Fabrication of crystalline submicro-to-nano carbon wire for achieving high current density and ultrastable current

Jufeng Deng; Chong Liu; Dian song; Marc Madou

doi:10.1038/s41378-021-00345-z

Fabrication of crystalline submicro-to-nano carbon wire for achieving high current density and ultrastable current

Deng, Jufeng; Liu, Chong; song, Dian; Madou, Marc 2022-02-04 00:00:00 Crystalline carbon nanowire arrays were fabricated taking advantage of near-ﬁeld electrospinning and stress decyanation. A novel fabrication method for carbon nanowires with radii ranging from ~2.15 µm down to ~25 nm was developed based on implementing nitrogen pretreatment on the silica surface and then aligning polymer nanoﬁbers during near-ﬁeld electrospinning at an ultralow voltage. Stress decyanation was implemented by subsequently pyrolyzing a polymer nanoﬁber array on the silica surface at 1000 °C for 1 h in an N atmosphere, thus obtaining a crystalline carbon nanowire array with a nanostructured surface. Various crystalline nanostructures were fabricated on the nanowire surface, and their electrochemical performance was evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Crystalline carbon wires with diameters ranging from micrometers to submicrometers displayed carbon nanoelectrode-like behavior with their CV curve having a sigmoidal shape. A highly crystalline carbon nanowire array showed distinct behavior, having a monotonically increasing straight line as its CV curve and a semicircular EIS spectrum; these results demonstrated its ultrastable current, as determined by electron transfer. Furthermore, nanocrystalline-structured carbon wires with diameters of ~305 nm displayed at least a fourfold higher peak current density during CV (4000 mA/m ) than highly crystalline carbon nanowires with diameters of ~100 nm and porous microwires with diameters of ~4.3 µm. Introduction case of conventional carbon nanowires with glassy carbon The attractive merit of carbon wires derives from car- structures, the principal philosophy emphasizes the bon microstructures integrating high-performance with interconnected graphitic structure, which provides an 1,2 abundant functionalities . There are several available extraordinary combination of mechanical, electrical, and microstructures of carbon materials, such as glassy car- thermal properties (strength up to 20 GPa, electrical 6 6 7 bon, diamond, and graphite. Carbon nanowires with conductivity of 1.5 × 10 S/m and thermal conductivity glassy carbon structures have been used in many different of 5000 W/m·K). Compared to glassy carbon nanowire applications, such as in high-power supercapacitors , conﬁgurations, graphitized carbon nanowires offer several electrochemical biosensors and high-energy rechargeable advantages, such as their extraordinary mechanical 5 8 batteries . To upgrade all of the intrinsic properties in the properties, which includes their elastic modulus (1.1 TPa), tensile strength (~130 GPa) and notable ﬂex- ibility, and their excellent electron transport perfor- Correspondence: Chong Liu (chongl@dlut.edu.cn)or mance , which includes their extremely high electric Marc Madou (mmadou@uci.edu) 1 8 2 School of Mechanical Engineering, Dalian University of Technology, Dalian conductivity (~10 S/m), carrier mobility (2000000 cm / 116023, China V·s) and ampacity (1–2 GA/cm ). These advantages Mechanical and Aerospace Engineering, University of California, Irvine, CA demonstrate the abundant functionalities of graphitized 92617, USA Full list of author information is available at the end of the article © The Author(s) 2022 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to theCreativeCommons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 2 of 13 carbon nanowires for use in various applications, such as programmed movement of a linear stage during near-ﬁeld 10 11 in electrical conductors , supercapacitors , actuators, electrospinning (NFES), the polymer nanoﬁbers are and solar devices , showing more applicability than arrayed on a silicon substrate coated with silica (500 nm conventional carbon nanowires. Regarding electro- thickness). Before depositing the polymer nanoﬁber chemical energy storage, graphitized carbon nanowires arrays, the silica surface deposited by polymer ﬁbers is are promising for enhancing energy and power densities. pretreated at 1000 °C in a nitrogen atmosphere. To con- vert the polymer ﬁbers into carbon nanowires, a silicon The thermal reduction and high-temperature anneal- ing of graphite oxide wire made by a wet-spinning chip with carbon dioxide and polymer ﬁbers is pyrolyzed process have been proposed to produce highly graphi- at 1000 °C in a nitrogen atmosphere. The microstructure tized carbon wires but are limited to micron-scale fab- of the carbon wires is evaluated by atomic force micro- rication . For the electrochemical applications scopy and Raman spectroscopy. Additionally, the fabri- mentioned above, the performance is typically improved cated HCCNs are characterized electrochemically with by increasing the surface area of graphitized carbon cyclic voltammetry and electrochemical impedance nanowires, which can be achieved by decreasing the spectroscopy (ESI). diameter of graphitized carbon nanowires. Most of the techniques that have been proposed for fabricating Results and discussion graphitized nanoscale carbon wires include mechanical Fabrication of polymer nanoﬁber arrays using near-ﬁeld 14 15 stress pyrolysis and chemical vapor deposition .The electrospinning (NFES) doping approaches involved in these techniques cause To attain the thinnest polymer ﬁbers at the lowest additional substances to be added into the carbon wires, applied voltages in NFES, one important procedure which affects the electrochemical behavior of the carbon concerns the protocol for preparing the ink by the dis- wire. This has led to extensive research into the pyr- solution of polyacrylonitrile (PAN) in N,N-dimethylfor- olysis of near-ﬁeld electrospun ﬁbers (PNFEFs), which mamide (DMF). A heat treatment from 60 to 126 °C is enables advanced applications in electrochemical sen- found to be accompanied by a conductivity change, sing, energy storage, and stem cells. which indicates the highest conductivity at 106 °C .The PNFEFs are dedicated to manufacturing new classes of conductivity data can be collected by a conductivity well-deﬁned carbon nanostructures. In this process, meter (OAKTON CON 510 Series) with the ability to polymer nanoﬁbers are arrayed by the near-ﬁeld elec- show both conductivity and solution temperature. The trospinning of a precursor solution and subsequently conductivity plays a role in inducing sufﬁciently large converted into pyrolytic carbon nanowires through electrical stress, which makes it possible to initiate a high-temperature treatment (>800 °C) in an inert nano jet during near-ﬁeld electrospinning. With a drum- atmosphere. This method offers excellent control of to-needle distance of 5 mm, the polymer jet does not feature sizes, density per unit area, and the capability to initiate even at 1500 V because the electrostatic force pattern carbon nanowires. By introducing carbon scaf- cannot overcome the surface tension at the droplet-air folds into this process, a dramatic decrease in the ﬁber interface. However, touching the ink droplet at the diameter leads to ultrathin carbon nanowires (~5 nm) . ejector needle tip with the rotating drum causes the This technique is mostly established because the high- electrical stress to become large enough to counter- throughput fabrication of carbon nanowires can be balance the surface tension stress, giving rise to the easily achieved. formation of a Taylor cone and jet initiation at a voltage Recently, highly graphitized carbon nanowires have as low as 500 V (see Fig. 1a). When using a PAN ink been developed on carbon scaffolds using near-ﬁeld heated to either 60 °C or 126 °C, jet initiation at 500 V is electrospinning, pyrolysis, electrodeposition, and chemi- not achieved by the same touching procedure, which cal vapor deposition. Clearly, these techniques are still demonstrates the key role of conductivity for initiating inseparable from the participation of nickel catalysts, the jet during NFES. resulting in an inability to remove the nickel from the A porous absorbent paper mounted around the base of carbon wire. Thus, to date, the electrochemical behavior the ejector needle thins the liquid layer around the needle of highly crystalline carbon nanowires with graphitic at low feed rates (1 nL/min). The resulting electric ﬁeld structures have not been improved. Here, we present the intensity at the liquid-air interface is substantially higher fabrication and characterization of highly crystalline car- than that of the nozzle without droplet shaping at the bon nanowire (HCCN) arrays with well-controlled wire- same voltage . This in turn results in the decrease in the to-wire spacing. For this purpose, a novel method for the applied voltage from 500 to 35 V, which is far below any fabrication of highly crystalline carbon nanowires is current low-voltage NFES practice (see Fig. 1a). During implemented, permitting control of the wire diameter, NFES, continuous jetting at an ultralow applied voltage of wire-to-wire spacing, and degree of graphitization. By the 35 V becomes possible (see Fig. 1a). This reduction in the Voltage Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 3 of 13 35 V Polymer fibers Polymer 300 V droplet Silicon substrate Taylor 500 V Needle Groove cone Porous absorbent Convexity paper 300 μm 500 μm Silica PAN fiber Oxidized PAN fiber Carbon fiber 115 °C 1000 °C Silicon substrate Silicon substrate Silicon substrate Oxidization Carbonization in air in nitrogen C C HC CH CH C C N N N N N 40% 30% 20% O OH CN 200 nm 900 nm Polymer fiber Carnon fiber 10% Experiment Carbon fibers Theory 35 V 100 V 0 nm 711 nm B 42.5 nm 0 0 31 nm 0 800 1600 2400 3200 0 200 400 600 52.7 nm 45 nm 72.6 nm Applied voltage (V) Rotational Speed (r/min) Fig. 1 Nanoscale carbon wires from PAN ﬁbers. a Thin PAN nanoﬁber array on a silicon substrate coated with silicon dioxide: continuous deposition of NFES PAN nanoﬁbers on substrates mounted on a rotating drum and electric jet ejection at different applied voltages from 500 to 35 V. b Fabrication of carbon nanowires from PAN ﬁbers at 300 V. c Atomic force microscopy (AFM) images of the carbon nanowires at 100 and 35 V. d Carbon wire diameter as a function of applied voltage; n > 30. e Highly uniform spacing between the carbon wires in an array obtained by varying the rotational speed at a constant low voltage of 35 V; n >50 Diameter (nm) Spacing (um) Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 4 of 13 working voltage causes a decrease in the jet diameter, as movement of the stage, polymer nanoﬁbers with dia- described by Eq. (1): meters as low as 50 nm diameter are aligned and depos- () "# 2 2 2 2 2 E K Inx E K InðÞ 2I þ EKx 1 EK Ex pﬃﬃﬃ h ¼ InverseFunction þ þ þ þ C ð1Þ 3 3 4 2 2 8I 16I 8Ix 8I x 2Q β ited on silicon substrates ﬁxed in the convex areas of the where h is the cross-sectional ﬁber radius, E is the electric drum. The limitations of the proposed method lie in the ﬁeld strength, K is the electric conductivity, I is the total dependency of initiating the jet and arraying ﬁbers on current of the electriﬁed jet, x is the distance between the the rotating drum. Instead of arraying ﬁbers, making needle and drum along the axis of the needle, Q is the three-dimensional ﬁber scaffolds or ﬁlms with this fabri- 3/2 volume ﬂow rate, β = ϵ -1 is the dimensionless con- cation method has to undergo breaks in the lines due to ductivity of the ﬂuid , ϵ is the dielectric constant and C the instability of the ﬂuid ﬂow inside the needle during is a constant. long-term jetting. After the polymer jet is initiated at various voltages from Pyrolysis at 1000 °C leads to radial and longitudinal 500 to 35 V, the alignment of polymer nanoﬁbers on the contraction as well as an enhanced tensile strength of the silicon substrate is implemented by the programmed stabilized PAN nanoﬁbers because of the increased car- 21–23 movement of a linear stage on which the needle is afﬁxed bon content . As a result, a radial shrinkage of less (see Fig. 1a). In this automated positioning system, the than 15% in Fig. 1b is observed, which is lower than the rotational speed ω of a drum and the linear speed ν of the 47.7 to 90% shrinkage from freely suspended polymeric nozzle moving along the surface of that drum can readily nanoﬁbers. This is most likely due to the strong attach- tune