
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13589-X
10.1016/j.heliyon.2024.e37558
e37558
Research Article
Influence of Ag doping on structural, morphological, and optical characteristics of sol-gel spin-coated TiO2 thin films
Al Amin Syed Muhammad syedalamin@duet.ac.bd
a⁎
Kowser Md. Arefin b
a Department of Nanomaterials and Ceramic Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh
b Department of Mechanical Engineering, Dhaka University of Engineering & Technology, Gazipur, Bangladesh
⁎ Corresponding author. syedalamin@duet.ac.bd
06 9 2024
30 9 2024
06 9 2024
10 18 e3755828 3 2024
4 9 2024
5 9 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Pure and Ag-doped Titanium dioxide (TiO2) thin films have been synthesized via the sol-gel spin coating method, and their structural, morphological, and optical properties have been investigated. X-ray diffraction analysis verifies that the polycrystalline anatase TiO2 phase is retained up to 2 % Ag doping. However, lattice expansion and grain refinement down to 25.345 nm are observed with increasing Ag content. Scanning electron microscopy (SEM) reveals the formation of large, isolated TiO2 aggregates separated by drying-induced cracks across the film surface. Optical studies show the undoped TiO2 films demonstrate excellent visible transparency (>75 % transmittance) that slightly decreases upon Ag doping, though it remains suitably high for device applications. Meanwhile, optical absorption and defect states improve with more Ag incorporation, narrowing the TiO2 bandgap from 3.83 eV (undoped) to 3.51 eV (2 % Ag-doping) due to the incorporation of Ag-induced electronic states just below the conduction band minimum. The capability to control optical and structural properties through Ag doping highlights the potential of these TiO2 films suitably tailored for photovoltaic devices or self-cleaning coatings.
==== Body
pmc1 Introduction

Thin film nanomaterials have recently garnered significant research interest due to their expanding importance across scientific fields and technological applications. Semiconductor thin films, in particular, have found extensive use in areas including environmental remediation, photovoltaics, sensing, biomaterials, pharmaceuticals, optoelectronics, and antimicrobials [1]. The unique physicochemical properties of nanostructured semiconductor thin films enable their integration into diverse technologies. For instance, their high surface area to volume ratio, tunable band gap, and facile charge transfer facilitate pollutant degradation, solar energy conversion, and antimicrobial functionality. The rich surface chemistry and engineering potential of thin film nanomaterials present opportunities for continuously emergent technologies to address worldwide challenges in sustainability, healthcare, and electronics. Elucidating structure-property-performance relationships in semiconductor thin films remains an active arena of nanomaterials research.

Among several transition metal oxides, TiO2 has attracted interest due to its appealing combination of properties, including non-toxicity, chemical stability, biocompatibility, and favorable charge transfer characteristics. This inorganic semiconducting oxide is characterized by a wide bandgap (3.0–3.4 eV) [2]. It is crystallized into three main polymorphs – anatase (tetragonal), rutile (tetragonal), and brookite (orthorhombic) – in addition to an amorphous phase. As a versatile material, TiO2 is incorporated into technologies such as photovoltaics, optoelectronics, gas sensing, photocatalysis, and anti-reflective coatings [[3], [4], [5]]. Under UVA irradiation, the anatase and rutile phases demonstrate promising antibacterial and photocatalytic activities. However, drawbacks like low surface area, high charge recombination rates, and a wide bandgap motivate the modification of TiO2. Doping with suitable elements can enhance the quality and performance of TiO2 thin films [6]. Ag, a well-documented antimicrobial capable of eliminating bacteria, fungi, and certain viruses without adverse human health effects, has been widely utilized in medical devices. The Ag+ ion is particularly effective in conferring antibacterial properties [7]. Therefore, doping TiO2 with silver represents a viable approach to augment the intrinsic antimicrobial activity of TiO2 thin films.

