The Potential of rGO@TiO₂ Photocatalyst for the Degradation of Organic Pollutants in Water

Abstract

The availability of clean water is essential for humans wellbeing and the diverse biotic population in the environment. Menkind imposes a significant pressure on food supplies, natural resources, and other commodities. Large-scale anthropogenic activities, such as agriculture and industry, which are practiced to ensure population growth and survival, have caused several harmful environmental effects, including the discharge of pollutants into the aquatic environment. Among them organic micropollutants (OMPs) are considered a significant threat to aquatic ecosystems. The efficient removal of this persistent pollutants from wastewater is nowadays commonly considered in water treatment technologies. Utilizing photocatalysis by harvesting solar energy as an inexhaustible source, represents a facile and easy to upscale solution, for removing persistent pollutants and other emerging contaminants. In the recent decade, graphene-based titanium dioxide catalyst (rGO@TiO₂) has received a lot of attention as an effective material for the degradation in the enviroment. This review summarizes the recent progress in preparing rGO@TiO₂ nanocomposites and their utilization for purification purposes. Its main advantage over many other photocatalysts is its easy preparation, low toxicity, and reasonable photo-activity in a specific solar light spectrum.

1. Introduction

Today, clean water sources represent the highest priority for the sustainability of the entire ecosystem [1,2,3]. Environmental issues that are related to water pollution raise a serious concern over the last couple of decades due to the expanding industrial and agriculture activities [4,5,6,7]. Toxic organic pollutants amassed in our proximity have a negative effect on the biotic habitat and even human health. It is still extremely difficult to envision the elimination of these persistent pollutants which were found virtually in all water bodies [8,9,10]. Among them, persistent organic micropollutants (OMPs) such as organic dyes (i.e., methylene blue (MB), methylene orange, congo red), pesticides, pharmaceuticals, and their metabolites were proven extremely toxic [11]. The waste consisting of OMPs is also deemed problematic due to its difficulty for removal, low biodegradability, and long degradation lifetime in the ecosystem [12].

Conventional wastewater methods such as adsorption, biological treatment, filtration, chemical treatment, flocculation, etc., are not efficient enough to degrade especialy aromatic and heterocyclic groups in their moleculear structure [13,14]. The remediation should consider degradation and mineralization of this contaminants to its highest stable oxidation state such as: water, carbon dioxide, and the oxidized inorganic anions or into less toxic and also easily degradable species, that can be remediated through naturally pathways [15]. Therefore, extensive efforts have been directed globally toward the development of suitable, efficient, and eco-friendly processes for the removal of persistent pollutants. Nevertheless, strategies are being continuously developed for their remediation [16,17]. The task at hand is to choose an efficient and environmentally benign method, which would clean wastewater of its toxic ingredients. Recent research is focused on the advanced oxidation process (AOP) utilizing semiconductor nanoparticles with appropriate physical and chemical properties which would enable photocatalysis [18,19,20].

1.1. Challenges of TiO₂ Semiconductor Photocatalysts

Titanium dioxide (TiO₂) represents a well-studied photocatalyst that has been widely considered in research activities and is also commercialized. Because of its appealing physical and chemical properties, it was found useful in a wide spectrum of applications. Its thermal and chemical stability makes it resistant and unproblematic in the environment, it has good mechanical properties, and with respect to photocatalytic processes, it exhibits great activity under ultraviolet (UV) light irradiation (<387 nm) [21,22]. However, utilization of TiO₂ in a wider sunlight spectrum, which has its maximum in the visible range, requires a decrease of its bandgap energy, whilst inhibiting the recombination of photogenerated electron (e⁻)—hole (h⁺) pairs. Among the many approaches composite based on graphene and graphene-like materials (GO, rGO) addresses major limitations of TiO₂ [23,24,25,26]. Coupling TiO₂ with narrow bandgap materials with the visible light response is important to obtain an effective nanocomposite for photocatalytic applications. The synergistic effect between materials constituting nanocomposite improves visible light response, charge separation, and thus photocatalytic activity. On top of all both species were found way less harmful to the environment.

The mechanism of semiconductor photocatalysis can be divided into three steps (Figure 1). Firstly, using irradiation with the required energy (wavelength), electron (e⁻)—hole (h⁺) pairs are produced within the TiO₂ semiconductor particles. Furthermore, photo-generated charges must diffuse to the surface of the TiO₂ hence the low recombination rate. Lastly, generated electrons (e⁻) and holes (h⁺) must have the right reductive and oxidative potential to initiate chemical reactions driven through the surface of the catalyst. These reactions subsequently transform OMPs into low or non-toxic compounds [27,28].

