photocatalytic hydrogen evolution rate (359.97 μmol h-1; apparent quantum efficiency (AQE): 12.86% at 420 nm) was achieved using CC3N5-500, which was 15-fold higher than that of pristine C3N5. Furthermore, CC3N5-500 exhibited improved activity for simultaneous benzyl alcohol oxidation and hydrogen production, as well as H2O2 production (AQE: 9.49% at 420 nm), signifying its multitudinous photoredox capabilities. Moreover, the recyclability tests of the optimal CC3N5-500 on a 3D-printed substrate also showed a 92% performance retention after 4 cycles (16 h). This highlights that crystalline C3N5 significantly augmented the reaction performance for diverse multifunctional solar-driven applications. As such, these results serve as a guide toward the structural tuning of 2D metal-free carbon nanomaterials with tunable crystallinity toward achieving boosted photocatalysis.

Engineering band structuring via dual atom modification for an efficient photoanode

Efficient carrier separation is important for improving photoelectrochemical water splitting. Here, the morphology modification and band structure engineering of Ta3N5 are accomplished by doping it with Cu and Zr using a two-step method for the first time. The initially interstitially-doped Cu atoms act as anchors to interact with subsequently doped Zr atoms under the influence of differences in electronegativity. This interaction results in Cu,Zrg-Ta3N5 having a dense morphology and higher crystallinity, which helps to reduce carrier recombination at grain boundaries. Furthermore, the gradient doping of Zr generates a band edge energy gradient, which significantly enhances bulk charge separation efficiency. Therefore, a photoanode based on Cu,Zrg-Ta3N5 delivers an onset potential of 0.38 VRHE and a photocurrent density of 8.9 mA cm-2 at 1.23 VRHE. Among all the Ta3N5-based photoanodes deposited on FTO, a Cu,Zrg-Ta3N5-based photoanode has the lowest onset potential and highest photocurrent. The novel material morphology regulation and band edge position engineering strategies described herein provide new ideas for the preparation of other semiconductor nanoparticles to improve the photoelectrochemical water splitting performance.

Aminosilanized Interface Promotes Electrochemically Stable Carbon Nitride Films with Fewer Trap States on FTO for (Photo)electrochemical Systems

We have demonstrated the direct growth of a CNx layer on a plasma-cleaned and aminosilanized F-doped SnO2 (FTO) electrode to study the CNx|FTO interface that is critical for (photo)electrocatalytic systems. The (3-aminopropyl)triethoxysilane (APTES) was chosen as a bifunctional organosilane, with the amino end incorporating into CNx and the silane end connecting to the hydroxylated FTO surface. Plasma cleaning and aminosilanization resulted in a highly hydrophilic surface, which leads to better contact of melted thiourea to the aminosilanized FTO (p-FTONH2) during CNx polymer condensation, thus generating a thinner and more compact CNx layer. The modification at the interface was shown to influence the CNx growth on length scales of tens of micrometers. We grew CNx thin films on p-FTONH2 (CNx/p-FTONH2) and nonaminosilanized p-FTO (CNx/p-FTO). CNx/p-FTONH2 had a smaller density of trap states and passed 2.4 times the charges before failure compared to CNx/p-FTO. Additionally, a 40% decrease in interfacial charge transfer resistance at the CNx|electrolyte interface was measured for CNx/p-FTONH2 compared to CNx/p-FTO under -0.5 V vs RHE in 0.1 M Na2SO4. Furthermore, with the CNx surface coated with a Pt cocatalyst, Pt/CNx/p-FTONH2 exhibited faster hydrogen evolution rates and larger current densities than Pt/CNx/p-FTO. The highest Faraday efficiency toward electrochemical hydrogen evolution (FEH2) in 0.1 M Na2SO4 (pH = 7) was 46.1%, 37.3%, 57.7%, and 70.5% for Pt/CNx/p-FTONH2, Pt/CNx/p-FTO, CNx/p-FTONH2, and CNx/p-FTO, respectively. The increase in hydrogen evolution rate did not follow the magnitude of the current density change, indicating electrochemical processes other than proton reduction. Overall, we have carefully investigated the CNx|FTO interface and suggested potential solutions to make CNx films better (photo)electrodes for (photo)electrochemical systems.

