The Photocatalytical Properties of RGO / TiO 2 Coated Fabrics

: The aim of this work was to immobilize reduced graphene oxide (RGO) and titanium dioxide (TiO 2 ) on the surface of selected ﬁbrous structures. Textile fabrics made of cotton (CO) and polyamide (PA) were used as a carrier. The following modiﬁcation methods were applied: coating for modiﬁcation of PA and dip-coating for modiﬁcation of CO. In the dip-coating method, no auxiliaries were used, which is a huge advantage. The RGO / TiO 2 coated fabrics were characterized using several techniques: ultraviolet–visible (UV–VIS) spectroscopy, scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The obtained results showed the immobilization of RGO and TiO 2 on the fabrics. Raw fabrics absorb much less radiation than coated ones, which is associated with strong absorption of radiation by applied modiﬁers (RGO and TiO 2 ). Photocatalytic activity of functionalized textiles was determined using aqueous phenol solutions. Phenol removal e ﬃ ciency obtained for RGO / TiO 2 coated CO and RGO / TiO 2 coated PA was 51% and 46%, respectively. The hydroxyl radicals play a major role in the phenol photocatalytic degradation. The phenol removal e ﬃ ciency in the ﬁfth cycle was higher (about 14% and 8% for RGO / TiO 2 coated CO and RGO / TiO 2 coated PA, respectively) compared to the ﬁrst cycle. This paper describes research on the immobilization of RGO and TiO 2 on the surface of selected ﬁbrous structures. The following modiﬁcation methods were used: coating and dip-coating. As a result of the conducted experiments, RGO / TiO 2 coated fabrics were obtained. The RGO / TiO 2 coated fabrics were characterized by UV–VIS spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). Photocatalytic activity of functionalized fabrics was determined using aqueous phenol solutions.


Introduction
In recent years, surface modification, including surface modification of various textile structures, has attracted the attention of scientists. Surface modification of textiles usually leads to changes in their properties. Mention should be made of multifunctional fabrics that have electroconductive properties [1][2][3][4], antibacterial properties [5][6][7][8][9], self-cleaning [10], flame retardant properties [11], ultraviolet blocking properties [3,4,9] and also photocatalytic properties [5,6,12]. The development of photocatalytic textiles is mainly associated with their covering with photocatalyst coatings. Due to the immobilization of photocatalysts on a proper carrier, the problem of their removal from the reaction solution after the photocatalytic process can be solved. One of the best known and most studied photocatalysts is titanium dioxide (TiO 2 ) which is characterized by relatively low toxicity, chemical stability and low cost [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27]. Easy and fast recombination of photogenerated electron-hole pairs and limited photoactivity in the visible light range, which leads to lower photocatalytic performance, are the disadvantages of this catalyst. To increase the photocatalytic activity of TiO 2 , doping with good recycling properties-the performance of this fabric has not been reduced after eight cycles of photodegradation of rhodamine B [44].
This paper describes research on the immobilization of RGO and TiO 2 on the surface of selected fibrous structures. The following modification methods were used: coating and dip-coating. As a result of the conducted experiments, RGO/TiO 2 coated fabrics were obtained. The RGO/TiO 2 coated fabrics were characterized by UV-VIS spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR). Photocatalytic activity of functionalized fabrics was determined using aqueous phenol solutions.

