ZnO thick films as prepared by flame spray pyrolysis

Sensors and Actuators B 152 (2011) 155–161

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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Acetylene sensor based on Pt/ZnO thick films as prepared by flame spray pyrolysis Nittaya Tamaekong a , Chaikarn Liewhiran b , Anurat Wisitsoraat c , Sukon Phanichphant a,∗ a b c

Nanoscience Research Laboratory, Department of Chemistry, Faculty of Science, Chiang Mai University, 239 Huaykaew Road, Suthep District Changpuek, Chiang Mai 50200, Thailand Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand National Electronics and Computer Technology Center, Pathumthani 12120, Thailand

a r t i c l e

i n f o

Article history: Received 4 July 2010 Received in revised form 8 November 2010 Accepted 26 November 2010 Available online 4 December 2010 Keywords: Pt/ZnO nanoparticles Flame spray pyrolysis Acetylene (C2 H2 ) Gas sensor

a b s t r a c t ZnO nanoparticles loaded with 0.2–2.0 at.% Pt have been successfully produced in a single step by flame spray pyrolysis (FSP) technique using zinc naphthenate and platinum(II) acetylacetonate, as precursors dissolved in xylene and their acetylene sensing characteristics have been investigated. The particle properties were analyzed by XRD, BET, TEM, SEM and EDS. Under the 5/5 (precursor/oxygen) flame condition, ZnO nanoparticles and nanorods were observed. The crystallite sizes of ZnO spherical and hexagonal particles were found to be ranging from 5 to 20 nm while ZnO nanorods were seen to be 5–20 nm in width and 20–40 nm in length. In addition, very fine Pt nanoparticles with diameter of ∼1 nm were uniformly deposited on the surface of ZnO particles. From gas-sensing characterization, acetylene sensing characteristics of ZnO nanoparticles is significantly improved as Pt content increased from 0 to 2 at.%. The 2 at.% Pt loaded ZnO sensing film showed an optimum C2 H2 response of ∼836 at 1% acetylene concentration and 300 ◦ C operating temperature. A low detection limit of 50 ppm was obtained at 300 ◦ C operating temperature. In addition, Pt loaded ZnO sensing films exhibited good selectivity towards hydrogen, methane and carbon monoxide. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Recently, semiconducting metal oxide nanostructures have earned much attention due to their advantageous nanoscale properties such as huge specific surface area. Consequently, gas sensors based on metal oxide nanostructures exhibit better sensing properties than those based on bulk or thin films [1,2]. ZnO [3–6] is among the most widely studied gas-sensing materials due to their excellent chemical stability, low cost and fabrication flexibility. In addition, it is highly sensitive to toxic and combustible gases especially acetylene (C2 H2 ) at moderate temperature. Various types of ZnO-based gas sensors, such as single crystal [7–9], sintered pellet [10–13], heterojunctions, thin film [14–17] and thick films [4,5,12,18,19] have been demonstrated. The physical and sensing properties of ZnO gas sensors are directly related to their preparation method, which greatly influences morphology, size, defect density [17] and film thickness [4,20–27]. One research interest in this field is the search for materials that exhibit high sensitivity and fast response time while its preparation method should be simple, low-cost and highly reproducible.

∗ Corresponding author. Tel.: +66 53 943345; fax: +66 53892277. E-mail address: [email protected] (S. Phanichphant). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.11.058

Flame spray pyrolysis (FSP) is a promising technique for the synthesis of high purity nano-sized materials with controlled size and crystallinity in a single step. It has been systematically investigated using an external-mixing gas-assisted atomizer supported by six premixed methane–oxygen flamelets, which can produce metal oxide nanoparticles with very large specific surface area [28,29]. Moreover, loading or doping with metals or other metal oxides can easily be performed by FSP technique. The addition of noble metal to semiconducting oxide is known to be an effective mean to enhance detection of specific gases. Platinum (Pt) is known to be the most effective catalyst for various reducing gases including carbon monoxide and hydrocarbon gases. Recently, Pt-loaded ZnO nanoparticles have been successfully produced by FSP and the effects of Pt loading on the specific surface area of the ZnO nanoparticles and crystalline sizes have been investigated [30]. Acetylene is an important gas for a number of industrial applications. The use of nanomaterials is a promising approach to achieve acetylene gas sensors with high sensitivity and excellent gas adsorption characteristics. However, there has been no prior study on acetylene gas-sensing behaviors of Pt/ZnO nanoparticles synthesized by FSP. In this work, Pt loaded ZnO nanoparticles produced by FSP method is characterized for acetylene sensing applications. In addition, the effects of Pt loading on acetylene sensing behaviors are characterized in terms of response, response/recovery times and selectivity towards hydrogen, methane and carbon monoxide.

