Hydrothermal synthesis of hierarchical SnO2 nanostructures made of superfine nanorods for smart gas sensor

Accepted Manuscript Title: Hydrothermal synthesis of hierarchical SnO2 nanostructures made of superfine nanorods for smart gas sensor Author: Xinliang Kuang Tianmo Liu Dongfeng Shi Wenxia Wang MingPing Yang Shahid Hussain Xianghe Peng Fusheng Pan PII: DOI: Reference:

S0169-4332(15)03177-3 http://dx.doi.org/doi:10.1016/j.apsusc.2015.12.172 APSUSC 32155

To appear in:

APSUSC

Received date: Revised date: Accepted date:

19-6-2015 10-12-2015 21-12-2015

Please cite this article as: X. Kuang, T. Liu, D. Shi, W. Wang, M.P. Yang, S. Hussain, X. Peng, F. Pan, Hydrothermal synthesis of hierarchical SnO2 nanostructures made of superfine nanorods for smart gas sensor, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.12.172 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.

Abstract

te

d

M

an

us

cr

ip t

Graphic

Highlights

Ac ce p

☆ Hierarchical SnO2 nanostructures made of superfine nanorods were controlled hydrothermal synthesized.

☆ The diameter and density of the nanorods can be tailored by adding NaOH.

☆ The as-prepared SnO2 nanostructures were found showing enhanced gas–sensing activity to

ethanol.

Hydrothermal synthesis of hierarchical SnO2 nanostructures made of superfine nanorods for smart gas sensor * Corresponding author. E-mail: [email protected] Tel.: +86 65102465. Fax: +86 65102465.

Page 1 of 26

Xinliang Kuang1, Tianmo Liu1,*, Dongfeng Shi1, Wenxia Wang1, MingPing Yang1, Shahid Hussain1, Xianghe Peng2, Fusheng Pan1 College of Materials Science and Engineering, Chongqing University, Chongqing, 400044, China 2

ip t

1

Chongqing Key Laboratory of Heterogeneous Material Mechanics, Chongqing University,

us

cr

Chongqing, 400044, China

Abstract: We report synthesis of hierarchical SnO2 nanostructures by a facile hydrothermal

an

method. Extensive structural characterizations demonstrate that the well-defined hierarchical nanostructures are composed of numerous one-dimensional nanorods, the diameter and density of

M

which can be precisely tailored by adjusting the dosage of NaOH. Interestingly, with more NaOH added, smaller and denser nanorods are formed, which is consistent with the assumption. We

te

d

proposed that the nucleation process was facilitated in such case leading to all-direction rapider growth. Moreover, the nucleation process could be started by the decomposition of preformed

Ac ce p

ZnSn(OH)6 induced by alkali etching. Based on the comparative experiments, a possible growth

mechanism for hierarchical SnO2 nanostructures has been proposed and discussed in detail. The gas sensing properties of the as-prepared hierarchical SnO2 nanostructures were all tested. It was

found that the S3 sample which assembled with smallest and densest nanorods showed the excellent sensitivities toward ethanol. Keywords: SnO2, Hydrothermal method, Nanorods, Gas sensor, Ethanol

1. Introduction As a typical n-type semiconductor with a wide band gap of 3.6 eV, Tin dioxide (SnO2) has

Page 2 of 26

been deeply studied and proved to be one of the most suitable suitable metal oxide materials for gas sensing owing to its high sensitivity toward combustible gases [1-3]. The gas sensing relies on

ip t

the chemical reaction between the reducing gases and oxygen molecules that adsorbed on the surface of SnO2 nanomaterials (NMs), which results in the change of electrical conductivity by

cr

releasing electrons trapped by oxygen molecules back to the SnO2 NMs [4]. Hence, small-sized

us

nanostructures can greatly improve the sensitivity of sensors due to their large surface area [5, 6]. Xu et al. reported that decreasing the crystallite size of SnO2 into the nano scale would increase

an

the sensor response to hydrogen and carbon monoxide [7]. Suematsu et al. also reported the synthesis of clustered SnO2 nanoparticles of ~5 nm improved sensor responses [8]. The crystal

M

size effect on the sensor response has recently been remodeled by Yamazoe et al. [9, 10]. They

te

sensitivity.

