- Email: [email protected]
Cotton fibers nano-TiO2 composites prepared by as-assembly process and the photocatalytic activities
Materials Research Bulletin 47 (2012) 3943–3946
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Cotton fibers nano-TiO2 composites prepared by as-assembly process and the photocatalytic activities J.H. Xia *, C.T. Hsu, D.D. Qin School of Engineering, The Hong Kong University of Science and Technology, Hong Kong, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 November 2011 Received in revised form 19 June 2012 Accepted 28 July 2012 Available online 14 August 2012
This paper describes photocatalytic cotton fibers produced by a TiO2 nanoparticle self-assembly process with the assistance of carboxylic groups. The carboxylic group was introduced by a displacement reaction, the molecular structure of the glucose unit was studied by utilizing solid 13C NMR. The appearance of the prepared fibers was observed by scanning electron microscopy, it was found that nanoTiO2 coated uniformly on the fiber surface. The loading amount of nano-TiO2 was depended on the displacement degree of C-6-OH. UV–Vis experiments showed these coated fibers undergo photocatalysis efficiently. The degradation reaction of Rhodamine 6G under UV light obeys the zero-order rate law. ß 2012 Elsevier Ltd. All rights reserved.
Keyword: A. Composites A. Surface A. Nanostructures C. Nuclear magnetic resonance (NMR)
1. Introduction Titanium dioxide nanoparticles (nano-TiO2) in the anatase form are an effective type of photocatalyst. They efficiently absorb solar energy to produce active superoxide anions and hydroxyl radicals, which can decompose organic and biological species. Nano-TiO2 is utilized by the textile industry to produce clothes with excellent anti-bacteria and anti-fungus properties. For this purpose, nanoTiO2 needs to be attached to the fiber firmly. Cotton fiber is widely used to make soft textile. The main ingredient of cotton fiber is cellulose, a linear polymer of D-glucose with 3 hydroxyl groups on C-2, C-3 and C-6 in the 6-member cycle chain. These polar groups can adsorb the nanoparticles via oxygen bonding [1]. However, the carboxylic group is more polar than the hydroxyl one, as well it has a stronger interaction with the hydroxylic group on the nanoparticle surface [2,3]. The oxygen from the hydroxyl group is a basic atom and a nucleophilic center, it easily attacks the carbon and then a displacement reaction is carried out after leaving the halide ion. This characteristic can be utilized to convert –OH into –COOH. The newly formed COOH can be utilized to anchor TiO2 to produce more stable nano-TiO2–polymer composites. The general strategy of coating nano-TiO2 on the fiber surface is by a common dipping or dripping technique [4–6]. The coating thickness is at the mm-scale even without repeated dipping operation. For practical applications, these methods will lead to
* Corresponding author. Tel.: +852 23588808; fax: +852 23588719. E-mail address: [email protected] (J.H. Xia). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.07.022
nano-TiO2 quite heterogeneous distribution, this is because bundles and bundles fibers are used like a filter. However, it is very important that the fibers retain their natural features after the surface modification. Particle-loading is an important parameter for this kind of material process. It is necessary to develop a technique that can be carried out in room temperature without heating to produce a thin film and tune the TiO2-loading. In this paper, a thin layer of nano-TiO2 was developed by selfassembling nanoparticles on the fiber surface. 2. Experiment In this paper, TiOx (0 < x 2) nanoparticles were produced by the arc discharge method. Nanoparticles were previously prepared using arc discharge in a liquid medium, such as deionized water [7,8], or ethanol [9]. The experiment was conducted with the following parameters, a 1 mm diameter titanium wire as the consumable anode, graphite cathode with a 3 mm diameter, and an applied voltage of 130 V. The two electrodes were submerged into a liquid medium in a glass beaker. The arc discharge was controlled by a DC power and a stepping motor. Before the experiment, cotton fiber was dipped into ethanol and deionized water respectively in an ultrasonic bath for 3 times. The carboxylic acid groups were introduced to the D-glucose unit via chloroacetic acid reaction. Typically, cotton fiber was treated by a strong alkali and chloroacetic acid. After the alkoxide has attacked the carbon-chlorine bonds and left the chlorine atoms, carboxymethyl cellulose was formed. Before further experiments, these as-prepared fibers were washed 3 times in a deionized water bath.
