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Structural, optical and gas sensing properties of pure and Mn-doped In2O3 nanoparticles
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Structural, optical and gas sensing properties of pure and Mn-doped In2O3 nanoparticles Kanica Anand, Jasmeet Kaur, Ravi Chand Singh, Rengasamy Thangaraj n Department of Physics, Guru Nanak Dev University, Amritsar 143005, India
art ic l e i nf o
a b s t r a c t
Article history: Received 26 November 2015 Received in revised form 22 March 2016 Accepted 30 March 2016
Pure and (1%, 3% and 5% Mn-doped In2O3 nanoparticles have been synthesised by co-precipitation. XRD pattern suggests the cubic bixbyte phase for the In2O3 nanoparticles and the decrease in lattice constant with Mn-doping confirms the incorporation of Mn3 þ in the In2O3 nanoparticles. Transmission electron microscopy also reveals the presence of bixbyte phase only. Red shift in the band gap due to Mn doping is observed, the hump in the reflectance spectra corresponding to 5Eg → 5T2g transitions confirms the incorporation of Mn3 þ in the In2O3 nanoparticles. This is also corroborated by Raman absorption in the range of 600–700 nm. The 3% Mn-doped In2O3 nanoparticles exhibit relatively high photoluminescence intensity due to large oxygen vacancies/defects and possess relatively higher sensor response towards methanol, ethanol, acetone, ammonia vapours and LPG, as higher oxygen vacancies/defects provide enhanced adsorption sites for target gas. & 2016 Published by Elsevier Ltd.
Keywords: In2O3 Doping Sensor response Optical
1. Introduction Gas sensors have been exploited in recent years owing to their promise for improving our living standard in detecting toxic, explosive, harmful, colourless, odourless and combustible gases. Many kinds of semiconductor metal oxide (SMO) gas sensors such as ZnO, SnO2, In2O3, Co3O4, NiO, CuO and WO3 have been evidenced to assist this purpose and have attracted much attention due to their good reproducibility, better sensitivity, low cost, nontoxicity, great stability and easy to use [1–6]. The large and fast change in the resistance of oxide semiconductors owing to gas adsorption on the surface of sensor, forms the basis of detection mechanism of SMO gas sensor. The gas sensing mechanism is prominently a surface phenomenon and the surface area to volume ratio of the sensor has significant impact on the sensor performance. As a consequence, nanostructured metal oxide materials are promising candidates for gas-sensing applications because of their higher surface area to volume ratio as compared to bulk counterparts. This favours the adsorption of gases on the surface of sensor leading to better sensing properties due to more interaction between the gas and the sensor. Thus, the sensor material is pivotal for the gas sensing performance [7]. Among many kinds of SMO, indium oxide (In2O3) is an n-type semiconductor, usually in non-stoichiometric form due to native n
Corresponding author. E-mail address: [email protected] (R. Thangaraj).
or intrinsic defects (oxygen vacancies and In interstitial) [8]. The nature and basis of these defects mainly depend upon the approach used during the material synthesis. These defects/vacancies release electrons to the In2O3 conduction band and induce a remarkable change in band structure as compared to perfect one and a change in the resistance of the material [9,10]. Due to these reasons, In2O3 has been extensively used as a gas sensor. So far, many In2O3 nanostructures have been prepared and examined as gas sensing material. Wang et al. fabricated In2O3 nanoparticles, which show excellent gas sensing performance towards acetone. They attributed this to small sizes and the presence of abundant oxygen vacancies [11]. Ayeshamariam et al. prepared In2O3 nanoparticles, that show enhanced gas sensing property towards ethanol [3]. Han et al. enlightened that the flower-like In2O3 nanostructures exhibited a superior response and better selectivity to ethanol gas. The enhancement in sensing properties is credited to their large surface area and more active sites on the surface [12]. Song et al. fabricated 3D porous In2O3 nanospheres by hydrothermal method that revealed higher sensor response towards ethanol gas compared to bulk In2O3 particles [13]. To further modify the gas sensing properties of In2O3 nanostructures, introduction of the dopant in the host In2O3 is one of the effective and proficient methods. Plenty of work has been carried out for improving the sensing properties through various additives like rare earth and noble metals. Zhang et al. revealed the improved alcohol sensing properties of In2O3 hollow spheres with rare earth (La, Er, Yb) doping and attributed the improvement to the change of the lattice and to the extent of adsorbed oxygen on doping [14]. Zheng et al. enhanced the gas sensing properties of
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Table 1 The synthesis parameter of In2O3 and Mn-doped In2O3 nanoparticles. Sample
Mol% of Mn
In2O3 0 In2O3:1% Mn 1 In2O3:3% Mn 3 In2O3:5% Mn 5
Amount of In(NO3)3 (g) Amount of dopant MnCl2 4H2O (g)
Experimental at% of Mn (EDX)
1.2033 1.1928 1.1640 1.1431
0 0.97 2.48 4.18
0 0.0079 0.0237 0.0395
In2O3 nanoparticles with Ag doping [6]. Xu et al. investigated the ethanol sensing properties of Au-loaded In2O3 nanofibers sensors with different Au concentrations and observed that the 0.2 wt% Au-loaded In2O3 sensors possessed high sensor response and, fast response and recovery [15]. These dopants have been proved to be very effective for modifying the gas sensing properties, but their high cost restricts their scope of practical applications. Many transition metals such as Fe [16], Ni [17] and Cu [18,19] are used as cost effective substitutes and also for improving sensing properties. To the best of our knowledge, no work has been reported on transition metal Mn-doped In2O3 nanoparticles as a gas sensor. In this study, we report the synthesis and structural, optical characterisation and gas-sensing properties of In2O3 and (1%, 3%, 5%) Mn-doped In2O3 nanoparticles sensor towards methanol, ethanol, acetone, ammonia vapours and Liquified Petroleum Gas (LPG). The comparison between In2O3 and Mn-doped In2O3 nanoparticles sensor reveals that 3% Mn-doped In2O3 sensor shows superior gas sensing properties. This is attributed to the increase in the number of defects in In2O3 nanoparticles with increase in Mn doping. This facilitates the reaction between oxygen species and target gas and hence the increase in sensor response.
Fig. 2. (a) XRD patterns of In2O3 and Mn-doped In2O3 nanoparticles with various Mn content and (b) Williamson Hall (W–H) plot of In2O3 and Mn-doped In2O3 nanoparticles.