Methods
The applied modification methods included coating and dip-coating. Modification of CO was made by dip-coating method using a water dispersion containing GO (5 cm −3 water-based GO solution at the concentration of 10 g·dm −3 ) and TiO 2 (10 g). The dispersion was acidified with hydrochloric acid (0.2 cm −3 HCl solution with concentration of 37%), next mechanically mixed (60 min) and sonicated (60-90 min). The fabric samples were placed in the water dispersion, and then were squeezed (automatic padding machine, Ernst Benz Textilmaschinen, Rumlang-Zurich, Switzerland) at a nip pressure of 30 kg·cm −2 . Three padding cycles were applied. Then the samples were dried (20 min, 60 • C) and heated (3 h, 200 • C) in a KTF-350 S coating-heating machine (Mathis) which caused a thermal reduction of GO. The deposition of the TiO 2 and GO on CO by using the dip-coating method gave satisfactory results (data presented in the results and discussion), therefore the coating method was not applied for CO. As the melting point of polyamide is in the range 200-230 • C [45,46], it makes the application of the above described dip-coating method followed by thermal reduction of GO for PA modification difficult. Therefore, for modification of PA only coating method was applied.
XPS/AES Microlab 350 spectrometer (Thermo Electron, Waltham, MA, USA) was used to analyze the chemical composition of the surface of the obtained materials. Non-monochromated X-ray source (Al K α ) with excitation energy at 1486.6 eV and 300 W of power was used for XPS analysis. For determination of the chemical state of the individual elements and quantitative chemical composition, the high-resolution XPS spectra were recorded in narrow binding energy ranges with a resolution of 0.1 eV. For deconvolution of the recorded XPS spectra, an Avantage software (ver. 5.9911, Thermo Fisher Scientific, Waltham, MA, USA) was used.
SEM-EDX analyses were performed using a scanning electron microscope equipped with a BSE detector (backscattered electrons signal detection) (LEO Electron Microscopy Ltd., Cambridge, UK) FTIR spectra were obtained using Fourier-transform infrared spectrophotometer (Jasco 4200 series) equipped with an ATR attachment (Pike Gladi ATR, Fitchburg, WI, USA). The study was carried out in the range of 400-4000 cm −1 .
UV-VIS spectra were assessed using a double beam Jasco V-550 UV/VIS spectrophotometer (Tokyo, Japan) with integrating sphere attachment in the range of 200-800 nm.
The photocatalytic efficiency of the prepared fabric-based composites was investigated by phenol degradation. The experiments were carried out in a 400 cm −3 quartz photoreactor placed in a merry-go-round device located between UV lamps. Three UV A (11 W, Philips, Eindhoven, The Netherlands) and one UV ABC (125 W, Philips, Eindhoven, The Netherlands) lamps were used as a source of ultraviolet light. A roll of rectangular RGO/TiO 2 coated fabric (dimensions 14 cm × 22 cm) was immersed in a photoreactor filled with an aqueous phenol solution. The content of the photoreactor was irradiated for 6 h and 2 cm −3 samples were taken continuously. The photolysis experiment was carried out in a similar way, but without a coated fabric placed in the photoreactor. The adsorption experiment was also carried out in the above-mentioned equipment, but without irradiation. In case of scavenger test, t-BuOH in concentration of 0.1 mol dm −3 was added to the aqueous phenol solution. After the experiments, the coated fabrics were rinsed with demineralized water and dried at ambient temperature.
The progress of phenol degradation was monitored by liquid chromatographic analysis using a chromatograph (Nexera-I LC-2040C, 3D Plus, LCMS-8045, Shimadzu, Kyoto, Japan) equipped with a Kinetex C18 column (100 mm × 3.0 mm, 1.7 µm) (Phenomenex, Aschaffenburg, Germany). A linear H 2 O/acetonitrile gradient (10% acetonitrile at 1 min to 100% at 4 min) was used during the analysis. The analyses were performed at a constant temperature of 30 • C. The injection volume of the samples was 20 µL. Phenol was detected at 272 nm.

UV-VIS Analysis
At the beginning, the light-absorbance properties of the raw and coated fabrics were analyzed by UV-VIS spectroscopy. The results of this analysis are presented in Figure 1. It can be clearly seen that both coated and raw fabrics absorb UV-VIS radiation. Raw fabrics, however, absorb much less radiation than coated ones, which is associated with strong absorption of radiation by the applied modifiers (RGO and TiO 2 ). In general, TiO 2 absorbs radiation in the UV region, what can be assigned to the intrinsic band gap absorption of this photocatalyst resulting from the electron transitions from the valence band to the conduction band (O2p → Ti3d) [22]. GO absorbs the UV and VIS radiation, which leads to changing of its physicochemical properties and the toxicity [47]. GO shows characteristic absorbance peaks at 230 and 300 nm due to π-π* transitions of C=C and n-π* transitions of C=O, O-C=O and C-OH, respectively [47]. According to the studies performed by Chen et al., the transmittance of the RGO/water nanofluids is quite less than of the GO/water one, what indicate that the change from GO to RGO enhanced optical absorption [48]. UV-VIS diffuse reflectance spectra measurement performed by Landi Jr. et al. [43] showed that the addition of more RGO coatings onto the cotton fabric induce the increase of absorbance in the UV region. Moreover, the above studies confirmed the strong absorption of radiation in the visible region by RGO [43]. Coatings 2020, 10, x 5 of 16

