Effect of Pd-Sensitization on Poisonous Chlorine Gas Detection Ability of TiO2: Green Synthesis and Low-Temperature Operation

Ganoderma lucidum mushroom-mediated green synthesis of nanocrystalline titanium dioxide (TiO2) is explored via a low-temperature (≤70 °C) wet chemical method. The role of Ganoderma lucidum mushroom extract in the reaction is to release the ganoderic acid molecules that tend to bind to the Ti4+ metal ions to form a titanium-ganoderic acid intermediate complex for obtaining TiO2 nanocrystallites (NCs), which is quite novel, considering the recent advances in fabricated gas sensing materials. The X-ray powder diffraction, field emission scanning electron microscopy, Raman spectroscopy, and Brunauer–Emmett–Teller measurements etc., are used to characterize the crystal structure, surface morphology, and surface area of as-synthesized TiO2 and Pd-TiO2 sensors, respectively. The chlorine (Cl2) gas sensing properties are investigated from a lower range of 5 ppm to a higher range of 400 ppm. In addition to excellent response–recovery time, good selectivity, constant repeatability, as well as chemical stability, the gas sensor efficiency of the as-synthesized Pd-TiO2 NC sensor is better (136% response at 150 °C operating temperature) than the TiO2 NC sensor (57% at 250 °C operating temperature) measured at 100 ppm (Cl2) gas concentration, suggesting that the green synthesized Pd-TiO2 sensor demonstrates efficient Cl2 gas sensing properties at low operating temperatures over pristine ones.


Introduction
Recently, controlling air pollutants and monitoring toxic gases have become increasingly important [1]. Concerns about the environment's health need stringent regulations on the harmful gas emissions from industrial and social sectors. Chlorine (Cl 2 ), one such toxic gas with a pungent odor, has been a great concern in the past few decades because it is extremely harmful to humans and the environment [2]. Cl 2 molecules can combine with water in the lung mucosa to form hydrofluoric acid when contacted with Cl 2 gas by breathing, causing serious respiratory difficulties. Exposure to high concentrations of Cl 2 is fatal to the human body, irritates the eyes, causes skin infection, psychological disorders, and so on [3]. Cl 2 is widely used in disinfectants, bleach, and raw materials to produce hydrochloric acid, and as part of various industrial processes, including tap water treatment, room extract in the reaction is to release the ganoderic acid molecules, which tend to bind the Ti 4+ metal ions to form a titanium-ganoderic acid intermediate complex for obtaining TiO 2 NCs. The Pd-sensitization is carried out on the TiO 2 sensor surface by impregnating the Pd-TiO 2 NC sensor. Both TiO 2 and Pd-TiO 2 NC sensors are characterized for their structure and morphology using standard material characterization tools. The gas sensing behaviors of TiO 2 and Pd-TiO 2 NC sensors are investigated for various gases such as ammonia (NH 3 ), hydrogen sulfide (H 2 S), carbon dioxide (CO 2 ), nitrogen dioxide (NO 2 ), and chlorine (Cl 2 ), out of which, Cl 2 has revealed an optimum performance, and therefore, the Cl 2 sensing properties are performed as a function of operating temperature and concentration with error limit. The transient responses for both sensors are also studied to find the respective response and recovery periods. The repeatability and stability tests of both sensor materials are recorded and reported.

An Extract Preparation
The medicinal Ganoderma lucidum mushroom collected from the forest of Tamhini of Pune District, Maharashtra, India, was washed with distilled water (Milli-Q water; 18.2 MΩ cm) a few times to remove dust and any other parasites that could be present. It was then dried under shade at room-temperature (27 • C) for about fifteen days under dust-free conditions. The mushroom was then divided into fine pieces, grounded, and sieved to get a fine powder. For 15 min, 1.0 gm of fine mushroom powder was boiled in 100 mL of distilled water at 85 • C. The resulting solution was filtered through Whatman paper no. 1 of 42 pore size and stored as a stock solution at 4 • C before use.

Synthesis of TiO 2 NC Powder
To prepare Ti-precursor, the 0.5 M titanium (IV) isopropoxide was added into 50 mL of ethyl alcohol and distilled water (1:1 proportion). The mushroom extract solution (5 mL) was added into the Ti-precursor solution, and the resulting mixture was stirred for 5 h and maintained at 70 • C by using a hot plate magnetic stirrer. The solution was allowed to cool down to room temperature (27 • C) after vigorous stirring, and the supernatants were removed, leaving a brown-yellow precipitate as a residue. The precipitate was washed several times with ethyl alcohol and distilled water. The color of the precipitate was brownyellow due to the formation of the titanium-ganoderic acid intermediate compound. The synthesized precipitate was dried at 50 • C overnight. Finally, this fried precipitate was air-annealed at 450 • C for 2 h to improve the crystallinity of white TiO 2 powder before being kept in an airtight container for later usage (Scheme 1). The solid-state sensor film was prepared by dissolving 1.0 g of polyvinyl alcohol (PVA) as a binder in 10 mL of distilled water followed heating the solution to 90 • C while constantly stirring until a transparent viscous gel. The TiO 2 NCs powder was mixed with a few drops of viscous PVA solution in a pestle and mortar. The excess water and binder were vaporized by coating the mixture on the conventional soda-lime glass as a substrate and drying it at 200 • C for 1h. Half of the obtained TiO 2 film sample was considered pristine TiO 2, whereas the remaining half was used for Pd-sensitization as Pd-sensitized TiO 2 .

