Thermally Stable SnO2 Nanocrystals - American Chemical Society

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Thermally stable SnO nanocrystals: Synthesis and application to gas sensors Masayoshi Yuasa, Koichi Suematsu, Kiyomi Yamada, Ken Watanabe, Tetsuya Kida, Noboru Yamazoe, and Kengo Shimanoe Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00087 • Publication Date (Web): 24 Jun 2016 Downloaded from http://pubs.acs.org on June 25, 2016

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COVER PAGE

Thermally stable SnO2 nanocrystals: Synthesis and application to gas sensors Masayoshi Yuasa

*,†

Koichi Suematsu,† Kiyomi Yamada,‡ Ken Watanabe,

†

Tetsuya Kida,

†

Noboru Yamazoe,† Kengo Shimanoe† †

Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, JAPAN

‡

Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, JAPAN

We prepared the thermally stable SnO2 nanocrystals (ca. 4 nm) in the mixture of SnCl4, tetraethylene glycol (TEG) and a tetrabutyl ammoniumhydroxide (TBAH) under refluxing and obtained a highly sensitive semiconductor gas sensor. It has been elucidated theoretically and experimentally that the synthesis of oxide semiconductor nanoparticles is an important factor for the highly sensitive semiconductor gas sensor. However, as-synthesized nanocrystals generally grow in large size during calcination at high temperature. Such a thermal growth of crystals reduces the sensor response. Therefore, to develop the sensor response of semiconductor gas sensor, thermally stable SnO2 nanocrystals were synthesized via the heating of SnCl4-TBAH-TEG mixture up to 15 200oC under refluxing. The SnO2 nanocrystals obtained exhibited highly thermal stability even if SnO2 nanocrystal via 10 they were calcined. The gas sensing films hydrothermal Treatment fabricated from the thermally stable SnO2 (conventional) 本法 nanocrystals exhibited high sensor response to 5 hydrogen due to their small crystalline size, small SnO2 nanocrystal donor density and change in the surface property as in this work 0 compared to the conventional SnO2 nanocrystals via 0 200 400 600 800 1000 hydrothermal treatment. Calcination temperature (oC) Crystallite size (nm)

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Crystal Growth & Design

Corresponding author (Current contact details): Masayoshi Yuasa, Department of Biological & Environmental Chemistry, Faculty of Humanity-Oriented Science and Engineering, Kindai University, Iizuka, Fukuoka 820-8555, Japan, Phone: +81-948-22-5659 (Ext.253), Fax: +81-948-23-0536, E-mail: [email protected]

*

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TITLE PAGE

Thermally stable SnO2 nanocrystals: Synthesis and application to gas sensors

Masayoshi Yuasa,*,†,§ Koichi Suematsu,†,∥ Kiyomi Yamada,‡ Ken Watanabe,† Tetsuya Kida, †, #

†

Noboru Yamazoe,† Kengo Shimanoe†

Department of Energy and Material Sciences, Faculty of Engineering Science, Kyushu

University, Kasuga, Fukuoka 816-8580, Japan ‡

Department of Molecular and Material Sciences, Interdisciplinary Graduate School of

Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan


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ABSTRACT In this study, we prepared thermally stable SnO2 nanocrystals (ca. 4 nm) in a mixture of SnCl4, tetraethylene glycol (TEG) and tetrabutyl ammoniumhydroxide (TBAH) under reflux and obtained a highly sensitive semiconductor gas sensor. It has been determined both theoretically and experimentally that the synthesis of oxide semiconductor nanoparticles is an important factor in highly sensitive semiconductor gas sensors. However, as-synthesized nanocrystals generally grow large during calcination at high temperature, and this thermal crystal growth reduces the sensor response. Therefore, to refine the response of the semiconductor gas sensor, we synthesized thermally stable SnO2 nanocrystals by heating under reflux a SnCl4-TBAH-TEG mixture. The obtained SnO2 nanocrystals exhibited high thermal stability even when calcined at a temperature up to 600oC. The gas-sensing films fabricated from the thermally stable SnO2 nanocrystals exhibited a high sensor response to hydrogen due to their small crystal size, small donor density, and a change in their surface property as compared with conventional SnO2 nanocrystals synthesized via hydrothermal treatment.

