Thickness and Morphology Effects on Optical Gas-Sensing Response

stylus profilometer. The sensor response was monitored with a UV-vis spectrophotometer as a transmittance change at 625 nm, between 200 ppm CO gas and...
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J. Phys. Chem. C 2007, 111, 9105-9109

9105

Thickness and Morphology Effects on Optical Gas-Sensing Response Using Nanostructured Cobalt Oxide Films Prepared by Pulsed Laser Ablation Hyun-Jeong Nam, Takeshi Sasaki, and Naoto Koshizaki* Nanoarchitectonics Research Center (NARC), National Institute of AdVanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed: January 4, 2007; In Final Form: March 20, 2007

We investigated the influence of deposition time on optical gas-sensing response using cobalt oxide films. The films were deposited by pulsed laser ablation in Ar at 13.3 and 133 Pa for 5-120 min and were characterized with field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), and a stylus profilometer. The sensor response was monitored with a UV-vis spectrophotometer as a transmittance change at 625 nm, between 200 ppm CO gas and air at 350 °C. The cobalt oxide films prepared at 13.3 Pa were smooth and adhered firmly to substrates. The films prepared at 133 Pa consisted of nanoparticle aggregates and an underlying thin film, and the amount of nanoparticle aggregates increased with deposition time. The highest sensor response for the film prepared at 133 Pa was obtained by the deposition for 30 min with 70% coverage of nanoparticle aggregates on the underlying thin film. Further increase of nanoparticle aggregates as an active material for sensing degraded the sensor response because of the light-scattering effect of aggregates. In contrast, the film prepared at 13.3 Pa for 30 min (160 nm thick) showed twice higher sensor response than that prepared at 133 Pa deposited for 30 min and was suitable for optical gas sensing with large response and good mechanical stability.

1. Introduction Cobalt oxide is a promising transition-metal oxide material for many industrial applications including use as a solar selective absorber,1 a pigment for glass and ceramics,2,3 and a catalyst for oxygen evolution and oxygen reduction reaction.4-6 It is also desirable as a material for an optical gas sensor for flammable gases such as CO, NO2, and H2.7-10 The optical gas sensor has several advantages over the more commonly used semiconductor gas sensor. We previously reported that an optical gas sensor using cobalt oxide film prepared by pulsed laser deposition (PLD) exhibited very high sensitivity.11 The sensitivity of common semiconductor gas sensor was reported to depend on morphology, surface area, film thickness, and particle size.12-15 The sensitivity of the optical gas sensor is also expected to depend on such film properties, although these effects on optical sensitivity have not been sufficiently investigated. PLD has attracted attention as a method for thin film preparation because of its simple experimental setup for deposition and the possibility to prepare a wide variety of materials in thin-film form.15,16 The properties of the films prepared by the PLD method depend on the following process variables: substrate temperature,17,18 target composition,19 arrangement between the substrate and the target,20 ambient gas in the chamber,21 and laser parameters22 such as power density, wavelength, and pulse repetition rate.23,24 Moreover, it was also confirmed that fine cobalt oxide films with smaller and more uniform particles were prepared under Ar gas ambient compared with other ambient gases. Our previous paper reported that Ar gas pressure in the PLD chamber significantly affected the morphology and particle size of cobalt oxide film; the films deposited at low and middle Ar pressures (0.07, 0.67, and 13.3 * To whom correspondence [email protected].

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Figure 1. Transmittance change at 625 nm for the sensitivity-measuring process of cobalt oxide film prepared by laser ablation at 13.3 Pa of Ar for 120 min.

Pa) were smooth and adhered firmly to substrates, while those prepared at high Ar pressures (80.0 and 133 Pa) consisted of two parts: nanoparticle aggregates and a thin film. This difference of film morphology is due to the extent of expanding plume confinement by gas pressure.25,26 These morphologies of films induced by ambient Ar pressure during deposition were reported to affect the optical gas-sensing properties.11,27 In this paper, we investigate the influence of film thickness on gas sensitivity by varying the deposition time for the cobalt oxide films prepared in Ar at 13.3 and 133 Pa, which have extremely different morphology suitable for studying the sensor response mechanism. 2. Experimental Section A third harmonic of the Nd:YAG laser (Continuum, Precision 8000, wavelength: 355 nm, pulse width: 7 ns, repetition rate:

10.1021/jp070065p CCC: $37.00 © 2007 American Chemical Society Published on Web 06/02/2007

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Figure 2. High-resolution FE-SEM images of films prepared at 13.3 Pa for 5-120 min.

