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polyaniline nanocomposites for efficient humidity sensing
Surfaces and Interfaces 18 (2020) 100410
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Surfaces and Interfaces journal homepage: www.elsevier.com/locate/surfin
Structurally optimized cupric oxide/polyaniline nanocomposites for efficient humidity sensing Pratibha Singha,b, S.K. Shuklab, a b
T
⁎
Department of Chemistry, University of Delhi, Delhi-110007, India Department of Polymer Science, Bhaskaracharya College of Applied Sciences, University of Delhi, Delhi-110075, India
A R T I C LE I N FO
A B S T R A C T
Keywords: CuO/PANI nanocomposite Guided interaction Humidity sensing and sensing mechanism
Structural synergism and interfacial optimization of organic and inorganic materials has exponentially advances several properties for different applications in sensing sciences. In this regard, present paper reports the synthesis of copper oxide and its nano composite with polyaniline by chemical oxidative polymerization route using ammonium persulphate as a polymerizing agent. The characterization was done by infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM) and UV–VIS spectroscopy. The result reveals the formation of nano composite with optimized crystallinity, improved thermal stability and electrical conductivity. Further, a film of composite was explored for resistive type electrochemical humidity sensing of a closed chamber in the range of 10–95% RH. The observed sensing parameters were sensitivity 4.5 ohm/RH, response time 40 s and recovery time 55 s. On the basis of trends in resistance and surface interaction the expected sensing mechanism has been proposed. This indicates that the CuO/PANI nanocomposite will be a promising tool for other molecules to develop perspective material for humidity monitoring with good sensing parameters due to guided interaction between composite and water molecules.
1. Introduction Humidity monitoring is an important parameter for food processing, instrumentation, agriculture, climatology and human comforts. Although humidity monitoring is in practice since long time using different devices including natural observation like pine cone, which opens in low humidity to release their spores [1]. But, currently its industrial importance compelled scientists to develop different types of advanced humidity sensors using different materials e.g. ceramic, conducting polymer, carboneous nanostructure and their composite [2]. Among the above quoted humidity sensitive materials, the nano composite of conducting polymer and metal oxides are reported important due to low cost, heterogenous surface structure, excellent sensitivity and selectivity. The basic reason for this evolution of optimized electronic, magnetic, wetting, optical properties, mechanical, and microwave-absorption for sensing purpose due to chemical and structural synergisms [3,4]. In this context the several researchers have explained the sensing prepared the different metal and conducting polymer explained the sensitivity on the basis synergized electrical and optical properties but chemical synergism is not studied so far [5]. Among different conducting polymer, PANI has been used in wide range of application like sensors [6], support catalysts [7], water
⁎
purifications [8], corrosion protection in organic coatings [9], electronic devices [10] and energy devices due to its good electrical conductivity, high environmental stability, low cost, oxygen and moisture stability and facile fabrication [11,12]. However, the limitations of PANI are processibility, stability and mechanical properties. The basic steps adopted in this regards are formation blending and formation of nano composite with metal oxide, carbon nano particle and metal particle. The addition of metal oxide in PANI matrix develops catalytic and adsorption sites along with improved thermal stability and electrical conductivity. Some of the metal oxide used for the preparation of PANI nanocomposite are ZnO, TiO2, SnO2, TiO2, Fe2O3 and CuO [13,14]. The addition of CuO in PANI optimizes magnetic properties, electrical, catalytic, gas and humidity sensing properties [15]. The various composites of PANI have been reported with improved their sensor properties and a list of PANI composite materials, along with their specification, used for the humidity sensor is given in Table 1. The addition of copper oxide in PANI optimizes magnetic properties, catalytic and guided adsorption sites due to presence vacant d orbital in aligned polymer chain [21]. Thus CuO and PANI composites are used for glucose sensing [22], super capacitor [23], solar selective coating [24] and antibacterial applications [25]. However, the use of CuO/PANI in humidity sensing is not used as per our observation.
Corresponding author. E-mail address: [email protected] (S.K. Shukla).
https://doi.org/10.1016/j.surfin.2019.100410 Received 17 August 2019; Received in revised form 8 November 2019; Accepted 19 November 2019 Available online 20 November 2019 2468-0230/ © 2019 Elsevier B.V. All rights reserved.
Surfaces and Interfaces 18 (2020) 100410
P. Singh and S.K. Shukla
Table 1 Comparison of PANI based nanocomposite humidity sensors. Sample
Sensing type
Humidity Range (%RH)
Response time
Recovery time
Temperature (°C)
Ref.
PANI CuO nanowires CuO/PANI Chitosan and CuMn2O4 spinel nano powder PANI/NiO CuO/PANI
Resistive Amperometric Resistive Resistive Resistive Resistive
3–8 0 20–80 25–95 22–94 5–90 10–95
30 s 120 s —20 s 60 s 40 s
80 s …. ….. 47 s 90 s 70 s
RT RT RT RT RT RT
[16] [17] [18] [19] [20] Present work
Initially a thin film of gold was made on sample in order to minimize charge accumulation. Thermo gravimetric analysis was performed on Linseis TA, thermo-gravimetric analyzer. The analysis was made in temperature range of 30 to 1000 °C at heating rate of 10 °C/min under continuous flow of nitrogen ~ 20 °C mL/min. However, UV–vis spectra were recorded on Shimadzu 260 spectrometer after due calibration of instrument. The percentage composition of CuO in prepared composite was determined by thermo oxidative gravimetric analysis after thermal removal of PANI in the presence oxygen. 2.4. Device fabrication and humidity sensing measurement Fig. 1. Humidity sensing set up.
