Hybrid nanostructures based on titanium dioxide for enhanced photocatalysis

Applied Catalysis A: General 489 (2015) 1–16

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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata

Hybrid nanostructures based on titanium dioxide for enhanced photocatalysis Kakarla Raghava Reddy, Mahbub Hassan, Vincent G. Gomes ∗ School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney, NSW 2006, Australia

a r t i c l e

i n f o

Article history: Received 19 June 2014 Received in revised form 11 September 2014 Accepted 3 October 2014

a b s t r a c t The design and development of efficient TiO2 -based, hybrid, nanostructured photocatalysts has recently been receiving substantial attention for environmental remediation due to their excellent physiochemical properties. This article provides an overview of the synthesis strategies and characteristics of the nextgeneration TiO2 -based hybrid photocatalysts, produced in combination with polymers (e.g., polyaniline, polypyrrole, polythiophene) and carbon nanomaterials (e.g., graphene, GO, CNT, carbon quantum dots, carbon nitride). The structural aspects, nanostructure formation process, parameters affecting catalytic activity, photocatalytic mechanisms and photocatalytic applications of TiO2 -based catalysts for efficient photocatalytic degradation of gaseous/volatile organic pollutants in water/air are reviewed. Further, current research trends, means to increase catalytic performance and future prospects of high-performance TiO2 -based hybrid photocatalytic materials, are briefly summarized. © 2014 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . TiO2 -based composite systems as photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. TiO2 /polymer nanohybrid composite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. TiO2 /poly(o-phenylenediamine)-based composite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. TiO2 /poly(thiophene)-based composite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. TiO2 /poly(aniline)-based composite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.4. TiO2 /poly(pyrrole)-based composite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.5. TiO2 /aniline-pyrrole copolymer-based composite catalysts and others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. TiO2 /carbon material-based hybrid composite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. TiO2 /carbon nanotubes-based composite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. TiO2 /graphene oxide-based composite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. TiO2 /graphene-based composite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4. TiO2 /carbon and graphene quantum dots-based composite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5. TiO2 /carbon nitride-based composite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.6. TiO2 /amorphous nanocarbon-based composite catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 2 2 2 3 4 6 7 8 8 9 10 11 12 13

Abbreviations: AO7, Acid orange 7; BDE209, decabrodiphenyl ether; CQDs, carbon quantum dots; CNFs, carbon nanofibers; CNTs, carbon nanotubes; CPs, conjugated polymers; 2-CP, 2-chlorophenol; CQDs, carbon quantum dots; CTAB, cetyltrimethylammonium bromide; FAC, fly ash cenosphere; F-TiO2 , fluorinated TiO2 ; GF, graphene film; GO, graphite oxide; rGO, reduced graphite oxide; Gr, graphene; GQDs, graphene quantum dots; g-C3 N4 , graphitic carbon nitride; IP-TiO2 , interpenetrating anatase TiO2 tablets; EDTA, disodium ethylenediamine tetraacetate; LBL, layer-by-layer; MB, methylene blue; MWCNTs, multiwall carbon nanotubes; MO, methyl orange; Ms, magnetic saturation; 4NPh, 4-nitrophenols; NPs, nanoparticles; NS, nanosheet; NRA, nanorod array; NSA, nanosheet array; N-RGO, N-doped reduced graphene; NT, nanotube; NTA, nanotube arrays; OA, oxalic acid; OL-TiO2 , overlapping anatase TiO2 nanosheets; PANI, poly(aniline); PEC, photoelectrocatalytic; PPy, poly(pyrrole); PPy-co-PANI, poly(pyrrole)-copoly(aniline) copolymer; PVP, poly(vinylpyrrolidone); P3HT, poly(3-hexylthiophene); PoPD, poly(o-phenyleneediamine); Poly(EA-co-MMA), poly(ethyl acrylate-co-methyl methacrylate); RGO, reduced graphite oxide; RhB, rhodamine; SAM, self-assembled monolayers; SRB, sulforhodamine B; SWCNTs, single-walled carbon nanotubes; TBA, tert-butyl alcohol; TCN, titanium carbon nitride; TCP, 2,4,6-trichlorophenol; TPC, temperature-programmed carbonization. ∗ Corresponding author. Tel.: +61 2 9351 4868; fax: +61 2 9351 2854. E-mail address: [email protected] (V.G. Gomes). http://dx.doi.org/10.1016/j.apcata.2014.10.001 0926-860X/© 2014 Elsevier B.V. All rights reserved.

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Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Semiconducting materials are invaluable in various photoinduced applications: decomposition of organic pollutants, generation of hydrogen gas by splitting water, conversion of CO2 , and production of coatings, gas sensors, catalysts, electronic devices (solar cells, laser diodes, photodetectors), and energy-harvesting devices (nanogenerators) [1–5]. Titanium dioxide (TiO2 ), a key semiconductor with a wide band gap, offers high exciton binding energy, tunable crystal structure (rutile, anatase, brookite), and effective photocatalytic activity [6,7]. The band gaps of these three TiO2 phases are 3.0, 3.2, and 3.25 eV, respectively. Among its useful attributes, TiO2 is: (i) insoluble in aqueous media, (ii) chemically and biologically inert, (iii) photostable, (iv) nontoxic, (v) inexpensive, and (vi) readily available [6,8–16]. Compared to bulk forms (single crystals and thin films) TiO2 nanoparticles show exceptional properties and can be prepared in a range of morphologies for targeted applications [17–29]. In particular, nanostructured TiO2 has been proving to be an efficient photocatalyst for selective elimination of environmental pollutants [6,7,30–38]. However, it responds primarily to UV light which only accounts for less than 5% of total solar radiation. Under visible light, the catalytic activity is limited by low electron transfer and high electron/pair recombination rates. For technoeconomic reasons, it is important that visible-light-driven photocatalysts with high efficiency developed. TiO2 is currently synthesized as nanoparticles, core shells, nanotubes, nanorods, nanofibers, nanocubes, and porous spheres using relatively low temperatures and inexpensive methods [2,39–48]. To overcome the poor photocatalytic performance under solar light, TiO2 is a prime candidate for fruitful modifications and functionalization to enhance photocatalytic activity. Organic dyes are among the largest group of harmful pollutants discharged into wastewater streams from industries [49–56]. During dye manufacturing and application, wastewater is typically contaminated by more than 15 wt% dye content. Thus, low-cost, efficient, and robust techniques are desirable to remove dyes from contaminated water prior to their discharge and to produce clean water on a large scale. Photocatalysts such as TiO2 are being employed to mineralize and thereby decontaminate dyes in wastewater due to their low cost, non-toxicity, and stability. For TiO2 -hybrid nanomaterials, the aim is to enhance quantum efficiency, environmental stability under variable pH conditions, and harness visible light to degrade organic pollutants. The aim of this review is to present recent advances with TiO2 –polymerand TiO2 -carbon-based hybrid nanostructures, their design, synthesis, properties, and corresponding photocatalytic applications. Reaction mechanisms will be discussed to explain the effects of synthesis and properties of key TiO2 -hybrids on photocatalysis. Finally, the future outlook of such nanostructured catalysts will be discussed. 2. TiO2 -based composite systems as photocatalysts The catalytic activity of nanostructured TiO2 is tunable by varying their size, shape, composition, and support material [57–61]. Their high photocatalytic activity mainly derives from the band-edge position, band gaps, and charge-carrier mobility. The four key steps underpinning photocatalysis are: (i) chargecarrier generation, (ii) charge-carrier separation, (iii) charge-carrier

