Lightweight, Strong, and Transparent Wood Films Produced by Capillary Driven Self-Densification.

Wood delignification and densification enable the production of high strength and/or transparent wood materials with exceptional properties. However, processing needs to be more sustainable and besides the chemical delignification treatments, energy intense hot-pressing calls for alternative approaches. Here, this study shows that additional softening of delignified wood via a mild swelling process using an ionic liquid-water mixture enables the densification of tube-line wood cells into layer-by-layer sheet structures without hot-pressing. The natural capillary force induces self-densification in a simple drying process resulting in a transparent wood film. The as-prepared films with ≈150 µm thickness possess an optical transmittance ≈70%, while maintaining optical haze >95%. Due to the densely packed sheet structure with a large interfacial area, the reassembled wood film is fivefold stronger and stiffer than the delignified wood in fiber direction. Owing to a low density, the specific tensile strength and elastic modulus are as high as 282 MPa cm3 g-1 and 31 GPa cm3 g-1. A facile and highly energy efficient wood nanotechnology approach are demonstrated toward more sustainable materials and processes by directly converting delignified wood into transparent wood omitting polymeric matrix infiltration or mechanical pressing.

Wet-Stable Lamellar Wood Sponge with High Elasticity and Fatigue Resistance Enabled by Chemical Cross-Linking.

The excessive consumption of fossil-based plastics and the associated environmental concerns motivate the increasing exploitation of sustainable biomass-based materials for advanced applications. Natural wood-derived lamellar wood sponges via a top-down approach have recently attracted significant attention; however, the insufficient compressive fatigue resistance and lack of structural stability in water limit their wide applications. Here, we report a facile chemical cross-linking strategy to tackle these challenges, by which the cellulose fibrils in the lamellas are covalently bridged to enhance their connectivity. The cross-linked wood sponges demonstrate high compressibility up to 70% strain and exceptional compressive fatigue resistance (∼5% plastic deformation after 10,000 cycles at 50% strain). The interfibrillar cross-linking inhibits the swelling of cellulose fibrils and preserves the arch-shaped lamellas of the sponge in water, endowing the wood sponge with excellent wet stability. Such highly elastic and wet-stable lamellar wood sponges offer a sustainable alternative to synthetic polymer-based sponges used in diverse applications.

Passive climate regulation with transpiring wood for buildings with increased energy efficiency.

Buildings are significant end-users of global energy. About 20% of the energy consumption worldwide is used for maintaining a comfortable indoor climate. Therefore, passive systems for indoor temperature and humidity regulation that can respond to environmental changes are very promising to reduce buildings' energy consumption. We developed a process to improve the responsiveness of wood to humidity changes by laser-drilling microscopic holes and incorporating a hygroscopic salt (calcium chloride). The resulting "transpiring wood" displays superior water adsorption capacity and high moisture exchange rate, allowing regulation of humidity and temperature by the exchange of moisture with the surrounding air. We proved that the hygrothermal performance of transpiring wood can be used to regulate indoor climate, with associated energy savings, for various climate types, thus favoring its application in the building sector. The reduction of temperature fluctuations, thanks to the buffering of temperature peaks, can lead to an indirect energy saving of about 10% for cooling and between 4-27% for heating depending on the climate. Furthermore, our transpiring wood meets different sustainability criteria, from raw materials to the fabrication process, resulting in a product with a low overall environmental impact and that is easy to recycle.

Review on design strategies and applications of metal-organic framework-cellulose composites.

Metal-organic frameworks (MOFs) are among the most attractive functional porous materials. However, their processability and handling remains a substantial challenge because MOFs generally occur in powder form due to their crystalline nature. Combining MOFs and cellulose substrates to fabricate engineered materials offers an ideal solution to broaden their utilization as functional materials. MOF/cellulose composites further provide remarkable mechanical properties, tunable porosity, and accessible active sites of MOFs. In this review, we summarize current state-of-the-art fabrication routes for MOF/cellulose composites, with a specific focus on the unique potential of utilizing three-dimensional bio-based cellulosic scaffolds. We highlight their utilization as adsorbents in the gas and liquid phase, for antibacterial and protein immobilization, chemical sensors, electrical energy storage, and other emerging applications. In addition, we discuss current limitations and potential future research directions in the field of MOF/cellulose composites for advanced functional materials.

