3D printing of cellulose nanocrystals based composites to build robust biomimetic scaffolds for bone tissue engineering.

Cellulose nanocrystals (CNC) are drawing increasing attention in the fields of biomedicine and healthcare owing to their durability, biocompatibility, biodegradability and excellent mechanical properties. Herein, we fabricated using fused deposition modelling technology 3D composite scaffolds from polylactic acid (PLA) and CNC extracted from Ficus thonningii. Scanning electron microscopy revealed that the printed scaffolds exhibit interconnected pores with an estimated average pore size of approximately 400 µm. Incorporating 3% (w/w) of CNC into the composite improved PLA mechanical properties (Young's modulus increased by ~ 30%) and wettability (water contact angle decreased by ~ 17%). The mineralization process of printed scaffolds using simulated body fluid was validated and nucleation of hydroxyapatite confirmed. Additionally, cytocompatibility tests revealed that PLA and CNC-based PLA scaffolds are non-toxic and compatible with bone cells. Our design, based on rapid 3D printing of PLA/CNC composites, combines the ability to control the architecture and provide improved mechanical and biological properties of the scaffolds, which opens perspectives for applications in bone tissue engineering and in regenerative medicine.

Design and Manufacturing of Si-Based Non-Oxide Cellular Ceramic Structures through Indirect 3D Printing.

Additive manufacturing of Polymer-Derived Ceramics (PDCs) is regarded as a disruptive fabrication process that includes several technologies such as light curing and ink writing. However, 3D printing based on material extrusion is still not fully explored. Here, an indirect 3D printing approach combining Fused Deposition Modeling (FDM) and replica process is demonstrated as a simple and low-cost approach to deliver complex near-net-shaped cellular Si-based non-oxide ceramic architectures while preserving the structure. 3D-Printed honeycomb polylactic acid (PLA) lattices were dip-coated with two preceramic polymers (polyvinylsilazane and allylhydridopolycarbosilane) and then converted by pyrolysis respectively into SiCN and SiC ceramics. All the steps of the process (printing resolution and surface finishing, cross-linking, dip-coating, drying and pyrolysis) were optimized and controlled. Despite some internal and surface defects observed by topography, 3D-printed materials exhibited a retention of the highly porous honeycomb shape after pyrolysis. Weight loss, volume shrinkage, roughness and microstructural evolution with high annealing temperatures are discussed. Our results show that the sacrificial mold-assisted 3D printing is a suitable rapid approach for producing customizable lightweight highly stable Si-based 3D non-oxide ceramics.

Improved electrochemical conversion of CO2 to multicarbon products by using molecular doping.

The conversion of CO2 into desirable multicarbon products via the electrochemical reduction reaction holds promise to achieve a circular carbon economy. Here, we report a strategy in which we modify the surface of bimetallic silver-copper catalyst with aromatic heterocycles such as thiadiazole and triazole derivatives to increase the conversion of CO2 into hydrocarbon molecules. By combining operando Raman and X-ray absorption spectroscopy with electrocatalytic measurements and analysis of the reaction products, we identified that the electron withdrawing nature of functional groups orients the reaction pathway towards the production of C2+ species (ethanol and ethylene) and enhances the reaction rate on the surface of the catalyst by adjusting the electronic state of surface copper atoms. As a result, we achieve a high Faradaic efficiency for the C2+ formation of ≈80% and full-cell energy efficiency of 20.3% with a specific current density of 261.4 mA cm-2 for C2+ products.

Investigation of polymer-derived Si-(B)-C-N ceramic/reduced graphene oxide composite systems as active catalysts towards the hydrogen evolution reaction.

Hydrogen Evolution Reaction (HER) is an attractive technology for chemical conversion of energy. Replacement of platinum with inexpensive and stable electrocatalysts remains a major bottleneck hampering large-scale hydrogen production by using clean and renewable energy sources. Here, we report electrocatalytically active and ultra-stable Polymer-Derived Ceramics towards HER. We successfully prepared ultrathin silicon and carbon (Si-C) based ceramic systems supported on electrically conducting 2D reduced graphene oxide (rGO) nanosheets with promising HER activity by varying the nature and the composition of the ceramic with the inclusion of nitrogen, boron and oxygen. Our results suggest that oxygen-enriched Si-B-C-N/rGO composites (O-SiBCN/rGO) display the strongest catalytic activity leading to an onset potential and a Tafel slope of - 340 mV and ~ 120 mV dec-1 respectively. O-SiBCN/rGO electrodes display stability over 170 h with minimal increase of 14% of the overpotential compared to ~ 1700% for commercial platinum nanoparticles. Our study provides new insights on the performance of ceramics as affordable and robust HER catalysts calling for further exploration of the electrocatalytic activity of such unconventional materials.

