Preparation and characterization of dopamine-decorated hydrophilic carbon black

Applied Surface Science 258 (2012) 5387–5393

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Preparation and characterization of dopamine-decorated hydrophilic carbon black Lijun Zhu a,b , Yonglai Lu a,b , Yiqing Wang a,c , Liqun Zhang a,b , Wencai Wang a,c,∗ a b c

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing 100029, China Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 17 November 2011 Received in revised form 31 January 2012 Accepted 4 February 2012 Available online 13 February 2012 Keywords: Carbon black Dopamine Surface Hydrophilic

a b s t r a c t Inspired by the bio-adhesive proteins secreted by mussels for attachment to almost all wet substrates, a facile method involving oxidative polymerization of dopamine was proposed to prepare highly hydrophilic carbon black (CB) particles. A self-assembled polydopamine (PDA) ad-layer was formed via the oxidative polymerization of dopamine on the surface of CB simply by dipping the CB into an alkaline dopamine solution and mildly stirring at room temperature. The process is simple, controllable, and environment-friendly. The surface composition and structure of the CB were characterized by Xray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The surface morphology of the CB was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the PDA ad-layer was successfully deposited on the CB surfaces. The PDA-functionalized CB (CB-PDA) gave a stable colloidal dispersion in water. Contact angle measurement results indicated that the hydrophilicity of CB was significantly improved after dopamine modification. TGA results confirmed that the modified CB maintained good heat resistance. The method provided a facile route to prepare hydrophilic CB having terminal hydroxyl groups. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Carbon black (CB) are structurally complex particles and composed of irregularly shaped aggregates ranging in size from tens of nanometers to several hundred nanometers. CB have excellent chemical stability, heat resistance, electrical conductivity, darkness, and safe features; therefore, they are good ingredients for dark paints used for coatings [1], plastics [2], ink [3], and inkjet [4] applications. Most of these applications require a good dispersion of CB, and the solvent system is often water-based for environmental and cost reasons. However, the stability and dispersity of CB in aqueous solution are notoriously poor. Because of their low hydrophilicity, CB particles are prone to aggregate. Therefore, surface modification for improving the hydrophilicity of CB is of great significance. Various methods have been reported for the surface modification and functionalization of CB, including oxidation modification, graft polymerization, surface coating, and dispersant modification [5–8]. Jeguirim et al. studied the oxidation mechanism of carbon black by NO2 [5]. Paredes et al. enhanced the hydrophilicity

∗ Corresponding author at: State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China. Tel.: +86 10 64443413; fax: +86 10 64456158. E-mail address: [email protected] (W.C. Wang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.016

of CB by plasma treatment [6]. Li et al. synthesized hydrophilic polyacrylate-g-carbon black by the free radical polymerization of acrylate monomers on the CB surface, and the grafted CB exhibited excellent hydrophilicity and dispersity [1]. Li et al. prepared PVAencapsulated hydrophilic CB and PAM-encapsulated hydrophilic CB [7]. Qi used peptides as dispersants and binders to disperse CB [8]. However, most of the above methods have limitations such as tedious steps, complex instruments, drastic reaction conditions, and high toxicity. Thus, the development of a facile and environment-friendly strategy for the surface hydrophilic modification of CB is desirable. Recently, biological methods used in materials preparation and modification have attracted scientists’ keen interests [9–12]. Inspired by the adhesion of proteins in mussels and geckos to universal substrates [13–15], researchers studied the composition of these adhesive proteins, which are rich in 3,4-dihydroxy-lphenylalanine (DOPA) and lysine amino acids [16]. DOPA, as the main ingredient of these adhesive proteins, and other catechol compounds are expected to perform well as binding agents for coating various inorganic and organic substrates (including such materials as metals, ceramics, semiconductors, and polymers), and the adhesion of DOPA and other catechol compounds to substrates can be enhanced for the strong covalent and non-covalent interactions between DOPA and substrates [17]. Dopamine (3,4-dihydroxyphenethylamine), which is a derivative