Various deposition techniques have been utilized to synthesize TiO2 thin films on assorted substrates, including electron beam/thermal evaporation, ion-beam-assisted deposition (IBAD), pulsed laser deposition (PLD), magnetron sputtering, hydrothermal synthesis, atomic layer deposition (ALD), chemical vapor deposition (CVD), spray pyrolysis, and sol-gel processing (dip and spin coating) [8,9]. Of these methods, the sol-gel process offers a simple, cost-effective, and adaptable approach for fabricating thin films. The liquid sol precursor enables coating substrates of various sizes, shapes, and compositions with uniform films. Specifically, spin coating rapidly produces thin films on flat substrates in a single step with precise thickness control [10]. Thus, considering the unique advantages, the sol-gel spin coating process is well-suited for fabricating high-quality TiO2 thin films.

This study aims to synthesize TiO2 thin films and characterize their structural, morphological, and optical properties for potential application as coatings in advanced optoelectronic, photocatalytic, and other applications. This research will explore the influence of Ag doping on the properties of TiO2-based thin films.

2 Materials and methods/experimental/methodology

Thin films of pure TiO2 and Ag-doped TiO2 were synthesized utilizing the sol-gel spin coating technique. Titanium isopropoxide (TTIP) served as the precursor, while ethanol was employed as the primary solvent for dissolving TTIP and as a stabilizer for the TiO2 particles within the solution. Hydrochloric acid (HCl) promotes the hydrolysis and condensation reactions of the TTIP and helps to maintain the pH of the solution. Silver nitrate (AgNO3) was utilized as the source for Ag doping. All chemicals used were of commercial grade and were utilized as received without any additional purification. The deposition of TiO2 thin film requires a clean glass substrate to achieve the desired film properties. An ultrasonic cleaner was used to clean the glass surface effectively.

The precursor solution was prepared by taking 1000 μl of TTIPs using a micropipette and dissolving in the 10 ml ethanol. This combination resulted in the formation of a white-colored solution. In this stage, 50 μl of hydrochloric acid (HCl) was added to the solution. Then, the mixture was stirred for 10 min, eliminating the white solution and yielding a clear, homogeneous solution. Silver nitrate (AgNO3) was utilized as the source for Ag doping in Ag doped films. The films from the prepared solution are deposited on a glass substrate using a spin coating process at 3000 rpm for 30 s. The wet films are then heated at 60 °C for 10 min for drying. The thickness of the films can be adjusted by repeating the coating process. To improve the crystallinity of the deposited films, the specimen underwent an annealing process at 400 °C for 1 h, followed by cooling inside the furnace.

X-ray diffraction (XRD) analysis was conducted to identify the phases present in the thin films. These measurements were performed using a Philips 3040 XPert PRO diffractometer with Cu-Kα radiation at a wavelength (λ) of 1.5418 Å at a scan rate of 2° per minute within a 2θ range of 10–80°.

The lattice parameter is calculated using the following equation1d2=h2+k2a2+l2c2

here a, c is the lattice parameter, hkl is the miller indices of the peaks and d is the spacing between the planes.

The crystallite sizes were estimated using equation (2):D=Kλβcosθ

where D is the mean value of crystallite size, k is the dimensionless shape factor, λ is the Xray emission wavelength (Cu, Kα = 1.54178 Å), β is the line broadening at half the maximum, in radians, and θ is Bragg's angle. The morphology and phase distribution of TiO2 thin films were analyzed using a Field Emission Scanning Electron Microscope (FE-SEM). This advanced imaging technique employed a high-energy electron beam to scan the sample surface in a raster pattern, providing detailed insights into the film's structure. To comprehensively assess the morphology and size distribution of the thin films, SEM images were captured at three distinct magnification levels. Transmittance spectra were acquired at room temperature utilizing the PerkinElmer Lambda 35 UV–Vis spectrophotometer, covering a wavelength range from 300 to 1200 nm. A blank glass slide was used as the reference during the optical absorption measurement of thin films. The optical bandgap of the material was determined using Tauc's relation, which relates the absorption coefficient (α) to the energy of the incident photons (hν) through the following equation:αhv=B(hv−Eg)2

In this equation, Eg represents the bandgap energy, and B is a constant that depends on the transition probability. The linear regions of a plot (αhv)2 versus hν were extrapolated to the energy axis to estimate the bandgap value.

