UV-Enhanced Acetone Gas Sensing of Co 3 O 4 -Decorated ZnS Nanorod Gas Sensors

Transcription

1 Electron. Mater. Lett., Vol. 11, No. 4 (2015), pp DOI: /s UV-Enhanced Acetone Gas Sensing of Co 3 O 4 -Decorated ZnS Nanorod Gas Sensors Sunghoon Park, 1 Gun-Joo Sun, 1 Soohyun Kim, 1 Sangmin Lee, 2 and Chongmu Lee 1, * 1 Department of Materials Science and Engineering, Inha University, Incheon , Korea 2 Department of Mechanical Engineering, Inha University, Incheon , Korea (received date: 29 January 2015 / accepted date: 25 February 2015 / published date: 10 July 2015) Co 3 O 4 -decorated ZnS nanorods were synthesized by the thermal evaporation of ZnS powders followed by a sol-gel process for Co 3 O 4 -decoration. The acetone gas sensing properties of multiple-networked pristine and Co 3 O 4 - decorated ZnS nanorod sensors were examined. The diameters of the Co 3 O 4 nanoparticles ranged from 4 to 20 nm. The multiple networked pristine ZnS nanorods and Co 3 O 4 - decorated ZnS nanorod sensors showed responses of % and 198-1,650% to ppm of acetone at room temperature under UV illumination at 2.2 mw/cm 2, respectively. The response and recovery times of the ZnS nanorod sensor at 500 ppm of acetone was reduced from 52 s to 26 s and from 192 s to 110 s, respectively, by Co 3 O 4 -decoration. The responses of the sensors exhibited strong dependence on the UV illumination intensity. The responses of the pristine ZnS nanorod and Co 3 O 4 -decorated ZnS nanorod sensors to 500 ppm of acetone at room temperature increased from 112 to 364% and from 132 to 1650%, respectively. This paper discusses the underlying mechanisms of the enhanced response of the ZnS nanorod sensor to acetone gas by Co 3 O 4 - decoration and UV irradiation. Keywords: ZnS, nanorod, sensor, Co 3 O 4, UV 1. INTRODUCTION Zinc sulfide (ZnS) is an important 2-6 compound semiconductor with a wide range of applications, such as shortwavelength light-emitting diodes (LEDs), lasers, flat panel displays, infrared windows, and biodevices. [1] In addition, ZnS can be applied to gas and ultraviolet (UV) light sensing. A range of ZnS nanostructures, such as ZnS nanowires, individual ZnS nanobelts, ZnS microspheres, and ZnS nanotube arrays, have been assessed for sensing hydrogen, oxygen, acetone, ethanol, humidity, etc. [2-6] On the other hand, ZnS has seldom been used in gas sensors despite exhibiting comparable sensing properties to oxide semiconductors. *Corresponding author: cmlee@inha.ac.kr KIM and Springer One-dimensional (1D) nanostructures are expected to have considerably enhanced gas sensing performance owing to their ultrahigh surface-to-volume ratios and a Debye length comparable to their dimensions, which makes their electrical properties extremely sensitive to surface-adsorbed species. [7,8] The small cross-section of the 1D nanostructure sensor can lead to the complete depletion or accumulation of charge carriers in the bulk of the 1D nanostructure, resulting in a stronger response to the target gas. Moreover, the distance between the tunable conducting channel along the axis of the 1D nanostructure and the surface of the 1D nanostructure in contact with the gas is very small, leading to a short response time upon exposure to the target gas, as well as a significant increase in the sensitivity of the device due to the high surface-to-volume ratio of 1D nanostructures. [9] A variety of techniques have been used to synthesize ZnS 1D