KEY WORDS: semiconductor gas sensor, tin oxide, nanocrystal, thermal growth, organic solvent 3 ACS Paragon Plus Environment


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INTRODUCTION Tin dioxide (SnO2) is suitable for application in semiconductor gas sensors due to its high stability, high gas sensitivity and low cost.1,2 In air atmosphere, a depletion layer is formed on the SnO2 crystal by the adsorption of oxygen species such as O- and O2-. When target gases, such as combustion gases, are mixed with the atmospheric air around a gas-sensing film composed of SnO2 crystals, the target gases diffuse into the porous sensing film and are oxidized by the absorbed oxygen onto the SnO2 surface. This results in a change in the electric resistance of the gas-sensing film. In this gas-sensing mechanism of the SnO2-based semiconductor gas sensor, the particle size of the SnO2 nanocrystals significantly affects the sensor response.3,4 Xu et al. have reported that the electric resistance of a gas-sensing film as well as the sensor response to hydrogen (the ratio of the electric resistance in air to that in hydrogen) increases drastically when the particle size of SnO2 nanocrystals is less than a critical value.5 Yamazoe et al. have provided a theoretical basis for a semiconductor gas sensor exposed to target gases by coupling the theory of the semiconductor depletion state with the chemistry of gas absorption and reaction.6 Moreover, the authors proposed that the conventional type of depletion state (regional depletion) transforms into a new type of depletion state (volume depletion) as the partial pressure of oxygen increases or the crystal size of the semiconductor oxide decreases,7 and that the square of the sensor response to hydrogen is inversely proportional to the donor density and particle size of the SnO2 4 ACS Paragon Plus Environment

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nanocrystals in the volume depletion state.8 Their theoretical approach to these gas-sensing properties was verified experimentally by the use of size- and donor-density-controlled SnO2 nanocrystals prepared by controlling the calcination temperature and Fe3+-doping.9 As mentioned above, minimizing of the size of SnO2 nanocrystals is a significant factor in highly sensitive SnO2-based semiconductor gas sensors. Thus far, SnO2 nanocrystals have been synthesized

via

sol-gel,10,11

hydrolysis,12,13

microemulsion,14,15

spray-pyrolysis,16,17

surfactant-mediated solvent,18 and hydrothermal19,20 processes. Of these, a stable solution of size-uniformed SnO2 nanocrystals has been synthesized via hydrothermal treatment of a stannic acid gel in an ammonium solution and its gas-sensing properties investigated.21-23 However, as-synthesized SnO2 nanocrystals via hydrothermal treatment tend to grow large during calcination. This thermal growth of the nanocrystals reduces the sensor response and obstructs our experimental understanding of the gas-sensing properties of nano-sized SnO2 crystals. Therefore, SnO2 nanocrystals with high thermal stability against crystal growth during calcination at high temperature is required to optimize the performance of highly sensitive SnO2-based semiconductor gas sensors and to gain a fundamental understanding of the gas-sensing properties of SnO2-based semiconductor gas sensors. Several approaches have been reported regarding the synthesis of SnO2 nanocrystals with thermal stability against crystal growth. Kida et al. prepared SnO2 nanocrystals by heating 5 ACS Paragon Plus Environment


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tin(IV) acetylacetonate in a mixture of dibenzyl ether, oleic acid, and oleylamine at 280 oC. The obtained SnO2 nanocrystals showed thermal stability against grain growth due to the presence of a capping agent on the surface of the SnO2 nanocrystals.24 Shoyama et al. prepared SnO2 nanocrystals by the sol-gel method with polyethylene glycol and successfully obtained thermally stable SnO2 due to the presence of polyethylene glycol in the SnO2 precursor solution.25 In this work, we used a new method to synthesize single nano-sized SnO2 nanocrystals that have high thermal stability against crystal growth. In this new method, SnO2 nanocrystals are synthesized by heating an SnCl4-TBAH-TEG mixture under reflux. We obtained single nano-sized SnO2 nanocrystals to fabricate a gas-sensing film (thickness: ca. 10µm) and we also investigated the film’s gas-sensing properties that have not yet been clearly elucidated.

