3.1 Powder X-Ray Diffraction analysis
The crystalline phase, purity, and lattice parameters of the prepared AC, AC/Ag nanocomposite, and AC/Ag/TiO2 nanocomposites were confirmed using the powder X-ray diffraction (XRD) technique. Fig. 1 shows the X-ray diffraction patterns of the AC, AC/Ag nanocomposite, and AC/Ag/TiO2 nanocomposites. The characteristic peaks at 25.45° and 44.39° in Fig. 1 corresponds to the Bragg reflections of (002) and (101) planes, respectively. The broad and amorphous jasmine flower-derived activated carbon (AC) is due to pseudo-crystalline graphitic carbon formation. The obtained AC characteristics peaks exhibit the hexagonal crystal system with a point group of P63mc (186), and it well matched with the JCPDS card number 89-8487 [20]. The measured lattice parameters ‘a’ and ‘c’ were 2.459 Å and 6.708 Å. The diffraction peaks located in Fig. 1 depict the AC/Ag nanocomposite, and the diffraction angle 2θ at 38.09° and 44.41° can be indexed to (111) and (200) planes. These diffraction peaks indicate that the prepared composite contains a metallic silver phase with a cubic crystal structure. The lattice constant (a) of 4.086 Å was well-matched with JCPDS File Card No. 89-3722 with a group of Fm3m (225) [21]. It is noticed that, compared with AC, the diffraction peak at 44.41° of AC/Ag nanocomposite becomes higher, which is attributed to the overlapping of AC (101) and Ag (200) planes. From fig. 1 the diffraction peaks at 25.54°, 38.31°, 44.54°, and 54.70° are assigned to the planes of (101), (004), (200), and (105). Here all the diffraction peaks were in good agreement with the anatase phase of TiO2 (JCPDS Card No: 21-1272) [22]. Also which exhibits the tetragonal crystalline system with the lattice constants of 4.560 Å (a) and 2.950 Å. This anatase crystalline phase is may also be responsible for the higher photocatalytic activity. The diffraction crystal plane of (004) for anatase TiO2 and (111) for Ag was overlapped, but the crystalline structure of TiO2 is not affected [23,24]. The presence of diffractions from carbon, Ag, and TiO2 in Fig. 1 suggests that the prepared materials are mainly composed of graphitic, metallic silver, and anatase TiO2 phases. The average crystallite size, dislocation density, and lattice strain of the AC, AC/Ag nanocomposite, and AC/Ag/TiO2 nanocomposites were measured using the following Debye – Scherrer’s and Williamson – Smallman relations [25] (Eq. 2-4)
Where, K is the Debye Scherrer constant, λ is the wavelength of the Cu-Kα X-ray (λ = 1.5406 Å), θ is the diffraction angle, and β is the full width at half maximum (in radian). In Table.1 predicts the average crystallite size, dislocation density, and lattice strain of the prepared photocatalysts. The average crystallite size of AC/Ag/TiO2 nanocomposites decreases with increasing the lattice strain as well as dislocation density, which may increase the surface area of the AC/Ag/TiO2 nanocomposites. The large surface area can facilitate more surface sites and lattice disorders, enhancing the AC/Ag/TiO2 nanocomposites photocatalytic activity. The smaller crystalline size can also migrate the photoexcited charge carriers to the photocatalyst surface, which may reduce the electron-hole pair recombination rate, where it has played a significant role in refining the photodegradation performance.
3.2 Fourier Transform Infra-Red Spectrum analysis
The functional group vibrational modes present in the prepared material were identified using Fourier Transform Infrared Spectroscopy. Fig. 2 shows the FTIR spectra of AC, AC/Ag nanocomposite, and AC/Ag/ TiO2 nanocomposite. The sharp band at 512 cm-1 is attributed to P vibration [18]. The modes at 980 cm-1 and 1120 cm-1 correspond to C-O asymmetric and symmetric stretching vibration [26]. The peak at 1384 cm-1 is ascribed to the carboxylic group absorption (COOH) present at the surface. The wavenumber at 1640 cm-1 is assigned to the bending mode of O-H vibration, which can occur due to the surface adsorbed water molecules [27]. The vibration peak of 2900 cm-1 is consigned to C-H stretching vibration mode. The broad peak at 3400 cm-1 is attributed to the O-H stretching vibration of water molecules. The vibrational mode at 777 cm-1 is indorsed to the vibrational mode of Ag–O–C [28]. The vibration at 622 cm-1 is described as the Ti-O stretching mode, which is distinctive of the formation of TiO2 on activated carbon [29]. Compared to AC and AC/Ag nanocomposite, the O-H vibrational bands at 1640 cm-1 and 3400 cm-1 are presented in AC/Ag/TiO2 nanocomposite is very broad and intense, which confirms that the AC/Ag/TiO2 nanocomposite contains a large number of surface adsorbed hydroxyl (OH) species. These hydroxyl species play a vital part in the photodegradation process, and they can increase the degradation efficiency during the photocatalytic process.
