**2.3. TiO2-based plasmonic photocatalysts**

Plasmonic photocatalysis has offered a new opportunity to solve the problem of the limited efficiency of photocatalysts [71–73]. In these photocatalysts, the nanostructured plasmonic metals are often combined with a semiconductor-based material (e.g., TiO2), and the photo‐ catalytic activity is greatly enhanced due to the local surface plasmon resonance (LSPR) effect

the induced electronic states of those dopants and reconstructing favorable surface structure

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‐

Graphene, a two-dimensional carbonaceous material, can be used in many applications due to its unique and remarkable properties such as high conductivity, large surface area, and good chemical stability [56–59]. Tremendous interest is devoted to fabricating numerous graphene–semiconductor composites to aid charge separation and migration and improve the performance of the photocatalysts [60–64]. Graphene works as an electron acceptor or transporter to induce electron transfer, leading to an efficient charge separation. Thus, an appropriate integration of graphene and TiO2 could give rise to a nanocomposite that combines the desirable properties of graphene and TiO2, e.g., the photocatalytic activity of TiO2 can be improved. In the past few years, there were some reports about graphene–

Recently, Liu et al. synthesized the graphene oxide–TiO2 nanorod composites [65, 66]. Wu et al. reported the synthesis and application of graphene–TiO2 nanorod hybrid nanostructures in microcapacitors [67]. As shown in Fig. 3, the assembling of TiO2 nanocrystalline with exposed {001} facets on graphene sheets reported in our previous work showed a higher photocatalytic activity than the other normal TiO2/graphene composites [68]. In another work (Fig. 4), graphene/rod-shaped TiO2 nanocomposite was synthesized by the solvothermal method [69]. In a one-pot system, the rod-shaped TiO2 can be homogeneously dispersed on the surface of graphene sheets by syngraphenization strategy. Owing to the combination of graphene and rod-shaped TiO2, the graphene/rod-shaped TiO2 nanocomposite shows a significant enhancement in the photocatalytic performance compared with that of the gra‐ phene/spherical TiO2 nanocomposite, which can be attributed to the high electronic mobility of graphene, higher Brunauer-Emmett-Teller (BET) surface area, and rod-shaped structures of TiO2. In our recent work [70], a series of B-doped graphene/rod-shaped TiO2 nanocomposites were synthesized via one-step hydrothermal reaction. The photocatalytic activity of the obtained nanocomposites for the oxidative photodestruction of NO*x* gas showed better photocatalytic properties than pure TiO2 and graphene/TiO2 nanocomposites. This work provides new insight into the fabrication of TiO2–carbon nanocomposites as high-performance photocatalysts and facilitates their application in addressing environmental protection issues.

Plasmonic photocatalysis has offered a new opportunity to solve the problem of the limited efficiency of photocatalysts [71–73]. In these photocatalysts, the nanostructured plasmonic metals are often combined with a semiconductor-based material (e.g., TiO2), and the photo‐ catalytic activity is greatly enhanced due to the local surface plasmon resonance (LSPR) effect

for photocatalysis.

excited electron–hole pairs [41].

340 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**2.2. TiO2-graphene composite**

TiO2 composites [13–17].

**2.3. TiO2-based plasmonic photocatalysts**

**Figure 3.** Scanning electron micrograph (SEM) and transmission electron micrograph (TEM) of (a1 and a3) GO and (a2 and a4) GS; (b1 and b2) TEM images of TiO2/GS with different Ti:C ratios; (b3 and b4) high-resolution transmission electron microscopic (HRTEM) images of sample b1; (c) photocatalytic degradation of MB and MO under the irradia‐ tion of UV light and visible light over the TiO2/GS composites; (d) a proposed schematic illustration showing the reac‐ tion mechanism for photocatalytic degradation of organic pollutants over the TiO2/GS composites. Reproduced with permission from ref. 68 © 2012 RSC.

[74]. The LSPR effect endows the metal nanocrystals with very large absorption and scattering cross sections and local electromagnetic field enhancement in the near-field region near the surface of plasmonic metal nanocrystals, which is promising in manipulating light absorption in photocatalytic systems. The intensity of the local electromagnetic field is several orders of magnitude larger than that of the far-field incident light, and the highest charge carrier formation is observed at the semiconductor/liquid interface, which benefits the photocatalytic reactions [71–72].

Recently, noble metal nanoparticle-deposited TiO2 has attracted significant attention. The main advantages of these noble metal NPs can be attributed to their chemically inert properties

**Figure 4.** TEM images of (a1) graphene oxide, (a2) graphene, (a3) 0.48% GR/RT, and (a4) HRTEM image of a single rod-shaped TiO2 nanocrystal in the composite; (b) photocatalytic degradation of MO solution over graphene/rod-shap‐ ed TiO2 nanocomposites with various graphene contents compared with that of GR/ST; (c1) SEM image, (c2) TEM im‐ age, (c3) EDX, and (c4) HRTEM image of 7.5% BG/RT; (d) photocatalytic activity for the destruction of NO*x* gas under UV irradiation using different photocatalysts. Reproduced with permission from ref. 69 & ref. 70 © 2012 & 2014 RSC.

toward (photo) oxidation and LSPR effect on the surface [75, 76]. Meanwhile, our previous work found the plasmonic enhancement of the photocatalytic activity of semiconductor–metal nanocomposite materials [77–79]. Thus, noble metal NP-deposited TiO2 could be an appro‐ priate approach to improve the photocatalytic performance of TiO2.

