High-Response Room-Temperature NO2 Sensor and Ultrafast

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Functional Inorganic Materials and Devices

High response room temperature NO2 sensor and ultrafast humidity sensor based on SnO2 with rich oxygen vacancy Yujia Zhong, Weiwei Li, Xuanliang Zhao, Xin Jiang, Shuyuan Lin, Zhen Zhen, Wenduo Chen, Dan Xie, and Hongwei Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01737 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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ACS Applied Materials & Interfaces

High response room temperature NO2 sensor and ultrafast humidity sensor based on SnO2 with rich oxygen vacancy Yujia Zhong1, WeiWei Li2,3, Xuanliang Zhao1, Xin Jiang1, Shuyuan Lin1, Zhen Zhen1, Wenduo Chen1, Dan Xie3, Hongwei Zhu1*

1State

Key Laboratory of New Ceramics and Fine Processing, and Center for Nano and Micro Mechanics, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

2Department of Basic Sciences, Air Force Engineering University, Xi’ an 710051, China 3Tsinghua

National Laboratory for Information Science and Technology, Institute of Microelectronics, Tsinghua University, Beijing 100084, China *Corresponding

author. Email: [email protected].

Abstract SnO2 nanosheets with abundant vacancies (designated as SnO2-x) have been successfully prepared by annealing SnSe nanosheets in Argon via a hot injection method. The TEM results of prepared SnO2 nanosheets indicated that high-density SnO2-x nanoplates with the size of 5~10 nm were distributed on the surface of the amorphous carbon. After annealing, the acquired SnO2-x/amorphous carbon retained the square morphology. The stoichiometric ratio of Sn: O=1:1.55 confirmed that oxygen vacancies were abundant in SnO2 nanosheets. The prepared SnO2-x exhibited excellent performance of sensing NO2 at room-temperature. The response of the SnO2-x-based sensor to 5 ppm NO2 was determined to be 16 with the response time and recovery time of 331 and 1057 s, respectively, which is superior to those of most reported room-temperature NO2 sensors based on SnO2 and other materials. When the humidity varied from 30 % to 40 %, the ΔR/R was 0.025. The ultrafast humidity response (52 ms) and recovery (140 ms) are competitive compared with other state-of-art humidity sensors. According to the mechanistic study, the excellent sensing performance of SnO2-x is attributed to its special structure. Keywords: tin dioxide; tin selenide; nanosheet; oxygen vacancy; NO2 gas sensor; humidity sensor

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Introduction Among the materials studied for gas sensor, SnO2 has been extensively studied and exhibits superior characteristics to other metal oxides. SnO2 is an n-type semiconductor with a bandgap of Eg=3.6 eV 1. It has been widely reported that the gas-sensing performance of SnO2 is recorded in the non-stoichiometric form. Although SnO2-based sensors have various advantages includes low cost and high sensitivities to different gas species, their high electrical resistance (100 MΩ or above)

2

severely limits their detection and sensitivity. To further

improve sensing performances of SnO2 and reduce the operating temperature, various technologies have been developed, such as SnO2 nanostructure 3, SnO2-based hybrid doping SnO2

7-9,

and introducing other elements

addition of reduced graphene oxide (rGO)

3-4, 11

10.

2, 4-6,

Among them, hybrids constructed by

and carbon nanotubes (CNTs)

2

have been

studied intensively because these carbon materials not only adsorb NO2 but also conduct electricity. SnO2 and ZnO heterojunction is another type of gas sensing hybrid 12. Most of the existing SnO2-based sensors require high operating temperature (>200 °C) 13 to enhance their surface adsorption and reaction kinetics. High operating temperature increases sensing cost and is undesirable in some application conditions. However, the response of SnO2 is unsatisfactory when sensors work at room temperature. The response of rGO/CNTs/SnO2, Ag/rGO/SnO2, and many kinds of SnO2/rGO sensors to 5 ppm NO2 at room temperature are only 1.2~2.6 11. Therefore, enhancing the gas sensor response at room temperature is of great significance in optimizing SnO2-based sensor. Oxygen vacancies (OVs) determines the physical and chemical properties of metal oxides. In the case of SnO2, OVs play a role of donor and provide free electrons, making SnO2 an ntype semiconductor. OVs with positive charge form the active sites for absorbing NO2. It has been reported that surface OVs can significantly improve the charge transferability of SnO2. The existence of OVs in NO2 sensing material leads to significantly enhanced sensing stability and specific surface area. For example, the response of the rGO/SnO2 hybrid to 1 ppm NO2 is increased from 2.13 to 4.98 by introducing OVs 14. Similarly, the response of ZnO sensor (0.9 ppm NO2) is increased from 1.75 to 5.26 by introducing OVs 15. Herein, we report an ultrasensitive room temperature NO2 sensor based on SnO2 with rich oxygen vacancy. By using tin selenide nanosheet instead of common tin tetrachloride as an 2 ACS Paragon Plus Environment

