Photocatalytic activity of wool fabrics deposited at low temperature with ZnO or TiO2 nanoparticles: Methylene blue degradation as a test reaction
Graphical abstract
Introduction
Zinc oxide (ZnO) is a known semiconductor, which was frequently used either as bulk or as thin layer due to its remarkable properties like high electron mobility, wide band gap, and strong luminescence at room temperature. Many applications were developed last years not only as solar cells and different sensors, in opto-electronics or for transparent conducting films but also as photocatalyst [1], [2], [3], [4], [5]. Titanium dioxide (TiO2) is another semiconductor widely recognized from a basic research point of view and due to its technological applications, based on its UV blocking, antibacterial or/and photocatalytic [6], [7], [8], [9], [10], [11], [12], [13], [14] properties. The general mechanism of photocatalysis should include the excitation with photons of energy higher than the band gap and the generation of the exciton pair with holes in the valence band and electrons in the conduction band. These charge carriers may either recombine, dissipating the energy as heat, or undergo an interfacial charge transfer process [3], [14]. Attempts were done to improve each of the steps in (heterogeneous) photocatalysis by tailoring the semiconductor and the reaction conditions. Moreover, its terminology, relative photonic efficiencies and quantum yields were discussed [15], [16], [17].
ZnO was settled upon various supports, including different textiles made from natural or synthetic fibers like cotton [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], polyester (e.g. [29], [30]), silk [31] or wool [32], [33]. Coating the fibers by TiO2 particles was also performed by many techniques: thus, synthetic fibers, e.g. polyester ones, were covered with TiO2 at temperature lower than the natural fibers [34], [35], [36], [37], [38], [39], [40]. Photocatalysis onto TiO2 deposited fabrics was discussed in several works, for example [14], [41], [42], [43].
The photocatalytic performances of different semiconductor catalysts including ZnO and TiO2 [44], [45], [46], [47] were tested by decolorization of different dyes, one of them being methylene blue (MB). Being water soluble, MB can be easily applied directly to different (textile) stuffs: wool [48], cotton [48], [49], polyvinyl alcohol (PVA) [50], silica gel [51], metal oxides [52] etc. Due to the fact thatthe reversibility of changes after exposure might be stopped or due to the ambiguous reaction mechanism though under ultraviolet irradiation where MB photoabsorption may be disregarded see for example [45], [53], MB was considered less suitable probe molecule [14].
This paper reports on ZnO and TiO2 nanoparticles deposition onto the surface of wool fabrics and model samples and on the photocatalytic properties of these particles. On one hand, the oxide deposition improves the textile features and on the other hand they might find different applications [54] as interesting substrates, which are cheap, non-toxic, washable and foldable. Our wool materials have different mesh sizes and fabrication mode. Pretreatments were already performed industrially to bleach them, to arrive up to effective anti-felting, to receive shrink-resistance etc. During the actual works plasma (pre)treatment was performed that is known to have the effect of cystine bond cleavage and forming of the cysteic acid. Two methods were then applied for oxide deposition leading to (nano)particles: electroless technique or sputtering for ZnO and sol-gel or sputtering for TiO2. UV photocatalytic degradation of methylene blue was finally used for evaluation of photocatalytic properties of coated wool fabrics. Photocatalytic kinetics was discussed as well. MB is degraded upon semiconductor oxide catalytic systems made from alike supports, the same deposited semiconductor particles and similar irradiation: we were in the case when MB photocatalytic degradation can serve as test reaction for fabrics. These reaction conditions rather indicate those in which the self cleaning of fabrics is manifested: thus the decolorization of this dye might be recommended to evaluate fabric photocatalytic properties at least at the self cleaning level.
Section snippets
Textiles samples
Pieces of sheep wool fabrics originated from industry or home-made were our samples. They differ by the chemical nature of the yarns, the size and the 2D texture elements and the pretreatment type (see Table 1). In Table 1 there are also given the images (made at the same magnification by optical microscopy) of the samples allowing to roughly estimate some morphological differences. Other samples made from cotton or deposited glass plates were used as model for comparison.
Textile functionalization
The pieces of the
Results and discussion
The presentation of the obtained results follows up the two main aims of the work: oxide particle deposition and the photocatalytic properties given by these particles. Thus, deposited samples were investigated in order to demonstrate the deposition of the oxides, the formation of new chemical bonds and the appearance of ZnO/TiO2 aggregates. Though some coated layers are amorphous and the oxide presence cannot be revealed by XRD investigations, the PCC2 checker and MB as the dye to be bleached
Conclusions
Wool textile and model glass plate samples were successfully coated with TiO2 and ZnO (nano)particles at rather low temperature using either a sputtering or a chemical sol-gel/electro less technique. The coated samples were structurally and morphologically characterized by XRD, SEM and XPS. Sputtering leads to wurtzite type ZnO crystallites but deposited TiO2 does not appear crystalline.
TiO2 or ZnO powders deposited upon the fabrics acted as photocatalysts under UV light to decolorize methylene
Acknowledgements
The authors gratefully thank the Romanian Authority of the Education Ministry for the financial support under the project ID 281/2011 and are indebted to Dr. N. Preda (NIMP, Magurele, Romania) for the experiments of ZnO electroless deposition upon the samples.
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