Morphological Evolution Induced through Heterojunction of W

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Surfaces, Interfaces, and Applications

Morphological Evolution Induced through Heterojunction of W-decorated NiO Nanoigloos: Synergistic Effect on High-performance Gas Sensors Seung Yeop Yi, Young Geun Song, Jae Yeol Park, Jun Min Suh, Gwang Su Kim, Young-Seok Shim, Jong Min Yuk, Sangtae Kim, Ho Won Jang, Byeong-Kwon Ju, and Chong-Yun Kang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18678 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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ACS Applied Materials & Interfaces

Morphological Evolution Induced through Heterojunction of W-decorated NiO Nanoigloos: Synergistic Effect on High-performance Gas Sensors Seung Yeop Yi,†, ‡, # Young Geun Song,‡, §, # Jae Yeol Park,∥ Jun Min Suh,∇ Gwang Su Kim,†, ‡ Young-Seok Shim,∥ Jong Min Yuk,∥ Sangtae Kim,†, ‡ Ho Won Jang,∇ Byeong-Kwon Ju,§ and Chong-Yun Kang*,†, ‡ †KU-KIST

Graduate School of Converging Science and Technology, Korea University, Seoul

02841, Republic of Korea ‡Center

for Electronic Materials, Korea Institute of Science and Technology (KIST), Seoul,

02791 Republic of Korea §Display

and Nanosystem Laboratory, School of Electrical Engineering, Korea University, Seoul

02841, Republic of Korea ∥Department

of Materials Science & Engineering, Korea Advanced Institute of Science and

Technology (KAIST), Daejeon, 34141 Republic of Korea ∇Department

of Materials Science and Engineering, Research Institute of Advanced Materials,

Seoul National University, Seoul 08826, Republic of Korea

ACS Paragon Plus Environment

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

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authors contributed equally.

*Corresponding author: [email protected] KEYWORDS: Nanostructure, Heterojunction, Morphological evolution, Gas sensor, NO2


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ABSTRACT

Morphological evolution accompanying a surface roughening and the preferred orientation is an effective way to realize a high-performance gas sensor because of its significant potential as a chemical catalyst by chemical potentials and atomic energy states. In this work, we investigated the heterojunction of both-sided W-decorated NiO nanoigloos fabricated through RF sputtering and a soft-template method. Interestingly, the morphological evolution characterized by a pyramidal rough surface and the preferred orientation of the (111) plane was observed upon decorating the bare NiO nanoigloos with W. The underlying mechanism of the morphological evolution was precisely demonstrated based on a van der Drift competitive growth model originating from the oxygen transport and chemical strain in a lattice. The gas sensing properties of W-decorated NiO show an excellent NO2 response and selectivity when compared to other gases. In addition, high response stability was evaluated under interference gas and humidity condition. The synergistic effects on the sensing performance were interpreted based on the morphological evolution of W-decorated NiO nanoigloos.


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INTRODUCTION High-dimensional nanostructured materials composed of nano-sized building blocks have been widely explored because their properties are no longer dominated by the bulk of the materials, but rather by the surface atoms.1,2 In addition, the large surface-to-volume ratio of a nanostructure opens up a new dimension in the material design for a multitude of applications including electronic, magnetic, photonic, chemical, and micromechanical devices.3,4 Recently, structural engineers have attempted to synthesize functional nanostructures matched with a specific application by controlling the physico-chemical characteristics such as the crystallic orientation, curvature, surface polarity, and chemical potential.5,6 To realize an effective functional nanostructure, a variety of approaches have been attempted based on the nanostructures through metal doping, a catalyst decoration, heterojunction, and morphological evolution.7,8 Among them, a morphological evolution accompanying a surface roughening and the preferred orientation is an effective method because it has extensive potential as a chemical catalyst by chemical potentials and different atomic energy states as compared with conventional nanostructures.9 Thompson et al.10 suggested morphological evolution induced through a minimization of the interface, surface or strain energy during the grain growth of pholycrystalline thin films. The degree of these three energy contributions differs under certain preparation conditions, consequently leading to a morphological evolution based on the van der Drift competitive growth model.11 Also, Consonni et al.12 thermodynamically reported the texture evolution of fluorine-doped SnO2 thin films along with the film thickness based on experimental results and theoretical modeling.


