“Au/TiO2 nanoparticles/Si-Al capacitive sensor for selective detection of VOCs (volatile organic compounds) and method for preparation thereof”

Abstract

The present invention discloses Au/TiO2 nanoparticles/Si-Al capacitive sensor for selective detection of VOCs (volatile organic compounds) at different frequency regimes. A method for synthesizing a sensing layer of titanium dioxide (TiO2) nanoparticles, comprising the steps of: (a) preparing a precursor solution by mixing 60 ml of de-ionized water, 10 ml of ethanol (99.9% pure), and 30 ml of acetic acid (99.5% pure) in a conical flask; (b) adding 2 ml of titanium tetrachloride to the precursor solution and stirring the mixture for a duration of 4 hours; (c) adding 10 ml of 2M ammonia hydroxide dropwise to the stirred mixture; (d) allowing the resulting mixture to rest in a dark room for a period of three days; (e) dip-coating a cleaned p-Si wafer (<100>) in the prepared solution using a programmable dip coating system for three consecutive cycles, wherein each cycle comprises dipping the wafer for 5 seconds and maintaining a gap of 10 seconds between cycles; (f) thermally depositing a gold (Au) top electrode on the grown TiO2 layer; and (g) establishing a bottom electrical contact using aluminum (Al) metal. Figures 1 to 5.

Specification

Description:TECHNICAL FIELD
The present invention relates to the field of nanoparticles. More specially, the present invention discloses Au/TiO2 nanoparticles/Si-Al capacitive sensor for selective detection of VOCs (volatile organic compounds) at different frequency regimes.

BACKGROUND ART
Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
Recent sensor development based on nanostructure material such as ZnO, TiO2, SnO2, WO3 etc. proved their potentiality for the detection of trace number of gases [2-4]. Among them, TiO2 found to be stable n-type material with large number of active sites available for gas interaction [7]. Rutile crystallinity of TiO2 is the dominant phase often found at higher temperature, whereas more stable anatase crystallinity observed at temperature <500°C [8]. Rutile phase is preferred for high temperature application while anatase phase has wide application in gas sensing fields due to high sensitivity, enhanced electron mobility and faster response.
Advancement of nanotechnology made it possible to design cost effective, low power portable sensor devices for the detection of hazardous gases [1-4]. However, sensitivity and selectivity of toxic and flammable gases is still remaining a challenge to the researchers [2-10]. Short time exposure of VOCs like methanol, ethanol and ketone possibly does not have much effect on human health but long-time exposure may develop allergies, headache, nausea, respiratory related diseases [5]. Detection of methanol, ethanol, and 2-propanol in exhaled breath of patient having lung cancer can be an alternative prediction route than that of the painful invasive procedure [6]. Similarly, concentrations of acetone in exhaled breath exceeding 18 ppm could be correlated with diabetic disease [6]. Therefore, trace detection of such VOCs for monitoring the quality of air grade has immense importance as far as human health is concern.
Most of the reported titania oxide-based sensors are resistive type where two contact electrodes were deposited on the sensing layer [8]. Electrical conductivity of such type of sensor is governed by the availability of adsorption sites on the sensor layer surfaces [9]. For example, Wisitsoraat et al. reported n-type and p-type semiconducting behavior of TiO2 towards ethanol and acetone at 300°C [10]. The n-type TiO2 were converted to p-TiO2 through the amalgamation of 10 wt% NiOx into TiO2 [10]. Similar ethanol and acetone were detected at 350°C and 400°C, respectively employing TiO2 nanoparticles deposited through matrix assisted pulsed laser evaporation (MAPLE) method [11]. A very stable response even at low concentrations (1-5 ppm) of acetone was reported by Pokhrel et al. [12]. Such stability of TiO2 was achieved through the incorporation of Cr3+ into the crystal structure of TiO2. However, Cr3+ enhances the sensor response time (10 s) and recovery time (10s) but at the cost of higher operating temperature 400°C [12].
An attempt of reducing operating temperature using 3D hierarchical flower like TiO2 microstructures was carried out by Wang et al. [13]. Such sensor showed room temperature ethanol sensitivity but poor selectivity towards similar interfering gases makes such sensor less practical in reality. Similarly, detection of many gases employing single sensor was also evident in the work of Bhowmik et al. [2]. But no authors clearly address the selectivity measurement issues towards similar interfering gases. Therefore, despite of much favorable properties such as ease of fabrication, large scale production and simple design, the resistive sensor unable to compete in commercial viability due to the poor sensitivity and selectivity [4].
Further, requirement of higher operating temperature ranging from 300°C-500°C make it's less practical for use in monolithic integrated Si platform [4, 9]. In contrast, nanostructured capacitive sensor is one step ahead of resistive sensor. It offers highly sensitive and selective gas detection in stable and repeatable manner [1, 4]. In general, capacitive sensor fabricated in vertical configuration where between the two electrodes a dielectric layer is incorporated which act as sensing layer [3]. Upon gas exposure, capacitive change resulted from the effective dielectric (ɛeff) variation of sensing material and its associated void region is used as an indicative mark for the detection of trace number of gases [14-17]. Resonance under gas exposure occurs at a particular frequency where device showed maximum capacitive change than that of the air ambient. Therefore, sensitivity of the device towards specific gas or selectivity can be enhanced by finding resonant peak over the tested frequency range [14, 17].
