**4. Composites systems**

Interestingly, Zhang [332] found that the photoactivity of BiOCl nanosheets shows a highly exposed facet-dependent effcet. The BiOCl nanosheets with exposed {001} facets showed higher direct semiconductor photoexcitation activity towards pollutant degradation due to both the surface atomic structure and suitable internal electric fields under UV light irradiation. Under visible light, highly exposed {010} facet BiOCl nanosheets shows superior indirect dye photosensitization activity for methyl orange degradation, which is due to the larger surface area and open channel characteristic of BiOCl nanosheets. It is belived that the enlarged surface area and open channel could enhance the adsorption capacity of methyl orange molecules as well as provide more contact sites between the photocatalyst and dye molecules, thereby facilitating the indirect dye photosensitization process because more efficient electron injection from the photoexcited dye into the conduction band of the catalyst happened (Fig. 27). These findings not only clarified the origin of facet-dependent photoreactivity of BiOCl nanosheets but also provided effective guidance for the design and fabrication of highly efficient bismuth

**Figure 26.** Schematic illustration of the fabrication of flower−like BiOCl hierarchical nanostructures by an in situ oxida‐

**Figure 27.** Schematic illustration of facet-dependent photoreactivity of BiOCl single-crystalline nanosheets [332].

oxyhalide photocatalyst.

tion process [331].

202 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

In contrast to one individual semiconductor photocatalyst, semiconductor composites are more intriguing for their interfacial heterostructures, which are formed at their junctures and have an important effect on their photocatalytic performances. There are usually three types of band positions in semiconductor heterojunctions: straddling gap (type I), staggered gap (type II) and broken gap (type III), as presented in Fig. 28. Among them, semiconductor composites with the staggered gap (type II) have drown much attention in the field of heterogeneous photocatalysis [336,337]. In this system, the photoinduced electrons and holes can be easily separated at the interface of the two semiconductors via effective interfacial charge transfer, thereby enhances the photocatalytic performance of the semiconductor composites.

### **4.1. Binary composite**

In recent years, tremendous efforts have been made in surface modification of TiO2 nanoma‐ terials with other semiconductors. Most of these systems possess a high dye adsorption capacity, an extended light absorption range, enhanced charge separation, promoted masstransfer and thus improved photocatalytic efficiency. This semiconductor provides the best compromise between catalytic performance and stability in aqueous media. Therefore, the magnetic iron oxide/TiO2 composite photocatalysts have become the research focus in recent years. Using the magnetic properties of iron oxide itself for obtaining the magnetic recoverable photocatalyst has become an important issue in the magnetic iron oxide/TiO2 composite photocatalyst system [339-342]. For instance, Wang and coworkers have reported the fabrica‐ tion of core−shell Fe3O4@SiO2@TiO2 microspheres through a wet−chemical approach. The microspheres possess both ferromagnetic and photocatalytic properties. The TiO2 nanoparti‐ cles on the surfaces of the microspheres degraded organic dyes under the illumination of UV light. Furthermore, the microspheres were easily separated from the solution after the photocatalytic process due to the ferromagnetic Fe3O4 core. The photocatalysts were recycled for further use and the degradation rate of methyl orange still reached 91% after six cycles of reuse [343]. As shown in Fig. 29, Chalasani and Vasudevan have demonstrated waterdispersible photocatalytic Fe3O4@TiO2 core−shell magnetic nanoparticles by anchoring carboxy−methyl beta−cyclodextrin (CMCD) cavities to the TiO2 shell, and photocatalytically destroyed endocrine-disrupting chemicals, bisphenol A (BPA) and dibutyl phthalate, present in water. The particles, which were typically 12 nm in diameter, were magnetic and removed from the dispersion by magnetic separation and then reused. The concentration of BPA solution was determined by liquid chromatography, and then irradiated under UV light for 60 min. After photodegradation of BPA, the CMCD−Fe3O4@TiO2 nanoparticles that were separated from the mixtures by a magnet, and can be reused for the photodegradation of newly prepared BPA solutions. The recycle photocatalytic performance of CMCD−Fe3O4@TiO2 for the photodegradation of BPA was excellent and stable, retaining 90% efficiency after 10 cycles [345]. For obtaining the magnetically recovered photocatalysts, Fe3O4 and *γ*-Fe2O3 were often employed due to their higher saturation magnetization and good magnetic separation ability.

