Oxygen vacancy engineering for enhanced sensing performances: A case of SnO2 nanoparticles-reduced graphene oxide hybrids for ultrasensitive ppb-level room-temperature NO2 sensing

Accepted Manuscript Title: Oxygen vacancy engineering for enhanced sensing performances: A case of SnO2 nanoparticles-reduced graphene oxide hybrids for ultrasensitive ppb-level room-temperature NO2 sensing Authors: Ziying Wang, Tong Zhang, Tianyi Han, Teng Fei, Sen Liu, Geyu Lu PII: DOI: Reference:

S0925-4005(18)30667-1 https://doi.org/10.1016/j.snb.2018.03.169 SNB 24454

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

21-12-2017 8-3-2018 27-3-2018

Please cite this article as: Ziying Wang, Tong Zhang, Tianyi Han, Teng Fei, Sen Liu, Geyu Lu, Oxygen vacancy engineering for enhanced sensing performances: A case of SnO2 nanoparticles-reduced graphene oxide hybrids for ultrasensitive ppb-level room-temperature NO2 sensing, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2018.03.169 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Oxygen vacancy engineering for enhanced sensing performances: A case of SnO2 nanoparticles-reduced graphene oxide hybrids for

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ultrasensitive ppb-level room-temperature NO2 sensing Ziying Wang, Tong Zhang, Tianyi Han, Teng Fei, Sen Liu*, Geyu Lu

State Key Laboratory of Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, P. R. China

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*Corresponding author: E-mail: [email protected] (S. Liu); Fax: +86 431 85168270; Tel:

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+86 431 85168385

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Highlights

SnO2 nanoparticles decorated reduced graphene oxide hybrids with abundant oxygen

Novel NO2 sensors have been fabricated SnO2-RGO-OVs hybrids as sensing materials.

SnO2-RGO-OVs-based NO2 sensor exhibit excellent room-temperature NO2 sensing

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vacancies have been prepared by two-step synthesis method.

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properties, including high sensitivity, fast response and recovery rate, and low

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detection limit.

Abstract

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In this paper, SnO2 nanoparticles (NPs) decorated reduced graphene oxide hybrids with abundant vacancies (designated as SnO2-RGO-OVs) have been successfully prepared by a combined hydrothermal synthesis and chemical solution deposition method. It is found

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that high density SnO2 NPs with the size of 3-5 nm are uniformly distributed on the surface of RGO nanosheets. Most importantly, SnO2-RGO-OVs hybrids exhibit excellent room-temperature NO2 sensing properties with the low detection limit of 50 ppb. When

nO2-RGO-OVs-based sensor was exposed to 1 ppm NO2, the response is 3.80 and response time and recovery time are 14 s and 190 s, respectively. These sensing

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performances are superior to those of most reported room-temperature NO2 sensors based

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on RGO-based materials and other materials. The excellent sensing performances of

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SnO2-RGO-OVs hybrids can be attributed to their specific structure, e.g., RGO that could

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facilitate transferring carriers during sensing progress, and abundant OVs that could

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facilitate adsorption of more NO2 molecules onto SnO2 NPs in SnO2-RGO-OVs hybrids.

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Keywords

Oxygen vacancies; Reduced graphene oxide; SnO2 nanoparticles; NO2 sensor; Room

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temperature

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1. Introduction

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Oxygen vacancies (OVs) dominate the physical and chemical properties of metal

oxides, which play an important role in various fields of catalyst, energy storage, sensors, etc [1-3]. Surprisingly, the electric features of metal oxides are strongly depended on the bulk defects such as OVs, and consequently oxygen vacancy engineering has been considered as an effective strategy for fabrication of high-performance metal

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oxides-based gas sensors [4,5]. For gas sensing, the existence of OVs not only act as an electronic charge carrier to significantly enhance the electronic conductivity, but also adsorb more oxygen molecules to form active sites [6,7]. However, in-depth investigation

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of oxygen vacancy engineering strategy is still required to achieve excellent sensing performances of metal oxides-based gas sensors for practical applications. In particular,

the relevant work that the effect of OVs of graphene-based materials on gas sensing performances is currently missing.

Graphene, since the first reported by Novoselov in 2004, has been demonstrated as a

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promising candidate for NO2 sensors mainly due to its high carrier mobility and

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room-temperature sensitivity [8,9]. Reduced graphene oxide (RGO) has been proven as

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an ideal candidate for fabrication of room-temperature NO2 sensors due to the possibility

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for chemical modification and binding sites (such as oxygen-containing groups) for adsorption of NO2 molecules [10,11]. Nevertheless, pristine RGO displays poor

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room-temperature NO2 sensing performances, such as low sensitivity, slow response and

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recovery rate and poor reversibility [12,13]. To improve NO2 sensing performances at

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room temperature, metal oxides have been used to modify RGO nanosheets. For instance, SnO2 [14-16], ZnO [17,18] and In2O3 [19,20] have been successfully used to hybridize

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with RGO, and the resulting RGO-based materials showed enhanced room-temperature NO2 sensing performances. As a typical n-type semiconductor, SnO2 has been widely

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investigated to modify RGO for enhancing NO2 room-temperature sensing performances [14-16]. However, SnO2 modified RGO hybrids (SnO2-RGO) still exhibit some shortcomings of low sensitivity and poor response-recovery properties required to be overcome [21].

