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Selective oxidizing gas sensing and dominant sensing mechanism of n-CaO-decorated n-ZnO nanorod sensors Gun-Joo Sun, Jae Kyung Lee, Seungbok Choi, Wan In Lee, Hyoun Woo Kim, and Chongmu Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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ACS Applied Materials & Interfaces

Selective oxidizing gas sensing and dominant sensing mechanism of n-CaO-decorated n-ZnO nanorod sensors Gun-Joo Suna, Jae Kyung Leea, Seungbok Choib, Wan In Lee*,c, Hyoun Woo Kimd, Chongmu Lee*,a a

Department of Materials Science and Engineering, Inha University, 253 Yonghyun-dong,

Nam-gu, Incheon 402-751, Republic of Korea b

Department of Mechanical Engineering, Inha University, 253 Yonghyun-dong, Nam-gu,

Incheon 402-751, Republic of Korea c

Department of Chemistry, Inha University, 253 Yonghyun-dong, Nam-gu, Incheon 402-751,

Republic of Korea d

Division of Materials Science and Engineering, Hanyang University, Seoul 133-791,

Republic of Korea KEYWORDS: CaO, ZnO, decoration, gas sensor, NO2 ABSTRACT In this work, we investigated the NO2 and CO sensing properties of n-CaO-decorated n-ZnO nanorods and the dominant sensing mechanism in n–n heterostructured one-dimensional (1D) nanostructured multi-networked chemiresistive gas sensors utilizing the nanorods. The CaOdecorated n-ZnO nanorods showed stronger response to NO2 than most other ZnO-based nanostructures including the pristine ZnO nanorods. Many researchers have attributed the enhanced sensing performance of heterostructured sensors to the modulation of the conduction channel width or surface depletion layer width. However, the modulation of the conduction channel width is not the true cause of the enhanced sensing performance of n–n heterostructured 1D gas sensors because the radial modulation of the conduction channel 1

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width is not intensified in these sensors. In this work, we demonstrate that the enhanced performance of the n-CaO-decorated n-ZnO nanorod sensor is mainly due to a combination of the enhanced modulation of the potential barrier height at the n–n heterojunctions, the larger surface-area-to-volume ratio and the increased surfaicial defect density of the decorated ZnO nanorods, not the enhanced modulation of the conduction channel width. 1. INTRODUCTION Semiconducting metal oxides (SMOs) are widely used as sensor materials because of their high sensitivity, small dimensions, simple sensing mechanism, simplicity of fabrication, low detection limits, and low power consumption1-4. However, they still have some drawbacks such as insufficient sensitivity at room temperature and low selectivity. The fabrication of n– n and p–n heterostructures by simply mixing two constituents, forming core–shell nanostructures, or decorating one type of SMO nanorods or thin films with another type of SMO nanoparticles is a widely accepted technique to enhance the sensitivity and selectivity of SMO sensors.5-10 In this study, n–n heterostructures were prepared by adopting n-ZnO and n-CaO as decorated and decorating SMOs, respectively. CaO-decorated ZnO nanorods were prepared using a two-step process: thermal evaporation of ZnO powders and solvothermal decoration of the ZnO nanorods with CaO nanoparticles. Multi-networked chemiresistive sensors were fabricated by connecting the pristine and CaO-decorated ZnO nanorods with interdigitated electrodes, and their NO2 and CO gas sensing properties were investigated. Although CaO has not previously been used as a main sensor material, it has been used as a decorating material or an additive.11 The chlorine gas sensing properties of a CaO–ZnO mixture have been reported previously, as well.12 CaO and Na2CO3 additives have also been observed to be effective in increasing the gas sensitivity13. Regarding the sensing mechanism of heterostructured SMO gas sensors, two main 2


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mechanisms have been reported: radial modulation of the conduction channel formed along the nanorod axis and modulation of the potential barrier formed at the heterojunction interface14. Of these two competing mechanisms, the dominant one is still controversial. The former has been considered as the main mechanism for the enhanced sensing performance of heterostructured gas sensors by many researchers,15-20 whereas the latter has been cited less often.14,21 Nevertheless, in this study, we demonstrate that the former mechanism is not appropriate for n-CaO-decorated n-ZnO nanorod gas sensors. The latter mechanism, however, as well as the higher surface-to-volume ratio is determined to contribute to the enhanced NO2 and CO gas sensing performance of these sensors. The sensing mechanism related to the heterostructures is quite complicated. To be able to draw an accurate energy band diagram, carrier trapping at the interface states at the heterojunctions and information about the band gap as well as electron affinity and work function of the two materials forming the heterostructures must be considered23. However, because of the difficulty in correctly determining the surface states, the band structure is often approximated, ignoring these effects.24,25 The authors of the current study recently reported that the optimal shell layer thickness in core–shell nanowire sensors was 2λD of the shell material when considering the interface states.26 If the effects of the interface states had been ignored, the optimal shell layer thickness would have been determined to be λD. Thus, ignoring the effects of the interface states could lead to an incorrect conclusion concerning the sensing mechanism of nanostructured sensors, particularly heterostructured sensors, because they trap a large number of carriers. In this study, both band structure models with and without considering the effects of carrier trapping at interface states were used to determine the dominant gas sensing mechanism in n–n heterostructured one-dimensional (1D) gas sensors. 3



