Effects of graphene on the microstructures of SnO2@rGO nanocomposites and their formaldehyde-sensing performance

Accepted Manuscript Title: Effects of graphene on the microstructures of SnO2 @rGO nanocomposites and their formaldehyde-sensing performance Authors: Xiaoru Rong, Deliang Chen, Geping Qu, Tao Li, Rui Zhang, Jing Sun PII: DOI: Reference:

S0925-4005(18)30890-6 https://doi.org/10.1016/j.snb.2018.04.176 SNB 24652

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

13-1-2018 6-4-2018 28-4-2018

Please cite this article as: Xiaoru Rong, Deliang Chen, Geping Qu, Tao Li, Rui Zhang, Jing Sun, Effects of graphene on the microstructures of SnO2@rGO nanocomposites and their formaldehyde-sensing performance, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2018.04.176 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.

Effects of graphene on the microstructures of SnO2@rGO nanocomposites and their formaldehyde-sensing performance

a

SC RI PT

Xiaoru Rong a, Deliang Chen a,b,*, Geping Qu a, Tao Li b, Rui Zhang a,c, Jing Sun d*

School of Materials Science and Engineering, Zhengzhou University, 100 Science Road, Zhengzhou 450001,

P.R.China

School of Chemical Engineering and Energy Technology, Dongguan University of Technology, Dongguan

U

b

Laboratory of Aeronautical Composites, Zhengzhou Institute of Aeronautical Industry Management, University

A

c

N

523808, P.R. China

d

M

Centre, Zhengdong New District, Zhengzhou 450046, China

The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of

*Corresponding authors:

TE

D

Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P.R.China

EP

Deliang Chen, Professor

School of Materials Science and Engineering, Zhengzhou University, 100 Science Road, Zhengzhou 450001,

CC

P.R. China. E-mail address: [email protected] (D.L. Chen) Jing Sun, Professor

A

The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of

Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, P.R.China; E-mail address: [email protected] (J. Sun)

1

Highlights •Porous SnO2/graphene (G) nanocomposite is synthesized by a solvothermal process. •Graphene amount highly affects the formaldehyde-sensing performance of SnO2/G.

SC RI PT

•High surface area and electron transfer tunnel improve formaldehyde-sensing behavior.

Abstract

Performance modulation of formaldehyde (HCHO) sensing active nanomaterials is of great significance in

environmental monitoring and disease diagnosis. This paper reports a simple but robust solvothermal process to synthesize SnO2@rGO nanocomposites for HCHO sensors with rGO mass fractions of 02%. The phases,

U

chemical compositions, microstructures and surface states of the as-obtained SnO2@rGO nanocomposites are

N

well characterized. The results indicate that the addition of GO overcomes the agglomeration of SnO2

A

nanocrystals (35 nm) and highly enhances the specific surface area (SSA) of the SnO2@rGO nanocomposites,

M

leading to higher response and lower operating temperature in the HCHO-sensing application. The SSA of SnO2@rGO is 133.1 m2/g, much larger than that (58.3 m2/g) of pure SnO2 nanocrystals. The SnO2@rGO

D

nanocomposites exhibit highly selective and sensitive to HCHO vapors at a relatively low operating temperature

TE

range of 100200 oC. The amount of GO added has a key effect on the HCHO-sensing performance, and the

EP

sample of [email protected]% exhibits the highest response at 100160 oC. Their recovery / response times are shorter than 20 s to HCHO vapors (less than 25 ppm). The enhanced HCHO-sensing performance is attributed to

A

CC

the formation of porous SnO2@rGO nanostructures with high SSAs and suitable electron transfer channels.

Keywords: Formaldehyde detection; Tin oxide; Graphene; SnO2@graphene nanocomposite; Chemical sensor

1. Introduction The Internet of Things has provoked extensive research into the variety of chemical sensor devices that enable us to collect and exchange gas-sensing data and thereby has opened a new way to improve efficiency,

2

accuracy, and economic benefit in smart technologies, such as smart automobiles, buildings, health, and environment [1]. Recently chemical sensors and related materials have attracted increasing attention because of their environmental monitoring applications [24]. Environmental quality has been one of the most important aspects tightly related to the people's health problems, and toxic gases in air can cause serious respiratory illnesses [5]. Volatile organic compounds (VOCs) are common reagents used in industry and daily life, and some

SC RI PT

of them (i.e., formaldehyde) are toxic to human body. Therefore, the detection of VOCs has attracted considerable attention due to its application in environmental and indoor air quality monitoring, as well as in noninvasive disease diagnosis [67]. For the purposes of VOCs detection, gas sensors based on various active

materials and techniques have recently been developed [8-12]. Metal oxide semiconductors (MOS) are widely investigated because of their stable sensibility and cost-effective production. Various MOSs, including tin oxide

U

(SnO2) nanofibers [13], WO3 nanocrystals [1418], In2O3 nanowires [19], SnO2/ZnO heterojunction nanofibers

N

[20], Co3O4 modified Pd-SnO2 yolk-shell microreactors [21], are also extensively investigated for gas sensing

A

applications. However, such MOS-based sensors require a relative high operating temperature, usually higher

M

than 200 oC.

Gas sensors with low operating temperature, high sensitivity and high selectivity are requisite in practical

D

applications [22]. To develop high-performance and low-temperature sensors, scientists have taken measures in

TE

various aspects during the synthesis of MOSs, including controlling particle-sizes & morphologies (i.e., onedimensional nanorods, two-dimensional nanoplates) [2324], doping and hybridizing with another functional

EP

compositions to form heterojunctions [2530]. Zhang et al. [31] synthesized Ag-LaFeO3 (ALFO) fibers, spheres and cages using a hydrothermal method combined with a template method, and the ALFO-based formaldehyde

CC

sensors exhibit a high response at an operating temperature below 100 oC. Graphene is one of the most promising materials for sensor applications since all of its atoms in the honeycomb lattice are exposed to the environment,

A

and the single layer graphene has the largest surface-to-volume ratio, possessing the potential ability to detect a single molecule or ion [3233]. Graphene or reduced graphene oxide (rGO) are found as efficient components to improve the gas sensing performance of MOS based sensors [3435], for example, RGO modified ZnO NRsAlGaN Schottky diode sensor for HCHO detection [36], NiO-rGO composited with SnO2 nanoplates for NO2 detection [37], Pd/SnO2/RGO nanocomposites for H2 detection [38], WO3-rGO porous nanocomposite for NO2 detection [39] , and SnO2/graphene quantum dots (GQDs) nanocomposites for acetone sensing [40]. Also, we

3

developed an efficient method on the basis of microwave heating process to synthesize SnO2/rGO nanostructures for H2S-sensing application, and found that the addition of graphene oxide could reduce operating temperature and enhance gas-sensing performance of the MOS based gas sensors [41]. Feng et al. [42] physically mixed rGO and SnO2 nanocrystals to form rGO-SnO2 nanocomposites for room-temperature ammonia sensing application, and the rGO-SnO2 composites exhibited a switch from an n-type semiconductor response behavior to a p-type

SC RI PT

semiconductor behavior as the rGO content increased from 0.1 wt% to 1 wt%. Abideen et al.[43] reported graphene nanosheet (NS)-loaded SnO2 nanofibers (NFs) synthesized by a low-cost facile electrospinning process for C6H6,C7H8, CO, CO2, and H2S sensing applications. Song et al. [44] designed a highly sensitive room-

temperature H2S gas sensor by anchoring SnO2 quantum wires on reduced graphene oxide (rGO) nanosheets. However, the effect of graphene on the gas-sensing performance is not clear yet.

U

Formaldehyde (HCHO), widely used as an important precursor in many industrial materials and chemical

N

compounds, has been classified as a highly toxic compound to living organisms [45], because its strong

A

interactions with proteins, nucleic acids and other biomolecules can lead to the inactivation of their biological

M

activities, and excess exposure of exogenous HCHO to the human body will cause a serious health threat and many diseases (i.e., various cancers, heart disorders and diabetes) [46, 47]. Great efforts have been made to

D

synthesize special MOS nanostructures for HCHO detection [34, 36, 46, 48, 49], including hollow ZnSnO3 Cubes

TE

[50], flower-like NiO hierarchical architectures [51], Fe-doped In2O3 hollow microspheres [52], ordered largepore mesoporous Cr2O3 [53].

EP

Tin oxide (SnO2) is well-known as a wide bandgap n-type semiconductor (Eg=3.6 eV) for gas sensors to detect toxic gases [54]. Graphene (GO or rGO) has been found as an efficient additive to enhance the gas-sensing

CC

performance of MOSs-based sensors [41]. We herein report a simple but robust solvothermal method to synthesize tin oxide nanocrystals modified with a small amount of GO for low-temperature formaldehyde sensing

A

applications. The addition of GO obviously deceases the operating temperature and enhances the HCHO-sensing performance when compared with the pure SnO2 sensors. 2. Experimental 2.1. Chemical regent All the chemical reagents were analytical grade and used as received without further purification. SnCl2•2H2O and ammonium hydroxide (NH3•H2O) were purchased from Tianjin Wind Boat Chemical Reagent

4

Technology Co. Ltd, China. Ethanol was purchased from Tianjin Kaitong Chemical Reagent Co. Ltd, China. Graphene oxide was purchased from Wuxi City Huicheng Graphene Technology Application Co. Ltd, China. Deionized water (resistivity of 18.2 MΩcm at 25 oC) was used through all experiments. 2.2 Synthesis of SnO2@rGO nanocomposites SnO2@rGO nanocomposites were synthesized via an in-situ solvothermal process (Fig. 1a). The mass ratio

SC RI PT

of GO to SnO2 was kept theoretically to be 0, 0.25%, 0.5%, 1%, 2%, and the resulting samples were named as SnO2, [email protected]%, [email protected]%, SnO2@rGO-1%, SnO2@rGO-2%, respectively. Typically, 0.1 g/L of GO aq. solution was prepared by thoroughly dispersing 1.7 mg GO in 17 mL of H2O under an ultrasonic

condition for 1 h, and 0.1 mol/L of SnCl2 was prepared by dissolving 512 mg SnCl2·2H2O in 22.7ml ethanol. The above GO aq. solution and Sn2+ ethanol solution were then mixed together by magnetic stirring for about 30 min.

