Dual DBD plasma treatments for synthesis of Ag-TiO2 functionalized

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Applied Chemistry

Dual DBD plasma treatments for synthesis of Ag-TiO2 functionalized polypropylene fabrics Peimei Dong, Xiaoxiao Nie, Zhi Jin, Zhengfeng Huang, Xiya Wang, and Xiwen Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00047 • Publication Date (Web): 28 Apr 2019 Downloaded from http://pubs.acs.org on April 29, 2019

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Industrial & Engineering Chemistry Research

Dual DBD plasma treatments for synthesis of Ag-TiO2 functionalized polypropylene fabrics

Peimei Dong1, Xiaoxiao Nie2, Zhi Jin3, Zhengfeng Huang4, Xiya Wang5, Xiwen Zhang*

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, 7th West Chemistry Building, Xixi Campus, Zhejiang University, Xihu District, Hangzhou City, 310027, Zhejiang Province, China

*[email protected]; Telephone: + 86 13606513237.

[email protected]; [email protected]; [email protected]; [email protected]; [email protected]

Abstract The immobilization of Ag-TiO2 nanoparticles (NPs) on polymer fabrics creates many additional functions for conventional fabrics such as water purification, self-cleaning, UV blocking and antibacterial properties and has been of increasing interest to researchers. However, it is difficult to load functional NPs on some fabrics with low surface energy; therefore, an environmentally friendly and effective technology is urgently needed to address this problem. In this work, dual DBD plasma treatment was

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applied to synthesis of Ag-TiO2 functionalized polypropylene (PP) fabrics for the first time. Compared with other methods of loading Ag NPs on the fiber cloth substrates such as chemical reduction, UV reduction and melt mixing, the technique utilized in this work has the characteristics of being environmentally friendly, of high efficiency, firm loading and economical. Air DBD plasma treatment was applied to introduce polar groups onto PP fiber surfaces for the subsequent hydrothermal reaction to load TiO2 NPs on the PP fabric surface. Ag NPs with the desired particle size and homogeneously distribution were then in situ reduced by H2 DBD plasma treatment. The corresponding experimental mechanisms are also discussed. Keywords: PP fabric; TiO2; DBD plasma treatment; pollutant degradation

1. Introduction The immobilization of nanoparticles (NPs) such as TiO2, Ag or Cu NPs on polymer fabrics such as polypropylene (PP), polyester (PET), cellulose, polyamide (PA) and polyacrylonitrile (PAN) brings new properties to the final fabric products and has been of increasing interest to both the academic and the industrial sectors.1-3 Ag or TiO2 NPs finished fabrics are widely studied because of their broad and diverse applications, including wastewater treatment, self-cleaning, UV blocking and antibacterial applications.4 Soft fabrics count as good hosts for the semiconductors to improve their efficiency and to employ their benefits in daily life.5 There is a wide variety of synthetic methodologies available to obtain Ag or Ag/TiO2 NPs finished fabrics using a combination of one or more methods, such as chemical reduction,6-8 UV reduction,9 melt

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mixing,10-12 the pad-dry-cure method,13 sonoprocessing1,14 and grafting pretreatment to create an easy adsorption environment on the surface of fabrics and facilitate postprocessing by UV light reduction15 or dip deposition.16-18 Nevertheless, most of the reported immobilization methods rely heavily on the use of cross-linking agents, reductant or stabilizers such as dimethylaminoborane (DMAB), aldehydes, NaBH4 or butane tetra carboxylic acid (BTCA), which are generally expensive, toxic or environmentally and biologically hazardous. In addition, most of the current methods for stabilizing of inorganic nanostructured materials on the textile surfaces require several steps including preparation, functionalization, final treatment, drying and curing.19 Some methods are time-consuming or of high cost for high-scale manufacturing production. Although there are some green preparation methods reported,20-22 more economical, efficient and environmentally friendly methods are demanded. Normally, there is no attraction between inorganic particles and polymeric materials such as textiles.19 Some polymer fabrics such as PP and PET are innately hydrophobic, low surface energy materials, and are not attractive to Ag or TiO2 NPs. The difference between surface energies of the two aforementioned organic and inorganic materials causes a kind of repellency in their interfaces.19 The polymer fibers presents a nonpolar characteristic which hinders their association in aqueous media.23 It is therefore often necessary to modify their surfaces to increase the surface energy without changing in their bulk properties before loading Ag or TiO2 NPs.24 Different methods such as UV irradiation, plasma treatment, gamma irradiation, and chemical

