Understanding the Poly (Triazine Imide) Crystals Formation Process: The Conversion from Heptazine to Triazine.

Poly (triazine imide) (PTI) generally obtained via ionothermal synthesis features extended π-conjugation and enhanced crystallinity. However, in-depth investigation of the polycondensation process for PTI is an onerous task due to multiple influencing factors and limited characterization techniques. Herein, to simplify the polymerization route and exclude non-essential factors, PTI was prepared by calcining only melamine and LiCl. This study aims to identify the pivotal role of LiCl in PTI formation, which can convert heptazine-based intermediates into more stable triazine-based PTI framework. Based on this discovery, we demonstrate the transformation process of the prepared samples from amorphous Bulk g-C3 N4 to regular PTI, and further prove that the reaction with LiCl causes disruption of heptazine covalent organic frameworks. Additionally, the PTI exhibits higher photocatalytic water splitting performance due to efficient charge carrier mobility and separation, as well as faster reaction kinetics. This discovery deepens understanding of the polycondensation process of PTI crystals and provides insights toward the rational design of crystalline carbon nitride-based semiconductors.

In Situ Growth Facilitating the Piezo-Photocatalytic Effect of Zn1-xCdxS/ZnO Nanorods for Highly Efficient H2 Production.

Photocatalytic H2 production holds promise for alleviating energy and environmental issues. The separation of photoinduced charge carriers plays vital roles in enhancing the activity of photocatalytic H2 production. The piezoelectric effect has been proposed to be effective in facilitating the separation of charge carriers. However, the piezoelectric effect is usually restricted by the noncompact contact between the polarized materials and semiconductors. In this study, Zn1-xCdxS/ZnO nanorod arrays on stainless steel for piezo-photocatalytic H2 production are fabricated by an in situ growth method, achieving an electronic-level contact between Zn1-xCdxS and ZnO. The separation and migration of photogenerated charge carriers in Zn1-xCdxS are significantly improved by the piezoelectric effect induced by ZnO under mechanical vibration. Consequently, under solar and ultrasonic irradiation, the H2 production rate of Zn1-xCdxS/ZnO nanorod arrays achieves 20.96 µmol h-1 cm-2, which is 4 times higher than that under solar irradiation. Such a performance can be attributed to the synergies of the piezoelectric field of bent ZnO nanorods and the built-in electric field of the Zn1-xCdxS/ZnO heterostructure, which efficiently separate the photoinduced charge carriers. This study provides a new strategy to couple polarized materials and semiconductors for highly efficient piezo-photocatalytic H2 production.

Photocatalytic Z-scheme Overall Water Splitting: Insight into Different Optimization Strategies for Powder Suspension and Particulate Sheet Systems.

Achieving solar light-driven photocatalytic overall water splitting is the ideal and ultimate goal for solving energy and environment issues. Photocatalytic Z-scheme overall water splitting has undergone considerable development in recent years; specific approaches include a powder suspension Z-scheme system with a redox shuttle and a particulate sheet Z-scheme system. Of these, a particulate sheet has achieved a benchmark solar-to-hydrogen efficiency exceeding 1.1 %. Nevertheless, owing to intrinsic differences in the components, structure, operating environment, and charge transfer mechanism, there are several differences between the optimization strategies for a powder suspension and particulate sheet Z-scheme. Unlike a powder suspension Z-scheme with a redox shuttle, the particulate sheet Z-scheme system is more like a miniaturized and parallel p/n photoelectrochemical cell. In this review, we summarize the optimization strategies for a powder suspension Z-scheme with a redox shuttle and particulate sheet Z-scheme. In particular, attention has been focused on choosing appropriate redox shuttle and electron mediator, facilitating the redox shuttle cycle, avoiding redox mediator-induced side reactions, and constructing a particulate sheet. Challenges and prospects in the development of efficient Z-scheme overall water splitting are also briefly discussed.

Enhanced Photocatalytic Hydrogen Production of ZnIn2S4 by Using Surface-Engineered Ti3C2Tx MXene as a Cocatalyst.

