Supercritical CH3OH-Triggered Isotype Heterojunction and Groups in g-C3N4 for Enhanced Photocatalytic H2 Evolution.

The structure tuning of bulk graphitic carbon nitride (g-C3N4) is a critical way to promote the charge carriers dynamics for enhancing photocatalytic H2-evolution activity. Exploring feasible post-treatment strategies can lead to effective structure tuning, but it still remains a great challenge. Herein, a supercritical CH3OH (ScMeOH) post-treatment strategy (250-300 °C, 8.1-11.8 MPa) is developed for the structure tuning of bulk g-C3N4. This strategy presented advantages of time-saving (less than 10 min), high yield (over 80%), and scalability due to the enhanced mass transfer and high reactivity of ScMeOH. During the ScMeOH post-treatment process, CH3OH molecules diffused into the interlayers of g-C3N4 and subsequently participated in N-methylation and hydroxylation reactions with the intralayers, resulting in a partial phase transformation from g-C3N4 into carbon nitride with a poly(heptazine imide)-like structure (Q-PHI) as well as abundant methyl and hydroxyl groups. The modified g-C3N4 showed enhanced photocatalytic activity with an H2-evolution rate 7.2 times that of pristine g-C3N4, which was attributed to the synergistic effects of the g-C3N4/Q-PHI isotype heterojunction construction, group modulation, and surface area increase. This work presents a post-treatment strategy for structure tuning of bulk g-C3N4 and serves as a case for the application of supercritical fluid technology in photocatalyst synthesis.

Base- or acid-assisted polystyrene plastic degradation in supercritical CO2.

Plastic has caused serious "white pollution" to the environment, and the highly inert characteristics of plastic bring a major challenge for degradation. Supercritical fluids have unique physical properties and have been widely used in various fields. In this work, supercritical CO2 (Sc-CO2) with mild conditions was selected and assisted by NaOH/HCl solution to degrade polystyrene (PS) plastic, and the reaction model was designed using response surface methodology (RSM). It was found that, regardless of the types of assistance solutions, the factors affecting PS degradation efficiencies were reaction temperature, reaction time, and NaOH/HCl concentration. At the temperature of 400 °C, time of 120 min, and base/acid concentration of 5% (in weight), 0.15 g PS produced 126.88/116.99±5 mL of gases with 74.18/62.78±5 mL of H2, and consumed 81.2/71.5±5 mL of CO2. Sc-CO2 created a homogeneous environment, which made PS highly dispersed and uniformly heated, thus promoting the degradation of PS. Moreover, Sc-CO2 also reacted with the degradation products to produce new CO and more CH4 and C2Hx (x=4, 6). Adding NaOH/HCl solution not only improved the solubility of PS in Sc-CO2, but also provided a base/acid environment that reduced the activation energy of the reaction, and effectively improved the degradation efficiencies of PS. In short, degrading PS in Sc-CO2 is feasible, and better results are obtained with the assistance of base/acid solution, which can provide a reference for the disposal of waste plastics in the future. Supplementary Information: The online version contains supplementary material available at 10.1007/s42768-023-00139-1.

Ultrathin porous graphitic carbon nitride from recrystallized precursor toward significantly enhanced photocatalytic water splitting.

Structure regulation (including electronic structure and morphology) for graphitic carbon nitride (g-C3N4) is an effective way to promote the photocatalytic activity. Herein, an ultrathin porous g-C3N4 (BCN-HT100) was synthesized by calcination of biuret hydrate. Hydrothermal treatment induced biuret recrystallization to form biuret hydrate precursor with regular morphology and large crystal size, thus promoting the polymerization of melem to form g-C3N4 network. Accordingly, BCN-HT100 possessed ultrathin nanosheet structure, higher polymerization degree, larger surface area and more pores than biuret-derived g-C3N4. BCN-HT100 behaved high-efficiency photocatalytic H2-productin activity with an apparent quantum yield (AQY) of 58.7% at 405 nm due to the enhanced utilization efficiency for photo-generated charge carriers and abundant reactive sites. Furthermore, Pt-NiCo2O4 dual cocatalysts were employed on BCN-HT100 for achieving photocatalytic overall water splitting, and the AQY reached 4.9% at 405 nm. This work provides a meaningful reference to designing g-C3N4 to achieve efficient solar energy conversion into hydrogen.

