Enhancement of electrosorption capacity of activated carbon fibers by grafting with carbon nanofibers

Electrochimica Acta 56 (2011) 3164–3169

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Enhancement of electrosorption capacity of activated carbon fibers by grafting with carbon nanofibers Yankun Zhan, Chunyang Nie, Haibo Li, Likun Pan ∗ , Zhuo Sun Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry of Education, Department of Physics, East China Normal University, Shanghai 200062, China

a r t i c l e

i n f o

Article history: Received 12 October 2010 Received in revised form 31 December 2010 Accepted 5 January 2011 Available online 22 January 2011 Keywords: Activated carbon fibers Carbon nanofibers Electrosorption Sodium chloride

a b s t r a c t The composite films of activated carbon fibers (ACFs) and carbon nanofibers (CNFs) are prepared via chemical vapor deposition of CNFs onto ACFs in different times from 0.5 to 2 h and their electrosorption behaviors in NaCl solution are investigated. The morphology, structure, porous and electrochemical properties are characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, N2 adsorption at 77 K, contact angle goniometer and electrochemical workstation, respectively. The results show that CNFs have been hierarchically grown on the surface of ACFs and the as grown ACF/CNF composite films have less defects, higher specific capacitances, more suitable mesoporous structure and more hydrophilic surface than the pristine ACFs, which is beneficial to their electrosorption performance. The ACFs/CNFs with CNFs deposited in 1 h exhibit an optimized NaCl removal ratio of 80%, 55% higher than that of ACFs and the NaCl electrosorption follows a Langmuir isotherm with a maximum electrosorption capacity of 17.19 mg/g. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction As a result of the rise in population rates and the expansion of industrial and agricultural activities, currently many countries in the world are suffering from a shortage of fresh drinking water. Seawater desalination has emerged as an important source of fresh water because about 97% of the earth’s water is seawater. The conventional desalination technologies such as membrane separation and thermal separation require either high-pressure pumps, membranes, distillation columns, or thermal heaters, resulting in high capital or operational expenditure [1,2]. Therefore, further exploration of low-cost desalination technology is neccesary. Electrosorption, also called as capacitive deionization (CDI), has been recently receiving great interests and offers an attractive, energy-efficient alternative to thermal and membrane desalination processes [3–6]. This electrochemical process operates by adsorbing ions in the double layer formed at the electrode surface by applying a low direct current (DC) potential (normally less than 2 V) and exhibits several advantages, such as high capacity, no secondary waste and good reversibility which render the electrosorption process attractive for water desalination. The efficiency of electrosorption strongly depends upon the properties of electrode materials. Conventionally, activated carbon (AC) materials including AC powders [7–9], AC sheets [10], AC cloths

∗ Corresponding author. Tel.: +86 21 62234132; fax: +86 21 62234321. E-mail address: [email protected] (L. Pan). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.01.059

[11–14] and AC fibers (ACFs) [15,16] have been widely used as electrosorption electrodes due to their large surface area, high chemical stability, relatively low cost and environmental friendliness. However, intrinsic drawbacks of conventional AC electrodes for electrosorption have been encountered recently, such as irregular pore structures and high mass transfer resistance. In order to improve the desalination capacity, two strategies are taken into consideration: the exploration of novel electrode materials such as carbon aerogels (CAs) [17–19] with small (<50 nm) interstitial pores, ordered mesoporous carbons (OMCs) [20–23] with regular mesopore arrangement, carbon nanofibers (CNFs) or carbon nanotubes (CNTs) with high mechanical strength [24,25] and graphene with remarkable electrical conductivity [26–28]; the modification of AC materials by strong acid oxidation, alkaline treatment, alkoxides reaction [29–31] or the combination with nanoscale materials [32,33]. Though, in the latter method, the electrosorption performance of AC materials is significantly enhanced after modification, chemical reaction to increase the adsorption sites may easily introduce other impurities even toxic substances and damage the mechanical strength and electrical conductivity of the pristine ACs. The composite electrodes proposed by Dai et al. [32,33] were fabricated by directly mixing ACs and CNTs while the uniform distribution of CNTs within ACs is difficult to obtain and the contact resistance between ACs and CNTs is innegligible. In this paper, we use chemical vapour deposition (CVD) method to deposit CNFs onto ACFs to form ACF/CNF composite materials. Such a method can form good contact between CNFs and ACFs. The ACF/CNF composite materials are employed as electrosorption electrodes and exhibit

