A Hydrogen Gas Sensor Based on TiO2 Nanoparticles on ... - MDPI

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A Hydrogen Gas Sensor Based on TiO2 Nanoparticles on Alumina Substrate Siti Amaniah Mohd Chachuli 1,2, *, Mohd Nizar Hamidon 1 , Md. Shuhazlly Mamat 3 , Mehmet Ertugrul 4 and Nor Hapishah Abdullah 1 1 2 3 4

*

Institute of Advanced Technology, University Putra Malaysia, Serdang 43400, Selangor, Malaysia; [email protected] (M.N.H.); [email protected] (N.H.A.) Faculty of Electronic & Computer Engineering, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, Durian Tunggal 76100, Melaka, Malaysia Faculty of Sciences, University Putra Malaysia, Serdang 43400, Selangor, Malaysia; [email protected] Engineering Faculty, Ataturk University, 25240 Erzurum, Turkey; [email protected] Correspondence: [email protected]; Tel.: +60-019-297-7732

Received: 2 May 2018; Accepted: 24 June 2018; Published: 1 August 2018

Abstract: High demand of semiconductor gas sensor works at low operating temperature to as low as 100 ◦ C has led to the fabrication of gas sensor based on TiO2 nanoparticles. A sensing film of gas sensor was prepared by mixing the sensing material, TiO2 (P25) and glass powder, and B2 O3 with organic binder. The sensing film was annealed at temperature of 500 ◦ C in 30 min. The morphological and structural properties of the sensing film were characterized by field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). The gas sensor was exposed to hydrogen with concentration of 100–1000 ppm and was tested at different operating temperatures which are 100 ◦ C, 200 ◦ C, and 300 ◦ C to find the optimum operating temperature for producing the highest sensitivity. The gas sensor exhibited p-type conductivity based on decreased current when exposed to hydrogen. The gas sensor showed capability in sensing low concentration of hydrogen to as low as 100 ppm at 100 ◦ C. Keywords: gas sensor; TiO2 -B2 O3 ; hydrogen; nanoparticles; p-type TiO2

1. Introduction Detection of hydrogen in fuel cell, combustion engines and monitoring faults in transformer have gained incredible interest from many researchers especially from gas sensing area. Hubert et al. reported that 1400 publications have been published in gas sensing from 1975 until 2010 [1]. Hydrogen which is known as a colorless, odorless, tasteless, and flammable gas, cannot be detected by human senses [2], thus its presence should be detected and analyzed. With a mixture of oxygen, leakage of hydrogen can cause explosions and degradation of many types of steels [3]. Hydrogen can also become flammable and explosive if the concentration is higher than 4% in air [4]. Different sensing technologies have been employed to detect hydrogen, such as catalyst, thermal conductivity, electrochemical, resistance based, work function based, mechanical, and optical [2]. Among them, electrochemical and resistance-based technologies are the most preferred due to their ability to detect low hydrogen concentration and acceptable selectivity [2]. It has been reported that effective sensing materials to sense hydrogen are based on palladium (Pd) [5–15] and metal-oxide semiconductors (MOX) such as SnO2 [16–19], ZnO [20–23], TiO2 [24–30], WO3 [31], and NiO [32] because of their capability to detect hydrogen with low concentration and ability to work at room temperature. Palladium is high sensitive to hydrogen; however it also has drawbacks such as hysteresis behavior in electrical resistance because of adsorption of hydrogen in the structure of Pd [5].

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[5]. Recently, a hydrogen gas sensor based on carbon-based materials such as carbon nanotubes [33– 35], graphene [36–39], and reduced graphene oxide (RGO) [40,41] has also attracted high attention Recently, a hydrogen gas sensor based on carbon-based materials such as carbon nanotubes [33–35], because it is highly sensitive to the changes in the chemical environments [42,43], and offers high graphene [36–39], and reduced graphene oxide (RGO) [40,41] has also attracted high attention because performance, label free chemical sensing [44]. it is highly sensitive to the changes in the chemical environments [42,43], and offers high performance, TiO2 has been chosen in this work because it is known as a chemically stable, nontoxic, label free chemical sensing [44]. biocompatible, inexpensive, wide band gap semiconducting material [45]. Due to being inexpensive, TiO2 has been chosen in this work because it is known as a chemically stable, nontoxic, hydrogen gas sensors based on TiO2 also can become affordable and safe hydrogen gas sensors [46]. biocompatible, inexpensive, wide band gap semiconducting material [45]. Due to being inexpensive, Among the metal-oxide semiconductors, the TiO2 gas sensor has been reported to be able to work hydrogen gas sensors based on TiO2 also can become affordable and safe hydrogen gas sensors [46]. under low operating temperatures, up to as low as room temperature [28,47,48], with fast response Among the metal-oxide semiconductors, the TiO2 gas sensor has been reported to be able to work [29]. These criteria have made TiO2 a practical material for gas sensing applications. under low operating temperatures, up to as low as room temperature [28,47,48], with fast response [29]. In this paper, a TiO2 gas sensor was fabricated on the alumina substrate using screen-printed These criteria have made TiO2 a practical material for gas sensing applications. method and was tested to different concentration of hydrogen from 100–1000 ppm at three different In this paper, a TiO2 gas sensor was fabricated on the alumina substrate using screen-printed operating temperatures: 100 °C, 200 °C, and 300 °C. method and was tested to different concentration of hydrogen from 100–1000 ppm at three different operating temperatures: 100 ◦ C, 200 ◦ C, and 300 ◦ C. 2. Materials and Methods 2. Materials and Methods 2.1. Preparation and Fabrication of Gas Sensor 2.1. Preparation Fabrication of Gas Sensorof two layers, which are an interdigitated electrode (IDE) Gas sensorand used in this work consist