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Later, considerable attention has been focused on development of high-performance SnO2-RGO-based room-temperature NO2 sensors. For example, Zhou et al. fabricated NO2 sensors with enhanced sensing performances by deposition RGO/SnO2

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nanocomposites on microporous substrates, which exhibits poor response and recovery property [15]; Xie and co-authors reported enhanced room-temperature NO2 sensing performances via formatting heterojunction NiO-RGO composited with SnO2 nanoplates

with the response time and recovery time of ~220s/835s toward 60 ppm NO2 [22];

Additionally, our group have also developed several efficient strategies to tailor sensing

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materials structure for enhanced sensing performances, such as construction of

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three-dimensional (3D) structure by introduction of multi-walled carbon nanotubes

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(MWCNTs) [23], surface modification by Ag NPs [24], tuning RGO structure by nitrogen

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doping [25] or introduction of sulfonic group [26]. It is seen that the responses of these sensors to 5 ppm NO2 are ranging from 1.203 to 2.53 and all these sensors show a

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relatively high detection limit of 1 ppm. Although sensing performances of

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SnO2-RGO-based NO2 sensors have been significantly improved by these novel

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technologies, the present sensors are still not satisfied for their practical applications, where poor sensing performances are main challenges still to be overcome.

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It is well known that OVs in metal oxides play important role in gas sensing application, and enhanced sensing performances could be achieved by increasing OVs

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providing more active sites [27-30]. Recently, some researchers have pointed out that surface oxygen vacancy defects can significantly improve the adsorptivity for NO2 and charge transfer ability of SnO2 [31-33]. Consequently, this unique function of surface defects could be applied to boost the sensing performances of SnO2-RGO-based NO2

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sensors. Our previous works have revealed that there are two important factors for achievement of enhanced sensing performances: good stabilizing ability of RGO-based materials and small particle size of metal oxides [23-25]. Therefore, it is still a challenge

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to synthesize SnO2-RGO hybrids with abundant OVs for enhanced gas sensing properties.

Herein, we develop a two-step deposition strategy to prepare SnO2-RGO hybrids with abundant OVs by using SnCl4·5H2O as Sn precursor (designated as

SnO2-RGO-OVs). Firstly, hydrothermal synthesis method was carried out to prepare

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SnO2-RGO hybrids. Then, another batch of SnO2 NPs onto SnO2-RGO hybrids further

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were deposited on SnO2-RGO by chemical solution deposition method. The gas sensing

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results indicate that NO2 sensors based on SnO2-RGO-OVs hybrids exhibit better sensing

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performances than that of SnO2-RGO hybrids.

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2.1. Materials.

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2. Experimental

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SnCl4·5H2O, KMnO4, H2O2 (30 wt%), NaNO3, H2SO4 (98 %) were purchased from Beijing Chemical Corp (Beijing, China). Graphite powder was purchased from Aladdin

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Ltd. (Shanghai, China). Thioacetamide (TAA) were purchased from Sinopharm Chemical Reagent Co. Ltd. All chemicals were used as received without any further purification.

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The water used throughout all experiments was purified through a Millipore system. 2.2. Synthesis of SnO2-RGO-OVs hybrids Hummers’ method was deployed to synthesize GO from graphite powder with the details described in the literature [34]. SnO2-RGO hybrids were prepared by

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hydrothermal synthesis method, according to our previous publication [35]. SnO2-RGO-OVs hybrids were performed by further deposition of SnO2 NPs onto SnO2-RGO hybrids by a wet-chemical method. In a typical

synthesis of

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SnO2-RGO-OVs-2, 1 mL of SnO2-RGO dispersion was added into 48 mL of H2O, followed by addition of 250 µL of SnCl4·5H2O solution (3.5 mg/mL). After stirring for 10

min, 250 µL of TAA solution (1.5 mg/mL) was added into the mixture. Then, the mixture was stirring at 95 ºC for 8 h. The product was collected by centrifugation and washed several times using water. The resulting precipitates were dispersed in water for

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characterization and further use.

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2.3. Characterization.

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Powder X-ray diffraction (XRD) data were recorded on a RigakuD/MAX 2550

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diffractometer with Cu Kα radiation (λ=1.5418 Å). A Perkin-Elmer thermal gravimetric analysis (TGA) 7 unit was used to carry out the TGA in air at a heating rate of 10 ºC/min.

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X-ray photoelectron spectroscopy (XPS) analysis was measured on an ESCALABMK II

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X-ray photoelectron spectrometer using Mg as the exciting source. Transmission electron

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microscope (TEM) micrographs were taken with a Tecnai G220S-Twin transmission electron microscope operating at an accelerating voltage of 120 kV. Elemental mapping

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was conducted by TEM (JEM-2100F) with an energy-dispersive X-ray spectrometer (EDX).

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2.4 Fabrication and measurement of gas sensors. The gas sensors were prepared by drop-coasting method, where aqueous dispersion

of sensing materials was dropped to form a sensing film on the ceramic substrate coated with two pairs of Au interdigitated electrodes (finger width of 180 μm and interfinger

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spacing of about 200 μm), which were printed on top side as signal electrode. Additionally, two parallel heating electrodes are on the back side (length of 820 μm and width of 360 μm).

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The gas sensing properties of sensors were measured by a CGS-8 intelligent test meter (Beijing Elite Tech. Co., Ltd, China). Target vapor was injected into a test chamber

(about 1 L in volume) by a microinjector through a rubber plug. The gas sensing

properties of the samples were determined under specific condition (relative humidity was about 25%). The response of a sensor was defined as the ratio (response: S = Ra/Rg)

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of the sensor resistance in air (Ra) to that in the target gas (Rg). The time taken by the

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sensor to achieve 90% of the total resistance change was defined as the response time in

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3. Results and discussion

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the case of adsorption and recovery time in the case of desorption.

