# Optical CO2 Gas Sensing Based on TiO2 Thin Films of Diverse Thickness Decorated with Silver Nanoparticles

Authors: Muhammad Akram Raza; Anam Habib; Zakia Kanwal; Syed Sajjad Hussain; Muhammad Javaid Iqbal; Murtaza Saleem; Saira Riaz; Shahzad Naseem
Journal: Advances in Materials Science and Engineering
 (2018)

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## Abstract

The fabrication, characterization, and CO2 gas detection performance of single component-based and hetero-nanostructure-based optical gas sensors are reported in the present work. Single component-based structures include (i) TiO2 thin films with varied film thickness (37.45 nm, 51.92 nm, and 99.55 nm) fabricated via the RF sputtering system for different deposition times and (ii) silver nanoparticles (AgNPs) deposited on the glass substrate by the wet chemical method. Hetero-nanostructures were achieved by decorating the AgNPs on the predeposited TiO2 thin films. The structural, morphological, and optical characteristics of prepared samples were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and ellipsometry, respectively. XRD analysis of AgNPs confirmed the crystalline nature of prepared particles with average crystallite size of 21 nm, however, in the case of TiO2 films XRD results suggested amorphous structure of all as-deposited films. size 21 nm. The SEM micrographs confirmed the deposition of AgNPs on the TiO2 thin films. With increasing sputtering time, TiO2 films were found to be denser and more compact, indicating a reduced porosity and higher film thickness. CO2 gas-sensing properties were investigated by measuring the optical transmission spectra in alone air and in CO2 gaseous atmosphere at room temperature. It was observed that neither TiO2 thin films even with higher thickness nor alone AgNPs could demonstrate any substantial gas-sensing activity. Nevertheless, TiO2/AgNP hetero-nanostructured substrates exhibited excellent CO2 gas-sensing performance as indicated by a huge change in the transmission spectra. The enhanced sensing efficiency of TiO2/AgNP nanostructures owing to synergistic effects suggests a promising role of our manufactured sensors in practical applications.


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## Body

## 1. Introduction

To meet the needs of rapidly increasing human population, industry production has drastically increased. During this process, a lot of industrial waste is causing environmental pollution due to improper disposal and management. The most common forms of industrial contaminants are pollutant gases such as CO, C6H4, NOx, NH3, CH4, SO2, and CO2. The exhaust fumes from automobiles are further intensifying the quantity of such pollutants in air making them more toxic and harmful for humans and for the natural ecosystem [1, 2]. The increasing public health issues demand strong efforts to reduce the impact of these noxious gases. The development of efficient sensors for gas monitoring could be the first step to address this challenge.For the quantitative detection of various gases in the environment, different types of traditional gas sensors are used including pellistors, semiconductor devices, and electrochemical gas sensors. Although these sensors are widely employed in several applications, they suffer from different drawbacks. For example, pellistors being robust in nature respond to combustion on a catalyst bead. They are suitable for flammable gases detection around the lower explosive limit but face zero drift problems at parts per million (ppm) levels. Semiconductor gas sensors can work effectively at the low ppm levels; however, they also suffer from drift and humidity variations and cross-respond to other gases. Similarly, electrochemical gas devices can be sensitive at ppm or ppb levels and comparatively explicit to individual gases; nevertheless, they have issues such as limited lifetimes, known cross-response, and changing humidity levels [3, 4].Owing to aforementioned limitations and drawbacks of conventional gas-sensing devices, optical gas sensors are attracting more attention. They are based on materials that can change their optical response when exposed to target gaseous atmosphere. They are fast in action (even below 1 sec) and have negligible drift and high gas specificity, with minimal cross-response. They provide a wide range of operative parameters, and any change in intensity, frequency, polarization, and phase of reflected or transmitted light as compared to incident light can be used in the sensing mechanism. Measurements made by optical gas sensors are self-referenced and inherently reliable because the intensity of incident and transmitted/reflected light can be determined. Furthermore, contactless readout measurements and independency from electromagnetic noise make them suitable for optical fibers and integrated photonic devices for possibly fast and easy signal transport and in situ measurements with a compact, flexible system [1, 5, 6].Carbon dioxide (CO2), a colorless, generally inert in nature, and highly oxidizing gas, belongs to those greenhouse gases that occur both naturally (by decomposition of organic matter) and from human activities (including burning fossil fuels for power generation, oil refining, natural gas production, and transportation). Higher concentration of CO2 in the atmosphere is not only dangerous for living beings but also has severe impact on global climate. For example, maximum limit of CO2 required by safety regulations in a mixture of gases for mines, wells, and sewers operations is 5–20%. Beyond this limit, it becomes hazardous for humans health. Likewise, the anomalous increase of CO2 can cause huge global climate changes such as greenhouse effect, global warming, possible expansion of subtropical deserts, and increase in sea levels [7–9]. Therefore, the qualitative and quantitative detection of CO2 is of substantial interest in various fields such as biotechnology, food packaging and beverage, marine and environmental science, medicine and health, air quality, and industrial monitoring [10, 11].Titanium dioxide (TiO2) owing to low cost, low oxidation potential, and nontoxic in nature along with high refractive index, high dielectric constant, and chemical stability is of much interest in various applications such as catalysis, optoelectronic devices, photocatalysis, and sensors [12–14]. TiO2-based gas sensors are becoming popular for their potential use in medicine, biology, self-safety, transportation industries, and environmental protection purposes [15, 16]. To improve the sensing performance of these sensors, various techniques have been employed by the researchers. Manufacturing of TiO2-based hetero-nanostructures using the metallic nanoparticles is a promising approach in the gas-sensing mechanism. The combined effect of the functional TiO2 matrix and large active surface areas of nanoparticles could boost the sensing efficiency as compared to single component-based sensors [15, 17, 18].The aim of this work was to explore the synergistic effect of TiO2/AgNP hetero-nanostructures for CO2 gas sensing. TiO2 thin films of varied thickness were fabricated via the RF magnetron sputtering system, and AgNPs were decorated on them by the wet chemical method. The optical CO2 gas detection experiments were carried out by measuring the transmission signals in a normal incident configuration of a variable-angle ellipsometer at room temperature. CO2 gas-sensing performance of alone AgNP films, pristine TiO2 thin films having different thickness, and AgNP-loaded TiO2 films was investigated. The effect of enhanced TiO2 film thickness on the AgNP deposition and CO2 gas-sensing efficacy was also studied. The focus of this study was to work out the best possible composite morphology for most effective and reliable CO2 gas sensors.


