Formation and Characterization of Hollow Microtubes by Thermal

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Formation and Characterization of Hollow Microtubes by Thermal Treatment of Human Hair Dong Su Im, Min Hee Kim, Hyeong-Seop Jung, and Won Ho Park* Department of Advanced Organic Materials and Textile Engineering System, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, South Korea

ABSTRACT: In this study, hollow microtubes were prepared via stepwise thermal treatment of human hair waste in the middle temperature range from 200 to 450 °C. The effect of the treatment temperature on the thermal decomposition of the hair was examined in terms of the dimension and wall thickness of the microtubes. The thermal decomposition behavior of the hair was analyzed via differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The structure and morphology of the hollow microtubes were examined using scanning electrochemical microscopy (SEM) and elemental analysis (EA). The thermal and morphological observations reveal that the thermal decomposition of the hair started from the core medulla section with the melting of the α-helical structure in the cortex at about 250 °C. As the heat treatment temperature further increased to 450 °C, the degradation of the cortical structure expanded to the outside, and the wall thickness of the hollow microtube decreased. Finally, the hollow microtube structure was completed, and the scaled structure in the cuticle layer became compact and smooth. Therefore, the hollow microtube structure might be a result of the melting and degradation of the cortex and medulla. The adsorption capacity of the hollow microtubes was also evaluated using n-hexane and water. KEYWORDS: Human hair, Hollow microtube, Medulla, Cortex, Cuticle, Wall thickness

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INTRODUCTION Hair mainly exists in the skin of vertebrates, including both humans and animals. It is a keratinous protein that is arranged in filaments.1 In most vertebrates, hair increases the sensitivity of the skin surface and forms an insulative, protective coating layer. Human hair is periodically shaved away because it grows constantly in the growth process of mammals.2,3 The hair has a composite structure, and it is mainly divided into three regions. The outermost region is the cuticle layer that is a thick sheath of several cell-like scales. The cuticle tightly protects the cortex, accounting for most of the hair volume that contains components such as lipids, keratins, and melanin. The third region is the medulla, which is close to the center of the hair. In human hair, there are various cells piled up leaving large empty spaces in the medulla. The cortex has a lot of micro- and macrofibril structures, such as the macrofibril, intermediate filament, inter-macrofibrillar matrix, and so on (Figure 1A).4 An enormous amount of human hair is treated as waste, and it can be easily obtained from barber shops or beauty salons. Therefore, using waste hair is very advantageous from an © XXXX American Chemical Society

economic and practical perspective. Waste hair can have various uses due to its fundamental properties, such as slow degradation, high tensile strength, high thermal insulation, and high elastic recovery.5−7 It has been utilized as a fertilizer due to its high nitrogen content (about 16%).8 Also, it has been used as a clay-reinforcing material for construction due to its high tensile strength and friction coefficient.9 In particular, hair is known to provide a life extension to construction by protecting against cracks.10,11 Furthermore, the applicability of hair can increase through various modifications including pyrolysis. Waste hair could be applied for the synthesis of functional carbon-rich materials via pyrolysis because it contains more than 50% carbon. Pramanick and co-workers obtained hollow carbon fibers from human hair after pyrolysis at 900 °C. The diameter of hair significantly decreased during the pyrolysis, and the resultant carbon-rich fibers had a hollow Received: January 11, 2018 Revised: February 20, 2018 Published: March 14, 2018 A

DOI: 10.1021/acssuschemeng.8b00099 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. (A) Schematic structure of the hair fiber composed of macrofibrils, microfibrils, intermediate filaments, tetramers, and keratin proteins;4 (B) Preparation procedure of hollow microtubes via heat treatment.