the ﬁber-to-ﬁber spacing d as follows: ment of the polymeric nanoﬁber to the silicon substrate, which derives from the impact of the polymer ﬁber on the d ¼ v=ω ð2Þ silicon substrate during deposition. A combination of the effects of surface wetting and thermal expansion coefﬁ- where ν is the linear speed of the nozzle moving along the cient mismatch at the interface contributes to the possible interaction between polymeric nanoﬁbers and a silicon surface of the drum, and ω is the rotational speed of the drum. In comparison with traditional electrospinning, substrate. The height of the carbon wire is much smaller shear force spinning during NFES affords a more uniform than the width in Fig. 1c, showing ﬁber deformation ﬁber-to-ﬁber spacing, as indicated by an ~2-fold decrease caused by strong attachment. Due to lower shrinkage, the in the relative standard deviation of the achieved fabrication of a thin carbon wire largely depends on spacings . Therefore, a rotating drum is introduced to reducing the diameter of the polymer ﬁber. Ultralow- add shear force into our electrospinning system, ulti- voltage (35 V) NFES allows for the smallest polymer ﬁber mately resulting in the more uniform alignment of diameter, achieving thin carbon nanowires with diameters polymer ﬁbers shown in Fig. 1a. A comparison between down to 45 nm, as shown in Fig. 1c. The effect of the the theoretical and experimental results is illustrated in applied voltage on the thickness of the carbon nanowires Fig. 1e, revealing that maximizing the rotational speed (ω) in Fig. 1d clearly reveals a decrease in the average dia- and minimizing the linear stage speed (ν) makes it meter of the carbon nanowires with decreasing applied possible to lower the ﬁber spacing to as small as ~2.5 µm; voltage during NFES. Since we initiate jetting with an thus, the density per unit area of the carbon nanowires on ultralow voltage (35 V), the resulting carbon nanowires the silicon substrate can be maximized. Controlling the have an average diameter of 60 nanometers, which is far wire-to-wire spacing and carbon nanowire diameter thinner than typical carbon nanowire diameters . The makes it possible to obtain materials with higher dependency of the electrical properties on the size sensitivity and stability for biosensing applications and demonstrates that thinner carbon nanowires tend to far kinetic studies . exceed the electrical properties of traditional carbon At a constant voltage of 35 V, polymer ﬁbers in the ﬁbers . convex parts of the drum are thinner than those in the grooved parts (see the right image in Fig. 1a). This is most Effect of aligning carbon nanowires and implementing a likely due to the increase in the mechanical stretching of nitrogen pretreatment on the nanocrystalline structuring of carbon nanowires the nanoﬁbers between the point of contact on the silicon substrate and droplet. By implementing a very low NFES Many reports conclude that the degree of a graphitic working voltage of 35 V and programming the linear microstructure in carbon wires is related to the physical Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 5 of 13 synthesis conditions, such as stress-induced routes , After jet initiation at 500 V in Fig. 1a, an increase in the torque-applied stretching , and mechanical and electro- applied voltage to 1200 V allows for the continuous static stretching . A novel method for improving the electrospinning of a single, stable ﬁlament from a droplet graphitic microstructure of carbon wires on silica surfaces at a ﬂow rate of 1 nl/min. In this case, increasing the is developed by structuring nanocrystalline materials in needle-drum distance from 1.85 mm to 10 mm during this paper, which lays a foundation for studying the NFES results in the formation of disorderly polymer ﬁbers electrochemical properties of graphitic microstructures. and partially aligned polymer ﬁbers onto the rotating To understand the mechanism of structuring nanocrys- drum. Subsequent stabilization in the air at 115 °C and talline materials, disorderly carbon nanowires and par- then carbonization in a nitrogen atmosphere at 1000 °C tially aligned carbon wires are introduced as reference (at a heating rate of 15 °C/min from 260 to 1000 °C) objects. transform such polymer ﬁbers into carbon nanowires, as Silica Carbon fiber Limits of radial and Alignment axial shrinkage Silicon substrate 600 nm 300 nm 45 28 D-peak D-peak D-peak G-peak G-peak G-peak 10 I /I =1.38 I /I =1.04 7 D D G G I /I =0.96 0 0 200 nm 1200 1400 1600 1800 1000 1250 1500 1750 2000 1000 1250 1500 1750 2000 –1 –1 –1 Raman shift (cm ) Raman shift (cm ) Raman shift (cm ) Nitrogen Carbon fiber at 300V D-peak G-peak I /I =0.74 Silica Silicon substrate 100 nm 1200 1400 1600 1800 –1 Raman shift (cm ) 1.5 Porous structuring 1200V 800V 1200 V 800 V 500 V 500V 35 V 35V 1.2 Nano-crystalline structuring 0.9 Highly crystalline 0.6 50 nm structuring 200 nm 150 nm 2 μm 0 400 400 1200 Voltage (V) Fig. 2 Effect of voltage-dependent nanowire size on the crystalline structure in regard to nitrogen pretreatment. a Limiting the radial and axial shrinkage to develop tensile stress. b Crystalline carbon nanowires on silica surfaces processed with nitrogen pretreatment at 300 V. c AFM phase images of carbon nanowires at 1200, 800, 500, and 35 V. d Transformation of the crystalline structure from porous to highly crystalline; n >35 I /I D G Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 6 of 13 shown in Fig. 2a. In general, the graphitic nature of PAN- Nitrogen pretreatment is performed by placing the derived carbons is evaluated based on Raman spectroscopy, silicon chip with the carbon dioxide surface (left side of a standard nondestructive analysis tool. As shown in Fig. Fig. 2b) in a quartz glass tube ﬁlled with nitrogen for 2a, we observe the D and G peaks centered at approxi- heat treatment at 1000 °C to dramatically overcome the −1 mately 1350 and 1590 cm , respectively. The spectrum of breaking of ﬁber into nanograins during pyrolysis. Once pure graphite shows a strong G-peak due to the in-plane the polymer ﬁbers are deposited on the silica surface pretreated with nitrogen, carbonization transforms the bond-stretching motion of pairs of Csp -bonded atoms, while the D-peak is more pronounced in the presence of polymer ﬁber structure into a defect-free carbon wire defects such as bond-angle disorder, bond-length disorder, (AFM phase images in the middle of Fig. 2b). The vacancies, and edge defects . The intensity ratio of the D- transition from a nanograin-based microstructure to a and G-peaks (I /I ) is proportional to the in-plane corre- defect-free microstructure is accompanied by a decrease D G lation length and corresponds to the amount of disorder in the I /I ratio from 1.38to0.74(Fig. 2a, b), thus D G present in the carbon wire . The lower the value of I /I forming highly crystalline carbon wires. A previous D G is, the higher the crystalline microstructure or the lower study showed that the graphitic structure in carbon the disordered (amorphous) nature of the carbon. wires showed a remarkable increase with a decrease in Because of the disordered arrangement of carbon wires the I /I ratio from 1.26 to 0.69 . The highly crystal- D G (Fig. 2a), the tensile force during the carbonization pro- line carbon wires are believed to have a much more cess does not effectively stretch the ﬁbers along the graphitic structure. longitudinal direction, resulting in a high-intensity ratio of 1.04 (Fig. 2a). After aligning the carbon wires along the Effect of nanowire diameter on crystalline structuring longitudinal direction (Fig. 2a), the resulting intensity From Fig. 2c, d, it is clear that stress management is ratio of 0.96 (Fig. 2a) is slightly lower than 1.04, showing a crucial to yield highly crystalline carbon nanowires. The higher crystallinity or a lower amorphous nature. tensile stress on the cross-section of the ﬁbers becomes Although the alignment of ﬁbers demonstrates the pos- more pronounced with decreasing diameters of the sibility of developing tension to improve crystallinity, the ﬁbers, so we also exploited thinning the wires to further microstructure of this carbon is still glassy in nature based improve the graphitic structure. The effect of the on the results (Fig. 2a). nanowire diameter, which is dependent on the applied The formation of glassy carbon during pyrolysis derives voltage in Fig. 1d, on the microstructure and crystallinity from the curved structures of the penta- and hepta- of the carbon nanowires on the silica surface pretreated carbon rings . To implement this alignment process for with nitrogen is illustrated in Fig. 2c, d. Further inves- curved structures, a mechanical treatment that changes tigation into the decrease in wire diameter from axial stress from compressive stress to tensile stress is ~5.05 μmto ~45nm (Fig. 2c, d) reveals an evolution applied to the PAN ﬁbers for a remarkable increase in the from a porous microstructure to a highly crystalline graphitic structure of the resulting carbons. Here, the nanostructure via nanocrystalline structuring. A Raman deformation introduced by aligning the polymer nanoﬁ- spectrometer with a DXR microscope and equipped with bers on a silica surface (Fig. 2a) plays a somewhat similar a 532 nm excitation laser was selected to evaluate the role to that of mechanical treatment, as described by crystallinity of the microstructures in the carbon nano- Maziar et al. . The resulting strong attachment of the wires fabricated at 1200, 500, and 35 V. The intensity polymeric nanoﬁber to the silicon substrate limits radial ratio of 0.78 in Fig. 2d is much lower than that of 1.41 in shrinkage during stabilization and carbonization, thus Fig. 2d, corresponding to the higher degree of crystal- forming tension stress in the axial and radial directions. linity of the nanocrystalline microstructure. This Upon increasing the rotational speed of the drum, the improvement in crystallinity may be attributed to the bond of the nanoﬁber to the silica surface becomes strong increase in tensile stress in the axial direction due to the enough to counterbalance the tension developed during decrease in diameter from ~5.05 μm to ~305 nm pyrolysis. Atomic force microscopy allows for a detailed (Fig. 2c). Upon further reduction of the carbon nanowire study of the resulting carbon wire microstructure. Visual diameter from ~305 nm to ~45 nm (Fig. 2c), the micro- examination of the atomic-resolution micrographs illus- structure is transformed into an even more crystalline trates the evolution of the developed carbon wire micro- microstructure, which is gleaned from the decrease in structures. Since these two forces have opposite effects on I /I from 0.78 to 0.64 (Fig. 2d). Figure 2d illustrates the D G the radial shrinkage of ﬁbers, polymer ﬁbers are trans- correlation between the crystallinity of the micro- formed into nanograin-based carbon wires (AFM phase structures and the applied voltage. Clearly, the polymer images on the left side of Fig. 2a). The resulting intensity ﬁbers obtained at the lowest voltages result—after pyr- ratio of 1.38 is much higher than 0.96 and 1.04, thus olysis—in the highly crystalline carbon nanowires owing poorly crystalline carbon wires are formed. to their highly graphitized structures. Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 7 of 13 Effect of decyanation on crystalline structuring impurity elements during carbonization (Fig. 3a) resulted The effect of tension stress on crystalline structuring is in the formation of carbon sheet-like layers from carbon- accompanied by decyanation reactions (named stress like ribbons . To correlate the inﬂuence of the deni- decyanation). A previous report showed that relying on trogenation and decyanation reactions with the graphitic denitrogenation and decyanation reactions to remove structure, Raman spectroscopy, and X-ray photoelectron 40% 30% N NN N CH N N N CH N N OH –N –HCN N NN NN N Carbonization 20% b c N 1s 33,000 N 1s 22,000 N N O NH NH 20,000 30,000 18,000 50% 50% 70% 27,000 30% 16,000 24,000 14,000 396 399 402 405 396 398 400 402 Binding energy (eV) Binding energy (eV) de N 1s N 1s N N O N O NH 67% 82% 34% 18% 398 399 400 401 396 400 404 408 Binding energy (eV) Binding energy (eV) fg N 1s 1.5 N O N 1.0 5% N 95% 0.5 0.0 396 399 402 405 0 24 48 72 Binding energy (eV) C (%) NH Fig. 3 Effect of the decyanation reaction on the crystalline structuring of wires. a Model reaction path from stabilized PAN to carbon. N 1 s X-ray photoelectron spectroscopy (XPS) spectra of b disordered carbon wires at 1200 V and of arrayed carbon nanowires at c 1200 V, d 500 V, and f 35 V with pretreatment of the silica surface in a nitrogen environment at 1000 °C. e N 1 s XPS spectra of arrayed carbon nanowires at 35 V in the absence of nitrogen pretreatment of the silica surface. g I /I as a function of the intensity of the acridine rings (C ); n >5 D G NH Counts per second Counts per second Counts per second Counts per second I /I D G Counts per second Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 8 of 13 spectroscopy characterization was performed. The full N scan rate or the EIS frequencies. Increasing the frequency 1 s spectra presented in Fig. 3b–f demonstrate two typical to the characteristic value ω = D/d , where D is the dif- peaks that are assigned to nitrogen atoms in the acridine fusion coefﬁcient of the analyte and d is the distance 34,35 ring and nitrogen atoms in the naphthyridine and between neighboring electrodes , causes the transition hydronaphthyridine rings. For the porous microstructure from overlapping (Fig. 4a) to nonoverlapping (Fig. 4b) of carbon wires with a diameter of ~4.3 µm (Fig. 2c), the diffusion hemispheres. By further increasing the fre- corresponding N 1 s spectrum in Fig. 3c has two com- quency to the characteristic value ω = D/a , where a is the ponents, which are assigned to nitrogens in the acridine radius of a single disc electrode, a planar diffusion regime ring bonds (50%) and in the naphthyridine and hydro- is obtained; thus, a single electrode can be taken as a naphthyridine rings (50%). In comparison with that of planar electrode. In the interval given by these two carbon wires fabricated at 1200 V (Fig. 2a), the intensity of characteristic frequencies, the electrode array shows the acridine rings is weakened from 70% (Fig. 3b) to 50%, steady-state behavior with a sigmoidal cyclic voltammetry revealing a more extensive decyanation reaction for the curve. carbon wires in an array fabricated at 1200 V. With Cyclic voltammetry (CV) curves of the ferri/ferrocya- 3−/4− copious decyanation, most of the reaction products that nide ([Fe(CN) ] ) redox couple in aqueous solutions leave the carbonizing ﬁber are in the form of gases, have often been used as a reference method to evaluate including HCN, H O, O ,H , CO, NH , and CH , ulti- the electrochemical performance of a carbon elec- 2 2 2 3 4 20,36,37 mately leading to a porous microstructure. The resulting trode . The CV curve from the disordered carbon intensity ratio of 1.41 (Fig. 2d) is much higher than the wire mat in Fig. 2a shows macroelectrode behavior with 0.96 in Fig. 2a but is comparable to the 1.38 from anodic and cathodic peak currents in Fig. 4c, indicating nanograin-based structuring, demonstrating that defects partially overlapping diffusion hemispheres. Aligning the in the carbon wire reduce the degree of crystallization. disordered carbon wires to a certain distance (~2.5 μm) at After the decrease in the nitrogen intensity of the 1200 V results in a typical steady-state sigmoidal vol- acridine ring from 34 to 5% (Fig. 3d, f), the resulting tammogram with a steady-state current in Fig. 4d. This intensity ratio (I /I ) decreases from 0.78 to 0.64 (Fig. 2d), demonstrates the dependency of the nonoverlapping-to- D G demonstrating the possibility of enhancing the decyana- overlapping diffusion regime on the distance of carbon tion reactions to improve the crystallinity. In the absence wires in the characteristic frequency of ω = D/d .By of nitrogen pretreatment of the silica surface, the resulting reducing the carbon wire diameter further from ~4.3 µm increase in the nitrogen intensity of the acridine ring from to ~512 nm, the interwire spacing further widens, leading 5 to 18% (Fig. 3e, f) demonstrates that the presence of to a decrease in the characteristic frequency (ω = D/d ). nitrogen at the surface facilitates more extensive decya- The resulting CV curve in Fig. 4e still shows steady-state nation reactions during carbonization, which leads to the behavior but has a higher steady-state current compared improved crystallinity of carbon wire; this is similar to the to that of carbon wires arrayed at 1200 V, which is likely results shown in Fig. 2a, b. Regardless of the porous derived from increasing the resistance of the carbon wire defects in Fig. 2c, the effect of I /I on the intensity of the due to the decrease in diameter. Theoretical and experi- D G acridine rings indeed reveals the improved crystallinity of mental work on the diffusion of microelectrode arrays the carbon wire with more extensive decyanation reac- show that, in the case of a scan rate of 100 mV/s for d ≥ tions, as shown in Fig. 3g. 100a, the dominating mode of diffusion is determined by Theeffectofnanowirediameterand decyanation on the transition between the planar and hemispherical dif- crystalline structuring reveals further insight into the fusion layers, leading to a sigmoidal shape in the CV underlying mechanism of graphitization at relatively low curve. Clearly, the transition from the overlapping of the temperatures (1000 °C). Several previous studies have individual diffusion layers to planar diffusion layers over demonstrated that enhancing the degree of graphitiza- the entire electrode array in Fig. 4f is very distinct for a tion can, to a large extent, improve the physical and highly crystalline carbon nanowire array. The weakening chemical nature of carbon nanowires . Thus, the of peak currents in the CV shows a monotonically improvement in crystallinity with extensive decyanation increasing straight line (Fig. 4f), which is expected due to reactions provides an attractive pathway for tailoring the the large resistance in the thin carbon nanowires (Fig. 5d). effect of a crystalline surface structure on electro- Lowering the applied voltage during NFES increases the chemical properties. speciﬁc surface area of carbon nanowires and subse- quently improves the degree of graphitization, thus Electrochemical characterization using cyclic voltammetry inﬂuencing the peak currents of carbon wires, as shown in In the case of very long experimental and theoretical Fig. 4h. The nanocrystalline-structured wires at 500 V time scales, hemispherical diffusion to microdisk elec- clearly display at least a fourfold higher current than the 30–33 trodes shows diffusion regimes depending on the CV porous-structured microwires at 1200 V and the highly Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 9 of 13 ab Silica Electrolyte solution Electrolyte solution Carbon fiber Silicon substrate Silicon substrate cd –35 –80 –70 –1.0 –0.5 0.0 0.5 1.0 –1.5 0.0 1.5 3.0 Potential (V vs Ag/AgCI) Potential (V vs Ag/AgCI) ef –90 –4 –180 –8 –6 –3 0 3 6 –1.2 –0.6 0.0 0.6 1.2 Potential (V vs Ag/AgCI) Potential (V vs Ag/AgCI) gh 0 300 600 900 1200 0 300 600 900 1200 Applied voltage (V) Applied voltage (V) Fig. 4 Electrochemical characterization of crystalline carbon nanowire arrays based on cyclic voltammetry. Schematics showing the diffusion of carbon nanowires in the case of a overlapping and b nonoverlapping diffusion hemispheres. Cyclic voltammetry (CV) curves of 5 mM K Fe(CN) / 4 6 5mM K Fe(CN) in 0.1 M phosphate buffer (pH 7.4) (1:1 mixture) for carbon wire mats disordered at c 1200 V and arrayed at d 1200 V, e 800 V, and 3 6 f 35 V, respectively; n >8. g Surface area and h peak current density of a carbon nanowire array as functions of applied voltage during NFES; n >8 crystalline nanowires at 35 V. The higher surface area nanowire array at 100 V shows the smallest surface area in derived from the appearance of the nanocrystalline Fig. 4g but has a peak current equivalent to that of the structure on the surface of the carbon nanowires is well carbon nanowire array at 1200 V (Fig. 4h). This is most correlated with the highest peak current of the carbon likely due to the highly graphitized structure increasing nanowire arrays at 500 V. The highly crystalline carbon the electrical conductivity . Current (µA) 2 Current (µ µA) Surface area (mm ) Current (µA) Peak current density (µA/mm ) Current (µA) Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 10 of 13 a b Disordered carbon fibers 1200 V Clustered and aligned 800 V carbon fibers 140 5 200 400 600 800 0 10 20 30 Z (real) (ohms) Z (real) (kohms) cd 300 V 100 V ct 35 V s 200 dl 0 100 200 300 0 400 800 1200 Z (real) (kohms) Applied voltage (V) Fig. 5 Electrochemical characterization of crystalline carbon nanowire arrays based on impedance spectroscopy. Impedance spectra of the a disordered carbon mat at 1200 V and clustered and aligned carbon ﬁber mat at 1200 V along with the near-ﬁeld carbon ﬁber arrays at b 1200 and 800 V and c at 300, 100, and 35 V. R , R , and C in c represent the solution resistance, electron transfer resistance, and double layer capacitance, s ct dl respectively. AC amplitudes of 1000 mV are applied to the carbon nanoﬁbers in (a–c). Impedance spectra of near-ﬁeld carbon ﬁber arrays at 35 V for AC amplitudes of 1000 and 25 mV; n >6. d Electron charge transfer resistance as a function of the applied voltage during NFES; n >10 By arraying all the carbon nanowires and maintaining a Electrochemical characterization using impedance spectroscopy wire-to-wire spacing of ~2.5 μm, the impedance spectrum Electrochemical impedance spectroscopy (EIS) of the shows two semicircles connected together at high fre- carbon nanowire array was performed using a solution of quencies and a relatively short straight line with a similar 5mM K Fe(CN) /5 mM K Fe(CN) in a 0.1 M phosphate slope (diffusion limitation) at the lowest frequencies, as 4 6 3 6 buffer (pH 7.4). Impedance spectra were obtained at fre- shown in Fig. 5b. Theoretical and experimental work on quencies from 0.1 Hz to 1000 kHz and at a 0 V dc potential the impedance of ultramicroelectrode arrays exhibit a vs. Ag/AgCl. The impedance spectra of the disordered car- semicircle at low frequencies in addition to the well- bon wire mat and clustered and aligned carbon wire mat are known semicircle at higher frequencies, demonstrating showninFig. 5a. Monotonically increasing straight lines overlap of the diffusion gradients. The change from with similar slopes at low frequencies are observed, reﬂect- overlapping diffusion to diffusion limitation is shown for ing a diffusion-limited process with the overlapping diffusion carbon nanowire arrays at 1200 V. This demonstrates the ﬁelds of all the participating wires. Upon the formation of weakening of the overlap of the diffusion gradients com- the wire-to-wire distance derived from clustered and aligned pared to that of the clustered and aligned carbon wires. carbon wires in Fig. 2a, the impedance spectroscopy in By reducing the applied voltage from 1200 to 800 V, Fig. 5a shows a quarter circle at high frequencies and a which is accompanied by a decrease in the wire diameter straight line at low frequencies. This is due to the weakening from ~4.3 µm to ~512 nm, the impedance spectrum of the diffusion gradients (f) according to Eq. (3): changes to a large semicircle with a small straight line, as shown in Fig. 5b. The semicircle diameter at higher fre- 4α ~ quencies is governed by the charge transfer resistance ð3Þ f ¼ πd (R ) related to the electron transfer rate of the redox et species on the electrode surface. The region at low fre- where f is the diffusion gradient in front of each individual quencies again represents a line with a similar slope, –ultramicroelectrode and ᾶ denotes the arithmetical showing mass transfer control or a diffusion limitation. average of the oxidation and reduction diffusion coefﬁ- Previously, in the case of three-dimensional hemispherical 3−/4− 31–34 cients of [Fe(CN) ] . diffusion to an array of microdisk electrodes , the Z (imaginary) (kohms) Z (imaginary) (ohms) Electron charge transfer resistance (kohms) Z (imaginary) (kohms) Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 11 of 13 impedance spectrum showed a semicircle for frequencies theoretical and experimental work on carbon microelec- between the characteristic frequencies of ω = D/d and trodes that the smaller the electrode surface area is, the 2 41 ω = D/a . The transition from overlapping diffusion to greater the charge transfer resistance . The monotonic three-dimensional hemispherical diffusion at high fre- correlation between the charge transfer resistance and quencies is shown to observe the changes between the wire diameter in Fig. 5d is expected in accordance with impedance spectra of carbon nanowire arrays at 1200 and the results of carbon microelectrodes. Clearly, by chan- at 800 V. The impedance spectrum of the carbon nano- ging the wire diameter in the carbon nanowire arrays, the wire array at 800 V is expected due to the lower density of semicircle scale at high frequencies can be adjusted. 