Realization of a portable H2S sensing instrument based on SnO2 nanowires

Given that the voltage signal from the sensor was measured across a 100 K U digital potentiometer in series with the sensor, the response to H 2 S gas was de ﬁned as V gas /V air , where[r]

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Original Article

Realization of a portable H2S sensing instrument based on SnO2

nanowires

Nguyen Xuan Thaia, Nguyen Van Duya,**, Chu Manh Hunga, Hugo Nguyenb,

Tran Manh Hungc, Nguyen Van Hieud,e, Nguyen Duc Hoaa,*

aInternational Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No 1, Dai Co Viet, Hanoi, Viet Nam bDepartment of Engineering Sciences, Uppsala University, L€agerhyddsv€agen 1, 751 21 Uppsala, Sweden

cSchool of Electrical Engineering, Hanoi University of Science and Technology (HUST), No Dai Co Viet Road, Hanoi, Viet Nam

dFaculty of Electrical and Electronic Engineering, Phenikaa Institute for Advanced Study (PIAS), Phenikaa University, Yen Nghia, Ha-Dong District, Hanoi,

Viet Nam

ePhenikaa Research and Technology Institute (PRATI), A&A Green Phoenix Group, 167 Hoang Ngan, Hanoi, Viet Nam

a r t i c l e i n f o

Article history:

Received December 2019 Received in revised form 14 January 2020 Accepted 16 January 2020 Available online xxx

Keywords: Portable gas sensors SnO2nanowires

Real-time monitoring H2S gas

a b s t r a c t

Monitoring of toxic gas in air is important because air pollution, especially in developing countries, has rapidly become severe The high cost of installation and maintenance of a stationary analysis system by using methods such as gas chromatography limits its applications Low-power, portable devices with relatively low-cost gas sensors are effective for mapping pollution levels in real-time in urban areas and in other living environmentts Herein, the realization of a portable H2S sensing instrument based on SnO2 nanowires is reported The sensor chip was prepared by the on-chip growth of SnO2nanowires directly from the edges of Pt electrodes The electronic system and software for signal acquisition, data pro-cessing, data storage, and output of the instrument were developed A prototype for zero series of the instrument was also realized The instrument is capable of monitoring H2S gas in air at ppm level and in biogas production with satisfation

© 2020 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

H2S is a gaseous constituent of environmental pollution It is a colorless, poisonous, corrosive,ﬂammable, and explosive gas [1] Exposure to this gas can shave a negative effect on heath, such as headache, coughing, and eye irritation [2] It is also one of the gases that cause corrosion on electronic components and change in the Earth's climate system [3] In Vietnam, for example, H2S gas comes from different sources, but it mainly comes from biogas production at high concentration ranging from 50 ppm to 5000 ppm The reason for this is the large number of biogas plants built for con-verting agricultural wastes and animal manures into coking gas without purifying it from H2S [4] In addition, the emission of H2S gas from decomposition of organic compounds containing sulfur and sulﬁde-oxidizing bacteria in the sediment of the sewerage that

goes out to the open waters, such as Tolich river in Hanoi, is very high, thereby making air quality in the riparian areas severely low [5] The conventional air pollution analyzers are relatively large, heavy, and expensive; the price for each unit can beV30,000 or higher [6] Therefore, a compact, user friendly, and low-cost device for air quality monitoring is highly desired [6,7] Different types of gas sensors for detection and measurement of H2S gas have been developed [8e10] Among them, the metal oxide gas sensors have the best potential for air monitoring due to their low cost, high sensitivity, compact size, real-time operation, ease-to-use, porta-bility, and low power consumption [11] Most scientiﬁc reports focus on the synthesis of new materials and the different methods of enhancement of gas sensor performance [12e16] For instance, porous CuO nanosheets were synthesized by a hydrothermal method for H2S gas sensing [17] SnO2quantum wire/rGO nano-composites were synthesized for H2S gas sensing, and this device had a response value of 33 to H2S at 50 ppm at 22C [18] SnO2 quantum dots synthesized by a hydrothermal method modiﬁed with CuO had a response value of 1755 to H2S at 50 ppm and at 70C [19] Mesoporous crystalline SnO2material with high surface

* Corresponding author ** Corresponding author

E-mail addresses:nguyenvanduy@itims.edu.vn(N Van Duy),ndhoa@itims.edu vn(N.D Hoa)

