Unlocking epoxy thermal management capability via hierarchical Ce-MOF@MoS2 hybrid constructed by in-situ growth method.

This study demonstrates the preparation of needle-like Ce-MOF crystals on molybdenum disulfide (MoS2) nanosheets using in-situ growth technology. This hybrid structure significantly enhances the thermal management and mechanical properties of thermosetting epoxy resin (EP). Specifically, EP/Ce-MOF@MoS2-3 exhibits a notable increase in tensile strength (TS) to 50.87 MPa and elongation at break (EB) to 10.84 %. Moreover, Ce-MOF@MoS2 provides synergistic flame retardant benefits, reducing the peak heat release rate (pHRR) and total heat release (THR) of EP/Ce-MOF@MoS2-3 by 38 % and 12.64 %, respectively, compared to EP-0. Additionally, Ce-MOF@MoS2 suppresses smoke and reduces toxic emissions; at a 3 % loading, it decreases CO and CO2 production in EP nanocomposites by 48.8 % and 38.7 %, respectively. Thus, this Ce-MOF@MoS2 hybrid, synthesized via in-situ growth, offers a novel approach for developing EP nanocomposites with superior thermal management and mechanical properties, along with effective flame retardancy and reduced hazardous emissions during thermal events.

Salvinia natans-Based Hierarchical Structures for Solar Thermal Clean Water Production from High-Salinity Wastewater.

Biomaterial-based solar-driven evaporation has great potential for wastewater treatment and seawater desalination with a high energy conversion and utilization efficiency. However, technology gaps still exist for effectively and directly applying multiscale structures and intrinsic water transport channels of natural materials to enhance high-efficiency photothermal evaporation. In this study, a high-performance biomass-derived photothermal evaporative material was obtained using Salvinia natans, a common aquatic floating plant, together with simple poly(m-phenylenediamine) oxidation modification, building a hybrid biomass evaporator. With advantageous natural features of adequate water transport, microscale-nanoscale hierarchical structures, effective water activation, and antisalt-fouling function, the hybrid biomass evaporator achieves a high evaporation rate of 2.24 kg m-2 h-1 under one sun radiation (1 kW m-2). In addition, modified Salvinia natans also demonstrate certain ability to remove heavy metals during the photothermal evaporation of wastewater. This work offers a new perspective on the synthesis of an environmentally friendly and cost-effective solar-driven evaporator material, which has the advantages of low cost, simple process, and high photothermal conversion efficiency, and can be widely applied to seawater desalination and the treatment of wastewater with high salt concentrations.

Hierarchical Branched TiO2 Photo/Photoelectrocatalyst with Directed Charge Transfer for Efficient Hydrogen Production from Seawater.

The directed electron transport channel design in semiconductors, which could promote charge utilization, is attractive but rarely reported. Hierarchical branched titanium dioxide (HB-TiO2), possessing a charge cascade transfer channel, was constructed by assembling titanium-defected TiO2 nanobranches on oxygen-defected TiO2 nanobelts. The interfacial Ti/O vacancies have been detected by X-ray photoelectron and electron paramagnetic resonance spectroscopies, and the vacancies act as the "bridge" of photogenerated carrier transport. This structure maintained high photoactivity in H2 production in different mass fractions of NaCl solutions. The photocurrent density of the HB-TiO2 photoanode in natural seawater is 3.9, 2.1, and 2.6 times that of oxygen-defected TiO2 nanobelts, titanium-defected TiO2 nanobranches, and their mixture, respectively. Besides, the charge transport mechanism from the inner lattice to the TiO2 surface is proposed.

Laser-Printed All-Carbon Responsive Material and Soft Robot.

Responsive materials and actuators are the basis for the development of various leading-edge technologies but have so far mostly been designed based on polymers, incurring key limitations related to sensitivity and environmental tolerance. This work reports a new responsive material, laser-printed carbon film (LPCF), produced via direct laser transformation of a liquid organic precursor and consists of graphitic and amorphous carbons. The high activity of amorphous carbon combined with the dual-gradient structure enables the LPCF to have a actuation speed of 9400° s-1 in response to the stimulus of organic vapor. LPCF exhibits a conductivity of 950 S m-1 and excellent resistance to various extreme environmental conditions, which are unachievable for polymer-based materials. Additionally, an LPCF-based all-carbon soft robot that can mimic the complex continuous backward somersaulting motions without manual intervention is constructed. The locomotion velocity of the robot reaches a value of 1.19 BL s-1, which is almost one to two orders of magnitude faster than that of reported soft robots. This work not only offers a new paradigm for highly responsive materials but also provides a great design and engineering example for the next generation of biomimetic robots with life-like performance.

