Enhancing Structural Integrity, Optical Properties, and Room Temperature Formaldehyde Sensing Through Optimized Spray Deposition Rates

This study explores the impact of deposition rate on the properties of TiO2 thin films produced via spray pyrolysis, focusing on their application in gas sensors. The analysis covers structural, morphological, optical, and gas sensing characteristics of TiO2 films deposited at rates between 1 and 2.5 ml min−1. Studies show optimizing TiO2 film deposition rates at 2 ml min−1 significantly enhances formaldehyde detection, improving selectivity and achieving a rapid response of 7.52 at 20 ppm concentration. This study underscores the pivotal role of deposition rate optimization in augmenting the gas-sensing efficacy of TiO2 films, particularly for formaldehyde detection at ambient conditions. Optimal deposition rates are instrumental in enhancing sensor performance. The synergistic application of XRD and Raman spectroscopy unequivocally confirmed the presence of the TiO2 anatase phase, which is of paramount significance in gas sensing applications. FESEM furnished high-resolution insights into the surface morphology, revealing a spherical architecture. Furthermore, UV–vis spectroscopy was employed to assess the optical band gap of the films, which exhibited a decrement correlating with the rate of deposition. Notably, a deposition rate of 2 ml min−1 markedly improved the TiO2 films’ sensing performance. These insights are critical for developing cost-effective, high-performance gas sensors for cutting-edge applications.

