Patent Application
20190210881
The currently claimed embodiments of the current invention relate to electro-hydrodynamic stimulated assembly of hierarchically porous, functional nanostructures from 2D layered soft materials.
In aerosol assembly, solvent evaporation drives the anisotropic crumpling process of graphene contained in aerosol droplets. As solvent is rapidly lost, sheets begin to aggregate due to strong intermolecular forces, ultimately clumping into crumpled balls. Although the resultant crumpled balls proved to be resistant to compressive forces and can be packed into high-density configurations, the overall performance is still far from ideal as a result of graphite-like walls. This also in turn generates monolithic materials with less desirable properties. Thus it is highly desirable to develop strategies that allow one to effectively harness the extraordinary material properties of single-to-few layered crumpled graphene nanostructures (CGNs), especially when assembled in a monolithic fashion. Indeed, the ability to reduce the number of layers of 3D structures upon assembling into macroscopic composites can not only be crucial for building new types of capacitors, batteries, sensors, and even actuators, but also may be paramount for future development of new generations of scaffolds with catalytically active, energetically favorable, and chemically defined interfaces.
There thus remains a need for improved methods for producing nanostructures and for improved nanostructures obtained by improved methods of production.
A method for producing a nanostructure or an article having at least a nanostructured portion according to some embodiments of the current invention includes obtaining a colloidal suspension of sheets of material for forming nanoparticles, the sheets being less than four atomic layers thick and the colloidal suspension having a preselected concentration of the sheets of material suspended therein; supplying the colloidal suspension to an electro-hydrodynamic system, the electro-hydrodynamic system including a spray nozzle, a ground electrode spaced apart from the spray nozzle, and a high voltage DC power supply electrically connected to the spray nozzle and the ground electrode, the high voltage DC Power supply being suitable for supplying at least a 0.05 kV/cm electric field between the spray nozzle and the ground electrode; providing a substrate arranged between the spray nozzle and the ground electrode such that droplets from the spray nozzle are directed to the substrate to deposit nanostructures thereon; and applying a DC voltage using the high voltage DC power supply between the spray nozzle and the ground electrode such that charged droplets from the spray nozzle are repelled from the spray nozzle and attracted towards the substrate. The DC voltage is selected such that the droplets have sizes sufficiently small to result in substantially isolated sheets within each droplet.
A nanostructured article or nanostructured article portion according to some embodiments of the current invention is produced using a method according to an embodiment of the current invention.
A nanostructure or an article having at least a nanostructured portion according to an embodiment of the current invention includes a plurality of crumpled nanoparticles formed into a self-supporting structure. The crumpled nanoparticles have walls having thicknesses of less than four atomic layers.
Further objectives and advantages will become apparent from a consideration of the description, drawings, and examples.
Some embodiments of the current invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. A person skilled in the relevant art will recognize that other equivalent components can be employed and other methods developed without departing from the broad concepts of the current invention. All references cited anywhere in this specification, including the Background and Detailed Description sections, are incorporated by reference as if each had been individually incorporated.
The term “nanoparticle” is intended to include any particles that have a longest dimension that is submicron in size down to about 1 nm, i.e., 1 nm to 999 nm.
The term “crumpled nanoparticles” is intended include the resultant nanoparticles following a change in morphology of substantially 2D nanoparticles. This change in morphology can result from a curving, bending, folding, wrinkling, creasing, crumpling, or compressing of the substantially 2D nanoparticles. The crumpled nanoparticles thus result in nanoparticles that have structure extending out of the original plane of the substantially planar precursor 2D nanostructure.
The term “single-to-few layered” structure is intended to refer to exfoliated molecular structures of single atomic layer thickness in some embodiments, up to two atomic layers in some embodiments, or up to three atomic layers in some embodiments. Such structures will also be referred to as 2D layered soft sheets. Three layers of graphene are the upper limit of CGNs. Beyond three layers, the material property of graphene will become graphite-like.
The term “substrate” is intended to have a broad meaning that can include any surface on which it is intended to form nanoparticles. The surface can serve for producing individual nanostructures, a self-supporting final structure consisting essentially of the nanostructures (e.g., but not limited to, a filter, a catalyst, an element of a battery, a supercapacitor, an ultra-capacitor and/or a fuel cell), or could be the portion of a device, such as, but not limited to an electronic device (e.g., but not limited to, a capacitor, a diode, a transistor, and/or a photovoltaic cell). The substrate can be a hydrophobic surface in at least portions and/or hydrophilic in portions. The substrate can also have a structure, such as, but not limited to a nanostructure in at least portions thereof.
