rGO@TiO2 Photocatalyst for Degradation of Organic Pollutants: History
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Contributor:
Martina Kocijan
,
Lidija Ćurković
,
Gil Gonçalves
,
Matejka Podlogar

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. 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. 

  • graphene-based titanium dioxide catalyst
  • rGO@TiO2
  • nanocomposite
  • water treatment
  • organic pollutants

1. Introduction

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) [1][2]. 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 [3][4][5][6]. 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 [7][8].
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Figure 1. Photocatalytic reaction mechanism on TiO2 semiconductor material.
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) [6][9][10]. 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 [9][11][12][13].
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 [13][14][15]. 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 [16]. By this, scholars can control the concentration of defects in the carbon structure Ref. [17]. By exploring several chemical [18] or physical treatments, ref. [19] 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.
/media/item_content/202211/637d6953dac13sustainability-14-12703-g002.png
Figure 2. Graphical representation of the molecular structures of graphene material and its derivates [20].

2. Photodegradation of Organic Pollutants

Many studies using rGO@TiO2 nanocomposite were made to improve photocatalytic efficiency. Liu [21] 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. [22] who got maximum degradation of MB equal to 91.3% within 30 min of sunlight irradiation. In another study by Mohammadi et al. [23], 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. 

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. [5] 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. 

3. Factors Affecting the Photodegradation of Organic Pollutants

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.

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. [24] 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 [24].

3.3. Effect of Initial Pollutant Concentration

Li et al. [25] 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 [26][27]. The effect of initial concentration of RhB dyes on the photodegradation was studied by Maruthamani et al. [24].

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. [26]. For instance, Deepthi et al. [28] 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.

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

Deepthi et al. [28] 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 people want to commercialize the process. Such a system should be functional under natural sunlight as the irradiation source [29]. 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 [30]. 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 [31].

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 [32]. 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.

4. Conclusions

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. 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.

This entry is adapted from the peer-reviewed paper 10.3390/su141912703

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