structure with wall thickness of 2−4 μm.12 The tubular morphological structure with high carbon content demonstrates that the hair waste may be useful as a valuable industrial functional material for oil and chemical absorption. For example, carbonized hair, which was obtained via pyrolysis at 900 °C in N2 atmosphere, can be used in electrochemical applications including electrodes, due to its high electrical conductivity and good chemical stability.12,13 The adsorption capacity of hollow microtubes can be improved by the chemical activation process with H3PO4, ZnCl2, and K2CO3, which provide a highly porous structure.14 The formation of a tubular structure from human hair via thermal treatment was first reported by Kim.15 The tubular structure was observed from human hair when it was thermally treated under inert gas at the intermediate temperature range 200−450 °C. Kim studied various fibrous ash-forming biological materials such as mushroom grills, cotton, silk fiber, spider silk, dog hair, and human hair. Among the fibrous materials, ashes from dog hair and human hair form tubes. He suggested that the medulla in human hair might be responsible for tube formation.15 The pyrolysis mechanism of human hair is very complex because it is composed of three major components (cuticle, cortex, and medulla), and each component has a different structure depending on the composition and sequence of 20 amino acids. During the pyrolysis, the crystalline melting and partial degradation were also concurrent in the keratin fibers, resulting in destroying the unique protein structure and fibrous nature.16 Recently, Istrate and co-workers reported that keratin microtubes could be obtained by heating keratin fibers at a temperature from 230 to 300 °C under a nitrogen atmosphere.17 They demonstrated that the resulting hollow tubes were caused by significant differences in the thermal stability between the cortex and the cuticle. The differences in thermal behavior in spite of the

similar amino acid compositions of the cortex and cuticle are interesting. Although there were the above fundamental findings on the formation of hair microtubes, the formation and further progress of hollow microtubes from human hair need to be systematically established via a stepwise thermal treatment. For this, we first investigated the role of medulla cells, which are loosely packed in the core of human hair, on the initial formation of hollow microtubes in the heating temperature range 200−300 °C, at which the crystalline melting and partial degradation coincided. Next, the dimensional change of hollow microtubes was examined in the heating temperature range 300−450 °C because the difference in thermal stability between the cortex and the cuticle steadily affected the hollow structure of microtubes. The morphology and dimension of hair microtubes were particularly interpreted in terms of the wall thickness. Also, we proposed a formation and progress mechanism of hollow microtubes from hair via thermal treatment. Finally, the adsorption capacity of the microtubes was evaluated using reference liquids.

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EXPERIMENTAL SECTION

Materials. Hair from males in their early twenties was supplied from a hair salon. Hair was purified by washing with ethanol (99.5%, Samchun Chemicals) for 30 min and was then dried in an oven at 60 °C for 24 h. Merino wool (Hanshinwool Co.) was used as a reference sample. n-Hexane (95%, Sanchun Chemicals) and distilled water were used to evaluate the oil and water absorptivity (%) of the hollow microtubes, respectively. Preparation of the Hollow Microtubes via Thermal Treatment. Dried hair (1 g) was heated to a specific temperature (200−450 °C) at a healing rate of 5 °C/min under an argon (Ar) atmosphere and was then held for 2 h in the furnace. The thermal treatment process is presented in Figure 1B. After the thermal treatment, the morphology and composition of the hair microtubes were examined, B


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ACS Sustainable Chemistry & Engineering and the sample codes and thermal treatment conditions are summarized in Table 1.

