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Photocatalytic self-cleaning properties of cellulosic fibers modified by nano-sized zinc oxide
Thin Solid Films 519 (2011) 3641–3646
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Photocatalytic self-cleaning properties of cellulosic fibers modified by nano-sized zinc oxide Hadi Fallah Moafi, Abdollah Fallah Shojaie ⁎, Mohammad Ali Zanjanchi Department of Chemistry, Faculty of Science, University of Guilan, P.O. Box 1914, Rasht, Iran
a r t i c l e
i n f o
Article history: Received 18 May 2010 Received in revised form 23 January 2011 Accepted 25 January 2011 Available online 2 February 2011 Keywords: Cellulosic fiber Coating Photocatalysis Self-cleaning Nano-sized zinc oxide
a b s t r a c t Nano-sized zinc oxide was synthesized and deposited onto cellulosic fibers using the sol–gel process at ambient temperature. The prepared materials were characterized using several techniques including scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy, X-ray diffraction and thermogravimetric analysis. X-ray diffraction studies of the ZnO-coated fiber indicate formation of the hexagonal crystal phase which was satisfactory crystallized on the fiber surface. The electron micrographs show formation of zinc oxide nanoparticles within 10–15 nm in size which have been homogeneously dispersed on the fiber surface. The prepared materials show significant photocatalytic selfcleaning activity, which was monitored by diffuse reflectance spectroscopy. The photoactivity was studied upon measuring the photodegradation of methylene blue and eosin yellowish under UV–Vis irradiation. The photocatalytic activity of the treated fabrics was fully maintained performing several cycles of photodegradation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Self-cleaning applications using semiconducting photocatalysts have become a subject of increasing interest especially in the last 10 years [1–4]. The photocatalytic properties of semiconductors such as TiO2 and ZnO originate decomposition of organic pollutants that come into contact with the surface and thus prevent them from building up [5–7]. Zinc oxide is an important alternative oxide semiconductor photocatalyst which its mechanism of photocatalysis has been proven to be similar to that of TiO2 [8–10]. ZnO also acquires many advantages, such as low price, many active sites with high surface reactivity, high absorption efficacy of light radiations, and environmental-safety [11]. ZnO thin films have been prepared by a variety of techniques such as pulsed laser deposition [12], magnetron sputtering [13,14], chemical vapor deposition [15], and sol–gel processing [16–19]. The sol–gel process is one of the versatile methods to prepare thin filmsupported nano-sized particles without complicated instruments [18]. The most important advantages are the simplicity of equipments, the ability of accurate control of stoichiometry, high homogeneity and relatively low process temperature. Therefore, this technique could be a suitable method for the preparation of self-cleaning materials. There are studies on the formation of titania nanoparticles onto polymeric fibers via different approaches [20–33]. Among those, preparation cotton textiles modified with TiO2 at low temperatures [24], deposition of TiO2 nanoparticles onto cotton fabrics by sol–gel ⁎ Corresponding author. Fax: +98 0131 3233262. E-mail address: [email protected] (A.F. Shojaie). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.347
process [28,30] and self-cleaning modified TiO2-cotton pretreated by UV-C light and RF-plasma [31] could be mentioned. The studies on the construction of ZnO nanoparticles onto cotton fibers are more recent [34–44]. Although, some properties such as UV protection [34–41], antibacterial properties [39–41], superhydrophobicity [42,43] and stain eliminating [44] have been explored, but, a solid-phase selfcleaning inspection is lacking in all of them. Direct examinations of the self-cleaning property or appropriate test of reusability are very important parameters to be considered in a comprehensive study. In this contribution, we describe a facile and effective sol–gel method to produce the nanoparticles ZnO on cellulosic fibers with good crystallinity at low temperature. The photocatalytic self-cleaning abilities of our prepared fibers were investigated by studying the solid-phase photodegradation of the dyes which are pre-adsorbed onto the ZnO-modified cotton fibers. Diffuse reflectance spectroscopy (DRS) was used for the monitoring of the dyes photodegredation and discoloration. This technique provides direct data on the photocatalytic self-cleaning properties of the prepared fibers. The reusability of the modified fiber can also be assessed easily using the diffuse reflectance technique. The reusability for the self-cleaning coatings is vital to make these coatings attractive in construction industrial products. 2. Experimental details 2.1. Materials Pure cellulosic fiber from cotton was used for the entire process. Methylene blue (MB) and eosin yellowish (EY) of AR grade (Merck,
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Germany) were used for the experiments. Zinc acetate dihydrate, methanol and sodium hydroxide were obtained from Merck (Germany) and used without further purification. 2.2. Synthesis procedure The typical procedure for the synthesis of ZnO sol is based on the method described in the literature with minor changes in details [45]. Zinc acetate dihydrate was used as zinc oxide source. In a typical procedure, 0.01 mol of zinc acetate dihydrate was dissolved in 50 ml of methanol and heated at 50 °C along with stirring for half an hour, thus making precursor solution A. Then, 0.02 mol of sodium hydroxide was dissolved in 50 ml of methanol and heated at 50 °C along with stirring for 1 h, making precursor solution B. In order to make ZnO nano-sol, solution B was added into the solution A dropwise under constant stirring for half an hour and then mixture was heated at 50 °C for further half an hour. Subsequently, after continuous stirring for 2 h and cooling at room temperature, a homogenous and transparent sol was obtained. For the impregnation, the cellulosic fibers were washed first by water and detergent at 80 °C for 30 min to remove the impurities such as wax, fat, etc. and then washed several times by a large amount of deionized water. They were further cleaned in acetone (Merck) for 60 min and dried at room temperature for 24 h. The cellulosic fibers after being dried in a preheated oven were immersed in the ZnO containing liquid sol for 5 min. The prepared samples were then placed at 70 °C preheated oven to remove the solvent from the fibers and then heated at 150 °C for 15 min, to complete the formation of zinc oxide from the precursor. Finally, the impregnated fibers were rinsed in deionized water. During this step the unattached ZnO particles were removed out from the fiber surface. 