Photocatalytic Water Splitting Reaction Catalyzed by Ion-Exchanged Salts of Potassium Poly(heptazine imide) 2D Materials

Metal Poly(heptazine imides) as Multifunctional Photocatalysts for Solar Fuel Production

Solar-driven photocatalysis employing particulate semiconductors represents a promising approach for sustainable production of valuable chemical feedstock. Metal poly(heptazine imide) (MPHI), a novel 2D ionic carbon nitride, has been recognized as an emerging photocatalyst with distinctive properties. In this minireview, we first delineate the forefront innovations of MPHI photocatalysts, spanning from synthetic strategies and solving structures to the exploration of novel properties. We place special emphasis on the structural design principles aimed at developing high-performance MPHI systems toward photocatalytic solar fuel production such as H2 evolution, H2O oxidation, H2O2 production and CO2 reduction. Finally, we discuss crucial insights and challenges in leveraging highly active MPHIs for efficient solar-to-chemical energy conversion.

2D Janus ZrSSe/SnSSe Heterostructure: A Promising Candidate for Photocatalytic Water Splitting

The distinctive physical characteristics and wide range of potential applications in optoelectronic and photovoltaic devices have ignited significant interest in two-dimensional materials. Intensive research attention has been focused on Janus transition metal dichalcogenides due to their unique properties resulting from symmetry disruption and their potential in photocatalysis applications. Motivated by the current fascination with Janus TMD heterostructures, we conducted first-principles calculations to examine the stability, electronic, and optical properties of monolayers consisting of ZrSSe, SnSSe, and the ZrSSe/SnSSe heterostructure. The results indicate that the Janus ZrSSe/SnSSe heterostructure exhibits a structural and mechanical stability. Using the HSE06 functional, the ZrSSe/SnSSe heterostructure shows an indirect band gap of 1.20 eV, and band edge analysis reveals a type-II band alignment. The potential for photo/electrocatalysis in the ZrSSe/SnSSe heterostructure for water splitting or generating reactive oxygen species (ROS) has been explored, and it was found that the oxygen evolution reaction (OER) can spontaneously activate in acidic (pH = 0) media under light irradiation, with a potential of U = 1.82 eV. Additionally, the ZrSSe/SnSSe heterostructure exhibits strong light absorption across a wide range, from visible light to the ultraviolet region, at various levels. These findings open up possibilities for the application of ZrSSe/SnSSe-based materials in optoelectronic devices.

Single-Atoms on Crystalline Carbon Nitrides for Selective C─H Photooxidation: A Bridge to Achieve Homogeneous Pathways in Heterogeneous Materials

Single-atom catalysis is a field of paramount importance in contemporary science due to its exceptional ability to combine the domains of homogeneous and heterogeneous catalysis. Iron and manganese metalloenzymes are known to be effective in C─H oxidation reactions in nature, inspiring scientists to mimic their active sites in artificial catalytic systems. Herein, a simple and versatile cation exchange method is successfully employed to stabilize low-cost iron and manganese single-atoms in poly(heptazine imides) (PHI). The resulting materials are employed as photocatalysts for toluene oxidation, demonstrating remarkable selectivity toward benzaldehyde. The protocol is then extended to the selective oxidation of different substrates, including (substituted) alkylaromatics, benzyl alcohols, and sulfides. Detailed mechanistic investigations revealed that iron- and manganese-containing photocatalysts work through a similar mechanism via the formation of high-valent M═O species. Operando X-ray absorption spectroscopy (XAS) is employed to confirm the formation of high-valent iron- and manganese-oxo species, typically found in metalloenzymes involved in highly selective C─H oxidations.

Extent of carbon nitride photocharging controls energetics of hydrogen transfer in photochemical cascade processes

Graphitic carbon nitride is widely studied in organic photoredox catalysis. Reductive quenching of carbon nitride excited state is postulated in many photocatalytic transformations. However, the reactivity of this species in the turn over step is less explored. In this work, we investigate electron and proton transfer from carbon nitride that is photocharged to a various extent, while the negative charge is compensated either by protons or ammonium cations. Strong stabilization of electrons by ammonium cations makes proton-coupled electron transfer uphill, and affords air-stable persistent carbon nitride radicals. In carbon nitrides, which are photocharged to a smaller extent, protons do not stabilize electrons, which results in spontaneous charge transfer to oxidants. Facile proton-coupled electron transfer is a key step in the photocatalytic oxidative-reductive cascade - tetramerization of benzylic amines. The feasibility of proton-coupled electron transfer is modulated by adjusting the extent of carbon nitride photocharging, type of counterion and temperature.