One key objective in the development of sensors is to make them more accessible to a wide range of applications by reducing their cost.Cost-effective sensors would enable their widespread adoption across industries and facilitate the integration of sensing capabilities into everyday devices and systems.By achieving economies of scale and optimizing manufacturing processes, the overall production cost can be significantly reduced without compromising the performance and reliability of the sensors. 1 Gas sensors are indispensable tools across multiple industries, offering versatile applications.3][4] These sensors play a vital role in maintaining safety, adhering to environmental standards, and optimizing industrial processes.Their ability to detect target gases and issue warnings when threshold values are exceeded ensures efficient operations and effective environmental monitoring.
The advancement in gas sensing technology has been greatly influenced by the development of nanostructured materials.These materials offer unique advantages over bulk materials, primarily due to their large surface area and small size.Among the various types of MO x -thin film based gas sensors investigated, such as SnO 2 , ZnO, WO 3 , CuO, NiO and Fe 2 O 3 , TiO 2 -thin film [5][6][7][8][9][10][11] based sensors have gained widespread popularity in numerous applications.This is primarily attributed to their exceptional sensing response, excellent selectivity, ease of manufacturing, affordability, thermal and chemical stability, and non-toxic nature. 12,13he effectiveness of gas sensors is heavily influenced by the size of the nanostructures utilized. 14,15Therefore, various synthesis methods have been employed to produce nanostructures with different morphologies.][18][19][20] The spray pyrolysis technique has garnered significant interest among the various chemical methods due to its notable advantages, including simplicity, cost-effectiveness, and the ability to incorporate different materials with high stoichiometry.Additionally, this technique proves advantageous for large-area applications.Moreover, during the spray process, we have the capability to control and manipulate the surface microstructure and morphological characteristics of the thin films, offering further flexibility and precision in material engineering. 21he efficacy of current gas sensors is hindered by high power demands, limited selectivity, and the necessity for elevated operating temperatures, limiting their application in portable and energyefficient settings.Particularly, the performance of TiO 2 sensors, crucial for detecting compounds like formaldehyde, hinges on their structural characteristics shaped by film deposition rates.A significant market gap exists for affordable, highly sensitive, and selective gas sensors operable at room temperature, essential for monitoring indoor air quality and ensuring industrial safety.This highlights a research void in investigating optimal TiO 2 film deposition rates to boost gas-sensing efficiency, with a focus on selectivity, sensitivity, and rapid response in ambient conditions.
In this study, the impact of the deposition rate on the characterization of TiO 2 films fabricate using the spray-pyrolysis method is investigated.The properties of the produced films are influenced by several important factors, including the composition and concentration of the starting solution, the type of precursors and dopants used, the temperature of the substrate, the choice of carrier gas, and the rate at which the deposition takes place.
However, there is limited research on the role of deposition rate in gas sensor applications.For instance, R.S. Gaikwad et al., 22 nanocrystalline ZnO films were deposited using spray pyrolysis and studied the effect of gas flow rate ranging from 3 litres per minute (lpm) to 7 lpm.Qutaiba A. Abduljabbar et al., 23 Ni-doped SnO 2 films were prepared via chemical spray method at 450 °C with varying spray rates of 0.8, 1, 1.33, and 2 ml min −1 for the detection of NO 2 at operating temperature 100 °C.Hence, this study focuses on thoroughly analysing the structural, morphological, optical, and gas sensing characteristics of TiO 2 films with varying deposition rates 1 to 2.5 ml min −1 .The results obtained from this study offer significant observations into the gas sensing properties of low-cost TiO 2 sensors, specifically for the detection of toxic formaldehyde vapours at room temperature.Table I presents a summary of existing literature related to the use of TiO 2 in the detection of formaldehyde (HCHO) at different operating temperatures.
To prepare the required precursor solution, TTIP (99.9% pure, Sigma Aldrich) was dissolved in methanol and continuously stirred.After stirring for five minutes, Acetyl acetone was added to the precursor in a 1:2 ratio based on TTIP.The prepared precursor solution was then applied onto ultrasonicated glass substrates.The deposition process was carried out at a temperature of 400 °C using a solution concentration of 0.1 M, ensuring precise control and accuracy.Different deposition rates were utilized: 1 ml min −1 , 1.5 ml min −1 , 2 ml min −1 , and 2.5 ml min −1 .The utilization of these different rates led to the fabrication of nanocrystalline TiO 2 thin films, designated as T1, T2, T3, and T4, respectively.
Characterization of TiO 2 thin-film.-TheXRD technique using the Bruker D8 advance system and Cu Kα radiation source is utilized for analysing deposited films.Thickness of the deposited films calculated using weight difference approach method (weigh balance instrument: Kerro P7B BL-2204).Optical bandgaps were calculated using a UV-visible spectrophotometer (Lambda 650).Raman spectra were acquired using a Lab Ram HR800 Raman spectrometer equipped with a high-power green laser (532 nm, 20 mW).FESEM analysis was conducted on the prepared thin films to examine their surface characteristics.Gas sensor efficiency was tested using a home-made gas sensing unit designed by VR Technologies, Bangalore.All these characterisations were employed on the deposited films at RT.

