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Semiconductors Application Forms and Doping Benefits to Wastewater Treatment: A Comparison of TiO2, WO3, and g-C3N4

University of Coimbra, CIEPQPF—Chemical Engineering Processes and Forest Products Research Center, Department of Chemical Engineering, Faculty of Sciences and Technology, Rua Sílvio Lima, Polo II, 3030-790 Coimbra, Portugal
Author to whom correspondence should be addressed.
Catalysts 2022, 12(10), 1218;
Submission received: 22 September 2022 / Revised: 7 October 2022 / Accepted: 8 October 2022 / Published: 12 October 2022


Photocatalysis has been vastly applied for the removal of contaminants of emerging concern (CECs) and other micropollutants, with the aim of future water reclamation. As a process based upon photon irradiation, materials that may be activated through natural light sources are highly pursued, to facilitate their application and reduce costs. TiO2 is a reference material, and it has been greatly optimized. However, in its typical configuration, it is known to be mainly active under ultraviolet radiation. Thus, multiple alternative visible light driven (VLD) materials have been intensively studied recently. WO3 and g-C3N4 are currently attractive VLD catalysts, with WO3 possessing similarities with TiO2 as a metal oxide, allowing correlations between the knowledge regarding the reference catalyst, and g-C3N4 having an interesting and distinct non-metallic polymeric structure with the benefit of easy production. In this review, recent developments towards CECs degradation in TiO2 based photocatalysis are discussed, as reference catalyst, alongside the selected alternative materials, WO3 and g-C3N4. The aim here is to evaluate the different techniques more commonly explored to enhance catalyst photo-activity, specifically doping with multiple elements and the formation of composite materials. Moreover, the possible combination of photocatalysis and ozonation is also explored, as a promising route to potentialize their individual efficiencies and overcome typical drawbacks.

1. Introduction

Society and industry are subject to constant evolution and increasing complexity. Consequently, the consumption of crucial resources for their expansion also progressively grows. Water, as the most vital of these resources, becomes an endangered commodity, making the discussion regarding its renewability against the growing usage and degradation a high-interest topic. Fresh and drinkable water scarcity is caused by different factors, such as climate change, loss of biodiversity, and pollution, which can be due to the direct irregular disposal of sewage, effluents, and other contaminated sources into water bodies, or by the inefficient treatment of those in wastewater treatment plants (WWTPs) [1]. Nowadays, an estimated 1.2 billion people live in locations facing water scarcity and 780 million have no access to basic water services, resulting in a variety of diseases associated with contaminated water consumption [2].
The WWTPs are currently responsible for the provision of proper sanitary living conditions for more than 5.2 billion people, but as industries evolve, a large number of different products and chemicals are consumed by the population, eventually making their way into these facilities, which are not designed to treat them [3,4]. There is a great number of these pollutants, but a certain group is particularly important, the contaminants of emerging concern (CECs), which are a variety of chemicals, such as pesticides, pharmaceutical and personal care products (PPCPs), hormones, and stabilizers [5]. Due to their ineffectiveness, WWTPs become a major source of CECs, which may accumulate in the environment and have already been detected, at ng L−1 and µg L−1 ranges, in surface and groundwater, as well as in remote places, such as high-altitude rivers and the Antarctic Peninsula [6,7,8,9].
Different treatments and techniques have been researched by the scientific community to solve the problem associated with CECs, but a feasible and broad-range solution is yet to be found [10]. Advanced oxidation processes (AOPs) are presented as suitable alternatives for the degradation of a wide range of pollutants, based upon the formation of reactive oxidative species (ROS), most importantly hydroxyl radicals (·OH). AOPs include an array of treatments, vastly explored towards CECs abatements, such as ultraviolet-, ozone-, photocatalysis-, Fenton-, and sulfate-based processes [11,12,13,14] (Figure 1). Ozone-based water disinfection treatments are well-established, but also currently one of the most applied AOP for micropollutants removal, being already implemented as tertiary wastewater treatment in some countries [15].
Ozone is a very strong oxidant, with a redox potential of 2.08 eV, capable of directly reacting with microorganisms and different organic compounds, but it may also be responsible for indirect reactions, producing hydroxyl radicals, which then can interact with the microcontaminants [16]. Nevertheless, this process faces some disadvantages, namely the low solubility of ozone in water, the high energy requirement for its production, low mineralization, and the formation of byproducts potentially more toxic than the parent compounds [17,18]. To enhance the single ozone process, a very common strategy is to integrate or couple it with other AOPs, for instance, O3/UV, O3/H2O2, O3/Cl, and O3/Photocatalysis [11,19,20]. Photocatalytic ozonation is then a promising combined technique, boosting the treatment efficacy and suppressing the individual processes’ disadvantages. Photocatalysis is based upon the activation of a catalyst through photon absorption, which then promotes multiple radical’s production reactions that attack a large range of contaminants. The contaminants may also be eliminated directly by the catalyst through adsorption [14]. The photocatalysis, when in combination with ozone, can promote a higher decomposition of the gas in water, diminishing ozone demand and increasing the production of ROS. Besides, ozone may also enhance photocatalysis by acting as an electron receiver, reducing electron-hole recombination, which is one of the major disadvantages of this process [21].
Photocatalytic systems are very adaptable and, currently, there is a great variety of catalysts available to be used, the most common being TiO2. Notwithstanding, TiO2 presents some disadvantages that challenge its large-scale application, but as these materials are easily tunable, recent studies present multiple adaptations to enhance photocatalysts’ performances, such as doping using metal and non-metal elements, immobilization onto other materials, as well as the coupling of different catalysts and materials with distinct natures. Another route is the exploration of alternative catalysts, other than TiO2, that present superior performances over visible/solar radiation, lowering the process overall cost, which nowadays corresponds to a high-interest field of exploration [22].
Regarding other catalysts, visible-light-driven (VLD) materials became the aim of an increasing number of studies, presenting a better and easier alternative regarding their activation. g-C3N4 is a currently very significant catalyst, possessing a metal-free polymeric structure, with simple and very adaptable synthesis methods. Another example, WO3, also possesses lower bandgap energy and broader absorption of solar radiation while sharing the metal oxide characteristic of TiO2, which allows an easier correlation between possible known mechanisms and adaptations that can be made. These alternative materials, although with interesting light absorption properties, still face some drawbacks, some also found in TiO2, such as high electron-hole recombination, low specific surface areas, and improper energy band positions, which hinder the potential of the redox reactions responsible for contaminants elimination.
This current review focuses on the critical analysis of current TiO2 data as a reference catalyst, and its comparison with VLD materials, g-C3N4 and WO3. Graphitic carbon nitride was selected due to its high recent relevance among this group of alternative catalysts, possessing a high density of recent scientific studies and distinct composition, while WO3 also has appealing features and structures, and may represent an important connection between the broad existent knowledge of TiO2 and metal oxides, with visible light active catalysts. Advantageously, these materials may also be involved in modification techniques to boost their efficiencies and overcome disadvantages, as will be further discussed, focusing on doping techniques and composite formation.

2. General Features of Catalysts for Photo-based Treatment Processes

Titanium dioxide is certainly the most applied catalyst for photocatalytic water treatment and possesses different crystalline phases, anatase, rutile, and brookite, with anatase having a higher general photocatalytic activity [23]. Degussa P25 is a commercial TiO2 catalyst, vastly used due to its optimized characteristics, presenting a mixed anatase-rutile crystal phase.
The benchmarked P25 catalyst was applied by Gomes et al. [24] for the degradation of a complex solution of insecticides, namely azoxystrobin, buprofezin, imidacloprid, procymidone, simazine, terbutryn, and thiamethoxam. Under solar radiation for 120 min, Degussa P25 TiO2 completely removed 100 µg L−1 of almost all pesticides, excluding thiamethoxam (~90%) and procymidone (~50%). The authors also evaluated the scale-up effect, in a pilot-scale 120 L photoreactor, P25 achieved lower degradation yields, expected due to the larger volume, but still favorable, with removal rates higher than 70% with a photon flux of approximately 8 kJ L−1. Photocatalytic ozonation was also assessed, and the higher production of ROS improved the degradation of the insecticides by more than 80%, while single ozonation had considerably lower removal rates for some contaminants, such as Procymidone (~50%) and Imidacloprid (~50%). However, the high electronic density groups characteristics of the contaminants that promote a fast reaction with molecular ozone still allow a noticeably efficient treatment by the single process.
Even with broad usage, TiO2 faces compromising disadvantages that challenge its full-scale application. Primarily, its high bandgap energy, ~3.2 eV for anatase and ~3.0 eV for rutile TiO2, makes its photoactivation possible only under UV radiation, with λ ≤ 390 nm, hampering its activation through sunlight as the UV portion represents only 4–6% [14]. Moreover, TiO2 also presents a high recombination rate of e/h+ and a weak separation of photocarriers, reducing its photocatalytic activity [25]. Thus, other catalysts have been constantly developed and investigated to overcome the disadvantages of TiO2 and fulfill the need for a feasible material. In the last years, a few catalysts have been the focus of an increasing number of publications, e.g., g-C3N4 and WO3 materials, which will be more extensively discussed in this review.
Graphitic carbon nitride (g-C3N4) is a metal-free semiconductor catalyst, with high photochemical stability and photoelectric properties, that can be obtained through the thermal polymerization of numerous low-price nitrogen-rich organic precursors. It emerges as a promising visible-light-driven photocatalyst, as it possesses a typical absorption edge at 450–470 nm which corresponds to a bandgap energy of approximately 2.7 eV [26]. The graphitic C3N4 is the most stable among the multiple allotropic forms of C3N4 (e.g., cubic, beta, alpha), due to its particular 2D structure formed by triazine or heptazine rings.
As mentioned, g-C3N4 can be synthesized through the polymerization of different precursors, which will intrinsically affect the properties of the photocatalyst. Nguyen et al. [27] evaluated this parameter using dicyandiamide, melamine, urea, and thiourea as g-C3N4 precursors. Their physical and optical properties were considerably distinct, with urea-based g-C3N4 having noticeably better values, a specific surface area (SBET) more than three times higher than the others (78.9 m2 g−1), and lower bandgap energy (2.72 eV). The authors also conducted a chemical oxidation treatment over the catalysts, a method used for the exfoliation of g-C3N4, improving their properties, such as the surface area, hydrophilicity, and the addition of reactive functional groups (e.g., hydroxyl). The exfoliation treatment resulted in generally higher SBET, pore density, and a slower recombination rate of the photogenerated electron-hole pairs, indicated by the photoluminescence spectra and electrochemical impedance spectroscopy (EIP). However, the Ebg of the treated g-C3N4 catalyst was also higher (3.4–3.6 eV), attributed to the exfoliation of the bulk structure into thin layers and the quantum confinement effect. Nonetheless, when applied for methylene blue (MB) photodegradation under visible light, the exfoliated catalyst had a significantly better performance, with urea- and thiourea-based catalysts achieving a complete abatement in 180 min. It is important to mention that the use of thiourea as a precursor also inflicts the addition of sulfur in the catalyst structure, which will be further discussed in the next sections.
The g-C3N4 exfoliation may also be obtained through other methods, such as ultrasonication in water or alcohol solutions, or thermal treatment [26]. Fernandes et al. [28] compared Degussa P25 TiO2 with dicyandiamide-based g-C3N4, subjected to a posterior thermal treatment, for the removal of methyl-, ethyl-, and propylparaben (0.08 mM), individually and in the mixture. Under visible light, the polymeric catalyst achieved complete elimination of parabens within 20 min, individually, and 30 min when in a mixture, while P25 TiO2 obtained the same results in 120 min. The g-C3N4 also proved capable to maintain its stability and efficiency when tested in real water matrices, tap and river water. The thermal exfoliation may also partially remove amino groups and thus introduce defects in the catalyst structure, which reduce electron and hole recombination [29].
The combination of g-C3N4 photocatalysis and ozonation is still very minimally investigated, yet some studies have been conducted to demonstrate a propitious tertiary wastewater treatment. The more negative conduction band of g-C3N4, in comparison to TiO2, makes it easier for the excited electrons to be captured by ozone. This system was detailed by Orge et al. [29], applying a dicyandiamide as a catalyst precursor with a thermal post-treatment for exfoliation, for the elimination of oxamic acid (OMA), a common by-product of the oxidation of nitrogenous compounds, using a LED system. The exfoliated catalyst, which had an increase of more than 10 times its surface area, presented a much faster removal rate and full elimination of OMA in 120 min, compared with the same system using a benchmark TiO2 or single ozonation, which resulted in near 80% and 5% removals. Thus, the beneficial synergetic effect was proposed to be due to the interaction between ozone and the graphitic layers of g-C3N4. However, in the catalyst’s reusing evaluation, the authors indicated that ozone may also deactivate the catalyst, causing a decrease in its activity, pointed to be due to the insertion of defects on the g-C3N4 structure and oxygenated groups at its surface.
The benefits of g-C3N4 have also been explored for disinfection proposes in recent years. Liu et al. [30] achieved a 4.80 and 4.24 log reduction of E. coli and Staphylococcus aureus, respectively, using ultrathin urea-based g-C3N4 with nitrogen vacancies under visible light, which kept its disinfection efficiency even after five runs. The presence of nitrogen vacancies coupled with a higher specific surface area was shown to be determinant of the catalyst’s bactericidal activity. Bacteriophage MS2 virus (Emesvirus zinderi) elimination using g-C3N4 was assessed by Li et al. [31], achieving a nearly 8-log reduction of the pathogen under 360 min. The photogenerated electrons and superoxide radicals were indicated as the main ones responsible for the virus inactivation, through the oxidation of their proteins.
Tungsten trioxide (WO3) is a transition metal oxide that also appears as a promising photocatalyst to be used in chemical and biological CECs removal. The high photostability, corrosion resistance, low-cost fabrication, and bandgap of 2.5–2.8 eV are some of its benefits for photocatalytic usage [26]. Bulk WO3 possesses a cubic perovskite structure and may be obtained in multiple crystalline forms, such as cubic, hexagonal, and monoclinic, the last being more stable at room conditions, formed between 17 °C and 330 °C.
The synthesis methods and procedures are determinants of the type of structure and the characteristics of the catalyst. The morphology and phases of WO3 are especially relevant as it possesses multiple possible structures, affected by the synthesis method or post-treatments. Nagy et al. [32] found a direct influence of the pH of the solution during catalyst production on its obtained morphology, with a lower pH resulting in a cuboid shape (0.10), changing into nanorod (0.51), nanoneedles (1.52), and nanowires (2.01) with the respective increase of this parameter. Crystallinity may also be intentionally altered, resulting in different single or mixed phases, which present variations in the final product performance. Besides the pH, the addition of some compounds can alter WO3 structures, such as polyethylene glycol and citric acid (Figure 2) [33]. This easy customization of the structure characteristics is a key advantage of WO3, and their balanced effect can considerably increase the catalyst photoactivity for targeted applications.
Different forms of WO3 catalysts, synthesized through sol-gel and hydrothermal methods, have also been applied for photocatalytic ozonation by Mena et al. [34] for N,N-diethyl-meta-toluamide (DEET) abatement under visible light. Sol-gel monoclinic WO3 calcinated at higher temperatures (600–700 °C) presented faster removals, eliminating DEET in 10 min. In comparison, over 2 h, the photocatalytic oxidation of DEET achieved only a 22% removal. The presence of ozone in the reaction medium can significantly decrease the typical high recombination rate of the photogenerated species.
The tungsten trioxide catalyst has also been tested for the treatment of more complex effluents. Razali et al. [35] used a WO3 catalyst synthesized through the coprecipitation method, for the color and suspended solid removal of palm oil mill effluent (POME), retrieved from different ponds of the conventional treatment. The applied catalyst was able to obtain up to 65% and 91% color and suspended solids removal, respectively. The number of active surface sites and their adsorptive characteristics are appointed to be determinants over the treatment process.
Even with a higher visible-light photoactivity, the alternative catalysts still face compromising drawbacks for their application in more rounded contaminant treatment technologies. The lower specific surface areas, high electron-hole recombination, the limited number of active sites, and low electron reduction potential are some examples of those disadvantages that still need to be circumvented [26].
Among different modifications, catalyst doping, and the production of different composites based on the commented catalysts appear as practical techniques to overcome these disadvantages. In the following sections, a recent overview of the general aspect of studies regarding the application of doped TiO2 and composites of TiO2 and the alternative catalysts for the photocatalytic degradation of contaminants will be given.