Stability and Crystallinity of Sodium Poly(Heptazine Imide) in Photocatalysis

Poly(heptazine imide) (PHI) salts, as crystalline carbon nitrides, exhibit high photocatalytic activity and are being extensively researched, but its photochemical instability has not drawn researchers' attention yet. Herein, sodium PHI (PHI-Na) ultrathin nanosheets with increased crystallinity, synthesized by enhancing contact of melamine with NaCl functioning as a structure-induction agent and hard template, exhibits improved photocatalytic hydrogen evolution activity, but low photochemical stability, owing to Na+ loss in the photocatalytic process, which, interestingly, can be enhanced by the common ion effect, e.g., addition of NaCl that is also able to remarkably increase the photoactivity with the apparent quantum yield at 420 nm reaching 41.5 %. This work aims at attracting research peers' attention to photochemical instability of PHI salts and provides a way to enhance their crystallinity.

Potassium Poly(heptazine imide) Coupled with Ti3C2 MXene-Derived TiO2 as a Composite Photocatalyst for Efficient Pollutant Degradation

The photocatalytic degradation of pollutants is an effective and sustainable way to solve environmental problems, and the key is to develop an efficient, low-cost, and stable photocatalyst. Polymeric potassium poly(heptazine imide) (K-PHI), as a new member of the carbon nitride family, is a promising candidate but is characterized by a high charge recombination rate. To solve this problem, K-PHI was in-situ composited with MXene Ti₃C₂-derived TiO₂ to construct a type-II heterojunction. The morphology and structure of composite K-PHI/TiO₂ photocatalysts were characterized via different technologies, including TEM, XRD, FT-IR, XPS, and UV-vis reflectance spectra. Robust heterostructures and tight interactions between the two components of the composite were verified. Furthermore, the K-PHI/TiO₂ photocatalyst showed excellent activity for Rhodamine 6G removal under visible light illumination. When the weight percent of K-PHI in the original mixture of K-PHI and Ti₃C₂ was set to 10%, the prepared K-PHI/TiO₂ composite photocatalyst shows the highest photocatalytic degradation efficiency as high as 96.3%. The electron paramagnetic resonance characterization indicated that the^·OH radical is the active species accounting for the degradation of Rhodamine 6G.

Heptazine-Based Ordered-Distorted Copolymers with Enhanced Visible-Light Absorption for Photocatalytic Hydrogen Production

Poly(heptazine imide) (PHI), one of the attractive allotropes of polymeric carbon nitride, has recently received extensive attention in photocatalysis due to its extended conjugation for fast separation and transfer of the charges. However, pristine PHI bears an intrinsic optical absorption band edge at 460 nm, which largely restrains the visible light utilization. Herein, the narrow-bandgap PHI (N-PHI) with an ordered-distorted interface was fabricated from polycondensation of the mixture of NaSCN, cyanuric chloride, and LiCl. Results revealed that the enhanced optical absorption and the promoted separation and transfer of the charge carriers at the interface greatly improved the photocatalytic performance, which endowed N-PHI with an apparent quantum yield of 20 % for hydrogen production at 450 nm.

Spiroheterocyclic Photocatalyst for Reducing QHIn-Persistent Pollutants, Dyes, and Transition-Metal Ions Cocatalyzed with Electrolytes

