Nanohybrid TiO2/carbon black sensor for NO2 gas

Availableonlineatwww.sciencedirect.come Science directChinaParticuologyELSEVIERChina Particuology 5(2007)225-229www.elseviercom/locate/cNanohybrid T1O /carbon black sensor for NO2 gasWei-Jen Liou, Hong-Ming LinDepartment of Materials Engineering, Tatung University, Taipei 104, Taiwan, ChinaReceived 15 September 2006; accepted 2 March 2007Abstracta nanohybrid sensor of nanosized TiO2-coated carbon black particles, prepared by sol-gel technology for the detection of NO2 gas, has beendeveloped. The response of the electric resistance of the hybrid sensor to NOz concentration is investigated, showing that the sensitivity of thehybrid sensor is raised as certain ratio of the TiO2 content in the sensor. Easy and cheap to fabricate, the hybrid TiO /carbon black promises to bectical sensor for detecting@2007 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B VAll rights reserved.Keywords: Carbon black: TiOz; Gas sensing; Hybrid1. Introductionlarge number of sites for reaction; ( ii)their fabrication is sim-ple and low-cost; (ii)they can be made into different types ofEnvironmental monitoring of industrial gaseous pollutants, sensors because of ready variation of their propertiessuch as CO and NO, must be observed for theireffective controlIn this study, TiO2 is introduced into the carbon black sensorNanosized materials, owing to their immense surface areas, have in order to prevent the oxidation of carbon black and improve thebeen widely used for making new gas sensors. Carbon black, the sensitivity of the hybrid sensor Nanohybrid TiO2-coated carboncheapest and earliest nanosized material used by man in rubber, black can be prepared by various methods, including physicalplastics, dielectric materials, printing ink, paint and batteries, evaporation, metallo-organic decomposition(MOD)(Wei et alcan be made into composites which have recently attracted wide 2004), and sol-gel method ( SG)(Liou, Lin, Yang, Lin, 2006)interest because of their sensing properties( Dong, Fu, Zhang, The present study uses the sol-gel method to coat TiOz on theZhang, Rong, 2004; Sotzing, Phend, Grubbs, Lewis, 2000; surface of carbon blackSwann, Lidle, Cui, Barker, Cooper, 1998). Besides their usemperature, pressure and pH sensors( Chen, Hu, Zhang,rong, 2005; Job, Oliveira, Alves, Giacometti, mattoso,2. Experimental2003: Leventis et al., 2004), carbon black composites can alsope used as chemical sensors and vapor detectors because their 2.1. Sol-gel preparationelectrical resistance changes when exposed to gases or vaporsNanohybrid TiO2/carbon black materials are synthe-(Chen, Iwata, Tsubokawa, Maekawa, Yoshida, 2002; ChenTsubokawa, 2001; Covington, Gardner, Briand, de rooij,sized via a sol-gel process. Titanium tetra-isopropoxide[(CH3)2CHOJ4Ti, solution is used as the precursor for the2001; Tsubokawa, Tsuchida, Chen, Nakazawa, 2001). In that sol-gel synthesis of TiO2 sol. Carbon black, bought from ECHOsense, carbon black composites can serve as electronic noses NANO-BIO Co, Ltd, Taipei, is about 30 nm in diameter(Doleman& Lewis, 2001; Sotzing, Briglin, Grubbs,& Lewis, 273/L in density, 10.4 MPa in tensile strength, and 59 Shore2000;zee&Judy,2001)A Hardness(ASTM). Titanium tetra-isopropoxide(TIP)andCarbon black composite sensors possess certain special fea-tures:(i) their nanoscale size with large surface areas provide propyl alcohol are obtained from Wako Pure Chemical Indus-tresWako first grade and JIS special grade中国煤化工 ide of02,0.8,and32gCorresponding author. TeL. +886 2 2586; fax: +8862 2593 6897is dilE-mail address: hmlin @ttu.edu. tw(HM. LindenotMHGOL,(CH3)2CHOH, and is-Mr--tively. The details of the1672-2515/S-see front matter 0 2007 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserveddoi:10.l016 j cpart2007.03.005W-J. Liou, H.M. Lin/China Particuology 5(2007)225-229Table 1Concentrations of titanium tetra-isopropoxide and carbon black in isopropyalcohol solutions used to prepare carbon black and Hybrid 1, 2, and 3 sampleMass FlowTitaniumCarbon black IsopropylController能EE:488ktetra-isopropoxide(g) (g)alcohol (mL)合Hybrid 23.20.2Heaterprepared concentrations are shown in Table 1. ThesoF-gel stion is subsequently peptized by vigorous stirring. A preliminaryFig. 