d

claimed that decreasing the crystallite size of SnO2 is one of the efficient ways to obtain high

Particularly, the morphologies of SnO2 nanomaterials have an effect on their applications. Up

Ac ce p

till now, various SnO2 nanostructures have been reported, for instance, zero-dimensional (0D)

nanoparticles [8, 11], one-dimensional (1D) nanorods [12-14] and nanotubes [15], and two-dimensional (2D)

nanosheets

[16]. Recently,

three-dimensional (3D)

hierarchical

architectures assembled by low-dimensional nanostructure have been widely explored to improve gas sensing properties [17-19]. These 3D hierarchical nanostructures are deemed to be the most effective because of their porous nanostructures formed by the adjacent building blocks [20]. Previously, Liu and the co-workers have reported synthesis of hierarchical SnO2 nanostructures made of ultrathin nanosheets for smart gas sensor [21]. Guan et al. also prepared Zn-doped SnO2

hierarchical architectures to enhance the sensing properties [22]. Above all, these researches have

Page 3 of 26

concentrated on the assembly of nanosheets as bricks into 3D hierarchical architectures. Lately, a novel coral-like porous SnO2 hollow architecture has been synthesized for dye-sensitized solar

ip t

cells [23]. This coral-like structure was considered to possess a high surface area and a densely packed microstructure for fast electron transport. Hence, 3D hierarchical architectures assembled

cr

from dense nanorods are proposed to be synthesized for exploring the gas sensing properties.

us

In this paper, we reported a facile synthesis of unique SnO2 nanostructures assembled of dense nanorods via a Zn(CH3COO)2-assisted hydrothermal process. The density and diameter of

an

the nanorods can be controlled by adjusting the dosage of NaOH. Systematic experiments were performed to understand the step-by-step formation mechanisms of the as-prepared SnO2

M

nanostructures. The sensors based on obtained samples were used for ethanol gas detection. Our

te

2. Experimental

d

results indicated that the S3 sample showed the highest response.

2.1 Synthesis of SnO2 Nanostructures

Ac ce p

All chemicals (98% purity, analytical grade) were purchased from Keshi Chendu

Development Co. Ltd. and used without further purification. In a typical synthesis, 0.05 g zinc acetic (Zn(CH3COO)2) was dissolved in a mixture of 15 ml of de-ionized (DI) water and equal

quantity of ethanol and stirred for 10 min. Then a certain amount of NaOH was added to the above mentioned solution. After the entire solid dissolved, 1 mmol stannic chloride pentahydrate (SnCl4·5H2O) was added into the solution while vigorous stirring for 45 min to form a homogeneous solution. After stirring, the resultant solution was transferred to a 50 ml teflon lined stainless steel autoclave and then sealed and maintained at 180 oC for 24 h. After the heating treatment, the autoclave was naturally cooled to room temperature and the obtained precipitates

Page 4 of 26

were washed several times with DI water and ethanol, respectively. Furthermore, the products were dried at 70 oC overnight. Then, different amounts of NaOH (The products are summarized in

ip t

Table 1) were added to the solution. In addition, in order to explore the formation processes of SnO2 nanostructure, more experiments with different reaction time were performed.

cr

2.2 Characterization

us

As-prepared products were all examined by X-ray diffraction (XRD) with a Rigaku D/Max-1200X diffractometry having Cu Ka radiation (30 kV, 100 mA) and by focused ion beam

an

field emission scanning double beam electron microscopy (Zeiss AURIGA FIB) and transmission electron microscopy (Zeiss LIBRA 200 FEG TEM), respectively.

M

2.3 Gas-sensing properties

The gas sensor was fabricated as follows: the as-prepared powder was dissolved in ethanol to

te

d

form a paste by ultrasonication. Then, the achieved paste was coated onto an Al2O3 tube by a small brush to form about 100–200 µm thick film between two parallel Au electrodes, which were fixed

Ac ce p

in advance at both end points. After sintering at 400 oC for 2 h in air, a Ni–Cr heating wire was

inserted into the alumina tube to control the operating temperature by tuning the voltage. Finally, the gas sensor prepared was aged at 240 oC for 72 h to improve stability and repeatability. Afterwards, its gas-sensing properties were measured with a static system controlled by a computer (HW-30A, Hanwei Electronics Co. Ltd.) under lab conditions. In this work, the response of the sensor (S) was defined as S=Rg/Ra for oxidizing gas or Ra/Rg for reducing gas, where Ra and Rg are resistances of the sensors in air and target gases, respectively. The response and recovery times are defined as time taken by the sensor to achieve 90% of the total resistance change in terms of adsorption and desorption, respectively.

