Highly Sensitive and Selective Ethanol Sensor ... - ACS Publications

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Highly Sensitive and Selective Ethanol Sensor Fabricated with InDoped 3DOM ZnO Zhihua Wang, Ziwei Tian, Dongmei Han, and Fubo Gu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *

ABSTRACT: ZnO is an important n-type semiconductor sensing material. Currently, much attention has been attracted to finding an effective method to prepare ZnO nanomaterials with high sensing sensitivity and excellent selectivity. A threedimensionally ordered macroporous (3DOM) ZnO nanostructure with a large surface area is beneficial to gas and electron transfer, which can enhance the gas sensitivity of ZnO. Indium (In) doping is an effective way to improve the sensing properties of ZnO. In this paper, In-doped 3DOM ZnO with enhanced sensitivity and selectivity has been synthesized by using a colloidal crystal templating method. The 3DOM ZnO with 5 at. % of In-doping exhibits the highest sensitivity (∼88) to 100 ppm ethanol at 250 °C, which is approximately 3 times higher than that of pure 3DOM ZnO. The huge improvement to the sensitivity to ethanol was attributed to the increase in the surface area and the electron carrier concentration. The doping by In introduces more electrons into the matrix, which is helpful for increasing the amount of adsorbed oxygen, leading to high sensitivity. The In-doped 3DOM ZnO is a promising material for a new type of ethanol sensor. KEYWORDS: macroporous, ZnO, indium, sensor, ethanol sensitivity for ethanol sensing. Xu et al.13 synthesized an excellent acetone sensor based on La-doped ZnO nanofibers. Among the doping elements, indium (In), a versatile transition metal, has attracted considerable attention. The advantages of In-doping are the following: (1) In2O3 itself is an important gas sensing material. (2) In3+ can easily replace for Zn2+ due to their similar ionic radii. (3) Because In3+ has a higher valence than Zn2+, In-doping can release more free electrons and contribute to a high electron carrier concentration in ZnO nanomaterials, which probably leads to an improvement in the sensitivity. Therefore, In-doped ZnO is expected to present an enhanced sensing performance. However, the role of In dopants in the gas sensing process is not well-understood. In addition, the gas sensing performance of ZnO strongly depends on its morphology. To date, In-doped ZnO has been exploited in several forms such as thin films,14 nanowires,15 and nanobelts.16 However, there exist many limitations in the sensitivity of the present morphologies, due to the lack of pores. Therefore, it is highly desired to seek a special porous Indoped ZnO for high sensing performance. More recently, three-dimensionally ordered macroporous (3DOM) materials have received significant attention. 3DOM materials are composed of well-interconnected pore and wall structures with wall thicknesses of a few tens of nanometers.

1. INTRODUCTION ZnO, a multifunctional n-type semiconductor, has been extensively studied as an important sensing material because of its excellent characteristics, such as low cost, nontoxic nature, abundant availability, and good biocompatibility.1 The sensing mechanism of the ZnO sensors follows the surface-controlled type.2−4 The sensitivity to reductive gases is defined as Ra/Rg, wherein Rg is the electrical resistance of the sensor in the test gas and Ra is the electrical resistance in air. When the ZnO sensors are exposed to air, oxygen molecules will be adsorbed on the surface of the material and form adsorbed oxygen by capturing free electrons from the conduction band, resulting in an increase in the electrical resistance. In contrast, when the ZnO sensors are exposed to reductive gases, the reductive gases will react with the adsorbed oxygen and release free electrons, resulting in a decrease in the electrical resistance.5 Therefore, large surface area, high amount of the adsorbed oxygen, and high amount of the free electrons facilitate the enhancement of sensitivity. It has been reported that ZnO nanomaterials of different morphologies showed good sensitivities to reductive gases such as ethanol,5 CO,6,7 H2,6 and H2S.8,9 Nevertheless, drawbacks still exist in gas sensitivity and selectivity, so it is very urgent to find an effective method to prepare ZnO nanomaterials with high sensing sensitivity and excellent selectivity. Doping by metal elements is a frequently applied strategy to modify ZnO nanomaterials for an enhanced gas sensing performance.10,11 For example, Noel et al.12 reported that 3%-Gd-modified ZnO fiberoptic sensor exhibited the best © XXXX American Chemical Society

Received: January 11, 2016 Accepted: February 4, 2016

A

DOI: 10.1021/acsami.6b00339 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the experimental procedure.