Boron and Sodium Doping of Polymeric Carbon Nitride Photoanodes for Photoelectrochemical Water Splitting

Polymeric carbon nitride is a promising photoanode material for water-splitting and organic transformation-based photochemical cells. Despite achieving significant progress in performance, these materials still exhibit low photoactivity compared to inorganic photoanodic materials because of a moderate visible light response, poor charge separation, and slow oxidation kinetics. Here, the synthesis of a sodium- and boron-doped carbon nitride layer with excellent activity as a photoanode in a water-splitting photoelectrochemical cell is reported. The new synthesis consists of the direct growth of carbon nitride (CN) monomers from a hot precursor solution, enabling control over the monomer-to-dopant ratio, thus determining the final CN properties. The introduction of Na and B as dopants results in a dense CN layer with a packed morphology, better charge separation thanks to the in situ formation of an electron density gradient, and an extended visible light response up to 550 nm. The optimized photoanode exhibits state-of-the-art performance: photocurrent densities with and without a hole scavenger of about 1.5 and 0.9 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE), and maximal external quantum efficiencies of 56% and 24%, respectively, alongside an onset potential of 0.3 V.

Direct Growth of Polymeric Carbon Nitride Nanosheet Photoanode for Greatly Efficient Photoelectrochemical Water-Splitting

A general method for the direct synthesis of highly homogeneous and dense polymerized carbon nitride (PCN) nanosheet films on F: SnO2 (FTO) is developed. Detailed photoelectrochemical (PEC) water-splitting studies reveal that the as-synthesized PCN films exhibit outstanding performance as photoanode for PEC water-splitting. The optimal PCN photoanode exhibits excellent photocurrent density of 650 µA cm-2 , and monochromatic incident photon-to-electron conversion efficiency (IPCE) value up to 30.55% (λ = 400 nm) and 25.97% (λ = 420 nm) at 1.23 VRHE in 0.1 m KOH electrolyte. More importantly, the PCN photoanode has an excellent hole extraction efficiency of up to 70 ± 3% due to the abundance of active sites provided by the PCN photoanode nanosheet, which promotes the transport rates of OER-relevant species. These PCN films provide a new benchmark for PCN photoanode materials.

Adaptable graphitic C6N6-based copper single-atom catalyst for intelligent biosensing

Self-adaptability is highly envisioned for artificial devices such as robots with chemical noses. For this goal, seeking catalysts with multiple and modulable reaction pathways is promising but generally hampered by inconsistent reaction conditions and negative internal interferences. Herein, we report an adaptable graphitic C₆N₆-based copper single-atom catalyst. It drives the basic oxidation of peroxidase substrates by a bound copper-oxo pathway, and undertakes a second gain reaction triggered by light via a free hydroxyl radical pathway. Such multiformity of reactive oxygen-related intermediates for the same oxidation reaction makes the reaction conditions capable to be the same. Moreover, the unique topological structure of Cu_SAC₆N₆ along with the specialized donor-π-acceptor linker promotes intramolecular charge separation and migration, thus inhibiting negative interferences of the above two reaction pathways. As a result, a sound basic activity and a superb gain of up to 3.6 times under household lights are observed, superior to that of the controls, including peroxidase-like catalysts, photocatalysts, or their mixtures. Cu_SAC₆N₆ is further applied to a glucose biosensor, which can intelligently switch sensitivity and linear detection range in vitro.

Developing extended visible light responsive polymeric carbon nitrides for photocatalytic and photoelectrocatalytic applications

Polymeric carbon nitride (CN) has emerged as an attractive material for photocatalysis and photoelectronic devices. However, the synthesis of porous CNs with controlled structural and optical properties remains a challenge, and processable CN precursors are still highly sought after for fabricating homogenous CN layers strongly bound to a given substrate. Here, we report a general method to synthesize highly dispersed porous CN materials that show excellent photocatalytic activity for the hydrogen evolution reaction and good performance as photoanodes in photoelectrochemical cells (PEC): first, supramolecular assemblies of melem and melamine in ethylene glycol and water are prepared using a hydrothermal process. These precursors are then calcined to yield a water-dispersible CN photocatalyst that exhibits beneficial charge separation under illumination, extended visible-light response attributed to carbon doping, and a large number of free amine groups that act as preferential sites for a Pt cocatalyst. The optimized CN exhibits state-of-the-art HER rates up to 23.1 mmol h^-1 g^-1, with an AQE of 19.2% at 395 nm. This unique synthetic route enables the formation of a homogeneous precursor paste for substrate casting; consequently, the CN photoanode exhibits a low onset potential, a high photocurrent density and good stability after calcination.

Polymeric carbon nitride as a platform for photoelectrochemical water-splitting cells

Polymeric carbon nitride (CN) materials are promising low-cost photocatalysts that exhibit a combination of chemical and physical properties suitable for converting light into redox activity on their surface. In this perspective, we describe our experience with this family of materials as light absorbers that serve as an anode in photoelectrochemical cells toward water-splitting. We describe some of the CN deposition techniques and procedures established in our lab. The knowledge gained from powder-based photocatalysis is implemented in photoelectrochemical scenarios and is used to determine the merits and shortcomings of resulting layers. We show how the preparation methods are oriented based on these factors and how high photoelectrochemical water-splitting activity develops in photoanodes we developed where CN(s) act as photoabsorbers. Lastly, we present our view on the future prospects of this field.

