Vertically Aligned Carbon Nanofibers Research Articles

Deciphering the effect of nucleation/growth kinetics on the morphology of Li plating/stripping on vertically aligned carbon nanofibers - A low-tortuosity 3-D nanostructured carbon host

Reversible lithium metal anodes (LMAs) are the holy grail for future rechargeable lithium metal batteries. Three-dimensional (3-D) conductive hosts have been extensively explored as an effective approach to suppressing dendrite formation and enabling reversible Li plating/stripping. However, the microscopic morphologies of Li plating and their correlation with the cell performance are not clear. Herein we unravel these issues using the vertically aligned carbon nanofiber (VACNF) array as a model 3-D conductive carbon host which has a well-defined vertical low-tortuosity structure allowing observation of the intrinsic Li morphologies infiltrated into the 3-D host. The VACNF array indeed provides much higher stability and reversibility for Li plating/stripping due to its high surface area and lithiophilic properties. We found that Li plating on both VACNF array and planar Cu electrodes follows the classical nucleation and growth model. Though the low plating current density (≤0.10 mA/cm2) provides better cycling stability consistent with the Sand’s equation, it forms sparse irregular grains stacked with dendrite-like long Li fibers. In contrast, the moderate to high plating current densities (1.0 − 5.0 mA/cm2) produce more uniform Li morphologies consisting of smaller micro-columns or micro-spheres. By decoupling the plating and stripping current densities, we unravel that the more uniform micro-columnar Li infiltrated in the VACNF array obtained at the moderate plating current density (∼1.0 mA/cm2) indeed exhibits the highest cycling performance. This provides new insights into the relationship between macroscopic electrochemical tests and microscopic Li morphologies, aiding in optimizing the performance of LMAs based on 3-D conductive hosts.

Binder-free Li-O 2battery cathodes using Ni- and PtRu-coated vertically aligned carbon nanofibers as electrocatalysts for enhanced stability

The development of an ideal cathode for Li-O₂ battery (LOB) has been a great challenge in achieving high discharge capacity, enhanced stability, and longevity. The formation of undesired and irreversible discharge products on the surface of current cathode materials limit the life span of the LOB. In this study, we report the systematic electrochemical study to compare the performance of LOB employing a unique graphitic nanostructured carbon architecture, i.e., vertically aligned carbon nanofiber (VACNF) arrays, as the cathode materials. Transition metal (Ni) and noble metal alloy (PtRu) are further deposited on the VACNF array as electrocatalysts to improve the discharge/charge processes at the cathode. The structure of as-prepared electrodes was examined with the field emission scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). The LOB with VACNF-Ni electrode delivered the highest specific and areal discharge capacities (14.92 Ah·g⁻¹, 4.32 mAh·cm⁻²) at 0.1 mA·cm⁻² current density as compared with VACNF-PtRu (9.07 Ah·g⁻¹, 2.62 mAh·cm⁻²), bare VACNF (5.55 Ah·g⁻¹, 1.60 mAh·cm⁻²) and commercial Vulcan XC (3.83 Ah·g⁻¹, 1.91 mAh·cm⁻²). Cycling stability tests revealed the superior performance of VACNF-PtRu with 27 cycles as compared with VACNF-Ni (13 cycles), VACNF (8 cycles), and Vulcan XC (3 cycles). The superior cycling stability of VACNF-PtRu can be attributed to its ability to suppress the formation of Li₂CO₃ during the discharge cycle, as elucidated by XPS analysis of discharged samples. We also investigated the impact of carbon cloth and carbon fiber as cathode electrode substrate on the performance of LOB.