Page 5 of 26

3. Results and discussion 3.1 Structural and morphological characteristics

ip t

Fig. 1 shows the typical XRD patterns of the SnO2 hierarchical nanostructures (S1-S3) synthesized with different dosage of NaOH. All the diffraction peaks for each sample correspond

cr

to the pure SnO2 with a tetragonal rutile structure matching the standard JCPDS file card No.

us

41-1445, which indicates the high crystallinity of SnO2 samples. Moreover, the diffraction peaks of the as-prepared samples with different dosage of NaOH tend to become slightly broader from

an

S1 and S3, which suggests that S3 sample presents the smallest crystalline size. This result also matches the diameter of nanorods observed from the SEM (Fig. 2a, b and c) and TEM (Fig. 3a, c

M

and e) images.

The morphologies and microstructures of SnO2 samples (S1-S3) were shown in Fig. 2,

te

d

respectively. As confirmed by the FESEM observations, all as-synthesized samples (S1-S3) possess hierarchical urchin-like morphologies. The typical diameter of a single hierarchical

Ac ce p

urchin-like structure is 1-2 µm. Interestingly, it is observed that the structures consist of many

superfine nanorods. Additionally, the surface of the nanorods is extremely smooth and clean. Compared with other two samples, sample S3 has the highest density. Fig. 3(a, c and e) shows the typical TEM images of the samples (S1-S3). It can be confirmed that the diameters of nanorods are 70-80 nm, 40-50 nm and 15-20 nm, respectively. The detailed structural and crystallographic properties of the three samples (S1-S3), investigated by HRTEM images (Fig. 3b, d and f), exhibit well-defined lattice fringes along three directions, respectively. All the three samples (S1-S3) grow along [110] direction. To investigate the detailed formation mechanism of the hierarchical SnO2 nanostructures,

Page 6 of 26

time-based experiments for S3 Sample were conducted at 180 oC. At the reaction time of 4 h, spherical microstructure with a diameter of ~1 µm are obtained (Fig. 5(a)). Based on the typical

ip t

diffraction pattern shown in Fig. 4, it can be confirmed that the spherical microstructure is ZnSn(OH)6 indeed. When the reaction time was 8 h, hierarchical nanostructures assembled by

cr

various plates were formed (Fig. 5(b)). When reaction time was 16 h, nanoplates decomposed and

us

some nanaorods started to form (Fig. 5(c)). Further, when the reaction time was increased up to 24 h, hierarchical SnO2 nanostructures with uniform and dense nanorods were obtained.

an

Based on the above experimental observations and previously reported literatures [12, 14, 18, 24], chemical reactions of synthesis of SnO2 nanostructures can be described as follow:

Zn 2+ + Sn 4+ + 6OH − → ZnSn(OH)6  4+ − 2−  Sn + 6OH → Sn(OH) 6 ZnSn(OH)6 + 4OH − → Zn(OH)24− + Sn(OH) 26−

M

(1a ) (1b)

   → SnO 2 + 3H 2 O + 2OH

d

Hydrothermal

(2) −

(3)

te

Sn(OH)

2− 6

A possible formation mechanism was proposed (Fig. 6). Firstly, ZnSn(OH)6 crystal nucleus

Ac ce p

formed by the hydrolysis of SnCl4 and Zn(CH3COO)2 (Eq. 1a). Subsequently, the crystal nucleus

further grew into ZnSn(OH)6 spheres due to the minimization of the surface energy. Meanwhile, a large amount of Sn(OH)62- remained in the solution because of the excess of Sn4+ and OH- ions (Eq. 1b). Due to the etching action of OH-, ZnSn(OH)6 spheres were decomposed into Zn(OH)42-

and Sn(OH)62- (Eq. 2), which entered the solution and further formed numerous SnO2 nanocrystals (Eq. 3). Through this process, the residual structure decomposed into Zn-doped SnO2 and Zn-poor

