Solution-Processed Gas Sensors Employing SnO2 Quantum Dot

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Solution-Processed Gas Sensors Employing SnO Quantum Dot/MWCNT Nanocomposites 2

Huan Liu, Wenkai Zhang, Haoxiong Yu, Liang Gao, Zhilong Song, Songman Xu, Min Li, Yang Wang, Haisheng Song, and Jiang Tang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10188 • Publication Date (Web): 14 Dec 2015 Downloaded from http://pubs.acs.org on December 22, 2015

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ACS Applied Materials & Interfaces

Solution-Processed Gas Sensors Employing SnO2 Quantum Dot/MWCNT Nanocomposites Huan Liu,*, † Wenkai Zhang,† Haoxiong Yu,† Liang Gao,‡ Zhilong Song,† Songman Xu,† Min Li,† Yang Wang,† Haisheng Song,‡ and Jiang Tang‡ †

School of Optical and Electronic Information, and ‡Wuhan National Laboratory for

Optoelectronics, Huazhong University of Science and Technology, 1037 Luoyu Rd., 430074 Wuhan, China ABSTRACT: Solution-processed SnO2 colloidal quantum dots (CQDs) have emerged as an important new class of gas-sensing materials due to their potential for low-cost and high-throughput fabrication. Here we employed the design strategy based on the synergetic effect from highly sensitive SnO2 CQDs and excellent conductive properties of multiwalled carbon nanotubes (MWCNTs) to overcome the transport barrier in CQD gas sensors. The attachment and coverage of SnO2 CQDs on the MWCNT surfaces were achieved by simply mixing the pre-synthesized SnO2 CQDs and MWCNTs at room temperature. Compared to the pristine SnO2 CQDs, the sensor based on SnO2 quantum dot/MWCNT nanocomposites exhibited higher response upon exposure to H2S, and the response toward 50 ppm of H2S at 70 oC was 108 with the response and recovery time being 23 and 44 s. Because of the favorable energy band alignment, the MWCNTs can serve as the acceptor of the electrons that are injected from H2S into SnO2 quantum dots in addition to the charge transport highway to direct the electron flow to the electrode, thereby enhancing the sensor response. Our research results open an easy pathway for developing highly sensitive and low-cost gas sensors.

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KEYWORDS: tin oxide, colloidal quantum dot, MWCNT, gas sensor, hydrogen sulfide

1. INTRODUCTION Solution-processed semiconductor nanocrystals or colloidal quantum dots (CQDs) have been investigated extensively due to the convenient fabrication of solids directly from the solution phase on any reasonable substrate.1-3 Many solution-processing approaches use low temperatures and large, potentially flexible and lightweight substrates, reducing the manufacturing cost.4 The CQD-based optoelectronic devices continue to attract increasing attention,5,6 and a number of breakthroughs in gas sensors have been recently achieved with metal chalcogenide CQDs.7-11 The environmental friendly SnO2 CQDs have also been utilized to develop low-temperature chemiresistive H2S gas sensors;12 their enhanced sensing performance were attributed to the extremely large and sensitive area of CQDs with the benefit of their extremely small crystal size that was well preserved in sensor devices owing to the room-temperature fabrication of the solution-processed gas sensors. However, the low mobility of solution-processed inorganic semiconductors remains a challenge to be overcome. The mobility of CQD film is usually low at 10–2–10–3 cm2V–1s–1,13-15 with the highest reported being 0.1 cm2V−1s−1.16 On the other hand, the one-dimensional nanomaterials such as multiwalled carbon nanotubes (MWCNTs) have been shown excellent charge transport with a mobility of 104


