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Enhanced relative humidity sensing property of porous Al:ZnO thin films
Materials Today: Proceedings xxx (xxxx) xxx
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Enhanced relative humidity sensing property of porous Al:ZnO thin films Soumalya Kundu a, Rahul Majumder a, Ria Ghosh a, Manish Pal Chowdhury a,⇑ a
Department of Physics, IIEST Shibpur, Howrah 711103, India
a r t i c l e
i n f o
Article history: Received 12 March 2019 Received in revised form 29 March 2019 Accepted 31 March 2019 Available online xxxx Keywords: Relative Humidity (RH) Positive humidity sensor Al-doped zinc oxide (AZO) Responsivity Nebulized spray pyrolysis
a b s t r a c t High responsive, reliable and low-cost positive type relative humidity (RH) sensors have been fabricated with aluminium (Al) doped zinc oxide (AZO) thin films synthesized by nebulized spray pyrolysis technique. The Al doping concentration in the material is varied from 1 to 5 at.%. The resistive humidity responses of the samples are measured in the range of 22–90% of RH with respect to the base humidity level 10%. Maximum responsivity of 820% is observed for 5 at.% AZO thin film at 90% RH. The sensing mechanism and the positive humidity change are explained on the ground of surface porosity and water adsorption process. Ó 2019 Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on Nano Science & Engineering Application (ICONSEA-2018) Centre for Nano Science and Technology, ICONSEA2018.
1. Introduction Measurement of relative humidity with high accuracy is an inevitable task due to its large impact on many areas like agriculture, biological, environmental monitoring and meteorological services [1]. Hence, constant effort is given by the researchers to achieve more advanced, reliable and highly sensitive humidity sensor. The metal oxide semiconductors are the good option to the researchers for the fabrication of humidity sensors due to their high chemical and thermal stability [2], large surface-to-volume ratio and high sensitivity with fast response [3]. ZnO is a popular n-type metal oxide semiconductors which has direct band gap of 3.3 eV [4] and large exciton binding energy of 60 meV [5]. ZnO is an extremely useful material for humidity sensing due to its morphological diversity [6], high crystallinity, surface porosity [7]. Besides, it is much easier to get higher conductivity of ZnO by simple doping of gr III elements [8]. ZnO can also be synthesized by some facile techniques e.g. chemical vapour deposition (CVD) [9], laser ablation [10], hydrothermal method [11]. In this paper, positive resistive response (positive change in resistance) of porous aluminium doped ZnO (AZO) thin film is reported under exposure to different humidity ambient. In this work AZO thin films synthesized by low-cost nebulized spray pyrolysis technique as compared to conventional synthesis ⇑ Corresponding author. E-mail address: [email protected] (M.P. Chowdhury).
processes [9–11]. Humidity sensing properties are investigated and finally an explanation is proposed in support of positive change in resistance. The electrical resistivity of the as-synthesized films is measured by Van der Pauw four-probe technique. Surface morphology of the samples is studied using FE-SEM (INSPECT F50, Netherland). Crystal structures of the samples are computed by X-Ray Diffractometer (Model No.: X’Pert Powder, PANalytical) data analysis. Band gaps of the ZnO and AZO thin films are found out by UV–VIS (UV-1800, SHIMADZU). 2. Experimental details 2.1. Synthesis of AZO thin film The synthesis of AZO thin films is carried out with the spray deposition of the precursor solution, prepared by mixing of dopant material aluminium nitrate monohydrate [Al(NO3)3H2O, Fisher Scientific] into 0.2 (M) aq. solution of zinc nitrate hexahydrate [Zn(NO3)26H2O, Fisher Scientific]. The amount of addition of the dopant material is decided by the required percentage of Al doping which varies from 1 to 5 at.%. The soda-lime glass substrates are chosen to be spray deposited by the nebulizer from a height of 2 cm above the substrates. The deposition is continued for 60 min at a stretch at 100 °C. Thereafter, the substrates are subjected to anneal at 500 °C in a split-open furnace at atmospheric pressure for 15 min. The spray coating and the annealing steps are repeated for second times to achieve the film thickness of
https://doi.org/10.1016/j.matpr.2019.03.235 2214-7853/Ó 2019 Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on Nano Science & Engineering Application (ICONSEA-2018) Centre for Nano Science and Technology, ICONSEA-2018.