3 Results and discussion

3.1 Structural properties

The X-ray diffraction (XRD) patterns for Pure TiO2, 1 %, and 2 % Ag-doped TiO2 thin films deposited on a glass substrate at 400 °C are illustrated in Fig. 1. The patterns reveal the polycrystalline nature of the prepared films. All the peaks have been indexed using the ICDD database, correlating them with samples bearing the JCPDS card no. 01-083-2243. It has been observed that all the samples are composed of the anatase phase of TiO2. The absence of additional peaks in the Ag-doped samples indicates that Ag incorporation does not induce the formation of secondary phases [11]. The consistency in peak positions and intensities across the pure and Ag-doped films suggests that Ag doping has little effect on the host anatase lattice over the doping levels examined.Fig. 1 XRD patterns of pure and Ag-doped TiO2 thin films.

Fig. 1

The presence of (200) and (105) peaks in the XRD patterns of Ag-doped TiO2 samples, which are absent in the un-doped sample, is due to the effects of Ag doping on the TiO2 crystal structure. Ag doping improves the crystallinity of specific planes, particularly the (200) and (105) orientations. This enhancement results in the amplification of these weak or undetectable peaks, making them more pronounced and easily observable in the XRD patterns of Ag-doped TiO2 thin films.

The doping of Ag into the TiO2 lattice results in a shift of the 2θ diffraction angle value for the (101) peak towards lower angles. This peak shift indicates that the Ag ions have been substituted into Ti sites in the TiO2 host lattice. The larger ionic radius of Ag+ (1.26 Å) compared to Ti4+ (0.60 Å) leads to an expansion of the TiO2 lattice parameters, as evidenced by the shift to lower 2θ values. The incorporation of the larger Ag ions distorts the lattice, modifying the Bragg diffraction conditions and causing the observed peak displacement [12].

The lattice parameters of the TiO2 and Ag-doped TiO2 thin films were calculated using equation (1) and are tabulated in Table 1. The pure TiO2 film has a lattice parameter of a = b = 3.7785 Å, c = 9.4742 Å consistent with standard reported values [13]. As the Ag doping level increases, the measured lattice parameter expands for the 1 % and 2 % Ag-TiO2 film. This lattice expansion can be attributed to the larger ionic radius of substitutions Ag+ (1.26 Å) compared to the replaced Ti4+ (0.60 Å). Incorporating the bigger Ag+ ions strains the crystal lattice, requiring an expansion of the unit cell dimensions to accommodate the size mismatch. The gradual increase in lattice parameters correlates directly with the increasing Ag content.Table 1 Main peak position and Lattice parameter of Pure Ag doped TiO2.

Table 1Sample	Main Peak, 2θ	Lattice parameter	
a = b	c	
Pure TiO2	25.462	3.7785	9.4742	
1 % Ag doped TiO2	25.409	3.7843	9.4992	
2 % Ag doped TiO2	25.357	3.7912	9.4580	

The crystallite sizes of undoped and Ag-doped TiO2 thin films were calculated, and the trends with Ag doping levels are shown in Fig. 2. The pure TiO2 film has the largest crystallite size at 30.125 nm. There is a decreasing trend in grain size with increasing Ag doping, with the 2 % Ag-TiO2 film having the smallest crystallites at 25.364 nm. This reduction can be attributed to the Ag + ion incorporation hindering crystallite growth during nucleation, resulting in a higher density of grain boundaries. The larger Ag ions strain the lattice, potentially pinning grain boundaries and restricting grain coarsening and growth [14].Fig. 2 Variation of crystallite size with Ag doping concentration.

Fig. 2

The dislocation density, which relates to the amount of defects in the film, was determined using the formula d = 1/D2, where D represents the crystallite size. The internal strain, arising from lattice misfit due to synthesis conditions, was calculated using the equation ε = β/4tanθ, where β is the full width at half maximum (FWHM) of the XRD peak, and θ is the Bragg angle. As illustrated in Fig. 3, both the dislocation density and internal strain exhibited an increasing trend with higher Ag doping percentages. This observed increase can be attributed to the incorporation of larger Ag atoms into the TiO2 lattice likely introduces local distortions and mismatches due to differences in ionic radii between Ag and Ti; the formation of Ag nanoparticles or clusters within the TiO2 matrix may create additional strain at particle-matrix interfaces; and the potential for Ag to occupy interstitial sites or form substitutional defects could further contribute to lattice distortions and increased dislocation density [15].Fig. 3 Variation of dislocation density and internal strain with doping percentage.

