http://dx.doi.org/10.5012/bkcs.2012.33.6.1993
CO Gas-Sensor Based on Pt-Functionalized Mg-Doped ZnO Nanowires
Changhyun Jin, Sunghoon Park, Hyunsu Kim, Soyeon An, and Chongmu Lee*
Department of Materials Science and Engineering, Inha University, Incheon 402-751, Korea. *E-mail: cmlee@inha.ac.kr Received December 2, 2011, Accepted March 19, 2012
Mg-doped ZnO one-dimensional (1D) nanostrutures were synthesized by using a thermal evaporation technique. The morphology, crystal structure, and sensing properties of the Mg-doped ZnO nanostructures functionalized with Pt to CO gas at 100oC were examined. The diameters of the 1D nanostructures ranged from 80 to 120 nm and that the lengths were up to a few tens of micrometers. The gas sensors fabricated from multiple networked Mg-doped ZnO nanowires functionalized with Pt showed enhanced electrical response to CO gas. The responses of the nanowires were improved by approximately 70, 69, 111, and 81 times at CO concentrations of 10, 25, 50, and 100 ppm, respectively. Both the response and recovery times of the nanowire sensor for CO gas sensing were not nearly changed by Pt functionalization. It also appeared that the Mg doping concentration did not influence the sensing properties of ZnO nanowires as strongly as Pt-functionalization. In addition, the mechanism for the enhancement in the CO gas sensing properties of Mg-doped ZnO nanowires by Pt functionalization is discussed.
Key Words : ZnO, Nanowires, Mg doping, Pt functionalization, CO
Introduction
In recent years, one-dimensional (1D) nanostructure-based sensors have become a focus of intensive research owing to the advantages of higher sensitivity, superior spatial resolution, and rapid response associated with individual 1D nanostructures due to the high surface-to-volume ratios compared to thin film gas sensors.1-3 However, it still remains a challenge to enhance their sensing performance and detection limit. A range of techniques such as doping,4-6 surface functionalization,7-9 and fabrication of heterostruc- tures10-12 have been developed to improve the sensitivity, stability, response, recovery speed of the 1D nanostructure- based sensors. Among these techniques, the functionali- zation of 1D nanostructure surfaces with catalysts such as Pd and Pt may be the simplest and most effective technique because the resistance of the sensor changes greatly upon exposure to target gas at low temperatures. The optical and electrical properties of the 1D nanostructures coated with catalyst can change upon exposure to gas and can be restored upon reexposure to air even at room tempera- ture.13-20 Such low temperature-processes are very desirable in detecting toxic gases safely.
Mgnesium(Mg)-doped Zinc oxide (ZnO) has been studied intensively because of its bandgap engineering effect. It is well known that addition of Mg to ZnO increases band-gap energy (Eg) of ZnO.21 For this reason Mg-doped ZnO 1D nanostructures also have been synthesized by using various techniques such as thermal evaporation,22 wet chemical method,23 and ion-beam sputtering technique on Mg-doped ZnO thin films,24 metal-organic chemical vapor deposition (MOCVD).25 In this paper, we report the synthesis of Mg- doped ZnO nanowires using a simple thermal evaporation technique, the structure of the Mg-doped ZnO nanowires,
and the enhanced sensing properties of the Mg-doped ZnO nanowires functionalized with Pt in detecting CO gas at 100
oC. Since CO is colorless, odorless, and tasteless despite being highly toxic, development of a sensitive CO gas sensor is essential to protect humans exposed to CO gas.
Experimental
Two types of Mg-doped ZnOnanowires with different compositions were synthesized by using an evaporation technique. Au-coated Si was used as a substrate for the synthesis of 1D Mg-doped ZnO nanostructures. Au was deposited on a (100) Si substrate by direct current (dc) sputtering. A quartz tube was mounted horizontally inside a tube furnace. A mixture of 99.99% pure ZnO (300 mg), 99.99% graphite (300 mg), and MgB2 (30 mg for sample 1, 60 mg for sample 2) powders were used as a source material for evaporation. The temperature was maintained at 950 oC for 20 min in an N2/O2 atmosphere with constant flow rates of oxygen (O2) (25 sccm) and N2 (135 sccm). The total pre- ssure was set to 0.95 Torr. Next, a thin Pt film was deposited onto the surfaces of some of the Mg-doped ZnOnanowire sample 1 using a direct current (dc) sputtering technique (substrate temperature: room temperature, power: 20 mA, working pressure: 1.9 × 10−2 Torr, and process time: 180 sec). Subsequently, the Pt-coated nanowires were annealed at 800 °C for 30 min in an Ar atmosphere. The Ar gas flow rate and process pressure were 100 standard cubic centi- meters per minute (sccm) and 0.8 Torr, respectively. The collected nanowire samples were characterized by scanning electron microscopy (SEM, Hitachi S-4200), transmission electron microscopy (TEM, Philips CM-200) equipped with an energy dispersive X-ray spectrometer (EDXS).