2 S. Park et al. 573 nanostructures including thermal evaporation, chemical vapor deposition, solvothermal process, electrochemical deposition, and template-assisted process, [10-15] etc. Of these methods, thermal evaporation might be the most attractive technique for the synthesis of ZnS nanostructures because of its simplicity, and various nanostructures can be synthesized at low substrate temperatures compared to other techniques. Over the past several decades, metal oxide semiconductorbased gas sensors have been studied extensively because of their high sensitivity. One of the limitations of metal oxide semiconductor-based gas sensors is the high operating temperature. Several techniques to enhance the sensing performance of 1D nanostructured metal oxide semiconductorbased gas sensors, such as surface functionalization or doping, [16-22] heterostructure formation, [23-28] and UV light irradiation [29,30] have been studied extensively. On the other hand, lowering their operating temperature and enhancing their sensing performance further is still a challenge. The combined effects of two other techniques on the sensing properties of 1D nanostructure sensors has attracted little attention. In this study, multiple-networked Co 3 O 4 -decorated ZnS nanorod sensors were fabricated and their acetone (CH 3 COCH 3 ) gas sensing properties under UV illumination were examined to observe the combinational effects of Co 3 O 4 -decoration and UV irradiation on the gas sensing properties of ZnS 1D nanostructures. The development of a sensor for detecting acetone gas with high sensitivity is required because acetone is a good breath marker for the non-invasive diagnosis of diabetes. [31,32] Over the past decade, heterostructure formation techniques to enhance the sensing performance of MOS sensors have been intensively studied, but there are few reports of heterostructure formation by Co 3 O 4 decoration. Recently, several study results on the enhanced sensing properties of gas sensors decorated with Co 3 O 4 have been published. [33,34] 2. EXPERIMENTAL PROCEDURE Co 3 O 4 -decorated ZnS nanorods were synthesized using a two-step process: thermal evaporation of ZnS powders and sol-gel process. First, Au-coated sapphire was used as a substrate for the synthesis of ZnS nanorods. Au was deposited on a silicon (100) substrate by direct current (dc) magnetron sputtering. A quartz tube was mounted horizontally inside a tube furnace. An alumina boat containing 99.99% pure ZnS powders and silicon substrates were placed separately in a two-heating zone-tube furnace, where the ZnS powders and Si substrates were in the first and second heating zones, respectively. The substrate temperatures of the first and second heating zones were set to 850 C and 700 C, respectively, with an ambient nitrogen gas pressure and flow rate maintained at 1 Torr and 50 cm 3 /min, respectively, throughout the synthesis process. The thermal evaporation process was carried out for 1 h and the furnace was then cooled to room temperature at 1 mtorr, after which the products were removed. The Co 3 O 4 precursor solution was prepared by dissolving 0.1 M cobalt acetate tetrahydrate (Co(COOCH 3 ) 2 4H 2 O) in 2-methoxyethanol (2ME) and the solution was then stirred magnetically for 1 h. Subsequently, 10 ml of a 28% ammonia solution was added and the mixed solution was then ultrasonicated for 1 h. To decorate the ZnS nanowires with Co 3 O 4 nanoparticles, the Co 3 O 4 precursor solution was dripped onto the ZnS nanowires on a substrate rotated at 500 rpm for 30 s. After the spin-coating process, the ZnS nanowire sample was dried at 150 C for 1 min and then annealed in air at 500 C for 1 h. The morphology of the collected nanowire samples was examined by scanning electron microscopy (SEM, Hitachi S-4200, 10 kv). The crystal structure of the samples was investigated by glancing angle x-ray diffraction (XRD, Philips X pert MRD) with Cu-K α radiation (λ = nm). The Co 3 O 4 -decorated and undecorated ZnS nanorods (50 mg) were dispersed ultrasonically in ethanol in separate beakers. Two sets of multiple-networked nanowire sensors were fabricated by spreading