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3.1 Materials characterizations

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SnO2-RGO-OVs hybrids were prepared by a two-step wet-chemical deposition

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method, and the synthesis progress is shown in Scheme 1. Firstly, SnO2-RGO hybrids were prepared by hydrothermal treatment of mixture containing GO and SnCl4 in an

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autoclave at 180 ºC for 12 h, according to our previous publication [35]. During the hydrothermal process, two reactions were carried out: (i) GO was reduced into RGO; (ii)

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SnCl4 was converted into SnO2 NPs. Our previous publications have demonstrated that such hydrothermal synthesis method has been proven as an effective strategy for preparation of SnO2-RGO with good NO2 sensing performances [23-26,35]. Secondly, a wet-chemical deposition method was developed to further deposition of SnO2 NPs onto

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SnO2-RGO hybrids, where SnO2 NPs were obtained by hydrolysis of SnCl4·5H2O at 95 ºC in the presence of TAA. It is deduced that SnO2 NPs content plays an important role in NO2 sensing

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performances for SnO2-RGO-OVs-based sensors, where SnO2 NPs content is depended on the amount of Sn salt added during the two deposition stages. In our previous work,

the effect of SnO2 NPs content in SnO2-RGO hybrids on sensing performances has been

examined detailed [35], and thus the effect of synthesis conditions for SnO2-RGO hybrids on sensing properties was not examined in this work. During the second-step deposition

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progress, SnO2 NPs content in SnO2-RGO-OVs hybrids could be tailored by changing the

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amount of Sn salt. Thus, four SnO2-RGO-OVs hybrids were synthesized by increasing

named

as

SnO2-RGO-OVs-1,

SnO2-RGO-OVs-2,

SnO2-RGO-OVs-3

and

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are

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the volume of SnCl4 solution (3.5 mg/mL) from 125 μL to 500 μL, where these samples

SnO2-RGO-OVs-4 (see detailed synthesis condition in Table 1). The structure of

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SnO2-RGO hybrids and SnO2-RGO-OVs hybrids was firstly examined by XRD

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technique. Fig. 1 shows the XRD patterns of SnO2-RGO hybrids and all SnO2-RGO-OVs

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hybrids. The XRD pattern of SnO2-RGO hybrids shows seven diffraction peaks at 2θ of 26.22°, 33.88°, 37.84°, 51.88°, 65.46°, 71.06°, and 78.52°, which are corresponding to

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the (110), (101), (200), (211), (301), (202) and (321) planes of tetragonal rutile SnO2 [37]. After further deposition of SnO2 NPs on SnO2-RGO hybrids, XRD patterns of these

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SnO2-RGO-OVs hybrids exhibit no appreciable, compared to SnO2-RGO hybrids, indicating the formation of SnO2 NPs in the hybrids after additional hydrolysis progress. Although two deposition progresses were adopted to prepare SnO2-RGO-OVs hybrids, all these diffraction peaks of SnO2-RGO-OVs hybrids are very broad, which could be

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attributed to the small crystalline size and poor crystalline structure of SnO2 NPs. Recently, Wang and co-authors have reported the preparation of SnS2 nanocrystals by hydrolysis of SnCl4·5H2O in the presence of TAA [36], but no obvious diffraction

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peaks attributed to SnS2 were obtained in the present work. This observation may be attributed to that the Sn4+ tends to react with the oxygen to form SnO2 rather than react

with sulfur, especially at mild condition. Indeed, Qian and co-authors have reported that SnO2 crystals are also observed during the synthesis progress for SnS2 crystals using Sn

metal as precursor due to the much lower value of GfSnO2 (−519.65 kJ mol−1) than that

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of GfSnS2 (−198.95 kJ mol−1) [38]. To examine the production structure, three

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referenced samples were prepared and corresponding synthesis condition and XRD

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patterns are shown in Table S1 and Fig. S1. The samples produced by heat treatment of

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RGO and SnCl4·5H2O at 95 ºC show typical diffraction peaks attributed to SnO2 crystals,

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where the samples are designated as SnO2-RGO-OP (Fig. S1), indicating the formation of

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SnO2 crystals. The heat treatment of SnCl4·5H2O and TAA in the absence of RGO also results in formation of SnO2 crystals (designated as SnO2-TAA-OP), not SnS2 crystals.

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To further investigate the effect of hydrolysis of TAA, the samples also prepared by

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hydrolysis of SnCl4·5H2O in the absence of TAA (designated as SnO2-OP), and the XRD pattern indicates formation of SnO2 crystals. From XRD analysis, the diffraction peaks of

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SnO2 NPs in SnO2-TAA-OP become weaker and possess poor crystalline structure, compared to SnO2-OP. It is suggested that production of second-step deposition progress is SnO2 NPs resulting from hydrolysis of SnCl4·5H2O, while the existence of TAA leadings to decreasing the crystalline and forming more OVs. Another batch of SnO2 NPs was introduced by second-step deposition progress, and

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thus TGA measurement carried out in the air was used to determine chemical composition of SnO2-RGO hybrids and SnO2-RGO-OVs-2 hybrids. As shown in Fig. 2, the weight loss from room temperature to 200 ºC is 4.00 % and 2.36 % for SnO2-RGO

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hybrids and SnO2-RGO-OVs-2 hybrids, respectively, which are attributed to the removal of adsorbed water [39]. The weight loss from 200 ºC and 500 ºC can be ascribed to the

removal of O-containing groups and decomposition of the carbon framework [40]. The weight losses are 6.89 % and 7.16 % for SnO2-RGO hybrids and SnO2-RGO-OVs-2

hybrids, respectively. A weak weight loss is also observed for these two hybrids by

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heating from 500 ºC and 700 ºC, which is attributed to further crystalline of SnO2 NPs.

