Mesoporous Ultrathin SnO2 Nanosheets in Situ Modified by Graphene

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Functional Nanostructured Materials (including low-D carbon)

Mesoporous Ultrathin SnO2 Nanosheets In-situ Modified by Graphene Oxide for Extraordinary Formaldehyde Detection at Low Temperature Ding Wang, Liang Tian, Huijun Li, Kechuang Wan, Xin Yu, Ping Wang, Aiying Chen, Xianying Wang, and Junhe Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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ACS Applied Materials & Interfaces

Mesoporous Ultrathin SnO2 Nanosheets In-situ Modified by Graphene Oxide for Extraordinary Formaldehyde Detection at Low Temperature Ding Wang1,2†, Liang Tian1†, Huijun Li1, Kechuang Wan1, Xin Yu1, Ping Wang1, Aiying Chen1, Xianying Wang1,2*, Junhe Yang1,2* 1School

of Material Science & Engineering, University of Shanghai for Science and Technology,

Shanghai, 200093, China. 2Shanghai

Innovation Institution for Materials, Shanghai, 200444, China.

KEYWORDS: SnO2 nanosheets, graphene oxide, formaldehyde gas sensors, two dimensional structure, nanocomposites

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Abstract In this article, we demonstrated an extremely high sensitivity formaldehyde (HCHO) gas sensor, where the graphene oxide (GO) in-situ modified two dimensional (2D) SnO2 nanosheets with inplane mesopores was utilized as the sensing materials. The sensor response (Ra/Rg) was larger than 2000 toward 100 ppm HCHO at 60 °C. In addition, the selectivity for detecting HCHO was excellent against other interferences including ethanol, acetone, methanol, toluene, ammonia and water etc. The outstanding sensing performance of 2D mesoporous GO/SnO2 nanosheets was attributed to the synergism of the sensitizer effect of GO, large surface areas of 2D nanostructure, suitable particle size and abundant in-plane mesopores. The high sensitivity, high selectivity and low working temperature of the sensor reported here endowed it a great potential in selective detection of HCHO. Meanwhile, the design and synthesis of GO/SnO2 nanocomposites will provide new paradigms in future development of HCHO sensitive materials.


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1. Introduction As a kind of industrial chemicals, formaldehyde (HCHO) is widely used in medical treatment, home decoration, agricultural breeding, chemical industry, wood industry, textile industry and so on 1. It is reported that the HCHO has strong carcinogenic and cancer-promoting effects even at low levels for short periods 2. Hence, the HCHO has become a major health threat due to the unremitting release from various above sources 3. On the other hand, with the development of internet of things, smart home/city and other emerging industries, the demand for HCHO sensors is also increasing day by day, and the detection of HCHO has become an important research topics. There are many techniques for detection of HCHO, such as phenol reagent

method,

gas

chromatography,

polarography,

colorimetry,

fluorescence,

and

spectrophotometry method, etc. Compared with these methods that require collection before analysis, gas sensors with real-time analysis function are undoubtedly the optimal choice. Actually, conductometric sensor, chemical/electrochemical sensor, mass sensor and piezolelectric sensor have reported for gases detection

4-7.

Among them, semiconductor gas

sensor is the most commonly used. Metal oxide semiconductor (MOS) sensitive material is widely used in gas sensors because of its high specific surface area, abundant active sites and oriented electron conduction 8. However, for some special gas detection, especially for HCHO, the poor sensitivity and low selectivity of MOS sensors are insufficient to practical application. The structural regulation and composite sensitization have been widely utilized to improve their sensitive property. Various nanostructures have been synthesized sequentially for HCHO gas detection,

such

as

nanowires,

nanotubes,

homogeneous/heterogeneous hierarchical structure

9-12.

nanosheets,

nano-flowers

and

Meanwhile, many reports have shown



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that carbon materials such as graphene oxide (GO), reduced graphene oxide (RGO), carbon nanotubes and g-C3N4 as additives can greatly improve their sensitive property

13-17.

