Selective hydrogen gas sensor using CuFe2O4 nanoparticle based thin film

Selective hydrogen gas sensor using CuFe2O4 nanoparticle based thin film

Accepted Manuscript Title: Selective hydrogen gas sensor using CuFe2 O4 nanoparticle based thin film Author: Mohammad Abu Haija Ahmad I. Ayesh Sadiqa ...

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Accepted Manuscript Title: Selective hydrogen gas sensor using CuFe2 O4 nanoparticle based thin film Author: Mohammad Abu Haija Ahmad I. Ayesh Sadiqa Ahmed Marios S. Katsiotis PII: DOI: Reference:

S0169-4332(16)30272-0 http://dx.doi.org/doi:10.1016/j.apsusc.2016.02.103 APSUSC 32621

To appear in:

APSUSC

Received date: Revised date: Accepted date:

1-9-2015 24-1-2016 9-2-2016

Please cite this article as: M.A. Haija, A.I. Ayesh, S. Ahmed, M.S. Katsiotis, Selective hydrogen gas sensor using CuFe2 O4 nanoparticle based thin film, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.02.103 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highlights

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Hydrogen gas sensors based on CuFe2O4nanoparticle thin film were fabricated. The thin films were produced by dc sputtering inside a high vacuum system. The sensors were selective to hydrogen. The sensors exhibited enhanced sensitivity at low temperatures. The sensors hada linear response with hydrogen concentration.

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*Graphical Abstract (for review)

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Selective hydrogen gas sensor using CuFe2O4 nanoparticle based thin film Mohammad Abu Haija1, Ahmad I. Ayesh2,*, Sadiqa Ahmed3, and Marios S. Katsiotis4

Department of Math., Stat. and Physics, Qatar University, Doha, Qatar 3

Department of Physics, United Arab Emirates University, Al Ain, United Arab Emirates

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Department of Chemistry, The Petroleum Institute, Abu Dhabi, United Arab Emirates

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Department of Chemical Engineering, The Petroleum Institute, Abu Dhabi, United Arab

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* Corresponding author. Email: [email protected], Tel.: +974-4403-6592, P. O. Box 2713, Doha,

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Qatar

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Abstract:

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Hydrogen gas sensors based on CuFe2O4 nanoparticle thin films are presented in this work. Each gas sensor was prepared by depositing CuFe2O4 thin film on a glass substrate by dc sputtering

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inside a high vacuum chamber. Argon inert gas was used to sputter the material from a composite sputtering target. Interdigitated metal electrodes were deposited on top of the thin films by thermal evaporation and shadow masking. The produced sensors were tested against hydrogen, hydrogen sulfide, and ethylene gases where they were found to be selective for hydrogen. The sensitivity of the produced sensors was maximum for hydrogen gas at 50 °C. In addition, the produced sensors exhibit linear response signal for hydrogen gas with concentrations up to 5%. Those sensors have potential to be used for industrial applications because of their low power requirement, functionality at low temperatures, and low production cost.

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Keywords:

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CuFe2O4, nanoparticle, hydrogen sensor, thin film

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Introduction:

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Hydrogen is an environment friendly fuel that can be used to power electric motors, and it has a potential to decrease human consumption of fossil fuel [1]. However, it is odorless, colorless,

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highly flammable, and its leakage poses explosion hazards [2]. Therefore, development of sensitive, selective, and cost effective safety and control hydrogen sensors is highly demanded

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by hydrogen fuel community [3, 4].

Spinel ferrites with a general composition of MFe2O4 (M = Cu, Mg, Zn, etc.) have received much

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attention due to their practical applications in different fields [5-8] including catalysts [9-11] and

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gas sensing [8, 12-16]. Nevertheless, limited information is available about the gas sensing

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properties of CuFe2O4 nanoparticle based thin films [15, 17]. The gas sensing properties of CuFe2O4 are affected by different parameters including chemical composition, morphology, and operation temperature. Therefore, investigating the gas sensing properties of CuFe2O4 is demanded to enhance our fundamental understanding of its properties before its utilization for practical applications. In addition, efficient gas sensors require low density and large surface area to increase gas exposure, thus, gas sensing devices that utilize nanostructured ferrite are expected to exhibit enhanced performance. Many previous studies have reported the fabrication of ferrite nanostructures by chemical methods including [18]: ball-milling [19], co-precipitation [20], and solid state reaction [21]. These methods suffer from deficiencies such as the purity of nanoparticle as well as the

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uniformity of their size. In addition, the assembly of such nanoparticles on a substrate requires further experimental steps that involve the addition of adhesive material to attach the ferrite to the substrate.