Peer review under responsibility of Vietnam National University, Hanoi

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2020.01.003

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area (98 m2/g) was used to fabricate the H2S sensor, and this device had a response value of approximately 240 to H2S at 100 ppm and at 350C [20] SnO2nanowires were synthesized by a hydrothermal method and then spin-coated to form a SnO2nanowire thinfilm, and subsequently, they were ligand exchange-treated by Cu(NO3)2 to improve the H2S sensing capability [21] SnO2 nanocrystals prepared by the solvothermal method were also used to fabricate a H2S gas sensor, wherein the sensitivity to ppm H2S was 357, thereby indicating the possibility of a practical application to the real-time monitoring of trace of H2S from the leaking biogas [22] ZnOecarbon nanofibers, which were prepared by a facial electro-spinning route followed by an annealing treatment, showed the excellent selectivity and response to H2S gas [23] Recently, the high-performance H2S gas sensor chips employing ZnFe2O4/rGO nanofibers [12], CuO nanoplates [26], and thin SnO2films [10] as sensing materials have been successfully fabricated The nano-structured metal oxides are excellent candidates for the H2S gas sensors, because they are highly sensitive to this gas at low detection limit (ppb level) [24] Recent studies have focused on the materials and/or sensor chip fabrication but the realization of a measurement system [25]

In this paper, the development of a portable H2S sensing instru-ment using SnO2nanowire sensors is presented The SnO2nanowires were grown from the edge of a pair of Pt electrodes on a glass sub-strate via the chemical vapor deposition (CVD) method A micro-heater was also integrated on this substrate The design of the sensing instrument architecture, including a printed circuit board (PCB), microprocessor, data processing, output signal, data storage,

power supply, and source code, was developed A complete instru-ment was also built and tested

2 Experimental

2.1 Sensor chip fabrication

Fig 1(a and b) shows the design of a SnO2nanowire sensor chip, which includes a microheater and a pair of electrodes composed of Pt/Au/Cr layers deposited on a glass wafer as reported previously [27] Both the microheater and the electrodes were patterned using photolithography, deposition, and lift-off process [28] To promote the SnO2nanowires grown from the edges of the Pt electrodes and to prevent them from growing on the surface of the microheater, a thin SiO2layer (25 nm) was deposited as the last layer before the lift-off step The SiO2film was deposited in a reactive sputtering process by using a magnetron sputter (von Ardenne CS 730 S, Germany) To verify the thickness of SiO2, a piece of Kapton tape was brought on the wafer as a shadow mask After the SiO2 depo-sition and removal of the tape, the step (i.e thickness) of thefilm was measured by using a profilometer (Dektak 150 Stylus Profiler, Veeco Instruments Inc., USA) Additional lithography, SiO2 deposi-tion and lift-off steps (the bright part inFig 4(a)) were conducted to mask the edges of the microheater to prevent SnO2from growing from them, thereby leaving only an open window of 20 20mm2at the tips of the electrodes The SnO2nanowires were then grown by CVD, as shown inFig 1(b) [29] Sn powder (0.2 g) was loaded into an alumina boat as the source of precursor materials for the

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synthesis of SnO2 nanowires The glass substrates were located approximately cm from the material source Then, the quartz tube was evacuated down to 1.0 101 Torr by a rotary pump and purged with argon at aﬂowrate of 300 sccm several times The furnace was programmed to increase temperature from room temperature to 730 C within 20 As the furnace reached 730C, 0.5 sccm of oxygen gas was bled into the quartz tube As a result, the SnO2nanowire growth started and was maintained at this temperature for 15 Then, all gas supplies and power were switched off to let the tube furnace cool down naturally to room temperature [30]

2.2 Sensor calibration

A configuration of the circuit for signal measurement is shown in Fig Given that the voltage signal from the sensor was measured across a 100 KUdigital potentiometer in series with the sensor, the response to H2S gas was defined as Vgas/Vair, where Vgas and Vairare the dropped voltages on the digital potentiometer upon exposure to analytic gas and to dry air, respectively The measured voltage on the digital potentiometer is defined as follows: Voutẳ

( Rref

Rrefỵ Rs

)

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where Vout, Rref, Rsare the measured voltage and resistance value of the digital potentiometer and the sensor, respectively