Initially a thin film (~0.25 μm) sensing electrode was made after spin coating the suspension of 200 mg of CuO/PANI nanocomposite in 2 mL analytical grade acetone on ITO collated glass plate of diameter 1.0 cm X 0.5 cm.Thus obtained film was placed in humidity controlled sensing chamber as well as connected to a Rish Max, multimeter after making contact with silver paste at side through copper wires. The complete experimental set up is given in Fig. 1. A reference hygrometer and thermometer were also kept inside the humidity chamber to monitor the absolute relative humidity and temperature. For humidity sensing the resistance of sensing electrode film was measured at room temperature against different relative humidity level. The relative humidity was controlled by putting different saturated salt solution like LiCl for 11%, CH3COOK for 23%, MgCl2 for 33%, K2CO3 for 43%, Mg(NO3)2 for 52%, CuCl2 for 67%, NaCl for 75%, KCl for 86%, and K2SO4 for 97% RH. The measurements were carried out three times and their mean is reported. The sensing properties of proposed sensors i.e. sensitivity, response and recovery time were also determined on same setup using earlier reported methods at defined temperature and RH [6].
However, the copper bears own selective way to interact with water molecules. Thus, in this work efforts have been CuO/PANI has been synthesized and explored for humidity sensing purposes. 2. Materials and methods 2.1. Materials CuSO4 (99.95%), NaNO3 (99.5%), KNO3 (99.9%), (NH4)2S2O8 (99.85%),CH3OH and aniline(99.5%)were purchased from Central Drug House(CDH), India. These chemicals were used without any further purification. All other aqueous solutions were prepared with Mili-Q water and AR grade solvents were used in entire investigations. 2.2. Synthesis of CuO and CuO/PANI nanocomposites Initially nano size CuO was prepared using eutectic melt method, in brief heating 2.0 g of CuSO4•5H2O in 5 gm NaNO3-KNO3 eutectic melt at 500 °C for 30 min in a muffle furnace [26]. Further, 2.0 mL of aniline was dissolved in 15 mL of 1 N hydrochloric acid. After that 200 mg of fine grinded powder of prepared CuO nano particles were dispersed after stirring for 10 min on a magnetic stirrer with speed of 1000 r.p.m. In resultant solution 25 mL of 0.1 M aqueous [(NH4)2S2O8] was added drop wise in 30 min along with continuous stirring. Further, stirring was continued for 3 h and finally black color precipitate was obtained. The precipitate was allowed to settle for 24 h and filtered using wattman no 1 filter paper. Thus, obtained precipitate was washed thoroughly and dried in vacuum oven for 24 h at 60 °C before further use.
3. Result and discussion 3.1. Selection of material and preparation of composite Generally ceramic, semiconductors and polymers are used in humidity sensing purpose, basically on the principle of competitive adsorption of water molecule. Each class of humidity sensing material bears its own merit and demerits. In this regards, the metal oxides allow the adsorption of water molecules [27]. The surface interaction of water molecule on metal oxide generates proton, which causes protonic conducting. Among the different metal oxides cupric ion is very sensitive to interact water molecules due presence of vacant d orbital. However, the protonic conduction is not possible on oxide at room temperature but occur at higher temperature. However, the polymers like PANI are favorable to protonic conduction of electricity at room temperature due low band gap and its structure with slow adsorption due to hydrophobic nature of organic compounds [28]. Thus, composite of metal oxide and conducting polymer with optimized structure, hydrophilicity and low temperature electricity will be is a promising material. The efficiency of a process is depending on different parameter and it also control the properties of materials and yield. The parameters recorded in present composite formation method are listed in Table 2.
2.3. Characterization The structural analysis of representative prepared samples were made by a RK-1310 model perkin elmer infrared spectrometer. The analysis was performed in KBr phase after making a pellet after mixing prepared sample in dry potassium bromide at ten ton pressure in a hydraulic press. The spectra were recorded with accumulation of 16 scan at resolution of 2 cm−1 in the range of 4000 to 400 cm−1. X-ray diffraction analysis was performed with Bruker D-8 model X-ray diffractometer at rate of 2° per min using Cu Kα1 (λ = =1.5405 Å) radiation generated at 50 KV and 40 mA. Further, microscopic study was done with the help of JSM-6610, JEOL, scanning electron microscope. 2
Surfaces and Interfaces 18 (2020) 100410
P. Singh and S.K. Shukla
Table 2. Observed parameters during polymerization. Composite
Aniline (ml)
CuO (mg)
Polymerization time (min)
Change in Temperature ( °C)
Yield (%)
PANI CuO/PANI
2 2
Nil 200
16 9
5 6
75.2 97.5
Fig. 2. Schematic representation for the preparation of CuO/PANI nanocomposite.
The data are indicating that the presence vacant d orbital of Cu ions allow the interaction between copper ions and nitrogen atom of aniline. This makes aniline molecules more prone for polymerization and composite formation with improved yield with CuO/PANI nanocomposite having 8.6% copper oxide by mass. The overall mechanism of composite formation is illustrated in Fig. 2.
Table 3. Peak position of PANI and CuO/PANI nano composite.
