Improved formaldehyde gas sensing properties of well-controlled Au nanoparticle-decorated In₂O₃ nanofibers integrated on low power MEMS platform

1. Introduction

Among various toxic gases, volatile organic compounds (VOCs) can lead to serious health problems such as sick house syndrome, skin atopy, mutagen and carcinogen [[1], [2], [3], [4]]. Formaldehyde (HCHO) is one of representative toxic indoor gases able to coexist with toluene and CO gases. Although a gas chromatography/mass spectroscopy has been typically employed to detect HCHO gases, the method is too huge and expensive to utilize as a rapid mobile and personal monitoring system. The research for detecting toxic gases with gas sensors installed in mobile and personal devices such as smartphone, drone, and tablet PC has extensively performed with the increasing concern about the health and safety of homes, industries and environment. Electrochemical gas sensors and metal oxide semiconducting gas sensors have been proposed as approaches for the development of the mobile and personal monitoring system to detect VOCs [5]. The electrochemical gas sensors still have challenging issues such as the sensor size, electrolyte and power consumption. The metal oxide semiconducting gas sensors should heat up the sensing part for the adsorption and desorption of VOC target gases at the surface of the sensing material at high temperatures (200-400 °C) [[6], [7], [8]]. The high power consumption due to high operating temperatures in the metal oxide gas sensors has limited the application of the gas sensor to the portable and wireless devices such as smartphones, tablet PCs and drones. Additionally, the selectivity on target gases with the metal oxide semiconducting gas sensors has been still a critical issue.

Various researches for the fabrication of low power-operable gas sensors equipped in the mobile and personal devices have been widely performed. Utilization of nanostructured materials with the increased surface area and junction site such as grain boundaries, Schottky contacts as sensing materials is one approach to exhibit the enhanced sensing properties with the lower temperature [[9], [10], [11], [12]]. The enhanced sensing behavior of nanostructured materials at even room temperature has been reported [13,14]. The employment of noble metal nanocatalyst such as Pt, Au, Pd, and Ag with semiconducting metal oxides has also improved the gas sensing properties with the high sensitivity and low sensing temperature [[15], [16], [17], [18], [19], [20], [21], [22], [23]]. Furthermore, the employment of appropriate catalysts with sensing materials can improve the selectivity on specific target gases [[24], [25], [26]].

Additionally, the application of a micro-platform to gas sensing devices is another approach to reach high temperature with low power consumption. The fabrication of micro-scaled platforms consisting of the micro-heater, insulating layer, micro-electrodes has been realized by a MEMS technique [[27], [28], [29], [30]]. The dimension of micro-platforms has been gradually miniaturized to reach high temperature with the power of the range from hundreds of milliwatts to several tens of milliwatts. Although the efficiency of micro-heater in the platform is improved by the miniaturization, the reliable integration of nanostructured sensing materials on the micro-scaled sensing part of the micro-platform has been required. The reliability and durability of the nanostructured sensing material-based gas sensors detectable at low temperature is challenging issues because of the unstable base lines after exposure to target gases, slow recovery time and reduced life time.

In this work, the chemiresistive HCHO gas sensing properties with controlled power consumption was schematically investigated in the micro-platform integrated with Au-decorated 1D nanostructured In₂O₃ nanofibers as sensing materials for the potential applications to portable gas sensors. The bridge-type micro-platform reachable to high temperature with low power consumption was fabricated by a typical MEMS process. The 1D nanostructured In₂O₃ fibers and Au-In₂O₃ nanofibers as sensing materials were prepared by an electrospinning technique and a chemical reduction method. The size and the density of the Au nanoparticles adsorbed on the surface of In₂O₃ nanofibers were controlled by the surfactant employed in the chemical reduction reaction. The In₂O₃ nanofiber-based sensing materials were reliably integrated on the sensing part with area of 167 by 167 μm² in the micro-platform by the injection of the ink with the dispersed sensing materials. The sensing properties of the fabricated sensing devices including bare In₂O₃ nanofibers and Au-In₂O₃ nanofibers were measured as a function of the input power at the micro-heater for HCHO gases with different concentration and the size and density of Au nanoparticles decorated on In₂O₃ nanofibers. Furthermore, the selectivity of Au-In₂O₃ nanofiber-based sensing materials on the HCHO gases were investigated.