Figure 2. (a) XRD pattern of 3DOM samples. (b) Position of the (101) peak and full width at half-maxima (fwhm), obtained from the XRD pattern of 3DOM samples. allowed to dry overnight and then calcinated to remove the PMMA template. The obtained powder was subsequently heated in nitrogen gas at 300 °C for 3 h and cooled to 50 °C in the same atmosphere, and next the solid was calcined in air at 300 °C for 3 h and then was heated to 550 °C and maintained for 3 h. The as-obtained 3DOM-structured samples were denoted as ZnO, ZnO/1%In, ZnO/3%In, ZnO/5%In, ZnO/7%In, and ZnO/10%In for the initial In/Zn molar ratios of 0, 0.01, 0.03, 0.05, 0.07, and 0.10, respectively. 2.2. Characterizations. The crystal phase and crystallinity of the synthesized 3DOM samples were recorded via X-ray diffraction (XRD, Bruker-D8 Advance) by using Cu Kα radiation (λ = 0.154 06 nm) at a scanning rate of 10°/min in the range 5−90°. The morphologies of the samples were observed by using scanning electron microscopy (SEM, S-4700). The elemental mapping of the 3DOM samples was performed by using transmission electron microscopy (TEM, Tecnai F20, FEI) equipped with an energy dispersive X-ray spectroscopy (EDS) in the scanning transmission electron microscopy (STEM) mode. The elemental analyses of the samples were carried out by using inductively coupled plasma atomic emission spectroscopy (ICP, Thermo Scientific iCAP 6000 series). The textural characteristics and the specific surface areas were obtained on a Micromeritics ASAP2020 analyzer at liquid nitrogen temperature (77 K). The pore size distribution was calculated from the desorption branch of the nitrogen desorption isotherm by using Barrett−Joyner−Halenda (BJH) formula. The Raman spectroscopy (Renishaw Invia) was obtained operating with an excitation wavelength of 514 nm. The UV−vis diffuse reflectance spectra of the products were obtained by using a UV−vis spectrophotometer equipped with an integrated sphere reflectance accessory (UV−vis, JMNU-3010). The electrical properties were recorded by using a Hall effect measurement system (RH2030, Phys Tech). X-ray photoelectron spectroscopy (XPS, VG Scientific ESCALAB 250X) was used to characterize the surface properties.

Because of the special nanostructure, 3DOM materials provide large surface areas and many active sites, which indicates their potential applications in the fields of sensors,17 catalysts,18 and battery materials.19,20 To the best of our knowledge, there have been no reports on the fabrication of the gas sensor based on In-doped 3DOM ZnO. Herein, In-doped 3DOM ZnO samples were prepared, and the influence of different In-doping concentrations on the gas sensing properties of the 3DOM ZnO was explored. Our experimental results indicated that appropriate In-doping concentration remarkably improved sensing sensitivity and selectivity. The synthesized In-doped 3DOM ZnO has potential applications in gas sensors.

