*Article* **Innovative Green Chemistry Approach to Synthesis of Sn2+-Metal Complex and Design of Polymer Composites with Small Optical Band Gaps**

**Shujahadeen B. Aziz 1,2,\* , Muaffaq M. Nofal <sup>3</sup> , Mohamad A. Brza <sup>4</sup> , Niyaz M. Sadiq <sup>1</sup> , Elham M. A. Dannoun <sup>5</sup> , Khayal K. Ahmed <sup>1</sup> , Sameerah I. Al-Saeedi <sup>6</sup> , Sarkawt A. Hussen <sup>1</sup> and Ahang M. Hussein <sup>1</sup>**


**Abstract:** In this work, the green method was used to synthesize Sn2+-metal complex by polyphenols (PPHs) of black tea (BT). The formation of Sn2+-PPHs metal complex was confirmed through UV-Vis and FTIR methods. The FTIR method shows that BT contains NH and OH functional groups, conjugated double bonds, and PPHs which are important to create the Sn2+-metal complexes. The synthesized Sn2+-PPHs metal complex was used successfully to decrease the optical energy band gap of PVA polymer. XRD method showed that the amorphous phase increased with increasing the metal complexes. The FTIR and XRD analysis show the complex formation between Sn2+-PPHs metal complex and PVA polymer. The enhancement in the optical properties of PVA was evidenced via UVvisible spectroscopy method. When Sn2+-PPHs metal complex was loaded to PVA, the refractive index and dielectric constant were improved. In addition, the absorption edge was also decreased to lower photon. The optical energy band gap decreases from 6.4 to 1.8 eV for PVAloaded with 30% (*v*/*v*) Sn2+ - PPHs metal complex. The variations of dielectric constant versus wavelength of photon are examined to measure localized charge density (*N*/*m\**) and high frequency dielectric constant. By increasing Sn2+-PPHs metal complex, the *<sup>N</sup>*/*m\** are improved from 3.65 <sup>×</sup> <sup>10</sup><sup>55</sup> to 13.38 <sup>×</sup> <sup>10</sup><sup>55</sup> <sup>m</sup>−<sup>3</sup> Kg−<sup>1</sup> . The oscillator dispersion energy (*E<sup>d</sup>* ) and average oscillator energy (*Eo*) are measured. The electronic transition natures in composite films are determined based on the Tauc's method, whereas close examinations of the dielectric loss parameter are also held to measure the energy band gap.

**Keywords:** Sn2+-PPHs metal complex; UV-Vis; XRD and FTIR analyses; optical property; bandgap analysis

#### **1. Introduction**

According to a recent study, the optical properties of polymer composites (PCs) have piqued the interest of a lot of academics because of their extensive application in a variety of sectors, including solar cells, optoelectronic device and light-emitting diode (LED) [1,2].

**Citation:** Aziz, S.B.; Nofal, M.M.; Brza, M.A.; Sadiq, N.M.; Dannoun, E.M.A.; Ahmed, K.K.; Al-Saeedi, S.I.; Hussen, S.A.; Hussein, A.M. Innovative Green Chemistry Approach to Synthesis of Sn2+-Metal Complex and Design of Polymer Composites with Small Optical Band Gaps. *Molecules* **2022**, *27*, 1965. https://doi.org/10.3390/ molecules27061965

Academic Editor: Tersilla Virgili

Received: 11 January 2022 Accepted: 8 March 2022 Published: 18 March 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Inorganic particles are commonly found in polymers, which are thought to be an outstanding host material. Studies on the optical characteristics of PVA based on metal complexes have been conducted in the literature [3,4].The insertion of inorganic particles into the host polymer might result in a significant alteration in the host's characteristics due to their high surface to bulk ratio [5].Green techniques have been widely reported as potential approaches for the synthesis of inorganic particles, with the results being shown to be safe and environmentally benign [6].

Alkaloids, amino acids, catechins, theavins, isomers of theavins, and other elements make polyphenols (PPHs) in black tea (BT). The most obvious molecular or chemical structures of the ingredients of BT have also been described in other investigations [7,8]. Dryan et al. [8] recently published a study which discovered that PPHs components are abundant in the BT aqueous mixture. PPHs conjugate, PPHs, and polymerized phenolic structure are the key elements of BT. In addition, black, green, and white tea all have a unique blend of conjugated flavonoids [9]. In earlier researches, it was found that extract solutions of black and green tea play a key role in lowering the polar polymers optical band gap including PMMA and PVA [10,11]. The extract tea solution contained PPHs, carboxylic acid groups, and hydroxyl group, according to the FTIR study [11]. As a result of the discoveries of experimental studies, the tea extract solution contains a large number of active ligands and functional groups, which are essential for complex formation with polymers and/or transition metal salts.