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Fig. 2. The specific surface area (SSABET ), dBET of ZnO and 0.2–2.0 at.% Pt/ZnO nanoparticles. Error bars indicate the variation over five samples for each Pt concentration.

Fig. 1. XRD patterns of flame-spray-made (5/5) 0.2–2 at.% Pt/ZnO as-prepared (P0–P5), Au/Al2 O3 substrate (S0), and samples P0, P1, P3, and P5 were spin-coated on Au/Al2 O3 substrate after annealing and sensing test (S1, S2, S3, and S4) ((䊉) ZnO; () Al2 O3 ; () Au).

2. Experimental 2.1. Particles synthesis and characterization Zinc naphthenate (Aldrich, 8 wt% Zn) and platinum(II) acetylacetonate (Pt (acac)2 , Aldrich, 97% Pt) were used as precursors because they possessed high specific heat of combustion. A 0.5 mol/l pre-

cursor solution was prepared by dissolving required amounts of precursor in xylene (Carlo Erba, 98.5%). In a typical run, the precursor solution was fed into a FSP reactor by a syringe pump at a rate of 5 ml/min while 5 l/min of O2 was distributed (5/5 flame condition). The gas flow rates of methane and O2 supporting flamelets were 1.19, and 2.46 l/min, respectively. The pressure drop at the capillary tip was kept constant at 1.5 bars by adjusting the orifice gap area at the nozzle. The flame height was observed to be approximately 10–12 cm and was increased slightly by increasing the combustion enthalpy. The combustion enthalpies were directly dependent on the particular solvent, starting materials and dopants. All samples produced yellowish-orange flames. The temperatures for the spray flame were typically in the range from 2000 K to 2500 K [31]. The liquid precursor mixture was rapidly dispersed by a gas stream and

Fig. 3. SEM and EDS mapping images of the 2 at.% Pt-doped ZnO nanoparticles.

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Fig. 4. High resolution TEM images of 1.0 (a) and 2.0 (b) at.% Pt-doped ZnO nanoparticles. ZnO particle sizes and morphology were spherical, hexagonal and rod-like. The darker spots on TEM images are Pt deposited on ZnO nanoparticles.

ignited by a premixed methane/oxygen flame. After evaporation and combustion of precursor droplets, particles were formed by nucleation, condensation, coagulation, coalescence and Pt deposition on ZnO support. Finally, the nanoparticles were collected on glass microfibre filters with the aid of a vacuum pump. The unloaded ZnO nanoparticles was designated as P0 while the ZnO nanoparticles loaded with 0.2–2.0 at.% Pt were designated as P1–P5, respectively. Powders of the various ZnO samples were characterized by X-ray diffraction (XRD, Siemens, D5000) and transmission electron microscope (TEM and HTEM, JEOL JEM-2010) while the specific surface area (SSABET ) of the nanoparticles was measured by nitrogen adsorption (BET analysis, Micromeritics Tristar 3000). 2.2. Sensing films preparation and characterization of the gas sensing properties Sensing films were prepared by mixing the nanoparticles into an organic paste composed of ethyl cellulose (Fluka, 30–60 mPa S) and terpineol (Aldrich, 90%), which acted as a vehicle binder and solvent, respectively. The resulting paste was spin-coated on Al2 O3 substrates (Semiconductor Wafer, Inc., 96%) with predeposited interdigitated Au electrodes. The films were then annealed at 400 ◦ C for 2 h (with heating rate of 2 ◦ C/min) for binder removal. The par-

