Multifunctional carbon nitride nanoarchitectures for catalysis

Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.

Ultrafast dynamics in polymeric carbon nitride thin films probed by time-resolved EUV photoemission and UV-Vis transient absorption spectroscopy

The ground- and excited-state electronic structures of four polymeric carbon nitride (PCN) materials have been investigated using a combination of photoemission and optical absorption spectroscopy. To establish the driving forces for photocatalytic water-splitting reactions, the ground-state data was used to produce a band diagram of the PCN materials and the triethanolamine electron scavenger, commonly implemented in water-splitting devices. The ultrafast charge-carrier dynamics of the same PCN materials were also investigated using two femtosecond-time-resolved pump-probe techniques: extreme-ultraviolet (EUV) photoemission and ultraviolet-visible (UV-Vis) transient absorption spectroscopy. The complementary combination of these surface- and bulk-sensitive methods facilitated photoinduced kinetic measurements spanning the sub-picosecond to few nanosecond time range. The results show that 400 nm (3.1 eV) excitation sequentially populates a pair of short-lived transient species, which subsequently produce two different long-lived excited states on a sub-picosecond time scale. Based on the spectro-temporal characteristics of the long-lived signals, they are assigned to singlet-exciton and charge-transfer states. The associated charge-separation efficiency was inferred to be between 65% and 78% for the different studied materials. A comparison of results from differently synthesized PCNs revealed that the early-time processes do not differ qualitatively between sample batches, but that materials of more voluminous character tend to have higher charge separation efficiencies, compared to exfoliated colloidal materials. This finding was corroborated via a series of experiments that revealed an absence of any pump-fluence dependence of the initial excited-state decay kinetics and characteristic carrier-concentration effects that emerge beyond few-picosecond timescales. The initial dynamics of the photoinduced charge carriers in the PCNs are correspondingly determined to be spatially localised in the immediate vicinity of the lattice-constituting motif, while the long-time behaviour is dominated by charge-transport and recombination processes. Suppressing the latter by confining excited species within nanoscale volumes should therefore affect the usability of PCN materials in photocatalytic devices.

Novel Triangulenes: Computational Investigations of Energy Thresholds for Photocatalytic Water Splitting

Organic materials with Inverted Singlet-Triplet (INVEST) gaps are interesting for their potential use in photocatalytic small molecule transformations such as the entirely solar-driven water splitting reaction. However, only a few INVEST emitters are thermodynamically able to split water requiring a first singlet excited dark state, S1 , above 1.27 or 1.76 eV, and absorption near solar the maximum, 2.57 eV. These requirements and the INVEST character are key for achieving a long-lived photocatalyst for water splitting. The only known INVEST emitters that conform to these criteria are large triangular boron carbon nitrides with unknown synthesis pathways. Using ADC(2), a quantum-mechanical method, we describe three triangulenes. 3 a is a cyano azacyclopenta[cd]phenalene derivative while 3 b and 3 c are cycl[3.3.3]azine derivatives. 3 b has a previously undescribed disulfide bridge. Overall, 3 a fulfills requirements for photocatalytic four-electron reduction of water while the S1 states of 3 b and 3 c are likely slightly low for the two-electron reduction process. By analyzing impacts of ligands, we find that there are guidelines describing how S1 -S5 energies and oscillator strengths, T1 energies, and ΔES1T1 gaps are affected, requiring deep-learning algorithms for which studies will be presented by us in due time. The impact of ground-state geometries, solvation effects, as well as reduced-cost ADC(2) algorithms on our findings are also discussed.

