Single crystal cupric oxide nanowires: Lengthand density-controlled growth and gas-sensing characteristics Le Duy Duc, Dang Thi Thanh Le, Nguyen Van Duy, Nguyen Duc Hoa, Nguyen Van Hieu
Received date: 26 August 2013 Revised date: 7 November 2013 Accepted date: 12 November 2013 Cite this article as: Le Duy Duc, Dang Thi Thanh Le, Nguyen Van Duy, Nguyen Duc Hoa, Nguyen Van Hieu, Single crystal cupric oxide nanowires: Lengthand density-controlled growth and gas-sensing characteristics, Physica E, http: //dx.doi.org/10.1016/j.physe.2013.11.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Single crystal cupric oxide nanowires: Length- and density-controlled growth and gas-sensing characteristics Le Duy Duc, Dang Thi Thanh Le*, Nguyen Van Duy, Nguyen Duc Hoa, Nguyen Van Hieu*
International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No. 1 Dai Co Viet, Hanoi, Vietnam.
Corresponding author * Dang Thi Thanh Le, PhD * Nguyen Van Hieu, Professor International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST) No.1, Dai Co Viet Road, Hanoi, Vietnam Tel: 84 4 38680787; Fax: 84 4 38692963 [email protected]/[email protected] Address: No.1 Dai Co Viet, Hanoi, Vietnam
Abstract: Nanowire structured p-type CuO semiconductor is a promising material for gassensing applications because of its unique electrical and optical properties. In this study, we demonstrate the length and density controlled synthesis of single crystal CuO nanowires (CuO NWs) by a simple and convenient thermal oxidation of high-purity copper foils in ambient atmosphere. The density and length of the CuO NWs are controlled by varying the oxidation temperature and heating duration to investigate their growth mechanism. Assynthesized materials are characterized by different techniques, such as X-ray diffraction, field emission-scanning electron microscopy, and high-resolution transmission electron microscopy. The gas-sensing characteristics of the CuO NWs are tested using hydrogen and ethanol gases. The results show that the CuO NWs could potentially sense hydrogen and 1
ethanol gases given a working temperature of 400 °C. Keywords: cupric oxide nanowires, growth mechanism, ethanol sensor
1. Introduction The development of a low-cost and scalable gas sensor for the detection of toxic, combustible, explosive and flammable gases with fast response and high sensitivity is extremely important because of its huge potential applications in various fields such as environmental monitoring and disease diagnosis [1]. For instance, receiving early warning of explosive gas leakage could potentially prevent disaster, and the ability to trace VOCs at low concentrations can help diagnose early state of lung cancer [2]. In the early 1960s, Seiyama, and Taguchi introduced a sensor that operated based on the variations in the resulting electrical resistance (conductance) of a chemical reaction and/or the absorption of the analytical species and the surface of metal oxide semiconducting layers [3]. Since then, research on resistive gas sensors has received enormous attention [4]. Up until now, minimization of production cost and the improvement of sensor performance still garner massive interest in the field of gas sensor technology. On the other hand, nanowire-structured cupric oxide material has also received a lot of attention because of its facile and low cost synthesis and potential applications in various fields, such as heterogeneous catalysts, electrochemical capacitors, photovoltaic cells, field emission nano-devices and gas sensors [5]. Cupric oxide is known as an environmentfriendly and thermally stable p-type semiconductor with a bandgap of ~1.2 eV; thus, it is believed to be an excellent candidate for long-term stable and low-power consumption sensing devices. Indeed, the particles, plates [6], thin film [7], nanosheets [8], and nanowires 2
of CuO were fabricated for CO, NO2, H2, and H2S sensing applications [9],[10]. According to the gas-sensing mechanism, the gas adsorption/desorption processes take place on the surface of sensing materials which lead to the expansion or reduction of the electron depletion (ntype) or accommodation (p-type) regions and result in varying the sensing conductance, thus large sensing sites are advantages for enhancement of sensor performances. In addition, small size compatible with the Debye length of the sensing crystals was believed to significantly enhance the sensitivity of devices [11]. Therefore, the nanowire structures exhibit significant advantages, such as high length-to-diameter ratio and small-diameter compatibility with Debye length [12]. These characteristics promise excellent sensitivity as well as cost efficiency with regards to device fabrication. Reports on the synthesis and application of CuO NWs for detecting toxic and flammable gases are increasing, where the CuO NWs could be grown by various methods, such as chemical routes [13], template ways [14] and eletrospinning techniques [15]. CuO NWs were also grown by simply heating the copper foil/wire in ambient atmosphere [16]. This method is one of the most convenient and simple ways to synthesize CuO NWs with high-quality single crystals [17]. However, large quantities of NWs are needed for mass production and the cost of producing these sensing devices must be lowered. Therefore, controlling the length and density towards the scalable synthesis of high-quality CuO NWs is still an interesting undertaking. In addition, the proper application of CuO NWs for gas sensors is also important. In this study, we demonstrate the density- and length-controlled synthesis of the CuO NWs by varying the oxidation temperatures and heating times. The growth mechanism of the CuO NWs is obtained by directly heating Cu foils in ambient atmosphere and is also clarified by comparing the surface morphology of the Cu foil and the Cu thin film after growth. The growth of CuO NWs is believed to obey the vapor solid (VS) model, where a higher surface roughness is preferred for the growth of the CuO NWs. The CuO NWs obtained by directly 3
heating Cu foils are then used for low-concentration ethanol and hydrogen sensor applications.
