Ag-NPs/TiO2-NTs samples characterization results
The fig.1.a displays a side-view SEM image of the 10 min Ag-NPs decorated TiO2-NTs. It is discernably observable that the nanotubes are perfectly aligned in vertical order with an identical diameter and length. Hypothetically speaking, the visible spherically shaped things attached to the external, internal walls and even the holes of the tubes are believed to be the deposited silver nanoparticles. These particles seem as if they grow in size and shape with the increase of the photoreduction time, which is remarked from the top-view SEM images (fig.1.b.c). For each sample, the size distribution was determined to back up the assumption of Ag-NPs agglomeration. The 20 min Ag-NPs decorated TiO2-NTs manifest 12 times more significant in the number of silver nanoparticles with almost ~200 nm in diameter. However, this could be beneficial for the electrons trapping but could slightly affect badly on the TiO2 photocatalytic performances since the agglomerated particles are blocking the TiO2-NTs. To assure that these particles are, in fact, the deposited Ag-NPs, an energy dispersive X-ray EDX analysis for the 10 min Ag-NPs/TiO2-NTs sample was carried out.
Fig. 2 depicts the sample's chemical composition, where no elements other than Ti, O, Ag, C, and a tiny amount of Cl were detected, proving the purity of the prepared samples and the mapping sum spectrum recounts that 50% of weight belongs to the Ag-NPs.
Moreover, the TEM images (fig.3) of the 10 min Ag-NPs/TiO2-NTs sample shows that the Ag nanoparticles randomly deposited with an average size estimated to be between 27.8 nm and 120 nm. The overall diameter of the TiO2-NTs was estimated to be about 130 nm, and the internal diameter was about 50 nm. The viewed TEM images displayed a variety of Ag-NPs sizes and shapes that could result to an undefined LSPR peaks. Apparently varying the reduction time can change the Ag-NPs size and shape vigorously as it is shown in Figure 1.
Fig. 4 shows the X-ray diffraction patterns of the Ag- decorated samples at 10 min and 20 min compared to the pure TiO2-NTs. The peaks at 25° attributed to the anatase phase (101) are intensely sustained for all the samples due to their annealing at 400 °C. However, for the decorated samples, the XRD patterns show an emerging peak at 2θ = 43.6°, and 63.8° related to Ag nanoparticles (200) and (220) diffraction planes, the limitation in the intensity of the silver peaks is probably due to the small deposited amount and/or the small particles size which could be below the XRD sensitivity threshold.
Figure 5a exhibits the survey spectra of Ag–TiO2, and the appearance of peaks, which are attributed to Ag 3p, Ti 2p and O 1 reveal the existence of Ag, Ti and O in the Ag–TiO2 surfaces. The obtained XPS results validated the formation of Ag species on the surface of TiO2-NTs, the signals in figure 5d at 374.3 and 368.3 eV substantiate distributive the existence of Ag 3d3/2 and Ag 3d5/2 of Ag-NPs which are frequently assigned to metallic Ag (Waterhouse et al. 2001).
Figure 5b displays a unique peak of O1s at 530.8 eV imputed to the oxide. The main and major occurrence of Ti+4 was proven by the manifestation of Ti-2p peak at binding energy of 459 eV (Figure 5c), these allocated values are endorsed in the NIST database for anatase TiO2. The procedure of photoreduction was sustained effective by these results.
The photoluminescence PL of TiO2-NTs could inform us of the life duration and transport of the photogenerated charges (Wang et al. 2012). Fig. 6 manifests the photoluminescence spectra of Ag- NPs decorated and pure TiO2-NTs. The peak located at 365 nm corresponds to the electronic transition between the conduction and the valence bands of the TiO2. The peaks presented at 459 nm, 483 nm, and 531 nm are merely attributed to the oxygen vacancies occurring at the TiO2 surface (Tachikawa et al. 2009; Mercado et al. 2012). However, the Ag-NPs decorated TiO2-NTs exhibited a much lower PL intensity than the pure TiO2, resulting from the Ag-NPs reducing the density of the radiative recombination centers. Moreover, these results could be explained by the migration of the photogenerated electrons under UV irradiation (λexcitation = 340 nm) from the conduction band to the Ag-NPs, reducing the radiative recombination within the TiO2 (Takai et al. 2011). This result of photoluminescence has already been mentioned previously in our previously published work (Gaidi et al. 2018; Ashraf et al. 2022).
Sample’s photocatalytic ability testing results
The photocatalytic process consists of total mineralization of any organic or inorganic pollutant into H2O and CO2. Under light irradiation, the oxygen species are activated along with the photodegradation of the adsorbed amido black staining is triggered (Saquib et al. 2004).
The fig.7. shows the absorbance spectra of amido black solution after 10 min photoreduction time Ag-NPs/TiO2-NTs; it depicts a contracting in the absorption peak located at 618 nm during the irradiation time is obviously due to the diminution of the amido black concentration. The quantity of degradation can be determined using the following equation.
I is the intensity at 618 nm and I
nitial is the initial intensity of the amido black solution.
The fig.7 (inset) shows that at 150 min, a total degradation of the amido black staining is detected, but in reality, the calculated percentage of degradation at 150 min is 75 %. At 270 min, the degradation is estimated to reach 96.4 %. However, the proven linearity between the absorbance and the concentration of amido black allows us to trace these plots of pure TiO2-NTs. The Ag-NPs decorated ones with 10 and 20 min photoreduction time. The slope of these plots fig.8.a, b, and c represent the photodegradation reaction rate k that outlooks the photocatalytic performances of each sample.
The fig.8.d represents the evolution of the kinetic constant at different photoreduction times; as shown, a significant increase of the photodegradation rate of AB was observed as the photoreduction time increases but drastically decreases with the further elevation of time.
Thus, the constant rate k reaches a maximum of 0.0122 min-1 for the 10 min photoreduction time, which is higher than the other samples. This effect could be caused by the transport of the photogenerated electrons from the conduction band of TiO2 to the Ag nanoparticles since the flat band potential of titanium dioxide is lower than that of metallic silver (Yu et al.2005)[41]. The flowed electrons are accumulated at Ag-NPs to form a Schottky barrier between Ag-NPs and TiO2-NTs, as is illustrated in fig.9. On the other hand, the accumulated holes in the valence band of TiO2-NTs that are responsible for the production of surface hydroxyl radical OH° these active species assure the mineralization of AB. Therefore, in this heterojunction Ag-NPs/TiO2-NTs, the efficient amount of metallic Ag delays the recombination of the photoinduced e-/h+ pairs, hence enhancing the photocatalytic activity. However, the increase in the number and size of the Ag-NPs, like the 20 min Ag-NPs/TiO2-NTs, has a high possibility of the holes capturing in the valence band. In addition to decrease the amount of received photons with their enormous size, it lowers the efficiency of photocatalytic reactions (Lan et al. 2009).
To study the effect of the Amido black concentration on the rate of its photodegradation, it was variated between 1, 2, 3, and 4 mL in 500 mL of deionized water. Then, the amido black solution was irradiated under UV light for 270 min with the 10 min Ag-NPs/TiO2-NTs catalyst. The fig.10 depicts that the reaction rate gets slower with the increase of the AB concentration. Thus, increasing amido black concentration clogs many active sites for the OH° generation. Thus, the excessive amount of the adsorbed staining molecules at the surface of Ag-NPs/TiO2-NTs catalyst reduces the radiation path hence the reduction of light intensity for the excitation of the photocatalyst (Subramani et al. 2007).