Human hair-derived nitrogen and sulfur co-doped porous carbon materials for gas adsorption

Abstract

Human hair, a biowaste composed of protein, is converted into nitrogen and sulfur co-doped porous carbonaceous materials via a facile degradation and carbonization/activation process. The resulting carbon materials possess a large specific surface area value (2700 m² g⁻¹) as well as high nitrogen and sulfur content (around 8.0 and 4.0 wt%, respectively). The morphology, composition and porous structure of the obtained materials were thoroughly characterized using scanning and transmission electron microscopy, elemental analysis, nitrogen and carbon dioxide sorption analysis, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy, etc. It is confirmed that both the degradation and the carbonization/activation procedures play important roles in the porous structure formation. Furthermore, these materials are proven to exhibit good performances in gas adsorption: carbon dioxide uptake (up to 24.0 wt%, at 273 K and 1.0 bar), methane adsorption (up to 3.04 wt%, at 273 K and 1.0 bar), and hydrogen adsorption (up to 2.03 wt%, at 77 K and 1.0 bar). The high gas adsorption capacities could be attributed to the microporous structure combined with the surface functionalities. In addition, we believe that this synthesis process offers a facile and effective way for transforming protein-containing biowastes into functionalized porous carbonaceous materials.

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Introduction

Carbon material is a kind of widely used material that has been commonly applied in various fields for thousands of years.^1,2 Since the discovery of fullerenes and carbon nanotubes,^3,4 the material science associated with valuable carbon materials, especially porous carbons, has attracted intense attention. Porous carbon materials possess many specific merits, such as a porous structure, large surface area, and thermal and chemical stability, which allow them to be successfully applied in many fields, such as energy storage,^5–7 catalysis,^2,8,9 and gas storage, etc.^10–12 Although porous carbon materials are promising candidates for some important environmental and industrial applications, for some specific applications, chemical functionalization is required.^13,14 Doping of carbon materials with heteroatoms (e.g. N, S, etc.) is a popular route to tune both the structural and physicochemical properties of such materials for various demands, such as the doping of nitrogen and sulfur into a carbon framework which can improve the adsorption properties for acidic gases.¹⁵ Generally, nitrogen can be introduced into a carbon material structure predominantly in two ways.^16,17 One way is the treatment of pre-synthesized carbon materials at high temperatures with nitrogen-containing gases or liquid.^18–21 The other way is carbonizing nitrogen-containing organic compounds.^22–25 The traditional synthesis methods for sulfur-containing materials mainly involve the pyrolysis or the arc vaporization of sulfur-containing compounds or polymers.^26–29

Although these nitrogen- or sulfur-doped carbon materials have attracted particular attention, the production methods normally rely on chemically harsh and multistep processes, typically involving high temperature treatment and often leading to the generation of quantities of waste. Furthermore, the conventional precursors are non-sustainable and relatively expensive as compared with biomass-derived materials, especially biowastes, such as watermelon,⁷ silk waste,³⁰ waste tea-leaves,³¹ animal bone,³² wood resources,^33,34 and corncob.³⁵

Human hair, as we all know, is a biowaste consisting of carbon, nitrogen, oxygen, sulfur, and hydrogen elements owing to its abundant proteins. Therefore, human hair could be treated as scrap material for the preparation of valuable carbon materials.^36–39 For instance, Qian et al. have reported the production of carbon materials from human hair with carbonization (300 °C) and sequent KOH activation (800 °C). The obtained HMC-800 had a BET specific surface area of 1306 m² g⁻¹ and a high specific capacitance of 340 F g⁻¹ in 6 M KOH at a current density of 1 A g⁻¹ as well as good stability over 20 000 cycles.³⁶ Similarly, Si and co-workers have reported the production of carbon materials from human hair and glucose via a hydrothermal carbonization procedure at 180 °C and KOH activation at 600 °C, the supercapacity properties were also tested.³⁸ Yu and co-workers have synthesized human hair-derived carbon via a three-step process: pre-carbonization, sequent NaOH activation (600 °C) and further graphitization (900 °C). The obtained HC-900 had a larger BET specific surface area (about 1810 m² g⁻¹), and was a good electrocatalyst for the oxygen reduction reaction.³⁹