Mechanistic understanding of Li metal anode processes in a model 3D conductive host based on vertically aligned carbon nanofibers

The use of three dimensional (3D) conductive hosts is a promising strategy to alleviate dendrite formation in lithium metal anodes. However, the fundamental issues associated with the 3D hosts, such as formation of excessive solid electrolyte interphase (SEI) and complicated Li deposition morphologies, have not been well studied. This study focuses on understanding these critical issues using vertically aligned carbon nanofiber (VACNF) array as a model 3D host. Various characterization tools are employed to elucidate the morphology and composition of electrodeposited Li (e-Li) and its associated SEI. The e-Li on VACNFs exhibits two different morphologies – a dominant micron-scale columnar infiltrative Li and a small amount of nanoscale coaxial Li sheath on VACNFs in the unfilled interstitial area between infiltrative grains. During Li intercalation, an organic-rich loose SEI film forms on the top of VACNF array and an inorganic-rich SEI sheath forms around individual VACNFs. Li plating transforms them into inorganic dominant SEIs directly in contact with the e-Li surface. The SEI skins on the infiltrative Li remain loosely attached to the VACNF array after stripping Li and irreversibly accumulate during Li plating/stripping cycling, while the coaxial SEI on the nanoscale Li remains as a stable elastic sheath. The loose SEI skins cause electrolyte consumption and increased heterogeneity, eventually leading to cell failure.

An efficient and broadly applicable method for transient transformation of plants using vertically aligned carbon nanofiber arrays.

Transient transformation in plants is a useful process for evaluating gene function. However, there is a scarcity of minimally perturbing methods for gene delivery that can be used on multiple organs, plant species, and non-excised tissues. We pioneered and demonstrated the use of vertically aligned carbon nanofiber (VACNF) arrays to efficiently perform transient transformation of different tissues with DNA constructs in multiple plant species. The VACNFs permeabilize plant tissue transiently to allow molecules into cells without causing a detectable stress response. We successfully delivered DNA into leaves, roots and fruit of five plant species (Arabidopsis, poplar, lettuce, Nicotiana benthamiana, and tomato) and confirmed accumulation of the encoded fluorescent proteins by confocal microscopy. Using this system, it is possible to transiently transform plant cells with both small and large plasmids. The method is successful for species recalcitrant to Agrobacterium-mediated transformation. VACNFs provide simple, reliable means of DNA delivery into a variety of plant organs and species.

Theoretical Investigation of the Oxygen Reduction Reaction over Platinum Catalysts Supported by Multi‐Edged Vertically Aligned Carbon Nanofiber for Electrocatalyst Preparation

AbstractVertically aligned carbon nanofibers (VACNFs) are promising supports for oxygen reduction reaction (ORR) electrocatalysts in fuel cells. Although experimentally these catalytic systems have shown great potential, there is lack of molecular understanding of the catalytic sites and reaction mechanisms. This work investigated the origin of the ORR reactivities of the platinum catalysts on multi‐edged VACNFs (Pt/VACNF) using a multiscale modeling approach combining Density Functional Theory (DFT) and classical Molecular Dynamics (MD) simulations. Based on the ReaxFF potential, all nanoscale Pt particles (Pt55, P100, and Pt147) are stabilized by the open edges located axially along the VACNF walls. The calculated first‐shell coordination numbers, , of surface Pt atoms are 6.63, 7.27, and 7.85, respectively, suggesting that the percentage of low coordination sites increases as the particle size decreases. The adsorption energies of OOH, O, and OH on Pt55 were systematically probed using DFT calculations. These adsorption energies retain a linear correlation against the generalized coordination numbers ( ). For Pt nanoparticles supported on VACNF, we found that the OOH and OH bind stronger than on Pt (111) by 0.14 and 0.17 eV, respectively, which can hinder the ORR activity with lower limiting potential than Pt (111). Our theoretical prediction is in good agreement with the linear sweeping voltammetry that revealed a left shift of the half‐wave potential.