2. Experimental

All the chemical reagents for the synthesis of sensing materials were analytical grade and used without further purification. The electrolyte for an electrospinning process to synthesize In₂O₃ nanofibers consists of 0.4 g Indium (III) nitrate hydrate (In(NO₃)₃·xH₂O) (Aldrich, 99.9%) and 0.8 g polyvinyl pyrrolidone (PVP) (Aldrich, molecular weight M_W = 130,000 g/mol) dissolved in 2.2 mL N,N-Dimethylformamide (DMF) (Aldrich, anhydrous) and 6.6 mL ethanol (Aldrich, absolute). A needle with an inner diameter of 0.26 mm (25 gauge) and an aluminum foil as a counter electrode to emit perpendicularly to the ground were employed in the electrospinning system. The electrolyte was electrospun by applying a voltage of 20 kV at a working distance of 10 cm. The electrospun nanofibers were calcinated for 4 h at 500 °C in the air. Au nanoparticles were decorated on the surface of the calcinated In₂O₃ nanofibers. 0.03 g In₂O₃ nanofibers was dispersed in 50 ml distilled water. 1.5 ml of 0.01 M HAuCl₄ (Aldrich, 99.99%) was added to the dispersed In₂O₃ solution. Furthermore, 0.01 M lysine (Alfa Aesar, 97%) with varied quantity of 3 mL and 15 mL was added to control the morphologies of Au-In₂O₃ nanofibers, respectively. Au nanoparticles on the surface of In₂O₃ nanofibers were formed by the addition of 5 ml of 0.01 M NaBH₄ (Aldrich, 98%) solution, respectively. The solution including Au-In₂O₃ nanofibers were centrifuged with the distilled water and ethanol several times, dried at 80 °C following by annealed at 300 °C for 30 min in the air. The characterization of In₂O₃ nanofibers and Au-In₂O₃ nanofibers were performed by field emission scanning electron microscope (FE-SEM) (JEOL, jsm-6700f), X-ray diffraction (XRD) (Rigaku, D/Max 2500) and transmission electron microscopy (TEM) (FEI, TF30ST).

The bridge-type micro-platforms including the interdigit electrodes/insulating layer/micro-heater/insulating layer/substrate were fabricated by a typical MEMS process as previously reported [31,32]. The four-inch Si wafers were utilized as a substrate for the fabrication of micro-platforms. The 2 μm thick low-stress SiN_x layer was deposited on both sides of a Si wafer by a low pressure chemical vapor deposition technique. The micro-heater with 10 nm thick Ta and 200 nm thick Pt layers was created by the sputtering process following by the typical lift-off and dry etching technique. The insulating layers consisting of 500 nm thick SiO₂, 250 nm thick SiN_x, and 250 nm thick SiO₂ (ONO) were deposited on the micro-heater by a plasma-enhanced chemical vapor deposition. The interdigit electrode with 10 nm thick Ti and 200 nm thick Pt layers was also prepared by the sputtering process following by the typical lift-off and dry etching technique. The bridge-type structure in the micro-platform was fabricated by the reactive ion etching process. The fabricated micro-platforms were mounted on metal can packages with four pins. Pads of electrode and micro-heater in the micro-platform were wire-bonded with the pins.

The ink for the integration of sensing materials on micro-platforms was prepared by the gentle sonication for one hour with 0.02 g In₂O₃ nanofiber-based sensing materials dispersed in 4 mL ethanol. The injection syringe, optical microscope and microfluidic pump were utilized to integrate nanostructured sensing materials on the micro-scaled sensing part of a micro-platform. The In₂O₃ nanofiber-based sensing materials were integrated by injecting the prepared ink with the droplet of 0.1 μL per a cycle and drying in sequence. The thickness and electrical resistance of the integrated sensing materials on the sensing part of a micro-platform were measured with the surface profiler (Veeco, Dektak 150) and multimeter (Keithley, model 2000).