Please cite this article as: S. Kundu, R. Majumder, R. Ghosh et al., Enhanced relative humidity sensing property of porous Al:ZnO thin films, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.235
2
S. Kundu et al. / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 1. Experimental set-up showing synthesis of AZO material.
500 nm. The film thickness has been measured with Bruker DektakXT stylus profilometer. Schematic diagram of the experimental set-up of synthesis is shown in Fig. 1. 2.2. Humidity sensing measurement Regarding the humidity sensing technique, humidity inside the sensing chamber is controlled by two bubblers (one is used for humidification and the other is for desiccation) connected to the chamber. The air flow through the bubblers is operated by computer controlled solenoidal electrical valves. Saturated solutions of NaOH, CH3COOK, MgCl2, Mg(NO3)2, NaCl, KCl, KNO3 are used to get the stable RH levels as 10, 22, 37, 53, 70, 80, 90% respectively. A standard RH meter is kept inside the chamber as a reference to indicate the RH level and temperature of the chamber. A digital multimeter (Keithley 2100) and a DAC (Labjack U6-Pro) are used to acquire the dynamic resistance of the sample and to control the valves respectively. 3. Results and discussions 3.1. Characterization of AZO samples The surface morphology of the AZO films is studied with FESEM micrographs shown in Fig. 2. It is quite clear from the images that the film surfaces are porous in nature and the surface porosity increases with the increase of Al doping concentration. The crystal structure is investigated by XRD using Cu-Ka radiation (k = 1.5418 Å). XRD spectra of AZO films is shown in Fig. 3. All the diffraction peaks have been identified with the standard value of ZnO crystal (JCPDS No. 36-1451) [12]. The diffraction peaks exhibit polycrystalline wurtzite [13] ZnO structure. The dominant peak intensities are observed along (1 0 0), (0 0 2) and (1 0 1) planes. UV–Vis spectroscopy of the AZO samples is studied with the absorbance spectra. For the allowed direct band transition, the relation between the absorption co-efficient a and incident photon energy (hm) is as follows [14],
Fig. 3. XRD spectra of (a) 1 at.%, (b) 3 at.%, (c) 5 at.% AZO thin films.
ahv ¼ B hv Eg
1=2
ð1Þ
where B is a constant, h is Planck constant, m is the frequency of the photon absorbed, Eg is optical band gap. Tauc plot is shown in Fig. 4 for the three AZO samples with 1 at.%, 3 at.% and 5 at.% Al doping. The band gaps are calculated by drawing tangents to the linear portion of the curves and extrapolating the tangents to the x-axis. It can be seen that the obtained band gaps decrease from 3.24 to 3.22 eV with increase in doping from 1 at.% to 5 at.%. Resistivity of the AZO thin films is measured at room temperature with Van der Pauw four probe measurement technique. For this purpose, square pattern of 5 mm dimension area is developed on the films by wet chemical etching. The electrical contacts are made at the corners of the pattern by silver paste. The lowest value of resistivity found is 14 O. cm obtained for 3 at.% doped AZO sample. Fig. 5 shows the resistivity plot of the different doped AZO samples which are annealed at 500 °C. Resistivity decreases initially due to the increase of free carrier concentration up-to doping level 3 at.%. When the doping concentration increases further, the Al dopants enter into the interstitial position of AZO lattice and form non-conducting clusters. Then they behave like carrier traps instead of electron donors [15]. This drives the AZO thin film resistivity to increase when the doping goes above 3 at.%.
Fig. 2. FESEM images of (a) 1 at.%, (b) 3 at.% and (c) 5 at.% AZO thin films.
Please cite this article as: S. Kundu, R. Majumder, R. Ghosh et al., Enhanced relative humidity sensing property of porous Al:ZnO thin films, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.235
S. Kundu et al. / Materials Today: Proceedings xxx (xxxx) xxx
3
Fig. 4. Tauc plot for 1 at.%, 3 at.% and 5 at.% AZO samples.
