Green synthesis of dual-surface nanocomposite films using Tollen’s method

2.1 Method of preparation

The materials used for this study are: PVA (99–100% hydrolyzed), silver nitrate (Avonvhem, UK), β-D-glucose (Interchem, UK) and distilled water. The PVA bulk solution was prepared by dissolving PVA powder (5 wt%) in distilled water under controlled temperature at 90°C and continued stirring for 3 h. After the solution cooled down to ambient temperature, Tollen’s reagent solution (0.1 M Ag) was added with stirring and then a solution of β-D-glucose as the reducing agent was added dropwise and stirred. The final ratio of Ag to glucose is 1/2 mole. The blend solution was poured into Petri dishes, 10 ml/dish, and allowed to dry to form the films by casting under ambient temperature for 1 day in a dark room. Tollen’s reagent was prepared by adding drops of NH₄OH to a solution of AgNO₃ until a brown precipitate formed and continuing to add NH₄OH until the solution became clear.

2.2 X-ray analysis

The XRD pattern was recorded in a continuous scanning mode at room temperature on an Empyrean diffractometer system operated at 45 kV and a current of 30 mA using a Cu tube with Cu Kα radiation (λ=1.5406 Å). The diffraction intensities were recorded from 4° to 80°, in 2θ angles in a step of 0.026° with a scan step time of 19 s. The diffraction intensity was compared with the standard inorganic crystal structure database (ICSD) collection files. SEM for samples was performed using a Quanta 250 field emission gun scanning electron microscope with an EDX unit attached, with an accelerating voltage of 30 kV and magnification 14× up to 1,000,000.

3.1 Mechanism for the formation of nanocomposite films with two surface layers

The basic method for synthesis of nanoparticles in PVA is to disperse the metal ion solution in the polymer and reduce them to a zero valent state [44].

In the case of using Tollen’s reagent, Ag(NH₃)²⁺ ions are reduced by β-D-glucose in the presence of ammonia, yielding Ag nanoparticles. Here, excess ammonia and Ag nitrate are reacted in the presence of the aldehyde, which oxidizes to form carboxylic acid and release Ag, which deposits on the inner surface of the glass container wall to give an Ag mirror [45]. The Ag mirror test is commonly used to detect the presence of an aldehyde group in organic compounds. The general reaction can be written as:

Following this reaction, the process was modified to get Ag nanoparticles. The proposed reaction mechanism for this reaction is very similar to that of the Ag mirror test. When the ammonia is dissolved in water, it withdraws hydrogen ions by leaving hydroxyl ions in solution. The ammonium ion further reacts with Ag nitrate to form a complex of Ag. The remaining hydroxyl ions oxidize the aldehyde group here in glucose to form gluconic acid and an electron is released in the process. This electron reduces the Ag complex to get metallic Ag [45]:

The Tollen’s synthesis method gives Ag nanoparticles with a controlled size in a one-step process [31]. PVA is appropriate as a stabilizer and polymeric medium for reducing the AgNO₃ using β-D-glucose as a reducing agent. PVA is a bio-friendly polymer since it is water soluble and has extremely low cytotoxicity [46].

The blend solution was poured into Petri dishes. The gradual color change of Ag in PVA solutions from colorless to light brown and then to brown and finally dark brown, indicated the formation of Ag nanoparticles in the bulk of the PVA solution which finally formed Ag layers on the inner side of the glass of the Petri dish. The blend solution was allowed to dry to form the nanocomposite films with dual-surface, (a) Ag nanoparticles layers and (b) Ag-PVA nanocomposite bulk layers, by casting under ambient temperature as shown in Scheme 1. The new in this technique in comparison to other techniques [44] is the formation of Ag nanocomposite films with dual-surface.

Scheme 1

Nanocomposite films with dual-surface.

3.2 XRD

XRD has a good potential for the analysis of nanostructures, because the width and shape of reflections yield information about the substructure of the materials such as crystal structure, crystallite size and strain [42]. The XRD of a pure PVA sample shown in Figure 1 exhibits a broad diffraction peak located at 19.66°. The broadening of the peaks is due to the amorphous nature of the polymer.

Figure 1

X-ray diffraction (XRD) of polyvinyl alcohol (PVA).

The XRD patterns nanocomposite films with dual-surface, Ag layers and Ag-PVA bulk layers, prepared by Tollen’s reagent are presented in Figures 2 and 3, respectively. XRD shows the prominent peaks at 2θ values of about 38°, 44°, 64° and 77°. This observation is a confirmation of reduction of Ag⁺ ions to metallic Ag [47]. No peaks of other impurity crystalline phases were detected. There was no diffraction peak for Ag₂O suggesting that no oxidation occurred during synthesis. The XRD peaks exactly matched 111, 200, 220 and 331 crystal planes of Ag with a face-centered cubic (FCC) crystal structure in the ICSD collection (code: 53762) as shown in Table 1.