2.3. Photocatalytic test The photoactivity of the zinc oxide coated cellulosic fibers has been investigated by exposing the samples containing adsorbed MB and EY to UV–Vis light. For this purpose, 100 ml aqueous solutions (1.0 × 10−5 mol L−1) of MB and EY were prepared. Both bare and coated fibers were treated in MB and EY solutions. For this purpose the same amount of each sample was immersed under mild stirring in the dye solution and remained overnight to complete the adsorption. The solution was then removed and the samples dried at room temperature. The so-obtained samples were then exposed to UV–Vis to test their photoactivity. For photocatalytic reactions, the irradiation were carried out on dry samples, by means of a highpressure mercury lamp (HPMV 400 W, Germany). Details of the experimental manipulation were described in our previous work [33]. The photocatalytic efficiency was expressed in terms of percent of degradation from the following equation. Percent of degradation = ½ðC0 −CÞ = C0 × 100 = ½ðA0 −AÞ = A0 × 100 where, C0 represents the initial concentration of the dyes on fiber surface, C is the final concentration after illumination by UV light, A0 is the initial absorbance, and A is the variable absorbance. The reusability of the ZnO-coated fiber was checked by consecutive repeated adsorption of the dyes and testing the photoactivity of the sample for the new degradation cycle. This procedure was repeated three times on the same sample as described above for both MB and EY solutions. 2.4. Characterization techniques To investigate the morphology of the pure and ZnO-modified cellulosic fibers, scanning electron microscopy (SEM) images were
obtained on a Philips, XL30 equipped with energy dispersive (EDS) microanalysis system for compositional analysis of the ZnO-coated cellulosic fibers. For the SEM and EDS analyses, samples were covered with gold layer. The ZnO particle sizes were obtained by transmission electron microscopy (TEM) images on a Philips CM10 instrument with an accelerating voltage of 100 kV. For photodecomposition reaction, the UV–Vis reflectance spectra were recorded at room temperature by a UV-2100 Shimadzu Spectrophotometer in the reflectance mode by investigating the evolution of the absorbance. X-ray diffraction (XRD) patterns were recorded by a D8 Bruker Advanced diffractometer with Cu-Kα radiation, scan rate 0.02 2θ/s and within range of 2θ of 10 to 70 at room temperature. Thermogravimetric analysis (TGA) was performed in air flow (ramp of 10°K/min) by TGA V5.1A DuPont 2000. 3. Results and discussion 3.1. Morphological and compositional analyses In order to investigate the morphology of the prepared coated samples, SEM images of samples were recorded (Fig. 1). Fig. 1a shows that the surface of pristine cellulosic fiber is clean and smooth. Fig. 1b and c shows the images of the treated fibers followed by a washing treatment. The surface of the fibers is covered by a continuous and relatively homogeneous deposited ZnO coating. The micrograph indicates that the particle sizes of the deposited zinc oxide on the fibers surface are less than 50 nm and they are spherical (Fig. 1b–c). Fig. 1d and e shows the SEM image of solderived pure ZnO nanoparticles. It is evident that the morphology of the ZnO particle is similar to that of ZnO nanoparticles on the fiber surface. In Fig. 2 the EDS analysis of ZnO-covered fibers following the washing step is reported. On the basis of this result, it is noteworthy to observe that the deposited material consisted of zinc and oxygen. This shows that even after washing, remarkable amount of zinc oxide is still present on the cellulosic fiber surface. This means that ZnO nanoparticles have sufficient adhesion towards the cellulosic fibers in order to resist a washing process. In order to investigate the size of ZnO nanoparticles formed on the fibers surface, the particles were analyzed by TEM, as showed in Fig. 3. TEM images shows that the deposited zinc oxide consisted of uniform spherical particles of average diameter 10–15 nm. The data of the particle sizes would be supported by the following XRD analysis. It is noticeable that the spherical ZnO particles are distributed homogenously on the fiber surface. 3.2. XRD analysis The XRD patterns of pure, ZnO-coated cellulosic fiber and solderived ZnO powder are reported in Fig. 4. Fig. 4a shows two broad peaks and one intense peak at 13.16°, 15.12° and 21.4°, respectively, which comprise the typical XRD pattern of cellulosic fibers [46]. Two of three major peaks at 13.16° and 15.12°, are related to amorphous phase of cotton fiber, while the peak at 21.4° is due to the crystalline phase. Fig. 4b shows XRD pattern of ZnO-modified fiber. The XRD pattern of this sample indicates that the synthesized ZnO particles on the fiber surface are hexagonal wurtzite crystal phase due to the presence of attributive peaks at 2θ = 31.71°, 34.48°, 36.23°, 47.53°, 56.47°, 62.78°, and 67.92°. These are associated with the (100), (002), (101), (102), (110), (103) and (112) planes of the ZnO hexagonal wurtzite structure. Fig. 4c shows XRD patterns of the solderived ZnO powder. It can be seen that all the diffraction peaks could be identified to ZnO peaks with hexagonal wurtzite structure. No other peak related to impurities was detected in the pattern which further confirms that the synthesized powder was single phase of ZnO with hexagonal wurtzite structure. Crystalline size can
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Fig. 1. SEM images of: (a) pure fiber (× 10,000), (b) ZnO-modified fiber after washing (×15,000), (c) enlarged ZnO-modified fiber after washing (×30,000), (d) ZnO powder (×15,000) and (e) enlarged ZnO powder (×30,000).
