Synthesis and Nanostructure Characterization of Azo-Polymer, Modified with Trichloroethylene, Containing Metal-Oxide Nanoparticles for Improved Optoelectronic Properties

INTRODUCTION
Nanotechnology is an evolving sector of science and engineering that investigates the ability to manipulate and control the properties of materials at the nanoscale, between 1-100 nm [1, 2]. Nanosized materials exhibit distinctly different behaviors than bulk-scale substances. The reason for this lies in extensive surface exposure, quantum effects, and altered atomic/electronic structures [3]. Nanomaterials have been greatly studied, with metal oxide being one of the most studied types of this category. Nano-sized metal oxides possess a wide range of properties, they demonstrate high structural strength, and are applicable in numerous ways [4]. Employing the nano-science can effectively improve the physic-chemical properties of metal oxides, making them promising in a wide range of applications such as catalysis [5], energy storage [6], environmental protection [6], and biomedical engineering [7]. The increase in the surface-to-volume ratio of metal oxides is one of the greatest benefits of nanotechnology. In bulk metal oxides, the majority of the atoms are located inside the bulk structure; however, in metal oxide nanoparticles, a large percentage of the atoms are located on the surface [8]. This increase in surface area has a strong effect on the physical properties of metal oxides, including: electrical conductivity, thermal conductivity, optical absorption, and mechanical strength [9, 10]. For example; nano-metal oxides such as TiO2 and ZnO showed improved optical properties against their bulk states, offering the great transparency and absorption of light, which can be an option for photocatalytic and photovoltaic devices [11-13]. The morphology and crystal structures of metal oxides are major factors in in altering their physical behavior [14]. To date, a series of nano-metal oxides having different morphologies including nanoparticles, nanowires, nanotubes, nanosheets and thin films have been achieved through employing different advanced synthesis techniques such as sol-gel processing, hydrothermal, co-precipitation, sonochemical and chemical vapour deposition [15, 16]. The morphological characteristics of metal oxides impact the electrochemical properties in lithium-ion batteries [17, 18], supercapacitors [19], and photoelectrochemical cells [20]. Nanotechnology not only affects the physical attributes of metal oxides; but also it has a very strong effect on their chemical attributes. Of all the metal oxide nanomaterials that researchers have investigated, ZnO and TiO2 are particularly prominent. Due to their excellent chemical stability and tunable electronic properties, these compounds have been used in photocatalysis [21, 22], sensors [23, 24], energy storage devices [25, 26], and biomedical applications [27, 28]. Although both are wide-bandgap semiconductors, they have very different phase behavior, structural characteristics, and morphology-dependent properties that impact their performance for different applications. The hexagonal wurtzite structure (space group P63mc) is the only naturally occurring form of ZnO and is the most stable crystal phase under standard conditions [29]. Previous studies indicated that alternative crystal forms (e.g., cubic zinc blende and cubic rocksalt ZnO) can be produced under extreme synthesis conditions; however, these other phases of ZnO are typically metastable and not common [30]. On the other hand, TiO2 exists in three main types of crystal structures: anatase, rutile, and brookite. Of these, anatase and rutile are the two most commonly studied and used for many applications [31]. Anatase has a tetragonal structure (I41/amd) with distortions of the [TiO6] octahedra. Rutile with tetragonal structure (P42/mnm) is the densest of the three forms. Rutile is also the thermodynamically stable form of TiO2 and is found as larger size crystals and produced at higher temperatures [32]. Although brookite has an orthorhombic structure (Pbca) and is a metastable form, it is a very rare form of TiO2 produced in synthesis reactions [33]. The relative stability of the three forms of TiO2 and what phase transitions occur between them will depend greatly on several factors, including synthesis method, temperature of synthesis, and size of particles. ZnO and TiO2 can produce multiple types of morphologies such as nanoparticles, nanorods, nanowires, nanosheets, nanotubes, and hierarchical/porous structures. The surface area value, exposed facets, defect density and charge transport pathways are key factors that affect by morphological characteristics of nano-metal oxides. Although ZnO and TiO2 exhibit similar basic characteristics as wide bandgap semiconductor materials, they can differ with respect to their bandgap energy and their degree of excitonic binding. ZnO has a bandgap of ~3.37 eV with an associated excitonic binding energy of ~60 meV [34]. As a result, ZnO can be used as a UV emitter, laser diode and as a transparent conducting oxide [35-37]. The reports showed bandgap energies of ~3.2 eV for anatase phase and ~3.0 eV for rutile phase of TiO2 as an effective UV absorber. Anatase phase generally shows superior photocatalytic activity due to high charge separation and slow recombination rates of photogenerated electron–hole pairs [38]. To design and optimize the nanomaterials as high-performance compounds, it is important to understand how the crystal structure, morphology, and surface reactivity can all influence each other. In this paper we report the crystal phases, structural, and morphological features of ZnO and TiO2 nanostructures prepared via citric acid-assisted pechini method.