Reaction Site Designation by Intramolecular Electric Field in Tröger's-Base-Derived Conjugated Microporous Polymer for Near-Unity Selectivity of CO2 Photoconversion

To facilitate solar-driven overall CO₂ and H₂ O convsersion into fuels and O₂ , a series of covalent microporous polymers derived from Tröger's base are synthesized featuring flexural backbone and unusual charge-transfer properties. The incorporation of rigid structural twist Tröger's base unit grants the polymers enhanced microporosity and CO₂ adsorption/activation capacity. Density function theory calculations and photo-electrochemical analyses reveal that an electric dipole moment (from negative to positive) directed to the Tröger's base unit is formed across two obliquely opposed molecular fragments and induces an intramolecular electric field. The Tröger's base unit located at folding point becomes an electron trap to attract photogenerated electrons in the molecular network, which brings about suppression of carrier recombination and designates the reaction site in synergy with the conjugated network. In response to the discrepancy in reaction pathways across the reaction sites, the product allocation in the catalytic reaction is thereby regulated. Optimally, CMP-nTB achieves the highest photocatalytic CO production of 163.53 µmol g^-1 h^-1 with approximately unity selectivity, along with H₂ O oxidation to O₂ in the absence of any photosensitizer or co-catalyst. This work provides new insight for developing specialized artificial organic photocatalysts.

Inequivalent Solvation Effects on the N 1s Levels of Self-Associated Melamine Molecules in Aqueous Solution

This work shows how the N 1s photoemission (PE) spectrum of self-associated melamine molecules in aqueous solution has been successfully rationalized using an integrated computational approach encompassing classical metadynamics simulations and quantum calculations based on density functional theory (DFT). The first approach allowed us to describe interacting melamine molecules in explicit waters and to identify dimeric configurations based on π-π and/or H-bonding interactions. Then, N 1s binding energies (BEs) and PE spectra were computed at the DFT level for all structures both in the gas phase and in an implicit solvent. While pure π-stacked dimers show gas-phase PE spectra almost identical to that of the monomer, those of the H-bonded dimers are sensibly affected by NH···NH or NH···NC interactions. Interestingly, the solvation suppresses all of the non-equivalences due to the H-bonds yielding similar PE spectra for all dimers, matching very well our measurements.

Recent Progress on Photoelectrochemical Water Splitting of Graphitic Carbon Nitride (g−CN) Electrodes

Graphitic carbon nitride (g−CN), a promising visible-light-responsive semiconductor material, is regarded as a fascinating photocatalyst and heterogeneous catalyst for various reactions due to its non-toxicity, high thermal durability and chemical durability, and “earth-abundant” nature. However, practical applications of g−CN in photoelectrochemical (PEC) and photoelectronic devices are still in the early stages of development due to the difficulties in fabricating high-quality g−CN layers on substrates, wide band gaps, high charge-recombination rates, and low electronic conductivity. Various fabrication and modification strategies of g−CN-based films have been reported. This review summarizes the latest progress related to the growth and modification of high-quality g−CN-based films. Furthermore, (1) the classification of synthetic pathways for the preparation of g−CN films, (2) functionalization of g−CN films at an atomic level (elemental doping) and molecular level (copolymerization), (3) modification of g−CN films with a co-catalyst, and (4) composite films fabricating, will be discussed in detail. Last but not least, this review will conclude with a summary and some invigorating viewpoints on the key challenges and future developments.

Photochemical Hydrogen Storage with Hexaazatrinaphthylene (HATN)

When irradiated with violet light, hexaazatrinaphthylene (HATN) extracts a hydrogen atom from an alcohol forming a long-living hydrogenated species. The kinetic isotope effect for fluorescence decay in deuterated methanol (1.56) indicates that the lowest singlet excited state of the molecule is a precursor for intermolecular hydrogen transfer. The photochemical hydrogenation occurs in several alcohols (methanol, ethanol, isopropanol) but not in water. Hydrogenated HATN can be detected optically by an absorption band at 1.78 eV as well as with EPR and NMR techniques. Mass spectroscopy of photoproducts reveal di-hydrogenated HATN structures along with methoxylated and methylated HATN molecules which are generated through the reaction with methoxy radicals (remnants from alcohol splitting). Experimental findings are consistent with the theoretical results which predicted that for the excited state of the HATN-solvent molecular complex, there exists a barrierless hydrogen transfer from methanol but a barrier for the similar oxidation of water.

