Qifeng Zheng

High-performance green flexible electronics based on biodegradable cellulose nanofibril paper

Todays consumer electronics, such as cell phones, tablets and other portable electronic devices, are typically made of non-renewable, non-biodegradable, and sometimes potentially toxic (for example, gallium arsenide) materials. These consumer electronics are frequently upgraded or discarded, leading to serious environmental contamination. Thus, electronic systems consisting of renewable and biodegradable materials and minimal amount of potentially toxic materials are desirable. Here we report high-performance flexible microwave and digital electronics that consume the smallest amount of potentially toxic materials on biobased, biodegradable and flexible cellulose nanofibril papers. Furthermore, we demonstrate gallium arsenide microwave devices, the consumer wireless workhorse, in a transferrable thin-film form. Successful fabrication of key electrical components on the flexible cellulose nanofibril paper with comparable performance to their rigid counterparts and clear demonstration of fungal biodegradation of the cellulose-nanofibril-based electronics suggest that it is feasible to fabricate high-performance flexible electronics using ecofriendly materials.

Green synthesis of polyvinyl alcohol (PVA)–cellulose nanofibril (CNF) hybrid aerogels and their use as superabsorbents

Cross-linked polyvinyl alcohol (PVA)–cellulose nanofibril (CNF) hybrid organic aerogels were prepared using an environmentally friendly freeze-drying process. The resulting PVA/CNF aerogel was rendered both superhydrophobic and superoleophilic after being treated with methyltrichlorosilane via a simple thermal chemical vapor deposition process. Successful silanization on the surface of the porous aerogel was confirmed by various techniques including scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDS), Fourier transform infrared spectroscopy (FTIR), and contact angle measurements. The silane-treated, cross-linked PVA/CNF aerogels not only exhibited excellent absorption performance for various types of oils (e.g., crude oil) or organic solvents (with a typical weight gain ranging from 44 to 96 times of their own dry weight), but also showed a remarkable scavenging capability for several types of heavy metal ions tested (e.g., Pb2+, Hg2+), making them versatile absorbents for various potential applications including water purification. Furthermore, these PVA/CNF aerogels demonstrated excellent elasticity and mechanical durability after silane-treatment as evidenced by the cyclic compression tests.

Cellulose Nanofibril/Reduced Graphene Oxide/Carbon Nanotube Hybrid Aerogels for Highly Flexible and All-Solid-State Supercapacitors

A novel type of highly flexible and all-solid-state supercapacitor that uses cellulose nanofibril (CNF)/reduced graphene oxide (RGO)/carbon nanotube (CNT) hybrid aerogels as electrodes and H2SO4/poly(vinyl alcohol) (PVA) gel as the electrolyte was developed and is reported here. These flexible solid-state supercapacitors were fabricated without any binders, current collectors, or electroactive additives. Because of the porous structure of the CNF/RGO/CNT aerogel electrodes and the excellent electrolyte absorption properties of the CNFs present in the aerogel electrodes, the resulting flexible supercapacitors exhibited a high specific capacitance (i.e., 252 F g(-1) at a discharge current density of 0.5 A g(-1)) and a remarkable cycle stability (i.e., more than 99.5% of the capacitance was retained after 1000 charge-discharge cycles at a current density of 1 A g(-1)). Furthermore, the supercapacitors also showed extremely high areal capacitance, areal power density, and energy density (i.e., 216 mF cm(-2), 9.5 mW cm(-2), and 28.4 μWh cm(-2), respectively). In light of its excellent electrical performance, low cost, ease of large-scale manufacturing, and environmental friendliness, the CNF/RGO/CNT aerogel electrodes may have a promising application in the development of flexible energy-storage devices.

Polyvinyl Alcohol-Cellulose Nanofibrils-Graphene Oxide Hybrid Organic Aerogels

Hybrid organic aerogels consisting of polyvinyl alcohol (PVA), cellulose nanofibrils (CNFs), and graphene oxide nanosheets (GONSs) were prepared using an environmentally friendly freeze-drying process. The material properties of these fabricated aerogels were measured and analyzed using various characterization techniques including compression testing, scanning electron microscopy, thermogravimetric (TGA) analysis, Brunauer-Emmet-Teller (BET) surface area analysis, and contact angle measurements. These environmentally friendly, biobased hybrid organic aerogels exhibited a series of desirable properties including a high specific compressive strength and compressive failure strain, ultralow density and thermal conductivity, good thermal stability, and moisture resistance, making them potentially useful for a broad range of applications including thermal insulation.

Poly(vinyl alcohol)/Cellulose Nanofibril Hybrid Aerogels with an Aligned Microtubular Porous Structure and Their Composites with Polydimethylsiloxane

Superhydrophobic poly(vinyl alcohol) (PVA)/cellulose nanofibril (CNF) aerogels with a unidirectionally aligned microtubular porous structure were prepared using a unidirectional freeze-drying process, followed by the thermal chemical vapor deposition of methyltrichlorosilane. The silanized aerogels were characterized using various techniques including scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, and contact angle measurements. The structure of the aerogels fully filled with polydimethylsiloxane (PDMS) was confirmed by SEM and optical microscopy. The mechanical properties of the resulting PDMS/aerogel composites were examined using both compressive and tensile tests. The compressive and tensile Youngs moduli of the fully filled PDMS/aerogel composites were more than 2-fold and 15-fold higher than those of pure PDMS. This study provides a novel alternative approach for preparing high performance polymer nanocomposites with a bicontinuous structure.