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cm2V−1s−1.17,18 Therefore, recent investigations have been logically extended to the nanocomposites of quantum dots and MWCNTs. Indeed, the presence of carbon nanotubes in PbS CQD-based optoelectronic devices has been shown to be associated with efficient and fast photocharge separation and collection.19-21 We thereby took the view that both the charge transfer at the quantum dot/gas interfaces and the electron transport to electrode for collection might be enhanced through the incorporation of nanotubes in CQD gas sensors. In the work reported here, we demonstrated for the first time a solution-processed gas sensor based on SnO2 quantum dot/MWCNT nanocomposites. While the nanocomposites of MWCNTs and metal oxides have exhibited better gas-sensing performance compared to either pristine carbon nanotubes or metal oxides, most of those materials and sensors were prepared via the chemical vapor deposition (CVD),22 electron beam (e-beam) evaporation23 or atomic layer deposition (ALD),24 respectively. Their high-temperature processing and high-vacuum conditions were not only accompanied by high-cost equipment, but also limiting the chemical activity of the nanocomposites and thereby necessitating high sensor operating temperature. From both the fundamental and technological viewpoints, the fabrication temperature of gas sensors and their operating temperature need to be decreased. Instead, we constructed the SnO2 CQD/MWCNT nanocomposite-based gas sensors in air ambient at room temperature. The SnO2 CQDs and MWCNTs were simply mixed under magnetic stirring, followed by a spin-coating deposition onto alumina ceramic substrates with electrodes. The sensors were highly sensitive and



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selective toward H2S gas at 70 oC, and they were fully recoverable upon gas release. For the sensing mechanism, we proposed that in addition to the excellent access of gas molecules to SnO2 CQD surfaces as well as the superb electron transport in MWCNTs, the favorable energy level alignment of SnO2 quantum dot/MWCNT was crucial for the rapid and sensitive H2S detection. Our solution-processed gas sensors were highly attractive due to their superior sensing performance as well as the possibility for large-scale production.

2. EXPERIMENTAL SECTION Materials Synthesis. The SnO2 CQD/MWCNT nanocomposites were synthesized by mixing the pre-synthesized SnO2 CQDs and MWCNTs under magnetic stirring. In a typical synthesis of SnO2 CQDs,25 0.6 g tin (IV) chloride pentahydrate (SnCl4·5H2O) was dissolved in the mixture of 20 mL oleic acid (OA), 2.5 mL oleylamine (OLA) and 120 µL deionized water. The mixed solution was heated to 80 oC for 6 h under a N2 gas flow. Then, 10 mL of anhydrous ethanol was added and the mixture was transferred into a Teflon-lined stainless steel autoclave to react at 180 °C for 3 h. The products were centrifuged at 8000 rpm for 10 min. The precipitate was dispersed in toluene and centrifugally washed with ethanol for three times. The precipitate was dispersed in 14 mL toluene to form a SnO2 CQD solution. MWCNTs synthesized by traditional chemical vapor deposition method with the length and diameter of 5–15 µm and 40–80 nm, respectively21 were purified and then dispersed in chloroform (1 mg/mL). For the synthesis of SnO2 CQD/MWCNT nanocomposites, 2 mL SnO2 CQD toluene solution and 1 mL of MWCNT chloroform suspension were mixed together


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under magnetic stirring at room temperature for 3 days. Sensor

Fabrication.

The

film

deposition

of

SnO2

CQD/MWCNT

nanocomposites was carried out in air ambient at room temperature using the spin-coating method. We employed the alumina ceramic substrates pre-patterned with a pair of interdigital Ag electrode. Typically, 120 µL SnO2 CQD/MWCNT solution was dropped onto the substrates and spin-coated at 1100 rpm for 30 s, repeated this procedure to get a two-layered film. Next, four drops of diluted AgNO3 in methanol (10 mg/mL) were dropped onto the film, waited for 45 s and spun dry at 1100 rpm for 30 s, repeated the AgNO3 treatment twice. Finally, anhydrous methanol was added to flush the film and spun dry at 1100 rpm for 30 s, repeated the methanol wash twice. Characterization. High-resolution transmission electron microscopy (HRTEM) images were recorded with a JEOL-2100 microscope operating at an accelerating voltage of 200 kV. UV–vis absorption spectra were measured by a PerkinElmer Lambda 950 UV/vis/NIR spectrophotometer. The Raman spectra were measured by using a LabRAM HR800 Microlaser Raman spectrometer in backscattering geometry using the 514.5 nm line of Ar+-laser as an excitation source. SEM images were obtained by a FEI Sirion 200 scanning electron microscope. Work functions were measured by a KP 020 Kelvin probe (KP Technology, UK). The ultraviolet photoelectron spectroscopy (UPS) measurement was performed using an Omicron Nanotechnology system with a base pressure of 2×10−10 Torr. It was carried out by using an unfiltered HeI (21.21 eV) gas discharge lamp source with an energy resolution of 0.1 eV.



