D-A Conjugated Polymer/CdS S-Scheme Heterojunction with Enhanced Interfacial Charge Transfer for Efficient Photocatalytic Hydrogen Generation.

Owing to the improved charge separation and maximized redox capability of the system, Step-scheme (S-scheme) heterojunctions have garnered significant research attention for efficient photocatalysis of H2 evolution. In this work, an innovative linear donor-acceptor (D-A) conjugated polymer fluorene-alt-(benzo-thiophene-dione) (PFBTD) is coupled with the CdS nanosheets, forming the organic-inorganic S-scheme heterojunction. The CdS/PFBTD (CP) composite exhibits an impressed hydrogen production rate of 7.62 mmol g-1  h-1 without any co-catalysts, which is ≈14 times higher than pristine CdS. It is revealed that the outstanding photocatalytic performance is attributed to the formation of rapid electron transfer channels through the interfacial Cd─O bonding as evidenced by the density functional theory (DFT) calculations and in situ X-ray photoelectron spectroscopy (XPS) analysis. The charge transfer mechanism involved in S-scheme heterojunctions is further investigated through the photo-irradiated Kelvin probe force microscopy (KPFM) analysis. This work provides a new point of view on the mechanism of interfacial charge transfer and points out the direction of designing superior organic-inorganic S-scheme heterojunction photocatalysts.

Molecularly Tunable Heterostructured Co-Polymers Containing Electron-Deficient and -Rich Moieties for Visible-Light and Sacrificial-Agent-Free H2O2 Photosynthesis.

As an alternative to hydrogen peroxide (H2O2) production by complex anthraquinone oxidation process, photosynthesis of H2O2 from water and oxygen without sacrificial agents is highly demanded. Herein, a covalently connected molecular heterostructure is synthesized via sequential C-H arylation and Knoevenagel polymerization reactions for visible-light and sacrificial-agent-free H2O2 synthesis. The subsequent copolymerization of the electron-deficient benzodithiophene-4,8-dione (BTD) and the electron-rich biphenyl (B) and p-phenylenediacetonitrile (CN) not only expands the π-conjugated domain but also increases the molecular dipole moment, which largely promotes the separation and transfer of the photoinduced charge carriers. The optimal heterostructured BTDB-CN0.2 manifested an impressive photocatalytic H2O2 production rate of 1920 µmol g-1 h-1, which is 2.2 and 11.6 times that of BTDB and BTDCN. As revealed by the femtosecond transient absorption (fs-TA) and theoretical calculations, the linkage serves as a channel for the rapid transfer of photogenerated charge carriers, enhancing the photocatalytic efficiency. Further, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) uncovers that the oxygen reduction reaction occurs through the step one-electron pathway and the mutual conversion between C=O and C-OH with the anchoring of H+ during the catalysis favored the formation of H2O2. This work provides a novel perspective for the design of efficient organic photocatalysts.

Atomic Interface Engineering of Single-Atom Pt/TiO2 -Ti3 C2 for Boosting Photocatalytic CO2 Reduction.

Solar-driven CO2 conversion into valuable fuels is a promising strategy to alleviate the energy and environmental issues. However, inefficient charge separation and transfer greatly limits the photocatalytic CO2 reduction efficiency. Herein, single-atom Pt anchored on 3D hierarchical TiO2 -Ti3 C2 with atomic-scale interface engineering is successfully synthesized through an in situ transformation and photoreduction method. The in situ growth of TiO2 on Ti3 C2 nanosheets can not only provide interfacial driving force for the charge transport, but also create an atomic-level charge transfer channel for directional electron migration. Moreover, the single-atom Pt anchored on TiO2 or Ti3 C2 can effectively capture the photogenerated electrons through the atomic interfacial PtO bond with shortened charge migration distance, and simultaneously serve as active sites for CO2 adsorption and activation. Benefiting from the synergistic effect of the atomic interface engineering of single-atom Pt and interfacial TiOTi, the optimized photocatalyst exhibits excellent CO2 -to-CO conversion activity of 20.5 µmol g-1  h-1 with a selectivity of 96%, which is five times that of commercial TiO2 (P25). This work sheds new light on designing ideal atomic-scale interface and single-atom catalysts for efficient solar fuel conversation.

Strong sequentially bridged MXene sheets.