ticle size of films was grown slightly after annealing at 400 ◦ C. The fabricated sensors using P0, P1, P3, and P5 powders were now labeled as S1, S2, S3, and S4 respectively. The morphology and the cross section of sensing films were analyzed by scanning electron microscope (SEM) and energy dispersive X-ray spectroscope (EDS) (JEOL JSM-6335F). The gas-sensing characteristics of metal oxide nanoparticles were characterized towards C2 H2 . A standard flow through technique was used to test the gas-sensing properties of thin films. A constant flux of synthetic air of 2 l/min was mixed with desired concentrations of pollutants. All measurements were conducted in a temperature-stabilized sealed chamber at 20 ◦ C under controlled humidity. The external NiCr heater was heated by a regulated dc power supply to different operating temperatures. The operating temperature was varied from 200 ◦ C to 350 ◦ C. The resistances of various sensors were continuously monitored with a computercontrolled system by voltage-amperometric technique with 5 V dc bias and current measurement through a picoammeter. The sensor was exposed to a gas sample for ∼5 min for each gas concentration testing and then the air flux was restored for 15 min. The C2 H2 concentration was varied from 0.05% to 1% in volume percentage of concentration. For selectivity characterization, the sensors were tested to C2 H2 , H2 , CO and CH4 at 1000 ppm (0.1%).

Fig. 5. SEM micrographs of flame-made ZnO thick films as a sensor S1 (a) and S4 (b) on an Al2 O3 substrate interdigitated with Au electrodes after annealing and sensing test at 300 ◦ C in dry air. The film thickness was approximately 10 ␮m.

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Fig. 6. Variation operating temperature of sensitivity with concentration of C2 H2 (0.05–1% in volume percentage of concentration) for S1, S2, S3 and S4 at 200 ◦ C (a), 300 ◦ C (b) and 350 ◦ C (c).

3. Results and discussion 3.1. Particles properties Fig. 1 shows the XRD patterns of as-prepared Pt/ZnO nanoparticles (P0–P5), Au/Al2 O3 substrate (S0), and Pt/ZnO sensing films after annealing (S1–S4). It can be seen that P0–P5 samples are highly crystalline, and all peaks can match to the hexagonal struc-

ture of ZnO (Inorganic Crystal Structure Database [ICSD] Coll. Code: 067454 [32,33]). The diffraction peaks of Al2 O3 (ICSD Coll. Code: No. 085137 [32,34]) (filled diamonds) and Au (ICSD Coll. CAS No. 7440-57-5 [32,35]) (filled rectangular) from the substrates are also visible from S0 to S4 samples. It should be noted that differences in intensity between sensing films (S0–S4) and their corresponding nanoparticles (P0–P4) are due to change in texturization of the crystal plane orientation while preparing sensing films. The average BET equivalent particle diameter (dBET ) as shown in Fig. 2 are calculated using the average of the density of ZnO and Pt/ZnO taken into account for their weight content of different loading. It can be seen that SSABET monotonically increases while dBET decreases with increasing Pt concentration from 0 to 2 at.%. The result can be explained as follows. When Pt particles are formed and deposited on ZnO supports in the flame, Pt creates a new nucleation center, which in turn changes the nucleation type from homogeneous to heterogeneous. dBET of Pt loaded ZnO nanoparticles will be the average size of the combined Pt and ZnO nanoparticles. With the increasing Pt loading, the number of Pt particles increases and hence the average particle size decreases because the size of Pt nanoparticles is expected to be much smaller than that of ZnO nanoparticles. Fig. 3 shows EDS mapping images of all elements in the 2 at.% Pt-loaded ZnO nanoparticles. It can be seen that Zn, O and Pt elements are quite evenly distributed over the area. In addition, the density of Pt sites is approximately a few percents of those of Zn and O sites. This is consistent with expected elemental composition. The average Pt concentrations of 1% and 2 at.% Pt-loaded ZnO nanoparticles are estimated by EDS quantitative analysis software (Oxford Instrument) to be ∼0.5 at.% and ∼1.3 at.%, respectively. The differences between measured concentrations and intended concentrations are due possibly to losses during FSP processing steps such as precursor injection and dispersion as well as inaccuracy of EDS measurements. Nevertheless, EDS data confirm the existence of Pt and indicate that Pt is uniformly dispersed in the mixture of nanoparticles. Fig. 4(a) and (b) shows transmission electron microscopic (TEM) images of 1.0 and 2.0 at.% Pt-loaded ZnO nanoparticles, respectively. It can be seen that very small Pt nanoparticles seen as darker spots are deposited on larger ZnO nanoparticles. The ZnO particles have spherical, hexagonal and rod-like morphologies. The sizes of spherical and hexagonal ZnO particles are found to be ranging from 5 to 20 nm while ZnO nanorods are 5–20 nm in width and 20–40 nm in length. All Pt nanoparticles dispersed on the surface of ZnO particles have spherical morphology. The average diameter of Pt nanoparticles deposited on ZnO is estimated to be 1 nm for both 1.0 and 2.0 at.% Pt/ZnO. In addition, it can be noticed that Pt nanoparticles are not agglomerated at all and this interesting feature would make them effective for gas-sensing. The result is in contrast to Pt/ZnO nanoparticles prepared by FSP in Pawinrat’s work, in which Pt particles’ size increases with increasing Pt content [30]. The different observations between Pawinrat’s and our work should be due to different flame conditions used. The flame condition in Pawinrat’s work was 8/3 (precursor/oxygen) while ours was 5/5 (precursor/oxygen). In their case, high precursor flow was used and hence Pt particles were produced with higher density and particles would agglomerate into large particles at high Pt concentration. In our case, precursor flow is moderately low so that Pt particles were not agglomerated at the concentrations up to 2 at.%. Moreover, Pt peaks in XRD were seen at 3 wt% (∼1.25 at.%) Pt-loaded ZnO in Pawinrat’s work while Pt peaks were not seen from our 2 at.% Pt-loaded ZnO. This may be explained from TEM data that larger Pt particles in Prawinrat’s work could have higher X-ray diffraction sensitivity than our smaller Pt particles.