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Sensor Measurement. The sensor measurement was conducted following similar steps described in our prior work where the details of station configuration and gas-sensing test steps were described.26 The sensor devices were placed over the test-board with controlled temperature with the accuracy of ±2 oC. The sensor resistance was continuously recorded upon gas exposure or release using a Keithley 2400 source meter (Keithley Instrument, USA). All gas sensing tests throughout the work were performed under atmospheric pressure with the relative humidity of (45±1)%. The static method was employed and the gas concentration was determined by the volume ratio.

3. RESULTS AND DISCUSSION Figure 1 illustrated our pathway to the solution-processed gas sensors based on SnO2 quantum dot/MWCNT nanocomposites. Colloidal quantum dots always have surfaces capped with long-chain organic ligands and/or molecules which commonly prevent further growth/aggregation in synthesis and favor their solubility in polar or nonpolar solvents.25 The as-synthesized SnO2 CQDs were capped by 18-carbon-atom-chained OA and OLA ligands through the interaction of Sn atom with carboxylic acid and amine groups, respectively. It should be noted that the MWCNTs were directly used without any surface functionalization. In contrast, the pre-functionalization of carbon nanotubes with OLA were found to be crucial to the efficient attachment of OLA-capped PbS CQDs19 and OLA-capped CdS nanoparticles onto the CNTs.27 We thus assumed that upon mixing with the OA and OLA capped SnO2 CQDs, the MWCNT surface might be spontaneously functionalized with the OA and/or OLA


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molecules excessive in the SnO2 CQD solution, and then the strong hydrophobic interaction19,28 of the OA and OLA ligands both on SnO2 CQD and MWCNT surfaces led to the noncovalent binding of SnO2 quantum dots to MWCNTs. Similar phenomenon has been observed in the anchoring of OA-capped PbS CQDs onto non-functionalized MWCNTs in our prior study.21

Figure 1. Schematic illustration for the materials synthesis and sensor fabrication of the SnO2 CQD/MWCNT nanocomposites. The molecular structures of oleic acid (OA) and oleylamine (OLA) are shown as insets. Colors are grey (carbon), yellow (hydrogen), red (oxygen) and green (nitrogen).

While the detailed mechanism showing how the oleic acid and oleylamine ligands and/or molecules take part in the synthesis of SnO2 CQD/MWCNT nanocomposites still needs further investigation, the efficient attachment and coverage of the SnO2 quantum dots onto the MWCNTs can be indicated by the HRTEM image (Figure 2a). The image obtained at higher magnification (Figure 2b) clearly indicated the bonding of SnO2 quantum dot to the MWCNT. The strong stability of the SnO2/MWCNT



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nanocomposites against the vigorous stirring and sonication during the sample preparation suggested the effective binding of the SnO2 quantum dots to MWCNTs. It is possible that because of the robust hydrophobic-hydrophobic association of the long carbon chains capped on surfaces, the SnO2 quantum dots could be stably bound to surface of MWCNT by the Van der Waals interaction through the OA and/or OLA bridging. It was observed that SnO2 quantum dots were well crystallized. The plane distances of (110) and (101) were estimated to be 0.334 and 0.264 nm, respectively.

Figure 2. The HRTEM image of the SnO2 CQD/MWCNT nanocomposites at different magnifications (a) ×500000, (b)×1000000, respectively.

The presence of MWCNT and SnO2 in the nanocomposites was further confirmed by spectrum studies. According to the Raman spectra (Figure 3a), the SnO2 CQD/MWCNT nanocomposites, similar to the pristine MWCNTs, exhibited three major peaks at 1337, 1585 and 2674 cm−1, ascribed to the D, G and the second-order D band of MWCNTs, respectively. The Raman spectrum of the SnO2 CQD/MWCNT nanocomposites at low frequency phase was consistent with that of the pristine SnO2 quantum dots; their peak at 2925 cm-1 was ascribed to the C-H bands from the OA and OLA ligands surrounding SnO2 CQDs. The quantum confinement in the SnO2 CQD/MWCNT nanocomposites was investigated by the UV-vis absorption spectra (Figure 3b). Both the nanocomposites and the pristine SnO2 CQDs had an excitonic peak at ~269 nm that corresponds to a bandgap of 4.61 eV and the MWCNT exhibited


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a featureless broad absorption spectrum. The spectrum studies confirmed the stability of the SnO2 CQD/MWCNT nanocomposites, in which the quantum confinement effect of SnO2 quantum dots were well conserved. \

Figure 3. (a) UV-Vis absorption and (b) Raman spectra of SnO2 CQD/MWCNT nanocomposites, SnO2 CQDs and MWCNTs, respectively.