**Figure 29.** Scheme for the reuse of cyclodextrin-functionalized Fe3O4@TiO2 for photocatalytic degradation of endocrinedisrupting chemicals in water supplies [344].

On the other hand, α-Fe2O3 has often been introduced into the magnetic iron oxide/TiO2 composite photocatalyst in order to use its narrow band gap properties and to obtain magnetic iron oxide/TiO2 composite heterostructures [344-348]. For example, Peng and coworkers have synthesized Fe2O3/TiO2 heterostructural photocatalysts by impregnation of Fe3+ on the surface of TiO2 and annealing at 300°C, the composites possess different mass ratios of Fe2O3 vs. TiO2. The photocatalytic activities of Fe2O3/TiO2 heterocomposites, pure Fe2O3 and TiO2 were studied by the photocatalytic degradation of Orange II dye in aqueous solution under visible-light (*λ* > 420 nm) irradiation. The Fe2O3/TiO2 heterogeneous photocatalysts exhibited an enhanced photocatalytic ability for Orange II, higher than either pure Fe2O3 or TiO2. The best photoca‐ talytic performance for Orange II could be obtained when the mass ratio in Fe2O3/TiO2 is 7 : 3. The results illustrate that the generation of heterojunctions between Fe2O3 and TiO2 is key for improving movement and restraining the recombination of photoinduced charge carriers, and finally improving the photocatalytic performance of Fe2O3/TiO2 composites [348]. Recently, Palanisamy and coworkers have prepared Fe2O3/TiO2 (10, 30, 50, 70 and 90 wt% Fe2O3) photocatalysts by a sol-gel process. Mesoporous Fe2O3/TiO2 composites exhibited excellent photocatalytic degradation ability for 4−chlorophenol in aqueous solution under sunlight irradiation. The author claimed that the photogenerated electrons in the VB of TiO2 are transferred to Fe(III) ions resulting in the reduction of Fe(III) ions to Fe(II) ions. Thus, the photoinduced holes in the VB of Fe2O3/TiO2 cause an oxidation reaction and decompose the 4 chlorophenol to CO2 and H2O. Meanwhile the transferred electrons in Fe(III) ions could trigger the reduction reaction [349].

**4.1. Binary composite**

204 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

In recent years, tremendous efforts have been made in surface modification of TiO2 nanoma‐ terials with other semiconductors. Most of these systems possess a high dye adsorption capacity, an extended light absorption range, enhanced charge separation, promoted masstransfer and thus improved photocatalytic efficiency. This semiconductor provides the best compromise between catalytic performance and stability in aqueous media. Therefore, the magnetic iron oxide/TiO2 composite photocatalysts have become the research focus in recent years. Using the magnetic properties of iron oxide itself for obtaining the magnetic recoverable photocatalyst has become an important issue in the magnetic iron oxide/TiO2 composite photocatalyst system [339-342]. For instance, Wang and coworkers have reported the fabrica‐ tion of core−shell Fe3O4@SiO2@TiO2 microspheres through a wet−chemical approach. The microspheres possess both ferromagnetic and photocatalytic properties. The TiO2 nanoparti‐ cles on the surfaces of the microspheres degraded organic dyes under the illumination of UV light. Furthermore, the microspheres were easily separated from the solution after the photocatalytic process due to the ferromagnetic Fe3O4 core. The photocatalysts were recycled for further use and the degradation rate of methyl orange still reached 91% after six cycles of reuse [343]. As shown in Fig. 29, Chalasani and Vasudevan have demonstrated waterdispersible photocatalytic Fe3O4@TiO2 core−shell magnetic nanoparticles by anchoring carboxy−methyl beta−cyclodextrin (CMCD) cavities to the TiO2 shell, and photocatalytically destroyed endocrine-disrupting chemicals, bisphenol A (BPA) and dibutyl phthalate, present in water. The particles, which were typically 12 nm in diameter, were magnetic and removed from the dispersion by magnetic separation and then reused. The concentration of BPA solution was determined by liquid chromatography, and then irradiated under UV light for 60 min. After photodegradation of BPA, the CMCD−Fe3O4@TiO2 nanoparticles that were separated from the mixtures by a magnet, and can be reused for the photodegradation of newly prepared BPA solutions. The recycle photocatalytic performance of CMCD−Fe3O4@TiO2 for the photodegradation of BPA was excellent and stable, retaining 90% efficiency after 10 cycles [345]. For obtaining the magnetically recovered photocatalysts, Fe3O4 and *γ*-Fe2O3 were often employed due to their higher saturation magnetization and good magnetic separation ability.