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Fig. 1. (a) Optical photograph of ACF cloth (dimensions: 80 mm × 80 mm); (b) and (c) FESEM images of ACFs; (d–f) FESEM images of ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNTs-2 h. Insets are corresponding high magnification images.

highly enhanced efficiency to remove sodium chloride (NaCl) from water by comparing with the pristine ACFs.

ACFs/CNFs-1 h and ACFs/CNFs-2 h) were obtained. The Ni–Al catalyst particles were removed before electrosorption experiment by immersing the electrodes in acid solution.

2. Experimental 2.2. Characterization 2.1. Preparation of ACFs/CNFs ACF films (80 mm width × 80 mm length × 3 mm thickness) were purchased from Nantong Senyou Carbon Fiber Co. Ltd., China. The ACFs were washed by deionized water, and then dried at 373 K before use. Subsequently, the ACFs were immerged in a 300 mL Ni–Al catalyst precursor solution with Ni(OH)2 and Al(OH)3 obtained by an in situ chemical co-precipitation method and the molar ratio of Ni–Al was 4:1 [34]. The reduction of the Ni–Al catalyst was performed in the thermal CVD system at 823 K for 30 min with a hydrogen gas flow rate of 100 mL/min. In situ growth of CNFs was carried out on the surface of ACFs by introducing acetylene and hydrogen mixture gas into the CVD chamber at a flow rate of 50 and 100 mL/min at 823 K, respectively. The ACF/CNF electrodes with CNFs deposited in 0.5, 1 and 2 h (named as ACFs/CNFs-0.5 h,

The surface morphology and structure were characterized by field emission scanning electron microscopy (FESEM, JEOL4700), transmission electron microscope (TEM, JEM-2100) and Raman spectroscopy (Renishaw inVia, resolution: 1 cm−1 ), respectively. The contact angles of water on the surface of electrodes were measured by the contact angle goniometer (JC2000D, Powereach) using digital micrographs of deionized water droplets. The Brunauer–Emmett–Teller (BET) specific surface area was determined by the surface analyzer (Quantachrome, O2108-KR-1) using N2 as adsorbate at 77 K. The potential sweep cyclic voltammetry (CV) measurement was examined in 1 M NaCl solution by using Autolab PGSTAT 302N electrochemical workstation in a three-electrode mode, including a standard calomel electrode as reference electrode and a platinum foil as counter electrode.

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Fig. 2. TEM images of (a) ACFs/CNFs-0.5 h, (b) ACFs/CNFs-1 h and (c) ACFs/CNTs-2 h; (d) high-magnification TEM image of ACFs/CNFs-1 h.

2.3. Electrosorption The ACF/CNF electrodes were assembled in holders with an area of 80 mm × 80 mm and the distance between the electrodes was 2 mm. The pristine ACF electrodes were also assembled for comparison. Electrosorption experiments were conducted in a continuously recycling system consisting of the removal cell attached to a DC power supply, in series with the measuring cell by means of a peristaltic pump. The analytical pure NaCl was used for the aqueous solutions. The solution temperature was kept at 298 K and a flow rate 40 mL/min was applied. The electrosorption was performed by a DC power supply at a constant voltage (1.2 V) and the variation of NaCl concentration was measured by a conductivity meter (DDS-308, Precision & Scientific Instrument, China) in situ. The relationship between conductivity and concentration was obtained according to a calibration table made prior to the experiments [26].

