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

Department of Chemistry, The University of Lahore, 1-Km Defence Road, Off Raiwind Road, Lahore 54000, Pakistan
Department of Chemical Engineering, COMSATS University Islamabad, Lahore Campus, Defence Road, Off Raiwind Road, Lahore 54000, Pakistan
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(11), 1331;
Submission received: 9 September 2022 / Revised: 24 October 2022 / Accepted: 26 October 2022 / Published: 31 October 2022


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). 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.
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). 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), 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.
Table 5. Summary of N-doped TiO2 photocatalysts synthesized by variety of methods and source precursor materials.
Year of StudyMethodTiO2 PrecursorNitrogen SourceRef.
2017Addition of N source to the TiO2 precursor solutionTBOTTetra methyl-ethylene-diamine[148]
2020CVDTICl4Tert-butylamine, benzyl amine[149]
2019HydrolysisTTIPNH4Cl, pyridine[151]
2016ElectrochemicalTitania nanotubesDiethylenetriamine, ethylenediamine, hydrazine[152]
2019Sol-gelTTIP, TBOT, TiCl4, Titanic acidUrea, 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 [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 O2- 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.
Table 7. Summary of P/TiO2 photocatalysts synthesized by variety of methods and source precursor materials.
Year of StudyMethodTiO2 PrecursorPhosphorous SourceRef.
2020Chemical vapor depositionTitanium (IV) butoxide (Ti(OC4H9)4)Red phosphorous[171]
2014Sol-gel methodTBOPhosphorous acid[172]
2011Sol-gel processSilicate/TiO2 NPsPhosphoric acid (H3PO4)[164]

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.
Table 8. Summary of B/TiO2 photocatalysts synthesized by variety of methods and source precursor materials.
Year of StudyMethodTiO2 PrecursorBoron SourceRef.
2020Sol-gel Titanium isopropoxideBoric acid [179]
2015Electrochemical anodization Titanium sheets NaBF4[178]
2011HT (NH4)2TiF6H3BO3[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 [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).

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 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.

Author Contributions

Conceptualization, P.A. and M.H.; methodology, A.A.; software, A.S.; validation, A.A.; A.S.; formal analysis, A.A.; investigation, P.A.; resources, M.H.; data curation, A.A.; writing—original draft preparation, P.A.; writing—review and editing, M.H.; visualization, P.A.; supervision, P.A.; project administration, P.A.; funding acquisition, M.H. All authors have read and agreed to the published version of the manuscript.


This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.