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A Semiconductor based chemiresistive gas sensor has been considered one of the most promising candidates for a functional nanostructure due to its surface-dependent sensing mechanism.13 Universally, the gas sensing principle has been elucidated as the adsorption and desorption of gas molecules on the surface of the sensing materials, which is indicated through a change in electric conductivity.14 Therefore, a functional nanostructure achieved through a morpholoigical evolution can catalytically enhance the gas sensing performance by improving the reaction with a single target gas (chemical sensitization) and the resistance modulation (electronic sensitization).15 For example, the morphological evolution of Rh-decorated WO316 and Fe2O3decorated NiO nanostructures17 have been reported as a promising substance for acetone and toluene detection, respectively. Despite these extensive efforts, the underlying mechanisms of the morphological evolution and their effects on the gas sensing properties remain unclear. Therefore, an understanding of the functional nanostructure induced through a morphological evolution is essential, not only for a sensor-based society but also for next-generation engineering materials. Herein, we investigated the heterojunction of both-sided W-decorated NiO nanoigloos fabricated through RF sputtering using 750 nm-diameter polystyrene (PS) beads as a soft-template. NiO is a representative p-type oxide that is advantageous because of its sensor stability and humidity tolerance from multiple stable oxidation states and a high concentration of positive holes.18 In additon, the WO3 clearly shows a catalytic effect on NO2, as indicated in our previous reports.19 The morphology of W-decorated NiO nanoigloos and their crystallographic orientation were clarified using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Interestingly, the morphological evolution characterized by a pyramidal rough surface and the preferred orientation of the (111) plane was observed when the NiO nanoigloos were decorated with metallic W. The underlying mechanism of the morphological


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evolution was explained using the van der Drift competitive growth model originating from the oxygen transport and chemical strain in a lattice as based on X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The gas sensing performance including the gas response, selectivity, and response stability was evalutated upon exposure to the target gases. The synergistic effect of W-decorated NiO nanoigloos on the gas sensing properties was demonstrated in a stepwise manner based on three main factors: the utility factor, transducer function, and receptor function.


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RESULTS AND DISCUSSION Morphological and chemical characteristics. To synthesize highly ordered and porous Wdecorated NiO nanoigloos, we employed a simple soft-template method using 750 nm PS nanobeads. First, PS nanobeads were drop-coated onto Pt-interdigitated electrodes (Pt-IDEs), and then sonicated for the formation of a close-packed monolayer, as illustrated in Figures 1a and S1a. Because the agglomeration of W metal followed by annealing into WO3 nanoparticles is an effective way to entirely decorate the surface of NiO films, we used metallic W instead of WO3. Decorated W thickness from 1 nm to 6 nm is defined as an ideally deposited thickness with a constant disposition rate of 15 nm/min. To maximize the electrical interaction between W and NiO, we deposited W before and after the deposition of a NiO film using RF sputtering. Thereafter, all specimens were annealed at 550°C for 2 h (Figure 1b). Owing to the evaporation of PS beads, W-decorated NiO nanoigloos show hollow hemispheres, as indicated in Figure S1b, which could be obtained through an inverted portion after scratching the sample with a tweezer. Figures 1c–g show SEM images of NiO nanoigloos as a function of the W decoration thickness. Interestingly, 1–3 nm W-decorated NiO nanoigloos have a rough surface with a pointed pyramid shape, whereas bare and 6 nm W-decorated NiO show relatively smooth surface. Cross-sectional SEM images of bare, 2 nm, and 6 nm W-decorated NiO are shown in Figures S1c–e. The crystallinity of each specimen was characterized using XRD. Figure 2a indicates that all diffraction peaks are indexed into NiO (JCPDS no. 47-1049). There are no significant impurity phases, whereas WO3 peaks are not observed. We assumed that WO3 peaks are beneath those of NiO because of the extremely small amount of WO3 present in the NiO nanoigloos. Figure 2b shows the preferred crystallographic orientation of NiO nanoigloos based on three main diffraction peaks: (111), (200), and (220). The orientation factor is calculated according to Song et al.20 As the thickness of W