and sensing film, as shown in Figure 1. of The usedwhich in this was a silver-conductive paste Gas sensor used in this work consist twoIDE layers, arework an interdigitated electrode (IDE) and (DGP80 TESM8020) provided by 1.Sigma-Aldrich am Albuch, Germany) and the sensing sensing film, as shown in Figure The IDE used(Steinheim in this work was a silver-conductive paste (DGP80 ® P25) provided by Sigma-Aldrich (Steinheim am material used in this work was TiO2 (Aeroxide TESM8020) provided by Sigma-Aldrich (Steinheim am Albuch, Germany) and the sensing material ® Albuch, IDE and sensing film were deposited the alumina substrate using screen used in Germany). this work was TiO2the (Aeroxide P25) provided by on Sigma-Aldrich (Steinheim am aAlbuch, printing method. Initially, IDE was as first on the substrate alumina substrate, followed by Germany). IDE and the sensing film deposited were deposited onlayer the alumina using a screen printing annealing in the furnace at temperature of 120 °C for 30 min. Air was used as a carrier gas in the method. Initially, IDE was deposited as first layer on the alumina substrate, followed by annealing in furnace. the furnace at temperature of 120 ◦ C for 30 min. Air was used as a carrier gas in the furnace. In TiO 2 on on alumina substrate, TiO2 powder was prepared as a paste. In order ordertotodeposit deposit TiO alumina substrate, TiO2 powder was prepared as aFirstly, paste. 2 the the 90 wt %90 of wt TiO%2 powder mixedwas withmixed 10 wt % of glass powder, boronpowder, oxide (B2boron O3), using m-xylene Firstly, of TiO2 was powder with 10 wt % of glass oxide (B2 O3 ), as a medium in an 90 min. Then, it was driedThen, in anitoven ground a using m-xylene as aultrasonic medium bath in anfor ultrasonic bath for 90 min. was and driedwas in an oven in and mortar. The purpose of glass powder is to hold the nanoparticles of TiO 2 on the substrate and to was ground in a mortar. The purpose of glass powder is to hold the nanoparticles of TiO2 on the ensure good TiO2 andbetween the alumina B2O3 was chosen as glasschosen powder substrate andadhesion to ensurebetween good adhesion TiO2 substrate. and the alumina substrate. B2the O3 was as ◦ C. in this work because haswork a lowbecause meltingitpoint 450melting °C. Thispoint method have been in [49,50]. the glass powder in it this has aoflow of 450 Thispresented method have been Organic binder was Organic preparedbinder by mixing m-xylene, linseed m-xylene, oil, and α-terpineol. The α-terpineol. paste was presented in [49,50]. was prepared by mixing linseed oil, and prepared mixing the TiO -B2O3 with organic binder homogeneous paste was obtained. Then, The pasteby was prepared by 2mixing the TiO organic binder until homogeneous paste was 2 -B2 O 3 withuntil TiO 2 -B 2 O 3 paste was deposited on the top of IDE and it was annealed in the furnace at temperature obtained. Then, TiO2 -B2 O3 paste was deposited on the top of IDE and it was annealed in the furnace ◦ Cfabricated of °C for 30 min. sensor is shown Figure The size of the 2. sensing filmofwas at 500 temperature of 500The for 30 min.gas The fabricated gas in sensor is 2. shown in Figure The size the 4.2 × 4.2 film mm,was while size of while the IDE × 4.2 TiO 2-B2O3 sensing 4.2 the × 4.2 mm, thewas size9.25 of the IDEmm. was The 9.25 IDE × 4.2was mm.fully Thecovered IDE wasby fully covered paste in2 -B order increase theto sensitivity of the gas sensor. Black onBlack the sensing film gas sensor by TiO paste in order increase the sensitivity of the gascolor sensor. color on theofsensing film 2 O3 to might caused by be diffusion (IDE) TiOinto 2 (Figure 2).2 It(Figure was reported the silver of gas be sensor might caused of by silver diffusion of into silverthe (IDE) the TiO 2). It wasthat reported that ◦ diffused into TiO 2 at a temperature of 400 °C [51]. the silver diffused into TiO2 at a temperature of 400 C [51].