U

The as-obtained GO/Sn2+ mixture was transferred into a Teflon-lined steel autoclave with a volume of 100 mL,

N

and heated at 180 oC for 12 in an electric heat oven. After naturally cooled down, the solid sample was collected

A

by centrifuging and washing the suspension. The as-obtained solid was then freeze-dried and the SnO2@rGO

synthesized using the similar process.

M

nanocomposite was [email protected]%. The other SnO2@rGO nanocomposites with various contents of GO were

D

2.3 Characterization of SnO2@rGO nanocomposites

TE

The phases of the SnO2@rGO nanocomposites were determined by X-ray diffraction (XRD) on a DX-2700 XRD diffractometer (Dandong Haoyuan Instrument Co., Ltd, China) with a Cu Kα radiation (λ =1.5406 Å )

EP

operated at 40 kV and 30 mA, with a scanning rate of 4o min-1 in 2θ = 10 ~ 80º. Raman spectra were obtained using a Raman spectrometer (LabRAM HR Evolution, HORIBA JobinYvon, France) at room temperature using

CC

the 633 nm line as the excitation source. The microstructures were observed by scanning electron microscopy (SEM, JSM-7001F JEOL, Japan) and transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN, FEI,

A

America). X-ray photoelectron spectroscopy (XPS) spectra were performed on an XPS spectroscope (Escalab 250xi, Thermo Scientific Ltd., England) equipped with an Al Ka (1486.6 eV) radiation source. The specific surface areas were measured by Brunauer-Enmet-Teller (BET) method according to the N2 adsorption-desorption isotherms obtained on ASAP 2460 (Micromeritics, USA) machine at a liquid nitrogen temperature (-196 oC). 2.4 Gas-sensing test of SnO2@rGO nanocomposites

5

The sensors based on the SnO2@rGO nanocomposites were fabricated according to the previous reports [14-18]. A schematic diagram of the SnO2@rGO sensor was shown in Fig. 1(a). The sensors with SnO2 nanocrystals were fabricated used the similar process (see Supplementary Material for detailed information). The gas-sensing test was conducted using a commercial computer-controlled WS-30A system (Zhengzhou Winsen Electronics Technology Co., LTD, Zhengzhou, China) under a static testing condition. An equivalent circuit of

SC RI PT

the gas-sensing test system is shown in Fig. 1b [17]. The test system was placed in a ventilating cabinet with a large draught capacity. Formaldehyde (HCHO) was used as the target gas to evaluate the gas-sensing

performance of the SnO2@rGO sensors. To investigate the selectivity of the SnO2@rGO sensors, some organic vapors (e.g., methanol, ethanol, and acetone) and H2 were also used as the target sensing substances. The target gases were sampled using a syringe-like sampler with a volume range of 110 L. The concentrations (0.5100

U

ppm) of the target sensing substances were calculated according to the densities of liquid organic substrates and

N

the volume of the chamber [15]. The operating temperatures varied from 100 oC to 250 oC. The relative humidity

A

(RH) of the test environment was ~40%. For reducing gases (or vapors) and n-type semiconductor sensors, the

M

response (Sr) is defined as the ratio of Ra to Rg, where Ra is the baseline resistance in presence of clean air and Rg is the resistance of the sensor device in presence of a target gas.

D

3. Results and discussion

TE

3.1 Synthesis of SnO2@rGO nanocomposites

The formation of SnO2@rGO nanocomposites experiences two chemical reactions: one is redox reactions, and

EP

the other is hydrolytic precipitation reactions. The redox reactions include the reduction of graphene oxide (GO) and the oxidization of Sn2+ ions, i.e., GO + Sn2+ → rGO + Sn4+. The hydrolytic precipitation reactions include the

CC

nucleation and growth of Sn(OH)4 and SnO2 nanocrystals under the solvothermal condition, i.e., Sn4+ + H2O → Sn(OH)4→ SnO2. Fig. 1a shows the schematic description of the process for synthesizing SnO2@rGO

A

nanocomposites as the formaldehyde (HCHO)-sensing active material. There are two key steps for the formation of SnO2@rGO nanocomposites: (1) the adsorption of Sn2+ ions on the surface of GO nanosheets and the redox reaction between Sn2+ and GO to form Sn4+ and rGO during the magnetic stirring; (2) the in-situ hydrolysis of Sn4+ ions to form Sn(OH)4 nanocrystals on rGO surface and the dehydration of Sn(OH)4 species to form SnO2@rGO nanocomposites during the solvothermal treatment at 180 oC. The in-situ solvothermal process is favorable in forming SnO2 nanocrystals on the rGO surface tightly, which is helpful for efficient transfer of

6

electrons during the gas-sensing process. To investigate the effect of rGO on HCHO-sensing performance, we synthesized a series of SnO2@rGO nanocomposites with various mass ratios (0.25−2%) of rGO to SnO2 using the above similar in-situ solvothermal process. We also prepared pristine SnO2 sample without GO for comparison to identify the role of rGO in the in-situ synthesis and HCHO-sensing performance. 3.2 Phase, composition and microstructure characterization of SnO2@rGO nanocomposites

SC RI PT

We conducted XRD, Raman and XPS analyses to determine the phases and chemical compositions of the SnO2@rGO nanocomposites. Fig. 2 shows the XRD patterns of the pristine GO, SnO2 and SnO2@rGO nanocomposites with various amounts of rGO. According to the XRD patterns, SnO2 and SnO2@rGO

nanocomposites have the same peak positions at Bragg angles (2θ) of 26.4°, 33.8°, and 51.8° regardless of the rGO ratio, corresponding to the (110), (101) and (211) planes of the tetragonal rutile SnO2 structure (JCPDS 41-

U

1445), respectively [42]. The XRD peaks at 37.8°, 54.7° and 61.8° corresponding to the (200), (220) and (310)

N

planes of SnO2 become more clear as the mass ratio of rGO to SnO2 increases from 0.25% to 2%. It is clear that

A

the major XRD peaks of the tetragonal rutile SnO2 structure become sharper and stronger as the mass ratio of

M

rGO to SnO2 increases, suggestive of the increased diameter and crystallinity. The observations from XRD patterns suggested that the addition of rGO does not change the lattice structure, but improves the growth and

D

crystallinity of SnO2 nanocrystals. During the solvothermal growth of SnO2 nanocrystals, the rGO nanosheets can

TE

serve as the crystal nucleus via the heterogeneous nucleation and growth mechanism. There are more rGO nanosheets in the case of with a larger rGO@SnO2 ratio, and the more rGO nanosheets are therefore favorable in

EP

forming SnO2 nanocrystals. For the pristine GO (Fig. 2a), the widened XRD peak at about 23° corresponds to the characteristic (002) plane of graphene oxide, and the sharp peak at about 10° should result from the layered

CC

structure formed by re-stacking GO nanosheets in a solid state [42]. One can also find a weak peak at 43°. These XRD peaks of GO do not occur in the SnO2@rGO nanocomposites, due to its small amounts and high dispersion

A

in the nanocomposites. The presence of rGO in the SnO2@rGO nanocomposites was further confirmed by Raman spectra. Fig. 3

and Table 1 show the detailed Raman data of GO, SnO2 and SnO2@rGO nanocomposites. Both the pristine GO and the SnO2@rGO nanocomposites exhibit two major peaks belonging to the D band at 1340−1352 cm−1 and the G band at 1597−1608 cm−1, corresponding to the defect formation and the first-order scattering of the E2g mode of sp2 domains in the GO or rGO nanosheets, respectively [32]. For the pristine GO, the intensity ratio (ID/IG) of the

7

D (at 1346 cm−1) and G (at 1598 cm−1) band is 1.04. After the in-situ solvothermal synthesis of SnO2@rGO nanocomposites, the ID/IG ratio increases to more than 1.36 (Table 1). The D and G bands of [email protected]% locate at 1352 cm−1 and 1608 cm−1, respectively. When the rGO amount increases from 0.25% to 2%, the D band decreases from 1352 cm−1 to 1340 cm−1, and the D band decreases from 1608 cm−1 to 1597 cm−1. The occurrence of Raman peaks (D and G band) verifies the presence of rGO in the SnO2@rGO nanocomposites [34]. The

SC RI PT

changes in both intensity and position of D and G bands are suggestive of some obvious interaction between rGO and SnO2 nanocrystals, and this hybridization between rGO nanosheets and SnO2 nanocrystals is expected to be favorable in enhancing the electron transfer at the SnO2@rGO interfaces to improve the gas-sensing performance [42]. For the SnO2@rGO-x% nanocomposites (x = 0.5, 1 and 2), the second-order D (2D) and G (2G) bands of rGO can be discerned at 2672 cm−1 and 2935 cm−1, respectively, further reconfirming the formation of rGO

U

during the in-situ solvothermal process. The 2D and 2G bands are not obvious in the [email protected]%

can also be found in the SnO2@rGO nanocomposites.