part. The inner part of the broken microtubes shows that the degradation started at the core and expanded in a circular shape to the outside direction, mainly in the cortex. Also, circular degradation occurred in irregular periods through the hair axis.17 In H350, a hollow structure was clearly observed, and the wall thickness of the hair decreased. In particular, the crosssection of the hollow structure was similar to the cross-section of the bamboo joint. In H450, the residual scaled surface of the cuticle layer became smooth, and the complete hollow structure without a joint formed with a further thinning of the wall thickness. The hollow structure was reduced to ashes with a thermal treatment above 450 °C. The hair microtube was somewhat friable because the heat treatment involved protein degradation and carbonization. For comparison, Merino wool was heat-treated in an identical temperature range to examine the formation of hollow microtubes. Merino wool is composed of several layers of cuticle covering the cortex layer and does not have a medulla core, unlike human hair.18,19 Degradation was not observed inside the wool in the temperature range 200−450 °C, except that the fiber diameter gradually decreased as the heating temperature increased (Figure 3). This difference in thermal degradation between human hair and wool might be due to the existence of the medulla core. The medulla core with a lower density is composed of air and dead cells. The degradation of human hair started in the medulla, and it subsequently expanded to the outside, resulting in a hollow microtube.17 DSC measurements were conducted to investigate the melting of α-helices and the initial degradation of hair. Figure 4A shows a DSC thermogram of human hair. The broad endothermic peak at 239 °C corresponded to the melting of the α-helical structure in the microfibrils (thermal denaturation of keratin fibers), and the small endothermic peak at 245 °C was due to the decomposition of cystine linkages (the thermal degradation of keratin fibers).17 Therefore, the partial degradation observed in the H250 sample reflected both the melting of α-helical structure and the cystine decomposition in the cortex. The weight loss of hair according to the heating temperature was analyzed using an isothermal TGA thermogram (Figure 4B). All hair samples were treated under identical conditions following a heating profile (heating rate, 5 °C/min; holding temperature and time, 200−450 °C and 2 h). The initial weight loss in all TGA thermograms was about 5% due to the evaporation of water, and the practical degradation of hair occurred at a holding time of about 40 min, irrespective of the heating temperature. The residual weights were 68.2%, 36.9%, and 26.8% at isothermal holding temperatures of 250, 350, and 450 °C for 2 h, respectively. Figure 5A shows the change in the wall thickness according to the heating temperature. The wall thickness gradually decreased from ∼40 to ∼5 μm as the heating temperature increased up to 450 °C, although the hollow structure that formed at a lower heating temperature (200−300 °C) was incomplete. The wall thickness (∼40 μm) of untreated hair was obtained from the distance between the medulla and the outmost cuticle. However, the whole thickness of the hair and the microtube nearly did not change with the heating temperature. Figure 5B shows the relationship between the weight loss and the wall thickness. The wall thickness of the microtube decreased linearly with an increase in weight loss of up to ∼75%. Therefore, the weight loss of hair was closely associated with the wall thickness, and inversely the hollow thickness of the microtube. Also, the wall thickness of the

Table 1. Sample Codes and Heat Treatment Conditions hair type

sample identifier (ID)

human hair

control H200 H250 H350 H450 H500 control wool W200 W250 W350 W450

merino wool

treatment condition (°C, h) untreated 200 °C, 2 250 °C, 2 350 °C, 2 450 °C, 2 500 °C, 2 untreated 200 °C, 2 250 °C, 2 350 °C, 2 450 °C, 2

h h h h h h h h h

Absorptivity Evaluation of Hollow Microtubes. n-Hexane and distilled water were used as reference liquids to evaluate the absorptivity for oil and water, respectively. The hollow microtubes (0.1 g) were immersed in n-hexane or distilled water (5 mL) for up to 10 min, and the absorptivity (%) was obtained using the following equation.

absorptivity (%) = (liquid volume absorbed by sample /initial liquid volume) × 100 For an evaluation of the absorptivity for the n-hexane/water mixed liquids, the hollow microtubes (0.1 g) were immersed in mixed liquids (10 mL) for up to 10 min. Characterization. The surface and hollow structures of the hair and microtubes were observed via scanning electron microscopy (SEM, Hitachi, S-4800). The wall thickness of the microtubes was obtained by measuring the SEM images of at least 50 microtubes using an image analyzer (Scope Eye, VI Technology). The thermal transition of the hair was examined at a heating rate of 5 °C/min using differential scanning calorimetry (DSC, Mettler-Toledo, DSC1). The formation temperature of the hollow microtubes was observed by the exothermic peak generated in the heating temperature range. The thermal degradation behavior according to the heating temperature was also observed by thermogravimetric analysis (TGA, MettlerToledo, DSC1). The weight loss of hair due to the microtube formation during thermal treatment was indirectly obtained by a TGA thermogram measured using an identical condition with the heating profile (heating rate, 5 °C/min; holding temperature and time, 200− 450 °C and 2 h) in the furnace. The change in the D and G bands of the hair and microtubes was analyzed using Raman spectroscopy (LabRAM, HR-800, Jobin Yvon). The change in the elemental composition of the hair and microtubes according to the thermal treatment was measured using an elemental analyzer (Thermo Scientific, EA 1112).