Mechanistic Understanding of Lithium Plating/Stripping Processes on the 3D Conductive Host Based on Vertically Aligned Carbon Nanofibers for Li Metal Anode

The highest theoretical capacity (3860 mAh/g) and lowest redox potential (-3.04 V vs standard hydrogen electrode) makes lithium metal as a promising anode for next generation high energy density batteries.1 Despite theoretical advantages, there are challenges in commercializing Li metal anodes such as low coulombic efficiency, dendrite formation and poor cycling life. Dendrite formation can cause short circuit and thermal runaway, which is a safety concern. Among the various ways to mitigate the Li dendrite formation, using three-dimensional (3D) current collectors with lithiophilic properties and low tortuous pore architecture is an effective way. It would decrease the local current density owing to high active surface area while accommodating the large volume change during prolonged charge/discharge cycles.Vertically aligned carbon nanofibers (VACNF) have been demonstrated as a promising candidate as a lithiophilic 3D host for lithium metal anodes as they possess unique conically stacked lithiophilic graphitic edges with O containing functional groups and high N doping (~6.5 at%).2 However, a fundamental, deeper understanding of the Li plating/stripping processes on such 3D electrodes has not been developed yet. Here in, we investigate the lithium nucleation and growth processes and its morphological changes using ex-situ structural characterization techniques such as SEM and cryo-TEM. The composition of the solid electrolyte interface is investigated using XPS. In-situ Raman spectroscopic studies provided further information about the changes that occur in the VACNFs during Li plating/stripping processes and Li intercalation/deintercalation processes. Ion-beam sputtering of an ultrathin gold layer onto the VACNFs was found to further improve the lithiophilicity of the 3D electrode and lead to more reversible Li metal plating/stripping. The new insights on the electrodeposition of lithium in 3D electrodes and its relationship with the enhanced cycle life acquired using various analytical tools will be discussed.Reference:(1) Manthiram, A.; Chung, S.-H.; Zu, C. Lithium–Sulfur Batteries: Progress and Prospects. Advanced Materials 2015, 27 (12), 1980–2006.(2) Chen, Y.; Elangovan, A.; Zeng, D.; Zhang, Y.; Ke, H.; Li, J.; Sun, Y.; Cheng, H. Vertically Aligned Carbon Nanofibers on Cu Foil as a 3D Current Collector for Reversible Li Plating/Stripping toward High-Performance Li–S Batteries. Advanced Functional Materials 2020, 30 (4), 1906444.

Nanoscale geometry determines mechanical biocompatibility of vertically aligned nanofibers

Vertically aligned carbon nanofibers (VACNFs) are promising material candidates for neural biosensors due to their ability to detect neurotransmitters in physiological concentrations. However, the expected high rigidity of CNFs could induce mechanical mismatch with the brain tissue, eliciting formation of a glial scar around the electrode and thus loss of functionality. We have evaluated mechanical biocompatibility of VACNFs by growing nickel-catalyzed carbon nanofibers of different lengths and inter-fiber distances. Long nanofibers with large inter-fiber distance prevented maturation of focal adhesions, thus constraining cells from obtaining a highly spread morphology that is observed when astrocytes are being contacted with stiff materials commonly used in neural implants. A silicon nanopillar array with 500 nm inter-pillar distance was used to reveal that this inhibition of focal adhesion maturation occurs due to the surface nanoscale geometry, more precisely the inter-fiber distance. Live cell atomic force microscopy was used to confirm astrocytes being significantly softer on the long Ni-CNFs compared to other surfaces, including a soft gelatin hydrogel. We also observed hippocampal neurons to mature and form synaptic contacts when being cultured on both long and short carbon nanofibers, without having to use any adhesive proteins or a glial monoculture, indicating high cytocompatibility of the material also with neuronal population. In contrast, neurons cultured on a planar tetrahedral amorphous carbon sample showed immature neurites and indications of early-stage apoptosis. Our results demonstrate that mechanical biocompatibility of biomaterials is greatly affected by their nanoscale surface geometry, which provides means for controlling how the materials and their mechanical properties are perceived by the cells. Statement of significanceOur research article shows, how nanoscale surface geometry determines mechanical biocompatibility of apparently stiff materials. Specifically, astrocytes were prevented from obtaining highly spread morphology when their adhesion site maturation was inhibited, showing similar morphology on nominally stiff vertically aligned carbon fiber (VACNF) substrates as when being cultured on ultrasoft surfaces. Furthermore, hippocampal neurons matured well and formed synapses on these carbon nanofibers, indicating high biocompatibility of the materials. Interestingly, the same VACNF materials that were used in this study have earlier also been proven to be capable for electrophysiological recordings and sensing neurotransmitters at physiological concentrations with ultra-high sensitivity and selectivity, thus providing a platform for future neural probes or smart culturing surfaces with superior sensing performance and biocompatibility.

