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Facile synthesis of nanostructured CuO for low temperature NO2 sensing
Physica E 54 (2013) 40–44
Contents lists available at SciVerse ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Facile synthesis of nanostructured CuO for low temperature NO2 sensing A. Das a,n, Bonu Venkataramana a, D. Partheephan b, A.K. Prasad a, S. Dhara a, A.K. Tyagi a a b
Surface and Nanoscience Division, Indira Gandhi Center for Atomic Research, Kalpakkam 603102, India Sensor System Technology, VIT University, Vellore 632014, India
H I G H L I G H T S
G R A P H I C A L
Single step synthesis of nanosized crystallites CuO. Sensing low concentration of toxic NO2 gas at only 50 1C without any interference from oxygen. Temperature dependence sensing offered low activation energy of 0.18 eV. Adsorption kinetics is detailed based on the Elovich model.
Nanostructured p-type CuO exhibited a quick sensing reaction (TR) as well recovery (TREC) of NO2 toxic gas at low temperature of 50 1C.
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2013 Accepted 3 June 2013 Available online 13 June 2013
Detection of environmental pollutant and health hazardous, nitrogen dioxide (NO2) is reported using nanostructured CuO particulates (NPs). Powder X-ray diffraction and field emission scanning electron microscopy were used to probe crystalline phase and morphological details, respectively. Small crystallites of ∼10–12 nm and a strain of 4% were found in the leafy structure of CuO. Raman studies further supported the presence of nanosized CuO phase. This is the first instance of utilizing CuO NPs to detect 5 ppm of NO2 even at a low operating temperature of 50 1C. The highest sensitivity for NO2 was observed at 150 1C, for the first time, in CuO NPs. A low activation energy of 0.18 eV was found for sensing process. The CuO NPs sensor responded to NO2 within a few seconds and recovered totally under a minute. The kinetics of the NO2 gas adsorption on the CuO film surface was described following the Elovich model. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanostructured copper oxide Nitrogen dioxide Sensor Adsorption kinetics
A B S T R A C T
1. Introduction Monitoring NO2 level is of considerable interest due to apprehensions of NO2 toxicity on our health and environment [1,2]. Low concentration of 5 ppm can anesthetize the nose and creates possibility of an overexposure. A major source of NOx emissions is automobile exhaust gas and incidentally, the number of automobiles worldwide is increasing rapidly. Metal phthalocyanines were used for sensing such electron rich, electrophilic gases [3]. Polymer based field effect transistor was employed for quick and low concentration detection of NO2 at room temperature [4]. Semiconductor gas sensors based on the metal oxides, however,
n
Corresponding author. Tel.: +91 4427480500x22409; fax: +91 4427480081. E-mail addresses: [email protected], [email protected] (A. Das).
1386-9477/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2013.06.007
offered a long life, low cost, and easy large-scale production and were used extensively for the detection of toxic pollutant gases, combustible gases and volatile organic compounds (VOCs) [5,6]. Among the metal oxides, n-type oxides like SnO2 [7,8], ZnO [9,10] and WO3 nanoparticles [11] were reported for sensing NOx gases, of course, at higher temperatures. Similarly, p-type cuprous oxide (Cu2O) had shown a good detection of NO2 at a temperature of 150 1C [12]. In contrast, cupric oxide (CuO), a more stable form of copper oxide used for sensing toward reducing gases like NH3 and H2S [13–15], had scarcely beenfde reported for NO2 detection at low temperature of below 150 1C. CuO nanoplates were used to detect NO2 at 200 1C [16] while CuO NWs, grown by thermally oxidizing the Cu wire, showed response to 60 ppm of NO2 at 250 1C [17]. CuO, a monoclinic crystal system, is a p-type semiconductor with a narrow band gap of 1.2 eV. Such semiconducting properties
A. Das et al. / Physica E 54 (2013) 40–44
make it a potential candidate for solar cell fabrications [18,19], catalytic applications [20] and also in gas sensors [13–17]. Low operating temperature, a cost effective option, offers stability to the active sensing materials and becomes an automatic choice for sensing application. However, for metal oxides, including surface reaction by an adsorption of toxic gas, the recovery process of the sensing materials depends strongly on the operating temperature. Therefore the demand for low operating temperature sensor faces strong challenge that needs further developments, in particular, synthesis and process control of suitable materials as well as understanding the reaction kinetics governing the sensing output. With the advent of synthesis of nanomaterials which provide high surface area and high energy, manipulation of the operating temperature and obtaining a superior response, a basic requirement for sensing, becomes feasible. In this context, CuO nanowires grown at 400 1C by the thermal method were successfully tested for NO2 toxic gas at high operating temperature of 300 1C and above [2,17]. Morphology, thus played a crucial role in detection of NO2 [16,17]. Here we present a simple chemical synthesis process for the growth of nanostructured CuO which shows leafy morphology made of nanosized crystalite. This nanostructured CuO responds towards NO2 gas with extraordinarily high sensor response with a reduction in the resistance at relatively low temperature of 50 1C. Analysis of sensor response reveals details on the adsorption kinetics and reaction rate of NO2.
