Wearable, Flexible Ethanol Gas Sensor Based on TiO2 Nanoparticles

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Wearable, Flexible Ethanol Gas Sensor Based on TiO2 Nanoparticles-Grafted 2D-Titanium Carbide Nanosheets Appu Vengattoor Raghu, Karthikeyan K Karuppanan, Jayakrishnan Nampoothiri, and Biji Pullithadathil ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01975 • Publication Date (Web): 07 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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ACS Applied Nano Materials

Wearable, Flexible Ethanol Gas Sensor Based on TiO2 Nanoparticles-Grafted 2D-Titanium Carbide Nanosheets Appu Vengattoor Raghu†, Karthikeyan K Karuppanan†, Jayakrishnan Nampoothiri† and Biji Pullithadathil#†* †Nanosensor

Laboratory, PSG Institute of Advanced Studies, Coimbatore-641004, INDIA

#Department

of Chemistry, PSG college of Technology, Coimbatore-641004, INDIA

ABSTRACT: Herein, we demonstrate a novel approach for development of TiO2 grafted 2D-TiC nanosheets (TiO2@2D-TiC) based room temperature operable, flexible ethanol gas sensor. The homogeneous distribution, unique composition and crystalline microstructure of TiO2 nanoparticles grafted 2D-TiC nanosheets have found to enhance the surface reactivity and efficiency of its transducer-receptor functions. The electron-hole recombination at the TiO2/2DTiC interfaces offered superior sensor performance with fast response and recovery times. Moreover, TiO2@2D-TiC nanosheets based flexible sensor exhibited high selectivity towards trace-level ethanol gas (10 ppb - 60 ppm) with extremely low noise-to–signal ratio and excellent stability. The results suggest that the development of low-cost flexible sensors based on TiO2@2DTiC nanosheets could be applied for potential applications, such as printed/wearable electronics, biomedical sector and environmental monitoring.

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KEYWORDS: ethanol gas sensors, flexible sensors, 2D-nanosheets, titanium carbide, titanium dioxide, wearable electronics INTRODUCTION Flexible gas sensors have gained great interest over rigid sensors owing to their low-cost, biocompatibility, softness and transparency properties. These sensors also have emerged as crucial components in future generation portable and foldable devices, including consumer electronics, health care, home land security, robotics and environmental monitoring. Especially, ethanol gas detection has been widely recognized in various fields, such as industrial emission control, medical diagnosis, food processing, environmental pollution monitoring etc.1–4 Most commonly used ethanol gas sensing materials are wide bandgap n-type semiconductors (eg.,TiO2, ZnO,etc),5–7 which generally work at very high operating temperature (> 150°C). These semiconducting metal oxide gas sensing materials also exhibits high resistance, and low reactive surfaces at room temperature which adversely affects their gas sensing performances resulting in poor sensitivity, long response and recovery times under atmospheric conditions.8,9

Among various

semiconducting metal oxide materials, nanostructured TiO2 has been widely used for ethanol gas sensing application due to its low cost, less power consumption, wide band gap (3.1eV), high chemical and mechanical stability.10 However, TiO2 based gas sensors are less efficient to satisfy the prerequisites of sensor response, selectivity, stability and low working temperature.11–13 Therefore, such sensing materials have been expanded from single component to multi-component hybrid nanostructures in order to enhance their sensing performances. As an alternative to metal oxide based sensing materials, two-dimensional (2D) nanostructures are currently being regarded as promising materials for gas sensors due to their high surface area, versatile surface chemistry and sensitivity at room temperature.14,15 So far, 2D-materials like,


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graphene sheets, metal oxide nanosheets, transition metal dichalcogenides, MoS2, black phosphorus (BP), transition metal carbides etc. have received widespread attention in sensors due to their unique electronic properties. Especially, the surface properties of 2D materials significantly contribute towards the molecular adsorption/desorption process and formation of non-covalent interaction with analyte molecules. Recently, 2D MX systems (where, M is an early transition metal, and X is carbon and/or nitrogen) have shown excellent sensing properties towards reducing gases (ammonia, acetone, ethanol, propanol, etc.). Particularly, titanium carbide (TiC) is recognized as one of the promising material owing to its unique physical properties such as, excellent electrical conductivity (3 x107 Ω.cm-1), superior chemical and thermal stability, relatively low density (4.91 g cm-3), high melting temperature (3067 °C) etc.

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MXenes are either metallic

or semiconducting, depending on surface functional groups (-F, -O and -OH) associated to TiC blocks, the electron transport by capturing surface free electrons and enhanced surface reactivity. Pristine TiC systems are demonstrated to be metallic with a high electron density near the Fermi level, wherein the electronic transport property of the conductive TiC systems was found to be strongly influenced by surface terminations that could make TiC systems as a potential candidate for gas sensing applications. Theoretical computations have demonstrated the tunable band gap of TiC which can be opened up for O-terminated Ti2C (Ti2CO2) monolayer.14,16 The presence of atomic layers of carbon was found to enable stronger adhesion, lower the Schottky barriers and enhance electron transport. These carbon layers with the abruptness and coherency of the contact interface are critical for understanding the origin of ohmic nature. Consequently, the surface chemistry and catalytic properties of TiC are relatively unknown.16–18,19 Recent study reported that TiC based sensors showed high sensitivity and selectivity towards various reducing gases.14 However, the TiC nanostructures generally suffer from selectivity issues among reducing gases


