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Original Article
DOI: 10.1016/j.jmrt.2018.06.025
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Available online 24 February 2019
Photocatalytic performance of N-doped TiO2nano-SiO2-HY nanocomposites immobilized over cotton fabrics
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Salmon Landia,b,
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salmon.landi@ifgoiano.edu.br

Corresponding author.
, Joaquim Carneiroa, Olivia S.G.P. Soaresc, Manuel F.R. Pereirac, Andreia C. Gomesd, Artur Ribeiroe, António M. Fonsecae,f, Pier Parpote,f, Isabel C. Nevese,f
a Department of Physics, University of Minho, Azurém Campus, Guimarães 4800-058, Portugal
b Instituto Federal Goiano, 75901-970 Rio Verde, Goiás, Brazil
c Laboratory of Catalysis and Materials – Associate Laboratory LSRE/LCM, Faculty of Engineering, University of Porto, 4200-465 Porto, Portugal
d Centre of Molecular and Environmental Biology, University of Minho, 4710 – 057 Braga, Portugal
e Centre of Biological Engineering, University of Minho, 4710-057 Braga, Portugal
f Centre of Chemistry, Chemistry Department, University of Minho, Gualtar Campus, Braga 4710-057, Portugal
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Received 15 January 2018, Accepted 04 June 2018
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Table 1. Physical properties of the pristine and nanocomposite materials.
Abstract

This work reports the synthesis of nanocomposite photocatalytic materials based on nitrogen-doped TiO2nano, SiO2 and different percentages of HY zeolite (0, 12, 25 and 50%). These materials were characterized by using Fourier transformed infrared spectroscopy, X-ray diffraction, N2 adsorption–desorption, UV-vis diffuse reflectance spectroscopy and scanning electron microscopy. The nanocomposites, which presented an energy band gap of about 3.03eV, were immobilized on cotton fabric and their self-cleaning properties were investigated by decolourization of rhodamine B (RhB) dye in aqueous solution under simulated solar irradiation. The fabrics coated with the photocatalysts, containing and not containing zeolites, showed the same RhB decolourization (about 95%) after 5h, excluding the situation where a large amount of HY (50%) was used in the nanocomposites. However, results obtained from high performance liquid chromatography analysis depicted that in the presence of the HY zeolite a more effective RhB degradation was achieved. In fact, even after the use of five consecutive cycles, the RhB decolourization remained high (about 85%). Generally, the photodegradation of RhB solution in the presence of cotton fabrics functionalized with TiO2nano, TiO2nano-SiO2 and TiO2nano-SiO2-0.25 HY resulted in the formation of products that exhibited a similar cytotoxic effect when compared to the untreated RhB solution and subjected to the same tested concentrations and incubation times.

Keywords:
Photocatalysis
Textile functionalization
TiO2-SiO2
HY zeolite
Rhodamine B
Cytotoxic
Full Text
1Introduction

Several works have been reported in the literature on heterogeneous photocatalysis, which usually explores the degradation of pollutants by redox reactions that begins near the surface of semiconductors after being subjected to light irradiation [1]. Due to its electronic structure, characterized by the presence of a nearly filled valence band (VB) and an practically empty conduction band (CB) separated by a band gap energy (Eg), the semiconductors can absorb photons whose energy is equal to or greater than Eg and as a consequence, electron–hole (e/h+) pairs are generated. The charge carriers which did not undergo the recombination process can react with electron donors (e.g. H2O or pollutant) or electron acceptors (e.g. O2) adsorbed on the semiconductor surface creating free radicals [2]. These species are highly reactive leading to the degradation of different pollutants, such as pesticides [3], pharmaceuticals [4], dyes [5], gases [6], or removing toxic heavy metal, such as Hg0 from flue gas [7] or even inactivating microorganisms (virus, bacteria, spores and protozoa) [8]. It is important to note that according to Derikvandi and Nezamzadeh-Ejhieh the major limitation of heterogeneous photocatalysis is the recombination of e/h+ pairs, and that this problem can be relatively mitigated by doping, supporting and coupling of semiconductors [9].

Particularly, the dyes from the textile industries represent an important class of pollutants because of their huge world production (about 105tonnes/year [10]) and due to its frequent discharge in water courses [11]. It is estimated that about 15% of the dyes are lost in the industrial effluents during manufacturing process [12]. In addition, some dyes are non-biodegradable and other can be metabolized into carcinogenic by-products [13]. Since the conventional methods used in the wastewater treatments, such as flocculation, chemical precipitation and adsorption on activated carbon do not promote the degradation of these pollutants, but only transfer them to other media (causing secondary pollution) [14], the photocatalysis enabled by semiconductor materials has been considered an efficient technology for remediation of wastewater [15].