SEM-EDX Analysis
RGO/TiO2 coated fabrics were examined using SEM. This analysis confirmed the immobilization of RGO/TiO2 onto fabrics ( Figure 2). The energy-dispersive X-ray spectroscopy (EDX) mapping showed the aggregation of TiO2 (Figures 3 and 4), which is particularly evident in RGO/TiO2 coated PA ( Figure 4). The aggregation of TiO2 nanoparticles on the RGO coatings was also noticed by Molina et al. during the modification of polyester fabrics [6]. Cai et al. observed the laminar GO layers on the SEM image of the cotton fabric treated with GO [4]. These laminar structures were also seen on the cotton fabrics surface coated with GO after thermal treatment [4]. SEM analysis performed by Zulan et al. showed that the morphology of RGO coated silk fabric was similar to the morphology of GO coated silk fabric, what suggests that the thermal GO reduction did not change the surface conformation [49]. The modification of cotton fabric by graphene and TiO2 performed by Karimi et al. lead to achieving the cotton surface completely coated by graphene/TiO2 nanocomposite layer [7]. The similar observation was made by Landi Jr. et al. [43].

SEM-EDX Analysis
RGO/TiO 2 coated fabrics were examined using SEM. This analysis confirmed the immobilization of RGO/TiO 2 onto fabrics ( Figure 2). The energy-dispersive X-ray spectroscopy (EDX) mapping showed the aggregation of TiO 2 (Figures 3 and 4), which is particularly evident in RGO/TiO 2 coated PA ( Figure 4). The aggregation of TiO 2 nanoparticles on the RGO coatings was also noticed by Molina et al. during the modification of polyester fabrics [6]. Cai et al. observed the laminar GO layers on the SEM image of the cotton fabric treated with GO [4]. These laminar structures were also seen on the cotton fabrics surface coated with GO after thermal treatment [4]. SEM analysis performed by Zulan et al. showed that the morphology of RGO coated silk fabric was similar to the morphology of GO coated silk fabric, what suggests that the thermal GO reduction did not change the surface conformation [49]. The modification of cotton fabric by graphene and TiO 2 performed by Karimi et al. lead to achieving the cotton surface completely coated by graphene/TiO 2 nanocomposite layer [7]. The similar observation was made by Landi Jr. et al. [43].       Whereas the presence of Ti 2p signals was associated with the TiO 2 nanoparticles deposits. Additionally, in case of RGO/TiO 2 coated PA the N 1s peak appears, what is associated with nitrogen functional groups being part of this fabric. In Figures 5b and 6b it can be observed that C 1s, O 1s and Ti 2p spectra can be deconvoluted respectively into few peaks, which correspond to characteristics bonds of analyzed materials like: C-C, C-O, C=O, C-N, C-F, Ti-O (TiO 2 ). The XPS results are summarized in Table 2, which show the most probable origin of the peaks with their binding energies and atomic percentage of each group. Our XPS results are consistent with the literature data [4,6,32]. The reduction of GO should be indicated by the decrease in oxygen to carbon (O/C) ratio of XPS spectra. Gao [48]. After irradiation of GO by UV light, they observed the decrease in area percentage of C=O group and the increase in area percentage of the C-C group [48]. The calculated O/C ratio for RGO/TiO 2 coated CO is higher (0.69) than O/C ratio for RGO/TiO 2 coated PA (0.44) which may suggest that the RGO/TiO 2 coated CO contains more oxygen functional groups than RGO/TiO 2 coated PA. spectra. Gao et al. obtained the O/C ratio equal to 0.4 and 0.59 for GO irradiated under UV and Vis light, respectively [47]. Chen et al. performed the reduction of GO under UV irradiation [48]. After irradiation of GO by UV light, they observed the decrease in area percentage of C=O group and the increase in area percentage of the C-C group [48]. The calculated O/C ratio for RGO/TiO2 coated CO is higher (0.69) than O/C ratio for RGO/TiO2 coated PA (0.44) which may suggest that the RGO/TiO2 coated CO contains more oxygen functional groups than RGO/TiO2 coated PA.        Figure 7 presents FTIR spectra of modifiers as well as raw and coated fabrics. The FTIR spectra of raw and coated fabrics displayed a broad absorption band of around 3280 cm −1 , which is attributed to the O-H stretching vibration of C-OH groups [4]. The presence of peaks at 1640 and at 1514 cm −1 is related to C=O stretching of COOH groups and C=C groups, respectively [4,43]. The broad absorption band in the range of 100-400 cm −1 is attributed to Ti-O and Ti-O-Ti vibrations [42,50]. In addition, for cotton fabrics there is a peak of around 1057 cm −1 corresponding to C-O stretching [4]. For coated fabrics, a decrease in the intensity of characteristic peaks can be observed compared to the raw fabrics. Cai et al. did not observe any significant changes in the FTIR spectrum of GO coated CO fabric. However, in the case of CO fabric coated with RGO, in the FTIR spectrum the peak related to C=O stretching was absent and the peaks associated with O-H stretching and asymmetrical C-H stretching became relatively weak [4].

