Temperature dependent selectivity towards ethanol and acetone of Dy3+-doped In2O3 nanoparticles

Chemical Physics Letters 670 (2017) 37–45

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Research paper

Temperature dependent selectivity towards ethanol and acetone of Dy3+-doped In2O3 nanoparticles Kanica Anand, Jasmeet Kaur, Ravi Chand Singh, Rengasamy Thangaraj ⇑ Department of Physics, Guru Nanak Dev University, Amritsar 143005, India

a r t i c l e

i n f o

Article history: Received 8 October 2016 In final form 25 December 2016 Available online 3 January 2017 Keywords: In2O3 Nanoparticles Gas sensor Selectivity Dy-doping

a b s t r a c t In this paper, the influence of Dy3+ doping on the sensor response (SR) and selectivity of In2O3 sensors for selective detection of ethanol and acetone has been studied. (0, 1, 5, 10%) Dy3+-doped In2O3 nanoparticles has been prepared employing a co-precipitation method and characterized by XRD, RAMAN, TEM, EDS and Photoluminescence studies. It has been observed that Dy3+ doping inhibits the nanoparticles growth and increases the lattice constant, structural disorder, activation energy, defect concentration and surface basicity. The highest SR for 10% Dy3+-doped In2O3 sensor has been ascribed to small size, high defects, high surface basicity and large lattice distortion. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor metal oxide (SMOx) such as SnO2, TiO2, ZnO, In2O3, WO3 have been the subject of a widespread investigation as a resistive gas sensor due to the sensitivity of their electrical resistance towards oxidising and reducing gas [1–5]. The majority of SMOx are n-type and the change in resistance, mainly arises due to the charge transfer interactions between the conduction band of SMOx and reactive gases adsorbed on their surface like O2, N2, CO, H2S, hydrocarbons and volatile organic compounds, etc. [6]. As gas sensing is a surface phenomenon, so for the enhancement of the sensor performance, great emphasis has been employed in research towards the nano sized material because of their large surface area to volume ratio and great miniaturization potential that permit quick diffusion of gas molecule on its surface [7]. Also, the sensitivity of nano-sized metal oxide begins to increase sharply as crystallite size decreases below critical value, which is equal to twice the thickness of schottky barrier/depletion region penetrating in the metal oxide grains. Under this condition the whole crystalline is depleted of electrons. As a result, the sensitivity will increase. Among many SMOx, In2O3 is a promising candidate as a gas sensor, due to their good thermal stability, chemical stability, high electrical conductivity and wide band gap energy of 3.5–3.75 eV [8]. Extensive methods have been reported in the literature for the improvement of the sensor performance of In2O3 based gas ⇑ Corresponding author. E-mail address: [email protected] (R. Thangaraj). http://dx.doi.org/10.1016/j.cplett.2016.12.057 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.

sensors like doping and modification of surface properties etc. [9–11]. Doping has gained widespread interest, since it can significantly affect the sensitivity and selectivity of the sensor by tuning the properties of host by creating oxygen vacancies/defects, lattice disorder, strain, grain size refinement and surface area to volume ratio amendment etc. Doping via noble metals and transition metals has been extensively studied to enhance the sensing properties of the sensor. For example. Inyawilert et al. prepared Pt-doped In2O3 nanoparticles by flame spray pyrolysis for NO2 sensing [12]. Huang et al. have fabricated Pd-loaded In2O3 nanowire–like network using carbon nanotube as a template for enhancing NO2 sensing performance [13]. Zhou et al. examined the gas-sensing properties of Fe-doped In2O3 nanotubes synthesized by electrospinning technique [14]. Yang et al. have fabricated ordered mesoporous Ni-doped In2O3 low operating temperature gas sensor selective to NO2 [15]. Anand et al. reported Mn-doped In2O3 nanoparticles gas sensor sensitive towards various volatile organic compounds [16]. But the rare earth doping can exert an active effect on sensing performance because of their properties like highly efficient catalyst, high surface basicity and fast oxygen ion mobility [17]. Han et al. have prepared rare earth (Tm, Er, La, Yb and Ce) doped In2O3 nanostructures with three-dimensionally ordered macro porous structure by colloidal crystal templating method and investigated their ethanol sensing properties [18]. Qina et al. have prepared Er-doped In2O3 nanoribbon and discussed their alcohol sensing properties [19]. In the present study, we report for the first time, the improvement of In2O3 sensor by rare earth dysprosium doping that shows