Suppression of Charge Recombination by Auxiliary Atoms in Photoinduced Charge Separation Dynamics with Mn Oxides: A Theoretical Study

Charge separation is one of the most crucial processes in photochemical dynamics of energy conversion, widely observed ranging from water splitting in photosystem II (PSII) of plants to photoinduced oxidation reduction processes. Several basic principles, with respect to charge separation, are known, each of which suffers inherent charge recombination channels that suppress the separation efficiency. We found a charge separation mechanism in the photoinduced excited-state proton transfer dynamics from Mn oxides to organic acceptors. This mechanism is referred to as coupled proton and electron wave-packet transfer (CPEWT), which is essentially a synchronous transfer of electron wave-packets and protons through mutually different spatial channels to separated destinations passing through nonadiabatic regions, such as conical intersections, and avoided crossings. CPEWT also applies to collision-induced ground-state water splitting dynamics catalyzed by Mn₄CaO₅ cluster. For the present photoinduced charge separation dynamics by Mn oxides, we identified a dynamical mechanism of charge recombination. It takes place by passing across nonadiabatic regions, which are different from those for charge separations and lead to the excited states of the initial state before photoabsorption. This article is an overview of our work on photoinduced charge separation and associated charge recombination with an additional study. After reviewing the basic mechanisms of charge separation and recombination, we herein studied substituent effects on the suppression of such charge recombination by doping auxiliary atoms. Our illustrative systems are X–Mn(OH)₂ tied to N-methylformamidine, with X=OH, Be(OH)₃, Mg(OH)₃, Ca(OH)₃, Sr(OH)₃ along with Al(OH)₄ and Zn(OH)₃. We found that the competence of suppression of charge recombination depends significantly on the substituents. The present study should serve as a useful guiding principle in designing the relevant photocatalysts.

Photochemistry of carbon nitrides and heptazine derivatives

We explore the photochemistry of polymeric carbon nitride (C₃N₄), an archetypal organic photocatalyst, and derivatives of its structural monomer unit, heptazine (Hz). Through spectroscopic studies and computational analysis, we have observed that Hz derivatives can engage in non-innocent hydrogen bonding interactions with hydroxylic species. The photochemistry of these complexes is influenced by intermolecular nπ*/ππ* mixing of non-bonding orbitals of each component and the relative energy of intermolecular charge-transfer (CT) states. Coupling of the former to the latter appears to facilitate proton-coupled electron transfer (PCET), resulting in biradical products. We have also observed that Hz derivatives exhibit an extremely rare inverted singlet/triplet energy splitting (ΔE_ST). In violation of Hund's multiplicity rules, the lowest energy singlet (S₁) is stabilized relative to the lowest triplet (T₁) electronic excited state. Exploiting this unique inverted ΔE_ST character has obvious implications for transformational discoveries in solid-state OLED lighting and photovoltaics. Harnessing this inverted ΔE_ST, paired with light-driven intermolecular PCET reactions, may enable molecular transformations relevant for applications ranging from solar energy storage to new classes of non-triplet photoredox catalysts for pharmaceutical development. To this end, we have explored the possibility of optically controlling the photochemistry of Hz derivatives using ultrafast pump-push-probe spectroscopy. In this case, the excited state branching ratios among locally excited states of the chromophore and the reactive intermolecular CT state can be manipulated with an appropriate secondary "push" excitation pulse. These results indicate that we can predictively redirect chemical reactivity with light in this system, which is an avidly sought achievement in the field of photochemistry. Looking forward, we anticipate future opportunities for controlling heptazine photochemistry, including manipulating PCET reactivity with a diverse array of substrates and optically delivering reducing equivalents with, for example, water as a partial source of electrons and protons. Furthermore, we wholly expect that, over the next decade, materials such as Hz derivatives, that exhibit inverted ΔE_ST character, will spawn a significant new research effort in the field of thin-film optoelectronics, where controlling recombination via triplet excitonic states can play a critical role in determining device performance.

