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Silver nanoparticle-loaded filter paper: Innovative assembly method by nonthermal plasma and facile application for the reduction of methylene blue
Surface & Coatings Technology 366 (2019) 7–14
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Silver nanoparticle-loaded filter paper: Innovative assembly method by nonthermal plasma and facile application for the reduction of methylene blue Pan Lu, Dong-Wook Kim, Dong-Wha Park
T
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Department of Chemistry and Chemical Engineering and Regional Innovation Center for Environmental Technology of Thermal Plasma (RIC-ETTP), Inha University, 100 Inha-Ro, Michuhol-gu, Incheon 22212, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Dielectric barrier discharge Nonthermal plasma Silver nanoparticles Nanoparticles synthesis
In this study, silver nanoparticles (AgNPs) were loaded onto commercial filter paper using a simple “suspendingdischarging-washing” process. Nanosecond pulse dielectric barrier discharge (NP-DBD) plasma was used to perform the in-situ reduction-loading process in a plate-to-plate DBD reactor during the five-minute fabrication process. Gaseous ethanol was used as the reducing agent under an argon gas plasma treatment. There was no thermal treatment, which prefers the formation of small nanoparticles. Transmission electron microscopy showed that the synthesized AgNPs were well-dispersed over the surface of the filter paper. The synthesized nanoparticles were hemispherical and crystalline. Fourier transform infrared spectroscopy showed that the cellulose skeletons had been mostly maintained after the plasma discharge. The catalytic activity of the AgNPs paper was evaluated by the reduction reaction of methylene blue (MB) with sodium borohydride (NaBH4) using a straightforward filtration process.
1. Introduction Noble metal nanoparticles are used widely in numerous fields owing to their unique remarkable properties compared to their bulk counterparts [1]. The novel properties of nanoparticles are affected significantly by their size and dispersity [2]. Therefore, a considerable volume of literature has been published on size-controlled synthesis methods of nanoparticles [3], such as single-atom-scaled catalyst [4]. As a relatively inexpensive noble metal, silver nanoparticles (AgNPs) have attracted interest because of their vast applications on selective NOx reduction [5], methanol oxidation [6], and antibacterial area [7]. Currently, solution-based synthesis is generally used to develop small nanoparticles by forming a well-dispersed colloid. On the other hand, the large specific area of nanoparticles also raises problems of agglomeration due to the high surface energy. In addition, the subsequent collection and assembly of individual AgNPs from a colloid are major challenges limiting their applications. Therefore, AgNPs colloids are usually used for catalysis directly by dispersing in the liquid phase without further recollection but the emission of AgNPs is a potential pollution source for the environment [8,9]. The introduction of AgNPs on/into various substrates is considered a good strategy to settle the problem of recollection [10]. Those
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composites can realize the anti-agglomeration effects without decreasing the catalytic performance. For example, magnetic composites can allow convenient catalyst recycling using an external magnetic field [11]. On the other hand, a sophisticated synthesis process and the use of toxic chemical reagents are essential during the fabrication process. In addition, an additional protective layer is always applied to stabilize the unstable magnetic substrate, which requires more toxic chemical reagents and more complicated synthesis process [12]. Recent developments in material synthesis have increased the need for green chemistry to avoid the excessive use of organic solvents and toxic chemical agents. For example, a range of physical deposition methods is eco-friendly because of the non-use and non-emission of chemical reagents. Nevertheless, the physical processes for nanomaterial synthesis are generally performed under vacuum, which requires complex apparatus and strict experimental conditions. In a view of the facility, studies on the reducibility of substrates have attracted considerable attention. At this point, macromolecules have become potential ideal substrates for metal nanoparticles. Both natural and man-made macromolecules have been adopted to synthesize silver nanoparticles without additional reductants [13,14]. On the other hand, prolonged thermal treatment is generally applied to activate the reduction process. In a view of the facility, atmospheric-pressure cold plasma, which
Corresponding author. E-mail address: [email protected] (D.-W. Park).
https://doi.org/10.1016/j.surfcoat.2019.03.019 Received 28 November 2018; Received in revised form 28 February 2019; Accepted 9 March 2019 Available online 11 March 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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respectively. The wet paper was then placed onto the dielectric surface of the DBD reactor. Note that the paper should be placed onto the dielectric surface carefully to avoid the existence of air between the paper and surface. The dielectric was then assembled into the reactor. The discharge gas was flowed into the reactor at 30 mL/min while the carrier gas was flowed at 20 mL/min through the ethanol bubbling bottle. The plasma discharge was generated by the power generator with an output voltage and pulse-per-second (PPS) of 22 kV and 1000, respectively. The plasma treatment time was 5 min. After the discharge treatment, the sample was immersed in a 7:3 (v/v) mixture of DI-water and ethanol to wash out the unreacted precursor and organic by-products. The sample was then dried under 40 °C.
can be generated by dielectric barrier discharge (DBD), has been used as an alternative method to fabricate metal catalyst composites instead of thermal treatments [15]. A plasma is a partially ionized gas, consisting of electrons, ions, molecules, radicals, photons, and excited species. Therefore, it is a highly reactive mixture that can induce a reaction that would not occur under normal conditions. It has been reported that a plasma pre-treatment was introduced to reduce silver ions to metallic silver nanoparticles in a polyacrylonitrile (PAN) solution without additional reducing agents, followed by electrospinning to fabricate Ag/PAN hybrid nanofibers [16]. On the other hand, the existence of nanoparticles affected the viscosity and conductivity of the PAN solution, which varies the morphology of the synthesized nanofibers by electrospinning method [17]. Therefore, direct deposition is preferred to avoid such morphological distortion. The direct deposition of nanoparticles onto a polymer is normally realized by immersing the polymer into the metal colloidal solution or precursor-reductant solutions [18]. In this study, a facile, innovative and low-cost in situ reduction-loading process was realized by applying DBD plasma. Although electrons from the plasma can reduce metal ions directly, reductive reagents, such as CO and H2, are usually added to the plasma to generate highly reactive species for reduction process, which increases the reaction significantly [19,20]. In the present study, ethanol was used as the reductant to avoid the use of toxic or explosive chemical reducing reagents. The low temperature process not only allows the use of thermal-sensitive substrates, but also suppresses the growth and aggregation of nanoparticles. The feasibility of this method was assessed using commercial filter paper as the target substrate. Filter paper, which normally consists of cellulose fibers, is a semi-permeable paper barrier that is abundant and inexpensive. The hydrophilic surface of cellulose allows filter paper to absorb suspensions of the nanoparticles, which helps prevent aggregation [21]. The use of filter paper also allows a straightforward catalytic reaction by a simple filtration process without the additional assembly of the catalyst.