Zn2SnO4 [25]. Afterwards, SnO2 nanocrystals found in the solution came onto the surface of the plates and grew there. It was reported that the centrosymmetric structure of low-axial ratios

Page 7 of 26

(c/a=0.673) increases the probability of anisotropic growth of the SnO2 crystals along [110] direction [26]. Thus, the SnO2 nanorods grow along the [110] direction, which has been confirmed

ip t

by the HRTEM (Fig. 1(b, d and e)). Finally, rough nanorods were changed to the thorns of the nanoflower. Particularly, as shown in Fig. 3, with the increase of the dosage of NaOH, the order of

cr

the nanorod diameter is found to be S1 > S2 > S3, while the density of the assembled-nanorods

us

follow the reverse order, which may be attributed to an assumption that the nucleation process is facilitated in such case, leading to all-direction rapider growth. In the previous chemical reactions

an

(Eq. 1a, Eq. 1b and Eq. 2), OH- plays a key role in the formation of urchin-like SnO2 nanostructures, and the higher the concentration of OH-, the faster reaction rate of Eq. 1a, Eq. 1b

M

and Eq. 2. On the other hand, the etching action of OH- was stronger because there was more dosage of NaOH in the reaction liquid, which resulted in decomposition of ZnSn(OH)6 spheres

te

d

into Zn(OH)42- and Sn(OH)62- more rapidly. Thus, more SnO2 crystal nuclei formed, a higher nucleation rate and a smaller crystalline size were presented to sample S3. So, the nanostructures

Ac ce p

of the sample S3 exhibit the finest nanorods with a highest density. 3.2 Gas-sensing properties

We fabricated three samples (S1-S3) into gas sensors and tested their gas sensing

performances. The operating current has a significant effect on gas response of the sensors [27-28]. Fig. 7(a) shows the sensitivities of three samples (S1-S3) at different operating current toward

ethanol of 50 ppm. It can be seen that the response of three sensors varied with operating current. According to the graph, we can make a conclusion that the current value of 150 mA is the most suitable operating current to investigate gas-sensing properties. Fig. 7(b, c and d) shows the dynamic gas response characteristics of SnO2 gas sensors with

Page 8 of 26

different ethanol gas concentrations (5–100 ppm) at 150 mA. It is obvious that the sensor S3 has a significantly high response. The responses of sensors S3 with six different ethanol concentrations

ip t

(5, 10, 25, 50, 75 and 100 ppm) are 7, 13, 32, 44, 59 and 72, respectively. In addition, sensitivities of other two sensors toward 100 ppm ethanol are 29.2 and 38, respectively. It can be seen that the

cr

response increased with the rise of ethanol concentrations. The comparison of the response of the

us

three sensors with the ethanol concentration ranging from 5 to 100 ppm at 150 mA are shown in Fig. 7(e). Apparently, sample S3 has the best sensitivity while S1 exhibits the worst gas sensitivity.

an

The response and recovery time of the three sensors toward 50 ppm ethanol at 150 mA are tested, as shown in Fig. 7(f). The response time is 2, 3 and 4 s, respectively. The recovery time is

M

all about 1 ~ 2 s. The sensor S3 exhibits the shortest response, recovery times and best sensitivity, which indicates that sensor S3 has the great potential for gas sensing application.

te

d

A comparison between the sensing performances of the sensors and literature reports is summarized in Table 2. It can be seen that the sensor S3 shows the higher response compared with

Ac ce p

other reports [29-31].

3.3 Gas sensing mechanism

Up to now, the most widely accepted gas sensing mechanism is the model based on an

adsorption-oxidation-desorption process [32], which can be summarized as follows. Firstly, as the as-made SnO2 sensors are exposed to air, oxygen molecule will absorb on the surface of the urchin-like SnO2 nanostructures and then capture free electrons from the conductance band of the 2－

SnO2 nanorods, resulting in forming chemisorbed oxygen species (O2 , O , O ) on the surface. －

－

Secondly, these absorbed O can generate a depletion layer and the band bending on the surface, which leads to energy barrier and resistance of the sensors increasing. This process can be

Page 9 of 26

described as follows: → O2 (ads)

