A Review on Metal Ions Modified TiO2 for Photocatalytic Degradation of Organic Pollutants

TiO2 is a semiconductor material with high chemical stability and low toxicity. It is widely used in the fields of catalysis, sensing, hydrogen production, optics and optoelectronics. However, TiO2 photocatalyst is sensitive to ultraviolet (UV) light; this is why its photocatalytic activity and quantum efficiency are reduced. To enhance the photocatalytic efficiency in the visible light range as well as to increase the number of the active sites on the crystal surface or inhibit the recombination rate of photogenerated electron–hole pairs electrons, various metal ions were used to modify TiO2. This review paper comprehensively summarizes the latest progress on the modification of TiO2 photocatalyst by a variety of metal ions. Lastly, the future prospects of the modification of TiO2 as a photocatalyst are proposed.


Introduction
In the past, industrial development has been accompanied by the discharge of a large number of organic pollutants, which has caused great harm to the environment and living beings in general [1,2]. Research on the purification of wastewater is currently available and ongoing. Various traditional methods including physical adsorption, biodegradation, catalytic oxidation, and high-temperature incineration have been proposed [3]. Table 1 presents the traditional treatment methods and their limitations. These treatment methods have some shortcomings, and the treatment effect on organic wastewater is not very ideal. Recent studies have been devoted to a promising approach, the advanced oxidation processes (AOP) for the degradation of organic pollutants from wastewaters [3][4][5][6][7][8][9][10], due to its ability to completely mineralize the targeted pollutants [11]. There are various types of AOPs including ozone (photo-ozonation, ozonation, O 3 +Fe 2+ /Fe 3+ , and ozonation + catalyst and O 3 +H 2 O 2 ), photolysis (VUV or UV), hydrogen peroxide (photo-Fenton: H 2 O 2 +Fe 2+ /Fe 3+ +UV, Fenton-like reagents: H 2 O 2 +Fe 2+ -solid/Fe 3+ -solid, Fenton: H 2 O 2 +Fe 2+ /Fe 3+ , and H 2 O 2 +UV), and photocatalysis [12,13].
The ordinary chemical oxidation process has limitations in degrading some harmful substances because of its poor oxidation ability. The intermediate products in the advanced oxidation process can react with hydroxyl radicals to oxidize harmful substances into carbon dioxide and water. AOPs are a chemical purification treatment used to remove inorganic/organic materials in wastewater and water via oxidation through reactions with hydroxyl radicals (·OH). AOPs can be categorized as heterogeneous systems (semiconductor photocatalytic process) and homogeneous systems (Fenton and photo-Fenton systems, Table 1. Traditional photodegradation methods and limitations.

Approach Limitations
Biological treatment Chlorinated phenols are resistant to biodegradation and can accumulate in sediments. They transfer the contaminants from one medium to another and require disposal or further treatment The biodegradation of organic pollutants such as 4-CP is slow and incomplete, and its byproducts are more toxic compared to other pollutants.
Biological processes usually require considerable processing time to decompose 4-CP. By use of the biological treatment, it cannot be degraded due to a large number of aromatic structures in the dye molecules and the stability of modern dyes. Azo bond is reduced to form a colorless but toxic and potentially carcinogenic aromatic amine.
Adsorption technology and activated carbon adsorption method The post-treatment of wastewater and regeneration of adsorbent materials requires an expensive operation. Activated carbon adsorption requires the safe disposal of carbon. During the adsorption process, the system cannot tolerate the suspended solids in the influent water as a result of blockage. The operating cost is high due to the requirements of the carbon system. The treatment may be problematic if the polluted carbon is not regenerated.

Chemical precipitation
Requirements for a large amount of chemicals and a large amount of sludge produced. Requires further treatment or disposal. Due to the large amount of sludge needing to be treated, the method is not feasible economically.

Air stripping
It is susceptible to pollution. Aesthetic limitations due to tower height. Challenges involving mechanical reliability.

Membrane adsorption
The purchasing cost of membranes and residues (very concentrated filtrate) is high and must be collected or may require further processing. The physical method is not destructive, but only transfers pollutants to other media, causing secondary pollution.