3.2. FTIR spectra Fig. 3. shows the FT-IR spectra of PANI and CuO/PANI nanocomposite. The spectrum of PANI in Fig. 3(a) is showing standards reported peaks for PANI. However, the IR spectrum of CuO/PANI nanocomposite is shown in Fig. 3(b). In the additional peaks in spectrum observed at 680 and 497 cm−1 are associated with the characteristic vibrational mode of CuO [17]. Further the all peaks of PANI are also present in Fig. 1b. The comparison of peaks of PANI and its composite along with expected functional groups are given in Table 3. The comparison of peaks position of PANI with CuO/PANI are indicating significant shift of peaks position of PANI. Primarily, the peaks for quinoid and benzenoid are showing blue shift from its original position. It reveals the CuO has electrostatic interaction between charged structure of PANI prominently and CuO. It brings both polymer and
S.N.
PANI(cm−1)
CuO/PANI (cm−1)
Inference
1 2 3 4 5 6 7 8
3193 1555 1426 1322 1282 1024 774 ……
3207 1540 1414 1302 1239 1050 767 460–680
-NH stretching Benzenoid ring vibration Quinoid ring vibration C-N C = =N C-H C-H plane deformation Metallic stretching
CuO together with structural charge synergism along with presence of metallic sites. The presence of addition metallic site also improves the adsorption behavior of water molecules for better sensing. 3.3. XRD XRD pattern of PANI(Fig. 4b) is showing the characteristics peaks for PANI at two theta value of 14.72, 20.22 and 25.22 due to the (011), (020) and (200) planes. However, diffraction pattern of CuO is showing ͦ ͦ ͦ ͦ most of the reported peaks for CuO at 32 for 111, 39 for 200, 49 , 53 ͦ and 66 for 220 planes [17]. Further, diffractogram of composite is ͦ ͦ ͦ showing peaks for PANI at 26.09 and for CuO 37.43 , 43.39 , and 62.92
Fig. 3. FT-IR spectra of (a) PANI and (b) CuO/PANI nanocomposite.
Fig. 4. XRD graphs of (a) CuO, (b) PANI and (c) CuO/PANI nanocomposite. 3
Surfaces and Interfaces 18 (2020) 100410
P. Singh and S.K. Shukla
Table 4. Crystal parameters of CuO, PANI and CuO/PANI nanocomposite. PANI Peak
plane
d-spacing (Å)
CuO/PANI nanocomposite Peak
Plane
d-spacing (Å)
CuO Peak
Plane
d-spacing (Å)
14.65 20.32 25.17 ……. …… ……
011 020 200 ……. ……. ……
6.042 4.367 3.535 …….. …….. …….
15.47 20.65 26.27 36.34 …… …….
011 020 200 111 ……. …….
5.723 4.297 3.390 2.470 ……. ……..
32 36.5 38.7 48.8 53.4 66.1
002 111 138 202 020 202
2.794 2.460 2.345 1.865 1.714 1.412
two theta values. The comparison of peak positions of virgin CuO and composite is revealing significant shifts in its position of two theta value. It indicates the formation of composites with some interactions between the chain of PANI chain and CuO as well change in inter planner spacing. Further, average particle size of CuO was calculated by using Scherrer equation is given D = =Kλ/βcosƟ, where K is the shape factor for the average crystallite, λ is the X-rays wavelength, β is full width at half maximum of the diffraction line and Ɵ is Braggs angle. The calculated particle size, d values and positions are given in Table 4. The data is indicating significant impact on PANI after composite formation. The comparisons of structural parameters are indicating that d spacing of PANI has changed but of CuO are still almost same. It indicates that the electrostatic interaction between constituents of composite optimize the surface structure without altering the lattice structure of PANI during composite formation and also improves the properties of PANI.
Fig. 6. UV spectra of PANI and CuO/PANI nanocomposite.
3.4. SEM SEM image indicates the binary type structure of CuO/PANI nanocomposite in Fig. 5c with particle size approximately 100 to 120 nm. The micrograph copper oxide is revealing the uniform sphere-like nano size shape (Fig. 5b). The micrograph of PANI indicates the bigger size of particle (Fig. 5a). The smaller size of composite is revealing large surface and better adsorption capacity.
3.5. UV spectra The electronic spectra of PANI and CuO/PANI nanocomposite are given in Fig. 6. The spectra of PANI as well as composites are indicating the both the characteristic peaks of for ᴫ to ᴫ * and polaron/ bipolaron. The graph is indicating the change in position and intensity of peak after composite formation. It shift is may be due to the interaction
Fig. 5. SEM image of (a) PANI (b) CuO/PANI and (b) CuO. 4
Surfaces and Interfaces 18 (2020) 100410
P. Singh and S.K. Shukla
the influence of the composite formation on the thermal stability of PANI. Several studies have also been made on the thermal stability of PANI/inorganic composites [29]. TG curves are shown in Fig. 7. TG curve of pure PANI is showing three step weight loss: i) upto 100 ° C due to elimination of solvent molecules, ii) between 230 and 380 ° C is due to loss of the dopant molecules and iii) from 400−600 ° C attributed for thermal degradation of polymeric backbone. Further, the TG curve of composite is also indicating similar pattern with formation more residue along and shift in degradation temperature towards higher temperature. The formation of higher residue is indicating the presence non degradable CuO and better thermal stability of CuO/PANI polymer matrix. The nature of decomposition pattern of composite is also very sharp due to formation more regular lattice than pure PANI. 3.7. Humidity sensing
Fig. 7. TGA graphs of PANI and PANI/CuO nanocomposite.