Table 2 Crystallite size (D), Lattice constant (a), Strain (ε), Band gap energy (Eg), Raman peak intensity ratio (I367 cm 1/I132 cm 1) of In2O3 and Mn3 þ -doped In2O3 nanoparticles. Sample
2. Experimental
a (Å)
Crystallite size D (nm) DS
2.1. Synthesis Analytical grade chemical reagents were used in the experiment without any further purification. In(NO3)3 and MnCl2 4H2O were used as indium and manganese sources, respectively. To prepare In2O3 and 1%, 3% and 5% Mn-doped In2O3 nanoparticles, an appropriate
In2O3 10.109 11.59 In2O3:1% Mn 10.105 14.37 In2O3:3% Mn 10.085 13.38 In2O3:5% Mn 10.080 12.93
Strain ε *10 4
Eg (eV) (I367 cm 1/I132 cm 1)
0.132 0.053 0.016 þ 3.88
3.64 3.55 3.47 3.46
DW 12.33 14.59 13.57 13.69
Fig. 1. The schematic diagram of sensing unit (a) Test chamber and (b) data acquisition unit.
0.53 0.61 0.76 0.71
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Fig. 3. (a–c) Low resolution TEM images; (d–f) high-resolution TEM images; (g–i) SAED pattern and (j–l) particle size histogram of In2O3, 3% and 5% Mn-doped In2O3 nanoparticles.
amount (Table 1) of In(NO3)3 and MnCl2 4H2O were dissolved in distilled water to form 20 ml of 0.2 M aqueous solution under vigorous stirring at room temperature for 8 h. Then, ammonium hydroxide was added to the above solution with continued stirring till pH 9. The obtained hydroxide (In(OH)3) and Mn-doped In(OH)3)) precipitates were separated by filtration and washed with ethanol and then dried at 80 °C for 5 h. These precipitates were then calcined at 500 °C for 5 h to get In2O3 and Mn-doped In2O3 nanoparticles.
2.2. Characterisation X-ray diffraction (XRD) patterns were taken on X-ray diffractometer (D8 Focus, Bruker, Germany) in the 2θ range of 20–70° using Cu Kα radiation. The morphology, size and detail about the structure of nanoparticles were explored using transmission electron microscope images (TEM), high resolution transmission electron microscope images (HRTEM) and selected area electron
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Fig. 4. EDX of (a) In2O3 and (b) Mn-doped In2O3 nanoparticles.
Fig. 5. Raman spectra at room temperature of In2O3 and Mn-doped In2O3 nanoparticles in the range of 100–900 cm 1.
diffraction patterns (SAED) that were taken on a JEOL TEM-2100. Field emission scanning electron microscope (FE-SEM) (Carl Zeiss Supra 55) equipped with an Energy-dispersive X-ray spectrometer (EDX) was used to determine elemental compositions of nanoparticles. Optical diffuse reflectance spectra were recorded with a Shimadzu UV-3600 spectrophotometer at room temperature using BaSO4 powder as a standard (100% reflectance). The
Fig. 7. Room temperature PL spectra under the 310 nm excitation wavelength of In2O3 and Mn-doped In2O3 nanoparticles.
photoluminescence (PL) spectra were carried out on Lambda 45, Perkin Elmer fluorescence spectrometer with an excitation wavelength of 310 nm. The Raman spectra of In2O3 nanoparticles were recorded using Renishaw Invia Microscope using 488 nm laser line as excitation source.
Fig. 6. (a) Room temperature diffuse reflection spectra; (b) F(R) versus energy (hν) plots; (c) [F(R)*hν)2 versus hν plots of In2O3 and Mn-doped In2O3 nanoparticles. The inset of (a) shows the hump related to transition of Mn3 þ ion and inset of (c) shows red shift in the band gap of the In2O3 nanoparticles with increase in Mn content.
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Fig. 8. Sensor response versus operating temperature histogram of In2O3 and Mn-doped In2O3 sensors towards methanol, ethanol, acetone, ammonia vapours and LPG.
2.3. Measurement of gas sensing properties For the synthesis of sensors (i.e. In2O3 sensor and 1%, 3% and 5% Mn-doped In2O3 sensor) an appropriate amount of as-prepared nanoparticles (In2O3 and 1%, 3% and 5% Mn-doped In2O3) were ground in a mortar and pestle and mixed with deionised water (about few microliters) to make a paste. The paste was painted on alumina substrate on which a pair of gold electrodes (electrode distance 2 mm) have been previously printed and then sintered in air at 400 °C for 1 h. The gas-sensing properties of sensors were measured using an experimental set up that include an indigenously built glass chamber having a 40 L capacity, temperature controlled oven, mixing fan, sample holder, Keithley Data
Acquisition Module KUSB-3100 and computer [20,21]. In the measuring electric circuit, a reference resistance (R) is connected in series with a sensor having resistance (Rs) and the circuit voltage VI (12 V) (Fig. 1). By monitoring the voltage signal variation (Vo) across the reference resistance (R) with time in the presence of the target gas (such as methanol, ethanol, acetone, ammonia vapours and LPG) and air, sensor response of target gas was measured in the temperature range of 250–450 °C. The sensor response (SR) is defined as the ratio of Rair/Rgas, where, Rair and Rgas are the resistance of sensor in air and air-target gas ambience, respectively. The gas sensor response time has been defined as the time taken to get 90% of the maximum SR value, and recovery time as the time taken to recover 10% of the base value.
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Fig. 9. Sensing curves of In2O3 and 3% Mn-doped In2O3 sensors to methanol, ethanol, acetone, ammonia vapours and LPG. Table 3 Response and recovery time of In2O3 and 3% Mn-doped In2O3 sensors. Target gas Operating temperature (°C)
In2O3 Response time (s)
Recovery time (s)
Response time (s)
Recovery time (s)
Methanol Ethanol Acetone Ammonia LPG
10 40 15 11 12
34 38 17 32 32
15 32 20 6 8
31 49 38 33 32
300 300 350 400 400
In2O3: 3%Mn
XRD peak (222) of doped In2O3 to higher 2θ value relative to those of undoped In2O3, indicating the possible reduction of lattice parameter ‘a’ probably due to the incorporation of Mn3 þ (0.65 Å) [22] which has smaller ionic radii than In3 þ (1.06 Å) [23]. The extent of the shift or lattice distortion depend upon the manganese content in In2O3. The lattice parameter ‘a’ is calculated by using Bragg's law. Moreover the crystallite size is calculated from the width of the reflection peak in the XRD pattern by Debye Scherrer formulae and given in Table 2. [24].