TiO 2 Photocatalysis
Photocatalysis is a general term for a photoinduced reaction that is accelerated in the presence of a catalyst. Heterogeneous photocatalysis is the most widely used and effective process for the degradation of organic pollutants and does not produce harmful intermediates at ambient temperature and pressure [17,18]. This process starts with electron excitation, which is transferred from the valence band (VB) to the empty conduction band (CB). In the presence of light, the catalyst absorbs adequate energy to become excited from the light which is equal to its energy bandgap. The excited electrons move toward the conduction band via the holes acting as a positive charge in the valence band, owing to which photogenerated species, including e − /h + pair, are created in the photocatalytic system. The generated species react with -OH groups or oxygen molecules to further produce reactive oxygen species such as superoxide anion radicals (O 2 • − ) and hydroxyl radicals (•OH). Thereafter, these reactive oxygen species attack the organic molecules, decomposing them via oxidation. Finally, the e-/h + pairs are formed on the photoexcited catalyst's surface [19,20]. These materials are generally N-type semiconductors with discontinuous band structures different from metals, usually composed of low-valence bands full of electrons. Common semiconductor photocatalysts include ZnO, TiO 2 , Bi 20 Ti 20 , Bi 2 WO 6 , Nb 2 O 5 , Fe 2 O 3 , SrTiO 3 , BiTiO 3 , CuS/ZnS, ZnWO 4 , WO 3 , ZnS, Ag 2 CO 3 , and SnO 2 , etc., but most photocatalysts have an impact on the photocatalytic performance due to the photocorrosion phenomenon. Among the aforementioned photocatalysts, Titanium dioxide (TiO 2 ) has been widely studied by researchers due to its low cost, high efficiency, and stability. The disadvantage of TiO 2 is that it cannot be activated by visible irradiation, but by UV. Its advantages over other semiconductors include being biologically and chemically inert, relatively easy to use and produce, photo-catalytically stable, relatively cheap, and able to efficiently catalyze reactions, with no risk to humans or the environment [11]. TiO 2 exists in three polymorphs such as rutile, anatase, and brookite. Both rutile and anatase have tetragonal crystal lattices while brookite appears as orthorhombic crystal lattices [21]. Rutile form is more stable than the other polymorphs among these three phases. TiO 2 has applications in various products including sunscreen lotions, capacitors, paint pigments, toothpaste, food coloring agents, solar cells, and electrochemical electrodes [22].
In 1967, Kenichi Honda and Akira Fujishima jointly discovered that light irradiating a titanium dioxide electrode can carry out the electrolysis of water. The ionized water or oxygen was converted into photo-living groups with oxidizing ability. Their energy was equivalent to a high temperature of 3600 K and is highly oxidizing. In 1972, Fujishima et al. first proposed the theory of photocatalytic water splitting on TiO 2 electrodes, and then in 1973, they proposed the use of titanium dioxide photocatalyst for environmental remediation, thus promoting the development of photocatalytic technology [23]. In 1995, Fujishima et al. [24] discovered that a film containing a certain amount of TiO 2 was super-hydrophilic under ultraviolet light, which promoted the derivation of the field of photocatalyst and film binding. This technology utilizes some special photocatalyst powder into sewage, which could decompose toxic metal substances in the water to obtain pure water under the irradiation with ultraviolet rays. This technology was also used to remediate polluted rivers and found harmless to the environment. Since then, researchers have found ways to modify the TiO 2 photocatalyst in a large amount. By 2004, Sonawane reported that Fe-TiO 2 film could degrade methyl orange solution within 4 h under sunlight, with a degradation efficiency (95%) [25].
The bandgap of pure nano-titanium dioxide powder is about 3.2 eV, which can only absorb ultraviolet light below 400 nm. In the natural environment, ultraviolet light is less than 10%, so pure nano-titanium dioxide does not have the function of a photocatalyst. This greatly affects the utilization rate of the catalyst to sunlight. Other key factors that limit the practical application of photocatalysts are the low quantum yield of the catalyst and the faster recombination of electrons and photogenerated holes. Generally, the recombination efficiency of photogenerated carriers determines the quantum quality of the photocatalyst, which affects carrier recombination. The main factors of the surface charge transfer process are the surface morphology, grain size, crystal phase structure, and surface lattice defects of the catalyst, as well as the intensity of light radiation.
To make the TiO 2 photocatalyst absorb visible light and far-infrared light and improve the photocatalytic efficiency, the photocatalyst needs to be modified. At present, researchers have proposed two ways to enhance the photocatalytic performance of TiO 2 : one is to prevent photogenerated electrons from recombining with photogenerated holes so that they can effectively participate in the catalytic degradation reaction process; the other is to introduce other elements into TiO 2 lattice, thereby reducing the bandgap energy of the catalyst and expanding its photoresponse range. Generally, the activated CB electrons and VB holes recombine into a neutral body to generate energy, which is lost in the form of light energy or heat. Therefore, the photogenerated electrons can be captured by the catalyst or the mobility of the surface charge of the catalyst can be improved. To slow down the recombination of hole-electron pairs, modification of TiO 2 lattice with metal ions is an effective way to improve photocatalytic performance. In the subsequent sections, recent studies on metal ion modified TiO 2 (with morphologies such as nanoparticles, microspheres, nanofibers, nanocrystals, nanosheets, nanotubes, nanopowders) were summarized and reported.

Mechanisms of TiO 2 Photocatalysts for Organic Pollutants
The mechanism of the photocatalytic reaction in the presence of TiO 2 photocatalysts consists of a free radical reaction initiated by light irradiation (photons) [26]. When the energy of solar radiation exceeds the bandgap of TiO 2 (i.e., photon energy reaches or exceeds its bandgap energy), the surface of the photocatalyst becomes excited, and the electrons transit from the valence band (VB) to the conduction band (CB). In the CB, corresponding electron holes are derived in the VB at the same time, forming electron-hole pairs (i.e., generating electron (e − ) and hole (H + ) pairs). VB holes have strong oxidation reaction activity (1.0~3.5 V) because they lose electrons and act as reducing agents, and electrons in the conduction band have good reducibility 0.5~1.5 V when they undergo reduction. Under light irradiation, positive holes and electrons are generated in the VB (hv + vb ) and CB (e − cb ) of TiO 2 as presented in Equation (1) [27]. These holes can either form hydroxyl radicals (Equation (3)) or react directly with organic molecules (Equation (5)) which subsequently oxidize the organic molecules (Equation (6)) [28]. The electrons can also react with organic compounds to produce reduction products (Equations (1)- (7)). The role of oxygen is important as it reacts with the photogenerated electrons. Organic compounds can then undergo oxidative degradation through their reactions with hydroxyl and peroxide radicals, VB holes, as well as reductive cleavage via reactions with electrons yielding various byproducts and finally mineral end-products [29].
In the presence of light irradiation using metal ion modified TiO 2 and because the metal ions create intermediate states in the TiO 2 structure, the surface electrons in the intermediate states become excited, having sufficient energy to access more light absorption and transfer electrons to the TiO 2 surface that stimulate more electrons under light, which as a consequence advances the redox reactions [31][32][33]. These properties being displayed by the metal ions help to improve the photocatalytic reactions. The transfer of electrons takes place at the VB to the CB via the creation of holes in the former and these holes subsequently react with H 2 O molecules present in the pollutant as well as helping to form the OH radicals [31][32][33][34]. The formed OH radicals are supportive and found significant success in degrading various dyes under light irradiation [35][36][37]. The photocatalytic mechanism of metal-ion modified TiO 2 for organic pollutants is illustrated in Figure 1.