10 Hz) was used as a light source for deposition. A laser beam with a fluence of 60 mJ/pulse was irradiated within an area of 36 mm2 on a cobalt monoxide (CoO) single-crystal target, rotating at 30 rpm. A substrate (quartz glass or silicon wafer) was perpendicularly placed to the target (off-axis geometry, center-to-center distance: 35 mm) and was rotated at 30 rpm. Before ablation, the chamber was pumped to 2.66 × 10-4 Pa by a turbomolecular pump and then was filled with Ar gas to 13.3 or 133 Pa. PLD was performed at room temperature for times ranging from 5 to 120 min. Two Ar pressures, 13.3 and 133 Pa, were selected to obtain different product morphologies on the basis of our previous study. The structural characterization of the films was performed using an X-ray diffractometer (XRD, Rigaku RAD-C), with Cu KR radiation. A field emission scanning electron microscope (FE-SEM, Hitachi S-4800) was used to observe morphology using the film deposited on a silicon wafer substrate. Film thickness was measured by a stylus profilometer (Kosaka, ET 4000A) at force of 100 µN. Optical gas-sensing responses were measured by the transmittance change of films in dry air and in 200 ppm of CO ambient at 350 °C. A cobalt oxide film deposited on quartz glass substrate was placed in a quartz glass cell installed in a UVvis spectrophotometer (Shimadzu UV-3100PC). The volume of the quartz glass cell was 10 cm3. The cell was heated from room temperature to 350 °C in dry air and was kept at this temperature during sensor property measurement. Transmittance change for the sensitivity-measuring process of cobalt oxide film prepared by laser ablation at 13.3 Pa of Ar for 120 min is shown in Figure 1. The transmittance of the film decreased with increasing temperature. After the transmittance change due to the temperature increase ceased and the transmittance became stable, dry air and 200 ppm of CO gas were alternately introduced into the quartz glass cell at a flow rate of 0.5 L/min. The transmittance of cobalt oxide film in CO and in dry air was monitored at 625 nm.

Figure 3. Thickness of the films prepared at 13.3 Pa for various ablation times measured by a stylus profilometer.

3. Results and Discussion 3.1. Morphology of Films Prepared at 13.3 and 133 Pa. Our previous paper reported the ambient Ar pressure effect on the morphology of film deposited only for 13 min.11 The film prepared at 13.3 Pa was smooth and adhered firmly to substrates. In contrast, the film prepared at 133 Pa consisted of nanoparticle aggregates and an underlying thin film. The underlying thin film was firmly fixed like the film prepared at 13.3 Pa. The nanoparticle aggregates on the film, however, did not adhere firmly to underlying thin films and were easily removed, even by a gentle wipe with soft paper. Figure 2 presents high-resolution FE-SEM images of the films prepared at 13.3 Pa for different deposition times. The film surface deposited for 5 min was uniform and dense, with spherical particles smaller than 5 nm in diameter. However, the deposition for 30 min or longer led to the larger grain formation with sharp crystal edges several tens of nanometers in size. Many deep cracks were also observed. The nanoparticle aggregates previously observed at 133 Pa for 13 min were not observed at 13.3 Pa for any deposition time from 5 to 120 min.

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Figure 4. Low-resolution FE-SEM images of films prepared at 133 Pa for 5-120 min.

The thicknesses of films prepared for longer deposition times from 60 to 180 min by stylus profilometer measurement are presented in Figure 3. The thicknesses of films prepared for shorter deposition times from 5 and 30 min were estimated to be 30 and 160 nm from the linear relationship in Figure 3. In contrast, the film prepared at 133 Pa exhibited different morphology (Figure 4). The amount of snowflakelike aggregates observed by 5 min deposition increased with the deposition time. The underlying thin film deposited at 133 Pa for 5 min was observed through the nanoparticle aggregates, although the nanoparticle aggregates after 30 and 120 min deposition covered 70% and 100% of the films underneath. From the side view image of the film, thicknesses of these nanoparticle aggregates were estimated to be on the order of micrometers. 3.2. Deposition Time Dependence of Sensitivity. The sensing response was compared using the following sensitivity value.

sensitivity )

(Tr(CO) - Tr(air)) Tr(CO)

× 100

where Tr(CO) and Tr(air) are transmittance in CO gas and dry air ambient at 350 °C. Figure 5 graphs the deposition time dependence of the sensitivity of the films prepared at 13.3 and 133 Pa for 5-120 min. All the films prepared at 13.3 Pa had higher sensitivity than those prepared at 133 Pa. The sensitivity of the film prepared at 13.3 Pa increased with the deposition time until 30 min, and the sensitivity reached as high as 70%. Gas sensitivity is considered to increase with the amount of deposited cobalt oxide that reacts with CO gas. The amount of cobalt oxide (film thickness) increases with the ablation time, and therefore the sensitivity depended on the film thickness. However, sensitivity of the films did not increase over 70% despite the longer deposition time. Here, a question arises as to why the films