A needle was used to scrape the treated surface of the paper. The scraped fragments were then collected. The fragments were dispersed in ethanol, and the mixture was treated by ultrasonication for several seconds. The treated mixture was dropped onto a copper mesh and dried for TEM observation. The morphology of the AgNP papers was observed by field emission transmission electron microscopy (FE-TEM, JEM2100F/JEOL). Sample S2 was adopted as the representative specimen to do the following characterizations. Fourier transform infrared (FT-IR, VERTEX 80 V/Bruker) vacuum spectroscopy was performed with specimens prepared from the scraped fragments using KBr methods. X-ray diffraction (XRD, Rigaku D/max) was performed using Cu Kα radiation. The XRD patterns were collected at room temperature over the 2θ ranges of 10 to 90°. The treated papers were used directly as the specimens for X-ray photoelectron spectroscopy (XPS, K-Alpha/ Thermo Fisher Scientific) using Al Kα radiation to access the bonding state on the surface. Raman spectra were recorded using a laser Raman spectrometer (HORIBA LabRAM Revolution) with an excitation of 532nm laser light. The papers were used as the specimens directly.
2. Experimental
2.4. Catalytic reduction of methylene blue (MB)
2.1. Materials
The catalytic reduction of methylene blue (MB) using sodium borohydride (NaBH4) is a representative reaction used to assess the catalytic activity of AgNPs [22–26]. Methylene blue became colorless in the presence of NaBH4 indicating the reduction of methylene blue to leucomethylene blue (LMB), as shown in Fig. 2. Sample S2 was used to perform the catalysis with sample S0 as the control group. The treated filter paper was cut into a circle, 15 mm in diameter. The paper was then put onto the inner bottom of a syringe. A 1.0 mL sample of a freshly prepared 10 mM NaBH4 solution was mixed with 1.5 mL of 1 mM MB, and the mixture was made up to 10 mL using DI-water and then stirred for 1 min. The mixture solution was transferred to the syringe and the filtration process was performed under gravity. The UV–Vis spectrum of the filtered solution was recorded between 500 and 750 nm using a UV/VIS/NIR spectrometer (Lambda 750, PerkinElmer).
2.3. Characterization
Silver nitrate (AgNO3, ACS reagent, ≥ 99.0%) was purchased from Sigma-Aldrich, Co., India. Ethanol (ethyl alcohol anhydrous, 99.9%) was obtained from Daejung Co., Ltd. Korea. The filter paper (No.20, αcellulose 99.9%, ash 0.01%) was acquired from Hyundai Micro Co., Ltd., Korea. The basic weight of the filter paper was 85 g/m2 and the thickness was 0.20 mm. Sodium borohydride (NaBH4) power was supplied by Daejung Chemicals and Metals Co., Ltd., Korea. The methylene blue solution (MB, 1.5%, aqueous) was purchased from Sigma-Aldrich, India. 2.2. Method Fig. 1 shows a schematic diagram of the experimental setup. A plateto-plate DBD reactor was adopted for plasma discharge. The electrode system was comprised of one 2-mm thick quartz glass (Φ 75 mm) and two stainless steel electrodes. The diameters of the upper and lower electrodes were 60 and 70 mm, respectively. Argon gas was used as both the discharge gas and carrier gas. The discharge gas was controlled by a mass flow controller (MFC), shown as white in Fig. 1. The carrier gas was controlled by another MFC, shown as blue in Fig. 1, and flowed through a bubbling bottle filled with ethanol. The discharge gas and carrier gas were mixed by a T-connector and then flowed into the reactor. The DBD discharge was induced by nanosecond pulsed power generator (PPM1000S-1KES, P, Suematsu Electronics Co., Ltd., Japan). A filter paper was cut into a circle, 60 mm in diameter. The cut paper was then immersed in an AgNO3 solution and stirred for several seconds. Five samples, marked S0, S1, S2, S3, and S4 were prepared from 0 mM, 5 mM, 10 mM, 15 mM, and 20 mM AgNO3 solutions,
3. Results and discussion 3.1. Plasma reduction When the paper was immersed in the aqueous AgNO3, silver ions were readily penetrated into cellulose fiber through their pores. The absorbed Ag+ were bound to cellulose fibers via electrostatic interactions, because the electron-rich oxygen atoms of the polar hydroxyl and ether groups of cellulose interact with the electropositive transition metal cations [21]. The fixation of silver ions is preferred for the generation of AgNPs in a good dispersion by in situ reductions. Argon discharge was used for the plasma treatment on the surface of the paper. Note that the ionization energy of Ar is 15.6 eV. Under atmospheric pressure, the charge conservation in the case of an Ar 8
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Fig. 1. Schematic diagram of the experimental setup for fabricating AgNPs on the filter paper by NP-DBD plasma system. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
eaq + Ag + → Ag
discharge with Ar2+ as the dominant ion is governed by the following reactions:
e + Ar2+ → Arm + Ar
(1)
e + Arm → Ar + + 2e
(2)
Ar + + 2Ar → Ar2+ + Ar
(3)
The plasma only occurred between the upper electrode and paper. Therefore, the generated radicals can only approach the surface above it; hence, the AgNPs were synthesized onto the upper surface of the paper. Previous studies have reported that the addition of ethanol can cause a dramatic increase in the reduction rate [29,31]. Therefore, reaction (6) is the dominating reaction during the reduction process. Although H radicals have a higher reducing potential than CH3CHOH radicals, the ultrahigh reactivity makes its lifetime very short. Therefore, the hydrogen radicals generated react with other species immediately before reducing the precursor at low concentrations. In contrast, the longer lifetime of the CH3CHOH radical improves the possibility of reacting with silver ions [32]. After reduction, AgNPs were generated from silver ions fixed on the cellulose fiber. The hydrophilic fiber surface of cellulose allows the filter paper to absorb the suspensions of nanoparticles, yielding a high loading of nanoparticles after drying.