## 2. Materials and Methods

### 2.1. Preparation of Samples

Three types of substrates were prepared: (i) alone TiO2 thin films having thickness of 37.45 nm, 51.92 nm, and 99.55 nm, (ii) alone silver nanoparticles deposited on glass substrates, and (iii) TiO2/AgNP hetero-nanostructure substrates with varied abovementioned TiO2 film thickness. Titanium dioxide (TiO2) thin films were fabricated on soda lime glass substrates via the RF magnetron sputtering technique. The glass slides were cleaned ultrasonically for 20 min first in acetone and then for further 20 min in IPA and dried in nitrogen (N2) flow, prior to film deposition. TiO2 films were sputtered using the titanium dioxide target, in the argon (Ar) environment with a fixed flow rate of 50 sccm at room temperature and 10−6 Pa of pressure. To achieve a varied film thickness, the deposition time was changed as 30 min, 60 min, and 90 min, keeping all other parameters unchanged. The TiO2 thin film samples were used as-deposited without any annealing for further application.In the next step, deposition of sliver nanoparticles (AgNPs) was carried out on bare glass substrates and on TiO2 thin film samples. AgNPs were first synthesized following a chemical reduction route described in detail in our previous study [19]. Briefly, 100 mL of 1 mM aqueous solution of silver nitrate (AgNO3) was heated to boiling temperature under vigorous stirring and refluxing conditions. Then, 10 mL of aqueous solution of 1% trisodium citrate (Na3C6H5O7) was added dropwise with the help of a dropper. The solution changed color from transparent color to pale yellow to bright yellow and finally to greenish yellow, indicating the completion of the reaction as shown in Figure 1. Dissolving of silver precursors produced silver ions (Ag+) in the aqueous solution. These Ag+ ions were reduced to free silver atoms (Ag0) by the reducing agent (trisodium citrate) which resulted in the initial color change (pale yellow). Owing to nucleation and growth processes, produced free silver atoms (Ag0) gathered into the oligomeric clusters and finally into silver nanoparticles [20]. The color change from bright yellow to greenish yellow indicated the different phases taking place in the reaction. Trisodium citrate served as a reducing agent and a stabilizing agent in this case. The colloidal solution was cooled to room temperature for further characterization and applications.Figure 1
Color change during the synthesis of AgNPs depicting different stages of the reaction. (a) Dissolving of silver precursors into water remained transparent. (b–d) Introduction of the reducing agent (trisodium citrate) caused the reduction of precursors as appeared in color change from transparent to greenish yellow.Before decorating with AgNPs, all substrates (bare glass and TiO2 thin films) were functionalized with 3-aminopropyltriethoxysilane (APTES) to obtain a high surface affinity for the colloidal AgNPs adopting a solution-based approach [21]. Briefly, bare cleaned glass substrates and TiO2 film samples were immersed into a 10% APTES methanolic solution for 60 min, followed by thoroughly rinsing with pure methanol and drying in N2 flow. Then, the samples were immersed in water for 30 min and dried in N2 flow after rinsing with water. Finally, APTES-derivatized substrates were immersed in the silver colloidal suspension for the whole night, rinsed with water, and dried in N2 flow. All of these procedures were performed at room temperature.


### 2.2. Characterization of Prepared Samples

All three types of fabricated samples were characterized with different characterization techniques for their morphological, optical, and structural analyses. Structural investigations were carried out by using X-ray diffraction (XRD) methods with an X-ray diffractometer (D-MaxIIA; Rigaku, Tokyo, Japan) operated at 40 kV and 25 mA using Cu Kα line radiation (λ = 1.5406 Å). Few drops of the silver colloidal sample were dried on the clean glass substrate to form a thick film for XRD measurements. A field-emission scanning electron microscope (FEI Nova NanoSEM 450; Hillsboro, USA) was used to study the morphology of obtained structures at different magnifications. A thin gold layer (10 nm) was also deposited, if needed, to avoid any possible charging on the sample. The thickness and the optical constants of TiO2 thin films fabricated at different deposition times were determined by spectroscopic ellipsometry. The measurements were carried out using a variable-angle ellipsometer (M-2000; J.A. Woollam, USA) in air (without chamber) at a fixed incidence angle of 65° with respect to the surface normal operating in the wavelength range of 300–900 nm at room temperature.


### 2.3. Optical Gas Sensing

In order to examine the optical CO2 gas-sensing performance of all three aforementioned categories of samples, optical transmission measurements were performed (in air and in CO2 gas) within a custom-made transparent airtight gas-sensing chamber (8 cm × 6 cm × 12 cm) using a spectroscopic ellipsometer in the vertical configuration (as shown in Figure 2) in the wavelength (λ) range of 300 nm to 900 nm at room temperature. CO2 gaseous environment was established by a constant flow rate of 50 sccm. Measurements were carried out by exposing each sample to air and to CO2 gaseous atmosphere for 2 min.Figure 2
Experimental setup for CO2 gas sensing.The light beam was allowed to fall on the substrate at right angle (as schematically shown in Figure2) which covered an area of about 2.8 mm2 on the substrate. The change in transmission due to CO2 gas was recorded for each sample.


## 2.1. Preparation of Samples

Three types of substrates were prepared: (i) alone TiO2 thin films having thickness of 37.45 nm, 51.92 nm, and 99.55 nm, (ii) alone silver nanoparticles deposited on glass substrates, and (iii) TiO2/AgNP hetero-nanostructure substrates with varied abovementioned TiO2 film thickness. Titanium dioxide (TiO2) thin films were fabricated on soda lime glass substrates via the RF magnetron sputtering technique. The glass slides were cleaned ultrasonically for 20 min first in acetone and then for further 20 min in IPA and dried in nitrogen (N2) flow, prior to film deposition. TiO2 films were sputtered using the titanium dioxide target, in the argon (Ar) environment with a fixed flow rate of 50 sccm at room temperature and 10−6 Pa of pressure. To achieve a varied film thickness, the deposition time was changed as 30 min, 60 min, and 90 min, keeping all other parameters unchanged. The TiO2 thin film samples were used as-deposited without any annealing for further application.In the next step, deposition of sliver nanoparticles (AgNPs) was carried out on bare glass substrates and on TiO2 thin film samples. AgNPs were first synthesized following a chemical reduction route described in detail in our previous study [19]. Briefly, 100 mL of 1 mM aqueous solution of silver nitrate (AgNO3) was heated to boiling temperature under vigorous stirring and refluxing conditions. Then, 10 mL of aqueous solution of 1% trisodium citrate (Na3C6H5O7) was added dropwise with the help of a dropper. The solution changed color from transparent color to pale yellow to bright yellow and finally to greenish yellow, indicating the completion of the reaction as shown in Figure 1. Dissolving of silver precursors produced silver ions (Ag+) in the aqueous solution. These Ag+ ions were reduced to free silver atoms (Ag0) by the reducing agent (trisodium citrate) which resulted in the initial color change (pale yellow). Owing to nucleation and growth processes, produced free silver atoms (Ag0) gathered into the oligomeric clusters and finally into silver nanoparticles [20]. The color change from bright yellow to greenish yellow indicated the different phases taking place in the reaction. Trisodium citrate served as a reducing agent and a stabilizing agent in this case. The colloidal solution was cooled to room temperature for further characterization and applications.Figure 1
Color change during the synthesis of AgNPs depicting different stages of the reaction. (a) Dissolving of silver precursors into water remained transparent. (b–d) Introduction of the reducing agent (trisodium citrate) caused the reduction of precursors as appeared in color change from transparent to greenish yellow.Before decorating with AgNPs, all substrates (bare glass and TiO2 thin films) were functionalized with 3-aminopropyltriethoxysilane (APTES) to obtain a high surface affinity for the colloidal AgNPs adopting a solution-based approach [21]. Briefly, bare cleaned glass substrates and TiO2 film samples were immersed into a 10% APTES methanolic solution for 60 min, followed by thoroughly rinsing with pure methanol and drying in N2 flow. Then, the samples were immersed in water for 30 min and dried in N2 flow after rinsing with water. Finally, APTES-derivatized substrates were immersed in the silver colloidal suspension for the whole night, rinsed with water, and dried in N2 flow. All of these procedures were performed at room temperature.


## 2.2. Characterization of Prepared Samples

All three types of fabricated samples were characterized with different characterization techniques for their morphological, optical, and structural analyses. Structural investigations were carried out by using X-ray diffraction (XRD) methods with an X-ray diffractometer (D-MaxIIA; Rigaku, Tokyo, Japan) operated at 40 kV and 25 mA using Cu Kα line radiation (λ = 1.5406 Å). Few drops of the silver colloidal sample were dried on the clean glass substrate to form a thick film for XRD measurements. A field-emission scanning electron microscope (FEI Nova NanoSEM 450; Hillsboro, USA) was used to study the morphology of obtained structures at different magnifications. A thin gold layer (10 nm) was also deposited, if needed, to avoid any possible charging on the sample. The thickness and the optical constants of TiO2 thin films fabricated at different deposition times were determined by spectroscopic ellipsometry. The measurements were carried out using a variable-angle ellipsometer (M-2000; J.A. Woollam, USA) in air (without chamber) at a fixed incidence angle of 65° with respect to the surface normal operating in the wavelength range of 300–900 nm at room temperature.


