Metalized Carbon Nitrides for Efficient Catalytic Functionalization of CO2

Enhancing photocatalytic CO2 reduction with TiO2-based materials: Strategies, mechanisms, challenges, and perspectives

The concentration of atmospheric CO2 has exceeded 400 ppm, surpassing its natural variability and raising concerns about uncontrollable shifts in the carbon cycle, leading to significant climate and environmental impacts. A promising method to balance carbon levels and mitigate atmospheric CO2 rise is through photocatalytic CO2 reduction. Titanium dioxide (TiO2), renowned for its affordability, stability, availability, and eco-friendliness, stands out as an exemplary catalyst in photocatalytic CO2 reduction. Various strategies have been proposed to modify TiO2 for photocatalytic CO2 reduction and improve catalytic activity and product selectivity. However, few studies have systematically summarized these strategies and analyzed their advantages, disadvantages, and current progress. Here, we comprehensively review recent advancements in TiO2 engineering, focusing on crystal engineering, interface design, and reactive site construction to enhance photocatalytic efficiency and product selectivity. We discuss how modifications in TiO2's optical characteristics, carrier migration, and active site design have led to varied and selective CO2 reduction products. These enhancements are thoroughly analyzed through experimental data and theoretical calculations. Additionally, we identify current challenges and suggest future research directions, emphasizing the role of TiO2-based materials in understanding photocatalytic CO2 reduction mechanisms and in designing effective catalysts. This review is expected to contribute to the global pursuit of carbon neutrality by providing foundational insights into the mechanisms of photocatalytic CO2 reduction with TiO2-based materials and guiding the development of efficient photocatalysts.

Bi-directional charge transfer channels in highly crystalline carbon nitride enabling superior photocatalytic hydrogen evolution

Introducing a donor-acceptor (D-A) unit is an effective approach to facilitate charge transfer in polymeric carbon nitride (PCN) and enhance photocatalytic performance. However, the introduction of hetero-molecules can lead to a decrease in crystallinity, limiting interlayer charge transfer and inhibiting further improvement. In this study, we constructed a novel D-A type carbon nitride with significantly higher crystallinity and a bi-directional charge transfer channel, which was achieved through 2,5-thiophenedicarboxylic acid (2,5-TDCA)-assisted self-assembly followed by KCl-templated calcination. The thiophene and cyano groups introduced serve as the electron donor and acceptor, respectively, enhancing in-plane electron delocalization. Additionally, introduced potassium ions are intercalated among the adjacent layers of carbon nitride, creating an interlayer charge transfer channel. Moreover, the highly ordered structure and improved crystallinity further facilitate charge transfer. As a result, the as-prepared photocatalyst exhibits superior photocatalytic hydrogen evolution (PHE) activity of 7.449 mmol h-1 g-1, which is 6.03 times higher than that of pure carbon nitride. The strategy of developing crystalline D-A-structured carbon nitride with controlled in-plane and interlayer charge transfer opens new avenues for the design of carbon nitride with enhanced properties for PHE.

Developing Polymeric Carbon Nitrides for Photocatalytic H2O2 Production

Hydrogen peroxide (H2O2) plays a crucial role in various applications, such as green oxidation processes and the production of high-quality fuels. Currently, H2O2 is primarily manufactured using the indirect anthraquinone method, known for its significant energy consumption and the generation of intensive by-products. Extensive research has been conducted on the photocatalytic production of H2O2 via oxygen reduction reaction (ORR), with polymeric carbon nitride (PCN) emerging as a promising catalyst for this conversion. This review article is organized around two approaches. The first part main consists of the chemical optimization of the PCN structure, while the second focuses on the physical integration of PCN with other functional materials. The objective is to clarify the correlation between the physicochemical properties of PCN photocatalysts and their effectiveness in H2O2 production. Through a thorough review and analysis of the findings, this article seeks to stimulate new insights and achievements, not only in the domain of H2O2 production via ORR but also in other redox reactions.