Figure Figure 1. 1. Front Front view view of of the the TiO TiO22-B -B22OO33gas gassensor. sensor.

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Figure2.2.Fabricated FabricatedTiO TiO2-B 2-B 2O gassensor sensoron onalumina aluminasubstrate substrateusing usingaascreen-printing screen-printingmethod. method. Figure 2O 3 3gas Figure 2. Fabricated TiO2 -B2 O3 gas sensor on alumina substrate using a screen-printing method.

2.2. Characterization Method TiO -B O33 2.2. 2.2.Characterization CharacterizationMethod Methodofof ofTiO TiO222-B -B22O 2 O3 Characterizations of TiO 2-B22O O33were were made made using using aa thermogravimetric thermogravimetric analyzer analyzer (TGA), (TGA), field field Characterizations 2-B Characterizationsof ofTiO TiO 2 -B2 O3 were made using a thermogravimetric analyzer (TGA), emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), and emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), and field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD). Thermal analysis ofTiO TiO2of 2-B -B2TiO 2O pastewas wastested testedusing usingTGA TGA(Brand: (Brand:Mettler Mettler X-ray diffraction (XRD). Thermal analysis of O 33paste and X-ray diffraction (XRD). Thermal analysis 2 -B2 O3 paste was tested using TGA (Brand: ◦ Toledo (Greifensee, Switzerland, Model: TGA/DSC 1 HT)) with heating rate 10 °C/min and air asand the Toledo (Greifensee, Switzerland, Model: TGA/DSC 1 HT)) with heating rate 10 °C/min and air as the Mettler Toledo (Greifensee, Switzerland, Model: TGA/DSC 1 HT)) with heating rate 10 C/min ◦ carrier for the temperature range: 25–1000 °C. The surface morphology of the thick films was carrier for the temperature range: 25–1000 °C. The surface morphology of the thick films was air as the carrier for the temperature range: 25–1000 C. The surface morphology of the thick films was analyzed usingFESEM FESEM(Model: (Model:Nova Nova Nanosem 230 (Thermo Fisher Scientific, Oregon City,USA), OR, analyzed FESEM (Model: Nova Nanosem 230 (Thermo Fisher Scientific, Oregon OR, analyzed using using Nanosem 230 (Thermo Fisher Scientific, Oregon City, City, OR, USA), and element composition was examined by EDX inside the FESEM. XRD (Brand: Philips USA), and element composition was examined by EDX inside the FESEM. XRD (Brand: Philips and element composition was examined by EDX inside the FESEM. XRD (Brand: Philips (Almelo, (Almelo, The Netherlands), Netherlands), Model: PWMPD 3040/60 MPD X’pert High Pro Pro Panalytical) Panalytical) studies were (Almelo, The Model: PW 3040/60 MPD X’pert High studies were The Netherlands), Model: PW 3040/60 X’pert High Pro Panalytical) studies were carried out ◦ ◦ carried out for powder and thick film over a 2θ range from 20° to 80°. The scanning time for TiO carried out for powder and thick film over a 2θ range from 20° to 80°. The scanning time for TiO 22 for powder and thick film over a 2θ range from 20 to 80 . The scanning time for TiO2 (P25) and (P25) and TiO 2-B -B 2O O 3powder powder were 5 min, and one hour for TiO 2-B2O 2O 3thick thick films. (P25) and TiO 2 2 3 were 5 min, and one hour for TiO 2 -B 3 films. TiO2 -B2 O3 powder were 5 min, and one hour for TiO2 -B2 O3 thick films. 2.3.Gas GasResponse ResponseMeasurement Measurement 2.3. Gas sensing sensingmeasurements measurements were performed inchamber gas chamber chamber with different different ppm levels of of Gas sensing measurements were performed in gas with ppm were performed in gas with different ppm levels of levels hydrogen hydrogen fromppm. 100–1000 ppm.As As carrier gas, 500sccm sccm nitrogen wasused. used.Experimental Experimental setupof of hydrogen from 100–1000 aacarrier gas, 500 was setup from 100–1000 As a ppm. carrier gas, 500 sccm nitrogen wasnitrogen used. Experimental setup of gas chamber is gas chamber shown in Figure3.3. Gas chamber wasLinkam obtained fromLinkam Linkam Scientific (Tadworth, UK, gas chamber isisshown Figure Gas chamber was obtained from Scientific UK, shown in Figure 3. Gasin chamber was obtained from Scientific (Tadworth, UK, (Tadworth, Model: HFS600). Model: HFS600). The gas chamberwas was connected tothe themass massflow flow controller,controller, temperature controller, Model: gas chamber connected to controller, temperature controller, The gasHFS600). chamberThe was connected to the mass flow controller, temperature and Kiethley and Kiethley487 487Picoammeter/Voltage Picoammeter/Voltage source. Three different operatingtemperatures temperatures were tested and Three different operating were 487 Kiethley Picoammeter/Voltage source. Threesource. different operating temperatures were tested on gas tested sensor ◦ ◦ ◦ on gas sensor which are 100 °C, 200 °C, and 300 °C to find the optimum operating temperature that on gas sensor which are 100 °C, 200 °C, and 300 °C to find the optimum operating temperature that which are 100 C, 200 C, and 300 C to find the optimum operating temperature that can produce can produce highest sensitivity to hydrogen. Formeasurement, measurement, 10VVvoltage voltage sourcetowas was applied to can produce highest hydrogen. For 10 source to highest sensitivity tosensitivity hydrogen.to For measurement, 10 V voltage source was applied theapplied IDE of gas the IDEand ofgas gas sensor and currentwas was observed as the response ofgas gassensor. sensor. the IDE of sensor and current observed sensor current was observed as the responseas ofthe gasresponse sensor. of