As a green technique, BT plant extract solutions can be utilized to synthesize Sn2+ - PPHs metal complexes. These solutions are high in PPHs, which have a significant interaction with the Sn2+ ion, forming a Sn2+-PPHs metal complex. Zielinski et al. described the primary ingredients and uses of tea leaves, for instance, PPHs and caffeine [12].

Earlier research has shown that functional groups and PPHs in tea extract solutions can capture the cations of heavy metals to crate metal complexes [3,4]. Metal complexes are combined with PVA polymer to create PCs with high-performance optical characteristics in the current work. This process is a new green technique to make PCs with adjustable optical band gaps. Electrical and optical properties of polymers have attracted researchers' interest in recent years due to their widespread application in optical systems and their superior interference, reflection, anti-reflection, and polarization capabilities [13].In recent studies, it has been discovered that PCs with low band gap energy (*Eg*) and large absorption play a key role in photonics and optoelectronic device applications [14]. Hasan et al. [15] reported that the use of nanotube-PCs in photonics is due to the composites' good optical absorptions, which cover a wide spectrum range from UV to near IR [15]. Organic–inorganic hybrid (PCs) serve as an active or passive layer in optoelectronic devices for instance large refractive index films, protective coatings, thin films, LEDs, solar cells, transistors, and waveguide materials play a vital role in various applications [16].

The goal of this research is to create PCs with a low energy bandgap (*Eg*). Because of the good optical properties, the green method might be used to make PCs with low *Eg*. The findings of this research can be regarded as a novel PCs approach. The optical dielectric function was accurately used in this study to experimentally detect the different types of optical transition between the conduction band and the valence band. Sn2+-polyphenol complex has a strong effect on the decrease of optical band gap in comparison with the other fillers for example nanoparticles (NPs). Aziz et al. [17] prepared PCs based on polystyrene. In their research, copper (Cu) powder was loaded into the polystyrene from 0 to 6 wt.%. Upon the incorporation of 6 wt.% Cu, the *E<sup>g</sup>* decreased from 4.05 to 3.65 eV. Aziz et al. [18], in another work, added copper monosulfide (CuS) NPs into methyl cellulose (MC) polymer to prepare polymer nanocomposites based on MC. The *E<sup>g</sup>* of MC decreased from 6.2 to 2.3 eV by the incorporation of 0.08 M of CuS NPs. In the current work, we observed that the *E<sup>g</sup>* decreased from 6.4 to 1.8 eV for PVA loaded with 30% (*v*/*v*) of Sn2+-polyphenol complex. Thus, based on the band gap analysis result, the green method is an appropriate for fabricating PCs with low value of *Eg*.

#### **2. Methodology** Sigma-Aldrich provided PVA powder (MW ranging from 85,000 to 124,000) and

**2. Methodology**  *2.1. Materials* 

#### *2.1. Materials* Tin(II) chloride (SnCl2) (MW = 189.6 g/mol). The BT leaf was bought from a nearby mar-

Sigma-Aldrich provided PVA powder (MW ranging from 85,000 to 124,000) and Tin(II) chloride (SnCl2) (MW = 189.6 g/mol). The BT leaf was bought from a nearby market. ket. *2.2. Sample Preparation* 

with 30% (*v*/*v*) of Sn2+-polyphenol complex. Thus, based on the band gap analysis result,

the green method is an appropriate for fabricating PCs with low value of *Eg*.

