# 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  
Numerous approaches have been developed to improve the photocatalytic response of TiO\({}_{2}\), in terms of both broadening the reaction spectrum to the entire visible spectral range [11] and mitigating the rapid recombination phenomena of electron-lacuna pairs [36]. The incorporation of non-noble metals (such as Fe, Cu, Co, Ni, and Cr) or metallic oxides onto TiO\({}_{2}\) represents a valid alternative to noble metals. In particular, the use of this approach can effectively hinder the recombination of electron-hole pairs and promote the separation of charge carriers [37]. [PERSON] et al. [38] synthesized a photocatalyst system based on TiO\({}_{2}\) doped with Cu and Zn (CuZn-TiO\({}_{2}\)). TEM images and XPS spectra showed the presence of Cu\({}_{2}\)O, CuO, ZnO, and Cu\({}^{0}\) on the TiO\({}_{2}\) surface. Metal-semiconductor junctions and heterojunctions between semiconductors were formed due to the growth of Cu\({}_{2}\)O, CuO, ZnO, and Cu\({}^{0}\) on the surface of TiO\({}_{2}\). Effective charge separation due to the heterojunctions and high electron mobility due to Cu\({}^{0}\) improved the H\({}_{2}\) production. In addition, Cu\({}^{0}\), CuO, and Cu\({}_{2}\)O allowed the absorption of TiO\({}_{2}\) to shift to visible light. In particular, the CuZn-TiO\({}_{2}\) with an optimum loading of Cu and Zn of 0.5 wt% achieved a H\({}_{2}\) production of about 14,521 umol h\({}^{-1}\) g\({}^{-1}\)\({}_{\text{cat}}\) under solar light, using glycerol as sacrificial reagent. A plausible mechanism for H\({}_{2}\) production is reported in the work of [PERSON] et al. [38] and is schematically represented in Figure 1.  
A cobalt (Co)-doped TiO\({}_{2}\) photocatalyst was synthesized by [PERSON] et al. [36] through the impregnation method. The presence of cobalt species on the TiO\({}_{2}\) surface of the synthesized photocatalyst was observed. The Co/TiO\({}_{2}\) photocatalyst, with 1 wt% of Co doped on TiO\({}_{2}\) P25, achieved a maximum hydrogen production of 11,021 umol h\({}^{-1}\) g\({}^{-1}\) from aqueous glycerol solutions under solar light irradiation, showing that Co\({}^{2+}\) ions can extend the photo-response of TiO\({}_{2}\) into the visible region [36]. [PERSON] et al. [11] prepared a series of photocatalyst systems consisting of TiO\({}_{2}\) P25 combined with Fe, Co, Ni, Cu, and Zn (denoted as Fe/TiO\({}_{2}\), Co/TiO\({}_{2}\), Ni/TiO\({}_{2}\), Cu/TiO\({}_{2}\), and Zn/TiO\({}_{2}\), respectively) using the impregnation method. The TiO\({}_{2}\) was loaded with 2 wt% of the selected transition metal and evaluated under both visible and UV light irradiation. During visible light exposure, with methanol serving as a sacrificial reagent, the photocatalysts Ni/TiO\({}_{2}\), Co/TiO\({}_{2}\), Zn/TiO\({}_{2}\), and Fe/TiO\({}_{2}\) exhibited lower H\({}_{2}\) production rates than the pure TiO\({}_{2}\). Conversely, Cu/TiO\({}_{2}\) demonstrated a substantial enhancement in photocatalytic activity, yielding H\({}_{2}\) production rates equal to 220 umol h\({}^{-1}\) g\({}^{-1}\)[11]. [PERSON] et al. [39] developed a photocatalyst system comprising bimetallic Cu-Ni alloy NPs decorated on TiO\({}_{2}\) P25. The Cu-Ni/TiO\({}_{2}\) photocatalyst, with a Cu loading of 2 mol% and Ni loading of 5 mol%, achieved a remarkable H\({}_{2}\) evolution rate of 35.4 mmol g\({}^{-1}\) h\({}^{-1}\) under solar light conditions, employing methanol as a sacrificial reagent. Additionally, the monometallic photocatalysts Cu-doped TiO\({}_{2}\) (Cu/TiO\({}_{2}\)) and Ni-doped TiO\({}_{2}\) (Ni/TiO\({}_{2}\)) were also synthesized and assessed. It was observed that the monometallic photocatalysts exhibited lower H\({}_{2}\) yields compared to the Cu-Ni/TiO\({}_{2}\) systems [39]. [PERSON] et al. [40] fabricated mesoporous nano TiO\({}_{2}\) and introduced Fe/TiO\({}_{2}\) catalysts by synthesizing and introducing Fe dopants onto the TiO\({}_{2}\) surface through the impregnation process.