Fig. 3 exhibits typical Raman spectra of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h. The spectra are dominated by two peaks at 1340 and 1570 cm−1 , which are referred as D band and G band, respectively. The D band is attributed to the presence of amorphous carbons or defects in curved graphite sheets. The G band corresponds to high degree of symmetry and order of carbon materials and is generally used to identify well-ordered CNTs

3. Results and discussion Fig. 1(a) shows optical photograph of ACF cloth. The ACF cloth is flexible and easily curled. The FESEM images in Fig. 1(b) and (c) display the morphology of the ACFs. The diameter of ACFs is about 8 ␮m and the inside wrinkles on the ACF surface mainly consist of micropores. Fig. 2(d)–(f) shows FESEM images of ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h, respectively. It can be observed that sparse CNFs appear on the ACF surface when the growth time is only half hour. The shapeless CNFs are observed apparently from the high-magnification FESEM image in the inset. As the growth time is increased to 1 h, the whole surface of ACFs is covered by a compact layer of CNFs. When the growth time reaches 2 h, the ACFs are crowded with a large amount of CNFs. It should be mentioned that no obvious destruction of ACF structure is found after CVD process. Fig. 2(a)–(c) displays TEM images of ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h, respectively. The average diameters of CNFs in the three electrodes are approximately 20, 25 and 40 nm. The high-magnification TEM image of ACFs/CNFs-1 h in Fig. 2(d) confirms the typical structure of CNF.

Fig. 3. Raman spectra of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h.

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Fig. 4. CV curves of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h in 1 M NaCl solution at a scan rate of 5 mV/s.

or CNFs [35–37]. The ratio of the intensity of the D peak and G peak (ID /IG ) is related to the amount of disorder in the carbon products. The ID /IG ratios of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h are 1.02, 0.76, 0.65 and 0.71, respectively, which indicates a less defect density and a better quality structure of ACFs/CNFs than ACFs and among composite materials, ACFs/CNFs1 h exhibits better ordered structure. The measured contact angle decreases from about 130◦ for ACFs to 125◦ , 100◦ and 110◦ for ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h, respectively, indicating ACFs/CNFs tend to more hydrophilic than ACFs and ACFs/CNFs-1 h owns best wettability. The BET surface areas, micropore areas and average pore sizes of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h are displayed in Table 1. It can be observed that ACFs exhibit highest specific surface area and about 74%, 62%, 41% and 53% of specific surface areas of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h are contributed by micropores, respectively, indicating that the specific surface area and micropore decrease dramatically after the grafting of CNFs. Such a phenomenon should be due to the blocking of micropores in ACFs by CNFs. The average pore sizes of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs2 h are 1.88, 1.99, 2.41, 2.15 nm, respectively. Based on IUPAC classification, the pore size is divided into three groups: micropores (pore width < 2 nm), mesopores (between 2 and 50 nm), and macropores (>50 nm). As a result, ACFs/CNFs-1 h consists of more mesopores which is beneficial to electrosorption because the overlapping effect caused by micropores can be minimized [38–41]. Fig. 4 shows the CV curves of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs1 h and ACFs/CNFs-2 h electrodes at a scan rate of 5 mV/s. The current increases and decreases steadily with the electric potential with symmetric CV curves, indicating that the electrosorption process is reversible. No obvious redox peaks are observed, which means that ions adsorb on the electrode surface by forming an electric double layer due to Coulombic interaction rather than electrochemical reaction [30]. The specific capacitance (Csp in F/g) can be obtained from the CV process according to Eq. (1): Csp =

¯i

v×M

(1)

where i (A) is the average current,  (V/s) is the scan rate, and M (g) is the total mass of electrodes. The specific capacitances of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h are 2.80, 17.5, 29.16 and 7.48 F/g, respectively. The result indicates that the

Fig. 5. (a) The electrosorption process for ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h in NaCl solution. Initial 30 min: physical absorption; subsequent 120 min: electrosorption; final 30 min: desorption; (b) recycle electrosorption experiment for ACFs/CNFs-1 h. C0 and Ct are the initial NaCl concentration and the concentration at time t, respectively.

