Highly Selective Sensing of CO, C6H6, and C7H8 Gases by Catalytic

Mar 7, 2016 - Caihui Feng , Fei Teng , Yang Xu , Yajun Zhang , Tiehu Fan , Tingting Lin. International Journal of Applied Ceramic Technology 2018 15 (...
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Highly Selective Sensing of CO, C6H6, and C7H8 Gases by Catalytic Functionalization with Metal Nanoparticles Jae-Hun Kim,† Ping Wu,*,‡ Hyoun Woo Kim,*,§ and Sang Sub Kim*,† †

Department of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Entropic Interface Group, Singapore University of Technology & Design, Singapore 487372, Singapore § Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea ‡

S Supporting Information *

ABSTRACT: We have fabricated multiple networked SnO2 nanowires and subsequently decorated them with uniformly distributed metal nanoparticles (NPs). The sensing tests indicated that the Pt-, Pd-, and Au-decorated SnO2 nanowires exhibited excellent sensing behavior, specifically for C7H8, C6H6, and CO gases, respectively. We discussed the associated sensing mechanisms in regard to the selective catalytic effects of metal NPs. In addition, by means of d-band theory, we explained the catalytic capabilities of each metal and proposed design principles for exploring new catalytic metals. The present study will pave the way for further development of high-selectivity sensors. KEYWORDS: selective sensing, gas, metal functionalization, catalyst, d-band theory

1. INTRODUCTION An emerging application of chemiresistive sensors is the diagnosis of diseases by detecting trace amounts of disease markers contained in human exhaled breath. A variety of compounds are contained in human breath. The gases in exhaled breath are known to be closely related with specific diseases.1−3 For example, benzene (C6H6), toluene (C7H8), and carbon monoxide (CO) are known to be the disease markers for leukemia,4,5 lung cancer,6−8 and asthma,9,10 respectively. An increase in the concentration of a specific gas is indicative of a specific disease. Accordingly, accurate and selective detection of such gases in exhaled breath is essential for accurate diagnoses. In addition, C6H6, C7H8, and CO are typical indoor gases. CO is produced whenever a material burns and comes from fuel-burning appliances and devices. C6H6 and C7H8, which are typical volatile organic compounds, will come from building materials and home/personal care products. The concentration of those gases should be suppressed below certain values; otherwise, they will increase peoples’ risk of health problems. Accordingly, the high-performance sensors of C6H6, C7H8, and CO should be developed not only for indoor harmful gas sensing but also for prognosis of disease markers. The unique physical and chemical properties of onedimensional oxide nanowires (NWs), which are significantly different from their bulk or thin-film counterparts, have attracted attention for a range of applications. Metal oxide NWs have the potential for use as highly sensitive chemical gas sensors.11−14 Their high surface-to-volume ratios, single crystallinity, and nanoscale size, equivalent to the Debye © XXXX American Chemical Society

length, explain why oxide NW sensors are more sensitive than those made from conventional materials. Regardless of the great advances both in miniaturizing singleNW gas sensors, designing sensor devices, and evaluating their sensing capability, their practical usability remains challenging largely due to difficulties in the fabrication process and poor reproducibility.15 One approach to overcoming the shortcomings of single-NW gas sensors is the use of networked NWs, in which multiple-junction NWs are involved in the sensing process.16−19 Although networked NW sensors have advantages in regard to the fabrication process and reliability, they exhibit inferior sensing performance (decreased sensitivity and prolonged response and recovery times) compared with single-NW gas sensors. Therefore, improving the properties of networked NWs is essential to their implementation as chemical gas sensors. Attachment or functionalization of noble metal nanoparticles (NPs) has often been used to improve the sensing abilities of oxide NWs.20−23 The improvement is attributed to electronic and/or chemical sensitizations by the metal NPs.24,25 For that purpose, noble metals are usually attached in the form of NPs to the NW surfaces using a variety of methods, such as photochemistry,26 aerosol-assisted chemical vapor deposition,27 silanization processes,28 wet-chemical reduction,29 γ-ray radiolysis,30,31 and chemically driven self-assembly methods.32 Received: January 30, 2016 Accepted: March 7, 2016

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DOI: 10.1021/acsami.6b01116 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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NPs were examined by FE-SEM (Hitachi, S-4200) and TEM (Philips CM-200). The phases were determined by XRD (Philips X’pert MRD diffractometer). The sensing performances were investigated using a gas sensing system and shown in terms of various gases (C6H6, C7H8, CO, C2H5OH, and H2S). The fabricated sensors were placed in a horizontal-type tube furnace and the measurement temperature, determined after preliminary experimentation, was fixed at 300 °C. Gas concentrations were controlled by altering the ratios of dry airbalanced target gases to dry air using mass flow controllers. The sensor response (R) was estimated according to the following equation: R = Ra/Rg, where Ra and Rg are resistances in the absence and presence of a target gas, respectively. The response and recovery times were defined as the times required to attain a 90% change in the resistance upon the supply or removal of the target gas, respectively. To investigate the effect of humidity in sensing test, we compared the sensing behaviors of the networked SnO2 nanowires in dry and humid air (RH 30%). In addition, to study the long-term stability, we maintained the sensors in the humid environment (RH 60% @ 25 °C) for 12 months and compared the sensing behaviors of the preserved sample with the original one.