Fig. 6. Variation of (a) responsivity and (b) sensitivity with relative humidity. The error bars are smaller than the size of the symbols.
Fig. 5. Resistivity of the AZO samples annealed at 500 °C.
3.2. Humidity sensing property The humidity sensing properties of the AZO samples are investigated in the range of 22–90 RH% at room temperature with respect to 10 RH% base humidity level. The samples of 3 mm 3 mm dimension are prepared for humidity sensor by drawing two parallel electrodes with silver paste with a separation of 2 mm. The responsivity is calculated from the dynamic resistance (R) data of the samples measured with time. The responsivity (Nr) and sensitivity (S) are calculated from Eqs. (4) and (5) [16] respectively using the dynamic resistance data.
Nr ¼ DR=R0 ¼ ðRRH R0 Þ=R0 100%
ð2Þ
S ¼ DR=DRH
ð3Þ
where RRH and R0 are the electrical resistances of the sensor in the given RH and dry air respectively. Fig. 6(a) and (b) show the plot of responsivity and sensitivity variation respectively against RH for the AZO samples. It is clearly observed in Fig. 6 that the responsivity gradually increases with the increase of relative humidity and the increase of such responsivity value is more prominent in the samples of higher doping concentration. The maximum amount of responsivity obtained is 820% which is shown by 5 at.% AZO sample at 90% RH. The sensitivity also increases with increase of relative
Fig. 7. Repeatability plot of 5 at % AZO sample.
humidity and it attains maximum value of 0.08 MX/RH% at 90% RH for 5 at.% AZO sample. Fig. 7 shows the repetitive response plot of 5 at.% AZO sample for five consecutive cycles. The positive responses are observed in all cycles which implies that the resistance of the sensor increases with relative humidity. The matter of getting higher responsivity for higher doped AZO material is in good agreement with the images obtained from FESEM analysis. It is evident from the FESEM study that the porosity of the AZO material increases with increase of Al doping percentage and the porosity itself enhances the water adsorption process due to the higher surface area of capillary nanopores [17].
Please cite this article as: S. Kundu, R. Majumder, R. Ghosh et al., Enhanced relative humidity sensing property of porous Al:ZnO thin films, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.235
4
S. Kundu et al. / Materials Today: Proceedings xxx (xxxx) xxx
4. Conclusion Positive type relative humidity sensors have been fabricated successfully with porous AZO thin films synthesized by an unique nebulized spray pyrolysis method. The XRD peaks indicate good polycrystalline wurtzite structure of the AZO material. AZO thin films with 5 at.% Al doping show highest responsivity of 820% at 90% humidity level. The responses are all positive for all the AZO samples. The positive change in resistance is explained in detail with the aid of electron trapping phenomena and renowned chemisorption and physisorbtion processes. Acknowledgement Authors would like to give thanks to Department of Science & Technology (DST), India for their continuous support to this research. Fig. 8. Sensing mechanism of positive humidity sensing.
References 3.3. Explanation of positive humidity sensing The proposed mechanism of positive humidity sensing is schematically presented in Fig. 8. The water (H2O) molecule when comes in immediate contact of the polar Zn2+/Al3+ sites, it dissociates into hydroxyl (–OH) and hydrogen ion (H+). The OH radicals create permanent bonds with the Zn2+/Al3+ sites. This irreversible reaction is known as chemisorption. Now, the second layer of water molecule get physically attached to the first chemisorbed OH through hydrogen (H) bonding. This is called as physisorbtion. Again, due to the presence of higher electronegative oxygen (O) atom in the water molecule, partial positive (d+) and negative (d) charges develop on the H and –OH site respectively. The partial negative charge on –OH of the water molecule is electrically nullified due to their active participation in physisorbtion. The uncompensated free-end Hd+ ion electro-statically [18] holds the free electrons of the AZO surface. Thus, the conductivity decreases with increase of RH. That is why, the resistance increases upon adsorption of water.