2. EXPERIMENTAL SECTION All of the chemicals were analytical-grade reagents and used as received without further purification. 3DOM samples were prepared by using a colloidal crystal templating method. The schematic illustration of the experimental procedure is shown in Figure 1. 2.1. Synthesis. Well-arrayed hard template PMMA microspheres with an average diameter of 330 nm were synthesized according to the procedures described elsewhere.21 In-doped 3DOM ZnO materials were synthesized through a one-step colloidal crystal templating method. In a typical process, Zn(NO3)2·6H2O (20 mmol) and a certain amount of In(NO3)3·4.5H2O (with In/Zn molar ratios of 0, 0.01, 0.03, 0.05, 0.07 and 0.10) were dissolved in anhydrous methanol (10 mL) to obtain a transparent solution, and then citric acid was added as the chelating agent. The PMMA hard template (2.0 g) was soaked in the obtained mixed solution for 4 h, and the excess solution was removed by using vacuum filtration. The obtained sample was B


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ACS Applied Materials & Interfaces Table 1. Structural Parameters, In-Doping Concentrations, and Surface Areas of 3DOM Samples lattice parama

a

sample

DZnO (nm)

a = b (Å)

c (Å)

lattice strain εa (%)

mole percentage of Inb (%)

surface areac (m2/g)

ZnO ZnO/1%In ZnO/3%In ZnO/5%In ZnO/7%In ZnO/10%In

26.135 18.351 14.751 12.741 11.126 10.523

3.229 3.232 3.233 3.235 3.240 3.241

5.192 5.201 5.215 5.226 5.232 5.236

0.430 0.634 0.791 0.863 0.990 1.048

0 0.86 2.60 4.27 6.03 8.57

18.759 20.996 28.192 32.160 33.254 34.501

a

Determined via XRD. bDetermined via ICP-AES. cDetermined via BET.

Figure 3. SEM images of the 3DOM samples: (a and b) ZnO; (c and d) ZnO/1%In; (e and f) ZnO/3%In; (g and h) ZnO/5%In; (i and j) ZnO/7% In; (k and l) ZnO/10%In. 2.3. Gas Sensing Measurements. In order to fabricate a gas sensor, the obtained 3DOM samples were mixed with several drops of ethanol in an agate mortar to form a homogeneous paste. Next, the paste was coated on the outer surface of an alumina ceramic tube equipped with a pair of Au electrodes and four Pt wires. Then, the sensor was aged at 350 °C in air for 24 h. A Ni−Cr coil was employed as the heater to control the operation temperature by tuning the heating voltage. The sensor sensitivity to gas was defined as Ra/Rg (when the test gas was a reductive gas) or Rg/Ra (when the target gas was an oxidative gas), wherein Rg was the electrical resistance of the sensor in the test gas and Ra was the electrical resistance in air. After the test, the chamber was opened to diffuse the test gas away. The time spent by the sensor achieving 90% of the total electrical resistance change was defined as the response time in the case of adsorption, or as the recovery time in the case of desorption. The sensing properties of the sensor toward test gas were measured on a sensor test system (WS-30A, Winsen Electronics Technology Co. Ltd., Zhengzhou, China) at a relative humidity (RH) of 20 ± 5%.

no diffraction peaks of other impurities such as In2O3 are found. Additionally, with the increase of the In concentration, an obvious shift can be observed to (101) and (100) diffraction peaks. Especially, the position of the (101) peak shifts toward low angle, and the full width at half-maximum (fwhm) of the diffraction peaks broadens as shown in Figure 2b. The crystallite sizes, the lattice constants, and the lattice strains ε of the 3DOM samples are listed in Table 1. The crystallite sizes (D) of the 3DOM samples have been determined by using Debye−Scherrer formula 1 D = 0.9λ /β cos θ

(1)

where λ is the X-ray wavelength (0.154 nm), θ is the Bragg diffraction angle, and β (radians) is the full-width at halfmaximum. The lattice constants, a, b, and c, have been calculated by using formula 2

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterizations of the 3DOM Samples. XRD analysis was carried out to investigate the crystal structures of the pure and the In-doped 3DOM ZnO. The XRD patterns of the samples are shown in Figure 2a. All of the diffraction peaks can be indexed to typical wurtzite structure ZnO (JCPDS PDF 75-0576). For the In-doped 3DOM ZnO,