For decades, it has been known that the electro-hydrodynamic (EHD) process can atomize liquid media for high throughput production of thin film specimens. A high voltage applied between the nozzle and a conductive support plate creates an electro-hydrodynamic phenomenon that drives the flow of colloidal dispersions out of the nozzle. An ultra-high D.C. voltage (kV) is applied between the nozzle tip and the metal plate using a computer controlled power supply to generate an electric field that causes charged species within the liquid medium to accumulate near the surface of the pendent meniscus. The escalating columbic repulsions between charged species induce a tangential stress on the liquid surface, thereby deforming the meniscus into a conical shape, known as a Taylor cone. At a sufficiently high electric field, the electrostatic stress overcomes the capillary tension at the apex of the liquid cone, giving rise to fine, charged droplets. The electric field can be greater than 0.05 kV/cm in some embodiments. In some embodiments, the electric field can be greater than 0.1 kV/cm. In some embodiments, the electric field can be, for example, 0.575 kV/cm. However, the electric field is not limited to these values. This unique feature can be significantly useful in the case of graphene and its derivatives in colloidal dispersions since the highly charged microenvironment first and foremost electrostatically stabilizes 2D layered soft sheets owing to the much-enhanced electrostatic repulsion spanning from the needle to collecting substrates. Next, the largest droplet just after separation from liquid jets has a charge density that exceeds the Rayleigh limit. At this point, large droplets will undergo a fission process to disseminate into highly charged, self-dispersing droplets with nearly monodispersed diameter distribution in sub-micron to nanometer ranges. In contrast to the aerosol process where shrinkage of droplets induces the folding of 2D layered soft sheets, the fission process readily reduces the loading of 2D layered soft sheets in each droplet. In this light, individual fine droplets will only contain a limited number of sheets, further reducing the possibility of irreversible aggregation.
Results Obtained Using High Resolution Scanning Electron Microscopy (HRSEM) Image of Spatially Separated 2D Layered Soft Sheets Deposited Via EHD Process at Room Temperature
The resultant 2D layered soft sheets appear to separate from each other without agglomerations, underscoring the importance of the electrostatically stabilizing microenvironment. SEM, atomic force microscopy (AFM), and a 3D profile scanned across a myriad of 2D layered soft sheets further reveals a step height of ˜1 nm, confirming the single layer identity. The ability to create single layer 2D layered soft sheets not only supports the hypothesis of electrostatically charged nanoreactors but also provides a means to obtain single layer specimens for device fabrication through a room temperature EHD process. Finally, upon annealing, metastable and adaptable droplets can act as individual nanoreactors to facilitate capillarity induced compressive forces introducing networks of ridges, ripples, folds and vertices to initiate the deformation process on the basal plane.
As a result of the EHD process, hierarchically porous, functional 3D nanostructures can be formed. Embodiments of the invention can use a variety of layered or substantially 2D soft materials, including graphene, clay, semiconductors, metals, and metal chalcogenides, dichacolgenides and transitional metal dichalcogenides (TMDs). For example, molybdenum disulfide can be used according to an embodiment. Embodiments of the invention are not limited to the materials listed, and may include any 2D materials. The resulting 3D or crumpled nanostructures can have walls that are single- to a few-layers thick. These crumpled nanostructures, and microscopic monoliths assembled from them, provide many useful material properties, including properties relevant to energy harvesting and storage. These material properties include high surface area, good electrical conductance, preserved capacity, and excellent photochemical properties, which can be effectively harnessed for macroscopic applications.
According to some embodiments, multicomponent crumpled structures and the encapsulation of guest species with dissimilar solubility into 3D nanostructures are possible. Thus, it is possible to form hybrid nano-building blocks that have the advantage of combining the complementary strength from both chemical worlds.
The combination of electrostatic and capillary cues stemmed from EHD processes collectively decouples exceptional properties from the layer dependent electronic structures of 2D soft layered derivatives. This embodies an important step to end the chasm between academic prototype and industrial implementation of graphene-based composites where the difficulties lie in the design of a hierarchically functional architecture that allows for extraordinary material properties of individual sheets to be effectively harnessed.
According to some embodiments of the current invention, the crumpled nanostructures and microscopic monoliths assembled from the crumpled nanostructures can be adapted for use in a variety of applications, including desalination, water remediation, chemistry, fluid dynamics, materials science, engineering, environmental remediation, health and sanitation, catalytic elements, actuators, medical devices, composite materials, biomedical sciences, agriculture, energy and infrastructure applications, and space applications, for example. An embodiment of the invention is able to provide a scalable platform for mass production of single to few layered crumpled graphene nanostructures for these applications.
According to some embodiments of the invention, the collecting substrate can be textured and/or chemically heterogeneous. As used herein, “chemically heterogeneous” refers to dissimilar chemical properties stemming from spatially distributed chemical functional groups. In some embodiments, the collection substrate is hydrophobic or super-hydrophobic, and/or can be nano-textured. These features, alone or in combination, can affect the hydrodynamics of droplets containing 2D materials. Whether or not the 2D materials undergo dimensional transition, or the extent to which dimensional transition occurs, upon solvent evaporation can be impacted by the direction of capillary forces. For example, when the surface of the substrate is hydrophobic, droplets will tend to remain in spheres or more spherical shapes, as opposed to spreading out on the surface. Such spherical shapes can be considered 3D platforms that exert omnidirectional capillary forces upon drying, thus forming 3D crumpled structures. On the other hand, water droplets on hydrophilic surfaces will tend to spread out on the surface, and will therefore generate capillary forces mostly in the lateral directions, or parallel to the plane of the surface. As a result, 2D sheets collected on hydrophilic surfaces are more likely to form sheets that are only wrinkled or creased, as opposed to being crumpled.
