Ultrafast hydrogen production in boron/oxygen-codoped graphitic carbon nitride revealed by nonadiabatic dynamics simulations

Graphitic carbon nitride (g-C3N4 or GCN) shows promise in photocatalytic water splitting, despite facing the challenge of rapid electron-hole recombination. In this study, we investigated the influence of boron/oxygen codoping on the photocatalytic performance of GCN systems for hydrogen generation. First-principles calculations and nonadiabatic molecular dynamics (NAMD) simulations were employed to reveal that the recombination time of photogenerated carriers could be increased by 16% to 64% in the codoped systems compared to the pristine GCN. The time-dependent density functional theory (TDDFT) scheme was utilized to select energy windows and initiate dynamics in cluster models of B/O co-doped heptazine with water molecules. Notably, we observed efficient direct photodissociation of hydrogen atoms from water molecules within 60 fs and proton hops within the hydrogen-bonded network within 80 fs in the co-doped system, diverging from the previously proposed mechanism for pristine heptazine in NAMD simulations. This discovery underscores the significant role of faster proton-coupled electron transfer (PCET) reactions and rapid radiationless relaxation in achieving high photocatalytic efficiency in water splitting. Our work enhances the understanding of the internal mechanism of highly efficient photocatalysts for water splitting and provides a new design strategy for doped GCN.

Vibrational dynamics and spectroscopy of water at porous g-C3N4 and C2N surfaces

Porous graphitic materials containing nitrogen are promising catalysts for photo(electro)chemical reactions, notably water splitting, but can also serve as "molecular sieves". Nitrogen increases the hydrophilicity of the graphite parent material, among other effects. A deeper understanding of how water interacts with C- and N-containing layered materials, if and which differences exist between materials with different N content and pore size, and what the role of water dynamics is - a prerequsite for catalysis and sieving - is largely absent, however. Vibrational spectroscopy can answer some of these questions. In this work, the vibrational dynamics and spectroscopy of deuterated water molecules (D2O) mimicking dense water layers at room temperature on the surfaces of two different C/N-based materials with different N content and pore size, namely graphitic C3N4 (g-C3N4) and C2N, are studied using ab initio molecular dynamics (AIMD). In particular, time-dependent vibrational sum frequency generation (TD-vSFG) spectra of the OD modes and also time-averaged vSFG spectra and OD frequency distributions are computed. This allows us to distinguish "free" (dangling) OD bonds from OD bonds that are bound in a H-bonded water network or at the surface - with subtle differences between the two surfaces and also to a pure water/air interface. It is found that the temporal decay of OD modes is very similar on both surfaces with a correlation time near 4 ps. In contrast, TD-vSFG spectra reveal that the interconversion time from "bonded" to "free" OD bonds is about 8 ps for water on C2N and thus twice as long as for g-C3N4, demonstrating a propensity of the former material to stabilize bonded OD bonds.

Untangling the Effect of Carbonaceous Materials on the Photoelectrochemical Performance of BaTaO2N

The water oxidation reaction is a rate-determining step in solar water splitting. The number of surviving photoexcited holes is one of the most influencing factors affecting the photoelectrochemical water oxidation efficiency of photocatalysts. The solar-to-hydrogen energy conversion efficiency of BaTaO2N is still far below the benchmark efficiency set for practical applications, notwithstanding its potential as a 600 nm-class photocatalyst in solar water splitting. To improve its efficiency in photoelectrochemical water splitting, this study offers a straightforward route to develop photocatalytic materials based on the combination of BaTaO2N and carbonaceous materials with different dimensions. The impact of diverse carbonaceous materials, such as fullerene, g-C3N4, graphene, carbon nanohorns, and carbon nanotubes, on the photoelectrochemical behavior of BaTaO2N has been examined. Notably, the use of graphene and g-C3N4 remarkably improves the photoelectrochemical performance of the composite photocatalysts through a higher photocurrent and acting as electron reservoirs. Consequently, a marked reduction in recombination rates, even at low overpotentials, leads to a higher accumulation of photoexcited holes, resulting in 2.6- and 1.7-fold increased BaTaO2N photocurrent densities using graphene and g-C3N4, respectively. The observed trends in the dark for the oxygen reduction reaction (ORR) potential align with the increase in the photocurrent density, revealing a good correlation between opposite phenomena. Importantly, the enhancement observed implies an underlying accumulation phenomenon. The verification of this concept lies in the evidence provided by oxygen reduction and is in line with photoredox flux matching during photocatalysis. This research underscores the intricate interplay between carbonaceous materials and oxynitride photocatalysts, offering a strategic approach to enhancing various photocatalytic capabilities.

