Highly Sensitive Ethanol Gas Sensor Using Pyramid-Shaped ZnO

Article Cite This: J. Phys. Chem. C 2018, 122, 7353−7360

pubs.acs.org/JPCC

Highly Sensitive Ethanol Gas Sensor Using Pyramid-Shaped ZnO Particles with (0001) Basal Plane Noriko Saito,*,† Ken Watanabe,‡ Hajime Haneda,† Isao Sakaguchi,† and Kengo Shimanoe‡ †

National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan

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‡

ABSTRACT: For monitoring of air quality and medical diagnosis, metal-oxide-semiconductor particles with high sensitivity to detect small amount of gases are desirable. Herein, we report the fabrication of ZnO pyramid-shaped particles with remarkably high sensitivity to ethanol gas. The ZnO pyramid-shaped particles were synthesized solvothermally under agitation from the solution of zinc acetate anhydride, hexamethylenetetramine, ethylene glycol, and water. Gas sensing response was evaluated as the ratio of electrical resistance of the ZnO particulate layer in air to that in ethanol. The agitation during the solvothermal process resulted in dispersion of the pyramidshaped particles rather than spherical aggregates. TEM studies revealed that the base of the pyramid-shaped particles is the (0001) plane and that the six side surfaces are the {101̅1̅} plane. The highest gas sensing response value to 50 ppm ethanol gas was about 10 000, which is remarkably higher than that of previously reported ZnO particles. The influence of the crystal facets and the polarity is discussed.

1. INTRODUCTION

To improve the performance of gas sensors, the design and control of the particle size, porosity, and the catalyst additives have been studied.18,19 Furthermore, the particle shape and the crystal facets have attracted interest due to their effect on the gas sensing properties.13−17,20,21 Past studies showed that the (0001) plane was the most efficient surface for ethanol gas sensing by ZnO14−16 because ethanol molecules are well adsorbed on the (0001) surface.14,22 Although high sensing properties on higher surface energy facets with higher Miller index have been expected,20 successful results have not yet been obtained. In this article, we report a remarkably sensitive ethanol sensor using pyramid-shaped ZnO particles. The particles were synthesized by a solvothermal procedure in water−ethylene glycol (EG) solvents and characterized by focusing on the crystal facets and the polarity. The influence of the morphology on the ethanol gas sensing properties is discussed.

Highly sensitive and reliable gas sensors are useful for monitoring of air quality, detection of flammable or toxic gases, and medical diagnosis. Various types of gas sensors such as metal-oxide-semiconductor, electrochemical, and catalytic combustion types have been used.1 The gas sensors using metal-oxide-semiconductor such as SnO2, ZnO, and In2O3 have advantages including simple structure, low cost, rapid response, and high sensitivity.2 Their ability to detect gas is based on the reversible change in the electrical conductivity of the metal oxide induced by gas−surface interactions. When the sensor device is exposed to reduced gases, the gas molecules are oxidized by the oxygen ions adsorbed on the metal oxide surface, and the electrons are released back into the conduction band, which increases the conductivity.3 Zinc oxide (ZnO) is an n-type semiconductor and one of the metal-oxide-semiconductor materials for gas sensors. It is also widely used in electronic applications, such as varistors, surface acoustic wave filters, transparent electrodes, and phosphors.4 Control of the morphology of ZnO nanostructures has been intensively investigated, and a wide variety of morphologies (e.g., nanowires, rods, plates, flowers, rings, and spheres) have been observed for particles and thin films.5,6 Various ZnO nanostructures have been studied for gas sensor applications.7−17 In our research, we found that the best result for ZnO ethanol sensors was brought by nanoparticles of about 15 nm synthesized via a vapor phase method.8 The sensor response (ratio of electrical resistance in air to that in ethanol gas) to 50 ppm ethanol gas in air was about 500 at 400 °C. © 2018 American Chemical Society

2. EXPERIMENTAL PROCEDURES Zinc acetate anhydride (2.202 g; Wako Pure Chemical Industries) and hexamethylenetetramine (HMT) (1.682 g; Wako Pure Chemical Industries) were dissolved separately in 20 mL of solvent of 87.5 vol % EG (Nacalai Tesque, Inc.) and 12.5 vol % water. The two solutions were mixed and placed in Teflon-lined stainless steel cylindrical chambers of 50 mL capacity and were then heated at 90 °C for 3 h using a heating Received: February 25, 2018 Revised: March 18, 2018 Published: March 20, 2018 7353