The use of a hydrophobic and/or nano-textured surface can enhance low-temperature processing capabilities of embodiments of the invention. Low-temperature processing can enable the use of a wider range of materials and applications, including polymers and flexible substrates.
The ability to reduce the number of layers of CGNs upon assembling into macroscopic composites will not only be crucial for building new types of capacitors, batteries, sensors, and even actuators, but also will be paramount for future development of a new generation of scaffolds with catalytically active, energetically favorable, and chemically defined interfaces. The discovery is extremely significant and is a very high-priority development opportunity representing the most effective solution for bulk implementation of graphene based materials as well as other 2D soft sheets made of either metallic, semi-metallic or insulating elements.
The following examples describe some embodiments in more detail. The broad concepts of the current invention are not intended to be limited to the particular examples. Further, concepts from each example are not limited to that example, but may be combined with other embodiments of the system.
The structures and methods according to various embodiments can facilitate dimensional transition of 2D layered soft materials into 3D porous and hierarchically functional nanostructures. Taking graphene as an example, graphene colloidal dispersions (0.5 mg/ml, 40 ml) made from a modified Hummers' approach were mixed with 0.1 ml hydrazine (35 wt % in water) and 0.56 ml ammonia (28 wt % in water) to adjust pH to 11 in a flask and stirred in a water bath at 95° C. for 1 hour. Flat graphene papers were prepared by vacuum filtrating of 8 ml as obtained graphene colloidal dispersion through an isopore membrane filter paper (100 nm pore size). To synthesize 3D nanostructures, graphene dispersions (50 ug/mL) were fed through a customized EHD setup. Note that pH of graphene dispersions are preferably maintained at 11 to obtain desired electrostatic force for isolating individual graphene sheets. In a typical experiment, solutions are fed to the spray head (gauge 23 TW needle) by a syringe pump. Electric fields are generated through a high power supply (ES 40P-20 W/DAM, Gamma high voltage research) with a distance of 10 cm measured from the tip of spinneret to collecting substrates. Computerized multi-pass deposition is achieved through the integration of x-y translational stage (Newport, moving speed 2 mm/sec) and micro-heating plate. A table of detailed operating parameters, including concentration, solution feed rate, and annealing temperature, to afford 3D graphene nanostructures can be found in Table 1. In an analogous fashion, other 2D metal chalcogenides and clays can be assembled, synthesized and processed to afford 3D porous nanostructures.
In some examples, 2-D transition metal dichacolgenides and clay nano-sheets have been shown to form crumpled structures when using a collecting substrate with a hydrophobic surface.
Electrohydrodynamic-Stimulated Assembly of Crumpled Graphene Nanostructures
Here we describe the convergence of stimuli-responsive graphene sheets with new insights into the decade-old electrohydrodynamic processes leading to the formation of electrocapacitively active and photoelectrochemically functional crumpled graphene nanoparticles (CGNs). This strategy conceptually mimics charge-stabilized colloidal systems that concurrently introduce electrostatic and capillary cues to initiate a dimensional transition of planar graphene sheets into spherical CGNs comprised of only single-to-few layered walls. We demonstrate that the resultant CGNs and their assembly into microscopic monoliths allows for extraordinary material properties, especially those relevant to energy harvesting and storage, such as high surface area, good electrical conductance, preserved capacity and excellent photochemical properties to be effectively harnessed for macroscopic applications. This general, yet versatile strategy also enables the creation of clay nanosheets, and metal dichalcogenides (molybdenum disulfide, MoS2) based 3D crumpled structures in tandem with the encapsulation of guest species with dissimilar solubility into CGNs, leading to the formation of hybrid nano-building blocks that can have the advantage of combining the complementary strengths from both chemical worlds.
