Heterojunction Based on Rh-Decorated WO3 Nanorods for

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Heterojunction based on Rh-decorated WO Nanorods for Morphological Change and Gas Sensor Application using Transition Effect Young Geun Song, Jae Yeol Park, Jun Min Suh, Young-Seok Shim, Seung Yeop Yi, Ho Won Jang, Sangtae Kim, Jong Min Yuk, Byeong-Kwon Ju, and Chong-Yun Kang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04181 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018

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Chemistry of Materials

Heterojunction based on Rh-decorated WO3 Nanorods for Morphological Change and Gas Sensor Application using Transition Effect Young Geun Song, †, ‡ Jae Yeol Park, § Jun Min Suh, ⊥ Young-Seok Shim, § Seung Yeop Yi, †, ∥ Ho Won Jang, ⊥ Sangtae Kim, † Jong Min Yuk, § Byeong-Kwon Ju‡,*, and Chong-Yun Kang†, ∥,* †Center

for Electronic Materials, Korea Institute of Science and Technology, Seoul 02792, 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 ⊥Research

Institute of Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea

∥KU-KIST

Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Republic of Korea

ABSTRACT: The use of heterojunctions based on Rh-decorated WO3 nanorods is an effective strategy for achieving highperformance gas sensors for volatile organic compounds (VOCs), especially acetone (CH3COCH3). Herein, we successfully fabricated Rh-decorated WO3 nanorods with one-dimensional (1D) structures by glancing angle deposition (GLAD). Interestingly, morphological changes characterized by anomalous surfaces with numerous regions of negative curvature were observed upon decoration of the bare WO3 nanorods with Rh, which were systematically investigated on the basis of impeded surface diffusion and trapping effects. The improvements of gas sensing properties were stepwisely demonstrated by synergistic effects involving transition of Rh, high chemical potential of the negative curvature, primary decomposition at the top side, and highly ordered nanostructures. We are confident that the results provide new insight into the synthesis of effective nanostructures, and contributes to a variety of applications including battery, solar water splitting, and sensor devices.

I. INTRODUCTION Healthcare is undergoing a paradigm shift from reactive medicine, which involves waiting for the patient to be sick, to personalized, predictive, preventive, and participatory (P4) medicine.1 This change has been promoted by a new approach to disease through the convergence of effective measurement, visualization techniques, and new computational tools. 2 Indoor air quality has been recently reported to be closely related to the quality of health because human activities are mainly conducted indoors, and many harmful gases are emitted from a variety of objects present inside buildings.3,4 In particular, volatile organic compounds (VOCs) with high vapor pressures, which exist as vapors at room temperature, are the most significant harmful substances that influence indoor air quality.5 Although VOCs, including acetone (CH3COCH3), ethanol (C2H5OH), toluene (C7H8), xylene (C8H10), and methane (CH4), are generally emitted in low concentrations, their emissions are continuous and prolonged, resulting in serious damage to human health, with symptoms that often include dizziness, paralysis, and dyspnea, eventually

leading to a comatose state.6 Therefore, the detection and monitoring of contaminants is an essential element in nextgeneration healthcare. In order to detect these harmful gases, metal-oxide based chemiresistive gas sensors have attracted significant attention due to their simple operation, cost-effectiveness, and flexibilities that allow them to be applied to existing circuits.7,8 Over the past few decades, a variety of approaches have been used in attempts to sensitively and selectively distinguish VOCs through nanostructure, metal doping, and formation of heterostructure.9 Among them, the heterostructure involving two different materials has been reported to effectively enhance gas sensing properties by promoting adsorption of target gases (chemical sensitization) and by resistance modulation upon gas adsorption (electronic sensitization).10 For instance, heterojunctions between rhodium (Rh) and metal-oxide have been studied as an effective substance for acetone detection through Rh-doped SnO2 nanofibers11 and Rh-decorated WO3 hollow-spheres12. Despite these extensive efforts, discrimination within VOCs remains challenging due to their high reactivities and similar compositions.

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Figure. 1 (a) Illustrating the preparation of Rh-decorated WO3 nanorods by the GLAD method. Top and cross-sectional SEM images of (b) as-deposited and (c) annealed WO3 nanorods, and (e) as-deposited and (f) annealed Rh-decorated WO3 nanorods. (d) XRD data for bare and Rh-decorated WO3 nanorods.