While papers related to rGO-based TiO₂ photocatalyst are continuously being published, a review in front of you is aimed at summarizing recent work on nanocomposites. More precisely, the main objective is to highlight few synthesis methods of graphene-based TiO₂ nanocomposites formation and their operation characteristics for efficient photocatalytic water purification treatment. This work also summarizes few research conducted on diverse water bodies, which represents an important step towards practical application.

1.2. Why Coupling TiO₂ with Graphene?

Graphene is a single layer of sp²-bonded carbon atoms firmly packed into a two-dimensional honeycomb structure. It is an allotrope of carbon, with extraordinary properties, such as large theoretical specific surface area (2630 m² g⁻¹), optical transparency, high Young’s modulus (~1 TPa), high carrier mobility at room temperature (~10,000 cm² V⁻¹ s⁻¹), and excellent thermal conductivity (3000–5000 W m⁻¹ K⁻¹) [26,29,30]. Graphene has been widely used to improve the catalytic efficiency of photocatalysts. The photogenerated charge carriers are separated and the recombination rate of electrons/holes is reduced, thus the lifespan of charge carriers is extended. As a result, more reductive electrons and oxidative holes are accessible for the reaction, and the photocatalytic activity is enhanced [29,31,32,33].

Graphene Oxide (GO) is a valuable derivative of graphene. It consists of also oxygen and hydrogen and their functional groups which are attached to the hexagonal carbon skeleton thus disrupting the perfect sp² hybridization. The process of partial sp² bond recovery produces another important hybrid of carbon named reduced graphene oxide (rGO). Ideally, GO consists of a single carbon layer structure with oxygen containing functional groups positioned in or out of the hexagonal skeleton plane. However, most often GO is composed of multilayers. In contrast to perfect graphite consisting of single graphene layers connected through van der walls bonds, carbon layers are intercalated with oxygen functional groups and their bonds. GO is also considered two-dimensional carbon material; however, its properties are far from that of graphene. It does not absorb visible light, has very low electric and thermal conductivity compared to graphene, and demonstrates significantly higher chemical reactivity due to the presence of oxygen species [33,34,35]. Graphene can be oxidized to consist of several different oxygen functional groups such as epoxy, carbonyl, carboxyl, and hydroxyl, which play a relevant role in catalytic applications. The physicochemical properties of GO can be significantly changed according to the oxidation process applied [36]. By this, we can control the concentration of defects in the carbon structure Ref. [37]. By exploring several chemical [38] or physical treatments, ref. [39] GO can be converted in rGO which is in terms of physic-chemical properties an intermediate material of GO and graphene and deserve its own labeling. The molecular structures of graphene and its derivates (GO and rGO) are shown in Figure 2.

GO and rGO have been usually synthesized using natural graphite flakes as a precursor material and in the following section, the most common approaches to synthesizing them will be discussed.

1.2.1. Synthesis of Graphene Oxide (GO)

Carbon has fascinated chemists already a very long time ago and in 1859 Benjamin Brody synthesized GO from graphite flakes using KClO₃ and HNO₃ [41], even though graphene was not discovered at that time yet. In 1898, Staudenmaier [42] has slightly modified Brody’s method by using a concentrated H₂SO₄ in the presence of KClO₃ and HNO₃. In 1958, Hummers [43] has improved the existing process of GO production by using strong oxidizing agents such as KMnO₄ and NaNO₃ and the obtained product was a higher yield and better purity in comparison to the product from previous methods. The reported methods are very simple for synthesis, however, a major drawback is the formation of several toxic components (NO₂, N₂O₄, and ClO₂) during the process [44]. In 2010, Marcano et al. [45], prepared GO material by implementing several changes to Hummers’ method, called modified Hummers’ method and improved Hummers’ method, using natural graphite flakes (~150 µm).

In the last decade, many researchers worldwide have given attention to GO used for many applications obtained via different approaches of the Hummers´ method due to its higher yield in the production of GO [46,47,48,49,50]. All these reports implicate that excellent knowledge of the synthesis of GO material is of paramount importance for obtaining a material with desirable properties for targeted applications.

1.2.2. Reduction of GO to rGO

Through the reduction of GO to rGO, functional groups of oxygen (hydroxyl, carbonyl, carboxyl, and epoxy) can be removed. Since these methods can greatly transform the microstructure and properties of GO, several successful reductions have been performed by chemical [51,52], thermal [51,53], sonochemical [54], electrochemical [55], and photochemical [56] methods.

The most common transformation of GO into rGO is through thermal annealing. The oxygen-containing functional groups decompose during high temperatures into gases. These gases can produce high pressure between the stacked layers. According to the reported research [57], pressures of 40 MPa and 130 MPa are produced at 300 °C and at 1000 °C, respectively.

