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Improved selectivity of SnO2:C alloy nanoparticles towards H2 and ethanol reducing gases; role of SnO2:C electronic interaction
Accepted Manuscript Title: Improved selectivity of SnO2 :C alloy nanoparticles towards H2 and ethanol reducing gases; role of SnO2 :C electronic interaction Authors: Mehar Bhatnagar, Shivani Dhall, Vishakha Kaushik, Akshey Kaushal, Bodh Raj Mehta PII: DOI: Reference:
S0925-4005(17)30142-9 http://dx.doi.org/doi:10.1016/j.snb.2017.01.135 SNB 21660
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
24-8-2016 11-1-2017 21-1-2017
Please cite this article as: Mehar Bhatnagar, Shivani Dhall, Vishakha Kaushik, Akshey Kaushal, Bodh Raj Mehta, Improved selectivity of SnO2:C alloy nanoparticles towards H2 and ethanol reducing gases; role of SnO2:C electronic interaction, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2017.01.135 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Improved selectivity of SnO2:C alloy nanoparticles towards H2 and ethanol reducing gases; role of SnO2:C electronic interaction Mehar Bhatnagar, Shivani Dhall, Vishakha Kaushik, Akshey Kaushal, Bodh Raj Mehta* Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi- 110016, India
Abstract In the present study, changes in the sensing properties of SnO2 on Carbon incorporation have been investigated in detail. The gas sensing response of size-selected SnO2 and SnO2:C alloy nanoparticles prepared by gas phase deposition method have been investigated for H2 and ethanol over a varied temperature range (50°C- 200°C). The incorporation of carbon into SnO2 lattice results in a large change in the sensing behaviour towards the two gases both having reducing nature. SnO2:C nanoparticles show positive sensing response for H2 and negative sensing response for ethanol, whereas SnO2 nanoparticles show a normal sensing response of an n-type semiconductor towards both the reducing gases. Observed values of activation energy of sensing and energy levels of O-vacancies observed in the PL spectra of SnO2 and SnO2:C are consistent with these results. (i) Catalytic C-H interaction and (ii) modified work function of SnO2 and C on hydrogenation resulting in alteration of electronic exchange between SnO2 and C, and (iii) passivation effect of carbon during SnO2-ethanol interaction along with a possibility of reduction in SnO2 sites in SnO2:C nanoparticles, are responsible for the observed behaviour. The present study shows that the incorporation of C in SnO2 nanoparticles results in excellent selectivity towards H2 and ethanol (both having reducing nature) in the low temperature range, normally not observed in oxide based resistive sensors.
Key words: size-selected SnO2:C alloy nanoparticles, PL, selectivity, reducing gases, catalytic effect
* Author to whom all correspondence must be addressed. Electronic mail: [email protected]
Highlights: 1. Gas sensing properties of size-selected SnO2 and SnO2:C alloy nanoparticles over a wide temperature range (50°C- 200°C). 2. Effect of incorporation of carbon in the alloy nanoparticles towards selectivity between two gases both having reducing nature (H2 and ethanol) at low temperatures. 3. Change over from n-type to p-type behaviour in H2 ambience. 4. Retention of n-type behaviour in ethanol ambience. 5. Correspondence between energy values from activation energy with PL and hence sensing. 6. Catalytic C-H interaction and passivating effect of carbon in SnO2:C alloy NPs in different reducing gas ambience.
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1. Introduction Oxide semiconductor materials in nanoparticle form have been extensively investigated because of their unique size dependent electronic, optical, and electrochemical properties [1], [2]. SnO2 is an n- type semiconductor with a direct optical band gap (3.8−4.3𝑒𝑉) and an indirect band gap (2.7−3.1𝑒𝑉), and has been widely used for gas sensing applications [3]. Tin oxide (SnOx) is generally available in two oxidation states, divalent (tin (II) oxide, SnO) and tetravalent (tin (IV) oxide, SnO2) with their behaviour characterised as p-type and n-type respectively [4]. Tin (II) oxide has attracted significant attention due to its native p-type conductivity and stability in both its structure and electronic properties. SnO2 being a surface sensitive material, high mobility of the charge carrier concentration results in a stronger change in electrical conductance of the material in the presence of gas molecules. SnO2 based sensors usually operate at temperatures above 200°C due to the activation energy required for reaction of analyte gases with O species [5- 6]. This poses an obvious limitation. One of the drawbacks of oxide resistive sensors is the poor selectivity to different gases having similar (reducing/oxidising) nature. Carbon nano-structures are known to be sensitive to charge transfer and chemical doping effects during interaction with H2 [78]. The resistance of sensors based on carbon nano-structures is observed to increase upon exposure to H2 gas as H2 molecules act as reducing agents having an e- donating nature. On exposure of these sensors to H2, electron transfer takes place from H2 molecules to carbon materials resulting in recombination of the electron-hole pairs, thereby increasing the electrical resistance [9- 13]. Gas sensing properties of SnO2:C with a combined effect due to carbon and metal oxide is thus an important area of investigation [7]. Though several studies on gas sensing viz. SnO2 coated CNTs, SnO2- filled CNTs and SnO2 doped CNTs have been reported [7-8,14-17, 18-21], no definitive reports on sensing of SnO2:C alloy structures to H2 and ethanol have been reported to the best of our knowledge. Modification of surface properties of SnOx by alloying [1], [22-29] with carbon is expected to alter the surface reactivity, electronic and optical properties [30]. In the present study, the gas sensing properties of size selected and well-sintered crystalline SnO2:C alloy nanoparticles have been investigated and compared with SnO2 nanoparticles grown under identical conditions. The effect of carbon incorporation on the sensing behaviour of SnO2:C nanoparticles has been studied. It is found that SnO2:C nanoparticles
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exhibit p-type behaviour in H2 gas with improved sensitivity as compared to SnO2 nanoparticles, where an opposite trend is observed. The increased sensitivity and selectivity of SnO2:C nanoparticles is explained in terms of the catalyst action of carbon and modified electronic properties of SnO2 and C on hydrogenation.