Recently, there have been some reports on heterojunctions accompanied by morphological changes with anomalous surfaces that contain numerous regions of negative curvature.13,14 Importantly, since the regions of negative curvature exhibit unique electronic and chemical properties, they have great potential in not only gas sensitivities and selectivities, but also a variety of fields including mesoscopic physics, chemistry, and materials science.15 However, to the best of our knowledge, there have been no reliable interpretations of these morphological changes. Hence, the elucidation of the mechanism associated with morphological change and its effects on gas sensing properties is required. Herein, we report a highly sensitive and selective acetone gas sensor based on heterojunctions of vertically ordered WO3 nanorods decorated with Rh fabricated by glancing angle deposition (GLAD). The Rh-decorated WO3 nanorods were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. In order to check the gas selectivities of our sensors, principal component analysis (PCA) was employed. Moreover, we not only clearly identify the mechanism involved in the morphological change by surface-diffusion inhibition and trapping effects, but also demonstrate the gas sensing mechanism through the transition of Rh on the surface of WO3. Our results suggest that the decoration of WO3 nanorods with Rh is an effective method for the fabrication of highly sensitive and selective gas sensors for the detection of acetone. II. RESULTS AND DISCUSSION Vertically ordered nanorods fabricated by GLAD method have been reported to effectively improve gas sensing

properties because of their large surface-to-volume ratio and nano-sized narrow necks.16,17 When initial nucleuses grow, self-shadowing effect is developed according to the incident angle of vapor flux (Figure S1). Therefore, highly porous and ordered nanorods successfully deposited on the Pt-IDEs. As described in our previous reports,17-21 we systematically investigated the gas sensing properties by controlling the incident angle of the vapor flux, orientation of the substrate, deposition rate, and vacuum pressure using the GLAD method with an e-beam evaporator. On the basis of these studies, vertically ordered Rh-decorated WO3 nanorods were fabricated by GLAD (off-axis mode) at glancing angle of 80° to optimize porosity and resistance of the structure, as shown in Figure 1a. Since agglomeration of the Rh metal followed by annealing into Rh or Rh2O3 nanoparticles is an effective method for entirely decorating the surfaces of WO3 nanorods,13 we used Rh instead of Rh2O3. Figure 1b and c show SEM images of the bare as-deposited and annealed WO3 nanorods, respectively, which reveal highly porous onedimensional structures. Compared to the as-deposited WO3, the annealed WO3 exhibited smooth surface because of interdiffusion and agglomeration during annealing. On the other hand, it is interesting that both the as-deposited and annealed Rh-decorated WO3 nanorods exhibit highly porous and extremely rough surfaces, as shown in Figure 1e and f. Accordingly, we can infer that the morphologies of the WO3 nanorods are predominately influenced by the presence, or absence, of Rh or Rh2O3 nanoparticles. The crystallinities of the bare and Rh-decorated WO3 nanorods were characterized by XRD. Figure 1d reveals that all diffraction peaks are indexed to WO3 (JCPDS no. 43-1035). The XRD results show no significant impurity phases exist, confirming that the WO3 nanorods were well crystallized during the annealing process, while peaks corresponding to Rh or Rh2O3 were not visible.


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Figure. 2 Cross-sectional TEM images of (a) bare and (b) Rh-decorated WO3 nanorods. EDS mapping of (c) W, (d) O, and (e) Rh for the Rh-decorated WO3 nanorods. High-resolution TEM images of (f–h) Rh-decorated WO3 nanorods in different regions: (f) WO3, (g) Rh, and (h) Rh2O3 nanoparticles.