Tailoring surface-supported water-melamine complexes by cooperative H-bonding interactions

The water-splitting photo-catalysis by carbon nitride heterocycles has been the subject of recent theoretical investigations, revealing a proton-coupled electron transfer (PCET) reaction from the H-bonded water molecule to the CN-heterocycle. In this context, a detailed characterization of the water-catalyst binding configuration becomes mandatory in order to validate and possibly improve the theoretical modeling. To this aim, we built a well-defined surface-supported water/catalyst interface by adsorbing water under ultra-high vacuum (UHV) conditions on a monolayer of melamine grown on the Cu(111) surface. By combining X-ray photoemission (XPS) and absorption (NEXAFS) spectroscopy we observed that melamine adsorbed onto copper is strongly tilted off the surface, with one amino group dangling to the vacuum side. The binding energy (BE) of the corresponding N 1s component is significantly higher compared to other N 1s contributions and displays a clear shift to lower BE as water is adsorbed. This finding along with density functional theory (DFT) results reveals that two adjacent melamine molecules concurrently work for stabilizing the H-bonded water-catalyst complex: one melamine acting as a H-donor via the amino-N (NH⋯OHH) and another one as a H-acceptor via the triazine-N (C[double bond, length as m-dash]N⋯HOH).

Thermodynamically stable polymorphs of nitrogen-rich carbon nitrides: a C3N5 study

We have carried out an extensive search for stable polymorphs of carbon nitride with C₃N₅ stoichiometry using the minima hopping method. Contrary to the widely held opinion that stacked, planar, graphite-like structures are energetically the most stable carbon nitride polymorphs for various nitrogen contents, we find that this does not apply for nitrogen-rich materials owing to the high abundance of N-N bonds. In fact, our results disclose novel morphologies with moieties not previously considered for C₃N₅. We demonstrate that nitrogen-rich compounds crystallize in a large variety of different structures due to particular characteristics of their energy landscapes. The newly found low-energy structures of C₃N₅ have band gaps within good agreement with the values measured in experimental studies.

Photooxidation of water with heptazine-based molecular photocatalysts: Insights from spectroscopy and computational chemistry

We present a conspectus of recent joint spectroscopic and computational studies that provided novel insight into the photochemistry of hydrogen-bonded complexes of the heptazine (Hz) chromophore with hydroxylic substrate molecules (water and phenol). It was found that a functionalized derivative of Hz, tri-anisole-heptazine (TAHz), can photooxidize water and phenol in a homogeneous photochemical reaction. This allows the exploration of the basic mechanisms of the proton-coupled electron-transfer (PCET) process involved in the water photooxidation reaction in well-defined complexes of chemically tunable molecular chromophores with chemically tunable substrate molecules. The unique properties of the excited electronic states of the Hz molecule and derivatives thereof are highlighted. The potential energy landscape relevant for the PCET reaction has been characterized by judicious computational studies. These data provided the basis for the demonstration of rational laser control of PCET reactions in TAHz-phenol complexes by pump-push-probe spectroscopy, which sheds light on the branching mechanisms occurring by the interaction of nonreactive locally excited states of the chromophore with reactive intermolecular charge-transfer states. Extrapolating from these results, we propose a general scenario that unravels the complex photoinduced water-splitting reaction into simple sequential light-driven one-electron redox reactions followed by simple dark radical-radical recombination reactions.

Photoinduced water oxidation in pyrimidine-water clusters: a combined experimental and theoretical study

The photocatalytic oxidation of water with molecular or polymeric N-heterocyclic chromophores is a topic of high current interest in the context of artificial photosynthesis, that is, the conversion of solar energy to clean fuels. Hydrogen-bonded clusters of N-heterocycles with water molecules in a molecular beam are simple model systems for which the basic mechanisms of photochemical water oxidation can be studied under well-defined conditions. In this work, we explored the photoinduced H-atom transfer reaction in pyrimidine-water clusters yielding pyrimidinyl and hydroxyl radicals with laser spectroscopy, mass spectrometry and trajectory-based ab initio molecular dynamics simulations. The oxidation of water by photoexcited pyrimidine is unequivocally confirmed by the detection of the pyrimidinyl radical. The dynamics simulations provide information on the time scales and branching ratios of the reaction. While relaxation to local minima of the S₁ potential-energy surface is the dominant reaction channel, the H-atom transfer reaction occurs on ultrafast time scales (faster than about 100 fs) with a branching ratio of a few percent.