3944
J.H. Xia et al. / Materials Research Bulletin 47 (2012) 3943–3946
TiO2-coated cotton fibers (TCFs) were produced by soaking the cotton fibers into the TiO2 suspension, and then left to dry in air for 24 h; the same procedure is typically repeated for 3 times. The experiments of Rhodamine 6G (R6G) degradation by TCFs were carried out in a UV irradiation chamber. Samples with an initial concentration of 5 105 mol/L of R6G, the catalyst loading 1 g/kg, were irradiated with a 4 W UV lamp (peak wavelength 370 nm), bubbled air was fed into the dye solution at a flow rate of 100 mL/min. The environment temperature throughout the experiments was maintained at around 25 8C. Samples for UV– Vis testing were obtained at an interval of 30 min after centrifugation. The intensity change of the absorption maximum indicated the degree of R6G concentration varied according to Beer’s law. The final concentrations of R6G were obtained from the calibration curve of the standard solution. Characterization. Scanning electron microscopy (SEM) was carried out using ultra-high resolution SEM (1 nm at 15 kV and 2.2 nm at 1 kV, a cold field emission gun, and the model JSM-6700F). The structural properties of TiO2 particles were analyzed by Xray diffraction (XRD). A PAnalytical powder X-ray diffractometer was equipped with an X’pert Pro model, and a monochromatized X-ray beam from the nickel-filtered CuKa radiation. For XRD testing, the samples were deposited onto glass slides, followed by vacuum-drying at room temperature. The experiments were performed on a solid state nuclear magnetic resonance spectrometer (Solid 13C NMR, Model NMESH40MU, JEOL, frequency range 40–162 MHz). The degree of dye decoloration was measured with a spectrophotometer (Perkin Elmer, Lambda 20, range 190– 1100 nm, resolution 1 nm) at the peak intensity l = 526 nm. 3. Results and discussion 3.1. XRD analysis Powder samples were produced by vaporizing the medium followed by rinsing with ethanol and then deionized water, and finally drying in a 50 8C vacuum oven to remove the water. The powder samples were sintered at different temperatures to study the amount of carbon removed and titanium atom oxidation. After thermal post-treatment, crystals were formed and compositions of as-synthesized nanoparticles were measured by XRD. In this paper, the experiment temperature was raised to 450 8C and 600 8C and maintained for 30 min. 2u were collected from 208 to 708, and the
XRD patterns are shown in Fig. 1. Before thermal treatment, the starting powder was assigned to TiC. The 450 8C-samples had main composition mixing TiO2 and TiC, indicating that the sintering temperature of 450 8C was not high enough to remove the carbon and anatase TiO2 formation. However, the 600 8C-samples showed a mixing phase of anatase and rutile. The results indicate that a heating temperature of 600 8C is sufficient to oxidize Ti to TiO2 and remove the carbon. The rutile appearance suggests transformation from anatase to rutile occurred [10]. However, anatase TiO2 has a better photocatalytic activity and stronger antibacterial property than rutile. So in this paper, the treatment temperature was no more than 600 8C. 3.2. NMR studies Cotton fiber consists of many glucose units, 13C NMR can give the spectrum of chemical shifts of C-1 to C-6 in the glucose units [11]. If a chemical reaction occurs in the backbone unit to link some chemical groups, there will be new peaks or a change in the intensity of the signal. In this study, the spectra recorded by a solid state 13C NMR spectrometer were studied by comparing before and after the treatment of chloroacetic acid to characterize the formation of carboxymethyl groups. Fig. 2.1 shows the typical spectra of cellulose obtained, four main peaks were assigned to the chemical shifts of C1, C-4, C-2–C-5, and C-6. Comparing the two samples (Figs. 2.1 and 2.2), the appearance of two new peaks with chemical shifts at 175.2 ppm and 44.5 ppm is noticed. The signal intensity of C-2 and C-3 decreased slightly, and C-6 at 64.3 ppm shows a like broad peak from 59.6 to 65.9 ppm; there is much difference to the original fibers without treatment. For the R-COOH compounds, a typical position of the carbonyl carbon is in the region of 160–180 ppm. The 175.2 ppm signal was assigned to the contribution of the carboxyl groups [12]. The new peak of 44.5 ppm was assigned to methylene. By comparison, in the original cotton fiber (Fig. 2.1), the NMR spectra changes correspond to the results that the displacement reaction of hydroxyl mainly on C-2 and C-3 (C–OH) with carboxylmethyl group underwent. By this way, the carboxylic groups (–O–C–COOH) were introduced to the cotton fiber surface. 3.3. TEM observation The main purpose of cotton fiber modification is to introduce new features to the fabric. For a good anti-bacteria and anti-fungus effect, as well as retaining the macroscopic properties of soft textile, the fibers should be coated with no more than a single layer C 2, 3, 5
7 6
Signal Intensity
5
C6
4
C1
65.9 ppm
C4
59.6 ppm
3
-COOH 175.2 ppm
CH2
2
2
1 0
1
C6 64.3 ppm 0
20
40
60
80
100
120
140
160
180
200
Chemical Shift (ppm) Fig. 1. XRD patterns of TiO2. Reflections from anatase and rutile TiO2 are indicated by A, R.