A

2

N

nanocomposite, possibly due to the too small amount of rGO. The wide band at about 580 cm−1 belonging to SnO-

To understand some detailed information of SnO2 nanocrystals, we dissected the Raman spectrum of pure

M

SnO2 nanocrystals, as shown as Fig. 3b. The band at about 300 cm−1 belongs to the Eu mode, including TO

D

(transverse optical) and LO (longitudinal optical) [55]. The wide band at 580 cm−1 can be divided into 5 peaks,

TE

locating at 401, 501, 586, 655 and 701 cm−1, which correspond to A2g, S2, S1, A1g and S3 modes, respectively [55]. The modes of Eu, A1g and A2g correspond to the classical vibration modes, while the bands of S1, S2 and S3 appear

EP

as a consequence of disorder activation [55]. The strong bands of S1, S2 and S3 indicate that the SnO2 sample is composed of nanocrystals with very small particle sizes or amorphous species.

CC

X-ray photoelectron spectroscopy (XPS) analysis was carried out to analyze the surface compositions and chemical states of the SnO2@rGO nanocomposite. Fig. 4 shows the typical XPS spectra of the [email protected]%

A

sample. Fig. 4a shows the survey spectrum, indicating that Sn, O, C are the major elements of the SnO2@rGO sample, and small amounts of elements Cl and Al should be impurities, resulted from the precursor SnCl2 or contamination during the course of test. Fig. 4b shows the Sn 3d5/2 and 3d3/2 spectra. The binding energies of Sn 3d5/2 and 3d3/2 are identified at 486.6 and 495.0 eV, respectively. Fig. 4c shows the C 1s spectrum. The wide band can be subdivided to four XPS peaks centered at 284.6, 284.9, 286.3, and 289.1 eV, corresponding to C-C/C=C, C−O, C−O−C/C=O and O−C=C groups of rGO, respectively [56][57]. Fig. 4d shows the O 1s spectrum of the

8

[email protected]% sample. The O 1s XPS peak can be decomposed into three Gaussian components centered at about 531.6, 532.9, and 533.4 eV, respectively; the above three components can be indexed to O2− ions in SnO2 lattice (OL), O2− ions in oxygen-deficient regions (Ov) and chemisorbed oxygen (OC) species and –OH groups, respectively [58]. We can conclude that the [email protected]% sample obtained via the in-situ solvothermal process consists of rGO and SnO2 species with some −OH groups and oxygen vacancies.

SC RI PT

Taking XRD, Raman and XPS results into account, we can safely conclude that the SnO2@rGO nanocomposites are composed of tetragonal rutile SnO2 nanocrystals with small particle sizes and partly reduced graphene oxide (rGO) with some -OH groups and oxygen vacancies.

SEM observations were conducted to investigate the morphology and microstructure of the GO, SnO2 and SnO2@rGO nanocomposites. Fig. 5a shows a typical SEM image of the pristine GO, suggestive of a restacking

U

layer structure with a large area. The typical SEM image of the pure SnO2 sample synthesized by the

N

solvothermal process is shown in Fig. 5b, and it consists of apparent spherical particles with a size range of 14

A

m. However, the weak and highly widened XRD peaks (Fig. 2b) of the pure SnO2 sample suggests that the

M

apparent spherical particles should be assembled of small SnO2 nanocrystals. Figs. 5cd show the typical SEM images of the SnO2@rGO-x% nanocomposites with x = 0.25, 0.5, 1 and 2, respectively. One can find that the

D

addition of small amounts of GO obviously changes the morphologies of the SnO2 samples from micrometer

TE

spherical particles to small nanocrystals, and that the SnO2 nanocrystals in-situ grow on the surfaces of GO nanosheets. We can identify some sheet-like particles (marked by arrows) in the SnO2@rGO-x% samples even

nanoparticles.

EP

though most of them are SnO2 species; the sheet-like particles should be rGO species combined with SnO2

CC

TEM and high-resolution TEM (HRTEM) observations were further conducted to characterize the microstructure of the SnO2@rGO nanocomposites. Fig. 6 shows the typical TEM, and HRTEM and selected area

A

electron diffraction (SAED) pattern of the [email protected]% sample. The low-magnification TEM image in Fig. 6a indicates that the SnO2 nanocrystals grow on the surface of rGO nanosheets and the clear edge outlines (marked by arrows) give some convincing evidence. The enlarged TEM image in Fig. 6b suggests that [email protected]% sample consists of SnO2 nanoparticles aggregated together to form a sheet. The HRTEM image (Fig. 6c) clearly shows the SnO2 nanoparticles have a size range of 35 nm, and the distinct lattice fringes of the SnO2 nanoparticles are suggestive of a high degree of crystallinity. Such small particles with a high degree of

9

crystallinity are helpful to improve their gas-sensing performance. The SAED pattern (Fig. 6d) with a series of diffraction spots further verifies that the small SnO2 nanoparticles in the SnO2@rGO nanocomposite are of a tetragonal rutile phase and a high degree of crystallinity. In order to quantitatively determine and compare the specific surface areas and pore sizes of the SnO2@rGO nanocomposite and pure SnO2 sample, the multipoint BET surface area and pore volume analyses were carried

SC RI PT

out based on the nitrogen adsorption−desorption isotherms. The N2 physisorption isotherms of the [email protected]% and pure SnO2 samples and their corresponding Barret-Joyner-Halenda (BJH) pore-size distribution curves are shown in Fig. 7. As Fig. 7a (A) shows, the [email protected]% sample exhibits a type IV isotherm with a hysteresis loop which resembles the H2 type as per the IUPAC classification [59]. The hysteresis loop is

associated with capillary condensation taking place in mesopores, and the limiting uptake over a range of high

U

relative pressure (P/P°). The initial part of the type IV isotherm is attributed to monolayer-multilayer adsorption.

N

The N2 physisorption isotherm of the pure SnO2 sample is shown in Fig. 7a(B). It exhibits the reversible type I

A

isotherm, which is concave to the relative pressure (P/P°) axis and approaches a limiting value as P/P°→1; type I

M

isotherms are given by microporous solids having relatively small external surfaces, the limiting uptake being governed by the accessible micropore volume rather than by the internal surface area [59]. Therefore, the N2

D

isotherms indicate that the [email protected]% sample is of a mesoporous structure while the SnO2 sample is of a

TE

microporous structure. From the N2 physisorption measurements, the BET specific surface areas of [email protected]% and SnO2 are 133.1 and 58.3 m2/g, respectively. On the basis of BJH model and the desorption data, the

EP

adsorption average pore diameter (4V/A by BET) of the [email protected]% and SnO2 samples are to be 3.2 nm and 2.2 nm, respectively. Fig. 7b shows the pore-size distribution curves of [email protected]% (A) and SnO2 (B). One

CC

can find that the SnO2 sample has some micropores (D  2 nm), while the [email protected]% sample has a large amount of mesopores (D  2 nm) besides the micropores.

A

According to the SEM, TEM and nitrogen adsorption-desorption isotherm analyses, we can safely draw a

conclusion that the addition of GO nanosheets dramatically modulate the morphology and microstructure of the SnO2 species, and the SnO2@rGO nanocomposites are of a mesoporous structure consisting of SnO2 nanocrystals with a size range of 35 nm, while the pure SnO2 sample is composed of large spherical particles (14 m) without mesopores. The SnO2@rGO nanocomposites with a mesoporous structure have a high specific area, being favorable in improving their gas-sensing performance.

10

3.3 Formaldehyde-sensing performance of SnO2@rGO nanocomposites The SnO2@rGO-x% nanocomposites with various amounts of rGO (x = 0, 0.25, 0.5 ,1 and 2) obtained via the in-situ solvothermal process were used as active materials to manufacture sensors for gas-sensing applications (e.g., formaldehyde detection). Firstly, we checked the formaldehyde (HCHO)-sensing performance of these sensors under different

SC RI PT

operating temperatures (100 - 300 oC) upon exposure to HCHO vapors with various concentrations (5100 ppm). Fig. 8 shows the typical U-t curves, and Fig. 9 shows the corresponding response (Ra/Rg)-C curves. As Fig. 8a and 9a show, when the operating temperature is 100 oC, the [email protected]% and [email protected]%

nanocomposites have obvious response to the HCHO vapors with a high concentrations of 25 ppm, while the pure SnO2 shows no response to HCHO vapors even with a concentration of 100 ppm. When the operating

U

temperature increases to 130 and 160 oC, the HCHO-sensing response of the SnO2@rGO-x% (x=0.25, 0.5 and 1)

N

nanocomposites exhibit an extreme increase, especially for the [email protected]% nanocomposite (Fig. 8b-c and

A

Fig. 9bc). When the operating temperature increases to 180 and 200 oC, the [email protected]% nanocomposite

M

shows the highest HCHO-sensing response, and the pure SnO2 also shows a increasing HCHO-sensing response (Fig. 8de and Fig. 9de). When the operating temperature increases to more than 240 oC, as Fig. 8fh and Fig.

