Synthesis and characterization of zinc oxide nanoparticles: application to textiles as UV-absorbers
We report the synthesis and characterization of nanosized zinc oxide particles and their application on cotton and wool fabrics for UV shielding. The nanoparticles were produced in different conditions of temperature (90 or 150 °C) and reacting medium (water or 1,2-ethanediol). A high temperature was necessary to obtain small monodispersed particles. Fourier transformed infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and X-ray powder diffractometry (XRD) were used to characterize the nanoparticles composition, their shape, size and crystallinity. The specific surface area of the dry powders was also determined. ZnO nanoparticles were then applied to cotton and wool samples to impart sunscreen activity to the treated textiles. The effectiveness of the treatment was assessed through UV–Vis spectrophotometry and the calculation of the ultraviolet protection factor (UPF). Physical tests (tensile strength and elongation) were performed on the fabrics before and after the treatment with ZnO nanoparticles.
KeywordsZinc oxide Nanoparticle(s) Textile UV UPF Sunscreen Nanocomposites Dispersions
The application of nanoparticles to textile materials has been the object of several studies aimed at producing finished fabrics with different performances. For example nano-Ag has been used for imparting antibacterial properties (Lee et al. 2003; Durán et al. 2007), nano-TiO2 for UV-blocking and self-cleaning properties (Xin et al. 2004; Fei et al. 2006; Qi et al. 2007) and ZnO nanoparticles for antibacterial and UV-blocking properties (Wang et al. 2004; Baglioni et al. 2003; Wang et al. 2005; Vigneshwaran et al. 2006). Inorganic UV blockers are more preferable than organic UV blockers (Riva et al. 2006; Scalia et al. 2006). In fact, zinc oxide and titanium dioxide are non-toxic and chemically stable under exposure to both high temperatures and UV. Furthermore, nanoparticles have a large surface area-to-volume ratio that results in a significant increasing of the effectiveness in blocking UV radiation when compared to bulk materials (Yadav et al. 2006).
The use of nanotechnology in the textile industry has increased rapidly. This is mainly due to the fact that conventional methods used to impart different properties to fabrics often do not lead to permanent effects, and will lose their functions after laundering or wearing. Nanoparticles can provide high durability for treated fabrics, with respect to conventional materials, because they possess large surface area and high surface energy that ensure better affinity for fabrics and lead to an increase in durability of the textile functions. Washfastness is a particular requirement for textile and it is strongly correlated with the nanoparticles adhesion to the fibers. In order to increase the washfastness, the nanoparticles can be applied by dipping the fabrics in a solution containing a specific binder (Vigneshwaran et al. 2006; Yadav et al. 2006). Washfastness can be further improved with the formation of covalent bonding between nanoparticles and the fabrics surface. In these cases the excellent UV blocking properties are still maintained after 55 home laundering (Daoud and Xin 2004; Xin et al. 2004; Lu et al. 2006).
Nanoparticle coating may affect other fabric properties like dyeing capacity, tensile strength, bursting strength, bending rigidity, air permeability (comfort) and fabric friction that play a crucial role in textile industries. Cotton fabrics treated with bulk-ZnO or nano-ZnO show different physical and mechanical properties. This reflects the improved properties of nano-sized particles with respect to conventional materials. Air permeability of the fabric is reduced when the coating process is carried out with bulk-ZnO, while it is improved when nano-ZnO is used. This basic difference has great consequences on the garment’s breathability, and eventually on the comfort of the treated fabrics. As opposed to bulk-ZnO coating, in the case of nano-ZnO treated fabric, the small size and uniform distribution of the particles results in a significant lowering of the friction (Yadav et al. 2006). Titania nanocoating slightly enhances bursting strength of treated cotton and the hand effect remains unchanged when compared to the untreated fabric. Again, this is mainly due to the nano-scaled structure of the titania layer (Xin et al. 2004).
Zinc oxide is widely used in different areas because of its unique photocatalytic, electrical, electronic, optical, dermatological, and antibacterial properties (Turkoglu and Yener 1997; Pan et al. 2001; Arnold et al. 2003; Sawai 2003; Xiong et al. 2003; Behnajady et al. 2006; Li et al. 2006; Tang et al. 2006c). For these applications, the nanoparticles need to be dispersed homogeneously in the different matrices, and a number of new synthetic strategies have been developed in order to prevent particles agglomeration, and increase the stability of ZnO nanoparticles dispersions (Guo et al. 2000; Kwon et al. 2002; Wang et al. 2002; Liufu et al. 2004; Tang et al. 2006a; Tang et al. 2006b).