The deployment of dimensional transitions is ubiquitous in nature, ranging from the Venus Flytrap, beating of a heart, sounds shaped by the vocal folds and zooming of focal length by the human eye. External stimuli in the form of chemical or mechanical cues arising from the environment result in the deformation of materials. Such a dimensional transition leads to new functionalities which cannot be found in their original formats.1 One of such fascinating examples in molecular material science is carbon. At the molecular level, carbon atoms placed in sp3 tetrahedral arrangement lead to the formation of diamond, the hardest naturally occurring material. In contrast, when pieced together in a planar sp2 network, rather soft two-dimensional (2D) graphene sheets are formed that can be re-stacked to create three-dimensional (3D) graphite. On the nanoscale, curled sp2 networks lead to strained and deformed structures such as fullerenes and carbon nanotubes. Crumpled graphene nanoparticles (CGNs) are the newest addition to the family and have already stimulated immense interests across different disciplines for widespread applications.2-5 In particular, 3D particle-like membranes represent a unique type of nano-building block in that they possess distinctly different assembling behaviors from parent graphene by virtue of the weak intermolecular forces that have been known to scale with the geometries between two interacting bodies (i.e., ˜1/d2 along with planar surfaces while 1/d6 between spheres). Closely resembling metallic lattices, the resulting CGNs, in theory, can be processed in a macroscopic bulk form without significantly compromising the intrinsic material properties, such as high free volume, accessible surface area, and specific capacity.2,5-8
A number of approaches, such as template-directed synthesis, chemical vapor deposition (CVD) over a porous catalyst, and sugar blowing, have been developed to fabricate highly porous, 3D interconnected CGN-like composites.9-11 However, these strategies all require harsh processing conditions or laborious removal of the sacrificing molds, inevitably introducing complexity and high possibilities to contaminate the functional interfaces.6,12,13 While these top-down synthetic approaches hold some promise, it is ultimately a much facile and scalable aerosol assembly that emerges as the most well developed and characterized approach.2,4,5,8,14 In essence, solvent evaporation drives the anisotropic crumpling process of graphene containing aerosol droplets.14 As solvent is rapidly lost, sheets begin to aggregate due to strong intermolecular forces, ultimately clumping into crumpled balls. Although the resultant crumpled balls proved to be compressive resistant and can be packed into high-density configuration, the overall performance is still far from ideal as a result of graphite-like walls. This also in turn generates monolithic materials with less desired properties.15-17 Thus it is highly desirable to develop strategies that allow us to effectively harness the extraordinary material properties of single-to-few layered CGNs, especially when assembled in a monolithic fashion. Indeed, the ability to reduce the number of layers of CGNs upon assembling into macroscopic composites will not only be crucial for building new types of capacitors, batteries, sensors, and even actuators but also will be paramount for future development of new generations of scaffolds with catalytically active, energetically favorable, and chemically defined interfaces.7,18,19
Recent advances in reduced graphene oxide (rGO), especially new insights into its colloidal chemistry and mechanical properties, open up new avenues to address this formidable challenge.20-22 rGO can be well dispersed in water without the need for foreign stabilizers by controlling its surface chemistry.23 For instance, ionizable edges of rGO were found to be pH sensitive, thus enabling the tuning of surface charge density.22,24 Under high pH values, the high negatively charged edges render rGO sheets repulsive with respect to each other, thus preserving the single layer conformation in colloidal dispersions. On the other hand, previous studies on the deformation of rGO sheets also suggest that the seemingly strongest materials on earth can be distributed over a large area when drop casting from rGO dispersions7,25-27 and can conform onto curvilinear foreign objects,21,28-30 and self-fold into various shapes9,28 upon isotropically capillary compression by virtue of the much reduced flexural rigidity.7,31,32 Therefore, rGO is indeed a stimulus-responsive, soft material with electrostatically ionizable edges and a mechanically deformable basal plane. We thus surmise that if external stimuli in the form of electrostatic and capillarity-induced-mechanical cues can be concurrently introduced, we may simultaneously tune the colloidal property and strain engineering of rGOs, ultimately transforming into single-to-few layered CGNs with much improved material properties.
In the present examples, we demonstrate the synthesis of mono-to-few layered CGNs and their assembly into multi-functional monoliths through a general, low-cost, rapid and scalable electrohydrodynamic (EHD) process. The resulting CGNs are found to exhibit a combination of high surface area, good conductivity, and largely preserved intrinsic capacity in a bulk form, while the thin and vertically structured walls can be used as energetically favorable 3D scaffolds to facilitate efficient electron transport. In particular, the unique bottom-up and low temperature characteristics of EHD process makes it possible to integrate CGNs onto flexible substrates, such as carbon fiber electrodes, representing a significant step further toward high throughput reel-to-reel production of graphene based flexible electronics. Moreover, incorporating a core/shell spinneret into the EHD approach allows for simultaneous synthesis and entrapment of inorganic guest species with dissimilar solubility into CGNs. This leads to the formation of hybrid nano-building blocks that have the advantage of combining the complementary strengths from both chemical worlds.
General Description of the EHD Process
Experiments were performed using a customized EHD setup. A complete diagram of the apparatus is illustrated in
Results
Synthesis of CGNs Via EHD Stimuli.
We explored a myriad of approaches and are particularly intrigued by the versatile, readily accessible and scalable EHD process (well known for its use in electrospinning and -spraying,
Complete transition of planar sheets into crumples occurred when supporting substrates were annealed at 255° C. using a programmable hot plate. To systematically explore the dynamic interaction between electrostatic and capillary stimuli, SEM images of samples captured along with the spraying pathway under combined effects of a constant electric field (0.575 kV/cm), concentration (50 μg/mL) and flow rate (4 μL/min) permits us to closely monitor the morphological evolution as a function of elevating temperatures. It was discovered that individual rGO sheets, unlike the aggregation seen in aerosol methods, begin to develop a ridge like morphology on the basal plane and folded edges induced by the increasing capillary force when the surrounding temperature is raised to 75° C. (
Meanwhile, the radii of the resulting CGNs were found to be relatively smaller than those predicted by theoretical modeling as the harsh chemical exfoliation often introduces high levels of defects on the basal plane of rGO.41 The rupture of the π-π conjugations not only leads to the reduction of the intrinsic flexural rigidity but makes the CGNs fold or even compress into a tightly packed configurations.4 Intriguingly, unlike most of the carbon foams and porous carbon structures that are prone to collapse when subjected to deformation, CGNs maintain a spherical shape and a largely accessible volume even after depositing onto hard substrates as shown in false colored cross-sectional SEM image (
Synthesis of the Geometrically Engineered rGO Sheets.