3. Result and discussion

One-dimensional In₂O₃ nanofibers with high surface area as a sensing material to detect HCHO gases were prepared by an electrospinning technique following by calcination. The bare In₂O₃ nanofibers were drawn from the electrolyte including PVP and In(NO₃)₃. The electrolyte droplet deformed into a conical shape, called Taylor cone, was formed by the high electric field between a spinneret and a collector. The ejection of the charged liquid jet from the tip of the Taylor cone was created by the repulsive electrostatic forces due to the electrical charges on the surface of droplet overcoming the surface tension. The formation of PVP/In(NO₃)₃ composite nanofibers can be controlled by the operating parameters, such as the applied voltage, electrolyte conductivity, electrolyte viscosity, electrolyte feeding rate, distance between a spinneret and a collector, and needle size. The bare In₂O₃ nanofibers were definitively synthesized by the calcination of the polymeric nanofibers electrospun in gentle experimental conditions. Fig. 1(a)-(c) shows SEM images of the prepared homogeneous In₂O₃ nanofibers with the diameter of 96.6 ± 27.2 nm. The inset in Fig. 1(a) described the diameter profile ranging from 47 nm to 167 nm of In₂O₃ nanofibers analyzed with 100 samples. The cross-sectional image of In₂O₃ nanofibers showed the dense surface morphology like the sintered particles with the developed grain boundaries. The XRD analysis of Fig. 1(b) exhibits the diffraction pattern peaks corresponding with the standard spectrums of cubic structural In₂O₃ (JCPDS #060416). Furthermore, the grain size of 14.8 nm was calculated from a Scherrer equation with the full width half maximum of the (222) peak.

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Fig. 1.   Characterization of bare In₂O₃ nanofibers: (a) SEM image of low magnification;(b) SEM image of high magnification; (c) cross-sectional image of nanofibers; (d) X-ray diffraction pattern of In₂O₃ nanofibers.

The decoration of Au nanoparticles on bare In₂O₃ nanofibers can enhance the performance of gas sensing as a catalyst. The decoration of Au nanoparticles on the prepared bare In₂O₃ nanofibers was performed by the chemical reduction of Au precursors. Lysine as a capping agent was utilized to synthesize homogeneously distributed Au nanoparticles on In₂O₃ nanofibers with the tailored size in the chemical reduction reaction. The morphologies of Au nanoparticles anchored on the surface of In₂O₃ nanofibers were observed as a function of the concentration of the employed lysine. Fig. 2(a)-(c) and Fig. 2(d)-(f) exhibit the TEM analysis of the Au-In₂O₃ nanofibers synthesized by adding lysine of 3 mL and of 15 mL, respectively. The bright field TEM images of Fig. 2(a) and (d) show homogeneously distributed Au nanoparticles on In₂O₃ nanofibers with the developed grain boundaries. In the high-resolution TEM images of Au nanoparticles with about 10 nm and 5 nm diameter on the surface of In₂O₃, the d-spacing analysis with well-developed lattice fringes can be defined as (111), (200) lattice planes of Au and (222), (400) lattice planes of In₂O₃ as shown in the insets of Fig. 2. The size profiles as described in other insets of Fig. 2 had the whole size distribution of 6.1 ± 4.1 nm and 3.2 ± 0.97 nm in the cases of Au nanoparticles synthesized by adding lysine of 3 mL and 15 mL, respectively. Au nanoparticles prepared with 3 mL lysine partially included particles with the size over 15 nm showing two groups with less homogeneous size distributions of 5.4 ± 0.6 nm and 21.0 ± 2.0 nm. However, the size of Au nanoparticles synthesized with 15 mL lysine has better homogenous size distribution of 3.2 ± 0.97 nm with the smaller particles size under 6 nm. The distribution of Au nanoparticles on an In₂O₃ nanofiber depending on the amounts of the applied lysine was clearly observed in the high-angle annular dark-field scanning TEM (HAADF STEM) images and EDS mappings for O, In and Au elements as shown in Fig. 2(b), (c), (e) and (f).

