Recent Development in Non-Metal-Doped Titanium Dioxide Photocatalysts for Different Dyes Degradation and the Study of Their Strategic Factors: A Review

Abstract

Semiconductor titanium dioxide in its basic form or doped with metals and non-metals is being extensively used in wastewater treatment by photocatalysis due to its versatile nature. Other numerous characteristics including being environmentally friendly, non-pernicious, economical, multi-phase, highly hydrophilic, versatile physio-chemical features, chemical stability, suitable band gap, and corrosion-resistance, along with its low price make TiO₂ the best candidate in the field of photocatalysis. Commercially, semiconductor and synthesized photocatalysts—which have been investigated for the last few decades owing to their wide band gap—and the doping of titania with p-block elements (non-metals) such as oxygen, sulfur, nitrogen, boron, carbon, phosphorus, and iodine enhances their photocatalytic efficiency under visible-light irradiation. This is because non-metals have a strong oxidizing ability. The key focus of this review is to discuss the various factors affecting the photocatalytic activity of non-metal-doped titania by decreasing its band gap. The working parameters discussed are the effect of pH, dyes concentration, photocatalyst’s size and structure, pollutants concentration and types, the surface area of photocatalysts, the effect of light intensity and irradiation time, catalyst loading, the effect of temperature, and doping impact, etc. The mechanism of the photocatalytic action of several non-metallic dopants of titanium dioxide and composites is a promising approach for the exploration of photocatalysis activity. The various selected synthesis methods for non-metallic-doped TiO₂ have been reviewed in this study. Similarly, the effect of various conditions on the doping mode has been summarized in relation to several sorts of modified TiO₂.

1. Introduction

Water is one of the most precious and non-replaceable commodities of mankind. The per capita usage of water varies between countries and its demand includes domestic, industrial, and public, but here lies a problem of the misuse of water on a larger scale. Obviously, photocatalysis is the most fundamental technique for the degradation of organic pollutants and their conversion into valuable products, the removal of industrial effluents, and energy utilization [1,2]. Appropriate water supplies are the most important and key parameters for human consumption, industrial, agricultural, and domestic purposes. Generally, a wide range of natural and artificial contaminations has deleteriously affected the environment directly or indirectly, in which textile dye removal is of major concern [3,4,5]. However, ample research has been carried out on TiO₂ as a paramount photocatalyst for a variety of applications, such as the degradation of organic pollutants, hydrogen production from water splitting, the purification of air and water, self-cleansing surfaces, food cosmetics, the paint industry, etc. Titania is extensively employed in energy-associated industries, specifically, in water splitting under visible-light irradiation [6]. However, different compounds have been used for the doping of TiO₂ with CdS [7]. SnO₂-doped TiO₂ [8,9,10], WO₃-doped TiO₂ [11], ZrTiO₄ [12,13,14], and are employed in the heterogeneous photocatalysis. Among these very endearing materials, non-metal-doped TiO₂ photocatalysts have gained significant attention due to their greater synergistic effect, which leads to the reduction in the band gap of titania from various metal-doped TiO₂ [14,15,16]. Many conventional techniques or pathways have been adopted for the wastewater treatment, but unfortunately, the problem remains unsolved. However, different measures such as adsorption, coagulation, flocculation, and oxidation have been employed to cast off dyes from wastewater. The most documented approaches for the removal of dyes from wastewater are precipitation, filtration, and electrochemical procedures. However, these methods also have disadvantages; moreover, some of them do not have the ability to break down the dye completely. Therefore, photocatalysis is now thought to be one of the best ways to remove dyes from contaminated water [17]. Additionally, to overcome this issue, the photocatalytic process of various dyes degradation has shown to be a more efficient, cost-effective, and convenient way to eliminate abundant contaminants and organic pollutants from water. This method has drawn the attention of researchers all over the world to the development of more effective techniques of wastewater treatment. For this mechanism, simple TiO₂, doped TiO₂, or composite of TiO₂ have been widely discussed in the literature [18,19,20,21]. Some modifications in titania can help in resolving the mistreatment of unadorned TiO₂. However, doping the TiO₂ would enhance the photocatalytic activity for a range of mechanisms, as well as reduce the band gap in TiO₂ materials [22]. The well-known polymorphs of TiO₂ are rutile, anatase, and brookite, which are naturally occurring. It has recently been demonstrated that photo-activity towards organic degradation depends on the phase composition and oxidizing agent; for example, when the performance of different crystalline forms was compared, it was discovered that rutile shows the highest photocatalytic activity with H₂O₂, whereas anatase shows the highest with O₂. A good photocatalyst should be photoactive, able to utilize visible and/or near UV light, and be biologically and chemically inert, photostable, inexpensive, and non-toxic. However, it was previously reported that the anatase phase of TiO₂ is more efficient owing to its excellent photocatalytic properties [19]. Rutile is stable and easy to fabricate in setting conditions, while brookite is metastable and the preparation is troublesome. Both rutile and anatase hold a tetragonal crystal structure with a band gap of approximately 3.0–3.2 eV, respectively [23]. Titania has many advantages; foremost important is its use as a photocatalyst, and it is innocuous and chemically stable, etc. However, it also has a few disadvantages, for instance, it has a higher energy gap around 3.2 eV, which is why its use is limited to UV light only. Moreover, it cannot be activated in visible light or sunlight; therefore, researchers have been working on other ways to use it. Few methods are being developed for this purpose; one of the methods is to dope non-metals or metals impurity, and the other is to introduce light-sensitive semiconductors such as WO₃ in order to enhance the photocatalytic activity of TiO₂ [24]. Below visible-light irradiation, metal doping enhances the performance of the doped TiO₂ by moving the absorption spectra into a low energy field. Metal doping, on the other hand, has significant disadvantages, including the thermal instability of doped TO₂, electron capture by metal centers, and the need for more expensive ion implants. Non-metallic doping increases the percentage of anatomical TiO₂, which slows down the formation of TiO₂ crystals and increases the specificity of titania [25]. The most important approach that is used widely is the sol-gel method, which produces high surface area TiO₂ by controlling the hydrolysis of titanium tetraisopropoxide (TTIP) [26]. Therefore, the re-integration of electron–hole pairs produced in the valence and conduction bands enhances the image performance of TiO₂ [27]. However, there is also provision for dye photosensitization and proper TiO₂ support with the help of an efficient TiO₂ modification method [28]. Likewise, mesoporous titania (mp-TiO₂) is considered the best due to its higher surface area, tunable pore-size distribution, and well-defined pore orientation [29]. Furthermore, nitrogen/fluorine (N/F) co-doped mp-TiO₂ has been shown to exhibit improved visible-light adsorption and photoactivity [30]. TiO₂ graphene (Gr) porous microspheres were prepared by ultrasonic spray pyrolysis [31]. However, N/F co-doped mp-TiO₂ showed the best photocatalytic activity compared with single-element doping. The solvothermal method was adopted for the synthesis of N/F-mp-TiO₂, using urea as a nitrogen source and ammonium fluoride as a source for fluorine. There has been a great change in band gaps by the doping of TiO₂ with non-metals constituents, which are reported as F-TiO₂, N-TiO₂, B/N-TiO₂ [32], and C-N-S [33,34]. Moreover, thin-film TiO₂ with a different nano size is being extensively used for solar cell, design sensors, displays, automobiles rearview mirrors, and many other gas purification applications [35,36,37].

In this review, we have unfolded all the possible prospects and aspects of TiO₂ photo-catalyst doped with non-metals for light sensitivity. By the end of the paper, almost all the precarious applications, challenges, and future recommendations will have been proposed.

