Selective H2S-sensing performance of Si nanowires through the formation of ZnO shells with Au functionalization

Sensors & Actuators: B. Chemical 289 (2019) 1–14

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Selective H2S-sensing performance of Si nanowires through the formation of ZnO shells with Au functionalization

T

Myung Sik Choia, Ali Mirzaeib,c, Jae Hoon Banga, Wansik Ouma, Yong Jung Kwond, ⁎ ⁎⁎ Jae-Hun Kime, Sun-Woo Choif, Sang Sub Kime, , Hyoun Woo Kima,b, a

Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea The Research Institute of Industrial Science, Hanyang University, Seoul 133-791, Republic of Korea c Department of Materials Science and Engineering, Shiraz University of Technology, Shiraz, Iran d Non-Ferrous Materials Group, Korea Institute of Industrial Technology (KITECH), 137-41 Gwahakdanji-ro, Gangneung-si 25440, Republic of Korea e Department of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea f Department of Materials and Metallurgical Engineering, Kangwon National University, Samcheok, Gangwon-do 25913, Republic of Korea b

A R T I C LE I N FO

A B S T R A C T

Keywords: ZnO Si nanowires Au Shell Gas sensor Sensing mechanism

A novel gas sensor fabricated from ZnO-shelled Si nanowires (SiNWs) is presented. After coating a thin layer of Au on the surfaces of Si NWs, ZnO layers were formed on the surfaces of p-SiNWs by thermal evaporation of Zn powders and a subsequent oxidation process. Microscopic analysis confirmed the successful formation of ZnO-Si core-shell NWs with Au nanoparticles present on the shell surface. The gas sensing performance of the gas sensor fabricated using the p-Si/n-ZnO core-shell NWs was evaluated for various gases. The sensor exhibited outstanding response and selectivity to H2S gas. The gas sensing mechanism was evaluated in detail and attributed to various factors, including the formation of ZnO/Si and Au/ZnO heterojunctions and the chemical attraction between ZnO and Au. The results demonstrate a new sensing material for H2S detection in various fields that can be easily incorporated into Si-based devices.

1. Introduction Silicon nanowires (SiNWs) are attractive materials in electronic applications due to their noticeable structural and optical properties. As their surface area is significantly larger than other advanced materials, they have different applications in the fields of high-performance transistors [1], photovoltaics [2], chemical and biological sensors [3], thermoelectrics [4], energy storage and conversion [5], and electrochemical resonators [6]. Another advantage of SiNWs is its flexibility in synthesis methods including chemical vapor deposition [7], laser ablation [8], thermal evaporation [9], and metal-assisted chemical etching (MACE) [10,11]. Since MACE is facile, effective, compatible with microelectric technology, and enables obtaining aligned NWs with a high density, it is particularly important for the low-cost fabrication of SiNWs [12,13]. NW-based sensors have high sensitivity, high stability, and a rapid response time [14]. Therefore, NWs are among the best candidates for realization of gas sensors. Owing to the aforementioned advantages, the gas sensing features of SiNWs for the detection of different toxic gases

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has been investigated [12,15]. However, the main disadvantage of SiNWs in gas sensing applications is their poor sensitivity, which limits their practical application as a gas sensor [16]. For enhancing the gas sensing characteristics of SiNW-based gas sensors, the creation of heterojunctions with metal oxides in the form of core-shells or decoration has recently been reported [3,15]. However, reports of these structures are rare and they need to be explored for better understanding of gas sensing properties and mechanisms. Therefore, we designed p-SiNWs/ n-ZnO core-shell nanocomposites. One of the most attractive semiconducting oxides for gas sensing is ZnO [17] due to its wide band gap (3.37 eV), non-toxicity, easy processing, and high mobility of electrical carriers [18,19]. Therefore, it is expected that the combination of SiNWs with ZnO, which has a high intrinsic sensing performance, can result in the realization of a highly sensitive and selective gas sensor. One of the most dangerous gases for human beings is H2S. At 50 ppm, it is highly irritating to eyesight, skin, and the respiratory system. When its concentration is > 100 ppm, it can paralyze the olfactory nerves in the human body. Inhalation of high concentrations (0.1%) of H2S can lead to suffocation [20]. Furthermore, the presence

Corresponding author at: 100 Inha-ro, Nam-gu, Incheon 402-751, Korea. Tel.: +82 32 960 7546; fax: +82 32 862 5546. Corresponding author at: 222 Wangsimni-ro, Seongdong-gu, Seoul 133-791, Korea. Tel.: +82 2 2220 0382; fax: +82 2 2220 0389. E-mail addresses: [email protected] (S.S. Kim), [email protected] (H.W. Kim).

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https://doi.org/10.1016/j.snb.2019.03.047 Received 28 July 2018; Received in revised form 8 January 2019; Accepted 9 March 2019 Available online 11 March 2019 0925-4005/ © 2019 Elsevier B.V. All rights reserved.