3. Catalyst Doping

3.1. TiO2

TiO2 is the most used semiconductor catalyst for the photocatalytic elimination of contaminants, but still presents some typical characteristics that hinder the process scale-up. Its high bandgap energy typically implies the use of external UV radiation sources, with λ ≤ 390 nm, hampering its activation through sunlight as the UV portion represents only 4–6% [14]. The high recombination rate of e/h+ and a weak separation of photocarriers also reduces its photocatalytic activity [25]. Among different modifications, catalyst doping thus appears as a practical technique to overcome these disadvantages. In this section, an overview of the general aspect of recent studies regarding the application of doped TiO2 for the photocatalytic degradation of contaminants will be presented.

3.1.1. Transition Metals Doping

Both metal and non-metal elements may be incorporated in the catalyst structure and are capable of altering light absorption capacity and, more importantly, visible light, preventing the recombination of electron-hole [36]. Regarding metal doping, transition metals are more commonly applied, due to their partially filled d states, which promote the creation of intra-bandgap energy states and the absorption shift [37]. Table 1 summarizes the results and main conditions of the application of metal-doped TiO2 found in the literature.
In reference to the catalyst synthesis, the sol-gel method is one of the main alternatives and allows better control of the product characteristics, such as porosity, structure, composition, and homogeneity, which is especially important in the incorporation of dopants. Moreover, a calcination step post-synthesis is usually applied as a simple technique to transform an amorphous structure into a crystalline (Anatase, Rutile, and Brookite). Karuppasamy et al. [38] attested that, during the investigation of the doping effect of different transition metals (Zn, Cu, and Zr) on TiO2, the presence of doping elements may also alter the crystalline phase formation, reducing the temperature at the calcination stage needed to achieve the different structures, which was confirmed by other studies [39,40]. Regarding the different elements used and their photocatalytic activity, Zn-TiO2 presented the best performance in methylene blue elimination under visible light. The interactions between Zn correspondent electronic states with the TiO2 conduction band may provoke a red shift in the bandgap, improving the material light absorption between 400 nm and 700 nm.
The amount of dopant is also a crucial parameter to be studied, as excess material can lead to the formation of metallic oxides on the catalyst surface during thermal treatment, which may hinder the catalyst’s exposure to photon irradiation. Lee et al. [39] applied hollow TiO2 spheres doped with copper for the degradation of phenol under visible irradiation. To remove the copper oxides (CuO and Cu2O), as detected in XRD and TEM analysis, an acid treatment was applied, capable of dissolving these species. The untreated catalyst presented minimal photodegradation, even lower than bare TiO2, while acid-treated Cu-TiO2 completely removed the contaminant in 4 h.
Table 1. Application of metal-doped TiO2 for the photocatalytic removal of contaminants.
Table 1. Application of metal-doped TiO2 for the photocatalytic removal of contaminants.
CatalystPrecursorMethodEbg (eV)Radiation SourceContaminantRemoval (Time)Ref.
Precipitation3.65UVRhodamine B,
5 mg L−1
97% (20 min)[41]
Ion-exchange-UV-VisibleMethyl Orange,
1 mg L−1
80% (150 min)[42]
S. aureus,
106 CFU mL−1
100% (2 h)
E. coli,
106 CFU mL−1
100% (1 h)
Au-TiO2TBOTGold (III) ChlorideSolvothermal3.70UV-VisibleMethylene Blue and Diuron
0.03 mM each
65% MB (180 min) and 95% DIU (120 min)[43]
EDTA-Citrate2.50SolarCiprofloxacin and
10 mg L−1 each
93.2% CPR and 93.6% NOR
(180 min)
E. coli,
108 CFU mL−1
95.0% (180 min)
Sol-gel3.70VisibleMethylene Blue, 0.05 M27.5% (60 min)[38]
Sol-gel and
2.38VisiblePhenol, 5 mg L−1100% (4 h)[39]
Eu-TiO2TTIPEuropium OxideSol-gel2.86VisibleMethylene Blue and Methyl
Orange, 5 mg L−1
72.1% MB and 71.8% MO
(180 min)
Sol-gel2.80VisibleAcid Orange Azo Dye,
10 mg L−1
80% (60 min)[46]
TEOTIron NitrateSol-gel-UV-VisibleNitrobenzene, 2.45 × 10−4 M97.3% (240 min)[47]
Electrospinning2.68VisibleCiprofloxacin and Methylene Blue,
10 mg L−1 each
91% MB and 99.5% CIP
(300 min)
1.65UVProzac®, 10 mg L−195% (30 min)[40]
Sol-gel3.00VisibleAcid Orange Azo Dye, 10 mg L−153% (60 min)[46]
Sol-gel2.83VisibleMethylene Blue, 0.05 M99.6 (60 min)[38]
Sol-gel3.30VisibleMethylene Blue, 0.05 M81.9% (60 min)[38]
Various factors are determinants of the performance and characteristics of doped catalysts, but regarding the element’s properties, both the ionic radius and electronegativity are very important, due to their role over the element’s solubility in TiO2 lattice [49]. If the radius and electronegativity of the metal ion are close to Ti4+, doping tends to be substitutional, meaning that the substitution of the existing Ti4+ ions for the dopant will occur, but if the ion radius is lower than the existing elements, interstitial doping may occur, as they will occupy the interstitial sites of the catalyst lattice. Moreover, if the radius of the doping element is higher, it will not be able to penetrate the catalyst structure and will be mostly present at its surface [50].
Different analytical techniques may be applied to better detect the possible doping types. Through the study of a Mn-TiO2 catalyst, Moreira et al. [40] detected a negative shift of the typical 144 cm−1 Eg mode of anatase TiO2 in Raman spectroscopy results, up to 5 cm−1 with the increase of Mn%. This mode is representative of the symmetrical stretching of O-Ti-O bonds, and the presented shift was suggested to be indicative of the Mn3+ substitution of Ti4+. Moreover, Lee et al. [39] conducted an XPS analysis of Cu-TiO2, and the results indicated a peak shift of the binding energy in both O 1s and Ti 2p spectrums, which was associated with the incorporation of Cu over the TiO2 lattice, through substitution of Ti4+ by Cu2+ atoms, forming Cu-O-Ti bonds, also corroborated by Cu 2p spectra, resulting in the modification of the electron density and charge distribution. Regarding interstitial and surface doping, Kayani et al. [51] studied vanadium doping TiO2 and suggested that, in the XRD analysis, the reduction in the intensity of the anatase phase diffraction peak (101) is an indication of the interstitial doping of vanadium, which was expected due to the lower ionic radius of V5+ in comparison to Ti4+.
The efficiency of photocatalytic systems may also be improved by combination with other AOPs, such as ozonation, where the incorporation of ozone can increase radicals’ formation and minimize photocatalytic drawbacks [52]. Ozone, as an excellent electron acceptor, can diminish electron-hole recombination by adsorbing onto the catalyst and retrieving the formed electrons, producing the ozonide radical (·O3) and engaging in multiple reactions, including the formation of hydroxyl and other radicals (Equations (1)–(3)).
O 3   ( ads ) + e   · O 3
· O 3 + H +   · O 3 H
· O 3 H O 2 + · OH
Considering that one of the main advantages of catalyst doping is the reduction of Ebg and the resultant easier activation, their use in photocatalytic ozonation induces a higher electronic excitation, and thus an even higher radical generation. The ozone present in the medium may also directly react with the contaminants, especially those with high electron density, such as aromatic chemicals [53].
Catalyst doping is also an approach to turn photocatalytic ozonation into a more feasible technology by not only improving ozone decomposition but compensating the ozone production cost by using solar irradiation. Mecha et al. [54] demonstrated that, when applied for the photocatalytic ozonation treatment of phenol in SWW, Cu-TiO2, Fe-TiO2, and Ag-TiO2 had an up to almost 40% lower energy requirement compared to the undoped catalyst, due to their increased activity. Moreover, notably, Fe-TiO2 also maintained an appreciable activity under UV irradiation, indicating a broader active spectrum. Still, photocatalytic ozonation is not very much studied for the doped and codoped TiO2 catalysts and there is plenty of space for developing this interesting and efficient technology. Moreover, the possibility of sunlight radiation usage, which is a natural resource available for a great part of the year, in such an approach is considered.

3.1.2. Noble and Rare-Earth Metals Doping

Noble metals (e.g., Au, Ag, Pd, Pt) represent a high-interest group of transition metals currently being vastly explored for TiO2 doping due to their light-gathering capability and their role as electron trappers. These metals present, in general, a larger ionic radius compared to Ti4+, which difficult their penetration into the catalyst lattice, being deposited mostly on the surface. Saber et al. [43] detected multiple peaks corresponding to the presence of an Au crystalline phase in Au-TiO2, prepared by solvothermal method, that can be indicative of surface modifications. Additionally, visible light absorption was considerably increased, again attributed to surface plasmon resonance.
Noble metal doping is known to produce a surface plasmon resonance effect, which induces a higher photoactivity of the catalyst under visible irradiation. This effect can be identified through diffuse reflectance spectroscopy (UV-Vis DRS), with a broader shoulder-like peak attributed to the heteroatom addition and the resulting oscillation of the conduction band electrons on its surface during photoirradiation [41]. Moreover, Ellouzi et al. [41] identified such a phenomenon, which resulted in considerable faster degradation of RhB, with an observed rate constant of 0.1827 min−1 using a Ag-TiO2 catalyst, while non-doped TiO2 led to an apparent constant rate of 0.1034 min−1. The lack of general and more specific legislation and regulation challenges the classification of CECs, but different synthetic dyes are also considered by many researchers as examples of CECs, due to their potential toxicity, with some studies already appointing their carcinogenic and mutagenic characteristics and ubiquitous behavior [55].
The use of noble metals in catalyst doping, especially silver, us also known to potentialize the antimicrobial activity of catalysts for disinfection purposes. In the medium, the metal particles present on the surface or even some that are released from the catalyst may interact with the cell membrane of microorganisms, damaging it and causing it to rupture, with the consequent oxidization of its proteins and genetic material (Figure 3) [42].
Wu et al. [42] proved the capacity of Ag-TiO2 nanofibers to inactivate S. aureus and E. coli bacteria. In 1 h, the number of bacterial colonies, initially ~106 CFU mL−1, was substantially reduced when in contact with Ag-TiO2 under visible light and eliminated in the next hour, while no significant inactivation was found for the undoped catalyst. A synergic effect between Ag2O, AgO, and metallic silver, present in the catalyst surface, and the photogenerated radical species can considerably boost the disinfection capacity of the material, as it is able to rupture the bacteria cell membranes and degenerate their genetic material.
The surface plasmon resonance phenomenon and other positive effects of noble metals can also be an important ally in photocatalytic ozonation. However, specific properties of each element can greatly affect the final product performance and need to be evaluated for a better selection. For instance, the high electronegativity of Au (2.54) entails a higher capacity to withhold the photogenerated electrons. Although it could contribute to the mitigation of electron-hole recombination, it also impedes ozone reduction and ozonide radical formation, reducing the overall efficiency of the ozone-based process [56]. Thus, other elements with lower electronegativities can produce a more balanced effect, such as Ag (1.93), and boost contaminants removal while decreasing ozone consumption.
Rare-earth metals (e.g., Eu, Ce, Pr) also possess a high capacity to enhance the solar (visible and NIR) photocatalytic activity of TiO2, in addition to its UV activity [23]. These elements are particularly interesting due to the partially filled 4f states, and the f-f electronic transitions present within. These electronic features combined with the interactions between the f and other orbitals, and with the TiO2 CB and VB are responsible for the formation of new electronic states and other beneficial doping effects regarding luminescence properties [50]. The 5s and 5p orbitals also promote a shielding effect of the 4f orbital, producing a narrowing of the transition emission bands [57]. As their ionic radius is overall larger than Ti4+, the surface doping and formation of their respective oxide layers are expected, but multiple doping forms may occur simultaneously [57].
These metals can also considerably increase the number of defects over the catalyst surface, which may act as photogenerated species trapping sites. Moreover, even though, due to their ionic radius and symmetric difference, the impregnation of such elements occurs typically on the material surface, some studies have detected their presence within the catalyst structure [58].
The rare-earth metal doping can also influence the crystalline phase transition of TiO2, which is known to inhibit the transition to rutile from the anatase phase, which possesses a higher photoactivity [58]. More specifically, Pascariu et al. [48] detected an increase from 13% to 70% in the contribution of the anatase phase as a result of La3+ doping (Figure 4). Additionally, due to the crystalline phase composition, a higher number of surface defects, and OH groups when La3+ was applied, the doped catalyst performed a higher degradation of contaminants, ciprofloxacin, and methylene blue, under visible light. An optimum concentration of dopant was also found, with a further increase of lanthanum concentration higher than 0.1% possibly leading to the blocking of active sites and an excess of trap sites, with a shorter distance between them, having a controversial result, increasing electron-hole recombination [59,60].
Overall, noble and rare-earth metals are able to produce significant alterations in the catalyst light absorption characteristics and its charge separation. However, in a more practical application, other circumstances must be considered which may hinder their use. These elements possess typically high price precursors and, even if used in low concentration, in comparison to other earth-abundant materials, will elevate the material and consequently the final process cost, which is counter to what is needed to broaden the photocatalysis application. Moreover, they present intricate or distinct impregnations, necessitating a case-by-case analysis of their introduction to evaluate whether it will in fact produce the desired effect in more specific materials and morphologies. The high heterogenicity of studies involving such elements, with different impregnations, irradiation sources, contrasting results, and other parameters, challenge a proper selection of a more promising element.