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and so far no reports are available related to selective gas sensors based on TiC. Hence, there is a high demand for development of hybrid materials for addressing the selectivity issues of TiC based gas sensors. Recent reports suggest that wide band gap metal oxides suffer from high operating temperature and usually exhibits intrinsically high signal-to-noise ratio, whereas TiC based materials exhibits reduced noise in the sensing signals while working at room temperature. In this work, we have developed a hybrid material; TiO2 nanoparticles grafted 2D-TiC Nanosheets using a single-step thermal decomposition method and demonstrated its superior sensing properties as flexible sensor device for trace-level detection of ethanol gas. A thin layer of TiO2 nanoparticles were grown on highly reactive 2D-TiC sheets which exhibited excellent sensitivity with reduced noise-to-signal ratio and enhanced selectivity towards ethanol. The low adsorption energy towards ethanol on TiO2 grafted on 2D-TiC nanosheets precisely controls the fast charge transfer mechanisms between ethanol gas and ionosorbed oxygen species at the interface. A comprehensive analysis showed that the development of TiO2 @2D -TiC nanosheets is endowed with high surface area which would be more beneficial for tuning the sensitivity and selectivity towards ethanol gas with short response time. To the best of our knowledge, there are no reports existing on TiO2@2D-TiC nanosheets which are deployed for gas sensor applications. An integrated platform of TiO2@2D-TiC nanosheets based flexible gas sensor exhibited outstanding sensing characteristics with low noise-to-signal ratio at room temperature which is one of the major challenge faced by conventional gas sensor. The fabrication of TiO2@2D-TiC nanosheets based chemi-resistive flexible gas sensors offers lightweight and portability which are preferably needed for wearable electronics applications.


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Scheme1. Schematic illustration of the fabrication process for the TiO2@2D TiC nanosheets coated PET based flexible gas sensor (a) cleaning of PET substrates and (b) fabrication of TiO2@2D TiC coated IDA electrodes on cleaned PET.

RESULTS AND DISCUSSION The TiO2@2D-TiC nanosheets were synthesized by a single step thermal decomposition as shown in Scheme 1. The pre-cleaned flexible PET substrate were dried under argon at 100°C and further subjected to etching with HF using a SS mask. The etched PET substrates were used for fabrication of gold IDA electrodes. The TiO2@2D-TiC nanosheets were spin coated on these IDA electrodes for the development of sensor devices. The reaction temperature of TiO2@2D-TiC nanosheets were analyzed by differential scanning calorimetry (DSC) studies(Figure S1). The anatase structure of synthesized TiO2 nanoparticles at 660 °C was transformed into rutile to structure. Meanwhile, the fluorinated content vanishes away from the surface of TiC from 500°C onwards. Therefore, the suitable temperature for the formation of anatase TiO2 on TiC surface was fixed as 500°C.


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Figure 1. X-ray diffraction patterns of the synthesized TiO2, 2D-TiC and TiO2@2D-TiC nanosheets. XRD patterns were recorded to identify the crystal structure of TiO2, 2D-TiC and TiO2@2D-TiC nanosheets, as shown in Figure 1. The TiO2 nanoparticles exhibited the planes of (101), (004) (200), (211), (204), (116), (301) and (224) corresponding to the formation of anatase tetragonal body centered l41/amd[141] structure, as indexed based on JCPDS database (89-4921). The lattice parameters of TiO2 were calculated to be a = b = 0.37 nm and c = 0.95 nm. On the other hand, 2D-TiC nanosheets showed the planes corresponding to (111), (200), (220), (311) and (222) revealing the formation of octahedral cubic face-centered Fm3m [225] (JCPDS No: 89-3828) structure. The lattice parameters of 2D-TiC nanosheets were calculated as a=b=c= 4.3 nm respectively. Moreover, the XRD pattern of TiO2@2D-TiC showed similar peaks as observed for TiO2 and 2D-TiC and no additional peaks were appeared, indicating the successful formation of


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heterostructures. The peaks were slightly shifted to higher anglesfor TiO2@2D-TiC compared to TiC. This shift is mainly because of the contraction in the lattice constant due to the incorporation of TiO2 on TiC surface.

Figure 2. TEM images, corresponding SAED patterns and plane orientation Fourier-masked micrographs of (a) TiO2, and (b) 2D-TiC nanosheets. In order to investigate the structure and morphology of TiO2 nanoparticles (Figure 2a), TiC sheets (Figure 2b) and TiO2@2D-TiC nanosheets (Figure 3) were analysed through TEM image analysis. As evident from Figure 2b, the TiC nanosheets were found to be remarkably transparent which indicated the formation of thin nanosheets. The plane orientation Fourier-masked


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micrographs of TiO2 and TiC showed lattice spacing of 0.35 and 0.24 nm, corresponding to (101) and (111) planes of anatase TiO2 and octahedral TiC crystal structures, respectively. The HRTEM images (Figure 3) clearly confirmed the formation heterostructures of TiO2@2D-TiC nanosheets. The SAED patterns revealed the nanocrystalline structure of TiO2@2D-TiC nanosheets. It is evident that TiO2@2D-TiC nanosheets exhibited an average TiO2 particle size of 14-18 nm, homogeneously distributed on the surface of TiC nanosheets. As can be seen from Figure S2 (AFM), TiC nanosheets showed thickness of ~4 nm, clearly implying the formation of thin sheets that would be more advantageous for effective surface reactions.