Results and Discussion
Thickness measurement.-Thedetermination of the thin film thickness was accomplished using the weight difference technique, employing the following formula. 29Notably, an increase in the deposition rate resulted in a corresponding increase in the thickness of the prepared thin films.The observed thickness augmentation in the TiO 2 films could be attributed to the excessive mass transfer phenomena.However, excessive mass transfer towards the substrate may also lead to the formation of amorphous films.Consequently, it becomes imperative to optimize the deposition rate to achieve highquality crystalline films.The calculated thickness values are provided in Table II.
where Δw is the weight difference of the sample before and after the deposition, ρ is the standard density of the titanium dioxide material, and "a" is the area of the substrate.
Structural characteristics.-GIXRDpattern were used to analyze the phase and structural attributes of TiO 2 films.The XRD diffraction pattern in Fig. 1 showed identical positions of diffraction peaks for T1, T2, T3, and T4 samples.These peaks corresponded to anatase TiO 2 crystal planes indexed as (101), (004), ( 200), (105), (204), (220), and (215).The analysis revealed that the films exhibited a tetragonal body-centered polycrystalline structure characteristic of anatase-TiO 2 , with space group I41/amd, as per JCPDS card No. 21-1272.This indicates the presence of pristine titania films with a single-phase composition, without the detection of any additional phases or materials.
The structural formation of a film during the deposition process is significantly influenced by the deposition rate.Elevating the deposition rate results in enhanced mass transfer towards the substrate. 22As a result, there is an observed augmentation in the film thickness, accompanied by an intensification of the anatase peaks.Specifically, the primary diffraction peak at (101) with a 2θ value of 25.3°exhibits a higher intensity in T3 compared to the other analyzed samples.An upward trend in the deposition rate corresponds to a higher intensity of the (101) peak, denoting the preferential crystal orientation with sharpened peaks.These observations suggest an improvement in the crystalline structure of the TiO 2 thin films. 30e have calculated particle size using Scherrer's equation 31 along anatase characteristic peak (101).Furthermore, structural parameters derived from X-ray diffraction data, like micro strain and dislocation density, 31 were computed to gain insights into the crystal lattice characteristics, and tabulated in Table II.
where k represents a constant, λ denotes the wavelength of the utilized X-rays (0.1540 nm), and β signifies the integral breadth of the diffraction peak.ECS Sensors Plus, 2024 3 025201 Micro strain can be quantified using techniques like X-ray diffraction or neutron diffraction.When a crystalline material is subjected to stress or temperature changes, its lattice spacing may change, leading to the generation of micro strain.Mathematically, micro strain (ε) is defined as: In thin film fabrication, the deposition rate significantly influences micro strain, which affects the material's uniformity and strain.Higher deposition rates can increase micro strain by altering atom and molecule arrangement during film growth.This understanding is vital for optimizing thin films' mechanical and electrical properties, crucial for electronics, coatings, and sensor applications. 32Controlling the deposition rate allows for micro strain adjustments, thereby enhancing material performance in specific applications.
The dislocation density (δ) is defined as the length of dislocation lines per unit volume of the crystal and was determined using the following relation Raman spectroscopy, as illustrated in Fig. 2, provided compelling evidence confirming the pristine anatase phase of the TiO 2 thin film with 2 ml min −1 deposition rate, under investigation.This characterization technique offered valuable insights into the structural composition of the deposited thin film, with Raman modes specifically linked to the bending vibrations within the O-Ti-O bonding. 33n the context of this study, the Raman spectroscopy analysis of as-deposited Anatase TiO 2 films revealed the presence of distinct Raman active modes at specific wavenumbers, namely 145.6 cm −1 , 198 cm −1 , 398.1 cm −1 , 518.5 cm −1 , and 638.9 cm −1 .It is worth noting that the observed peak positions may deviate from the expected Anatase phase peak positions due to various external factors, including the deposition method and the specific measuring instrument employed. 34urthermore, it is important to recognize that the intensity of Raman bands is highly sensitive to a range of variables, including oxygen concentration, deposition temperature, and the size of crystallites within the thin film.Controlling the deposition parameters that affect these structural characteristics, researchers can tailor TiO 2 thin films to optimize their performance in gas sensor applications.This involves not just achieving the anatase phase but also fine-tuning its microstructure to enhance selectivity, sensitivity, and response time towards specific target gases.
Morphology using FESEM.-Aroom temperature FESEM investigation was conducted to analyse the morphology and growth mechanisms of the synthesized films.Figure 3 shows the morphology of the films fabricated at different deposition rates (1, 1.5, 2, and 2.5 ml min −1 ).The influence of the deposition rate on the stability and shape of TiO 2 thin films was observed, in agreement with the findings of Govindasamy et al. 35 The FESEM images revealed random growth and agglomeration of grains, indicating varying degrees of crystallinity in the films.At lower deposition rates, the films exhibited well-defined multi-grain agglomerates with irregular shapes and sizes.As the deposition rate increases, all the films have shown increased crystallinity and surface morphology appeared in spherical more in clear. 36The fabricated film T3 has shown proper arrangement of grains with smaller crystallites which indicates the highest crystallinity whereas film T4 appeared with decreased crystallinity due to more amount of material deposition leads to improper growth mechanism due to lack of available time for crystal growth. 37tical characteristics.-Theoptical characteristics of the spray pyrolysis-deposited TiO 2 films were evaluated using a UV-Vis spectrophotometer.The bandgap values are influenced by parameters such as the thickness of the thin film, the crystal structure, and the arrangement of atoms within the crystal lattice. 38To determine the optical energy gap of the TiO 2 thin film, a Tauc's plot was employed.