3.1.3. Non-Metals Doping

Non-metal elements (e.g., N, S, B, C) are vastly explored for TiO2 doping, with facile production methods, and capable of enhancing the catalyst stability and photoactivity, forming new energy levels with the consequent bandgap shortening, and inducing the formation of oxygen vacancies [37]. Contrarily to metals, in non-metal doping, it is expected that the introduced elements will have an influence over the valence band through interactions with O 2p states, even though cationic interactions may also take place [25]. Table 2 summarizes the application of different non-metals in TiO2 doping.
Nitrogen doping is one of the most explored techniques, due to its promising red shift of the absorption edge and a similar ionic radius to oxygen, facilitating its substitution, although it may be inserted in the catalyst structure in different forms (Figure 5) [61]. Because of the broad exploration, different preparation methods have been applied, such as pulsed laser deposition [62], thermal annealing [63], hydrothermal [64,65], solvothermal [66], and sol-gel [67,68,69].
The sol-gel method is the most widely applied, due to its facile, flexible, and controllable operation, allowing different modifications of the basic processes to achieve the desired characteristics [50]. Using such a method, Assayehegn et al. [71] synthesized multiple N-TiO2 catalysts applying different ratios of guanidium chloride (GUA), an environmentally friendly N precursor, to Ti, and indicated that the incorporation of N precursor showed a direct effect over the crystalline phase of N-TiO2 formation post calcination. This can be caused by the perturbance of N3− ions on the lattice orientation and density of charges, due to its higher negative charges and ionic radius compared to O2−. Moreover, the found optimum mixed crystalline phase, 42% anatase and 58% rutile, is pointed out to have a beneficial synergic effect with nitrogen doping, which was also concluded by other research groups [39,68]. When applied for MB degradation under visible light, the best N-TiO2 had an apparent reaction rate constant of 0.0325 min−1, which is almost 17 times higher than the undoped catalyst, and represented 97% of MB removal within 100 min.
Dopant precursor is a key parameter, especially due to the large variety of compounds to be used. Bakre et al. [68] used semicarbazide, N,N’-dimethyl urea, and urea as nitrogen precursors in the sol-gel synthesis of N-TiO2. Regarding the crystalline phase composition, urea led to a pure anatase phase catalyst, while the other dopants resulted in rutile-anatase mixed phases. Following the characterization of the N-TiO2 catalysts, the urea catalyst presented a higher absorption on the visible range, seconded by N,N’-dimethyl urea. FTIR spectroscopy results showed proper nitrogen incorporation over all catalysts lattice due to an indication of N-Ti-N bonds existence, possibly interstitially as suggested by N1s XPS spectra binding energies peaks. Additionally, other substances containing nitrogen have been vastly applied for doping, such as ammonia, ammonium hydroxide, ethylenediamine, and hydrazine hydrate [66,72,73,74].
Table 2. Application of non-metal doped TiO2 for photocatalytic removal of contaminants.
Table 2. Application of non-metal doped TiO2 for photocatalytic removal of contaminants.
CatalystPrecursorMethodEbg (eV)Radiation SourceContaminantRemoval (Time)Ref.
B-TiO2* P25Boric AcidEDTA-Citrate2.87SolarCiprofloxacin and
10 mg L−1
93.2% CPR and 93.0% NOR (180 min)[44]
E. coli, 108 CFU mL−199.9% (180 min)
TBOTBoric AcidSol-gel3.01VisibleCatechol, 10 mg L−1100% (60 min)[75]
TTIPBoric AcidSol-gel2.98UVDiclofenac, 50 mg L−198% (180 min)[67]
F-TiO2TiCl4Ammonium FluorideNebulizer Spray2.79VisibleMalachite Green,
300 mg L−1
90% (60 min)[76]
I-TiO2TBOTIodic AcidSol-gel3.18SolarMethylene Blue,
4.8 mg L−1
30% (60 min)[77]
N-TiO2TTIPUreaSol-gel2.27UVDiclofenac, 50 mg L−195% (180 min)[67]
TBOTGuanidinium ChlorideSol-gel2.91VisibleMethylene Blue,
10 mg L−1
97% (100 min)[71]
TTIPUreaSol-gel-SolarMethylene Blue,
10 mg L−1
99% (100 min)[68]
TTIPN,N’-dimethyl ureaSol-gel-98% (120 min)
TTIPSemicarbazideSol-gel-98% (80 min)
TBOTUreaSol-gel2.58VisibleMicrocystis aeruginosa,
3 × 106 cells mL−1
99.1% (12 h)[78]
TTIPAmmonium HydroxideSol-gel2.31VisibleE. coli, 105 CFU mL−1~100% (12 h)[79]
S. aureus, 105 CFU mL−1~100% (12 h)
Mycobacterium avium,
105 CFU mL−1
~100% (12 h)
Candida albicans,
105 CFU mL−1
99.9% (12 h)
S-TiO2TTIPSulfuric AcidFlame Spray
0.5 mM
65% (300 min)[80]
-VisibleMethyl Orange,
5 mg L−1
38% (300 min)[81]
* Commercial Degussa P25 TiO2.
Sulfur emerges as an interesting dopant, presenting in both cationic and anionic species, as in the form of S6+ and S4+ it can substitute Ti4+, although the O2− substitution by S2− can also occur. Through the study of sulfur-doped TiO2 films, Bento et al. [81] suggested that the cationic substitution by S6+ would be more advantageous for photocatalytic purposes in comparison to the anionic, due to the formation of impurity energy levels located above the TiO2 valence band, decreasing the Ebg and improving activation by visible light. Furthermore, the authors verified the formation of SO42− on the surface of the catalyst, known to occur during S doping, which may act as electron trap centers and improve radicals’ formation. The replacement of O2− by S2− is may be less chemically favorable due to the larger ionic radius of the sulfur anion, hampering its penetration into the catalyst lattice and inducing higher thermodynamic energy for the S-Ti-S bond to be formed [80].
Boron has been found to promote a higher reduction of Ti4+ into Ti3+, which induces the formation of oxygen vacancies, a broader catalyst spectrum response, and improved photocatalytic activity. Yadav et al. [67] identified Ti3+ correspondent peaks in interstitially boron-doped TiO2 Ti2p2/3 XPS spectra, indicating its existence in the catalyst lattice. Moreover, the full-width half maxima (FWHM) was used as a correlation of the presence of Ti3+, and when compared to a N-TiO2 catalyst, it was found that B-TiO2 presented a higher density of Ti at a less oxidized state. In addition, both N-TiO2 and B-TiO2 were compared regarding diclofenac degradation, and B-TiO2 presented a higher photocatalytic activity, with an apparent constant rate value 30% higher than N-TiO2. Zhang et al. [75] also confirmed the benefits of boron doping regarding Ti4+ reduction and suggested that, besides its role as electron trapping sites, Ti3+ altered the adsorptive properties of the catalyst. The formed O-Ti3+ bonds may weaken the water adsorption and enhance the formation of cooperative hydrogen bonds on the catalyst surface, promoting a higher interaction with contaminants, such as phenolic compounds.
Halogens, such as chlorine, iodine, and fluorine, possess attractive properties, such as high electronegativity and the capacity to occupy both titanium and oxygen sites [82]. Ravidhas et al. [76] successfully synthesized F-TiO2 thin films and suggested that fluorine, the most electronegative element, could substitute oxygen atoms in the catalyst structure, leading to the formation of Ti-F-Ti bonds and the reduction of Ti4+, promoting an adjustment of the electron density of titanium and an increased concentration of oxygen vacancies. This incorporation of impurities into the structure also caused a decrease of the Ebg down to 2.90 eV, from an initial 3.30 eV of the undoped catalyst. F-TiO2 also presented an improved degradation of malachite green dye under visible and solar radiation, attributed to its higher surface area and number of active sites, oxygen vacancies, and lower Ebg. The disturbance of charges was also found by Hwang et al. [77] after iodine doping of TiO2. In the I 3d XPS spectra, multiple valence states were found, with peaks corresponding to I5+, I7+, and I being detected in coexistence. The authors pointed out that I5+ had substituted Ti4+ atoms, due to their similar radius, with the ensuing formation of Ti3+ and Ti2+ due to compensation for the disproportionate charges. Meanwhile, the larger ionic radius of I induces its dispersion on the TiO2 surface.
Apart from chemical contaminants, photocatalytic processes using non-metal doped catalysts have been applied for microorganisms’ elimination, as a facile disinfection method that does not produce secondary pollutants [83]. The disinfection of a series of pathogenic contaminants, S. aureus, E. coli, Mycobacterium avium, and Candida albicans, was studied by Tzeng et al. [79] using a N-TiO2 catalyst under visible light. Starting with a 105 CFU mL−1 individual solutions of each species, in 12 h treatment, N-TiO2 obtained a final survival rate, C/C0, of approximately 10−1, 10−2, 10−4, and 10−5 of, respectively, C. albicans, M. avium, E. coli, and S. aureus. The order of the disinfection yields is directly related to the complexity of the cells, as C. albicans is a unicellular fungus, it presents a higher structural complexity and contains substances less prominent to oxidative radicals’ attacks, whereas S. aureus is a Gram-positive bacteria, which has no lipopolysaccharide layer.
The precursors may also deeply affect the disinfection characteristics of the material. Using urea and N2 as nitrogen precursors, Zhou et al. [78] demonstrated that even with the catalyst only calcinated in the presence of N2 having the highest incorporation of nitrogen, the urea doped catalyst calcinated in the same atmosphere presented a removal of 99% of chlorophyll-a, related to the Microcystis aeruginosa algae cells degradation, in 12 h.
The doping process may also lead to the formation of other compounds within the catalyst structure, such as the elements’ respective oxides, which may provide additional mechanisms for chemical and biological contaminant removal. For instance, regarding boron doping, the formation of a B2O3 can enhance the catalyst bacterial inactivation ability, as it can dissolve in water and lead to the formation of boric acid [44,84].
The typical low cost of non-metal precursors can be an interesting aspect to reduce photocatalytic ozonation costs. As said previously, the combined process may present an increased cost due to ozone production energy requirement and possible use of artificial radiation sources, in the case of typical TiO2. Thus, the lower cost associated with non-metal doping and the produced benefits, such as the increase in photocatalytic activity under solar radiation, may render the photocatalytic ozonation process more feasible. In previous studies, Fernandes et al. [18,21] developed multiple nitrogen-doped catalysts for application in the photocatalytic ozonation removal of a mixture of parabens. The doped catalyst, especially ammonia doped, led to an improved degradation of all five contaminants (methyl-, ethyl-, propyl-, butyl-, and benzylparaben), completely removing them faster and with a lower consumption of ozone. As ozone has low solubility in water, the photocatalyst may enhance its solubilization and decomposition, allowing a more efficient consumption, leading to a lower concentration needed through the production of other reactive species.

3.1.4. Co-Doping

The application of two or more dopants can be highly beneficial, as it can enhance the catalyst performance through multiple mechanisms. Multiple combinations of elements are possible and have been explored, even with different classifications, such as metal and non-metal co-doping. The co-doped Fe-Pr-TiO2 catalyst, studied by Mancuso et al. [46], presented an improved performance regarding AO dye removal compared to the single and undoped catalyst, with 87% removal of the AO and 80% TOC removal in 60 min under visible light. In the studied case, the co-doping was accounted to considerably decrease the Ebg (2.7 eV) and enhance the formation of oxygen vacancies, confirmed in photoluminescence spectra, acting as electron trappers, and reducing the e/h+ recombination. The authors also investigated in another study the co-doping of Fe and N, and again it showed a superior performance as it can benefit from both the substitution of Ti4+ by Fe3+, acting as electron acceptors and inhibiting e/h+ recombination, and the nitrogen replacement of oxygen sites, producing Ti3+ and oxygen vacancies [59].
Jahdi et al. [85] explored the combination of fluorine with palladium for the degradation of sulfamethoxazole (SMX), using a hydrothermal-produced F-Pd-TiO2 catalyst. The addition of F was suggested to control the crystalline phase growth, while Pd significantly improved the photocatalytic activity under higher wavelengths, with its increasing concentration (0–10%) leading to Ebg down to 0.54 eV. The best catalyst achieved 98.4% of SMX removal under direct sunlight in 40 min. It should be noticed though that the presence of fluorine was found to lead to the formation of white fluorine-based polymeric substances floating in the treated solution.
Different precursors may be used to incorporate more than one element simultaneously [86]. Thiourea is commonly used as a source of carbon, sulfur, and nitrogen, and the three dopants’ simultaneous incorporation may promote an even more significant narrowing of the bandgap of TiO2 through the formation of mid-gap levels due to interactions with the O 2p orbitals. Additionally, Khedr et al. [86] indicated that sulfate and carbonaceous species can also be formed and may improve the photocatalytic activity, acting as electron trapping sites, inducing visible-light absorption, and enhancing radicals’ formation. In this specific study, N-C-S-TiO2 had substantially better performance over ibuprofen degradation under visible light irradiation, almost fully eliminating the contaminant under 240 min of reaction, against a 12.2% degradation reached by the undoped catalyst. Table 3 presents some key studies involving the use of co-doped TiO2 catalysts.