EXPERIMENTAL SECTION Chemicals.

Tin(IV) chloride pentahydrate was purchased from Wako Pure Chemical

Industries and used as a precursor of SnO2 nanocrystals.

Tetraethylene glycol (TEG) was

purchased from Kanto Chemical and used as a solvent.

1.0 mol.L-1 tetrabutylammonium

hydroxide (TBAH) in methanol solution was purchased from Aldrich.

Ethanol and α-terpineol

was purchased from Kishida Chemical and used as a solvent for SnO2 synthesis and a binder for the gas sensing film, respectively.

All chemicals were used as received without further 6

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purifications. Materials synthesis and characterization. Tin(IV) chloride pentahydrate (0.001mol) was dissolved into the mixture of TEG (50 mL) and ethanol (30 mL).

Separately, a 1.0 mol.L-1

TBAH in methanol solution (30 mL) was mixed with TEG (50 mL).

Each solution was heated

at 135oC for 4 h in air in order to remove water and alcohols in the solution, and mixed together in a round bottom flask (100 mL) and then re-reheated at 135 oC for 1 h.

Then the mixture was

further heated at 160-250 oC under refluxing for 12 h, producing a brown-colored precipitate. After the solution was cooled to room temperature, the precipitated was collected by centrifuge (20000rpm for 10 min) and washed by ethanol and water.

Finally, SnO2 nanocrystals were

obtained after the precipitate was dried at 120oC for 12 h.

As a comparison, a stable sol

suspension of SnO2 was synthesized via hydrothermal treatment in line with the process reported elsewhere.21

The stable sol suspension of SnO2 was evaporated to dryness at 120oC for 12 h.

As-synthesized SnO2 nanocrystals were calcined at 600-900 oC in order to investigate the thermal stability of SnO2 nanocrystals against crystal growth.

The crystalline phase were analyzed by

means of X-ray diffractmetry with Cu Κα radiation (λ = 1.54056 Å) (XRD; RINT 2100, Rigaku). The crystalline size of as-synthesized and calcined SnO2 nanocrystals was calculated by Scherer’s formula from their XRD patterns.

The morphology of as-synthesized SnO2 nanocrystals was

observed by means of transmission electron microscopy working at 200 kV (TEM; TECNAI-F20, 7 ACS Paragon Plus Environment


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FEI.).

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The presence of organic compound was confirmed by Fourier transform infrared

spectroscopy (FT-IR; FT/IR-4100, JASCO) and thermogravimetric/differential thermal analysis (TG/DTA; EXSTAR 6000, Seiko Instruments), respectively.

For the TG/DTA measurement, the

heating rate was 5oC/min and the flow rate of air was 100 ml/min. Sensor fabrication and measurements.

A powder of SnO2 nanocrystals was mixed

mechanically with α-terpineol as a binder to form a paste for gas sensing films.

The paste

obtained was fabricated to thick film (Ca. 10 µm) through a patterned-screen on alumina substrates attached with a pair of comb-type Au electrodes (electrode gap: 90 µm).

The sensor

devices obtained were settled in quarts tube and heated by a tube-type electric furnace.

The

sensor devices were connected with a standard resistor in series, and the voltage across the standard resistor was measured under an applied voltage of dc 4 V to evaluate the electrical resistance of the device.

Before the gas sensing measurement, sensor devices were pre-treated

by pure oxygen at 580 oC for 3 h and at operating temperature for approximately 12 h in order to eliminate the impurities such as –OH groups or hydrocarbons on the surface of SnO2 nanocrystal. Then, dependence of the electric resistance on the partial pressure of oxygen (PO2) and the sensor response to hydrogen was measured.

Sensor response to hydrogen (S = Rair/Rgas) was defined as

the ratio of the electric resistance in air (Rair) to in hydrogen containing air (Rgas).


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RESULTS AND DISCUSSION Preparation of thermally stable SnO2 nanocrystals. First, we investigated the effect of the heating temperature on SnO2 nanocrystal formation. Figure 1 shows the XRD patters of samples synthesized at 160, 200 and 250 oC. The patterns of the samples synthesized at 200 and 250 oC were tetragonal SnO2 (JCPDS: 41-1445).