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reactions have been employed to modify the fabric surfaces.25-27 Plasma processing is another promising materials modification method developed in recent years, especially noted for its revolutionary effects in nanoscale science and engineering technology.28 Plasma treatment is now a widely used industrial technique for modifying polymer surfaces.29 Recently, ambient dielectric barrier discharge (DBD) plasma has attracted considerable attention because it is cost effective, environment friendly, can be easily implemented in a continuous production line and can offer a variety of highly reactive chemical species at atmospheric pressure contributing to the surface processing.23,30 Plasma is an ionized gas containing a mixture of ions, electrons, neutral and excited molecules and photons. Its content and effect on the material surface depend on the composition of the gas in the discharge, the composition of the sample treated and all the process parameters.31 It is reported that PP fabric that has undergone ambient DBD plasma treatment has polar groups generated on the PP fabric surface such as hydroxide radicals, carboxyl and acylamino groups that greatly improve its wettability and surface energy. This makes it possible for some catalysts that would otherwise not have been able to be firmly supported on the fiber surface. Hence, we supposed that the enhanced wettability and the polar functional groups of the plasma-treated PP fabrics are beneficial for attracting titanium ions and consequently promote the in situ nucleation and firm loading of the TiO2 particles on the fabric surface by the following hydrothermal method. It is reported that H2 plasma has an excellent ability for reducing metal ions due to the strong reducing nature of the H radicals or atoms that are produced in the

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plasma.28,32,33 In our previous studies, the H2 DBD cold plasma process, a green method with no environmentally hazardous reducing agents, was developed to in situ reduce silver NPs with average size 5.8 nm and homogeneously distribution on TiO2-FTO substrate28 and average size 3.7 nm on P25 powder.34 This technique is expected to be applied on the fabric substrate to in situ reduce Ag NPs based on green manufacturing considerations and it has not previously been studied by researchers before. Therefore, in this work, we have prepared the Ag-TiO2 functionalized PP fabrics via dual ambient DBD plasma surface modification techniques and the hydrothermal method for the application of photocatalytic degradation of dyes in wastewater. The purpose of the first plasma treatment was to introduce polar groups onto the PP fiber surface for the subsequent hydrothermal reaction to load TiO2 on the PP fabric surface. The second plasma treatment was to in situ reduce silver NPs with desirable sizes and homogeneous distribution on the PP substrate by H2 plasma. We expect the as-prepared functionalized fabrics to perform as an efficient, low cost and eco-friendly treatment technology in the wastewater industry. This strategy can be extended to other fabrics such as PET and polyamide (PA). 2. Experimental 2.1 Materials Commercially available polypropylene (PP) fabric (the average fiber diameter of 20 μm, area 5 cm * 15 cm) was used for the present experiments. The fabric was ultrasonically cleaning with ethanol solution and finally washed repeatedly with distilled water to remove the impurities and dried in air before use. Tetrabutyl titanate

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(Ti(OBu)4, CP), hydrochloric acid (HCl, AR, 36 ~ 38% by weight), ethanol (AR), acetone (AR), AgNO3 (AR) and ammonium hydroxide solution (AR) were supplied by Sinopharm Chemical Reagent Co., Ltd.. All of the chemical reagents were used without further purification.

Figure 1. Schematic of the synthesis of PP-DBD-Ag-TiO2.

2.2 Surface modification of PP by ambient air DBD plasma The whole experimental process includes the surface pretreatment of PP by ambient air DBD plasma, hydrothermal treatment and precipitation reaction followed by reduction of Ag NPs by H2/Ar DBD plasma. A schematic diagram of the preparation procedure is shown in Figure 1. The surface pretreatment was carried out by a homemade dielectric barrier discharge system, as shown in part (a) of Figure 1. The 6