3.2. WO3

The WO3 photocatalysts appear as an interesting alternative to typical TiO2, with good visible-light activity, but also chemical and electronic properties [91]. However, some drawbacks are still found, principally the high electron-hole recombination rate and a more positive position of the CB, which compromises the superoxide production [50]. The introduction of heteroatoms to the WO3 structure has not been widely explored compared with other catalysts, but some studies have been conducted showing that this adaptation may be responsible for promoting the enhancement of the catalyst performance and its typical disadvantages. Studies regarding the application of doped WO3 catalysts are summarized in Table 4.

3.2.1. Transition Metals Doping

WO3 doping can considerably interfere with basic catalyst characteristics, such as crystallinity, morphology, and optical properties, which will then produce variations in their photocatalytic activity. Cu-WO3 was found to have a lower Ebg (2.78 to 2.60 eV) and higher crystallinity compared to the studied undoped catalyst by Quyen et al. [95]. Besides, changes in its surface properties were also detected, with the incorporation of Cu being mainly at this level, promoting the formation of a more porous structure, meaning a higher surface area and number of active sites capable of enhancing contaminants’ interaction and degradation. The presence of the heteroatom also promoted a higher presence of the W5+ state, which may imply the equivalent formation of oxygen vacancies due to electronic rebalance, which may increase the catalyst electroconductivity and pollutants adsorption.
Mehmood et al. [94] also presented a broad study of the incorporation of another metallic element, cobalt, over the WO3 lattice. The Co atoms were appointed to be doped substitutionally, by the replacement of W6+ by Co2+, which promoted an expansion of the lattice due to the slightly higher ionic radius of Co2+. Thus, excessive addition of the dopant could degrade the catalyst crystallinity due to the collapse of its structure. The dopant also supported the creation of localized states within the catalyst bandgap, reducing the Ebg. The Co-WO3 presented incredibly better performance for the degradation of a methyl red solution, increasing the removal rate from less than 5% to 90% under 2 h. Additionally, the use of this catalyst was also applied for the inactivation of cancerous cells, which reduced the viability of MCF-7 breast cancer and Hep-2 liver cancer cells down to 60%, possibly through the attack of their mitochondria and rupture of the cell wall.
The doped catalysts must also be subjected to lixiviation tests, especially when more dangerous metal elements are used, as their continuous reutilization may be responsible for the release of this species into the medium. The use of cadmium (Cd) as a metal dopant has been indicated to diminish e/h+ pairs recombination and increase the catalyst crystallinity and porosity, resulting in a higher photocatalytic degradation [93]. However, although the dopant may substitute W atoms on the catalyst structure, implying stronger bounds, Cd lixiviation can still occur, especially at acidic conditions. It must be noted that US EPA regulations, for example, establish a maximum of 0.005 ppm of cadmium in drinking water, with serious negative health effects when in contact with higher concentrations.

3.2.2. Noble and Rare-Earth Metals Doping

The use of different precursors during the photocatalyst and doping synthesis can alter the mechanisms and the configuration of the formed product. Palharim et al. [92] studied the alteration caused by the addition of HCl or HNO3 during Ag-WO3 synthesis, using AgNO3 as the dopant precursor. It was found that HNO3 promoted the incorporation of the dopant metallic silver, while HCl reacted with AgNO3 to form AgCl, being detected in the catalyst structure. This promoted differences in the catalyst, as an increase in Ag concentration led to lower Ebg in HNO3/Ag-WO3 due to the particle agglomeration, and the opposite occurred for HCl/Ag-WO3. Regardless of the acid used, Ag-WO3 had better removal rates of acetaminophen under visible light than the undoped catalyst, because of the combination of factors such as the localized surface plasmon resonance, characteristic of noble metal doping, the insertion of new energy level within the catalyst bandgap and the reduction of Ebg. Moreover, HCl doped presented a better performance, possibly due to the interaction between Ag-WO3 and the AgCl clusters formed, and the contribution towards decreasing the WO3 conduction band to more negative values, allowing oxygen reduction.
The f-orbitals of rare-earth metals can also be beneficial for the improvement of WO3 performance. Tahir and Sagir [102] conducted a wide study of the application of these metals by synthesizing lanthanum-, erbium- and gadolinium-doped WO3 through a hydrothermal method. As these rare-earth metals possess considerably higher ionic radius, the substitution of W atoms is not favored, and the dopants were found to occupy interstitial sites. The catalyst crystallite size decreased with the incorporation of the heteroatoms, having advantages regarding the reduction of photogenerated species recombination, and the specific surface areas were also greatly increased, especially for Gd-WO3, 106 m2 g−1 compared to 30.7 m2 g−1 of the undoped catalyst. Moreover, the higher adsorption locations and catalytic center of Gd-WO3, combined with lower recombination of electron-hole, presented a faster and almost complete removal of multiple single contaminants (methotrexate, tetracycline, methyl orange, methylene blue, and crystal violet).

3.2.3. Non-Metals Doping

The combination of the non-metal elements and WO3 can also promote variations of the CB and VB of the catalyst, hinder the recombination factor of photoinduced charges, and increase photocatalytic activity under visible light. The S-WO3 nanowires catalyst was investigated by Han et al. [101] using thiourea as the sulfur precursor. The non-metal element was able to substitute W6+ in the form of S6+ in the WO3 lattice, as opposed to the anionic replacement of O2−, due to the higher energy form of W-S bonds than W-O. The doping method also accounted for the formation of surface hydroxyl groups because of oxygen adsorption, which may act as electron trapping centers. Sulfur also was responsible for creating intermediate energy levels above the VB of WO3, enhancing the catalyst’s photoresponse. Even if not mentioned by the authors, the use of thiourea as a precursor could also involve the parallel incorporation of nitrogen over the structure.
Similar effects were found regarding the positive effects of non-metallic heteroatoms by Zheng et al. [99], in interstitially carbon-doped WO3. The location of the C atoms may denote its interaction with WO3 and consequent distortion of the WO6 octahedron structure and an induced increased dipole, possibly more conducive for the transmission of photocarriers and photocatalytic activity. Comparably with other non-metals, new C 2p induced levels were formed in the interior of the bandgap of WO3, acting as transfer stations to the photocarriers.

3.2.4. Co-Doping

As dopants may have different mechanisms, their combination can severely upgrade the photocatalytic activity through the synergic combined effect, being able to attain higher removal rates, even at complex effluents. Tijani et al. [100] proposed a green synthesis of I-P-WO3, by applying a plant extract in bulk catalyst production. The I and P3+ atoms acted as structure-directing agents, affecting the morphology of the rod-like catalyst, with P conducing to more elongated shapes and I with shorter and more uniform. Alterations of lattice parameters indicated distortions of the catalyst structure, which may be a result of the partial substitution of W6+ and O2−, by respectively P3+, which has a smaller ionic radius than W, and I that has a higher ionic radius compared to O. This substitution is pointed out to have detrimental effects as it may occur the collapsing of the WO3 structure. However, due to the low concentration of the incorporation, it only resulted in beneficial decreased grain sizes and enhanced surface areas, up to 416.3 m2 g−1. The optical properties of I-P-WO3 were also improved, decreasing the Ebg from 2.61 to 2.02 eV, due to acceptor energy levels below the CB of WO3. The codoped catalyst was applied for the treatment of dyeing wastewater under sunlight, achieving high color and odor removal, as well as 93.4%, 95.1%, and 92.0% removals of total organic carbon (TOC), chemical, and biological oxygen demand (COD and BOD), respectively, accounting for values below the regulated international standards. Thus, codoping may lead to the formation of a more feasible photocatalyst regarding more complex applications.

3.3. g-C3N4

Even with a typically lower Ebg compared to TiO2, g-C3N4 still exhibits a fast recombination rate of the photogenerated e/h+ pairs and a limited light absorption range, which diminishes the visible and solar radiation conversion. Thus, the doping of g-C3N4 may contribute to charge separation and concurrently a shift of light absorption. As g-C3N4 has gotten much attention recently, investigation of the incorporation of multiple elements over the catalyst structure has been reported.

3.3.1. Metals Doping

The doping using metallic elements is known to promote the formation of oxygen vacancies and considerably improve the catalyst activity. The existence of various unbounded electrons in the hexazine structures of g-C3N4 makes it susceptible for metallic elements to be inserted in its lattice [103]. The results regarding the use of metal-doped g-C3N4 are present in Table 5. For the removal of paracetamol under visible light, doping using cobalt (II) nitrate was made over a melamine-based g-C3N4 through a calcination method [104]. The detected Co2+ and Co3+ in the catalyst and the O 1s XPS spectra indicated the formation of oxygen vacancies. Moreover, the doping affected the crystallinity of g-C3N4 and its morphology, presenting a mixed structure of cobalt oxide closely attached to carbon nitride layers. Optical characterization also showed indirect recombination of charge carriers and smaller charge transfer resistance, which indicates a higher charge transport kinetics and separation. Ultimately, the Co-doped g-C3N4 presented a reaction rate almost three times higher than the undoped catalyst, 0.0382 min−1, completely removing 1 mg L−1 of paracetamol within 120 min. No leaching of Co ions was detected after the oxidation experiments.
Pham et al. [105] studied Ni-doped g-C3N4 for volatile organic compounds (VOCs), nitrobenzene, and toluene, removal under natural sunlight. The presence of Ni promoted a red shift of the absorption edge, suggesting a higher photoactivity over the visible range, and Ebg was estimated as 2.25 eV, noticeably lower than the undoped g-C3N4, 2.76 eV. The doped catalyst also presented a higher specific surface area and pore volume, which promote a higher interaction with the different compounds involved in the oxidative reactions. The doping treatment finally improved VOCs elimination, with reaction rate constants of 0.0170 and 0.0173 min−1 for, respectively, toluene and nitrobenzene, compared with 0.0049 and 0.0072 min−1 of the undoped catalyst.
The incorporation of iron over g-C3N4 can also improve the oxidation potential and may act as electron trap centers. With a relatively larger ionic radius, Fe atoms tend to be doped into the interlayers of the catalyst, forming strong bonds with the N atoms present in the aromatic rings, which decreases the Ebg. However, an excess amount of Fe may also cause the collapse of the sheet structure. The dopant exists mainly as Fe3+, and as the reduction potential of Fe3+/Fe2+ lies between the CB and VB of g-C3N4, the photogenerated e may easily react with Fe3+, reducing it into Fe2+ and diminishing e/h+ recombination. The instability of Fe2+ makes it prone to suffer oxygen reduction and return to the Fe3+ form [103].
Although not often, the use of alkaline and alkaline-earth metals (such as K, Mg, and Ca) for g-C3N4 doping has been explored. Studies regarding the use of these elements in more surveyed catalysts present mixed results, with possibly poisoning effects, but also an increase in photoactivity and surface area [50]. Tripathi and Narayanan [106] tested potassium doping of melamine-based g-C3N4. The presence of K during the thermal polymerization was suggested to inhibit the formation of hydrogen bonds between the polymeric melon strings of the intra-layer infrastructure, reducing the layer’s stacking, but also the Ebg, from 2.7 to 2.5 eV, with changes of the VB energy and Fermi level towards more positive potentials. This condition may promote an increase in the oxidation capacity of the K-doped catalyst. Furthermore, under natural sunlight, the doped catalyst obtained 56.5% elimination of phenol present in real wastewater.

3.3.2. Noble and Rare-Earth Metals Doping

Noble metals, such as Ag and Au, are electron-rich transition metal elements that can capture photogenerated electrons and provide the excess energy of plasmonic states, provoking a shift of the Fermi levels to less positive potentials due to an excess of negative charges, reducing electron-hole recombination and boosting its catalytic performance. Tri et al. [107] explored Ag-doped g-C3N4 for the treatment of tetracycline, an antibiotic, in hospital wastewater under solar irradiation. Besides the faster production and higher separation of the e/h+, Ag significantly altered the optical properties of the catalyst, decreasing its Ebg down to 2.19 eV, and possibly increasing the number of active sites and surface area, culminating in an optimum removal of 96.8% of tetracycline under 120 min.
Bawazeer et al. [108] also proposed a facile ball mill-assisted synthesis method for Au-doped g-C3N4, without the use of solvents. Even with no significant modification of the Ebg and a decrease in specific surface area, the presence of gold atoms boosted the catalyst performance over Arsenazo III dye decolorization, eliminating the contaminant within 50 min and with a reaction rate constant 2.5 times higher. The role of Au as recombination centers and a higher charge photogeneration due to the surface plasmon resonance is appointed to be responsible for the improved efficiency.
Due to their unfilled f-orbitals, rare-earth metals may act as trapping sites, distributing the photogenerated carriers, but can also form complexes with Lewis bases by bonding with their respective functional groups, promoting a higher interaction with organic pollutants at the catalyst surface. An Er-doped g-C3N4 was produced by Li et al. [109], with the dopant existing in the Er3+ state. The foreign element promoted a higher degree of polymerization and crystallization of the g-C3N4 network, but when in excess may also weaken tri-s-triazine bonds. The doped catalyst showed great efficiencies towards the degradation of three different contaminants, i.e., RhB, tylosin, and tetracycline, under simulated solar irradiation, with kinetics rate constants, respectively, 3.6, 1.6, and 1.7 times higher than the bulk g-C3N4. Additionally, Er3+ presented a detrimental effect over the specific surface area, which indicates that changes in the optoelectronic properties may play more important roles in the photodegradation performance.
Table 5. Application of metal-doped g-C3N4 for photocatalytic removal of contaminants.
Table 5. Application of metal-doped g-C3N4 for photocatalytic removal of contaminants.
CatalystPrecursorMethodEbg (eV)Radiation SourceContaminantRemoval (Time)Ref.
Ag-g-C3N4Melamine and UreaSilver
Polymerization and
2.19SolarTetracycline, 20 mg L−196.8% (120 min)[107]
2.46VisibleOxytetracycline, 10 mg L−1 and 98.6%
(120 min)
Antibiotics Wastewater:
101.5 mg L−1;
Tetracycline, 85.3 mg L−1;
Gatifloxacin, 89.4 mg L−1
94.5% OTC, 81.8% TC,
67.3% GFA
(120 min)
Au-g-C3N4MelamineAu NPsThermal
2.86VisibleArsenazo, 4 mg L−1100%
(60 min)
2.38VisibleParacetamol, 10 mg L−196.3%
(120 min)
Er-g-C3N4MelamineEuropium NitrateThermal
2.50SolarTetracycline, 25 mg L−185% (90 min)[109]
Tylosin, 25 mg L−170% (90 min)
Rhodamine B, 5 mg L−190% (30 min)
Cyanuric Acid and MelamineEuropium ChlorideThermal
2.70VisibleRhodamine B, 10 µg L−1100%
(90 min)
2.73VisibleRhodamine B, 10 mg L−1100%
(150 min)
K-g-C3N4MelaminePotassium ChlorideThermal
Polymerization and
2.50SolarPhenolic Effluent, Ph = 980 mg L−1 and COD = 6300 mg L−156.5%
(300 min)
ThioureaPotassium BromideThermal
2.15VisibleNitric Oxide, 600 ppb 36.8%
(30 min)
2.25SolarToluene and
5 mg L−1
85.8% TOL 98.6% NBZ (120 min)[105]
V-g-C3N4MelamineAmmonium MetavanadateThermal
2.63SolarTetracycline, 10 mg L−181.9%
(240 min)