## 2.3. Optical Gas Sensing

In order to examine the optical CO2 gas-sensing performance of all three aforementioned categories of samples, optical transmission measurements were performed (in air and in CO2 gas) within a custom-made transparent airtight gas-sensing chamber (8 cm × 6 cm × 12 cm) using a spectroscopic ellipsometer in the vertical configuration (as shown in Figure 2) in the wavelength (λ) range of 300 nm to 900 nm at room temperature. CO2 gaseous environment was established by a constant flow rate of 50 sccm. Measurements were carried out by exposing each sample to air and to CO2 gaseous atmosphere for 2 min.Figure 2
Experimental setup for CO2 gas sensing.The light beam was allowed to fall on the substrate at right angle (as schematically shown in Figure2) which covered an area of about 2.8 mm2 on the substrate. The change in transmission due to CO2 gas was recorded for each sample.


## 3. Results and Discussion

Figure3 shows TiO2 thin film samples before and after AgNP deposition as an upper panel and a lower panel, respectively. It can be seen that the shade of the color changes from lighter to darker as the deposition times enhanced from 30 min to 90 min, which could be an indication of increased film thickness. After decorating with silver NPs, a further change in the color was noticed that depicts the successful adsorption of AgNPs.Figure 3
A CCD image showing the fabricated TiO2 thin films on glass substrates before and after AgNP deposition.


### 3.1. X-Ray Diffraction Analysis

The crystalline nature of as-deposited TiO2 thin films with sputtering times of 30 min, 60 min, and 90 min (without any further heat treatment) and synthesized AgNPs was determined by the XRD technique at room temperature.The XRD indexed pattern of alone AgNPs deposited on the glass substrate by the drop-casting method is shown in Figure4. Four major characteristic peaks at 2θ values of 38.45°, 46.35°, 64.75°, and 78.05° can clearly be observed. These peaks can be discerned to the reflection planes (111), (200), (220), and (311) of the face-centered cubic (FCC) pure metallic silver according to JCPDS file number 04-0784 [22]. Thus, XRD results confirmed the pure crystalline metallic nature of our synthesized AgNPs.Figure 4
XRD pattern of the AgNP sample.The broadening of the major diffraction peak (111) was utilized to calculate the crystallite size of prepared AgNPs using Debye–Scherrer’s formula [23] as follows:(1)D=kλβhklcosθ,where λ is the wavelength of X-ray used (1.54 Å), k is the shape factor (0.9), βhkl (taken in radians) is the full width at half maximum (FWHM) of the diffraction peak, and θ is the diffraction angle at that intensity. All measured vales are listed in Table 1. The average crystalline size of AgNPs was found to be 21 ± 1 nm as listed in Table 1.Table 1
Calculated crystallite size of AgNPs.
Peak position (2θ)
Diffraction plane (hkl)
FWHM (radians)
Crystallite sizeD (nm)
38.45°
(111)
0.0069
21 ± 1The XRD spectra obtained for TiO2 thin films for deposition times of 30 min, 60 min, and 90 min are shown in Figure 5(a). No sharp peak can be observed in all three spectra which can help to identify the crystalline nature or any phase of deposited TiO2 films. Absence of any strong reflection peak revealed apparently the amorphous nature of as-deposited TiO2 thin films in our case. This indicated that, without any further annealing or heat treatment, there was not enough crystalline structure developed which could be detected by the XRD. Perhaps, proper annealing could be helpful to develop sufficient crystalline nature of the films. However, a slight increase in the intensity of the spectra can be noticed by increasing the film deposition time which might be due to increased film thickness.Figure 5
XRD (a) and ellipsometry spectra (b) of TiO2 thin films for different deposition times.
(a)
(b)For the measurements of fabricated film thickness, refractive index, and film roughness, spectroscopic ellipsometry was employed, and calculations were made using the well-known Tauc–Lorentz oscillator model [24, 25]. The obtained results are presented in Figure 5(b) by plotting the refractive indices as a function of wavelength and are listed in Table 2. It can be observed that, by increasing the deposition time, TiO2 film thickness, refractive index, and film roughness increased which can be attributed to enhanced atomic packing density and decrease in the porosity of the deposited film [25, 26].Table 2
Calculated refractive indices for different deposition times of TiO2 thin films.
Film deposition time (min)
Refractive index (atλ = 550 nm)
Film thickness (nm)
Film roughness (nm)
30
1.832
37.54
0.02
60
2.301
51.92
6.05
90
2.459
99.55
8.81


### 3.2. Scanning Electron Microscopy Analysis

In order to determine the size and the shape of prepared AgNPs and the morphology of the TiO2 thin films with and without AgNP deposition, a field-emission scanning electron microscope (FE-SEM) was used.Figure6 shows the SEM micrographs of AgNPs deposited on glass substrates (by drop-casting methods). It is evident from SEM images that shapes of the silver particles were not perfectly spherical, although some round-shaped particles can also be noticed in magnified view (Figure 6(a)). The size of synthesized particles was found in the range of 20 nm to 60 nm. The observed agglomeration and clustering of particles could be the result of depositing and drying processes of silver colloids on glass by the drop-casting method.Figure 6
SEM images of AgNPs at different magnifications.SEM images of TiO2 thin films prepared at different deposition times (30, 60, and 90 min) on glass substrates with and without AgNPs are presented in Figure 7. It can be observed that TiO2 thin films with lower thickness (Figure 7(a)) comprised many islands of tiny particles having size in the range of 20 to 30 nm. At some places, TiO2 cluster formation can also be observed, and the overall film structure seemed to be porous. After the deposition of AgNPs, the morphology of the substrate appeared as TiO2 films decorated with randomly scattered clusters of AgNPs (Figure 7(b)).Figure 7
SEM micrographs of TiO2 thin films having different thickness: (a) 37.54 nm, (c) 51.92 nm, and (e) 99.55 nm, without (left) and with AgNPs (right).
(a)
(b)
(c)
(d)
(e)
(f)For the TiO2 samples with moderate film thickness of 51 nm (Figure 7(c)), it appeared that TiO2 film became dense and compact, indicating the decreased porosity and enhanced film thickness. The occurrence of TiO2 clusters at many places can also be noticed increasing the film roughness. The introduction of AgNPs (Figure 7(d)) over the 51 nm thick TiO2 films depicted that density of scattered AgNP clusters appeared to increase. Perhaps, the agglomeration of AgNPs occurred over the already developed TiO2 clusters which increased by increasing film deposition time. In the case of 90 min deposition time, the film thickness and roughness further increased, and more clusters and bump-like structures can be observed on the TiO2 films (Figure 7(e)). The film morphology after depositing the AgNPs revealed that hill- and valley-like structures covered the TiO2 films (Figure 7(f)).The deposition of AgNPs in the form of clusters and bunches instead of uniform distribution of individual nanoparticles could be due to nonuniform adsorption of APTES layers on TiO2 thin films by various chemisorption and physisorption mechanisms leading to scattered binding sites for AgNPs [27]. Besides, the formation of TiO2 clusters in the films might be another reason of AgNP agglomeration during the deposition process that enhanced the manufactured structure roughness.It can be concluded from the SEM results that, by increasing the film deposition times from 30 to 90 min, the film morphology clearly changed by improving the thickness and roughness of the deposited film. The deposition of AgNPs not only developed the hetero-nanostructures but also enhanced the roughness by nonuniform decoration of silver clusters and hill- and valley-like assemblies on TiO2 films.


### 3.3. CO2 Gas-Sensing Analysis

In order to describe the measured optical response of all the samples in the air and in CO2 atmosphere, the substrates were divided into two categories:(a)
Substrates without silver nanoparticles (AgNPs)(b)
Substrates with silver nanoparticles (AgNPs).


