Environmental applications of titania-graphene photocatalysts

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Environmental applications of titania-graphene photocatalysts Marisol Faraldos ∗ , Ana Bahamonde Instituto de Catálisis y Petroleoquímica, ICP-CSIC, C/Marie Curie, 2, 28049 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 5 October 2016 Received in revised form 26 December 2016 Accepted 17 January 2017 Available online xxx Keywords: Graphene Titania Water treatment Air pollution Dye Heterogeneous photocatalysis

a b s t r a c t Nowadays, graphene is considered one important achievement as a consequence of its high potential in nanotechnologyand in the development of new environmental and energy processes. Reciently, graphene is receiving great attention in the area of photocatalysis, where is emerging in the next generation of photocatalysts, as a tool for enhancing photocatalytic performance and solar photoefficiency. Titanium dioxide hybridization with graphene has an effect on band gap energy decrease, shifting its absorption threshold to the visible light region and allowing to harness solar energy. So, the conjugation of graphene with semiconductor solid particles such as TiO2 , results in photocatalysts with improved charge separation, reduced recombination of the photogenerated electron-hole pairs, increased specific surface area, and introduces an adequate quantity and quality of adsorption sites, given that enhances their electronic, optoelectronic, electrocatalytic and photocatalytic properties. This critical review sumarizes the recent progress in the design and synthesis of graphene-based titania semiconductor photocatalysts. Moreover, their applications in wastewater treatments, disinfection and air pollution control have been also discussed. Finally, some perspectives and challenges considered essential to extend the photoefficiency of these new photocatalysts in the visible region, to harvest directly solar light, have been suggested to introduce and elucidate new improvements in environmental photocatalytic processes. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In 2003, two researchers at The University of Manchester, Prof. Andre Geim and Prof. Kostya Novoselov, were managed to isolate from graphite “a magnificent new wonder material that is a million times thinner than paper, stronger than diamond and more conductive than copper”. This strictly two-dimensional material that exhibited exceptionally high crystal and electronic properties was called graphene, and has revolutionized the physics community when the first paper appeared published in Science in 2004 [1]. Six years later, they were awarded the 2010 Nobel Prize in physics for this work. Graphene is the name given to a single layer of carbon atoms densely packed into a benzene-ring structure, and is widely used to describe properties of many carbon-based materials, including graphite, large fullerenes, nanotubes, etc. (e.g., carbon nanotubes are usually thought of as graphene sheets rolled up into nanometersized cylinders) [2–4]. Fascination with this material stems from its remarkable physical properties and the potential applications these properties offer for the future. Nowadays, graphene is emerging on

∗ Corresponding author. E-mail address: [email protected] (M. Faraldos).

materials sience and condensed-matter physic fields, aiming for a wide range of technological applicacions [5–9],. Its high potential in nanotechnology applications converts to graphene in an important achievement which could change our lifes with the development of new processes in environmental and energy disciplines. Various methods, including thermal expansion [1] micromechanical exfoliation [10], epitaxial growth [11], chemical vapor deposition [12], chemical and electro-chemical reduction of graphite oxide [13,14] and bottom-up organic synthesis have been developed [15,16] from the first report on graphene isolated by manual mechanical cleavage of graphite with a Scoth tape [1]. Reduction of exfoliated graphene oxide (GO) has proven to be an effective and reliable way to obtain graphene nanosheets at low cost and great stability. Besides, via chemical modification can be well adjusted surface properties of graphene, which offers excellent chances for the progress of functionalized graphene-based materials [17,18]. They are very attractive for many potential applications, such as energy storage [19], catalysis [20], biosensors [21], molecular imaging [22], drug delivery [23], nanomedicine and more concretely stem cellbased tissue engineering [24], and recently nanomotors [25]. All of them are posible, fundamentally, because graphene shows distinctive electronic and optical properties with good biocompatibility. Concretely, this material is receiving great attention in the area of

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Fig. 1. Proposed mechanisms of phenol degradation in oxygen by TiO2 -graphene composites: A) Without e− transfer from TiO2 conduction band (CB) to graphene. B) With e− transfer. (from [48]).

nanotechnology and photocatalysis due to its outstanding properties, especially those which exploit its high electron mobility and chemical stability [26,27], and its high adsorption capacity [28,29]. In photocatalysis area, semiconductor materials have been the focus of numerous investigations due to its application to the quantitative destruction of undesirable chemical contaminants in water and air [30,31], and to solar energy conversion [32]. Although several oxide semiconductors have photocatalytic properties, the polycrystalline powders of titanium dioxides present unique properties since it is a stable material, high relative photoactivity, high chemical inertness, nontoxicity and low cost [33,34]. However, many problems need to be solved in the TiO2 photocatalyst system for practical applications, such as narrow UV spectrum response range because of a large band gap energy (∼3.0 eV for rutile and 3.2 eV for anatase) and relatively fast recombination of the photo-generated e− /h+ pairs [32]. Holeelectron recombination is a serious problem for the development of photo-catalytically based technologies since it severely limits the quantum yields achievable [35]. Therefore, various methods have been developed to extend the absorption range of titania into the visible region and to improve its photocatalytic activity, and there are strategies to reduce electron-hole recombination rates and increase photocatalyst efficiency. For instance, via suitable textural design, doping with noble metals, such as Pt, Ag, Au, Pd, etc, transition metal cations, non-metallic doping or immobilization on adsorbent surface and forming semiconductor composites have been employed [36–44]. In this line, also carbon nanomaterials such as fullerenes, carbon nanotubes and graphene are recently receiving a great attention in photocatalityc process, predominantly graphene, because of its unique properties, including large specific surface area, flexible structure, excellent mobility of charge carriers at room temperature and good electrical and thermal conductivities [45,46]. So, graphene is emerging as one of the most promising materials in the next generation of photocatalysts [47], given that has been embraced in the design of new photodegradation catalysts as a tool for enhancing their photocatalytic performance [48]. The excellent properties of graphene are important features when dealing with the preparation and use of graphene-based materials, thus graphene-based TiO2 composites are being developed and successfully applied as photocatalysts for the abatement of pollutants, hydrogen production and others applications [44,49–51]. Where the most widely used synthesis procedures with semiconductors such are TiO2 are in situ growth, solution mixing, sol-gel, hydrothermal and/or solvothermal methodologies [44]. Titanium dioxide hybridization with graphene has an effect on band gap energy decrease, thus shifting the absorption threshold to the visible light region and allowing utilization of solar energy [30]. The

excited electrons could be transfered from the conduction band of TiO2 to the surface of graphene, hence improving the separation of the electron-hole pairs and preventing their recombination [52,53], because it has been already proven that the electron accepting and transport properties of graphene provide a convenient way to direct the flow of photo-generated charge carriers, which thus increases the lifetime of e− /h+ pairs generated by TiO2 upon light irradiation [54,55]. Therefore, graphene is a suitable alternative to noble metals because it has a high conductivity, high surface area, and the ability to favor the electron transfer from the conduction band of TiO2 to delocalized aromatic structure of graphene [56]. So, the conjugation of graphene with semiconductor solid particles, such as TiO2 , results in a photocatalyst with improved charge separation, reduced recombination of the photogenerated electron-hole pairs, increased specific surface area, and introduces an adequate quantity and quality of adsorption sites, that lead to enhance their electronic [57], optoelectronic [58], electrocatalytic [59] and photocatalytic properties [60]. 2. Mechanistic aspects of titania-graphene photocatalysts It is known carbonaceous materials play an important role in photocatalytic processes given that usually present exceptional adsorption ability to many type of pollutants. In this line, TiO2 graphene based catalysts can in turn, if photo-generated charges are transferred toward graphene, improve the final efficiency of the process [30,31]. However, there is not a complete agreement on the real role of the presence of graphene on these types of composites along photocatalytic process. Often, different operational parameters and some of the most relevant properties of TiO2 -graphene composites can even affect their final photocatalytic efficiency, such as type of substrates, irradiation light (UV–vis or Vis), etc. In the work of Minella et al. [61] a complete summary of the operational mechanism alternatives acting on TiO2 -rGO composites is presented. They discuss three types of photo-mechanism, concluding as follow: 1) When substrates are not absorbing light, and are hardly being adsorbed on the catalyst surface, a typical UV-based photocatalytic process can happen, supported by UV-activated e− transition from valence band (VB) to conduction band (CB), which originates e− /h+ pairs (Fig. 1A). In this case, no electron is transferred between TiO2 and rGO, and rGO can act only as competitive light absorber. The extension of light absorption to visible range could happen due to the presence of graphene which can even create states between the VB and CB of TiO2 .

