Room Temperature Hydrogen Gas Sensor Based on Pd-SnO2 Nanomaterials with Electro-spinning
JangDongjun1
KimSangwan1
KwonMin-Woo2,*
-
(Department of Electronics Engineering, Sogang University, Seoul 04107, Republic of
Korea)
-
(Department of Electronic Engineering, Seoul National University of Science and Technology,
Seoul 01811, South Korea)
Copyright © The Institute of Electronics and Information Engineers(IEIE)
Index terms
Hydrogen gas sensor, SnO2 nanowires, electro-spinning, Pd nanodots
I. INTRODUCTION
Recently, a hydrogen (H2) has begun to receive attention as an eco-friendly smart energy source due to the
depletion of fossil fuels and the advancement of automotive technology. Although the
H2 gas is recognized as an abundant, renewable, and efficient energy source, it is colorless,
odorless, tasteless, and becomes extremely flammable and explosive when its concentration
exceeds 4% in the air [1]. In addition, the H2 features a low ignition energy (0.02 mJ), a high diffusion coefficient (0.61 cm2/s), and a wide explosion limit (4-75%), which can lead to the severe consequences
in the event of an accidental leak [2,3]. Therefore, there is a demand for the highly sensitive H2 gas sensors capable of reliably detecting H2 gas and providing real-time monitoring under the various environmental conditions.
Over the past few years, several H2 detection technologies have been explored, including thermal detection sensors, electrochemical
sensors, resistive metal-oxide semiconductor (MOS) sensors, work function-based detection
techniques, catalytic sensors, optical sensors, and acoustic sensors [4-7]. Among them, the resistive MOS gas sensors are advantageous in terms of a low cost,
a stable sensing capability, and a simple fabrication process [8-10]. A MOS based on tin dioxide (SnO2) is well known for its high electron mobility and chemical stability and is widely
used to detect reducing gases such as H2 [11,12]. Furthermore, the SnO2 nanowires (NWs) offer a large specific surface area and rapid gas accessibility to
the reactive sites [13,14]. Several fabrication methods for metal oxide nanostructures, including hydrothermal
synthesis, thermal oxidation, and electro-spinning, have been developed [15]. Among these methods, the electro-spinning process has been reported to produce uniform
nanostructures with controlled diameters [16]. In addition, a noble metal nanodots (NDs)-embedded technology presents as a catalyst
for sensors, allowing for easy adsorption of H2 gas due to the spillover effect [17]. In case of the H2 sensing noble materials, the sensors decorated with Pd have been reported to show
enhanced gas sensing performance [18].
As the critical issues in H2 gas sensing technologies, the detect operation at high temperature can lead to considerable
energy consumption and reduced device’s stability due to heat generation. Also, the
sensors capable of detecting low concentrations at room temperature offer advantages
in an energy efficiency, a sensor’s lifetime, and reliability. Recent gas sensor studies
have been investigated a nanocomposite structure and nanoparticle-based catalysts
to develop H2 gas sensors with high sensitivity at low concentration and room temperature [19,20]. Zhang et al. have reported a Pd-SnO2 nanocomposite sensor with an operation temperature (280℃) and a detection range (20-1,000
ppm) [21]. Additionally, Ling et al. have demonstrated on H2 gas sensing at room temperature with a high limit of detection in the range of 500-10,000
ppm [22].
In this study, we investigate the effects of electro-spinning time on the deposition
of the SnO2 NWs and their gas sensing performance. By adjusting the electro-spinning times to
5, 10, and 20 seconds, the influence of the NWs density on the response and sensitivity
of h2 gas sensors is analyzed. Despite extensive research on MOS-based hydrogen sensors,
limited attention has been paid to the direct impact of electro-spinning duration
on nanowire density and subsequent gas sensing characteristics. This study uniquely
addresses this gap by systematically exploring how electro-spinning time alters the
surface morphology and sensing performance at room temperature. Furthermore, the decoration
of Pd NDs onto the SnO2 NWs surface is examined to elucidate the H2 sensing mechanism through the catalytic effect of Pd NDs and H2. The proposed Pd-SnO2 based sensor demonstrates enhanced sensing characteristics at room temperature for
low H2 concentrations (4-50 ppm), offering an innovative approach for practical H2 detection applications.