Modification with Metal Ions
Modification with metal-ion can act as electron/hole traps and alter e − /h + recombination rate, according to the below mechanisms [39]: where energy level for M n+ /M (n−1)+ lies below the CB edge (e cb ) and the energy level for M n+ /M (n+1)+ above the VB edge (e vb ). The presence of metal ions allows visible light absorption and introduces trap/recombination sites within the TiO 2 bands which may as a result increase the life span of photoinduced charge carriers as well as the reduction in quantum efficiency. Modifications with metal ions can influence TiO 2 photocatalysis according to these principles: (i) improve the electron-hole separation (strength) by selective trapping [40,41]; (ii) due to their ability to act as recombination centers, they reduce carriers' lifespan (weakness) [42,43], and (iii) enhance optical absorption in visible light range. The commonly used metal ions are the metals, transition metal, rare earth metal, or noble metal [44]. Modification of TiO 2 by metal ions is generally carried out via sputtering [45], ion implantation [46], or via chemical processes (for example sol-gel) [47][48][49][50]. Figure 2 presents a table of commonly used metallic ions to enhance the photocatalytic degradation of organic pollutants.
In general, various types of metals (Transition, rare earth, or other metals) have been used to modify TiO 2 photocatalysts. Table 2 presents the general properties and principal applications of the metals. In the subsequent sections, the modifications using metal ions TiO 2 photocatalysts are described and discussed. The following section starts with the modification using transition metal, which is subsequently followed by lanthanides or rare earth metal modifications. At the end of this section, the modifications of other metals were presented.  Table 2. General properties and principal applications of the metals.

Transition metals
They have incompletely filled d orbitals. The transition metals are more electronegative than the other metals and form stable compounds with neutral molecules (such as water or ammonia). The main advantages of these metals are malleability and ductility. Examples of transition metals used to modify TiO 2 photocatalysts include vanadium (V), nickel (Ni), copper (Cu), manganese (Mn), zirconium (Zr), iron (Fe), chromium (Cr), molybdenum (Mo), cobalt ( where M is the metal. The energy necessary for reducing the metal ion should be less negative than the conduction band edge of titania while its oxidation energy should be less positive than the valence band edge of TiO 2 . Additionally, photocatalytic reactions can occur only if the trapped holes and electrons are transferred to the catalyst's surface. Therefore, metal ions should be situated near the surface of TiO 2 crystallites for a better charge transfer [55]. If the amount of metal ions is more than optimal, the metal ions tend to behave as recombination centers and the transfer of h + /e − pairs to the interface is more complex. Generally, the metal ions are precipitated on the surface as oxides forming a MO x /TiO 2 nanocluster and not incorporated in the structure of TiO 2 . If the energetic positions of CB and VB of components forming the composite are suitable (i.e., a type II heterojunction), beneficial charge transfer occurs within the photocatalyst where e − is accumulated in the CB edge of one component and h + is accumulated in the VB edge of the other component. Therefore, the charge carriers are separated efficiently, leading to improved photocatalytic property [56,57].