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humidity of the laboratory air ranged between 30% and 60% as measured before each experiment. The fabric was attached to an electro-mechanical system which can slide back-and forth in the horizontal plane at a controlled speed. The pulsed voltage applied between A (ceramic electrode) and B (glass plate) created a macroscopically homogeneous, diffuse, thin (3 mm) layer of high power-density plasma, with the input power of 37.5W. The fabric passes through this plasma strip via the back and forth movement. After 3 minutes of ambient air DBD plasma treatment, the fabrics obtained were labeled as PP-DBD and placed into stainless steel autoclaves. 2.3 Hydrothermal treatment The hydrothermal treatment has been introduced in our previous article.35 Briefly, 0.04 M HCl was added in deionized water to reach a total volume of 40 mL. A 4 mL aliquot of tetrabutyl titanate (TBT) was then added dropwise into the prepared solution and stirred vigorously until clear. The obtained solutions were poured into the autoclaves and the fabrics were completely immersed in the solutions. Subsequently, the autoclaves were sealed and maintained at 90 ˚C for 9 h. After synthesis, the autoclaves were cooled to room temperature naturally and the resulting PP-TiO2 samples were rinsed repeatedly with deionized water to remove the unattached TiO2 particles and finally dried in air. The as-prepared samples were denoted as samples PPDBD-TiO2. For comparative studies, pure PP fabric was also treated by hydrothermal process without the surface pretreatment by ambient air DBD plasma, and the other experimental conditions were kept unchanged (designated as samples PP-TiO2). 2.4 Precipitation reaction and reduction of Ag NPs by H2/Ar DBD plasma

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The obtained sample of PP-DBD-TiO2 was immersed into AgNO3 solution followed by 0.1M ammonium hydroxide solution added dropwise into the solution with magnetic stirring.28 The samples changed from white to dark due to the deposition of AgOH particles, refer to part (c) of Figure 1. The obtained samples were subsequently dried at 60 ℃ for 30 min, followed by a second DBD plasma treatment in a homemade reactor to reduce the Ag NPs (Figure 1 (d)). Before the DBD plasma treatment, the working gas (a mixture of Ar and H2 gases with the volume ratio of 1:1) was introduced into the reactor at a flow rate of 50 mL/min for 1 min to purge the reactor. The duration of the plasma treatment was limited to 5 s with the flow rate controlled at 30 mL/min. The input power was 22.5W. The final sample obtained was denoted as PP-DBD-AgTiO2. 2.5 Characterization of the catalysts Field emission scanning electron microscopy (FESEM, Hitachi SU-70) was used to characterize the microstructures of the PP fibers and TiO2 particles. Energy dispersive spectroscopy (EDS) was performed on the SEM. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical changes arising out of the plasma treatment, using a VG ESCALAB MARK II spectrometer with an Mg Kα (1253.6 eV) X-ray source and the collected spectral data were calibrated by the C 1s peak at 284.6 eV. The functional groups on the surface of the samples were examined using an ATR-FTIR spectrometer (VERTEX 70, Bruker, Germany). Hydrophilicity of these polymer film surfaces was studied by contact angle measurements using a video-based, contact angle measuring device (OCA 20, Dataphysics, Germany). De-ionized water was used as the

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probe liquid in all measurements. Each reported average contact angle value is the arithmetic mean value of 10 angle measurements collected from 10 different locations on the sample surface. Thermogravimetric analysis (TGA) was conducted on an SDT Q600 V20.9 Build 20 instrument was used to measure the TiO2 contents of the samples. Pieces of 10.000~12.000 mg were cut from the samples for full calcination, and the final residual mass defaults to the quantity of residual titanium dioxide. The measurements were conducted from 30 °C to 800 °C under an air environment with a temperature ramp rate of 10 °C/min. 2.4 Photocatalytic activity measurements Photocatalytic degradation of adsorbed methylene blue (MB) was carried out in a quartz beaker of 150 mL MB solution (5 mg L-1) with a cooling water system and magnetic stirring at room temperature. The device is an open system in which the MB solution is continuously stirred to capture oxygen from the air. A Xe-lamp (PLSSXE300/300UV, Beijing Perfectlight Technology Co. Ltd) was used as the simulated sunlight source. The distance between the Xe-lamp and the sample was set to 30 cm. Optical power was measured using a UV light meter (PA05-00UV51X-01, Century Optronics Co. Ltd) utilizing about 25-30 mW cm−2 in the light wave range of 280-420 nm. At first, the as-prepared samples of PP-TiO2, PP-DBD-TiO2 and PP-DBD-Ag-TiO2 were immersed into the MB solution and set in the dark for 90 min to reach the absorption stripping balance. The reactor was then exposed to the simulated sunlight illumination, and the UV-vis spectrum was measured by UV-Visible spectrometer (UV2100, Jinghua Technology Instrument Co. Ltd., China) at regular intervals to determine