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transmission electron microscopy (TEM, JEOL JEM-2010 JEOL Ltd., Tokyo, Japan) were used. An energy dispersive X-ray spectrometer in combination with TEM was employed to evaluate the chemical components of the products.

of even a low concentration of sulfur compounds can corrode pipelines and equipment [21,22]. To determine the existence of this noxious gas in an environment, some traditional methods such as colorimetry [23], gas chromatography [24], and the ion-chromatography method [25] can be used due to their high sensitivity to low concentrations of H2S. However, they incur high operation costs and require a long detection time. Accordingly, precise detection of H2S using a cheap, fast, and user-friendly detector is of importance. Herein, we report the synthesis of SiNWs-ZnO core-shells for gas sensing applications. As mentioned previously, a Si-based sensor would be extremely useful and efficient due to well-established Si fabrication and device technology. However, a Si structure alone cannot promise sufficient sensing performance, particularly for H2S gas. ZnO is known to exhibit excellent sensing properties for H2S gas and a shell of ZnO on the surfaces of SiNWs can be a promising way to enhance the sensitivity to H2S gas. To the best of our knowledge, no study has investigated the gas sensing performances of SiNWs-ZnO core-shells. Herein, SiNWs were formed by MACE and after deposition of a thin Au layer, they were coated by ZnO through thermal evaporation of zinc nanopowders. The response of the fabricated SiNWs-ZnO core-shell gas sensor toward H2S was measured at 300 °C and it was revealed that the sensor has superb sensitivity and selectivity towards H2S gas. Further, the underlying sensing mechanism was explained in details.

2.3. Gas sensing tests The gas sensing set up employed is similar to that reported in our previous papers [27–29]. The top-top electrode configuration was used, where a bi-layered electrode composed of an Au (300 nm thick) and a Ti (50 nm thick) was deposited on the synthesized products. The gas flow was controlled by varying the mixing ratio of dry air to the target gas using mass flow controllers (MFCs) with a total flow of 500 sccm. All evaluations were carried out in a gas chamber inside a horizontal tube furnace at the optimum sensing temperature and the sensor resistances in the presence of air (Ra) and the target gas (Rg) were recorded. The sensor response for NO2, which is an oxidizing gas, was calculated as R = Rg/Ra, and for reducing gases (H2S and C2H5OH), it was calculated as R = Ra/Rg. The response time was calculated as the time required for the sensor to reach 90% of its final resistance in the presence of the target gas. The recovery time was calculated in the same manner but after stopping the target gas. 3. Results and discussion

2. Experimental procedure

3.1. Morphological and structural analyses

2.1. Synthesis of SiNWs/ZnO core-shells A modified Peng’s method was used to synthesize the Si NWs [26]. The Si pieces were cut from Si wafers into 2 × 2 cm2 squares. Before use, the Si pieces were cleaned by using a Piranha solution (98% H2SO4, 30% H2O2) and rinsed with deionized (DI) water. Then, they were completely dipped into a solution containing water and HF at an equal volume ratio for 5 min to remove the native oxide films. After this process, they were rinsed with DI water. Ag was used as the catalyst for Si etching. Fresh Si pieces were vertically immersed into an Ag deposition solution comprised of HF (5 ml), 1–4 ml of AgNO3 (98%), and H2O (42.5 ml) for 60 s at room temperature. Following the formation of a uniform layer of Ag NPs, the coated Si wafers were cleaned with DI water to remove the extra Ag ions. During the etching process, which was performed in a dark room, the coated Si wafers were vertically immersed into an etchant comprised of HF (5 ml), H2O (53 ml), and H2O2 (0.5–2 ml). After etching for 1 h and washing with DI water, the Ag-coated Si samples were dipped in a HNO3 solution for 60 s to get rid of the remaining Ag NPs. For the detachment of SiNWS, the etched Si wafer was dipped in ethanol and sonicated for 0.5 h. The floating SiNWs were accumulated and then sprayed onto hot (˜80 °C) quartz substrates. By putting the SiNWs in a vacuum oven at 50 °C for 5 h, the synthesized SiNWs were dried and cleaned. Before coating with ZnO, the as-prepared SiNWs were coated with Au (3 nm) by sputtering using a turbo sputter coater (Emitech K575X, Emitech Ltd., Ashford, Kent, UK). The deposition time and the sputtering current were set to 1 min and 10 mA, respectively. Then, in a vertical furnace, ZnO was deposited on the SiNWs by heating the metallic Zn powders to 500 °C. The synthesis steps of the Si NWs-ZnO coreshells are schematically represented in Fig. 1. After the completion of ZnO deposition, we demonstrated that Au nanoparticles were generated on the ZnO surface. We surmise that the initial Au shell on the Si NW surface was agglomerated and pushed up to the exterior of the ZnO shell during the ZnO deposition process.