Gaseous ethanol molecules, under plasma treatment, can be decomposed to various radicals, such as CH3CHOH radicals, hydrogen radicals, etc. [27,28]. CH3CHOH radicals have been reported to be effective species in the reduction process. The CH3CHOH radicals were generated by the following reactions [29]:
e + C2 H5 OH → CH3 CHOH + ˙H + e
(4)
H + C2 H5 OH → CH3 CHOH + H2
(5)
When CH3CHOH radicals reached the surface of filter paper, the silver ions are reduced according to the following reaction:
CH3 CHOH + Ag + → CH3 CHO + Ag + H+
(6)
3.2. Structure of the Ag/FP papers
On the other hand, the energetic electrons collided with water molecules, leading to the generation of hydrogen radicals:
e + H2 O → ˙H + ˙OH + e
Fig. 3 presents photographs of the AgNPs papers from S0 to S4. No color change was observed after the plasma treatment without the precursor. Under a low precursor concentration, the treated surface of the paper had a yellowish color, which indicates the typical surface plasmon resonance of AgNPs. With increasing precursor concentration, the color became deeper and then turned gray, indicating an increase in Ag content. During the plasma treatment, the electrons and radicals
(7)
Both hydrogen radicals and solvated electrons can reduce silver ions directly [30]:
Haq + Ag + → Ag + H+
(9)
(8)
Fig. 2. Catalytic reduction of MB by NaBH4 with AgNPs as the catalyst. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.) 9
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Fig. 3. Photographs of the treated samples S0 to S4.
Fig. 5. Single particles from sample (a)S1, (b)S2, (c)S3, and (d)S4. The characteristic d-spacings of 0.236 nm, 0.240 nm, and 0.253 nm are corresponding to the facets (1 1 1), (0 0 2), and (1 0 0) of Ag NPs, respectively.
Fig. 4. Typical TEM images of sample (a)S0, (b)S1, (c)S2, (d)S4, and (e)S5.
could barely penetrate the inner paper due to the frequent collision and short lifetime. Therefore, the reduction of Ag+ occurred mainly on the upper surface of the paper. The synthesized AgNPs were loaded onto the surface upon drying, which can be regarded as a stabilizing method to prevent the aggregation of nanoparticles. More AgNPs were generated at higher Ag+ concentrations. As a result, nanoparticles tended to aggregate to minimize their surface energy. The dispersity of the synthesized nanoparticles was observed by FETEM, as shown in Fig. 4. The size and yield of AgNPs increased with increasing precursor concentration. In addition, AgNPs aggregation became more severe with the increasing number of nanoparticles. With the exception of some severely aggregated particles, the mean diameters of the AgNPs for samples S1, S2, S3, and S4 were 4.0 ± 1.0 nm, 7.0 ± 2.0 nm, 9.5 ± 3.5 nm, and 12.5 ± 5.0 nm, respectively. Fig. 5 presents the single AgNPs extracted from Fig. 4 to analyze the morphology. The as-grown AgNPs were hemispherical and lattice fringes
Fig. 6. FT-IR spectra of the (a) S0 and (b) S2. The marked absorption bands, from left to right, locate at 3391 cm−1, 2906 cm−1, 1427 cm−1, 1382–1375 cm−1, 1317 cm−1, 1105 cm−1, 1054 cm−1, and 898 cm−1, respectively.
10
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thermal methods. 3.3. Properties of the Ag/FP papers The identification of possible organic groups responsible for the reduction of synthesized metallic nanoparticles can be achieved by IR spectra analysis. Cellulose contains several functional groups, which can be used to reduce Ag+ to Ag by a thermal treatment. To determine if the groups of cellulose participate in the reduction process during the plasma treatment, the S0 and S2 samples were analyzed by FT-IR spectroscopy, as shown in Fig. 6. The absorption bands at 3391 cm−1 and 2906 cm−1 were assigned to the stretching vibrations of hydroxyl groups stretching and CeH group stretching in the glucose units, which is a typical characterization of commercial cellulose [33]. Other spectral bands inside the glucose units, including CH bending (1382–1375 cm−1), CH2 wagging (1317 cm−1), CeOeC pyranose ring stretching (1317 cm−1), and CeC ring stretching (1105 cm−1) vibrations, could be observed in the spectrum [34]. The invariability of such groups after plasma discharge confirmed that the cellulose skeletons were maintained mostly during the plasma treatment. Minor variabilities occur on 1427 cm−1 and 898 cm−1. The spectral band at 1427 cm−1 was assigned to the asymmetric CH2 bending vibration, which is called the “crystallinity band”. The variability in its intensity indicates the change in the degree of crystallinity of the sample. The spectral band at 898 cm−1 corresponded to the CeOeC stretching at β(1 → 4)-glyosidic linkages, which is also called the “amorphous” band. The intensity indicates the existence of amorphous structures [35]. The change in those bands could be related to the scraping methods, which might destroy the crystalline structure of cellulose. Overall, the reduction process is dominated by the plasma-induced radicals instead of the organic groups of cellulose. The low temperature and short treatment time could not activate the reducing potential of the functional groups. In addition, the water layer played a protective role in suppressing the energetic electron collisions onto the cellulose. Therefore, the groups on the cellulose were well preserved after the plasma treatment. In this experiment, the paper was still hydrophilic after the plasma treatment, which allowed the catalytic process to proceed by a filtration process. Fig. 7 presents XRD patterns of the S0 and S2 samples. The filter
Fig. 7. XRD pattern of the sample (a) S0 and (b) S2.