Spatially Confined Assembly and Immobilization of Hierarchical Nanoparticle Architectures inside Microdroplets in Magnetic Fields.

Magnetic field-directed colloidal interactions offer facile tools for assembly of structures that range from linear chains to multidimensional hierarchical architectures. While the field-driven assembly of colloidal particles has commonly been investigated in unbounded media, a knowledge gap remains concerning such assembly in confined microenvironments. Here, we investigate how confinement of ferromagnetic nanoparticles in microspheres directs their magnetic assembly into hierarchical architectures. Microdroplets from polydimethylsiloxane (PDMS) liquid precursor containing dispersed iron oxide magnetic nanoparticles (MNPs) were placed in a static magnetic field leading to the formation of organized assemblies inside the host droplets. By changing the MNP concentrations, we revealed a sequence of microstructures inside the droplets, ranging from linear chains at a low MNP loading, transitioning to a combination of chains and networked bundles, to solely 3D bundles at high MNP loading. These experimental results were analyzed with the aid of COMSOL simulations where we calculated the potential energy to identify the preferred assembly conformations. The chains at high MNP loading minimized their energy by aggregating laterally to form bundles with their MNP dipoles being out-of-registry. We cured these PDMS droplets to immobilize the assemblies by forming soft microbeads. These microbeads constitute an "interaction toolbox" with different magnetic macroscale responses, which are governed by the structuring of the MNPs and their magnetic polarizability. We show that thanks to their ability to rotate by field-induced torque under a rotating field, these microbeads can be employed in applications such as optical modulators and microrollers.

Combinatorial Bionic Hierarchical Flexible Strain Sensor for Sign Language Recognition with Machine Learning.

Flexible strain sensors have been widely researched in fields such as smart wearables, human health monitoring, and biomedical applications. However, achieving a wide sensing range and high sensitivity of flexible strain sensors simultaneously remains a challenge, limiting their further applications. To address these issues, a cross-scale combinatorial bionic hierarchical design featuring microscale morphology combined with a macroscale base to balance the sensing range and sensitivity is presented. Inspired by the combination of serpentine and butterfly wing structures, this study employs three-dimensional printing, prestretching, and mold transfer processes to construct a combinatorial bionic hierarchical flexible strain sensor (CBH-sensor) with serpentine-shaped inverted-V-groove/wrinkling-cracking structures. The CBH-sensor has a high wide sensing range of 150% and high sensitivity with a gauge factor of up to 2416.67. In addition, it demonstrates the application of the CBH-sensor array in sign language gesture recognition, successfully identifying nine different sign language gestures with an impressive accuracy of 100% with the assistance of machine learning. The CBH-sensor exhibits considerable promise for use in enabling unobstructed communication between individuals who use sign language and those who do not. Furthermore, it has wide-ranging possibilities for use in the field of gesture-driven interactions in human-computer interfaces.

Bioinspired Nanolayered Structure Tuned by Extensional Stress: A Scalable Way to High-Performance Biodegradable Polyesters.

The nacre-inspired multi-nanolayer structure offers a unique combination of advanced mechanical properties, such as strength and crack tolerance, making them highly versatile for various applications. Nevertheless, a significant challenge lies in the current fabrication methods, which is difficult to create a scalable manufacturing process with precise control of hierarchical structure. In this work, a novel strategy is presented to regulate nacre-like multi-nanolayer films with the balance mechanical properties of stiffness and toughness. By utilizing a co-continuous phase structure and an extensional stress field, the hierarchical nanolayers is successfully constructed with tunable sizes using a scalable processing technique. This strategic modification allows the robust phase to function as nacre-like platelets, while the soft phase acts as a ductile connection layer, resulting in exceptional comprehensive properties. The nanolayer-structured films demonstrate excellent isotropic properties, including a tensile strength of 113.5 MPa in the machine direction and 106.3 MPa in a transverse direction. More interestingly, these films unprecedentedly exhibit a remarkable puncture resistance at the same time, up to 324.8 N mm-1, surpassing the performance of other biodegradable films. The scalable fabrication strategy holds significant promise in designing advanced bioinspired materials for diverse applications.