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2. EXPERIMENTAL 2.1 Synthesis of pristine and CaO-decorated ZnO nanorods. First, a 3-nm-thick gold (Au) thin film was deposited on (100) Si by dc magnetron sputtering to prepare a substrate for the synthesis of the ZnO nanorod structures. A quartz tube was mounted horizontally inside a tube furnace. An alumina boat with a mixture of ZnO (purity: 99.99%) and graphite (purity: 99.99%) powders was placed at the center of the quartz tube. The Au-deposited Si substrate was placed ~5 mm away from the ZnO/graphite powder mixture. The furnace was heated to 950 °C and maintained at that temperature for 1 h in a N2/3 mol%-O2 atmosphere with a constant flow rate of 500 cm3/min for both oxygen and nitrogen. The total pressure of the furnace was maintained at 1.0 Torr. On the other hand, 50 ml of a 50 mM Ca(OH)2 precursor solution was prepared by dissolving calcium acetate [Ca(CH3COO)2] in distilled water. To prepare the Ca(OH)2 precursor solution, 10 ml of a 28% NH4OH solution and 40 ml of a H2O2 solution were mixed together and then agitated using a magnetic bar for 30 min. The mixed solution was then ultrasonicated for 30 min to form a uniform solution and then rotated using a centrifuge at 3,000 rpm for 2 min to precipitate Ca(OH)2 powders. The precipitated powders were collected by removing the liquid, leaving the powders behind. The collected powders were rinsed in a 1:1 solution of isopropyl alcohol and distilled water to remove the impurities. The rinsing process was repeated five times. A 100-ml Ca(OH)2 precursor solution was prepared by adding only isopropyl alcohol to the Ca(OH)2 powders. Subsequently, the Ca(OH)2 precursor solution was dropped onto the ZnO nanorods on a substrate, and the substrate was rotated at 1,000 rpm for 30 s to precipitate the CaO nanoparticles on the nanorods. After the spin-coating process, the Ca(OH)2-decorated ZnO nanorod sample was dried at 150 °C for 1 min. This process was repeated three times, and then, the synthesized Ca(OH)2-nanoparticle4


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decorated ZnO nanorods were annealed in air at 600 °C for 1 h to convert the Ca(OH)2 nanoparticles to CaO nanoparticles and to enhance the adhesion of the CaO nanoparticles to the ZnO nanorods. During these processes the decorated product after being immersed into the precursor solution might be Ca(OH)2-ZnO due to the dissolution of ZnO nanorods in the alkaline precursor solution and it would be converted into CaO-ZnO after calcination at higher temperature. Consequently, the microstructure of ZnO nanorods would be altered and many pores or defects would be produced in the CaO-decorated ZnO nanorods. 2.2 Materials characterization. The phase and crystallinity of the pristine ZnO nanorods and CaO-decorated ZnO nanorods were examined using X-ray diffraction (XRD; Philips X’Pert MRD) using Cu Kα radiation (1.5406 Å). The data were collected over the 2θ range of 20–80° with a step size of 0.05° 2θ at a scan speed of 0.05°/s. Assignment of the XRD peaks and identification of the crystalline phases were performed by comparing the obtained data with those of the reference compounds in the JCPDS database. The surface morphology and size of the synthesized nanorods and nanoparticles were examined using field-emission scanning electron microscopy (FESEM; Hitachi S-4200) at an accelerating voltage of 5 kV. The structures and phases of the nanostructures were further examined by transmission electron microscopy (TEM; Philips CM-200). 2.3 Multi-networked chemiresistive sensor fabrication. The SiO2 film (~200 nm) was grown thermally on single-crystalline Si (100), and the pristine ZnO nanorods and CaOdecorated ZnO nanorods were dispersed in a 1:1 mixture of deionized water and isopropyl alcohol by ultrasonication. The pristine ZnO nanorod and CaO-decorated ZnO nanorod chemiresistive sensors were fabricated by pouring a few drops of nanorod-suspended ethanol onto the SiO2-coated Si substrates equipped with a pair of interdigitated Ni (~10 nm)/Au (~100 nm) electrodes with a 20-µm gap. 5