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Fig. 2. FT-IR spectra of as-prepared Brodie-GO (black) and Hummers-GO (red). (from [73]). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

2) Two possible mechanisms can operate when substrates are not absorbing light and movement of photogenerated charges from TiO2 and rGO are possible. a. Electron transfer from TiO2 to rGO (Fig. 1B). The e− photogenerated in the TiO2 jump into the CB and are immediately injected to the graphene aromatic structure avoiding their recombination with the photogenerated h+ that remains in the VB of TiO2 . The transitions are favored by graphene redox potential that is less negative than the conduction band edge of titanium dioxide. b. Electron transfer from rGO to TiO2 . This less probable and more specific mechanism describes the transference of e− from photoexcited rGO to TiO2 structure with further dissipation of energy excess. In this case, rGO would be a merely sensitizer. 3) When dyes are involved in the photocatalytic process, a visibleactivated dye-sensitized pathway can be found, with the single or multi-electron injection from the photo-excited dye to the delocalized empty states of rGO or TiO2 to form the oxidized dye molecules, and following photodegradation. 3. Methods for preparation of titania-graphene photocatalysts In general, the route to obtain graphene oxide starts from graphite that is oxidized to graphite oxide, then delaminated to render graphene oxide and further reduced to form graphene as it can be illustrated in Fig. 2 [62]. Graphite oxide chemistry is quite old and was first studied by Brodie in 1859 [63]. Afterwards, several authors such as Staudenmaier [64], Hofmann and Frenzels [65], Hamdi and Hummers [66], among others, have reported synthesis of graphite oxide from graphite, under very strong oxidizing agents presence, with slight modifications in reaction conditions [67]. An important property of GO, brought about by the hydrophilic nature of the oxygenated graphene layers, is its easy exfoliation in aqueous media. As a result, GO readily forms stable colloidal suspensions of thin sheets in water. After a suitable ultrasonic treatment, such exfoliation can produce stable dispersions of very thin graphene oxide sheets in water. These sheets are, however, different from graphitic nanoplatelets or pristine graphene sheets due to their low electrical conductivity [68]. Brodie was the first to describe a graphite oxidation method [61], which drives through the mixture of graphite with potassium chlorate (3:1 proportion), with the addition of the strongest fuming nitric acid. This reaction evolves at a temperature of 60◦ C in a water-bath during 3-4 days. The obtained material was through

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into water, washed, and re-oxidized under the same conditions, until no further change in C:H:O ratio was observed, usually up to four repetitions. Finally a light yellow solid was obtained upon drying in vacuum and then at 100 ◦ C. Variations with shorter reaction times and higher temperature of final calcination have been described [69]. Staudenmaier [62] and later Hofmann [63] slightly modified Brodie’s oxidation method by progressively dosage of potassium chlorate and further acidification with concentrated sulphuric acid or using non-fuming nitric acid, respectively. The result was a material with similar C/O ratio in shorter time. Graphite oxidation by Hummers’ method [64] is based on the reaction of commercial graphite powder with H2 SO4 /NaNO3 and KMnO4 (safer method than preceding since nitric acid is generated in situ), followed by the addition of H2 O2 to neutralize the excess of MnO4 − that remains unreacted. The brownish solid was filtered and washed repeatedly with HCl and water, then dried under vacuum at room temperature. The graphite oxide was heavily oxygenated, as such; it has been frequently adopted by many researchers for subsequent preparation of photocatalytic composites. Recently, in situ production of nitric acid was replaced by phosphoric acid, trying to reduce risks, provide more oxidized forms of graphite oxide and keep unaltered 2D graphitic structure [70]. Successive improvements on synthesis methods of graphite oxide pursue safer procedures that led to materials with high C/O ratio, but able to maintain the crystallinity upon reduction and exfoliation steps [71]. As mentioned above, Hummers’ method approaches are the preferred for graphite oxidation specially for further photocatalytic applications. This is mostly due to the smaller amount of oxygen introduced by Brodie’s method compared to Hummers’, and the difficulty to recover the Csp2 structure of the carbon lattice after treatment at moderate temperatures; however residual oxygen remains even after treatment at 2000 ◦ C. In contrast, the presence of less conjugated oxygen groups in Hummers graphite oxide facilitates their thermal removal resulting in a better restoration of the pristine graphite 2D structure [72]; and added reason for the preference of Hummers-GO in photocatalysis applications, essentially those where water or high relative humidity environments are involved. The ability of GO to disperse in aqueous media on separate graphene oxide sheets has recently attracted strong attention to these materials. It is known that GO sheets undergo swelling when immersed into water (or other liquid solvent) provoked by solvent molecules intercalation (solvation) between GO layers which results in an increase of the distance. The different functionalities of Brodie and Hummers GO samples originate different hydration/solvation behavior. In particular, the presence of higher number of C O groups in Hummers-based GO materials (see Fig. 3) could be responsible for higher separation of graphene oxide sheets, thus achieving higher degree of hydration/solvation. In contrast, C OH groups, more abundant in Brodie-based GO samples, leads to better ordering of graphene oxide sheets in the stacks and consequently, better crystallinity of the solvated GO structure. The better crystallinity of the Brodie-based GO sample makes it difficult to disperse Brodie-based GO on single sheets in water solution even after intense sonication. Indeed, experiments with samples prepared by both methods showed that Hummers-based GO samples are very easily dispersed in water after mild sonication, while Brodie-based GO does not delaminate well even when sonicated for tens of hours [73]. The reduction of graphene oxide to graphene is visually recognized by a color change of the suspension from brown (graphene oxide) to black (graphene), and an increase of hydrophobicity/aggregation of the material due to loss of oxygenated functional groups. The efficiency of a reduction method can be evaluated by increase of C/O ratio (to quantify the decrease of oxygen content)

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Fig. 3. Schematic route of GO reduction and in situ growth of TiO2 nanoparticles on RGO sheets via hydrothermal process. (Adapted from [62] and [110]).