Modification with Vanadium (V)
Vanadium exists in several oxidation states (V 2+ to V 5+ ) and a variety of species. The types of species and oxidation state are a function of the redox potential, pH, concentration, and other factors [58]. The radius of the V ion is nearly identical to that of titanium (Ti) and can be conveniently introduced into titania [59]. V ions with different valence states (V 3+ to V 5+ ) can transfer between V 3+ to V 5+ under oxidizing and reducing conditions. For example, the tetragonal crystal structure of VO 2 is similar to that of titania, which is responsible for an increase in visible light absorption and photogenerated holes and electrons [60]. The photogenerated holes and electrons can be migrated, trapped, and released on the TiO 2 surface by V 4+ ions, and play a role in charge transfer species. It is very difficult for V 5+ ions to enter the lattice of TiO 2 , making it appropriate to form V 2 O 5 on the surface and thus responsible for h + and e − separation [61]. Subsequently, this photogenerated e − and h + can be accepted by the adsorbed O 2 and surface hydroxyl group (OH − ), and therefore transform OH − and O 2 into hydroxyl radical (•OH) and superoxide radicals (•O 2 − ) active species, respectively. Vanadium-TiO 2 (V-TiO 2 ) at low concentrations has been found to improve TiO 2 photocatalytic behavior via the existence of the photogenerated charge and the more efficient expansion of absorption spectrum as compared to other metals [62,63]. V-TiO 2 has an enhanced photocatalytic activity due to the following reasons: (i) improving the absorption in the visible light range, (ii) improved quantum efficiency due to effective h + to e − pair separation, and (iii) presence of both V 5+ and V 4+ species in the V-TiO 2 . Additionally, the second reason can contribute to the increased electron transfer and visible light absorption, whereas the latter enhances e − -h + separation and is a potential electron acceptor [64][65][66]. V-TiO 2 have been extensively studied in experiments and theory due to evidence of ferromagnetism above room temperature or at room temperature [67,68]. Vanadium with a narrow bandgap can be coupled with TiO 2 by various approaches such as metal-ion implantation, sol-gel, hydrothermal, and coprecipitation approach. Generally, the sol-gel method is frequently used for depositing the V-TiO 2 films while the coprecipitation approach is used to prepare V-TiO 2 powders. Among these approaches, (compared with the hydrothermal approach), the sol-gel and coprecipitation approach is quite harsh and complicated; however, the hydrothermal approach is easy to execute and involves low cost [69,70]. Table 3 presents the summary of recent progress on V-TiO 2 photocatalysts for organic pollutants degradation. Nickel (Ni) has good activity and it is less expensive than noble metals [78]. This element is used in many applications due to its physicochemical properties. Ni is a good candidate for substituting Ti atoms in the TiO 2 structure [79]. The incorporation of Ni ions into the TiO 2 lattice can actively modify the TiO 2 physical properties via the creation of an impurity energy level. Upon modifying TiO 2 with Ni, the recombination of photogenerated hole-electron pairs is suppressed effectively, thus resulting in an enhanced photocatalytic activity [79][80][81]. The structure of the Ni 2+ valence band is 3d8. When the Ni ions trap the photogenerated hole-electron pairs, the valence layer (d) is converted from a high to low spin state, and thus results in a significant spin energy loss. Based on the crystal field theory, charge carriers that are being trapped by Ni ions tend to migrate to the H 2 O molecules (adsorbed on the surface) to restore their energy, and as a consequence will prevent the recombination of hole-electron pairs [82]. Furthermore, Ni 2+ plays an important role in the improvement of thermal stability as well as controlling the morphology of TiO 2 photocatalysts. Table 4 presents the summary of recent progress on Ni-TiO 2 photocatalysts for organic pollutants degradation.

Modification with Copper (Cu)
The introduction of Cu ion can directly trap generated electrons by excitation of light, and thus prevents the rapid recombination of hole-electron pairs [95]. It can increase the surface diffusivity of the TiO 2 [96], and thereby enables the interaction of holes and electrons with other compounds (e.g., H 2 O) more quickly [97], generating feasible reactive species, and subsequently bypassing the recombination of the hole-electron pair [98]. Cu with redox potentials of 0.16 V (Cu 2+ /Cu + ) and 0.52 V (Cu 2+ /Cu) has been used as a suitable modifier for various visible light responsive photocatalysts. Ti 4+ and Cu 2+ have similar ionic radii and therefore Cu 2+ can easily penetrate into TiO 2 matrixes as a deep acceptor in conjunction with neighboring oxygen vacancies or substitute the positions of Ti 4+ [99]. In addition, modification with Cu shifts the absorption edges of both the photocatalysts towards the visible region [100].
The behavior involved in modifying TiO 2 with Cu is strongly linked to parameters of synthesis and diverse approaches used for fabrication of materials as well as analysis, and has led to various experimental findings in the past. Studies have observed the diverse speciation of Cu-TiO 2 with the majority of studies reporting the presence of Cu in a Cu 2+ valence. These species are frequently reported to substitute for Ti 4+ to give a solution phase with the composition Cu x O 2 Ti 1-x and an increased lattice density of oxygen vacancies [101], and reportedly may also occupy interstices in anatase [102]. In addition to solid solutions, Cu 2+ has been found to exist in amorphous or crystalline CuO nanoclusters as well as surface localized Cu(OH) 2 in titania [103][104][105]. The presence of monovalent Cu + (less common as compared to Cu 2+ ) has also been reported, both in Cu 2 O nanoclusters and in substitutional positions [102,106]. The CB edges of CuO 2 and CuO are suitable for the enhancement of TiO 2 photocatalytical effect [107][108][109]. Generally, an efficient photocatalytic activity is understood to be driven by a reduced charge carrier recombination in Cu-TiO 2 photocatalyst. This was found to arise as a result of photogenerated electrons which facilitates the reduction of Cu 2+ + e − → Cu + , and thus extends the valence band hole lifetimes at the surfaces, which are able to react with adsorbed species to form active radical species [103,104]. Alternatively, the presence of Cu 2 O or CuO phase in TiO 2 may improve exciton lifetime through electron capture in the secondary phase [110] or improve the activity via the increment in surface area [111]. Table 5 presents the summary of recent progress on Cu-TiO 2 photocatalysts for organic pollutants degradation.

