## 3 Results and discussion
### Effect of the Photocatalyst Load  
Determining the optimum photocatalyst concentration is of crucial importance since it allows scaling up the process, with relevant impact on the economic aspects of the whole discoloration process. Accordingly, we have investigated the effect of the photocatalyst load (0.10, 0.33, 0.66 or 1.00 mg mL\({}^{-1}\)) on the dye degradation efficiency. Proper amounts of _a_-TiO\({}_{2}\) or ZnO NFs were dispersed in the MB solution and activity was evaluated at RT under UV irradiation. Figure 4a-c displays the absorbance spectra recorded at different time intervals in the presence of 0.10, 0.66 and 1.00 mg mL\({}^{-1}\)_a_-TiO\({}_{2}\) NFs (photocatalyst TZ10); the corresponding time-evolution of the degradation rates is shown in Figure 4d and a larger time interval is displayed in Figure S9. Sample TZ10 exhibits outstanding degradation performance, compared with both electrospun titania NFs and TiO\({}_{2}\) photocatalysts prepared by different techniques (compare entries 17-20 with 1, 3, 6, 16 and 32-35 in Table S1). Just to cite some \(D\) values, at the lowest concentration (0.1 mg mL\({}^{-1}\)), the percentage of dye degraded after 4 h (94%) exceeds by a factor greater than 4.5 that reported by [PERSON] et al. [26] for a higher load of electrospun titania NFs in the same time interval (see entry 1 in Table S1). At 0.66 and 1.00 mg mL\({}^{-1}\) loads, respectively 97% and 99% MB decomposes in less than 2 h, while, according to reports by [PERSON] et al. [27], 93% dye removal from a solution with the same initial concentration is achieved in a longer time by the use of a largely higher concentration of electrospun titania NFs (see entry 3 in Table S1). The time-evolution of the degradation rates for ZnO NFs (photocatalyst TZ01) is shown in Figure 4e. The absorbance spectra recorded in the presence of 0.66 mg mL\({}^{-1}\) are displayed in Figure S10. Those relative to 0.1 and 1.0 mg mL\({}^{-1}\) loads were reported in previous work [21]. At the lower concentrations (0.10 and 0.33 mg mL\({}^{-1}\)), 89% and 97% MB degrades in less than 1.5 h, while in the same interval lower \(D\) values are reported by other authors (see entry 7 in Table S1). By using higher loads of ZnO NFs (0.66 mg mL\({}^{-1}\)), the MB solution fully discolors in less 1 h.  
In order to calculate the reaction rate constants, the \(A_{\text{t}}/A_{0}\) ratios between absorbance measured at 664 nm, at equilibrium, at times \(t\) and 0, were plotted in a semi-logarithmic scale as a function of the irradiation time, as shown in Figure S11 for \(a\)-TiO\({}_{2}\) NFs at different loads. Regardless of the photocatalyst concentration, linear trends were found, as in the  
Figure 4: Effect of the load of the electrospun NFs on the photocatalytic performance of \(a\)-TiO\({}_{2}\) NFs and ZnO NFs. (**a–c**) Time-evolution of the absorbance spectra of an aqueous MB solution with initial 15 uM concentration and different concentrations of \(a\)-TiO\({}_{2}\) NFs, under exposure to 350 nm UV radiation. (**d,e**) Degradation rates of photocatalysts (**d**) TZ10 and (**e**) TZ01 as a function of time. (**f**) Concentration-dependence of the corresponding rate constants. case of UV radiation alone. This finding indicates that the kinetics of the process can be modeled via a pseudo first-order law [5; 21]. The values of the rate constants (\(k\)), obtained as the slope of the curves \(\ln(A_{t}/A_{0})=-k\ t\) via a best-fit procedure to the data, are reported in Table S1 (entries 17-20 and 27-30). The \(k\)-values for \(a\)-TiO\({}_{2}\) (1.24 \(\times\) 10\({}^{-2}\)-1.12 \(\times\) 10\({}^{-1}\) min\({}^{-1}\)) and ZnO NFs (2.86 \(\times\) 10\({}^{-2}\)-7.94 \(\times\) 10\({}^{-2}\) min\({}^{-1}\)) well compare with those available in the literature for titania and zinctic photocatalysts (see entries 12, 16 and 32-35 in Table S1). Figure 4 displays the load dependence of the reaction rate constants for photocatalysts TZ10 and TZ01.