Impact of surfactant-assisted synthesis on the structural, optical, and dielectric characteristics of ZnO nanoparticles

In this study, the enhancement of ZnO’s dielectric properties is pursued through the manipulation of its particle size using a surfactant-based approach. The synthesis of ZnO nanoparticles with the aid of surfactants is achieved through the sol–gel method. Through x-ray diffraction analysis, the formation of the desired Wurtzite structure is confirmed, with no secondary phases detected. The surface characteristics of the synthesized powders are examined through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), both of which highlight the even distribution of the surfactant-assisted nanoparticles. The crystallite size is quantified using Scherrer’s formula. UV-visible spectroscopy and photoluminescence are employed to delve into the optical properties. Dielectric behavior is assessed across a frequency range of 20 Hz to 4000 kHz at room temperature. Comparative analysis between ZnO samples grown with and without surfactants reveals that those with surfactant assistance display heightened capacitance, a notably higher dielectric constant, increased AC conductivity, and longer transit time.


Introduction
In recent times, there has been a notable surge in the exploration of the dielectric and electronic attributes of materials due to their pivotal roles in advanced applications like LEDs, sensors, rechargeable batteries, electroluminescent devices, and as intermetallic dielectrics in ICs [1][2][3][4][5][6].Nano dielectrics have found extensive utility across communication, electronics, electrical systems, and nanoscale devices like dynamic random access memory [7][8][9][10].Among the key parameters defining insulator materials for microelectronic design, the dielectric constant and dissipation factor stand out [11].The focus of researchers has now shifted towards comprehending the microscopic mechanisms responsible for dielectric relaxation and long-range conduction in such materials [12,13].Additionally, the exploration of dielectric constant and loss factor, contingent on frequency, remains one of the most sensitive and convenient approaches for probing metal oxide characteristics [13,14].Amid an array of metal oxides, ZnO holds immense significance with a wide band gap (3.37 eV) and substantial excitation binding energy (60 meV) [15,16], ZnO emerges as a crucial material.Its nanoparticles find applications encompassing UV absorption, catalysis [17], storage, solar cells [18,19], ceramics, biosensors [20], chemical sensors [21][22][23], light emitting diodes [24], and photocatalysts [21].However, the characteristics of ZnO nanocrystals are significantly influenced by factors such as particle sizes, morphology, surface area, and activity.Various synthesis methods have been developed, including sol-gel [25,26], electrospinning [27], hydrothermal [28,29], and many more.Several techniques exist for synthesizing ZnO nanoparticles, the sol-gel method stands out as a straightforward and uncomplicated approach rooted in the hydrolysis of reactive metal precursors [30][31][32].Several investigations have been conducted to regulate the shape and structure of ZnO.The Morphology of ZnO is influenced by various factors, including the composition of the precursor composition, its concentration, pH levels, temperature, and the incorporation of surfactants.The use of surfactants allows precise control over ZnO nanoparticle size and distribution.This control is crucial for customizing the optical, electrical, chemical, and magnetic properties of nanoparticles to suit specific applications.The introduction of surfactants lowers the surface tension and energy of liquid particles, consequently minimizing their aggregation.This process effectively manages the size and morphology of nanoparticles.This study unveils surfactantassisted nanosized ZnO powder, synthesized using a non-aqueous sol-gel method.The investigation delves into the influence of nanoparticle size achieved through surfactant assistance on vital dielectric properties like dielectric constant, AC conductivity, and transient time.The nomenclature are used S1, S2, and S3 for pure ZnO, CTAB, and PEG-assisted ZnO respectively.

Experimental
2.1.Synthesis of ZnO nanoparticles A 0.25 M concentration solution was prepared by dissolving 0.0125 moles of Zinc acetate in 50 ml of deionized water.The mixture was stirred at 30 °C for an hour until complete dissolution occurred.Separate aqueous solutions of the surfactants CTAB and PEG were prepared, each at a concentration of 0.25 M, by dissolving the specified amounts in 50 ml of deionized water.These solutions were then added to the prepared precursor solution, followed by another hour of stirring.Subsequently, a 2 M solution of NaOH was introduced into the mixture, resulting in the formation of a white aqueous solution.This solution was then passed through a membrane filter, yielding powders that were subjected to calcination at 400 °C for 2 h.The resulting calcined powders were pressed into pellets with a diameter of 12 mm and a thickness of approximately 0.6 mm using a uniaxial hydraulic press.These pellets were further sintered at 450 °C for 4 h in a conventional furnace.

Characterization of ZnO nanoparticles
For crystal phase analysis, a Bruker x-ray diffractometer (D8) operating at 40 kV and 30 mA, scanning from 20°t o 70°with a rate of 2°/s, was employed.Surface morphology studies were conducted using scanning electron microscopy (SEM -model no.JEOL JSM-6610lV) and transmission electron microscopy (TEM -model no.FEI TecnaiG2).The emission properties of the samples were investigated at room temperature, exciting them at 310 nm using an LS-55 (Perkin) spectrophotometer.UV-vis absorbance spectra were obtained in the wavelength range of 200 to 900 nm using a UV-160A (Shimadzu) spectrophotometer.Dielectric measurements were performed at room temperature across a frequency range of 100 Hz to 3000 kHz, employing a HIOKI 3532-50 LCR meter with an oscillation level set at 1 volt.