Modification with Manganese (Mn)
Manganese (Mn) has a great potential to permit significant optical absorption in infrared solar light or visible light, via the introduction of intermediate bands within the forbidden gap as well as the combined effects of narrowed bandgap [129,130]. The intermediate bands provide adequate carrier mobility and hence significant curvature [131]. Mn ions can be easily incorporated into the lattice of TiO 2 to obtain a deformed structure. It has been reported that Mn in 3d states has some contributions to the TiO 2 conduction band, which will further impact the bandgap of TiO 2 . Modifying with Mn cations has been reported to favor the charge separation acting like electrons trap as Mn 4+ or Mn 3+ [129] and holes trap as Mn 2+ [132,133], thus prolonging the separation of photoinduced carriers and increasing the photocatalytic activity [134].
In Mn-photocatalytic materials with a mixture of oxidation states could be seen as a limitation because it is impossible to easily control the amount of one specific oxidation state [135]. However, this can be approached as a benefit since the oxidation state of Mn can act as a charge separator, attracting photogenerated holes in the Mn 2+ /Mn 3+ state or as a photogenerated electron in the Mn 4+ /Mn 3+ state, the oxidation states mixture can improve the photocatalytic activity [133]. When TiO 2 semiconductors are modified with Mn and applied in the photocatalytic degradation of organic dyes, the reduction in bandgap can be achieved, thereby improving the performance [136][137][138]. This is due to the following reasons: when Mn is introduced in the TiO 2 network at different oxidation states (2+, 3+, or 4+), there is a deformation of the TiO 2 crystalline structure and optimization of its optical properties [139][140][141]. Ref. [142] confirmed a high stability feature when TiO 2 was modified with Mn 1+ to Mn 6+ . Mn 2+ can enter the lattice of TiO 2 , replacing the position of the original titanium atom while forming a new chemical bond. In addition, modifying with Mn 2+ can also increase the number of hydroxyl groups on the surface of TiO 2 in the aqueous solution, forming an active center for the adsorption of water [143].

Modification with Zirconium (Zr)
Zirconium (Zr) has a wide bandgap with a more positive (4.0 V vs. NHE) VB and a more negative (−1.0 V vs. NHE) CB than titania [155]. The tetravalent cations of both Ti and Zr have a comparable atomic radius, and their oxides have similar physicochemical properties. It has been previously reported that the modification of TiO 2 with Zr can improve physicochemical characteristics including an increase in the specific surface area [156], decrease in crystallite size as a result of dissimilar coordination geometry and nuclei [157]; enhance surface acidity [158], the transition temperature between anatase and rutile [156,158], and the adsorption and the hydrophilic properties [159], and enhance thermal stability and imbalance of charge resulting from the formation of capture traps as well as the formation of Ti-O-Zr bonds. These features favor a higher efficiency while separating photogenerated carriers, thus improving the photocatalytic efficiency [160,161]. Moreover, the hydroxyl groups that are trapped on the catalyst surface by holes improve the quantum yield and reduce the recombination reactions [159,162]. Modifying with Zr conquers charge recombination by electron trapping [163].
Liu et al. [164] prepared mesoporous Zr-TiO 2 NPs via the solvothermal approach and observed a higher photocatalytic activity in comparison with the commercialized P25 titania. Li et al. [165] reported a macro-mesoporous Zr-TiO 2 material via the facile surfactant self-assembly approach and observed a remarkable photocatalytic activity towards RhB decomposition under UV light radiation as compared to pure ZrO 2 and unmodified TiO 2 materials. A study by [166] found that modifying with Zr 4+ can increase the surface area, and suppress crystal growth. Table 7 presents the summary of recent progress on Zr-TiO 2 photocatalysts for organic pollutants degradation.   [174,175], and it has been documented that modifying the crystal lattice of TiO 2 with Fe 3+ ions weakens the surrounding oxygen atoms bonding. Consequently, the oxygen atoms are readily released from the lattice causing an oxygen vacancy, hence more oxygen/H 2 O/OH-can be adsorbed onto the TiO 2 surface resulting in the decrease in CB electrons. In addition, Fe 3+ ions can perform as a shallow charge trap in the TiO 2 lattice, because the energy level (redox potential) of Fe 2+ /Fe 3+ lies close to that of Ti 3+ /Ti 4+ , which favors the development of charge carrier separation, further causing a decrease in the e − /h + pair recombination and expanding the photoresponse of TiO 2 into the visible light range [175,176].
Ali et al. [174] used the sol-gel method for the successful synthesis of Fe-TiO 2 NPs which could be activated in the presence of visible light. Naghibi et al. [177] also studied the properties of TiO 2 modified with four metals including Cu, Fe, Cd, and Ce, and observed that the Fe-TiO 2 presents the smallest crystal size with forbidden bandwidth among these four metals-modified TiO 2 . [178] found that the Fe 3+ -TiO 2 powder showed strong absorption characteristics in the visible region and [179] observed that introducing Fe 3+ into the TiO 2 lattice facilitates the formation of a redox site, enabling absorption of the visible portion. The semi-filled electronic structure of Fe 3+ ions not only facilitates the separation of photoelectrons and holes but also reduces the bandgap of TiO 2 [180]. In addition, modifying with Fe 3+ can introduce more oxygen vacancies in TiO 2 surface and lattice [181], which is beneficial for increasing the number of surface OH groups and improvement in photocatalytic degradation. Table 8 presents the summary of recent progress on Fe-TiO 2 photocatalysts for organic pollutants degradation.     Chromium (Cr) has received many considerations owing to its partially filled d shell, and optically active nature. Cr 3+ is suitable with an abundant electron shell structure [78]. Cr 3+ ions have fewer valence electrons in comparison with Ti 4+ . TiO 2 crystalline lattice deformation is evoked by substitution of Ti 4+ by Cr 3+ , creating an extra energy level between the VB and the CB of TiO 2 , which permits those photons which have lower energy to accomplish the photocatalytic activity in the visible light range [218]. Because of the narrowing of the bandgap between valence and acceptor, the electron movement is better even at a lower temperature. In addition, such substitution restrains the crystal growth of TiO 2 as well as reducing TiO 2 crystallization, which may improve the absorption of light derived from the size effect [219]. Peng et al. [220] studied the influence of modification with Cr on the photocatalytic activity of TiO 2 and observed a shift in absorption edges of TiO 2 towards high wavelength region with the increasing Cr content. Various approaches are being employed to fabricate Cr-TiO 2 such as the spray pyrolysis [221], sol-gel method [220], and sputtering [45]. Table 9 presents the summary of recent progress on Cr-TiO 2 photocatalysts for organic pollutants degradation. Modification of TiO 2 using metals with a higher oxidation state (e.g., Mo 6+ and Mo 5+ ) increases the photocatalytic activity due to improved transfer and separation of photogenerated holes and electrons [225]. Moreover, Mo 6+ is also a suitable transition metal to be introduced into the lattice of TiO 2 due to similarity in the ionic radii of Ti 4+ and Mo 6+ [226], so Mo can easily be incorporated into TiO 2 crystal structure, producing the impurity levels, which as a consequence narrows the bandgap of TiO 2 [77]. In addition, Mo 6+ has no electron in the d orbital and can also be reduced to lower oxidation states (Mo 4+ , Mo 5+ ), implying its several oxidation states in the TiO 2 matrix can act as a superficial potential trap for the photoinduced e − to h + pairs, thus lengthening the lifetime of carriers and increasing the photocatalytic activity. Furthermore, the redox potential of Mo 6+ /Mo 5+ (vs. NHE) is 0.4 V, allowing Mo 6+ to capture photoinduced electrons, inhibiting the recombination of charge carriers [227]. Mo-TiO 2 photocatalysts can be synthesized using evaporation-induced self-assembly, hydrothermal method, and sol-gel process [228][229][230][231][232][233]. Table 10 presents the summary of recent progress on Mo-TiO 2 photocatalysts for organic pollutants degradation. Co 2+ has a similar radius as Ti 4+ , which easily enters into lattice interstitial sites or replaces Ti 4+ to broaden its photocatalytic activity [236,237]. Co-TiO 2 is a capable candidate owing to its excellent properties such as stability, optically active nature, transparency, low cost and high n-type carrier mobility [238,239]. Furthermore, the presence of cobalt (Co 2+ or Co 3+ ) enhances crystallization due to the increased amounts of oxygen vacancies in the lattice of TiO 2 , created by replacing the Ti 4+ sites of the TiO 2 lattice [182,240,241]. Ebrahimian et al. [242] prepared Co-TiO 2 NPs with a great absorption range in the visible light range. Iwasaki et al. [243] synthesized Co-TiO 2 and observed that Co 2+ could shift the edge of light absorption of TiO 2 to the visible light range and enhance photocatalytic activity in both visible and UV regions. Additionally, [92,240] reported that modifying TiO 2 lattice with Co enhances the photocatalytic activity of MO and carbamazepine under both visible and UV-A irradiations in comparison with unmodified TiO 2 .
Some studies have greatly emphasized the contradictory influence of cobalt: some authors have reported its incorporation as detrimental for photocatalytic activity, while some have observed a slight improvement in photodegradation or selectively for some organic compounds. Studies by [244,245] show that that the photocatalytic activity of Co-TiO 2 is lower than unmodified TiO 2 in the presence of UV light. Meanwhile, 0.5% Co-TiO 2 presents the highest activity in the presence of visible light. Generally, most authors have concluded on the following facts (i) Co is present as the divalent form; (ii) Co (II) states are located within the bandgap; (iii) cobalt can be found in interstitial positions of TiO 2 [246]. Table 11 presents the summary of recent progress on Co-TiO 2 photocatalysts for organic pollutants degradation.