be calculated from Scherrer formula, which can be described as follows: D=
Kλ β cosθ
where D is the grain size, K = 0.89, λ the X-ray wavelength (0.15406 nm), θ the Bragg diffraction angle, and β is the peak width at half maximum (FWHM). The grain size of our sample was calculated with the (101) diffraction peak. The grain size of samples was 10 nm. This is very close to that found from TEM analysis as mentioned earlier. 3.3. Thermogravimetric analysis
Fig. 2. EDS result of ZnO-modified cellulosic fiber after washing.
The TG analysis of the pure and treated cellulosic fiber was carried out in air. DTG and TG curves of the untreated cellulosic fibers (Fig. 5) reveal that its pyrolysis includes three stages: initial, main, and char decomposition steps [47]. The related temperature, speed and mass loss of every stage can be found from the TG curve. In the initial stage, where the temperature range is below 300 °C, little mass loss happens. Here, the damage to cellulose occurs mostly in its
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Fig. 3. TEM image of ZnO nanoparticles formed on the fiber.
amorphous region. The main pyrolysis stage occurs in the temperature range of 300–370 °C. In this stage, the mass loss is very fast and significant. Most of the pyrolysis products are produced in this stage. Referring to the literature, L-glucose is one of the major products, together with all kinds of combustible gases [47]. The char pyrolysis occurs at the temperature above 370 °C. During this process, dehydration and charring reactions compete with the production of glucose, with the dehydration and charring reactions being more obvious. The mass decomposition continues to dehydration and decarboxylization, releasing more water and carbon dioxide. Although the exact temperature ranges of cellulose pyrolysis may vary
Fig. 5. TGA and DTGA (derivative of TGA) curves of pure cellulosic fiber and ZnOmodified fiber.
depending on different cellulosic materials and experimental conditions, the three steps always exist in pyrolysis of cellulose. The TG curve concerning the ZnO-modified sample (Fig. 5) shows similar main three stages but with lower decomposition temperatures and lower total mass loss. The pyrolysis stage is widespread and that may be due to inhomogeneity of the ZnO particles coated on the fiber. After combustion of all organic parts in the ZnO-modified fiber, the residual amount (~16% by weight) corresponds to ZnO. From this result, it is evident that the thermogravimetric analysis technique in air allows evaluating the presence of ZnO incorporated in the cellulosic fiber. 3.4. Photocatalytic degradation
Fig. 4. XRD patterns of: (a) pure cellulosic fiber, (b) ZnO-modified fiber, and (c) sol–gelderived ZnO powder.
The photocatalytic activity of the prepared ZnO-modified fibers was studied by the solid-phase photodegradation of MB and EY dyes as model compounds. These dyes are suitable because of strong adsorption characteristics on many surfaces, good resistance to light degradation and a well defined optical absorption band in the visible region. To investigate the photocatalytic self-cleaning performance of the samples, the delicate technique of DRS was used. ZnO-modified fibers were capable to decompose dyes upon UV– Vis irradiation. The UV–Vis reflectance spectra obtained from the modified fibers are shown in Figs. 6 and 7 prior (spectrum a) and after illumination (spectra b–e). The absorption peaks, corresponding to dye, diminished under reaction which indicated that the dye had been degraded. No new absorption bands appear in the visible regions. From Fig. 6, it can be observed that the absorption band intensities in the 500–700 nm region which are due to adsorption of MB decrease rapidly, because supported ZnO promotes the catalytic photodegradation (spectra b–e). This is not unexpected since the photocatalytic activity of ZnO is well known [48]. The disappearance rate of the band due to MB adsorbed on the ZnO-modified fiber is much higher than that observed in case of untreated fiber (Fig. 8a and d). These experiments demonstrated that both UV–Vis light and a photocatalyst, such as ZnO are needed for the effective decomposition of dyes.