the suspensions of the nanowires in alcohol (ethanol) over thermally oxidized Si substrates with interdigitated Pt electrodes, as described elsewhere. [33] The gas sensing properties were measured using the flow-through technique in a tube furnace with a resistance heater. The pristine ZnS nanorods or Co 3 O 4 - decorated ZnS nanorods were inserted in the chamber. Acetone gas diluted with dry synthetic air was introduced into the quartz tube at a flow rate of 200 cm 3 /min at room temperature. The sensing tests were carried out at room temperature under 50% RH using a voltamperometric method. The electrical resistance of the gas sensors was determined by measuring the electric current using a Keithley source meter-2612 with a source voltage of 1 V. 3. RESULTS AND DISCUSSION Figure 1 shows SEM images of the Co 3 O 4 -decorated ZnS nanorods synthesized in this study. The nanorods ranged from 100 to 300 nm in diameter and from a few tens to a few hundreds of micrometers in length (Fig. 1(a)). An enlarged SEM image of a typical 1D nanostructure confirmed its rodlike morphology (Fig. 1(b)). Figure 1(c) shows the XRD patterns of the pristine ZnS nanorods and Co 3 O 4 -decorated ZnS nanorods. All XRD peaks for the pristine ZnS nanorods were assigned to a wurtzite-structured ZnS phase (JCPDS card No ). In contrast, several small reflection peaks assigned to a face-centered cubic-structured Co 3 O 4 phase were also observed in the pattern of the Co 3 O 4 -decorated ZnS nanorods. Figures 2(a) and 2(b) show the sensing transients of the

3 574 S. Park et al. Fig. 1. (a) SEM image of Co 3O 4-decorated ZnS nanorods. (b) Enlarged SEM image of a typical Co 3O 4-decorated ZnS nanorod. (c) XRD pattern of the pristine ZnS nanorods and Co 3O 4-decorated ZnS nanorods. Fig. 2. Sensing transients of (a) the pristine ZnS nanorods and (b) Co 3O 4-decorated ZnS nanorods towards acetone gas at room temperature under UV illumination at 2.2 mw/cm 2.

4 S. Park et al. 575 Fig. 3. (a) Response, (b) response time and (c) recovery time of the pristine ZnS nanorods and (b) Co 3O 4-decorated ZnS nanorods towards acetone gas at room temperature under UV illumination at 2.2 mw/cm 2. Table 1. Responses of the different nanomaterials to acetone gas. Materials T ( C) Acetone concentration (ppm) Response (%) Comment Reference Co 3O 4-decorated ZnS NWs under UV Present work Co 3O 4-decorated ZnS NWs ,650 under UV Present work Y-doped ZnO nanorods [36] SnO 2 nanoflowers [37] ZnO nanomaterials [38] In 2O 3 nanoparticles [39] WO 3 nanocrystals [40] ZnO nanoflowers [41] Ni-doped SnO 2 hollow nanofibers [42] Fe 3O 4-Co 3O 4 core-shell microspheres [43] InN thin films [44] TiO 2 thin films [45] Ce-doped CoFe 2O [46] pristine ZnS nanorods and Co 3 O 4 -decorated ZnS nanorods, respectively, towards acetone gas under UV illumination at 2.2 mw/cm 2 at room temperature. The sensing transients exhibited stable and reproducible response and recovery characteristics. Figure 3(a) shows the responses of multiplenetworked pristine ZnS nanorod and Co 3 O 4 -decorated ZnS nanorod sensors calculated from Figs. 2(a) and 2(b). The response was defined as (R a -R g )/R g for acetone gas, where R a

5 576 S. Park et al. and R g are the electrical resistance of the sensors in air and acetone gas, respectively. The former ranged from 156% to 364% and the latter ranged from 198 to 1,650% to ppm of acetone. Figures 3(b) and 3(c) show that the response time and recovery time of the Co 3 O 4 -decorated ZnS nanorods were almost half of those of the pristine ZnS nanorods, respectively, for the same acetone concentration. In this study, the response time was defined as the time required for a change in the electrical resistance to reach 90% of the equilibrium value after injecting acetone gas, and the recovery