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The mass percentages of SnO2 NPs in SnO2-RGO hybrids and SnO2-RGO-OVs-2 hybrids

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are 88.61% and 89.73%, respectively. This result indicates the further deposition of SnO2

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crystals on SnO2-RGO hybrids by the second-step deposition progress. The XPS characterization was applied to reveal the surface chemical bond

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configuration, composition and OVs in hybrids. Fig. 3a shows the survey spectra of

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SnO2-RGO hybrids and SnO2-RGO-OVs-2 hybrids, revealing that the peaks of Sn3d,

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Sn4d, Sn3p, O1s and C1s were observed, which confirm the presence of three elements of Sn, O and C in hybrids and no traces of impurities were observed. Additionally, no

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peaks attributed to S element are observed for SnO2-RGO-OVs-2 hybrids, indicating no SnS2 is formed during the second-step deposition progress. Fig. S2 shows the S2p

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spectrum of SnO2-RGO-OVs-2 hybrids, further confirming no S-containing composition. Fig. 3b shows the Sn3d spectra of SnO2-RGO hybrids and SnO2-RGO-OVs-2 hybrids. It is seen that two strong peaks at 487.1 eV and 495.5 eV are observed, which are attributed to binding energy of Sn3d5/2 and Sn3d3/2, indicating the formation of SnO2 [40]. As

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shown in Fig. 3c, the peaks of C1s spectra exhibit three types of C-containing groups, including C-C (284.7 eV), C-O (286.1 eV) and C=O (carboxyl, 288.7 eV) bands for both hybrids. The intensity of C-C band is much higher than that of O-containing groups in

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SnO2-RGO hybrids and SnO2-RGO-OVs-2 hybrids, indicating the successful reduction of GO by the hydrothermal process. Additionally, the O1s feature is wide and asymmetric, which can be divided into five well-defined Gaussian-like peaks, revealing the presence

of five types of O-related species, including Sn-O-Sn lattice O atoms in rutile SnO2 structure (530.8 eV for OL) [41], OVs (531.4 eV for OV) [42-44], the chemisorbed

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oxygen related species (532.3 eV for OC), such as hydroxyl group (OH-) or other radicals

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(CO, CO2) at the samples’ surface [43,45], C=O band (532.9 eV) and C-O band (533.9

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eV). As can be seen from data collected in Table 2, the relative percentage of peak areas

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for OV is 30.4 % for SnO2-RGO-OVs-2 hybrids, whereas the percentage is 27.3 % for SnO2-RGO hybrids, indicating that SnO2-RGO-OVs-2 contains more OVs than

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SnO2-RGO hybrids. It is possible that this difference could account for the enhanced gas

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sensing performance of SnO2-RGO-OVs-2 hybrids in comparison with SnO2-RGO

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hybrids. Based on XPS results, the chemical composition of SnO2-RGO-OVs-2 hybrids is C 25.23 %, O 52.75 % and Sn 22.02 %, while the chemical composition of SnO2-RGO

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hybrids is C 33.01 %, O 46.61 % and Sn 20.38 %. It is concluded that abundant OVs

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were obtained by the second-step deposition progress. Fig. 4a shows the low magnification TEM image of SnO2-RGO-OVs-2 hybrids,

indicating

formation

RGO-based

materials.

The

chemical

composition

of

SnO2-RGO-OVs-2 hybrids examined by EDX is shown in Fig. S3, where the signals of C, Sn, O and Cu are observed (the peak of Cu is attributed to Cu substrate used for TEM

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characterization). To confirm the distribution of C, O and Sn onto the surface, elemental mapping of the area was carried out, and the results are depicted in Fig. 4b-d. Notably, O element and Sn element are uniformly distributed on RGO nanosheets, indicating the

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formation of SnO2 crystals on RGO nanosheets. However, C element is distributed over the whole region for TEM image, which is associated with the carbon film in the substrate for TEM characterization.

The detailed structure of SnO2-RGO-OVs-2 hybrids was further examined by TEM

images. It is seen that the loading SnO2 NPs on RGO nanosheets was found in the low

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magnification TEM image (Fig. 5a). The formation of NPs over the surface of RGO has

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been further confirmed by high magnification TEM image, as shown in Fig. 5b. Neither

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overlapped NPs, nor isolated NPs outside of the RGO nanosheets were observed, which

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indicated that growth of ultrafine SnO2 NPs only occurred on the GO surface from the adsorbed Sn4+ by the two deposition progresses, suggesting the strong binding and

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anchoring ability of GO. Fig. 5c shows the high-resolution TEM (HR-TEM) image of

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SnO2-RGO-OVs-2 hybrids. The NPs show lattice distance of 0.33 nm ascribed to the

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(110) plane of SnO2, indicating the formation of SnO2 NPs on RGO surface. The corresponding particle size distribution histogram is shown in Fig. 5d, indicating that the

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particle size of SnO2 NPs is mainly distributed at 3-5 nm.

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3.2 Sensing properties All these observations indicate that ultrafine SnO2 NPs modified RGO hybrids have

been successfully prepared by the two-step deposition method. It is well known that the surface structure of sensing materials plays an important role in sensing performances of gas sensors. As shown in the above results, due to the SnO2 NPs with ultrafine particle

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size prepared by the second step, which make the large amount of OVs and surface active sites on surface of RGO. Thus, it is deduced that excellent sensing performances could be obtained using such ultrafine SnO2 NPs to modify RGO. sensing

application

of

SnO2-RGO-OVs

hybrids

was

tested

for

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The

room-temperature NO2 sensing. The SnO2 NPs content tailored by the second-step

hydrolysis reaction has an effect on amount of OVs, and thus the effect of SnO2 NPs content on NO2 sensing performances of SnO2-RGO-OVs-based sensors was firstly examined. The response and recovery properties for the sensors based on the five samples

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(SnO2-RGO hybrids and four SnO2-RGO-OVs hybrids) were evaluated by exposing to 1

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ppm NO2 at room temperature, as shown in Fig. 6a. It is seen that SnO2-RGO hybrids

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give a response of 2.13 to 1 ppm NO2 with response time and recovery time of 20 s and

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43 s, indicating the good sensing performances for room-temperature NO2 sensing. Notably, after further deposition of SnO2 NPs, all SnO2-RGO-OVs hybrids show higher

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response to 1 ppm NO2, compared to SnO2-RGO hybrids, indicating that sensitivity to

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NO2 was enhanced by further deposition SnO2 NPs. For instance, the responses of the

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sensors to 1 ppm NO2 are 2.97, 3.80, and 4.98 for SnO2-RGO-OVs-1, SnO2-RGO-OVs-2, and SnO2-RGO-OVs-3 prepared by adding 125 µL, 250 µL and 375 µL of SnCl4 solution

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into reaction mixture. This remarkable response change confirms that a slight change of the amount of OVs in SnO2 NPs could dramatically influence the sensitivity of the sensor.