Platinum Deposited Nitrogen-Doped Vertically Aligned Carbon Nanofibers as Methanol Tolerant Catalyst for Oxygen Reduction Reaction with Improved Durability

Nitrogen doping in carbon materials can modify the employed carbon material’s electronic and structural properties, which helps in creating a stronger metal-support interaction. In this study, the role of nitrogen doping in improving the durability of Pt catalysts supported on a three-dimensional vertically aligned carbon nanofiber (VACNF) array towards oxygen reduction reaction (ORR) was explored. The nitrogen moieties present in the N-VACNF enhanced the metal-support interaction and contributed to a reduction in the Pt particle size from 3.1 nm to 2.3 nm. The Pt/N-VACNF catalyst showed better durability when compared to Pt/VACNF and Pt/C catalysts with similar Pt loading. DFT calculations validated the increase in the durability of the Pt NPs with an increase in pyridinic N and corroborated the molecular ORR pathway for Pt/N-VACNF. Moreover, the Pt/N-VACNF catalyst was found to have excellent tolerance towards methanol crossover.

Enhancing Methanol Oxidation Reaction with Platinum‐based Catalysts using a N‐Doped Three‐dimensional Graphitic Carbon Support

AbstractNitrogen‐doped graphitic carbon materials have been widely used as a catalyst support in the methanol oxidation reaction (MOR). In this study, we report the role of three‐dimensionally architectured in‐situ N‐doped vertically aligned carbon nanofibers (VACNF) as a catalyst support for MOR in acidic and alkaline media. The abundant graphitic edge sites at the sidewall of N‐doped VACNF strongly anchor the deposited platinum group metal (PGM) catalysts and induce a partial electron transfer between the PGM catalysts and support. Density Functional Theory (DFT) calculations reveal that the strong metal‐support interaction substantially increases the adsorption energy of OH, particularly near the N‐doping sites, which helps to compete and remove the adsorbed intermediate species generated during MOR. The PGM catalysts on N‐doped VACNF support exhibits CO stripping at lower potentials comparing to the commercial Vulcan carbon support and presents an enhanced electrocatalytic performance and better durability for MOR.

Fundamental Electrochemical Insights of Vertically Aligned Carbon Nanofiber Architecture as a Catalyst Support for ORR

Three-dimensionally (3D) architectured carbon supports have received great attention for fuel cell catalysts. However, the complicated structure makes the assessment of catalytic properties difficult using the conventional rotating disk electrode (RDE) method. This paper reports a systematic study on oxygen reduction reaction (ORR) with ion-beam sputtered Pt catalyst (at Pt loadings of 6.5–43 μg cm−2) on a vertically aligned carbon nanofiber (VACNF) array, consisting of conically stacked graphitic microstructures. The RDE studies reveal that thick 3D architecture of VACNFs exhibits enhanced limiting current density that deviates from the Levich equation for conventional thin-film catalysts. Nevertheless, useful information can be derived from RDE experiments with such systems. Molecular models representing VACNFs have been constructed to explore their capacity as catalyst supports for ORR. Platinum atoms form strong bonds at the open graphitic edges in VACNF, corroborating the role of VACNF in stabilizing Pt. Density Functional Theory (DFT) calculations further elucidate the two-electron and four-electron ORR pathways on the bare VACNF and Pt/VACNF catalysts, respectively. Furthermore, the Pt/VACNF catalysts show enhanced tolerance to methanol oxidation and a higher ability to recover from carbon monoxide poisoning in comparison to the benchmark Pt/C catalysts. These results provide critical insights for developing future high-performance electrocatalyst supports.