All in all,

these strategies are implemented by regulating the particle size, increasing specific surface area, constructing mesoporous structure, or compound modification, etc. Recently, 2D MOS nanosheets with quasi-graphene structures have been successfully prepared, which provide new concepts in enhancing the performance of gas-sensitive materials 18-20.

Among these 2D MOS sensitive materials, SnO2, ZnO, TiO2, Co3O4 and WO3 nanosheets

are popular sensing materials 20. 2D MOS nanosheets can be prepared via metal ions adsorption on GO and a subsequent heat treatment of GO/metal precursor. Due to unique crystal nucleation and growth process, mesopores can be formed in the in-plane surface of the 2D MOS. Although 2D MOS prepared by GO templated method had been reported in a recent, most of them focused on preparation of 2D MOS rather than 2D GO/MOS composites18-21. In addition, more attention had been paid to the application of 2D MOS in energy storage, Li-ion battery anodes, catalysis and so on, while research about their sensing performance was neglected

18-21.

Furthermore, the

adhesion between MOS and GO are more tight than that of composites prepared by traditional physical mixing or hydrothermal methods, which is conductive to electron transport 22. Therefore, through controlling the thermal treatment conditions, in-situ modification of 2D MOS with GO of different contents can be realized and 2D MOS mesoporous nanosheets modified by GO could be simply synthesized. The new 2D nanocomposite, having large specific surface area, abundant in-plane mesopores and unique electrical properties, is a promising gas sensitive material. In view of the advantages of 2D MOS nanosheets, herein, 2D mesoporous GO/SnO2 nanosheets were prepared by utilizing GO as both template and sensitizer. A series of GO/SnO2


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nanosheets were obtained by varying the heat-treated temperature. The structure, morphology and composition of 2D GO/SnO2 nanosheets were studied through the corresponding characterization methods. Sensitive properties of a series of GO/SnO2 nanosheets were comparatively studied based on response/recovery characteristic curves. The effects of calcination process on the sensing performance were also comprehensively studied. Finally, the sensitive mechanism was explored from structural characteristics of GO/SnO2 and GO regulation. 2. Experimental Details All reagents were purchased from Aladdin Reagent (Shanghai) Co. Ltd., and were used as received and without further purification. The preparation of GO can be seen from the Supporting Information (S1). The mesoporous GO/SnO2 nanosheets were prepared via a facile template method. In a typical process, 100 mg of GO was dispersed in 200 mL absolute ethanol with the aid of ultrasonic. After ultrasonic treatment for 1 h, 2.5 mol of dibutyltin dilaurate (DBTDL) was putted in the mixture solution by pipette and kept stirring overnight. The products were centrifuged and washed with absolute alcohol 4-5 times for removing excessive DBTDL. Then the centrifugation products were dried for overnight at 80 °C. Subsequently, the gas sensitive materials were obtained by calcinating the dried powder in a muffle furnace at 475 °C for 2 h. In addition, the dried powder was also calcined at other temperatures (425 °C, 450 °C, 500 °C and 525 °C) to regulate the content of GO. These samples were named as GO/SnO2 NS-T (T= 425, 450, 475, 500 and 525 °C). 3. Results and Discussion 3.1 Structure and morphology characterization



The preparation process of 2D mesoporous GO/SnO2 nanosheets is shown in Figure 1. The negative charges near the surface of GO were confirmed by Zeta potential measurements. The negative charges could be attributed to functional groups at the surface of GO. Therefore, the templates and DBTDL were attracted due to the electrostatic force, resulting in that the DBTDL firmly attached and uniformly distributed on the surface of GO and cannot be departured by ultra-sonication or stirring. The mesoporous ultrathin SnO2 nanosheets in-situ modified by GO sensitive material was obtained after calcination treatment. The thermal behavior of the GO/DBTDL was researched using TG-DSC. As shown in Figure 2(a), four distinct stages with a total weight loss of 93 wt% could be observed. The first stage (∼10 wt% weight loss) below 150 °C corresponded to the desorption of adsorbed water. And then, the abrupt weight loss stage (∼25 wt%) between 150~250 °C might be related to the decomposition of organotin23. The third stage (from 250 °C to 425 °C) could be subdivided into two parts, decomposition of Sn precursor and GO. The subsequent weight loss (39 wt%) at around 510 °C was the combustion of GO. No obvious weight loss above 600 °C indicated the completely removal of GO. Thus, the thermal decomposition process includes the evaporation of adsorbed water, the decomposition of DBTDL and the combustion of GO. The weight percent of the adsorbed water, DBTDL and GO in the precursor were estimated from TG-DSC as 10 wt%, 36.33 wt% and 53.67 wt%, respectively. The crystalline structure of the as-synthesized GO/SnO2 NS-475 was confirmed by XRD. In Figure 2(b), these diffraction peaks were fitted well with the JCPDS card (No. 41-1445) of tetragonal rutile SnO2 24. Three strong diffraction peaks at 2θ = 26.61︒, 33.89︒, and 51.78 ︒ could be assigned to the characteristic (110), (101), and (211) crystalline planes of SnO2, respectively 2. No GO related peaks have been detected in XRD patterns, which was because of