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Sputtering is a well-established technique of producing granular thin films [22, 23] of high purity, as they are prepared inside a high vacuum chamber. The sputtering technique of producing

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granular thin films has a high yield, and it has been adopted by industry for many fields such as

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producing thin metallic films for digital optical storage discs. In addition, sputtered material is assembled directly on a substrate placed facing the sputter source without any further

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experimental steps, and the composition of the produced film can be identical to the composition of the sputtering target [24]. Controlling the sputtering conditions (such as:

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sputtering discharge power, inert gas flow rate, and distance between the sputtering target and substrate) would control the size of the produced nanoparticles as well as their yield [3].

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In this work, we report on the fabrication of conductometric hydrogen gas sensors that are

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based on CuFe2O4 nanoparticle thin films. The thin films were prepared by dc sputtering inside a

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high vacuum chamber. The crystal structure of the fabricated films was tested by x-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). The morphology of the fabricated films was investigated by scanning electron microscopy (SEM) as well as HRTEM. The composition of the fabricated films was confirmed by energy dispersive Xray spectroscopy (EDS). Electrical properties of the fabricated sensors were tested by dc electrical measurements, and their sensitivity was verified to different gases including hydrogen.

Experimental: Gas sensor devices were fabricated on glass substrates with an area 1 cm2 each. The substrates were cleaned by acetone in an ultrasonic bath, rinsed with IPA and deionized water, then dried

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with nitrogen gas. CuFe2O4 thin films were fabricated by dc sputtering inside a Torr International high vacuum system. The substrates were fixed on a rotating table to assure the uniformity of the film thickness. Thin films with a thickness of 300 nm were deposited on the substrates, as

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measured using a quartz crystal monitor (QCM) fixed close to the substrates. Argon (Ar) inert gas with a flow rate of 115 sccm was used to introduce the plasma and to sputter the material

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from a composite CuFe2O4 target (99.99% purity, purchased from Testbourne ltd, UK) fixed on a

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water cooled sputter head. The base pressure of the vacuum system was approximately 10-6 Torr. Electrical electrodes were fabricated by thermal evaporation of Au through a shadow mask

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[25] on the CuFe2O4 thin films. The electrodes exhibit an interdigitated structure with finger separation of 50 μm as shown schematically in Fig. 1.

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The response of the device to different gases was characterized inside a temperature controlled chamber under different concentrations relative to nitrogen gas. The target gases were

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introduced into a gas mixer, and their flow rates were controlled using a Bronkhors mass flow

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236 source measuring unit.

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meters. The response signal of the device was measured using a computer controlled Keithley

The crystallographic identity of the thin film specimen was measured by XRD using a Panalytical Diffractometer (X’Pert PRO) with Cu-Kα radiation (λ = 1.5418 Å). Morphological characteristics were observed by an FEI-Quanta-250 (FEG) SEM at 2kV. EDS measurements were performed using SEM at 30kV. The specimen was attached as prepared to a standard SEM holder (stab) using conductive carbon tape. Additional morphological and crystallographic identifications were performed by an FEI-Tecnai-G20 HRTEM at 200kV, equipped with EDS (for further confirmation of the composition measured using the EDS attached to the SEM). Sections of films were removed from the glass substrates using an ultramicrotome and were subsequently deposited on standard TEM copper grids.

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Results and Discussion: Figure 2 shows the XRD of a produced thin film that was annealed 300 °C for 30 min. The background was subtracted from the spectrum, and it was matched with the standard CuFe2O4

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spectrum [26]. The following Miller indices were identified ([311], [400], and [440]), and could

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be assigned to cubic structure of CuFe2O4 [27, 28]. In addition few peaks with low intensities could be matched with the structure of monoclinic phase of CuO [29]. Herein, the low intensity

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of the CuO peaks indicates the low percentage of monoclinic phase of CuO. The composition of the thin films was also examined by EDS measurements, as shown in the inset of Fig. 2.