DOI: 10.1021/acs.jpcc.8b01936 J. Phys. Chem. C 2018, 122, 7353−7360

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The Journal of Physical Chemistry C oven with stirrer (RDV-TMS, San-Ai Kagaku Co. Ltd.). The resultant precipitates were separated by centrifugation, washed with ethanol via ultrasonication three times, and dried at room temperature. For the sensor device, the ZnO powder was dispersed in EG, and the slurry was drop-deposited on a gold interdigital electrode with a 100 μm pitch sputtered on a silica glass substrate and dried. Au wire electrodes were attached to the device using Au paste, and then the device was heated in air at 400 °C. The electrical resistance was measured under gas flow at 100 sccm of synthetic air and 1−50 ppm ethanol gas. The size of the chamber was about 500 cm3. The device was heated with an external furnace. The sensor device was connected in series to a reference resistor, and the voltage across the reference resistor, Vref, was measured under an applied voltage of DC 4.0 V. The electrical resistance of samples was evaluated using the equation Rs = Rref(4.0 − Vref)/Vref, where Rref is the resistance of the reference resistor.11 The sensor response, S, was defined as S = Rair/Rgas, where Rair and Rgas are the sample resistance in the synthetic air and ethanol gas, respectively. For flushing the adsorbed gas from the particles, the samples were annealed in air at 400 °C for 3 h after each experiment. The particles were observed by scanning electron microscopy (SEM; Hitachi S-5500). Cross sections of the particles were observed by transmission electron microscopy (TEM; Jeol JEM-2100F) with a field emission gun, operated at 200 kV. The facets of the particles were determined by selected area electron diffraction (SAED). For the determination of the polarity, convergent-beam electron diffraction (CBED) was used, with the incident azimuth along the m-axis, ⟨11̅00⟩. The relative surface area of the powder annealed at 400 °C for 2 h was measured by a surface area analyzer (BELSORP-max, MicrotracBEL Corp.). Thermogravimetry differential thermal analysis (TG-DTA; Rigaku) measurement of preannealed ZnO particles was conducted in air at a heating rate of 10 °C/min.

(CH 2)6 N4 + 6H 2O → 6HCHO + 4NH3 NH3 + H 2O → NH4 + + OH−

Zn(CH3COO)2 → Zn 2 + + 2CH3COO−

Zn 2 + + 2OH2 − → ZnO + H 2O

When aqueous HMT solutions are heated, the HMT decomposes to formaldehyde and ammonia, which acts as a base and induces ZnO precipitation.23−25 Without agitation, spherical particles of about 2 μm were precipitated (Figure 1a) by reaction for 3 h as observed in previous studies.23−25 The particles were composed of small pyramid-shaped crystallites. At the initial stage of growth, spherical particles made of nanocrystallites were precipitated. With time, the composing nanoparticles transformed into pyramid-shaped particles with coalescence. Eventually, the spheres became cracked and broke into wedge-shaped pieces made of the pyramid-like particles, but not into dispersed pyramidal crystallites.24 Figure 1b shows the SEM photograph of wedge-shaped fragments of spherical particles precipitated by long aging (12 h) without agitation. Under agitation, dispersed pyramid-shaped particles of about 20 nm were obtained by reaction for 3 h (Figure 1c). The SEM photographs show that the crystallites were the same size as in the spherical particles produced by reaction for 12 h without agitation.23,24 The coherent lengths along the a- and c-axis estimated from the peak width in XRD diffractograms were 22.4 and 24.9 nm for the pyramid-shaped particles and 22.0 and 31.5 nm for the spherical particles,23 respectively. The longer coherent length along c-axis for the spherical particles is due to the hierarchical structure where the crystallites were aligned along the c-axis. The dispersion of the crystallites under agitation suggests that the agitation prevented the growth of spheres due to the oriented attachment of nanoparticles, which allowed the random growth of crystallites. The yield of ZnO particles with agitation and without agitation was 3% and 9%, respectively. The increase in yield means that nucleation was promoted by the agitation. The relative surface area of the ZnO particles annealed at 400 °C was 23.1 m2/g. Figure 2 shows the result of TG-DTA for the ZnO pyramidal precipitates. Weight loss due to the evaporation and combustion of residual organic substances from room temperature to 400 °C was 4%. Over 400 °C, slight weight loss continued. Figure 1d shows SEM photographs of ZnO particles after annealing at 400 °C for 2 h. The size of the particles was about 20 nm, the same as that before annealing. The shape of the particles was changed by the annealing as the top of the pyramid became a rounded shape. 3.2. Crystal Facets and Polarity of Pyramid-Shaped Particles. The crystal facets of the ZnO pyramid particles were determined by TEM diffraction. The image in Figure 3a shows that the ZnO particle is a pyramid-shaped single crystal. The lattice fringes with spacing of 0.52 nm shown in Figure 3b correspond to the distance between the (0001) planes. The SAED pattern shown in Figure 3c confirms that the pyramidal tip direction is along [0001], which is consistent with the lattice shown in Figure 3b. The polarity of the pyramid-like crystallites was previously confirmed two times by CBED.23,24 The sample for the CBED experiment was made by long aging (12 h) without agitation. As shown in Figure 4, the experimental CBED patterns were similar to the simulated patterns, which