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a H

C2H2 H

H+

+

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ZnO

OO

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OO-

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Fig. 7. Gas sensing mechanisms based on spillover effect of Pt loaded ZnO nanoparticles (a) low Pt concentration (0.2 at.%) and (b) high Pt concentration (>1 at.%).

3.2. Morphology of sensing film The cross-section, film thickness and surface morphology of the sensing film layer (S1 and S4) after annealing and sensor tested at 300 ◦ C were characterized using SEM analysis as shown in Fig. 5. The film thickness of sensing film was approximately 10 ␮m (side view) as shown in Fig. 5(a) and (b), which benefited tremendously to C2 H2 gas sensing properties. Regularities in the film thickness (top view) stem from the spin coating technique. The high density Al2 O3 substrate interdigitated with Au electrodes was also visible. After annealing process, a denser film layer was formed. 3.3. Gas sensing properties In this study, the gas sensing properties of Pt loaded ZnO sensing films are characterized in terms of response, response time, and recovery time as a function of operating temperature, gas concentration and Pt loading. The gas-sensing response, S, is defined as: S=

Ra (for reducing gas) Rg

(1)

where Ra is the resistance in dry air, and Rg is the resistance under a reducing gas. The response time, Tres is defined as the time required to reach 90% of the steady response signal. The recovery times, Trec denotes the time needed to recover 90% of the original baseline resistance. Fig. 6(a)–(c) shows the response and response time of Pt/ZnO sensing films as a function of C2 H2 concentration between 0.05 and 1% at 200, 300 and 350 ◦ C, respectively. It is evident that the response of 2 at.% Pt-loaded ZnO nanoparticles increases by more than one order of magnitude compared to unloaded, 0.2 at.% Ptloaded and 1 at.% Pt-loaded ones for all operating temperatures from 200 to 350 ◦ C. As the operating temperature increases from 200 ◦ C to 300 ◦ C, the response increases from 520 to 836 while the response time decreases from 9 to 6 s as seen in Fig. 6(a) and (b). The corresponding recovery time decreases from ∼2.05 to ∼1.05 min, respectively. However, the response decreases to 720 while the response time remains at ∼6 s as operating temperatures increases further to 350 ◦ C as shown in Fig. 6(c). The corresponding recovery time is also approximately the same at 1.35 min. Therefore, 2 at.% Pt-loaded ZnO nanoparticles gas sensor has optimum response at operating temperature of 300 ◦ C, which is lower than optimum operating temperature of undoped ZnO ones at around 350–400 ◦ C. This lower optimum operating temperature is attributed to Pt catalyst’s behavior that can be best reduced acetylene at relatively low temperature. From the response vs. concentration plot of 2% Pt-loaded ZnO film, a detection limit at 300 ◦ C operating temperature is estimated

