Highly Sensitive, Temperature-Independent Oxygen Gas Sensor based on Anatase TiO 2 Nanoparticles-grafted, 2D Mixed Valent VO x Nanoflakelets

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1 Highly Sensitive, Temperature-Independent Oxygen Gas Sensor based on Anatase TiO 2 Nanoparticles-grafted, 2D Mixed Valent VO x Nanoflakelets Appu Vengattoor Raghu, Karthikeyan K Karuppanan and Biji Pullithadathil #*. Nanosensor Laboratory, PSG Institute of Advanced Studies, Coimbatore, , INDIA # Department of Chemistry, PSG College of Technology, Coimbatore , INDIA Figure S1. XRD patterns of VO X materials prepared at 300, 400 and 600 C under nitrogen atmosphere. XRD analysis (Figure S1) was performed to investigate the crystal structures of synthesized VO x at various temperatures 300, 400 and 600 C under nitrogen atmosphere. The VO x prepared at 300 C clearly indicated the formation of NH 4 VO 3 /VO 2 mixed phases (VO 2 and un-reacted state of vanadium (JCPDS No )) and the sample prepared at 400 C and 500 C under N 2 atmosphere exhibited two mixed valent states, VO 2 (JCPDS ) and V 4 O 9 (JCPDS ).

2 Figure S2. TEM images of TiO 2 nanoparticles prepared by hydrothermal method The size and morphology of TiO 2 was determined by transmission electron microscopy, as shown in Figure S2. The inter-planer spacing of TiO 2 was estimated to be 0.35 nm and corresponding SAED confirmed the anatase crystal structure of TiO 2 nanoparticles. Figure S3. EDS analysis of VO x -500N (a), VO x -500A (b), TiO 2 nanoparticles (c) and VO x /TiO 2 nanoflakelets (d).

3 The presence of TiO 2 on vanadium oxide flakeletes was confirmed by energy dispersive spectrum of VO x /TiO 2 nanocomposite, as shown in Figure S3. Figure S3 (a, b, c and d) shows the spectrum of VO x -400N, VO x -500N, VO x /TiO 2 and VO x -500A respectively. Figure S4. Raman spectra of VO x -500-A, TiO 2 and hybrid VO x /TiO 2 nanoflakelets. Figure S5. AFM images (left) and measured thickness (right) of VO x -500N (a-b) and VO x -500A (c-d)

4 The semi-contact mode AFM (Figure S5) imaging was carried out to study the surface morphology and measure the thickness of VO x nanoflakelets in VO x -500N and VO x -500A samples. The calculated thickness of VO x nanoflakelets was nearly about 4 nm in both the samples that facilitating the surface reactivity. Figure S6. DSC curves of VO x -300, VO x -500A, TiO 2 and VO x /TiO 2 composites at different conditions. The growth formation of phase transitions from V 4 O 9 to V 2 O 5 in VO x nanoflakelets was studied through DSC curves, shown in figure S6. The peaks appeared at 426 C is indicated the transformation of V 4 O 9 to V 2 O 5. Figure S7. Current-Voltage (I-V) Characteristics of (a) TiO 2 nanoparticles (b)vo X -500A and (c) VO X /TiO 2 nanoflakelets

5 The temperature dependant electrical measurements were carried out by I-V measurements using Keithley 2420 electrometer. The resistance values were calculated from I-V plot and detailed analysis is depicted in Figure 6a. Figure S8. Oxygen sensing performance of VO x /TiO 2 nanoflakelets (100 ppm concentration at 500 C). Figure S8 shows the oxygen sensing performance of the VO X /TiO 2 nanoflakelets for the concentration of 100 ppm at 500 C. After the saturation level of oxygen adsorption, the Fermi level bears a constant relationship with the bottom of the bulk conduction band which reduces to the same energy level of LUMO.

6 Figure S9. Temperature dependent ( C) oxygen gas sensing performance of VO x nanoflakelets at various concentrations (100, 200 and 300 ppm) Figure S10. Oxygen gas sensing performances of VO x and VO x /TiO 2 nanoflakelets at 100 C. The performance of the VO x nanoflakelets based sensor was performed and shown in Figure S9. Due to the lack of active surface sites in VO x nanoflakelets, the sensitivity of VO x was found to be lower (S % ~1-2%). The dynamic response of VO x /TiO 2 nanoflakelets based oxygen sensor is compared with VO x nanoflakelets as shown in Figure 7, S9 and S10.

7 Figure S11. (a-c) Elovich plot and their parameters, (d) response time and recovery time towards O 2 gas adsorption on VO x /TiO 2 nanoflakelets. Figure S12. (a) Elovich plot for O 2 (500 ppm) adsorption on VO x /TiO 2 surfaces at different temperature ( C). The detailed understanding of the VO x /TiO 2 nanoflakelets based gas sensor was studied using Elovich plots. The sensor showed faster recovery and response at temperatures above 150 C.