Growth Mechanism
TiO 2 NCs were synthesized through the hydrolysis of titanium (IV) isopropoxide in the presence of ganoderic acid at 70 • C as follows. The Ti(OC 3 H 7 ) 4 reacts with water. The amount of water determines the degree of hydrolysis (i.e., solubility and ionic products), which is high at the degree of saturation where both are nearly the same, and the type of initial species formed, such as ganoderic acid molecules for binding metal ions, thus influencing the condensation reaction that involves the polymerization of hydrolyzed titanium alkoxides in alcoholic solution to form Ti +4 ions in the solution. Thus, the Ti +4 ions bound to ganoderic acid legends to form the titanium-ganoderic acid intermediate complex.
The titanium-ganoderic acid complex molecules most likely produce a steric hindrance at specific places via hydrogen bonding, helping to control both the particle size and the porous nature of the aggregates. Subsequently, the as-developed titanium-ganoderic acid complex powder was annealed at 450 • C to obtain a very fine-sized TiO 2 NCs powder.

Growth Mechanism
TiO2 NCs were synthesized through the hydrolysis of titanium (IV) isopropoxide in the presence of ganoderic acid at 70 °C as follows. The Ti(OC3H7)4 reacts with water. The amount of water determines the degree of hydrolysis (i.e., solubility and ionic products), which is high at the degree of saturation where both are nearly the same, and the type of initial species formed, such as ganoderic acid molecules for binding metal ions, thus influencing the condensation reaction that involves the polymerization of hydrolyzed titanium alkoxides in alcoholic solution to form Ti +4 ions in the solution. Thus, the Ti +4 ions bound to ganoderic acid legends to form the titanium-ganoderic acid intermediate complex. The titanium-ganoderic acid complex molecules most likely produce a steric hindrance at specific places via hydrogen bonding, helping to control both the particle size and the porous nature of the aggregates. Subsequently, the as-developed titaniumganoderic acid complex powder was annealed at 450 °C to obtain a very fine-sized TiO2 NCs powder.

Pd-Sensitized TiO2 NCs
In the present work, Pd-sensitized TiO2 film was fabricated as follows. The 0.01M PdCl2 in an aqueous solution was used as metal salts for the sensitization of TiO2 film. The TiO2 film prepared using the doctor blade method was dipped into a 0.01M aqueous solution of the respective salt for 5 s, and then dried under an IR lamp. Ten such cycles of dipping were carried out for each film. The sensitized TiO2 film was then heated at 200 °C for 1 h to remove the chloride from the deposited sensor film.

Characterization Techniques
The field emission scanning electron microscope (FE-SEM, Nova-SEM 200-FEI) was used to confirm the surface morphology of TiO2, and Pd-TiO2 sensor film surfaces A FEI TECNAI G2 20 STWIN equipment was used to capture the High-Resolution Transmission Electron Microscopy (HR-TEM) image and electron mapping configuration. The crystal Scheme 1. Schematic presenting a process for obtaining bio-inspired TiO 2 NCs.

Pd-Sensitized TiO 2 NCs
In the present work, Pd-sensitized TiO 2 film was fabricated as follows. The 0.01M PdCl 2 in an aqueous solution was used as metal salts for the sensitization of TiO 2 film. The TiO 2 film prepared using the doctor blade method was dipped into a 0.01M aqueous solution of the respective salt for 5 s, and then dried under an IR lamp. Ten such cycles of dipping were carried out for each film. The sensitized TiO 2 film was then heated at 200 • C for 1 h to remove the chloride from the deposited sensor film.

Characterization Techniques
The field emission scanning electron microscope (FE-SEM, Nova-SEM 200-FEI) was used to confirm the surface morphology of TiO 2, and Pd-TiO 2 sensor film surfaces A FEI TECNAI G2 20 STWIN equipment was used to capture the High-Resolution Transmission Electron Microscopy (HR-TEM) image and electron mapping configuration. The crystal structure was elucidated using an X-ray diffraction (XRD) spectrum (XRD-6000, Shimadzu X-ray diffractometer) and a Cu-K radiation tube. The elemental contents were identified by using an X-ray photoelectron spectroscopy profile obtained on a PHI 5000 Versa Probe (XPS, Ulvac-PHI). Additionally, the Belsorp II, BET, and Japan Inc. equipment measured the surface area and pore-size distribution.

