Effect of Ag ion Implantation on the Crystallinity and Optical Property of Spin-coated TiO2 Thin Films

Introduction

Metal oxide-based semiconductors are classified as a key class of substance because of their extraordinary basic mechanical, chemical, electrical, optical, and luminous capabilities ^1,2. The ability of nanosized TiO₂ to act as a photocatalyst was initially identified in 1967 by Akira Fuji Shima, who then published his findings in 1972 ³. This is primarily a result of its advantageous characteristics, such as non-toxicity, molecular inertness, and persistence across a broad range of pH values under illumination circumstances ⁴. TiO₂, a semiconductor material, is utilized in a few processes, according to studies, TiO₂ can be utilized in many different applications, such as dielectric layers, solar cells, self-cleaning windows, biohybrid interfaces lithium batteries, optoelectronics devices, gas detection, and air-water purification ^5-11.

Due to present environmental issues, TiO₂ has emerged as the prime popular semiconductor for heterogenous photocatalysis and has generated significant interest from scientists. Anatase-rutile-brookite are the three phases of TiO₂ crystallization that are most often known. As a phase, rutile is the most stable although anatase is the most active.

With the help of UV light (wavelength of 380nm), the anatase band gap (E_g > 3.22eV) is activated, permitting electrons to move from the valence band to the conduction region¹². As an outcome, photocatalytic reactions on TiO₂ are constrained in their use of visible light as an excitation source ¹³.

The photons absorption on the surface to produce electron-hole pairs is one of the photocatalytic reaction’s most significant characteristics. The shape of the films, which affects their photocatalytic activities, significantly impacts this pair’s production ^14,15. By altering the surface of semiconductor particles, the electron transport efficacy may be increased. Numerous techniques, such as the incorporation of TiO₂ with noble metal ions and transition metal ions as well as the addition of nonmetal ions (C, B, S, F, Cl) ^16,19, connected semiconductors systems ²⁰, dye sensitization ²¹, can be used to alter the surface of the semiconductor. It was discovered that native metals including silver, gold, platinum, and palladium, when coated on a TiO₂ surface, improve the photocatalytic efficiencies in a variety of ways, including by trapping electrons and preventing the reunify of Ele-hole pairs and improving the e/h pairs ^22,23. Additionally, noble metals were shown to enhance visible light absorption, enabling surface electron activation by plasmon resonances ²⁴. The alteration of the photocatalyst surface’s characteristic is another result of the noble metal addition ²⁵.

 Ag metal doping of TiO₂ thin films has got a lot of focus since it has so many intriguing qualities, such as outstanding effectiveness, relatively inexpensive in comparison to other noble metals, antibacterial capabilities ²⁶, and strong oxygen binding response²⁷.

The preparation procedure has a substantial influence on all physical attributes and photocatalytic activity of thin films. Several techniques for preparing TiO₂ thin films have been developed during the last two decades, including atmospheric pressure chemical vapor deposition (CVD)²⁸, sputtering²⁹, electron beam evaporation³⁰, and sol-gel approach³¹.

The sol-gel spin-coating method is the best choice for creating vitreous or multilayer films on a range of substrates when excellent consistency, pureness, and homogeneity are required.

The main objective of the study was to develop silver-doped TiO₂ (phase-anatase) opaque thin films backed by glass substrate by applying the sol-gel process. In addition, the impact of silver ions on the evolution of the crystalline structure, the mechanism of grain growth, and the characteristics of the optical and photocatalytic activity prior to and subsequent to surface enhancement with metallic silver were investigated as well.

Materials and methods 

Chemicals

TTIP [Ti(O-i-C₃H₇)₄], ethanol (CH₃CH₂OH), DI water, and HCl (Hydrochloric acid) all materials purchased from CDH Private Limited, Delhi. All the materials are AR grade, which is used to prepare TiO₂ sol 

Cleanup of Substrates

We cleaned substrates made of glass in isopropanol, Deionized water (DI), and acetone for a total of 30 minutes. The Glass substrates were lastly stored in Deionized water (DI water) before use. 

