Abstract Tin-based compounds, particularly SnO2-derived catalysts, are extensively studied for selective electrochemical reduction of carbon dioxide (eCO2RR) to formate. Compared with tin oxides, tin chalcogenides such as SnTe remain relatively unexplored for eCO2RR, in spite of having desired electronic properties, often combined with native surface oxide layers. In this work, we report the catalytic behavior of finely powdered polycrystalline SnTe showing high activity toward CO2 reduction as well as the hydrogen evolution reaction (HER). We show that SnTe exhibits selective eCO2RR toward formate in 0.5 M CsHCO3, with a partial current density of −35 mA cm–2 at −1.1 V vs RHE, similar to SnO2. Concurrently, SnTe exhibits high activity toward HER, in contrast to SnO2. Comprehensive potential-dependent structural characterizations and operando SEIRAS measurements suggest that the chemical transformation of SnTe and SnO2 to a Sn-rich active surface under high reductive potential may be the reason for their similar eCO2RR activity. On the other hand, control experiments on elemental Te and SnO2 as well as XPS and operando SEIRAS data point to a possible role of residual tellurium on the surface of the SnTe precatalyst to drive the HER. This work underscores the significance of understanding the in situ transformation of the precatalyst to the active species during the eCO2RR to rationalize its activity and product selectivity.

The selective and sustainable synthesis of epoxides remains a critical challenge in chemical industry, as conventional oxidation routes often rely on transition metals, hazardous oxidants, and energy-intensive conditions, resulting in poor atom economy and limited sustainability. Herein, we report a transition-metal-free photo-catalyst system capable of promoting olefin epoxidation under mild conditions. Poly(triazine imide) (PTI), a highly crystalline carbon nitride, achieves near-quantitative styrene conversion with good selectivity toward styrene oxide under visible-light irradiation, using molecular oxygen (O2) as the sole oxidant and without the need for stoichiometric oxidants, chemical additives and harsh conditions typical of traditional protocols. A comprehensive structural and optoelectronical characterization was performed to correlate catalytic performance with the intrinsic properties of the material. Furthermore, mechanistic investigations were conducted to identify the key reactive species involved in the reaction. In situ EPR spin-trapping experiments reveal the cooperative involvement of photogenerated charge carriers and radical superoxide (O2 center dot-), driving the selective epoxidation process and singlet oxygen as the responsible for the byproducts. Overall, these findings highlight PTI as an efficient metal-free photocatalyst for O2-driven olefin epoxidation and demonstrate the potential of crystalline poly(triazine imide) materials as promising platforms for sustainable oxidation processes.

ABSTRACT Developing strategies to enhance the utilization efficiency of catalytic sites in molecular catalysts has garnered increasing research interest in the field of molecular heterogeneous catalysis. The primary challenges in achieving this goal lie in the aggregation‐induced site inaccessibility in molecular catalysts. Here, we present the synthesis of covalent organic framework‐carbon nanotube (COF‐CNT) core‐shell nanohybrids as a platform to improve the site utilization of molecular catalysts in electrochemical CO 2 reduction. COF shells with a thickness of 50–80 nm are uniformly grown on CNTs, ensuring a well‐defined morphology with pores oriented perpendicularly to the CNT basal plane. The incorporation of molecular catalysts with COF‐CNT nanohybrids enables their application as scaffolds in the electrochemical CO 2 reduction. The best‐performing sample exhibits a two‐orders‐of‐magnitude increase in CO turnover frequency (TOF) compared to both pristine CoTPyP molecular catalyst and COF‐366‐Co, thus underscoring the effectiveness of the COF‐CNT hybrid structure in optimizing catalytic site accessibility. The enhanced site utilization is further validated in other molecular catalyst systems, where exceptionally high TOF values—among the highest reported to date for electrochemical CO 2 ‐to‐CO conversion—were achieved. Collectively, this study establishes COF‐CNT nanohybrids as a promising strategy for advancing COF‐based electrocatalysts and facilitating molecular catalyst applications in electrochemical energy conversion.

