**Recent Advances in Visible-Light Driven Photocatalysis**

Yuhua Wang, Xinlong Ma, Hao Li, Bin Liu, Huihui Li, Shu Yin and Tsugio Sato

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61864

### **Abstract**

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Semiconductor photocatalysis has been considered a potentially promising approach for renewable energy and environmental remediation with abundant solar light. However, the currently available semiconductor materials are generally limited by either the har‐ vesting of solar energy or insufficient charge separation ability. To overcome the serious drawbacks of narrow light-response range and low efficiency in most photocatalysts, many strategies have been developed in the past decades. This article reviews the recent advancements of visible-light-driven photocatalysts and attempts to provide a compre‐ hensive update of some strategies to improve the efficiency, such as doping, coupling with graphene, precipitating with metal particles, crystal growth design, and heterostruc‐ turing. A brief introduction to photocatalysts is given first, followed by an explanation of the basic rules and mechanisms of photocatalysts. This chapter focuses on recent progress in exploring new strategies to design TiO2-based photocatalysts that aim to extend the light absorption of TiO2 from UV wavelengths into the visible region. Subsequently, some strategies are also used to endow visible-light-driven Ag3PO4 with high activity in photo‐ catalytic reactions. Next, a novel approach, using long afterglow phosphor, has been used to associate a fluorescence-emitting support to continue the photocatalytic reaction after turning off the light. The last section proposes some challenges to design high efficiency of photocatalytic systems.

**Keywords:** Photocatalysts, TiO2, Ag3PO4, graphene, long afterglow phosphor

## **1. Introduction**

It is well known that environmental pollution is affecting human survival and development. Photocatalytic technology is considered as an efficient, stable, and environmentally friendly method in the field of environmental pollution control [1]. In general, a photocatalytic reaction includes three steps [2] (Fig. 1). First, electrons (e−) and holes (h+) are generated through

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

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

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 to harvest visible light.

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].

**Figure 2.** Schematic illustration of band alignments corresponding to five kinds of heterostructured photocatalysts, in‐ cluding the straddling alignment (type I), staggered alignment (type II), Z-scheme system, p–n heterojunctions, and homojunctions. Reproduced with permission from ref. 30–32 © 2011 RSC & 2011 ACS & 2012 Wiley.
