First-Principles Evaluation of Volatile Organic Compounds Degradation in Z-Scheme Photocatalytic Systems: MXene and Graphitic-CN Heterostructures

Ferroelectric polarization promotes a CdS/In2Se3 heterostructure for photocatalytic water splitting

The rapid recombination of photogenerated electrons and holes greatly limits the efficiency of photocatalyst based on semiconductor. In order to address this issue, we predicted a novel ferroelectric polarized heterojunction photocatalyst, CdS/In2Se3, which enables the spontaneous overall water splitting reaction. The CdS/In2Se3 heterojunction exhibits a band-edge staggered alignment and it is easy to form a direct Z-scheme charge transfer pathway. Besides, the built-in electric field (Eint) in the CdS/In2Se3 heterojunction promoted the charge transfer of CdS/In2Se3, leading to an improved separating efficiency of photo-generated carriers. Moreover, the vertical intrinsic polarized electric field (Ep) not only alters the position of the band edge but also reduces the bandgap limitations commonly associated with photocatalytic materials. Furthermore, the CdS/In2Se3 heterojunctions demonstrate separate catalytic activity for the hydrogen evolution reaction (HER) on the surface of the CdS monolayer and oxygen evolution reaction (OER) on the surface of In2Se3, respectively. Notably, the CdS/In2Se3-down configuration enables spontaneous photocatalytic water splitting in pH = 7, while the CdS/In2Se3-up configuration efficiently facilitates the HER process. This study highlights the significant advantages of CdS/In2Se3 heterojunctions as photocatalytic materials, offering unique insights into the development and research of this promising heterojunction architecture.

Efficient photocatalytic degradation of antibiotics by binary heterojunction complex boron subphthalocyanine bromide/bismuth oxychloride

The development of stable and efficient heterojunction photocatalysts for wastewater environmental purification exhibits a significant challenge. Herien, a promising binary heterojunction complex comprising boron subphthalocyanine bromide/bismuth oxychloride (SubPc-Br/BiOCl) was successfully synthesized using the hydrothermal method, which involved the self-assembled of SubPc-Br on the surface of BiOCl via intermolecular π-π stacking interactions to compose an electron-transporting layer. The photocatalytic efficiency of SubPc-Br/BiOCl for the degradation of tetracycline and the minocycline exhibited a substantial improvement of 29.14% and 53.72%, respectively, compared to the original BiOCl. Experimental characterization and theoretical calculations elucidated that the enhanced photocatalytic performance of the SubPc-Br/BiOCl composite photocatalysts stemmed from the S-scheme electron transport mechanism at the interface between BiOCl and SubPc-Br supramolecules, which broadened the visible light absorption range, increased the carrier molecular efficiency, and accelerated the carriers. Furthermore, molecular dynamic (MD) simulations provided insights into the action trajectories of the two semiconductors, revealing that the presence of SubPc-Br enhances the water and organic pollutant adsorption capabilities of the BiOCl surface within the supramolecular array system. In conclusion, the synthesis and analysis of the binary heterojunction complex SubPc-Br/BiOCl yield valuable insights into the efficient photocatalytic degradation of antibiotics, holding great promise for diverse environmental applications.

Unveiling the intrinsic role of water in the catalytic cycle of formaldehyde oxidation: a comprehensive study integrating density functional theory and microkinetic analysis

Previous research is predominantly in consensus on the reaction mechanism between formaldehyde (HCHO) and oxygen (O2) over catalysts. However, water vapor (H2O) always remains present during the reaction, and the intrinsic role of H2O in the oxidation of HCHO still needs to be fully understood. In this study, a single-atom catalyst, Al-doped C2N substrate, Al1/C2N, can be adopted as an example to investigate the relationship and interaction among O2, H2O, and HCHO. Density functional theory (DFT) calculations and microkinetic simulations were carried out to interpret the enhancement mechanism of H2O on HCHO oxidation over Al1/C2N. The outcome demonstrates that H2O directly breaks down a surface hydroxyl group on Al1/C2N, considerably lowering the energy required to form crucial intermediates, thus promoting oxidation. Without H2O, Al1/C2N cannot effectively oxidize HCHO at ambient temperature. During oxidation, H2O takes the major catalytic responsibility, delaying the entrance of O2 into the reaction, which is not only the product but also the crucial reactant to initiate catalysis, thereby sustaining the catalytic cycle. Moreover, this study predicts the catalytic behavior at various temperatures and presents feasible recommendations for regulating the reaction rates. The oxidation mechanism of HCHO is explained at the molecular level in this study, emphasizing the intrinsic role of water on Al1/C2N, which fills in the relevant studies for HCHO oxidation on two-dimensional carbon materials.

