V. Manikandan*a,
Iulian Petrilab,
S. Vigneselvanc,
R. S. Mane
d,
Bogdan Vasile
e,
Raghu Dharmavarapuf,
Stefan Lundgaard
f,
Saulius Juodkazis
f and
J. Chandrasekarang
aDepartment of Physics, Kongunadu Arts and Science College, Coimbatore – 641 029, India. E-mail: manikandan570@gmail.com
bFaculty of Automatic Control and Computer Engineering, Gheorghe Asachi Technical University of Iasi, Str. Dimitrie Mangeron, Nr. 27, 700050 Iasi, Romania
cDepartment of Physics, Government College of Technology, Coimbatore – 641 013, India
dCenter for Nanomaterial & Energy Devices, Swami Ramanand Teerth Marathwada University, Dnyanteerth, Vishnupuri, Nanded-431606, India
eUniversity Politehnica of Bucharest, Gh. Polizu Street No. 1-7, 011061 Bucharest, Romania
fCenter of Micro-Photonics, Swinburne University of Technology, Victoria 3122, Australia
gDepartment of Physics, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641 020, India
First published on 22nd January 2020
Herein, we report the chemiresistive gas and humidity sensing properties of pristine and nickel-doped tin oxide (Ni-SnO2) gas sensors prepared by a microwave-assisted wet chemical method. The structural and optical properties are characterised using X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy, ultraviolet spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The structural elucidation and morphology analyses confirm a particle size of 32–46 nm, tetragonal rutile crystal structure and small cauliflower-type surface appearance. Nickel doping can tune the structure of NPs and morphology. The tested carbon dioxide gas and humidity sensing properties reveal a rapid sensing performance with high-to-moderate sensitivity. Also, the materials favour gas sensing because their sensitivity is enhanced with the increase in nickel concentration. The sensing results suggest that nickel is a vibrant metal additive to increase the gas sensitivity of the sensor. However, nickel doping decreases the electron density and increases the oxygen vacancies. Ultimately, the gas sensor produces highly rapid sensing with a response time of 4 s.
SnO2 being an n-type metal oxide is a chemiresistive sensor because of its large band gap (3.40 eV), d-10 electronic configuration, high reactivity against pollutant gases, moderate chemical stability, high reactive surface, and quantum confinement.12–14 It has been widely used in oxidation catalysts,15 gas sensors,16 conducting electrodes17 and opto-electronic devices.18 The electronic structure of tin oxide changes in accordance with doping, the influence of surfactants and thermal annealing processes.19 Also, the low electrical resistance of tin oxide favours better electrical conduction in the process of charge/electron/ion transportation.20 SnO2 has been considered as the most potent sensor material as far as sensitivity, selectivity and response are concerned.21,22 The doping of nickel according to its size, oxidation state and concentration in tin oxide alters the sensitivity and selectivity, and nickel ions can easily occupy the tin sites because of their close ionic radii: Ni2+ = 69 nm and Sn2+ = 71 nm. Furthermore, the doping level decreases the electron density and increases the oxygen vacancies, which elevates the sensitivity greatly.23,24 In the past, the specific area of SnO2 was found to increase upon the addition of additives such as Pt, Pd, Co, Fe and Ni. Of these additives, nickel has been widely employed to enhance the sensing performance of tin oxide sensors for various target gases.25
In this work, nickel-doped tin oxide (Ni-SnO2) nanoparticles were successfully synthesized using a microwave-assisted wet chemical method. The effects of Ni doping on the structural, morphological and sensing properties of the SnO2 nanoparticles with different concentrations were analysed and their sensor performance was studied and reported. The Ni-SnO2 sensor showed the highest sensitivity and response for CO2 gas, where the sensitivity increased with the concentration of Ni doping because the electron density decreased. On the other hand, the tested humidity performance also showed an increasing trend. From the results for the optimization of the sensor, the nickel-doped tin oxide sensor was found to be an appropriate material for carbon dioxide gas detection. The results for the gas sensor are compared to recently reported results, as shown in Table 1. The literature survey concludes that the present sensor demonstrates high sensitivity and rapid detection.