3.3.3. Non-Metals Doping

Non-metal and metalloid elements have a larger record of use in g-C3N4 doping and are able to overcome the disadvantageous surface characteristics of the catalyst and improve its electronic and optical properties. Table 6 presents the results of the use of non-metal doped g-C3N4.
Zhang et al. [114] synthesized a mesoporous g-C3N4 which presented a high specific surface area, 91.1 m2 g−1, due to its porous structure, providing a larger number of active sites. However, due to the quantum size effect, the UV-Vis absorption spectrum suffered a blue shift as bandwidth decreased with the concurrent decrease of particle size. It was found that oxygen doping, besides the reduction of the photo-induced carriers’ recombination, may compensate for the absorption blue shift and enhance photocatalytic activity under simulated solar irradiation. The oxygen-doped mesoporous catalyst attained incredibly faster quasi-first-order kinetic constants for RhB and MO degradation, 64 and 24 times higher than simple mesoporous g-C3N4.
Significant surface alterations may also be found when doping with oxygen, as investigated by Praus et al. [115]. The authors presented a facile synthesis of the doped catalyst, by the combination of melamine and cyanuric acid, an oxygen-rich compound, in the thermal polymerization step. The modification increased the catalyst specific surface area from 14 to up to 41 m2 g−1 and a lowering of protonated nitrogen groups at the catalyst surface occurred, by means of substitution by -OH and -O- groups. Under LED lights, the O-doped g-C3N4 presented faster kinetic rates regarding ofloxacin and RhB, i.e., ~30 mol L−1 min−1 and ~3 min−1, respectively. The conduction and valence bands potentials at pH 7 were also calculated against normal hydrogen electrode (NHE), with the best doped catalyst presenting values of −0.93 and 1.69 eV, respectively. As the redox potential of hydroxyl radicals, E0(·OH/H2O), is 2.74 eV, which means the oxidation of water to form commented radicals by the positive holes at the VB is not energetically favored, the direct contaminant oxidation by h+ and superoxide radicals are the main degradation mechanisms (Figure 6). An interesting approach to overcome the low production of ·OH due to the respective low VB potential would be the incorporation of ozone in the reaction. The ozonide racial redox potential, E0(·O3/O3), is within the catalyst band potential, 0.89 eV, which can improve the production of radicals including •OH through consecutive reactions (Equations (1)–(3)) [116].
Other elements, such as silicon and phosphorous, have been indicated to substantially alter the bandgap energy and/or position of CB and VB of g-C3N4 catalysts, due to orbitals interaction. Wang et al. [117] produced P-doped g-C3N4 nanobelts and showed a significant decrease of the CB compared to bulk g-C3N4, –0.92 to –0.04 eV, with P successfully substituting C atoms, and even with a less negative CB, e/h+ recombination was reduced and p-hydroxybenzoic acid degradation increased from 33.1% to 77.3%, by comparison of undoped and P-doped nanobelt catalysts. Once more, as the CB is less negative than E0(·O2/O2), −0.33 eV, the superoxide may not be generated, and the presence of ozone would be beneficial as it possesses higher E0 and may act as the main electron acceptor.
Liang et al. [118] suggested that, when a silicon-doped catalyst was synthesized, N-Si bonds may act as electron trap centers, as it forms impurity levels close to the conduction band of g-C3N4, facilitating the interaction between e and respective acceptors, oxygen or ozone.
Table 6. Application of non-metal-doped g-C3N4 for photocatalytic removal of contaminants.
Table 6. Application of non-metal-doped g-C3N4 for photocatalytic removal of contaminants.
CatalystPrecursorMethodEbg (eV)Radiation SourceContaminantRemoval (Time)Ref.
B-P-g-C3N4MelamineBoric and Phosphoric AcidThermal
Polymerization and
10 mg L−1
(90 min)
Cl-g-C3N4DicyandiamideCyanuric ChlorideSolvothermal1.78VisibleRhodamine B,
10 mg L−1
(125 min)
MelamineAmmonium ChlorideThermal
2.71VisibleRhodamine B,
10 mg L−1
(30 min)
100 mg L−1
100% (5 h)[122]
UreaHydrogen PeroxideThermal Polymerization and
2.94SolarRhodamine B,
10 mg L−1
(20 min)
Methyl Orange,
10 mg L−1
70% (4 h)
UreaCyanuric AcidThermal
2.62VisibleRhodamine B,
10 mg L−1
(120 min)
20 mg L−1
(120 min)
P-g-C3N4MelaminePhosphoric AcidThermal
2 mg L−1
40.6% (5 h)[123]
MelaminePhosphoric AcidSolvothermal1.66Visiblep-Hydroxybenzoic Acid, 1 mg L−177.3%
(120 min)
2.51VisibleRhodamine B,
10 mg L−1
(30 min)
S-Cl-g-C3N4MelamineThiourea and
Ammonium Chloride
2.55VisibleRhodamine B,
10 mg L−1
(30 min)
Si-g-C3N4UreaAmmonium FluorosilicateThermal
2.75VisibleRhodamine B,
10 mg L−1
75% (50 min)[118]
Halogen doping of g-C3N4 has also been reported in the literature and shows interesting results. In the case of chlorine doping of g-C3N4, interesting aspects regarding the roles of interstitial and substitutional incorporation have been studied [120]. Substitutional chlorine doping may correspond to a higher specific surface area and oxidation performance, while the interstitial presence of chlorine contributes to a higher separation of photogenerated species. Thus, as both can occur simultaneously, their balanced doping may correspond to an optimization of the catalyst activity. The synthesis method applied and other parameters, such as the agitation and dopant ratio, can be fundamental to allow better control of the catalyst’s characteristics. Moreover, as indicated previously during the discussion of other catalysts, halogen doping studies are very scarce, possibly due to their more difficult impregnation, being highly electronegative elements, and producing toxic by-products.

3.3.4. Co-Doping

The use of multiple elements simultaneously for the doping of g-C3N4 has already been explored, albeit with fewer studies, and has shown a great boost in the material photoactivity. The most significant feature of g-C3N4 co-doping is appointed to be an improvement in electron mobility [119]. Moreover, additional alterations also promote the increase in the material photocatalytic performance, such as modifications of its morphology. The use of different amounts of dopants may dictate the final structure of the catalyst, as found by Ganganboina et al. [119], where an increasing amount of phosphoric acid mL in the B-P-g-C3N4 synthesis transformed a solid into a sponge-like, and finally, a hexagonal tubular structure, which possessed a higher photoactivity (Figure 7). The hydrolysis of melamine, the g-C3N4 precursor, led to the production of cyanuric acid, which may form a supramolecular precursor complex involving both chemicals, and finally aggregate to form the catalyst structure. The dopants introduced may then adsorb on these supramolecular complexes and alter their configuration.

3.4. Overall Considerations

The application of catalyst doping is a well-researched subject, noticeably producing positive effects regarding the improvement of photocatalytic activity. However, even with promising results, some considerations must be taken.
The similarities between the structures of WO3 and TiO2 as metal oxides allow analogous modification mechanisms to occur, providing great support as TiO2 is already the subject of a great amount of research and information as it has been intensively optimized.
Rare-earth and noble metals provide excellent alterations, especially on the crystalline and electronic structure, due to the existence of more electron-rich orbitals, as well as their optical properties, e.g., through surface plasmon resonance. However, the sometimes complex and difficult impregnation of such elements, and their typically higher cost, need to be evaluated, although their use in small quantities can already provide significant results.
Other metallic elements can be an easier alternative, with comparatively lower costs regarding materials and impregnation techniques. In the case of g-C3N4, the exploration of transition metals doping is more widely explored, possibly due to the more distinct nature between the foreign and existent elements, which can provide more evident effects. These elements may lead to the easier formation of new energy sublevels and shift the light absorption to higher wavelengths. Such electronic interaction, especially as it occurs mainly with the conduction band of the catalysts, is also a great improvement for the less negative conduction band of WO3, which difficult its reduction potential. The ozonide radical formation can also be enhanced by the modification of the conduction band, in the case of photocatalytic ozonation, facilitating the capture of electrons by ozone.
Nonetheless, transition metals doping has been reported to occasionally cause the lixiviation of such elements, e.g., iron and some heavy metals, presenting dangerous toxic effects and strict limits. Some progress in this matter has been made, with encapsulation techniques and methods that result in a stronger fixation, although it may increase the complexity of the process.
As the most abundant elements in nature, non-metals are the most broadly used elements in catalyst doping. Another great advantage is that a one-step method can usually be applied, simplifying the material’s synthesis. Especially for g-C3N4, already being usually synthesized through a simple thermal polymerization, some compounds possess structures containing carbon and nitrogen together with other elements, which can be singularly applied, such as thiourea. Their electronic alteration may result in modification regarding the valence band, contributing, for example, to the less positive VB of g-C3N4 and improving hydroxyl radical production. Nonetheless, interaction with both conduction and valence bands can occur, even simultaneously, e.g., in sulfur doping.
In such a context, doping techniques represent a simple approach to overcome typical catalyst disadvantages and improve their performance. The difference in overall studies can challenge the conceptualization of standard knowledge, as multiple features can be altered and conflicting data exist. Thus, a more precise study may be necessary to evaluate each case.

