The Effect of Heat Treatment on Crystalline, Electrical and Optical Properties of ZnO/PVA Nanocomposites

ZnO/PVA nanocomposites have been successfully synthesized using the spin coating method by using synthesized ZnO, ZnO nanoparticles as filler, and PVA as a matrix. The synthesized films were treated at temperatures of 70, 90, and 120 °C. Some characterizations, such as I-V, UV-Vis, XRD, and SEM, have been carried out to study the electrical, crystalline, and optical properties of ZnO/PVA nanocomposites. The characterization of electrical properties using I-V measurements showed that ZnOsynthesized/PVA had a high current value of 1.55 nA, while ZnOnano/PVA had a smaller current value of 1.49 nA when treated at 120 °C. In addition, the higher the treatment temperature, the greater the resistance of ZnOsynthesized/PVA and ZnOnano/PVA. UV-Vis characterization showed that ZnOnano/PVA experienced a red shift when the temperature increased, while ZnOsynthesized/PVA and ZnOnano/PVA when treated at 120 °C experienced a blue shift. XRD characterization of ZnOnano/PVA showed that the higher the given temperature, the lower the crystallinity, from 45.77 to 34.81%. SEM characterization showed that ZnOnano/PVA at room temperature agglomerated and formed larger particles.


INTRODUCTION
Nanocomposites are inorganic (polymer) or organic composites that are nanometer in size and act as fillers and matrices [1,2,3].The development of nanocomposites is currently widely found in biomedicine, sensors, and computing.Because they have a small size and the nanocomposites have different properties to adjust the material [4].
The polymer used in this study was polyvinyl alcohol (PVA).PVA is a polymer made of polyvinyl esters or polyvinyl ethers by going through a transesterification reaction using methyl or ethyl alcohol, such as polyvinyl acetate.It is known that polyvinyl acetate is crystalline and has a relatively small and adjacent hydroxyl group size, so this polyvinyl alcohol is easily soluble in water [5].In addition, PVA also has the ability to form films with high optical qualities, judging by their luminescent properties, which can be used for nonlinear optical devices and optical sensors.In this study, the filler used was zinc oxide (ZnO).ZnO is currently emerging as a promising material for semiconductor, piezoelectric and optoelectronic devices, due to its optical and electrical properties.In addition, ZnO has a wide band gap (3.3 eV) and a large excitation binding energy (60 meV) [6].The combination of ZnO and PVA nanoparticles results in better electrical, mechanical, and optical properties [7,8].
Research on ZnO/PVA nanocomposites has been widely carried out for the application of optical sensors.One of them is a study conducted by Kandulna where ZnO/PVA nanocomposites with various concentrations are used as electron transfers in OLED [9].In the study, ZnO/PVA nanocomposites were fabricated into new hybrid materials with low band gaps, high dielectric permittivity, high voltage breakdown, and energy storage density for applications in OLEDs and capacitors as electrical energy storage devices.It is known that concentration can affect the physical properties of ZnO/PVA nanocomposites.However, only a few of the studies reported on the thermal treatment of the ZnO/PVA nanocomposite.The thermal effect of synthesized ZnO/PVA can alter the properties of the nanocomposite.It is necessary to study the thermal treatment of ZnO/PVA nanocomposites to degrade thermo-mechanical and thermo-oxidative materials and increase the solubility of PVA during and after the ZnO/PVA nanocomposite synthesis process [10,11].This mechanism can control the size of ZnO and PVA nanocomposite particles [12].The change in the properties of the nanocomposite after the thermal treatment can also change its optical properties.It is indicated that this has the potential to be applied as the optical sensor in flexible devices.

