Fast formaldehyde gas sensing response properties of ultrathin SnO₂ nanosheets

Abstract

Ultrathin SnO₂ nanosheets have been successfully synthesized through a simple low temperature hydrothermal strategy and developed for formaldehyde gas detection. Morphology characterizations are confirmed by the results of FESEM, TEM and HRTEM. The ultrathin SnO₂ nanosheets are configured as high performance sensors to detect formaldehyde, and show very fast response times (<1 s), recovery times (6 s), good repeatability and selectivity at a relatively low working temperature. The high sensitivity performances are attributed to the morphology characterizations of ultrathin SnO₂ nanosheets for affording large specific surface areas and more direct conduction pathways. The growth mechanism of the ultrathin SnO₂ nanosheets and a morphology dependent sensing mechanism are proposed.

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1. Introduction

It is well known that formaldehyde (HCHO), as an enormously valuable industrial chemical, is one of the most extensive chemicals applied in the manufacture of numerous industrial and building materials and consumer products of our daily lives.^1–4 So in indoor and environmentally sensitive areas to human health, formaldehyde is also one of the most serious pollutant sources in volatile organic compounds (VOCs).⁵ It has been recognized as a suspected carcinogen, an allergen, and an intense irritant of eyes and respiratory organs even at a low concentration of sub ppm level.^1,2 Until now, an abundance of detecting approach for formaldehyde concentration have been reported or utilized, including piezoelectric approach,⁶ optical methods,⁷ colorimetric approach,⁸ electrochemical approach,⁹ spectrophotometry approach,¹⁰ chromatography approach,¹¹ biosensors,¹² potentiometric sensor,¹³ chemical sensor approach¹⁴ and etc. Among them, the chemical sensor approach based on metal oxide semiconductors, utilizing the resistance changes of sensor upon exposing to target gases, have been a subject of intensive research as the detection harmful gases (e.g., formaldehyde), due to its simple, inexpensive, and efficient.¹⁵

Tin dioxide (SnO₂) with a wide direct energy band gap of 3.6 eV at 300 K, as an important n-type metal oxide semiconductor, have attracted great attention for a long time as a strong candidate for chemical sensors with wide applications in transparent conductive films.^16,17

The amelioration of SnO₂-based gas sensors performances have been mainly focused on the preparation of the micro- or nanoscaled sensing materials with diverse morphological in respect that their advantage of the higher specific surface area and simplicity in fabrication. So diverse morphological hierarchical micro- or nanostructures of SnO₂-based gas sensors have stimulated great research interest and been intensively explored due to their prominent superiority of high sensitivity, such as nanoparticles,¹⁸ nanowires,¹⁹ nanorods,²⁰ nanotubes,²¹ nanobelts,²² nanosheets,²³ nanopillar,²⁴ nanocubes,²⁵ mesoporous,²⁶ hollow spheres,²⁷ core–shell structure²⁸ flowers-like,²⁹ urchins-like,³⁰ aurelia-like,³¹ and so on. Nevertheless, to date, various techniques with synthesizing diverse morphologies of SnO₂ micro- or nanostructures, for instance template method,³² hydrothermal strategy,³³ chemical vapor deposition (CVD),³⁴ and electrochemical deposition,³⁵ are still a challenge to rapidly detect formaldehyde gas with high sensitivity in a broad linear range and a low detection limit.³⁶ Related to SnO₂, the hydrothermal strategy has been most widely used.³⁷ To accomplish high sensitivity and fast response-recovery capacities for real-time monitoring of formaldehyde gases to prevent possible disasters,³⁸ the abundant research has mainly fabricate the hierarchical micro- or nanostructures in an expected manner, which possess good dispersity and surface accessibility,³⁸ besides higher specific surface areas. Recently, some efforts of the synthesis with rapidly detecting formaldehyde gas have been have been reported. Cao et al. has fabricated SnO₂–grapheme nanocomposite for photocatalysis and formaldehyde gas sensing by solid-state synthesis.¹⁶ Lin et al. has synergistically improved formaldehyde gas sensing properties of SnO₂ microspheres by indium and palladium co-doping.⁵ Huang et al. has prepared porous SnO₂ microcubes with enhanced gas-sensing property.³⁹ All these results above in the literatures show the SnO₂ micro- or nanostructures remarkably promote the property for detecting formaldehyde gas in certain range and low concentration by changing SnO₂ hosts, doping or controlling crystalline phase of the materials.^40,41 However, those processes for the synthesis of SnO₂ micro- or nanostructure are in general multistep, expensive and time-consuming. In contrast, the advantages process ought to meet the following rules: (a) simple, cheaper and convenient; (b) involve less solvent and reduce contamination; (c) give high yields of products.⁴² Therefore, it is essential to take challenges and find better route for the synthesis of SnO₂-based gas sensing material to fast detect formaldehyde at a relatively low optimum working temperature.