41
XRD patterns of the synthesized CuO are shown in Fig. 2. All the diffraction peaks can be indexed according to monoclinic crystalline phase of CuO (JCPDS #45-0937). Intense peaks observed at 35.51, 38.71 and 48.81 correspond to (hkl) values of (002), (111) and (−202) planes, respectively. The crystallite size around 18 nm was then deduced from the broadening of diffraction peaks following Scherrer's equation [22]. The major reason for observing broadening in XRD peaks, apart from finite size of the crystallite is the presence of non-uniform strain. The Williamson–Hall equation [23] as described below takes into account the effect of strain as well as size of the NPs. βcosθ 1 ηsinθ ¼ þ λ D λ
ð1Þ
where β is a measurement of full width at half maximum (FWHM) of a diffraction peak at θ value, D and η are effective particle size and effective strain, respectively and λ¼ 1.5406 Å is the wavelength of the X-ray. Plot of βcosθ=λ vs. sinθ=λ is shown in the inset of Fig. 1. The slope of the graph provides a total strain in the NPs whereas the inverse of interception on the Y axis offers effective particle size. The tensile strain is obviously seen in Fig. 1 for the asprepared CuO particles amounting as much as 4% which contributed strongly to the broadening of the XRD pattern. The deduced effective particles size from the same plot is of 10 70.2 nm. As the broadening of the XRD pattern was influenced by the strain, the average size of CuO NPs was found be smaller than the deduced particle size from Scherrer's formula.
2. Experimental Copper nitrate (Cu(NO3)2) and ammonium hydroxide (NH4OH) were the starting materials used in the chemical synthesis technique. 1 M of NH4OH was added drop wise to an aqueous solution of 0.1 M Cu(NO3)2. The reaction temperature was kept at 80 1C. The resulting solution was stirred vigorously to obtain a homogeneous mixture. A black precipitate was formed which was thoroughly washed with ample amount of double distilled water followed by drying on hot plate to get pure CuO nanoparticles. The surface topography of copper oxide powder sample was studied by field emission scanning electron microscopy (FESEM, SUPRA 40). The phase identification was performed using an X-ray powder diffraction (XRD, BRUKER) technique with λ¼1.5406 Å of CuKα line. Micro-Raman spectroscopy (InVia; Reinshaw) was used in the backscattering configuration using 100 objective, 514.5 nm excitation, 1800 g/mm grating and thermoelectrically cooled CCD detector. This provides a spectral resolution of 0.5 cm−1. The interdigitated gold electrodes (IDE) were deposited on the SiO2 (100 nm)/Si substrates. 2 mg of CuO powder in 5 ml of ethanol was thoroughly stirred to make homogeneous dispersion of NPs. Thin film of CuO was spin coated at 2000 rpm on to the surface of the IDE. The resistance in presence of air and NO2 gas atmosphere at room temperature (300 K) has been measured using Agilent 34,401 A digital multimeter having in built source of constant voltage power supply. Gas response (S) of NO2 is defined as S ¼(Ra−Rg)/Ra where Rg and Ra are CuO film resistance, measured in NO2 and air atmosphere, respectively. Details of gas exposure and measurement process were reported elsewhere [21].
Fig. 1. FESEM image of synthesized CuO NPs.
3. Results and discussion The FESEM image of the synthesized CuO NPs is displayed in Fig. 1. It exhibits stacking of small flakes made of CuO giving leafy shapes. The needle-like flakes have very small width. The average size of the small flakes is 200–300 nm with thickness around 10–12 nm.
Fig. 2. XRD patterns of CuO NPs. The inset shows strain present in the CuO NPs.
42
A. Das et al. / Physica E 54 (2013) 40–44
Raman spectroscopy provided further evidences of nanosized CuO crystallites. Fig. 3 shows the Raman spectra corresponding to CuO synthesized using Cu(NO3)2 as precursor. In monoclinic CuO crystal with a space group of C2/c the identified Raman peaks for the Ag, B1g, and B2g modes appear at 303, 350, and 636 cm−1, respectively [24]. The Raman spectrum for the as-prepared CuO revealed all three phonon modes at 285, 333 and 618 cm−1, corresponding to A1g, B1g and B2g, respectively. Obviously Raman modes of CuO NPs exhibit a red–shift. In addition, peaks are broad in comparison to the reported FWHM for a single crystalline CuO [25]. The broad feature as well as red–shift may be ascribed to the quantum confinement effect of nanosized CuO where non-zone center phonon also contributes in the total spectra causing the broadening [26]. Similar red–shift was observed in the tip of CuO nanowires due to the reduction in size and subsequent quantum confinement [27]. Thus single step synthesis of CuO at low temperature was found to possess perspective structural aspects conformed to the nanomaterials prominently. These nanoparticulates by offering large surfaces as well as high surface energy can strongly influence the sensing of NO2 which is detailed below.