II. EXPERIMENTS
1. Synthesis of SnO2 Nanowires
To synthesize SnO2 NWs, a precursor solution is prepared by adding 0.5 g of tin (II) chloride dihydrate
and 1.0 g of polyvinyl pyrrolidone (PVP) to a homogeneously dissolved solution of
3.8 g of N, N-dimethyl methanamide (DMF) and 1.5 g of ethyl alcohol (EtOH) and stirring
until the solution becomes transparent. The SiO2/Si substrate is cleaned sequentially for 1 minute using acetone, deionized water,
and ethanol (EtOH, 99.9%) and then placed on a collector heated to 70℃. As shown in
Fig. 1(a), a voltage of 20 kV is applied to the prepared precursor solution, and it is dispensed
at a constant flow rate of 1 mL/h. This allows SnO2 NWs to be electro-spinning onto the prepared SiO2/Si substrate for 5 seconds (5SnO2), 10 seconds (10SnO2), and 20 seconds (20SnO2), respectively. The electro-spinning samples are then calcined at 500℃ for 3 hours
in a furnace with a ramping speed of 2℃/min to induce crystallization.
Fig. 1. The schematic diagram of (a) the synthesis of SnO2 nanowires by electro-spinning method, (b) fabrication process of Pd-SnO2 gas sensor, (c) the gas probe station measurement system, and (d) three-phase curve
for gas injection protocol.
2. Sensor Device Fabrication Process
The fabrication process of the H2 gas sensors based on Pd-SnO2 is illustrated in Fig. 1(b). The process begins with substrate preparation, where organic solvents are removed
from p-type silicon substrate (10 mm × 10 mm) through sequential cleaning steps using
acetone, isopropyl alcohol, and deionized water. After N2 gas drying, a 300 nm-thick SiO2 layer is grown through a dry oxidation process. The SnO2 NWs are synthesized through an electro-spinning process for durations of 5, 10, and
20 seconds. To deposit the two-terminal electrodes, a 100 nm-thick titanium (Ti) is
formed by a direct current (DC) sputtering. The deposition conditions for the DC sputtering
system are maintained consistently, with an argon gas flow rate of 30 SCCM, a DC power
of 100 W, and the deposition process conducted at a temperature of 2℃. Finally, the
noble Pd NDs are decorated onto the surface of SnO2 NWs using DC sputtering method.
3. Gas Sensing Measurements
To evaluate the H2 gas detection of the fabricated gas sensor under conditions of temperature (27℃)
and humidity (31%), the H2 gas sensing measurements are performed using a gas probe station and analysis equipment
(Keithley 2450 SMU) as illustrated Fig. 1(c). The measurement setup implements a two-terminal configuration with 1 V applied bias.
Initial gas flow validation is performed in unsealed chamber conditions, followed
by 600 seconds stability assessment and three-phase gas injection protocol [Fig. 1(d)]. The protocol consists of: (1) H2 gas flow in unsealed chamber, (2) sealed chamber H2 gas injection, and (3) nitrogen gas chamber purge followed by air exposure for recovery
analysis. In the first step, the chamber remains unsealed while H2 gas is flowed for 150 seconds. During the second step, the chamber is sealed, and
H2 gas is injected at varying concentrations from 4 to 50 ppm. For the final step, nitrogen
gas is used to clean the chamber, and the gas sensor is exposed to air to establish
a recovery environment. To minimize pressure-induced current fluctuations, the gas
injection flow rate is maintained at 200 SCCM. The electrical characteristics of the
fabricated Pd-SnO2 sample are analyzed by monitoring the current variation over time in this mixed gas
environment. The gas sensing current is defined as Igas when exposed to H2 gas and Iair when exposed to air. Therefore, the sensor response is calculated according to Eq. (1).
III. RESULTS AND DISCUSSION
1. Morphological and structural of Pd-SnO2
The morphology of the fabricated Pd-SnO2 samples is illustrated in FE-SEM images at a magnification of 1 µm (Figs. 2(a)-2(c)). The average diameter of the SnO2 is 115 nm and the number of NWs is increased as the electro-spinning process time
increases.