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decoration increases up to 3 nm, the orientation factor of the (111) plane is increased, whereas that of the (200) plane is decreased. The orientation factors of the (111) and (200) planes are decreased and increased from 3 nm to 6 nm W thickness, respectively, which is in agreement with the behavior of the morphological evolution in the SEM images. In this respect, it is inferred that the preferred orientation of NiO is correlated with the morphological evolution of the W-decorated NiO nanoigloos. Cross-sectional TEM images of the bare, 2 nm, and 6 nm W-decorated NiO nanoigloos were acquired to investigate the structure and crystallinity, as shown in Figures 3a–c, respectively. A rough pyramidal surface was confirmed for 2 nm W-decorated NiO nanoigloos, whereas the bare and 6 nm W-decorated NiO exhibit a relatively smooth surface. The chemical maps indicates a uniform distribution of the elemental Ni and O across the NiO nanoigloos (Figures 3d and e). Although W is decorated on both sides of the NiO nanoigloos, it is uniformly distributed across the nanostructure (Figure 3f). High-resolution TEM images of 2 nm W-decorated NiO nanoigloos show the existence of well-crystallized NiO (cubic, Fm3m) nanoigloos and WO3 (monoclinic, P21 /c) nanoparticles (Figures 3g–i). The nanoparticle size of WO3 is confirmed about 4–6 nm for 2 nm W-decroated NiO (Figure S2a). In addition, Figure S2b exhibits coexistance of crystallized Ni(WO4)2 (triclinic, P1), indicating the strong interaction between Ni and W. To accurately investigate the chemical states of O, W, and Ni, XPS was performed using Wdecorated NiO nanoigloos. Figure 4a shows the binding energies for the oxygen-related peaks including NiO lattice oxygen (OL-Ni) at 529.4 eV, WO3 lattice oxygen (OL-W) at 530.15 eV, an oxygen vacancy (OV) at 531.28 eV, and chemisorbed oxygen (OC) at 532.53 eV.22,23 The atomic compositions of the oxygen-related peaks are shown for a different W thickness in Figure 4b. When the thickness of W increases in the NiO nanoigloos, the atomic composition of OL-W is


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significantly increased, whereas that of OL-Ni is decreased. Figure 4c exhibits the W 4f7/2 and 4f5/2 peaks of W3+ occur at approximately 35.7 and 37.9 eV, respectively. Compared with bare NiO, the intensities of these peaks increase as the thickness of W increases. In addition, it was observed that the decorated-W was entirely oxidized into WO3 during the annealing process. The chemical states of Ni are shown in Figure 4d. Ni 2p3/2 and 2p1/2 peaks of NiO occur at 853.9 and 872.4 eV, respectively, and their satellite peaks appear at 860.8 and 879.4 eV. As the thickness of W increases, the intensities of the NiO peaks do not significantly change, indicating that the deposited W was uniformly distributed on the NiO surface as WO3 nanoparticles rather than as a thin film. Additionally, Raman spectroscopy reveals one-phonon scattering (LO mode) of NiO at 570 cm–1 pronounced by defects or the surface effect (Figure 4e).21 The surface effect is improved by the large surface-to-volume ratios through the morphological evolution, whereas NiO nanoigloos with a 6 nm-thick W have a surface similar to bare NiO. Therefore, the phonon scattering is mainly ascribed to the defects on the surface of the NiO nanoigloos. Based on these results, it is inferred that the oxygen nearby interfaces moved to W from bonding with Ni. To verify the thermodynamic preference, enthalpies for the oxygen transport are expressed in Equations (1)–(3).24 1

ΔH = – 239.74 kJ/mol

(1)

W (s) + 2 O2 (g) → WO3 (s)

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ΔH = – 842.91 kJ/mol

(2)

3 NiO (s) + W (s) → Ni (s) + 2WO3 (s)

ΔH = – 126.93 kJ/mol

(3)

Ni (s) + 2 O2 (g) → NiO (s)

Because of the negative total enthalpy presented in Equation (3), oxygen transport from the bonding of Ni to W is energetically preferred. Therefore, the oxygen vacancy in NiO is developed,


