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Synthesis of p-type semiconducting cupric oxide thin films and their application to hydrogen detection
Sensors and Actuators B 146 (2010) 239–244
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Synthesis of p-type semiconducting cupric oxide thin films and their application to hydrogen detection Nguyen Duc Hoa, Sea Yong An, Nguyen Quoc Dung, Nguyen Van Quy, Dojin Kim ∗ Department of Materials Science and Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 18 November 2009 Received in revised form 28 January 2010 Accepted 12 February 2010 Available online 19 February 2010 Keywords: Cupric oxide Thin film sensor Hydrogen sensor Sensing mechanism
a b s t r a c t Nanostructured CuO thin films are synthesized by deposition and thermal oxidation of Cu on SiO2 substrates. The effects of oxidation temperatures on the morphologies and crystallinity of the CuO thin films are investigated by SEM, XRD, and XPS. The electrical and hydrogen sensing properties of CuO are studied by electrical resistance measurements. In addition, the effects of carrier gases on the gas response of CuO are investigated. The results showed that Cu was oxidized into monoclinic cupric oxide in the investigated temperature range from 300 to 800 ◦ C. The p-type semiconducting CuO thin film showed increases in electrical resistance upon exposure to hydrogen. The CuO thin film oxidized at 400 ◦ C showed the highest response as compared to others, and it was 3.72 for 6% H2 at an operating temperature of 250 ◦ C. The carrier gases played very important roles in the hydrogen sensing by CuO. That is, the sensor showed good response-and-recovery with a carrier gas of air but not with nitrogen. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The demand in environmental pollution monitoring, industrial monitoring and process control, medical diagnosis, public security, agriculture, and a variety of other industries requires the development of gas sensors for the early detection of chemical species and flammable and toxic gases [1]. A wide range of metal oxides have been used for gas sensors including TiO2 , ZnO, SnO2 , and WO3 [2–5]. Ordinary and commercial metal oxide-based gas sensors are usually in the form of thick or thin oxide films, and they exhibit a short lifetime, low sensitivity, and poor selectivity. Therefore, many researchers have attempted to find more suitable materials for better gas sensors that have high sensitivity and selectivity [6–8]. The metal oxides used for gas sensor application are mostly n-type semiconductors, and focus was made on improving their gas sensing performance [2–8]. Opposite to n-type semiconducting metal oxides, cupric oxide (CuO) is a p-type semiconductor with a band gap of 1.2–1.9 eV. Its applications may be for catalysis, lithium–copper oxide electrochemical cells, solar cells, and gas sensors [9–11]. The nanoparticles, plates, and nanowires of CuO were also reported to sense NO2 , H2 S, and CO [10,12]. CuO was also used to coat on tin titanate thick film sensor and enhance the
∗ Corresponding author. Tel.: +82 42 821 7648; fax: +82 42 823 7648. E-mail addresses: [email protected] (N.D. Hoa), [email protected] (S.Y. An), [email protected] (N.Q. Dung), [email protected] (N. Van Quy), [email protected] (D. Kim). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.02.045
response to hydrogen [13]. On the other hand, the thin film type has some advantages as compared to nanostructures. For instance, it is more stable and can be integrated with microelectronic silicon devices. In addition, the CuO thin film can be synthesized by the sol–gel or sputter methods [14,15]. The latter allows for the easy control of film thickness and combination with silicon semiconductor technology. However, the hydrogen sensing properties of CuO deposited by the sputter method has rarely been reported. In the present work, we report on the synthesis, characteristics, and gas sensing properties of a CuO thin film deposited by the sputter method. The CuO thin film is synthesized by the deposition of the Cu metal layer on SiO2 substrate followed by thermal oxidation at high temperatures. The effects of oxidation temperatures on the morphology, crystalline quality, and hydrogen sensing of the materials are investigated. In addition, the response to hydrogen for different carrier gases (dry air and nitrogen) is compared, and the sensing mechanism is discussed. 2. Experiment The nanostructured cupric oxide thin film was synthesized by the deposition of a copper layer on thermally oxidized silicon substrate-supported Pt electrodes by using a dc sputter followed by the thermal oxidation process. In brief, a two-inch copper (99.9%) target was used for Cu deposition. The sputtering conditions were as follows: based pressure of 10−6 Torr, Ar working pressure of 2 × 10−3 Torr, and DC plasma power of 10 W. Copper thin film with a thickness of 100 nm was deposited on the silicon substrate through a shadow mask to make a 2mm × 3 mm pattern.
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Fig. 1. Schematic of the sensor property measurement system.