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RESULTS AND DISCUSSION Formation Mechanism of the Hollow Microtubes from Human Hair by Heat Treatment. Figure 2 shows SEM images of the hair and hollow microtubes according to the heating temperature. In the untreated hair (control), a scaled structure of the cuticle was clearly observed on the hair surface, and the cortex and medulla were observed in the hair crosssection. In hair treated at 200 °C (H200), both the surface and cross-section of the hair were very similar to those of untreated hair. However, in hair treated at 250 °C, the scaled structure was blurred, and the cuticle layer became compact; notably the medulla and cortex layer seemed to be partially degraded. The hair sample was broken to closely observe the degraded inner C


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Figure 2. SEM images of surface, cross-section, and inner part of untreated hair (control) and heat-treated hair (H200, H250, H350 and H450) samples (scale bar = 50 μm). The tubular structure was observed in the H250, H350, and H450 samples.

cuticle. As seen in the DSC thermogram, the α-helix melted to flow at above 240 °C and was thermally degraded with an amorphous domain, leading to hair microtubes. The cuticle contains some crystalline β-sheet structures and an amorphous matrix with a higher cross-linking density, and it thus has a higher thermal stability than the medulla and cortex.17 On the contrary, merino wool was only shrunk through a thermal treatment because it contains large amounts of an amorphous domain without the medulla core.20 Therefore, the thermal degradation of the hair led to a hollow microtube structure due to a complex thermal effect depending on the cystine content and the morphological nature of the hair components. Also, the wall thickness of the hollow microtubes gradually decreased

microtube was found to be tuned by controlling the heating conditions. The difference in the thermal degradation of the hair components can be explained by the composition of the amino acids. The medulla, in which the initial degradation began, only has traces of cystine linkages providing the matrix with thermal stability, and it has large amounts of ionic amino acids including glutamic acid, lysine, and leucine, when compared to the cortex.5 Therefore, the initial degradation started at the medulla with a lower thermal stability and higher blank spaces. The cortex neighboring the medulla consisted of large amounts of crystalline α-helical structures and amorphous domains, and it also has a lower cystine content compared to the outmost D


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Figure 4. (A) DSC thermogram of human hair and (B) isothermal TGA thermogram of hair. Isothermal treatment started at the dotted line.

D band indicates an amorphous active band with a low torsion of the sp3 carbon, whereas the G band generally indicates a Raman active band with well-aligned crystallization.26 The appearance of the D and G band demonstrated that the carbonization was partially processed in the H450 sample. Also, the thermal treatment induces a change in the elemental composition of the hair. Figure 8 shows a change in the elemental composition of the hair according to the heating temperature. The hair is composed of about 49% carbon, 17% nitrogen, 21% oxygen, 6% hydrogen, 5% sulfur, and trace amounts of various minerals. The untreated hair contains about 49% carbon, and the elemental composition did not change in the H200 sample because thermal degradation did not occur at that temperature. The oxygen content abruptly decreased in the H250 sample, and the hydrogen and sulfur content decreased with an increase in the carbon content of up to 350 °C. The carbonization through stabilization of hair was predominant at 350 °C, but after that temperature, the carbon content decreased because of a predominant combustion of hair.27 In contrast, the nitrogen content relatively increased during heat treatment because of its higher thermal stability. Absorption Capacity of the Hollow Microtubes. In general, human hair is a good absorbent for oil. Several studies have reported that waste hair could be used to absorb oil spilled at sea.28,29 Hollow microtubes have great potential as oil absorbents because they contain a higher carbon content and a