Among diverse semiconductors, titanium dioxide (TiO2) is widely used in photocatalytic reactions and it is also considered as a potential candidate in industrial applications for remediation applications because it is biologically and chemically inert, inexpensive, harmless and non-toxic [10]. However, according to Nezamzadeh-Ejhieh TiO2 presents low surface area, rapid recombination of e/h+ pairs and in order to be light activated, TiO2 requires the absorption of photons with energy in the ultraviolet (UV) region due to its large value of band gap energy [16]. Therefore, to better exploit the solar radiation that reaches Earth's surface (UV, 1–380nm, 5%; visible, 380–780nm, 46%; near infrared, 780–3000nm, 49% [17]), researchers have tried to implement different strategies. Among them, the doping process has been applied because it is capable to generate new energy levels inserted between the original VB and CB [18] and also to decrease the recombination process already mentioned [19]. Another strategy concerns the production of hybrid systems with noble metals, which can absorb the visible light by surface plasmon resonance phenomena. This absorbed energy can then be transferred to the CB by charge transfer, promoting, subsequently, the occurrence of reduction reactions [17].

Since the effectiveness of the photocatalysis process is highly dependent upon the charge transfer between the semiconductor and the dissolved species, it is strongly desirable to use materials that are characterized by having a large surface area and acting as dispersive support of the photocatalysts. In this sense, it has been reported that the supporting TiO2 on activated carbon [20], graphene-based materials [21], SiO2[22] can improve the photocatalytic activity of TiO2 semiconductor material. Currently, zeolites are also widely exploited in the preparation of composites having photocatalytic properties [23–28]. The properties of the zeolites that are interesting for the photocatalysis reactions, mainly include a high internal surface area formed by cannels or cavities and also the presence of ions and acid sites [29]. In fact, in a recent published article, we have shown the potential use of the HY zeolite in promoting the photocatalytic activity of cotton textiles functionalized with SiO2-TiO2 and SiO2-TiO2-HY composites [30]. In that work, several experimental parameters were evaluated and the best catalytic performance was achieved for the SiO2-TiO2 and SiO2-TiO2-HY composites that were subjected to a calcination treatment at a temperature of 400°C.

However, for a scenario of industrial applications, there are some drawbacks related with the use of the photocatalysts in the form of directly added powder to promote the remediation of wastewater. For example, powders present a tendency to form agglomerates and, as consequence, they can precipitate, thus decreasing the material availability to participate into photodegradation reactions. Therefore, unless some kind of stirring system is used, which can undoubtedly lead to increased energy consumption, the direct addition of photocatalyst powder should be avoided. Other inconvenient is related to the photocatalyst reuse, since the separation of powders from treated effluents will certainly increase the treatment costs. Therefore, the photocatalysts impregnation on a solid support can be an important and smart alternative to overcome the previous mentioned difficulties [31]. Moreover, several works have also chosen textile materials to obtain self-cleaning photocatalytic surfaces because they are cheap, present low weight and high mechanical flexibility and, in addition, they can also be easily adjusted to the different geometries of treatment tanks that would have already been installed in the original industrial layouts. Recently, Fan et al. demonstrated a remarkable photocatalytic activity and removal of stains using g-C3N4 onto cotton fabrics under sunlight irradiation [32]. El-Naggar et al. synthetized SiO2-ZnO nanocomposites over cotton fabrics with antibacterial and UV protection properties [33]. Similar materials were obtained by Mehrez et al. from in situ synthesis of TiO2 on cotton fabrics using urea nitrate as a peptizing agent [34] and by Mohamed et al. from functionalization of cotton textiles with Ag and SiO2 nanoparticles [35]. Katoueizadeh et al. applied N-doped TiO2 over cotton textiles to produce samples that presents photocatalytic and super hydrophobic behaviour under visible light irradiation [36]. In order to follow up on our previous work, a photocatalytic composite based on the TiO2 nanoparticles, SiO2, and HY zeolite was impregnated on the cotton fabrics and a lamp that simulated the solar radiation was used to promote the degradation of rhodamine B (RhB), a widely used dye in textile industry effluents [37]. In particular, RhB (C28H31N2O3Cl) is a dye of the xanthene class that causes irritation to the skin, eyes and respiratory tract and even possess carcinogenicity, reproductive and progressive toxicity [37].

