Opto-structural Insights Into Zno-doped Ps Nanocomposites: Photoluminescence Spectroscopy and Defect Analysis

doi:10.21203/rs.3.rs-4303369/v1

Download PDF

Research Article

Opto-structural Insights Into Zno-doped Ps Nanocomposites: Photoluminescence Spectroscopy and Defect Analysis

https://doi.org/10.21203/rs.3.rs-4303369/v1

This work is licensed under a CC BY 4.0 License

You are reading this latest preprint version

In this study, zinc oxide nanoparticles (ZnO NPs) were successfully incorporated into polystyrene (PS) using a combination of solution casting and hot-pressing, yielding a range of PS/ZnO nanocomposites. These nanocomposite films were characterized using ultraviolet visible spectroscopy (UV-Visible), photoluminescence (PL), and X-ray diffraction (XRD) techniques. Hexagonal wurtzite structure of ZnO nanoparticles was confirmed by X-ray diffraction (XRD) results. XRD analysis demonstrated a transition in crystallinity from 3.88 nm to 1.58 nm, further confirming the impact of ZnO concentration on structural properties. The optical absorption spectra were used to determine optical energy band gaps, where their values indicated an improvement in the optical characteristics of the nanocomposite films. PL spectra interpretation elucidated optically active defects and relaxation pathways, with wide emission bands in the violet and blue regions suggesting transitions from excitonic levels and/or zinc interstitials (Zni) to the conduction band, alongside contributions from defects like zinc interstitials (Zni) and natural zinc vacancies (Znv). These findings deepen our understanding of the intricate relationship between ZnO nanoparticle concentration and the optical and structural characteristics of PS/ZnO nanocomposite films, offering insights essential for diverse applications.

In recent decades, numerous research groups have concentrated on organic-inorganic nanocomposite materials [1-3]. These composites display distinctive mechanical, optical, chemical, and physical properties compared to their parent polymers and nanofillers. Polymers serve as an ideal host matrix for nanocomposite materials due to their straightforward synthesis procedures, affordability, lightweight nature, and resistance to moisture and corrosion [4,5]. Nanocomposites find diverse applications, including solar cells, UV-shielding materials, sensors, photocatalysis, hybrid OLEDs, and optoelectronics [6-9]. Polystyrene (PS) stands out among other polymers due to its lightweight, cost-effectiveness, transparency, and optical stability. It boasts high biocompatibility and stability in chemical, electrical, and mechanical aspects. PS is a rigid, easily processable, recyclable, and reusable material [10, 11]. The structure of PS polymer comprises an aromatic ring and a connected aliphatic chain. These aromatic rings contain delocalized electrons, facilitating control over the polymer's conductivity. Additionally, due to its superior optical characteristics, PS can serve as an excellent UV absorber [12-14]. To enhance the characteristics of polystyrene (PS), nanoparticles such as SiO2, CNTs, NiO, TiO2, Fe2O3, CuO, Cu, and ZnO can be incorporated as fillers into the matrix. Consequently, polystyrene (PS) composites have garnered significant interest across industrial and academic fields, particularly when nanoparticles are integrated into the PS matrix, resulting in nanocomposites with versatile applications. These additions provide PS with new attributes like low density, thermal conductivity, processability, and electrical conductivity. Furthermore, the dispersion of nanoparticles within the matrix has negligible effects on the quality of PS nanocomposites [2,7]. In this study, we utilized ZnO nanoparticles to enhance the properties of polystyrene (PS). ZnO is considered one of the prime alternatives for TiO₂ as a semiconductor due to its similar band gap energy (3.37 eV) to TiO2 (3.2 eV) [15-17]. However, ZnO's photocatalytic activity is limited to UV light irradiation, utilizing only 4-5% of the solar spectrum. Modifications of ZnO with metals [16-18], nonmetals [15,19,20], or linking semiconductors [21,22] have been proposed to enhance its absorption of broad solar light, thereby reducing its band gap energy. Stoichiometric zinc oxide exhibits a wurtzite structure and behaves as an insulator. The structure contains significant voids capable of accommodating interstitial atoms, making the production of pure crystals challenging, which also tend to lose oxygen at high temperatures [23]. ZnO possesses inherent flaws, including oxygen deficiency and zinc excess, contributing to its n-type semiconductor characteristics. Typically, at room temperature, ZnO exhibits a UV emission peak due to the recombination of free excitons and possibly one or more peaks in the visible spectral range attributed to defect emissions [34-36]. Investigating the photoluminescence (PL) properties of ZnO is therefore intriguing, as it can offer significant insights into the quality and purity of the materials. Moreover, owing to the high surface-to-volume ratio of ZnO, it facilitates enhanced interfacial coordination between polymers and nanoparticles, resulting in improved optical characteristics and crystallization degree [15, 24]. The unique properties of ZnO nanoparticles make them attractive for technological optoelectronic applications when incorporated into polymer matrices. The interaction between ZnO and the polymer can also lead to several new radiative recombination mechanisms, potentially enabling broadband light emission or wavelength-tunable emission. Since these mechanisms are influenced by various parameters, including the individual features and relative concentrations of both organic and inorganic components, emission properties can be adjusted in multiple ways. In this study, ZnO/PS nanocomposites were synthesized by embedding varying weight percentages of ZnO in polystyrene membranes using the solution cast method and hot-pressing. The optical-structural properties of these nanocomposites were investigated based on the concentration of zinc oxide nanoparticles in the polymer matrix. The influence of nanoparticle content on the mechanism of UV and PL emissions from PS/ZnO nanocomposite thin films is explored.