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Table 1 The synthesis parameter of In2O3 and Dy3+-doped In2O3 nanoparticles.

a

Sample

Dopant %age

Amount of In(NO3)3
xH2Oa (g) (±0.0001)

Amount of dopant Dy(NO3)3
5H2O (g) (±0.0001)

Experimental at% Dy EDX

In2O3 In2O3:1% Dy3+ In2O3:5% Dy3+ In2O3:10% Dy3+

0 1 5 10

1.2033 1.1912 1.1431 1.0829

0.0175 0.0877 0.1754

0 1.05 3.98 8.84

x=0.

temperature dependent sensitivity and selectivity towards ethanol at 300 °C and acetone at 350 °C. In2O3 and (1%, 5% and 10%) Dy3+doped In2O3 nanoparticles are prepared via a facile, feasible and economical co-precipitation method. Structural, optical and electrical properties and, gas sensing properties towards methanol, ethanol, acetone, ammonia, LPG, H2 gas of the synthesized nanoparticles has been investigated and correlated. The comparison between In2O3 and Dy3+-doped In2O3 nanoparticles sensor reveals that 10% Dy3+-doped In2O3 sensor shows superior gas sensing properties. The highest sensor response may be ascribed to small size, high oxygen vacancies/defects, high surface basicity and large lattice distortion of doped In2O3 nanoparticles gas sensor.

2. Experimental details 2.1. Chemicals used

2.4. Gas sensor fabrication and response test The basic fabrication process is as follows: In2O3 nanoparticles were mixed with water and ground in an agate mortar to form a gas-sensing paste. The paste was coated on an alumina substrate on which a pair of Au electrodes was previously printed, then dried at room temperature for 5 min in the air, and then sintered at 400 °C for 1 h. The gas sensing properties of sensors has been investigated by employing home-made apparatus comprise of 40L test chamber contain sample holder, temperature controlled oven, and mixing fan. In the measuring electric circuit, load resistance is connected in series with the sensor, circuit voltage is 12 V, and output voltage is the voltage across the load resistance [16]. By monitoring the output voltage as a function of time using Keithley Data Acquisition Module KUSB-3100 and computer, the resistance of the sensor in the air or test gas can be measured at working temperature varying from 250 to 450 °C. The sensor response (SR) magnitude has been defined as Ra/Rg, where Ra is

All the chemical reagents, indium (III) nitrate (In(NO3)3), dysprosium (III) nitrate pentahydrate (Dy(NO3)3
5H2O and ammonium hydroxide (NH4OH) used in our research are of analytical grade and used without further purification.

2.2. Synthesis process Pure indium oxide and (1%, 5% and 10%) dysprosium doped indium oxide nanoparticles were prepared by employing a coprecipitation technique. In a typical synthesis, (In(NO3)3) was dissolved in distilled water to form 0.2 M solution. The NH4OH was added drop wise to the formed solution with stirring until the pH of the solution reached 9. The prepared precipitates were separated by filtration, washed with ethanol several times, and dried at 80 °C for 5 h. The dried precipitates were calcined at 500 °C for 3 h. Likewise, Dy3+-doped In2O3 nanoparticles with varying dysprosium concentration (1, 5 and 10%) were prepared by adding the appropriate amount of Dy(NO3)3
5H2O into the In(NO3)3 solution in water (Table 1.).

2.3. Characterization The prepared nanoparticles were characterized by X-ray diffraction (XRD, D8 Focus, Bruker, Germany, with Cu Ka radiation, wavelength 1.54056 Å) in the 2h range of 20–70° with the step size of 0.02°, transmission electron microscopy (TEM, JEOL TEM-2100), RAMAN spectroscopy (Renishaw Invia Microscope using argon ion laser at 488 nm as excitation wavelength in the wavenumber range of 100–900 cm1 at spectral resolution of 1 cm1), photoluminescence spectroscopy (PL, Lambda 45, Perkin Elmer fluorescence spectrometer at 310 nm as excitation wavelength in the wavelength range of 350–500 nm with the resolution of 0.5 nm and energy dispersive spectroscopy (EDS) was attained on a scanning electron microscope (FE-SEM, Carl Zeiss Supra 55).