2. Experimental 2.1.
Preparation of SnO2 and SnO2: C nanoparticles
An integrated gas phase synthesis setup has been used to grow SnO2 and SnO2:C nano particles. As illustrated in Fig.1, the setup consists of an aerosol generator (GFG 1000 manufactured by PALAS Gmbh) for forming SnO2,and SnO2:C agglomerates, a neutraliser (Kr 85) for charging the agglomerates, a sintering furnace for converting the agglomerates into compact spherical particles (an
in-flight sintering process) and an electrostatic
precipitator (ESP) to deposit charged nanoparticles onto desired substrates. To produce SnO2 nanoparticles both the electrodes employed in the spark generator are of Sn, while for SnO2:C alloy nanoparticles, one of the electrodes is of Sn and the other of graphite. In addition, a 2.5 litre/min flow of O2 is deliberately introduced in the spark generator to oxidize tin. The growth process and the detailed conversion of the primary nano-agglomerates to wellsintered crystalline nanoparticles has been described in detail elsewhere (“Structural and photoluminescence properties of Tin oxide and Tin oxide: C core–shell and alloy nanoparticles synthesised using gas phase technique, by Bhatnagar, et.al., communicated, AIP Advances). Gas sensing properties of these well-sintered monocrystalline and sizeselected nanoparticles are studied in the present study. For sensor fabrication, a pattern of inter-digitated electrodes (IDEs) with finger width of 5 µm, finger length of 100 µm and a gap between the two fingers of 5 µm were made for carrying out gas sensing measurements. The optical image of the fabricated IDEs and the schematic showing the nanoparticles deposited on the IDEs is shown in Fig 2. 2.2.Structural and optical characterisation SnO2 and SnO2:C nanoparticles were deposited on the carbon coated Cu TEM grids for HRTEM analysis and on Si substrates for XRD measurements. The structural, morphological, and compositional properties were evaluated for the deposited nano particles as a function of alloy formation. HRTEM (FEI- Technai-G20 with a LaB6 filament, operated at 200kV) analysis of the deposited SnO2:C nanoparticles was done to study the size and structural
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properties. The structural properties of the deposited nano particles on Si substrate were analyzed by GAXRD (Philips X‟Pert, PRO-PW 3040, CUKa) at a glancing angle of 0.5°. The PL spectra of the as-prepared nanoparticle samples deposited on Si were recorded using a Horiba Scientific LabRAM HR evolution Raman spectrometer and a laser with excitation wavelength of 325 nm was used as the source. 2.3. Measurement of gas sensing properties Gas sensing properties were determined by measuring the changes in the electric resistance on repeatedly changing the gas environment between 2% H2 (balanced N2) (prepared apriory) and pure N2 with substrates (schematic of the device as shown in Fig 2) maintained at a constant temperature between RT and 200°C. The electrical resistance was measured using an electrometer (2000 multimeter, Keithley). The magnitude of the gas response (S) was defined as the ratio
, where Rn is the resistance in inert gas (N2) and Rh is
the resistance in the combustible gas (H2). Selectivity response with respect to ethanol was also measured.
3. Results and Discussion: 3.1.Structural properties For investigating the role of carbon incorporation in the gas sensing properties, SnO2 and SnO2:C nano particles prepared using the size selection based gas- phase method have been prepared in the present study. As described earlier, the nano- agglomerates produced in the spark generator are size selected by DMA and are converted to monocrystalline particles by in-flight sintering process. The effect of in-flight sintering temperature on the morphology and crystal structure of SnO2 and SnO2:C nanoparticles was investigated and optimised. These results are described elsewhere. The nanoparticle samples investigated in this study were prepared with an average electrical mobility equivalent diameter of 20 nm using a DMA voltage of 0.667 kV. Spherical and mono crystalline nanoparticles with an average size of ~ 20 nm are obtained on sintering at higher temperatures. Sintering temperature of about 1100°C was used to prepare SnO2 (sample SN) and SnO2:C alloy nanoparticles (sample SC). TEM images of SnO2 and SnO2:C nanoparticles samples sintered at 1100°C are shown in Fig. 3 (a) &(b) respectively. Fig 4 shows the results of XRD measurements on the nanoparticle samples deposited on Si substrates. The high intensity diffraction peak observed at 2θ =26.9° was noted to occur for
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both the samples (SN and SC samples), corresponding to the lattice plane {110} of the tetragonal phase of SnO2 (JCPDS data card no. 77-0452) with lattice parameters of a= 4.755 and c= 3.199. For SnO2 nanoparticles sintered at 1100°C (SN), an excellent match of the other XRD peaks is observed with the lattice planes of the SnO2 tetragonal phase. The peak positions for the SnO2 samples thus confirm the majority SnO2 phase. In the case of SC sample, an alloy formation is observed between SnO2 and C. Tetragonal phase of the alloy structure is concluded using the PANalytical X‟Pert High Score Plus software with lattice constants: a= 4.732, c= 3.186, very close to the tetragonal phase of SnO2. The average crystallite size calculated using the Scherrer equation for SnO2:C nanoparticles (sample SC) ~ 20 nm, corresponds very well with the size estimated from the HRTEM images. 3.2. Gas – sensing properties SN and SC samples were deposited on inter-digitated electrodes for sensing measurements. The chemi-resistive gas sensing response of oxide material is characterised by % change in resistance on exposure to a particular gas. For studying the sensing properties, the sensing response (S) is plotted as a function of time during gas ON and OFF cycles. Gas sensing response of SN and SC samples to 2% H2 and ethanol at a sensing temperature of 50°C are shown in Fig. 5 (a) and (b) respectively. As shown in these response transients, the samples respond reversibly to both H2 and ethanol gases over a number of gas ON and OFF cycles indicating the repeatability of the sensor response. A trend of decrease in the resistance on exposing the SnO2 samples (SN) to gas with 2% concentration of H2 and ethanol can be seen. A sensing response of 6% and 11% sensitivity has been observed for both H2 and ethanol gases, respectively. This behaviour is typical of an n-type semiconductor. On the other hand, for sample SC, a completely opposite sensing behaviour is observed. An increase in resistance with 5% sensitivity on exposure to H2 and a decrease in resistance for the same sensor with 6% sensitivity is noticed for ethanol under identical measurement conditions. The difference in the sensitivity behaviour of SN and SC samples, highlight the effect of incorporation of carbon during alloy formation.