Thin films deposition

For undoped thin films of TiO₂ First, we combine 0.1 Mol of TTIP [Ti(O-i-C₃ H₇)₄] and 25 ml of ethanol (CH₃CH₂OH) in a beaker. 2.30 hrs were used for stirring at fifty degrees Celsius. In order to lessen evaporation, the precursor solution was covered with an aluminum sheet carefully. Following that, remove the aluminum sheet and add 2gm ethyl-cellulose (as a binder) and continue stirring. After that, a solution consisting of 25 ml of ethanol, 0.1 mol of HCL, and 0.1 mol of DI water was added by dropper and stirred for 36 hours at 50 °C. then we obtained a light yellowish TiO₂ solution. 

For doped thin films, a solution containing 25 ml of ethanol, 0.1 mol of HCl, 0.1 mol of DI water, and silver nitrate (AgNO₃– according to doping) was added and stirred for 36 hours at 500°C. then a light white TiO₂ solution was sent to us. 

The resulting mixture was then spin-coated at 1800 r.p.m. for 25 seconds onto a non-conducting glass substrate. To eliminate the solvents and other impurities, the deposited TiO₂ thin films were heated to 110°C and baked for 3 minutes. After that, the baked film was heated for 90 minutes at 450°C. 

Characterization Techniques

The X-Ray Diffraction technology “Panalytical X Pert Pro” using CuK radiation technology with CuK radiation was performed to figure out the crystal framework of pure, Ag ion implanted TiO₂ thin films at the surrounding temperature. Scherrer’s equation is used to calculate the crystal size. 

Using the Nova Nano SEM 450 and “scanning electron microscopy”, the surface morphology was studied. AFM using a Multimode Scanning Probe Microscope (Bruker) was used to examine the surface topography. 

Spin-coated pure and Ag-implanted films served as samples and standards, and the spectral absorbance of TiO₂ thin films was figured out using a LAMBDA-750 (version name) UV-Vis-NIR Spectrophotometer.  

Results and analysis

XRD approach- Structural examination

To explore the structure of crystalline materials, XRD, a trustworthy and widely used identification method, is applied ³². After the analysis of the XRD data, plot a graph of undoped, 1% Ag-implanted and 2% Ag-implanted TiO₂ thin films were shown in Figure 1A. Anatase is the most known phase of TiO₂. The indices of the phases are listed in parenthesis and denoted by the letter “A”. These patterns clearly show that for an undoped TiO₂ thin film, the diffraction peak is observed at 2θ = 25.43⁰, which corresponds to the diffraction plane with Miller indices (101). While all of these peaks are associated with TiO₂‘s anatase phase. By correlating these spikes with PDF cards #00-21-1272 and #00-21-1276, respectively, it was determined that the tetragonal anatase phase had formed. As a result, with a minor contribution from the rutile phase, these thin films largely frame out in the anatase phase. The fact that when 1% and 2% Ag were incorporated into TiO₂ thin film, neither a new peak nor a change in peak position was created proved the stability of the anatase and rutile phases upon Ag doping. Instead of forming a chemical link with the Ag atoms in the TiO₂ matrix, the Ag atoms simply exist there in nanoclusters ^33,34. 

Figure 1: (1a) XR-D plot of the pure, (1b) 1% Ag doped & (1c) 2% Ag implanted TiO2 thin films Click here to View Figure

The lattice parameters of the tetragonal phase were derived utilizing the following calculation ³⁵; 

The lattice parameters, in this case, are a and c, the interplanar pitch is d, and the miller indices are (h k l). 

Lattice constant measurements for undoped TiO₂ are determined to be a = 3.7412 and c = 9.4676. The creation of the tetragonal phase of TiO₂ was validated by such features because they matched previously published literature [PDF cards #00-21-1272 and #00-21-1276]. By adding 1% & 2% Ag to TiO₂, the lattice parameters marginally grew (a = 3.74, c = 9.52), which is consistent according to the diffracted hikes relocating down to the lower 2 values.  

Scherer’s formula was employed to calculate the crystallite size ^36,37; 

where K is the form component, with a magnitude of 0.9, λ is the CuK radiation’s wavelength, 1.55, and β is full width at half maxima. In contrast, θ provides the diffraction angle. 

The size of the crystallite was measured by employing the biggest intensity XRD peak (XRD spike-101). Undoped TiO₂ is found to have crystallite sizes of 35.3 nm, which are reduced to 33.03 nm and 33.96 nm when doped with 1% and 2% Ag, respectively. 