#### 3.3.1. Substrates without AgNPs

In Figure8, transmission spectra of all four samples, bare glass and TiO2 thin films, having different film thickness (37.54 nm, 51.92 nm, and 99.55 nm), in air and in the presence of CO2, are presented. It can be seen that, in case of air (Figure 8(a)), bare glass exhibited the typical spectrum with maximum transmission. Apparently, the transmission value of the glass substrate (∼60%) seems lower than the reported value (<90%) [28]. All optical measurements were carried out inside a transparent box to generate a gaseous environment around the sample (Figure 2). The lower values of the bare glass substrate might be due to reflections off the windows of the gas cell apparatus. For the TiO2 thin film samples, transmission was found to decrease by increasing the film deposition. This can be understood by considering the improvement in the film thickness and density which absorbed more light causing a decrease in transmission. In the case of CO2 gaseous atmosphere, transmission spectra (Figure 8(b)) of all four samples without AgNPs appeared similar to those as were in the case of alone air. Thus, no clear difference could be noticed in the presence of CO2 gas.Figure 8
Transmission spectra of TiO2 thin films with thickness of 37 nm, 51 nm, and 99 nm in air (a) and CO2 gas flow (b).
(a)
(b)


#### 3.3.2. Substrates with AgNPs

The optical transmission spectra of all samples decorated with AgNPs in air and in the presence of CO2 gas are demonstrated in Figure 9. A slight change in the shape of all spectra can be noticed. As evident from SEM images (Figure 7), deposition of AgNPs formed different-sized Ag aggregates along with some scattered distinctive AgNPs. These Ag aggregates could act as scattering centers causing a decrease in the transmission signal which are discussed in the next section.Figure 9
Transmission spectra of TiO2 thin films decorated with AgNPs of varied film thickness in air (a) and CO2 gas flow (b).
(a)
(b)


#### 3.3.3. Comparison of Gas-Sensing Performance

A detailed comparison of optical characteristics in the presence and absence of CO2 gaseous atmosphere for all samples is reported in Figure 10.Figure 10
Comparison of optical responses of all samples in air and CO2 gas flow.
(a)
(b)
(c)
(d)Figure10(a) represents the transmission response of bare glass and alone AgNP-deposited glass substrate in air and in CO2 gas flow. A huge decrease in the transmission spectra (in air) of the glass substrate decorated with AgNPs as compared to bare glass indicates the improved surface coverage of the glass substrate by the deposition of AgNPs. However, no significant change in the optical spectra due to the CO2 gas atmosphere can be noticed except a slight decrease in case of AgNP-loaded samples. This revealed substrates covered with AgNPs alone were not sufficient enough for CO2 gas detection.In the case of TiO2 thin film (37 nm thickness) samples with and without AgNP deposition (Figure 10(b)), the overall decrease in the optical response was found to be larger as compared to samples without TiO2 films (Figure 10(a)). This depicts that morphology and thickness of the material on glass substrates had changed due to TiO2 thin films. However, absence of any prominent change in the transmission in the CO2 gaseous environment proved that TiO2 thin films with insufficient thickness despite the presence of AgNPs were also incapable of noticeable CO2 gas sensing.For the TiO2 samples with 51 nm film thickness (Figure 10(c)), a significant decrease in the transmission spectra, starting from 600 nm to higher wavelength values, was noticed when TiO2 thin films decorated with AgNPs were exposed to CO2 gas flow. This indicates that increased TiO2 content in hetero-nanostructures (TiO2/AgNPs) had improved the CO2 gas-sensing properties of the samples. This argument was further strengthened when TiO2 thin films of higher thickness (99 nm) loaded with AgNPs were analyzed in the CO2 gaseous environment as shown in Figure 10(d). A huge decrease in the transmission of hetero-nanostructured samples exposed to CO2 gas (green line) was noticed as compared to air alone (blue line). These optical results revealed that neither alone silver nanoparticles nor alone TiO2 films of various thicknesses were proficient CO2 gas sensors nevertheless; hetero-nanostructures (TiO2/AgNPs) with higher TiO2 content provided the maximum CO2-sensing properties.


#### 3.3.4. Dynamic Gas Sensing

Figure11 illustrates the dynamic CO2 gas-sensing response of the hetero-nanostructured sample TiO2/AgNPs with maximum TiO2 film thickness (99 nm). The transmission signal measurements were carried out at room temperature for a fixed wavelength (λ = 600 nm) by flowing air and CO2 gas alternatively. The response time (air-to-CO2 gas transient) is defined as the time required to achieve 10% to 90% of maximum transmission signal, while the recovery time (CO2 gas-to-air transient) is defined as the time required to achieve 90% to 10% of maximum transmission. The response time was found to be about 10 sec, while recovery time was about 110 sec. The dynamic gas-sensing results (Figure 11) revealed the stability and reversibility of our prepared CO2 gas sensors.Figure 11
Dynamic CO2 gas-sensing response of TiO2 films decorated with AgNPs with maximum thickness (99 nm).


#### 3.3.5. Transmission Change Ratio (TCR)

The sensitivity of various samples toward CO2 gas can more clearly be observed in Figure 12, which reports the calculated transmission change ratio (TCR) values. TCR is defined as the difference between the transmissions measured during CO2 gas exposure (Tgas) and in air (Tair), normalized to the transmissions level in air, and is given by the following formula [17]:(2)TCR=Tgas−TairTair.Figure 12
TCR of TiO2 thin films without (a) and with AgNPs (b).
(a)
(b)The TCR plots of substrates without AgNPs and with AgNPs are presented in Figures12(a) and 12(b), respectively. The almost same behavior of TCR around zero line for all the samples without AgNPs (Figure 12(a)) indicates that increased TiO2 film thickness alone was not enough to produce any sharp change in transmission as exposed to CO2 gas. However, in the case of hetero-nanostructured morphology, a combination of TiO2 thin films of higher thickness with AgNPs, a substantial variation in TCR trend was noted. A perturbation in TCR (blue line) of 51 nm thickness (TiO2/AgNPs) sample starting from 400 nm showing peak value around 550 nm and ending at higher wavelength values can clearly be seen. Similarly, a negative trend in TCR for 99 nm thickness (TiO2/AgNPs) substrates indicates a large decrease in the transmission when exposed to CO2 gas atmosphere. Again a perturbation starting from 460 nm and ending at higher wavelength values can be observed. From these TCR results, it can be inferred that enhanced TiO2 film thickness decorated with AgNPs confirmed effectiveness of hetero-nanostructures for good CO2 gas sensors.