Fig. 1. (a) XRD patterns and (b) WH plot of pure In2O3 and Dy3+-doped In2O3 nanoparticles.

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K. Anand et al. / Chemical Physics Letters 670 (2017) 37–45 Table 2 Crystallite size (D), Lattice constant (a), Strain (e), Activation energy (Ea) of In2O3 and Dy3+-doped In2O3 nanoparticles. Sample

a (Å) (±0.002 Å)

Crystallite size D (nm) DS ( ± 0.3 nm)

DW

In2O3 In2O3:1% Dy3+ In2O3:5% Dy3+ In2O3:10% Dy3+

10.109 10.122 10.132 10.136

11.6 9.4 7.8 4.8

12.3 10.9 9.7 7.6

Strain e * 104

Ea (eV)

0.13 17.12 18.41 30.21

0.19 0.22 0.25 0.27

Fig. 2. Raman shift of pure In2O3 and Dy3+-doped In2O3 nanoparticles and and the inset shows the magnified view of (132 cm1) peak.

the sensor resistance in air ambience and Rg is resistance in air-gas mixture. 3. Result and discussion 3.1. X-ray diffraction The crystallinity and structure of the synthesized pure In2O3 and Dy3+-doped In2O3 nanoparticles are investigated by X-ray diffraction patterns shown in Fig. 1a. All the diffraction peaks are matched with the standard XRD data (JCPDS) indicating the cubic bixbyte phase of bulk In2O3. No additional diffraction peaks are observed even for 10% Dy3+-doped In2O3. Furthermore, peaks of doped In2O3 nanoparticles shifts towards lower 2h as compared to pure In2O3 nanoparticles, indicating the increase of the lattice constant ‘a’ and lattice distortion which is due to the doping of Dy3+ having larger ionic radii than In3+ into the In2O3 lattice [20]. The lattice parameter ‘a’ and crystallite size ‘d’ have been calculated by employing the Bragg’s law and Debye Scherrer equation and are given in Table 2. The decrease in the crystallite size with increase in Dy3+ doping is mainly accredited to the distortion of the host In2O3 lattice by larger Dy3+, which inhibits the nucleation and growth rate of In2O3 nanoparticles. Besides the crystallite size, strain present in the lattice also contributes in the broadening of the XRD peaks and is calculated by Williamson Hall plot (Fig. 1b) using the following equation [21]

bcosh=k ¼ k=DW þ 4esinh=k where k, 2h, b, k, DW and e are the X-ray wavelength, Bragg’s diffraction angle, Full width at half maxima, Scherer’s constant (k = 0.9), crystallite size and micro-strain respectively. The crystallite size calculated from the WH plot follows the similar trend as obtained by Scherer formulae given in Table 2. The calculated tensile strain value (Table 1) increases with Dy3+ doping in In2O3 and is mainly

Fig. 3. (a, b) Low resolution TEM images; (c, d) high-resolution TEM images; (e, f) SAED pattern and (g, h) particle size histogram of In2O3 and 10%Dy3+-doped In2O3 nanoparticles.

attributed to the lattice expansion, which is confirmed from the increased lattice parameter with doping [22].

3.2. Raman study Room temperature Raman spectra were recorded in the spectral range of 100–900 cm1 to investigate the structural properties of

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Fig. 4. EDS spectra of (a) In2O3 and (b) 10%Dy3+-doped In2O3 nanoparticles.

were observed. The relative intensity of the peak at 367 cm1 related to the defect due to oxygen vacancies with respect to maximum intensity peak at 132 cm1 increases with the Dy3+ content [24]. Also, appreciable broadening and shifting of Raman peak is observed with increase in doping content, which is generally observed if the size of the particles is in the nanometre range and this behaviour is explained by a phonon confinement model [25]. In addition, the intensity of the Raman peaks (132, 308, 497, 630 cm1) decreases with increase in Dy3+ concentration, which may be due to loss of crystallinity and increase of structural disorder also confirmed by XRD study [26]. 3.3. TEM study

Fig. 5. Photoluminescence spectra of In2O3 and Dy3+-doped In2O3 nanoparticles.