The oxidation process was performed in a horizontal tube furnace of air ambient. The as-deposited Cu thin films were placed in the center of the quart tube, and then the temperature was increased to oxidation temperatures (Tox ) of 300, 400, 500, 600, 700, and 800 ◦ C in 30 min. The oxidation was then carried out for 2 h. Field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) measurements were carried out to examine the morphology and crystal structure of the films. The X-ray diffraction (XRD) measurements were carried out using Cu K␣ -X-radiation with wavelength = 1.54178 Å (Model: D/max 2500, Rigaku, Japan). XPS measurements were carried out using an AXIS-NOVA (Kratos) system (X-ray source: monochromated Al K␣ radiation) to investigate the stoichiometry of CuO thin films in the base pressure of ∼10−9 Torr. The electrical and gas sensing properties of the nanostructured CuO thin films were studied by measuring their resistances in a test chamber with different gases ambient as reported in Ref. [16]. Fig. 1 shows the schematic of used gas supply and measurement system. Dry air or high-purity nitrogen (5N) was used as references, and concentrated H2 was employed as the test gas. The test gases regulated to the desired concentrations were introduced into the test chamber using a series of mass flow controllers. The gas sensing properties were measured in a dynamic condition in which the gases continuously flowed through the chamber with a total flow rate of 500 sccm during the measurements. Prior to the test, the sensors were pre-heated at 250 ◦ C for 1 h for stable references, and then the sensing property measurements were performed. For each measurement, hydrogen was introduced into the test chamber for 10 min and then stopped. The sensors resistances were automatically recorded using a programmable electrometer (Keithley 2400) controlled by a personal computer using the Labview program. The sensor response, S, is defined as follows: S = R/Ro where Ro and R are the resistance of the sensor in reference gas (air or nitrogen) and the test gases (H2 ), respectively. 3. Results and discussion The surface FE-SEM images of the CuO thin films formed by oxidation at different temperatures are shown in Fig. 2(a)–(f). They can be compared with the smooth surface of the as-deposited Cu thin film shown as the inset in Fig. 2(a). The very rough and bumpy CuO surfaces after the oxidation process are due to the volume expansion that occurs during Cu to CuO conversion. The surfaces
of the CuO thin films became rougher and rougher with the oxidation temperatures due to the increasing crystal growth rates. The average grain size of ∼15 nm for the thin film oxidized at 300 ◦ C increased up to several hundreds of nm for 800 ◦ C oxidation. The crystal structures of cupric oxide formed during the oxidation process were characterized by XRD as shown in Fig. 3. The XRD patterns showed two strong peaks at 2 = 35.5 and 38.8◦ corresponding to (0 0 2) and (1 1 1) planes of monoclinic CuO, respectively, according to the standard JCPDS card no. 80-1917. This result is well coincided with other XRD reports [17,18]. The oxidation of Cu took place according to the following equations: Cu + O2 = CuO2 , 3CuO2 + (1/2)O2 = 2Cu3 O2 , and Cu3 O2 + (1/2)O2 = 3CuO [19]. Since monoclinic CuO forms at around 270 ◦ C, the single phase was observed at all investigated oxidation temperatures. The crystalline size of materials can be roughly estimated by using Scherrer’s formula, D = /ˇ cos , where ˇ is the full width half maximum (FWHM) of the strongest diffraction peak, is the diffraction angle, is the X-ray wavelength (0.145 nm), and (∼0.89) is Scherrer’s constant [10]. The crystalline size was 20, 25, 29, 33, 37, and 40 nm for the CuO thin films oxidized at 300, 400, 500, 600, 700, and 800 ◦ C, respectively. This increase in crystalline size with the oxidation temperature coincides with the trend observed by SEM. However, the values are much smaller than the SEM observation particularly at high oxidation temperatures. For example, the average grain size of SEM increased from 166 to 180 nm (∼7.7%) whereas the average crystalline size of XRD increased from 37 to 40 nm (∼7.5%) for the oxidation temperature change from 700 to 800 ◦ C. The estimation via SEM observation is influenced by the sampling of the viewing window, but in addition, the difference suggests that the visually observed one grain is possibly formed by several crystallites of different crystallographic orientations. The XPS spectra of CuO thin films oxidized at 300, 400, and 600 ◦ C are measured. The survey scans of all CuO thin films show photoelectron lines for Cu, O, Si, and C (not shown). There were not any detectable impurity elements other than the commonly observed carbon contamination and Si from the silicon dioxide substrate. To examine the stoichiometry of the cupric oxides, the high resolution scans for O1s were analyzed. The fittings of the O1s peaks with the Gaussian functions are shown in Fig. 4(a)–(c) assuming that the oxygen bindings come from O2− (529.1 eV), Cu–O (530 eV), C O (531.36 eV), Si–O (532.36 eV), and O–C O (533.51 eV). The fitting results are excellent and the compositions estimated as such are Cu0.82 O, Cu0.76 O, and Cu0.70 O for the oxidation at 300, 400, and 600 ◦ C, respectively. While the trend of the highly excess incorporation of oxygen at higher temperatures is expected, the huge ı values in Cu1−ı O expression as Cu vacancy are rather unexpected. Without additional confirmation it is difficult to take the whole Cu deficiency values purely as the Cu vacancy concentrations in the lattice. Furthermore, the examinations do not explain the trend in sensing results as will be shown if Cu vacancy should play the key role in charge transport. Therefore, the XPS analysis alone seems to not clearly explain the Cu vacancy concentrations. The responses of the CuO thin films to 6% hydrogen gas were measured at 250 ◦ C and are summarized in Fig. 5. The responses for the films oxidized at different temperatures are summarized as the inset. All sensors showed an increase in resistance upon exposure to hydrogen and recovered to the initial values with a stop in the flow of H2 . This resistance increase upon exposure to the reducing gas indicates the p-type conduction of CuO. The p-type semiconducting property is due to the Cu vacancies in the crystal structure (Cu1−ı O) [20]. Thin film oxidized at 300 ◦ C showed the lowest response (S ∼ 1.3), whereas that oxidized at 400 ◦ C showed the highest response (S ∼ 3.7). One may consider the parameters that could affect the response: (i) grain size, (ii) porosity, and (iii) the density of Cu vacancy in the material.
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Fig. 2. FE-SEM images of CuO thin films formed by oxidation at (a) 300 ◦ C, (b) 400 ◦ C, (c) 500 ◦ C, (d) 600 ◦ C, (e) 700 ◦ C, and (f) 800 ◦ C. Inset of (a) is for the as-deposited Cu thin film.
Fig. 3. XRD of CuO thin film oxidized for 2 h in air at (a) 300 ◦ C, (b) 400 ◦ C, (c) 500 ◦ C, (d) 600 ◦ C, (e) 700 ◦ C, and (f) 800 ◦ C.
The porosity, or the size and density of the pores, is not a simple parameter to evaluate the effect since it controls the diffusion kinetics of the gases into the films. However, grain growth will reveal a monotonic decrease in response, and a decrease at temperatures above 400 ◦ C should partly be due to this effect. Similarly, the porosity may increase with the grain size, and the increase up to 400 ◦ C can partly be due to this effect. The tradeoff between grain size and porosity may reveal such change in the response with the oxidation temperature. If another very probable cause, the density of Cu vacancy, is to be the dominant parameter, the density of Cu vacancy should be the highest for the 400 ◦ C oxidation. However, as discussed in Fig. 4, the stoichiometry analysis via XPS does not properly explain the results at present. Furthermore, measurements revealed that the resistances do not remarkably change with the oxidation temperatures in 300–700 ◦ C. These results are different from the SnOx thin films, where the oxidation temperature greatly affects the film stoichiometry or the layer resistance, which became the major parameter to determine the response of the sensor [21]. We tentatively conclude that the change of response in CuO thin films was mainly caused by the combined effect of the grain size and porosity while the details need to be further explored.
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Fig. 5. The change in response (R/Ro) of CuO thin films oxidized at different temperatures upon exposure to 6% H2 at 250 ◦ C.
(Fig. 6(b)). When the sensor was first exposed to the diluted hydrogen, the sensor resistance increased abruptly from 1.3 × 105 to 4.1 × 108 in ∼5 min and then decreased to 1.3 × 101 in the following 5 min. After these dramatic orders of magnitude change at the first exposure, the sensor showed no response at all from the second exposure to hydrogen.