Figure 5. Ablation time dependence of sensitivity at 625 nm for films prepared at 13.3 and 133 Pa.

deposited for 30 min and longer presented similar sensitivity despite the different thicknesses. Figure 6 displays the XRD patterns of various films to investigate the effect of film thickness on sensitivity. Curves a and b are XRD patterns of films deposited for 30 and 120 min (160 and 600 nm thick) and cooled in dry air after CO exposure at 350 °C. If a whole section of thicker film was oxidized to Co3O4 in dry air ambient at 350 °C, the film should be Co3O4 single phase as observed in the film deposited for 30 min (160 nm, curve a). However, the film deposited for 120 min (600 nm) and cooled in air exhibited Co3O4 peaks and small CoO peaks at 42.5°. This suggests that the film deposited for 120 min as thick as 600 nm thick was not fully oxidized to Co3O4 at 350 °C in air. Moreover, curve c is the XRD pattern of the as-deposited film for 120 min. This clearly reveals that CoO phase remaining in the film cooled in dry air comes from the as-deposited film. Thus, the oxidation occurred in the whole

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Figure 6. Curves (a) and (b) are the XRD patterns of films deposited at 13.3 Pa for 30 and 120 min (160 and 600 nm) and cooled in dry air after CO sensing measurement at 350 °C. Curve (c) is the XRD pattern of as-deposited film for 120 min. Reference peak positions of Co3O4 and CoO are also shown from the JCPDF database.

Figure 7. Low-resolution FE-SEM image of film prepared at 13.3 Pa for 30 min.

film when the film was 160 nm thick but could not completely take place in the whole film when the film was as thick as 600 nm. In contrast, sensitivity of films prepared at 133 Pa increased with the ablation time up to 30 min (Figure 5). The ablation time increase exceeding 30 min gradually degraded the sensitivity, although the amount of nanoparticle aggregates increased with deposition time. In our previous paper,11 we reported that the sensor properties of film prepared at 133 Pa depended primarily on the nanoparticle aggregates. Therefore, this sensitivity degradation with deposition time over 30 min is probably due to the light-scattering influence by nanoparticle aggregates in the micrometer scale. Clearly supporting this explanation is the fact that low-resolution FE-SEM images (Figure 7) of the film prepared at 13.3 Pa did not present any features and were very smooth. Figure 8 plots the transmittance of films prepared at 13.3 and 133 Pa for various ablation times after long exposure in CO ambient at 350 °C. The cobalt oxide films should be reduced to nearly transparent CoO by long exposure in CO, and hence this plot provides the structural and morphological information of the films. The transmittance of the films prepared at 133 Pa decreased with increasing ablation time, although transmittance of films prepared at 13.3 Pa was almost constant around 80%. The film prepared at 133 Pa had the highest sensitivity until 30 min deposition with transmittance at 63%, while that prepared

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Figure 8. Transmittance at 625 nm of films prepared at 13.3 and 133 Pa for various ablation times measured after long exposure in CO ambient at 350 °C.

for 120 min with very low sensitivity exhibited an extremely low transmittance of 6% in CO. This result clearly indicates significant light scattering for film deposited longer than 30 min with many nanoparticle aggregates. Therefore, the film prepared at 133 Pa seems to have the highest sensitivity when the amounts of nanoparticle aggregates with higher sensitivity and light scattering to reduce sensitivity are appropriately balanced. The above results suggest that the morphology for acceptable optical sensor response does not include formation of nanoparticle aggregates, and the suitable film thickness depends on the film morphology. In this study, an optical gas sensor with high sensitivity and good adherence with the substrate was achieved when the film was prepared at 13.3 Pa for 30 min (160 nm thick) without the formation of nanoparticle aggregates. This reveals that important requirements for a good optical gas sensor are a large amount of active materials available for sensing and a small loss due to the light scattering induced by thicker film and nanoparticle aggregates. 4. Conclusion The deposition time dependence of optical gas sensitivity to CO using films prepared at 13.3 and 133 Pa of Ar pressures by laser ablation was investigated. Films prepared at 13.3 Pa were smooth and adhered firmly to the substrate for all deposition times. The sensitivity of films increased with the thickness and saturated at 160 nm. This is because the sensing process could not completely occur in the whole film when the film became thicker than 160 nm. In contrast, films prepared at 133 Pa were composed of an underlying thin film that strongly adhered to the glass substrate, and the nanoparticle aggregates connected weakly to the underlying thin film. The amount of nanoparticle aggregates increased with increasing deposition time, resulting in the large light-scattering effect and sensitivity degradation. Thus, the cobalt oxide film 160 nm thick without nanoparticle aggregates prepared at 13.3 Pa by laser ablation at off-axis configuration is considered to be the best optical CO gas sensor in terms of high sensor response and good adherence with the substrate. References and Notes (1) Avila, A. G.; Barrera, E. C.; Huerta, L. A.; Huhl, S. Sol. Energy Mater. Sol. Cells 2004, 82, 269-278. (2) Sulcova, P.; Trojan, M. Dyes Pigm. 1999, 40, 83-86. (3) Pisareva, S. Stud. ConserV. 2005, 50, 190-192. (4) Gulari, E.; Gu¨ldu¨r, C.; Srivannavit, S.; Osuwan, S. Appl. Catal., A: Gen. 1999, 182, 147-163.