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However, when the volume of SnCl4·5H2O solution was further increased to 500 μL, the response of the sensor based on SnO2-RGO-OVs-4 decreases to 3.55, which could be attributed to the aggregation of SnO2 NPs. Additionally, the response and recovery rate is also depended on the SnO2 NPs content (as shown in Table 1). All SnO2-RGO-OVs

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hybrids exhibit the response time from 14 s to 25 s, while the recovery times are changed from 190 s to 729 s. Among all these hybrids, SnO2-RGO-OVs-2 hybrids exhibit the best sensing properties, where the response to 1 ppm NO2 is 3.80 and the response time and

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effect

of

operating

temperature

on

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recovery time are 14 s and 190 s, respectively. sensing

performances

for

SnO2-RGO-OVs-2-based NO2 sensors is also examined. As shown in Fig. 6b, the

response decreased with increasing the operating temperature from 30 ºC to 100 ºC, which is similar to our previous publication [35]. The reason for this result could be

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attributed to that increasing temperature can speed up NO2 molecular motion to weaken

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the infirm coordination adsorption between the gas molecules and gas-sensing film. As a

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result, the sensor responses were decreased due to decreasing amounts of NO2 adsorbed

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and electrons transferred from NO2 to gas-sensing film [46]. All these observations indicate that enhanced sensing performances have been successfully achieved by

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second-step deposition SnO2 NPs leading to increasing the amount of OVs. Additionally,

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the sensing performances were also compared with SnO2-RGO-based NO2 sensors

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fabricated by our group, as shown in Table 1. It is clearly seen that SnO2-RGO-OVs-2-based NO2 sensor exhibits low detection limit (50 ppb) and high

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response (3.80 to 1 ppm NO2), compared to other SnO2-RGO-based NO2 sensors

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(1.203-2.53 to 5 ppm NO2) [23-26]. The SnO2-RGO-OVs-2-based sensor possesses the appropriate amount of OVs

exhibits the superior sensing performances. To further evaluate its sensing performances, Fig. 7a shows the response and recovery curve of SnO2-RGO-OVs-2-based sensor to various NO2 concentrations ranging from 50 ppb to 2 ppm. Interestingly, the response of

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the sensor based on SnO2-RGO-OVs-2 hybrids to 50 ppb NO2 is 1.47, indicating that the NO2 sensor thus constructed can be used to detect low concentration NO2. Fig. 7b shows the relationship curve between the responses toward NO2 and corresponding

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concentrations from 50 ppb to 2 ppm, revealing that the response of sensor increases with increasing NO2 concentrations. The reproducibility of temporal response of SnO2-RGO-OVs-2 hybrids exposed to 1 ppm NO2 at room temperature, as shown in Fig. 7c. It is seen that the sensor maintains its initial response amplitude upon three successive

sensing tests to 1 ppm NO2, indicating that SnO2-RGO-OVs-2 hybrids possess good

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repeatability. Finally, to investigate the charge transfer ability from SnO2-RGO-OVs-2

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hybrids to the adsorbed gas molecules, besides the acceptor NO2, the sensing

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performance of SnO2-RGO-OVs-2-based sensor towards another common donor gas (Cl2)

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and acceptor gas (NO) were evaluated, as shown in Fig. 7d. It is observed that the response of the sensor to NO2 is much higher than that of Cl2 and NO, indicating that

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SnO2-RGO-OVs-2 hybrids exhibit high selectivity for NO2 sensing. Additionally, the

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responses of SnO2-RGO-OVs-2-based sensor were also examined toward the typical

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receptor gas NH3 (100 ppm), O2 (1000 ppm) volatile organic compounds (VOCs) including ethanol (100 ppm), formaldehyde (100 ppm), toluene (100 ppm), revealing that

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the response of the sensor to NO2 is larger than all these gases. All these observations suggest that SnO2-RGO-OVs hybrids exhibit good sensing performances for

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room-temperature NO2 sensing. In addition, SnO2-RGO-OVs-2-based sensor exposed to 1 ppm NO2 at different relative humidity (RH) levels was also examined (Fig. S4), where the humidity level was increased from 20% RH to 80% RH. It is seen that the response to NO2 decreased with increasing RH (the response from 4.40 to 2.55), indicating that

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humidity has an effect on the sensitivity of the sensor. 3.3 The effect of synthesis conditions on sensing performances Based on the above discussion, the second-step deposition progress plays an

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important role in the sensing performances of SnO2-RGO-OVs-based NO2 sensors. The effect of synthesis condition on sensing performances is further evaluated. To further explore the role of SnCl4·5H2O and TAA solution, several referenced experiments were

carried out and the results are shown in Table 3 and Fig. 8. Firstly, it is seen that the response of SnO2-RGO-OVs-2-based sensor to 1 ppm NO2 is 3.80, which is higher than

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that of SnO2-RGO hybrids (2.13). Obviously, the response time and recovery time of the

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SnO2-RGO-OVs-2-based sensor are 14 s and 190 s, respectively (Fig. 8a and 8b). The

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result of XPS has confirmed that the content of OVs increased by second-step deposition

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through the hydrolysis of SnCl4·5H2O in the presence of TAA. Due to OVs as active sites for adsorption of NO2, along with the amount of OVs increasing, the response of sensor

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to 1 ppm NO2 is enhanced by further modification of SnO2 NPs. Besides, the recovery

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time of SnO2-RGO-OVs-2 hybrids becomes longer. That is because the active sites for