We assume that peaks corresponding to Rh or Rh2O3 are underneath those corresponding to WO3 because of the very small amount of Rh present in the WO3 nanorods. To investigate the structures of the bare and Rh-decorated WO3 nanorods in detail, cross-sectional TEM images were acquired, as shown in Figure 2a and b, respectively. The chemical maps reveal uniform distributions of elemental W and O across the WO3 nanorods (Figure 2c and d). Although Rh is deposited on the tops of the WO3 nanorods, it is uniformly distributed over the entire nanorod structure because of its high porosity and the diffusion of Rh during annealing (Figure 2e). High-resolution TEM images confirm the existence of well crystallized WO3 (Monoclinic, P21/c) nanorods, and Rh and Rh2O3 nanoparticles (Figure 2f–h). Although Rh cannot form an oxide easily, even at a high annealing temperatures,[22] the very small sizes of the Rh nanoparticles facilitate the partial oxidation of the Rh, resulting in the co-existence of Rh (Cubic, Fm3m) and Rh2O3

(Trigonal, R3C) nanoparticles. Figure S3 exhibits dark-field TEM to verify the grain size of the bare and 0.5 nm Rhdecorated WO3 nanorods. Average grain sizes were calculated as 38.28 nm and 24.79 nm for bare and Rh-decorated WO3, respectively. Upon decorating the Rh nanoparticle on the WO3 surface, the hetero-phase nanoparticle act as grain-growth pinning sites during annealing, resulting in reduced grain size. In order to precisely investigate the chemical states of W and Rh, the bare and Rh-decorated WO3 nanorods were subjected to XPS. Figure 3a displays binding energies for Rh0 (308.5 eV for 3d5/2 and 313.2 eV for 3d3/2) and Rh+3 (310.4 eV for 3d5/2 and 315.1 eV for 3d3/2). As shown in Table S1, the Rh phase makes up approximately 80% of our sample, with the Rh2O3 phase present in about 20%, which leads to the formation of heterojunctions with the WO3, respectively. Figure 3b shows that the W 4f7/2 and 4f5/2 peaks of the bare WO3 nanorods exist at around 35.7 eV and 37.9 eV, respectively. Compared to bare WO3, the intensities of these



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Figure. 3 XPS spectra of bare and Rh2O3-decorated WO3 nanorods with different Rh contents: (a) Rh 3d and (b) W 4f. (c) Raman spectra of bare and Rh-decorated WO3 nanorods with different Rh contents.

peaks decrease with increasing Rh content, indicative of uniform Rh and Rh2O3 nanoparticle decoration on the WO3 surface, and strong interactions between the W and Rh phases. In addition, the Raman spectra of the bare and Rh-decorated WO3 nanorods with varying Rh contents reveal four wellresolved peaks at 272, 326, 717, and 807 cm-1, as shown in Figure 3c. The peaks centered at 717 and 807 cm-1 are attributed to the O-W-O stretching-vibration mode, and the two lower-frequency peaks, at 272 and 326 cm-1, are associated with O-W-O bending. Raman analysis reveals that the intensities of these peaks decrease with increasing Rh content, which is consistent with the XPS results. The gas sensing properties were evaluated in a quartz tube using a furnace for external heating, as illustrated in Figure S2. The gas sensing properties of semiconducting gas sensors based on metal-oxides are significantly affected by the operating temperature.23 In particular, gas sensitivity is closely related to the number of ionized oxygens (O2–, O–, and O2–) on

the surface, which leads to the formation of depleted regions when the ionized oxygens are adsorbed at the active sites.24 For instance, it is well known that superoxide (O2–) is physisorbed on surfaces at temperatures lower than 150°C. With increasing operating temperature, the oxygen species chemisorbed on the surface, in the forms of chemisorbed oxygen (O–), and lattice oxygen (O2–) appears in excess of 400°C. This pre-adsorbed oxygens react with the target molecules, resulting in the electron transport from the oxygen to sensing materials. On the basis of these reactions, and in order to optimize the operating temperature, the response of the bare WO3 nanorods to 5 ppm CH3COCH3 was examined over the 100–400°C range, as shown in Figure 4a. Generally, as the operating temperature increases, the base resistance decreases owing to enhanced electrical conductivity. However, chemisorbed oxygens (O–) develop on the surfaces of sensing materials at temperatures over 150°C, leading to an increase in base resistance (Figure 4d, inset). Interestingly, the material

Figure. 4 (a) Response transients and (d) response of the bare WO3 nanorods to 5 ppm CH3COCH3 measured at different temperatures in the 100–400°C. The inset in (d) shows the base resistance of the bare WO3 nanorods, as a function of the operating temperature, in ambient air. (b, c) Response transients and (e, f) responses of Rh-decorated WO3 nanorods with different Rh contents to 5 ppm CH3COCH3 and C2H5OH at 300°C, respectively. The inset in (e) shows the base resistance of the Rh decorated WO3 nanorods, as a function of Rh thickness.