3.1. NO2 sensing Gas sensing characteristics of spin coated CuO NPs films at operating temperatures of 50 and 150 1C are shown in Figs. 4 and 5, respectively. The dynamic gas sensing characteristics towards NO2 were recorded for a series of concentrations also. The resistance of the NPs films decreased upon exposure of NO2, regardless of the operating temperatures. In metal oxide semiconductors, generally, a reaction takes place between a target gas and the negatively charged surface-adsorbed oxygen species (O− or O2−) and subsequent change in the resistance of the system
Fig. 3. Raman spectrum of CuO exhibiting all allowed Raman modes.
Fig. 4. Change in resistance of CuO gas sensor for different concentrations of NO2 at 50 1C.
Fig. 5. Sensor response from the CuO NPs on exposing to different concentrations of NO2 at 150 1C. The inset shows linear variation in sensitivity with concentrations.
is monitored for measuring the sensor response [1,5]. An n-type oxide semiconductor such as SnO2 and ZnO upon exposure to oxidizing NO2 shows generally an increase in the resistance due to withdrawal of electrons from the semiconducting surfaces. In contrast, p-type CuO semiconductor exhibits a decrease in the resistance which is attributed to the increase of the hole concentration as a matter of fact of utilizing electrons from the CuO NPs and contributing to the formation of negatively charged oxygen species upon exposure to the oxidizing gas such as NO2. The sensing mechanism is therefore presented for surface reaction of NO2 target gas [5] as NO2ðgasÞ þ e− -NOðgasÞ þ O−ðsurf Þ
ð2Þ
Fig. 4 shows the ability of detection of NO2 in the concentration range of 5–50 ppm successfully at 50 1C. The inset in Fig. 4 displays temperature dependent measurements of sensor response, for a typical concentration of 50 ppm. The highest sensor response was obtained at 150 1C followed by saturation of sensitivity which was related to adsorption and desorption kinetics of NO2 and chemisorbed oxygenated species occurred at the surfaces of CuO NPs [5]. Fig. 5 demonstrates the variation of sensor response at 150 1C. As depicted in the inset in Fig. 5, the sensitivity varied linearly with concentrations. This linear variation offers a simple calibration procedure which can be utilized for manipulation to detect an unknown concentration precisely. The CuO NPs also responded quickly within few seconds while the recovery of the system was also very fast within a minute. The CuO plates were reported to have a recovery time for NO2 sensing more than a minute even at 200 1C [16]. Similarly, recovery for the phthalocyanines based sensor was reported to be very slow [28]. In this context, fast response followed by a quick recovery of the CuO NPs sensor delivered a better option for NO2 sensing. For understanding the activated reaction process that allowed the sensor response even at low temperature, activation energy was calculated from the measurements of temperature dependent sensitivities. The activation energy (A.E.) was estimated from the Arrhenius plot of ln (S) against 1/T (K) which is shown in Fig. 6. The A.E. for the NO2 reaction was only 0.18 eV which was reasonably low for the metal oxides surfaces. The low activation energy and high surface area arising from the CuO NPs thus allowed a detection of low concentration of NO2 even at 50 1C. Adsorption kinetics plays an important role in devising and developing adsorbent materials for industrial application. In this context, understanding the activated chemical adsorption of a gas on sensor surface becomes crucial and such study can be followed using the Elovich model [3,14]. The general form of the model is given below, where q describes the change in conductance, C as a
A. Das et al. / Physica E 54 (2013) 40–44
43
Fig. 6. Arrhenius plot for activation energy calculation. Fig. 7. Elovich plot for NO2 adsorption on CuO NPs surfaces. The inset indicates the first order rate for NO2 adsorption.