The experimental findings revealed a notable decrease in the optical band gap of the TiO 2 thin films as the deposition rate increased, as illustrated in Fig. 4. Specifically, the band gap values decreased from 3.38 eV to 3.26 eV.This observation is consistent with the findings reported in prior studies, 39,40 indicating a convergence of results.The reduction in the band gap associated with elevated deposition rates can be ascribed to the spatial confinement of electrons and holes, leading to energy level quantization. 41s sensing properties.-For the investigation of gas sensing properties, spray-deposited TiO 2 thin films were employed to detect volatile gases at room temperature.The experimental arrangement consisted of a cylindrical gas sensing chamber.A highly sensitive Keithley electrometer (6517B, USA) was connected to a personal computer for data acquisition.To establish a baseline, the resistance (R a ) of the entire sample was initially measured in the presence of regular airflow.Subsequently, the target gas vapours were  ECS Sensors Plus, 2024 3 025201  introduced and sensing chamber was sealed.This led to a significant alteration in the resistance of the TiO 2 film.After achieving stabilization of the sensor element's resistance (R g ) in the presence of the target gas, the chamber door was immediately opened, exposing the element to normal air.During this process, the sensor element gradually regained its initial resistance level.To determine the sensor's response, the following equation 42 can be used: Furthermore, it is crucial to note that higher humidity levels can negatively impact the stability of the sensor element.To ensure accurate and reliable gas sensing studies, we maintained a controlled relative humidity of ∼60% within the testing chamber.This controlled humidity level helps mitigate the potential effects of humidity on the sensor element's performance and stability.
Gas sensing mechanism.-Gassensing mechanisms involve the interaction between the test gas and the solid surface. 43For n-type semiconducting metal oxide surfaces, a two-step mechanism has been proposed.It starts with oxygen adsorption on the surface, followed by the reduction of surface area by the detected gas.In the presence of chemisorbed oxygen, charge transfer occurs, leading to band bending and a shift in the electrostatic potential towards the metal oxide surface.Oxygen molecules adsorb on the TiO 2 surface and withdraw electrons from its conduction band, forming − O , Exposure to formaldehyde gas led to a discernible reduction in the electrical resistance of the TiO 2 material.This decrease can be attributed to the oxidizing nature of formaldehyde gas.Upon the interaction between formaldehyde and chemisorbed oxygen species, it undergoes oxidation to produce CO 2 , H 2 O, and an electron.In the presence of formaldehyde in the air, the process of oxygen release can be summarized as follows: 44 The ejected electrons from this process interact with the TiO 2 thin-film surface and return to it.Consequently, when the TiO 2 thinfilm surface interacts with formaldehyde gas, its conductivity increases due to an elevated electron concentration. 45lectivity.-TiO 2 film sensors deposited at different rates were tested for their gas sensing responses to formaldehyde, toluene, ethanol, xylene, and butanol.The sensors showed excellent selectivity towards formaldehyde due to small-scale crystallites and its lower dissociation energy 46 compared to other tested gases, shown in Fig. 5.The sensor prepared at a deposition rate of 2 ml min −1 exhibited a strong response of 7.52 at 20 ppm formaldehyde, indicating potential for low-cost, ambient-operating formaldehyde sensors with high selectivity.
Response and recovery characteristics.-Theroom temperature operated TiO 2 film response towards increased formaldehyde concentration with deposition rate is shown in Fig. 6.The findings clearly demonstrate that an elevated carrier concentration can significantly improve the conductivity of the films, resulting in a higher gas response.This observation further supports that the 2 ml min −1 deposition rate can effectively enhance the gas response.
This improvement in gas sensing properties due to the structural and morphological changes resulting from the deposition rate.The results indicate that the TiO 2 film sensor deposited at a rate of 2 ml min −1 exhibits a strong response to formaldehyde gas at RT, with a least detection limit of 1 ppm.Figure 7a illustrates the transient response curve of TiO 2 film resistance in the presence and absence of 20 ppm formaldehyde vapours at room temperature.The curve shows the fall and rise behaviour of resistance, indicating the sensor's response to the gas.The response time, which is the time required for the sensor to reach 90% of its saturation resistance in the presence of the test gas, is determined to be 44 s.The recovery time, which refers to the duration required for the sensor to reach 10% of its saturated resistance 47 following the removal of the test gas, was measured to be 102 s.The results indicate that the optimized deposition rate is well-suited for formaldehyde detection at room temperature.
In Fig. 7b, the repeatability of the fabricated sensor is depicted.The gas-sensing experiment involved four consecutive cycles, with each cycle aiming for 20 ppm of HCHO detection at room temperature.The response values displayed only slight variations  throughout these repeated cycles.As a result, it can be concluded that the created sensor exhibits excellent repeatability attributes.
Figure 7c, the stability of the fabricated sensor was observed over a 25-day period starting from the day of deposition, with measurements taken every 5 days.Throughout this period, the sensor element consistently maintained nearly stable response values, confirming its excellent stability.
Figure 7d shows the sensor's response linearity with respect to HCHO gas concentrations, which falls within the range of 5 to 25 ppm.A notable observation is the sensor's response demonstrating a clear and direct correlation with increasing HCHO concentrations.This rise in sensor response is primarily attributable to the chemical interaction between the HCHO gas and the sensing material incorporated within the sensor.This finding underscores the sensor's sensitivity to varying HCHO concentrations and its potential for accurate gas detection in this concentration range.