2. Effect of Non-Metal Doping

Different approaches are investigated to prevent the recombination of electron–hole pairs that are photogenerated by undoped TiO₂. Doping adjusts the electronic assembly of TiO₂, which boosts it from the UV region toward visible spectra. The doping of titania with any species enhances their ability to confer the chemistry of that particular material; for example, a high degree of crystallinity is mainly due to a large surface area and a large crystal size, which ultimately increases the photocatalytic activity and decreases the charge separation of the photogenerated e⁻/h⁺ pair recombination [38,39]. Nevertheless, the doping or modification of TiO₂ using non-metal/anion has been carefully adopted to overcome the use of maximum energy, high photocatalytic activity, reduced time span of charge separation, and higher efficiency of TiO₂, and the effect of various parameters has been discussed here. Moreover, non-metallic dopants such as carbon (C) [40,41,42], nitrogen (N) [43,44,45,46,47], sulfur (S) [47,48], boron (B) [49,50], iodine (I) [51], N/F-doped TiO₂ [30,52], l-amino acids (C–N co-doped or C–N–S tri-doped)-TiO₂, C-N-TiO₂ /CNT composites, etc., have also been discussed (

Table 1. Summary of non-metal-doped TiO₂ photocatalyst under varied circumstances.

Year of Study	Type of Non-Metal Dopants	Synthesis Route/Method	Type of Dye	Characterization Techniques	Ref.
2017	C-TiO2	Hydrothermal	Methylene blue, Rhodamine B, p nitrophenol	XRD, SEM, TEM, STEM, XPS, UV-vis	[56]
2019	C-TiO2	Hydrothermal	Methylene blue	XRD, FTIR, N2 adsorption-desorption isotherm, SEM, UV-vis	[57]
2020	C-TiO2	Sol-gel	Methylene blue	EDX, UV-vis DRS analysis, SEM	[58]
2020	C-TiO2 double-layer hollow microsphere	Hydrolysis of thermal expandable microsphere	Rhodamine B	XRD, FTIR, TGA, SEM, Raman N2 adsorption-desorption isotherm, XPS, UV-vis	[59]
2021	Carbon-doped TiO2 nanoparticles	Sol-gel	Methylene blue	XRD, TEM, XPS, DRS,138	[60]
2022	C-TiO2 nanoflakes (C-TNFs)	Facile hydrothermal	Methylene blue	XRD, FTIR, SEM, UV-vis	[40]
2007	N-TiO2	Microemulsion-hydrothermal	Rhodamine B	XRD, Raman, XPS, PL emission spectra	[61]
2010	N-TiO2	Sol-gel/acidic media	Lindane	XRD, SEM, TEM, Raman, XPS, GC-MS	[62]
2015	N and C-co-doped porous TiO2 nanofibers	Electrospinning and calcination	Methylene blue	XRD, FESEM, TEM, XPS, DRS,	[63]
2017	N-TiO2	Solvothermal	Rhodamine B	XRD, SEM, TEM, BET, XPS, UV-vis	[64]
2020	TiN/N-doped TiO2 composites	Sputtering process	Methylene blue	Raman, XPS, UV-vis	[65]
2020	C-N-TiO2 composite fibers	Hydrolysis and calcination	Rhodamine B	XRD, SEM, TEM, FTIR, Raman, XPS, UV-vis	[66]
2021	N-TiO2 nanotubes	Hydrothermal	Methyl orange	XRD, SEM, XPS, UV-vis	[67]
2009	S-TiO2	Hydrothermal	Methylene orange	XRD, TEM	[68]
2015	S-TiO2	Wet-impregnation method.	Humic acid Humic acid	EDX, SEM, EEM fluorescence	[69]
2016	(S–TiO2), (N–S–TiO2)	Sol-gel	Phenol and MB	BET, FESEM, FTIR, XPS, DRS	[70]
2011	S-TiO2, N-S-TiO2	Sol-gel	Methyl orange	XRD, TEM, UV-vis DRS	[71]
2021	NS/TiO2	Sol-gel	Methylene blue, methyl red	XRD, BET, SEM, FTIR, Raman, UV-vis	[21]
	P/TiO2	Hydrothermal/sol-gel			[72]
	Ag-P/TiO2 nanofibers	One-pot electrospinning	Methylene blue	XRD, XPS, FE-SEM, TEM, UV-vis	[73]
2022	P/TiO2/MWCNTs	Sol-gel	Methylene blue	XRD, FE-SEM, FTIR, UV-vis	[74]
2017	Si/TiO2	Solvothermal	Methyl orange	XRD, SEM, EDS, BET, XPS	[75]

). The compatibility of (N) was investigated, and the improved photocatalytic performance and morphology of TiO₂ was observed [23], as well as its composites [15]. Furthermore, GO/TiO₂ nanocomposite showed an upgraded photocatalytic efficiency [53]. Attention is being diverted to the treatment of contaminated water with porous mineral composite materials. The growth of non-metallic spongy minerals has resulted in a high specific surface area, strong adsorption capabilities, and tailored chemical accumulation [54]. Montmorillonite-supported-TiO₂ (MMT/TiO₂) will solve the cohesion problem of TiO₂ particles. Contaminants can be adsorbed on the surface of nano TiO₂ to improve the probability of ion exchange and contact between the catalysts and contaminants deterioration rate. Therefore, a porous MMT/TiO₂ complex system can improve its photodecomposition efficiency by reducing the agglomeration and intensifying the photocatalytic response. The comparatively improved absorbance of the composite material in visible light is (390–780 nm); therefore, MMT/TiO₂ also improves the optical absorption capacity from 70 to 87%, instead of using TiO₂ individually [55].

3. Treatment Opportunities for Dyes

Dyes have been a constant source of contamination for decades; textile and industrial dyes in the wastewater are one of the major contributors of toxic organic pollutants. The ever-increasing production of dyes is because of rapid industrialization; therefore, it is urgently necessary to focus on the proper treatment of these dyes [76]. In industries, most of the fabrication and processing operations excrete dyes in the wastewaters; the range of dyes varies from 2% to 50% from basic to reactive colorants, respectively. Therefore, the toxins produced are harmful for ground as well as surface water, as most of the compounds present in these hazardous dyes are non-biodegradable and likely to cause cancer [77]. Now, the primary concern is the sufficient treatment of these dyes; for their removal, various chemical and physical methods are being reported. Some of these methods are: ozonation [78], activated carbon [79], bio-degradation [80], and photocatalytic degradation using TiO₂, etc. [81]. The removal of dyes from contaminated water has been accomplished through biological processes that include microbiological or enzymatic decomposition and environmental degradation. Anaerobic conditions also contribute to the deterioration of the azo bond, which leads to the removal of pigment, but also leads to an insufficient mineral formation of harmful and carcinogenic products. A variety of synthetic dyes have been extensively tested in recent years to develop a more promising method based on advanced oxidation processes (AOPs) that can release pollutants quickly and extensively. AOPs rely on the formation of highly active hydroxyl radicals (OH^•), which can release any chemical present in a liquid matrix, usually with a reaction rate controlled by diffusion [82].