Figure 3. SEM images of surface and cross-section of untreated wool (control wool) and heat-treated Merino wool (W200, W250, W350, and W450) samples (scale bar = 10 μm). The tubular structure was not observed irrespective of heating temperature.

with an increase in the heating temperature to up to 450 °C. The formation and progress of the hollow microtubes from human hair by thermal degradation are presented in Figure 6. Structural and Compositional Change of Hair via Heat Treatment. Thermal treatment of hair involved both thermal degradation and partial carbonization, which generally occurred in carbon-rich materials.21−23 Figure 7 shows Raman spectra of hair and microtube according to the heating temperature. A broad peak at 382 cm−1 was observed in untreated hair, and it shifted toward the red part of the spectrum with an increase in the heating temperature. The broad peak at the lower range in the spectra corresponded to the cystine (disulfide) linkage of protein. As the heating temperature increased, the cystine linkage was broken, and the peak disappeared. Particularly, two peaks at 1335 and 1546 cm−1 newly appeared in the H450 sample corresponding to the D and G band, which were associated with the carbonization of carbon materials.24,25 The E


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Figure 7. Raman spectra of untreated hair (control) and heat-treated hair (H200, H250, H350 and H450) samples according to heating temperature.

Figure 5. (A) Change in wall thickness of hollow microtube according to heating temperature. (B) Relationship between weight loss (%) and wall thickness of microtube. Figure 8. Change in the elemental composition of untreated hair (UT) and heat-treated hair (H200, H250, H350 and H450) samples according to heating temperature.

partially carbonized structure.30 n-Hexane was chosen as a model compound for oil. Figure 9A shows the oil adsorption capacity of the hair and hollow microtubes according to the heating temperature. Untreated hair showed a maximum absorption capacity of 16.3 g/g at 300 s. The absorption capacities of the H250, H350, and H450 samples were 19.3, 24.4, and 32.6 g/g, respectively, at 300 s. The absorption capacity increased gradually with the heating temperature. This

result might be mainly associated with the formation of microtubes, i.e., increase in the surface area due to decrease in the wall thickness. Figure 9B shows the water adsorption capacity of the hair and hollow microtubes according to the

Figure 6. Schematic illustration on formation of hollow microtube structure by heat treatment of hair. F


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range of temperature from 200 to 450 °C. The thermal decomposition behavior of the hair was confirmed by conducting thermal analyses, including differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The morphology and composition of the hollow microtubes were examined using scanning electron microscopy (SEM) and elemental analysis (EA). The thermal decomposition of the hair started from the medulla core at around 250 °C, and also the cortex was molten at about 250 °C, forming the hollow microtubes. As the heating temperature further increased to 450 °C, the degradation of the inner cortex expanded, and the empty space of the hollow tubes increased, while the cuticle became compact and smooth. Therefore, the formation of the hollow microtubes might be a result of the melting and degradation of the cortex and the medulla. Also, the different thermal stability of the hair components was closely associated with their cystine content. Finally, the adsorption capacity of the microtubes was higher against hexane than water.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +82-42-821-6613. Fax: +82-42-823-3736. E-mail: [email protected]. ORCID

Won Ho Park: 0000-0003-1768-830X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Nuclear R&D Program (NRF2015M2A2A6A03044942) through the National Research Foundation funded by the Ministry of Science, ICT and Future Planning Korea.