Fig. 2 shows SEM images of pristine and ZnO-coated SiNWs. In both cases, the final morphology is comprised of long NWs with a length of several micrometers, as presented in the low magnification SEM images in Fig. 2A and C. Enlarged micrographs are presented in Fig. 2B and D, clearly showing that after deposition of the ZnO shell, the smooth morphology of the surfaces of the pristine Si NWs changes into a rugged and rough morphology, indicating the presence of ZnO on the surfaces of the SiNWs. For deeper insight, TEM analysis was carried out and the results are presented in Fig. 3A–F. Figs. 3A–C shows the TEM micrographs in order of increasing magnification. The porous ZnO shell is deposited on the SiNWs with a diameter of ˜100 nm. Fig. 3D and Fig. 3E shows lattice-resolved TEM images for the ZnO shell and the SiNW core, respectively. The observed interplanar spacings are ˜0.247 and 0.260 nm, which correspond to the separation between the neighboring (101) and (002) lattice planes of ZnO, respectively. In addition, the lattice fringes of the (111) plane can be observed with an interplanar spacing of 0.235 nm. Fig. 3F shows the associated selected area electron diffraction (SAED) pattern, revealing ring-shaped spots that clearly indicate the polycrystalline nature of ZnO, Si, and Au. The crystal diffraction rings were indexed as wurtzite ZnO, including (101), (103), (002), and (201) reflections, and (300), (330), and (030) diffraction planes of Si as well as the (111) plane of Au. Fig. 4 shows EDS color mapping of the synthesized products, in which the presence of Si, Au, Zn, and O is confirmed. Si, Zn, and O are distributed relatively uniformly on the surface whereas Au is discontinuously distributed. The EDS line mapping results are displayed in Fig. 5. Fig. 5A presents the TEM micrograph and Fig. 5B displays the corresponding EDS line mapping along the diameter. As indicated in more detail in Fig. 5C−F, the atomic percent of Si in the core part is higher than in the shell section whereas the reverse is true for Zn and O. It can be concluded that ZnO is principally located in the shell part and Si is in the core part. The thickness of the ZnO shell is estimated to be ˜45 nm. Trace amounts of Au are also observed in the EDS analysis with a relatively constant distribution over the diameter of the SiNWs.

2.2. Materials characterization

3.2. Gas sensing studies

To study the morphology of the products, scanning electron microscopy (FESEM, JSM-6700, JEOL Ltd., Tokyo, Japan) and

Firstly, the optimal sensing temperature of the gas sensor was found to be 300 °C based on a series of preliminary tests. Fig. 6A provides the 2

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Fig. 1. Schematic illustration of the synthesis steps of the SiNWs/ZnO core-shells.

H2S, the signal returns to its initial baseline. The responses for 10, 20, and 50 ppm H2S gas are 4.82, 6.92, and 11.22, respectively. The response time was also calculated (Fig. 6C). The expected trend was observed where the response time decreases with increasing H2S gas concentration. In more detail, the response times for 10, 20, and 50 ppm H2S gas were 61, 52, and 48 s, respectively. This decrease is due to the fact that a higher gas concentration means faster gas proliferation and adsorption onto the surfaces of the sensor. Fig. 6D shows the

dynamic resistance changes of the sensor to 10, 20, and 50 ppm H2S at 300 °C, and Fig. 6B presents the dynamic response changes to different concentrations of H2S. Based on Fig. 6A, upon exposure to H2S gas, the conductance increases, which indicates n-type semiconducting behavior of the gas sensor. This is an interesting finding as the sensing is principally governed by the ZnO shell rather than the p-type Si core, indicating that the conduction of electrons occurs inside the ZnO shell. Furthermore, the sensor possesses reversibility and upon the removal of

Fig. 2. SEM micrographs of (A)-(B) pristine SiNWs and (C)-(D) SiNWs/ZnO core-shells. 3

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Fig. 3. TEM analyses of the SiNWs/ZnO core-shells: (A) TEM image, (B)-(C) magnified TEM images, (D)-(E) lattice-resolved TEM images, and (F) SAED pattern.

performance for H2S compared to the other gas sensors. To evaluate the selectivity of the gas sensor, it was exposed to H2S, ethanol, acetone, and NO2 at the optimal working temperature. The dynamic resistance curves and corresponding dynamic response values are provided in Fig. S1 and Fig. 7, respectively. Ethanol and acetone are reducing gases whereas NO2 is an oxidizing gas. It can be seen that at all concentrations, the response to H2S gas is higher than the response to the other interfering gases (Table S1). However, at higher concentrations, the difference increases. As an example, the responses to 10 ppm of H2S, NO2, ethanol, and acetone gases were 4.82, 4.49, 1.15, and 1.13, respectively. The responses to 50 ppm of the same gases were

recovery times for the mentioned concentrations. The response times are 72, 68, and 64 s, which are applicable for practical use. It is worth noting that the time for gas recovery is longer than that for gas response. The adsorption and desorption kinetics of the gas molecules during adsorption and diffusion into the sensing layer determine the dynamic characteristics of the gas sensor. While gas molecules can quickly adsorb on the surface of the gas sensor, the desorption process is rather slow since the chemical connections between gas molecules and the sensor’s surface are strong. Table 1 compares the H2S sensing performance of some metal oxide gas sensors with the results obtained in this study [30–60]. Clearly, the present sensor has better sensing 4

