Twinned Growth of Metal-Free, Triazine-Based Photocatalyst Films as Mixed-Dimensional (2D/3D) van der Waals Heterostructures

The Directional Crystallization Process of Poly (triazine imide) Single Crystals in Molten Salts

Poly (triazine imide) photocatalysts prepared via molten salt methods emerge as promising polymer semiconductors with one-step excitation capacity of overall water splitting. Unveiling the molecular conjugation, nucleation, and crystallization processes of PTI crystals is crucial for their controllable structure design. Herein, microscopy characterization was conducted at the PTI crystallization front from meso to nano scales. The heptazine-based precursor was found to depolymerize to triazine monomers within molten salts and KCl cubes precipitate as the leading cores that guide the directional stacking of PTI molecular units to form aggregated crystals. Upon this discovery, PTI crystals with improved dispersibility and enhanced photocatalytic performance were obtained by tailoring the crystallization fronts. This study advances insights into the directional assembling of PTI monomers on salt templates, placing a theoretical foundation for the ordered condensation of polymer crystals.

Constitutional isomerism of the linkages in donor–acceptor covalent organic frameworks and its impact on photocatalysis

When new covalent organic frameworks (COFs) are designed, the main efforts are typically focused on selecting specific building blocks with certain geometries and properties to control the structure and function of the final COFs. The nature of the linkage (imine, boroxine, vinyl, etc.) between these building blocks naturally also defines their properties. However, besides the linkage type, the orientation, i.e., the constitutional isomerism of these linkages, has rarely been considered so far as an essential aspect. In this work, three pairs of constitutionally isomeric imine-linked donor-acceptor (D-A) COFs are synthesized, which are different in the orientation of the imine bonds (D-C=N-A (DCNA) and D-N=C-A (DNCA)). The constitutional isomers show substantial differences in their photophysical properties and consequently in their photocatalytic performance. Indeed, all DCNA COFs show enhanced photocatalytic H₂ evolution performance than the corresponding DNCA COFs. Besides the imine COFs shown here, it can be concluded that the proposed concept of constitutional isomerism of linkages in COFs is quite universal and should be considered when designing and tuning the properties of COFs.

Systematic investigation of isomerism in covalent organic frameworks (COFs) can provide key insights into their properties. Here, the authors reveal that the constitutional isomerism of the linkage i.e., linkage orientations distinctly impact COFs’ structural and photophysical properties.

Covalent Triazine Frameworks with Defective Accumulation Sites: Exceptionally Modulated Electronic Structure for Solar-Driven Oxidative Activation of Peroxymonosulfate

Precisely tailoring the electronic structure and surface chemistry of metal-free covalent triazine frameworks (CTFs) for efficient photoactivation of oxyanions is environmentally desirable but still challenging. Of interest to us in this work was to construct artificial defective accumulation sites into a CTF network (CTF-SD_x) to synchronously modulate both thermodynamic (e.g., band structure) and kinetic (e.g., charge separation/transfer/utilization and surface adsorption) behaviors and probe how the transformation affected the subsequent activation mechanism of peroxymonosulfate (PMS). With the incorporation of terminal cyano (-CN) groups and boron (B) dopants, the delocalized CTF-SD underwent a narrowed electronic energy gap for increased optical absorption as well as a downshifted valence band position for enhanced oxidation capacity. Moreover, the localized charge accumulation regions induced by the electron-withdrawing -CN groups facilitated the exciton dissociation process, while the adjacent electron-deficient areas enabled strong affinity toward PMS molecules. All of these merits impelled the photoactivation reaction with PMS, and a 15-fold enhancement of bisphenol-A (BPA) removal was found in the CTF-SD₂/PMS/vis system compared with the corresponding pristine CTF system. Mechanistic investigations demonstrated that this system decomposed organics primarily through a singlet oxygen-mediated nonradical process, which originated from PMS oxidative activation over photoinduced holes initiated by an electron transfer process, thereby opening a new avenue for designing an efficient PMS activation strategy for the selective oxidation of organic pollutants.