Effects on Optical Gas-Sensing Response (5) Lin, H.-K.; Chin, H.-C.; Tsai, H.-C.; Chien, S.-H.; Wang, C.-B. Catal. Lett. 2003, 88, 169-174. (6) Kohse-Ho¨inghaus, K.; Brechling, A.; Kleineberg, U. Appl. Catal., B: EnViron. 2004, 53, 245-255. (7) Kobayashi, T.; Haruta, M.; Ando, M. Sens. Actuators, B 1993, 1314, 545-546. (8) Ando, M.; Kobayashi, T.; Haruta, M. J. Chem. Soc., Faraday Trans. 1994, 90, 1011-1013. (9) Ando, M.; Kobayashi, T.; Iijima, S.; Haruta, M. J. Mater. Chem. 1997, 7, 1779-1783. (10) Ando, M.; Kobayashi, T.; Iijima, S.; Haruta, M. Sens. Actuators, B 2003, 96, 589-595. (11) Nam, H.-J.; Sasaki, T.; Koshizaki, N. In Nanoparticles and Nanostructures in Sensors and Catalysis, Materials Research Society Symposium Proceedings 900E; Zhong, C-J., Kotov, N. A., Daniell, W., Zamborini, F. P., Eds.; Warrendale, PA, 2005; p 0900-O09-17. (12) Baˆrsan, N.; Weimar, U. J. Phys. Condens. Matter 2003, 15, R813R839. (13) Sharma, R. K.; Chan, P. C. H.; Tang, Z.; Yan, G.; Hsing, I-M.; Sin, J. K. O. Sens. Actuators, B 2001, 81, 9-16. (14) Sahner, K.; Moos, R.; Matam, M.; Tunney, J. J.; Post, M. Sens. Actuators, B 2005, 108, 102-112. (15) Min, B.-K.; Choi, S.-D. Sens. Actuators, B 2004, 98, 239-246.

J. Phys. Chem. C, Vol. 111, No. 26, 2007 9109 (16) Geretovszky, Zs.; Haraszti, T.; Szo¨re´nyi, T.; Antoni, F.; Fogarassy, E. Appl. Surf. Sci. 2003, 208-209, 566-574. (17) Inoue, N.; Yuasa, H.; Okoshi, M. Appl. Surf. Sci. 2002, 197-198, 393-397. (18) Henley, S. J.; Ashfold, M. N. R.; Nicholls, D. P.; Wheatley, P.; Cherns, D. Appl. Phys. A 2004, 79, 1169-1173. (19) Beck, K. M.; Sasaki, T.; Koshizaki, N. Chem. Phys. Lett. 1999, 301, 336-342. (20) Wang, Z.; Sasaki, T.; Shimizu, Y.; Kirihara, K.; Kawaguchi, K.; Kimura, K.; Koshizaki, N. Appl. Phys. A 2004, 79, 891-893. (21) Inada, M.; Umezu, I.; Sugimura, A. J. Vac. Sci. Technol., A 2003, 21, 84-86. (22) Dwivedi, R. K.; Thareja, R. K. Surf. Coat. Technol. 1995, 73, 179176. (23) Ossi, P. M.; Bottani, C. E.; Miotello, A. Appl. Surf. Sci. 2005, 248, 334-339. (24) Zbroniec, L.; Sasaki, T.; Koshizaki, N. Appl. Phys. A 2004, 79, 1783-1787. (25) Geohegan, D. B. Appl. Phys. Lett. 1992, 60, 2732-2734. (26) Geohegan, D. B.; Puretzky, A. A. Appl. Phys. Lett. 1995, 67, 197199. (27) Nam, H.-J.; Sasaki, T.; Koshizaki, N. J. Phys. Chem. B 2006, 110, 23081-23084.