[1] D. Zhang, N. Yin, B. Xia, Y. Sun, Y. Liao, Z. He, S. Hao, J. Mater. Sci.: Mater. Electron. 27 (2016) 2481–2487. [2] M.V. Arularasu, R. Sundaram, Sens. Bio-Sens. Res. 11 (2016) 20–25. [3] N.M. Kiasari, S. Soltanian, B. Gholamkhass, P. Servati, Sens. Actuators, A 182 (2012) 101–105. [4] R.M. Mohite, R.R. Kothawale, Indian J. Chem. 54A (2015) 872–876. [5] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Hew, T. Steiner, Superlattices Microstruct. 34 (2003) 3–32. [6] Z.L. Wang, J. Phys.: Condens. Matter 16 (2004) R829. [7] X. Wang, Y. Ding, Z. Li, J. Song, Z. Wang, J. Phys. Chem. C 113 (2009) 1791–1794. [8] K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1–46. [9] S.K. Shukla, A. Tiwari, G.K. Parashar, A.P. Mishra, G.C. Dubeye, Talanta 80 (2009) 565–571. [10] A.B. Hartanto, X. Ning, Y. Nakata, T. Okada, Appl. Phys. A 78 (2004) 299–301. [11] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430–4431. [12] N.F. Hsu, M. Chang, C.H. Lin, Microsyst. Technol. 19 (2013) 1737–1743. [13] J. Fan, T. Li, H. Heng, Bull. Mater. Sci. 39 (2016) 19–26. [14] M. Yilmaz, D. Tatar, E. Sonmez, C. Cirak, S. Aydogan, R. Gunturkun, Synth. React. Inorg. Met.-Org. Nano-Met. Chem. 46 (2016) 489–494. [15] K. Mahmood, S.B. Park, Electron. Mater. Lett. 9 (2013) 161–170. [16] D. Zhang, Y. Sun, P. Li, Y. Zhang, ACS Appl. Mater. Interfaces 8 (2016) 14142– 14149. [17] K.S. Chou, T.K. Lee, F.J. Liu, Sens. Actuators, B 56 (1999) 106–111. [18] N. Uzar, G. Algun, N. Akcay, D. Akcan, L. Arda, J. Mater. Sci.: Mater. Electron. 28 (2017) (1870) 11861–11871.
Please cite this article as: S. Kundu, R. Majumder, R. Ghosh et al., Enhanced relative humidity sensing property of porous Al:ZnO thin films, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.235
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Enhanced relative humidity sensing property of porous Al:ZnO thin films Soumalya Kundu a, Rahul Majumder a, Ria Ghosh a, Manish Pal Chowdhury a,⇑ a
Department of Physics, IIEST Shibpur, Howrah 711103, India
a r t i c l e
i n f o
Article history: Received 12 March 2019 Received in revised form 29 March 2019 Accepted 31 March 2019 Available online xxxx Keywords: Relative Humidity (RH) Positive humidity sensor Al-doped zinc oxide (AZO) Responsivity Nebulized spray pyrolysis
a b s t r a c t High responsive, reliable and low-cost positive type relative humidity (RH) sensors have been fabricated with aluminium (Al) doped zinc oxide (AZO) thin films synthesized by nebulized spray pyrolysis technique. The Al doping concentration in the material is varied from 1 to 5 at.%. The resistive humidity responses of the samples are measured in the range of 22–90% of RH with respect to the base humidity level 10%. Maximum responsivity of 820% is observed for 5 at.% AZO thin film at 90% RH. The sensing mechanism and the positive humidity change are explained on the ground of surface porosity and water adsorption process. Ó 2019 Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on Nano Science & Engineering Application (ICONSEA-2018) Centre for Nano Science and Technology, ICONSEA2018.
1. Introduction Measurement of relative humidity with high accuracy is an inevitable task due to its large impact on many areas like agriculture, biological, environmental monitoring and meteorological services [1]. Hence, constant effort is given by the researchers to achieve more advanced, reliable and highly sensitive humidity sensor. The metal oxide semiconductors are the good option to the researchers for the fabrication of humidity sensors due to their high chemical and thermal stability [2], large surface-to-volume ratio and high sensitivity with fast response [3]. ZnO is a popular n-type metal oxide semiconductors which has direct band gap of 3.3 eV [4] and large exciton binding energy of 60 meV [5]. ZnO is an extremely useful material for humidity sensing due to its morphological diversity [6], high crystallinity, surface porosity [7]. Besides, it is much easier to get higher conductivity of ZnO by simple doping of gr III elements [8]. ZnO can also be synthesized by some facile techniques e.g. chemical vapour deposition (CVD) [9], laser ablation [10], hydrothermal method [11]. In this paper, positive resistive response (positive change in resistance) of porous aluminium doped ZnO (AZO) thin film is reported under exposure to different humidity ambient. In this work AZO thin films synthesized by low-cost nebulized spray pyrolysis technique as compared to conventional synthesis ⇑ Corresponding author. E-mail address: [email protected] (M.P. Chowdhury).