13 14 14

recombination, and (iv) degradation of organic pollutants into small molecules. For single-component TiO2 , intrinsic features such as wide band gap and photo-induced corrosion limit their large-scale application. Thus, hybrid photocatalysts, with one or more active components and functional support are recommended [43,45,60,62–76]. Some improvements in photocatalytic performance by doping with metals and non-metals have been reported in the literature [77–82]. Such hybrid photocatalysts integrate the synergistic effects between the individual components for increased light harvesting, prolonged lifetimes, enhanced photocatalytic performance as well as higher chemical and environmental stability. TiO2 -based hybrid photocatalysts can be classified into two main categories according to the type of species added: polymers and carbon nanomaterials (Tables 1 and 2). 2.1. TiO2 /polymer nanohybrid composite catalysts Recent reports show that conjugated polymers (CPs) have outstanding electrochemical properties with excellent electron mobility compared with ceramic and metal oxide nanomaterials [83–88]. CPs can improve electrical conductivity, corrosion resistance, environmental stability, solar energy transfer, and photocatalytic activity of TiO2 and other semiconducting materials [89–93]. They also improve environmental stability and photo-corrosion resistance. Such nanostructured polymers are efficient electron donors, electron transporters, and are suitable as stable photosensitizers to modify inorganic semiconductors such as TiO2 to fabricate optical, electronic, and photo-conversion devices. Photo-sensitizers in composites greatly improve the charge separation efficiency at the inorganic (TiO2 ) and organic (polymer) interface and provide synergetic interactions between polymer and TiO2 . Recently, a variety of TiO2 –polymer-based nano-/microstructures, such as 0D (zero-dimensional) nanospheres/particles, 1D nanofibers/wires, 2D nanosheets, and 3D hierarchical architectures have been synthesized to maximize photoactivity. The key productive composites shown in Table 1 are briefly discussed. 2.1.1. TiO2 /poly(o-phenylenediamine)-based composite catalysts Muthirulan et al. [94] reported that core-shell TiO2 -poly(ophenylenediamine) (PoPD) composites with 1.5 wt% TiO2 nanoparticles (NPs) prepared via in-situ chemical oxidative polymerization method shows much higher photocatalytic activity for the degradation of RhB than that of pure TiO2 NPs in the presence of solar light. These particles degraded RhB via N-deethylation process into small molecules such as CO2 , H2 O, NO3 − , and NH4 + , as PoPD transport holes and oxidized the absorbed RhB molecules efficiently. This process enhances charge separation and promotes photocatalytic activity of the composite catalyst. Another TiO2 composite modified by aromatic polyamide dendrimer showed improved photocatalytic efficiency in degrading phenol compared to pure TiO2 through the formation of donor–acceptor type p-conjugated complexes stimulated by visible light [95]. Further, the activity of these particles for degrading phenol was increased from 66.8% (pH 7) to 81.6 wt% with acid treatment (pH 3) due to transformation of spirolactam (switch off, closed ring structure) to a ring-opened amide form (switch on). The closed ring form of hybrid catalyst was tested for stability by conducting catalytic experiments for eight successive cycles. It was found that the phenol degradation rate was still 50%, indicating that they have good photocatalytic stability under solar light irradiation.

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Table 1 Effect of TiO2 /polymer composite on photocatalytic degradation of organic pollutants. Polymer with TiO2

Operating condition

Pollutants

Degradation rate

Reference

PPy

90 min sunlight irradiation 3 h natural light irradiation 80 min visible-light irradiation 3 h visible-light irradiation 2 h UV-light irradiation 20 min visible-light irradiation

MB

>90%

[107]

MO MB

>80% 99%

[115] [116]

Gaseous acetone MO

99%

[117]

>90%

[118]

99% 60% RhB

[119] [94]

99% >60% >90%

[96] [6] [120]

99%

[105]

RhB

96% 90%

[121] [103]

MB 4-CP MO

>80% 82.4% 96%

[122] [104] [98]

MO

88.5%

[123]

70% 96.4% 80.3% 66.8% phenol degraded at pH 7 70%

[99] [124] [124] [95]

RhB PoPD

60 min solar light irradiation 240 min visible light irradiation

PANI

200 min UV-light irradiation 1 h UV light irradiation

2 h visible-light irradiation 6 h white-light illumination 50 min visible-light irradiation 320 min winter-sunlight 7 h visible light

P3HT Poly(thiophene)

60 min visible light irradiation 10 h visible light irradiation 180 min visible light irradiation 150 min UV light irradiation 10 h visible light irradiation

RhB MB MO Organic vapors (Benzene, toluene, ethyl benzene, and xylene) RhB RhB

Aromatic poly(amide) dendrimer

8 h visible light irradiation

MO MO MO Phenol

Hyperbranched polyester

40 min UV-light irradiation

Phenol

TiO2 -PoPD nanohybrid particles with high activity and ease in separation were synthesized using UV light photoinitiating method, without any external oxidants, surfactants, or organic dopants [96]. In this method, hydroxyl radicals generated in the reaction system by TiO2 under the UV light irradiation induced polymerization of oPD monomer molecules on the surface of TiO2 particles leading to the formation of TiO2 -PoPD composite having core-shell structure for degrading methylene blue (MB) under visible light irradiation. These particles also showed good photocatalytic stability at low acidic conditions after several runs. 2.1.2. TiO2 /poly(thiophene)-based composite catalysts Zhu et al. [97] reported the synthesis of poly(3-hexylthiophene) (P3HT) through chemical oxidative polymerization using iron chloride as oxidant in chloroform solvent. TiO2 -P3HT composite particles were prepared by blending the polymer and filler. UV–vis DRS measurements confirmed that composite particles have a broad and strong absorption in the visible range, indicating that the formation of a polymer layer on TiO2 surface can extend the photoresponse of TiO2 particles. In the presence of both UV and visible light, the composite particles showed better catalytic activity in removing dye molecules than pure TiO2 particles. The chromophoric groups of methyl orange (MO) molecules were dominantly cleaved under UV light; while, with visible irradiation, competitive photodegradation reactions occurred between the chromophoric group intermediates and MO. After 2.5 h of UV irradiation, composite particles prepared with 10 wt% P3HT degraded 88.8 wt% of dye molecules, which was greater than that of pure TiO2 (65 wt%). This is due to high electron mobility of ␲conjugated P3HT which facilitate the separation of electron–hole pairs generated under UV irradiation and improve the photocatalytic activity of TiO2 . During the irradiation process, electrons were