1.2.3. Preparation of rGO-Based TiO₂ Photocatalyst

rGO-based TiO₂ material is commonly used in light-driven photocatalysis of dyes in an aqueous medium. Because of exceptional properties, rGO-based oxide semiconductors promote electron separation, which results in boosting photo-driven reactions such as the degradation of carcinogenic dyes (e.g., methylene blue) and solar-fuel (hydrogen) production. Preparation of rGO-based TiO₂ photocatalysts increases the specific surface area of the nanocomposite, consequently increasing the photocatalytic activity, which is why rGO-based semiconductor photocatalysts have been found to be promising in several applications. In the last decade, many studies have reported on improved photocatalytic activity on OMPs degradation. In these studies, rGO has been coupled with TiO₂, the well-recognized semiconductor material, to enhance their performance in the photodegradation processes of OMPs from water bodies [58,59,60].

Nowadays, the rGO-based TiO₂ nanoparticles are synthesized by physical as well as chemical methods such as sol-gel, hydrothermal and solvothermal techniques and by calcination treatment, hydrolysis, etc. The sol-gel method is a simple and cheap synthesis process that provides homogeneity to the prepared nanocomposite and is performed at very low temperatures. Preetha et al. [61] has reported the synthesis of rGO@TiO₂ nanocomposite by the sol-gel method using titanium (IV) isopropoxide (TTIP) as a precursor in an ethanol medium followed by calcination treatment. Different weight percentages of GO were dispersed before the sol-gel reaction. The hydrothermal/solvothermal technique is one of the simplest synthesis techniques widely adopted for the preparation of nanocomposite photocatalysts. The morphology and physicochemical properties are significantly affected by the hydrothermal conditions such as duration of synthesis, temperature, cooling conditions, used precursors, etc. [26]. The high pressure used in the hydrothermal/solvothermal technique leads to good interactions between used precursors and can be performed at even lower temperatures. The synthesized nanocomposite exhibits synergy in properties of individual components which are attractive in photocatalytic degradation. During the hydrothermal/solvothermal technique, GO is mixed with a precursor in solution, where the latter transforms into small crystals on the surface of graphene [62]. Several authors reported how enhanced degradation of organic pollutants can be achieved using such composite prepared by this simple and cost-effective hydrothermal technique followed by calcination treatment (

Table 1. Photocatalytic degradation of organic pollutants by rGO@TiO₂ nanocomposite.

Synthesis Method	Catalyst Dosage	Light Source	Pollutant and Initial Concentration	Irradiation Time	Kinetic /10−3	Removal Efficiency	Reference
Solvothermal	0.8 g/L	UV light	Methylene blue, 10 mg/L	90 min	-	100.0%	[25]
Sol-gel method	0.5 g/L	Natural sunlight	Alizarin Yellow GG, 10 mg/L	150 min	-	100.0%	[61]
Hydrothermal	0.5 g/L	Simulated solar light	Rhodamine B, 10 mg/L	120 min	53.1 min−1	99.8%	[63]
Hydrothermal	0.5 g/L	Simulated solar light	Methylene blue, 10 mg/L	120 min	32.84 min−1	98.1%	[63]
Sonochemical	2.0 g/L	Natural sunlight	Methylene blue, 20 mg/L	30 min	18.8 min−1	91.3%	[65]
Solvothermal	0.1 g/L	Natural sunlight	Methylene blue, 15 mg/L	60 min	21.2 min−1	85.0%	[66]
Solvothermal	0.1 g/L	UV lamp	Methylene blue, 15 mg/L	60 min	89.1 min−1	100.0%	[66]
Hydrothermal	0.075 g/L	Natural sunlight	Diclofenac, 25 mg/L	60 min	50.4 min−1	~100.0%	[67]
Hydrothermal	0.1 g/L	UV light	Methylene blue, 12 mg/L	180 min	13.1 min−1	~90.0%	[68]
Hydrothermal	0.0004 g/L	UV light	Methyl orange, 30 mg/L	30 min	122.0 min−1	95.0%	[69]
Hydrothermal	0.5 g/L	Visible light	Methyl orange, 10 mg/L	240 min	126.0 min−1	87.4%	[70]
Hydrothermal	0.1 g/L	UV-A lamp	Clofibric acid, 20 mg/L	360 min	11.2 min−1	100.0%	[71]
Hydrothermal	0.18 g/L	Simulated sunlight	Formalin, 40 mg/L	90 min	17.9 min−1	93.8%	[72]
Freeze-drying	0.4 g/L	Simulated solar lamp	Rhodamine B, 10 mg/L	240 min	-	84.6%	[73]
Ultrasonication	0.75 g/L	UV light source	Methylene blue, 30 mg/L	120 min	-	99.3%	[74]
Solvothermal	0.1 g/L	Visible light	Methyl orange, 10 mg/L	180 min	-	94.1%	[75]
Hydrothermal	0.25 g/L	Simulated sunlight	4-nitrophenol, 45 mg/L	360 min	11.8 min−1	95.0%	[76]
Solvothermal	1.0 g/L	UV light	Rhodamine B, 0.125 mM	300 min	6.3 min−1	94.8%	[77]
Hydrothermal	0.5 g/L	UV-A lamp	Methylene blue, 6.4 mg/L	60 min	23.0 min−1	86.0%	[78]
Hydrothermal	0.5 g/L	Natural sunlight	Methylene blue, 6.4 mg/L	60 min	20.0 min−1	81.0%	[78]
Sol-anaerobic calcination	0.4 g/L	UV light	Phenol, 10 mg/L	180 min	-	97.9%	[79]
Hydrothermal	1.2 g/L	Visible light	Humic acid, 30 mg/L	180 min	-	88.0%	[80]
Hydrothermal	0.5 g/L	Visible light	Methylene blue, 0.01 mM	130 min	37.4 min−1	~99.0%	[81]
Hydrothermal	0.7 g/L	UV light	Ethylparaben, 0.3 mM	40 min	96.0 min−1	98.6%	[82]
Hydrothermal	0.5 g/L	Natural sunlight	Bisphenol A, 0.01 mol/L	30 min	-	100.0%	[83]
Hydro-solvothermal	1.0 g/L	Solar light	2,4-Dichlorophenoxyacetic acid, 0.5 mM	180 min	9.6 min−1	97.0%	[84]