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of DOPA, contains catechol groups and ethylamino groups and can form a hydrophilic polymer layer around the surface of almost any substrate [17–19]. Besides, the polymerization of dopamine can be controlled by dopamine concentration, reaction time, and pH. Thicker polydopamine layers were formed with higher dopamine concentrations and longer reaction times [17,20]. The excellent adhesive ability of dopamine has been reported before. Lee et al. successfully coated various substrates (PTFE, PC, NC, SiO2 , TiO2 , Cu, Au) with oxide-polydopamine (PDA) [17]. Xi et al. modified hydrophobic membranes by DOPA and dopamine, and the membranes’ hydrophilicity was significantly improved after the modification [21]. The adhesive property of dopamine and the hydrophilicity of PDA provide a convenient and effective method for the hydrophilic modification of substrates. However, to the best of our knowledge, the preparation of hydrophilic CB by the method of oxidative polymerization and deposition of dopamine has not yet been reported. In this work, the polymerization of dopamine was used to form a hydrophilic surface layer for CB particles. The hydrophilic CB was simply made by dipping the CB into dopamine aqueous solution for a period of time. A PDA ad-layer was formed and deposited on the surface of the CB as a permanent and hydrophilic coating by the dopamine’s self-polymerization. 2. Material and methods

or FWHM) of Gaussian peaks was kept constant for all components in every spectrum. 2.4. Fourier transform infrared spectroscopy (FTIR) The chemical structure of dopamine-modified CB was characterized by FTIR. FTIR spectra were obtained by using a TENSOR 27 spectrometer under ambient conditions. The test samples were prepared by dispersing the CB in KBr uniformly and pressing the powder into pellets. The spectra were obtained in the wavenumber range from 600 cm−1 to 4000 cm−1 . Typically, 32 scans at a resolution of 4 cm−1 were accumulated to obtain one spectrum. 2.5. Scanning electron microscopy (SEM) The surface morphologies of the CB were observed by using a scanning electron microscope (S-4700, Hitachi, Japan). The CB powder was dispersed in distilled water and ultrasonic treated for 30 min. A drop of the CB dispersion was dripped on a small piece of microscopic glass, which was then mounted on a sample stud by means of double-sided adhesive tapes. A thin layer of platinum was sputtered on the surface of the sample prior to the SEM measurement. The SEM measurements were performed at an accelerating voltage of 20 kV.

2.1. Materials

2.6. Transmission electron microscopy (TEM)

CB (N330) with a mean diameter of about 30 nm was obtained from Shandong Bestar Carbon Black Factory, Shandong Province, China. The CB particles were extracted by methylbenzene for 36 h before use. Dopamine and Tris(hydroxymethyl)aminomethene (Tris) were purchased from Alfa Aesar Company, USA. Hydrochloric acid was offered by Beijing Chemical Plant, China. All the chemicals were analytical reagents and used without further purification.

The morphologies and distribution of CB nanoparticles were studied by using a transmission electron microscope (H-800 and JEM-3010, Hitachi, Japan) operating at a voltage of 200 kV. For electron microscopy, CB was dispersed in distilled water and ultrasonic treated for 30 min. Then a drop of the dispersion was dripped on a copper screen and allowed to dry. 2.7. Water contact angle measurements

2.2. Dopamine oxidation polymerization on CB surface CB powder was dispersed in distilled water followed by ultrasonic treatment for 30 min. Tris was added into the dopamine solution to form a homogeneous dopamine/Tris solution. Then the CB dispersion was added into the dopamine/Tris solution, and 1 M HCl was added into the CB/dopamine/Tris solution dropwise until the pH was 8.5. The final CB concentration was 3 g/L and that of Tris was 10 mM. The concentrations of dopamine were 1.0 g/L, 2.0 g/L, 3.0 g/L, and 4.0 g/L. After a predetermined time (10 h, 16 h, 20 h, or 24 h), the PDA-coated CB was rinsed by distilled water for several times until the washing liquid became colorless. Then the CB was dried in a vacuum oven at 40 ◦ C for 48 h. The PDA-coated CB is denoted as CB-PDA. The samples of CB-PDA prepared under a dopamine concentration of 2.0 g/L and a reaction time of 10 h. 2.3. X-ray photoelectron spectroscopy (XPS) The composition of the CB was evaluated by X-ray photoelectron spectroscopy (ESCALAB 250, Thermo Electron Corporation, USA) with an Al K␣ (1486.6 eV photons) X-ray source. The CB powder was mounted on standard sample studs by means of double-sided adhesive tapes. During the measurements, the X-ray source was run at a reduced power of 150 W, and the pressure in the chamber was maintained at about 10−8 Torr or lower. The core-level signals were received at a photoelectron take-off angle of 45◦ with respect to the sample surface. The C 1s hydrocarbon peak at 284.6 eV was used as the reference. Gaussian peaks and linear baseline were used in the data analysis, and the line width (full width at half maximum