In situ fabrication of Z-scheme C3N4/Ti3C2/CdS for efficient photocatalytic hydrogen peroxide production

Photocatalysis is a potential technology to produce hydrogen peroxide with low energy consumption and no pollution. However, when using traditional photocatalysts it is hard to meet the requirements of wide visible light absorption, high carrier separation rate and sufficient active sites. Graphitic carbon nitride (g-C3N4) has great potential in the photocatalytic production of hydrogen peroxide, but its photocatalytic performance is limited by a high carrier recombination ratio. Here, we fabricated the Z-Scheme heterojunction of C3N4/Ti3C2/CdS in situ. The large specific surface area of C3N4 can provide plenty of reactive sites, and the absorption efficiency under visible light is improved with the addition of Ti3C2 and CdS. The better conductivity of Ti3C2 reduces the charge transfer resistance. With the increase of surface charge carriers, the width of the space charge region decreases and the photocurrent density increases significantly. Under visible light irradiation, the H2O2 yield of the ternary photocatalyst reaches 256 μM L-1 h-1, which is about 6 times that of pristine C3N4. After three cycles, the high photocatalytic efficiency can still be maintained. In this paper, the reaction mechanism of photocatalytic hydrogen peroxide production by the C3N4/Ti3C2/CdS composite material is proposed through an in-depth study of energy band theory, which provides a new reference for the design and preparation of high-performance materials for photocatalytic hydrogen peroxide production.

A novel broken-gap chemical-bonded SiC/Ti2CO2 heterojunction with band to band tunneling: first-principles investigation

A broken-gap heterojunction is a bright approach for designing tunneling field-effect transistors (TFETs) due to its distinct quantum tunneling mechanisms. In this study, we investigate the electronic structure and transport characteristics of a SiC/Ti2CO2 heterojunction, as well as the impacts of electric field and strain on the electronic properties via density functional theory. We determine that the interfacial atoms of the heterojunction are covalently bonded, forming a type-III heterojunction with a broken-gap. There exists band-to-band tunneling (BTBT) from the valence band (VB) of SiC to the conduction band (CB) of Ti2CO2. The creation of the heterojunction also enhances the carrier mobility arising from the large elastic modulus and the decrease of deformation potential. The current-voltage (I-V) characteristics of the device demonstrate a pronounced negative differential resistance (NDR) effect, along with a current that is about ten times greater than that of the vdW type-III heterojunction. Moreover, the tunneling window of SiC/Ti2CO2 is only slightly altered when subjected to an external electric field and vertical strain, demonstrating the remarkable stability of its type-III band alignments. Our results indicate that the SiC/Ti2CO2 heterojunction is useful to construct high-performance TFETs, and also introduces new ideas to design TFETs by using type-III covalent-bond heterojunctions.

Stable Loading of TiO2 Catalysts on the Surface of Metal Substrate for Enhanced Photocatalytic Toluene Oxidation

To promote the practical application of TiO2 in photocatalytic toluene oxidation, the honeycomb aluminum plates were selected as the metal substrate for the loading of TiO2 powder. Surface-etching treatment was performed and titanium tetrachloride was selected as the binder to strengthen the loading stability. The loading stability and photocatalytic activity of the monolithic catalyst were further investigated, and the optimal surface treatment scheme (acid etching with 15.0 wt.% HNO3 solution for 15 min impregnation) was proposed. Therein, the optimal monolithic catalyst could achieve the loading efficiency of 42.4% and toluene degradation efficiencies of 76.2%. The mechanism for the stable loading of TiO2 was revealed by experiment and DFT calculation. The high surface roughness of metal substrate and the strong chemisorption between TiO2 and TiCl4 accounted for the high loading efficiency and photocatalytic activity. This work provides the pioneering exploration for the practical application of TiO2 catalysts loaded on the surface of metal substrate for VOCs removal, which is of significance for the large-scaled application of photocatalytic technology.