well-designed hierarchical ACF/CNF structure should exhibit better electrosorption performance than ACFs. The electrosorption experiments of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h were conducted in 50 mL NaCl aqueous solution. The initial conductivity was 50 ␮S/cm. The system was maintained to circularly flow for 30 min to investigate physically adsorbed amounts of NaCl. And then a constant potential of 1.2 V was imposed and maintained for 120 min to measure electrosorptive amounts of NaCl. Subsequently, short-circuiting was employed in the next 30 min for the electrode regeneration. As shown in Fig. 5(a), the NaCl concentration decreases very little during the physical adsorption process. Once the voltage is imposed, ions are driven onto the electrodes and the NaCl concentration decreases dramatically and finally reaches equilibrium at about 120 min. Among the electrodes, ACFs/CNFs-1 h exhibits highest NaCl removal ratio of 80%, 55% higher than that of ACFs (only 25%). The result clearly shows that ACFs/CNFs are more suitable electrode materials than ACFs and 1 h is optimal time for the CVD deposition of CNFs. Table 2 compares the electrosorption capacity among different electrode materials including OMCs, ACs, CNT/CNF composite, CAs and ACFs/CNFs in NaCl solution with the same initial conductivity. It is observed that the electrosorption capacity of ACFs/CNFs is comparable to OMCs and much higher than ACs although specific surface area of ACFs/CNFs is smaller. The very

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Table 1 BET surface areas, micropore areas and average pore sizes of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h.

2

BET surface area (m /g) Micropore area (m2 /g) Average pore size (nm)

ACFs

ACFs/CNFs-0.5 h

ACFs/CNFs-1 h

ACFs/CNTs-2 h

1416 1053 1.88

790 490 1.99

558 262 2.41

646 340 2.15

Table 2 Comparison of electrosorption capacity among different electrode materials. Electrode

Applied voltage (V)

Initial solution conductivity (␮S/cm)

Electrosorption capacity (mg/g)

Specific surface area (m2 /g)

OMCs [20] ACs [20] ACFs/CNFs OMCs [22] ACs [22] CNTs/CNFs [24] CAs [17] ACFs/CNFs

1.2 1.2 1.2 0.8 0.8 1.2 1.2 1.2

51 51 50 100 100 100 100 100

0.68 0.25 0.62 0.93 0.27 3.32 3.33 1.3

968 844 558 1491 845 211 400–1100 558

high value of CNTs/CNFs and CAs should be due to their optimal pore structure or good electrical conductivity [17,24]. As known, good regeneration is essential for electrode materials. It is observed that in 30 min after short-circuiting, adsorbed NaCl is rapidly desorbed. Finally, around 90% of the initial NaCl amount returns back to the solution for ACFs, ACFs/CNFs-0.5 h and ACFs/CNFs-2 h while 85% for ACFs/CNFs-1 h. However, the desorbed rate of ACFs/CNFs-1 h reaches average 2.2%/min, much higher than those of ACFs (0.5%/min), ACFs/CNFs-0.5 h (1%/min) and ACFs/CNFs-2 h (1.7%/min). Such a rapid desorption is helpful in practical application of CDI. Fig. 5(b) shows the variation of NaCl concentration over several adsorption (1 h)–desorption (1 h) cycles using ACFs/CNFs-1 h electrode. Obviously, the repeatability of electrosorption process can be realized in our unit cell. In the practical experiment, electrosorption capacity declination has not been observed in our unit cell after over 30 charge–discharge cycles. To further compare electrosorption behavior of the electrodes, the batch experiments with different initial conductivities were carried out. The linear forms of Langmuir isotherm (Eq. (2)) and Freundlich isotherm (Eq. (3)) are used to fit the experimental data [25,42]: Ce 1 Ce = + qe qm KL qm lg qe = lg KF +

1 lg Ce n

are grafted by CNFs. The qm of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h are 1.85, 8.77, 17.19 and 14.14 mg/g, respectively, indicating that ACF/CNF composites exhibit more optimal electrosorption performance than ACFs and may be potential candidate as electrode materials for electrosorption.