2. Apparatus for gas sensing measurementssolution of carbon black (0.2 g)diluted in 50 mL isopropyl alcohol(IPA)is added to the sol-gel solution during its preparation. and to record the data. The electrical properties of the sen-This produces a uniform mixture of carbon black in the sol-gel sors are measured at different temperatures(50, 100, 150 andsolution. Deionized water is added to accelerate the sol-gel 200 C)and under various concentrations of NO2 gas(from 1 toreaction100 ppm of NO2 in pure air). The sensing properties of hybridThe gel is stirred to turn it into TiO /carbon black nanoballs of TiO /carbon black are measured by the changes of the electricalgray color. The gray TiO2/carbon black nanoballs are collected resistances at different temperaturesand calcined in a furnace under argon atmosphere. The furnacetemperature is first raised slowly to 600C which is maintained 3. Results and discussionfor 1.5 h for TiO to form the anatase phase3.1. Structure analysis2.2. Electrical characterizationig. 3(a) is the XRD pattern of carbon black used in thisHybrid TiO /carbon black nanoballs are mixed in alcohol study, and Fig 3(b) -(d) show the XRD patterns of Hybrid 1, 2,nd directly coated onto the surface of Au electrodes which are and 3 samples, respectively, revealing both the carbon black andprinted on alumina substrates. Fig. I shows the top and sec- TiO2 anatase phases. Fig 3(e)shows the XRD spectrum of theof theas sensorThe sensing element TiO2 powders prepared by the sathe same sol-gel process indicatiis heated up to 250C for 2 days to stabilize the structure of the the anatase phase of TiO2 through comparison with its standardnanohybrid sensing film. The sample is examined in a chamber pattern. These results indicate that the surface of carbon blackwith flowing NO2 gas. a mass flow controller controls the NO2 is covered by the anatase structure of TiO2gas at a flow rate of 300 mL per minute and maintains a dynamicig. 4(a)shows the Raman spectrum of pure carbon blackequilibrium at 1 atmafter heat treatment at 600C for 1.5 h in argon. Raman spec-The detecting system shown in Fig. 2 is linked to a com- trum of nano-TiO2 powder is also shown in Fig. 4(e)forputer with an IEEE-488 interface to adjust the system parameters comparison. The peaks at 394 and 630 cm are characteristic spectra of TiO2. Raman spectra(b), (c), and( d)in Fig 4 forHybrid 1, 2 and 3 samples, respectively, indicate that the two10mm合010(a)carbon blackAL O3substrate(c)Hybrid 2(d)Hybrid 31011124105;211(o2中国煤化工sensingAuPt wireCNMHG 60 70 80materiaelectrode2 thetaFig 3. XRD patterns of (a)carbon black, (b)Hybrid 1, (c)Hybrid 2,(d)HybridFig. 1. Top and sectional views of a gas sensor3, and(e)pure TiO(anatase phase)W.-J. Liou, H.-M. Lin/China Particuology 5(2007)225-229sities of the Raman spectra for carbon black are enhanced bythe TiOz contents as shown in Hybrids 1, 2 and 3, possiblyD mode G modecaused by the resonance Raman effect between TiOz and car-(a) carbon blackbon black that enhances the intensities of the d and g modesof carbon black. The Raman scattering spectra of TiO2/carbonblack illustrate that TiOz is formed in hybrid systems butit does not induce bond angle disorder, implying that TiOHybrid 2nanoparticles have stable crystal structure in hybrid with carbon(d)Hybrid 33. 2. Morphological analysis(e)TioIn Fig. 5(a), the HRTEM micrograph of pure carbon black150020002500Raman Shift(cmrtshows that commercial carbon black is nanosized in geometryand clear on the surface. Fig. 5(b)(d)shows respectively theFig 4. Raman spectra of(a)carbon black, (b)Hybrid I, (c)Hybrid 2, (d) Hybrid HRTEM images of the Hybrid 1, 2, and 3 fabricated by the3, and(e)pure TiO(anatase phsol-gel method. In Fig. 5(b), it is difficult to find TiO2 particleson the carbon black surface but the size of the nanoballs isweak Raman peaks at about 391 and 627 cm-l are the TiOz somewhat larger than that of pure carbon black. The HRTEManatase phase. Compared to Fig. 4(a), the Raman-active peaks results indicate that nanosized TiO2 particles can be coatedb)(d) are con- the surfaces of carbon black by the sol-gel method. Furthermore,at 1346(D mode)and 1573( G mode)in wt the structures of as the ratio of TIP to carbon black increases during fabrication,tributed by the carbon black, indicating thatcarbon black are still stable after calcination at 600C in argon. the amount and size of TiO2 increase on the surfaces of carbonFurthermore, an interesting phenomenon indicates that the inten- black, as shown in Fig. 5(c)and(d)40 nm40 nm中国煤化工40 nmCNMHGFig. 