function of time for sensor responses on exposing NO2. q ¼ 1=αlnða αÞ þ 1=αlnðtÞ
ð3Þ
where ‘q’ is the quantity of gas adsorbed during time t. α and á are the constants. The constant á is the initial adsorption rate and α is related to a measure of potential barrier for successive adsorption. These constants can be obtained from the slope and the intercept of a straight line plot of (q ¼Ct−Co) against ln (t). Co and Ct are conductances of CuO NPs at times of t¼0 and t¼t on exposing NO2, respectively. Such plot is shown in Fig. 7 for NO2 adsorption on the CuO NPs at 150 1C where the response was the highest. The linear part as shown in the Fig. 7 confirms the validity of the Elovich model (Eq. (3)). Details are presented in Table 1 below for various temperatures. Constant α is related to a measure of the extent to which the surface has been screened by potential barrier for successive adsorption. The decreasing trend with increasing temperatures formally endorsed the concepts that high temperature favored the adsorption till a temperature of 150 1C and then saturation is observed at higher temperature. On the other hand, the constant á is regarded as the initial adsorption rate, and it depends on the activation energy. The increasing trend of á with increasing operating temperature substantiated the observed enhancement in the adsorption of NO2 which allowed chemical response as an output of the sensor signal. This value was found to get saturated also and indicated a limitation of the process to convert further the sensor signal to a better output. Both, á and α values pointed toward a high sensor response at 150 1C which clearly matched well the observed sensitivity output at the same temperature (Fig. 4). In addition, rate determination of this sensing process related to CuO NPs and NO2 was also focused in details following the above model. As the reaction kinetics progressed with time, increasing sensor output was observed to vary linearly depending on the adsorbed NO2 concentrations (Fig. 5). In such case, first order rate is predicted when the rate depends primarily on the exposure gas. For understanding the first order rate [3], a plot of log (Ce−Ct) vs. t was shown in the inset of Fig. 7. Here Ce was the conductance before saturation in the sensor response for a particular exposure. A linear fitting was carried out for the plot which was exhibited in the inset of Fig. 7. The linear fitting confirmed that a first order type reaction took place between NO2 and CuO NPs. The sensing mechanism for the surface reaction of NO2 target gas is therefore solely determined by the initial concentration of NO2 and its adsorption kinetics. The reactive sites, which initiated the reaction by providing electronic charge to NO2 following the reaction mechanism as given in Eq. (2) were the active surfaces of CuO NPs. This process helped in creating hole carrier in CuO NPs leading to a decrease in resistance. During sensing studies interference effect from oxidizing gas like O2 was also exposed separately however no significant change in resistance could be observed in the whole operating temperature
Table 1 Values of constants α and á from Elovich plot. Temperature (1C)
α (MΩ)
á (10−6)
50 100 150 250
2.3 1.4 0.32 0.34
0.1027 0.04 0.460 7 0.05 0.380 7 0.05 0.4177 0.05
range. The detection of NO2 is therefore found not to be disturbed by oxygen.
4. Conclusion Single step synthesis of CuO nanoparticles of 10–12 nm was presented. These NPs were found to be highly sensitive to toxic NO2 gas with a high recovery rate. Nanosized CuO offering large surface area and high surface energy made it possible to detect low concentration of NO2 at relatively low temperature. A maximum response was obtained at an operating temperature of 150 1C where adsorption kinetics favored the system as evidenced from the Elovich model. The mechanistic aspect on the p-type CuO was explained based on the enhancement of hole carriers. The overall reaction rate was found to be a first order kinetics. Moreover, no interference from oxygen was detected.
Acknowledgments We are thankful to Dr. R. Pandian of SND, IGCAR for the FESEM study and Mr R.M. Sarguna of CMPD, IGCAR, Kalpakkam for the help in recording XRD. Reference [1] A. Afzal, N. Cioffi, L. Sabbatinia, L. Torsi, Sensors and Actuators B 171–172 (2012) 25. [2] Y. Kim, I. Hwang, S. Kim, C. Lee, J. Lee, Sensors and Actuators B 135 (2008) 298. [3] R. Tongpool, S. Yoriya, Thin Solid Films 477 (2005) 148. [4] A. Das, R. Dost, T. Richardson, M. Grell, J.J. Morrison, M.L. Turner, Advanced Materials 19 (2007) 4018. [5] K. Wetchakunet, T. Samerjai, N. Tamaekong, C. Liewhiran, C. Siriwong, V. Kruefu, A. Wisitsoraat, A. Tuantranont, S. Phanichphant, Sensors and Actuators B 160 (2011) 580. [6] G. Korotcenkov, Material Science and Engineering B 139 (2007) 1. [7] J. Zhang, S. Wang, Y. Wang, Y. Wang, B. Zhu, H. Xia, X. Guo, S. Zhang, W. Huang, S. Wu, Sensors and Actuators B 135 (2009) 610. [8] B.G. Kim, D.G. Lim, J.H. Park, Y.J. Choi, J.G. Park, Applied Surface Science 257 (2011) 4715. [9] J.H. Jun, J. Yun, K. Cho, I.S. Heang, J.H. Lee, S. Kim, Sensors and Actuators B 140 (2009) 412.