2Experimental2.1Materials

Tetraethoxysilane (TEOS) was used as precursor for SiO2 and purchased from Aldrich. The HY zeolite (CBV 400) was obtained from Zeolyst International in powdered form. Commercial TiO2 nanoparticles were acquired from Quimidroga and consisted in a mixture of anatase (80%) and rutile (20%) crystalline phases. Ethanol and water were used as solvents and ammonium hydroxide was utilized as a catalyst for hydrolysis reactions.

2.2Preparation and characterization of composites and self-cleaning fabrics

The photocatalysts preparation method was similar to the one described in our previous and recent published work [30]. Typically, the hydrolysis of the SiO2 precursor (2.30g) was achieved in the presence of TiO2 nanoparticles (1.44g), ethanol (1.20mL) and H2O (1.00mL) in alkaline media. However, in this work a substantial amount (0.5mL) of ammonium hydroxide (NH4OH) was used in order to obtain nitrogen-doped TiO2 nanoparticles. Hereafter, the HY zeolite was added in the refluxed solution, which was magnetically stirred for 6h and finally dried at 70°C in an oven. Different amounts of zeolite (0, 12, 25 and 50% taken in relation to the sum of the weights of SiO2 precursor and TiO2 nanoparticles) were employed. The nanocomposites containing an increasing amount of the HY zeolite were denoted as TiO2nano-SiO2, TiO2nano-SiO2-0.12HY, TiO2nano-SiO2-0.25HY and TiO2nano- SiO2-0.50HY and their properties were compared to bare TiO2. Since the TiO2 nanoparticles present crystalline phases and the zeolite is already acidic, the calcination step was not performed in this work [30].

The photocatalyst nanocomposites were characterized by different techniques. Fourier transform infrared (FTIR) spectra of the nanocomposites in KBr pellets were performed to investigate the vibrations bands on a Bomem MB104 spectrophotometer (range: 4000–500cm−1, resolution: 4cm−1 using 32 scans). The powder X-ray diffraction (XRD) was performed to analyze the crystal structure of the nanocomposites on a Bruker D8 Discover (scan range: 5–60° at a scan rate of 0.1°/s). The textural properties of the nanocomposites were determined by adsorption–desorption of N2 at −196°C on a Nova 4200e (Quantachrome Instruments) equipment. The micropores volumes (Vmicro) and mesoporous surface areas (Smeso) were calculated by using the t-method, the BET surface areas (SBET) were determined via BET equation and the total pore volume was obtained for P/P0=0.95 (VP). Transformed Kubelka–Munk function, obtained from the ultraviolet-visible diffuse reflectance spectra (UV-vis DRS) acquired on a ScanSpecUV-vis spectrophotometer equipped with an integrating sphere, was used to calculate the band gap energy (Eg) of the as-prepared photocatalyst nanocomposites.

The functionalization of the cotton fabrics (3cm×3cm) with the photocatalyst nanocomposites, essentially consisted of the following three steps: (i) initially, a methanol/composite suspension was homogenized by sonication during 15min; (ii) the cotton fabrics were dipped in the suspension and stayed there for 15min under sonication and (iii) after that time, the sonicator was switched off but the cotton fabrics still remained in the suspension during 1min and then removed to let them to dry at room temperature. Actually, step (ii) consists of an important modification to the methodology performed by Xu et al. [38] because it enables a better interaction between the cotton fibres and the nanocomposite particles, thus improving the fabrics functionalization process. The step (iii) was repeated two times in the same suspension and, finally, the samples were cured in an oven at 100°C for 10min. The micrographs of the nanocomposites and the self-cleaning fabrics were examined by scanning electron microscopy (SEM) using a FEI Nova 200 FEG-SEM microscope at a magnification of 5000× and 50,000×.

2.3Photocatalytic activity

The photocatalytic performance of the different samples was evaluated by the degradation of RhB aqueous solutions (5mg/L) under similar sunlight irradiation. The experiments were conducted in beakers with 20mL of dye aqueous solution together with the cotton samples. The beakers were sealed with flexible film (at least having 90% of transmittance in range 300–800nm) to avoid the evaporation of the solution and then placed in a region, delimited by a circular mark, and whose centre was directly exposed to the light emitted by a 300W lamp (Ultra-Vitalux E27) and placed at a distance of about 30cm above the solution surface. The UVA/B and visible light irradiance was around 1.0 and 2.0mW/cm2, respectively. The solutions were not subjected to any type of stirring process. In order to distinguish the adsorption and the photocatalytic degradation process, the cotton samples remained in the solutions during 1h in the dark and, after that time, the lamp (simulating the sunlight spectrum) has been switched on for 5h. The recyclability of the performance of the cotton fabrics coated with TiO2nano, TiO2nano-SiO2 and TiO2nano-SiO2-0.25HY composites was analyzed up to five cycles. Experiments were performed in duplicate and the error bars refer to the standard deviations.