The direct mixing of colloidal quantum dots with MWCNT is simple and efficient in producing stable nanocomposites, and it also promises the resulting nanocomposites will be solution-processable, which enables the construction of gas sensors via the spin-coating deposition in air ambient at room temperature. The room-temperature sensor fabrication not only offers possibilities for scale-up production with low cost, but also opens more room for the benefits of nanostructured materials to be well maintained and fully utilized in real sensor devices. Hydrogen sulfide (H2S) is a corrosive and flammable gas often present in natural and industrial environments. Human body also produces small amount of H2S gas, which could act as a breath marker of halitosis in exhaled breath analysis. While the H2S-sensitive semiconductor materials have been extensively investigated, most of the sensors have to be operated at high temperatures above 150 oC to ensure sufficient



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sensing magnitude and kinetics.29 Owing to the enhanced grain size effect, the SnO2 CQD gas sensors exhibited sensitive response when operated at a relatively low temperature around 70 oC.12 We therefore sought to examine and compare the SnO2 CQD/MWCNT nanocomposites with the pristine SnO2 CQDs for H2S gas detection. We conducted six successive cycles of H2S gas exposure/release with increasing concentration and recorded the dynamic response curves. Each cycle consists of three consecutive steps that include 1) exposure of the device to clean air to record a base value of the sensor resistance (Ra), 2) injection of the target gas of certain concentrations to register a response signal of the resistance (Rg), and 3) exposure of the device to clean air aided by vacuum pumping to recover the sensor. The sensor response was defined as the ratio of Ra to Rg. The response time (T90) and the recovery time (T10) were defined as the time required for a 90% change in the full magnitude change of the response.

Figure 4. (a) Response curves of the sensors based on SnO2 CQD/MWCNT nanocomposites and pristine SnO2 CQDs upon H2S exposure/release cycles at 70 oC. (b) Selectivity of the SnO2 CQD/MWCNT gas sensor at 70 oC.


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As shown in Figure 4a, gas sensors based on the SnO2 CQD/MWCNT nanocomposites and pristine SnO2 CQDs both exhibited sensitive response and fast recovery upon H2S exposure/release cycles at 70 oC. At the gas concentration of 3.3, 10, 16.7, 33.6, 50 and 100 ppm, the response of the SnO2 CQD/MWCNT sensor was 5.0, 15, 26, 50, 108 and 181, while the response of pristine SnO2 CQD sensor was 2.9, 7.2, 16, 29, 37 and 47, respectively. Therefore, the SnO2 CQD/MWCNT nanocomposites were more sensitive than the pristine SnO2 CQDs for H2S gas detection. Owing to this improvement, a lower limit of detection (LOD) could be achieved: the theoretical LOD was calculated to be 43 and 71 ppb in the case of SnO2 CQD/MWCNT nanocomposites and pristine SnO2 CQDs, respectively (Figure S1). Moreover, while their response increased as the gas concentration increased in the range of 3.3-100 ppm, the response of SnO2 CQD/MWCNT sensors increased approximately linearly at higher concentrations, unlike the pristine SnO2 CQD sensor that gradually saturated. Our results suggest that the H2S sensing capability of the SnO2 CQD gas sensors was enhanced by the incorporation of MWCNTs. Selectivity is another critical parameter of the gas sensor for practical applications. We compared the response of the SnO2 CQD/MWCNT gas sensors toward several hazardous air pollutants in Figure 4b. The inset shows the dynamic response curves upon gas exposure and release of NH3, SO2 and NO2, respectively. At 70 oC, the sensors were strongly selective toward H2S: they had no response to SO2 and the response to NH3 and NO2 were rather low yet recoverable. Specifically, the sensor resistance increased in the presence of NO2 that generally acts as an oxidizing agent,