Titanium carbide (Ti3C2Tx) MXene has great potential for use in aerospace and flexible electronics due to its excellent electrical conductivity and mechanical properties. However, the assembly of MXene nanosheets into macroscopic high-performance nanocomposites is challenging, limiting MXene's practical applications. Here we describe our work fabricating strong and highly conductive MXene sheets through sequential bridging of hydrogen and ionic bonding. The ionic bonding agent decreases interplanar spacing and increases MXene nanosheet alignment, while the hydrogen bonding agent increases interplanar spacing and decreases MXene nanosheet alignment. Successive application of hydrogen and ionic bonding agents optimizes toughness, tensile strength, oxidation resistance in a humid environment, and resistance to sonication disintegration and mechanical abuse. The tensile strength of these MXene sheets reaches up to 436 MPa. The electrical conductivity and weight-normalized shielding efficiency are also as high as 2,988 S/cm and 58,929 dB∙cm2/g, respectively. The toughening and strengthening mechanisms are revealed by molecular-dynamics simulations. Our sequential bridging strategy opens an avenue for the assembly of other high-performance MXene nanocomposites.

Promoting Solar-driven Hydrogen Peroxide Production over Thiazole-based Conjugated Polymers via Generating and Converting Singlet Oxygen.

Solar-driven synthesis of hydrogen peroxide (H2 O2 ) from water and air provides a low-cost and eco-friendly alternative route to the traditional anthraquinone method. Herein, four thiazole-based conjugated polymers (Tz-CPs: TTz, BTz, TBTz and BBTz) are synthesized via aldimine condensation. BBTz exhibits the highest H2 O2 production rate of 7274 µmol g-1 h-1 in pure water. Further, the reaction path is analyzed by electron paramagnetic resonance (EPR), in situ diffuse reflectance infrared Fourier transform (DRIFT) and theoretical calculation, highlighting the prominent role of singlet oxygen (1 O2 ). The generation of 1 O2 occurs through the oxidation of superoxide radical (⋅O2 - ) and subsequent conversion into endoperoxides via [4+2] cycloaddition over BBTz, which promotes charge separation and reduces the barrier for H2 O2 production. This work provides new insight into the mechanism of photocatalytic O2 reduction and the molecular design of superior single-polymer photocatalysts.

Rapid Charge Transfer Endowed by Interfacial Ni-O Bonding in S-scheme Heterojunction for Efficient Photocatalytic H2 and Imine Production.

Cooperative coupling of H2 evolution with oxidative organic synthesis is promising in avoiding the use of sacrificial agents and producing hydrogen energy with value-added chemicals simultaneously. Nonetheless, the photocatalytic activity is obstructed by sluggish electron-hole separation and limited redox potentials. Herein, Ni-doped Zn0.2 Cd0.8 S quantum dots are chosen after screening by DFT simulation to couple with TiO2 microspheres, forming a step-scheme heterojunction. The Ni-doped configuration tunes the highly active S site for augmented H2 evolution, and the interfacial Ni-O bonds provide fast channels at the atomic level to lower the energy barrier for charge transfer. Also, DFT calculations reveal an enhanced built-in electric field in the heterojunction for superior charge migration and separation. Kinetic analysis by femtosecond transient absorption spectra demonstrates that expedited charge migration with electrons first transfer to Ni2+ and then to S sites. Therefore, the designed catalyst delivers drastically elevated H2 yield (4.55 mmol g-1 h-1 ) and N-benzylidenebenzylamine production rate (3.35 mmol g-1 h-1 ). This work provides atomic-scale insights into the coordinated modulation of active sites and built-in electric fields in step-scheme heterojunction for ameliorative photocatalytic performance.

High-strength scalable graphene sheets by freezing stretch-induced alignment.

ABSTRACCT: Efforts to obtain high-strength graphene sheets by near-room-temperature assembly have been frustrated by the misalignment of graphene layers, which degrades mechanical properties. While in-plane stretching can decrease this misalignment, it reappears when releasing the stretch. Here we use covalent and π-π inter-platelet bridging to permanently freeze stretch-induced alignment of graphene sheets, and thereby increase isotropic in-plane sheet strength to 1.55 GPa, in combination with a high Young's modulus, electrical conductivity and weight-normalized shielding efficiency. Moreover, the stretch-bridged graphene sheets are scalable and can be easily bonded together using a commercial resin without appreciably decreasing the performance, which establishes the potential for practical applications.