3.2. Discussion In sample SN, surface of SnO2 nanoparticles is generally covered with adsorbed oxygen ions, which take e- s from n-type SnO2. These negatively charged O- ions generate an electrical field at the surface causing band-bending of the energy bands in SnO2 [19]. A –ve
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surface charge with upward band-bending, results in an electron depletion layer just below the surface due to the following reaction: O2 + 2 e- 2 O- ----- (i) The surface interaction between the adsorbed O species and the target gas (in the present case, H2 or ethanol) is what governs the sensing mechanism. It is reported that a reducing molecular gas adsorbed at the surface can either accept or donate e- s, depending on whether the LUMO /HOMO of the adsorbate lies above/below the Fermi energy level of SnO2 [19]. On exposure to H2, surface O species are consumed; thereby electrons are released back to the oxide, resulting in an increase in the electrical conduction as per the following reaction: H2 + O- H2O + e- ----- (ii) The proposed mechanism is elaborated in Fig 6(a). A comparison between the sensing response of the sintered SnO2 nanoparticles samples (SN) to H2 and ethanol as a function of sensing temperature is shown in Fig 7(a). A higher sensitivity of SnO2 nanoparticles towards ethanol in comparison to H2 is clearly observed even at lower temperatures of 50°C. Similar results for SnO2 nanoparticles have also been reported at measurement temperature of 220°C [19]. Furthermore, in the presence of ethanol, electrons are released back to the conduction band of SnO2 by either / both of the two processes at elevated temperatures [18] Process 1:
C2H5OH + 2 O2- 2 C2H4O- + O2 + 2 H2O + 2eC2H4O- (unstable) CH3CHO + e2 CH3CHO + 5 O2- 4 CO2 + 4 H2O + 10 e-
Process 2:
C2H5OH C2H4 + H2O C2H4 + 3 O22- 2CO2 + 2 H2O + 6 e-
The formation of the electron depleted region within the entire volume of the nano-sized crystals is known to enhance the sensing response. For SnO2:C alloy nanoparticles, the sensing towards ethanol, is directly due to the removal of the O- species similar to SnO2 for reducing gases [31]. On nanoparticle samples, a catalytic effect of carbon, facilitating the interaction between the analyte gases and the adsorbed oxygen along with spill-over effect resulting in dissociation of the analyte is quite possible [5,7,19]. Therefore, there are two sensing mechanisms involved in SnO2:C nanoparticles (sample SC). The first mechanism is similar to that observed in SnO2 nanoparticles (sample SN) in which
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the adsorbed oxygen species are removed by the reducing action of the sensing gas (H2 and ethanol). It is possible that the catalytic action of C may affect or enhance this sensing mechanism, due to any electron exchange between SnO2 and C. The work function values of pristine SnO2 and C are similar indicating no significant changes in the electronic properties of SnO2:C nanoparticles in comparison to SnO2. A decrease in the work function of SnO2 has been suggested on exposure to H2 environment [5]. This modification of SnO2 work function could result in modified electronic properties of SnO2:C. This becomes a second sensing route for H2 in which H species interacting with SnO2 and carbon, which initiates electron exchange due to modified work function. This is similar to the claim made by Lu, et al. [5]. Change in the relative work functions of SnO2 and C result in change over of n-type nature to p-type. Release of e-s due to the twin effects compensates holes in p-SnO2 thereby increasing the resistance. The increase in resistance of SC sample is less than the decrease in resistance of SN sample on exposure to H2. Thus, the response of SnO2:C alloy nanoparticles towards H2 in comparison to SnO2 nanoparticles is attributed to a combined effect of C-H interaction and conversion of n-type SnO2:C nanoparticles to p-type SnO2:C nanoparticles. It has also been reported that the electronic properties of carbon nano-structures are known to change dramatically in response to various molecular adsorbates at RT [5]. The resistance of sensors based on carbon materials (CNTs and carbon nanoparticles) has been reported to increase upon exposure to H2 gas as H2 molecules act as reducing agents having an e- donating nature. On exposure of these sensors to H2, electron transfer takes place from H2 molecules to carbon materials resulting in modification of the electronic properties of C [9-13] further accelerated by the low binding energy between H2 molecule and the outer surface of C promoting splitting to H atoms, as mentioned earlier. Thus, H2 interacts at carbon surface and gets converted to atomic H which interacts with O2 adsorbates in SnO2 even at lower temperatures. Similar transitions from p to n and n to p in gas sensing have also been reported for other oxides (iron oxides and their composites) [32, 33].
Surface
modification of SnO2 nanoparticles on exposure to reducing gases leading to improved selectivity has also been explained by Dai, et.al. based on Janox redox reaction. Change in carrier concentration type on surface modification due to exposure to certain reducing gases is also a plausible mechanism suggested for such switchiing transitions [34]. Oxygen vacancies and defects are also reported to play a key role in preferable gas adsorption [35]. Thus an interplay of defects (O- vacancies) and surface modification of the nanoparticles plays a pivotal role in understanding the gas sensing mechanism of oxides.
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For ethanol, the sensing behaviour remains unaltered due to the lack of C- ethanol interaction. The improvement of selectivity in the SnO2:C alloy nanoparticles is clearly observed even at temperatures as low as 50°C as given in Fig 7. It may be noted that the sensitivity values for ethanol are slightly lowered in SnO2:C (for example, 6% at 50°C), in comparison to SnO2 (11% at 50°C). This may be due to the decrease in the SnO2 sites due to the presence of C in SnO2:C nanoparticles. The ability of a reacting species to overcome a free energy barrier pertaining to a reaction is commonly modelled by the Arhenius equation. For most of the chemical reactions, the Arhenius equation gives the dependence of rate, K (
) of a chemical reaction on the
absolute temperature, T (iii) [32-33]. K= A𝑒 ln K = ln A – Ea/kT
.... (iii)
with Rg being the resistance of the sensor in the particular analyte gas and T being the temperature in Kelvin, a plot between ln(1/Rg) with (1/T) is shown in Fig 8. The Arhenius plots for samples SN towards both H2 and ethanol give a similar value of activation energy (Ea) of 0.53 eV. Different values of 0.52 eV for ethanol and 0.37 eV for H2 is observed in the case of sample SC. The similarity in the values of Ea in case of H2 for sample SN and ethanol gas for sample SN and SC, suggests a common operational sensing mechanism involving O-vacancies. However, the drastically different Ea values for sample SC towards H2 confirms that a different controlling mechanism is involved. The activation energy of sample SC (0.37 eV) in H2 is lower than that observed for sample SN (0.53 eV). Lower activation energy seems to be related to the mechanism of electron exchange between SnO2 and C in the hydrogen environment [9-13] modifying its electronic nature to p-type. One of the novelties of the present work is the enhanced selectivity for reducing gases namely H2 and ethanol observable at temperatures as low as 50° C due to SnO2:C alloy formation. The role of O- vacancies in SnO2 nanoparticles in determining the sensing response to H2/ ethanol can be further understood by comparing the PL properties of SN and SC. Strong luminescence observed in nano-SnO2 has been attributed to structural defects, Sn interstitials, and O- vacancies near surface region [1],[14]. PL emission spectra of NP samples with 320 nm excitation are shown
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in Fig 9. A detailed study of the PL properties of SnO2 and SnO2:C nanoparticles has been discussed elsewhere (“Structural and photoluminescence properties of Tin oxide and Tin oxide: C core–shell and alloy nanoparticles synthesised using gas phase technique, by Bhatnagar, et.al., communicated, AIP Advances).The peak observed at „a1‟: 416.7 (2.98 eV) in case of sintered sample SN, is quite close to the band-edge emission of SnO2 NPs. While the emission peak „a2‟ centered at 511.3 nm (2.4 eV) in case of SN has been attributed to oxygen vacancies [14], minor intensity peak „a3‟ has been attributed to Sn interstitials [1, 3435]. For SC sample, the particularly broad intensity peak „b2‟ in SnO2:C nanoparticles can be de-convoluted into three peaks: b1, b2 and b3 as shown in the Fig. While the origin of „b1‟and „b2‟ is similar to that of sample SN, as discussed above, new peak „b3‟ seems to be a characteristic of sample SC. The large intensity in the case of sample SC is attributed to the effect of carbon acting as a reducing agent during SnO2:C alloy formation thereby increasing the number of oxygen vacancies. Moreover, the observed response of the SnO2:C alloy nanoparticle samples at lower temperatures can be attributed to the enhanced density of defects and oxygen vacancies, indicated by the significant change in the intensity of the PL peak „-2‟ in SN („a2‟) and SC sample („b2‟) (Fig 9) , an indicator of defects and oxygen vacancies. These changes in oxygen vacancies in case of SnO2:C on sintering are very large due to the reducing effects of carbon. It can also be noted that the value of activation energy ~0.5 eV observed in sample SN (for H2 and ethanol) and sample SC (for ethanol), bears a direct correspondence to the energy difference between the conduction band (2.98 eV) and the position of the defect levels (O- vacancies) (2.42 eV) as revealed by the PL spectra. As mentioned earlier, the presence of carbon during synthesis has a reducing effect, increasing the relative concentration of O-vacancies. The ratio of intensities of PL peaks due to Ovacancies w.r.t. to band-edge emission for sample SC (b2:b1) is observed to be 13.7, in comparison to the intensity ratio of 5.8 for these peaks observed in sample SN (a2:a1). It is interesting to note that the sensitivity of SN towards ethanol has significantly reduced despite the increase in O-vacancies. A decrease from 11% in sample SN to 6% in sample SC has been observed. This shows that the presence of carbon sites, which are unreactive towards ethanol may have a passivating effect on SnO2- ethanol interaction, in addition to the relative decrease in SnO2 sites. It is also noted that, although a new defect peak related to the alloy is observed in SC sample, gas sensing is not related to this peak as Sn interstitals do not take part in the gas sensing mechanism.