Fig. 2. 13C NMR spectra of cotton fiber before or after chloroacetic acid treatment. (1) Original fiber, (2) the fiber after chloroacetic acid treatment.
J.H. Xia et al. / Materials Research Bulletin 47 (2012) 3943–3946
of nano-TiO2. The appearance of the experimental fibers before and after nano-TiO2 was coated on the cotton fibers was analyzed and compared using SEM images. In Fig. 3, the SEM images show the fibers were coated uniformly with nano-TiO2. There were also some uncoated surfaces on the fibers. This is beneficial for lowering the nano-TiO2-loading and retaining the original physical properties of the fibers. The nanoTiO2 covered not only the surface of the fiber sheet but also the area between individual fibers. This phenomenon can be explained by the fact that the carboxylic groups have existed along the polymer fibers. The strong polar carboxylic groups could anchor the nanoTiO2 anywhere on the fibers’ surface via chemical interaction. This is quite different from the simple soaking method. The method adopted in this paper contributed to the nano-TiO2 self-assembly process on the fiber surface. At the same time, this chemical
3945
interaction led to monolayer formation and mitigated the problem of large amounts of excess TiO2 nanoparticle deposition on the fiber surface. From the NMR results, we found that –O–C–COOH was formed by HCl elimination. The hydroxylic groups were more active in adsorbing TiO2 by interaction with Ti–OH and –O–C–COOH. As a result of this reaction, inorganic nanoparticles attached easily onto the fiber surface. The schematic of the process is shown in Fig. 4. One particle can react with a few carboxylic groups. The conversion yield of C–OH to –O–C–COOH is not 100%, this is the reason why there was an uncoated area (Fig. 3A). Increasing the treatment time of chloroacetic acid reaction increased the conversion yield of C–OH to –O–C–COOH. So, there were more –COOH per unit surface area of fiber, which resulted in an increase in the amount of attached particles (Fig. 3E and B). This is a practical and easy way to
Fig. 3. SEM images of (A) naked cotton fibers, (B, C, E) TiO2-coated cotton fibers, (D) enlarged image of selected area in image B.
Fig. 4. Schematic model of TiO2 self-assembled on the surface of cotton fibers.
J.H. Xia et al. / Materials Research Bulletin 47 (2012) 3943–3946
3946 R6G Decoloration under UV 0.5 h
In the same figure, degradation degree y ¼ ðC 0 C t Þ=C 0 , x axis is the degradation time. The degradation degree increased with time. A linear trend line was simulated and can be described by the equation y ¼ k x þ b. The results show the degradation rate is almost constant, k = 20.3 h1. That means the degradation reaction obeys the zero-order rate law. After 3.5 h, nearly 70% of the R6G was decomposed.
R6G Degradation (%)
80
y = 20.357x - 4.5 60
2
R = 0.9646 3.5 h
40
4. Conclusions 500
20
y = 2.8204x - 0.2635 2 R = 0.9516
600
0 0
0.5
1
1.5
2
2.5
3
3.5
4
Time (h) Fig. 5. Kinetics of the photocatalytic degradation of aqueous solutions of R6G under UV irradiation. (a, ^ TCFs), (b, & naked cotton fiber without TiO2). Absorbance values at 526 nm were measured to calculate the concentration of R6G. The inset shows the absorption spectra of the R6G solution at various time intervals after ultracentrifugation (105 rpm).
Cotton fiber modified by a polar carboxylic group can plant nano-TiO2 by a self-assembly process to produce TiO2 composites. The most interesting feature of this process is nano-TiO2 loading can be controlled by the number of carboxylic groups per unit area of the fiber surface. Photocatalyst performance was studied using R6G as a photodegradation goal, under 370 nm of UV light. After 3.5 h, around 70% of the R6G was decomposed. By calculation, the degradation reaction obeys the zero-order rate law. The photocatalytic activities show potential for practical applications in the antibacterial textile industry. Acknowledgements
control the inorganic nanoparticle loading by changing the degree of C–OH displacement.