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Fig. 6. UV–Vis reflectance spectra of MB-treated ZnO-modified fiber: spectra a–e for 0, 2, 4, 6 and 8 h irradiation respectively.
Similar results have been obtained for the adsorbed EY (Figs. 7 and 8). As a matter of fact the absorption of dye (in the region at 450– 600 nm) decreases rapidly under the exposure to UV–Vis light. The degradation rate of pre-adsorbed EY on the ZnO-covered fiber is much higher than that observed in case of untreated fiber (Fig. 8b and c). The photocatalytic activity of ZnO-modified fiber is attributed to the dispersed ZnO nanoparticles which satisfactorily were crystallized on the fiber surface. During photocatalysis, conduction-band electrons and valence-band holes would be generated on its surface when ZnO is illuminated by light with energy greater than its band gap energy (Eq. (1)). Holes can then react with water molecules adhering to the surface of the photocatalyst to form highly reactive hydroxyl radicals (OH•) (Eq. (2)). Oxygen here acts as an electron acceptor by forming a super-oxide radical anion (O−• 2 ) on the catalyst surface (Eq. (3)). The super-oxide radical anions may act as oxidizing agents or as an additional source of hydroxyl radicals via the subsequent formation of hydrogen peroxide. The powerful oxidants associated with hydroxyl radicals are able to oxidize organic materials (Eq. (4)). −
þ
ZnO þ hν → ZnOðeCB þ hVB Þ −
ð2Þ
eCB þ O2 → •O2
−
−
ð3Þ
•
ð4Þ
OH þ dye → degradation of the dye:
Fig. 8. Time dependence of the surface concentration of adsorbed MB and EY upon light exposure: (a) MB on untreated fiber, (b) EY on untreated fiber (c), EY on ZnO-modified fiber, and (d) MB on ZnO-modified fiber.
Therefore, the objective for cleaning and sterilization can be achieved by the assistance of a photocatalyst [48,49]. In order to elucidate the durability of photocatalytic activity of ZnO deposited on the cellulosic fabrics, the photodegradation process under the UV–Vis illumination was repeated three more times. Fig. 9 shows the photocatalytic efficiency of the ZnO-modified fiber for the degradation of MB and EY, which are unchanged upon repeating cycles. These results suggest that ZnO losses from the fiber surface for each impregnation of stains (MB and EY) are ignorable and ZnO particles were adhered compactly to the fibers. It is evident that ZnO-
ð1Þ
þ
hVB þ H2 OðOH Þ → •OH
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Fig. 7. UV–Vis reflectance spectra EY-treated ZnO-modified fiber: spectra a–e for 0, 2, 4, 6 and 8 h irradiation respectively.
Fig. 9. Photocatalytic efficiency of ZnO-modified cellulosic fiber upon repeated MB and EY adsorption–illumination cycles.
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modified fibers promote the photodegradation process and the high surface area associated with the small particle size ensures a favorable condition. 4. Conclusion Self-cleaning cellulosic fibers have been prepared by grafting ZnO nanoparticles on the surface of fiber by simple method via sol–gel process at ambient temperature. Based on the TEM images, the ZnO coating constitutes crystallites of about 10–15 nm particle size. The thermogravimetric data showed that about 18% of the weight of the modified fiber is ZnO which are strongly adhered to the surface of the support. ZnO-modified cellulosic fibers with small particles size distribution possess significant photocatalytic self-cleaning properties, upon exposing the pre-adsorbed MB and EY to UV–Vis light. Selfcleaning properties were monitored by DRS. The photoactivity of ZnOmodified fiber is attributed to the dispersed ZnO nanoparticles which were crystallized and have adhered on the fiber surface. ZnO nanoparticles supported on cellulosic fiber promote the photodegradation process and the high surface area associated with the small particle size ensures a favorable condition for self-cleaning purposes. The nano-ZnO coated fibers showed excellent reusability self-cleaning properties that are much essential to formed intelligent textiles toward stain eliminating. This innovation is important because it may allow its practical use for industrial applications. Acknowledgements The authors are grateful to the University of Guilan for the financial assistance of this research project. References [1] E. Allain, S. Besson, C. Durand, M. Moreau, T. Gacoin, J.P. Boilot, Adv. Funct. Mater. 17 (2007) 549. [2] Z. Liu, X. Zhang, T. Murakami, A. Fujishima, Sol. Energy Mater. Sol. Cells 92 (2008) 1434. [3] H. Yaghoubi, N. Taghavinia, E.K. Alamdari, Surf. Coat. Technol. 204 (2010) 1562. [4] N.P. Mellott, C. Durucan, C.G. Pantano, M. Guglielmi, Thin Solid Films 502 (2006) 112. [5] K. Guan, Surf. Coat. Technol. 191 (2005) 155. [6] Y.C. Lee, Y.P. Hong, H.Y. Lee, H. Kim, Y.J. Jung, K.H. Ko, H.S. Jung, K.S. Hong, J. Colloid Interface Sci. 267 (2003) 127. [7] Q. Liu, X. We, B. Wang, Mater. Res. Bull. 37 (2002) 2255. [8] H.F. Lin, S.C. Liao, S.W. Hung, J. Photochem. Photobiol. A Chem. 174 (2005) 82.