Zinc oxide is actually one of the best biofriendly absorbers of UV radiation that mainly come on Earth from the Sun through the atmospheric ozone layer (Sato and Ikeya 2004). UV radiation is responsible for the generation of free radical species (Bednarska et al. 2000; Takeshita et al. 2006) that are supposed to participate in the development of various pathologies such as cancer, ageing, Alzheimer’s disease, inflammatory disorders, and so on (Liebler 2006). In order to decrease the health risks due to overexposure to UV radiation, the World Health Organization (WHO) has also recommended the use of loose-fitting, full-length clothes with a high protection factor (Algaba et al. 2007). However most sportwear and light clothing commonly worn in the summertime do not guarantee an efficient shield against UV. The protection provided by fabrics depends on several parameters: fiber type, color, the presence of UV absorbers and additives, the porosity, thickness, mass per unit surface, other finishing processes and laundering, and the wearing conditions (stretched, dry or wet) (Gamblicher et al. 2002; Wang et al. 2005; Algaba et al. 2007).
The present work addresses the synthesis and characterization of ZnO nanoparticles obtained through a homogeneous phase reaction between zinc chloride and sodium hydroxide at high temperature. In order to evaluate the effects of the experimental conditions on the particle size and morphology, the temperature (90 or 150 °C) and the reaction medium (water or 1,2-ethanediol) were changed. The particles were then characterized, by evaluating their chemical composition through FTIR spectroscopy, their crystallinity through X-ray diffractometry, their shape and size via TEM microscopy, and the specific surface area.
ZnO nanoparticles were then applied to cotton and wool fabrics in order to evaluate the sunscreen activity in the treated textiles through UV spectrophotometry. The thermal behavior of the fabrics was assessed through thermogravimetric analysis (DTGA), and the mechanical resistance was evaluated through tensile strength and elongation tests.
ZnCl2 (min. 98%), NaOH (pellet min. 99%), 1,2-ethanediol (min. 99.5%), and 2-propanol (min. 99.5%) were purchased from Merck (Darmstadt, Germany). All products were used as received. Water was purified by a Milli-RO6 Plus Millipore Organex system (Resistance > 18 MΩ cm).
Zinc oxide nanoparticles were synthesized following a procedure reported elsewhere (Moroni et al. 2005). The synthesis was carried out at a high degree of supersaturation, in order to achieve a nucleation rate much greater than the growth rate (Ambrosi et al. 2001). ZnCl2 (5.5 g) was dissolved in 200 mL of water at 90 °C in an oil bath. 16 mL of 5 M NaOH aqueous solution were added dropwise to the zinc chloride solution, with a gentle stirring over a period of 10 min at 90 °C. The particles were separated from the supernatant dispersion by sedimentation. The supernatant solution was discarded, and the remaining suspension washed five times with distilled water to lower the concentration of NaCl below 10−6 M. Each time, the dilution ratio between the concentrated suspension and washing solution was about 1:10. The complete removal of NaCl from the suspension was checked with a solution of AgNO3. The purified particles were then peptized with 2-propanol in an ultrasonic bath for 10 min at room temperature. The peptization process is necessary to disrupt the microagglomerates and release the nanounits of zinc oxide (Perez-Maqueda et al. 1998; Salvadori and Dei 2001). The particles were then collected by centrifugation at 6,000 rpm for 15 min. The washing procedure was repeated three times. Thermal treatment of the particles at 250 °C for 5 h lead to the formation of ZnO. The synthesis in 1,2-ethanediol (ED) was carried out in the same way, but at 150 °C.
White wool and cotton fabrics, kindly provided by Grado Zero Espace (Florence, Italy), were used as received. The mass per unit surface was 146 g/m2 for cotton and 339 g/m2 for wool.
The fabric samples were conditioned at constant relative humidity (33%) and temperature (20 °C). The wool and cotton samples (10 cm × 10 cm) were soaked for 10 min in a 2-propanol dispersion of ZnO nanoparticles (5% w/w), under gentle magnetic stirring. The clothes were then squeezed to remove the excess dispersion, and dried in a oven at 130 °C for 15 min at atmospheric pressure (dry heat). In order to evaluate the nanoparticles adhesion to the textile fibers, the treated fabrics were washed five times according to a standard method (UNI EN ISO 26330:1996). A model A1 Wascator Electrolux automatic laundry machine (internal drum diameter 51.5 cm, internal drum depth 33.5 cm, heating capacity 5.4 kW) was used, and the washing cycles were performed at 30 °C, with an ECE phosphate reference detergent (B) without optical brighteners. The drying step was carried out on a horizontal flat surface. The fabric specimen were tested before and after the washing cycles via TEM and UV spectrophotometry.
UV absorption properties
E(λ) is the relative erythemal spectral effectiveness, S(λ) is the solar spectral irradiance in W m−2 nm−1, and T(λ) is the spectral transmission of the specimen obtained from UV spectrophotometric experiments. The values of E(λ) and S(λ) were obtained from the National Oceanic and Atmospheric Administration database (NOAA). The UPF value was calculated for UV-A in the range 315–400 nm, and for UV-B in between 295 and 315 nm. The percent UV transmission, obtained from Eq. 2, was determined for UV-A and UV-B radiation from the transmission spectra of the fabric samples.