D

9fh, the pure SnO2 sample shows the highest HCHO-sensing response, while the SnO2@rGO-x% (x=0.25, 0.5

TE

and 1) nanocomposites decrease their HCHO-sensing response. From Fig. 8 and Fig. 9, one can find that the addition of a small amount of rGO highly enhances the HCHO-sensing performance of the SnO2@rGO-x%

EP

nanocomposites (x=0.25 and 0.5) at a low operating temperature (i.e., 100160 oC), while the SnO2@rGO-2% nanocomposite exhibits a poor HCHO-sensing performance in the test operating temperatures of 100-300 oC.

CC

When the operating temperature is higher than 240 oC, the HCHO-sensing performance of the pure SnO2 sensor is superior to the SnO2@rGO nanocomposites (Fig. 9).

A

Fig. 10 compares the HCHO-sensing response (Ra/Rg) of the SnO2@rGO nanocomposites as a function of

operating temperature. One can see that the [email protected]% sample has the highest HCHO-sensing response (~35) operating at 130 oC, and the [email protected]% sample has the highest HCHO-sensing response (~40) operating at 160 oC. The nanocomposites of SnO2@rGO-1% and SnO2@rGO-2% show a low HCHO-sensing response during the test temperature range (100300 oC). For the pure SnO2, it shows an obvious HCHO-sensing response at more than 200 oC.

11

Taking Fig. 8, Fig. 9 and Fig. 10 into consideration, we can conclude the following points: (1) The addition of GO nanosheets can improve the HCHO-sensing performance of SnO2 nanocrystals at a relatively low operating temperature (100160 oC); (2) The amount of GO has a crucial effect on the HCHO-sensing performance and a large amount of GO (i.e., 2%) will deteriorate the HCHO-sensing performance of SnO2 nanocrystals; (3) Increasing operating temperature to more than 200 oC deteriorates the HCHO-sensing

SC RI PT

performance of the SnO2@rGO-x% nanocomposites (x=0.25 and 0.5). We investigated the cycling stability of the SnO2@rGO-x% nanocomposites (x=0.25 and 0.5) upon exposure to HCHO vapors with various concentrations (1100 ppm) operating at 160 oC. Fig. 11 shows the typical U-t and (Ra/Rg)-C curves of the [email protected]% and [email protected]% samples. One can find that the SnO2@rGO nanocomposites exhibit a quick, significant and repeatable response to HCHO vapors even with a concentration

U

as low as 1 ppm. For the [email protected]% sample, its response (Ra/Rg) shows a good linear relation with the

N

HCHO concentration (CHCHO / ppm), Ra/Rg = 2.45 + 0.35CHCHO, according to the linear fitting result (R2 = 0.99).

A

For the [email protected]% sample, its response (Ra/Rg) shows a quadratic function relation with the HCHO

M

concentration (CHCHO / ppm), Ra/Rg = 3.4 + 0.45CHCHO - 0.0016C2HCHO, according to the second linear fitting data (R2 = 0.996). Such stable function relation between the response and HCHO concentration is useful for

D

quantitative analysis in detection of formaldehyde.

TE

The response and recovery times are another important aspect of a gas sensor. We calculated the response and recovery times according to their U-t response curves (Fig. 12). Fig. 12a shows the response and recovery

EP

times of the HCHO sensors based on the SnO2@rGO nanocomposites with various amounts of GO upon exposure to 10 ppm HCHO at 160 oC according their U-t curves in Fig. 8c. One can find that their response times

CC

are less than 10 s, while their recovery times lie in a range of 15-30 s. Figs. 12b-c show the response and recovery times of the [email protected]% and [email protected]% samples, respectively, operating at 160 oC upon exposure

A

to formaldehyde vapors with various concentrations according to the U-t curves in Fig. 11. One can find that the response times of the SnO2@rGO nanocomposites upon exposure to HCHO vapors with a concentration range of 1100 ppm are kept less than 20 s, and their recovery times upon exposure to HCHO vapors with a low concentration of less than 10 ppm are also kept less than 20 s. But for the high-concentration formaldehyde vapors (i.e., 25100 ppm), their recovery times exhibit a large value more than 30 s, because of the low desorption rate of formaldehyde molecules at a low temperature of 160 oC.

12

To check the effect of operating temperature on the HCHO-sensing performance of the [email protected]% sample, we placed an emphasis on comparing the U-t curves of its HCHO-sensing performance under various operating temperatures, and Fig. 13a shows the detailed results. One can see that the sensor based on the [email protected]% nanocomposite exhibits obvious response to HCHO vapors during the operating temperature range of 100240 oC, but the optimum operating temperature is 130200 oC. For the HCHO vapor with a large

SC RI PT

concentration of 25 ppm, the [email protected]% sensor can work at 100 oC. At the optimum operating temperature of 160 oC, the detection limit of HCHO concentration of the [email protected]% sensor can be expected to extend to ppb-level if the measuring accuracy of sampling HCHO is allowed. The response of the [email protected]% sensor under different operating temperatures to a 100 ppm HCHO vapor is shown in Fig. 13b. One can see that the HCHO-sensing response operating at 160 oC reaches a summit of 138, much higher than that obtained at the

U

other operating temperatures.

N

Selectivity is one of the essential requirements for a good sensor. We checked the selective response of the

A

[email protected]% sensor towards the common VOCs (i.e., methanol, acetone, ethanol and formaldehyde) and H2. Fig. 14a shows the typical U-t curves of the [email protected]% sensor upon exposure to the above vapors with a

M

concentration of 100 ppm operating at 160 oC. The [email protected]% sensor exhibits a highly rapid and

D

significant sensing response to formaldehyde (i.e., HCHO), when comparing it with the other vapors. Fig. 14b

TE

gives a comparison of their response upon exposure to these vapors. The typical response to formaldehyde is more than 35, much higher than that of methanol, acetone, H2 and ethanol. Therefore, the SnO2@rGO

EP

nanocomposite is an efficient candidate active material to develop high-performance, low operating temperature HCHO sensors for indoor environment-monitoring applications. It is notable that the above selectivity is based on

CC

simple comparisons of the sensing properties of the SnO2@rGO sensors to various vapors with the same concentration under similar operating conditions in a separated manner. If a mixture of these substances is used as

A

the target vapors, the SnO2@rGO sensors are difficult to distinguish one (e.g., HCHO) from the others, because of their similar sensing mechanism (discussed below). 3.3 Mechanism understanding of HCHO-sensing behavior of the SnO2@rGO nanocomposites The SnO2@rGO nanocomposite obtained consists of SnO2 nanoparticles attached on the surfaces of rGO nanosheets, as shown as Fig. 15a. SnO2 is a typical n-type semiconductor in air, and the most accepted mechanism for gas sensing in a n-type semiconductor material involves the adsorption of oxygen species on its

13

surface [60]; the gas-sensing mechanism is then based on changes of the resistance before and after being exposed to the test gas [31, 6162]. The addition of GO highly enhanced the specific surface area (Fig. 7), which is favorable in promoting the adsorption of oxygen species and test gases on SnO2@rGO surfaces. The oxygen adsorbed on the surface directly influences the resistance of the SnO2@rGO-based sensors. When the sensors based on SnO2@rGO are exposed to the air during heating, the oxygen chemically adsorbed on the surface

SC RI PT

undergoes the following reactions according to eq. 1 [63]. When the operating temperature increases, the equilibrium shifts to the right. The oxygen captures electrons from the material, and the resistance of the sensors increases due to the enlarged space-charged layers, forming high-resistance state. On the other hand, when the sensors were exposed to reducing gases such as formaldehyde, the interaction between the reducing gas and the oxygen adsorbed on the surface of the sensors can be represented as eq. 2 [63]. In many studies, it was found that

U

formaldehyde gas can be decomposed into CO2 and H2O, producing electrons at the same time [61, 64]. The

N

possible reaction that takes place on the surface of SnO2@rGO can be shown as eq. 3. The electrons trapped by

A

the adsorbed oxygen are released, forming a low-resistance state. Figs. 15b and c show the possible cases. − 2− O2(ad) ↔ O− 2(ad) ↔ 2O(ad) ↔ 2O(ad)

M

(1)

′ R + O𝑛− (ad) ↔ RO + 𝑛e

(2)

D

′ HCHO(ad) + 2O2− (ad) → CO2 + H2 O(ad) + 4e (3)

TE

In the SnO2@rGO nanocomposite, the conduction path is mainly through SnO2 NPs due to the small amount of rGO, but rGO affects the electrical transport properties of SnO2. When rGO and SnO2 are in contact, electrons

EP

are transferred from SnO2 NPs to rGO because of the difference between their work functions, which are 4.55 eV

CC

for SnO2 and 4.75 eV for rGO [65], resulting in a band-bending of 0.2 eV at the rGO@SnO2 heterojunction. Thus, rGO reduces the quantity of electrons on SnO2 surface (i.e., increases the electron depletion layer), leading to an

A

enhanced resistance changes after the exposure to HCHO vapors (Fig. 15dc). As Fig. 8 and Fig. 9 show, the HCHO-sensing properties of SnO2 can be enhanced by modifying it with a small amount of graphene (rGO). The possible reasons for this may lie in the following aspects: (1) The addition of rGO reduces the particle size of SnO2 nanocrystals and forms a mesoporous structure (Fig. 5 and Fig. 7), and the enhanced specific surface area is helpful to improve the HCHO-sensing performance; (2) The good electric conductance of rGO and the