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annealing precursor to SnO2, OVs increased substantially. In the meantime, SnO2 was doped by the remaining trace amounts of selenium element. The two-dimensional (2D) nanosheets assembled by SnO2 nanocrystal with rich OVs are denoted as SnO2-x, which is sensitive to H2O and exhibits high selectivity for NO2 sensing. The response of assembled SnO2 nanocrystal to 5 ppm NO2 at room temperature was determined to be 16. In addition, SnO2-x sensor exhibited an ultrafast response to humidity. The response time and recovery time were 52 and 140 ms, respectively.

Results and Discussion Hot injection (Figure 1a) is a common method of synthesizing IIA-VIA, IIIB-VA, and IVAVIA semiconductor nanocrystals with optimal homogeneous dimension and crystallizing degree

16.

Specifically, after Sn source was added to a round-bottom flask, the system was

evacuated and flushed for one hour with nitrogen to remove the dissolved oxygen and water. Then, the Sn precursor was heated to 240 °C, followed by injection of the prepared HMDSTOP-Se. After allowing the solution to sit for 30 min, dark precipitate SnSe nanosheets (SnSeNS) was generated and collected. For further usage, the SnSeNS was suspended in ethanol after collecting and washing. The crystal structure of SnSeNS is illustrated in Figure 1b. As shown in Figure S1a, the side length of SnSeNS square was 500-1000 nm. HRTEM images and XRD results (Figures S1b and S1c) indicated that it was an orthogonal crystal (PDF#48-1244) with a lattice constant of 11.5×4.2×4.4 Å and a normal direction of . Assembly macroscale film is important in nanocrystal application. Dispensing, spin coating, dip coating, knife coating, and liquid surface assembly fit for different materials and application situations. Herein, we developed a procedure to coat uniform SnSeNS films (Figure 1d) on various substrates by centrifuging SnSeNS/ethanol solution (Figure 1c) and denoted this procedure as centrifuge coating. The size of the film was 1×2 cm2 and easy to be further enlarged by changing the centrifugal tube and substrate size. The straightforward forming film method is not only suitable for mass production but also can be integrated with centrifugal washing procedure without introducing other steps. The hot-injection and centrifuge coating procedure reduce the sample difference in the same batch, which offers the 3 ACS Paragon Plus Environment


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possibility for quantity production. (b)

(a)

(c)

(e)

(d)

Ar Hot injection

(h)

SnSe

Centrifuging

SnSe film

Annealing

(f)

(g)

O2

Se

SnO2-x

Figure 1. Schematic of the preparation and structure of SnO2-x. (a) Schematic of hot injection. (b) SnSe crystal structure. (c) Centrifuge coating. (d) SnSe film on mica substrate. (e) Annealing in Ar. (f) Generation of SnO2-x film. (g) Square plates consisting of small SnO2-x nanocrystals and amorphous carbon. (h) SnO2-x nanocrystal with rich OVs. Off-white tin oxide films are acquired by annealing SnSeNS films (500 °C, holding 1 h) in Ar (Figure 1e). Because of the organic residue introduced by hot injection ligand, oxygen in organics reacted with tin to form SnO2 and carbon in organics to form amorphous carbon. The melting points of Sn, Se, and SnSe are 232, 221, and 860 °C, respectively. When the annealing temperature was higher than 400 °C, the Se of SnSeNS began to be removed gradually and tin oxide was acquired (Figure 1f). The DTA result (Figure S1d) of SnSeNS exhibited a wide endothermic peak at 418 °C, corresponding to the oxidation of SnSeNS. Thus, the annealing temperature was preferably above 418 °C to obtain SnO2, which was consistent with the experimental results. When SnSeNS was annealed in Ar, the mass decreased with increasing temperature until 686 °C, as shown in TG result (Figure S1e). When the temperature was higher than 686 °C, the mass increased, indicating that further oxidation occurred. Therefore, in order to obtain tin oxide with OVs, the annealing temperature should be lower than 686 °C to prevent complete oxidation. After annealing, the film was still composed of square nanosheets, but the material changed from pure SnSe with some organic residue to small SnO2 nanocrystal and amorphous carbon (Figure 1g). The atomic ratio of oxygen to tin (O: 4 ACS Paragon Plus Environment