High-Throughput Screening of Gas Sensor Materials for Decomposition Products of Eco-Friendly Insulation Medium by Machine Learning

Nowadays, trifluoromethyl sulfonyl fluoride (CF3SO2F) has shown great potential to replace SF6 as an eco-friendly insulation medium in the power industry. In this work, an effective and low-cost design strategy toward ideal gas sensors for the decomposed gas products of CF3SO2F was proposed. The strategy achieved high-throughput screening from a large candidate space based on first-principle calculation and machine learning (ML). The candidate space is made up of different transition metal-embedded graphic carbon nitrides (TM/g-C3N4) owing to their high surface area and subtle electronic structure. Four main noteworthy decomposition gases of CF3SO2F, namely, CF4, SO2, SO2F2, and HF, as well as their initial stable structure on TM/g-C3N4 were determined. The best-performing ML model was established and implemented to predict the interaction strength between gas products and TM/g-C3N4, thus determining the promising gas-sensing materials for target gases with the requirements of interaction strength, recovery time, sensitivity, and selectivity. Further analysis guarantees their stability and reveals the origin of excellent properties as a gas sensor. The high-throughput strategy opens a new avenue of rational and low-cost design principles of desirable gas-sensing materials in an interdisciplinary view.

TiO2/graphitic carbon nitride nanosheet composite with enhanced sensitivity to atmospheric water

Understanding the fundamentals of transport properties in two-dimensional (2D) materials is essential for their applications in devices, sensors, and so on. Herein, we report the impedance spectroscopic study of carbon nitride nanosheets (CNNS) and the composite with anatase (TiO₂/CNNS, 20 atom% Ti), including their interaction with atmospheric water. The samples were characterized by X-ray diffraction, N₂ adsorption/desorption, solid state ¹H nuclear magnetic resonance spectroscopy, thermogravimetric analysis, and transmission electron microscopy. It is found that CNNS is highly insulating (resistivity ρ ∼ 10¹⁰ Ω cm) and its impedance barely changes during a 20 min-measurement at room temperature and 70% relative humidity. Meanwhile, incorporating the semiconducting TiO₂ nanoparticles (∼10 nm) reduces ρ by one order of magnitude, and the decreased ρ is proportional to the exposure time to atmospheric water. Sorbed water shows up at low frequency (<10² Hz) with relaxation time in milliseconds, but the response intrinsic to CNNS and TiO₂/CNNS is evident at higher frequency (>10⁴ Hz) with relaxation time in microseconds. These two signals apparently correlate to the endothermic peak at ≤110 °C and >250 °C, respectively, in differential scanning calorimetry experiments. Universal power law analysis suggests charge hopping across the 3D conduction pathways, consistent with the capacitance in picofarad typical of grain response. Our work demonstrates that the use of various formalisms (i.e., impedance, permittivity, conductivity, and modulus) combined with a simple universal power law analysis provides insights into water-induced transport of the TiO₂/CNNS composite without complicated curve fitting procedure or dedicated humidity control.

Graphene and g-C3N4-Based Gas Sensors

The efficient monitoring of the environment is currently gaining a continuous growing interest in view of finding solutions for the global pollution issues and their associated climate change. In this sense, two-dimensional (2D) materials appear as one of highly attractive routes for the development of efficient sensing devices due, in particular, to the interesting blend of their superlative properties. For instance, graphene (Gr) and graphitic carbon nitride g-C3N4 (g-CN) have specifically attracted great attention in several domains of sensing applications owing to their excellent electronic and physical-chemical properties. Despite the high potential they offer in the development and fabrication of high-performance gas-sensing devices, an exhaustive comparison between Gr and g-CN is not well established yet regarding their electronic properties and their sensing performances such as sensitivity and selectivity. Hence, this work aims at providing a state-of-the-art overview of the latest experimental advances in the fabrication, characterization, development, and implementation of these 2D materials in gas-sensing applications. Then, the reported results are compared to our numerical simulations using density functional theory carried out on the interactions of Gr and g-CN with some selected hazardous gases’ molecules such as NO2, CO2, and HF. Our findings conform with the superior performances of the g-CN regarding HF detection, while both g-CN and Gr show comparable detection performances for the remaining considered gases. This allows suggesting an outlook regarding the future use of these 2D materials as high-performance gas sensors.

