Heliyon 10 (2024) e40776

Contents lists available at ScienceDirect

Heliyon
journal homepage: www.cell.com/heliyon

Research article

Investigation of structural, optical, antibacterial, and dielectric

properties of sol-gel and biosynthesized TiO2 nanoparticles
Abdullah Al Moyeen a, Raiyana Mashfiqua Mahmud a , Durjoy Datta Mazumder b,
Sondip Ghosh a, Orchi Datta a, Anik Molla a , M. Esmotara Begum a,*
a
Department of Glass & Ceramic Engineering, Rajshahi University of Engineering & Technology (RUET), Rajshahi-6204, Bangladesh
b
Department of Materials Science & Engineering, Rajshahi University of Engineering & Technology (RUET), Rajshahi-6204, Bangladesh

A R T I C L E I N F O A B S T R A C T

Keywords: This study explored the structural, optical, antibacterial, and dielectric properties of TiO2
Sol-gel nanoparticles synthesized using two distinct approaches: sol-gel and biosynthesis. Density func­
Biosynthesis tional tight binding (DFTB+) and density functional theory (DFT) calculations were employed
Allium sativum
alongside experimental techniques to gain a comprehensive understanding of the electronic-
Band gap
Antibacterial activity
property relationships. Allium sativum peel extract was utilized for the biosynthesis method. X-
Dielectric properties ray diffraction (XRD) affirmed the anatase phase formation for both nanoparticles. Rietveld
technique was employed for a detailed structural analysis. The FESEM analysis revealed the
diminutive particle size of TiO2 nanoparticles with a comparable size distribution for both vari­
ants. However, the biosynthesized variant exhibited smaller average particle size (26.74 nm) than
the sol-gel variant (32.22 nm). Optical studies showed an absorption redshift for the bio­
synthesized variant (352 nm) relative to the sol-gel variant (347 nm). The band gap energy is
higher for the sol-gel variant (3.17 eV) compared to the biosynthesized variant (3.02 eV). The
biosynthesized nanoparticles showed strong antibacterial activity, with inhibition zones of 15 mm
against E. coli and S. flexneri bacteria. Dielectric analysis revealed that the sol-gel synthesized
nanoparticles exhibited a higher dielectric permittivity of 27.80 and a lower dielectric loss of
0.37 at 1 kHz, compared to the biosynthesized nanoparticles, which showed a dielectric
permittivity of 19.48 and a dielectric loss of 0.69.

1. Introduction

Nanoparticles, sizes in the range of 100 nm exhibit significant changes in optical, magnetic, and electrical properties compared to
their bulk counterparts. These distinct properties of nanoparticles are driven by quantum confinement effects, a high surface-to-
volume ratio, minimal gravitational forces, and dominating electromagnetic forces [1,2]. These remarkable properties of nano­
particles have revolutionized numerous fields, including nanotechnology, biology, optics, photocatalysis, sensing, and electronic
systems [3]. Among different nanoparticles, metal oxides like ZnO, CuO, TiO2, MgO etc., have demonstrated their potential in diverse
applications. Among these, TiO2 stands out for its versatility, non-toxicity, and exceptional electrical properties, finding its applica­
tions in the fields of environmental remediation, photovoltaics, energy storage devices, self-cleaning surfaces, and biomedical devices.
TiO2 exists in three distinct crystalline phases (anatase, brookite, and rutile), offers a range of properties that can be tailored for

* Corresponding author.
E-mail addresses: abdullah264253@gmail.com (A.A. Moyeen), meb@gce.ruet.ac.bd (M.E. Begum).

https://doi.org/10.1016/j.heliyon.2024.e40776
Received 12 September 2024; Received in revised form 2 November 2024; Accepted 27 November 2024
Available online 28 November 2024
2405-8440/© 2024 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
A.A. Moyeen et al. Heliyon 10 (2024) e40776

specific applications [4]. Notably, anatase TiO2 nanoparticles (NPs) possess excellent optical and dielectric properties along with
biocompatibility, and high thermal stability [5]. These attributes have led to their widespread use in photocatalysis, dye-sensitized
solar cells (DSSCs), lithium-ion batteries, optoelectronic devices, hydrogen peroxide (H2O2) reduction, and solar cells [6–8]. How­
ever, the selection of synthesis methods for TiO2 nanoparticles plays a crucial role in determining their crystalline phases, which in turn
affects their structural, optical, antibacterial, and electrical characteristics [9]. In recent years, a variety of synthesis methods have
been developed to tailor the structure and properties of TiO2 nanostructures, including the sol-gel process, solvothermal synthesis,
electrodeposition, direct oxidation, chemical vapor deposition, hydrothermal techniques, and biosynthesis [10–14]. Table 1 sum­
marizes the influence of various synthesis approaches and calcination conditions on the size and optical properties of TiO2 NPs.
It is noteworthy that, most of the synthesis techniques require a controlled atmosphere, expensive instrumentation, and the use of
chemical components to reduce and control nanoparticle sizes. Among these methods, the sol-gel process stands out for its simplicity
and ease of handling, yet it can produce TiO2 NPs with excellent photocatalytic activity, high crystalline quality, well-controlled
particle size and shape [26]. The conventional sol-gel method involves the hydrolysis and polycondensation of metal alkoxide,
leading to the formation of oxopolymers, which are then converted into an oxide network. The structural and characteristic properties
of the resulting metal oxides are significantly affected by the rates of hydrolysis and polycondensation during synthesis [27]. This
process often requires the use of additional chemicals to control particle sizes, which are eventually released into the atmosphere.
Another promising approach, the biosynthesis method, has gained wide acceptance due to its ability to eliminate the toxic effects
associated with various traditional chemical synthesis methods [28]. Utilizing plant elements for nanoparticle synthesis is particularly
desirable because it is a one-step green reduction method that requires less energy, resulting in eco-friendly nanoparticles with diverse
properties, high stability, and suitable dimensions. The use of plant extracts in the biosynthesis of TiO2 nanoparticles has emerged as a
promising alternative due to its environmentally friendly and cost-effective nature. Biologically synthesized TiO2 NPs exhibit not only
excellent antibacterial activity but also diverse biological properties such as anti-inflammatory, antifungal, and antimicrobial func­
tionalities. Additionally, the photo-semiconductor capabilities of these nanoparticles enhance their biological activity, making them
highly effective in various applications [29]. Several plant extracts have been explored for the synthesis of TiO2 NPs, including Acorus
calamus leaf, Aloe vera leaf, Jatropha curcas L., Lemon peel, and more [15,18,25,30]. In addition, Garlic peels (Allium sativum) hold
immense promise for eco-friendly synthesis of TiO2 NPs. It is recognized for their exceptional physiological activity [31]. The presence
of bioactive components such as phenolics, flavonoids, alliin, and allicin in garlic peel extract contributes to its high ability to scavenge
free radical [32]. Although commonly seen as waste, the peels of Allium sativum can be repurposed as a reducing agent, making their
use a sustainable approach in nanoparticle production.
In this study, the authors aimed to synthesize TiO2 NPs using two methods: a traditional sol-gel approach and a novel biosynthesis
technique utilizing garlic peel extract. A detailed comparison was then conducted on the TiO2 NPs, in terms of their structural, optical,
antibacterial, and dielectric properties. The study integrated experimental techniques with DFTB+ and DFT calculations to provide an
in-depth analysis of the relationships between electronic properties and nanoparticle characteristics.