Homayoonnia S and Zeinali S 2016 Design and fabrication of capacitive nanosensor based on MOF nanoparticles as sensing layer for VOCs detection Sens. Actuators B Chem. 237 776-86, Due to high porosity and increased surface area; metal organic frameworks (MOFs) have been widely used in gas storage applications as well as volatile organic compounds (VOCs) detection. Most of these sensors are electromechanical or electrochemical based; however there are few works in which capacitive sensors were developed using micrometer MOFs. In the present study, for the first time, MOF (Cu-BTC) nanoparticles were used to develop capacitive sensor device for detecting VOCs (e.g. methanol, ethanol, isopropanol, and acetone). The studied capacitive sensors were performed in a moderate environment (10% relative humidity and 25 °C). Capacitive sensors were fabricated in a sandwich form or parallel plate by using copper plate as a back electrode, MOF nanoparticles layer as the dielectric, and interconnected silver spots as the upper electrode of the capacitor. Linearity of the response versus LCR meter frequency, reusability, reversibility, response time, and limit of detection (LOD) of the capacitive sensor were determined to evaluate the sensor performance. The results showed high ability of Cu-BTC nanoparticles synthesized as dielectric layer of a capacitive nanosensor to determine VOCs.
Hosseini M S, Zeinali S and Sheikhi M H 2016 Fabrication of capacitive sensor based on Cu-BTC (MOF-199) nanoporous film for detection of ethanol and methanol vapors Sens. Actuators B Chem 230 9-16, Metal-organic framework (MOF), a nanoporous compound, has been used as sensing material to fabricate capacitive nanosensor. The proposed nanosensor was fabricated by growing a Cu-BTC (MOF-199) film on a copper substrate using electrochemical method. 1-Methyl-3-octylimidazolium chloride, an ionic liquid (IL), was used as conducting salt in the electrochemical cell. Scanning electron microscopy (SEM), FTIR spectroscopy, X-ray diffraction analysis, and BET techniques were used to characterize the prepared MOF which shows a thin film (about 5 μm) of Cu-BTC layer with particle size of 2-3 μm. In order to fabricate the upper electrode of capacitor some interconnected Ag paste dots were patterned on the MOF layer which was coated on the copper surface as back electrode. This fabricated sensor was used for investigation of the capacitance variations in the presence of different amounts of introduced ethanol and methanol vapors. The capacitive sensing parameters were measured by a LCR meter. Relative capacitance variations were measured to verify the potential of the Cu-BTC films for using as dielectric layer in this capacitive sensor. The linear range of the signal vs. concentration is 0-1000 ppm for ethanol and methanol. Limit of detection of the fabricated sensors were 130.0 ppm and 39.1 ppm for ethanol and methanol, respectively. The selectivity of the sensor for polar and nonpolar VOCs was examined by introducing n-hexane in sensing chamber.
OBJECTS OF THE INVENTION
The principal object of the present invention is about impedance analysis of Au/TiO2 nanoparticles/Si-Al capacitive sensor for selective detection of VOCs at different frequency regimes.
An object of the present invention is that the TiO2 nanoparticles (NP) were synthesized through solution process and characterized by Field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) analysis, Photoluminescence spectroscopy, and atomic force microscopy (AFM).
An object of the present invention is the fastest response time and recovery time were found to be 32/21s, 31.2/8s, 32.5/9s, and 40/26s for acetone, 2-propanol, ethanol, methanol, respectively.
Another object of the present invention shows the Optimum VOCs detection was achieved through finding resonant peak over the range of frequencies.

Another object of the present invention shows the mechanism of frequency tuned selectivity of different VOCs were correlated with the dielectric variation of the NPs and its associated void region under gas exposure.

Another object of the present invention shows an equivalent circuit model of TiO2 nanoparticle grains, grain boundaries, Au-TiO2 nanoparticles contact and Si-Al contact to correlate sensor response.
Another object of the present invention is to develop an Au/TiO2 nanoparticles/Si-Al capacitive sensor that selectively detects various volatile organic compounds (VOCs) across multiple frequency regimes, improving the specificity and sensitivity of VOC detection compared to existing sensors.
Another object of the present invention is to provide a novel synthesis method for the sensing layer of titanium dioxide (TiO2) nanoparticles, ensuring consistent quality and enhanced performance of the sensor through a precise and reproducible dip-coating technique.
Another object of the present invention is to create a compact and integrated sensor design that can be easily embedded in portable electronic devices, facilitating real-time monitoring of VOCs in various environments, including indoor air quality assessments and industrial applications.
Another object of the present invention is to enhance the stability and durability of the Au/TiO2 nanoparticles/Si-Al capacitive sensor, ensuring reliable operation over extended periods and under varying environmental conditions.
Another object of the present invention is to design a cost-effective sensing solution by utilizing readily available materials and scalable synthesis techniques, making it accessible for widespread commercial applications in environmental monitoring and safety.
Another object of the present invention is to achieve a faster response time in VOC detection, enhancing the sensor's applicability in dynamic environments where real-time data is critical for safety and compliance.
Another object of the present invention is to enable the sensor to operate efficiently across different frequency regimes, providing versatility in detection capabilities for various VOCs with differing dielectric properties.
Another object of the present invention to minimize the environmental footprint of the sensor production and operation, utilizing sustainable materials and processes wherever possible, thereby aligning with contemporary green technology trends.
SUMMARY
The present invention relates to Au/TiO2 nanoparticles/Si-Al capacitive sensor for selective detection of VOCs (volatile organic compounds) at different frequency regimes.
