3.1 XRD analysis
Figure 1(a-e) shows the XRD pattern of CdO nanoparticles prepared under four different surfactants such as n-heptane, polyimide, PVA, PVP, and SDS. The spectrum of the samples indicates the strong diffraction peaks at the angle as 33.1o, 38.4o, 55.4o, 66.0o and 69.3o. The corresponding reflection peaks belong to the (hkl) values (110), (200), (220), (311), and (222) [JCPDS 05- 0640]. These peaks belong to the simple cubic structure [3].The main peak of 33.1o of (111) crystallographic plane, which predominantly specifies the good crystalline nature for all the surfactant-based CdO nanoparticles. There are no additional diffraction peaks for representing other spices in the samples. It confirmed that these prepared cadmium oxide nanoparticles with the help of n-heptane, polyimide, PVA, PVP, and SDS are free from impurities. The crystallinity level of CdO nanoparticles of chemical surfactant of n-heptane during the preparation is helped well to achieve good crystallinity in a curve (a) in Fig [1]. The influence of the next surfactant of polyimide for the preparation of CdO nanoparticles is also analyzed from the XRD pattern and it is shown in the curve (b) of Fig. 1(b). The reflection peaks of polyimide-based CdO nanoparticles are observed as (111), (200), (220), (311), and (222) which is also indicated with good crystalline nature. Similar peaks such as (111), (200), (220), (311), and (222) are observed for CdO nanoparticles prepared under n-heptane and polyimide-based samples the curve (c, d, and e) are also possessed similar hkl value and indicate the crystalline nature of the sample. The change in crystallite size is varied for all the samples. This change is occurred due to the influence of chemical surfactants like n-heptane, polyimide, PVA, PVP, and SDS. These chemical surfactants are played as a capping agent [4] and decreased the grain size. The addition of these chemical polymeric surfactants helps to give the confirmed environment for the nucleation of growth of particles. The sharp diffraction peaks at 33.1oand 38.4o indicated that the crystallite size of CdO nanoparticles is very small. The remaining diffraction peaks (55.4o, 66.0o, and 69.3o) indicate the high crystalline. The average particle size of the CdO nanoparticles is determined using Debye- Scherrer’s formula [12]. The crystallite sizes are predicted as 70 nm for n-heptane based CdO nanoparticles, 84 nm for polyimide-based CdO nanoparticles, 89 nm for PVA-based CdO nanoparticles, 54 nm for PVP-based CdO nanoparticles, and 71 nm for SDS based CdO nanoparticles. The usage of PVP as surfactant is well to form the less particle size than the other above-mentioned surfactants.
3.2 UV-Vis-DRS Spectra
Figure 2(a-e) shows the UV absorption spectra of CdO nanoparticles prepared under four different surfactants such as n-heptane, polyimide, PVA, PVP, and SDS. The UV absorption spectrum of n-heptane based CdO nanoparticles shown a peak in the UV region but the absorption band edge was gradually decreased compared to other spectra. The surfactants like n-heptane, polyimide, PVA, PVP, and SDS in CdO nanoparticles formation have indicated the same band at 240 nm. This occurrence of absorption at 240 nm is due to the intrinsic bandgap absorption of CdO nanoparticles due to the electron transitions from the valence band to the conduction band (Cd3d -O2p) [5]. Further, the adding of chemical surfactant solutions of n-heptane, polyimide, PVA, PVP, and SDS possess a strong absorption band in the visible region at 516 nm within an absorption edge of 400–800 nm. The surfactants-based CdO nanoparticles lead to a gradual increase in absorption intensity. The various surfactants which obtaining the different absorption of the CdO nanoparticles are identified from these spectra. The reflectance (R) is taken from absorption by Kubelka-Munk function F(R) [6].
The bandgap energies of n-heptane, polyimide, PVA, PVP, and SDS-based CdO nanoparticles of samples are predicted from the DRS spectra and it is shown in Fig. 3(a-e). The optical bandgap energy value varies are 2.37 eV for n-heptane CdO nanoparticles 2.28 eV for polyimide CdO nanoparticles, 2.21 eV for PVA CdO nanoparticles, 2.39 eV for PVP CdO nanoparticles and 2.30 eV for SDS CdO nanoparticles respectively. A good agreement of the bandgap energy value with standard CdO nanoparticles is found for these samples [7]. This process of change in a bandgap due to chemical surfactants is happened because of surface modification of CdO nanoparticles. Thus, this difference in bandgap energy (Eg) value is caused only by various chemical surfactants. The increase in bandgap energy value for PVP-based CdO nanoparticles has also evidenced the decrease of particle size [21].