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REFERENCES

(1) Yang, F. C.; Zhang, Y.; Rheinstadter, M. C. The structure of people’s hair. PeerJ 2014, 2, e619. (2) Zheljazkov, V. D. Assessment of wool waste and hair waste as soil amendment andnutrient source. J. Environ. Qual. 2005, 34, 2310− 2317. (3) Cavello, I. A.; Hours, R. A.; Cavalitto, S. F. Bioprocessing of ″Hair Waste″ by Paecilomyces lilacinus as a source of a bleach-stable, alkaline, and thermostable keratinase with potential application as a laundry detergent additive: Characterization and wash performance analysis. Biotechnol. Res. Int. 2012, 2012, 369308. (4) Cruz, C. F.; Costa, C.; Gomes, A. C.; Matama, T.; Paulo, A. C. Human hair and the impact of cosmetic procedures: A review on cleansing and shape-modulating cosmetics. Cosmetics 2016, 3, 26. (5) Robbins, C. R. Chemical composition of different hair types-In Chemical and physical behavior of human hair; Springer, 2012; pp 105− 176; DOI: 10.1007/978-3-642-25611-0. (6) Aboushwareb, T.; Eberli, D.; Ward, C.; Broda, C.; Holcomb, J.; Atala, A.; Van Dyke, M. A keratin biomaterial gel hemostat derived from human hair: evaluation in a rabbit model of lethal liver injury. J. Biomed. Mater. Res., Part B 2009, 90B, 45−54. (7) Srinivasan, B.; Kumar, R.; Shanmugam, K.; Sivagnam, U. T.; Reddy, N. P.; Sehgal, P. K. Porous keratin scaffold−promising biomaterial for tissue engineering and drug delivery. J. Biomed. Mater. Res., Part B 2010, 92B, 5−12. (8) Zheljazkov, V. D.; Silva, J. L.; Patel, M.; Stojanovic, J.; Lu, Y.; Kim, T.; Horgan, T. Human hair as a nutrient source for horticultural crops. HortTechnology 2008, 18 (4), 592−596. (9) Ahmad, S. Preparation of eco-friendly natural hair fiber reinforced polymeric composite (FRPC) material by using of polypropylene and fly ash: a review. Int. J. Sci. Eng. Res. 2014, 5 (11), 969−972.

Figure 9. (A) n-Hexane absorption capacity and (B) water absorption capacity of hollow microtubes with various heating temperatures. (C) Absorption capacity of a hollow microtube in mixed solution.

heating temperature. Untreated hair showed a lower absorption capacity (8.0 g/g) for water than that (16.3 g/g) for oil at 300 s. All the hollow microtubes also had a lower absorption capacity, and the H450 sample had the highest adsorption capacity of 20.0 g/g because of its higher surface area per weight. Figure 9C represents the adsorption capacity for the mixture (1/1, v/ v) of oil and water for 10 min. The difference in the absorption capacity between oil and water became higher in the mixture solution, indicating that the hair and microtubes preferentially absorb oil from the mixture. Therefore, the hollow microtubes have a great potential as oil absorbents.

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CONCLUSIONS This study examined the formation procedure of hollow microtubes from human hair via heat treatment in the middle G