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the weak diffraction intensity of GO with low content

25.

Raman spectra of GO and GO/SnO2

NS-475 sample were shown in Figure 2(c). The Raman shift at 1580 cm-1 (G band) was caused by the C=C vibration of sp2 bonding, and the 1378 cm-1 peak (D band) could be ascribed to the C-C vibration of sp3 bonding which indicate that the existence of defects or disorder in the hexagonal graphitic layers

26.

These scattering peaks of sp3-bonded carbon and sp2-bonded

carbon proved the presence of GO in the GO/SnO2 NS-475 sample. In addition, bands at around 310 cm-1, 475 cm-1 and 629 cm-1 were assigned to the Eu, A2u and A1g vibrational modes of SnO2, respectively

27.

The porous structure of the GO/SnO2 NS-475 was characterized. The N2

adsorption-desorption isotherms in Figure 2(d) belonged to type IV adsorption isotherms with obvious hysteresis loops at high relative pressures, which indicated the existence of mesoporous structure. The specific surface area of GO/SnO2 NS-475 is 69.84 m2/g-1, which is much larger than that of the reported SnO2 nanomaterials

7,13.

The abundant mesoporous and large specific

surface area facilitated the adsorption, desorption, diffusion of oxygen and HCHO, which was helpful to improving the sensitive property. Figure 2(d) showed the pore-size distribution curve of the sample. The average pore size of GO/SnO2 NS-475 is 15.49 nm, which might be ascribed to the aggregated porous among SnO2 nanoparticles. XPS can used to study the chemical composition and chemical states of the sample according to the measured electron binding energy. As shown in Figure 3(a), the wide-survey XPS spectrum indicated the coexist of C, O and Sn elements in the GO/SnO2 NS-475. The C 1s spectrum in Figure 3(b) could be de-convoluted to three independent spectra, positioned at 284.6 eV, 286.1 eV and 289.4 eV, which could be ascribed to non-oxygenated C, carbon in C-O and carbonyl carbon (O=C-OH), respectively. The XPS result also proves that GO existed in



GO/SnO2 nanocomposite, which is in agreement with Raman analysis. Figure 3(c) showed that the wide O 1s spectrum of GO/SnO2 NS-475 included three peaks centered at 531.5 eV, 532.6 eV and 533.3 eV, which belonged to Sn-O and/or C=O bonds, Sn-O-C bonds, and C-OH and/or C-O-C groups 28. As shown in Figure 1, the GO nanosheets interacted with the DBTDL by the electrostatic force. After calcination at 475 °C, the Sn-C-O bond was formed between SnO2 and GO, indicating the existence of strong interaction between them, which was the key to improve the sensing performances

28.

