J Inorg Organomet Polym DOI 10.1007/s10904-017-0632-7
Crosslinked PVA/PVP Supported Silver Nanoparticles: A Reusable and Efficient Heterogeneous Catalyst for the 4-Nitrophenol Degradation Wael H. Eisa1 · T. A. Abdel‑Baset2 · Elalyaa M. A. Mohamed2 · S. Mahrous2
Received: 25 April 2017 / Accepted: 24 July 2017 © Springer Science+Business Media, LLC 2017
Abstract Well-organized and stable silver nanoparticles (AgNPs) were successfully prepared within polymeric film of polyvinyl alcohol and Polyvinylpyrrolidone as supporting matrix. The free-standing film was obtained by using the casting technique. Electron microscopy techniques confirmed the formation of spherical AgNPs of 15 nm size. The prepared AgNPs were crystallized in face-centered cubic structure as indicated by X-ray diffraction. The asprepared nanocomposite film exhibited good catalytic properties in the complete catalytic degradation of 4-nitrophenol using sodium borohydride as a reducing agent. The AgNPs are tightly held within a polymeric matrix which facilitates their recovery and reuse for several cycles. Keywords Silver · Polyvinyl alcohol · Polyvinylpyrrolidone · Nanocomposite · Catalyst
1 Introduction Silver nanoparticles (AgNPs) have attracted a lot of researchers’ attention due to their unique physical and chemical properties. AgNPs are widely used in many applications in different fields like electronics , anti-microbial agents , sensing applications , energy conversion , and water purification . As the demand of using silver
* Wael H. Eisa [email protected]
Physics Division, Spectroscopy Department, National Research Centre (NRC), Cairo, Egypt
Physics Department, Faculty of Science, Fayoum University, El Fayoum, Egypt
nanoparticles in many applications is increasing the need for cheap, easy and safer synthesis methods is increasing. AgNPs show outstanding catalytic efficiency in various organic reactions, such as the oxidation of olefins  reduction of nitro compounds , oxidation of phenylsilanes , Diels–Alder cycloadditions  and organic dyes . The AgNPs-based catalyst is to somewhat more favored as compared with other metals such as gold, palladium, and platinum because of the low cost and ease of preparation. However, the practical use of AgNPs in catalytic reactions is faced with a serious obstacle which is the lake of an efficient strategy for recovery and reuse. In this regard, the efforts of the researchers were focused on loading the AgNPs to solid matrices such as metal oxides [11–13], carbon nanotubes , and silica spheres . All these approaches suffered from multi-steps preparation, expensive precursors, and the need for magnetic separation, filtration and/or centrifugation for the catalyst recovery. Thus, the development of a solid catalyst which is easy to prepare, inexpensive, recoverable and reusable remain a challenging area of research. Polymeric matrices supported AgNPs represent an effective pathway to overcome this problem. The literature is witnessed for the large varieties of synthetic routes for loading of AgNPs to the polymeric matrices [16–21]. These synthetic routes were processed in liquid environments. Hence, the obtained nanoparticles suffer from instability problems and can not be stored for long intervals due to aggregation, agglomeration, and precipitation. In addition, the presence of nanoparticles in solution and in powder form limits their real applications. In our preceding work, a polymeric film of PVA/PVP loaded with monodispersed and well distributed AgNPs was prepared successfully without the need for surfactant and/or reducing agents . In this method, the reduction
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of the silver ions was performed in the solid phase which provides high restriction against the growth and aggregation of the reduced nanoparticles. In addition, the delay of the reduction process after film drying was found to achieve homogenous dispersion and narrow particle size distribution of nanoparticles within the polymeric matrix. However, the main disadvantage of this route was the too long time needed for the reduction and growth of AgNPs which was reached to 40 days. In addition, the polymeric matrix of PVA/PVP is known to be soluble in water which represents another obstacle for efficient removal of the catalyst from water. This work is concerned with making low-cost polymeric films loaded with silver nanoparticles with efficient catalytic activity for the degradation of 4-nitrophenol (4-NP). The present method overtakes the disadvantages of matrix solubility and slow AgNPs reduction. The polymeric film was crosslinked using glutaraldehyde which limited and/or prevented the solubility of the film. The AgNPs were reduced effectively and quickly by dipping the polymeric film within N aBH4 so the issue of time-consuming was treated. The crosslinked PVA/PVP/ Ag offers several benefits as potential catalyst including using cheap matrices, simplified preparation steps, easy to recover by handling, and fewer environmental problems.