). Another case of enhanced photocatalytic degradation, of methylene blue and rhodamine B in an aqueous medium [63], was observed using self-assembled rGO@TiO₂ nanocomposite synthesized by a one-pot hydrothermal technique using TTIP and GO at 180 °C for 6 h followed by calcination treatment at 300 °C for 2 h. Wanag et al. [25] reported a two-step solvothermal technique followed by calcination treatment for the preparation of rGO@TiO₂ photocatalyst nanocomposites with improved photocatalytic degradation. Sohail et al. [64] reported a simple hydrolysis method for preparing the same composite using titanium butoxide as a precursor in ethanol which was added to the GO suspension after which the obtained precipitates were calcined at 500 °C for 2 h.

2. Photodegradation of Organic Pollutants

In the recent decade, photocatalytic degradation of persistent OMPs such as dyes, pharmaceuticals, and pesticides into non-toxic and innoxious compounds (water and carbon dioxide) has become a hot research topic. Conventional wastewater treatment techniques present significate limitations in terms of complete degradation of the harmful OMPs as well as its high cost associated with the operational conditions. Indeed, with these techniques, it is very difficult to achieve the quality of treated water to a level required by the WHO organization [85].

Many studies using rGO@TiO₂ nanocomposite were made to improve photocatalytic efficiency. Liu [86] treated Methylene Orange (MO) using rGO@TiO₂ nanocomposite for 240 min exposing it to visible light (λ > 400 nm) irradiation and reported ~90% photodegradation of overall organic pollutant. Several authors report the photocatalytic degradation of methylene blue (MB) using rGO@TiO₂ nanocomposites, among them Deshmukh et al. [65] who got maximum degradation of MB equal to 91.3% within 30 min of sunlight irradiation. In another study by Mohammadi et al. [87], photocatalytic degradation of MB using rGO@TiO₂ composite was even better. 95% and 93% of the overall organic pollutant were removed within 30 min using irradiation from a 200 W Mercury short arc and Osram 500 W Xenon lamp with a cut-off UV filter at 400 nm, respectively. Furthermore, Garrafa-Gálvez et al. [66] studied the photocatalytic degradation of MB while using the same rGO@TiO₂ nanocomposites under natural sunlight and UV irradiation. They reported high effectiveness, 100% and 85% of MB of overall organic content under 60 min of UV and natural sunlight irradiation, respectively. Kusiak-Nejman et al. [88] investigated the same system but used UV-Vis light with higher UV intensity. The authors reported 91.48% of MB degradation after 60 min of UV-Vis light irradiation. Wu et al. [89] studied photocatalytic degradation of rhodamine B (RhB) using rGO@TiO₂ and reported ~95% removal within 90 min of UV irradiation (250 W high-pressure Hg lamp) at room temperature. The authors also studied nanocomposite reusability; the results obtained after five cycles indicate that the prepared nanocomposite is rather stable, with above 90% removal capabilities.