2.4Analytical methods

The photocatalytic degradation of the RhB solutions was monitored by examining its maximum absorbance values acquired at specific intervals of time. For this purpose, the lamp was switched off and dye solution aliquots (3.0mL) were extracted and centrifuged at 6000rpm for 10min to obtain the supernatant. The absorbance of the supernatant was monitored using a spectrophotometer (ScanSpecUV-Vis) in the 300–700nm wavelength range and its corresponding concentration was determined from the calibration curve. After performing the absorbance measurements, the supernatants were re-inserted into the beakers and the lamp was switched on again according to Zhang et al. [39].

The N-de-ethylated intermediates from the photodegradation of RhB were identified by electrospray ionization (ESI) mass spectral technique in the positive ion mode and confirmed by high liquid chromatography performance (HPLC) technique on a Thermo Finnigan Lxq equipped with a UV-vis diode array detector in series with a mass detector using a C18 reverse phase column. The mobile phase consisted in a mixture of an acetic acid solution (0.2%) and acetonitrile at a flow rate of 0.4mL/min.

2.5Cellular viability assay2.5.1Cell culture maintenance

NCTC 2544 cell line was used as general model of cytotoxicity. The NCTC 2544 cell line (human skin keratinocytes) was cultured in DMEM media, and supplemented with 7% FBS and 1% (v/v) penicillin/streptomycin solution. Cells were maintained in 75cm2 tissue culture flasks at 37°C in a humidified atmosphere with 5% CO2. The cell culture medium was renewed twice a week.

2.5.2Cell viability assessed by MTT assay

The MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide) assay was used to measure the cytotoxicity of the RhB solutions before and after photocatalytic treatment in cell cultures [40]. Cells were seeded at a previously optimized density of 1×105cells/mL on a 48-well tissue culture plate, the day before the experiments. NCTC 2544 cells were exposed to four concentrations (0.015, 0.025, 0.050 and 0.075ppm) of RhB solutions obtained before and after photocatalytic treatment. Cells incubated with DMSO (30% of the total volume) and cells without the addition of the compounds were used as positive and negative controls of cytotoxicity, respectively. Cells were incubated at 37°C in a humidified atmosphere with 5% CO2. At the end of 24 and 48h of contact, cell metabolic activity was assessed by MTT viability assay [40]. After incubation, the medium with RhB samples was removed and medium with MTT (5mg/mL) was added to each well, and cells were further incubated at 37°C for 2h. The MTT solution was carefully decanted, and formazan crystals were dissolved in DMSO/EtOH (1:1 (v/v)) mixture. Colour was measured with 96-well plate reader at 570nm in a microplate reader SpectraMax Plus (Molecular Devices). Cellular viability was determined in relation to the life control from the three independent experiments.

2.5.3Statistical analysis

Data are presented as average standard deviation (SD), n=3. Statistical comparisons were performed by one-way ANOVA with GraphPad Prism 7.0 software (La Jolla, CA, U.S.A.). Tukey's post hoc test was used to compare all the results between them, and a Dunnet's test was used to compare the results with a specific control. A P-value of <0.05 was considered to be statistically significant.

3Results and discussion3.1Characterization of the photocatalyst nanocomposites

In this work, the nanocomposites were prepared with TiO2 nanoparticles and the zeolite HY. This zeolite belongs to faujasite structure [41] and it was used as received in the proton form with a total ratio Si/Al 2.80 and presents a pH value of 4.5 at the point of zero charge (pHPZC), which confirms the acidic behaviour of the structure. The pHPZC of the zeolite was determined by the procedure describe in [42].

FTIR spectra of the nanocomposites show the strong bands attributed to the HY zeolite structure (Fig. S1) [43]. The vibration mode detected between 570 and 600cm−1 is very sensitive to changes in the HY zeolite structure and can be used to calculate the framework Si/Al ratio [30]. This band did not undergo any shift and therefore, all composites showed the same framework Si/Al ratio (2.75) suggesting that the nanocomposite preparation did not change the HY zeolite structure, in agreement to the expected value (2.80). On the other hand, only for the nanocomposite without the presence of HY zeolite, it was possible to identify a band at about 960cm−1 confirming the existence of linkages of Si–O–Ti [38,44]. Probably, in the nanocomposite spectra, there was a superposition of this band with the strong bands of the HY zeolite precluding its identification.