Herein, we establish the synthesis of the ultrathin SnO₂ nanosheets, using a simple, inexpensive, easy one-step, low temperature and efficient hydrothermal strategy. Then we systematically investigate morphology characterizations and gas sensing properties of the ultrathin SnO₂ nanosheets. A comprehensive gas sensing study has demonstrated excellent performances for detection of formaldehyde in terms of very fast response time/recovery time, high sensitivity, good repeatability and good sensing selectivity at a lower working temperature. Moreover, the possible formation mechanism of the ultrathin SnO₂ nanosheets is discussed from the viewpoint of nucleation, self-assembly growth by the experimental results. The possible sensing mechanism about the effects on the formaldehyde sensing properties is proposed. Our results further prompt that the ultrathin SnO₂ nanosheets could provide more potential for applications as fast detect harmful gases sensor at a relatively low optimum working temperature in a broad linear concentration range and a low concentration detection limit.

2. Experimental section

2.1 Synthesis of ultrathin SnO₂ nanosheets

The reagents such as tin(II) chloride dihydrate (SnCl₂·2H₂O), sodium hydroxide (NaOH), cetyltrimethylammonium bromide (CTAB) were of analytical-grade purity and used as received without further purification. Distilled (DI) water was used throughout the experiments. Ultrathin SnO₂ nanosheets were synthesized followed our reported method with some modification.³¹ In brief, the same as mass of SnCl₂·2H₂O and NaOH, 0.8 g of CTAB were dissolved in 35 mL of distilled water with magnetic stirring for 2 h at room temperature. During this process, the mixed solution quickly turned colour. The milky precipitate gradually turned black under the final stage, and then the solution was transferred to a Teflon-lined stainless-steel autoclave with 50 mL capacity. The autoclave was put into an oven, which was already heated to 180 °C and kept at that temperature for 5 h. After cooling to room temperature, the yellowish powders were obtained and washed with distilled water and absolute ethanol for several times and dried in air at 60 °C for 12 h.

2.2 Fabrication of gas sensors and response test

A stationary gas distribution method is used for testing gas response in dry air. The test is operated in a measuring system of a CGS-8 intelligent gas sensing analysis system. The ultrathin SnO₂ nanosheets (0.2 g) are uniformly dispersed into distilled water (about 0.2 mL) and milled to get a homogeneous paste. Then the paste is coated onto the surface of a ceramic tube (outer diameter = 1.35 mm, length = 4 mm), which forms a pair of gold electrodes on the surface by the dip-coating method to form a sensor. A Ni–Cr heating wire (diameter = 0.5 mm, resistance = 35 Ω) is inserted into the tube to heat the gas sensor directly. The sensor is welded on a socket and the electrical properties of the sensor are measured by a CGS-8 intelligent gas sensing analysis system. In order to improve the long-term stability and reusability, the gas sensor is kept at the operating temperature (∼100 °C) for 12 h in air. To detect gases, a test compound such as formaldehyde vapor, is injected into a test chamber or gas bottle mixed with air. The sensor response in this paper is defined as S = R_a/R_g (reductive gases). Here R_a and R_g are the sensor resistance in dry air and a mixture of the target gas, respectively. The response time (t_res) and the recovery time (t_rec) is expressed as the time taken for the sensor output to change from R_a to R_a −90% (R_a − R_g) and change from R_g to R_g +90% (R_a − R_g) of the total resistance change in case of adsorption and desorption, respectively.⁴³