S. no. | Materials | Gas | Gas concentration | Operating temperature | Response | Sensitivity | Ref. |
---|---|---|---|---|---|---|---|
1 | BaTiO3 | CO2 | 10![]() |
550 °C | — | 1.25 | 1 |
2 | Ca-ZnO | CO2 | 5000 ppm | 300 °C | 120 s | 10 | 6 |
3 | CoAl2O4 | CO2 | 100 ppm | 400 °C | 46 s | 0.43 | 62 |
4 | SnO2 | CO2 | 5000 ppm | 279 °C | 180 s | 48 | 63 |
5 | La2O3 | CO2 | 350 ppm | 321 °C | 105 s | 65 | 2 |
6 | Pd-La2O3 | CO2 | 500 ppm | 250 °C | 50 s | 28 | 64 |
7 | Ni-SnO2 | CO2 | 100 ppm | 275 °C | 04 s | 73.29 | Present work |
Material Ni-SnO2 | CO2 gas sensor | Humidity sensor | |||||
---|---|---|---|---|---|---|---|
Concentration | Particle size (nm) | Res. time (s) | Rec. time (s) | Sensitivity (Rg/Ra) | Res. time (s) | Rec. time (s) | Sensitivity |
SnO2 (x = 0.0) | 46.70 | 29 | 40 | 62.31 | 41 | 68 | 81.98 |
Ni-SnO2 (x = 0.2) | 32.17 | 21 | 24 | 71.53 | 49 | 71 | 78.11 |
Ni-SnO2 (x = 0.4) | 42.32 | 04 | 13 | 73.29 | 54 | 84 | 72.03 |
Fig. 2a–c highlight the SEM images of the pristine SnO2 and Ni-SnO2 surfaces at different Ni-doping levels. The surface of pristine SnO2 is like brick stone; after Ni doping, it changes to a cauliflower type. This indicated that the morphology of the Ni-doped SnO2 NPs significantly changed. Pristine nanoparticles had no interesting morphology, but the defined shape of a cauliflower structure with a fine gap was observed due to the addition of nickel, which enabled the target gas to simply penetrate through the fine gap, yielding a high resistance change in sensor analysis. Fig. 3a–f show the STEM images of pristine SnO2 and Ni-SnO2 NPs, where the brick stone-like surface of high-surface-energy pristine sample is observed. The surface energy was reduced due to Ni doping and the particles showed a few needles in addition to chain formation. However, nickel acted as an energy barrier and this energy barrier reduced the surface energy, leading to good crystallization. The selected area electron diffraction patterns of the Ni-SnO2 nanoparticles afford a continuous ring pattern without additional diffraction spots, suggesting the nanocrystalline feature of Ni-SnO2.
To verify the presence of chemical elements, XPS analysis was carried out for pristine SnO2 and Ni-SnO2 NPs. The binding energies of the O 1s peak of oxygen are centred at 528.94, 529.07 and 529.96 eV (Fig. 4). From the binding energies of O 1s, a shift in the position of a small peak due to the doping of nickel ions was confirmed.28,29 The increase in binding energy is associated with OH in the oxygen-deficient regions.30 Also, it causes O2− adsorption on the surface of the sensor and increases oxygen adsorption as a result of nickel doping, which would eventually enhance the gas-sensing performance.30,31 In Fig. 4, the XPS spectrum of tin shows similar peaks. The binding energies of Sn in pristine SnO2 NPs were compared with the binding energies of Ni-SnO2 NPs. From this comparison, the obtained binding energies of Ni-SnO2 showed an extremely small shift due to oxygen deficiencies caused by nickel doping.32,33 The binding energies of Ni 2p3/2 were 852.76 and 858.58 eV for a concentration of x = 0.2 and 853.03 and 858.81 eV for a concentration of x = 0.4 with a small peak shift. These results suggest that nickel ions can be doped into the tin lattice without structural deviation.34,35
Fig. 5a–c display the FTIR transmittance spectra of Ni-SnO2 NPs containing different concentrations of nickel. The transmittance spectra of NPs present changes in the position and shape of the IR bands with respect to the concentration, suggesting the incorporation of nickel in the SnO2 lattice. The IR band at 450 cm−1 can be assigned to the O–Sn–O antisymmetric stretching vibrations and that at 693 cm−1 can be assigned to the symmetric stretching vibrations.36 The IR band at around 1380–1647 cm−1 is due to the C–O bonds.37 The O–H stretching of water molecules contributed to bands at 2558–3465 cm−1.38 Pristine SnO2 NPs contributed to IR bands and these were compared to that of Ni-SnO2, where a shift in the IR band as a result of the particle size and shape change was evidenced by nickel substitution. Due to the variation in bond length on nickel doping, the band shift extended the position of the IR band towards a higher wavenumber. The intensities of the IR bands increased due to the increase in the nickel doping level, which decreased the particle size due to the high surface area and the continuous adsorption of water molecules from the atmosphere.39 In all the samples, the IR bands related to the O–H bonds suggested the presence of active sites, fast adsorption and surface modification.