Vertically Aligned Carbon Nanofibers on Cu Foil as a 3D Current Collector for Reversible Li Plating/Stripping toward High‐Performance Li–S Batteries

AbstractA vertically aligned carbon nanofiber (VACNF) array with unique conically stacked graphitic structure directly grown on a planar Cu current collector (denoted as VACNF/Cu) is used as a high‐porosity 3D host to overcome the commonly encountered issues of Li metal anodes. The excellent electrical conductivity and highly active lithiophilic graphitic edge sites facilitate homogenous coaxial Li plating/stripping around each VACNF and forming a uniform solid electrolyte interphase. The high specific surface area effectively reduces the local current density and suppresses dendrite growth during the charging/discharging processes. Meanwhile, this open nanoscale vertical 3D structure eliminates the volume changes during Li plating/stripping. As a result, highly reversible Li plating/stripping with high coulombic efficiency is achieved at various current densities. A low voltage hysteresis of 35 mV over 500 h in symmetric cells is achieved at 1 mA cm−2 with an areal Li plating capacity of 2 mAh cm−2, which is far superior to the planar Cu current collector. Furthermore, a Li–S battery using a S@PAN cathode and a lithium‐plated VACNF/Cu (VACNF/Cu@Li) anode with slightly higher capacity (2 mAh cm−2) exhibits an excellent rate capability and high cycling stability with no capacity fading over 600 cycles.

Recent Mechanistic Understanding of Oxygen Reduction Reaction over Pt-Supported 3D Carbon Nanofiber Catalysts

For both proton exchange membrane and direct methanol fuel cells, oxygen reduction reaction (ORR) is a performance-determining step. While Pt has been established as a standard bearer in ORR catalyst development, practical long-term applications require continuing effort to pursue active, stable, as well as affordable materials as alternatives to precious metals. In this talk, carbon-based vertically aligned carbon nanofibers (VACNFs), essentially a stack of conical graphitic structures, will be discussed as highly promising ORR catalysts. In an alkaline medium, at -0.4 V applied potential, the current density measured on VACNF cathode is twice more than that measured with commercial Pt catalyst (Pt/C). Molecular insights have been gained through the use of Density functional theory (DFT) calculations to: (1) reveal interactions of ORR intermediate species at the catalytically active sites; and (2) elucidate the origin of catalytic activities of VACNF under reaction conditions. Specifically, semi-periodic fishbone-like stacked graphene sheets, bare or terminated with H and OH, have been employed to understand the impact on ORR mechanism and corresponding reactivities. Bindings of O2, O, and OH at various configurations obtained from DFT are then used as descriptors to assess catalyst performance against simple Pt (111) and Pt (211) surfaces. In addition, Pt nanocatalysts supported on VACNF (Pt/VACNF), which can push the ORR current density even higher, will be discussed as well. Again, with DFT, a model with Pt anchored at the VACNF edges was proposed to investigate the functionalities of Pt/VACNF for alkaline ORR reactions. In this talk, both associative and dissociative ORR pathways will be analyzed based on the established models. In particular, the overall computed ORR thermodynamics are shown to be favored toward Pt/VACNF than Pt (111), consistent with experimental results. Nevertheless, the unique interactions of ORR intermediates such as O*, OOH*, and OH* with catalytic active VACNF edge sites, revealed by DFT calculations, suggest intriguing but also complex electrochemistries.