Figure 3. Experimental setup of gas sensing measurement. Figure3.3.Experimental Experimentalsetup setupof ofgas gassensing sensingmeasurement. measurement. Figure

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3. Results 3. Results and and Discussion Discussion 3.1. Characterization 3.1. Characterization of of TiO TiO22-B -B22O O33Using UsingTGA, TGA,FESEM, FESEM,EDX EDXand andXRD XRD TGA analysis was was performed performed to to determine determine the the thermal thermal behavior behavior of of the theTiO TiO22-B -B22O O33 paste to TGA analysis paste and and to find an optimum calcination temperature. Figure 4 shows total mass loss of TiO 2 -B 2 O 3 paste over a find an optimum calcination temperature. Figure 4 shows total mass loss of TiO2 -B2 O3 paste over ◦ ◦ temperature range of 25 °C. At loss was approximately 49.61%,49.61%, which a temperature range of to 25 1000 to 1000 C.400 At °C, 400mass C, mass lossmeasured was measured approximately indicated that the organic binder was not fully evaporated at this temperature. It became decreased which indicated that the organic binder was not fully evaporated at this temperature. It became ◦ C which again when temperature reached reached at 500 °C which approximately 25.12%. Composition ratio of decreased again when temperature at 500 approximately 25.12%. Composition ratio TiO 2 -B 2 O 3 powder and organic binder used in this work was 30:70. It can be seen that the organic of TiO2 -B2 O3 powder and organic binder used in this work was 30:70. It can be seen that the organic binder was was fully fully evaporated evaporatedatattemperature temperatureofof500 500◦ C. °C.Therefore, Therefore,this this temperature been chosen binder temperature hashas been chosen as as the annealing temperature sensing film. the annealing temperature forfor thethe sensing film.

Figure 4. 4. Thermal Thermal Behavior Behavior of of TiO TiO22-B -B22O Figure O33 paste paste using using thermogravimetric thermogravimetric analysis analysis (TGA). (TGA).

The morphology of the TiO TiO22-B22O O33 nanoparticles nanoparticles (NP) (NP) structure structure at at aa temperature temperature of 500 ◦°C C is shown in Figure 5. It can can be be seen seen that that the the nanoparticles nanoparticles of of TiO TiO22 was wasclearly clearlyseen seenat at200k 200kmagnification. magnification. emissionscanning scanning electron microscopy images theof uniformity of Field emission electron microscopy (FESEM)(FESEM) images shown the shown uniformity nanostructures nanostructures due to the of Average prepareddiameter paste. Average diameterwas of nanoparticles was due to the homogeneity of homogeneity prepared paste. of nanoparticles observed to be in ◦ C as shown observed be in 40–70 nm.showed The EDX result that peak ofatTitemperature was detected temperature of 40–70 nm.toThe EDX result that peakshowed of Ti was detected ofat 500 500Figure °C as 6. shown Figureconfirmed 6. These results confirmed that TiOat 2 was at this temperature. in Theseinresults that TiO this crystalline temperature. Thus, it has been 2 was crystalline Thus, itashas chosen asthe thegas sensing thebegas sensor, will be tested with chosen thebeen sensing film of sensor,film andof will tested with and hydrogen exposure in gashydrogen chamber. exposure in 7gas chamber. Figure shows the XRD pattern of TiO2 (P25) and TiO2 -B2 O3 without heat treatment. XRD analyses were examined using X’Pert HighScore software. From XRD spectra, it can be seen that both figures (TiO2 and TiO2 -B2 O3 ) consisted of rutile and anatase phases. It can be observed that pure titanium had a peak at 2θ = 25.35◦ (Figures 7 and 8). According to literature, this peak was attributed to the anatase (101) TiO2 phase. Whereas, a high peak of rutile phase (110) was located at 2θ = 27.49◦ (Figures 7 and 8). It was reported that crystallinity of TiO2 was decreased with the addition of boron content [52]. This work also showed that the intensity of TiO2 was decreased when added with B2 O3 . It can also be seen that the peak of anatase (101) in TiO2 -B2 O3 was lower than peak of anatase (101) in TiO2 at 2θ = 25.35◦ . Meanwhile, a small peak of B2 O3 was observed at 2θ = 36.04◦ in TiO2 -B2 O3 . This phase also contributed to the rutile phase (101).