20 14 arrayed carbon wire , which is derived from widening the A previous study on the correlation between micro- wire-to-wire distance with decreasing wire diameter. structures and properties showed that enhancing graphi- Reducing the diameter of the arrayed carbon wire from tic microstructures could improve the bulk characteristics ~512 to ~40 nm with decreasing applied voltage from 800 of carbon fabrics, resulting in an increase in conductivity to 35 V results in changes in the impedance spectrum from ~200 S/m to ~5000 S/m with improving graphiti- from a large semicircle with a small straight line to just a zation. The charge transfer resistance of highly crystalline single semicircle without a Warburg impedance element carbon nanowire arrays at 35 V is unusually low and is (Fig. 5c). This case allows for the process from the over- half of that of carbon nanowire arrays at 100 V (Fig. 5d). lapping diffusion of all the participating wires to the This is most likely due to the further improvement of the complete reaction kinetics determined by electron trans- graphitic microstructure as the voltage is decreased from fer. Theoretical work has shown that a smaller electrode 100 to 35 V (Fig. 2d). Thus, the combination of these radius leads to a larger electrode admittance per unit area, factors, including the wire-to-wire spacing, nanoscale which allows the susceptance at low frequencies to remain diameter, and graphitic microstructure, leads to a distinct negligible. In addition, decreasing the nanowire diameter semicircle of highly crystalline carbon nanowire arrays increases the lower frequency limit at which the suscep- in EIS. tance (imaginary part of the impedance) becomes lower than the conductance (real part of the impedance). Thus, Conclusion the impedance spectrum of the highly crystalline carbon In the state-of-the-art fabrication of carbon nanowires nanowire array at 35 V shows a semicircle for ω = D/a » with the carbon-nano-electro-mechanical system D/d . These results demonstrate the ability to control the approach, the growth of a graphitic microstructure is typically limited to the use of a Ni catalyst. In this work, diffusion regimes by altering the wire spacing and nanowire size. we demonstrated a novel catalyst-free fabrication process In most studies, the semicircle spectrum from carbon for arrays of highly graphitized carbon nanowires with nanowire arrays is meaningful only when ac signals with various surface nanostructures. It was developed by 38,39 amplitudes ≤25 mV are applied . By reducing the minimizing the polymer ﬁber diameter with ultralow- amplitude from 1000 mV to 25 mV, the impedance voltage NFES. The linear speed of the spinneret and the spectrum of the highly crystalline carbon nanowires in rotational speed of the collector allowed for better control Fig. 5c still shows a single semicircle without scattered of the ﬁber-to-ﬁber distance in the ﬁber arrays. The sta- data points, demonstrating the high signal-to-noise ratio bilization at 115 °C and subsequent carbonization con- compared to that of typical carbon nanowire electrode verted these polymer nanoﬁbers arrayed on a silica surface arrays . Therefore, highly crystalline carbon nanowire into carbon nanowires. The thickness control of carbon arrays can be selected as potential candidates for DNA nanowires was conveniently adjusted by tuning the switching . applied voltage during NFES. Aligning the wires on a The impedance spectra were ﬁtted by means of an nitrogen-pretreated silica surface allowed for stress equivalent Randles circuit with capacitance, charge decyanation to improve the graphitic content of the car- transfer resistance, solution resistance, and Warburg ele- bon nanowires. The presented diameter control showed ments. The charge transfer resistance related to the the ability to transform a porous-microstructured carbon overall surface area of the carbon nanowire arrays corre- wire into a highly crystalline nanostructure via nano- sponds to the diameter of the semicircle in the high- crystalline structuring. frequency domain of the impedance spectra. Figure 5d Various electrochemical behaviors were observed in the shows the increase in charge transfer resistance as the obtained CV curves when reducing the wire diameter applied voltage is decreased from 1200 to 100 V; notably, because that altered the structure on the surface of the this is accompanied by a decrease in the wire diameter carbon wire and improved the graphitic structure. The from ~4.3 µm to ~120 nm and the microstructural typical CV curve showing macroelectrode behavior with transformation from a porous microstructure to a nano- anodic and cathodic peak currents was altered to a sig- crystalline microstructure. It is well known from moidal CV curve with the steady-state current dominated Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 12 of 13 by radial diffusion. This is a characteristic behavior In general, the graphitic nature of PAN-derived carbons observed for carbon nanowire arrays where there is no was evaluated based on Raman spectroscopy, a standard overlap of the diffusion hemispheres from neighboring nondestructive analysis tool. The Raman spectrometer electrodes. Compared to three-dimensional carbon nano- with a DXR microscope (Thermo Fisher) and equipped wire electrode arrays with wire diameters of ~100 nm, with a 532 nm excitation laser assessed the corresponding two-dimensional carbon nanowire arrays with diameters carbon wire arrays. By moving the platform on which the sample was located, the wire was moved to a location just of ~512 nm easily obtained low steady-state currents during cyclic voltammetry, which has been regarded as a below the marked laser spot. Linear scanning was potential advantage for biosensing applications. –The implemented with a step size of 100 nanometers. For each nanocrystalline structure on the surface of graphitized step, data were collected by exciting the laser, ultimately carbon wires rendered extremely high peak currents in resulting in obtaining the Raman spectrum of a carbon their CV curve. More importantly, highly crystalline car- nanowire. We used the lens for an average of approxi- bon nanowire arrays showed a linear CV curve without mately 20 times per sample. To correlate the inﬂuence of anodic and cathodic peak currents, representing the spe- denitrogenation and the decyanation reaction on the cial characteristics of the highly graphitized structures in graphitic structure, X-ray photoelectron spectroscopy was the nanoscale carbon wires. The charge transfer resistance performed. could be controlled by varying the diameter of the carbon Atomic force microscopy (AFM) (Bruker Dimension nanowires. In the EIS spectra of the highly crystalline Icon) and scanning electron microscopy (SU8220) carbon nanowire arrays, the abnormal decrease in the allowed for a detailed study of the surface nanostructures charge transfer resistance conﬁrmed the inﬂuence of the in the resulting carbon ﬁbers. The topology of carbon graphitized structure on the electrochemical behavior. ﬁbers was studied with the tapping mode to investigate The electrochemical performance of the crystalline the inﬂuence of the applied voltage on various surface carbon nanowire arrays with various surface nanos- nanostructures. tructures makes them potential candidates for biochem- A traditional three-electrode conﬁguration consisting of ical sensors with lower detection limits and as devices for an Ag/AgCl reference electrode, Pt counter electrode, and electrochemical energy storage. Graphitized carbon sub- carbon working electrode was used. To probe the carbon/ microwires with a nanocrystalline structure could allow electrolyte interface, a GPSTAT12 potentiostat/galvano- for ampliﬁed biosensing via redox cycling and enhanced stat equipped with a frequency response analyzer module capacitive energy storage in microsupercapacitors. The was employed. Cyclic voltammetry (CV) of the ferri/fer- 3−/4− linear CV curve of highly crystalline carbon nanowires rocyanide ([Fe(CN) ] ) redox couple in aqueous shows the ability to permit stable data collection for solutions have often been used as a reference method to ultrasensitive biological detection. In particular, a further evaluate the electrochemical performance of a carbon 20,36,37 increase in the surface area of –nanocrystalline-structured electrode . carbon wires could allow for high-power supercapacitors. Electrochemical impedance spectroscopy (EIS) of the carbon nanoﬁber array was performed using a solution Materials and methods containing 5 mM K Fe(CN) /5 mM K Fe(CN) in a 0.1 M 4 6 3 6 The electrospinning solution (9% PAN) was prepared by phosphate buffer (pH 7.4). Impedance spectra were taken dissolving PAN (150000 mw, Sigma Aldrich, St. Louis, at frequencies from 0.1 Hz to 1000 kHz and at a dc MO) in DMF. Using vortex mixing at 30 RPM, mixtures of potential of 0 V vs. Ag/AgCl. The number of experiments PAN/DMF were allowed to freely diffuse at different (n) is indicated in the ﬁgure legends. temperatures. For the NFES experiments, we used a 3 mL Acknowledgements syringe mounted on a syringe pump to dispense the This research was supported by the National Key R&D Program of China highest conductivity ink at a feed rate below 10 nL/min. (2020YFB2009002) and the National Natural Science Foundation of China (51875084). The support provided by the China Scholarship Council (CSC) Silicon substrates coated with silicon dioxide were during the visit of Jufeng Deng to the University of California, Irvine is mounted in the grooves and convex areas on the drum acknowledged. with carbon tape. The NFES voltage was applied between the dispensing needle and the grounded drum. It should Author details School of Mechanical Engineering, Dalian University of Technology, Dalian be noted that regular arrays were achieved as long as ω ≥ 116023, China. Mechanical and Aerospace Engineering, University of 400 RPM and ν ≥ 80 µm/s were achieved at the same time. 3 California, Irvine, CA 92617, USA. Chemical and Biomolecular Engineering, Fabrication of carbon nanowire arrays derived from PAN University of California, Irvine, CA 92517, USA. School of Engineering and Science, Tecnologico de Monterrey, Monterrey, NM 64849, Mexico nanoﬁber arrays consisted of oxidization in the air at 115 °C (named stabilization) and subsequent carbonization Author contributions in a furnace with an inert nitrogen atmosphere at 1000 °C JD, CL, DS, and MM conceived the project. JD, CL, DS, and MM developed the (heating ramp rate of 15 °C/min) (Fig. 1b). process for growing carbon nanowires. JD, CL, DS, and MM interpreted the Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 13 of 13 data and developed the ﬁgures. JD, CL, DS, and MM wrote the manuscript. JD 19. Dotivala, A. C.,Puthuveetil, K.P. & Tang,C.Shear force ﬁber spinning: process transferred and prepared the AFM samples. JD performed the AFM parameter and polymer solution property considerations. Polymer 11,294 characterization and data analysis. JD performed the AFM, XPS, CV, and EIS (2019). characterization tests and the data analysis. JD, CL, DS, and MM edited the 20. Siddiqui, S.,Arumugam, P. U.,Chen, H.,Li, J. &Meyyappan, M.Characterization manuscript. All authors discussed the data and contributed to the ﬁnal of carbon nanoﬁber electrode arrays using electrochemical impedance manuscript. spectroscopy: effect of scaling down electrode size. ACS Nano 4,955–961 (2010). 21. Liu, J. et al. Study on the oxidative stabilization of polyacrylonitrile ﬁbers by Conﬂict of interest microwave heating. Polym. Degrad. Stab. 150,86–91 (2018). The authors declare no competing interests. 22. Khayyam, H. et al. PAN precursor fabrication, applications and thermal stabi- lization process in carbon ﬁber production: experimental and mathematical Received: 29 July 2021 Revised: 25 November 2021 modelling. Prog. Mater. Sci. 107, 100575 (2020). Accepted: 3 December 2021 23. Zhao, R.-X. et al. Inﬂuence of heating procedures on the surface structure of stabilized polyacrylonitrile ﬁbers. Appl. Surf. Sci. 433,321–328 (2018). 24. 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Publisher: Springer Journals
Copyright: Copyright © The Author(s) 2022
eISSN: 2055-7434
DOI: 10.1038/s41378-021-00345-z
Publisher site: See Article on Publisher Site

Abstract

