Recent Advances in Graphene Based TiO₂ Nanocomposites (GTiO₂Ns) for Photocatalytic Degradation of Synthetic Dyes

Abstract

Synthetic dyes are widely used in textile, paper, food, cosmetic, and pharmaceutical industries. During industrial processes, some of these dyes are released into the wastewater and their successive release into rivers and lakes produces serious environmental problems. TiO₂ is one of the most widely studied and used photocatalysts for environmental remediation. However, it is mainly active under UV-light irradiation due to its band gap of 3.2 eV, while it shows low efficiency under the visible light spectrum. Regarding the exploration of TiO₂ activation in the visible light region of the total solar spectrum, the incorporation of carbon nanomaterials, such as graphene, in order to form carbon-TiO₂ composites is a promising area. Graphene, in fact, has a large surface area which makes it a good adsorbent for organic pollutants removal through the combination of electrostatic attraction and π-π interaction. Furthermore, it has a high electron mobility and therefore it reduces the electron-hole pair recombination, improving the photocatalytic activity of the semiconductor. In recent years, there was an increasing interest in the preparation of graphene-based TiO₂ photocatalysts. The present short review describes the recent advances in TiO₂ photocatalyst coupling with graphene materials with the aim of extending the light absorption of TiO₂ from UV wavelengths into the visible region, focusing on recent progress in the design and applications in the photocatalytic degradation of synthetic dyes.

1. Introduction

One of the biggest problems that the world is facing today is environmental pollution, which increases every year and causes serious and irreparable damage to the Earth [1]. Therefore, environmental protection and a new approach in environmental remediation are important factors for a real improvement in quality of life and for sustainable development.

In recent years, a large number of research activities have been dedicated to environmental protection and remediation as a consequence of special attention from social, political, and legislative authorities, which has led to the delivery of very stringent regulations for the environment [2].

Due to industrialization and the lack of effective treatments of the effluents at the source, a severe deterioration of freshwater resources caused by the release of a wide range of hazardous substances into water bodies has occurred. Of these substances, synthetic dyes represent a large group and therefore deserve particular attention, due to the high quantity—more than 800,000 tons—that is produced annually worldwide [3]. About one third of these is released into receiving waters every year through industrial wastewater discharges [4], which may have a severe influence on both the environment and human health, affecting photosynthetic activity as well as dissolved oxygen concentration, and therefore causing serious ecologic problems [5].

Dyes present high toxicity, are carcinogenic and mutagenic in nature, and can also result in bioaccumulation in the food chain [6]. Because their origin is synthetic, these compounds are very stable in the presence of light and biodegradation [7]. Furthermore, very low concentrations of these are easily visible in contaminated waters; therefore, the total effect of the presence of dyes in the water ecosystem is both environmentally and aesthetically unacceptable.

Taking account of these considerations, there is an urgent need to develop new methods for the treatment of industrial wastewater containing synthetic dyes.

In recent years, photocatalysis studies have focused on the use of semiconductor materials as photocatalysts for environmental remediation. Semiconductor photocatalysis is a versatile, low-cost, and environmentally friendly treatment technology for many pollutants [8,9], and the use of heterogeneous photocatalysis is an emerging application in water decontamination.

Within a great number of oxide semiconductors, TiO₂ is the most widely used as promising photocatalyst, demonstrating a very important role in environmental remediation [10,11], solar energy [12], and other fields [13,14] due to its nontoxicity, low cost, corrosion resistance, abundant resources, and high photocatalytic efficiency [15,16,17].

A high number of studies concerning TiO₂ regard wastewater treatments [18,19,20,21] and, in many cases, total mineralization of pollutants without any waste disposal problem have been published [22]. On the other hand, the practical applications of this technique are limited to a narrow excitation wavelength because of a large band gap energy (3.2 eV), high recombination rate of the photo-produced electron–hole pair, and poor adsorption capacity [23,24,25]. In recent decades, various attempts have been applied to improve the catalytic efficiency of TiO₂ [18,26,27].

Due to its wide band-gap, TiO₂ is active only under UV-light irradiation; considering that the percentage of UV-light is less than 5% of the total incident solar spectrum on the Earth, in recent years research has focused on extending the light absorption of TiO₂ under visible light. The research goal of TiO₂ photocatalysis, as for other semiconductors, is represented by the combination of TiO₂ with other nanomaterials to achieve both visible light activation and adsorption capacity improvement, with the simultaneous limitation of the electron-hole recombination rate. For this reason, to improve the photocatalytic performances in the use of TiO₂, different approaches have been investigated, such as the use of co-catalysts, and loading with noble metal particles, dye, metallic or non-metallic doping [28,29,30,31,32,33,34,35,36]. Recently, enhancements of the photocatalytic activity of TiO₂ by visible light have been also demonstrated by the modification of TiO₂ with carbonaceous substances such as fullerenes, carbon nanotubes, and graphene to form carbon-TiO₂ composites [37,38,39,40].

In particular, graphene nanomaterials in combination with TiO₂ highlight new prospectives in the field of photocatalysis for their large specific surface area, flexible structure, extraordinary mobility of charge carriers at room temperature, high thermal and electrical conductivities, and high chemical stability [41,42,43,44,45,46,47,48], emerging as one of the most promising materials for enhancing the photocatalytic performance in the new generation of photocatalysts [49,50,51,52,53,54,55,56].

In this short review, the recent research advances in graphene-TiO₂ employed in photocatalysis are presented with the aim of extending the light absorption of TiO₂ from UV wavelengths into the visible region, as well as increasing the photocatalytic activity of the compound, focusing on the applications of synthetic dye degradation.

2. Graphene Materials: Concepts and Properties

Graphene (G) is the term used to indicate carbon atoms tightly packed in a plane monolayer into a two-dimensional (2D) “honeycomb lattice” that represents the building block for all graphitic materials such as fullerenes, nanotubes, and graphite [48,57].

The isolation of graphene from graphite in 2003, by Prof. Andre Geim and Prof. Kostya Novoselov, which was the first paper published in Science in 2004 [42] and was awarded the Nobel Prize in Physics in 2010, has fascinated and revolutionized the scientific community [42,58,59]. This is due to graphene’s notable properties and to the high range of applications that these properties offer to technologies such as energy storage and photocatalysis [60,61,62,63,64,65,66,67].

The IUPAC commission proposed the term “graphene” to substitute the older term “graphite layers”, which was inappropriate in the research of two-dimensional (2D) monolayers of carbon atoms because the three-dimensional (3D) stacking structure is identified as “graphite” [59].

After the first report on graphene, which was obtained manually by mechanical cleavage with a Scotch tape sample of graphite, various techniques developed to produce thin graphitic films have been reported [55,68,69]. These can be divided into two main groups: bottom-up and top-down methods [55,69], as schematized in Figure 1.

In the bottom-up growth of graphene sheets (

Table 1. Graphene synthesis with the bottom-up approach.

	Results	Highlight	Refs.
Epitaxial growth	- General	Experimental and theoretical aspect	[71]
	- Flower defects and effects of G grown on SiC substrate	Enhancements in the electronic properties of G-based devices	[72]
	- G nanoribbons (GNR) on surface facets of SiC(0001)	Influence of the substrate step height on the energy barrier	[73]
Chemical vapor deposition (CVD)	- Large-size, single-crystal, twisted bilayer G	Raman spectra measured as a function of the rotation angle	[74]
	- G	Review: challenges and future perspective	[75]
	- G on Ni and Cu substrates	Representative applications	[76]
	- 200 mm G on Ge(001)/Si(001) wafers	Absence of metallic contaminations	[77]
	- Multilayer G films	Educational experiments: economical, safe, and simple technique in 30–45 min	[78]
	- Large-scale G films on thin Ni layers	Two methods to shape and transfer films to specific substrates	[79]
	- Monolayer G films on SiC(0001)	Ex-situ graphitization in argon atmosphere	[80]
	- Centimeter-scale single- to few-layer G on Ni foils	Efficient roll-to-roll process	[81]
	- N-doped graphene	N-type behavior useful to modulate G electrical properties	[82]
Electrochemical reduction of CO and CO2	- Several types of nanocarbons of controlled shape	Direct reaction of CO2 with Mg metal	[83]
	- G flakes	Room-temperature synthesis method on copper foil from different carbon sources using external charges	[84]
Arc discharge	- Few-layered G	The reactivity of buffer gases (helium, oxygen-helium, and hydrogen-helium) is the key factor	[85]
	- Few-layered G	Different mechanisms in the presence and absence of TiO2 and ZnO catalysts	[86]
Unzipping carbon-nanotubes (CNT)	- Chemical-free G	Radial and shear loading unzipping modes with cryomill method at 150 K	[70]
	- G nanoribbons	Linear longitudinal opening of the Multi Wall CNT (MWCNT)	[87]
Organic synthesis	- Two-dimensional G nanoribbons	Highly ordered monolayers (2D crystal) of larger G ribbons	[88]
Pyrolysis	- General description	Pyrolysis of organic matter	[89]
	- Metal-free catalyst of G sheets	Spray pyrolysis of iron carbonyl and pyridine	[90]
	- G nanoporous with high specific surface area	Spray pyrolysis at different temperatures, graphene oxide-based precursor, nitrogen carrier gas	[91]

), the synthesis of graphene [70] can be obtained via epitaxial growth [71,72,73], chemical vapor deposition (CVD) [74,75,76,77,78,79,80,81,82], electrochemical reduction of CO and CO₂ [83,84], arc discharge [85,86], unzipping carbon nanotubes [70,87], organic synthesis [88], and pyrolysis [89,90,91].