To understand the impact of nickel doping on the optical properties of SnO2 at various concentrations, the UV-Vis absorption and photoluminescence spectra were recorded (Fig. 6a). Upon nickel doping, the absorption band of SnO2 was deduced. In particular, due to photo-excitation, the absorption band at 300 nm shifted to 291 nm for Ni-SnO2.40 Also, the absorption spectra showed an ultra-violet cut-off around 275–325 nm. With photo-excitation, the energy of the atoms was lifted to a certain level by the absorption of photons of a particular wavelength. In this case, the energy of the atoms decreased due to Ni doping as the excited electrons attained transition from the valence band to the conduction band. Usually, the absorption spectrum of wide-band materials depends on oxygen deficiencies, surface roughness and impurities. In addition, the absorption decreased with the increase in the nickel concentration. The optical energy band gap obtained from the diffuse reflectance spectra using the Kubelka–Munk function F(R) is as follows:41
![]() | (1) |
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Fig. 6 (a) Absorption, (b) band gap energy and (c) photoluminescence spectra of pristine SnO2 and Ni-SnO2 NPs. |
Fig. 7a–c demonstrate the quick response of the SnO2 and Ni-SnO2 sensors for CO2 gas from 0 to 200 ppm. To gain a first insight into the electrical behaviour of the pristine SnO2 and Ni-SnO2 sensors, the resistance change of the sensor with respect to various operating temperatures, i.e., 100–400 °C was measured in dry air. An increase in resistance with respect to the operating temperature was observed in all sensors, which was adopted for stabilizing the resistance (Ra).1,44 The decrease in the sensor resistance (Rg) of the Ni-SnO2 sensor on exposure to CO2 was evidenced. Both the resistance values, i.e., Ra and Rg increased with the Ni-doping level. However, the sensitivity was very resistive due to the decrease in conductivity. Also, the resistance values of the sensor in air and gas decreased with the increase in the operating temperature as the band gap of the sensor increased with the increase in nickel concentration; this suggested that electrons get sufficient thermal energy to excite from the valence band to the conduction band, thus continuously decreasing the resistance.45 The sensor confirmed rapid detection of the gas with an ultra-fast response time (Fig. 7c). The response times of the pristine SnO2 and Ni-SnO2 sensors were 29, 21 and 04 s, whereas the recovery time values were 40, 24 and 13 s. Both the response time and recovery time decreased with the increase in the Ni-doping level, which could be the result of a faster reaction between CO2 and the chemisorbed oxygen molecules.46
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Fig. 7 (a and b) Resistance of pristine SnO2 and Ni-SnO2 sensors in air and gas versus temperatures. (c) Dynamic response of pristine SnO2 and Ni-SnO2 sensors. |
Fig. 8a–c demonstrate the sensitivity of the pristine SnO2 and Ni-SnO2 sensors obtained at various Ni-doping levels versus the concentration of the CO2 gas, time and operating temperature. The sensitivity of the pristine SnO2 and Ni-SnO2 sensors increased with the increase in the CO2 gas concentration. Here, oxygen vacancies play a leading role in sensitivity estimation as they increase with the increase in gas concentration, which increases the sensitivity.47,48 Fig. 8b demonstrates the sensitivity curves with respect to time. From this figure, it can be seen that the sensitivity of the Ni-SnO2 sensors increases with the nickel-doping level, suggesting a moderate repeatability signature.13,49 The sensitivity of the pristine SnO2 and Ni-SnO2 sensors with the operating temperature increased and then decreased (Fig. 8c). Initially, the sensitivity increased up to 275 °C and then decreased, which depended on the adsorption and desorption of gas molecules on the sensor surface. In this case, the sensitivity might be controlled through adsorption from 100 to 275 °C, whereas desorption started from 275–400 °C. At a particular temperature, adsorption and desorption were equal and the equilibrium position was reached.2,50 In the present case, the optimum sensing temperature of the pristine SnO2 and Ni-SnO2 sensors for CO2 gas was 275 °C. The sensitivity varied from 62 to 73.29 and the maximum sensitivity with a 4 s response time was obtained when x = 0.4 for 100 ppm CO2 gas, which was comparable with other values reported recently (Table 1).