However, the pattern of samples synthesized at 160

o

C were not tetragonal SnO2, but can be regarded as those of an organic compound derived from

either the TEG or TBAH, since the pattern did not match that of tin oxide (SnO2, Sn2O3, SnO) or SnCl4. Therefore, we consider that heating the TEG-TBAH mixture higher than 200 oC is required to synthesize SnO2 nanocrystals. The sizes of the SnO2 nanocrystals obtained at 200 and 250 oC, as calculated by the Scherer’s formula from their XRD patterns, were 3.8 nm and 4.1 nm, respectively, which is quite small and not dependent on the heating temperature. In the synthesis of SnO2 using an aqueous solution, such as with hydrothermal treatment, the sol-gel method, and the hydrolysis method, we obtained SnO2 nanocrystals via an amorphous phase (Sn(OH)4) as a precursor. In contrast, by heating the SnCl4-TBAH-TEG mixture, SnO2 crystal can be obtained directly from the SnCl4-TBAH-TEG mixture. Caruntu et al. reported that the metal ion in glycol can be complexed with glycol and Cl-, and that Cl- ligands in the metal-glycol-Cl- complex can be substituted by OH-.26 Therefore, in this study, it seems that SnCl4 and TEG formed an SnCl4-TEG-Cl- complex, and then the complex changed to an SnCl4-TEG-OH complex by the 9 ACS Paragon Plus Environment


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reaction between OH- in the TBAH and SnCl4-TEG-Cl- complex. The SnCl4-TEG-OH complex then thermally decomposed into SnO2 nanocrystals when the mixture was heated to 200 oC. In addition, it seems that the SnO2 crystals were immediately capped with molecules in the solution (Cl-, tetrabutylammonium or TEG), thereby preventing further crystal growth and nanocrystal formation. Figures 2(a) and 2(b) show TEM images of as-synthesized SnO2 nanocrystals synthesized by heating of the SnCl4-TBAH-TEG mixture at 200 oC. For comparison, Figures 2(c) and 2(d) show TEM images of SnO2 nanocrystals synthesized via hydrothermal treatment at 200 o

C. We obtained samples for the TEM observation by dispersing SnO2 nanocrystal in ethanol and

depositing the suspension on a carbon-supported Cu mesh grid. We estimated the average particle

size of the SnO2 nanocrystals via the heating of the SnCl4-TBAH-TEG mixture to be 4.0 nm. This value is in good agreement with the crystal size calculated from the peak width of the XRD patterns (3.8 nm) in Figure 1. This tendency indicates that the obtained SnO2 particles are mono-crystal. As shown in Figure 2, the particle sizes of the SnO2 nanocrystals via heating of the SnCl4-TBAH-TEG mixture and those via hydrothermal treatment are almost the same. However, the thermal stability of the SnO2 nanocrystals in these two samples was quite different. Figure 3 shows XRD patterns of the as-synthesized and calcined SnO2 nanocrystals via heating of the SnCl4-TBAH-TEG mixture. The peak width of the patterns is not significantly changed even 10 ACS Paragon Plus Environment

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after calcination at 600−900 oC. The particle size of the SnO2 nanocrystals, as calculated by Scherer’s formula from their peak width, is summarized in Figure 4, in which the particle sizes of the calcined SnO2 nanocrystals synthesized via hydrothermal treatment are also shown for comparison.

With respect to the SnO2 nanocrystals synthesized via hydrothermal treatment,

those calcined at 600 oC grew three times as large as the as-synthesized SnO2 nanocrystals. In contrast, the particle size of the SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture had not changed even after calcination at 600−900 oC. We note that the SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture also have high thermal stability. The reason for their high thermal stability against crystal growth can be attributed to the absorbed molecules on the surface of the SnO2. Cl-, TBAH, and TEG have the potential to adsorb onto the SnO2 surface during synthesis. It has been reported that residual Clon oxide particles tends to hinder the agglomeration of oxides during calcination.27 However, Clions seems to damage the sensor response of SnO2 nanocrystals and it is well known that Cl- has negative effects on the surface properties of oxides particles.28,29 However, as mentioned in the section below, SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture have a higher sensor response to H2 as compared with those synthesized via hydrothermal treatment. Therefore, Cl- ions in the reaction system might not affect the thermal stability of the obtained SnO2 nanocrystals. Accordingly, TEG, TBAH, and their decomposition products can be 11 ACS Paragon Plus Environment