O2 (gas) + e

→ 2O (ads)

－

－

O (ads)

－

+e

(4)

－

－

(5)

－

→ O (ads)2

(6)

－

ip t

O2 (gas) + e

cr

Finally, the pre-adsorbed oxygen will react with the reductive gas molecules such as ethanol once

us

it is introduced and this process will release the trapped electrons back to the conduction band of SnO2. Subsequently, electron concentration will rise in the SnO2 sensor and hence enhance its

an

conductivity. This can be simply described as

4C2H5OH + O2- → 4CH3CHO + 2H2O + e

M

2C2H5OH + O- → 2CH3CHO + H2O + e

－

－

2C2H5OH + O2- → 2CH3CHO + H2O + 2e

－

(7) (8) (9)

te

d

It is reported that the size and dimension of the sensing materials play a crucial role on the response of the metal-oxide gas sensors, and those materials with the size close to Debye

Ac ce p

screening length (D ~ 2λD) can often exhibit excellent sensing responses [5, 33-34]. For SnO2, the

reported value of λD is 7 nm. Thereby, the crystals with sizes close to or less than 14 nm (2 λD) contribute more to the improvement of SnO2 sensitivity upon ethanol exposure [35]. Figure 8 shows the cross-sectional view of different SnO2 nanorods based on samples S1-S3 under

exposure to oxidizing gases, the inner core corresponds to the conduction channel and the outer shell to the space charge region, respectively [18]. Thus, as shown in Figure 8, when the nanorod is exposed to air, the electron acceptor molecules (O2ads−) species are accumulated at the surface of

the nanorod. The electron captured by oxygen molecules derives from the space charge region, thus the diameters of the nanorods will affect the diameter of the conduction channel. And as

Page 10 of 26

aforementioned, the order of the nanorods diameters is found to be S1 > S2 > S3. Once the nanorods exposure to air, the area of conduction channel in S1 is larger than that in S2. Therefore

ip t

S2 has the higher resistance than S1. The diameters of nanorods of sample S3 is 15-20 nm, which indicates all the nanorods body may be electron depleted in air. Moreover, the specific area of S3

cr

with the highest density of nanorods is increased. This means that the amount of oxygen that can

us

easily be absorbed and ionized. As reported in literature, the ability of the sensing material to absorb and ionize oxygen species is fundamental to the sensor performance [36]. Owing to

an

electron depleted, more oxygen species adsorb to the surface of SnO2 leading to further increase of the sensor responses. Thus, the sensor S3 show an excellent sensitivity. Beyond that, the good gas

te

4. Conclusions

d

unique hierarchical 3D nanostructures.

M

sensor properties of urchin-like SnO2 would be also attributed to the small crystallite size and

In summary, a facile Zn(CH3COO)2-assisted hydrothermal method was developed to prepare

Ac ce p

3D hierarchical SnO2 nanostructures. The results of XRD indicated the nucleation was initiated by formation of ZnSn(OH)6. The prepared 3D hierarchical architectures were made of numerous

smooth and superfine nanorods. Moreover, the diameters and density of the nanorods can be controled by adjusting the dosage of NaOH. The prepared hierarchical nanomaterials were fabricated into high-sensitive ethanol gas sensors. The sensor S3 with the smallest nanorods presented the highest sensitivity toward ethanol. It should be attributed to the small crystallite size, the size of nanorods that is closing to Debye screening length (D ~ 2λD), and the unique hierarchical 3D nanostructures which provides the quick passages for absorb and desorb of gases. Acknowledgements

Page 11 of 26

This work was supported by National Natural Science of China No. 11332013 and

ip t

Fundamental Research Funds for Central Universities (Grant no. 106112015CDJXY130013).

References

cr

[1] L.V. Thong, L.T.N. Loan, N. Van Hieu, Comparative study of gas sensor performance of SnO2

us

nanowires and their hierarchical nanostructures, Sens. Actuators B: Chem. 150 (2010) 183-187. [2] L.M. Yu, X.H. Fan, L.J. Qi, L.H. Ma, W. Yan, Dependence of morphologies for SnO2

an

nanostructures on their sensing property, Appl. Surf. Sci. 257 (2011) 3140-3144.