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Figure 1. Major classes of dyes.
Figure 1. Major classes of dyes.
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Scheme 1. Reaction scheme for the degradation of dyes using TiO2 [90].
Scheme 1. Reaction scheme for the degradation of dyes using TiO2 [90].
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Figure 2. The process of dye-degradation mechanisms by photocatalysis, depicted graphically [91].
Figure 2. The process of dye-degradation mechanisms by photocatalysis, depicted graphically [91].
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Figure 3. Different methodologies adopted for the removal of dyes.
Figure 3. Different methodologies adopted for the removal of dyes.
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Figure 4. Photocatalysis mechanism of C-doped TiO2 nanorods under visible light, adopted from [56].
Figure 4. Photocatalysis mechanism of C-doped TiO2 nanorods under visible light, adopted from [56].
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Figure 5. Photocatalytic mechanism of C-TiO2/rGO adopted from [127].
Figure 5. Photocatalytic mechanism of C-TiO2/rGO adopted from [127].
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Figure 6. N-doped TiO2 photocatalytic process driven under the sun is schematically represented [154].
Figure 6. N-doped TiO2 photocatalytic process driven under the sun is schematically represented [154].
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Figure 7. N-doped TiO2 photocatalytic [157].
Figure 7. N-doped TiO2 photocatalytic [157].
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Figure 8. (a) Response of different concentrations of sulfur in S-doped TiO2; (b) photocatalytic degradation of 4-chlorophenol by pure TiO2, SdT prepared by ThU and SdT by hydrothermal process studied via UV-vis [160].
Figure 8. (a) Response of different concentrations of sulfur in S-doped TiO2; (b) photocatalytic degradation of 4-chlorophenol by pure TiO2, SdT prepared by ThU and SdT by hydrothermal process studied via UV-vis [160].
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Figure 9. The photocatalytic degradation of phenol using SdST-0.5 at various wavelength irradiation [161].
Figure 9. The photocatalytic degradation of phenol using SdST-0.5 at various wavelength irradiation [161].
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Figure 10. Photocatalytic mechanism of P-doped Fe2O3-TiO2 against SMHZ [171].
Figure 10. Photocatalytic mechanism of P-doped Fe2O3-TiO2 against SMHZ [171].
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Figure 11. (a) Photocatalytic degradation of 4-nitrophenol using B/TiO2 and undoped TiO2. (b) First-order plot of photocatalytic degradation of 4-nitrophenol using UV-visible-light irradiation in the presence of BdT and undoped TiO2 [171].
Figure 11. (a) Photocatalytic degradation of 4-nitrophenol using B/TiO2 and undoped TiO2. (b) First-order plot of photocatalytic degradation of 4-nitrophenol using UV-visible-light irradiation in the presence of BdT and undoped TiO2 [171].
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Figure 12. Suggested photocatalytic degradation for H2 production and dye degradation [180].
Figure 12. Suggested photocatalytic degradation for H2 production and dye degradation [180].
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Scheme 2. Reaction scheme for the degradation of RhB dye using I/TiO2.
Scheme 2. Reaction scheme for the degradation of RhB dye using I/TiO2.
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Table 1. Summary of non-metal-doped TiO2 photocatalyst under varied circumstances.
Table 1. Summary of non-metal-doped TiO2 photocatalyst under varied circumstances.
Year of StudyType of Non-Metal
Synthesis Route/MethodType of DyeCharacterization TechniquesRef.
2017C-TiO2HydrothermalMethylene blue,
Rhodamine B, p nitrophenol
XRD, SEM, TEM, STEM, XPS, UV-vis[56]
2019C-TiO2HydrothermalMethylene blueXRD, FTIR, N2 adsorption-desorption isotherm, SEM, UV-vis[57]
2020C-TiO2Sol-gelMethylene blueEDX, UV-vis DRS analysis, SEM[58]
2020C-TiO2 double-layer hollow microsphereHydrolysis of thermal expandable microsphereRhodamine BXRD, FTIR, TGA, SEM, Raman N2 adsorption-desorption isotherm, XPS, UV-vis[59]
2021Carbon-doped TiO2 nanoparticlesSol-gelMethylene blueXRD, TEM, XPS,
2022C-TiO2 nanoflakes
Facile hydrothermalMethylene blueXRD, FTIR, SEM, UV-vis[40]
2007N-TiO2Microemulsion-hydrothermalRhodamine BXRD, Raman, XPS, PL emission spectra[61]
2010N-TiO2Sol-gel/acidic mediaLindaneXRD, SEM, TEM, Raman, XPS, GC-MS[62]
2015N and C-co-doped porous TiO2 nanofibersElectrospinning and calcinationMethylene blueXRD, FESEM, TEM, XPS, DRS,[63]
2017N-TiO2SolvothermalRhodamine BXRD, SEM, TEM, BET, XPS, UV-vis[64]
2020TiN/N-doped TiO2
Sputtering processMethylene blueRaman, XPS, UV-vis[65]
2020C-N-TiO2 composite fibersHydrolysis and calcinationRhodamine BXRD, SEM, TEM, FTIR, Raman, XPS, UV-vis[66]
2021N-TiO2 nanotubesHydrothermalMethyl orangeXRD, SEM, XPS, UV-vis[67]
2009S-TiO2HydrothermalMethylene orangeXRD, TEM[68]
2015S-TiO2Wet-impregnation method.Humic acid
Humic acid
EDX, SEM, EEM fluorescence[69]
2016(S–TiO2), (N–S–TiO2)Sol-gelPhenol and MBBET, FESEM, FTIR, XPS, DRS[70]
2011S-TiO2, N-S-TiO2Sol-gelMethyl orangeXRD, TEM, UV-vis DRS[71]
2021NS/TiO2Sol-gelMethylene blue, methyl redXRD, BET, SEM, FTIR, Raman, UV-vis[21]
P/TiO2Hydrothermal/sol-gel [72]
Ag-P/TiO2 nanofibersOne-pot electrospinningMethylene blueXRD, XPS, FE-SEM, TEM, UV-vis[73]
2022P/TiO2/MWCNTsSol-gelMethylene blueXRD, FE-SEM, FTIR, UV-vis[74]