EP

adsorption of NO2 molecules increase, so the time of desorption for NO2 molecules is prolonged. As shown in Fig. 8c, the samples prepared by heating SnO2-RGO dispersion

CC

in the presence of SnCl4 (designated as SnO2-RGO-SnO2) exhibit poor sensing performances, compared to SnO2-RGO-OVs-2 hybrids. Interestingly, SnO2-RGO-SnO2

A

hybrids turned into n-type semiconductor. It is demonstrated that SnO2 content increased by further hydrolysis of SnCl4·5H2O at low temperature, leading to changing semiconductor type. The samples prepared by heating SnO2-RGO dispersion only in the presence of TAA (designated as SnO2-RGO-TAA) exhibit p-type semiconductor similar

16

with SnO2-RGO hybrids, and the basis resistance became lower a lot (from the level of about 50 MΩ to 320 KΩ). However, no obviously enhanced sensing performances were observed. It is clearly seen that the existence of TAA could tailor the surface structure of

SC RI PT

SnO2 NPs during the second-step deposition progress. In our previous work, we also prepared SnO2-RGO hybrids with high SnO2 NPs content by one-pot hydrothermal synthesis method [35]. However, poor sensing performances were obtained by further increasing SnO2 NPs content, indicating the second-step deposition SnO2 NPs is very important for enhanced sensing performances.

U

Moreover, we also prepare the referenced samples using RGO dispersion instead of

N

SnO2-RGO dispersion as precursor. The production was obtained by heating RGO

A

dispersion (obtained by hydrothermal treatment of 0.5 mL of GO solution) in the

M

presence of 250 µL of SnCl4·5H2O solution (3.5 mg/mL) and 250 µL of TAA solution (1.5 mg/mL), designated as SnO2-RGO-OP hybrids. As shown in Fig. S5, the response of

D

SnO2-RGO-OP hybrids to 1 ppm NO2 is only 1.12, which is much lower than that of

TE

SnO2-RGO-OVs-2 hybrids, indicating that the first-step deposition of SnO2 NPs also

EP

plays an important role in improving sensing performances. It is demonstrated that the SnO2 NPs on the RGO nanosheets synthesized by the first-step synthesis is necessary,

CC

which supplied the platform for the second-step of hydrolysis to produce SnO2 NPs with rich OVs.

A

3.4 Sensing mechanism As discussed above, SnO2-RGO-OVs-2-based NO2 sensor exhibits excellent sensing

performances, especially compared to previous NO2 sensors based on SnO2-RGO hybrids prepared by one-step method. The good sensing performances could be attributed to the

17

increased OVs. The possible sensing mechanism could be concluded as follow: In the air, the O2 is absorbed on the surface of SnO2-RGO-OVs hybrids and electrons captured from the conduction band of SnO2 to form the first depletion layer (O2-, O2- or O-). In addition,

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the improvement of sensing performance of SnO2-RGO-OVs hybrids may be attributed to p-n junction formed at the interface between SnO2 (n-type) and RGO (p-type). The

electrons will transfer from SnO2 to RGO, because the work function of SnO2 (4.5 eV) is

lower than that of RGO (4.7 eV) [47]. When the sensor based is exposed to NO2, O2exists on the surface of SnO2 NPs with the reaction (2NO2 + O2- + e- → 2NO3-). NO2

U

can directly capture free electrons from the SnO2. Most importantly, the OVs can

N

significantly enhance the adsorption of O2 molecules, and electrons will transfer from the

A

OVs in SnO2 to the O2 molecules, resulting in more negative oxygen ions (especially O2-)

M

[48]. As a result, there will be in a more abrupt change in the resistance of the sensor.

D

Fortunately, the attachment of SnO2 NPs onto SnO2-RGO hybrids leads to more active

TE

sites (rich OVs), which prepared by hydrolysis of SnCl4·5H2O in the presence of TAA. The OVs make the grain surface possess special chemistry state thereby improving the

EP

NO2 adsorption at low operating temperatures and enhancing the charge transfer from the surface to NO2 [31,42].

CC

3.5 Sensing performances comparison The sensing performances of SnO2-RGO-OVs-based NO2 sensors are also compared

A

with the other recently reported room-temperature NO2 sensors and the sensing performance comparison is shown in Table 4. It is interestingly seen that the detection limit of SnO2-RGO-OVs-based NO2 sensor is 50 ppb, which is much lower than that of other RGO-based sensors, such as CeO2-RGO (10 ppm) [48], 3D crumpled RGO

18

nanosheets (1 ppm) [49], 3D chemically functionalized RGO hydrogel (0.2 ppm) [50,51], Ag-NA-RGO (1 ppm) [52], ZnO nanorods-RGO (1 ppm) [53], NiO-SnO2-RGO (5 ppm) [22], and MoS2 QDs-RGO CY (0.45 ppm) [54]. The response of SnO2-RGO-OVs-based

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sensor to 1 ppm NO2 is 3.80, which is also higher than most of RGO-based sensors and graphene-based sensors [53,55,56]. Moreover, the NO2 sensors thus constructed also exhibit faster response and recovery rate than the most of RGO-based sensors reported recently. However, recently reported other 2D materials, such as black phosphorus (BP)

and AgNWs-WS2 exhibit long response time and difficulty in recovery entirely [57,58].

U

In addition, the preparation of SnO2-RGO-OVs hybrids by two-step wet-chemical method

N

is simple and cost-effective compared to preparation of special morphology for

A

fabrication of NO2 sensors, such as PbS colloidal quantum dots (CQD), PANI Nanofiber

M

and NiO/WO3 nanoplates [59-61]. All these observations indicate that the NO2 sensors based on SnO2-RGO-OVs hybrids exhibit superior sensing performances than most

D

recently reported RGO-based NO2 sensors, paving the way for preparation of

TE

high-performance room-temperature NO2 sensors for practical applications. Recently, a

EP

review about flexible graphene-based gas sensors has been published, where several RGO-based NO2 sensors exhibit excellent sensing performances [62]. It is well known

CC

that the sensor structure plays an important role in sensing performances, and thus the sensing performances of SnO2-RGO-OVs-based NO2 sensor were compared with the

A

RGO-based NO2 sensors prepared by the similar method in this review, as shown in Table 4. Although SnO2-RGO-OVs-based NO2 sensor is not the best one among all these sensors, all the sensing performances (including response, response time and recovery time, detection limit) have been improved at the same time, especially compared to pure

19

RGO-based NO2 sensors.