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Figure 5. Field emission scanning electron microscopy (FESEM) image of TiO2-B2O3 on the alumina Figure 5. Field emission scanning electron microscopy (FESEM) image of TiO2 -B2 O3 on the alumina Figure 5. at Field substrate T = emission 500 °C. scanning electron microscopy (FESEM) image of TiO2-B2O3 on the alumina substrate at T = 500 ◦ C. substrate at T = 500 °C.

Figure 6. Energy-dispersive X-ray (EDX) results of TiO2-B2O3 on the alumina substrate at T = 500 °C. Figure 6. Energy-dispersive X-ray (EDX) results of TiO2-B2O3 on the alumina substrate at T = 500 °C. Figure 6. Energy-dispersive X-ray (EDX) results of TiO2 -B2 O3 on the alumina substrate at T = 500 ◦ C.

Figure 7 shows the XRD pattern of TiO2 (P25) and TiO2-B2O3 without heat treatment. XRD ◦ C. It can Figure shows theusing XRD pattern of2 -B TiO 2 (P25) TiO 3 spectra, without treatment. XRD analyses were examined X’Pert XRD itheat can seenthat, that XRD both Figure 87 shows XRD pattern of HighScore TiO thick and filmFrom at T2-B =2O 500 bebe seen 2 O3 software. analyses were examined using X’Pert HighScore software. From XRD spectra, it can be seen that both figures (TiO 2 and and TiO2rutile -B2O3)phases consisted of 2rutile anatase phases. It as canXRD be observed pure pattern of anatase in TiO -B2 O3and thick film was similar pattern inthat Figure 7. ◦ ◦ ◦ ◦ figures (TiO 2 and TiO 2 -B 2 O 3 ) consisted of rutile and anatase phases. It can be observed that pure titanium had 2θ =also 25.35° (Figures and film 8). According to literature, this peak attributed Whereas, B2 Oa3 peak peaksatwere detected in 7thick at 2θ = 27.76 , 36.04 , 48.37 andwas 54.58 . It was titanium had athat peak at 2θ (Figures andat 8).similar According to literature, this peak was attributed to theobserved anatase (101) TiO 2 phase. a 7high peak of rutile phase was located at◦ )2θ = 27.49° also peaks of= 25.35° B2 O3Whereas, were detected location of (110) anatase (2θ = 48.37 and rutile ◦ ◦ ◦ to the anatase (101) TiO 2 phase. Whereas, a high peak of rutile phase (110) was located at 2θ = 27.49° (Figures(2θ 7 and 8). It, was that).crystallinity ofalso TiO2indicated was decreased the addition of boron phases = 27.76 36.04reported and 54.58 This analysis that thewith XRD pattern of TiO 2 was (Figures 7 and 8). Itpresence wasalso reported crystallinity of TiO 2 was decreased with the addition of boron content [52]. This showed that the intensity of TiO 2 was decreased when added with B2O3. not affected by thework of B2 Othat . 3 content [52]. alsopeak showed that the intensity waslower decreased whenof added with B2Oin 3. It can also beThis seenwork that the of anatase (101) in TiOof 2-BTiO 2O3 2was than peak anatase (101) It can also be seen that the peak of anatase (101) in TiO 2 -B 2 O 3 was lower than peak of anatase (101) in TiO2 at 2θ = 25.35°. Meanwhile, a small peak of B2O3 was observed at 2θ = 36.04° in TiO2-B2O3. This TiO 2 atalso 2θ =contributed 25.35°. Meanwhile, a small of B2O3 was observed at 2θ = 36.04° in TiO2-B2O3. This phase to the rutile phasepeak (101). phase also contributed to the rutile phase (101).