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Fig. 2.   TEM analysis of Au catalyst-decorated In₂O₃ nanofibers prepared by adding lysine with controlled amount of 3 mL (a, b, c) and 15 mL (d, e, f): (a, c) bright field TEM images (inset: HRTEM images and diameter profiles of the decorated Au catalysts); (b, e) high-angle annular dark field STEM images; (c, f) EDS mapping for O, In, and Au elements.

As shown in Fig. 2, size and homogeneous distribution of Au nanoparticles on the surface of In₂O₃ nanofibers were controlled by the addition of a lysine capping agent for the chemical reduction reaction. Lysine with dual roles as a linker and a surfactant in the reaction has functional groups consisting of -NH₂ and -COOH. The primary amine (-NH₂) in one end has been reported to be able to bind with Au nanoparticles restricting the growth and the aggregation of Au [33]. The amino acid including both of -NH₂ and -COOH in the other end of lysine can coordinate with metals in metal oxides such as In₂O₃ nanofibers [34]. As described in Fig. 3, the surfaces of In₂O₃ nanofibers and Au nanoparticles can be functionalized with lysine, respectively. Therefore, lysine linked with Au can restrict the growth and aggregation of Au nanoparticles. Additionally, the strong anchoring of Au nanoparticles on In₂O₃ nanofibers can be achieved by the amine and amino acid ends of the functionalized surfaces. Consequently, lysine as a bridge between Au and In₂O₃ resulted in the uniform distribution of the size-restricted Au nanoparticles on In₂O₃ nanofibers.

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Fig. 3.   Schematic illustration for the decoration of Au nanoparticles on the surface of In₂O₃ nanofibers.

The reduction of power consumption is a critical factor for the extended applications of gas sensors in various fields including portable devices. The fabrication of micro-platforms for the gas sensing devices with low power consumption has been recently conducted by microelectromechanical system (MEMS) techniques. The bridge-type micro-platforms as the structure to minimize the heat loss can reduce the power consumption to reach high temperature. As shown in Fig. 4(a), the bridge-type micro-platform consists of the interdigit sensing electrodes on ONO insulation layer and the micro-heater between SiN_x and ONO insulation layers. The dimension of a membrane sensing part including the interdigit electrodes and micro-heater indicated 600 μm × 600 μm, and the width of bridges in the micro-platform displayed 60 μm. The micro-platform with the dimension of 3.1 mm × 3.1 mm was mounted on the metal can package with diameter of 9 mm. The insets show the optical images of a micro-platform with different magnifications. The calibration graph in Fig. 4(b) described the local temperature of the sensing part depending on the applied power to micro-heater. The temperature of the sensing part was linearly proportional to the applied power indicating 133°C, 205°C and 277°C in 5 mW, 10 mW and 15 mW, respectively. For the reliable integration of sensing materials on the electrodes of micro-platforms, the inks of In₂O₃ nanofiber-based materials dispersed in ethanol were injected by a 25 G syringe with a microfluidic pump. The cycles including injection and drying processes for the integration of sensing materials on the micro-platform were controlled with a droplet of about 0.5 μL volume. The droplets were restrictively positioned on the micro-scaled interdigit electrodes in the membrane as shown in Fig. 4(c). The resistance and thickness of In₂O₃ nanofiber-based films integrated on the electrode of a micro-platform were analyzed. As the number of the cycle increase, the resistance decrease and the thickness increase, as shown in Fig. 4(d). So, this method can be proposed as a reliable method to integrate the sensing materials on the electrode of micro-platform.

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Fig. 4.   (a) Illustration of a MEMS micro-platform (inset: the images of a micro-platform), (b) the local temperature of a sensing part on the micro-platform depending on the applied power to micro-heater, (c) images for the reliable integration of sensing material-based ink on micro-platform depending on the droplet cycles and (d) the thickness and resistance of the integrated sensing materials as a function of droplet cycles.