As shown in Figure 3(d), the existence form of SnO2 could be

obtained by analyzed the Sn 3d spectrum. In Figure 3(d), two distinct peaks were ascribed to Sn 3d5/2 and Sn 3d3/2. Moreover, XPS spectrum of the Sn 3d region could be de-convoluted into three peaks around 486.7 eV, 487.4 eV and 488.1 eV, which confirmed the existence of both Sn4+ (SnO2) and Sn2+ (SnO) 29. The small amount of Sn2+ might be derived from the reducibility of GO in heat treatment. The typical SEM image in Figure 4(a) showed that the GO/SnO2 NS-475 sample possessed smooth surface and a pleated structure which is similar to graphene. The size of nanosheets ranged from several microns to dozens of microns. The TEM image of GO/SnO2 NS-475 in Figure 4(b) indicates a 2D nanosheets feature that consisted of in-plane mesopores with interconnected superlattices. The measured particles size is about 6 nm for GO/SnO2 NS-475. The SAED pattern in the inset of Figure 4(b) is also demonstrative of the polycrystalline nature of the GO/SnO2 NS-475. The diffraction rings could be indexed to (110), (101), (210) and (211) planes of the rutile SnO2 phase from the inside out, respectively, which were consistent with the XRD pattern. The HRTEM image was showed in Figure 4(c). The interplanar spacing of nanoparticles had been measured as 0.33 nm, 0.26 nm and 0.24 nm, respectively, corresponding


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to the (110), (101) and (200) planes of tetragonal SnO2 2. Furthermore, the lattice fringe of graphene could also be observed in the inset of Figure 4(c), where the crystalline nature of graphene with the nearness of a six-sided shell, i.e. honeycomb lattice of graphene could be observed. Figure 4 (d) shows the AFM characterization of GO/SnO2 NS-475. Obviously, the thickness of the 2D material was uniform and the thickness value was about 6 nm. Figure 4(e) highlighted the elemental dispersion of C, O and Sn of GO/SnO2 NS-475. These data confirmed the 2D ultrathin morphology and mesoporous structure of the GO/SnO2 NS-475. 3.2 Gas sensing performance Sensing properties of GO/SnO2 NS-475 were studied through a gas sensor evaluation system. The optimal operation temperature was obtained by testing the sensing performance at 40 °C to 100 °C. The response of the sensor against the operation temperature was displayed in Figure 5(a). For the detection of 100 ppm HCHO, the response values increased from 40 °C to 60 °C and then decreased from 60 °C to 100 °C, with a maximum value of 2275 at 60 °C. Real-time changes based on resistance/response of GO/SnO2 NS-475 sensor to 100 ppm HCHO gas at 60 °C were displayed in Figure 5(b). The response/recovery times were 81.3 and 33.7 s for GO/SnO2 NS-475. For the 2D GO/SnO2 NS, ultrathin nanosheet structures shortened the transport path, which greatly enhanced the response/recovery of gas sensor. In addition, the abundant mesopores in 2D SnO2 were conducive to gas diffusion, which also helped to improve the recovery performance. Therefore, the ultrathin nanosheet and porous structure of 2D GO/SnO2 made the gas desorption and re-adsorption of oxygen species more easy, which leaded to a fast response/recovery speed. The selectivity of sensor was further investigated. The selectivity of HCHO was defined as the ratio of the response of HCHO over that of the



interfering gas. In Figure 5(c), the selectivity of HCHO to the same concertation of ethanol, acetone, NH3, toluene and H2O was 32, 105, 373, 1750 and 989. The comparison implied that these sensors can be effectively resistant to drink, some VOCs and humidity in daily life. Considering the practical application of selective detection the HCHO from environmental atmosphere, the GO/SnO2 NS-475 sensor was exposed to mixed gases which were prepared by a certain amount of HCHO solution and interfering reagents including acetone, ammonia, toluene, ethanol, and water. The concentration of HCHO in the mixed gases testing maintained to 10 ppm and the operation temperature was 60 ℃. Figure 5(d) indicated that the GO/SnO2 NS-475 presented good anti-interference performance in a complicated atmosphere. Obviously, the response towards interfering vapors was much less than toward pure HCHO. The stability and repeatability of GO/SnO2 NS-475 had been studied by alternate testing the sensor in 10 ppm HCHO vapor and fresh air for several cycles. As shown in Figure 5(e), the sensitivity were remained the same magnitude in the 5 cycles and the curve can fully recovered to the pristine level, which revealed good stability and repeatability. The long-term stability of GO/SnO2 NS475 was further studied by repeatedly testing the sensor every 3 days within a month. Figure 5(f) showed that the sensor response value decreased from 2270 to 1975 after a month of testing. Noteworthy, the performance decay mainly occurred during the first week. To characterize the thermal stability of GO, Raman spectra were collected before and after operating at 60 °C for a week. By comparing the IG and ID, the structural stability of GO in the GO/SnO2 composite can be reflected. Obviously, the D bands and G bands in Figure 5(f) indicated that the microstructure of GO could be maintained due to the low operation temperature and preheating treatment. In addition, the ID/IG decreased slightly from 0.97 to 0.95 after a week of operation, which might be due to the aging of gas sensors. Although the response of the sensor decreased slightly in the first