Fig. 4. The XPS O1s peaks of high magnification for samples oxidized at (a) 300 ◦ C, (b) 400 ◦ C, and (c) 600 ◦ C. Fittings with bindings of O2− , Cu–O, C O, Si–O, and O–C O were done.
The hydrogen sensing of CuO was further examined in order to study the sensing mechanism. The cyclic response of the sensor to hydrogen was measured using either dry air or nitrogen as the dilution gas, and the comparison results are shown in Fig. 6. It is indicated that a good response-and-recovery behavior was repeated when dry air was used for dilution (Fig. 6(a)). However, a very interesting result was observed when nitrogen was used
Fig. 6. The response-and-recovery behaviors of CuO thin film oxidized at 400 ◦ C in different dilution gases of (a) dry air and (b) nitrogen at 250 ◦ C.
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One may explain this interesting difference by considering the interactions occurring in the CuO grains. First, the sensing mechanism of CuO may be explained as follows for the response in dry air. The pre-adsorbed oxygen may establish forms of O− and/or O2− at the CuO surface through charge exchange interactions with CuO following Eqs. (1)–(4). When such CuO is exposed to hydrogen molecules, the adsorbed hydrogen interacts with the pre-adsorbed oxygen as Eqs. (5)–(7) show. The free electrons released via the reaction between the H2 molecules and the pre-adsorbed O− or O2− neutralize the holes, or the majority carrier in p-type CuO. This compensation results in a decrease in the hole carriers in CuO, and consequently, an increase in sensor resistance. Note here that the oxygen molecules are continuously supplied from the dilution gas (dry air) and are adsorbed on the CuO surface while the interaction with hydrogen continuously forms water molecules to escape from the surface. When the hydrogen flowing is stopped for recovery, the oxygen molecules in air will adsorb on the surface of CuO, and the capture of electrons through the processes indicated in Eqs. (1)–(4) will reduce the sensor resistance towards the initial stable surface state of CuO. O2 (gas) ↔ O2 (ads) −
(1) −
O2 (ads) + e ↔ O2 (ads) −
−
(2)
−
(3)
(ads)
(4)
O2 (ads) + e ↔ 2O (ads) −
−
O (ads) + e ↔ O
2−
H2 (gas) ↔ H2 (ads) −
H2 (ads) + O (ads) ↔ H2 O + e H2 (ads) + O
2−
(5) −
(ads) ↔ H2 O + 2e
e− + h◦ ↔ Null
(6) −
(7) (8)
However, the situation becomes different when the refreshing gas does not contain oxygen as in the case when N2 was used as the dilution gas. At the first cycle of exposure, H2 molecules interact with the pre-adsorbed oxygen on the surface of CuO, and the sensor resistance increases as shown above. Note, however, that the resistance increase is about 3 orders of magnitude higher with the dilution in N2 . The pre-adsorbed oxygen again forms H2 O molecules via Eqs. (6) and (7) as indicated above, and is removed from CuO. However, in this case, no oxygen molecules are supplied during the exposure to the CuO surfaces from flowing gases. This sudden total exhaust in surface oxygen resulted in the initial dramatic increase in resistance at the first cycle. Furthermore, the continuous hitting of hydrogen on the CuO surface reduces the oxide to Cu elements via H2 + CuO ↔ Cu + H2 O [22]. This metallic Cu formation in the grains may be the cause for the dramatic decrease in resistance in the first cycle. The reduction process might be accelerated due to the elevated temperature in measurements at 250 ◦ C. Now, the recovery in N2 does not occur due to the stop of oxygen supply, and the metallic Cu layer is maintained. Also, when hydrogen gas diluted in N2 is again introduced in the second cycle, the reaction of oxygen with hydrogen does not further take place on the CuO surfaces, so the sensor showed no response. Therefore, the study dramatically demonstrates the key role of oxygen adsorbed on the surfaces of sensor materials particularly of metal oxides. In other words, the interfacing and reactions between the sensor materials and analyte gases are in most cases mediated by oxygen, the chemically active element in the sensing environment. Cupric oxide is a dramatic example showing no reaction with the analyte gas, hydrogen, without mediation via oxygen. Providing air environments is essential for development of engineering sensors. However, intentional environment controls are often useful and required for studying of the intrinsic reactions between the material and the surroundings.
Fig. 7. The hydrogen sensing properties of the CuO thin film oxidized at 400 ◦ C. (a) The sensor response upon exposure to 6% H2 measured at different operating temperatures and (b) the sensor response measured with varying H2 concentrations at operating temperature of 250 ◦ C.
The sensing properties of CuO at different H2 concentrations and operating temperatures were also investigated for the oxidized at 400 ◦ C (showing the highest response). Fig. 7(a) shows the change in sensor response (R/Ro ) upon exposure to 6% H2 measured at different temperatures. The sensor showed the highest response of 3.7 at 250 ◦ C. The response measured at different H2 concentrations at the optimum working temperature of 250 ◦ C is shown in Fig. 7(b). The sensor response changed from 1.4 to 3.7 in the investigated range of 1–6%, but the trend is opposite to the calculated power law of the concentration dependence [23]. 4. Conclusion The synthesis, characterization, and gas sensing properties of granular cupric oxide thin films were investigated. A single phase monoclinic crystal structure of CuO was obtained by oxidizing the Cu thin films in air for 2 h at a temperature range of 300–800 ◦ C. The CuO thin film oxidized at 400 ◦ C showed the highest response of 3.72 for 6% H2 at 250 ◦ C. The effect of carrier gases on the sensing was also studied. The null response of CuO to the hydrogen in an oxygen-free environment was dramatically observed. The results confirmed that the pre-adsorbed oxygen plays a crucial role in the hydrogen sensing of CuO. These provide useful information in discussing the sensing mechanism in CuO and other oxide materials.