Solvated Electrons Generated on the Surface of Na-SPHI for Boosting Visible Light Photocatalytic Hydrogen Evolution with Ultra-High AQE

Enhancing the utilization of visible-light-active semiconductors with an excellent apparent quantum efficiency (AQE) remains a significant and challenging goal in the realm of photocatalytic water splitting. In this study, a fully condensed sulfur-doped poly(heptazine imide) metalized with Na (Na-SPHI) is synthesized by an ionothermal method by using eutectic NaCl/LiCl mixture as the ionic solvent. Comprehensive characterizations of the obtained Na-SPHI reveal several advantageous features, including heightened light absorption, facilitated exciton dissociation, and expedited charge transfer. More importantly, solvated electron, powerful reducing agents, can be generated on the surface of Na-SPHI upon irradiation with visible light. Benefiting from above advantage, the Na-SPHI exhibits an excellent H2 evolution rate of 571.8 µmol·h-1 under visible light illumination and a super-high AQE of 61.7% at 420 nm. This research emphasizes the significance of the solvated electron on the surface of photocatalyst in overcoming the challenges associated with visible light-driven photocatalysis, showcasing its potential application in photocatalytic water splitting.

A novel 2D g-C3N4 material applied for Paraquat adsorbing and detoxifying in vitro and in vivo

In the environmental safety area, the widespread use of the herbicide Paraquat (PQ) poses a great threat to hydrobionts and mammals. Due to the lack of specific antidote, it may lead to irreversible pulmonary fibrosis with a mortality rate of 60%. Therefore, it is necessary to develop an effective and specific PQ antidote. The g-C3N4 (HPCN) with excellent surface physicochemical properties was prepared by a two-step calcination method using urea and dicyandiamide as raw materials, showing a significant photocatalyst against environmental PQ pollution. The SEM results showed that HPCN possesses a porous layered structure. X-ray diffraction and infrared spectroscopy indicated that the conjugated aromatic rings were orderly stacked, forming a 2D layered structure of g-C3N4. The HPCN had a larger specific surface area (56.84 m2 g-1) and pore volume (0.2718 cm3 g-1), which enhanced its adsorption capacity and photocatalytic activity. HPCN exhibited an effective adsorption rate of 38.25% for PQ in water under light. Compared with the PQ group (54.8%), the cell viability of the HPCN group (91.4%) significantly increased by 36.6%, and the SEM observation revealed the restoration of normal cell morphology. The HPCN effectively reduced PQ content in zebrafish and mice in vivo, resulting in an approximately 70% increase in survival rate. The UV-Vis results indicated that the adsorption rate of HPCN for PQ in zebrafish was 43.5%. The enhanced catalytic performance of HPCN provides a promising solution for the detoxification of PQ and of other environmental pollutants.

Graphitic Carbon Nitride as Photocatalyst for the Direct Formylation of Anilines

The use of graphitic carbon nitride (g-CN) for the photocatalytic radical formylation of anilines, which represents a more sustainable and attractive alternative to the currently used approaches, is reported herein. Our operationally simple method occurs under mild conditions, employing air as an oxidant. In particular, the chemistry is driven by the ability of g-CN to reach an electronically excited state upon visible-light absorption, which has a suitable potential energy to trigger the formation of reactive α-amino radical species from anilines. Mechanistic investigations also proved the key role of the g-CN to form reactive superoxide radicals from O2 via single electron transfer. Importantly, this photocatalytic transformation provides a variety of functionalized formamides (15 examples, up to 89 % yield).

Understanding the unique S-scheme charge migration in triazine/heptazine crystalline carbon nitride homojunction

Understanding charge transfer dynamics and carrier separation pathway is challenging due to the lack of appropriate characterization strategies. In this work, a crystalline triazine/heptazine carbon nitride homojunction is selected as a model system to demonstrate the interfacial electron-transfer mechanism. Surface bimetallic cocatalysts are used as sensitive probes during in situ photoemission for tracing the S-scheme transfer of interfacial photogenerated electrons from triazine phase to the heptazine phase. Variation of the sample surface potential under light on/off confirms dynamic S-scheme charge transfer. Further theoretical calculations demonstrate an interesting reversal of interfacial electron-transfer path under light/dark conditions, which also supports the experimental evidence of S-scheme transport. Benefiting from the unique merit of S-scheme electron transfer, homojunction shows significantly enhanced activity for CO₂ photoreduction. Our work thus provides a strategy to probe dynamic electron transfer mechanisms and to design delicate material structures towards efficient CO₂ photoreduction.