One of the important advantages brought by the metal NP functionalization is the enhancement of the selective detectability for specific gases due to the special catalytic sensitization with regards to the type of noble metals used. In the present work, we fabricated networked SnO2 NWs using a selective growth technique and subsequently functionalized their surfaces with various noble metal NPs such as Pd, Pt and Au using the γ-ray radiolysis technique. The γ-ray radiolysis has many advantages over conventional chemical or photochemical techniques in preparation of metal nanoparticles.33,34 Controlled reduction of metal ions can be performed without surplus reducing agents or undesired oxidation products, and thus, the process is uncontaminated. The highly reproducible process can be carried out at room temperature and ambient pressure. A uniform-sized metal nanoparticles can be produced in a relatively short exposure time. The process is uncomplicated and harmless. Their sensing properties were investigated for various gases: C6H6, C7H8, CO, ethanol (C2H5OH), and H2S. We find that each noble metal NP provides exceptional selectivity for a specific gas; Pd for C6H6, Pt for C7H8, and Au for CO. The origin of the selective detection is proposed. The findings in this work will pave the way for development and rational material design for exhaled breath sensors. Furthermore, this study will provide a definite solution for obtaining the sufficient sensing selectivity for future high-performance sensors.

3. RESULTS AND DISCUSSION 3.1. Materials Characterization of SnO2 NWs Functionalized with Metal NPs. The microstructure of NWs functionalized with metal NPs was examined. Figure 1a shows a representative top-view field-emission scanning electron microscopy (FE-SEM) image of a sensor of networked SnO2 NWs on a patterned IDE-prepared Si substrate. The stripe pattern clearly indicates that SnO2 NWs selectively grew on the Au catalytic layers. The inset in the figure shows a magnified image that more clearly shows the NW growth. Namely SnO2 NWs, grown on a Au layer, become entangled with those grown on adjacent Au layers. This results in a networked morphology. Figure 1b shows a cross-sectional image of networked SnO2 NWs that more obviously demonstrates the selective growth of SnO2 NWs on the Au layer and the formation of networks. On these nanowires, metal NPs were synthesized using the γ-ray radiolysis technique. The microstructures of SnO2 NWs functionalized with metal NPs were investigated by transmission electron microscopy (TEM) and are shown in Figure 1c−h. Figure 1c is a typical lowmagnification TEM image of a SnO2 NW with Pd NP functionalization. The Pd NPs, with an average diameter of ∼6 nm, are uniformly distributed on the surface of NWs. The highresolution lattice image (Figure 1d) shows the attachment of Pd NPs on the SnO2 NW and the single-crystalline nature of the Pd NPs. As shown in Figure 1e,f, Pt NPs with an average diameter ∼8 nm, were well formed and attached to the SnO2 NW. As shown in Figure 1g, Au NPs were synthesized via the γ-ray radiolysis technique. They exhibited a slightly elongated morphology aligned with the length of the NW. The lattice image in Figure 1h demonstrates that the Au NPs are nearly single crystalline without any noticeable structural defects like dislocation and stacking faults. The crystalline phases of the functionalized NWs were investigated using X-ray diffraction (XRD), and the results are summarized in Figure S2 in the Supporting Information. The XRD pattern of pure SnO2 NWs is included as a reference, showing that the SnO2 phase has a tetragonal rutile structure (JCPDS Card No. 88-0287). In conjunction with the microstructural results shown in Figure 1, the XRD patterns of the functionalized SnO2 NWs with Pd, Pt, and Au NPs reveal that the desired NPs were successfully synthesized and successfully attached onto the surface of multiple-networked SnO2 NWs using the γ-ray radiolysis method.