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2.4 Sensing tests. The gas sensing properties of the pristine ZnO nanorods and CaOdecorated ZnO nanorods were examined at different temperatures in a quartz tube inserted in an electrical furnace. During the sensing tests, the nanorod gas sensors were placed in a sealed quartz tube with an electrical feed through. Predetermined amounts of NO2 and CO (>99.99%) gas were injected into the testing tube to obtain NO2 and CO concentrations of 10, 20, 50, 100, and 200 ppm while the electrical resistances of the nanorods were monitored. Detailed sensing test procedures are described in the earlier papers by the present authors27-29. The response was defined as Rg/Ra and Ra/Rg for oxidizing and reducing gases, respectively, where Rg and Ra are the electrical resistances of sensors in analyte gas and ambient air, respectively. The response and recovery times were defined as the durations required to reach 90% of the resistance change upon exposure to the analyte gas and air, respectively. 3. RESULTS AND DISCUSSION 3.1 Structure and morphology of the pristine and CaO-decorated ZnO nanorods. Figures 2a, b present FESEM images of the pristine and CaO-decorated ZnO nanorods, respectively, synthesized using thermal evaporation and solvothermal techniques. The ZnO nanostructures (Figure 2a) exhibited a rod-like morphology with a gold (Au) nanoparticle at one end of the rod. The diameter of the rods tended to increase gradually with increasing distance from the rod tip to the Au nanoparticle. In some ZnO nanorods, the diameter of the nanorod end without the Au nanoparticle was much larger than that with an Au nanoparticle. Statistical analysis of various FESEM images of the synthesized ZnO nanorods reveals that the nanorod diameters ranged from 40 to 180 nm as shown in Figure 1. This diameter uniformity is good enough to obtain stable and reproducible gas sensing performance. In contrast, the mean diameter of the CaO nanoparticles on the CaO-decorated ZnO nanorods was ~50 nm (Figure 2b). The distribution of the CaO nanoparticles on the ZnO nanorods was 6


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nonuniform. In addition to the CaO nanoparticles decorating the ZnO nanorods, a few CaO clusters were observed, separated from the nanorods. Low-magnification TEM analysis confirmed the nonuniform distribution of the nanoparticles on the nanorods (Figure 2c). The fringe patterns in the high-resolution TEM image (Figure 2d) and spotty pattern in the selected area electron diffraction pattern (Figure 2e) indicated that both the ZnO nanorods and CaO nanoparticles were single crystals. XRD patterns of the pristine and CaO-decorated ZnO nanorods are presented in Figure 3. The pristine ZnO nanorods exhibited sharp tall peaks assigned to the (100), (002), (101), (102), (110), (103), and (112) reflections from the wurtzite-structured ZnO (JCPDS Card No. 89-1397) with lattice constants of a = 0.3253 nm and c = 0.5213 nm. In contrast, the CaOdecorated ZnO nanorods exhibited far taller peaks assigned to the (111), (200), (220), (311), (222), and (400) reflections from face-centered-cubic-structured CaO (JCPDS Card No. 821691) with a lattice constant of a = 0.4796 nm in addition to the reflections from the wurtzitestructured ZnO. The XRD patterns confirm that both the ZnO nanorods and CaO nanoparticles were crystalline. 3.2 Gas sensing properties of the pristine and CaO-decorated ZnO nanorods. In the preliminary step before the main sensing tests, the optimal operating temperature at which the highest response was obtained was determined. Figures 4a, b show the responses of the two sensors to CO and NO2 at different sensor operating temperatures. Both sensors exhibited maximum responses to CO and NO2 at 300 °C, suggesting that the optimal operating temperature is 300 °C. Based on this result, all the other sensing tests in this study were conducted at 300 °C. Figures 5a,b show the dynamic responses of the pristine and CaO-decorated ZnO nanorod sensors to the reducing gas CO. Both sensors exhibited typical electrical behavior of 7