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Acknowledgment This work was supported by the National Research Laboratory program, Korea. References [1] T. Zhang, S. Mubeen, N.V. Myung, M.A. Deshusses, Recent progress in carbon nanotube-based gas sensors, Nanotechnology 19 (2008) 332001 (14 pp.). [2] M.R. Mohammadi, D.J. Fray, M.C. Cordero-Cabrera, Sensor performance of nanostructured TiO2 thin films derived from particulate sol–gel route and polymeric fugitive agents, Sens. Actuators B 124 (2007) 74–83. [3] (a) D.R. Patil, L.A. Patil, Cr2 O3 -modified ZnO thick film resistors as LPG sensors, Talanta 77 (2009) 1409–1414; (b) A. Umar, M.M. Rahman, Y.B. Hahn, Ultra-sensitive hydrazine chemical sensor based on high-aspect-ratio ZnO nanowires, Talanta 77 (2009) 1376–1380. [4] J. Arbiol, E. Comini, G. Faglia, G. Sberveglieri, J.R. Morante, J. Cryst. Growth 310 (2008) 253. [5] T. Siciliano, A. Tepore, G. Micocci, A. Serra, D. Manno, E. Filippo, WO3 gas sensors prepared by thermal oxidization of tungsten, Sens. Actuators B 133 (2008) 321–326. [6] H.R. Kim, K.I. Choi, J.H. Lee, A. Sheikh, Akbar, Highly sensitive and ultra-fast responding gas sensors using self-assembled hierarchical SnO2 spheres, Sens. Actuators B 136 (2009) 138–143. [7] M.W. Ahn, K.S. Park, J.H. Heo, D.W. Kim, K.J. Choi, J.G. Park, On-chip fabrication of ZnO-nanowire gas sensor with high gas sensitivity, Sens. Actuators B 138 (2009) 168–173. [8] N. Izu, S. Nishizaki, T. Itoh, M. Nishibori, W. Shin, I. Matsubara, Gas response, response time and selectivity of a resistive CO sensor based on two connected CeO2 thick films with various particle sizes, Sens. Actuators B 136 (2009) 364–370. [9] J. Zhang, J. Liu, Q. Peng, X. Wang, Y. Li, Nearly monodisperse Cu2 O and CuO nanospheres: preparation and applications for sensitive gas sensors, Chem. Mater. 18 (2006) 867–871. [10] A. Cruccolini, R. Narducci, R. Palombari, Gas adsorption effects on surface conductivity of nonstoichiometric CuO, Sens. Actuators B 98 (2004) 227–232. [11] Y.S. Kim, I.S. Hwang, S.J. Kim, C.Y. Lee, J.H. Lee, CuO nanowire gas sensors for air quality control in automotive cabin, Sens. Actuators B 135 (2008) 298–303. [12] Y. Li, J. Liang, Z. Tao, J. Chen, CuO particles and plates: synthesis and gas-sensor application, Mater. Res. Bull. 43 (2008) 2380–2385. [13] S.A. Patil, L.A. Patil, D.R. Patil, G.H. Jain, M.S. Wagh, CuO-modified tin titanate thick film resistors as H2 -gas sensors, Sens. Actuators B 123 (2007) 233–239. [14] D. Chauhan, V.R. Satsangi, S. Dass, R. Shrivastav, Preparation and characterization of nanostructured CuO thin films for photoelectrochemical splitting of water, Bull. Mater. Sci. 29 (2007) 709–716. [15] A. Parretta, M.K. Jayaraj, A.D. Nocera, S. Loreti, L. Quercia, A. Agati, Electrical and optical properties of copper oxide films prepared by reactive af magnetron sputtering, Phys. Status Solidi (a) 155 (1996) 399–404.
[16] N.D. Hoa, N.V. Quy, Y.S. Cho, D.J. Kim, Porous single-wall carbon nanotube films formed by in-situ arc-discharge deposition for gas sensors application, Sens. Actuator B 135 (2009) 656–663. [17] Z.H. Liang, Y.J. Zhu, Single-crystalline CuO nanosheets synthesized from a layer precursor, Chem. Lett. 34 (2005) 214–215. [18] W.W. Wang, Y.J. Zhua, G.F. Cheng, Y.H. Huang, Microwave-assisted synthesis of cupric oxide nanosheets and nanowhiskers, Mater. Lett. 60 (2006) 609–612. [19] D.L. Cocke, R. Schennach, M.A. Hossain, D.E. Mencer, H. McWhinney, J.R. Parga, M. Kesmez, J.A.G. Gomes, M.Y.A. Mollah, The low-temperature thermal oxidation of copper, Cu3 O2 , and its influence on past and future studies, Vacuum 79 (2005) 71–83. [20] D. Wu, Q. Zhang, M. Tao, LSDA+U study of cupric oxide: electronic structure and native point defects, Phys. Rev. B 73 (2006) 235206 (6 pp.). [21] N. D. Hoa, N. V. Quy, D.J. Kim, Tin-oxide thin films sensors: oxidation temperature effect on NOx sensing, unpublished, 2009. [22] J.A. Rodriguez, J.Y. Kim, J.C. Hanson, M. Perez, A.I. Frenkel, Reduction of CuO in H2 : in situ time-resolved XRD studies, Catal. Lett. 85 (2003) 247–254. [23] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electroceram. 7 (2001) 143–167.