could be observed clearly. The nucleation of AgNPs occurred mainly on the plasma-liquid interface, where the silver ions reacted with the reductive radicals from the plasma. The existence of water was due only to the absorption of hydrophilic groups of the cellulose fibers, the precursor solution was confined within the filter paper. With a high concentration precursor, some of the silver ions were not fixed and existed as the free species in the aqueous layer. Therefore, fixed silver ions produced fixed silver nuclei while free silver ions produced free silver nuclei. Growth occurred after nucleation to decrease the large surface energy. Spontaneous aggregation occurred with large volumes, forming large particles. The cellulose fiber contained AgNPs upon drying but the fibers were stationary. The unfixed nanoparticles kept growing by agglomeration or Oswald ripening before reaching the cellulose fiber. Very large particles cannot be deposited tightly onto the cellulose surface so that they can be washed out by water/ethanol mixture. Because it is a low-temperature process, Ostwald ripening was suppressed to preserve small nanoparticles, which is preferable over
Fig. 8. (a) Ag 3d core level XPS spectrum of sample S2; (b) Raman spectra of the sample S2 and the paper. The characteristic bands of Ag2O locate at about 80, 146, 220, 342, 420, 487, 950, and 1195 cm−1, labeled by “+” symbols. There are also three peaks, labeled by “*” symbols, from the impurities of the paper, according to the Raman spectrum of the paper. 11
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Fig. 9. (a) and (c) show the UV–Vis absorption spectra recorded during the filtration process for sample S0 and S2, respectively. The corresponding relationships between the absorption intensity and filtration number are shown on the (b) and (d). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
radicals, as shown in reaction (7), with subsequent generation of %HO2 radicals, and H2O2 molecules:
paper showed three characteristic XRD peaks at 2θ = 14.7°, 16.8° and 22.7°, corresponding to the assignments of (1 1 0) (110), and (200) planes, respectively. Therefore, the filter paper consisted of typical semi-crystalline α-cellulose structures. After loading the AgNPs, new peaks at 46.18° and 67.44° 2θ were noted corresponding to (111) and (220) planes of silver crystalline, respectively. As mentioned previously, the synthesized AgNPs can only be generated on the surface exposed to the plasma. As a result, the Ag peaks were minor, indicating that the amount of the AgNPs was low compared to cellulose. XPS of Ag3d was performed to investigate the bonding state of Ag. The structural evolution was studied by XPS and Raman spectroscopy, as presented in Fig. 8. The XRD patterns have indicated the existence of metallic Ag, of which the XPS peak locates at 368.1 eV. As shown in Fig. 8(a), the XPS spectrum indicated that there was also a minor peak of Ag2O, located at 367.7 eV. The Raman spectra, as presented in Fig. 8(b), showed a series of bands, which also identify the existence of Ag2O [36]. The Ag2O came from the oxidation of Ag, because energetic electron collisions on the water molecules led to the production of ·OH
OH + ˙OH → H2 O2
(10)
OH + H2 O2 → ˙HO2 + H2 O
(11)
Those species, which have very strong oxidizing potential, can oxidize Ag to Ag2O, whereas %OH radicals can react with ethanol molecules immediately:
OH + C2 H5 OH → CH3 CHOH + H2 O
(12)
Therefore, the addition of ethanol can suppress the oxidation reaction by consuming oxidizing species. In addition, the Ag2O generated could also be reduced back to Ag by CH3CHOH species. 3.4. Catalytic activities In aqueous medium, MB molecules show an absorption band at 664 nm with a shoulder at 614 nm. Fig. 9 shows variations in the 12
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intensity of the absorption band at 664 nm for samples filtered by S0 and S2 after several filtration processes under room temperature. The process took 80 s to finish the filtration process of the 10 mL solution. In the reaction mixture, a higher concentration of NaBH4 compared to MB was used, which resulted in an increase in pH of the entire system. This, in turn, retarded the degradation of BH4− ions. The liberated H+ ions produce a purging environment over the reaction mixture thereby effectively inhibiting the aerial oxidation of reduced MB products. A decrease in the absorption band was observed, but the paper turned blue after the filtration process, as shown in Fig. 9(a). Such a phenomenon is associated with the absorption effect of the filter paper. The porous structure and hydrophilic fiber surface allowed the filter paper to absorb MB molecules, which led to a decrease in the absorption of the filtered solution until the absorption of the filter paper was saturated. On the other hand, Fig. 9(c) shows the catalytic nature of the AgNPs in the reduction of MB by NaBH4. Almost no MB molecules were absorbed on the filter paper after the filtration process. This suggests that the deposited AgNPs can catalyze the reaction to completion after the 20th filtration process. The catalytic reduction aided by metallic nanoparticles was carried out effectively by transferring electrons from the electron donor species, BH4−, to electron acceptor species MB mediated by AgNPs, which reduces the activation energy and helps stabilize the system. The color of the AgNPs turned lighter after catalysis possibly due to the migration of AgNPs from the surface to the inner paper.
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4. Conclusion Silver nanoparticles were synthesized and loaded onto the upper surface of filter paper by a simple “immersing- discharging-washing” process. Ethanol was used as a green reagent for the production of reactive species under the plasma treatment. The reactive species have a high reducing potential to reduce silver ions. The hydrophilic surface of the paper plays as the role of fixing the nanoparticles for good dispersion. The synthesized AgNPs were hemispherical with a crystal structure. The functional groups did not participate in the reduction process and were well preserved after the plasma treatment. The NP-DBD plasma reduction process did not destroy the micro-skeletons of the paper. Metallic AgNPs were the main product with little oxidized Ag2O. The oxidative species were generated from the aqueous layer, but the ethanol molecules consumed the oxidative species to a certain extent, which impeded the oxidation reaction. The utility of filter paper allows the deposited AgNPs to catalyze the reduction of MB by NaBH4 through a straightforward filtration process. Acknowledgments This work was supported by the Regional Innovation Center for Environmental Technology of Thermal Plasma (RIC-ETTP) at Inha University. References [1] L. Di, J. Zhang, X. Zhang, A review on the recent progress, challenges, and perspectives of atmospheric-pressure cold plasma for preparation of supported metal catalysts, Plasma Process. Polym. 15 (2018) 1700234, , https://doi.org/10.1002/ ppap.201700234. [2] S. Agnihotri, S. Mukherji, S. Mukherji, Size-controlled silver nanoparticles synthesized over the range 5–100 nm using the same protocol and their antibacterial efficacy, RSC Adv. 4 (2014) 3974–3983, https://doi.org/10.1039/c3ra44507k. [3] X. Zhang, M.Z. Yates, Controllable synthesis of hydroxyapatite-supported palladium nanoparticles with enhanced catalytic activity, Surf. Coat. Technol. 351 (2018) 60–67, https://doi.org/10.1016/J.SURFCOAT.2018.07.075. [4] A. Wang, J. Li, T. Zhang, Heterogeneous single-atom catalysis, Nat. Rev. Chem. 2 (2018) 65–81, https://doi.org/10.1038/s41570-018-0010-1. [5] M. Männikkö, X. Wang, M. Skoglundh, H. Härelind, Silver/alumina for methanolassisted lean NOx reduction—on the influence of silver species and hydrogen formation, Appl. Catal. B Environ. 180 (2016) 291–300, https://doi.org/10.1016/J. APCATB.2015.06.002.
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P. Lu, et al. [31] X. Gong, Y. Ma, J. Lin, X. He, Z. Long, … Q.C.-J. of T., undefined 2018, Tuning the formation process of silver nanoparticles in a plasma electrochemical system by additives, Jes.Ecsdl.Org. (n.d.). http://jes.ecsdl.org/content/165/11/E540.short (accessed January 8, 2019). [32] T. Sudare, T. Ueno, A. Watthanaphanit, N. Saito, Verification of radicals formation in ethanol–water mixture based solution plasma and their relation to the rate of reaction, J. Phys. Chem. A 119 (2015) 11668–11673, https://doi.org/10.1021/acs. jpca.5b07224. [33] F. Carrillo, X. Colom, J. Suñol, J. Saurina, Structural FTIR analysis and thermal characterisation of lyocell and viscose-type fibres, Eur. Polym. J. 40 (2004) 2229–2234, https://doi.org/10.1016/J.EURPOLYMJ.2004.05.003.
