Letter pubs.acs.org/NanoLett
Hydrogen Sensing under Ambient Conditions Using SnO2 Nanowires: Synergetic Effect of Pd/Sn Codeposition Seung Ho Jeong,† Sol Kim,‡ Junho Cha,† Min Soo Son,† Sang Han Park,† Ha-Yeong Kim,‡ Man Ho Cho,† Myung-Hwan Whangbo,§,‡ Kyung-Hwa Yoo,†,* and Sung-Jin Kim*,‡ †
Department of Physics, Yonsei University, Seoul, 120-749, Republic of Korea Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea § Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States ‡
S Supporting Information *
ABSTRACT: Semiconducting SnO2 nanowires deposited with Pd and Sn nanoparticles on their surface are shown to be a highly sensitive hydrogen sensor with fast response time at room temperature. Compared with the SnO2 nanowire deposited with Pd or Sn nanoparticles alone, the Pd/Sn-deposited SnO2 nanowire exhibits a significant improvement in the sensitivity and reversibility of sensing hydrogen gas in the air at room temperature. Our investigation indicates that two factors are responsible for the synergistic effect of Pd/Sn codeposition on SnO2 nanowires. One is that in the presence of Pd the oxidation of Sn nanoparticles on the surface of the SnO2 nanowire is incomplete leading only to suboxides SnOx (1 ≤ x < 2), and the other is that the surface of the Pd/Sn-deposited SnO2 nanowire is almost perfectly hydrophobic. KEYWORDS: Room-temperature hydrogen sensor, SnO2 nanowire, synergetic effect of Pd/Sn codeposition
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origin of such a high sensitivity is not clearly understood.32 In the present work, we prepare SnO2 single-nanowire sensors deposited with both Sn and Pd nanoparticles on their surface (hereafter Sn/Pd-deposited SnO2 nanowire sensors), which can sense hydrogen gas in the air at room temperature with high sensitivity and reversibility and examine probable causes for the synergistic effects of Sn/Pd codeposition in some detail. These sensors exhibit much higher sensitivity than do the SnO2 nanowire sensors with Pd or Sn nanoparticles alone. To explore the reasons for the synergistic effects of Pd/Sn nanoparticle codeposition, we investigate the oxidation states of Sn in the presence and absence of Pd using X-ray photoelectron spectroscopy (XPS) and evaluate the hydrophobicity of the surface of nanowire networks since water molecules form during the sensing of H2 and hence influence the sensor characteristics.25−29 We synthesize SnO2 nanowires by thermal evaporation of SnO as previously reported.30−33 Two different kinds of SnO2 nanowires are obtained depending on the zone of the furnace. At the center of a tube furnace where the temperature is highest, a large amount of SnO2 nanowires with uniform morphology is obtained (Figure 1a). In the region of the furnace with lower temperature (i.e., away from the center), SnO2 nanowires with Sn nanoparticles attached on their surface
n recent years, hydrogen (H2) has become one of the most important and promising energy sources because it is renewable, abundant, efficient, and eco-friendly.1−3 However, H2 gas is flammable in the presence of oxygen and is explosive especially when its concentration exceeds 4% in air.3 Therefore, detecting H2 leakage from storage and transporting equipment is very important.2 Many different types of H2 sensors have been developed, which include electrochemical, electrical, optical, and acoustic sensors.4−11 Those H2 sensors based on semiconducting metal oxides, particularly tin dioxide (SnO2) widely used in both industrial and domestic applications,12−16 change their conductance when exposed to H2. Huge efforts have been directed toward improving the performance of SnO2based hydrogen sensors, for example, by using catalytic additives such as Pd and Pt. Incorporation of Pd nanoparticles onto SnO2 thin films,17 nanobelts,18 and nanowires19,20 enhances the sensitivity of these materials to H2. Compared to the bare SnO2 based sensors, the Pd-incorporated SnO2 sensors provide higher sensitivity and faster response.4,21−23 However, the operation of these sensors requires high temperatures to promote their reaction with H2 gas and also remove water formed during the process of H2 sensing. Recently, In2O3-doped SnO2 thin films24 and mesoporous PdO-SnO2 films13 were reported to respond to H2 at room temperature, but their response to H2 is very slow at room temperature. We have reported a ultrahigh sensitive H2 sensor at room temperature, which is fabricated from SnO2 nanowire networks by decorating with Pd and Sn nanoparticles, but the © 2013 American Chemical Society
Received: August 10, 2013 Revised: October 14, 2013 Published: November 13, 2013 5938
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Figure 1. Field-emission SEM (FESEM) images of (a) pristine and (b) Sn-decorated SnO2 nanowires. The PXRD patterns of (c) the pristine and (d) Sn-decorated SnO2 nanowires. In (d), the peaks arising from Sn metal are labeled with stars.