Three different kinds of samples, i.e. as-grown bare (with
no Pt-functionalization) Mg-doped ZnO nanowire samples 1 and 2 and Pt-functionalized Mg-doped ZnO nanowire sample 1 were dispersed in a mixture of deionized water (5 mL) and isopropyl alcohol (5 mL) by ultrasonication. A SiO2 film 200 nm thick was grown thermally on the single crystalline Si (100). A slurry droplet containing Mg-doped ZnO nanowires (10 µL) was dropped onto the SiO2-coated Si substrates equipped with a pair of interdigitated (IDE) Ni (~200 nm)/Au (~50 nm) electrodes with a gap of 20 μm.
The gas sensing properties of the as-synthesized and Pt- functionalized Mg-doped ZnO nanowires were measured at 100 °C in a quartz tube inserted in an electric furnace. A given amount of CO (> 99.99%) gas was injected into the testing tube through a microsyringe to obtain a CO concentration of 10, 25, 50, or 100 ppm while the electrical resistance of the nanowires was monitored. The electrical resistance of gas sensors was determined by measuring the electric current that flowed when a potential difference of 0.5 V was applied between the IDE Ni/Au electrodes. The response of the n-type ZnMgO nanowire sensors was defined as (Ra-Rg)/Rg for a reducing gas CO, where Ra and Rg are the electrical resistances of sensors in air and target gas, respectively. The response time was defined as the time required for the variation in electrical resistance to reach 90% of the equilibrium value after the gas was injected, and the recovery time was defined as the time needed for the sensor to return to 90% above the original resistance in air after the gas was removed.
Results and Discussion
A typical SEM image of the Mg-doped ZnO 1D nano- structures (sample 1) synthesized on the Si (100) substrate using a thermal evaporation technique shows that the diameters of the 1D nanostructures ranged from 80 to 120 nm and that the lengths were up to a few tens of micrometers (Fig. 1(a)). Both of the two EDX spectra (Figs. 1(b) and (c)) taken from Pt-functionalized Mg-doped ZnO nanowire samples 1 and 2 exhibit Zn, Mg, and O peaks. The concen- tration of Mg was far lower than those of Zn and O in both types of nanowire samples. The EDX measurements clearly showed that sample 2 has a higher Mg concentration than sample 1 due to the larger amount of Mg in the source material for sample 2. The XRD pattern of the Pt-function- alized Mg-doped ZnO nanowires (sample1) is displayed in Figure 2. The main diffraction peaks in the pattern of the as- synthesized nanowires (Fig. 2) can be indexed to a hexa- gonal wurtzite-structured crystalline ZnO with lattice con- stants a = 0.3249 nm and c = 0.5206 (JCPDS No. 36-1451), indicating that the nanomaterial mainly comprises a ZnO phase. Besides the reflections from ZnO, one from Pt was also observed. According to the XRD raw data in this study, the ZnO (002) peak position is 35.36o which agrees well with the previous report that the ZnO (002) peak position is shifted from 34.40o to 35.40o with increasing the Mg concentration in Mg-doped ZnO nanostructures. The shift of the ZnO peak position to a larger Bragg angle implies the
existence of the lattice shrinkage in the c-axis orientation.26 The low-magnification TEM image (Fig. 3(a)) shows that Pt nanoparticles with diameters in a range from 30 to 80 nm are distributed at the surface of a Mg-doped ZnO nanowire. The HRTEM image (Fig. 3(b)) exhibits a fringe pattern clearly, indicating that the Mg-doped ZnO nanowires are poly- crystalline. The resolved distance between two neighboring parallel fringes in the Mg-doped ZnOnanowire region was Figure 1. (a) SEM image of Pt-functionalized Mg-doped ZnO nanowires (Mg: 1.44 at%) (Sample 1). Inset, enlarged SEM image of a typical Pt-functionalized Mg-doped ZnO nanowire. (b) EDX spectrum of Pt-functionalized Mg-doped ZnOnanowires (Mg:
1.44 at%). (c) EDX spectrum of bare-functionalized Mg-doped ZnOnanowires (Mg: 4.31 at%) (Sample 2).