Physical and physico-chemical characterization
Physical tests (tensile strength and elongation) of the treated fabrics were performed according to the standard methods (UNI EN ISO 13934-1:1999).
The chemical composition of the synthesized materials was checked by FTIR spectroscopy with a Biorad FTS-40 spectrometer. The crystallinity was determined by XRD using a Bruker D8 Advance X-rays Diffractometer equipped with a Cu Kα (λ = 1.54 Å) source (applied voltage 40 kV, current 40 mA). About 0.5 g of the dried particles were deposited as a randomly oriented powder onto a Plexiglass sample container, and the XRD patterns were recorded at angles between 20° and 80°, with a scan rate of 1.5°/min.
The shape and size of the particles were obtained through TEM, using a Philips EM201C apparatus operating at 80 kV. The samples for TEM measurements were placed on carbon-coated copper grids (supplied by Agar Scientific Ltd, U.K.). The samples for TEM measurements were prepared from very diluted dispersions of the particles in 2-propanol. Surface area measurements were determined from BET on a Coulter SA 3100 surface area analyzer, under N2 flow.
The ZnO-treated fabrics were analyzed through scanning electron microscopy (SEM), using a Stereoscan S360 Oxford–Cambridge. The samples were previously coated with a thin layer of gold deposited by sputtering under vacuum.
Thermogravimetric analysis was performed with an SDT 2960, series Q600 apparatus (TA Instruments, Milan, Italy). The temperature range was between 25 and 500 °C, with a scan rate of 20 °C/min. All runs were performed with a nitrogen flux of 100 mL/min.
Results and discussion
Synthesis and characterization of ZnO nanoparticles
Crystallite diameter (nm)
Average diameter (nm)
19.25 ± 0.002
21 ± 5
24.8 ± 5%
2 × 105
9.61 ± 0.001
9 ± 1
60.1 ± 5%
1.5 × 104
The specific surface areas obtained from BET experiments are 24.8 m2/g in the case of synthesis 1, and 60.1 m2/g in the case of synthesis 2, respectively (see Table 1). These data, agree very well with the expected values calculated from the particle diameters, within the experimental error, and confirm the effects induced by the experimental conditions on the nucleation and growth rates of the crystal nuclei.
Sunscreen activity of fabrics
The application of nanosized ZnO particles on the cotton fabric increases the absorption of UV light over the entire investigated UV spectrum. Higher values of UV absorbance were obtained when ZnO nanoparticles from synthesis 2 were applied on cotton (Fig. 7a). Similar results were obtained for the treated wool samples. The application of nanosized ZnO on wool fabric increases UV light absorbance in the region between 300 and 400 nm if compared to the untreated wool, in both cases (Fig. 7b). The results imply that the effectiveness in shielding UV radiation is due to the UV absorption capacity of ZnO nanoparticles on the fabrics surface. UV transmission and reflection spectra confirm these conclusions, as the ZnO-treated cotton and wool samples do not transmit or reflect the radiation over the entire UV spectrum (Fig. 7c, d, e and f).
selecting a more compact fabric,
selecting another kind of textile material (polyester, viscose, etc.),
using a different procedure for curing the fabric with the ZnO dispersion,
spreading an appropriate filming additive on the fabric in order to increase the amount of zinc oxide embedded into the textile structure.
UV transmission (%)
Tensile strength (N) and elongation (%) of the different samples
Maximum force (N)
We report the synthesis of zinc oxide nanoparticles through a homogeneous phase reaction starting from zinc chloride and sodium hydroxide at high temperature, in water or in 1,2-ethanediol. The reaction in 1,2-ethanediol at 150 °C results in the formation of smaller nanoparticles with respect to the reaction carried out in water at 90 °C. In both cases, the nanoparticles appear to be nearly spherical and with a quite narrow size range. Nanoparticles were analyzed through electron microscopy, X-ray diffraction, FTIR, and specific surface area experiments. The homogeneous phase reaction processes offer a valid alternative for an industrial-scale production of ZnO nanoparticles for many applications.
The peculiar performance of ZnO nanoparticles as UV-absorbers, can be efficiently transferred to fabric materials through the application of ZnO nanoparticles on the surface of cotton and wool fabrics. The UV tests indicate a significant increment of the UV absorbing activity in the ZnO-treated fabrics. Such result can be exploited for the protection of the body against solar radiation and for other technological applications.
Further studies are currently being carried out in order to enhance the UV-shielding activity of ZnO-loaded textiles.
The authors greatly acknowledge partial financial support from CSGI (Florence, Italy) and MIUR (PRIN-2003, Rome, Italy).
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