Ds=kλ /β cos θ
(1)
where β is the full width at half maximum, DS is the crystallite size, k is shape factor and 2θ is the diffraction angle. The crystallite size of undoped In2O3 is 11.57 nm and no appreciable change has been observed with Mn doping in In2O3 nanoparticles. Apart from the crystallite size, lattice strain also plays a role in the broadening of the XRD peaks. Strain is calculated by using Williamson Hall plot based on the following equation [24]
β cos θ /λ=k/D W +4ϵ sin θ /λ
(2)
where DW, β, λ, ε, and 2θ are the crystallite size, full width at half maximum, X-ray wavelength, strain and Bragg's diffraction angle respectively. The calculated strain value for Mn doped In2O3 nanoparticles are given in Table 2. The increase in strain is due to lattice contraction as revealed from the decrease in lattice parameter ‘a’ with an increase in Mn content.
Fig. 10. Stability curves for In2O3 and Mn-doped In2O3 sensors.
3. Result and discussion 3.1. Structural properties 3.1.1. X-ray diffraction analysis The XRD pattern of In2O3 and 1%, 3% and 5% Mn-doped In2O3 nanoparticles is shown in Fig. 2. All the diffraction peaks are assigned to cubic bixbyte type crystalline phase of indium oxide and are confirmed from JCPDS card no. 06–0416. No characteristic peak of any other possible phases such as manganese oxide or indium oxide phase has been observed, indicating the substitution of Mn3 þ in In2O3. This is further evidenced by the shifting of strong
3.1.2. Transmission electron microscopy Detailed structural analysis of the pure and Mn-doped In2O3 nanoparticles have been done using TEM. The low resolution and high resolution TEM images of In2O3 and Mn-doped In2O3 nanoparticles are shown in Fig. 3(a–f). Nearly spherical shaped nanoparticles are observed with some agglomeration. The average particle size has been calculated by plotting a histogram for 100 nanoparticles for In2O3 and Mn-doped In2O3 nanoparticles and is nearly same. This is in agreement with the XRD data. The spotted ring like SAED pattern shown in Fig. 3(g–i), further confirms the presence of crystalline cubic bixbyte phase of In2O3 nanoparticles. The rings correspond to (211), (222), (400), (444), and (622) planes of In2O3 nanoparticles [25]. The lattice spacing of 0.289 nm and 0.41 nm measured in HRTEM images corresponds to interplanar distance of (222) and (211) plane of bcc- In2O3 nanoparticles [26]. Absence of any secondary phase in HRTEM images is in agreement with XRD data and indicates that Mn ions are diluted in In2O3 matrix. EDX spectra of In2O3 and Mn-doped In2O3 nanoparticles shown in Fig. 4, indicates that In2O3 nanoparticles are doped with
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Fig. 11. The reproducibility of the In2O3 and 3% Mn-doped In2O3 sensors on successive exposure (4 cycles) to 50 ppm test gas (methanol, ethanol, acetone, ammonia vapours and LPG) at optimum operating temperature.
Mn element and free of any other impurity. The average value of concentration of Mn as determined from EDX analysis are listed in Table 1. The EDX analysis is taken from different part of the samples and showed almost the same value indicating the homogeneity of the prepared nanoparticles. 3.1.3. Raman analysis Room temperature Raman spectra of the In2O3 and Mn-doped In2O3 nanoparticles have been recorded in the 100–900 cm 1 spectral range (Fig. 5). The Raman spectrum of In2O3 nanoparticles revealed vibrational modes pertaining to E1g (132, 308 cm 1), E2g (630 cm 1), A1g (497 cm 1) and 367 cm 1 associated with the bcc-In2O3 as reported in literature [27]. The mode at 132 cm 1 can be assigned to In-O stretching vibrations of InO6 structure units/ octahedron, the 306 cm 1 can be attributed to the bending (δ) vibration of InO6 octahedron, the 490 cm 1 and 631 cm 1 are usually interpreted as the v stretching modes of InO6 octahedrons. The mode at 367 cm 1 corresponds to In–O–In stretching vibrations. It is also related to the oxygen vacancies in the In2O3 nanoparticles [26]. The above modes i.e. 132, 306 and 490 cm 1 are also observed in the Mn-doped In2O3 nanoparticles spectra, but their intensity decreases with an increase in Mn content. This decrease in intensity corresponds to increase in structural disorder with doping as supported by XRD. Also, the higher frequency peak in the Mn doped In2O3 spectrum approx. at 645 cm 1 is due to the superposition of the contribution of the In–O vibrational modes with frequency 631 cm 1, and the Mn–O vibrational modes with frequency 665 cm 1. The intensity of this peak increases with increase in Mn content and corresponds to Mn3 þ O6 octahedron coordination and confirms the incorporation of Mn3 þ in In2O3 nanoparticle lattice [28]. Moreover, the relative intensity of the In–O–In defects peak (367 cm 1) with respect to maximum intensity peak (132 cm 1) given in Table 2 is maximum for 3% Mn-doped In2O3 nanoparticles, implying 3% Mn-doped In2O3 nanoparticles possess maximum oxygen vacancies [27].
3.2. Optical properties 3.2.1. Diffuse reflectance spectroscopy study In order to confirm that Mn3 þ has been substituted for In3 þ , optical measurements have been performed at room-temperature. Diffuse reflectance spectra of pure In2O3 and Mn-doped In2O3 nanoparticles are shown in Fig. 6(a). The absorption spectra of nanoparticles shown in Fig. 6(b) is obtained by employing the Kulbeka Munk method [29]. A significant red shift in the absorption edge and broad absorption hump in visible spectral range of approx. 20000 cm 1 (450–630 nm) has been observed, which increases with an increase in Mn content. This broad hump is mainly attributed to crystal field d–d transitions of Mn3 þ in a crystalline environment of In2O3 [22,30–32]. As, Mn3 þ has d4 electronic configuration and the 5D ground state of high spin octahedral Mn3 þ ions is split into 5T2g and 5Eg terms and this splitting leads to 5Eg→ 5 T2g spin allowed transitions, which appear in the visible spectral region at approx. 20000 cm 1. The direct band gap energy (Eg) of In2O3 nanoparticles was determined by plotting [F(R)* hν]2 vs hν and extrapolating the linear part of the curve [F(R)* hν]2 to zero as shown in Fig. 6(c) and the obtained Eg values are given in Table 2. The band gap narrowing observed with manganese doping is attributed to the sp-d exchange interactions between band e–s and localised d electrons of transition metal ions [33]. 3.2.2. Photoluminescence study Fig. 7 shows the room temperature PL spectra of In2O3 and Mndoped In2O3 nanoparticles using 310 nm as an excitation wavelength. The spectra show a series of PL peaks at 376, 420, 441, 455, 480 nm. The peak near 376 nm represents the NBE (near band emission) and the peak at higher wavelength are assigned to DLE (deep level emission) [34,35]. These deeper levels are mainly attributed to In, O, and/or O/In vacancies/defect centres already reported in literature [36–39]. Moreover, it is clear from the figure that the intensity of the blue emission in the range 420–480 nm increases considerably with the Mn doping up to 3% and suggests maximum vacancies/defects centres in 3% Mn-doped In2O3 nanoparticles.