2.1. Materials

All chemicals were utilized as received without undergoing purification. Polystyrene (PS) with CAS code 9002-86-2 exhibits a density of 1.04 g/cm3, a melting temperature range of 150–220°C, a spark temperature of 625°C, an ignition temperature of 500°C, and an auto-ignition temperature exceeding 1100°C. Tetrahydrofuran (THF) with PLC 143537 was also used as received. Zinc oxide nanoparticles with a particle size ranging from 10 to 30 nm were purchased from SkySpring Nanomaterials, Inc.

2.2. Synthesis of Polymer Nanocomposites

Polymer nanocomposite materials were prepared as follows: Polystyrene was dissolved in its organic solvent, tetrahydrofuran (THF). The precise weight of zinc oxide nanoparticles with varying volume contents (1%, 5%, 10%) was then added to the solution under vigorous stirring for 2 hours to ensure the complete dispersion of ZnO nanoparticles in the PS–THF solution. The resulting mixture was transferred to a Petri dish and air-dried throughout the day. To remove the solvent completely from the polymeric matrix, the nanocomposites were subsequently dried in a vacuum oven at room temperature for an hour. Thin films of nanocomposites were then obtained from these samples using the hot pressing method at a melting temperature of PS (180°C) and a pressure of 10 MPa. Cooling of the films after hot pressing was achieved by immersion in water.

2.3. Instrumentation

X-ray diffraction analysis was conducted using a Rigaku Mini Flex 600 XRD diffractometer at ambient temperature. For all experiments, Cu Kα radiation operating at 15 mA and 30 kV was employed. The samples were scanned within the 10–90° range of the Bragg angle 2θ.

The absorption spectra of the prepared films were recorded using a B Specord 250 Plus UV-vis spectrophotometer, enabling research within the wavelength range of 250–600 nm.

Photoluminescent properties of the nanocomposite films were examined utilizing a spectrofluorimeter Varian Cary Eclipse, covering a wavelength range of 200–900 nm.

3.1. X‑Ray Diﬀraction Technique

X-ray diffraction (XRD) is a valuable technique for studying the structure of polymeric materials, providing insights into parameters such as crystallite size, degree of crystallinity, dislocation density, and microstrain. In this study, XRD analysis was conducted to investigate the influence of ZnO nanoparticles on the structure of the PS polymer matrix. Figure 1 presents the diffraction pattern of pristine PS and PS/ZnO nanocomposites. The spectrum exhibits a broad peak around 2θ value of 20°, indicating the amorphous nature of the PS film [25, 26].