2. Computational and experimental methods

2.1. Calculation model and method

In the present work, density functional theory and density functional tight binding methods were used. In the context of the
implementation of DFT, Schrödinger equation (Eqn. (1)) remains all-important of this calculation.

Table 1
Influence of different synthesis techniques and calcination conditions on TiO2 NPs size and optical properties.
Methods Calcination Particle Size Absorption Maxima Band Gap Ref.

Biosynthesis 600 ◦ C (3 h) 11–30 nm 355 nm 3.20 [15]

Acorus calamus leaf
Sol-gel 400 ◦ C (3 h) ~13 nm 432 nm 2.90 [16]
Hydrothermal 500 ◦ C (2 h) 32–48 nm 385 nm – [17]
Biosynthesis 500 ◦ C (2 h) 80–140 nm 325 nm 3.08 [18]
Lemon peel
Biosynthesis 500 ◦ C (4 h) 8–13 nm 330 nm 3.13 [19]
Gum Kondagogu
Simple precipitation 500 ◦ C (1 h) 7–21 nm 475 nm 3.30 [20]
Biosynthesis 600 ◦ C (3 h) 11–30 nm 360 nm 3.29 [21]
Acorus calamus leaf
Biosynthesis 550 ◦ C (2 h) 80–100 nm 300 nm 3.22 [22]
Citrus Limetta
Solvothermal 200 ◦ C (2 h) ~13.4 nm 381 nm 3.20 [23]
Sol-gel 450 ◦ C (3 h) ≥80 nm 350 nm 3.20 [24]
Biosynthesis 500 ◦ C (5 h) 10–80 nm 344 nm 3.19 [25]
Aloe vera leaf
Sol-gel 500 ◦ C (3 h) 20–45 nm 347 nm 3.17 This work
Biosynthesis 500 ◦ C (3 h) 15–35 nm 352 nm 3.02 This work
Allium sativum peel

2
A.A. Moyeen et al. Heliyon 10 (2024) e40776

HΨ
̂ = EΨ (1)

where E represents the energy of the system, H ̂ is the Hamiltonian operator, and Ψ denotes the wavefunction. To simplify this equation,
the Born–Oppenheimer approximation is often applied. The methods of the DFT, based upon the Kohn-Sham theorem, make it possible
to solve problems concerned with molecular and solid material electronics. It adeptly transforms the daunting many-electron problem
into a more manageable single-electron framework. This is achieved through the Hohenberg–Kohn theorems, which state that the
ground-state properties of a system are determined by its electron density, ρ(r). The Kohn–Sham equation (Eqn. (2)) then permit the
computation of the total energy (Eν [ρ]) via a non-interacting reference system.
∫ ∫
1 ρ(r)ρ(rʹ)
Eν [ρ] = Ts [ρ] + Exc [ρ] + ρ(r)vext (r) dr + dr drʹ (2)
2 |r − rʹ|

In this expression, Ts [ρ] signifies the kinetic energy of the non-interacting electrons, vext represents the external potential, and Exc
denotes the exchange-correlation energy. To approximate the exchange-correlation energy, methodologies such as the Generalized
Gradient Approximation (GGA) and the Local Density Approximation (LDA) were employed. Specifically, for GGA, the Perdew-Burke-
Ernzerhof (PBE) functional defines the Exc as Eqn. (3).
∫
EGGA
XC [ρ] = ρ(r)ϵhom
XC (ρ(r), ∇ρ(r) ) dr (3)

Conversely, LDA presupposes that the exchange-correlation energy density at any given point is determined solely by the local
electron density (ρ(r)), as shown in Eqn. (4).
∫
ELDA
XC [ρ] = ρ(r)ϵhom
XC (ρ(r)) dr (4)

The Ceperley-Alder/Perdew-Zunger (CA-PZ) functional represents a specific LDA formulation. Collectively, these methods
constitute the foundation of DFT, facilitating the efficient exploration of the electronic structures inherent in complex systems [33].
In our alternative computational approach, we employed DFTB+ within Materials Studio, which is an efficient implementation of
the Density Functional based Tight Binding (DFTB) method. DFTB is a streamlined version of DFT, where the total energy of a system is
expanded using a Taylor series derived from Kohn-Sham density functional theory. The electron density (ρ(r)) is expressed as the sum
of a reference density (ρ0 (r)) and a small fluctuation (δρ(r)), as shown in Eqn. (5).

ρ(r) = ρ0 (r) + δρ(r) (5)

The total energy (E[ρ]) in DFTB is then expanded as Eqn. (6).

[ ] [ ]
E[ρ] = E0 [ρ0 ] + E1 [ρ0 , δρ] + E2 ρ0 , (δρ)2 + E3 ρ0 , (δρ)3 (6)

In this expansion, E0 and E1 form the basic DFTB (1) model, which is the simplest version of DFTB. The term E2 incorporates second-
order corrections, defining the DFTB2 model, while E3 adds third-order corrections, leading to the more accurate DFTB3 model. The
different DFTB models have reasonably clear areas of application, with each level (DFTB (1), DFTB2, and DFTB3) balancing
computational efficiency and precision, making DFTB highly effective for large-scale materials simulations due to its significant speed
advantage over DFT [34]. This makes DFTB ideal for systems involving weak interactions, such as Van der Waals complexes [35].
In this study, Geometry optimization of the structure was carried out using the DFTB+ approach within the Materials Studio with
the “Smart” technique, 0.05 kcal/mol energy convergence, and 0.5 kcal/mol/Å force convergence over a maximum of 500 cycles. To
solve the Schrödinger equation for bonded electrons, the ‘tiorg’ Slater-Koster library was used for SCC calculations, with a tale of 1 ×
10− 5 and 50 maximum iterations. DFT calculations as a comparison were implemented with Materials Studio CASTEP software using
the LDA and the CA-PZ functional and Koelling-Harmon relativistic treatment. Pseudopotentials were in reciprocal disposed space, and
300 eV plane wave cut-off, total electrons 48, non-spin-polarized conditions, and 24 bands. Energy minimization converged with a 0.2
× 10− 5 eV tolerance, total energy per atom, and 0.5 × 10− 6 eV eigen energy over a maximum of 100 SCF cycles, with the Pulay scheme
for density mixing. Band structure calculations included 46 bands per k-point, with a 0.1 × 10− 4 eV convergence tolerance. In addition,
Perdew-Burke-Ernzerhof (PBE)/Generalized Gradient Approximation (GGA) functional DFT was calculated with similar conditions
300 eV cut-off, 48 electrons, non-spin-polarized, 24 bands, and Monkhorst-Pack grid size 4 × 4 × 5 with 26 k-points. For the
computational analysis of dielectric properties, we employed DFT calculations, utilizing the Generalized Gradient Approximation with
the Perdew-Burke-Ernzerhof functional. The computations were performed along the [1 0 0] polarization axis, with a scissors operator
of 0 eV applied. An instrumental smearing of 0.5 eV was used to account for broadening effects, while the plasma frequency was set to
0 eV.