The top-down approach (

Table 2. Graphene synthesis with the top-down approach.

	Results	Highlight	Refs.
Mechanical	- Monocrystalline graphitic films	Exfoliation of small mesas of highly oriented pyrolytic graphite	[42]
	- G	Review: general description	[94]
	- G sheets	Dispersion and exfoliation of graphite in N-methyl-pyrrolidone	[95]
	- Stably single-layer G sheets in organic solvents	Exfoliation–reintercalation–expansion of graphite	[96]
	- G nanoplatelet	Thin films on a low-density polyethylene substrate	[97]
Electrochemical expansion	- Functional G sheets	Hyperexpanded graphite by electrolysis in a Li+ containing electrolyte and in situ electrochemical diazonium functionalization	[98]
	- G flakes	Cathodic graphite expansion in dimethylformamide (DMF) and functionalization by reducing aryl diazonium salts in organic solution	[99]
	- G	Exfoliation temperature increase from 25 to 95 °C reuslt in decrease of defects and increase of thermal stability (with H2O2 addition)	[100]
Chemical synthesis	- G	Review: general description	[103]
	- Carbon nanoscrolls	Low-temperature, catalyst-free graphite intercalation with alkali metals	[104]
	- G and chemically modified G	From colloidal suspensions	[105]
Unzipping nanotube	- Few-layer nanoribbons	Mechanical sonication and gas-phase oxidation in organic solvent of multiwalled carbon nanotubes	[106]
	- G nanoribbons	Lengthwise cutting of MWCNTs by a solution-based oxidative process	[107]
	- G nanoribbons	Plasma etching of CNT partly embedded in a polymer film	[108]
Microwave synthesis	- Flower-like G on hexagonal boron nitride crystals (h-BN)	Microwave G growth on polymethyl methacrylate (PMMA)-coated h-BN flakes	[109] [110]
Chemical and/or thermal graphene oxide (GO) reduction	- Polylactic acid (PLA)/rGO nanocomposites	Glucose and polhyvinilpyrrolidone (PVP) reduction of GO mixed with PLA	[111]
Reduction	- rGO - rGO	Two different reducing mixed reagents: HI/NH and NH/HI. Review: 50 types of reducing agents	[114] [115]
Other	- G - G networks	Review: different sizes and chemical compositions. Controlled segregation of G chemically modified on liquid interfaces	[116] [117]

) offers considerable economic advantages over bottom-up methods [92,93], producing high quality graphene. In this case, the graphene is derived from the exfoliation of graphite, from mechanical [42,94,95,96,97], electrochemical expansion [98,99,100], thermal [101], electrostatic deposition [102], and chemical synthesis [103,104,105]. Some other techniques are unzipping nanotube [106,107,108] and microwave synthesis [109,110], or by chemical and/or thermal graphene oxide (GO) reduction [111]. This last method allows the production of low-cost and large-scale graphene, though with oxygen-containing groups and defects [112,113]. Much interest is directed at the study of proceedings to obtain graphene in the form of highly reduced graphene oxide (rGO) [114,115] or chemically modified graphene [116,117] from the oxidation and exfoliation of graphite and successive chemical reduction.

The strong oxidation of graphite produces bulk solid graphite oxide that can be exfoliated in water or other suitable organic solvents to form GO [118,119,120]; several recent reviews report the synthesis of graphite oxide [121,122,123]. Graphite oxide can be prepared with Brodie [124], Staudenmaier [125], and Hummer’s [126] methods. The synthetic technique and the extent of the reaction influence the degree of graphite oxidation; the most oxidized graphite oxide is produced with the Staudenmaier method, but this reaction may take several days. However, because both Staudenmaier and Brodie methods generate highly toxic ClO₂ gas that can decompose in air violently; thus, the most widely used method to prepare graphite oxide is Hummer’s method.

With Hummer’s method, the GO is prepared by exfoliating graphite oxide obtained from the oxidation of graphite powder with strong chemical oxidants, such as HNO₃, KMnO₄, and H₂SO₄ [112,113,126]. GO can be successively reduced to graphene by the partial restoration of the sp²- hybridization by thermal [127], chemical [128], electrochemical [129], photothermal [130], photocatalytic [69,131], sonochemical [132,133], and microwave reduction methods [134]. The high number of oxygen-containing groups of the obtained GO product permits the interaction with the cations, providing important reactive molecules for the nucleation and growth of nanoparticles and therefore the formation of various graphene-based composites.

3. The Photocatalytic Process: Fundamentals of Graphene-TiO₂ Photocatalysts

Photocatalysis is based on the activation of a semiconductor (SC) by the sun or artificial light. When an SC material is irradiated with photons whose energy is higher or equal to its band gap energy, a promotion of an electron from the valence band (VB) to the conduction band (CB) occurs with the concomitant generation of a hole in the valence band VB [135].

In a water system, oxygen adsorbed on the surface of the semiconductor acts as an electron acceptor, while the adsorbed water molecules and hydroxyl anions act as electron donors, leading to the formation of a very powerful oxidizing ^•OH radical. The electrons react with oxygen molecules to form the superoxide anion, O₂^•-. In the presence of a contaminant organic molecule (M), adsorbed on the catalyst surface, the ^•OH radical is the primary oxidant that reacts to produce adducts, followed by the fragmentation of the molecular structure into several intermediates species until the total mineralization of contaminant is completed with the formation of CO₂ and H₂O [136,137,138].

The successful of photocatalytic process is therefore strictly dependent on the competition between the reaction of the electron with water on the SC surface and the electron-hole recombination process that releases heat or radiation.

The overall process can be described by the following reactions:

• $electron-hole pair generation:$
  $SC+hv\to SC\left(e_{CB}^{-}+h_{VB}^{+}\right)$
• $hydroxil radicals formation:$
  $SC\left(h_{VB}^{+}\right)+H_2{O}_{ads}\to SC+{}^{•}\mathit{OH}_{ads}+H_{ads}^{+}$
• $superoxyde formation from electrons in CB and oxygen:$
  $SC(e_{CB}^{-})+O_{2 ads}\to SC+O_{2ads}^{•-}$
• $adsorption of M on catalyst:$
  $SC\left(h_{VB}^{+}\right)+M_{ads}\to SC+M_{ads}^{•+}$
• $the hydroxyl radical is the primary oxidant:$
  $O{H}_{ads}+M_{ads}\to intermediate products\to C{O}_2+H_{2}O$

As an example, Khan et al. [23] defined the different intermediate degradation products of Methyl Orange by using TiO₂ photocatalysts that are resumed in Figure 2. The reported compounds are derived from the reaction of ^•OH radicals with the final mineralization in CO₂ and water.

It is important to consider that, because the photocatalytic process is strictly dependent on the kinetics of charge carriers and redox reactions, the knowledge of the electronic processes that occur at the level of the SC surface are of great importance. In the SC, the oxidation fromphotogenerated holes and the reduction fromphotogenerated electrons may occur simultaneously and, to keep the photocatalyst electrically neutral, should occur at the same rate.

The photocatalytic activity is greatly limited by electron–hole recombination, and therefore several methods are used to increase the efficiency of charge carrier separation and therefore to improve the photocatalytic performance of the SC photocatalyst [139,140,141]. In this context, the combination of graphene with the SC photocatalyst represents an innovative strategy [55,69,142].

Titanium dioxide (TiO2) is generally considered the best SC material that can be used as a photocatalyst [18,139,143], showing almost all of the required properties for an efficient photocatalytic process, except for activity under visible light irradiation.

From these considerations, TiO2/graphene nanocomposites (GTiO₂Ns) are widely used for photocatalytic applications, exploiting their potential in environmental applications.

The effective role of G in a photocatalytic event on GTiO₂Ns has not been completely investigated, or is not completely understood. In order to explain the possible mechanism, the light source (UV or VIS light) and the presence/absence of dye molecules, or rather the presence of a compound able to absorb visible light, are fundamental.