Please cite this article as: K. Anand, et al., Structural, optical and gas sensing properties of pure and Mn-doped In2O3 nanoparticles, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.233i
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Fig. 12. The sensor response variation of In2O3 and 3% Mn-doped In2O3 gas sensors as a function of operating temperature and test gas concentration (methanol, ethanol, acetone, ammonia vapours and LPG).
Please cite this article as: K. Anand, et al., Structural, optical and gas sensing properties of pure and Mn-doped In2O3 nanoparticles, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.233i
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3.3. Gas sensing properties
2NH3 +3O− (ads)⟶N2 +3H2 O+3e−
Gas sensing characteristics were carried out systematically to study the effect of Mn doping, operating temperature, and gas concentration on the sensor response of In2O3 gas sensor towards reducing test gases such as methanol, ethanol, acetone, ammonia vapours and LPG. As we know, the gas sensing is a surface phenomenon and depends upon the variation in electrical resistance of the sensor due to adsorption and desorption of test gas molecule on the surface of sensing film. Fig. 8 shows the sensor response of In2O3 and Mn-doped In2O3 nanoparticles sensors as a function of operating temperature in the range from 250–450 °C towards target gases. As shown in Fig. 8, 300 °C is the optimum operating temperature for methanol, ethanol, 350 °C is for acetone, and 400 °C is for ammonia vapours and LPG and this is due to the reason that sensor shows the maximum sensor response at corresponding temperature. Fig. 9 shows the variation of sensor response with time for In2O3 and 3% Mn-doped In2O3 gas sensor and response and recovery time calculated from this are given in Table 3. Also, from Fig. 8, it is clear that higher sensor response value is obtained for 3% Mn-doped In2O3 nanoparticles for 50 ppm methanol, ethanol, acetone, ammonia vapours and LPG. The significant enhancement of sensor response of 3% Mn doping in In2O3 is due to the introduction of high oxygen vacancies, surface defects and metal interstitials within the lattice of In2O3 nanoparticles due to Mn doping as confirmed by the photoluminescence spectra. From Fig. 7 it is clear that 3% Mn doped In2O3 nanoparticles shows maximum vacancy centres and these vacancies and surface defect centres are the key factors promoting the adsorption of gas molecules on the surface of the sensor and hence the increase in sensor response [35,40]. Another reason for the increase of sensor response is large lattice distortion and high strain of Mn doped In2O3 nanoparticle, due to the presence of lattice mismatch between In3 þ and Mn3 þ ions confirmed by XRD and Raman studies and this lattice distortion is advantageous for the interaction between test gas molecule and the sensor surface [41]. The difference in gas sensor response towards different gases might be due to difference in adsorption and the reaction process. In2O3 is n type metal oxide semiconductor (SMO) and when it is exposed to air, oxygen molecule O2 adsorbed on the sensor surface in the form of oxygen species such as ( O−2 , O , O2 ) formed by capturing electrons from the In2O3 conduction band depending upon operating temperature. As a result, electron concentration in In2O3 conduction band decreases and hence resistance of In2O3 sensor increases. The state at which oxygen molecules are adsorbed on the sensor surface are given in the following reactions [15]:
Cn H2n +2 +2O− (ads)⟶Cn H2n −O+H2 O+2e−
O2 (gas)⟷O2 (ads)
References
O2 (ads)+e−⟷O2− (ads)
O−2 (ads)+e−⟷2O− (ads) O− (ads)+e−⟷O2 − (ads) In the presence of Methanol, Ethanol, Acetone, Ammonia vapours and LPG, the sensing mechanism can be explained through the following reactions [15,42–44].
CH3 OH+3O− (ads)⟶CO2 +2H2 O+3e− C2 H5 OH+6O− (ads)⟶2CO2 +3H2 O+6e−
CH3 COCH3 +8O− (ads)⟶3CO2 +3H2 O+8e−
9
The interaction of target gas with surface adsorbed oxygen species results in releasing electrons back to In2O3 conduction band, thereby decreasing the resistance of the sensor. Hence, by monitoring the resistance of sensor in the presence or absence of target gas, the sensor response magnitude is determined. Moreover, to check the stability of In2O3 and 3% Mn-doped In2O3 gas sensors, the SR towards ethanol have been measured at different intervals for 60 days (Fig. 10) and have been found that sensors show stable SR. Fig. 11 illustrates the reproducibility of In2O3 and 3% Mn-doped In2O3 gas sensors, revealing that the sensor maintains its initial response amplitude upon four successive sensing tests to 50 ppm of methanol, ethanol, acetone, ammonia vapours and LPG, indicating that the sensor has a good stability throughout the cycle test. The sensor response of In2O3 and 3% Mn-doped In2O3 gas sensors as a function of operating temperature (250–450 °C) and test gas (ethanol, methanol, acetone, ammonia vapours and LPG) concentration (50–1250 ppm) is shown in Fig. 12. Sensor response is found to exhibit similar behaviour with temperature at all concentration of test gas.
4. Conclusion Pure and (1%, 3% and 5%) Mn-doped In2O3 nanoparticles have been prepared by co-precipitation. All the prepared In2O3 nanoparticles possessed cubic bixbyite crystal phase and decrease in lattice constant with Mn doping owing to decrease in ionic radius of Mn3 þ as compared to In3 þ . The absorption band in visible region corresponds to 5Eg → 5T2g spin allowed transitions and confirms the incorporation of Mn3 þ in host In2O3 lattice. This is supported by Raman spectra also. Photoluminescence spectra of In2O3 nanoparticles shows an increase in the emission intensity related to increase in oxygen vacancies/defects with Mn doping. An enhanced SR is achieved for 3%Mn doping, has been attributed to oxygen vacancies/defects generated due to Mn doping.