4. Composite Catalysts

With the increasing conceptualization of different catalysts, the combination of different materials emerges to present a wide number of possibilities to improve the overall catalytic properties [124]. By mixing materials with different characteristics, their individual disadvantages may be overcome, and a final and more robust photocatalyst can be obtained. These composites are then a combination of materials with distinct natures, such as mixed metal oxides, carbon-semiconductors, polymeric structures, porous materials, and many others. In this section, an overview of the principal applications of the previously discussed photocatalysts in composite materials will be discussed [125]. The use of WO3, g-C3N4, and TiO2 combined in different composite catalysts is evaluated in Table 7.
The mixing of WO3 and TiO2 is a clear example of mixed metal oxides, a well-known group of photocatalysts defined by the combination of different semiconductors. The coupling of these photocatalysts aims to improve the photocatalytic activity of the resultant material by mostly increasing the charges separation efficiency and visible-light sensitization for the complete system [125]. Mugunthan et al. [126] proved the higher efficiency of the coupled TiO2/WO3 catalyst for diclofenac elimination under visible radiation. The group synthesized the photocatalyst through a hydrothermal method and studied the variation of TiO2:WO3 molar ratios. The composites presented higher surface areas and smaller particles compared to bare TiO2, with the increasing amount of TiO2 leading to larger SBET, bus also particle sizes, and Ebg. Thus, an optimum TiO2:WO3 molar ratio (10:1), presented more balanced properties and higher removal of diclofenac (~90%). An excess amount of WO3 is also appointed to possibly act as recombination centers for the photoinduced charges, reducing the process efficiency.
The surplus addition of one of the applied semiconductors can also promote the agglomeration of photocatalyst particles, which may hinder the absorption of light and CECs photodegradation, as it was suggested by El-Yazeed and Ahmed [127] during the impregnation of WO3 particles onto TiO2, which had a detrimental effect over a concentration of 10 wt%. This was also found by Wang et al. [128] for hollow spherical TiO2/WO3 composite, with uniform spheres being formed with a 5 wt% incorporation of WO3, but with the further increase in concentration and particle agglomeration causing changes in the catalyst morphology and leading to flat and unequal forms. However, in this study, the catalyst containing 10 wt% WO3 had a better performance in methylene blue and metoprolol degradation, due to the photoelectronic and surface area improvements.
The TiO2-WO3 heterojunction can greatly improve the photo-mechanisms involved in the photocatalytic process. Considering their bandgap positions, the charge separation can follow two mechanisms: type-II heterojunctions or Z-scheme mechanisms (Figure 8). For the type-II mechanism, the excited electrons from the CB of TiO2, which has a more negative potential, will migrate to the CB of WO3. Meanwhile, the opposite occurs in the positive holes, which tend to be transferred to the TiO2 VB [129]. Nonetheless, the Z-scheme mechanism is appointed to be more favorable due to the larger redox potential, as the e of the CB of WO3 and h+ of the VB of TiO2 tend to recombine fast, leading to the accumulation of e on the CB of TiO2 and h+ on the VB of WO3, potentializing its respective reductive and oxidative potential [130].
Moreover, Wei et al. [131] indicated that, during the degradation of malachite green under visible light using TiO2-WO3, the superoxide radical (·O2) was one of the main mechanisms of the contaminant removal, even with the characteristic disadvantage of the more positive CB potential of WO3 (0.74 eV vs. NHE) comparatively to the redox potential of ·O2/O2 (–0.33 eV vs. NHE). Thus, the composite material may also take advantage of alterations of the carriers’ transference through interactions with TiO2, allowing the adsorption and reduction of oxygen.
The use of doped catalysts in the coupled systems can further improve the overall efficiency through the synergic effect between all species. For instance, the use of noble metals, which promote the surface plasmon resonance effect, or non-metals that can increase visible light absorption, alongside the lower e/h+ recombination and electronic bands interaction of the heterojunction can potentialize contaminants removal [133,134].
More complex composites may be synthesized by the continuous arrangement of materials with different natures. Chen et al. [135] combined carbon-based and ceramic materials with metal oxides, producing silica-TiO2 microspheres coupled with graphene oxide (GO) and WO3 quantum dots. Each material has a principal role, and the arrangement is conducted to overcome their individual drawbacks. GO possesses large surface areas and good electrical properties and may enhance the photocatalytic activity of semiconductors as it does not present one on its own. Meanwhile, silica is a low-cost and non-toxic template, and WO3 and TiO2 may harness the beneficial effects of their coupled mechanisms. Finally, the complex composite presented enhanced optoelectrical properties and higher photocatalytic activity under natural sunlight, removing 98% of RhB under 60 min.
The polymeric characteristics of g-C3N4 can also be greatly advantageous when coupled with other semiconductors and materials. The metal-free catalyst exhibits narrower bandgap energy, making the photoexcitation of the electrons in the VB under visible light possible. Nonetheless, one of its main drawbacks is the low VB potential, which typically lies below the redox potential for the reaction between positive holes and water to occur, producing hydroxyl radicals.
Following computational density functional theory (DFT) calculations, Deng et al. [136] investigated the g-C3N4/TiO2 heterostructure. The study proposed that the composite may present a lower Ebg than the single catalysts. Moreover, it was found that an accumulation of photogenerated electrons over the CB of g-C3N4 occurred, while positive holes tended to accumulate on the VB of TiO2, meaning that reduction reactions will favorably take place on g-C3N4, and oxidation will occur on TiO2. These mechanisms may also reduce the recombination of the photogenerated pairs and lead to higher radical production, especially ·OH, due to the well-constructed heterojunction [137].
Table 7. Application of WO3, g-C3N4, and TiO2 mixed composites for photocatalytic removal of contaminants.
Table 7. Application of WO3, g-C3N4, and TiO2 mixed composites for photocatalytic removal of contaminants.
(m2 g−1)
Ebg (eV)Radiation SourceContaminantRemoval (Time)Ref.
g-C3N4/TiO2200.02.70UVFormic Acid, 50 mg L−190.0% (5.5 h)[138]
40.22.81SolarMethylene Blue, 10 mg L−194.9% (80 min)[137]
Rhodamine B, 15 mg L−193.1% (80 min)
--VisibleE. coli, 107 CFU mL−1100% (180 min)[139]
-2.58SolarE. coli, 103 CFU mL−196.8% (30 min)[140]
WO3/TiO288.43.06VisibleMalachite Green, 50 mg L−199.0% (60 min[131]
11.72.40VisibleMethylene Blue, 10 mg L−187.8% (150 min)[128]
Metoprolol, 2 mg L−167.1% (150 min
103.92.95VisibleDiclofenac, 10 mg L−1100% (150 min)[126]
WO3/g-C3N4--UV-A +
Visible + NIR
Ciprofloxacin, 10 mg L−198.6% (180 min)[141]
Tetracycline, 10 mg L−198.5% (180 min)
28.62.53UV + VisibleTartrazine, 25 mg L−195.0% (20 min)[142]
-3.00VisibleTetracycline, 10 mg L−198.1% (60 min)[143]
N-doped CHS/g-C3N4/TiO278.0-VisibleTetracycline, 20 mg L−185.0% (120 min)[144]
-2.07VisibleMetronidazole, 8.2 mg L−197.2% (60 min)[145]
C-doped WO3/TiO293.02.98SolarDiclofenac, 10 mg L−1100% (250 min)[133]
Ag-doped WO3/TiO2-3.07VisibleMethylene Blue, 3.2 g L−172% (60 min)[134]
GO/WO3/TiO2-2.89SolarRhodamine B, 143.7 mg L−198.2% (5 h)[146]
--SolarE. coli, 2 × 103 CFU mL−197.3% (80 min)[147]
CQDs/WO3/TiO296.22.61SolarCephalexin, 10 mg L−1100% (90 min)[148]
GO/CQDs/WO3/TiO2/SiO2202.6-SolarRhodamine B, 14.4 mg L−198% (60 min)[135]
Cd-doped WO3/g-C3N48.51.53VisibleMethylene Blue, 10 mg L−196.0 (80 min)[93]
The WO3/g-C3N4 can thus be an interesting coupled system, as both catalysts can benefit from the synergic effect regarding their energy band potential and radical production. Huang et al. [141] produced g-C3N4 nanosheets coupled with WO3 quantum dots (QDs), and they indicated that the combination of a strong reducing capacity of the CB of g-C3N4 to react with electron acceptors, such as O2 and O3, combined with the oxidation ability of the VB of WO3 to form ·OH, promoted a greatly improved photocatalytic activity towards tetracycline and ciprofloxacin, with removals higher than 98%. Furthermore, the WO3 QDs induced a local surface plasmon resonance, enhancing the charge carrier’s separation efficiency.
Besides the reduction of Ebg and the synergic effect over photogenerated carriers, morphology modification may also be obtained to suppress the usual low specific surface area of g-C3N4. The coupling of g-C3N4 with TiO2, WO3, or other materials may lead to the formation of different porous structures and the existence of multiple functional groups that may interact with a wider range of organic pollutants [93,149].
In the following sections, the use of TiO2, g-C3N4, and WO3 in different composite materials as well as their main and most recently found advantages and mechanisms will be assessed.

4.1. TiO2

Over the years, TiO2 was vastly employed in the production of different composite catalysts, allowing a better understanding of their overall mechanisms and advantages. The coupling of TiO2 with other photocatalytic materials appeared as a vast family of mixed catalysts, with combined improved properties. Due to the existence of multiple photocatalysts, numerous combination possibilities are the aim of a rising number of investigations. Some studies involving TiO2 composite catalysts are present in Table 8.
Zinc Oxide (ZnO) is, jointly with TiO2, one of the most investigated semiconductors for contaminants removal, sharing their good photochemical characteristics but with relatively better electrical properties. Nonetheless, their heterostructures have been explored in different forms and systems for pollutant elimination. The increasing addition of ZnO in ZnO/TiO2 fibers has been shown to also promote changes in the physical properties of the material, creating a rougher surface with a higher specific surface area and faster interaction with contaminants [150]. Changes in the morphology of the material have been also attested. Das et al. [151], indicated the transformation of uniform rod-like structures of ZnO into more of a flower-like form by the addition of TiO2, with the enhancement of the BET surface until an optimal amount. A lower recombination rate of the electron-hole pairs was also demonstrated, increasing the lifespan of the photoinduced charge carriers.
The combination of TiO2 with bismuth-based materials is a topic that gained much attention, as this family of compounds presents a series of attractive features, such as their low toxicity, easy functionalization, cost-effectiveness, and ability to absorb in near infra-red regions. Some examples of the most studied Bi-based photocatalysts are bismuth oxide (Bi2O3), bismuth vanadate (BiVO4), and bismuth oxyhalide (BiOX), involving combination with halogen elements (X) [152,153,154].
The BiVO4/N-TiO2 heterostructure synthesized by Cipagauta-Díaz et al. [152] presented increased specific surface areas and absorption of visible light with the increasing BiVO4 content. The catalyst also presented zones of heterogeneity that allow contact between the semiconductors and improve the separation of photogenerated charges and the lifetime of charge carriers. Ultimately, the best composite catalyst resulted in 98% removal of ofloxacin under 90 min. Even with the morphology and light absorption properties improvement, an excessive amount of BiVO4 (>5 wt%) was demonstrated to be prejudicial to the photocatalytic performance, possibly due to the formation of BiVO4 agglomerates in the catalyst surface, which hamper the homogeneous light absorption.
The electron-accepting nature of Bi2O3 was also proven to enhance the lifetime of the photocarriers and considerably increase the visible light absorption, even if the Ebg of the resulting materials did not suffer great alteration. Sharma et al. [153] indicated that Bi2O3 creates heterojunctions that facilitate the transport of photogenerated carriers and improve the visible light-capturing capacity in a CuO/Bi2O3/TiO2 ternary composite catalyst. However, in high concentrations, Bi2O3 can reduce the catalyst efficiency, as pure Bi2O3 possesses less activity and can act as a recombination center.
Table 8. Application of TiO2 composites for photocatalytic removal of contaminants.
Table 8. Application of TiO2 composites for photocatalytic removal of contaminants.
CompositeSBET (m2 g−1)Ebg (eV)Radiation SourceContaminantRemoval (Time)Ref.
ZnO/TiO2-3.15UVEriochrome Black T, 6.4 × 103 mg L−182% (6 h)[155]
84.73.15SolarMethylene Blue, 6.4 mg L−195% (60 min)[151]
Methyl Orange, 6.5 mg L−199% (60 min)
BiVO4/N-TiO292.02.56VisibleOfloxacin, 20 mg L−198% (90 min)[152]
Rhodamine B, 20 mg L−192% (90 min)
Bi2O3/TiO2102.9-VisibleRhodamine B, 10 mg L−1100% (100 min)[153]
CuO/Bi2O3/TiO283.6-VisibleRhodamine B, 10 mg L−1100% (60 min)
Fe2O3/TiO2-2.49SolarMethylene Blue, 10 mg L−192% (180 min)[156]
-Phenol, 10 mg L−152% (180 min)
56.93.08VisibleNaproxen and Ibuprofen,
10 mg L−1 each
100% NPX (15 min) and 91% IBF (240 min)[157]
Au/Fe2O3/TiO2-1.55Visible2,4 Dichlorophenol, 10 mg L−194% (90 min)[158]
-1.55Visible4-Bromophenol, 10 mg L−197% (60 min)
SiO2/Fe3O4/Sn-TiO2-1.32UVTetracycline, 10 mg L−198.2% (40 min)[159]
P/Ag/Ag2O/Ag3PO4/TiO2307.22.98VisibleE.coli, 107 CFU mL−1100% (20 min)[160]
Salmonella, 107 CFU mL−1100% (30 min)
Enterococcus sp., 107 CFU mL−1100% (6 h)
S. aureus, 107 CFU mL−1100% (3 h)
CNT/Au-TiO2-1.95SolarMethylene Blue, 3 mg L−180% (30 min)[161]
GO/TiO2-3.02SolarE. coli, 107 CFU mL−199.9% (30 min)[162]
Chitosan/N-TiO252.02.82UVPatulin, 500 µg kg−1100% (35 min)[163]
Perlite/F-Ce-TiO214.82.96VisibleMicrocystis aeruginosa, 2.7 × 106 cell mL−198.1% (9 h)[164]
Iron oxides (e.g., Fe2O3, Fe3O4) have been successfully used in photocatalytic systems, as highly chemically stable and non-toxic materials with narrow bandgaps and interesting magnetic properties [124]. α-Fe2O3 possesses a broad absorption over the entire UV-Vis region. Thus, their use in composite photocatalytic can significantly extend the material’s optical absorption, allowing it to preserve a good photocatalytic activity in both UV and visible range, and fully harness solar radiation (Figure 9) [165]. Bouziani et al. [156] demonstrated the expansion of absorption range in sol-gel synthesized α-Fe2O3/TiO2, as single TiO2 presented minimal absorption over 400 nm while the composite catalyst showed good absorption over all UV-Vis spectra. Furthermore, the 10 wt% α-Fe2O3/TiO2 had better crystallization and a greater number of active sites, providing a faster degradation of methylene blue and phenol, i.e., 96% and 52%, within 120 min of sunlight irradiation.
The synthesis of α-Fe2O3/TiO2 composites may also occur using metal-organic frameworks (MOFs) as sacrificial templates. These materials are highly porous and lead to the formation of homogeneous structures with excellent electronic mobility and active sites. Li et al. [158] confirmed the potential of these catalysts through the study of MOF-derived Au-doped α-Fe2O3/TiO2 and its reaction with 2,4 dichlorophenol and 4-bromophenol. The ternary system obtained degradation rates higher than 95% in 90 min, accounted for the high surface area and the number of reactive sites, and the movement of excited electrons between the more negatively charged CB of TiO2 to the CB of Fe2O3 in the formed heterojunction. Peña-Velasco et al. [157] also concluded that, when synthesizing a Fe2O3/TiO2 composite derived from a Fe-based MOF (MIL-101), Fe3+ may partially substitute Ti4+ in the TiO2 structure, which may introduce lattice defects and vacancies that boost the photocatalytic activity, achieving 91% and 100% removal of ibuprofen and naproxen.
Asgari et al. [53] reported the efficient use of magnetic Fe3O4/TiO2 composite catalyst in photocatalytic ozonation reactions. The presence of the photocatalyst enhanced ozone decomposition in the medium and the generation of oxidative radicals, boosting the degradation of ceftazide, eliminated under 15 min with a reaction rate constant of 0.1682 min−1, almost two times faster than single ozonation. The magnetic property of the composite may also facilitate the posterior removal of the catalyst from the medium, a key parameter for the scale-up of the process. Remarkably, Lu et al. [159] appointed that interference may occur in the Fe3O4/TiO2 catalyst as a result of Fe ion diffusion to the surrounding TiO2, causing excessive doping. The authors utilized a SiO2 coating, as a ceramic material able to shield the Fe3O4, but also reduce particle agglomeration, forming a ternary composite.
Carbonaceous materials, such as carbon nanotubes (CNTs), carbon quantum dots (CQDs), activated carbon (AC), and graphene oxide (GO), can provide support for the photocatalyst, but also great mechanical and electrical properties that may enhance its photoactivity. CNTs possess high mechanical strength and conductivity, but also present an important charge storage capacity and can act as a receiver of the photogenerated electrons from the catalyst CB. Mohammed [161] indicated an incredibly higher specific surface area of CNT/Au-TiO2 hybrid catalysts, in simultaneous to higher absorption of visible light, effective transport of charges between CNTs and TiO2, reduced electron-hole pair recombination, and better adsorption of molecules. CNTs may also promote a higher ozone decomposition in photocatalytic ozonation reactions, as ozone present a high affinity towards basic carbons due to its high density of π electrons on the basal planes. Orge et al. [166] verified the higher efficiency of CNT/TiO2 in the photocatalytic ozonation of oxamic acid, which was completely removed under 60 min. The group also pointed out that ozone decomposition may be favored by CNTs with low acidic characteristics.
Graphene oxide (GO) also shows appealing hydrophilic features, tunable attributes, and particular optical properties that may be applied to composite materials. Different GO-hybrid catalysts have shown better exploitation of visible and solar irradiation, fast electron transference, and suppressed agglomeration. Thomas et al. [162] exploited the antibacterial properties of GO in GO/BiVO4 and GO/TiO2 composites, with a higher production of ROS and the characteristic morphology, small size, and sharp edges. Both binary composites presented promising results regarding their application in disinfection treatment. However, GO/TiO2 had a better performance with a 3-log reduction of E. coli k12. It was indicated that Bi ions interactions with the surface of bacteria cells are lengthier than with Ti ions, which can bind to the glycoproteins and penetrate the cell.
In ozone-assisted photocatalysis, Chávez et al. [167] combined the magnetic properties of Fe3O4 and the high surface area of activated carbon (AC) with a TiO2 catalyst for the treatment of a real secondary effluent spiked with different CECs. The study proved the higher efficiency of photocatalytic ozonation, as single ozonation presented low mineralization levels and photocatalytic oxidation did not achieve the full elimination of CECs. Thus, the combined process showed faster removal, lower presence of intermediates, and better usage of ozone. Nonetheless, it was appointed that interactions between ozone and AC may alter the latter surface chemical properties, as a pHZPC decrease was found because of acidic oxygen groups on the surface of the carbon structure. This modification may affect the adsorptive behavior of the material. Other studies involving the application of TiO2 composites in photocatalytic ozonation treatments are present in Table 9.