#### 3.3.6. Optical Sensing Mechanism for CO2 Gas

The interaction of gases with solid surfaces depends on various factors such as the type of the gas (reducing or oxidizing in nature), chemical nature of solid surface (n-type or p-type), surface morphology, and the surrounding atmosphere including temperature and humidity [29–31]. Furthermore, type of sensing layers’ configuration, whether a single component-based nanostructure or a multimaterial hetero-nanostructure, also plays a vital role to control surface reactions and thus gas-sensing mechanism [15].TiO2 is an n-type semiconducting material having d0 electronic configuration and belongs to the binary transition metal oxide category which is considered potential candidate for real gas-sensing applications. The gas-sensing mechanism of TiO2-based sensors can be described by considering the adsorption of oxygen on the surface of sensors which could be due to physisorption and chemisorption processes. These processes are found to be surface temperature and activation energy dependent. When TiO2-based sensors are exposed to air at room temperature, oxygen molecules may adsorb on the surface of TiO2-based materials by physisorption process due to Van der Waals and dipole interactions. These adsorbed oxygen molecules, then, generate chemisorbed oxygen species (O2−) on the sensor surfaces. The reaction process can be described as follows [15, 30]:(3)O2g⟶O2adsO2ads+e⟶O2−Carbon dioxide (CO2) is an oxidizing gas having a linear bonded atoms’ stable structure with no lone pair of electrons. When TiO2-based surfaces are exposed to CO2 gaseous atmosphere at room temperature, the moisture present on the surface of sensor come into action. The water molecules split into hydroxyl and hydrogen ions which subsequently react with gaseous CO2 to form carbonate ions as follows [31]:(4)CO2gas+OHadsorbed−⇆HCO3surface−CO2gas+Hadsorbed++2esurface−⟶HCO2surface−HCO2surface−⟶CO↑+OHsurface−HCO3surface−+OHabsorbed−⇆CO3bulk2−+H2OgasTiO2-based gas sensor properties are also influenced by the TiO2 crystal phases, and rutile and anatase phases are found to be more promising for the sensing applications.In our case, alone pristine TiO2 thin films even with increased film thickness (37–99 nm) exposed to CO2 gaseous flow could not exhibit any notable sensing performance (Figures 10(a)–10(c)). One of the probable reasons of this little or no sensing response of as-deposited TiO2 films could be the lack of TiO2 crystalline phases (Figure 5(a)) or higher sensitivity of TiO2 polymorph toward reducing gases (such as CO and H2) than oxidizing gases such as CO2 [17, 29]. Besides, negligible CO2 gas-sensing properties of alone AgNP substrates (Figure 10(a)) suggested that neither alone as-deposited TiO2 thin films nor alone AgNPs were substantial materials for CO2 gas-sensing mechanism. Instead, hetero-structure fabricated by decorating AgNPs on the predeposited TiO2 thin films demonstrated excellent CO2 gas sensing (Figure 12(b)). Manera et al. [12] in the case of alcohol vapor sensors using TiO2/gold nanocomposite and Wang et al. [30] during their CO gas-sensing studies have already reported the significance of hetero-nanostructures in improving the gas-sensing properties.This enhanced gas-sensing abilities of our TiO2/AgNPs hetero-structured sensors can be explained in the way that introduction of silver nanoentities on the TiO2 films generated high oxygen dissociation active sites which increased the probability of CO2 adsorption and reaction on the sensor surfaces [15, 32]. This perhaps provoked the prominent change in optical transmission when sensors exposed to in CO2 gaseous environment. Furthermore, enhanced adsorption of CO2 near the AgNPs could lead the changes in the refractive index of TiO2 [33] which caused a change in optical response. Another reason of improved sensing effect due to AgNPs on the TiO2 films could be the surface plasmon resonance (SPR) effect which is influenced by the variation in thickness and refractive index of the active sensing layer. Moreover, the size and density of AgNP islands on the TiO2 films could also affect the SPR gas sensitivity [33, 34]. The increased CO2 sensing properties of TiO2 films decorated with AgNPs with higher film thickness could be due to the enhanced SPR effect owing to increasing the thickness of TiO2 film along with growing size and density of AgNP islands.Thus findings of this work proved that TiO2/AgNPs hetero-nanostructures were more efficient CO2 gas optical sensors rather than alone AgNPs or as-deposited TiO2 films. The excellence performance of prepared hetero-nanostructured CO2 gas sensors makes them promising candidates for practical gas-sensing applications.


## 3.1. X-Ray Diffraction Analysis

The crystalline nature of as-deposited TiO2 thin films with sputtering times of 30 min, 60 min, and 90 min (without any further heat treatment) and synthesized AgNPs was determined by the XRD technique at room temperature.The XRD indexed pattern of alone AgNPs deposited on the glass substrate by the drop-casting method is shown in Figure4. Four major characteristic peaks at 2θ values of 38.45°, 46.35°, 64.75°, and 78.05° can clearly be observed. These peaks can be discerned to the reflection planes (111), (200), (220), and (311) of the face-centered cubic (FCC) pure metallic silver according to JCPDS file number 04-0784 [22]. Thus, XRD results confirmed the pure crystalline metallic nature of our synthesized AgNPs.Figure 4
XRD pattern of the AgNP sample.The broadening of the major diffraction peak (111) was utilized to calculate the crystallite size of prepared AgNPs using Debye–Scherrer’s formula [23] as follows:(1)D=kλβhklcosθ,where λ is the wavelength of X-ray used (1.54 Å), k is the shape factor (0.9), βhkl (taken in radians) is the full width at half maximum (FWHM) of the diffraction peak, and θ is the diffraction angle at that intensity. All measured vales are listed in Table 1. The average crystalline size of AgNPs was found to be 21 ± 1 nm as listed in Table 1.Table 1
Calculated crystallite size of AgNPs.
Peak position (2θ)
Diffraction plane (hkl)
FWHM (radians)
Crystallite sizeD (nm)
38.45°
(111)
0.0069
21 ± 1The XRD spectra obtained for TiO2 thin films for deposition times of 30 min, 60 min, and 90 min are shown in Figure 5(a). No sharp peak can be observed in all three spectra which can help to identify the crystalline nature or any phase of deposited TiO2 films. Absence of any strong reflection peak revealed apparently the amorphous nature of as-deposited TiO2 thin films in our case. This indicated that, without any further annealing or heat treatment, there was not enough crystalline structure developed which could be detected by the XRD. Perhaps, proper annealing could be helpful to develop sufficient crystalline nature of the films. However, a slight increase in the intensity of the spectra can be noticed by increasing the film deposition time which might be due to increased film thickness.Figure 5
XRD (a) and ellipsometry spectra (b) of TiO2 thin films for different deposition times.
(a)
(b)For the measurements of fabricated film thickness, refractive index, and film roughness, spectroscopic ellipsometry was employed, and calculations were made using the well-known Tauc–Lorentz oscillator model [24, 25]. The obtained results are presented in Figure 5(b) by plotting the refractive indices as a function of wavelength and are listed in Table 2. It can be observed that, by increasing the deposition time, TiO2 film thickness, refractive index, and film roughness increased which can be attributed to enhanced atomic packing density and decrease in the porosity of the deposited film [25, 26].Table 2
Calculated refractive indices for different deposition times of TiO2 thin films.
Film deposition time (min)
Refractive index (atλ = 550 nm)
Film thickness (nm)
Film roughness (nm)
30
1.832
37.54
0.02
60
2.301
51.92
6.05
90
2.459
99.55
8.81


## 3.2. Scanning Electron Microscopy Analysis

In order to determine the size and the shape of prepared AgNPs and the morphology of the TiO2 thin films with and without AgNP deposition, a field-emission scanning electron microscope (FE-SEM) was used.Figure6 shows the SEM micrographs of AgNPs deposited on glass substrates (by drop-casting methods). It is evident from SEM images that shapes of the silver particles were not perfectly spherical, although some round-shaped particles can also be noticed in magnified view (Figure 6(a)). The size of synthesized particles was found in the range of 20 nm to 60 nm. The observed agglomeration and clustering of particles could be the result of depositing and drying processes of silver colloids on glass by the drop-casting method.Figure 6
SEM images of AgNPs at different magnifications.SEM images of TiO2 thin films prepared at different deposition times (30, 60, and 90 min) on glass substrates with and without AgNPs are presented in Figure 7. It can be observed that TiO2 thin films with lower thickness (Figure 7(a)) comprised many islands of tiny particles having size in the range of 20 to 30 nm. At some places, TiO2 cluster formation can also be observed, and the overall film structure seemed to be porous. After the deposition of AgNPs, the morphology of the substrate appeared as TiO2 films decorated with randomly scattered clusters of AgNPs (Figure 7(b)).Figure 7
SEM micrographs of TiO2 thin films having different thickness: (a) 37.54 nm, (c) 51.92 nm, and (e) 99.55 nm, without (left) and with AgNPs (right).
(a)
(b)
(c)
(d)
(e)
(f)For the TiO2 samples with moderate film thickness of 51 nm (Figure 7(c)), it appeared that TiO2 film became dense and compact, indicating the decreased porosity and enhanced film thickness. The occurrence of TiO2 clusters at many places can also be noticed increasing the film roughness. The introduction of AgNPs (Figure 7(d)) over the 51 nm thick TiO2 films depicted that density of scattered AgNP clusters appeared to increase. Perhaps, the agglomeration of AgNPs occurred over the already developed TiO2 clusters which increased by increasing film deposition time. In the case of 90 min deposition time, the film thickness and roughness further increased, and more clusters and bump-like structures can be observed on the TiO2 films (Figure 7(e)). The film morphology after depositing the AgNPs revealed that hill- and valley-like structures covered the TiO2 films (Figure 7(f)).The deposition of AgNPs in the form of clusters and bunches instead of uniform distribution of individual nanoparticles could be due to nonuniform adsorption of APTES layers on TiO2 thin films by various chemisorption and physisorption mechanisms leading to scattered binding sites for AgNPs [27]. Besides, the formation of TiO2 clusters in the films might be another reason of AgNP agglomeration during the deposition process that enhanced the manufactured structure roughness.It can be concluded from the SEM results that, by increasing the film deposition times from 30 to 90 min, the film morphology clearly changed by improving the thickness and roughness of the deposited film. The deposition of AgNPs not only developed the hetero-nanostructures but also enhanced the roughness by nonuniform decoration of silver clusters and hill- and valley-like assemblies on TiO2 films.