Utilizing bifunctional materials capable of simultaneous light harvesting and charge storage presents a compelling direct light-to-charge (electron/hole) storage strategy to circumvent the intermittency of solar energy, thereby enabling reliable power supply under dark conditions. Most of the bifunctional materials reported focus on metal oxides and metal-organic compounds. In contrast, only a few fully organic, metal-free systems exhibiting such bifunctionality have been reported, despite their inherent advantages of rich structural diversity and facile structural tunability, which enable systematic structure–property–function investigations. Here, we report a two-dimensional (2D) 2,2’-bipyridine (bpy) based crystalline covalent triazine framework (CTF), CTF-Bpy, that reversibly stores photogenerated electrons via one-electron reduction of the bpy units. Mechanistic studies suggest that the reduction of the bpy units in CTF-Bpy proceeds via a proton-couped electron transfer (PCET) process, yielding bpyH • as the predominant reduction product within the framework. Photoanodes based on CTF-Bpy show a stable photopotential of –0.92 V vs Ag/AgCl (–0.72 V vs NHE), which is so far the most negative photopotential realized in metal-free bifunctional materials. Under oxygen-free conditions, photogenerated electrons remain stable and can be controllably released on-demand in the dark, either by electrical discharge (for aqueous solar battery applications) or to reduce O 2 to H 2 O 2 . Moreover, this work also establishes CTF-Bpy as the first photochargeable covalent organic framework (COF) and CTF with trapped electrons sufficiently reducing to drive H 2 evolution in the dark, highlighting the promise of using COFs/CTFs for dark photocatalysis. The systematic investigation of the light-induced charge storage properties of CTF-Bpy with a set of spectroscopic and spectro-electrochemical techniques provides a framework for studying optoionic materials and to guide their rational design for tailored dark photocatalysis.

The electrochemical reduction of CO2 to valuable chemicals and fuels is a promising strategy for mitigating climate change, but it requires the development of efficient and selective catalysts. We report the synthesis and characterization of a series of Cu x Al1-layered double hydroxides (LDH) (x = 1, 1.5, 2, 3), yielding a robust structural model of Cu1Al1-LDH. Combining structural, elemental, and thermal investigations leads to a precise determination of the LDH's compositions as well as an understanding of the thermal decomposition of the LDHs into (mixed-) metal oxides. During the electrochemical CO2 reduction reaction (CO2RR), we observe an operando reduction to catalytically active copper dendrites. The Cu:Al ratio is found to play a crucial role in determining the size and activity of these dendrites, enabling a tunable catalyst system with Faradaic efficiencies (FEs) for formic acid (HCOOH) reaching up to 30.5% and FE for syngas (hydrogen and carbon monoxide) of up to 72%. This study demonstrates the potential of CuAl-LDH phase engineering as a cheap and easily accessible pathway for the development of efficient electrochemical CO2 reduction catalysts.

Covalent organic frameworks (COFs) have been developed as photosensitizers for photocatalytic energy conversion over the past decade; however, COF photocatalysts have yet to demonstrate the ability to harvest near-infrared light (above 760 nm, approximately 53% of the solar spectrum) for fuel or chemical conversion. In this work we introduce a post-synthetic functionalization strategy for COFs by incorporating a palladacycle directly into the COF backbone, extending the light absorption of an azobenzene-based COF into the near-infrared region. This approach enables homogeneous, atomically distributed palladium functionalization with a high loading of 12 wt% and without noticeable formation of palladium nanoparticles. The cyclopalladated COF, TpAzo-CPd, was used as a catalyst for photocatalytic hydrogen peroxide production under 810 nm illumination. This study demonstrates the use of COFs for near-infrared photocatalysis and opens the door to palladium-single-site COF catalysts for a wide range of chemical transformations.