3. RESULTS AND DISCUSSION 3.1. Synthesis of ZnO Pyramid-Shaped Particles. A schematic diagram of the preparation of ZnO particles is shown in Scheme 1. The chemical reactions of zinc acetate and HMT in aqueous solutions are as follows: Scheme 1. Schematic Diagram of ZnO Particle Preparation

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Figure 1. SEM photographs of ZnO particles. (a) Spherical particles precipitated under no-agitation, (b) wedge-shaped fragments of spherical particles precipitated without agitation, (c) pyramid-shaped particles precipitated under agitation, and (d) ZnO particle after annealing the pyramidshaped particles at 400 °C.

Several cases of pyramid-shaped ZnO particles have been previously reported. Capping agents were often used to stabilize side planes with high surface energy. Zhou et al. synthesized ZnO pyramid-shaped particles from zinc acetate, oleic acid, trioctylamine, and ethylenediamine.16,17,28 The particles were about 1 μm in size and exhibited smooth facets. Ionic liquids worked as the capping agents to stabilize the pyramid side surfaces. The polarity of the pyramidal base was determined to be the (0001)̅ plane by CBED. The pyramid particles had the opposite polarity of those prepared in the present study. Chang et al. synthesized ZnO pyramidal particles about 100 nm in size from anhydrous zinc acetate, benzyl ether, and benzylamine.29,30 The base of the pyramid was assumed to be (0001̅), but the polarity was not determined. Yang et al. fabricated pyramid-shaped ZnO nanoparticles (10−200 nm in size) from simple solutions of zinc acetate, KOH, methanol, and water.31,32 They also assumed that the base of the pyramid was (0001̅), but the polarity was not determined. It is noted that the synthesis method was similar to that in our study except for the use of HMT. Some assemblies, where the pyramid particles are connected in the same direction (head to tail connection), look the same as the fragments of spheres observed in our previous study.23,24 In addition to the pyramid-shaped particles, variations of the pyramid surfaces have also been reported.31−35 Javon et al. prepared coupled pyramids, where two pyramids are joined across the basal plane.33 The polarity was determined by highangle annular-dark-field scanning transmission electron microscopy (HAADF-STEM), and the base of the pyramid was the (0001) plane. Conical-shaped particles and hourglass structures have also been reported.34 Han et al. fabricated pagoda-like and hexagonal pyramidal ZnO nanostructures through the synthesis of ZnO rods followed by the etching process.35 Tang et al. fabricated ZnO wires covered by nanoscale pyramids, where the base of the pyramids was identified as the (0001) plane by HAADF-STEM.36 Huang et al. solvothermally synthesized

Figure 2. TG-DTA for the pyramid-shaped ZnO powder.

were calculated under the assumption that the base of the pyramid is the (0001) plane. Consequently, it was concluded that the base of the pyramids was the (0001) plane. The experimental fact that the direction of the diffraction spots of (101̅1̅) and (1̅011̅) shown in Figure 3c agrees with the direction of the side surfaces of the pyramid particles (Figure 3a) means that the pyramid side surfaces are the {101̅1̅} planes. Thus, the TEM results revealed that the pyramid-shaped particles have the (0001) plane on the bottom and the {101̅1̅} planes on the sides (Figure 3d). This is the first report of synthesizing dispersed pyramid-shaped ZnO particles with (0001) and {1011̅ }̅ surfaces, where the polarity is clarified. ZnO has a wurtzite-type crystal structure and shows spontaneous electrical polarization along its c-axis. The (0001) and (0001̅) surfaces are rich in zinc and oxygen atoms, respectively. Therefore, there are two types of ZnO pyramid particles with opposite polarity, as shown in Scheme 2. For CdSe, which also has wurtzite crystal structure, pyramid particles with opposite polarity were synthesized by different solution conditions.26,27 7355


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Figure 3. (a) TEM image, (b) high-resolution TEM image, (c) electron diffraction pattern, and (d) corresponding scheme of a ZnO pyramid.