Figure 3. TEM images, SAED patterns and corresponding plane orientation Fourier-masked micrographs of TiO2@2D-TiC nanosheets. The XPS analysis was performed to investigate the chemical composition and oxidation states of various elements of the TiO2 nanoparticles, TiC nanosheets and TiO2@2D-TiC nanosheets. The full survey spectrum of TiC (Figure S3a) shows the existence of titanium (Ti), carbon (C) and oxygen (O). High-resolution XPS spectra of Ti 2p and C 1s core levels of TiC and TiO2@2D-TiC


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nanosheets were analyzed and presented in Figure 4a-c. The deconvolution of high resolution peaks was carried out using Gaussian-Lorentzian for the precise calculation of chemical states of these atoms present in TiC, TiO2 and TiO2@TiC nanosheets.20-21 The Ti 2p core level spectrum consists of six peaks centered at an energy level of 455.05, 455.8, 457, 458.5, 461.3 and 464.2 eV as shown Figure 4a. The main component at 455.05 and 461.3 eV originate from Ti2p3/2 and Ti2p1/2 in Ti-C bond (Ti+), respectively. The peaks appeared at 458.5 and 464.2 eV are attributed to Ti2p3/2 and Ti2p1/2 associated with Ti ions with a formal 4+ valence. The components appeared at lower binding energy 455.5 eV corresponds to Ti−X (sub-stoichiometric titanium carbide (TiCx) or titanium oxycarbides (TiCxOy)).23 As presented in Figure 4b, the carbon core level spectrum was resolved into four components at the typical binding energy values of 281.9, 284.3, 284.7, 285.9 and 288.5 eV. The predominant peaks located at 284.3 eV and 285.9 eV represent the formation of sp2 C-C bonds and sp3 C-C bonds, respectively. The low intensity peaks at 285.4 eV and 281.9 eV is evident for C-O bonds and Ti-C, respectively. Figure 4c shows two bands contributed to the O1s signal; the main component is located at a binding energy of 532.5 eV assigned to the characteristics of the adsorbed oxygen species (carbonates and hydroxyl groups), whereas the minor component peak appeared at 530.5 eV is ascribed to Ti–O bonds. Analysis of C1s and O1s spectra results indicated that the TiC surface was predominantly functionalized with two type of surface functional groups, such as oxide (-O), and hydroxyl (-OH).


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Figure 4. High resolution XPS spectra of (a-Ti2p, b-C1s, c-O1s) TiC and (d-Ti2p, e-C1s, f-O1s) TiO2@2D-TiC nanosheets. Figure S4 (supporting information) shows the XPS survey spectra of TiO2, where the peaks located at 285.8, 399.6, and 537.2 eV clearly indicated the presence of C, O and Ti elements. As indicated in Figure 4a, the high-resolution Ti 2p core level peak was fitted into two well resolved peaks observed at 464.2 and 458.5 eV with a separation of 5.7 eV that can be attributed to Ti 2p1/2 and Ti 2p3/2 spin-orbital components (characteristic Ti4+ oxidation state) present in TiO2. The appearance of less intense peaks located at a lower binding energy value of 465.2 and 458.3 eV indicated the presence of small amount of Ti3+ in the TiO2 nanoparticles, due to the surface oxygen vacancies originated from high temperature annealing process under nitrogen atmosphere. Figure S3 shows the asymmetric O1s core spectrum fitted into three peaks. The first major band at the binding energy value of 529.9 eV is ascribed to the O2− ions in the crystal lattice of anatase structure (O - Ti4+), the second peak observed at 531.6 eV is assigned to O2− ions in the oxygen deficient regions (lattice oxygen vacancies, Ov) and the third peak at 533.2 eV is associated with


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the presence of chemisorbed oxygen on the surface, demonstrating the presence of many more TiOH species on the surface. The high resolution C1s spectrum of TiO2 nanoparticles was deconvoluted as to two peaks, as shown in Figure S3. The binding energies of 284.5 and 285.7 eV could be related to adventitious carbon C=C and chemisorbed surface carbonates C–O bonds respectively. Therefore, the results clearly implied that these oxygen vacancies and surface adsorbed oxygen species tend to form more number of active sites that can enhance the sensing properies.24,25 Figure S3, F1s spectra (Supporting information) demonstrate that there was no fluorinated groups on the TiC surface (eliminated during thermal treatment) Figure 4d-f shows the deconvoluted peaks of high resolution XPS spectrum of the TiO2@2DTiC nanosheets. The peaks presented on the full spectra corresponding to Ti2p, O1s and C1s. The deconvoluted peaks of Ti2p3/2 spectra located at 455.12, 457.25, and 459.26 eV indicate the presence of Ti-C, Ti2+/Ti3+ and Ti4+ respectively (Figure 4d). A prominent peak observed at 459.6 in TiO2@2D-TiC nanosheets confirms the strong interaction of TiO2 on 2D-TiC nanosheet interfaces. Similarly, the Figure 4e, C1s spectra depicts the deconvoluted peaks of 282.15, 284.59 and 285.67 eV which confirmed the evolution of Ti-C, C=C and the C-O bonds in TiO2@2D-TiC nanosheets. The O1s core-level spectra displayed in Figure 4f shows the adsorption of surface oxide groups on TiO2@2D-TiC nanosheets similar to O1s spectra of TiO2. The peak appeared at 531.23 eV corresponds to lattice oxygen (OL) and 533.25 eV is related to the lattice oxygen vacancy (Ovac) created in TiO2 during the heat treatment process. The relative shift (0.12eV) in Ti4+ peaks in TiO2@2D-TiC nanosheets indicates the formation of heterojunctions at the interface of TiO2 and TiC. Hence, the presence of adsorbed oxide groups due to successful integration of TiO2 on TiC nanosheets and oxidation states of Ti were clearly understood from XPS spectra of the TiO2@2D-TiC nanosheets.