## 3.3. CO2 Gas-Sensing Analysis

In order to describe the measured optical response of all the samples in the air and in CO2 atmosphere, the substrates were divided into two categories:(a)
Substrates without silver nanoparticles (AgNPs)(b)
Substrates with silver nanoparticles (AgNPs).


### 3.3.1. Substrates without AgNPs

In Figure8, transmission spectra of all four samples, bare glass and TiO2 thin films, having different film thickness (37.54 nm, 51.92 nm, and 99.55 nm), in air and in the presence of CO2, are presented. It can be seen that, in case of air (Figure 8(a)), bare glass exhibited the typical spectrum with maximum transmission. Apparently, the transmission value of the glass substrate (∼60%) seems lower than the reported value (<90%) [28]. All optical measurements were carried out inside a transparent box to generate a gaseous environment around the sample (Figure 2). The lower values of the bare glass substrate might be due to reflections off the windows of the gas cell apparatus. For the TiO2 thin film samples, transmission was found to decrease by increasing the film deposition. This can be understood by considering the improvement in the film thickness and density which absorbed more light causing a decrease in transmission. In the case of CO2 gaseous atmosphere, transmission spectra (Figure 8(b)) of all four samples without AgNPs appeared similar to those as were in the case of alone air. Thus, no clear difference could be noticed in the presence of CO2 gas.Figure 8
Transmission spectra of TiO2 thin films with thickness of 37 nm, 51 nm, and 99 nm in air (a) and CO2 gas flow (b).
(a)
(b)


### 3.3.2. Substrates with AgNPs

The optical transmission spectra of all samples decorated with AgNPs in air and in the presence of CO2 gas are demonstrated in Figure 9. A slight change in the shape of all spectra can be noticed. As evident from SEM images (Figure 7), deposition of AgNPs formed different-sized Ag aggregates along with some scattered distinctive AgNPs. These Ag aggregates could act as scattering centers causing a decrease in the transmission signal which are discussed in the next section.Figure 9
Transmission spectra of TiO2 thin films decorated with AgNPs of varied film thickness in air (a) and CO2 gas flow (b).
(a)
(b)


### 3.3.3. Comparison of Gas-Sensing Performance

A detailed comparison of optical characteristics in the presence and absence of CO2 gaseous atmosphere for all samples is reported in Figure 10.Figure 10
Comparison of optical responses of all samples in air and CO2 gas flow.
(a)
(b)
(c)
(d)Figure10(a) represents the transmission response of bare glass and alone AgNP-deposited glass substrate in air and in CO2 gas flow. A huge decrease in the transmission spectra (in air) of the glass substrate decorated with AgNPs as compared to bare glass indicates the improved surface coverage of the glass substrate by the deposition of AgNPs. However, no significant change in the optical spectra due to the CO2 gas atmosphere can be noticed except a slight decrease in case of AgNP-loaded samples. This revealed substrates covered with AgNPs alone were not sufficient enough for CO2 gas detection.In the case of TiO2 thin film (37 nm thickness) samples with and without AgNP deposition (Figure 10(b)), the overall decrease in the optical response was found to be larger as compared to samples without TiO2 films (Figure 10(a)). This depicts that morphology and thickness of the material on glass substrates had changed due to TiO2 thin films. However, absence of any prominent change in the transmission in the CO2 gaseous environment proved that TiO2 thin films with insufficient thickness despite the presence of AgNPs were also incapable of noticeable CO2 gas sensing.For the TiO2 samples with 51 nm film thickness (Figure 10(c)), a significant decrease in the transmission spectra, starting from 600 nm to higher wavelength values, was noticed when TiO2 thin films decorated with AgNPs were exposed to CO2 gas flow. This indicates that increased TiO2 content in hetero-nanostructures (TiO2/AgNPs) had improved the CO2 gas-sensing properties of the samples. This argument was further strengthened when TiO2 thin films of higher thickness (99 nm) loaded with AgNPs were analyzed in the CO2 gaseous environment as shown in Figure 10(d). A huge decrease in the transmission of hetero-nanostructured samples exposed to CO2 gas (green line) was noticed as compared to air alone (blue line). These optical results revealed that neither alone silver nanoparticles nor alone TiO2 films of various thicknesses were proficient CO2 gas sensors nevertheless; hetero-nanostructures (TiO2/AgNPs) with higher TiO2 content provided the maximum CO2-sensing properties.


### 3.3.4. Dynamic Gas Sensing

Figure11 illustrates the dynamic CO2 gas-sensing response of the hetero-nanostructured sample TiO2/AgNPs with maximum TiO2 film thickness (99 nm). The transmission signal measurements were carried out at room temperature for a fixed wavelength (λ = 600 nm) by flowing air and CO2 gas alternatively. The response time (air-to-CO2 gas transient) is defined as the time required to achieve 10% to 90% of maximum transmission signal, while the recovery time (CO2 gas-to-air transient) is defined as the time required to achieve 90% to 10% of maximum transmission. The response time was found to be about 10 sec, while recovery time was about 110 sec. The dynamic gas-sensing results (Figure 11) revealed the stability and reversibility of our prepared CO2 gas sensors.Figure 11
Dynamic CO2 gas-sensing response of TiO2 films decorated with AgNPs with maximum thickness (99 nm).


### 3.3.5. Transmission Change Ratio (TCR)

The sensitivity of various samples toward CO2 gas can more clearly be observed in Figure 12, which reports the calculated transmission change ratio (TCR) values. TCR is defined as the difference between the transmissions measured during CO2 gas exposure (Tgas) and in air (Tair), normalized to the transmissions level in air, and is given by the following formula [17]:(2)TCR=Tgas−TairTair.Figure 12
TCR of TiO2 thin films without (a) and with AgNPs (b).
(a)
(b)The TCR plots of substrates without AgNPs and with AgNPs are presented in Figures12(a) and 12(b), respectively. The almost same behavior of TCR around zero line for all the samples without AgNPs (Figure 12(a)) indicates that increased TiO2 film thickness alone was not enough to produce any sharp change in transmission as exposed to CO2 gas. However, in the case of hetero-nanostructured morphology, a combination of TiO2 thin films of higher thickness with AgNPs, a substantial variation in TCR trend was noted. A perturbation in TCR (blue line) of 51 nm thickness (TiO2/AgNPs) sample starting from 400 nm showing peak value around 550 nm and ending at higher wavelength values can clearly be seen. Similarly, a negative trend in TCR for 99 nm thickness (TiO2/AgNPs) substrates indicates a large decrease in the transmission when exposed to CO2 gas atmosphere. Again a perturbation starting from 460 nm and ending at higher wavelength values can be observed. From these TCR results, it can be inferred that enhanced TiO2 film thickness decorated with AgNPs confirmed effectiveness of hetero-nanostructures for good CO2 gas sensors.