Crystalline carbon nanowire arrays were fabricated taking advantage of near-ﬁeld electrospinning and stress decyanation. A novel fabrication method for carbon nanowires with radii ranging from ~2.15 µm down to ~25 nm was developed based on implementing nitrogen pretreatment on the silica surface and then aligning polymer nanoﬁbers during near-ﬁeld electrospinning at an ultralow voltage. Stress decyanation was implemented by subsequently pyrolyzing a polymer nanoﬁber array on the silica surface at 1000 °C for 1 h in an N atmosphere, thus obtaining a crystalline carbon nanowire array with a nanostructured surface. Various crystalline nanostructures were fabricated on the nanowire surface, and their electrochemical performance was evaluated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Crystalline carbon wires with diameters ranging from micrometers to submicrometers displayed carbon nanoelectrode-like behavior with their CV curve having a sigmoidal shape. A highly crystalline carbon nanowire array showed distinct behavior, having a monotonically increasing straight line as its CV curve and a semicircular EIS spectrum; these results demonstrated its ultrastable current, as determined by electron transfer. Furthermore, nanocrystalline-structured carbon wires with diameters of ~305 nm displayed at least a fourfold higher peak current density during CV (4000 mA/m ) than highly crystalline carbon nanowires with diameters of ~100 nm and porous microwires with diameters of ~4.3 µm. Introduction case of conventional carbon nanowires with glassy carbon The attractive merit of carbon wires derives from car- structures, the principal philosophy emphasizes the bon microstructures integrating high-performance with interconnected graphitic structure, which provides an 1,2 abundant functionalities . There are several available extraordinary combination of mechanical, electrical, and microstructures of carbon materials, such as glassy car- thermal properties (strength up to 20 GPa, electrical 6 6 7 bon, diamond, and graphite. Carbon nanowires with conductivity of 1.5 × 10 S/m and thermal conductivity glassy carbon structures have been used in many different of 5000 W/m·K). Compared to glassy carbon nanowire applications, such as in high-power supercapacitors , conﬁgurations, graphitized carbon nanowires offer several electrochemical biosensors and high-energy rechargeable advantages, such as their extraordinary mechanical 5 8 batteries . To upgrade all of the intrinsic properties in the properties, which includes their elastic modulus (1.1 TPa), tensile strength (~130 GPa) and notable ﬂex- ibility, and their excellent electron transport perfor- Correspondence: Chong Liu (chongl@dlut.edu.cn)or mance , which includes their extremely high electric Marc Madou (mmadou@uci.edu) 1 8 2 School of Mechanical Engineering, Dalian University of Technology, Dalian conductivity (~10 S/m), carrier mobility (2000000 cm / 116023, China V·s) and ampacity (1–2 GA/cm ). These advantages Mechanical and Aerospace Engineering, University of California, Irvine, CA demonstrate the abundant functionalities of graphitized 92617, USA Full list of author information is available at the end of the article © The Author(s) 2022 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to theCreativeCommons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 2 of 13 carbon nanowires for use in various applications, such as programmed movement of a linear stage during near-ﬁeld 10 11 in electrical conductors , supercapacitors , actuators, electrospinning (NFES), the polymer nanoﬁbers are and solar devices , showing more applicability than arrayed on a silicon substrate coated with silica (500 nm conventional carbon nanowires. Regarding electro- thickness). Before depositing the polymer nanoﬁber chemical energy storage, graphitized carbon nanowires arrays, the silica surface deposited by polymer ﬁbers is are promising for enhancing energy and power densities. pretreated at 1000 °C in a nitrogen atmosphere. To con- vert the polymer ﬁbers into carbon nanowires, a silicon The thermal reduction and high-temperature anneal- ing of graphite oxide wire made by a wet-spinning chip with carbon dioxide and polymer ﬁbers is pyrolyzed process have been proposed to produce highly graphi- at 1000 °C in a nitrogen atmosphere. The microstructure tized carbon wires but are limited to micron-scale fab- of the carbon wires is evaluated by atomic force micro- rication . For the electrochemical applications scopy and Raman spectroscopy. Additionally, the fabri- mentioned above, the performance is typically improved cated HCCNs are characterized electrochemically with by increasing the surface area of graphitized carbon cyclic voltammetry and electrochemical impedance nanowires, which can be achieved by decreasing the spectroscopy (ESI). diameter of graphitized carbon nanowires. Most of the techniques that have been proposed for fabricating Results and discussion graphitized nanoscale carbon wires include mechanical Fabrication of polymer nanoﬁber arrays using near-ﬁeld 14 15 stress pyrolysis and chemical vapor deposition .The electrospinning (NFES) doping approaches involved in these techniques cause To attain the thinnest polymer ﬁbers at the lowest additional substances to be added into the carbon wires, applied voltages in NFES, one important procedure which affects the electrochemical behavior of the carbon concerns the protocol for preparing the ink by the dis- wire. This has led to extensive research into the pyr- solution of polyacrylonitrile (PAN) in N,N-dimethylfor- olysis of near-ﬁeld electrospun ﬁbers (PNFEFs), which mamide (DMF). A heat treatment from 60 to 126 °C is enables advanced applications in electrochemical sen- found to be accompanied by a conductivity change, sing, energy storage, and stem cells. which indicates the highest conductivity at 106 °C .The PNFEFs are dedicated to manufacturing new classes of conductivity data can be collected by a conductivity well-deﬁned carbon nanostructures. In this process, meter (OAKTON CON 510 Series) with the ability to polymer nanoﬁbers are arrayed by the near-ﬁeld elec- show both conductivity and solution temperature. The trospinning of a precursor solution and subsequently conductivity plays a role in inducing sufﬁciently large converted into pyrolytic carbon nanowires through electrical stress, which makes it possible to initiate a high-temperature treatment (>800 °C) in an inert nano jet during near-ﬁeld electrospinning. With a drum- atmosphere. This method offers excellent control of to-needle distance of 5 mm, the polymer jet does not feature sizes, density per unit area, and the capability to initiate even at 1500 V because the electrostatic force pattern carbon nanowires. By introducing carbon scaf- cannot overcome the surface tension at the droplet-air folds into this process, a dramatic decrease in the ﬁber interface. However, touching the ink droplet at the diameter leads to ultrathin carbon nanowires (~5 nm) . ejector needle tip with the rotating drum causes the This technique is mostly established because the high- electrical stress to become large enough to counter- throughput fabrication of carbon nanowires can be balance the surface tension stress, giving rise to the easily achieved. formation of a Taylor cone and jet initiation at a voltage Recently, highly graphitized carbon nanowires have as low as 500 V (see Fig. 1a). When using a PAN ink been developed on carbon scaffolds using near-ﬁeld heated to either 60 °C or 126 °C, jet initiation at 500 V is electrospinning, pyrolysis, electrodeposition, and chemi- not achieved by the same touching procedure, which cal vapor deposition. Clearly, these techniques are still demonstrates the key role of conductivity for initiating inseparable from the participation of nickel catalysts, the jet during NFES. resulting in an inability to remove the nickel from the A porous absorbent paper mounted around the base of carbon wire. Thus, to date, the electrochemical behavior the ejector needle thins the liquid layer around the needle of highly crystalline carbon nanowires with graphitic at low feed rates (1 nL/min). The resulting electric ﬁeld structures have not been improved. Here, we present the intensity at the liquid-air interface is substantially higher fabrication and characterization of highly crystalline car- than that of the nozzle without droplet shaping at the bon nanowire (HCCN) arrays with well-controlled wire- same voltage . This in turn results in the decrease in the to-wire spacing. For this purpose, a novel method for the applied voltage from 500 to 35 V, which is far below any fabrication of highly crystalline carbon nanowires is current low-voltage NFES practice (see Fig. 1a). During implemented, permitting control of the wire diameter, NFES, continuous jetting at an ultralow applied voltage of wire-to-wire spacing, and degree of graphitization. By the 35 V becomes possible (see Fig. 1a). This reduction in the Voltage Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 3 of 13 35 V Polymer fibers Polymer 300 V droplet Silicon substrate Taylor 500 V Needle Groove cone Porous absorbent Convexity paper 300 μm 500 μm Silica PAN fiber Oxidized PAN fiber Carbon fiber 115 °C 1000 °C Silicon substrate Silicon substrate Silicon substrate Oxidization Carbonization in air in nitrogen C C HC CH CH C C N N N N N 40% 30% 20% O OH CN 200 nm 900 nm Polymer fiber Carnon fiber 10% Experiment Carbon fibers Theory 35 V 100 V 0 nm 711 nm B 42.5 nm 0 0 31 nm 0 800 1600 2400 3200 0 200 400 600 52.7 nm 45 nm 72.6 nm Applied voltage (V) Rotational Speed (r/min) Fig. 1 Nanoscale carbon wires from PAN ﬁbers. a Thin PAN nanoﬁber array on a silicon substrate coated with silicon dioxide: continuous deposition of NFES PAN nanoﬁbers on substrates mounted on a rotating drum and electric jet ejection at different applied voltages from 500 to 35 V. b Fabrication of carbon nanowires from PAN ﬁbers at 300 V. c Atomic force microscopy (AFM) images of the carbon nanowires at 100 and 35 V. d Carbon wire diameter as a function of applied voltage; n > 30. e Highly uniform spacing between the carbon wires in an array obtained by varying the rotational speed at a constant low voltage of 35 V; n >50 Diameter (nm) Spacing (um) Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 4 of 13 working voltage causes a decrease in the jet diameter, as movement of the stage, polymer nanoﬁbers with dia- described by Eq. (1): meters as low as 50 nm diameter are aligned and depos- () "# 2 2 2 2 2 E K Inx E K InðÞ 2I þ EKx 1 EK Ex pﬃﬃﬃ h ¼ InverseFunction þ þ þ þ C ð1Þ 3 3 4 2 2 8I 16I 8Ix 8I x 2Q β ited on silicon substrates ﬁxed in the convex areas of the where h is the cross-sectional ﬁber radius, E is the electric drum. The limitations of the proposed method lie in the ﬁeld strength, K is the electric conductivity, I is the total dependency of initiating the jet and arraying ﬁbers on current of the electriﬁed jet, x is the distance between the the rotating drum. Instead of arraying ﬁbers, making needle and drum along the axis of the needle, Q is the three-dimensional ﬁber scaffolds or ﬁlms with this fabri- 3/2 volume ﬂow rate, β = ϵ -1 is the dimensionless con- cation method has to undergo breaks in the lines due to ductivity of the ﬂuid , ϵ is the dielectric constant and C the instability of the ﬂuid ﬂow inside the needle during is a constant. long-term jetting. After the polymer jet is initiated at various voltages from Pyrolysis at 1000 °C leads to radial and longitudinal 500 to 35 V, the alignment of polymer nanoﬁbers on the contraction as well as an enhanced tensile strength of the silicon substrate is implemented by the programmed stabilized PAN nanoﬁbers because of the increased car- 21–23 movement of a linear stage on which the needle is afﬁxed bon content . As a result, a radial shrinkage of less (see Fig. 1a). In this automated positioning system, the than 15% in Fig. 1b is observed, which is lower than the rotational speed ω of a drum and the linear speed ν of the 47.7 to 90% shrinkage from freely suspended polymeric nozzle moving along the surface of that drum can readily nanoﬁbers. This is most likely due to the strong attach- tune the ﬁber-to-ﬁber spacing d as follows: ment of the polymeric nanoﬁber to the silicon substrate, which derives from the impact of the polymer ﬁber on the d ¼ v=ω ð2Þ silicon substrate during deposition. A combination of the effects of surface wetting and thermal expansion coefﬁ- where ν is the linear speed of the nozzle moving along the cient mismatch at the interface contributes to the possible interaction between