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Figure 5a shows the AFM topographic images and surface potential images of TiO2 and TiC. The two-pass scanning was performed using Kelvin probe microscopy (KPM), in which the topography was obtained during the first pass scan corresponding to semi-contact mode AFM imaging. During second pass scan, the conductive probe was lifted upto a fixed height, for measuring the surface potential difference on the material. The surface potential imaging of TiO2 and TiC was performed to understand the work function of the materials. The electrons in both TiO2 and TiC possess different chemical binding energies when they are in electrically contact. The electrons usually transfer from the material with lower work function (weak binding) to the material with the higher work function (strong binding). The diffusion current builds up a double layer at the interface resulting in the electrostatic contact potential difference between the two species. The potential difference at the interface shifts the bulk electron energy levels until the Fermi levels of these two materials are matched. When this equilibrium is reached, the electrostatic force cancels the diffusion current at the interface and the contact potential equals to the difference in the both work functions. The electrostatic contact potential difference (VCPD) is created due to the difference in work functions of the electrically connected conductive tip and the material surface which can be defined as,25 𝜑𝑡𝑖𝑝 ― 𝜑𝑠 𝑒

(1)

𝜑𝑠 = 𝜑𝑡𝑖𝑝 ― 𝑒𝑉𝐶𝑃𝐷

(2)

𝑉𝐶𝑃𝐷

=

Where, e is the electronic charge, φtip and φs are the work functions of the tip and the sample respectively. The observed surface potential difference between TiO2 and TiC indicated that the electrons get transferred from TiO2 to TiC until the equilibrium Fermi level was achieved. The


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electron transport occurs from TiO2 to TiC and was mainly due to the higher work function of TiC compared to TiO2.

Figures 5. (a)Topographic and surface potential images of TiO2 nanoparticles and TiC nanosheets (b) proposed band diagram corresponding to TiO2-TiC hetrojunctions The work function of TiO2 and TiC was calculated from the equations (1) and (2), and the values were found to be 4.23 and 4.465 eV, respectively. As a result, Schottky-type junctions were formed across the TiO2/TiC interfaces, which was characterized with an upward band bending and depletion region in TiO2 near the surface. The gas sensor response characteristics of TiO2, TiC and TiC@2D-TiO2 nanosheets toward different ethanol concentrations ranging from 10 to 60 ppm were thoroughly investigated at room temperature by monitoring the change in resistance (Figure 6c). The generally accepted mechanism for n-type semiconductor (TiO2) systems involves initial dissociative chemisorption of oxygen, resulting in the formation of negatively charged oxygen species (O2−, O− and O2−) on the surface of sensing material to compensate their deficiencies.26 Upon the introduction of


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reductive ethanol gas to the test chamber, the gas molecules react with the surface adsorbed oxygen species releasing the pre-captured electrons back to the surface (Eqns. (3) and (4)) that promote their oxidation of ethanol into H2O and CO2. Such reactions retrieve the trapped electrons back to the conduction band of the sensing material leading to a decrease in resistance of the gas sensor. In the case of TiC nanosheets, the sensing behavior shows a p-type characteristic response by increasing the resistance during ethanol gas exposure and that is due to the presence of more surface functional groups. The above mentioned free electrons leading to the formation of electronhole recombination followed by an increase of electrical resistance.14 The reaction between ethanol and oxide ion species can follow two different pathways, the overall reactions can be represented as Eqns (3-4)1,26,27 ― 𝑂2𝑔𝑎𝑠 + 𝑆𝑎𝑚𝑝𝑙𝑒𝑎𝑑𝑠 ↔ 𝑂2𝑎𝑑𝑠 + 2𝑒 - ↔2𝑂2𝑎𝑑𝑠

(3)

2𝐶2𝐻5𝑂𝐻 + 5𝑂2―(𝑎𝑑𝑠)→4𝐶𝑂2 + 4𝐻2𝑂 +5𝑒 ―

(4)

As can be seen from Figure 6c, TiO2@2D-TiC nanosheets showed excellent response towards ethanol gas at room temperature. This is potentially superior than that of pristine TiC and mainly attributed to unique physico-chemical properties, such as high surface area, superior electrical conductivity and synergetic interactions between TiC and TiO2. The integration of TiO2 nanoparticles on the surface of TiC nanosheets forms hole-depletion region at the interface between TiC nanosheet and n-type TiO2 that leads to the decrease in electrical conductivity of TiO2@TiC. Figure 6c illustrates the adsorption/desorption cycles of ethanol gas molecules adsorbed on the sensor. Upon exposure, ethanol gas molecules readily react with pre-adsorbed oxygen species on the surface of TiO2@2D-TiC nanosheets. The integration of TiO2 nanoparticles on TiC nanosheets introduces additional receptor sites by modulating the conductivity of TiC which is liable to sense trace level ethanol gas. Moreover, complete recovery of the base resistance