2.2. Experimental

2.2.1. Synthesis of TiO2 NPs by sol-gel method

For the preparation of TiO2 NPs by the sol-gel process, 80 mL ethanol was mixed with 10 mL deionized water under constant
stirring using a magnetic stirrer. Then 5 mL titanium isopropoxide (TTIP) was added by dropwise to the above-mixed solution under

3
A.A. Moyeen et al. Heliyon 10 (2024) e40776

constant stirring and followed by a 15 h stirring period. Subsequently, the resulting gel was dried in the hot-air oven at a temperature of
80 ◦ C for 8 h. Further, the calcination was performed for 3 h at 500 ◦ C and the prepared sample was named ‘TSol’.

2.2.2. Synthesis of TiO2 NPs by biosynthesis method

The outer peels of Allium sativum were cleaned with tap and deionized water, cut into small pieces, and mixed with 100 mL of
deionized water. This mixture was boiled at 80 ◦ C for 30 min to extract bioactive compounds. After cooling to room temperature, the
extract was filtered using Whatman No. 1 filter paper to remove impurities. The extract was then refrigerated for further use.
A 0.1 M solution of TTIP in 80 mL of deionized water was prepared. Then, 40 mL of Allium sativum peel extract was added, and the
mixture was stirred at 600 rpm for 3 h at 60 ◦ C, with pH controlled using a few drops of NH3 (aq.). The solution was kept for 24 h to
complete gelation and aging. Afterward, the mixture was centrifuged and rinsed with ethanol and deionized water. The TiO2 NPs were
dried at 80 ◦ C for 8 h and then calcined at 500 ◦ C for 3 h. The prepared sample was named ‘TBio’.

2.2.3. Pellet preparation

The TiO2 NPs were formed into pellets for electrical characterization. Each nanoparticle sample weighing 1 g was mixed with a
polyvinyl alcohol binder and compacted into disk-shaped pellets with a diameter of 12 mm under a pressure of 2.5 tons. Subsequently,
the pellets underwent sintering in an air environment at 750 ◦ C.

2.2.4. Characterization
Crystallographic analysis of TiO2 NPs was performed using a SHIMADZU LabX XRD-6100 (Japan) instrument. The X-ray diffraction
(XRD) data was analyzed using the FullProf software via Rietveld refinement. Field emission scanning electron microscopy (FESEM)
was employed to investigate the surface morphology of the TiO2 NPs using a JEOL JSM-7600F (Germany) instrument. ImageJ software
was then utilized to assess the avg. particle size and distribution from the FESEM images.
The UV–vis absorption spectrum of TiO2 NPs was measured using a Shimadzu UV-2600i (Japan) spectrophotometer over wave­
lengths from 250 to 800 nm. An ethanol suspension of the nanoparticles was utilized for the analysis.
The antibacterial activity of the synthesized TiO2 NPs was evaluated using the agar well diffusion method against four bacterial
strains: gram-positive Staphylococcus aureus (ATCC25923), and gram-negative Escherichia coli (ATCC25922), Pseudomonas aeruginosa
(ATCC27853), and Shigella flexneri (clinical isolate). Fresh bacterial cultures were uniformly spread on agar plates. Paper discs

Fig. 1. (a) XRD patterns of TiO2 NPs and Rietveld refined pattern of (b) TSol and (c) TBio.

4
A.A. Moyeen et al. Heliyon 10 (2024) e40776

containing TiO2 NPs at concentrations of 100, 200, and 300 μg/mL were placed on the agar surface and incubated for 24 h at 30 ◦ C. The
zones of inhibition, represented as circular areas around the discs where bacterial growth was suppressed, were measured in
millimeters.
The dielectric measurements of the synthesized TiO2 nanoparticle pellets were investigated using the conventional parallel plate
capacitor method. Measurements were performed in the frequency range of 40 Hz to 10 kHz with an Agilent 4294A Precision
Impedance Analyzer (USA).

3. Results and discussion

3.1. Structural characteristics

Fig. 1a presents the XRD patterns of TSol and TBio samples acquired in 2θ range of 20–80◦ . According to JCPDS card No. 01-084-
1286, the intense peaks of TiO2 NPs are identified along the (101), (004), (200), (105), (211), (204), (116), (220), and (215) planes.
Table 2 compares the XRD peak positions (2θ) of TSol and TBio NPs with the standard values. The indexed planes confirm the pure
tetragonal anatase phase of TiO2, characterized by octahedral and trigonal planar coordination geometry [16]. Interestingly, the TBio
sample have higher intensity peaks compared to the TSol sample. This observation might be attributed to the existence of polyphenolic
compounds within the plant extract used for biosynthesis [17]. These biomolecules can potentially influence the crystallization
process, leading to the formation of NPs with improved crystallinity.
For a more quantitative analysis of the crystallographic structure, the obtained XRD data were subjected to Rietveld refinement
using the FullProf software. A Pseudo-Voigt fitting function was employed. The final refined parameters, including lattice parameters,
cell volume, and profile R-factors, are summarized in Table 3. The good agreement between the calculated and observed XRD patterns
(Fig. 1b and c) suggests a high-quality fit and supports the presence of a pure anatase phase. Furthermore, the crystallographic
structure of the anatase TiO2 NPs was visualized using the VESTA software (inset of Fig. 1b, violet and green spheres represent titanium
and oxygen atoms, respectively).
The crystallite size of the TiO2 NPs was determined using Scherrer’s equation based on the prominent diffraction peak corre­
sponding to the (101) plane. Scherrer’s equation is expressed as: d = 0.89λ/βcosθ, where d represents the crystallite size (nm), λ is the
X-ray wavelength (nm), β is the full width at half maximum (FWHM), and θ is the Bragg diffraction angle [36]. The estimated crystallite
sizes were ~9.54 and ~11.14 nm for the TSol and TBio samples, respectively. These values are quite comparable with the reported
literature [37,38]. Furthermore, the observed peak broadening in the XRD pattern of the TSol sample also support the presence of a
smaller crystallite size compared to TBio [16].
The crystallinity of the TiO2 NPs was further quantified using the XRD data. This analysis involved calculating the percentage
contribution of the crystalline and amorphous phases within the samples. By this method, the crystallinity of the sol-gel synthesized
TiO2 NPs was determined to be 75.76 %, while the biosynthesized NPs exhibited a higher crystallinity of 78.56 %. All the crystal­
lographic parameters are presented in Table 3.