Prior to analyzing sensing behaviors of Au-In₂O₃ nanofibers, the sensing properties of bare In₂O₃ nanofibers on HCHO gases with different concentration were evaluated as a function of working temperature controlled with applied powers to micro-platforms as shown in Fig. 5(a). The electron depletion layer in the surface of bare n-type semiconducting In₂O₃ nanofibers on the platform heated with the applied power was formed by the temperature-dependent chemisorbed oxygen species. The dominantly chemisorbed oxygen species on metal oxides were determined by surrounding temperature. O₂^- at the temperature below 150°C, and the chemisorbed O^- and O^2- at the temperature above 150°C dominantly exist, respectively [35]. When HCHO reducing gases was exposure to the bare In₂O₃ nanofibers, the depletion layer can be narrowed by chemical reaction between the temperature-dependent chemisorbed oxygen species and HCHO gases as described in Eq.s (1,2, and 3) [[36], [37], [38]]. The exposure HCHO gases can be detected by the reduced electrical resistance of the bare In₂O₃ nanofibers due to the varied depletion layer. The working temperature is one of critical factors to control the gas sensing behavior because the sensing response can be hugely affected by the surface reactions between chemisorbed oxygen species and target gases. The micro heater of the platforms with bare In₂O₃ nanofibers was heated to 277°C, 205°C and 133°C with the applied power of 15 mW, 10 mW, and 5 mW, respectively, to investigate the working temperature-dependent sensing behaviors. The bare In₂O₃ nanofibers heated with the applied power of 10 mW and 15 mW exhibited the sensing behavior on HCHO gases of even 1 ppm. As the temperature of the platform decreased with the reduction of power, the sensitivity on HCHO gases was decreased and the response/recovery time was also decreased. The bare In₂O₃ nanofibers heated with the 5 mW power didn’t clearly show the sensing behavior in HCHO gases of even 10 ppm. The graph of Fig. 5(b) described sensitivities of bare In₂O₃ nanofibers depending on the concentration of HCHO gases and the applied power (or temperature) of micro platforms. The sensitivities of bare In₂O₃ nanofibers linearly increased with the increase of HCHO concentration in the micro platforms heated to higher temperature of 277°C and 205°C with the powers of 15 mW and 10 mW, although the bare In₂O₃ nanofibers at 133°C with 5 mW indicated the unclear sensing behavior with non-linear response.

HCHO + $O_2^-$→ 2HCOOH + e^- (1)

HCHO + O^- → HCOOH + e^- (2)

HCHO + 2O^2- → CO₂ + H₂O + 4e^- (3)

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Fig. 5.   (a) Sensing behavior of bare In₂O₃ nanofibers as a function of HCHO gas concentration for different heating powers and (b) sensitivity for gas concentrations of HCHO and heating powers.

Sensing properties of In₂O₃ nanofibers decorated with Au nanoparticles were investigated as shown in Fig. 6. Sensing behaviors of bare In₂O₃ nanofiber, Au-In₂O₃ nanofiber prepared with 3 mL lysine, and Au-In₂O₃ nanofiber prepared with 15 mL lysine were compared in the successive exposure of HCHO gases with the controlled concentration heating the sensing part with 15 mW (Fig. 6(a) and (b)). Sensitivities of the In₂O₃ nanofiber-based sensing materials linearly increased with the increase in the concentration of HCHO gases. Au-In₂O₃ nanofibers (15 mL lysine) including Au nanoparticles with homogenous size distribution of average 3.2 ± 0.97 nm as shown in Fig. 2 describes the highest sensitivities on each concentration of HCHO gases and the biggest increase rate of sensitivity depending on the increase of gas concentration. Furthermore, the sensitivities of Au-In₂O₃ nanofibers (15 mL lysine) on HCHO gases rose in proportion to a rise in heating power as described in Fig. 6(c) and (d). The sensitivity in the exposure of 10 ppm HCHO drastically increased from 1.46 to 11.30 with the increase of heating power from 5 mW to 15 mW. The sensing behavior on 1 ppm HCHO were clearly demonstrated at even low temperature of 133°C with the heating power of 5 mW, as contrasted with the unclear sensing behavior of the bare In₂O₃ nanofibers in 5 mW heating power. Additionally, the response time and recovery time were improved with increase in the heating power due to the kinetically enhanced reaction on surface at high temperature. Response time, T₉₀, and recovery time, D₁₀, denote the time elapsed to reach 90% of the steady state response and the steady state recovery, respectively. In the case of exposure to 5 ppm HCHO gases, T₉₀ and D₁₀ were reduced from 1331s to 109 s, and from 2499s to 493 s, respectively, when the heating power for the sensing part increased from 5 mW to 15 mW. The slow response/recovery time at low temperature with small power consumption need to improve for practical applications to commercial devices. As described in Table 1, the sensing properties, such as sensitivity, detection limit, working temperature and power consumption, of the gas sensor fabricated with Au-In₂O₃ nanofibers (15 mL lysine) were compared with the recently reported results of various MEMS platform-based gas sensors. The Au-In₂O₃ nanofiber (15 mL lysine)-based MEMS gas sensor demonstrated relatively remarkable performance with the low working temperature of 133 °C and the low power consumption of 5 mW.