Fig. 3

3.2 Morphological properties

Scanning electron microscopy (SEM) analysis of the TiO2 films annealed at 400 °C is shown in Fig. 4, revealing an inhomogeneous coverage of the substrate. The surface consists of large TiO2 aggregates, with smooth well-defined edges. These aggregates are separated by cracks spanning several microns, likely originating from drying-induced stresses. During solvent evaporation, capillary forces build-up from the liquid-air interface of the drying gel film. These capillary forces can overcome the intermolecular bonds between adjacent TiO2 grains, fracturing the continuous wet gel network and resulting in crack formation as the film solidifies. The process leaves isolated TiO2 aggregates separated by drying cracks across the substrate surface rather than a homogeneous microstructural film [16].Fig. 4 SEM images of Pure TiO2 thin films.

Fig. 4

3.3 Optical properties

Optical transmission spectra of undoped and Ag-doped TiO2 films were recorded in the 300–1200 nm wavelength range, as shown in Fig. 5. The pure TiO2 film exhibits over 75 % transmittance in the visible region. As the Ag doping concentration increases, the optical transmittance of the films decreases to 70 % for the highest doping level. This reduction in transparency can be attributed to increased light scattering effects originating from oxygen vacancies as well as grain boundaries. The incorporation of Ag ions likely disrupts the lattice oxygen sites, leading to a higher density of oxygen vacancies that can scatter propagating light waves. Additionally, the grain refinement induced by Ag doping introduces more grain boundaries, which can also serve as scattering centers to attenuate overall transmittance. The combined effects of these optically active defects and interfaces lead to a decline in transparency with increasing Ag content [17,18].Fig. 5 Transmittance of Pure and Ag-doped TiO2 thin films.

Fig. 5

Variations in the optical absorption edge wavelengths were observed with different Ag doping levels. The absorption edges exhibit a redshift to longer wavelengths as the Ag content increases. This redshift can be attributed to sub-bandgap electronic transitions facilitated by incorporating Ag ions [19].

The optical absorption spectra of pure and Ag-doped TiO2 thin films in the visible wavelength range are presented in Fig. 6. Across the scanned 400–800 nm visible range, all the fabricated TiO2 films demonstrate low optical absorption with values under 4 %. The undoped TiO2 film exhibits the smallest absorbance among the samples. Upon Ag doping, slight increments in the light absorption are observed.Fig. 6 Absorbance of Pure and Ag-doped TiO2 thin films.

Fig. 6

The optical bandgaps of the deposited TiO2 and Ag-doped TiO2 thin films were determined from analysis of the spectral absorption data using Tauc's relation. This involved plotting (αhν)2 as a function of photon energy (hν) and extrapolating the linear region to the energy axis, as shown in Fig. 7. From this analysis, the bandgaps for the TiO2 films are as follows: 3.83 eV for the undoped film, 3.80 eV for the 1 % doped film, and 3.51 eV for the 2 % doped film. As depicted in the figure, the bandgap exhibits a downward trend with increasing Ag doping percentages. Specifically, the bandgap progressively narrows from the undoped value as more Ag is added. This bandgap narrowing effect suggests that the introduced Ag atoms provide intermediate energy states just below the conduction band edge of the band structure. Electrons likely occupy these lower Ag-derived levels, needing less energy for excitation to the TiO2 conduction band and resulting in the apparent band gap reduction. Further theoretical study is warranted to confirm the origin of these in-gap states and quantify their impacts on optical transitions [20].Fig. 7 Tauc plot for Pure and Ag-doped TiO2 thin films.