[34] A. Kumar, Y. Singh Negi, V. Choudhary, N. Kant Bhardwaj, Characterization of cellulose nanocrystals produced by acid-hydrolysis from sugarcane bagasse as agrowaste, J. Mater. Phys. Chem. 2 (2014) 1–8, https://doi.org/10.12691/jmpc-2-1-1. [35] D. Ciolacu, F. Ciolacu, V.I. Popa, Amorphous cellulose-structure and characterization, 2011. http://www.cellulosechemtechnol.ro/pdf/CCT1-2(2011)/p.13-21.pdf (accessed January 8, 2019). [36] I. Martina, R. Wiesinger, D. Jembrih-Simbürger, M. Schreiner, Micro-Raman Characterisation of Silver Corrosion Products: Instrumental Set up and Reference Database, E-Preservation Science: Morana RTD, 2012, http://www.morana-rtd. com/e-preservationscience/2012/Martina-05-03-2012.pdf.
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Silver nanoparticle-loaded filter paper: Innovative assembly method by nonthermal plasma and facile application for the reduction of methylene blue Pan Lu, Dong-Wook Kim, Dong-Wha Park
T
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Department of Chemistry and Chemical Engineering and Regional Innovation Center for Environmental Technology of Thermal Plasma (RIC-ETTP), Inha University, 100 Inha-Ro, Michuhol-gu, Incheon 22212, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Dielectric barrier discharge Nonthermal plasma Silver nanoparticles Nanoparticles synthesis
In this study, silver nanoparticles (AgNPs) were loaded onto commercial filter paper using a simple “suspendingdischarging-washing” process. Nanosecond pulse dielectric barrier discharge (NP-DBD) plasma was used to perform the in-situ reduction-loading process in a plate-to-plate DBD reactor during the five-minute fabrication process. Gaseous ethanol was used as the reducing agent under an argon gas plasma treatment. There was no thermal treatment, which prefers the formation of small nanoparticles. Transmission electron microscopy showed that the synthesized AgNPs were well-dispersed over the surface of the filter paper. The synthesized nanoparticles were hemispherical and crystalline. Fourier transform infrared spectroscopy showed that the cellulose skeletons had been mostly maintained after the plasma discharge. The catalytic activity of the AgNPs paper was evaluated by the reduction reaction of methylene blue (MB) with sodium borohydride (NaBH4) using a straightforward filtration process.
1. Introduction Noble metal nanoparticles are used widely in numerous fields owing to their unique remarkable properties compared to their bulk counterparts [1]. The novel properties of nanoparticles are affected significantly by their size and dispersity [2]. Therefore, a considerable volume of literature has been published on size-controlled synthesis methods of nanoparticles [3], such as single-atom-scaled catalyst [4]. As a relatively inexpensive noble metal, silver nanoparticles (AgNPs) have attracted interest because of their vast applications on selective NOx reduction [5], methanol oxidation [6], and antibacterial area [7]. Currently, solution-based synthesis is generally used to develop small nanoparticles by forming a well-dispersed colloid. On the other hand, the large specific area of nanoparticles also raises problems of agglomeration due to the high surface energy. In addition, the subsequent collection and assembly of individual AgNPs from a colloid are major challenges limiting their applications. Therefore, AgNPs colloids are usually used for catalysis directly by dispersing in the liquid phase without further recollection but the emission of AgNPs is a potential pollution source for the environment [8,9]. The introduction of AgNPs on/into various substrates is considered a good strategy to settle the problem of recollection [10]. Those
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composites can realize the anti-agglomeration effects without decreasing the catalytic performance. For example, magnetic composites can allow convenient catalyst recycling using an external magnetic field [11]. On the other hand, a sophisticated synthesis process and the use of toxic chemical reagents are essential during the fabrication process. In addition, an additional protective layer is always applied to stabilize the unstable magnetic substrate, which requires more toxic chemical reagents and more complicated synthesis process [12]. Recent developments in material synthesis have increased the need for green chemistry to avoid the excessive use of organic solvents and toxic chemical agents. For example, a range of physical deposition methods is eco-friendly because of the non-use and non-emission of chemical reagents. Nevertheless, the physical processes for nanomaterial synthesis are generally performed under vacuum, which requires complex apparatus and strict experimental conditions. In a view of the facility, studies on the reducibility of substrates have attracted considerable attention. At this point, macromolecules have become potential ideal substrates for metal nanoparticles. Both natural and man-made macromolecules have been adopted to synthesize silver nanoparticles without additional reductants [13,14]. On the other hand, prolonged thermal treatment is generally applied to activate the reduction process. In a view of the facility, atmospheric-pressure cold plasma, which
Corresponding author. E-mail address: [email protected] (D.-W. Park).
https://doi.org/10.1016/j.surfcoat.2019.03.019 Received 28 November 2018; Received in revised form 28 February 2019; Accepted 9 March 2019 Available online 11 March 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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respectively. The wet paper was then placed onto the dielectric surface of the DBD reactor. Note that the paper should be placed onto the dielectric surface carefully to avoid the existence of air between the paper and surface. The dielectric was then assembled into the reactor. The discharge gas was flowed into the reactor at 30 mL/min while the carrier gas was flowed at 20 mL/min through the ethanol bubbling bottle. The plasma discharge was generated by the power generator with an output voltage and pulse-per-second (PPS) of 22 kV and 1000, respectively. The plasma treatment time was 5 min. After the discharge treatment, the sample was immersed in a 7:3 (v/v) mixture of DI-water and ethanol to wash out the unreacted precursor and organic by-products. The sample was then dried under 40 °C.
can be generated by dielectric barrier discharge (DBD), has been used as an alternative method to fabricate metal catalyst composites instead of thermal treatments [15]. A plasma is a partially ionized gas, consisting of electrons, ions, molecules, radicals, photons, and excited species. Therefore, it is a highly reactive mixture that can induce a reaction that would not occur under normal conditions. It has been reported that a plasma pre-treatment was introduced to reduce silver ions to metallic silver nanoparticles in a polyacrylonitrile (PAN) solution without additional reducing agents, followed by electrospinning to fabricate Ag/PAN hybrid nanofibers [16]. On the other hand, the existence of nanoparticles affected the viscosity and conductivity of the PAN solution, which varies the morphology of the synthesized nanofibers by electrospinning method [17]. Therefore, direct deposition is preferred to avoid such morphological distortion. The direct deposition of nanoparticles onto a polymer is normally realized by immersing the polymer into the metal colloidal solution or precursor-reductant solutions [18]. In this study, a facile, innovative and low-cost in situ reduction-loading process was realized by applying DBD plasma. Although electrons from the plasma can reduce metal ions directly, reductive reagents, such as CO and H2, are usually added to the plasma to generate highly reactive species for reduction process, which increases the reaction significantly [19,20]. In the present study, ethanol was used as the reductant to avoid the use of toxic or explosive chemical reducing reagents. The low temperature process not only allows the use of thermal-sensitive substrates, but also suppresses the growth and aggregation of nanoparticles. The feasibility of this method was assessed using commercial filter paper as the target substrate. Filter paper, which normally consists of cellulose fibers, is a semi-permeable paper barrier that is abundant and inexpensive. The hydrophilic surface of cellulose allows filter paper to absorb suspensions of the nanoparticles, which helps prevent aggregation [21]. The use of filter paper also allows a straightforward catalytic reaction by a simple filtration process without the additional assembly of the catalyst.