Robust Carbon Nitride Homojunction Photoelectrode for Solar-Driven Water Splitting

Achieving intimate particle-to-particle and particle-to-substrate contacts is the first priority for fabricating high-quality photoelectrodes to ensure sufficient visible light absorption and efficient charge separation/transport. To achieve this goal, a seeding strategy is designed to construct a robust carbon nitride (CN) homojunction photoelectrode, in which vaporized precursors are condensed into a compact seeding layer at low temperatures, inducing the further deposition of the top layer. This optimized photoelectrode displays an excellent photocurrent density of 320 μA cm^-2 in 0.1 M NaOH electrolyte at 1.23 V_RHE (V vs reversible hydrogen electrode) under AM 1.5G illumination, with H₂ and O₂ evolution rates of 2.98 and 1.47 μmol h^-1 cm^-2, respectively. Characterizations show that both the robust contact and the homojunction of the double-layered CN film contribute to enhanced photoelectrochemical performance. This work may provide a new strategy for the design of high-performing CN photoelectrodes.

Electron flux at the Schottky junction of Bi NPs and WO3-supported g-C3N4: an efficient ternary S-scheme catalyst for removal of fluoroquinolone-type antibiotics from water

Recently, global-scale attempts have been conducted to develop clean technologies and affordable materials to remediate pharmaceutical contaminants of water resources that are resistant to the biodegradation. In line with global efforts, this study reports a facile method to fabricate Bi nanocrystals in situ decorated on WO₃ nanoplates and its composite with graphitic carbon nitride (WO₃/Bi/g-C₃N₄) for photocatalytic degradation of fluoroquinolone-type antibiotics (ciprofloxacin and ofloxacin). The designed ternary S-scheme WO₃/Bi/g-C₃N₄ composite material was fully characterized by physicochemical and electrochemical analysis. Depositing the cost-effective and earth-abundant Bi nanocrystals onto WO₃ via a facile reduction route has been shown to increase the boosting of electron flux at their interface (Schottky junction). The S-scheme separation is confirmed by the calculation of band positions and the analysis of photogenerated hydroxyl radicals and holes. The complete removal of contaminants was obtained over the WO₃/Bi/g-C₃N₄ photocatalyst after 90 min under visible light irradiation. The present work would provide a rational route for developing Bi NP-based photocatalysis to replace metallic Au, Pt, and Ag NPs.

Partial Sulphidation to Regulate Coordination Structure of Single Nickel Atoms on Graphitic Carbon Nitride for Efficient Solar H2 Evolution

To develop a non-precious highly efficient cocatalyst to replace Pt on graphitic carbon nitride (g-C₃ N₄ ) for solar H₂ production is great significant, but still remains a huge challenge. The emerging single-atom catalyst presents a promising strategy for developing highly efficient non-precious cocatalyst owing to its unique adjustability of local coordination environment and electronic structure. Herein, this work presents a facile approach to achieve single Ni sites (Ni₁ -N₂ S) with unique local coordination structure featuring one Ni atom coordinated with two nitrogen atoms and one sulfur atom, confirmed by high-angle annular dark-field scanning transmission electron microscopy, X-ray absorption spectroscopy, and density functional theory calculation. Thanks to the unique electron structure of Ni₁ -N₂ S sites, the 1095 µmol g^-1 h^-1 of high H₂ evolution rate with 4.1% of apparent quantum yield at 420 nm are achieved. This work paves a pathway for designing a highly efficient non-precious transition metal cocatalyst for photocatalytic H₂ evolution.

Rational Design of Carbon Nitride Photoelectrodes with High Activity Toward Organic Oxidations

Carbon nitride (CN_x ) is a light-absorber with excellent performance in photocatalytic suspension systems, but the activity of CN_x photoelectrodes has remained low. Here, cyanamide-functionalized CN_x (^NCN CN_x ) was co-deposited with ITO nanoparticles on a 1.8 Å thick alumina-coated FTO electrode. Transient absorption spectroscopy and impedance measurements support that ITO acts as a conductive binder and improves electron extraction from the ^NCN CN_x , whilst the alumina underlayer reduces recombination losses between the ITO and the FTO glass. The Al₂ O₃ |ITO : ^NCN CN_x film displays a benchmark performance for CN_x -based photoanodes with an onset of -0.4 V vs a reversible hydrogen electrode (RHE), and 1.4±0.2 mA cm^-2 at 1.23 V vs RHE during AM1.5G irradiation for the selective oxidation of 4-methylbenzyl alcohol. This assembly strategy will improve the exploration of CN_x in fundamental and applied photoelectrochemical (PEC) studies.