Coordination tailoring of Cu single sites on C3N4 realizes selective CO2 hydrogenation at low temperature

CO₂ hydrogenation has attracted great attention, yet the quest for highly-efficient catalysts is driven by the current disadvantages of poor activity, low selectivity, and ambiguous structure-performance relationship. We demonstrate here that C₃N₄-supported Cu single atom catalysts with tailored coordination structures, namely, Cu–N₄ and Cu–N₃, can serve as highly selective and active catalysts for CO₂ hydrogenation at low temperature. The modulation of the coordination structure of Cu single atom is readily realized by simply altering the treatment parameters. Further investigations reveal that Cu–N₄ favors CO₂ hydrogenation to form CH₃OH via the formate pathway, while Cu–N₃ tends to catalyze CO₂ hydrogenation to produce CO via the reverse water-gas-shift (RWGS) pathway. Significantly, the CH₃OH productivity and selectivity reach 4.2 mmol g^–1 h^–1 and 95.5%, respectively, for Cu–N₄ single atom catalyst. We anticipate this work will promote the fundamental researches on the structure-performance relationship of catalysts.

CO₂ hydrogenation has attracted intense scientific attention yet suffers from the disadvantage of poor activity and low selectivity. Here, the authors report that Cu single atom catalysts with tailored coordination environments on C₃N₄ serve as highly selective catalysts for CO₂ hydrogenation.

Band alignment tuning of heptazine-g-C3N4/g-ZnO vdW heterostructure as a promising water-splitting photocatalyst

Van der Waals (vdW) heterostructures of two-dimensional monolayers are a relatively new class of materials with highly tunable band alignment, bandgap energy, and bandgap transition type. In this study, we performed density functional theory calculations to investigate how a vdW heterostructure of heptazine-based graphitic carbon nitride (hg-C₃N₄) and graphitic zinc oxide (g-ZnO) monolayers is formed (hg-C₃N₄/g-ZnO). This heterostructure is a potential solar-driven photocatalyst for the water-splitting reaction. Upon the formation of the heterostructure, a type-I indirect bandgap (E_g = 2.08 eV) is created with appropriate conduction band minimum and valence band maximum levels relative to the oxidation/reduction potentials for the water-splitting reaction. In addition, a very large electrostatic potential difference of 11.18 eV is generated across the heterostructure, leading to a large, naturally-formed, built-in electric field directing from hg-C₃N₄ to g-ZnO. The produced electric field forces photogenerated electrons in g-ZnO to transfer toward hg-C₃N₄, leading to a decrease in the electron-hole recombination rate. We also found that both g-ZnO and hg-C₃N₄ synergistically lead to higher light absorption of the heterostructure (λ_max = 387 nm). Furthermore, band alignment, bandgap energy, and transition type of the heterostructure can be tuned by applying external perpendicular electric fields and biaxial strains. It was found that a strain of +2% leads to a Z-scheme band alignment (E_g = 2.34 eV, direct) and an electric field of 1 V Å^-1 leads to a type-II heterostructure (E_g = 2.29 eV, indirect), which are both beneficial for efficient water-splitting photocatalysis.

Nonadiabatic Dynamics of Photocatalytic Water Splitting on A Polymeric Semiconductor

To elucidate the nature of light-driven photocatalytic water splitting, a polymeric semiconductor-graphitic carbon nitride (g-C₃N₄)-has been chosen as a prototype substrate for studying atomistic water spitting processes in realistic environments. Our nonadiabatic quantum dynamics simulations based on real-time time-dependent density functional theory reveal explicitly the transport channel of photogenerated charge carriers at the g-C₃N₄/water interface, which shows a strong correlation to bond re-forming. A three-step photoreaction mechanism is proposed, whereas the key roles of hole-driven hydrogen transfer and interfacial water configurations were identified. Immediately following photocatalytic water splitting, atomic pathways for the two dissociated hydrogen atoms approaching each other and forming the H₂ gas molecule are demonstrated, while the remanent OH radicals may form intermediate products (e.g., H₂O₂). These results provide critical new insights for the characterization and further development of efficient water-splitting photocatalysts from a dynamic perspective.