When the target molecule does not adsorb light and is efficiently adsorbed on the photocatalyst surface:

(1)

UV light excitation of GTiO₂Ns photogenerates the electron–hole pairs and the electrons are then injected into graphene due to its more positive Fermi level [144]. This process is favored by the position of the work function of graphene that is −4.42 eV, with respect the conduction band of TiO₂ that is located at −4.20 eV [145]; from this consideration, the electron in the CB of TiO₂ is injected to G. Graphene scavengesphotogenerated electrons by dissolved oxygen; facilitates the hole-electron separation, reducing the recombination of e-(CB) and the holes (VB); and, due to its high carrier mobility, accelerates the electron transport, thus enhancing the photocatalytic performance [142,146].

(2)

When the operational mechanism takes place via visible light, the electron transfer of thephotogenerated electron is promoted from the G photoexcited state and then delocalized to the TiO₂ surface. M.T Silva et al. indicated, by rGO photoluminescence study, that the photogenerated electrons under Vis or NIR laser can be transferred to the surface of TiO₂ with a consequent quenching of photoluminescence; also in this case, charge recombination is inhibited with a consequent increasing of photocatalytic activity under visible light [147]. It is important to know that the presence of G in GTiO₂Ns photocatalyst produces a red shift in the absorption, reducing its band gap and thus extending the photoresponse to a longer wavelength [148]. The explanation of visible light activation in the GTiO₂Ns composites is not clear, but it is possible to attribute this phenomena to the sensitization of TiO₂ due to the presence of graphene [147,149,150]. In this case, in the visible light excitation of GTiO₂Ns, graphene absorbs the light, and the photoexcited electrons in high energy G states are delocalized into the CB of the TiO₂ surface with the dissipation of excess energy due to electron vibrational interaction [151]; successively electrons react with oxygen, resulting in the formation of superoxide radicals. In Figure 3, the activation mechanisms of GTiO₂Ns under UV and visible light are reported.

When a dye (D) as a target molecule is efficiently adsorbed onto the photocatalyst surface, the UV mechanism is the same as that reported before (1) while, under visible light, different activation pathways can be found. In this case, the dye acts both as a sensitizer for visible-activation and as a pollutant during the photocatalytic process, promoting the electron transfer from the excited dye molecules state to the TiO₂ CB while G acts as an electron scavenger, as in the UV mechanism [147].

The other possible pathway under visible light involves G which acts as visible light sensitizer promoting the electron transfer to the TiO₂ CB (as in 2) [152,153,154] (Figure 4). This process is less probable due to the competition between G and D as light sensitizers, but in any case is possible.

4. Preparation Methods of GTiO₂Ns

According to the facts that graphene doping of TiO₂ generally contributes to the bathochromic shift of the absorption band, hinders the recombination of h⁺/e⁻ by transferring the photoexcited electron to the graphene surface, which also enhances the surface area of TiO₂ for better adsorptive properties [155], the investigation of GTiO₂Ns photoactivity under visible light seems to be crucial. The improvement of performance due to the presence of G in GTiO₂Ns photocatalyst is primarily attributed to the extension of the light absorption range, the increase of absorptivity, and the efficient charge separation and transportation. With GTiO₂Ns photocatalyst, the rate of h⁺/e⁻ recombination after light excitation decreases [156], while the charge transfer rate of electrons increases in addition to the surface-adsorption of chemical species thanks to π-π interactions [157].

However, the properties GTiO₂Ns and different operational parameters, such as the characteristics of substrates, UV-vis or Vis light irradiation, etc., can affect the photocatalytic efficiency [157].

GTiO₂Ns can be generally categorized into three kinds: TiO₂-mounted activated graphene [92], graphene-doped TiO₂, and graphene-coated TiO₂ [158]. Each of these types exhibits good photocatalytic activity. In order to improve the efficiency, the surface properties of graphene could be adjusted via chemical modification, which facilitates its use in composite materials [156,159,160].

GTiO₂Ns can be generally realized in two different ways by “ex situ hybridization” or “in situ crystallization”. In Figure 5, a schematic representation of ex situ hybridization and in situ crystallization in the synthesis of GTiO₂NPs is reported.

Ex situ hybridization (

Table 3. The synthesis methods of Graphene Based TiO₂ nanocomposites.

Ex-Situ Hybridization
Photocatalyst	Synthetic Route	Starting Graphite	Starting Semiconductor	Refs.
rGO/TiO2	Mechanical mixing and ultrasonication	rGO	TiO2NPs in 1-butyl alcohol	[163]
G/TiO2	Mechanical mixing and ultrasonication	G in PVP/water	TiO2NPs anatase in water	[164]
GO/TiO2	Mechanical mixing	GO	Titanium isopropoxide	[165]
In-Situ Crystallization
rGO/TiO2	M with GO and PR reduction	GO	TiO2NPs	[174]
	M and GO and ST reduction	GO	TiO2NPs	[175]
G/TiO2	ST	GO	Tetrabutyl titanate	[145]
G/TiO2	SG	rGO	Titanium isopropoxide	[146]
G/TiO2	HD	GO	P25	[147]
rGO/TiO2	SG	GO	Tetrabutyl titanate	[176]
G/TiO2	SG	G dispersion	Titanium isopropoxide	[177]
rGO/TiO2	Solvothermal SG	GO sheets	TiO2 sol	[178]
rGO/TiO2	SG	GO	Titanium isopropoxide	[179]
G/TiO2	HD	GO suspension	TiO2 sol	[180]
G/TiO2	one-step HD	GO water/ethanol	TiO2	[181]
G/TiO2	HD	GO	Different Ti sources	[182]
G/TiO2	Wet impregnation and thermal reduction (H2)	GO	TiO2 nanotubes from HD of TiO2 P25	[183]
rGO superlong TiO2	HD and heating reflux	GO	Super long TiO2	[184]
GO and rGO/TiO2 (nanocristals)	Elevated pressure HD	GO and rGO	TiO2	[185]
rGO/TiO2 sandwich-like structure	HD/ST	rGO	Butyl titanate	[186]
nanocrystalline anatase TiO2-GO	SG and GO decoration by ST	GO	Anatase TiO2NPs by SG	[187]
TiO2/rGO-X nanocomposites	One-step surfactant (X)-assisted HD	GO	TiO2 (P25)	[188]
rGO films on TiO2 nanotubes	Two-step ED and carbonation techniques	rGO	Two-step anodized TiO2 nanotubes from Ti foils	[189]
Ti plate deposited TiO2 and GO film	ED	GO from nanographite	Ti plate and K2TiF6 aqueous solution	[190]
G/TiO2	MW	GO	TiCl4	[191]
rGO/TiO2	MW	GO	Ti powder	[192]
rGO/TiO2	MW	GO	TiO2NPs	[193]
rGO/TiO2	PR	GO	TiO2NPs	[194]
rGO/TiO2	PR	GO	Colloidal TiO2	[156]

) involves the mixing of graphene dispersions with pre-synthesized TiO₂ nanoparticles (TiO₂NPs) [147]. To enhance their solubility and improve the quality of GTiO₂Ns composites, before mixing, TiO₂NPs and/or graphene sheets can be functionalized by covalent C–C coupling [59,161] or non-covalent π-π stacking reactions [162]. For example, Morawski et al. [163] prepared a visible light-active TiO₂-reduced GO photocatalyst by mechanically mixing TiO₂ with an appropriate mass ratio of reduced GO in 1-butyl alcohol and successive ultrasonication. Yong Liu et al. [164] prepared a graphene aqueous dispersion by sonicating a mixture of graphene raw materials with polyvinylpyrrolidone (PVP) as surfactant and a suspension of TiO₂ by dispersing TiO₂ powder in deionized water with ultrasonication. In this preparation, graphene-TiO₂ composite material was obtained by simple mechanical mixing and sonication using anhydrous ethanol to improve the wettability of the dispersion, while spray-coating was carried out both on polycarbonate and on rubber substrates to facilitate the testing of photocatalytic performance. Ramesh Raliya et al. [165] also prepared nanomaterial/composite solutions by mixing GO at different concentration ratios to TiO₂ and tested their effects on photocatalytic performance.

However, with ex situ hybridization, in some cases it is possible to obtain a low-density and non-uniform coverage of nanostructures by G sheets [113,166,167,168,169].

The most common strategy to synthetize GTiO₂Ns nanocomposites is represented by in situ crystallization (Table 3). In this case, GO or rGO are usually used as starting materials for the presence of oxygen-containing functional groups on their surface. In fact, they act as a nucleation point for grooving and anchoring semiconductor nanocrystals. The successive reduction of GO generates GTiO₂Ns with homogeneous distribution on 2D nanosheets, thereby promoting the direct interaction between semiconductor nanocrystals and G. For this reason, various methods, such as mixing, hydrothermal/solvothermal methods, sol-gel, electrochemical deposition, combustion, microwave, photo-assisted reduction, and self-assemble approaches, can be applied for the synthesis of GTiO₂Ns [66,170,171].

In the mixing method (M), GO and semiconductors are mixed with stirring and ultrasonication in order to obtain the exfoliation of GO and, successively, GO/TiO₂ product with uniform distribution; in a second step the reduction of GO is performed [51,172,173,174,175].