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Figure. 5 Response transients of (a) bare and (b) 0.5 nm Rh-decorated WO3 nanorods to various gases (5 ppm) at 300°C. (c) Polar plots depicting responses to various gases (5 ppm) at 300°C.

exhibits abnormal sensing behavior, similar to that of a p-type semiconductor, at an operating temperature of 100°C (Figure S4). According to Li et al.,25 WO3 shows abnormal p-type sensing behavior due to the effect of humidity on the surface at low operating temperatures, which is consistent with the sensing behavior observed here. As the operating temperature increases, the WO3 nanorods exhibit n-type semiconducting behavior upon exposure to CH3COCH3, and the response increases with temperature up to 300°C. After then, the base resistance and response decrease as the operating temperature is increased above 300°C due to the thermally induced electron-hole pairs. The effect of base resistance on the gas response is precisely investigated in our previous report.18 Hence, all subsequent experiments were conducted at 300°C. In order to maximize catalytic effect including electronic and chemical sensitization, the size and distribution of the metal or metal-oxide nanoparticles required optimization, which is achieved by controlling the deposition thickness of the metal film, as described in our previous reports.20,26 Therefore, we deposited Rh of varying thickness (0.5, 1, 2, and 3 nm) in order to determine the optimal distribution and size of the Rh and Rh2O3 nanoparticles, and measured their gas responses to 5 ppm CH3COCH3 and C2H5OH at 300°C, the results of which are shown in Figure 4b and c. The base resistance of each sample increased due to the formation of a depletion region from heterojunctions with Rh compounds, compared with bare WO3 nanorods, and the maximum resistance was observed at Rh thickness of 0.5 nm (inset in Figure 4e). The sizes and distributions of the Rh and Rh2O3 nanoparticles increased with increasing Rh-layer thickness, while the WO3 surface area to react with oxygen molecules decreased, leading to a decrease in base resistance. Figure 4e and f show volcano-shaped correlations between gas response and Rh thickness. The maximum responses toward CH3COCH3 and C2H5OH were observed when the WO3 nanorods were decorated with 0.5 nm-thick Rh. Therefore, we determined that the WO3 nanorods decorated with 0.5 nmthick Rh films are optimal for high VOC-sensing performance. Response and recovery times are also critical factor to determine the sensor performance. These are defined as the time to reach 90% of the maximum response and original state, respectively. Figure S5 shows the response and recovery times of WO3 nanorods with different Rh contents. Although catalytically enhanced high response delays the response time at Rh 0.5 nm, it is extremely short as 11 s, while the minimum recovery time appears due to reduced activation energy barriers. The sensitivities and selectivities of WO3 nanorods decorated with different Rh contents toward a variety of VOCs,

