## 3 Results and Discussion  
### Photocatalytic Activity Tests  
#### 3.1.1 Influence of Cu Content on the Hydrogen Production under Visible Light  
The photocatalytic hydrogen production was evaluated for undoped ZnO and Cu-doped ZnO photocatalysts under visible light from a glycerol aqueous solution at 5 wt % initial concentration and with a catalyst dosage equal to 1.5 g/L. Figure 1 reports the results obtained after 4 h of visible light irradiation. It is worthwhile to note that the hydrogen production obtained with undoped ZnO (612 umol/L) was comparable to that obtained by photolysis alone (592 umol/L). This result means that the undoped ZnO was not active in the presence of visible light for the hydrogen production due to its large band gap energy [46]. On the other hand, all of the Cu-doped ZnO samples showed positive results towards hydrogen production under visible light irradiation. In detail, from the data presented in Figure 1, it is possible to observe that the hydrogen production increased with the increase of Cu content up to 1.08 mol %, but a further increase of the dopant level resulted in a decrease of photocatalytic hydrogen production. In particular, the 1.08 Cu_ZnO photocatalyst showed a hydrogen production equal to 4180 umol/L. Possibly, the improved photocatalytic performances observed up to 1.08 mol % Cu content were due to the inhibition of the recombination rate of the electron-hole pairs [47]. However, the increase in Cu content beyond the optimal value induced a worsening of photocatalytic performances, indicating that an excess of dopant content may act as a recombination center of the electron gap pairs, as reported in previous works [37; 48; 49]. Regarding the comparison with the available literature data, it should be pointed out that in many cases, besides glycerol, in the solution there is the presence of a further sacrificial agent such as methanol or the addition of noble metals, which act as cocatalysts for hydrogen production [23]. A hydrogen production of 2600 umol/L is reported starting from a glycerol solution with 6 wt % initial concentration and using a Pt/TiO\({}_{2}\) photocatalyst [15]. In particular, the hydrogen production reported in [15] was lower than that achieved with the 1.08 Cu_ZnO catalyst, which showed good efficiency under visible light and in the absence of noble metals, allowing it to achieve a hydrogen production equal to twice that found in the literature. It is well known that the parameter that defines the ability of a photocatalyst in hydrogen production is the electronic structure [50]. Considering that the copper content in the 1.08 Cu_ZnO photocatalyst was very low (1.2 mol %), it is possible to assume its electronegativity value was equal to that of the undoped ZnO (5.94 eV) [46]. From the band gap value (\(E_{g}\)) of 1.08 Cu_ZnO (2.92 eV), it was possible to calculate the values of conduction band (\(E_{\mathrm{CB}}\)) and valence band (\(E_{VB}\)) energy through the Mulliken relationship [51]:  
\[E_{VB}=X-4.5+(0.5\times E_{g})\]  
\[E_{\mathrm{CB}}=E_{VB}-E_{g}\]  
where X is the semiconductor electronegativity. The obtained results are presented in Figure 2.  
As it is possible to observe from the E\({}_{\mathrm{CB}}\) (-0.05 eV vs. NHE) and E\({}_{\mathrm{VB}}\) (2.86 eV vs. NHE) values, the electronic structure of the 1.08 Cu_ZnO photocatalyst satisfied the thermodynamic requirements for the water splitting reactions, with respect to the potentials for water oxidation/reduction reactions, reported also by [PERSON] et al. [52]. In fact, the E\({}_{\mathrm{VB}}\) value was equal to 2.89 eV versus NHE, which was more positive than the oxidation potential O\({}_{2}\)/H\({}_{2}\)O (+1.23 eV vs. NHE), and the E\({}_{\mathrm{CB}}\) value was equal to -0.05 eV versus NHE, which was more negative than the reduction potential H\({}^{+}\)/H\({}_{2}\) (0 eV).