Acknowledgement The authors are thankful to the Guru Nanak Dev University's Central Instrumental Facility for providing Raman Spectrophotometer, TEM and XRD facilities.
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Structural, optical and gas sensing properties of pure and Mn-doped In2O3 nanoparticles Kanica Anand, Jasmeet Kaur, Ravi Chand Singh, Rengasamy Thangaraj n Department of Physics, Guru Nanak Dev University, Amritsar 143005, India
art ic l e i nf o
a b s t r a c t
Article history: Received 26 November 2015 Received in revised form 22 March 2016 Accepted 30 March 2016
Pure and (1%, 3% and 5% Mn-doped In2O3 nanoparticles have been synthesised by co-precipitation. XRD pattern suggests the cubic bixbyte phase for the In2O3 nanoparticles and the decrease in lattice constant with Mn-doping confirms the incorporation of Mn3 þ in the In2O3 nanoparticles. Transmission electron microscopy also reveals the presence of bixbyte phase only. Red shift in the band gap due to Mn doping is observed, the hump in the reflectance spectra corresponding to 5Eg → 5T2g transitions confirms the incorporation of Mn3 þ in the In2O3 nanoparticles. This is also corroborated by Raman absorption in the range of 600–700 nm. The 3% Mn-doped In2O3 nanoparticles exhibit relatively high photoluminescence intensity due to large oxygen vacancies/defects and possess relatively higher sensor response towards methanol, ethanol, acetone, ammonia vapours and LPG, as higher oxygen vacancies/defects provide enhanced adsorption sites for target gas. & 2016 Published by Elsevier Ltd.
Keywords: In2O3 Doping Sensor response Optical
1. Introduction Gas sensors have been exploited in recent years owing to their promise for improving our living standard in detecting toxic, explosive, harmful, colourless, odourless and combustible gases. Many kinds of semiconductor metal oxide (SMO) gas sensors such as ZnO, SnO2, In2O3, Co3O4, NiO, CuO and WO3 have been evidenced to assist this purpose and have attracted much attention due to their good reproducibility, better sensitivity, low cost, nontoxicity, great stability and easy to use [1–6]. The large and fast change in the resistance of oxide semiconductors owing to gas adsorption on the surface of sensor, forms the basis of detection mechanism of SMO gas sensor. The gas sensing mechanism is prominently a surface phenomenon and the surface area to volume ratio of the sensor has significant impact on the sensor performance. As a consequence, nanostructured metal oxide materials are promising candidates for gas-sensing applications because of their higher surface area to volume ratio as compared to bulk counterparts. This favours the adsorption of gases on the surface of sensor leading to better sensing properties due to more interaction between the gas and the sensor. Thus, the sensor material is pivotal for the gas sensing performance [7]. Among many kinds of SMO, indium oxide (In2O3) is an n-type semiconductor, usually in non-stoichiometric form due to native n
Corresponding author. E-mail address: [email protected] (R. Thangaraj).
or intrinsic defects (oxygen vacancies and In interstitial) [8]. The nature and basis of these defects mainly depend upon the approach used during the material synthesis. These defects/vacancies release electrons to the In2O3 conduction band and induce a remarkable change in band structure as compared to perfect one and a change in the resistance of the material [9,10]. Due to these reasons, In2O3 has been extensively used as a gas sensor. So far, many In2O3 nanostructures have been prepared and examined as gas sensing material. Wang et al. fabricated In2O3 nanoparticles, which show excellent gas sensing performance towards acetone. They attributed this to small sizes and the presence of abundant oxygen vacancies [11]. Ayeshamariam et al. prepared In2O3 nanoparticles, that show enhanced gas sensing property towards ethanol [3]. Han et al. enlightened that the flower-like In2O3 nanostructures exhibited a superior response and better selectivity to ethanol gas. The enhancement in sensing properties is credited to their large surface area and more active sites on the surface [12]. Song et al. fabricated 3D porous In2O3 nanospheres by hydrothermal method that revealed higher sensor response towards ethanol gas compared to bulk In2O3 particles [13]. To further modify the gas sensing properties of In2O3 nanostructures, introduction of the dopant in the host In2O3 is one of the effective and proficient methods. Plenty of work has been carried out for improving the sensing properties through various additives like rare earth and noble metals. Zhang et al. revealed the improved alcohol sensing properties of In2O3 hollow spheres with rare earth (La, Er, Yb) doping and attributed the improvement to the change of the lattice and to the extent of adsorbed oxygen on doping [14]. Zheng et al. enhanced the gas sensing properties of
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Table 1 The synthesis parameter of In2O3 and Mn-doped In2O3 nanoparticles. Sample
Mol% of Mn
In2O3 0 In2O3:1% Mn 1 In2O3:3% Mn 3 In2O3:5% Mn 5
Amount of In(NO3)3 (g) Amount of dopant MnCl2 4H2O (g)
Experimental at% of Mn (EDX)
1.2033 1.1928 1.1640 1.1431
0 0.97 2.48 4.18
0 0.0079 0.0237 0.0395
In2O3 nanoparticles with Ag doping [6]. Xu et al. investigated the ethanol sensing properties of Au-loaded In2O3 nanofibers sensors with different Au concentrations and observed that the 0.2 wt% Au-loaded In2O3 sensors possessed high sensor response and, fast response and recovery [15]. These dopants have been proved to be very effective for modifying the gas sensing properties, but their high cost restricts their scope of practical applications. Many transition metals such as Fe [16], Ni [17] and Cu [18,19] are used as cost effective substitutes and also for improving sensing properties. To the best of our knowledge, no work has been reported on transition metal Mn-doped In2O3 nanoparticles as a gas sensor. In this study, we report the synthesis and structural, optical characterisation and gas-sensing properties of In2O3 and (1%, 3%, 5%) Mn-doped In2O3 nanoparticles sensor towards methanol, ethanol, acetone, ammonia vapours and Liquified Petroleum Gas (LPG). The comparison between In2O3 and Mn-doped In2O3 nanoparticles sensor reveals that 3% Mn-doped In2O3 sensor shows superior gas sensing properties. This is attributed to the increase in the number of defects in In2O3 nanoparticles with increase in Mn doping. This facilitates the reaction between oxygen species and target gas and hence the increase in sensor response.
Fig. 2. (a) XRD patterns of In2O3 and Mn-doped In2O3 nanoparticles with various Mn content and (b) Williamson Hall (W–H) plot of In2O3 and Mn-doped In2O3 nanoparticles.