Hydrophobic surface efficiently boosting Cu2O nanowires photoelectrochemical CO2 reduction activity

The limited mass transfer of CO₂ and the competitive hydrogen evolution reaction (HER) during photoelectrochemical (PEC) CO₂ reduction usually result in low CO₂ reduction activity. Here, we constructed a Cu₂O/Sn/PTFE photocathode with a hydrophobic surface based on Cu₂O by physical vapor deposition and a dipping method. The CO faradaic efficiency (FE) increased from 34.5% (Cu₂O) to 95.1% (Cu₂O/Sn/PTFE) at -0.7 V vs. RHE, and the FE_H2 decreased from 27.9% (Cu₂O) to 3.8% (Cu₂O/Sn/PTFE). The introduction of the hydrophobic layer enhances the local CO₂ concentration on the electrode surface and effectively isolates H⁺ from the aqueous electrolyte, thereby enhancing the CO₂ reduction activity.

Nanoarchitecture Manipulation by Polycondensation on KCl Crystals toward Crystalline Lamellar Carbon Nitride for Efficient H2O2 Photoproduction

A lamellar carbon nitride (CN) framework is one of the most promising materials for solar-driven hydrogen peroxide production. The low dielectric constant of the organic CN framework leads to severe recombination of the excitons, and the photon-to-chemical conversion efficiency is thus unsatisfactory. In this work, by polycondensation of the small molecules on the KCl crystal surface, K⁺-incorporated crystalline CN (CNK) frameworks show significantly extended periodicity of the stacking layers and in-plane orderly organized heptazine/triazine units. The crystalline CNK frameworks exhibit a series of favorable photophysical properties, such as enhanced photon absorption, negatively shifted LUMO potentials, and attenuated emissive decay of the excitons. The CNK frameworks thus present remarkable performance in the photocatalytic selective oxygen reduction reaction for hydrogen peroxide production, e.g., CNK framework from the polycondensation of NH₄SCN on the KCl surface could produce hydrogen peroxide at a remarkable reaction rate of 26.7 mmol h^-1 g^-1 with a high apparent quantum yield of 25.0%, which is 23.5 times that on its counterpart synthesized in the absence of KCl. This method is generally applicable to all of the precursors for CN synthesis.

Flux-assisted synthesis of bismuth nanoparticle decorated carbon nitride for efficient photocatalytic degradation of endocrine disrupting compound

Traditional approaches to synthesizing bismuth nanoparticle decorated carbon nitride (C₃N₄) materials suffer from the complex synthesis process and the addition of a surfactant, which is not conducive to environmental protection. To address these problems, we adopted a simple and green flux-assisted approach for the first time to fabricate metallic bismuth nanoparticle decorated C₃N₄ (BiCCN). Electron microscopy results suggested that bismuth vanadate was converted into small bismuth nanoparticles via the flux-assisted approach. Highly dispersed Bi nanoparticles dramatically intensify light absorption, facilitate spatial charge separation as electron acceptors, shorten the charge diffusion length, and reserve more active sites for generating reactive species via surface photo-redox reactions. Consequently, the derived optimized photocatalyst BiCCN-15 rendered around 26 times higher photocatalytic degradation efficiency toward an endocrine disrupting compound (bisphenol A) than C₃N₄. This work provides a novel approach for developing non-precious metal decorated photocatalytic materials for sustainable water decontamination.

Facile Chemical Vapor Modification Strategy to Construct Surface Cyano-Rich Polymer Carbon Nitrides for Highly Efficient Photocatalytic H2 Evolution

The surface grafting of electro-negative cyano groups on polymer carbon nitrides (PCNs) is an effective way to tail their electronic structure. Despite the significant progress in the synthesis of cyano group-enriched PCN, developing a simple and efficient method remains challenging. Here, a facile strategy was developed for fabricating surface cyano-rich PCN (PCN-DM) with a porous structure via chemical vapor modification using diaminomaleonitrile. The cyano groups of diaminomaleonitrile substituted the amino groups on PCN surface via a deamination. The hydrogen production rate of the PCN-DM was approximately 17 times higher than that of pristine PCN. This significant increase in photocatalytic performance could be assigned to the fusion of cyano groups in the surface of PCN, forming new gap states that broadened the visible-light harvesting and accelerated charge separation for photoredox reactions. This study unveils a promising approach for incorporating functional units in the design of novel photocatalysts for efficient hydrogen production.