A strong mixing and the best chemical interaction between GO and TiO₂ can be obtained by applying the sol-gel method (SG). As an example, Zhang et al. [176] used a sol-gel method to prepare a series of TiO₂ and graphene sheet composites by using tetrabutyl titanate and graphite oxide and the obtained precursors were calcinated at 450 °C for 2 h under air or nitrogen atmospheres. Zabihi et al. [177] prepared photocatalytic composite thin films of GTiO₂Ns with a low-temperature synthetic process by using G dispersion and titanium isopropoxide bis (acetylacetonate) solution by using a sol-gel chemical method. Spin- and spray-coating was successively used for films deposition; the use of ultrasonic vibration demonstrated a positive effect from both sol-gel and deposition processes that produced rutile and anatase nanoparticles inclused in a matrix of few-layered graphene thin film.

Gopalakrishnan et al. [178] prepared TiO₂-reduced GO nanocomposites in situ from the incorporation of TiO₂ sol into GO sheets and the successive solvothermal method without the addiction of any chemicals. Shao et al. [179] reported a novel so-called gel-sol process for the synthesis of TiO₂ nanorods combining rGO composites by utilizing the triethanolamine as a shape controller and under specific conditions. The as-fabricated hybrid composites presented distinct advantages over the traditional methods in terms of the well-confined TiO₂ morphology and the formation of Ti-C bonds between TiO₂ nanoroads and rGO simultaneously. Long et al. [180] used the hydrothermal method to prepare efficient visible light active GTiO₂Ns by using a GO suspension and TiO₂ sol with undergrown TiO₂ nanoparticles at 413 K. The study of obtained materials demonstrated high chemical interaction at the interface of GO sheets and the polymeric Ti-O-Ti structure that facilitated the retaining of more alkoxyl groups; the induced crystal disorders and oxygen vacancies contributed positively to the performance of the obtained photocatalyst.

In the hydrothermal/solvothermal methods (HD/SD), semiconductor nanoparticles or their precursors are loaded on GO sheets that are successively reduced to rGO involving reactions under controlled temperature and or pressure. For example, Hao Zhang et al. [181] obtained a GTiO₂Ns (P25) nanocomposite photocatalyst with a high adsorptivity of dyes and a greater light absorption range by using a one-step hydrothermal reaction obtaining both GO reduction and P25 cover. Hu et al. [182] successfully synthesized different structures of TiO₂-graphene composites through the hydrothermal reaction by using GO and different titanium sources in hydrothermal conditions. The results showed the larger interfacial contact between TiO₂ and G, in addition to the greater surface area of the poriferous composite. Hamandi et al. [183] studied the influence of the photocatalytic performance of TiO₂ nanotubes (NT) resulting from the addition of GO. TiO₂ nanotubes were prepared using alkaline hydrothermal treatment of TiO₂ P25 followed by calcination at 400 °C under air, while GO-NT composites were obtained by the wet impregnation of the as-prepared TiO₂ nanotubes onto graphene oxide before reduction under H₂ at 200 °C. Sijia et al. [184] synthesized a novel composite photocatalyst, reduced graphene oxide (rGO)-modified superlong TiO₂ nanotubes with a length of about 500 nm, by an improved hydrothermal process and a heating reflux method.

A hybrid nanocomposite containing nanocrystalline TiO₂ and graphene-related materials (GO and rGO) was prepared from Kusiak et al. [185] by using the hydrothermal method under elevated pressure. In this preparation, the presence of graphitic carbon depends on temperature, the type of modification method, and the preparation conditions, obtaining multilayer GO flakes in comparison to rGO flakes. This multilayer character produced the most intensive time resolved microwave conductivity signals under both UV and visible illumination for materials modified with GO, indicating that more electrons are induced in the conduction band of hybrid nanocomposites. Li et al. [186] synthesized reduced graphene oxide-TiO₂ (rGO-TiO₂) composites with a sandwich-like structure by using a simple solvothermal method. Hemraj et al. [187] synthesized nanocrystalline anatase TiO₂-GO nanocomposites by two steps with the initial preparation of anatase TiO₂NPs by a simple sol-gel method and the successive decoration of GO on its surface by using a facile solvothermal method. Ju Hu et al. [188] synthesized TiO₂/reduced graphene oxide nanocomposites by using a one-step surfactant-assisted hydrothermal method. Compared with the control TiO₂/rGO nanocomposite, TiO₂/rGO-X (X = sodium dodecyl benzene sulfonate, Triton X-100, and acetyl trimethyl ammonium bromide) presented excellent photocatalytic activity. The study indicated that the surfactant-assisted hydrothermal method is an effective approach to improve the structure, morphology, and photocatalytic performance of TiO₂/rGO composites.

Semiconductor nanostructures can be electrolytically deposited on the surface of graphene-based substrates, at low temperature condition, from an aqueous solution containing the semiconductor. For example, Ming-Zheng Ge et al. [189] adopted both electrodeposition and carbonation methods to deposit rGO films on TiO₂ nanotubes obtained with two-step electrochemical anodization. Recently, Guimaraes de Oliveira et al. [190] used the electrochemical method to deposite GTiO₂Ns composite film on a Ti substrate by using GO and Ti(IV) aqueous solution, demonstrating a photocatalytic activity almost twice as high as that observed for the TiO₂ only film.

The microwave-assisted strategy, in this context, addresses the synthesis of semiconductor nanomaterials of a controlled size and shape. For example, Shanmugama et al. [191] synthesized GTiO₂Ns nanocomposites by a novel surfactant-free, environmentally friendly one-pot in situ microwave method. This method leads to the uniform distribution of TiO₂ nanoparticles on G sheets with the binding nature of TiO₂. Wang et al. [192] reduced GO with both direct and microwave-assisted reduction in the presence of Ti powders, showing that the microwave effect decreased the reduction time. In this case, Ti ions derived from the reaction of Ti powder with GO were hydrolytically transferred to TiO₂ with the formation of rGO-TiO₂, which was proven to be an active material in the removal of methylene blue. Yang et al. [193] prepared nanocomposites of TiO₂ and rGO, with a fast and simple microwave irradiation method by the reaction of graphene-oxide and commercial TiO₂ nanoparticles in water/ethanol solvent, examining different microwave powers with different time intervals. The effect of time and power on absorption wavelength and the creation of defects in the graphene layer was examined, obtaining an optimum irradiation time in which the nanocomposite presented the highest absorption wavelength (the smallest band gap), and the optimum value for microwave power in which the nanocomposite had the lowest number of defects. First, GO was prepared using a modified Hummer’s method, and then TiO₂ and the mixture of water/ethanol and graphene-oxide (GO) was used for the hydrolysis of TiO₂, followed by its mounting on graphene-oxide. In this way, microwave irradiation reduced GO to graphene with the formation of TiO₂/rGO nanocomposites.

Other methods have been investigated, such as that proposed by Pu et al. [194] regarding the synthesis of TiO₂-rGO nanocomposites, conducted simultaneously with the photoreduction (PR) of GO nanosheets in a simple GO and TiO₂ ethanol system, demonstrating a facile and environmentally friendly strategy for the in situ preparation of the TiO₂-rGO hybrid “dyade”. In Table 3, the principal synthesis methods of GTiO₂Ns are summarized.

5. Photocatalytic Degradation of Dyes with GTiO₂Ns

Dye molecules, which are solubilized during several application processes, present different colors derived from the selective absorption of light. Coloring properties of dyes depend on their chemical structure consisting of two fundamental components: the chromophore and the auxochrome. The chromophore is a covalently unsaturated group, which is responsible for the color production for the absorption in the UV or visible region; the auxochrome is supplementary to the chromophore, and influences the solubility in water and the affinity towards a particular support [195]. Concerning their uses, dyes can be anionic, cationic, and non-ionic; anionic dyes are divided into direct, acid, and reactive dyes, while cationic dyes are basic [196]. Water treatments by using GTiO₂Ns in the photocatalytic degradations of dyes became very important due to the danger of dyes as environmental pollutants [142].

Methylene blue (MB) was first prepared by Caro in 1876 as an aniline-derived dye for textiles, and is largely used as a water-soluble cationic dye in the dyeing of paper, wood, plastic, silk, and cotton, as well as in scientific research and pharmaceutical industries [197,198]. Kyeong Min Cho et al. [8], to obtain MB photodegradation, used hydrothermal methods in the self-assembly of TiO₂ precursors, GO, and a surfactant to prepare rGO mesoporous TiO₂; this material showed high photocatalytic performances in the MB degradation test, good ability in the charge separation, and a large surface area compared to typical GTiO₂Ns.