Table 2 Crystallite size (D), Lattice constant (a), Strain (ε), Band gap energy (Eg), Raman peak intensity ratio (I367 cm 1/I132 cm 1) of In2O3 and Mn3 þ -doped In2O3 nanoparticles. Sample
2. Experimental
a (Å)
Crystallite size D (nm) DS
2.1. Synthesis Analytical grade chemical reagents were used in the experiment without any further purification. In(NO3)3 and MnCl2 4H2O were used as indium and manganese sources, respectively. To prepare In2O3 and 1%, 3% and 5% Mn-doped In2O3 nanoparticles, an appropriate
In2O3 10.109 11.59 In2O3:1% Mn 10.105 14.37 In2O3:3% Mn 10.085 13.38 In2O3:5% Mn 10.080 12.93
Strain ε *10 4
Eg (eV) (I367 cm 1/I132 cm 1)
0.132 0.053 0.016 þ 3.88
3.64 3.55 3.47 3.46
DW 12.33 14.59 13.57 13.69
Fig. 1. The schematic diagram of sensing unit (a) Test chamber and (b) data acquisition unit.
0.53 0.61 0.76 0.71
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Fig. 3. (a–c) Low resolution TEM images; (d–f) high-resolution TEM images; (g–i) SAED pattern and (j–l) particle size histogram of In2O3, 3% and 5% Mn-doped In2O3 nanoparticles.
amount (Table 1) of In(NO3)3 and MnCl2 4H2O were dissolved in distilled water to form 20 ml of 0.2 M aqueous solution under vigorous stirring at room temperature for 8 h. Then, ammonium hydroxide was added to the above solution with continued stirring till pH 9. The obtained hydroxide (In(OH)3) and Mn-doped In(OH)3)) precipitates were separated by filtration and washed with ethanol and then dried at 80 °C for 5 h. These precipitates were then calcined at 500 °C for 5 h to get In2O3 and Mn-doped In2O3 nanoparticles.
2.2. Characterisation X-ray diffraction (XRD) patterns were taken on X-ray diffractometer (D8 Focus, Bruker, Germany) in the 2θ range of 20–70° using Cu Kα radiation. The morphology, size and detail about the structure of nanoparticles were explored using transmission electron microscope images (TEM), high resolution transmission electron microscope images (HRTEM) and selected area electron
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Fig. 4. EDX of (a) In2O3 and (b) Mn-doped In2O3 nanoparticles.
Fig. 5. Raman spectra at room temperature of In2O3 and Mn-doped In2O3 nanoparticles in the range of 100–900 cm 1.
diffraction patterns (SAED) that were taken on a JEOL TEM-2100. Field emission scanning electron microscope (FE-SEM) (Carl Zeiss Supra 55) equipped with an Energy-dispersive X-ray spectrometer (EDX) was used to determine elemental compositions of nanoparticles. Optical diffuse reflectance spectra were recorded with a Shimadzu UV-3600 spectrophotometer at room temperature using BaSO4 powder as a standard (100% reflectance). The
Fig. 7. Room temperature PL spectra under the 310 nm excitation wavelength of In2O3 and Mn-doped In2O3 nanoparticles.
photoluminescence (PL) spectra were carried out on Lambda 45, Perkin Elmer fluorescence spectrometer with an excitation wavelength of 310 nm. The Raman spectra of In2O3 nanoparticles were recorded using Renishaw Invia Microscope using 488 nm laser line as excitation source.
Fig. 6. (a) Room temperature diffuse reflection spectra; (b) F(R) versus energy (hν) plots; (c) [F(R)*hν)2 versus hν plots of In2O3 and Mn-doped In2O3 nanoparticles. The inset of (a) shows the hump related to transition of Mn3 þ ion and inset of (c) shows red shift in the band gap of the In2O3 nanoparticles with increase in Mn content.
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Fig. 8. Sensor response versus operating temperature histogram of In2O3 and Mn-doped In2O3 sensors towards methanol, ethanol, acetone, ammonia vapours and LPG.
2.3. Measurement of gas sensing properties For the synthesis of sensors (i.e. In2O3 sensor and 1%, 3% and 5% Mn-doped In2O3 sensor) an appropriate amount of as-prepared nanoparticles (In2O3 and 1%, 3% and 5% Mn-doped In2O3) were ground in a mortar and pestle and mixed with deionised water (about few microliters) to make a paste. The paste was painted on alumina substrate on which a pair of gold electrodes (electrode distance 2 mm) have been previously printed and then sintered in air at 400 °C for 1 h. The gas-sensing properties of sensors were measured using an experimental set up that include an indigenously built glass chamber having a 40 L capacity, temperature controlled oven, mixing fan, sample holder, Keithley Data
Acquisition Module KUSB-3100 and computer [20,21]. In the measuring electric circuit, a reference resistance (R) is connected in series with a sensor having resistance (Rs) and the circuit voltage VI (12 V) (Fig. 1). By monitoring the voltage signal variation (Vo) across the reference resistance (R) with time in the presence of the target gas (such as methanol, ethanol, acetone, ammonia vapours and LPG) and air, sensor response of target gas was measured in the temperature range of 250–450 °C. The sensor response (SR) is defined as the ratio of Rair/Rgas, where, Rair and Rgas are the resistance of sensor in air and air-target gas ambience, respectively. The gas sensor response time has been defined as the time taken to get 90% of the maximum SR value, and recovery time as the time taken to recover 10% of the base value.
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Fig. 9. Sensing curves of In2O3 and 3% Mn-doped In2O3 sensors to methanol, ethanol, acetone, ammonia vapours and LPG. Table 3 Response and recovery time of In2O3 and 3% Mn-doped In2O3 sensors. Target gas Operating temperature (°C)
In2O3 Response time (s)
Recovery time (s)
Response time (s)
Recovery time (s)
Methanol Ethanol Acetone Ammonia LPG
10 40 15 11 12
34 38 17 32 32
15 32 20 6 8
31 49 38 33 32
300 300 350 400 400
In2O3: 3%Mn
XRD peak (222) of doped In2O3 to higher 2θ value relative to those of undoped In2O3, indicating the possible reduction of lattice parameter ‘a’ probably due to the incorporation of Mn3 þ (0.65 Å) [22] which has smaller ionic radii than In3 þ (1.06 Å) [23]. The extent of the shift or lattice distortion depend upon the manganese content in In2O3. The lattice parameter ‘a’ is calculated by using Bragg's law. Moreover the crystallite size is calculated from the width of the reflection peak in the XRD pattern by Debye Scherrer formulae and given in Table 2. [24].
