Nanoparticles for the Degradation of Water Pollutants
3.3 Results and Discussion
3.3.2 Diphenhydramine Photocatalytic Degradation
to a bandgap narrowing [47, 48]. Moreover, the addition of gold nanoparticles into the TiO2 reveals the presence of one band in the visible region with a wavelength maximum at around 550 nm, typical of the gold surface plasmon [73, 74]. The increase in absorption in the visible region depended on the type of nanocarbon used, a higher absorption in the visible region being obtained for nanocomposites prepared with GO, then with CNT, and finally with ND.
Figure 3.6 DRUV–Vis spectra of Au/TiO2 and Au/carbon–TiO2 composites.
3.3.2 Diphenhydramine Photocatalytic
resistant pollutant in the absence of a catalyst, since the DP conversion observed under non-catalytic conditions is less than 5% in 60 min (Figure 3.7a). Moreover, in the dark, the adsorption capacity was not higher than 9%
of the initial DP concentration for all the photocatalysts tested.
Figure 3.7 Photocatalytic degradation of DP under near-UV–Vis (a) and visible light (b) irradiation over Au/TiO2 and Au/carbon–TiO2 composites.
Curves represent the fitting of the pseudo-first-order equation to the experimental data.
Table 3.3 Pseudo-first-order kinetic rate constant (k) of DP degradation for
different experimental conditions and respective coefficient of variation (CV), expressed as a percentage (kCV) and regression coefficient (r2) under UV–Vis and visible light irradiation.
The photocatalytic activity of the gold catalysts follows the order (Figure 3.7a): Au/GO–TiO2 (23.3 × 10–3 min–1) ~ Au/ND–TiO2 (20.1 × 10–3 min–
1) > Au/CNT–TiO2 (10.0 × 10–3 min–1) > Au/C60–TiO2 (6.0 × 10–3 min–
1) > Au/TiO2 (4.8 × 10–3 min–1), where the values in brackets refer to the pseudo-first-order rate constants (Table 3.3). These results indicate that the presence of any carbon material leads to an increase in the efficiency for DP removal in comparison to Au/TiO2, suggesting a synergistic effect between the carbon phase and TiO2 particles which depends on the nature of the carbon material [47]. In fact, the highest photocatalytic performances under near-UV–Vis irradiation were found for the GO composite (Au/GO–TiO2) and for the ND composite (Au/ND–TiO2).
The photocatalytic activity of these composites (but without the presence of gold) has been already studied for the degradation of DP [45, 54]. The good performance of GO–TiO2 (containing 4% wt. of GO) has been attributed to the good TiO2 distribution in the composite prepared with this GO content, leading to a good assembly and interfacial coupling between the GO sheets and TiO2 nanoparticles, acting as an efficient electron acceptor and donor [54]. In the case of the ND–TiO2 catalyst (also without gold), the best
photocatalytic performance was observed when the composite was prepared with 15% wt. of NDs, due to the presence of significant amounts of oxygen surface species on ND and to the increased purity of the ND constituent after the oxidation treatment [45]. In fact, the better performance for the composites containing GO and NDs, in comparison with those prepared with CNT or C60, may be related to the large amounts of surface functional groups in both GO and ND materials [45, 47, 59], since these groups can facilitate dispersion in the solution during the preparation of the composites, as well as good anchoring of the TiO2 particles on the carbon material. These effects can be responsible for the higher photocatalytic performance of both Au/GO–TiO2 and Au/ND–TiO2 composites.
It can be seen that, in this case, contrarily to what is often reported in the literature for gold catalysis in general [66, 67], the gold nanoparticle size has no influence in the catalytic activity, since the sample with the largest size (Au/GO–TiO2, 23.7 nm) is the most active, and that with the lowest size (Au/CNT–TiO2, 13.8) has an intermediate behaviour.
The photocatalytic activity of Au/TiO2 and the Au/carbon–TiO2 composites for the degradation of DP under visible light irradiation (λ = 420 nm) was also evaluated for a longer reaction time (240 min, Figure 3.7b), and the respective pseudo-first-order rate constants are shown in Table 3.3. Once again, the results show that the Au/GO–TiO2 composite exhibited the highest photocatalytic activity (0.90 × 10–3 min–1). The observed enhancement of the photocatalytic activity in the Au/GO–TiO2 composite could be in principle accounted for by the interfacial charge transfer process from GO to TiO2, attributed to the role of GO as photosensitizer [54].
On the other hand, both Au/TiO2 and Au/ND–TiO2 have the lowest photocatalytic activity for DP degradation under visible light illumination, which may be attributed to their low absorption in the visible spectral range (Figure 3.6).
Therefore, the results obtained showed that, under the tested conditions, both Au/GO–TiO2 and Au/ND-TiO2 presented the best photocatalytic activity under near-UV–Vis irradiation, while the composites prepared with GO and CNT exhibited the highest activity under visible light illumination.
Gold particles have been used as surface modifiers because they possibly inhibit charge recombination by accelerating transfer of photoexcited electrons from titania to substrates. Another advantage of materials containing gold is their absorption to induce photocatalytic reactions under visible light irradiation [74]. Nevertheless, the results obtained for the Au photocatalysts suggest the existence of different activation mechanisms depending on the type of carbon material and on the irradiation wavelength used [75].