Hanan H. Mohamed [199] recently described the simultaneous photoassisted oxidation of MB by using new synthesized hierarchical structures of anatase/rutile TiO₂ microsphere-rGO nanocomposites. Interestingly, rutile/anatase phase transformation was observed upon changing the GO content. The synthesized nanocomposites showed enhanced photocatalytic activity for the dye degradation compared to pristine TiO₂ nanoparticles that was attributed to the synergetic effect between anatase and rutile in the synthesized TiO₂ microsphere composites. Also, Zhang et al. [200] synthesized graphene-supported mesoporous titania nanosheets (GTiO₂Ns) by using liquid-phase exfoliated G as a template and sandwich-like G-silica as intermediates. GTiO₂Ns showed a mesoporous structure formed by crystalline TiO₂ nanoparticles anchored on G nanosheets that showed high surface areas. These characteristics significantly enhanced the photocatalytic activity of TiO₂ in the MB degradation.

Najafi et al. [201] fabricated TiO₂-GO nanocomposites with different nanowire and nanoparticle morphologies of TiO₂ by using a one-step hydrothermal method. The different morphologies of TiO₂ were grown on the surface of the GO sheets, previously synthesized by the modified Hummer’s method. The obtained TiO₂-GO nanocomposites showed a tetragonal structure and covalent bonds between TiO₂ nanostructures and GO sheets. The best photodegradation rate of MB was found by using TiO₂ nanowire-GO nanocomposites. Divya et al. [202] used microwave irradiation of different GO weights and tetrabutyl titanate in isopropyl alcohol to obtain TiO₂ hybridation with the formation of GTiO₂Ns nanocomposites. In this method, the irradiation with microwaves increase the temperature in a short time, favoring the GO reduction to G and the growth of TiO₂ nanoparticles on the G surface. The obtained GTiO₂Ns nanocomposites presented efficient electron conductivity in G, therefore showing a reduced electron-hole recombination rate.

Another example is that of Sohail et al. [203], who prepared GO nanosheets by using a modified Hummer’s technique with the successive conversion into particles by a spray-drying method. TiO₂ nanoparticles were grown on the surface of GO particles to obtain rGO-TiO₂ nanocomposites that showed better photocatalytic activities upon the degradation of MB compared to pure TiO₂, also showing in this case that rGO played an important role in the charge recombination to enhance the electron-hole separation.

Tseng et al. [204] studied a facile process to fabricate two-dimensional titania nanosheets (t-NS) by using GO as a support with in situ growth of anatase TiO₂ on GO suspended in butanol by combining the sol-gel and solvothermal processes without further calcination. Significant amounts of MB molecules can easily be adsorbed on the intrinsic graphitic structure. An optimum rate of hole titration by ethylenediaminetetraacetic acid was essential to efficiently reduce the recombination of charge carriers and consequently produce more active radicals to decompose MB. The strong interaction between graphitic and titania structures, rather than the crystallite size of anatase, dominated the photoreduction capability of t-NS.

Similarly, Minella et al. [205] obtained rGO-TiO₂ and rGO-SiO₂ hybrid materials by reducing different quantities of GO with hydrazine in the presence of TiO₂ and SiO₂ nanoparticles; the photocatalytic activity of these nanomaterials was evaluated in the degradation of MB under both UV-vis and only visible light irradiation. The results showed that MB was strongly adsorbed on these new materials with an increase of degradation rates with respect to pristine semiconductors. The visible light degradation of MB was attributed to the dye-sensitization mechanism, while the UV-vis degradation was considered to be the typical semiconductor photocatalytic mechanism.

“Biphasic TiO₂ nanoparticles” and their G nanocomposites were synthesized by Raja et al. [206] by the hydrothermal method. Introducing high surface area, G suppressed the electron-hole pair recombination rate in the nanocomposites. Further, the nanocomposites showed a red-shift of the absorption edge with a contraction of the band gap from 2.98 eV to 2.85 eV. The characterization of photocatalytic activities under natural sunlight and UV-filtered sunlight irradiation revealed that the GTiO₂Ns composite exhibited about a 15- and 3.5-fold increase in the degradability of Congo red and MB dyes, respectively, in comparison to pristine TiO₂. The authors therefore developed a visible light active G composite catalyst that can degrade both cationic and anionic dyes, making it potentially useful in environmental remediation and water splitting applications under direct sunlight.

The ex situ hydrothermal method was proposed from Verma et al. [207] to synthesize rGO mixed TiO₂ nano-composites. The study demonstrated that the cooperation between the optimal phase ratio of TiO₂ (anatase/rutile) and rGO produced a positive system in the photocatalytic degradation test with MB other than antibacterial activity. Atchudan et al. [208] used a solvothermal method to prepare GO grafting TiO₂ nanocomposites and studied it as a photoactive material under UV-light irradiation in the degradation of MB and methyl orange. The obtained degradation efficiency, compared with only TiO₂, demonstrated the important function that GO played in the increase of performance due to the increase of light absorption and reducing charge recombination.

Darvishi et al. [209] instead used titanium butoxide (TBO) as a TiO₂ precursor, commercial TiO₂, and GO in a water/ethanol mixture to produce hydrolytically, and under microwave irradiation, rGO-TiO₂ nanocomposites; these showed high conductive and light absorption properties, demonstrating an enhancement of photocatalytic MB degradation. In these nanocomposites, the content of G improved the photocatalytic performances of the photocatalysts.

An example of an eco-friendly method was studied by Rezaei et al. [210], who synthesized TiO₂-graphene nanocomposites by the addition of the “blackberry juice” to GO as a reducing agent to produce rGO nano-sheets. The obtained TiO₂ (anatase)-rGO materials exhibited an excellent photocatalytic activity toward MB degradation due to the enlarged surface area and the collaborative effect of rGO. Shanmugam et al. [191] synthesized GTiO₂Ns nanocomposites by a surfactant-free, environmentally friendly one-pot in situ microwave method. TiO₂ nanoparticles of 5–10 nm were distributed on the G sheets. The photocatalytic activity of pure TiO₂ and GTiO₂Ns nanocomposites was studied under UV and visible light irradiation sources with MB. The studies revealed the highest degradation efficiency of 97% with UV light and 96% with visible light irradiation by using the GTiO₂Ns photocatalyst, opening the possibility of using this material for industrial wastewater treatment.

The sol-gel technique was used by Rezaei [211], who prepared GO-TiO₂ nanocomposites by evaluating the photocatalytic activities of MB aqueous solution under sunlight irradiation. The results showed that the nanocomposite containing 9.0 wt % of GO had the highest photocatalytic performance in the MB degradation to either “single-phase anatase” or other composites containing different GO quantities. The improvement of the photocatalytic activity was attributed to the favorable effect of the distribution of TiO₂ nanoparticles of less than 20 nm on GO sheets.

UV-assisted photocatalytic reduction of GO by TiO₂ nanoparticles in ethanol was used by Charoensuk et al. [212] in the preparation of rGO-TiO₂ nanocomposites. The photocatalytic activity of prepared rGO/TiO₂ and GO/TiO₂ nanocomposites was evaluated by the kinetics of the photocatalytic degradation of MB under UV irradiation. Significant roles in the MB photodegradation were played by important factors such as the bandgap, electron-hole recombination, characteristics of surface (area and functional groups), and adsorption capacity of nanocomposites. The results revealed that rGO/TiO₂ and GO/TiO₂ nanocomposites exhibited efficient charge separation, showing about 500% improvement of photocatalytic activity in MB photodegradation compared to pristine TiO₂.

The solvothermal technique was used by Wang et al. [213], who prepared an efficient photocatalyst of nano TiO₂-functionalized GO nanocomposites. This nanomaterial showed an quadruple increase in MB photodegradation activity with respect to that obtained by using the P25-graphene composite photocatalyst. Liu et al. [214] instead used an interesting impregnation-hydrothermal method to prepare highly distributed TiO₂ nanoparticles with in situ growth on functional G (FG), which was before obtained by modifying GO by using triethanolamine. The obtained FGTiO₂Ns photocatalyst showed better photocatalytic activity under UV light irradiation with respect to pure TiO₂ and GO-TiO₂ prepared by other similar methods, also revealing considerable photocatalytic skill under visible light irradiation.

A fruitful method to produce expanded exfoliated GO (EGO) was used by Baldissarelli et al. [215]. The use of ozone exposure and thermal treatment transformated the graphite surface to a G-like surface containing oxygen and sp³ carbon. The EGO, obtained through thermal treatment, produced, with the addition of TiO₂ nanoparticles, photocatalytic TiO₂-EGO nanocomposites that demonstrated higher activity with respect to that produced by the common Hummer’s method. In particular, TiO₂-EGO showed enhanced MB photodegradation under UV light compared to TiO₂ P25.