According to an embodiment of the present invention a method for synthesizing a sensing layer of titanium dioxide (TiO2) nanoparticles, comprises the steps of: (a) preparing a precursor solution by mixing 60 ml of de-ionized water, 10 ml of ethanol (99.9% pure), and 30 ml of acetic acid (99.5% pure) in a conical flask; (b) adding 2 ml of titanium tetrachloride to the precursor solution and stirring the mixture for a duration of 4 hours; (c) adding 10 ml of 2M ammonia hydroxide dropwise to the stirred mixture; (d) allowing the resulting mixture to rest in a dark room for a period of three days; (e) dip-coating a cleaned p-Si wafer (<100>) in the prepared solution using a programmable dip coating system for three consecutive cycles, wherein each cycle comprises dipping the wafer for 5 seconds and maintaining a gap of 10 seconds between cycles; (f) thermally depositing a gold (Au) top electrode on the grown TiO2 layer; and (g) establishing a bottom electrical contact using aluminum (Al) metal.
According to an embodiment of the present invention, TiO2 nanoparticles (NP) were synthesized through solution process and characterized by Field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) analysis, Photoluminescence spectroscopy, and Atomic force microscopy (AFM).
According to an embodiment of the present invention, the optimum sensor response of 136%, 63%, 152%, and 174% was found at resonant frequencies of 0.38 kHz, 0.22 kHz, 0.15 kHz, and 0.1 kHz for the exposure of 2-propanol, acetone, ethanol, and methanol, respectively.
According to an embodiment of the present invention, the fastest response time and recovery time were found to be 32/21s, 31.2/8s, 32.5/9s, and 40/26s for acetone, 2-propanol, ethanol, methanol, respectively. Selective detection of different VOCs at various resonant frequencies has correlated with the dielectric variation of the NPs and its associated void region under gas exposure
BRIEF DESCRIPTION OF DRAWINGS
The accompanying illustrations are incorporated into and form a part of this specification in order to aid in comprehending the current disclosure. The pictures demonstrate exemplary implementations of the current disclosure and, along with the description, assist to clarify its fundamental ideas.
Fig. 1 illustrates the (a) Vertical capacitive sensor with requiste dimension, (b) FESEM images of the grown film (c) Atomic force microscopy study of the grown film, (d) X-Ray diffraction analysis of the grown film.
Fig. 2 illustrates the (a) Current voltage (I-V) characteristics of device, (b) Impedance magnitude as a function of frequency of the nanoparticle based capacitive devices, (c) Nyquist plot deduced from bode plot showing distributed parameters, (d) Phase plot as a function of frequency
Fig 3 illustrates the (a) Imaginary impedance (ǀZSinθǀ) as function of frequency in air and tested VOCs (b) Impedance selectivity window of the device at resonant frequency of 0.1 kHz, 0.15 kHz, 0.22 kHz and 0.38 kHz, (c) capacitance as a function of operating temperature for the input voltage 0.5V peak to peak and frequency was set 0.1 kHz, 0.15 kHz, 0.22 kHz and 0.38 kHz for methanol, ethanol, acetone and 2-propanol, respectively, and (d) capacitive response magnitude as a function of operating temperature (25-250°C) towards acetone, 2-propanol, ethanol and methanol at their respective resonant frequency.
Fig. 4 illustrates the (a) Capacitive response magnitude as a function of VOCs concentrations at their respective optimum temperatures and resonant frequencies, (b) response time and recovery time of the device towards tested VOCs, (c) Transient response of the fabricated sensor devices towards methanol (at 150°C), ethanol (at 150°C), 2-propanol (at 175°C) and acetone (at 175°C), and (d) stability study for the span of 14 days.
Fig. 5 illustrates the (a) TiO2 nanoparticles thin film capacitive sensor (b) TiO2 nanoparticle with Au electrode and Si-Al electrode (c) Equivalent circuit model of TiO2 nanoparticle grains, grain boundaries, Au-TiO2 nanoparticles contact and Si-Al contact (d) corresponding energy band diagram of fig 5 (b).
It should be noted that the figures are not drawn to scale, and the elements of similar structure and functions are generally represented by like reference numerals for illustrative purposes throughout the figures. It should be noted that the figures do not illustrate every aspect of the described embodiment sand do not limit the scope of the present disclosure.
Other objects, advantages, and novel features of the invention will become apparent from the following detailed description of the present embodiment when taken in conjunction with the accompanying drawings.
DETAILED DESCRIPTION OF THE INVENTION
While the present invention is described herein by way of example using embodiments and illustrative drawings, those skilled in the art will recognize that the invention is not limited to the embodiments of drawing or drawings described and are not intended to represent the scale of the various components. Further, some components that may form a part of the invention may not be illustrated in certain figures, for ease of illustration, and such omissions do not limit the embodiments outlined in any way. It should be understood that the drawings and the detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the scope of the present invention as defined by the appended claim.
As used throughout this description, the word "may" is used in a permissive sense (i.e. meaning having the potential to), rather than the mandatory sense, (i.e. meaning must). Further, the words "a" or "an" mean "at least one" and the word "plurality" means "one or more" unless otherwise mentioned. Furthermore, the terminology and phraseology used herein are solely used for descriptive purposes and should not be construed as limiting in scope. Language such as "including," "comprising," "having," "containing," or "involving," and variations thereof, is intended to be broad and encompass the subject matter listed thereafter, equivalents, and additional subject matter not recited, and is not intended to exclude other additives, components, integers, or steps. Likewise, the term "comprising" is considered synonymous with the terms "including" or "containing" for applicable legal purposes. Any discussion of documents acts, materials, devices, articles, and the like are included in the specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention.
