Comparative study of the photocatalytic activity of g-C3N4/MN4 (M = Mn, Fe, Co) for water splitting reaction: A theoretical study

In this study, nanocomposites of g-C3N4/MN4 (where M is Mn, Fe and Co) have been designed using advanced density functional theory (DFT) calculations. A comprehensive analysis was conducted on the geometry, electronic, optical properties, work function, charge transfer interaction and adhesion energy of the g-C3N4/MN4 heterostructures and concluded that g-C3N4/FeN4 and g-C3N4/CoN4 heterojunctions exhibit higher photocatalytic performance than individual units. The better photocatalytic activity can be attributed mainly by two facts; (i) the visible light absorption of both g-C3N4/FeN4 and g-C3N4/CoN4 interfaces are higher compared to its isolated analogs and (ii) a significant enhancement of band gap energy in g-C3N4/FeN4 and g-C3N4/CoN4 heterostructures limited the electron-hole recombination significantly. The potential of the g-C3N4/MN4 heterojunctions as a photocatalyst for the water splitting reaction was assessed by examining its band alignment for water splitting reaction. Importantly, while the electronic and magnetic properties of MN4 systems were studied, this is the first example of inclusion of MN4 on graphene-based material (g-C3N4) for studying the photocatalytic activity. The state of the art DFT calculations emphasis that g-C3N4/FeN4 and g-C3N4/CoN4 heterojunctions are half metallic photocatalysts, which is limited till date.

Hydrogen Bonds and In situ Photoinduced Metallic Bi0/Ni0 Accelerating Z-Scheme Charge Transfer of BiOBr@NiFe-LDH for Highly Efficient Photocatalysis

For heterojunction system, the lack of stable interfacial driving force and definite charge transfer channel makes the charge separation and transfer efficiency unsatisfactory. The photoreaction mechanism occurring at the interface also receives less attention. Herein, a 2D/2D Z-scheme junction BiOBr@NiFe-LDH with large-area contact featured by short interface hydrogen bonds and strong interfacial electric field (IEF) is synthesized, and in situ photoinduced metallic species assisting charge transfer mechanism is demonstrated. The hydrogen bonds between O atoms from BiOBr and H atoms from NiFe-LDH induce a significant interfacial charge redistribution, establishing a robust IEF. Notably, during photocatalytic reaction, Bi0 and Ni0 are in situ performed in heterojunction, which separately act as electron transport mediator and electron trap to further accelerate charge transfer efficiency up to 71.2 %. Theoretical calculations further demonstrate that the existence of Bi0 strengthens the IEF. Therefore, high-speed spatial charge separation is realized in Bi0/BiOBr@Ni0/NiFe-LDH, leading to a prominent photocatalytic activity with a tetracycline removal ratio of 88.3 % within 7 min under visible-light irradiation and the presence of persulfate, far exceeding majority of photocatalysts reported previously. This study provides valid insights for designing hydrogen bonding heterojunction systems, and advances mechanistic understanding on in situ photoreaction at interfaces.

Theoretical Study on Photocatalytic Reduction of CO2 on Anatase/Rutile Mixed-Phase TiO2

The construction of anatase/rutile heterojunctions in TiO2 is an effective way of improving the CO2 photoreduction activity. Yet, the origin of the superior photocatalytic performance is still unclear. To solve this issue, the band edges between anatase and rutile phases were theoretically determined based on the three-phase atomic model of (112)A/II/(101)R, and simultaneously the CO2 reduction processes were meticulously investigated. Our calculations show that photogenerated holes can move readily from anatase to rutile via the thin intermediated II phase, while photoelectrons flowing in the opposite direction may be impeded due to the electron trapping sites at the II phase. However, the large potential drop across the anatase/rutile interface and the strong built-in electric field can provide an effective driving force for photoelectrons' migration to anatase. In addition, the II phase can better enhance the solar light utilization of (112)A/(100)II, including a wide light response range and an intensive optical absorption coefficient. Meanwhile, the mixed-phase TiO2 possesses negligible hydrogenation energy (CO2 to COOH*) and lower rate-limiting energy (HCOOH* to HCO*), which greatly facilitate CH3OH generation. The efficient charge separation, strengthened light absorption, and facile CO2 reduction successfully demonstrate that the anatase/rutile mixed-phase TiO2 is an efficient photocatalyst utilized for CO2 conversion.