A new nanomaterial composed of ultrasmall TiO₂ nanoparticles and rGO nanosheets was prepared by Gu et al. [216], who used glucosamine in alkaline conditions under hydrothermal treatment. In this synthesis, glucosamine regulated the growth and homogeneity of TiO₂ nanoparticles dispersed on the G structure, permitting the formation of the expected GTiO₂Ns products, demonstrating that TiO₂ nanoparticles about 13 nm in diameter were strongly anchored on G. Different GTiO₂Ns samples were calcined at several temperatures; in particular, the GTiO₂Ns sample treated at 700 °C showed good photocatalytic activity in MB photodegradation compared to those produced at other calcination temperatures.

Moreover, Fan et al. [217] used the hydrothermal method to synthesize TiO₂ nanospindles, which are featured by large exposed {0 0 1} facets, and then fabricated TiO₂ nanospindles/reduced GO nanocomposites (rGO-TiO₂). The photocatalytic activity in the photodegradation of MB showed an impressive photocatalytic enhancement of rGO-TiO₂ with respect to pure TiO₂ nanospindles. In addition, Sun et al. [218] used a one-step in situ hydrothermal method to prepare chemically bonded GTiO₂ nanorods hybrid composites with high dispersity by using GO and TiO₂ (P25) as the starting materials without using reducing agents. Ultraviolet-visible diffuse reflectance measurements of the composites showed an enhanced light absorption and a red shift of absorption edge. When GTiO₂Ns nanorods hybrid composites were used as photocatalysts, they showed an enhancement of photocatalytic performance in the photodegradation of MB under visible light irradiation compared to that of pristine TiO₂ nanorods.

Rong et al. [219] prepared different GTiO₂Ns photocatalysts with a simple hydrothermal method. First, GO (obtained from graphite oxidation) was dispersed in a water/ethanol solution with ultrasonic treatment, after which TiO₂ was added and a thermal treatment at 120° for 12 h was used. A series of GTiO₂Ns photocatalysts obtained with different oxidation times (in the GO production from graphite) and GO contents were obtained. When these materials were applied to the photodegradation of MB under visible light irradiation, they exhibited excellent photocatalytic activities.

Gupta et al. [220] proposed a strategy to fabricate G quantum dots (GQDs) infilled TiO₂ nanotube arrays hybrid structure (GQDs-TiO₂NTAs) for the application of MB degradation under UV light irradiation. In particular, anodic oxidation of a Ti sheet by using an impregnation method produced the fill of GQDs into TiO₂ NTAs. The application of this material in the adsorption and photodegradation of MB in an aqueous solution under UV light irradiation showed high photocatalytic efficiency, which was attributed to the favorable visible light absorption and to efficiency in the transfer of photogenerated electrons from the TiO₂NTAs to GQDs that produced large photoinduced charge separation. No less important was the strong adsorption capacity of the GQDs to MB molecules.

GP-wrapped TiO₂ (GwTiO₂) hybrid material was fabricated by Ni et al. [221] with the simultaneous reduction and wrapping of GO on the surface of high-reactive anatase TiO₂, based on the surface negatively charged property of GO. GwTiO₂ gave a strong red-shifted light absorption edge and a contracted bandgap compared to that of GP randomly supported TiO₂; this behavior was attributed to the positive chemical interaction between TiO₂ and G. The photocatalytic MB degradation under a xenon lamp and visible light irradiation confirmed the best quality of this nanomaterial. Furthermore, Yang et al. [222] prepared TiO₂/G porous composites with rGO by using a template of G microsphere colloidal crystals that showed high light absorbing properties, and good MB adsorption properties, other than a fast ability in the charge transportation and separation. These porous composites showed high degradation activities in MB degradation under visible light irradiation with a constant rate almost 6.5 times higher than that of P25 under the same conditions.

Also, Suave et al. [223] prepared photocatalytic TiO₂ nanocomposites with different loads of ozonated graphene (OGn) and evaluated the performances in photocatalytic MB degradation and other dyes using UVC irradiation. Although the photocatalytic activity decreased with each cycle, it is possible to reuse the nanocomposite that facilitates its practical application for water treatment.

In

Table 4. Photocatalytic applications of Graphene Based TiO₂ nanocomposites in the degradation of Methylene Blue.

Photoactive Nanomaterials	Dye Conc. (mg/L)	Catalyst Quantity (g/L)	Light Source	Irradiation Time (min)	Degradation (%)	Refs.
G/TiO2	9.60	0.33	Visible	180	90	[8]
P25-G	8.64	0.6	UV	60	85	[199]
P25-G	8.64	0.6	Visible	60	65	[199]
G/TiO2	10	0.6	UV	120	100	[200]
G/TiO2	10	0.2	UV	40	85	[202]
TiO2@rGO	--	0.1	UV	120	92	[203]
GO/TiO2	15	0.2	UV	350	92	[204]
TiO2-G	0.13	0.5	UV-Vis	450	100	[205]
rGO/TiO2	320	0.5	Visible	90	95	[207]
TiO2/GO	--	0.2	UV	25	100	[208]
TiO2/G	0.13	0.4	UV	60	96	[209]
TiO2/G	3	0.48	Visible	90	100	[210]
G/TiO2	3.2	0.2	UV	180	97	[191]
G/TiO2	3.2	0.2	Visible	240	96	[191]
TiO2/GO	3	0.48	Visible	60	94	[211]
TiO2/GO	5	0.1	UV	40	93	[213]
TiO2/GO	5	0.1	Visible	40	70	[213]
TiO2-Graphite Oxide	10	0.5	UV	60	100	[215]
TiO2/G	10	0.2	UV	20	97	[216]
TiO2/rGO	10	0.17	UV	60	100	[217]
G/TiO2	5	0.5	Visible	100	70	[218]
TiO2/G	10	0.8	Visible	100	98.8	[219]
Graphene quantum dots/TiO2	6.4	--	UV	180	100	[220]
TiO2/GO	10	0.1	UV	180	100	[221]
TiO2/G	10	0.01	Visible	150	100	[222]
TiO2/G	10	0.5	UV	90	100	[223]
Graphene quantum dots/TiO2	6.4	--	UV	180	100	[220]

, the different preparation methods of GTiO₂Ns applied to MB photodegradation are listed.

Methyl orange (MO) is a well-known anionic azo dye largely used in the textile, printing, paper, food, pharmaceutical, and research fields [224]. As an example, MO photodegradation was studied by Lavanya et al. [225], who combined novel reduced graphene oxide (rGO) with anatase/rutile mixed phase TiO₂ nanofibers (MPTNFs), by using electrospinning and easy chemical methods, to enhance the photocatalytic performance. The photocatalytic activity in the photodegradation of MO showed a significant increase in the reaction rate for the synergistic effect of anatase/rutile mixed phase in one-dimensional nanostructures, and the electronic interaction between TiO₂ and rGO that provided the improvement of electron transfer reduced the charge recombination for the enhancement of catalytic efficiency.

Xu et al. [63] used a two-step method to prepare G-pasted TiO₂ spheres. In the first step, the total cover of the surface of TiO₂ spheres with small GO sheets was obtained, while in the second step, GO was photocatalytically reducted in situ. G-pasted TiO₂ spheres presented more interfaces between G and TiO₂ with favorable interaction under the light irradiation compared to that of typical GTiO₂Ns composites. As a result, these materials demonstrated higher photocatalytic activity in MO degradation. Hou et al. [226] instead used hydrothermal synthesis to obtain rGO-TiO₂ composites, and part of these were successively calcined at 450 °C (CS-rGO-TiO₂). Photocatalytic activities in the decomposition of MO under UV, visible, and solar light by using both RGO-TiO₂ and calcinated composites were tested. The amount of G apparently influenced the photocatalytic activities of both rGO-TiO₂ composites and CS-rGO-TiO₂, which showed better photocatalytic activity compared to that of rGO-TiO₂. These results were due to the calcination that favored the oxidation of residual organics in the rGO-TiO₂ and a better crystallization of TiO₂ nanoparticles with smaller diameters.

Zhao et al. [227] synthetized three-dimensional nanocomposites with TiO₂ nanotubes (TNTs) and rGO nanosheets by using the hydrothermal method. The obtained rGO/TNTs nanocomposites presented sufficient active sites and gave the path of good electron-transport. These nanomaterials showed better photocatalytic activity in MO degradation under UV-light irradiation with respect to traditional TiO₂ nanotubes. Also, Lu et al. [228] combined TiO₂ and GO and used UV irradiation to synthesize rGO-TiO₂ composites. The influence of TiO₂ quantity and UV irradiation time on the rGO formation during the composite synthesis was investigated. The results demonstrated that a longer UV irradiation time corresponded to a higher reduction degree of GO, and therefore to a higher photodegradation efficiency of the composites in MO photodegradation. However, the results also showed that excessive UV irradiation times gave a negative effect on the photodegradation efficiency and therefore proposed a mechanism that correlated the reduction degree of GO with the photodegradation efficiency of composites.

Chemisorption and successive heating at different temperatures was applied by Xia et al. [229] to combine TiO₂ in different anatase and rutile phases with G sheets. The photoactivities of these nanomaterials were tested by using MO under UV and visible light irradiation, demonstrating an enhanced respect to only TiO₂. Notably, the different crystallite phases of TiO₂ reacted with different behaviors under UV and visible light irradiation due to the different charge transfer mechanisms.