[125]

excited for transfer from the valence band to conduction band with generation of electron–hole pairs. The separated electrons and holes can migrate to the interface of polymer and TiO2 and transfer to P3HT. The transfer process is more efficient due to the intrinsic conjugated structure of P3HT and the preadsorption of P3HT on TiO2 surface. The separated holes and electrons were captured by H2 O and O2 dissolved in water to generate hydroxyl and superoxide radicals which attack the azo-bond of dye molecules and oxidize them to the mineral end products resulting in complete degradation of the dye molecules. This stems from the strong oxidizing nature of hydroxyl radicals (standard redox potential of +2.8 V). Another nanohybrid catalyst with different oxidation degree of P3HT was investigated for the influence of degree of oxidation of P3HT on the photocatalytic activity of TiO2 [98] under visible light. It was found that moderate oxidation degree of conjugated polymer is beneficial for removing dye molecules from solution under visible light (>450 nm). Huang et al. [99] prepared TiO2 -P3HT nanohybrid catalyst (Fig. 1) by graft polymerization of thiophene monomer molecules on the surface of 2-thiophenecarboxylic acid-modified TiO2 as self-assembled monolayers (SAM) and studied the effects of SAM interface between the TiO2 NPs and P3HT polymer on the photocatalytic ability of composite for the degradation of MO under visible light. Polymerization of 3-hexylthiophene monomer was carried out on the surface of modified TiO2 in chloroform solvent using FeCl3 as an oxidizing agent in nitrogen atmosphere. The catalytic activity of nanohybrid TiO2 /P3HT catalysts was found to be 2.7 times greater than that of pure TiO2 and 1.6 times that of unmodified TiO2 -P3HT composite prepared through chemical oxidative polymerization of 3HT in the presence of as-received TiO2 particles without SAM pretreatment. This was attributed to decrease in barrier for charge transfer between P3HT photosensitizer and TiO2

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Table 2 TiO2 -carbon nanomaterial-based photocatalysts for treatment of various pollutants. Photocatalyst TiO2 /CNTs

Preparation method Hydrolysis

TiO2 /SWCNTs

Hydration/dehydration

TiO2 /CNFs TiO2 NSA/CF

Sol-gel/heat treatment

3D TiO2 /GO TiO2 /RGO

Electrospinning/carbonization Sonication/heating Hydrothermal process Solvothermal process

Light source 3 h UV-light irradiation 2 h Solar light-irradiation (␭ > 350 nm) 20 min UV light (3.39 eV) 1 h UV-light irradiation 60 min Visible light 80 min visible-light irradiation 50 min UV-light irradiation 50 min UV-light 20 min UV-light irradiation 40 h visible light 7 h Visible light

Pollutant degraded 68.6% OA and 41.4% Phenol 99% Caffeine

12 h UV-light irradiation 1 h Visible light irradiation

99.9% MO

Reference [126] [126] [43] [31]

Acetaldehyde 85% MB

[46] [173]

88% MB 98% MO

[128] [150]

99.9% MO >90% RhB and AO7

[174] [175]

TiO2 /graphene TiO2 NTA/GF

TiO2 NRA/graphene TiO2 /N-RGO

In-situ assembly Self-assembly at the water/toluene interface Hydrolysis In-situ growth/oxidation UV-assisted photocatalytic reduction method Solvothermal synthesis

TiO2 /N-doped carbonaceous composite Spray pyrolysis Solvo/hydrothermal process Solvothermal Sonication/hydrothermal One step solvothermal Spark plasma sintering Hydrothermal

1 h solar light 30 min UV-light 1 h UV-light 1 h visible light 80 min UV-light 250 min UV-light irradiation 60 min sun light 90 min UV-light 10 min UV-light 2 h visible-light irradiation

Gaseous butane 90% MB

[176] [177]

72% BDE209 [178] 99% RhB [179] 98% MB 95% MB 80% MO 99.8% MO 99% MB 35% MB 75% MO

Calcination Hydrothermal process

2.5 h Vis light 5 h UV light 6 h Visible light

91% RhB nearly 100% MB

Anodization/electrodeposition

40 min Visible-light irradiation

65.9% MB

[180] [181] [182] [183] [184] [185] [186] [187] [152] [145]

TiO2 /amorphous carbon

One step annealing LBL assembly Hummers/anodization

80% MB 70% MB 70% MB and 60% 2-CP

Low temperature hydrothermal-calcination

Escherichia coli K-12

High temperature hydrothermal method Liquid phase deposition method Electrospinning

60 min UV-light irradiation 60 min UV-light irradiation 12 h UV-light irradiation 2 h UV-light irradiation

Pyrolysis

TiO2 nanosheet/carbon quantum dots TiO2 /C60 TiO2 /g-C3 N4

>90% diphenhydramine 99% RhB 60% methythionine chloride

[191] [192]

[193] [194]

Low temperature calcination

60 min visible-light irradiation

98% RhB

[195] [196]

Hydrothermal method Solvent evaporation method

Sodium lamp (400 W) with UV cutoff filter 25 min a high-pressure mercury lamp (100 W)–UV light 120 min 300 W UV–vis light Visible light (50 W Halogen lamp/400 nm cut-off) 410 LED light Visible-light irradiation 5 h Visible-light with 108 W lamp (␭>420 nm) 1 h Visible-light with 1000 W Xenon lamp (a cut-off filter at 420 nm)

76% TCP 100% MB

[129] [169]

85% phenol 68%MB

[166] [171]

89%MB 90% MO 56.3% SRB

[170] [168] [197]

87.8% TCP and 94.3% MO

[198]

Impregnation method Direct heating in the presence of Ar gas Hydrothermal method Calcination Exfoliation/Calcination TiO2 /porous carbon fibers

98% RhB

[188] [189] [190]

Temperature-programmed carbonization (TPC)

due to SAM, and the flow of electrons in the valence band of TiO2 to ␲-orbital of P3HT. 2.1.3. TiO2 /poly(aniline)-based composite catalysts Reddy et al. [100] used an electrospinning technique for fabricating 1D TiO2 nanofibers. In this process, an ethanol solution containing polymer solution [poly(vinylpyrrolidone), PVP] and

titanium precursor were electrospun by exposure to an electric field. At a critical voltage, the electric field overcomes the surface tension of the mixed solution, a jet is extruded and electrospun fibers are collected as nanowoven mats. These mats were calcined at 500 ◦ C to produce high-quality 1D TiO2 nanofibers aligned in a regular orientation (Fig. 2a). The average diameter of these nanofibers was 30–50 nm, and the length up to several microns. The

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Fig. 1. Synthesis of TiO2 -P3HT nanohybrid catalyst in the presence of SAM-modified TiO2 . Reproduced with permission from publisher [99].

Fig. 2. FE-SEM images of (a) electrospun TiO2 nanofiber and (b) TiO2 -PANI nanocable catalyst. Reproduced with permission from publisher [100].

synthesis of 1D core-sheath nanocable hybrid catalyst [100] (Fig. 2b) by uniformly coating electrospun TiO2 nanofibers with 1D PANI without any external agents such as surfactants and organic dopants was described. Superior catalytic activity compared to pure TiO2 nanofibers was determined for removing MO in aqueous solution under UV light. This was attributed to PANI acting as an efficient electron donor, stimulation by UV light, and transfer of electrons generated from PANI to TiO2 conduction band. If the electrons are trapped at the cable-structured composite interface, the efficient charge separation of photo-created electron and hole pair occurs. In this process, the reactive electrons reduce oxygen adsorbed on the surface of the composite to • O− 2 , which further transform into H2 O2 and • OH, resulting in the oxidation of dye molecules into small molecules such as H2 O and CO2 , thereby enhancing the photocatalytic activity of fibrous nanocomposites. Nanostructured anatase TiO2 -PANI (3:1) composite, prepared via in-situ deposition polymerization, showed good photocatalytic activity [84]. The weight of composites decreased with irradiation time in air with 6.8% weight loss during 60 h irradiation. With addition of TiO2 particles, higher absorption in UV region and faster photocatalytic degradation rates were achieved due to the sensitizing effect of PANI [101]. The weight loss of the composite particles was due to volatilization in air during irradiation. Nanopapilla-structured TiO2 -PANI composites, prepared via hydrothermal method in the presence of PANI provided rate constant (␬) values 2.5 times greater with UV light, and 8 times with visible-light irradiation than that of pure TiO2 while degrading gaseous pollutants (acetone) [102]. Gu and co-workers [103] reported that the catalytic activity (under both UV and visible light irradiation) of TiO2 nanorods-coated 1D PANI fibrous catalysts (b) prepared by hydrothermal method is much higher than that of TiO2 -PANI aggregated composite particles (a) prepared via conventional method (Fig. 3). Similarly, TiO2 -PANI nanohybrid catalyst prepared by hydrothermal method showed that the rate constant of photocatalytic degradation of RhB and 4-CP under visible light was 5.01 and 2.03 times higher than that of the mixed TiO2 -PANI composites prepared by conventional method [104].