After the Pd NDs-decoration of SnO2, their surfaces exhibit different morphologies. As shown in Fig. 2(d), the protruding Pd particles are observed on the surface of the SnO2, which are uniformly distributed throughout the NWs. For a more precise analysis,
the FE-SEM images at a magnification of 100 µm are presented in Figs. 2(e)-2(f).
Fig. 2. The morphological characterization of Pd-SnO2 samples through FE-SEM images: (a) Pd-5SnO2, (b) Pd-10SnO2, (c) Pd-20SnO2 at a magnification of 1 µm, (d) Pd nanodots observed at a magnification of 100 nm,
and (e)-(f) comparison of SnO2 and Pd-SnO2 at a magnification of 100 µm.
Fig. 3 shows the XRD patterns of the directly electro-spinning SnO2. The XRD analysis is conducted using CoKα radiation (λ = 1.78 Å) under operating
conditions of 40 kV and 40 mA. The scanning range is set to 2θ = 20◦-70◦ with a step
increment of 0.02◦. For the SnO2 NWs, the peaks corresponding to the (110), (101), (200), (211), and (220) planes
are observed at 30.918◦, 39.467◦, 44.290◦, 60.803◦, and 64.429◦, respectively, indicating
a SnO2 tetragonal crystallized structure.
Fig. 3. The XRD pattern of the synthesized SnO2 nanowires.
2. Chemical Bonding of Pd-SnO2
Figs. 4(a)-4(c) present the XPS spectra of the fabricated Pd-5SnO2, Pd-10SnO2, and Pd-20SnO2 samples. Fig. 4(a) shows the full scan XPS survey of Pd-SnO2. All fabricated sensor samples exhibit peaks assigned to Sn, O, Pd, and C, consistent
with previous studies on Pd-SnO2 spectra [23]. In Fig. 4(b), the Sn peaks are split into Sn 3d5/2 and Sn 3d3/2, observed at 486.8 eV and 495.2 eV, respectively. High-resolution spectrum analysis
revealed a peak separation of 8.4 eV, confirming the presence of Sn4+ ions bonded to oxygen within SnO2 [24]. Additionally, a 0.2 eV shift phenomenon is observed in Pd-5SnO2, which aligns with previous findings that reported a chemical shift between the charge
states of Sn2+ and Sn4+ [25]. Fig. 4(c) presents the XPS analysis of the Pd peaks. The Pd peaks are detected at 335.44 eV
and 340.74 eV, corresponding to Pd 3d5/2 and Pd 3d3/2, with a peak separation of 5.3 eV. Additionally, the O 1s XPS spectra (not shown
here) revealed peaks centered at ~ 530.1 eV and ~ 531.6 eV, corresponding to lattice
oxygen (O2−) and surface adsorbed oxygen species, respectively. These findings support the existence
of reactive surface oxygen crucial for H2 sensing.
Fig. 4. The XPS analysis data for Pd-SnO2 samples (electro-spinning process times: 5, 10, and 20 seconds): (a) The full can
survey spectra, (b) Sn 3d spectra, and (c) Pd 3d spectra.
3. Gas Sensing Properties
The H2 gas sensing characteristics of Pd-SnO2 switching layer are conducted through various measurements at room temperature. Figs. 5(a)-5(c) show the gas sensing performance of Pd-SnO2 samples with electro-spinning for 5, 10, and 20 seconds under H2 gas concentrations of 4, 10, and 50 ppm. The current in ambient conditions (Iair) increases upon H2 gas exposure, resulting in enhanced sensing current (Igas) attributed to the H2 adsorption induced electrical mechanism within the Pd-SnO2 switching channel. This operational behavior reflects the semiconductor characteristics
of the Pd-SnO2 interface, as depicted in Fig. 5(d).
The sensing response of all fabricated devices shows proportional enhancement with
increasing H2 concentration in the current-time (I-T) characteristics. Among the fabricated sensors,
the Pd-10SnO2 device exhibits superior responsivity at 50 ppm H2 concentration. This enhanced performance is attributed to the optimized NWs density,
whereas the Pd-20SnO2 sample demonstrates reduced H2 adsorption due to nanosheet formation with extended electro-spinning time. At the
room operating temperature, the H2 sensing response of the Pd-10SnO2 sensor is measured at 10.