3.4 PL Spectra
Figure 4(a-e) shows the PL spectra of CdO nanoparticles prepared under various surfactants such as n-heptane, polyimide, PVA, PVP, and SDS. The n-heptane based CdO nanoparticles of luminescence spectrum of the curve (a) in Fig. 4 show (a) strong emission peak at 310 nm corresponding to the UV region of CdO nanoparticles. Similarly, luminance spectra of the curve (b-e) the polyimide, PVA, PVP, and SDS-based CdO nanoparticles showed in Fig. 4 (b-e). This spectrum shows the emission peak at 312 nm with excitation of 250 nm and slight variation is observed in wavelength. A small shift appears for other surfactants such as PVA, PVB, and SDS than n-heptane. This shift can be revealed the change in surface morphology of CdO nanoparticles [8]. This emission is assigned to the transition between photo-generated holes and singly ionized oxygen vacancies [9]. The second prominent peak at 372 nm and 375 nm belongs to the violet region of curve (c) and curve (d). The prominent peak in the curve (b) and (e) is observed only for the polyimide and SDS-based CdO nanoparticles. These peaks reveal the smoothness of the surface of spherical particles with no crystal defects in the crystal lattice [10].
3.5 FT-IR Spectra
Figure 5(a-e) shows the FT-IR spectrum of CdO nanoparticles prepared under various surfactants such as n-heptane, polyimide, PVA, PVP, and SDS. These samples display prominent transmittance spectra with respective significant vibration bands in the IR region of 4000-1- 400 cm-1. The curve shows strong and weak bands at 3782 cm-1, 2348 cm-1, 1590 cm-1, 1382 cm-1, and 433 cm-1 emerge all the samples. The strong band at 3782 cm-1 is O-H stretching vibration of the hydroxyl group is appeared in all these samples [11]. The presence of O-H vibration in the calcinated samples may be due to the adsorption of moisture in the atmosphere. The tiny bend at 2348 cm-1correspond to the O = C = O stretching vibrations of instrumental carbon-di-oxide [12]. The band located at 1590 cm-1 is represented as the C-C stretching vibration of aromatics in the chemical surfactants [13]. The sharp band at 1382 cm-1 is indicated surface coated carbon present in the annealed samples [14]. The trace of carbon is evidenced in the EDAX spectra. This carbon procedure is unavoidable while preparing surfactant-based nanoparticles [ 15]. The main metal band region at 433 cm-1 is indicating the vibration between cadmium (Cd) and oxygen (O) [15].
3.6 FESEM and HRTEM analysis
Figure 6.1(a1-a4) shows the FESEM and HRTEM images of n-heptane based CdO nanoparticles. This n-heptane is the composition of carbon and hydrogen molecules. Figure 6.1(a1) shows the FESEM image of n-heptane based CdO nanoparticles. This image depicts a huge bunch of nanowires with some smooth agglomerated spherical particles. These nanowires and spherical particles appear as a mixture of both. It is visualized from the following in the HRTEM image. Figure 6.1(a2) shows the HRTEM image of h-heptane based CdO nanoparticles. This image exhibits specifically the formation of spherical particles with very fine nanowires are formed. The average diameter of the spherical particle is 37 nm to 74 nm and the width of the nanowire is 3 nm. These sizes are shown in TEM images. Figure 6.1(a3) shows the higher magnification of the two spherical CdO nanoparticles [16] already reported CdO nanoparticles prepared using n-heptane as a surfactant to the preparation of CdO nanoparticles. The author achieved very fine nanowires in the size of 25-30nm. Figure 6.1 (a4) shows the SAED pattern with bright and dark spots of CdO nanoparticles. This image reveals good crystallinity and matches well with the diffraction plans of the respective XRD spectrum.
Figure 6.2(b1-b4) shows the FESEM and HRTEM images of polyimide-based CdO nanoparticles. Figure 8.3.6 (b1) shows the FESEM image of CdO nanoparticles prepared using polyimide. This image shows the almost equally distributed spherical structure of CdO nanoparticles. Figure 6.2 (b2) shows the HRTEM image of polyimide-based CdO nanoparticles predicts spherical-shaped nanoparticles with a fine edge. The diameter of the spherical particles calculated as 76 nm. Figure 6.2 (b3) shows another image of the sample with higher magnification of the two spherical particles attached one to another. This image showed that the polyimide surfactant protects from more agglomeration of CdO nanoparticles. Figure 6.2 (b4) shows the SAED pattern of dark and bright rings. It shows significant single crystallinity of CdO nanoparticles and also these spots have good agreement with XRD hkl planes.