Developing efficient and stable photocatalysts is crucial for photocatalytic hydrogen production. Cocatalyst loading is one of the effective strategies for improving photocatalytic efficiency. Here, Ti3C2Tx (Tx = F, OH, O) nanosheets have been adopted as promising cocatalysts for photocatalytic hydrogen production due to their metallic conductivity and unique 2D characterization. In particular, surface functionalized Ti3C2(OH)x and Ti3C2Ox cocatalysts were synthesized through the alkalization treatment with NaOH and a mild oxidation treatment of Ti3C2Fx, respectively. ZnIn2S4/Ti3C2Tx composites, which were fabricated by the in-situ growth of ZnIn2S4 nanosheets on the Ti3C2Tx surface, exhibited the promoted photocatalytic performance, compared with the parent ZnIn2S4. The enhanced photocatalytic performance can be further optimized through the surface functionalization of Ti3C2Fx. As a result, the optimized ZnIn2S4/Ti3C2Ox composite with oxygen functionalized Ti3C2Ox cocatalyst demonstrated excellent photocatalytic hydrogen evolution activity. The characterizations and density functional theory calculation suggested that O-terminated Ti3C2Ox could effectively facilitate the transfer and separation of photogenerated electrons and holes due to the formation of a Schottky junction, with the largest difference in work function between ZnIn2S4 and Ti3C2Ox. This work paves the way for photocatalytic applications of MXene-based photocatalysts by tuning their surface termination groups.

Construction of TiO2 /SrTiO3 Heterojunction Derived from Monolayer Ti3 C2 MXene for Efficient Photocatalytic Overall Water Splitting.

Construction of heterojunction at the atomic scale to ensure efficient charge separation for improvement of photocatalytic water splitting is challenging. Herein, a facile hydrothermal method has been applied for the in situ fabrication of TiO2 /SrTiO3 heterojunction, using the monolayer Ti3 C2 MXene as the template and reactant. It is found that the sample with the hydrothermal reaction time of 60 min exhibits the highest H2 evolution rate with the sacrificial reagent, due to the efficient charge separation of TiO2 /SrTiO3 heterojunction as Ti3 C2 derivative. In addition, the sample shows the best overall water splitting performance at a hydrothermal reaction time of 120 min, where TiO2 is nearly converted to SrTiO3 , due to the fast kinetic process and low structural defects of SrTiO3 . This work not only provides a simple strategy for the fabrication of heterojunction photocatalysts but also demonstrates the difference in optimization of half-reaction and overall water splitting reaction.

Piezoelectric built-in electric field advancing TiO2 for highly efficient photocatalytic air purification.

Photocatalytic air purification is a promising technology; however, it suffers from a limited rate of photocatalytic mineralization (easily inactivated surfactant sites of hydroxyls) and poor kinetics of degradation. Herein, we report a ferroelectric strategy, employing a polyvinylidene fluoride (PVDF) layer embedded with TiO2, where the polarization field of stretched PVDF dramatically enhances and stabilizes active adsorption sites for the promotion of charge separation. The F (-) and H (+) atomic layers with distinct local structures in stretched PVDF increase the electron cloud density around Ti which simultaneously promotes the dissociation of water to form hydroxyl groups which are easier to activate for adsorption of formaldehyde molecules. Besides, the ferroelectric field of stretched PVDF effectively separates the photogenerated charge carriers and facilitates the carriers' transportation of TiO2/PVDF. The optimal stretched TiO2/PVDF exhibits excellent photocatalytic mineralization for formaldehyde with considerable stability. This work may evolve the polarization field as a new method to enhance adsorption and activation of hydroxyls and disclose the mechanism by which hydroxyl radicals mineralize gaseous formaldehyde for photocatalytic air purification.

Priority Occupation of C-Sites by N-Confining P-Implantation in Pyrrodic N-Sites in NCNT@P,N-Mo2C for Highly Efficient Electrocatalytic Hydrogen Evolution.

Optimizing the electronic configuration of Mo2C by activating heteroatom(s)-neighboring carbon atoms to enhance the activity of hydrogen evolution reaction (HER) has been demonstrated. However, the development of heteroatom-doped Mo2C to fabricate a water electrolyzer is still a challenge because of the limitation of a well-defined electronic structure of hybridization of Mo with heteroatom(s). Here, nitrogen (N) and phosphor (P) codoped Mo2C embedded carbon nanotubes (NCNT@P,N-Mo2C) with the priority occupation of C-sites by N, which well confines the P-implantation at the pyrrodic N-sites and brings out N-O bonding on the surface, which favorably modifies the electronic configuration of adjacent Mo, resulting in highly efficient pH-tolerant HER activity. This study not only presents a potential HER electrocatalyst candidate but also provides a strategy for the construction of a well-defined electronic structure of heteroatom(s)-neighboring carbon-based materials.