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4. Conclusion The results of the present study clearly show that the incorporation of carbon in SnO2 nano particles has a large impact on the gas sensing behaviour. SnO2 nanoparticles show similar sensing response, whereas SnO2:C nanoparticles show opposite sensing behaviour for H2 and ethanol. The catalytic properties of carbon in SnO2:C, and modification in the relative work function values of SnO2 and C in H2 ambient, are responsible for the change over from ntype (SnO2) to p-type (SnO2:C alloy nanoparticles). Sensing behaviour of SnO2:C nanoparticles in ethanol remains unaffected due to lack of C – ethanol interaction, although a decrease in resistance is observed. Similar values of activation energies (0.52 eV) are observed for H2 and ethanol in SnO2 nanoparticles and for ethanol in SnO2:C nanoparticles. Whereas, the activation energy value in case of SnO2:C nanoparticles for H2 is observed to be lower (0.37 eV). The value of energy levels for O-vacancies in SnO2 and SnO2:C obtained from PL (0.5-0.6 eV) support the proposed mechanism. On incorporation of carbon, a selective response of SnO2:C alloy nanoparticles thereby resulting in selective response of SnO2 particles to H2 and ethanol gases, normally not observed in oxide based resistive sensors. Acknowledgements MB would like to acknowledge the financial support from DST INSPIRE (IF130231) for Senior Research Fellowship. BRM would like to acknowledge the support from DRDO project (RP02781).
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Figures
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List of figures: Fig.1 Schematic illustrating synthesis of SnO2 and SnO2:C nanoparticles prepared using the Integrated gas phase synthesis setup. Filled green circles denote SnO2 nucleates and open red circles represent C nucleates in samples SN1 and SC1. Shaded circles represent well sintered SnO2:C alloy nanoparticles (sample SC). Fig.2 (a) Optical image of the IDE pattern for the fabricated sensor device (b) TEM of the nanoparticles deposited (c) Schematic of the deposited nanoparticles on the IDE device fabricated. Fig.3 HRTEM micrographs of well-sintered monocrystalline spherical (a) SnO2 (sample SN) and (b) SnO2:C alloy nanoparticles(SC). Fig.4 XRD plot of SN and SC samples. (*) denotes peaks corresponding to SnO2 cassiterite – tetragonal. (.) denotes peaks corresponding to graphite. (#) denotes alloy peaks. Tetragonal structure symmetry of SnO2 is still retained in the alloy in sample SC. The broad peak (~) in samples SC is due to the overlap of peaks due to Si substrate and shifted SnO2 and C peaks on alloy formation. Fig.5 (a) Sensing response of SnO2 nanoparticles (SN) and SnO2:C alloy nanoparticles (SC) measured at 50°C on exposure to 2% H2. Solid and dotted lines represent time at which H2 gas was switched ON and OFF, respectively. (b) Sensing response of SnO2 nanoparticles (SN) and SnO2:C alloy nanoparticles (SC) measured at 50°C on exposure to ethanol. Solid and dotted line represent time at which ethanol gas was switched ON and OFF, respectively. Fig.6 Schematic illustrating (a) the n- type response of SnO2 nanoparticles( SN) on exposure to analyte gases H2 / ethanol resulting in a negative sensing response (b) response of SC on exposure to H2 and thechange-over from n- to p- type in H2 ambient, +ve sensing response and (c) n- type response of SC on exposure to ethanol. The catalytic role of carbon in H2 gas and passivating effect in ethanol is also shown. Fig.7 Comparison between the sensing response of (a) SN and (b) SC samples to H2 and ethanol as a function of sensing temperature.
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Fig.8 Arrhenius plots of SN and SC samples on exposure to (a) H2 and (b) ethanol highlighting the activation energy and hence the different sensing mechanisms involved. Fig.9 Comparison drawn between the PL spectra of samples SN and SC. With peak „-1: a1 & b1‟ reported to be quite close to the band edge, peak „-2: a2 & b2‟ attributed to oxygen vacancies and peak „a3‟ to Sn interstitials, a new peak „b3‟ seems to be characteristic of the SnO2:C alloy formed. Broad peak “-2” of sample from 416 nm to 747 nm with high intensity in SC is shown in the form of de-convoluted peaks b1,b2 and b3.This peak broadening and increased intensity is attributed to an increase in the number of oxygen vacancies on addition of carbon and sintering.