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week, almost 90 % of the response value could still be maintained after a month of testing. The effect of moisture was further considered. Responses of GO/SnO2 NS-475 to 10 ppm HCHO and H2O vapor under different relative humidity (RH) were measured. Figure S1 showed that the response of GO/SnO2 NS-475 to moisture increased from 1 to 2.3 when the RH changed from 30 % to 70 %. In addition, for 10 ppm HCHO detection, the response value was relatively stable below 50 RH%. And then, the response was slightly reduced when further increasing the RH. The effect of RH on HCHO detection could be attributed to that the adsorption sites was blocked by the introducing of H2O molecules 13. 3.3 Discussion The outstanding sensing property might be originated from the unique morphology and modification of GO. To investigate the dominant influencing factors, samples with different crystallite, pore sizes and residue amounts of GO were obtained by adjusting the thermal treatment temperatures. As demonstrated by the TG-DSC analysis in Figure 2(a), calcination temperatures (425 -525 °C) had been selected since the combustion of GO mainly occurred at this temperature range. The TGA curve of GO/SnO2 NS-425 in Figure S2(a) showed an abrupt weight loss in the range of 425 °C to 700 °C, indicative of the combustion or decomposition of GO. Most of the GO were combusted between 425 °C and 500 °C, consistent with the TG-DSC analyses in Figure 2(a). Therefore, the weight change before and after the combustion of GO corresponded to the contents of GO in these materials. In the TGA curves, the mass fraction of GO was about 41.7 wt% in GO/SnO2 NS-425, 3.6 wt% in GO/SnO2 NS-450, 2.2 wt% in GO/SnO2 NS-475, and 1.3 wt% in GO/SnO2 NS-500, respectively. Raman spectra of GO/SnO2 NS-T were displayed in Figure S2(b). The intensity of GO Raman peaks gradually decreased



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with calcination temperature. These peaks could not be detected when the calcination temperature exceeds 500 °C, indicating that the GO could be removed after calcination over 500 °C for 2 hours. The intensity ratio (ID/IG) of GO/SnO2 NS-425 was calculated as 1.04; however, for GO/SnO2 NS-475, the ID/IG decreased to 0.97, which could as because of the different stability of sp3 and sp2 bonded carbon atoms 2. These results were in good agreement with previous reports on GO thermal reduction

30.

FT-IR spectra of GO/SnO2 NS-T were shown in

Figure S2(c). Strong absorption bands at wavenumbers of 3440 cm-1 and 1620 cm-1 came from O-H bonds, which could be thought of as the surface adsorbed H2O

31.

The wide absorption at

500-2000 cm-1 could be assigned to the presence of GO in GO/SnO2 NS-425

32.

When the

calcination temperature was 450 °C, the wide absorption band was gradually weakened due to the decrease of GO. In FT-IR spectra, the new absorption at 450-750 cm-1 appeared and increased with heat treatment temperature, which corresponded to Sn-O-Sn and SnO2 stretching, clarifying the existence of SnO2 nanomaterials 33. These bands further confirmed that the SnO2 phase was formed after calcination above 450 °C. Figure S2(d) showed XRD patterns of the assynthesized GO/SnO2 NS-T. No diffraction peak was found in XRD pattern of GO/SnO2 NS425, which indicated the poor crystallization performance of SnO2 after calcination at 425 °C. All diffraction peaks accorded well with the tetragonal rutile SnO2 (JCPDS card No. 41-1445) 2. The morphology of GO/SnO2 NS-T was investigated by SEM and TEM. Figure S3 (a1, b1, c1, d1, e1) showed that all GO/SnO2 NS-T samples possessed smooth surface and a pleated structure that is similar to graphene. With increasing the heat treatment temperature from 425 to 525 °C, the size of nanosheets decreased from 50 to 10 µm. Obviously, the structure of nanosheet was still maintained well even after calcination at 525 °C. Morphological characterization played an