### 3.3.6. Optical Sensing Mechanism for CO2 Gas

The interaction of gases with solid surfaces depends on various factors such as the type of the gas (reducing or oxidizing in nature), chemical nature of solid surface (n-type or p-type), surface morphology, and the surrounding atmosphere including temperature and humidity [29–31]. Furthermore, type of sensing layers’ configuration, whether a single component-based nanostructure or a multimaterial hetero-nanostructure, also plays a vital role to control surface reactions and thus gas-sensing mechanism [15].TiO2 is an n-type semiconducting material having d0 electronic configuration and belongs to the binary transition metal oxide category which is considered potential candidate for real gas-sensing applications. The gas-sensing mechanism of TiO2-based sensors can be described by considering the adsorption of oxygen on the surface of sensors which could be due to physisorption and chemisorption processes. These processes are found to be surface temperature and activation energy dependent. When TiO2-based sensors are exposed to air at room temperature, oxygen molecules may adsorb on the surface of TiO2-based materials by physisorption process due to Van der Waals and dipole interactions. These adsorbed oxygen molecules, then, generate chemisorbed oxygen species (O2−) on the sensor surfaces. The reaction process can be described as follows [15, 30]:(3)O2g⟶O2adsO2ads+e⟶O2−Carbon dioxide (CO2) is an oxidizing gas having a linear bonded atoms’ stable structure with no lone pair of electrons. When TiO2-based surfaces are exposed to CO2 gaseous atmosphere at room temperature, the moisture present on the surface of sensor come into action. The water molecules split into hydroxyl and hydrogen ions which subsequently react with gaseous CO2 to form carbonate ions as follows [31]:(4)CO2gas+OHadsorbed−⇆HCO3surface−CO2gas+Hadsorbed++2esurface−⟶HCO2surface−HCO2surface−⟶CO↑+OHsurface−HCO3surface−+OHabsorbed−⇆CO3bulk2−+H2OgasTiO2-based gas sensor properties are also influenced by the TiO2 crystal phases, and rutile and anatase phases are found to be more promising for the sensing applications.In our case, alone pristine TiO2 thin films even with increased film thickness (37–99 nm) exposed to CO2 gaseous flow could not exhibit any notable sensing performance (Figures 10(a)–10(c)). One of the probable reasons of this little or no sensing response of as-deposited TiO2 films could be the lack of TiO2 crystalline phases (Figure 5(a)) or higher sensitivity of TiO2 polymorph toward reducing gases (such as CO and H2) than oxidizing gases such as CO2 [17, 29]. Besides, negligible CO2 gas-sensing properties of alone AgNP substrates (Figure 10(a)) suggested that neither alone as-deposited TiO2 thin films nor alone AgNPs were substantial materials for CO2 gas-sensing mechanism. Instead, hetero-structure fabricated by decorating AgNPs on the predeposited TiO2 thin films demonstrated excellent CO2 gas sensing (Figure 12(b)). Manera et al. [12] in the case of alcohol vapor sensors using TiO2/gold nanocomposite and Wang et al. [30] during their CO gas-sensing studies have already reported the significance of hetero-nanostructures in improving the gas-sensing properties.This enhanced gas-sensing abilities of our TiO2/AgNPs hetero-structured sensors can be explained in the way that introduction of silver nanoentities on the TiO2 films generated high oxygen dissociation active sites which increased the probability of CO2 adsorption and reaction on the sensor surfaces [15, 32]. This perhaps provoked the prominent change in optical transmission when sensors exposed to in CO2 gaseous environment. Furthermore, enhanced adsorption of CO2 near the AgNPs could lead the changes in the refractive index of TiO2 [33] which caused a change in optical response. Another reason of improved sensing effect due to AgNPs on the TiO2 films could be the surface plasmon resonance (SPR) effect which is influenced by the variation in thickness and refractive index of the active sensing layer. Moreover, the size and density of AgNP islands on the TiO2 films could also affect the SPR gas sensitivity [33, 34]. The increased CO2 sensing properties of TiO2 films decorated with AgNPs with higher film thickness could be due to the enhanced SPR effect owing to increasing the thickness of TiO2 film along with growing size and density of AgNP islands.Thus findings of this work proved that TiO2/AgNPs hetero-nanostructures were more efficient CO2 gas optical sensors rather than alone AgNPs or as-deposited TiO2 films. The excellence performance of prepared hetero-nanostructured CO2 gas sensors makes them promising candidates for practical gas-sensing applications.


## 3.3.1. Substrates without AgNPs

In Figure8, transmission spectra of all four samples, bare glass and TiO2 thin films, having different film thickness (37.54 nm, 51.92 nm, and 99.55 nm), in air and in the presence of CO2, are presented. It can be seen that, in case of air (Figure 8(a)), bare glass exhibited the typical spectrum with maximum transmission. Apparently, the transmission value of the glass substrate (∼60%) seems lower than the reported value (<90%) [28]. All optical measurements were carried out inside a transparent box to generate a gaseous environment around the sample (Figure 2). The lower values of the bare glass substrate might be due to reflections off the windows of the gas cell apparatus. For the TiO2 thin film samples, transmission was found to decrease by increasing the film deposition. This can be understood by considering the improvement in the film thickness and density which absorbed more light causing a decrease in transmission. In the case of CO2 gaseous atmosphere, transmission spectra (Figure 8(b)) of all four samples without AgNPs appeared similar to those as were in the case of alone air. Thus, no clear difference could be noticed in the presence of CO2 gas.Figure 8
Transmission spectra of TiO2 thin films with thickness of 37 nm, 51 nm, and 99 nm in air (a) and CO2 gas flow (b).
(a)
(b)


## 3.3.2. Substrates with AgNPs

The optical transmission spectra of all samples decorated with AgNPs in air and in the presence of CO2 gas are demonstrated in Figure 9. A slight change in the shape of all spectra can be noticed. As evident from SEM images (Figure 7), deposition of AgNPs formed different-sized Ag aggregates along with some scattered distinctive AgNPs. These Ag aggregates could act as scattering centers causing a decrease in the transmission signal which are discussed in the next section.Figure 9
Transmission spectra of TiO2 thin films decorated with AgNPs of varied film thickness in air (a) and CO2 gas flow (b).
(a)
(b)


## 3.3.3. Comparison of Gas-Sensing Performance

A detailed comparison of optical characteristics in the presence and absence of CO2 gaseous atmosphere for all samples is reported in Figure 10.Figure 10
Comparison of optical responses of all samples in air and CO2 gas flow.
(a)
(b)
(c)
(d)Figure10(a) represents the transmission response of bare glass and alone AgNP-deposited glass substrate in air and in CO2 gas flow. A huge decrease in the transmission spectra (in air) of the glass substrate decorated with AgNPs as compared to bare glass indicates the improved surface coverage of the glass substrate by the deposition of AgNPs. However, no significant change in the optical spectra due to the CO2 gas atmosphere can be noticed except a slight decrease in case of AgNP-loaded samples. This revealed substrates covered with AgNPs alone were not sufficient enough for CO2 gas detection.In the case of TiO2 thin film (37 nm thickness) samples with and without AgNP deposition (Figure 10(b)), the overall decrease in the optical response was found to be larger as compared to samples without TiO2 films (Figure 10(a)). This depicts that morphology and thickness of the material on glass substrates had changed due to TiO2 thin films. However, absence of any prominent change in the transmission in the CO2 gaseous environment proved that TiO2 thin films with insufficient thickness despite the presence of AgNPs were also incapable of noticeable CO2 gas sensing.For the TiO2 samples with 51 nm film thickness (Figure 10(c)), a significant decrease in the transmission spectra, starting from 600 nm to higher wavelength values, was noticed when TiO2 thin films decorated with AgNPs were exposed to CO2 gas flow. This indicates that increased TiO2 content in hetero-nanostructures (TiO2/AgNPs) had improved the CO2 gas-sensing properties of the samples. This argument was further strengthened when TiO2 thin films of higher thickness (99 nm) loaded with AgNPs were analyzed in the CO2 gaseous environment as shown in Figure 10(d). A huge decrease in the transmission of hetero-nanostructured samples exposed to CO2 gas (green line) was noticed as compared to air alone (blue line). These optical results revealed that neither alone silver nanoparticles nor alone TiO2 films of various thicknesses were proficient CO2 gas sensors nevertheless; hetero-nanostructures (TiO2/AgNPs) with higher TiO2 content provided the maximum CO2-sensing properties.