2.2 Characterization of the Nanocomposite Films
swelling% = (m2 /m1 ) × 100
2.1 Preparation of PVA/PVP/Ag Nanocomposite Film
Gel% = (m3 /m1 ) × 100
Polyvinyl alcohol, (PVA, Mwt=31,000 g/mol, Roth GmbH) and Polyvinylpyrrolidone, (PVP, M wt=40,000 g/mol, Molekula, United Kingdom) were dissolved separately in deionized water to prepare solutions of polymer/water with a concentration of 6% (w/v). Equal volumes of PVA and PVP were mixed using a magnetic stirrer till the solutions become homogenous. 10 mL of the above mixture was acidified with 10 µL of (0.1 M) H NO3. Thereafter, 250 µL of AgNO3 (0.1 M) was added to the polymeric solution under continuous stirring. 10 µL of 2.5% glutaraldehyde solution was added as a crosslinking agent and the stirring process was continued for 2 h. The metallo-polymeric solution was poured onto dried glass petri dish and the solvent was allowed to evaporate at room temperature. After drying, the as-prepared film was peeled from petri dish to obtain free-standing film. Finally, the obtained film was dipped in an ice-cold solution of 0.1 M sodium borohydride (NaBH4, Merck Co.) for 10 s. Thereafter, the nanocomposite film was washed thoroughly using distilled water to get rid off the uncrosslinked and/or unreacted chemicals.
UV–Vis absorption spectra of the films were recorded on Jasco V-630 UV–Vis spectrophotometer. The FTIR analysis was performed on a Jasco spectrometer of the model FT/IR 6100. The size and shape of the nanoparticle were studied by scanning electron microscopy (Quanta FEG 250 electron microscope) equipped with EDX unit and transmission electron microscopy (TEM), using Jeol JeM-2100. X-ray diffraction (XRD) of the type Brucker axis D8 instrument was used to perform the crystalline study and to determine phase structures of the samples. Thermal gravimetric analysis (TGA) was studied using TGA Q500, TA Instruments. The silver concentration was determined by using atomic absorption spectrometer of the type Varian SpectrAA (220). 2.3 Swelling/Gel Ratio Measurements The classical gravimetric method was used to evaluate the swelling and gel percentages of the nanocomposite film. The nanocomposite film of mass (m1) was soaked in distilled water at room temperature. The polymeric film was removed from water and the excess water on their surface was wiped quickly by filter papers and weighed again (m2). Finally, the film was dried in an oven at 50 °C for 24 h and the weight was measured (m3). The following relations was employed
2.4 Catalytic Experiment A rectangular piece of nanocomposite film was soaked in quartz cuvette containing a solution of 5 × 10−5 M 4-nitrophenol (4-NP, Loba Chemie, India) and 5 × 10−2 M NaBH4. The degradation of 4-nitrophenol to 4-amino phenol (4-AP) was monitored through vanishing of the absorption peak at 400 nm recorded by the successive UV–Vis spectra.
3 Results and Discussion 3.1 UV–Vis Absorption Analysis The growth of AgNPs within PVA/PVP matrix was validated by visually observing the polymeric films containing AgNO3 before and after dipping within NaBH4 solution for 10 s. Before dipping, the polymeric film was observed to retain its transparent color, whereas it turned
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to brownish yellow color after dipping. The color variation of the polymeric film from transparent to yellow appearance is considered as a clear indication of the formation of AgNPs within the polymeric matrix . Figure 1 show the UV–Vis spectra of PVA/PVP loaded with AgNO3 before and after dipping within NaBH4. Before dipping, nearly zero absorption in the visible range (400–700 nm) was recorded i.e. the two types of polymers (PVA and PVP) were unable to reduce the Ag+ ions under these conditions. When the polymeric films were brought in contact with the reducing agent N aBH4, a strong and symmetric absorption peak was recorded at 437 nm indicating the formation of AgNPs. The color change was observed directly after dipping the polymeric film into N aBH4 solution. The 437 nm absorption peak is attributed to the surface plasmon resonance (SPR) of AgNPs . In the light of Mie theory, spherical AgNPs show an absorption spectrum with only single SPR peak, whereas multiple peaks are recorded as the anisotropy increases. Herein, SPR band points out that the prepared AgNPs are spherical in shape which is further assured by TEM analysis. The size of the AgNPs is calculated using modified Mie theory as follow :
𝛾(R) = 𝛾0 + (A𝜈F )/R where γ(R) is SPR broadening, A is scattering process (3/4 for Ag), γo is the bulk velocity scattering (5 × 1012 s−1 for Ag) and νF is the Fermi velocity (1.39 × 106 ms−1 for Ag). The calculated size of the prepared AgNPs is 12 nm.