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the concentration change of the MB solution. Every 30 minutes, 3 mL of the MB solution was transferred to a quartz cuvette with a pipetting gun and the absorbance was tested by UV-Visible spectrometer. The characteristic peak of MB was observed at 664 nm. As a comparison, the DBD-PP sample was used as a blank control. In order to obtain insight into adsorption rate of MB on the samples, the described procedure was conducted in the dark, without illumination, but the beakers with MB solution were vigorously stirred. The adsorption rate of the PP-DBD sample to MB within 4 hours was 3.3% and we think this result is acceptable. The percentage of MB removal was calculated according to following expression: MB removal, (%) =

𝐶0 ― 𝐶 𝐶0

（1）

× 100%

where 𝐶0 is the initial concentration of MB solution and 𝐶 is the concentration of MB solution at the time of sampling. 3. Results and discussion

Figure 2. (a) Curve-fitted C 1 s peaks of (a) pristine PP and (b) PP treated by ambient air DBD plasma.

XPS was used to investigate the chemical composition of the PP fabric surfaces

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before and after plasma treatment. The decomposition of the C1s peak can be used to investigate the principal functional groups introduced onto the PP surface caused by air DBD plasma treatment and accordingly, high-resolution XPS analysis of C1s peak was performed. As shown in Figure 2(a), the spectrum of untreated PP fabric indicate the presence of two peaks with binding energy of 285.7 eV for C1 (C―H or C―C bonds) and 286.5 eV for C2 (C–O). The concentration of C2 as estimated from survey spectra is 4.1%. However, the spectrum of plasma-treated PP fabric mainly contains three asymmetrical three peaks at 286.5 eV, 288.0 eV and 289 eV which may be attributed to C2 (C―O or C―N bonds), C3 (C=O or O–C–O) and C4 (O―C=O) respectively.30,36 The concentration of elements estimated from the survey spectra are: 77.1%, 13.8%, 5.3% and 3.8% for C1, C2, C3 and C4, respectively. There are very significant trends in the measured atomic percentages, which are consistent with the expected compositional changes due to oxidation and removal of carbon contamination.

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Figure 3. ATR-FTIR spectra of untreated and ambient air DBD-treated PP fabrics.

The results from the XPS analysis are further corroborated by ATR-FTIR analysis. The functional groups on the surfaces of the samples were examined using a FTIR-ATR spectrometer. As shown in Figure 3, the pristine and plasma-treated PP fabrics both exhibit some peaks at 840, 970, 996, 1167, 1254, 1304, 1376, 1457, 2725 and 2900 cm−1. In contrast to the untreated PP fabric, the DBD-treated PP fabric showed an extra peak appearing at around 1720 cm−1 (Figure 3(b)). Combined with the XPS analysis results, this peak can be attributed to C=O stretching.36-39 It is reported that the plasma treatments at atmospheric pressure affect only the outermost layer of the surface such

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that is not an easy task to observe surface chemical modification by FTIR analysis.36 Since PP fabric is treated in air plasma, some nitrogen-containing groups may be incorporated onto the surface, but it is difficult to find them in the corresponding peaks of the spectra. The broad band which appears between 3100 and 3600 cm-1 which is due to hydroxyl groups.38,40 The absence of carboxyl peaks may be due to their extreme low content as mentioned in the XPS analysis. This result indicates the introduction of functional groups, such as hydroxyl and carbonyl-containing groups, on the PP membrane surface. However, we think that these polar and soluble substances on the surface of the fibers are low in content and tend to disappear when exposed to air for a certain period of time. The impartment of polar functionalities on the fabric surfaces correlated as the surface free energy change and their wettability improvement, as shown in Figure 4.37

Figure 4. Water contact angle of PP fabrics (a) before and (b) after ambient air DBD plasma treatment, (c) average contact angle and standard deviation of pure PP fabric and PP-DBD.