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Fig. 4. EDS color mapping of SiNWs/ZnO core-shells: (A) Si, (B) Au, (C) Zn, and (D) O.

the presence of humidity can be attributed to the occupation of adsorption sites on the surface of the gas sensor by water vapor. Thus, lower concentrations of H2S gas can be adsorbed on the surface of the gas sensor, leading to a decrease of the gas response [61]. Fig. 10A and B show the transient response curves to 50 ppm H2S gas at 300 °C for a fresh gas sensor and a sensor kept for 1 year in the laboratory, respectively. The laboratory environment was kept in a humid environment (RH 60% @25 °C). As shown in Fig. 10C, after 1 year, the response of gas sensor decreased to 10.58 from an initial value of 11.22. Both tests were carried out in a RH0% humidity condition in the chamber. This corresponds to only a 5.7% decrease of the response, demonstrating its excellent stability even after 1 year.

11.22, 6.62, 1.72, and 1.33, respectively. To study the response of the gas sensor to lower concentrations of H2S gas, it was exposed to 50, 20, 10, and 5 ppm H2S gas where the transient response curves are depicted in Fig. 8A-D, respectively. The responses of the gas sensor to 5, 10, 20, and 50 ppm H2S gas are 2.29, 4.82, 6.99, and 11.22, respectively. This demonstrates the potential of the gas sensor for the detection of low concentrations of H2S gas. To investigate the effect of humidity on the response of gas sensor, it was exposed to 50 ppm H2S at 40% RH. The dynamic responses are shown in Fig. 9A and B for 0% RH and 40 RH%, respectively, showing that the response clearly decreases in the presence of humidity. Fig. 9C shows that the initial response of gas sensor at 0% RH (11.22) decreases to 8.83 at 40% RH. The reason for decrease of the sensor’s response in 5

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Fig. 5. EDS line mapping of the section shown in (A) and (B) full line mapping of all elements. Individual line mapping of (C) Zn, (D) O, (E) Si, and (F) Au.

Fig. 6. (A) Dynamic response to different concentrations of H2S gas at 300 °C and the (B) corresponding response changes, (C) response times, and (D) recovery times. 6

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sensing behavior is mainly governed by the ZnO shell with n-type semiconducting behavior. The value of λD for ZnO can be obtained as follows [63]:

Table 1 Metal oxide gas sensors for the detection of H2S. Materials

H2S Conc. (ppm)

T(°C)

Response

Ref.

SiNWs/ZnO core-shell

50

300

CuO NRs ZnO films ZnO thin film Cu-doped ZnO thin films Mo-doped ZnO NWs network

100 20 20 20 5

300 500 300 250 300

This Work [30] [31] [32] [33] [34]

SnO2 NWs WO3 hemitubes/graphene ZnO NFs Cu-doped ZnO NFs ZnO NRs Pd-embedded WO3NFs Pd-NPs/WO3 NFs Pd-NPs/Pd-embedded WO3 NFs 0.03 wt% Au-doped WO3 NPs WO3 NWs CuO-functionalized WO3 NWs 3 at% In-doped ZnO α-Fe2O3 nanochains 2 wt% Pt-doped α-Fe2O3 NPs 2 wt% Ag-doped α-Fe2O3 NPs Porous α-Fe2O3 NPs Fe2O3-loaded NiO nanoplates 1 at% Au-modified ZnO NWs 3D rGO/h-WO3 nanocomposites

20 1 10 10 0.05 5 5 5 2 100 100 100 5 10 100 10 10 5 1

300 300 230 230 25 350 350 350 300 300 300 250 285 160 160 350 200 25 330

3D Fe2(MoO4)3 10 wt.% Co-doped CdIn2O4 CdIn2O4 In2O3: 2.5 wt.% Co/0.2 wt.% Pt ZnO nanoflowers ZnO film WO3 NPs CuO-ZnO composite hollow spheres YMnO3 La0.7Pb0.3Fe0.4Ni0.6O3 V-doped In2O3 Pd-doped ZnO:TiO2

50 500 500 50 100 50 20 5

25 200 200 100 300 300 200 350

11.22 (Ra/ Rg) 4 (ΔR/Ra) 2 (Ra/Rg) 0.22 (ΔR/Ra) 0.38 (ΔG/Ga) 14.11 (Ra/ Rg) 11 (Ra/Rg) 5.6 (Ra/Rg) 1.57 (Ra/Rg) 18.7 (Ra/Rg) 1.7 (Ra/Rg) 3.1 (Ra/Rg) 3.5 (Ra/Rg) 3.2 (Ra/Rg) 12.4 (Ra/Rg) 1.84 (Ra/Rg) 6.72 (Ra/Rg) 60 (Ra/Rg) 4.7 (Ra/Rg) 46.6 (Ra/Rg) 220 (ΔG/Ga) 12.9 (Ra/Rg) 7.35 (Ra/Rg) 80 (Ra/Rg) 10.80 (Ra/ Rg) 13.5(Ra/Rg) 0.8 (ΔR/Ra) 0.25 (ΔR/Ra) 0.9 (ΔR/Ra) 9 (Vg/Va) 0.85 (ΔG/Ga) 3 (Ra/Rg) 13.3 (Ra/Rg)