Conclusions
The study investigated TiO 2 thin films deposited at varying deposition rates and their structural, morphological, optical, and gas sensing characteristics.The films exhibited a tetragonal bodycentered polycrystalline anatase structure with preferred crystal orientation.Increasing the deposition rate led to thicker films, increased crystallinity, and a reduction in the optical band gap.The films showed good gas sensing capabilities towards formaldehyde gas at room temperature, with selectivity towards formaldehyde and a quick response 7.52 at 20 ppm and response/recovery times 44 s/102 s.The optimized deposition rate resulted in films with enhanced crystallinity, proper grain arrangement, and high response to formaldehyde with low detection limit of 1ppm, suggesting their potential for low-cost and ambient-operating formaldehyde sensors.

Figure 1 .
Figure 1.XRD spectra of TiO 2 films deposited at different rates.

Figure 3 .
Figure 3. FESEM images of the TiO 2 thin films at deposition rates.

Figure 4 .
Figure 4. Optical band gaps of spray pyrolyzed TiO 2 films at various deposition rates.

Figure 5 .
Figure 5. Responses of the fabricated sensors for different target gases (at 20 ppm) with different deposition rates.

Figure 6 .
Figure 6.Responses of the fabricated sensors at different formaldehyde concentration.

Figure 7 .
Figure 7. (a) Transient response curve, (b) Repeatability of the sensor (T3) at 20 ppm of HCHO (c) stability of TiO 2 sensor with 2 ml min −1 and (d) Response linearity of T3 sensor over different HCHO concentration.

Table I .
TiO 2 sensors for detection of formaldehyde gas at different operating temperature.

Table II .
Structural information of TiO 2 films deposited at varied rates.