1 4 ⎛ h2 + hk + k 2 ⎞ l2 = + ⎟ ⎜ 2 3⎝ ⎠ c2 dhkl a2

(2)

where dhkl is the interplanar spacing, and hkl are the Miller indices of the plane of diffraction. C


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ACS Applied Materials & Interfaces The lattice strains (ε) of the 3DOM samples have been determined by using tangent formula 3:22 ε = β /4 tan θ

(3)

It is observed that the crystallite sizes gradually decrease, and the lattice constants as well as the lattice strains ε gradually increase with the increase of the In concentration. Those mentioned phenomena are all related to the In-doping.9,23 The doping by In will introduce more nucleation sites, which leads to the formation of the smaller crystallite size. Because the ionic radius of In3+ (0.80 Å) is bigger than that of Zn2+ (0.74 Å),24,25 the lattice constants increase with the increasing of the In concentration. Moreover, the doping by In will result in structure defects and reduce the crystallinity degree of the host ZnO crystal.22 Therefore, the diffraction peaks become weaker and broader. The actual In concentrations of the 3DOM ZnO/ x%In were determined by using the ICP technique, as shown in Table 1. It can be seen that the actual In concentrations of the samples are lower than the initial theoretical values. Figure 3 shows the SEM images of the 3DOM samples. It can be seen that all of the samples exhibit a highly ordered macroporous structure that is obtained via the long-range replicating of the 3D close-packed PMMA template (Figure S1).26,27 The macropores that are adjacent are interconnected with each other, which will contribute to the gas diffusion into the inner space of the materials, leading to the improvement in the gas sensing performance. The pure ZnO displays a relatively poor 3DOM structure. With the doping by In, high-quality 3DOM samples with a pore size of 150 nm and a wall thickness of 25 nm are generated. Combined with the result from XRD, it is evident that the doping by In of ZnO lattice is a benefit to obtain a perfect 3DOM structure due to the small crystallite size. Typical HR-TEM images of ZnO, ZnO/5%In, and ZnO/ 10%In (Figure S2) indicate that the macropores of the 3DOM samples are highly ordered. The doping by In improves the integrity of the 3DOM structure, which is consistent with the SEM images. Moreover, the crystallite size decreases, and the interplanar spacing of the ZnO (101) facet increases with increasing the In concentration, which is consistent with the results from XRD. Figure S3 shows the STEM images and EDS elemental mappings of ZnO, ZnO/5%In, and ZnO/10%In, which indicate that In is uniformly doped. 3.2. Gas Sensing Properties. The gas sensing sensitivities of the 3DOM samples to 100 ppm ethanol as the function of working temperature are shown in Figure 4. As can be seen, the sensitivity enhances with the increase of the operating temperature, and reaches a maximum. Then, the sensitivity decreases with further increasing the operating temperature. At low temperatures, the adsorbed ethanol molecules are not sufficiently activated to overcome the energy barrier to react with the adsorbed oxygen. Nonetheless, when the temperature is higher than the optimum, the adsorbed ethanol molecules may be desorbed in a large quantity before their reaction, causing a lower gas sensitivity.3,28 Figure 4 shows that the Indoping is an effective way to increase ZnO sensing sensitivity and decrease the operating temperature. In particular, the gas sensing sensitivity value increases gradually with the In concentration from 0 at. % to 5 at. %, but decreases when the In concentration exceeds 5 at. %. The ZnO/10%In shows a lower sensitivity to ethanol, while the ZnO/5%In has the highest sensitivity (∼88), which is about 3 times higher than that of the pure ZnO (∼28). Additionally, the optimum operating temperatures are 285 °C for ZnO, 265 °C for ZnO/

Figure 4. Sensitivities of the 3DOM samples to 100 ppm ethanol at various operating temperatures; the error bars represent the sensitivity variations of each sample, and the error bars represent the standard deviation (SD) of the determinations for five independently fabricated sensors.