In this disclosure, whenever a composition or an element or a group of elements is preceded with the transitional phrase "comprising", it is understood that we also contemplate the same composition, element, or group of elements with transitional phrases "consisting of", "consisting", "selected from the group of consisting of, "including", or "is" preceding the recitation of the composition, element or group of elements and vice versa.
The present invention is described hereinafter by various embodiments with reference to the accompanying drawing, wherein reference numerals used in the accompanying drawing correspond to the like elements throughout the description. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiment set forth herein. Rather, the embodiment is provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art. In the following detailed description, numeric values and ranges are provided for various aspects of the implementations described. These values and ranges are to be treated as examples only and are not intended to limit the scope of the claims. In addition, several materials are identified as suitable for various facets of the implementations.
In the present invention, Au/TiO2-NP/Si-Al capacitive gas sensor device is reported for selective detection of VOCs via frequency (f) tuning and dielectric variation (ɛ). Resonance of the device under exposure of methanol, ethanol, acetone, and 2-propanol were found at 0.1 kHz, 0.15 kHz, 0.22 kHz, and 0.38 kHz, respectively. Dissimilar response magnitude upon exposure to different VOCs is illustrated with the dielectric variation of the sensing layer and correlated with equivalent Fermi energy band model
In an embodiment of the present invention the present invention discloses Au/TiO2 nanoparticles/Si-Al capacitive sensor for selective detection of VOCs (volatile organic compounds) at different frequency regimes.
In another embodiment of the present invention a method for synthesizing a sensing layer of titanium dioxide (TiO2) nanoparticles, comprises the steps of: (a) preparing a precursor solution by mixing 60 ml of de-ionized water, 10 ml of ethanol (99.9% pure), and 30 ml of acetic acid (99.5% pure) in a conical flask; (b) adding 2 ml of titanium tetrachloride to the precursor solution and stirring the mixture for a duration of 4 hours; (c) adding 10 ml of 2M ammonia hydroxide dropwise to the stirred mixture; (d) allowing the resulting mixture to rest in a dark room for a period of three days; (e) dip-coating a cleaned p-Si wafer (<100>) in the prepared solution using a programmable dip coating system for three consecutive cycles, wherein each cycle comprises dipping the wafer for 5 seconds and maintaining a gap of 10 seconds between cycles; (f) thermally depositing a gold (Au) top electrode on the grown TiO2 layer; and (g) establishing a bottom electrical contact using aluminum (Al) metal.
In another embodiment of the present invention, it is about impedance analysis of Au/TiO2 nanoparticles/Si-Al capacitive sensor for selective detection of VOCs at different frequency regimes. TiO2 nanoparticles (NP) were synthesized through solution process and characterized by Field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) analysis, Photoluminescence spectroscopy, and atomic force microscopy (AFM).
In an embodiment of the present invention the gas sensitivity of Au/TiO2-NP/Si-Al was investigated, with the effect of temperature modulation (25-250°C) and dielectric variation at the vicinity of nanoparticles. Impedance spectroscopy of TiO2-NP was carried-out to obtain resonant peaks over the frequency ranging from 0.05-225 kHz and fitted with a complex nonlinear least-squares method.
The optimum sensor response of 136%, 63%, 152%, and 174% was found at resonant frequencies of 0.38 kHz, 0.22 kHz, 0.15 kHz, and 0.1 kHz for the exposure of 2-propanol, acetone, ethanol, and methanol, respectively. The fastest response time and recovery time were found to be 32/21s, 31.2/8s, 32.5/9s, and 40/26s for acetone, 2-propanol, ethanol, methanol, respectively. Selective detection of different VOCs at various resonant frequencies has correlated with the dielectric variation of the NPs and its associated void region under gas exposure.
In another embodiment of the present invention all the chemicals were analytical grade and used without further purification. TiO2 nanoparticles were prepared by solution process. In typical process, a mixed solution of 60 ml de-ionized water (resistivity 18MΩ-cm), 10 ml ethanol (99.9% pure) and 30 ml acetic acid (99.5% pure) were transferred in a conical flask. Conical flask were mounted on ice bath and then 2 ml 0.5M Titanium tetrachloride (TiCl4) (sigma Aldrich, 99.9% pure) mixed in the solution under stirring condition (Magnetic stirrer, 1-MLH, Remi Electrotechnic Ltd., India). The stirring was continued for 4 hours at the rate of 300 rpm. Finally, 2M ammonia hydroxide (10 ml) is added drop wise. The rate of reaction was enhanced by ammonia hydroxide which makes TiO2 formation faster. Complete solution was aged for 72 hour in dark environment. Cleaned p-Si wafer (thickness 380 µm, resistivity 8 Ω-cm, <100>)) then fully dipped into the prepared solution using programmable dip coating system ( xdip-sv1, Apex Instruments, India). Dipping was continued for three consecutive cycles. In each cycle sample were dipped for 5s and gap between the cycles was maintained 10s. Au top electrode (dimension 1mm×1mm× 50nm) was thermally deposited (using thermal vacuum evaporation system under vacuum pressure of 10-5 Torr) on the grown TiO2 and the bottom contact was taken using Al metal (Shown in Fig 1 (a)). Electrical wiring from the electrodes (Au, Al) was taken using Cu wire and silver paste.