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n-type semiconductors. The resistance of the pristine ZnO nanorods (Figure 5a) first decreased rapidly when the CO gas was supplied and then slowly recovered to the original resistance value when the CO supply was stopped. The sensor exhibited reversible and stable response and recovery behaviors. The resistance change of the sensors in a cycle increased with increasing CO concentration. The CaO-decorated ZnO nanorod sensor (Figure 5b) exhibited much higher initial resistance than the pristine one. This difference might be attributed to the high intrinsic resistivity of CaO resulting from the low mobility of electrical carriers. In other words, ZnO has a very low resistivity compared with that of CaO, which leads to a lower initial resistance of the pristine ZnO nanorod sensor. The dynamic responses of the pristine and CaO-decorated ZnO nanorod sensors to an oxidizing gas NO2 are shown in Figures 5c, d, respectively. Both sensors also exhibited typical electrical behavior of n-type semiconductors toward NO2 by exhibiting the reverse response and recovery behavior to the reducing gas CO. The resistance change of the sensors in a cycle increased slightly with increasing NO2 concentration. The responses of the sensors determined from the dynamic response curves are plotted as a function of the CO and NO2 concentrations in Figures 6a, b, respectively. The responses of both sensors to NO2 were far higher than those to CO. The response of the decorated sensor to 200 ppm NO2 was more than 10 times larger than that to 200 ppm CO. The response of the CaO-decorated ZnO nanorods synthesized in this work to NO2 is compared with those of other ZnO-based nanostructures in the literature in Table 130-35. The CaO-decorated ZnO nanorod sensor exhibited a stronger response to NO2 than the other ZnO-based nanostructured sensors. This result suggests that CaO is a promising decorating SMO, even though CaO itself is not a good sensor material. The response/recovery times of the two sensors are shown in Figures 7a, b, respectively. The decorated ZnO nanorods 8


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exhibited shorter response/recovery times than the pristine ZnO nanorods. 3.3 Gas sensing mechanism of the pristine ZnO nanorod sensor. Figures 8a, b36-40 show the CO and NO2 gas sensing mechanism of the pristine ZnO nanorod sensor. In the pristine ZnO nanorods, before introducing CO or NO2, the ZnO nanorods absorb O2 from ambient air, and the adsorbed oxygen molecules are ionized into O-, O2-, or O2- depending on the temperature by capturing electrons from the conduction band of ZnO41, resulting in the formation of a depletion layer in the oxide surface region and an increase in the resistivity of the oxide. When the reducing gas CO is supplied, the CO molecules react with the adsorbed oxygen species to form CO2 molecules. This process causes the captured electrons to return to the conduction band of the ZnO, thinning the electron-depletion layer and expanding the conduction channel formed in the central region of the nanorod along the nanorod axis. Consequently, the resistance of the nanorods is decreased. In contrast, when the oxidizing gas NO2 is supplied, the depletion layer in the oxide surface region is expanded, and the conduction channel in the central region of the ZnO nanorod shrinks. That is, the radial modulation of the conduction channel and thereby the modulation of the resistance occur during a cycle of supply and no supply of either CO or NO2 gas, which mainly determines the response of the pristine ZnO nanorods because the response is defined as Ra/Rg and Rg/Ra for CO and NO2 gases, respectively. Another contribution to the response of the pristine ZnO nanorod sensor is the modulation of the potential barrier height at n–n homojunctions, i.e., the contact points of two ZnO nanorods. As illustrated in Figure 8b, depletion layers are formed on both sides of the n–n junction in the ZnO–ZnO interface region and a higher potential barrier (V1) is formed at the interface in ambient air because of significant adsorption of oxygen molecules. When CO gas is supplied, thinner depletion layers are formed on both sides of the n–n junction, and a smaller potential barrier (V2) is formed at the n-n junction 9



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because of less significant oxygen adsorption. In contrast, when NO2 is supplied, further thicker depletion layers are formed on both sides of the n–n junction and a further higher potential barrier (V3) is formed at the n–n junction. Accordingly, the modulation of the potential barrier height as well as the radial modulation of the conduction channel width occurs, leading to the modulation of the resistance of the nanorod sensor. However, the contribution of this modulation is relatively small because of the small number of the ZnOZnO contact points in the multi-networked ZnO nanorod sensor. 3.4 Gas sensing mechanism of the CaO-decorated ZnO nanorod sensor. The CO and NO2 gas sensing mechanism of the CaO-decorated ZnO nanorod sensor is illustrated in Figures 9a–e. The enhanced sensing performance of the decorated ZnO nanorods compared with that of the pristine ZnO nanorods was reported to be mainly due to the larger radial modulation of the conduction channel width and thereby the intensified modulation of the resistance of the decorated ZnO nanorods.18-20 Both ZnO and CaO are n-type semiconductors, and the work function (qΦ) of ZnO is larger than that of CaO (Figure 8a). Accordingly, as observed in the energy band diagram of the CaO–ZnO couple, electrons tend to transfer from CaO to ZnO in the CaO-decorated ZnO nanorods even in vacuum until electronic equilibrium is attained between CaO and ZnO. Because of this electron transfer, electron-depletion and accumulation layers are formed on the CaO and ZnO sides, respectively, in the CaO–ZnO interfacial region (Figure 8b). In ambient air, oxygen molecules are adsorbed by CaO and ZnO surfaces and ionized by consuming the free electrons in the electron-accumulation layer. The higher electron concentration on the ZnO side in the CaO–ZnO interfacial region, i.e., in the accumulation layer induces additional oxygen adsorption.42 All the electrons in the accumulation layer and some electrons in the vicinity of the accumulation layer might be consumed (Figure 8c). Consequently, the width of the depletion layer newly formed on the 10