Multiscale engineered artificial compact bone via bidirectional freeze-driven lamellated organization of mineralized collagen microfibrils.

Bone, renowned for its elegant hierarchical structure and unique mechanical properties, serves as a constant source of inspiration for the development of synthetic materials. However, achieving accurate replication of bone features in artificial materials with remarkable structural and mechanical similarity remains a significant challenge. In this study, we employed a cascade of continuous fabrication processes, including biomimetic mineralization of collagen, bidirectional freeze-casting, and pressure-driven fusion, to successfully fabricate a macroscopic bulk material known as artificial compact bone (ACB). The ACB material closely replicates the composition, hierarchical structures, and mechanical properties of natural bone. It demonstrates a lamellated alignment of mineralized collagen (MC) microfibrils, similar to those found in natural bone. Moreover, the ACB exhibits a similar high mineral content (70.9 %) and density (2.2 g/cm3) as natural cortical bone, leading to exceptional mechanical properties such as high stiffness, hardness, and flexural strength that are comparable to those of natural bone. Importantly, the ACB also demonstrates excellent mechanical properties in wet, outstanding biocompatibility, and osteogenic properties in vivo, rendering it suitable for a broad spectrum of biomedical applications, including orthopedic, stomatological, and craniofacial surgeries.

The promotion of vascular reconstruction by hierarchical structures in biodegradable small-diameter vascular scaffolds.

Tissue engineering of small-diameter vessels remains challenging due to the inadequate ability to promote endothelialization and infiltration of smooth muscle cells (SMCs). Ideal vascular graft is expected to provide the ability to support endothelial monolayer formation and SMCs infiltration. To achieve this, vascular scaffolds with both orientation and dimension hierarchies were prepared, including hierarchically random vascular scaffold (RVS) and aligned vascular scaffold (AVS), by utilizing degradable poly(ε-caprolactone)-co-poly(ethylene glycol) (PCE) and the blend of PCE/gelatin (PCEG) as raw materials. In addition to the orientation hierarchy, dimension hierarchy with small pores in the inner layer and large pores in the outer layer was also constructed in both RVS and AVS to further investigate the promotion of vascular reconstruction by hierarchical structures in vascular scaffolds. The results show that the AVS with an orientation hierarchy that consists with the natural vascular structure had better mechanical properties and promotion effect on the proliferation of vascular cells than RVS, and also exhibited excellent contact guidance effects on cells. While the dimension hierarchy in both RVS and AVS was favorable to the rapid infiltration of SMCs in a short culture time in vitro. Besides, the results of subcutaneous implantation further demonstrate that AVS achieved a fully infiltrated outer layer with wavy elastic fibers-mimic strips formation by day 14, ascribing to hierarchies of aligned orientation and porous dimension. The results further indicate that the scaffolds with both orientation and dimension hierarchical structures have great potential in the application of promoting the vascular reconstruction.

A Hierarchical Hydrogel Impregnation Strategy Enables Brittle-Failure-Free 3D-Printed Bioceramic Scaffolds.

3D-printed bioceramic scaffolds offer great potential for bone tissue engineering (BTE) but their inherent brittleness and reduced mechanical properties at high porosities can easily result in catastrophic fractures. Herein, this study presents a hierarchical hydrogel impregnation strategy, incorporating poly(vinyl alcohol) (PVA) hydrogel into the macro- and micropores of bioceramic scaffolds and synergistically reinforcing it via freeze-casting assisted solution substitution (FASS) in a tannic acid (TA)-glycerol solution. By effectively mitigating catastrophic brittle failures, the hydrogel-impregnated scaffolds showcase three- and 100-fold enhancement in mechanical energy absorption under compression (5.05 MJ m-3) and three-point bending (3.82 MJ m-3), respectively. The reinforcement mechanisms are further investigated by experimental and simulation analyses, revealing a multi-scale synergy of fracture and fragmentation resistance through macro and micro-scale fiber bridging, and nano and molecular-scale hydrogel reinforcement. Also, the scaffolds acquire additional antibacterial and drug-loading capabilities from the hydrogel phase while maintaining favorable cell biocompatibility. Therefore, this study demonstrates a facile yet effective approach for preparing brittle-failure-free bioceramic scaffolds with enhanced biological functionalities, showcasing immense potential for BTE applications.