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Fig. 3. Encapsulation of nano-TiO2 in PANI via (a) conventional approach and (b) hydrothermal method. Reproduced with permission from publisher [103].

Li et al. [105] prepared a catalyst by sensitizing anodic TiO2 nanotube (NT) arrays with PANI through electrodeposition method. The catalyst was used as a visible-light ( > 400 nm)driven photocatalyst for the degradation of RhB. The composite NT electrode-degraded RhB 100% in 120 min in the presence of H2 O2 (Fig. 4a), and the kinetic constant for the composite electrode (4.01 × 10−2 min−1 ) was higher than that of the pure TiO2 NT electrode (9.3 × 10−4 min−1 ). This has been attributed to the alignment of energy bands between PANI and TiO2 NT for efficient generation and separation of photoinduced charge under visible light. Extended absorption in the visible-light region by PANI and effective separation of photogenerated carriers driven by photoinduced potential difference occurred at TiO2 /PANI NT interface (Fig. 4b). In addition, H2 O2 performed as an efficient electron scavenger and a source of oxidative hydroxyl radicals generated by photoelectrochemical decomposition of H2 O2 under visible light, thereby preventing recombination of electron–hole pairs and leading to faster RhB degradation.

2.1.4. TiO2 /poly(pyrrole)-based composite catalysts Deng et al. [106] reported preparation of TiO2 /poly(pyrrole) (PPy) hybrid catalyst by surface molecular imprinting technique (Fig. 5) using organic dye (MO) molecules as a soft template. They achieved narrower band gap for the composite catalyst than pure TiO2 and enhanced activity for molecularly imprinted TiO2 /PPy catalyst for removing MO from wastewater under stimulated solar radiation. Molecularly imprinted PPy possesses specific recognition sites for template molecules, has strong affinity towards target contaminants, and is unaffected by problems due to mass transfer. The activity of these special composite catalysts is two times higher than that of in-situ TiO2 /PPy composite catalysts prepared in the absence of template dye molecules. Similarly, new hierarchically structured porous TiO2 /PPy composites were synthesized by polymerization of pyrrole vapor in TiO2 macropores. These catalysts showed higher catalytic efficiency for the degradation of MB under sunlight compared to pure TiO2 and mixed composites prepared by physical blending of PPy and TiO2 particles [107]. To understand the

Fig. 4. (a) Photoelectrochemical (PEC) degradation of RhB in the presence of different electrodes under visible-light irradiation; (b) mechanism for the decomposition of RhB on PANI/TiO2 nanotube electrode under visible-light irradiation. Reproduced with permission from publisher [105].

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Fig. 5. Preparation of TiO2 -PPy composite catalyst via molecular imprinting. Reproduced with permission from publisher [106].

role of PPy in the porous composite catalysts, further studies were conducted on electron and hole trapping by adding oxidizing agent tert-butyl alcohol (TBA) or disodium ethylenediamine tetraacetate (EDTA) to the dye solution. They found that the activity of composites over MB was slightly decreased in the presence of EDTA (hole scavenger), and the activity was greatly reduced with TBA (an efficient scavenger). The decomposition rate of pollutant under sunlight depends on the porous structure of catalyst and photogenerated electrons in porous composites during irradiation [108]. TiO2 -PPy composite catalysts prepared by polymerization of pyrrole with FeCl3 as an initiator and poly(sodium 4-styrenesulfonate) or PSS as organic dopant, and fly ash cenosphere (FAC) as substrate, showed that the extent of degradation of organic pollutants (phenol and MB) is only about 70% even after four successive cycles under irradiation [109]. Another PPy-based catalyst, a belt-structured TiO2 , was prepared by sol–gel technique using cotton absorbent as a template. This was followed by calcination and coating with PPy by polymerization of Py monomer molecules on the surface of TiO2 nanobelt. This structured TiO2 -PPy catalyst degraded over 95 wt% MO dye molecules under UV light [110,111]. The composite catalyst TiO2 -PPy prepared in different forms (films and particles) by combining pure rutile TiO2 and PPy through in situ chemical polymerization showed good catalytic activity compared to pure rutile TiO2 [112].

2.1.5. TiO2 /aniline-pyrrole copolymer-based composite catalysts and others Synthesis of conductive TiO2 /PPy-co-PANI composite catalysts has been performed via copolymerization of aniline and pyrrole (Py) in the presence of TiO2 NPs [113]. The photocatalytic activities of pristine TiO2 NPs and TiO2 /PPy-PANI catalysts were evaluated by comparing their rate constants. The introduction of a small amount of copolymer to TiO2 NPs enhanced photocatalytic activity under visible light for degrading 4-nitrophenols (4NPh) compared to pure TiO2 , TiO2 /PANI, and TiO2 /PPy. With increasing molar proportion of comonomers, the degradation rate of NPh increased and then decreased. The highest degradation rate occurred (10 mg/L 4NPh) at an optimized pyrrole:aniline:TiO2 molar ratio of 0.75:0.25:100 due to the conjugated structure and synergistic effect of TiO2 with copolymers. The penetration of light decreased with increase in 4NP concentration, leading to lower number of active sites. The superior photocatalytic performance of TiO2 /PPy-PANI is due to its narrower band gap energy (2.97 eV) than that of the mixed-phase TiO2 (3.10 eV) as depicted in Fig. 6. An enhancement of photoelectrochemical properties of a composite (current density of 25.7 ␮A cm−2 ) by combining TiO2 with PPy-PANI copolymer was observed under visible light (Fig. 7). Peng et al. [114] prepared polymer nanohybrid photocatalyst by chemically grafting conjugated polymer structures onto the surface of TiO2 nanoparticles through controlled thermal degradation

Fig. 6. Mechanism of visible-light photocatalysis for TiO2 /PPy-PANI nanohyrbid. Reproduced with permission from publisher [113].