MATERIALS AND METHODS
Materials 
For synthesis of ZnO and TiO2 nanoparticles, zinc nitrate hexahydrate (Zn(NO3)2.6H2O), ethylene glycol (C2H6O2), citric acid (C6H7O8), ethanol (C2H5OH) and titanium IV isopropoxide (Ti[OCH(CH3)2]4) were achieved from Merck company. 

Synthesis of ZnO nanostructures 
To synthesize the ZnO nanostructures, the role of citric acid and ethylene glycol as chelating and polymerizing agents in pechini reaction was examined, respectively. Adjusting the molar ratio of 1:1 for citric acid:ethylene glycol and 3:1 for citric acid: Zn metal allowed for good chelation and a homogeneous metal ion distribution. In this reaction, the certain amount of Zn precursor was first dissolved in distilled water under stirring. A solution from citric acid was added to the above Zn solution to generate Zn-citrate complex. After that, polymerizing agent was introduced and a polyesterification reaction was occurred. The mixture was then heated at 120 °C for 2 h to allow evaporation of the solvent. Finally, the formed gel was dried at 65 °C for 18 h and then calcined at 500 °C for 2h, resulting in complete removal of organic material, resulting in crystalline ZnO nanoparticles.

Synthesis of TiO2 nanostructures 
TiO2 nanoparticles were fabricated via a modified Pechini technique with citric acid serving as a chelating agent and ethylene glycol functioning as a polymerizing agent; a molar ratio of 1:1 for citric acid to ethylene glycol was used, while retaining a molar ratio of 3:1 for citric acid: Ti ion for optimum chelation and uniformity of metal species distribution. Ethanol was served as the solvent for the reaction. To begin the synthesis, the citric acid and ethanol were mixed together under a magnetic stirrer until a clear and homogeneous solution was produced. When the citric acid was completely solubilized, the Ti precursor was added to the solution at a controlled rate in order to achieve the maximum amount of interaction between Ti ions and citric acid. After that, ethylene glycol was added slowly as the final component to trigger the polyesterification reaction, resulting in the development of a polymeric network. The formed sol was subsequently exposed to heat for solvent evaporation as well as polymerization. This sol was converted into a gel. The resulting gel was dried at 50 °C for 18 h and then underwent heat treatment to remove organic substrates within the gel and to produce crystallized TiO2 nanocrystals at 400 °C for 2 h.

Characterization methods 
For evaluation of the crystalline structure and phase composition of ZnO and TiO2 nanoparticles, X-ray diffraction (XRD, Philips-X’Pert-Pro – monochromatized Cu Kα radiation; λ=1.54Å) was employed. The fabricated nano-metal oxides were identicated through Fourier transform infrared (FT-IR, Nicolet Magna- 550 spectrophotometer) to illustrate the functional groups of samples. Furthermore, Field emission scanning electron microscopy (FE-SEM, MIRA3 TESCAN) was followed for analyzing the morphological properties of samples.