Charge separation and successive reconfigurations of electronic and protonic states in a water-splitting catalytic cycle with the Mn4CaO5 cluster. On the mechanism of water splitting in PSII

Charge separation, reloading of electrons and protons, and O₂ generation in a catalytic cycle for water splitting with Mn₄CaO₅ in PSII.

UV-induced hydrogen transfer in DNA base pairs promoted by dark nπ* states

Formation of an excited-state complex enables ultrafast photorelaxation of dark nπ* states in GC and HC base pairs.

On the Elementary Chemical Mechanisms of Unidirectional Proton Transfers: A Nonadiabatic Electron-Wavepacket Dynamics Study

We propose a set of chemical reaction mechanisms of unidirectional proton transfers, which may possibly work as an elementary process in chemical and biological systems. Being theoretically derived based on our series of studies on charge separation dynamics in water splitting by Mn oxides, the present mechanisms have been constructed after careful exploration over the accumulated biological studies on cytochrome c oxidase (CcO) and bacteriorhodopsin. In particular, we have focused on the biochemical findings in the literature that unidirectional transfers of approximately two protons are driven by one electron passage through the reaction center (binuclear center) in CcO, whereas no such dissipative electron transfer is believed to be demanded in the proton transport in bacteriorhodopsin. The proposed basic mechanisms of unidirectional proton transfers are further reduced to two elementary dynamical processes, namely, what we call the coupled proton and electron-wavepacket transfer (CPEWT) and the inverse CPEWT. To show that the proposed mechanisms can indeed be materialized in a molecular level, we construct model systems with possible molecules that are rather familiar in biological chemistry, for which we perform the ab initio calculations of full-dimensional nonadiabatic electron-wavepacket dynamics coupled with all nuclear motions including proton transfers.

Understanding Charge Transport in Carbon Nitride for Enhanced Photocatalytic Solar Fuel Production

Photocatalytic solar fuel production, for example, production of hydrogen via water-splitting, is an effective means of chemical storage of solar energy and provides a potential option for achieving a zero-emissions energy system. Conveniently, hydrogen can be converted back to electricity either via fuel cells or through combustion in gas turbines, or it can be mixed in low concentrations with natural gas or biogas for combustion in existing power plants. The cornerstone of a practical solar fuel production process is a stable, efficient, and scalable photocatalyst (a semiconductor material that accommodates photon absorption, charge carrier generation and transport, and catalytic reactions). Therefore, the quest for suitable photocatalyst materials is an ongoing process. Recently, carbon nitride (CN) has attracted widespread interest as a metal-free, earth-abundant, and highly stable photocatalyst. However, the catalytic efficiency of CN is not satisfactory because of its poor charge transport attributes. There is a direct relation between the photocatalytic efficiency and charge transport because the basic principle of light-promoted overall photodecomposition of water into H₂ and O₂ molecules (or, generally speaking, photoredox reactions) relies on separation and subsequent transfer of excited-state electron-hole pairs to relative redox couples. However, the excited states last for a very short time, typically nanoseconds to microseconds in liquids, and unless they are separated within this time frame, the excited-state electron-hole pairs undergo recombination with release of the captured light energy as heat or photon emission. To utilize light in a form other than heat or emitted photons by avoiding the recombination of excited-state electron-hole pairs, charged excitons must be scavenged before the absorption of subsequent photons to sustain a multielectron photoredox reaction. Otherwise, the extraction of charges becomes more difficult. This imposes a potential efficiency-limiting factor. An enhancement in water to hydrogen conversion efficiency in CN therefore requires the use of precious-metal cocatalysts (e.g., Pt) and sacrificial electron donor/acceptors to facilitate multielectron/multiproton transfers to overcome the high kinetic barriers. The use of Pt and sacrificial agents is not consistent with the notion of low-cost and sustainable hydrogen production from water. CN must overcome this dependence to stand out as a truly scalable photocatalyst. To make progress, the foremost requirement is to attain an in-depth understanding of the fundamental charge transport phenomena needed for the rational design of CN-based photocatalysts. In this Account, therefore, our aim is to provide a synopsis of current understanding and progress regarding charge-transport-related phenomena (e.g., recombination, trapping, transfer of charge carriers, etc.) and to discuss the effects of charge transport in enhancing the apparent quantum yield of hydrogen production in CN. This understanding is necessary to broaden the scope of CN for other catalytic applications, for example, efficient CO₂ reduction to methanol or methane, fixation of nitrogen to ammonia, and use as an active material in solar cells. We also identify research gaps and issues to be addressed for a more clear elucidation of charge-transport-related phenomena in CN. Thus, this Account may inspire new research opportunities for tuning the extrinsic/intrinsic photophysicochemical properties of CN by rational design to attain the most favorable properties for improved catalytic efficiency.