Biographies Nguyen Duc Hoa received his PhD in Materials Science and Engineering at Chungnam Nat. Univ., Korea, in 2009. He is now at a post-doctor in NanoMaterials and Application Laboratory at Chungnam Nat. Univ. His current interests are functionalizing of CNTs, nanowires structure materials, and their applications to field emitter, filed effect transistor, sensor, and solar cell devices. Sea Yong An received his BS in Materials Science and Engineering at Chungnam National University, Korea, in 2009. He is now in Master course in Materials Science and Engineering, Chungnam National University, Korea. His current interests are synthesis of graphene using CVD technique. Nguyen Quoc Dung received his Master of Materials Science in International Training Institute for Materials Science (ITIMS) at Hanoi University of Technology, Vietnam, in 2006. He is now in PhD course in Materials Science and Engineering, Chungnam National University, Korea. His current interests are nano-structure materials and their applications to bio- and chemical-sensors. Nguyen Van Quy received the degree of Master of Materials Science and Engineering at Chungnam Nat. Univ., Korea, in 2005. He is now in PhD Scholar course in Materials Science and Engineering, Chungnam Nat. Univ., Korea. His current interests include fabrication, characterization of carbon nanotube, and its application to nanoelectronic devices of field emitter and sensor. Dojin Kim received his PhD in Materials Science and Engineering at University of Southern California, USA, in 1989. He is a professor of School of Nanotechnology and Department of Materials Science and Engineering at Chungnam Nat. Univ., Korea. His current research interests are carbon nanotube synthesis and applications to electronic devices of field emitters, solar cells, gas sensors, etc. Also interested in III–V magnetic semiconductors growth and electrical characterizations.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Synthesis of p-type semiconducting cupric oxide thin films and their application to hydrogen detection Nguyen Duc Hoa, Sea Yong An, Nguyen Quoc Dung, Nguyen Van Quy, Dojin Kim ∗ Department of Materials Science and Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 18 November 2009 Received in revised form 28 January 2010 Accepted 12 February 2010 Available online 19 February 2010 Keywords: Cupric oxide Thin film sensor Hydrogen sensor Sensing mechanism
a b s t r a c t Nanostructured CuO thin films are synthesized by deposition and thermal oxidation of Cu on SiO2 substrates. The effects of oxidation temperatures on the morphologies and crystallinity of the CuO thin films are investigated by SEM, XRD, and XPS. The electrical and hydrogen sensing properties of CuO are studied by electrical resistance measurements. In addition, the effects of carrier gases on the gas response of CuO are investigated. The results showed that Cu was oxidized into monoclinic cupric oxide in the investigated temperature range from 300 to 800 ◦ C. The p-type semiconducting CuO thin film showed increases in electrical resistance upon exposure to hydrogen. The CuO thin film oxidized at 400 ◦ C showed the highest response as compared to others, and it was 3.72 for 6% H2 at an operating temperature of 250 ◦ C. The carrier gases played very important roles in the hydrogen sensing by CuO. That is, the sensor showed good response-and-recovery with a carrier gas of air but not with nitrogen. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The demand in environmental pollution monitoring, industrial monitoring and process control, medical diagnosis, public security, agriculture, and a variety of other industries requires the development of gas sensors for the early detection of chemical species and flammable and toxic gases [1]. A wide range of metal oxides have been used for gas sensors including TiO2 , ZnO, SnO2 , and WO3 [2–5]. Ordinary and commercial metal oxide-based gas sensors are usually in the form of thick or thin oxide films, and they exhibit a short lifetime, low sensitivity, and poor selectivity. Therefore, many researchers have attempted to find more suitable materials for better gas sensors that have high sensitivity and selectivity [6–8]. The metal oxides used for gas sensor application are mostly n-type semiconductors, and focus was made on improving their gas sensing performance [2–8]. Opposite to n-type semiconducting metal oxides, cupric oxide (CuO) is a p-type semiconductor with a band gap of 1.2–1.9 eV. Its applications may be for catalysis, lithium–copper oxide electrochemical cells, solar cells, and gas sensors [9–11]. The nanoparticles, plates, and nanowires of CuO were also reported to sense NO2 , H2 S, and CO [10,12]. CuO was also used to coat on tin titanate thick film sensor and enhance the
∗ Corresponding author. Tel.: +82 42 821 7648; fax: +82 42 823 7648. E-mail addresses: [email protected] (N.D. Hoa), [email protected] (S.Y. An), [email protected] (N.Q. Dung), [email protected] (N. Van Quy), [email protected] (D. Kim). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.02.045
response to hydrogen [13]. On the other hand, the thin film type has some advantages as compared to nanostructures. For instance, it is more stable and can be integrated with microelectronic silicon devices. In addition, the CuO thin film can be synthesized by the sol–gel or sputter methods [14,15]. The latter allows for the easy control of film thickness and combination with silicon semiconductor technology. However, the hydrogen sensing properties of CuO deposited by the sputter method has rarely been reported. In the present work, we report on the synthesis, characteristics, and gas sensing properties of a CuO thin film deposited by the sputter method. The CuO thin film is synthesized by the deposition of the Cu metal layer on SiO2 substrate followed by thermal oxidation at high temperatures. The effects of oxidation temperatures on the morphology, crystalline quality, and hydrogen sensing of the materials are investigated. In addition, the response to hydrogen for different carrier gases (dry air and nitrogen) is compared, and the sensing mechanism is discussed. 2. Experiment The nanostructured cupric oxide thin film was synthesized by the deposition of a copper layer on thermally oxidized silicon substrate-supported Pt electrodes by using a dc sputter followed by the thermal oxidation process. In brief, a two-inch copper (99.9%) target was used for Cu deposition. The sputtering conditions were as follows: based pressure of 10−6 Torr, Ar working pressure of 2 × 10−3 Torr, and DC plasma power of 10 W. Copper thin film with a thickness of 100 nm was deposited on the silicon substrate through a shadow mask to make a 2mm × 3 mm pattern.
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Fig. 1. Schematic of the sensor property measurement system.