Density functional theory-based modeling of the half-metallic g-C3N4/CoN4 heterojunction for photocatalytic water splitting reaction

Using density functional theory (DFT), we have investigated the structural, optical, electronic and magnetic properties of a graphitic carbon nitride (g-C3N4) and CoN4 composite to explore the effect of the heterojunction on the photocatalytic performance of g-C3N4. The structure of g-C3N4 is modified while complexing with CoN4 and the corresponding stabilization is confirmed through adhesion energy calculation. The phonon spectra analysis furthermore guaranteed the lattice-dynamic stability of the CoN4 bulk and the CoN4 slab. Pristine g-C3N4 is a wide band gap semiconductor, which becomes half metallic upon CoN4 inclusion. The metallicity in the g-C3N4/CoN4 composite originates from the spin down channel, keeping the spin up channel in a semiconducting state. The charge density analysis and work function calculation suggest a substantial amount of charge transfer from g-C3N4 to the CoN4 unit in the g-C3N4/CoN4 heterojunction. The model heterojunction of the g-C3N4/CoN4 composite can enhance the utilization ratio of visible light for the g-C3N4 photocatalyst. In g-C3N4/CoN4, the valence band maximum (VBM) has a more positive potential compared to O2/H2O (+1.23 V) on the normal hydrogen electrode (NHE) scale. However, the conduction band minimum (CBM) displays a more negative potential compared to H+/H2 (0 V) on the NHE scale. The details of the band structure, density of states and band edge position determining calculations confirm that the g-C3N4/CoN4 composite forms a type 1 heterojunction, making it a suitable photocatalyst for water splitting reaction. The practical application of the g-C3N4/CoN4 heterostructure as a photocatalyst was substantiated in the presence of polar solvent (water) by calculating the band gap, charge transfer interaction and charge density difference. There is a significant decrease of charge transfer and thereby charge density difference in the g-C3N4/CoN4 heterojunction in the presence of water; however, it still holds potential for use as a photocatalyst for water splitting reaction. The state-of-the-art theoretical modeling of the g-C3N4/CoN4 heterojunction is the first theoretical study incorporating the CoN4 crystal.

Study of CuSb bimetallic flow-through gas diffusion electrodes for efficient electrochemical CO2 reduction to CO

Electrochemical CO2 reduction (eCO2R) to industrially important feedstock has received great attention, but it faces different challenges. Among them, the poor CO2 mass transport due to low intrinsic CO2 solubility significantly limits the rate of reduction reactions, leading to lower catalytic performance; thereby, commercially relevant current densities can't be achieved. Moreover, the poor activity and selectivity of high-cost monometallic catalysts, including Cu, Zn, Ag, and Au, undermine the efficiency of eCO2R. Flow-through gas diffusion electrodes (FTGDE), a newly developed class of GDEs, can potentially solve the issue of poor CO2 mass transport because they directly feed the CO2 to the catalyst layer. In addition, abundant surface area, porous structure, and improved triple-phase interface make them an excellent candidate for extremely high rate eCO2R. Antimony, a low-cost and abundant metalloid, can be effectively tuned with Cu to produce useful products such as CO, formate, and C2H4 through eCO2R. Herein, a series of porous binary CuSb FTGDEs with different Sb compositions are fabricated for the electrocatalytic reduction of CO2 to CO. The results show that the catalytic performance of CuSb FTGDEs improved with increasing Sb content up to a certain threshold, beyond which it started to decrease. The CuSb FTGDE with 5.4 g of antimony demonstrated higher current density (206.4 mA/cm2) and faradaic efficiency (72.82 %) at relatively lower overpotentials. Compared to gas diffusion configuration, the poor catalytic activity and selectivity achieved by CuSb FTGDE in non-gas diffusion configuration signifies the importance of improved local CO2 concentration and improved triple-phase interface formation in GDE configuration. The several hours stable operation of CuSb FTGDEs during eCO2R demonstrates its potential for efficient electrocatalytic conversion applications.