The morphology and structural parameters of the pure and Dy3+-doped In2O3 nanoparticles have been investigated by TEM and HRTEM images. Fig. 3(a)–(d) shows the low resolution (TEM) images and high resolution HRTEM images of In2O3 and 10% Dy3+-doped In2O3 which clearly reveals the spherical shape nanoparticles with agglomeration. The average particle size is about 11.1 ± 0.2 nm and 5.8 ± 0.1 nm calculated by plotting a histogram for 100 particles (Fig. 3(g, h)). The corresponding spotted ring like SAED pattern (Fig. 3(e, f)) confirms that the In2O3 nanoparticles are of crystalline cubic bixbyte phase [27]. In addition, the interplanar distance calculated using IMAGE J software from HRTEM images is 0.29 nm and 0.41 nm corresponds to (2 2 2) and (2 1 1) plane of c-In2O3 nanoparticles for both pure In2O3 and Dy3+-doped In2O3 nanoparticles respectively [28]. 3.4. Energy dispersive spectra EDX spectra of In2O3 and 10%Dy3+-doped In2O3 nanoparticles is shown in Fig. 4. The presence of Dy, O and In in doped In2O3 samples indicates the purity of the samples. The EDX analysis is taken from different parts of the sample and the average value of concentration of Dy is listed in Table 1. 3.5. Photoluminescence study

3+

Fig. 6. Temperature dependence of conductance of In2O3 and Dy -doped In2O3 nanoparticles.

In2O3 and Dy3+-doped In2O3 nanoparticles. The pure In2O3 spectrum (Fig. 2) exhibits the vibrational modes at 132, 308, 367, 497 and 630 cm1 which is an unambiguous signature of bcc-In2O3 [11,23]. On doping Dy3+ into In2O3 several interesting features

Fig. 5 represents the room temperature PL spectra of pure In2O3 and Dy3+-doped In2O3 nanoparticles taken at an excitation wavelength of 310 nm. The PL emission peak at 376 nm corresponds to NBE (near band emission) originate due to the recombination of free excitons [16]. The deep level (DL) emission peaks at higher wavelengths (420, 441, 455 and 480 nm) are related to defects such as O vacancy (Vo), O interstitial (Oi), In vacancy (VIn), In

K. Anand et al. / Chemical Physics Letters 670 (2017) 37–45

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Fig. 7. Histogram of sensor response versus operating temperature of In2O3 and Dy3+-doped In2O3 sensors towards methanol, ethanol, hydrogen gas, acetone, ammonia and LPG.

interstitial (Ini) already reported in the literature [29–32]. Also, it is clear from the Fig. that the intensity of DL emissions increases with increase of Dy3+ in In2O3, indicates the enhancement of defects with doping.

3.6. Electrical properties The intrinsic activation energy for In2O3 and Dy3+-doped In2O3 sensors has been calculated from Ln G versus 1000/T plot (Fig. 6)

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Fig. 8. Sensor response versus time graph for In2O3 and 10% Dy3+-doped In2O3 sensors towards 50 ppm of methanol, ethanol, hydrogen gas, acetone, ammonia and LPG at their respective optimum operating temperatures.

Table 3 Response and recovery time of In2O3 and 10% Dy3+-doped In2O3 sensors. Target gas

Operating temperature (°C) (±1 °C)

Methanol Ethanol Hydrogen Acetone Ammonia LPG

300 300 300 350 400 400

In2O3: 10%Dy3+

In2O3 Response time (s) (±1 s)

Recovery time (s) (±1 s)

Response time (s) (±1 s)

Recovery time (s) (±1 s)

10 40 8 15 11 12

34 38 25 17 32 32

12 31 6 5 10 9

36 32 29 18 37 31

Fig. 9. Schematic diagram of gas sensing mechanism.

K. Anand et al. / Chemical Physics Letters 670 (2017) 37–45

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Fig. 10. Histogram showing the selectivity of 10% Dy3+-doped In2O3 sensors towards (a) ethanol at 300 °C and (b) acetone at 350 °C.