Tin-based compounds, particularly SnO2-derived catalysts, are extensively studied for selective electrochemical reduction of CO2 (eCO2RR) to formate. As compared to Sn oxides, chalcogenides such as SnTe are relatively unexplored in the domain of eCO2RR, in spite of having desired electronic properties, often combined with native surface oxide layers. In this work, we report dual catalytic behavior of finely powdered polycrystalline SnTe showing high activity towards CO2 reduction as well as the hydrogen evolution reaction (HER). We show that SnTe exhibits selective eCO2RR to formate with partial faradic current densities of −35 mAcm-2 at −1.1 V vs. RHE in 0.5 M CsHCO3 solution, similar to SnO2. Concurrently, SnTe exhibits high activity towards HER, in contrast to SnO2. Comprehensive potential dependent structural characterizations and SEIRAS measurements suggest that the chemical transformation of SnTe and SnO2 to reduced Sn under high reductive potentials may be the reason for their similar eCO2RR activity. On the other hand, control experiments on elemental Te and SnO2 as well as XPS data point towards the important role of residual tellurium on the surface of the SnTe pre-catalyst to drive the HER. This work underscores the significance of understanding the in-situ transformation of the pre-catalyst to the active species during the eCO2RR to rationalize its activity and product selectivity.

The direct coupling of light harvesting and charge storage in a single material opens new avenues to light storing devices. Here we demonstrate the decoupling of light and dark reactions in the two-dimensional layered niobium tungstate (TBA)+(NbWO6)- for on-demand hydrogen evolution and solar battery energy storage. Light illumination drives Li+/H+ photointercalation into the (TBA)+(NbWO6)- photoanode, leading to small polaron formation assisted by structural distortions on the WOx sublattice, along with a light-induced decrease in material resistance over 2 orders of magnitude compared to the dark. The photogenerated electrons can be extracted on demand to produce solar hydrogen upon the addition of a Pt catalyst. Alternatively, they can be stored for over 20 h under oxygen-free conditions after 365 nm UV illumination for only 10 min, thus featuring a solar battery anode with promising capacity and long-term stability. The optoionic effects described herein offer new insights to overcome the intermittency of solar irradiation, while inspiring applications at the interface of solar energy conversion and energy storage, including solar batteries, "dark" photocatalysis, solar battolyzers, and photomemory devices.

This study reports the synthesis and crystal structure determination of a novel CrTe3 phase using various experimental and theoretical methods. The average stoichiometry and local phase separation of this quenched high-pressure phase were characterized by ex situ synchrotron powder X-ray diffraction and total scattering. Several structural models were obtained using simulated annealing, but all suffered from an imperfect Rietveld refinement, especially at higher diffraction angles. Finally, a novel stoichiometrically correct crystal structure model was proposed on the basis of electron diffraction data and refined against powder diffraction data using the Rietveld method. Scanning electron microscopy-energy-dispersive X-ray spectrometry (EDX) measurements verified the targeted 1:3 (Cr:Te) average stoichiometry for the starting compound and for the quenched high-pressure phase within experimental errors. Scanning transmission electron microscopy (STEM)-EDX was used to examine minute variations of the Cr-to-Te ratio at the nanoscale. Precession electron diffraction (PED) experiments were applied for the nanoscale structure analysis of the quenched high-pressure phase. The proposed monoclinic model from PED experiments provided an improved fit to the X-ray patterns, especially after introducing atomic anisotropic displacement parameters and partial occupancy of Cr atoms. Atomic resolution STEM and simulations were conducted to identify variations in the Cr-atom site-occupancy factor. No significant variations were observed experimentally for several zone axes. The magnetic properties of the novel CrTe3 phase were investigated through temperature- and field-dependent magnetization measurements. In order to understand these properties, auxiliary theoretical investigations have been performed by first-principles electronic structure calculations and Monte Carlo simulations. The obtained results allow the observed magnetization behavior to be interpreted as the consequence of competition between the applied magnetic field and the Cr-Cr exchange interactions, leading to a decrease of the magnetization towards T = 0 K typical for antiferromagnetic systems, as well as a field-induced enhanced magnetization around the critical temperature due to the high magnetic susceptibility in this region.