Toluene decomposition by non-thermal plasma assisted CoOx - γ-Al2O3: The relative contributions of specific energy input of plasma, Co3+ and oxygen vacancy

Cobalt oxide (CoO_x) is a common catalyst for plasma catalytic elimination of volatile organic compounds (VOCs). However, the catalytic mechanism of CoO_x under radiation of plasma is still unclear, such as how the relative importance of the intrinsic structure of the catalyst (e.g., Co³⁺ and oxygen vacancy) and the specific energy input (SEI) of the plasma for toluene decomposition performance. CoO_x - γ-Al₂O₃ catalysts were prepared and evaluated by toluene decomposition performance. Changing the calcination temperature of the catalyst altered the content of Co³⁺ and oxygen vacancies in CoO_x, resulting in different catalytic performance. The results of the artificial neural network (ANN) models presented that the relative importance of three reaction parameters (SEI, Co³⁺, and oxygen vacancy) on the mineralization rate and CO₂ selectivity were as follows: SEI > oxygen vacancy > Co³⁺ , and SEI > Co³⁺ > oxygen vacancy, respectively. Oxygen vacancy is essential for mineralization rate, and CO₂ selectivity is more dependent on Co³⁺ content. Furthermore, a possible reaction mechanism of toluene decomposition was proposed according to the analysis results of in-situ DRIFTS and PTR-TOF-MS. This work provides new ideas for the rational design of CoO_x catalysts in plasma catalytic systems.

Biomass derived reduced-graphene-oxide supported α-Fe2O3/ZnO S-scheme heterostructure: Robust photocatalytic wastewater remediation

The emergence of new diseases and the unplanned industrialization of cities have led to new diseases and the subsequent use of antibiotics. Hence the remediation of wastewater containing antibiotics and their severe pollution has raised serious concerns in recent years. Herein coral-shaped α-Fe₂O₃/ZnO/reduced graphene oxide (r-GO)-like carbon heterojunction in-situ were prepared from basil seed as a sustainable biomass resource and applied for the photodegradation of the oxytetracycline (OTC) as a typical antibiotic in a helical plug flow photoreactor (HPFPR) via persulfate activation under visible light irradiation. Spectroscopy and electrochemical results confirmed the tunable band structure and quick light absorption, superior charge separation and transfer, satisfactory charge carrier lifetime, and long-term stability for the prepared photocatalyst. The 98% degradation efficiency was achieved for OTC within 90 min fitted by a first-order kinetic model with the rate constant of 0.1248 min^-1. The finding proves that HPFPR exhibited a higher degradation rate of OTC by 2.3 times compared to the batch reactor. The 3D computational fluid dynamics (CFD) model confirmed the outstanding performance of the HPFPR. Scavenging experiments integrated with mott Schottky and DRS results revealed that rGO intensifies the S-scheme charge carrier transfer and built-in electric field and reduces the recombination. Finally, this work has substantial potential for the in-situ synthesis of environmental-friendly and large-scale metal oxide heterojunctions in natural carbon supports as well as scale-up and gives novel insights from molecular and engineering points of view into the wastewater remediation processes and clean water production.

Computational study on persulfate activation by two-dimensional carbon materials with various nitrogen proportions for carbamazepine oxidation in wastewater: The essential role of graphitic N atoms

Two-dimensional carbon materials with various N atom proportions (2D-CNMs) are constructed to clarify the optimal catalyst for carbamazepine (CBZ) oxidation and the inner mechanism for persulfate-based advanced oxidation processes (P-AOPs). Results show that peroxydisulfate (PDS) can be activated by all 2D-CNMs with the order of C₃N > C₇₁N > graphene > C₂N > CN, while C₃N is the only catalyst for peroxymonosulfate (PMS) activation. The C₃N with the maximum graphitic N can activate PDS and PMS in a wide temperature range at any pH, and demonstrates the optimal CBZ oxidation performance. Notably, the graphitic N atoms promote P-AOPs from five aspects: (i) electron structure, (ii) electrical conductivity, (iii) electron transfer from persulfate to catalysts, (iv) electron jump of co-system before and after activation, (v) interaction between catalyst and persulfate. The most vigorous activity of C₃N is attributed to the greatest number of graphitic N. This work clarifies the essential role of graphitic N atoms with implications for the catalyst design, and facilitates the environmental applications of P-AOPs for micropollutant abatement.