The exploration of metal-free catalyst g-C3N4 for NO degradation

We propose a new metal-free scheme of the reaction between the molecules CO and NO on a g-C₃N₄ monolayer. We first investigate the electronic properties of the related molecules CO, NO, N₂, and CO₂ adsorbed g-C₃N₄ systems, and then figure out the possible reaction pathways. It is shown that all the molecules will be physisorbed above the triangular cavity. Also, we find the NO binding on g-C₃N₄ is stronger than CO. The NO dissociation will be the rate-determining step of the reaction, and the formation of NCO· intermediate will play a critical role for the reaction process. This research presents a new route of applying g-C₃N₄ as a catalyst in the NO catalytic degradation reaction.

DFT-Guided Design and Fabrication of Carbon-Nitride-Based Materials for Energy Storage Devices: A Review

Carbon nitrides (including CN, C₂N, C₃N, C₃N₄, C₄N, and C₅N) are a unique family of nitrogen-rich carbon materials with multiple beneficial properties in crystalline structures, morphologies, and electronic configurations. In this review, we provide a comprehensive review on these materials properties, theoretical advantages, the synthesis and modification strategies of different carbon nitride-based materials (CNBMs) and their application in existing and emerging rechargeable battery systems, such as lithium-ion batteries, sodium and potassium-ion batteries, lithium sulfur batteries, lithium oxygen batteries, lithium metal batteries, zinc-ion batteries, and solid-state batteries. The central theme of this review is to apply the theoretical and computational design to guide the experimental synthesis of CNBMs for energy storage, i.e., facilitate the application of first-principle studies and density functional theory for electrode material design, synthesis, and characterization of different CNBMs for the aforementioned rechargeable batteries. At last, we conclude with the challenges, and prospects of CNBMs, and propose future perspectives and strategies for further advancement of CNBMs for rechargeable batteries.

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.

From Triazine to Heptazine: Origin of Graphitic Carbon Nitride as a Photocatalyst

Graphitic carbon nitride (g-CN) has emerged as a promising metal-free photocatalyst, while the catalytic mechanism for the photoinduced redox processes is still under investigation. Interestingly, this heptazine-based polymer optically behaves as a "quasi-monomer". In this work, we explore upstream from melem (the heptazine monomer) to the triazine-based melamine and melam and present several lines of theoretical/experimental evidence where the catalytic activity of g-CN originates from the electronic structure evolution of the C-N heterocyclic cores. Periodic density functional theory calculations reveal the strikingly different electronic structures of melem from its triazine-based counterparts. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy also provide consistent results in the structural and chemical bonding variations of these three relevant compounds. Both melam and melem were found to show stable photocatalytic activities, while the photocatalytic activity of melem is about 5.4 times higher than that of melam during the degradation of dyes under UV-visible light irradiation. In contrast to melamine and melam, the frontier electronic orbitals of the heptazine unit in melem are uniformly distributed and well complementary to each other, which further determine the terminal amines as primary reduction sites. These appealing electronic features in both the heterocyclic skeleton and the terminated functional groups can be inherited by the polymeric but quasi-monomeric g-CN, leading to its pronounced photocatalytic activity.

Enhanced Photocatalytic Activity of Aerogel Composed of Crooked Carbon Nitride Nanolayers with Nitrogen Vacancies

Self-supported aerogel composed of carbon nitride nanolayers can act as a bifunctional photocatalyst and show enhanced photoreduction and photooxidation performance due to the large surface areas and nitrogen vacancies. The carbon nitride aerogel can catalyze hydrogen evolution at a rate of nearly 4.2 mmol h^-1 g^-1 and oxidize benzyl alcohols with a high conversion efficiency and selectivity under milder conditions. Note that the activity of carbon nitride aerogel for photochemical alcohol oxidation shows outstanding performance compared with carbon nitride based photocatalysts. Both density functional theory and experimental results demonstrate that the introduction of nitrogen vacancies within the carbon nitride aerogel contributes to the formation of a crooked structure and enhanced adsorption of oxygen compared with a bulk sample.

Theoretical insight into the mechanism of photoreduction of CO2 to CO by graphitic carbon nitride

The assistance of the proton plays a key role in the photoreduction of CO₂ to CO.