The XRD patterns of the photocatalysts and the HY zeolite are depicted in Fig. S2. The diffraction peaks of the nanocomposites show that the TiO2 nanoparticles consist in a mixture of anatase (JCPDS, no. 21-1272) and rutile (JCPDS, no. 21-1276) phases and their intensities confirm that anatase is the dominant crystalline phase. The unit cell parameter of the HY zeolite (face centred cubic structure) was determined from the angular positions of the (533), (642) and (555) diffractions peaks by using the ASTM D 3942-80 method. Subsequently, the framework Si/Al ratio of the zeolite HY was determined by using the Breck and Flanigen equation [30]. These three Miller diffraction peaks, which were previously mentioned, presented the same diffraction angles in the diffractograms of the HY zeolite and the nanocomposites, which confirms that there was not dealumination of the zeolite structure during the composites preparation process. The obtained value from the XRD (Si/Al=2.87) is in good agreement with the expected one.

The nitrogen adsorption–desorption equilibrium isotherms at −196°C reveal that the presence of the HY zeolite enhances the surface area as well as the total pore volume of the SiO2-TiO2-HY composites, which is related to the high surface area of the HY zeolite, as can be observed in Table 1. It can be observed that TiO2nano-SiO2 material presents a surface area of 53m2/g whereas the TiO2nano-SiO2-0.12HY show a surface area of 430m2/g, evidencing that even using a small amount of HY zeolite, it significantly enhances the SBET. As expected, the nanocomposite synthesis leads to a decrease of the microporosity comparing to HY, which should be related to the mesoporous nature of the TiO2nano and TiO2nano-SiO2 materials.

Table 1.

Physical properties of the pristine and nanocomposite materials.

Sample  SBET (m2/g)  Smeso (m2/g)  Vmicro (cm3/g)  VP P/P0=0.95 (cm3/g) 
TiO2nano  41  41  0.000  0.079 
TiO2nano-SiO2  53  49  0.001  0.111 
HY  665  25  0.302  0.349 
TiO2nano-SiO2-0.12HY  430  317  0.050  0.386 
TiO2nano-SiO2-0.50HY  497  263  0.097  0.348 

DRS were thoroughly investigated such that all the conditions required by the Kubelka–Munk theory were satisfied. From the reflectance (R∞) measurements as a function of wavelength of the incident photon the Eg value of the nanocomposites, for indirect optical transitions can be determined by using the equation: [F(R∞)]1/2=C(E−Eg)[45,46]. In this equation, the variables, F, E and C are Kubelka–Munk function, incident photon energy and proportionality constant, respectively, which depends on the material properties. The usual procedure consists in plotting the left side of the previous equation against the E variable (that is, the energy of the incident photon) fitting the linear portion of this curve by a straight line (Fig. 1). Therefore, the linear coefficient divided by the slope (taken in module) provides the numerical value of Eg. Fig. 1 shows the determined Eg value of the nanocomposites, which is compared with the Eg value of TiO2 nanoparticles.

Fig. 1.

Kubelka–Munk transform versus photon energy of the prepared nanocomposites compared to TiO2 nanoparticles.

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The Eg of TiO2 nanoparticles was determined to be 3.19eV, which is a value in accordance to the literature [47]. On the other hand, considering the uncertainties (calculated from the linear fit parameters), all nanocomposites presented the same Eg, which is substantially lower than that of TiO2. The reason for this Eg decrease probably results from the effect of N-doping of TiO2 nanoparticles occurred during the preparation method of the nanocomposites. In addition, besides extending the light absorption range (for longer wavelengths) of TiO2 nanoparticles, the N-doping effect also leads to less a recombination rate of electron–hole pairs, thus enhancing the photocatalytic activity of these photocatalysts [48]. Fig. 1 also shows the R2 (0.998) obtained from the linear fit as previously explained, showing an excellent linearity, which confirms the consistency of the obtained optical properties for these nanocomposites.

Fig. 2(a)–(c) shows the SEM micrographs of the TiO2nano, TiO2nano-SiO2 and TiO2nano-SiO2-0.25HY photocatalysts on the fabric's surface. It is noted that these three photocatalysts are differently dispersed on the cotton substrate. TiO2 nanoparticles coated better the cotton fibres (Fig. 2(a)) in comparison with the other nanocomposites. In particular, those ones that include the HY zeolite in their compositions tend to form bigger agglomerates (Fig. 2(c)), which is due to the tendency of the zeolite particles to agglomerate and therefore affecting the corresponding catalytic properties.