Fig. 5. The results of I-T curve for H2 gas sensing performance of Pd-SnO2 (concentration: 4, 10, and 50 ppm): (a) Pd-20SnO2, (b) Pd-10SnO2, (c) Pd-5SnO2, and (d) Response of Pd-SnO2 gas sensors at varying concentrations.
Additionally, the gas sensing performance is further evaluated based on the response
and recovery times. These temporal metrics are essential parameters for evaluating
the practical applicability of H2 gas sensors. Figs. 6(a)-6(b) present the response recovery characteristics of the fabricated sensors under varying
H2 gas concentrations. The response time is defined by the duration required for sensor
resistance to achieve 90% of equilibrium value upon target gas exposure, while recovery
time corresponds to the period needed for resistance reduction to 10% of equilibrium
value. The increased H2 gas concentration accelerates the surface adsorption kinetics, thereby reducing the
response time through enhanced resistance or current modulation.
The Pd-5SnO2 sensor demonstrates response and recovery times of 208 and 223 seconds, respectively,
at 50 ppm concentration. In comparison, the Pd-20SnO2 device exhibits enhanced temporal characteristics with response and recovery times
of 112 and 204 seconds, attributed to increased NWs density resulting from extended
electro-spinning duration. The response-recovery analysis indicates improved H2 detection capability with increased electro-spinning process time. The proposed Pd-SnO2 based sensors demonstrate enhanced H2 sensing performance within the low concentration range of 4-50 ppm.
Fig. 6. (a) Response time and (b) recovery time of the fabricated Pd-5SnO2, Pd-10SnO2, and Pd-20SnO2 sensors exposed to H2 gas concentrations of 4-50 ppm at room temperature.
4. Gas Sensing Mechanisms
The H2 sensing mechanism of Pd-SnO2 has been investigated through analysis of spillover effect and electron transport
characteristics [26,27]. The detection mechanism of H2 gas sensor based on MOS-type relies on the alteration of the sensing material’s electrical
conductivity when exposed to H2 gas. This change in conductivity is due to several physical and chemical interactions
between the H2 molecules and the sensing material. This section presents fundamental principles
of H2 detection in MOS-based sensors, emphasizing surface reactions, charge transfer processes,
and associated physicochemical transformations.
Figs. 7(a)-7(b) illustrate the spillover effect in H2 gas sensing mechanisms. In ambient conditions, oxygen (O2) molecules are adsorbed onto SnO2 NWs surface through electro-static attraction [Eq. (2)]. The subsequent electron capture by adsorbed O2 results in O− ion formation [Eqs. (3)-(4)]. This ionization process reduces majority carrier (e−) concentration at Pd-SnO2 junction, leading to depletion layer (DL) expansion and high-resistance state formation.
Upon H2 gas exposure, the sensor exhibits H2 molecular adsorption at Pd-SnO2 interface [Eq. (5)]. The interaction between O− ions and H atoms releases electrons into SnO2 NWs [Eqs. (6)-(7)]. This redox reaction induces DL region reduction within SnO2 channel, resulting in transition to low-resistance state. The increased nanowire
density in the Pd-SnO2 sample provides more active surface sites and continuous electron transport pathways,
thereby enhancing the sensor’s conductivity change upon gas exposure. Furthermore,
the catalytic activity of Pd accelerates the dissociation of H2 molecules into atomic hydrogen, facilitating faster reaction kinetics with surface
oxygen species. Pd nanoparticles provide additional active sites for hydrogen adsorption
and catalyze the dissociation of H2 into atomic hydrogen, even at relatively low temperatures. This process, often referred
to as the spillover effect, allows dissociated hydrogen atoms to migrate from the
Pd surface to adjacent SnO2 sites, where they react more readily with chemisorbed oxygen. This dual-site interaction
enhances both the speed and magnitude of the sensor response.
The resistive H2 sensing mechanism is further enhanced by Pd-SnO2 heterojunction characteristics. The work function difference between noble metal
Pd (Φ = 5.12 eV) and n-type semiconductor SnO2 (Φ = 4.51 eV) establishes metal-semiconductor junction [28,29]. Fig. 7(c) demonstrates Schottky barrier formation and DL region expansion at Pd-SnO2 interface in ambient conditions. H2 exposure induces Pd and O− reaction, forming palladium hydride (PdHx) [Eq. (8)] [30,31].
