Highly Sensitive and Selective PbTiO3 Gas Sensors with Negligible

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Functional Inorganic Materials and Devices

Highly Sensitive and Selective PbTiO3 Gas Sensors with Negligible Humidity Interference in Ambient Atmosphere Xinghua Ma, Hua-Yao Li, Sang Hyo Kweon, Seong-yong Jeong, Jong-Heun Lee, and Sahn Nahm ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18428 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 13, 2019

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ACS Applied Materials & Interfaces

Highly Sensitive and Selective PbTiO3 Gas Sensors with Negligible Humidity Interference in Ambient Atmosphere

Xing-Hua Maa,§, Hua-Yao Lia,§, Sang-Hyo Kweona, Seong-Yong Jeonga, Jong-Heun Leea,*, Sahn Nahma,b,*

aDepartment

of Materials Science and Engineering, Korea University, Seoul 02841, Republic

of Korea. bDepartment

of Nano Bio Information Technology, KU-KIST Graduate School of Converging

Science and Technology, Korea University, Seoul 02841, Republic of Korea. *Authors

to whom correspondence should be addressed.

Email: [email protected]; Fax: +82-2-928-3584; Tel: +82-2-3290-3279 Email: [email protected]; Fax: +82-2-928-3584; Tel: +82-2-3290-3282 §These

authors contribute equally to this work.

ACS Paragon Plus Environment

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ABSTRACT: Three PbTiO3 nanostructures were synthesized using a one-step hydrothermal reaction with different TiO2 powders as Ti sources, and their gas sensing properties were investigated. The sensor comprising PbTiO3 nanoplates exhibited a high response (resistance ratio = 80.4) to 5 ppm ethanol at 300 °C and could detect trace concentrations of ethanol down to 100 ppb. Moreover, the sensor showed high ethanol selectivity and nearly the same sensing characteristics despite the wide range of humidity variation from 20% RH to 80% RH. The mechanism for humidity-independent gas sensing was elucidated using the diffuse reflectance infrared Fourier transform spectra. PbTiO3 nanoplates are new and promising sensing materials that can be used for detecting ethanol in a highly sensitive and selective manner with negligible interference from ambient humidity.

KEYWORDS: PbTiO3 nanoplate; gas sensor; ethanol; humidity interference; selectivity


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1. INTRODUCTION In the past few years, much attention has been focused on gas sensors used for detecting combustible, flammable, and toxic gases as they can be widely used in various industrial and domestic fields. Among the various candidate materials for gas sensors, metal oxides comprise an important class of chemiresistive sensors owing to their high response, fast responding speed, simple structure, and facile integration into small devices.1-3 In this category, binary metal oxides are normally used as the gas sensing material, and the representative ones are the n-type semiconductors such as SnO2,4,5 ZnO,6 In2O3,7 TiO2,8 and WO39 as well as the p-type semiconductors10 such as Co3O4,11 NiO,12 and CuO.13 Besides these binary metal oxides, ternary oxides, such as perovskite14,15 and spinel16 materials with distinctive physico-chemical and catalytic properties, are being considered as new sensing materials to detect various gases for monitoring environment, controlling food quality, and diagnosing the diseases from exhaled breath. In particular, perovskite structured materials with the general formula of ABO3 are excellent platforms for gas sensors because of compositional diversity, synergistic catalytic effect of constituting elements, and effective control of nonstoichiometry via the aliovalent doping in both A- and B-sites.17 Accordingly several perovskites have been employed for gas sensors, which include SrTiO3 as an oxygen sensor,18 CuO-loaded BaTiO3 as a CO2 sensor,19,20 and LaFeO3 as an ethanol sensor.21,22 Among the perovskite oxides, lead titanate (PbTiO3, PT) with a wide band gap of around 3.45 eV23 is an important ferroelectric and piezoelectric material