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Figure 7. X-ray diffraction pattern of TiO2 (P25) and TiO2-B2O3.

Figure 8 shows XRD pattern of TiO2-B2O3 thick film at T = 500 °C. It can be seen that, XRD pattern of anatase and rutile phases in TiO2-B2O3 thick film was similar as XRD pattern in Figure 7. Whereas, B2O3 peaks were also detected in thick film at 2θ = 27.76°, 36.04°, 48.37° and 54.58°. It was also observed that peaks of B2O3 were detected at similar location of anatase (2θ = 48.37°) and rutile phases (2θ = 27.76°, 36.04° and 54.58°). This analysis also indicated that the XRD pattern of TiO2 was not affected by the presence of B2O3. Figure 7. 7. X-ray diffraction diffraction pattern pattern of of TiO TiO22 (P25) and TiO TiO22-B22O Figure O33..

Figure 8 shows XRD pattern of TiO2-B2O3 thick film at T = 500 °C. It can be seen that, XRD pattern of anatase and rutile phases in TiO2-B2O3 thick film was similar as XRD pattern in Figure 7. Whereas, B2O3 peaks were also detected in thick film at 2θ = 27.76°, 36.04°, 48.37° and 54.58°. It was also observed that peaks of B2O3 were detected at similar location of anatase (2θ = 48.37°) and rutile phases (2θ = 27.76°, 36.04° and 54.58°). This analysis also indicated that the XRD pattern of TiO2 was not affected by the presence of B2O3.

Figure ◦ C. Figure 8. 8. X-ray X-ray diffraction diffraction pattern pattern of of TiO TiO22-B -B22O O33thick thickfilm filmatatTT==500 500°C.

3.2. Electrical Characteristics of TiO2-B2O3 Gas Sensor 3.2. Electrical Characteristics of TiO2 -B2 O3 Gas Sensor Electrical characteristics of the TiO2-B2O3 gas sensor that annealed at 500 °C were◦ studied. Figure Electrical characteristics of the TiO2 -B2 O3 gas sensor that annealed at 500 C were studied. 9 shows the resistance of TiO2-B2O3 gas sensor at the operating temperatures: 100 °C, 200 °C, and 300 Figure 9 shows the resistance of TiO2 -B2 O3 gas sensor at the operating temperatures: 100 ◦ C, 200 ◦ C, °C. The graph showed that resistance was approximately 8.36 TΩ at 100 °C. This caused the range of and 300 ◦ C. The graph showed that resistance was approximately 8.36 TΩ at 100 ◦ C. This caused current to be below than 1 pA. The resistance was dropped sharply at a temperature of 200 °C and the range of current to be below than 1 pA. The resistance was dropped sharply at a temperature 8. X-ray pattern of TiO2-B2O3 thick film at Tand = 500 °C. MΩ respectively. ◦ C, where of 200 ◦ C and 300Figure thediffraction values were approximately 39.59 GΩ 33.74 This phenomenon can be caused by the conversion of silver (electrode) to metallic silver at operating 3.2. Electrical Characteristics of TiO2-B2O3 Gas Sensor temperatures of 200 ◦ C and 300 ◦ C, where it was decomposed into silver and oxygen [53]. This metallic characteristics of theof TiO 2-B 2O3sensor gas sensor that annealed 500 °C wereofstudied. silverElectrical has decreased the resistivity the gas and improved the at conductivity the gas Figure sensor. 9 shows the resistance of TiO2-B2O3 gas sensor at the operating temperatures: 100 °C, 200 °C, and 300 °C. The graph showed that resistance was approximately 8.36 TΩ at 100 °C. This caused the range of current to be below than 1 pA. The resistance was dropped sharply at a temperature of 200 °C and

300 °C, where the values were approximately 39.59 GΩ and 33.74 MΩ respectively. This phenomenon can be caused by the conversion of silver (electrode) to metallic silver at operating temperatures of 200 °C and 300 °C, where it was decomposed into silver and oxygen [53]. This metallic silver has Sensors 2018, 18, 2483 decreased the resistivity of the gas sensor and improved the conductivity of the gas sensor. 7 of 13

Figure 9. Electrical Characteristics operatingtemperature. temperature. Figure 9. Electrical CharacteristicsofofTiO TiO22-B -B22O O33at at different different operating

3.3. Performance of TiO -B32Gas O3 Gas Sensor at DifferentOperating Operating Temperatures Temperatures 3.3. Performance of TiO 2-B22O Sensor at Different ◦