Synergistic Effect of Doping and Compositing on Photocatalytic Efficiency: A Case Study of La2Ti2O7

Charge generation and separation are two key issues in developing a high-efficiency semiconductor for the visible-light-driven photocatalysis. Here, we use the layered perovskite-type wide-gap semiconductor La₂Ti₂O₇ (LTO) as a model to systematically explore the synergistic effect of doping (with sulfur or nitrogen) and heterojunction (with graphitic C₃N₄) on improving visible light absorption and photoexcited charge separation by means of density functional theory calculations. It is found that the anion (N or S) doping into the LTO(010) surface can not only shift the optical absorption edge to the visible region, but also creates some partially occupied or unoccupied states in the band gap that would facilitate the formation of recombination centers. For the purpose of promoting electron-hole separation, the (N or S-doped) LTO(010) surfaces were hybridized with the monolayer g-C₃N₄. Interestingly, we found that the (S-doped) LTO/g-C₃N₄ heterostructure forms a type-II heterojunction, with the valence band maximum residing in the (S-doped) LTO and the conduction band minimum in g-C₃N₄, respectively. This band alignment feature facilitates efficient electron-hole separation. Moreover, we found that the S-doped LTO/g-C₃N₄ composite has a short interfacial distance (about 2.1 Å), implying that the interfacial interaction of this composite might be a chemical bond rather than a weak van der Walls interaction. The chemical bonding can enhance charge separation. Our theoretical findings provide design principles for optimizing the photocatalytic performance of the wide-gap photocatalysts and demonstrate that the S-doped LTO/g-C₃N₄ composite would be a potential candidate for the photocatalysis of water splitting.

Electronic and Optical Properties of 2D Materials Constructed from Light Atoms

Boron, carbon, nitrogen, and oxygen atoms can form various building blocks for further construction of structurally well-defined 2D materials (2DMs). Both in theory and experiment, it has been documented that the electronic structures and optical properties of 2DMs are well tunable through a rational design of the material structure. Here, the recent progress on 2DMs that are composed of B, C, N, and O elements is introduced, including borophene, graphene, h-BN, g-C₃ N₄ , organic 2D polymers (2DPs), etc. Attention is put on the band structure/bandgap engineering for these materials through a variety of methodologies, such as chemical modifications, layer number and atomic structure control, change of conjugation degree, etc. The optical properties, such as photoluminescence, thermoluminescence, single photon emission, as well as the associated applications in bioimaging and sensing, are discussed in detail and highlighted.

Where do photogenerated holes at the g-C3N4/water interface go for water splitting: H2O or OH-?

Graphitic carbon nitride (g-C3N4), a metal-free two-dimensional photocatalyst, has drawn increasing attention due to its application in photocatalytic water splitting. However, its quantum efficiency is limited by the poor performance of the oxygen evolution reaction (OER). Therefore, it is important to clarify the behavior of photogenerated holes in the OER. In this work, we investigate the energy level alignment using the GW method and the exciton properties using the Bethe-Salpeter equation within the ab initio many-body Green's function theory at the g-C3N4/water interface. We found that the g-C3N4 substrate can elevate energy levels of OH- and H2O molecules at the interface by up to 0.6 eV. This effect can make the electronic levels of OH- surpass the valence band maximum (VBM) of g-C3N4. However, orbital energies of H2O molecules remain far below the VBM of g-C3N4. This indicates that a photogenerated hole after exciting g-C3N4 can relax to OH- instead of neutral H2O. Moreover, OH- could be directly oxidized through electron transfer from OH- to g-C3N4 by light near the optical absorption edge of g-C3N4, which is beneficial for efficient carrier separation at the interface.

Exploring the formation and electronic structure properties of the g-C3N4 nanoribbon with density functional theory

The optical properties and condensation degree (structure) of polymeric g-C₃N₄ depend strongly on the process temperature. For polymeric g-C₃N₄, its structure and condensation degree depend on the structure of molecular strand(s). Here, the formation and electronic structure properties of the g-C₃N₄ nanoribbon are investigated by studying the polymerization and crystallinity of molecular strand(s) employing first-principle density functional theory. The calculations show that the width of the molecular strand has a significant effect on the electronic structure of polymerized and crystallized g-C₃N₄ nanoribbons, a conclusion which would be indirect evidence that the electronic structure depends on the structure of g-C₃N₄. The edge shape also has a distinct effect on the electronic structure of the crystallized g-C₃N₄ nanoribbon. Furthermore, the conductive band minimum and valence band maximum of the polymeric g-C₃N₄ nanoribbon show a strong localization, which is in good agreement with the quasi-monomer characters. In addition, molecular strands prefer to grow along the planar direction on graphene. These results provide new insight on the properties of the g-C₃N₄ nanoribbon and the relationship between the structure and properties of g-C₃N₄.