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because of its high Curie temperature (~490C), low aging rate of the dielectric constant,24 excellent ability of sustaining the ferroelectricity down to ultra-thin thickness.25 Since the existence of defects in perovskite oxides is normal,26 PbTiO3 can be considered as good candidate material for gas sensing applications. However, to the knowledge of authors, PbTiO3 (PT) has not been studied for gas sensors. One of the greatest obstacles to the widespread and reliable applications of oxide semiconductor gas sensors is the humidity dependence of the gas sensing characteristics.27-29 The ambient moisture is known to react with negatively charged adsorbed oxygen (O- or O2-) on oxide chemiresistors, which releases electrons.30,31 Thus, the majority of oxide semiconductors suffer a change in their sensor resistance and gas response with a variation in humidity, and thus, the elimination of moisture interference is of crucial importance because the ambient humidity changes dynamically according to variations in weather, climate, season, and location. Moreover, the measurement of ethanol or biomarker gases in the exhaled breath of intoxicated drivers or patients also requires reliable gas sensing at high or dynamically changing humidity conditions.32 In order to screen intoxicated drivers, the sensor should detect 200 ppm ethanol in exhaled breath, which is equivalent to approximately 0.5 g ethanol per liter of blood.33 It should be noted that the breath immediately after exhalation is highly humid and the humidity level of exhaled gas tends to decrease as the distance between the mouth and sensor


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increases because the exhaled breath is diluted by ambient air. Thus, the ethanol has to be detected at either a high or dynamically changing humidity. In this contribution, PbTiO3 nanoplates are suggested as a new perovskite-based gas sensing material with humidity-independent gas sensing characteristics over a wide range of humidity from 20% RH to 80% RH as well as a high response and selectivity to ethanol. Thus, three PbTiO3 nanostructures were prepared via a hydrothermal reaction of slurries containing Pbprecursors and different TiO2 powders, and their gas sensing characteristics were investigated. The mechanism underlying sensitive, selective, and humidity-independent gas sensing was studied in relation to the morphology of sensing materials and the interaction between adsorbed oxygen and water vapor.

2. EXPERIMEANTAL SECTION 2.1. Material Synthesis. PbTiO3 (PT) nanoplates (NPs) were synthesized using high-purity chemicals (> 99%) via a one-step hydrothermal method.34 Lead (II) nitrate (Pb(NO3)2, High Purity Chemicals, Osaka, Japan) and three types of titanium dioxide powders (Rutile, High Purity Chemicals, Osaka, Japan; Anatase, Sigma, USA; Degussa P25 TiO2, Evonik Corporation, Parsippany, USA, respectively) were used as the source materials. Analytical grade solid potassium hydroxide (KOH, > 99%, High Purity Chemicals, Osaka, Japan) was used as the mineralizer. For the specific synthesis processes, 5 mmol TiO2 powders were first added into


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30 ml of deionized (DI) water mixed with 0.24 mol KOH to form a milk-white suspension. Then, 10 ml of DI water with a 6.25 mmol Pb(NO3)2 solution was injected into the suspension drop by drop. After thoroughly mixing the solution using a magnetic stirrer, it gradually turned red. It was then transferred to a Teflon-lined stainless autoclave and reacted at 200 °C for 10 h. The samples thus obtained were filtered and washed using DI water several times and dried for the following characterizations. 2.2. Characterization. The phase structure of the as-synthesized PT products was identified using an X-ray diffractometer (XRD; Rigaku D/max-RC, Tokyo, Japan) with Cu Kα radiation (λ = 1.5418 Å). The morphology observations were performed using a scanning electron microscope (SEM; Hitachi S-4800, Tokyo, Japan). High-resolution transmission electron microscopy (HRTEM; Tecnai F20, FEI, the Netherlands) was performed to analyze the oriented direction of the exposed crystal plane of the nanoplates. 2.3. Gas-Sensing Measurement. The slurry containing PT products was drop-coated onto an alumina substrate with two Au electrodes using a micropipette. The sensors were named based on their Ti sources. That is, the PbTiO3 synthesized from rutile, anatase, and P25 TiO2 powders were referred as “rutile-PT,” “anatase-PT,” and “P25-PT,” respectively. Before performing the measurement, the sensor was annealed at 600 °C for 3 h to stabilize the sensors. The gas concentrations were controlled by adjusting the mixing ratio of the parent gases (5 ppm ethanol, xylene, acetone, toluene, HCHO, benzene, NH3, CO, and NO2 all in dry synthetic air