2. Experimental CuO NWs were synthesized by directly heating Cu foils in ambient atmosphere with humidity of about 60-90% [20]. In a general synthesis process, the commercial Cu foils were cut into small pieces (1×1 cm2), washed with acetone for 15 min, and then rinsed in distilled water to remove all organic contamination. The small Cu-foil pieces were dipped in hydrochloric acid solution (1 M) solution for a few minutes to clean the unwanted oxide layer on the surface. The samples were then rinsed once more with distilled water and blown with clean, compressed gas to dry. Thereafter, the Cu foils were loaded into the center of a quartz tube to grow the CuO NWs. The CuO NWs were grown by increasing the temperature of the quartz tube to 400 °C, 500 °C, or 600 °C and maintained for various time periods. The furnace temperature was ramped up at a rate of 15oC/min. After growth, the quartz tube furnace was naturally cooled down by switching off the furnace to avoid the cracking of the oxide layers on the surface of the Cu foils. None of the air or N2/O2 mixture was flown through the system during CuO NWs growth, but the quartz tube was kept with two ends opened. The CuO NWs were extracted as a black oxide layer on the surface of the Cu foil. The morphology and crystal structure of the Cu foils after growing the CuO NWs were characterized via field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD, Cu Ka). To determine its gas-sensing characteristics, the CuO NWs were collected and then deposited into a pre-fabricated interdigitated Pt electrode substrate. The Pt-interdigitated electrode with an area of 800 µm × 1600 µm was fabricated by sputtering deposition of 50 4
nm Cr and 200 nm Pt on thermally oxidized silicon substrate using a conventional photolithographic method. The width of the finger and the gap between two fingers was 20 µm. For deposition of CuO NWs, the silicon substrate supported the interdigitated Pt electrode was cleaned in ethanol solution under ultrasonic vibration to remove all contaminations. After that, CuO NWs dispersed in butyl alcohol solution were dropped onto the substrate and then dried in a hot plate at temperature of ~ 200oC for 2 h. The sensing measurements were taken using H2 and ethanol at various temperatures by a flow-through technique [18]. In our experiment, we used the standard gas concentration of Cstd(ppm)=10,000 ppm diluted in nitrogen. To obtain a lower concentration, we mixed the standard gas with dry air as carrier by using a series of mass flow controllers. The gas concentration was calculated as C(ppm)=Cstd(ppm)×f/(f+F); where f, and F is the flow rate of analytic gas and dry air, respectively. The total gas flow rate was kept constantly at 400 sccm during sensing measurement. For electrical measurement, two tungsten tips were used to make contact with Pt electrodes [18]. The effect of metal-metal contact resistance between tungsten tips and Pt electrodes was ignored because it was smaller compared with the resistance of CuO NW. Prior to these measurements; dry air was blown through the sensing chamber until the desired stability of sensor resistance was reached. During this process, the resistance of the sensors was continuously measured using a Keithley (2700) instrument connected to a computer while dried air and analytic gases were switched on and off during each cycle. Sensor response was defined as S=R/Ro, where Ro and R are the resistances of the sensor in dry air and analytic gas, respectively. 3. Results and discussion The morphology of the Cu foils examined through SEM images after heating in ambient atmosphere at different temperatures is shown in Figure 1. The SEM images 5