Graphitic Aza-Fused π-Conjugated Networks: Construction, Engineering, and Task-Specific Applications

2D π-conjugated networks linked by aza-fused units represent a pivotal category of graphitic materials with stacked nanosheet architectures. Extensive efforts have been directed at their fabrication and application since the discovery of covalent triazine frameworks (CTFs). Besides the triazine cores, tricycloquinazoline and hexaazatriphenylene linkages are further introduced to tailor the structures and properties. Diverse related materials have been developed rapidly, and a thorough outlook is necessitated to unveil the structure-property-application relationships across multiple subcategories, which is pivotal to guide the design and fabrication toward enhanced task-specific performance. Herein, the structure types and development of related materials including CTFs, covalent quinazoline networks, and hexaazatriphenylene networks, are introduced. Advanced synthetic strategies coupled with characterization techniques provide powerful tools to engineer the properties and tune the associated behaviors in corresponding applications. Case studies in the areas of gas adsorption, membrane-based separation, thermo-/electro-/photocatalysis, and energy storage are then addressed, focusing on the correlation between structure/property engineering and optimization of the corresponding performance, particularly the preferred features and strategies in each specific field. In the last section, the underlying challenges and opportunities in construction and application of this emerging and promising material category are discussed.

Hybrid Inorganic-Organic Visible-Light-Driven Microrobots Based on Donor-Acceptor Organic Polymer for Degradation of Toxic Psychoactive Substances

Light-driven microrobots based on organic semiconductors have received tremendous attention in the past few years due to their unique properties, such as ease of reactivity tunability, band-gap modulation, and low cost. However, their fabrication with defined morphologies is a very challenging task that results in amorphous microrobots with poor motion efficiencies. Herein, we present hybrid inorganic-organic photoactive microrobots with a tubular shape and based on the combination of a mesoporous silica template with an active polymer containing thiophene and triazine units (named as Tz-Th microrobots). Owing to their well-defined tubular structure, such Tz-Th microrobots showed efficient directional motion under fuel-free conditions. Depending on the accumulation of the polymer coating, these microdevices also exhibited stand-up and rotation motion. As a proof-of-concept, we use these hybrid microrobots for the capture and degradation of toxic psychoactive drugs commonly found in wastewater effluents such as methamphetamine derivatives. We found that the microrobots were able to decompose the drug into small organic fragments after 20 min of visible light irradiation, reaching total intermediates removal after 2 h. Therefore, this approach represents a versatile and low-cost strategy to fabricate structured organic microrobots with efficient directional motion by using inorganic materials as the robot chassis, thereby maintaining the superior photocatalytic performance usually associated with such organic polymers.

Development of metal-free layered semiconductors for 2D organic field-effect transistors

To this day, the active components of integrated circuits consist mostly of (semi-)metals. Concerns for raw material supply and pricing aside, the overreliance on (semi-)metals in electronics limits our abilities (i) to tune the properties and composition of the active components, (ii) to freely process their physical dimensions, and (iii) to expand their deployment to applications that require optical transparency, mechanical flexibility, and permeability. 2D organic semiconductors match these criteria more closely. In this review, we discuss a number of 2D organic materials that can facilitate charge transport across and in-between their π-conjugated layers as well as the challenges that arise from modulation and processing of organic polymer semiconductors in electronic devices such as organic field-effect transistors.

Direct growth of crystalline triazine-based graphdiyne using surface-assisted deprotection-polymerisation

Graphdiyne polymers have interesting electronic properties due to their π-conjugated structure and modular composition. Most of the known synthetic pathways for graphdiyne polymers yield amorphous solids because the irreversible formation of carbon–carbon bonds proceeds under kinetic control and because of defects introduced by the inherent chemical lability of terminal alkyne bonds in the monomers. Here, we present a one-pot surface-assisted deprotection/polymerisation protocol for the synthesis of crystalline graphdiynes over a copper surface starting with stable trimethylsilylated alkyne monomers. In comparison to conventional polymerisation protocols, our method yields large-area crystalline thin graphdiyne films and, at the same time, minimises detrimental effects on the monomers like oxidation or cyclotrimerisation side reactions typically associated with terminal alkynes. A detailed study of the reaction mechanism reveals that the deprotection and polymerisation of the monomer is promoted by Cu(ii) oxide/hydroxide species on the as-received copper surface. These findings pave the way for the scalable synthesis of crystalline graphdiyne-based materials as cohesive thin films.

