**2.1. Doping**

photoexcitation. Then, these electrons and holes migrate to the surface of photocatalysts and subsequently react with adsorbed electron acceptors and donors, respectively. Thus, an efficient photocatalyst requires a semiconductor with a suitable band gap for harvesting light, facile separation and transportation of charge carriers, and proper valence band (VB) and conduction band (CB) edge potential for redox reaction being feasible. To date, much effort is currently focused on how to improve the light absorption, charge separation, and surface reactivity in order to achieve outstanding photocatalytic performance, such as doping, coupled with graphene, precipitated with metal particles, crystal growth design, and heterostructuring. In the past few decades, TiO2 nanomaterials have attracted tremendous interest in environ‐ mental pollution removal and photocatalytic hydrogen generation. However, all of them have large electronic band gaps of 3.0–3.2 eV, which means only less than 5% of the entire solar energy can be utilized. Thus, much effort has been devoted to bring about the absorption of

**Figure 1.** Schematic illustration of the reactions following light absorption by a photocatalyst. (a) Electron–hole pair formation; (b) donor oxidation by hole; (c) reduction by electron; (d) and (e) electron–hole recombination on the sur‐

Despite the fact that there are many research articles on modified TiO2, the exploration of new active and efficient visible-light-driven photocatalysts attracts much attention [3–22]. In the effort to exploit novel photocatalyst systems working under visible light, it has been revealed that orbitals of some p-block metals with a d10 configuration [22], such as Ag 4d in Ag(I), could hybridize O 2p levels to form a new preferable hybridized VB, thus narrowing the band gap

Compared with single-phase semiconductor photocatalysts, hybrids of two or more semicon‐ ductor systems, i.e., heterostructures, seem to possess advantages in more efficiently utilizing solar light. Besides metal/semiconductor and carbon group materials/semiconductor-hetero‐ structured photocatalysts [23–28], semiconductor/semiconductor-heterostructured photoca‐ talysts with diverse models have been developed (Fig. 2), including type-I and type-II heterojunctions, Z-scheme, p–n heterojunctions, and homojunction band alignments [29–32].

TiO2 into the visible-light region and improved photocatalytic activity.

338 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

face or in the bulk. Adapted with permission from ref. 2. © 1995 ACS.

to harvest visible light.

Doping is an available strategy to tune the absorption band of wide-band photocatalysts. Several studies have investigated nonmetal doping. Asahi *et al.* reported nitrogen-doped TiO2 of nonmetals, such as N [33, 34], C [35–37], S [38, 39], B [40–42], F [43–45], Br [46], I [47– 50], P [51], in 2001 [33]. In our study [34], nitrogen-doped titania nanoparticles (NPs) were successfully prepared by a microwave-assisted solvothermal process in a very short time, and the prepared samples showed visible-light absorption in the range of 400–550 nm, indicating its potential applications as visible-light-induced photocatalyst. The chemical states and locations of dopants are considered to be key factors in adjusting the spectral distribution of the induced electronic states of those dopants and reconstructing favorable surface structure for photocatalysis.

Co-doping with two suitable heteroatoms can also achieve substantial synergistic effects [41, 52–55]. For example, B, N co-doped TiO2, benefits from the B–N bonds formed, which can increase the amount of doped N on the TiO2 surface and the promoted separation of photo‐ excited electron–hole pairs [41].