21 Author Biographies :
Mehar Bhatnagar : A senior research fellow (PhD student) at Indian Institute of Technology Delhi, Working on core shell and alloy semiconductor- metal nanoparticles for gas sensing, plasmonic and hot carrier solar cell applications. Shivani Dhall : A National Fellow for Post Doctoral Research at Indian Institute of Technology Delhi, Working on gas sensing of oxide nanostructures and thin films. Vishakha Kaushik: PhD student at Indian Institute of Technology Delhi. Working on Raman and PL studies of MoS2 thin film devices. Akshey Kaushal: Project scientist working on HRTEM. Bodh Raj Mehta : Schlumberger Chair Professor and Dean of Research and Development at Indian Institute of Technology Delhi. Academic Interests include Physics and Technology of Nanostructured Materials and Thin Films ( Rare-earth Metal and Palladium Nanoparticles based Sensing and Switching Devices, Nanostructured Solar Cell, Resistive Memory and Thermoelectric Materials and Devices , Size-Selected Growth of Metal and Semiconductor Nanoparticles, Organic-Inorganic Semiconductor Interfaces, Scanning Probe Microscopy and High Resolution Transmission Electron Microscopy
S0925-4005(17)30142-9 http://dx.doi.org/doi:10.1016/j.snb.2017.01.135 SNB 21660
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
24-8-2016 11-1-2017 21-1-2017
Please cite this article as: Mehar Bhatnagar, Shivani Dhall, Vishakha Kaushik, Akshey Kaushal, Bodh Raj Mehta, Improved selectivity of SnO2:C alloy nanoparticles towards H2 and ethanol reducing gases; role of SnO2:C electronic interaction, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2017.01.135 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Improved selectivity of SnO2:C alloy nanoparticles towards H2 and ethanol reducing gases; role of SnO2:C electronic interaction Mehar Bhatnagar, Shivani Dhall, Vishakha Kaushik, Akshey Kaushal, Bodh Raj Mehta* Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi- 110016, India
Abstract In the present study, changes in the sensing properties of SnO2 on Carbon incorporation have been investigated in detail. The gas sensing response of size-selected SnO2 and SnO2:C alloy nanoparticles prepared by gas phase deposition method have been investigated for H2 and ethanol over a varied temperature range (50°C- 200°C). The incorporation of carbon into SnO2 lattice results in a large change in the sensing behaviour towards the two gases both having reducing nature. SnO2:C nanoparticles show positive sensing response for H2 and negative sensing response for ethanol, whereas SnO2 nanoparticles show a normal sensing response of an n-type semiconductor towards both the reducing gases. Observed values of activation energy of sensing and energy levels of O-vacancies observed in the PL spectra of SnO2 and SnO2:C are consistent with these results. (i) Catalytic C-H interaction and (ii) modified work function of SnO2 and C on hydrogenation resulting in alteration of electronic exchange between SnO2 and C, and (iii) passivation effect of carbon during SnO2-ethanol interaction along with a possibility of reduction in SnO2 sites in SnO2:C nanoparticles, are responsible for the observed behaviour. The present study shows that the incorporation of C in SnO2 nanoparticles results in excellent selectivity towards H2 and ethanol (both having reducing nature) in the low temperature range, normally not observed in oxide based resistive sensors.
Key words: size-selected SnO2:C alloy nanoparticles, PL, selectivity, reducing gases, catalytic effect
* Author to whom all correspondence must be addressed. Electronic mail: [email protected]
Highlights: 1. Gas sensing properties of size-selected SnO2 and SnO2:C alloy nanoparticles over a wide temperature range (50°C- 200°C). 2. Effect of incorporation of carbon in the alloy nanoparticles towards selectivity between two gases both having reducing nature (H2 and ethanol) at low temperatures. 3. Change over from n-type to p-type behaviour in H2 ambience. 4. Retention of n-type behaviour in ethanol ambience. 5. Correspondence between energy values from activation energy with PL and hence sensing. 6. Catalytic C-H interaction and passivating effect of carbon in SnO2:C alloy NPs in different reducing gas ambience.
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1. Introduction Oxide semiconductor materials in nanoparticle form have been extensively investigated because of their unique size dependent electronic, optical, and electrochemical properties [1], [2]. SnO2 is an n- type semiconductor with a direct optical band gap (3.8−4.3𝑒𝑉) and an indirect band gap (2.7−3.1𝑒𝑉), and has been widely used for gas sensing applications [3]. Tin oxide (SnOx) is generally available in two oxidation states, divalent (tin (II) oxide, SnO) and tetravalent (tin (IV) oxide, SnO2) with their behaviour characterised as p-type and n-type respectively [4]. Tin (II) oxide has attracted significant attention due to its native p-type conductivity and stability in both its structure and electronic properties. SnO2 being a surface sensitive material, high mobility of the charge carrier concentration results in a stronger change in electrical conductance of the material in the presence of gas molecules. SnO2 based sensors usually operate at temperatures above 200°C due to the activation energy required for reaction of analyte gases with O species [5- 6]. This poses an obvious limitation. One of the drawbacks of oxide resistive sensors is the poor selectivity to different gases having similar (reducing/oxidising) nature. Carbon nano-structures are known to be sensitive to charge transfer and chemical doping effects during interaction with H2 [78]. The resistance of sensors based on carbon nano-structures is observed to increase upon exposure to H2 gas as H2 molecules act as reducing agents having an e- donating nature. On exposure of these sensors to H2, electron transfer takes place from H2 molecules to carbon materials resulting in recombination of the electron-hole pairs, thereby increasing the electrical resistance [9- 13]. Gas sensing properties of SnO2:C with a combined effect due to carbon and metal oxide is thus an important area of investigation [7]. Though several studies on gas sensing viz. SnO2 coated CNTs, SnO2- filled CNTs and SnO2 doped CNTs have been reported [7-8,14-17, 18-21], no definitive reports on sensing of SnO2:C alloy structures to H2 and ethanol have been reported to the best of our knowledge. Modification of surface properties of SnOx by alloying [1], [22-29] with carbon is expected to alter the surface reactivity, electronic and optical properties [30]. In the present study, the gas sensing properties of size selected and well-sintered crystalline SnO2:C alloy nanoparticles have been investigated and compared with SnO2 nanoparticles grown under identical conditions. The effect of carbon incorporation on the sensing behaviour of SnO2:C nanoparticles has been studied. It is found that SnO2:C nanoparticles
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exhibit p-type behaviour in H2 gas with improved sensitivity as compared to SnO2 nanoparticles, where an opposite trend is observed. The increased sensitivity and selectivity of SnO2:C nanoparticles is explained in terms of the catalyst action of carbon and modified electronic properties of SnO2 and C on hydrogenation.