2. EXPERIMENTAL DETAILS 2.1. Growth of Networked SnO2 NWs. Networked SnO2 nanowires were prepared by the vapor−liquid−solid growth method on a deliberately patterned interdigital electrode (IDE) that had been made on SiO2-grown Si (100) substrates using a conventional photolithographic process. The IDE was comprised of a trilayer; Au (3 nm)/Pt (100 nm)/Ti (100 nm) in sequence. Each layer was deposited by sputtering the corresponding metal targets. The top Au layer was used to trigger the selective growth of SnO2 NWs on the Au pad. The Pt layer was used for electrical conductance. The Ti layer was used to enhance the adhesion between the Pt layer and the substrate. The size and geometry of the IDE used in this study are described in detail in our earlier reports.19,35 The IDE-prepared substrate was put into a horizontal tube furnace, in which an alumina crucible containing Sn powder (Aldrich, 99.9%) was placed. SnO2 NWs, selectively grown on an Au-layer pad, became entangled with the ones being grown on a neighboring pad, eventually producing networked junctions as is schematically shown in Figure S4. The experimental procedure for the growth of networked SnO2 nanowires used in this study is described in more detail in our earlier reports.16,36,37 2.2. Functionalization of Metal NPs. For the functionalization of metal NPs, the γ-ray radiolysis method was employed. The prepared precursor solutions were illuminated with 60 Co γ-rays at room temperature at the Korea Atomic Energy Research Institute. The used γ-ray radiolysis conditions were as follows: For Pd NPs, exposure time was 1 h, illumination intensity was 10 kGy h−1 with a PdCl2 concentration of 0.028 mM; for Pt NPs, the exposure time was 2 h, illumination intensity was 10 kGy h−1, and the H2PtCl6·nH2O (n = 5.8) concentration was 1.0 mM; for Au NPs, exposure time was 3 h, illumination intensity was 10 kGy h−1, and the HAuCl4·nH2O (n = 3.5) concentration was 0.248 mM. After the γ-ray illumination, all samples were taken out of the solution container and heat-treated at 500 °C for 1 h in an air atmosphere to remove any remaining solvent. The γ-ray radiolysis procedure and resulting morphologies are shown in Figure S4. The detailed conditions of the γ-ray radiolysis method used for synthesis of Pd, Pt and Au NPs are described in our earlier reports.38−40 2.3. Materials Characterization and Sensing Measurements. The microstructures of the networked SnO2 NWs with attached metal B

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the electron-depletion layer thereby decreasing the resistance of the NWs. This is the resistance change contributed by the radial modulation. The other contribution is related to the potential barrier modulation. At the NW junctions, the potential barrier is built up. This is similar to grain boundaries in bulk sensor materials. In air, the upward band bending occurs due to the adsorption of oxygen species. Reducing gases trigger desorption of preadsorbed oxygen species, lowering the height of the potential barrier. However, the contribution of the relatively small amount of NW-NW junctions to the total variation of the resistance change is not significant. One important feature that should be noted in Figure S2 is the obvious difference in the resistances for specific gases with respect to the kind of metal NPs used. For clarity, we compare the normalized resistance curves of the sensors for specific gases. These results are summarized in Figure 2. The resistance curve obtained from pristine SnO2 NWs without any metal functionalization is included as a reference. As is evident in Figure 2a, the functionalization of Pd NPs exceptionally improves the C6H6-sensing response of SnO2 NWs, demonstrating the selective C6H6 detection among the tested reducing gases. For the detection of CO, Au functionalization is most effective, as shown in Figure 2b. The Pt functionalization leads to a prominent enhancement in C7H8-sensing ability, as is shown in Figure 2c. In contrast, as shown in Figure 2d,e, although the functionalization of Pd, Pt, or Au NPs greatly enhances the pristine SnO2 NWs’ sensing properties for H2S and C2H5OH, selective detection is not evident. The selective sensing properties of functionalized SnO2 NWs are highlighted again with a bar-graph in Figure 3. The functionalization of pristine SnO2 NWs with Pd-, Pt-, and AuNPs provided the high responses of 25.5, 40.0, and 20.1 for 1 ppm of C6H6, C7H8, and CO gases, respectively, definitively demonstrating the selective detection of disease markers for leukemia,4,5 lung cancer,6−8 and asthma,9,10 respectively. In addition to the enhancement of the sensor response to the disease markers, the functionalization was effective in reducing the response and recovery times. Figure S4 summarizes the response and recovery times with regards to the tested gases, showing that these times were greatly shortened by the attachment of metal NPs. For clarity, we present Figure 4, plotting the sensor responses, response times, and recovery times of networked SnO2 NWs functionalized with noble metal NPs for various reducing gases. Along with the results in Figures 2 and 3, it is obvious that gas-sensing capabilities such as response, selectivity, and times for response and recovery of networked SnO2 NWs were enhanced enough to diagnose specific diseases by attaching suitable metal NPs. 3.3. Sensing Mechanisms: Investigations on the Catalytic Mechanisms of Noble Metals. Exhaled breath sensors have recently attracted a significant amount of attention as a means for obtaining information that can be used for the diagnosis of early stages of diseases. A number of the gases contained in human exhaled breath are known to be markers for specific diseases.1−3 Therefore, the selective detection of such gases is essential for the realization of exhaled breath sensors. In this study, it was found that subppm concentrations C6H6, C7H8, or CO, typical disease markers, can be selectively detected by multiple-networked SnO2 NWs sensors functionalized with Pd, Pt or Au NPs, respectively. The functionalization of these NWs with metal NPs provides two contributions to the sensing properties. One is the electronic sensitization (ES). The ES originates from the flow

Figure 1. Microstructures of networked SnO2 NWs functionalized with metal NPs. (a) Top-view and (b) side-view images of networked pristine SnO2 NWs. The inset in panel a shows a magnified microstructure. (c, e, g) Low-magnification and (d, f, g) highresolution TEM images of SnO2 NWs functionalized with (c, d) Pd NPs; (e, f)Pt NPs; (g, h) Au NPs.