to be as low as 50 ppm. The detection limit is defined as the concentration at which the response is 1.1 (10% change of resistance). However, the ultimate optimal Pt content for C2 H2 sensing cannot yet be determined from this study. From the result trend, the response for C2 H2 should be further increased if Pt content increases more than 2 at.% and it should eventually maximize at an optimal concentration. 2% Pt loading was chosen as the limit of this study because most other reports found the optimal Pt loading on ZnO for various gases at Pt content below 2%. Further study will be conducted to determine the ultimate optimal Pt content of Pt-loaded ZnO nanoparticles for C2 H2 sensing. It is widely believed that Pt catalyst enhances reducing gas sensing of metal oxide via spillover mechanism [36]. Chemical sensitization via spill-over effect of Pt and Pd nanoparticles on SnO2 film has been reported for H2 and CH4 [37,38]. This interaction is a chemical reaction by which additives assist the redox process of metal oxides. The term spillover refers to the process, illustrated in Fig. 7, namely the process where the metal catalysts dissociates the molecule, then the atom can ‘spillover’ onto the surface of the semiconductor support. At appropriate temperatures, reactants are first adsorbed on to the surface of additive particles and then migrate to the oxide surface to react there with surface oxygen species, affecting the surface conductivity. For the above processes to dominate the metal oxide resistance, the spilled-over species must be able to migrate to the interparticle contact. Thus, for a catalyst to be effective there must be a good dispersion of the catalysts so that catalyst particles are available near all contacts. Only then can the catalysts affect the important interparticle contact resistance. From the experimental results, the response of Pt/ZnO at 2 at.% is higher than that of 0.2 at.% and the dBET of 2 at.% Pt/ZnO nanoparticles is smaller. From TEM images in Fig. 4, the size of Pt nanoparticles is not considerably depending on Pt loading concentration. Thus, the density of Pt nanoparticle increases with Pt concentration as depicted in Fig. 7. It can be seen that the low Pt concentration case will have larger average particle diameter, dBET , than high Pt concentration one because the average particle size of Pt and ZnO mixed nanoparticles will decrease as the number of smaller Pt nanoparticles increases. For low Pt concentration, the number of Pt nanoparticles is low and spill-over mechanism is not effective because Pt catalyst particles are not available near all contacts. The spill-over action of Pt becomes considerably more effective when the number of Pt nanoparticles is sufficiently high so that they can control interparticle contact resistance. This is the reason why gas response significantly decreases and the average particle size measured by dBET decreases as Pt concentration decreases. The gas sensing selectivity of ZnO gas sensor has been characterized towards three other common reducing gases, hydrogen (H2 ), carbon monoxide (CO) and methane (CH4 ) as shown in Fig. 8. Fig. 8

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the Ph.D. scholarship. S.P. would like to acknowledge the financial support from the Research, Development and Engineering (RD&E) fund through National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA), Thailand (Project No. NN-B-22-FN5-11-52-18) granted to Chiang Mai University. The National Research University Project under Thailand’s Office of the Higher Education Commission, Graduate School, Department of Chemistry, Faculty of Science, Chiang Mai University, Thailand and National Electronics and Computer Technology Center, Pathumthani, Thailand are gratefully acknowledged. References

Fig. 8. Variation of sensitivity with concentration of C2 H2 , H2 , CO and CH4 (at 300 ◦ C, 0.1% in volume percentage of concentration) for sensor S1 (pure ZnO) as compared to S2 (0.2 at.% Pt-doped ZnO).