0.28 nm, which is in good agreement with the interplanar spacing of the (100) planes in ZnO.The reflection spots on concentric circles in corresponding SAED pattern (Fig. 3(c)) reveals that the nanowire is not monocrystalline but poly- crystalline. Dim reflection spots from Pt are also observed on the concentric circles in the SAED pattern in addition to the strong reflection spots from ZnO. The dim spotty pattern Figure 2. XRD pattern of Pt-functionalized Mg-doped ZnO nanowires (Mg: 1.44 at%).
Figure 3. (a) Low-magnification TEM image of a typical of Pt- functionalized Mg-doped ZnO nanowire (Mg: 1.44 at%). (b) Local HRTEM image of the nanostructure at the interface region of a Mg-doped ZnO nanowire (Mg: 1.44 at%) and a Pt nanoparticle.
(c) SAED pattern of the nanomaterial at the same region as in (b).
Figure 4. Electrical responses of the gas sensors to 10, 25, 50, and 100 ppm CO gas at 100 oC fabricated from bare- and Pt-functionalized Mg-doped ZnOnanowires (Mg: 1.44 at%) (Sample 1): (a) Dynamic response curve and (b) enlarged part of the response curve to 100 ppm CO gas of pure Mg-doped ZnO nanowires. (c) Dynamic response curve and (d) enlarged part of the response curve to 100 ppm CO gas of Pt-functionalized Mg-doped ZnOnanowires.
on concentric circles indicates that Pt has a polycrystalline face-centered cubic structure with a lattice constant a = 0.3923 nm. The resolved distance between two neighboring parallel fringes in the Pt nanoparticle region was 0.23 nm, which is in good agreement with the interplanar spacing of the (111) planes in bulk Pt.
The CO gas sensing properties of bare Mg-doped ZnO nanowire sensors (Samples 1 and 2) and Pt-functionalized Mg-doped ZnOnanowire sensors (Sample 1) were examined at 100oC. The curves in Figs. 4(a) and 4(c) show the sensing characteristics of bare Mg-doped ZnOnanowires and Pt- coated Mg-doped ZnOnanowires, respectively, to a typical reducing gas CO with concentrations of 10, 25, 50, and 100 ppm. The resistance decreased upon exposure to CO and recovered completely to the initial value upon removal of CO. Figures 4(b) and 4(d), respectively, show the enlarged parts of the data in Figures 4(a) and 4(c) measured at a CO concentration of 100 ppm both for bare Mg-doped ZnO nanowires and Pt-functionalized Mg-doped ZnOnanowires to reveal the moments of gas input and gas stop. The response to CO was greatly enhanced by Pt functionalization (Table 1). Bare Mg-doped ZnOnanowires showed responses of 4.62, 4.62, 4.95, and 4.63% at CO concentrations of 10, 25, 50, and 100 ppm, respectively. In contrast, Pt- functionalized Mg-doped ZnOnanowires showed responses of 325.24, 319.68, 550.05, and 374.30% at corresponding concentrations, respectively. Thus, the responses of the nanowires were improved by approximately 70, 69, 111, and 81 times at CO concentrations of 10, 25, 50, and 100 ppm, respectively. On the other hand, it appears in Table 1 that both the response and recovery times of the nanowire sensor for CO gas sensing were not nearly changed by Pt function- alization.
Figure 5 shows the sensing characteristics of bare Mg- doped ZnOnanowires to 10, 25, 50, and 100 ppm CO gas.
As can be seen in Table 1, the bare Mg-doped ZnOnano- wires (Mg: 1.44 at%) was almost equal to that of bare Mg- doped ZnOnanowires (Mg: 4.31 at%) in response at the same CO concentration, but the former was somewhat shorter than the latter in response and recovery times at the same CO concentration. This result suggests that the Mg doping concentration does not influence the sensing properties of ZnO nanowires as strongly as Pt-functionalization.