Figure 6.3(c1-c4) shows the FESEM and HRTEM images of PVA-based CdO nanoparticles. Figure 6.3 (c1) shows the FESEM image of PVA-based CdO nanoparticles. This image displays a conglomeration of bulk CdO nanoparticles with very few plate structures. Also, this image shows carbon presence as noticed from FT-IR with CdO nanoparticles. It is nothing but the trace of the surfactant even after annealing treatment. It also reflects the polymeric nature of PVA surfactant. Figure 6.3 (c2) shows the HRTEM image of CdO nanoparticles. This image reveals small spherical particles combined to form a DNA-like structure.
Figure 6.3 (c3) shows the higher magnification of spherical particles with an average size at 19 nm- 55 nm. Figure 6.3 (c4) shows the SAED image of CdO nanoparticles. It also reveals that the polycrystalline of the CdO nanoparticles with simple cubic structure [17]. The bright dots over the circular ring indicate corresponding plane such as (110), (200), (220), (311), and (222) which is identified from XRD pattern sample C. the agglomeration is more between the spherical particles than other surfactants used here for the PVA is not much supported to prevent agglomeration like other chemical surfactants such as n-heptane and polyimide.
Figure 6.4 (d1-d4) shows the FESEM and HRTEM images of PVP-based CdO nanoparticles. Figure 6.4 (d1) shows the FESEM image of PVP surfactants-based CdO nanoparticles. This image shows agglomerated spherical-shaped CdO nanoparticles. It appears like a coral surface. Most of the particles are irregular with some spherical-shaped particles. Figure 6.4 (d2) shows the HRTEM image of PVP-based CdO nanoparticles. This image exhibits coral shape with embedded spherical particles of the average particle size of 78 nm. It creates a network of CdO spherical particles. Figure 6.4 (d3) shows the next magnification of CdO nanoparticles which displays particles combines to another like chain pasted structure. This combination may be occurring due to PVP surfactant. Figure 6.4 (d4) shows the SAED pattern of dark and bright fringes circular rings indicated good polycrystalline nature.
Figure 6.5(e1-e4) shows the FESEM and HRTEM images of SDS-based CdO nanoparticles. The FESEM image of Fig. 6.5 (e1) shows more number of spherical and platelet-shaped CdO nanoparticles with smooth edges. The HRTEM image of Fig. 6.5 (e2) shows the spherical and square platelets shaped by CdO nanoparticles. The size of particles is measured from this image is between 71 nm and 92 nm. Also, this image confirms less agglomeration. Figure 6.5 (e4) shows the next magnification of the above image (e3). This image shows the hexagonal crystalline form with smooth cutting edges. Figure 6.5 (e4) shows the SAED pattern of CdO nanoparticles with significant polycrystalline nature. The bright spots representation in the pattern has good agreement with the crystallinity as recorded from XRD spectra.
3.7 EDAX analysis
Figure 7(a-e) shows the EDAX spectra of n-heptane, polyimide, PVA, PVP, and SDS-based CdO nanoparticles. The compositions of elements present in the prepared surfactant-based CdO samples are identified. The well-maintained stoichiometry is identified for most of the samples. It also reveals that the synthesized CdO nanoparticles using surfactants like polyimide and PVP have shown the metallic ions impurity as Au. The presence of Au in the spectra is identified as the golden grid which is used as a sample holder. The emission of strong signals belongs to cadmium and oxygen as 84.79 % and 10.27 % for n-heptane based CdO nanoparticles, 42.18 % and 17.28 % for polyimide-based CdO nanoparticles, 82.91 % and 10.8 % for PVA based CdO nanoparticles, 75.35 % and 22.69 % for PVPbased CdO nanoparticles 84.79 % and 10.27 % for SDS based CdO nanoparticles. The presence of the elemental composition of these samples is listed in Table 1.
Table 1
shows the composition of elements of various surfactant based CdO nanoparticles.