We are grateful to Hong Kong ITC Project (GHP/028/08SZ) for financial support. We would also like to thank MCPF for material properties characterization.
3.4. Photocatalytic activities experiments
References
Optical absorption depends on the concentration of conjugated chromophore in a molecular structure. R6G is a common dye to determine the nano-TiO2 photodegradation activities [10]. For R6G, absorption at 526 nm was assigned to the response of the conjugated xanthene ring in a molecule structure. So the reduction in the absorption maximum indicates partial decomposition of the R6G. In this case, under UV illumination, hydroxyl radicals were formed on the nano-TiO2 surface. The active hydroxyl radicals are a far stronger oxidizing agent and have the ability to react with many species [13]. A useful method to determine the degradation rate of R6G is to measure absorbance values at the peak position at series time intervals of UV irradiation. In Fig. 5 the inset shows the absorbance change as a function of UV irradiation time. The R6G concentration decreased with illumination time during the degradation process.
[1] E.M. Rockafellow, X. Fang, B.G. Trewyn, K. Schmidt-Rohr, Chem. Mater. 21 (2009) 1187. [2] D. Zhao, C. Chen, Y. Wang, W. Ma, J. Zhao, T. Rajh, L. Zang, Environ. Sci. Technol. 42 (2008) 308. [3] W. Zhao, Y. Sun, F.N. Castellano, J. Am. Chem. Soc. 130 (2008) 12566. [4] N.J. Peill, M.R. Hoffmann, Environ. Sci. Technol. 29 (1995) 2974. [5] W. Choi, J.Y. Ko, H. Park, J.S. Chung, Appl. Catal. B: Environ. 31 (2001) 209. [6] A. Danion, J. Disdier, C. Guillard, O. Paısse, N. Jaffrezic-Renault, Appl. Catal. B: Environ. 62 (2006) 274. [7] N. Sano, H. Wang, M. Chhowalla, I. Alexandrou, G.A. Amaratunga, Nature 414 (2001) 506. [8] J.K. Lung, J.Ch. Huang, D.Ch. Tien, C.Y. Liao, K.H. Tseng, T.T. Tsung, W.S. Kao, T.H. Tsai, C.S. Jwo, H.M. Lin, L.J. Stobinski, J. Alloys Compd. 434–435 (2007) 655. [9] H. Chang, M.K. Liu, J. Cryst. Growth 304 (2007) 244. [10] S.C. Pillai, P. Periyat, R. George, D.E. McCormack, M.K. Seery, H. Hayden, J. Colreavy, D. Corr, S.J. Hinder, J. Phys. Chem. C 111 (2007) 1605. [11] R. Ek, H. Lennholm, R. Davidson, C. Nystro¨m, G. Ragnarsson, Int. J. Pharm. 122 (1995) 49. [12] S. Montanari, M. Roumani, L. Heux, M.R. Vignon, Macromolecules 38 (2005) 166. [13] K. Woan, G. Pyrgiotakis, S. Wolfgang, Adv. Mater. 21 (2009) 1.
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Cotton fibers nano-TiO2 composites prepared by as-assembly process and the photocatalytic activities J.H. Xia *, C.T. Hsu, D.D. Qin School of Engineering, The Hong Kong University of Science and Technology, Hong Kong, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 November 2011 Received in revised form 19 June 2012 Accepted 28 July 2012 Available online 14 August 2012
This paper describes photocatalytic cotton fibers produced by a TiO2 nanoparticle self-assembly process with the assistance of carboxylic groups. The carboxylic group was introduced by a displacement reaction, the molecular structure of the glucose unit was studied by utilizing solid 13C NMR. The appearance of the prepared fibers was observed by scanning electron microscopy, it was found that nanoTiO2 coated uniformly on the fiber surface. The loading amount of nano-TiO2 was depended on the displacement degree of C-6-OH. UV–Vis experiments showed these coated fibers undergo photocatalysis efficiently. The degradation reaction of Rhodamine 6G under UV light obeys the zero-order rate law. ß 2012 Elsevier Ltd. All rights reserved.