1%In and ZnO/3%In, and 250 °C for ZnO/5%In, ZnO/7%In, and ZnO/10%In. The optimum operating temperature decreases gradually with increasing the In concentration. Furthermore, compared with other work reported in the literature (shown in Table 2), the 3DOM ZnO/5%In sensor exhibits much higher gas sensitivity and lower operating temperature. Table 2. Sensing Properties to 100 ppm Ethanol of the Different Materials in Our Present Study and in Literature Studies material 6.4 at. % Cd-doped ZnO nanoparticles Sn-doped ZnO microrods 3 wt % V-doped burger-like ZnO 3 wt % W-doped donut-like ZnO 5 at. % Ce-doped ZnO thin-film 1 at. % Fe-doped lotus-like ZnO 2 wt % In-doped ZnO nanorods 5 at. % In-doped 3DOM ZnO a

sensitivity

T (°C)

5.5

250

29

52.4 10 2.2 75 24 17 88 ± 2.2a

300 350 350 320 400 550 250

30 31 31 32 33 34 present work

ref

The result of sensitivity was the mean of five determinations ± SD.

Figure 5a shows the dynamic sensitivities of the typical sensors to different concentrations of ethanol at their respective optimum working temperatures. It is clearly seen that all of the sensors exhibit excellent repeatability and stability. Figure 5b exhibits the corresponding sensitivities of the sensors to ethanol in the concentration range 5−500 ppm, and the inset shows their corresponding log S versus log C curves. It is apparent that the sensitivities of the sensors increase with the increasing gas concentration, and the rate slows down gradually. The logarithm of the sensitivity presents superior linearity with the logarithm of the ethanol concentration. The linear trend can be related to the conductance model of the semiconductor. The gas absorption on the surface of the sensors can be described empirically as33,35 S = 1 + Ag (Pg)β

(4)

where Pg is the partial pressure of the test gas, which is proportional to the gas concentration, Ag is a prefactor, and β is the response order depending on the charge state of the surface D


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Figure 5. (a) Dynamic sensitivity curves of typical 3DOM samples to different concentrations of ethanol at the optimum working temperature. (b) Gas sensitivities of sensors to ethanol concentration, and the error bars represent SD of the determinations for five independently fabricated sensors. Inset of part b shows the corresponding log S versus log C curves.

Figure 6. (a) Sensitivities of the 3DOM samples to 100 ppm toxic gases and 100 ppm of NO2. Inset shows sensitivities of the 3DOM ZnO and 3DOM ZnO/5%In sensors to ethanol and acetone. (b) Sensitivities of 3DOM samples to 100 ppm acetone at various operating temperatures. Inset show sensitivities of the 3DOM ZnO/5%In sensors to ethanol and acetone at various operating temperatures.

Figure 7. (a) Nitrogen adsorption−desorption isotherms and (b) pore size distribution of the 3DOM samples.

reaction species. The β value is usually 0.5 or 1, which implies that the oxygen species adsorbed on the ZnO is almost O2− or O−, respectively.2,36 In our case, the β values of the ZnO and ZnO/5%In are 0.75 and 1.02, respectively, indicating that the oxygen ions adsorbed on the surface are O− and O2− for pure ZnO, and mainly O− for ZnO/5%In. The β of 3DOM ZnO/5% In is the largest, which corresponds to the gas sensing tests. The sensor selectivity was tested by using 100 ppm of different gases. The testing temperatures of pure 3DOM ZnO, ZnO/5%In, and ZnO/10%In are their respective optimum operating temperatures to ethanol. As shown in Figure 6a, for the pure 3DOM ZnO sensor, the sensitivities to ethanol and acetone are similar (∼28), and much higher than those to other gases. However, for the In-doped 3DOM ZnO sensors, the sensitivity to ethanol is much higher than those to acetone and