14

rGO@SnO2 heterojunction may accelerate the electric transfer, promoting the low-temperature HCHO-sensing performance. The HCHO selective sensing performance of the SnO2@rGO sensors can be understood from the point of the oxidation-reduction reaction capacity of the target vapors [31]. Compared with ethanol, methanol and toluene, formaldehyde is much more reductive and thus causes a larger resistance change by reacting with a larger amount

SC RI PT

of adsorbed oxygen. The reducibility of H2 at a low temperature (e.g., 160 oC) is also lower than the HCHO vapor. The amount of rGO has a critical effect on the HCHO-sensing performance of the SnO2@rGO sensors. The suitable amount (i.e., 0.250.5%) of rGO can enhance their specific surface areas and speed up the electron

transferring, thus the HCHO-sensing performance of SnO2@rGO sensors is improved. When the amount of rGO is too much, the resistances of SnO2@rGO sensors before and after exposure to HCHO vapors are small because

U

of the good conductivity of rGO, therefore the resistance change is also small, reducing their response under the

N

same conditions. The hybridization of SnO2 and rGO may improve the absorption of HCHO molecules and the

A

enhance the HCHO-sensing properties of the SnO2 and rGO sensors [66]. Graphene oxide–tin oxide (GO–SnO2)

M

nanocomposites reported by Han et al. [67] showed a similar effect of GO amounts on their gas-sensing performance, and the 0.3%GO@SnO2 sample exhibited the highest response to ethanol vapors. They thought that

D

doping a little of GO in SnO2 could improve capability of oxygen adsorption, decrease the electric resistance and

TE

promote its sensing performance, while excessive amount of GO undermines its sensing capability because the too many adsorbed oxygen ions occupy most active sites on the surface of SnO2 and block the test gas molecules

EP

[67]. Graphene nanosheet (NS)-loaded SnO2 nanofibers (NFs) obtained by an electrospinning process also exhibited an optimal amount (0.5%) of graphene in the detection of C6H6,C7H8, CO, CO2, and H2S [43].

CC

We compare some recent research results about the HCHO-sensing active materials, operating temperatures and response values, and the typical cases are listed in Table 2. Metal semiconductors, such as SnO2, ZnO, In2O3,

A

NiO and Cr2O3, are the major active materials for the detection of HCHO vapors [68-78]. Synthesizing porous nanostructures with high specific surface areas, designing various heterojunctions, and doping metal elements and graphene are the efficient strategies to improve their HCHO-sensing performance. From Table 2, one can find that most of the HCHO sensors require an operating temperature higher than 200 oC, and their responses are not high enough for the practical applications. The [email protected]% sensor reported in the present work shows a

15

better overall performance (i.e., low operating temperature and high response) in the detection of HCHO vapors than most of the other materials listed in Table 2. 4. Conclusions We have developed a simple but robust solvothermal process to synthesize a series of SnO2/rGO nanocomposites with various rGO mass fractions of 02% by using the redox reaction between Sn2+ and

SC RI PT

graphene oxide (GO) at 120 oC for 12 h in H2O/ethanol solution. The addition of GO relieved the agglomeration of SnO2 nanocrystals (3-5 nm) and highly enhanced the specific surface area (SSA) of the SnO2@rGO

nanocomposites. The SSA of [email protected]% was up to 133.1 m2/g, much larger than that (58.3 m2/g) of SnO2 nanocrystals. The SnO2@rGO nanocomposites exhibited a highly selective and sensitive to formaldehyde

(HCHO) vapors at a relatively low operating temperature range of 100-200 oC. The amount of GO added had an

U

obvious effect on the HCHO-sensing performance of the SnO2@rGO nanocomposites, and the [email protected]%

N

sample showed the highest response to HCHO vapors at 100-160 oC. Their recovery / response times were shorter

A

than 20 s when exposed to HCHO vapors with a low concentration of less than 25 ppm. The enhanced HCHO-

M

sensing performance is attributed to the formation of porous SnO2@rGO nanocomposites with high SSAs and suitable electron transfer channels. The present work provides an successful case in designing high-performance

TE

Acknowledgements

D

sensors for harmful gas detection using a simple and safe process.

EP

This work was supported by the National Natural Science Foundation of China (Grant No.51574205, Grant No. 51172211), the National Key Research and Development Program of China (2016YFA02030000), Program

CC

for Science & Technology Innovation Talents in Universities of Henan Province (14HASTIT011), Special Support Program for High-End Talents of Zhengzhou University (ZDGD13001), Program from Dongguan

A

University of Technology (G200906-17), and Plan for Scientific Innovation Talent of Henan Province (154100510003).

16

References

[1] R, Potyrailo, Multivariable sensors for ubiquitous monitoring of gases in the era of Internet of Things and industrial Internet, Chem. Rev. 116 (2016) 11877−11923. [2] J. Kong, N. R. Franklin, C. Zhou, M. G. Chapline, S. Peng, K. Cho, H. Dai, Nanotube molecular wires as

SC RI PT

chemical sensors, Science 287 (2000) 622–625. [3] P. Grundler, Chemical Sensor: An Introduction for Scientists and Engineers, Springer, Berlin , 2007.

[4] D. Chen, Topochemical conversion of inorganic–organic hybrid compounds into low-dimensional inorganic nanostructures with smart control in crystal-sizes and shapes, in: E. Borisenko (EDs.), Crystallization and Material Science of Modern Artificial and Natural Crystals, InTech, Croatia, 2012, pp. 99–138.

U

[5] Y. M. Zhang, J. H. Zhao, T. F. Du, Z. Q. Zhu, J. Zhang, Q. J. Liu, A gas sensor array for the simultaneous detection of multiple VOCs, Sci. Rep. 7 (2017) 1960.

N

[6] F. J. Zhang, G. Qu, E. Mohammadi, J. G. Mei, Y. Diao, Solution-processed nanoporous organic

A

semiconductor thin films: toward health and environmental monitoring of volatile markers, Adv. Funct.

M

Mater. 27 (2017) 1701117.

D

[7] X. Y. Xu, B. Yan, Eu(III)-functionalized ZnO@MOF heterostructures: integration of pre-concentration and efficient charge transfer for the fabrication of a ppb-level sensing platform for volatile aldehyde gases in

TE

vehicles, J. Mater. Chem. A 5 (2017) 2215–2223.

1567.

EP

[8] Y. M. Zhang, J. Zhang, Q. J. Liu, Gas sensors based on molecular imprinting technology, Sensors 17 (2017)

CC

[9] G. Zilberstein, R. Zilberstein, S. Zilberstein, U. Maor, E. Baskin, S. M. Zhang, P. G. Righetti, A miniaturized sensor for detection of formaldehyde fumes, Electrophoresis 38 (2017) 2168–2174.

A

[10] X. S. Wang, H. Y. Li, M. C. Ni, L. Y. Wang, L. Liu, H. Wang, X. X. Guo, Excellent formaldehyde gassensing properties of ruptured Nd-Doped In2O3 porous nanotubes, J. Electron. Mater. 46 (2017) 363–369.

[11] H. Wang, Y. D. Chi, X. H. Gao, S. Lv, X. F. Chu, C. Wang, L. Zhou, X. T. Yang, Amperometric formaldehyde sensor based on a Pd nanocrystal modified C/Co2P electrode, J. Chem. (2017) 2346895. [12] I. Potzelberger, C. C. Mardare, L. M. Uiberlacker, S. Hild, A. I. Mardare, A. W. Hassel, Electrocatalysis on copper-palladium alloys for amperometric formaldehyde sensing, RSC Adv. 7 (2017) 6031–6039.

17

[13] L. Wang, X. Luo, X. Zheng, R. Wang, T. Zhang, Enhanced H2S sensor based on electrospun mesoporous SnO2 nanotubes, RSC Adv. 3 (2013) 9723–9728. [14] D. Chen, L. Yin, L. Ge, B. Fan, R. Zhang, J. Sun, G. Shao, Low-temperature and highly selective NOsensing performance of WO3 nanoplates decorated with silver nanoparticles, Sen. Actuators B: Chem. 185 (2013) 445–455.

SC RI PT

[15] D. Chen, X. Hou, T. Li, L. Yin, B. Fan, H. Wang, X. Li, H. Xu, H. Lu, R. Zhang, J. Sun, Effects of morphologies on acetone-sensing properties of tungsten trioxide nanocrystals. Sens. Actuators B: Chem. 153 (2011) 373–381.

[16] L. Yin, D. Chen, M. Feng, L. Ge, D. Yang, B. Fan, G. Shao, R. Zhang, G. Shao, Hierarchical Fe2O3/WO3 nanostructures with ultrahigh specific surface areas: microwave-assisted synthesis and enhanced H2S-sensing

U

performance, RSC Adv. 5 (2015) 328–337.

N

[17] D. Chen, X. Hou, H. Wen, Y. Wang, H. Wang, X. Li, R. Zhang, H. Lu, H. Xu, S. Guan, J. Sun, L. Gao, The

A

enhanced alcohol-sensing response of ultrathin WO3 nanoplates. Nanotechnology 21 (2010) 035501.

M

[18] D. Chen, L. Ge, L. Yin, H. Shi, D. Yang, J. Yang, R. Zhang, G. Shao, Solvent-regulated solvothermal synthesis and morphology-dependent gas-sensing performance of low-dimensional tungsten oxide

D

nanocrystals, Sens. Actuators B: Chem. 205 (2014) 391–400.

25692–25697.