Interfacial Charge-Modulated Multifunctional MoS2/Ti3C2Tx Penetrating Electrode for High-Efficiency Freshwater Production

Freshwater production is critical in terms of solving the global water shortage. Aiming at improving freshwater production capability and ensuring its quality, an interfacial charge-modulated MoS₂/Ti₃C₂T_x-modified carbon fiber (CF/MoS₂/Ti₃C₂T_x) penetrating electrode is designed. To maximize the desalination and degradation efficiencies of CF/MoS₂/Ti₃C₂T_x, a photocatalytic component is introduced into the membrane capacitive deionization (PMCDI) device. High desalination capability is derived from the lamellar architecture structure of MoS₂/Ti₃C₂T_x. Meanwhile, excellent degradation performance is due to the formation of two photoelctrocatalytic activity centers, directionally generating singlet oxygen (¹O₂) and hydroxyl radical (^•OH). The intercalated Cl^- (desalination) as the electron transfer bridge optimizes the charge distribution of MoS₂/Ti₃C₂T_x, reinforcing the photoelectrocatalytic activity (degradation). The formation of the electron-deficient (desalination) and electron-rich (regeneration) regions at the terminated O atom of Ti₃C₂T_x accelerate the generations of ^•OH and ¹O₂, respectively. In perspective, a mutual promotion process of desalination and degradation is achieved for high-efficiency production of high-quality freshwater.

Mechanism of the two-dimensional WSeTe/Zr2CO2 direct Z-scheme van der Waals heterojunction as a photocatalyst for water splitting

The direct Z-scheme van der Waals (vdW) heterojunctions based on biomimetic artificial photosynthesis are a promising strategy for enhancing photocatalytic activity. Therefore, the search for superior direct Z-scheme photocatalysts is an urgent task. Herein, we predicted the WSeTe/Zr₂CO₂ heterostructure as a potential direct Z-scheme photocatalyst based on density functional theory (DFT). The bands of the WSeTe/Zr₂CO₂ heterojunction follow a typical Type-II arrangement, where the interlayer band gap is smaller than that of the individual molecular layers, and staggered alignment of the large band-edge creates conditions that allow for a direct Z-scheme. The position of the Fermi energy levels of the two monolayers determines the formation of the built-in electric field pointing from WSeTe to Zr₂CO₂, promoting the desired interlayer electron-hole (e^--h⁺) recombination and suppressing the undesired carrier recombination. Finally, in-plane biaxial strain can effectively modulate the optoelectronic properties of the catalyst, while compressive strain has a more pronounced effect on the overpotential driving force of the material. Therefore, the WSeTe/Zr₂CO₂ heterojunction is an effective new photocatalyst satisfying the direct Z-scheme charge transfer mechanism with its specific application.

Hybrid MXene-Graphene/Hexagonal Boron Nitride Structures: Electronic and Molecular Adsorption Properties

Recent advances in experimental techniques allow for the fabrication of hybrid structures. Here, we study the electronic and molecular adsorption properties of the graphene (G)/hexagonal boron nitride (h-BN)-MXenes (Mo2C) hybrid nanosheets. We use first-principles calculations to explore the structure and electronic properties of the hybrid structures of G-2H-Mo2C and h-BN-2H-Mo2C with two different oxygen terminations of the Mo2C surface. The embedding of G or h-BN patches creates structural defects at the patch-Mo2C border and adds new states in the vicinity of the Fermi energy. Since this can be utilized for molecular adsorption and/or sensing, we investigate the ability of the G-M-O1 and BN-M-O1 hybrid structures to adsorb twelve molecules. Generally, the adsorption on the hybrid systems is significantly higher than on the pristine systems, except for N2 and H2, which are weakly adsorbed on all systems. We find that OH, NO, NO2, and SO2 are chemisorbed on the hybrid systems. COOH may be chemisorbed, or it may dissociate depending on its location at the edge between the G/h-BN and the MXene. NH3 is chemisorbed/physisorbed on the BN/G-M-O1 systems. CO, H2S, CO2, and CH4 are physisorbed on the hybrid systems. Our results indicate that the studied hybrid systems can be used for molecular filtration/sensing and catalysis.