The aspect ratios of geometrically well-defined rGO sheets can be systematically engineered through the unraveling of commercially available multiwalled carbon nanotubes (MWCNTs) (Kosynkin, D. V. et al. Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons. Nature 458, 872-U875, doi:Doi 10.1038/Nature07872 (2009)). To study the effect of aspect ratios on the final morphology, MWCNTs with various diameters (˜25 nm, and ˜170 nm) were selected. Unzipping of MWCNTs with the diameter of ˜25 nm results in ribbon like rGOs with high aspect ratios (width of 157 nm, length of ˜5 μm, and aspect ratios of ˜32). In contrast, unzipping of MWCNTs with a larger diameter (˜170 nm) often produces rectangular shaped rGOs (width of 1067.6 nm, length of ˜2 μm, and aspect ratios of ˜1.88). To start, a 150 mg portion of MWCNTs was suspended in 36 mL of H2SO4 by stirring the mixture for a period of a minimum 1 h to 12 h. Next, H3PO4 (85%, 4 mL) was then added, and the mixture was allowed to stir another 15 min before the addition of KMnO4 (750 mg). The reaction mixture was then heated at 65° C. for 2 h, and then allowed to cool to room temperature before product isolation as described below. The reaction mixture was poured onto 100 mL of ice containing H2O2 (30%, 5 mL). The product was allowed to coagulate (no stirring) for 14 h. The top portion was decanted from the solid, and the remaining portion was filtered over a 200 nm pore size PTFE membrane (5 μm pore size also works). The brown filter cake was washed 2 times with 20% HCl (20 mL each), re-suspended in Acetone (60 mL). The product was filtered on the same PTFE membrane and then dispersed in ethanol (100%, 40 mL) for 2 h with stirring, followed by filtration. The resulting solid was dispersed in a mixture of H2O and MeOH (v/v, 9:1) in a 1 mg/mL ratio and sonicated for 1 hour. Subsequently, the mixture was placed inside the hood overnight. Decant the supernatant and then centrifuge at 2,000 rpm for 1 hour to further remove any agglomerations.
Conformational Evolution of CGNs Through Geometrically Engineered rGO Sheets
MD simulations were performed using LAMMPS software (Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular-Dynamics. J Comput Phys 117, 1-19, doi:Doi 10.1006/Jcph.1995.1039 (1995)). The initial geometry of rGO configurations were optimized using the conjugate gradient method and the folding simulations were conducted with a time step of 1 fs. The crumpling process of the relatively hydrophobic (lower surface energy) rGO sheets can be deemed as a process to minimize the mechanical instability in colloidal dispersions. In particular, the presence of water is expected to change the folding dynamics of CGNs as rGO sheets will first self-fold to increase the contact area of π-π interactions which scale with van der Waals (vdW) forces, thus minimizing the overall potential energy (E) (Patra, N., Song, Y. B. & Kral, P. Self-Assembly of Graphene Nanostructures on Nanotubes. Acs Nano 5, 1798-1804, doi:Doi 10.1021/Nn102531h (2011); Qin, Z., Taylor, M., Hwang, M., Bertoldi, K. & Buehler, M. J. Effect of Wrinkles on the Surface Area of Graphene: Toward the Design of Nanoelectronics. Nano Lett 14, 6520-6525, doi:Doi 10.1021/N1503097u (2014); Tang, C., Oppenheim, T., Tung, V. C. & Martini, A. Structure-stability relationships for graphene-wrapped fullerene-coated carbon nanotubes. Carbon 61, 458-466, doi:Doi 10.1016/J.Carbon.2013.04.103 (2013)). From an energetic point of view, the crumpling dynamics in this case can be briefly expressed as the competition between van der Waals energy (EvdW) and elastic bending energy (Ebend). In the former case, EvdW scales with the contacting area while Ebend predominately hinges on the aspect ratios. While the presence of water is expected to change the rate at which folding or wrinkling occurs on the hydrophobic basal plane, the aspect ratios of the rGO sheets derived from chemically unraveling of MWCNTs will likely dominate the preferred direction of folding. In other words, the final geometrical conformation of these freestanding rGO sheets can be controlled by the length-to-width ratio on the preferred direction of folding. As shown in our previous study, Ebend increases with rGO size, and the rate of that increase is faster with width than with length. In addition, the energy ratio between bending from longitudinal and width direction derived from the MD simulation sheds lights on predicting the energetically favorable direction for folding (Tang, C., Oppenheim, T., Tung, V. C. & Martini, A. Structure-stability relationships for graphene-wrapped fullerene-coated carbon nanotubes. Carbon 61, 458-466, doi:Doi 10.1016/J.Carbon.2013.04.103 (2013)).