Reported studies also showed that the tailoring of rGO@TiO₂ nanocomposite morphology promotes improved photocatalytic efficiency for pharmaceutical and pesticide degradation. Balsamo et al. [84] reported 97% degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) in 180 min using rGO@TiO₂ under solar light irradiation (Figure 3). The ion-spray ionization mass spectrometric (ISI-MS) analysis was utilized to show which by-products are formed. Figure 4. displays the ISI-MS spectra of prepared 2.4-D solution before (upper) and after performed photocatalytic test (bottom) using rGO@TiO₂ solar catalyst. After 3 h of continuous irradiation, 96% of removal was confirmed with the Total Organic Carbon (TOC) analysis. The ISI-MS spectra are done to investigate the formation of by-products. After 3 h of continuous irradiation, 96% of removal was detected with the Total Organic Carbon (TOC) analysis. As confirmed by the ion-spray ionization mass spectrometry (ISI-MS) 2,4-dichlorophenol (2,4-DCP) and 2,4-dichlororesorcinol (2,4-DCR) formation was observed as the main by-products.

In another study, Luna-Sanguino et al. [90] prepared a rGO@TiO₂ nanocomposite that was applied for the removal of methomyl, isoproturon, and alachlor. It was found that rGO@TiO₂ exhibits a high degradation rate after 25 min of natural sunlight irradiation. Deepthi et al. [67] investigated the application of synthesized rGO@TiO₂ nanocomposite for solar-driven degradation of diclofenac (DCF). Besides high effectiveness (>98% DCF degradation and 65% mineralization of organic content within 60 min irradiation), the synthesized rGO@TiO₂ nanocomposite catalyst mineralizes DCF completely in ~100 min. Similar efficiency can be found also for many other degradations [64,68,69,91,92].

Reusability of rGO@TiO₂

The investigation of the long-term stability and reusability of prepared rGO@TiO₂ photocatalyst is a crucial parameter for its practical application. The stability of prepared rGO@TiO₂ photocatalyst is investigated between several consecutive cycles with the same photocatalytic tests. Wanag et al. [25] investigated the stability of prepared rGO@TiO₂ photocatalyst under seven cycles. The obtained results show very high activity after five cycles. A substantial decrease in the photoactivity is noted after seventh cycles. Prepared rGO@TiO₂ photocatalyst showed high stability during the photodegradation of MB dye. In another study, Balsamo et al. [84] investigated the reusability of prepared rGO@TiO₂ solar catalyst under five cycles with very high photodegradation of pesticide. A slight decrease in photoactivity is attributed to the loss of catalyst powder between cycles. Deepthi et al. [67] prepared rGO@TiO₂ catalyst with excellent reusability after six cycles of DCF degradation. Similar reusability of rGO@TiO₂ photocatalyst has been reported in other photodegradation investigations [88,93].

According to published results on photocatalytic degradation of OMPs (dyes, pharmaceuticals, pesticides) and high reusability after several cycles, rGO-based TiO₂ photocatalysts are expected to be commercialized in the near future.

3. Factors Affecting the Photodegradation of Organic Pollutants

Several parameters which affect the photodegradation process are GO to TiO₂ weight ratio, catalyst loading, initial pollutant concentration, pH, water matrix composition, the intensity of irradiation, scavengers, and wavelength of light irradiation. The impact of these parameters is shown in

Table 2. The impact of parameters on the rGO@TiO₂ photocatalytic process.

Parameter	Impact on Photocatalysis
Catalyst concentration	The photodegradation rate increases with the increase of the amount of catalyst. Above a certain amount of catalyst, the photodegradation rate decreases as the catalyst amount increases.
Initial pollutant concentration	The photodegradation rate increases with the increase of the initial concentration of pollutant. Above a certain initial concentration of pollutant, the photodegradation rate decreases as the initial concentration of pollutant increases.
pH	The photodegradation rate depends significantly on the pH value.
Light source	Light source supplies irradiation of different wavelengths (UV-C, UV-B, UV-A, visible light, simulated solar light, natural sunlight).
Intensity of irradiation	Light intensity depends on the light source and enhances the photocatalytic reaction.
Water matrix	Presence of different pollutants/organic matter that can act as inhibitors or competitors of the photodegradation rate.
Weight ratio of graphene oxide to TiO2	The photodegradation rate increases with the increasing amount of GO to TiO2. Above a certain threshold the photodegradation rate decreases as the amount of GO increases.
Scavengers	Improve the separation rate of photogenerated holes and electrons and enhance the photocatalytic reaction.

.