2.3 Characterization

The crystal phase of the synthesized products is characterized by a Rigaku D/max-2500 X-ray powder diffraction (XRD) with Cu Kα₁ radiation (λ = 1.54056 Å) in the range of 20–80° (2θ) at a scanning rate of 6° min⁻¹. Field-emission scanning electron microscopy (FESEM) images are obtained by using a Magellan 400, FEI microscope operating at 20 kV. Transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and high resolution TEM (HRTEM) images are obtained on a JEOL JEM-2200FS microscope operated at 200 kV. Nitrogen (N₂) adsorption–desorption isotherms are obtained using a Micromeritics ASAP 2420 N₂ adsorption apparatus. The Brunauer–Emmett–Teller (BET) specific surface areas (S_BET) were calculated using the BET equation.⁴⁴

3. Results and discussion

3.1 Structural and morphological characteristics

The crystal phases of the products with yellowish powders are investigated by the XRD pattern. Fig. 1 provides further insight into the crystallinity of the products. All the diffraction peaks can be very well indexed to the standard tetragonal rutile phase SnO₂, which is consistent with the standard data file (JCPDS Card no. 77-452), space group P42/mnm (no. 136) and lattice parameters of a = b = 0.47552 nm, c = 0.31992 nm. No diffraction peaks corresponding to any other impurities are detected, which further confirm the excellent purity and crystallinity of these products. As shown in Fig. 2, the typical FESEM images clearly show the morphology and structure of the SnO₂ products at different magnifications. The low magnification FESEM image (Fig. 2a) displays that the products are consisted of nanosheets, indicating the high uniformity of the products and no other morphologies can be found in Fig. 2a. The detailed morphology information about the ultrathin SnO₂ nanosheets is presented in an enlarger-magnification FESEM (Fig. 2b and c). It further confirms that the thicknesses of such unique ultrathin SnO₂ nanosheets are about 10–30 nm. More structural information about the ultrathin SnO₂ nanosheets is provided by TEM, HRTEM and SAED. The representative HRTEM image(Fig. 3b) shows that the planar space of the lattice fringes is about 0.357 nm, corresponding to the (110) plane of tetragonal rutile SnO₂ structure (JCPDS Card no. 77-452).

Fig. 1 XRD patterns of the SnO₂ products.

Fig. 2 (a) A low-magnification, (b) and (c) an enlarged FESEM image of the ultrathin SnO₂ nanosheets.

Fig. 3 (a) TEM image and (b) HRTEM image of ultrathin SnO₂ nanosheets. The inset of (a) is the corresponding SAED patterns.

The lattice planes which continuously extend in each nanocrystal show the good crystallinity of the sample. The SAED pattern is shown in the inset of Fig. 3a. The lattice fringes in the HRTEM image and the corresponding SAED pattern indicate the monocrystalline nature of the ultrathin SnO₂ nanosheets. The sample is further characterized using N₂ adsorption and desorption. Fig. 4 shows the N₂ adsorption/desorption isotherm of the ultrathin SnO₂ nanosheets. Such structures possess a high surface area of 17.98 m² g⁻¹.

Fig. 4 The N₂ adsorption/desorption isotherm of the ultrathin SnO₂ nanosheets.

3.2 Formation mechanism of the ultrathin SnO₂ nanosheets

To explore the formation mechanism of the ultrathin SnO₂ nanosheets, time-dependent experiments are carried out with other conditions keeping constant, as illustrated by the SEM images in Fig. 5. It can be seen from Fig. 5a–c that the FESEM images of the samples have been prepared for different hydrothermal reaction times (1 h, 3 h, and 5 h). After the hydrothermal reaction for 1 h, nanoparticles grow and many flakelet nano-protrusions formed the rough surface of nanomaterials, as indicated in Fig. 5a. Extending the hydrothermal reaction for 3 h, a large quantity of SnO₂ nanosheets with partially aggregated gradually grew in Fig. 5b.

Fig. 5 FESEM image of the ultrathin SnO₂ nanosheets product synthesized at 180 °C for different reaction time: (a) 1, (b) 3, and (c) 5 h.

When the hydrothermal reaction time is prolonged to 5 h, the ultrathin SnO₂ nanosheets are finally formed (Fig. 5c). According to the time-dependent experiments, it is demonstrated that the size of the ultrathin SnO₂ nanosheets can be controlled by adjusting the reaction time. We believe that the possible fabrication mechanism and morphological evolution of the ultrathin SnO₂ nanosheets can be proposed and depicted as Fig. 6, which are schematically illustrated. On the basis of the chemical reactions of the our previous reported,³¹ SnCl₂·2H₂O, NaOH and CTAB are dissolved in distilled water with stirring under the condition of normal temperature during the initial process, in which each reagents and process is essential and indispensable to the final formation of the ultrathin SnO₂ nanosheets.