Calcination-regulated Microstructures of Donor-Acceptor Polymers towards Enhanced and Stable Photocatalytic H2 O2 Production in Pure Water.

Regulating molecular structure of donor-acceptor (D-A) polymer is a promising strategy to improve photoactivity. Herein, a porous nanorod-like D-A polymer is synthesized via a strategy of supramolecular chemistry combined with subsequent calcination treatment. This polymer consists of benzene rings (D) and triazine (A) that are linked by amido bond (-CONH-). -CONH- further partially cracks into cyano groups (-C≡N) (A) under calcination. The ratio of benzene to triazine could be tuned to adjust the -C≡N content by varying the calcination atmosphere. Such regulation of molecular structure could modulate the band structure of D-A polymer and endow it with unique porous nanorod-like morphology, leading to the achievement of two-electron oxygen reduction and two-electron water oxidation and the improvement of exciton splitting, O2 adsorption and activation. These merits synergistically ensure a highly efficient and stable photocatalytic H2 O2 production in pure water.

Sequentially bridged graphene sheets with high strength, toughness, and electrical conductivity.

We here show that infiltrated bridging agents can convert inexpensively fabricated graphene platelet sheets into high-performance materials, thereby avoiding the need for a polymer matrix. Two types of bridging agents were investigated for interconnecting graphene sheets, which attach to sheets by either π-π bonding or covalent bonding. When applied alone, the π-π bonding agent is most effective. However, successive application of the optimized ratio of π-π bonding and covalent bonding agents provides graphene sheets with the highest strength, toughness, fatigue resistance, electrical conductivity, electromagnetic interference shielding efficiency, and resistance to ultrasonic dissolution. Raman spectroscopy measurements of stress transfer to graphene platelets allow us to decipher the mechanisms of property improvement. In addition, the degree of orientation of graphene platelets increases with increasing effectiveness of the bonding agents, and the interlayer spacing increases. Compared with other materials that are strong in all directions within a sheet, the realized tensile strength (945 MPa) of the resin-free graphene platelet sheets was higher than for carbon nanotube or graphene platelet composites, and comparable to that of commercially available carbon fiber composites. The toughness of these composites, containing the combination of π-π bonding and covalent bonding, was much higher than for these other materials having high strengths for all in-plane directions, thereby opening the path to materials design of layered nanocomposites using multiple types of quantitatively engineered chemical bonds between nanoscale building blocks.

Ultra-Tough Inverse Artificial Nacre Based on Epoxy-Graphene by Freeze-Casting.

Epoxy nanocomposites combining high toughness with advantageous functional properties are needed in many fields. However, fabricating high-performance homogeneous epoxy nanocomposites with traditional methods remains a great challenge. Nacre with outstanding fracture toughness presents an ideal blueprint for the development of future epoxy nanocomposites. Now, high-performance epoxy-graphene layered nanocomposites were demonstrated with ultrahigh toughness and temperature-sensing properties. These nanocomposites are composed of ca. 99 wt % organic epoxy, which is in contrast to the composition of natural nacre (ca. 96 wt % inorganic aragonite). These nanocomposites are named an inverse artificial nacre. The fracture toughness reaches about 4.2 times higher than that of pure epoxy. The electrical resistance is temperature-sensitive and stable under various humidity conditions. This strategy opens an avenue for fabricating high-performance epoxy nanocomposites with functional properties.

Graphene-based artificial nacre nanocomposites.

With its extraordinary properties as the strongest and stiffest material ever measured and the best-known electrical conductor, graphene could have promising applications in many fields, especially in the area of nanocomposites. However, processing graphene-based nanocomposites is very difficult. So far, graphene-based nanocomposites exhibit rather poor properties. Nacre, the gold standard for biomimicry, provides an excellent example and guidelines for assembling two-dimensional nanosheets into high performance nanocomposites. The inspiration from nacre overcomes the bottleneck of traditional approaches for constructing nanocomposites, such as poor dispersion, low loading, and weak interface interactions. This tutorial review summarizes recent research on graphene-based artificial nacre nanocomposites and focuses on the design of interface interactions and synergistic effects for constructing high performance nanocomposites. This tutorial review also focuses on a perspective of the dynamic area of graphene-based nanocomposites, commenting on whether the concept is viable and practical, on what has been achieved to date, and most importantly, what is likely to be achieved in the future.