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Fig. 8 Sensitivity change of pristine SnO2 and Ni-SnO2 sensors with (a) gas concentration, (b) time and (c) temperature. |
The fabricated pristine SnO2 and Ni-SnO2 sensors were basically chemiresistive, where the sensor resistance was controlled by the CO2 gas molecules and the chemisorbed oxygen molecules. The free electrons from the sensor surface were arrested using the adsorbed oxygen molecules and then converted to oxygen anions, such as O2 or O−, as follows:51
O2(gas) ↔ O2(ads) | (2) |
O2(ads) + e− ↔ O2(ads)− | (3) |
O2(ads)− + e− ↔ 2O(ads)− | (4) |
The adsorption of oxygen molecules was influenced by the operating temperature. Also, the oxygen vacancies played a leading role at the chemisorption sites under the influence of oxygen. The concern oxygen molecules were adsorbed on the sensor surface by a chemisorption process. During adsorption, bonding occurred between the chemisorbed oxygen and the surface of the sensor. Also, weak bonding was observed at a low operating temperature with a low response. However, a strong bond produced at higher operating temperatures allows a higher response under the influence of a gas.52 Carbonate ions were formed during the exposure of the CO2 gas to the SnO2 and Ni-SnO2 sensors, and the adsorbed gas molecules could act as acceptors as per the following reaction:53
2CO2(gas) + 2O2(ads)− → 2CO3(gas)− + O2− | (5) |
The resistance returned to its original value due to the removal of carbon dioxide gas, whereas the oxygen molecules (air) were adsorbed on the surface of the sensor again.54
Fig. 9a–c demonstrate the humidity sensing response of the pristine SnO2 and Ni-SnO2 sensors measured at room temperature (27 °C). The humidity measurement was implemented in the range of 0–100% RH. The resistance of the sensor changed in accordance with the humidity (Fig. 7a). Here, the resistance to humidity increased with the Ni-doping level, suggesting that the sensor response occurred in a good manner with respect to humidity. Fig. 9b demonstrates the sensitivity of the sensor in accordance with the humidity. From this figure, the sensitivity of each sensor can be seen to have improved with the humidity. Here, the sensitivity of the pristine SnO2 sensor was high compared to that of the Ni-SnO2 sensor. Notably, the sensitivity of the sensor to humidity decreased after Ni doping with the increase in the elemental concentration (Ni); this can be used to support the use of the material as a gas sensor (CO2) because the sensitivity to gas increases with the increase in the elemental concentration (Ni). Also, their sensitivity followed a descending order with Ni doping. Usually, humidity changes the sensitivity of metal-oxide-based sensors. In this case, the carbon dioxide gas molecules adsorbed on the surface of the sensor previously, thus limiting the availability of the active sites for the adsorption of water molecules. As a consequence of the limited active sites, the sensitivity was reduced since the active sites are used for gas sensing.6,55 Although, the pristine SnO2 sensor surface could hold water vapour for a long duration during the exposure to humidity, which resulted in high sensitivity, the Ni-SnO2 sensor failed to hold water vapour for a long duration under humidity, thus reducing the sensitivity.56 On further increasing Ni doping, the surface of the sensor failed to hold water vapour again, thus decreasing the overall sensitivity.56 Fig. 9c demonstrates the response time and recovery time performance of the pristine SnO2 and Ni-SnO2 sensors, where a fast response and quite long recovery to humidity are noted. The response time and recovery time of the pristine and Ni-SnO2 sensors were 41, 49, 54 and 68, 71, 84 s, respectively, due to Ni doping. The response time for adsorption and the recovery time for desorption were relatively longer.57 Moreover, in each case, the response time was lower than the recovery time because of the fact that the adsorption could be faster than the desorption of adsorbed water molecules.58,59 The figure presents the repeatability curves for all the sensors, which signify their good sensing ability for humidity. The humidity sensing mechanism was based on a Grotthuss chain reaction. Humidity sensing was characterized by the change in resistance in accordance with the humidity. Initially, a few water vapour molecules could chemically adsorb on the surface of the sensor, yielding hydroxyl groups on the sensor surface.60 The surface hydroxyl groups underwent hydrogen bonding with the physisorption of water molecules; the primary adsorbed layer was then localized using hydrogen bonds to the hydroxyl group and hydronium ions were produced due to the proton transfer from the hydroxyl groups to the water molecules. More water molecules formed a liquid network similar to a chain-type network.61 Also, the hydrogen ions increased with the increase in the moisture content. Thus, the hydrogen ions easily moved to the liquid network, which produced a change in resistance in accordance with the variation in humidity.
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Fig. 9 (a) Resistance towards humidity, (b) sensitivity towards RH (%), (c) sensitivity towards time of pristine SnO2 and Ni-SnO2 NPs. |
In summary, the fabricated sensor played a dual role in gas and humidity sensing. In accordance with sensitivity, the sensor was favourable for gas sensing as the sensitivity increased with the increase in the Ni-doping level, whereas it decreased under humidity tests.
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