3.1. Types of Dyes

The method by which photo-degradation occurs is determined by the products generated, as these product molecules would be adsorbed on the surface of the semiconductor by modifying the layout of its electronic and active sites. In photo-catalytic degradation, it has been discovered that dyes with a positive charge (cationic) adsorb more on unaltered TiO₂ than dyes with a negative charge (anionic) [83]. Azo dyes (AzD) are attributed as the major (60–70%) class of synthetic dye stuff used in the textile, leather, oil, additives, and food industry, etc., and the resulting byproducts carry both the metal ion and the dyes. All the major classes of dyes are shown in Figure 1 and

Table 2. Types and characteristics of Azo dyes [76].

Category of Dye	Features	Fiber	Pollutant	Dyes Fixation
Acidic	Water-soluble anionic compounds	Wool, nylon, cotton blends, acrylic, and protein fibers	Organic acids, unfixed dyes, color	80–93
Basic	Water-soluble, applied in weakly acidic dye baths, very bright dyes	Acrylic, cationic, polyester, nylon, cellulosic, and protein fibers	NA	97–98
Direct	Water-soluble, anionic compounds, applied without mordant	Cotton, rayon, and other cellulosic fibers	Surfactant, defoamer, leveling and retarding agents, finish, diluents	70–95
Dispersive	Insoluble in water	Polyester, acetate, modacrylic, nylon, polyester, triacetate, and olefin fibers	Phosphates, defoamer, lubricants, dispersants, diluents	80–92
Val	Oldest dyes, chemically complex, water-insoluble	Cotton, wool, and other cellulosic fibers	Alkali, oxidizing agents, reducing agents, color	60–70

.

These pigments comprise of a double bond between two nitrogens (–N=N–), which is further linked to two aromatic group moieties such as naphthalene/benzene rings [84]. AzDs are positively charged at pH 6.8 and negatively charged at a higher pH, which affects their adsorption on semiconductor surfaces. The pH of the effluent is not neutral, and the mixture of substances that would have dissolved in water varies the surface characteristics of the semiconductor. When charged species are present in solution, an electrical double layer forms, affecting electron–hole pair separation and dye adsorption properties on the semiconductor surface. pH and the amount of dye present influence the rate of photocatalytic dye abasement [85]. They have amphoteric properties. Depending on the pH of the medium, AzDs can be anionic (deprotonation in the acidic group), cationic (extension in the amino group), or non-ionic [86]. The most known AzDs are acid dyes, basic dyes (cationic dyes), direct dyes (substantive dyes), disperse dyes (non-ionic dyes), reactive dyes, vat dyes, and sulfur dyes. Acidic dyes acquire their name from the fact that they are frequently employed to dye nitrogenous textiles or fabrics in inorganic or organic acid solutions. Cations that are extensively utilized in the manufacturing of acrylic and modacrylic fibers are produced by basic dyes in the solution. Electrostatic forces are used to apply direct dyes to fibers/fabrics in an aqueous medium containing ionic salts and electrolytes [87]. Anaerobic conditions can also contribute to the deterioration of the azo bond, which leads to the removal of pigments, but also leads to an insufficient mineral formation of harmful and carcinogenic products [88].

3.2. Dye-Degradation Mechanisms

The degradation mechanism had been discussed in detail in the literature in various citations; only one is mentioned here [89]. When aqueous titania is subjected to the visible-light irradiation greater than 3.2 eV (band gap), the electrons of the conduction band (electrons _CB⁻) and the holes of the valence band (holes _VB⁺) are created. These light-generated electrons (e⁻) can either react with e^- acceptor O₂ adsorbed on the surface of TiO₂, or they can reduce the dye; they can also dissolve in water, causing reduction and producing an anion which could be a superoxide radical—O₂^•−. The holes that are generated have the tendency to oxidize any organic molecule to R⁺, and they can also oxidize water or hydroxyl ion to OH^• radicals. All these radicals along with titania have the potential to decompose dyes photocatalytically. The series of reaction schemes is given below [90] by Scheme 1. The pictorial representation of the dye-degradation mechanism is represented in Figure 2.

On the other hand, indirect methods can be used for the degradation mechanism, such as electron spin, resonance test scavenging, and radical trapping [92,93].

3.3. Technologies or Methods for the Removal of Dyes

There are innumerable dye-elimination techniques, which can be categorized as physical, chemical, and biological. The physical methods comprise of adsorption, ion exchange, and filtration/coagulation [94,95,96,97,98,99,100], whereas the biological include anaerobic deterioration, bio-sorption, and many more, while the chemical consist of ozonation, Fenton reagent, and photocatalytic processes, respectively [101,102]. The methodologies adopted for the removal of dyes is described in Figure 3.

In the physical method, different processes are used in the treatment of dyes, such as testing, mixing, precipitation, adsorption, and membrane filtration [97,103,104,105] (

Table 3. Illustration of various methods for removal of organic dyes.

Strategies	Methods	Advantages	Disadvantages	Ref.
Chemical	Electro-Fenton reagent Ozonation Photocatalysis	Effective decolorization of soluble or insoluble dyes No sludge production initiates and accelerates Azo bond cleavages	No diminution of COD values by extra costs Sludge formation Formation of byproducts release of aromatic amines High costs	[108,111]
Physical	Ion exchange Adsorption Filtration/coagulation	Good removal of wide variety of dyes Regeneration No absorbent loss Good elimination of insoluble dyes Low-pressure process	Non-selective to absorbate Non-effective for all dyes High costs of sludge treatment Quality not high enough for re-using the flood	[103,104,105]
Biological	Enzymes Microbes Aerobic and anaerobic degradation Biosorption	Reduces the amount of waste that is delivered to landfills or incinerators Manufacturing requires less energy When it breaks down, it releases less hazardous compounds	Low biodegradability of dye Salt concentration stays constant	[98]

). Studies have shown that adsorption is the best method for the removal of dyes as it is simple, easily operative, highly efficient, cheap, and has been effective for toxic substances, whereas for membrane filtration, the contamination of the membranes and the cost of changing them are the biggest filtering limitations [106]. On the other hand, chemical treatments include coagulation-flocculation, oxidation, ozonation, Fenton oxidation, photocatalytic oxidation, irradiation, ion exchange, and electrochemical treatment [107,108]. These chemical methods are very effective in enabling the possibility of dyes removal, but excessive use of the chemicals causes difficulty in disposing of them into mud. Their disposal is too costly, and a reasonable amount of electrical power is required for these processes [109]. Biological analysis includes aerobic, anaerobic, and anaerobic–aerobic investigation, in which natural contamination is reversed into harmless and solid objects. The physicochemical and biological treatment of polluted water are the standard for dye-removal methods [110].

3.4. Stability of Non-Metal-Doped Titania

Non-metal-doped TiO₂ possesses higher catalytic activity in comparison to the undoped titania, as they have the enhanced capability of absorbing visible light, hence, more enhanced photocatalytic activity. Furthermore, the band gap becomes narrow because the lattice of titania is being substituted by oxygen. Almost all the non-metals reduce the re-connectivity of the e⁻/h⁺ duos but also decrease the band gap energy of titania via the formation of aperture energy. Therefore, the doping of non-metals in TiO₂ is an approach to increase the catalytic activity and the visible-light harvesting capacity. Therefore, it is safe to say that non-metal-doped titania can turn out to be the active catalyst for a visible-light catalyst for the degradation of different dyestuff [112]. Practical applications of nitrogen-doped TiO₂ for dye-sensitized solar cells were reported by Wei Guo and his co-workers, as N-doped titania dye-sensitized solar cells have shown 10.1% more conversion efficacy and are more stable because of the induction of N into the titania photo-electrode [113]. Carbon (C) and boron (B) co-doped photosensitizers have also been reported in the literature for efficient applications [114].