Material Characterizations
In another embodiment of the present invention Fig.1 (b) shows the FESEM image of the TiO2 thin film. FESEM image reveals that the surface is porous in nature. The porosity of deposited TiO2 thin film was calculated following equation Φ= [1-(bulk film thickness/deposited film thickness)) [20]. The calculated porosity was found to be 68-70%. The average size of the nanoparticles is found to be 12-15 nm whereas film thickness in the order of 400±5 nm. Further, nanoparticles are uniformly distributed over the surfaces without agglomeration. From the XRD data, major peaks were found at 27.22°, 52.76° and 62.77° attributed to the anatase crystallinity whereas peaks at 34.73° corresponds to rutile phase. Mainly, reactive anatase facet of <111>, <202>, <204> were found to be dominant, however a single rutile facet <212> were also evident in XRD. The obtained crystallinity of the grown TiO2 NP found to be similar with the standard JCPDS file no. 21-1272 [21-22]. The crystalline size was calculated following the Scherrer's equation D = 0.9λ/βCosθ, where λ = 0.1540 nm is the wavelength of X-ray radiation, θ is the incidence angle of radiation, and β is the full width at half maxima.
The average crystalline size found to be 11 nm. Atomic force microscopy was carried out to analyze the surface roughness of the TiO2 nanoparticles (Shown in Fig 1 (c)). The average surface roughness, peak to peak roughness and RMS roughness were found to be 33.96 nm, 396.73 nm and 45.32 nm, respectively. Photoluminescence study was carried out for the scanning wavelength ranging from 300-550 nm for the excitation wavelength of 320 nm (not shown). From PL spectra, two prominent peaks were evident at 407 nm and 455 nm, respectively. The peak at 407 nm correlated with the band-to-band emission whereas at 455 nm correspond to oxygen vacancy related peak [8]. Band gap of the sample calculated from the band-to-band emission (λ=407) following the formula Eg=1240/λ was found to be 3.04eV.
As elucidates from the XPS study (Fig 1 (f)), the peak of Ti (2p) confirms the different suboxide formation of titania viz.; TiO2, TiO, Ti2O3 etc. Such suboxide (TiO, Ti2O3) formation with TiO2 suggest the presence of oxygen vacancies (OV) in the grown material. Inset of Fig 1 (f) shows the O (1s) spectrum of TiO2 in the wavelength range of 525-540eV. Three peaks at 530.30 nm, 531.34 nm and 532.65 atrributed to the lattice oxygen (Ti-O), Ti2O3 and surface oxygen (non lattice oxygen) [2]. Surface oxygen was originated either from the adsorbed oxygen ions (atmosphere) or from the hydroxyl ion (water).
Electrical and Impedance Spectroscopy analysis
Validation of claims through Electrical and Impedance Spectroscopy analysis
Current voltage characteristics (DC measurement) were carried out in the voltage range of -2V to 2V at four (27°C, 50°C, 100°C and 200°C) different operating temperature. At all temperature (Fig 2 (a), TiO2 shows linear property confirming through the linear increase in current. Rate of increase in current were found to be slightly higher in relatively higher temperature attributed to the generation of electron-hole pairs. Impedance spectroscopy was used to extract resistive and capacitive information at the grain boundaries, electrode-grain boundaries interfaces. Impedance of the TiO2 NP was measured by LCR meter (LCR-821, GWINSTEK). The measurement (impedance and phase) were carried out at room temperature (27°C) in air ambient for the input sinusoidal signal (v=0.5sin(2πft)) with frequency (f) ranging from 0.05-225 kHz. The impedance is defined as Z(ω) = v(t)/i(t) = VοSin(ωt)/IοSin(ωt+θ), where, Z(ω) = Z' + Z'', Z' = |Z|Cosθ and Z'' = |Z|Sinθ [18]. Fig 2 (b)-(d) shows the impedance (Z) and phase (θ) plot as a function of frequency (Bode plot). From the impedance data, cole-cole curve (shown in Fig 2 (c)) has been plotted considering imaginary impedance at Y axis and real impedance at X axis. The physical interface influencing shape of the cole-cole plot are bulk resistive-capacitive effects, surface generation-recombination effects and type of contact electrodes [23-24]. As revealed from the cole-cole plot, in the tested frequency range (0.05-225 kHz), a semicircle found to depress below the real axes (Z′). The depressed semicircle suggests the existence of the distributed parameters consisting of resistive and capacitive part in the device configurations [18, 24].
These distributed elements are formulated from the constant phase element (Z=Y(jω)-α) method, where Y is the admittance, ω is angular frequency of signal, j = √(-1) and α = 2θ/π is the measure of depression (θ is angular distance from real axis to the center of semicircle in radian) [18, 23-24]. For α=0, constant phase element is purely resistive whereas for α=1 it is purely capacitive. The frequency dependent capacitive and resistive part may exist when 0<α<1 [18, 23-24]. Device resistive and capacitive part from the impedance data (Z, θ) were calculated following the reference [18]. In cole-cole plot Fig 2 (c), the black line represents the measured impedance and blue line resembles fitted curve. As evident in Fig 2 (c), fitted curve (blue line) and experimentally obtained curve (black line) are well matched. From the experimentally obtained curve (black line), single semicircle attributed to the impedance of TiO2 layer (Zn=Rn‖Cn). It is worth mentioning that, the fitted curve (blue line) crosses the real axis (higher frequency side) but not at the origin suggesting the existence of contact resistance (Rc) originated from Au/TiO2 interfaces. The contact resistance (Rc) was found to be 956.9 Ω. The impedance Zn consist of resistance Rn and Cn, where Rn and Cn is the resistance and capacitance of the TiO2 nanostructure, respectively. The calculated resistive and capacitive elements were found to be Rn =630 kΩ, Cn = 0.45 pF for α=0.85 at 0.7 kHz. Using extracted data further simulation was also carried out with the ZSimDemo 3.20d software for R-(RC) circuit configuration. The simulation study showed a similar cole-cole plot as shown by magenta line in Fig 2(c). However, TiO2 layer consist of TiO2 nanoparticles and TiO2 seed layer (between the NP and substrate layer). Therefore, impedance Zn (Rn‖Cn) further can be model considering resistance and capacitance contribution from the nanoparticles (NP) and seed layer differently.