3.2. Gas Sensing Performances of the Networked SnO2 NWs Functionalized with Different Metal NPs. We have investigated the gas-sensing performances of the networked SnO2 NWs functionalized with different metal NPs in terms of their capability for detecting various reducing gases. Typical resistance curves measured at 300 °C for 0.1−10 ppm of reducing gases such as C6H6, C7H8, C2H5OH, CO, and H2S are shown in Figure S3 in the Supporting Information. All sensors clearly tracked changes in gas concentration. The resistance decreased/increased during the supply/stoppage of the tested reducing gases, respectively. This overall sensing behavior can be explained from the perspective of n-type oxide semiconductors based on the joint effect; first the radial modulation in electron-depletion layer along the NW length direction,36,41 and second, the potential barrier modulation at the NW junctions.15,16 In air, oxygen species adsorb onto the surface of SnO2 NWs that are n-type semiconductors, capturing electrons in the conduction band of SnO2, resulting in an electron-depleted surface on the NWs. When a reducing gas is supplied, the reducing gas molecules are likely to react with the preadsorbed oxygen species and form a volatile chemical compound, subsequently releasing the captured electrons back into the conduction band of SnO2, and consequently thinning C

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Figure 2. Normalized resistance curves of networked SnO2 NWs functionalized with metal NPs for a specific gas: (a) C6H6, (b) CO, (c) C7H8, (d) H2S, and (e) C2H5OH.

contrast, the CS is responsible for the selective detection due to different degrees of catalytic interaction between a noble metal and a specific gas. Prior to investigations on the catalytic mechanisms of noble metals that may result with chemical and electrical changes, we first studied the thermodynamics of the system. Although the sensors were placed in a furnace to fix the gas measuring temperature at 300 °C, the initial temperature of the inlet gas was at room temperature which is expected to be heated up to 300 °C when reaching the sensor surfaces. Therefore, prior to investigations on catalytic mechanism of noble metals that may result in chemo-resistivity changes, we first study the thermodynamics of the system at the range of 35−400 °C that covers the whole possible gas temperature range. A commercial software package, FactSage,42 was used to calculate the boundary chemical equilibrium at 35 and 400 °C, respectively, for a system involving 1 mol Au, 1 mol Pd, and 1 mol Pt (the three most commonly used metal catalysts) and 0.01 mol CO, 0.01 benzene, 0.01 mol toluene (the three typical chemicals) and air (0.8 mol N2 and 0.2 mol O2), based on the

of electrons between the n-type oxide NWs and the metal NPs, resulting in the generation of the potential barrier. In particular, in the present study, the work function of n-SnO2 (4.6−5.7 eV) is supposed to be lower than those of Pd, Pt, and Au.37 Herein, electrons flow from the n-type oxide NWs to the metal NPs, resulting in the suppression of the conduction channel created inside the NWs, eventually intensifying the resistance modulation of the NWs when gas molecules adsorb and desorb onto the NWs. The other contribution to the sensing capabilities is the chemical sensitization (CS), which is the contribution from the catalytic effect of the metal NPs. This contribution either facilitates the adsorption of particular gas molecules onto the metal NPs and the efficient transport to the NWs. Furthermore, it induces more interactions with preadsorbed oxygen species, facilitating the relevant chemical reactions. This is the source of the intensified resistance modulation seen in NWs during the supply/stoppage of gas molecules. The ES has nothing to do with the selectivity since the ES contribution is not significantly affected by changing the type of gases once the noble metal has been selected. In D