5. HRTEM micrographs of(a)carbon black, (b) Hybrid 1, (c)Hybrid 2, and (d) Hybrid 3, after sintering at 600 C for 1.5h.W.J. Liou, H.-M. Lin/China Particuology 5(2007)225-229ing through the testing chamber for the examination is shown inFig 6, which can be regarded to be composed of a succession offinite pulses. Each pulse with progressively increased magnitudeis held for 200 s and followed by 200 s of zero magnitude, corre-50ponding to pure air, in order to observe the recovery conditionsnal changes of sensors, defined as(Ro-Rgas)/Ro)X 100%,in1234567汨joresponse to NO2 concentration variation for carbon black andnanohybrids. When the sensitivity increases upon introducing00000000000000the NOz gas, the sensor demonstrates the p-type sensing behavior, and otherwise the n-type sensing behavior. The results in800 1600 2400 3200 4000 4800 5600 6400 7200 8000 Fig. 7(a)show at 50C that the sensitivities of Hybrids I and 2Time(s)consisting of TiO2 are higher than that of pure carbon black atFig. 6. NO2 concentrations in detecting system for gas sensing.50C. Furthermore, the positive drifts of signal baselines existobviously for all sensors. It results from the absorption of Noon the sensing materials, and the operating temperature is not3.3. Electrical propertieshigh enough for NO to be desorbed from the surfaces of thematerials during the recovery period. According to the results,The electrical properties for carbon black, Hybrid 1, 2 and 3 the recovery conditions of all sensors are not ideal at 50"C, andsensors were examined by exposing the sensing element to atmo- Hybrid 3 has the least drift of baseline, which is covered by mostspheres of various NO2 concentrations in the testing chamber at amount of TiO2different temperatures and measuring the corresponding sensorSimilar sensitivities at 100 C are shown in Fig. 7(b), indioutputs. The NO2 concentration variation of the gas stream flow- cating the same tendency as at 50.C, that is, the sensitivities(b)fa)carbon back-.-(a) carbon black5050"(c) Hybrid 2(c) Hybrid(d) Hybrid 3(d) Hybrid 3202800160024003200400048005600640072008000800160024003200400048005600640072008000Time(s)Time(s)(a) carbon black-.-fa)carbon black-(b) Hybrid 1(b)Hybrid 1. -(c)Hybrid 2(d) Hybrid 3(d)Hybrid 3AA>c0246中国煤化工CNMHG800160024003200400048005600640072008000800160024003200400048005600640072008000ig. 7. Sensitivities of carbon black and nanohybrids under various NO2 concentrations at different temperatures: (a)50,(b)100, (c)150, and(d)200C.W-J Liou, H.-M. Lin/China Particuology 5(2007)225-229of sensors are increased after hybridization of nano-TiO2. The Acknowledgementdrifts of signal baselines are shown to reduce obviously withthe increase of operating temperature, possibly because of The authors appreciate the grant from the National Sciencedecrease of residual NO2 absorbed on the surface of the materials Council for supporting this researchle to more desorptionprovided by heatingever, the states of sensors have not returned to the original Referencescondition for introducing pure air into the detecting chamber.Lo, Harris, J, McConigly, B, Cunniffeof the electrical behaviour of surfactantThe sensitivities in response to NO2 concentration variationtreated polymer/caosite gas sensors. Composites Part A, 36,487491.for carbon black and hybrid sensors at 150 and 200"C are shown Chen, I, Tsubokawa, N.