TE

[19] Y. Tang, J. Ma, In2O3 nanostructures: synthesis and chlorobenzene sensing properties, RSC Adv. 4 (2014)

EP

[20] W. Tang, Sensing mechanism of SnO2/ZnO nanofibers for CH3OH sensors: heterojunction effects, J. Phys. D: Appl. Phys. 50 (2017) 475105.

CC

[21] S. Y. Jeong, J. W. Yoon, T. H. Kim, H. M. Jeong, C. S. Lee, Y. C. Kang, J. H. Lee, Ultra-selective detection of sub-ppm-level benzene using Pd-SnO2 yolk-shell micro-reactors with a catalytic Co3O4 overlayer for

A

monitoring air quality, J. Mater. Chem. A 5 (2017) 1446–1454. [22] X. Xing, X. Xiao, L. Wang, Y. Wang, Highly sensitive formaldehyde gas sensor based on hierarchically porous Ag-loaded ZnO heterojunction nanocomposites, Sens. Actuators B: Chem. 247 (2017) 797–806. [23] W. Zhang, X. Cheng, X. Zhang, Y. Xu, S. Gao, H. Zhao, L. Huo, High selectivity to ppb-level HCHO sensor based on mesoporous tubular SnO2 at low temperature, Sens. Actuators B: Chem. 247 (2017) 664–672.

18

[24] Y. J. Kwon, H. G. Na, S. Y. Kang, M. S. Choi, J. H. Bang, T. W. Kim, A. Mirzaei, H. W. Kim, Attachment of Co3O4 layer to SnO2 nanowires for enhanced gas sensing properties, Sens. Actuators B: Chem. 239 (2017)180–192. [25] Y. Gao, Q. H. Kong, J. H. Zhang, G. C. Xi, General fabrication and enhanced VOC gas-sensing properties of hierarchically porous metal oxides, RSC Adv. 7 (2017) 35897–35904.

SC RI PT

[26] Z. Ul Abideen, J.-H. Kim, S. S. Kim, Optimization of metal nanoparticle amount on SnO2 nanowires to achieve superior gas sensing properties, Sens. Actuators B: Chem. 238 (2017) 374–380.

[27] N. Quan Thi Minh, D. Nguyen Van, P. Nguyen Thi, T. Nguyen Ngoc, H. Chu Manh, H. Nguyen Duc, H. Nguyen Van, Superior enhancement of NO2 gas response using n-p-n transition of carbon nanotubes/SnO2 nanowires heterojunctions, Sens. Actuators B: Chem. 238(2017) 1120–1127.

U

[28] Y. J. Kwon, S. Y. Kang, A. Mirzaei, M. S. Choi, J. H. Bang, S. S. Kim, H. W. Kim, Enhancement of gas

A

Actuators B: Chem. 249 (2017) 656–666 .

N

sensing properties by the functionalization of ZnO-branched SnO2 nanowires with Cr2O3 nanoparticles, Sens.

M

[29] J. H. Kim, Y. F. Zheng, A. Mirzaei, H. W. Kim, S. S. Kim, Synthesis and selective sensing properties of

D

rGO/Metal-Coloaded SnO2 nanofibers, J. Electron. Mater. 46 (2017) 3531–3541.

TE

[30] S. Cui, Z. Wen, X. Huang, J. Chang, J. Chen, Stabilizing MoS2 nanosheets through SnO2 nanocrystal decoration for high-performance gas sensing in air, Small 11 (2015) 2305–2313.

EP

[31] Y. M. Zhang, J. Zhang, J. H. Zhao, Z. Q. Zhu, Q. J. Liu, Ag-LaFeO3 fibers, spheres, and cages for

CC

ultrasensitive detection of formaldehyde at low operating temperatures, Phys. Chem. Chem. Phys. 19 (2017) 6973–6980.

[32] X. H. Tang, N. Mager, B. Vanhorenbeke, S. Hermans, J. P. Raskin, Defect-free functionalized graphene

A

sensor for formaldehyde detection, Nanotechnology 28 (2017) 055501.

[33] D. Cortes-Arriagada, N. Villegas-Escobar, S. Miranda-Rojas, A. Toro-Labbe, Adsorption/desorption process of formaldehyde onto iron doped graphene: a theoretical exploration from density functional theory calculations, Phys. Chem. Chem. Phys. 19 (2017) 4179–4189.

19

[34] X. Li, J. Wang, D. Xie, J. L. Xu, Y. Xia, W. W. Li, L. Xiang, Z. M. Li, S. W. Xu, S. Komarneni, Flexible room-temperature formaldehyde sensors based on rGO film and rGo/MoS2 hybrid film, Nanotechnology 28 (2017) 325501. [35] L. Guo, Z. Yang, Y. Li, B. Zu, X. Dou, Sensitive, real-time and anti-interfering detection of nitro-explosive vapors realized by ZnO/rGO core/shell micro-Schottky junction, Sens. Actuators B:Chem. 239(2017) 286–

SC RI PT

294. [36] N. M. Triet, L. T. Duy, B. U. Hwang, A. Hanif, S. Siddiqui, K. H. Park, C. Y. Cho, N. E. Lee, High-

performance schottky diode gas sensor based on the heterojunction of three-dimensional nanohybrids of reduced graphene oxide-vertical ZnO Nanorods on an AIGaN/GaN layer, ACS Appl. Mater. Interfaces 9 (2017) 30722–30732.

U

[37] J. Zhang, J. Wu, X. Wang, D. Zeng, C. Xie, Enhancing room-temperature NO2 sensing properties via

N

forming heterojunction for NiO-rGO composited with SnO2 nanoplates, Sens. Actuators B:Chem. 243 (2017)

A

1010–1019.

M

[38] P. Yitian, Z. Lulu, Z. Kun, L. Cong, Enhancing performances of a resistivity-type hydrogen sensor based on Pd/SnO2/RGO nanocomposites, Nanotechnology 28 (2017) 215501.

D

[39] Q. Hao, T. Liu, J. Y. Liu, Q. Liu, X. Y. Jing, H. Q. Zhang, G. Q. Huang, J. Wang, Controllable synthesis and

EP

14192–14199.

TE

enhanced gas sensing properties of a single-crystalline WO3-rGO porous nanocomposite, RSC Adv. 7 (2017)

[40] X. F. Chu, J. L. Wang, J. Zhang, Y. P. Dong, W. Q. Sun, W. B. Zhang, L. S. Bai, Preparation and gas-

CC

sensing properties of SnO2/graphene quantum dots composites via solvothermal method, J. Mater. Sci. 52 (2017) 9441–9451.

A

[41] L. Yin, D. Chen, X. Cui, L. Ge, J. Yang, L. Yu, B. Zhang, R. Zhang, G. Shao, Normal-pressure microwave rapid synthesis of hierarchical SnO2@rGO nanostructures with superhigh surface areas as high-quality gassensing and electrochemical active materials. Nanoscale 6 (2014) 13690–13700. [42] Q. Feng, X. Li, J. Wang, Percolation effect of reduced graphene oxide (rGO) on ammonia sensing of rGOSnO2 composite based sensor, Sens. Actuators B: Chem. 243 (2017) 1115–1126.

20

[43] Z. U. Abideen, J. Y. Park, H. W. Kim, S. S. Kim, Graphene-loaded tin oxide nanofibers: optimization and sensing performance, Nanotechnology 28 (2017) 035501. [44] Z. L. Song, Z. R. Wei, B. Wang, Z. Luo, S. M. Xu, W. K. Zhang, H. X. Yu, M. Li, Z. Huang, J. F. Zang, F. Yi, H. Liu, Sensitive Room-temperature H2S gas sensors employing SnO2 quantum wire/reduced graphene oxide nanocomposites, Chem. Mater. 28 (2016) 1205–1212.

SC RI PT

[45] K. Dou, G. Chen, F. B. Yu, Y. X. Liu, L. X. Chen, Z. P. Cao, T. Chen, Y. L. Li, J. M. You, Bright and

sensitive ratiometric fluorescent probe enabling endogenous FA imaging and mechanistic exploration of indirect oxidative damage due to FA in various living systems, Chem. Sci. 8 (2017) 7851–7861.

[46] Z. Li, Y. Q. Xu, H. L. Zhu, Y. Qian, Imaging of formaldehyde in plants with a ratiometric fluorescent probe, Chem. Sci. 8 (2017) 5616–5621.

U

[47] K. Vellingiri, A. Deep, K.-H. Kim, D. W. Boukhvalov, P. Kumar, Q. Yao, The sensitive detection of

N

formaldehyde in aqueous media using zirconium-based metal organic frameworks, Sens. Actuators B:Chem.

A

241 (2017) 938–948.

M

[48] Z. Li, M. Fang, M. K. LaGasse, J. R. Askim, K. S. Suslick, Colorimetric recognition of aldehydes and ketones, Angew. Chem. Int. Ed. 56 (2017) 9860–9863.

D

[49] S. Ishihara, J. Labuta, T. Nakanishi, T. Tanaka, H. Kataura, Amperometric detection of sub-ppm

TE

formaldehyde using single-walled carbon nanotubes and hydroxylamines: A referenced chemiresistive

EP

system, ACS Sens. 2 (2017) 1405–1409. [50] T. Zhou, T. Zhang, R. Zhang, Z. Lou, J. Deng, L. Wang, Hollow ZnSnO3 cubes with controllable shells

CC

enabling highly efficient chemical sensing detection of formaldehyde vapors, ACS Appl. Mater. Interfaces 9 (2017) 14525–14533.