Figure 10 shows the response of TiO2 -B2 O3 gas sensors at operating temperatures: 100 C, Figure 10 shows the response of TiO2-B2O3 gas sensors at operating temperatures: 100 °C, 200 °C 200 ◦ C and 300 ◦ C. It can be seen that in Figure 10a, the measurement was quite sensitive to noise and 300 °C. It can be seen that in Figure 10a, the measurement was quite sensitive to noise when the when the experiment was carried out at 100 ◦ C. It was observed that the response was not as smooth experiment was carried out at 100 °C. It was observed that the response was not as smooth as the as the response in Figure 10b,c. This environment occurred because the measured current was response Figure This environment the measured current wascurrent very low, very in low, which10b,c. is below than 1 pA. As theoccurred operatingbecause temperature increased, the observed whichstarted is below than 1and pA.showed As thehigh operating temperature the observed current started to to increase response to hydrogen.increased, From experiments have been conducted, increase high able response to hydrogen. From experiments been conducted, TiO◦2C. -B2O3 TiO2and -B2 Oshowed to sense low concentration of hydrogenhave as low as 100 ppm at 100 3 gas sensor gas sensor able to sense low concentration of hydrogen as low as 100 ppm at 100 °C. However, it However, it also has been observed that the TiO2 -B2 O3 gas sensor was unable to operate at roomalso temperature. Response showed observed current decreased when exposed hydrogen has been observed that the TiO2that -B2Othe 3 gas sensor was was unable to operate at roomto temperature. and it was increased when exposed to the nitrogen. It also means that resistance of TiO -B O 2 2 3 increased Response showed that the observed current was decreased when exposed to hydrogen and it was when exposed to the hydrogen and decreased when exposed to the nitrogen. This behavior indicated increased when exposed to the nitrogen. It also means that resistance of TiO2-B2O3 increased when that TiO2 -B2 O3 gas sensor is a p-type gas sensor based on its response. P-type responses might be exposed to the hydrogen and decreased when exposed to the nitrogen. This behavior indicated that caused by diffusion of silver into TiO2 . Sheini and Rohani [51] have compared the sensing mechanism TiO2-B2O3 gas sensor is a p-type gas sensor based on its response. P-type responses might be caused of TiO2 to reducing gas before and after silver diffusion into TiO2 and found that sensing mechanism by diffusion of silver into TiO2. Sheini and Rohani [51] have compared the sensing mechanism of TiO2 of gas sensor has been changed to p-type when silver diffused into TiO2 . The sensitivity of p-type gas to reducing gasbe before and after silver[54]: diffusion into TiO2 and found that sensing mechanism of gas sensor can calculated as follows R into TiO2. The sensitivity of p-type gas sensor sensor has been changed to p-type when silver diffused S = H2 RN can be calculated as follows [54]: where RH2 is resistance in hydrogen flow and RN is initial resistance in nitrogen flow.

=

where RH2 is resistance in hydrogen flow and RN is initial resistance in nitrogen flow.


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(a)

(b)

(c) Figure 10. Response of TiO2-B2O3 gas sensor to hydrogen at different operating temperature Figure 10. Response of TiO2 -B2 O3 gas sensor to hydrogen at different operating temperature (a) T = 100 ◦ C (a) T = 100 °C (b) T = 200 °C (c) T = 300 °C. (b) T = 200 ◦ C (c) T = 300 ◦ C.

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Comparison of sensor response at different operating temperature is shown in Figure 11. The gas sensor responded well to hydrogen. It also found that the responses values were unable to return Comparison sensorisresponse different operating temperaturewere is shown Figure 11. The gas to the original value, of which 1. This at indicated that the responses not in fully recovered when sensor responded well to hydrogen. It also found that the responses values were unable to was return nitrogen was flowed to the gas chamber. It can be seen that the value of sensor response very to the original value, which is 1. This indicated that the responses were not fully recovered when low at an operating temperature of 100 °C compared to the operating temperature at 200 °C and 300 nitrogen was flowed to the gas chamber. It can be seen that the value of sensor response was very °C. The highest peak of sensor response was achieved at an operating temperature of 300 °C. The low at an operating temperature of 100 ◦ C compared to the operating temperature at 200 ◦ C and sensitivity was increased when operating temperature was The sensitivity ofof100 ◦ C. of 300 ◦ C. The highest peak of the sensor response was achieved at higher. an operating temperature 300ppm H2 was the lowest at an operating temperature 200 °C due to the sensor response being the lowest The sensitivity was increased when the operating temperature was higher. The sensitivity of 100 ppm at ◦ C due to the sensor this temperature 10). different highest sensitivity obtained of H2 was the(Figure lowest at an Among operatingthree temperature 200temperatures, response beingwas the lowest at an operating temperature of10). 300Among °C andthree the sensitivity values werehighest 2.30, 7.28, and 9.68 100 ppm, at this temperature (Figure different temperatures, sensitivity was at obtained at an and operating of 300 ◦ CFrom and the sensitivity values 7.28, and 9.68resistance at 100 ppm,was 500 ppm, 1000 temperature ppm respectively. observation, it canwere be 2.30, concluded that 500 ppm, 1000 ppm respectively. From observation, can flow be concluded that resistance decreased whenand temperature was increased. These indicateditthat of current will become was higher decreased when temperature was increased. These indicated that flow of current will become higher as temperature increased, where more electrons can pass through the gas sensor and increase the as temperature increased, where more electrons can passgas through the gas sensor and increase the conductivity. Overall, it can improve the sensitivity of the sensor. conductivity. Overall, it can improve the sensitivity of the gas sensor.