processes [9–11]. Humidity sensing properties are investigated and finally an explanation is proposed in support of positive change in resistance. The electrical resistivity of the as-synthesized films is measured by Van der Pauw four-probe technique. Surface morphology of the samples is studied using FE-SEM (INSPECT F50, Netherland). Crystal structures of the samples are computed by X-Ray Diffractometer (Model No.: X’Pert Powder, PANalytical) data analysis. Band gaps of the ZnO and AZO thin films are found out by UV–VIS (UV-1800, SHIMADZU). 2. Experimental details 2.1. Synthesis of AZO thin film The synthesis of AZO thin films is carried out with the spray deposition of the precursor solution, prepared by mixing of dopant material aluminium nitrate monohydrate [Al(NO3)3H2O, Fisher Scientific] into 0.2 (M) aq. solution of zinc nitrate hexahydrate [Zn(NO3)26H2O, Fisher Scientific]. The amount of addition of the dopant material is decided by the required percentage of Al doping which varies from 1 to 5 at.%. The soda-lime glass substrates are chosen to be spray deposited by the nebulizer from a height of 2 cm above the substrates. The deposition is continued for 60 min at a stretch at 100 °C. Thereafter, the substrates are subjected to anneal at 500 °C in a split-open furnace at atmospheric pressure for 15 min. The spray coating and the annealing steps are repeated for second times to achieve the film thickness of
https://doi.org/10.1016/j.matpr.2019.03.235 2214-7853/Ó 2019 Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on Nano Science & Engineering Application (ICONSEA-2018) Centre for Nano Science and Technology, ICONSEA-2018.
Please cite this article as: S. Kundu, R. Majumder, R. Ghosh et al., Enhanced relative humidity sensing property of porous Al:ZnO thin films, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.235
2
S. Kundu et al. / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 1. Experimental set-up showing synthesis of AZO material.
500 nm. The film thickness has been measured with Bruker DektakXT stylus profilometer. Schematic diagram of the experimental set-up of synthesis is shown in Fig. 1. 2.2. Humidity sensing measurement Regarding the humidity sensing technique, humidity inside the sensing chamber is controlled by two bubblers (one is used for humidification and the other is for desiccation) connected to the chamber. The air flow through the bubblers is operated by computer controlled solenoidal electrical valves. Saturated solutions of NaOH, CH3COOK, MgCl2, Mg(NO3)2, NaCl, KCl, KNO3 are used to get the stable RH levels as 10, 22, 37, 53, 70, 80, 90% respectively. A standard RH meter is kept inside the chamber as a reference to indicate the RH level and temperature of the chamber. A digital multimeter (Keithley 2100) and a DAC (Labjack U6-Pro) are used to acquire the dynamic resistance of the sample and to control the valves respectively. 3. Results and discussions 3.1. Characterization of AZO samples The surface morphology of the AZO films is studied with FESEM micrographs shown in Fig. 2. It is quite clear from the images that the film surfaces are porous in nature and the surface porosity increases with the increase of Al doping concentration. The crystal structure is investigated by XRD using Cu-Ka radiation (k = 1.5418 Å). XRD spectra of AZO films is shown in Fig. 3. All the diffraction peaks have been identified with the standard value of ZnO crystal (JCPDS No. 36-1451) [12]. The diffraction peaks exhibit polycrystalline wurtzite [13] ZnO structure. The dominant peak intensities are observed along (1 0 0), (0 0 2) and (1 0 1) planes. UV–Vis spectroscopy of the AZO samples is studied with the absorbance spectra. For the allowed direct band transition, the relation between the absorption co-efficient a and incident photon energy (hm) is as follows [14],
Fig. 3. XRD spectra of (a) 1 at.%, (b) 3 at.%, (c) 5 at.% AZO thin films.