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balance) and dry synthetic air with a fixed flow rate of 200 cm3/min. For measuring the humidity dependence of the gas sensing characteristics, the room-temperature (25 C) ambient gases with various values of humidity were provided to the locally heated sensor. The sensor configuration and sensing system to measure gas sensing characteristics were shown elsewhere35 and the detailed procedure to control the humidity in the gas chamber was given in supporting information and Figure S1. The gas responses (S = Rg/Ra; Rg = resistance in gas, and Ra = resistance in air) of the sensors to the corresponding gases were measured by switching the gas atmosphere.

3. RESULTS AND DISCUSSION 3.1. Effect of TiO2 Source. Three types of TiO2 powders, rutile-TiO2, anatase-TiO2, and P25-TiO2 were used as the Ti source materials for the hydrothermal synthesis of PbTiO3. The average particle sizes of rutile-TiO2, anatase-TiO2, and P25-TiO2 powders were approximately 800 nm, 200 nm, and 35 nm, respectively. After the hydrothermal reaction, all the powders were yellow, which indicated the successful formation of the PbTiO3 products. However, their morphology depended closely on the source TiO2 powders. The rutile-PT sample showed cubic granular aggregates (Fig. 1a), while the anatase-PT and P25-PT specimens consisted of nanoplates. The formation of large cubes in rutile-PT (Fig. 1a) can be attributed to the coarse rutile TiO2 precursors of almost micron sizes (inset in Fig. 1a), while the uniform and small nanoplates in P25-PT (Fig. 1c) can be attributed to the fine TiO2 source powders (inset in Fig. ACS Paragon Plus Environment

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1c). Moreover, P25 is a hydrophilic fumed titanium dioxide powder that can be dispersed well in an aqueous solution. This promotes the hydrothermal reaction with the Pb source material, which can also facilitate the formation of uniform and fine P25-PT nanoplates. The lateral size of these P25-PT nanoplates is estimated to be approximately 1 μm, and their thickness ranges from 100 to 200 nm. 3.2. Characterization of the Sensing Materials. The XRD patterns of the rutile-PT, anatase-PT, and P25-PT nanostructures prepared using a hydrothermal reaction at 200 °C for 10 h are shown in Fig. 2a. All the patterns exhibit a pure perovskite phase without any other secondary phase. The intensity ratio of (001) and (100) peaks (I(001)/I(100)) in the rutile-PT nanocubes, anatase-PT nanoplates, and P25-PT nanoplates are 0.32, 0.77, and 1.03, respectively. The increase of the I(001)/I(100) value indicates the preferential growth of nanoplates and suggests that the dominant basal plane of the P25-PT nanoplate is the {001} crystal plane.34 The stronger (002) peak of P25-PT than the other two samples also supports this explanation. A low-resolution TEM image shows a typical single P25-PT nanoplate with a square morphology (Fig. 2b). The corresponding high-resolution TEM (HRTEM) image (Fig. 2c) shows well-ordered lattice fringes and the fast Fourier transform pattern (FFT, inset of Fig. 2c). Both of the lattice distances along the two yellow-lined directions are determined to be approximately 0.39 nm, and the intersection angle between them is 90°, which matched well with the (100) and (010) planes of the tetragonal PbTiO3 crystal. This indicates that each