A broad suppressed peak at 20° is observed for pristine PS, which diminishes as ZnO is introduced into the PS matrix. With increasing ZnO content from 1 to 10 wt%, new sharp and intense peaks emerge at 31.71°, 34.53°, 36.26°, 48°, 56.67°, 62.85°, 66°, and 68.07°, as depicted in the spectra of the nanocomposite films. These peaks correspond to the presence of ZnO within the PS polymer matrix, aligned with the Miller indices (100), (002), (101), (102), (110), (103), (200), (112), and (201), respectively. They become more prominent with increasing ZnO wt%. Comparison of these peaks with the XRD peaks of ZnO nanopowder and JCPDS file confirms the presence of ZnO.

The average crystallite size (D) for the nanocomposites was estimated using Scherrer’s formula, where D represents the grain size (or crystalline size) of the material, λ is the wavelength of the X-ray beam, θ is the diffraction angle, β is the full width at half maximum (FWHM) of the observed peak in radians, and K is a constant called the shape factor, equal to 0.89 [27, 28, 29, 30]. The average grain sizes (or crystalline sizes) were found to be 3.88 nm, 3.41 nm, and 1.58 nm, respectively, for the polystyrene/1% ZnO NC, polystyrene/5% ZnO NC, and polystyrene/10% ZnO NC. The addition of ZnO resulted in a gradual increase in crystalline size due to its large surface area.

3.2. UV–VIS Spectroscopy

The effect of zinc oxide nanoparticles on optical absorbance of PS was studied through UV-vis spectra. UV–Visible spectra of nanocomposite ﬁlms was recorded in the wavelength range of 250–600 nm. Figure 1 illustrates the absorption spectra of pristine PS and PS/ZnO nanocomposite films. The presence of ZnO induces an absorption band at 370 nm, consistent with findings from previous studies [31, 32]. As the filler content increased, this peak expanded and moved to a longer wavelength (red-shifted). This change clearly shows that nanoparticles agglomerate as their content increases. The spectra reveal that the absorbance of the polymer increases with rising concentrations of ZnO nanoparticles, likely attributed to the increased number of charge carriers within the nanocomposite [33].

The absorption coefficient (αhν)² versus the wavelength (nm) for PS/ZnO nanocomposites is shown in Figure 3.

For a clear comparison, the calculated E_g values for the investigated PS/ZnO nanocomposites are listed in Table 1.

Table 1. Pure polystyrene (PS) and PS/ZnO based nanocomposites band gap (E_g)

Samples	E_g(Calculated using conventional extrapolation of Tauc’s plot) (eV)
PS	2,3 eV (539 nm)
PS/3%ZnO	3,2 eV (387 nm)
PS/5% ZnO	3,15 eV (393nm)
PS/10% ZnO	3,14 eV (391 nm)

The table shows that as the concentration of nanoparticles grew, the band gap E_g of the nanocomposites decreases (i.e., red-shifted), which is caused by the growing particle size. [34] In other words, as the nanoparticle content in the matrix increases, nanoparticle agglomeration occurs, leading to the formation of larger clusters and consequently a decrease in the optical band gap. The band gap reduction is consistent with the findings from PL investigations.

3.3. Photoluminescence (PL) Spectroscopy

Photoluminescence (PL) is a significant property that provides insights into the optically active defects and relaxation pathways for excited states. This study is essential for identifying the source of sub-band-gap luminescence. Figure 5 illustrates the photoluminescence (PL) spectra of PS/ZnO-based nanocomposites, depending on the volume content of ZnO nanoparticles. The presence of wide emission bands in the violet and blue regions suggests transitions from excitonic levels and/or zinc interstitials (Zn_i) to the conduction band, as well as contributions from defects such as zinc interstitials (Zn_i) and natural zinc vacancies (Zn_v) [36,37]. It is suggested that the origin of the violet emission at λ ~420 nm (2.95 eV) is due to the transitions from conduction band (CB) to the holes localized at defect level associated with zinc vacancy (V Zn ) [38]. The blue luminescence bands situated in (430–480) nm (2.8–2.68 eV) domain are due to transitions from extended donor Zn_i states and the holes in the valence band [38]. The blue-green emissions [480–510 nm] may be due to the transition from the oxygen inertial (O_i) and zinc antisites (O_Zn) to the valence band [39].