Gas Sensor Setup
The gas sensing operation was performed in a stainless-steel chamber with 250 mL volume capacity. To adjust the constant temperature, a PID-controlled heater was installed at the base of the cylindrical chamber. To mitigate against changes in operating temperature, a dimmer stat was employed as a stable voltage supply. After adding the target gases to the stainless-steel chamber, a Keithley 6514 programmable computer-connected electrometer was utilized to identify the sensor film resistance. Thin films with an area of 1.5 × 1.5 cm 2 were made using a doctor-blade technique on a non-conducting glass substrate in presence of PVA as the binder material which was air-annealed for sensor activity measurement (200 • C, 1 h). The commercial silver paste was used to draw the electrical contacts. The following equation was used to calculate the gas sensor response: where R a represents the stabilized resistance of ambient air, R g represents the stabilized resistance of the target gas.

Surface Morphology Appearance
The FE-SEM micrographs of the as-obtained TiO 2 and Pd-TiO 2 NC film sensors at two different magnifications are shown in Figure 1a,d. The FE-SEM image of the TiO 2 film sensor demonstrates the formation of small-sized spherical nanocrystallites with an agglomerated-type architecture. The Pd nanoparticles are uniformly dispersed over the TiO 2 film sensor surface in the form of soft agglomerations. Pd nanoparticles show a spillover effect on the surface of the TiO 2 film, which is discussed in the below section. The crystal structure and phase analysis of TiO 2 and Pd-TiO 2 NC sensors characterized by using HR-TEM and HR-TEM techniques are shown in Figure 1b,e. Figure 1b presents the HR-TEM image of TiO 2 , where the particle size is about 8.0 nm, which was an exact estimation of the XRD (101) peak (i.e., 7.2 nm) using Scherrer relation. Figure 1e

Gas Sensor Setup
The gas sensing operation was performed in a stainless-steel chamber with 250 mL volume capacity. To adjust the constant temperature, a PID-controlled heater was installed at the base of the cylindrical chamber. To mitigate against changes in operating temperature, a dimmer stat was employed as a stable voltage supply. After adding the target gases to the stainless-steel chamber, a Keithley 6514 programmable computer-connected electrometer was utilized to identify the sensor film resistance. Thin films with an area of 1.5×1.5 cm 2 were made using a doctor-blade technique on a non-conducting glass substrate in presence of PVA as the binder material which was air-annealed for sensor activity measurement (200 °C, 1 h). The commercial silver paste was used to draw the electrical contacts. The following equation was used to calculate the gas sensor response: where Ra represents the stabilized resistance of ambient air, Rg represents the stabilized resistance of the target gas.

Surface Morphology Appearance
The FE-SEM micrographs of the as-obtained TiO2 and Pd-TiO2 NC film sensors at two different magnifications are shown in Figure 1a,d. The FE-SEM image of the TiO2 film sensor demonstrates the formation of small-sized spherical nanocrystallites with an agglomerated-type architecture. The Pd nanoparticles are uniformly dispersed over the TiO2 film sensor surface in the form of soft agglomerations. Pd nanoparticles show a spillover effect on the surface of the TiO2 film, which is discussed in the below section. The crystal structure and phase analysis of TiO2 and Pd-TiO2 NC sensors characterized by using HR-TEM and HR-TEM techniques are shown in Figure 1b,e. Figure 1b presents the HR-TEM image of TiO2, where the particle size is about 8.0 nm, which was an exact estimation of the XRD (101) peak (i.e., 7.2 nm) using Scherrer relation. Figure