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important role in understanding the formation mechanism of materials. Figure S3 (a1, b1, c1, d1, e1) indicated that atomically dispersed metal species or ultrafine clusters on GO nanosheet were first produced in the heat treatment of GO/DBTDL. As shown in Figure S3(a1, a2, a3), the SnO2 nanoparticles (NPs) with particles, of which the size were about 2 nm, dispersed uniformly on GO, which implied that the GO nanosheets could efficaciously prevent the aggregation of SnO2 NPs. Thereby, they could contribute to the formation of well dispersed SnO2 NPs on GO. The SAED pattern in the insert of Figure S3(a3) indicates the amorphous nature of SnO2. Furthermore, the nanoparticles and part of precursors were in-plane migrated with the increase of temperature due to the existence of GO template. The sintering and growing of SnO2 nanoparticles produced 2D interconnected nanoparticles and in-plane mesopores that constituted the basic building blocks of the final SnO2 nanosheets. As shown in Figure S3(b2, c2, d2, e2), all GO/SnO2 NS-T materials had 2D nanosheets features consisted of pores with interconnected superlattices. The average diameter of the nanoparticles were measured about 2 nm for GO/SnO2 NS-425, 5 nm for GO/SnO2 NS-450, 6 nm for GO/SnO2 NS-475, 8 nm for GO/SnO2 NS-500 and 10 nm for GO/SnO2 NS-525, respectively. The increase of particle size might be due to the Ostwald ripening of SnO2 in heating treatment 34. For SEAD patterns of GO/SnO2 NS-475 and GO/SnO2 NS-525, the typical polycrystalline structures (Figure S3(c3, e3)) became clear with the decrease of GO content, which also indicated better crystallization performance as stated in the XRD characterization. The electrical property of sensing materials was usually effected by the thermal treatment process. I-V curves of GO/SnO2 NS-T were measured and displayed in Figure 6(a). The nearly linear I-V relationships of all sensors indicated the Ohmic contacts between sensing materials



and electrodes 35. The conductance decreased gradually with the calcination temperature, which indicated that the calcination temperature could regulate the electric property of GO/SnO2 NS-T to get different GO contents in products. Responses of sensors against the operation temperature based on GO/SnO2 NS-T samples were displayed in Figure S4(a). Due to the good conductivity of the GO/SnO2 NS-425 sensor, no response to HCHO was found during the test. The optimum temperatures of other sensors toward 100 ppm HCHO were 60 °C. The sensing performance was increased initially and then decreased at the calcination temperature, and the optimal calcination parameter was found to be 475 °C. The influence of the calcinations temperatures on the selectivity of the sensors was also investigated (Figure S4(b)). All GO/SnO2 NS-T sensors showed excellent selectivity to HCHO, which was almost impervious to heat treatment temperature. According to Figure S4(c), the response time and recovery time were 81.3 s and 33.7 s for GO/SnO2 NS-475, which were a little bit faster than that of samples calcinated at other temperature. The response-HCHO concertation curves in Figure 6(b, c) showed that responses of sensor increased with the HCHO concentration in the 0.25-100 ppm. In addition, the response of GO/SnO2 NS-475 sensor were higher than that of GO/SnO2 NS-T (T= 450, 500, and 525 °C) sensors in full concentration range. Thus, the HCHO gas sensor based on 2D mesoporous GO/SnO2 composite could be used in ppm and sub-ppm levels. The relationship between response value (S) and HCHO concentration (Cg) was studied for better application. As shown in Figure 6(d), the linearity of S-1 and HCHO concentration was fitted in a log-log coordinate system. The good linear relation in the whole concentration range (0.25-100 ppm) indicated that it is easy to be applied in practice. More importantly, the adsorbed oxygen species could be inferred from the following equations