3.1. The Effect of GO to TiO₂ Weight Ratio

The weight ratio of GO in the rGO@TiO₂ nanocomposite plays a crucial role in obtaining a high surface area. Good TiO₂-decorated rGO sheets can effectively absorb irradiated light and convert it to electron-hole pairs by decreasing the bandgap. The photocatalyst with the most suitable TiO₂ nanoparticles distributed on rGO sheets has the highest charge transfer rate and consequently improves pollutant degradation. Kocijan et al. [63] reported that rGO@TiO₂ nanocomposite with 15 wt.% of GO shows the fastest photodegradation rate of MB and RhB dyes under simulated solar light irradiation. According to Kusiak-Nejman et al. [88], the rGO@TiO₂ with 8 wt.% rGO shows a high rate of MB removal under UV light irradiation.

3.2. Effect of Catalyst Loading

The amount of prepared rGO@TiO₂ photocatalyst can significantly influence the photodegradation rate of the organic pollutant from the aqueous medium. Maruthamani et al. [77] have investigated different concentrations of 20% rGO@TiO₂ in the range of 0.5 to 2.0 g/L to observe the effect of photocatalyst concentration on the degradation rate of RhB (1.25 × 10⁻⁴ M) under UV irradiation. The obtained experimental results showed that the catalyst amount has both positive and negative impacts on the photodecomposition rate. The catalysts amount up to 1.5 g/L demonstrated increased photodegradation rate of RhB from 70 to 93%, which could be explained by an increased number of active sites from the catalyst [77]. On the other side, increasing the catalyst amount above, from 1.5 to 2.0 g/L, resulted in a significant drop in photodegradation of RhB. According to their agglomeration of catalyst nanoparticles coupled with increase of suspension turbidity caused light scattering that obstructed UV light penetration during the reaction [83]. Similar results can also be found elsewhere [55,63,84].

3.3. Effect of Initial Pollutant Concentration

Li et al. [93] studied the effect of initial concentration on the degradation of acid orange 7 (AO7) at different concentrations such as 10, 20, and 30 mg/L. The degradation rate decreased with increase of the initial AO7 concentration of up to 30 mg/L. Other papers also reported similar results [68,74]. The effect of initial concentration of RhB dyes on the photodegradation was studied by Maruthamani et al. [77]. They modified the initial concentration of RhB between 0.625 × 10⁻⁴ and 5.0 × 10⁻⁴ M. They found that the decolourisation percentage of RhB initially increased and later decreased with increase of initial dye concentration. Deepthi et al. [67] investigated the removal rate of diclofenac (DCF) in the initial concentration range of 10–30 mg/L. It was found that the degradation rate of DCF increased steadily for up to 25 mg/L of DCF and thereafter was stabilized or slightly decreased. Hence, as the organic pollutant concentration increases, more molecules of pollutant are adsorbed on the photocatalyst surface which reduces the surface of active sites, thus fewer photons can reach the catalyst surface. This phenomenon can significantly decrease photocatalytic degradation efficiency due to the reduced production of oxygen reactive species such as ^•OH and O₂^•− radicals [94].

3.4. Effect of Initial pH

The pH value can influence the photodegradation efficiency through several possible reaction mechanisms. Parameters responsible for the changes are substrate and surface chemistry, extent of adsorption, catalyst surface charge, types of surface interactions, substrate nature, solvent molecules, the numerous intermediates formed during the progress of the reaction, etc. [68]. For instance, Deepthi et al. [67] investigated DCF degradation under natural sunlight irradiation, using different initial pH of the DCF solution. A maximum of 68.4% degradation was obtained (using the same rGO@TiO₂ nanocomposite) at pH 6. The point of zero charge (PZC) of the rGO@TiO₂ nanocomposite was found to be at 4.59, meaning that above this pH value the catalyst surface is negatively charged and capable of adsorbing cationic species, whereas below it is positively charged and hence attracts anionic species onto the surface. Thus, the ionic state of the substrate species is vital in determining the adsorption degree and consequently the photodegradation rate. Furthermore, it was observed that pH changes during photocatalytic reactions. According to Balsamo et al. [84], at 3.5 pH (surface with a small positive potential) rGO@TiO₂ nanocomposite showed the highest rate of 2,4-D degradation under solar irradiation. Maruthamani et al. [77] investigated RhB degradation between pH 3.5 and 10.0 and noted that the highest RhB decolorization is under acidic conditions. For pH 3.5 after 3 h under UV light irradiation, the RhB decolorization was at 93%. At pH 10 and above there was also an increase in RhB photodegradation, which was attributed to the fact that the catalyst surface is sufficiently negatively charged. At pH 7 RhB decolorization value was the lowest, which was attributed to the decrease of TiOH₂⁺ concentration. Deshmukh et al. [65] reported MB degradation using four pH values: 2.3, 4.0, 12.1, and 13.2, as shown in Figure 5. According to the obtained results, as the pH increased from 2.3 to 13.2, the amount of decolorization/degradation of MB dye increased from 17.4 to 91.3% after 30 min of reaction time. The point of zero charge (PZC) of rGO@TiO₂ nanocomposite suspension was measured with the help of zeta potential values at several different pH values and is shown in Figure 5 (inset). The point of zero charge was found to be 2.62 for the ultrasonically prepared rGO@TiO₂ catalyst. At lower pH, the ultrasonically prepared nanocomposite surface got positively charged which led to the protonation of active sites. MB dye is a cationic dye; thus, electrostatic repulsion is generated between the positively charged MB dye molecules and the catalyst surface. Contrarily, at higher pH, the surface of the nanocomposite becomes alkaline in nature which leads to deprotonation. In this case, there is a higher number of negative sites on the surface of the photocatalyst, leading to the interaction of the dye with one of these negative sites on the surface of the nanocomposite.