Static water contact angles of the CB were measured at 25 ◦ C and 50% relatively humidity by the sessile drop method using a 2 ␮L water droplet in a telescopic goniometer (OCA15 EC, Dataphysics, Germany). The telescope with a magnification power of 23× was equipped with a protractor with 1◦ graduations. The CB pellet was obtained by pressing the carbon powder (about 20 mg) with a mold at room temperature under a load of 10 MPa for about 4 min. For each sample, at least fifteen measurements obtained from different surface locations were averaged. The angles measured were repeatable to ±3◦ . 2.8. Stability measurements The stability of CB in aqueous solution was measured by natural sedimentation. The CB powder was dispersed in distilled water with ultrasonic treatment for 30 min, and the CB dispersion (0.06 wt.% CB) was laid up on the shelf without movement at room temperature. The sedimentation of CB particles was observed for a predetermined time. 2.9. Particle size distribution measurements The particle size distribution of CB was measured by a laser particle analyzer (Zetasizer Nano-ZS90, Malvern, Great Britain). The CB powder was dispersed in distilled water with ultrasonic treatment for 30 min, and the dispersion (0.005 wt.% CB) was laid up in the laser particle analyzer.

L.J. Zhu et al. / Applied Surface Science 258 (2012) 5387–5393

wide-scan

(a)

C 1s

wide-scan

(d) C 1s

Table 1 Compositions of pristine CB, PDA homopolymer, dopamine, and CB-PDA nanoparticles.

O 1s

O 1s

C 1s (at.%)

O 1s (at.%)

N 1s (at.%)

N/C

90.39 68.78 72.73

9.61 25.28 18.18

0 5.94 9.09

– 0.086 0.125

Reaction time = 10 h, pH 8.5 83 Cdopamine a = 1.0 g/L 85.05 Cdopamine = 2.0 g/L Cdopamine = 3.0 g/L 79.67 Cdopamine = 4.0 g/L 81.7

14.48 12 16.53 13.92

2.52 2.95 3.8 4.38

0.030 0.035 0.048 0.054

Cdopamine = 2.0 g/L, pH 8.5 Reaction time = 10 h Reaction time = 16 h Reaction time = 20 h Reaction time = 24 h

12 15.88 15.53 18.57

2.95 3.74 4.17 4.75

0.035 0.047 0.052 0.062

N 1s

Intensity (A.U.)

Pristine CB PDA Dopamine (theoretical)

260

390

520

260

C 1s

(b)

282

(e)

C-O C=O O-C=O

C-C

285

288

291 N 1s

(c)

390

520

C 1s

C-C C-O C-N

282

C=O

285

(f)

O-C=O

288

291 N 1s

399

402

a

85.05 80.37 80.30 76.68

Cdopamine is the concentration of dopamine).

-N-H -N=

396

5389

396

399

402

Binding Energy (eV) Fig. 1. XPS wide-scan, C 1s, and N 1s core-level spectra of ((a), (b), (c)) pristine CB and ((d), (e), (f)) CB-PDA.

2.10. Thermal gravimetric analysis (TGA) The thermal gravimetric analysis (TGA) of CB was carried out with a TGA analyzer (STARe System, Mettler-Toledo, Switzerland). Each sample weighing about 10 mg was heated from 30 ◦ C to 800 ◦ C at a rate of 10 ◦ C/min in a nitrogen atmosphere. 3. Results and discussion XPS was used to determine the surface chemical compositions of the pristine CB and the CB-PDA nanoparticles. Fig. 1 shows the XPS wide-scan, C 1s, and N 1s core-level spectra of the pristine CB (Fig. 1(a)–(c)) and the CB-PDA (Fig. 1(d)–(f)) nanoparticles. The wide-scan spectrum given in Fig. 1(a) for the pristine CB nanoparticles shows two peak components: C 1s and O 1s. The XPS C 1s core-level spectrum of the pristine CB can be curved-fitted with four peak components at the binding energies (BEs) of about 284.6, 286.4, 287.8, and 288.9 eV, attributable to the C H and C C species, C O species, C O species, and O C O species, respectively (Fig. 1(b)). On the other hand, the XPS C 1s core-level spectrum of the CB-PDA can be curved-fitted with five peak components at the BEs of 284.6, 285.5, 286.4, 287.8, and 288.9 eV, attributable to the C H and C C, C N, C O, C O, and O C O species, respectively (Fig. 1(e)). It is noted that the peak component at the BE of 285.5 eV comes from the C N species of PDA. The presence of PDA on the surface of CB can be deduced from the appearance of the N 1s peak at the BE of about 399 eV in Fig. 1(d) and (f). Fig. 1(f) shows two additional peak components at the BE of 398.5 eV for the N species and the BE of 399.5 eV for the N H species in the N 1s spectrum for CB-PDA, which are absent in the N 1s spectrum for the pristine CB (Fig. 1(c)). The N H species and the N species may be attributed to PDA. All these results confirm the successful deposition of PDA on the CB surface. Table 1 shows that the self-polymerization of dopamine depends on dopamine concentration and reaction time. Table 1 summarizes the chemical compositions of the pristine CB and the