Keyword: A. Composites A. Surface A. Nanostructures C. Nuclear magnetic resonance (NMR)
1. Introduction Titanium dioxide nanoparticles (nano-TiO2) in the anatase form are an effective type of photocatalyst. They efficiently absorb solar energy to produce active superoxide anions and hydroxyl radicals, which can decompose organic and biological species. Nano-TiO2 is utilized by the textile industry to produce clothes with excellent anti-bacteria and anti-fungus properties. For this purpose, nanoTiO2 needs to be attached to the fiber firmly. Cotton fiber is widely used to make soft textile. The main ingredient of cotton fiber is cellulose, a linear polymer of D-glucose with 3 hydroxyl groups on C-2, C-3 and C-6 in the 6-member cycle chain. These polar groups can adsorb the nanoparticles via oxygen bonding [1]. However, the carboxylic group is more polar than the hydroxyl one, as well it has a stronger interaction with the hydroxylic group on the nanoparticle surface [2,3]. The oxygen from the hydroxyl group is a basic atom and a nucleophilic center, it easily attacks the carbon and then a displacement reaction is carried out after leaving the halide ion. This characteristic can be utilized to convert –OH into –COOH. The newly formed COOH can be utilized to anchor TiO2 to produce more stable nano-TiO2–polymer composites. The general strategy of coating nano-TiO2 on the fiber surface is by a common dipping or dripping technique [4–6]. The coating thickness is at the mm-scale even without repeated dipping operation. For practical applications, these methods will lead to
* Corresponding author. Tel.: +852 23588808; fax: +852 23588719. E-mail address: [email protected] (J.H. Xia). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.07.022
nano-TiO2 quite heterogeneous distribution, this is because bundles and bundles fibers are used like a filter. However, it is very important that the fibers retain their natural features after the surface modification. Particle-loading is an important parameter for this kind of material process. It is necessary to develop a technique that can be carried out in room temperature without heating to produce a thin film and tune the TiO2-loading. In this paper, a thin layer of nano-TiO2 was developed by selfassembling nanoparticles on the fiber surface. 2. Experiment In this paper, TiOx (0 < x 2) nanoparticles were produced by the arc discharge method. Nanoparticles were previously prepared using arc discharge in a liquid medium, such as deionized water [7,8], or ethanol [9]. The experiment was conducted with the following parameters, a 1 mm diameter titanium wire as the consumable anode, graphite cathode with a 3 mm diameter, and an applied voltage of 130 V. The two electrodes were submerged into a liquid medium in a glass beaker. The arc discharge was controlled by a DC power and a stepping motor. Before the experiment, cotton fiber was dipped into ethanol and deionized water respectively in an ultrasonic bath for 3 times. The carboxylic acid groups were introduced to the D-glucose unit via chloroacetic acid reaction. Typically, cotton fiber was treated by a strong alkali and chloroacetic acid. After the alkoxide has attacked the carbon-chlorine bonds and left the chlorine atoms, carboxymethyl cellulose was formed. Before further experiments, these as-prepared fibers were washed 3 times in a deionized water bath.
3944
J.H. Xia et al. / Materials Research Bulletin 47 (2012) 3943–3946
TiO2-coated cotton fibers (TCFs) were produced by soaking the cotton fibers into the TiO2 suspension, and then left to dry in air for 24 h; the same procedure is typically repeated for 3 times. The experiments of Rhodamine 6G (R6G) degradation by TCFs were carried out in a UV irradiation chamber. Samples with an initial concentration of 5 105 mol/L of R6G, the catalyst loading 1 g/kg, were irradiated with a 4 W UV lamp (peak wavelength 370 nm), bubbled air was fed into the dye solution at a flow rate of 100 mL/min. The environment temperature throughout the experiments was maintained at around 25 8C. Samples for UV– Vis testing were obtained at an interval of 30 min after centrifugation. The intensity change of the absorption maximum indicated the degree of R6G concentration varied according to Beer’s law. The final concentrations of R6G were obtained from the calibration curve of the standard solution. Characterization. Scanning