Ultrastrong MXene films via the synergy of intercalating small flakes and interfacial bridging.

Titanium carbide MXene combines high mechanical and electrical properties and low infrared emissivity, making it of interest for flexible electromagnetic interference (EMI) shielding and thermal camouflage film materials. Conventional wisdom holds that large MXene is the preferable building block to assemble high-performance films. However, the voids in the films comprising large MXene degrade their properties. Although traditional crosslinking strategies can diminish the voids, the electron transport between MXene flakes is usually disrupted by the insulating polymer bonding agents, reducing the electrical conductivity. Here we demonstrate a sequential densification strategy to synergistically remove the voids between MXene flakes while strengthening the interlayer electron transport. Small MXene flakes were first intercalated to fill the voids between multilayer large flakes, followed by interfacial bridging of calcium ions and borate ions to eliminate the remaining voids, including those between monolayer flakes. The obtained MXene films are compact and exhibit high tensile strength (739 MPa), Young's modulus (72.4 GPa), electrical conductivity (10,336 S cm-1), and EMI shielding capacity (71,801 dB cm2 g-1), as well as excellent oxidation resistance and thermal camouflage performance. The presented strategy provides an avenue for the high-performance assembly of other two-dimensional flakes.

High-strength scalable MXene films through bridging-induced densification.

MXenes are a growing family of two-dimensional transition metal carbides and/or nitrides that are densely stacked into macroscopically layered films and have been considered for applications such as flexible electromagnetic interference (EMI) shielding materials. However, the mechanical and electrical reliabilities of titanium carbide MXene films are affected by voids in their structure. We applied sequential bridging of hydrogen and covalent bonding agents to induce the densification of MXene films and removal of the voids, leading to highly compact MXene films. The obtained MXene films show high tensile strength, in combination with high toughness, electrical conductivity, and EMI shielding capability. Our high-performance MXene films are scalable, providing an avenue for assembling other two-dimensional platelets into high-performance films.

Strong, Conductive, Foldable Graphene Sheets by Sequential Ionic and π Bridging.

The goal of this work is to develop an inexpensive low-temperature process that provides polymer-free, high-strength, high-toughness, electrically conducting sheets of reduced graphene oxide (rGO). To develop this process, we have evaluated the mechanical and electrical properties resulting from the application of an ionic bonding agent (Cr3+ ), a π-π bonding agent comprising pyrene end groups, and their combinations for enhancing the performance of rGO sheets. When only one bonding agent was used, the π-π bonding agent is much more effective than the ionic bonding agent for improving both the mechanical and electrical properties of rGO sheets. However, the successive application of ionic bonding and π-π bonding agents maximizes tensile strength, toughness, long-term electrical stability in various corrosive solutions, and resistance to mechanical abuse and ultrasonic dissolution. Using a combination of ionic bonding and π-π bonding agents, high tensile strength (821 MPa), high toughness (20 MJ m-3 ), and electrical conductivity (416 S cm-1 ) were obtained, as well as remarkable retention of mechanical and electrical properties during ultrasonication and mechanical cycling by both sheet stretch and sheet folding, suggesting high potential for applications in aerospace and flexible electronics.

Fatigue Resistant Bioinspired Composite from Synergistic Two-Dimensional Nanocomponents.

Portable and wearable electronics require much more flexible graphene-based electrode with high fatigue life, which could repeatedly bend, fold, or stretch without sacrificing its mechanical properties and electrical conductivity. Herein, a kind of ultrahigh fatigue resistant graphene-based nanocomposite via tungsten disulfide (WS2) nanosheets is synthesized by introducing a synergistic effect with covalently cross-linking inspired by the orderly layered structure and abundant interfacial interactions of nacre. The fatigue life of resultant graphene-based nanocomposites is more than one million times at the stress level of 270 MPa, and the electrical conductivity can be kept as high as 197.1 S/cm after 1.0 × 105 tensile testing cycles. These outstanding properties are attributed to the synergistic effect from lubrication of WS2 nanosheets for deflecting crack propagation, and covalent bonding between adjacent GO nanosheets for bridging crack, which is verified by the molecular dynamics (MD) simulations. The WS2 induced synergistic effect with covalent bonding offers a guidance for constructing graphene-based nanocomposites with high fatigue life, which have great potential for applications in flexible and wearable electronic devices, etc.