including 5 ppm C2H5OH, CH3COCH3, C7H8, C8H10, and CH4, were investigated at 300°C. Figure 5a and b display the response transients of the bare and Rh-decorated WO3 nanorods, respectively. In order to determine variations in sensitivity and selectivity as functions of Rh content (Figure S6), we examined the overall responses by constructing polar plots (Figure 5c). For 0.5 nm Rh-decorated WO3 nanorods, we demonstrated the strong selective detection of a single target gas among the various gases examined. Further investigation is demonstrated through PCA in the Supporting Information with detailed description (Figure S8). High sensor stability is required to apply for a practical application. The stability includes many characteristics such as humidity dependency, response linearity, repeatability, and long-term operation. Generally, water molecules interfere with the gas reaction by eliminating the pre-adsorbed oxygen, which refers to water poisoning. To evaluate the gas sensing properties as a function of relative humidity from RH 0% to 80%, we exposed 0.5 nm Rh-decorated WO3 nanorods to 5 ppm CH3COCH3 at 300°C (Figure 6a). Inset in Figure 6b exhibits that base resistance is slightly decreased as the relative humidity increases due to desorbed pre-adsorbed oxygen. Figure 6b shows a linear-relationship between the response and relative humidity and high linearity is confirmed by R-square of 0.99091. When the relative humidity increases, the response is linearly decreased, implying that the response can be sufficiently predictable in a certain relative humidity. In addition, gas sensors are required to have response linearity and low detection limits. To evaluate these sensing properties, the bare and 0.5 nm Rh-decorated WO3 were exposed to CH3COCH3 at concentrations of 0.2–1 ppm at 300°C (Figure 6c and d). The linear relationship between response and CH3COCH3 concentration indicates reliable operation over the tested concentration range. Although 0.2 ppm CH3COCH3 was the lowest concentration examined experimentally in the present study, the theoretical detection limit (signal-to-noise ratio > 3) is calculated to be 1.45 ppb for bare and 131 ppt for 0.5 nm Rh-decorated WO3 nanorods. This sub-ppt level detection limit for CH3COCH3 demonstrates the potential for Rh-decorated WO3 nanorods in high-performance VOC sensors. An effect of the system noise on the detection limit at high resistance is described in the Supporting Information (Figure S9). Furthermore, the response to high concentration CH3COCH3 (10–1000 ppm) was measured and its description is presented in Figure S10. To evaluate the repeatability and long-term stability, we measured 0.5 nm Rh-decorated WO3 nanorods that have been left in ambient air for 8 months after the operation. Figure 6e and f show response transients and



Figure. 6 Sensor stability of 0.5 nm Rh-decorated WO3 nanorods including (a, b) humidity dependency, (c, d) response linearity, (e, f) repeatability, and long-term stability.

response 5 ppm CH3COCH3, respectively. The dotted line in Figure 6f indicates the initial response of this sensor. Although the Rh-decorated WO3 nanorods have been left for 8 months, it shows very similar response, fast response and recovery without degradation. The electrical properties of our Rh-decorated WO3 nanorods are greatly influenced by their highly porous nanostructures that are linked with narrow necks, as well as heterojunctions.24 Hence, nanostructure control is a crucial aspect that determines gas sensing performance. During the annealing process, crystallization, including nucleation and grain growth, takes place by inter-diffusion between as-deposited WO3 nanorods. Theoretically, this can be expressed as a WO3 flux (J) according to Fick’s first law: J = ―D∇μ (1) where, D is the diffusion coefficient and μ is the chemical potential. According to the Gibbs-Thomson equation, the chemical potential is proportional to the surface energy (𝛾) and curvature (𝜅): μ ∝ 𝛾𝜅 (2) Generally, a nanostructure has numerous regions of positive and negative curvature on its surface, and the negatively curved surfaces have much higher surface energies than those of positive curvature.15 In this respect, the surface energy gradient between regions of positive and negative curvature acts as a driving force for WO3 flow during the annealing process. As mentioned above, we observed morphological changes, from the rough surface of the as-deposited WO3 to the smooth surface of the WO3 nanorods, which is in good agreement with the theoretically expected behavior. On the