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unlike H2S and NH3 leading to the resistance decrease. The H2S-sensing selectivity of the SnO2 CQD/MWCNT gas sensors against NH3, SO2 and NO2 may be electronic and chemical, where the operating temperature, the gas adsorption and the electronegativity of gas molecules, as well as the surface states of the SnO2 quantum dots are key factors. But the exact mechanism showing why the sensor was selective to H2S when operated at 70 oC still needs further investigation. We further summarized the performance of our SnO2 CQD/MWCNT gas sensors and compared it with other H2S gas sensors based on SnO2 and/or carbon nanotubes that have impressive operating temperatures (≤ 150 oC) reported in recent literatures (Table 1). At a moderately elevated temperature around 70

o

C, the SnO2

CQD/MWCNT sensors we reported here provided one order of magnitude higher response with rapid response and recovery kinetics, suggestive of relatively low energies required for H2S gas adsorption and desorption. These chemiresistive gas sensors were made by the simple spin-coating approach onto substrates at room temperature without sintering and/or high vacuum conditions required in the screen-printing and CVD fabrications. The SnO2 CQD/MWCNT nanocomposites were synthesized via the direct mixing in solution phase. The highly sensitive and fast response combined with the ease of fabrication make the solution-processed SnO2 CQD/MWCNT gas sensors particularly attractive for portable and wearable gas monitoring systems.


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Table 1. H2S-sensing performance of different sensor devices based on SnO2 and/or carbon nanotubes (CNTs). Materials

Operating temperature (oC)

Gas concentration (ppm)

Response

Response /recovery time

Film preparation method

Reference

SnO2/MWCNT

70

50

108

23 / 44 s

Spin coating

This work 30

SnO2/CNT

25

50

1.33

~60 / 60s

Hot filament CVD

SnO2 /MWCNT

90

20

1.36

~600 s / -(70oC~2h for recovery)

Drop coating

31

Pt/MWCNT

150

1500

1.03

~5 / >25 min

Spin coating

32

Ag/SWCNT

25

2

~1.29

~45 s/ --

Drop casting

33

SnO2

150

10

54

N/A

RF sputtering

34

The sensitive response of the SnO2 CQD/MWCNT gas sensors may be firstly attributed to the sensitive SnO2 CQD surfaces with excellent accessibility to gas molecules due to their extremely small crystal size that were well maintained in the sensor devices. A preliminary attempt for optimizing the amount of MWCNTs (Figure S2) indicated that the pristine MWCNTs had no response and excessive amount of MWCNTs even decreased the response, suggesting the role of SnO2 quantum dots as the key sensing materials in the nanocomposites. On the other hand, insufficient amount of MWCNT resulted in limited sensor performance enhancement. Furthermore, the SnO2 CQD/MWCNT film had a compact surface morphology, unlike the porous SnO2 CQD film with some cracks (Figure S3). We inferred that the sensing-performance enhancement brought by MWCNT was not due to morphology improvement since a porous film should be more favorable for gas adsorption and desorption. We thereby proposed that the enhancement might be due to a synergetic effect between the highly sensitive SnO2 CQDs and the conductive MWCNTs achieved through a moderate incorporation of MWCNTs in the SnO2 quantum dot



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matrix. We employed energy band diagram to investigate further the sensing mechanism of SnO2 quantum dot/MWCNT nanocomposites. The Kelvin Probe measurement carried out in air ambient indicated that the work function of MWCNT (WMWCNT) and SnO2 CQD films (WSnO2) was approximately 4.70 and 4.92 eV, respectively. It was noted that the incorporation of MWCNTs did not alter the work function since the average value of the work function of the SnO2 CQD/MWCNT nanocomposites was measured to be 4.92 eV as well (Figure 5), once again suggesting the role of the SnO2 as the matrix in the SnO2 CQD/MWCNT nanocomposites. For simplicity, we took the MWCNT as a conductive material and depicted the schematic band structure of the SnO2 CQD/MWCNT nanocomposites in thermodynamic equilibrium according to the Schottky-Mott model of a metal-semiconductor junction in the case where the metal work function is less than that of the semiconductor.

Figure 5. Work functions of (a) the SnO2 CQD/MWCNT film and (b) pristine SnO2 CQD film.