[3] Y.C Her, B.Y. Yeh, S.L. Huang, Vapor–solid growth of p-Te/n-SnO2 hierarchical

M

heterostructures and their enhanced room-temperature gas sensing properties, ACS Appl. Mater. Interfaces 6 (2014) 9150-9159.

te

d

[4] W. Zeng, H. Zhang, Y.Q. Li, W.G. Chen, Z.C. Wang, Hydrothermal synthesis of hierarchical folwer-like SnO2 nanostructures with enhanced ethanol gas sensing propertied, Mater. Research

Ac ce p

Bulletin 57 (2014) 91-96.

[5] J. Zhang, J. Guo, H.Y. Xu, B.Q. Cao, Reactive-template fabrication of porous SnO2 nanotubes

and their remarkable gas-sensing performance, ACS Appl. Mater. Interfaces 5 (2013) 7893–7898. [6] C. Gu, X. Xu, J. Huang, W. Wang, Y. Sun, J. Liu, Porous flower-like SnO2 nanostructures as

sensitive gas sensors for volatile arganic compound detection, Sens. Actuators B: Chem. 174 (2012) 31-38. [7] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Grain size effects on gas sensitivity of porous SnO2-based elements, Sens. Actuators B: Chem. 3 (1991) 147-155.

[8] K. Suematsu, Y.K. Shin, Z.Q. Hua, K.H. Yoshida, M.S. Yuasa, T.Y. Kida, K. Shimanoe,

Page 12 of 26

Nanoparticle cluster gas sensor: controlled clustering of SnO2 nanoparticles for highly sensitive toluene detection, ACS Appl. Mater. Interfaces 6 (2014) 5319-5326.

gas sensors I. response to oxygen, J. Electrochem. Soc. 155 (2008) 85-92.

ip t

[9] N. Yamazoe, K. Shimanoe, Roles of shape and size of component crystals in semiconductor

cr

[10] N. Yamazoe, K. Shimanoe, Roles of shape and size of component crystals in semiconductor

us

gas sensors II. response to NO2 and H2, J. Electrochem. Soc. 155 (2008) 93-98.

[11] H. Wang, Y. Qu, H. Chen, Z.D. Lin, K. Dai, Highly selective n-butanol gas sensor based on

an

mesoporous SnO2 prepared with hydrothermal treatment, Sens. Actuators B: Chem. 201 (2014) 153-159.

M

[12] O. Lupan, L. Chow, G. Chai, A. Schulte, S. Park, H. Heinrich, A rapid hydrothermal synthesis of rutile SnO2 nanowires, Mater. Sci. Eng. B 157 (2009) 101-104.

te

d

[13] D.F. Zhang, L.D. Sun, J.L. Yin, C.H. Yan, Low-temperature fabrication of highly crystalline SnO2 nanorods, Adv. Mater. 15 (2003) 1022-1025.

Ac ce p

[14] O. Lupan, L, Chow, G. Chai, H. Heinrich, S. Park, A. Schulte, Synthesis of one-dimensional SnO2 nanorods via a hydrothermal technique, Physica E 41 (2009) 533-536.

[15] L. Shi, H.L. Lin, Preparation of band gap tunable SnO2 nanotubes and their ethanol sensing

properties, Langmuir 27 (2011) 3977-3981. [16] Y.L. Wang, X.C. Jiang, Y.N. Xia, A solution-phase, precursor route to polycrystalline SnO2 nanowires that can be used for gas sensing under ambient conditions, J. Am. Chem. Soc. 125 (2003) 16176-16177. [17] H.B. Wu, J.S. Chen, X.W. Lou, H.H. Hng, Synthesis of SnO2 hierarchical structures

assembled from nanosheets and their lithium storage properties, J. Phys. Chem. C 115 (2011)

Page 13 of 26

24605-24610. [18] I. Paulowicz, V. Hrkac, S. Kaps, V. Cretu, O. Lupan, T. Braniste, Multifunctional applications:

ip t

from high-temperature stretchable ceramics to ultraresponsive sensors, Adv. Electron. Mater. 1 (2015) 1500081.

cr

[19] Y.Q. Zhu, H.Z. Guo, H.Z. Zhai, C.B. Cao, Microwave-assisted and gram-scale synthesis of

us

ultrathin SnO2 nanosheets with enhanced lithium storage properties, ACS Appl. Mater. Interfaces, 7 (2015) 2745–2753.

an

[20] M.Y. Wu, W. Zeng, Y.Q. Li, Hydrothermal synthesis of novel SnO2 nanoflowers and their gas-sensing properties, Mater. Lett. 104 (2013) 34-36.