polymeric nanoﬁbers and a silicon surface of the drum, and ω is the rotational speed of the drum. In comparison with traditional electrospinning, substrate. The height of the carbon wire is much smaller shear force spinning during NFES affords a more uniform than the width in Fig. 1c, showing ﬁber deformation ﬁber-to-ﬁber spacing, as indicated by an ~2-fold decrease caused by strong attachment. Due to lower shrinkage, the in the relative standard deviation of the achieved fabrication of a thin carbon wire largely depends on spacings . Therefore, a rotating drum is introduced to reducing the diameter of the polymer ﬁber. Ultralow- add shear force into our electrospinning system, ulti- voltage (35 V) NFES allows for the smallest polymer ﬁber mately resulting in the more uniform alignment of diameter, achieving thin carbon nanowires with diameters polymer ﬁbers shown in Fig. 1a. A comparison between down to 45 nm, as shown in Fig. 1c. The effect of the the theoretical and experimental results is illustrated in applied voltage on the thickness of the carbon nanowires Fig. 1e, revealing that maximizing the rotational speed (ω) in Fig. 1d clearly reveals a decrease in the average dia- and minimizing the linear stage speed (ν) makes it meter of the carbon nanowires with decreasing applied possible to lower the ﬁber spacing to as small as ~2.5 µm; voltage during NFES. Since we initiate jetting with an thus, the density per unit area of the carbon nanowires on ultralow voltage (35 V), the resulting carbon nanowires the silicon substrate can be maximized. Controlling the have an average diameter of 60 nanometers, which is far wire-to-wire spacing and carbon nanowire diameter thinner than typical carbon nanowire diameters . The makes it possible to obtain materials with higher dependency of the electrical properties on the size sensitivity and stability for biosensing applications and demonstrates that thinner carbon nanowires tend to far kinetic studies . exceed the electrical properties of traditional carbon At a constant voltage of 35 V, polymer ﬁbers in the ﬁbers . convex parts of the drum are thinner than those in the grooved parts (see the right image in Fig. 1a). This is most Effect of aligning carbon nanowires and implementing a likely due to the increase in the mechanical stretching of nitrogen pretreatment on the nanocrystalline structuring of carbon nanowires the nanoﬁbers between the point of contact on the silicon substrate and droplet. By implementing a very low NFES Many reports conclude that the degree of a graphitic working voltage of 35 V and programming the linear microstructure in carbon wires is related to the physical Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 5 of 13 synthesis conditions, such as stress-induced routes , After jet initiation at 500 V in Fig. 1a, an increase in the torque-applied stretching , and mechanical and electro- applied voltage to 1200 V allows for the continuous static stretching . A novel method for improving the electrospinning of a single, stable ﬁlament from a droplet graphitic microstructure of carbon wires on silica surfaces at a ﬂow rate of 1 nl/min. In this case, increasing the is developed by structuring nanocrystalline materials in needle-drum distance from 1.85 mm to 10 mm during this paper, which lays a foundation for studying the NFES results in the formation of disorderly polymer ﬁbers electrochemical properties of graphitic microstructures. and partially aligned polymer ﬁbers onto the rotating To understand the mechanism of structuring nanocrys- drum. Subsequent stabilization in the air at 115 °C and talline materials, disorderly carbon nanowires and par- then carbonization in a nitrogen atmosphere at 1000 °C tially aligned carbon wires are introduced as reference (at a heating rate of 15 °C/min from 260 to 1000 °C) objects. transform such polymer ﬁbers into carbon nanowires, as Silica Carbon fiber Limits of radial and Alignment axial shrinkage Silicon substrate 600 nm 300 nm 45 28 D-peak D-peak D-peak G-peak G-peak G-peak 10 I /I =1.38 I /I =1.04 7 D D G G I /I =0.96 0 0 200 nm 1200 1400 1600 1800 1000 1250 1500 1750 2000 1000 1250 1500 1750 2000 –1 –1 –1 Raman shift (cm ) Raman shift (cm ) Raman shift (cm ) Nitrogen Carbon fiber at 300V D-peak G-peak I /I =0.74 Silica Silicon substrate 100 nm 1200 1400 1600 1800 –1 Raman shift (cm ) 1.5 Porous structuring 1200V 800V 1200 V 800 V 500 V 500V 35 V 35V 1.2 Nano-crystalline structuring 0.9 Highly crystalline 0.6 50 nm structuring 200 nm 150 nm 2 μm 0 400 400 1200 Voltage (V) Fig. 2 Effect of voltage-dependent nanowire size on the crystalline structure in regard to nitrogen pretreatment. a Limiting the radial and axial shrinkage to develop tensile stress. b Crystalline carbon nanowires on silica surfaces processed with nitrogen pretreatment at 300 V. c AFM phase images of carbon nanowires at 1200, 800, 500, and 35 V. d Transformation of the crystalline structure from porous to highly crystalline; n >35 I /I D G Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Intensity (a.u.) Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 6 of 13 shown in Fig. 2a. In general, the graphitic nature of PAN- Nitrogen pretreatment is performed by placing the derived carbons is evaluated based on Raman spectroscopy, silicon chip with the carbon dioxide surface (left side of a standard nondestructive analysis tool. As shown in Fig. Fig. 2b) in a quartz glass tube ﬁlled with nitrogen for 2a, we observe the D and G peaks centered at approxi- heat treatment at 1000 °C to dramatically overcome the −1 mately 1350 and 1590 cm , respectively. The spectrum of breaking of ﬁber into nanograins during pyrolysis. Once pure graphite shows a strong G-peak due to the in-plane the polymer ﬁbers are deposited on the silica surface pretreated with nitrogen, carbonization transforms the bond-stretching motion of pairs of Csp -bonded atoms, while the D-peak is more pronounced in the presence of polymer ﬁber structure into a defect-free carbon wire defects such as bond-angle disorder, bond-length disorder, (AFM phase images in the middle of Fig. 2b). The vacancies, and edge defects . The intensity ratio of the D- transition from a nanograin-based microstructure to a and G-peaks (I /I ) is proportional to the in-plane corre- defect-free microstructure is accompanied by a decrease D G lation length and corresponds to the amount of disorder in the I /I ratio from 1.38to0.74(Fig. 2a, b), thus D G present in the carbon wire . The lower the value of I /I forming highly crystalline carbon wires. A previous D G is, the higher the crystalline microstructure or the lower study showed that the graphitic structure in carbon the disordered (amorphous) nature of the carbon. wires showed a remarkable increase with a decrease in Because of the disordered arrangement of carbon wires the I /I ratio from 1.26 to 0.69 . The highly crystal- D G (Fig. 2a), the tensile force during the carbonization pro- line carbon wires are believed to have a much more cess does not effectively stretch the ﬁbers along the graphitic structure. longitudinal direction, resulting in a high-intensity ratio of 1.04 (Fig. 2a). After aligning the carbon wires along the Effect of nanowire diameter on crystalline structuring longitudinal direction (Fig. 2a), the resulting intensity From Fig. 2c, d, it is clear that stress management is ratio of 0.96 (Fig. 2a) is slightly lower than 1.04, showing a crucial to yield highly crystalline carbon nanowires. The higher crystallinity or a lower amorphous nature. tensile stress on the cross-section of the ﬁbers becomes Although the alignment of ﬁbers demonstrates the pos- more pronounced with decreasing diameters of the sibility of developing tension to improve crystallinity, the ﬁbers, so we also exploited thinning the wires to further microstructure of this carbon is still glassy in nature based improve the graphitic structure. The effect of the on the results (Fig. 2a). nanowire diameter, which is dependent on the applied The formation of glassy carbon during pyrolysis derives voltage in Fig. 1d, on the microstructure and crystallinity from the curved structures of the penta- and hepta- of the carbon nanowires on the silica surface pretreated carbon rings . To implement this alignment process for with nitrogen is illustrated in Fig. 2c, d. Further inves- curved structures, a mechanical treatment that changes tigation into the decrease in wire diameter from axial stress from compressive stress to tensile stress is ~5.05 μmto ~45nm (Fig. 2c, d) reveals an evolution applied to the PAN ﬁbers for a remarkable increase in the from a porous microstructure to a highly crystalline graphitic structure of the resulting carbons. Here, the nanostructure via nanocrystalline structuring. A Raman deformation introduced by aligning the polymer nanoﬁ- spectrometer with a DXR microscope and equipped with bers on a silica surface (Fig. 2a) plays a somewhat similar a 532 nm excitation laser was selected to evaluate the role to that of mechanical treatment, as described by crystallinity of the microstructures in the carbon nano- Maziar et al. . The resulting strong attachment of the wires fabricated at 1200, 500, and 35 V. The intensity polymeric nanoﬁber to the silicon substrate limits radial ratio of 0.78 in Fig. 2d is much lower than that of 1.41 in shrinkage during stabilization and carbonization, thus Fig. 2d, corresponding to the higher degree of crystal- forming tension stress in the axial and radial directions. linity of the nanocrystalline microstructure. This Upon increasing the rotational speed of the drum, the improvement in crystallinity may be attributed to the bond of the nanoﬁber to the silica surface becomes strong increase in tensile stress in the axial direction due to the enough to counterbalance the tension developed during decrease in diameter from ~5.05 μm to ~305 nm pyrolysis. Atomic force microscopy allows for a detailed (Fig. 2c). Upon further reduction of the carbon nanowire study of the resulting carbon wire microstructure. Visual diameter from ~305 nm to ~45 nm (Fig. 2c), the micro- examination of the atomic-resolution micrographs illus- structure is transformed into an even more crystalline trates the evolution of the developed carbon wire micro- microstructure, which is gleaned from the decrease in structures. Since these two forces have opposite effects on I /I from 0.78 to 0.64 (Fig. 2d). Figure 2d illustrates the D G the radial shrinkage of ﬁbers, polymer ﬁbers are trans- correlation between the crystallinity of the micro- formed into nanograin-based carbon wires (AFM phase structures and the applied voltage. Clearly, the polymer images on the left side of Fig. 2a). The resulting intensity ﬁbers obtained at the lowest voltages result—after pyr- ratio of 1.38 is much higher than 0.96 and 1.04, thus olysis—in the highly crystalline carbon nanowires owing poorly crystalline carbon wires are formed. to their highly graphitized structures. Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 7 of 13 Effect of decyanation on crystalline structuring impurity elements during carbonization (Fig. 3a) resulted The effect of tension stress on crystalline structuring is in the formation of carbon sheet-like layers from carbon- accompanied by decyanation reactions (named stress like ribbons . To correlate the inﬂuence of the deni- decyanation). A previous report showed that relying on trogenation and decyanation reactions with the graphitic denitrogenation and decyanation reactions to remove structure, Raman spectroscopy, and X-ray photoelectron 40% 30% N NN N CH N N N CH N N OH –N –HCN N NN NN N Carbonization 20% b c N 1s 33,000 N 1s 22,000 N N O NH NH 20,000 30,000 18,000 50% 50% 70% 27,000 30% 16,000 24,000 14,000 396 399 402 405 396 398 400 402 Binding energy (eV) Binding energy (eV) de N 1s N 1s N N O N O NH 67% 82% 34% 18% 398 399 400 401 396 400 404 408 Binding energy (eV) Binding energy (eV) fg N 1s 1.5 N O N 1.0 5% N 95% 0.5 0.0 396 399 402 405 0 24 48 72 Binding energy (eV) C (%) NH Fig. 3 Effect of the decyanation reaction on the crystalline structuring of wires. a Model reaction path from stabilized PAN to carbon. N 1 s X-ray photoelectron spectroscopy (XPS) spectra of b disordered carbon wires at 1200 V and of arrayed carbon nanowires at c 1200 V, d 500 V, and f 35 V with pretreatment of the silica surface in a nitrogen environment at 1000 °C. e N 1 s XPS spectra of arrayed carbon nanowires at 35 V in the absence of nitrogen pretreatment of the silica surface. g I /I as a function of the intensity of the acridine rings (C ); n >5 D G NH Counts per second Counts per second Counts per second Counts per second I /I D G Counts per second Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 8 of 13 spectroscopy characterization was performed. The full N scan rate or the EIS frequencies. Increasing the frequency 1 s spectra presented in Fig. 3b–f demonstrate two typical to the characteristic value ω = D/d , where D is the dif- peaks that are assigned to nitrogen atoms in the acridine fusion coefﬁcient of the analyte and d is the distance 34,35 ring and nitrogen atoms in the naphthyridine and between neighboring electrodes , causes the transition hydronaphthyridine rings. For the porous microstructure from overlapping (Fig. 4a) to nonoverlapping (Fig. 4b) of carbon wires with a diameter of ~4.3 µm (Fig. 2c), the diffusion hemispheres. By further increasing the fre- corresponding N 1 s spectrum in Fig. 3c has two com- quency to the characteristic value ω = D/a , where a is the ponents, which are assigned to nitrogens in the acridine radius of a single disc electrode, a planar diffusion regime ring bonds (50%) and in the naphthyridine and hydro- is obtained; thus, a single electrode can be taken as a naphthyridine rings (50%). In comparison with that of planar electrode. In the interval given by these two carbon wires fabricated at 1200 V (Fig. 2a), the intensity of characteristic frequencies, the electrode array shows the acridine rings is weakened from 70% (Fig. 3b) to 50%, steady-state behavior with a sigmoidal cyclic voltammetry revealing a more extensive decyanation reaction for the curve. carbon wires in an array fabricated at 1200 V. With Cyclic voltammetry (CV) curves of the ferri/ferrocya- 3−/4− copious decyanation, most of the reaction products that nide ([Fe(CN) ] ) redox couple in aqueous solutions leave the carbonizing ﬁber are in the form of gases, have often been used as a reference method to evaluate including HCN, H O, O ,H , CO, NH , and CH , ulti- the electrochemical performance of a carbon elec- 2 2 2 3 4 20,36,37 mately leading to a porous microstructure. The resulting trode . The CV curve from the disordered carbon intensity ratio of 1.41 (Fig. 2d) is much higher than the wire mat in Fig. 2a shows macroelectrode behavior with 0.96 in Fig. 2a but is comparable to the 1.38 from anodic and cathodic peak currents in Fig. 4c, indicating nanograin-based structuring, demonstrating that defects partially overlapping diffusion hemispheres. Aligning the in the carbon wire reduce the degree of crystallization. disordered carbon wires to a certain distance (~2.5 μm) at After the decrease in the nitrogen intensity of the 1200 V results in a typical steady-state sigmoidal vol- acridine ring from 34 to 5% (Fig. 3d, f), the resulting tammogram with a steady-state current in Fig. 4d. This intensity ratio (I /I ) decreases from 0.78 to 0.64 (Fig. 2d), demonstrates the dependency of the nonoverlapping-to- D G demonstrating the possibility of enhancing the decyana- overlapping diffusion regime on the distance of carbon tion reactions to improve the crystallinity. In the absence wires in the characteristic frequency of ω = D/d .By of nitrogen pretreatment of the silica surface, the resulting reducing the carbon wire diameter further from ~4.3 µm increase in the nitrogen intensity of the acridine ring from to ~512 nm, the interwire spacing further widens, leading 5 to 18% (Fig. 3e, f) demonstrates that the presence of to a decrease in the characteristic frequency (ω = D/d ). nitrogen at the surface facilitates more extensive decya- The resulting CV curve in Fig. 4e still shows steady-state nation reactions during carbonization, which leads to the behavior but has a higher steady-state current compared improved crystallinity of carbon wire; this is similar to the to that of carbon wires arrayed at 1200 V, which is likely results shown in Fig. 2a, b. Regardless of the porous derived from increasing the resistance of the carbon wire defects in Fig. 2c, the effect of I /I on the intensity of the due to the decrease in diameter. Theoretical and experi- D G acridine rings indeed reveals the improved crystallinity of mental work on the diffusion of microelectrode arrays the carbon wire with more extensive decyanation reac- show that, in the case of a scan rate of 100 mV/s for d ≥ tions, as shown in Fig. 3g. 100a, the dominating mode of diffusion is determined by Theeffectofnanowirediameterand decyanation on the transition between the planar and hemispherical dif- crystalline structuring reveals further insight into the fusion layers, leading to a sigmoidal shape in the CV underlying mechanism of graphitization at relatively low curve. Clearly, the transition from the overlapping of the temperatures (1000 °C). Several previous studies have individual diffusion layers to planar diffusion layers over demonstrated that enhancing the degree of graphitiza- the entire electrode array in Fig. 4f is very distinct for a tion can, to a large extent, improve the physical and highly crystalline carbon nanowire array. The weakening chemical nature of carbon nanowires . Thus, the of peak currents in the CV shows a monotonically improvement in crystallinity with extensive decyanation increasing straight line (Fig. 4f), which is expected due to reactions provides an attractive pathway for tailoring the the large resistance in the thin carbon nanowires (Fig. 5d). effect of a crystalline surface structure on electro- Lowering the applied voltage during NFES increases the chemical properties. speciﬁc surface area of carbon nanowires and subse- quently improves the degree of graphitization, thus Electrochemical characterization using cyclic voltammetry inﬂuencing the peak currents of carbon wires, as shown in In the case of very long experimental and theoretical Fig. 4h. The nanocrystalline-structured wires at 500 V time scales, hemispherical diffusion to microdisk elec- clearly display at least a fourfold higher current than the 30–33 trodes shows diffusion regimes depending on the CV porous-structured microwires at 1200 V and the highly Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 9 of 13 ab Silica Electrolyte solution Electrolyte solution Carbon fiber Silicon substrate Silicon substrate cd –35 –80 –70 –1.0 –0.5 0.0 0.5 1.0 –1.5 0.0 1.5 3.0 Potential (V vs Ag/AgCI) Potential (V vs Ag/AgCI) ef –90 –4 –180 –8 –6 –3 0 3 6 –1.2 –0.6 0.0 0.6 1.2 Potential (V vs Ag/AgCI) Potential (V vs Ag/AgCI) gh 0 300 600 900 1200 0 300 600 900 1200 Applied voltage (V) Applied voltage (V) Fig. 4 Electrochemical characterization of crystalline carbon nanowire arrays based on cyclic voltammetry. Schematics showing the diffusion of carbon nanowires in the case of a overlapping and b nonoverlapping diffusion hemispheres. Cyclic voltammetry (CV) curves of 5 mM K Fe(CN) / 4 6 5mM K Fe(CN) in 0.1 M phosphate buffer (pH 7.4) (1:1 mixture) for carbon wire mats disordered at c 1200 V and arrayed at d 1200 V, e 800 V, and 3 6 f 35 V, respectively; n >8. g Surface area and h peak current density of a carbon nanowire array as functions of applied voltage during NFES; n >8 crystalline nanowires at 35 V. The higher surface area nanowire array at 100 V shows the smallest surface area in derived from the appearance of the nanocrystalline Fig. 4g but has a peak current equivalent to that of the structure on the surface of the carbon nanowires is well carbon nanowire array at 1200 V (Fig. 4h). This is most correlated with the highest peak current of the carbon likely due to the highly graphitized structure increasing nanowire arrays at 500 V. The highly crystalline carbon the electrical conductivity . Current (µA) 2 Current (µ µA) Surface area (mm ) Current (µA) Peak current density (µA/mm ) Current (µA) Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 10 of 13 a b Disordered carbon fibers 1200 V Clustered and aligned 800 V carbon fibers 140 5 200 400 600 800 0 10 20 30 Z (real) (ohms) Z (real) (kohms) cd 300 V 100 V ct 35 V s 200 dl 0 100 200 300 0 400 800 1200 Z (real) (kohms) Applied voltage (V) Fig. 5 Electrochemical characterization of crystalline carbon nanowire arrays based on impedance spectroscopy. Impedance spectra of the a disordered carbon mat at 1200 V and clustered and aligned carbon ﬁber mat at 1200 V along with the near-ﬁeld carbon ﬁber arrays at b 1200 and 800 V and c at 300, 100, and 35 V. R , R , and C in c represent the solution resistance, electron transfer resistance, and double layer capacitance, s ct dl respectively. AC amplitudes of 1000 mV are applied to the carbon nanoﬁbers in (a–c). Impedance spectra of near-ﬁeld carbon ﬁber arrays at 35 V for AC amplitudes of 1000 and 25 mV; n >6. d Electron charge transfer resistance as a function of the applied voltage during NFES; n >10 By arraying all the carbon nanowires and maintaining a Electrochemical characterization using impedance spectroscopy wire-to-wire spacing of ~2.5 μm, the impedance spectrum Electrochemical impedance spectroscopy (EIS) of the shows two semicircles connected together at high fre- carbon nanowire array was performed using a solution of quencies and a relatively short straight line with a similar 5mM K Fe(CN) /5 mM K Fe(CN) in a 0.1 M phosphate slope (diffusion limitation) at the lowest frequencies, as 4 6 3 6 buffer (pH 7.4). Impedance spectra were obtained at fre- shown in Fig. 5b. Theoretical and experimental work on quencies from 0.1 Hz to 1000 kHz and at a 0 V dc potential the impedance of ultramicroelectrode arrays exhibit a vs. Ag/AgCl. The impedance spectra of the disordered car- semicircle at low frequencies in addition to the well- bon wire mat and clustered and aligned carbon wire mat are known semicircle at higher frequencies, demonstrating showninFig. 5a. Monotonically increasing straight lines overlap of the diffusion gradients. The change from with similar slopes at low frequencies are observed, reﬂect- overlapping diffusion to diffusion limitation is shown for ing a diffusion-limited process with the overlapping diffusion carbon nanowire arrays at 1200 V. This demonstrates the ﬁelds of all the participating wires. Upon the formation of weakening of the overlap of the diffusion gradients com- the wire-to-wire distance derived from clustered and aligned pared to that of the clustered and aligned carbon wires. carbon wires in Fig. 2a, the impedance spectroscopy in By reducing the applied voltage from 1200 to 800 V, Fig. 5a shows a quarter circle at high frequencies and a which is accompanied by a decrease in the wire diameter straight line at low frequencies. This is due to the weakening from ~4.3 µm to ~512 nm, the impedance spectrum of the diffusion gradients (f) according to Eq. (3): changes to a large semicircle with a small straight line, as shown in Fig. 5b. The semicircle diameter at higher fre- 4α ~ quencies is governed by the charge transfer resistance ð3Þ f ¼ πd (R ) related to the electron transfer rate of the redox et species on the electrode surface. The region at low fre- where f is the diffusion gradient in front of each individual quencies again represents a line with a similar slope, –ultramicroelectrode and ᾶ denotes the arithmetical showing mass transfer control or a diffusion limitation. average of the oxidation and reduction diffusion coefﬁ- Previously, in the case of three-dimensional hemispherical 3−/4− 31–34 cients of [Fe(CN) ] . diffusion to an array of microdisk electrodes , the Z (imaginary) (kohms) Z (imaginary) (ohms) Electron charge transfer resistance (kohms) Z (imaginary) (kohms) Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 11 of 13 impedance spectrum showed a semicircle for frequencies theoretical and experimental work on carbon microelec- between the characteristic frequencies of ω = D/d and trodes that the smaller the electrode surface area is, the 2 41 ω = D/a . The transition from overlapping diffusion to greater the charge transfer resistance . The monotonic three-dimensional hemispherical diffusion at high fre- correlation between the charge transfer resistance and quencies is shown to observe the changes between the wire diameter in Fig. 5d is expected in accordance with impedance spectra of carbon nanowire arrays at 1200 and the results of carbon microelectrodes. Clearly, by chan- at 800 V. The impedance spectrum of the carbon nano- ging the wire diameter in the carbon nanowire arrays, the wire array at 800 V is expected due to the lower density of semicircle scale at high frequencies can be adjusted. 