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Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Photocatalytic self-cleaning properties of cellulosic fibers modified by nano-sized zinc oxide Hadi Fallah Moafi, Abdollah Fallah Shojaie ⁎, Mohammad Ali Zanjanchi Department of Chemistry, Faculty of Science, University of Guilan, P.O. Box 1914, Rasht, Iran
a r t i c l e
i n f o
Article history: Received 18 May 2010 Received in revised form 23 January 2011 Accepted 25 January 2011 Available online 2 February 2011 Keywords: Cellulosic fiber Coating Photocatalysis Self-cleaning Nano-sized zinc oxide
a b s t r a c t Nano-sized zinc oxide was synthesized and deposited onto cellulosic fibers using the sol–gel process at ambient temperature. The prepared materials were characterized using several techniques including scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy, X-ray diffraction and thermogravimetric analysis. X-ray diffraction studies of the ZnO-coated fiber indicate formation of the hexagonal crystal phase which was satisfactory crystallized on the fiber surface. The electron micrographs show formation of zinc oxide nanoparticles within 10–15 nm in size which have been homogeneously dispersed on the fiber surface. The prepared materials show significant photocatalytic selfcleaning activity, which was monitored by diffuse reflectance spectroscopy. The photoactivity was studied upon measuring the photodegradation of methylene blue and eosin yellowish under UV–Vis irradiation. The photocatalytic activity of the treated fabrics was fully maintained performing several cycles of photodegradation. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Self-cleaning applications using semiconducting photocatalysts have become a subject of increasing interest especially in the last 10 years [1–4]. The photocatalytic properties of semiconductors such as TiO2 and ZnO originate decomposition of organic pollutants that come into contact with the surface and thus prevent them from building up [5–7]. Zinc oxide is an important alternative oxide semiconductor photocatalyst which its mechanism of photocatalysis has been proven to be similar to that of TiO2 [8–10]. ZnO also acquires many advantages, such as low price, many active sites with high surface reactivity, high absorption efficacy of light radiations, and environmental-safety [11]. ZnO thin films have been prepared by a variety of techniques such as pulsed laser deposition [12], magnetron sputtering [13,14], chemical vapor deposition [15], and sol–gel processing [16–19]. The sol–gel process is one of the versatile methods to prepare thin filmsupported nano-sized particles without complicated instruments [18]. The most important advantages are the simplicity of equipments, the ability of accurate control of stoichiometry, high homogeneity and relatively low process temperature. Therefore, this technique could be a suitable method for the preparation of self-cleaning materials. There are studies on the formation of titania nanoparticles onto polymeric fibers via different approaches [20–33]. Among those, preparation cotton textiles modified with TiO2 at low temperatures [24], deposition of TiO2 nanoparticles onto cotton fabrics by sol–gel ⁎ Corresponding author. Fax: +98 0131 3233262. E-mail address: [email protected] (A.F. Shojaie). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.347
process [28,30] and self-cleaning modified TiO2-cotton pretreated by UV-C light and RF-plasma [31] could be mentioned. The studies on the construction of ZnO nanoparticles onto cotton fibers are more recent [34–44]. Although, some properties such as UV protection [34–41], antibacterial properties [39–41], superhydrophobicity [42,43] and stain eliminating [44] have been explored, but, a solid-phase selfcleaning inspection is lacking in all of them. Direct examinations of the self-cleaning property or appropriate test of reusability are very important parameters to be considered in a comprehensive study. In this contribution, we describe a facile and effective sol–gel method to produce the nanoparticles ZnO on cellulosic fibers with good crystallinity at low temperature. The photocatalytic self-cleaning abilities of our prepared fibers were investigated by studying the solid-phase photodegradation of the dyes which are pre-adsorbed onto the ZnO-modified cotton fibers. Diffuse reflectance spectroscopy (DRS) was used for the monitoring of the dyes photodegredation and discoloration. This technique provides direct data on the photocatalytic self-cleaning properties of the prepared fibers. The reusability of the modified fiber can also be assessed easily using the diffuse reflectance technique. The reusability for the self-cleaning coatings is vital to make these coatings attractive in construction industrial products. 2. Experimental details 2.1. Materials Pure cellulosic fiber from cotton was used for the entire process. Methylene blue (MB) and eosin yellowish (EY) of AR grade (Merck,
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Germany) were used for the experiments. Zinc acetate dihydrate, methanol and sodium hydroxide were obtained from Merck (Germany) and used without further purification. 2.2. Synthesis procedure The typical procedure for the synthesis of ZnO sol is based on the method described in the literature with minor changes in details [45]. Zinc acetate dihydrate was used as zinc oxide source. In a typical procedure, 0.01 mol of zinc acetate dihydrate was dissolved in 50 ml of methanol and heated at 50 °C along with stirring for half an hour, thus making precursor solution A. Then, 0.02 mol of sodium hydroxide was dissolved in 50 ml of methanol and heated at 50 °C along with stirring for 1 h, making precursor solution B. In order to make ZnO nano-sol, solution B was added into the solution A dropwise under constant stirring for half an hour and then mixture was heated at 50 °C for further half an hour. Subsequently, after continuous stirring for 2 h and cooling at room temperature, a homogenous and transparent sol was obtained. For the impregnation, the cellulosic fibers were washed first by water and detergent at 80 °C for 30 min to remove the impurities such as wax, fat, etc. and then washed several times by a large amount of deionized water. They were further cleaned in acetone (Merck) for 60 min and dried at room temperature for 24 h. The cellulosic fibers after being dried in a preheated oven were immersed in the ZnO containing liquid sol for 5 min. The prepared samples were then placed at 70 °C preheated oven to remove the solvent from the fibers and then heated at 150 °C for 15 min, to complete the formation of zinc oxide from the precursor. Finally, the impregnated fibers were rinsed in deionized water. During this step the unattached ZnO particles were removed out from the fiber surface. 