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Therefore, under these conditions, CO is oxidized into CO2, but C6H6 and C7H8 are dehydrogenized and further oxidized to H2O. Only Pd metal is partially (∼6%) oxidized within the temperature range of 35−400 °C, whereas the other metals remain in a nonoxidized state. We shall focus mainly on the oxidation of the three gas species on pure metal Au, Pd, and Pt surfaces rather than their oxides. Among the three target gas species (CO, C6H6, and C7H8), the oxidation of CO on Au, Pd, and Pt catalytic surfaces is mostly studied via either experiments or density functional theory (DFT) calculations, due to its small molecular size and few reactionary pathways. The reported reaction barriers (from DFT calculations) for CO + O = CO2 are 0.21,43 0.87,43 and 1.20 eV44 for Au, Pd, and Pt, respectively. Therefore, the highest sensor response for CO sensing can be obtained using Au NPs. Besides the sensor response, another crucial design goal in these sensors will be to obtain sufficient gas selectivity. We need to understand why Au NPs preferentially sense CO, rather than C6H6 or C7H8. It is observed that all C6H6 and their dissociated biphenyl species desorb from Au surfaces at 127 °C.45 Because adsorption of gas species onto metal surfaces is the first necessary step of any possible catalytic reactions, no signals from C6H6 or C7H8 gas species will be detected from the Au surfaces. Thereafter, it is reasonable to predict that Au NPs offer both high sensitivity and selectivity in sensing CO in the presence of C6H6 and C7H8 at higher temperatures (>127 °C). Dehydrogenation of C6H6 has also been widely studied for hydrogen applications. Koel46 suggested that the initial C6H6 decomposition step on the Pt and Pd surfaces is the C−H bond breaking that forms the C6Hx species. By DFT calculations of C6H6 on Pt(111), Saeys47 found that dehydrogenation of C6H6 is neither thermodynamically nor kinetically feasible; C6H6 will desorb from Pt rather than dehydrogenate to a phenyl species, which is consistent with the experimental observations that all C6H6 desorbs from the Pt surfaces at 227 °C.48 Meanwhile, trends in C6H6 adsorption onto noble metals have been widely reported.49 Experimental adsorption energies data indicate that C6H6 weakly interacts with Au (0.64 eV in adsorption energy) but strongly interacts with Pd (1.35 eV) and Pt (1.49−1.83 eV) surfaces.49 In other words, the adsorption energy of C6H6 onto Au is significantly lower than its adsorption energy with either Pt or Pd. Since the adsorption energy of C6H6/Pd (1.35 eV) is in between those of the other two noble metal couplets (C6H6/Pt (1.49−1.83 eV) and C6H6/Au (0.64 eV)), Pd NPs may achieve higher catalytic activities based on volcano theory.50 Volcano theory suggests that the search for the perfect transition metal to catalyze a certain reaction is always a compromise.51 Furthermore, although the adsorption energy of C6H6/Pd is slightly lower than that of C6H6/Pt, the C6H6 can easily desorb from the Pt surface at sensing temperatures higher than 237 °C. Accordingly, we surmise that the higher sensor response for C6H6 can be obtained using Pd NPs, rather than Pt or Au-based systems. Oxidation of C7H8 on both Pd(111) and Pt(111) (supported by various oxides)50,52−54 has shown that a low C 7H 8 conversion rate is seen with SnO2 supported Pd(111) at approximately 300 °C. Unfortunately no conversion data for SnO2 supported Pt(111) is available. In general, C7H8 becomes more strongly bonded than C6H6 on Pt(111) at 300 K.55 Furthermore, C7H8 is more strongly bonded onto Pt(111) than on Pd(111).56 Studies of desorption kinetics for iodobenzene

Figure 3. Selective sensing properties of functionalized SnO2 NWs: (a) Pd NPs, (b) Pt NPs, and (c) Au NPs.

first-principles thermodynamics. At 30 °C, the following reaction will be estimated: 0.2 O2 + 0.8 N2 + 0.01 CO + 0.01 C6H6 + 0.01 C7H8 + Au + Pd + Pt = 1.00 Gas + Au + Pt + 0.94 Pd + 0.06 PdO, where the 1.00 mol gas phase (Gas) has a composition of 0.06 H2O, 0.14 CO2 and 0.80N2. If we repeat the above calculation at 400 °C, we obtained the following reactrions: 0.2 O2 + 0.8 N2 + 0.01 CO + 0.01 C6H6 + 0.01 C7H8 + Au + Pd + Pt = 1.01 Gas + Au + Pt + 0.94 Pd + 0.06 PdO, where the 1.01 mol gas phase (Gas) has a composition of 0.07 H2O, 0.14 CO2 and 0.79N2. Further calculations show that no obvious changes in the gas phase chemistry are expected in the tested temperature range (35− 400 °C), whereas PdO reduction (to Pd) begins at approximately 400 °C. E

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Figure 4. Sensor responses, response times, and recovery times of networked SnO2 NWs functionalized with noble metal NPs for various reducing gases: (a) C6H6, (b) C7H8, and (c) CO.