Herein, the human hair-derived materials were synthesized via a facile degradation and carbonization/activation process. During the degradation process, the protein of the human hair is degraded into small peptide chains and amino acids, and then the small peptide chains and amino acids are transformed into porous carbon through the carbonization/activation process. The as-prepared porous material possesses a much larger specific surface area value (2700 m² g⁻¹) as compared with other human hair-derived carbon and the material without degradation. It is important to note that the materials exhibit good performance in carbon dioxide uptake (24.0 wt%, at 273 K and 1.0 bar), methane adsorption (3.04 wt%, at 273 K and 1.0 bar), and hydrogen storage (2.03 wt%, at 77 K and 1.0 bar).

Experimental section

Materials

Human hair fiber was obtained from a barbershop. Hydrochloric acid (36–38 wt%), potassium hydroxide, and ethanol were purchased from Beijing Chemical Reagents Company. Ultra-pure water (18 MΩ cm) was produced by a Millipore-ELIX water purification system. All chemicals were used without further purification.

Preparation of the nitrogen and sulfur co-doped carbon

The hair fiber was washed thoroughly with ultra-pure water and ethanol three times in order to remove dust and adhering grease. Then, the hair (6.0 g) was added into aqueous KOH solution (1.12 M, 48 mL) and stirred for 6 h at room temperature. After being dissolved absolutely, the obtained dark aqueous dispersion was placed into an oven at 95 °C for 24 h for drying. Then, part of the black solid (3.0 g) was transferred to a horizontal quartz tube furnace for carbonization/activation and heated (with the heating rate of 2 °C min⁻¹) under a steady nitrogen flow at 750, 650, or 550 °C for 1 hour. After carbonization, the obtained black solid powder was washed with HCl solution (0.05 M), water, and ethanol until the pH value reached approximately 7, and dried at 80 °C for 12 h.⁴⁰ The resulting human hair-derived porous carbon (HPC) materials are denominated as HPC-x, where x indicates the carbonization temperature. As a control experiment, the material without KOH degradation (direct mixing of the human hair with KOH, and carbonization at 750 °C) was synthesized and named as HCA-750.

Instrumental characterization

The nitrogen sorption, hydrogen sorption, carbon dioxide sorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) spectra were measured by the same instruments and under the same conditions as ref. 41. Methane sorption was investigated on a Micromeritics TriStarII 3020 surface area and porosity analyzer (Micromeritics, USA) at 273 K. Before measurement, the samples were degassed for 12 h under vacuum at 120 °C. The specific surface area of the materials was calculated based on the Brunauer–Emmett–Teller (BET) model, while the pore size distribution was calculated by the density functional theory (DFT) method. Infrared (IR) spectra were obtained by using a Spectrum One Fourier transform infrared (FT-IR) spectrometer (Perkin-Elmer Instruments Co. Ltd, USA) in KBr pellets. Elemental analysis was carried out using a Flash EA 1112 Elemental Analyzer (Thermo Scientific, Italy).

Results and discussion

It is well-known that human hair mainly consists of proteins, or to be more specific, keratin.³⁹ Keratin, a kind of fibrous scleroprotein that is composed of polypeptide chains in parallel,⁴² is not soluble in water, salt solution, dilute acid or alkaline solution. Keratin contains about 18 kinds of amino acid such as cysteine, glutamic acid, and arginine. These components make human hair rich in carbon (about 45.22 wt%), nitrogen (about 14.99 wt%), sulfur (about 4.38 wt%), and hydrogen (about 6.95 wt%) (determined by elemental analysis, as shown in Table 1). Therefore, in this work, human hair is treated as carbon, nitrogen, and sulfur sources to prepare nitrogen and sulfur co-doped carbon materials. The HPC-x materials are obtained by two simple steps: degradation and carbonization/chemical activation, as shown in Scheme 1. Since the main component of human hair is protein, when human hair comes into contact with a highly concentrated alkali solution such as potassium hydroxide, the alkali medium will promote the degradation of peptide bonds from proteins into small peptide chains and amino acids,⁴³ the amino acids are easily dissolved in solution, and then transformed into carbon materials. The carbonization process is performed under a nitrogen flow with a steady heating rate at different temperatures. During the procedure, KOH not only acts as a hydrolysis/degradation agent, but also as an activating agent in the carbonization/activation process.