Origin of transparency in scattering biomimetic collagen materials.

Living tissues, heterogeneous at the microscale, usually scatter light. Strong scattering is responsible for the whiteness of bones, teeth, and brain and is known to limit severely the performances of biomedical optical imaging. Transparency is also found within collagen-based extracellular tissues such as decalcified ivory, fish scales, or cornea. However, its physical origin is still poorly understood. Here, we unveil the presence of a gap of transparency in scattering fibrillar collagen matrices within a narrow range of concentration in the phase diagram. This precholesteric phase presents a three-dimensional (3D) orientational order biomimetic of that in natural tissues. By quantitatively studying the relation between the 3D fibrillar network and the optical and mechanical properties of the macroscopic matrices, we show that transparency results from structural partial order inhibiting light scattering, while preserving mechanical stability, stiffness, and nonlinearity. The striking similarities between synthetic and natural materials provide insights for better understanding the occurring transparency.

Palladium/Carbon Nanofibers by Combining Atomic Layer Deposition and Electrospinning for Organic Pollutant Degradation.

As organic dyes are a major source of pollution, it is important to develop novel and efficient heterogeneous catalysts with high activity for their degradation. In this work, two innovative techniques, atomic layer deposition and electrospinning, were used to prepare palladium nanoparticles (Pd NPs) supported on carbon nanofibers (CNFs). The sample morphology was investigated using scanning and transmission electron microscopy. This showed the presence of nanofibers of several micrometers in length and with a mean diameter of 200 nm. Moreover, the size of the highly dispersed Pd NPs was about 7 nm. X-ray photoelectron spectroscopy visually validated the inclusion of metallic Pd. The prepared nano-catalysts were then used to reduce methyl orange (MO) in the presence of sodium borohydride (NaBH4). The Freundlich isotherm model was the most suitable model to explain the adsorption equilibrium for MO onto the Pd/CNF catalysts. Using 5 mL MO dye-solution (0.0305 mM) and 1 mL NaBH4 (0.026 mM), a 98.9% of catalytic activity was achieved in 240 min by 0.01 g of the prepared nano-catalysts Pd/C (0.016 M). Finally, no loss of catalytic activity was observed when such catalysts were used again. These results represent a promising avenue for the degradation of organic pollutants and for heterogeneous catalysis.

Atomic layer deposition of Pd nanoparticles on self-supported carbon-Ni/NiO-Pd nanofiber electrodes for electrochemical hydrogen and oxygen evolution reactions.

The most critical challenge in hydrogen fuel production is to develop efficient, eco-friendly, low-cost electrocatalysts for water splitting. In this study, self-supported carbon nanofiber (CNF) electrodes decorated with nickel/nickel oxide (Ni/NiO) and palladium (Pd) nanoparticles (NPs) were prepared by combining electrospinning, peroxidation, and thermal carbonation with atomic layer deposition (ALD), and then employed for hydrogen evolution and oxygen evolution reactions (HER/OER). The best CNF-Ni/NiO-Pd electrode displayed the lowest overpotential (63 mV and 1.6 V at j = 10 mA cm-2), a remarkably small Tafel slope (72 and 272 mV dec-1), and consequent exchange current density (1.15 and 22.4 mA cm-2) during HER and OER, respectively. The high chemical stability and improved electrocatalytic performance of the prepared electrodes can be explained by CNF functionalization via Ni/NiO NP encapsulation, the formation of graphitic layers that cover and protect the Ni/NiO NPs from corrosion, and ALD of Pd NPs at the surface of the self-supported CNF-Ni/NiO electrodes.

Enhanced sieving from exfoliated MoS2 membranes via covalent functionalization.