3.3 Surface Morphological and Elemental analysis
The surface morphological and average size of the prepared materials were infra-red from SEM analysis. Fig. 3 represents the SEM images of prepared AC, AC/Ag nanocomposite and AC/Ag/TiO2 nanocomposites. The SEM image (Fig. 3 (a)) reveals that the prepared activated carbon exhibits the irregular-shaped flake/sheet-like surface morphology with an average size of 50 to 100 nm. The dark and bright regions of AC/Ag composite in Fig. 3 (b) show the spherical-shaped particle aggregation due to silver nanoparticles presence over the surface of AC. The average size of the prepared AC/Ag nanocomposite is found to be around 30 nm. The homogeneous, smoother and compact needle-like particles are distributed on the surface of AC were observed in Fig. 3 (c) is attributed to the existence of TiO2 on the AC surface. Also, some of the Ag particles are anchored on the surface of TiO2 needles. The average diameter and size of the TiO2 needles are in the range of 30 - 15 nm [30]. From the SEM image of AC/Ag/TiO2 nanocomposite, the observed flake, sphere and one dimensional (1D) needles are responsible for the migrations of photo excited charge carriers on the surface of AC/Ag/TiO2 nanocomposite; it can inhibit the electron-hole pair recombination rate during the degradation process. The element distribution of this AC/Ag/TiO2 nanocomposite can be determined by EDX mapping on an EDX microanalysis system (Fig. 4 b). The signals for Si, Ag, and Ti elements are associated with the MMT, Ag and TiO2 nanoparticles, respectively [31, 32]. Fig. 4 a, b shows EDX spectra of nanocomposites.
3.4 UV-Visible spectroscopy analysis
UV-Visible spectra of AC, AC/Ag and AC/Ag/TiO2 nanocomposite were shown in fig. 5 a). The maximum absorption spectra of activated carbon reveal λmax as 257 nm reveals π-π* transition in carbon material [20]. The presence of silver nanoparticles reveals the absorption spectrum around 400 nm [33]. AC/Ag spectra do not reveal surface Plasmon resonance due to the incorporation of activated carbon. The AC/Ag/TiO2 nanocomposite shows a wide absorption peak at 608 nm [14]. The observed result is well-matched with the previous report. The addition of titanium ions incorporated with silver ions reveals strong interaction between the orbit of TiO2 and Ag. Strong absorption spectra were observed due to strong hybridization between the silver orbitals and the Ti 3d and O 2p bands near the Fermi level [34]. The optical bandgap of the investigated nanocomposite was calculated using the Tauc plot. AC, AC/Ag and AC/Ag/TiO2 nanocomposite reveals optical band gap value of 2.75 eV, 2.25 eV, and 1.99 eV respectively. The narrow bandgap of AC/Ag/TiO2 nanocomposite acts as a better candidate material for active, visible light heterojunction with activated carbon [16]. Lower band gap value observed at AC/Ag/TiO2 nanocomposite confirmed due to quantum confinement effect caused by nano dimensional state materials [35]. The bandgap values were well-matched with the crystallite sizes and particle sizes values. The obtained values are tabulated in table 2.
3.5 Transmission Electron Microscope analysis
TEM images of AC/Ag/TiO2 nanocomposite are shown in Fig. 6. Morphology, and particle size values can be identified using the TEM image. TEM images of AC/Ag/TiO2 nanocomposite reveal the needle-shaped structure with spherical pores due to presence of silver nanoparticles. The slight approximate surface area obtained on the surface of needles indicates the presence of carbon. The AC/Ag/TiO2 nanocomposite expresses a particle size value of 24 to 36 nm. The intended results are highly correlated with the mean crystallite size value calculated from XRD. The spotted rings found in the SAED pattern shows the crystalline nature of composite.