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considered as candidate sources of the thermal stability of the SnO2 nanocrystals. Therefore, we investigated the structure of the absorbed molecules on SnO2 by TG/DTA and FT-IR analyses to confirm the source of the thermal stability of the SnO2 nanocrystals. Figure 5 shows the TG/DTA curve of SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture. In the TG curve, we see two large weight losses at 150−300 oC and 600−700 oC. The large weight loss between 150 and 300 oC seems to correspond with the decomposition of TEG or TBAH, because the weight loss occurred simultaneously with an exothermic peak in the DTA curve. Figure 6 shows the FT-IR spectra of SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture. We attribute the broad absorption band at 600 cm-1 in all spectra to Sn-O and Sn=O stretching.30,31 For the FT-IR spectrum of the as-synthesized SnO2 nanocrystals, we observed the same kind of absorption bands, and we attribute a broad absorption band around 3500 cm-1 to hydroxide stretching. We attribute the absorption bands at 2850 and 2920 cm-1 to symmetric and asymmetric methylene stretching, respectively, and the absorption bands at 1450, 1330, and 850 cm-1 to scissoring, wagging, and rocking of methylene, respectively. We attribute the absorption band at 1620 cm-1 to carboxylate stretching. The source of the carboxylate seems to be oxidation of the CH2OH group in TEG molecules during synthesis. Therefore, the as-synthesized SnO2 nanocrystals contain decomposition compounds of TEG as major contaminants. In the spectra of the SnO2 nanocrystals calcined at 600 oC, we confirmed residuals of methylene and carboxylate, 12 ACS Paragon Plus Environment

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which means that some organic compounds remained after calcination at 600 oC. It seems that these remaining organic compounds inhibited the agglomeration of SnO2 nanocrystals during calcination, resulting in the high thermal stability of the SnO2 nanocrystals against crystal growth. As yet, we cannot elucidate the details of the mechanism of the effect of the decomposition compound of TEG. However, Shoyama et al. reported that the addition of polyethylene glycol (PEG) in the precursor solution of SnO2 is effective in improving the thermal stability of SnO2 during calcination due to the presence of a –CH2-OH group on the surface of the SnO2 particles.25 Kato et al. reported that HCO3- and CO32- species derived from the thermal decomposition of PEG can modify the behavior of TiO2 crystal growth during calcination.32 Based on these previous reports, it is probable that the methylene group and carboxylate absorbed onto the SnO2 nanocrystals synthesized in this study improved their thermal stability against crystal growth during calcination. The organic compound seems to be eliminated at 600−900 oC, as we observed no absorption bands of the organic compound in the spectra of the SnO2 nanocrystals calcined at 900 oC. It seems that the weight loss indicated by the TG curve between 600 and 700 oC in Figure 5 corresponds with the desorption of the remaining organic compounds. This desorption of organic compounds caused a slight increase in the crystallne size from 4.2 nm (calcined at 600 o

C) to 7.1 nm (calcined at 900 oC). Gas-sensing properties of SnO2 nanocrystals. Figure 7 shows the electric resistance of a 13 ACS Paragon Plus Environment


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gas-sensing film as a function of the partial pressure of hydrogen (PH2) of the gas-sensing film of SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture and via hydrothermal treatment. The electric resistance of the gas-sensing film fabricated from SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture is much higher than those synthesized via hydrothermal treatment. We calculated the sensor response to hydrogen (S = Rair/Rgas) of the gas-sensing films from the value of the electric resistance in Figure 7 and S as a function of the partial pressure of hydrogen (PH2) shown in Figure 8. The gas-sensing film using the SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture exhibited a higher sensor response than those synthesized via hydrothermal treatment. In order to understand the reason for this increase in the sensor response, we analyzed the dependence of S on PH2 according to the theoretical approaches reported by Yamazoe et al.,33 for which S2 is in proportion to PH2 at a slope of 3C/aND when the SnO2 nanocrystal is so small as to be in the volume depletion state, and where C, a, and ND are an equilibrium reaction constant of the H2 oxidation on the SnO2 surface, the radius of the SnO2 nanocrystal, and the donor density of SnO2, respectively. Therefore, we re-plotted the S vs. PH2 plot in Figure 8 as shown in Figure 9. The correlation between S2 and PH2 had slopes of 1.23x109 and 1.12x108 atm-1 for the SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture and those synthesized via hydrothermal treatment, respectively. Their slope ratio (10.9) was far greater than the ratio of 1/a 14 ACS Paragon Plus Environment