confirmed that the CuO NWs were successfully grown on the Cu foils after thermal oxidation at 400 °C, 500 °C, and 600 °C for 2 h. The CuO NWs were quite aligned and grew nearly perpendicular to the surface of the Cu foil. The CuO NW was not cylindrical; rather, it took the shape reminiscent of a needle or a blade of grass where the stump is a litter bigger than the tip. The tip of the CuO NWs has an average diameter of about 30 nm, whereas its stump has an average diameter of about 100 nm for samples grown at 400 °C (Figure 1(A, B)). The thermal oxidation temperature significantly affected the diameter, length and density of the CuO NWs. The diameters of the CuO NWs increased proportionately with the increase in thermal oxidation temperature, whereas their length and density decreased. The CuO NWs grown at 600 °C (Figure 1(E, F)) have the largest diameter and lowest density, whereas at 400 °C, the CuO NWs have the smallest diameter and the greatest density (Figure 1(A, B)). Figure 1 The effect of growth time on the morphology of the CuO NWs was examined by keeping the temperature at 400 °C and increasing the heating time from 2 h to 36 h. The length of the CuO NWs was longer and their density became higher with increasing growth time, as shown in Figure 2. The length of the CuO NWs can reach an average value of hundreds of micrometers with the prolonged heating time. Moreover, further prolonging the growth time no longer had any significant effects on the length of the CuO NWs (data not shown). Higher density and longer CuO NWs are very effective for sensor applications because they provide a larger quantity of CuO NWs for sensor fabrication, which reduces the cost of the devices. Figure 2 The crystal structures of the synthesized materials examined via XRD are shown in Figure 3. From the XRD pattern of the NWs on the Cu foil (Figure 3(A)), the main peaks are indexed at the monoclinic phase of CuO (JCPDS, 48-1548), whereas few peaks correspond to the 6
cubic Cu2O (JCPDS, 05-0667) and face-centered cubic Cu (JCPDS, 04-0836) phases. Based on literature, NWs are single phased CuO, and Cu2O is only located in the underlying layer if the temperatures are within the range of this experiment [19]. The growth mechanism of the NWs was based on the VS model but not on VLS because the oxidation temperature at 400 °C was significantly lower than the evaporation and melting points of Cu metal; thus, no liquid can be formed [20]. The top Cu layer was also believed to be easily oxidized to form Cu2O because Cu2O has a lower oxidization Gibb free energy than CuO. The Cu2O was then oxidized into CuO phase when the heating time was further increased. The reactions for the formation of the CuO NWs are presented by Eqs. (1)−(2):
4Cu + O2 → 2Cu2O
(1)
2Cu 2O + O2 → 4CuO (2)
To confirm the single phase of the synthesized CuO NWs, XRD patterns were recorded from the products collected from the Cu foils after heating in air for 4 h, as shown in Figure 3(B). The results revealed that the synthesized CuO NWs was a single phase of monoclinic crystal structure, where all the diffraction peaks were well indexed in the profile of monoclinic CuO (JCPDS, 48-1548). The full widths at half of the diffraction peaks were also very broad as a result of the nano-crystalline structure of the CuO NWs. The monoclinic crystal structure of CuO NWs was also confirmed by Raman data, as shown in Figure 3(C). The Raman spectrum shows three active modes at around 297, 347, and 632 cm-1, where the peak at 297 cm-1 belongs to the Ag mode and the two latter active modes at 347 and 632 cm-1 are assigned to the B g modes of monoclinic CuO [21],[22]. Figure 3
In our previous report, the sputtered Cu thin film with a thickness of about 100 nm was heat7
treated at a temperature of 400°C at similar conditions with those in this study. However, no CuO NWs were obtained in the 100 nm Cu thin film after heat treatment [17]. The surface roughness and grain size of the Cu layer are believed to determine the growth of CuO NWs, where a higher surface roughness and a larger grain size are favorable for the growth of CuO NWs [23],[24]. To confirm this hypothesis, the surface of the Cu thin film and the Cu foil were examined via SEM, as shown in Figure 4(A) and (B), respectively. After heat treatment, the surface of the Cu thin film was much smoother than that of the Cu foil. The grains observed on the surface of the Cu thin film has an average size of 60 nm, whereas that value is about 500 nm for the Cu foil. The grains are easily oxidized and formed on the Cu2O phase when the duration of heating time in air was prolonged because of the high level of porosity of the Cu foil. Thereafter, the Cu2O were then transformed to the CuO phase and grew along the easy axis to form the CuO NWs by a continuous supply of both Cu from the Cu films