electron microscopy (SEM) was carried out using ultra-high resolution SEM (1 nm at 15 kV and 2.2 nm at 1 kV, a cold field emission gun, and the model JSM-6700F). The structural properties of TiO2 particles were analyzed by Xray diffraction (XRD). A PAnalytical powder X-ray diffractometer was equipped with an X’pert Pro model, and a monochromatized X-ray beam from the nickel-filtered CuKa radiation. For XRD testing, the samples were deposited onto glass slides, followed by vacuum-drying at room temperature. The experiments were performed on a solid state nuclear magnetic resonance spectrometer (Solid 13C NMR, Model NMESH40MU, JEOL, frequency range 40–162 MHz). The degree of dye decoloration was measured with a spectrophotometer (Perkin Elmer, Lambda 20, range 190– 1100 nm, resolution 1 nm) at the peak intensity l = 526 nm. 3. Results and discussion 3.1. XRD analysis Powder samples were produced by vaporizing the medium followed by rinsing with ethanol and then deionized water, and finally drying in a 50 8C vacuum oven to remove the water. The powder samples were sintered at different temperatures to study the amount of carbon removed and titanium atom oxidation. After thermal post-treatment, crystals were formed and compositions of as-synthesized nanoparticles were measured by XRD. In this paper, the experiment temperature was raised to 450 8C and 600 8C and maintained for 30 min. 2u were collected from 208 to 708, and the
XRD patterns are shown in Fig. 1. Before thermal treatment, the starting powder was assigned to TiC. The 450 8C-samples had main composition mixing TiO2 and TiC, indicating that the sintering temperature of 450 8C was not high enough to remove the carbon and anatase TiO2 formation. However, the 600 8C-samples showed a mixing phase of anatase and rutile. The results indicate that a heating temperature of 600 8C is sufficient to oxidize Ti to TiO2 and remove the carbon. The rutile appearance suggests transformation from anatase to rutile occurred [10]. However, anatase TiO2 has a better photocatalytic activity and stronger antibacterial property than rutile. So in this paper, the treatment temperature was no more than 600 8C. 3.2. NMR studies Cotton fiber consists of many glucose units, 13C NMR can give the spectrum of chemical shifts of C-1 to C-6 in the glucose units [11]. If a chemical reaction occurs in the backbone unit to link some chemical groups, there will be new peaks or a change in the intensity of the signal. In this study, the spectra recorded by a solid state 13C NMR spectrometer were studied by comparing before and after the treatment of chloroacetic acid to characterize the formation of carboxymethyl groups. Fig. 2.1 shows the typical spectra of cellulose obtained, four main peaks were assigned to the chemical shifts of C1, C-4, C-2–C-5, and C-6. Comparing the two samples (Figs. 2.1 and 2.2), the appearance of two new peaks with chemical shifts at 175.2 ppm and 44.5 ppm is noticed. The signal intensity of C-2 and C-3 decreased slightly, and C-6 at 64.3 ppm shows a like broad peak from 59.6 to 65.9 ppm; there is much difference to the original fibers without treatment. For the R-COOH compounds, a typical position of the carbonyl carbon is in the region of 160–180 ppm. The 175.2 ppm signal was assigned to the contribution of the carboxyl groups [12]. The new peak of 44.5 ppm was assigned to methylene. By comparison, in the original cotton fiber (Fig. 2.1), the NMR spectra changes correspond to the results that the displacement reaction of hydroxyl mainly on C-2 and C-3 (C–OH) with carboxylmethyl group underwent. By this way, the carboxylic groups (–O–C–COOH) were introduced to the cotton fiber surface. 3.3. TEM observation The main purpose of cotton fiber modification is to introduce new features to the fabric. For a good anti-bacteria and anti-fungus effect, as well as retaining the macroscopic properties of soft textile, the fibers should be coated with no more than a single layer C 2, 3, 5
7 6
Signal Intensity
5
C6
4
C1
65.9 ppm
C4
59.6 ppm
3
-COOH 175.2 ppm
CH2
2
2
1 0
1
C6 64.3 ppm 0
20
40
60
80
100
120
140
160
180
200
Chemical Shift (ppm) Fig. 1. XRD patterns of TiO2. Reflections from anatase and rutile TiO2 are indicated by A, R.
Fig. 2. 13C NMR spectra of cotton fiber before or after chloroacetic acid treatment. (1) Original fiber, (2) the fiber after chloroacetic acid treatment.