3.2. Morphological analysis

Fig. 2 displays the FESEM micrographs of TSol and TBio nanoparticles. The FESEM observations indicate that both samples
exhibited predominantly spherical nanoparticles. Interestingly, the biosynthesized TiO2 NPs displayed a more uniform size distri­
bution compared to the TSol NPs. The sol-gel synthesized nanoparticles showed a tendency to form irregular, aggregated structures,
possibly due to the rapid hydrolysis and condensation reactions during the synthesis process. In contrast, the TBio NPs appeared
clearer, lighter, and less prone to agglomeration. This suggests that the garlic peel extract acted as a stabilizing agent during the
synthesis process, influencing the growth and dispersion of the nanoparticles.
The inset histograms in Fig. 2a, b depict the particle size distribution for each sample. The majority of particles in the TSol sample
fall within the range of 20–45 nm, while the TBio sample exhibits a narrower size distribution with particles ranging from 15 to 35 nm.
The avg. particle size was calculated to be 32.22 nm for TSol and 26.74 nm for TBio. This confirms the effectiveness of the garlic peel
extract in reducing the particle size of the biosynthesized TiO2 NPs.

Table 2
Shows comparison of XRD data of TiO2 NPs with standard value.
Miller Plane (hkl) JCPDS Card No: TSol (2θ) TBio (2θ)
01-084-1286 (2θ)

101 25.32 25.34 25.30

004 37.84 37.93 37.94
200 48.07 48.09 48.02
105 53.95 54.03 54.02
211 55.10 55.11 55.05
204 62.75 62.76 62.75
116 68.84 69.04 69.04
220 70.34 70.30 70.29
215 75.12 75.30 75.27

5
A.A. Moyeen et al. Heliyon 10 (2024) e40776

Table 3
Crystallographic parameters of TSol and TBio NPs.
Crystallographic parameters TSol TBio

Lattice parameters a=b 3.785 Å 3.787 Å

c 9.487 Å 9.484 Å
Cell Volume 135.886 Å3 135.980 Å3
R-factors Rp 3.37 3.41
Rwp 4.26 4.35
Rexp 3.99 4.05
Rf-factor 1.53 2.71
Bragg R-factor 2.23 4.07
GOF 1.1 1.1
X-ray density 4.160 gcm− 3 4.044 gcm− 3
FWHM 0.8377 0.7157
Crystallite size For (101) plane 9.536 nm 11.139 nm
Crystallinity 75.76 % 78.56 %

Fig. 2. FESEM image of (a) TSol and (b) TBio and inset showing particle size distribution.

3.3. Optical properties

Fig. 3a shows the absorption spectra of TiO2 NPs fabricated using sol-gel and biosynthesis methods. Understanding optical
properties is crucial for photocatalysis study, as they determine the light absorption efficiency of the material during the photocatalysis
process [22]. The UV–visible absorption profile of the synthesized nanoparticles spans from 250 nm to 800 nm. Both types of

Fig. 3. (a) UV–vis absorption spectrum and (b) the Tauc plot of TiO2 NPs.

6
A.A. Moyeen et al. Heliyon 10 (2024) e40776

nanoparticles exhibited a characteristic absorption spectrum with a single broad peak in the UV region. The observed absorption peak
originates from the excitation of electrons from the valence band of the TiO2 NPs to the conduction band. The absorption maxima
(λmax) for TSol and TBio NPs were found to be 347 nm and 352 nm, respectively, further confirming the presence of TiO2 [25]. The
observed slight red shift in the absorption peak of TBio NPs compared to TSol NPs could be attributed to variations in particle size and
configuration.
The direct band gap energy (Eg ) of the TiO2 NPs was evaluated by employing the Tauc relation (Fig. 3b) [39]. The Eg of TSol NPs
was found to be 3.17 eV, which is higher than that of TBio NPs (3.02 eV). The Tauc plot was employed here to more accurately
determine the band gap energy, as broadened absorption peaks can sometimes obscure precise absorption values. The Eg values re­
ported by Alhalili et al. [25] (3.19 eV with an absorption peak at 344 nm) and Zedek et al. [40] (3.02 eV with a peak near 350 nm) are
quite similar to those observed in our samples. Furthermore, the difference in Eg between TSol and TBio NPs can be primarily explained
by the quantum confinement effect [41]. According to this, the confinement of electrons and holes within the smaller crystallites of
TSol NPs results in discrete energy levels and a consequent widening of the band gap. Moreover, the reduced band gap of TBio NPs
could be ascribed to the existence of localized gap states induced by Ti3+ ions [42,43]. Additionally, oxygen vacancies within the
crystal lattice might also contribute to the reduction in band gap energy, potentially enhancing their photocatalytic activity [42]. Since
a narrower band gap corresponds to improved light absorption in the visible region, the TBio NPs with their lower Eg value (3.02 eV)
are expected to demonstrate improved photocatalytic activity compared to the TSol NPs (3.17 eV) [44].

3.3.1. Computational analysis of the band gap

In the present work, we used a variety of computational approaches to study the band gap of TiO2 NPs. We compared the obtained
results with experimental findings to determine their validity in this section. The band gap was experimentally determined based on
the UV–vis spectroscopy to be 3.17 eV for TSol NPs, and 3.02 eV for TBio NPs. From the DFTB+ simulations the band gap was
determined to be 3.189 eV (See Fig. 4a), demonstrating excellent agreement with the experimental band gap of TSol NPs as well as
TBio NPs. Conversely, DFT calculations using various functionals produced significantly lower values. For DFT calculations, different
computational settings were employed. Using the GGA/PBE functional with a 4 × 4 × 5 k-point grid and a plane wave cutoff of 300 eV,
the convergence process yielded total energies of − 429.2137 eV for oxygen after 23 iterations and − 1596.0330 eV for titanium after 32
iterations. The resulting band gap was 2.091 eV (See Fig. 4b). For Local Density Approximation (LDA) with CA-PZ functional, similar
parameters were used. Convergence was attained after 24 iterations for oxygen, resulting in a total energy of − 428.3332 eV, and after
32 iterations for titanium, yielding a total energy of − 1594.8364 eV. The band gap was 2.048 eV, as depicted in Fig. 4c. Both studies
demonstrated high computational efficiency, with the GGA/PBE method obtaining an impressive parallel efficiency of 89 % for G-
vector and k-point parallelization. Similarly, the LDA/CA-PZ method achieved a commendable efficiency of 87 %, highlighting the
effective distribution of data by G-vector (6-way) and k-point (2-way).
Additionally, the density of states (DOS) calculations provide crucial insights into the electronic behavior of TiO2. Fig. 5 illustrates
the distribution of electronic states near energy levels in the DOS, essential for understanding the material’s conductivity and optical

Fig. 4. Bandgap visualization of TiO2 calculated by (a) DFTB+, (b) GGA/PBE, and (c) LDA/CA-PZ functional.

7
A.A. Moyeen et al. Heliyon 10 (2024) e40776

properties.
In summary, DFTB+ outperformed other methods, accurately predicting the band gap of TiO2 (3.189 eV) close to experimental
values. On the other hand, DFT techniques that employed GGA/PBE and LDE/CA-PZ functionals showed a notable underestimation of
the band gap, with respective values of 2.091 eV and 2.048 eV. These results highlight the limitations of GGA/PBE and LDA-based
functionals in accurately predicting the band gap of TiO2 and underscores the superiority of DFTB+ for investigating the electronic
properties of TiO2 and similar materials.