The structure of the first lithium containing bismuth ortho-thiophosphate was solved using a combination of powder X-ray, neutron, and electron diffraction. Li60–3xBi16+x(PS4)36 with x in the range of 4.2 to 6.7 possesses a complex monoclinic structure (space group C2/c, no. 15) and a large unit cell with lattice parameters a = 15.487Å, b = 10.323Å, c = 33.767Å, and = 85.394° for Li44.4Bi21.2(PS4)36. The disordered distribution of lithium ions within the interstices of the dense host-structure as well as the Li ion dynamics and diffusion pathways have been investigated by X-ray and neutron PDF analysis, solid-state NMR spectroscopy, PFG-NMR diffusion measurements, and BVS calculations. The total lithium ion conductivities range from 2.6 × 10−7 to 2.8 × 10−6 S cm−1 at 20 °C with activation energies between 0.29 and 0.32 eV, depending on the bismuth content. Despite the highly disordered nature of lithium ions in Li60–3xBi16+x(PS4)36, the underlying dense host-framework appears to limit the dimensionality of the lithium diffusion pathways and emphasizes once more the necessity of a close inspection of structure-property relationships in solid electrolytes.

The structure of the first lithium-containing bismuthortho (o)-thiophosphate was determined using a combinationof powderX-ray, neutron, and electron diffraction. Li60-3x Bi16+x (PS4)(36) with x in the range of 4.1-6.5possesses a complex monoclinic structure [space group C2/c (No. 15)] and a large unit cell with the latticeparameters a = 15.4866 & ANGS;, b = 10.3232 & ANGS;, c = 33.8046 & ANGS;, and & beta;= 85.395 & DEG; for Li44.4Bi21.2(PS4)(36), in agreement with the structure as observed by X-rayand neutron pair distribution function analysis. The disordered distributionof lithium ions within the interstices of the dense host structureand the Li ion dynamics and diffusion pathways have been investigatedby solid-state nuclear magnetic resonance (NMR) spectroscopy, pulsedfield gradient NMR diffusion measurements, and bond valence sum calculations.The total lithium ion conductivities range from 2.6 x 10(-7) to 2.8 x 10(-6) S cm(-1) at 20 & DEG;C with activation energies between0.29 and 0.32 eV, depending on the bismuth content. Despite the highlydisordered nature of lithium ions in Li60-3x Bi16+x (PS4)(36), the underlying dense host framework appears to limit the dimensionalityof the lithium diffusion pathways and emphasizes once more the necessityof a close inspection of the structure-property relationshipsin solid electrolytes. Different analyticaltechniques were used to characterizethe structure and to investigate the ionic transport properties. Thecombination of these techniques allowed us to elucidate the complexstructure of the first lithium-containing bismuth o-thiophosphate and provide insights into the electrochemical andionic diffusion properties.

The prevalent global energy crisis calls for searching viable pathways for generating green hydrogen as an alternative energy resource. Dye-sensitized photocatalytic water splitting is a feasible solution to produce green hydrogen. However, identifying suitable catalysts has been one of the bottlenecks in driving dye-sensitized photocatalysis efficiently. In this work, we report a new class of electrocatalysts based on the layered Weyl semimetals MIrTe4 (M = Nb, Ta) for the Eosin Y (EY)-sensitized hydrogen evolution reaction (HER) under visible light illumination. NbIrTe4 and TaIrTe4 exhibit HER activities of ~ 18000 and ~ 14000 mol.g-1, respectively after 10h of irradiation with visible light. Time-dependent UV-Vis spectroscopy and high-pressure liquid chromatography coupled with mass spectroscopy analysis shed light on the reaction dynamics and enable deeper understanding of the observed trend in hydrogen evolution rates for MIrTe4 materials. MIrTe4 (M = Nb, Ta) semimetals outperform related catalysts including transition metal dichalcogenides and other Weyl semimetals in terms of HER activity using EY as photosensitizer and triethanolamine as the sacrificial agent. We hypothesize that the topology-related band inversion in MIrTe4 Weyl semimetals promotes a high density of metal d-states near the Fermi level, driving their high catalytic performance. This study introduces a new class of layered Weyl semimetals as efficient catalysts, and provides perspectives for designing topology-enhanced catalysts.