8 The oxygen gas sensing properties in trace level (minimum 100 ppm) were validated by Elovich plots and the fast adsorption kinetics analysis was studied at various temperatures 100,150, 200, 300, 400 and 500 C. Figure S13. Plot of sensitivity (%) versus relative humidity (%) at 100 C Figure S14. (a) The reproducibility studies of VO x /TiO 2 nanoflakelets towards 100 ppm oxygen gas.(b) Oxygen sensing performances of 100 ppm oxygen gas exposure times of 5 minutes and 9 minutes. The VO x /TiO 2 nanocomposite based sensor fabricated 6 months with same sensitivity (~0.7% drift) before have shown very good stability as well as reproducibility. Negligible drift (0.5 %)

9 was found in the base resistance during reproducibility tests and the continuous adsorptiondesorption cycles indicating reproducibility are shown in figure S14. The long term stability was also tested by extending exposure time upto 5 and 9 minutes in order to understand the stability of gas sensor at longer time exposure time. The results showed that the sensor showed a better stability towards oxygen gas even after prolonged exposure time. Figure S15. (a) Oxygen sensing performances of VO x /TiO 2 nanoflakelets towards percentage level oxygen gas concentration (1-4 %) In order to understand the performance comparison at higher concentrations of oxygen at % levels (1% to 4%), experiments were carried out as shown in Figure S15. The developed sensor based on VO x /TiO 2 nanoflakelets described in the present study mainly focuses on trace level detection (ppm level) of oxygen gas. However, the sensor can also be used to detect oxygen gas at percentage levels which was further demonstrated. The results suggest that the sensor shows rapid response at trace-level concentrations (ppm) compared to % levels.

10 Figure S16. The selectivity studies of VO x /TiO 2 nanocomposites with various gases, such as oxygen (100 ppm), NO 2 (100ppm), H 2 (0.1%) and NH 3 (100ppm). The selectivity of VO x /TiO 2 sensor towards exposure to oxygen, nitrogen dioxide (NO 2 ), hydrogen (H 2 ) and ammonia (NH 3 ) gases were tested as shown in Figure S16. The results suggest that though VO x /TiO 2 nanoflakelets showed minimal sensitivity towards oxidizing gas, such as NO 2 gas, it exhibited high selectivity towards oxygen compared to reducing gases, such as H 2 and NH 3. Table S1: Comparison of sensor properties of VO x /TiO 2 nanoflakelets with reported temperature independent materials. Sl.No. Material Temperature independent range( C) Oxygen sensing factor Reference 1 (La0:7Sr 0:3 )(AlxFe 1 x )O (not a sensor) 2 BaFe 0.70 Ta 0.30 O 3-δ Not specified value 3 SrTi 0.7 Fe 0.3 O 3-δ Not specified value F. Zaza, AIP Conference Proceedings 1603, 53 (2014); doi: / Murat ektasa, Sensors and Actuators B,190 (2014) Ralf Moos, Sensors and actuators B 67

11 4 CNT/carbon black composite (not a sensor) Kunmo Chu, Sung- Chul Lee, RSC, Nanoscale, 2015,7, VO x nanoflakelets Sensitivity factor S%=2.5% for 100ppm Present Work 6 VO x /TiO 2 nanocomposites Sensitivity factor S%=35% for 100ppm Present Work Characterization techniques X-ray diffraction (XRD) analysis of the materials were acquired using X-ray diffractometer (Rigaku ULTIMA IV,,Japan) with a Cu-Kα radiation (λ= Å) source. Morphology and structure of the materials were examined using high-resolution transmission electron microscopy (HRTEM, JEOL-JEM 2000, Japan). The samples were drop-casted on holey carbon film supported copper grids (200-mesh) and TEM analysis was carried out at an accelerating voltage of 200 kev. The microstructural parameters were determined using JCPDS software. AFM analysis of the samples was performed using Multimode Scanning Probe Microscope (NTMDT-NTEGRA, Russia) using semi-contact imaging mode. Raman spectra were acquired using Horiba-Jobin (LabRAM HR, France) with an excitation wave length of 514 nm. Differential scanning calorimetry (DSC) was conducted to study the thermal properties of the synthesized material using NETZSCH (Germany) thermal analyzer. X-ray photoelectron spectroscopy (XPS) analysis was performed using Thermoscientifc, K-Alpha, with a monochromatic Al Kα (hυ = ev) radiation source. The electrical conductivities of the samples were studied using digital multimeter

12 (Keithely Instruments, model-smu-2420, USA) using two-probe method at room temperature under ambient conditions.