E
bend
L
/E
bend
W=0.34w−2π(R+1.35)2/0.34l+2π(R+135)2 (1)
where w is the width, and l is the length of rGO. R is radius of the supporting substrates and will not be used in calculation since rGOs are freestanding. It becomes apparent that when w<l, the ratio of EbendL/EbendW is less than unity, suggesting that rGO will be energetically prone to bending along the L direction (tubular structures). With decreasing aspect ratios (w>l), the energy ratio dramatically reduces, leading to the formation of short side rolling and all side folding as shown in the schematic diagram of folding as a function of length and width.
Characterization of CGNs.
Similar to many rGO based colloidal experiments, including our previous work on counter ion stabilized rGO-hydrazine dispersions, the stability of the electrostatically charged colloidal microenvironment strongly hinges on pH, the content of dispersants, and the concentration of electrolyte.22,24,43 Among these variables, pH value plays a vital role by drastically altering the surface charge density (Zeta potential) of rGO sheets as it affects the degree of ionizable carboxylic groups.
Synergistic combination of the stimuli responsive nature of rGO and external stimuli arising from the electrohydrodynamically generated droplets unlocks a new approach to harness superlative material properties at the nanoscale for microscopic integration and macroscopic applications. While the 2D configuration of rGO is well suited for constructing electrically conductive, and spatially interconnected networks, it is the propensity to aggregate when processed in a bulk form that adversely affects permeability, ionic transport, accessible surface area and most importantly the intrinsic capacity.19 On the contrary, the EHD process reported here alleviates the geometry dependent constraints for the effective and direct assembly of highly conductive yet porous monoliths. Large-area CGN deposition is made possible by employing a “multi-pass” technique. After iterative cycles of deposition, densely packed CGN assemblies were found to uniformly distribute throughout the entire substrates. Alternatively, CGNs can be selectively registered in a way similar to mask-assisted photolithography. Upon deposition, the motion of the charged droplets can be preferentially guided (including deflection or focusing) by a directional electric field, enabling simultaneous transformation and selective patterning of CGNs monoliths.
3D Interconnected CGN Monolithic Capacitors
To this end, an ultracapacitor using CGN monoliths obtained through EHD spraying was examined by a symmetrical two-electrode coin-cell configuration. Cells can be directly configured as collecting substrates because the conductive stainless steel electrostatically guides the preferential deposition of CGNs. It is noted that a thin corrugated rGO papers made by room temperature EHD process was employed as conductive scaffolds to further enhance both the efficiency and density of CGNs. Thickness of the CGN monoliths was controlled by the parameters, e.g., concentration, deposition time, and flow rate, of EHD deposition. In a typical deposition to achieve a thin film thickness of 10 μm, a solution of rGO (concentration 50 μg/mL; pH 11; electric field of 0.575 kV/cm and flow rate of 4 μL/min) was continuously sprayed via the automated EHD process for 60 to 80 hours. Note that the throughput can be readily scaled up via using multiplexing nozzles.34 Upon continuous deposition, the initially loose and sporadically distributed CGNs gradually transform into spatially connected, structurally adaptable and tightly packed monoliths as shown in tilted (
The binder-free feature also manifests in improved ion flow and electron transport for increased mass loading. Unlike the rGO counterpart, where the ion flow and transport of electrons are normal to the direction of stacking sheets, free space inside and between neighboring CGNs synergistically establishes dual pathways for ion flow while the seemingly joined walls facilitate efficient electron transport pathways, closely resembling the holey graphene based composites albeit in a 3D configuration (
A similar trend is observed in the volumetric capacitance (Cvol). As with most of 3D graphene foams, the once advantageous properties, such as high surface area and porosity, adversely affect the packing density. Although aerosol assembled crumpled balls can be tightly packed into electrode stacks, it is the irreversible clumping of rGOs that generates hard and rigid textures prevents preferential packing in the densest fashion. This leads to a relatively low packing density (˜0.5 g/cm3) when compared to the rGO papers (˜1 g/cm3)3, 47 As shown in the previous section, the shape-adaptable CGNs can be deformed into non-spherical, polyhedral shapes under gravitational compression, mimicking the assembly of individual bubble cells into foams in a highly dense-packing fashion. This leads to the increase of overall packing density of CGN monoliths to be as high as 0.68 g/cm3 and 0.62 g/cm3 on average. As a result, the corresponding Cvol of CGN monoliths remains relatively high when compared to that of the rGO counterparts as summarized in
3D Interconnected CGN/TiO2 Photoanodes
The tantalizing utility of these CGNs is further demonstrated by their successful integration as vertically extended, and energetically favorable 3D scaffolds for photoanodes in photoelectrochemical (PEC) applications. A formidable challenge in achieving competitive power conversion efficiency is the efficient transport of electrons across the entire photoanode. Thus far, nanostructured titanium dioxide (TiO2) represented the widely used material system because of their commercial availability and solution processability. Unfortunately, the spatial distribution of grain boundaries throughout the particulate TiO2 layers imposes energetic hurdles for charge carriers, leading to increasing numbers of recombination and trap sites. While the 2D graphene derivatives, including both GO and rGO, have been extensively used to improve electrical contacts, and energetics at interfaces for quite some time, sheets tend to aggregate during co-assembly with TiO2 nanoparticles, thus leading to the formation of metal-semiconductor Schottky junctions.53,54 ENREF 46 In addition, multilayers of horizontally stacking graphene sheets is detrimental to the carrier transport, which prefers the direction perpendicular to the current collecting electrodes. As a result, the overall performance still falls short of the crystalline TiO2 counterparts. In this light, to form continuous pathways for efficient carrier transport, it is highly desirable to assemble rGO modification layers with thicknesses of just 1-2 monolayers while percolating within the nanostructured TiO2 active layers to ensure sufficient vertical conductivity. 3D CGNs should be well suited to address this challenge. Unlike the lamellar counterparts, the non-planar contour of CGNs first and foremost suppresses the formation of unwanted Schottky junctions while the thin and vertically protruded walls that well extend into hundreds of nanometers align well with the flow of electrons.