4. C-Doped TiO₂ (C/TiO₂)

The exploration of carbonaceous-doped materials with TiO₂ plays a significant role in photocatalysis, and they have shown spectacular growth due to their very simple method of synthesis [115]. Three dimensional cariogenic dot/TiO₂ nanoheterojunctions photocatalyst was synthesized by the hydrolysis process for the degradation of RhB, with different weight percent of C-dots for enhanced photocatalytic activity w [116]

The C/TiO₂ was immobilized with polyamide fibers with different weight percentages (1 wt.%, 2 wt.%, and 3 wt.%); the surface area of TiO₂ increased and the band gap energy (2.38 eV) decreased measurably after doping with non-metal for the degradation of MB [57]. The addition of C atoms to the TiO₂ structure can improve visible-light absorption by narrowing the band gap. However, organic chemicals, different dyes, and medicines were removed using C/TiO₂ photocatalyst matrices derived from aqueous [117]. The C/TiO₂ with several forms of carbon precursors was reported [118]. The most utilized carbon precursors are difficult to detect since the researchers use a variety of chemicals for titanium precursors, such as titanium carbide, titanium (IV) oxacetylacetonate, and titanium (IV) butoxide (TBOT) to act as TiO₂. The recombination of e⁻/h⁺ pairs is reduced by doping titania with carbon. Titanium dioxide (TiO₂) nanoparticles (NPs) were synthesized by sol-gel synthesis and doped with polydiallyldimethyl ammonium chloride (PDADMAC), as the carbon precursor caused a significant decrease in the band gap from 2.96 eV to 2.37 eV [41].

Recently, C/TiO₂ with anatase/rutile multi-phases coated on granular activated carbon was used under visible light for the removal of nonylphenol. A significant doping effect was observed in the band gap, which saw a decrease from 3.17 eV to 2.72 eV, respectively [119,120]. Indeed, carbon-based (nano) composites have improved photocatalytic activity due to the coupling effect from the incorporation of carbon species. However, several types of carbon–TiO₂ composites such as C-doped TiO₂, N–C-doped TiO₂, metal–C-doped TiO₂, and other co-doped C/TiO₂ composites have been reported (

Table 4. Methods and precursors used for the synthesis of C-doped TiO₂ photocatalysts.

Methods	TiO2 Precursor	Carbon Source	References
Chemical bath deposition (CBD)	Titanium isopropoxide (TTIP)	Melamine borate	[122]
Sol-gel	Titanium isopropoxide (TTIP)	Microcrystalline cellulose (MCC)	[58]
Hydrothermal	TiC	-	[123]
Sol-gel	TTIP, TBOT, TiCl4, TiCl3	Ethanolamine (ETA), glycine, polyacrylonitrile (PAN), polystyrene (PS), starch, TBOT	[124,125]
Solvothermal treatment and calcinations	TiCl4	Alcohols (benzyl alcohol and anhydrous ethanol)	[126]
Solvothermal	TTIP	Acetone	[127]
Electrospinning followed by heat treatment	TTIP	Acetic acid	[128]
Hydrolysis	TBOT	Glucose	[129]
Sol-gel	Titanium butoxide	-	[30]
Hydrothermal route	-	Various carbon sources	[42]

), which were synthesized by the solvothermal/HT method and sol-gel process [121].

The C/TiO₂ photocatalyst exhibits enhanced photocatalytic activity in comparison to titania because the catalyst alters the crystal structure, lowers the pH, and narrows the band gap. C/TiO₂ supported by granular activated carbon for photocatalytic degradation of nonylphenol and anatase ratio is much better for the degradation in comparison to rutile [130]. Furthermore, it was discovered that annealing can increase the crystallinity of C/TiO₂ nanotubes, as shown in Figure 4 in the presence of Argon (Ar), instead of oxygen or nitrogen [131]. In addition, Ar or N₂ has proven beneficial in increasing the photocatalytic activity. The recombination of e⁻/h⁺ pairs is reduced by doping titania with carbon. TiO₂ nanoflakes (TNFs) and C/TiO₂ nanoflakes (CTNFs) were synthesized by the HT method, which is superior (92.7%) in the degradation of MB [40].

C/TiO₂ composites were further modified with nitrogen (as N–C-doped TiO₂ composites), and it was observed that N–C-doped TiO₂ composites exhibit improved photocatalytic activity as compared to only C/TiO₂ nano-formulations. High meso-porosity and a well-defined large surface area (102 m² g⁻¹) were obtained by N–C-doped TiO₂ composites synthesized by the solvothermal method with high photocatalytic evaluation [132,133,134,135]. In visible light, the catalytic image functions of non-metal-doping photocatalysts with different colors as model compounds are commonly used to analyze irradiation; however, this strategy has already been reported in the literature as an ineffective method [136]. This is due to the dye’s ability to absorb visible light, which means that the photocatalytic process may not be driven solely by visible-light absorption. However, this is not only by the photocatalyst, but also by the dye’s absorption of light (that is, dye sensitization). The most employed dyes in the literature are on visible-light active photocatalysts. In addition, Song and his colleagues reported a C-doped TiO₂/carbon nanofiber film (CTCNF) under visible-light irradiation for the breakdown of rhodamine B (RhB). The dye started to degrade, and its discoloration rate was 66.4–94.2% after 150 min of visible-light irradiation [137].

In addition, the removal of other organic substances from pharmaceuticals, personal care items, and even microbes from water and hydrogen production are the best examples of compounds being removed with the help of a C/TiO₂ photocatalyst. C-TiO₂/rGO is used to form a hybrid nanocomposite that exhibits excellent photocatalytic activity for the better production of H₂ instead of pure TiO₂, as shown in Figure 5 [127].

The C/TiO₂ photocatalyst was prepared by the sol-gel process by using microcrystalline cellulose (MCC) as a carbon source material. In comparison to pure and C-TiO₂ a reduced band gap was observed. On the other hand, when Fe is co-doped with C-TiO₂ further reduction in the band gap exhibit stronger visible light absorption [138]. Some other carbon material, such as graphdiyne, plays a significant role in a variety of applications [139,140].

5. N-Doped TiO₂ (N/TiO₂)

Nitrogen is the most often-utilized non-metal dopant among all non-metal dopants [130]. N/TiO₂ has been reported (Table 5) for various photocatalytic applications [141,142,143,144,145]. Nitrogen (N) doping in TiO₂ nanotubes was prepared by the hydrothermal process for the degradation of dye and H₂ gas evolution. Urea was used as a N source, and optimized N/TiO₂ nanotubes (TiO₂ nanotubes vs. urea at 1:1 ratio) showed the highest degradation efficiency over methyl orange (MO) dye (~91% in 90 min) and manifested the highest rate of H₂ evolution (~19,848 μmol h⁻¹·g⁻¹) under solar-light irradiation [67].

N/TiO₂ was synthesized by the direct hydrolysis of titanium tetraisopropoxide and ammonia as a nitrogen source for the photocatalytic degradation of organic dyes in liquid phase with visible light. A batch photo-reactor was used to study the photocatalytic evaluation of methylene blue (MB) dye degradation under the optimum operating parameters to obtain the maximum efficiency toward dye degradation. The band gap energy of titanium dioxide was shifted toward a lower band gap, i.e., in the visible range from 3.3 to 2.5 eV. This is because of its low ionization energy and the atomic size being comparable to the size of oxygen [146]. As proven within Table 5, quite a few nitrogen precursors were used in the synthesis of N-doped TiO₂.