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[10] M.W. Ahn, K.S. Park, J.H. Heo, D.W. Kim, K.J. Choi, J.G. Park, Sensors and Actuators B 138 (2009) 168. [11] D. Meng, T. Yamazaki, Y. Shen, Z. Liu, T. Kikuta, Applied Surface Science 256 (2009) 1050. [12] S.T. Shishiyanu, T.S. Shishiyanu, O.I. Lupan, Sensors and Actuators B 113 (2006) 468. [13] S. Wang, Y. Xiao, D. Shi, H. Kun Liu, S.X. Dou, Materials Chemistry and Physics 130 (2011) 1325. [14] I. Singh, R.K. Bedi, Solid State Sciences 13 (2011) 2011. [15] N.S. Ramgir, S.K. Ganapathi, M. Kaur, N. Datta, K.P. Muthe, D.K. Aswal, S.K. Gupta, J.V. Yakhmi, Sensors and Actuators B 151 (2010) 90. [16] J. Yueming, Z. Liang, J. Tao, Chen, Materials Research Bulletin 43 (2008) 2380. [17] N. Hoa, N. Quy, M. Tuan, N. Hieu, Physica E 42 (2009) 146. [18] Y.W. Zhu, T. Yu, F.C. Cheong, X.J. Xu, C.T. Lim, V.B.C. Tan, J.T.L. Thong, C.H. Sow, Nanotechnology 16 (2005) 88. [19] Y.P. Sukhorukov, B.A. Gizhevskii, E.V. Mostovshchikova, A. Yermakov, S.N. Tugushev, E.A. Kozlov, Technical Physics Letters 32 (2006) 132.
[20] K. Zhou, R. Wang, B. Xu, Y. Li, Nanotechnology 17 (2006) 3939. [21] P. Sahoo, S. Dhara, S. Dash, S. Amirthapandian, Arun K. Prasad, A.K. Tyagi, International Journal of Hydogen Energy 38 (2013) 3513. [22] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, third ed., Addison-Wesley Publisher, USA, 1956. [23] Khorsand ZaK, W.H. Abd. Majid, M.E. Abrishami, Ramin Yousefi, Solid State Sciences 13 (2011) 251. [24] J. Chrzanowski, J.C. Irwin, Solid State Communications 70 (1989) 11. [25] S. Sahoo, S. Dhara, S. Mahadevan, A.K. Arora, Journal of Nanoscience and Nanotechnology 9 (2009) 5604. [26] H. Liu, C. Chen, C. Chia, C. Yeh, C. Chen, M. Yu, S. Keller, S.P. DenBaars, Chemical Physics Letters 345 (2001) 245. [27] M.H. Chou, S.B. Liu, C.Y. Huang, S.Y. Wu, C.L. Cheng, Applied Surface Science 254 (2008) 7539. [28] N. Padma, A. Joshi, A. Singh, S.K. Deshpande, D.K. Aswal, S.K. Gupta, J.V. Yakhmi, Sensors and Actuators B 143 (2009) 246.
Contents lists available at SciVerse ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Facile synthesis of nanostructured CuO for low temperature NO2 sensing A. Das a,n, Bonu Venkataramana a, D. Partheephan b, A.K. Prasad a, S. Dhara a, A.K. Tyagi a a b
Surface and Nanoscience Division, Indira Gandhi Center for Atomic Research, Kalpakkam 603102, India Sensor System Technology, VIT University, Vellore 632014, India
H I G H L I G H T S
G R A P H I C A L
Single step synthesis of nanosized crystallites CuO. Sensing low concentration of toxic NO2 gas at only 50 1C without any interference from oxygen. Temperature dependence sensing offered low activation energy of 0.18 eV. Adsorption kinetics is detailed based on the Elovich model.
Nanostructured p-type CuO exhibited a quick sensing reaction (TR) as well recovery (TREC) of NO2 toxic gas at low temperature of 50 1C.
art ic l e i nf o
a b s t r a c t
Article history: Received 11 April 2013 Accepted 3 June 2013 Available online 13 June 2013
Detection of environmental pollutant and health hazardous, nitrogen dioxide (NO2) is reported using nanostructured CuO particulates (NPs). Powder X-ray diffraction and field emission scanning electron microscopy were used to probe crystalline phase and morphological details, respectively. Small crystallites of ∼10–12 nm and a strain of 4% were found in the leafy structure of CuO. Raman studies further supported the presence of nanosized CuO phase. This is the first instance of utilizing CuO NPs to detect 5 ppm of NO2 even at a low operating temperature of 50 1C. The highest sensitivity for NO2 was observed at 150 1C, for the first time, in CuO NPs. A low activation energy of 0.18 eV was found for sensing process. The CuO NPs sensor responded to NO2 within a few seconds and recovered totally under a minute. The kinetics of the NO2 gas adsorption on the CuO film surface was described following the Elovich model. & 2013 Elsevier B.V. All rights reserved.
Keywords: Nanostructured copper oxide Nitrogen dioxide Sensor Adsorption kinetics
A B S T R A C T
1. Introduction Monitoring NO2 level is of considerable interest due to apprehensions of NO2 toxicity on our health and environment [1,2]. Low concentration of 5 ppm can anesthetize the nose and creates possibility of an overexposure. A major source of NOx emissions is automobile exhaust gas and incidentally, the number of automobiles worldwide is increasing rapidly. Metal phthalocyanines were used for sensing such electron rich, electrophilic gases [3]. Polymer based field effect transistor was employed for quick and low concentration detection of NO2 at room temperature [4]. Semiconductor gas sensors based on the metal oxides, however,
n
Corresponding author. Tel.: +91 4427480500x22409; fax: +91 4427480081. E-mail addresses: [email protected], [email protected] (A. Das).
