**1. Introduction**

Ever-increasing environmental issues and consumption of fossil fuels have stimulated extensive research on the utilization of sustainable solar energy [1]. The extensive use of fossil fuels has led to a serious energy crisis and environmental pollution, which are the two major challenges facing the world in the 21st century. Among the many advanced technologies available today, heterogeneous photocatalysis in view of semiconductors taking advantage of regeneration solar energy has been identified as one of the most prospective strategies for resolving both the environmental and energy problems and has thus caused much attention during the recent decades [2–4]. In the past decades, numerous results have been reported for photocatalysis and their applications to produce hydrogen from water (see Fig.1 a) [5–8], convert solar energy into electric energy (see Fig.1 b) [9,10], degrade organic pollutants (see

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Fig.1 c) [11–13] and reduce CO2 into organic fuels (see Fig.1 d) [13–15]. Besides the naturally abundant in nonrenewable energy sources such as solar energy can be renewed into chemical or electrical and thermal energies by using semiconductors having persisting materials in the process of photocatalysis [17-20]. Generally speaking, the mechanism of a typical powerdriven photocatalysis process is mainly owing to three critical related synergistic steps: (i) light absorption and charge excitation; (ii) charge separation and transport from the semiconductor particle to its surface active sites; (iii) surface photocatalytic chemical reactions, and this process is similar to the fundamental mechanism of photocatalysis in power systems.

**Figure 1.** Photocatalytic mechanisms of water splitting, solar cell, degradation of pollutants, CO2 reduction via onestep photoexcitation. CB and VB represent the conduction and valence bands, respectively [16].

Typically, the electron-hole pair with specific reduction and oxidation potential will be created on its conduction band (CB) and valence band (VB) under the irradiation of incident light with energy greater than the band gap of a given semiconductor. Here, the band gap of the semiconductor determines the utilization rate of the energy of the incident light, and the CB and VB values are the origin of the reduction and oxidation abilities of the photoexcited electrona and holes [21]. However, in practical process, the performance of photocatalysts is mainly related to two conditions: (i) the energy (*hv*) of the incident photon should be larger than the energy gap (*E*g) of the photocatalyst; (ii) the redox potential of reactants should be located between the CB and VB of the semiconductor photocatalyst. One the one hand, the former condition indicates a narrow band gap, which can facilitate the efficient utilization of incident solar-light. On the other hand, the latter condition demonstrate that a more higher CB potential and a more lower VB potential, which are thermodynamically beneficial for the reduction and oxidation reactions of the reactants, respectively. But a high CB and low VB potential means a broad band gap of a photocatalyst, which leads to the poor solar−light utilization as discussed in condition (i). It is obvious that these conditions above are mutually contradiction, and it is important to find the balance point to design the photocatalyst. However, for a single component photocatalyst, it is difficult to possess both wide light −absorption range and strong redox ability concurrently. Besides, in the single-component structure, the photogenerated electrons in the CB can easily return to the VB or trap in the defect state and recombine with the holes, which seriously reduces the utilization efficiency of solar energy [22-24]. Hence, designing appropriate heterogeneous photocatalytic systems should be an effective way to overcome this problem.

The chapter is divided into four main sections. In the first part, we describe the importance of binary oxide system photocatalytic matericals in the case of two prominent and widely studied matel oxides: Titania (TiO2), hematite (α-Fe2O3). In the second part, we do focus on materials with a specific ternary oxide photocatalytic matericals, such as Bi systems photocatalytic materials. In the third part, we discuss the semiconducting materials and its composites which have promising applications in the area of energy and environment especially in photocatal‐ ysis. In the end, we highlight the challenges and opportunities on the way to implement photocatalytic materials to help on the development of energy research and finding ways to approach for the major problems. Hence, we believe that a comprehensive chapter on ad‐ vanced nanomatericals for solar photocatalytic is desirable for the further development of the novel photocatalytic materials and deeper understanding of photocatalytic mechanisms will be achieved in the near future, through more fundamental interdisciplinary research.