Pd/Sn-decorated SnO2 nanowire sensors. In addition, we fabricate the Sn/Pd-deposited SnO2 nanowire sensor by depositing a 1 nm thick Sn film on the Pd-deposited nanowire (see Figure S1 of Supporting Information). Exposure to the 1.1% H2 gas increases the conductance of the Pd-deposited SnO2 nanowire by about 9% (Figure 2), and that of the Pd/Sndecorated SnO2 nanowire by 170% (Figure 2). Moreover, after each cycle of reduction/oxidation, the conductance of the Pd/ Sn-decorated SnO2 nanowire sensor is restored nearly to the initial value G0, but that of the Pd-deposited SnO2 nanowire sensor is not, as observed in the pristine and Sn-decorated SnO2 ones. This synergistic effects of Sn/Pd codeposition is further confirmed with a Sn/Pd-deposited single nanowire sensor (Figure 2e). The conductance of this Sn/Pd-deposited SnO2 nanowire rises strongly during the reduction (by about 500%) and decreases strongly during the oxidation. The change in the conductance during the reduction/oxidation cycles is much greater and faster for the Sn/Pd-deposited than the Pd/ Sn-decorated SnO2 nanowire. Furthermore, at the end of each reduction/oxidation cycle a nearly complete recovery of the conductance is observed, as in the case of the Pd/Sn-decorated sensor. The above results show that Sn/Pd codeposition on the SnO2 nanowire surface significantly enhances its sensor performance at room temperature. To gain insight into the role of Sn/Pd codeposition in enhancing the H2 sensing capability of the SnO2 nanowire, we measure the current versus gate voltage (I−VG) curves in vacuum for the pristine, Pd-deposited, and Sn/Pd-deposited SnO2 nanowires (Figure 2f) by using a Si substrate as the gate electrode. The pristine SnO2 nanowire exhibits an n-type
are obtained (hereafter Sn-decorated SnO2, Figure 1b). The powder X-ray diffraction (PXRD) data obtained from SnO2 (Figure 1c) indicate that SnO2 nanowires have a rutile (P42/ mnm) structure with lattice parameters a = 3.186 Å and c = 4.738 Å (ICSD #9163), whereas the PXRD data of Sndecorated SnO2 nanowires (Figure 1d) show the presence of both Sn metal and SnO2 phases. A single-nanowire sensor using the pristine or Sn-decorated SnO2 nanowire was fabricated on a Si substrate with a 200 nm thick thermally grown SiO2 layer (inset of Figure 2a). We measured the conductance at room temperature of the pristine and Sn-decorated SnO2 nanowire sensors while they are exposed to the alternating flow of the reducing gas (i.e., a mixture of 1.1% of H2 + 98.9% dry air, hereafter referred to as the 1.1% H2 gas unless mentioned otherwise) and the oxidizing gas (i.e., 100% dry air). Given G(t) as the conductance of the nanowire at the gas-exposure time t, we report the change in the conductance as a function of t in terms of the percentage change, ΔG(t)/G0, where G0 is the conductance at t = 0, that is, G(0), and ΔG(t) = 100 × [G(t) − G0]. On exposure to the 1.1% H2 gas, the conductance of the pristine SnO2 nanowire sensor increases by about 5%, and that of the Sn-decorated SnO2 nanowire sensor by about 7.5%. After each cycle of the reduction and oxidation (i.e., the exposure to 1.1% H2 gas and then to dry air), the conductance is not restored to the initial value G0, as reported by others.20,34−36 To increase their sensitivity to H2 sensing, we deposit Pd nanoparticles on the pristine and Sn-decorated SnO2 nanowires, by first depositing a 1 nm thick Pd film and then annealing at 500 °C for 1 min. This results in Pd-deposited and 5939
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Figure 2. (a) Conductance change, ΔG/G0, measured at room temperature as a function of the gas flow time t for the pristine and Sn-decorated SnO2 nanowires. The inset shows a FESEM image of the pristine SnO2 single nanowire sensor. (b) FESEM image of the Pd-deposited SnO2 nanowire. The inset shows a zoomed-out view. (c) ΔG/G0 versus t plot for Pd-deposited SnO2 nanowire, (d) that for Pd/Sn-decorated SnO2 nanowire, and (e) that for Pd/Sn-deposited SnO2 nanowire. In (d), the plot shows two reducing/oxidation cycles at four different H2 concentrations in the reducing gas; in addition to 1.1%, we considered 0.8, 0.6, and 0.5% H2 in the mixture of H2 and dry air. (f) I−VG transfer curves measured in vacuum for pristine, Pd-deposited, and Sn/Pd-deposited SnO2 nanowires.