1386-9477/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physe.2013.06.007
offered a long life, low cost, and easy large-scale production and were used extensively for the detection of toxic pollutant gases, combustible gases and volatile organic compounds (VOCs) [5,6]. Among the metal oxides, n-type oxides like SnO2 [7,8], ZnO [9,10] and WO3 nanoparticles [11] were reported for sensing NOx gases, of course, at higher temperatures. Similarly, p-type cuprous oxide (Cu2O) had shown a good detection of NO2 at a temperature of 150 1C [12]. In contrast, cupric oxide (CuO), a more stable form of copper oxide used for sensing toward reducing gases like NH3 and H2S [13–15], had scarcely beenfde reported for NO2 detection at low temperature of below 150 1C. CuO nanoplates were used to detect NO2 at 200 1C [16] while CuO NWs, grown by thermally oxidizing the Cu wire, showed response to 60 ppm of NO2 at 250 1C [17]. CuO, a monoclinic crystal system, is a p-type semiconductor with a narrow band gap of 1.2 eV. Such semiconducting properties
A. Das et al. / Physica E 54 (2013) 40–44
make it a potential candidate for solar cell fabrications [18,19], catalytic applications [20] and also in gas sensors [13–17]. Low operating temperature, a cost effective option, offers stability to the active sensing materials and becomes an automatic choice for sensing application. However, for metal oxides, including surface reaction by an adsorption of toxic gas, the recovery process of the sensing materials depends strongly on the operating temperature. Therefore the demand for low operating temperature sensor faces strong challenge that needs further developments, in particular, synthesis and process control of suitable materials as well as understanding the reaction kinetics governing the sensing output. With the advent of synthesis of nanomaterials which provide high surface area and high energy, manipulation of the operating temperature and obtaining a superior response, a basic requirement for sensing, becomes feasible. In this context, CuO nanowires grown at 400 1C by the thermal method were successfully tested for NO2 toxic gas at high operating temperature of 300 1C and above [2,17]. Morphology, thus played a crucial role in detection of NO2 [16,17]. Here we present a simple chemical synthesis process for the growth of nanostructured CuO which shows leafy morphology made of nanosized crystalite. This nanostructured CuO responds towards NO2 gas with extraordinarily high sensor response with a reduction in the resistance at relatively low temperature of 50 1C. Analysis of sensor response reveals details on the adsorption kinetics and reaction rate of NO2.
41
XRD patterns of the synthesized CuO are shown in Fig. 2. All the diffraction peaks can be indexed according to monoclinic crystalline phase of CuO (JCPDS #45-0937). Intense peaks observed at 35.51, 38.71 and 48.81 correspond to (hkl) values of (002), (111) and (−202) planes, respectively. The crystallite size around 18 nm was then deduced from the broadening of diffraction peaks following Scherrer's equation [22]. The major reason for observing broadening in XRD peaks, apart from finite size of the crystallite is the presence of non-uniform strain. The Williamson–Hall equation [23] as described below takes into account the effect of strain as well as size of the NPs. βcosθ 1 ηsinθ ¼ þ λ D λ
ð1Þ
where β is a measurement of full width at half maximum (FWHM) of a diffraction peak at θ value, D and η are effective particle size and effective strain, respectively and λ¼ 1.5406 Å is the wavelength of the X-ray. Plot of βcosθ=λ vs. sinθ=λ is shown in the inset of Fig. 1. The slope of the graph provides a total strain in the NPs whereas the inverse of interception on the Y axis offers effective particle size. The tensile strain is obviously seen in Fig. 1 for the asprepared CuO particles amounting as much as 4% which contributed strongly to the broadening of the XRD pattern. The deduced effective particles size from the same plot is of 10 70.2 nm. As the broadening of the XRD pattern was influenced by the strain, the average size of CuO NPs was found be smaller than the deduced particle size from Scherrer's formula.
2. Experimental Copper nitrate (Cu(NO3)2) and ammonium hydroxide (NH4OH) were the starting materials used in the chemical synthesis technique. 1 M of NH4OH was added drop wise to an aqueous solution of 0.1 M Cu(NO3)2. The reaction temperature was kept at 80 1C. The resulting solution was stirred vigorously to obtain a homogeneous mixture. A black precipitate was formed which was thoroughly washed with ample amount of double distilled water followed by drying on hot plate to get pure CuO nanoparticles. The surface topography of copper oxide powder sample was studied by field emission scanning electron microscopy (FESEM, SUPRA 40). The phase identification was performed using an X-ray powder diffraction (XRD, BRUKER) technique with λ¼1.5406 Å of CuKα line. Micro-Raman spectroscopy (InVia; Reinshaw) was used in the backscattering configuration using 100 objective, 514.5 nm excitation, 1800 g/mm grating and thermoelectrically cooled CCD detector. This provides a spectral resolution of 0.5 cm−1. The interdigitated gold electrodes (IDE) were deposited on the SiO2 (100 nm)/Si substrates. 2 mg of CuO powder in 5 ml of ethanol was thoroughly stirred to make homogeneous dispersion of NPs. Thin film of CuO was spin coated at 2000 rpm on to the surface of the IDE. The resistance in presence of air and NO2 gas atmosphere at room temperature (300 K) has been measured using Agilent 34,401 A digital multimeter having in built source of constant voltage power supply. Gas response (S) of NO2 is defined as S ¼(Ra−Rg)/Ra where Rg and Ra are CuO film resistance, measured in NO2 and air atmosphere, respectively. Details of gas exposure and measurement process were reported elsewhere [21].