EXPERIMENTAL METHOD
This research was conducted to determine the effect of heat treatment on the crystalline, electrical, and optical properties of ZnO/PVA nanocomposites.The research begins with the preparation of tools, substrates, and solutions.The process is continued by using two types of ZnO: synthesized ZnO, which is ZnO synthesized using simple chemical methods, and ZnO nanoparticles (Sigma Aldrich) that have been purchased in the market.The two types of ZnO were used to make ZnO/PVA nanocomposites using the in situ method.Then, the nanocomposite is given a temperature treatment after going through the synthesis process, and the solution has been printed on the glass substrate.After being given temperature treatment, the thin films were characterized by their physical properties (crystal, morphological, electrical, and optical properties).
In the synthesis of ZnO, a simple chemical method was used.0.1 M zinc acetate is stirred into 100 ml of methanol at room temperature with a magnetic stirrer until it dissolves, then 0.2 mM NaOH is added with a pipette.The stirring process was carried out for 2 hours, then filtered using filter paper.Then it is dried in an oven at 50 °C for 5-6 hours [13].
The in-situ method was used to synthesize the ZnO/PVA nanocomposite.0.1g of synthesized ZnO and 0.1 ZnO nanoparticles were dissolved in 10 ml of distilled water each, then sonicated for 30 minutes.After that, mix the ZnO solution with 10% w/v PVA nanocomposite and stir using a magnetic stirrer for 2 hours at room temperature or 80°C.Then, the ZnO/PVA solution was sonicated for 30 minutes.After sonication, the solution is deposited onto a glass substrate and leveled.In forming a thin film from a roomtemperature ZnO/PVA solution, drying is carried out at room temperature in a closed room for 24-48 hours.As for the ZnO/PVA solution at certain temperatures (70°C, 90°C, and 120°C), drying is carried out with a hot plate stirrer for 10 minutes at 80°C.Giving heat treatment at this temperature is carried out based on the PVA thermal degradation area at 70°C-120°C, where evaporation of O bonds occurs [14].
After the ZnO/PVA nanocomposite synthesis process, the thin films were then characterized by their properties.It is characterized by its crystal structure, electrical properties, morphology, and optical properties.The crystal structure of ZnO/PVA was then characterized by XRD analysis, in which the pattern was taken using an X-ray diffractometer (Cu K = 0.154 nm) [7,9].The electrical properties of the ZnO/PVA nanocomposite were then characterized by measuring the current-voltage (I-V) characteristics using a Keithley 2400 [15].Morphological analysis of ZnO/PVA nanocomposite thin films was used by SEM to study the morphology of the samples and the particle size distribution using a particle size analyzer [7].The optical properties of the ZnO/PVA nanocomposite were then studied by using UV-Vis analysis with a wavelength range of 200-800 nm [9].