2.3 Design and manufacture of the instrument

The portable H2S sensing instrument was designed as a handheld mobile device with the dimensions of 135 mm 67 mm 35 mm The popular software package Altium designer was used to make the design of the PCB of the instru-ment The PCB design proceeded through 10 main steps, as fol-lows: creating a schematic design; creating and setting up the PCB design; linking the schematic design to the PCB design; deﬁning the board size; placing the components; inserting drill holes; routing traces; verifying circuit board layout; adding labels on the board; and generating the designﬁle.Fig 3(a) shows the sche-matics of the instrument with its main components A require-ment on the instrurequire-ment was the ability to manage and control the external and internal signal processing, such as analog signals from the sensor, signals for calibrations and control of

microheater, display, and storage of sensing data To meet this requirement, six function blocks were assigned to the electronic system, as shown inFig 3(b) The core of the instrument is a low-power 32-bit processor, the STM32F103 (STMicroelectronics, Dallas, TX, USA) based on ARM Cortex-M3 architecture micro-controller that operates at standard low voltage of 3.3 V (Fig 3(b)e1) The processor includes 512 K Flash, 64 KB SRAM, 64 I/O (input/output) ports, two 12-bit analog to digital converter (ADCs), one 12-bit digital to analog converter (DAC), an advanced control timer, three general purpose 16-bit timer, and pulse width-modulated timer It also provides two Inter-Integrated Circuit and Serial Peripheral Interface, three Universal Asynchro-nous Receiver/Transmitter (UARTs), a USB port, and a controller area network as a communication interface system

To calibrate the sensor, a V DC voltage is applied to it, so that the corresponding current from it can be acquired Thus, the actual resistance of the sensor is measured A schematic diagram for recording the input signal from the sensor is shown inFig 3(b)e2 Further measurement of the change in current is realized through a voltage divider by using a MCP41100 digital potentiometer (100 kU) connected in series with the sensor chip

Given that the sensor has an optimal working temperature, powering the on-chip heater and maintain it at the working temperature is important For this purpose, a real-time heater controller is implemented, as can be seen in Fig 3(b)e2 The voltage, which is calculated using an internal 12-bit DAC of the STM32 microcontroller chip, is supplied to the heater through a shunt resistor To stabilize the voltage, a buffer operational ampliﬁer circuit with unity gain is used With this approach, different stable voltages ranging from to Ve5 V can be created to power the heater In this study, the heater is powered with 70, 90, 110, and 130 mW to select the optimal working condition The entered calibration data are saved on the EFROM of the MCU With a simple set ofﬁve buttons (left, right, up, down, and enter), the information on calibration data for the sensor chip, turning on/off the instrument, and sampling pump can be easily handled A detailed schematic diagram of the button system is shown in Fig 3(b)e3

The instrument can be powered by a 3.7 Ve2800 mAh rechargeable polymer lithium battery and/or a Ve1A DC-power source adapter, as shown inFig 3(b)e5 A switched-mode power supply is used to enable the operation with two power sources Voltage from the power supply is regulated to 3.3 Vdc to power the STM32F103 microcontroller through the HT7833 chip and to adjust to 0e5 Vdc for supplying the microheater through the TPS55340 chip (Texas Instrument, USA) The lithium battery can be charged via a small circuit equipped with a TP405 charge controller IC When the instrument is powered by a V DC adapter, the charge control circuit will operate in a low-dropout mode (the low power consumption mode)

Gas concentration is calculated from the sensor signals based on the calibration curve by the microprocessor and then displayed on a graphic liquid-crystal display GLCD5110 that has a resolution of 84 48 pixels The schemes of the display and storage units are shown inFig 3(b)e4, andFig 3(b)e6, respectively The measured data are displayed in a real-time mode The instrument is equipped with a piezoelectric buzzer that is set on alert when the gas con-centration exceeds a settled value As for data storage, a micro SD memory card is assigned for this task, as shown inFig 3(b)e6 The measurement results (gas concentration, data logger) are calcu-lated and analyzed by the MCU and recorded on the SD memory The processed data are saved on the SD card in the form of a textﬁle by using data package format separated by commas in the following order: header message, date/time, voltage of heater, and gas concentration