RGO/TiO2 Coated CO
For coated fabrics, a decrease in the intensity of characteristic peaks can be observed compared to the raw fabrics. Cai et al. did not observe any significant changes in the FTIR spectrum of GO coated CO fabric. However, in the case of CO fabric coated with RGO, in the FTIR spectrum the peak related to C=O stretching was absent and the peaks associated with O-H stretching and asymmetrical C-H stretching became relatively weak [4].

Photocatalytic Activity
The photocatalytic activity of RGO/TiO 2 coated fabric was evaluated for the photodegradation of phenol under UV light irradiation. Figure 8a shows the results of phenol degradation using various processes: photocatalytic degradation as well as adsorption and photolysis. In the case of RGO/TiO 2 coated CO, after 6 h of degradation process, the removal efficiency reached 4% and 51% for adsorption and photocatalysis, respectively. The application of RGO/TiO 2 coated PA leads to the phenol removal efficiency of 8% and 46% for adsorption and photocatalysis, respectively. The photolysis efficiency after the same time was 30% which indicates that this process plays an important role in phenol degradation during photocatalytic degradation using RGO/TiO 2 coated fabrics, while the adsorption process has a small effect. The studies of photocatalytic degradation of phenol under visible light irradiation and using AgBr/BiOBr/graphene as a photocatalyst showed that photolysis had no effect on phenol removal, and the adsorption process remarkably influenced degradation of this compound. In these studies, simultaneous adsorption and photocatalysis led to complete mineralization of phenol within 6 h [51].
In order to better understand the mechanism of the phenol photodegradation, the scavenger tests were performed. In these studies, the well-known hydroxyl radical scavenger-t-BuOH [52] was applied. The obtained results (Figure 8a) suggested that hydroxyl radicals (OH • ) play a major role in the phenol photocatalytic degradation. The phenol removal efficiencies in the presence of t-BuOH (about 25% and 29%, respectively for CO and PA) were slightly lower than efficiency of the photolysis process, which indicates that radicals take part in the phenol photolysis process. According to the literature, absorption of UV radiation by phenol leads to its excitation [53]. The excited state of phenol can undergo relaxation to the ground state or transformation to the phenoxyl radicals and solvated electrons. Then, the phenoxyl radicals can react with oxygen leading to the formation of benzoquinone. While solvated electrons can undergo a few reactions leading to the hydroxyl radicals formation, which can then react with phenol [53]. The disadvantage of this work is low phenol removal efficiency; therefore, our future work will be focused on the improvement and optimization of the photodegradation efficiency. The removal efficiency can be improved by the addition of a higher amount of TiO2 to the dispersion or paste used in the dip-coating and coating process, respectively, which may increase the absorption of the UV light by this photocatalyst. According to the Chun et al. studies, an appropriate catalyst dosage can avoid direct photolysis of phenol and possibly other compounds, as well as increase the mineralization rate of phenol [54]. On the other hand, the amount of RGO can also influence the photodegradation. The experiments performed by Lin et al. indicated that the photocatalytic activity increased with increasing concentration of RGO in composites from 0% to 2.7%, but the degradation was inhibited when the RGO concentration was larger than 2.7% [24]. The increasing of RGO amount in the composite may be achieved by application of another GO reduction method after dip-coating process. There are several methods of GO reduction: thermal reduction applied in this and other works [4,49,55], hydrothermal [32,56], UV photoreduction [28,48,57], chemical reduction [1,6,7,43], electrochemical reduction [58,59]. Moreover, the application of GO prepared by Brodie's method instead of the GO prepared by Hummers' method can increase the photodegradation efficiency. According to the literature, GO prepared by Brodie's method has a much higher photocatalytic activity compared to GO prepared by Hummers' method [29].