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leading to a negative shift of the O1s peak in the XPS and one-phonon scattering in the Raman spectroscopy. Morphological evolution mechanism. The oxygen vacancy induced by the heterojunction generates a chemical strain in the lattice, indicating that the internal energy of the structure is relatively high compared with a strain-free structure.10,25 To reduce the chemical strain energy, the deposited film is preferably orientated toward a crystal plane with the lowest strain energy during the deposition and grain growth. For rock-salt NiO, the lowest strain energy appears in the (111) plane because of its relatively low atomic packing density.26,27 Thus, the NiO is energetically preferred to grow toward the [111] direction for minimization of the chemical strain energy. Polycrystalline growth was phenomenologically proposed by van der Drift competitive growth model.10 In this model, randomly orientated nuclei competitively grow on the substrate, and grains with a higher growth rate along the out-of-plane direction survive during the thickening of the film. In contrast, the other planes are buried below and their evolution is frozen. Therefore, the growth toward the (111) plane dominates for a minimization of the chemical strain energy, leading to surface roughening through the formation of a pyramid structure according to the van der Drift model. To understand the smooth surface of NiO with 6 nm-thick W, we focused on the oxygen transport in detail. During the crystallization and grain growth, inter-diffusion takes place from a region of higher chemical potential to that of a lower potential.28 The diffusivity determines how quickly equilibrium is reached from non-equilibrium. In solid-state materials, diffusion can occur through lattice (bulk or volume diffusion) and surface diffusion.29 The surface diffusion mechanism is generally much faster than the lattice diffusion mechanism, resulting in high diffusivity paths. Therefore, the oxygen transport is predominantly developed through diffusion


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from surface of NiO to that of W. From this perspective, the perimeters of the as-deposited W nanoparticles or islands become a limiting factor in determining the surface diffusivity. Figure S3 shows SEM images of WO3 with different W amounts of 0 to 6 nm on a SiO2/Si wafer. As the thickness of W increases, the nuclei grows into islands, and eventually form a thin film that covers the entire substrate, which is coincident with the general thin film growth mechanism.30 The total perimeters of the WO3 nanoparticles or islands are calculated based on the SEM images, as shown in Figure S4. It was clearly confirmed that the total perimeter steadily increases up to 2 nm, and thereafter decreases as it grows into islands. This reduction in the total perimeter at a W thickness of 6 nm inhibits the oxygen transport and chemical strain, reducing the orientation factor of the (111) plane in the XRD result. Accordingly, the morphological evolution does not occur through the restricted van der Drift competitive growth. Additionally, SEM images of W-decorated NiO nanoigloos for inside and outside is observed as shown in Figure S5, respectively. Inside-decorated sample has pyramidal rough surface with a pointed shape, whereas outside decorated one shows relatively smooth surface. Therefore, the van der Drift growth is developed for inside-decorated sample because of bottom-up growth mechanism. In contrast, outside-decorated W could not influence on the growth mechanism, leading to smooth surface. To confirm the oxygen diffusion in another way, we synthesized NiO decorated with WO3 instead of metallic W. Figure S6a shows an SEM image of WO3-decorated NiO nanoigloos. Due to the oxygen species in the decorated WO3, the oxygen transport is reduced between Ni and W. Hence, a morphological evolution did not occur, and simultaneously the (111) plane of NiO was not enhanced upon decorating the WO3 from the XRD results (Figure S6b). Gas sensing properties of W-decorated NiO nanoigloos. The sensing properties of semiconducting gas sensors are significantly affected by the catalyst contents because of electronic


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and chemical sensitization. To maximize the catalytic effect, the size and distribution of the heterophase nanoparticles require optimization by controlling the thickness of the metallic W. Hence, we deposited W of varying thicknesses (1, 2, 3, and 6 nm) to optimize size and distribution of the WO3 nanoparticles, and measured their gas sensing properties for 5 ppm NO2 at 300°C. Upon exposure to the target gas, the resistance of all samples abruptly decreased, which is well matched with a p-type semiconducting behavior (Figure 5a). Compared with bare NiO nanoigloos, the base resistance increases because of a depletion region induced by the heterojunctions with WO3 nanoparticles, and a maximum resistance appears at a W thickness of 2 nm (inset in Figure 5d). In addition, the surface coverage of WO3 increases with the increase in the thickness of W, whereas the WO3 decreases in its base resistance, which can be attributed to a current path (a detailed description of which is given in Figure S8). Figure 5d exhibits a volcano-shaped correlation between the response and W thickness. The maximum response to NO2 is observed when the NiO nanoigloos are decorated with 2 nm-thick W. We therefore determined that the 2 nm W-decorated NiO nanoigloos are optimized condition for the catalytic effect. In addition, the response time (90% of the response) and recovery time (90% of the original state) are significant factors to evalutate the sensing properties. Figure S9 exhibits the response time and recovery time of NiO nanoigloos as a function of W thickness to 5 ppm NO2. The 2 nm W-decorated NiO, that is optimized condition, exhibits the fastest response time and recovery time through the catalytic effect of WO3 toward NO2. The catalytic effect effectively reduces activation energy barrier in a reaction with the target molecules, leading to acceleration of response and recovery. Furthermore, the operating temperature is an important parameter to determine the sensing performance. For ptype semiconductor, sensor conductivity is continuously enhanced because of thermally induced electron-hole pair and adsorbed oxygen species. Figure S7 shows response transients of 2 nm W-