and oxygen from air. The inset of Figure 4(B) shows a CuO tip grown on the Cu foil by directly heating in air. The observations are consistent with the proposed growth mechanism that was attributed to the grain boundary diffusion of Cu ions through the Cu2O and oxygen ions through the CuO layer. In thermal heating conditions, the grain boundary diffusion dominates the process, and most copper ions reach the top surface of the Cu2O layer on the same spots where these new NW grew [25], [26]. Figure 4 To determine the materials and gas-sensing characteristics, the as-synthesized CuO NWs were immersed in ethanol by using an ultrasonic vibration to break the NWs from its substrate. The solution of CuO NWs was then dropped onto the interdigitated Pt electrode substrate. A scheme designing the CuO nanowire sensor and an equivalent electrical circuit of the device are shown in Figs. 5(A) and (B) respectively. The sensor involved the contact resistance between the platinum surface and the first nanowires, the contact resistance between nanowires (nanowire-nanowire junctions), and surface resistance of nanowires. CuO is a p-type semiconductor with a work function of 5.2 eV, whereas that value for metallic Pt electrode is 6.35 eV, thus the Ohmic contact was made between CuO NWs and Pt electrode. Therefore, the contact resistance between CuO NWs and Pt electrode is ignorable. The contribution on the resistance of sensor can be considered involving (i) the nanowirenanowire junctions, and (ii) the surface resistance of nanowires. Figures 5(C)–(F) show the 8
SEM and TEM images of the device and the CuO NWs, respectively. In Figure 5(C), a thin layer network of CuO NWs was deposited on the substrate and functioned as a sensing layer for the sensor. A further magnified SEM image (Figure 5(D)) shows that the CuO NWs were broken down into the shortened NWs (500 nm to 3 µm in length) after ultrasonic dispersion and dropping onto the substrate. The device fabrication processes shortened the CuO NWs but did not change their diameter. Figure 5(E) shows the TEM image of CuO NWs having different diameters and lengths as a result of the dispersion process that broke down the CuO NWs. Higher magnification TEM images (Fig. 5(F), and inset) show the clear lattice fringes ¯
with an interspace of 0.25 nm belonging to the atomic planes ( 1 11) of monoclinic CuO. Furthermore, no significant grain boundary was observed indicating that the CuO NW is a high-quality single crystal [20]. Figure 5 The gas-sensing characteristics of the synthesized CuO NWs were studied for the detection of ethanol and hydrogen at different temperatures. Figure 6 shows the hydrogen sensing characteristics of the CuO NWs measured at 300 °C, 350 °C, and 400 °C. The resistance of the sensor increased abruptly when exposed to a reducing gas such as H2. The increase in the electrical resistance of the sensor upon exposure to a reducing gas (H2) indicated the p-type semiconducting property of CuO, which was caused by Cu vacancies in the crystal structure (Cu1-δO). The sensor showed good response and recovery characteristics across all test temperatures. The 90% response (τres) and recovery (τre) times were about 40 and 35 sec, respectively (inset, Figure 6(C)). The fast response and recovery times also suggested the physical adsorption of analytic gas molecules and sensing layer. The sensor response (R/Ro) as a function of hydrogen concentration measured at different temperatures is shown in Figure 6(D). When the working temperature was increased, the responsivity of the CuO NWs sensor also increased at all concentrations of hydrogen. In addition, a nearly linear dependence of the sensor response to hydrogen concentration was observed. The sensor response was highest at 400 °C followed by 350 °C and 300 °C. The sensor response could be improved by increasing the working temperature; however, a higher working temperature meant the device may require higher power consumption in practical application. In this study, at a working temperature of 400 °C, the responses were 1.16, 1.35, 1.53, and 1.79 at 9