In consideration of the stability and hydrophobic recovery of the modified PP fabric surface, the static contact angle test was conducted after 5 hours of plasma

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treatment. The contact angle in Figure 4(a) illustrates that the pure PP fabric is hydrophobic while Figure 4(b) indicates that PP-DBD is hydrophilic. As can be seen from Figure 4(c), after 3 minutes of ambient air DBD plasma treatment, the average water contact angle of the PP fabric abruptly dropped from 138.15° to 25.51°, better than that observed in other reports, indicating that the hydrophilicity of PP fabric has been greatly improved (the standard deviation was 4.8 and 6.2 for pure PP fabric and PP-DBD, respectively). It is worth mentioning that the hydrophilicity of PP-DBD is uneven, and that some parts of the surface are super-hydrophilic, as shown in video S1. The decrease in contact angle indicates the formation of hydrophilic groups on the plasma treated PP fabric as has been proved by the XPS and ATR-FTIR analyses. This was undoubtedly beneficial to the following in situ growth of titanium dioxide on the PP fabric surface. We supposed that the enhanced wettability and the polar functional groups of the plasma-treated PP fabrics are beneficial for attracting titanium ions, possibly even forming valence bonds and consequently promoting the in situ nucleation and firm loading of the TiO2 particles on the fabric surface during the hydrothermal process, as can be further confirmed by the following tests.

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Figure 5. FESEM images of (a) pristine PP fabric, (b) PP fabric treated by ambient air DBD plasma, (c) and (d) PP-TiO2 and (e) and (f) PP-DBD-TiO2.

Figure 5(a) shows the surface of the pristine PP fabric which is very smooth, while the DBD-treated PP fabric suffered morphological alterations on the surface, with the formation of unequal-sized “drop-like” or the nodules structure, which turn out to be related to treatment dose and time, as seen in Figure 5(b).41 Plasma treatment affects the polymer surface to the depths from several hundred to several thousand angstroms but the bulk properties of the polymers remain unaltered.42 It is reported that the nodules on intrinsically hydrophobic polymer surfaces can be explained by the formation of polar 15



and soluble low molecular weight oxidized material (LMWOM), which agglomerates into spherically shaped structures.36 Although the LMWOM can dissolve in water, the experimental results show that titanium dioxide can still be loaded on the fabric surface. Introduction of polar groups on the surface of PP fibers would enhance the binding of hydrophilic TiO2 NPs, as shown in Figures 5(c) and (d) and Figures 5(e) and (f).6,24 Figures 5(c) and (d) showed the sample PP-TiO2 which did not undergo the surface pretreatment by ambient air DBD plasma, and it is clear that the number of TiO2 particles on this surface was very limited compared to that of sample PP-DBD-TiO2 which underwent the plasma pretreatment, and were probably adsorbed by physical absorption.

Figure 6. TGA results of samples PP-TiO2 and PP-DBD-TiO2.

As can be shown from the TGA results in Figure 6, the specific contents of TiO2 were only 1.13 wt% for sample PP-TiO2 and 1.72 wt% for sample PP-DBD-TiO2. The enlarged versions of the selected areas in Figure 5(f) show a relatively denser titania thin film than that of Figure 5(d). The TiO2 nanocrystals were attracted by the polar functional groups of the plasma-treated PP fabrics and started to agglomerate on the PP 16


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fabrics, forming a thin film layer. Notably, the micron-sized TiO2 particles in both samples PP-TiO2 and PP-DBD-TiO2 (see Figure 5(c) and (d) and Figure 5(e) and (f)) were easy to dislodge from the fabric surface during the second DBD plasma treatment, as will be noted again in the following analysis. As mentioned in the experimental part, the obtained sample PP-DBD-TiO2 was subjected to a precipitation reaction to deposit AgOH NPs which were unstable and easily oxidized into Ag2O NPs. The H2 DBD plasma treatment was then adopted to in situ reduce the Ag NPS. The whole process can be described by the following reactions: AgNO3 + NH3H2O → AgOH + NH4NO3

(1)

2AgOH→Ag2O + H2O

(2)

e* + Ar → Ar* + e

(3)

e* + H2 → H2* + e

(4)

2Ag+ + H2* + 2e → 2Ag + H2

(5)

Figure 7. (a) SEM images and corresponding EDX mapping diagrams of sample PP-DBD- Ag-TiO2, (b) C element, (c) O element (d) Ti element and (e) Ag element with scale bar of 40 μm. 17