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(2.85). Therefore, the reason for the greater sensor response of SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture seems to involve reasons in addition to the decrease in the crystalline size, including the surface impurities on the SnO2 nanocrystals. As discussed above, the SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture have organic compounds on their surfaces.

As discussed above, the SnO2 nanocrystals via the

heating of SnCl4-TBAH-TEG mixture has organic compounds on the surface.

These organic

compounds on the surface of SnO2 nanocrystals seemed to act as acceptor-like surface impurities. The acceptor-like surface impurities compensated the carrier in the SnO2 nanocrystals, reading to the increase in the sensor response.34

CONCLUSIONS In this study, we obtained SnO2 nanocrystals that have highly thermal stability against crystal growth via heating of an SnCl4-TBAH-TEG mixture up to 200 oC. These SnO2 nanocrystals were so stable against crystal growth that the crystalline size did not change after calcination at 600 oC, due to the presence of carboxylate and methylene organic compounds on the surface of the SnO2 nanocrystals. The gas-sensing films fabricated from the thermally stable SnO2 nanocrystals exhibited high sensor response to hydrogen as compared with those fabricated from SnO2 nanocrystals synthesized via conventional hydrothermal treatment. The reason for this 15 ACS Paragon Plus Environment


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high sensor response seems to be a decrease in the crystalline size, a decrease in the donor density, and a change in the surface property of SnO2.


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FIGURES

(c)

（321）

（301）

（112）

（220）

（200）

（211）（220）

（101）

（110）

: SnO2 (JCPDS 41-1445)

Intensity (a.u.)

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(b) (a) 20

30

40

50 60 2θ (degree)

70

80

Figure 1. XRD patterns of samples synthesized via heating of the SnCl4-TBAH-TEG mixture at (a) 160 oC, (b) 200 oC, and (c) 250 oC under reflux.



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(a)

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(b) 0.34 nm SnO2 (110)

20 nm

5 nm

(c)

(d)

0.34 nm SnO2 (110)

5 nm

20 nm

Figure 2. TEM images of SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture at (a) low magnification and (b): high magnification; and via hydrothermal treatment at 200 oC for 3 h in an ammonia solution at (c) low magnification and (d): high magnification.


(c)

（321）

（301）

（211）（220）（002）（220）（112）

（200）

（110）

Intensity (a.u.)

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（101）

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(b)

(a) 20

30

40 50 60 2θ (degree)

70

80

Figure 3. XRD patterns of SnO2 nanocrystals synthesized by heating the SnCl4-TBAH-TEG mixture: (a) as-synthesized SnO2, (b) after calcination at 600 oC, and (c) after calcination at 900 o

C.



15

Crystallite size (nm)

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10

(b) 本法 (a)

5

0

0

200 400 600 800 Calcination temperature (oC)

1000

Figure 4. Dependence of the particle size of SnO2 nanocrystals on the calcination temperature: (a) SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture and (b) SnO2 nanocrystals synthesized via hydrothermal treatment.


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80 Programming rate : 5o C.min -1

0

10 40 20

Weight loss (%)

60

DTA (µV)

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20 30 0 0

200

400

600

800

Temperature (o C)

Figure 5. TG/DTA curve of as-synthesized SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture.



(a)

Transmittance (a.u.)

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(b)

(c)

(d)

4000

2000

3000

1000

Wave number (cm-1) Figure 6. FT-IR spectra of (a) commercial SnO2, (b) as-synthesized SnO2 nanocrystals, (c) after calcination at 600 oC, and (d) after calcination at 900 oC.