Another example was described by Ge et al. [230], who combined electrodeposition and carbonation techniques to obtain the deposition of rGO films on TiO₂ nanotube arrays (TiO₂ NTAs); these materials, prepared in ethylene glycol by using two-step electrochemical anodization, showed an enhanced MO photocatalytic degradation with respect to both pristine TiO₂ NTAs and annealed TiO₂ NTAs under the same conditions. The increase of efficiency in photocatalytic degradation was due to the increase of separation efficiency of photoinduced electrons and holes, to red-shift in the UV absorption and to the profitable adsorbent skill of rGO towards the MO dye.

GTiO₂Ns nanocomposites were instead synthetized by Han et al. [231] by using the hydrothermal method and a successive annealing process. In this preparation, the loading of flocculent-like TiO₂ nanostructures onto G sheets was obtained. The evaluation of photoelectrochemical activities and the photocatalytic degradation of MO under UV light irradiation showed that the flocculent-like GTiO₂Ns composites presented the greatest photocatalytic activity compared to that of commercial anatase TiO₂, due to the increase of light absorption and fruitful charge separation of the nanocomposite structure.

In order to synthesize rGO wrapped with anatase mesoporous TiO₂ nanofibers, Lavanya et al. [232] used electrospinning and easy chemical methods. The interface between rGO and TiO₂ nanofibers in these composites favored an efficient photogenerated charge carrier separation with an enhanced efficiency of photocatalytic MO degradation of 96% instead of only 43% of TiO₂ nanofibers. Wang et al. [233] applied a low-temperature solvothermal method using graphite oxide and TiCl₃ as precursors to prepare GTiO₂Ns nanocomposites. During this solvothermal transformation, the synthesis of TiO₂ nanoparticles and G formation occurred. Compared to only TiO₂ nanoparticles, also in this case, the results revealed a greatly enhanced visible light photocatalytic MO degradation.

Athanasekou et al. [234] prepared rGO-TiO₂ composite membranes for incorporation into a water purification device for the application in the hybrid photocatalytic/ultrafiltration process. For this, GO sheets were decorated with TiO₂ nanoparticles and deposited into the pores of ultrafiltration mono-channel monoliths using the dip-coating technique and water filtration conducted in the dark, as well as UV and visible light irradiation. The obtained results, compared with that of standard nanofiltration, showed that the synergetic effects of GO on MO adsorption and photocatalytic degradation demonstrated that this material can be positively applied in nanofiltration technology.

Liu [235] prepared rGO-TiO₂ with the hydrothermal method by using GO and Ti(OH)₄ in ethanol/water. The results showed that rGO-TiO₂ presented a stratified structure composed of dispersed anatase TiO₂ on the surface of rGO; the larger surface area of the composite was favorable for the MO absorption; in addition, the extension of the visible light region absorption range was observed. All these factors positively favored the photocatalytic performances of the composites in the photodegradation of MO under visible light irradiation with respect to that of pure TiO₂.

In

Table 5. Photocatalytic applications of Graphene Based TiO₂ nanocomposites in the degradation of Methyl Orange.

Photoactive Nanomaterials	Dye Conc. (mg/L)	Catalyst Quantity (g/L)	Light Source	Irradiation Time (min)	Degradation (%)	Refs.
rGO/TiO2 (mix anatase/rutile nanofibers)	10	0.4	UV	120	97	[225]
G-pasted TiO2 spheres	12	0.5	UV	75	95	[63]
GO/TiO2	13	1	UV	60	88	[226]
		1	Vis	60	80
rGO/TiO2		1	UV	60	70
		1	Vis	60	99
rGO/TiO2 anotube	20	0.25	UV	210	100	[227]
rGO/TiO2	10	0.5	UV	75	70	[228]
rGO/TiO2 mix anatasio/rutilio	6.55	0.6	UV	100	100	[229]
			Vis	100	50
G/TiO2/Magnetite	9.6	0.16	UV	90	99	[230]
Flocculent likeTiO2/G	20	0.8	UV	60	70	[231]
rGO/TiO2 nanofibers	15	0.4	UV	120	100	[232]
G/TiO2	10	0.6	UV	60	80	[233]
TiO2/rGO	10	0.5	Vis	240	90	[235]

, the different preparation methods of GTiO₂Ns applied in MO photodegradation are summarized.

In addition, other authors explored photocatalytic materials in the photodegradation of Rhodamine-B (RB), which is a brilliant soluble basic dye, most used in the dyeing of various products of the textile industry such as cotton, silk, paper, leather, and others. Notably, RB imparts an intense color to polluted wastewaters derived from different industries and from scientific laboratories [236]. In particular, for RhB photodegradation, Chen Q. et al. [237] used electrospinning and the hydrothermal reaction in mixed ethanol/water solution to prepare nanocomposites composed from rGO and TiO₂ nanotubes. In these nanocomposites, TiO₂ nanotubes combined with rGO sheets between Ti-C and Ti-O-C bonds and the absorption edge shifted to higher wavelengths, improving the visible light absorption with respect to pure TiO₂ nanotubes. The measurements of photocatalytic activities regarding RhB photodegradation under xenon lamp irradiation showed that the highest photocatalytic activities were obtained when the rGO ratio increased to 10%.

Furthermore, Chen Y. et al. [238] used a hydrothermal method to prepare rGO and TiO₂ hybrid of 10–20 nm, by using TiO₂ P25 nanoparticles and “liquid-exfoliated” GO. Also in this case, rGO-TiO₂, compared to only TiO₂, showed an increased photocatalytic RhB degradation under Xe lamp irradiation. The rGO-TiO₂ characterization confirmed the enhancement of photocatalytic and could be attributed to two reasons. The first was that rGO extended the path and lifetime of photogenerated electrons of TiO₂, minimizing the recombination of electron–hole pairs. while the second was that rG expanded the light absorption spectrum versus the visible light range. These effects were explained by the change of the energy gap and the likelihood of the up-conversion photoluminescence mechanism.

Another example was reported by Wang et al. [239] about the immobilization of anatase TiO₂ nanosheets on the magnetically actuated artificial cilia film by using rGO as the contact medium. The artificial cilia film was optimal in the immobilization of more powder of photocatalysts and, when used under s rotating magnetic field, exhibited an improvement of RhB degradation efficiency due to high mass transfer and to the efficiencies of photoproducts desorption. Biris et al. [240] synthesized core-shell nanostructural materials with multi-component architectures from TiO₂ and layers of graphitic materials by application CVD for 5, 10, 30, and 45 min with methane as the carbon source. The reaction time linearly regulated the quantity of G shells covering the TiO₂ surfaces, obtaining nanostructured materials with excellent stability and photocatalytic activity in the UV RhB degradation.

Moreover, Kim et al. [241] used synthesized composites of flower-like TiO₂ spheres and rGO (FTS-G). First TiO₂ spheres, with high specific surface area and good pore structure, and rGO without the use of strong reducing agents, were separately prepared; next, FTS-G were synthesized by using a solvothermal method. The photocatalytic performance of FTS-G was evaluated for RhB photodegradation under solar light irradiation, establishing that the rGO quantity was very important for the effects on photocatalytic activity. This preparation method could also be used to prepare other photocatalysts composed of flower-like TiO₂ spheres with carbon materials. Liu et al. [242] prepared a photocatalyst based on TiO₂ nanotubes and rGO with the hydrothermal method that was successively tested in the photocatalytic degradation of RhB under UV-light irradiation. Also with this photocatalyst, thanks to the introduction of rGO on TiO₂ nanotubes, the adsorption capacity as well as the photocatalytic activity in RhB photodegradation increased compared to the same without rGO.

Sedghi et al. [243], by using Titanium(IV) chloride as a photocatalyst precursor, prepared different nanocomposites of porous and magnetic porous GO in the mix with TiO₂ (anatase) and TiO₂ (mix phase), respectively. Between these nanocomposites, the mix of TiO₂ with magnetic porous GO showed enhanced efficiency in the RhB degradation under visible light irradiation compared with that of the other photocatalysts, obtaining 100% degradation in less than 20 min. Similarly, Zhang et al. [244] used the one-pot hydrothermal process, by using tetrabutyl titanate and GO, to prepare a series of self-assembled composites of anatase TiO₂ nanocrystals and three-dimensional graphene aerogel. The obtained composites showed higher adsorption capacities and good visible light efficiency in the RhB photodegradation. The results of this study clarified that the chemical-physical properties of these composites could be attributed to their 3D nanoporous structure with high surface areas and to the contemporary action of G nanosheets and TiO₂ nanoparticles.