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values during each sensing pulses was revealed the reversible interaction of TiO2@2D-TiC nanosheets with ethanol gas mainly because of the physisorption process without noticeable change in base resistance. These results indicated that the chemisorption process is not involved during sensing events and is a key factor for technological applications. The concentration-dependant sensing profiles exhibited a linear relationship, indicating excellent ethanol sensing properties of TiO2@2D-TiC nanosheets. The TiO2 nanoparticles exhibited a lower sensing response (n-type behavior resistance decreases) compared to TiC and TiO2@2D-TiC nanosheets (p-type behavior - resistance increases) at room temperature. The TiO2@2D-TiC nanosheets could even detect 10 ppm concentration of ethanol and their measured response time was found to be 2.0 s, whereas TiC showed 3.2 s (Figure 6 d). As shown in Figure 6f, TiO2@2D-TiC nanosheets based sensor demonstrated reproducible and superior response/recovery characteristics. The process of adsorption/desorption mechanism is shown in the following schematic diagram (Figure 7). The high surface area and superior electrical property of TiC nanosheets allows rapid transport of the charge carriers that greatly promotes the transducer functions of the TiO2@2DTiC nanosheets. This involves two main competing mechanisms; namely (i) changes in the potential energy barrier encountered by electron mobility across a hetero-junction interface and (ii) shortening of the charge conduction channel along the nanosheets via the p–n depletion region impinging on the core. The selectivity of the TiO2@2D-TiC nanosheets based sensor was studied by upon exposing of various analytes of both reducing [ethanol (60 ppm), isoprene (60 ppm), acetone (60 ppm), hydrogen (10000 ppm), di isopropyl methyl phosphonate-DMP (60 ppm), hydrogen sulfide (60 ppm), ammonia (100 ppm), formaldehyde (100ppm), & methanol (100ppm)] and oxidizing gases [(oxygen (1000 ppm), nitrogen dioxide (100 ppm)].


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Figure 6. (a) Gas sensor test station used for sensor-property evaluation (b) digital photograph of the fabricated flexible sensor on PET substrate. (c) dynamic sensor response of TiO2, TiC and TiC@2D-TiO2 nanosheets at room temperature at various concentrations (10, 20, 30, 40, 50 and 60 ppm) towards ethanol gas, (d) the typical response and recovery curves of pure TiC and TiO2@2D-TiC nanosheets exposed towards 10 ppm of ethanol. (e) Selectivity of TiO2@2DTiC nanosheets and (f) reproducibility of TiO2@2D-TiC based sensors upon successive ethanol exposure (10 cycles) at RT.


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In view of these observations, the TiO2@2D-TiC nanosheets based sensor manifested its high sensitivity and selectivity towards ethanol compared to other gases, as displayed in Figure 6e. It is reported that TiC nanosheets show a remarkable response towards reducing gases, such as ammonia, isoprene, ethanol etc. The presence of Ti2CO2 (involved in the surface of TiC) monolayer are highly selective towards NH3 among reducing species.16 Introduction of TiO2 nanoparticles on 2D-TiC nanosheets tend to provide more catalytic active sites (oxygen ions on the surface) and the decreased thickness of the surface depletion region and improves the selectivity towards ethanol gas. The large size of ethanol molecules can easily adsorb on TiO2@2D-TiC nanosheets surface thereby release more electrons to the surface (large number of donor characteristics of ethanol) by forming a thin charge depletion layer. In order to further validate the noise to signal ratio (NSR), the electrical noise of each sensor was initially determined by measuring the average resistance fluctuations during nitrogen gas condition. The noise to signal ratio level was obtained by I-V measurements (Figure S5) and the values for TiO2, TiC, and TiO2@2D-TiC nanosheets were calculated to be 0.12, 0.002948, and 0.005274%, in which TiC nanosheets displayed the lowest noise level. The TiO2@2D-TiC nanosheets showed very less noise to signal ratio compared to TiO2 and the negligible noise to signal ratio is more desirable for practical applications. The chemical adsorption of the analyte gas on sensor material can be explained by the Elovich model and the general form of the model is expressed as follows,30

𝑞=

1 𝑎𝑙𝑛(𝑎'𝑎)

1

+ 𝑎𝑙𝑛(𝑡)

(5)

Where, q describes the change in conductance during time, t, α is related to the measure of potential barrier for successive adsorption and á is the initial adsorption rate. These constants can be derived


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from the slope and the intercept of plot. C0 and Ct are conductance of TiC and TiO2@2D-TiC nanosheets at time, t=0 and t=t during the adsorption of ethanol gas molecules, respectively.