20 14 arrayed carbon wire , which is derived from widening the A previous study on the correlation between micro- wire-to-wire distance with decreasing wire diameter. structures and properties showed that enhancing graphi- Reducing the diameter of the arrayed carbon wire from tic microstructures could improve the bulk characteristics ~512 to ~40 nm with decreasing applied voltage from 800 of carbon fabrics, resulting in an increase in conductivity to 35 V results in changes in the impedance spectrum from ~200 S/m to ~5000 S/m with improving graphiti- from a large semicircle with a small straight line to just a zation. The charge transfer resistance of highly crystalline single semicircle without a Warburg impedance element carbon nanowire arrays at 35 V is unusually low and is (Fig. 5c). This case allows for the process from the over- half of that of carbon nanowire arrays at 100 V (Fig. 5d). lapping diffusion of all the participating wires to the This is most likely due to the further improvement of the complete reaction kinetics determined by electron trans- graphitic microstructure as the voltage is decreased from fer. Theoretical work has shown that a smaller electrode 100 to 35 V (Fig. 2d). Thus, the combination of these radius leads to a larger electrode admittance per unit area, factors, including the wire-to-wire spacing, nanoscale which allows the susceptance at low frequencies to remain diameter, and graphitic microstructure, leads to a distinct negligible. In addition, decreasing the nanowire diameter semicircle of highly crystalline carbon nanowire arrays increases the lower frequency limit at which the suscep- in EIS. tance (imaginary part of the impedance) becomes lower than the conductance (real part of the impedance). Thus, Conclusion the impedance spectrum of the highly crystalline carbon In the state-of-the-art fabrication of carbon nanowires nanowire array at 35 V shows a semicircle for ω = D/a » with the carbon-nano-electro-mechanical system D/d . These results demonstrate the ability to control the approach, the growth of a graphitic microstructure is typically limited to the use of a Ni catalyst. In this work, diffusion regimes by altering the wire spacing and nanowire size. we demonstrated a novel catalyst-free fabrication process In most studies, the semicircle spectrum from carbon for arrays of highly graphitized carbon nanowires with nanowire arrays is meaningful only when ac signals with various surface nanostructures. It was developed by 38,39 amplitudes ≤25 mV are applied . By reducing the minimizing the polymer ﬁber diameter with ultralow- amplitude from 1000 mV to 25 mV, the impedance voltage NFES. The linear speed of the spinneret and the spectrum of the highly crystalline carbon nanowires in rotational speed of the collector allowed for better control Fig. 5c still shows a single semicircle without scattered of the ﬁber-to-ﬁber distance in the ﬁber arrays. The sta- data points, demonstrating the high signal-to-noise ratio bilization at 115 °C and subsequent carbonization con- compared to that of typical carbon nanowire electrode verted these polymer nanoﬁbers arrayed on a silica surface arrays . Therefore, highly crystalline carbon nanowire into carbon nanowires. The thickness control of carbon arrays can be selected as potential candidates for DNA nanowires was conveniently adjusted by tuning the switching . applied voltage during NFES. Aligning the wires on a The impedance spectra were ﬁtted by means of an nitrogen-pretreated silica surface allowed for stress equivalent Randles circuit with capacitance, charge decyanation to improve the graphitic content of the car- transfer resistance, solution resistance, and Warburg ele- bon nanowires. The presented diameter control showed ments. The charge transfer resistance related to the the ability to transform a porous-microstructured carbon overall surface area of the carbon nanowire arrays corre- wire into a highly crystalline nanostructure via nano- sponds to the diameter of the semicircle in the high- crystalline structuring. frequency domain of the impedance spectra. Figure 5d Various electrochemical behaviors were observed in the shows the increase in charge transfer resistance as the obtained CV curves when reducing the wire diameter applied voltage is decreased from 1200 to 100 V; notably, because that altered the structure on the surface of the this is accompanied by a decrease in the wire diameter carbon wire and improved the graphitic structure. The from ~4.3 µm to ~120 nm and the microstructural typical CV curve showing macroelectrode behavior with transformation from a porous microstructure to a nano- anodic and cathodic peak currents was altered to a sig- crystalline microstructure. It is well known from moidal CV curve with the steady-state current dominated Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 12 of 13 by radial diffusion. This is a characteristic behavior In general, the graphitic nature of PAN-derived carbons observed for carbon nanowire arrays where there is no was evaluated based on Raman spectroscopy, a standard overlap of the diffusion hemispheres from neighboring nondestructive analysis tool. The Raman spectrometer electrodes. Compared to three-dimensional carbon nano- with a DXR microscope (Thermo Fisher) and equipped wire electrode arrays with wire diameters of ~100 nm, with a 532 nm excitation laser assessed the corresponding two-dimensional carbon nanowire arrays with diameters carbon wire arrays. By moving the platform on which the sample was located, the wire was moved to a location just of ~512 nm easily obtained low steady-state currents during cyclic voltammetry, which has been regarded as a below the marked laser spot. Linear scanning was potential advantage for biosensing applications. –The implemented with a step size of 100 nanometers. For each nanocrystalline structure on the surface of graphitized step, data were collected by exciting the laser, ultimately carbon wires rendered extremely high peak currents in resulting in obtaining the Raman spectrum of a carbon their CV curve. More importantly, highly crystalline car- nanowire. We used the lens for an average of approxi- bon nanowire arrays showed a linear CV curve without mately 20 times per sample. To correlate the inﬂuence of anodic and cathodic peak currents, representing the spe- denitrogenation and the decyanation reaction on the cial characteristics of the highly graphitized structures in graphitic structure, X-ray photoelectron spectroscopy was the nanoscale carbon wires. The charge transfer resistance performed. could be controlled by varying the diameter of the carbon Atomic force microscopy (AFM) (Bruker Dimension nanowires. In the EIS spectra of the highly crystalline Icon) and scanning electron microscopy (SU8220) carbon nanowire arrays, the abnormal decrease in the allowed for a detailed study of the surface nanostructures charge transfer resistance conﬁrmed the inﬂuence of the in the resulting carbon ﬁbers. The topology of carbon graphitized structure on the electrochemical behavior. ﬁbers was studied with the tapping mode to investigate The electrochemical performance of the crystalline the inﬂuence of the applied voltage on various surface carbon nanowire arrays with various surface nanos- nanostructures. tructures makes them potential candidates for biochem- A traditional three-electrode conﬁguration consisting of ical sensors with lower detection limits and as devices for an Ag/AgCl reference electrode, Pt counter electrode, and electrochemical energy storage. Graphitized carbon sub- carbon working electrode was used. To probe the carbon/ microwires with a nanocrystalline structure could allow electrolyte interface, a GPSTAT12 potentiostat/galvano- for ampliﬁed biosensing via redox cycling and enhanced stat equipped with a frequency response analyzer module capacitive energy storage in microsupercapacitors. The was employed. Cyclic voltammetry (CV) of the ferri/fer- 3−/4− linear CV curve of highly crystalline carbon nanowires rocyanide ([Fe(CN) ] ) redox couple in aqueous shows the ability to permit stable data collection for solutions have often been used as a reference method to ultrasensitive biological detection. In particular, a further evaluate the electrochemical performance of a carbon 20,36,37 increase in the surface area of –nanocrystalline-structured electrode . carbon wires could allow for high-power supercapacitors. Electrochemical impedance spectroscopy (EIS) of the carbon nanoﬁber array was performed using a solution Materials and methods containing 5 mM K Fe(CN) /5 mM K Fe(CN) in a 0.1 M 4 6 3 6 The electrospinning solution (9% PAN) was prepared by phosphate buffer (pH 7.4). Impedance spectra were taken dissolving PAN (150000 mw, Sigma Aldrich, St. Louis, at frequencies from 0.1 Hz to 1000 kHz and at a dc MO) in DMF. Using vortex mixing at 30 RPM, mixtures of potential of 0 V vs. Ag/AgCl. The number of experiments PAN/DMF were allowed to freely diffuse at different (n) is indicated in the ﬁgure legends. temperatures. For the NFES experiments, we used a 3 mL Acknowledgements syringe mounted on a syringe pump to dispense the This research was supported by the National Key R&D Program of China highest conductivity ink at a feed rate below 10 nL/min. (2020YFB2009002) and the National Natural Science Foundation of China (51875084). The support provided by the China Scholarship Council (CSC) Silicon substrates coated with silicon dioxide were during the visit of Jufeng Deng to the University of California, Irvine is mounted in the grooves and convex areas on the drum acknowledged. with carbon tape. The NFES voltage was applied between the dispensing needle and the grounded drum. It should Author details School of Mechanical Engineering, Dalian University of Technology, Dalian be noted that regular arrays were achieved as long as ω ≥ 116023, China. Mechanical and Aerospace Engineering, University of 400 RPM and ν ≥ 80 µm/s were achieved at the same time. 3 California, Irvine, CA 92617, USA. Chemical and Biomolecular Engineering, Fabrication of carbon nanowire arrays derived from PAN University of California, Irvine, CA 92517, USA. School of Engineering and Science, Tecnologico de Monterrey, Monterrey, NM 64849, Mexico nanoﬁber arrays consisted of oxidization in the air at 115 °C (named stabilization) and subsequent carbonization Author contributions in a furnace with an inert nitrogen atmosphere at 1000 °C JD, CL, DS, and MM conceived the project. JD, CL, DS, and MM developed the (heating ramp rate of 15 °C/min) (Fig. 1b). process for growing carbon nanowires. JD, CL, DS, and MM interpreted the Deng et al. Microsystems & Nanoengineering (2022) 8:15 Page 13 of 13 data and developed the ﬁgures. JD, CL, DS, and MM wrote the manuscript. JD 19. Dotivala, A. C.,Puthuveetil, K.P. & Tang,C.Shear force ﬁber spinning: process transferred and prepared the AFM samples. JD performed the AFM parameter and polymer solution property considerations. Polymer 11,294 characterization and data analysis. JD performed the AFM, XPS, CV, and EIS (2019). characterization tests and the data analysis. JD, CL, DS, and MM edited the 20. Siddiqui, S.,Arumugam, P. U.,Chen, H.,Li, J. &Meyyappan, M.Characterization manuscript. All authors discussed the data and contributed to the ﬁnal of carbon nanoﬁber electrode arrays using electrochemical impedance manuscript. spectroscopy: effect of scaling down electrode size. ACS Nano 4,955–961 (2010). 21. Liu, J. et al. Study on the oxidative stabilization of polyacrylonitrile ﬁbers by Conﬂict of interest microwave heating. Polym. Degrad. Stab. 150,86–91 (2018). The authors declare no competing interests. 22. Khayyam, H. et al. PAN precursor fabrication, applications and thermal stabi- lization process in carbon ﬁber production: experimental and mathematical Received: 29 July 2021 Revised: 25 November 2021 modelling. Prog. Mater. Sci. 107, 100575 (2020). Accepted: 3 December 2021 23. Zhao, R.-X. et al. Inﬂuence of heating procedures on the surface structure of stabilized polyacrylonitrile ﬁbers. Appl. Surf. Sci. 433,321–328 (2018). 24. 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Fabrication of crystalline submicro-to-nano carbon wire for achieving high current density and ultrastable current

Fabrication of crystalline submicro-to-nano carbon wire for achieving high current density and ultrastable current

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Fabrication of crystalline submicro-to-nano carbon wire for achieving high current density and ultrastable current

Fabrication of crystalline submicro-to-nano carbon wire for achieving high current density and ultrastable current

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