An ultrasonication-assisted reduction process was used from He et al. [245] to produce rGO-TiO₂ nanocomposites. In this process, simultaneous GO reduction and TiO₂ crystals formation were obtained; photocatalytic studies demonstrated that the quantities rGO in these nanocomposites influenced the RhB degradation efficiency under visible light irradiation. The obtained increase in the photocatalytic activity was attributed in this case to the harvesting under visible-light irradiation and to the efficiency in the separation of the photogenerated charge carriers, also obtaining an improvement in the photoelectric conversion efficiency.

Fang et al. [246] exfoliated GO sheets from graphite and wrapped on the surfaces of polymer microspheres of about 2.5 μm in diameter. Successively, the solvothermal method permitted the nucleation and growth of anatase TiO₂ nanoparticles on the GO surfaces and the contemporary GO reduction to rGO. The as-prepared GO-skined GO/polymer hybrid microspheres composites tested in the photocatalytic activity with RhB solutions gave 96% degradation in 30 min under visible light irradiation, demonstrating an improvement with respect to commercial TiO₂. The results were attributed to the favorable interactions between rGO of singular electrical conductivity and TiO₂ nanoparticles.

Liang et al. [247] used a one-step hydrothermal approach to synthesize chemically bonded TiO₂/rGO nanocomposites by using Ti(SO₄)₂ and GO as precursors and ethanol/water solvent as a reducing agent, obtaining well-dispersed TiO₂ nanocrystals on the surface of rGO sheets with optimal interfacial contact. The obtained nanocomposites demonstrated higher photocatalytic activity with respect to only TiO₂ nanoparticles and to a simple mixing of TiO₂ and rGO samples in the RhB degradation which was attributed, also in this case, to the interaction between TiO₂ and rGO, with excellent electron trapping and transportation properties.

Li et al. [248] obtained different carbon materials-TiO₂ hybrid nanostructures incorporated into TiO₂, activated carbon, G, carbon nanotubes, and fullerene by using a solvothermal method and thermal annealing. The relationships between the interactions of carbon materials with TiO₂ nanoparticles, morphologies, structures, and increasing photodegradation performances in RhB degradation were clarified. The results of these photocatalysts were due to higher adsorption properties, the favorable formation of chemical bonds of Ti-O-C, reduced band gap, lower particle size, and charge-carrier qualities. In

Table 6. Photocatalytic applications of Graphene Based TiO₂ nanocomposites in the degradation of Rhodamin B.

Photoactive Nanomaterials	Dye Conc. (mg/L)	Catalyst Quantity (g/L)	Light Source	Irradiation Time (min)	Degradation (%)	Refs.
TiO2/rGO	30	0.2	Vis	40	100	[238]
rGO/TiO2 nanosheets onmagnetically cilia film rGO/TiO2-Au	20	0.2	UV	180	83 100	[239]
Core-shell TiO2/G	4.79	0.6	UV	270	100	[240]
Flower-like TiO2 sphere /rGO	15	0.4	Simulated solar	120	100	[241]
TiO2 nanotubes/rGO	10	0.5	UV	20	100	[242]
TiO2/magnetic porous GO	10	0. 1	Vis	20	100	[243]
3D TiO2/G aerogel	20	0.2	Visible light	180	99	[244]
TiO2/rGO	4.79	0.4	Visible light	180	100	[245]
TiO2/rGO/polymer	8	0.2	Visible light	30	96	[246]
TiO2/rGO	20	0.5	UV	30	100	[247]
GTiO2	5	0.5	Visible light	60	80	[248]

, the different preparation methods of GTiO₂Ns applied in RhB photodegradation are listed.

Rhodamine 6G (Rh6G) is dark reddish-purple dye, mostly used in the textile industries, in biochemistry research laboratories, and in other applications. Rh6G is a non-volatile compound that presents high solubility in water causing skin, eye, and respiratory system irritations, and is also carcinogenic and poisonous to living organisms [249].

In order to obtain the photocatalytic degradation of Rh6G, Pu et al. [194] demonstrated a facile and environmentally friendly strategy for the simultaneous in situ preparation of the rGO-TiO₂ “dyade hybrid” obtained by the photoreduction of GO nanosheets in ethanolic solutions of GO and TiO₂ (P25) mix. Successively, the photodegradation efficiency of the resultant composite by utilizing Rh6G as the target pollutant was systematically investigated. The obtained rGO-TiO₂ presented a significant enhancement in photo energy adsorption, leading to the effective photocatalytic degradation reactions which exhibited more than triple the higher photodegradation rate compared to commercial TiO₂ (P25) nanoparticles.

Reactive Black 5 (RB5), which has two azo groups, belongs to the class of azo reactive dyes, which are abundantly used in textile industries for dyeing.

The photocatalytic degradation of RB5 was studied by Day et al. [250], who prepared different types of GTiO₂Ns nanocomposites with G and P25 or Titanium (IV) n-butoxide as precursors by using the hydrothermal method. To investigate the differences of the photocatalytic properties of composites with various TiO₂ shapes, nanotubes, nanosheets, and nanoparticles of TiO₂ were prepared and combined with G. The photocatalytic activities of these composites were tested in the degradation of RB5 under UV lamp irradiation, indicating that the differences in the TiO₂ morphology and the characteristics of heterojunction between G and TiO₂ played an important role in the photocatalytic abilities. The results demonstrated that these composites exhibited excellent adsorption capacity and higher photocatalytic activity with respect to P25, and that the photocatalytic activity of GTiO₂Ns nanosheets was higher than those of the other tested photocatalysts. Another example was proposed by Liang et al. [251], who obtained an increase of photocatalysis efficiency for RB5 by using TiO₂ nanotubes and G nanocomposites prepared with the hydrothermal method. In particular, Zhang et al. [252] deciphered the mechanism of RB5 photodegradation, which was used as a model for other azo dyes by using G-loaded TiO₂ as a photocatalyst. They demonstrated that electronic transfer groups affect the degradability of azo dyes, that ^•OH radicals are the major oxidized species, and that substituents with high electron-density in the dye structure were more favorably attacked by ^•OH radicals.

The photodegradation of the non-biodegradable azo dye acid orange 7 (AO7) in aqueous solution was studied by Posa et al. [253] who, by using GTiO₂Ns nanocomposites, obtained an improvement of photocatalytic performance compared with that of pure TiO₂, examining also the reusability of the photocatalyst. They suggested a reaction mechanism on the basis of the obtained results. Furthermore, Gao et al. [254] synthesized different TiO₂ nanostructures: one-dimensional TiO₂ nanotubes and nanowires, three-dimensional spheres assembled by nanoparticles and by nanosheets. The results of photodegradation activity in the AO7 degradation indicated that the photodegradation efficiency of TiO₂ spheres assembled by nanosheets was the highest with respect to the other TiO₂ nanostructures, while its specific surface area was lower than that of TiO₂ nanotubes. The results were attributed to the highest light harvesting capacity under solar light derived from the multiple reflections of light, and from the hierarchical mesoporous structure. The last example regards the study of Muthirulan et al. [255], who fabricated GTiO₂Ns by using a simple one-step chemical process, mixing TiO₂ nanoparticles suspended in ethanol with G followed by ultrasonication and successive treatment in a rotary evaporator under vacuum. The dried GTiO₂Ns showed improvement in the photocatalytic ability in the degradation of AO7 under both UV and solar light irradiations. The obtained results of photodegradation indicated that GTiO₂Ns nanocomposites exhibited higher photocatalytic activity than that of TiO₂, which was attributed to the role of G in the suppression of charge recombination and in the promotion of the charge transfer, permitting also a possible reaction pathway for the degradation of AO7.

6. Conclusions

In this short review, we have described the characteristics of both TiO₂ semiconductors and graphene materials and reported several examples focusing on the recent progress in the design, synthesis, and applications of these GTiO₂Ns nanocomposites used as photocatalysts in the photocatalytic degradation of synthetic dyes.

We have described the important effects of G in GTiO₂Ns nanocomposites, as shown in Figure 6. In fact, in conjunction with TiO₂ nanomaterials, G can act as a co-adsorbent thanks to the increase of adsorption surface and to π-π interactions with dye and as a sensitizer due to the electron transfer from the G photoexcited state, with delocalization and energy excess dissipation, to the TiO₂ surface with the extension of the light absorption towards the visible region. Notably, both the functions of conductive materials and co-catalysts favor the stabilization of excited electrons, thus limiting the electron-hole recombination in the photocatalytic process. In addition to these properties, TiO₂ band-gap narrowing due to the interaction between TiO₂ and G was demonstrated.

Thanks to these features, in order to obtain the best benefit of graphene in the enhancment of the photoactivity of TiO₂ nanomaterials, all of these factors that dominate the photocatalytic process must to be taken into consideration and optimized with the purpose of obtaining the best dye absorption and its complete mineralization.

Much research has been conducted regarding the photocatalytic degradation of synthetic dyes, and the high-quality results of these studies demonstrate that these materials could be promising for large-scale applications in wastewater depuration.