The photocatalytic activity of N/TiO₂ and N-doped TiO₂ with transition metals (Fe, Cu) was reported for the degradation of MB under sunlight [147]. N/TiO₂ showed the highest activity among the doped TiO₂ nanoparticles (0.006 min⁻¹). Titanium nitride/nitrogen-doped titanium oxide (TiN/N-doped TiO₂) composite films were synthesized by the sputtering process. A Raman spectral analysis revealed the formation of TiO₂ with anatase, which later transformed to the rutile phase, but the results showed that the photodegradation efficiency of MB was higher in the case of titania anatase after exposure to visible light [65].

Table 5. Summary of N-doped TiO₂ photocatalysts synthesized by variety of methods and source precursor materials.

Year of Study	Method	TiO2 Precursor	Nitrogen Source	Ref.
2017	Addition of N source to the TiO2 precursor solution	TBOT	Tetra methyl-ethylene-diamine	[148]
2020	CVD	TICl4	Tert-butylamine, benzyl amine	[149]
2017	Hydrothermal	TBOT	KNO3	[150]
2019	Hydrolysis	TTIP	NH4Cl, pyridine	[151]
2016	Electrochemical	Titania nanotubes	Diethylenetriamine, ethylenediamine, hydrazine	[152]
2019	Sol-gel	TTIP, TBOT, TiCl4, Titanic acid	Urea, NH3, nitro methane, n-butyl amine, N2, hydrazine, HNO3,	[153]

Table 5. Summary of N-doped TiO₂ photocatalysts synthesized by variety of methods and source precursor materials.

Year of Study	Method	TiO2 Precursor	Nitrogen Source	Ref.
2017	Addition of N source to the TiO2 precursor solution	TBOT	Tetra methyl-ethylene-diamine	[148]
2020	CVD	TICl4	Tert-butylamine, benzyl amine	[149]
2017	Hydrothermal	TBOT	KNO3	[150]
2019	Hydrolysis	TTIP	NH4Cl, pyridine	[151]
2016	Electrochemical	Titania nanotubes	Diethylenetriamine, ethylenediamine, hydrazine	[152]
2019	Sol-gel	TTIP, TBOT, TiCl4, Titanic acid	Urea, NH3, nitro methane, n-butyl amine, N2, hydrazine, HNO3,	[153]

For the solar-driven photocatalysis over Ti³⁺ and N co-doped photo catalysts, Cao et al. [154] proposed a modified mechanism. The materials were made by reducing urea-modified mesoporous TiO₂ spheres with NaBH₄ at 350 °C in an Ar environment. The above-mentioned N-doping of TiO₂ resulted in the emergence of a new impurity level above the VB. In addition, by introducing Ti³⁺ and O below the CB, an intermediate energy level was created. As a result, the band gap was decreased, which increased photocatalytic effectiveness in the visible light. The schematic diagram of N-doped TiO₂ is shown in Figure 6.

The activity of N-doped TiO₂ as a photocatalyst has been examined in various studies. In general, N/TiO₂ photocatalysts have been utilized to remove organic chemicals from water, namely dyes and medicines, but phenol, furfural parabens, surfactants, and herbicides were among the other pollutants that were destroyed using doped TiO₂ with N [155]. It was also suggested that N/TiO₂ photocatalyst could be used to remove pollutants from the gaseous phase, such as acetaldehyde, benzene, ethylbenzene, NH₃, and NOx. Antibacterial characteristics were found in some of the photocatalysts, such as against Escherichia coli and oral cariogenic biofuels. Furthermore, the use of N-doped TiO₂ for human breast cancer diagnostics, and cancer treatment such as for melanoma, has been reported previously in Figure 7 [156]. N/F co-doped mp TiO₂ has been shown to have the highest adsorption and photocatalytic activity. The solvothermal method was adopted for the synthesis of N/F mp TiO₂ by using urea as a nitrogen source and ammonium fluoride as a fluorine source [29] The L-amino acid (C-N co-doped or C-N-S tri-doped) -TiO₂ photocatalyst was used for dyes removal under visible dye. The photodegradation of methyl orange and direct red 16 was studied by the first order kinetics, and the rate constant for DR16 photocatalytic removal using l-Arginine (1 wt.%)-TiO₂ was 2.9 and 4.3 times greater compared to those of l-Methionine (1.5 wt.%)-TiO₂ and l-Proline (2 wt.%)-TiO₂, respectively [157].

6. S-Doped TiO₂ (S/TiO₂ or SdT)

Amongst all the non-metals, sulfur would have been difficult to dope with titania because a large amount of formation energy is required for the replacement of S with O in titanium dioxide [158]. In contrast to the previous statement, S/TiO₂ (SdT) (anatase) was synthesized using TiS₂ (titanium disulfide) by the oxidative annealing. Therefore, this doping led to the red shift in the absorption band of SdT rather than the undoped titania_. Furthermore, it was used for the photocatalytic degradation of methylene blue (MB). SdT instigated MB’s irradiation quite efficiently in the visible-light region, as shown in Figure 8 [159]. SdT was also prepared by the heating of powder TiS₂ in the solution of HCl at 180 degrees, a comparatively lower temperature in comparison to the conventional methods of synthesizing SdT. Traditionally, it has been prepared by the thermal decomposition of thiourea (ThU) at elevated temperatures, whereas ThU is the source of sulfur. The formulated SdT at a demoted temperature was used for the desolation of 4-chlorophenol; it was concluded that the one-step hydrothermal process for the production of SdT was successful for the irradiation of 4-chlorophenol by visible-light photocatalytic activity [160].

Sulfur-doped and sulfate TiO₂ (SdST) were synthesized using the solvothermal method; potassium per sulfate (KPS) was taken as a source of sulfur and the irradiation rate of phenol was studied. As we know, titania showed lower photocatalytic activity in the visible light because it has a broad band gap energy, which limits the absorbance of UV light of less than 387 nm. However, the SdST catalyst showed higher degradation activity for phenol instead of pure TiO₂, especially in the range greater than 450 nm (longer visible-light range). Almost 51.3% of the containment (phenol) was degraded at a 0.5 ratio of the catalyst (SdST) for 10 h under the ranges, as mentioned in Figure 9 and

Table 6. Summary of S-doped TiO₂ photocatalysts synthesized by variety of methods and source precursor materials.

Year of Study	Method	TiO2 Precursor	Sulfur Source	Ref.
2003	Oxidative heating	Anatase	TiS2	[159]
2006	Low-temp hydrothermal	Anatase	TiS2 powder with HCl solution	[160]
2012	Solvothermal	TBOT	Potassium per sulfate	[161]
2016	Free oxidant peroxide method	Anatase	Thiourea (ThU)	[162]
2018	HT	Titanium sulfate (TiOSO4)	TiOSO4	[163]

[161].

7. P-Doped TiO₂ (P/TiO₂ or PdT)

In the recent years, P/TiO₂ (PdT) has gained attention because of their potential to increase the surface area, restrain the transformation of anatase to rutile, enhance the absorption of visible light and decrease grain growth [164] (Table 7). Therefore, many studies have been conducted for the generation of PdT and to study their properties regarding water splitting [165], dye-degradation, etc. [166]. Similarly, the synthesis of PdT nanoparticles using the sol-gel process was performed. The results showed a remarkable increased activity of the degradation of MB because of high visible-light pursuit, and the ESR spectroscopy (electron spin resonance) deduced its refined charge separation [167].