Confining Metal-Organic Framework in the Pore of Covalent Organic Framework: A Microscale Z-Scheme System for Boosting Photocatalytic Performance

The search for building hierarchical porous materials with accelerated photo-induced electrons and charge-carrier separation is important because they hold great promise for applications in various fields. Here, a facile strategy of confining metal-organic framework (MOF) in the 1D channel of the 2D covalent organic framework (COF) to construct a novel COF@MOF micro/nanopore network is proposed. Specifically, a nitrogen-riched COF (TTA-BPDA-COF) is chosen as the platform for in-situ growth of a Co-based MOF (ZIF-L-Co) to form a TTA-BPDA-COF@ZIF-L-Co hybrid material. The hierarchical porous structure endows TTA-BPDA-COF@ZIF-L-Co with superior adsorption capacity. In addition, the integration of TTA-BPDA-COF and ZIF-L-Co forms a Z-scheme photocatalytic system, which significantly improved the redox properties and accelerated the separation of photogenerated charges and holes, achieving great improvement in photocatalytic activity. This confinement engineering strategy provides a new idea to construct a versatile molecular-material photocatalytic platform.

"Capture-Backdonation-Recapture" Mechanism for Promoting N2 Reduction by Heteronuclear Metal-Free Double-Atom Catalysts

Facing the increasingly serious energy and environmental crisis, the development of heteronuclear metal-free double-atom catalysts is a potential strategy to realize efficient catalytic nitrogen reduction with low energy consumption and no pollution because it could combine the advantages of flexible active sites in double-atom catalysts while also being pollution-free and have high Faraday efficiency in metal-free catalysts simultaneously. However, according to the existing mechanism, the finite orbits of other nonmetallic atoms, except the boron atom, reduce the possibility of metal-free catalysis and hinder the development of heteronuclear metal-free double-atom catalysts. Herein, we propose a new "capture-backdonation-recapture" mechanism, which skillfully uses the electron capture-backdonation-recapture between boron, the substrate, and other nonmetallic elements to solve the above problems. Based on this mechanism, by means of the first-principle calculations, the material structure, adsorption energy, catalytic activity, and selectivity of 36 catalysts are systematically investigated to evaluate their catalytic performance. B-Si@BP1 and B-Si@BP3 are selected for their good catalytic performance and low limiting potentials of -0.14 and -0.10 V, respectively. Meanwhile, the "capture-backdonation-recapture" mechanism is also verified by analyzing the results of adsorption energy and electron transfer. Our work broadens the ideas and lays the theoretical foundation for the development of heteronuclear metal-free double-atom catalysts in the future.

Designing oxygen vacancy mediated bismuth molybdate (Bi2MoO6)/N-rich carbon nitride (C3N5) S-scheme heterojunctions for boosted photocatalytic removal of tetracycline antibiotic and Cr(VI): Intermediate toxicity and mechanism insight

Polymeric N-rich carbon nitride of C₃N₅ is being utilized as a new visible-light-driven catalyst due to its narrower bandgap (∼2.0 eV). Building step-scheme (S-scheme) heterojunction by coupling with other semiconductors especially those own oxygen vacancies (OVs) can further upgrade the photocatalytic performance of C₃N₅-based photocatalysts. Herein, a novel S-scheme heterojunction of OVs mediated Bi₂MoO₆/C₃N₅ was fabricated by in-situ growing Bi₂MoO₆ nanoparticles with OVs on C₃N₅ nanosheets. Benefiting from the efficient separation and transfer of high energetic charge carriers by S-scheme charge migration, enriched structural defects, as well as the close contact by the in-situ growth, the heterojunction exhibited superior visible-light photocatalytic performance toward the removal of tetracycline (TC) and Cr(VI) than C₃N₅, Bi₂MoO₆, and their mechanical mixture under visible light. The TC degradation routes and the bio-toxicity evolution of TC were explored. Moreover, the photocatalytic mechanism for TC decomposition and Cr(VI) reduction over Bi₂MoO₆/C₃N₅ with OVs were elucidated. This work presents a newfangled vision for designing promising C₃N₅-based S-scheme heterojunction photocatalysts for pollution control.

Type-II Band Alignment and Tunable Optical Absorption in MoSSe/InS van der Waals Heterostructure

In this work, we study the electronic structure, the effective mass, and the optical properties of the MoSSe/InS van der Waals heterostructures (vdWHs) by first-principles calculations. The results indicate that the MoSSe/InS vdWH is an indirect band gap semiconductor and has type-Ⅱ band alignment in which the electrons and holes located at the InS and the MoSSe side, respectively. The band edge position, the band gap and the optical absorption of the MoSSe/InS vdWH can be tuned when biaxial strains are applied. In addition, compared with MoSSe and InS monolayers, the optical absorption of the MoSSe/InS vdWH is improved both in the visible and the ultraviolet regions. These findings indicate that the MoSSe/InS vdWHs have potential applications in optoelectronic devices.