2.2. Experimental Details
2.3.5. Visible Light Photodegradation Studies
2.3.5.4. Mechanism of Enhanced Visible Light Photocatalytic
TiO2 is known to be a large band gap (~3.2 eV) semiconductor and is optically active mostly at the UV region (<400 nm). In this work, the band gap of pure TiO2(B) NRs, as well as its HSs, is
tuned so that it can absorb broader window of the solar spectrum. In pure TiO2(B) NRs, the range of photoactivity has been enhanced (band gap 2.8 eV) as compared to bulk TiO2 and found to be capable for the light absorption up to ~440 nm. Thus, under visible light irradiation pristine TiO2(B) NRs show reasonably high photodegradation efficiency (see Table 2.2). Pristine Ag2O NPs also show high visible light photodegradation efficiency, as shown in Table 2.2 due to the lower optical band gap. When Ag2O NPs are decorated on pure B-phase TiO2 NRs, a massive enhancement in the photodegradation efficiency has been observed. Here, the Ag2O NPs might act as a visible light sensitizer leading to the enhanced visible light photocatalytic activity. There are several factors that may contribute to this enhancement, such as
i. Enhanced surface area
ii. Tunable band gap and enhanced optical absorption range iii. High adsorption of dye and dye-catalyst interaction iv. Efficient charge separation at the interface of the HS
v. Ionic condition of the dye as well the heterostructured material
High surface area is generally desirable to have enhanced photocatalytic activity of a semiconductor photocatalyst since higher surface area allows greater interfacial contact points with organic dyes leading to enhanced photodegradation efficiency. However, in our case, the specific surface area of the pristine TiO2(B) NRs is reduced after the formation of HS with Ag2O NPs, as confirmed from the BET analysis, although the photodegradation efficiency of the sample TA2 is much higher than the bare TiO2(B) NRs, as shown in Table 2.2. Thus, it can be concluded that the surface area is not the primary factor for the enhanced photocatalytic activity observed here.
The rate of photocatalysis depends on the relative concentration of dye and the catalyst in the solution. In the present case, 100 mL dye solution (10 mg/L) was used with 20 mg catalyst.
Thus, 1 mg dye was degraded up to 76% within 20 min by 20 mg of TA2 in the presence of visible light. Zhou et al. have reported ~77% degradation of MO in 20 min by Ag2O and anatase TiO2 HS with 1:1 weight ratio.6 However, the authors have used only 0.4 mg MO in the aqueous solution, which is 2.5 times lower than the amount used in the present study. Thus, in the present case, the fabricated HS shows 2.5 times stronger photocatalysis than that reported earlier in the presence of visible light.
Enhancement of photocatalytic efficiency requires high absorption of light to create enough e-h pairs and easy separation of the photoexcited e-h pairs with minimum recombination.
The band gap of the as-synthesized TiO2(B) NRs and its HSs with Ag2O NPs clearly falls in the visible region (in Fig. 2.7(b) and Table 2.1) and is likely to be effective for the generation of enough photoexcited carriers. The broadband optical absorption with high intensity for the HS samples allows generation of the high density of photoinduced carriers that can facilitate certain redox reactions to degrade the organic pollutants. The sample TA2 shows visible absorption with the highest intensity as compared to the other HS samples, and it is extended to the NIR region, which is possibly one of the important reasons behind its highest photocatalytic efficiency among all HS samples.
In a homogeneous semiconductor, fast recombination of the photoexcited e-h pairs is mostly observed in the form of PL, and their separation with minimum recombination is highly challenging. Efficient separation of photogenerated e-h pairs is essential for the generation of highly active superoxide and hydroxyl radicals to facilitate enough redox reaction for the degradation of the dye molecules. On the basis of the enhanced photodegradation by Ag2O/TiO2(B) HSs under visible light irradiation, the following mechanism can be proposed. A possible band diagram of the Ag2O/TiO2(B) HS at the interface and the carrier transfer mechanism are schematically illustrated in Fig. 2.14. In the HS, Ag2O NPs are well coupled to the porous sites of TiO2(B) NRs and energy bands of Ag2O and TiO2(B) are aligned suitably to facilitate charge transfer from Ag2O to TiO2 NRs. Previous reports suggest that in the Ag2O/TiO2(B) HSs, both the CB and VB of Ag2O NPs lie above the corresponding bands of TiO2(B).16, 23 Therefore, the migration of the photogenerated charge carriers through the interface is thermodynamically favourable.16 The decrease in the PL intensity of TiO2(B) NRs and the slower decay rate in TRPL after Ag2O NPs loading on to it strongly suggest that the efficient charge separation indeed takes place in the HS samples. Ag2O NPs are fully visible light sensitive due to its narrow band gap (~1.34 eV). When the Ag2O/TiO2(B) HSs are exposed to the visible light, both the components produce electrons in the conduction band leaving behind holes in the valence band. Due to the band positions and their bending at the interface24, photo-excited electrons from the CB of the Ag2O migrate to that of TiO2 and holes from VB of TiO2 move to that of Ag2O.
Fig. 2.14. A schematic representation of Ag2O/TiO2(B) NRs HS and the corresponding band alignment at the interface showing the charge separation and the corresponding photodegradation mechanism.
This charge transfer process promotes the charge carriers a longer lifetime reducing their recombination probability, which is consistent with the TRPL results. Adsorbed O2 and H2O molecules react with the photogenerated electrons at the interface of the HS to form superoxide radicals (strong reducing agent), and holes can be trapped by hydroxyl groups to form hydroxyl radicals (strong oxidizing agent). Thus a sufficient number of powerful superoxide and hydroxyl radicals are generated, and they decompose the adsorbed organic pollutants. The reaction mechanism goes as follows:6
1. TiO2(B) ℎν → e + h+ 2. Ag2O ℎν → e + h+ 3. O2 (at interface) + e-
→ •O2
4. H2O
→ H+ + OH 5. OH + h+
→ •OH 6. Organic dye + •OH
→ degradation products
7. Organic dye + •O2
→ degradation products
Under visible light illumination, the holes are accumulated on the Ag2O surface by transferring electrons to the TiO2(B) surface. Thus, the TiO2(B) becomes anionic by gathering electrons, and Ag2O becomes cationic in nature by accumulating holes. In the HS, Ag2O NPs are uniformly coated on the TiO2(B) surface, and thus the HS as a whole behaves cationic in nature from the
surface. As the dye MO is anionic in nature, in aqueous solution, the anionic dye radicals feel columbic attraction to the cations (h+) on the HS surfaces. Thus, the anionic dye radicals get adsorbed very promptly to the HS surface and react with the hydroxyl radicals (step 6). As the hydroxyl radicals are mainly generated at the Ag2O surface, the concentration of this radical is very high at the Ag2O surfaces. Thus, the MO gets decomposed very rapidly with time under the visible light irradiation. Note that MB is slightly cationic in nature. So, in aqueous solution, there is columbic repulsion between the cationic MB radicals and cationic Ag2O surface in the HS samples. Thus, the adsorption rate of MB dye radicals at the HS surface is slower than that of MO radicals, which results in the slower degradation rate of MB as shown in Table 2.2.25 Nevertheless, the degradation rate is considerably high due to the high density of photoinduced carriers present in the HSs, resulting in the significant degradation of MB by the superoxide and hydroxyl radicals. Thus, the Ag2O/TiO2(B) HSs are highly efficient for the visible light photodegradation of dyes irrespective of their ionic nature.