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nanoplate is a single crystal. Furthermore, the spot pattern of the FFT is proved to be the {hk0} set reflections. These analyses confirm that the PT nanoplates are bounded by the {100} planes and orientated with the exposed plane of {001} of the perovskite tetragonal structure.34 3.3. Gas-Sensing Characteristics. The gas sensing transients of the sensors to 5 ppm ethanol at 300 °C are shown in Fig. 3. Three sensors were fabricated using rutile-PT, anatase-PT, and P25-PT nanostructures. For the purpose of comparison, a sensor was also fabricated using PbTiO3 powders prepared using the conventional solid-state reaction between P25 and PbO powders (named as “P25-PTpowder”). The Degussa P25 TiO2 and PbO powders (purity > 99%, High Purity Chemicals, Osaka, Japan) were mixed in a nylon jar filled with zirconia balls for 24 h. The resulting powders were subsequently dried and calcined at 880oC for 4 h. Finally, the calcined powders were re-milled for 24 h to obtain fine powders. The SEM image is shown in Fig. S2 (Supporting Information). All the four sensors showed p-type sensing characteristics. That is, the sensor resistance increased upon exposure to ethanol and returned to its original value in air atmosphere. It has been reported that in PbTiO3, the Pb vacancies act as shallow acceptors, leading to p-type semi-conductivity.26,36 Because of the high volatility of Pb, the evaporation of Pb might have been appreciable at the annealing temperature (600 °C).37 Thus, the p-type semiconducting behavior in the present study can be explained by the Pb vacancies. In p-type oxide semiconductors, a hole accumulation layer (HAL) is formed near the sensing surface by the ionized adsorption of oxygen, which facilitates the conduction along the shells.10


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When the reducing gas reacts with the adsorbed oxygen ions, the electrons are released and the sensor resistance increases owing to the decrease in the hole concentration in the HAL due to electron–hole recombination. Considering the sensing temperature (300 – 500C) in the present study, O- and O2- are feasible adsorbed oxygen species27 and the following reaction can be considered for ethanol sensing.

C2H5OH (g) + 6O-(ad) (or 6O2-(ad)) + 6h (or 12 h) 

2CO2 (g) + 3H2O (g)

(1)

The sensor response (S) of the P25-PT sensor to 5 ppm ethanol was 80.4, which was 5.8 times and 14.4 times higher than those of the rutile-PT (S = 13.8) and anatase-PT sensors (S = 5.6), respectively. The BET surface area of P25-PT, rutile-PT, and anatase-PT specimens were 1.11, 0.56, and 0.67 m2/g, respectively. Accordingly, the high gas response can be attributed to the wider sensing area resulting from the small and uniform P25-PT nanoplates. It should be noted that the P25-PTpowder sensor also showed a relatively high response (S = 42.5). However, its baseline resistance is in the range of giga ohm and is difficult to measure using the conventional electric circuit. The Ra value of the rutile-PT sensor is also very high (>100 M), which can be explained by the narrow contact area between the large cubes. It should be noted that the P25-PT sensor shows the lowest sensor resistance probably due to the wide contact area between the nanoplates, which indicates that the nanoplate morphology is also advantageous for measuring the sensor resistance. The 90% response and recovery times (res and recov), the times to show 90% resistance variation upon exposure to gas and air, were


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measured from the sensing transients. The res and recov values of sensors using the P25-PT nanoplates and P25-PT powders were shorter than those of the sensors using rutile-PT and anatase-PT. Accordingly, the P25-PT sensor was selected as the optimal one because of its highest gas response, lowest resistance in air, and relatively fast responding/recovering speed. The responses of the P25-PT sensor to various gases at 5 ppm between 300 °C and 500 °C are shown in Fig. 4a. The gas sensing characteristics below 300 °C was not measured because the sensor resistance was too high to measure and responding kinetics was sluggish. The response to ethanol was markedly higher than those of the other interference gases at 300– 450 °C. Although the sensor also responded to acetone and formaldehyde, these responses were significantly lower than that in the case of ethanol. The oxide semiconductor gas sensors often show high response to ethanol with high reactivity.5,21 Although the further systematic study is necessary, the ethanol selectivity in the present study can be also explained by high reactivity of ethanol. This indicates that PT nanoplates can be employed to detect ethanol in a highly selective manner for safety alarm systems or other related applications over the wide range of sensing temperature. The gas responses tend to decrease when the temperature increases to 500 °C. This can be explained either by the decrease in oxygen adsorption or by the oxidation of analyte gases into non-reactive species at the upper part of the sensing film before the sensing reaction. The sensing transients to 5 ppm ethanol at 300 °C–500 °C are shown in Fig. 4b. As the temperature increased, the resistance in air and response of the sensor decreased. Thus, the