Fig. 11. Curves showing the stability of sensors towards 50 ppm (a) ethanol at 300 °C and (b) acetone at 350 °C.

for temperature range (300–450 °C) using the following equation [33] and listed in Table 2.

LnG ¼ LnGo þ

  eVs kT

where k the Boltzmann’s constant, T the absolute temperature, Go the bulk intragranular conductance, and eVs the activation energy. It is clear from the Table 2 that as the size of particles decreases the activation energy increases. This is due to the reason that the depletion region extends deeper into the grains as the size of the particle decreases which leads to the narrowing of the conduction region and hence increase of activation energy or intergranular energy barrier and its detailed discussion is already reported in the literature [34,35]. 3.7. Sensing properties The sensor response of In2O3 and Dy3+-doped In2O3 sensors towards 50 ppm methanol, ethanol, ammonia, acetone vapours, LPG and H2 gas as a function of operating temperature in the tem-

perature range of 250–300 °C is shown in Fig. 7. The optimum operating temperature of In2O3 and Dy3+-doped In2O3 sensor for methanol, ethanol, H2 is 300 °C, for acetone is 350 °C, for ammonia and LPG is 400 °C. From the Fig. 8 that represents the sensor response versus time plot for In2O3 and 10% Dy3+-doped In2O3 sensor for methanol, ethanol, ammonia, acetone vapours, LPG and H2 gas, the response and recovery time are calculated and listed in Table 3. It is clear from the Fig. 7 that 10% Dy3+-doped In2O3 sensor exhibits a maximum sensor response towards 50 ppm methanol, ethanol, ammonia, acetone vapours, LPG and H2. The gas sensing mechanism and the enhancement in sensor response due to Dy3+ doping into In2O3 is explained below: Sensing mechanism of n-type SMOx sensor depends upon the adsorption of oxygen molecule (O2) from air onto SMOx surface 2 and ionisation of O2 into O or O by capturing free electrons 2, O from the conduction band of SMOx. During this process the econcentration of conduction band decreases and thereby increases the resistance of SMOx sensor. This adsorption can be represented by chemical equations as follows [36]:

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3.7.2. Stability In order to check long term stability, the sensors were tested at regular interval for 60 days for ethanol and acetone and found to have an almost stable response (Fig. 11). 3.7.3. Variation of sensor response magnitude versus test gas concentration The SR of In2O3 and 10% Dy3+-doped In2O3 at different concentration (50-1250 ppm) of methanol, ethanol, acetone, ammonia vapours, LPG and H2 gas is shown in Fig. 12. It is found that SR increases with an increase in the concentration. 4. Conclusion

Fig. 12. Variation of sensor response of 10% Dy3+-doped In2O3 sensor towards different concentration ethanol and acetone at their optimum operating temperature of 300 °C and 350 °C respectively.

O2 ðgasÞ $ O2 ðadsÞ O2 ðadsÞ þ e $ O2 ðadsÞ O2 ðadsÞ þ e $ O2 ðadsÞ O ðadsÞ þ e $ O2 ðadsÞ After this, the target reducing gas such as Methanol, Ethanol, Acetone, Ammonia, LPG and H2 is exposed to the surface of SMOx senor that reacts with the adsorbed oxygen ions and releasing captured e-s back to the SMOx conduction band and decrease the resistance of SMOx as explained through Fig. 9 [37–42] and hence increases the SR. The sensor response enhancement of Dy3+-doped In2O3 sensor is mainly due to the augmentation in the interaction between the sensor surface and target gas due to following reasons:  Decrease in the crystallite size due to increase of dopant concentration (confirmed from XRD and TEM), results in increase of surface area, which provides a large number of gas adsorption surface sites and hence high sensor response.  The enhancement of the defects due to Dy3+ doping into In2O3 (as confirmed from PL and Raman studies) are the key factors enhancing the adsorption of gas molecule on the surface and hence sensor response [43,44].  Due to differences in the ionic radii of Dy3+ and In3+, there is large lattice distortion and strain (confirmed form XRD and Raman studies). This distortion is beneficial for the interaction between target gas and sensor surface which leads to high sensor response [19].  Because of the basic nature of rare earth oxide, the surface basicity of rare earth Dy3+-doped In2O3 increases, which enhance the sensor response towards reducing gases [8,17].