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Fig. 7. Photocurrent transient response of TiO2 catalysts under visible-light irradiation. Reproduced with permission from publisher [113].

of coacervated polymer layer. The composite catalyst degraded phenol and MO by >90 wt% under visible light with 500 W halogen lamp ( > 450 nm using a cutoff filter). In composite catalysts, the conjugated polymer chains act as electron donors, while TiO2 nanoparticles act as electron acceptors. The interfacial covalent C–O–Ti bonds formed between TiO2 and conjugated structures perform as pathways to transfer excited electrons rapidly from conjugated polymer structures to TiO2 , thereby contributing to high photocatalytic efficiency under visible light. 2.2. TiO2 /carbon material-based hybrid composite catalysts TiO2 composites with carbon materials as supporting material are intriguing for their interfacial heterostructures, which display unique structural variability that affect their photocatalytic performance. With TiO2 /nanocarbon hybrid catalysts, photoinduced electrons and holes are readily separated between the two semiconductors via effective interfacial charge transfer, thereby boosting photocatalytic performance. Recently, various composites of titanium precursors [43,46,126–129] with carbon materials (e.g. fullerene, CNTs, GO, graphene, graphene/carbon quantum dots, carbon nitride, etc.) were converted into nanostructured titanium dioxide to form TiO2 /carbon nanocomposites. The photocatalytic properties of TiO2 -carbon nanohybrids depend strongly on the size, morphology, preparation conditions, and chemical composition of the hybrids (Table 2). 2.2.1. TiO2 /carbon nanotubes-based composite catalysts Modification of nanostructured TiO2 with carbon nanomaterials enhances their charge separation efficiency and sunlight absorption. For example, TiO2 /CNT composites, prepared via CVD process [128] with flow of ethene over Ni(NO3 )2 /TiO2 substrate in a horizontal quartz tube, demonstrated that the stability and activity of nanohybrids are tunable by changing the CNT content in composites. After loading 2 wt% Ru-based co-catalyst on the surface of TiO2 /CNT nanohybrids, the conversion rate of benzyl alcohol to benzyl aldehyde increased by 95% with an increase in CNT content. The results were superior compared to Ru/TiO2 and Ru/CNT catalysts. Reti et al. [126] prepared TiO2 /MWCNT composite catalysts via hydrolysis of Ti-precursor adsorbed on the surface of MWCNTs, followed by annealing, thereby converting amorphous titanium-hydroxide to crystalline TiO2 NPs decorated on the surface of nanotubes. The CNT content and model compound were found to play major roles for their catalytic activity. Catalysts with

5 wt% MWCNT was the most efficient for oxalic acid decomposition, while ones with 1 wt% performed well for phenol decomposition under UV light. In particular, CNT-, C60 - and graphene-modified TiO2 catalysts were shown to degrade 2,4,6-trichlorophenol (TCP) by 72, 76, and 90% under visible light, respectively [129]. An and coworkers [130] synthesized a series of sub-micrometer anatase TiO2 /MWCNT composite photocatalyst spheres by onestep hydrothermal method using titanium tetrafluoride as titanium source and purified MWCNTs as structure regulator. These composite catalysts consisted of CNTs wrapping around TiO2 spheres with controllable crystal facets for which the aggregated particle average diameter were 200–600 nm. The size was tunable on changing the process parameters such as the concentration of Ti precursor, CNT concentration, autoclave temperature, and reaction time. They found that these parameters affected the composite characteristics, including structural, dimensional, compositional, crystal facet, and physicochemical properties. Among the composites tested, the ones with 18.9 wt% CNTs showed the best photocatalytic performance for decolorization of MO (91.6 wt%), and for the complete photocatalytic degradation of styrene (in 330 min) with a 365 nm UV-LED lamp having UV intensity of 70 mW/cm2 . This was attributed to the formation of 001 and 101 crystal facets during the hydrothermal process, which contributes to an increase in synergistic effect and enhancement of photocatalytic efficiency. Chen et al. [131] introduced 3–8 nm TiO2 nanoparticles into CNT channels with titanium isopropoxide as the precursor under a nitrogen atmosphere using the capillary force of the tubes and obtained composites (denoted TC-in), where most of the TiO2 nanoparticles were neatly aligned in the channel of the nanotubes. For comparison, particles with the same composition were dispersed on the outer surface of CNTs and obtained composites (denoted TC-out), where all of the TiO2 nanoparticles were located outside the nanotubes. Interestingly, both TC-in and TC-out showed paramagnetic behavior, and Ms values at 50 kOe and 5 K were estimated as 0.9 and 0.35 emu/g; the Ms values of reference TiO2 and commercial P25 were much smaller (<0.1 emu/g). Higher Ms values for TC-in in comparison with TC-out is due to the formation of oxygen vacancies facilitated for the TiO2 confined inside the nanotube channels in comparison with the ones outside. In addition to paramagnetic property, both TC-in and TC-out showed excellent catalytic activity for the degradation of MB under visible light ( > 420 nm). More than 95% of the initial dye was decomposed within 90 min of irradiation by TC-in and TC-out catalysts, whereas nearly 62 and 65% of the initial dye still remained after 90 min in the reference TiO2 and P25. The average rate constant () for TC-in and TC-out catalysts were 0.0585 and 0.0448 min−1 , respectively, which were ∼10 times those of bare TiO2 ( for reference TiO2 was 0.0059 min−1 , and  for P25 0.0045 min−1 ). Furthermore, the activity of TC-in catalyst was 31% higher than that of TC-out catalyst during 15 min irradiation under visible light. This is due to the confinement environment within the nanotube channels, the formation of Ti3+ and oxygen vacancies facilitated in TiO2 encapsulated inside the nanotubes. Takenaka et al. [132] prepared a catalyst by deposition of TiO2 nanoparticles on the surface of carbon nanotubes using the hydrolysis of titanium tetraisopropoxide (Ti(OPr)4 ) in the presence of urea or glycine amide as linker molecules between the TiO2 nanoparticles and the nanotube surface, followed by calcination in air at 603 K. The TiO2 -coated CNTs showed higher catalytic activity for the photodegradation of organic molecules (acetic acid) than TiO2 alone under an ultra-high pressure Hg lamp (500 W) at room temperature. Further, the catalytic activity of the composite was improved by inserting Pt noble metal particles in the cavities of nanotubes for TiO2 -coated CNT composite catalyst. The Pt and TiO2 loading in the composites were 1.9 and 72 wt%, respectively. The superior catalytic activity is due to the deposition of Pt metal