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The source code is written in C programming language and loaded into the MCU through Keil C MDK V5 software package, which is designed to create embedded applications for ARM Cortex-M Processor-based devices The software schematic algo-rithm for the sensing instrument is presented inFig Upon using the instrument, several tasks have to be accomplished, such as hardware platform initialization, system clock initialization, UART configurations, LCD port initialization, timer configurations, and ADC/DAC configurations The software can be divided into two blocks, namely, one with the main task for data acquisition and one with the subtasks for entering calibration data and for changing the system information The main task has higher priority than the subtasks After powering the microheater with a given voltage, the MCU reads the sensor and processes the signals These signals are combined with the calibrated data to interpolate the gas

concentration displayed on the GLCD and packaged and stored on the SD card During data acquisition, a setup mode can be activated through the button system (press up and down buttons simulta-neously and navigate to setup menu by using the left and right buttons) In the setup mode, the calibrated data for the sensor and system parameter, such as thresholds for alarming, real-time clock, and sample time, can be changed

3 Results and discussion

3.1 Characterization of the SnO2nanowire sensor

The fabricated sensor was characterized by optical and scanning electron microscopies.Fig 5(a) shows an optical microscopy image of the sensor center part, including bar-type electrodes and the

Fig (a) Architecture of the H2S gas sensing instrument; and (b) Schematic diagram of functional blocks of the H2S sensor measurement system: (1) Block center processor- Chip

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surrounding microheater A sensor chip after dicing has a dimen-sion of 4 mm2(Fig 1(a)).Fig 5(b) shows a scanning electron microscopy image of the SnO2nanowires at the center of the sensor chip, where the two electrodes underneath can be inferred Notably, the aspect ratio of the SnO2nanowire is very high with a typical length of few micrometers and an average diameter of approximately 70 nm The nanowires that grew from the edges of the electrodes are sufﬁciently long to bridge the gap of 6mm be-tween the two Pt electrodes and form a quite sparse andﬂuffy SnO2 nanowire network above them This network is important for the analytic gas molecules to easily come into contact with and adsorb on the surface of the nanowires, thereby enabling good sensitivity and accelerating the response/recovery time of the sensor [31] High resolution TEM image of the SnO2 nanowire is shown in Fig 5(c) The TEM image exhibits clear lattice fringes with an interspace of about 0.26 nm, which corresponded to the (101) planes of SnO2tetragonal rutile structure The composition of the synthesized SnO2 nanowire sensor was analyzed by energy dispersive spectrometry (EDS), as shown inFig 5(d) The sensor chip is composed of Pt, O, and Sn The presence of Pt was original

from the Pt electrodes, whereas O and Sn were the composition of SnO2nanowires The composition analyzed by EDS was close to the stoichiometric SnO2material, thereby indicating the high quality of the grown nanowires

3.2 Calibration of the instrument

The H2S gas sensing characteristics of the sensor were tested at different supplied powers (70, 90, 110, and 130 mW).Fig 6(aed) show the dynamic sensor response to 0.25, 1, 4, and 10 ppm H2S under varying supplied powers (at an environmental temperature of ~25C and relative humidity of ~80%) The sensor showed the sensing behavior of a n-type semiconductor, because the decrease of its resistance increased the measured voltage on the digital potentiometer when exposed to reducing gases (here H2S) The sensor also exhibited good reproducibility with a relatively short response time and a recovery time ranging from 50 s to 200 s As shown inFig 6(e), the sensor response increased with increasing H2S concentration at all supplied power For a constant H2S con-centration (here ppm), the sensor response also increased with increasing supplied power from 70 mW to 130 mW (Fig 6(f)) This phenomenon was a result of the increasing reaction between the activated H2S molecules and the preabsorbed oxygen species (O2; O; or O2) on the sensor material as the power supplied to the microheater increases [32] In general, the temperature-dependent response of the metal oxide-based sensors had a bell shape This condition means that the response increased with increasing temperature (to the sensor optimal working temperature Topt.) and then decreased with further increase of the temperature In this study, the response increased with increasing heating power in the measured range, which meant before the sensor reached its optimal working temperature High temperature could easily cause the microheater to break Thus, in this study, the sensor was tested only up to the supplied power of 130 mW Among the various supplied power levels, the sensor response, as a function of H2S concentrations, showed the steepest slope at 90 mW Therefore, this response line was used for the instrument's calibration