Construction of 2D S-Scheme Heterojunction Photocatalyst

Semiconductor photocatalytic technology holds immense promise for converting sustainable solar energy into chemically storable energy, with significant applications in the realms of energy and the environment. However, the inherent issue of rapid recombination of photogenerated electrons and holes hinders the performance of single photocatalysts. To overcome this challenge, the construction of 2D S-scheme heterojunction photocatalysts emerges as an effective strategy. The deliberate design of dimensionality ensures a substantial interfacial area; while, the S-scheme charge transfer mechanism facilitates efficient charge separation and maximizes redox capabilities. This review commences with a fresh perspective on the charge transfer mechanism in S-scheme heterojunctions, followed by a comprehensive exploration of preparation methods and characterization techniques. Subsequently, the recent advancements in 2D S-scheme heterojunction photocatalysts are summarized. Notably, the mechanism behind activity enhancement is elucidated. Finally, the prospects for the development of 2D S-scheme photocatalysts are presented.

Ultrasound-Driven Non-Metallic Fenton-Active Center Construction for Extensive Chemodynamic Therapy

Chemodynamic therapy (CDT) is an emerging tumor microenvironment-responsive cancer therapeutic strategy based on Fenton/Fenton-like reactions. However, the effectiveness of CDT is subject to the slow kinetic rate and non-homogeneous distribution of H2 O2 . In this study, a conceptual non-metallic "Fenton-active" center construction strategy is proposed to enhance CDT efficiency using Bi0.44 Ba0.06 Na0.5 TiO2.97 (BNBT-6) nanocrystals. The separated charge carriers under a piezoelectric-induced electric field synchronize the oxidation of H2 O and reduction of H2 O2 , which consequently increases hydroxyl radical (·OH) yield even under low H2 O2 levels. Moreover, acceptor doping induces electron-rich oxygen vacancies to facilitate the dissociation of H2 O2 and H2 O and further promote ·OH generation. In vitro and in vivo experiments demonstrate that BNBT-6 induces extensive intracellular oxidative stress and enhances cell-killing efficiency by activating necroptosis in addition to the conventional apoptotic pathway. This study proposes a novel design approach for nanomaterials used in CDT and presents a new treatment strategy for apoptosis-resistant tumors.

2D Bi2MoO6/Zn3V2O8 heterojunction photocatalyst for efficient photocatalytic reduction of CO2 to CO and CH4

Two-dimensional (2D) "face-to-face" heterojunctions promote interfacial charge transfer and separation in composite photocatalysts. Here, we report an efficient 2D/2D step-scheme (S-scheme) photocatalyst composed of Bi2MoO6/Zn3V2O8 (BMO/ZVO), which has been designed and prepared via the self-assembly of BMO and ZVO nanoflakes. The heterojunction with an optimized composition of 30% BMO/ZVO showed extended light absorption capacity and enhanced separation efficiency of photogenerated carriers. Density functional theory (DFT) calculation on work function and charge density revealed the presence of a built-in electric field at the interface region, which should facilitate the separation of photogenerated electron-hole pairs. This work showed that it is essential to select two photocatalysts with interlaced band arrangement and to fine-tune the heterojunction interface for the preparation of S-scheme heterojunctions to achieve high photocatalytic efficiency.

A DFT study on the probability of using the heteroatom decorated graphitic carbonitride (g-C3N4) species for delivering of three novel Multiple sclerosis drugs

In this project, the possibility of drug delivery application of three anti-Multiple sclerosis (MS) agents (containing diroximel fumarate (DXF), dimethyl fumarate (DMF), and mono methyl fumarate (MMF)) by using some heteroatom decorated graphitic carbonitride (g-C3N4) (as nano-sized carriers) have been systematically investigated. The results of the study have indicated that As-g-C3N4 QD is not a suitable candidate for drug delivery (at least in the cases of DMF, and DXF drugs); while, it would be an accurate semiconductor sensor for selective detection of each mentioned agents. Also, the use of the P-doped as well as pristine g-C3N4 QD could lead to weak electronic signals with relatively same values (in electronvolts). It means that P-g-C3N4, and g-C3N4 QDs are not good sensors for detection of each of the three considered drugs. However, those two sorbents would be suitable carriers for delivering of all three mentioned pharmaceuticals.