4.2. WO3

To overcome WO3 drawbacks and obtain a more robust photocatalytic material, various hybrid structures have been explored. The coupling with other metal oxides and semiconductors is more substantially pursued, possibly as a more facile approach to surpass its high electron-hole recombination rate, and the less negative conduction band and compromised reductive potential, as it can benefit from the electron-hole interaction of the formed heterojunctions. The use of these WO3 composites in the degradation of different contaminants is demonstrated in Table 10.
In recent years, phosphate-based photocatalysts, especially Ag3PO4, have been the focus of numerous studies due to their superior quantum efficiency under visible light irradiation, 90% [22]. However, it still faces relatively large particle sizes (0.5–2 µm), instability, and photo-corrosion, hindering a highly efficient photoactivity and its recyclability. Thus, Ag3PO4 and WO3 composites may benefit from their heterostructures and be presented as more robust photocatalysts. Liu et al. [170] demonstrated that Ag3PO4/WO3 had relatively stronger absorption in the long-wavelength range (>500 nm), while single WO3 presented no activity in the same range. Regarding the ratio between the semiconductors, when Ag3PO4 exceeded 60%, a decrease in the photocatalytic activity towards RhB degradation, related to an excessive number of Ag3PO4 particles deposited at the WO3 surface that may block light penetration. However, other studies showed that the formation of metallic Ag during the composite synthesis may occur and possess a negative effect, inhibiting the transport of positive holes from the Ag3PO4 VB to its surface, and the deposition of inactive substances over the composite surface, blocking active sites [171].
The instability of Ag3PO4 can be further increased by adding other components with better electronic properties. Graphene, which can be doped to boost its characteristics, is an excellent electron mediator and has been demonstrated to significantly increase the recyclability of Ag3PO4/WO3 composites, providing higher chemical stability, surface area, and electron mobility by the organic material on the ternary composite [172].
Other semiconductors have been combined with WO3, as mentioned, effectively obtaining improved features. Sodium niobate (NaNbO3) is a semiconductor with a perovskite structure, low density, and high crystallinity, but with a broad bandgap energy (3.0–4.7 eV). The respective NaNbO3/WO3 has been reported to possess great photocatalytic activity, with a higher separation of photogenerated charges and reductive potential, benefiting from the considerably more negative conduction band of NaNbO3 [173]. ZnWO4 is another photocatalyst that presents higher bandgap energy (3.5–5.7 eV), but its formed composites with WO3 can broaden its visible light absorption and form built-in electric fields as a result of a fast electron transference between both materials [174].
Table 10. Application of WO3 composites for photocatalytic removal of contaminants.
Table 10. Application of WO3 composites for photocatalytic removal of contaminants.
(m2 g−1)
Ebg (eV)Radiation SourceContaminantRemoval (Time)Ref.
Ag3PO4/WO323.9-VisibleRhodamine B, 20 mg L−198% (90 min)[170]
Ag3PO4/WO3·H2O-2.43VisibleMethylene Blue, 10 mg L−1 and
Tetracycline 20 mg L−1
98.9% MB and
70.4% (35 min)
Ag3PO4/NG/WO3-2.36VisibleIndomethacin, 5 mg L−199.3% (50 min)[172]
NaNbO3/WO37.22.60Visible2,4-Dichlorophenoxyacetic acid,
10 mg L−1
60% (210 min)[173]
Bi2S3/WO353.81.90VisibleRhodamine B, 10 mg L−190.7% (100 min)[175]
Ag/ZnWO4/WO3--UV-VisibleMethylene Blue, 200 mg L−194% (80 min)[174]
GO/WO318.72.32VisibleRhodamine B, 20 mg L−196% (120 min)[176]
Ciprofloxacin, 20 mg L−190% (120 min)
Ag/GO/Chitosan/WO326.42.40VisibleMethylene Blue, 10 mg L−199% (8 min)[177]
SBA-15/Ag-WO32081.70VisibleAtrazine, 20 mg L−168% (18 min)[178]
Gondal et al. [178] reported efficient herbicide removal using SBA-15, a highly porous siliceous material, as a template for a g-C3N4 composite catalyst, to obtain a structure with a high surface area and facilitate the reutilization of the material. The composite also benefits from the presence of the noble metal, with the surface plasmon resonance effect, which can increase light absorption, but it can also act as an electron trap center, as its Fermi level is lower than the semiconductor and electrons tend to accumulate on the Ag particles. Finally, the SBA-15/Ag-WO3 presented a ~70% removal of atrazine, with a reaction rate constant of 0.065 min−1.

4.3. g-C3N4

The characteristics of graphitic carbon nitride that hinder its broader utilization, such as its low specific surface area, obstructed active sites, and visible light utilization, can be strongly surpassed by the construction of composite materials. Its polymeric structure may be linked to other materials, with their own photocatalytic properties or characteristics that can boost the g-C3N4 performance. Their application in photocatalytic studies is shown in Table 11.
There is an expanding number of studies regarding the combination of g-C3N4 with other polymeric materials. Their typically low-cost production, large surface areas, and presence of different functional groups and electrostatic charges can enhance pollutants interaction. Polyethyleneimine (PEI), for example, is a cationic polymer containing many amino groups that have been used in combination with catalyst and carbon-based materials to enhance the electrochemical properties and overall activity of composites. Yan et al. [186] synthesized in one step a PEI/g-C3N4 composite through the thermal copolymerization of urea mixed with PEI, obtaining a tremella-like structure containing -NHX groups, that may be beneficial for water dispersion and photon absorption, and an increased BET surface area, up to 250%. The best composite catalyst also presented 80% removal of tetracycline with a reaction rate constant of 0.0226 min−1, 3.2 times higher than g-C3N4. The efficiency of PEI/g-C3N4 has also been attested for disinfection purposes, by Zeng et al. [187], resulting in 6.2 and 4.2 log reductions of E. coli and E. faecalis in 45 and 60 min, respectively. PEI can increase O2 reduction and alter the surface charges of the catalyst, promoting the adhesion of bacteria through electrostatic attraction.
Polyacrylonitrile (PAN) is also used for the synthesis of nanowires and other hybrid materials, with excellent flexibility and adsorption capacities. Alias et al. [185] applied electrospun PAN/g-C3N4 nanofibers for oilfield-produced water (OPW) treatment. The PAN matrix can adsorb organic compounds such as oil and accumulate them near g-C3N4, which allows the treatment of even more diluted solutions. The proposed composite obtained 96.6% and 85.4% degradation of OPW under UV and visible irradiation, respectively.
g-C3N4 has been used in combination with other metal oxides and semiconductor materials to merge its advantages and obtain heterojunctions with synergic effects. LaVO4 and other vanadate complex oxides such as BiVO4 present interesting features, especially their surface catalytic and optical properties, with good visible light absorption due to the V 3d orbital state. Jing et al. [182] indicated a high matching degree of LaVO4 and g-C3N4 band structures in their respective hybrid catalyst, promoting an efficient migration of photocarriers and improving the photocatalytic activity. LaVO4 also presents a more negative CB, while g-C3N4 has a more positive VB, potentializing the redox reactions in the respective bands of the heterostructure.
Dong et al. [180] demonstrated the advantages of BiVO4 in combination with g-C3N4, which resulted in a material with a larger specific surface area, from 1.6 to 16.3 m2 g−1, and better visible light absorption. The presence of bismuth precursors may cause the compression of g-C3N4, which facilitates electron transference to the catalyst surface and the photogenerated pair recombination [181].
The matching bands of Bi-based oxides and g-C3N4 catalysts, and their better reductive/oxidative potential and ROS production have been reported as efficient pathogens disinfection treatment. Zhang et al. [179] demonstrated the capacity of Bi4O7/g-C3N4 to eliminate germinated fungi spores of Aspergillus fumigatus under visible light, attaining 81% disinfection of an initial 1 × 104 CFU mL−1. The boosted production of ·O2 and ·OH by the composite were fundamental for better performance, as they were appointed to be the main active species on the decomposition of the fungus cell wall and cytoplasm.
The significant improvement of treatment performance in photocatalytic ozonation using g-C3N4 composites has been also attested, and some studies are present in Table 12. Jourshabani et al. [190] investigated the Fe2O3 and S-g-C3N4 combination in a composite catalyst and obtained substantially higher mineralization of Bisphenol A (BPA) under visible radiation, 97.8% TOC removal in 3 h, compared with the photocatalytic oxidation and photolytic ozonation treatments, 41.1%, and 40.1% respectively. The presence of the Fe-based material enhances the visible light absorption of g-C3N4 and the adsorption of both contaminants and ozone, which leads to faster elimination, higher production of ROS, and lower dosages of ozone, consequently reducing the process cost.
Zhang et al. [184] reported the use of a metal-free heterostructure consisting of oxygen-doped g-C3N4 microspheres integrated with hydrothermal carbonation carbon (HTCC) for human adenovirus disinfection. HTCC is an interesting carbon-based material recently found to act as a visible-light semiconductor, with a lower cost production compared to other carbonaceous compounds, such as graphene, although it suffers from a low photocatalytic activity due to fast recombination of electron-hole pairs [191]. The group applied a facile solvo-hydrothermal method, and the resulting composite presented a robust photocatalytic performance and biocompatibility towards human cells, achieving 5-log inactivation of the virus within 120 min.
Table 12. Application of g-C3N4 composites in photocatalytic and ozone-based processes for contaminants removal.
Table 12. Application of g-C3N4 composites in photocatalytic and ozone-based processes for contaminants removal.
CompositeRadiation SourceContaminantOxidantRemoval (Time)References
GO/g-C3N4SolarOxalic Acid, 10 mg L−1O249.5% (40 min)[192]
O393.2% (40 min)
-82% (40 min)
SBA-15/Ag-g-C3N4SolarOxalic Acid, 10 mg L−1O216.8% (11 min)[193]
O3100% (11 min)
-4% (11 min)
Fe2O3/S-g-C3N4VisibleBisphenol A, 50 mg L−1O241.1% TOC (3 h)[190]
O397.8% TOC (3 h)
40.1% TOC (3 h)
Ceramic materials may be employed in composite catalysts as highly porous materials with excellent mechanical properties. These materials also act as great templates for photocatalytic species, producing high surface areas and recycling properties. Akulinkin et al. [189] used β-SiAlON, an aluminosilicate porous material, as a base for a g-C3N4 composite. The semiconductor was loaded in the pores of the ceramic substrate and the catalyst was obtained in plate forms. The composite presented great removal of murexide dye, as a consequence of its extended surface area and stability, maintaining its photocatalytic activity after seven cycles.

5. Future Perspectives

TiO2 is by far the most used photocatalyst and possesses its basic properties already optimized, with a proper position of its CB and VB, which allow good redox potentials, high crystallinity, and overall photocatalytic performance. Although some disadvantages still prevent its full application, such as fast electron-hole pair recombination and, more especially, its broad bandgap energy, which implies the use of UV irradiation.
Thus, the development of new visible light active photocatalysts has been extremely encouraged and studied by the scientific community. Among them, g-C3N4 and WO3 appear as promising candidates, presenting lower bandgap energies, higher absorption of visible light, and still maintaining the required properties common to TiO2, such as good chemical stability and facile and low-cost production. Other alternative catalysts that have gotten more attention in recent years include the Bi-based (BiO3, BiVO4, BiOX) and phosphate (Ag3PO4, CePO4) catalysts, which have interesting visible-light and overall catalytic activities.
Even with a better performance in the visible region, these selected catalysts still suffer from low specific surface areas, high electron-hole pair recombination, and unfavorable band potential, as g-C3N4 typically presents low VB and WO3 less negative CB potentials, that diminish, respectively, ·OH production from water and O2 reduction into ·O2.
To overcome these obstacles, modifications of the photocatalysts can be explored, such as the introduction of multiple elements in catalyst doping, and the combination with materials of different natures to obtain composite photocatalysts. The incorporation of foreign elements in the structure of the catalyst may take place in different positions, with all possessing respective advantages. By the application of characterization methods, a specific analysis of each modification needs to be conducted to better understand its mechanisms. Generally, doping can alter the bandgap properties and may lead to shifts in the optical response of the catalysts, allowing it to present better responses towards higher wavelengths. These techniques may also reduce the photogenerated pairs recombination, modification of surface properties, and functional groups, which highly influence the interaction with CECs.
Composite materials encompass the use of mixed metallic oxides and other semiconductors, ceramics, polymers, and other structures to obtain a combination of their individual strengths and advantages. Outstanding properties may be achieved, such as high specific surface areas, electron mobility, different morphologies, easier recyclability, posterior separation, and heterojunctions which may increase the reduction and oxidation reactions, an important aspect for the commented alternative catalysts.
Even so, each alteration may also have side effects. Possible poisoning, lixiviation, active site blockage, and loss of photoactivity may occur. Some compounds, such as noble metal or rare-earth elements, and more elaborated materials can also represent a high cost involved and short availability. The precise evaluation of the concentration of elemental dopants and added materials are crucial, with excessive doping resulting in the opposite effect, acting as recombination centers, or the extreme agglomeration of the structures in hybrid materials. The combination of different materials in composite catalysts may also lead to a faster deactivation of the final product, shortening their lifetime and increasing the process cost. Thus, a case-by-case analysis is fundamental to better understand these side effects and obtain a more tailored material, which can be an exhaustive task. Additionally, the catalysts’ reuse and possible post-treatments for their reactivation need to be more profoundly explored.
The proper application and reactor to be used are also fundamental in the conceptualization of the photocatalytic treatment apparatus and need to be evaluated alongside the production of the catalytic material. This task is one of the greatest challenges, as it encompasses the optimization of multiple parameters, to ensure efficient light penetration, interactions between contaminants and the catalyst, proper residence time, etc. Even being a cost-free radiation source, sunlight photocatalyst activation may require special reactor designs, presenting additional costs that need to be evaluated case-by-case. Nonetheless, great advancements have been obtained in compound parabolic collector (CPC) reactor developments, allowing proper light penetration within the equipment and better exploitation of the photon source. A facile retrieval of the catalyst after the reaction is also a key factor and dependent on the selected reaction equipment. Some achievements have been obtained in this post-recovery phase, such as the exploitation of magnetic properties of some materials, which can assist in the separation and reutilization of the photocatalyst.
The number of studies involving the application of such materials in real effluents is considerably limited. There are different common parameters and compounds present in these complex water matrices that compromise and interact with the species participants in the photocatalytic ozonation reaction. Broader investigations are thus vital to understand these effects and construct a more robust system.