The electrochemical humidity is based on monitoring of induces resistance or capacitance of a materials against the different humidity level. Generally, the change in electrical properties is based adsorption of water molecules. The existing literatures are indicating the nonguided adsorption of water molecules due to porosity or surface behavior. The change in electrical resistance against different relative humidity (RH) values is shown in Fig. 8. The curve reveals a continuous decrease in resistance of composites with an increase in humidity range from 10 to 95% RH. This trend in resistance reveals the suitability of materials in humidity sensing. This regular trend is being attributed to the guided dissociative adsorption of H2O on the nano-composite surface of PANI containing cupric ions [30]. It seems that the interactive existence cupric ion with PANI reduces the adsorption energy but still the presence vacant d orbital guides the adsorption of water molecules at atomic level. It proposes the humidity concentration. Initial layer of adsorptions reveals the formation of non-ionized H2O layer due to chemisorption; however the subsequent layer formation produces ions due to guided physio-sorption. The presence of ions initiates the protonic conduction in CuO/PANI nanocomposite, which was not feasible in cupric ions. Thus the formation of CuO/PANI nanocomposite synergizes the interaction of water with copper ions with protonic conduction of PANI, which was not feasible individually. Thus this synergism generates an effective humidity sensing platform for low level humidity sensing. At higher RH, the water vapor condensation occurs in the capillary-quasi apertures to form liquid-like layers. The electrolytic conduction leads to the occurrence of further conductivity. Further, on this basis above postulate, the expected sensing mechanism is proposed in Fig. 9. The adsorption of H2O generates ions and subsequent causes
Fig. 8. Trends of resistance against% RH of CuO/PANI nanocomposite and PANI (inset).
between metal oxide and PANI. The change in degree of polymerization due to the presence metal oxide also alters the population of polaron and bipolaron, which changes the peek area as well as conductivity of the composites.
3.6. Thermo gravimetric analysis Thermal degradation is a very important parameters to determine
Fig. 9. Schematic illustration of sensing mechanism over CuO/PANI nanocomposite. 5
Surfaces and Interfaces 18 (2020) 100410
P. Singh and S.K. Shukla
Fig. 10. (a) Change in resistance of CuO/PANI nanocomposite with time at 60% RH and (b) Percentage sensing response of CuO/PANI at different relative humidity.
assistance. The authors are also thankful Dr. Balaram Pani, Principal, Bhaskaracharya College of Applied Sciences, University of Delhi for maintaining socio academic culture in the college.
efficient directional charge conduction. Therefore, linear decrease in resistance with increasing humidity is confirms the working sensor. 3.8. Sensing parameter
References The sensitivity of developed sensor towards humidity was calculated by using Eq. (1).
S=
R0 − R RH1 − RH2
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(1)
Where, Ro and R are the initial and final resistance of the sensing film at particular humidity, respectively. RH1 is the% humidity at resistance Ro and RH2 is the% humidity at resistance. It was found to be 4.932 kΩ/ RH and 0.879 kΩ at 50% RH for composite and PANI respectively. Further to estimate response and recovery time the change in resistance was measured with time at fix RH values of 50%. The obtained graph is shown in Fig. 10(a), which regularly reproducible nature for a suitable sensing substrate. The curve shows the 80% change in resistance is in 40 s. The resistance was again recovering its original values in 55 s after putting in dry condition. Thus, the response and recovery times of proposed sensor are 40 and 55 s respectively. The lifetime of the CuO/ PANI nanocomposite-based sensor was also tested by monitoring the induced resistance against humidity with regular interval of time (seven days) up to 6 month and found consistent. 4. Conclusions In summary, we have synthesized the structurally synergized and humidity responsive CuO/PANI nanocomposite by in situ polymerization composite formation technique at optimized conditions. The film of CuO/PANI nanocomposites. on ITO coated glass has been used for efficient humidity sensing with better sensitivity and better sensing parameters than pristine PANI for nine months due structure optimization. Further improvement in sensing parameter were discussed on the basis synergistic effect between structure of CuO and PANI, presence metallic sites and hydrophilicity. The finding is indicating the improvement in sensitivity at lower RH value due to composing metal oxide, which explores the possibility for designing better sensing substrate at lower RH values in portable manner. Declaration of Competing Interest We do not have any conflict of interest. Acknowledgements PS is thankful university grant commission, New Delhi for financial 6
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polyaniline nanocomposite, ACS Appl. Mater. Interfaces 7 (2015) 1955–1966. [26] R.P. Rastogi, N.B. Singh, S.K. Shukla, Synthesis of NiO nano crystals through nitrate eutectic melt, Ind. J. Eng. Mater Sci. 7 (2010) 477–480. [27] S.K. Shukla, A. Tiwari, G.K. Parashar, A.P. Mishra, G.C. Dubey, Exploring fiber optic approach to sense humid environment over nano-crystalline zinc oxide film, Talanta 80 (2009) 565–571. [28] C.Y. Lee, G.B. Lee, Humidity sensors: a review, Sensor Lett. 3 (2005) 1–5. [29] T. Sampreeth, M.A. Al-Maghrabi, B.K. Bahuleyan, M.T. Ramesan, Synthesis, characterization, thermal properties, conductivity and sensor application study of polyaniline/cerium-doped titanium dioxide nanocomposites, J. Mater. Sci. 53 (2018) 591–603. [30] I.S. Yakubu, U. Muhammad, A.M. A'isha, Humidity sensing study of polyaniline/ copper oxide nanocomposites, Int. J. Adv. Acad. Res. Sci. Technol. Eng. 4 (2018) 49–61.