3.5. Effect of Water Matrix on the Photocatalytic Degradation of Pollutant

Usually, the photocatalytic degradation of organic pollutants throughout the literature was carried out with distilled or ultrapure water, which contains a minimum number of ions, hence also the conductivity was very low. The conductivity of ultrapure and distilled water is between 0.15 and 0.055 µS/cm and 0.5 and 3 µS/cm, respectively [95,96]. The presence of these ions may negatively or positively affect the photodegradation efficiency. For industrial scale-up, the photodegradation efficiency of the system should be tested in various natural water matrices, such as tap water, river water, lake water, seawater, industrial wastewater, etc.

Deepthi et al. [67] investigated the photocatalytic degradation of diclofenac in several water matrices (ultrapure water, river water, dug well water, filtered effluent, and unfiltered effluent) under sunlight irradiation using rGO@TiO₂ nanocomposite. According to their report, they found that DCF can effectively be degraded in all investigated water systems. The photodegradation efficiency decreases in the following order: ultrapure water  >  river water  >  dug well water  >  filtered effluent  >  unfiltered effluent. Substantial photocatalytic degradation in ultrapure water was finished in 60 min, while it took 90 and 120 min for river and well water, respectively. Four hours were required for the degradation efficiency of only 90% in effluent medium. According to their analysis of the water matrices, the effluent water contained significant amounts of ions (Cl⁻, SO₄²⁻, and PO₄³⁻). These ions inhibited the photodegradation process. As they argued, ions from effluent water were adsorbed on the catalyst surface, blocking the reactive sites. Furthermore, these ions scavenged holes and hydroxyl radicals producing less powerful oxidants such as NO₃⁻, Cl⁻, PO₄²⁻, and CO³⁻.

3.6. Effect of Intensity and Wavelength of Light Irradiation

TiO₂ has a wide band gap energy (3.0–3.20 eV) which limits its absorption only in the UV region of the solar spectrum. The wavelengths and intensities of UV light irradiation significantly affect the photodegradation of pollutants in an aqueous medium. UV irradiation is thus more frequently practiced than sunlight as it has higher efficiency in the degradation of pollutants. Expanding the photocatalytic degradation of pollutants to visible irradiation is an important aspect to recon with if we want to commercialize the process. Such a system should be functional under natural sunlight as the irradiation source [97]. The intensity of the light also affects the transition rate of electrons from the valence band (VB) to the conduction band (CB). Higher intensity usually leads to significantly higher degradation rates of the photocatalytic process. After saturation when the amount of photons is equal to TiO₂ active sites, the rate of photogeneration becomes less dependent on the increase of the light intensity. Therefore, appropriate photon energy distribution contributes to the photodegradation rate [9]. A surplus of photons of given energy cannot contribute to a higher photocatalytic degradation rate because of the limited amounts of active sites on the surface of the catalyst [98].

Coupling TiO₂ with materials such as graphene which can shift the response to visible light is crucial for obtaining an effective nanocomposite for photocatalytic applications using natural sunlight irradiation as a cost-effective irradiation source. The synergistic effect between materials in nanocomposites can improve visible light response, charge separation, and photodegradation rate. The reported investigations based on rGO@TiO₂ photocatalyst with different light irradiation are shown in Table 1.

3.7. Effect of Scavengers

The photocatalytic reaction can be enhanced by increasing the number of radicals, which leads to a better separation of charge carriers. These conditions can be created on rGO material which also alters the recombination rate of charge carriers. Furthermore, free oxidative holes can directly react with the organic pollutant compounds or indirectly through the ^•OH radicals, which are strong oxidants [99]. In an aqueous medium, active components such as hydroxyl radical (^•OH), holes (h⁺), and electrons (e⁻) can play significant roles in the photodegradation of organic pollutants.