Table 1 The BET specific surface area, pore volume, and the chemical compositions of the HPC-x materials

Sample	SBET^a (m^2 g^−1)	Vtotal^b (cm^3 g^−1)	Vmicro^c (cm^3 g^−1)	Vultramicro^d (cm^3 g^−1)	Elemental analysis (wt%)
					C	N	S^e
a Specific surface area calculated from the nitrogen adsorption branch using the BET method in a relative pressure (P/P0) range from 0.05 to 0.20.b Single point adsorption total pore volume of pores less than 105 nm diameter at P/P0 = 0.97.c The pore volume of pores less than 2.0 nm based on the density functional theory (DFT) method.d The pore volume of pores less than 0.8 nm based on the DFT method.e The sulfur content was calculated based on the barium salt titration method.
Hair	—	—	—	—	45.22	14.99	4.36
HPC-550	1230	0.90	0.43	0.35	66.41	8.33	4.00
HPC-650	2380	1.64	0.94	0.60	77.93	4.94	1.12
HPC-750	2700	1.33	0.82	0.52	80.95	3.45	1.29
HCA-750	420	0.47	0.09	—	—	—	—

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Scheme 1 Schematic procedure for the formation of the HPC-x materials.

Fig. 1 shows the SEM and TEM images of the obtained material (HPC-750), all the HPC-x materials show similar results to HPC-750 (not shown here). From the SEM images of HPC-750, an aggregation of small particles with a monolith structure is observed (Fig. 1a and b), which is very different from the morphology of human hair. The TEM images at both low and high magnification are shown in Fig. 1c and d, the images reveal disordered worm-like structures, which are representative of the microporous structures within these materials.

Fig. 1 The SEM (a and b) and TEM (c and d) images of HPC-750.

The porosity properties of the HPC-x materials were further investigated by a nitrogen adsorption–desorption experiment. The isotherms of the HPC-x materials are shown in Fig. 2a. As seen from the isotherms, all the samples exhibit rapid uptakes at low relative pressure (P/P₀ 0–0.10) with type-I sorption isotherms, which are characteristic of microporous materials. Taking into account the rapid uptake at very low relative pressure, ultramicropores should exist in the carbon materials. The carbon dioxide adsorption isotherms at low pressure are used to accurately analyze the micropores; the isotherms and pore size distribution of the materials are shown in Fig. S1 (in ESI†), the carbon dioxide isotherms confirm the microporous nature of these materials. The pore size distribution (PSD) profile of the materials from nitrogen sorption and carbon dioxide adsorption is shown in Fig. 2b and S1b (in ESI†). As seen from the figures, the dominant pore diameters are around 0.5–0.9 nm, which display a typical microporous, in particular an ultramicroporous structure, these results are consistent with the HR-TEM observation (Fig. 1b). The PSD profile from nitrogen sorption shows that these materials also contain some mesopores with the pore diameter between 2.0 nm and 4.0 nm. As seen from Table 1, the BET specific surface area and pore volume values of HPC-x increase with the carbonization/activation temperature, the BET specific surface area values of HPC-550, HPC-650, and HPC-750 are 1230, 2380, and 2700 m² g⁻¹, respectively. The BET specific surface area value of HPC-750 is the largest value among the human hair-derived carbon materials.^36,38,39 As we expected, on one hand, the carbonization/chemical activation process contributes a lot to the large specific surface area of the obtained materials, since the micro/mesoporous structure is easily obtained by potassium hydroxide activation.⁴⁴ On the other hand, as shown in Table 1, the material synthesized under the same conditions (750 °C) but without degradation in KOH has a much smaller BET specific surface area as well as pore volume (420 m² g⁻¹, 0.47 cm³ g⁻¹), which means that the degradation process also plays an important role in the porous structure formation.