Indeed, the versatile CGNs can be readily configured as 3D textured scaffolds with energetically favorable interfaces for TiO2 nanoparticle based photoanodes through substantially improving both carrier diffusion and collecting efficiency. CGNs with varied spatial distribution and densities can be simply obtained by adjusting the concentration of the starting rGO dispersion in tandem with the deposition time. Flexible and conductive carbon fiber electrodes (CFEs) were used as both the modification layer and the current collecting substrates. CFEs have been used as the back contact because of their highly conductive, chemically inert and mechanical robust nature.55 HRSEM images reveal the formation of CGN based scaffolds as shown in
As for the planar rGO/TiO2 case, the metal-semiconducting Schottky contact primarily accounts for the electron transport. Although both CGNs and rGOs exhibit similar work function around 4.5 eV (
The combination of electrostatic and capillary cues stemmed from EHD processes collectively helps to decouple those exceptional properties from the layer dependent electronic structures of graphene derivatives when processed in a bulk form. This embodies an important step to end the chasm between academic prototype and industrial implementation of graphene based composites where the difficulties lie in the design of a hierarchically functional architecture that allows for extraordinary material properties of individual sheets to be effectively harnessed.62 Further, the EHD strategy reported here should be universal and applicable to many emerging inorganic 2D sheets. Indeed, we have achieved the dimensional transition of clay nanosheets (CNS), and molybdenum disulfide (MoS2). As shown in
The utility of the EHD process is further demonstrated via the direct entrapment of inorganic nanoparticles within CGNs through the incorporation of coaxial needles as schematically illustrated in
Methods
Synthesis of CGN Monoliths from rGO Dispersions
rGO dispersions was synthesized based on the published method.22 In essence, GO colloids (0.5 mg/ml, 40 ml) made from the modified Hummers' approach was mixed with 0.1 ml hydrazine (35 wt % in water) and 0.56 ml ammonia (28 wt % in water) to adjust pH to 11 in a flask and stir in a oil bath at 95° C. for 1 hour. Flat rGO papers were prepared by vacuum filtrating of 8 ml as obtained rGO colloidal dispersion through an isopore membrane filter paper (100 nm pore size). To synthesize CGNs, rGO dispersions (50 μg/mL) were fed through a customized EHD setup (
Characterization of CGNs
The morphologies of CGNs were examined by field emission SEM integrated with energy dispersive X-ray spectroscopy (EDX, ULTRA-55), atomic force microscopy (Multimode, DI) and optical microscopy (Leica DM-2500). Zeta potential was measured with Malvern Instruments' Zetasizer Nanosystem. The conductivity measurements of CGN networks were made by depositing CGNs for 40 hours on a pre-cleaned Si substrate with a thermally grown 300 nm SiO2 and were analyzed with a field effect transistor configuration using a semiconductor analyzer (Keithley 2400). Electrical contact was made possible by thermally evaporating a combination of gold and chromium electrodes (100 nm) under vacuum of 5×10−8 torr. The channel length (200 μm) between two electrodes is defined by using a shadow mask. The surface area measurements were carried out at a liquid nitrogen temperature on a Tristar II series. The volume of the CGN films was calculated through multiplying the thickness by area (0.8 cm×0.8 cm). Thickness of CGN films was determined through cross sectional SEM images. As a control experiment, thickness of rGO films was calculated in a similar manner.