ahv ¼ B hv Eg
1=2
ð1Þ
where B is a constant, h is Planck constant, m is the frequency of the photon absorbed, Eg is optical band gap. Tauc plot is shown in Fig. 4 for the three AZO samples with 1 at.%, 3 at.% and 5 at.% Al doping. The band gaps are calculated by drawing tangents to the linear portion of the curves and extrapolating the tangents to the x-axis. It can be seen that the obtained band gaps decrease from 3.24 to 3.22 eV with increase in doping from 1 at.% to 5 at.%. Resistivity of the AZO thin films is measured at room temperature with Van der Pauw four probe measurement technique. For this purpose, square pattern of 5 mm dimension area is developed on the films by wet chemical etching. The electrical contacts are made at the corners of the pattern by silver paste. The lowest value of resistivity found is 14 O. cm obtained for 3 at.% doped AZO sample. Fig. 5 shows the resistivity plot of the different doped AZO samples which are annealed at 500 °C. Resistivity decreases initially due to the increase of free carrier concentration up-to doping level 3 at.%. When the doping concentration increases further, the Al dopants enter into the interstitial position of AZO lattice and form non-conducting clusters. Then they behave like carrier traps instead of electron donors [15]. This drives the AZO thin film resistivity to increase when the doping goes above 3 at.%.
Fig. 2. FESEM images of (a) 1 at.%, (b) 3 at.% and (c) 5 at.% AZO thin films.
Please cite this article as: S. Kundu, R. Majumder, R. Ghosh et al., Enhanced relative humidity sensing property of porous Al:ZnO thin films, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.235
S. Kundu et al. / Materials Today: Proceedings xxx (xxxx) xxx
3
Fig. 4. Tauc plot for 1 at.%, 3 at.% and 5 at.% AZO samples.
Fig. 6. Variation of (a) responsivity and (b) sensitivity with relative humidity. The error bars are smaller than the size of the symbols.
Fig. 5. Resistivity of the AZO samples annealed at 500 °C.
3.2. Humidity sensing property The humidity sensing properties of the AZO samples are investigated in the range of 22–90 RH% at room temperature with respect to 10 RH% base humidity level. The samples of 3 mm 3 mm dimension are prepared for humidity sensor by drawing two parallel electrodes with silver paste with a separation of 2 mm. The responsivity is calculated from the dynamic resistance (R) data of the samples measured with time. The responsivity (Nr) and sensitivity (S) are calculated from Eqs. (4) and (5) [16] respectively using the dynamic resistance data.
Nr ¼ DR=R0 ¼ ðRRH R0 Þ=R0 100%
ð2Þ
S ¼ DR=DRH
ð3Þ
where RRH and R0 are the electrical resistances of the sensor in the given RH and dry air respectively. Fig. 6(a) and (b) show the plot of responsivity and sensitivity variation respectively against RH for the AZO samples. It is clearly observed in Fig. 6 that the responsivity gradually increases with the increase of relative humidity and the increase of such responsivity value is more prominent in the samples of higher doping concentration. The maximum amount of responsivity obtained is 820% which is shown by 5 at.% AZO sample at 90% RH. The sensitivity also increases with increase of relative
Fig. 7. Repeatability plot of 5 at % AZO sample.
humidity and it attains maximum value of 0.08 MX/RH% at 90% RH for 5 at.% AZO sample. Fig. 7 shows the repetitive response plot of 5 at.% AZO sample for five consecutive cycles. The positive responses are observed in all cycles which implies that the resistance of the sensor increases with relative humidity. The matter of getting higher responsivity for higher doped AZO material is in good agreement with the images obtained from FESEM analysis. It is evident from the FESEM study that the porosity of the AZO material increases with increase of Al doping percentage and the porosity itself enhances the water adsorption process due to the higher surface area of capillary nanopores [17].