Electrochemiluminescence resonance energy transfer biosensing platform between g-C3N4 nanosheet and Ru-SiO2@FA for dual-wavelength ratiometric detection of SARS-CoV-2 RdRp gene

Rational detection of syndrome coronavirus 2 (SARS-CoV-2) is crucial to prevention, control, and treatment of disease. Herein, a dual-wavelength ratiometric electrochemiluminescence (ECL) biosensor based on resonance energy transfer (RET) between g-C₃N₄ nanosheets and Ru-SiO₂@folic acid (FA) nanomaterials was designed to realize ultrasensitive detection of SARS-CoV-2 virus (RdRp gene). Firstly, the unique g-C₃N₄ nanosheets displayed very intense and stable ECL at 460 nm, then the triple helix DNA was stably and vertically bound to g-C₃N₄ on electrode by high binding affinity between ssDNA and g-C₃N₄. Meanwhile, trace amounts of target genes were converted to a large number of output by three-dimensional (3D) DNA walker multiple amplification, and the output bridged a multifunctional probe Ru-SiO₂@FA to electrode. Ru-SiO₂@FA not only showed high ECL at 620 nm, but also effectively quenched g-C₃N₄ ECL. As a result, ECL decreased at 460 nm and increased at 620 nm, which was used to design a rational ECL biosensor for detection of SARS gene. The results show that the biosensor has excellent detection sensitivity for RdRp gene with a dynamic detection range of 1 fM to 10 nM and a limit of detection (LOD) of 0.18 fM. The dual-wavelength ratio ECL biosensor has inestimable value and application prospects in the fields of biosensing and clinical diagnosis.

Designing a 0D/1D S-Scheme Heterojunction of Cadmium Selenide and Polymeric Carbon Nitride for Photocatalytic Water Splitting and Carbon Dioxide Reduction

Constructing photocatalysts to promote hydrogen evolution and carbon dioxide photoreduction into solar fuels is of vital importance. The design and establishment of an S-scheme heterojunction system is one of the most feasible approaches to facilitate the separation and transfer of photogenerated charge carriers and obtain powerful photoredox capabilities for boosting photocatalytic performance. Herein, a zero-dimensional/one-dimensional S-scheme heterojunction composed of CdSe quantum dots and polymeric carbon nitride nanorods (CdSe/CN) is created and constructed via a linker-assisted hybridization approach. The CdSe/CN composites exhibit superior photocatalytic activity in water splitting and promoted carbon dioxide conversion performance compared with CN nanorods and CdSe quantum dots. The best efficiency in photocatalytic water splitting (10.2% apparent quantum yield at 420 nm irradiation, 20.1 mmol g−1 h−1 hydrogen evolution rate) and CO2 reduction (0.77 mmol g−1 h−1 CO production rate) was achieved by 5%CdSe/CN composites. The significantly improved photocatalytic reactivity of CdSe/CN composites primarily originates from the emergence of an internal electric field in the zero-dimensional/one-dimensional S-scheme heterojunction, which could greatly improve the photoinduced charge-carrier separation. This work underlines the possibility of employing polymeric carbon nitride nanostructures as appropriate platforms to establish highly active S-scheme heterojunction photocatalysts for solar fuel production.

Self-assembly of colloidal single-layer carbon nitride

We introduce a new concept of a "bottom-to-top" design of intercalate carbon nitride compounds based on the effects of self-assembly of colloidal single-layer carbon nitride (SLCN) sheets stabilized by tetraethylammonium hydroxide NEt₄OH upon ambient drying of the water solvent. These effects include (i) formation of stage-1 intercalates of NEt₄OH during the ambient drying of SLCN colloids on glass substrates and (ii) the spontaneous formation of layered hexagonally-shaped networks of SLCN sheets on freshly-cleaved mica surfaces. The dynamics of the intercalate formation was followed by in situ X-ray diffraction allowing different stages to be identified, including the deposition of a primary "wet" intercalate of hydrated NEt₄OH and the gradual elimination of excessive water during its ambient drying. The intercalated NEt₄⁺ cations show a specific "flattened" conformation allowing the dynamics of formation and structure of the intercalate to be probed by vibrational spectroscopies. The two-dimensional self-assembly on mica is assumed to be driven both by the internal hexagonal symmetry of heptazine units and by a templating effect of the mica surface.