As can be seen from the PL spectra, the luminescence intensity increases depending on the concentration. At a certain volume content of the filler, it stops increasing and saturation occurs. This tendency is due to the increase in the size of the luminescent centers. The decrease in the luminescence intensity at a high amount of filler is explained by the fact that as the nanoparticle content increases, the probability of their agglomeration increases, which in turn significantly reduces the number of crystallization centers.

In summary, PS/ZnO nanocomposite films were successfully fabricated through solution-casting and hot-pressing techniques, incorporating varying concentrations of ZnO nanoparticles. Analysis of the films' opto-structural characteristics was conducted employing X-ray diffraction (XRD), photoluminescence (PL), and ultraviolet-visible spectroscopy (UV-Vis). The investigation revealed a reduction in the energy band gap of the nanocomposite films from 3.2 to 3.14 eV with increasing ZnO nanoparticle content. This decrease indicates a narrowing of the optical band gap due to the coagulation of nanoparticles as their concentration in the matrix rises. Furthermore, the interpretation of PL spectra provides additional insights into the optically active defects and relaxation pathways for excited states within the nanocomposite films. The presence of wide emission bands in the violet and blue regions suggests transitions from excitonic levels and/or zinc interstitials (Zn_i) to the conduction band, as well as contributions from defects such as zinc interstitials (Zn_i) and natural zinc vacancies (Zn_v). These findings enhance our understanding of the optical behavior of the nanocomposite films. The change in crystallinity, as evidenced by XRD analysis, exhibited a transition from 3.88 nm to 1.58 nm, further corroborating the impact of ZnO concentration on the nanocomposite films' structural properties. These findings underscore the intricate relationship between ZnO nanoparticle concentration and the optical and structural characteristics of PS/ZnO nanocomposite films, offering insights crucial for their potential applications in various fields.