Eng. 58 (2019) 139–147. [21] P. Zarrintaj, R. Khalili, H. Vahabi, M.R. Saeb, M.R. Ganjali, M. Mozafari, Polyaniline/metal oxides nanocomposites, Fundam. Emerg. Appl. Polyaniline (2019) 131–141, https://doi.org/10.1016/b978-0-12-817915-4.00008-7. [22] A. Esmaeeli, A. Ghaffarinejad, A. Zahedi, O. Vahidi, Copper oxide-polyaniline nanofiber modified fluorine doped tin oxide (FTO) electrode as non-enzymatic glucose sensor, Sensor Actuat B-Chem. 266 (2018) 294–301. [23] A. Viswanathan, A.N. Shetty, Facile in-situ single step chemical synthesis of reduced graphene oxide-copper oxide-polyaniline nanocomposite and its electrochemical performance for supercapacitor application, Electrochim. Acta 257 (2017) 483–493 (2017). [24] L. Cindrella, CuO-PANI nanostructure with tunable spectral selectivity for solar selective coating application, App. Surf. Sci. 378 (2016) 245–252. [25] U. Bogdanovic, V. Vodnik, M. Mitric, S. Dimitrijevic, S.D. Skapin, V. Zunic, M. Budimir, M. Stoiljkovic, Nanomaterial with high antimicrobial efficacy-copper/
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Surfaces and Interfaces journal homepage: www.elsevier.com/locate/surfin
Structurally optimized cupric oxide/polyaniline nanocomposites for efficient humidity sensing Pratibha Singha,b, S.K. Shuklab, a b
T
⁎
Department of Chemistry, University of Delhi, Delhi-110007, India Department of Polymer Science, Bhaskaracharya College of Applied Sciences, University of Delhi, Delhi-110075, India
A R T I C LE I N FO
A B S T R A C T
Keywords: CuO/PANI nanocomposite Guided interaction Humidity sensing and sensing mechanism
Structural synergism and interfacial optimization of organic and inorganic materials has exponentially advances several properties for different applications in sensing sciences. In this regard, present paper reports the synthesis of copper oxide and its nano composite with polyaniline by chemical oxidative polymerization route using ammonium persulphate as a polymerizing agent. The characterization was done by infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM) and UV–VIS spectroscopy. The result reveals the formation of nano composite with optimized crystallinity, improved thermal stability and electrical conductivity. Further, a film of composite was explored for resistive type electrochemical humidity sensing of a closed chamber in the range of 10–95% RH. The observed sensing parameters were sensitivity 4.5 ohm/RH, response time 40 s and recovery time 55 s. On the basis of trends in resistance and surface interaction the expected sensing mechanism has been proposed. This indicates that the CuO/PANI nanocomposite will be a promising tool for other molecules to develop perspective material for humidity monitoring with good sensing parameters due to guided interaction between composite and water molecules.
1. Introduction Humidity monitoring is an important parameter for food processing, instrumentation, agriculture, climatology and human comforts. Although humidity monitoring is in practice since long time using different devices including natural observation like pine cone, which opens in low humidity to release their spores [1]. But, currently its industrial importance compelled scientists to develop different types of advanced humidity sensors using different materials e.g. ceramic, conducting polymer, carboneous nanostructure and their composite [2]. Among the above quoted humidity sensitive materials, the nano composite of conducting polymer and metal oxides are reported important due to low cost, heterogenous surface structure, excellent sensitivity and selectivity. The basic reason for this evolution of optimized electronic, magnetic, wetting, optical properties, mechanical, and microwave-absorption for sensing purpose due to chemical and structural synergisms [3,4]. In this context the several researchers have explained the sensing prepared the different metal and conducting polymer explained the sensitivity on the basis synergized electrical and optical properties but chemical synergism is not studied so far [5]. Among different conducting polymer, PANI has been used in wide range of application like sensors [6], support catalysts [7], water
⁎
purifications [8], corrosion protection in organic coatings [9], electronic devices [10] and energy devices due to its good electrical conductivity, high environmental stability, low cost, oxygen and moisture stability and facile fabrication [11,12]. However, the limitations of PANI are processibility, stability and mechanical properties. The basic steps adopted in this regards are formation blending and formation of nano composite with metal oxide, carbon nano particle and metal particle. The addition of metal oxide in PANI matrix develops catalytic and adsorption sites along with improved thermal stability and electrical conductivity. Some of the metal oxide used for the preparation of PANI nanocomposite are ZnO, TiO2, SnO2, TiO2, Fe2O3 and CuO [13,14]. The addition of CuO in PANI optimizes magnetic properties, electrical, catalytic, gas and humidity sensing properties [15]. The various composites of PANI have been reported with improved their sensor properties and a list of PANI composite materials, along with their specification, used for the humidity sensor is given in Table 1. The addition of copper oxide in PANI optimizes magnetic properties, catalytic and guided adsorption sites due to presence vacant d orbital in aligned polymer chain [21]. Thus CuO and PANI composites are used for glucose sensing [22], super capacitor [23], solar selective coating [24] and antibacterial applications [25]. However, the use of CuO/PANI in humidity sensing is not used as per our observation.