We present a one-pot deprotection/polymerisation protocol for the synthesis of crystalline graphdiynes on top of a copper surface starting with stable trimethylsilylated alkyne monomers.

The Prospect of Dimensionality in Porous Semiconductors

With the advent of silicon-based semiconductors, a plethora of previously unknown technologies became possible. The development of lightweight low-dimensional organic semiconductors followed soon after. However, the efficient charge/electron transfers enabled by the non-porous 3D structure of silicon is rather challenging to be realized by their (metal-)organic counterparts. Nevertheless, the demand for lighter, more efficient semiconductors is steadily increasing resulting in a growing interest in (metal-)organic semiconductors. These novel materials are faced with a variety of challenges originating from their chemical design, their packing and crystallinity. Although the effect of molecular design is quite well understood, the influence of dimensionality and the associated change in properties (porosity, packing, conjugation) is still an uncharted area in (metal-)organic semiconductors, yet highly important for their practical utilization. In this Minireview, an overview on the design and synthesis of porous semiconductors, with a particular emphasis on organic semiconductors, is presented and the influence of dimensionality is discussed.

Bi2O3/BiVO4@graphene oxide van der Waals heterostructures with enhanced photocatalytic activity toward oxygen generation

The van der Waals (vdW) integration enables to create heterostructures with intimate contact and bring new opportunities. However, it is not confined to layered materials but can also be generally extended to 3D materials. Multidimensional Bi₂O₃/BiVO₄@graphene oxide (GO) van der Waals heterostructures are synthesized by one-pot wet chemistry method. Bi₂O₃/BiVO₄ composite nanoparticles are self-assembled with GO framework by vdW interaction to form vdW heterostructures, in which GO framework allows short electron transport distance and rapid charge transfer and provides massive reactive sites. Such self-assembled heterostructures show a superior high photoactivity towards oxygen evolution with an enhanced oxygen generation rate of 1828 µmol h^-1 g^-1, nearly 3 times than that of pure BiVO₄, attributed to the accelerated charge separation and transfer processes of Bi₂O₃/BiVO₄@GO vdW heterostructures. This study indicates that our strategy provides a new avenue towards fabricating multi-dimensional vdW heterostructures and inspiring more innovative insights in oxygen evolution field.

Toward the synthesis, fluorination and application of N-graphyne

The discovery of properties and applications of unknown materials is one of the hottest research areas in materials science. In this work, we navigate a route towards these goals by the development of a new type of graphyne nanostructure. It is synthesised by a Sonogashira cross-coupling reaction of 1,3,5-triethynylbenzene with cyanuric chloride resulting in an extended carbon-based material called TCC. Also, we modify the obtained TCC via fluorination using XeF2 at various concentrations to investigate the effect of fluorination on the triple bonds and the conjugated structure of graphyne. In this study, we put special emphasis on the determination of the impact of the fluorine content and the type of CF functionalities on the morphology, chemical and electronic structure, biocompatibility, electrical conductivity and possible applicability as anode materials for Li-ion batteries. The obtained results indicate that the character of C-F bonds influences the final properties of fluorinated materials. The polar C-F bonds are preferable for cell proliferation while CF2 groups are most suitable for battery devices, however, the appearance of PTFE-like units may have a negative impact on battery specific capacitance as well as on cell viability.

Revisiting the Limiting Factors for Overall Water-Splitting on Organic Photocatalysts

In pursuit of inexpensive and earth abundant photocatalysts for solar hydrogen production from water, conjugated polymers have shown potential to be a viable alternative to widely used inorganic counterparts. The photocatalytic performance of polymeric photocatalysts, however, is very poor in comparison to that of inorganic photocatalysts. Most of the organic photocatalysts are active in hydrogen production only when a sacrificial electron donor (SED) is added into the solution, and their high performances often rely on presence of noble metal co-catalyst (e.g. Pt). For pursuing a carbon neutral and cost-effective green hydrogen production, unassisted hydrogen production solely from water is one of the critical requirements to translate a mere bench-top research interest into the real world applications. Although this is a generic problem for both inorganic and organic types of photocatalysts, organic photocatalysts are mostly investigated in the half-reaction, and have so far shown limited success in hydrogen production from overall water-splitting. To make progress, this article exclusively discusses critical factors that are limiting the overall water-splitting in organic photocatalysts. Additionally, we also have extended the discussion to issues related to stability, accurate reporting of the hydrogen production as well as challenges to be resolved to reach 10 % STH (solar-to-hydrogen) conversion efficiency.