2. Experimental 2.1.
Preparation of SnO2 and SnO2: C nanoparticles
An integrated gas phase synthesis setup has been used to grow SnO2 and SnO2:C nano particles. As illustrated in Fig.1, the setup consists of an aerosol generator (GFG 1000 manufactured by PALAS Gmbh) for forming SnO2,and SnO2:C agglomerates, a neutraliser (Kr 85) for charging the agglomerates, a sintering furnace for converting the agglomerates into compact spherical particles (an
in-flight sintering process) and an electrostatic
precipitator (ESP) to deposit charged nanoparticles onto desired substrates. To produce SnO2 nanoparticles both the electrodes employed in the spark generator are of Sn, while for SnO2:C alloy nanoparticles, one of the electrodes is of Sn and the other of graphite. In addition, a 2.5 litre/min flow of O2 is deliberately introduced in the spark generator to oxidize tin. The growth process and the detailed conversion of the primary nano-agglomerates to wellsintered crystalline nanoparticles has been described in detail elsewhere (“Structural and photoluminescence properties of Tin oxide and Tin oxide: C core–shell and alloy nanoparticles synthesised using gas phase technique, by Bhatnagar, et.al., communicated, AIP Advances). Gas sensing properties of these well-sintered monocrystalline and sizeselected nanoparticles are studied in the present study. For sensor fabrication, a pattern of inter-digitated electrodes (IDEs) with finger width of 5 µm, finger length of 100 µm and a gap between the two fingers of 5 µm were made for carrying out gas sensing measurements. The optical image of the fabricated IDEs and the schematic showing the nanoparticles deposited on the IDEs is shown in Fig 2. 2.2.Structural and optical characterisation SnO2 and SnO2:C nanoparticles were deposited on the carbon coated Cu TEM grids for HRTEM analysis and on Si substrates for XRD measurements. The structural, morphological, and compositional properties were evaluated for the deposited nano particles as a function of alloy formation. HRTEM (FEI- Technai-G20 with a LaB6 filament, operated at 200kV) analysis of the deposited SnO2:C nanoparticles was done to study the size and structural
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properties. The structural properties of the deposited nano particles on Si substrate were analyzed by GAXRD (Philips X‟Pert, PRO-PW 3040, CUKa) at a glancing angle of 0.5°. The PL spectra of the as-prepared nanoparticle samples deposited on Si were recorded using a Horiba Scientific LabRAM HR evolution Raman spectrometer and a laser with excitation wavelength of 325 nm was used as the source. 2.3. Measurement of gas sensing properties Gas sensing properties were determined by measuring the changes in the electric resistance on repeatedly changing the gas environment between 2% H2 (balanced N2) (prepared apriory) and pure N2 with substrates (schematic of the device as shown in Fig 2) maintained at a constant temperature between RT and 200°C. The electrical resistance was measured using an electrometer (2000 multimeter, Keithley). The magnitude of the gas response (S) was defined as the ratio
, where Rn is the resistance in inert gas (N2) and Rh is
the resistance in the combustible gas (H2). Selectivity response with respect to ethanol was also measured.
3. Results and Discussion: 3.1.Structural properties For investigating the role of carbon incorporation in the gas sensing properties, SnO2 and SnO2:C nano particles prepared using the size selection based gas- phase method have been prepared in the present study. As described earlier, the nano- agglomerates produced in the spark generator are size selected by DMA and are converted to monocrystalline particles by in-flight sintering process. The effect of in-flight sintering temperature on the morphology and crystal structure of SnO2 and SnO2:C nanoparticles was investigated and optimised. These results are described elsewhere. The nanoparticle samples investigated in this study were prepared with an average electrical mobility equivalent diameter of 20 nm using a DMA voltage of 0.667 kV. Spherical and mono crystalline nanoparticles with an average size of ~ 20 nm are obtained on sintering at higher temperatures. Sintering temperature of about 1100°C was used to prepare SnO2 (sample SN) and SnO2:C alloy nanoparticles (sample SC). TEM images of SnO2 and SnO2:C nanoparticles samples sintered at 1100°C are shown in Fig. 3 (a) &(b) respectively. Fig 4 shows the results of XRD measurements on the nanoparticle samples deposited on Si substrates. The high intensity diffraction peak observed at 2θ =26.9° was noted to occur for
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both the samples (SN and SC samples), corresponding to the lattice plane {110} of the tetragonal phase of SnO2 (JCPDS data card no. 77-0452) with lattice parameters of a= 4.755 and c= 3.199. For SnO2 nanoparticles sintered at 1100°C (SN), an excellent match of the other XRD peaks is observed with the lattice planes of the SnO2 tetragonal phase. The peak positions for the SnO2 samples thus confirm the majority SnO2 phase. In the case of SC sample, an alloy formation is observed between SnO2 and C. Tetragonal phase of the alloy structure is concluded using the PANalytical X‟Pert High Score Plus software with lattice constants: a= 4.732, c= 3.186, very close to the tetragonal phase of SnO2. The average crystallite size calculated using the Scherrer equation for SnO2:C nanoparticles (sample SC) ~ 20 nm, corresponds very well with the size estimated from the HRTEM images. 3.2. Gas – sensing properties SN and SC samples were deposited on inter-digitated electrodes for sensing measurements. The chemi-resistive gas sensing response of oxide material is characterised by % change in resistance on exposure to a particular gas. For studying the sensing properties, the sensing response (S) is plotted as a function of time during gas ON and OFF cycles. Gas sensing response of SN and SC samples to 2% H2 and ethanol at a sensing temperature of 50°C are shown in Fig. 5 (a) and (b) respectively. As shown in these response transients, the samples respond reversibly to both H2 and ethanol gases over a number of gas ON and OFF cycles indicating the repeatability of the sensor response. A trend of decrease in the resistance on exposing the SnO2 samples (SN) to gas with 2% concentration of H2 and ethanol can be seen. A sensing response of 6% and 11% sensitivity has been observed for both H2 and ethanol gases, respectively. This behaviour is typical of an n-type semiconductor. On the other hand, for sample SC, a completely opposite sensing behaviour is observed. An increase in resistance with 5% sensitivity on exposure to H2 and a decrease in resistance for the same sensor with 6% sensitivity is noticed for ethanol under identical measurement conditions. The difference in the sensitivity behaviour of SN and SC samples, highlight the effect of incorporation of carbon during alloy formation.