The fabrication of P doped over titania nanofibers (PdTNFs) was first reported by Zhu. Y. et al., from the method known as chemical vapor deposition (CVD), which showed remarkable results for electrochemical water splitting [168]. The doping of phosphorous decreases the band gap of titania and creates disproportion between O₂ and Ti⁴⁺ charges; therefore, the recombination of charge carriers is hindered. In TiO₂ (anatase type), the substitution of P³⁻ over O^2- is much greater than substituting P⁵⁺ onto Ti⁴⁺ (1.32 eV) because it requires high formation energy (15.48 eV). Hence, it shows that the incorporation of P⁵⁺ into titania lattice is achievable by forming the Ti-O-P bond rather than the Ti-P bond [169]. The separation of charges in the photocatalytic phenomenon is facilitated by the phosphate ions, which act as an electron-withdrawing species [170].

The photocatalytic activity of sulfamethazine (SMHZ) was investigated using mixed oxide novel-doped Fe₂O₃ and TiO₂. The weight percentage of 1.2 of mixed oxide had successfully degraded 30% of SMHZ; the percentage degraded is much higher than Fe₂O₃-TiO₂ or pure TiO₂ (Figure 10). [171].

Table 7. Summary of P/TiO₂ photocatalysts synthesized by variety of methods and source precursor materials.

Year of Study	Method	TiO2 Precursor	Phosphorous Source	Ref.
2020	Chemical vapor deposition	Titanium (IV) butoxide (Ti(OC4H9)4)	Red phosphorous	[171]
2014	Sol-gel method	TBO	Phosphorous acid	[172]
2011	Sol-gel process	Silicate/TiO2 NPs	Phosphoric acid (H3PO4)	[164]
2009	Sol-gel	TBO	H3PO4	[173]

Table 7. Summary of P/TiO₂ photocatalysts synthesized by variety of methods and source precursor materials.

Year of Study	Method	TiO2 Precursor	Phosphorous Source	Ref.
2020	Chemical vapor deposition	Titanium (IV) butoxide (Ti(OC4H9)4)	Red phosphorous	[171]
2014	Sol-gel method	TBO	Phosphorous acid	[172]
2011	Sol-gel process	Silicate/TiO2 NPs	Phosphoric acid (H3PO4)	[164]
2009	Sol-gel	TBO	H3PO4	[173]

8. B-Doped TiO₂ (B/TiO₂ or BdT)

The non-metal doping of anatase TiO₂ was further doped with B, C, N, and F [174,175,176]. The B/TiO₂ hybrid hollow microspheres were synthesized by hydrothermal treatment. This boron-doped titania photocatalyst was used for MB degradation in aqueous solution and served as a target pollutant to evaluate the photocatalytic activity under sunlight. A higher photocatalytic activity of boron-doped hybrid hollow microspheres was observed (Table 8), which was comparatively much greater than undoped titania [177].

B/TiO₂-nanotubes (B/TiO₂NTs) were synthesized by the electrochemical anodization method with boron concentrations of 70, 140, 280, and 560 ppm and activated via UV-visible irradiation. The results showed that the dye-degradation rate of Acid Yellow1 (AY1) was twice greater at the doped electrode, which contained 280 ppm of B, and the activation of the electrode was maximum there, as observed from the UV-visible light. When the AY1 (100 ppm) was treated at the B/TiO₂NTs electrode for 120 min at +1.2 V, pH 2 and in 0.01 mol L⁻¹ of sodium sulfate solution (Na₂SO₄), 100% discoloration of the dye was observed. Therefore, the synthesized amalgamation has the potential to become a stable electrochemical catalyst [178].

The synthesized BdT nanostructures was carried out using the sol-gel method. Studies have shown that, after the doping of boron, the band gap decreased from 2.98 eV of undoped titania to 2.95 eV of B/TiO₂ (7% boron content) (Figure 11a). Boron has the tendency to occupy the interstitial sites in the crystal lattice of TiO₂ and forms a Ti-O-B bond. Therefore, the degradation studies of 4-nitrophenol were studied using the said nanoformulations. The results showed that BdT (7%) displayed a 90% degradation efficacy compared to the undoped titania (79%) because the Ti-O-B linkage has a synergistic effect on supplementing the catalytic activity, as shown in Figure 11b [179].

Table 8. Summary of B/TiO₂ photocatalysts synthesized by variety of methods and source precursor materials.

Year of Study	Method	TiO2 Precursor	Boron Source	Ref.
2020	Sol-gel	Titanium isopropoxide	Boric acid	[179]
2015	Electrochemical anodization	Titanium sheets	NaBF4	[178]
2011	HT	(NH4)2TiF6	H3BO3	[177]

Table 8. Summary of B/TiO₂ photocatalysts synthesized by variety of methods and source precursor materials.

Year of Study	Method	TiO2 Precursor	Boron Source	Ref.
2020	Sol-gel	Titanium isopropoxide	Boric acid	[179]
2015	Electrochemical anodization	Titanium sheets	NaBF4	[178]
2011	HT	(NH4)2TiF6	H3BO3	[177]

9. Halogens-Doped TiO₂ (X = F, Cl, Br, and I)

The hydrogenated F/TiO₂ (FdT) nanocrystals were synthesized using the physicochemical method. F-doping has the potential to increase the surface area of TiO₂; further, hydrogenation plays a pivotal role in forming the F-H and O-H bonds on the surface of titania and creating vacancies of Ti³⁺ and O²⁻, which increases the range of absorption alongside the light utilization capacity of TiO₂. The bonds O-H and F-H can favor trapping the holes and can also react with water to produce an active species (OH^• ). Now, these hole and electron pairs can easily be separated to participate in the photocatalysis. Irradiation studies of MB were conducted and showed that the degradation rate constant of hydrogenated FdT is 0.146 min⁻¹, which is almost twice the rate of pure TiO₂ (0.063 min⁻¹) [180].

Hydrogen and fluoride co-doped TiO₂ nanostructures have been made from annealing. The doping of halogens decreases the band gap and bestows the oxide material a greater absorbance capacity. The results demonstrated that FdT is a better photocatalyst, as shown in Figure 12 and

Table 9. Summary of halogens-doped TiO₂ photocatalysts synthesized by variety of methods and source precursor materials.

Year of Study	Method	TiO2 Precursor	Fluorine Source	Ref.
2020	Oxidative annealing	Titanium isopropoxide	NH4F	[181]
2019	Physicochemical	TTIP	NH4F	[180]
2017	Sol-gel	Titanium isopropoxide	Trifluroacetic acid	[182]
2014	Sol-gel	Tetrabutyl titanate	NH4F	[183]
Year of Study	Method	TiO2 Precursor	Chloride Source	Ref.
2020	Oxidative annealing	Titanium isopropoxide	NH4Cl	[181]
2012	Sonochemical synthesis	Tetraisopropyl titanate	NaCl	[184]
2008	Hydrolysis	Tetrabutyl titanate	HCl	[185]
Year of Study	Method	TiO2 Precursor	Bromide Source	Ref.
2017	HT	TBOT	NH4Br	[186]
2009	Sol-gel	TBOT	Cetyl trimethyl Ammonium bromide (CTAB)	[187]
2004	HT	Titanium chloride	Hydrobromic acid	[188]
Year of Study	Method	TiO2 Precursor	Iodide Source	Ref.
2017	Sol-gel	Titanium (IV) ter-butoxide	Potassium iodide	[189,190]

[181].

A Cl/TiO₂ (CldT) photocatalyst was prepared, which shows greater absorption in the range of visible light. The doping of chlorine decreases the band gap of titania; therefore, the absorption spectra are extended. CldT shows better degradation of phenol than the pure one that is 42.5% after 120 min [185].