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Systematic EDS measurements for different samples reveal that the atomic ratio of Cu/Fe is 0.54

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 0.02 which is close to the theoretical ratio. Nevertheless, the films are rich more with Cu which is consistent with the above XRD measurements.

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Representative SEM images of a deposited film used for hydrogen gas sensor devices are shown

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in Figs. 3(a) and 3(b). A homogeneous granular film of nanoparticles can be observed. A representative TEM image is shown in Fig. 3(c), while HRTEM images are shown in Figs. 3(d) and

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3(e). TEM furthermore confirms the particle homogeneity, with an average size of 40 nm. The distance between lattice spaces (d) were calculated and used to identify Miller indices, which were in turn used to identify the crystal structure of the fabricated nanoparticles using Fourier Transform (FFT). The indices were consistent with those determined from the XRD measurements, and were assigned to cubic structure of CuFe2O4 which is in agreement with the previous reports [27, 28].

Current-voltage (I(V)) measurements as a function of temperature for a fabricated gas sensor are shown in Fig. 4. The figure reveals linear I(V) characteristics where increasing the temperature increases the slope of the I(V) curves (negative temperature coefficient of the resistance). The linear I(V) characteristics is a result of the homogenous electrode/film 5 Page 7 of 21

interfaces [30]. Herein, the external bias voltage is distributed across grains within the granular film, thus, each pair of grains within the granular film network will remain under low voltage which causes the linear I(V) characteristics [31]. It should be noted here that nonlinear I(V)

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characteristics would be expected for systems with electronic transport by quantum tunneling through nanogaps. The decrease of resistance (increase in the slop) with temperature has been

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observed for different systems [32], and it is common for granular films [33]. Increasing the

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temperature normally will increase the grains size, thus, the interconnection among grains increases resulting in fewer scattering center for charge carrier and lower resistance [33].

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Figure 5(a) depicts the electrical current response of a representative device as a function of hydrogen concentration, measured at 50°C and using a constant voltage of 60 mV. The figure

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reveals that as the device is exposed to hydrogen, the electrical current response signal decreases, and this decrease proportional to hydrogen concentration. When hydrogen flow is

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stopped, the current signal returns back to its original reference value. The fabricated sensors

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were tested for their sensitivity to the following gases: ethylene (C2H4), hydrogen sulfide (H2S),

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and hydrogen (H2). Figure 5(b) shows sensor’s response for the above three gases with concentration of 1% at 50°C. The figure reveals that the response of a fabricated sensor is low for C2H4 and H2S, thus, the produced sensors are selective to H2. Exposing a ferrite material to a reducing gas changes the resistance of the material due to interaction between the reducing gas and oxygen spices on the surface of ferrite nanoparticle [17]. The decrease in electrical current upon exposure to hydrogen is an indication the film holds p-type semiconductor properties which might implies that the film is CuO rich [34], which is consistent with the above XRD and EDS measurements. The observed sensing behavior results from the chemisorption of H2 and adsorption of oxygen on the surface of nanoparticles [35]. Once nanoparticles are exposed to H2 gas, H2 molecules interact with the surface of nanoparticle

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through oxygen ions that were predesorbed onto nanoparticle surface according the following equations [34]: H2(ads) +O-(ads) ↔ H2O + e- (1)

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H2(ads) +O-2(ads) ↔ H2O + 2e- (2)

The reactions imbue electrons into the p-type CuFe2O4 which leads to a decrease in conductivity

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(increase in the resistance).

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The sensitivity (S) of a gas sensor is defined as: (3)

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where R0 is the resistance of the sensor without hydrogen, and R is the maximum value of the resistance after exposing the sensor to hydrogen. The sensitivity is highly dependent on the

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operation temperature since the measurement temperature determine: i) oxygen contents

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within nanoparticles which in turns control the amount of the reaction between H2 and

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bounded oxygen spices [17], and ii) the reference resistance of the device according to its temperature coefficient of the resistance [32]. The effect of operating temperature on the

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sensitivity of the fabricated sensors for H2 gas is shown in Fig. 5(c). The figure demonstrates that the optimum operation temperature of the presented sensors is around 50 °C. The figure also shows that the sensitivity of the sensor at 50 °C varies linearly with H2 concentration within the range of the experimental measurements, while the sensitivity of the sensor decreases and becomes nonlinear.