2017Si/TiO2SolvothermalMethyl orangeXRD, SEM, EDS, BET, XPS[75]
Table 2. Types and characteristics of Azo dyes [76].
Table 2. Types and characteristics of Azo dyes [76].
Category of DyeFeaturesFiberPollutantDyes Fixation
AcidicWater-soluble anionic compoundsWool, nylon, cotton blends, acrylic, and protein fibersOrganic acids, unfixed dyes, color80–93
BasicWater-soluble, applied in weakly acidic dye baths, very bright dyesAcrylic, cationic, polyester, nylon, cellulosic, and protein fibersNA97–98
DirectWater-soluble, anionic compounds, applied without mordantCotton, rayon, and other cellulosic fibersSurfactant, defoamer, leveling and retarding agents, finish, diluents70–95
DispersiveInsoluble in waterPolyester, acetate, modacrylic, nylon, polyester, triacetate, and olefin fibersPhosphates, defoamer, lubricants, dispersants, diluents80–92
ValOldest dyes, chemically complex, water-insolubleCotton, wool, and other cellulosic fibersAlkali, oxidizing agents, reducing agents, color60–70
Table 3. Illustration of various methods for removal of organic dyes.
Table 3. Illustration of various methods for removal of organic dyes.
ChemicalElectro-Fenton reagent
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
PhysicalIon exchange
Good removal of wide variety of dyes
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
Aerobic and
anaerobic degradation
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
Table 4. Methods and precursors used for the synthesis of C-doped TiO2 photocatalysts.
Table 4. Methods and precursors used for the synthesis of C-doped TiO2 photocatalysts.
MethodsTiO2 PrecursorCarbon SourceReferences
Chemical bath deposition (CBD)Titanium isopropoxide (TTIP)Melamine borate[122]
Sol-gelTitanium isopropoxide (TTIP)Microcrystalline cellulose (MCC)[58]
Sol-gelTTIP, TBOT, TiCl4, TiCl3Ethanolamine (ETA), glycine, polyacrylonitrile (PAN), polystyrene (PS), starch, TBOT[124,125]
Solvothermal treatment and calcinationsTiCl4Alcohols (benzyl alcohol and anhydrous ethanol)[126]
Electrospinning followed by heat treatmentTTIPAcetic acid[128]
Sol-gelTitanium butoxide-[30]
Hydrothermal route-Various carbon sources[42]
Table 6. Summary of S-doped TiO2 photocatalysts synthesized by variety of methods and source precursor materials.
Table 6. Summary of S-doped TiO2 photocatalysts synthesized by variety of methods and source precursor materials.
Year of StudyMethodTiO2 PrecursorSulfur SourceRef.
2003Oxidative heatingAnataseTiS2[159]
2006Low-temp hydrothermalAnataseTiS2 powder with HCl solution[160]
2012SolvothermalTBOTPotassium per sulfate[161]
2016Free oxidant peroxide methodAnataseThiourea (ThU)[162]
2018HTTitanium sulfate (TiOSO4)TiOSO4[163]
Table 9. Summary of halogens-doped TiO2 photocatalysts synthesized by variety of methods and source precursor materials.
Table 9. Summary of halogens-doped TiO2 photocatalysts synthesized by variety of methods and source precursor materials.
Year of StudyMethodTiO2 PrecursorFluorine SourceRef.
2020Oxidative annealingTitanium isopropoxideNH4F[181]
2017Sol-gelTitanium isopropoxideTrifluroacetic acid[182]
2014Sol-gelTetrabutyl titanateNH4F[183]
Year of StudyMethodTiO2 PrecursorChloride SourceRef.
2020Oxidative annealingTitanium isopropoxideNH4Cl[181]
2012Sonochemical synthesisTetraisopropyl titanateNaCl[184]
2008HydrolysisTetrabutyl titanateHCl[185]
Year of StudyMethodTiO2 PrecursorBromide SourceRef.
2009Sol-gelTBOTCetyl trimethyl Ammonium bromide (CTAB)[187]
2004HTTitanium chlorideHydrobromic acid[188]
Year of StudyMethodTiO2 PrecursorIodide SourceRef.
2017Sol-gelTitanium (IV) ter-butoxidePotassium iodide[189,190]
Table 10. Summary of Si/TiO2 photocatalysts synthesized by variety of methods and source precursor materials.
Table 10. Summary of Si/TiO2 photocatalysts synthesized by variety of methods and source precursor materials.
Year of StudyMethodTiO2 PrecursorSilicon SourceRef.
2019HydrothermalCommercial TiO2SiO2 commercial[191]
2017SolvothermalTitanium oxysulfateTetraethoxysilane[75]
Table 11. Different types of dyes under optimum pH.
Table 11. Different types of dyes under optimum pH.
Dye TypeLight SourcePhotocatalystpH RangeOptimum pHRef.
Orange G (OG)UVSn/TiO2/Ac1.0–12.02.0[196]
Bromo-cresol purple (BCP)UVTiO24.5 & 8.04.5[198,199]
Methyl Red (MR)Visible3%Ag+1.5%Ni-TiO23–104[199]
Malachite Green (MC)Sun lightNi/MgFe2O42.0–10.04[200]
Indigo Carmine (IC)UVTiO24.0–11.04[201]
Textile dye (TD)UVTiO23.0–7.05[202]
Basic Yellow 28 (BY28)UVTiO23.0–9.05[203]
Methylene Blue (MB)UVTiO2ZnO1.0–6.02[204]
Reactive Blue 4 (RB4)UVAnatase TiO23.0–13.03–7[205]
Procion Yellow
Acid Orange (AO)UVWO3-TiO21.0–9.03[84]
Methyl Orange (MO)UVTiO22.0–10.08[207]
Rhodamine B (RhB)UVZnO2.0–12.012[208]
MO, RhBUVZnO2.0–10.0Basic medium[209]
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Akhter, P.; Arshad, A.; Saleem, A.; Hussain, M. Recent Development in Non-Metal-Doped Titanium Dioxide Photocatalysts for Different Dyes Degradation and the Study of Their Strategic Factors: A Review. Catalysts 2022, 12, 1331.

AMA Style

Akhter P, Arshad A, Saleem A, Hussain M. Recent Development in Non-Metal-Doped Titanium Dioxide Photocatalysts for Different Dyes Degradation and the Study of Their Strategic Factors: A Review. Catalysts. 2022; 12(11):1331.

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Akhter, Parveen, Abdullah Arshad, Aimon Saleem, and Murid Hussain. 2022. "Recent Development in Non-Metal-Doped Titanium Dioxide Photocatalysts for Different Dyes Degradation and the Study of Their Strategic Factors: A Review" Catalysts 12, no. 11: 1331.

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