A variety of nanostructured Au/TiO2 with different morphologies have been synthesized, such as highly stable mesoporous Au/TiO2 spheres (~500 nm) [80] and mesoporous Au–TiO2 nanocomposites using a simple spray hydrolytic method [81]. In our study, Au NPs are precipitated on the highly porous one-dimensional (1D) TiO2 nanotubes (NTs), and the plasmonic photocatalytic properties of the material are investigated [82]. Compared with nanoparticles, there are some advantages of 1D NT structures, such as favorable recycling characteristics and the vectorial transport of photogenerated charge carriers [83, 84], which have great potential for superior photocatalytic performance. The Au NPs/TiO2 NTs were synthesized by emulsion electrospinning followed by deposition–precipitation (DP) method. The results in Fig. 5 show that the modified porous TiO2 NTs with the presence of Au NPs increased photocatalytic destruction of methylene blue (MB) solution under visible-light irradiation. Furthermore, the migration of Au NPs from the rutile phase to the interface of rutile/anatase was found when the calcination temperature changed from 250 °C to 350 °C. The optimal photocatalytic activity was obtained in the sample Au3(DP350)/TiO2, due to the plasmon activation of the Au NPs followed by consecutive electron transfer that induced efficient charge separation. Therefore, such a highly porous Au/TiO2 heterojunction structure provides a new pathway for the design and fabrication of other energy- and environmentrelated applications.

toward (photo) oxidation and LSPR effect on the surface [75, 76]. Meanwhile, our previous work found the plasmonic enhancement of the photocatalytic activity of semiconductor–metal nanocomposite materials [77–79]. Thus, noble metal NP-deposited TiO2 could be an appro‐

**Figure 4.** TEM images of (a1) graphene oxide, (a2) graphene, (a3) 0.48% GR/RT, and (a4) HRTEM image of a single rod-shaped TiO2 nanocrystal in the composite; (b) photocatalytic degradation of MO solution over graphene/rod-shap‐ ed TiO2 nanocomposites with various graphene contents compared with that of GR/ST; (c1) SEM image, (c2) TEM im‐ age, (c3) EDX, and (c4) HRTEM image of 7.5% BG/RT; (d) photocatalytic activity for the destruction of NO*x* gas under UV irradiation using different photocatalysts. Reproduced with permission from ref. 69 & ref. 70 © 2012 & 2014 RSC.

A variety of nanostructured Au/TiO2 with different morphologies have been synthesized, such as highly stable mesoporous Au/TiO2 spheres (~500 nm) [80] and mesoporous Au–TiO2 nanocomposites using a simple spray hydrolytic method [81]. In our study, Au NPs are precipitated on the highly porous one-dimensional (1D) TiO2 nanotubes (NTs), and the plasmonic photocatalytic properties of the material are investigated [82]. Compared with nanoparticles, there are some advantages of 1D NT structures, such as favorable recycling characteristics and the vectorial transport of photogenerated charge carriers [83, 84], which have great potential for superior photocatalytic performance. The Au NPs/TiO2 NTs were synthesized by emulsion electrospinning followed by deposition–precipitation (DP) method. The results in Fig. 5 show that the modified porous TiO2 NTs with the presence of Au NPs increased photocatalytic destruction of methylene blue (MB) solution under visible-light irradiation. Furthermore, the migration of Au NPs from the rutile phase to the interface of

priate approach to improve the photocatalytic performance of TiO2.

342 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

**Figure 5.** (a) SEM and (b) TEM images of as-prepared TiO2 NTs and (c) SEM and (d) TEM images of the representative sample Au3(DP350)/TiO2 NTs; TEM images of Au3(DPy)/TiO2 (*y* = 150 (e11), 250 (e12), 350 (e13), and 450 °C (e14), respec‐ tively) and the dispersion of Au co-catalysts (e21), (e22), (e23), and (e24), respectively; (f1) effect of calcination tempera‐ ture on the location and size of the Au NPs on TiO2 NTs; (f2 and f3) TEM and HRTEM images of Au3(DP350)/TiO2. Variation of normalized C/C0 of MB concentration as a function of visible-light irradiation time for (g1) Aux(DP350)/ TiOy and (g2) Au3(DPy)/TiO2 (*y* = 150, 250, 350, 450 °C, respectively); (g3) schematic diagram for the possible mecha‐ nism for photocatalytic degradation of MB over Au3(DP350)/TiO2 under visible-light irradiation. Reproduced with per‐ mission from ref. 82 © 2013 RSC.