For instance, Mohammadi et al. [87] investigated photodegradation mechanisms of MB by rGO@TiO₂ under visible light. Several scavengers were introduced. Among them were EDTA for holes (h⁺) generation, AgNO₃ and K₂S₂O₈ for electrons, p-benzoquinone for O₂^•−, t-butanol for ^•OH radical scavenger, and NaN₃ for singlet oxygen, ¹O₂ species. According to their results, adding t-butanol, EDTA and NaN₃ had no considerable influence on the photodegradation of MB dye. They subsequently concluded that OH radicals, h⁺, and ¹O₂ do not play a significant role as active species in MB photodegradation. In contrast, adding e⁻ through introduction of K₂S₂O₈ proved to be the main acetifier of photocatalytic degradation of MB dye. Photocatalytic degradation efficiency decreased in the presence of p-benzoquinone, superoxide radical ion, O₂^•−, which also represents an important finding. Pastrana-Martínez et al. [70] investigated the photodegradation of diphenhydramine under near-UV/Vis and visible light irradiation. EDTA or t-BuOH were used as hole and radical trapping agents. It was found that photogenerated holes and hydroxyl radicals are the main reactive species in the investigated process.

The photocatalytic mechanism of the rGO@TiO₂ nanocomposite is shown in Figure 6. rGO in the rGO@TiO₂ nanocomposite can improve photocatalytic degradation of pollutants by promoting separation and charge transfer. The electron-hole pairs have been excited within the TiO₂ molecule after irradiation. The e⁻ quickly shifted from the CB of the TiO₂ molecule to the surface of the rGO material. Transferred e⁻ react with dissolved oxygen resulting in the high generation of ^•OH radicals. Furthermore, the h⁺ from the VB of the TiO₂ molecule has the potential to oxidize H₂O/OH⁻ to form ^•OH radicals. From the oxidation and reduction process hydroxyl (^•OH) and superoxide radical (O₂^•−), are obtained respectively. They react with pollutants (MB dye) to transform pollutants into non-hazardous compounds such as H₂O and CO₂ [66,88,100]. Thus, because of multiple responses of rGO, a careful investigation is crucial to determine under what photocatalytic reaction conditions (such as type of irradiation exposure, pH solution, the concentration of rGO in the nanocomposite, and nature of oxidizing agent), the photodegradation mechanism of the rGO@TiO₂ nanocomposites can be improved [26,101].

4. Challenges and Future Perspectives

With the growth of population and various industries, many pollutants which can have a negative influence on the environment and human health, enter the ecosystem. There is much ongoing research to partially remove pollutants from water and finding an appropriate and effective method is of vital importance. Nowadays, conventional treatments have many drawbacks such as high cost or the formation of toxic by-products. The use of catalysts represents an appealing method to remove pollutants from the water, however, there are still many costs related issues in terms of energy spending.

Advanced oxidation processes especially heterogeneous photocatalytic degradation can be utilized as an appropriate remediation strategy. Heterogeneous catalysts have many advantages over homogeneous catalysts, such as easy separation from the reaction mixture and their reusability. As mentioned above and shown in Table 1, various organic pollutants can be efficiently degraded in natural waters or wastewaters. Among them, rGO@TiO₂ nanocomposite proved to be highly efficient for photodegradation of these contaminants. These novel nanocomposite materials are considered green photocatalysts, and they can be activated by solar light, which avoids pitfalls in consuming energy for the generation of UV light.

In order to realize their full potential, the research gap needs to be closed. First, optimize the integration process of rGO with TiO₂ to further increase the photocatalytic activity using solar irradiation. Fine-tuning the synthesis and photocatalytic parameters is crucial for the large-scale synthesis required for technological implementation. In addition, further research is needed to determine the reversibility of the electronic properties of rGO, as its influence on the photochemical process has not yet been fully elucidated. The use of theoretical models to support empirical models could contribute to obtaining new inputs into the interfacial analysis of the rGO@TiO₂ nanocomposite. Finally, future research should be devoted to investigate the photodegradation of pollutants under realistic conditions and their toxicity in contrast to many published studies under ideal experimental conditions in a short reaction time.

5. Conclusions

This review summarized the progress performed on rGO@TiO₂ catalysts for the photodegradation of organic pollutants from different types of water bodies. Introduced photocatalyst nanocomposite show several advantages. Many studies report different synthesis conditions and there are new strategies for further development. rGO@TiO₂ catalyst is very versatile in terms of removing different types of contaminants, but also from the applications point of view since it can be prepared as thin films, coatings, etc. Such immobilization of the rGO@TiO₂ catalyst plays an important role for easy recyclability, which is important from the economic aspects. Although many efforts and studies have been made using rGO@TiO₂ as a catalyst there are still some challenges that need to be addressed in the future. Among them and probably the most relevant is the conversion of this technology from a lab to a pilot scale project to finally industrialize this important application.