Ds=kλ /β cos θ
(1)
where β is the full width at half maximum, DS is the crystallite size, k is shape factor and 2θ is the diffraction angle. The crystallite size of undoped In2O3 is 11.57 nm and no appreciable change has been observed with Mn doping in In2O3 nanoparticles. Apart from the crystallite size, lattice strain also plays a role in the broadening of the XRD peaks. Strain is calculated by using Williamson Hall plot based on the following equation [24]
β cos θ /λ=k/D W +4ϵ sin θ /λ
(2)
where DW, β, λ, ε, and 2θ are the crystallite size, full width at half maximum, X-ray wavelength, strain and Bragg's diffraction angle respectively. The calculated strain value for Mn doped In2O3 nanoparticles are given in Table 2. The increase in strain is due to lattice contraction as revealed from the decrease in lattice parameter ‘a’ with an increase in Mn content.
Fig. 10. Stability curves for In2O3 and Mn-doped In2O3 sensors.
3. Result and discussion 3.1. Structural properties 3.1.1. X-ray diffraction analysis The XRD pattern of In2O3 and 1%, 3% and 5% Mn-doped In2O3 nanoparticles is shown in Fig. 2. All the diffraction peaks are assigned to cubic bixbyte type crystalline phase of indium oxide and are confirmed from JCPDS card no. 06–0416. No characteristic peak of any other possible phases such as manganese oxide or indium oxide phase has been observed, indicating the substitution of Mn3 þ in In2O3. This is further evidenced by the shifting of strong
3.1.2. Transmission electron microscopy Detailed structural analysis of the pure and Mn-doped In2O3 nanoparticles have been done using TEM. The low resolution and high resolution TEM images of In2O3 and Mn-doped In2O3 nanoparticles are shown in Fig. 3(a–f). Nearly spherical shaped nanoparticles are observed with some agglomeration. The average particle size has been calculated by plotting a histogram for 100 nanoparticles for In2O3 and Mn-doped In2O3 nanoparticles and is nearly same. This is in agreement with the XRD data. The spotted ring like SAED pattern shown in Fig. 3(g–i), further confirms the presence of crystalline cubic bixbyte phase of In2O3 nanoparticles. The rings correspond to (211), (222), (400), (444), and (622) planes of In2O3 nanoparticles [25]. The lattice spacing of 0.289 nm and 0.41 nm measured in HRTEM images corresponds to interplanar distance of (222) and (211) plane of bcc- In2O3 nanoparticles [26]. Absence of any secondary phase in HRTEM images is in agreement with XRD data and indicates that Mn ions are diluted in In2O3 matrix. EDX spectra of In2O3 and Mn-doped In2O3 nanoparticles shown in Fig. 4, indicates that In2O3 nanoparticles are doped with
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Fig. 11. The reproducibility of the In2O3 and 3% Mn-doped In2O3 sensors on successive exposure (4 cycles) to 50 ppm test gas (methanol, ethanol, acetone, ammonia vapours and LPG) at optimum operating temperature.
Mn element and free of any other impurity. The average value of concentration of Mn as determined from EDX analysis are listed in Table 1. The EDX analysis is taken from different part of the samples and showed almost the same value indicating the homogeneity of the prepared nanoparticles. 3.1.3. Raman analysis Room temperature Raman spectra of the In2O3 and Mn-doped In2O3 nanoparticles have been recorded in the 100–900 cm 1 spectral range (Fig. 5). The Raman spectrum of In2O3 nanoparticles revealed vibrational modes pertaining to E1g (132, 308 cm 1), E2g (630 cm 1), A1g (497 cm 1) and 367 cm 1 associated with the bcc-In2O3 as reported in literature [27]. The mode at 132 cm 1 can be assigned to In-O stretching vibrations of InO6 structure units/ octahedron, the 306 cm 1 can be attributed to the bending (δ) vibration of InO6 octahedron, the 490 cm 1 and 631 cm 1 are usually interpreted as the v stretching modes of InO6 octahedrons. The mode at 367 cm 1 corresponds to In–O–In stretching vibrations. It is also related to the oxygen vacancies in the In2O3 nanoparticles [26]. The above modes i.e. 132, 306 and 490 cm 1 are also observed in the Mn-doped In2O3 nanoparticles spectra, but their intensity decreases with an increase in Mn content. This decrease in intensity corresponds to increase in structural disorder with doping as supported by XRD. Also, the higher frequency peak in the Mn doped In2O3 spectrum approx. at 645 cm 1 is due to the superposition of the contribution of the In–O vibrational modes with frequency 631 cm 1, and the Mn–O vibrational modes with frequency 665 cm 1. The intensity of this peak increases with increase in Mn content and corresponds to Mn3 þ O6 octahedron coordination and confirms the incorporation of Mn3 þ in In2O3 nanoparticle lattice [28]. Moreover, the relative intensity of the In–O–In defects peak (367 cm 1) with respect to maximum intensity peak (132 cm 1) given in Table 2 is maximum for 3% Mn-doped In2O3 nanoparticles, implying 3% Mn-doped In2O3 nanoparticles possess maximum oxygen vacancies [27].
3.2. Optical properties 3.2.1. Diffuse reflectance spectroscopy study In order to confirm that Mn3 þ has been substituted for In3 þ , optical measurements have been performed at room-temperature. Diffuse reflectance spectra of pure In2O3 and Mn-doped In2O3 nanoparticles are shown in Fig. 6(a). The absorption spectra of nanoparticles shown in Fig. 6(b) is obtained by employing the Kulbeka Munk method [29]. A significant red shift in the absorption edge and broad absorption hump in visible spectral range of approx. 20000 cm 1 (450–630 nm) has been observed, which increases with an increase in Mn content. This broad hump is mainly attributed to crystal field d–d transitions of Mn3 þ in a crystalline environment of In2O3 [22,30–32]. As, Mn3 þ has d4 electronic configuration and the 5D ground state of high spin octahedral Mn3 þ ions is split into 5T2g and 5Eg terms and this splitting leads to 5Eg→ 5 T2g spin allowed transitions, which appear in the visible spectral region at approx. 20000 cm 1. The direct band gap energy (Eg) of In2O3 nanoparticles was determined by plotting [F(R)* hν]2 vs hν and extrapolating the linear part of the curve [F(R)* hν]2 to zero as shown in Fig. 6(c) and the obtained Eg values are given in Table 2. The band gap narrowing observed with manganese doping is attributed to the sp-d exchange interactions between band e–s and localised d electrons of transition metal ions [33]. 3.2.2. Photoluminescence study Fig. 7 shows the room temperature PL spectra of In2O3 and Mndoped In2O3 nanoparticles using 310 nm as an excitation wavelength. The spectra show a series of PL peaks at 376, 420, 441, 455, 480 nm. The peak near 376 nm represents the NBE (near band emission) and the peak at higher wavelength are assigned to DLE (deep level emission) [34,35]. These deeper levels are mainly attributed to In, O, and/or O/In vacancies/defect centres already reported in literature [36–39]. Moreover, it is clear from the figure that the intensity of the blue emission in the range 420–480 nm increases considerably with the Mn doping up to 3% and suggests maximum vacancies/defects centres in 3% Mn-doped In2O3 nanoparticles.