Photoelectron Storages in Functionalized Carbon Nitrides for Colorimetric Sensing of Oxygen

Colorimetric sensors have been widely used for centuries across diverse fields, thanks to their easy operation and uncompromisingly high sensitivity with no need for electricity. However, it is still a great challenge for conventional chromogenic systems to perform multiple measurements meanwhile maintaining high robustness. Here, we reported that carbon nitrides (CNs), the raw materials that are abundant, structure-tunable, and stable semiconductors with photoelectron storage capability, can be developed as a chromogenic system for colorimetric sensors. Beyond conventional metal oxides that only demonstrated a single blue-color switch after photoelectron storage, CN exhibited a multicolor switch under identical conditions owing to the unusual multiple photoelectron storage pathways. Mechanism studies revealed cyano and carbonyl groups in CN crucially elongated the centroid distance of electrons/holes, which exclusively stabilized the specific excited states that have different light absorption; meanwhile, the counter cations strengthened these processes. As a result, O₂, a proof-of-concept analyte, was quantitatively detected by the CN-derived colorimetric sensor, showing high reversibility in hundreds of cycles and adaptable sensitivity/detection range, outperforming most reported and commercial oxygen sensors. These intriguing features of CN are highly envisioned for the next generation of colorimetric sensors, especially in developing countries or fieldworks, to improve the detection reliability and lower the sensing cost.

Photoelectrochemical alcohols oxidation over polymeric carbon nitride photoanodes with simultaneous H2 production

The photoelectrochemical oxidation of organic molecules into valuable chemicals is a promising technology, but its development is hampered by the poor stability of photoanodic materials in aqueous solutions, low faradaic efficiency, low product selectivity, and a narrow working pH range. Here, we demonstrate the synthesis of value-added aldehydes and carboxylic acids with clean hydrogen (H₂) production in water using a photoelectrochemical cell based solely on polymeric carbon nitride (CN) as the photoanode. Isotope labeling measurements and DFT calculations reveal a preferential adsorption of benzyl alcohol and molecular oxygen to the CN layer, enabling fast proton abstraction and oxygen reduction, which leads to the synthesis of an aldehyde at the first step. Further oxidation affords the corresponding acid. The CN photoanode exhibits excellent stability (>40 h) and activity for the oxidation of a wide range of substituted benzyl alcohols with high yield, selectivity (up to 99%), and faradaic efficiency (>90%).

Coordination Chemistry Engineered Polymeric Carbon Nitride Photoanode with Ultralow Onset Potential for Water Splitting

Construction of an intimate film/substrate interface is of great importance for a photoelectrode to achieve efficient photoelectrochemical performance. Inspired by coordination chemistry, a polymeric carbon nitride (PCN) film is intimately grown on a Ti-coated substrate by an in situ thermal condensation process. The as-prepared PCN photoanode exhibits a record low onset potential (Eonset ) of -0.38 V versus the reversible hydrogen electrode (RHE) and a decent photocurrent density of 242 μA cm-2 at 1.23 VRHE for water splitting. Detailed characterization confirms that the origin of the ultralow onset potential is mainly attributed to the substantially reduced interfacial resistance between the Ti-coated substrate and the PCN film benefitting from the constructed interfacial sp2 N→Ti coordination bonds. For the first time, the ultralow onset potential enables the PCN photoanode to drive water splitting without external bias with a stable photocurrent density of ≈9 μA cm-2 up to 1 hour.

Recent Progress on Photoelectrochemical Water Splitting of Graphitic Carbon Nitride (g-CN) Electrodes

Graphitic carbon nitride (g-CN), a promising visible-light-responsive semiconductor material, is regarded as a fascinating photocatalyst and heterogeneous catalyst for various reactions due to its non-toxicity, high thermal durability and chemical durability, and "earth-abundant" nature. However, practical applications of g-CN in photoelectrochemical (PEC) and photoelectronic devices are still in the early stages of development due to the difficulties in fabricating high-quality g-CN layers on substrates, wide band gaps, high charge-recombination rates, and low electronic conductivity. Various fabrication and modification strategies of g-CN-based films have been reported. This review summarizes the latest progress related to the growth and modification of high-quality g-CN-based films. Furthermore, (1) the classification of synthetic pathways for the preparation of g-CN films, (2) functionalization of g-CN films at an atomic level (elemental doping) and molecular level (copolymerization), (3) modification of g-CN films with a co-catalyst, and (4) composite films fabricating, will be discussed in detail. Last but not least, this review will conclude with a summary and some invigorating viewpoints on the key challenges and future developments.