Fig. 4. Routes for the synthesis of graphene oxide-TiO2 composites.

and enhancement of current conductivity (to evaluate the crystallinity of graphitic basal plane) [69]. The reduction of graphite oxide could be afforded by physical procedures, traditional redox reactants or green chemical agents. i) Among physical processes, temperature treatment in vacuum or inert atmosphere is a simple way [43,74–76], microwave [77–82], sonication [83–86] or irradiation [49,87–89]; last three could be carried out directly on the suspension and permit to afford reduction and partial exfoliation simultaneously. ii) The traditional redox chemicals are frequently toxic; however, use to follow well-known reduction routes, which permit to control the reduction degree and tune the properties of final compounds. A wide range of agents with different nature and chemical behavior have

been reported, although collateral processes could happen like adsorption or doping of resulting graphene. The chemical agents most used are [69]: a) Borohydrides (commonly NaBH4 ) soluble in aqueous but limited efficiency with some types of carbonyl groups [90,91]; b) Aluminum hydrides, strongest reducing agents than borohydride salts; c) Sulphur-containing reducing agents (Thiourea dioxide in alkaline conditions); d) Nitrogen-containing reducing agents (Hydrazine, Hydroxylamine, Pyrrole, Benzylamine, p-Phenylene diamine, Ethylenediamine, Urea, Dimethyl ketoxime, Hexamethylenetetramine, Polyelectrolyte, Poly(amido amine)) [59,88,92]; e) Oxygen-containing reducing agents (Alcohol, Hydroquinone, L-Ascorbic acid, Saccharides, Gallic acid) [88,93]; f) Sulphur-containing reducing agents (namely Na2 SO3 , Na2 S2 O3 ,

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Thiourea, Thiophene); g) Metal–acid (Al, Fe, Zn, Mg, Sn); h) Metal–alkaline (Zn, Al, Na); etc. iii) The last years, growing efforts to achieve safer and greener synthesis have arisen the use of natural or biocompatible reduction agents like: a) Amino acid (L-Cysteine, Glycine, L-Lysine, L-Glutathione) [94]; b) Plant extracts (green tea, reach in polyphenols) [95]; c) Microorganisms; d) Proteins; e) Hormones; etc. Usually, the combination of chemical and physical processes is convenient to minimize the synthesis time. In a typical procedure, GO is suspended in water and sonicated to yield a homogeneous yellow brown dispersion. The reduction chemical agent in adequate proportion was added to the solution and heat treated (or microwave, sonication, UV) for determined time (from min in some cases to hours, correspondingly). Reduced GO was gradually produced as a black solid. The product was filtered and washed copiously with water (and/or any other solvent). The product could be additionally calcined under vacuum (or inert atmosphere) to complete the desired reduction grade [96]. Once known how to oxidize, exfoliate and reduce the graphite, the strategy to prepare the photocatalytic composite with titanium dioxide or the selected semiconductor could be afforded. Two general cases are generally found, as it has been graphically described in Fig. 4: i) synthesis route based on commercial or synthetized crystalline TiO2 or ii) synthesis procedure using titanium dioxide precursors. In both cases the role of pH is important to favor the optimal dispersion of solids in the reaction media and the interaction among graphene oxide and titania particles in suspension. A simple route consists on physical mixing of crystalline TiO2 (anatase or anatase/rutile phases) with previously exfoliated graphene oxide, but weak interactions arise and any evidence of photocatalytic efficiency enhancement takes place [97–99]. Several other synthesis of graphene oxide-titanium dioxide describe that crystalline TiO2 and exfoliated graphene oxide are suspended in an adequate media (water, alcohol o mixture or both, generally), stirred, heat treated and dried, accomplishing the reduction of GO to rGO and the interaction between rGO and TiO2 at the same time. This heat treatment could be carried out at vacuum or inert atmosphere in an oven [76] or hydro/solvo-thermal reaction [100–103]. In other cases the reduction could be carried out by UV irradiation, then a strictly control of time is recommended [104] to avoid composite photodegradation. Prolonged times of UV irradiation on TiO2 -GO composites can result in excessive GO reduction degree and further photodegradation of rGO sheets [105], what could be a drawback in long-driven photocatalytic applications. This characteristic has been used for production of a novel structure of graphene called graphene nanomesh [106]. In general, the initially higher oxygen-containing functional groups and surface properties of graphene oxide obtained by Hummers’ method leads to more stable TiO2 -GO composites with better performance in photocatalytic applications. Therefore, when an intimate link between reduced graphene oxide sheets and titanium dioxide is pursued, most of the researchers follow a sol-gel route where the formation of TiO2 nanoparticles and deposition on Hummers-GO layers happens simultaneously, where a further calcination step is needed [107–112]. However some authors tried hydro/solvo-thermal approaches to crystalize TiO2 , deposit the nanoparticles and reduce GO in one-pot [78], or combination of two stages: sol-gel followed by hydro/solvo-thermal procedures [113–115]. The synthesis of doped TiO2 or GO (out of the focus of this short review) uses the same described routes with the incorporation of the dopant (metal, non-metal, anion) in the initial steps, followed by sol-gel and/or hydro/solvo-thermal reaction to complete the formation of the doped graphene oxide-titania composite. Most of the authors are conscious that one of the main factors for having a good graphene-titania composite is the formation of

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Fig. 5. PL signals for G0-TiO2 , Mixing, and G2.5-TiO2 under excitation at 325 nm. (The lowest PL intensity indicates the lowest recombination rate of photoinduced electron–hole pairs. PL results demonstrate that only the TiO2 /graphene composite with the chemically bonding interface can effectively facilitate charge transportation and separation. From [119]).

Ti C and/or Ti O C between TiO2 and graphene. In the aim to improve this interaction, new developments have been designed in the recent years employing different molecules to functionalize rGO and facilitate the anchorage of TiO2 nanoparticles, leading to more efficient and durable photocatalyst composites [94,116–118]. 4. Characterization of titania-graphene photocatalysts The photocatalytic behavior of graphene oxide-titanium dioxide photocatalysts becomes directly related to their physico-chemical properties. The knowledge of these specific features is desirable to optimize the design of photoefficient composites, where the modification of synthesis parameters could be associated to some characteristic variation and finally to photocatalytic activity. An exhaustively use of many different characterization techniques are usually employed trying to control the oxidation and reduction degree obtained during graphite to graphene route (Raman spectroscopy), crystallinity of components (X-ray diffraction), understand the nature of GO and TiO2 interaction (X-ray Photoelectronic Spectroscopy, XPS; Fourier Transform Infra-red spectroscopy, FTIR; Ultraviolet-Visible spectroscopy, UV–vis), etc. All of them facilitate the complementary information required to argue the improvement on photocatalytic efficiency when graphene oxide is incorporated to the photocatalyst composition by chemical bonding. While loosely interactions have no effect on photocatalytic activity; even the photoluminescence (PL) signal decrease due to recombination inhibition, usually detected for GO-TiO2 composites (Fig. 5), was not observed in a simple mixing of materials [91,119–125]. Another useful techniques are the determination of photocurrent induced when a GO-TiO2 film deposited over a conductive substrate, acting as working electrode, is irradiated; or electric impedance spectroscopy (EIS) to detect the limiting interfaces in the electronic transfer [109], [126–129]. On the other hand, the information obtained by scanning (SEM) and transmission (TEM) electron microscopy are valuable tools for morphological, structural and composition (when associated to EDX) properties of GO-TiO2 composites [46,130,131]. As key values when chemical bonding between GO and TiO2 takes place, could be mentioned: reduction of band gap, UV–vis continuous absorption, appearance of Ti O C signal in FTIR par-

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Preparation method

Initial Operating Conditions

Lamp

Photocatalytic efficiency

Reference

Solvothermal

Phenol

Visible and UV light

[73]

Sol-gel followed by calcination in air or treatment under vacuum Hydrothermal

[phenol] = 10 mg L−1 [catalyst] = 100 mg L−1

Medium Hg 150-W ␭ = 200–600 nm ␭max = 366 nm Halogen Lamp (400–900 nm) ␭max = 650 nm