3.2 SEM and TEM Analysis The morphology of the AgNPs is given in Fig. 2a. The SEM micrograph obviously shows a number of homogenously distributed grains with smaller sizes and semispherical shaped particles. EDX spectrum (Fig. 2b) shows that the major component of the sample is carbon (73.11%) compared to oxygen (24.88%) and silver (2.01%). AgNPs show typical peak approximately at 3 keV as reported previously . To confirm the distribution of AgNPs in the surface of the PVA/PVP film, elemental mapping of SEM was carried out and displayed in Fig. 2c–f. The collected images displayed the elemental mapping in different colors for Ag (green) and C (red). The images of elemental mapping confirmed the uniform distribution of AgNPs in the polymeric film. The external shape and size distribution of the as-prepared AgNPs are further demonstrated using TEM micrographs as displayed in Fig. 3. A number of well distributed AgNPs could be seen throughout the whole images. The external shape of AgNPs is spherical with an average diameter of 15 nm (specified as the peak position of the Gaussian fitting of the histogram). The narrowness of the Gaussian peak gives strong evidence to a uniform shape and a very narrow size distribution of the as-prepared AgNPs. It is worth to mention that the TEM micrograph is nearly free from aggregation and/or agglomeration i.e. the AgNPs are to a large extent well-separated from one another. 3.3 XRD Analysis
Fig. 1 The UV–Vis absorption spectra of PVA/PVP loaded with AgNO3 before and after dipping into NaBH4 solution
The formation, crystalline structure, and mean diameter of Ag nanoparticles were also studied by XRD technique. Figure 4 showed the XRD patterns of PVA/PVP and PVA/ PVP/Ag nanocomposites. XRD pattern of PVA/PVP film showed the existence of reflexes at 2θ = 19.31° and 22.21°, which can be indexed to PVA and PVP matrices, respectively. The presence of such broad reflexes was attributed to the semi-crystalline nature of the two polymers. The remarkable structural changes in terms of the appearance of new peaks were observed for PVA/PVP/Ag nanocomposite diffraction pattern as shown in Fig. 4. Four new diffraction peaks were recorded at 2θ = 38.13°, 44.2°, 65.6°, and 77° representing the (111), (200), (220), and (311) Bragg’s reflections, respectively. All the diffraction peaks can be indexed to FCC crystalline structure of Ag nanoparticles on comparing with the standard JCPDS No. 04-0783. The broadening of the XRD peaks is associated to the small grain size and low degree of crystallinity. In comparison with bulk material, the nanocrystals have lesser lattice planes, which contribute to the peak width in the diffraction pattern .
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Fig. 2 a SEM, b EDX and the elemental mapping of PVA/PVP/Ag nanocomposite film c reference image, d Ag, e C, and f the overlay of the Ag and C maps
The average crystallite diameter (D) of Ag nanoparticles was estimated using Debye–Scherrer relation as follows, D = 0.89𝜆∕𝛽 cos 𝜃, where λ is the X-rays wavelength, θ is the diffraction angle and β is the full width at
half maximum (FWHM; in radians) . The crystallite size was found to be 13 nm based on the broadening of the diffraction peak at 2θ = 38°.