High-performance potassium poly(heptazine imide) films for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is an appealing approach by which to convert solar energy into hydrogen fuel. Polymeric semiconductors have recently attracted intense interest of many scientists for PEC water splitting. The crystallinity of polymer films is regarded as the main factor that determines the conversion efficiency. Herein, potassium poly(heptazine) imide (K-PHI) films with improved crystallinity were in situ prepared on a conductive substrate as a photoanode for solar-driven water splitting. A remarkable photocurrent density of ca. 0.80 mA cm-2 was achieved under air mass 1.5 global illumination without the use of any sacrificial agent, a performance that is ca. 20 times higher than that of the photoanode in an amorphous state, and higher than those of other related polymeric photoanodes. The boosted performance can be attributed to improved charge transfer, which has been investigated using steady state and operando approaches. This work elucidates the pivotal importance of the crystallinity of conjugated polymer semiconductors for PEC water splitting and other advanced photocatalytic applications.

Flowery ln2MnSe4 Novel Electrocatalyst Developed via Anion Exchange Strategy for Efficient Water Splitting

Oxygen and hydrogen generated by water electrolysis may be utilized as a clean chemical fuel with high gravimetric energy density and energy conversion efficiency. The hydrogen fuel will be the alternative to traditional fossil fuels in the future, which are near to exhaustion and cause pollution. In the present study, flowery-shaped In₂MnSe₄ nanoelectrocatalyst is fabricated by anion exchange reaction directly grown on nickel foam (NF) in 1.0 M KOH medium for oxygen evolution reaction (OER). The physiochemical and electrical characterization techniques are used to investigate the chemical structure, morphology, and electrical properties of the In₂MnSe₄ material. The electrochemical result indicates that synthesized material exhibits a smaller value of Tafel slope (86 mV/dec), lower overpotential (259 mV), and high stability for 37 h with small deterioration in the current density for a long time. Hence, the fabricated material responds with an extraordinary performance for the OER process and for many other applications in the future.

Thermal conductivity temperature dependence of water confined in nanoporous silicon

Recently, it has been shown that high density nanoconfined water was the reason of the important enhancement of the effective thermal conductivity up to a factor of 50% of a nanoporous silicon filled with water. In this work, using molecular dynamics simulations, we further investigate the role of the temperatureT(from 285 to 360 K) on the thermal conductivity enhancement of nanohybrid porous silicon and water system. Furthermore, by studying and analysing several structural and dynamical parameters of the nanoconfined water, we give physical insights of the observed phenomena. Upon increasing the temperature of the system, the thermal conductivity of the hybrid system increases reaching a maximum forT= 300 K. With this article, we prove the existence of new heat flux channels between a solid matrix and a nanoconfined liquid, with clear signatures both in the radial distribution function, mean square displacements, water molecules orientation, hydrogen bond networks and phonon density of states.

One-Pot Synthesis of CoS2 Merged in Polymeric Carbon Nitride Films for Photoelectrochemical Water Splitting

Polymeric carbon nitride (PCN) has attracted intensive interest as sustainable, metal-free semiconductor for photoelectrochemical (PEC) water splitting. Charge transfer along the films acts as the main concern to restrict the performance due to the amorphous nature of polymer. Herein, gradient concentration of cobalt disulfide (CoS₂ ) merged in PCN films was realized as CSCN photoanode by a one-pot synthesis. Owing to the unique properties of CoS₂ , namely high conductivity, the charge transfer of the CSCN photoanode was promoted, and thus the performance for PEC water oxidation was improved. The optimal photoanode exhibited a photoanodic current of 200 μA cm^-2 at 1.23 V versus reversible hydrogen electrode under air mass 1.5 global (AM 1.5G) illumination, which was approximately 4 times that of the pristine PCN photoanode. This work provides a new design of metal-free photoanodes to improve the performance of water splitting.

The Influence of Metal-Doped Graphitic Carbon Nitride on Photocatalytic Conversion of Acetic Acid to Carbon Dioxide