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decorated NiO to 5 ppm NO2 at 300ºC, 350ºC, and 400ºC. Although we measured sensing properties at 250ºC, sensor resistance could not observed due to extremely high resistance over 109 Ω. With increasing the operating temperature, the base resistance and response to NO2 are significantly decreases. Hence, subsequent experiments were performed at 300ºC. To verify our approach, we deposited the W-decorated NiO both inside and outside by changing the deposition sequence of the metallic W, as described in our previous report.19,31 Compared with bare NiO, the base resistances of all sensors are increased through a direct electronic interaction with the WO3 near the interfaces, which is referred to as electrical sensitization15 (Figure 5b). The outside decorated sensor exhibits a higher resistance than the inside decorated sensor. As shown in Figure S5, the inside-decorated sample has pyramidal rough surface with a pointed shape, whereas the outside-decorated one shows relatively smooth surface. In this respect, the insidedecorated sample significantly interacts with oxygen species because of its large surface-tovolume ratio, leading to decrease in base resistance for p-type semiconductor. In addition, hemispherical NiO has an internal area smaller than the external area (Figure 3a). When the same amount of W is decorated with NiO surface, the outside-decorated sample provides wider area for heterojunction than inside, resulting in a much strong electronic coupling. Furthermore, the bothside WO3-decorated NiO nanoigloos show response to NO2 between both-side and outside Wdecorated sensors (Figure S6c). Accordingly, the excellent sensing properties of the both-sided Wdecorated NiO nanoigloos is attributed to the synergistic effect of the electrical, chemical sensitization, and morphological evolution (Figure 5e). To examine a response linearity and detection limit of our sensor, the 2 nm W-decorated NiO was exposed to NO2 within the range of 0.4–1 ppm at 300°C (Figures 5c and f). The linearity between the response and concentration implies a reliable operation over the range of examined concentration. The theoretical detection


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limit (signal-to-noise ratio > 3) is calculated to be extremely low as 106.73 ppt. This sub-ppt level detection limit implies the potential for high-performance NO2 sensors. The sensitivity and selectivity of bare and W-decorated NiO nanoigloos were investigated toward various gases such as 5 ppm , NO2, CH3COCH3, C2H5OH, C7H8, NH3, H2S, and CH4 at 300°C (Figure S10 and S11). To check variations of the response and selectivity, we plotted the overall response as a bar graph (Figure 6a). Upon decorating W on the NiO, the response to all exposed gases changes toward a positive direction, namely direction in which the resistance decreases. As illustrated in Figure S8, coexistence of current paths toward NiO and WO3 is generated for 2 nm W-decorated NiO nanoigloos. When an oxidizing gas is introduced to Wdecorated NiO, p-type conduction path is enhanced by trapped electrons. In contrast, a reducing gas improves the NiO based p-type conduction path by released free electrons from pre-adsorbed oxygen. Therefore, the sensor resistance changes a direction to decrease whatever types of gas is exposed upon decorating W on the NiO nanoigloos. This unique characteristic of the heterojunctions enables the strongly sensitive and selective detection of NO2. Furthermore, we conducted principal component analysis (PCA) on the bare and W-decorated NiO nanoigloos by determining the gas response, response time, and recovery time upon exposure to three pulses of each gas examined in this study. PCA is an effective data patterning technique that extracts useful information from the data and analyzes the data structure, the relationship between objects and features, and the global correlation of the features. Figures 6 b and c clearly show that NO2 is completely separable using W decoration compared to the bare NiO, indicating that the fabricated W-decorated NiO nanoigloos have excellent selectivity. The response stability of the W-decorated NiO nanoigloos was evaluated by mixing 5 ppm of NO2 with other gases including CH3COCH3, C2H5OH, C7H8, NH3, H2S, and CH4. Compared to