Vis: Composite (TGC3-3) Xphenol = 62% <> P25, Xphenol = 20% UV: Composite (TGC3-3), Xphenol = 81% <> P25, Xphenol = 48% Degree of phenol decomposition: Composite (T-G–15 V), 0.92 <> P25, 0.58

[152]

One-pot synthesis + Heat treatment N2 /300 ◦ C/1 h Sol-gel and heat treatment 100 ◦ C/1 h

[phenol] = 50 mg L−1 [catalyst] = 147 mg L−1

400-W Heraeus Lamp

P25 < Police < Composite (1-RGO-T) Best Composite: (single-layer 1-RGO-TiO2 ) (TiO2 -0.25 wt.% TGO), Xphenol = 96%

[phenol] = 1 mM [catalyst] = 0.5 g dm−3

UV and Vis radiation

[59]

Heterocoagulation from aqueous dispersions

[phenol] = 1 mM [catalyst] = 1 g L−1

High pressure Hg-Lamp ␭ = 240–580 nm

Poor phenol photodegradation under UV and scare under Vis light Worse conversion than P25. Study cycles photodegradation

Hydrothermal

[TCP] = 60 mg L−1 [catalyst] = 1000 mg L−1

Composite (GR-1.5 TiO2 ), 90% TCE and 77% TOC

[122]

Electrostatic self-assembly method + hydrothermal process

[BPA] = 100 mg L−1 [catalyst] = 200 mg L−1

400-W high-pressure Na-Lamp. UV cut-off filter ␭max > 420 nm Hg-Lamp UV Cut-off filter ␭max = 400 nm

[156]

Hydrothermal

[BPA] = 100 mg L−1 [catalyst] = 200 mg L−1

Composite photodegradation increases: (GR/TiO2 ) nanoparticle < (GR/TiO2 ) nanorod < (GR/TiO2 ) nanowire (80%) pH 8, Composite, 84% UV, 71% Vis pH 11, Composite, 100% UV, 85% Vis

Electrospun + Heat Treatment Air/500 ◦ C/2h

[4-CP] = 5.33·10−5 M [catalyst] = 222.22 mg L−1

Bare Titania, Xphenol = 23% P25, Xphenol = 41% Composite (TiO2 /4%GO), Xphenol = 88%

[155]

Phenol

Different TiO2 concentrations

0–5 wt.% graphene oxide 1–10 wt.% graphene oxide Phenolic compounds 0.2/0.5/1.5/3 wt.% graphene oxide

1/2/3/4/5 wt.% graphene oxide

500-W Hg-Lamp UV␭max = 365 nm Vis-␭ max = 400 nm 500-W Xenon-Lamp ␭ < 420 nm

[46]

[96]

[121]

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0.25-0.5-1.0 wt.% graphene oxide

[phenol] = 10 mg L−1 [catalyst] = 100 mg L−1

[74]

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Few, one (1), two (2) graphene layers

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Table 1 Summary of photocatalytic applications of graphene-titania composites in wastewaters.

GO/TiO2 weight ratio: (1:10, 1:100) Trichloroethylene (TCE) wt. ratio GO-WTiO2 (0-0.01: 1)

Initial Operating Conditions

Lamp

Photocatalytic efficiency

Reference

Solvothermal

[2,4-DCP] = 10 mg L−1 [catalyst] = 0.04 g [Na-PCP] = 50 mg L−1 [catalyst] = 100 mg

TiO2 nanosheets/grapheme best performance Cycles photodegradation Best Composite (GO/TiO2 ) XNa-PCP > 97% <> P25, XNa-PCP = 31.1%

[153]

Hydrothermal

High pressure Hg-Lamp ␭max = 365 nm High pressure Hg-Lamp

Chemical Reduction+ Thermal Treatment Hydrothermal

[TCE] = 500 mg L−1 [catalyst] = 80 mg L−1

Solar Simulator Vis-cut-off filter ␭ > 440 nm Xe solar simulator + AM 1.5G filter

Best Composite (WTiO2 -d-graphene-20), 75% TCE

[80]

Best Composite (FTS-G-1), 80% TCE

[157]

Hydrothermal

[ATL] = 5–25 mg L−1 [catalyst] = 0.52 mg L−1 [RS] = 2 mg L−1 [catalyst] = 100 mg L−1

1000-W Xe arc Lamp + AM 15G filter 1500-W Xe arc Lamp Vis − ␭ = 430 nm

Best Composite, 72% ATL and 100% TOC

[162]

[163]

High-Hg-Lamp − ␭max > 365 nm Low-Hg Lamp − ␭max > 254 nm Vis fluorescent Lamp (simulated natural sunlight) Xe Lamp UV␭ > 320 nm Vis-cut-off filter − ␭ > 400 nm UV lamp ␭= 254 nm

Higher photocatalytic activity of TiO2 -rGO composites compared to TiO2 -P25 Influence of water matrix: photocatalytic efficiency decreases: distilled water < tap water < river water < lake water Best Composite (TiO2 -2.7% rGO), 54% carbamazepine, 81% Ibuprofen, 92% sulfamethoxazole

Best Composite: (G-TiO2 -0.86 wt.%) XTOC = 86%

[165]

Study of operating conditions: [pollutant], pH Cycles of photodegradation.

[161]

[TCE] = 500 mg L−1 [catalyst] = 200 mg L−1

[155]

Pharmaceutical compounds

GO: TiO2 ratio (1:10)

Hydrothermal

GO: 0.1–10 wt.%

Hydrothermal deposition method

[pharmaceuticals] = 5 mg L−1 10 cm photocatalyst-coated SOFs were placed in a glass petri

0.11-11.45 wt.% graphene oxide

Solvothermal

[norfloxacin] = 20 mg L−1 [catalyst] = 1000 mg L−1

1–20 wt.% graphene oxide

Hydrothermal

[acetaminophen] = 5 mg L−1 [catalyst] = 0.1 g L-1

[164]

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0.5–20 wt.% graphene oxide

Preparation method

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Fraction of Graphene/TiO2

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Table 1 (Continued)

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Lamp

Photocatalytic efficiency

Reference

4 wt.% graphene oxide

Immobilized in membrane

[Diphenylhydramine] = 3.4·10−5 M

Mediumpressure Hg-Lamp cut-off filtered ␭ >350 nm

Best performance composite (M-GOT) under vis light

[91,112,158–160]

Solvothermal method

[aldicarb] = 10 mg L−1 [catalyst] = 1000 mg L−1

Best Composite (G-TiO2-0.86 wt.%) XTOC = 36.8%

[165]

4 wt.% graphene oxide

Liquid phase deposition

[isoproturon] = [alachlor] = 12.5 mg L−1 , [diuron] = 7.5 mg L−1 , [atrazine] = 6.25 mg L−1 [catalyst] = 500 mg L−1

Best performance: Composite (GO-TiO2 ) Vis: Composite (GO-TiO2 ) higher photodegradation than TiO2 -P25

[151]

80 mg·L−1 graphene oxide

TNA by in situ anodization. GR-TNA by electrodeposition

[alachlor] = 2–10 mg L−1

Xe Lamp − UV␭> 320 nm Vis-cut filter − ␭ > 400 nm FluorescentLamp UV: 6BBL + 4DL, ␭max > 365 nm Vis 10 DL − ␭ > 400 nm Xe Lamp

Best photodegradation: Composite (GR/TNA) Xalachlor > 99.5% after 15 cycles

[146]