Hierarchical syntax model of music predicts theta power during music listening.

Linguistic research showed that the depth of syntactic embedding is reflected in brain theta power. Here, we test whether this also extends to non-linguistic stimuli, specifically music. We used a hierarchical model of musical syntax to continuously quantify two types of expert-annotated harmonic dependencies throughout a piece of Western classical music: prolongation and preparation. Prolongations can roughly be understood as a musical analogue to linguistic coordination between constituents that share the same function (e.g., 'pizza' and 'pasta' in 'I ate pizza and pasta'). Preparation refers to the dependency between two harmonies whereby the first implies a resolution towards the second (e.g., dominant towards tonic; similar to how the adjective implies the presence of a noun in 'I like spicy … '). Source reconstructed MEG data of sixty-five participants listening to the musical piece was then analysed. We used Bayesian Mixed Effects models to predict theta envelope in the brain, using the number of open prolongation and preparation dependencies as predictors whilst controlling for audio envelope. We observed that prolongation and preparation both carry independent and distinguishable predictive value for theta band fluctuation in key linguistic areas such as the Angular, Superior Temporal, and Heschl's Gyri, or their right-lateralised homologues, with preparation showing additional predictive value for areas associated with the reward system and prediction. Musical expertise further mediated these effects in language-related brain areas. Results show that predictions of precisely formalised music-theoretical models are reflected in the brain activity of listeners which furthers our understanding of the perception and cognition of musical structure.

Anisotropic V-Groove/Wrinkle Hierarchical Arrays for Multidirectional Strain Sensors with High Sensitivity and Exceptional Selectivity.

Flexible strain sensors have been continuously optimized and widely used in various fields such as health monitoring, motion detection, and human-machine interfaces. There is a higher demand for sensors that can sensitively identify both the strain amplitude and direction in real-time to adapt to complex human movements. This study proposes a flexible strain sensor construction strategy based on V-groove/wrinkle hierarchical structures via a facile and scalable prestretching approach. A gold film is sputtered on a V-groove structure soft substrate under a vertical biaxial prestrain. When the strain is released, a variety of wondrous V-groove/wrinkle hierarchical structures are formed. The microstructure and the properties of the resulting sensor can be controlled by adjusting the prestrain, which has obvious anisotropic response characteristics and exhibits high sensitivity (maximum gauge factor up to 20,727.46) and a wide sensing range (up to 51%). In addition, the resulting multidirectional sensor based on double-sided microstructures has an exceptional directional selectivity of 67.39, at an advanced level among all stretchable multidirectional strain sensors reported so far. The sensor can detect human motion signals and distinguish motion patterns, proving its great potential in the field of human motion detection and laying a foundation for high-performance wearable devices.

Bioinspired Structures Made of Silicone Nanofilaments for Upcycling Waste Masks to Reusable N95 Respirators.

The increasing demand for personal protective equipment such as single-use masks has led to large amounts of nondegradable plastic waste, aggravating economic and environmental burdens. This study reports a simple and scalable approach for upcycling waste masks via a chemical vapor deposition technique, realizing a trichome-like biomimetic (TLB) N95 respirator with superhydrophobicity (water contact angle ≥150°), N95-level protection, and reusability. The TLB N95 respirator comprising templated silicone nanofilaments with an average diameter of ∼150 nm offers N95-level protection and breathability comparable to those of commercial N95 respirators. The TLB N95 respirator can still maintain its N95-level protection against particulate matter and viruses after 10 disinfection treatment cycles (i.e., ultraviolet irradiation, microwave irradiation, dry heating, and autoclaving), demonstrating durable reusability. The proposed strategy provides new insight into upcycle waste masks, breaking the existing design and preparation concept of reusable masks.

Rational Electrochemical Design of Cuprous Oxide Hierarchical Microarchitectures and Their Derivatives for SERS Sensing Applications.