**Figure 29.** Scheme for the reuse of cyclodextrin-functionalized Fe3O4@TiO2 for photocatalytic degradation of endocrine-

disrupting chemicals in water supplies [344].

Wang's group successfully synthesized Bi2WO6-TiO2 hierarchical heterostructure through a simple and practical electrospinning-assisted route (Fig. 30 (A) and (B)) [350]. As shown in Fig. 30 (A), Bi2WO6 nanoplates grew aslant on the primary TiO2 nanofibers. These three dimensional (3D) Bi2WO6−TiO2 hierarchical heterostructures exhibited enhanced visible−light −driven photocatalytic activity for the decomposition of CH3CHO, which was almost eight times higher than that of the Bi2WO6 sample, and the decomposition rate by the bare TiO2 could be neglected under visible light irradiation. This high photocatalytic activity was ascribed to the reduced probability of electron−hole recombination and the promoted migration of photogenerated carriers. Similarly, Wang et al. [352] fabricated SnO2−TiO2 heterostructured photocatalysts based on TiO2 nanofibers by combining the electrospinning technique with the hydrothermal method (Fig 30C and D). This SnO2−TiO2 composite possessed a high photoca‐ talytic activity for the degradation of rhodamine B (RhB) dye under UV light irradiation, which was almost 2.5 times higher than that of the bare TiO2. The enhanced photocatalytic efficiency was attributed to the improvement of the separation of photogenerated electrons and holes. Wang's group [352] prepared a graphene−Bi2WO6 composite via an in situ hydrothermal reaction (Fig. 31(A) and (B)). This graphene−Bi2WO6 photocatalyst showed significantly enhanced photocatalytic activity for the degradation of RhB under visible light (*λ* > 420 nm), which was three times greater than that of the pure Bi2WO6. The enhanced photocatalytic activity could be attributed to the negative shift in the Fermi level of graphene−Bi2WO6 and the high migration efficiency of photoinduced electrons; these electrons may not only be effectively involved in the oxygen reduction reaction but also suppress the charge recombi‐ nation. Kudo et al. [353] reported the composite of reduced graphene oxide (RGO) with BiVO4, where an significantly improved PCE activity of a near 10−fold enhancement was observe compared with pure BiVO4 under visible-light irradiation. The longer photoexcited electron lifetime of BiVO4 is mainly responsible for this improvement as the electrons are injected to RGO instantly at the site of generation, leading to a significant reduction in charge recombination.

**Figure 30.** SEM (A) and HRTEM (B) images of Bi2WO6/TiO2 [343]; SEM (C) and HRTEM (D) images of SnO2/TiO2 heter‐ ostructures [351]. was almost 2.5 times higher than that of the bare TiO2. The enhanced photocatalytic efficiency was attributed to the improvement of the separation of photogenerated electrons and holes. Wang's

**Figure 31.** (A and B) TEM images of graphene decorated with Bi2WO6 composite[352]. **Figure 31.** (A and B) TEM images of graphene decorated with Bi2WO6 composite [352].

generation, leading to a significant reduction in charge recombination.

exposure to a 3W LED light[354].