Figure 11. Sensor response of TiO sensoratatdifferent different operating 2 -B 3 gas Figure 11. Sensor response of TiO 2-B 2O23Ogas sensor operatingtemperature. temperature.

In term of stability and repeatability properties of TiO -B O gas sensor, the same sample has been

2 2 3 In term of stability and repeatability properties of TiO 2-B2O3 gas sensor, the same sample has exposed to hydrogen at optimal operating temperature, which is at 300 ◦ C. The cycle time of hydrogen been exposed to hydrogen at optimal operating temperature, which is at 300 °C. The cycle time of and nitrogen was increased to 1200 s for this measurement. The sensor response of TiO2 -B2 O3 gas hydrogen nitrogen was increased toseen 1200that s for thissensor measurement. sensorwell response sensorand is shown in Figure 12. It can be the gas was unableThe to recover when of TiO2-Bexposed 2O3 gas to sensor is shown in Figure It can seen the gastosensor was unable to recover hydrogen even though the12. cycle timebe has beenthat increased two-fold from the previous well when exposed However, to hydrogen though the time to two-fold from the measurement. thiseven measurement hascycle shown thehas gas been sensorincreased has repeatability properties without large drift, based on its similar behavior when hydrogen. In terms stability previous measurement. However, this measurement hasexposed showntothe gas sensor has of repeatability properties, the large gas sensor be considered to have good stability, since thetosensitivity reduced to of properties without drift,can based on its similar behavior when exposed hydrogen. In terms 61.16% after six months. Sensitivity decreases with time have also been reported in [55,56]. stability properties, the gas sensor can be considered to have good stability, since the sensitivity reduced to 61.16% after six months. Sensitivity decreases with time have also been reported in [55,56].


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Figure 12. Sensor response of TiO2-B2O3 gas sensor to hydrogen at 300 °C. Figure 12. Sensor response of TiO2 -B2 O3 gas sensor to hydrogen at 300 ◦ C.

4. 4. Conclusions Conclusions ◦ C 500 O33 gas gassensor sensor calcined °C hasgood shown good performance to low A TiO2-2-BB22O A TiO thatthat calcined at 500at has shown performance to low concentrations concentrations of hydrogen, as low as 100 ppm at different operating temperatures. The gas sensor of hydrogen, as low as 100 ppm at different operating temperatures. The gas sensor also showed an ◦ also showed an ability to perform at low operating temperatures, to as low as 100 °C. Responses ability to perform at low operating temperatures, to as low as 100 C. Responses showed that the showed that the TiO2-B2O3 gas sensor behaved as a p-type gas sensor, based on decreased currents TiO2 -B2 O 3 gas sensor behaved as a p-type gas sensor, based on decreased currents when exposed to when exposed to hydrogen. Results showed thatwas highest sensitivity was achieved at an operating hydrogen. Results showed that highest sensitivity achieved at an operating temperature of 300 ◦ C temperature of 300 °C with sensitivity at 100 1.44,ppm, 4.60,500 andppm, 8.90 for ppm, ppm, and 1000 with sensitivity values at 1.44, 4.60, andvalues 8.90 for and100 1000 ppm500 respectively. ppm respectively.

Author Contributions: Formal Analysis, N.H.A.; Writing-Original Draft Preparation, S.A.M.C.; Supervision, Author Formal Analysis, N.H.A.; Writing-Original Draft Preparation, S.A.M.C.; Supervision, M.N.H.,Contributions: M.S.M., and M.E. M.N.H., M.S.M., and M.E. Funding: This research was funded by Research Management Centre, Universiti Putra Malaysia. Funding: This research was funded by Research Management Centre, Universiti Putra Malaysia. Acknowledgments: The author would like to thank members of Physic Laboratory, Science Faculty, Ataturk University for providing facilitieswould for gas sensor measurement. Acknowledgments: The author like to thank members of Physic Laboratory, Science Faculty, Ataturk University forInterest: providing gas sensor measurement. Conflicts of Thefacilities authors for declare no conflict of interest.

Conflicts of Interest: The authors declare no conflict of interest.

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