36:


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S=1+αCgβ

(1)

lg(S-1)=lgα+βlgCg

(2)

Where α is prefactor, and β is the surface species charge parameter. Empirically, the chemisorbed oxygen species might be O2- or O-, when the β was 0.5 or 1, respectively 36. Thus, the adsorbed oxygen species could be evaluated based on β values. As shown in Figure 6(d), the β values for GO/SnO2 NS-T (T= 450, 500, and 525 °C) sensors were calculated to be 1.26, 1.13, 1.19, and 1.29, respectively, which were comparatively close to 1. This indicated that O- and/or O2- might be the dominating absorbed oxygen species on the samples. In addition, the order of α values was: GO/SnO2 NS-475 > GO/SnO2 NS-500 > GO/SnO2 NS-525 > GO/SnO2 NS-450. These results indicated that the heat treatment could effectively regulate the sensing performances. The interrelations between response values and the particle sizes were presented in Figure 6(e). Obviously, the effect of particle size on gas response was significant. With the increase of particle size, the response increased first and then decreased, and the maximum value was obtained around 6 nm. This phenomenon could be explained by the surface control model 37. When the diameter of the sensitive material lesser or equal to the double thickness of the depletion layer, the whole surface of grains tended to become the depletion region, and then the conductance could be grain-controlled 37. As reported, the thickness of SnO2 depletion layer was about 3 nm, which indicated that the SnO2 crystallites in the ultrathin nanosheets have been depleted completely

37.

These results implied that a majority of SnO2 were active, providing

abundant active sites for oxygen species and HCHO gas. The relationship of responses vs. GO content in GO/SnO2 NS-T was studied in Figure 6(f). The suitable GO content could improve the electronic interaction among the SnO2 nanoparticles and make for excellent gas sensitivity.



Besides, defects and functional groups on the GO surface, acting as adsorption sites of gas molecules, could contributed to enhance their response. Furthermore, the strong interaction between the SnO2 and GO confirmed by the formation of Sn-O-C bond in XPS characterization, is helpful to improve their sensing performances 28. The comparison of the response to HCHO gas among mesoporous GO/SnO2 NS-475 sensor and other reported SnO2 based on MOS sensors (SnO2 microspheres, SnO2 microtubes, 3D SnO2 inverse opals), MOS composite sensors (In2O3/SnO2 nanospheres, NiO/SnO2 microspheres, TiO2@SnO2 nanofibers, octahedral-like Zn2SnO4/SnO2, ZnO/ZnSnO3), MOS/graphene-based sensors (mesoporous SnO2/graphene, GO/SnO2 nanofibers, SnO2/graphene, SnO/graphene) and noble metal sensitized MOS sensors (PtO2/SnO2, Pd/SnO2) were shown in Table 1 and Figure 7 3,10,11,13 ,38-47.

In contrast to most of the MOS, MOS composite, MOS/graphene-based materials,

GO/SnO2 NS-475 gas sensor has lower operation temperature and extremely high sensitivity. It was worth mentioned that the GO/SnO2 NS-475-based sensor also has a fast response/recovery speed to HCHO gas at 60 °C (Figure 3(b)). In short, the gas sensors based on mesoporous ultrathin SnO2 nanosheets in-situ modified by GO showed excellent sensing properties, especially in sensitivity, selectivity and working temperature. 3.4 The sensitive mechanism The sensitive mechanism of MOS sensing materials could be elucidated from the interaction between materials surface and HCHO 48. As shown in Figure 8, chemisorbed oxygen (O2-, O-, or O2-) was formed on the surface of GO/SnO2 NS-T due to its high electronegativity. The electron depletion layer was formed at the same time, which results in the resistance increase of sensitive


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materials 49. The chemisorbed oxygen species (O2-, O-, or O2-) were depended on their operation temperature (Top) 50: O2 (gas) → O2 (ads)

(3)

O2 (ads) + e- → O2- (ads) (Top

Mesoporous Ultrathin SnO2 Nanosheets in Situ Modified by Graphene

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