The oxidation process was performed in a horizontal tube furnace of air ambient. The as-deposited Cu thin films were placed in the center of the quart tube, and then the temperature was increased to oxidation temperatures (Tox ) of 300, 400, 500, 600, 700, and 800 ◦ C in 30 min. The oxidation was then carried out for 2 h. Field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) measurements were carried out to examine the morphology and crystal structure of the films. The X-ray diffraction (XRD) measurements were carried out using Cu K␣ -X-radiation with wavelength = 1.54178 Å (Model: D/max 2500, Rigaku, Japan). XPS measurements were carried out using an AXIS-NOVA (Kratos) system (X-ray source: monochromated Al K␣ radiation) to investigate the stoichiometry of CuO thin films in the base pressure of ∼10−9 Torr. The electrical and gas sensing properties of the nanostructured CuO thin films were studied by measuring their resistances in a test chamber with different gases ambient as reported in Ref. [16]. Fig. 1 shows the schematic of used gas supply and measurement system. Dry air or high-purity nitrogen (5N) was used as references, and concentrated H2 was employed as the test gas. The test gases regulated to the desired concentrations were introduced into the test chamber using a series of mass flow controllers. The gas sensing properties were measured in a dynamic condition in which the gases continuously flowed through the chamber with a total flow rate of 500 sccm during the measurements. Prior to the test, the sensors were pre-heated at 250 ◦ C for 1 h for stable references, and then the sensing property measurements were performed. For each measurement, hydrogen was introduced into the test chamber for 10 min and then stopped. The sensors resistances were automatically recorded using a programmable electrometer (Keithley 2400) controlled by a personal computer using the Labview program. The sensor response, S, is defined as follows: S = R/Ro where Ro and R are the resistance of the sensor in reference gas (air or nitrogen) and the test gases (H2 ), respectively. 3. Results and discussion The surface FE-SEM images of the CuO thin films formed by oxidation at different temperatures are shown in Fig. 2(a)–(f). They can be compared with the smooth surface of the as-deposited Cu thin film shown as the inset in Fig. 2(a). The very rough and bumpy CuO surfaces after the oxidation process are due to the volume expansion that occurs during Cu to CuO conversion. The surfaces
of the CuO thin films became rougher and rougher with the oxidation temperatures due to the increasing crystal growth rates. The average grain size of ∼15 nm for the thin film oxidized at 300 ◦ C increased up to several hundreds of nm for 800 ◦ C oxidation. The crystal structures of cupric oxide formed during the oxidation process were characterized by XRD as shown in Fig. 3. The XRD patterns showed two strong peaks at 2 = 35.5 and 38.8◦ corresponding to (0 0 2) and (1 1 1) planes of monoclinic CuO, respectively, according to the standard JCPDS card no. 80-1917. This result is well coincided with other XRD reports [17,18]. The oxidation of Cu took place according to the following equations: Cu + O2 = CuO2 , 3CuO2 + (1/2)O2 = 2Cu3 O2 , and Cu3 O2 + (1/2)O2 = 3CuO [19]. Since monoclinic CuO forms at around 270 ◦ C, the single phase was observed at all investigated oxidation temperatures. The crystalline size of materials can be roughly estimated by using Scherrer’s formula, D = /ˇ cos , where ˇ is the full width half maximum (FWHM) of the strongest diffraction peak, is the diffraction angle, is the X-ray wavelength (0.145 nm), and (∼0.89) is Scherrer’s constant [10]. The crystalline size was 20, 25, 29, 33, 37, and 40 nm for the CuO thin films oxidized at 300, 400, 500, 600, 700, and 800 ◦ C, respectively. This increase in crystalline size with the oxidation temperature coincides with the trend observed by SEM. However, the values are much smaller than the SEM observation particularly at high oxidation temperatures. For example, the average grain size of SEM increased from 166 to 180 nm (∼7.7%) whereas the average crystalline size of XRD increased from 37 to 40 nm (∼7.5%) for the oxidation temperature change from 700 to 800 ◦ C. The estimation via SEM observation is influenced by the sampling of the viewing window, but in addition, the difference suggests that the visually observed one grain is possibly formed by several crystallites of different crystallographic orientations. The XPS spectra of CuO thin films oxidized at 300, 400, and 600 ◦ C are measured. The survey scans of all CuO thin films show photoelectron lines for Cu, O, Si, and C (not shown). There were not any detectable impurity elements other than the commonly observed carbon contamination and Si from the silicon dioxide substrate. To examine the stoichiometry of the cupric oxides, the high resolution scans for O1s were analyzed. The fittings of the O1s peaks with the Gaussian functions are shown in Fig. 4(a)–(c) assuming that the oxygen bindings come from O2− (529.1 eV), Cu–O (530 eV), C O (531.36 eV), Si–O (532.36 eV), and O–C O (533.51 eV). The fitting results are excellent and the compositions estimated as such are Cu0.82 O, Cu0.76 O, and Cu0.70 O for the oxidation at 300, 400, and 600 ◦ C, respectively. While the trend of the highly excess incorporation of oxygen at higher temperatures is expected, the huge ı values in Cu1−ı O expression as Cu vacancy are rather unexpected. Without additional confirmation it is difficult to take the whole Cu deficiency values purely as the Cu vacancy concentrations in the lattice. Furthermore, the examinations do not explain the trend in sensing results as will be shown if Cu vacancy should play the key role in charge transport. Therefore, the XPS analysis alone seems to not clearly explain the Cu vacancy concentrations. The responses of the CuO thin films to 6% hydrogen gas were measured at 250 ◦ C and are summarized in Fig. 5. The responses for the films oxidized at different temperatures are summarized as the inset. All sensors showed an increase in resistance upon exposure to hydrogen and recovered to the initial values with a stop in the flow of H2 . This resistance increase upon exposure to the reducing gas indicates the p-type conduction of CuO. The p-type semiconducting property is due to the Cu vacancies in the crystal structure (Cu1−ı O) [20]. Thin film oxidized at 300 ◦ C showed the lowest response (S ∼ 1.3), whereas that oxidized at 400 ◦ C showed the highest response (S ∼ 3.7). One may consider the parameters that could affect the response: (i) grain size, (ii) porosity, and (iii) the density of Cu vacancy in the material.
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Fig. 2. FE-SEM images of CuO thin films formed by oxidation at (a) 300 ◦ C, (b) 400 ◦ C, (c) 500 ◦ C, (d) 600 ◦ C, (e) 700 ◦ C, and (f) 800 ◦ C. Inset of (a) is for the as-deposited Cu thin film.
Fig. 3. XRD of CuO thin film oxidized for 2 h in air at (a) 300 ◦ C, (b) 400 ◦ C, (c) 500 ◦ C, (d) 600 ◦ C, (e) 700 ◦ C, and (f) 800 ◦ C.
The porosity, or the size and density of the pores, is not a simple parameter to evaluate the effect since it controls the diffusion kinetics of the gases into the films. However, grain growth will reveal a monotonic decrease in response, and a decrease at temperatures above 400 ◦ C should partly be due to this effect. Similarly, the porosity may increase with the grain size, and the increase up to 400 ◦ C can partly be due to this effect. The tradeoff between grain size and porosity may reveal such change in the response with the oxidation temperature. If another very probable cause, the density of Cu vacancy, is to be the dominant parameter, the density of Cu vacancy should be the highest for the 400 ◦ C oxidation. However, as discussed in Fig. 4, the stoichiometry analysis via XPS does not properly explain the results at present. Furthermore, measurements revealed that the resistances do not remarkably change with the oxidation temperatures in 300–700 ◦ C. These results are different from the SnOx thin films, where the oxidation temperature greatly affects the film stoichiometry or the layer resistance, which became the major parameter to determine the response of the sensor [21]. We tentatively conclude that the change of response in CuO thin films was mainly caused by the combined effect of the grain size and porosity while the details need to be further explored.
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Fig. 5. The change in response (R/Ro) of CuO thin films oxidized at different temperatures upon exposure to 6% H2 at 250 ◦ C.
(Fig. 6(b)). When the sensor was first exposed to the diluted hydrogen, the sensor resistance increased abruptly from 1.3 × 105 to 4.1 × 108 in ∼5 min and then decreased to 1.3 × 101 in the following 5 min. After these dramatic orders of magnitude change at the first exposure, the sensor showed no response at all from the second exposure to hydrogen.