2.3. Photocatalytic test The photoactivity of the zinc oxide coated cellulosic fibers has been investigated by exposing the samples containing adsorbed MB and EY to UV–Vis light. For this purpose, 100 ml aqueous solutions (1.0 × 10−5 mol L−1) of MB and EY were prepared. Both bare and coated fibers were treated in MB and EY solutions. For this purpose the same amount of each sample was immersed under mild stirring in the dye solution and remained overnight to complete the adsorption. The solution was then removed and the samples dried at room temperature. The so-obtained samples were then exposed to UV–Vis to test their photoactivity. For photocatalytic reactions, the irradiation were carried out on dry samples, by means of a highpressure mercury lamp (HPMV 400 W, Germany). Details of the experimental manipulation were described in our previous work [33]. The photocatalytic efficiency was expressed in terms of percent of degradation from the following equation. Percent of degradation = ½ðC0 −CÞ = C0 × 100 = ½ðA0 −AÞ = A0 × 100 where, C0 represents the initial concentration of the dyes on fiber surface, C is the final concentration after illumination by UV light, A0 is the initial absorbance, and A is the variable absorbance. The reusability of the ZnO-coated fiber was checked by consecutive repeated adsorption of the dyes and testing the photoactivity of the sample for the new degradation cycle. This procedure was repeated three times on the same sample as described above for both MB and EY solutions. 2.4. Characterization techniques To investigate the morphology of the pure and ZnO-modified cellulosic fibers, scanning electron microscopy (SEM) images were
obtained on a Philips, XL30 equipped with energy dispersive (EDS) microanalysis system for compositional analysis of the ZnO-coated cellulosic fibers. For the SEM and EDS analyses, samples were covered with gold layer. The ZnO particle sizes were obtained by transmission electron microscopy (TEM) images on a Philips CM10 instrument with an accelerating voltage of 100 kV. For photodecomposition reaction, the UV–Vis reflectance spectra were recorded at room temperature by a UV-2100 Shimadzu Spectrophotometer in the reflectance mode by investigating the evolution of the absorbance. X-ray diffraction (XRD) patterns were recorded by a D8 Bruker Advanced diffractometer with Cu-Kα radiation, scan rate 0.02 2θ/s and within range of 2θ of 10 to 70 at room temperature. Thermogravimetric analysis (TGA) was performed in air flow (ramp of 10°K/min) by TGA V5.1A DuPont 2000. 3. Results and discussion 3.1. Morphological and compositional analyses In order to investigate the morphology of the prepared coated samples, SEM images of samples were recorded (Fig. 1). Fig. 1a shows that the surface of pristine cellulosic fiber is clean and smooth. Fig. 1b and c shows the images of the treated fibers followed by a washing treatment. The surface of the fibers is covered by a continuous and relatively homogeneous deposited ZnO coating. The micrograph indicates that the particle sizes of the deposited zinc oxide on the fibers surface are less than 50 nm and they are spherical (Fig. 1b–c). Fig. 1d and e shows the SEM image of solderived pure ZnO nanoparticles. It is evident that the morphology of the ZnO particle is similar to that of ZnO nanoparticles on the fiber surface. In Fig. 2 the EDS analysis of ZnO-covered fibers following the washing step is reported. On the basis of this result, it is noteworthy to observe that the deposited material consisted of zinc and oxygen. This shows that even after washing, remarkable amount of zinc oxide is still present on the cellulosic fiber surface. This means that ZnO nanoparticles have sufficient adhesion towards the cellulosic fibers in order to resist a washing process. In order to investigate the size of ZnO nanoparticles formed on the fibers surface, the particles were analyzed by TEM, as showed in Fig. 3. TEM images shows that the deposited zinc oxide consisted of uniform spherical particles of average diameter 10–15 nm. The data of the particle sizes would be supported by the following XRD analysis. It is noticeable that the spherical ZnO particles are distributed homogenously on the fiber surface. 3.2. XRD analysis The XRD patterns of pure, ZnO-coated cellulosic fiber and solderived ZnO powder are reported in Fig. 4. Fig. 4a shows two broad peaks and one intense peak at 13.16°, 15.12° and 21.4°, respectively, which comprise the typical XRD pattern of cellulosic fibers [46]. Two of three major peaks at 13.16° and 15.12°, are related to amorphous phase of cotton fiber, while the peak at 21.4° is due to the crystalline phase. Fig. 4b shows XRD pattern of ZnO-modified fiber. The XRD pattern of this sample indicates that the synthesized ZnO particles on the fiber surface are hexagonal wurtzite crystal phase due to the presence of attributive peaks at 2θ = 31.71°, 34.48°, 36.23°, 47.53°, 56.47°, 62.78°, and 67.92°. These are associated with the (100), (002), (101), (102), (110), (103) and (112) planes of the ZnO hexagonal wurtzite structure. Fig. 4c shows XRD patterns of the solderived ZnO powder. It can be seen that all the diffraction peaks could be identified to ZnO peaks with hexagonal wurtzite structure. No other peak related to impurities was detected in the pattern which further confirms that the synthesized powder was single phase of ZnO with hexagonal wurtzite structure. Crystalline size can
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Fig. 1. SEM images of: (a) pure fiber (× 10,000), (b) ZnO-modified fiber after washing (×15,000), (c) enlarged ZnO-modified fiber after washing (×30,000), (d) ZnO powder (×15,000) and (e) enlarged ZnO powder (×30,000).