Quantitative Electrochemical Analysis of Cathepsin Bactivity Using Carbon Nanofiber Nanoelectrode Arrays Withoptimized Peptide Substrate Length and Temperature

It is well known that the over-expression, increased activity and altered localization of many proteases are associated with tumor progression. Many clinical diagnoses target proteases as biomarkers. However, the quantitative analyses of the activity of tumor-related protease are challenging, because of the network interaction among the large set of proteases. Hence, there is a strong demand to develop a bioanalytical technique that can rapidly detect protease activities with high sensitivity and specificity. In this presentation, I will report an efficient electrochemical quantitative analyses method for detecting the cathepsin B activity by using a peptide-functionalized nanoelectrode array. The nanoelectrode array (NEA) was fabricated with vertically aligned carbon nanofibers (VACNFs), which serves as a unique electrochemical platform to enhance the protease detection. VACNFs of ~100-150 nm in diameter and ~5 mm in length are grown on chromium coated silicon wafer in uniform vertical alignment and are fully separated from each other to form a brush-like structure. With further processes to encapsulate the VACNFs in SiO2 by chemical vapor deposition (CVD) followed by mechanical polishing and reactive ion etching (RIE), ~100 nm long VACNF tips can be exposed above the SiO2 matrix, forming a stable NEA. By covalently attaching the exposed tip with proper peptide probes containing a ferrocene (Fc) redox tag at the distal, the VACNF NEA can be used to detect proteolysis with AC voltammetry. Such NEAs showed improved temporal resolution and reduced steric hindrance. As a result, reliable real-time proteolytic kinetics are recorded, from which the protease activity is quantitatively derived based on the heterogeneous Michalis-Menten model. In this work, we systematically investigated the cathepsin B activity with peptides of different length (including tetrapeptide, hexapeptide, and octapeptide) and at different temperatures (at 19.3, 29.5, 38.6, and 44.2 °C) to reveal their effects on the kinetic proteolysis rate by cathepsin B. The hexapeptide gave the highest proteolysis rate over tetrapeptide and octapeptide, as reflected by the smallest exponential decay time constant t. Meanwhile, 38.6 °C gave the highest proteolysis rate over the other 3 temperatures. At 38.6 °C, the optimized temperature, the limit of detection of cathepsin B activity and concentration has been determined to be 2.49 X 10-4 s-1 and 0.32 nM, respectively. Furthermore, the Fc-hexapeptide functionalized VACNF NEA shows very high selectivity, with essentially no measurable cross-reaction with 6.0 nM of other two cancer-related proteases, ADAM10 and ADAM17. This study demonstrates the potential to develop this method into a multiplex electronic chip for rapidly profiling the activity of up to 9 proteases simultaneously, which would facilitate accurate diagnoses of cancers and fast careening effective protease inhibitors as drug candidates. It would be a useful tool toward developing precision medicine.

Electrochemical Activity Assay for Protease Analysis Using Carbon Nanofiber Nanoelectrode Arrays.

There is a strong demand for bioanalytical techniques to rapidly detect protease activities with high sensitivity and high specificity. This study reports an activity-based electrochemical method toward this goal. Nanoelectrode arrays (NEAs) fabricated with embedded vertically aligned carbon nanofibers (VACNFs) are functionalized with specific peptide substrates containing a ferrocene (Fc) tag. The kinetic proteolysis curves are measured with continuously repeated ac voltammetry, from which the catalytic activity is derived as the inverse of the exponential decay time constant based on a heterogeneous Michaelis-Menten model. Comparison of three peptide substrates with different lengths reveals that the hexapeptide H2N-(CH2)4-CO-Pro-Leu-Arg-Phe-Gly-Ala-NH-CH2-Fc is the optimal probe for cathepsin B. The activity strongly depends on temperature and is the highest around the body temperature. With the optimized peptide substrate and measuring conditions, the limit of detection of cathepsin B activity and concentration can reach 2.49 × 10-4 s-1 and 0.32 nM, respectively. The peptide substrates show high specificity to the cognate proteases, with negligible cross-reactions among three cancer-related proteases cathepsin B, ADAM10, and ADAM17. This electrochemical method can be developed into multiplex chips for rapid profiling of protease activities in cancer diagnosis and treatment monitoring.