In Figures 7(d) and (e), the SEM images and corresponding EDX mapping diagrams of sample PP-DBD-Ag-TiO2 revealed that the Ti and Ag elements were both evenly distributed on the fiber surface, indicating the uniform distribution of titania and metallic silver NPs. Compared to Figures 5(e) and (f), it can also be concluded that the number of TiO2 particles of sample PP-DBD-Ag-TiO2 was much fewer than that of sample PP-DBD-TiO2. The loose micron-sized TiO2 particles on the fabric surface were knocked off by the plasma during the reduction of the Ag NPs, leaving a dense layer of titanium dioxide NPs. In fact, the recombination rate of the photon generated carriers in the micron-sized TiO2 particles was relatively high, so that the peeling was insignificant to the final utilization. It is worth mentioning that, in the process of reducing silver, it is necessary to accurately control the discharge intensity and time so as not to seriously damage the dense titania film that has been loaded.

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Figure 8. (a) HRTEM image of TiO2 particles collected from the sample of PPDBD-TiO2 after ultrasonic oscillations processing of 1 h, (b) TEM image of Ag NPs 19



collected from the sample of PP-DBD-Ag-TiO2 after 10 min of ultrasonic processing and (c) HRRTEM image of one of the Ag NPs in (b). The illustration is a statistical picture of silver NPs size distribution for (b).

In order to further investigate the morphologies and sizes of the titanium dioxide and silver particles, TEM and HRTEM tests were carried out. To facilitate the collecting of statistics to distinguish the titanium dioxide and silver particles, titanium dioxide particles were collected from the sample PP-DBD-TiO2 after ultrasonic oscillation processing of 1 h, while the silver particles were collected from the sample PP-DBDAg/TiO2 after 10 min of ultrasonic processing. The results are shown in Figure 8. It can be seen that most of the nanosized TiO2 particles shown in Figure 8(a) were anatase phase with lattice spacing of 3.52 Å, which is consistent with our previous report.35 As shown in Figure 8(b), after 5 s of H2/Ar DBD plasma treatment, Ag NPs with diameter ranging from 4 nm to 11 nm were obtained. The TRTEM image of Figure 8(c) shows one of the Ag NPs of Figure 8(b) with the lattice fringes of 2.35 Å that were wellindexed to the (111) plane of Ag. The illustration in Figure 8(c) indicates that most silver particles were 7 - 7.99 nm in size, with the average size of Ag NPs of 7 nm. The above results show that the procedure employed in the present work which took only 5 s reduced Ag+ to Ag0. The results also indicate that this technique is a very promising and straightforward way to prepare metal NPs on the fabric substrate surfaces.

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Figure 9. High-resolution XPS spectra of (a) Ti 2p and (b) Ag 3d of sample PPDBD-Ag-TiO2.

In order to further verify the existence of TiO2 and metallic Ag, high-resolution XPS spectral measurements of Ti 2p and Ag 3d were performed on sample PP-DBDAg-TiO2. Peaks centered at 458.5 eV and 464.2 eV were observed as shown in Figure 9(a), which conform to the Ti 2p3/2 and Ti 2p1/2 binding energies respectively with the spin separation of 5.7 eV, indicating that the Ti atoms are stable with the H2 cold plasma treatment. As shown in Figure 9(b), the Ag 3d spectrum is fitted to two symmetrical peaks at 367.9 eV and 373.9 eV which can be attributed to the Ag 3d3/2 and Ag 3d5/2 binding energies respectively with spin separation of 6.0 eV. These results indicate that the silver was completely reduced by the DBD plasma treatment and Ag exists in its metallic state on the PP fabric surface.

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Figure 10. (a) Concentration of MB solution as a function of the UV absorbance intensity; (b) Photocatalytic degradation of MB under simulate sunlight illumination.