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nanoparticles in the cavities of CNTs; this prevented backward reactions because the photogenerated electrons on Pt particles were separated from the holes on TiO2 in the TiO2 -CNT@Pt composite catalyst. These composites also showed excellent catalytic activity towards decomposition of methanol, propanal, butanal, and hexanal. Li et al. [133] used micro-emulsion method for uniformly coating TiO2 nanoparticles on the surface of nanotubes. TiO2 -coated CNT composite catalysts showed higher photoactivity than that of pure TiO2 and the commercial photocatalyst P25 for the degradation of MB. This is because the addition of CNTs increased the surface area and amount of hydroxyl groups on the composite surface—a process that suppressed the recombination of photogenerated electron/hole pairs (excitons). 2.2.2. TiO2 /graphene oxide-based composite catalysts Stengl et al. [134] adopted the same route for synthesizing both TiO2 /GO and TiO2 /rGO composites through homogeneous hydrolysis of titanium oxo-sulfate (TiOSO4 ) with urea. These hybrid nanomaterials were used as catalysts to degrade Orange II and Reactive Black 5 dyes in aqueous media under irradiation at wavelengths of 365 and >400 nm. To further enhance the photocatalytic activity, they doped these catalysts with noble metals such as gold, palladium, and platinum. A catalyst, prepared by dispersing Degussa TiO2 in colloidal GO from Hummers method, followed by reflux and evaporation to obtain a fine powder of TiO2 /GO nanohybrid [135] was used for photocatalytic gas-phase oxidation of ethanol and benzene vapors. The photocatalytic efficiency of the hybrid catalyst was found to be enhanced 4-fold compared to commercial P25 catalyst. Perera et al. [136] prepared TiO2 nanotube (TNT)/deoxygenated graphene oxide (hGO) composite via alkaline hydrothermal process. This was achieved by decorating GO layers with commercially available TiO2 NPs (P90) followed by hydrothermal synthesis, which converted the TiO2 NPs to small diameter (∼9 nm) TNTs on the surface of hGO. The alkaline medium used to synthesize TNTs simultaneously converted GO to deoxygenated graphene oxide (hGO) having ∼70% reduction in oxygenated species. The composites prepared with 10% hGO showed the highest photocatalytic activity, with threefold enhancement in photocatalytic efficiency over pure TNTs for degrading malachite green (MG). The extent of degradation was 80 wt% with 30 min irradiation in a quartz reactor. This was due to the formation of unique “high surface area–small diameter” TNTs and simultaneous conversion of GO to graphene-like hGO without the use of strong reducing agents during the hydrothermal process. Sha and co-workers [137] synthesized two types of nanostructured TiO2 : interpenetrating anatase TiO2 tablets (IP-TiO2 ), and overlapping anatase TiO2 nanosheets (OL-TiO2 ) with exposed (001) facets by a one-step hydrothermal reaction of titanium tetrachloride with hydrofluoric acid as a structure directing agent. The composites were prepared with GO using the same method in the presence of GO dispersions. The GO-supported TiO2 hybrid catalysts were found to completely decompose MB molecules within 3 h under Xe lamp with much greater efficiency than that of IP and OL-TiO2 . This is because GO provides excellent contact with large surface area TiO2 nanosheets, promotes electron–hole pair separation, and extends the life of electrons. Similarly, TiO2 -GO composites [138], prepared by thermal hydrolysis of a suspension of GO and a titania-peroxy complex, showed high photo-degradation activity of butane gas under UV and visible light. Dai et al. [139] prepared surface-fluorinated TiO2 (FTiO2 )/GO hybrid nanosheet catalyst by a hydrothermal process. The composite catalyst with 3 wt% GO was found to degrade 96 wt% MB in 60 min under 365 nm UV light with a rate coefficient of 4.93 × 10−2 min−1 , which is 3–4 times greater than that with pure TiO2 nanosheets and commercial P25. The two semiconductors

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in this catalyst created junctions and promoted charge collection and separation at their interface. When GO combines with F-TiO2 nanosheet with exposed (001) facet, stronger electron–hole separation is obtained, and the photoelectrochemical current is enhanced for hybrid catalyst relative to pure F-TiO2 and P25 catalysts. Thus, the favorable electron–hole separation for F-TiO2 /GO interface junction was attributed to the enhancement of photoelectrochemical current. The suggested photocatalytic mechanism for MB decomposition over F-TiO2 /GO catalyst under UV-light irradiation is shown in Fig. 8. When an optimal amount of GO was incorporated into the TiO2 , the CB of anatase TiO2 is −0.24 V (vs. SHE), while the potential of GO is −0.08 V (vs. SHE). Thus, the photoinduced electrons of the CB of TiO2 can be smoothly transferred to GO sheets under UV irradiation, which effectively retards the recombination of photoinduced e− and h+ . The photogenerated h+ left in TiO2 VB can react with adsorbed water molecules or surface hydroxyl groups and generate hydroxyl radicals (• OH). These hydroxyl radicals are highly reactive toward MB degradation. Owing to the high specific surface area and superior electron mobility of GO, an integration of GO and TiO2 provides a hybrid catalyst that can achieve high photodegradation activity. Liu and coworkers [140,141] prepared TiO2 nanorod-coated graphene oxide (TiO2 -GO NRCs) by a water/toluene two-phase process. Compared with pristine TiO2 nanorods and GO-P25 composites, TiO2 -GO NRC catalyst showed superior activity for the decomposition of MB and acid orange (AO7) under UV light. This difference in photocatalytic activity was attributed to their nanostructure and larger specific surface area. Kim et al. [142] encapsulated GO nanosheets into 1D TiO2 NFs to prepare TiO2 NF-GO hybrid catalyst by using a sol–gel method and an electrospinning technique. It was found that this nanohybrid is a better catalyst than pure TiO2 NFs for the photodecomposition of water. This process provides two photocatalysts, one for the production of H2 and another for generating O2 . A typical case is using photoreduced GO NSs for transferring the photo-generated electrons to large surface area TiO2 NFs on evolving O2 and H2 gases. Hierarchically ordered mesoporous TiO2 -graphene composite films were prepared by confinement self-organization process for which GO suspension was added dropwise to TiO2 colloidal crystals assembled from monodispersed poly(styrene) (PSt) spheres having controlled size and subsequently reducing GO in-situ with the films. These hierarchically structured TiO2 -graphene films showed enhanced capacity to rapidly adsorb and degrade organic pollutants under UV light [143]. Byeon et al. [144] used gas-phase self-assembly process for preparing hybrid nanoflake composite catalyst with a combination of rGO nanoflakes and highly ordered ultrafine TiO2 particles. In this process, a spark discharge produced TiO2 nanoparticles, and the particle-laden flow passed over a collision atomizer orifice, where they mixed with the atomized rGO solution to form hybrid droplets. These were then passed through a heated tube reactor to remove the solvent from the droplets, resulting in TiO2 /rGO hybrid nanoflakes with 89.4 wt% yield. During this process, synthesized TiO2 nanoparticles (∼35 nm in equivalent mobility diameter) were quantitatively incorporated with nanoscale rGO (∼36 nm in equivalent mobility diameter) in the form of TiO2 /rGO hybrid nanoflakes (∼31 nm in equivalent mobility diameter). The TiO2 /rGO hybrid nanoflakes showed enhanced photocatalytic performance for both production of hydrogen and ∼100% degradation of RhB in 4 h under irradiation with a high pressure mercury lamp (160 W, cutoff filter  > 400 nm). This is due to the formation of unique flake nanostructures with enhanced interfacial contact between TiO2 and rGo, resulting in photogenerated conduction band electrons in TiO2 , which could be more easily transferred to rGO nanoflakes.

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Fig. 8. Photocatalytic mechanism for MB degradation over F-TiO2 /GO nanosheet photocatalyst under UV-light irradiation. Reproduced with permission from publisher [139].