Structural Elucidation
The XRD patterns of TiO 2 and Pd-TiO 2 film sensors are shown in Figure 2a. The appearance of the diffraction peaks in the XRD patterns confirms the formation of pyramidal crystal structure with anatase TiO 2 phase (JCPDS card No. 21-1272) [25]; however, due to the infinitesimal amount, there is no evolution of diffraction peaks in the XRD pattern for Pd. The sharp diffraction peaks indicate good crystallinity and involvement of no impurity peaks [26]. The metal oxide gas sensor properties can be influenced by structural parameters such as crystallite size, texture coefficient (TC), and dislocation densities; hence, there is a need to study these parameters of given sensor materials [27][28][29]. The estimated structural parameters of the as-obtained TiO 2 film sensor are tabulated in Table 1. The dislocation densities calculated for each diffraction peak are also given in Table 1. The TC values corresponding to every diffraction peak were estimated. If the TC value is greater than one, sugegsting the preferred orientation. From Table 1, the maximum TC value of 1.22 is associated with the (101) plane, which is the preferred growth direction of the present TiO 2 film sensor. The negative value of micro-strain observed in Table 1 infers the existence of compressive type strain. The dislocation densities corresponding to each diffraction peak are mentioned in Table 1.
The TC values corresponding to every diffraction peak were estimated. If the TC value is greater than one, sugegsting the preferred orientation. From Table 1, the maximum TC value of 1.22 is associated with the (101) plane, which is the preferred growth direction of the present TiO2 film sensor. The negative value of micro-strain observed in Table 1 infers the existence of compressive type strain. The dislocation densities corresponding to each diffraction peak are mentioned in Table 1.    Raman spectroscopy is a powerful technique to detect molecules in TiO 2 film sensor obtained from the Ganoderma lucidum mushroom. The significant Raman shift peaks at E g (144), E g (205), B 1g (400), B 1g /A 1g (519), and E g (635) cm −1 , as shown in Figure 2b, are attributed to the anatase crystal phase of TiO 2 [30]. In the Raman spectrum, no additional peaks, except distinctive vibrations, are found, thus confirming the formation of the pure phase of Pd-TiO 2 . The nitrogen adsorption and desorption isotherms obtained during the BET surface area measurement for pristine TiO 2 and Pd-TiO 2 films are shown in Figure 2c and 170.9 m 2 /g. It seems that Pd incorporation decreases the particle size of TiO 2 , which, in turn, enhances the specific surface area [31]. The insets of Figure 2c,d present the poresize distribution curves for the pristine TiO 2 and Pd-TiO 2 NCs sensors. The pore-size distribution is narrow, nearly nano-sized pores, resulting in an average pore volume of 0.15 cm 3 /g for pristine TiO 2 and 0.17 cm 3 /g for Pd-TiO 2 NCs. The average pore diameter of the pristine TiO 2 and Pd-TiO 2 NCs are found to be 7.13 and 4.25 nm, respectively. The higher surface area of pristine TiO 2 and Pd-TiO 2 NCs and their porous nature may provide access to active sites and a surface for adsorption of target gas molecules to enhance the gas sensing activity.

Elemental Analysis
The surface states of Pd-TiO 2 NCs were investigated by using X-ray photoelectron spectroscopy analysis. The survey XPS spectrum of the Pd-TiO 2 film sensor is shown in Figure 3a, confirming the presence of Pd, Ti, and O elements. The high-resolution XPS spectra for Ti2p, O1s, and Pd 3d are shown in Figure 3b-d.
Eg (144), Eg (205), B1g (400), B1g/A1g (519), and Eg (635) cm −1 , as shown in Figure 2b, are attributed to the anatase crystal phase of TiO2 [30]. In the Raman spectrum, no additional peaks, except distinctive vibrations, are found, thus confirming the formation of the pure phase of Pd-TiO2. The nitrogen adsorption and desorption isotherms obtained during the BET surface area measurement for pristine TiO2 and Pd-TiO2 films are shown in Figure  2c,d. The specific surface areas for TiO2 and Pd-TiO2 NC film sensors are respectively 161.5 m 2 /g and 170.9 m 2 /g. It seems that Pd incorporation decreases the particle size of TiO2, which, in turn, enhances the specific surface area [31]. The insets of Figure 2c,d present the pore-size distribution curves for the pristine TiO2 and Pd-TiO2 NCs sensors. The poresize distribution is narrow, nearly nano-sized pores, resulting in an average pore volume of 0.15 cm 3 /g for pristine TiO2 and 0.17 cm 3 /g for Pd-TiO2 NCs. The average pore diameter of the pristine TiO2 and Pd-TiO2 NCs are found to be 7.13 and 4.25 nm, respectively. The higher surface area of pristine TiO2 and Pd-TiO2 NCs and their porous nature may provide access to active sites and a surface for adsorption of target gas molecules to enhance the gas sensing activity.

Elemental Analysis
The surface states of Pd-TiO2 NCs were investigated by using X-ray photoelectron spectroscopy analysis. The survey XPS spectrum of the Pd-TiO2 film sensor is shown in Figure 3a, confirming the presence of Pd, Ti, and O elements. The high-resolution XPS spectra for Ti2p, O1s, and Pd 3d are shown in Figure 3b-d. The Ti2p3/2 peak has a binding energy of 463.9 eV, whereas the low-intensity Ti2p1/2 peak has a binding energy of 458.1 eV, thus indicating the presence of a Ti 4+ oxidation state [32]. The O1s spectrum of the pristine TiO2 film sensor noted, as seen in Figure 3c at 529.3 eV, is of TiO2 which is ascribed to the surface oxygen atoms [33]. The Pd3d spectrum, as shown in Figure 3d, adduces two peaks: one at 341.1 eV corresponding to Pd3d3/2, and the other at 335.8 eV for Pd3d5/2. The appearance of Pd 2+ peak may be due to the partial oxidation of the Pd surface since the Pd particles are very minute in the Pd-TiO2 film sensor [34]. The Ti2p 3/2 peak has a binding energy of 463.9 eV, whereas the low-intensity Ti2p 1/2 peak has a binding energy of 458.1 eV, thus indicating the presence of a Ti 4+ oxidation state [32]. The O1s spectrum of the pristine TiO 2 film sensor noted, as seen in Figure 3c at 529.3 eV, is of TiO 2 which is ascribed to the surface oxygen atoms [33]. The Pd3d spectrum, as shown in Figure 3d, adduces two peaks: one at 341.1 eV corresponding to Pd3d 3/2 , and the other at 335.8 eV for Pd3d 5/2 . The appearance of Pd 2+ peak may be due to the partial oxidation of the Pd surface since the Pd particles are very minute in the Pd-TiO 2 film sensor [34].