Figure 4. (a) TEM photograph of ZnO pyramidal particles. (b) Experimental and (c) simulated CBED patterns for the circled area. The two arrows show the same direction. Adapted with permission from ref 23.

The ZnO crystal planes {1011̅ }̅ have been observed on the etched surface of single crystals or thin films. Pyramid-shaped pits with {101̅1̅} were found when the (0001̅) surface was etched with acidic38 and basic solutions.39 Zuniga-Perez et al. reported ZnO thin films covered by rhombohedral pyramids with side walls on the {1011̅ } and {1011̅ }̅ planes grown by metal−organic vapor phase epitaxy, where the polarity was confirmed by CBED.40,41 3.3. Ethanol Gas Sensing of ZnO Pyramid-Shaped Particles. Figure 5a shows the dynamic response of the sensor device to 50 ppm ethanol gas in air at an operating temperature of 350 °C, where the ZnO sensor device was annealed at 400 °C before the sensing measurement. The resistance of the ZnO powder decreased upon the change from air to ethanol gas, and then recovered by the reverse exchange. The sensor response, the ratio of electrical resistance in air to that in ethanol gas (Rair/Rgas), was about 11 186. The response values for the ZnO spherical particles synthesized without agitation (Figure 1a) and the wedge-shaped particles synthesized by long aging without agitation (Figure 1b) were 88 and 466, respectively.

Scheme 2. Schematics of (a) ZnO Wurtzite Crystal Structure and (b) Two Pyramids with Opposite Polarity

polyhedron particles using zinc acetate, HMT, EG, and water.37 The chemicals were similar to those used in the present study, but pyramidal particles were not obtained, and the polarity was not determined. 7356


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previous reports, where Pt-doped SnO2 hollow nanospheres were used and the Pt doping increased the response 15 times.42 Figure 5b shows the sensor response values when the annealing temperature before the sensing measurement was changed. The best results were obtained for the samples annealed at 400−450 °C. At lower temperatures, some organic substances remained, as shown in Figure 2. At higher temperatures, the activity decreased due to coalescence. Hereafter, the annealing temperature before sensor measurement was 400 °C. The operating temperature dependence of the electrical resistance in air and in 50 ppm ethanol, and the sensing response are shown in Figure 6. The data in air was the value before the ethanol gas was introduced. The data in ethanol gas was the value after it became a constant. A high sensing response was obtained at 310−350 °C. This agrees with the suitable operating temperature range of 250−400 °C for ethanol sensors using ZnO.7−17 As shown in Figure 7, more time was needed to maintain a constant resistance in ethanol gas at lower temperatures, where the oxidation reaction of ethanol molecules on the ZnO surface is slower compared to that at higher temperatures. At higher temperatures, the sensor response value decreased due to desorption of ethanol. The sensor response was 200 at 400 °C, which was 1/50 the value at 350 °C. Figure 8 shows the sensor response when the concentration of ethanol gas was changed. The response value for 1 ppm ethanol was 50, which is sufficient for detecting low concentrations of ethanol gas. The unique features of the present ZnO particles are the pyramidal shape and the crystal planes, which most likely brought about the exceptionally high gas sensing properties. Although the gas sensing properties of the pyramid-shaped ZnO particles have been studied previously, satisfactory results were not obtained,16,17 as the response to 300 ppm ethanol gas was about 10 at 350 °C. These pyramids had (0001̅) on the bottom and {1011̅ } on the side surfaces; they had the opposite polarity to that of the particles in the present study. This suggests that the crystal facets including the polarity influence the gas sensing properties. Lattice defects and kinks can depend on the facet and polarity. The facets of the present pyramid particles, not as smooth as the previous ones,16,17,28 can influence the property. The polarity of ZnO is known to affect some properties, such as catalytic properties,45 electronic properties,46 and chemical stability, where (0001) and {101̅1̅} were found to be stable planes under etching conditions.39,41 It is suggested that the polarity of the pyramidal particles

Figure 5. (a) Dynamic response in resistance of ZnO particles annealed at 400 °C to 50 ppm ethanol measured at 350 °C. (b) Annealing temperature dependence of sensing response (Rair/Rgas) to 50 ppm ethanol in air measured at 350 °C.