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As shown in Figure 6a, the valence-band edge (EV) of SnO2 CQD films measured from the UPS spectrum was -8.22 eV; the conduction-band edge (EC) was then calculated to be -3.61 eV according to the bandgap (~4.61 eV) of SnO2 CQDs which was significantly broadened compared to bulk SnO2 due to the quantum confinement effect. When the conductive MWCNT and the n-type SnO2 semiconductor are brought into contact, the electrons pass from the MWCNT to the SnO2 quantum dot: this creates a positive charge layer at the MWCNT surface and a negative one near the SnO2 surface, until there is only a single common Fermi level. Because of the existence of these charges, there is an electric field and hence a downward band bending of 0.22 eV (WSnO2- WMWCNT) near the SnO2 surface (Figure 6b).

Figure 6. Schematic band structure of SnO2 CQD/MWCNT junction. (a) no contact, and (b) in contact. E0 denotes the vacuum level, EF denotes the Fermi level, W denotes the work function, EC and EV denote the conduction-band edge and valence band edge, respectively.

When the n-type SnO2 quantum dots are exposed the H2S gas which acts as a reducing regent in general, a direct electron injection from the H2S into the conduction band of SnO2 quantum dots may occur due to the adsorption of H2S which forms donor-like surface states and induces the charge transfer. It is also possible that



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the H2S reacts with the pre-adsorbed surface oxygen and thereby releases the electrons trapped by the oxygen adsorbates back to SnO2 quantum dots. In any case, the resistance of the n-type SnO2 quantum dots will decrease due to the increased amount of electrons in the conduction band, shown as the response. According to the energy band alignment at the SnO2 quantum dot/MWCNT interfaces, the MWCNTs can accept the electrons that are injected from H2S into SnO2 quantum dots. Furthermore, the conductive MWCNTs can serve as the charge transport highway to direct the electron flow to the electrode for collection, thereby overcoming the transport barrier that generally present in CQD films due to the low mobility. Therefore, the transport of the H2S-induced electrons in the whole sensor was promoted because of the favorable energy band alignment and the high electron mobility of MWCNTs. It was also possible that the SnO2 quantum dots were not all bound onto MWCNTs and their contribution to the sensing behavior could not be neglected. Overall, the superior performance of this type of sensors over pristine CQD-based ones was attributed to the synergetic effect that incorporates the benefits of sensitive SnO2 CQDs and conductive MWCNTs, as well as their favorable band energy alignment. The room-temperature fabrication and the low-temperature operating of our solution-processed gas sensors offer excellent compatibilities with MEMS and C-MOS technologies for integration and miniaturization, potentially promising novel gas sensors with interesting functionalities for the Internet of Things.

4. CONCLUSIONS We have achieved enhanced H2S-sensing performance by employing SnO2


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quantum dot/MWCNT nanocomposites that were synthesized through a simple mixing under magnetic stirring. Owing to the solution processibility of the nanocomposites, we were able to construct the gas sensors in air ambient at room temperature via the spin-coating method. The room-temperature fabrication of solution-processed gas sensor not only promises low cost, but also enables the extremely small size and high surface activity of semiconductor nanocrystals to be well preserved in real gas sensors. In addition to the excellent access of gas molecules to highly sensitive SnO2 CQD surfaces as well as the superb electron transport in MWCNTs, the synergetic effect of SnO2 CQD/MWCNT nanocomposites was attributed to the favorable energy band alignment of SnO2 quantum dot/MWCNT for electron transport, thereby enhancing the gas-sensing performance.

ASSOCIATED CONTENT Supporting Information Calculation of the limit of detection (Figure S1). Response curves of the SnO2 CQD/MWCNT sensors with different volumetric ratio of SnO2 CQDs to MWCNTs (Figure S2). SEM images of gas sensor devices based on the SnO2 CQD/MWCNT nanocomposites and SnO2 CQDs (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (H. Liu).



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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Research described in this paper was supported by the National Natural Science Foundation of China (61571206 and 61274055). H. L. acknowledges the Program for New Century Excellent Talents in University (NCET-12-0216) and the Natural Science Foundation of Hubei Province (2015CFA055). We thank the Analytical and Testing Center of HUST and the Center for Nanoscale Characterization & Devices of WNLO for the characterization support.

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TOC

H2S

e-

e-

e-

n-SnO2

MWCNT

200

Response

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SnO 2 CQD/MWCNT

100 ppm

SnO 2 CQD

150

50 ppm

100 50 3.3 ppm

0 0

33.6 ppm 16.7 ppm 10 ppm

200

400

600

800 1000 1200

Time (s)


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Solution-Processed Gas Sensors Employing SnO2 Quantum Dot

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