Bioinspired Graphene-Based Nanocomposites and Their Application in Flexible Energy Devices.

Graphene is the strongest and stiffest material ever identified and the best electrical conductor known to date, making it an ideal candidate for constructing nanocomposites used in flexible energy devices. However, it remains a great challenge to assemble graphene nanosheets into macro-sized high-performance nanocomposites in practical applications of flexible energy devices using traditional approaches. Nacre, the gold standard for biomimicry, provides an excellent example and guideline for assembling two-dimensional nanosheets into high-performance nanocomposites. This review summarizes recent research on the bioinspired graphene-based nanocomposites (BGBNs), and discusses different bioinspired assembly strategies for constructing integrated high-strength and -toughness graphene-based nanocomposites through various synergistic effects. Fundamental properties of graphene-based nanocomposites, such as strength, toughness, and electrical conductivities, are highlighted. Applications of the BGBNs in flexible energy devices, as well as potential challenges, are addressed. Inspired from the past work done by the community a roadmap for the future of the BGBNs in flexible energy device applications is depicted.

Nacre-inspired integrated strong and tough reduced graphene oxide-poly(acrylic acid) nanocomposites.

Inspired by the relationship between interface interactions and the high performance mechanical properties of nacre, a strong and tough nacre-inspired nanocomposite was demonstrated based on graphene oxide (GO) and polyacrylic acid (PAA) prepared via a vacuum-assisted filtration self-assembly process. The abundant hydrogen bonding between GO and PAA results in both high strength and toughness of the bioinspired nanocomposites, which are 2 and 3.3 times higher than that of pure reduced GO film, respectively. In addition, the effect of environmental relative humidity on the mechanical properties of bioinspired nanocomposites is also investigated, and is consistent with previous theoretical predictions. Moreover, this nacre-inspired nanocomposite also displays high electrical conductivity of 108.9 S cm(-1). These excellent physical properties allow this type of nacre-inspired nanocomposite to be used in many applications, such as flexible electrodes, aerospace applications, and artificial muscles etc. This nacre-inspired strategy also opens an avenue for constructing integrated high performance graphene-based nanocomposites in the near future.

Use of Synergistic Interactions to Fabricate Strong, Tough, and Conductive Artificial Nacre Based on Graphene Oxide and Chitosan.

Graphene is the strongest and stiffest material, leading to the development of promising applications in many fields. However, the assembly of graphene nanosheets into macrosized nanocomposites for practical applications remains a challenge. Nacre in its natural form sets the "gold standard" for toughness and strength, which serves as a guide to the assembly of graphene nanosheets into high-performance nanocomposites. Here we show the strong, tough, conductive artificial nacre based on graphene oxide through synergistic interactions of hydrogen and covalent bonding. Tensile strength and toughness was 4 and 10 times higher, respectively, than that of natural nacre. The exceptional integrated strong and tough artificial nacre has promising applications in aerospace, artificial muscle, and tissue engineering, especially for flexible supercapacitor electrodes due to its high electrical conductivity. The use of synergistic interactions is a strategy for the development of high-performance nanocomposites.

Synergistic toughening of graphene oxide-molybdenum disulfide-thermoplastic polyurethane ternary artificial nacre.

Inspired by the ternary structure of natural nacre, robust ternary artificial nacre is constructed through synergistic toughening of graphene oxide (GO) and molybdenum disulfide (MoS2) nanosheets via a vacuum-assisted filtration self-assembly process. The synergistic toughening effect from high mechanical properties of GO and lubrication of MoS2 nanosheets is successfully demonstrated. Meanwhile, the artificial nacre shows high electrical conductivity. This approach for constructing robust artificial nacre by synergistic effect from GO and MoS2 provides a creative opportunity for designing and fabricating integrated artificial nacre in the near future, and this kind of ternary artificial nacre has great potential applications in aerospace, flexible supercapacitor electrodes, artificial muscle, and tissue engineering.