shows that 2 at.% Pt-loaded ZnO gas sensor has a good gas selectivity for 0.1% in volume percentage of C2 H2 concentration of 43 at 300 ◦ C. The response of 2 at.% Pt-loaded ZnO gas sensor of H2 , CO and CH4 are 8.2, 2.4 and 1.4, respectively at 0.1 vol. % concentration and 300 ◦ C operating temperature. Thus, the acetylene-sensing response of 2 at.% Pt-loaded ZnO is higher than that to H2 , CO and CH4 . The acetylene selectivity of 2 at.% Pt-loaded ZnO is also substantially higher compared to undoped ZnO gas sensor whose acetylene response is on the same order as those of H2 , CO and CH4 . The results indicate that Pt has higher catalytic activity to C2 H2 than H2 , CO and CH4 . Finally, the reproducibility, stability and reversibility of the sensor were assessed. Five sensors from the same batch were found to have response variation of less than 15%. The sensors were also seen to have good stability with less than 20% drift in response over 1 month of operation. Moreover, the sensors were highly reversible with base line shift of less than 5% after recovery from several repeated gas sampling at high concentration. 4. Conclusions The acetylene sensing characteristics of the flame-made (5/5) unloaded ZnO and Pt-loaded ZnO nanoparticles have been studied. The XRD characterizations showed that Pt loaded ZnO nanoparticles and their corresponding sensing films were highly crystalline with a hexagonal phase of ZnO. From BET measurement, SSABET increased and dBET decreased with increasing Pt concentration from 0 to 2 at.%. From TEM characterization, ZnO nanoparticles was seen to be mainly spherical particles (5–20 nm) with occasional hexagonal (5–20 nm) and rod-like particles (5–20 nm in width and 20–40 nm in length). In addition, very fine Pt nanoparticles with diameter of ∼1 nm were uniformly deposited on the surface of ZnO nanoparticles. Moreover, the presence of Pt element was confirmed by EDS analysis. The C2 H2 sensing behaviors were found to improve with high Pt content of 2 at.%. A high response of 836 was obtained from 2 at.% Pt-loaded ZnO sensing film at 1% C2 H2 concentration and 300 ◦ C operating temperature. The response times was generally well within 6 s in the regime of high response. Thus, ZnO nanoparticles loaded with 2 at.% Pt shows excellent C2 H2 response with a low acetylene detection limit of 50 ppm at 300 ◦ C operating temperature. Moreover, Pt loaded ZnO sensing films exhibited good selectivity towards hydrogen, methane and carbon monoxide. Acknowledgements N.T. would like to acknowledge the financial support from the Office of the Higher Education Commission, under the program Strategic Scholarships for Frontier Research Network, Thailand for

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Biographies

Nittaya Tamaekong received her B.Sc. with first class honors from Meajo University in 2006 on Chemistry. At present, she is a Ph.D. student at the Department of Chemistry, Chiang Mai University. Her current research interests involve the Effect of Platinum on the Gas Sensing Properties of Zinc Oxide Nanoparticles Synthesized by Flame Spray Pyrolysis.

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Chaikarn Liewhiran received his B.Sc. from Srinakharinwirot University in 2002 on Physics, M.S. and Ph.D. degrees of Materials Science from the Chiang Mai University in 2004 and 2006, respectively. He was currently a lecturer in the Department of Physics and Materials Science at Chiang Mai University until the present. There he has lectured the Physics for Engineering and Agro-industry students, Physics and Materials Science Laboratories since 2008. His research program focuses on the Nanoscience and Nanotechnology, the fundamentals of Physical and Chemical synthesis of metal oxide and metal–ceramic nanoparticles and their applications in nanocomposites, and the development of novel nanomaterials in selective bio- and chemical gas sensing for environmental monitoring. Anurat Wisitsoraat received his Ph.D., M.S. degrees from Vanderbilt University, TN, U.S.A., and B.Eng. degree in Electrical Engineering from Chulalongkorn University, Bangkok, Thailand in 2002, 1997, and 1993, respectively. His research interests include microelectronic fabrication, semiconductor devices, electronic and optical thin film coating, gas sensors, and micro-electromechanical systems (MEMS).

Sukon Phanichphant is an associate professor in chemistry at Department of Chemistry, Faculty of Science, Chiang Mai University, since 1977. Currently she is a senior researcher at the Materials Science Research Center, Faculty of Science. Her research interests include synthesis and characterization of nanomaterials for use in medical and sensor applications, photocatalysts, solar cells and light-emitting devices.