Fig. 2.

SEM micrographs of (a) TiO2nano, (b) TiO2nano-SiO2 and (c) TiO2nano-SiO2-0.25HY deposited on cotton fibres at a magnification of 5000×, and morphology of (d) TiO2 nanoparticles and (e) TiO2nano-SiO2-0.25HY nanocomposite at a magnification of 50,000×. (f) EDX analysis of TiO2nano-SiO2-0.25HY.

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Fig. 2(d) and (e) shows the morphology of TiO2 nanoparticles and TiO2nano-SiO2-0.25HY nanocomposite, respectively, at a magnification of 50,000×. The histogram (Fig. S3) represents the size distribution of TiO2 nanoparticles and it was obtained from analysis of Fig. 2(d) and the respective average particle size was found to be 40±7nm. This value is much higher than that of the pore diameter of HY zeolite (0.74nm), thus these TiO2 nanoparticles can be only impregnated onto the surface of the zeolite particles, as is shown in the Fig. 2(e). Fig. 2(f) depicts the EDX results from the square region highlighted in the Fig. 2(e). Although only TiO2 nanoparticles are visible in this highlighted region (Fig. 2(e)), the presence of the zeolite and the SiO2 is confirmed through the quantification of Si and Al atoms in the EDX analysis. This confirms that for the nanocomposites based in TiO2, SiO2 and HY zeolite, the TiO2 nanoparticles are impregnated at surface of the zeolite particles.

The essential idea of this work consists in proving that the photodegraded products near to TiO2 surface can be better degraded by the action of the acid sites present in the HY zeolite, whereas the role of SiO2 is to act as a bond between these two different materials.

3.2Photocatalytic degradation of RhB dye

The photocatalytic ability of the fabrics was evaluated by decolourization of RhB solution calculated by equation: Decolourization (%)=(1−C/C0)×100 The RhB concentration also was analyzed from the pseudo-first-order kinetic model: ln(C/C0)=−kappt. In these equations, C0 is the RhB dye concentration after its adsorption during 1h in dark conditions, C represents its concentration after being subjected to irradiation of light performed at different interval of times (t) and kapp is the pseudo-first-order kinetic reaction constant [49]. As can be observed in Fig. 3(a), the decolourization of RhB bare solution was about 10% after having spent 5h in light exposure conditions, demonstrating the photolysis occurrence of the RhB molecules. Meanwhile, within this same period of time, the RhB solution in the presence of pristine cotton experienced a decolourization equal to 26%. This increase can be probably related with two main factors: the presence of OH groups on the cellulose fibres and also due to O2 trapped between the fibres. These species may form hydroxyl and superoxide radicals, respectively, contributing to improve the decolourization of RhB solution when compared with the results obtained from its bare solution (that is, without the presence of any fabric).

Fig. 3.

Decolourization of RhB solution under similar sunlight. Experimental conditions: 35°C; pHinitial=5.9; [RhB]=5.0mg/L.

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On the other hand, the cotton fabrics coated with TiO2nano, TiO2nano-SiO2, TiO2nano-SiO2-0.12HY and TiO2nano-SiO2-0.25HY induced the same solution decolourization (about 95%) after 5h under simulated solar irradiation condition. According the SEM micrographs shown in Fig. 2, it can be observed that the cotton fibres are better coated by the photocatalysts that does not contain the HY zeolite. Even with less nanocomposites deposited on the cotton fibres, the decolourization of the RhB solution in the presence of the cotton substrate coated with TiO2nano-SiO2-(0.12 and 0.25)HY remained high after 5h of exposure to light. However, from our experimental conditions, it was found that when a large amount of HY is present in the nanocomposites, the percentage decolourization of RhB solution decreases significantly, as can be observed from the curve shown in Fig. 3(a), which is represented by empty triangular symbols. Fig. 3(b) shows the variations in ln(C/C0) with reaction time for the different treatments, in which the linear relationship between these variables indicates that RhB decolourization follows the pseudo-first-order kinetic model. It is also possible to observe that the decolourization rates for the treatments performed with the functionalized fabrics present the same order of magnitude, nevertheless the ones related to TiO2nano-SiO2-0.50 is substantially lower. These results suggest that smaller amounts of HY added to the composites have not a negative effect on the RhB degradation during the studied time interval.