Fig. 1. FESEM image of synthesized CuO NPs.
3. Results and discussion The FESEM image of the synthesized CuO NPs is displayed in Fig. 1. It exhibits stacking of small flakes made of CuO giving leafy shapes. The needle-like flakes have very small width. The average size of the small flakes is 200–300 nm with thickness around 10–12 nm.
Fig. 2. XRD patterns of CuO NPs. The inset shows strain present in the CuO NPs.
42
A. Das et al. / Physica E 54 (2013) 40–44
Raman spectroscopy provided further evidences of nanosized CuO crystallites. Fig. 3 shows the Raman spectra corresponding to CuO synthesized using Cu(NO3)2 as precursor. In monoclinic CuO crystal with a space group of C2/c the identified Raman peaks for the Ag, B1g, and B2g modes appear at 303, 350, and 636 cm−1, respectively [24]. The Raman spectrum for the as-prepared CuO revealed all three phonon modes at 285, 333 and 618 cm−1, corresponding to A1g, B1g and B2g, respectively. Obviously Raman modes of CuO NPs exhibit a red–shift. In addition, peaks are broad in comparison to the reported FWHM for a single crystalline CuO [25]. The broad feature as well as red–shift may be ascribed to the quantum confinement effect of nanosized CuO where non-zone center phonon also contributes in the total spectra causing the broadening [26]. Similar red–shift was observed in the tip of CuO nanowires due to the reduction in size and subsequent quantum confinement [27]. Thus single step synthesis of CuO at low temperature was found to possess perspective structural aspects conformed to the nanomaterials prominently. These nanoparticulates by offering large surfaces as well as high surface energy can strongly influence the sensing of NO2 which is detailed below.
3.1. NO2 sensing Gas sensing characteristics of spin coated CuO NPs films at operating temperatures of 50 and 150 1C are shown in Figs. 4 and 5, respectively. The dynamic gas sensing characteristics towards NO2 were recorded for a series of concentrations also. The resistance of the NPs films decreased upon exposure of NO2, regardless of the operating temperatures. In metal oxide semiconductors, generally, a reaction takes place between a target gas and the negatively charged surface-adsorbed oxygen species (O− or O2−) and subsequent change in the resistance of the system
Fig. 3. Raman spectrum of CuO exhibiting all allowed Raman modes.
Fig. 4. Change in resistance of CuO gas sensor for different concentrations of NO2 at 50 1C.
Fig. 5. Sensor response from the CuO NPs on exposing to different concentrations of NO2 at 150 1C. The inset shows linear variation in sensitivity with concentrations.
is monitored for measuring the sensor response [1,5]. An n-type oxide semiconductor such as SnO2 and ZnO upon exposure to oxidizing NO2 shows generally an increase in the resistance due to withdrawal of electrons from the semiconducting surfaces. In contrast, p-type CuO semiconductor exhibits a decrease in the resistance which is attributed to the increase of the hole concentration as a matter of fact of utilizing electrons from the CuO NPs and contributing to the formation of negatively charged oxygen species upon exposure to the oxidizing gas such as NO2. The sensing mechanism is therefore presented for surface reaction of NO2 target gas [5] as NO2ðgasÞ þ e− -NOðgasÞ þ O−ðsurf Þ
ð2Þ
Fig. 4 shows the ability of detection of NO2 in the concentration range of 5–50 ppm successfully at 50 1C. The inset in Fig. 4 displays temperature dependent measurements of sensor response, for a typical concentration of 50 ppm. The highest sensor response was obtained at 150 1C followed by saturation of sensitivity which was related to adsorption and desorption kinetics of NO2 and chemisorbed oxygenated species occurred at the surfaces of CuO NPs [5]. Fig. 5 demonstrates the variation of sensor response at 150 1C. As depicted in the inset in Fig. 5, the sensitivity varied linearly with concentrations. This linear variation offers a simple calibration procedure which can be utilized for manipulation to detect an unknown concentration precisely. The CuO NPs also responded quickly within few seconds while the recovery of the system was also very fast within a minute. The CuO plates were reported to have a recovery time for NO2 sensing more than a minute even at 200 1C [16]. Similarly, recovery for the phthalocyanines based sensor was reported to be very slow [28]. In this context, fast response followed by a quick recovery of the CuO NPs sensor delivered a better option for NO2 sensing. For understanding the activated reaction process that allowed the sensor response even at low temperature, activation energy was calculated from the measurements of temperature dependent sensitivities. The activation energy (A.E.) was estimated from the Arrhenius plot of ln (S) against 1/T (K) which is shown in Fig. 6. The A.E. for the NO2 reaction was only 0.18 eV which was reasonably low for the metal oxides surfaces. The low activation energy and high surface area arising from the CuO NPs thus allowed a detection of low concentration of NO2 even at 50 1C. Adsorption kinetics plays an important role in devising and developing adsorbent materials for industrial application. In this context, understanding the activated chemical adsorption of a gas on sensor surface becomes crucial and such study can be followed using the Elovich model [3,14]. The general form of the model is given below, where q describes the change in conductance, C as a
A. Das et al. / Physica E 54 (2013) 40–44
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Fig. 6. Arrhenius plot for activation energy calculation. Fig. 7. Elovich plot for NO2 adsorption on CuO NPs surfaces. The inset indicates the first order rate for NO2 adsorption.