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108 107

Electric resistance / Ω

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


106

(a)

105 104 103

(b)

102 0

200

600 400 800 Concentration of H2 (ppm)

1000

Figure 7. Electric resistance of gas-sensing film as a function of H2 concentration at 350 oC: (a) and (b) are for SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture and those synthesized via hydrothermal treatment, respectively.



1400 1200

Sensor response (Ra/Rg )

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

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(a)

1000 800 600

(b)

400 200 1 0

200

400

600

800

1000

Concentration of H2 (ppm)

Figure 8. Sensor response as a function of H2 concentration at 350 oC: (a) and (b) are for SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture and those synthesized via hydrothermal treatment, respectively.


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16 14

Square of sensor response : S2 ( x105)

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


12

(a)

10 8 6 4

(b)

2 1 0

0.2

0.4

0.6

0.8

1.0

Pertial pressure of H2: PH2 (×10-3 atm)

Figure 9. Square of the sensor response as a function of H2 partial pressure: (a) and (b) are for SnO2 nanocrystals synthesized via heating of the SnCl4-TBAH-TEG mixture and those synthesized via hydrothermal treatment, respectively.



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AUTHOR INFOMATION Corresponding author *E-mail: [email protected] Present Address §Department of Biological & Environmental Chemistry, Faculty of Humanity-Oriented Science and Engineering, Kindai University, Iizuka, Fukuoka 820-8555, Japan ∥Chemical and Texture Research Institute, Fukuoka Industrial Technology Center, Chikushino, Fukuoka 818−8540, Japan. # Department of Applied Chemistry and Biochemistry, Kumamoto University, Kumamoto, 860-8555, Japan Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was partially supported by Grant-in-Aid for Scientific Research (B) (22350064) from the Japan Society for the Promotion of Science (JSPS).


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RFERENCES (1) Göpel, W.; Schierbaum, K. D. Sens. Actuators, B 1995, 26-27, 1-12. (2) Yamazoe, N.; Sakai, G.; Shimanoe, K. Catal. Surv. Asia 2003, 7, 63-75. (3) Yamazoe, N.; Fuchigami, J.; Kishikawa, M.; Seiyama, T. Surf. Sci. 1972, 86 335-344. (4) Yamazoe,N. Sens. Actuators, B 1991, 5, 7-19. (5) Xu, C.; Tamaki, J.; Miura, N.; Yamazoe, N. Sens. Actuators, B 1991, 3, 147-155. (6) Yamazoe, N.; Shimanoe, K. Sens. Actuators, B 2008, 128, 566-573. (7) Yamazoe, N.; Shimanoe, K. J. Electrochem. Soc. 2008, 155, J85-J92. (8) Yamazoe, N.; Shimanoe, K.; J. Electrochem. Soc. 2008, 155, J93-J98. (9) Suematsu, K.; Yuasa, M.; Kida, T.; Yamazoe, N.; Shimanoe, K. J. Electrochem. Soc.2012, 159, J136-J141. (10) Rella, R.; Serra, A.; Siciliano, P.; Vasanelli, L.; De, G.; Licciulli, A. Thin Solid Films 1997, 304, 339-343. (11) Acciarri, M.; Canevali, C.; Mari, C. M.; Mattoni, M.; Ruffo, R.; Scotti, R.; Morazzoni, F. Chem. Mater. 2003, 15, 2646-2650. (12) Sangaletti, L.; Depero, L. E.; Allieri, B.; Pioselli, F.; Angelucci, R.; Poggi, A; Tagliani, A.; Nicoletti, S. J. Eur. Ceram. Soc. 1999, 19, 2073-2077. (13) Dos Santos, O.; Weiller, M. L.; Junior, D. Q.; Medina, A. N. Sens. Actuators, B 2001, 75, 27 ACS Paragon Plus Environment