time was defined as the time required for the sensor to return to more than 90% of its original resistance in air after removing the acetone gas. The Co 3 O 4 -decoration reduced the response time and recovery times of the ZnS nanorod sensor to 500 ppm of acetone from 52 s to 26 s and 192 s to 110 s, respectively. Table 1 compares the responses of the Co 3 O 4 -decorated ZnS nanorod sensor to acetone measured in this study with those of other nanomaterial sensors in the literature. [36-46] When the sensing test was conducted under UV irradiation, the Co 3 O 4 -decorated ZnS nanorod sensor showed a significantly stronger response to acetone than the other nanomaterials despite being performed at room temperature. Figures 4(a) and 4(b) present the sensing transient curves, showing the strong dependence of the sensitivities of the pristine ZnS nanorod and Co 3 O 4 -decorated ZnS nanorod sensors on the UV illumination intensity. The responses of the two different sensors exhibited a strong dependence on the UV light intensity. Figure 5(a) shows that the responses of the pristine ZnS nanorod and Co 3 O 4 -decorated ZnS nanorod sensors to 500 ppm of acetone at room temperature, which were measured from Figs. 4(a) and 4(b), increased from 112 to 364% and from 132 to 1,650%, respectively, with increasing UV illumination intensity from 0 to 2.2 mw/ cm 2. Figure 5(b) compares the electrical responses of the two sensors to acetone gas with those to other gases. Both the pristine ZnS nanorod and Co 3 O 4 -decorated ZnS nanorod sensors showed a stronger response to acetone gas than the other gases. The latter showed superior selectivity for acetone gas than the former. The reason why the pristine ZnS nanowires are selectively sensitive to acetone gas is not Fig. 4. Sensing transients of (a) the pristine ZnS nanorods and (b) the Co 3O 4-decorated ZnS nanorods to 500 ppm of acetone at room temperature under UV illumination at different intensities. Fig. 5. (a) Responses of the pristine ZnS nanorods and Co 3O 4-decorated ZnS nanorods to 500 ppm of acetone at room temperature as a function of UV illumination intensity (b) Comparison of the response of the pristine ZnS nanorods and Co 3O 4-decorated ZnS nanorods to acetone with those to other volatile organic compond gases.

6 S. Park et al. 577 understood completely. This might be related to the different optimal operating temperatures of the sensor for different target gases. The oxidation rate of a gas might depend on many factors, such as the solid solubility of the gas in the material, the decomposition rate of the adsorbed molecule at the material surface, the charge carrier concentration in the material, the Debye length in the material, the catalytic activity of the material, and the orbital energy of the gas molecule. [47,48] Therefore, the response of a sensor material to a certain gas is determined by these factors, and each gas has a characteristic optimal operating temperature at which its oxidation rate is maximized. The optimal operating temperature of ZnS for the maximal oxidation rate of acetone might be closer to room temperature than that of the other sensor material. The reason why the decoration of Co 3 O 4 nanoparticles is extremely efficient in enhancing the sensor response of acetone and LPG (Liquefied Petroleum Gas) is unclear. The excellent catalytic property of Co 3 O 4 is widely known. One possible reason is the superior catalytic activity of Co 3 O 4 for the oxidation of acetone and LPG compared to that for the oxidation of other gases at room temperature. ZnS is an n-type semiconductor. When a pristine ZnS nanorod is exposed to air at room temperature, it interacts with oxygen by transferring electrons from the conduction band to the adsorbed oxygen atoms, forming ionic species, such