A needle was used to scrape the treated surface of the paper. The scraped fragments were then collected. The fragments were dispersed in ethanol, and the mixture was treated by ultrasonication for several seconds. The treated mixture was dropped onto a copper mesh and dried for TEM observation. The morphology of the AgNP papers was observed by field emission transmission electron microscopy (FE-TEM, JEM2100F/JEOL). Sample S2 was adopted as the representative specimen to do the following characterizations. Fourier transform infrared (FT-IR, VERTEX 80 V/Bruker) vacuum spectroscopy was performed with specimens prepared from the scraped fragments using KBr methods. X-ray diffraction (XRD, Rigaku D/max) was performed using Cu Kα radiation. The XRD patterns were collected at room temperature over the 2θ ranges of 10 to 90°. The treated papers were used directly as the specimens for X-ray photoelectron spectroscopy (XPS, K-Alpha/ Thermo Fisher Scientific) using Al Kα radiation to access the bonding state on the surface. Raman spectra were recorded using a laser Raman spectrometer (HORIBA LabRAM Revolution) with an excitation of 532nm laser light. The papers were used as the specimens directly.
2. Experimental
2.4. Catalytic reduction of methylene blue (MB)
2.1. Materials
The catalytic reduction of methylene blue (MB) using sodium borohydride (NaBH4) is a representative reaction used to assess the catalytic activity of AgNPs [22–26]. Methylene blue became colorless in the presence of NaBH4 indicating the reduction of methylene blue to leucomethylene blue (LMB), as shown in Fig. 2. Sample S2 was used to perform the catalysis with sample S0 as the control group. The treated filter paper was cut into a circle, 15 mm in diameter. The paper was then put onto the inner bottom of a syringe. A 1.0 mL sample of a freshly prepared 10 mM NaBH4 solution was mixed with 1.5 mL of 1 mM MB, and the mixture was made up to 10 mL using DI-water and then stirred for 1 min. The mixture solution was transferred to the syringe and the filtration process was performed under gravity. The UV–Vis spectrum of the filtered solution was recorded between 500 and 750 nm using a UV/VIS/NIR spectrometer (Lambda 750, PerkinElmer).
2.3. Characterization
Silver nitrate (AgNO3, ACS reagent, ≥ 99.0%) was purchased from Sigma-Aldrich, Co., India. Ethanol (ethyl alcohol anhydrous, 99.9%) was obtained from Daejung Co., Ltd. Korea. The filter paper (No.20, αcellulose 99.9%, ash 0.01%) was acquired from Hyundai Micro Co., Ltd., Korea. The basic weight of the filter paper was 85 g/m2 and the thickness was 0.20 mm. Sodium borohydride (NaBH4) power was supplied by Daejung Chemicals and Metals Co., Ltd., Korea. The methylene blue solution (MB, 1.5%, aqueous) was purchased from Sigma-Aldrich, India. 2.2. Method Fig. 1 shows a schematic diagram of the experimental setup. A plateto-plate DBD reactor was adopted for plasma discharge. The electrode system was comprised of one 2-mm thick quartz glass (Φ 75 mm) and two stainless steel electrodes. The diameters of the upper and lower electrodes were 60 and 70 mm, respectively. Argon gas was used as both the discharge gas and carrier gas. The discharge gas was controlled by a mass flow controller (MFC), shown as white in Fig. 1. The carrier gas was controlled by another MFC, shown as blue in Fig. 1, and flowed through a bubbling bottle filled with ethanol. The discharge gas and carrier gas were mixed by a T-connector and then flowed into the reactor. The DBD discharge was induced by nanosecond pulsed power generator (PPM1000S-1KES, P, Suematsu Electronics Co., Ltd., Japan). A filter paper was cut into a circle, 60 mm in diameter. The cut paper was then immersed in an AgNO3 solution and stirred for several seconds. Five samples, marked S0, S1, S2, S3, and S4 were prepared from 0 mM, 5 mM, 10 mM, 15 mM, and 20 mM AgNO3 solutions,
3. Results and discussion 3.1. Plasma reduction When the paper was immersed in the aqueous AgNO3, silver ions were readily penetrated into cellulose fiber through their pores. The absorbed Ag+ were bound to cellulose fibers via electrostatic interactions, because the electron-rich oxygen atoms of the polar hydroxyl and ether groups of cellulose interact with the electropositive transition metal cations [21]. The fixation of silver ions is preferred for the generation of AgNPs in a good dispersion by in situ reductions. Argon discharge was used for the plasma treatment on the surface of the paper. Note that the ionization energy of Ar is 15.6 eV. Under atmospheric pressure, the charge conservation in the case of an Ar 8
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Fig. 1. Schematic diagram of the experimental setup for fabricating AgNPs on the filter paper by NP-DBD plasma system. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
eaq + Ag + → Ag
discharge with Ar2+ as the dominant ion is governed by the following reactions:
e + Ar2+ → Arm + Ar
(1)
e + Arm → Ar + + 2e
(2)
Ar + + 2Ar → Ar2+ + Ar
(3)
The plasma only occurred between the upper electrode and paper. Therefore, the generated radicals can only approach the surface above it; hence, the AgNPs were synthesized onto the upper surface of the paper. Previous studies have reported that the addition of ethanol can cause a dramatic increase in the reduction rate [29,31]. Therefore, reaction (6) is the dominating reaction during the reduction process. Although H radicals have a higher reducing potential than CH3CHOH radicals, the ultrahigh reactivity makes its lifetime very short. Therefore, the hydrogen radicals generated react with other species immediately before reducing the precursor at low concentrations. In contrast, the longer lifetime of the CH3CHOH radical improves the possibility of reacting with silver ions [32]. After reduction, AgNPs were generated from silver ions fixed on the cellulose fiber. The hydrophilic fiber surface of cellulose allows the filter paper to absorb the suspensions of nanoparticles, yielding a high loading of nanoparticles after drying.