concentrations of 100, 250, 500 and 1000 ppm of hydrogen, respectively. The hydrogen gassensing characteristic of the fabricated sensor originates from the interaction between the hydrogen molecules and the pre-absorbed oxygen on the surface of CuO NWs [17]. When Cu1-δO was exposed to hydrogen, the H2 molecule interacts with Cu1-δO through the oxygen ions pre-absorbed on the Cu1-δO surfaces, as described in Eqs. (3) and (4). The reactions between H2 molecules and the pre-absorbed oxygen ions release electrons to Cu1-δO. These free electrons recombine with the holes in Cu1-δO(Eq. (5)), which leads to a decrease in the whole carrier density and increases the sensor resistance. − H 2 ( asd ) + O ads ↔ H 2O + e −
(3)
2− H 2 ( asd ) + O ads ↔ H 2 O + 2 e − (4)
h • + e − = Null (5)
Figure 6 The ethanol sensing properties of the fabricated CuO NWs sensor were also studied at 350 oC and 400 °C, as shown in Figure 7(A) and (B), respectively. The resistance of the sensor also increased when exposed to ethanol gas because of the reducing behavior of ethanol, which is similar to hydrogen. The CuO NWs sensor was able to detect very low concentrations of ethanol down to 12.5 ppm with significantly high response. However, graph of sensor resistance was more wrinkled at saturation value upon exposure to 12.5 ppm Ethanol compared with other concentrations (Fig. 7(B)). The origin of this was not clear yet. Despite that, this characteristic disappeared at higher Ethanol concentrations ranging from 25 to 200 ppm. The responses of the sensors at 400 °C were higher than those at 350 °C. At ethanol concentrations of 25, 50, 100 and 200 ppm, the sensor responses were 1.27, 1.45, 1.64, and 1.89, respectively. The sensor response as a function of hydrogen and ethanol measured at 400 °C is shown in Figure 7(B). The sensors could detect ethanol at a much lower 10
concentration with a higher response compared with hydrogen. This result indicates that the CuO NWs sensor is more effective for the detection of ethanol than the detection of hydrogen at the measured temperatures. The interactions between Ethanol molecules and pre-adsorbed oxygen can be expressed as below. C2 H 5OH ( ads ) + 6O − ( ads ) → 2CO2 + 3H 2O + 6e − (7) C2 H 5OH ( ads ) + 6O 2 − ( ads ) → 2CO2 + 3H 2O + 12e − (8)
Form the above reactions; it is clearly that the interactions between ethanol molecules and pre-adsorbed oxygen released more electrons than that of the hydrogen, thus results in a higher response. Despite that, the interaction between analytic molecules and sensing surface is complex and thus this needs further investigation for confirmation. The fundamental study on the gas-sensing mechanism of CuO-based sensors was examined for the CuO thin film, as reported in [17]. The mechanism involved the adsorption of oxygen to form the accommodation layer and the interaction between analytic molecules and the pre-adsorbed oxygen. The exchange in electrons that happened during gaseous interactions led to the variation in sensor resistance. Figure 7 4. Conclusions High-quality CuO NWs were synthesized by directly heating Cu foils in air. The heating conditions were varied to control the length and density of the CuO NWs. The growth mechanism of the CuO NWs was also confirmed obeying the self-catalytic growth of VS by comparing the surface morphology of the Cu thin film and the Cu foil heat treated in the same conditions. The hydrogen and ethanol sensing characteristics of the synthesized CuO NWs were examined by depositing it into an interdigitated Pt electrode substrate and by measuring the resulting temperatures. The results showed that the CuO NW-based gas sensor 11
could potentially be used for ethanol and hydrogen sensing applications, where it exhibited responses to 1.27 and 1.16 for 12.5 ppm ethanol and 100 ppm H2, respectively.
Acknowledgement This research was funded by the Vietnam National Foundation for Science and Technology Development (Grant number 103.02-2013.23).
References [1].
N. V. Hieu, N. D. Khoang, D. D. Trung, L. D. Toan, N. V. Duy, N. D. Hoa, Comparative study on CO2 and CO sensing performance of LaOCl-coated ZnO nanowires, J. Hazardous Materials, 244-245 (2013) 209–216
[2].
G. Peng, U. Tisch, O. Adams, M. Hakim, N. Shehada, Y. Y. Broza, S. Billan, R. Abdah-Bortnyak, A. Kuten, H. Haick, Diagnosing lung cancer in exhaled breath using gold nanoparticles, Nature Nanotechnology 4 (2009) 669 – 673
[3].