Recent Advances in Conjugated Polymers for Visible-Light-Driven Water Splitting

With the ambition of solving the challenges of the shortage of fossil fuels and their associated environmental pollution, visible-light-driven splitting of water into hydrogen and oxygen using semiconductor photocatalysts has emerged as a promising technology to provide environmentally friendly energy vectors. Among the current library of developed photocatalysts, organic conjugated polymers present unique advantages of sufficient light-absorption efficiency, excellent stability, tunable electronic properties, and economic applicability. As a class of rising photocatalysts, organic conjugated polymers offer high flexibility in tuning the framework of the backbone and porosity to fulfill the requirements for photocatalytic applications. In the past decade, significant progress has been made in visible-light-driven water splitting employing organic conjugated polymers. The recent development of the structural design principles of organic conjugated polymers (including linear, crosslinked, and supramolecular self-assembled polymers) toward efficient photocatalytic hydrogen evolution, oxygen evolution, and overall water splitting is described, thus providing a comprehensive reference for the field. Finally, current challenges and perspectives are also discussed.

Al cluster oxide modified a hematite/P3HT ternary Z-scheme photocatalyst with excellent photocatalytic performance: A discussion of the mechanism

The Z-scheme photocatalyst is valuable for use in polluted water purification, and there is a need to develop a cheap and efficient photocatalyst. This study presents a new type of Z-scheme photocatalytic material composed of hematite and P3HT, also, the Al cluster oxides were first used as a charge transfer mediator to construct Z-scheme photocatalysis system. In addition, by analyzing the electron rearrangement phenomenon exhibited by the XPS spectrum and the experimental results of the in-situ illumination XPS, it is found that Al cluster is beneficial to the electron multi-step charge transfer process and Al cluster favors the construction of Z-scheme structure between hematite and P3HT. This Poly-3-hexylthiophene (P3HT)/Al cluster oxide/hematite ternary photocatalyst exhibited an excellent degradation effect on bisphenol A and tetracycline. This study demonstrates the feasibility of using the Al cluster as a charge mediator in the Z-scheme, providing new Z-scheme catalyst construction idea which is beneficial to the popularization and use of Z-scheme system in practical applications, and providing a new material for water pollution purification.

A novel immobilized Z-scheme P3HT/α-Fe2O3 photocatalyst array: Study on the excellent photocatalytic performance and photocatalytic mechanism

The Z-scheme photocatalyst is valuable for use in polluted water purification. To improve the practical application value of the Z-scheme, in this study, a nano α-Fe₂O₃ and P3HT composite photocatalyst array was synthesized by electrophoretic deposition for the first time. This material was named F1P. The EPR, VBXPS and in situ illumination XPS analyses indicated that the charge transfer path of F1P conforms to the Z-scheme, and F1P array has a high light energy conversion efficiency. Due to the advantages of the Z-scheme system and the good light harvesting ability, F1P exhibited a good degradation effect on different types of pollutants under visible light irradiation. The degradation efficiency of tetracycline and bisphenol A by F1P can reach 97 % and 99 %, respectively. Moreover, F1P showed a certain resistance to water quality changes. After four cycles, F1P can maintain the structural stability and good pollutant treatment effect. Due to the high redox ability of F1P, the complex structure of pollutants can be destroyed and the degradation path of pollutants in F1P system was also analyzed. This study provides a new way of synthesizing Z-scheme systems and immobilizing Z-scheme photocatalysts. F1P is a new material with high practicability.

Metal-free photocatalysts for hydrogen evolution

This article provides a comprehensive review of the latest progress, challenges and recommended future research related to metal-free photocatalysts for hydrogen productionviawater-splitting.

Porous Cationic Covalent Triazine-Based Frameworks as Platforms for Efficient CO2 and Iodine Capture

Porous cationic covalent triazine-based frameworks (CTFs) with imidazolium salts as tectons were prepared via ionothermal synthesis. The high-PF₆ ^- -content CTF shows the CO₂ adsorption of 44.7 cm³  g^-1 and I₂ capture capacity of 312 wt %. The influence of counterion species and contents on the porosities, CO₂ adsorptions, and I₂ capture capacities of the CTFs has been investigated.