Despite some of the samples showed the same decolourization over a period of 5h, their absorbance spectra presented some peculiarities, as can be observed in Fig. 4. In fact, the solutions treated with cotton fabrics coated with the nanocomposites that hold 12% and 25% of HY zeolite material revealed a very similar optical behaviour. However, concerning the RhB solutions submitted to the treatment with photocatalysts without the zeolite material, it was observed that the absorbance of these aqueous solutions presented significant differences, mainly for the one that was treated with the sample that only contains TiO2 nanoparticles.

Fig. 4.

Degradation absorbance curves of RhB solution after 5h of exposure to sunlight irradiation for the different nanocomposites compared to curves related with solutions treated with pristine cotton and without fabric (bare solution). Experimental conditions: 35°C; pHinitial=5.9; [RhB]=5.0mg/L.

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As it is known, the shift of the maximum absorbance peak of the RhB towards lower wavelengths is related to the intermediates formed from the N-de-ethylation process [50]. Thus, the obtained UV-vis spectra for the RhB dye solutions treated with cotton fabrics coated with TiO2nano, TiO2nano-SiO2 and TiO2nano-SiO2-HY photocatalysts suggest the presence of products that still preserve the chromophore structure of RhB dye. The relative amounts of these products will be discussed in detail in the next section.

In order to investigate the stability of self-cleaning fabrics, recycled photocatalytic degradation assays were performed for the cotton fabrics coated with TiO2nano, TiO2nano-SiO2 and TiO2nano-SiO2-0.25HY photocatalysts (Fig. 5). From the analysis of Fig. 5 it can be stated that the RhB decolourization slightly decreased only after performing five consecutive cycles, denoting that these fabrics possess high recyclability and stability.

Fig. 5.

Stability tests of cotton fabrics coated with photocatalysts under simulated solar irradiation over a time period of 5h. Experimental conditions: 35°C; pHinitial=5.9; [RhB]=5.0mg/L.

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3.3Transformation products and photocatalytic mechanism

The dye solutions subjected to different treatments were analyzed on the ion-trap mass spectrometer linked to the HPLC system (HPLC-MS) in positive ionization mode. Due to the lack of the appropriate standard compounds, these analyses enables to perform a comparative study concerning the N-de-ethylated intermediates present in different solutions, through the ratios of the corresponding peak areas. Fig. 6 depicts the chromatograms of the photodegraded solutions in the presence of cotton fabrics coated with TiO2nano, TiO2nano-SiO2 and TiO2nano-SiO2-0.25HY photocatalysts.

Fig. 6.

Total ion current chromatograms of the dye solutions after having been photocatalytically treated for 5h with cotton fabrics coated with three different photocatalysts. The peaks (a–e) refer to the RhB ion and the intermediates that lost ethyl groups in sequence.

(0.27MB).

The ion peaks (a–e) differ exactly by 28mass units in sequence, which is consistent with the loss of the N-ethyl groups from the original RhB molecule. The areas of the peak “a” calculated for these three solutions were very similar denoting that the RhB concentration for these treated solutions is almost equal. On the other hand, the areas for the other peaks (b–e) showed significant differences, namely (i) the relative abundance of the full N-de-ethylated ion (m/z=331, peak “e”) for the solution, which was only treated with TiO2 nanoparticles, is higher than that the one observed for the other two dye solutions, (ii) the concentration of the intermediates having a m/z=387 (peak “c”) and a m/z=415 (peak “b”) is higher for the solution treated with TiO2nano-SiO2 composite, and (iii) the bottom chromatogram, which correspond to the solution treatment carried out with the TiO2nano-SiO2-0.25HY composite, shows that all N-de-ethylated products of RhB exist in a lower concentration when compared with those observed for the other two solutions, showing that the presence of the HY zeolite material in the composite enhanced the RhB molecule degradation.

Since the aeration does not exist, it is expected that the superoxide radicals do not have much influence on the RhB degradation, being the photogenerated holes the main active species that participate in the photocatalytic mechanism. Based on this plausible supposition and from the results obtained above, the proposed mechanism for the RhB photocatalytic degradation, promoted by the cotton fabrics coated with the synthetized nanocomposites, can be the following: (i) electron–hole pairs are generated by the photocatalysts under UV and visible light irradiation; (ii) holes can be transferred to the water molecules producing hydroxyl radicals [51]; (iii) RhB molecules can also transfer electrons into the semiconductor CB by absorption of visible light at maximum of absorbance (553nm) [52]; and (iv) acid sites present in the HY zeolite contribute for the degradation of the formed intermediate products.