Metal-doped graphitic carbon nitride (MCN) materials have shown great promise as effective photocatalysts for the conversion of acetic acid to carbon dioxide under UV–visible irradiation and are superior to pristine carbon nitride (g-C₃N₄, CN). In this study, the effects of metal dopants on the physicochemical properties of metal-doped CN samples (Fe-, Cu-, Zn-, FeCu-, FeZn-, and CuZn-doped CN) and their catalytic activity in the photooxidation of acetic acid were investigated and discussed for their correlation, especially on their surface and bulk structures. The materials in the order of highest to lowest photocatalytic activity are FeZn_CN, FeCu_CN, Fe_CN, and Cu_CN (rates of CO₂ evolution higher than for CN), followed by Zn_CN, CuZn_CN, and CN (rates of CO₂ evolution lower than CN). Although Fe doping resulted in the extension of the light absorption range, incorporation of metals did not significantly alter the crystalline phase, morphology, and specific surface area of the CN materials. However, the extension of light absorption into the visible region on Fe doping did not provide a suitable explanation for the increase in photocatalytic efficiency. To further understand this issue, the materials were analyzed using two complementary techniques, reversed double-beam photoacoustic spectroscopy (RDB-PAS) and electron spin resonance spectroscopy (ESR). The FeZn_CN, with the highest electron trap density between 2.95 and 3.00 eV, afforded the highest rate of CO₂ evolution from acetic acid photodecomposition. All Fe-incorporated CN materials and Cu-CN reported herein can be categorized as high activity catalysts according to the rates of CO₂ evolution obtained, higher than 0.15 μmol/min⁻¹, or >1.5 times higher than that of pristine CN. Results from this research are suggestive of a correlation between the rate of CO₂ evolution via photocatalytic oxidation of acetic acid with the threshold number of free unpaired electrons in CN-based materials and high electron trap density (between 2.95 and 3.00 eV).

Trioctylphosphine Oxide (TOPO)-Assisted Facile Fabrication of Phosphorus-Incorporated Nanostructured Carbon Nitride Toward Photoelectrochemical Water Splitting with Enhanced Activity

Designing nanostructured arrays of two-dimensional surfaces and interfaces is a versatile approach to increasing their photoelectrochemical activity. Here, phosphorus (P)-incorporated nanostructured carbon nitride (h-PCN) with an enlarged surface area is fabricated by employing trioctylphosphine oxide (TOPO) as a dopant precursor for visible-light-driven photoelectrochemical water splitting to produce hydrogen. The structural, morphological, and electronic properties of the photocatalyst have been characterized through various physicochemical techniques. We show that the incorporation of P into the g-C₃N₄ framework enhances light absorption over broad regimes, charge separation, and migration, as well as the specific surface area, showing excellent photocurrent enhancement (5.4 folds) in the cathodic direction as compared to bulk g-C₃N₄. Moreover, the photocathode shows 3.3-fold enhancement in current at zero biased potential. Without using any cocatalyst, the photoelectrodes produced 27 μmol h^-1 of H₂ and 13 μmol h^-1of O₂ with 95% faradic efficiency. The excellent photoelectrochemical behavior toward water-splitting reactions by the photoelectrode is attributed to the synergistic effect of P incorporation and active sites emerging from the nanostructured architecture of the material. This work demonstrates the facile fabrication of nanostructured P-incorporated g-C₃N₄ toward water-splitting reactions to produce hydrogen without using a cocatalyst in a simple and cost-effective way.

Synergic Photocatalytic CH4 Conversion to C1 liquid products using Fe oxides species-modified g-C3N4

CH4 direct conversion into high valued liquid oxygenated products at mild experimental conditions is of great significance to solve both environment and energy problems. Although great effort has been made,...

Enhanced photocatalytic performance of carbon fiber paper supported TiO2 under the ultrasonic synergy effect

In the present work, TiO₂ rutile nanorods and anatase nanoflakes have been grown on carbon fiber paper (CFP) by the hydrothermal method. Their photoelectrochemical properties and photocatalytic performances have been investigated. The introduction of CFP is found to improve visible light absorption intensity and effective surface areas apparently, and also make TiO₂ photocatalysts easier to recycle from aqueous waste. An ultrasonic field was employed during the process of photocatalysis. Sono-photocatalytic efficiency is found to be enhanced significantly in comparison with those of photocatalysis and sonocatalysis, which indicates a positive ultrasonic synergy effect. The scavenger experiments reveal that superoxide radicals (˙O₂⁻) and hydroxyl (˙OH) are the predominant active species during the dye degradation sono-photocatalytic process assisted by CFP-supported TiO₂ catalysts. To investigate the ultrasonic synergy photocatalytic effect, the generated amount of reactive oxygen species (ROS) was detected and quantitatively evaluated under visible light, ultrasound, and the combined condition of visible light and ultrasound. As a result, the present work provides an efficient way to improve photocatalytic performance and to realize easy recovery of photocatalyst, which will be helpful for better design of advanced photocatalysts for practical applications.

SEM images of TiO₂(R) nanorods and TiO₂(A) nanoflakes grown on CFP. And the corresponding catalytic performances under solely visible light, solely ultrasonic field, and the combined conditions of visible light and ultrasonic field.