Fig. 7

The observed reduction in the bandgap of TiO2 with increasing Ag doping concentration opens up exciting possibilities for bandgap engineering. This narrowing of the bandgap, suggests that Ag-doped TiO2 thin films could exhibit enhanced light absorption, particularly in the visible region. Consequently, this bandgap tuning holds the potential to improve the photocatalytic efficiency of these materials, making them more effective for applications such as environmental remediation and renewable energy technologies.

4 Conclusion

In the present work, undoped and Ag-doped TiO2 thin films are synthesized using the sol-gel spin coating method. All the fabricated TiO2 thin films have a polycrystalline anatase phase, regardless of Ag doping. Upon Ag incorporation, the (101) main diffraction peak shifts to lower 2θ angles. This shift implies an expansion of the lattice parameters, which can be attributed to the larger ionic radius of substituted Ag ions for the smaller Ti ions. Additionally, there is a declining trend in crystallite size with more Ag doping, with grain refinement down to 25.345 nm for 2 % Ag doping. The SEM analysis of TiO2 films annealed at 400 °C indicates non-uniform substrate coverage. The surface exhibits large, well-defined TiO2 aggregates separated by micron-scale cracks induced during drying. As-deposited TiO2 thin films exhibit excellent optical transparency, with over 75 % transmittance across the visible spectral region. Upon Ag doping, there is a slight decrease in transmittance, however, overall visible transparency remains high even at the highest doping levels. Meanwhile, optical absorption shows marginal improvements with increasing Ag incorporation. Bandgap analysis indicates a reduction of the TiO2 bandgap from 3.83 eV in the undoped case down to 3.51 eV at 2 % Ag doping. This bandgap narrowing can be attributed to incorporating Ag-derived electronic defect states just below the conduction band minimum of TiO2.

Data availability statement

Data will be made available upon request.

CRediT authorship contribution statement

Syed Muhammad Al Amin: Writing – review & editing, Writing – original draft, Visualization, Validation, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Md. Arefin Kowser: Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Formal analysis, Conceptualization.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:Syed Muhammad Al Amin reports a relationship with Vice-Chancellor Research Fund, DUET that includes: funding grants. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The author gratefully acknowledges the funding received from the Vice-Chancellor Research Fund and DUET, Gazipur. The author would also like to acknowledge the Department of Mechanical Engineering, DUET, Gazipur, and Department of Nanomaterials and Ceramic Engineering, BUET Bangladesh for permitting to use of their testing facilities.
==== Refs
References

