3.1.Elemental and Structural analysis
A Scanning Electron Microscope (SEM) was utilized to study the morphology of the sensing material. Figure 4(a-c) shows representative SEM micrographs of C1Z4, C4Z4, and C4Z1 nanocomposites having densely agglomerated spherical morphology and their corresponding elements contained in the samples as seen in EDS spectra (Fig. 4(d-f)). Likewise,
Table 1.The At (%) various elements present in C1Z4, C4Z4, and C4Z1 before and after coating upon the optical fiber.
As synthesized CeO2 doped ZnO
|
Zn (at.%)
|
Ce(at.%)
|
O(at.%)
|
C1Z4
|
64.30
|
16.38
|
19.12
|
C4Z4
|
45.42
|
31.78.
|
12.80
|
C4Z1
|
17.10
|
68.35
|
14.55
|
Optical fiber coated CeO2 doped ZnO
|
Zn (at.%)
|
Ce(at.%)
|
O(at.%)
|
C1Z4
|
36.72
|
04.11
|
30.27
|
C4Z4
|
20.42
|
19.30
|
30.48
|
C4Z1
|
13.83
|
62.13
|
18.62
|
3.2) VOCs Gas sensing Analysis to determine its sensitivity and selectivity
The C1Z4, C4Z4 and C4Z1 samples were evaluated with various concentrations of test VOC gases at low ppm to understand their potential as a breath analyzer, utilising a clad modified optic fibre gas sensor setup, and their individual spectrum responses are shown in Fig. 5. With varying quantities of VOC test gases in 10 ppm increments, the spectral peaks were observed at 670, 757 and 935 nm wavelengths. All the gases displayed spectral peaks at these wavelengths, however their intensities differed in incremental and decremental orders. The spectral response peaks clearly depicted that the sensor's consistency with ammonia was better compared to other VOC test gases at room temperature.
Further, the gas sensitivity was measured for all the samples by plotting a graph with intensity along y-axis and concentration of various ppm’s of VOC test vapours of ammonia, ethanol and methanol along the x-axis for the spectral points at 690 nm as shown in Fig. 5. An encompassed Table has been presented to display the gas sensitivity of the samples in the presence of various test gases as shown in Table 2. The gas sensitivity was observed to be the highest for C4Z1 sample with a negative slope observed as -2.6 counts/10 ppm for ammonia, compared to other test gas vapours;
the negative sign indicating the larger leakage of light. Figures 6(a-i) give the plot between the spectral peak intensity gases ammonia,ethanol and methanol at 690 nm, 757 nm and 935 nm.. The results show that CeO2 doped ZnO is highly selective to ammonia as for other gases the sensitivity is very less.(Table.2)
Table 2
Gas sensitivity along with increase and decrease in spectral intensity of various samples towards the test gases at room temperature.
Test Gas Vapours
|
Gas Sensitivity (10 counts/ppm)
|
C4Z1
|
C4Z4
|
C1Z4
|
Ammonia
|
2.6
|
2.0
|
0.7
|
↓
|
1.3
|
1.0
|
0.8
|
↑
|
1.4
|
1.1
|
0.6
|
↓
|
Ethanol
|
1.6
|
0.9
|
0.3
|
↑
|
1.4
|
1.2
|
0.2
|
↑
|
0.7
|
0.5
|
0.3
|
↓
|
Methanol
|
1.7
|
1.4
|
1.3
|
↑
|
2.1
|
1.8
|
0.9
|
↓
|
1.0
|
0.8
|
0.7
|
↓
|
A bar diagrammatic representation of the gas sensitivity is displayed in Fig. 7(a) for various test gases at maximum spectral peak intensities 690 nm, 757 nm and 935 nm respectively. Interestingly, C4Z1 showed decrease in the magnitudes of spectral intensities from its reference value (0 ppm), which is due to more evanescent light absorption at the boundary between C4Z1 and the test gas ammonia, resulting in changes in its refractive index. Rare earth CeO2 helped in enhancing the gas sensing ability of ZnO for C4Z1, as the concentration of CeO2 was larger than ZnO. As ZnO is a wide band gap semiconductor, inclusion of more concentration of dopant RE state of CeO2 into host ZnO, narrowed the Fermi energy level and reduced the band gap between the valence and conduction band, thus enhancing better conduction mechanism. Upon bonding, every rare earth CeO2 loses its electrons, and thus the 4f electrons occupy gradually the majority spin states thereby increasing the total magnetic moments and its conduction state. Likewise, the sensitivity percentage was calculated for C4Z1 sample towards the low concentrations (0-100 ppm) of test gas ammonia and the sensitivity (%) was observed to be 12.5% at the maximum of 100 ppm as shown in Fig. 7(b).
The sensitivity (%) was calculated using the formula ((I-I0)/ I0 )* 100 wherein the maximum spectral peak intensity of ammonia at 100 ppm (denoted as I) was subtracted with the maximum spectral peak intensity of reference (0 ppm) (denoted as I0).
3.3 UV absorption spectra
The C4Z1 sample was studied using UV–Vis Absorption Spectroscopy since it had a greater sensing response than the other samples. Optical analysis was performed to determine the absorption capability of the material in various test gas conditions. UV–Vis absorption spectroscopy was performed by depositing the sample on one of the cuvette's inner walls and pouring 1 ml of the solution (ethanol, methanol, and ammonia) into its bottom at a time to generate vapour [34]. A comparable cuvette was used to capture the same quantity of fluid without the sample coating for comparison, and the spectra were recorded.
Fig. 8 depicts variations in the UV–Vis absorption spectra for C4Z1 in the presence of air, ethanol, and ammonia gas environments, with visible light wavelengths ranging from 300 nm to 400 nm. In the case of ammonia, absorbance increases as the wavelength increases, however in the case of air and ethanol, absorbance decreases. C4Z1 had a high interaction, which can be ascribed to its smaller particle size, which affects its surface to volume ratio, enhancing absorption.