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.
for the treatment of tetracycline, an antibiotic, in hospital wastewater under solar irradiation. Besides the faster production and higher separation of the e
, Ag significantly altered the optical properties of the catalyst, decreasing its E
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.
Non-metal and metalloid elements have a larger record of use in g-C3
doping and are able to overcome the disadvantageous surface characteristics of the catalyst and improve its electronic and optical properties. Zhang et al. 
synthesized a mesoporous g-C3
which presented a high specific surface area, 91.1 m2
, 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-C3
The use of multiple elements simultaneously for the doping of g-C3
has already been explored, albeit with fewer studies, and has shown a great boost in the material photoactivity. The most significant feature of g-C3
co-doping is appointed to be an improvement in electron mobility 
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.
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 
. 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.
The mixing of WO3
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 
. Mugunthan et al. 
proved the higher efficiency of the coupled TiO2
catalyst for diclofenac elimination under visible radiation. The group synthesized the photocatalyst through a hydrothermal method and studied the variation of TiO2
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
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 
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. 
for hollow spherical TiO2
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 the 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.
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. 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
. 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
of the VB of TiO2
tend to recombine fast, leading to the accumulation of e−
on the CB of TiO2
on the VB of WO3
, potentializing its respective reductive and oxidative potential 
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.
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 
. Changes in the morphology of the material have been also attested. Das et al. 
, 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 (Bi2
), bismuth vanadate (BiVO4
), and bismuth oxyhalide (BiOX), involving combination with halogen elements (X) 
heterostructure synthesized by Cipagauta-Díaz et al. 
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.
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.
In recent years, phosphate-based photocatalysts, especially Ag3
, have been the focus of numerous studies due to their superior quantum efficiency under visible light irradiation, 90% 
. 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, Ag3
composites may benefit from their heterostructures and be presented as more robust photocatalysts.
The instability of Ag3
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 Ag3
composites, providing higher chemical stability, surface area, and electron mobility by the organic material on the ternary composite 
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.
There is an expanding number of studies regarding the combination of g-C3
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. 
synthesized in one step a PEI/g-C3
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-C3
. The efficiency of PEI/g-C3
has also been attested for disinfection purposes, by Zeng et al. 
, 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.
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.