Photoinduced electron-driven proton transfer from water to an N-heterocyclic chromophore: nonadiabatic dynamics studies for pyridine–water clusters

We performed the excited-state dynamics simulations for pyridine–water clusters and found the more water molecules involved in the cluster, the higher efficiency the water-splitting reaction has, which is qualitatively in consistent with a recent gas-phase experimental observations.

UV-Vis, Fluorescence, and Resonance Raman Spectroscopic and Density Functional Theoretical Studies on 3-Amino-1,2,4-triazole: Microsolvation and Solvent-Dependent Nonadiabatic Excited State Decay in Solution

The microsolvation and photophysics of 3-amino-1,2,4-triazole (3AT) after excitation to the light-absorbing S₂(nπ*) state were studied by using resonance Raman spectroscopy and single component artificial force-induced reaction (SC-AFIR) in a global reaction route mapping (GRRM) strategy. The vibrational spectra were assigned on the basis of experimental data and density functional theory (DFT) calculations. The resonance Raman spectra of 3AT were measured to probe the excited state structural dynamics in the Franck-Condon region. The conformations of 3AT(CH₃CN)₁, 3AT(CH₃OH)₂, and 3AT(H₂O)₂ clusters were determined by combining vibrational spectrum experiments and B3LYP/6-311++G(d,p) computations. DFT calculations were carried out to obtain the minimal excitation energies of the lower-lying singlet excited states, and the curve-crossing points. It was revealed that the short-time structural dynamics of 3AT were dominated by the N-N stretching coordinates. An excited state decay mechanism is proposed: 3AT is initially excited to the S₂(nπ*) state, then the conical intersection (CI) of the S₂(nπ*)/S₁(ππ*) potential energy surfaces is crossed, and 3AT then decays to the lower solvent-dependent excited state S₁(ππ*). It subsequently returns to the S₀ state, accompanied by a large Stokes fluorescence shift, which was interpreted as the stabilized S₁(ππ*) excited state bonding to several water molecules via intermolecular hydrogen bonding.

Proton-Coupled Electron Transfer from Water to a Model Heptazine-Based Molecular Photocatalyst

To gain mechanistic understanding of heptazine-based photochemistry, we synthesized and studied 2,5,8-tris(4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a model molecular photocatalyst chemically related to carbon nitride. On the basis of time-resolved photoluminescence (TR-PL) spectroscopy, we kinetically reveal a new feature that emerges in aqueous dispersions of TAHz. Using global target analysis, we spectrally and kinetically resolve the new emission feature to be blue shifted from the steady-state luminescence, and observe a fast decay component exhibiting a kinetic isotope effect (KIE) of 2.9 in H₂O versus D₂O, not observed in the steady-state PL. From ab initio electronic-structure calculations, we attribute this new PL peak to the fluorescence of an upper excited state of mixed nπ*/ππ* character. In water, the KIE suggests the excited state is quenched by proton-coupled electron transfer, liberating hydroxyl radicals that we detect using terephthalic acid. Our findings are consistent with recent theoretical predictions that heptazine-based photocatalysts can participate in proton-coupled electron transfer with H₂O.