2.2.3. TiO2 /graphene-based composite catalysts Cheng et al. [145] prepared electrode catalyst by decorating graphene film (GF) on the surface of TiO2 nanotube array (NTA) photoelectrode through anodization process, followed by electrodeposition. They used the TiO2 NTA/GF hybrid electrode as a visible-light-driven photocatalyst for the degradation of MB. It was found that GF in hybrid electrode catalyst, increases charge separation and light absorption both in the UV and visible range. On the other hand, with graphene-coated TiO2 nanoparticles, the rate coefficient of MB degradation was measured as 3.4 × 10−2 min−1 , which is an order of magnitude greater than that measured for anatase TiO2 (3.28 × 10−3 min−1 ) [146]. The superior performance of nanohybrid catalyst has been attributed to its narrower band gap (2.8 eV) relative to anatase TiO2 (3.2 eV). As a result, the energy level difference between TiO2 and graphene was modified and allowed electrons to flow from excited MB to the conduction band of TiO2 via graphene. A 3D TiO2 -graphene hydrogel has been used as a photocatalyst for the degradation of MB but also as a reusable adsorbent [147]. Lee et al. [148] prepared composite catalyst by graphene wrapped entirely on the surface of amorphous TiO2 nanoparticles (NPs) through co-assembly of positively charged TiO2 NPs with negatively charged graphene oxide nanosheets, followed by hydrothermal treatment for the reduction of graphene oxide to graphene and the crystallization of amorphous TiO2 . These graphene-coated TiO2 NPs exhibited superior photoactivity under visible light for the degradation of MB because of bandgap narrowing of composite catalyst, enhanced absorption of visible light and efficient transfer of photogenerated electrons from excited MB to TiO2 NPs through graphene nanosheets. Yang et al. [149] reported that TiO2 -carbon materials (Gr, GO, CNT) prepared via a combination of sol–gel and hydrothermal method has been used for the selective photocatalytic oxidation of benzyl alcohol to benzaldehyde. In the sol–gel process, multiple stages are involved such as sol formation, gelling, creation of gel, calcination and drying. This process allows homogeneous doping at the molecular level, resulting in superior photocatalytic properties. Two different approaches have been used in this process: sol–gel preparation of TiO2 followed by addition of carbon material dispersion and preparation of sol–gel from the precursor solutions of titanium and carbon nanomaterials. Guo et al. [150] used solvothermal route to prepare ordered and uniform single-crystalline TiO2 nanosheet (NS) on the surface of CFs by introducing CFs as a substrate for growing TiO2 NSs. The TiO2 NS/CF arrays were found to enhance MO degradation by 3.38-fold with excellent stability compared to pure TiO2 NS under UV–Vis light. Li et al. [151] used one-step hydrothermal method to prepare two types of catalysts: TiO2 -graphene and TiO2 -nanotube with the same carbon content. They found that the introduction of carbon materials caused band gap narrowing of TiO2 , which is attributed to chemical bonding between TiO2 and carbon material, and formation of giant 2D planar structured composites. The TiO2 -carbon nanocomposites showed enhanced photocatalytic activity toward

the photodegradation of MB compared to pristine TiO2 under both UV and visible light irradiation due to enhancement of dye adsorption and charge carrier transport by carbon nanomaterial. Zou et al. [152] used nanocrystal-seed-directed hydrothermal route for large scale synthesis of sandwich-like TiO2 /graphene/TiO2 heterostructures where TiO2 was grown as nanorod arrays on both sides of flexible graphene. These heterostructured catalysts have much higher photocatalytic activity than pristine TiO2 nanorods with a catalytic degradation rate of MB that is four times faster than that of the TiO2 nanorods under UV-light with a 500 W Xe lamp. Further, TiO2 -graphene composite with 64% (001) reactive facets were prepared through a facile solvothermal synthetic route [153] during which nano-sized anatase TiO2 nanosheets were grown insitu on graphene sheet surface (Fig. 9A andB). In this process, GO nanosheets were initially prepared using Hummer’s method, followed by pyrolysis of mixed reaction solution containing Ti(OBu)4

Fig. 9. (A, B) Scheme for synthesis of TiO2 -graphene nanosheet catalysts; (C) photocatalytic degradation of RhB in the presence of different photocatalysts under visible light. Reproduced with permission from publisher [153].

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Fig. 10. (a) Photoelectrochemical degradation rate curves and (b) cycling stability for catalysis of MB over TNA, TNA-GQD, and TNA-CdS as photoelectrocatalysts under visible light; (c) SEM image of electrophoretically deposited GQD on TNA walls. Reproduced with permission from publisher [157].

and GO using 50-mL Teflon-lined stainless steel autoclave kept in an electric oven at 200 ◦ C for 24 h. The autoclave was removed from the oven and left to cool naturally to room temperature. The black precipitated composites obtained were washed with distilled water several times to produce purified composites. During solvothermal process, the reduction of GO to graphene and the growth of TiO2 nanosheets can be achieved simultaneously. In comparison with commercial P25 and pure TiO2 nanosheets, TiO2 -grafted graphene nanosheet catalyst showed superior photocatalytic degradation for azo dye RhB under visible light (Fig. 9C). This is due to effective charge anti-recombination of graphene and high catalytic activity of 001 facets. Besides, these hierarchical heterostructures with highly ordered nanoscale texturing on flexible substrates could provide opportunities for field emission displays, and high efficiency photocatalytic and energy-harvesting devices. Sun and Huang et al. [154,155] adopted the same method for preparing TiO2 /graphene composites and used them as photocatalysts for the degrading HCHO, RhB, and 2,4-dichlorophenol under UV irradiation. The enhanced catalytic performance of composites in comparison to pure TiO2 is due to the interfacial charge transfer between TiO2 and graphene nanosheets. Another catalyst [156], TiO2 /graphene composite prepared through a modular synthesis process, showed both magnetic behavior and catalytic activity for the degradation of MB and pharmaceutical compounds such as caffeine and carbamazepine. 2.2.4. TiO2 /carbon and graphene quantum dots-based composite catalysts Yu et al. [63] reported carbon quantum dots (CQDs) and modified Degussa TiO2 with a “dyad”-like structure prepared via a singlestep hydrothermal process. These catalysts exhibited enhanced photocatalytic activity under UV–Vis and visible light ( > 450 nm) compared to pure P25. CQDs play dual roles for enhanced photocatalytic activity of hybrid catalyst. Under UV light, CQDs act as electron reservoirs to improve the separation efficiency of photoinduced electron–hole pairs of P25. Under visible light, CQDs act as photosensitizers to sensitize P25 to “dyad” structure for improving catalytic activity. TiO2 /CQDs prepared [138] via facile hydrothermal reaction of TiO2 with bidentate complex of a green carbon

source (vitamin C) exhibited 9.7 times higher catalytic activity than that of pure TiO2 . This is due to the synergetic effect, favorable electron transfer ability, and upconverted photoluminescence of CDs. Yang and coworkers [149] prepared a series of TiO2 -based composites with several carbon nanomaterials (graphene, carbon nanotubes, and fullerene) via a combination of sol–gel and hydrothermal methods. They found that catalytic activity of composite catalysts is significantly higher than pure TiO2 under visible light. Pan and coworkers [157] prepared vertically ordered TiO2 nanotube arrays (TNAs) and modified graphene quantum dot heterojunction films via combination of controllable electrophoretic assembly, anodic oxidation, and annealing process. Ultra-small size GQDs were homogeneously dispersed on the surface of long vertically oriented TNA walls (length 12 ␮m, inner diameter 110 nm) (Fig. 10c). By modification with GQDs, light absorption of TNA-GQD heterojunction film is enhanced from UV and visible to near IR (400–600 nm) range. The visible-light photoelectrocatalytic (PEC) activity of unfilled TNA and TNA-GQDs were evaluated via measurement of rate constant (k) for MB degradation under simulated solar light at a low intensity of 1.5 mW cm−2 . The heterojunction films exhibited a 5.7-fold enhancement in PEC activity compared with unfilled TNAs. The MB degradation rate of catalyst upon 100 min irradiation was 90% and remained almost the same after four continuous cycles (Fig. 10a and b). This high cycling stability of hybrid photoanodes is due to the excellent photostability of GQDs and TNAs as well as the firm binding of the two constituents via electrophoretic assembly. They also found that the activity of TNA-GQDs for degrading MB is superior to that of TNA-CdS prepared by a sequential chemical bath deposition process where cubic CdS QDs (10 nm) were homogeneously deposited on TNAs films. Sun et al. [158] adopted a simple strategy for fabricating a nanocomposite of TiO2 nanotubes with carbon quantum dots (CQDs). CQDs were obtained by electrochemically etching graphite electrodes, and TiO2 nanotube arrays by anodization. Subsequently, CQDs were assembled on the surface of the vertically aligned TiO2 nanotube arrays. Composites showed 2.7 times greater photocurrent density than that of pristine TiO2 nanotubes due to