Disordered Bilayered V2O5 ⋅ nH2O Shells Deposited on Vertically Aligned Carbon Nanofiber Arrays as Stable High‐Capacity Sodium Ion Battery Cathodes ⋅

AbstractDue to the large radius of Na+ ions, it has been challenging to find suitable host materials for sustainable sodium ion batteries (SIBs). This study investigates sputter‐coated thin V2O5 shells on vertically aligned carbon nanofiber (VACNF) arrays as a novel three‐dimensional (3D) core‐shell material for SIB cathodes. SEM, TEM, XRD and Raman spectroscopy revealed that the as‐deposited V2O5 shell has a highly disordered bilayered V2O5 ⋅ nH2O structure with a large interlayer spacing of 11.0 Å, which can accommodate Na+ ions better than orthorhombic α‐V2O5 crystals. This hydrated metastable structure has been systematically characterized for Na+ storage. A high initial insertion capacity of 277 mAh g−1 can be achieved at a current density of 250 mA g−1 in the potential window of 4.0–1.0 V (vs Na/Na+). Using higher charge‐discharge rate or narrower potential windows, the electrode becomes more reversible, able to reach a coulombic efficiency of ∼99 %. Cyclic voltammetry, galvanostatic charge‐discharge and electrochemical impedance spectroscopy measurements indicate that the Na+ storage is dominated by a large pseudocapacitive contribution due to fast surface reactions, which facilitates the improved stability and high power density. Such highly disordered bilayered V2O5 ⋅ nH2O material in the 3D core‐shell architecture provides new insights for developing future SIB materials.

High Performance Tin-coated Vertically Aligned Carbon Nanofiber Array Anode for Lithium-ion Batteries

ABSTRACTThis study reports a high-performance tin (Sn)-coated vertically aligned carbon nanofiber array anode for lithium-ion batteries. The array electrodes have been prepared by coaxial sputter-coating of tin (Sn) shells on vertically aligned carbon nanofiber (VACNF) cores. The robust brush-like highly conductive VACNFs effectively connect high-capacity Sn shells for lithium-ion storage. A high specific capacity of 815 mAh g-1 of Sn was obtained at C/20 rate, reaching toward the maximum value of Sn. However, the electrode shows poor cycling performance with conventional LiPF6 based organic electrolyte. The addition of fluoroethylene carbonate (FEC) improve the performance significantly and the Sn-coated VACNFs anode shows stable cycling performance. The Sn-coated VACNF array anodes exhibit outstanding capacity retention in the half-cell tests with electrolyte containing 10 wt.% FEC and could deliver a reversible capacity of 480 mAh g-1 after 50 cycles at C/3 rate.

(Invited) Dielectrophoretic Capture and Detection of Microbial Pathogens Using Nanoelectrode Arrays