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other hand, the morphology did not change when Rh was deposited on the surfaces of the WO3 nanorods. Based on Fick’s law and the Gibbs-Thomson equation, we infer that the hetero-phase particles, namely the metal or metal-oxide impurity, affect the diffusion coefficient and the gradient of the surface energy between regions of positive and negative curvature, leading to retardation of WO3 flux during the annealing process. First, the hetero-phase nanoparticle acts as a stabilizer against coarsening of WO3 nanorods via surface diffusion. Most nanostructures undergo coarsening due to high surface energy and fast surface diffusion. The presence of metallic phase Rh significantly impedes the surface diffusion of WO3, due to the limited diffusivity of oxygen inside metallic Rh.27 Also, it has been reported that there exists a linear relationship between the grain size of the host material and the diameter of the hetero-phase nanoparticle.28 We thus effectively reduce the diffusion coefficient of WO3 via employing extremely small-sized Rh and Rh2O3 nanoparticles (diameters < 3 nm). Second, the hetero-phase particles affect the gradient of the surface energy between regions of positive and negative curvature through the trapping effect.29,30 Ouyang et al.15 theoretically and experimentally reported that the Gibbs free energy for nucleation of a metallic impurity at a region of negative curvature is negative, which indicates that recrystallization of a metallic impurity at a negatively curved site is energetically preferred compared to a positively curved or flat surface. In other words, regions of negative curvature in host materials act as sinks, and hetero-phases spontaneously flow into these regions during the annealing process. Therefore, regions of negative curvature on WO3 nanorods with relatively high surface energies are covered by Rh nanoparticles, leading to a decrease in surface energy gradient (∇μ ). As a result of the impeded surface diffusion and trapping effect, the flux on the WO3 surface is limited since both the diffusion coefficient and the chemical-potential gradient are reduced, resulting in Rh-decorated WO3 nanorods with rough surface morphologies, as illustrated in Figure 7a-1 and a-2. These nanostructures, with anomalous surfaces that contain numerous regions of negative curvature, have enormous potential as chemical catalysts through different atomic energy states and chemical potential compared to those with flat or positive curvatures, thereby providing active sites that are significantly more responsive. In terms of the gas sensing mechanism associated with the superior sensitivity and selectivity towards CH3COCH3, we focused on the near-surface region of the Rh-decorated WO3. Figure 7b schematically depicts the Rh-decorated WO3 nanorods with local suppression in the radial nanorod direction. The metallic Rh nanoparticle, about 80% of the Rh compounds, mainly enhance direct electronic interaction with the WO3 throughout the nanostructure. This electronic interaction develops a heterojunction near the interfaces, and the conduction channel created inside the WO3 nanorods is suppressed by the depletion region. Therefore, the resistance modulation is intensified by electronic sensitization of the metallic Rh nanoparticles when the target gas adsorb and desorb on the surface. According the operando DRIFT results reported Staerz et al.,12 decreases in tungsten-oxygen overtones (W-O bond) and increases in tungsten-oxygen double bonds (W=O bond) were exhibited for bare WO3 upon exposure to CH3COCH3, which imply the presence of reduced chemisorbed oxygen on the WO3 surface, and the developed oxygen vacancies are


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Figure. 7 Schematic illustrations of (a) p-n heterojunctions, (b) surface morphological changes, and (c) surface chemistry change in Rhdecorated WO3 nanorods.

compensated for by the formation of tungsten-oxygen double bonds. In contrast, the Rh-decorated WO3 exhibits increased tungsten-oxygen overtones and decreased tungsten oxygen double bonds. This represents an increase in WO3 surface oxidation, and a significant decrease in sensor resistance implies a strong catalytic effect of Rh2O3. Further underlying mechanism of the catalytic effect is demonstrated in the Supporting Information. On the basis of these results, we reveal the sensing mechanism of the Rh-decorated WO3 nanorods to CH3COCH3 in detail. In ambient air, depletion layers induced by the negatively charged oxygen ions (chemisorbed oxygen) and heterojunctions are developed on the surface of the Rh-decorated WO3 nanorods, as shown in Figure 7c-1. Upon exposure to CH3COCH3, the Rh2O3 reacts with CH3COCH3 to produce CO2 and H2O, and is itself reduced to Rh2O3-x. The reduction of Rh2O3 decreases the concentration of holes and its work function. Therefore, depletion layers induced by p-n heterojunctions are decreased (Figure 7c-2). The availability of extra electrons in the WO3, a result of the reduction of the heterojunction-related depletion layer, facilitates the adsorption of more oxygen at its surface (Figure 7c-3), after which the adsorbed oxygens on the WO3 surface migrate to the Rh2O3-x to recover its initial state. Despite the uniform distribution of Rh compounds over the entire nanorods, there are slightly more Rh compounds on the top side due to limitations of the deposition process. When CH3COCH3 introduces to the nanostructure, it firstly approaches the top side, and is selectively decomposed by catalytic effect of Rh2O3 as a more reactive molecule (CH3COOH). This process is facilitated by the slightly more Rh compound, the possible reaction path is as follows (Equation 3). CH3COCH3 +4O ― → CH3COOH + CO2 + H2O + 4e ―

(3)

