An Improved Metal-to-Ligand Charge Transfer Mechanism for Photocatalytic Hydrogen Evolution

Atomistic understanding of enhanced selectivity in photocatalytic oxidation of benzyl alcohol to benzaldehyde using graphitic carbon nitride loaded with single copper atoms

The loading of graphitic carbon nitride (gCN) with transition metals has received significant attention for efficient light-driven catalysis. However, the contribution of the loaded metals to enhanced performance remains unclear. In this study, Cu is loaded onto gCN to understand how photocatalytic activity is regulated by the loaded metals. Loading gCN with 3 wt% of Cu increases the electron population by 8.1 and 4.6 times under UV (λ < 370 nm) and visible light (390 < λ < 740 nm), respectively. This sample shows nearly 100% selectivity for oxidizing benzyl alcohol to benzaldehyde and a high yield-to-power ratio, reaching 0.35 mmol g-1 h-1 W-1. The loaded Cu species exist as single atoms with a +1-oxidation state. Each Cu+ cation is coordinated to two (at 3 wt% Cu) or four (at 6 wt% Cu) N atoms within the cavity of the gCN framework. Doubling the Cu loading results in a smaller electron population and coordinatively more saturated Cu+ cations, making it catalytically less reactive. Ab initio molecular dynamics simulations show that Cu+ cations produce filled mid-gap states above the valence band, which function as hole traps and hence oxidation centers. The Cu+ cation and the neighboring N atoms are electron-depletion and electron-accumulation sites due to Cu → N electron transfer, making it highly reactive for oxidative transformations via the hole transfer pathway. The role of Cu as a hole-transfer site updates the received understanding that surface-loaded Cu serves as an electron-accumulation site. A strong correlation is observed between the electron population at steady-state and the product yield, indicating that it could serve as a promising performance indicator for the design of future photocatalysts.

Ag-based coordination polymer-enhanced photocatalytic degradation of ciprofloxacin and nitrophenol

Transition-metal coordination complexes have attracted wide attention in molecular chemistry, but their applications still confront a tremendous challenge. Herein, a novel silver coordination polymer with a formula of {[Ag9(TIPA)6](NO3)9·12H2O}n (Ag-TIPA) was prepared by a solvothermal reaction of silver nitrate with triangular tris(4-imidazolylphenyl)amine (TIPA). The crystalline molecular structure was determined by single-crystal X-ray diffraction, which showed that each Ag(I) was coordinated with two nitrogen atoms of TIPA ligands. Such Ag-TIPA was used as a catalyst for the photodegradation of ciprofloxacin and 4-nitrophenol under UV-visible light irradiation. The results exhibited excellent photocatalytic performance and reusability due to high structure stability in an acidic, neutral and alkaline environment. The experimental findings and density functional theory calculations revealed that metal-ligand charge transfer in Ag-TIPA extended the absorption range of light and improved the charge transfer properties of TIPA. To further understand the photodegradation process, the intermediates were predicted and analysed through electrostatic potential, orbital weighted dual descriptor, and liquid chromatography-mass spectrometry techniques. Based on these findings, a possible degradation mechanism was proposed. This study provides new insights into the design and synthesis of Ag-based coordination polymers with novel structures, excellent catalytic activity, and good durability.

Ultrafast plasma method allows rapid immobilization of monatomic copper on carboxyl-deficient g-C3N4 for efficient photocatalytic hydrogen production

Transition-metal monometallic photocatalysts have received extensive attention owing to the maximization of atomic utilization efficiency. However, in previous related works, single-atom loading and stability are generally low due to limited anchor sites and mechanisms. Recently, adding transition-metal monatomic sites to defective carbon nitrides has a good prospect, but there is still lack of diversity in defect structures and preparation techniques. Here, a strategy for preparing defect-type carbon-nitride–coupled monatomic copper catalysts by an ultrafast plasma method is reported. In this method, oxalic acid and commercial copper salt are used as a carboxyl defect additive and a copper source, respectively. Carbon nitride samples containing carboxyl defects and monatomic copper can be processed within 10 min by one-step argon plasma treatment. Infrared spectroscopy and nuclear magnetic resonance prove the existence of carboxyl defects. Spherical aberration electron microscopy and synchrotron radiation analysis confirm the existence of monatomic copper. The proportion of monatomic copper is relatively high, and the purity is high and very uniform. The Cu PCN as-prepared shows not only high photo-Fenton pollutant degradation ability but also high photocatalytic hydrogen evolution ability under visible light. In the photocatalytic reaction, the reversible change of Cu⁺/Cu²⁺ greatly promotes the separation and transmission of photogenerated carriers and improves the utilization of photoelectrons. The photocatalytic hydrogen evolution rate of the optimized sample is 8.34 mmol g⁻¹·h⁻¹, which is 4.54 times that of the raw carbon nitride photocatalyst. The cyclic photo-Fenton experiment confirms the catalyst has excellent repeatability in a strong oxidation environment. The synergistic mechanism of the photocatalyst obtained by this plasma is the coordination of single-atom copper sites and carboxyl defect sites. The single copper atoms incorporated can act as an electron-rich active center, enhancing the h+ adsorption and reduction capacity of Cu PCN. At the same time, the carboxyl defect sites can form hydrogen bonds to stabilize the production of hydrogen atoms and subsequently convert them to hydrogen because of the unstable hydrogen bond structure. This plasma strategy is green, convenient, environment-friendly, and waste-free. More importantly, it has the potential for large-scale production, which brings a new way for the general preparation of high-quality monatomic catalysts.