## 3.3.4. Dynamic Gas Sensing

Figure11 illustrates the dynamic CO2 gas-sensing response of the hetero-nanostructured sample TiO2/AgNPs with maximum TiO2 film thickness (99 nm). The transmission signal measurements were carried out at room temperature for a fixed wavelength (λ = 600 nm) by flowing air and CO2 gas alternatively. The response time (air-to-CO2 gas transient) is defined as the time required to achieve 10% to 90% of maximum transmission signal, while the recovery time (CO2 gas-to-air transient) is defined as the time required to achieve 90% to 10% of maximum transmission. The response time was found to be about 10 sec, while recovery time was about 110 sec. The dynamic gas-sensing results (Figure 11) revealed the stability and reversibility of our prepared CO2 gas sensors.Figure 11
Dynamic CO2 gas-sensing response of TiO2 films decorated with AgNPs with maximum thickness (99 nm).


## 3.3.5. Transmission Change Ratio (TCR)

The sensitivity of various samples toward CO2 gas can more clearly be observed in Figure 12, which reports the calculated transmission change ratio (TCR) values. TCR is defined as the difference between the transmissions measured during CO2 gas exposure (Tgas) and in air (Tair), normalized to the transmissions level in air, and is given by the following formula [17]:(2)TCR=Tgas−TairTair.Figure 12
TCR of TiO2 thin films without (a) and with AgNPs (b).
(a)
(b)The TCR plots of substrates without AgNPs and with AgNPs are presented in Figures12(a) and 12(b), respectively. The almost same behavior of TCR around zero line for all the samples without AgNPs (Figure 12(a)) indicates that increased TiO2 film thickness alone was not enough to produce any sharp change in transmission as exposed to CO2 gas. However, in the case of hetero-nanostructured morphology, a combination of TiO2 thin films of higher thickness with AgNPs, a substantial variation in TCR trend was noted. A perturbation in TCR (blue line) of 51 nm thickness (TiO2/AgNPs) sample starting from 400 nm showing peak value around 550 nm and ending at higher wavelength values can clearly be seen. Similarly, a negative trend in TCR for 99 nm thickness (TiO2/AgNPs) substrates indicates a large decrease in the transmission when exposed to CO2 gas atmosphere. Again a perturbation starting from 460 nm and ending at higher wavelength values can be observed. From these TCR results, it can be inferred that enhanced TiO2 film thickness decorated with AgNPs confirmed effectiveness of hetero-nanostructures for good CO2 gas sensors.


## 3.3.6. Optical Sensing Mechanism for CO2 Gas

The interaction of gases with solid surfaces depends on various factors such as the type of the gas (reducing or oxidizing in nature), chemical nature of solid surface (n-type or p-type), surface morphology, and the surrounding atmosphere including temperature and humidity [29–31]. Furthermore, type of sensing layers’ configuration, whether a single component-based nanostructure or a multimaterial hetero-nanostructure, also plays a vital role to control surface reactions and thus gas-sensing mechanism [15].TiO2 is an n-type semiconducting material having d0 electronic configuration and belongs to the binary transition metal oxide category which is considered potential candidate for real gas-sensing applications. The gas-sensing mechanism of TiO2-based sensors can be described by considering the adsorption of oxygen on the surface of sensors which could be due to physisorption and chemisorption processes. These processes are found to be surface temperature and activation energy dependent. When TiO2-based sensors are exposed to air at room temperature, oxygen molecules may adsorb on the surface of TiO2-based materials by physisorption process due to Van der Waals and dipole interactions. These adsorbed oxygen molecules, then, generate chemisorbed oxygen species (O2−) on the sensor surfaces. The reaction process can be described as follows [15, 30]:(3)O2g⟶O2adsO2ads+e⟶O2−Carbon dioxide (CO2) is an oxidizing gas having a linear bonded atoms’ stable structure with no lone pair of electrons. When TiO2-based surfaces are exposed to CO2 gaseous atmosphere at room temperature, the moisture present on the surface of sensor come into action. The water molecules split into hydroxyl and hydrogen ions which subsequently react with gaseous CO2 to form carbonate ions as follows [31]:(4)CO2gas+OHadsorbed−⇆HCO3surface−CO2gas+Hadsorbed++2esurface−⟶HCO2surface−HCO2surface−⟶CO↑+OHsurface−HCO3surface−+OHabsorbed−⇆CO3bulk2−+H2OgasTiO2-based gas sensor properties are also influenced by the TiO2 crystal phases, and rutile and anatase phases are found to be more promising for the sensing applications.In our case, alone pristine TiO2 thin films even with increased film thickness (37–99 nm) exposed to CO2 gaseous flow could not exhibit any notable sensing performance (Figures 10(a)–10(c)). One of the probable reasons of this little or no sensing response of as-deposited TiO2 films could be the lack of TiO2 crystalline phases (Figure 5(a)) or higher sensitivity of TiO2 polymorph toward reducing gases (such as CO and H2) than oxidizing gases such as CO2 [17, 29]. Besides, negligible CO2 gas-sensing properties of alone AgNP substrates (Figure 10(a)) suggested that neither alone as-deposited TiO2 thin films nor alone AgNPs were substantial materials for CO2 gas-sensing mechanism. Instead, hetero-structure fabricated by decorating AgNPs on the predeposited TiO2 thin films demonstrated excellent CO2 gas sensing (Figure 12(b)). Manera et al. [12] in the case of alcohol vapor sensors using TiO2/gold nanocomposite and Wang et al. [30] during their CO gas-sensing studies have already reported the significance of hetero-nanostructures in improving the gas-sensing properties.This enhanced gas-sensing abilities of our TiO2/AgNPs hetero-structured sensors can be explained in the way that introduction of silver nanoentities on the TiO2 films generated high oxygen dissociation active sites which increased the probability of CO2 adsorption and reaction on the sensor surfaces [15, 32]. This perhaps provoked the prominent change in optical transmission when sensors exposed to in CO2 gaseous environment. Furthermore, enhanced adsorption of CO2 near the AgNPs could lead the changes in the refractive index of TiO2 [33] which caused a change in optical response. Another reason of improved sensing effect due to AgNPs on the TiO2 films could be the surface plasmon resonance (SPR) effect which is influenced by the variation in thickness and refractive index of the active sensing layer. Moreover, the size and density of AgNP islands on the TiO2 films could also affect the SPR gas sensitivity [33, 34]. The increased CO2 sensing properties of TiO2 films decorated with AgNPs with higher film thickness could be due to the enhanced SPR effect owing to increasing the thickness of TiO2 film along with growing size and density of AgNP islands.Thus findings of this work proved that TiO2/AgNPs hetero-nanostructures were more efficient CO2 gas optical sensors rather than alone AgNPs or as-deposited TiO2 films. The excellence performance of prepared hetero-nanostructured CO2 gas sensors makes them promising candidates for practical gas-sensing applications.


## 4. Conclusion

In summary, CO2 gas-sensing characteristics of different substrates including alone AgNPs, TiO2 thin films with different thickness prepared by different sputtering times and TiO2/AgNPs hetero-nanostructures were studied. The surface morphology of all the samples was determined by SEM and TiO2 film thickness and roughness was found enhancing by increasing film sputtering times. The deposition of AgNPs introduced the Ag islands on the TiO2 films and thus further increased the surface roughness. XRD investigations confirmed the amorphous nature of TiO2 thin films and FCC crystalline structure of AgNPs. By ellipsometry measurement using Tauc–Lorentz model, film thickness was found to be 37 nm, 51 nm, and 99 nm for sputtering times of 30 min, 60 min, and 90 min, respectively. Optical CO2 gas detection studies, performed by ellipsometry in alone air and CO2 gas flow, showed that only TiO2/AgNPs hetero-nanostructures exhibited substantial sensing performance as compared to single component-based structures. These results revealed the vital role of hetero-nanostructures in improving the gas-sensing properties by virtue of synergistic effects.



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*Source: 2780203-2018-07-19.xml*