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ZnO side will be W41 (≅ W4) that is equal to λD (ZnO) ≅ 30 nm.43 In the outer surface regions of the ZnO nanorods and CaO nanoparticles exposed to air, electron-depletion layers with a width of W4 equal to λD (ZnO) are also formed. Consequently, the conduction channel width will be W1, because it is not almost affected by the existence of the CaO nanoparticle. When the reducing gas CO is supplied, the CO molecules are adsorbed by the surfaces of the ZnO nanorods and CaO nanoparticles. In the CO atmosphere, less oxygen molecules are adsorbed by the ZnO and CaO surfaces than in ambient air. A smaller portion of the electrons in the accumulation layer formed on the ZnO side are consumed to ionize the reduced number of adsorbed oxygen molecules, resulting in the formation of a thin depletion layer and a thicker accumulation layer compared to that in an air or NO2 atmosphere (Figure 8d). On the other hand, the adsorbed CO molecules react with the preadsorbed oxygen species to produce CO2 and electrons. The produced CO2 molecules are desorbed from the ZnO and CaO surfaces, and the produced electrons return to the conduction band of the ZnO and CaO, which makes the depletion layer thinner (W51≅ W5) and shrinks the accumulation layer to a width of W52 in the CaO-ZnO interfacial region. Consequently, a thick conduction channel (W2) is formed in the central region of the ZnO nanorod. In contrast, when NO2 gas is supplied, the NO2 molecules are adsorbed by the CaO and ZnO surfaces. In the NO2 atmosphere, the adsorbed NO2 molecules are converted to NO2-or NO44 and oxygen molecules are ionized into oxygen ions by consuming all the electrons in the accumulation layer and some electrons near the accumulation layer on the ZnO side of the CaO–ZnO interfacial region. Consequently, a depletion layer with a width of W61 (≅ W6) is formed on the ZnO side of the CaO–ZnO interfacial region, and a conduction channel with a width of W3 is formed in the central region of the ZnO nanorod (Figure 8e). A careful comparison of the radial modulation of the conduction channel width during a 11



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cycle of supply and no supply of CO gas between the pristine ZnO nanorods (Figure 8a) and CaO-decorated ZnO nanorods (Figures 9c–e) reveals that the modulation in the decorated ZnO nanorods is almost the same as that of the pristine ones. The difference in the conduction channel width between in air and in CO is W2-W1 for both the pristine ZnO nanorods and decorated ZnO nanorods (Figures 9c, d). On the other hand, the difference in the conduction channel width in air and in NO2 is W1-W3 for both the pristine ZnO nanorods and CaOdecorated ZnO nanorods (Figures 9c, e). From the above discussion, we may conclude that the modulation of the conduction channel width of the ZnO nanorods is not intensified either in reducing gas CO sensing or oxidizing gas NO2 sensing by CaO decoration. Therefore, the previous reports indicating that intensified radial modulation of the conduction channel width occurs in heterostructured 1D nanostructures18-20 is not applicable to n-CaO-decorated n-ZnO nanorods. The above discussion and the schematics and energy band diagrams in Figures 8a–e were approximated by ignoring the carrier trapping at the interface states in the CaO–ZnO interfacial region. However, a discussion on the sensing mechanism of the CaO-decorated ZnO nanorods to CO and NO2 considering the carrier trapping at the interface states is provided in Supporting Information (Figures S1a–e). The conclusion is that the modulation of the conduction channel width in the ZnO nanorods is not intensified by CaO decoration in either CO or NO2 gas sensing, even if carrier trapping at interface states is taken into consideration. That is, the same conclusion is obtained regardless of considering carrier trapping at interface states or not. Another mechanism is the modulation of the potential barrier height at the CaO–ZnO interface during a cycle of supply and no supply of CO or NO2 gas.21,22 A potential barrier is formed at the CaO–ZnO interface because of energy band bending. As illustrated in Fig. 8, 12


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less significant band bending occurs in the CO atmosphere than in ambient air because of the reduced oxygen adsorption in the CO atmosphere. Accordingly, the potential barrier height is smaller for the CO atmosphere (V2) than for ambient air (V1) (Figures 8c, d). The difference in the potential barrier height between the CO atmosphere and ambient air is V1-V2. Similarly, the difference in the potential barrier height between the NO2 atmosphere and ambient air is V3-V1 (Figures 8c, e). These differences can be more clearly observed in Figures S1c–e. This modulation of potential barrier height contributes to the enhanced response of the CaOdecorated ZnO nanorods via the resistance modulation. The resistance of the sensor depends on the potential barrier height according to the following equation45: R = R0 exp (qV/kT),