Conclusions
Textile fabrics made of cotton and polyamide have been modified by RGO and TiO2. The following two methods were used for modification: coating for modification of PA and dip-coating for modification of CO. In the dip-coating method, no auxiliaries were used, which is a huge advantage. The physicochemical studies (UV-VIS, SEM, XPS, FTIR) confirmed immobilization of RGO/TiO2 on fibrous structures. The raw fabrics absorb much less radiation than coated ones, which is associated with strong absorption of radiation by the applied modifiers (RGO and TiO2). The photocatalytic activity of functionalized textiles using aqueous solutions of phenol was determined. The 51% and 46% efficiency of phenol removal was obtained for RGO/TiO2 coated CO and RGO/TiO2 coated PA, respectively. The hydroxyl radicals play a major role in the phenol photocatalytic degradation. Phenol removal efficiency in the fifth cycle was higher (about 14 and 8% for RGO/TiO2 coated CO and RGO/TiO2 coated PA, respectively) compared to the first cycle. In summary, very promising results have been obtained, but more research is needed, particularly related to  Figure 8b shows the results of re-use tests of coated fabrics for photocatalytic degradation of phenol. After each photocatalytic experiment the coated fabric was washed with deionized water and dried at ambient temperature before re-use. As can be seen, the phenol removal efficiency in the fifth cycle was higher (about 14% and 8% for RGO/TiO 2 coated CO and RGO/TiO 2 coated PA, respectively) compared to the first cycle. On the surface of coated fabrics probably a small amount of GO is present (especially in the case of RGO/TiO 2 coated CO fabric) which has not been reduced during the thermal reduction process. The performed XPS analysis suggests that RGO/TiO 2 coated CO contains more oxygen functional groups than RGO/TiO 2 coated PA (the calculated O/C atomic ratio for RGO/TiO 2 coated CO is higher than O/C ratio for RGO/TiO 2 coated PA). According to the literature, GO can undergo the photoreduction process under UV radiation [28]. Therefore, the amount of RGO on coated fabrics may change, which can lead to changes in phenol removal efficiency. This phenomenon can be associated with the photoreduction of GO under UV irradiation, but this requires further experimentation studies.
The disadvantage of this work is low phenol removal efficiency; therefore, our future work will be focused on the improvement and optimization of the photodegradation efficiency. The removal efficiency can be improved by the addition of a higher amount of TiO 2 to the dispersion or paste used in the dip-coating and coating process, respectively, which may increase the absorption of the UV light by this photocatalyst. According to the Chun et al. studies, an appropriate catalyst dosage can avoid direct photolysis of phenol and possibly other compounds, as well as increase the mineralization rate of phenol [54]. On the other hand, the amount of RGO can also influence the photodegradation. The experiments performed by Lin et al. indicated that the photocatalytic activity increased with increasing concentration of RGO in composites from 0% to 2.7%, but the degradation was inhibited when the RGO concentration was larger than 2.7% [24]. The increasing of RGO amount in the composite may be achieved by application of another GO reduction method after dip-coating process. There are several methods of GO reduction: thermal reduction applied in this and other works [4,49,55], hydrothermal [32,56], UV photoreduction [28,48,57], chemical reduction [1,6,7,43], electrochemical reduction [58,59]. Moreover, the application of GO prepared by Brodie's method instead of the GO prepared by Hummers' method can increase the photodegradation efficiency. According to the literature, GO prepared by Brodie's method has a much higher photocatalytic activity compared to GO prepared by Hummers' method [29].

Conclusions
Textile fabrics made of cotton and polyamide have been modified by RGO and TiO 2 . The following two methods were used for modification: coating for modification of PA and dip-coating for modification of CO. In the dip-coating method, no auxiliaries were used, which is a huge advantage. The physicochemical studies (UV-VIS, SEM, XPS, FTIR) confirmed immobilization of RGO/TiO 2 on fibrous structures. The raw fabrics absorb much less radiation than coated ones, which is associated with strong absorption of radiation by the applied modifiers (RGO and TiO 2 ). The photocatalytic activity of functionalized textiles using aqueous solutions of phenol was determined. The 51% and 46% efficiency of phenol removal was obtained for RGO/TiO 2 coated CO and RGO/TiO 2 coated PA, respectively. The hydroxyl radicals play a major role in the phenol photocatalytic degradation. Phenol removal efficiency in the fifth cycle was higher (about 14 and 8% for RGO/TiO 2 coated CO and RGO/TiO 2 coated PA, respectively) compared to the first cycle. In summary, very promising results have been obtained, but more research is needed, particularly related to improvement of the photodegradation efficiency.