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Sn40%), tri-n0-octylphosphine (TOP, 90%) and hexamethyldisilazane (HMDS, >90%) were purchased from Aldrich. Colorless TOP-Se solution was prepared by dissolving 395 mg (5 mmol) selenium powder in 5 mL TOP. Tin precursor solution was prepared by ultrasonic dissolving 40 mg (0.21 mmol) SnCl2 in 20 mL (60 mmol) OLA. 0.25 mL of TOP-Se solution was mixed with 1mL (4.71 mmol) of HMDS to obtain Se precursor solution HMDS-TOP-Se. Sn source was added to a 4-neck round-bottom flask fitted with air inlet, condenser, thermometer, and rubber septum. The solution was evacuated and flushed for 1 h with nitrogen to remove dissolved oxygen and water. The Sn precursor solution was heated to 240 °C in 15 min followed by injecting prepared HMDS-TOPSe. The solution color gradually turned to dark, indicating formation of SnSe nanosheets (SnSeNS). The solution was aged for 30 min and rapidly cooled by removing the reaction flask from the heating mantle. The SnSe nanosheets were precipitated by adding 40 mL isopropanol 14 ACS Paragon Plus Environment

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and 8 mL methylbenzene. The solution was sonicated centrifuged at 10000 r/min for 10 min. The obtained black powder was washed three times with a 2:1 isopropanol/ethanol mixture with centrifugation at 10000 r/min for 5 min between each wash. The SnSe NSs were suspended in ethanol for further usage 16. Film Formation and Annealing: Black SnSe NS films were prepared on any substrates (mica, silicon, glass) by centrifuging at 5000 r/min for 10 min. Then SnSe films were annealed (500 °C, holding 1 h) in Ar to acquire offwhite SnO2-x films. NO2 Sensor Fabrication: Gold interdigitated electrodes with a finger width and an interfinger spacing of 10 μm were fabricated on SiO2/Si substrate. Spin coating polymethyl-methacrylate (PMMA) on SnO2-x/mica and solidification at 140 °C for 30 min provided protection. SnO2 film was transferred from mica to Si/SiO2 substrate with Au interdigitated electrode on water surface. After drying 12 h, PMMA protective layer was dissolved in acetone. NO2 Sensor Characterization: The performance of the NO2 sensor was characterized at room temperature. The signal was collected by measuring the changes of the electrical conductance of SnO2 film. The applied voltage was 1 V. The sensor was mounted in an air-tight sensor testing chamber with electrical feedthrough. Air was used as the carrier gas and the total flow rate of air and testing gas was 100 mL/min. The selectivity was measured with 5 ppm SO2, 25 ppm HCHO, 50 ppm NH3 and 100 ppm CO. Humidity Sensor Characterization: N2 (100 mL/min) was used as the carrier gas. The current was measured with a voltage bias of 1 V to detect the humidity change. Keithley 4200 was used to measure the electrical signal. The minimum time step of Keithley is 1s. When the response time was lower than 1 s, it was inaccuracy to acquire data from Keithley results. Oscilloscope is suitable for testing rapid signal changes. As oscilloscope can only measure voltage signal, thus a direct current of 1 μA was applied to change the resistance variation to voltage variation.

Supporting Information. Structural characterizations of SnSe nanosheets; Raman, XPS spectra and sensing performance of SnO2-x.

Acknowledgements 15 ACS Paragon Plus Environment


This work was supported by the National Natural Science Foundation of China (51672150, 51672154), and Tsinghua University Initiative Scientific Research Program.

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