Figure 3. Sn 3d5/2 XPS peaks, measured after exposing in air for 1 h, of a 1 nm thick Sn film deposited (a) on a bare Si substrate and (b) on a Si substrate covered with 10 nm thick Pd film. The experimental peaks were fitted with Gaussians representing the metallic Sn and three oxidized Sn species.
semiconducting behavior that the current increases with increasing VG. The threshold voltage (Vth) of the Pd-deposited SnO2 nanowire is higher than that of the pristine one due to the
occurrence of nanoscale depleted regions surrounding the Pd nanoparticles, which arise from a net electron transfer from the SnO2 nanowire to the deposited Pd nanoparticles.34 The Vth of 5940
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Figure 4. (a) I−VG transfer curves measured before and after H2 sensing for Sn-deposited and Sn/Pd-deposited SnO2 nanowires. Images of a water droplet before and after being dropped on the surface of (b) the pristine, (c) the Pd-deposited and (d) the Sn/Pd-deposited SnO2 nanowire networks.
→ 2H → 2H+ + 2e−. The electrons generated are expected to remove the electron-depleted regions of the SnO2 nanowire around the Pd nanoparticles hence enhancing the conductance of the nanowire. In addition, the SnOx suboxides should undergo reduction leading eventually to Sn metal and H2O, namely, SnOx + xH+ → Sn(OH)x, and Sn(OH)x + xH+ → Sn + xH2O. H2O molecules are also expected from the O2− species on the surface of the nanowire, namely, O2− + 2H+ → H2O. The oxidation of Sn to SnOx requires less energy and time than that of Sn to SnO2, and so does the reduction of SnOx to Sn than that of Sn to SnO2. This explains in part why the oxidation and reduction processes are faster on the Sn/Pddeposited SnO2 than on the pristine, Sn-deposited, or Pddeposited SnO2 nanowire. In explaining the enhanced sensitivity of the Sn/Pd-deposited SnO2 nanowire to the alternating flow of the reducing and oxidizing gases, it is necessary to note that the end product of the reduction is H2O molecules, which may wet the surface of the nanowires and hence prevent the reduced surface from becoming oxidized when exposed to dry air. Thus, the enhanced reversibility observed for the Sn/Pd-deposited SnO2 nanowire sensor suggests that its surface is not wetted by water molecules produced. The Sn/Pd-deposited SnO2 nanowire sensor exhibits nearly complete recovery at room temperature (Figure 2d,e), in contrast to the cases of the pristine, Sn-deposited, and Pddeposited SnO2 nanowires (Figure 2a,c). To understand this difference, we measure the I−VG transfer curves for the Sndeposited and Sn/Pd-deposited SnO2 nanowires before and after H2 sensing (Figure 4a). The Vth of the Sn-deposited SnO2 nanowire sensor shifts to a more negative VG value after H2 sensing, due probably to the adsorption of water molecules formed during the H2 sensing. In contrast, the Sn/Pd-deposited SnO2 nanowire sensor exhibits almost an identical I−VG transfer curve before and after the H2 sensing, confirming the reversible behavior of the sensor. To further examine how the water adsorption on the sensor surface influences the sensor characteristics, we also measure the I−VG transfer curves of the pristine, Pd-deposited, and Sn/Pd-deposited SnO2 nanowires under the ambient condition at 10 min, and 6 or 30 h after they
the Sn/Pd-deposited SnO2 nanowire lies slightly lower than that of the Pd-deposited nanowire but still much higher than that of the pristine nanowire (Figure 2f). This reflects the fact that the work function of Sn (4.4 eV) is smaller than that of Pd (5.3 eV), so the extent of charge depletion around the Sn/Pd nanoparticles in the Sn/Pd-dposited SnO2 nanowire is slightly lower than that of the Pd nanoparticles in the Pd-deposited nanowire. We now examine the chemical reactions that occur on the surface of the Sn/Pd codeposited SnO2 nanowire during the oxidation under dry air and the reduction under the 1.1% H2 gas. It is known that submonolayer Sn films deposited on a Pd substrate are oxidized to stoichiometric SnO at 300 K in the presence of oxygen.37,38 Thus, under dry air the surface of Sn nanoparticles on the Sn/Pd-deposited SnO2 nanowire are expected to undergo oxidation to form largely suboxides SnOx (1 ≤ x < 2), Sn + 1/2xO2 → SnOx. To verify this point we carry out XPS measurements for a 1 nm thick Sn film deposited on a bare Si substrate and also that deposited on a Si substrate covered with a 10 nm thick Pd film. The Sn 3d5/2 peaks measured for these Sn films after exposing them in dry air for 1 h at room temperature are presented in Figure 3a, where the peaks are deconvoluted into one metallic and three oxidized states. The Sn film on the bare Si substrate shows a much higher intensity for the oxidized states than for the metallic state (Figure 3a), indicating that Sn nanoparticles are largely oxidized to SnO2. In contrast, for the Sn film on the Pd-covered Si substrate, higher intensity peaks are found for the metallic Sn and suboxides SnOx (1≤ x