3.4. Antibacterial activity

The antibacterial efficacy of TiO2-based materials is attributed primarily to the generation of reactive oxygen species (ROS) [45,
46]. These ROS, including hydroxyl radicals as well as superoxide anions, are extremely reactive and can directly damage bacterial cell
walls [47]. The resultant damage to cellular components such as lipids, DNA, and proteins ultimately results in the loss of cellular
functions and cell death [48,49]. Fig. 6 illustrates the antibacterial activity mechanism of TiO2 NPs.
In this research, the antibacterial properties of TiO2 NPs synthesized via sol-gel and biosynthesis methods were evaluated against
both gram-positive and gram-negative bacteria. Fig. 7 illustrates the antibacterial efficacy of both TSol and TBio NPs against
Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, and Shigella flexneri. The histogram depicting the zones of inhibition
(ZOI) is presented in Fig. 8. The cell walls of gram-positive bacteria possess a thick peptidoglycan layer, whereas gram-negative
bacteria have a thin peptidoglycan layer. Microbial pathogens can cause various diseases in living organisms. The maximum ZOI
observed for TSol NPs against the gram-negative bacteria E. coli, P. aeruginosa, and S. flexneri were 12 mm, 14 mm, and 10 mm,
respectively. For the gram-positive bacterium S. aureus, the ZOI was 12 mm. Interestingly, P. aeruginosa demonstrated the most sig­
nificant zone of inhibition among the bacteria tested for TSol NPs.
The TBio NPs, however, demonstrated overall higher antibacterial activity compared to TSol. This enhanced efficacy can likely be
attributed to their smaller particle size and higher surface area than TSol NPs, which aligns with observations reported by Krishna R.
Raghupathi [50], suggesting that the antibacterial activity of nanoparticles is a size-dependent property that increases with particle
size reduction. The maximum ZOI for TBio NPs against E. coli, P. aeruginosa, and S. flexneri were 15 mm, 12 mm, and 15 mm,
respectively. The bacteria S. aureus showed similar susceptibility to TSol NPs (12 mm). Notably, E. coli and S. flexneri displayed the
highest susceptibility to TBio NPs.

3.5. Dielectric properties

The dielectric properties of a material are closely linked to its electro-optic response. In the microelectronics industry, materials
with low dielectric constants are particularly sought after for applications as interlayer dielectrics [51]. In this study, the frequency
dependence of the dielectric constant (ε′) and the loss tangent (tanδ) were measured for pelletized TiO2 nanoparticle samples. The

Fig. 5. Total density of states calculated by (a) DFTB+, (b) GGA/PBE, and (c) LDA/CA-PZ functional.

8
A.A. Moyeen et al. Heliyon 10 (2024) e40776

Fig. 6. Antibacterial activity mechanism of TiO2 NPs.

Fig. 7. Antibacterial activity of (a–d) TSol and (e–h) TBio NPs against E. coli, S. aureus, P. aeruginosa, and S. flexneri bacteria.

Fig. 8. Histogram of Zone of Inhibition of TSol and TBio NPs against E. coli, S. aureus, P. aeruginosa, and S. flexneri bacteria.

measurements were conducted at room temperature and across a range of frequencies (40 Hz–10 kHz). Fig. 9a, b depicts the frequency
dependence of the dielectric constant for the TSol and TBio sample, respectively. The insets of each figure show the corresponding loss
tangent values. As observed in Fig. 9a, the TSol sample exhibits the highest dielectric permittivity, reaching a value of 27.80 at 1 kHz.
This value is accompanied by a tanδ of 0.37 at the same frequency. In comparison, the TBio sample demonstrates a lower dielectric

9
A.A. Moyeen et al. Heliyon 10 (2024) e40776

constant of 19.48 at 1 kHz, with a higher dielectric loss of 0.69 (Fig. 9b).
As shown in Fig. 9a and b, the ε′ for both TSol and TBio sample exhibits a characteristic frequency dependence. At lower fre­
quencies, both samples exhibit high ε′ values that decrease with increasing frequency. The dielectric constant is fairly remaining
constant in the high-frequency region. The high ε′ at low frequency is likely due to the combined effect of multiple polarization
mechanisms within the material [52,53]. At low frequencies, all four main types (electronic, ionic, orientational, and space-charge)
can contribute, with space-charge polarization likely dominating and leading to the initial rise in ε’ [54,55]. However, as the fre­
quency increases, these polarization mechanisms become less efficient. They can no longer respond as effectively to the rapidly
changing electric field, resulting in a decrease in the overall dielectric constant observed at higher frequencies [56].
The inset of Fig. 9a, b reveals the dependence of tanδ on frequency. Both TSol and TBio samples exhibit a high tanδ at lower
frequencies (40 Hz), likely due to the space-charge polarization mechanisms contributing to both dielectric constant and loss [57]. As
frequency increases, tanδ decreases gradually, reaching a frequency-independent region above 3 kHz for TBio. This low tanδ value at
high frequencies for TBio could be attributed to the nanomaterial’s purity, as a purer material with greater crystallinity tends to have a
more ordered structure with fewer defects [58]. Notably, the absence of any dielectric loss peak for TBio at higher frequencies indicates
its potential suitability for high-frequency optoelectronic applications [59]. In contrast, the TSol sample exhibits a more complex
behavior with a second increase in tanδ at higher frequencies, suggesting a possible double relaxation process [60]. However, despite
this additional relaxation, the TSol NPs possess a lower tanδ value compared to TBio at 1 kHz.

3.5.1. Computational analysis of the dielectric properties

For analyzing the dielectric properties of the TSol and TBio samples in this computational study, we compared them with
experimental results for the dielectric function (ϵ (ω)). A key aspect of calculating the complex dielectric function involves using Eqn.
(7).
ϵ(ω) = ϵ1 (ω) + iϵ2 (ω) (7)
This dielectric function describes the electronic response of the material for the interaction with the electromagnetic wave [61].
The real part (Fig. 10a and b) of the dielectric function is ϵ1 (ω), and it can be obtained through Kramers-Kronig relations which relates
the real and imaginary part of dielectric function to maintain causality and consistency of optical response of the system. This makes it
possible to establish the characteristics of the material used when exposed to different intensities of electromagnetic waves and come
up with the theoretical background of the results observed in the experiments.
In the experimental findings, a high dielectric constant at low frequencies shows strong effects of polarization, while as the fre­
quency increases, the polarization effect reduces, as polarization mechanisms are less effective at high frequencies. The same can be
observed in the present computational study, where trends occurring below 10 eV coupled with marked peaks represent enhanced
dielectric response above this energy limit while beyond which (ϵ1 ) tends to reduce gradually. On this basis, it is concluded that, at
higher energies (corresponding to the higher frequencies), the material’s capacity to the weak polarization decreases, which correlates
with the decrease of value (ϵʹ) observed in the experiment. A comparison between the theoretical estimates and the simulation results
shows a good agreement where it emphasizes the strong polarization effects at low frequency or energy range and declines with an
increase in frequency or energy. This high dielectric response at lower frequencies as observed in both approaches supports the
authenticity of the material’s behavior across the different approaches.