Gas Sensing Properties
The gas sensing properties of the TiO 2 and Pd-TiO 2 film sensors were investigated. Metal oxide-based gas sensors usually work at higher operating temperatures (i.e., ≥150 • C). The adsorption/desorption of target gas molecules varies with the operating temperature. As a result, optimizing the current sensor's operating temperature is critical. These TiO 2 and Pd-TiO 2 sensors were studied for Cl 2 (100 ppm) at various operating temperatures (Figure 4a). It was observed that the TiO 2 film sensor reveals the highest Cl 2 response of 57%, at a 250 • C operating temperature. In contrast, the Pd-TiO 2 film sensor demonstrates the highest Cl 2 response at 136%, at a relatively low operating temperature (i.e., 150 • C). The involvement of oxygen vacancies, which act as defects, is responsible for an n-type semiconducting behavior of TiO 2 ; hence, electrons are the majority charge carriers in the conduction band. The basis for sensor working is the change of its resistance on the exposure of the target gas. The resistance of the TiO 2 NCs film sensor is increased after Cl 2 gas exposure. A typical Cl 2 gas sensing mechanism has been explained for n-type metal oxide-based gas sensor by Navale et al. [24]. The presence of the Pd catalyst on the TiO 2 surface is responsible for the higher Cl 2 gas response at the relatively low optimal operating temperature [15]. Pd (being noble metal) has better dissociation catalytic ability.
The gas sensing properties of the TiO2 and Pd-TiO2 film sensors were investigated. Metal oxide-based gas sensors usually work at higher operating temperatures (i.e., ≥150 °C). The adsorption/desorption of target gas molecules varies with the operating temperature. As a result, optimizing the current sensor's operating temperature is critical. These TiO2 and Pd-TiO2 sensors were studied for Cl2 (100 ppm) at various operating temperatures (Figure 4a). It was observed that the TiO2 film sensor reveals the highest Cl2 response of 57%, at a 250 °C operating temperature. In contrast, the Pd-TiO2 film sensor demonstrates the highest Cl2 response at 136%, at a relatively low operating temperature (i.e., 150 °C). The involvement of oxygen vacancies, which act as defects, is responsible for an n-type semiconducting behavior of TiO2; hence, electrons are the majority charge carriers in the conduction band. The basis for sensor working is the change of its resistance on the exposure of the target gas. The resistance of the TiO2 NCs film sensor is increased after Cl2 gas exposure. A typical Cl2 gas sensing mechanism has been explained for n-type metal oxide-based gas sensor by Navale et al. [24]. The presence of the Pd catalyst on the TiO2 surface is responsible for the higher Cl2 gas response at the relatively low optimal operating temperature [15]. Pd (being noble metal) has better dissociation catalytic ability. The presence of Pd catalyst on the sensor surface provides an active site and surface to enhance the catalytic chemical sensing reaction. The electrical and chemical effects of the Pd catalyst help to improve the gas sensing performance [35]. As well as Cl2, these sensors are further employed to check their responses to gases such as NH3, H2S, CO2, and NO2 ( Figure 4b). Both sensors feature good selectivity towards Cl2 gas, whereas the Pd-TiO2 NCs film sensor reveals higher selectivity over the TiO2 film sensor. The transient Cl2 gas responses for TiO2 and Pd-TiO2 film sensors are provided in Figure 4c,d. As the Cl2 gas is injected into the testing system, the target gas molecules diffuse through air and get adsorbed onto the sensor surface for catalytic sensing reaction over time. This sensor shows the response in increasing order as time progresses until saturation or equilibrium level for a given target gas concentration is achieved.
At the saturation level, the sensor shows a constant response. After the gas testing system is opened to an outer atmosphere, the target gas molecules start desorbing, and the corresponding sensor response also decreases. The time taken by the sensor to reach 90% of the change in response or resistance value for a given concentration of target gas is nothing but the response time. Similarly, recovery time can also be recorded. The recorded response time values for TiO2 and Pd-TiO2 sensors are respectively 96.5 s and 15.7 The presence of Pd catalyst on the sensor surface provides an active site and surface to enhance the catalytic chemical sensing reaction. The electrical and chemical effects of the Pd catalyst help to improve the gas sensing performance [35]. As well as Cl 2 , these sensors are further employed to check their responses to gases such as NH 3 , H 2 S, CO 2 , and NO 2 (Figure 4b). Both sensors feature good selectivity towards Cl 2 gas, whereas the Pd-TiO 2 NCs film sensor reveals higher selectivity over the TiO 2 film sensor. The transient Cl 2 gas responses for TiO 2 and Pd-TiO 2 film sensors are provided in Figure 4c,d. As the Cl 2 gas is injected into the testing system, the target gas molecules diffuse through air and get adsorbed onto the sensor surface for catalytic sensing reaction over time. This sensor shows the response in increasing order as time progresses until saturation or equilibrium level for a given target gas concentration is achieved.
At the saturation level, the sensor shows a constant response. After the gas testing system is opened to an outer atmosphere, the target gas molecules start desorbing, and the corresponding sensor response also decreases. The time taken by the sensor to reach 90% of the change in response or resistance value for a given concentration of target gas is nothing but the response time. Similarly, recovery time can also be recorded. of Cl 2 gas. Moreover, this may be due to the reaction products that are not leaving the interface immediately after the reaction, thus resulting in a decrease in desorption rate [36]. Figure 5a,b depict the dynamic gas responses of the TiO 2 and Pd-TiO 2 NCs' film sensors vs. time towards 5-400 ppm concentrations of Cl 2 gas. The response values of the TiO 2 sensor toward 5, 20, 40, 100, 200, and 400 ppm concentrations of Cl 2 gas at 250 • C are found to be 17.14, 40.8, 49.6, 56.9, 62.7, 68.2, and 70.4%, respectively. It is observed that, as the concentration of Cl 2 increases, the TiO 2 film sensor response also increases; this is because higher Cl 2 gas concentration covers the higher surface of the TiO 2 film sensor, eventually increasing the interaction between Cl 2 gas molecules and sensor surface. On the other hand, lower Cl 2 gas concentration mitigates the interaction between Cl 2 gas molecules and the sensor surface; a lower response is obtained due to the lower surface coverage [37]. Moreover, from Figure 5b, the Pd-TiO 2 NC film sensor can detect as low as 5 ppm Cl 2 gas with a phenomenal response of 40.3%. Both the sensors demonstrate a lower gas response for lower Cl 2 gas concentration. With an increase in Cl 2 concentration, corresponding, responses are also increased (168.5% at 400 ppm). The response to maximum Cl 2 gas concentration is restricted by the available area of the sensor surface. As the sensor surface area is limited, the sensor gets saturated, indicating a saturated response for higher gas concentrations [38].