Please cite this article as: K. Anand, et al., Structural, optical and gas sensing properties of pure and Mn-doped In2O3 nanoparticles, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.03.233i
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Fig. 12. The sensor response variation of In2O3 and 3% Mn-doped In2O3 gas sensors as a function of operating temperature and test gas concentration (methanol, ethanol, acetone, ammonia vapours and LPG).
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3.3. Gas sensing properties
2NH3 +3O− (ads)⟶N2 +3H2 O+3e−
Gas sensing characteristics were carried out systematically to study the effect of Mn doping, operating temperature, and gas concentration on the sensor response of In2O3 gas sensor towards reducing test gases such as methanol, ethanol, acetone, ammonia vapours and LPG. As we know, the gas sensing is a surface phenomenon and depends upon the variation in electrical resistance of the sensor due to adsorption and desorption of test gas molecule on the surface of sensing film. Fig. 8 shows the sensor response of In2O3 and Mn-doped In2O3 nanoparticles sensors as a function of operating temperature in the range from 250–450 °C towards target gases. As shown in Fig. 8, 300 °C is the optimum operating temperature for methanol, ethanol, 350 °C is for acetone, and 400 °C is for ammonia vapours and LPG and this is due to the reason that sensor shows the maximum sensor response at corresponding temperature. Fig. 9 shows the variation of sensor response with time for In2O3 and 3% Mn-doped In2O3 gas sensor and response and recovery time calculated from this are given in Table 3. Also, from Fig. 8, it is clear that higher sensor response value is obtained for 3% Mn-doped In2O3 nanoparticles for 50 ppm methanol, ethanol, acetone, ammonia vapours and LPG. The significant enhancement of sensor response of 3% Mn doping in In2O3 is due to the introduction of high oxygen vacancies, surface defects and metal interstitials within the lattice of In2O3 nanoparticles due to Mn doping as confirmed by the photoluminescence spectra. From Fig. 7 it is clear that 3% Mn doped In2O3 nanoparticles shows maximum vacancy centres and these vacancies and surface defect centres are the key factors promoting the adsorption of gas molecules on the surface of the sensor and hence the increase in sensor response [35,40]. Another reason for the increase of sensor response is large lattice distortion and high strain of Mn doped In2O3 nanoparticle, due to the presence of lattice mismatch between In3 þ and Mn3 þ ions confirmed by XRD and Raman studies and this lattice distortion is advantageous for the interaction between test gas molecule and the sensor surface [41]. The difference in gas sensor response towards different gases might be due to difference in adsorption and the reaction process. In2O3 is n type metal oxide semiconductor (SMO) and when it is exposed to air, oxygen molecule O2 adsorbed on the sensor surface in the form of oxygen species such as ( O−2 , O , O2 ) formed by capturing electrons from the In2O3 conduction band depending upon operating temperature. As a result, electron concentration in In2O3 conduction band decreases and hence resistance of In2O3 sensor increases. The state at which oxygen molecules are adsorbed on the sensor surface are given in the following reactions [15]:
Cn H2n +2 +2O− (ads)⟶Cn H2n −O+H2 O+2e−
O2 (gas)⟷O2 (ads)
References
O2 (ads)+e−⟷O2− (ads)
O−2 (ads)+e−⟷2O− (ads) O− (ads)+e−⟷O2 − (ads) In the presence of Methanol, Ethanol, Acetone, Ammonia vapours and LPG, the sensing mechanism can be explained through the following reactions [15,42–44].
CH3 OH+3O− (ads)⟶CO2 +2H2 O+3e− C2 H5 OH+6O− (ads)⟶2CO2 +3H2 O+6e−
CH3 COCH3 +8O− (ads)⟶3CO2 +3H2 O+8e−
9
The interaction of target gas with surface adsorbed oxygen species results in releasing electrons back to In2O3 conduction band, thereby decreasing the resistance of the sensor. Hence, by monitoring the resistance of sensor in the presence or absence of target gas, the sensor response magnitude is determined. Moreover, to check the stability of In2O3 and 3% Mn-doped In2O3 gas sensors, the SR towards ethanol have been measured at different intervals for 60 days (Fig. 10) and have been found that sensors show stable SR. Fig. 11 illustrates the reproducibility of In2O3 and 3% Mn-doped In2O3 gas sensors, revealing that the sensor maintains its initial response amplitude upon four successive sensing tests to 50 ppm of methanol, ethanol, acetone, ammonia vapours and LPG, indicating that the sensor has a good stability throughout the cycle test. The sensor response of In2O3 and 3% Mn-doped In2O3 gas sensors as a function of operating temperature (250–450 °C) and test gas (ethanol, methanol, acetone, ammonia vapours and LPG) concentration (50–1250 ppm) is shown in Fig. 12. Sensor response is found to exhibit similar behaviour with temperature at all concentration of test gas.
4. Conclusion Pure and (1%, 3% and 5%) Mn-doped In2O3 nanoparticles have been prepared by co-precipitation. All the prepared In2O3 nanoparticles possessed cubic bixbyite crystal phase and decrease in lattice constant with Mn doping owing to decrease in ionic radius of Mn3 þ as compared to In3 þ . The absorption band in visible region corresponds to 5Eg → 5T2g spin allowed transitions and confirms the incorporation of Mn3 þ in host In2O3 lattice. This is supported by Raman spectra also. Photoluminescence spectra of In2O3 nanoparticles shows an increase in the emission intensity related to increase in oxygen vacancies/defects with Mn doping. An enhanced SR is achieved for 3%Mn doping, has been attributed to oxygen vacancies/defects generated due to Mn doping.
Acknowledgement The authors are thankful to the Guru Nanak Dev University's Central Instrumental Facility for providing Raman Spectrophotometer, TEM and XRD facilities.
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