Figure 7. Elovich plots corresponding to ethanol gas adsorption on (a) TiC and (b). TiO2@2DTiC nanosheets. The derived plots from initial adsorption and potential barrier of (c) TiC and (d) TiO2@2D-TiC nanosheets. The linearity of plot confirms the validity of the Elovich model, as shown in eqn. (5) and constant α is related to a measure of potential barrier for successive adsorption (Figure 7). This trend indicates the diminishing of barrier potential in pure TiC (14.27 MΩ for 10 ppm and 1.16 MΩ for 60 ppm) and TiO2@2D-TiC nanosheets (5.06 MΩ for 10 ppm to 3.06 MΩ for 60 ppm). Similarly, the adsorption rate of TiO2@2D-TiC nanosheets is comparatively higher (4.75x10-6 Ω-


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1s-1)

than that of TiC (1.31x10-6 Ω-1s-1) towards 10 ppm ethanol concentration. Figure 7(b) and 7(d)

display the adsorption kinetics of TiC and TiO2@2D-TiC nanosheets towards various concentrations of ethanol gas. Elovich model clearly depicts the relationship between adsorption kinetics and concentration, where the potential barrier reduces with respect to increasing concentration. At higher concentration, the possibility of adsorption in TiO2@2D-TiC nanosheets is predominant as compared to TiC nanosheets, promoting the significant changes in the potential barrier. The trace-level limit up to 10 ppb (Figure S6a) and the linear response at higher concentrations (0.01-60 ppm) were systematically investigated (Figure S6b).

Figure 8. A Schematic representation of ethanol gas sensing mechanism of (a).TiO2@2D-TiC nanosheets, (b) adsorbed oxygen species under air atmosphere, (c) conversion O2 gas into O2- (d) introduction of ethanol gas on TiO2@2D-TiC nanosheets (e) ethanol gas reduction on the surface and its charge transfer (f) ethanol interaction scheme on TiO2@2D-TiC nanosheets.


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It is evident that the responses were enhanced linearly with increasing concentrations of ethanol (Figure S7). Figure S8 shows the sensing response, SR (%) as a function of ethanol concentration Pgas at room temperature (25°C) indicating good linearity between logarithm of SR (%) and logarithm of ethanol concentration (Figure S8a). Typically, the gas sensing response of metal oxide gas sensors can be empirically denoted as,28 SR=K(P)m

(6)

Where, K is a pre-factor, P is the partial pressure of the target gas which is proportional to the gas concentration and m is the exponent on P. Usually, the ideal m value of 1 is obtained for the reaction between O- ions and reducing ethanol gas on TiO2@2D-TiC nanosheets. The charge depletion layer model induced by the adsorption/desorption of oxygen molecules on the surface of sensing materials was used to further investigate the gas-sensing mechanism, as schematically depicted in Figure 8a. The chemisorbed oxygen on TiO2@2D-TiC nanosheets causes the depletion of accumulation layer resulting the formation of more O2–, as shown in Eq. (4) and Figure 8b and 8c. These adsorbed oxygen molecules has the tendency to trap electrons from the sensing materials because of their large electro-negativity and that creates large number of oxygen ion species (O2-, O- or O2-), as indicated in equations (3-4). These species can induce the formation of a thick charge depletion layer in n-type semiconductor or a thick accumulation layer in p-type semiconductor near the surface that corresponds to a high resistance state or low resistance state, respectively. The ethanol molecules readily react with the adsorbed oxygen species present in the TiO2@2DTiC nanosheets surface, consequently releasing the electrons back to the surface of the sensing materials, as shown in equation (4). Hence, this result reveals that the anchoring of TiO2 on TiC nanosheets is highly responsible for enhancing the gas sensing performance (Figure 8d-f). The p–n


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hetero-junctions formed between p-type TiC and n-type TiO2 showed significant effects on the resistance modulation of TiC@2D-TiO2 nanosheets that predominantly enhanced the response signal. The conduction band edge of TiC was found to be more negative than that of TiO2, which causes the electron transfer from TiC to lower-energy conduction band of TiO2 in order to equalize the Fermi level. An internal charge layer was formed in the depletion region across the TiO2/TiC interfaces. Upon exposing the ethanol molecules on TiO2@2D-TiC nanosheets, the electrons get transferred from TiO2 to adsorbed ethanol molecules leading to the increase of surface carrier concentration in TiO2. The increased number of charge carriers in TiO2 decreases the Ec/EF difference and therefore smaller Fermi level difference between TiO2 and TiC leading to an increment in the electric field of the interfacial junction.32 The upward band bending and enhanced potential barrier was established at the interface between TiO2 nanopartcles and TiC nanosheets. The TiO2@2D-TiC nanosheets shows high initial resistance as compared to the pristine TiC owing to the presence of enhanced potential barrier.29 Due to the increase in initial resistance, more resistance changes occurred during the exposure of ethanol gas which evidentially showed higher response characteristics. The ethanol gas reacts with surface adsorbed gas molecules (𝑂− and 𝑂H −)

of TiC nanosheets causes the generation of electrons, leading to hole-electron recombination.

The surface functional groups could change the metallic TiC nanosheets to a narrow band-gap semiconductor. In the case of TiO2@2D-TiC nanosheets during the exposure of ethanol gas, the charge carrier accumulation layer near the surface is thinned by the interaction between 𝑂2― (ads) and ethanol molecules; thereby it releases free electrons and consequently neutralizes the holes in the TiC. The released electrons on the surface of TiO2 generate more electron-hole null recombination on the interface between TiO2 and TiC. The electron transfer from the TiO2 to the TiC is impeded, which increases the resistance of the device rapidly. Moreover, the nanocrystalline