Corresponding author. E-mail address: [email protected] (S.K. Shukla).
https://doi.org/10.1016/j.surfin.2019.100410 Received 17 August 2019; Received in revised form 8 November 2019; Accepted 19 November 2019 Available online 20 November 2019 2468-0230/ © 2019 Elsevier B.V. All rights reserved.
Surfaces and Interfaces 18 (2020) 100410
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Table 1 Comparison of PANI based nanocomposite humidity sensors. Sample
Sensing type
Humidity Range (%RH)
Response time
Recovery time
Temperature (°C)
Ref.
PANI CuO nanowires CuO/PANI Chitosan and CuMn2O4 spinel nano powder PANI/NiO CuO/PANI
Resistive Amperometric Resistive Resistive Resistive Resistive
3–8 0 20–80 25–95 22–94 5–90 10–95
30 s 120 s —20 s 60 s 40 s
80 s …. ….. 47 s 90 s 70 s
RT RT RT RT RT RT
[16] [17] [18] [19] [20] Present work
Initially a thin film of gold was made on sample in order to minimize charge accumulation. Thermo gravimetric analysis was performed on Linseis TA, thermo-gravimetric analyzer. The analysis was made in temperature range of 30 to 1000 °C at heating rate of 10 °C/min under continuous flow of nitrogen ~ 20 °C mL/min. However, UV–vis spectra were recorded on Shimadzu 260 spectrometer after due calibration of instrument. The percentage composition of CuO in prepared composite was determined by thermo oxidative gravimetric analysis after thermal removal of PANI in the presence oxygen. 2.4. Device fabrication and humidity sensing measurement Fig. 1. Humidity sensing set up.
Initially a thin film (~0.25 μm) sensing electrode was made after spin coating the suspension of 200 mg of CuO/PANI nanocomposite in 2 mL analytical grade acetone on ITO collated glass plate of diameter 1.0 cm X 0.5 cm.Thus obtained film was placed in humidity controlled sensing chamber as well as connected to a Rish Max, multimeter after making contact with silver paste at side through copper wires. The complete experimental set up is given in Fig. 1. A reference hygrometer and thermometer were also kept inside the humidity chamber to monitor the absolute relative humidity and temperature. For humidity sensing the resistance of sensing electrode film was measured at room temperature against different relative humidity level. The relative humidity was controlled by putting different saturated salt solution like LiCl for 11%, CH3COOK for 23%, MgCl2 for 33%, K2CO3 for 43%, Mg(NO3)2 for 52%, CuCl2 for 67%, NaCl for 75%, KCl for 86%, and K2SO4 for 97% RH. The measurements were carried out three times and their mean is reported. The sensing properties of proposed sensors i.e. sensitivity, response and recovery time were also determined on same setup using earlier reported methods at defined temperature and RH [6].
However, the copper bears own selective way to interact with water molecules. Thus, in this work efforts have been CuO/PANI has been synthesized and explored for humidity sensing purposes. 2. Materials and methods 2.1. Materials CuSO4 (99.95%), NaNO3 (99.5%), KNO3 (99.9%), (NH4)2S2O8 (99.85%),CH3OH and aniline(99.5%)were purchased from Central Drug House(CDH), India. These chemicals were used without any further purification. All other aqueous solutions were prepared with Mili-Q water and AR grade solvents were used in entire investigations. 2.2. Synthesis of CuO and CuO/PANI nanocomposites Initially nano size CuO was prepared using eutectic melt method, in brief heating 2.0 g of CuSO4•5H2O in 5 gm NaNO3-KNO3 eutectic melt at 500 °C for 30 min in a muffle furnace [26]. Further, 2.0 mL of aniline was dissolved in 15 mL of 1 N hydrochloric acid. After that 200 mg of fine grinded powder of prepared CuO nano particles were dispersed after stirring for 10 min on a magnetic stirrer with speed of 1000 r.p.m. In resultant solution 25 mL of 0.1 M aqueous [(NH4)2S2O8] was added drop wise in 30 min along with continuous stirring. Further, stirring was continued for 3 h and finally black color precipitate was obtained. The precipitate was allowed to settle for 24 h and filtered using wattman no 1 filter paper. Thus, obtained precipitate was washed thoroughly and dried in vacuum oven for 24 h at 60 °C before further use.
3. Result and discussion 3.1. Selection of material and preparation of composite Generally ceramic, semiconductors and polymers are used in humidity sensing purpose, basically on the principle of competitive adsorption of water molecule. Each class of humidity sensing material bears its own merit and demerits. In this regards, the metal oxides allow the adsorption of water molecules [27]. The surface interaction of water molecule on metal oxide generates proton, which causes protonic conducting. Among the different metal oxides cupric ion is very sensitive to interact water molecules due presence of vacant d orbital. However, the protonic conduction is not possible on oxide at room temperature but occur at higher temperature. However, the polymers like PANI are favorable to protonic conduction of electricity at room temperature due low band gap and its structure with slow adsorption due to hydrophobic nature of organic compounds [28]. Thus, composite of metal oxide and conducting polymer with optimized structure, hydrophilicity and low temperature electricity will be is a promising material. The efficiency of a process is depending on different parameter and it also control the properties of materials and yield. The parameters recorded in present composite formation method are listed in Table 2.