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the bare NiO nanoigloos shown in Figure S12, the W-decorated NiO exhibits an excellent response similar to that of pure NO2 gas (Figure 7a). In addition, tolerance to humidity or the linearity between the response and relative humidity is a critical factor determining the sensor stability for practical applications. Thus, we exposed W-decorated NiO to 5 ppm NO2 with different relative humidity from RH 0% to 80% (Figure 7b). The inset in Figure 7c shows that the base resistance slightly increases with an increase in the relative humidity because of the desorbed pre-adsorbed oxygen. Because the adsorbed site of NO2 is identical with that of oxygen, the activated site allows NO2 to be adsorbed more effectively, resulting in a higher reponse under humid conditions (Figure 7c). The linearity between the response and relative humidity is verified by an R-squre of 0.98642. Hence, the response is sufficiently predictable under a certain relative humidity. Underlying sensing mechanism. The gas sensing properties of a metal oxide semiconductor are strongly dependent on three basic factors: the utility factor, the transducer function, and the receptor function.32 First, the utility factor implies how the target gases are effectively diffused through the surface of the oxide. Second, the transducer function refers to the ability to convert a signal caused by chemical adsorption on the oxide surface into an electrical signal. Third, the receptor function is the ability of the oxide surface to interact with the surrounding atmosphere containing oxygen and the target gases. Based on these three main factors, the underlying sensing mechanism of the W-decorated NiO nanoigloos was systematically elucidated. Figure 8 illustrates highly porous and ordered nanoigloos with a cross-sectional SEM image of W-decorated NiO nanoigloos. This pyramidal rough surface and hollow nanostructure are efficient in terms of accessibility to the target molecules on the entire surface with regard to the utility factor. The presence of WO3 enhances the direct electronic interaction with NiO accross the entire nanostructure by the formation of heterojunctions. In addition, each NiO nanoigloos connected


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through the pointed pyramidal sites lead to the formation of high double Schottky barriers, as confirmed through the approximately 105 fold increase in the base resistance, as shown in Figures 5a and b. Therefore, the resistance change is effectively enhanced by the transducer function upon exposure to the target molecules. Figure 9a illustrates the step-wise process of NO2 sensing on the surface of the W-decorated NiO nanoigloos. In ambient air, a depletion region through the heterojunction is generated on the surface of the W-decorated NiO (Figure 9a-1). When NO2 is introduced to the surface, the WO3 nanoparticles catalytically attract NO2 molecules, which is confirmed through our previous reports.19,31 Thus, negatively charged NO2 is generated on the WO3 surface, and leads to decreases in the depletion region induced by the heterojunction (Figure 9a-2). Subsequently, when extra NO2 molecules are adsorbed on the WO3, the pre-adsorbed NO2– migrate toward the NiO surface through a spill-over effect (Figure 9a-3)15. The adsorbed NO2 on the NiO surface traps electrons from the conduction band of the NiO, leading to increases in the electrical conductivity of the Wdecorated NiO nanoigloos. In addition, the rock-salt NiO surface of the (111) plane consists of layers containing only Ni2+ or O2– ions.33,34 These layers are stacked consecutively along the [111] direction, resulting in the formation of a surface polarity, which is energetically unstable because the electrostatic energy per atom increases linearly with the thickness of the sample. To reduce the energetic instability of NiO, Ebensperger et al.35 proposed a stabilizing reconstruction of such polar surfaces with a rock-salt structure. The reconstruction dominantly occurs though octopolar reconstruction including p(2 × 2) O- and Ni-octopolar structures with 3/4 and 1/4 atoms missing in the first and second surface layers, respectively. Although a p(2 × 2) O-octopolar has a stable structure with a surface energy of approximately 0.035 eV lower compared to that of a p(2 × 2) Ni-octopolar, this small energy difference can be sufficiently overcome by the lattice vibrations


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under our operating temperature of 300°C. Therefore, the surface reconstruction randomly generates O- and Ni-terminated surface for a neutral charge (Figure 9b). In terms of the gas sensing procedure, NO2 molecules are adsorbed only on the Ni-terminated surface as an active site. The adsorbed NO2 on the Ni-termination forms a local dipole, leading to a strong interaction and response to NO2. Hence, the catalytic effect of WO3 and the surface polarity of the NiO (111) plane improve the NO2 sensitivity and selectivity through the receptor function. Additionally, we examined the literature on metal-oxide based NO2 sensors (Table 1).36-46 In general, p-type sensors have poor NO2 sensing properties due to the lack of electrons to be attracted by the oxidizing gas. Although our W-decorated NiO have a p-type characteristic, the present study clearly exhibits a superior response to NO2 with advantages of p-type semiconductor including humidity tolerance and high stability. Accordingly, there is no doubt that our W-decorated NiO nanoigloos are promsing candidates for NO2 sensors. In addition, comparisons with recent approaches of twodimensional NO2 gas sensors47-51 are demonstrated in the Supporting information.