The 7-nitro-2'-phenyl-5',6',7',7a'-tetrahydrospiro[indeno[1,2-b]quinoxaline-11,3'-pyrrolizine]-1',1'(2'H)-dicarbonitrile (SIQPI), 2'-(4-cyanophenyl)-7-nitro-5',6',7',7a'-tetrahydrospiro[indeno[1,2-b]quinoxaline-11,3'-pyrrolizine]-1',1'(2'H) dicarbonitrile (SIQPII), and 2'-(4-methoxyphenyl)-7-nitro-5',6',7',7a'-tetrahydrospiro[indeno[1,2-b]quinoxaline-11,3'-pyrrolizine]-1',1'(2'H)-dicarbonitrile (SIQPIII) were used to photocatalyze quinonoid phenolphthalein (QHIn) in aq-ACN-EtOH (mixed solvent) with NaCl and KCl electrolytes. SIQPI, II, and III spiroindenoquinoxaline pyrrolidines (SIQPs) as spiroheterocyclic photocatalysts alone could not reduce QHIn, but with the addition of electrolytes they are reduced via π cationic interactions (PCI). SIQPI, II, and III with NaCl reduced QHIn in 120, 28, and 50 min, unlike in 138, 58, and 63 min with KCl in mixed solvent. SIQPI, II, and III alone have reduced methylene blue (MB) in 120, 45, and 70 min, unlike in 110, 27, and 55 min with graphene oxide (GO), whereas with NaCl and KCl hey are reduced in 82, 36, and 44 min and 89, 43, and 50 min, respectively. SIQPs with GO had reduced MB in less time than the SIQPs alone, and SIQPs with NaCl had reduced QHIn in a shorter time than KCl. The electrolytes have cocatalyzed a reduction of dyes under sunlight (SL). The electrolytes have reduced a quinonoid structure (QS) and dyes by generating negative and positive (e ^- and h ⁺) holes in a shorter time. SIQPII and magnetic nanoparticles (MNPs) of 58 nm with NaCl photocatalyzed the QHIn in 2880 min. The SIQPs also reduced methyl orange (MO) and brilliant blue R (BBR) at variable temperature (T) and pH range, whereas SIQPs have developed a molecular organic framework (MOF) with transition-metal salts (NiCl₂, CrO₃, KMnO₄, CuSO₄, and MnCl₂) on photocatalysis.

Modified Poly(Heptazine Imides): Minimizing H2O2 Decomposition to Maximize Oxygen Reduction

Photocatalysis provides a sustainable pathway to produce the consumer chemical H2O2 from atmospheric O2 via an oxygen reduction reaction (ORR). Such an alternative is attractive to replace the cumbersome traditional anthraquinone method for H2O2 synthesis on a large scale. Carbon nitrides have shown very interesting results as heterogeneous photocatalysts in ORR because their covalent two-dimensional (2D) structure is believed to increase selectivity toward the two-electron process. However, an efficient and scalable application of carbon nitrides for this reaction is far from being achieved. Poly(heptazine imides) (PHIs) are a more powerful subgroup of carbon nitrides whose structure provides high crystallinity and a scaffold to host transition-metal single atoms. Herein, we show that PHIs functionalized with sodium and the recently reported fully protonated PHI exhibit high activity in two-electron ORR under visible light. The latter converted O2 to up to 1556 mmol L-1 h-1 g-1 H2O2 under 410 nm irradiation using inexpensive but otherwise chemically demanding glycerin as a sacrificial electron donor. We also prove that functionalization with transition metals is not beneficial for H2O2 synthesis, as the metal also catalyzes its decomposition. Transient photoluminescence spectroscopy suggests that H-PHIs exhibit higher activity due to their longer excited-state lifetime. Overall, this work highlights the high photocatalytic activity of the rarely examined fully protonated PHI and represents a step forward in the application of inexpensive covalent materials for photocatalytic H2O2 synthesis.

NiPS3 ultrathin nanosheets as versatile platform advancing highly active photocatalytic H2 production

High-performance and low-cost photocatalysts play the key role in achieving the large-scale solar hydrogen production. In this work, we report a liquid-exfoliation approach to prepare NiPS₃ ultrathin nanosheets as a versatile platform to greatly improve the light-induced hydrogen production on various photocatalysts, including TiO₂, CdS, In₂ZnS₄ and C₃N₄. The superb visible-light-induced hydrogen production rate (13,600 μmol h^-1 g^-1) is achieved on NiPS₃/CdS hetero-junction with the highest improvement factor (~1,667%) compared with that of pure CdS. This significantly better performance is attributed to the strongly correlated NiPS₃/CdS interface assuring efficient electron-hole dissociation/transport, as well as abundant atomic-level edge P/S sites and activated basal S sites on NiPS₃ ultrathin nanosheets advancing hydrogen evolution. These findings are revealed by the state-of-art characterizations and theoretical computations. Our work for the first time demonstrates the great potential of metal phosphorous chalcogenide as a general platform to tremendously raise the performance of different photocatalysts.