Constructing mesoporous Zr-doped SiO2 onto efficient Z-scheme TiO2/g-C3N4 heterojunction for antibiotic degradation via adsorption-photocatalysis and mechanism insight

Novel modified-TiO₂/Zr-doped SiO₂/g-C₃N₄ ternary composite is fabricated via an in-situ grow of porous Zr-SiO₂ layer to TiO₂/g-C₃N₄ heterojunction, which exhibits well adsorption-photocatalytic performance under simulated solar light irradiation. The nano-size mesoporous TiO₂ are dispersed on the lamellar g-C₃N₄, and the Zr-SiO₂ is in-situ fabricated onto the surface of g-C₃N₄ sheets. The adsorption occurs on the SiO₂ layers, and doping Zr element to SiO₂ enhances the adsorption of pollutants, while the photocatalytic reaction occurs on the valence band (VB) of TiO₂ and conduction band (CB) of g-C₃N₄, which gives reactive oxygen species of ∙O₂^-, h⁺, and ∙OH for high efficient decomposition of antibiotics, i.e. berberine hydrochloride (98.11%), tetracycline (80.76%), and oxytetracycline (84.84%). The excellent adsorption capacity and Z-scheme photoinduced charge carrier migration behavior endowed the novel material with enhanced berberine hydrochloride (BH) removal in water, which approximately 2.5 and 3.8 folds than that of pure g-C₃N₄ and sole TiO₂, respectively. Three degradation pathways are unraveled by LC-MS and theoretical calculations. Furthermore, the toxicity of intermediates was evaluated by the Toxicity Estimation Software Tool (T.E.S.T.), the result demonstrated a good application potential of M-TiO₂/Zr-SiO₂/g-C₃N₄ as an novel adsorptive photocatalyst.

Two-Dimensional Covalent Heptazine-Based Framework Enables Highly Photocatalytic Performance for Overall Water Splitting

Screening high-efficiency 2D conjugated polymers toward visible-light-driven overall water splitting (OWS) is one of the most promising but challenging research directions to realize solar-to-hydrogen (STH) energy conversion and storage. "Mystery molecule" heptazine is an intriguing hydrogen evolution reaction (HER) building block. By covalently linking with the electron-rich alkynyl and phenyl oxygen evolution reaction (OER) active units, 10 experimentally feasible 2D covalent heptazine-based frameworks (CHFs) are constructed and screened four promising visible-light-driven OWS photocatalysts, which are linked by p-phenyl (CHF-4), p-phenylenediynyl (CHF-7), m-phenylenediynyl (CHF-8), and phenyltriynyl (CHF-9), respectively. Their HER and OER active sites achieve completely spatially separated, where HER active sites focus on heptazine units and OER active sites located on alkynyl or phenyl units. Their lower overpotentials allow them to spontaneously trigger the surface OWS reaction under their own light-induced bias without using any sacrificial agents and cocatalysts. Among them, CHF-7 shows the best photocatalytic performance with an ideal STH energy conversion efficiency estimated at 12.04%, indicating that it is a promising photocatalyst for industrial OWS. This work not only provides an innovative idea for the exploration of novel polymer photocatalysts for OWS but also supplies a direction for the development of heptazine derivatives.

COF-5/CoAl-LDH Nanocomposite Heterojunction for Enhanced Visible-Light-Driven CO2 Reduction

Photocatalytic conversion of CO₂ into value-added chemical fuels is an attractive route to mitigate global warming and the energy crisis. Reasonable design of optical properties and electronic behavior of the photocatalyst are essential to improve their catalytic activity. Herein, the 1D/2D heterojunction by direct in-situ synthesis of the covalent organic framework (COF)-5 colloid on the surface of CoAl layered double hydroxide (LDH) was used as the prospective photocatalyst for CO₂ reduction. COF-5/CoAl-LDH nanocomposite achieved 265.4 μmol g^-1 of CO with 94.6 % selectivity over CH₄ evolution in 5 h under visible light irradiation, which was 4.8 and 2.3 times higher than those of COF-5 colloid and CoAl-LDH, respectively. The enhanced catalytic activity was derived from the increased visible-light activity and the construction of type II-2 heterojunction, which greatly optimized visible light harvesting and accelerated the efficient separation of the photoinduced holes and electrons. This work paves the way for rational design of heterojunction catalysts in photocatalytic CO₂ reduction.

The tremendous boost for photocatalytic properties of g-C3N4: regulation from polymerization kinetics to crystal structure engineering

Graphite carbon nitride (g-C3N4) has become research hotspot owing to its special electronic structure and excellent chemical stability. Although g-C3N4 has made great progress in the field of photocatalysis, its...