A π-Conjugated, Covalent Phosphinine Framework

Structural modularity of polymer frameworks is a key advantage of covalent organic polymers, however, only C, N, O, Si, and S have found their way into their building blocks so far. Here, the toolbox available to polymer and materials chemists is expanded by one additional nonmetal, phosphorus. Starting with a building block that contains a λ5 -phosphinine (C5 P) moiety, a number of polymerization protocols are evaluated, finally obtaining a π-conjugated, covalent phosphinine-based framework (CPF-1) through Suzuki-Miyaura coupling. CPF-1 is a weakly porous polymer glass (72.4 m2  g-1 BET at 77 K) with green fluorescence (λmax =546 nm) and extremely high thermal stability. The polymer catalyzes hydrogen evolution from water under UV and visible light irradiation without the need for additional co-catalyst at a rate of 33.3 μmol h-1  g-1 . These results demonstrate for the first time the incorporation of the phosphinine motif into a complex polymer framework. Phosphinine-based frameworks show promising electronic and optical properties, which might spark future interest in their applications in light-emitting devices and heterogeneous catalysis.

Real-time optical and electronic sensing with a β-amino enone linked, triazine-containing 2D covalent organic framework

Fully-aromatic, two-dimensional covalent organic frameworks (2D COFs) are hailed as candidates for electronic and optical devices, yet to-date few applications emerged that make genuine use of their rational, predictive design principles and permanent pore structure. Here, we present a 2D COF made up of chemoresistant β-amino enone bridges and Lewis-basic triazine moieties that exhibits a dramatic real-time response in the visible spectrum and an increase in bulk conductivity by two orders of magnitude to a chemical trigger - corrosive HCl vapours. The optical and electronic response is fully reversible using a chemical switch (NH₃ vapours) or physical triggers (temperature or vacuum). These findings demonstrate a useful application of fully-aromatic 2D COFs as real-time responsive chemosensors and switches.

Synthesis and Applications of Graphdiyne-Based Metal-Free Catalysts

The development of carbon materials offers the hope for obtaining inexpensive and high-performance alternatives to substitute noble-metal catalysts for their sustainable application. Graphdiyne, the rising-star carbon allotrope, is a big family with many members, and first realized the coexistence of sp- and sp² -hybridized carbon atoms in a 2D planar structure. Different from the prevailing carbon materials, its nonuniform distribution in the electronic structure and wide tunability in bandgap show many possibilities and special inspirations to construct new-concept metal-free catalysts, and provide many opportunities for achieving a catalytic activity comparable with that of noble-metal catalysts. Herein, the recent progress in synthetic methodologies, theoretical predictions, and experimental investigations of graphdiyne for metal-free catalysts is systematically summarized. Some new perspectives of the opportunities and challenges in developing high-performance graphdiyne-based metal-free catalysts are demonstrated.

Direct functionalization of methane into ethanol over copper modified polymeric carbon nitride via photocatalysis

Direct valorization of methane to its alcohol derivative remains a great challenge. Photocatalysis arises as a promising green strategy which could exploit hydroxyl radical (·OH) to accomplish methane activation. However, both the excessive ·OH from direct H2O oxidation and the neglect of methane activation on the material would cause deep mineralization. Here we introduce Cu species into polymeric carbon nitride (PCN), accomplishing photocatalytic anaerobic methane conversion for the first time with an ethanol productivity of 106 μmol gcat-1 h-1. Cu modified PCN could manage generation and in situ decomposition of H2O2 to produce ·OH, of which Cu species are also active sites for methane adsorption and activation. These features avoid excess ·OH for overoxidation and facilitate methane conversion. Moreover, a hypothetic mechanism through a methane-methanol-ethanol pathway is proposed, emphasizing the synergy of Cu species and the adjacent C atom in PCN for obtaining C2 product.