T. Seiyama, A. Kato, K. Fujiishi and M. Negatani, A new detector for gaseous components using semiconductive thin films. Anal. Chem., 34 (1962) 1502–1503
[4].
P. R. Chung, C. T. Tzeng, M. T. Ke, C. Y. Lee, Formaldehyde Gas Sensors: A Review, Sensors 2013, 13(4), 4468–4484.
[5].
L. Liao, Z. Zhang, B. Yan, Z. Zheng, Q. L. Bao, T. Wu, C. M. Li, Z. X. Shen, J. X. Zhang, H. Gong, J. C. Li, and T. Yu, Multifunctional CuO nanowire devices: p-type field effect transistors and CO gas sensors, Nanotechnology 20 (2009) 085203
[6].
Y. Li, J. Liang, Z. Tao, J. Chen, CuO particles and plates: Synthesis and gas-sensor application, Materials Research Bulletin 43 (2008) 2380–2385.
[7].
N. S. Ramgir, S. K. Ganapathi, M. Kaur, N. Datta, K. P. Muthe, D. K. Aswal, S. K. Gupta, and J. V. Yakhmi, Sub-ppm H2S sensing at room temperature using CuO thin 12
films, Sensors and Actuators B: Chemical, 151 (2010) 90–96 [8].
F. Zhang, A. Zhu, Y.Luo, Y.Tian, J. Yang, Y. Qin, CuO Nanosheets for Sensitive and Selective Determination of H2S with High Recovery Ability, J. Phys. Chem. C, 114 (45) (2010) 19214–19219
[9].
P. Raksa, A. Gardchareon, T. Chairuangsri, P. Mangkorntong, N. Mangkorntong, S. Choopun, Ethanol sensing properties of CuO nanowires prepared by an oxidation reaction, Ceramics International, 35 (2009) 649–652
[10].
L. Liao, Z. Zhang, B. Yan, Z. Zheng, Q. L. Bao, T. Wu, C. M. Li, Z. X. Shen, J. X. Zhang, H. Gong, J. C. Li, T. Yu, Multifunctional CuO nanowire devices: p-type field effect transistors and CO gas sensors, Nanotechnology, 20 (2009) 085203 (6pp)
[11].
C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Grain size effects on gas sensitivity of porous SnO2-based elements, Sensors and Actuators B: Chemical, 3 (1991) 147-155
[12].
J. Chen, K. Wang, L. Hartman, W. Zhou, H2S detection by vertically aligned CuO nanowire array sensors, J. Phys. Chem. C, 112 (41) (2008) 16017–16021
[13].
W. Wang, Z. Liu, Y. Liu, C. Xu, C. Zheng, and G. Wang, A simple wet-chemical synthesis and characterization of CuO nanorods, Applied Physics A, 76 (2003) 417– 420
[14].
X. Wen, Y. Xie, C. L. Choi, K. C. Wan, X.-Y. Li, and S. Yang, Copper-based nanowire materials: templated syntheses, characterizations, and applications, Langmuir, 21 (2005) 4729–4737
[15].
H. Wu, D. Lin, and W. Pan, Fabrication, assembly, and electrical characterization of CuO nanofibers, Applied Physics Letters, 89 (2006) 133125
[16].
X. Jiang, T. Herricks, Y. Xia, CuO Nanowires can be synthesized by heating copper substrates in air, Nano Letters, 2 (2002) 1333–1338
[17].
N. D. Hoa, S. Y. An, N. Q. Dung, N. V. Quy, D. Kim, Synthesis of p-type 13
semiconducting cupric oxide thin films and their application to hydrogen detection, Sensors and Actuators B, 146 (2010) 239–244 [18].
N. V. Hieu, N. V. Duy, P. T. Huy, N. D. Chien, Inclusion of SWCNTs in Nb/Pt codoped TiO2 thin-film sensor for ethanol vapor detection, Physica E 40 (2008) 2950– 2958
[19].
M. L. Zhong, D. C. Zeng, Z. W. Liu, H. Y. Yu, X. C. Zhong, and W. Q. Qiu, Synthesis, growth mechanism and gas-sensing properties of large-scale CuO nanowires, Acta Materialia, 58 (2010) 5926–5932
[20].
N. D. Hoa, N. V. Quy, M. A. Tuan, N. V. Hieu, Facile synthesis of p-type semiconducting cupric oxide nanowires and their gas-sensing properties, Physica E, 42 (2009) 146–149
[21].