group[352]

prepared a grapheneBi2WO6 composite via an in situ hydrothermal reaction (Fig. 31(A) and (B)). This grapheneBi2WO6 photocatalyst showed significantly enhanced photocatalytic activity for the degradation of RhB under visible light (*λ* > 420 nm), which was three times greater than that of the pure Bi2WO6. The enhanced photocatalytic activity could be attributed to the negative shift in the Fermi level of grapheneBi2WO6 and the high migration efficiency of photoinduced electrons; these electrons may not only be effectively involved in the oxygen reduction reaction but also suppress the charge recombination. Kudo et al.[353] reported the composite of reduced graphene oxide (RGO) with BiVO4, A simple soft-chemical method was used to synthesize the BiOI/TiO2 heterostructures with different Bi to Ti molar ratios at low temperature of 80°C. The degradation of methyl orange under visible-light irradiation (*λ* > 420 nm) of the material revealed that the BiOI/TiO2 heterostructures exhibited much higher photocatalytic activities than pure BiOI or TiO2, where 50%BiOI/TiO2 showed the best activity among all these heterostructured photocatalysts [348]. BiOBr−Bi2WO6 mesoporous nanosheet composite enhanced photocatalytic activity is attribut‐ ed to well-matched band edge positions of BiOBr and Bi2WO6 and the large specific surface area of the mesoporous nanosheet composites in view of the incorporation of mesopores and

where an significantly improved PCE activity of a near 10fold enhancement was observe compared with pure BiVO4 under visible‐light irradiation. The longer photoexcited electron lifetime of BiVO4 is mainly responsible for this improvement as the electrons are injected to RGO instantly at the site of

 A simple soft‐chemical method was used to synthesize the BiOI/TiO2 heterostructures with different Bi to Ti molar ratios at low temperature of 80°C. The degradation of methyl orange under visible‐light irradiation (*λ*  > 420 nm) of the material revealed that the BiOI/TiO2 heterostructures exhibited much higher photocatalytic activities than pure BiOI or TiO2, where 50%BiOI/TiO2 showed the best activity among all these heterostructured photocatalysts[348]. BiOBrBi2WO6 mesoporous nanosheet composite enhanced photocatalytic activity is attributed to well‐matched band edge positions of BiOBr and Bi2WO6 and the large specific surface area of the mesoporous nanosheet composites in view of the incorporation of mesopores and the highly exposed BiOBr (001) facet, compared with pure BiOBr and Bi2WO6 under

 During the past few years, another promising carbon material, graphene, which possess many unique properties, has been used for corporation with BiOX (X = Cl, Br and I) and significantly improved photocatalytic efficiencies was achieved[356‐360]. Ai and coworkers[357] have developed a facile solvothermal route tosynthesize BiOBr/graphene hybrids using graphene oxide (GO), bismuth nitrite, and CTAB as the precursors. As shown in Fig. 32, BiOBr nanoplates with hundreds nanometers in size are dispersed randomly on the 2D graphene sheet surface. Evaluated by the removal of gaseous NO under visible‐light irradiation, the as‐prepared BiOBr/graphene hybrid displays a two times higher removal rate than that of pure BiOBr. It is evidenced that the strong chemical bonding between BiOBr and graphene is mainly responsible for the fast photogenerated electrons transfer from BiOBr to the highly exposed BiOBr (001) facet, compared with pure BiOBr and Bi2WO6 under exposure to a 3−W LED light [354].

During the past few years, another promising carbon material, graphene, which possess many unique properties, has been used for corporation with BiOX (X = Cl, Br and I) and significantly improved photocatalytic efficiencies was achieved [356-360]. Ai and coworkers [357] have developed a facile solvothermal route tosynthesize BiOBr/graphene hybrids using graphene oxide (GO), bismuth nitrite, and CTAB as the precursors. As shown in Fig. 32, BiOBr nanoplates with hundreds nanometers in size are dispersed randomly on the 2D graphene sheet surface. Evaluated by the removal of gaseous NO under visible-light irradiation, the as-prepared BiOBr/graphene hybrid displays a two times higher removal rate than that of pure BiOBr. It is evidenced that the strong chemical bonding between BiOBr and graphene is mainly responsible for the fast photogenerated electrons transfer from BiOBr to graphene, which further inhibities the unwanted recombination and leading to its enhanced photocatalytic activity.

**Figure 32.** Schematic illustration of the visible−light photocatalytic enhancement of BiOBr/graphene nanocomposites [357].