RESULT AND ANALYSIS
In this result, we will have studied the samples that are categorized into two types: the first is the samples using the ZnO synthesis (ZnO synthesis) and the ZnO nanoparticles from Sigma Aldrich (ZnOnano).The electrical properties of ZnO/PVA nanocomposite thin films used in ZnOnano/PVA and ZnOsynthesis/PVA samples are shown in Figure . 1 below.The I-V curve can be divided into two regions, namely the ohmic (high resistance) and non-ohmic (low resistance) regions.The particle size of the synthesized ZnO/PVA nanocomposite is related to the electrical properties; the smaller the particle size resulting from the higher temperature treatment given during the annealing process, the more effectively it improves the electrical properties [16].Synthesis of ZnO/PVA samples (A4, A5, A6) and ZnOnano/PVA samples (B4, B5, B6) show an ohmic curve where each sample has a lower current (nA) and the current increases linearly with the increase in the applied voltage.This is indicated to be caused by the electron mobility, which is based on the electronhole recombination process (increasing hole charge carriers) in the presence of ZnO in the PVA matrix.This conduction mechanism of polymer nanocomposites is slightly different from the intrinsic semiconductor's phenomenon [9].The thin films ZnOsynthesis/PVA and ZnOnano/PVA had the highest currents of 1.55 nA at a voltage of 5.01 V, which obtained a resistance of 3.23 G, and 1.49 nA at a voltage of 5.03 V, which obtained a resistance of 3.37 G, respectively.This result is clearly depicted in the thin film that was treated at 120 0 C, as shown in Figure 1.From these results, the particle size of the nanocomposite made using the as-synthesized ZnO is larger than the ZnO nanoparticles.For analysis of induced electrical properties, temperature treatment on ZnOsynthesis and PVA samples decreased resistance when given a temperature of 900 °C (A5), whereas in the ZnOnano and PVA sample it increased.The higher the temperature given, the higher the resistance.
The growth process and post-deposition treatment have strong effects on the structure and properties of ZnO/PVA nanocomposite, which can also significantly affect their electrical and piezoelectric properties [19].In figure 1, the I-V result does not start at V = 0, as predicted by the piezoelectric properties resulting for the research.Beside that, it is also due to the possibility of thermal agitation caused by the internal nature of ZnO, which is a piezoelectric material that occurs at room temperature.Due to the decrease in particles that is caused by the varying temperatures and semiconductor quantum confinement effects, the absorbance edge will shift to a higher energy [12,17].Therefore, the band gap energy of all the samples can be obtained from the UV-Vis spectra.In ZnOnano/PVA (B4, B5, and B6), the band gap energy decreased from 4.10 to 4.01 eV with an increased wavelength of 302-309 nm.For the synthesized ZnO (A6), it can be seen from the smaller band gap energy of 3.35 eV with a wavelength of 370 nm compared to ZnO nanoparticles (B6).It is clearly shown that the shifting at the absorption edge is caused by the quantum confinement effect.Therefore, the average particle size can be determined from the inflection point in the absorbance spectrum with the wavelength [17].The calculation of the band gap energy is shown in Table 1 below.In table 1, the higher the treatment temperature, the larger the particle size, it is indicated the absorption wavelength peaks on ZnOnano/PVA thin films (B4, B5, and B6).
The XRD analyses were studied for the samples synthesized in room temperature ZnO/PVA (B) and annealed at 120℃ ZnO/PVA samples (B6).The purpose of XRD results is to determine crystallite size and crystallinity and to be compared with the standard diffraction pattern of JCPDS No. 36-1451, which corresponds to the structures of pure ZnO and PVA [18].In the XRD results of samples B and B6, each has seven main peaks, which correspond to the hkl field and JCPDS peak No. 36-1451.From the XRD pattern, it has the FWHM for obtaining the size of the crystallite using the Scherrer equation, and the results are shown in Table 2.   2 above, the average of crystallite size of the ZnO/PVA thin film sample at room temperature (B) is ≈ 66.978 nm while the ZnO/PVA at temperature 120℃ (B6) is ≈ 37.897 nm.From this result the average crystallite size of B is almost twice bigger than sample B6.The crystallinity of ZnO/PVA can be calculated by comparing the area of the crystal peaks with the total integrated area of all peaks.The area calculation can be measured using the origin software.The crystallinity showed clearly in the equation (1) below.From the calculation it is found that sample B and B6 have a crystallinity of 45.77% and 34.81 %, respectively. ( According to Kandulna, there is a correlation between FWHM and crystallinity index with ZnO/PVA nanocomposites by incorporation of ZnO in the PVA polymer matrix [9].It is clearly seen that the higher the temperature treatment the crystallinity of ZnO nano/PVA decreases.This is due to the thermal degradation of PVA which has decreased in mass when it is given heat treatment at 70℃-120℃.It causes the evaporation of H2O bonds which are physically weak and chemically strong [17].In addition, the results of the crystallinity of ZnOnano/PVA showed that ZnO nanoparticles had a semi-crystalline structure and distribution of ZnO nanoparticles on the PVA surface. The SEM results of ZnOnano/PVA for samples B and B6 are shown in Figure 4. From these results, it is shown that the particles when treated at a given room temperature (B) are larger than at 120℃ (B6).For particle measurements from SEM results using the image software shown in Table 3.This is caused by the surface energy in sample B, which causes the particles to tend to agglomerate and form larger particles [7].The average particle size of the ZnO/PVA nanocomposite at room temperature is 639 nm, while at 120℃ is 603 nm.For crystallinity, the calculation is carried out using the largest particle sizes, 1966 nm (B) and 1821 nm (B6), resulting in the crystallinity of 30.77% at room temperature and 29.03% at 120°C, respectively.Figure 5 depicts the results of the EDS spectra of the ZnOnano/PVA for samples B and B6, with C, O, and Zn atomic compositions in the thin film and the percentage of each component shown in Tables 4 and 5.The percentage of Zn atoms in ZnOnano/PVA decreased from 2.86% (table 4) to 0.98% (table 5) and the percentage of C atoms in ZnOnano/PVA decreased from 67.05% to 66.05% because of the temperature treatment.

CONCLUSION
From the result, it shows that film ZnOsynthesis/PVA has a larger particle size than ZnOnano/PVA, which, in terms of the electrical properties of ZnOsynthesis/PVA has the highest current of 1.55 nA.ZnOnano/PVA has a current of 1.49 nA when given a temperature of 120°C.There was a decrease in resistance when given a temperature of 90 °C, while for ZnOnano/PVA (B4, B5, and B6), the resistance increased with the higher temperature treatment.ZnOsynthesis/PVA (B6) has a band gap energy of 3.35 eV, which is smaller than the band gap energy of ZnOnano/PVA (B6), while the particle size of ZnOsynthesis/PVA (B6) is larger.The absorbance spectra were shifted to the blueshift region.For the film of ZnOnano/PVA thin films, crystal size and crystallinity decreased from 66.978-37.897nm and 45.77-34.81%when given room temperature (B) and 120 °C (B6), respectively.Therefore, it can be seen from the SEM results that sample B undergoes agglomeration and forms larger particles.This result has the potential to be applied as an optical sensor for flexible devices, such as pulse oximeter that is usually applied in medical applications.

Figure 5 .
Figure 5. EDX results of sample B (a) and sample B6 (b)