Construction of Z scheme S-g-C3N4/Bi5O7I photocatalysts for enhanced photocatalytic removal of Hg0 and carrier separation

Photocatalysis has demonstrated the potential to solve challenges in various practical application fields such as energy and environmental science due to its environmental friendliness. However, the photocatalytic activity is mainly affected by the weak absorption of visible light and the low separation efficiency of photogenerated carriers. Herein, an S-doped g-C₃N₄/Bi₅O₇I heterojunction was designed by the calcination method. It was found that S doping not only reduces the band gap of g-C₃N₄, which raises the optical absorption boundary of g-C₃N₄ from 465 nm to 550 nm. At the same time, the introduction of S elements leads to new doping energy levels, which can act as photogenerated electron trapping centers and thus inhibit the complexation of photogenerated carriers. Second, the construction of the heterojunction greatly facilitates the transport of carriers and the separation of electrons and holes driven by the built-in electric field. Finally, the abundant oxygen vacancies in the system result in defective energy levels that not only promote the activation of molecular oxygen, but also act as photogenerated electron traps, which further boost the separation of electron-hole pairs. Benefiting from the optimized performance, the photocatalytic reaction rates of S-doped g-C₃N₄/Bi₅O₇I are 5.2 and 2.1 times higher than those of g-C₃N₄ and Bi₅O₇I, respectively. This work provides a viable idea for the potential development of non-metal doping combined with heterojunction photocatalytic systems.

Photocatalytic Cascade Reaction Driven by Directed Charge Transfer over VS -Zn0.5 Cd0.5 S/GO for Controllable Benzyl Oxidation

Photocatalysis is an important technique for synthetic transformations. However, little attention has been paid to light-driven synergistic redox reactions for directed synthesis. Herein, the authors report tunable oxidation of benzyl to phenylcarbinol with the modest yield (47%) in 5 h via singlet oxygen (¹ O₂ ) and proton-coupled electron transfer (PCET) over the photocatalyst Zn_0.5 Cd_0.5 S (ZCS)/graphene oxide (GO) under exceptionally mild conditions. Theoretical calculations indicate that the presence of S vacancies on the surface of ZCS/GO photocatalyst is crucial for the adsorption and activation of O₂ , successively generating the superoxide radical (^• O₂ ^- ) and ¹ O₂ , attributing to the regulation of local electron density on the surface of ZCS/GO and photogenerated holes (h⁺ ). Meanwhile, accelerated transfer of photogenerated electrons (e^- ) to GO caused by the π-π stacking effect is conducive to the subsequent aldehyde hydrogenation to benzyl alcohol rather than non-selective oxidation of aldehyde to carboxylic acid. Anisotropic charge transport driven by the built-in electric field can further promote the separation of e^- and h⁺ for multistep reactions. Promisingly, one-pot photocatalytic conversion of p-xylene to 4-methylbenzyl alcohol is beneficial for reducing the harmful effects of aromatics on human health. Furthermore, this study provides novel insights into the design of photocatalysts for cascade reactions.

Using the aluminum decorated graphitic-C3N4 quantum dote (QD) as a sensor, sorbent, and photocatalyst for artificial photosynthesis; a DFT study

In this project, we have investigated the possibility of mimicking the natural photosynthesis, as well as sensing and adsorption application of aluminum decorated graphitic C₃N₄ (Al-g-C₃N₄) QDs (toward some air pollutants containing CO, CO₂, and SO₂). The results of the potential energy surface (PES) studies show that in all three adsorption processes, the energy changes are negative (-10.70 kcal mol^-1, -16.81 kcal mol^-1, and -79.97 kcal mol^-1 for CO, CO₂, and SO₂ gasses, respectively). Thus, all of the adsorption processes (mainly SO₂) are spontaneous. Moreover, the frontier molecular orbital (FMO) investigations indicate that the Al-g-C₃N₄ QD could be used as a suitable semiconductor sensor for detection of CO, and CO₂ (as carbon oxides) in one hand, and SO₂ gaseous species on the other hand. Finally, the results reveal that those QDs could be applied for artificial photosynthesis (in presence of CO₂; Δμ^h-e = 1.43 V), and for water splitting process for the H₂ generation (Δμ^h-e = 1.23 V) as a clean fuel for near future.