3.7.1. Temperature dependence selectivity test The SR of In2O3 and 10% Dy3+-doped In2O3 sensors towards methanol, ethanol, acetone, ammonia vapours, LPG and H2 gas at 300 °C and 350 °C is shown in Fig. 10a and b respectively. It is clearly seen from the figure that at 300 °C the sensor is selective to ethanol and at 350 °C is selective to acetone.

In summary, pure In2O3 and (1%, 5% and 10%) Dy3+-doped In2O3 nanoparticles are prepared via co-precipitation method and consist of cubic bixbyte-type In2O3 phase. The rare earth Dy3+- doping has great influence on the structural, optical, and gas sensing properties of In2O3 nanoparticles sensor. With increase in Dy3+ concentration the decrease in crystalline size and increase in lattice constant has been observed, which is attributed to a large ionic radius of Dy3+ as compared to In3+. The Raman modes become broad, relative less intense and shifts as Dy3+ content in In2O3 increases. TEM micrographs display the spherical shape nanoparticles and EDX results supported the presence of Dy3+ in doped In2O3 nanoparticles. Photoluminescence spectra show an increase in the emission intensity of deep level emissions related to oxygen vacancies/defects. From electrical studies, it is observed that the activation energy increases as the size of the particle decreases. The gas sensing properties of pure In2O3 and Dy3+-doped In2O3 nanoparticles have been investigated towards 50 ppm methanol, ethanol, acetone, ammonia, LPG and H2 gas a different operating temperature.10% Dy3+-doped In2O3 sensor exhibits the highest sensor response (SR) value and temperature dependent selectivity towards ethanol at 300 °C and acetone at 350 °C and also show great stability. An enhanced SR for this sensor is mainly attributed to small size, high oxygen vacancies/defects, high surface basicity and large lattice distortion of 10% Dy3+-doped In2O3 nanoparticles gas sensor. Acknowledgment The authors are thankful to the Guru Nanak Dev University’s Central Instrumental Facility for providing TEM, EDS, Raman Spectrophotometer and XRD facilities. References [1] N. Ma, K. Suematsu, M. Yuasa, T. Kida, K. Shimanoe, Effect of water vapor on Pd-loaded SnO2 nanoparticles gas sensor, ACS Appl. Mater. Interfaces 7 (2015) 5863–5869. [2] K.W. Tae, K.I. Ho, C.W. Youl, Fabrication of TiO2 nanotube arrays and their application to a gas sensor, J. Nanosci. Nanotechnol. 15 (2015) 8161–8165. [3] E. Dilonardo, M. Penza, M. Alvisi, C.D. Franco, F. Palmisano, L. Torsi, Ni. Cioffi, Evaluation of gas-sensing properties of ZnO nanostructures electrochemically doped with Au nanophase, Beilstein J. Nanotechnol. 7 (2016) 22–31. [4] X. Xu, H. Zhang, X. Hu, P. Sun, Y. Zhu, C. He, S. Hou, Y. Sun, G. Lu, Hierarchical nanorod-flowers indium oxide microspheres and their gas sensing properties, Sens. Actuat. B 227 (2016) 547. [5] Y. Wang, B. Liu, S. Xiao, X. Wang, L. Sun, H. Li, W. Xie, Q. Li, Q. Zhang, T. Wang, Low-temperature H2S detection with hierarchical Cr-doped WO3 microspheres, ACS Appl. Mater. Interfaces 8 (2016) 9674–9683. [6] C. Wang, L. Yin, L. Zhang, D. Xiang, R. Gao, Metal oxide gas sensors: sensitivity and influencing factors, Sensors 10 (2010) 2088–2106. [7] G.J. Cadena, J. Riu, F.X. Rius, Gas sensors based on nanostructured materials, Analyst 132 (2007) 1083–1099. [8] K. Anand, J. Kaur, R.C. Singh, R. Thangaraj, Effect of terbium doping on structural, optical and gas sensing properties of In2O3 nanoparticles, Mater. Sci. Semicond. Process. 39 (2015) 476–483.

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