High-yield production of liquid fuels in CO2 hydrogenation on a zeolite-free Fe-based catalyst.

Catalytic conversion of CO2 to long-chain hydrocarbons with high activity and selectivity is appealing but hugely challenging. For conventional bifunctional catalysts with zeolite, poor coordination among catalytic activity, CO selectivity and target product selectivity often limit the long-chain hydrocarbon yield. Herein, we constructed a singly cobalt-modified iron-based catalyst achieving 57.8% C5+ selectivity at a CO2 conversion of 50.2%. The C5+ yield reaches 26.7%, which is a record-breaking value. Co promotes the reduction and strengthens the interaction between raw CO2 molecules and iron species. In addition to the carbide mechanism path, the existence of Co3Fe7 sites can also provide sufficient O-containing intermediate species (CO*, HCOO*, CO3 2*, and ) for subsequent chain propagation reaction via the oxygenate mechanism path. Reinforced cascade reactions between the reverse water gas shift (RWGS) reaction and chain propagation are achieved. The improved catalytic performance indicates that the KZFe-5.0Co catalyst could be an ideal candidate for industrial CO2 hydrogenation catalysts in the future.

Facile synthesis of self-assembled ultrathin α-FeOOH nanorod/graphene oxide composites for supercapacitors.

A one-pot facile, impurity-free hydrothermal method to synthesize ultrathin α-FeOOH nanorods/graphene oxide (GO) composites is reported. It is directly synthesized from GO and iron acetate in water solution without inorganic or organic additives. XRD, Raman, FT-IR, XPS and TEM are used to characterize the samples. The nanorods in composites are single crystallite with an average diameter of 6nm and an average length of 75nm, which are significantly smaller than GO-free α-FeOOH nanorods. This can be attributed to the confinement effect and special electronic influence of GO. The influences of experimental conditions including reaction time and reactant concentration on the sizes of nanorods have been investigated. It reveals that the initial Fe2+ concentration and reaction time play an important role in the synthetic process. Furthermore, a possible nucleation-growth mechanism is proposed. As electrode materials for supercapacitors, the α-FeOOH nanorods/GO composite with 20% iron loading has the largest specific capacitance (127Fg-1 at 10Ag-1), excellent rate capability (100Fg-1 at 20Ag-1) and good cyclic performance (85% capacitance retention after 2000 cycles), which is much better than GO-free α-FeOOH nanorods. This unique structure results in rapid electrolyte ions diffusion, fast electron transport and high charging-discharging rate. In virtue of the superior electrochemical performance, the α-FeOOH nanorods/GO composite material has a promising application in high-performance supercapacitors.

Considerations of the Effects of Naphthalene Moieties on the Design of Proton-Conductive Poly(arylene ether ketone) Membranes for Direct Methanol Fuel Cells.

Novel sulfonated poly(arylene ether ketones) (SDN-PAEK-x), consisting of dual naphthalene and flexible sulfoalkyl groups, were prepared via polycondensation, demethylation, and sulfobutylation grafting reaction. Among them, SDN-PAEK-1.94 membrane with the highest ion exchange capacity (IEC = 2.46 mequiv·g(-1)) exhibited the highest proton conductivity, which was 0.147 S· cm(-1) at 25 °C and 0.271 S·cm(-1) at 80 °C, respectively. The introduction of dual naphthalene moieties is expected to achieve much enhanced properties compared to those of sulfonated poly(arylene ether ketones) (SNPAEK-x), consisting of single naphthalene and flexible sulfoalkyl groups. Compared with SNPAEK-1.60 with a similar IEC, SDN-PAEK-1.74 membrane showed higher proton conductivity, higher IEC normalized conductivity, and higher effective proton mobility, although it had lower analytical acid concentration. The SDN-PAEK-x membranes with IECs higher than 1.96 mequiv·g(-1) also exhibited higher proton conductivity than that of recast Nafion membrane. Furthermore, SDN-PAEK-1.94 displayed a better single cell performance with a maximum power density of 60 mW·cm(-2) at 80 °C. Considering its high proton conductivity, excellent single cell performance, good mechanical stabilities, low membrane swelling, and methanol permeability, SDN-PAEK-x membranes are promising candidates as alternative polymer electrolyte membranes to Nafion for direct methanol fuel cell applications.