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ACS Sustainable Chemistry & Engineering (10) Gupta, A. Human hair “Waste” and its utilization: Gaps and possibilities. J. Waste Manage. 2014, 2014, 1−17. (11) Akhtar, J.; Ahmad, S. The Effect of Randomly Oriented Hair Fiber on Mechanical Properties of Fly-Ash Based Hollow Block for Low Height Masonry Structures. Asian Journal of Civil Engineering 2009, 10 (2), 221−228. (12) Pramanick, B.; Cadenas, L. B.; Kim, D. M.; Lee, W.; Shim, Y. B.; Martinez-Chapa, S. O.; Madou, M. J.; Hwang, H. Human hair-derived hollow carbon microfibers for electrochemical sensing. Carbon 2016, 107, 872−877. (13) Guo, Z.; Zhou, Q.; Wu, Z.; Zhang, Z.; Zhang, W.; Zhang, Y.; Li, L.; Cao, Z.; Wang, H.; Gao, Y. Nitrogen-doped carbon based on peptides of hair as electrode materials for supercapacitors. Electrochim. Acta 2013, 113, 620−627. (14) Chen, W.; Liu, X.; He, R. L.; Lin, T.; Zeng, Q. F.; Wang, X. G. Activated carbon powders from wool fibers. Powder Technol. 2013, 234, 76−83. (15) Kim, Y. Small structures fabricated using ash-forming biological materials as templates. Biomacromolecules 2003, 4, 908−913. (16) Senoz, E.; Wool, R. P.; McChalicher, C. W. J.; Hong, C. K. Physical and chemical changes in feather keratin during pyrolysis. Polym. Degrad. Stab. 2012, 97, 297−307. (17) Istrate, D.; Rafik, M. E.; Popescu, C.; Demco, D. E.; Tsarkova, L.; Wortmann, F. J. Keratin made micro-tubes: The paradoxical thermal behavior of cortex and cuticle. Int. J. Biol. Macromol. 2016, 89, 592−598. (18) Wortmann, F. J.; Deutz, H. Thermal analysis of ortho-and paracortical cells isolated from wool fibers. J. Appl. Polym. Sci. 1998, 68, 1991−1995. (19) Fan, J.; Yu, W. D. Fractal analysis of the ortho-cortex and paracortex of wool fiber. Adv. Mater. Res. 2011, 197−198, 86−89. (20) Hock, C.; Mcmurdie, H. Structure of the wool fiber as revealed by the electron microscope. J. Res. Natl. Bur. Stand. 1943, 31, 229− 236. (21) Kong, J.; Wang, M.; Zou, J.; An, L. Soluble and meltable hyperbranched polyborosilazanes toward high-temperature stable SiBCN ceramics. ACS Appl. Mater. Interfaces 2015, 7, 6733−6744. (22) Luo, C.; Duan, W.; Yin, X.; Kong, J. Microwave-absorbing polymer-derived ceramics from cobalt-coordinated poly(dimethylsilylene)diacetylenes. J. Phys. Chem. C 2016, 120, 18721− 18732. (23) Song, Y.; He, L.; Zhang, X.; Liu, F.; Tian, N.; Tang, Y.; Kong, J. Highly efficient electromagnetic wave absorbing metal-free and carbon-rich ceramics derived from hyperbranched polycarbosilazanes. J. Phys. Chem. C 2017, 121, 24774−24785. (24) Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Cançado, L. G.; Jorio, A.; Saito, R. Studying disorder in graphite-based systems by Raman spectroscopy. Phys. Chem. Chem. Phys. 2007, 9, 1276−1290. (25) Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Perspectives on carbon nanotubes and graphene Raman spectroscopy. Nano Lett. 2010, 10, 751−758. (26) Ferrari, A. C. Raman spectroscopy of graphene and graphite: disorder, electron−phonon coupling, doping and nonadiabatic effects. Solid State Commun. 2007, 143, 47−57. (27) Cho, C. W.; Cho, D. H.; Ko, Y. G.; Kwon, O. H.; Kang, I. K. Stabilization, carbonization, and characterization of PAN precursor webs processed by electrospinning technique. Carbon Lett. 2007, 8, 313−320. (28) Wasiuddin, N. M.; Tango, M.; Islam, M. R. A novel method for arsenic removal at low concentrations. Energy Sources 2002, 24, 1031− 1041. (29) Jadhav, A. S.; Naniwadekar, M. Y.; Shinde, N. H.; Anekar, S. V. Study of adsorption of oil from oily water using human hair. Int. J. Eng. Adv. Technol. 2011, 2 (2), 37−51. (30) Zhao, W.; Tang, Y.; Xi, J.; Kong, J. Functionalized graphene sheets with poly(ionic liquid)s and high adsorption capacity of anionic dyes. Appl. Surf. Sci. 2015, 326, 276−284.

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Formation and Characterization of Hollow Microtubes by Thermal

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