In the field of artificial photosynthesis with semiconductor light harvesters, the default cocatalyst morphologies are isotropic, 0D nanoparticles. Herein, the use of highly anisotropic 2D ruthenium oxide nanosheet (RONS) cocatalysts as an approach to enhance photocatalytic oxygen evolution (OER) rates on commercial WO3 nanoparticles (0D light harvester) is presented. At optimal cocatalyst loadings and identical photocatalysis conditions, WO3 impregnated with RONS (RONS/WO3) shows a fivefold increase in normalized photonic efficiency compared to when it is impregnated with conventional ruthenium oxide (rutile) nanoparticles (RONP/WO3). The superior RONS/WO3 performance is attributed to two special properties of the RONS: i) lower electrochemical water oxidation overpotential for RONS featuring highly active edge sites, and ii) decreased parasitic light absorption on RONS. Evidence is presented that OER photocatalytic performance can be doubled with control of RONS edges and it is shown that compared to WO3 impregnated with RONP, the advantageous optical properties and geometry of RONS decrease the fraction of light absorbed by the cocatalyst, thus reducing the parasitic light absorption on the RONS/WO3 composite. Therefore, the results presented in the current study are expected to promote engineering of cocatalyst morphology as a complementary concept to optimize light harvester-cocatalyst composites for enhanced photocatalytic efficiency.

As covalent organic frameworks (COFs) are coming of age, the lack of effective approaches to achieve crystalline and centimeter-scale-homogeneous COF films remains a significant bottleneck toward advancing the application of COFs in optoelectronic devices. Here, we present the synthesis of colloidal COF nanoplates, with lateral sizes of ∼200 nm and average heights of 35 nm, and their utilization as photocathodes for solar hydrogen evolution. The resulting COF nanoplate colloid exhibits a unimodal particle-size distribution and an exceptional colloidal stability without showing agglomeration after storage for 10 months and enables smooth, homogeneous, and thickness-tunable COF nanofilms via spin coating. Photoelectrodes comprising COF nanofilms were fabricated for photoelectrochemical (PEC) solar-to-hydrogen conversion. By rationally designing multicomponent photoelectrode architectures including a polymer donor/COF heterojunction and a hole-transport layer, charge recombination in COFs is mitigated, resulting in a significantly increased photocurrent density and an extremely positive onset potential for PEC hydrogen evolution (over +1 V against the reversible hydrogen electrode), among the best of classical semiconductor-based photocathodes. This work thus paves the way toward fabricating solution-processed large-scale COF nanofilms and heterojunction architectures and their use in solar-energy-conversion devices.

Crystals of Sc2Si2Te6 have been grown and its crystal, micro- and electronic structures were investigated. The layered character of the title compound exhibits stacking faults that impede a full structural characterization by single crystal X-ray diffraction due to diffuse scattering. Based on high resolution transmission electron micrographs and diffraction patterns, the stacking faulted nature of the real structure of Sc2Si2Te6 has been revealed. Different stacking models were derived from the idealized, faultless structure and the stacking disorder was quantitatively analyzed by Rietveld refinement of powder X-ray diffraction patterns. An energetic comparison of the stacking models by density functional theory is in line with the experimental observations. Further, the bonding situation was investigated by electronic structure calculations. Sc2Si2Te6 is a narrow gap semiconductor with an indirect band gap of 0.65 eV.

Carbon nitrides are among the most studied materials for photocatalysis, however, limitations arise from inefficient charge separation and transport within the material. Here, this aspect is addressed in the 2D carbon nitride poly(heptazine imide) (PHI) by investigating the influence of various counterions, such as M = Li+, Na+, K+, Cs+, Ba2+, NH4+ and tetramethyl ammonium, on the material’s conductivity and photocatalytic activity. These ions in the PHI pores affect the stacking of the 2D layers, which further influences the predominantly ionic conductivity in M-PHI. Na-containing PHI outperforms the other M-PHI in various relative humidity (RH) environments (0-42 %RH) in terms of conductivity, likely due to pore channel geometry and size of the (hydrated) ion. With increasing RH, the ionic conductivity increases by 4-5 orders of magnitude (for Na-PHI up to 10-5 S cm-1 at 42 %RH). At the same time, the highest photocatalytic hydrogen evolution rate is observed for Na-PHI, which is mirrored by increased photo-generated charge carrier lifetimes, pointing to efficient charge carrier stabilization by mobile ions. These results indicate that ionic conductivity is an important parameter that can influence the photocatalytic activity. Besides, RH-dependent ionic conductivity is of high interest for separators, membranes, or sensors.