The roles of buckled geometry and water environment in the excitonic properties of graphitic C3N4

The exciton plays a crucial role in two-dimensional materials involved in photocatalytic water splitting, where its properties are determined not only by the material itself, but also by the surrounding water environment. By the framework of many-body perturbation theory, we investigated the excitonic effects in pure and water-adsorbed g-C₃N₄. It is shown that the excitonic properties are very sensitive to the geometry of g-C₃N₄ and the adsorption of water molecules. Firstly, the optical band gap, i.e. the first bright excitonic energy of pure g-C₃N₄ decreases remarkably from a high symmetry planar structure (3.8 eV) to a P1 buckled configuration (2.7 eV). Secondly, the hydrogen bonds between water and g-C₃N₄ induce the generation of interface excitons. With a reduced binding energy (at least 0.2 eV), interface excitons can contribute to a more efficient separation of electrons and holes. Our work provides an insight into the excitation mechanism of 2D photocatalysts in a real environment.

In situ decoration of plasmonic Au nanoparticles on graphene quantum dots-graphitic carbon nitride hybrid and evaluation of its visible light photocatalytic performance

This work spotlights the development of a plasmonic photocatalyst showing surface plasmon induced enhanced visible light photocatalytic (PC) performance. Plasmonic Au nanoparticles (NPs) are decorated over the hybrid nanosystem of graphitic carbon nitride (GCN) and graphene quantum dots (GQD) by citrate reduction method. Surface plasmon resonance (SPR) induced enhancement of Raman G and 2D band intensity is encountered on excitation of the Plasmonic hybrid at 514.5 nm, which is near to the 532 nm absorption band of Au NPs. Time-resolved photoluminescence and XPS studies show charge transfer interaction between GQD-GCN and Au NPs. Plasmonic hybrid exhibits an enhanced PC activity over the other catalysts in the photodegradation of methylene blue (MB) under visible light illumination. Plasmonic photocatalyst displays more than 6 fold enhancement in the photodecomposition rate of MB over GQD and nearly 2 fold improvement over GCN and GQD-GCN. GQD-GCN absorbs mostly in the near visible region and can be photoexcited by visible light of wavelength ([Formula: see text]) < 460 nm. Plasmon activation in Au NPs decorated GQD-GCN could exploit the entire UV-visible light for photocatalysis. Furthermore, plasmonic Au act as antennas for accumulation and enhancement of localized electromagnetic field at the interface with GQD-GCN, and thereby promotes photogeneration of large numbers of carriers on GQD-GCN. The carriers are separated by charge transfer migration from hybrid to Au NPs. Finally, the carriers on the plasmonic Au nanostructures initiate MB degradation under visible light. Our results have shown that plasmon decoration is a suitable strategy to design a carbon based hybrid photocatalyst for solar energy conversion.

Mechanism of Photocatalytic Water Splitting with Graphitic Carbon Nitride: Photochemistry of the Heptazine-Water Complex

Impressive progress has recently been achieved in photocatalytic hydrogen evolution with polymeric carbon nitride materials consisting of heptazine building blocks. However, the fundamental mechanistic principles of the catalytic cycle are as yet poorly understood. Here, we provide first-principles computational evidence that water splitting with heptazine-based materials can be understood as a molecular excited-state reaction taking place in hydrogen-bonded heptazine-water complexes. The oxidation of water occurs homolytically via an electron/proton transfer from water to heptazine, resulting in ground-state heptazinyl and OH radicals. It is shown that the excess hydrogen atom of the heptazinyl radical can be photodetached by a second photon, which regenerates the heptazine molecule. Alternatively to the photodetachment reaction, two heptazinyl radicals can recombine in a dark reaction to form H₂, thereby regenerating two heptazine molecules. The proposed molecular photochemical reaction scheme within hydrogen-bonded chromophore-water complexes is complementary to the traditional paradigm of photocatalytic water splitting, which assumes the separation of electrons and holes over substantial time scales and distances.