Ultrafast Electron Transfer in InP/ZnSe/ZnS Quantum Dots for Photocatalytic Hydrogen Evolution.

InP/ZnS core/shell quantum dots have shown extraordinary application potential in photocatalysis. In this work, we demonstrated by ultrafast spectroscopy that the electron transfer ability of InP/ZnSe/ZnS core/shell/shell quantum dots was better than that of InP/ZnS quantum dots, because the introduction of ZnSe midshell resulted in improved passivation and greater exciton delocalization. The temperature-dependent PL spectra indicate that the exciton-phonon coupling strength and exciton binding energy of InP/ZnSe/ZnS quantum dots are smaller than those of InP/ZnS quantum dots. Further photocatalytic hydrogen evolution testing revealed that the photocatalytic activity of InP/ZnSe/ZnS quantum dots was significantly higher than that of InP/ZnS quantum dots, and InP/ZnSe/ZnS quantum dots even demonstrated improved stability. This research deepened our understanding of carrier dynamics and charge separation of InP/ZnSe/ZnS quantum dots, especially highlighting the application potential of InP/ZnSe/ZnS quantum dots in photocatalytic hydrogen evolution.

EDTA-dominated hollow tube-like porous graphitic carbon nitride towards enhanced photocatalytic hydrogen evolution.

Graphite carbon nitride (g-C3N4) as metal-free photocatalyst has been widely studied recently in photocatalytic water reduction, which is considered as one of the promising routes to realizing the hydrogen energy-based society in the future. The generally used preparation process based on thermal polymerization of precursors easily brought the formation of aggregated nanosheets morphology, severely limiting its photocatalytic activity. Herein, the hollow tube-like morphology with porous surface was elaborately obtained by ethylene diamine tetraacetic acid (EDTA)-involved hydrothermal treatment of melamine precursor. The hollow and porous features shortened the migration distance of photo-generated carriers, trapped the incident lights, and provided more photocatalytic reactive sites, then realizing the enhanced photocatalytic H2-evolution activity up to 7.1 times that of pristine g-C3N4. The presence of EDTA acted as the pivotal role to control the recrystallization process of melamine and its derivative, cyanuric acid, and thus to determine the framework formation of the hollow tube-like microstructure. Moreover, complete thermal decomposition of cyanuric acid during the thermal polymerization of precursors was responsible for the hollow and porous features. This work extends the morphology regulation cognition of g-C3N4 based on hydrothermal treatment of precursors, and is expected to bring deep understanding and feasible strategies to design morphology-dominated highly-efficient g-C3N4 photocatalysts.

Bridging regulation in graphitic carbon nitride for band-structure modulation and directional charge transfer towards efficient H2 evolution under visible-light irradiation.

The bridging N atom in g-C3N4 structure plays a decisive role in photo-generated charge transfer because it usually confines photo-generated electrons and holes in each heptazine, thus leading to severe recombination. In this work, a kind of 2-aminoterephthalic acid-derived benzene ring group with rich π-electrons was considered to integrate with bridging N to break the above-mentioned confining effect. On the basis of density-functional theory calculations and experimental analysis, this 2-aminoterephthalic acid-derived bridging structure facilitated to draw photo-generated charge out of heptazine unit, and its polarized asymmetric structure promoted the directional transfer of photo-generated charge carriers across adjacent heptazines, thus efficiently reducing the recombination. Meanwhile, the 2-aminoterephthalic acid-derived bridging structure also reinforced the connectivity of heptazine units in g-C3N4 framework and led to high degree of polymerization, which thus extended the π-conjugated electronic system of g-C3N4 and modulated the band structure favoring photocatalytic hydrogen production. Consequently, a high photocatalytic H2-production activity of 24,595 µmol h-1 gcat-1 was achieved on the bridging regulated g-C3N4 under visible light, with an apparent quantum yield of 48.7% at 425 nm.