3.2. Discussion In sample SN, surface of SnO2 nanoparticles is generally covered with adsorbed oxygen ions, which take e- s from n-type SnO2. These negatively charged O- ions generate an electrical field at the surface causing band-bending of the energy bands in SnO2 [19]. A –ve
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surface charge with upward band-bending, results in an electron depletion layer just below the surface due to the following reaction: O2 + 2 e- 2 O- ----- (i) The surface interaction between the adsorbed O species and the target gas (in the present case, H2 or ethanol) is what governs the sensing mechanism. It is reported that a reducing molecular gas adsorbed at the surface can either accept or donate e- s, depending on whether the LUMO /HOMO of the adsorbate lies above/below the Fermi energy level of SnO2 [19]. On exposure to H2, surface O species are consumed; thereby electrons are released back to the oxide, resulting in an increase in the electrical conduction as per the following reaction: H2 + O- H2O + e- ----- (ii) The proposed mechanism is elaborated in Fig 6(a). A comparison between the sensing response of the sintered SnO2 nanoparticles samples (SN) to H2 and ethanol as a function of sensing temperature is shown in Fig 7(a). A higher sensitivity of SnO2 nanoparticles towards ethanol in comparison to H2 is clearly observed even at lower temperatures of 50°C. Similar results for SnO2 nanoparticles have also been reported at measurement temperature of 220°C [19]. Furthermore, in the presence of ethanol, electrons are released back to the conduction band of SnO2 by either / both of the two processes at elevated temperatures [18] Process 1:
C2H5OH + 2 O2- 2 C2H4O- + O2 + 2 H2O + 2eC2H4O- (unstable) CH3CHO + e2 CH3CHO + 5 O2- 4 CO2 + 4 H2O + 10 e-
Process 2:
C2H5OH C2H4 + H2O C2H4 + 3 O22- 2CO2 + 2 H2O + 6 e-
The formation of the electron depleted region within the entire volume of the nano-sized crystals is known to enhance the sensing response. For SnO2:C alloy nanoparticles, the sensing towards ethanol, is directly due to the removal of the O- species similar to SnO2 for reducing gases [31]. On nanoparticle samples, a catalytic effect of carbon, facilitating the interaction between the analyte gases and the adsorbed oxygen along with spill-over effect resulting in dissociation of the analyte is quite possible [5,7,19]. Therefore, there are two sensing mechanisms involved in SnO2:C nanoparticles (sample SC). The first mechanism is similar to that observed in SnO2 nanoparticles (sample SN) in which
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the adsorbed oxygen species are removed by the reducing action of the sensing gas (H2 and ethanol). It is possible that the catalytic action of C may affect or enhance this sensing mechanism, due to any electron exchange between SnO2 and C. The work function values of pristine SnO2 and C are similar indicating no significant changes in the electronic properties of SnO2:C nanoparticles in comparison to SnO2. A decrease in the work function of SnO2 has been suggested on exposure to H2 environment [5]. This modification of SnO2 work function could result in modified electronic properties of SnO2:C. This becomes a second sensing route for H2 in which H species interacting with SnO2 and carbon, which initiates electron exchange due to modified work function. This is similar to the claim made by Lu, et al. [5]. Change in the relative work functions of SnO2 and C result in change over of n-type nature to p-type. Release of e-s due to the twin effects compensates holes in p-SnO2 thereby increasing the resistance. The increase in resistance of SC sample is less than the decrease in resistance of SN sample on exposure to H2. Thus, the response of SnO2:C alloy nanoparticles towards H2 in comparison to SnO2 nanoparticles is attributed to a combined effect of C-H interaction and conversion of n-type SnO2:C nanoparticles to p-type SnO2:C nanoparticles. It has also been reported that the electronic properties of carbon nano-structures are known to change dramatically in response to various molecular adsorbates at RT [5]. The resistance of sensors based on carbon materials (CNTs and carbon nanoparticles) has been reported to increase upon exposure to H2 gas as H2 molecules act as reducing agents having an e- donating nature. On exposure of these sensors to H2, electron transfer takes place from H2 molecules to carbon materials resulting in modification of the electronic properties of C [9-13] further accelerated by the low binding energy between H2 molecule and the outer surface of C promoting splitting to H atoms, as mentioned earlier. Thus, H2 interacts at carbon surface and gets converted to atomic H which interacts with O2 adsorbates in SnO2 even at lower temperatures. Similar transitions from p to n and n to p in gas sensing have also been reported for other oxides (iron oxides and their composites) [32, 33].
Surface
modification of SnO2 nanoparticles on exposure to reducing gases leading to improved selectivity has also been explained by Dai, et.al. based on Janox redox reaction. Change in carrier concentration type on surface modification due to exposure to certain reducing gases is also a plausible mechanism suggested for such switchiing transitions [34]. Oxygen vacancies and defects are also reported to play a key role in preferable gas adsorption [35]. Thus an interplay of defects (O- vacancies) and surface modification of the nanoparticles plays a pivotal role in understanding the gas sensing mechanism of oxides.
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For ethanol, the sensing behaviour remains unaltered due to the lack of C- ethanol interaction. The improvement of selectivity in the SnO2:C alloy nanoparticles is clearly observed even at temperatures as low as 50°C as given in Fig 7. It may be noted that the sensitivity values for ethanol are slightly lowered in SnO2:C (for example, 6% at 50°C), in comparison to SnO2 (11% at 50°C). This may be due to the decrease in the SnO2 sites due to the presence of C in SnO2:C nanoparticles. The ability of a reacting species to overcome a free energy barrier pertaining to a reaction is commonly modelled by the Arhenius equation. For most of the chemical reactions, the Arhenius equation gives the dependence of rate, K (
) of a chemical reaction on the
absolute temperature, T (iii) [32-33]. K= A𝑒 ln K = ln A – Ea/kT
.... (iii)
with Rg being the resistance of the sensor in the particular analyte gas and T being the temperature in Kelvin, a plot between ln(1/Rg) with (1/T) is shown in Fig 8. The Arhenius plots for samples SN towards both H2 and ethanol give a similar value of activation energy (Ea) of 0.53 eV. Different values of 0.52 eV for ethanol and 0.37 eV for H2 is observed in the case of sample SC. The similarity in the values of Ea in case of H2 for sample SN and ethanol gas for sample SN and SC, suggests a common operational sensing mechanism involving O-vacancies. However, the drastically different Ea values for sample SC towards H2 confirms that a different controlling mechanism is involved. The activation energy of sample SC (0.37 eV) in H2 is lower than that observed for sample SN (0.53 eV). Lower activation energy seems to be related to the mechanism of electron exchange between SnO2 and C in the hydrogen environment [9-13] modifying its electronic nature to p-type. One of the novelties of the present work is the enhanced selectivity for reducing gases namely H2 and ethanol observable at temperatures as low as 50° C due to SnO2:C alloy formation. The role of O- vacancies in SnO2 nanoparticles in determining the sensing response to H2/ ethanol can be further understood by comparing the PL properties of SN and SC. Strong luminescence observed in nano-SnO2 has been attributed to structural defects, Sn interstitials, and O- vacancies near surface region [1],[14]. PL emission spectra of NP samples with 320 nm excitation are shown
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in Fig 9. A detailed study of the PL properties of SnO2 and SnO2:C nanoparticles has been discussed elsewhere (“Structural and photoluminescence properties of Tin oxide and Tin oxide: C core–shell and alloy nanoparticles synthesised using gas phase technique, by Bhatnagar, et.al., communicated, AIP Advances).The peak observed at „a1‟: 416.7 (2.98 eV) in case of sintered sample SN, is quite close to the band-edge emission of SnO2 NPs. While the emission peak „a2‟ centered at 511.3 nm (2.4 eV) in case of SN has been attributed to oxygen vacancies [14], minor intensity peak „a3‟ has been attributed to Sn interstitials [1, 3435]. For SC sample, the particularly broad intensity peak „b2‟ in SnO2:C nanoparticles can be de-convoluted into three peaks: b1, b2 and b3 as shown in the Fig. While the origin of „b1‟and „b2‟ is similar to that of sample SN, as discussed above, new peak „b3‟ seems to be a characteristic of sample SC. The large intensity in the case of sample SC is attributed to the effect of carbon acting as a reducing agent during SnO2:C alloy formation thereby increasing the number of oxygen vacancies. Moreover, the observed response of the SnO2:C alloy nanoparticle samples at lower temperatures can be attributed to the enhanced density of defects and oxygen vacancies, indicated by the significant change in the intensity of the PL peak „-2‟ in SN („a2‟) and SC sample („b2‟) (Fig 9) , an indicator of defects and oxygen vacancies. These changes in oxygen vacancies in case of SnO2:C on sintering are very large due to the reducing effects of carbon. It can also be noted that the value of activation energy ~0.5 eV observed in sample SN (for H2 and ethanol) and sample SC (for ethanol), bears a direct correspondence to the energy difference between the conduction band (2.98 eV) and the position of the defect levels (O- vacancies) (2.42 eV) as revealed by the PL spectra. As mentioned earlier, the presence of carbon during synthesis has a reducing effect, increasing the relative concentration of O-vacancies. The ratio of intensities of PL peaks due to Ovacancies w.r.t. to band-edge emission for sample SC (b2:b1) is observed to be 13.7, in comparison to the intensity ratio of 5.8 for these peaks observed in sample SN (a2:a1). It is interesting to note that the sensitivity of SN towards ethanol has significantly reduced despite the increase in O-vacancies. A decrease from 11% in sample SN to 6% in sample SC has been observed. This shows that the presence of carbon sites, which are unreactive towards ethanol may have a passivating effect on SnO2- ethanol interaction, in addition to the relative decrease in SnO2 sites. It is also noted that, although a new defect peak related to the alloy is observed in SC sample, gas sensing is not related to this peak as Sn interstitals do not take part in the gas sensing mechanism.