Emerging S-Scheme Photocatalyst

Photocatalysis is a green technology to use ubiquitous and intermittent sunlight. The emerging S-scheme heterojunction has demonstrated its superiority in photocatalysis. This article covers the state-of-the-art progress and provides new insights into its general designing criteria. It starts with the challenges confronted by single photocatalyst from the perspective of energy dissipation by borrowing the common behaviors in the dye molecule. Subsequently, other problems faced by single photocatalyst are summarized. Then a viable solution for these problems is the construction of heterojunctions. To overcome the problems and mistakes of type-II and Z-scheme heterojunctions, S-scheme heterojunction is proposed and the underlying reaction mechanism is summarized. Afterward, the design principles for S-scheme heterojunction are proposed and four types of S-scheme heterojunctions are suggested. Following this, direct characterization techniques for testifying the charge transfer in S-scheme heterojunction are presented. Finally, different photocatalytic applications of S-scheme heterojunctions are summarized. Specifically, this work endeavors to clarify the critical understanding on curved Fermi level in S-scheme heterojunction interface, which can help strengthen and advance the fundamental theories of photocatalysis. Moreover, the current challenges and prospects of the S-scheme heterojunction photocatalyst are critically discussed.

Biomass-Induced Diphasic Carbon Decoration for Carbon Nitride: Band and Electronic Engineering Targeting Efficient N2 Photofixation

Boosting the replacement of traditional NH₃ production (Haber-Bosch process) with photocatalytic technology is of great importance for energy and environment remediation. Herein, to develop a photocatalyst with efficient charge separation and abundant reactive sites for photocatalytic N₂ fixation, a biomass-induced diphase-carbon doping strategy is proposed by adding lotus root starch which can be environmentally produced into the preparation of carbon nitride (CN). The adjustment to the CN framework by planar-fused carbon optimizes the band alignment of the catalyst, improving its response to sunlight. In particular, the in-plane-fused carbon in collaboration with the physically piled carbon initiates unique dual electron transfer pathways from different dimensions. The diphasic carbons can both function as qualified reactive sites according to the experimental explorations and further theoretical calculations, which effectively regulate the electron transfer and energy barrier associated with the N₂ reduction on catalyst. The bio-carbon-doped catalyst exhibits drastically enhanced photocatalytic N₂ fixation performance, and the NH₃ yield on the optimized DC-CN0.1 reaches 167.35 µmol g^-1 h^-1 , which is fivefold of g-C₃ N₄ and stands far out from the single-phase doped systems. These explorations expand the metal-free skeleton engineering toolbox and provide new guidance for the solar energy utilizations.

Recent advancements of g-C3N4-based magnetic photocatalysts towards the degradation of organic pollutants: a review

Heterogeneous photocatalysis premised on advanced oxidation processes has witnessed a broad application perspective, including water purification and environmental remediation. In particular, the graphitic carbon nitride (g-C₃N₄), an earth-abundant metal-free conjugated polymer, has acquired extensive application scope and interdisciplinary consideration owing to its outstanding structural and physicochemical properties. However, several issues such as the high recombination rate of the photo-generated electron-hole pairs, smaller specific surface area, and lower electrical conductivity curtail the catalytic efficacy of bulk g-C₃N₄. Another challenging task is separating the catalyst from the reaction medium, limiting their reusability and practical applications. Therefore, several methodologies are adopted strategically to tackle these issues. Attention is being paid, especially to the magnetic nanocomposites (NCs) based catalysts to enhance efficiency and proficient reusability property. This review summarizes the latest progress related to the design and development of magnetic g-C₃N₄-based NCs and their utilization in photocatalytic systems. The usefulness of the semiconductor heterojunctions on the catalytic activity, working mechanism, and degradation of pollutants are discussed in detail. The major challenges and prospects of using magnetic g-C₃N₄-based NCs for photocatalytic applications are highlighted in this report.