Nanolaminate membranes made of two-dimensional materials such as graphene oxide are promising candidates for molecular sieving via size-limited diffusion in the two-dimensional capillaries, but high hydrophilicity makes these membranes unstable in water. Here, we report a nanolaminate membrane based on covalently functionalized molybdenum disulfide (MoS2) nanosheets. The functionalized MoS2 membranes demonstrate >90% and ~87% rejection for micropollutants and NaCl, respectively, when operating under reverse osmotic conditions. The sieving performance and water flux of the functionalized MoS2 membranes are attributed both to control of the capillary widths of the nanolaminates and to control of the surface chemistry of the nanosheets. We identify small hydrophobic functional groups, such as the methyl group, as the most promising for water purification. Methyl- functionalized nanosheets show high water permeation rates as confirmed by our molecular dynamic simulations, while maintaining high NaCl rejection. Control of the surface chemistry and the interlayer spacing therefore offers opportunities to tune the selectivity of the membranes while enhancing their stability.

Role of Sulfur Vacancies and Undercoordinated Mo Regions in MoS2 Nanosheets toward the Evolution of Hydrogen.

Low-dimensional materials have been examined as electrocatalysts for the hydrogen evolution reaction (HER). Among them, two-dimensional transition metal dichalcogenides (2D-TMDs) such as MoS2 have been identified as potential candidates. However, the performance of TMDs toward HER in both acidic and basic media remains inferior to that of noble metals such as Pt and its alloys. This calls for investigating the influence of controlled defect engineering of 2D TMDs on their performance toward hydrogen production. Here, we explored the HER activity from defective multilayered MoS2 over a large range of surface S vacancy concentrations up to 90%. Amorphous MoS2 and 2H MoS2 with ultrarich S vacancies demonstrated the highest HER performance in acid and basic electrolytes, respectively. We also report that the HER performance from multilayered MoS2 can be divided into two domains corresponding to "point defects" at low concentrations of surface S vacancies (Stage 1) and large regions of undercoordinated Mo atoms for high concentrations of surface S vacancies (Stage 2). The highest performance is obtained for Stage 2 in the presence of undercoordinated Mo atoms with a TOF of ∼2 s-1 at an overpotential of 160 mV in 0.1 M KOH which compares favorably to the best results in the literature. Overall, our work provides deeper insight on the HER mechanism from defected MoS2 and provides guidance for the development of defect-engineered TMD-based electrocatalysts.

Molecular-Level Processing of Si-(B)-C Materials with Tailored Nano/Microstructures.

The design of Si-(B)-C materials is investigated, with detailed insight into the precursor chemistry and processing, the precursor-to-ceramic transformation, and the ceramic microstructural evolution at high temperatures. In the early stage of the process, the reaction between allylhydridopolycarbosilane (AHPCS) and borane dimethyl sulfide is achieved. This is investigated in detail through solid-state NMR and FTIR spectroscopy and elemental analyses for Si/B ratios ranging from 200 to 30. Boron-based bridges linking AHPCS monomeric fragments act as crosslinking units, extending the processability range of AHPCS and suppressing the distillation of oligomeric fragments during the low-temperature pyrolysis regime. Polymers with low boron contents display appropriate requirements for facile processing in solution, leading to the design of monoliths with hierarchical porosity, significant pore volume, and high specific surface area after pyrolysis. Polymers with high boron contents are more appropriate for the preparation of dense ceramics through direct solid shaping and pyrolysis. We provide a comprehensive study of the thermal decomposition mechanisms, and a subsequent detailed study of the high-temperature behavior of the ceramics produced at 1000 °C. The nanostructure and microstructure of the final SiC-based ceramics are intimately linked to the boron content of the polymers. B4 C/C/SiC nanocomposites can be obtained from the polymer with the highest boron content.

Molecular design of melt-spinnable co-polymers as Si-B-C-N fiber precursors.