20 150 50 10,000

100 200 90 250

0.49 0.85 0.14 0.62

(ΔR/Ra) (ΔR/Ra) (ΔR/Ra) (ΔR/Ra)

λD =

εkB T , q 2n c

(2)

where ε, kB, T, q, and nc are the dielectric constant of ZnO (8.75 × 8.85 × 10−12 F/m), Boltzmann’s constant (1.38 × 10-23 J/K), absolute temperature, electrical charge (1.6 × 10-19 C), and carrier charge concentration of ZnO (5.1 × 1016/cm3), respectively. At 300 °C, the λD value for bulk ZnO is 22–35 nm [37]. According to Fig. 5B, the approximate thickness of the ZnO shell layer is 45 nm. Therefore, it can be supposed that the ZnO shell will not be fully depleted at 300 °C. Since ZnO has a larger shell thickness than the Debye length of ZnO, the electrical movement will be mainly limited to the ZnO shell layer rather than the Si core [64]. Therefore, the resistance modulation of the gas sensor will be mainly governed by the radial modulation of the electron depletion layer in the ZnO shell rather than in the Si NW core. When the SiNWs-ZnO core-shell gas sensor is in an air atmosphere, the oxygen molecules will be adsorbed on the surface of the ZnO shell, abstract electrons from the conduction band of ZnO, and be converted into oxygen ions (O2−, O− and O2−). Accordingly, an electron depletion layer forms on the surface of the ZnO shell, where the resistance is high relative to the core region. When H2S is injected, the adsorbed oxygen ions will react with the H2S gas according to the following equation [65,66].

[35] [36] [37] [37] [38] [39] [39] [39] [40] [41] [41] [42] [43] [44] [45] [46] [47] [48] [49]

H2 S + 3O x − (ads ) → H2 O + SO2 + 3xe− x = 1,2

[50] [51] [51] [52] [53] [54] [55] [56]

(3)

Accordingly, the oxygen species react with H2S and release the electrons back to the conduction band of ZnO. As a result, the width of the electron depletion layer decreases and a response results. This mechanism is in place for pristine ZnO surfaces. However, in the SiNWsZnO core-shell, additional factors, namely (i) heterojunctions between ZnO/Au, (ii) the catalytic effects of Au, and (iii) ZnO/Si heterojunction, must be considered. Fig. 11 schematically shows the associated sensing mechanism. It can be reasonably assumed that Au NPs are uniformly dispersed on the surfaces of p-Si(core)/n-ZnO(shell) NWs. Regarding Au, chemical sensitization (CS) should be considered. In CS, the gas molecules are adsorbed on the surfaces of Au; here, due to the catalytic effect of Au, gas molecules are dissociated, in a so-called spill-over effect, and transferred onto the surfaces of the ZnO shell. In fact, Au acts as a very effective adsorption site to adsorb H2S gas (Fig. 12A). The catalytic activity of Au towards H2S was reported in a previous work [67]. As shown in Fig. 12B, the work function of Au (> 5.2 eV [68]) is larger than that of ZnO (5.2 eV [69]). The electrons must be transferred from ZnO to Au to equate the Fermi levels and as a result, a Schottky junction will be formed. Consequently, an additional depletion layer will form on the contact areas between ZnO and Au with a width of W1 (Fig. 12B). After exposure to H2S gas, the released electrons return to the surface of ZnO and the width of the depletion layer decreases significantly. This modulation of the resistance causes an increase of the response to H2S gas. The initial amount of electrons in ZnO decreases due to the presence of Au. Through the adsorption of H2S gas, the resistance will decrease. If we reasonably suppose that the amount of decrease in resistance is similar for both cases, the sensor response of ZnO in the presence of the ZnO/Au interfaces will be larger than that of ZnO without ZnO/Au interfaces. Accordingly, the modulation of the resistance by the ZnO/Au heterojunctions will contribute to improve the H2S sensing performance. Therefore, in the case of Au, both effects will enhance the response of the sensor. Subsequently, we discuss the effects of the p-Si(core)/n-ZnO(shell) NWs heterointerfaces. With regard to the role of p-Si, when p-Si and nZnO are in intimate contact (Fig. 12C), for the alignment of the Fermi levels, the electrons will flow from the region with the smaller work

[57] [58] [59] [60]

Fig. 7. Sensor responses of the SiNWs/ZnO core-shell sensor to H2S, ethanol, acetone, and NO2 gases at various concentrations.

3.3. Gas sensing mechanism The sensing mechanism of metal oxide-based gas sensors is based on the adsorption of gas molecules on the surfaces of the sensor, which results in modulation of the resistance [62]. As stated before, the 7

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Fig. 8. The transient response of the Si-Zn-Au gas sensor to (A) 50 ppm, (B) 20 ppm, (C) 10 ppm, and (D) 5 ppm H2S gas at 300 °C.

