Optical CO<sub>2</sub> Gas Sensing Based on TiO<sub>2</sub> Thin Films of Diverse Thickness Decorated with Silver Nanoparticles

Raza, Muhammad Akram; Habib, Anam; Kanwal, Zakia; Hussain, Syed Sajjad; Iqbal, Muhammad Javaid; Saleem, Murtaza; Riaz, Saira; Naseem, Shahzad

doi:https://doi.org/10.1155/2018/2780203

Advances in Materials Science and Engineering

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Abstract Introduction Materials and Methods Results and Discussion Conclusion Data Availability Disclosure Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2018 | Article ID 2780203 | https://doi.org/10.1155/2018/2780203

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

Muhammad Akram Raza,¹Anam Habib,¹Zakia Kanwal,²Syed Sajjad Hussain,¹Muhammad Javaid Iqbal,¹Murtaza Saleem,³Saira Riaz,¹and Shahzad Naseem¹

Academic Editor: Mikhael Bechelany

Received26 Mar 2018

Revised29 May 2018

Accepted14 Jun 2018

Published19 Jul 2018

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.

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, NO_x, 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.

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.

The light beam was allowed to fall on the substrate at right angle (as schematically shown in Figure 2) 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

Figure 3 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.

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 Figure 4. 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.

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:where is the wavelength of X-ray used (1.54 Å), k is the shape factor (0.9), (taken in radians) is the full width at half maximum (FWHM) of the diffraction peak, and 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.

The 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.

(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].

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.

Figure 6 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.

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)).

(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 Figure 8, 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.

(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.

(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.

(a)

(b)

(c)

(d)

Figure 10(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

Figure 11 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.

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 (T_gas) and in air (T_air), normalized to the transmissions level in air, and is given by the following formula [17]:

(a)

(b)

The TCR plots of substrates without AgNPs and with AgNPs are presented in Figures 12(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 () on the sensor surfaces. The reaction process can be described as follows [15, 30]:

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]:

TiO₂-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.

Data Availability

All the data and references cited to support our results are available and can be found on the internet.

Disclosure

Some part of this work has been presented at International Conference on Solid State Physics (ICSSP’17), December 10–14, Lahore, Pakistan.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

The authors highly acknowledge the financial support provided the Higher Education Commission (HEC) of Pakistan and Centre of Excellence in Solid State Physics, University of the Punjab, QAC, Lahore 54590, Pakistan.

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Copyright © 2018 Muhammad Akram Raza et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Advances in Materials Science and Engineering

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

Abstract

1. Introduction

2. Materials and Methods

2.1. Preparation of Samples

2.2. Characterization of Prepared Samples

2.3. Optical Gas Sensing

3. Results and Discussion

3.1. X-Ray Diffraction Analysis

3.2. Scanning Electron Microscopy Analysis

3.3. CO2 Gas-Sensing Analysis

3.3.1. Substrates without AgNPs

3.3.2. Substrates with AgNPs

3.3.3. Comparison of Gas-Sensing Performance

3.3.4. Dynamic Gas Sensing

3.3.5. Transmission Change Ratio (TCR)

3.3.6. Optical Sensing Mechanism for CO2 Gas

4. Conclusion

Data Availability

Disclosure

Conflicts of Interest

Acknowledgments

References

Copyright

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

3.3. CO₂ Gas-Sensing Analysis

3.3.6. Optical Sensing Mechanism for CO₂ Gas