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CONCLUSIONS We successfully fabricated an effective heterojunction of W-decorated NiO nanoigloos accompanying a morphological evolution. The underlying mechanism of the morphological evolution was precisely demonstrated using a van der Drift competitive growth model originating from the oxygen transport from bonding with Ni to W. In addition, the high-performance sensing properties of W-decorated NiO nanoigloos were elucidated based on three main factors: (i) the utility factor, (ii) the transducer function, and (iii) the receptor function. The unique nanostructures of W-decorated NiO through a morphological evolution enable the effective maximization of the utility factor and the transducer function. In addition, the receptor function is intensified through the catalytic effect of WO3 and the surface polarity of the NiO (111) plane. We strongly confidence that the method proposed in this study will contribute not only to a sensor-based society but also to next-generation enginnering materials.


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EXPERIMENTAL SECTION Fabrication. Interdigitated electrodes (IDEs) were fabricated by Pt/Ti (150/30 nm) on a SiO2/Si substrate using photolithography. The distances between the Pt-IDEs were about 5 μm and 20 electrodes were applied within a 1 mm × 1 mm area. To fabricate a uniform close-packed monolayer of WO3-decorated NiO nanoigloos, an aqueous suspension of 750-nm-diameter polystyrene beads (2.6 wt%, Polysciences, Warrington, US) was used as a soft-template. Before the drop coating of the aqueous suspension, the Pt-IDEs were treated with O2 plasma using a microwave plasma Asher (Plasma Finish V15-G) at an RF power of 150 W and a working pressure of 400 mTorr (O2) for 5 min to create a hydrophilic surface. Next, the Pt-IDEs were dried at room temperature with 60 Hz vibration to evaporate the aqueous suspension and form a monolayer of polystyrene beads. To synthesize both-sided W-decorated NiO, we deposited W nanoparticles before and after the deposition of the 150 nm-thick NiO film using RF sputtering with a base pressure, working pressure, and Ar flow rate of 3 × 10−6 mTorr, 10 mTorr, and 30 sccm, respectively. The RF power and growth rate of the W nanoparticles are 50 W and 6 nm/min, and the nickel oxide thin film is 100 W and 20 nm/min, respectively. The fabricated specimens were annealed in air at 550°C for 2 h to burn out the PS beads and polycrystalline W-decorated NiO nanoigloos. Characterization. Field-emission SEM (Inspect F50) was carried out to examine the morphologies of the fabricated samples with an aceelerator voltage of 15 kV and a working distance of 10 nm. In addition, field-emission TEM (JEM-2100F) was performed to obtain brightfield and high-rosolution (HR) images through mechnical polishing method, and energy-dispersive X-ray spectroscopy (EDS) was used for further analysis. To investigaed crystallity of the Wdecorated NiO nanoigloos, glancing angle XRD (D8 advance) was utilized over a range of 20–80°


19


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as the X-ray source with CuKα radiation (1.5418 Å wavelength) at a fixed incident angle of 2°. XPS (PHI 5000 VersaProbe) was used to check the chemical state of the elements, and the binding energy for the C 1s peak (284.6 eV) of carbon was calibrated using a monochromatic AlKα X-ray source (1486 eV). Gas sensor measurements. The gas sensing properties of the sample were measured while externally heating the quartz tube in a box furnace. The gas flow rate which was controlled using mass flow controllers, was constant at 1000 sccm, while varied from dry air to a calibrated target gas. The baseline resistance was measured under a DC bias voltage of 1 V (Keithley 2401) and all measurements were recorded on a computer using LabVIEW over a GPIB interface.