3.6 Photocatalytic dye degradation analysis:
The photocatalytic activity of activated carbon (AC), AC/Ag and AC/Ag/TiO2 nanocomposite was investigated against the photodegradation of methylene blue dye under Visible irradiation (Fig. 7-a). Initially the maximum absorbance peak obtained at 657 nm for methylene solution without catalyst. Maximum absorptionoccurs due to the n-π* transition of MB.Activated carbon shows a maximum degradation efficiency of 79 % at the end of 120 Minutes. When visible light pass through the heterogeneous catalyst, electronexcited from the valance band to the conduction band, generates positive holes and negative electrons on the surface of TiO2. The electrons from the conduction band of TiO2 react withAg/AC catalyst surface to form the free radical OH• and the superoxide radicals O2•− [37]. Free radicals such as O2•− and OH•are responsible for the degradation of MB. During the reduction reaction, the methylene blue dye is converted to Leuco Methylene blue (LMB) [38].On the other hand, the positive holes of titanium dioxide break down water molecules to form free radicals and negative electrons react with oxygen molecules to form superoxide anions. The carbon act as suitable electron acceptor, and titanium dioxide as a perfect donor under the light irradiations [39].
The decomposition of methylene blue dye after 120 minutes by Ag/Ac catalyst results degradation efficiency of 84% while its 96% for Ag/AC/TiO2 catalyst. The degradation efficiency of Ag/ACwas much higher than Ag/AC/TiO2 catalyst. Higher catalytic activity obtained due to larger surface area, less particle size and crystallite size, and excellent optical activity. The obtained result suggests that silver nanoparticles enhances the charge separation by trapping photoelectrons. Co-catalyst silver acts as electron sink and inhibit the recombination rate, quantum yield and enhances the absorption capacity for visible light due to Surface Plasmon resonance. The rate of kinetics of the catalyst were calculated form pseudofirst-order kinetics and their plotted spectrum was shown in Fig. 7.b. The possible mechanism of the catalyst against MB dye as shown in fig. 8. The addition of TiO2 helps prolong the lifetime of the electron, and it has a high ability to break molecular bonding. Photocatalytic activity of as prepared nanocomposites reveals potential application for the removal of methylene blue, environmental and wastewater treatment [40,41].
Compared to our present study, the degradation efficiency increases from 84% to 96% for AC/Ag/TiO2 composite. Photodegradation efficiency of AC/Ag/TiO2 nanocomposites (96%) > AC/Ag nanocomposite (84%) > Activated carbon (79%) respectively. The enhanced photocatalytic activity in the composite could be assigned to reduction of electron-hole pair recombination, size, morphology and crystal structure of the nanoparticles.The present work compared with previous reported metal and metal oxide catalyst as tabulated in table 3.
3.7 Anti-bacterial activity:
In ancient times silver and silver samples have been used for its microbial activity. The possible reaction mechanism may occur 1) electrostatic attraction takes place between negative charge cell wall and positive charge nanocomposite.2) effect of concentration may affect the cell wall of bacteria.In the present work, antibacterial activity was examined against Bacillus subtilis and Escherichia Coli. E-coliis a gram-negative pathogen and less susceptible to antibiotics and antimicrobial activity. Bacillus subtilis is a gram-positive bacterium found in soil. As expected no inhibition zone layers are observed in control. Among these bacteria reveals maximum zone inhibition value of 2.8 mm for Bacillus subtilisand E-Coli bacteria has minimum zone value.
Fig. 9 a, b shows antibacterial activity of activated carbon, AC/Ag and AC/Ag/TiO2 nanocomposite. Interaction takes place between positive ions from composite with negativebacterial cell wall enhances the antibacterial activity. The gram-positive bacteria displayed higher antibacterial activitythan gram-negative pathogen [53, 54]. Activated carbon combined with silver and titanium composite acts as an excellent antimicrobial agent. The excellent antibacterial activity reveals the strong interaction between metal and metal oxide ions with cell membrane and proteins leads to increase permeability and consequently distribution and extravasation of intercellular content [55,56]. Active materials exert their activity by contact with bacteria and get surface interaction between nanoparticles, increasing antibacterial activity. Antibacterial activity of investigated carbon and composites is applicable for biomedical application, wastewater treatment and other environmental applications.