Two series of co-polymers with the general formula [B(C2H4SiCH3(NH)x(NCH3)y)3]n, i.e., composed of C2H4SiCH3(NH)x and C2H4SiCH3(NCH3)y (C2H4 = CHCH3, CH2CH2) building blocks in a well defined x : y ratio, have been synthesized by hydroboration of dichloromethylvinylsilane with borane dimethyl sulfide followed by successive reactions with lithium amide and methylamine according to controlled ratios. The role of the chemistry behind their syntheses has been studied in detail by solid-state NMR, FT-IR and elemental analyses. Then, the intimate relationship between the chemistry and the melt-spinnability of these polymers was discussed. By keeping x = 0.50 and increasing y above 0.50, i.e., obtaining methylamine excess, the co-polymers contained more ending groups and especially more tetracoordinated boron, thus allowing tuning very precisely the chemical structure of the preceramic polymer in order to meet the requirements for melt-spinning. The curing treatment under ammonia at 200 °C efficiently rendered the green fibers infusible before their subsequent pyrolysis under nitrogen at 1000 °C to generate Si-B-C-N ceramic fibers. Interestingly, it could be possible to produce also low diameter hollow fibers with relatively high mechanical properties for a further exploration as membrane materials.

Molecular Chemistry and Engineering of Boron-Modified Polyorganosilazanes as New Processable and Functional SiBCN Precursors.

A series of boron-modified polyorganosilazanes was synthesized from a poly(vinylmethyl-co-methyl)silazane and controlled amounts of borane dimethyl sulfide. The role of the chemistry behind their synthesis has been studied in detail by using solid-state NMR spectroscopy, FTIR spectroscopy, and elemental analysis. The intimate relationship between the chemistry and the processability of these polymers is discussed. Polymers with low boron contents displayed appropriate requirements for facile processing in solution, such as impregnation of host carbon materials, which resulted in the design of mesoporous monoliths with a high specific surface area after pyrolysis. Polymers with high boron content are more appropriate for solid-state processing to design mechanically robust monolith-type macroporous and dense structures after pyrolysis. Boron acts as a crosslinking element, which offers the possibility to extend the processability of polyorganosilazanes and suppress the distillation of oligomeric fragments in the low-temperature region of their thermal decomposition (i.e., pyrolysis) at 1000 °C under nitrogen. Polymers with controlled and high ceramic yields were generated. We provide a comprehensive mechanistic study of the two-step thermal decomposition based on a combination of thermogravimetric experiments coupled with elemental analysis, solid-state NMR spectroscopy, and FTIR spectroscopy. Selected characterization tools allowed the investigation of specific properties of the monolith-type SiBCN materials.

Plasmon-mediated chemical surface functionalization at the nanoscale.

Controlling the surface grafting of species at the nanoscale remains a major challenge, likely to generate many opportunities in materials science. In this work, we propose an original strategy for chemical surface functionalization at the nanoscale, taking advantage of localized surface plasmon (LSP) excitation. The surface functionalization is demonstrated through aryl film grafting (derived from a diazonium salt), covalently bonded at the surface of gold lithographic nanostripes. The aryl film is specifically grafted in areas of maximum near field enhancement, as confirmed by numerical calculation based on the discrete dipole approximation method. The energy of the incident light and the LSP wavelength are shown to be crucial parameters to monitor the aryl film thickness of up to ∼30 nm. This robust and versatile strategy opens up exciting prospects for the nanoscale confinement of functional layers on surfaces, which should be particularly interesting for molecular sensing or nanooptics.

Boron nitride ceramics from molecular precursors: synthesis, properties and applications.

Hexagonal boron nitride (h-BN) attracts considerable interest because its structure is similar to that of carbon graphite while it displays different properties which are of interest for environmental and green technologies. The polar nature of the B-N bond in sp(2)-bonded BN makes it a wide band gap insulator with different chemistry on its surface and particular physical and chemical properties such as a high thermal conductivity, a high temperature stability, a high resistance to corrosion and oxidation and a strong UV emission. It is chemically inert and nontoxic and has good environmental compatibility. h-BN also has enhanced physisorption properties due to the dipolar fields near its surface. Such properties are closely dependent on the processing method. Bottom-up approaches consist of transforming molecular precursors into non-oxide ceramics with retention of the structural units inherent to the precursor molecule. The purpose of the present review is to give an up-to-date overview on the most recent achievements in the preparation of h-BN from borazine-based molecular single-source precursors including borazine and 2,4,6-trichloroborazine through both vapor phase syntheses and methods in the liquid/solid state involving polymeric intermediates, called the Polymer-Derived Ceramics (PDCs) route. In particular, the effect of the chemistry, composition and architecture of the borazine-based precursors and derived polymers on the shaping ability as well as the properties of h-BN is particularly highlighted.