A

[51] X. G. San, G. D. Zhao, G. S. Wang, Y. B. Shen, D. Meng, Y. J. Zhang, F. L. Meng, Assembly of 3D flowerlike NiO hierarchical architectures by 2D nanosheets: synthesis and their sensing properties to formaldehyde, RSC Adv. 7 (2017) 3540–3549. [52] R. Dong, L. P. Zhang, Z. Y. Zhu, J. D. Yang, X. L. Gao, S. R. Wang, Fabrication and formaldehyde sensing performance of Fe-doped In2O3 hollow microspheres via a one-pot method, CrystEngComm 19 (2017) 562– 569.

21

[53] C. Ding, Y. Ma, X. Lai, Q. Yang, P. Xue, F. Hu, W. Geng, Ordered large-pore mesoporous Cr2O3 with ultrathin framework for formaldehyde sensing, ACS Appl. Mat. Interfaces 9 (2017) 18170–18177. [54] F. Gyger, M. Hübner, C. Feldmann, N. Barsan, U. Weimar, Nanoscale SnO2 hollow spheres and their application as a gas-sensing material, Chem. Mater. 22 (2010) 4821–4827. [55] A. Diéguez, A. Romano-Rodríguez, A. Vilà , J. R. Morante, The complete Raman spectrum of nanometric

SC RI PT

SnO2 particles. J. Appl. Phys. 90 (2001) 1550–1557.

[56] J. Zhang, J. Wu, X. Wang, D. Zeng, C. Xie, Enhancing room-temperature NO2 sensing properties via

forming heterojunction for NiO-rGO composited with SnO2 nanoplates. Sens. Actuators B : Chem. 243 (2017) 1010–1019.

[57] S. Chen, Y. Qiao, J. L. Huang, H. L. Yao, Y. L. Zhang, Y. Li, J. P. Du, W. B. Fan, One-pot synthesis of

U

mesoporous spherical SnO2@graphene for high-sensitivity formaldehyde gas sensors, RSC Adv. 6 (2016)

N

25198–25202.

A

[58] Y. Yuan, Y. Wang, M. Wang, J. Liu, C. Pei, B. Liu, H. Zhao, S. Liu, H. Yang, Effect of unsaturated Sn

M

atoms on gas-sensing property in hydrogenated SnO2 nanocrystals and sensing mechanism, Sci. Rep. 7 (2017) 1231.

D

[59] K. S. W. Sing, D. H. Everett , R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol, T. Siemieniewska,

TE

Reporting physisorption data for gas/solid systems with special reference to the determination of surface area

EP

and porosity, Pure Appl. Chem. 57 (1985) 603–619. [60] Y.-X. Li, Z. Guo, Y. Su, X.-B. Jin, X. H. Tang, J.-R. Huang, X.-J. Huang, M.-Q. Li, J.-H. Liu, Hierarchical

CC

morphology-dependent gas-sensing performances of three-dimensional SnO2 nanostructures, ACS Sens. 2 (2017) 102–110.

A

[61] R. K. Mishra, G. Murali, T. H. Kim, J. H. Kim, Y. J. Lim, B. S. Kim, P. P. Sahay, S. H. Lee, Nanocube In2O3@RGO heterostructure based gas sensor for acetone and formaldehyde detection, RSC Adv. 7 (2017) 38714–38724. [62] Z. W. Chen, Y. Y. Hong, Z. D. Lin, L. M. Liu, X. W. Zhang, Enhanced formaldehyde gas sensing properties of ZnO nanosheets modified with graphene, Electron. Mater. Lett. 13 (2017) 270–276.

22

[63] C. Xiangfeng, L. Xingqin and M. Guangyao, Effects of CdO dopant on the gas sensitivity properties of ZnFe2O4 semiconductors, Sens. Actuators B: Chem. 65 (2000) 64–67. [64] T. Chen, Q. J. Liu, Z. L. Zhou, Y. D. Wang, The fabrication and gas-sensing characteristics of the formaldehyde gas sensors with high sensitivity, Sens. Actuators B: Chem. 131 (2008) 301–305. [65] C. A. Zito, T. M. Perfecto, D. P. Volanti, Impact of reduced graphene oxide on the ethanol sensing

SC RI PT

performance of hollow SnO2 nanoparticles under humid atmosphere, Sens. Actuators B: Chem. 244 (2017) 466–474.

[66] T. Zhang, H. Sun, F. D. Wang, W. Q. Zhang, J. M. Ma, S. W. Tang, H. W. Gong, J. P. Zhang, Reversible adsorption/desorption of the formaldehyde molecule on transition metal doped graphene by controlling the external electric field: first-principles study, Theor. Chem. Acc. 136 (2017) 134.

U

[67] M. M. Han, W. C. Liu, Y. Qu, L. Y. Du, H. S. Wei, Graphene oxide-SnO2 nanocomposite: synthesis,

N

characterization, and enhanced gas sensing properties, J. Mater. Sci. - Mater. Electron. 28 (2017) 16973–

A

16980.

M

[68] Y. He, H. Li, X. Zou, N. Bai, Y. Cao, Y. Cao, M. Fan, G.-D. Li, Platinum dioxide activated porous SnO2 microspheres for the detection of trace formaldehyde at low operating temperature, Sens. Actuators B:

D

Chem. 244 (2017) 475–481.

TE

[69] W. W. Guo, Design of gas sensor based on Fe-Doped ZnO nanosheet-spheres for low concentration of formaldehyde detection, J. Electrochem. Soc. 163 (2016) B517–B525.

EP

[70] C. Gu, Y. Cui, L. Wang, E. Sheng, J.-J. Shim J. Huang, Synthesis of the porous NiO/SnO2 microspheres and microcubes and their enhanced formaldehyde gas sensing performance, Sens. Actuators B:Chem. 241 (2017)

CC

298–307.

A

[71] Q. J. Fu, M. M. Ai, Y. Duan, L. M. Lu, X. Tian, D. D. Sun, Y. Y. Xu, Y. Q. Sun, Synthesis of uniform porous NiO nanotetrahedra and their excellent gas-sensing performance toward formaldehyde, RSC Adv. 7 (2017) 52312–52320.

[72] X. Li, J. Liu, H. Guo, X. Zhou, C. Wang, P. Sun, X. Hu, G. Lu, Au@In2O3 core–shell composites: a metal– semiconductor heterostructure for gas sensing applications, RSC Adv. 5 (2015) 545–551. [73] E.-X. Chen, H. Yang, J. Zhang, Zeolitic imidazolate framework as formaldehyde gas sensor, Inorg. Chem. 53 (2014) 5411–5413.

23

[74] X. Shen, L. Guo, G. Zhu, C. Xi, Z. Ji, H. Zhou, Facile synthesis and gas-sensing performance of Sr- or Fedoped In2O3 hollow sub-microspheres, RSC Adv. 5 (2015) 64228–64234. [75] H. Yang, S. Wang, Y. Yang, Zn-doped In2O3 nanostructures: preparation, structure and gas-sensing properties, CrystEngComm 14 (2012) 1135–1142. [76] H. Y. Du, P. J. Yao, Y. H. Sun, J. Wang, H. S. Wang, N. S. Yu, Electrospinning hetero-nanofibers

SC RI PT

In2O3/SnO2 of homotype heterojunction with high gas sensing activity, Sensors 17 (2017) 1822. [77] S. Bai, J. Guo, X. Shu, X. Xiang, R. Luo, D. Li, A. Chen, C. C. Liu, Surface functionalization of Co3O4 hollow spheres with ZnO nanoparticles for modulating sensing properties of formaldehyde, Sens. Actuators B:Chem. 245 (2017) 359–368.

[78] I. Castro-Hurtado, J. Gonzalez-Chavarri, S. Morandi, J. Sama, A. Romano-Rodriguez, E. Castano, G. G.

U

Mandayo, Formaldehyde sensing mechanism of SnO2 nanowires grown on-chip by sputtering techniques,

A

N

RSC Adv. 6(2016) 18558–18566.

M

Biographies

D

Deliang Chen has been a professor at Zhengzhou University since 2013. He received his BS and MS degrees

TE

from Central South University in 1999 and 2002, respectively. Then he moved to Shanghai Institute of Ceramics, Chinese Academy of Sciences, where he received his PhD degree in Materials Science and Engineering in 2005.

EP

From April 2005 to March 2007, he moved to Waseda University to do postdoctoral research as Visiting Research Associate. From April 2007 to December 2012, he worked at Zhengzhou University as an associate

CC

professor. From March 2011 to March 2012, he move to KAIST as a visiting scholar supported by China government. His current research interests focus on the controllable synthesis of low-dimensional inorganic

A

nanomaterials via intercalation chemistry, microwave chemistry and sonochemistry, and their applications in chemical sensors, photocatalysts, electrocatalysts and energy storage. He has published more than 100 ISI papers and his H-index is about 25 from the ESI citation record. Xiaoru Rong is currently a master course student at Zhengzhou University, majoring in Materials Science and Engineering. Her current research focuses on metal oxides/graphene nanocomposites for gas-sensing applications.

24

Geping Qu is currently a master course student at Zhengzhou University, majoring in Materials Science and Engineering. Her current research focuses on metal oxide semiconductor nanocomposites for gas-sensing applications. Tao Li is currently a researcher at Dongguan University of Science and Technology. He received his MS degree from Zhengzhou University in 2012, majoring in Materials Science and Engineering. Her current research

SC RI PT

focuses on functional nanocomposites for catalytic applications.