M

[21] Y. Liu, Y. Jiao, Z.l. Zhang, F.Y. Qu, U. Ahmad, X. Wu, Hierarchical SnO¬2 nanostructures made of intermingled ultrathin nanosheets for environmental remediation, smart gas sensor, and

te

d

supercapacitor applications, ACS Appl. Mater. Interfaces 6 (2014) 2174-2184. [22] Y. Guan, D.W. Wang, X. Zhou, P. Sun, H.Y. Wang, J. Ma, G.Y. Lu, Hydrothermal preparation

Ac ce p

and gas sensing properties of Zn-doped SnO2 hierarchical architectures, Sens. Actuators B: Chem.

191 (2014) 45-52.

[23] Z.D. Li, Y. Zhou, T. Yu, J.G. Liu, Z.G. Zou, Unique Zn-doped SnO2 nano-echinus with excellent charge transport and light harvesting properties as photoanode materials for high performance dye-sensitized solar cell, CrystEngComm. 14 (2012) 6462-6468. [24] O. Lupan, G.Y. Chai, L. Chow, Novel hydrogen gas sensor based on single ZnO nanorod, Microelectronic Engineering 85 (2008) 2220-2225. [25] X.C. Dou, D. Sabba, N. Mathews, L.H. Wong, Y.M. Lam, S. Mhaisalkar, Hydrothermal synthesis of high electron mobility Zn-doped SnO2 nanoflowers as photoanode material for

Page 14 of 26

efficient dye-sensitized solar cells, Chem. Mater. 23 (2011) 3938-3945. [26] F. Song, H.L. Su, J. Han, W.M. Lau, W.J. Moon, D. Zhang, Bioinspired hierarchical tin oxide

ip t

scaffolds for enhanced gas sensing properties, J. Phys. Chem. C 116 (2012) 10274-10281. [27] V.R. Shinde, T.P. Gujar, C.D. Lokhande, Enhanced response of porous ZnO nanobeads

cr

toward LPG: effect of Pd sensitization, Sens. Actuators B: Chem. 123 (2007) 701-706.

us

[28] Z. Cao, J.R. Stetter. A selective solid-state gas sensors for halogenated hydrocarbons, Sens. Actuators B: Chem. 1 (1991) 109-113.

an

[29] H. Wang, Q.Q. Liang, W.J. Wang, Y.R. An, J.H. Li, L. Guo, Preparation of flower-like SnO2 nanostrucetures and their applications in gas-sensing and lithium storage, Cryst. Growth Des. 11

M

(2011) 2942-2947.

[30] S. Peng, X.D. Mei, Y.X. Cai, J. Ma, Y.F. Sun, X.S. Liang, F.M. Liu, G.Y. Lu, Synthesis and

301–307.

te

d

gas sensing properties of hierarchical SnO2 nanostructures, Sens. Actuators, B: Chem. 187 (2013)

Ac ce p

[31] Z. Lou, L.L. Wang, R. Wang, T. Fei, T. Zhang, Synthesis and ethanol sensing properties of SnO2 nanosheets via a simple hydrothermal route, Solid-State Electronics 76 (2012) 91–94. [32] J. Guo, J. Zhang, D.X. Ju, H.Y. Xu, B.Q. Cao, Three-dimensional SnO2 microstructures assembled by porous nanosheets and their superior performance for gas sensing, Powder Technol. 250 (2013) 40-45.