4. Conclusions In conclusion, the rich OVs were intentionally created onto the surface of

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SnO2-RGO hybrids by controlling the hydrolysis of SnCl4·5H2O in the presence of TAA

at low temperature (95 ºC). The resulting SnO2-RGO-OVs-based sensor exhibits a response as high as 3.80 toward 1 ppm NO2 at room temperature. It is confirmed that the increased OVs possess strong gas-adsorbing and high electron-donating capability toward

NO2 molecules, and account for the high sensitivity of the sensor. Thus, this study

U

presents a totally new understanding for the sensor sensitivity by a systematical analysis

N

on the OVs of SnO2-RGO hybrids and it would help to shape a new method in improving

M

A

the performance of room temperature NO2 sensors.

D

Acknowledgments

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This research work was financially supported by National Natural Science Foundation of China (Grant No. 61671218), and Jilin Provincial Science & Technology

EP

Department (No. 20160520090JH), National Natural Science Foundation Committee

CC

(Granted No. 61673191), High Tech Project of Jilin Province (No. 20150204029GX).

A

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Biographies Ziying Wang received her BS degree from the College of Electronics Science and

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Engineering, Jilin University, China in 2013. During BS course, She studied in Tomsk and received her BS degree from the College of Institute of Non-Destructive Testing, Tomsk Polytechnic University, Russia in 2013. She received his MS degree from in 2016 in College of Electronics Science and Engineering from Jilin University. Now her research focuses on preparation of graphene-based gas sensors.

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Tong Zhang completed her MS degree in semiconductor materials in 1992 and her PhD

N

in the field of microelectronics and solid-state electronics in 2001 from Jilin University.

A

She was appointed as a full-time professor in the College of Electronics Science and

M

Engineering, Jilin University in 2001. Her research interests are sensing functional materials, gas sensors, humidity sensors and electrochemical sensors.

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Tianyi Han received her BS degree from the College of Science, Changchun University

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of Science and Technology, China in 2016. She entered the MS course in 2016. Now her

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research focuses on preparation of gas sensors. Teng Fei received his B.S. degree in 2005 in chemical engineering and technology and

CC

Ph.D. degree in 2010 in polymer chemistry and physics from Jilin University, China. He is presently an associate professor in the College of Electronic Science and Engineering,

A

Jilin University. His research interests include sensing functional materials and devices Sen Liu received his BS degree in 2005 in Chemistry and PhD degree in 2010 in Inorganic Chemistry from Jilin University. Now he is an associate professor in Jilin University and his current research is focused on the carbon-based functional materials

30

and chemical sensors. Geyu Lu received the BS degree in electronic sciences in 1985 and the MS degree in1988 from Jilin University in China and the Dr Eng degree in 1998 from Kyushu

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University in Japan. Now he is a professor of Jilin University, China. Now, he is

A

CC

EP

TE

D

M

A

N

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interested in the development of functional materials and chemical sensors.

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Table 1 The sensing performance comparison of the sensor based on SnO2-RGO-OVs

SnCl4·5H2O

TAA

NO2

solution

solution

concentration

(μL)a

(μL)b

(ppm)

SnO2-RGO

0

0

1

2.13

20/43

-

SnO2-RGO-OVs-1

125

125

1

2.97

18/402

-

SnO2-RGO-OVs-2

250

250

1

3.80

14/190

0.05

SnO2-RGO-OVs-3

375

375

1

4.98

25/729

-

SnO2-RGO-OVs-4

500

500

1

3.55

-

-

-

5

U

24/325

2.53

8/77

1

AgNPs-SnO2-RGOe

-

-

N

SC RI PT

hybrids and NO2 sensors-based on SnO2-RGO hybrids reported by our group previously.

2.17

49/339

1

SnO2-N-RGOf

-

-

5

1.38

45/168

1

SnO2-S-RGOg

-

-

5

1.203

40/357

1

5

A

SnO2-CNT-RGO

d

M

Samples

a

Response

tres/trec (s/s)c

Detection limit

(ppm)

The concentration of SnCl4·5H2O solution is 3.5 mg/mL The concentration of TAA solution is 1.5 mg/mL c Response time and recovery time d Ref. [23] Sens. Actuators B 211 (2015) 318-324 e Ref. [24] Sens. Actuators B 222 (2016) 893-903 f Ref. [25] Sens. Actuators B 242 (2017) 269-279 g Ref. [26] Sens. Actuators B 228 (2016) 134-143

A

CC

EP

TE

D

b

32

Table 2 The fitting results of O1s XPS spectrum of SnO2-RGO hybrids and SnO2-RGO-OVs-2 hybrids. Oxygen species

Binding energy (eV)

Relative percentage (%)

SnO2-RGO

OL (Sn-O)

530.8

30.8

OV (vacancy)

531.4

OC (chemisorbed)

532.3

C=O

532.9

C-O

533.9

OL (Sn-O)

530.8

OV (vacancy)

531.4

30.4

OC (chemisorbed)

532.3

24.1

532.9

11.1

U

N

SnO2-RGO-OVs-2

A

C=O

533.9

27.3 27.6 10.0 4.2

32.6

1.8

A

CC

EP

TE

D

M

C-O

SC RI PT

Samples

33

Table 3 The sensing performance comparison of the sensor based on SnO2-RGO hybrids

temperature.