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4. Conclusion The results of the present study clearly show that the incorporation of carbon in SnO2 nano particles has a large impact on the gas sensing behaviour. SnO2 nanoparticles show similar sensing response, whereas SnO2:C nanoparticles show opposite sensing behaviour for H2 and ethanol. The catalytic properties of carbon in SnO2:C, and modification in the relative work function values of SnO2 and C in H2 ambient, are responsible for the change over from ntype (SnO2) to p-type (SnO2:C alloy nanoparticles). Sensing behaviour of SnO2:C nanoparticles in ethanol remains unaffected due to lack of C – ethanol interaction, although a decrease in resistance is observed. Similar values of activation energies (0.52 eV) are observed for H2 and ethanol in SnO2 nanoparticles and for ethanol in SnO2:C nanoparticles. Whereas, the activation energy value in case of SnO2:C nanoparticles for H2 is observed to be lower (0.37 eV). The value of energy levels for O-vacancies in SnO2 and SnO2:C obtained from PL (0.5-0.6 eV) support the proposed mechanism. On incorporation of carbon, a selective response of SnO2:C alloy nanoparticles thereby resulting in selective response of SnO2 particles to H2 and ethanol gases, normally not observed in oxide based resistive sensors. Acknowledgements MB would like to acknowledge the financial support from DST INSPIRE (IF130231) for Senior Research Fellowship. BRM would like to acknowledge the support from DRDO project (RP02781).
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Figures
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List of figures: Fig.1 Schematic illustrating synthesis of SnO2 and SnO2:C nanoparticles prepared using the Integrated gas phase synthesis setup. Filled green circles denote SnO2 nucleates and open red circles represent C nucleates in samples SN1 and SC1. Shaded circles represent well sintered SnO2:C alloy nanoparticles (sample SC). Fig.2 (a) Optical image of the IDE pattern for the fabricated sensor device (b) TEM of the nanoparticles deposited (c) Schematic of the deposited nanoparticles on the IDE device fabricated. Fig.3 HRTEM micrographs of well-sintered monocrystalline spherical (a) SnO2 (sample SN) and (b) SnO2:C alloy nanoparticles(SC). Fig.4 XRD plot of SN and SC samples. (*) denotes peaks corresponding to SnO2 cassiterite – tetragonal. (.) denotes peaks corresponding to graphite. (#) denotes alloy peaks. Tetragonal structure symmetry of SnO2 is still retained in the alloy in sample SC. The broad peak (~) in samples SC is due to the overlap of peaks due to Si substrate and shifted SnO2 and C peaks on alloy formation. Fig.5 (a) Sensing response of SnO2 nanoparticles (SN) and SnO2:C alloy nanoparticles (SC) measured at 50°C on exposure to 2% H2. Solid and dotted lines represent time at which H2 gas was switched ON and OFF, respectively. (b) Sensing response of SnO2 nanoparticles (SN) and SnO2:C alloy nanoparticles (SC) measured at 50°C on exposure to ethanol. Solid and dotted line represent time at which ethanol gas was switched ON and OFF, respectively. Fig.6 Schematic illustrating (a) the n- type response of SnO2 nanoparticles( SN) on exposure to analyte gases H2 / ethanol resulting in a negative sensing response (b) response of SC on exposure to H2 and thechange-over from n- to p- type in H2 ambient, +ve sensing response and (c) n- type response of SC on exposure to ethanol. The catalytic role of carbon in H2 gas and passivating effect in ethanol is also shown. Fig.7 Comparison between the sensing response of (a) SN and (b) SC samples to H2 and ethanol as a function of sensing temperature.
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Fig.8 Arrhenius plots of SN and SC samples on exposure to (a) H2 and (b) ethanol highlighting the activation energy and hence the different sensing mechanisms involved. Fig.9 Comparison drawn between the PL spectra of samples SN and SC. With peak „-1: a1 & b1‟ reported to be quite close to the band edge, peak „-2: a2 & b2‟ attributed to oxygen vacancies and peak „a3‟ to Sn interstitials, a new peak „b3‟ seems to be characteristic of the SnO2:C alloy formed. Broad peak “-2” of sample from 416 nm to 747 nm with high intensity in SC is shown in the form of de-convoluted peaks b1,b2 and b3.This peak broadening and increased intensity is attributed to an increase in the number of oxygen vacancies on addition of carbon and sintering.
21 Author Biographies :
Mehar Bhatnagar : A senior research fellow (PhD student) at Indian Institute of Technology Delhi, Working on core shell and alloy semiconductor- metal nanoparticles for gas sensing, plasmonic and hot carrier solar cell applications. Shivani Dhall : A National Fellow for Post Doctoral Research at Indian Institute of Technology Delhi, Working on gas sensing of oxide nanostructures and thin films. Vishakha Kaushik: PhD student at Indian Institute of Technology Delhi. Working on Raman and PL studies of MoS2 thin film devices. Akshey Kaushal: Project scientist working on HRTEM. Bodh Raj Mehta : Schlumberger Chair Professor and Dean of Research and Development at Indian Institute of Technology Delhi. Academic Interests include Physics and Technology of Nanostructured Materials and Thin Films ( Rare-earth Metal and Palladium Nanoparticles based Sensing and Switching Devices, Nanostructured Solar Cell, Resistive Memory and Thermoelectric Materials and Devices , Size-Selected Growth of Metal and Semiconductor Nanoparticles, Organic-Inorganic Semiconductor Interfaces, Scanning Probe Microscopy and High Resolution Transmission Electron Microscopy