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Fig. 11. (A) Proposed model for sensitization of CQDs on TiO2 nanotube arrays; (B) photocatalytic degradation of MB catalyzed as a function of time with TNTs and TiO2 -CQDs; (C) decomposition ratio of MB over composite catalyst used thrice. Reproduced with permission from publisher [158].

the sensitization of CQDs on TiO2 nanotube arrays (Fig. 11a). The composite exhibited 14% enhanced degradation rate for decomposition of MB than pure TiO2 nanotubes under visible light (Fig. 11B) with 1–2 reuse while incurring no significant effect on activity (Fig. 11C). The higher activity of composite is due to sensitization. A model for the sensitization of CQDs on TiO2 nanotube arrays showed that visible light helps generate electrons and holes in CQDs. The conduction band of TiO2 nanotubes is more positive than that of CQDs, resulting in a local electric field. As a result, the excited electrons quickly transfer from the CQDs to TiO2 conduction band. The tubular structure of TiO2 is favorable for separating and transferring electrons to the Ti substrate foil, which contributes to increased photocurrent density and activity. The excellent activity of TiO2 -CQDs composites has been attributed to the sensitization of CQDs [159–162].

2.2.5. TiO2 /carbon nitride-based composite catalysts TiO2 nanotube array with a polymeric nitrogen-rich graphitic carbon nitride (g-C3 N4 ) was prepared by the anodic oxidation followed by electrodeposition [163]. In this process, dicyandiamide was ionized under high voltage electric field in organic solvents (electrolyte) where TiO2 nanotube array film was used as a positive electrode and platinum wire as a negative electrode. The sample was deposited under an applied potential of 120 V at room temperature with interelectrode separation time of 2 min. The composite catalyst (TiO2 /g-C3 N4 ) prepared via 5 min deposition time completely degraded MO (100%) under visible light after 2.5 h, which implies three-fold enhancement over pure g-C3 N4 . This is due to the effects of large surface area, heterojunction structure, and narrow band value of composite catalyst by the combination of hollow TiO2 nanotubes with C3 N4 (band gap 2.7 eV).

Fig. 12. (a) UV–vis absorption spectra of TiO2 /g-C3 N4 composite TCN300; (b) UV–vis absorption spectra of TiO2 /g-C3 N4 composite TCN500; (c) time-dependent photodegradation of MB under visible light. Reproduced with permission from publisher [164].

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and fine powders) have typically shown superior catalytic activity compared to pure TiO2 nanostructures [165–171]. 2.2.6. TiO2 /amorphous nanocarbon-based composite catalysts Reddy et al. [172] used electrospinning method to synthesize 1D TiO2 nanofibers of 30–50 nm diameter and then covalently grafted nanostructured carbon (3–5 nm thickness) on the surface of aminoterminated TiO2 nanofibers via silanization coupling followed by carbonization. C-functionalized TiO2 core-sheath nanofibers were used as re-usable efficient photocatalysts for decomposing MO. The UV-absorption spectra for C-grafted TiO2 hybrid catalyst are shown in Fig. 13A. Prior to photocatalytic reaction, the aqueous dye solutions containing catalysts were kept in the darkroom for 2 h to establish an adsorption–desorption equilibrium. The absorption intensity of MO decreased gradually with irradiation. The MO molecules were fully decomposed over C-grafted TiO2 hybrid catalyst in 120 min under UV radiation (Fig. 13B). The catalytic efficiency of covalently carbon-grafted TiO2 hybrid catalyst was found to be much superior to that of simply mixed or unfunctionalized TiO2 -carbon composite prepared in the absence of silanization (Fig. 13B). The carbon-grafted TiO2 catalysts were found to be catalytically stable at pH of 3–12 and degraded the pollutant by >90 wt% during tests for several cycles under UV irradiation. This is due to the formation of strong interactions between TiO2 and carbon in the functional catalyst through covalent bonds via silanization, resulting in carbon-promoted charge collection and separation at their interface. Also, due to higher surface area and one-dimensional characteristics of the core-sheath nanofibrous catalysts which increases the active sites, more oxygen molecules were adsorbed on the catalyst surface and greater • OH radicals were produced for dye degradation. These novel TiO2 -g-nanocarbonbased nanohybrid materials with a hierarchical structure and morphology could potentially provide an attractive opportunity for environmental remediation. 3. Concluding remarks

Fig. 13. (A) UV–Vis absorption spectra during photodegradation of MO under UVillumination at regular time intervals with functional C-grafted TiO2 catalyst; (B) normalized MO concentration as a function of time for photocatalytic degradation of MO under UV-light with: (a) no catalyst, (b) TiO2 NFs, (c) mixed TiO2 and carbon composite, and (d) functionalized TiO2 @C NFs. Reproduced with permission from publisher [172].

A related study on TiO2 /g-C3 N4 composite catalyst [164] used thermal transformation with cetyltrimethylammonium bromide (CTAB) as surfactant, and urea and D-glucose as dopant precursor. The hydrothermal product obtained had isocyanic acid adsorbed on the surface of TiO2 nanoparticles through hydroxyl ions (OH) present on TiO2 . This led to the formation of cyanamide in the presence of glucose molecules during carbonization process and was polymerized to TiO2 /g-C3 N4 composite nanoparticles. On formation of the composite, the color changed from pure white (TiO2 ) to orange powder. With irradiation, the absorption intensity of catalyst prepared at 300 ◦ C (Fig. 12a) decreased more than that for catalyst prepared at 500 ◦ C (Fig. 12b). Both composite catalysts (titanium carbon nitride, TCN) degraded MB almost completely in 90 min under visible light, while P25 achieved about 40% degradation (Fig. 12c). This is due to the formation of synergistic heterojunction, which facilitates fast electron transfer at the interface between TiO2 nanoparticles and g-C3 N4 . TiO2 /g-C3 N4 composites prepared via various strategies in suitable forms (films

This review presents recent efforts for synthesizing and enhancing TiO2 -based hybrid nanostructured materials with a focus on treatment techniques and applications as heterostructured catalysts. These TiO2 –polymer- and TiO2 –carbon-based hybrid catalysts have recently drawn considerable interest due to their exceptional photocatalytic activity. To enhance their photocatalytic performances, particularly as visible-light-driven hybrid catalysts, the focus has been on light-response and charge-transfer events. Catalysts have been synthesized having various nanostructures (0D to 3D) through strategies such as hydro/solvothermal synthesis, sol–gel processing, chemical polymerization, emulsion synthesis, sonochemical method, wet chemistry routes, electrospinning, and electrochemical deposition. By coupling TiO2 with nanocarbons or polymers having suitable bandgaps, composite nanocatalysts were formed with desirable bandgaps exhibiting greatly enhanced visible light photocatalytic activities for degrading organic dyes. The optimization of the synergistic effects and operational parameters for design, synthesis, and testing for deployment is important when selecting nanohybrid catalyst systems with suitable charge carrier transport properties in wastewater treatment. Since the photocatalytic activity of these materials depend on their light-responsive range and carrier-separation capacity, rational design of TiO2 -based nanostructures that meet the requirement is crucial. Despite much progress in photocatalysis, research on optimizing the properties of these photocatalysts is still in its infancy, as considerable challenges with developing accurate structure–property relationship need to be addressed. For scale-up, these catalysts require pilot-scale evaluation to

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