Polyvinyl alcohol (PVA)–cellulose nanofibril (CNF)–multiwalled carbon nanotube (MWCNT) hybrid organic aerogels with superior mechanical properties

Polyvinyl alcohol (PVA)–cellulose nanofibril (CNF)–multiwalled carbon nanotube (MWCNT) hybrid organic aerogels were prepared using an environmentally friendly freeze-drying process with renewable materials. The material properties of these “green” hybrid aerogels were characterized extensively using various techniques. It was found that adding a small amount of CNFs and MWCNTs increased the mechanical properties of the PVA aerogels drastically. The mechanical properties of the hybrid aerogels showed an exponential dependency on the relative aerogel densities. These low-density hybrid aerogels also exhibited very low thermal conductivities and high surface areas, thereby making them potentially useful for many applications including thermal insulation and structural components.

Synthesis of polyvinyl alcohol/cellulose nanofibril hybrid aerogel microspheres and their use as oil/solvent superabsorbents

Superhydrophobic and crosslinked poly(vinyl alcohol) (PVA)/cellulose nanofibril (CNF) aerogel microspheres were prepared via a combination of the water-in-oil (W/O) emulsification process with the freeze-drying process, followed by thermal chemical vapor deposition of methyltrichlorosilane. The oil phase and the cooling agent were judiciously selected to ensure that the frozen ice microspheres can be easily separated from the emulsion system. The silanized microspheres were highly porous with a bulk density ranging from 4.66 to 16.54mg/cm(3). The effects of the solution pH, stirring rate, and emulsifier concentration on the morphology and microstructure of the aerogel microspheres were studied. The highly porous structure of the ultralight aerogel microspheres demonstrated an ultrahigh crude oil absorption capacity (up to 116 times its own weight). This study provides a novel approach for the large-scale preparation of polymeric aerogel microspheres with well-controlled particle sizes that can be used for various applications including oil and chemical spill/leak clean-up.

Multi-stimuli-responsive self-healing metallo-supramolecular polymer nanocomposites

The self-healing capability is particularly desirable for multifunctional polymeric materials because it can make the materials more reliable, reduce repair costs, and extend their lifetime in many applications. Herein, a multi-stimuli responsive self-healing metallo-supramolecular polymer nanocomposite exhibiting superior mechanical properties was developed and demonstrated. Terpyridine ligand-terminated polyurethane (PU) was prepared by in situ polymerization on the surface of multi-walled carbon nanotubes (CNTs). The terpyridine ligand-terminated CNT/PU prepolymers were dynamically crosslinked with the metal ion Zn2+ to obtain a metallo-supramolecular polymer nanocomposite. The tensile strength and tensile strain-at-break of the CNT/PU polymer nanocomposite increased from 14.2 MPa and 620% to 22.8 MPa and 1076%, respectively, with the addition of Zn2+. The Zn2+-coordinated metallo-supramolecular polymer nanocomposite showed a rare combination of strong, tough, and elastic mechanical properties and was able to self-heal via multiple stimuli, including remotely controlled near infrared (NIR) light (4.2 mW mm−2, 30 min), relatively low temperatures (90 °C, 1 h), and/or solvents, all with excellent healing efficiencies (i.e., higher than 93%) and short healing times. Therefore, this unique multi-stimuli responsive self-healing metallo-supramolecular polymer nanocomposite has the potential to provide advances in structural component, microelectronic and sporting equipment applications, to name just a few.

Graphene/Phase Change Material Nanocomposites: Light-Driven, Reversible Electrical Resistivity Regulation via Form-Stable Phase Transitions

Innovative photoresponsive materials are needed to address the complexity of optical control systems. Here, we report a new type of photoresponsive nanomaterial composed of graphene and a form-stable phase change material (PCM) that exhibited a 3 orders of magnitude change in electrical resistivity upon light illumination while retaining its overall original solid form at the macroscopic level. This dramatic change in electrical resistivity also occurred reversibly through the on/off control of light illumination. This was attributed to the reversible phase transition (i.e., melting/recrystallization) behavior of the microscopic crystalline domains present in the form-stable PCM. The reversible phase transition observed in the graphene/PCM nanocomposite was induced by a reversible temperature change through the on/off control of light illumination because graphene can effectively absorb light energy and convert it to thermal energy. In addition, this graphene/PCM nanocomposite also possessed excellent mechanical properties. Such photoresponsive materials have many potential applications, including flexible electronics.

Mechanically strong fully biobased anisotropic cellulose aerogels

Fully biobased chemically crosslinked anisotropic carboxymethyl cellulose (CMC)/cellulose nanofibril (CNF) aerogels were prepared using an environmentally friendly directional freeze-drying method. The resulting cellulose aerogels were characterized using various techniques. It was found that the CMC/CNF aerogel exhibited a honeycomb structure, and thus possessed anisotropic properties. Moreover, the fully biobased crosslinked organic aerogel possessed excellent mechanical properties based on both compression and three-point bending tests. For instance, it exhibited a remarkable compressive modulus (up to 10 MPa) along the vertical direction (parallel to the freezing direction) as well as a high flexural modulus (up to 54 MPa) perpendicular to the freezing direction. The effects of different aerogel densities and CNF contents on the mechanical properties of CMC/CNF aerogels have been studied. With increasing aerogel density or CNF content, the modulus and strength of the CMC/CNF aerogel increased in both compression and three-point bending tests. In addition, these cellulose aerogels also exhibited relatively low thermal conductivities (<54 mW m−1 K−1). Considering their excellent mechanical properties, very low densities, and the “green” synthesis process, these CMC/CNF aerogels hold great promise for potential industrial applications such as green thermal insulation building materials.