2.3. Characterization The structural analysis of representative prepared samples were made by a RK-1310 model perkin elmer infrared spectrometer. The analysis was performed in KBr phase after making a pellet after mixing prepared sample in dry potassium bromide at ten ton pressure in a hydraulic press. The spectra were recorded with accumulation of 16 scan at resolution of 2 cm−1 in the range of 4000 to 400 cm−1. X-ray diffraction analysis was performed with Bruker D-8 model X-ray diffractometer at rate of 2° per min using Cu Kα1 (λ = =1.5405 Å) radiation generated at 50 KV and 40 mA. Further, microscopic study was done with the help of JSM-6610, JEOL, scanning electron microscope. 2
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Table 2. Observed parameters during polymerization. Composite
Aniline (ml)
CuO (mg)
Polymerization time (min)
Change in Temperature ( °C)
Yield (%)
PANI CuO/PANI
2 2
Nil 200
16 9
5 6
75.2 97.5
Fig. 2. Schematic representation for the preparation of CuO/PANI nanocomposite.
The data are indicating that the presence vacant d orbital of Cu ions allow the interaction between copper ions and nitrogen atom of aniline. This makes aniline molecules more prone for polymerization and composite formation with improved yield with CuO/PANI nanocomposite having 8.6% copper oxide by mass. The overall mechanism of composite formation is illustrated in Fig. 2.
Table 3. Peak position of PANI and CuO/PANI nano composite.
3.2. FTIR spectra Fig. 3. shows the FT-IR spectra of PANI and CuO/PANI nanocomposite. The spectrum of PANI in Fig. 3(a) is showing standards reported peaks for PANI. However, the IR spectrum of CuO/PANI nanocomposite is shown in Fig. 3(b). In the additional peaks in spectrum observed at 680 and 497 cm−1 are associated with the characteristic vibrational mode of CuO [17]. Further the all peaks of PANI are also present in Fig. 1b. The comparison of peaks of PANI and its composite along with expected functional groups are given in Table 3. The comparison of peaks position of PANI with CuO/PANI are indicating significant shift of peaks position of PANI. Primarily, the peaks for quinoid and benzenoid are showing blue shift from its original position. It reveals the CuO has electrostatic interaction between charged structure of PANI prominently and CuO. It brings both polymer and
S.N.
PANI(cm−1)
CuO/PANI (cm−1)
Inference
1 2 3 4 5 6 7 8
3193 1555 1426 1322 1282 1024 774 ……
3207 1540 1414 1302 1239 1050 767 460–680
-NH stretching Benzenoid ring vibration Quinoid ring vibration C-N C = =N C-H C-H plane deformation Metallic stretching
CuO together with structural charge synergism along with presence of metallic sites. The presence of addition metallic site also improves the adsorption behavior of water molecules for better sensing. 3.3. XRD XRD pattern of PANI(Fig. 4b) is showing the characteristics peaks for PANI at two theta value of 14.72, 20.22 and 25.22 due to the (011), (020) and (200) planes. However, diffraction pattern of CuO is showing ͦ ͦ ͦ ͦ most of the reported peaks for CuO at 32 for 111, 39 for 200, 49 , 53 ͦ and 66 for 220 planes [17]. Further, diffractogram of composite is ͦ ͦ ͦ showing peaks for PANI at 26.09 and for CuO 37.43 , 43.39 , and 62.92
Fig. 3. FT-IR spectra of (a) PANI and (b) CuO/PANI nanocomposite.
Fig. 4. XRD graphs of (a) CuO, (b) PANI and (c) CuO/PANI nanocomposite. 3
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Table 4. Crystal parameters of CuO, PANI and CuO/PANI nanocomposite. PANI Peak
plane
d-spacing (Å)
CuO/PANI nanocomposite Peak
Plane
d-spacing (Å)
CuO Peak
Plane
d-spacing (Å)
14.65 20.32 25.17 ……. …… ……
011 020 200 ……. ……. ……
6.042 4.367 3.535 …….. …….. …….
15.47 20.65 26.27 36.34 …… …….
011 020 200 111 ……. …….
5.723 4.297 3.390 2.470 ……. ……..
32 36.5 38.7 48.8 53.4 66.1
002 111 138 202 020 202
2.794 2.460 2.345 1.865 1.714 1.412
two theta values. The comparison of peak positions of virgin CuO and composite is revealing significant shifts in its position of two theta value. It indicates the formation of composites with some interactions between the chain of PANI chain and CuO as well change in inter planner spacing. Further, average particle size of CuO was calculated by using Scherrer equation is given D = =Kλ/βcosƟ, where K is the shape factor for the average crystallite, λ is the X-rays wavelength, β is full width at half maximum of the diffraction line and Ɵ is Braggs angle. The calculated particle size, d values and positions are given in Table 4. The data is indicating significant impact on PANI after composite formation. The comparisons of structural parameters are indicating that d spacing of PANI has changed but of CuO are still almost same. It indicates that the electrostatic interaction between constituents of composite optimize the surface structure without altering the lattice structure of PANI during composite formation and also improves the properties of PANI.
Fig. 6. UV spectra of PANI and CuO/PANI nanocomposite.
3.4. SEM SEM image indicates the binary type structure of CuO/PANI nanocomposite in Fig. 5c with particle size approximately 100 to 120 nm. The micrograph copper oxide is revealing the uniform sphere-like nano size shape (Fig. 5b). The micrograph of PANI indicates the bigger size of particle (Fig. 5a). The smaller size of composite is revealing large surface and better adsorption capacity.