be calculated from Scherrer formula, which can be described as follows: D=
Kλ β cosθ
where D is the grain size, K = 0.89, λ the X-ray wavelength (0.15406 nm), θ the Bragg diffraction angle, and β is the peak width at half maximum (FWHM). The grain size of our sample was calculated with the (101) diffraction peak. The grain size of samples was 10 nm. This is very close to that found from TEM analysis as mentioned earlier. 3.3. Thermogravimetric analysis
Fig. 2. EDS result of ZnO-modified cellulosic fiber after washing.
The TG analysis of the pure and treated cellulosic fiber was carried out in air. DTG and TG curves of the untreated cellulosic fibers (Fig. 5) reveal that its pyrolysis includes three stages: initial, main, and char decomposition steps [47]. The related temperature, speed and mass loss of every stage can be found from the TG curve. In the initial stage, where the temperature range is below 300 °C, little mass loss happens. Here, the damage to cellulose occurs mostly in its
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Fig. 3. TEM image of ZnO nanoparticles formed on the fiber.
amorphous region. The main pyrolysis stage occurs in the temperature range of 300–370 °C. In this stage, the mass loss is very fast and significant. Most of the pyrolysis products are produced in this stage. Referring to the literature, L-glucose is one of the major products, together with all kinds of combustible gases [47]. The char pyrolysis occurs at the temperature above 370 °C. During this process, dehydration and charring reactions compete with the production of glucose, with the dehydration and charring reactions being more obvious. The mass decomposition continues to dehydration and decarboxylization, releasing more water and carbon dioxide. Although the exact temperature ranges of cellulose pyrolysis may vary
Fig. 5. TGA and DTGA (derivative of TGA) curves of pure cellulosic fiber and ZnOmodified fiber.
depending on different cellulosic materials and experimental conditions, the three steps always exist in pyrolysis of cellulose. The TG curve concerning the ZnO-modified sample (Fig. 5) shows similar main three stages but with lower decomposition temperatures and lower total mass loss. The pyrolysis stage is widespread and that may be due to inhomogeneity of the ZnO particles coated on the fiber. After combustion of all organic parts in the ZnO-modified fiber, the residual amount (~16% by weight) corresponds to ZnO. From this result, it is evident that the thermogravimetric analysis technique in air allows evaluating the presence of ZnO incorporated in the cellulosic fiber. 3.4. Photocatalytic degradation
Fig. 4. XRD patterns of: (a) pure cellulosic fiber, (b) ZnO-modified fiber, and (c) sol–gelderived ZnO powder.
The photocatalytic activity of the prepared ZnO-modified fibers was studied by the solid-phase photodegradation of MB and EY dyes as model compounds. These dyes are suitable because of strong adsorption characteristics on many surfaces, good resistance to light degradation and a well defined optical absorption band in the visible region. To investigate the photocatalytic self-cleaning performance of the samples, the delicate technique of DRS was used. ZnO-modified fibers were capable to decompose dyes upon UV– Vis irradiation. The UV–Vis reflectance spectra obtained from the modified fibers are shown in Figs. 6 and 7 prior (spectrum a) and after illumination (spectra b–e). The absorption peaks, corresponding to dye, diminished under reaction which indicated that the dye had been degraded. No new absorption bands appear in the visible regions. From Fig. 6, it can be observed that the absorption band intensities in the 500–700 nm region which are due to adsorption of MB decrease rapidly, because supported ZnO promotes the catalytic photodegradation (spectra b–e). This is not unexpected since the photocatalytic activity of ZnO is well known [48]. The disappearance rate of the band due to MB adsorbed on the ZnO-modified fiber is much higher than that observed in case of untreated fiber (Fig. 8a and d). These experiments demonstrated that both UV–Vis light and a photocatalyst, such as ZnO are needed for the effective decomposition of dyes.
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Fig. 6. UV–Vis reflectance spectra of MB-treated ZnO-modified fiber: spectra a–e for 0, 2, 4, 6 and 8 h irradiation respectively.