Effect of Cl 2 Gas Concentration on Sensor Response
ppm. The decrease in response time is attributed to the abundance of vacant sites on the film surface for gas adsorption. At the same time, the increase in recovery time is assigned to the heavier nature of Cl2 gas. Moreover, this may be due to the reaction products that are not leaving the interface immediately after the reaction, thus resulting in a decrease in desorption rate [36]. Figure 5a,b depict the dynamic gas responses of the TiO2 and Pd-TiO2NCs' film sensors vs. time towards 5-400 ppm concentrations of Cl2 gas. The response values of the TiO2 sensor toward 5, 20, 40, 100, 200, and 400 ppm concentrations of Cl2 gas at 250 °C are found to be 17.14, 40.8, 49.6, 56.9, 62.7, 68.2, and 70.4%, respectively. It is observed that, as the concentration of Cl2 increases, the TiO2 film sensor response also increases; this is because higher Cl2 gas concentration covers the higher surface of the TiO2 film sensor, eventually increasing the interaction between Cl2 gas molecules and sensor surface. On the other hand, lower Cl2 gas concentration mitigates the interaction between Cl2 gas molecules and the sensor surface; a lower response is obtained due to the lower surface coverage [37]. Moreover, from Figure 5b, the Pd-TiO2 NC film sensor can detect as low as 5 ppm Cl2 gas with a phenomenal response of 40.3%. Both the sensors demonstrate a lower gas response for lower Cl2 gas concentration. With an increase in Cl2 concentration, corresponding, responses are also increased (168.5% at 400 ppm). The response to maximum Cl2 gas concentration is restricted by the available area of the sensor surface. As the sensor surface area is limited, the sensor gets saturated, indicating a saturated response for higher gas concentrations [38].  As shown in Figure 5c,d, the exponential relationship between gas response and gas concentration-tested three times-reveals good reproducibility for both sensors. At lower concentrations, the Cl 2 gas molecules cover a smaller portion of the surface of sensitized sensor films, resulting in fewer surface interactions and, as a result, a smaller response value is noted. Furthermore, a higher concentration of Cl 2 gas covers more sensor surface area, resulting in a higher response value due to greater surface contacts [39]. The Cl 2 gas sensing performance of the Pd-TiO 2 NCs sensor was compared with previously published data (shown in Table 2).