Tuning the Porosity and Photocatalytic Performance of Triazine-Based Graphdiyne Polymers through Polymorphism

Crystalline and amorphous organic materials are an emergent class of heterogeneous photocatalysts for the generation of hydrogen from water, but a direct correlation between their structures and the resulting properties has not been achieved so far. To make a meaningful comparison between structurally different, yet chemically similar porous polymers, two porous polymorphs of a triazine-based graphdiyne (TzG) framework are synthesized by a simple, one-pot homocoupling polymerization reaction using as catalysts Cu^I for TzG_Cu and Pd^II /Cu^I for TzG_Pd/Cu . The polymers form through irreversible coupling reactions and give rise to a crystalline (TzG_Cu ) and an amorphous (TzG_Pd/Cu ) polymorph. Notably, the crystalline and amorphous polymorphs are narrow-gap semiconductors with permanent surface areas of 660 m²  g^-1 and 392 m²  g^-1 , respectively. Hence, both polymers are ideal heterogeneous photocatalysts for water splitting with some of the highest hydrogen evolution rates reported to date (up to 972 μmol h^-1  g^-1 with and 276 μmol h^-1  g^-1 without Pt cocatalyst). Crystalline order is found to improve delocalization, whereas the amorphous polymorph requires a cocatalyst for efficient charge transfer. This will need to be considered in future rational design of polymer catalysts and organic electronics.

Hydrophilicity-Controlled Conjugated Microporous Polymers for Enhanced Visible-Light-Driven Photocatalytic H2 Evolution

To take advantage of high surface area of network conjugated microporous polymers, four linear or network conjugated polymers L-PDBT, L-PDBT-O, N-PDBT, and N-PDBT-O are designed in terms of water-compatibility, and it turned out that microporous network N-PDBT-O exhibited the highest hydrogen evolution rate (HER) at 366 µmol h^-1 under visible light irradiation (λ > 420 nm, one of best reported pristine polymer-based photocatalysts), which is three times higher than the corresponding linear L-PDBT-O. Water contact angle measurements revealed that benzothiophene-sulfone-based conjugated polymers display better water compatibility and adsorption, and the synergic effect of better hydrophilic surface and higher surface area of N-PDBT-O might eventually lead to more exposed active sites in comparison to linear L-PDBT-O in the H₂ evolution suspension system. The hydrophilicity-controlled strategy could be applied to design of other network conjugated microporous polymer photocatalysts in an attempt to improve the activity.

Two-Dimensional versus Three-Dimensional Self-Assembly of a Series of 5-Alkoxyisophthalic Acids

Physisorbed self-assembled monolayers (SAMs) have been suggested as potential models for three-dimensional (3D) crystallization. This work studies the effect of altering the chain length of 5-alkoxyisophthalic acid (C _nISA) on self-assembled morphology in both two-dimensional (2D) and 3D to explore the extent comparisons can be drawn between dimensions. Previous studies of 5-alkoxyisophthalic acid at solid-liquid interfaces (2D) reported different morphologies for C₅ISA and C₆ISA-alkoxy chains on the one hand and C₁₀ISA and C₁₈ISA on the other. Independently, also in 3D a dependence of morphology on chain length has been reported, including an unexpected inclusion of a solvent in the 3D morphology of C₆ISA, while the previous reports of 2D self-assembly were driven only by molecule-molecule and molecule-substrate interactions. However, a complete set of data for comparison has been missing. Here, we report scanning tunneling microscopy (STM) and molecular dynamics simulations performed for C₂ISA self-assembled monolayers (SAMs) and STM imaging of C₆ISA-C₉ISA SAMs, to further examine self-assembly behavior in 2D. In 3D, X-ray diffraction analysis of C₂ISA single crystals was carried out to complete the data set. With a complete set of data, it was observed that regardless of the dimension, short-chain-length C _nISAs formed H-bonding-dominated structures, mid-chain-length C _nISAs exhibited solvent-dependent morphologies, and long-chain-length C _nISAs displayed van der Waals-dominated solvent-independent structures. However, the transition point among morphologies occurred at different chain lengths in 2D and 3D regardless of the dominant interaction. The results of this study inform the design of 2D films and guide the application of knowledge from physisorbed SAMs to 3D systems, including mixed-dimensional (2D/3D) van der Waals heterostructures.

Molecular engineering of polymeric carbon nitride: advancing applications from photocatalysis to biosensing and more

Different designs and constructions of molecular structures of carbon nitride for emerging applications, such as biosensing, are discussed.