Barala, M.; Mohan, D.; Sanghi, S.; Siwach, B.; Kumari, S.; Yadav, S. Optical properties of PS/ZnO nanocomposites foils prepared by casting method. AIP Conf. Proc. 2019, 2142.
Sangawar, V.S.; Golchha, M.C. Evolution of the optical properties of Polystyrene thin films filled with Zinc Oxide nanoparticles. Int. J. Sci. Eng. Res. 2013, 4(6), 2700–2705.
Kattimuttathu, S.I.; Krishnappan, C.; Vellorathekkaepadil, V.; Nutenki, R.; Mandapati, V.R.; Černík, M. Synthesis, characterization and optical properties of graphene oxide–polystyrene nanocomposites. Polymer Adv. Technol. 2015.
Bhavsar, S.; Patel, G.B.; Singh, N.L. Investigation of optical properties of aluminium oxide doped polystyrene polymer nanocomposite films. Phys. B Phys. Condens. Matter 2018.
Alekseeva, O.V.; Barannikov, V.P.; Bagrovskaya, N.A.; Noskov, A.V. DSC investigation of the polystyrene films filled with fullerene. J. Therm. Anal. Calorim. 2012, 109, 1033–1038.
Nenna, G.; De Girolamo Del, A.; Mauro, E.; Massera, A.; Bruno, T.F.; Minarini, C. Optical Properties of Polystyrene-ZnO Nanocomposite Scattering Layer to Improve Light Extraction in Organic Light-Emitting Diode. J. Nanomater. 2012, 319398, 1–7.
Abutalib, M.M. Effect of zinc oxide nanorods on the structural, thermal, dielectric and electrical properties of polyvinyl alcohol/carboxymethyle cellulose composites. Physica B 2019, 557, 108–116.
Antolini, F.; Pentimalli, M.; Luccio, T.D.; Terzi, R.; Schioppa, M.; Re, M.; Mirenghi, L.; Tapfer, L. Structural characterization of CdS nanoparticles grown in polystyrene matrix by thermalytic synthesis. Mater. Lett. 2005, 59, 3181–3187.
Kassaee, M.Z.; Motamedi, E.; Majdi, M. Magnetic Fe3O4-Graphene Oxide/Polystyrene: Fabrication and Characterization of a Promising Nanocomposite. Chem. Eng. J. 2011, 172, 540–549.
Sabbar, A.N.; Mohammed, H.S.; Ibrahim, A.R.; Saud, H.R. Thermal and Optical Properties of Polystyrene Nanocomposites Reinforced with Soot. Oriental J. Chem. 2019, 35(1), 455–460.
Sharma, T.; Garg, M. Pristine, Irradiated and Nanocomposite Polystyrene: Recent Experimental and Theoretical Developments. Trans. Electr. Electron. Mater. 2021, 22, 394–418.
Jeeju, P.P.; Sajimol, A.M.; Sreevalsa, V.G.; Varma, S.J.; Jayalekshmi, S. Size‐dependent optical properties of transparent, spin‐coated polystyrene/ZnO nanocomposite films. Polymer Int. 2011, 60, 1263–1268.
Jeeju, P.P.; Jayalekshmi, S.; Chandrasekharan, K.; Sudheesh, P. Size dependent nonlinear optical properties of spin coated zinc oxide-polystyrene nanocomposite films. Opt. Commun. 2012, 285, 5433–5439.
Farha, A.H.; Al Naim, A.F.; Mansour, S.A. Thermal degradation of polystyrene (PS) nanocomposites loaded with sol gel-synthesized ZnO nanorods. Polymers 2020, 12, 1–12.
Mu, J.; Shao, C.; Guo, Z.; Zhang, Z.; Zhang, M.; Zhang, P.; Chen, B.; Li, Y. High Photocatalytic Activity of ZnO−Carbon Nanofiber Heteroarchitectures. ACS Appl. Mater. Interfaces 2011, 3, 590–596.
Mohan, R.; Krishnamoorthy, K.; Kim, S.J. Enhanced photocatalytic activity of Cu-doped ZnO nanorods. Solid State Commun. 2012, 152, 375–380.
Xu, C.; Cao, L.; Su, G.; Liu, W.; Qu, X.; Yu, Y. Preparation, characterization and photocatalytic activity of Co-doped ZnO powders. J. Alloys Compd. 2010, 497, 373–376.
Bouzid, H.; Faisal, M.; Harraz, F.A.; Al-Sayari, S.A.; Ismail, A.A. Synthesis of mesoporous Ag/ZnO nanocrystals with enhanced photocatalytic activity. Catal. Today 2014, 252, 20–26.
Zheng, M.; Wu, J. One-step synthesis of nitrogen-doped ZnO nanocrystallites and their properties. Appl. Surf. Sci. 2009, 255, 5656–5661.
Sun, Y.; He, T.; Guo, H.; Zhang, T.; Wang, W.; Dai, Z. Appl. Surf. Sci. 2010, 257, 1125–1128.
Habibi, M.H.; Rahmati, M.H. The effect of operational parameters on the photocatalytic degradation of Congo red organic dye using ZnO-CdS core-shell nano-structure coated on glass by Doctor Blade method. Spectrochim. Acta, Part A 2015, 137, 160–164.
Saravanan, R.; Gupta, V.K.; Mosquera, E.; Gracia, F. Preparation and characterization of V2O5/ZnO nanocomposite system for photocatalytic application. J. Mol. Liq. 2014, 198, 409–412.