RESULTS AND DISCUSSION
According to the XRD result of the ZnO nanostructures in Fig. 1a, the high-intensity peaks confirm its excellent crystalline quality. The characteristic peaks found at approximately 31.7°, 34.4°, 36.2°, 47.5°, 56.6°, 62.8°, and 67.9° correspond to the (100), (002), (101), (102), (110), (103), and (112) planes, respectively, of hexagonal wurtzite-type ZnO as defined in standard JCPDS data (01-080-0075). The comparatively intense (100), (002), and (101) peaks point to the fact that that there is a preference for preferential growth of the crystals in particular directions of crystalline ZnO structure. The XRD pattern of the TiO2 nanostructures in Fig. 1b indicates good crystallinity due to the presence of distinct peaks. Various major peaks located at around 25.3°, 37.8°, 48.0°, 54.0°, 55.1°, 62.7°, 68.6°, 70.3°, and 75.0 ° correspond to peaks found in the standard JCPDS data (00-004-0477) that belong to crystallographic planes (101), (004), (200), (105), (211), (204), (116), (220), and (215) of the anatase TiO2 phase. The dominant crystalline structure is anatase as indicated by the strong (101) peak at around 25.3° and no additional diffraction peaks indicating the presence of rutile or brookite. Therefore it can be seen that pure-phase TiO2 (anatase) was successfully synthesized using the Pechini method. 
The FTIR spectrum of ZnO nanoparticles in Fig. 2a shows broad peaks at 3000-3600 cm⁻1 due to the residual water on the surface of ZnO (stretching vibration). The absorption band at 1640 cm⁻1 indicates the presence of H-O-H groups (bending vibration). The strong peaks ranging from 400 to 550 cm⁻1 corresponds to the Zn–O bond’s stretching vibration, demonstrating the successful formation of ZnO nanostructures. Also, it should be noted that the observed peaks within 1000-1500 cm⁻1 may be related to the organic templates and asymmetric stretching vibration of nitrate ions [39, 40]. In the FTIR spectrum of TiO2 nanoparticles (Fig. 2b), there is a broad peak in the region of 3000-3600cm-1, which represents the stretching vibrations of –OH due to physically adsorbed water. In addition, there is weak peak due to the bending vibration of H–O–H, which occurs at 1636 cm-1. The most important features of the TiO2’s FTIR spectrum were below 800 cm-1 and corresponded to metal-oxygen vibrations. The absorption peaks, between 400 and 700 cm-1, can be assigned to the Ti–O or Ti–O–Ti stretching of the TiO2 lattice. Furthermore, the absence of peaks associated with organic functional groups implied that no organic residues remained after calcination of the nanomaterials [41].
The ZnO nanoparticle’s uniformity is extremely high. In FESEM image and particle size distribution plot of Fig. 3(a, b), the ZnO nanoparticles are spherical having narrow size distributions. The uniformity of shape and particle size indicates that the nanoparticles grow orderly. In some cases, slight clustering was found. A homogeneous precursor solution creates conditions to prevent uncontrolled nanocrystal growth and possess similar sizes and shapes. These results support the Pechini method as a reliable way to produce high-quality nanostructures with controlled shape. Morphological uniformity can improve their performance in catalytic, optical, and electronic applications.
The FESEM image and particle size distribution plot of TiO2 structures in Fig. 3(c, d) show the distribution of nanoparticles is uniform and there is a homogeneous distribution in the sample. The majority of these nanoparticles is quasi-spherical in shape and has the same size, indicating how the Pechini process can control the nucleation and growth phase of TiO2 nanoscale structures. Variations in the TiO2 nanoparticle’s size confirm the strong interaction between Ti ions and the polymeric precursor matrix. This level of synthetic uniformity ensures that synthesized TiO2 nanoparticles have a high degree of morphological uniformity, which is essential for applications in which the uniformity of the surface area and active sites on materials is important.

CONCLUSION
In summary, the single phased metal oxide nanostructures, including ZnO and TiO2 were rationally designed via modified pechini method. In the synthesis reaction, role of citric acid as a chelating or capping agent and role of ethylene glycol as polymerizing agent was followed in formation of homogenous oxide powder. XRD patterns validated the hexagonal wurtzite for ZnO and anatase phase for TiO2 structures. Observation data from FESEM analysis showed that quasi-spherical and well-dispersed particles were formed for both ZnO and TiO2 nanoparticles, describing the mean particle size of 55.19 nm and 94.22 nm, respectively. Homogeneous nucleation and controlled growth kinetics in pechini synthesis method resulted from metal (Ti and Zn)–citrate complexes and a polymeric network with ethylene glycol. These structural and morphological features can provide high-quality ZnO and TiO2 nanoparticles with effective efficiency in different catalytic and energetic fields. 

ACKNOWLEDGEMENTS
The authors thank the Department of Chemistry, University of Baghdad and College of Sciences, University of Al-Qadisiyah for laboratory facilities and technical support during this work.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.