Structural characterization of ZnO powder
Figure 1 illustrates the X-ray diffraction (XRD) pattern of the synthesized ZnO powders.The pattern distinctly displays Bragg's peaks corresponding to reflections from the (100), (002), ( 101), ( 102), ( 110), ( 103), ( 200), (112), and (201) crystallographic planes, respectively.Among the various samples, the S3 sample stands out with a significantly intense peak, indicating a pronounced crystalline structure.All of these diffraction peaks were unambiguously matched to the Wurtzite crystalline structure of ZnO (JCPDS card No.36-1451) [33].No impurity peaks were identified, affirming the purity of the ZnO crystals in all the products.This observation indicates that the use of various surfactants does not affect the oriented state of resulting ZnO nanostructures.The determination of the crystallite size (D) of the ZnO powder was conducted by employing the Debye-Scherrer's relation.This calculation involved utilizing the full width at half maximum (β) values of distinct major peaks [34].
The crystallite size of the surfactant-free sample was found to be 57 nm.Conversely, the samples prepared with the assistance of surfactants, CTAB and PEG, displayed reduced crystallite sizes, measuring 25 nm and 27 nm, respectively.This surfactant-mediated process effectively minimized the crystallite size of the ZnO nanoparticles.
Figure 2 illustrates the surface morphology of the synthesized powders post-calcination.SEM micrographs reveal distinct characteristics: the surfactant-free sample exhibits dense agglomeration, while those with surfactants demonstrate looser agglomeration.Specifically, figures 2(b) and (c) display the surface morphology of ZnO powders prepared using surfactants CTAB and PEG, respectively.These images reveal that the surfactant-assisted powders showcase reduced conglomeration and a more loosely aggregated structure.The introduction of surfactants serves a crucial role in maintaining minimal particle sizes.This effect is likely attributed to the modifying influence of capping agents like CTAB and PEG on the nanocrystal's morphology [35].
Figure 3 presents the utilization of TEM to unveil the shape and dimensions of the synthesized powders.The morphological analysis indicates that the particles exhibit a spherical morphology.Employing Image J software, the average particle sizes were quantified, yielding measurements of 52 nm, 26 nm, and 28 nm for samples S1, S2,   and S3, respectively.These findings align well with the results obtained from XRD analysis.Notably, figure 3 highlights a significant reduction in particle size facilitated by the inclusion of a surfactant.
Figure 4 illustrates the UV-vis absorption spectrum of the ZnO powder.Prior to testing, the samples were dispersed in absolute ethanol and subjected to ultrasonication.Notably, the absorption maxima were detected at 387 nm, 402 nm, and 427 nm for samples S1, S2, and S3, respectively.
Calculations of the band gap values resulted in 3.07 eV, 3.19 eV, and 3.12 eV for samples S1, S2, and S3, respectively.Intriguingly, a trend emerged where a reduction in particle size correlated with an increase in the band gap.This phenomenon, termed a blue shift, is likely attributed to the quantum confinement effect.Particularly, in sample S2, the most significant blue shift was observed, possibly due to its smallest particle size [36].

Photoluminescence study
To investigate the impact of nanoparticle structure on the photoluminescence (PL) spectrum, an analysis of the PL spectrum was conducted across the range of 320 nm to 600 nm, with excitation at a wavelength of 310 nm, as depicted in figure 5(a).Figures 5(b)-(d) displayed the Gaussian de-convolution of PL spectra of ZnO Powder S1, S2, and S3 respectively.Notably, surfactant-assisted ZnO samples exhibited alterations in emission intensity, accompanied by minor shifts towards longer wavelengths in the peaks.The involvement of surfactants played a pivotal role in modulating the emission spectra of these nanoparticles.This phenomenon is likely attributed to the presence of diverse low-energy defects on the surface of the nanostructures, introduced during growth via the hydrothermal method.Specifically, the peak at 451 nm, corresponding to blue emission, exhibited heightened intensity in the surfactant-assisted synthesized nanoparticles [37].Furthermore, within the CTABmodified sample, an amplified intensity of blue emission peaks was observed in comparison to the other samples [35].This enhancement can be attributed to the influence of various surface oxygen vacancies that alter the positions of defect levels within the forbidden band gap of ZnO [38,39].The fluctuations in peak intensity and shifts in emission peaks in surfactant-modified samples can also be indirectly associated with crystallite size.The reduction in crystallite size in these samples induces a greater degree of strain and augments defect density [37,40].Given that the smallest particle size, as determined through XRD analysis, was observed in the CTABmodified sample, the likelihood of defects was higher in this case [40].Consequently, the photoluminescence investigations of both the as-prepared and surfactant-assisted grown ZnO samples elucidated the influence of surfactants on the intrinsic properties of the ZnO samples.