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structure of 2D TiC nanosheets provides fast transport of the charge carriers promoting the transducer functions of the TiO2@2D-TiC nanosheets. The TiO2@2D-TiC nanosheets can adsorbs more O2 molecules because of its large surface area and existence of more active sites, such as oxygen vacancies, defects, oxygen functionalized groups related to TiO2 which is more beneficial for enhanced gas sensing properties. The comparison of performances such as sensitivity, response and recovery times were shown in Table S1. The TiO2@TiC 2D nanosheets exhibit better performances than the TiO2 nanoparticles and TiC nanosheets due to the transducer-receptor functions by electron-hole recombination at the interface of TiO2 and 2D-TiC nanosheets. The sensing mechanism of TiO2@2D-TiC nanosheets hetero-junction was further analyzed by studying the changes in surface band bending and sensor response (S), as shown in Figure S8b. In general, the Boltzmann distribution law relates the concentration of the surface electrons (ns) and the band energy (q∆V) can be expressed as,30,31

𝑞∆𝑉 = ― 𝐾𝐵.𝑇.ln

( ) 𝑅𝑔𝑎𝑠 𝑅𝑎𝑖𝑟

(7)

Where, KB is the Boltzmann constant, T is the working temperature of the sensor, Rgas is the sensor resistance in gas environment and Rair is the sensor resistance in air. The change in band bending (q∆V) of the TiC and TiO2@TiC nanosheets could be calculated from the sensor response towards various analyte gas concentrations. The results indicated that TiO2@2D-TiC nanosheets showed higher change in surface band bending than that of pure TiC due the high surface reactivity of TiO2. The TiO2@TiC 2D nanosheets based flexible ethanol gas sensor exhibited better sensing performances (in terms of sensitivity, selectivity, operating temperature and trace level detection limit at room temperature) as compared to other reported ethanol sensing materials listed in Table S2. In order to further speculate the long-term stability, the sensing experiments were repeated


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with five months old TiO2@TiC based sensor and their results are shown in Figure S9. The results clearly indicated that TiO2@TiC based sensors retained almost same sensitivity (~0.7% drift) during several adsorptions-desorption cycles. To investigate the effect of humidity on gas sensing performances of TiO2@TiC sensor, the experiments were carried out at different relative humidity (10-90%) levels towards 1 ppm level of ethanol gas. It is clearly seen that there is no large breakdown in sensitivity (changes of ~6%) during the relative humidity 10-60%, but, the sensitivity was drastically reduced beyond 60% relative humidity as shown in Figure S10 (sensitivity changes of ~ 32.4%). We surmise that the superior gas sensing properties on the TiO2@2D-TiC nanosheets can contribute to the development of low-cost, transparent, flexible gas sensors with high sensitivity, selectivity, fast adsorption/desorption and negligible noise to signal ratio which are suitable for wearable electronics applications.

CONCLUSIONS To summarize, TiO2@2D-TiC nanosheets were successfully synthesized via single step thermal decomposition method and its ethanol gas-sensing properties using a flexible platform was systematically investigated. The uniformly distributed TiO2 nanoparticles on 2D-TiC nanosheets created more number of chemisorbed oxygen surface groups leading to higher degree of accessible sites for high sensitivity and selectivity of ethanol gas molecules. The gas sensing analysis of TiC@2D-TiO2 nanosheets based sensor exhibited more attractive functional performances in terms of both responses and selectivity toward ethanol gas with a limit of detection upto 10 ppb with low noise to signal ratio. The sensor performance enhancement was mainly due to the formation of Schottky -type p-n junctions at the TiO2/TiC interfaces, which induced an extension of the electron depletion layer, where surface coarsening as well as formation of TiO2/TiC


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heterojunctions existed. The chemical sensitization mechanism was exploited for the formation of surface depletion model at the TiO2/TiC interfaces to improve the sensor performance. The excellent transducer functions and fast transport of charge carriers in TiO2@2D-TiC nanosheets predicted its potential as a promising candidate for flexible trace-level ethanol sensing applications. Moreover, the development of sensors based on such hybrid 2D inorganic materials paves a way for fabricating flexible and low-cost strip sensors for real time wearable electronics applications. EXPERIMENTAL SECTION The TiO2 nanoparticles, TiC sheets and TiO2@2D-TiC nanosheets were prepared by thermal decomposition method. The precursors of titanium isopropoxide (99.99 %, Sigma Aldrich), titanium carbide bulk powders (~200 µm, 99.96%, Alfa Aeser) purchased and used without further purification. The procedure adopted for synthesis of TiO2@2D-TiC nanosheets is described as follows: Titanium isopropoxide (5 mL) was stirred with the mixture of ethanol and de-ionised water (60:40 ratio) for 2 h, the collected product was washed with distilled water and dried in a oven at 80 °C for 8h. The resultant powder was further grounded for 30 min and annealed at 500 °C (heating rate, 10 °C min-1) for 2 h under argon atmosphere. For preparing TiC nanosheets, commercial TiC bulk powder (10 g) was annealed at 500 °C (heating rate, 10 °C min-1) for 2 h under argon atmosphere. Similarly, TiO2@2D-TiC nanosheets were prepared by mixing TiC bulk powder (10 g) with titanium isopropoxide (5 mL) in ethanol and de-ionised water (60:40 ratio) for 2 h. The reaction product was washed and dried under argon atmosphere. The final product was annealed at 500 °C (heating rate, 10 °C min-1) for 2 h under argon atmosphere. After completion of the