Due to the deformation of the lattice and keeping in mind that the rutile phase in addition to the anatase phase of TiO₂, the size of the crystallites has diminished.  

Optical properties

The UV-vis transmittance plot of Ag-implanted TiO₂ nanocomposite thin films is displayed in Figure 2. In the 400 nm to 900 nm wavelength range, Ag-doped TiO₂ films have transmittances of more than 50%. Both the transmission and reflectance spectra’s wavy characteristics in the 800-400 nm range are caused by the interference factor, and a rapid decline in transmittance may be connected to the excitonic transition. It is clear from the somewhat undulating transmittance curve that the Ag-implanted TiO₂ sheets are homogeneous and thin. The behavior of Ag-implanted TiO₂ films in terms of transmittance declined as the silver concentration rose, the wavelength of the films’ intake edge migrated towards longer wavelengths and the peak’s intensity has grown ^38,39. Based on the local surface plasmonic response (LSPR) effect, this type of change can be understood ⁴⁰. It is a result of the addition of silver. The gap between the energy level of Ag-doped TiO₂ should be less than TiO₂‘s (3.2eV/3.1eV).  

Figure 2: UV-Visible absorption plot of pure and Ag-implanted TiO2 thin films Click here to View Figure

Band-gap

The energy difference between the valance and conduction band (E_g) is studied using the Tauc formula. The energy axis is linearly met by extrapolating (hv)^1/2 v/s hv. The gap energy value is determined by this energy plot. The plot of (hv)^1/2 v/s hv is shown in Figure 3. Thin films of undoped TiO₂, 1% Ag, and 2% Ag doped-TiO₂ have corresponding band gaps of 3.16 eV, 2.96eV, and 2.88ev. The band-gap energy is varying between 3.2eV and 2.8 eV as a result of Ag doping into TiO₂. The appearance of lattice distortion brought on by silver doping, which generated fresh levels of energy levels inside the TiO₂ thin film’s forbidden band, is the potential cause of this band gap reduction ⁴¹. The development of metal groupings, which resulted in regional energy levels and surface absorption, was another factor in the band gap’s reduction ⁴².

Figure 3: Band-gap plot for undoped and Ag-implanted TiO2 thin films Click here to View Figure

SEM Analysis

pure and silver-implanted TiO₂ films grown by the sol-gel technique on Si substrates are depicted in SEM images in Fig. 4. The type of Ag incorporation (doping) into TiO₂ film may be identified using excellent quality SEM imaging and EDX examination. SEM pictures indicate dense clusters of nano craters with sizes between 60 and 170 nm. In excellent quality SEM imaging, the particular nanoparticle dispersion both inside and around the nano fissures is visible (Fig-5). 

Ag-nano-dots are nanoparticles that are around 30 nm in size when they are isolated, but they can aggregate to produce particles that are 200 nm in size. According to some research, increasing the Ag content may enhance the propensity to produce Ag-agglomerates. 

There are no traces of the Ag component seen in the Silver doped TiO₂ films’ EDX spectra (Fig. 5c). This was allocated to the increased dispersion of silver nanoparticles on the top area of the film, which limited EDX observation. The detected signal of underneath silver nanoparticles is in fact too tiny to be separated from the background signal when employing the standard acceleration voltages of 5 keV to 20 keV. 

Figure 4: SEM pictures showing the surface morphologies of undoped TiO2 film(a), 1% Ag-doped (b), and 2% Ag-implanted TiO2 film (c). Click here to View Figure

Figure 5: EDX plot of pure and Ag-doped TiO2 Click here to View Figure

Conclusions

The relatively low-cost sol-gel method has been used to create thin films of Ag-doped TiO₂ at a heating temperature of 500°C. 

In comparison to pure TiO₂ films, Ag-doped TiO₂ films have smaller crystallite domain sizes.  

According to X-ray diffractograms, TiO₂ developed into the anatase phases, whose lattice parameters initially grew up with Ag doping and then decreased when the fluence value was further raised. 

 Compared to undoped TiO₂ films, Ag-doped TiO₂ films have a greater photocatalytic function. The E_g is decreased with doping. 

Acknowledgments

We received funding for this study from the CSIR in New Delhi, and characterization resources came from the MNIT MRC and Manipal University in Jaipur. We appreciate the facilities for experimentation and characterization provided by USIC and the physics department at UOR, Jaipur.

Conflict of 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.

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