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Fig. 6.   (a) Sensing behavior of In₂O₃ nanofibers-based sensing materials as a function of HCHO gas concentration in the micro-platform heated with 15 mW for different heating powers, (b) sensitivity of In₂O₃ nanofibers-based sensing materials heated with 15 mW at different concentrations of HCHO gases, (c) sensing behavior of Au-In₂O₃ nanofibers (15 mL lysine) as a function of HCHO gas concentration for different heating powers and (d) sensitivity of Au-In₂O₃ nanofibers (15 mL lysine) for gas concentrations of HCHO and heating powers.

The improved sensing performance of Au-In₂O₃ nanofibers prepared with well-controlled Au nanoparticles was ascribed by the spillover effect and the enhanced surface area due to the homogeneously distributed small Au nanoparticles on the surface of the In₂O₃ nanofibers. As shown in Fig. 7(a), an electron depletion layer was typically created on the surface of the n-type semiconducting In₂O₃ as the result of chemisorbed oxygen species at high temperature. The surface of Au-In₂O₃ nanofibers can have a thicker depletion layer due to the increase of the oxygen species spilled from Au nanoparticles, compared to the thickness of a depletion layer in bare In₂O₃ nanofibers (Fig. 7(b)). The electrical resistance of Au-In₂O₃ nanofibers was increased by the conduction channel reduced from the expanded depletion layer. When the Au-In₂O₃ nanofibers with the increased oxygen species on the surface were exposed to reducing gases such as HCHO, the electrical resistance was decreased with the reduced depletion layer due to the increased chemical reactions as described in Eq. 1 Introduction, 2 Experimental and 3 (Fig. 7(c)). Consequently, the thicken depletion layer in the Au-In₂O₃ nanofibers can produce the relatively enhanced variation of the electrical resistance on the exposure of target gases. Compared to Au-In₂O₃ nanofiber (3 mL lysine)-based gas sensor, the improved sensing properties of Au-In₂O₃ nanofibers (15 mL lysine) might be attributed to the enhanced spillover effect due to the reduced size and improved distribution of Au nanoparticles on In₂O₃ nanofibers in the case of utilizing 15 mL lysine. The optimized sensing performance of Au-In₂O₃ nanofibers requires the homogeneously distributed Au nanoparticle decoration with the controlled size until the severe aggregation occurs. The distribution of Au nanoparticles on In₂O₃ nanofibers might be accurately tailored by quantity of lysine.

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Fig. 7.   Schematic diagram of sensing behaviors: (a) bare In₂O₃ nanofiber with the depletion layer formed by chemisorbed oxygen species; (b) Au-In₂O₃ nanofibers with the increased depletion layer due to the oxygen species spilled from Au as well as chemisorbed oxygen species; (c) Au-In₂O₃ nanofibers with the decreased depletion layer after the exposure on HCHO reducing gases.