Rational morphology control of inorganic microarchitectures is important in diverse fields, requiring precise regulation of nucleation and growth processes. While wet chemical methods have achieved success regarding the shape-controlled synthesis of micro/nanostructures, accurately controlling the growth behavior in real time remains challenging. Comparatively, the electrodeposition technique can immediately control the growth behavior by tuning the overpotential, whereas it is rarely used to design complex microarchitectures. Here, the electrochemical design of complex Cu2O microarchitectures step-by-step by precisely controlling the growth behavior is demonstrated. The growth modes can be switched between the thermodynamic and kinetic modes by varying the overpotential. Cl- ions preferably adhered to {100} facets to modulate growth rates of these facets is proved. The discovered growth modes to prepare Cu2O microarchitectures composed of multiple building units inaccessible with existing methods are employed. Polyvinyl alcohol (PVA) additives can guarantee all pre-electrodeposits simultaneously evolve into uniform microarchitectures, instead of forming undesired microstructures on bare electrode surfaces in following electrodeposition processes is discovered. The designed Cu2O microarchitectures can be converted into noble metal microstructures with shapes unchanged, which can be used as surface-enhanced Raman scattering substrates. An electrochemical avenue toward rational design of complex inorganic microarchitectures is opened up.

Construction of Hierarchical Films via Layer-by-Layer Assembly of Exfoliated Unilamellar Zeolite Nanosheets.

Zeolites have been widely applied as versatile catalysts, sorbents, and ion exchangers with unique porous structures showing molecular sieving capability. In these years, it is reported that some layered zeolites can be delaminated into molecularly thin 2-dimensional (2D) nanosheets characterized by inherent porous structures and highly exposed active sites. In the present study, two types of zeolite nanosheets with distinct porous structures with MWW topology (denoted mww) and ferrierite-related structure (denoted bifer) are deposited on a substrate through the solution process via electrostatic self-assembly. Alternate deposition of zeolite nanosheets with polycation under optimized conditions allows the layer-by-layer growth of their multilayer films with a stacking distance of 2-3 nm. Furthermore, various hierarchical structures defined at the unit-cell dimensions can be constructed simply by conducting the deposition of mww and bifer nanosheets in a designed sequence. Adsorption of a dye, Rhodamine B, in these films, is examined to show that adsorption is dependent on constituent zeolite nanosheets and their assembled nanostructures. This work has provided fundamental advancements in the fabrication of artificial zeolite-related hierarchical structures, which may be extended to other zeolite nanosheets, broadening their functionalities, applications, and benefits.

Impact of high temperature on microstructural changes and oil absorption of tigernut (Cyperus esculentus L.) starch: Investigations in the starch-oil model system.

This study was to explore the internal reasons for the changes in oil absorption performance of tigernut starch (TS) by revealing the high-temperature induced variations of structural and functional properties of TS. The results showed that as the temperature increased from 80 °C to 140 °C, the degree of starch gelatinization increased, while the proportion of double helix structures, the total proportion of B1 and B2 chains, the relative crystallinity and the molecular weight decreased, accompanied by the fragmentation and swelling of TS granules. The oxidation of tigernut oil (TNO) led to a decrease in oil density and an increase in total polar component content. These phenomena could result in an increase of oil absorption capacity of TS and starch-lipid complex index. With further increase in temperature from 170 °C to 200 °C, the disruption of the crystalline structure and chain structure increased, resulting in the melting and disintegration of TS granules. This caused a decrease in the starch-oil contact area and capillary absorption of TNO by the TS granules. The results will contribute to revealing the effect of high-temperature induced changes in the structural and functional properties of TS on its oil absorption properties.

Hierarchical Co2 P/CoS2 @C@MoS2 Composites with Hollow Cavity and Multiple Phases Toward Wideband Electromagnetic Wave Absorption.

The synergistic effect of hollow cavities and multiple hetero-interfaces displays huge advantages in achieving lightweight and high-efficient electromagnetic wave absorption, but still confronts huge challenges. Herein, hierarchical Co2 P/CoS2 @C@MoS2 composites via the self-sacrificed strategy and a subsequent hydrothermal method have been successfully synthesized. Specifically, ZIF-67 cores first act as the structural template to form core-shell ZIF-67@poly(cyclotriphosphazene-co-4,4'-sulfonyldiphenol) (ZIF-67@PZS) composites, which are converted into hollow Co2 P@C shells with micro-mesoporous characteristics because of the gradient structural stabilities and preferred coordination ability. The deposition of hierarchical MoS2 results in phase transition (Co2 P→Co2 P/CoS2 ), yielding the formation of hierarchical Co2 P/CoS2 @C@MoS2 composites with hollow cavities and multiple hetero-interfaces. Benefiting from the cooperative advantages of hollow structure, extra N,P,S-doped sources, lattice defects/vacancies, diverse incoherent interfaces, and hierarchical configurations, the composites deliver superior electromagnetic wave capability (-56.6 dB) and wideband absorption bandwidth (8.96 GHz) with 20 wt.% filler loading. This study provides a reliable and facile strategy for the precise construction of superior electromagnetic wave absorbents with efficient absorption attenuation.