as O, O 2 and O 2 depending on the temperature. In particular, among these oxygen species, O 2 forms mainly at a low temperatures, such as room temperature. [49] A depletion layer is created in the surface region of the ZnS nanorod due to the consumption of electrons in the ZnS nanorod. Upon exposure to acetone gas, acetone gas adsorbs on the ZnS nanorod surface, and electrons are released back to the conduction band of ZnS, as shown in the following reactions: CH 3 COCH 3 (gas) + O 2 CH 3 CO + CH 3 O + 2e (1) CH 3 CO CH 3 + CO (2) CO + O 2 CO 2 + 2e (3) The electrons released will decrease the depletion layer width, resulting in a decrease in the resistance of the nanorod sensor. Therefore, the depletion layer width and electrical resistance of the sensor tend to decrease with increasing acetone concentration and UV illumination intensity because of an increase in the number of electrons generated by the above reactions. On the other hand, in the Co 3 O 4 -decorated ZnS nanorod sensor, the n-type ZnS surface and p-type Co 3 O 4 nanoparticle surface respond differently to oxygen and acetone gases. The same reactions occur on the ZnS surface as those on the pristine ZnS nanorod sensor. In contrast, on the Co 3 O 4 nanoparticle surface, upon exposure to air, an accumulation layer forms via the following reaction: 1/2O 2 (g) = O (ad) + h + (4) Subsequently, upon exposure to acetone, the width of the accumulation layer near the Co 3 O 4 nanoparticle surface is reduced by the following reaction between CH 3 COCH 3 and O (ad), resulting in an increase in resistance: CH 3 COCH 3 (gas) + O 2 + 2h + CH 3 CO + CH 3 O (5) CH 3 CO CH 3 + CO (6) CO + O 2 + 2h + CO 2 (7) Overall, p-type Co 3 O 4 nanoparticles always behave in an opposite way to n-type ZnS nanorods, which must have a negative effect on the sensitivity of the nanorod sensor. Nevertheless, the sensitivity of the sensor was enhanced because the positive effect of the Co 3 O 4 nanoparticles expanding the depletion layer on the ZnS side upon exposure to air was more significant than the negative effect. In addition, a potential barrier might form at the Co 3 O 4 -ZnS interface due to carrier trapping at the interface, and modulation of the barrier height might occur during the adsorption and desorption of acetone gas. [24] The potential barrier modulation provides another positive effect on the sensitivity, even though the effect is insignificant compared to that of the Co 3 O 4 nanoparticles expanding the depletion Fig. 6. Energy band diagram of the Co 3O 4-decorated ZnS nanorods in the dark and und under UV illumination showing the depletion layer and potential barrier forming at the Co 3O 4-ZnS junction.

7 578 S. Park et al. layer on the ZnS side. Figure 6 presents the proposed mechanism for the enhanced response of the Co 3 O 4 -decorated ZnS nanorod sensor to acetone gas under UV illumination. The enhanced sensing performance of the Co 3 O 4 -decorated ZnS nanorods to acetone gas compared to that of pristine ZnS nanorods can be explained by expansion of the depletion layer through the formation of p-n junctions near the interface of n-type ZnS and p-type Co 3 O 4. [50] Modulation of the depletion layer width can occur in the Co 3 O 4 -decorated ZnS nanorods accompanying the adsorption and desorption of acetone gas, which results in an increased change in resistance and enhanced sensitivity. Upon exposure to UV light with a photon energy larger than the band gap of ZnS and that of Co 3 O 4, electron-hole pairs will be generated in the ZnS nanorods and Co 3 O 4 nanoparticles, respectively. The change in resistance of the sensor accompanying the adsorption and desorption of acetone gas will be increased under UV illumination because of these extra carriers. 