(2001). A novel gas sensor from polymerin Fig. 7(c)and(d), respectively. Operating at 150C, the drifts grafted carbon black: Effects of polymer, crystalline organic compound,of signal baselines for carbon black, Hybrids 2 and 3 becomend carbon black on clectric response to tetrahydrofuran vapor. Jour-much smaller, implying that the recovery properties of sensorsnal of Macromolecular Science Pure and Applied Chemistry, 38, 383-at 150"C are better than those at 50 and 100"C. Particularly, Chen. S.G. Hu, J W. Zhang, M.Q. Rong M.Z(2005). Effects of tem.the signal baseline of Hybrid 3 keeps almost at zero within the perature and vapor pressure on the gas sensing behavior of carbon blackhole span from 1 to 100 ppm NO2 gas. However, afterexposingfilled polyurethane composites. Sensors and Actuators B--Chemical, 105,to NOz gas to higher than 40 ppm at 150.C, the baselines of 187-19carbon black and Hybrid 2 become negative, that means the Chen, J, Iwata, H, Tsubokawa, N, Maekawa, Y, Yoshida, M(2002). Novelresistances of these sensors increase with introducing NO2 gasat this temperature. This result may be caused by the reaction hexane vapor. Polymers, 43, 2201-22a6 ponse of sensor material in cyclo-between carbon black, air and NO2 at 150C at which NOz gas Covington, J.A., Gardner, J W, Briand, D,& de Rooij, N F(2001). A polymerdestroys the structure of carbon black and changes the electronicgate FET sensor array for detecting organic vapours. Sensors and Actuatorsstructure of carbon blackB- Chemical,77,155-162At 200C, as shown in Fig. 7(d), the phenomenon of neg-Doleman, B J,& Lewis, N S (2001). Comparison of odor detection thresholdsand odor discriminablities of a conducting polymer composite clectronicative sensitivities becomes obvious, first appearing under nOnose versus mammalian olfaction. Sensors and Actuators B--Chemical, 72concentrations higher than 10 ppm for carbon black, Hybrid 141-50and 2. However, this is not the case for Hybrid 3 sensor, possibly Dong, X. M, Fu, R. W, Zhang, M.Q. Zhang, B.& Rong. M. Z(2004)due to the protection provided by TiO2 coating on the carbonblack surfaces. This coating of TiO2 also enhances the sensingcomposite sensors to organic vapors at low vapor concentrations, Carbon42,2551-259properties in detecting NOJob, A. E, Oliveira, F. A, Alves, N, Giacometti, J. A,& Mattoso, L. HThe electrical properties of carbon black and the hybrids indiC.(2003). Conductive composites of natural rubber and carbon black forcate that (i)the sensitivities of the sensing materials decreasepressure sensors. Synthetic Metals, 135/136, 99-100temperature increases; (i)the sensitivities of the sensing mate- Leventis, H C Streeter, I, Wildgoose, GG, Lawrence, N.S. Jiang. L, Jones,rials can be increased by adding nano-TiO2; (iii) recoverysensors. Talanta. 63. 1039-1051improved by increasing the operating temperature; (iv)without Liou, w.J., Lin, H. M, Yang, T. Y, Lin, K. N(2006). Hybrid MOS/CNTsadequate surface protection, the structures of sensing materialsmaterials for gas sensing. Solid State Phenomena, 111, 19-24will be damaged at high temperature. Summarizing the electrical Sotzing, G. A, Briglin, S. M, Grubbs, R. H,& Lewis, N. S(2000)measurements of the sensing materials studied, including sensi-tivities, recovery properties, and energy efficiency, the optimalpoly (3, 4-ethylenedioxy )thiophene-poly(styrene sulfonate) insulating poly-parameters appear to belong to Hybrid 3, with a weight ratio of Sotzing, GA, Phend, N. Grubbs, R. H,& Lewis, N S(2000). HighlyTIP to carbon black of 16: I and operating at a temperature ofsensitive detection and discrimination of biogenic amines utilizing arrays of150°C.polyaniline/carbon black composite vapor detectors. Chemistry of Materials,12,593-595Swann, M.J., Glidle, A, Cui, L, Barker, J. R,& Cooper, J. M. (1998). Theoncon of gaseous molecular density using a hybrid vapour sensor.Chemical Communications. 24. 2753-2754By using the sol-gel technology, successful coating of TiO2 Tsubokawa, N,Tsuchida,M, Chen,I,&Nakazawa, Y(2001).A novel con-on carbon black has been accomplished to make a new type of tamination sensor in solution: The response of the electric esistance of aNO2 gas sensor. The amount of TiOz on carbon black surfacecan be controlled by adjusting the amount of the precursor ofWei, B-Y Hsu, M.-C, Su, P.-G, Lin, H.-M, Wu, R.-J,& Lai, H.-J.the coating materials during preparation. This study also demon-(2ith carbon nanotubes operastrates that temperature can affect the sensitivities and recovery中国煤化工 gators B--Chemica,101,8lconditions of sensors. The new hybrid gas sensor shows adequate zee, E. L array. Sensors and Actuators B-Chemical, 72, 120-129mical gassensing properties of detecting NO2 gas and practical applicationCNMHG polymer-based cheron account of its easy and cheap fabrication