J.H. Xia et al. / Materials Research Bulletin 47 (2012) 3943–3946
of nano-TiO2. The appearance of the experimental fibers before and after nano-TiO2 was coated on the cotton fibers was analyzed and compared using SEM images. In Fig. 3, the SEM images show the fibers were coated uniformly with nano-TiO2. There were also some uncoated surfaces on the fibers. This is beneficial for lowering the nano-TiO2-loading and retaining the original physical properties of the fibers. The nanoTiO2 covered not only the surface of the fiber sheet but also the area between individual fibers. This phenomenon can be explained by the fact that the carboxylic groups have existed along the polymer fibers. The strong polar carboxylic groups could anchor the nanoTiO2 anywhere on the fibers’ surface via chemical interaction. This is quite different from the simple soaking method. The method adopted in this paper contributed to the nano-TiO2 self-assembly process on the fiber surface. At the same time, this chemical
3945
interaction led to monolayer formation and mitigated the problem of large amounts of excess TiO2 nanoparticle deposition on the fiber surface. From the NMR results, we found that –O–C–COOH was formed by HCl elimination. The hydroxylic groups were more active in adsorbing TiO2 by interaction with Ti–OH and –O–C–COOH. As a result of this reaction, inorganic nanoparticles attached easily onto the fiber surface. The schematic of the process is shown in Fig. 4. One particle can react with a few carboxylic groups. The conversion yield of C–OH to –O–C–COOH is not 100%, this is the reason why there was an uncoated area (Fig. 3A). Increasing the treatment time of chloroacetic acid reaction increased the conversion yield of C–OH to –O–C–COOH. So, there were more –COOH per unit surface area of fiber, which resulted in an increase in the amount of attached particles (Fig. 3E and B). This is a practical and easy way to
Fig. 3. SEM images of (A) naked cotton fibers, (B, C, E) TiO2-coated cotton fibers, (D) enlarged image of selected area in image B.
Fig. 4. Schematic model of TiO2 self-assembled on the surface of cotton fibers.
J.H. Xia et al. / Materials Research Bulletin 47 (2012) 3943–3946
3946 R6G Decoloration under UV 0.5 h
In the same figure, degradation degree y ¼ ðC 0 C t Þ=C 0 , x axis is the degradation time. The degradation degree increased with time. A linear trend line was simulated and can be described by the equation y ¼ k x þ b. The results show the degradation rate is almost constant, k = 20.3 h1. That means the degradation reaction obeys the zero-order rate law. After 3.5 h, nearly 70% of the R6G was decomposed.
R6G Degradation (%)
80
y = 20.357x - 4.5 60
2
R = 0.9646 3.5 h
40
4. Conclusions 500
20
y = 2.8204x - 0.2635 2 R = 0.9516
600
0 0
0.5
1
1.5
2
2.5
3
3.5
4
Time (h) Fig. 5. Kinetics of the photocatalytic degradation of aqueous solutions of R6G under UV irradiation. (a, ^ TCFs), (b, & naked cotton fiber without TiO2). Absorbance values at 526 nm were measured to calculate the concentration of R6G. The inset shows the absorption spectra of the R6G solution at various time intervals after ultracentrifugation (105 rpm).
Cotton fiber modified by a polar carboxylic group can plant nano-TiO2 by a self-assembly process to produce TiO2 composites. The most interesting feature of this process is nano-TiO2 loading can be controlled by the number of carboxylic groups per unit area of the fiber surface. Photocatalyst performance was studied using R6G as a photodegradation goal, under 370 nm of UV light. After 3.5 h, around 70% of the R6G was decomposed. By calculation, the degradation reaction obeys the zero-order rate law. The photocatalytic activities show potential for practical applications in the antibacterial textile industry. Acknowledgements
control the inorganic nanoparticle loading by changing the degree of C–OH displacement.
We are grateful to Hong Kong ITC Project (GHP/028/08SZ) for financial support. We would also like to thank MCPF for material properties characterization.
3.4. Photocatalytic activities experiments
References
Optical absorption depends on the concentration of conjugated chromophore in a molecular structure. R6G is a common dye to determine the nano-TiO2 photodegradation activities [10]. For R6G, absorption at 526 nm was assigned to the response of the conjugated xanthene ring in a molecule structure. So the reduction in the absorption maximum indicates partial decomposition of the R6G. In this case, under UV illumination, hydroxyl radicals were formed on the nano-TiO2 surface. The active hydroxyl radicals are a far stronger oxidizing agent and have the ability to react with many species [13]. A useful method to determine the degradation rate of R6G is to measure absorbance values at the peak position at series time intervals of UV irradiation. In Fig. 5 the inset shows the absorbance change as a function of UV irradiation time. The R6G concentration decreased with illumination time during the degradation process.
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