Fig. 9. Transient response of the Si-ZnO-Au gas sensor to 50 ppm H2S gas at 300 °C in the presence of (A) 0% and (B) 40% RH. (C) Gas response versus RH%. 8

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Fig. 10. The transient response curves to 50 ppm H2S gas at 300 °C for a (A) fresh gas sensor and (B) a sensor kept for 1 year in the laboratory (RH 60% @25 °C). (C) Responses of the gas sensor in the fresh state and after 1 year.

the Fermi level will bring about bending of the energy band, as shown in Fig. 12D (i). The potential barrier at the interface of ZnO and p-Si results from the difference of their work functions. Owing to the potential barriers of the ZnO/Si interfaces, control of electron transportation is as described previously [30–33]:

function to that with the larger one. According to previous research, the work function of ZnO is in the range of 4.7–5.4 eV. The electronic affinity of ZnO is about 4.35 eV and the work function of severely ndoped ZnO can be reduced to ˜4.4 eV. On the other hand, the electronic affinity and bandgap of Si are about 4.07 and 1.12 eV, respectively. Accordingly, if Si is p-type, the work function of p-type Si will be in the range of 4.63–5.19 eV. Therefore, it is more likely that the work function of ZnO is higher than that of p-Si. However, there is also the possibility that the work function of p-Si is higher than that of ZnO. We surmise that the work function of ZnO is higher than that of p-Si in which the electrons will be transferred from p-Si to ZnO. As a result, hole accumulation and electron accumulation layers will be created on the Si and ZnO sides, respectively, at the Si/ZnO interface, as revealed in Fig. 12D (i). Due to the adsorption of H2S gas molecules, the electrons will move from the molecules to the conduction band of ZnO. Due to the presence of Si/ZnO heterointerfaces, the relative change of the amount of electrons in the conduction band of ZnO is reduced and does not contribute to sensor enhancement. If we suppose that the work function of p-Si is higher than that of ZnO, the electrons flow from ZnO to Si, where an electron depletion layer and a hole depletion layer are created on the ZnO and Si sides, respectively. Being similar to the case of the ZnO/Au heterojunctions, the initial amount of electrons in ZnO decreases due to the presence of p-Si. As a result of the adsorption of H2S gas, the resistance decreases. If we reasonably surmise that the amount of the decrease of the resistance is similar for both cases, the sensor response of ZnO with the presence of ZnO/Si interfaces will be larger than that of ZnO without ZnO/Si interfaces. Accordingly, modulation of the resistance by ZnO/Si heterojunctions is an important factor, leading to a response of the gas sensor. Apart from the above-mentioned electron depletion effect of ZnO, the barriers of the ZnO/Si heterojunction will cause an improvement of the sensing performance. The flow of the charge carriers to equilibrate

R = Ro exp [ΔΦ/kBT],

(4)

where Ro is a constant and ΔΦ is the effective potential barrier. In air, the oxygen species are adsorbed on the ZnO/Si heterointerfaces. In a H2S atmosphere, the effective potential barrier is changed. Accordingly, it is possible that the presence of effective potential barriers at the ZnO/ Si heterointerfaces will result in modulation of the resistance upon the adsorption of H2S gas. The above equation for control of electron transportation is applicable only when the charge carriers pass through the heterojunctions. However, in the present case of the p-Si/ZnO coreshell NW structures, the current will mainly flow through the outer ZnO shell layers. Accordingly, the electrical currents through the ZnO/Si heterojunctions will not be significant. Although it is a rare case, it is possible that the electrons may flow from one part of the ZnO shell to pSi and subsequently flow to the other part of the ZnO shell. When the work function of ZnO is higher than that of p-Si, the established potential barrier corresponds to the step with ZnO being at a higher position (Fig. 12D (ii)). The adsorption of H2S gas will mainly occur on the ZnO shell surface and the addition of electrons to the ZnO surface will elevate the Fermi level of ZnO. Therefore, in the electron flow of ZnOSi-ZnO, by the introduction of H2S gas, the electron flow from Si to ZnO will become easier, contributing to the reduced resistance. This will further add to the reduced resistance through the introduction of H2S gas, contributing to an increased sensor response. When the work function of p-Si is higher than that of ZnO, the established potential barrier corresponds to the step with p-Si being at the higher position. Therefore, in the electron flow of ZnO-Si-ZnO, by the introduction of 9

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Fig. 11. Sensing mechanism in the Au-functionalized SiNWs/ZnO core-shell gas sensor. (A) Formation of the depletion layers in the pristine SiNWs/ZnO core-shell layer. (B) Expansion of the electron depletion layer in the presence of Au.

H2S gas, the electron flow from Si to ZnO will become difficult, contributing to the increase of resistance. This will oppose the reduction of resistance resulting from the introduction of H2S gas, contributing to a decrease in sensor response. To summarize, improvement of the sensor response by increased electron flow by ZnO-Si-ZnO will occur only when the work function of ZnO is higher than that of p-Si. In addition, being similar to the ZnO/Si heterojunction, the barriers of the ZnO/Au heterojunction will cause an improvement in the sensing performance. The selectivity depends on the bond energy of the target gases, porosity, and oxygen vacancies [70]. However, good selectivity to H2S can be explained as follows (Fig. 7). On one hand, H2S has high reactivity and can easily react with adsorbed oxygen ions on the surfaces of ZnO. Otherwise, the bond energy between HeSH in H2S is 381 KJ/ mol, which is smaller than other interfering gases (Table 2).