Calcium Poly(Heptazine Imide): A Covalent Heptazine Framework for Selective CO2 Adsorption

Potassium poly(heptazine imide) (KPHI) has recently garnered attention as a crystalline carbon nitride framework with considerable photoelectrochemical activity. Here, we report a Ca²⁺-complexed analogue of PHI: calcium poly(heptazine imide) (CaPHI). Despite similar polymer backbone, CaPHI and KPHI exhibit markedly different crystal structures. Spectroscopic, crystallographic, and physisorptive characterization reveal that Ca²⁺ acts as a structure-directing agent to transform melon-based carbon nitride to crystalline CaPHI with ordered pore channels, extended visible light absorption, and altered band structure as compared to KPHI. Upon acid washing, protons replace Ca²⁺ atoms in CaPHI to yield H⁺/CaPHI and enhance porosity without disrupting crystal structure. Further, these proton-exchanged PHI frameworks exhibit large adsorption affinity for CO₂ and exceptional performance for selective carbon capture from dilute streams. Compared to a state-of-the-art metal organic framework, UTSA-16, H⁺/CaPHI exhibits more than twice the selectivity (∼300 vs ∼120) and working capacity (∼1.2 mmol g^-1 vs ∼0.5 mmol g^-1) for a feed of 4% CO₂ (1 bar, 30 °C).

Asymmetric surfaces endow Janus bismuth oxyhalides with enhanced electronic and catalytic properties for the hydrogen evolution reaction

The electronic and catalytic properties of Janus bismuth oxyhalide (Bi₂O₂XY, where X/Y = Cl, Br, or I, and X ≠ Y) for the hydrogen evolution reaction (HER) are evaluated through first-principles calculations. Janus Bi₂O₂XY shows an enhanced separation efficiency of electron-hole pairs and an augmented utilization of solar energy due to Janus asymmetry. The asymmetric halogen surfaces on both sides of Janus Bi₂O₂XY induce an electrostatic potential difference, which leads to a staggered band alignment. The solar-to-hydrogen (STH) efficiencies of Janus Bi₂O₂BrI and Bi₂O₂ClI have greatly improved compared to those of pristine BiOBr and BiOCl. Additionally, Janus Bi₂O₂XY achieves stronger internal electric fields (IEFs) and a more suitable Gibbs free energy of hydrogen adsorption (ΔG_H) than pristine BiOX. Moreover, the halogen layer with a smaller electronegativity in Janus Bi₂O₂XY forms a stronger IEF with the oxygen layer; consequently, the ΔG_H of terminations value is closer to the ideal value for the HER. The localized edge states in the p-orbital density of states (DOS) projected onto O atoms are responsible for the HER activity of terminations. This work provides a comprehensive understanding of Janus Bi₂O₂XY for the HER and provides a strategy for improving photocatalysis.

Water on porous, nitrogen-containing layered carbon materials: The performance of computational model chemistries

Porous, layered materials containing sp2-hybridized carbon and nitrogen atoms, offer through their tunable properties, a versatile route towards tailormade catalysts for electrochemistry and photochemistry. A key molecule interacting with these...

Synthesis of Sulfonyl Chlorides from Aryldiazonium Salts Mediated by a Heterogeneous Potassium Poly(heptazine imide) Photocatalyst

Visible light photocatalysis is a tool in synthetic chemistry that allows us to utilize the energy of photons via photoinduced electron transfer to promote diverse organic reactions. Herein, a heterogeneous transition metal-free material, a type of carbon nitride photocatalyst, potassium poly(heptazine imide), is employed to produce sulfonyl chlorides from arenediazonium salts under mild conditions (visible light irradiation, room temperature) with 50-95% yields. The method is suitable for the synthesis of both electron rich and electron deficient compounds, and it shows high tolerance toward different functional groups (halides, ester, nitro, cyano groups). Thus, a sustainable photocatalytic alternative to the Meerwein chlorosulfonylation reaction is offered.