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83-87. (14) Song, K. C.; Kim, J. H. J. Colloid Interface Sci. 1999, 212, 193-196. (15) Yuasa, M.; Masaki, T.; Kida, T.; Shimanoe, K.; Yamazoe, N. Sens. Actuators, B 2009, 136, 99-104. (16) Hieda, K.; Hyodo, T.; Shimizu, Y.; Egashira, M. Sens. Actuators, B 2008, 133, 144-150. (17) Yuan, L.; Guo, Z. P.; Konstantinov, K.; Liu, H. K.; Dou, S. X. J. Power Sources 2006, 159, 345-348. (18) Wang, Y.; Ma, C.; Sun, X.; Li, H. Inorg. Chem. Commun. 2002, 5, 751-755. (19) He, Y.; Li, Y.; Yu, J.; Qian, Y. Mater. Lett. 1999, 40, 23-26. (20) Zhang, Y.; Li, L.; Zheng J.; Li, Q.; Zuo, Y.; Yang, E.; Li, G. J. Phys. Chem. C 2015, 119, 19505-19512. (21) Baik, N. S.; Sakai, G.; Miura, N.; Yamazoe, N. Sens. Actuators, B 2000, 63, 74–79. (22) Sakai, G.; Baik, N. S.; Miura, N.; Yamazoe, N. Sens. Actuators, B 2001, 77, 116-121. (23) Kida, T.; Fujiyama, S.; Suematsu, K.; Yuasa, M.; Shimanoe, K. J. Phys. Chem. C 2013, 117, 17574-17582. (24) Kida, T.; Doi, T.; Shimanoe, K. Chem. Mater. 2010, 22, 2662-2667. (25) Shoyama, M.; Hashimoto, N. Sens. Actuators, B 2003, 93, 585-589. (26) Caruntu, D.; Remond, Y.; Chou, N.H.; Jun, M.-J.; Caruntu, G.; He, J.; Goloverda, G.; 28 ACS Paragon Plus Environment

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O’Connor, C.; Kolesnichenko, V. Inorg. Chem. 2002, 41, 6137-6146. (27) Parra, R.; Ramajo, L. A.; Gòes M. S.; Varela, J. A; Castro, M. S. Mater. Research Bulletin 2008, 43, 3202-3211. (28) Bollinger, M. A.; Vannice, M. A. Applied Catalysis B: Environmental 1996, 8, 417-443. (29) Gélin, P.; Primet M. Appl. Catal., B 2002, 39, 1-37. (30) Jouen, S.; Lefez, B.; Sougrati, M. T.; Hannoyer, B. Mater. Chem. Phys. 2007, 105, 189-193. (31) Culha, O.; Ebeoglugil, M. F.; Birlik, E.; Toparli, M. J. Sol-Gel Sci. Technol. 2009, 51, 32-41. (32) Kato, K.; Niihara, K. Thin Solid Films 1997, 298, 76-82. (33) Yamazoe, N.;Suematsu, K.; Shimanoe, K. Sens. Actuators, B 2012, 163, 128-135. (34) Kissine, V.V.; Sysoev, V.V.; Voroshilov, S.A. Sens. Actuators, B 2001, 79, 163-170.



Table of contents graphic (For table of contents use only)

Masayoshi Yuasa,*,† Koichi Suematsu,† Kiyomi Yamada,‡ Ken Watanabe,† Tetsuya Kida,† Noboru Yamazoe,† Kengo Shimanoe†

†Department of Energy and Material Sciences, Faculty of Engineering Science, Kyushu University, Kasuga, Fukuoka 816-8580, Japan ‡Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan

SnCl4, Quaternary ammonium, Tetraethylene glycol

Heater

15

Crystallite size (nm)

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Thermally stable SnO2 nanocrystals

Conventional SnO2 nanocrystal (hydrothermal synthesis)

10

Heating (200℃)

本法

5

0 0

20 nm

Heater

SnO2 nanocrystal in this work

200 400 600 800 1000 Calcination temperature ( oC)

Synopsis Thermally stable SnO2 nanocrystals (ca. 4 nm) were synthesized in a mixture of SnCl4, tetraethylene glycol and tetrabutyl ammoniumhydroxide under reflux at 200 oC. The obtained SnO2 nanocrystals were so stable against crystal growth that the crystalline size did not change after calcination at 600 oC and exhibited excellent sensor response to hydrogen gas. 30 ACS Paragon Plus Environment

Thermally Stable SnO2 Nanocrystals - American Chemical Society

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