Agitation during solvothermal synthesis resulted in dispersion of crystallites and increase in the response value. The ethanol sensing response of the ZnO pyramid-shaped particles was significantly better than that previously reported for ZnO particles, as shown in Table 1. The response value was higher than the previous best result for ZnO particles despite their smaller particle size and higher relative surface area.8 For comparison, the best results for ethanol sensor using SnO242 and In2O343 particles are also listed in Table 1. The sensor responses to ethanol gas in previous studies were summarized for SnO2,42 In2O3,44 and other different sensing materials.44 The sensor response was comparable to that of the most highly sensitive ethanol sensor to 50 ppm ethanol gas found in

Table 1. Sensor Response of ZnO Particles to 50 ppm of Ethanol Gas and Comparison to SnO2 and In2O3 Particles shape

size (nm)

not specific sheet wire not specific thin film plate rod disk rod pyramid SnO2 (Pt-doped) In2O3 (nondoped)

15 10 (thickness) 25 (diameter) 30 50 (diameter) 50 (thickness) 200 (diameter) 100 (thickness) 1000 (diameter) 20 32 (crystallite) 50−200

relative surface area (m2/g) 41 52

14 7 6 5 23 23.6

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temp (°C)

sensor response

ref

400 400 300 350 400 300 300 330 330 350 325 300

400 180 15 30 20 18 10 90 20 10000 10000 100

8 9 10 11 12 14 14 15 15 present study 42 43 DOI: 10.1021/acs.jpcc.8b01936 J. Phys. Chem. C 2018, 122, 7353−7360

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Figure 6. (a) Electric resistance in air (Rair) and in gas (Rgas) and (b) sensing response (Rair/Rgas) to 50 ppm of ethanol in air at 250−400 °C.

Figure 8. (a) Dynamic response in resistance to different concentrations of ethanol in air at 350 °C. (b) Sensor response (Rair/Rgas) plotted with respect to the ethanol concentration.

activity.47 For gas sensors, pyramidal facets are known to lead to a high response. SnO2 octahedral particles were found to exhibit a higher sensitivity to ethanol gas than rod-shaped particles.20 Recently, the present authors made flat and rough epitaxial ZnO films with (0001) and (0001̅) surfaces by pulsed laser deposition (unpublished results). Although the sensor response to ethanol gas did not depend on the roughness for the (0001) surface, the rough film showed higher sensitivity than the frat one for the (0001̅) surface. The polarity dependence of the sensor response on the rough surfaces supports the idea that the polarity of ZnO pyramid particles affects the sensor response to ethanol gas. The present findings provide new insight for designing the exposed crystal facets for highly sensitive gas sensors. To understand the crystal facet/polarity dependence of the gas sensing response, the adsorption of oxygen ions and ethanol molecules and the catalytic effects of ethanol on oxidation should be examined with evaluation of surface defects and kinks in future studies. Application to other gases using a suitable selection of additives would be an interesting future challenge.

Figure 7. Dynamic response profiles in resistance measured at different operating temperatures.

previously reported by a number of groups29−37 would be examined. The ZnO pyramidal surfaces are known as active planes for dye-sensitized solar cells and catalysts, where the molecule adsorption and surface reactions are used as well as for gas sensors. Chang et al. investigated the effects of the exposed crystal facets and found that the activity of the side surfaces of the pyramid was higher than that of the surfaces of rod-shaped particles.29,30 Huang et al. synthesized polyhedrons with the same planes as pyramids and found that the photocatalytic activity depended on the crystal facets.37 Cha et al. found that ZnO pyramidal particles have a high ability to reduce enzyme

4. CONCLUSION We reported the fabrication of remarkably sensitive ethanol gas sensors using dispersed ZnO pyramid-shaped particles. The particles were synthesized solvothermally with agitation from solutions of zinc acetate anhydride, HMT, EG, and water. TEM 7358


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study revealed that the precipitated particles were pyramidshaped with the base being the (0001) plane and the six side surfaces being the {101̅1̅} planes. The polarity was identified by CBED. Using the pyramid-shaped ZnO particles, gas sensor devices were fabricated. The highest response value (ratio of electrical resistance in air to that in ethanol gas) to 50 ppm ethanol gas was more than 10000, which is significantly higher than previously reported for ZnO particles. The influence of the crystal facets and the polarity was discussed. The unique features of the present ZnO particles, i.e., the pyramidal shape and the crystal planes, most likely brought about the exceptionally high gas sensing properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Noriko Saito: 0000-0002-8104-0172 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Nos. 26420688 and 17K06807. REFERENCES

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Highly Sensitive Ethanol Gas Sensor Using Pyramid-Shaped ZnO

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