One-pot synthesis sol-gel method

[DCA] = 100–90 ␮M

Best (Layered-titanate-RGO) ∼ = XDCA = 72%

[102]

0.5–10 wt.% graphene oxide

Solvothermal

Liquid phase deposition

0.5–6.4 wt.% graphene oxide

Solvothermal

Composite (P25-GR), XNA > 90% <> P25, XNA = 80% Better performance under simulated and solar light: Composite (TiO2 -GO-4) Best Composite (T-3.2% G)

[148]

4–20% wt. graphene oxide

[Naftenic acid] = 0.1 g L−1 [catalyst] = 0.1 g L−1 [MicrocystinLA] = 0.2 ␮M [catalyst] = 0.5 g L−1 [Fulvic acid] = 100 mg L−1 [catalyst] = 0.5 g L−1

Pesticides 0.11-11.45 wt.% graphene oxide

Photodegradation of others pollutants 450-W Xe arc Lamp cut-off filter ␭ > 330 nm 12-W UV-Lamp, ␭ = 254 nm Xe Lamp − Vis

High pressure Hg-Lamp ␭ = 254 nm

[166]

[167]

Photo-reduction of metals Deposition TiO2 nanoparticles on GO nanosheets 20% TiO2 -rGH

[Cr(VI)] = 0.2 mM at pH 2 [catalyst] = 0.5 g L−1

20-W UV Lamp ␭max = 253.7 nm

Best Composite. (Heat −Treated GO-TiO2 ): 5.4 times more photo-reduction than P25

[168]

[Cr(VI)]o = 5–10 mg L−1 [catalyst] = 1000 mg L−1

250-W Hg-Lamp ␭max = 365 nm

Under Continuous flow conditions- Best performance composite (TiO2 -rGH), 100% photo-reduction

[171]

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Table 1 (Continued)

Initial Operating Conditions

Lamp

Photocatalytic efficiency

Reference

Hydrothermal method

[Zn2+ ] = 21 mg L−1 [FA] = 250 mg·L−1 [catalyst] = 2 g L−1 [BrO3 − ] = 10 mg L−1 [catalyst] = 0.1 g L−1

1000-W Xe arc Lamp + AM 1.5G filter 24-W Low Pressure-HgLamp (8 lamps) ␭max = 254 nm UV 230-W Hg-Lamp

Best Composite (TiO2 -G), 99.99% removal at pH 7.1 under solar light Best Composite (P25-GR 1%), 99% removal at pH 6.8

[172]

Best photo-reduction, composite (G-TiO2 )

[170]

[Cr (VI)] = 12 mg L−1 [catalyst] = 0.2 g L−1

125-W Hg-Lamp Vis ␭ > 450 nm

Improved Cr (VI) reduction (86.5%) composite (TiO2 -RGO) compared to bare TiO2 (54.2%)

[169]

Best graphene oxide/TiO2 film by a factor about 6 (7.5) relative to bare TiO2 Best disinfection activity: Composite (GO-TiO2 ) under solar light

[130]

Best Composite (TiO2 /4.2 wt% GSs)

[175]

Vis: E. Coli disinfection: Composite TiO2 -RGO better than P25 Vis: E. solani spores similar for composite and P25

[176]

0.5–10 wt.% graphene oxide

Hydrothermal method

30 mg GO

One-pot Solvothermal method Sol-Gel + heat treatment under vacuum

[Cr (VI)] = 10 mg L−1 [catalyst] = 500 mg L−1

[173]

Photo-disinfection Deposition on TiO2 thin films of GO platelets TiO2 − solvothermal GOTiO2 = Mixing

Antibacterial Test (E. coli, ATCC 25922)

Solar light

Standard E. coli test

1.4, 4.2, 7 wt.% graphene

Redox reaction

Standard E. coli test

5% w/v GO respect to TiO2

Mixing + Photoreduction

Antibacterial test E. coli K-12 (ATCC 23631) Fusarium solani (CECT 20232)

UV-UVP Pen-Ray Hg-Lamp ␭ = 254 nm Vis-Xe Arc Lamp Indoor natural light irradiation, ␭ = 400–700 nm Real Natural Solar light UVA-filtered solar radiation

[174]

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Table 1 (Continued)

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tially overlapped to Ti O Ti, wider XPS signal in the region of C1s and Ti2p due to Ti O C and Ti C contribution, shift of signal due to weight loss in ATR by stabilization of species when GO-TiO2 are strongly interacting, and attenuation of Ti OH signal in Raman assigned to Ti O C formation [132–135].

5. Photocatalytic applications It is a long concern that semiconductor-mediated photocatalysis are attracting a lot of attention among scientific community for its potential applications in environmental remediation [30]; where the presence of materials such as graphene in new titaniabased composites is undoubtedly improving their photocatalytic performances. The relatively high rate of photocatalytic degradation ability of graphene-titania nanocomposites is attributed to efficient charge separation and transfer of photo-induced electrons, decreasing h+ and e− recombination rates, increased light absorption ranges, ordered crystalline size, and the high oxidizing capacity of photo-generated h+ [121]. In this section, in turn subdivided in two parts (water and air pollution processes), a brief summary of the main applications of graphene-titania catalysts of the last ten years is reviewed.

5.1. Photocatalytic water treatment 5.1.1. Photocatalytic water decontamination Great deals of effort are being dedicated to address the widespread pollution of effluents from urban environment, agricultural and industries with very different bio-recalcitrant organic pollutants by means of photocatalytic processes. Concerning TiO2 graphene composites applied to hazardous waste remediation and water purification, Chen et al. [136], presented a possible way to fabricate graphene oxide/semiconductor composites (GOT) with different properties by using a tunable semiconductor conductivity type of graphene oxide (GO), where the concentration of GO in starting solution played an important role in photoelectronic and photocatalytic performance of composites on methyl orange photodegradation, and semiconductors formed by GO on the surface of GOT could act as a sensitizer and enhance the visible-light photocatalytic performance of GOT. However, since the creative work of Zhang et al. [98], chemically bonded TiO2 (P25)-graphene composite photocatalysts were prepared by using a hydrothermal method; many efforts have been made to utilize the unique properties of graphene in order to increase the efficiency of photocatalytic processes. But until now, most of the studied substrates have been mainly dyes, such as Methylene Blue (MB) [66,137–139], Rhodamine B (RhB) [140–142], Methyl Orange (MO) [143], Acridine Orange Brilliant Red X–3B [144], Congo Red [145], Acid Orange 7 (AO7) [146–148], Reactive Yellow 145 [149], and Disperse Red 1 (DR1) [150], under both UV–vis and Vis light irradiation. When dyes degradation is involved, the photocatalytic activity of GO-TiO2 composites is most of the times followed by UV–vis spectroscopy, where contributions of other organic molecules (e.g. original compound and intermediates) could happen, disturbing the evaluation of final photocatalytic conversion. Nevertheless, it is essential to extent these studies to different types of pollutants, to try to solve the widespread pollution of effluents where photocatalytic processes will be competitive, with the aim to reach new developments where this new generation of photocatalysts based on titania-graphene composites, can be applied. In this section the main environmental applications of graphene-titania composites without any metal doping, concerning treatment of pollutants different of dyes, are briefly summarized in Table 1, where proportion of graphene/TiO2 , prepa-