Lam, S.M.; Sin, J.C.; Abdullah, A.Z.; Mohamed, A.R. Sunlight responsive WO3/ZnO nanosheets for photocatalytic degradation and mineralization of chlorinated phenoxyacetic acid herbicides in water. J. Colloid Interface Sci. 2015, 450, 34–44.
Drozd, V.E.; Titov, V.V.; Kasatkin, I.A.; Basov, L.L.; Lisachenko, A.A.; Stroyuk, O.L.; Kuchmiy, S.Y. Structure, optical properties and visible-light-induced photochemical activity of nanocrystalline ZnO films deposited by atomic layer deposition onto Si(100). Thin Solid Films 2014, 573, 128–133.
Jangir, L.K.; Kumari, Y.; Kumar, A.; Kumar, M.; Awasthi, K. Optical and Structural Study of Polyaniline/Polystyrene Composite Films. Macromol. Symp. 2015, 357, 218.
Jeeju, P.P.; Sajimol, A.M.; Sreevalsa, V.G.; Varma, S.J.; Jayalekshmi, S. Size‐dependent optical properties of transparent, spin‐coated polystyrene/ZnO nanocomposite films. Polym. Int. 2011, 60, 1263.
Senthilkumar, N.; Vivek, E.; Shankar, M.; Meena, M.; Vimalan, M.; Potheher, I.V. Synthesis of ZnO nanorods by one step microwave-assisted hydrothermal route for electronic device applications. J. Mater. Sci.: Mater. Electron. 2018, 29, 2927.
Vijayabala, V.; Senthilkumar, N.; Nehru, K.; Karvembu, R. Hydrothermal synthesis and characterization of ruthenium oxide nanosheets using polymer additive for supercapacitor applications. J. Mater. Sci.: Mater. Electron. 2018, 29, 323.
Jeyachitra, R.; Senthilnathan, V.; Senthil, T.S. Studies on electrical behavior of Fe doped ZnO nanoparticles prepared via co-precipitation approach for photo-catalytic application. J. Mater. Sci.: Mater. Electron. 2018, 29, 1189.
Senthilkumar, N.; Ganapathy, M.; Arulraj, A.; Meena, M.; Vimalan, M.; Potheher, I.V. Two step synthesis of ZnO/Ag and ZnO/Au core/shell nanocomposites: Structural, optical and electrical property analysis. J. Alloys Compd. 2018, 750, 171.
Samuel, M.S.; Bose, L.; George, K.C. Optical properties of ZnO nanoparticles. SB Acad. Rev. 2009, 16, 57–65.
Soliman, T.S.; Rashad, A.M.; Ali, I.A.; Khater, S.I.; Elkalashy, S.I. Investigation of linear optical parameters and dielectric properties of polyvinyl alcohol/ZnO nanocomposite films. Physica Status Solidi (a) 2020, 217(19), 2000321.
El Sayed, A.M.; Morsi, W.M. α-Fe2O3/(PVA+PEG) Nanocomposite Films; Synthesis, Optical, and Dielectric Characterizations. J. Mater. Sci. 2014, 49, 5378–5387.
Sangawar, V.; Golchha, M. Evolution of the optical properties of Polystyrene thin films filled with Zinc Oxide nanoparticles. Int. J. Sci. Eng. Res. 2013, 4(6).
Zeng, H. et al. Blue luminescence of ZnO nanoparticles based on non-equilibrium processes: Defect origins and emission controls. Adv. Funct. Mater. 2010, 20, 561–572.
Mandal, S.; Goswami, M.L.N.; Das, Dhar, A.; Ray, S.K. Temperature dependent photoluminescence characteristics of nanocrystalline ZnO films grown by sol–gel technique. Thin Solid Films 2008, 516, 8702–8706.
Ma, J.G. et al. Preparation and characterization of ZnO particles embedded in SiO2 matrix by reactive Magnetron sputtering. J. Appl. Phys. 2005, 97, 103509.
Zeng, H.B.; Duan, G.T.; Li, Y.; Yang, S.K.; Xu, X.X.; Cai, W.P. Blue luminescence of ZnO nanoparticles based on non-equilibrium processes: defect origins and emission control. Adv. Funct. Mater. 20 (2010) 561–572.
Heo, Y.W.; Norton, D.P.; Pearton, S.J. Origin of green luminescence in ZnO thin film grown by molecular-beam epitaxy. J. Appl. Phys. 98 (7) (2005), 073502.

No competing interests reported.

Download PDF

Reviewers agreed at journal
02 Aug, 2024
Reviewers invited by journal
30 Jul, 2024
Editor assigned by journal
22 Apr, 2024
Submission checks completed at journal
22 Apr, 2024
First submitted to journal
22 Apr, 2024

You are reading this latest preprint version

Opto-structural Insights Into Zno-doped Ps Nanocomposites: Photoluminescence Spectroscopy and Defect Analysis

Status:

Version 1

Abstract

1. Introduction

2. Materials and Methods

3. Results and Discussions

4. Conclusion

References

Additional Declarations

Status:

Version 1