Reproducibility and Stability Studies
The reliability and quality studies of the film sensors are represented by two important parameters; reproducibility and stability. The capability of sensor material to reproduce the same experimental results measured for another synthesis batch under similar working environmental conditions is reproducibility. To study the reproducibility of the TiO 2 film sensor, three sensor scans were operated to 100 ppm of Cl 2 gas in the same working environment (Figure 6a). The reproducibility of a sensor is one of the measures of its product quality. To investigate the reproducibility of the Pd-TiO 2 NCs; film, response experiments were repeated three times towards a 100 ppm of Cl 2 . The results in Figure 6b demonstrate that upon repeated exposure to 100 ppm Cl 2 gas, the response of the Pd-TiO 2 NCs film sensor is almost constant, which confirms the excellent repeatability of the Pdsensitized sensor material [40]. Response stability is one of the most important parameters of metal oxide-based gas sensors; therefore, the stability studies of both TiO 2 and Pd-TiO 2 NCs film sensors were studied towards the fixed 100 ppm concentration of Cl 2 gas for thirty days at five days (Figure 6c). Both the sensors retain the responses significantly for thirty days; however, the Pd-TiO 2 film sensor has confirmed the highest stability response (92.0%) to 100 ppm Cl 2 gas concentration at 150 • C, thus signifying its excellent chemical and environmental stability.

Reproducibility and Stability Studies
The reliability and quality studies of the film sensors are rep important parameters; reproducibility and stability. The capability of reproduce the same experimental results measured for another synthesi ilar working environmental conditions is reproducibility. To study the the TiO2 film sensor, three sensor scans were operated to 100 ppm of C working environment (Figure 6a). The reproducibility of a sensor is on of its product quality. To investigate the reproducibility of the Pd-T sponse experiments were repeated three times towards a 100 ppm of Figure 6b demonstrate that upon repeated exposure to 100 ppm Cl2 ga the Pd-TiO2 NCs film sensor is almost constant, which confirms the exce of the Pd-sensitized sensor material [40]. Response stability is one of th parameters of metal oxide-based gas sensors; therefore, the stability st and Pd-TiO2 NCs film sensors were studied towards the fixed 100 ppm Cl2 gas for thirty days at five days (Figure 6c). Both the sensors retain nificantly for thirty days; however, the Pd-TiO2 film sensor has confirm bility response (92.0%) to 100 ppm Cl2 gas concentration at 150 °C, thu cellent chemical and environmental stability.

Gas Sensor Working Mechanism
The most widely prevalent gas sensor working mechanism for n-type metal oxide films is the sensor materials' noteworthy resistance change, which is caused by the adsorption/desorption process of the gas molecules on the sensing materials' surface during exposure to various gas environments [41]. Based on the energy band diagram, as given in Scheme 2, a gas sensing mechanism of the Pd-TiO 2 film sensor in the presence of air atmosphere and Cl 2 gas has been proposed. The majority charge carrier in the n-type Pd-TiO 2 semiconductor is electrons. Under ambient air conditions, oxygen molecules are adsorbed on the Pd-TiO 2 film's exposed surface by capturing electrons from the TiO 2 valance band. The process creates a surface depletion layer via adsorbed oxygen species (O − ).