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3.4 FTIR Analysis The FTIR spectra of PVA/PVP and PVA/PVP/Ag were displayed in Fig. 5. For PVA/PVP, the broad band at 3378 cm−1 is assigned to the stretching mode of –OH group of the polymeric chain . The two absorption bands at 2935 and 2860 cm−1 were attributed to the asymmetric and symmetric stretching vibration of C–H band. The intense absorption band at 1660 cm−1 was assigned to stretching vibration of C=O groups of PVP . The band at 1438 cm−1 attributed to the in-plane swaying vibration of C–H group. The band at 1286 cm−1 was assigned to the stretching vibration of C–N groups of tertiary amid present in the PVP backbone . The double peaks at 1110 and 1042 cm−1 corresponding to the C–N stretching . Comparing the spectra of PVA/PVP/Ag with that of PVA/PVP, the band of C=O was red shifted from 1660 to 1647 cm−1. The band intensity of C–N was weakened and shifted from 1042 to 1022 cm−1. The spectral changes suggested that the polymeric chains could be coordinated with AgNPs through N and/or O atoms. 3.5 TGA Analysis
Fig. 3 a TEM micrographs and b histogram of PVA/PVP loaded AgNPs obtained after dipping into NaBH4 solution for 10 s
Fig. 4 XRD patterns of PVA/PVP with and without loading of Ag nanoparticles
Figure 6 showed TGA and differential gravimetric analysis (DTG) thermographs of PVA/PVP and PVA/PVP/ Ag composite polymeric films during heating from 30 to 500 °C at a heating rate of 10 °C/min. TGA and DTG thermographs of the two polymeric films exhibited three main weight loss steps. The first degradation step for PVA/PVP film noticed at a temperature range of 30–130 °C with a weight loss of 8.5 wt% and a peak at 75 °C in the DTG curve. This could be attributed to the removal of residual
Fig. 5 FTIR spectra of PVA/PVP and PVA/PVP/Ag nanocomposite films
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Fig. 7 The degree of swelling of PVA/PVP versus PVA/PVP/Ag nanocomposite. The inset is the gel% of the two samples
Fig. 6 a TGA and b the derivative curves of the PVA/PVP and PVA/ PVP/Ag nanocomposite films heated from 30 to 600 °C
solvent i.e. the elimination of physically and chemically bounded water from the polymer matrix [32, 33]. In the second degradation step (250–380 °C), a total weight loss of about 61 wt% was observed with a sharp peak at 347 °C in the DTG curve. This is due to the structural decomposition of the polymeric system. Finally, the third degradation step (380–440 °C) showed a sharp peak at 411 °C and a total weight loss of about 97.5 wt% which was attributed to the disintegration of the carbon skeleton of the polymeric chains known as carbonation [34, 35]. In the case of PVA/PVP/Ag film, the first degradation step (30–130 °C) showed a weight loss of 8.5 wt%, just like PVA/PVP, while the DTG peak was shifted to 85 °C as compared with that of PVA/PVP. The second degradation step started earlier than that of PVA/PVP i.e., it was recorded at a temperature range of 190–350 °C with a total weight loss of about 52 wt%. Also, the DTG peak was shifted to the lower temperature side by 17 °C and was located at 330 °C. The presence of mineral acids within PVA matrix, lower the activation energy and consequently
catalyzes the thermal degradation . Hence, the reason behind the fast degradation rate of the polymeric film in the second stage could be attributed to the presence of HNO3 acid within PVA/PVP film during the preparation process. The DTG peak of the third stage (350–430 °C) was located at 399 °C i.e., 12 °C less than the corresponding peak in the case of PVA/PVP film. At the end of the heating course, the total weight loss was about 86%. The presence of Ag nanoparticles is responsible for minimizing the total weight loss of the PVA/PVP/Ag film. Hence, the thermal stability of polymeric film was enhanced as a result of growth the Ag nanoparticles within the PVA/PVP matrix. 3.6 Swelling, Gel, and Atomic Absorption Measurements PVA and PVP are water soluble polymers. When brought in contact with water, PVA/PVP/Ag film swelled and then dissolved in the aqueous medium. Hence, glutaraldehyde was added as a crosslinking agent in order to prevent the solubility of the polymeric matrix as well as to tightly held the AgNPs within the polymeric matrix [37, 38]. The swelling profiles the crosslinked polymeric matrices (Fig. 7) showed that the two samples under investigation had a high degree of swelling. The swelling degree of PVA/PVP/Ag nanocomposite was higher than that of PVA/PVP. The sorption rate was very fast in the first 2 h; thereafter the equilibrium was reached with a swelling degree ca. 414 and 481% for PVA/PVP and PVA/PVP/Ag, respectively. The gel% obtained from the two samples was very close and both of them were greater than 98%. These results indicated the