We have made a calibration curve of concentration of MB solution as a function of the UV absorbance intensity, as shown in Figure 10(a). The curve is linear at low MB concentrations (below 7 mg/L) and conforms to the formula y = 0.146x in this region. When the concentration of MB solution is higher than 7 mg/L, it cannot allow an accurate calculation of the degraded dye cannot be determined using a UV-Vis spectrometer. Considering that the adsorption of MB by fabrics at low concentration will cause a large error on the degradation results, the concentration of the MB solution was set at 5 mg/L. Figure 10 shows the comparison of photodegradation activities under simulated sunlight illumination. MB photolysis without any photocatalysts (PP-DBD fabric) is used as the reference. The adsorption rate of the PP-DBD sample to MB within 4 hours was 3.3% and we think this result is acceptable. As shown in Figure 10(b), samples of PP-TiO2, PP-DBD-TiO2 and PP-DBD-Ag-TiO2 degraded about 53%, 73% and 99% of the MB molecules respectively under simulated sunlight irradiation after 4 22


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hours. The fiber substrate helps adsorb dye molecules and an appropriate photocatalyst immobilization strategy can avoid the leakage and depletion of the catalyst during the photocatalytic process and provide a cost-effective solid–liquid separation, as well as minimizing the scale of the reactor.43 The sample of PP-DBD-Ag-TiO2 exhibited the highest activity for MB degradation as expected. TiO2 can be excited under UV light irradiation and the Ag NPs deposited on the TiO2 surface can be excited under visible light irradiation due to the LSPR effect, both creating electron-hole pairs.44,45 The excitation electrons created by Ag NPs then separate from the holes, and migrate into the conduction band (CB) of TiO2 due to the Schottky barrier between the Ag NPs and TiO2.46,47 The electron-hole pairs can then undergo further reactions with dissolved oxygen and water to form reactive radical species such as ·O2- and ·OH that can nonselectively oxidize and mineralize organic water pollutants.48-51 The Ag NPs play the critical role of electron traps, promoting interfacial charge transfer, hence restraining recombination of the electron–hole pairs. As a result, the improvements of 86.8% and 35.6% of MB degradation efficiency were observed with the PP-DBD-Ag-TiO2 compared to PP-TiO2 and PP-DBD-TiO2, respectively. 4. Conclusions In this work, dual DBD plasma treatment was for the first time applied to the synthesis of Ag-TiO2 functionalized PP fabrics with application to the degradation of water pollutants. The relationships between material surface processing and the corresponding material composition and properties have been explored. The air DBD plasma surface modification introduced polar functionalities such as hydroxyl and

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carbonyl-containing groups on the fabrics surface and greatly improved its wettability by decreasing contact angles from 130° to less than 30°, resulting in the increasing the amount of titanium dioxide loading from 1.13 wt% to 1.72 wt%. Ag NPs with average particle size of 7 nm and homogeneous distribution were completely reduced by the H2 plasma. The synergistic effect of TiO2 and Ag NPs of PP-DBD-Ag-TiO2 improved the MB degradation efficiency by 86.8% and 35.6% after 4 hours compared to PP-TiO2 and PP-DBD-TiO2, respectively. This work also provides a technical reference for the green, effective and economical synthesis of functional NPs finished fabrics which are normally difficult to prepare, such as PET and polyamide (PA). 5. Conflict of interest The authors declare no competing financial interest. Acknowledgements This work was supported by the National Natural Science Foundation of China [grant number 50772098]. References (1) Perelshtein, I.; Applerot, G.; Perkas, N.; Guibert, G.; Mikhailov, S.; Gedanken, A. Sonochemical coating of silver nanoparticles on textile fabrics (nylon, polyester and cotton) and their antibacterial activity. Nanotechnology 2008, 19, 245705. (2) Rehim, M. H. A.; Youssef, A. M.; Ghanem, A. Polystyrene/hydrophobic TiO2 nanobelts as a novel packaging material. Polym. Bull. 2015, 72, 2353. (3) Rezaie, A. B.; Montazer, M.; Rad, M. M. Low toxic antibacterial application with hydrophobic properties on polyester through facile and clean fabrication of nano copper with fatty acid. Mater. Sci. Eng., C 2019, 97, 177. (4) Dastjerdi, R.; Montazer, M. A review on the application of inorganic nano-structured materials in the modification of textiles: focus on anti-microbial properties. Colloids Surf. B. Biointerfaces 2010, 79, 5. (5) Komeily-Nia, Z.; Montazer, M.; Heidarian, P.; Nasri-Nasrabadi, B. Smart photoactive soft materials for environmental cleaning and energy production through incorporation of nanophotocatalyst on polymers and textiles. Polym. Adv. Technol. 2019, 30, 235. 24


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Dual DBD plasma treatments for synthesis of Ag-TiO2 functionalized

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