Natural Wood-Based Catalytic Membrane Microreactors for Continuous Hydrogen Generation.

The development of controlled processes for continuous hydrogen generation from solid-state storage chemicals such as ammonia borane is central to integrating renewable hydrogen into a clean energy mix. However, to date, most reported platforms operate in batch mode, posing a challenge for controllable hydrogen release, catalyst reusability, and large-scale operation. To address these issues, we developed flow-through wood-based catalytic microreactors, characterized by inherent natural oriented microchannels. The prepared structured catalysts utilize silver-promoted palladium nanoparticles supported on metal-organic framework (MOF)-coated wood microreactors as the active phase. Catalytic tests demonstrate their highly controllable hydrogen production in continuous mode, and by adjusting the ammonia borane flow and wood species, we reach stable productivities of up to 10.4 cmH23 min-1 cmcat-3. The modular design of the structured catalysts proves readily scalable. Our versatile approach is applicable for other metals and MOF combinations, thus comprising a sustainable and scalable platform for catalytic dehydrogenations and applications in the energy-water nexus.

Lightweight, strong, moldable wood via cell wall engineering as a sustainable structural material.

Wood is a sustainable structural material, but it cannot be easily shaped while maintaining its mechanical properties. We report a processing strategy that uses cell wall engineering to shape flat sheets of hardwood into versatile three-dimensional (3D) structures. After breaking down wood's lignin component and closing the vessels and fibers by evaporating water, we partially re-swell the wood in a rapid water-shock process that selectively opens the vessels. This forms a distinct wrinkled cell wall structure that allows the material to be folded and molded into desired shapes. The resulting 3D-molded wood is six times stronger than the starting wood and comparable to widely used lightweight materials such as aluminum alloys. This approach widens wood's potential as a structural material, with lower environmental impact for buildings and transportation applications.

Enhanced mechanical energy conversion with selectively decayed wood.

Producing electricity from renewable sources and reducing its consumption by buildings are necessary to meet energy and climate change challenges. Wood is an excellent "green" building material and, owing to its piezoelectric behavior, could enable direct conversion of mechanical energy into electricity. Although this phenomenon has been discovered decades ago, its exploitation as an energy source has been impaired by the ultralow piezoelectric output of native wood. Here, we demonstrate that, by enhancing the elastic compressibility of balsa wood through a facile, green, and sustainable fungal decay pretreatment, the piezoelectric output is increased over 55 times. A single cube (15 mm by 15 mm by 13.2 mm) of decayed wood is able to produce a maximum voltage of 0.87 V and a current of 13.3 nA under 45-kPa stress. This study is a fundamental step to develop next-generation self-powered green building materials for future energy supply and mitigation of climate change.

Sustainable and Biodegradable Wood Sponge Piezoelectric Nanogenerator for Sensing and Energy Harvesting Applications.

Developing low-cost and biodegradable piezoelectric nanogenerators is of great importance for a variety of applications, from harvesting low-grade mechanical energy to wearable sensors. Many of the most widely used piezoelectric materials, including lead zirconate titanate (PZT), suffer from serious drawbacks such as complicated synthesis, poor mechanical properties (e.g., brittleness), and toxic composition, limiting their development for biomedical applications and posing environmental problems for their disposal. Here, we report a low-cost, biodegradable, biocompatible, and highly compressible piezoelectric nanogenerator based on a wood sponge obtained with a simple delignification process. Thanks to the enhanced compressibility of the wood sponge, our wood nanogenerator (15 × 15 × 14 mm3, longitudinal × radial × tangential) can generate an output voltage of up to 0.69 V, 85 times higher than that generated by native (untreated) wood, and it shows stable performance under repeated cyclic compression (≥600 cycles). Our approach suggests the importance of increased compressibility of bulk materials for improving their piezoelectric output. We demonstrate the versatility of our nanogenerator by showing its application both as a wearable movement monitoring system (made with a single wood sponge) and as a large-scale prototype with increased output (made with 30 wood sponges) able to power simple electronic devices (a LED light, a LCD screen). Moreover, we demonstrate the biodegradability of our wood sponge piezoelectric nanogenerator by studying its decomposition with cellulose-degrading fungi. Our results showcase the potential application of a wood sponge as a sustainable energy source, as a wearable device for monitoring human motions, and its contribution to environmental sustainability by electronic waste reduction.