CB-PDA surface. The molar N/C ratio for the CB-PDA (Cdopamine (dopamine concentration) = 2.0 g/L, reaction time = 10 h, pH 8.5) is 0.035, significantly smaller than that for PDA (N/C = 0.086). The lower N/C ratio for CB-PDA suggests that the thickness of the PDA layer is approaching the probe depth (about 7.5 nm in an organic polymer matrix [22]) of the XPS technique. The value of N 1s and molar N/C ratio for CB-PDA increases with increasing dopamine concentration and reaction time. As the N 1s species can come from PDA only, the increase of the value of N 1s (at.%) and molar N/C ratio can confirm the growth of the PDA layer, and the results in Table 1 indicate that the PDA layer thickness was a function of dopamine concentration and reaction time. Although the method of dopamine self-polymerization has been reported as a facile and versatile way for surface modification and functionalization [17–21], the mechanism for dopamine selfpolymerization remains unknown. Moreover, the physicochemical details of dopamine-surface interaction remain elusive. On the basis of previous studies [17,18] and the XPS results presented above, a possible mechanism for dopamine oxidative selfpolymerization is presented in Fig. 2. Under an aerobic and alkaline condition, the dihydroxyl groups deprotonate and oxidize to form dopaminequinone. The dopaminequinone undergoes cyclization, oxidation, and rearrangement to form 5,6-dihydroxyindole which further oxidizes and polymerizes through deprotonation and inter-molecular Michael addition reaction to form a crosslinked homopolymer. Thus, the mechanism of dopamine polymerization is in good agreement with the XPS results in Fig. 1. We can now attribute the N H species to the amine groups of PDA, and the N species to the structure evolution during the oxidative polymerization of dopamine. FTIR was also employed to determine the chemical structures of the pristine CB and CB-PDA nanoparticles. Fig. 3 shows the FTIR spectra of dopamine monomer (Fig. 3(a)), PDA homopolymer (Fig. 3(b)), pristine CB (Fig. 3(c)), and CB-PDA (Fig. 3(d)). The absorption bands associated with the phenol structure near the wavenumbers of 3426 cm−1 (aromatic O-H stretching vibration) and 1626 cm−1 (aromatic C C stretching vibration) are present for all the samples. Comparing the FTIR spectra of dopamine, PDA, and CB-PDA with that of pristine CB, we found that the width of the O H bands of dopamine, PDA, and CB-PDA is much broader because of the contribution of N H stretching vibration. In addition, compared with the corresponding bands in the spectra of pristine CB and dopamine monomer, the bands between 1620 cm−1 and 1750 cm−1 in the spectra of PDA and CB-PDA are much wider as a result of the stretching vibration of C N in the range of 1620-1650 cm−1 , and this result further verifies the mechanism proposed in Fig. 2. The new absorption band at 1516 cm−1 noted in the CB-PDA spectrum

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Absorbance (A.U.)

Fig. 2. Possible reaction mechanism for dopamine polymerization.