other gases. To ethanol, the optimum operating temperature of pure ZnO is 285 °C. Both the optimum operating temperatures of ZnO/5%In and ZnO/10%In are 250 °C, as shown in Figure 4. To acetone, the optimum operating temperatures of pure 3DOM ZnO, ZnO/5%In, and ZnO/10%In are 285 °C, as shown in Figure 6b. In-doping observably decreases the optimum operating temperatures to ethanol, but not to acetone. This difference may be attributed to the variation of the composition and the structure of the 3DOM ZnO materials.37−39 Similar results were reported in the literature.28,32,34 At the high operating temperature of 285 °C, both ethanol and acetone will be oxidized easily; thus, the sensor shows bad selectivity. At the low operating temperature of 250 °C, the oxidation of acetone will become difficult, which results in the low sensitivity to acetone and the excellent selectivity to E


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conduction band becomes stronger, and consequently results in the decrease of the band gap.48,49 Hall measurements indicate that all the 3DOM samples exhibit n-type conductivity. The electron carrier concentrations, Hall mobilities, and resistivity values of the 3DOM samples are shown in Figure 8. The pure ZnO has a lower electron carrier

ethanol. The result indicates that In-doping is an effective way to improve the gas selectivity of the 3DOM ZnO materials via decreasing the optimum operating temperature. Therefore, the synthesized 3DOM ZnO/5%In can be used as a promising sensing material for detecting a mixture of ethanol and acetone by selecting different operating temperature. As shown in Figure 6b, in a comparison with the pure 3DOM ZnO sensor, the In-doped 3DOM ZnO sensor also exhibited enhanced sensitivity to acetone. That may be because In3+ has a higher valence than Zn2+: In-doping can release more free electrons and contribute to a high electron carrier concentration in ZnO nanomaterials, which probably leads to an improvement of the sensitivity. The response time and recovery time of 3DOM ZnO/5%In are ∼25 and ∼10 s at 250 °C, respectively (Figure S4). 3.3. Structural Characterizations. For the investigation of the influences of 3DOM structure and In-doping on the gas sensing property, a series of structural characterizations of the 3DOM samples were carried out. Figure 7 shows the nitrogen adsorption−desorption isotherms and the pore size distributions of the 3DOM samples. The nitrogen adsorption− desorption isotherms are characteristic of typical macropores, as confirmed by their pore size distributions, which is similar to other macropores materials.40,41 The surface areas of the 3DOM samples are listed in Table 1. The surface areas increase gradually with the increase of the In-doping concentration. Compared with some reported ZnO sensors,29−34 the 3DOM ZnO/5%In sensor exhibits a higher sensitivity to ethanol due to their larger surface areas. The large surface areas provide more active sites, which are beneficial to the enhancement of the gas sensitivity. In our case, the 3DOM structure has many active sites due to its well-developed pore structure and large surface area, which leads to the enhancement of the gas sensitivity. The Raman spectra of the different 3DOM samples are shown in Figure S5. The peak at 437 cm−1 is assigned as the vibration mode E2H from the oxygen atom, which is characteristic of the wurtzite crystal structure. The peak centered at 330 cm−1 is assigned to the modes E2H−E2L of the ZnO crystal. The peak at ∼580 cm−1 belongs to the E1L mode of ZnO, which is correlated to the structure defects.42,43 With the increase of the In-doping concentration, the relative intensity ratio of the E1L mode to the E2H mode increases. So, Raman spectra show that the defects increase with increasing the In-doping concentration. This result is in good agreement with the XRD results. The UV−vis DRS was used to investigate the band gaps of the pure and the In-doped 3DOM ZnO. As shown in Figure S6a, it can be observed that the pure 3DOM ZnO has a strong absorption around 390 nm in the UV region, corresponding to the band to band transition. In a comparison with the pure ZnO, the In-doped 3DOM ZnO samples display a red shift to the longer wavelength (λ > 400 nm). The band gap was determined by using the Taucs plot, from the cutoff wavelength obtained by intersecting the extrapolation in the linear region.44 As shown in Figure S6b, the obtained band gap (Eg) values are 3.21, 3.14, 3.03, 2.95, 2.90, and 2.84 eV for the pure ZnO, ZnO/1%In, ZnO/3%In, ZnO/5%In, ZnO/7%In, and ZnO/ 10%In, respectively. The higher the In-doping concentration, the smaller the Eg is. Other groups also found that the Eg decreased with the increase of the In-doping concentration.45−47 With the increase of the In-doping concentration, the hybridization level of the s orbitals between In and ZnO