Fig. 6 Schematic illustration of the formation process of the ultrathin SnO₂ nanosheets.

The solution shortly becomes turbid and gradually appears the black precipitates under vigorous stirring after 2 h. This phenomenon indicates that double decomposition reaction and multiple nucleation have produced (Fig. 6a). The black precipitate is confirmed by XRD as same as the intermediate of the our reported³¹ (Fig. 6b), as shown in Fig. 7.

Fig. 7 XRD patterns of the precursors.

All the diffraction peaks could be very well indexed to the standard tetragonal rutile phase SnO, which is consistent with the standard data file (JCPDS Card no. 1-902), space group P4/mnm (no. 129) and lattice parameters of a = b = 0.378 nm, c = 0.479 nm. No diffraction peaks corresponding to any other impurities are detected, which indicate the excellent purity and crystallinity of these SnO products. The black muddy solution with SnO is transferred to a Teflon-lined stainless-steel autoclave in the oven of 180 °C for 5 h by hydrothermal strategy. The oxidation of Sn(II) into Sn(IV), the oxidation of Sn(II) into Sn(IV), the oxidation into SnO₂ can be performed by dissolving oxygen in the precursor solution.³⁷ The oxidation process and the most stable (110) surface energies are crucial for the formation of the nanosheet structure, while the coproduct of SnO may be produced because of the limited dissolved oxygen.⁴⁵ Surfaces with high reactivity usually diminish rapidly during the crystal growth to minimize the total surface energy.⁴⁶ In addition, the organic long-chain surfactants (CTAB) are able to preferentially adsorb on certain crystal facets and change their relative stability in terms of the surface energy, tune the growth rates, and thus tailor into nanosheet structure of the growing crystals.^47,48 These flexible nonionic polymers containing numerous hydrophilic and hydrophobic sites direct the aggregation of SnO₂ nuclei and the following self-assembly growth^31,37 (Fig. 6c). Eventually the sample further grows into the ultrathin SnO₂ nanosheets (Fig. 6d). Such ultrathin nanosheets are considered to be an ideal factor for gas sensing application with a large specific surface area.

3.3 Gas sensing properties

As we know, the admissible concentration of formaldehyde gas for both environmental protection and human being is strictly limited in the OSHA (Occupational Safety and Health Administration, USA).⁴⁹ To fast detect formaldehyde in a broad linear range and a low concentration by semiconductor gas sensors with high sensitivity is becoming more and more urgent. We expect that fabricated gases sensor, based on the ultrathin SnO₂ nanosheets, will be investigated the gas sensing performances for fast detect formaldehyde.

Each sensor exhibits the highest response to the targeted gases at the optimal operating temperature. So the optimum operating temperature is one of the most important factors in the gas sensing study of semiconductor oxide sensors. To research the influence of operating temperature and to determine an optimum operating temperature, the response of the gas sensor toward 100 ppm formaldehyde vapor is examined as a function of operating temperature and the result is shown in Fig. 8. It is clear that the sensing response first increases with temperature (up to 240 °C) and then decreases from the curve (ranging from 160 °C to 280 °C), which present typical n-type gas sensing behaviors, a decrease in resistance on exposure to reducing gases. The maximum response is 7 at the optimum operating temperature. Thus the optimized 240 °C is chosen for further gas sensing tests of the ultrathin SnO₂ nanosheets sensor.

Fig. 8 The responses of the ultrathin SnO₂ nanosheets sensor upon exposure to formaldehyde (100 ppm) at the different operating temperatures (160–280 °C).

The selectivity of the ideal gas sensors is an important parameter from the view of the practical application. To get information about the gas sensing selectivity of the ultrathin SnO₂ nanosheets sensor, the responses of the sensing ability among various testing gases with a concentration of 100 ppm are compared, such as formaldehyde (HCHO), IPA ((CH₃)₂CHOH), methanol (CH₃OH), acetone (CH₃COCH₃) and toluene (C₇H₈) at the optimum operating temperature of 240 °C.

From the data shown in Fig. 9, the results imply that the sensor based on the ultrathin SnO₂ nanosheets, exhibits an obvious response for formaldehyde (HCHO) and a lesser effect for other aforementioned testing gases, where the sensor responses are 7 (formaldehyde), 3.5 (IPA), 2.6 (methanol), 2.6 (acetone) and 1 (toluene), respectively.

