# The Application of Nano Titanium Dioxide for Hydrogen Production and Storage Enhancement
## 2 Titanium Dioxide
### Hydrogen Production Exploiting TiO\({}_{2}\) under Visible and Solar Light
#### 2.1.1 Modification of TiO\({}_{2}\) with Non-Noble Metals and Metallic Oxides
These catalysts were tested under solar light irradiation and the H\({}_{2}\) production rate of 270 umol/h was attained with 0.5 wt% of Fe\({}^{3+}\) loading. A comparison between Fe doping on the TiO\({}_{2}\) surface and Fe doping into the TiO\({}_{2}\) lattice revealed that metal ions should be doped in the proximity to the TiO\({}_{2}\) surface to achieve efficient photocatalytic activity through enhanced charge transfer [40]. [PERSON] et al. [1] prepared a Cu\({}_{2}\)O/TiO\({}_{2}\)/Bi\({}_{2}\)O\({}_{3}\) ternary photocatalyst: the presence of Cu\({}_{2}\)O and Bi\({}_{2}\)O\({}_{3}\) allows the visible-light absorption to be extended and helps minimize the electron-hole recombination of TiO\({}_{2}\), respectively. When exposed to solar light irradiation, the ternary photocatalysts, with 2 wt% Cu and 2 wt% Bi, demonstrated an impressive H\({}_{2}\) production rate of 3678 umol/h in glycerol aqueous solution. Furthermore, the dependence of photocatalytic H\({}_{2}\) production on the glycerol and catalyst content was investigated. It was observed that under an optimal glycerol concentration and catalyst amount, a H\({}_{2}\) production rate of 6727 umol/h was achieved [1]. A photocatalyst consisting of mesoporous ZrO\({}_{2}\)-TiO\({}_{2}\) NPss anchored on reduced graphene oxide (rGO) was synthesized by [PERSON] et al. [41]. The ZrO\({}_{2}\)-TiO\({}_{2}\)/rGO photocatalyst, with a loading of 1.0 wt% of ZrO\({}_{2}\) and rGO on TiO\({}_{2}\), exhibited remarkable H\({}_{2}\) production (-7773 umol h\({}^{-1}\) g\({}^{-1}\)) in a glycerol aqueous solution under solar light irradiation. Furthermore, a ZrO\({}_{2}\)-TiO\({}_{2}\) photocatalyst, with 1.0 wt% of ZrO\({}_{2}\) loading on TiO\({}_{2}\), was also examined, and it achieved an H\({}_{2}\) production rate of 5781 umol h\({}^{-1}\) g\({}^{-1}\). The ZrO\({}_{2}\)-TiO\({}_{2}\) heterojunction anchored on rGO sheets demonstrated the suppression of electron-hole pair recombination and an enhancement in the efficiency of interfacial charge transfer [41]. [PERSON] et al. [42] synthesized a photocatalyst composed of CuO deposited onto TiO\({}_{2}\) nanotubes. The photocatalyst, with a copper loading of 1.5 wt%, exhibited  
Figure 1: Schematic representation of a possible mechanism (Z-scheme mechanism with double charge transfer mechanism) for H\({}_{2}\) production by CuZn-TiO\({}_{2}\) photocatalyst under solar light. the highest photocatalytic hydrogen evolution rate under solar light using glycerol as a sacrificial agent. It achieved an impressive H\({}_{2}\) production rate of 99,823 \(\upmu\)mol h\({}^{-1}\) g\({}^{-1}\)[42]. [PERSON] et al. [43] synthesized a Cu\({}_{\text{x}}\)O/TiO\({}_{2}\) nanocomposite loaded with layered Ni(OH)\({}_{2}\). Under solar light irradiation and in the presence of glycerol (5 vol%), the Ni(OH)\({}_{2}\)-Cu\({}_{\text{x}}\)O-TiO\({}_{2}\) photocatalyst, with an optimal loading of 1.0 wt% Ni(OH)\({}_{2}\) and 0.5 wt% Cu\({}_{\text{x}}\)O, showed a H\({}_{2}\) production of \(\sim\)15,789 umol h\({}^{-1}\)g\({}^{-1}\). Furthermore, Cu-doped TiO\({}_{2}\) photocatalysts (Cu\({}_{\text{x}}\)O/TiO\({}_{2}\)) were tested. It was shown that the photocatalyst with 0.5 wt% Cu loading on TiO\({}_{2}\) allowed an advancement in H\({}_{2}\) production (\(\sim\)7679 umol h\({}^{-1}\) g\({}^{-1}\)) [43].