Fig. 4. The XPS O1s peaks of high magnification for samples oxidized at (a) 300 ◦ C, (b) 400 ◦ C, and (c) 600 ◦ C. Fittings with bindings of O2− , Cu–O, C O, Si–O, and O–C O were done.
The hydrogen sensing of CuO was further examined in order to study the sensing mechanism. The cyclic response of the sensor to hydrogen was measured using either dry air or nitrogen as the dilution gas, and the comparison results are shown in Fig. 6. It is indicated that a good response-and-recovery behavior was repeated when dry air was used for dilution (Fig. 6(a)). However, a very interesting result was observed when nitrogen was used
Fig. 6. The response-and-recovery behaviors of CuO thin film oxidized at 400 ◦ C in different dilution gases of (a) dry air and (b) nitrogen at 250 ◦ C.
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One may explain this interesting difference by considering the interactions occurring in the CuO grains. First, the sensing mechanism of CuO may be explained as follows for the response in dry air. The pre-adsorbed oxygen may establish forms of O− and/or O2− at the CuO surface through charge exchange interactions with CuO following Eqs. (1)–(4). When such CuO is exposed to hydrogen molecules, the adsorbed hydrogen interacts with the pre-adsorbed oxygen as Eqs. (5)–(7) show. The free electrons released via the reaction between the H2 molecules and the pre-adsorbed O− or O2− neutralize the holes, or the majority carrier in p-type CuO. This compensation results in a decrease in the hole carriers in CuO, and consequently, an increase in sensor resistance. Note here that the oxygen molecules are continuously supplied from the dilution gas (dry air) and are adsorbed on the CuO surface while the interaction with hydrogen continuously forms water molecules to escape from the surface. When the hydrogen flowing is stopped for recovery, the oxygen molecules in air will adsorb on the surface of CuO, and the capture of electrons through the processes indicated in Eqs. (1)–(4) will reduce the sensor resistance towards the initial stable surface state of CuO. O2 (gas) ↔ O2 (ads) −
(1) −
O2 (ads) + e ↔ O2 (ads) −
−
(2)
−
(3)
(ads)
(4)
O2 (ads) + e ↔ 2O (ads) −
−
O (ads) + e ↔ O
2−
H2 (gas) ↔ H2 (ads) −
H2 (ads) + O (ads) ↔ H2 O + e H2 (ads) + O
2−
(5) −
(ads) ↔ H2 O + 2e
e− + h◦ ↔ Null
(6) −
(7) (8)
However, the situation becomes different when the refreshing gas does not contain oxygen as in the case when N2 was used as the dilution gas. At the first cycle of exposure, H2 molecules interact with the pre-adsorbed oxygen on the surface of CuO, and the sensor resistance increases as shown above. Note, however, that the resistance increase is about 3 orders of magnitude higher with the dilution in N2 . The pre-adsorbed oxygen again forms H2 O molecules via Eqs. (6) and (7) as indicated above, and is removed from CuO. However, in this case, no oxygen molecules are supplied during the exposure to the CuO surfaces from flowing gases. This sudden total exhaust in surface oxygen resulted in the initial dramatic increase in resistance at the first cycle. Furthermore, the continuous hitting of hydrogen on the CuO surface reduces the oxide to Cu elements via H2 + CuO ↔ Cu + H2 O [22]. This metallic Cu formation in the grains may be the cause for the dramatic decrease in resistance in the first cycle. The reduction process might be accelerated due to the elevated temperature in measurements at 250 ◦ C. Now, the recovery in N2 does not occur due to the stop of oxygen supply, and the metallic Cu layer is maintained. Also, when hydrogen gas diluted in N2 is again introduced in the second cycle, the reaction of oxygen with hydrogen does not further take place on the CuO surfaces, so the sensor showed no response. Therefore, the study dramatically demonstrates the key role of oxygen adsorbed on the surfaces of sensor materials particularly of metal oxides. In other words, the interfacing and reactions between the sensor materials and analyte gases are in most cases mediated by oxygen, the chemically active element in the sensing environment. Cupric oxide is a dramatic example showing no reaction with the analyte gas, hydrogen, without mediation via oxygen. Providing air environments is essential for development of engineering sensors. However, intentional environment controls are often useful and required for studying of the intrinsic reactions between the material and the surroundings.
Fig. 7. The hydrogen sensing properties of the CuO thin film oxidized at 400 ◦ C. (a) The sensor response upon exposure to 6% H2 measured at different operating temperatures and (b) the sensor response measured with varying H2 concentrations at operating temperature of 250 ◦ C.
The sensing properties of CuO at different H2 concentrations and operating temperatures were also investigated for the oxidized at 400 ◦ C (showing the highest response). Fig. 7(a) shows the change in sensor response (R/Ro ) upon exposure to 6% H2 measured at different temperatures. The sensor showed the highest response of 3.7 at 250 ◦ C. The response measured at different H2 concentrations at the optimum working temperature of 250 ◦ C is shown in Fig. 7(b). The sensor response changed from 1.4 to 3.7 in the investigated range of 1–6%, but the trend is opposite to the calculated power law of the concentration dependence [23]. 4. Conclusion The synthesis, characterization, and gas sensing properties of granular cupric oxide thin films were investigated. A single phase monoclinic crystal structure of CuO was obtained by oxidizing the Cu thin films in air for 2 h at a temperature range of 300–800 ◦ C. The CuO thin film oxidized at 400 ◦ C showed the highest response of 3.72 for 6% H2 at 250 ◦ C. The effect of carrier gases on the sensing was also studied. The null response of CuO to the hydrogen in an oxygen-free environment was dramatically observed. The results confirmed that the pre-adsorbed oxygen plays a crucial role in the hydrogen sensing of CuO. These provide useful information in discussing the sensing mechanism in CuO and other oxide materials.