3.4Cytotoxicity assay

To assess the potential cytotoxic effect of the RhB before and after the photocatalytic treatment with fabrics functionalized with TiO2nano, TiO2nano-SiO2 and TiO2nano-SiO2-0.25HY, human skin keratinocytes (NCTC 2544) cells were used as model of general cytotoxicity. Fig. 7 shows a decrease on cell viability with increasing concentrations of RhB for all the tested samples. However, this effect was more pronounced at the end of 48h of exposition and it was only significant for the highest concentration (0.075ppm) of RhB treated with TiO2nano-SiO2 and TiO2nano-SiO2-0.25HY when comparing with the negative control.

Fig. 7.

Relative viability of NCTC 2544 (human skin keratinocytes) evaluated with the MTT assay, after 24 and 48h incubation in medium containing four concentrations (0.015, 0.025, 0.05 and 0.075ppm) of RhB solutions obtained before and after treatment with textiles substrates functionalized with TiO2nano, TiO2nano-SiO2 and TiO2nano-SiO2-0.25HY. Cells incubated with culture medium were used as negative control and cells incubated with 30% DMSO as positive control of cytotoxicity. Data were determined relative to the negative control (life). αP0.05 when compared with negative control 24h, ξP0.01 when compared with negative control 48h, ΔP0.01 when compared with 0.075ppm concentration of TiO2nano-SiO2-0.25HY treatment, βP0.05 when compared with 0.075ppm concentration of TiO2nano treatment 24h, μP0.05 when compared with 0.075ppm concentration of TiO2nano treatment 48h.

(0.4MB).

Despite the cytotoxic effect for the highest concentrations of RhB-species resulting from the TiO2nano treatment, there was a slight increase on cell viability after 48h of incubation for the lowest concentration of TiO2 RhB-species. The RhB-species formed with this treatment seem to have a proliferative effect for the lowest tested concentration.

The effect of RhB (no treatment) on cell viability was more uniform for the tested concentrations when compared with the effect of the formed RhB-species, presenting cell viabilities ranging between 81±9% and 93.8±0.9%.

4Conclusions

Photocatalysts based on TiO2 nanoparticles, SiO2 and different amounts of HY zeolite were successfully produced. XRD and FTIR analysis confirmed that the crystalline structure of the zeolite material did not change even after SiO2-TiO2nano loading. DRS reveal that the TiO2 nanoparticles experienced N-doping during the syntheses, thus extending its light absorption range for longer wavelengths. The photocatalytic properties of cotton fabric functionalized with these nanocomposites were successfully confirmed via the RhB decolourization under simulated solar irradiation. It was observed that the presence of the HY zeolite in the nanocomposites does not change the RhB decolourization after 5h of photocatalytic experiments but favoured the degradation of N-de-ethylated intermediates, excluding when a large amount of zeolite was used in the synthesis of the nanocomposites. It was observed that the effect of RhB dye and their photodegraded products on cell viability is dependent on time of incubation and RhB concentration. In general, the photodegradation of RhB dye solutions in the presence of cotton fabrics functionalized with TiO2nano, TiO2nano-SiO2 and TiO2nano-SiO2-0.25HY resulted in the formation of species that were not much more cytotoxic than that of the original RhB solution, under the same tested concentrations and incubation times. Lastly, this work has shown that the photodegraded solutions by the fabrics coated with the synthesized composites presented a lower concentration of N-de-ethylated intermediates when compared with the ones that were only treated with cotton textiles coated by the TiO2 semiconductor material. In this sense, it is possible to state that the use of fabrics coated with the synthesized composites do not present disadvantages for the tested cells relative to the initial RhB solution, for the same experimental conditions; in fact, some of the coated samples present environmental vantages, since it was verified a slight increase on cell viability.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

Salmon Landi Jr. thanks the Capes from Brazil for supports his Doctoral Fellowship (Proc. no. 13346/13-0). The authors are also grateful to the financial support of the project “AIProcMat@N2020 – Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020”, with the reference NORTE-01-0145-FEDER-000006 and the project BioTecNorte (operation NORTE-01-0145-FEDER-000004), supported by Norte Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF). This work also has been funded by ERDF through COMPETE2020 – Programa Operacional Competitividade e Internacionalização (POCI), Project POCI-01-0145-FEDER-006984 – Associate Laboratory LSRE-LCM and by national funds through FCT – Fundação para a Ciência e a Tecnologia for project PTDC/AAGTEC/5269/2014 and Centre of Chemistry (UID/QUI/00686/2013 and UID/QUI/0686/2016). Artur Ribeiro thanks FCT for the SFRH\BPD\98388\2013 grant.

Appendix A
Supplementary data

The following are the supplementary data to this article:

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