Modification with Niobium (Nb)
The incorporation of niobium (Nb) can act as electrons sink as well as minimize the recombination of e − to h + pairs, and thus make more carriers available for reduction or oxidation processes on TiO 2 surface [258]. Modifying with Nb 5+ enables the release of one electron for every Nb introduced: the Fermi level of TiO 2 shifts upward into the CB and results in a typical n-type metallic characteristic in the electronic structure [259]. Furubayashi et al. [260] prepared an Nb 5+ -TiO 2 film and their results show a room temperature resistivity of 2 to 3 × 10 −4 Ω cm when the Nb concentration was >3%. Modifying of Nb5+ into TiO 2 lattice can extend the absorption that spans from UV to visible region as well as to the mid-infrared region. This allows TiO 2 to be active in the presence of both visible and UV light [261]. Some studies have also shown that charge transfer can be improved upon implantation with small Nb 5+ content. Upon modifying with Nb, Nb 5+ replaces Ti 4+ and the donor is formed at the CB of TiO 2 , providing electron for Ti 4+ as well as obtaining a high concentration of carrier [260]. Su et al. [262] have showed that modifying with Nb increases the electrical conductivity, decreases bandgap, and increases optoelectronic property. Khan et al. [263] also demonstrated that charge-compensating (NbTi-VTi) 3− complexes serve as the dominant defect in Nb-TiO 2 composite enabling the improvement in the photocatalytic activity via the formation of shallow defect level. Table 12 presents the summary of recent progress on Nb-TiO 2 photocatalysts for organic pollutants degradation.