Please cite this article as: S. Kundu, R. Majumder, R. Ghosh et al., Enhanced relative humidity sensing property of porous Al:ZnO thin films, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.235
4
S. Kundu et al. / Materials Today: Proceedings xxx (xxxx) xxx
4. Conclusion Positive type relative humidity sensors have been fabricated successfully with porous AZO thin films synthesized by an unique nebulized spray pyrolysis method. The XRD peaks indicate good polycrystalline wurtzite structure of the AZO material. AZO thin films with 5 at.% Al doping show highest responsivity of 820% at 90% humidity level. The responses are all positive for all the AZO samples. The positive change in resistance is explained in detail with the aid of electron trapping phenomena and renowned chemisorption and physisorbtion processes. Acknowledgement Authors would like to give thanks to Department of Science & Technology (DST), India for their continuous support to this research. Fig. 8. Sensing mechanism of positive humidity sensing.
References 3.3. Explanation of positive humidity sensing The proposed mechanism of positive humidity sensing is schematically presented in Fig. 8. The water (H2O) molecule when comes in immediate contact of the polar Zn2+/Al3+ sites, it dissociates into hydroxyl (–OH) and hydrogen ion (H+). The OH radicals create permanent bonds with the Zn2+/Al3+ sites. This irreversible reaction is known as chemisorption. Now, the second layer of water molecule get physically attached to the first chemisorbed OH through hydrogen (H) bonding. This is called as physisorbtion. Again, due to the presence of higher electronegative oxygen (O) atom in the water molecule, partial positive (d+) and negative (d) charges develop on the H and –OH site respectively. The partial negative charge on –OH of the water molecule is electrically nullified due to their active participation in physisorbtion. The uncompensated free-end Hd+ ion electro-statically [18] holds the free electrons of the AZO surface. Thus, the conductivity decreases with increase of RH. That is why, the resistance increases upon adsorption of water.
[1] D. Zhang, N. Yin, B. Xia, Y. Sun, Y. Liao, Z. He, S. Hao, J. Mater. Sci.: Mater. Electron. 27 (2016) 2481–2487. [2] M.V. Arularasu, R. Sundaram, Sens. Bio-Sens. Res. 11 (2016) 20–25. [3] N.M. Kiasari, S. Soltanian, B. Gholamkhass, P. Servati, Sens. Actuators, A 182 (2012) 101–105. [4] R.M. Mohite, R.R. Kothawale, Indian J. Chem. 54A (2015) 872–876. [5] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Hew, T. Steiner, Superlattices Microstruct. 34 (2003) 3–32. [6] Z.L. Wang, J. Phys.: Condens. Matter 16 (2004) R829. [7] X. Wang, Y. Ding, Z. Li, J. Song, Z. Wang, J. Phys. Chem. C 113 (2009) 1791–1794. [8] K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1–46. [9] S.K. Shukla, A. Tiwari, G.K. Parashar, A.P. Mishra, G.C. Dubeye, Talanta 80 (2009) 565–571. [10] A.B. Hartanto, X. Ning, Y. Nakata, T. Okada, Appl. Phys. A 78 (2004) 299–301. [11] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430–4431. [12] N.F. Hsu, M. Chang, C.H. Lin, Microsyst. Technol. 19 (2013) 1737–1743. [13] J. Fan, T. Li, H. Heng, Bull. Mater. Sci. 39 (2016) 19–26. [14] M. Yilmaz, D. Tatar, E. Sonmez, C. Cirak, S. Aydogan, R. Gunturkun, Synth. React. Inorg. Met.-Org. Nano-Met. Chem. 46 (2016) 489–494. [15] K. Mahmood, S.B. Park, Electron. Mater. Lett. 9 (2013) 161–170. [16] D. Zhang, Y. Sun, P. Li, Y. Zhang, ACS Appl. Mater. Interfaces 8 (2016) 14142– 14149. [17] K.S. Chou, T.K. Lee, F.J. Liu, Sens. Actuators, B 56 (1999) 106–111. [18] N. Uzar, G. Algun, N. Akcay, D. Akcan, L. Arda, J. Mater. Sci.: Mater. Electron. 28 (2017) (1870) 11861–11871.
Please cite this article as: S. Kundu, R. Majumder, R. Ghosh et al., Enhanced relative humidity sensing property of porous Al:ZnO thin films, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.03.235