(a) 3426

(b)

1626

(c) 1516

(d) 4000

3500

3000

2500

2000

1500

1000

-1

Wavenumber (cm ) Fig. 3. FTIR spectra of (a) dopamine monomer, (b) PDA homopolymer, (c) pristine CB, and (d) CB-PDA.

must be associated with the PDA coating because there is a similar band arising from N H shearing vibration in the PDA spectrum. It is also useful to compare the FTIR spectrum of dopamine monomer to that of PDA. There are broad absorption bands in the dopamine spectrum between 1500 cm−1 and 600 cm−1 , such as at 1345 cm−1 (CH2 bending vibration), 1321 cm−1 (C O H asymmetric bending vibration), 1191 cm−1 (C O asymmetric vibration), and 1025 cm−1

(C N stretching vibration), consistent with the FTIR character of dopamine, which contains unsaturated aromatic phenol, CH2 , and NH2 . In the spectra of PDA and CB-PDA, the broad bands are not discernible, confirming that the PDA and CB-PDA samples were free of dopamine and the absorption band at 1516 cm−1 comes from PDA. The surface morphology of the CB nanoparticles was observed by SEM. The SEM micrographs of the pristine CB and the CBPDA nanoparticles are shown in Fig. 4(a) and (b), respectively. It can be seen from Fig. 4(a) that the surface of the pristine CB (mean diameter is about 30 nm) is rough. The CB-PDA nanoparticles show a smoother and more ambiguous surface than the pristine CB nanoparticles, and the CB-PDA nanoparticles are larger than the pristine CB nanoparticles, both results indicating that the PDA formed a distinctive layer on the CB surface. Because of the high surface energy of nanoparticles, most pristine CB nanoparticles form agglomerates. However, after the encapsulation of CB particles with PDA, the number of relatively large agglomerates decreases significantly because large agglomerates were divided into small agglomerates, as shown in Fig. 4(b). The abundant hydrophilic groups introduced by PDA on the CB surface make the particles disperse homogeneously in water. Furthermore, the PDA layer around the CB nanoparticles acts as a steric hindrance to the CB nanoparticles and prevents the CB nanoparticles from aggregation. The morphology and dispersion of CB were further studied by TEM. The TEM image of the pristine CB is shown in Fig. 5(a). It can be seen from Fig. 5(a) that the particles tend to aggregate. The TEM images of the CB-PDA with different magnifications are shown in Fig. 5(b) and (c). Comparing the TEM image of CB-PDA in Fig. 5(b)

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Fig. 4. SEM images of (a) pristine CB and (b) CB-PDA.

with that of the pristine CB (Fig. 5(a)), we can see an increase in the particle size of CB after PDA coating. Moreover, a thin layer of PDA coating about 1–5 nm thick can be observed on the CB-PDA nanoparticles, as shown in Fig. 5(c). The above phenomena all suggest that the CB nanoparticles are well encapsulated by the PDA homopolymer and agglomerate less after the encapsulation. These results are well consistent with the XPS and SEM results. The hydrophilicity of the modified CB particles is one of the major concerns of this study. The surface hydrophilicity of the pristine CB nanoparticles and the CB-PDA nanoparticles was measured by the method of static water contact angle at room temperature, and the images are shown in Fig. 6. In this method, 2 ␮L of distilled water was placed on the surface of the CB pellets, and then the contact angles were calculated automatically by the software SCA20. The compact pellets of the pristine CB exhibited a hydrophobic surface with a contact angle of about 123◦ . After the CB particles were encapsulated by PDA, the water contact angle of the CB powder decreased significantly. The contact angle of the CB-PDA was about 28◦ , indicating that the surface of the CB-PDA nanoparticles was hydrophilic. This transformation is mainly due to the introduction of large numbers of hydrophilic groups such as the phenolic hydroxyl groups and imino groups of PDA on the CB surface. It should also be noted that during the measurements, the droplet remained stable on the pristine CB pellets for a long time. On the other hand, the droplet spread out quickly and was absorbed in a few seconds into the CB-PDA pellets, indicating the highly hydrophilic nature of the CB-PDA surface. The dispersivity and stability of the CB-PDA nanoparticles in aqueous solution were evaluated by natural placement. For comparison, the pristine CB particles were also placed under the same conditions. The results of comparison are shown in Fig. 7. It can be seen from Fig. 7(b) that the pristine CB nanoparticles (left bottle) settle to the bottom very quickly and precipitate almost completely after 3 days. In contrast, the CB-PDA nanoparticles (right bottle)

Fig. 5. TEM images of (a) pristine CB and ((b), (c)) CB-PDA.

became extremely hydrophilic and yielded a stable colloidal dispersion in water. As shown in Fig. 7(c), the CB-PDA nanoparticles form a homogeneous dispersion in water and are stable for at least one month before any noticeable flocculation. The results further indicate that the CB particles were encapsulated by PDA, which contains abundant amine and hydroxyl groups, leading to the excellent solubility of CB in water. Nanoparticles are prone to aggregate for their high surface energy, but the aggregation can be repressed and small aggregates are expected to form if the nanoparticles are hydrophilic. So particle size distribution is another major concern of this work, and the particle size here refers to the size of tens of aggregated CB-PDA nanoparticles. A laser particle analyzer was used

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280

Particle Size (nm)

270 260 250 240 230 220 210 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Dopamine Concentration (g/L) Fig. 6. Static water contact angle of (a) pristine CB and (b) CB-PDA.