Sensor characterizations were standardized as per following method; initially, impedance (Z) behavior as a function of frequency (0.05-225 kHz) for all the analytes was recorded. Such characterizations were employed to find out the device resonance for different VOCs i.e. to find the resonance frequency (fr) for optimal sensor response. Upon finding resonant frequency (fr), capacitive measurement was conducted at different temperature (25-250°C) and gas concentrations (1-50 ppm). Fig 3 (a) shows the imaginary impedance (|Z|Sinθ) as a function frequency. As can be seen from Fig 3 (a), the device shows resonant at 0.7 kHz, 0.38 kHz, 0.22 kHz, 0.15 kHz and 0.1 kHz in air, 50 ppm of 2-propanol, acetone, ethanol and methanol, respectively. Over the tested frequency range (0.05 kHz-225 kHz), resonant peak found to be at lower frequency regime (0.1-0.7 kHz) suggesting resistive and capacitive variation mostly due to the TiO2 surface nanoparticles [24]. It further ensures that gas penetration limited up to few layers of the nanoparticles and bulk have negligible influence [1, 5, 8].
However, these resonant frequencies were considered for rest of the sensor characterizations towards respective VOCs. Fig 3 (b) elucidates a comparative impedance analysis at resonant frequency of 0.1 kHz, 0.15 kHz, 0.22 kHz, and 0.38 kHz. Maximum impedance change towards methanol, ethanol, 2-propanol and acetone were found to be 183.17%, 147.13%, 91.18% and 72.63% at resonant frequency of 0.1 kHz, 0.15 kHz, 0.22 kHz and 0.38 kHz, respectively. Device capacitance (dielectric change) as a function of operating temperature (25-250°C) was studied under different gas exposure keeping input signal amplitude of 0.5V peak to peak and frequency was set as resonant frequency of corresponding VOCs. It is clear from the Fig 3 (c) that, temperature has negligible influence on device capacitance in air ambient. The same is found to increasing order upon exposure to acetone, 2-propanol, ethanol and methanol. The maximum capacitive change (∆C=Cg-Ca) for acetone was found at 175°C and the same is found at 175°C, 150°C, 150°C for the case of 2-propanol, ethanol and methanol, respectively. Fig 3 (d) shows the capacitive response magnitude as a function of temperature. At room temperature (25°C), response magnitude was 21%, 36%, 42% and 49% towards 50 ppm of acetone, 2-propanol, ethanol and methanol, respectively. Optimum capacitive response order were methanol (174% at 150°C)>ethanol (152% at 150°C)>2-propanol (136% at 175°C) >acetone (63% at 175°C).
Validation of claims through Sensing study
Response magnitude as a function of VOCs concentrations profile is shown in Fig 4 (a). The sensor showed detection limit down to 1 ppm with response magnitude of 32%, 56%, 67% and 87% towards acetone, 2-propanol, ethanol and methanol, respectively. Sensor did not show appreciable capacitive change beyond 50 ppm. From the transient response curve (Fig 4 (c)), sensor response time and recovery time were found to followed similar trends for all the tested VOCs. The response time and recovery time were found in the range of 31.2-45s and 8-30s, respectively at the temperature range of 25-250°C. However, The fastest response time and recovery time of 32/21s, 31.2/8s, 32.5/9s and 40/26s recorded at 200°C (Fig 4 (b)). The response time and recovery time of acetone and ethanol is found to be much better than the other two gases in the present study. Such phenomena are the reason of different gaseous interaction and its oxidation at the TiO2 NP surfaces [2, 8, 22].
The more gases interaction with fast subsequent oxidation resulted in faster response time and recovery time [2]. Gaseous interaction at the nanostructure interfaces and its core region depends on the (i) number of available sites for gaseous interaction and (ii) sticking probability of the gaseous molecules [8]. On the other hand, oxidation of the molecules depends on (i) bond dissociation energy (BDE) and (ii) molecular weight of gases [2, 8]. Number of available sites is similar for all VOCs in the present cases. Therefore, interaction depends on the sticking probability of gaseous molecules at the NP surfaces. Study reveals that, lighter molecules offers higher sticking co-efficient suggesting order of gaseous interaction found to be methanol>ethanol> acetone>2-propanol [8]. However, other influencing factor bond dissociation energy found to be higher in the molecules having maximum carbon chain length. Higher BDE leading to the easier bond breaking phenomena facilitates faster response time and recovery time. Possibly, such two processes make sensor fast responding towards ethanol and acetone, respectively.
Stability study of the sensor device was investigated for 14 days in presence of air, acetone, 2-propanol, ethanol and methanol as shown in Fig 4 (d). The resistance baseline was found to be almost constant with a drift of 0.75% in air whereas the same in 50 ppm of acetone, 2-propanol, ethanol and methanol were recorded to be 1.1%, 2.24%, 2.01% and 1.73%, respectively. The baseline drift of resistance suggests that TiO2 nanoparticle-based sensor device provides promising stability during the observation periods. Further, device stability may be improved through suitable annealing and control stoichiometry of the sample.