ration method, initial operating conditions, type of used lamps and photocatalytic efficiency are included, and commented following. The evolution of these pollutants along irradiation time have been generally followed by chromatographic system, complemented by determination of total organic carbon (TOC) to evaluate the effectiveness of mineralization or biodegradability of resultant effluents [59,151–156]. Phenol photodegradation is a frequently studied pollutant, used as target compound because of its presence in many residual industrial effluents. One of the first papers about TiO2 -graphene composites (TGC) with visible and UV light photocatalytic performance in phenol degradation was that presented by Jiang et al. [73], where the thermally treated composites, prepared by solvothermal process, exhibited higher photocatalytic efficiency than Evonik P25, under both visible and UV lights. Whereas, Aleksandrzak et al. first studied the influence of TiO2 loading and calcination treatment on TiO2 reduced graphene oxide nanocomposites in [74]; where higher TiO2 concentrations resulted in higher photo-catalytic activity; in [157], the influence of graphene thickness on its photoactivity was also explored by these authors, concluding that the highest photocatalytic activity was arisen by TiO2 supported on single-layered graphene (1-RGO-T), where an important decrease in photocatalytic activity was observed when the number of graphene layers was increased. Lastly, Adamu et al. [46] prepared, by a simple one-pot synthesis, TiO2 -graphene oxide (GO)/thermally reduced graphene oxide (TGO) composites, where the best performance was arisen with 0.25 wt% loadings of GO and TGO, which could be explained by hybridized adsorption-carrier charge separation cooperation. In contrast, Szabó et al. [96] and Minella et al. [59] reported worse phenol conversion when graphene oxide was incorporated to the photocatalyst composition. In the case of [96] GO was prepared by Brodie’s method with different ratios to TiO2 (P25) in the composite; besides photoactivity, a sedimentation study for catalyst recovery and reuse during four cycles was carried out without photoactivity loss. The GO-TiO2 composites prepared by [59] showed poorer conversion with increasing GO proportion under UV light, while scarce photoefficiency was observed when Vis light was employed. Other chlorophenolic compounds have been also studied as target pollutants in their photo-oxidation; graphene modified TiO2 composites prepared by Hu et al. [122] was used for the photocatalytic degradation of 2,4,6-trichlorophenol (TCP) under visible light, where a percentage of 1.5 wt% of graphene in a modified TiO2 nanocomposite showed the highest electron-hole separation rate, the highest photocatalytic activity (90%) and good stability after three cycles of photodegradation. Sun et al. [158] described the photodegradation of 2,4-dichlorophenol (2,4-DCP) with different TiO2 morphologies (nanosheets and nanorods) bonded to graphene oxide, achieving the maximum conversion with a loading of 0.5% GO. They studied the mechanism using scavengers and durability along five consecutive cycles. Continuing with studies of chlorophenols, Zhang et al. [159] photodegraded sodium pentachlorophenol (Na-PCP); both found improvement with the incorporation of graphene oxide by simple solvo/hydro-thermal routes. Finally, Zhang et al. [160] has recently reported the preparation of welldefined and structurally stable TiO2 nanofibers aggregated by GO with different contents (0–5 wt.%) by means of electrospinning technology. Their photocatalytic activity indicated that TiO2 /GO composite nanofibers possessed higher photocatalytic ability than TiO2 -P25 and the bare titania nanofibers for degradation of 4CP molecules under visible-light due to the enhanced separation efficiency of photogenerated electron-hole pairs showing higher mobility of charge carriers. The photodegradation of bisphenol-A (BPA) in water was studied by Hua et al. [161] who analyzed different graphene (GR)-

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modified morphologies of TiO2 , including nanoparticles, nanorods and nanowires, and Bai et al. [121] prepared graphene (GR)/TiO2 nanocomposites that presented significantly higher photocatalytic activity than TiO2 -P25, mostly attributed to effective charge transfer from TiO2 nanoparticles to GR, exhibiting good performance after five cycles of photodegradation. The photocatalytic performance of some graphene-titania composites were evaluated through the photodegradation of trichloroethylene (TCE) in aqueous suspensions, by Lee at al. [81], who improved the photocatalytic properties of TiO2 (P25) by using water-mediated titania decorated with graphene (WTiO2 d-graphene) composites, and Kim et al. [162] who synthesized novel flower-like TiO2 spheres (FTS)/reduced graphene oxide (rGO) composites (FTS-G) which also presented a good photocatalytic performance in TCE photodegradation. The destruction of emerging contaminants is one of the most suitable applications of photocatalysis technology; usually, relatively dilute multi-component solutions and fixed volumes are involved, that can be recirculated to achieve complete mineralization of organic carbon in the existing environment. In this context, the most recent papers focused on the photo-oxidation of antibiotics, pharmaceutical compounds, drugs, pesticides, etc. over titania-graphene composites can be considered very relevant in wastewater applications by heterogeneous photocatalysis. Pastrana-Martínez, Morales-Torres et al. [91,112,163–165] carried out a complete study about diphenilhydramine photodegradation utilizing the GOT composite immobilized in membranes, they found relevant the photoactivity employing Vis light. The photooxidation of acetaminophen [166] was aborded analyzing the effect of initial concentration of contaminant and catalyst loading, pH, scavengers and cycles, although using 254 nm UV light. Bathia et al. [167] synthesized, by a simple hydrothermal method, a graphene oxide-TiO2 (P25), evaluating its photoactivity in atenolol (ATL) photo-degradation, as model pharmaceutical pollutant under UV–vis light and “simulated Sun” irradiation conditions. One of the most novelty and complete papers has been presented by Calza et al. [168] where some graphene-based composites were synthesized through a hydrothermal method using graphene oxide and the commercial TiO2 -P25 as starting materials. Their photocatalytic activities were assessed through the photodegradation of risperidone (RS, an antipsychotic drug), under a variety of experimental conditions, such as, different aqueous matrices (distilled, tap, river and lake water) under artificial solar light and visible light. Concluding that the use of a TiO2 -rGO10 composite revealed the effectiveness and benefits, in terms of faster drug disappearance, earlier abatement of toxicity and easier photodegradation of the transformation products and mineralization; at the same time both reduction of toxicity and mineralization were faster achieved than with bare TiO2 -P25. The photocatalytic performance of a series of TiO2 -reduced graphene oxide (rGO) coated side-glowing optical fibers (SOFs), synthesized by polymer assisted hydrothermal deposition method (PAHD), in the degradation of three pharmaceuticals, including carbamazepine, ibuprofen, and sulfamethoxazole, under UV and visible light irradiation, was analyzed by Lin et al. [169]. Their photocatalytic activity increased with the concentration of rGO (0–2.7%) in composites, but the degradation was inhibited when the rGO concentration was larger than 2.7%. A pesticide, aldicarb, and an antibiotic, norfloxacin, were used as the model compounds to determine the photocatalytic performance in simulated sunlight and visible light irradiation [170] under a series of graphene/TiO2 composites using a single-step nonionic surfactant strategy combined with the solvothermal treatment technique. Cruz et al. [151] analyzed the photodegradation of a mixture of four pesticides: diuron, alachlor, isoproturon and atrazine with a GO-TiO2 composite under both UV–vis and visible light. Their photo-efficiencies were always better than bare TiO2 , with very

11

Fig. 6. Effect of water matrix on pesticides photodegradation: relation of initial reaction rates (r0,natural /r0,ultrapure ) for degradation of pesticides with bare TiO2 and GO-TiO2 catalysts. Operating conditions: [TOC]0 = 22.8 mgL−1 , irradiance: EUV-A = 40 Wm−2 and catalyst loading = 500 mgL−1 . (From [156]).