Samples
|
Element
|
Weight %
|
Atomic %
|
CdO- n-heptane
|
C K
|
4.94
|
22.75
|
O K
|
10.27
|
35.52
|
Cd L
|
84.79
|
41.73
|
CdO- Polyimide
|
C K
|
30.16
|
62.48
|
O K
|
17.28
|
26.87
|
Au M
|
10.38
|
1.31
|
Cd L
|
42.18
|
9.34
|
CdO- PVA
|
C K
|
6.29
|
27.04
|
O K
|
10.8
|
34.85
|
Cd L
|
82.91
|
38.1
|
CdO- PVP
|
O K
|
22.69
|
67.59
|
Au M
|
1.96
|
0.47
|
Cd L
|
75.35
|
31.94
|
CdO- SDS
|
C K
|
4.94
|
22.75
|
O K
|
10.27
|
35.52
|
Cd L
|
84.79
|
41.73
|
3.8 Photocatalytic activity
The photocatalytic activity of various surfactants-based CdO nanoparticles was investigated for the aqueous solution of the chromophoric structure of MB dye under solar light irradiation. The characteristics absorption peak of MB dye at ~ 664 nm is fixed to monitor the photo degeneration of dye molecules. Figure 9. (A-E) shows the degradation of MB dye with surfactant-based CdO nanoparticles at a different concentration under the wavelength, λ = 365 nm. From Fig. 9. (A-E) it is clear that after exposing the mixed solution of MB dye and photocatalyst such as CdO nanoparticle samples A- n-heptane, B- Polyimide, C- PVA, D- PVB, and E- SDS under solar radiation. Each sample MB solution is irradiated under solar irradiation for 120 min. The without extract CdO nanoparticles sample n-heptane degrade the MB dye up to 83 %, similarly, the degradation is observed in MB dye for 95.4 % for polyimide, 86 % for PVA, 90 % for PVB, and 78 % for SDS are observed from the solar light irradiation. The degradation of MB dye is not observed in the absence of photocatalyst. The SDS surfactant-based CdO nanoparticles show only 78 % of degradation. The highest level the 95.4 % of polyimide and 90 % of PVB. This result confirms that the spherical-shaped morphology like polyimide and PVB based CdO nanoparticles have shown an enhancement in intrinsic photocatalytic activity under solar light irradiation of their CdO nanoparticles. The comparison of degradation efficiency was made by considering the almost similar intensity of solar radiation [20].
3.9 Antibacterial and Antifungal activities
The antimicrobial activity of various surfactants-based CdO nanoparticles was investigated for an antibacterial agent were tested for respective antibacterial towards both gram-positive (Staphylococcus aureus) and gram-negative (Escherichia coli) bacteria. The inhibition of CdO nanoparticles over the bacterial and fungal are discussed as follows. The gram-positive bacteria cell wall is a peptidoglycan molecule. The gram-positive bacteria cell consists of a thick wall. As the Cd2 + ions are positively charged and peptidoglycan is negatively charged. So there is an electrostatic [21] attraction between these positive and negative ions. So, more positive Cd2 + ions are interacting over the surface of the peptidoglycan. The more accumulation of Cd2 + ions over the surface damages the cell wall and diffuses into the cells. In this way, the cell wall or membrane gets damaged.
Figure 9. (A-D) shows that the zone of inhibition for gram-positive bacteria Staphylococcus aureus is observed for the samples n-heptane, polyimide, PVA, PVB, and SDS. similarly, the zone of inhibition of CdO nanoparticles on gram-negative bacteria of Escherichia coli is observed for the samples n-heptane, polyimide, PVA, PVB, and SDS. Table 2 summarizes the antibacterial activity of extracts-based CdO nanoparticles. Based on the zone of inhibition, polyimide-based CdO nanoparticles proved the highest antibacterial activity against Staphylococcus aureus and Escherichia coli. The result from the antibacterial activity is presumed from the inhibition as low activity over the bacterial medium. Thus, the zones around CdO nanoparticles are formed when the growth of the bacteria is stopped by CdO nanoparticles cogently [22].
Table 2
Summarizes the zone inhibition values of chemical surfactants based CdO nanoparticles
Surfactant based CdO nanoparticles
|
Sample code
|
Staphylococcus aureus(mm)
|
Escherichia Coli(mm)
|
Candida albicans(mm)
|
Aspergillus niger(mm)
|
n-heptane
|
A1
|
7
|
18
|
6
|
9
|
Poly imide
|
B1
|
11
|
19
|
7
|
4
|
PVA
|
C1
|
8
|
14
|
4
|
3
|
PVP
|
D1
|
6
|
15
|
4
|
5
|
SDS
|
E1
|
7
|
17
|
6
|
8
|
Antifungal properties of surfactant-based CdO nanoparticles were also examined against selected fungal test pathogens such as Candida Albicans and Aspergillus niger are shown in Fig. 9. (A-D) shows the zone of inhibition of CdO nanoparticles concerning all pathogens are listed in Table 2. This test is proved that CdO nanoparticles possessed good antifungal activity towards Candida Albicans and Aspergillus niger is observed as n-heptane, polyimide, PVA, PVB, and SDS. The predominant zone inhibition due to CdO nanoparticles over the antifungal activity is observed. These results confirmed that the chemical surfactant-based CdO nanoparticles showed significant antibacterial capability against gram-positive and gram-negative bacteria as well as good antifungal activities. The negative strain of the Escherichia coli is the best inhibition of bacterial study. Also tested by the fungal organism Candida albicans is the best inhibition value compared to the other organism of Aspergillus niger.