In present capacitive sensor, TiO2 nanoparticles based thin film is sandwiched between the two electrodes. Ratio of titania ions to oxygen ions is not exactly 2:1 suggesting dominant oxygen or titania vacancies in TiO2 [2]. In TiO2 structure, VTi'''' (represents Kröger-Vink notation for titanium vacancy) have four negative charge and VO¨ (indicates Kröger-Vink notation of oxygen vacancy) contains two positive charge [2]. Each oxygen vacancy in TiO2 releases two electrons whereas Ti vacancy offers four holes following OOO↔5O2(gas)+V¨O+2e- and O2(gas)↔2 OOO+V''''Ti+4h+, respectively where OOO is the neutral oxygen atom on lattice oxygen [30]. Formation of such oxygen vacancies (as confirmed through PL spectra) ensure releases of the free electrons to the conduction band of TiO2 leading to the n-type conductivity of the film. Therefore, oxygen vacancies are act as donor type in the oxide nanostructure [8]. On the other hand, Ti metal ions capture extra electrons even though it has complete outer shell electrons and act as acceptor [2].
Gas sensing performance of the device depends on the following phenomena; (i) number of vacant sites for active adsorption and subsequent dissociation of gaseous molecules on the surfaces, (ii) type of gaseous interaction (physisorption or chemisorption) at semiconducting oxide surfaces, and (iii) barrier height optimization (for easy carrier transport) by promoting activation energy from the operating temperature) [8]. Role of gaseous interaction or number of adsorption center depends on the quality of the synthesized materials. Non-stoichiometric property of the material offers more actives sites by creating either oxygen vacancies or titania vacancy [2]. However, oxygen vacancies facilitate active sites for analytes adsorption [2]. Oxygen vacancies (VO¨ ) in TiO2 attracts ambient oxygen which are physisorbed on the surface of nanoparticles grains [2]. Such weakly bond O2 at the nanoparticle surfaces converted to O2-, O- and O2- at elevated temperature [8]. Formation of oxygen species O2-, O- and O2- on the surfaces is completed through chemisorption process where molecules make strong bond with TiO2 via electron acceptance following the equation 1-4 [5].
Such oxygen spill over on the TiO2 surfaces pulls most of the electrons from TiO2 grains leading to the formation of depletion layer (λd=(ɛοɛrKT/q2nb)1/2) around the grain boundary (as shown in Fig 5 (a)-(b)) [19]. This phenomenon reduces the conductivity of the sensor as well as the capacitance [2, 16]. Upon exposure of VOCs (methanol, ethanol, 2-propanol and acetone) and subsequent interaction with oxygen species O2-, O- and O2- at the TiO2 nanoparticles surfaces releasing the surface trapped electrons back to the conduction band [19]. Such process reduces the depletion width (λd) resulted in increase in conductivity and capacitance. Reduction in depletion width (λd) depends on the number of interactions on the surfaces (θ), and concentrations of the VOCs [8]. Oxidation of methanol, ethanol, acetone and 2-propanol produces byproduct CO2, H2O and releases electrons as shown in eq 5-8 [5, 30-31].
O2(gas) ↔O2 (ads) (Physisorbed) (1)
O2(ads) + e- ↔O2- (ads) (< 200°C)(Chemisorbed) (2)
O2- (ads) + e- ↔2O- (ads) (< 200°C)(Chemisorbed) (3)
O- (ads) + e- ↔ O2- (ads) (> 200°C)(Chemisorbed) (4)
CH3OH(gas) + 3O-(ads) →CO2 + 2H2O + 3e- (5)
CH3CH2OH(ads) + 6O- → 2CO2 + 3H2O + 6e- (6)
CH3 CH2CH2OH(ads) + 9O- → 3CO2 + 4H2O + 9e- (7)
CH3COCH3(ads) + 8O- →3CO2 +3H2O +8e- (8)

As already furnished in the impedance spectroscopy analysis, TiO2 layer consist of TiO2 nanoparticles and TiO2 seed layer, therefore, device capacitance must be calculated considering capacitance of the nanoparticles (Cgb) and seed layer (Cb) shown in Fig 5 (c (ii)). However, capacitance formation among the nanoparticles void region becomes prominent only under VOC exposure (as depicted in Fig 5 (c (iii)) as dielectric constant of test VOCs is much greater than air. Cb does not have too much effect on the sensitivity calculation possibly due to the less penetration of VOC molecules up to seed layer. Therefore, Cb does not vary under gas exposure; however, Cb has to be taken into account for device impedance calculation. On the other hand, formation of depletion capacitance occurs as charge carrier separation happens at the nanoparticles grain boundary and its associated interfaces [19]. Considering each nanoparticle grain as spherical (shown in Fig 5 (a-b)), the capacitance can be correlated as Cgb=4πɛοɛr[λp(λp - λd)/λd] where λp = grain size and λd = depletion width, εο= dielectric constant of air, εr=dielectric of TiO2 grains [32]. Therefore, capacitance in air due to nanoparticle grains is C=(m×n)Cgb assuming m×n number of grains accommodates in sensor volume as depicted in Fig 5 (a) [32]. However, depending on the oxide layer thickness gas can be penetrated fully or partially in the device vertical direction.