Gaseous ethanol molecules, under plasma treatment, can be decomposed to various radicals, such as CH3CHOH radicals, hydrogen radicals, etc. [27,28]. CH3CHOH radicals have been reported to be effective species in the reduction process. The CH3CHOH radicals were generated by the following reactions [29]:
e + C2 H5 OH → CH3 CHOH + ˙H + e
(4)
H + C2 H5 OH → CH3 CHOH + H2
(5)
When CH3CHOH radicals reached the surface of filter paper, the silver ions are reduced according to the following reaction:
CH3 CHOH + Ag + → CH3 CHO + Ag + H+
(6)
3.2. Structure of the Ag/FP papers
On the other hand, the energetic electrons collided with water molecules, leading to the generation of hydrogen radicals:
e + H2 O → ˙H + ˙OH + e
Fig. 3 presents photographs of the AgNPs papers from S0 to S4. No color change was observed after the plasma treatment without the precursor. Under a low precursor concentration, the treated surface of the paper had a yellowish color, which indicates the typical surface plasmon resonance of AgNPs. With increasing precursor concentration, the color became deeper and then turned gray, indicating an increase in Ag content. During the plasma treatment, the electrons and radicals
(7)
Both hydrogen radicals and solvated electrons can reduce silver ions directly [30]:
Haq + Ag + → Ag + H+
(9)
(8)
Fig. 2. Catalytic reduction of MB by NaBH4 with AgNPs as the catalyst. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.) 9
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Fig. 3. Photographs of the treated samples S0 to S4.
Fig. 5. Single particles from sample (a)S1, (b)S2, (c)S3, and (d)S4. The characteristic d-spacings of 0.236 nm, 0.240 nm, and 0.253 nm are corresponding to the facets (1 1 1), (0 0 2), and (1 0 0) of Ag NPs, respectively.
Fig. 4. Typical TEM images of sample (a)S0, (b)S1, (c)S2, (d)S4, and (e)S5.
could barely penetrate the inner paper due to the frequent collision and short lifetime. Therefore, the reduction of Ag+ occurred mainly on the upper surface of the paper. The synthesized AgNPs were loaded onto the surface upon drying, which can be regarded as a stabilizing method to prevent the aggregation of nanoparticles. More AgNPs were generated at higher Ag+ concentrations. As a result, nanoparticles tended to aggregate to minimize their surface energy. The dispersity of the synthesized nanoparticles was observed by FETEM, as shown in Fig. 4. The size and yield of AgNPs increased with increasing precursor concentration. In addition, AgNPs aggregation became more severe with the increasing number of nanoparticles. With the exception of some severely aggregated particles, the mean diameters of the AgNPs for samples S1, S2, S3, and S4 were 4.0 ± 1.0 nm, 7.0 ± 2.0 nm, 9.5 ± 3.5 nm, and 12.5 ± 5.0 nm, respectively. Fig. 5 presents the single AgNPs extracted from Fig. 4 to analyze the morphology. The as-grown AgNPs were hemispherical and lattice fringes
Fig. 6. FT-IR spectra of the (a) S0 and (b) S2. The marked absorption bands, from left to right, locate at 3391 cm−1, 2906 cm−1, 1427 cm−1, 1382–1375 cm−1, 1317 cm−1, 1105 cm−1, 1054 cm−1, and 898 cm−1, respectively.
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thermal methods. 3.3. Properties of the Ag/FP papers The identification of possible organic groups responsible for the reduction of synthesized metallic nanoparticles can be achieved by IR spectra analysis. Cellulose contains several functional groups, which can be used to reduce Ag+ to Ag by a thermal treatment. To determine if the groups of cellulose participate in the reduction process during the plasma treatment, the S0 and S2 samples were analyzed by FT-IR spectroscopy, as shown in Fig. 6. The absorption bands at 3391 cm−1 and 2906 cm−1 were assigned to the stretching vibrations of hydroxyl groups stretching and CeH group stretching in the glucose units, which is a typical characterization of commercial cellulose [33]. Other spectral bands inside the glucose units, including CH bending (1382–1375 cm−1), CH2 wagging (1317 cm−1), CeOeC pyranose ring stretching (1317 cm−1), and CeC ring stretching (1105 cm−1) vibrations, could be observed in the spectrum [34]. The invariability of such groups after plasma discharge confirmed that the cellulose skeletons were maintained mostly during the plasma treatment. Minor variabilities occur on 1427 cm−1 and 898 cm−1. The spectral band at 1427 cm−1 was assigned to the asymmetric CH2 bending vibration, which is called the “crystallinity band”. The variability in its intensity indicates the change in the degree of crystallinity of the sample. The spectral band at 898 cm−1 corresponded to the CeOeC stretching at β(1 → 4)-glyosidic linkages, which is also called the “amorphous” band. The intensity indicates the existence of amorphous structures [35]. The change in those bands could be related to the scraping methods, which might destroy the crystalline structure of cellulose. Overall, the reduction process is dominated by the plasma-induced radicals instead of the organic groups of cellulose. The low temperature and short treatment time could not activate the reducing potential of the functional groups. In addition, the water layer played a protective role in suppressing the energetic electron collisions onto the cellulose. Therefore, the groups on the cellulose were well preserved after the plasma treatment. In this experiment, the paper was still hydrophilic after the plasma treatment, which allowed the catalytic process to proceed by a filtration process. Fig. 7 presents XRD patterns of the S0 and S2 samples. The filter
Fig. 7. XRD pattern of the sample (a) S0 and (b) S2.