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thermal decomposition reaction, the resultant product were washed with distilled water in order to remove the impurities and subsequently dried in a vacuum oven at 80 °C for 8 h. EVALUATION OF GAS SENSING PROPERTIES TiO2, TiC and TiO2@2D-TiC nanosheets were deposited on gold iterdigitated array (IDA) in order to evaluate the sensing performance. Prior to the fabrication process, Polyethylene terephthalate sheets (PET) (2x6 cm) were ultrasonically cleaned in acetone and ethanol for about 5–10 min followed by sonication in millipore water (Scheme 1a). Further, the PET sheets were treated with Hydrofluoric acid in a mask covered form and washed with freshly prepared piranha solution (H2SO4 and H2O2 in 3: 1 v/v ratio) for about 2– 3 times. Finally, the sheets were washed with de-ionized water and dried at 100 oC under Ar atmosphere. The gold IDA electrodes were fabricated on flexible PET sheet by DC magnetron sputtering using a customized SS hard mask. The inter-finger gap between the electrodes was 50 μm. Prior to deposition, the synthesized materials were ultrasonicated to enable proper dispersion in the channels of IDA. Then, the samples were coated on PET sheets using spin coating method with 2000 rpm for 2 min. Finally, the fabricated electrodes were dried in vacuum oven at 60°C for 30 min to complete the fabrication (Scheme 1b). The gas sensing characteristics were studied on indigenously made testing chamber equipped with mass flow controller (MFC), (Alicat Scientific,USA), baratron capacitance manometer (MKS 660), Owlstone gas generator (OHG-4, UK), temperature controller (Eurotherm 3216). The resistance signal of the sensor was measured by a data acquisition module (Agilent Multimeter 34410A). The Electrical (I-V) measurements was carried out by Keithley source meter (2420),The dilution system in our testing apparatus was limited to 0.01-60 ppm for ethanol, acetone isoprene, hydrogen sulfide, ammonia, and nitrogen dioxide controlled by OHG. The other gases (hydrogen and oxygen) were fixed below 10% concentrations and were controlled by MFCs.


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The real-time resistance was monitored to calculate sensitivity factor (S%) (defined as ∆R/R0 x 100), ∆R = (Rt – R0)/R0, where Rt and R0 are the acquired resistance and initial resistances, respectively. CHARACTERIZATION TECHNIQUES The X-ray diffraction (XRD) patterns were collected with Cu-Kα radiation (λ=1.5406 Å) using a powder X-ray diffractometer (Panalytical X-pert) to identify the crystal structures of the synthesized materials. The instrument was equipped with an X’celerator detector and operated at 40 kV and 25 mA and the scanning was performed over the 2θ range of 10°− 90° at a scan rate 0.02. Morphology, size, distribution and crystal structure of the materials were examined through high-resolution transmission electron microscopy (HRTEM) on a JEOL JEM 2100, Japan, with an operating voltage of 200 kV. The synthesized samples were coated on carbon coated copper grid (200 mesh). The semi-contact mode AFM analysis of the prepared materials was performed using multimode scanning probe microscope (SPM) (NTMDT-NTEGRA, Russia) under ambient conditions (25 °C). X-ray photoelectron spectroscopy (Thermoscientific, k-Alpha) equipped with a monochromatic Al Kα (hυ = 1486.6 eV) radiation source. Wide energy survey scans were obtained with a binding energy range of 0–1360 eV, pass energy of 150 eV, and an energy step of 1 eV. The high resolution spectral data were obtained from the circular spot size, pass energy and step size were 400 µm, 30 eV and 0.1 eV, respectively. The synthesized samples were subjected to differential scanning calorimetry/thermogravimetry (DSC/TG) analysis using NETZSCH (model- Jupiter STA449F3), Germany.

ASSOCIATED CONTENT Supporting Information


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The AFM images of TiC nanosheets, XPS analysis (full spectra, C1s spectrum of TiO2, and Flourine groups of TiO2 and TiC), HRXPS spectra of TiO2 nanopartcles DSC results of TiO2 and TiC precursors, I-V Characteristics of prepared materials, Low concentration gas sensing analysis of TiO2 @ TiC nanosheets and performance comparisons of developed sensors. AUTHOR INFORMATION * Corresponding Author Dr. P. Biji, Associate Professor in Nanotechnology, Nanosensor Laboratory, PSG Institute of Advanced Studies, Post Box No: 1609, Avinashi Road, Peelamedu, Coimbatore-641 004, INDIA. E-mail: [email protected] Phone: 0091-422 4344000 Extn (4193) †Alternate

Address: Associate Professor, Department of Chemistry, PSG College of Technology,

Post Box No: 1609, Avinashi Road, Peelamedu, Coimbatore-641 004, INDIA.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors acknowledge DST-SERB (Ref: SB/S3/CE/038/2015) for financial support.The authors also wish to acknowledge the facilities and support provided by the management,


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PSG Sons and Charities, Coimbatore. Authors thank to Dr. Anuradha ashok and Mr. T. Vijayaragavan for TEM analysis.

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Table of Contents


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SYNOPSIS A facile approach for development of flexible, transparent and selective TiO2 grafted 2D-TiC nanosheets (TiO2@2D-TiC) based trace-level ethanol sensor with fast response/recovery time and negligible noise to signal ratio is reported.


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Wearable, Flexible Ethanol Gas Sensor Based on TiO2 Nanoparticles

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