function of time for sensor responses on exposing NO2. q ¼ 1=αlnða αÞ þ 1=αlnðtÞ
ð3Þ
where ‘q’ is the quantity of gas adsorbed during time t. α and á are the constants. The constant á is the initial adsorption rate and α is related to a measure of potential barrier for successive adsorption. These constants can be obtained from the slope and the intercept of a straight line plot of (q ¼Ct−Co) against ln (t). Co and Ct are conductances of CuO NPs at times of t¼0 and t¼t on exposing NO2, respectively. Such plot is shown in Fig. 7 for NO2 adsorption on the CuO NPs at 150 1C where the response was the highest. The linear part as shown in the Fig. 7 confirms the validity of the Elovich model (Eq. (3)). Details are presented in Table 1 below for various temperatures. Constant α is related to a measure of the extent to which the surface has been screened by potential barrier for successive adsorption. The decreasing trend with increasing temperatures formally endorsed the concepts that high temperature favored the adsorption till a temperature of 150 1C and then saturation is observed at higher temperature. On the other hand, the constant á is regarded as the initial adsorption rate, and it depends on the activation energy. The increasing trend of á with increasing operating temperature substantiated the observed enhancement in the adsorption of NO2 which allowed chemical response as an output of the sensor signal. This value was found to get saturated also and indicated a limitation of the process to convert further the sensor signal to a better output. Both, á and α values pointed toward a high sensor response at 150 1C which clearly matched well the observed sensitivity output at the same temperature (Fig. 4). In addition, rate determination of this sensing process related to CuO NPs and NO2 was also focused in details following the above model. As the reaction kinetics progressed with time, increasing sensor output was observed to vary linearly depending on the adsorbed NO2 concentrations (Fig. 5). In such case, first order rate is predicted when the rate depends primarily on the exposure gas. For understanding the first order rate [3], a plot of log (Ce−Ct) vs. t was shown in the inset of Fig. 7. Here Ce was the conductance before saturation in the sensor response for a particular exposure. A linear fitting was carried out for the plot which was exhibited in the inset of Fig. 7. The linear fitting confirmed that a first order type reaction took place between NO2 and CuO NPs. The sensing mechanism for the surface reaction of NO2 target gas is therefore solely determined by the initial concentration of NO2 and its adsorption kinetics. The reactive sites, which initiated the reaction by providing electronic charge to NO2 following the reaction mechanism as given in Eq. (2) were the active surfaces of CuO NPs. This process helped in creating hole carrier in CuO NPs leading to a decrease in resistance. During sensing studies interference effect from oxidizing gas like O2 was also exposed separately however no significant change in resistance could be observed in the whole operating temperature
Table 1 Values of constants α and á from Elovich plot. Temperature (1C)
α (MΩ)
á (10−6)
50 100 150 250
2.3 1.4 0.32 0.34
0.1027 0.04 0.460 7 0.05 0.380 7 0.05 0.4177 0.05
range. The detection of NO2 is therefore found not to be disturbed by oxygen.
4. Conclusion Single step synthesis of CuO nanoparticles of 10–12 nm was presented. These NPs were found to be highly sensitive to toxic NO2 gas with a high recovery rate. Nanosized CuO offering large surface area and high surface energy made it possible to detect low concentration of NO2 at relatively low temperature. A maximum response was obtained at an operating temperature of 150 1C where adsorption kinetics favored the system as evidenced from the Elovich model. The mechanistic aspect on the p-type CuO was explained based on the enhancement of hole carriers. The overall reaction rate was found to be a first order kinetics. Moreover, no interference from oxygen was detected.
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