4. Conclusions

In conclusion, this study successfully synthesized TiO2 NPs by sol-gel method and environmentally friendly biosynthesis technique
using Allium sativum peel extract. Comprehensive analysis through DFTB+, DFT calculations, and experimental techniques provided a

Fig. 9. Frequency dependence of the dielectric behavior for the (a) TSol and (b) TBio sample.

10
A.A. Moyeen et al. Heliyon 10 (2024) e40776

Fig. 10. Calculated real part of the dielectric function (ϵ1 (ω)) vs energy graph for the (a) TSol and (b) TBio sample.

detailed understanding of the electronic-property relationships of TiO2 NPs. Notably, the biosynthesized nanoparticles exhibited
smaller particle sizes compared to the sol-gel variant. Additionally, the biosynthesized nanoparticles displayed a redshift in their
optical absorption and a lower band gap energy of 3.02 eV. Furthermore, the biosynthesized nanoparticles exhibited superior anti­
bacterial activity, with zones of inhibition measuring 15 mm for both E. coli and Shigella flexneri, outperforming the sol-gel synthesized
nanoparticles, which showed maximum inhibition of 14 mm for P. aeruginosa. However, the sol-gel variant exhibited a higher
dielectric permittivity of 27.80 at 1 kHz, while maintaining a reduced dielectric loss of 0.37. These findings highlight the effectiveness
of synthesis method selection in controlling the properties of TiO2 NPs for diverse applications in optoelectronics, dielectrics, and
antibacterial materials.

CRediT authorship contribution statement

Abdullah Al Moyeen: Writing – original draft, Methodology, Formal analysis, Conceptualization, Project administration. Raiyana
Mashfiqua Mahmud: Writing – original draft, Visualization, Formal analysis. Durjoy Datta Mazumder: Writing – original draft,
Software. Sondip Ghosh: Investigation, Data curation. Orchi Datta: Investigation, Data curation. Anik Molla: Writing – review &
editing, Resources, Data curation. M. Esmotara Begum: Writing – review & editing, Validation, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to
influence the work reported in this article.

Acknowledgments

The authors sincerely acknowledge the Department of Glass & Ceramic Engineering, RUET for facilitating this research by
providing access to the laboratory facilities.

References

[1] J.M. Romo-Herrera, M. Terrones, H. Terrones, S. Dag, V. Meunier, Covalent 2D and 3D networks from 1D nanostructures: designing new materials, Nano Lett. 7
(2007) 570–576, https://doi.org/10.1021/nl0622202.
[2] C.B. Ng, L.S. Schadler, R.W. Siegel, Synthesis and mechanical properties of TiO2-epoxy nanocomposites, Nanostruct. Mater. 12 (1999) 507–510, https://doi.
org/10.1016/S0965-9773(99)00170-1.
[3] G. Nabi, Qurat-ul-Aain, N.R. Khalid, M.B. Tahir, M. Rafique, M. Rizwan, et al., A review on novel eco-friendly green approach to synthesis TiO2 nanoparticles
using different extracts, J. Inorg. Organomet. Polym. Mater. 28 (2018) 1552–1564, https://doi.org/10.1007/s10904-018-0812-0.
[4] R. Mohan, J. Drbohlavova, J. Hubalek, Water-dispersible TiO2 nanoparticles via a biphasic solvothermal reaction method, Nanoscale Res. Lett. 8 (2013) 503,
https://doi.org/10.1186/1556-276X-8-503.
[5] C.Y. Huang, P. Selvaraj, G. Senguttuvan, C.J. Hsu, Electro-optical and dielectric properties of TiO2 nanoparticles in nematic liquid crystals with high dielectric
anisotropy, J. Mol. Liq. 286 (2019) 110902, https://doi.org/10.1016/j.molliq.2019.110902.
[6] L.G. Devi, N. Kottam, B.N. Murthy, S.G. Kumar, Enhanced photocatalytic activity of transition metal ions Mn2+, Ni2+ and Zn2+ doped polycrystalline titania
for the degradation of Aniline Blue under UV/solar light, J. Mol. Catal. Chem. 328 (2010) 44–52, https://doi.org/10.1016/j.molcata.2010.05.021.
[7] M.Z. Iqbal, S. Khan, Progress in the performance of dye sensitized solar cells by incorporating cost effective counter electrodes, Sol. Energy 160 (2018) 130–152,
https://doi.org/10.1016/j.solener.2017.11.060.
[8] B. Xu, H.Y. Sohn, Y. Mohassab, Y. Lan, Structures, preparation and applications of titanium suboxides, RSC Adv. 6 (2016) 79706–79722, https://doi.org/
10.1039/C6RA14507H.
[9] M. Mhadhbi, H. Abderazzak, B. Avar, Synthesis and Properties of Titanium Dioxide Nanoparticles. Updates on Titanium Dioxide, IntechOpen, 2023, https://doi.
org/10.5772/intechopen.111577.