Electrochemical Measurements
The electrochemical characterization was conducted by applying constant current discharge/charge cycles and impedance measurements were done in a symmetrical coin-call configuration (MTI CR2016) using a similar procedure reported in the literature.3,46,47 Stainless steel current collectors were used to define device area and substrates for CGN deposition. The size of all electrode films were fixed to ˜0.8 cm×0.8 cm in accordance with the diameter of stainless steel collectors. Prior to deposition, a thin, corrugated rGO film created from room temperature EHD process (rGO concentration of 500 μg/mL, electric field of 0.575 kV/cm, pH at 11, and flow rate of 20 μL/min and deposition time of 5 hours) was used as a conductive scaffold to ensure the efficiency and density of subsequent CGN deposition. To rule out the effect of rGO and possible overestimation, the mass of rGO paper was subtracted from the mass loading of whole electrode stacks as well as calculation of capacitance. Aerial mass loading levels of 2 mg to 16 mg per electrode were achieved through iterative deposition from 10 to 80 hours, under the operating conditions of concentration of 50 μg/ml, flow rate of 4 μg/min and surrounding temperature of 255° C. rGO paper based electrodes of different loading mass were made by direct filtration of rGO dispersions of various concentrations. KOH solution (5M) was used as the electrolyte and a glassy fiber filter paper was used as the separator. It is noted that iterative pre-scans were performed with a scan rate of 50 mV/s to ensure the stabilization of the devices. The data presented were taken upon the superimposition of each current-voltage loops. The galvanostatic charge/discharge curves were conducted at different scan rates from 0.1 to 10 A/g while the electrochemical impedance spectroscopy measurements were performed under a sinusoidal signal over a frequency range from 103 to 10−2 Hz with a magnitude of 5 mV. Device performance and calculation were based on published reports. 10,46,47
Synthesis and Characterization of 3D CGN Scaffolds
In a typical preparation of 3D CGNs, CFEs were pretreated with UV/ozone for 15 min to remove any contamination. rGO dispersions in a mixture of isopropanol (IPA): deionized water (DI-H2O) (v/v, 3: 7), pH at 11, applied electric field of 0.575 kV/cm and a concentration of 50 μg/mL, a flow rate of 4 μL/min were directly deposited on CFE. The total deposition time is seven and half minutes and the substrate is pre-annealed at 200° C. Finally, the deposition of TiO2 particulate photoanode is prepared based on a published strategy.53,54 A 5 mg/mL suspension of TiO2 (Anatase, 25 nm in diameter, Sigma Aldrich) in methanol was prepared and sonicated using a VWR table top sonicator for 30 minutes to ensure stable dispersion. A total volume of 650 μL TiO2 colloidal suspensions was directly drop-casted. Pt wire and Ag/AgCl were used as counter and reference electrodes, respectively. To ensure electrical contact, the CFE/CGN/TiO2 working electrode was connected through a toothless alligator clip, which was then connected to a tandem working station comprised of a CH Instruments and a photovoltaic characterization setup (QE-5 IPCE, ENLI Tech, Taiwan). 1.2 mM KOH solution was used as the electrolyte, which was made from dissolving 61.5 mg KOH (reagent grade, Sigma-Aldrich) into 900 mL DI water and 100 mL ethylene glycol (anhydrous, Sigma-Aldrich). Ethylene glycol was added to adjust the pH value to 8 as well as increase the electrolyte conductivity. The working electrode was illuminated by a 150 W simulated Xenon light source with an AM 1.5 global illumination filter to get an intensity of 100 mW/cm2. Linear sweep voltammetry sequences were performed to identify the photocurrent density as well as the open circuit potential of the devices. In addition, photocurrent densities in response with light switch tests were measured through Bulk Electrolysis with Coulometry technique.
Coaxial EHD-Assembly of CGN Hybrid Nanocomposites
Chemically exfoliated MoS2 sheets were prepared followed the protocol as reported.66 TiO2 (˜25 nm in diameter, Sigma Aldrich), and silicon (Si) nanoparticles (˜5 nm in diameter in American Elements) were used as received unless specified elsewhere. Clay nanosheets was received from Rockwood Ltd. (Lapointe XLG) In a typical procedure for synthesis of CMoS2, solutions of MoS2 (55 ug/mL, DI:IPA=7:3, pH=6) were directly sprayed at the conditions of 0.575 kV/cm, 4 uL/min, 7.5 min, and 200° C. As for CNS, the processing conditions were identical to the CMoS2 except for concentration of 1 mg/mL. To constitute a stable dispersion, CNS first mixed with sodium polyacrylate. Upon mixing, the highly entangled clay nanosheets are exfoliated and dispersed homogenously owing to the mutual repulsion caused by site-specific wrapping of anionic sodium polyacrylate. Hybrid CGN composites were prepared separately in a mixture of IPA and DI-H2O (v/v, 3:7). The concentrations of Si and TiO2 nanoparticles are 200 μg/mL. Two phases were injected through the customized coaxial spinneret (100-10-COAXIAL, Ramé-hart Instrument Co.) under a feed rate of 4 μl/min, spraying time of 10 minutes, electric field Of 0.575 KV/cm, and surrounding temperature of 255° C. It is noted that coaxial EHD spinneret was used as rGO dispersion was fed through the shell. CGN/TiO2 and CGN/Si composites were synthesized in an analogous manner.
The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art how to make and use the invention. In describing embodiments of the invention, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. The above-described embodiments of the invention may be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described.
This application is a U.S. National Stage Application under 35 U.S.C. § 371 of PCT/US2016/058517, filed Oct. 24, 2016, the entire content of which is hereby incorporated by reference, and claims the benefit of U.S. Provisional Application No. 62/245,802 filed Oct. 23, 2015, and U.S. Provisional Application No. 62/245,806 filed Oct. 23, 2015, the entire contents of which are hereby incorporated by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2016/058517 | 10/24/2016 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62245802 | Oct 2015 | US | |
| 62245806 | Oct 2015 | US |