6. Conclusions

The application of semiconductors in photocatalytic based for CECs abatement has shown remarkable potential as a water treatment technology. The production of highly oxidative radicals may eliminate a variety of these chemical and biological compounds that represent a danger to human and environmental health. These treatments have been pointed out to be effective even in more complex matrices, which proves their capacity to be applied to real effluents under different conditions. The use of additional oxidants such as ozone can also boost the overall process efficiency, increasing the production of radicals and electron retrieval.
The investigations regarding new visible light active photocatalysts show promising results, but more complete studies still need to be conducted to collect more information. The applications of doping and composite materials open a great variety of possibilities for materials with more robust and feasible characteristics to be obtained. Thus, more elaborative comparisons between the new and standard semiconductors and their adaptations need to be performed, to better understand the advantages of further exploration of the already well-founded TiO2 based materials and the development of alternative materials. The addition of ozone in the photocatalytic process has been proven to enhance the overall efficiency, and more studies of its application with alternative and adapted catalysts will be valuable. Nonetheless, the photocatalytic-based process is a favorable route for the degradation of pathogens and CECs and future large-scale water reclamation technologies.

Author Contributions

Conceptualization, E.F., J.G. and R.C.M.; methodology, E.F., J.G. and R.C.M.; writing—original draft, E.F.; writing—review and editing, J.G. and R.C.M.; supervision, J.G. and R.C.M.; funding acquisition, E.F., J.G. and R.C.M. All authors have read and agreed to the published version of the manuscript.


This work was financed by European Structural and Investment Funds through Portugal2020 by the project PhotoSupCatal—Development of supported catalytic systems for wastewater treatment by photo-assisted processes (POCI-01-0247-FEDER-047545). The authors gratefully acknowledge FCT (Fundação para a Ciência e Tecnologia, Portugal) for the PhD Grant (2020.06130.BD) and the financial support (CEECIND/01207/2018). Thanks are due to FCT/MCTES for the financial support to CIEPQPF (UIDB/00102/2020).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. Water treatment scheme and routes of contaminants introduction in the environment.
Figure 1. Water treatment scheme and routes of contaminants introduction in the environment.
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Figure 2. SEM images of WO3 morphology alteration by variation of citric acid addition, (a) nanoflakes, (b) nanosheets, and (c) hollow spheres (Retrieved with permission from Ref. [33] Copyright (2020) Elsevier).
Figure 2. SEM images of WO3 morphology alteration by variation of citric acid addition, (a) nanoflakes, (b) nanosheets, and (c) hollow spheres (Retrieved with permission from Ref. [33] Copyright (2020) Elsevier).
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Figure 3. The mechanism for bacterial disinfection using Ag-TiO2 (Retrieved with permission from Ref. [42] Copyright (2019) Elsevier).
Figure 3. The mechanism for bacterial disinfection using Ag-TiO2 (Retrieved with permission from Ref. [42] Copyright (2019) Elsevier).
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Figure 4. Crystalline phase transformation through La3+ at different concentrations (0–1.0%) (Retrieved with permission from Ref. [48] Copyright (2022) Elsevier).
Figure 4. Crystalline phase transformation through La3+ at different concentrations (0–1.0%) (Retrieved with permission from Ref. [48] Copyright (2022) Elsevier).
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Figure 5. Schematics of different nitrogen incorporation over TiO2 lattice: (a) pure TiO2, (b) substitutional, and (c) interstitial doping (Retrieved with permission from Ref. [70] Copyright (2009) John Wiley and Sons).
Figure 5. Schematics of different nitrogen incorporation over TiO2 lattice: (a) pure TiO2, (b) substitutional, and (c) interstitial doping (Retrieved with permission from Ref. [70] Copyright (2009) John Wiley and Sons).
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Figure 6. Representation of the energy diagram of Ag-g-C3N4 CB and VB potentials [115].
Figure 6. Representation of the energy diagram of Ag-g-C3N4 CB and VB potentials [115].
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Figure 7. SEM images of (a) undoped g-C3N4, (b) 0.5 B-P-g-C3N4, (c) 1 B-P-g-C3N4, (d) 2 B-P-g-C3N4, (e) 3 B-P-g-C3N4, and (f) enlarged inside structure of 3 B-P-g-C3N4. (Retrieved with permission from Ref. [119] Copyright (2021) Elsevier).
Figure 7. SEM images of (a) undoped g-C3N4, (b) 0.5 B-P-g-C3N4, (c) 1 B-P-g-C3N4, (d) 2 B-P-g-C3N4, (e) 3 B-P-g-C3N4, and (f) enlarged inside structure of 3 B-P-g-C3N4. (Retrieved with permission from Ref. [119] Copyright (2021) Elsevier).
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Figure 8. Representation of the mechanisms of Z-scheme and type-II heterojunctions [130,131,132].
Figure 8. Representation of the mechanisms of Z-scheme and type-II heterojunctions [130,131,132].
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Figure 9. UV-vis spectra of Fe2O3/TiO2 composite and commercial TiO2 (Adapted and retrieved with permission from Ref. [162] Copyright (2020) Elsevier).
Figure 9. UV-vis spectra of Fe2O3/TiO2 composite and commercial TiO2 (Adapted and retrieved with permission from Ref. [162] Copyright (2020) Elsevier).
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Table 3. Application of co-doped TiO2 for photocatalytic removal of contaminants.
Table 3. Application of co-doped TiO2 for photocatalytic removal of contaminants.
CatalystPrecursorMethodEbg (eV)Radiation SourceContaminantRemoval (Time)Ref.
C-Co-TiO2TTIPGlucose and
Cobalt Chloride
100 mg L−1
(120 min)
C-N-TiO2TBOTExtrapallial Fluid of
Polyethylene, 0.4% w/v
(50 h)
20 mg L−1
(5 h)
Cu-N-TiO2TTIPUrea and
Copper (III)
Sol-gel-VisibleMethylene Blue,
12.5 mg L−1
(90 min)
Fluoride and
Solvothermal-Solar2-nitrophenol, 10 mg L−1~98.1%
(75 min)
Fluoride and
-Solar4-nitrophenol, 10 mg L−1~93.9%
(75 min)
F-Pd-TiO2TBOTTrifluoroacetic Acid and
30 mg L−1
(40 min)
Oxide and Iron
Sol-gel2.78VisibleMethylene Blue and Methyl
Orange, 5 mg L−1
97.9% MB and 99.7% MO
(180 min)
Fe-N-TiO2TTIPUrea and Iron AcetylacetonateSol-gel2.70VisibleAcid Orange Azo Dye, 10 mg L−190%
(60 min)
Fe-Pr-TiO2TTIPPraseodymium Nitrate and Iron AcetylacetonateSol-gel2.70VisibleAcid Orange Azo Dye, 10 mg L−187%
(60 min)
Table 4. Application of doped WO3 for photocatalytic removal of contaminants.
Table 4. Application of doped WO3 for photocatalytic removal of contaminants.
CatalystPrecursorMethodEbg (eV)Radiation SourceContaminantRemoval (Time)Ref.
Ag-WO3Sodium TungstateSilver NitrateHydrothermal, HCl2.63SolarAcetaminophen,
5 mg L−1
(120 min)
Ion-exchange1.85VisibleMethylene Blue,
10 mg L−1
(80 min)
Co-WO3Sodium TungstateCobalt
Co-precipitation-VisibleMethyl Red,
10 mg L−1
(120 min)
Cu-WO3-Copper NitratePrecipitation2.60VisibleTetracycline,
50 mg L−1
(120 min)
Fe-WO3Ammonium ParatungstateIron ChlorideSol-gel2.39VisibleMethylene Blue,
10 mg L−1
(120 min)
Gd-WO3Sodium TungstateGadolinium NitrateHydrothermal2.64VisibleRhodamine B,
20 mg L−1
(100 min)
Mn-WO3Tungstic AcidManganese ChlorideMicrowave-
2.00VisibleSulfamethoxazole, 1 mg L−1100%
(70 min)
C-WO3Sodium TungstateCarbonized GlucoseHydrothermal-UV-VisibleRhodamine B,
20 mg L−1
(180 min)
I-WO3Ammonium Paratungstate and Spondias mombin leaves
Ammonium IodideHydrolysis and
2.17SolarDyeing Wastewater, TOC = 576.8 mg L−1 COD = 991 mg L−188.2% TOC
89.1% COD
(240 min)
P-WO3Ammonium Phosphate2.4186.8% TOC
86.6% COD
(240 min)
P-I-WO3Ammonium Phosphate and
Ammonium Iodide
2.0293.4% TOC
95.1% COD
(240 min)
S-WO3Sodium TungstateThioureaHydrothermal-VisibleMethyl Orange,
20 mg L−1
97% (3 h)[101]
Table 9. Application of TiO2 composites in photocatalytic and ozone-based processes for contaminants removal.
Table 9. Application of TiO2 composites in photocatalytic and ozone-based processes for contaminants removal.
CompositeRadiation SourceContaminantOxidantResultsRef.
CNT/TiO2UVOxamic acid, 89 mg L−1O270% removal in 60 min[166]
O3100% removal in 60 min
-24% removal in 60 min
Fe3O4/TiO2UV-ACeftazide, 10 mg L−1O234.6% removal in 15 min[168]
O3100% removal in 15 min
-86.7% removal in 15 min
SolarMetoprolol, Ibuprofen,
Clofibric acid and DEET,
2 mg L−1 each
O256% and 45% removals of contaminants mixture in 120 min in, respectively,
synthetic and real secondary effluent, and up to 40% of DOC removal
O3100% removal of contaminants mixture in 15 min and up to 70% of DOC removal
-100% removal of contaminants mixture in 30 min and up to 25% of DOC removal
GO/Fe3O4/TiO2SolarCotinine, Caffeine, Ciproflaxin, Metoprolol, Sulfamethoxazole, Bezafibrate, Tritosulfuron, Ibuprofen, Clofibric acid, and DEET,
0.5 mg L−1 each
O370% of TOC removal in 120 min at pH = 4 in urban wastewater[169]
-63% of TOC removal in 120 min at pH = 4 in urban wastewater
Table 11. Application of g-C3N4 composites for photocatalytic removal of contaminants.
Table 11. Application of g-C3N4 composites for photocatalytic removal of contaminants.
(m2 g−1)
Ebg (eV)Radiation SourceContaminantRemoval (Time)Ref.
Bi4O7/g-C3N4109-VisibleAspergillus fumigatus, 106 CFU mL−181% (6 h)[179]
BiVO4/g-C3N472.43VisibleNonylphenol Ethoxylate, 50 ppm100% (120 min)[180]
BiVO4/Bi2O6/g-C3N495.9-VisibleRhodamine B, 20 mg L−1100% (60 min)[181]
Tetracycline, 20 mg L−1100% (60 min)
LaVO4/g-C3N4--VisibleTetracycline, 20 mg L−183.4% (30 min)[182]
Naproxen, 20 mg L−180% (120 min)
Ag-ZnO/S-g-C3N457.22.51SolarMethylene Blue, 10 mg L−197% (40 min)[183]
HTCC/O-g-C3N4-0.95VisibleHuman Adenovirus Type 2,
105 MPN mL−1
100% (120 min)[184]
PAN/g-C3N413.3-UVOilfield Produced Water96.6% (8 h)[185]
Visible85.4% (8 h)
PEI/g-C3N470.21.74VisibleTetracycline, 40 mg L−180% (120 min)[186]
PEI/g-C3N4--SolarE. coli, 2 × 106 CFU mL−1100% (45 min)[187]
Enterococcus faecalis,
2 × 106 CFU mL−1
67.7% (60 min)
Au-SiO2/g-C3N4365-VisibleRhodamine B, 10 mg L−199.8% (90 min)[188]
β-SiAlON/g-C3N4--VisibleMurexide, 250 mg L−190% (8 h)[189]
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Fernandes, E.; Gomes, J.; Martins, R.C. Semiconductors Application Forms and Doping Benefits to Wastewater Treatment: A Comparison of TiO2, WO3, and g-C3N4. Catalysts 2022, 12, 1218.

AMA Style

Fernandes E, Gomes J, Martins RC. Semiconductors Application Forms and Doping Benefits to Wastewater Treatment: A Comparison of TiO2, WO3, and g-C3N4. Catalysts. 2022; 12(10):1218.

Chicago/Turabian Style

Fernandes, Eryk, João Gomes, and Rui C. Martins. 2022. "Semiconductors Application Forms and Doping Benefits to Wastewater Treatment: A Comparison of TiO2, WO3, and g-C3N4" Catalysts 12, no. 10: 1218.

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