11
A.A. Moyeen et al. Heliyon 10 (2024) e40776

[10] P. Raveendran, J. Fu, S.L. Wallen, Completely “green” synthesis and stabilization of metal nanoparticles, J. Am. Chem. Soc. 125 (2003) 13940–13941, https://
doi.org/10.1021/ja029267j.
[11] K.N. Thakkar, S.S. Mhatre, R.Y. Parikh, Biological synthesis of metallic nanoparticles, Nanomedicine 6 (2010) 257–262, https://doi.org/10.1016/j.
nano.2009.07.002.
[12] A. Castro-Beltrán, P.A. Luque, H.E. Garrafa-Gálvez, R.A. Vargas-Ortiz, A. Hurtado-Macías, A. Olivas, et al., Titanium butoxide molar ratio effect in the TiO2
nanoparticles size and methylene blue degradation, Optik 157 (2018) 890–894, https://doi.org/10.1016/j.ijleo.2017.11.185.
[13] S. Musić, M. Gotić, M. Ivanda, S. Popović, A. Turković, R. Trojko, et al., Chemical and micro structural properties of TiO2 synthesized by sol-gel procedure,
Mater. Sci. Eng., B 47 (1997) 33–40, https://doi.org/10.1016/S0921-5107(96)02041-7.
[14] X. Chen, S.S. Mao, Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications, Chem. Rev. 107 (2007) 2891–2959, https://doi.org/
10.1021/cr0500535.
[15] A. Ansari, V.U. Siddiqui, W.U. Rehman, MdK. Akram, W.A. Siddiqi, A.M. Alosaimi, et al., Green synthesis of TiO2 nanoparticles using Acorus calamus leaf
extract and evaluating its photocatalytic and in vitro antimicrobial activity, Catalysts 12 (2022) 181, https://doi.org/10.3390/catal12020181.
[16] R.S. Dubey, K.V. Krishnamurthy, S. Singh, Experimental studies of TiO2 nanoparticles synthesized by sol-gel and solvothermal routes for DSSCs application,
Results Phys. 14 (2019) 102390, https://doi.org/10.1016/j.rinp.2019.102390.
[17] M. Aravind, M. Amalanathan, M.S.M. Mary, Synthesis of TiO2 nanoparticles by chemical and green synthesis methods and their multifaceted properties, SN
Appl. Sci. 3 (2021) 409, https://doi.org/10.1007/s42452-021-04281-5.
[18] G. Nabi, Q.-U.- Ain, M.B. Tahir, K. Nadeem Riaz, T. Iqbal, M. Rafique, et al., Green synthesis of TiO 2 nanoparticles using lemon peel extract: their optical and
photocatalytic properties, Int. J. Environ. Anal. Chem. 102 (2022) 434–442, https://doi.org/10.1080/03067319.2020.1722816.
[19] K.S. Saranya, V. Vellora Thekkae Padil, C. Senan, R. Pilankatta, K. Saranya, B. George, et al., Green synthesis of high temperature stable anatase titanium dioxide
nanoparticles using gum kondagogu: characterization and solar driven photocatalytic degradation of organic dye, Nanomaterials 8 (2018) 1002, https://doi.
org/10.3390/nano8121002.
[20] W. Buraso, V. Lachom, P. Siriya, P. Laokul, Synthesis of TiO 2 nanoparticles via a simple precipitation method and photocatalytic performance, Mater. Res.
Express 5 (2018) 115003, https://doi.org/10.1088/2053-1591/aadbf0.
[21] A. Ansari, V.U. Siddiqui, W.U. Rehman, MdK. Akram, W.A. Siddiqi, A.M. Alosaimi, et al., Green synthesis of TiO2 nanoparticles using Acorus calamus leaf
extract and evaluating its photocatalytic and in vitro antimicrobial activity, Catalysts 12 (2022) 181, https://doi.org/10.3390/catal12020181.
[22] G. Nabi, A. Majid, A. Riaz, T. Alharbi, M. Arshad Kamran, M. Al-Habardi, Green synthesis of spherical TiO2 nanoparticles using Citrus Limetta extract: excellent
photocatalytic water decontamination agent for RhB dye, Inorg. Chem. Commun. 129 (2021) 108618, https://doi.org/10.1016/j.inoche.2021.108618.
[23] R.S. Dubey, K.V. Krishnamurthy, S. Singh, Experimental studies of TiO2 nanoparticles synthesized by sol-gel and solvothermal routes for DSSCs application,
Results Phys. 14 (2019) 102390, https://doi.org/10.1016/j.rinp.2019.102390.
[24] M. Marycleopha, B. Yaou Balarabe, I. Adjama, H. Moussa, H. Anandaram, M.W. Abdoul Razak, Anhydrous sol-gel synthesis of TiO2 nanoparticles: evaluating
their impact on protein interactions in biological systems, Journal of Trace Elements and Minerals 7 (2024) 100114, https://doi.org/10.1016/j.
jtemin.2023.100114.
[25] Z. Alhalili, M. Smiri, The influence of the calcination time on synthesis of nanomaterials with small size, high crystalline nature and photocatalytic activity in the
TiO2 nanoparticles calcined at 500 ◦ C, Crystals 12 (2022) 1629, https://doi.org/10.3390/cryst12111629.
[26] J. Yan, G. Wu, N. Guan, L. Li, Z. Li, X. Cao, Understanding the effect of surface/bulk defects on the photocatalytic activity of TiO2: anatase versus rutile, Phys.
Chem. Chem. Phys. 15 (2013) 10978, https://doi.org/10.1039/c3cp50927c.
[27] A. Liza Kretzschmar, M. Manefield, The role of lipids in activated sludge floc formation, AIMS Environ Sci 2 (2015) 122–133, https://doi.org/10.3934/
environsci.2015.2.122.
[28] S. Iravani, Green synthesis of metal nanoparticles using plants, Green Chem. 13 (2011) 2638, https://doi.org/10.1039/c1gc15386b.
[29] L.K. Ruddaraju, S.V.N. Pammi, G sankar Guntuku, V.S. Padavala, V.R.M. Kolapalli, A review on anti-bacterials to combat resistance: from ancient era of plants
and metals to present and future perspectives of green nano technological combinations, Asian J. Pharm. Sci. 15 (2020) 42–59, https://doi.org/10.1016/j.
ajps.2019.03.002.
[30] S.P. Goutam, G. Saxena, V. Singh, A.K. Yadav, R.N. Bharagava, K.B. Thapa, Green synthesis of TiO2 nanoparticles using leaf extract of Jatropha curcas L. for
photocatalytic degradation of tannery wastewater, Chem. Eng. J. 336 (2018) 386–396, https://doi.org/10.1016/j.cej.2017.12.029.
[31] I.Y. Hernández-Montesinos, D.F. Carreón-Delgado, E. Ocaranza-Sánchez, C.E. Ochoa-Velasco, Á. Suárez-Jacobo, C. Ramírez-López, Garlic (Allium sativum) peel
extracts and their potential as antioxidant and antimicrobial agents for food applications: influence of pretreatment and extraction solvent, Int. J. Food Sci.
Technol. 58 (2023) 6794–6805, https://doi.org/10.1111/ijfs.16652.
[32] E. Subroto, Y. Cahyana, Mahani Tensiska, F. Filianty, E. Lembong, et al., Bioactive compounds in garlic (Allium sativum L.) as a source of antioxidants and its
potential to improve the immune system: a review, Food Res. 5 (2021) 1–11, https://doi.org/10.26656/fr.2017.5(6).042.
[33] P. Makkar, N.N. Ghosh, A review on the use of DFT for the prediction of the properties of nanomaterials, RSC Adv. 11 (2021) 27897–27924, https://doi.org/
10.1039/D1RA04876G.
[34] M. Gaus, Q. Cui, M. Elstner, Density functional tight binding: application to organic and biological molecules, WIREs Computational Molecular Science 4 (2014)
49–61, https://doi.org/10.1002/wcms.1156.
[35] M. Elstner, G. Seifert, Density functional tight binding, Phil. Trans. Math. Phys. Eng. Sci. 372 (2014) 20120483, https://doi.org/10.1098/rsta.2012.0483.
[36] A.L. Patterson, The scherrer formula for X-ray particle size determination, Phys. Rev. 56 (1939) 978–982, https://doi.org/10.1103/PhysRev.56.978.
[37] H. Kaur, S. Kaur, J. Singh, M. Rawat, S. Kumar, Expanding horizon: green synthesis of TiO 2 nanoparticles using Carica papaya leaves for photocatalysis
application, Mater. Res. Express 6 (2019) 095034, https://doi.org/10.1088/2053-1591/ab2ec5.
[38] N.K. Sethy, Z. Arif, P.K. Mishra, P. Kumar, Green synthesis of TiO2 nanoparticles from Syzygium cumini extract for photo-catalytic removal of lead (Pb) in
explosive industrial wastewater, Green Process. Synth. 9 (2020) 171–181, https://doi.org/10.1515/gps-2020-0018.
[39] P. Makuła, M. Pacia, W. Macyk, How to correctly determine the band gap energy of modified semiconductor photocatalysts based on UV–vis spectra, J. Phys.
Chem. Lett. 9 (2018) 6814–6817, https://doi.org/10.1021/acs.jpclett.8b02892.
[40] R. Zedek, H. Djedjiga, M. Megherbi, M.S. Belkaid, E. Ntsoenzok, Effects of slight Fe (III)-doping on structural and optical properties of TiO2 nanoparticles, J. Sol.
Gel Sci. Technol. 100 (2021) 44–54, https://doi.org/10.1007/s10971-021-05602-1.
[41] M.M. Ali, MdJ. Haque, M.H. Kabir, M.A. Kaiyum, M.S. Rahman, Nano synthesis of ZnO–TiO2 composites by sol-gel method and evaluation of their antibacterial,
optical and photocatalytic activities, Results in Materials 11 (2021) 100199, https://doi.org/10.1016/j.rinma.2021.100199.
[42] X. Pan, M.-Q. Yang, X. Fu, N. Zhang, Y.-J. Xu, Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications, Nanoscale 5 (2013)
3601, https://doi.org/10.1039/c3nr00476g.
[43] X. Liu, S. Gao, H. Xu, Z. Lou, W. Wang, B. Huang, et al., Green synthetic approach for Ti3+ self-doped TiO2− x nanoparticles with efficient visible light
photocatalytic activity, Nanoscale 5 (2013) 1870, https://doi.org/10.1039/c2nr33563h.
[44] M.M. Khan, S.A. Ansari, D. Pradhan, M.O. Ansari, D.H. Han, J. Lee, et al., Band gap engineered TiO 2 nanoparticles for visible light induced
photoelectrochemical and photocatalytic studies, J. Mater. Chem. A 2 (2014) 637–644, https://doi.org/10.1039/C3TA14052K.
[45] T.P. Dasari, K. Pathakoti, H.-M. Hwang, Determination of the mechanism of photoinduced toxicity of selected metal oxide nanoparticles (ZnO, CuO, Co3O4 and
TiO2) to E. coli bacteria, J. Environ. Sci. 25 (2013) 882–888, https://doi.org/10.1016/S1001-0742(12)60152-1.
[46] K. Sunada, T. Watanabe, K. Hashimoto, Studies on photokilling of bacteria on TiO2 thin film, J. Photochem. Photobiol. Chem. 156 (2003) 227–233, https://doi.
org/10.1016/S1010-6030(02)00434-3.
[47] B. Pant, M. Park, S.-J. Park, Recent advances in TiO2 films prepared by sol-gel methods for photocatalytic degradation of organic pollutants and antibacterial
activities, Coatings 9 (2019) 613, https://doi.org/10.3390/coatings9100613.
[48] B. Pant, H.R. Pant, D.R. Pandeya, G. Panthi, K.T. Nam, S.T. Hong, et al., Characterization and antibacterial properties of Ag NPs loaded nylon-6 nanocomposite
prepared by one-step electrospinning process, Colloids Surf. A Physicochem. Eng. Asp. 395 (2012) 94–99, https://doi.org/10.1016/j.colsurfa.2011.12.011.