could be observed clearly. The nucleation of AgNPs occurred mainly on the plasma-liquid interface, where the silver ions reacted with the reductive radicals from the plasma. The existence of water was due only to the absorption of hydrophilic groups of the cellulose fibers, the precursor solution was confined within the filter paper. With a high concentration precursor, some of the silver ions were not fixed and existed as the free species in the aqueous layer. Therefore, fixed silver ions produced fixed silver nuclei while free silver ions produced free silver nuclei. Growth occurred after nucleation to decrease the large surface energy. Spontaneous aggregation occurred with large volumes, forming large particles. The cellulose fiber contained AgNPs upon drying but the fibers were stationary. The unfixed nanoparticles kept growing by agglomeration or Oswald ripening before reaching the cellulose fiber. Very large particles cannot be deposited tightly onto the cellulose surface so that they can be washed out by water/ethanol mixture. Because it is a low-temperature process, Ostwald ripening was suppressed to preserve small nanoparticles, which is preferable over
Fig. 8. (a) Ag 3d core level XPS spectrum of sample S2; (b) Raman spectra of the sample S2 and the paper. The characteristic bands of Ag2O locate at about 80, 146, 220, 342, 420, 487, 950, and 1195 cm−1, labeled by “+” symbols. There are also three peaks, labeled by “*” symbols, from the impurities of the paper, according to the Raman spectrum of the paper. 11
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Fig. 9. (a) and (c) show the UV–Vis absorption spectra recorded during the filtration process for sample S0 and S2, respectively. The corresponding relationships between the absorption intensity and filtration number are shown on the (b) and (d). (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
radicals, as shown in reaction (7), with subsequent generation of %HO2 radicals, and H2O2 molecules:
paper showed three characteristic XRD peaks at 2θ = 14.7°, 16.8° and 22.7°, corresponding to the assignments of (1 1 0) (110), and (200) planes, respectively. Therefore, the filter paper consisted of typical semi-crystalline α-cellulose structures. After loading the AgNPs, new peaks at 46.18° and 67.44° 2θ were noted corresponding to (111) and (220) planes of silver crystalline, respectively. As mentioned previously, the synthesized AgNPs can only be generated on the surface exposed to the plasma. As a result, the Ag peaks were minor, indicating that the amount of the AgNPs was low compared to cellulose. XPS of Ag3d was performed to investigate the bonding state of Ag. The structural evolution was studied by XPS and Raman spectroscopy, as presented in Fig. 8. The XRD patterns have indicated the existence of metallic Ag, of which the XPS peak locates at 368.1 eV. As shown in Fig. 8(a), the XPS spectrum indicated that there was also a minor peak of Ag2O, located at 367.7 eV. The Raman spectra, as presented in Fig. 8(b), showed a series of bands, which also identify the existence of Ag2O [36]. The Ag2O came from the oxidation of Ag, because energetic electron collisions on the water molecules led to the production of ·OH
OH + ˙OH → H2 O2
(10)
OH + H2 O2 → ˙HO2 + H2 O
(11)
Those species, which have very strong oxidizing potential, can oxidize Ag to Ag2O, whereas %OH radicals can react with ethanol molecules immediately:
OH + C2 H5 OH → CH3 CHOH + H2 O
(12)
Therefore, the addition of ethanol can suppress the oxidation reaction by consuming oxidizing species. In addition, the Ag2O generated could also be reduced back to Ag by CH3CHOH species. 3.4. Catalytic activities In aqueous medium, MB molecules show an absorption band at 664 nm with a shoulder at 614 nm. Fig. 9 shows variations in the 12
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intensity of the absorption band at 664 nm for samples filtered by S0 and S2 after several filtration processes under room temperature. The process took 80 s to finish the filtration process of the 10 mL solution. In the reaction mixture, a higher concentration of NaBH4 compared to MB was used, which resulted in an increase in pH of the entire system. This, in turn, retarded the degradation of BH4− ions. The liberated H+ ions produce a purging environment over the reaction mixture thereby effectively inhibiting the aerial oxidation of reduced MB products. A decrease in the absorption band was observed, but the paper turned blue after the filtration process, as shown in Fig. 9(a). Such a phenomenon is associated with the absorption effect of the filter paper. The porous structure and hydrophilic fiber surface allowed the filter paper to absorb MB molecules, which led to a decrease in the absorption of the filtered solution until the absorption of the filter paper was saturated. On the other hand, Fig. 9(c) shows the catalytic nature of the AgNPs in the reduction of MB by NaBH4. Almost no MB molecules were absorbed on the filter paper after the filtration process. This suggests that the deposited AgNPs can catalyze the reaction to completion after the 20th filtration process. The catalytic reduction aided by metallic nanoparticles was carried out effectively by transferring electrons from the electron donor species, BH4−, to electron acceptor species MB mediated by AgNPs, which reduces the activation energy and helps stabilize the system. The color of the AgNPs turned lighter after catalysis possibly due to the migration of AgNPs from the surface to the inner paper.
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4. Conclusion Silver nanoparticles were synthesized and loaded onto the upper surface of filter paper by a simple “immersing- discharging-washing” process. Ethanol was used as a green reagent for the production of reactive species under the plasma treatment. The reactive species have a high reducing potential to reduce silver ions. The hydrophilic surface of the paper plays as the role of fixing the nanoparticles for good dispersion. The synthesized AgNPs were hemispherical with a crystal structure. The functional groups did not participate in the reduction process and were well preserved after the plasma treatment. The NP-DBD plasma reduction process did not destroy the micro-skeletons of the paper. Metallic AgNPs were the main product with little oxidized Ag2O. The oxidative species were generated from the aqueous layer, but the ethanol molecules consumed the oxidative species to a certain extent, which impeded the oxidation reaction. The utility of filter paper allows the deposited AgNPs to catalyze the reduction of MB by NaBH4 through a straightforward filtration process. Acknowledgments This work was supported by the Regional Innovation Center for Environmental Technology of Thermal Plasma (RIC-ETTP) at Inha University. References [1] L. Di, J. Zhang, X. Zhang, A review on the recent progress, challenges, and perspectives of atmospheric-pressure cold plasma for preparation of supported metal catalysts, Plasma Process. Polym. 15 (2018) 1700234, , https://doi.org/10.1002/ ppap.201700234. [2] S. Agnihotri, S. Mukherji, S. Mukherji, Size-controlled silver nanoparticles synthesized over the range 5–100 nm using the same protocol and their antibacterial efficacy, RSC Adv. 4 (2014) 3974–3983, https://doi.org/10.1039/c3ra44507k. [3] X. Zhang, M.Z. Yates, Controllable synthesis of hydroxyapatite-supported palladium nanoparticles with enhanced catalytic activity, Surf. Coat. Technol. 351 (2018) 60–67, https://doi.org/10.1016/J.SURFCOAT.2018.07.075. [4] A. Wang, J. Li, T. Zhang, Heterogeneous single-atom catalysis, Nat. Rev. Chem. 2 (2018) 65–81, https://doi.org/10.1038/s41570-018-0010-1. [5] M. Männikkö, X. Wang, M. Skoglundh, H. Härelind, Silver/alumina for methanolassisted lean NOx reduction—on the influence of silver species and hydrogen formation, Appl. Catal. B Environ. 180 (2016) 291–300, https://doi.org/10.1016/J. APCATB.2015.06.002.
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