*Molecules* **2022**, *27*, x FOR PEER REVIEW 3 of 23

#### *2.2. Sample Preparation* The use of distilled water (D.W.) in the extraction of tea leaves is required. The steps

The use of distilled water (D.W.) in the extraction of tea leaves is required. The steps are as follows: In the absence of sunshine, 50 g of BT leaf was placed in 250 mL D.W.at almost 90 ◦C. The resultant extract solution was filtered by (Whatman paper 41, cat. no. 1441) with a pore radius (20 µm) to thoroughly remove the residues after standing for 10 min. 200 mL HCl was diluted into 400 mL of D.W. and then used it to dissolve 10 g of SnCl<sup>2</sup> in a separate flask. The Sn2+-PPHs metal complex was then made by adding SnCl<sup>2</sup> solution to the extract tea leaf solution and stirring for 10 min at 80 ◦C. The complexation between Sn2+-metal ions and PPHs was confirmed by the color change of the extract solution from dark to green at the top of the beaker and formation of sediment as clouds at the bottom of the beaker. The complex solution was allowed to cool to room temperature. These complexes were detached in 100 mL of D.W. after numerous washings of the Sn2+-PPHs metal complexes with D.W. The solution cast approach was used to produce composite samples made up of PVA loaded with Sn2+-PPHs metal complex. To begin, a PVA solution was made by adding 1 g of PVA to 40 mL of D.W., stirring for 1 h at roughly 80 ◦C, then cooling to room temperature. Different volumes of the complex solution, ranging from 0 to 30% (*v*/*v*), were added to the homogenous PVA solution in 15% (*v*/*v*) increments. The resulting solutions were stirred for approximately 50 min. PVSN0, PVSN1 and PVSN2 were used to represent 0% (*v*/*v*), 15% (*v*/*v*) and 30% (*v*/*v*) of the loaded complex solution, respectively. To cast the manufactured films, the contents of the mixture were poured into petri dishes and allowed to dry at ambient temperature. The samples were dried more using blue silica gel desiccant prior to characterization. Pure PVA and composite films have thicknesses ranging from 0.012 to 0.015 cm. A pictorial sample preparation of PCs consists of Sn2+-PPHs metal complex and PVA is shown in Scheme 1. are as follows: In the absence of sunshine, 50 g of BT leaf was placed in 250 mL D.W.at almost 90 °C. The resultant extract solution was filtered by (Whatman paper 41, cat. no. 1441) with a pore radius (20 µm) to thoroughly remove the residues after standing for 10 min. 200 mL HCl was diluted into 400 mL of D.W. and then used it to dissolve 10 g of SnCl2 in a separate flask. The Sn2+-PPHs metal complex was then made by adding SnCl2 solution to the extract tea leaf solution and stirring for 10 min at 80 °C. The complexation between Sn2+-metal ions and PPHs was confirmed by the color change of the extract solution from dark to green at the top of the beaker and formation of sediment as clouds at the bottom of the beaker. The complex solution was allowed to cool to room temperature. These complexes were detached in 100 mL of D.W. after numerous washings of the Sn2+-PPHs metal complexes with D.W. The solution cast approach was used to produce composite samples made up of PVA loaded with Sn2+-PPHs metal complex. To begin, a PVA solution was made by adding 1 g of PVA to 40 mL of D.W., stirring for 1 h at roughly 80 °C, then cooling to room temperature. Different volumes of the complex solution, ranging from 0 to 30% (*v*/*v*), were added to the homogenous PVA solution in 15% (*v*/*v*) increments. The resulting solutions were stirred for approximately 50 min. PVSN0, PVSN1 and PVSN2 were used to represent 0% (*v*/*v*), 15% (*v*/*v*) and 30% (*v*/*v*) of the loaded complex solution, respectively. To cast the manufactured films, the contents of the mixture were poured into petri dishes and allowed to dry at ambient temperature. The samples were dried more using blue silica gel desiccant prior to characterization. Pure PVA and composite films have thicknesses ranging from 0.012 to 0.015 cm. A pictorial sample preparation of PCs consists of Sn2+-PPHs metal complex and PVA is shown in Scheme 1.

**Scheme 1. Scheme 1***.* Schematic diagram of sample preparation. schematic diagram of sample preparation*.* 

#### *2.3. Measurement Techniques*

X-ray diffraction (XRD) patterns were analyzed at room temperature using a Bruker AXS diffractometer (Billerica, MA, USA) with a 40-kV voltage and 45-mA current. The composite films were examined using a Nicolet iS10 FTIR spectrophotometer (Perkin Elmer,

Yokohama, Japan) with a resolution of 2 cm−<sup>1</sup> in the range of 450 and 4000 cm−<sup>1</sup> . A Jasco V-570 UV-vis-NIR spectrophotometer (JASCO, Tokyo, Japan) was used to record the samples UV-vis absorption spectra. For measuring UV-Vis for the liquid samples (Sn2+-PPHs complexes), firstly, two cuvettes filled with distilled water were used for correcting background and then one of the cuvettes was removed while another cuvette was left and used as a reference sample. The absorbance of the liquid samples (Sn2+-PPHs complexes) was measured in comparison to the reference sample. For measuring FTIR for the liquid samples (Sn2+-PPHs complex), Sn2+-PPHs complexes were coated on the standard glass slides and then dried at room temperature until evaporated. The dried Sn2+-PPHs complexes were scratched on the glass slides to create powders form. Then potassium bromide (KBr) (100 mg) was added to the Sn2+-PPHs (1 mg) powders and then the powders were combined in a mortar and finally turned to pellets in a sample holder.

When Sn2+-PPHs complexes were added to the dissolved PVA, PVA composite films were created. For measuring UV-Vis and FTIR for the solid composite films, firstly the UV-Vis spectroscopy and FTIR devices with air and without any samples were corrected for background and then the UV-Vis and FTIR spectra were measure for the composite films.

#### **3. Results and Discussion**