In such cases, capacitance has to be calculated following the formula C=[(m-p)×n])Cgb. Here n is the number of grains in each row, m is the total number of row and p is no of grains in the vertical direction where no gas adsoption takes place. The total capacitance Cn of the device has been calculated and furnished in the impedance spectroscopy section. The prime governing factor in capacitive measurement is the variation of effective dielectric constant (ɛeff) of the device. Effective dielectric constant of TiO2 nanoparticles is the combination of dielectric of TiO2 grains and its associated void region. Dielectric variation of TiO2 grains and associated capacitance change through depletion region has already been discussed in above section. However, dielectric of the void region among the grains plays a major role for determining the effective dielectric constant as the dielectric of the tested gas (methanol=32.7, ethanol=24.5, acetone=20.7, and 2-propanol=17.9) is higher than that of the dielectric constant of air [14-15]. In both cases (dielectric variation at the grain itself and associated void regions), dielectric constant of device is varied without change in device area (A) and film thickness (t).
Therefore, capacitive response magnitude of the device is mostly dominant by the dielectric variation of the gases itself. The methanol has highest response magnitude among the test gas showing maximum response at the resonant frequency. The equivalent circuit representation of the sensor consisting of resistance and capacitance of the metal contact, grain boundary and its interfaces is shown in Fig 5 (c). Rgb and Cgb represent TiO2 nanoparticle grain boundary resistance and capacitance. Rb and Cb reprsents the bulk (seed layer) resistance and capacitance, respectively. The same has already been calculated and illustrated in the impedance spectroscopy discussion. A detail modeling of the same has been carried out to correlate with energy band diagram. Total impedance of device is defined as Zn=Zgb+Zb+1/jωCgas where Zgb=(Rgb‖Cgb) (shown in equation 9) is impedance of the TiO2 nanoparticle grains consist of parallel connection of Rgb ≈ nbexp(qVS/KBT) and capacitance Cgb ≈ (ɛ/qVS)0.5 [22]. Rgb and Cgb are the influential factors for determining response time and recovery time of the sensor. On the other hand, impedance (Zb=Rb‖Cb) of seed layer is proportional to the majority charge carrier electrons and is the parallel combination of Rb and Cb (shown in eq. 10) [20, 22]. Assuming, ω2Cgb2Rgb2>>1 and ω2Cb2Rb2>>1 , resistance and capacitance expression of the device is depicted in equation 11-12 [15].
Zgb = Rgb (1-jωCgbRgb)/(1+ω2C2gb R2gb) (9)
Zb, = Rb(1-jωCbRb)/(1+ω2C2b R2b) (10)
Rn=[CgbRgbCair+CbRbCair+CgbRgbCbRb]2/RgbRbC2air(C2gbRgb+C2bRb) (11)
Cn=CgbRgbCbRbCair/(CgbRgbCair+CbRbCair+CgbRgbCbRb) (12)
Smaller size TiO2 nanoparticles in combination with oxygen vacancies resulted in more active site for O- chemisorption [2, 8]. As discussed such process causes depletion width of the nanoparticles wider leading to the upward band bending [7, 22]. Band bending qVs,air is proportional to the carrier concentrations and width of the depletion region following the relation qVs,air=Ndλd2e2/2εairλp [33]. Such barrier height (upward band bending) restricted the carrier transport through grain boundaries (shown by dotted line in energy band diagram) [19]. In gas medium, carrier accumulation at the sensing channel through carrier exchange leading to the reduction in depletion width and thus lower the barrier potential (solid line) [8, 31]. Therefore, dielectric constant εeff and depletion width (λd) strongly influenced the electrical properties of oxide nanostructure and carrier transport.
Further, the operations need not be performed in the disclosed order, although in some examples, an order may be preferred. Also, not all functions need to be performed to achieve the desired advantages of the disclosed system and method, and therefore not all functions are required.
Various modifications to these embodiments are apparent to those skilled in the art from the description and the accompanying drawings. The principles associated with the various embodiments described herein may be applied to other embodiments. Therefore, the description is not intended to be limited to the embodiments shown along with the accompanying drawings but is to be providing the broadest scope consistent with the principles and the novel and inventive features disclosed or suggested herein. Accordingly, the invention is anticipated to hold on to all other such alternatives, modifications, and variations that fall within the scope of the present invention and appended claims.

, C , Claims:I/We Claim
1. A Au/TiO2 nanoparticles/Si-Al capacitive sensor for selective detection of VOCs (volatile organic compounds) at different frequency regimes.
2. A method for synthesizing a sensing layer of titanium dioxide (TiO2) nanoparticles, comprising the steps of:
(a) preparing a precursor solution by mixing 60 ml of de-ionized water, 10 ml of ethanol (99.9% pure), and 30 ml of acetic acid (99.5% pure) in a conical flask;
(b) adding 2 ml of titanium tetrachloride to the precursor solution and stirring the mixture for a duration of 4 hours;
(c) adding 10 ml of 2M ammonia hydroxide dropwise to the stirred mixture;
(d) allowing the resulting mixture to rest in a dark room for a period of three days;
(e) dip-coating a cleaned p-Si wafer (<100>) in the prepared solution using a programmable dip coating system for three consecutive cycles, wherein each cycle comprises dipping the wafer for 5 seconds and maintaining a gap of 10 seconds between cycles;
(f) thermally depositing a gold (Au) top electrode on the grown TiO2 layer; and
(g) establishing a bottom electrical contact using aluminum (Al) metal.
3. The method as claimed in claim 2, wherein the stirring is conducted at room temperature.
4. The method as claimed in claim 2, wherein the p-Si wafer is cleaned using a chemical cleaning process prior to the dip-coating.
5. The method as claimed in claim 2, wherein the thickness of the TiO2 layer can be controlled by adjusting the number of dip-coating cycles.
6. The sensing layer as claimed in claim 2, wherein said sensing layer includes an electrical contact structure consisting of a gold (Au) top electrode and an aluminum (Al) bottom contact.

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