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sensor operation at 300°C is advantageous to detect ppm-level ethanol, whereas the sensing at higher temperature (e.g. 400 or 450°C) can be used to detect a wide range of ethanol from several ppm to several hundred ppm without scarifying gas selectivity. The sensing transients of the P25-PT sensor to 0.1–5 ppm ethanol were measured at 300 C (Fig. 5a). The response to 100 ppb ethanol was 1.8, which confirmed that the sensor could detect sub-ppm-level ethanol. The detection limit of ethanol was calculated to be 88 ppb when S > 1.5 is considered for the criterion of reliable gas sensing. The sensing transients showed stable and reversible sensing characteristics. The sensor response to the ethanol concentration showed a linear relationship on a double logarithmic scale, as shown in Fig. 5b, which is advantageous for real applications. And the P25-PT sensor showed relatively stable sensing characteristics for two months (Figure S3). It should be noted that the response of P25-PT sensor to 5 ppm ethanol at 300 °C is substantially higher than those of pure ternary oxides in the literature such as LaFeO3,21,38 BaSnO3,39 CdSnO3,40 LaAlO3,41 NiCo2O4,16 CoFe2O4,42 and ZnFe2O4.43 (Table 1) The humidity dependence of the gas sensing characteristics was examined (Fig. 6 and Table S1). On changing the atmosphere from dry to 20% RH, the Ra value increased but the gas response decreased. In general, the reaction between the oxide semiconductor surface and moisture is known to generate electrons owing to the formation of the hydroxyl group at the expense of adsorbed oxygen,30,31,44 which is in line with the increase in sensor resistance and a


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decrease in the sensor response at 20% RH in the present study. Interestingly, the present P25PT sensor showed nearly the same gas response as well as sensor resistance in air over a wide range of humidity values (20% RH–80% RH), which indicates that the sensing surface is hardly affected by a further increase in humidity. Thus, the P25-PT sensor in the present study satisfies the stringent requirements for reliable breath alcohol detectors as well as air quality monitoring such as high ethanol response and humidity independent gas sensing characteristics. In order to further investigate the surface reaction in humid conditions, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was used. The DRIFT spectra of P25-PT in different environments are shown in Fig. 7. The band from 3680 cm-1 to 3000 cm-1 was observed when the sensor was exposed to a humid atmosphere (red and blue lines), which corresponds to the hydroxyl groups related with Ti elements.45-47 The peak at approximately 1630 cm-1 (red and blue lines) corresponds to the bending vibration of water molecules, which indicates the adsorption of H2O.48,49 On taking into account the formation of the OH group in the DRIFT spectra, the increase in the sensor resistance, and the decrease in the gas response with the increase in humidity, it is considered that the H2O interacts with the adsorbed O- to form a hydroxyl group on the surface and release electrons. This is in line with the mechanism reported in the extant literature. It should be noted that the amplitude of this band in the DRIFT spectra remained similar despite a change in humidity from 40% RH to 80% RH. This indicates that the formation of the OH group on the sensing surface is nearly completed at a low RH, and a


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further increase in humidity does not affect the sensing surface. Furthermore, this is consistent with the observation that the sensor resistance and gas responses for 20%–80% RH remained almost unchanged.