The in-depth understanding of the reported photoelectrochemical properties of the layered carbon nitride, poly(triazine imide)/LiCl (PTI/LiCl), has been limited by the apparent disorder of the Li/H atoms within its framework. To understand and resolve the current structural ambiguities, an optimized one-step flux synthesis (470 oC, 36 h, LiCl/KCl flux) was used to prepare PTI/LiCl and deuterated-PTI/LiCl in high purity. Its structure was characterized by a combination of neutron/X-ray diffraction and transmission electron microscopy. The range of possible Li/H atomic configurations were enumerated for the first time and, combined with total energy calculations, reveals a more complex energetic landscape than previously considered. Experimental data were fitted against all possible structural models, exhibiting the most consistency with a new orthorhombic model (Sp. Grp. Ama2) that also has the lowest total energy. In addition, a new Cu(I)-containing PTI (PTI/CuCl) was prepared with the more strongly scattering Cu(I) cations in place of Li, and which also most closely matched with the partially-disorded structure in Cmc21. Thus, a complex configurational landscape of PTI is revealed to consist of a number of ordered crystalline structures that are new potential synthetic targets, such as with the use of metal-exchange reactions.

Solid electrolytes (SEs) with high ionic conductivities are prerequisites to establish solid state batteries on a broad basis. Here we report a novel approach to thiophosphate SEs with improved ionic conductivities based on the in situ formation of LGPS-type tetra-Li7SiPS8/lithium argyrodite Li6PS5X (X= Cl, Br, I) hybrid SEs. Quantitative phase analysis reveals the formation of halogen-poor argyrodites Li6+yPS5+yX1-y next to the tetra-Li7SiPS8 majority phase and an amorphous side phase. EIS measurements indicate ionic conductivities of up to 7 mS cm(-1), which exceed those of the parent tetra-Li7SiPS8 and Li6PS5X (X = Cl, Br, I) phases as well as those of simple physical mixtures whose conductivities are well described by the effective medium approximation. In contrast to previous reports, no evidence for halide substitution of the PS43- anions in tetra-Li7SiPS8 was found. Instead, the observed increase in ionic conductivity along with reduced grain boundary resistance is attributed to the directed growth of the tetra-Li7SiPS8 majority phase in the presence of an argyrodite side phase. As a result, a substantially increased isotropic Li diffusion radius is observed by PFG NMR, consistent with both more bulk-like Li transport within secondary particles and with reduced grain boundary resistance through more benign argyrodite interphases as compared to pristine tetra-Li7SiPS8. The microstructural changes induced by hybridization thus provide access to the bulk properties of tetra-Li7SiPS8.

In article number 2003016, Bettina V. Lotsch and co-workers demonstrate that covalent surface modifications of the 2D carbon nitride poly(heptazine imide) with melamine groups strongly influence its polarity and photo(electrochemical) properties. This potent tuning pathway also results in the boosting of photocatalytic hydrogen evolution due to increased donor interactions and enhanced hole extraction.

Carbon nitrides constitute a class of earth‐abundant polymeric semiconductors, which have high potential for tunability on a molecular level, despite their high chemical and thermal inertness. Here the first postsynthetic modification of the 2D carbon nitride poly(heptazine imide) (PHI) is reported, which is decorated with terminal melamine (Mel) moieties by a functional group interconversion. The covalent attachment of this group is verified based with a suite of spectroscopic and microscopic techniques supported by quantum–chemical calculations. Using triethanolamine as a sacrificial electron donor, Mel‐PHI outperforms most other carbon nitrides in terms of hydrogen evolution rate (5570 µmol h −1 g −1 ), while maintaining the intrinsic light storing properties of PHI. The origin of the observed superior photocatalytic performance is traced back to a modified surface electronic structure and enhanced interfacial interactions with the amphiphile triethanolamine, which imparts improved colloidal stability to the catalyst particles especially in contrast to methanol used as donor. However, this high activity can be limited by oxidation products of donor reversibly building up at the surface, thus blocking active centers. The findings lay out the importance of surface functionalization to engineer the catalyst–solution interface, an underappreciated tuning parameter in photocatalytic reaction design.