2.2. Experimental Details
2.3.4. Optical Analysis
2.3.4.1. UV-Vis Absorption Study
To measure the optical response of pristine TiO2(B) NRs, Ag2O NPs and their HSs, the UV- visible absorption spectra were studied from the measurement of the diffuse reflectance spectra (DRS) of the powdered samples. The absorbance of a material system is related to the diffuse reflectance (R) by the Kubelka-Munk (K-M) function, F(R), given by
F(R) = (100 − R)2 200R = α
S (2.1)
Where 𝛼 and S represent the absorption and scattering coefficients, respectively and R is the percentage reflectance of the respective samples. Fig. 2.7(a) shows the plot of the K-M function
of different samples depicting their absorption spectra and Fig. 2.7(b) shows the (F(R)hν)1/2 vs. hν plot for the calculation of effective band gap (indirect) of the HSs.
Extrapolation of the linear portion at (F(R)hν)1/2= 0 gives the effective band gap of the nanostructures. As evident from the data, the pristine TiO2(B) NRs exhibit a sharp absorption edge at ~400 nm and at higher wavelengths (visible region) the absorbance is negligibly low.
However, the pristine Ag2O NPs have strong absorption in the UV-visible-NIR range of 200-850 nm showing a broad absorption band centred at ~430 nm and the absorption tail extends to the NIR region.
Fig. 2.7. (a) K-M function (F(R)) of different samples plotted against wavelength. (b) The corresponding Tauc plot considering the indirect bandgap nature of the TiO2 samples. The effective band gaps of the respective samples are estimated from the intercept on the x-axis (extrapolated dashed lines).
The bandgap (direct) of Ag2O NPs estimated from the corresponding Tauc plot is ~1.34 eV, which is consistent with the literature report16. The absorption spectra of Ag2O/TiO2(B) HSs illustrate that all HS systems have extremely high absorption in the entire UV-visible-NIR region of the optical spectrum. Among these, the TA2 HS shows the highest absorption in visible to NIR region, may be due to the optimum size and number density of the Ag2O NPs decorated on the TiO2(B) NRs, see Fig. 2.2(b). For the visible light sensitization by Ag2O NPs, the HSs are expected to have high photocatalytic activity under the visible as well as UV and NIR irradiation.
Interestingly, the effective band gap of the TiO2(B) NRs has been reduced considerably after loading with Ag2O NPs. Considering the indirect nature of band gap in the pristine TiO2(B) NRs as well as its HSs, the effective band gap of the respective samples has been estimated using Tauc plot, as shown in Fig. 2.7(b).11 The bulk anatase TiO2 powder has a band gap 3.03 eV, which is reduced to 2.80 eV for pristine TiO2(B) NRs due to the presence of OV defects. This
reduction in the effective band gap helps the TiO2(B) NRs to be active under the visible light and expected to have significant effect on the visible light photodegradation capability. Interestingly, after the decoration of the Ag2O NPs, the effective band gap energy of TiO2(B) NRs has been tuned in the range 2.20-1.68 eV, as shown in Fig. 2.7(b). The detailed absorption range and the effective band gap of each catalyst are tabulated in Table 2.1. The modification of the band gap energy for the HS samples may be due to the efficient coupling between the Ag2O NPs and TiO2(B) NRs. The band structures of each of the nano-components are suitable enough to have proper band bending at the interface due to their close coupling. It is noteworthy that in case of TA2, the effective band gap is found to be lowest (1.68 eV), which implies that Ag2O NPs and TiO2(B) NRs HS has large band bending and may have high carrier concentration at room temperature. This turns out to be a very promising and an extremely beneficial approach for the efficient visible light photocatalysis by the HSs samples (discussed later).17
2.3.4.2. Photoluminescence Study
The advantage of the as-synthesized HSs in solar light-driven photocatalysis over the pristine TiO2(B) NRs has been confirmed from the PL studies. The characteristic PL spectra clarify the mode of separation and recombination of the photogenerated e-h pairs. A comparison of the room temperature steady state PL spectra for pristine TiO2(B) NRs, Ag2O NPs and TA2 HS are shown in Fig. 2.8(a). Interestingly, a significant decrease (by a factor of ~5.9) in PL intensity of the TiO2(B) NRs has been observed after the decoration with Ag2O NPs. However, the nature of the PL band remained unaltered. For a detailed understanding of the origin of the broad PL emission, each PL spectrum is deconvoluted with four symmetric Gaussian peaks, as shown in Fig. 2.8(b, c). For both the samples, the well fitted individual bands are labelled as Peak 1, 2, 3, 4. For pristine TiO2(B) NRs (Fig. 2.8(b)), Peak 1 (at 427.2 nm) is attributed to the self- trapped excitons located at TiO6 octahedra.11Peak 2 (at 491.2 nm) is due to the charge transfer transition from Ti 3d orbital to O 2p orbital in TiO62-octahedra18, while Peak 3 (at 560.6 nm) is attributed to the shallow traps associated with the oxygen vacancies (VO) in the TiO2 structure.19 Peak 4 (641.0 nm) is possibly due to the deep level emissions associated with VO states.20 Similarly, for TA2 HS (Fig. 2.8(c)), the four peaks are located at 434.9, 490.6, 561.0 and 648.6 nm, respectively, having the same identity. Note that the intensity ratio of the peaks in each sample is also almost the same in Fig. 2.8(b) and 2.8(c).