SC RI PT

prepared in the presence of SnCl4·5H2O or TAA solution to 1 ppm NO2 at room

SnCl4·5H2O

TAA solution

Type of

solution (μL)

(μL)

semiconductor

SnO2-RGO

0

0

p

SnO2-RGO-OVs-2

250

250

p

SnO2-RGO-SnO2

250

0

n

SnO2-RGO-TAA

0

250

p

SnCl4·5H2O solution (3.5 mg/mL)

b

TAA solution (1.5 mg/mL)

Response

Response time and recovery time (s/s)

2.13

20/43

2.97

18/402

3.80

14/190

4.98

25/729

A

CC

EP

TE

D

M

A

a

b

U

a

N

Samples

34

Table 4 Comparison of the room-temperature sensing performance of our proposed NO2

Concentration

Materials

tres/trec b

Detection

Ref.

(s/s)

limit (ppm)

30/85

10

[47]

-/-

1

[48]

-/-

0.2

[49]

12/11 600/2400

0.2 1

[50] [51]

50

1.25

5

1.60

10

1.07

4 5

1.24 1.74

ZnO nanorods-RGO

1

2.19

75/132

1

[52]

MoS2 QDs-RGO CY

0.45

1.39

-/-

0.45

[53]

CDs-RGO Mechanical exfoliation (ME)-graphene BP

5

1.74

100/150

0.05

[54]

1.05

240/-

0.12

[55]

1

~5

-/-

0.1

[56]

1

1.32

-/-

0.1

[57]

50

21.7

12/37

0.084

[58]

1

1.8

~55/~68

1

[59]

NiO/WO3 nanoplates

30

~4.5

2.5/1.1

5

[60]

RGO/Fe2O3 nanocomposite

90

2.50

-/1648

0.18

[63]

RGO/WO3 nanocomposite Graphene-aerogel-supported SnO2 NPs RGO/Cu2O nanowire mesocrystals Graphene/MoS2 hybrid aerogel

5

8.69

540/1080

0.5

[64]

50

-

190/224

10

[65]

2.0

~1.68

-/-

0.064

[66]

-

-

-

0.05

1

3.80

14/190

0.05

[67] This work

CC

EP

TE

PANI Nanofiber

D

AgNWs-WS2 PbS CQD

A

SnO2-RGO-OVs

A

M

1.5

U

CeO2-RGO 3D crumpled RGO nanosheets 3D chemically functionalized RGO hydrogel 3D sulfonated RGO hydrogel Ag-NA-RGO

N

(ppm)

Responsea

SC RI PT

sensor with recently reported other NO2 sensors.

a

Response: Ra/Rg or Rg/Ra (Ra: resistance of the sensor in air; Rg: resistance of the sensor in the target gas) b Response time and recovery time

35

Figure captions

SC RI PT

Scheme 1 Schematic illustration of the synthesis progress for SnO2-RGO-OVs hybrids.

Fig. 1 XRD patterns of SnO2-RGO hybrids and SnO2-RGO-OVs hybrids with various SnO2 NPs content.

Fig. 2 TGA curves of SnO2-RGO hybrids and SnO2-RGO-OVs-2 hybrids under air

U

atmosphere at a heating rate of 10 °C/min.

N

Fig. 3 (a) XPS spectra, (b) Sn3d XPS spectra, (c) C1s XPS spectra, and (d) O1s XPS

A

spectra of SnO2-RGO hybrids and SnO2-RGO-OVs-2 hybrids.

SnO2-RGO-OVs-2 hybrids.

M

Fig. 4 (a) Low magnification TEM image, and (b-d) typical elemental mapping of

D

Fig. 5 (a) Low magnification, (b) high magnification, (c) HR-TEM images of

TE

SnO2-RGO-OVs-2 hybrids and (d) the corresponding particle size distribution histogram of SnO2 Ns in SnO2-RGO-OVs-2 hybrids.

EP

Fig. 6 (a) The response and recovery curves of the sensors based on SnO2-RGO hybrids

CC

and all these SnO2-RGO-OVs hybrids to 1 ppm NO2 at room temperature, and (b) The responses of SnO2-RGO-OVs-2 toward 1 ppm NO2 at different operating temperatures

A

(30 oC, 40 oC, 60 oC, 80 oC and 100 oC). Fig. 7 The sensing performances of the SnO2-RGO-OVs-2-based NO2 at room temperature: (a) The response and recovery curve to NO2 concentrations of 50 ppb, 100 ppb, 200 ppb, 500 ppb, 1 ppm, and 2 ppm, (b) The relationship curve between responses

36

and NO2 concentrations, (c) The reproducibility of response exposed to 1 ppm NO2, and (d) The selectivity toward different kinds of gases, including NO2, Cl2, NO. Fig. 8 The response and recovery curves of the sensor based on (a) SnO2-RGO hybrids,

SC RI PT

(b) SnO2-RGO-OVs-2 hybrids, (c) SnO2-RGO-SnO2 hybrids and (d) SnO2-RGO-TAA

A

CC

EP

TE

D

M

A

N

U

hybrids.

37

D

TE

EP

CC

A

SC RI PT

U

N

A

M

Scheme 1

38

D

TE

EP

CC

A

SC RI PT

U

N

A

M

Figure 1

39

D

TE

EP

CC

A

SC RI PT

U

N

A

M

Figure 2

40

D

TE

EP

CC

A

SC RI PT

U

N

A

M

Figure 3

41

D

TE

EP

CC

A

SC RI PT

U

N

A

M

Figure 4

42

D

TE

EP

CC

A

SC RI PT

U

N

A

M

Figure 5

43

D

TE

EP

CC

A

SC RI PT

U

N

A

M

Figure 6

44

D

TE

EP

CC

A

SC RI PT

U

N

A

M

Figure 7

45

D

TE

EP

CC

A

SC RI PT

U

N

A

M

Figure 8

46