[33] X.H. Sun, X.D. Hu, Y.C. Wang, R. Xiong, X. Li, J. Liu, H.M. Ji, X.L. Li, S. Cai, C.M. Zheng, Enhanced gas-sensing performance of Fe-doped ordered mesoporous NiO with long-range periodicity, J. Phys. Chem. C 119 (2015) 3228–3237. [34] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sens. Actuators B:

Page 15 of 26

Chem. 5 (1991) 7–19. [34] J. Guo, J. Zhang, D.X. Ju, H.Y. Xu, B.Q. Cao, Three-dimensional SnO2 microstructures

ip t

assembled by porous nanosheets and their superior performance for gas sensing, Powder Technol. 250 (2013) 40-45.

cr

[35] D. Hu, B.Q. Han, S.J. Deng, Z.P. Feng, Y. Wang, J.K. Popovic, M. Nuskol, Y.D. Wang, I.

us

Djerdj, Novel mixed phase SnO2 nanorods assembled with SnO2 nanocrystals for enhancing gas-sensing performance toward isopropanol gas, J. Phys. Chem. C 118 (2014) 9832−9840.

an

[36] O. Lupan, T. Braniste, M. Deng, L. Ghimpu, I. Paulowicz, Rapid switching and ultra-responsive nanosensors based on individual shell-core Ga2O3/gaN: Ox@SnO2 nanobelt with

te

d

M

nanocrystalline shell in mixed phases, Sens. Actuators, B: Chem.221 (2015) 544-555.

Figures and Table captions

Ac ce p

Fig. 1. XRD patterns of S1-S3.

Fig. 2. FESEM images of (a) S1, (b) S2 and (c) S3. Fig. 3. Typical TEM and HRTEM images of (a-b) S1, (c-d) S2 and (e-f) S3. Fig. 4. XRD characterization of the products (S3) obtained at a reaction time of 4 h.

Fig. 5. SEM observation of the evolution process of hierarchical SnO2 nanostructures, the SEM images of the products (S3) obtained at a reaction time of (a) 4 h, (b) 8 h, (c) 16 h and (d) 24 h. Fig. 6. Schematic for the possible growth of S1-S3. Fig. 7. (a) Response versus operating current of the sensors to 50 ppm ethanol; (b-d) response of sensors to ethanol with different concentration; (e) The comparison of the response of sensors with

Page 16 of 26

different ethanol concentration at 150 mA; (f) Response and recover time of three sensors. Fig. 8. Schematic of mechanisms involved in gas sensing with different SnO2 nanorods

ip t

(cross-sectional view) under exposure to oxidizing gases. Central core region (with electrons-dots) is the conducting channel and the surrounding shell region denotes the space charge region (the

cr

grid area). O− at the surface are adsorbed oxygen species. The adsorbed oxygen species on the

us

nanorods surface acquire electrons from nanorods leading to the formation of space charge region and the diameters of the nanorods will affect the diameter of the conduction channel.

an

Table 1 Experimental setup of as-prepared samples.

Ac ce p

te

d

M

Table 2 Comparison of gas sensing properties of SnO2 nanostructures in previous reports.

Figure 1

Page 17 of 26

ip t cr us an M d te Ac ce p Figure 2

Page 18 of 26

ip t cr us an M d te Ac ce p Figure 3

Page 19 of 26

ip t cr us an M d te Ac ce p Figure 4

Page 20 of 26

ip t cr us an M d te Ac ce p Figure 5

Page 21 of 26

ip t cr us an M d te Ac ce p Figure 6

Page 22 of 26

ip t cr us an M d te Ac ce p Figure 7

Page 23 of 26

ip t cr us an M d te Ac ce p Figure 8

Page 24 of 26

ip t cr us an M d te Ac ce p Table 1

Page 25 of 26

SnCl4•5H2O (mmol)

Zn(CH3COO)2 (g)

NaOH (g)

Temperature (oC)

Time (h)

S1

1

0.05

0.30

180

24

S2

1

0.05

0.60

180

24

S3-1 S3-2 S3-3 S3

1 1 1 1

0.05 0.05 0.05 0.05

0.90 0.90 0.90 0.90

180 180 180 180

4 8 16 24

Table 2

SnO2 nanosheets

Reference

300

350 oC

35

[29]

100

300 oC

33

[30]

50

290 oC

25

[31]

150 mA

18 25 44

Present work

M

an

Sensor response

50

Ac ce p

S1 S2 S3

Hydrothermal route

Temperature (oC) / Current (mA)

d

Flower-like SnO2 nanostructures Hierarchical SnO2 nanostructures

Ethanol concentration (ppm)

te

Material

Fabrication approach

us

cr

ip t

Sample

Page 26 of 26