Figure 8. Electron carrier concentration, resistivity, and Hall mobility of the 3DOM samples, and the error bars represent SD of the determinations for three independently samples.

concentration than those of the In-doped ZnO. Upon the Indoping in ZnO, the ionized In3+ will replace the Zn2+ in the ZnO host lattice. That replacement contributes one free electron and thus increases the electron carrier concentration,25,50 which is beneficial to the improvement on the gas sensing properties. In our work, the electron carrier concentration increases with the In concentration from 0 at. % to 5 at. %. However, the electron carrier concentration decreases when the In concentration is more than 5 at. %. XPS analysis was conducted to test the surface chemical components and the electronic states of the elements on the surfaces of the 3DOM samples. The binding energy of the photoelectron was referenced to the C 1s peak of adventitious hydrocarbon at 284.8 eV. Figure S7a shows the survey XPS spectra of the 3DOM samples. Only Zn, O, and In related core levels are detectable in the spectra, indicating that no impurities are introduced. In Figure S7b, the Zn 2p peaks for the pure ZnO are observed at 1021.5 and 1044.9 eV, corresponding to Zn 2p3/2 and Zn 2p1/2, respectively, which suggests that Zn in the samples only exists in the divalent oxidation state.10,41 Furthermore, in a comparison with the pure ZnO, the binding energies of the Zn 2p peaks for the In-doped ZnO samples increase gradually.24,25 The shifts of the binding energies reflect the electronic interaction between the ZnO and the dopant, which is consistent with the UV−vis analysis. The higher the In-doping concentration, the stronger the electronic interaction is. In Figure S7c, the binding energies of 445.5 and 452.9 eV corresponding to In 3d5/2 and In 3d3/2, respectively, are higher than that of the pure In2O3, which indicates a successful In-doping into the ZnO lattice.9,22 The Gauss fitting curves of the O 1s spectra of the pure 3DOM ZnO and the In-doped 3DOM ZnO are shown in Figure S7d, and the corresponding data are listed in Table 3. Two species, centered at 530.0 eV (OI) and 532.0 eV (OII), are indexed.22,23 The main species of OI with a lower binding energy are attributed to the coordination oxygen in the ZnO, and the higher binding energy species of OII, centered at 532.0 eV, belong to the adsorbed oxygen species. As shown in Table 3, the In-doped ZnO has more OII, by comparison with the F


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ACS Applied Materials & Interfaces CH3CH 2OH(ads) + 6O−(ads)

Table 3. Content of the Oxygen Species of Different Samples sample

OI (%)

OII (%)

ZnO ZnO/1%In ZnO/3%In ZnO/5%In ZnO/7%In ZnO/10%In

62.26 60.91 56.53 52.68 53.29 55.31

37.74 39.09 43.47 47.32 46.71 44.69

→ 2CO2 (g) + 3H 2O(l) + 6e− CH3CH 2OH(ads) + 6O2 −(ads) → 2CO2 (g) + 3H 2O(l) + 12e−

(10)