good initial pesticide photodegradation rates under both natural and ultrapure water matrices as it can be seen in Fig. 6. The photocatalytic activity found for GO-TiO2 composite under visible light was remarkably higher than TiO2 -P25. Zheng et al. [146] developed a titanium dioxide nanotubes electrodes where was electrodeposited graphene oxide to demonstrate the improvement in alachlor photodegradation, maintaining a conversion higher than 99.5% after 15 cycles. Finally, others compounds such as e. g. dichloroacetic acid (DCA) was also studied as model pollutant in the photocatalytic degradation (Kim et al. [102]), under coupled nanocomposites of layered titanate and reduced graphene oxide (RGO), where the layered-titanate-RGO nanocomposite was much more active than uncomposited layered titanate under visible-light as a consequence of fairly strong electronic coupling. Liu et al. [148] prepared P25GR composites by solvothermal method, that achieved more than 90% of naftenic acids removal, much higher than P25 (XNAs < 80%). The effect of pH and graphene content was optimized and radicals trapping experiments pointed to • OH and holes as the crucial reactive species. Sampaio et al. [171] described the behavior of GOTiO2 with variable GO loading (4–20%) during photodegradation of microcystin-LA under Xe or Vis lamps, obtaining the best performing with 4% GO. However, Zhou et al. [172] found a slight improvement in the photodegradation of fulvic acids with T-G composites prepared by solvothermal procedure. Compared to other chemical methods, the photocatalytic method displays many advantages in removing toxic heavy metal ions, including high efficiency, low energy consumption, and mild reaction conditions, which are the key points for removing Cr (VI) pollution from water. Jiang [173], Zhao et al. [174], Wang et al. [175] and Li [176] studied the photocatalytic reduction of Cr (VI) with graphene-titania based composites. Whereas the synthesized graphite oxide/TiO2 composites [168] not only exhibited excellent photocatalytic ability for the oxidative degradation of organic pollutants, but also showed strong photocatalytic activity for the reductive conversion of inorganic pollutants such as Cr (VI). TiO2 graphene hydrogel with three-dimensional (3D) network structure [171] also presented an excellent adsorption-photocatalysis performance, removing 100% Cr (VI) under UV irradiation. The same optimal behavior was obtained for the sol-gel prepared TiO2 -RGO photocatalyst [169] followed by heat treatment at 600 ◦ C for 1 h, able to reduce 86.5% of Cr(VI) compared to 54.2% of bare TiO2 . Kumordzi et al. [177], synthesized a composite catalyst of TiO2 and graphene by a hydrothermal treatment method to photoreduce Zn2+ , the most abundant heavy metal found in combined

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Preparation method

Initial Operating Conditions

Lamp

Photocatalytic efficiency

Reference

0.2–30 wt.% graphene oxide

Solvothermal

[benzene] = 250 ppm

UV lamp, ␭ = 254 nm

[48]

0-0.2 wt.% graphene oxide

Hydrothermal

[acetone] = 300 ± 20 ppm [catalyst] = 0.3 g (dish-coated)

UV 15-W Lamp, ␭ = 365 nm

0.01-0.15 wt.% graphene oxide

Colloidal blending process

55% RH [BTEX] = 0.1–1 ppm

5–10 wt.% graphene oxide

Impregnation

96 vol.% ethanol/benzene [catalyst] = 0.05 g

Best performance: Composite (TiO2 -P25/H-GO)

[179]

Nanostructured Membrane: rGO/TiO2 ratio 1:10

Hydrothermal

[Methanol]

Vis: 8-W daylight lamp ␭ = 400–700 nm UV: 8-W black light ␭ = 352 nm 1000 W high-pressure Hg-Lamp cut-filtered, ␭ = 310–390 nm UV radiation

Best Composite (TiO2 -P25-0.5% GR), XBenzene = 76.2% <> P25, XBenzene = 5.8% Best Composite (TiO2 -0.05 wt.% GO) enhanced photoactivity 1.7 and 1.6 times more TiO2 -bare and TiO2 -P25 Best photodegradation Composite (TiO2 –GO-0.10)

[180]

0.5–3.0 wt.% graphene oxide

Solvothermal

[HCHO] = 200 ppm [catalyst] = 0.05 g

UV LED, ␭ = 365 nm

GO-TiO2 ratio: 0.033

Chemical mixing

Study of [catalyst], [2E1H] and percentage of humidity

Fluorescent lamp ␭ = 400–720 nm

0.5–2.0%

Solvothermal

20 W m−2 Iron Halogenide Lamp, ␭ = 315–400 nm

0.01/0.1/1.0 wt.% graphene oxide

Solvothermal

[NO] = 1000 ppb [catalyst] = 50 mg RH: 50% NOx Standard ISO/DIS-22197-1 procedure [NO] = 1 ppm Relative Humidity 50%

Total photodegradation of methanol composite (Graphene + TiO2 ) − Type B graphene 0.04·cm−2 Composite (G2.5-TiO2 ), 2.6 times more photodegradation than pure TiO2 Best composite (GO-TiO2), X2E1H = 99.3%, mineralization 55.1% Best composite GO-TiO2 XNOx = 100% Best composite (TiO2 /rGO-2) UV − XNOx = 41.51% Vis − XNOx = 22.34%

[183]

UV-A 15-W Lamps Vis 8-W Lamps

[177]

[178]

[181]

[149]

[182]

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Table 2 Summary of graphene-titania composites in photocatalytic applications in air pollution control.

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sewer overflows (CSOs). This TiO2 -Graphene composite showed high performance in the photo-reduction of Zn2+ under solar light compared to un-doped titania. The photodegradation of bromate (BrO3 − ) has attracted much attention as bromate is a carcinogenic and genotoxic contaminant in drinking water. Huang et al. [178], studied TiO2 -graphene composite (P25-GR) photocatalyst for BrO3 − reduction, prepared by a facile one-step hydrothermal method, where the composite of P25 with 1% GR loaded exhibited higher (BrO3 − ) removal efficiency. The physical characterization revealed that the photo-reduction instead of adsorption was predominantly responsible for the bromate removal. 5.1.2. Photocatalytic water disinfection It is widely known that the addition of a catalyst enhances the efficiency of solar disinfection. Heterogeneous photocatalysis utilizes light along with a semiconductor to produce reactive oxygen species (ROS) which can inactivate bacteria and degrade a wide range of chemical contaminants in water. One of the first applications reported of titania-graphene composites was the work of Akhavan et al. [179] who utilized graphene oxide/TiO2 thin films reduced at different irradiation times as nanocomposite photocatalysts for degradation of E. coli bacteria in an aqueous solution under solar light irradiation. The photocatalytic reduction of these graphene oxide platelets for four hours caused an improvement of the antibacterial activity of the TiO2 thin film by a factor of about 7.5. Gao et al. [180] successfully synthesized various TiO2 nanostructures including one-dimensional (1D) TiO2 nanotube, 1D TiO2 nanowire, three-dimensional (3D) TiO2 sphere assembled by nanoparticles (TiO2 sphere-P) and 3D TiO2 sphere assembled by nanosheets (TiO2 sphere-S). The combination of TiO2 sphere-S with graphene oxide (GO) sheets can further enhance the photodegradation efficiency of AO7 and disinfection activity of Escherichia coli (E. coli) under solar light, which is more energy efficient. Cao et al. [181] obtained composites of ultrafine TiO2 nanoparticles and graphene sheets (GSs) for the photodegradation of E. coli bacteria. With optimization of the content of GSs in the composite, the TiO2 /4.2 wt% GSs sample exhibited the best photocatalytic antibacterial activity under ambient visible light illumination. TiO2 (P25)-RGO composites have been applied in the disinfection of water contaminated with E. coli and Fusarium solani (F. ˜ et al. [182]. solani) spores under real sunlight by Fernandez-Ibanez An enhancement in the rate of inactivation of E. coli was observed with the TiO2 -RGO composite compared to P25 alone. The rate of inactivation of F. solani spores was similar for both TiO2 -RGO and P25. When the major part of the solar UVA was cut-off (k > 380 nm) using a methacrylate screen, there was a marked increase in the time required for inactivation of E. coli with P25 but no change in the inactivation rate for TiO2 -RGO was found. There is evidence of singlet oxygen production with visible light excitation of the TiO2 -RGO composites which would lead to E. coli inactivation. 5.2. Photocatalytic air pollution control On the contrary, a very few papers in the literature, based on TiO2 -graphene composites, have been applied to photocatalytic degradation of air pollutants compared to photocatalytic waste water applications. In Table 2 are summarized all these works, where again, proportion of graphene/TiO2 , preparation method, initial operating conditions, type of used lamp and photocatalytic efficiency are included. Some of them obtained titania-graphene composites using the known TiO2 -P25 from Evonik, such as Zhang et al. [48] who synthetized by a simple solvothermal method TiO2 P25-graphene (G) composites, and were compared with TiO2 P25-carbon nanotubes (CNT) during photocatalytic degradation of benzene vapor. The best photocatalytic performance was observed