(2)

(3)

where Ce (mg/L) is the equilibrium concentration, qe (mg/g) is the amount of adsorbed NaCl at equilibrium, qm (mg/g) is the maximum electrosorption capacity corresponding to complete monolayer coverage, KL (L/mg) and KF (L/g) are Langmuir constant that relates to the affinity of binding sites and Freundlich constant, respectively and 1/n is unitless Freundlich exponent. The linear plots of Ce /qe versus Ce for Langmuir model and lg qe versus lg Ce for Freundlich are given in Fig. 6(a) and (b), respectively. Values of qm , KL , KF and 1/n are calculated from the slope and intercept of the plots and presented in Table 3. From the regression coefficient r2 , Langmuir isotherm describes the experimental data of both ACFs and ACFs/CNFs better than Freundlich isotherm, suggesting that Na+ or Cl− monolayer coverage and equal activation energy during electrosorption process can be assumed. According to Langmuir model, KL associated with the heat of adsorption is used to determine the affinity between adsorbed ions and electrode. As a result, the affinity between adsorbed ions and binding sites is improved when ACFs

Fig. 6. (a) Langmuir isotherm and (b) Freundlich isotherm of ACFs, ACFs/CNFs-0.5 h, ACFs/CNFs-1 h and ACFs/CNFs-2 h in NaCl solution.

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Table 3 Parameters determined from Langmuir and Freundlich isotherms of different electrodes. Isotherm

Parameter

ACFs

ACFs/CNFs-0.5 h

ACFs/CNFs-1 h

ACFs/CNFs-2 h

Langmuir

qm (mg/g) KL (L/mg) r2

1.85 0.007 0.985

8.77 0.008 0.988

17.19 0.01 0.999

14.14 0.007 0.996

Freundlich

KF (L/g) 1/n r2

0.108 0.423 0.927

0.332 0.508 0.882

1.217 0.405 0.925

0.871 0.411 0.93

4. Conclusion The ACF/CNF composite films are successfully fabricated by growing CNFs onto ACFs via CVD in different times from 0.5 to 2 h. The as grown ACFs/CNFs have less defects, higher specific capacitances, more mesopores and better hydrophilicity to water than the pristine ACF, which is beneficial to their electrosorption performance. The electrosorption isotherms of ACFs/CNFs and ACFs show that both of them follow Langmuir adsorption, suggesting that Na+ or Cl− monolayer coverage and equal activation energy during electrosorption process can be assumed. The electrosorption behaviors of ACFs/CNFs and ACFs in NaCl solution are investigated. The ACFs/CNFs with CNFs deposited in 1 h exhibit the optimized electrosorption performance, and the maximum electrosorption capacity of NaCl reaches 17.19 mg/g, much better than that of pristine ACFs. ACFs/CNFs may be potential candidate as electrode materials for electrosorption. Acknowledgements This work was supported by Special Project for Nanotechnology of Shanghai (No. 1052nm02700) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars. References [1] [2] [3] [4] [5] [6]

T. Matsuura, Desalination 134 (2001) 47. R. Semiat, Environ. Sci. Technol. 42 (2008) 8193. Y. Oren, Desalination 228 (2008) 10. P. Xu, J.E. Drewes, D. Heil, G. Wang, Water Res. 42 (2008) 2605. M.A. Anderson, A.L. Cudero, J. Palma, Electrochim. Acta 55 (2010) 3845. S.J. Seo, H. Jeon, J.K. Lee, G.Y. Kim, D. Park, H. Nojima, J. Lee, S.H. Moon, Water Res. 44 (2010) 2267. [7] J.B. Lee, K.K. Park, S.W. Yoon, P.Y. Park, K.I. Park, C.W. Lee, Desalination 237 (2009) 155. [8] J.H. Choi, Sep. Purif. Technol. 70 (2010) 362. [9] E.J. Bain, J.M. Calo, R. Spitz-Steinberg, J. Kirchner, J. Axeˇın, Energy Fuels 24 (2010) 3415.