According to the above reactions, it can be clearly seen that richer pores and a larger specific surface area of the materials can provide more active sites for gas adsorption. In addition, higher electron concentration is helpful to increasing the adsorbed oxygen amount, leading to a high sensitivity. In our case, the 3DOM structure has a large amount of active sites due to its well-developed pore structure and large surface, which leads to an enhancement of the gas sensitivity. The huge improvement in the sensitivity to ethanol of the In-doped 3DOM ZnO can be attributed to the large surface area and high electron carrier concentration. As for In-doped 3DOM ZnO, the trivalent In3+ ions will replace the divalent Zn2+ ions and increase free electrons, which is helpful for increasing the electron carrier concentration and the amount of adsorbed oxygen.25,48 Hall measurements (as shown in Figure 8) show that the electron carrier concentration increases gradually when the In concentration is below 5 at. %. The 3DOM ZnO/5%In with the highest electron carrier concentration and adsorbed oxygen amount has the highest sensitivity. However, when the In concentration is over 5 at. %, the electron carrier concentration decreases gradually. The XRD analysis (as shown in Table 3) indicates that higher In-doping concentration reduces the crystallinity degree of the host ZnO crystal. Raman spectra (as shown in Figure S5) show that the defects caused by In-doping increase with the increase of the In-doping concentration. The decrease of the electron carrier concentration at the higher doping concentration is probably due to the poor crystallinity. When the doping concentration of In is higher than 5 at. %, the amount of adsorbed oxygen decreases with the electron carrier concentration, which is consistent with the XPS results. Therefore, the 3DOM ZnO with a higher In concentration has a lower sensitivity.

pure ZnO, and the percentage of OII increases gradually with the In concentration from 0 at. % to 5 at. %. However, it decreases when the In concentration is above 5 at. %. The ZnO/5%In with the most adsorbed oxygen exhibits the highest gas sensitivity. The trends of the changes of the percentage of OII and the electron carrier concentration stay consistent. The above data results are in agreement with the sensing tests. It indicates that the amount of the adsorbed oxygen is closely related to the electron carrier concentration. The In-doping can increase the electron carrier concentration, thus increasing the adsorbed oxygen, which is helpful to enhancing the gas sensitivity. 3.4. Sensing Mechanism. The gas sensing mechanism of the 3DOM ZnO sensors is interpreted by the electrical resistance change that originated from the adsorbed oxygen on the material surface.2−4 When the ZnO sensors are exposed to air, oxygen molecules will be adsorbed on the surface of ZnO and form adsorbed oxygen species (including O2−, O2−, and O−) by capturing free electrons from the conduction band.5 Then, a thick depletion layer is formed, resulting in an increase of the electrical resistance (as shown in Figure 9, wherein the

4. CONCLUSION In summary, the pure and the In-doped 3DOM ZnO materials have been prepared through a colloidal crystal templating method. The 3DOM structure is serviceable for improving the gas sensing properties of ZnO materials, because of its welldeveloped pore structure and large surface area. In-doping is an effective way to improve the gas sensing properties. The Indoping introduces more electrons into the ZnO matrix, which is helpful to increase the amount of adsorbed oxygen, leading to a higher sensitivity. The 3DOM ZnO/5%In with the highest electron carrier concentration and the highest amount of the adsorbed oxygen exhibits the highest gas sensitivity and excellent selectivity at 250 °C. Therefore, the prepared 3DOM ZnO/5%In can be used as a promising material for a new type of ethanol sensors.

Figure 9. Schematic diagram of ethanol sensing on the surface of the pure and the In-doped 3DOM ZnO.

thickness of the depletion layer is marked D). In contrast, when the ZnO sensors are exposed to ethanol, the ethanol molecules will react with the adsorbed oxygen and release free electrons, which leads to the decrease of the thickness of the depletion layer and the electrical resistance. The mechanism can be explained by several chemical reactions as follows: O2 (gas) → O2 (ads)

(9)

(5)

O2 (ads) + e → 2O (ads)

(7)

■

O−(ads) + e− → O2 −(ads)

(8)

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00339.

−

−

O2 (ads) + e → O2 (ads) −

−

−

(6)

ASSOCIATED CONTENT

S Supporting Information *

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SEM images, HR-TEM images, elemental mappings, sensor characteristics, Raman spectra, diffuse reflectance spectra, and XPS spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 010-64445927. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21275016, 21575011). REFERENCES

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