Rapid and highly sensitive detection of pathogenic microbes is critical for food safety and environmental protection. To achieve this, it requires combining novel sample preparation techniques and extremely sensitive detection methods. In this presentation, I summarize a series of studies in my lab on developing a nanoscale dielectrophoresis (DEP) device for effective capturing and concentrating pathogenic particles such as viral and bacterial particles in a fluidic chip. This technique was then integrated with various detection methods such as fluorescence imaging, surface enhanced Raman spectroscopy (SERS) and real-time impedance measurements for quantitative assessment of the pathogen concentration. Due to the small size (~1 micron for bacterial cells and ~100 nm for viral particles), it is challenging to capture and concentrating these pathogenic particles from dilute solutions. Here we demonstrate that the highly focused electric field at the tip of an embedded nanoelectrode array can be utilized to generate a large DEP force to effectively manipulate pathogenic particles. The nanoelectrode array was made of vertically aligned carbon nanofibers (VACNFs), about 5 micron in length and 100 nm in diameter, and embedded in a SiO2 film. It was placed at the bottom of a microfluidic channel against a macroscopic indium tin oxide (ITO) top electrode, forming a DEP device in “points-and-lid” configuration. We have shown that E. coli DHα5 cells flowing through the channel can be effectively captured and concentrated at the desired microscale areas inside the channel using a small AC voltage (~6-10 Vpp). This device is particularly effective in capturing viral particles (such as bacteriophage T4r) due to the comparable size between the viral particles and the exposed VACNF tip. To achieve specific detection, the DEP chip has been integrated with a Raman probe for SERS measurements of nanotag-labelled E. coli DHα5 cells. The SERS nanotags are based on iron oxide-gold (IO-Au) core-shell nanoovals (NOVs) of ~50 nm in size, which are coated with QSY21 Raman reporters and attached to E. coli cells through specific antibodies. The combination of the greatly enhanced Raman signal by the SERS nanotags and the effective DEP concentration significantly improves the detection limit and speed. The SERS signal has been fully validated with fluorescence microscopy measurements under all DEP conditions. A concentration detection limit as low as 210 cfu/mL using a portable Raman system has been obtained with a DEP capture time of only ~50 s. The detection has been validated in purified solutions as well as complex samples including chicken broth, soil solution and apple juice. Real-time impedance measurements has also been demonstrated as a feedback during DEP capture of vaccinia viral particles. By controlling the proper AC voltage, the nucleic acid contents in the viral particles can be released by opening the envelope with electroporation under the high electric field at the VACNF tips. These results demonstrate the potential to develop a compact portable system for rapid and highly sensitive detection of specific pathogens. Furthermore, this technique is based on physical method and the pathogenic particles are fully released after the AC voltage is turned off, making it a robust device that can be repeatedly used.

Supercapacitors: Battery‐like Supercapacitors from Vertically Aligned Carbon Nanofiber Coated Diamond: Design and Demonstrator (Adv. Energy Mater. 12/2018)

Supercapacitors: Battery‐like Supercapacitors from Vertically Aligned Carbon Nanofiber Coated Diamond: Design and Demonstrator (Adv. Energy Mater. 12/2018)

Integrated on-chip solid state capacitor based on vertically aligned carbon nanofibers, grown using a CMOS temperature compatible process

Complete miniaturized on-chip integrated solid-state capacitors have been fabricated based on conformal coating of vertically aligned carbon nanofibers (VACNFs), using a CMOS temperature compatible microfabrication processes. The 5 µm long VACNFs, operating as electrode, are grown on a silicon substrate and conformally coated by aluminum oxide dielectric using atomic layer deposition (ALD) technique. The areal (footprint) capacitance density value of 11–15 nF/mm2 is realized with high reproducibility. The CMOS temperature compatible microfabrication, ultra-low profile (less than 7 µm thickness) and high capacitance density would enables direct integration of micro energy storage devices on the active CMOS chip, multi-chip package and passives on silicon or glass interposer. A model is developed to calculate the surface area of VACNFs and the effective capacitance from the devices. It is thereby shown that 71% of surface area of the VACNFs has contributed to the measured capacitance, and by using the entire area the capacitance can potentially be increased.

Carbon nanospikes for biosensing applications.

In this paper, we discuss the structure and characteristics of carbon nanospikes. We also compare carbon nanospikes with previously grown carbon nanostructures known as vertically aligned carbon nanofibers (VACNF). Plasma enhanced chemical vapor deposition (PECVD) is used to fabricate both the nanospikes and VACNF. However, carbon nanospikes do not require a catalyst for the growth process, whereas VACNF requires a catalyst in the growth process. This facilitates batch fabrication with greater reproducibility. Scanning electron microscope images and Raman spectroscopy show that carbon nanospikes fabricated on silver wires will show superior performance.