Fig. 8. Effect of dopamine concentration on particle size distribution of CB-PDA nanoparticles.

Mass (%)

to analyze the particle size distribution of CB-PDA at different dopamine concentrations. It can be seen from Fig. 8 that with the increase of dopamine concentration up to 2.0 g/L, the average particle size of CB-PDA decreases from 277 nm to 212 nm because of the hydrophilic character of PDA on the surface of CB. PDA, which is rich in hydrophilic catechol and amine groups, contributes to the high dispersivity and stability of CB and disrupts the aggregation of CB. Although a further increase in dopamine concentration to 3.0 g/L did improve the hydrophilicity of the CB-PDA nanoparticles even more because of the increase in the number of hydrophilic groups on the surface of CB, the particle size of the CB-PDA nanoparticles also increased as a result of the thicker PDA layer. The thermal stability of the CB nanoparticles was evaluated by thermal gravimetric analysis (TGA). Fig. 9 shows the TG thermograms of pristine CB (curve (a)), CB-PDA (curve (b)), and PDA homopolymer (curve (c)). As shown in Fig. 9(c), the PDA homopolymer exhibits a two-step weight loss process. The onset of the first major weight loss starts at about 40 ◦ C, attributable to the elimination of moisture and other thermolabile residuals. The second major weight loss begins at about 300 ◦ C, corresponding to the decomposition of the PDA main chain. The weight loss of the PDA homopolymer is about 50% up to 800 ◦ C, indicating that the PDA homopolymer has excellent thermal stability. The pristine CB nanoparticles show no obvious weight loss during the thermogravimetric analysis, consistent with their high thermal stability. On the other hand, the CB-PDA nanoparticles show a weight loss of less than 15% up to 800 ◦ C, a significant result for some applications. 120 110 100 90 80 70 60 50 40 30 20 10 0

(a) (b)

(c)

0 Fig. 7. Natural sedimentation in water of (a) 0 day later, (b) 3 days later, and (c) one month later. (Pristine CB (left bottle), CB-PDA (right bottle)).

100

200

300

400

500

Temperature

600

700

800

900

(oC)

Fig. 9. TGA curves of (a) pristine CB, (b) CB-PDA, and (c) PDA homopolymer.

L.J. Zhu et al. / Applied Surface Science 258 (2012) 5387–5393

The mass of the residual CB-PDA powder also supports the conclusion that PDA was successfully deposited onto the CB nanoparticles, in accordance with all the results above. 4. Conclusion Highly hydrophilic CB nanoparticles were successfully prepared through the oxidative polymerization of dopamine. The hydrophilic catechol and indole groups of PDA provide the CB nanoparticles with good hydrophilicity and dispersion stability. The thickness of the PDA layer can be well controlled by varying the dopamine concentration and reaction time. The hydrophilic CB nanoparticles obtained by this method showed a water contact angle as low as 28◦ and exhibited excellent dispersion ability in aqueous solution, remaining stable for months. The TGA measurements confirmed that the heat resistance of the CB treated with dopamine was maintained even at high temperatures. The method demonstrated in this work provides a facile, controllable, and environment-friendly way for CB’s surface modification and functionalization, and this method is versatile because the PDA layer on CB surface constructs a platform for the functionalization of a substantial number of other materials. Acknowledgments The authors acknowledge the financial supports from the Natural Science Foundation of China (grant no. 51073013), the Program for New Century Excellent Talents in University (NCET), and the Program for Changjiang Scholars and Innovative Research Team in University (grant no. IRT0807), Ministry of Education, China. References [1] W. Li, Z.M. Xie, Z.M. Li, Synthesis, characterization of polyacrylate-g-carbon black and its application to soap-free waterborne coating, J. Appl. Polym. Sci. 81 (2001) 1100–1106. [2] J.C. Grunlan, W.W. Gerberich, L.F. Francis, Lowering the percolation threshold of conductive composites using particulate polymer microstructure, J. Appl. Polym. Sci. 80 (2001) 692–705.

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