The synthesized Br/TiO₂ (BrdT) hollow spheres are made by the hydrothermal method. Studies have revealed that the adsorption peak of BrdT hollow spheres is near 517 nm, which is greater than that of undoped titania. The band enhanced towards the visible-light range. The band gap of the fabricated spheres decreased (1.75 eV), whereas the band gap of undoped TiO₂ was around 2.85 eV; this illustrates that the doping of Br instigates the impurity level between the conduction and valence bands; therefore, the transition of electrons is promoted [186].

I/TiO₂ photocatalysts were prepared via the sol-gel method. They tend to have higher photo-degradation potential under direct-sunlight irradiation. Without catalysts, the photocatalytic degradation of RhB was not observed. The mechanism of the photocatalytic degradation of RhB using I/TiO₂ involves the following steps as shown in Scheme 2 [189]:

10. Si-Doped TiO₂ (SidTiO₂)

Scheme 2 is based on nanotubes for MO. The results showed that 5% SidTiO₂ has a higher catalytic efficacy for MO degradation than the tubes synthesized with titania [75]. Similarly, in another study, SidTiO₂ was synthesized using the hydrothermal method for the photocatalytic degradation of organic pollutant, phenol. The results showed a nine-times higher percentage degradation with SidTiO₂ than undoped titania nanotubes [191] (

Table 10. Summary of Si/TiO₂ photocatalysts synthesized by variety of methods and source precursor materials.

Year of Study	Method	TiO2 Precursor	Silicon Source	Ref.
2019	Hydrothermal	Commercial TiO2	SiO2 commercial	[191]
2017	Solvothermal	Titanium oxysulfate	Tetraethoxysilane	[75]

).

11. Factors Affecting the Degradation of Photocatalytic Activity

The photocatalytic degradation of dyes is strongly affected by consideration of the following parameters: pH, dye concentration, the size and structure of the photocatalyst, pollutants concentration and types, light intensity and irradiation time, dopants’ effect on dye concentration, etc. These factors and their impact on the dye-degradation performance are demonstrated with details in this section.

11.1. Effect of pH

pH is a pivotal parameter that impacts the photocatalytic activity. Some of the results on the effect of pH on dye degradation are presented in

Table 11. Different types of dyes under optimum pH.

Dye Type	Light Source	Photocatalyst	pH Range	Optimum pH	Ref.
Orange G (OG)	UV	Sn/TiO2/Ac	1.0–12.0	2.0	[196]
(OG)	Visible	N-TiO2	1.5–6.5	2.0	[197]
Bromo-cresol purple (BCP)	UV	TiO2	4.5 & 8.0	4.5	[198,199]
Methyl Red (MR)	Visible	3%Ag+1.5%Ni-TiO2	3–10	4	[199]
Malachite Green (MC)	Sun light	Ni/MgFe2O4	2.0–10.0	4	[200]
Indigo Carmine (IC)	UV	TiO2	4.0–11.0	4	[201]
Textile dye (TD)	UV	TiO2	3.0–7.0	5	[202]
Basic Yellow 28 (BY28)	UV	TiO2	3.0–9.0	5	[203]
Methylene Blue (MB)	UV	TiO2ZnO	1.0–6.0	2	[204]
Reactive Blue 4 (RB4)	UV	Anatase TiO2	3.0–13.0	3–7	[205]
Procion Yellow (PY)	UV	TiO2	2.0–10.0	7.8	[206]
Acid Orange (AO)	UV	WO3-TiO2	1.0–9.0	3	[84]
Methyl Orange (MO)	UV	TiO2	2.0–10.0	8	[207]
Rhodamine B (RhB)	UV	ZnO	2.0–12.0	12	[208]
MO, RhB	UV	ZnO	2.0–10.0	Basic medium	[209]

pH fluctuations in combination with calcinated non-metal-doped titania possess the best catalytic degradation results because of the synergistic effect of phase structure and crystallinity [192]. The results reported in Table 11 show that the facet TiO₂ can be deprotonated/protonated under alkaline/acidic conditions, respectively. Therefore, it can be concluded that pH alterations can result in the escalation efficacy of the dye degradation of titania without influencing the rate equation [193]. The effect of pH on the decomposition of MO was investigated by Guttai N et al., and it turned out to be the first-order reaction rate, which was 25 times higher at pH 2 than at pH 9 [194]. This also means that some types of dyestuff are preferentially photo-degraded on TiO₂ surfaces. Dyes can be degraded in three ways as a function of pH: directing hydroxyl radical, (ii) the direct involvement of a hole in the oxidation reaction, and (iii) a tête-à-têtes reduction in the participation of the activated electron in the steering band [195]. The effect of pH on the photo-degradation efficiency of the dyes must be considered in conjunction with several other parameters. The interaction of hydroxide ions can produce hydroxyl radicals. At a low pH, holes are the dominant forms of oxidation, and at a neutral or high pH, hydroxyl radicals are the dominant species, respectively.

Hydroxide (OH^-) is easy to produce in an alkaline solution with an oxidizing ion, which then penetrates through the semiconductor surface. Therefore, the efficiency of the process increases reasonably [210]. According to the findings, pH plays an important role in altering the charges on the dyes; for example, BCP dye degradation was better in acidic media than in alkaline media [198]. Azo dyes are positively charged at pH 6.8 and negatively charged at a higher pH, which affects their adsorption on semiconductor surfaces. When charged species are present in a solution an electrical double layer forms, which affects the electron–hole pair separation and adheres the adsorption properties of the dye on the surface of the semiconductor. pH and the amount of dye present influence the rate of photocatalytic activity [85].

11.2. Effect of Dye Concentration

The optimum concentration of the dye is very important for the photocatalytic reaction, as it is highly dependent on the type of dye being considered. Generally, by increasing the concentration of the colorant, the photocatalytic degradation efficiency of the dyestuff will decrease by suppressing the active sites, therefore, hindering the activity [211]. However, a small quantity of the dye is subjected to degradation and it only contributes to the process of photocatalysis. The initial concentration of the dye in the photocatalytic process is important to consider [81]. This is because as the dye concentration increases, more organic molecules are adsorbed on the surface of TiO₂, but fewer photons reach the catalyst surface, resulting in less OH production, and thus, lowering the percentage degradation [212]. On the other hand, the ionic behavior of dye molecules may result in either accelerating or slowing down the process of the degradation of dyestuff. Normally, metal ions are adsorbed on the surface of the photocatalyst and make them slightly positively charged. As this effect reduces the electrostatic repulsion for anionic dyes, they can be adsorbed and degraded readily in the presence of metal ions. On the contrary, a retarding effect can also be observed for cationic dyes due to the decrease in attraction between positively charged dyestuff and neutral or slightly positive catalyst surfaces [213].

11.3. Photocatalyst’s Size and Structure

Surface morphology, such as particle size and agglomerate size, is an important component to address while discussing photocatalytic degradation, since there is a direct relationship between organic molecules and photocatalyst surface coverage. The speed of the reaction is determined by the number of photons striking the photocatalyst; therefore, it can be suggested that the reaction happens solely in the phase absorbed by photocatalysts [214]. Titania and Cu-TiO₂ nanocomposites were used to degrade MR. Studies have suggested that the Cu-modified titania shows more promising results for the catalytic degradation of MR in comparison to the titania because the morphology of the latter is different than the modified version of titania, which facilitates the advancement of organic molecules towards the catalyst [215].