Accordingly, the HeSH bond can be easily collapsed at 300 °C during chemical adsorption [71]. Even though the bond energy of OeNO is lower than that of HeSH, the interaction strength between the sensor and the tested gas is another important issue, which helps determine the gas response [49]. In addition, H2S is an acidic gas and ZnO is a basic oxide. Therefore, the acid-base interactions on the surface of ZnO can enhance the response to H2S gas [72]. Furthermore, the pores can influence the diffusion and penetration of large gas molecules such as acetone and ethanol. Hence, it is complicated for large gas molecules to reach and diffuse into the deep parts of the gas sensor, resulting in lower gas responses. Also, a sulfurization reaction between ZnO and H2S can take place, which is a spontaneous and exothermic process.

ZnO + H2 S → ZnS + H2 O

10

(5)

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Fig. 12. (A) Schematic outline showing spillover effects due to the presence of Au NPs. (B) Energy band structure of ZnO/Au upon the formation of heterointerfaces. (C) Energy band structure of ZnO/p-Si, prior to contact. (D) (Top) When the work function of p-Si is higher than that of ZnO, electrons will flow from ZnO to Si and electron-depletion and hole-depletion layers will be created on the ZnO and Si sides, respectively. (Below) When the work function of ZnO is higher than that of p-Si, the established potential barrier corresponds to the step with ZnO being in the higher position.

the surfaces of Au NPs, which lowers the surface work function of the Au NPs up to 1 eV [79]. As a result of this change, the extent of electron exchange between the Au NPs and ZnO can be varied, which contributes to the enhancement of the sensor response to H2S gas. In the present work, we believe that the size of the Au NPs can be controlled by varying the thickness of the Au layer. The Au NP size will increase with increasing thickness of the deposited Au layer. We expect that the sensor response will be maximized at an appropriate Au NO size and Au layer thickness. This response behavior suggests that the size of the Au NPs must be optimized to obtain superior sensing performance when Au NPs are used for functionalization of oxide NWs. When the surface coverage of Au NPs is lower than the optimized value, the sensor response will decrease because a smaller amount of Au catalytic NPs participates in the sensing process. On the contrary, when the surface coverage of Au NPs is higher than the optimized value, the sensor response will be decreased because there is not enough sensor surface due to the steric hindrance of metal NPs. Of course, the present

This is another reason for the sensor to show higher sensitivity to H2S gas relative to other gases since the reactions between ZnO and other interfering gases are endothermic and non-spontaneous [73]. In addition, the Au nanoparticles will exert catalytic activities, particularly towards H2S gas. It is known that there is a strong chemical affinity between Au and S (D0298 = 418 ± 25 kJ mol−1) [74]. Accordingly, H2S molecules will adsorb efficiently and strongly on Au surfaces, which contributes to the enhancement of the sensor response to H2S gas. In fact, considering the strong chemical affinity between Au and S, interactions of sulfur with gold are highly probable [75–77].

H2 S + Au (s ) → AuS (s ) + H2 (g )

(6)

This has been confirmed in previous studies, where the metal-sulfur interaction in the form of HS or H2S adsorption on metals coincides to the S 2p binding energy in the range of 163.5–165 eV [78]. Adsorption of H2S molecules onto Au NPs can possibly produce Au-SH or Au-S type species on the surface of Au NPs. As a result, a sulfide shell will cover Table 2 Properties of the tested gas molecules [49]. Gas

H2S

C2H5OH

C3H6O

NO2

H-HS 381.0

H-OC2H5 436.0

H-CH2COCH3 393.0

O-NO 305.0

Structural Formula

Bond Bond Energy (KJ/mol)

11

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case is different from a previous report [80]. In a previous report, an Au layer was deposited on NWs and NWs were subsequently annealed to form Au NPs. However, in the present work, the Au layer was agglomerated during deposition of the ZnO shell at 500 °C. Au NPs were generated on the ZnO surface by pushing up Au to the exterior of the ZnO shell. Accordingly, variation of the sensor response as a function of the Au layer thickness needs to be investigated. Even though the present gas sensor showed enhanced response and selectivity towards H2S gas, the selectivity may be decreased if other sulfur containing gases such as methanethiol (CH4S) are present in the environment. In fact, Au atoms can react with sulfur and most sulfur containing gases such as methylsulfide (CH3S), dimethylsulfide, (CH3eSeCH3) dimethyldisulfide (S(CH3)2), methanethiol (CH4S), ethanethiol (C2H5S), hydrogen sulfide (HS), carbonyl sulfide (O]C] S), and carbon disulfide (CS2), which can be adsorbed on the surface of Au to some degree [81]. It has been confirmed that binding of organothiols and disulfides to Au is a result of RSeH or SeS bond cleavage followed by the formation of an AueS bond [82]. Roy et al. theoretically investigated the sensing mechanism of Au NWs in the presence of CH3S and other gases [83]. It was shown that CH3S gas with a large electron affinity had a large binding energy to Au and could strongly interact with Au NWs. Kisner et al. experimentally demonstrated good alkanethiol adsorption on Au NWs [84] and Vasumathi et al. also confirmed the adsorption of methanethiol molecules on the surfaces of Au NPs [85].