Modification with Tungsten (W)
Tungsten (W) can easily replace Ti in the TiO 2 matrix due to the similarities in ionic radii of Ti 4+ and W 6+ . W in its high oxidation state (+6) can act as an electron trap center and the substitutional modification of Ti 4+ with W 6+ can impart an almost small change in the TiO 2 matrix [270]. Modifying with W can only reduce the recombination in the TiO 2 bulk, whereas the defects present on the TiO 2 surface can act as recombination centers [271]. Furthermore, modifying with W can lead to the placement of electrons in a state below the CB or donor state [272]. These electrons can receive energy from visible light and be transferred to the CB. Finally, these excited electrons can generate O 2 • − , OH•, and OH• which are responsible for the heterogeneous photocatalytic treatment of organic pollutants [273]. When the anatase TiO 2 is modified with WO 3 , separation of charge is improved as a result of the coupling of both materials [274]. Moreover, the presence of WO 3 can increase the TiO 2 acidity, modifying the substrate's affinity for TiO 2 surface and thus affects the adsorption equilibrium and photooxidation activity of the catalyst [275,276]. Three major advantages of WO 3 include the thermodynamically favorable position of VB edge for the oxidation of H 2 O (3 V vs. NHE), visible light response with a bandgap energy of 2.6-2.8 eV, and photochemical stability in acidic media [277]. Superior photocorrosion resistance and chemical stability are the other advantages provided when TiO 2 is modified with WO 3 . Various techniques including sol-gel, chemical vapor deposition, impregnation, mechanical blending, and liquid phase plasma (LPP) method have been also applied to the modification of host photocatalyst with transition metals [278,279]. Table 13 presents the summary of recent progress on W-TiO 2 photocatalysts for organic pollutants degradation.

Modification with Zinc (Zn)
Zn 2+ ion can easily substitute Ti 4+ ion in TiO 2 lattice without destroying the crystal structure, thereby stabilizing the anatase phase. Zn-TiO 2 powder can be synthesized using various preparative methods such as sol-hydrothermal, homogeneous hydrolysis, sol-gel and solid-phase reaction, electrospinning, controlled hydrolysis of titanium (IV) butoxide, assembly process based on a Ligand exchange reaction; micro-emulsion and solgel method. Table 14 presents the summary of recent progress on Zn-TiO 2 photocatalysts for organic pollutants degradation.

Modification with Gold (Au)
Gold (Au) can act as an electron acceptor since the energy gap (Eg) of Au (5.1-5.47 eV) is greater than TiO 2 (~4.7 eV) [293]. Owing to its plasmon resonance effect, modifying TiO 2 with Au will allow the achievement of visible light-driven photocatalyst [294,295]. Au-TiO 2 can also enhance the functional abilities of the material, causing alteration of the chemical or photocatalytic properties. Thus, the interaction between the photogenerated active species and the substrate of the Au-TiO 2 interface is increased and easily distinguished [296][297][298]. The collective oscillating motion of CB electrons on the Au nanoparticles (NPs) surface is mainly responsible for the reductive properties of the NPs and originates from a wellknown surface plasmon resonance (SPR) [299]. Furthermore, modifying with Au promotes the gradual injection of hot electrons into TiO 2 , and as a consequence increases the life span of the hole-electron pair and improve the photocatalytic activity [300]. The presence of Au in TiO 2 lattice structure may cause the switching of Ti 4+ /Ti 3+ in defects, which effectively minimizes the wide bandgap of TiO 2 . The presence of Ti 4+ /Ti 3+ states inside the nanostructure enhances the electron-transfer mechanism inside the crystal lattice of TiO 2 , increasing the photocatalysis efficiency [301]. This property can overcome the limitations of using unmodified TiO 2 photocatalyst due to its low recycling capability and complications of recovery from H2O.
The key parameters for the Au-TiO 2 activity are the Au particle size and the crystal phase of TiO 2 . Additionally, Au-TiO 2 possesses a high performance coupled with a longer life span [302]. On the other hand, the dependence Au-TiO 2 activity is also influenced by the particle size of Au in the action mechanisms, which can be classified into the electronhole transfer mechanism [303]. Deposition of Au occurred without changing the TiO 2 crystal structure and also achieved effective separation of photogenerated electron-hole in the UV-visible (UV-vis) light conditions, due to the smaller Au particle size of 15 nm. Table 15 presents the summary of recent progress on Au-TiO 2 photocatalysts for organic pollutants degradation.

Modification with Silver (Ag)
Compared to gold and platinum, silver (Ag) is the cheapest coinage metal having a high charge carrier density [315]. It also has other advantages such as higher antibacterial activity, lower cost, and toxicity [316]. Modifying with Ag ions can create a new impurity band about 0.7 eV below the CB of titania which will thus narrow the energy bandgap of titania [317]; subsequently, Ag enhances the electron-hole separation by acting as an electron trapper to capture the electrons while reacting with the contaminants [318]. Hence, Ag assists in charge separation via the formation of a Schottky barrier between the metal and TiO 2 [319,320]. The incorporation of silver ion also extends the absorption of TiO 2 under visible light due to SPR effect, excited by visible light [321][322][323][324]. The presence of Ag ion can also contribute in two ways: (i) reinforcement of the photoinduced charge carriers as well as enhancing the electromagnetic field at the interfaces, (ii) promotion of interfacial charge-transfer process which limits the recombination rates. The electronic band-alignment at the Ag-clusters-TiO 2 interface favors the migrations of photogenerated electrons to metallic particles.
Zhou et al. [336] reported Ag decorated Ti 3+ self-doped porous black TiO 2 pillar using a combined oil bath and wet impregnation approach. They claimed that the synergistic effect between Ti 3+ self-doping and Ag decoration enhances photocatalytic activity. Liu et al. [337] presented the formation of Ag impurity band above the VB and observed the presence of AgO on the TiO 2 surface as well as the presence of Ag in the bulk. They noted that this Ag system will fail to show the energy level matching concept as the presence of Ag-ions is required on the TiO 2 surface. Contrary to various conventional reports, some reports have also confirmed the formation of impurity bands starting from 0.7 eV below CB [338]. Xiong et al. [339] also introduced 0.25 wt% Ag NPs into mesoporous TiO 2 and observed enhanced degradation of RhB in the presence of UV light irradiation. In addition, the Ag-TiO 2 catalysts present a higher photocatalytic efficiency for indigo carmine in the presence of visible light which was higher than the commercial P25 titania or unmodified TiO 2 [339]. Table 16 presents the summary of recent progress on Ag-TiO 2 photocatalysts for organic pollutants degradation.