1 Mosquera A.A. Albella J.M. Navarro V. Bhattacharyya D. Endrino J.L. Effect of silver on the phase transition and wettability of titanium oxide films Sci. Rep. 6 1 2016 10.1038/srep32171
2 Yilmaz H.Ç. Ilhan C. Akgeyik E. Erdemoğlu S. Preparation and characterization of Co doped TiO2 for efficient photocatalytic degradation of Ibuprofen Journal of the Turkish Chemical Society, Section A: Chemistry 8 2 2021 553 566 10.18596/jotcsa.855107
3 Huang M. Wu Y. Fan F. Lu S. Luo B. Li Y. Antibacterial and ultraviolet protective neodymium-doped TiO2 film coated on polypropylene nonwoven fabric via a sputtering method Journal of Engineered Fibers and Fabrics 16 2021 155892502110252 10.1177/15589250211025257
4 Kaya İ.C. Metal oksit nanopartikül esaslı P-N eklem UV fotodedektörlerin üretimi Konya Journal of Engineering Sciences 10 1 2022 240 248 10.36306/konjes.1057176
5 Mansor M.L.H.A. Hussin R. Kamdi Z. Ainuddin A.R. Ibrahim S.A. Hatta M.N.M. Yunos M.Z. Deposition and characterization of cobalt doped titanium dioxide thin films using sol-gel method Journal of Science and Technology/Journal of Science and Technology 10 2 2018 10.30880/jst.2018.10.02.010
6 Farooq M.H. Aslam I. Anam H.S. Tanveer M. Rizwan M. Defect engineering for improved photocatalytic performance of reduced lead titanate (PbTiO3) under solar light irradiation Bull. Chem. Soc. Ethiop. 33 2 2019 373 10.4314/bcse.v33i2.17
7 Chu Z. Zhao T. Li L. Fan J. Qin Y. Characterization of antimicrobial poly (lactic acid)/nano-composite films with silver and zinc oxide nanoparticles Materials 10 6 2017 659 10.3390/ma10060659 28773018
8 Munandar H. Doyan A. Susilawati S. Synthesis of SnO2 thin coatings by indium and aluminum mixed doping using the sol-gel spin-coating technique Jurnal Penelitian Pendidikan IPA 6 2 2020 152 156 10.29303/jppipa.v6i2.391
9 Rao M.C. Optical and electrical properties of MN2+ doped titanium oxide thin films Rasayan J. Chem. 10–10 3 2017 825 831 https://www.rasayanjournal.co.in/admin/php/upload/217_pdf.pdf
10 Bezir N.Ç. Evcin A. Kayali R. Özen M. Esen K. Comparison of five-layered ZRO2 and single-layered CE, EU, and Dy-doped ZRO2 thin films prepared by sol-gel spin coating method Acta Phys. Pol., A 132 3 2017 612 616 10.12693/aphyspola.132.612
11 Suciu R. Zagrai M. Popa A. Toloman D. Berghian-Grosan C. Tudoran C. Stefan M. The influence of AG+/TI4+ ratio on structural, optical and photocatalytic properties of MWCNT–TIO2–AG nanocomposites Inorganics 11 6 2023 249 10.3390/inorganics11060249
12 Schubert U. Photoenergy and Thin Film Materials 2019 Wiley eBooks 10.1002/9781119580546
13 Liaqat M.A. Hussain Z. Khan Z. Akram M.A. Shuja A. Effects of Ag doping on compact TiO2 thin films synthesized via one-step sol–gel route and deposited by spin coating technique J. Mater. Sci. Mater. Electron. 31 9 2020 7172 7181 10.1007/s10854-020-03288-9
14 Elfanaoui A. Elhamri E. Boulkaddat L. Ihlal A. Bouabid K. Laanab L. Taleb A. Portier X. Optical and structural properties of TiO2 thin films prepared by sol–gel spin coating Int. J. Hydrogen Energy 36 6 2011 4130 4133 10.1016/j.ijhydene.2010.07.057
15 Rajkumar S. Venkatraman M.R. Suguna K. Karuppasamy P. Pandian M.S. Ramasamy P. Synthesis of Ag-incorporated TiO2 nanoparticles by simple green approach as working electrode for dye-sensitized solar cells J. Mater. Sci. Mater. Electron. 33 8 2022 4965 4973 10.1007/s10854-021-07685-6
16 Wang X. Xi M. Zheng F. Ding B. Fong H. Zhu Z. Reduction of crack formation in TiO 2 mesoporous films prepared from binder-free nanoparticle pastes via incorporation of electrospun SiO 2 or TiO 2 nanofibers for dye-sensitized solar cells Nano Energy 12 2015 794 800 10.1016/j.nanoen.2015.01.045
17 Cuadra J. Molina-Prados S. Mínguez-Vega G. Estrada A. Trindade T. Oliveira C. Seabra M. Labrincha J. Porcar S. Cadena R. Fraga D. Carda J. Multifunctional silver-coated transparent TiO2 thin films for photocatalytic and antimicrobial applications Appl. Surf. Sci. 617 2023 156519 10.1016/j.apsusc.2023.156519
18 Ivanova T. Harizanova A. Koutzarova T. Vertruyen B. Characterization of nanostructured TiO2:Ag films: structural and optical properties J. Phys. Conf. 764 2016 012019 10.1088/1742-6596/764/1/012019
19 Shaban Z.M. Khlati J.A. Khadayeir A.A. Habubi N.F. Chiad S.S. Structural, morphology and optical properties of Ag-doped nanostructured CdS thin films J. Phys. Conf. 1999 1 2021 012063 10.1088/1742-6596/1999/1/012063
20 Gupta A.K. Srivastava P. Bahadur L. Improved performance of Ag-doped TiO2 synthesized by modified sol–gel method as photoanode of dye-sensitized solar cell Appl. Phys. Mater. Sci. Process 122 8 2016 10.1007/s00339-016-0241-2