J. F. Xu, W. Ji, Z. X. Shen, W. S. Li, S. H. Tang, X. R. Ye, D. Z. Jia, X. Q. Xin, Raman spectra of CuO nanocrystals, J. Raman Spectro., 30 (1999) 413–415
[22].
T. Yu, X. Zhao, Z. X. Shen,Y. H. Wu, W. H. Su, Investigation of individual CuO nanorods by polarized micro-Raman scattering, J. Crys. Growth, 268 (2004)590–595
[23].
L. Yuan, G.Zhouz, Enhanced CuO Nanowire Formation by Thermal Oxidation of Roughened Copper, J. Electrochem. Soc. 159(4)(2012) C205–C209
[24].
Y. W. Park,
N. J. Seong,
H. J. Jung,
A. Chanda, S. G. Yoon, Growth
Mechanism of the Copper Oxide Nanowires from Copper Thin Films Deposited on CuO-Buffered Silicon Substrate, J. Electrochem. Soc. 157(6) (2010) K119–K124 [25].
A. M. B. Goncalves, L. C. Campos, A. S. Ferlauto, and R. G. Lacerda, On the growth and electrical characterization of CuO nanowires by thermal oxidation, J. Applied Physics, 106 (2009) 034303 (5p)
[26].
X. Li, J. Zhang, Y. Yuan, L. Liao, and C. Pan, Effect of electric field on CuO nanoneedle growth during thermal oxidation and its growth mechanism, J. Applied 14
Physics, 108 (2010) 024308 (6p)
Figure 1. CuO NWs grown by heat-treated Cu foils in ambient atmosphere for 2 h at different temperatures: (A,B) 400 °C, (C,D) 500 °C, and (E,F) 600 °C. Figure 2. SEM images of CuO NWs grown by heat-treated Cu foils at 400 °C for (A,B) 2 h, and (C,D) 36 h Figure 3. XRD patterns of CuO NWs (A) on Cu foil, (B) after from Cu foil, and (C) Raman spectrum of CuO NWs Figure 4. SEM images of (A) Cu thin film (100 nm) and (B) Cu foil after heat treated at 400 °C for 2 h Figure 5. Designing of CuO NW sensor fabricaraed on silicon dioxide substate (A), and an equivalent electrical circuit (B); SEM (C, D) and TEM (E, F) images of CuO NWs after collecting from the Cu foil Figure 6. Resistance-time curves of the CuO NWs sensor measured at different temperatures: (A) 300 °C, (B) 350 °C, (C) 400 °C, and (D) sensor responses as a function of H2 concentration. Inset (C) shows the calculation of response and recovery times. Figure 7. Ethanol sensing characteristics of CuO NWs measured at (A) 350oC, (B) 400oC, and sensor response as a function of Ethanol (C); and (D) comparison of sensor response to ethanol and hydrogen measured at 400 °C
•
Large-scale CuO nanowires were controllably synthesized by a simple method.
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The CuO nanowires were clearly exhibited as p-type gas sensors.
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The CuO nanowires sensors have good response to C2H5OH and H2 gases.
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Novel gas sensors based on hetero-junction of p-type and n-type NWs can be developed.
15
Figure 1. CuO NWs grown by heat treated Cu foils in ambient atmosphere for 2 h at different temperatures: (A,B) 400oC, (C,D) 500oC, and (E,F) 600oC.
(B)
(A)
1 µm
(C)
200 nm
(D)
100 nm
1 µm
(E)
(F)
1 µm
200 nm
Figure 2. SEM images of CuO NWs grown by heat treated Cu foils at 400oC for (A,B) 2 h, and (C,D) 36 h.
(A)
(B)
200 nm
1 µm
(C)
(D)
1 µm
200 nm
Figure 3. XRD patterns of CuO NWs (A) on Cu foil, (B) after from Cu foil, and (C) Raman spectrum of CuO NWs.
25
30
CuO
Cu
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45
Cu
Cu2O
CuO
CuO
CuO
Cu2O
Intensity (a.u.)
Cu2O
( ) (A)
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2θ (deg.)
(B)
CuO
CuO (111))
CuO (-111)
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CuO NWs
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297
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Figure 4. SEM images of (A) Cu thin film (100 nm) and (B) Cu foil after heat treated at 400oC for 2 h.