Rui Zhang has been a professor at Zhengzhou University and Zhengzhou Institute of Aeronautical Industry Management. He received his BS and MS degrees from Tsinghua University in 1990 and 1995, respectively, and received his PhD degree from Shanghai Institute of Ceramics, Chinese Academy of Sciences in 2004. His current research interests include metal-ceramics, electronic ceramics and ceramic matrix composites.

U

Jing Sun has been a professor at Shanghai Institute of Ceramics, CAS since 2005. She received her MS degree

N

from Changchun Institute of Applied Chemistry, CAS in 1994, and received her PhD degree from Shanghai

A

Institute of Ceramics, CAS in 1997. Her current research interests are focused on photocatalysts to decompose

TE

D

M

VOCs and low-dimensional functional nanomaterials.

Table 1. A summary of Raman data, morphology based on SEM observation and HCHO-sensing properties of

EP

SnO2 and SnO2@rGO nanocomposites.

Position of D band / cm-1

Position of G band / cm-1

I /I

Morphology

Pristine GO

1346

1598

1.04

SnO2

/

/

/

1352

1608

1.44

1348

1602

1.36

1345

1601

1.36

1340

1597

1.40

Sheet Large spherical Small particle Small particle Small particle Small particle

A

CC

Sample

[email protected]% [email protected]% SnO2@rGO1% SnO2@rGO2%

D G

Optimal operating temperature /oC

Response at HCHO 100 ppm

/

/

200300

15.5

~200

38

~160

33-138

~160

17

/

3.6

25

Table 2. Comparison of formaldehyde-sensing performance between SnO2@rGO nanocomposites and other related MOS sensing materials.

Operating temperature/oC

Gas concentration /ppm

Response (Ra/Rg)

Reference

[email protected]%

160

100

138

This work

Mesoporous Cr2O3

230

9

GO-0.3%/SnO2

150

200

PtO2/SnO2-5 mol%

100

100

Nd-doped In2O3

240

100

Mesoporous SnO2/graphene

120

100

Hollow ZnSnO3 Cubes

220

100

5.5wt% Fe-doped ZnO spheres

300

10

Porous NiO/SnO2 microspheres

200

Porous SnO2 microtubes

92

Cone-shaped SnO2 nanostructures

325

NiO nanotetrahedra

250

Au@In2O3

200

ZIF-67

150

Fe-doped In2O3 hollow microspheres

90

[67]

70

[68]

46.8

[10]

45

[57]

37.2

[50]

33

[69]

100

27.6

[70]

100

26.2

[37]

100

24

[60]

200

22.7

[71]

100

17

[72]

100

14

[73]

260

100

12.9

[52]

200

100

12

[62]

200

100

9.4

[74]

260

100

9

[75]

In2O3/SnO2 heteronanofiber

275

10

8.7

[76]

In2O3 nanowires

430

100

6.6

[19]

ZnO nanoparticles/Co3O4 hollow spheres

160

10

5.8

[77]

SnO2 nanowires

270

10

2.5

[78]

In2O3@RGO heterostructures

225

100

1.8

[61]

Graphene/ZnO nanosheets Sr-doped In2O3

A

CC

EP

ZnO-doped In2O3

M

A

N

U

[53]

TE

119

D

SC RI PT

Material system

26

List of Figure Captions Fig. 1. (a) Schematic description of the process for synthesizing SnO2@rGO nanocomposites as formaldehyde (HCHO)-sensing active material; (b) Equivalent circuit diagram of the gas-sensing test system for HCHO)sensing evaluation.

SC RI PT

Fig. 2. XRD patterns of the SnO2@rGO nanocomposites: (a) GO, (b) SnO2, (c) [email protected]%, (d) [email protected]%, (e) SnO2@rGO-1%, and (f) SnO2@rGO-2%.

Fig. 3. (a) Raman spectra of the SnO2@rGO nanocomposites: (A) GO, (B) SnO2, (C) [email protected]%, (D) [email protected]%, (E) SnO2@rGO-1%, and (F) SnO2@rGO-2%; (B) Detailed Raman spectrum of SnO2

U

nanocrystals.

N

Fig. 4. XPS spectra of [email protected]%: (a) survey scan, (b) Sn 3d, (c) C1s, and (d) O1s.

A

Fig. 5. SEM images of the SnO2@rGO nanocomposites: (a) GO, (b) SnO2, (c) [email protected]%, (d)

M

[email protected]%, (e) SnO2@rGO-1%, and (f) SnO2@rGO-2%.

D

Fig. 6. Typical TEM observations of [email protected]%: (a-b) Low-magnification TEM images, (c) HRTEM

TE

image, and SAED pattern.

Fig. 7. (a) Nitrogen adsorption-desorption isotherms and (b) pore-size distribution curves of [email protected]%

EP

(A) and SnO2 (B).

CC

Fig. 8. HCHO sensing curves of the SnO2@rGO nanocomposites (SnO2@rGO-x%, x=0, 0.25, 0.5, 1 and 2) at different operation temperatures: (a) 100 oC, (b) 130 oC, (c) 160 oC, (d) 180 oC, (e) 200 oC, (f) 240 oC, (g) 260 oC,

A

and (h) 300 oC.

Fig. 9. Response of the SnO2@rGO nanocomposites (SnO2@rGO-x%, x=0, 0.25, 0.5, 1 and 2) upon exposure to HCHO vapors with various concentrations (5-100 ppm) at different operation temperatures: (a) 100 oC, (b) 130 o

C, (c) 160 oC, (d) 180 oC, (e) 200 oC, (f) 240 oC, (g) 260 oC, and (h) 300 oC.

27

Fig. 10. HCHO-sensing response as a function of operation temperature in the presence of SnO2@rGO nanocomposites: (a) SnO2, (b) [email protected]%, (c) [email protected]%, (d) SnO2@rGO-1%, and (e) SnO2@rGO-2%. Fig. 11. HCHO-sensing stability of the SnO2@rGO nanocomposites operating at 160 oC: (a-b) [email protected]%,

SC RI PT

(c-d) [email protected]%. Fig. 12. Response and recovery times of the HCHO sensors based on the SnO2@rGO nanocomposites: (a)

Various samples upon exposure to 10 ppm HCHO at 160 oC according the U-t curves in Fig. 8c; (b) [email protected]% and (c) [email protected]% upon exposure to HCHO vapors with various concentrations at 160 oC.

Fig. 13. Typical HCHO-sensing data of the [email protected]% sample: (a) U-t response curves obtained at various

N

U

operation temperatures, and (b) typical response to 100 ppm HCHO at various temperatures.

Fig. 14. Selective response to some typical VOC vapors (100 ppm at 160 oC) of the sensor based on the sample of

M

A

[email protected]%.

Fig. 15. Proposed HCHO-sensing mechanism for SnO2@rGO nanocomposites: (a) Schematic of the SnO2@rGO

D

nanocomposite; (b) Possible gas-sensing reaction and electron transfer in air; (c) Possible gas-sensing reaction

TE

and electron transfer in HCHO vapors; (d) Energy band structure diagrams of rGO and SnO2 before contacting with each other; (e) Energy band structure diagram of SnO2@rGO nanocomposite in air; (f) Energy band

A

CC

EP

structure diagram of SnO2@rGO nanocomposite in HCHO vapors.

28

A

CC

EP

TE

D

Fig. 1 by Xiaoru Rong, Deliang Chen, et al..

M

A

N

U

SC RI PT

List of Figures

29

SC RI PT U N A

A

CC

EP

TE

D

M

Fig. 2 by Xiaoru Rong, Deliang Chen, et al..

30

SC RI PT U N A M D TE EP CC A Fig. 3 by Xiaoru Rong, Deliang Chen, et al..

31

SC RI PT U N

A

CC

EP

TE

D

M

A

Fig. 4 by Xiaoru Rong, Deliang Chen, et al..

32

SC RI PT U

A

CC

EP

TE

D

M

A

N

Fig. 5 by Xiaoru Rong, Deliang Chen, et al..

33

SC RI PT U N A M D TE

A

CC

EP

Fig. 6 by Xiaoru Rong, Deliang Chen, et al..

34

SC RI PT U N A M A

CC

EP

TE

D

Fig. 7 by Xiaoru Rong, Deliang Chen, et al..

35

SC RI PT

A

CC

EP

TE

D

M

A

N

U

Fig. 8 by Xiaoru Rong, Deliang Chen, et al..

Fig. 9 by Xiaoru Rong, Deliang Chen, et al..

36

SC RI PT U N A M D

A

CC

EP

TE

Fig. 10 by Xiaoru Rong, Deliang Chen, et al..

37

SC RI PT U N A M

A

CC

EP

TE

D

Fig. 11 by Xiaoru Rong, Deliang Chen, et al..

38

SC RI PT U N A M D TE EP CC A Fig. 12 by Xiaoru Rong, Deliang Chen, et al..

39

SC RI PT U N A M D TE EP CC A Fig. 13 by Xiaoru Rong, Deliang Chen, et al..

40

SC RI PT U N A M D TE EP

A

CC

Fig. 14 by Xiaoru Rong, Deliang Chen, et al..

41

SC RI PT U N

A

CC

EP

TE

D

M

A

Fig. 15 by Xiaoru Rong, Deliang Chen, et al..

42