The CO gas sensing mechanism of Pt-functionalized Mg- doped ZnO nanowires can be described based on the models proposed for the metal catalyst-enhanced gas sensing of
nanomaterials previously.27-29 Mg-doped ZnO nanowires have relatively high electrical resistance at room temperature because Mg-doped ZnO is a semiconducting material with an energy band gap of 2.7 eV. At high temperatures reactive oxygen species such as O−, O2−, and O2− are chemosorbed by Mg-doped ZnO and electrons transfer as a result of the chemisorptions whereas no such chemisorption and the transfer of electrons occur at room temperature. In the case of Pt-functionalized Mg-doped ZnO nanowires, the adsorp- tion of reactive oxygen species is possible even at room temperature. Chemisorption of reactive oxygen species occurs on the Pt nanoparticle surface owing to the high conductive nature of Pt. The Pt nanoparticles spill the oxygen species over the Mg-doped ZnO nanowire surface, which is well known as the spill-over effect.30,31 Upon exposure of the Pt-functionalized Mg-doped ZnO nanowires to CO gas, CO molecules are adsorbed by the surface of the Pt nanoparticles on the nanowires. The electrons in the conduction band can be trapped by the oxygen species, resulting in an electron depletion layer on the surface of Mg- doped ZnO nanowire making the material highly resistive.
The adsorbed CO molecules react with the preadsorbed oxygen species as in the following equations.32:
CO(g) + O− (ad) → CO2(g) + e− (1) CO + 2O−→ CO32− → CO2 + 1/2O2 + 2e− (2) Table 1. CO gas sensing responses of bare and Pt-functionalized Mg-doped ZnOnanowires
CO Conc.
Response (%) Response time (sec) Recovery time (sec)
ZnO (Mg:
1.44 at%)
ZnO (Mg:
4.31 at%)
Pt-ZnO (Mg:
1.44 at%)
ZnO (Mg:
1.44 at%)
ZnO (Mg:
4.31 at%)
Pt-ZnO (Mg:
1.44 at%)
ZnO (Mg:
1.44 at%)
ZnO (Mg:
4.31 at%)
Pt-ZnO (Mg:
1.44 at%)
100 ppm 4.63 4.65 374.30 530 630 560 620 620 590
50 ppm 4.95 4.80 550.05 600 670 610 440 650 570
25 ppm 4.62 4.63 319.68 630 700 635 460 690 580
10 ppm 4.62 4.65 325.24 590 750 610 450 710 500
Figure 5. Electrical responses of the gas sensors fabricated Pt- functionalized Mg-doped ZnOnanowires (Mg: 4.31 at%) (Sample 2) to 10, 25, 50, and 100 ppm CO gas at 100 oC.
In the process of surface sensing reactions, the electrons trapped by the surface oxygen species will be fed back into the electron depletion layer, which will increase the con- ductivity or decrease the electrical resistance of Mg-doped ZnO. The enhanced chemisorption of oxygen species by oxygen spillover effect will promote the response of the sensor to the target gases, leading to the enhanced sensing properties of the nanowire sensor.
Conclusions
The morphology, crystal structure, and enhanced sensing characteristics of the Mg-doped ZnOnanostructures func- tionalized with Pt to CO gas at 100 oC were examined. The nanostructures of the Mg-doped ZnO 1D nanostrutures synthesized using an evaporation technique were of a wire- like morphology with diameters in a range from 80 to 120 nm and lengths up to a few tens of micrometers. The dia- meters of the Pt nanoparticles on the nanowires ranged from 30-80 nm. The gas sensors fabricated from multiple net- worked Mg-doped ZnOnanowires functionalized with Pt showed enhanced electrical responses to CO gas at 100 oC.
The responses of the nanowires were improved by approxi- mately 70, 69, 111, and 81 times at CO concentrations of 10, 25, 50, and 100 ppm, respectively. Both the response and recovery times of the nanowire sensor for CO gas sensing were not nearly changed by Pt functionalization. The en- hanced electrical response of the Pt-functionalized Mg- doped ZnO nanowire sensor to CO gas is due to the combi- nation of the spillover effect and the enhancement of chemisorption and dissociation of gas.
Acknowledgments. This study was supported by the 2010 Core Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.
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