Figure 2

X-ray diffraction (XRD) of silver layers of the nanocomposite films.

Figure 3

X-ray diffraction (XRD) of Ag-polyvinyl alcohol (PVA) bulk layers of the nanocomposite films.

The degree of crystallinity determines the amount of crystal phase (Ag nanoparticles) in the amorphous polymer (PVA), which can be calculated by dividing the total area of crystalline peaks by the total area of all peaks. The crystallinity of Ag layers was found to be 82%, indicating a higher concentration of Ag nanoparticles on the Ag layers. The crystallinity of Ag-PVA bulk layers was 59%, which indicates a lower concentration of Ag nanoparticles on the Ag-PVA bulk layers.

The Sherrer diffraction formula was used to estimate the crystallite domain size (D):

D=kλβcosθ

where D is the crystallite size in (nm), k is the so-called shape factor which usually takes a value of about 0.94, λ is the X-ray wavelength, (CuKa´=1.5406 Å), β is the full width half maximum (FWHM) of the main diffraction peak and θ is the diffraction angle corresponding to the maximum intensity peak in the XRD pattern. The crystallite size calculation from the FWHM of the (111) peak using Scherrer’s equation resulted in average crystallite sizes of about 43 nm for the Ag layers and about 29 nm for the Ag-PVA bulk layers.

3.3 EDX analysis

EDX microanalysis is a technique used for identification of the elemental composition of a specimen. The “fingerprint” energies of the emitted X-rays can then be used to identify an element. Also, EDX microanalysis is capable of generating a map of one or more chemical elements of interest [43]. Analysis of nanocomposite films with dual-surface with an EDX spectrometer confirmed the presence of elemental Ag signals of the Ag nanoparticles at 2.6 keV, 3.0 keV, 3.2 keV and 3.4 keV, which correspond to the binding energies of Ag lines. The EDX analysis of Ag layers represents the element percentage (%) of Ag, C and O as 93.06, 0.003 and 6.93, respectively, (Table 2 and Figure 4B), and indicates a higher concentration of Ag nanoparticles on the Ag layers. EDX analysis of the Ag-PVA bulk layers represents the element percentage (%) of Ag, C and O as 10.80, 24.11 and 65.06, respectively, (Table 2 and Figure 4A), which indicates a lower concentration of Ag nanoparticles on the Ag-PVA bulk layers. The carbon and oxygen in the examined samples are attributed to the matrix of PVA. The structure and morphology of the two surfaces were confirmed by SEM.

Figure 4

Energy dispersive X-ray (EDX) analysis of (A) Ag-polyvinyl alcohol (PVA) bulk layers and (B) silver layers.

3.4 SEM

The SEM images of the nanocomposite films with dual-surface are presented in Figures 5 and 6, respectively. SEM of the Ag layers showed concentrated and agglomerated nanoparticles of spherical shape with a diameter range of 30 nm-60 nm. SEM of Ag-PVA bulk layers showed that the Ag nanoparticles formed were predominantly cubical, uniform shape with no agglomeration and well-defined morphology observed in the micrograph. The average diameter of the Ag nanoparticles was found to be approximately 200 nm–350 nm on the Ag-PVA bulk layers. The function of PVA in the Ag-PVA nanocomposites prevents the process of agglomeration of Ag nanoparticles. The growth of the nanoparticles distributed uniformly between the networks of the PVA hydrogel.

Figure 5

Scanning electron microscopy (SEM) of silver layers of the nanocomposite films.

Figure 6

Scanning electron microscopy (SEM) of Ag-polyvinyl alcohol (PVA) bulk layers of the nanocomposite films.

XRD, EDX and SEM analysis confirmed the formation of nanocomposite films with dual-surface, Ag layers and Ag-PVA bulk layers shown in Scheme 1. Formation of Ag layers could be explained by an increase in reaction time, particle size and aggregation of Ag nanocrystals gradually increased together. Inside the beaker or the inner surface of the glass of a Petri dish finally turned into an Ag mirror. Ag particles deposited on a glass substrate to be Ag layers as shown in Scheme 1. Ag layers were formed very easily because the wetting-contact angle between two phases (substrate and solution) was greater than zero and reduced the energy needed. The Tollen’s reaction occurred as a heterogeneous nucleation [48].