(1)

where R is the resistance of the material, R0 is the initial resistance, q is the charge of an electron, V is the potential energy barrier height, k is Boltzmann’s constant, and T is the absolute temperature of the sensing material. Because the response is defined as Rg/Ra or Ra/Rg, the potential barrier modulation does contribute to the enhanced response of the decorated ZnO nanorods. In contrast, modulation of the potential barrier at the n–n heterojunctions in the pristine ZnO nanorod sensor is not observed because this device contains no n–n heterojunctions, i.e., no interfaces with other materials. Modulation of the potential barrier only occurs at the ZnO–ZnO contact points, i.e., n-n homojunctions; however, the contribution of this modulation is small as stated above. Figure S1 shows that the potential barrier height for NO2 is higher than that for air in the case of oxidizing gas NO2. Accordingly, the resistance, Rg due to the potential barrier at the CaO-ZnO interface for NO2 is larger than that Ra for air. On the other hand, the response, S to oxidizing gas is defined as S = Rg/Ra. Therefore, we may say that the resistance, Rg due to the potential barrier at the CaO-ZnO interface for NO2 has an effect of increasing the response of the CaO13



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decorated ZnO nanorod sensor. Another notable effect of CaO decoration on the enhanced response of the CaOdecorated ZnO nanorods to NO2 and CO is the increase in the surface-to-volume ratio. Brunauer–Emmett–Teller (BET) analysis was performed to compare the surface-area-tovolume ratio of the pristine and CaO-decorated ZnO nanorods. The measured specific surface areas of the pristine and CaO-decorated ZnO nanorods were 5.06 and 13.2m2/g, respectively (Figures S4a, b). This result confirms that the enhanced response of the decorated nanorods to NO2 or CO compared with that of the pristine one is partially attributed to the larger surfacearea-to-volume ratio. The increased specific surface area of the CaO-decorated ZnO nanorods might also be partially attributed to the pores or defects formed during the immersion of the ZnO nanorods into the alkaline precursor solution followed by the calcination at higher temperature in addition to the physical expansion of the surface area of the nanorods due to the decoration of the ZnO nanorods with many small CaO nanoparticles. In addition to the above two effects: the electronic sensitization effect and the effect of increased specific surface area of the ZnO nanorods due to the CaO decoration, the enhanced sensing performance of the CaO-decorated ZnO nanorods might be attributed to the surficial defects created as a result of corrosion of the ZnO nanorod by OH- in the precursor solution. These crystallographic defects provide preferential adsorption sites and diffusion paths for oxygen and ethanol molecules, which might also contribute to the enhanced ethanol gas sensing properties of the CaO-decorated ZnO nanorod sensor. We have demonstrated that the intensified modulation of the conduction channel width during a cycle of supply and no supply of reducing gas does not occur in n-CaO-decorated nZnO 1D nanostructures. Thus far, we have discussed CaO-decorated ZnO nanorods in which the work function of the decorating SMO (CaO) is smaller than that of the decorated one 14


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(ZnO). In addition to CaO-decorated ZnO nanorods, SnO2-decorated ZnO nanorods, In2O3decorated ZnO nanorods, Ga2O3-decorated ZnO nanorods, and TiO2-decorated ZnO nanorods belong to this category (Table 236,39,46-50). However, the modulation of the conduction channel width during a cycle of supply and no supply of analyte gas might be intensified in the heterostructured 1D nanostructures for n-SMO-decorated n-SMO 1D nanostructures for which the work function (qΦ)of the decorating SMO is larger than that of the decorated SMO, as discussed in Supporting Information (Figures S2a–e). This behavior occurs because the formation of the depletion layer on the decorated SMO side of the interface region reduces the conduction channel width and thereby enhances its modulation. For example, the radial modulation of the conduction channel width of the ZnO nanorod sensor is intensified by the decoration of MoO3 or WO3, the work functions of which are larger than that of ZnO (Table 2). Therefore, we may conclude that if we do not consider the effects of carrier trapping at the interface states, the dominant sensing mechanism between modulation of the conduction channel width and that of the potential barrier height at the n–n heterojunction in n–n heterostructured gas sensors might depend on the relative magnitude of work functions of decorating and decorated SMOs. In contrast, as discussed in Supporting Information (Figures S3a, b), if the effects of carrier trapping at the interface states are taken into consideration, even in the MoO3-decorated ZnO nanorods to CO and NO2, the modulation of the conduction channel width in the ZnO nanorods is not intensified by MoO3 decoration in either CO or NO2 gas sensing. Therefore, we reach a final conclusion that the intensified modulation of the conduction channel width does not occur in all n-SMO-decorated n-SMO 1D nanostructures, regardless of the relative magnitude of work functions of decorating and decorated SMOs. We estimate that the relative contribution of the three different effects to the enhanced sensing properties of the CaO nanoparticle-decorated ZnO nanowire sensor is as follows: the 15