12
A.A. Moyeen et al. Heliyon 10 (2024) e40776

[49] J. Hou, L. Wang, C. Wang, S. Zhang, H. Liu, S. Li, et al., Toxicity and mechanisms of action of titanium dioxide nanoparticles in living organisms, J. Environ. Sci.
75 (2019) 40–53, https://doi.org/10.1016/j.jes.2018.06.010.
[50] K.R. Raghupathi, R.T. Koodali, A.C. Manna, Size-dependent bacterial growth inhibition and mechanism of antibacterial activity of zinc oxide nanoparticles,
Langmuir 27 (2011) 4020–4028, https://doi.org/10.1021/la104825u.
[51] J.-Y. Zhang, X.-Y. Wang, M. Xiao, L. Qu, X. Peng, Lattice contraction in free-standing CdSe nanocrystals, Appl. Phys. Lett. 81 (2002) 2076–2078, https://doi.org/
10.1063/1.1507613.
[52] A.K. Abdul Gafoor, J. Thomas, M.M. Musthafa, P.P. Pradyumnan, Effects of Sm3+ doping on dielectric properties of anatase TiO2 nanoparticles synthesized by a
low-temperature hydrothermal method, J. Electron. Mater. 40 (2011) 2152–2158, https://doi.org/10.1007/s11664-011-1707-9.
[53] A.K. Abdul Gafoor, M.M. Musthafa, K. Pradeep Kumar, P.P. Pradyumnan, Effect of Ag doping on structural, electrical and dielectric properties of TiO2
nanoparticles synthesized by a low temperature hydrothermal method, J. Mater. Sci. Mater. Electron. 23 (2012) 2011–2016, https://doi.org/10.1007/s10854-
012-0695-8.
[54] X. Wang, B. Zhang, L. Xu, X. Wang, Y. Hu, G. Shen, et al., Dielectric properties of Y and Nb co-doped TiO2 ceramics, Sci. Rep. 7 (2017) 8517, https://doi.org/
10.1038/s41598-017-09141-0.
[55] A.K. Yadav, C. Gautam, Dielectric behavior of perovskite glass ceramics, J. Mater. Sci. Mater. Electron. 25 (2014) 5165–5187, https://doi.org/10.1007/s10854-
014-2311-6.
[56] S. Chao, V. Petrovsky, F. Dogan, Effects of sintering temperature on the microstructure and dielectric properties of titanium dioxide ceramics, J. Mater. Sci. 45
(2010) 6685–6693, https://doi.org/10.1007/s10853-010-4761-4.
[57] B. Ghosh, R.M. Tamayo Calderón, R. Espinoza-González, S.A. Hevia, Enhanced dielectric properties of PVDF/CaCu 3 Ti 4 O 12 :Ag composite films, Mater.
Chem. Phys. 196 (2017) 302–309, https://doi.org/10.1016/j.matchemphys.2017.05.009.
[58] P. Saravanan, M. Ganapathy, A. Charles, S. Tamilselvan, R. Jeyasekaran, M. Vimalan, Electrical properties of green synthesized TiO2 nanoparticles. Pelagia
Research Library, Adv. Appl. Sci. Res. 7 (2016) 158–168.
[59] R. Narzary, B. Dey, S.N. Rout, A. Mondal, G. Bouzerar, M. Kar, et al., Influence of K/Mg co-doping in tuning room temperature d0 ferromagnetism, optical and
transport properties of ZnO compounds for spintronics applications, J. Alloys Compd. 934 (2023) 167874, https://doi.org/10.1016/j.jallcom.2022.167874.
[60] X. Zhao, L. Chen, X. Zhang, P. Liu, C.-L. Xu, Z.-Y. Hou, et al., The abnormal multiple dielectric relaxation responses of Al3+ and Nb5+ co-doped rutile TiO2
ceramics, J. Alloys Compd. 860 (2021) 157891, https://doi.org/10.1016/j.jallcom.2020.157891.
[61] A.H. Reshak, S.A. Khan, Z.A. Alahmed, Investigation of electronic structure and optical properties of MgAl2O4: DFT approach, Opt. Mater. 37 (2014) 322–326,
https://doi.org/10.1016/j.optmat.2014.06.017.

13