The stability and repeatability of the sensor were checked by consecutively recording nine cycle exposure to ppm H2S and back

Fig Flow chart of the instrument's software

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to air (Fig 7(a)) Notably, the response of the sensor exhibited a good repeatability in the instrument conﬁguration Fig 7(b) showed the calibration line for varying H2S concentrations at the same heating power of 90 mW, where the measurement points indicated an almost linear line over the full scale of H2S concen-trations (between 0.25 and 10 ppm) The red line was a linearﬁtted H2S concentration as a function of the output signal from the sensor, which was expressed as follows:

C¼ 0:0272 V 3:899 (2)

where C and V are the H2S gas concentration and the measured voltage on the digital potentiometer in series with the sensor, respectively This line is only a fairly linear part of the concentration versus voltage line because further increase of H2S concentration would eventually saturate the sensor, and the output voltage would reach the maximum value giving a nonlinear line that resembles an inverted Arrhenius function

In practice, a new instrument assembled with a sensor from the same batch as the above tested one has to be calibrated in the same way, i.e., the instrument has to be exerted in the air with H2S concentrations of 0.25, 1, 4, and 10 ppm At each concentration, four measurements were conducted, and the output voltages entered (by pressing the button for SET) The software in the instrument automatically calculated their mean value For a better precision of the measurement, linear ﬁt was not applied for all four concen-trations (0.25, 1, 4, and 10 ppm) but only for two consecutive concentrations in this series After the calibration, the instrument still read the output voltage but displayed it as the H2S concen-tration Table in the inset in Fig 7(b) summarizes the measured values from the calibration of one instrument Notably, the readout values were stable, with an accuracy of 99.98%

Fig 8(aed) show how the sensor chip was packed and inte-grated into the instrument A sensor array prepared on a glass wafer was diced into the sensor chips of 4 mm2(Fig 8(a)) The en-velope of the sensor chip had two parts, namely, the top cap and the

Fig Calibration of the sensor chip (aed) Output signals of the instrument at various supplied powers and H2S gas concentrations; (e) Instrument response as a function of H2S gas

concentrations; and (f) Instrument response as a function of supplied power at ppm H2S gas

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bottom part For prototyping, those are 3D-printed using the commercial acrylonitrile butadiene styrene plastic material (Fig 8(b)) The chip is placed topedown in the shallow pocket of the bottom part in such a way that the four electric pads of the sensor make good contact with the four pogo-type pins of the bottom part (Fig 8(c)) Finally, the top cap was snapped tightly on the bottom part to enclose and secure the sensor chip inside (Fig 8(d)) The bottom part and the top cap had cuts on the sides, and they were aligned to each other to make a channel that enabled the ambient gas to reach the sensor

The instrument motherboard's front and backside, before and after mounting electronic components are shown inFig (e, f) and (g, h), respectively After its assembling and calibration, the instru-ment was tested at different places close to Tolich river under real conditions.Fig 8(i) shows a photo of the instrument at a location of approximately 200 m from the river The display shows a concen-tration of zero, which meant no H2S gas was detected When measured at a closer distance from the river (50 m), the instrument displayed a low H2S gas concentration of approximately 180 ppb (Fig 8(j)) At approximately m, the instrument displayed a high H2S gas concentration of approximately 356 ppb (Fig 8(k)) These results indicate that the developed sensing instrument can detect H2S gas in air very efﬁciently

4 Conclusion

A portable sensing instrument based on the on-chip grown SnO2 nanowires for monitoring H2S gas in air at low concentrations,

down to the ppb level, was successfully manufactured With the exception of the commercial electronic components, the rest of the instrument was developed and manufactured in-house The pro-cesses included developing the on-chip growth SnO2 nanowire sensor, packaging the sensor, designing and manufacturing the motherboard, software coding, designing and manufacturing the casing and interface of the instrument, and calibrating and testing of the instrument The instrument showed good sensitivity and repeatability when measuring H2S gas in air under various condi-tions The developed instrument was manufactured in small number, tested as zero series, and it is considered suitable for cosmetic upgrading, before its large-scale production and commercialization

Declaration of Competing Interest

The authors declared that they have no conﬂicts of interest to this work

Acknowledgments

This work wasﬁnancially supported by the Ministry of Science and Technology, under the Grant No ÐTÐLCN.21/17

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