Theoretical investigation on the photocatalytic activity of the Au/g-C3N4 monolayer

Two-dimensional optical catalysis materials have a wonderful potential application. Here, a new two-dimensional material consisting of the supported single-atom Au on a graphite carbon nitride (g-C3N[Formula: see text] single layer has been designed and its electronic and optical properties have been characterized by density functional calculations. The bandgap of 1.82[Formula: see text]eV calculated by the hybrid functional HSE06 shows that the Au/g-C3N4 is an indirect semiconductor, and the electron can easily be excited from the single-atom Au to the bottom of the conduction band. This material therefore has relatively strong optical properties in the visible region. Moreover, the process of Au insertion into the cavity of g-C3N4 single layer is energy-favorable. This work may provide insights and a new avenue for fabricating supported Au catalysts with high stability.

First-Principle Framework for Total Charging Energies in Electrocatalytic Materials and Charge-Responsive Molecular Binding at Gas-Surface Interfaces

Heterogeneous charge-responsive molecular binding to electrocatalytic materials has been predicted in several recent works. This phenomenon offers the possibility of using voltage to manipulate the strength of the binding interaction with the target gas molecule and thereby circumvent thermochemistry constraints, which inhibit achieving both efficient binding and facile release of important targets such as CO2 and H2. Stability analysis of such charge-induced molecular adsorption has been beyond the reach of existing first-principle approaches. Here, we draw on concepts from semiconductor physics and density functional theory to develop a first principle theoretical approach that allows calculation of the change in total energy of the supercell due to charging. Coupled with the calculated adsorption energy of gas molecules at any given charge, this allows a complete description of the energetics of the charge-induced molecular adsorption process. Using CO2 molecular adsorption onto negatively charged h-BN (wide-gap semiconductor) and g-C4N3 (half metal) as example cases, our analysis reveals that - while adsorption is exothermic after charge is introduced - the overall adsorption processes are not intrinsically spontaneous due to the energetic cost of charging the materials. The energies needed to overcome the barriers of these processes are 2.10 and 0.43 eV for h-BN and g-C4N3, respectively. This first principle approach opens up new pathways for a more complete description of charge-induced and electrocatalytic processes.

Interfacial electronic structure and charge transfer of hybrid graphene quantum dot and graphitic carbon nitride nanocomposites: insights into high efficiency for photocatalytic solar water splitting

The interfacial electronic structure and electron transfer of graphene quantum dot–graphitic carbon nitride composites are characterized by using hybrid functional calculations and include long-range dispersion corrections.

Isotype heterostructure of bulk and nanosheets of graphitic carbon nitride for efficient visible light photodegradation of methylene blue

Isotype heterostructure of bulk and nanosheets of graphitic carbon nitride with effective band gap of 2.62 eV and charge carrier mean lifetime of 21 ns exhibits an efficient visible light photocatalysis.

Heptazine-based graphitic carbon nitride as an effective hydrogen purification membrane

A graphitic C₃N₄ nanosheet with well-ordered sized intrinsic vacancies provides a natural porous diffusion pathway to separate H₂ from common gases.

Charge Modulation in Graphitic Carbon Nitride as a Switchable Approach to High-Capacity Hydrogen Storage

Electrical charging of graphitic carbon nitride nanosheets (g-C4 N3 and g-C3 N4 ) is proposed as a strategy for high-capacity and electrocatalytically switchable hydrogen storage. Using first-principle calculations, we found that the adsorption energy of H2 molecules on graphitic carbon nitride nanosheets is dramatically enhanced by injecting extra electrons into the adsorbent. At full hydrogen coverage, the negatively charged graphitic carbon nitride achieves storage capacities up to 6-7 wt %. In contrast to other hydrogen storage approaches, the storage/release occurs spontaneously once extra electrons are introduced or removed, and these processes can be simply controlled by switching on/off the charging voltage. Therefore, this approach promises both facile reversibility and tunable kinetics without the need of specific catalysts. Importantly, g-C4 N3 has good electrical conductivity and high electron mobility, which can be a very good candidate for electron injection/release. These predictions may prove to be instrumental in searching for a new class of high-capacity hydrogen storage materials.

Carbon nanodot decorated graphitic carbon nitride: new insights into the enhanced photocatalytic water splitting from ab initio studies

Density functional theory calculations reveal that hybrid carbon nanodots and graphitic carbon nitride can form a type-II van der Waals heterojunction, leading to significant reduction of band gap and enhanced visible light response.