Fig. 9 Responses of the sensors upon exposure to five kinds of testing gases (100 ppm) at 240 °C.

Thus, formaldehyde is first chosen as a representative reductive gas for investigate the gas sensing properties of the ultrathin SnO₂ nanosheets. The formaldehyde sensor response of the ultrathin SnO₂ nanosheets is evaluated. Fig. 10 presents the response versus formaldehyde concentration of the ultrathin SnO₂ nanosheets sensor at the low operating temperature of 240 °C, set as 5, 10, 30, 50, 100, 200, 300, 500, and 1000 ppm. The response sharply increase with increasing the formaldehyde concentration in the range of 5–500 ppm. When the concentration reaches above 500 ppm, the increase in response of the sensors becomes slight. It indicates that the sensor has become more or less saturated, and the sensor reaches saturation about 1000 ppm. Noticeably, the ultrathin SnO₂ nanosheets sensor exhibit good response to formaldehyde at low concentration of 5 ppm, the corresponding responses is about 3. Furthermore, the gas sensing response and recovery time (t_res/t_rec) is another very remarkable parameter to evaluate the practical application of the gas sensor. We investigate the response and recovery time of the ultrathin SnO₂ nanosheets sensor to formaldehyde with the concentrations of 100 ppm at the optimal temperature (240 °C), as shown in Fig. 11. Measured from the enlarged sensing curves of response and recovery time, it is found that the sensor exhibits a fast response–recovery characteristic for detection formaldehyde, with the response time within 1 s and the recovery time about 6 s, respectively, indicating the fast sensing ability of the ultrathin SnO₂ nanosheets sensor. Remarkably, the results demonstrate that the sensor immediately responds when formaldehyde gas is introduced and then rapidly recovers to its initial value after the formaldehyde is released. These times are short enough to meet efficient for formaldehyde gas detection completely.

Fig. 10 Dependencies of response on formaldehyde concentration for the ultrathin SnO₂ nanosheets sensor.

Fig. 11 The transient response of the ultrathin SnO₂ nanosheets sensors exposure to 100 ppm formaldehyde at 240 °C.

In the case of the sensors, reversibility and repeatability are important parameters for reliable applications. To examine the reversibility and repeatability of the sensor, we have also repeated the sensing test to 100 ppm formaldehyde at 240 °C.

As shown in Fig. 12, four periods are measured and the response of the ultrathin SnO₂ nanosheets sensor only exhibits a slight decrease compared with the former test of 100 ppm at the optimal temperature, however, the fast response and recovery speed is still maintained.

Fig. 12 Response and recovery repeatability curve to 100 ppm for the sensor at 240 °C.

Obviously, the sensor shows a good reversibility and repeatability from the curve. An overview from Fig. 13 presents the representative dynamic real-time response curves of the ultrathin SnO₂ nanosheets sensor for detecting formaldehyde with concentrations ranging from 5–100 ppm at the low operating temperature of 240 °C. From the curves, it can be seen that sensor resistance values descend or ascend when formaldehyde vapor is in or out from the gas chamber, revealing the n-type semiconductor characteristic of the sensor, and the response amplitude of the sensors increases gradationally with increasing the gas concentration. After five cycles between the test gas and clean air are successively recorded, corresponding to five different formaldehyde concentrations of 5, 10, 30, 50, and 100 ppm, the responses of the ultrathin SnO₂ nanosheets are about 2.9, 3.2, 4.2, 5, and 7, respectively. The resistance values could recover their initial state, which indicates that the sensors have good reversibility.

Fig. 13 Real-time response curves of the sensor upon exposure to different concentrations of formaldehyde at 240 °C.

3.4 Gas sensing mechanism

It is well-known that the sensing mechanism for SnO₂-based gas sensors, one of the most representative n-type oxide semiconductor gas sensing materials, is mainly explained as the electron depletion layer generated by the chemisorption of oxygen species on their surface.^5,50 The chemisorption of oxygen plays an important role in the electrical transport properties of SnO₂ by the surface-depletion model.⁵¹ The change in resistance is accompanied by the chemical interaction and charge-transfer between chemisorbed oxygen and the test-gas with their surface in this model.⁵²

In comparison with the previous gas sensor properties reports of hierarchical SnO₂ nanostructures in the literatures, it shows that the ultrathin SnO₂ nanosheets sensor of this work own very fast response/recovery time at a relatively low optimum working temperature of 240 °C (in this work), as shown in Table 1. In the case of the ultrathin SnO₂ nanosheets structure, the greatly improved sensitivity to formaldehyde may be due to the following factors.