The employment of catalysts on sensing materials has been reported to improve the selectivity on a specific target as well as the sensitivity. HCHO, toluene and CO gases are typical toxic indoor gases causing a sick building syndrome [44,45]. The selectivity of Au-In₂O₃ nanofibers (15 mL lysine) on HCHO gases was compared with the indoor interference gases such as CO and toluene gases able to coexist in houses or buildings as shown in Fig. 8(a), because those gases can be emitted from the coal combustion for heating and cooking, and household stuffs including adhesives, paints, flooring material, etc [[46], [47], [48]]. The sensing behavior of Au-In₂O₃ nanofibers working at the 5 mW heating power didn’t show notable difference with other gases. However, the selectivity on the target gas of HCHO evidently appeared with raising both the heating power and the concentration of gases. The enhanced response of Au-In₂O₃ nanofibers on HCHO gases may be contributed to the increased reactivity of HCHO gases with chemisorbed oxygen species on the surface, compared to the reaction of CO and toluene gases with chemisorbed oxygen species. The relative reactivity increased with raising the temperature and the gas concentration. The Au-In₂O₃ nanofibers (15 mL lysine) demonstrated 10 times higher selectivity on HCHO gases than the selectivity on CO and toluene gases in the gas concentration of 10 ppm at 15 mW. In addition to the selectivity test, the sensing properties of the fabricated HCHO gas sensor at different humidity conditions were evaluated as shown in Fig. 8(b). Humidity in the operation of semiconducting gas sensing devices is a critical factor to induce a malfunction, to deteriorate sensing performance, and to reduce lifetime. The humidity test of RH 30%, RH 50%, and RH 80% was conducted with Au-In₂O₃ nanofibers (15 mL lysine) on 10 mW powered micro-platform to detect 1 ppm HCHO gases. The sensitivities were 1.38, 1.35 and 1.34 at RH 30%, RH 50% and RH 80%, respectively. The sensing behavior of the fabricated gas sensor demonstrated stable working with insignificant variation under 3% degradation of sensitivity at varied humidity conditions.

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Fig. 8.   (a) Selective sensing properties of Au-In₂O₃ nanofibers (15 mL lysine) on the micro-platform heated with controlled powers in the exposure on HCHO, toluene and CO gases with different concentrations and (b) sensing properties to 1 ppm HCHO gases in the 10 mW powered micro-platform at various humidity conditions.

4. Conclusion

For the development of high sensitive and selective HCHO gas sensor operable with low heating power, In₂O₃ nanofiber-based sensing materials and micro-platforms were utilized. Homogeneous In₂O₃ nanofibers with the diameter of 96.6 ± 27.2 nm were synthesized by an electrospinning technique. Well-dispersed Au nanoparticles on the surface of the synthesized In₂O₃ nanofibers were prepared to enhance the sensing properties on HCHO gases. The size of adsorbed Au nanoparticles was controlled by the employed surfactant, lysine, during the chemical reduction reaction. The adsorbed Au nanoparticles with the size distribution of 6.1 ± 4.1 nm and 3.2 ± 0.97 nm was shown in the reaction adding lysine of 3 mL and 15 mL, respectively. Ink including the prepared sensing materials was reliably injected on a micro-platform with a sensing part membrane of 600 μm × 600 μm by utilizing a syringe with a microfluidic pump. The fabricated bridge-type micro-platform was designed to reach high temperature with low power. The sensing behavior of In₂O₃ nanofiber-based sensing materials integrated on the micro-platform was schematically investigated as a function of heating power and gas concentration. The bare In₂O₃ nanofibers showed the sensing behavior on HCHO gases of even 1 ppm at the heating power of 10 mW and 15 mW, although unclear sensing properties were exhibited in the micro-platform heated with 5 mW. Au-In₂O₃ nanofibers, on the other hand, demonstrated the definite sensing behavior at the low heating power of even 5 mW. Especially, In₂O₃ nanofibers decorated with smaller Au nanoparticles of avg. 3.2 nm, which prepared with 15 mL lysine, showed the enhanced sensitivity of 11.3 on 10 ppm HCHO gases at the 15 mW heating power. The gas concentration-dependent sensitivity of Au-In₂O₃ nanofibers (15 mL lysine) was increased with increase in the heating power. Additionally, The Au-In₂O₃ nanofibers (15 mL lysine) operated at 15 mW demonstrated the distinguished selectivity on HCHO gases among other gases.

Acknowledgments

This work was supported financially by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2017R1D1A1B03030796).