Fig. 2 Nitrogen adsorption–desorption isotherms (a) and the DFT pore size distribution profiles (b) of the HPC-x materials.

The doping of nitrogen and sulfur in the obtained carbon materials is confirmed by elemental analysis (Table 1). As seen from Table 1, the nitrogen content of the obtained materials reaches up to 8.33 wt%, while the weight percentage of nitrogen decreased with the increase of the activation temperature. The sulfur content of HPC-550 is 4.00 wt%, which is also the highest percentage among the HPC samples. This phenomenon is in accordance with other reports that the amount of nitrogen and sulfur decreased with the increase of the treatment temperature.^5,36,37,45

FT-IR spectroscopy was used to investigate the presence of functional groups on the surface of these materials. The infrared spectra is shown in Fig. 3. As seen from the spectra, the bands between 3200 and 3500 cm⁻¹ correspond to the stretching vibrations of O–H or N–H.⁴⁵ In the spectra of HPC-550 and HPC-650, the peaks at 1640 and 1580 cm⁻¹ correspond to the C=O stretching vibration³⁷ and N–H in-plane deformation,⁴⁵ respectively. The peaks between 1396 and 1496 cm⁻¹ are ascribed to the C–N and N–H groups.³⁷ These peaks are not well observed in HPC-750, which is probably because the primary amide decreased or disappeared at higher temperature. Sulfur functionalities are also well observed in these materials. The peak at 1150 cm⁻¹ can be attributed to the C–O–C, C–O, and –SO₃⁻ bonds.^37,46 The peak at 1040 cm⁻¹ is related to S=O stretching vibrations. In other words, nitrogen and sulfur functionalities are observed from the FT-IR spectra, which prove that the nitrogen and sulfur are successfully doped into the carbonaceous materials.

Fig. 3 FT-IR spectra of the HPC-x materials. The ordinates of the spectra are shifted for clarity.

More detailed information regarding the chemical and bonding environment of the obtained HPC-x materials is observed from XPS analysis. Fig. 4 shows the XPS spectra of the HPC-750 material. Carbon, nitrogen, and sulfur are observed from the survey spectra. In the high-resolution C_1s XPS spectrum (Fig. 4b), four peaks with the binding energies of about 284.6, 285.4, 286.4, and 289.2 eV, are attributed to sp² C–H, C–N/C–O/C–S, C–N/C–O and C–O species, respectively.^47,48 The N 1s spectrum in Fig. 4c exhibits three peaks at 398.5, 400.4, and 402.3 eV, which are attributed to the pyridinic N, pyrrolic/pyridonic N, and quaternary N, respectively.^49–52 For sulfur functionalities, sulfur existed in two forms as seen from the S 2p spectrum in Fig. 4d. The first form includes a components at 164.5 eV, which is in accordance with the reported S 2p position of the C–S–C species from thiophene-S.⁵³ The other form includes two components at 168.8 and 169.9 eV, resulting from oxidized sulfur, which may come from the reaction of surface sulfur with adjacent oxygen molecules.^46,52

Fig. 4 XPS spectra of the HPC-750 (a) XPS survey, (b) C 1s, (c) N 1s, and (d) S 2p.

As mentioned above, the BET specific surface area and the pore volume of the HPC-x materials are extremely large (Table 1), and it is already reported that microporous materials with large specific surface area interact attractively with small gas molecules,⁵⁴ we suppose that our materials could present high adsorption properties for some gases, such as carbon dioxide, methane and hydrogen. Herein, the behaviors of the materials in gas adsorption (carbon dioxide, methane and hydrogen) were investigated, and the results are shown in Fig. 5 and Table 2.