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with 0.5 wt% G, analogously to CNT. The authors suggest that G acts similarly to any other carbon-based (CNT, fullerenes, and activated carbon) composite on enhancement of photocatalytic activity of TiO2 , although structural and electronic characteristics make G unique compared to other carbon allotropes. The 2D structure with abundant oxygen-containing functional groups provides reactive sites to interact with organic/inorganic systems; while the induced photocurrent, recombination rate of photogenerated e− /h+ and response to visible radiation, lead in turn to the observation of much higher photocatalytic activity of G-TiO2 . The increase of G or CNT ratio into the composite results in higher adsorptivity of pollutants, consequently, the contact surface of TiO2 particles with the light irradiation becomes reduced, which would provoke lower photocatalytic activity. The stability and activity of TiO2 (P25)-G nanocomposites resulted much higher than bare TiO2 . Wang et al. [183] studied the effect of graphene content in the photocatalytic degradation of acetone in air, resulting an optimal concentration of 0.05 wt.% in the hierarchical macro/mesoporous TiO2 composite, at which the prepared materials improved the photocatalytic activity of bare TiO2 and commercial P-25 by a factor of 1.7 and 1.6, respectively. Jo et al. [184] developed a TiO2 (P25)-GO-0.10 composite by means of colloidal blending process which exhibited superior photocatalytic performance than unmodified P25-TiO2 for degradation of vaporous aromatic pollutants (BTEX) under both visible-light and UV irradiation, or Andryushina et al. [185] who also prepared composites by an impregnation method to be applied in the photocatalytic gas-phase oxidation of ethanol and benzene vapors. Roso et al. [186] deposited TiO2 (P25) and TiO2 (P25)-graphene composites on membranes, and found that graphene based photocatalysts revealed a higher reaction rate on methanol gas-phase degradation; moreover, the performance of the nanostructured membranes could be restored several times by stripping with an inert gas. Also, some home-titania synthesis have been analyzed to obtain graphene-based composites in air pollution control, such as paper of Huang [199] developed a chemically bonded G2.5TiO2 composite with 2.5 wt.% graphene that effectively enhanced the photocatalytic activity in photodegradation of formaldehyde in air, or Chun et al. [149] who concluded that a two-dimensional graphene oxide-coupled titania (GO–TiO2 ) hybrid post-treated at 400 ◦ C does not presented appreciable loss of photocatalytic efficiency after five cycles for the degradation of low-concentration gas-phase 2-ethyl-1-hexanol (2E1H) in continuous-flow mode under visible-light illumination. Neppolian et al. [187] demonstrated that GO-TiO2 was an excellent catalyst for the total decomposition of toxic NOx gas that was achieved within 50 min. Analogously, Trapalis et al. [188] studied the photocatalytic performance in the abatement of NOx with two types of composites TiO2 /G and TiO2 /rGO, each with graphene loadings 0.01-1%. The TiO2 /rGO at low 0.1% graphene loading, exhibited superior efficiency than the TiO2 /G under Vis radiation, due to interaction between TiO2 -nanoparticles and graphene sheets. On the basis of the observations could be concluded that the capability of graphene oxide for accelerating the photocatalytic transformations of pollutants in air depends on a balance between two factors, in particular, (i) interaction of graphene oxide with the titania nanocrystal surface via anchoring functional groups and (ii) efficiency of acceptance of the photogenerated TiO2 conduction band electrons [179]. 6. Future advances in the development of visible-light active photocatalysts based on TiO2 -graphene Photocatalytic process is a known technology efficient for treatment of reduced volume of effluents and moderately contaminated.

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Continuous efforts have been done to develop new photocatalysts more active in the use of solar light; in this sense, different strategies have been designed to dope the semiconductor obtaining colored photocatalyst sensitized in the Vis region, composites based on mixture of different semiconductors, oxides, carbonaceous materials or adsorbent structures [189]. New approaches and more creative synthesis routes should emerge trying to avoid the aggregation induced in TiO2 particles by reduction treatments necessary to improve the crystallinity of graphene basal plane. The use of UV radiation, microwave assisted or freeze-drying treatments is promising in the future photocatalysts development which combined with ultrasonication, dialysis and higher control of suspensions stability by addition of adequate compounds, adjust of ionic strength or shifting pH values, could arose more stable and efficient photo-composites. The use of functionalized graphene oxide seeks to improve the composite materials linkage, gaining in photoefficiency and stability. Currently, the discrimination of the amount of GO bonded to TiO2 by chemical interaction (Ti C and/or Ti O C) from the loosely interacting, and the different behavior of these both types of GO in the photocatalytic processes, still supposes a difficulty. In this sense, characterization techniques such as XPS, complemented by others (FTIR, UV–vis, TGA, etc.) previously mentioned in section 4, could deeply help to clarify the nature of interactions and determine the real loading of GO in TiO2 -rGO composites. One of the drawback for real applications is the recoverability of catalyst, and more developments are needed to define immobilized systems like described in the form of membranes [186,160,190] or fibers [191–193], but still low competitive compared to suspended photocatalytic devices. Applications of graphene-TiO2 photocatalysts are focused on dyes photodegradation and only some tens of different pollutants present in water and air, however very few was found up to now about self-cleaning uses [58,194], when the possibility to produce transparent films with the presence of highly exfoliated graphene, open an interesting field.

7. Conclusions The photocatalytic behavior is frequently difficult to understand because numerous simultaneous processes are interrelated. The more complex system the more difficult to find a linear or direct relationship between photoefficiency and composition of photocatalyst, or amount of graphene oxide present in the composite, or grade of oxidation of graphene surface, etc. Even the performance varies with the different pollutants involved. Takind into account that rate and efficiency of heterogeneous photocatalytic reactions is highly dependent on a certain number of parameters which govern the kinetic of the process, where it must be also taken into account that photocatalytic processes are mediated by photonic activation. In this context, and since every single research group has carried out their studies at different operating conditions, with different photoreactors and light sources, results a very difficult task to try to compare them and getting some comparative conclusions. However there are some points that seem to emerge along the majority of the papers revised:

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Please cite this article in press as: M. Faraldos, A. Bahamonde, Environmental applications of titania-graphene photocatalysts, Catal. Today (2017), http://dx.doi.org/10.1016/j.cattod.2017.01.029