4. CONCLUSIONS The response of the ZnS nanorod to acetone gas at room temperature was enhanced significantly by a combination of Co 3 O 4 -decoration and UV illumination. The response and recovery times of the ZnS nanorod sensor were decreased to a half that of the original ones by the combinational techniques. The substantial enhancement in the response of the Co 3 O 4 -decorated ZnS nanorod sensor under UV illumination might be due to the larger change in resistance caused by the increased number of carriers participating in the reactions with acetone molecules because of the photogenerated electron-hole pairs and the expanded depletion layer. These results show that a synergistic effect on the gas sensing properties of the nanostructure sensors can be achieved by a combination of two different techniques. Overall, this study is expected to contribute to the development of gas sensors with high performance at room temperature. ACKNOWLEDGEMENT This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education ( ). REFERENCES 1. T. Zhai, L. Li, Y. Ma, M. Liao, X. Wang, X. Fang, J. Yao, Y. Bando, and D. Golberg, Chem. Soc. Rev. 40, 2986 (2011). 2. X. S. Fang, T. Y. Zhai, U. K. Gautam, L. Li, L. M. Wu, Y. Bando, and D. Golberg, Prog. Mate. Sci. 56, 175 (2011). 3. Y. G. Liu, P. Feng, X. Y. Xue, S. L. Shi, X. Q. Fu, and C. Wang, Appl. Phys. Lett. 90, (2007). 4. L. Yang, J. Han, T. Luo, M. Li, J. Huang, and F. Meng, Chem. Asian. J. 4, 174 (2009). 5. Z. G. Chen, J. Zou, G. Liu, H. F. Lu, F. Li, and G. Q. Lu, Nanotechnol. 19, (2008). 6. X. Wang, Z. Xie, H. Huang, Z. Liu, D. Chen, and G. Shen, J. Mat. Chem. 22, 6845 (2012). 7. S. Park, S. An, H. Ko, C. Jin, and C. Lee, ACS Appl. Mater. Interfaces 4, 3650 (2012). 8. Y. J. Kwon, H. S. Kim, S. M. Lee, I. J. Chin, T. Y. Seong, W. I. Lee, and C. Lee, Sens. Actuat. B: Chem. 173, 441 (2012). 9. Y. Engel, R. Elnathan, A. Pevzner, G. Davidi, E. Flaxer, and F. Patolsky, Angew. Chem. 49, 6830 (2010). 10. Y. Q. Li, K. Zou, Y. Y. Shan, J. A. Zapien, and S. T. Lee, J. Phys. Chem. B 110, 6759 (2006). 11. T. V. Prevenslik, J. Lumin. 1210, 87 (2000). 12. L. Chai, J. Du, S. Xiong, H. Li, Y. Zhu, and Y. Qian, J. Phys. Chem. C 111, (2007). 13. X. J. Xu, G. T. Fei, W. H. Yu, X. W. Wang, L. Chen, and L. D. Zhang, Nanotechnol. 17, 426 (2006). 14. X. P. Shen, M. Han, J. M. Hong, Z. L. Xue, and Z. Xu, Chem. Vap. Dep. 11, 250 (2005). 15. X. Wang, Z. Xie, H. Huang, Z. Liu, D. Chen, and G. Shen, J. Mater. Chem. 22, 6845 (2012). 16. A. Kolmakov, D. Klenov, Y. Lilach, S. Stemmer, and M. Moskovits, Nano Lett. 5, 667 (2005). 17. H. Kim, C. Jin, S. Park, S. Kim, and C. Lee, Sens. Actuat. B: Chem. 161, 594 (2012). 18. N. Ramgir, I. Mulla, and K. Vijayamohanan, Sens. Actuat. B: Chem. 107, 708 (2005). 19. S. Park, S. An, H. Ko, S. Lee, and C. Lee, Sens. Actuat. B: Chem. 188, 1270 (2013). 20. Q. Wan and T. Wang, Chem. Commun. 1, 3841 (2005). 21. S. Kim, S. Park, S. Park, and C. Lee, Sens. Actuat. B: Chem. 209, 180 (2015). 22. G. D. Khuspe, S. T. Navale, D. K. Bandgar, R. D. Sakhare, M. A. Chougule, and V. B. Patil, Electron. Mater. Lett. 10, 191 (2014). 23. M. Rumyantseva, V. Kovalenko, A. Gaskov, E. Makshina, V. Yuschenko, I. Ivanova, A. Ponzoni, G. Faglia, and E. Comini, Sens. Actuat. B: Chem. 118, 208 (2006). 24. S. Park, H. Ko, S. Kim, and C. Lee, ACS Appl. Mater. Interfaces 6, 9595 (2014). 25. J. Tamaki, K. Shimanoe, Y. Yamada, Y. Yamamoto, N. Miura, and N. Yamazoe, Sens. Actuat. B: Chem. 49, 121 (1998). 26. S. Park, S. Park, S. Lee, H. W. Kim, and C. Lee, Sens. Actuat. B: Chem. 202, 840 (2014). 27. S. Park, S. Kim, S. Park, and C. Lee, RSC Adv. 4, (2014). 28. C.-G. Kuo, L.-R. Hwang, S. Hor, and J.-S. Chen, Electron. Mater. Lett. 9, 481 (2013). 29. E. Comini, A. Cristalli, G. Faglia, and G. Sberveglieri, Sens.

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