Fig. 5 Gravimetric adsorption isotherms of HPC-750 (square), HPC-650 (triangle), and HPC-550 (circle). (a) Carbon dioxide at 273 K, (b) methane at 273 K, and (c) hydrogen at 77 K.

Table 2 Carbon dioxide, hydrogen, and methane adsorption results of HPC-x

Sample	CO2^a (wt%)	CH4^a (wt%)	H2^b (wt%)
a CO2 and CH4 gravimetric uptake capacities at 273 K and 1.0 bar.b H2 gravimetric uptake capacities at 77 K and 1.0 bar.
HPC-750	18.8	2.38	1.84
HPC-650	24.0	3.04	2.03
HPC-550	22.6	2.86	1.26

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The carbon dioxide adsorption experiments were carried out at 273 K (Fig. 5a), the carbon dioxide adsorption capacity of HPC-650 shows a maximum value of about 24.0 wt% (5.5 mmol g⁻¹) at 1.0 bar, this is a relatively high value when compared with other biomass-derived materials (Table 3). Some researchers have investigated carbon dioxide uptake on different biomass-derived materials, such as chitosan, bamboo, grass, and sawdust, etc., as shown in Table 3. It is found that the carbon precursors as well as the synthesis methods have a significant influence on the carbon dioxide sorption properties. The human hair-derived carbon material HPC-650 in this study possesses a higher carbon dioxide adsorption capacity at 273 K than many other biomass-derived carbons except for those derived from sawdust and bamboo at 273 K and 1.0 bar. The results are also higher than or comparable with other reported materials, such as a carbazole-based porous organic polymer (CPOP-1, 21.2 wt% at 273 K),⁵⁴ zeolitic imidazolate frameworks (ZIF-69, 12.5 wt% at 273 K),⁵⁵ amine-functionalized silica nanospheres (7.5 wt% at 273 K),⁵⁶ and nitrogen-doped carbon (27.2 wt% at 273 K).⁵⁷

Table 3 Comparison of the carbon dioxide adsorption capacities on different biomass-derived carbon materials

Precursors	Sample code	CO2 adsorbed^a (mmol g^−1/wt%)	Ref.
a CO2 uptake capacities at 273 K and 1.0 bar.
Xylose	HTC-X-950	1.2/5.10	58
Alginate	Alginate-800 °C-KOH	4.8/21.1	63
Olive stone	GKOSA40	4.5/19.8	64
Sawdust	AS-2-650	6.0/26.4	60
Coffee ground	NCLK1	4.9/21.6	65
Bamboo	Bamboo-3-873	7.0/30.8	66
	Bamboo-1-973	5.3/23.3
Coconut shell	CACM32	3.7/16.4	67
Grass cutting	—	4.0/17.6	59
Human hair	HPC-650	5.5/24.0	This study

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The high carbon dioxide adsorption capacities could be attributed to the microporous structure as well as the nitrogen functionalities of the carbon materials. It has already been confirmed by many researchers that a large amount of micropores, especially ultramicropores, are advantageous for the adsorption of carbon dioxide.^58–60 In addition, the nitrogen-containing groups give Lewis basicity to the materials, which might be relevant for the capture of acidic gases, such as carbon dioxide.^12,61,62 From Table 2 and Fig. 5a, we can easily find out that HPC-650 displays the highest capacity among the HPC-x materials, this is probably because the HPC-650 material possess the largest specific surface area, and pore volume (especially the micro and ultramicropore volume), as well as a medium nitrogen content. The HPC-550 material shows a smaller surface area and pore volume than HPC-750, but a higher carbon dioxide sorption capacity, this is probably due to the higher nitrogen content of the HPC-550 material. These results confirm that both the micropores and the surface functionalities contribute to the high gas adsorption capacities.