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modulation of conduction channel : the modulation of potential barrier height : the increased surface-to-volume ratio: the increased surfacial defect density = 1 : 4: 3: 2. 3.5 Effective circuit model of the simplified CaO-decorated ZnO nanorod sensor Figure 10 is a simplified resultant equivalent circuit model for the two CaO-decorated ZnO nanorods contacting each other. The Rg can be considered as the shunt connection of channel resistance and the resistance induced by the potential barrier height in the CaO nanoparticledecorated ZnO nanowire sensor. All the carriers transferring through the CaO-ZnO junctions merge into the connection channel formed along the axis of each ZnO nanorod. Accordingly, all the contact resistances at the CaO-ZnO junctions , Rcz are connected to the reistances of Zn nanorods, Rz in series. If we simplify the CaO-decorated ZnO nanorod sensor into a sensor consisting of two ZnO nanorods each of which are decorated with two CaO nnanoparticles and two electrodes, the total resistance in the equivalent circuit can be expressed as follows: Rg = 2Rz + 4Rcz + Rzz ,

(2)

where Rg is the resistance of the CaO-decorated ZnO nanorod sensor in the analyte gas, ethanol, Rz is the resistance of a ZnO nanorod, is the resistance of a CaO-ZnO contact and Rzz is the resistance of a ZnO-ZnO contact.

3.6 Selectivity of the pristine and decorated ZnO nanorod sensors. To examine the selectivity of the sensors toward NO2 against other gases, selectivity tests were conducted on the sensors for NO2, CO, ethanol, acetone, benzene, and toluene gases at 300 °C. The concentrations of all the gases were 200 ppm. As demonstrated in Figure 11, the pristine and CaO-decorated ZnO nanorod sensors exhibited significantly stronger responses to the oxidizing gas NO2 than to the reducing gases CO, ethanol, acetone, benzene, and toluene, indicating that the sensors exhibit selectivity toward an oxidizing gas against reducing gases. 16


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The difference between the potential barrier height at the CaO–ZnO interface in ambient air and CO is V1-V2 (Figures 9b, c), whereas the difference between that in ambient air and NO2 is V3-V1 (Figures 9b,d), as stated above. Because V3-V1 > V1-V2, the stronger response of the decorated nanorod sensor to the oxidizing gas NO2 than to the reducing gas CO may be attributed mainly to the larger modulation of the potential barrier height at the CaO–ZnO interface for NO2 than for CO, not the larger modulation of the conduction channel width. Note that a surface-area-to-volume ratio does not apply to this case. On the other hand, in the pristine ZnO nanorods, the difference between the potential barrier height at the ZnO–ZnO interface in ambient air and CO is V1-V2 (Figure 8b), whereas the difference between that in ambient air and NO2 is V3-V1 (Figure 8b), and V3-V1 > V1-V2. Therefore, the stronger response of the pristine nanorod sensor to NO2 than to CO may be attributed to the larger modulation of the potential barrier height at the ZnO–ZnO interface for NO2 than for CO. 4. CONCLUSIONS Pristine and CaO-decorated ZnO nanorods were synthesized using facile thermal evaporation and solvothermal techniques. Multi-networked chemiresistive sensors were fabricated by connecting them with interdigitated electrodes, and their NO2 and CO gas sensing properties were examined. XRD and FESEM analyses revealed that both the ZnO nanorods and CaO nanoparticles on the nanorods were single crystals. The decorated ZnO nanorod sensor exhibited stronger and faster response to NO2 and CO than the pristine ZnO nanorod sensor. The CaO-decorated n-ZnO nanorods also showed stronger response to NO2 than most other ZnO-based nanostructures. In addition, the CaO-decorated ZnO nanorod sensor exhibited excellent selectivity toward the oxidizing gas NO2 against reducing gases such as CO, ethanol, methanol, benzene, and toluene. The improved sensing performance of the CaO-decorated ZnO nanorod sensor toward NO2 is mainly attributed to the larger modulation of the 17



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potential barrier height at the CaO–ZnO interface during a cycle of the supply and no supply of NO2, the larger surface-to-volume ratio and the increased surfacial defect density of the decorated sensor, not the larger radial modulation of the conduction channel width.