Constructing a WC/NCN Schottky Junction for Rapid Electron Transfer and Enrichment for Highly Efficient Photocatalytic Hydrogen Evolution

The low charge-transfer efficiency and slow surface reaction kinetics are the main factors affecting the performance of carbon nitride photocatalysts. Here, a Schottky heterostructure (WCN) was constructed by combining WC with porous carbon nitride nanosheets with a cyanide group (NCN). The Schottky junction provides a convenient way for photoinduced electrons to transfer and promotes the effective separation of photoinduced carriers. Furthermore, due to the good conductivity of WC and an electronic structure similar to Pt, the W atom in WC as the active site of hydrogen production can realize efficient reaction kinetics. In this way, the WCN Schottky heterostructure showed a 2.0- and 5.0-fold enhancement in photocatalytic H₂ evolution as compared to the single NCN component under visible-light and near-infrared light irradiation. By combining with theoretical simulations, as an electron acceptor in the WCN heterostructure, WC can effectively improve the charge-transfer efficiency and also act as an active site for hydrogen production.

Radical-Driven Decomposition of Graphitic Carbon Nitride Nanosheets: Light Exposure Matters

Understanding the transformation of graphitic carbon nitride (g-C₃N₄) is essential to assess nanomaterial robustness and environmental risks. Using an integrated experimental and simulation approach, our work has demonstrated that the photoinduced hole (h⁺) on g-C₃N₄ nanosheets significantly enhances nanomaterial decomposition under ^•OH attack. Two g-C₃N₄ nanosheet samples D and M2 were synthesized, among which M2 had more pores, defects, and edges, and they were subjected to treatments with ^•OH alone and both ^•OH and h⁺. Both D and M2 were oxidized and released nitrate and soluble organic fragments, and M2 was more susceptible to oxidation. Particularly, h⁺ increased the nitrate release rate by 3.37-6.33 times even though the steady-state concentration of ^•OH was similar. Molecular simulations highlighted that ^•OH only attacked a limited number of edge-site heptazines on g-C₃N₄ nanosheets and resulted in peripheral etching and slow degradation, whereas h⁺ decreased the activation energy barrier of C-N bond breaking between heptazines, shifted the degradation pathway to bulk fragmentation, and thus led to much faster degradation. This discovery not only sheds light on the unique environmental transformation of emerging photoreactive nanomaterials but also provides guidelines for designing robust nanomaterials for engineering applications.

Porous g-C3N4 with defects for the efficient dye photodegradation under visible light

Porous graphitic carbon nitride (p-C₃N₄) was fabricated via simple pyrolyzing treatment of graphitic carbon nitride (g-C₃N₄). The defects could be introduced into the structure of g-C₃N₄ by breakage of some bonds, which was beneficial for the generation of electron-hole pairs and inhibiting their recombination. Compared with g-C₃N₄, p-C₃N₄ showed a narrow band gap to promote the utilization of visible light. Furthermore, the porous structure also increased the specific surface area to maximize the exposure of active sites and promote mass transfer during photodegradation. As a result, the as-reported p-C₃N₄ exhibited considerably higher degradation efficiency for Rhodamine B (RhB) and Methyl Orange (MO) than that of the original g-C₃N₄. Moreover, the photocatalyst showed high durability and stability in recycling experiments.

Fabrication of graphitic carbon nitride functionalized P-CoFe2O4 for the removal of tetracycline under visible light: Optimization, degradation pathways and mechanism evaluation

In this study, nano-sized CoFe₂O₄ composites were prepared through co-precipitation process. Then the phosphorus-doped strong magnetic graphitic carbon nitride hybrids composites (P-CoFe₂O₄@GCN) was stemmed from the CoFe₂O₄ composites via the thermal polymerization method. The TEM results show that the CoFe₂O₄ nanoparticles have been successfully embedded into the graphitic carbon nitride (GCN). The BET specific surface area of P-CoFe₂O₄@GCN-1 could reach 36.91 m²/g, which was 5.38 times higher than that of GCN. Thus, it provided sufficient reaction active sites to enhance the photocatalytic activity for tetracycline (TC) decomposition. The results from the photocatalytic experiments showed that the degradation efficiency of TC by P-CoFe₂O₄@GCN-1 could reach 96.2% within 60 min, which is 3.19 times higher than that of GCN. The h⁺, O₂•^- and •OH radicals detected by the electron spin resonance (ESR) were responsible for the TC decomposition in the photocatalytic reaction system. Persulfate (PS) can further activate the hybrid mixture system, and the fitting model predicted by the response surface methodology (RSM) indicated that the maximum tetracycline removal could reach 99.6% within 30 min. In addition, the degradation intermediates of TC were detected by HPLC-MS and the photodegradation mechanism was discussed.