Luminescent and Hydrophobic Wood Films as Optical Lighting Materials.

Most materials used for optical lighting applications need to produce a uniform illumination and require high mechanical and hydrophobic properties. However, they are rarely eco-friendly. Herein, a bio-based, polymer matrix-free, luminescent, and hydrophobic film with excellent mechanical properties for optical lighting purposes is demonstrated. A template is prepared by turning a wood veneer into porous scaffold from which most of the lignin and half of the hemicelluloses are removed. The infiltration of quantum dots (CdSe/ZnS) into the porous template prior to densification resulted in almost uniform luminescence (isotropic light scattering) and could be extended to various quantum dot particles, generating different light colors. In a subsequent step, the luminescent wood film is coated with hexadecyltrimethoxysilane (HDTMS) via chemical vapor deposition. The presence of the quantum dots coupled with the HDTMS coating renders the film hydrophobic (water contact angle ≈ 140°). This top-down process strongly eliminates lumen cavities and preserves the orientation of the original cellulose fibrils to create luminescent and polymer matrix-free films with high modulus and strength in the direction of fibers. The proposed optical lighting material could be attractive for interior designs (e.g., lamps and laminated cover panels), photonics, and laser devices.

Green Synthesis of Hierarchical Metal-Organic Framework/Wood Functional Composites with Superior Mechanical Properties.

The applicability of advanced composite materials with hierarchical structure that conjugate metal-organic frameworks (MOFs) with macroporous materials is commonly limited by their inferior mechanical properties. Here, a universal green synthesis method for the in situ growth of MOF nanocrystals within wood substrates is introduced. Nucleation sites for different types of MOFs are readily created by a sodium hydroxide treatment, which is demonstrated to be broadly applicable to different wood species. The resulting MOF/wood composite exhibits hierarchical porosity with 130 times larger specific surface area compared to native wood. Assessment of the CO2 adsorption capacity demonstrates the efficient utilization of the MOF loading along with similar adsorption ability to that of pure MOF. Compression and tensile tests reveal superior mechanical properties, which surpass those obtained for polymer substrates. The functionalization strategy offers a stable, sustainable, and scalable platform for the fabrication of multifunctional MOF/wood-derived composites with potential applications in environmental- and energy-related fields.

Tunable Wood by Reversible Interlocking and Bioinspired Mechanical Gradients.

Elegant design principles in biological materials such as stiffness gradients or sophisticated interfaces provide ingenious solutions for an efficient improvement of their mechanical properties. When materials such as wood are directly used in high-performance applications, it is not possible to entirely profit from these optimizations because stiffness alterations and fiber alignment of the natural material are not designed for the desired application. In this work, wood is turned into a versatile engineering material by incorporating mechanical gradients and by locally adapting the fiber alignment, using a shaping mechanism enabled by reversible interlocks between wood cells. Delignification of the renewable resource wood, a subsequent topographic stacking of the cellulosic scaffolds, and a final densification allow fabrication of desired 3D shapes with tunable fiber architecture. Additionally, prior functionalization of the cellulose scaffolds allows for obtaining tunable functionality combined with mechanical gradients. Locally controllable elastic moduli between 5 and 35 GPa are obtained, inspired by the ability of trees to tailor their macro- and micro-structure. The versatility of this approach has significant relevance in the emerging field of high-performance materials from renewable resources.