(C6H5I) on Pt(111)48,57 and Pd(111)58 surfaces reveal that biphenyl (C12H10) formation (which may react with hydrogen to reform C6H6 or C7H8) occurs in the iodobenzene/Pd(111) system but not with the iodobenzene/Pt(111) system.48,57 Because the regeneration of C7H8 will not occur efficiently on Pt, the catalytic activity may be higher for the C7H8/Pt couplet compared with the activity seen at C7H8/Pd interfaces. Also, we already indicated that catalytic activity will be relatively low with the C7H8/Au system because the C7H8 will be desorbed at higher temperatures (>127 °C). Therefore, we expect to obtain a higher sensitivity for C7H8 on Pt than on either the Pd or Au surfaces. C7H8, among the gases studied, is unique in that it has an additional methyl group (−CH3). This group plays a crucial role in enhancing the sensitivity.59 It is important to consider electronic and steric effects in order to figure out the role that the methyl group plays during adsorption onto Pt surfaces.60 If one were to only consider steric effects, the steric repulsion between the methyl group and the Pt surface would prevent C7H8 from adsorbing onto the Pt surface.60 Adsorption of C7H8 onto the Pt surface, however, is significantly affected by electronic effects. The electronic effects lower the barrier for adsorption of C7H8 by making the donation of electrons from the πCH3 level to the Fermi level easier and by the making backdonation from the Fermi level to the πCH3* level more facile.60 Because the presence of the methyl substituent in the ring of

aromatic compounds increases the amount of available surface for strongly bonded retained species,59 the electronic effects surpass the steric effects, enhancing the adsorption of C7H8 gas, and thus the sensing behavior. The above analysis agrees with experimental results showing that (1) Au/SnO2 has the highest sensitivity and selectivity for CO gas; (2) Pd/SnO2 has the highest sensitivity and selectivity for C6H6 among all the three surfaces of Au/SnO2, Pd/SnO2 and Pt/SnO2; and (3) Pt/SnO2 has the highest sensitivity for C7H8 among the three surfaces. We surmise that the reason why Pt/SnO2 achieved the highest selectivity among the three surfaces for C7H8 sensing is associated with the relatively strong bonding of C7H8 onto Pt and the difficulties in the regeneration of C7H8 on Pt. The easier adsorption of C7H8 on Pt is related to electronic effects with respect to the methyl group in C7H8 gas. Close examination of Figure 4 reveals that Au, Pd, and Pt exhibit not only the high sensor response but also the short response time for CO, C6H6, and C7H8 gases, respectively. However, this is not the case for recovery time. Although Pd exhibits the shortest recovery time for C6H6 gas, Au and Pt do not have the shortest ones for CO and C7H8 gases, respectively. We surmise that CO will be efficiently adsorbed on Au catalyst for subsequent oxidation reaction, less tending to be desorbed. The difficulty of desorption will result in the longer recovery time. Also, while the strong bonding of C7H8 onto Pt provides F

DOI: 10.1021/acsami.6b01116 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces the high sensor response and short response time, it will result in the longer recovery time. By the way, the adsorption energy of C6H6/Pd is in between those of the other two noble metal couplets (C6H6/Pt and C6H6/Au). We surmise that C6H6 not only is adsorbed onto Pd efficiently, but also can be adequately desorbed to promise the short recovery time. The sensing properties of the SnO2 nanowires functionalized with a specific kind of metal nanoparticles are surely dependent on their amount. Usually, an optimized amount of metal nanoparticles provides us a maximized sensing performance, showing a volcano-shaped curve with respect to the amount of functionalization; namely in between the cases of small and large amounts. In fact, the amount of functionalization in this work was different in each metal. Functionalization of metal nanoparticles with the same amount is hard to be realized. However, the purpose of this work does not require such a synchronized functionalization. The purpose of this work is to identify which noble metal element is suitable for detection of a particular target gas. In Figure 3, for a sensor sample with a fixed amount of metal nanoparticles functionalized, it is possible to compare the response to various gases, and to determine which metal is better for a certain gas. We do not compare sensing properties of nanowires with different kind of metal nanoparticles functionalized for a specific gas. Instead, we compare sensing properties of nanowires with one kind of metal nanoparticles functionalized for various gases. 3.4. Study of Selective Sensing Based on d-Band Theory. Figure 5 shows the variations in sensor responses with changing d-band center for C6H6, C7H8, and CO gases. The dband model is suitable for explaining the trends in bond formation and reactivity among various transition metals that exhibit interactions between the adsorbate valence states and the s- and d-states of a transition metal surface.61−63 The binding energy has two components, coupling to the metal sstates and extra coupling to the d-states.61 Because the contribution from the coupling to the metal s-states is approximately the same for the various metals, the interaction with the broad s-band of metal will not significantly affect the bonding energy. Because the d-bands are narrow, the interaction of the adsorbate with the d electrons of the metal surface generates both bonding and antibonding states. In other words, hybridization of the metal d-band with the bonding orbital of the adsorbate forms both bonding and antibonding states. The characteristics of d-bands can be well-defined by the d-band center, εd.61 A higher d-band center (with respect to the Fermi level) corresponds to an increase in energy (relative to the Fermi level) and to less filling of the antibonding (d-σ)* state. Herein, the metal−adsorbate system is more stable, translating to stronger binding between the metal and the adsorbate. The increased filling of the unstable antibonding state, however, will result in the destabilization of the metal−adsorbate interaction and thus weaker bonding energy. For Au, Pt, and Pd, the d-band centers are below their respective Fermi levels. Accordingly, the εd values (d-band centers measured with respect to their Fermi levels) are negative. Because the εd of Au, Pt, and Pd are −4.32, −2.14, and −1.68 eV, respectively, we expect that the bonding energy, or chemisorption strength, will increase in the following order: Au, Pt, and Pd. Additionally, the other calculation, using the coupling matrix element between the molecular orbitals and the metal d-states, d-band centers, and electronic adsorbate levels,