4. CONCLUSIONS A new ethanol sensor based on the PbTiO3 nanoplates was suggested, which was synthesized using a facile one-step hydrothermal method using commercial Degussa P25 as the Ti source. The p-type gas sensing characteristics were observed, and it was found that the sensor exhibited a high response to 5 ppm ethanol (resistance ratio = 80.4) as well as excellent ethanol selectivity at 300 °C. Moreover, the sensor resistance and gas response remained nearly the same despite the wide variation in humidity from 20% RH to 80% RH, which demonstrates reliable ethanol sensing in a moisture-containing ambient atmosphere. The reaction between the atmospheric moisture and adsorbed oxygen to form an OH group at a low humidity and the maintaining of a similar configuration of the sensing surface over a range of 20%–80% RH have been suggested as the reasons for the negligible humidity interference to the gas sensing characteristics. The gas sensor comprising PbTiO3 nanoplates can be applied to a new application of gas sensing in high humidity and dynamically changing humidity environments.


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ASSOCIATED CONTENT Supporting Information. Experimental procedures to measure gas sensing characteristics; schematics of gas sensing system; SEM image of the P25-PT powder synthesized by the conventional solid state reaction; long-term stability of P25-PT sensor; the response of P25-PT nanoplates to 5 ppm C2H5OH in various humidity conditions (measured at 300 °C).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the National Research Council of Science & Technology (NST) Grant by the Korean government (MSIP) (No. CAP-17-04-KRISS) and by a grant from the National Research Foundation of Korea (NRF), which was funded by the Korean government (Ministry of Education, Science, and Technology (MEST), grant no. 2016R1A2A1A05005331). Furthermore, the authors also thank the KU-KIST Graduate School Program of Korea University.


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Figure 1. SEM images of (a) rutile-PbTiO3 nanocubes, (b) anatase-PbTiO3 nanoplates, (c) P25PbTiO3 nanoplates. The insets show the corresponding SEM images of the starting rutile, anatase, and P25 TiO2 powders.


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Figure 2. (a) XRD patterns of rutile-PT (a1), anatase-PT (a2), and P25-PT nanoplates (a3); (b) low-resolution and (c) high-resolution TEM images of P25-PT nanoplates. Inset image (c) shows the corresponding FFT pattern.


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Figure 3. Response-recovery curves of sensors using rutile-PT, anatase-PT, P25-PT, and P25PTpowder to 5 ppm C2H5OH at 300 °C.


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Figure 4. (a) Selectivity of P25-PT nanoplate sensor (gas concentration: 5 ppm); (b) responserecovery curves of the sensor to 5 ppm C2H5OH at different temperatures.


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Figure 5. (a) Response-recovery curves of the P25-PT sensor at different concentrations of C2H5OH at 300 °C; (b) the response of the sensor at various gas concentrations.


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Figure 6. Response-recovery curves of the P25-PT sensor to 5 ppm C2H5OH gas in air at different humidity levels at 300 °C.


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Figure 7. DRIFT spectra of P25-PT in different humidity conditions (dry air, 40% RH and 80% RH) at 300 °C.


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Table 1 Ethanol responses of ternary oxide semiconductor gas sensors in this present study and those reported in the literature.16,21,38-43 Sensing Materials

Conc. (ppm)

S (Ra/Rg or Rg/Ra)

Temp.(C)

Ref.

LaFeO3 2D IO* LaFeO3 NPs† BaSnO3 NPs CdSnO3 NPs LaAlO3 NPs NiCo2O4 NTs‡ CoFe2O4 NPs ZnFe2O4 NPs PbTiO3 Nanoplates

5 200 100 100 50 400 10 50 5

~15 46.1 34.3 11.2 3.23 7.63 4 8.4 80.4

450 112 350 267 350 300 150 270 300

[21] [38] [39] [40] [41] [16] [42] [43] this study

*IO:

Inverse Opal, †NP: Nanoparticles, ‡NT: Nanotube


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Highly Sensitive and Selective PbTiO3 Gas Sensors with Negligible

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