3.5. UV spectra The electronic spectra of PANI and CuO/PANI nanocomposite are given in Fig. 6. The spectra of PANI as well as composites are indicating the both the characteristic peaks of for ᴫ to ᴫ * and polaron/ bipolaron. The graph is indicating the change in position and intensity of peak after composite formation. It shift is may be due to the interaction
Fig. 5. SEM image of (a) PANI (b) CuO/PANI and (b) CuO. 4
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P. Singh and S.K. Shukla
the influence of the composite formation on the thermal stability of PANI. Several studies have also been made on the thermal stability of PANI/inorganic composites [29]. TG curves are shown in Fig. 7. TG curve of pure PANI is showing three step weight loss: i) upto 100 ° C due to elimination of solvent molecules, ii) between 230 and 380 ° C is due to loss of the dopant molecules and iii) from 400−600 ° C attributed for thermal degradation of polymeric backbone. Further, the TG curve of composite is also indicating similar pattern with formation more residue along and shift in degradation temperature towards higher temperature. The formation of higher residue is indicating the presence non degradable CuO and better thermal stability of CuO/PANI polymer matrix. The nature of decomposition pattern of composite is also very sharp due to formation more regular lattice than pure PANI. 3.7. Humidity sensing
Fig. 7. TGA graphs of PANI and PANI/CuO nanocomposite.
The electrochemical humidity is based on monitoring of induces resistance or capacitance of a materials against the different humidity level. Generally, the change in electrical properties is based adsorption of water molecules. The existing literatures are indicating the nonguided adsorption of water molecules due to porosity or surface behavior. The change in electrical resistance against different relative humidity (RH) values is shown in Fig. 8. The curve reveals a continuous decrease in resistance of composites with an increase in humidity range from 10 to 95% RH. This trend in resistance reveals the suitability of materials in humidity sensing. This regular trend is being attributed to the guided dissociative adsorption of H2O on the nano-composite surface of PANI containing cupric ions [30]. It seems that the interactive existence cupric ion with PANI reduces the adsorption energy but still the presence vacant d orbital guides the adsorption of water molecules at atomic level. It proposes the humidity concentration. Initial layer of adsorptions reveals the formation of non-ionized H2O layer due to chemisorption; however the subsequent layer formation produces ions due to guided physio-sorption. The presence of ions initiates the protonic conduction in CuO/PANI nanocomposite, which was not feasible in cupric ions. Thus the formation of CuO/PANI nanocomposite synergizes the interaction of water with copper ions with protonic conduction of PANI, which was not feasible individually. Thus this synergism generates an effective humidity sensing platform for low level humidity sensing. At higher RH, the water vapor condensation occurs in the capillary-quasi apertures to form liquid-like layers. The electrolytic conduction leads to the occurrence of further conductivity. Further, on this basis above postulate, the expected sensing mechanism is proposed in Fig. 9. The adsorption of H2O generates ions and subsequent causes
Fig. 8. Trends of resistance against% RH of CuO/PANI nanocomposite and PANI (inset).
between metal oxide and PANI. The change in degree of polymerization due to the presence metal oxide also alters the population of polaron and bipolaron, which changes the peek area as well as conductivity of the composites.
3.6. Thermo gravimetric analysis Thermal degradation is a very important parameters to determine
Fig. 9. Schematic illustration of sensing mechanism over CuO/PANI nanocomposite. 5
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Fig. 10. (a) Change in resistance of CuO/PANI nanocomposite with time at 60% RH and (b) Percentage sensing response of CuO/PANI at different relative humidity.
assistance. The authors are also thankful Dr. Balaram Pani, Principal, Bhaskaracharya College of Applied Sciences, University of Delhi for maintaining socio academic culture in the college.
efficient directional charge conduction. Therefore, linear decrease in resistance with increasing humidity is confirms the working sensor. 3.8. Sensing parameter
References The sensitivity of developed sensor towards humidity was calculated by using Eq. (1).
S=
R0 − R RH1 − RH2
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(1)
Where, Ro and R are the initial and final resistance of the sensing film at particular humidity, respectively. RH1 is the% humidity at resistance Ro and RH2 is the% humidity at resistance. It was found to be 4.932 kΩ/ RH and 0.879 kΩ at 50% RH for composite and PANI respectively. Further to estimate response and recovery time the change in resistance was measured with time at fix RH values of 50%. The obtained graph is shown in Fig. 10(a), which regularly reproducible nature for a suitable sensing substrate. The curve shows the 80% change in resistance is in 40 s. The resistance was again recovering its original values in 55 s after putting in dry condition. Thus, the response and recovery times of proposed sensor are 40 and 55 s respectively. The lifetime of the CuO/ PANI nanocomposite-based sensor was also tested by monitoring the induced resistance against humidity with regular interval of time (seven days) up to 6 month and found consistent. 4. Conclusions In summary, we have synthesized the structurally synergized and humidity responsive CuO/PANI nanocomposite by in situ polymerization composite formation technique at optimized conditions. The film of CuO/PANI nanocomposites. on ITO coated glass has been used for efficient humidity sensing with better sensitivity and better sensing parameters than pristine PANI for nine months due structure optimization. Further improvement in sensing parameter were discussed on the basis synergistic effect between structure of CuO and PANI, presence metallic sites and hydrophilicity. The finding is indicating the improvement in sensitivity at lower RH value due to composing metal oxide, which explores the possibility for designing better sensing substrate at lower RH values in portable manner. Declaration of Competing Interest We do not have any conflict of interest. Acknowledgements PS is thankful university grant commission, New Delhi for financial 6
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