■ ASSOCIATED CONTENT (S) Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Gas sensing mechanism of the CaO-decorated ZnO nanorod sensor – considering carrier trapping at the interface states (Figures S1a-e); gas sensing mechanism of the MoO3decorated ZnO nanorod sensor – considering carrier trapping at the interface states (Figures S2a-e); gas sensing mechanism of the MoO3-decorated ZnO nanorod sensor – considering the effects of carrier trapping at the interface states (Figures S3a, b); raw data for BET measurements- specific surface areas of the pristine and MoO3-decorated ZnO nanorods (Figures S4a, b)

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions C.L. conceived the project, G. J. S. and J. L. performed the experiments. The results were discussed and interpreted by all the authors. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the 18


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manuscript. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENT This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2015R1D1A1A01057029 and 2010-0020163).

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Table 1. Comparison of the response of the CaO-decorated ZnO nanorods synthesized in this work to NO2 with those of other ZnO-based nanostructures in the literature Materials

Concentration Temperature

Response

Reference

(ppm)

(°C)

(Ra/Rg)

200

300

141.66

This work

20

225

~90

30

5

200

45.4

31

ZnO NBs

8.5

250

~4

32

ZnO NRs

0.1

250

8.24

33

Au/ZnO

50

300

4.14

34

1

200

20.8

35

ZnO NRs

1

200

10.23

35

ZnO NWs

1

200

19.1

35

CaOdecorated ZnO NRs Defectcontrolled ZnO NRs Co3O4decorated ZnO NWs

NRs ZnO thin film

25



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Table 2. Work functions (qΦ)of semiconducting metal oxides Material

Work function (eV)

Reference

CaO

1.69

36

SnO2

4.5

42

In2O3

4.8

43

Ga2O3

5.0

44

TiO2

5.23

45

ZnO

5.3

39

WO3

6.4

46

MoO3

6.6

47

26


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Figure Captions Figure 1. The diameter distribution of the ZnO nanorods synthesized by the thermal evaporation of a mixture of ZnO and graphite powders in this study. Figure 2. SEM images of (a) the pristine and (b) CaO-decorated ZnO nanorods.

(c) Low-

magnification TEM image, (d) high-resolution TEM image and (e) corresponding selected area electron diffraction pattern of a typical CaO-decorated ZnO nanorods. Figure 3. X-ray diffraction pattern of the pristine and CaO-decorated ZnO nanorods. Figure 4. Responses of the pristine and CaO-decorated ZnO nanorods to (a) CO and (b) NO2 at various temperatures. Figure 5. Dynamic response curves of the (a) pristine and (b) CaO-decorated ZnO nanorods to CO. Dynamic response curves of the (c) pristine and (d) CaO-decorated ZnO nanorods to NO2. Figure 6. Responses of the pristine and CaO-decorated ZnO nanorods to (a) CO and (b) NO2 as a function of CO and NO2 concentrations, respectively. Figure 7. Response and recovery times of the pristine and CaO-decorated ZnO nanorods toward (a) CO and (b) NO2 as a function of CO and NO2 concentrations, respectively. Figure 8. (a) Schematics of the pristine ZnO nanorod and corresponding energy band diagrams in air, CO and NO2 atmosphere. The energy bandgap (Eg), electron affinity(qχ) and work function(qΦ) of CaO are 7.7 eV36, 0.7 eV37 and 1.69 eV38 and those of ZnO are 3.37 eV,39 4.3 eV39 and 5.2 eV40, respectively. (b) Schematics of two pristine ZnO nanorods in contact and corresponding energy band diagrams in air, CO and NO2 atmosphere. Figure 9. Energy band diagrams of a CaO-ZnO couple in vacuum: (a) before and (b) after contact. Schematics of the CaO-decorated ZnO nanorod and corresponding energy band diagrams in (c) air, (d) CO and (e) NO2 atmosphere, ignoring the effects of carrier trapping at 27



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interface states. Figure 10. Simplified resultant equivalent circuit model for the two CaO-decorated ZnO nanorods contacting each other. Figure 11. Responses of pristine and CaO-decorated ZnO nanorods to various gases at a temperature of 300 oC and a gas concentration of 200 ppm, which shows the selectivity of both nanorods toward an oxidizing gas NO2 against reducing gases.

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Fig. 1

29



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Fig. 2

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Fig. 3

31



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Fig. 4

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Fig. 5

33



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Fig. 6

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Fig. 7

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Fig. 8

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Fig. 9

37



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Fig. 10

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Fig. 11

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Table of Contents

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Selective Oxidizing Gas Sensing and Dominant ... - ACS Publications

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