Fig. 2.8. (a) A comparison of the PL spectra for pristine TiO2(B) NRs, Ag2O NPs and TA2 HS excited with 405 nm laser. (b, c) Gaussian fitted PL spectra of TiO2(B) NRs and TA2, respectively. (d) A comparison of the TRPL spectra for pristine TiO2(B) NRs and TA2 HS monitored at 490 nm (emission) with a 405 nm laser excitation. The symbols represent the experimental data, and solid lines represent the corresponding tri-exponential fit. The inset shows the average time constant for different samples.
However, the intensity of each peak is substantially reduced in TA2 HS as compared to that in pristine TiO2(B) NRs. The decrease in PL intensity after the decoration with Ag2O NPs may be due to the following reasons: (i) the deposited Ag2O NPs work as the trap centres of the photoexcited electrons and prevents their recombination;6 (ii) due to band bending at the interface, the charge transfer process is very efficient in case of HSs and thus charge carriers (e, h) are separated enough reducing the recombination probability, lowering the PL intensity; (iii) Ag2O NPs may partly passivate the luminescent centers on TiO2(B) NRs. All these factors may contribute to reduce the PL intensity of the sample TA2 as compared to the pristine TiO2(B) NRs. Thus, under visible light illumination, due to reduced recombination, plenty of excitons are
available near the interface of Ag2O/TiO2(B) HS, which may help in achieving high photocatalytic activity (discussed later).
2.3.4.3. Time-Resolved Photoluminescence Study
To investigate the recombination kinetics of photogenerated charge carriers, time-resolved photoluminescence (TRPL) measurement was performed on pristine TiO2(B) NRs, and its HS with 405 nm laser excitation and the emission was monitored at 490 nm. The TRPL decay profiles are fitted using a tri-exponential decay function expressed as follows:
I(t) = ∑ Aie(−t τ⁄ ) i (2.2)
3
i=1
Where 𝜏𝑖 is the lifetime of individual component, and 𝐴𝑖 is the corresponding amplitude.
The average excited state lifetime can be calculated by using the following relation21: τav =∑3i=1Aiτi2
∑3i=1Aiτi (2.3)
Fig. 2.8(d) shows a comparison of the TRPL decay curves for pristine TiO2(B) NRs and TA2 HS with the average lifetime (𝜏av) tabulated as the inset. Each TRPL spectral data can be fitted by a tri-exponential decay function, which implies that three different states contribute to the TRPL spectra in each sample, fully consistent with the steady-state PL spectra (Fig. 2.8(b, c)). It can be noted that the average lifetime is considerably increased from 2.70 ns to 3.90 ns after Ag2O loading and consequently, the decay becomes slower in TA2. The increase in carrier lifetime seems to be playing a critical role in the enhanced visible light photocatalytic activity in the HS samples. The photogenerated charge carriers first migrate to the TiO2/Ag2O interface, which is thermodynamically favourable, since the valence and conduction band position of Ag2O is above of that of TiO2(B) NR.16After photoexcitation, electrons from the conduction band of Ag2O NPs migrate to the conduction band of TiO2(B) NRs and holes from the valence band of TiO2(B) NRs migrate to that of Ag2O NPs. Thus the interfacial charge transfer prolongs the lifetime of the photogenerated charge carriers, which clearly manifest as longer av in the TRPL spectra. Surface decoration of TiO2(B) NRs with the Ag2O NPs reduces the probability of recombination facilitating higher density of carriers available near the surface of the HS, which promotes high visible light photocatalytic performance, as discussed below.