Figure 5. Variations of the gas sensor responses by varying the d-band center (i.e., Au, Pt, and Pd). (a) C6H6, (b) C7H8, and (c) CO.

predicts that the bonding energy will increase in the same order: Au, Pt, and Pd (Text S1, Supporting Information). Herein, we predict the sensing abilities using the volcano-plot approach,64 which is based on the chemisorption/adsorption strength of each metal. The volcano-plot approach usually represents the role of interfacial energy with surface catalytic reactions. First, strong bonding between the surface and gas molecules will decrease the catalytic efficiency since the binding will be too strong to break (to form the transition state). Second, weak bonding between the surface and gas molecules will also decrease the catalytic efficiency since the gas molecules G

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Figure 6. Stability of the networked SnO2 nanowires in humid environment (RH 60% @ 25 °C) for 12 months. Comparison of the sensing responses, which were preserved in the humid environment for 6 months, with that of the original sensor.

(εd = −4.32 ∼ −3.56). The use of Ag can be explored, since the εd of Ag is approximately −4.3. Although further investigation is necessary, for CO sensing, compress-strained Au NPs may lead to higher catalytic activities due to the down-shift of the Au d-band center which makes the CO/Au interface less stable.51 3.5. Long-Term Stability and Humidity Effects. First, we investigated the long-term stability of the networked SnO2 nanowires in humid environment. In Figure 6, we compared the sensing response curve of the sensor, which was preserved in a laboratory (RH 60% @ 25 °C) for 12 months, with that of the original sensor. Obviously, the sensor response was changed after 12 months. For pristine SnO2 nanowires, with the relatively low sensor responses, there is no significant degradation by keeping the samples in a humid laboratory environment for 12 months. For Pd NPs-functionalized SnO2 NWs, the sensor response to C6H6 gas was decreased by 15.7%, by preserving in a laboratory (RH 60% @ 25 °C) for 12 months. For Pt NPs-functionalized SnO2 NWs, the sensor response to C7H8 gas was decreased by 13.0%. Also, for Au NPs-functionalized SnO2 NWs, the sensor response to CO gas was increased by 2.0%. Accordingly, it is likely that keeping in a laboratory for 12 months does not significantly degrade the sensor response of the NPs-functionalized SnO2 NWs, particularly to their selectively detectable gases. It is possible that the water adsorption in the laboratory environment will not significantly affect the sensing capabilities. Second, we investigated the effect of humidity in sensing test on the sensing behaviors of the networked SnO2 nanowires. For these sensing tests, we employed the samples, which were preserved in a laboratory for 12 months (Figure 6). Figure 7

may not stick to the surface for the subsequently required reaction. With moderate bonding strength, the molecules stick to the surface but are flexible enough to move around and form transition states. The above arguments have been justified by DFT calculations for multiple cases. Figure 5a shows C6H6 sensitivity vs the d-band center curve, indicating that the C6H6 sensitivity increases with increasing dband center. There are, however, several methods to increase the sensitivity further. Using the volcano-plot approach, we can find a new noble metal whose d-band center is close to Pd but outside the Pd−Pt range. Also, it is possible to form tensile strained Pd layers. This will move the d-band center toward the Fermi level thus increasing the adsorption energy.65 Figure 5b shows C7H8 sensitivity vs the d-band center curve, indicating that the sensitivity of the Pt-functionalized sample is exceptionally high, whereas the Pd-functionalized sensor has an exceptionally low sensitivity from the viewpoint of d-band theory. From the shape of the curve, it is more probable that the peak is located between Pt and Au (i.e., according to volcano theory there is a peak following the volcano plot). We can find a new noble metal whose d-band center is close to Pt but between Pt and Au. Between Pt and Au (−4.32 < εd < −2.14), there is Cu, whose εd is approximately −2.67.61 In addition, the formation of compressed Pt will move the d-band center of Pt down from the Fermi level, decreasing the adsorption energy of C7H8 gas and thus enhancing its sensing behavior. Figure 5c shows CO sensitivity vs the d-band center curve, indicating that the sensitivity of the Au-functionalized sample is relatively high, from the viewpoint of d-band theory. It is possible that the peak may be located on either side of Au H