Highly Robust Conductive Organo-Hydrogels with Powerful Sensing Capabilities Under Large Mechanical Stress.

The low mechanical strength of conductive hydrogels (<1 MPa) has been a significant hurdle in their practical application, as they are prone to fracturing under complex conditions, limiting their effectiveness. Here, this work fabricates a strong and tough conductive hierarchical poly(vinyl alcohol) (PEDOT:PSS/PVA) organo-hydrogel (PPS organo-hydrogel) via a facile combining strategy of self-assembly and stretch training. With PVA/PEDOT:PSS microlayers and aligned PVA/PEDOT:PSS nanofibers, PVA and PEDOT:PSS nanocrystalline domains, and semi-interpenetrating polymer networks, PPS organo-hydrogels display outstanding mechanical performances (strength: 54.8 MPa, toughness: 153.97 MJ m-3 ). Additionally, PPS organo-hydrogels also exhibit powerful sensing capabilities (gauge factor (GF): 983) due to the aligned hierarchical structures and organic liquid phase of DMSO. Notably, with the synergy of such mechanical and sensing properties, organo-hydrogels can even detect objects as light as 1 gram, despite bearing a tensile strength of ≈23 MPa. By incorporating these materials into human-machine interfaces, such as controlling artificial arms for grabbing objects and monitoring sport behaviors in soccer training, this work has unlocked a new realm of possibilities for these high-performance hierarchical organo-hydrogels. This approach to designing hierarchical structures has the potential to lead to even more high-performance hydrogels in the future.

Micro-/Nanohierarchical Structures Physically Engineered on Surfaces: Analysis and Perspective.

The high demand for micro-/nanohierarchical structures as components of functional substrates, bioinspired devices, energy-related electronics, and chemical/physical transducers has inspired their in-depth studies and active development of the related fabrication techniques. In particular, significant progress has been achieved in hierarchical structures physically engineered on surfaces, which offer the advantages of wide-range material compatibility, design diversity, and mechanical stability, and numerous unique structures with important niche applications have been developed. This review categorizes the basic components of hierarchical structures physically engineered on surfaces according to function/shape and comprehensively summarizes the related advances, focusing on the fabrication strategies, ways of combining basic components, potential applications, and future research directions. Moreover, the physicochemical properties of hierarchical structures physically engineered on surfaces are compared based on the function of their basic components, which may help to avoid the bottlenecks of conventional single-scale functional substrates. Thus, the present work is expected to provide a useful reference for scientists working on multicomponent functional substrates and inspire further research in this field.

Wood-Derived Freestanding Carbon-Based Electrode with Hierarchical Structure for Industrial-Level Hydrogen Production.

The sustainable and scalable fabrication of low-cost, efficient, and durable electrocatalysts that operate well at industrial-level current density is urgently needed for large-scale implementation of the water splitting to produce hydrogen. In this work, an integrated carbon electrode is constructed by encapsulating Ni nanoparticles within N-doped carbonized wood framework (Ni@NCW). Such integrated electrode with hierarchically porous structure facilitates mass transfer process for hydrogen evolution reaction (HER). Ni@NCW electrode can be employed directly as a robust electrocatalyst for HER, which affords the industrial-level current density of 1000 mA cm-2 at low overpotential of 401 mV. The freestanding binder-free electrode exhibits extraordinary stability for 100 h. An anion exchange membrane water electrolysis (AEMWE) electrolyzer assembled with such freestanding carbon electrode requires only a lower cell voltage of 2.43 V to achieve ampere-level current of 4.0 A for hydrogen production without significant performance degradation. These advantages reveal the great potential of this strategy in designing cost-effective freestanding electrode with monometallic, bimetallic, or trimetallic species based on abundant natural wood resources for water splitting.