Similar results have been obtained for the adsorbed EY (Figs. 7 and 8). As a matter of fact the absorption of dye (in the region at 450– 600 nm) decreases rapidly under the exposure to UV–Vis light. The degradation rate of pre-adsorbed EY on the ZnO-covered fiber is much higher than that observed in case of untreated fiber (Fig. 8b and c). The photocatalytic activity of ZnO-modified fiber is attributed to the dispersed ZnO nanoparticles which satisfactorily were crystallized on the fiber surface. During photocatalysis, conduction-band electrons and valence-band holes would be generated on its surface when ZnO is illuminated by light with energy greater than its band gap energy (Eq. (1)). Holes can then react with water molecules adhering to the surface of the photocatalyst to form highly reactive hydroxyl radicals (OH•) (Eq. (2)). Oxygen here acts as an electron acceptor by forming a super-oxide radical anion (O−• 2 ) on the catalyst surface (Eq. (3)). The super-oxide radical anions may act as oxidizing agents or as an additional source of hydroxyl radicals via the subsequent formation of hydrogen peroxide. The powerful oxidants associated with hydroxyl radicals are able to oxidize organic materials (Eq. (4)). −
þ
ZnO þ hν → ZnOðeCB þ hVB Þ −
ð2Þ
eCB þ O2 → •O2
−
−
ð3Þ
•
ð4Þ
OH þ dye → degradation of the dye:
Fig. 8. Time dependence of the surface concentration of adsorbed MB and EY upon light exposure: (a) MB on untreated fiber, (b) EY on untreated fiber (c), EY on ZnO-modified fiber, and (d) MB on ZnO-modified fiber.
Therefore, the objective for cleaning and sterilization can be achieved by the assistance of a photocatalyst [48,49]. In order to elucidate the durability of photocatalytic activity of ZnO deposited on the cellulosic fabrics, the photodegradation process under the UV–Vis illumination was repeated three more times. Fig. 9 shows the photocatalytic efficiency of the ZnO-modified fiber for the degradation of MB and EY, which are unchanged upon repeating cycles. These results suggest that ZnO losses from the fiber surface for each impregnation of stains (MB and EY) are ignorable and ZnO particles were adhered compactly to the fibers. It is evident that ZnO-
ð1Þ
þ
hVB þ H2 OðOH Þ → •OH
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Fig. 7. UV–Vis reflectance spectra EY-treated ZnO-modified fiber: spectra a–e for 0, 2, 4, 6 and 8 h irradiation respectively.
Fig. 9. Photocatalytic efficiency of ZnO-modified cellulosic fiber upon repeated MB and EY adsorption–illumination cycles.
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modified fibers promote the photodegradation process and the high surface area associated with the small particle size ensures a favorable condition. 4. Conclusion Self-cleaning cellulosic fibers have been prepared by grafting ZnO nanoparticles on the surface of fiber by simple method via sol–gel process at ambient temperature. Based on the TEM images, the ZnO coating constitutes crystallites of about 10–15 nm particle size. The thermogravimetric data showed that about 18% of the weight of the modified fiber is ZnO which are strongly adhered to the surface of the support. ZnO-modified cellulosic fibers with small particles size distribution possess significant photocatalytic self-cleaning properties, upon exposing the pre-adsorbed MB and EY to UV–Vis light. Selfcleaning properties were monitored by DRS. The photoactivity of ZnOmodified fiber is attributed to the dispersed ZnO nanoparticles which were crystallized and have adhered on the fiber surface. ZnO nanoparticles supported on cellulosic fiber promote the photodegradation process and the high surface area associated with the small particle size ensures a favorable condition for self-cleaning purposes. The nano-ZnO coated fibers showed excellent reusability self-cleaning properties that are much essential to formed intelligent textiles toward stain eliminating. This innovation is important because it may allow its practical use for industrial applications. Acknowledgements The authors are grateful to the University of Guilan for the financial assistance of this research project. References [1] E. Allain, S. Besson, C. Durand, M. Moreau, T. Gacoin, J.P. Boilot, Adv. Funct. Mater. 17 (2007) 549. [2] Z. Liu, X. Zhang, T. Murakami, A. Fujishima, Sol. Energy Mater. Sol. Cells 92 (2008) 1434. [3] H. Yaghoubi, N. Taghavinia, E.K. Alamdari, Surf. Coat. Technol. 204 (2010) 1562. [4] N.P. Mellott, C. Durucan, C.G. Pantano, M. Guglielmi, Thin Solid Films 502 (2006) 112. [5] K. Guan, Surf. Coat. Technol. 191 (2005) 155. [6] Y.C. Lee, Y.P. Hong, H.Y. Lee, H. Kim, Y.J. Jung, K.H. Ko, H.S. Jung, K.S. Hong, J. Colloid Interface Sci. 267 (2003) 127. [7] Q. Liu, X. We, B. Wang, Mater. Res. Bull. 37 (2002) 2255. [8] H.F. Lin, S.C. Liao, S.W. Hung, J. Photochem. Photobiol. A Chem. 174 (2005) 82.
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