[10] K.K. Park, J.B. Lee, P.Y. Park, S.W. Yoon, J.S. Moon, H.M. Eum, C.W. Lee, Desalination 206 (2007) 86. [11] H.J. Ahn, J.H. Lee, Y. Jeong, J.H. Lee, C.S. Chi, H.J. Oh, Mater. Sci. Eng. A 449 (2007) 841. [12] H.J. Oh, J.H. Lee, H.J. Ahn, Y. Jeong, Y.J. Kim, C.S. Chi, Thin Solid Films 515 (2006) 220. [13] L. Zou, G. Morris, D. Qi, Desalination 225 (2008) 329. [14] E. Bayram, E. Ayranci, Carbon 48 (2010) 1718. [15] E. Avraham, B. Yaniv, A. Soffer, D. Aurbach, J. Phys. Chem. C 112 (2008) 7385. [16] M. Wang, Z.H. Huang, L. Wang, M.X. Wang, F. Kang, H. Hou, New J. Chem. 34 (2010) 1843. [17] J.C. Farmer, D.V. Fix, G.V. Mack, R.W. Pekala, J.F. Poco, J. Electrochem. Soc. 143 (1996) 159. [18] S.W. Hwang, S.H. Hyun, J. Non-Cryst. Solids 347 (2004) 238. [19] H.H. Jung, S.W. Hwang, S.H. Hyun, L. Kang-Ho, G.T. Kim, Desalination 216 (2007) 377. [20] L. Zou, L.X. Li, H.H. Song, G. Morris, Water Res. 42 (2008) 2340. [21] L. Zou, L. Li, H. Song, G. Morris, Water Sci. Technol. 61 (2010) 1227. [22] L.X. Li, L. Zou, H.H. Song, G. Morris, Carbon 47 (2009) 775. [23] X. Wang, J.S. Lee, C. Tsouris, D.W. DePaoli, S. Dai, J. Mater. Chem. 20 (2010) 4602. [24] X.Z. Wang, M.G. Li, Y.W. Chen, R.M. Cheng, S.M. Huang, L.K. Pan, Z. Sun, Appl. Phys. Lett. 89 (2006) 053127. [25] S. Wang, D.Z. Wang, L.J. Ji, Q.M. Gong, Y.F. Zhu, J. Liang, Sep. Purif. Technol. 58 (2007) 12. [26] H.B. Li, T. Lu, L.K. Pan, Y.P. Zhang, Z. Sun, J. Mater. Chem. 19 (2009) 6773. [27] H.B. Li, L. Zou, L.K. Pan, Z. Sun, Environ. Sci. Technol. 44 (2010) 8692. [28] H.B. Li, L. Zou, L.K. Pan, Z. Sun, Sep. Purif. Technol. 75 (2010) 8. [29] M.W. Ryoo, G. Seo, Water Res. 37 (2003) 1527. [30] M.W. Ryoo, J.H. Kim, G. Seo, J. Colloid Interface Sci. 264 (2003) 414. [31] I. Villar, S. Roldan, V. Ruiz, M. Granda, C. Blanco, R. Meneˇındez, R. Santamariˇıa, Energy Fuels 24 (2010) 3329. [32] K. Dai, L.Y. Shi, D.S. Zhang, J.H. Fang, Chem. Eng. Sci. 61 (2006) 428. [33] D.S. Zhang, L.Y. Shi, J.H. Fang, K. Dai, Mater. Lett. 60 (2006) 360. [34] Q.M. Gong, Z. Li, X.W. Zhou, J.J. Wu, Y. Wang, J. Liang, Carbon 43 (2005) 2426. [35] A. Sadezky, H. Muckenhuber, H. Grothe, R. Niessner, U. Pöschl, Carbon 43 (2005) 1731. [36] M. Khosravi, M.K. Amini, Carbon 48 (2010) 3131. [37] M.A. Pimenta, G. Dresselhaus, M.S. Dresselhaus, L.G. Cancado, A. Jorio, R. Saito, Phys. Chem. Chem. Phys. 9 (2007) 1276. [38] M. Kruk, M. Jaroniec, Chem. Mater. 13 (2001) 3169. [39] C.H. Hou, C.D. Liang, S. Yiacoumi, S. Dai, C. Tsouris, J. Colloid Interface Sci. 302 (2006) 54. [40] K.L. Yang, T.Y. Ying, S. Yiacoumi, C. Tsouris, E.S. Vittoratos, Langmuir 17 (2001) 1961. [41] Y. Gao, L.K. Pan, H.B. Li, Y.P. Zhang, Z.J. Zhang, Y.W. Chen, Z. Sun, Thin Solid Films 517 (2009) 1616. [42] H.B. Li, L.K. Pan, Y.P. Zhang, L.D. Zou, C.Q. Sun, Y.K. Zhan, Z. Sun, Chem. Phys. Lett. 485 (2010) 161.