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Acknowledgment This work was supported by the National Research Laboratory program, Korea. References [1] T. Zhang, S. Mubeen, N.V. Myung, M.A. Deshusses, Recent progress in carbon nanotube-based gas sensors, Nanotechnology 19 (2008) 332001 (14 pp.). [2] M.R. Mohammadi, D.J. Fray, M.C. Cordero-Cabrera, Sensor performance of nanostructured TiO2 thin films derived from particulate sol–gel route and polymeric fugitive agents, Sens. Actuators B 124 (2007) 74–83. [3] (a) D.R. Patil, L.A. Patil, Cr2 O3 -modified ZnO thick film resistors as LPG sensors, Talanta 77 (2009) 1409–1414; (b) A. Umar, M.M. Rahman, Y.B. Hahn, Ultra-sensitive hydrazine chemical sensor based on high-aspect-ratio ZnO nanowires, Talanta 77 (2009) 1376–1380. [4] J. Arbiol, E. Comini, G. Faglia, G. Sberveglieri, J.R. Morante, J. Cryst. Growth 310 (2008) 253. [5] T. Siciliano, A. Tepore, G. Micocci, A. Serra, D. Manno, E. Filippo, WO3 gas sensors prepared by thermal oxidization of tungsten, Sens. Actuators B 133 (2008) 321–326. [6] H.R. Kim, K.I. Choi, J.H. Lee, A. Sheikh, Akbar, Highly sensitive and ultra-fast responding gas sensors using self-assembled hierarchical SnO2 spheres, Sens. Actuators B 136 (2009) 138–143. [7] M.W. Ahn, K.S. Park, J.H. Heo, D.W. Kim, K.J. Choi, J.G. Park, On-chip fabrication of ZnO-nanowire gas sensor with high gas sensitivity, Sens. Actuators B 138 (2009) 168–173. [8] N. Izu, S. Nishizaki, T. Itoh, M. Nishibori, W. Shin, I. Matsubara, Gas response, response time and selectivity of a resistive CO sensor based on two connected CeO2 thick films with various particle sizes, Sens. Actuators B 136 (2009) 364–370. [9] J. Zhang, J. Liu, Q. Peng, X. Wang, Y. Li, Nearly monodisperse Cu2 O and CuO nanospheres: preparation and applications for sensitive gas sensors, Chem. Mater. 18 (2006) 867–871. [10] A. Cruccolini, R. Narducci, R. Palombari, Gas adsorption effects on surface conductivity of nonstoichiometric CuO, Sens. Actuators B 98 (2004) 227–232. [11] Y.S. Kim, I.S. Hwang, S.J. Kim, C.Y. Lee, J.H. Lee, CuO nanowire gas sensors for air quality control in automotive cabin, Sens. Actuators B 135 (2008) 298–303. [12] Y. Li, J. Liang, Z. Tao, J. Chen, CuO particles and plates: synthesis and gas-sensor application, Mater. Res. Bull. 43 (2008) 2380–2385. [13] S.A. Patil, L.A. Patil, D.R. Patil, G.H. Jain, M.S. Wagh, CuO-modified tin titanate thick film resistors as H2 -gas sensors, Sens. Actuators B 123 (2007) 233–239. [14] D. Chauhan, V.R. Satsangi, S. Dass, R. Shrivastav, Preparation and characterization of nanostructured CuO thin films for photoelectrochemical splitting of water, Bull. Mater. Sci. 29 (2007) 709–716. [15] A. Parretta, M.K. Jayaraj, A.D. Nocera, S. Loreti, L. Quercia, A. Agati, Electrical and optical properties of copper oxide films prepared by reactive af magnetron sputtering, Phys. Status Solidi (a) 155 (1996) 399–404.
[16] N.D. Hoa, N.V. Quy, Y.S. Cho, D.J. Kim, Porous single-wall carbon nanotube films formed by in-situ arc-discharge deposition for gas sensors application, Sens. Actuator B 135 (2009) 656–663. [17] Z.H. Liang, Y.J. Zhu, Single-crystalline CuO nanosheets synthesized from a layer precursor, Chem. Lett. 34 (2005) 214–215. [18] W.W. Wang, Y.J. Zhua, G.F. Cheng, Y.H. Huang, Microwave-assisted synthesis of cupric oxide nanosheets and nanowhiskers, Mater. Lett. 60 (2006) 609–612. [19] D.L. Cocke, R. Schennach, M.A. Hossain, D.E. Mencer, H. McWhinney, J.R. Parga, M. Kesmez, J.A.G. Gomes, M.Y.A. Mollah, The low-temperature thermal oxidation of copper, Cu3 O2 , and its influence on past and future studies, Vacuum 79 (2005) 71–83. [20] D. Wu, Q. Zhang, M. Tao, LSDA+U study of cupric oxide: electronic structure and native point defects, Phys. Rev. B 73 (2006) 235206 (6 pp.). [21] N. D. Hoa, N. V. Quy, D.J. Kim, Tin-oxide thin films sensors: oxidation temperature effect on NOx sensing, unpublished, 2009. [22] J.A. Rodriguez, J.Y. Kim, J.C. Hanson, M. Perez, A.I. Frenkel, Reduction of CuO in H2 : in situ time-resolved XRD studies, Catal. Lett. 85 (2003) 247–254. [23] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electroceram. 7 (2001) 143–167.
Biographies Nguyen Duc Hoa received his PhD in Materials Science and Engineering at Chungnam Nat. Univ., Korea, in 2009. He is now at a post-doctor in NanoMaterials and Application Laboratory at Chungnam Nat. Univ. His current interests are functionalizing of CNTs, nanowires structure materials, and their applications to field emitter, filed effect transistor, sensor, and solar cell devices. Sea Yong An received his BS in Materials Science and Engineering at Chungnam National University, Korea, in 2009. He is now in Master course in Materials Science and Engineering, Chungnam National University, Korea. His current interests are synthesis of graphene using CVD technique. Nguyen Quoc Dung received his Master of Materials Science in International Training Institute for Materials Science (ITIMS) at Hanoi University of Technology, Vietnam, in 2006. He is now in PhD course in Materials Science and Engineering, Chungnam National University, Korea. His current interests are nano-structure materials and their applications to bio- and chemical-sensors. Nguyen Van Quy received the degree of Master of Materials Science and Engineering at Chungnam Nat. Univ., Korea, in 2005. He is now in PhD Scholar course in Materials Science and Engineering, Chungnam Nat. Univ., Korea. His current interests include fabrication, characterization of carbon nanotube, and its application to nanoelectronic devices of field emitter and sensor. Dojin Kim received his PhD in Materials Science and Engineering at University of Southern California, USA, in 1989. He is a professor of School of Nanotechnology and Department of Materials Science and Engineering at Chungnam Nat. Univ., Korea. His current research interests are carbon nanotube synthesis and applications to electronic devices of field emitters, solar cells, gas sensors, etc. Also interested in III–V magnetic semiconductors growth and electrical characterizations.