Table 1 The performances of formaldehyde gas sensors based on various SnO₂ architectures described in the literatures compared to the ultrathin SnO₂ nanosheets prepared in this research

Sample	Formaldehyde (ppm)	S	tres/trec (s)	T (°C)	Ref.
Ag coated ZnO–SnO2	50	95	21/64	210	56
Porous SnO2 microcubes	100	57.4	13/35	280	39
SnO2 IO	100	629	12/58	215	15
Nanofragment SnO2	100	39.4	9/12	300
Nanocoral SnO2	100	43.6	7/8	300	55
Nanograss SnO2	100	55.6	6/7	300
Porous SnO2 rods	100	21.4	3/12	240	49
Ultrathin SnO2 nanosheets	100	7	<1/6	240	This work

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The ultrathin SnO₂ nanosheets are assembled according to a certain angle on the ceramic tube surface, which is favorable to provide a large percentage of certain facets exposed, and thus highly reactive surfaces have been widely explored.³⁷ In this specific structure, which presents a BET specific surface area of 17.98 m² g⁻¹, the diffusion and mass transport of gas molecules are facilitated.

Furthermore, the ultrathin SnO₂ nanosheets are so thin and afford direct conduction pathway, which can make electron transport more effective, and therefore improve the gas-sensing properties. When the ultrathin SnO₂ nanosheets sensor contact with air at the optimum working temperature, the more oxygen molecules can easily diffuse reach the relatively large sides of the ultrathin nanosheet, and trap mass electrons from the conductance band of SnO₂ to product more reactive oxygen ions species (O⁻), rapidly forming an electron depletion layer at the direct conduction pathway of their characteristic interface. The electronic concentration in the conduction band is immediately decreased which results the electron transport can be descend in the formation of potential barrier, and the ultrathin SnO₂ nanosheets sensor shows a high-resistance value in air at once. The reactive oxygen ions (O⁻) are deemed to be ascendant at the optimum working temperature of 240 °C.⁵³

In the reductive tested vapor, the reactive oxygen ions (O⁻) approach and react with the reductive tested vapor molecules such as (e.g., formaldehyde). The trapped electrons are rapidly oxidized and fed back into the conductance band of SnO₂, resulting in the decreasing potential barrier height and the thinning of the depletion layer by direct conduction pathway of their characteristic interface. Therefore the senor resistance of the ultrathin SnO₂ nanosheets sensor is decreased in this reaction process. The possible reaction of formaldehyde with the adsorbed oxygen can be explained as follows:⁵⁴

HCHO + 2O⁻_(ads) → CO₂ + H₂O + 2e⁻

When the reductive tested vapor is out, the ultrathin SnO₂ nanosheets sensor is exposed to air and rapidly shows an original high-resistance value again. Obviously, the gas sensing properties of rapidly detection formaldehyde will strongly dependent on the ultrathin SnO₂ nanosheets structure on the basis of this sensing mechanism.

4. Conclusions

In summary, the superior gas sensitive materials based on the ultrathin SnO₂ nanosheets are successfully obtained by a simple and facile hydrothermal strategy, and their outstanding performance to formaldehyde gas detection is also carefully studied. The ultrathin SnO₂ nanosheets sensors show fast response/recovery time (<1 s/6 s) for the formaldehyde detection of 100 ppm at a relatively low optimum working temperature. The excellent sensing performances are attributed to the unique morphology characterizations for affording large specific surface area and more direct conduction pathway. Morphology characterizations of the ultrathin SnO₂ nanosheets are confirmed by the results of FESEM, TEM and HRTEM. A formation growth mechanism is proposed on the basis of time-dependent experiments.

Moreover, our results also demonstrate that both this simple and economic synthesized approach and the synthesized unique nanosheets are also expected to be extended to other metal oxide semiconductor materials with unique morphologies and potential applications in the detection of environmental pollutants.

Acknowledgements

This work was funded by the National Science Foundation of China, no. 11174103 and 11474124, and Specialized Research Fund for the Doctoral Program of Higher Education of China, no. 20130061110012.

Notes and references

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