The optimum operation temperature is determined by reduction of metal and release of electrons to conduction band [26]. This process is a surface controlled process due to the adsorption of oxygen on the surface of nanoparticles, and it is maximized at the optimum operation temperature. The linear response is an important feature of the current sensors since it facilitates the calibration of the gas sensor. The low optimum operation temperature is an 7 Page 9 of 21

indication of the low operational power requirements of the presented sensors. In addition, low operation temperature is desired for hydrogen sensors to reduce the possibility of explosion hazard.

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Fast response is an important feature of any fabricated gas sensor. The response time of the current sensors was found to be slightly dependent on hydrogen concentration, and mainly

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dependent on the operation temperature. Figure 5(d) shows the dependence of response time

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on operating temperature. Response time is defined as the time required for the sensor response to reach 90% of its maximum value. At each temperature, the value of response time

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was taken as the average response at different hydrogen concentrations. The error bars in the figure are one standard deviation. The figure reveals that increasing the operation temperature

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decreases the response time between 23 °C and 100 °C, then the response time becomes almost constant after 100 °C. The small error bars (standard deviation) indicates that the values of

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response time are close to each other at a certain temperature, and they vary mainly due to the

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change in temperature. The decrease in response time with operation temperature could result

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from the negative temperature coefficient of the resistance (see Fig. 4) that causes faster charge transport at higher temperatures.

Conclusion:

In conclusion, novel and selective hydrogen gas sensors were fabricated using CuFe2O4 nanoparticle based thin films. The thin films were deposited on glass substrates by dc sputtering using argon inert gas to generate plasma and to establish discharge inside a high vacuum chamber. Interdigitated gold electrodes were fabricated by thermal evaporation and a shadow mask. The cubical structure was confirmed by set of X-ray diffraction measurements and high resolution transmission electron microscopy, while the composition was confirmed by X-ray

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diffraction and energy dispersive X-ray spectroscopy . The fabricated sensors were found to be selective for hydrogen gas, which was assigned to the chemisorption of H2 and adsorption of oxygen on the surface of nanoparticles. The fabricated sensors showed their maximum response

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at low temperature which indicates the low operational power requirement, and their compatibility with the safety requirement to be used as hydrogen gas sensors. In addition, those

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sensors exhibit linear response with hydrogen concentration which facilitate their calibration

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when used for practical applications. The response time of those sensors was reasonably low, and it is around 48 ± 11 s at 50 °C. The production of presented sensors is cost effective, thus

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they have the potential to be used for practical industrial applications.

Acknowledgments:

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This work was supported by the Petroleum Institute under a grant number RIFP-14312.

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Figure Captions:

Fig. 1: (a) Schematic diagrams of the electrical measurement circuit and a side view of the sample. (b) Top view of the gas sensor device.

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Fig. 2: Grazing angle X-ray diffraction patterns of a produced thin film annealed at 300 °C for 30 min. The background was subtracted from the spectrum. The inset is an EDS measurement for the prepared samples.

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Fig. 3: (a) Low-magnification SEM image of CuFe2O4 thin film showing homogeneous distribution of spattered material. The square indicates the location of the high resolution image. (b) Particle

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size and shape homogeneity can be observed at higher magnification SEM. (c) TEM image of thin

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film. (d) and (e) HRTEM images of nanoparticles within the thin film (the inset is Fast Fourier Transform).

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Fig. 4: Current-voltage measurement as a function of temperature of a CuFe2O4 nanoparticle based gas sensor device.

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Fig. 5: (a) Electrical current response curve of the fabricated gas sensor as a function of time for different hydrogen concentrations measured using a constant voltage of 60 mV and at 50 °C. (b)

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Sensor selectivity results for gas concentration of 1% at 50°C. (c) Sensitivity of hydrogen sensor

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as a function of hydrogen concentration at different temperatures. The solid line is a linear fit of

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the results measured at 50°C. (d) Dependence of the sensor response time on temperature. Each point represents the average value of response time of different hydrogen concentrations at a fixed temperature. The error bars are one standard deviation.

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