Dielectric study of ZnO powders
The sintered disks were polished and then silver paste was coated on both the surfaces of the disk.They were then annealed at 50°C for 15 min to establish a strong electrical contact for dielectric measurements.Within this setup, AC parameters encompassing capacitance and dissipation factor (tan δ) were measured across a spectrum of frequencies spanning from 20 Hz to 4000 kHz, all conducted under ambient room temperature conditions.Notably, an evident pattern emerged concerning the relationship between capacitance and increasing frequency.While the S1 sample (lacking surfactant) displayed a sharper decline, samples S2 and S3 (with surfactant) exhibited more gradual decreases, as visually demonstrated in figure 6.This observation underscores the discernible impact of particle size on capacitance-a phenomenon substantiated by earlier investigations on metal oxide samples [41].This phenomenon can be ascribed to the heightened charge density per unit volume within nanomaterials relative to their bulk counterparts, alongside an augmented polarization effect.Additionally, the behavior of capacitance revealed frequency-dependent nuances.It manifested rapid  oscillations at lower frequencies, transitioning into a relatively stable state as frequencies increased.This characteristic behavior can be attributed to the concept of dipole relaxation [42,43].Dipole relaxation pertains to the temporal lag experienced by dipoles when subjected to an electric field.As these dipoles realign and the electric field direction shifts, their phases become discordant, leading to energy dissipation [44].

Dielectric constant
The Dielectric constant (ε΄) of the material was determined using the relation [45].
In this relation, t represents the thickness of the pellet, C p signifies the experimentally obtained equivalent parallel capacitance, ε ο denotes the permittivity of vacuum and A represents the electrode area.The dielectric loss ε′′ was derived from the measured value of dissipation factor (D = tan δ), wherein ε′′ = D × ε΄.Here, both the dissipation factor (D) and equivalent parallel capacitance C p were directly extracted from the measurements.Figure 7 portrays the room temperature dielectric constant of the ZnO samples.It becomes evident through observation that, at lower frequencies, the dielectric constant of the ZnO samples decreases as frequency increases.Conversely, at higher frequencies, it exhibits a nearly constant behavior despite frequency variations.This phenomenon, known as dielectric dispersion, is likely attributed to the accumulation of space charges.This finding aligns well with prior research outcomes concerning ZnO samples [46,47].Upon closer examination of Figure 7, it becomes apparent that, at lower frequencies, the surfactant-assisted nanoparticles display higher dielectric constants.Notably, among the samples, S2 stands out with the highest dielectric constant value.This elevation in dielectric constant within sample S2 is possibly attributed to its smaller particle size, an effect that leads to greater polarization and enhanced dielectric response.The enhanced dielectric constant at room temperature renders surfactant-assisted ZnO a suitable material for charge storage applications.

A.C conductivity
The AC conductivity (σ ac ) was computed using the following relation [45].Here, ε ο represents the permittivity of vacuum, ω stands for the angular frequency, and ε′′ denotes the dielectric loss.The frequency-dependent behavior of the AC conductivity for the ZnO samples is depicted in figure 8. Notably, the AC conductivity showcased a linear response at lower frequencies and demonstrated an increase with ascending frequency.The linear response observed in the AC conductivity is known as long-range order or DC conductivity [48].The escalation in AC conductivity corresponded to ω n , where n takes an integer value within the range of 1 and 2 [49].Interestingly, the AC conductivity of the S1 sample was notably lower than that of samples S2 and S3.This emphasizes the significant role played by surfactants in enhancing the conductivity of the ZnO samples [50].The inset of figure 8 displays a closer overview of ac conductivity for samples S1 and S3.

Transit time
The fixed capacitance values, indicative of the geometric capacitance (C geo ), for the ZnO samples S1, S2, and S3 were determined to be 8.42 pF, 14.56 pF, and 12.32 pF, respectively.Calculations of charge mobility within the ZnO samples were conducted using plots of differential susceptance (ΔB = ω (C−C geo )) against frequency [51].This relationship is illustrated in figure 9, which portrays the frequency-dependent behavior of ΔB under a voltage of 1V.The frequency corresponding to the maximum ΔB value provides the relaxation time value, as outlined in relation.The obtained values of relaxation time for different samples are displayed in table 1. Evidently, the provided data underscores that surfactant-assisted grown samples exhibit notably longer relaxation times in comparison to samples prepared without the utilization of surfactants.

Conclusion
The successful synthesis of surfactant-assisted ZnO particles has been achieved.Both surfactant-assisted and non-surfactant samples exhibited the characteristic Wurtzite structure of ZnO.Notably, the utilization of surfactants resulted in a reduction in the particle size of the ZnO particles.These surfactants played a vital role in quenching emission spectra and enhancing blue emissions.In terms of dielectric behavior, the dielectric constant (ε΄) displayed a decreasing trend with increasing frequency, which can be attributed to polarization effects stemming from the molecules' longer relaxation time at lower frequencies.Moreover, a direct correlation was observed between decreasing nanoparticle size and an increase in dielectric constant.This enhancement in dielectric constant for nanoparticles is likely due to their higher charge density per unit volume.Furthermore, surfactant-assisted grown nanoparticles exhibited elevated AC conductivity and demonstrated a decrease in transit time as particle size decreased.

Table 1 .
The value of relaxation time for different samples.