To determine the interaction strength between the human hair-derived carbon and the carbon dioxide molecules, the isosteric heats of carbon dioxide adsorption (Q_st) by the human hair-derived porous carbon materials are calculated by applying the Clausius–Clapeyron equation to the adsorption isotherms obtained at 273 and 298 K (the results of the carbon dioxide adsorption capacities at 298 K on the same samples (from 8.1–11 wt%) are shown in Fig. S2 (in ESI†) and Table S1 (in ESI†)). The virial analysis of the carbon dioxide adsorption data and isosteric heat of carbon dioxide adsorption results are shown in Fig. S2b (in ESI†), S2c (in ESI†) and Table S1 (in ESI†). The calculated Q_st for HPC-550, HPC-650 and HPC-750 are in the range of 28.6–41.3, 26.5–30.4, and 29.7–30.7 kJ mol⁻¹, respectively. These values are relatively higher than or comparable with those previously reported porous carbon and other sorbents,^60,68–71 which is due to the presence of micropores and surface functionalities in the materials. However, these values are comparable or lower than mainly amine-functionalized materials.^61,72 These results confirm that there are interactions between the nitrogen functional groups and carbon dioxide. Among the three samples, HPC-550 has the highest Q_st, which results in the highest carbon dioxide sorption capacity at very low pressure. However, as discussed before, the HPC-650 sample has the most micropores, which leads to the highest carbon dioxide adsorption capacities at 1.0 bar. This phenomenon indicates that the microporous structure plays a more decisive role in the carbon dioxide adsorption than the surface and bulk functionalities.

Apart from finding some effective materials for carbon dioxide capture and storage, using cleaner fuels, such as natural gas (mainly methane) or hydrogen is another solution for the conservation of our environment. Herein, the adsorption properties of our biowaste-derived materials for methane (CH₄) and hydrogen (H₂) were also studied. Fig. 5b shows the methane adsorption isotherms of the HPC-x materials, the adsorption capacities of methane at 273 K and 1.0 bar are shown in Table 2, where HPC-650 shows a maximum value of about 3.04 wt% (1.9 mmol g⁻¹), which is a high value in the field of methane adsorption on carbon materials.^73,74 Furthermore, the hydrogen adsorption isotherms of the materials are shown in Fig. 5c, as seen from the isotherms, the HPC-650 material presents the highest performance adsorption behavior for H₂, the capacity reached as high as 2.03 wt% (at 77 K and 1.0 bar). It is a considerably high value when compared with previously reported materials.^54,75,76 The high hydrogen adsorption capacities should be related to the high micropore volume.

Conclusions

Sustainable nitrogen and sulfur co-doped porous carbonaceous materials were successfully synthesized from biowaste, human hair, via a facile degradation and carbonization/chemical activation process. The obtained carbon materials possess large BET specific surface area values (up to 2700 m² g⁻¹) as well as relatively high nitrogen and sulfur contents. What’s more important, owing to the synergetic effect of the porous structure as well as the surface functionalities, the HPC-x materials show excellent gas sorption capacities: carbon dioxide capture (up to 24.0 wt% at 273 K and 1.0 bar), methane adsorption (up to 3.04 wt% at 273 K and 1.0 bar), and hydrogen uptake (up to 2.03 wt%, at 77 K and 1.0 bar). Besides the gas adsorption, the electrochemical properties of these materials in supercapacitors are presently under investigation. Furthermore, the functionalized porous carbon materials are obtained from biowaste through a simple process, this process provides an effective way for transforming other biowastes into useful porous carbonaceous materials.

Acknowledgements

The financial support of the National Science Foundation of China (Grant No. 21201048) and the Ministry of Science and Technology of China (Grant No. 2013CB934200) is acknowledged. Dr Xiaoying Qi is thanked for the TEM measurement. Ms Peng Xu is thankfully acknowledged for the XPS measurement.

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Footnote

† Electronic supplementary information (ESI) available: The carbon dioxide adsorption isotherms (273 K) and pore size distribution, carbon dioxide adsorption isotherms at 298 K, virial analysis, and the calculation of isosteric heat. See DOI: 10.1039/c5ra15690d

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