[8]

[9] [10]

[11]

[12]

[13] [14]

[15]

[16] [17] [18] [19]

[20]

4. Conclusions [21]

In conclusion, SiNWs/ZnO core-shells were synthesized by MACE and the thermal evaporation of Zn powders. The results of the SEM, TEM, and EDS analyses collectively demonstrated the formation of the desired morphology and chemical composition, namely SiNWs-ZnO core-shells with Au NPs being generated on the surface. The gas sensing tests exhibited a high response and good selectivity to H2S gas along with a short response time. The small bond energy of HeHS, small size of H2S molecules, catalytic role of Au, intrinsically high sensitivity of ZnO, and formation of Si/ZnO and Au/ZnO heterojunctions are the primary causes for the good response to H2S gas. This new and inexpensive gas sensor is a good choice for H2S sensing in real applications.

[22]

[23]

[24] [25]

[26] [27]

Acknowledgements

[28]

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2016R1A6A1A03013422).

[29] [30] [31]

Appendix A. Supplementary data [32]

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.snb.2019.03.047. [33]

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Myung Sik Choi received his B. S. and M. S. degree in materials science and engineering from Inha University, Korea in 2014 and 2016. He is currently a graduate student in Hanyang University. His research work is to focus on nanomaterials and their application to gas sensors. Ali Mirzaei received his Ph.D. degree in Materials Science and Engineering from Shiraz University in 2016. He was visiting student at Messina University, Italy in 2015. From 2016–2018 he was postdoctoral fellow at Hanyang University in Seoul. Currently, he is faculty member of Shiraz University of Technology, Shiraz, Iran. He is interested in the synthesis and characterization of nanocomposites for gas sensing applications. Jae Hoon Bang received his B.S. degree in Materials Science and Engineering from Hanyang University, in 2016. He is currently a graduate student in Hanyang University. He has been working on metal oxide gas sensors and on the synthesis of metal nanoparticles. Wansik Oum received his B.S. degree in Materials Science and Engineering from Seokyeong University, in 2018. He is currently a graduate student in Hanyang University. He has been working on metal oxide gas sensors. Yong Jung Kwon received his B.S. degree in Materials Science and Engineering from Hanyang University, in 2016. He received his Ph.D. degree from Hanyang University in Materials Science and Engineering in 2017. He has been working on metal oxide gas sensors and on the synthesis of metal nanoparticles. Jae-Hun Kim received his B.S. degree from Gyeongsang National University, Repub-lic of Korea in 2013. In February 2015, he received M.S. degree from Inha University,Republic of Korea. He is now working as a Ph.D. candidate at Inha University, Republic of Korea. He has been working on oxide nanowire gas sensors. Sun-Woo Choi received his B.S. and M.S. degrees from Inha University, Republic ofKorea in 2008 and 2010, respectively. He received his Ph.D. degree from Inha University in Materials Science and Engineering in 2014. He is currently a professor at Department of

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M.S. Choi, et al.

research interests include the synthesis and applications of nanomaterials such as nanowires and nanofibers, functional thin films, and surface and interfacial characterizations.

Materials and Metallurgical Engineering, Kangwon National University, Korea. He has been working on nanomaterials based gas sensors and on the synthesis of metal/metal oxide nanoparticles.

Hyoun Woo Kim joined the Division of Materials Science and Engineering at Hanyang University as a full professor in 2011. He received his B.S. and M. S. degrees from Seoul National University and his Ph.D. degree from Massachusetts Institute of Technology (MIT) in Electronic Materials in 1986, 1988, and 1994, respectively. He was a senior researcher in the Samsung Electronics Co. Ltd. from 1994 to 2000. He has been a professor of materials science and engineering at Inha University from 2000 to 2010. He was a visiting professor at the Department of Chemistry of the Michigan State University, in 2009. His research interests include the one-dimensional nanostructures, nanosheets, and gas sensors.

Sang Sub Kim joined the Department of Materials Science and Engineering, Inha University, in 2007 as a full professor. He received his B.S. degree from Seoul National University and his M.S and Ph.D. degrees from Pohang University of Science and Technology (POSTECH) in Material Science and Engineering in 1987, 1990, and 1994, respectively. He was a visiting researcher at the National Research in Inorganic Materials (currently NIMS), Japan for 2 years each in 1995 and in 2000. In 2006, he was a visiting professor at Department of Chemistry, University of Alberta, Canada. In 2010, he also served as a cooperative professor at Nagaoka University of Technology, Japan. His

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