11.4. Pollutant Concentrations and Types

The rate of the photocatalytic destruction of a pollutant is determined by the type of pollutant employed, along with its concentration and other chemicals present in the aqueous matrix. Many researchers have found that the rate of reaction of TiO₂ is influenced by the concentration contaminants present in water. High levels of water contaminants fill the TiO₂ site, reducing photonic efficiency, and moving the image of the catalyst towards malfunctioning [216]. The chemical structure of the target component, in addition to the concentrations of the pollutant, alters the photocatalytic degradation performance of the catalyst being used because of its conversion into its respective intermediates. For example, studies have shown that 4-chlorophenol takes longer to release irradiation than oxalic acid because it converts directly into carbon dioxide and water [217].

11.5. Surface Area of the Photocatalyst

The surface of TiO₂ is crucial in its employment as a photocatalyst because its reactions occur at the surface. This usually increases its availability either by utilizing it in the form of very fine particles, molding it into a porous sheet, or suspending it in some liquids [218]. Nanomaterials with a crystallite size/grain of less than 20 nm have gained particular interest among researchers because their physical characteristics differ significantly from their bulk counterparts. This has also opened new opportunities for their use as photographic catalysts in various fields. As the surface area of the image catalyst grows, more active sites become prominent [219].

11.6. Effect of the Intensity of Light and Irradiation Time

The intensity of light plays an important role in photocatalytic dye degradation, which results in the formation of less toxic byproducts of the dye under consideration. A large amount of research has been conducted on these specific parameters [220]. The effect of the intensity of light on the degradation and decolorization of RY14 is studied in [221]. The dye degradation is affected by both the strength of the light and the time it takes to expose it to degrade the dye or pollutants [193]. As the intensity of light significantly increases, the dye degradation rate enhances; this is because of an increase in the number of photons striking per unit time per unit area of the photocatalyst. Conversely, by further increasing the intensity of light, there might be a chance of higher thermal effects [173]. It was confirmed that N/TiO₂ showed higher dye degradation (almost 60% in 6 h) under visible light. On the contrary, under sunlight irradiation, N/TiO₂ unveiled degradation efficiency that was a little higher as compared to the non-doped TiO₂ efficiency of degradation. A negative light intensity effect on Congo Red was studied when the light intensity varied from 50 to 90 J·cm⁻ [222].

11.7. Dopants’ Impact on Dye Degradation

The main goal of doping is to cause a chromic change in the optical properties, which is defined as the reduction in the band gap or the introduction of intra-band gap conditions that leads to an increase in the absorption of visible light. Non-metal dopants’ effects on photocatalytic activity is a challenging problem to tackle. The performance of TiO₂ can be improved by doping with non-metals to enhance the photocatalytic efficiency of titania. Depending on the type of dopants and its concentration, it can also allow light to be absorbed into the visible area at different levels. As a result, visible light can be used to stimulate photocatalysis on modified TiO₂. In mixed systems, dye deterioration is often much faster than systems alone, as the oxidation of dyes utilizes exciting holes quickly and effectively, reducing electron–hole regeneration [214]. The prime concentration of dyes for a photocatalytic reaction is a key parameter that is highly dependent on the type of dye employed. Typically, by increasing the pollutants concentration, the photocatalytic degradation efficiency deceases. The reason for this is that maximum or higher dye molecules compete for limited active sites along with turbidity increases [223].

11.8. Effect of Mass Loading on the Catalytic Activity

TiO₂ has been made to absorb lower-energy photons using a variety of approaches. Kaur et al., reported that under optimized conditions with the highest efficiency, the catalyst dosage for the maximum photocatalytic degradation of RR 198 is 0.3 g [224]. It is manifested that the photodegradation rate increases with the increase in the amount of photocatalyst and then decreases with the increase in the catalyst concentration [225]. In another study, an increase was reported in the weight of the catalyst from 1.0 to 4.0 g L⁻¹, which increases the dye decolorization sharply from 69.27% to 95.23% at 60 min and the dye degradation from 74.54% to 97.29% at 150 min. The optimum concentration of the catalyst for efficient solar photo decolorization and degradation is found to be 4 g/L [226].

12. Conclusions

This review focused on the comprehensive study of the fundamental aspects of non-metal-doped/TiO₂ nanoparticles. Titania is considered as the best visible-light-driven photocatalyst for the degradation of numerous dyes. Plentiful research has been carried out that encompasses the importance of non-metals-doped titania in comparison to the undoped TiO₂ for their better photocatalytic dye degradation of various anionic and cationic dyes, in order to overcome the environmental pollution issues via industrial effluents or other pollutants. We must incorporate most of the literature present on non-metals such as C, Si, N, P, B, halogens, etc. The effect of different parameters, for instance, pH, dyes concentration, photocatalyst’s size and structure, pollutants concentration and types, the surface area of photocatalysts, the effect of light intensity along with its irradiation time, catalyst loading, variation in temperature, and doping (non-metals) impact with optimization has a strong correlation with pollutants degradation rate, which is also discussed in this review. The photocatalytic performance of non-metal-doped titania can be further improved with the design of efficient synthesis methodologies.

13. Opportunities, Challenges and Future Prospects

Titanium dioxide has a wide range of properties associated to it; it is a semiconductor [227] with varied properties such as being non-toxic [228], highly efficient [229], cost-effective [230], highly reactive [231], and eco-friendly. It has been accepted globally as a photocatalyst because of its high pore size [232], notable band gap [233], and large surface area [234]. Nevertheless, it has certain drawbacks, of which the following are related: the rejoining of the photo-generated charge bearers, low adsorption range, and the ineffectual visible-light utilization which requires improvement for the intensified photocatalytic activity. Numerous researchers have been regulated to undermine the limitations related to the photocatalysis of titania; the incorporation of nanoparticles and the doping of metals and non-metals have helped in enhancing its process to a larger extent. The amalgamation of NPs has been of utmost importance lately because they produce the desired results by amending the particle shape and size along with the physicochemical properties of TiO₂. Moreover, doping can strengthen the photocatalytic property by minimizing the reposting of charge carriers and decreasing the band-shift towards the region of visible light.

The photooxidation of organic effluent in wastewater and the elimination of nitrates and sulfates, along with acidic, basic, VAT, and azo dyes, can be achieved by the non-metallic doping of TiO₂. Moreover, the sensitization of dyes can be achieved by the doping of non-metals with titania; it can help in the breakdown of pesticides, the industrial discharge of dyes which have toxic and malignant effects, and the generation of photocatalytic hydrogen [235].

It is important to understand the prospects and future aspects on improving the photocatalytic activity of TiO₂ by different means, either by doping or co-doping. The studies have shown that both manifest remarkable results. The subject of high noticeability here is that whose process has produced more beneficial and long-lasting outcomes. More studies focusing on the engineered edges via doping for the improvement of the conduction and valence bands are required for enhancement of the absorption band of titania. Furthermore, the movability of charge carriers should be increased by introducing an impurity to improve the working efficiency of TiO₂. Therefore, the use of photocatalysis along with some other technologies would improve its application, which would benefit the environment for a longer period.

Moreover, a gradual deactivation of the photocatalytic materials concerns all the potential industrial applications. A periodical regeneration of the photocatalytic materials would be required, which would also increase the overall cost. Therefore, cost would obviously remain an essential issue for commercialization. Hence, to overcome this obstacle, extensive research would be necessary to develop both economical reactors and photocatalytic materials. Concomitant, concerted, and extensive research progress to achieve these goals is necessary for the practical implementation of this technology.