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Figure 7. Effect of humidity in sensing test on the sensing behaviors of the networked SnO2 nanowires. The H2 concentrations were set to 0.1, 1, 5, and 10 ppm. Comparison of sensor responses of the networked SnO2 nanowires in humid air (RH 30%) with those in dry air (RH 0%).

reducing gas cannot sufficiently donate electrons to nanofibers, degrading the sensing capabilities.

compares the sensor responses of the networked SnO2 nanowires in humid air (RH 30%) with those in dry air (RH 0%). For all samples including pristine SnO2 NWs, functionalized SnO2 NWs with Pd, Pt, and Au NPs, the sensor responses in humid air are noticeably decreased, with respect to those in dry air. For example, in the case of functionalized SnO2 NWs with Pt NPs, the sensor responses to C6H6, CO, H2S, C7H8, and C2H5OH in humid air are decreased by 56.3, 37.0, 64.2, 64.41, and 20.0%, respectively, in comparison to those in dry air, respectively. In case of functionalized SnO2 NWs with Pd NPs, the sensor responses to C6H6, CO, H2S, C7H8, and C2H5OH in humid air are decreased by 60.0, 51.7, 7.7, 0.0, and 20.0%, respectively, in comparison to those in dry air, respectively. For Pd NPs-functionalized SnO2 NWs, the sensor response to C6H6 gas was decreased by 60.0%, by carrying out the sensing test in humid air. For Pt NPs-functionalized SnO2 NWs, the sensor response to C7H8 gas was decreased by 64.41%. Also, for Au NPs-functionalized SnO2 NWs, the sensor response to CO gas was decreased by 65.9%. Accordingly, it is likely that carrying out the sensing test in humid air significantly degrade the sensor response of the NPs-functionalized SnO2 NWs. During the present sensing tests, the reaction between the surface oxygen and the water molecules reduces baseline resistance of the gas sensor, decreasing the sensitivity.66 Because the degradation of the sensor response is significant, regardless of the species of metal nanoparticles (Figure 7), we surmise that the adsorption of water molecules overall affect the surface of the SnO2 NWs, rather than particularly on the metal catalyst. Therefore, the adsorbed water molecules will occupy the surfaces, leading to less chemisorption of oxygen species and/or gas molecules on the SnO2 surface. The incoming

4. CONCLUSION In summary, we functionalized the surface of networked SnO2 NWs with a variety of noble metal NPs, including Pd, Pt, and Au. XRD and TEM analyses indicate that the desired metal NPs, which are single crystalline, were uniformly distributed on the NW surfaces. Investigations of the sensing properties revealed that Au, Pd, and Pt NPs exhibit exceptional selectivity for CO, C6H6, and C7H8 gases, respectively. At a low concentration of 1 ppm, Pd-, Pt-, and Au-NP functionalization provided pristine SnO2 NWs with high responses of 25.5, 40.0, and 20.1 for C6H6, C7H8, and CO, respectively. We explained the selective sensing behaviors in regard to the selective catalytic effects of metal NPs. For CO gas, the catalytic oxidation on Au is carried out efficiently, and Co is stable at sensing temperatures. For C6H6 gas, the C6H6 strongly interacts with Pd, with C6H6 being stable at sensing temperatures. For C7H8 gases, C7H8 tends to bond strongly onto Pt. Using the volcano-plot theory and catalytic research, we can predict other metals that may be able to achieve even better sensing performance. While the present sensor is stable for 12 months in a humid environment, its sensing capabilities were significantly degraded by the humid air during the sensor operation. The present work is expected to provide the realistic and immediate solution to enhance the sensing selectivity, which is the most crucial problem in the current chemical sensor. I

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01116. XRD patterns of networked SnO2 NWs functionalized with metal NPs; dynamic resistance curves; response and recovery times; schematics illustrating the procedure of functionalizing networked NWs with metal nanoparticles; text with respect to the other calculation, using the coupling matrix element between the molecular orbitals and the metal d-states, d-band centers, and electronic adsorbate levels. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

S.S.K. and H.W.K. conceived the study, designed the experiments and prepared the manuscript. P.W. carried out the calculation and prepared the manuscript. J.-H.K. performed the experiments. All authors approved the final version of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Inha University. P.W.’s research is partially sponsored by the SUTD-ZJU grant (SUTD-ZJU/ RES/01/2012) on chemical thermodynamics and by Singapore MOE Tier 2 grant (MOE 2012-T2-1-097) on gas−solid reactions.

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REFERENCES

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