The formation of PdHx is not only a chemical transformation but also significantly alters the electronic
properties of the Pd-SnO2 junction. The reduced work function of PdHx compared to metallic Pd lowers the energy barrier for electron transfer into the
SnO2 conduction band, enabling more efficient carrier injection. This transition from
a Schottky to quasi-Ohmic contact promotes rapid resistance change upon H2 exposure, thereby improving both response and recovery dynamics of the sensor.
The work function modification of PdHx (Φ = 3.21 eV) facilitates Ohmic contact formation with SnO2, as illustrated in Fig. 7(d). The electron release from O− ions adsorbed on Pd NDs surface and subsequent transfer to SnO2 results in DL charge region reduction and low-resistance state transition [23]. Taken together, the combined effects of increased nanowire density, Pd-catalyzed
spillover and dissociation, and Schottky barrier modulation lead to a highly responsive
and sensitive gas sensing system. The Pd-SnO2 sensor particularly benefits from this synergy, achieving enhanced detection performance
even at room temperature.
Fig. 7. The schematic illustrations of the spillover effect in Pd-SnO2 nanostructures: (a) In air environment, and (b) In H2 environment. The band diagram images of Pd-SnO2 gas sensor: (c) In air environment (Schottky contact), and (d) In H2 environment (Ohmic contact).
IV. CONCLUSION
In summary, this study demonstrates fabrication and characterization of resistive
MOS gas sensors based on Pd-SnO2 nano-network channel on SiO2/Si substrate with varying electro-spinning times. Material characterization through
FE-SEM, XRD, and XPS analyses confirms tetragonal structure of Pd-SnO2. The Pd-10SnO2 sample, fabricated using 10-second electro-spinning process, exhibits superior H2 sensing characteristics including enhanced responsivity, rapid response/recovery
times, low detection limit (4-50 ppm), and a room temperature operation (27℃). The
improved H2 sensing performance has been attributed to optimized NWs density and Pd embedding
methodology. The sensing mechanism has been elucidated through analysis of Pd NDs
decoration effect, Schottky barrier formation, and PdHx-SnO2 interface Ohmic contact characteristics. These findings demonstrate critical correlation
between electro-spinning duration and H2 gas sensor performance optimization.
ACKNOWLEDGMENTS
This study was supported by the Research Program funded by the SeoulTech(Seoul National
University of Science and Technology).
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Dongjun Jang received his B.S., and M.S. degrees in electronic engineering from Gangneung-Wonju
University (GWNU, Republic of Korea) in 2022 and 2024. Since 2024, he has now been
working on Ph.D. course at the Department of Electronic Engineering, Sogang University,
Seoul, Republic of Korea. His current research interests include nanomaterial based
on next-generation memory devices, synaptic devices and MOS-type gas sensors.
Sangwan Kim received his B.S., M.S., and Ph.D. degrees in electrical engineering from
Seoul National University, Seoul, Republic of Korea, in 2006, 2008, and 2014, respectively.
He had been a postdoctoral scholar at the Department of Electrical Engineering and
Computer Sciences, University of California, Berkeley, USA, from 2014 to 2017. He
had been with the Department of Electrical and Computer Engineering, Ajou University,
Suwon, Republic of Korea, as Assistant/Associate Professor from 2017 to 2022. Since
2022, he has been a Faculty Member with Sogang University, Seoul, Republic of Korea,
where he is currently an Associate Professor with the Department of Electronic Engineering.
His current research interest includes, ultra-low power logic devices, future memory
devices, synaptic devices and their applications.
Min-Woo Kwon received his B.S. and Ph.D. degrees in electrical and computer engineering
from Seoul National University (SNU), in 2012 and 2019, respectively. From 2019 to
2021, he worked at Samsung Semiconductor Laboratories, where he contributed to the
development of 1x nm DRAM cell transistors and their characterization. From 2021 to
2024, he worked at Gangneung Wonju National University (GWNU) as an assistant professor
in the Department of Electric Engineering. Since 2024, he has been conducting research
in the Department of Electronic Engineering at Seoul National University of Science
and Technology, where he is currently a professor.