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success in producing crosslinked matrix which is highly resistant to solubility in water. In addition, the polymeric matrix is still capable of absorbing water which gives the opportunity for contact between AgNPs and reactants in the catalytic reaction. The atomic absorption data confirmed the presence of minor traces of Ag (0.73 mg/L) after immersion of PVA/ PVP/Ag nanocomposite film within deionized water for 4 h. This could give a clear evidence for the fixation of Ag particles within the crosslinked polymeric chains. 3.7 Catalytic Activity The catalytic degradation of 4-NP using borohydride ions (BH4−) as a reducing agent and Au nanoparticles as a catalyst is considered as the model reaction for estimating the catalytic activity of the metallic nanoparticles . Thermodynamically this reaction is suitable; however, in absence of a catalyst, it is kinetically restricted. This is due to the kinetic barrier between the mutually repelling negative ions (4-NP) and ( BH4−) is very high. Figure 8a showed the absorption spectra of 4-NP before and after addition of N aBH4. Before the addition of reducing agent, 4-NP showed a strong absorption at 317 nm, this peak underwent a red shift to 400 nm after addition of NaBH4 due to the formation of 4-nitrophenolate ions [40, 41]. The 4-nitrophenolate peak was highly stable with no considerable change in its profile and/or position for several days, confirming that the reduction does not occur without a catalyst. After dipping PVA/PVP/Ag nanocomposite film within reaction mixture, the 400 nm peak faded gradually and simultaneously with the appearance of a new peak at 294 nm. The solution color turned from yellow to colorless which was taken as a sign for the reduction of 4-NP to 4-AP (see Fig. 8b). The nanocomposite film was easy handled, ejected from the solution, and washed for the next run of catalysis. Hence, PVA/PVP/Ag nanocomposite film could be effectively recycled and reused for at least three times. In presence of excess N aBH4, the catalytic degradation reaction followed pseudo-first-order kinetics. Hence, the reaction rate (k) was determined from the plot of ln(At/Ao) against the time (t) where A0 and At are the concentration at time 0 and t, respectively. Figure 8c showed a linear relationship between ln(At/Ao) and t for the three successive cycles of the catalytic reaction. The rate constants were calculated from the slop of the straight lines and were found to equal 1.23 × 10−3, 7.15 × 10−4, and 5.4 × 10−4 sec−1 for the first, second, and third cycle, respectively. Figure 9 displayed the SEM micrograph of recycled PVA/PVP/Ag nanocomposite film after using for three successive cycles of degradation reaction. It could be seen that the AgNPs maintain their homogenous distribution on the PVA/PVP surface without considerable change. Herein, the
Fig. 8 a UV–Vis absorption spectra of 4-NP before and after immediate addition of NaBH4, b time-evolution of 4-NP degradation over PVA/PVP/Ag nanocomposite film, c Plot of ln(At/A0) against reaction time
polymeric matrix uses its chains to fix AgNPs and prohibit them from leaching out and/or aggregation.
4 Conclusions PVA/PVP/Ag nanocomposite film exhibiting well-organized structures were successfully prepared by simple casting technique. Spherical AgNPs of 15 nm size were synthesized using NaBH4 as a reducing agent. The addition of glutaraldehyde as crosslinking agent enabled the prepared film to swell but protect it from dissolving in water. The
Fig. 9 The SEM micrograph of recycled PVA/PVP/Ag nanocomposite film
atomic absorption measurements proved the successful immobilization of AgNPs within the polymeric matrix. These properties of PVA/PVP/Ag film paved the way for its application as an efficient heterogeneous catalyst in the degradation of 4-NP. The nanocomposite film could be used in the catalytic degradation with considerable activity for three cycles.
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