Construction of 2D layered phosphorus-doped graphitic carbon nitride/BiOBr heterojunction for highly efficient photocatalytic disinfection

Infectious diseases caused by bacteria intimidate the health of human beings all over the world. Although many avenues have been tried, various operating conditions limit their actual applications. Photocatalytic nanomaterials are becoming candidates to be competent for water purification. Here, a novel and more efficient S-scheme has been engineered between two dimensional (2D) layered phosphorus-doped graphitic carbon nitride (P-g-C 3 N 4 ) and BiOBr via hydrothermal polymerization to inhibit the recombination of charge and broaden light absorption. The as-prepared P-g-C 3 N 4 /BiOBr hybrids exhibits significantly improved photocatalytic disinfection contrast to g-C 3 N 4 /BiOBr in visible wavelengths, suggesting phosphorus doping which adjusts the band structure plays a significant role in the S-scheme system. And the sterilization rate of multidrug-resistant Acinetobacter baumannii 28 ( AB 28 ) was 99.9999% within 80 min and Staphylococcus aureus ( S. aureus ) was 99.9%.

Single Copper Atoms Enhance Photoconductivity in g-C3N4

Graphitic carbon nitride (g-C₃N₄) and its doped analogues have been studied over the past decade in part due to their promising applications in heterogeneous photocatalysis; however, the effect of doping on the photoconductivity is poorly understood. Herein, we investigate Cu doped g-C₃N₄ (Cu-g-C₃N₄) and demonstrate via extended X-ray absorption fine structure that Cu⁺ incorporates as an individual ion. Time-resolved optical pump terahertz probe spectroscopy was utilized to measure the ultrafast photoconductivity in response to a 400 nm pump pulse and showed that the Cu⁺ dopant significantly enhances photoconductivity of the as-prepared powdered sample, which decays within 10 ps. Furthermore, a film preparation technique was applied that further enhanced the photoconductivity and induced a longer-lived photoconductive state with a lifetime on the order of 100 ps. This study provides valuable insight into the ultrafast photoconductivity dynamics of g-C₃N₄ materials, which is essential toward developing efficient g-C₃N₄ photocatalysts.

A new Ni-diaminoglyoxime-g-C3N4 complex towards efficient photocatalytic ethanol splitting via a ligand-to-metal charge transfer (LMCT) mechanism

We report a novel Ni-diaminoglyoxime-g-C₃N₄ (Ni-DAG-CN) complex for H₂ evolution through photocatalytic ethanol splitting. Compared to that of pristine g-C₃N₄, Ni-DAG-CN exhibits a 21-fold enhancement of photocatalytic activity (296.1 μmol h^-1 g^-1) under irradiation with excellent stability. The enhanced photocatalytic activity can be attributed to a proposed ligand-to-metal charge transfer (LMCT) mechanism, which is illustrated both experimentally and theoretically. This work provides great potential in the future design of low-cost, high-performance photocatalysts for H₂ evolution from alcohol splitting.

Design of C3 N4 -Based Hybrid Heterojunctions for Enhanced Photocatalytic Hydrogen Production Activity

Semiconductors and metals can form an Ohmic contact with an electric field pointing to the metal, or a Schottky contact with an electric field pointing to the semiconductor. If these two types of heterojunctions are constructed on a single nanoparticle, the two electric fields may cause a synergistic effect and increase the separation rate of the photogenerated electrons and holes. Metal Ni and Ag nanoparticles were successively loaded on the graphitic carbon nitride (g-C₃ N₄ ) surface by precipitation and photoreduction in the hope of forming hybrid heterojunctions on single nanoparticles. TEM/high-resolution TEM images showed that Ag and Ni were loaded on different locations on C₃ N₄ , which indicated that during the photoreduction reaction Ag⁺ obtained electrons from C₃ N₄ in the reduction reaction, whereas oxidation reactions proceeded on Ni nanoparticles. Photocatalytic hydrogen production experiments showed that C₃ N₄ -based hybrid heterojunctions can greatly improve the photocatalytic activity of materials. The possible reason is that two heterojunctions could form a long-range electric field similar to the p-i-n structure in semiconductors. Most of the photogenerated carriers were generated and then separated in this electric field, thereby increasing the separation rate of electrons and holes. This further improved the photocatalytic activity of C₃ N₄ .