Subsequently, the highly ordered nanostructures, that improve the diffusivity of the target gas, allow the reactive molecules to effectively spread throughout the WO3 surface. Thereafter, the response to a particular gas is maximized due to a high acidity of reactive molecules, providing an excellent response and selectivity (Equation 4). CH3COOH + 4O ― →2CO2 + 2H2O + 4e ― (4) Additionally, we surveyed the literature for WO3 or Rh based CH3COCH3 sensors, and tabulated their results in Table S3. The present study clearly shows superior response through synergistic effect of not only the transition of Rh, but also the high chemical potential of the negative curvature, primary decomposition at the top side, and highly ordered nanostructures. III. CONCLUSION We systematically investigated high-performance p-nheterojunction gas sensors based on Rh-decorated WO3 nanorods. Rh decoration dynamically and effectively contributes to the sensing properties through (i) morphological changes, (ii) electronic sensitization, and (iii) chemical sensitization. The morphological change at the heterojunctions wes successfully investigated through retarded surfacediffusion and the trapping effect. In addition, the excellent sensing performance toward CH3COCH3 was demonstrated by the transition of Rh. We strongly believe that this study provides new insight into the synthesis of effective nanostructures and contributes to various applications in the sensor field and beyond. IV. EXPERIMENTAL SECTION



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Corresponding Author Fabrication: Pt/Ti (150 nm/30 nm-thick) interdigitated electrodes (IDEs) were fabricated on 4 inch SiO2/Si wafer substrates (1 μm/550 μm-thick) using photolithography (followed by a lift-off procedure). The distances between the Pt/Ti IDEs were approximately 5 μm and there were 20 electrodes in a 1 mm × 1 mm area. Prior to deposition of sensing films, the Pt/Ti IDE-patterned SiO2/Si substrates were cleaned in acetone, ethanol, and DI water, followed by drying under a flow of nitrogen gas. An electron-beam evaporator was subsequently used to deposit 450 nm-thick WO3 nanorods at a glancing angle of 80° and a rotation speed of 15 rpm, based on the off-axis mode. The substrate was located 30 cm away from the crucible and shadow masks were used to ensure deposition only onto the area containing the electrodes. The base pressure and growth rate were 5 × 10-6 mTorr and 1 Å s-1, respectively. After depositing 450 nm WO3 nanorods, the substrate was repositioned to 0° (on-axis mode), and 0.5, 1, 2, or 3 nm Rh was deposited at a rotation speed of 6 rpm. The fabricated samples were annealed at 550°C for 2 h in air to crystallize the WO3 and agglomerate the Rh. The annealing temperature was selected considering trade-off relationship between the sensor stability and response. Characterization: The morphologies of the fabricated samples were examined by field-emission SEM (Inspect F50) with an accelerator voltage of 15 kV and a working distance of 10 nm, as well as field-emission TEM (JEM-2100F). The TEM samples were mechanically polished. Bright-field and high-resolution (HR) TEM images were acquired, and energydispersive X-ray spectroscopy (EDS) was employed for further analysis. The Rh-decorated WO3 nanorods were crystallographically investigated by glancing angle XRD (DMax 2500) over the 20–80° range, with CuKα radiation (1.5418 Å wavelength) used as the X-ray source at a fixed incident angle of 2°. The chemical binding states were examined by XPS (PHI 5000 VersaProbe) and the binding energies were calibrated against the C 1s peak (284.6 eV) of adventitious carbon using a monochromated AlKα X-ray source (1486 eV). Raman spectroscopy (inVia Raman Microscope) was employed to characterize the O-W-O bonding in the WO3 nanorods using a 532 nm Nd:Yag laser. Sensor properties: Gas sensing properties were examined in a quartz tube using a furnace for external heating. The flow gas was changed from dry air to the calibrated target gas (balanced with dry air). A constant flow rate of 1000 sccm was used for the dry air and target gas. Resistance was measured at a DC bias voltage of 1 V with a source measurement unit (Keithley 2401). The gas flow was controlled with a massflow controller and all data were recorded on a computer through the general purpose interface bus (GPIB) using LabVIEW software.

■ ASSOCIATED CONTENT Supporting Information. Additional information of atomic composition, dark-field TEM, and supplementary gas sensing properties. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

* E-mail: [email protected] * E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors.

Notes The authors declare no competing financial interest.

■ 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 by the Korea government (MSIP; Ministry of Science, ICT & Future Planning) (NRF-2018H1A2A1060105-Global Ph.D. Fellowship Program, NRF-2018R1C1B6002624, NRF2018M3A7B4065625).

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