Gas Sensor Working Mechanism
The most widely prevalent gas sensor working mechanism for n-type metal oxide films is the sensor materials' noteworthy resistance change, which is caused by the adsorption/desorption process of the gas molecules on the sensing materials' surface during exposure to various gas environments [41]. Based on the energy band diagram, as given in Scheme 2, a gas sensing mechanism of the Pd-TiO2 film sensor in the presence of air atmosphere and Cl2 gas has been proposed. The majority charge carrier in the n-type Pd-TiO2 semiconductor is electrons. Under ambient air conditions, oxygen molecules are adsorbed on the Pd-TiO2 film's exposed surface by capturing electrons from the TiO2 valance band. The process creates a surface depletion layer via adsorbed oxygen species (O − ). Scheme 2. (a,b) Proposed sensing-based reaction mechanism for the Pd-TiO2 NC film sensor in air and Cl2 gas atmospheres.
The resistance of the sensor films changes when oxygen is absorbed on the sensor surface. Generally, the adsorbed oxygen ion species depend on the sensor film temperature, with O 2− below 100 °C, O − in between 100-300 °C, and O 2− above 300 °C [42]. O − is dominant in the present study because of the approximate Pd-TiO2 film's working temperature of 150 °C. From Scheme 2, it is seen that the potential barrier has a direct relationship with the oxygen ions on the film surface. The increase in potential barrier height eventually increases the film resistance. When the sensors are exposed to oxidizing gas, such as Cl2, Cl2 gas molecules react with oxygen ions to form ClO − species, and conduction electrons (drawn from Pd-TiO2 film) confirm Clad − . This decreases the film's carrier concentration, thus increasing the depletion width and film resistance. The proposed reaction is as follows: where subscripts adand O − denotes species adsorbed on the surface of the film sensor and species occupy lattice oxygen sites, respectively, and Vo is oxygen vacancy [27,43]. The proposed gas sensing mechanism of the Pd-TiO2 NCs' film sensor for the Cl2 gas atmosphere is shown in Scheme 2b. Noble metals such as Au, Pt, Ag, and Pd are frequently used as sensitizers in sensing materials to improve their gas sensing properties [44]. The enhanced gas sensing properties of nobel metal doped sensing materials are due to two effects; electronic sensitization and chemical sensitization [45]. To begin, in the case of the Scheme 2. (a,b) Proposed sensing-based reaction mechanism for the Pd-TiO 2 NC film sensor in air and Cl 2 gas atmospheres.
The resistance of the sensor films changes when oxygen is absorbed on the sensor surface. Generally, the adsorbed oxygen ion species depend on the sensor film temperature, with O 2− below 100 • C, O − in between 100-300 • C, and O 2− above 300 • C [42]. O − is dominant in the present study because of the approximate Pd-TiO 2 film's working temperature of 150 • C. From Scheme 2, it is seen that the potential barrier has a direct relationship with the oxygen ions on the film surface. The increase in potential barrier height eventually increases the film resistance. When the sensors are exposed to oxidizing gas, such as Cl 2 , Cl 2 gas molecules react with oxygen ions to form ClO − species, and conduction electrons (drawn from Pd-TiO 2 film) confirm Cl ad − . This decreases the film's carrier concentration, thus increasing the depletion width and film resistance. The proposed reaction is as follows: where subscripts ad − and O − denotes species adsorbed on the surface of the film sensor and species occupy lattice oxygen sites, respectively, and Vo is oxygen vacancy [27,43]. The proposed gas sensing mechanism of the Pd-TiO 2 NCs' film sensor for the Cl 2 gas atmosphere is shown in Scheme 2b. Noble metals such as Au, Pt, Ag, and Pd are frequently used as sensitizers in sensing materials to improve their gas sensing properties [44]. The enhanced gas sensing properties of nobel metal doped sensing materials are due to two effects; electronic sensitization and chemical sensitization [45]. To begin, in the case of the electron sensitization effect, free electron transfer between Pd and TiO 2 occurs due to their different work functions, resulting in the formation of a Schottky barrier and extra electron depleted layers near the contact interfaces. When the Cl 2 gas molecules come into contact with the sensing material, the extra electron depleted layer causes an increasing number of Cl 2 molecules to be adsorbed on the sensor surface, thus increasing the gas sensor response. Second, the active Pd nanoparticles accelerate the reaction by providing more active sites for Cl 2 gas molecules to interact with, which may be one of the factors responsible for improved sensor performance.

Conclusions
In summary, the nanocrystalline TiO 2 film sensor has been synthesized by a lowtemperature (≤70 • C) bio-inspired wet chemical method in Ganoderma lucidum mushroom sensitized with Pd as Pd-TiO 2 sensor by a simple dip-coating method. Both the sensors endow a good selectivity for Cl 2 gas. The slow response/recovery time and high operating temperature are also major drawbacks of Cl 2 -based gas sensors. To overcome this disadvantage, Pd doping in TiO 2 produces a synergetic effect that improves response/recovery time and lowers operating temperature. The TiO 2 sensor shows the highest Cl 2 gas response of 57% for 100 ppm at 250 • C operating temperature. The Pd-TiO 2 film sensor exhibits the highest Cl 2 gas response of 136% at the relatively lower operating temperature of 150 • C. The response/recovery time for the TiO 2 sensor is 97 s/56 s, whereas it is just 16 s/52 s for the Pd-TiO 2 film sensor. At a concentration of 5 ppm, Cl 2 detection is smaller, whereas, at a concentration of 400 ppm, Cl 2 detection is higher. The present sensors reveal a good response-recovery properties, selectivity, remarkable repeatability, and stability for Cl 2 sensing; therefore, bio-inspired wet chemical synthesis of TiO 2 in the presence of Ganoderma lucidum mushroom can be a potential candidate, over other expensive and rare-earth materials, for obtaining TiO 2 and Pd-TiO 2 film sensors which would be an easy way to increase the sensitivity of Cl 2 for producing scalable commercial gas sensors.