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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional information for morphological evolution (Figures S1–S6), supplementary gas sensing properties and mechanism (Figures S7–S12) (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Chong-Yun Kang: 0000-0002-4516-8160 Author Contributions S. Y. Yi and Y. G. Song contributed equally. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by an Institute for Information & Communications Technology Promotion (IITP) grant funded by the Korea government (MSIP; Ministry of Science, ICT & Future Planning) (No. 2015-0-0031, Olfactory Bio Databased Emotion Enhancement Interactive Content Technology Development); National Research Foundation of Korea (NRF) grant funded


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by the Korea government (MSIP; Ministry of Science, ICT & Future Planning) (NRF2018R1C1B6002624, NRF-2018M3A7B4065625, NRF-2018H1A2A1060105-Global Ph.D. Fellowship Program).


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25


TABLE OF CONTENTS

Bare NiO

200

2 nm W-decorated NiO

10

– 0.2 0.7

– 0.73

– 1.75

– 0.46 2.01

– 0.4

– 0.64

– 1.22 0.37

– 1.2 0.07

0

0.46

5 200

W-decorated NiO nanoigloos

Response (Ra/Rg-1)

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

Page 26 of 36

-5

500 nm


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Figure 1. Schematics illustrations of (a) well-aligned PS beads and (b) both-sided W-decorated NiO nanoigloos between Pt-electrodes. SEM images of W-decorated NiO nanoigloos with different thickness of W: (c) bare, (d) 1 nm, (e) 2 nm, (f) 3 nm, and (g) 6 nm.


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Figure 2. (a) XRD data and (b) orientation factor of bare and W-decorated NiO nanoigloos with different W thickness.


28

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Figure 3. Cross-sectional bright-field scanning TEM (BF-STEM) images of (a) bare, (b) 2 nm, and (c) 6 nm Wdecorated NiO nanoigloos. EDS mapping of (d) Ni, (e) O, and (f) W for W-decorated NiO nanoigloos. High-resolution TEM images of 2 nm W-decorated NiO nanoigloos: (g) middle NiO, (h) top, NiO as indicated through dotted squares in (b), and (i) WO3.


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Figure 4. XPS of bare and W-decorated NiO nanoigloos with different W thickness: (a) O 1s, (c) W 4f and (d) Ni 2p peaks, (b) atomic composition of oxygen species, and (e) Raman spectra of bare and W-decorated NiO nanoigloos.


30

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Figure 5. (a) Response transients and (d) response of NiO nanoigloos with different W thickness up to 5 ppm NO2 at 300 °C. Inset in (d) shows the base resistance of NiO nanoigloos with different W thickness. (b) Response transients and (e) response of NiO nanoigloos with different W decorated sequence. (c) Response to different NO2 concentrations and (f) theoretical detection limits.


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Figure 6. (a) Bar graph of bare and W-decorated NiO nanoigloos to various target gases at 300 °C. Principal component analysis for (b) bare and (c) W-decorated NiO nanoigloos.


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Figure 7. Response curves of (a) W-decorated NiO nanoigloos to NO2 mixed with other gases including CH3COCH3, C2H5OH, C7H8, NH3, H2S, and CH4. (b) Response transient and (c) response of W-decorated NiO nanoigloos to 5 ppm NO2 as a function of relative humidity at 300 °C.


33


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Figure 8. Schematic illustration of highly porous W-decorated NiO nanoigloos between Pt-IDEs with double Schottky barriers and cross-sectional SEM image.


34

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Figure 9. Schematic illustrations of (a) stepwise NO2 sensing procedure in the near surface region and (b) the effect of NiO (111) plane on NO2 response.


35


Table 1. Sensing properties of metal-oxide based NO2 gas sensors.

P-type

Type

N-type

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

Page 36 of 36

Materials

Response

Concentration (ppm)

Temperature (°C)

Response time (s)

Recovery time (s)

Ref.

W-decorated NiO

200

5

300

2

60

This work

NiO nanosheets

0.5

10

250

-

600

[36]

NiO films

0.23

200

200

20

498

[37]

NiO thin films

2.5

20

150

-

-

[38]

Ce doped NiO nanoparticles

0.04

10

150

313

399

[39]

Cr doped TiO2

3.5

100

500

-

< 300

[40]

ZnO nanorods

30

100

175

< 20

-

[41]

In2O3 nanosheets

164

50

250

5

14

[42]

ZnO nanowire forests

140

0.5

200

156

120

[43]

Ni doped ZnO films

1.08

5

200

11

123

[44]

RuO2-WO3

186.1

5

250

-

-

[45]

Fe doped In2O3

71

1

150

300

150

[46]


36

Morphological Evolution Induced through Heterojunction of W

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