Modification with Platinum (Pt)
Platinum (Pt) is a shiny silvery metal with low chemical activity. It is stable in air and a humid environment. The oxidation state of platinum is +2 to +6, which easily forms coordination compounds and has good ductility, thermal conductivity, and electrical conductivity. Pt is a noble metal that can increase the TiO 2 photoefficiency [352]. The energy level of Pt is lower than the one of the TiO 2 CB, so the energy required for the electron transition is lower and may be induced by the light with higher wavelengths. Therefore, the absorption spectrum can be broadened from the UV towards the VIS region. In the presence of UV light, the photogenerated electrons quickly transfer from the TiO 2 surface to the Pt particles, leading to effective separation of electron-hole, and as a consequence improving the photocatalytic activity [353]. The deposition of Pt nanoclusters onto TiO 2 surface serves as an electron sink, consequently slowing down charge-pair recombination as well as changing reaction pathways by providing catalytic sites where active intermediates can be stabilized [121]. Table 17 presents the summary of recent progress on Pt-TiO 2 photocatalysts for organic pollutants degradation.

Modification with Ruthenium (Ru)
Modifying with ruthenium (Ru) can reduce the Egap of TiO 2 , allowing the visible lightdriven photocatalysts, and acts as an electron acceptor/donor, which efficiently minimizes the recombination due to acceleration of electron transfer [360,361]. Ru-TiO 2 nanotubes have been widely used for the photocatalytic degradation of organic pollutants under visible or UV light [362,363]. Table 18 presents the summary of recent progress on Ru-TiO 2 photocatalysts for organic pollutants degradation.

Modification with Palladium (Pd)
Modification of TiO 2 with Palladium (Pd) has also received some attention due to their good stability coupled with high effectivity [367]. Pd is one of the most active elements for interacting with the surface of oxides as support. It has been revealed that the absorbability in the visible light region notably increases with the Pd incorporation; moreover, this metal can act as trapping sites for electrons and hence increase the photocatalytic activity of TiO 2 . Table 19 presents the summary of recent progress on Pd-TiO 2 photocatalysts for organic pollutants degradation.

Modification with Rare Earth Metals
Modifying with yttrium (Y) was found to be successful for improving the photocatalytic response of TiO 2 under visible light irradiations [372,373]. Y-TiO 2 can be synthesized using different techniques such as deposition-precipitation with urea, sol-gel, impregnation and chemical coprecipitation method. It has been reported that Y-TiO 2 gives improved photocatalytic response attributed to the visible light absorption, electron-hole pairs separation, higher interfacial charge transfer, lower crystallite size, and high specific surface area. Niu et al. [372] found that introducing yttrium shifted the absorption edge of TiO 2 towards visible light, reduced the crystallite size, inhibited anatase to rutile phase transformation and decreased the photogenerated electron-hole pair recombination. Table 20 presents the summary of recent progress on Y-TiO 2 photocatalysts for organic pollutants degradation. Modification with lanthanide ions can form complexes with various Lewis bases such as thiols amines, alcohols due to the interaction of their f-orbitals with different functional groups. Adding lanthanides in TiO 2 lattice suppresses e − to h + recombination and also increases the adsorptive capacity of the model pollutants [376]. It also stabilizes the mesoporous structure, prevents agglomeration, and increases thermal stability [53,377]. The common lanthanides include lanthanum (La), erbium (Er), neodymium (Nd), gadolinium (Gd), thulium (Tm), ytterbium (Yb), holmium (Ho), terbium (Tb), praseodymium (Pr), samarium (Sm), and europium (Eu).
Among the lanthanide ions, modifying TiO 2 with cerium (Ce) has received considerable attention due to the following: (1) it forms labile oxygen vacancies easily with the relatively high mobility of bulk oxygen species (2) the redox couple Ce 3 +/Ce 4+ with the ability of ceria to shift between Ce 2 O 3 and CeO 2 under reducing and oxidizing conditions [378][379][380]. This enables Ce to reduce e-to h + recombination within TiO 2 via the trapping of an electron. The cerium ions (Ce 3+ /Ce 4+ ) have variable valence states with multi-electron energy levels [381]. CeO 2 has a strong absorption ability to UV light with a small bandgap [382], its electrons are easy to jump and the 5d and 4f orbitals of Ce 4+ are without electrons, which leads to large deformation and strong polarization which allows for effective separation of photoinduced carriers and the improvement of the photocatalytic activity of TiO 2 [383]. Ce-TiO 2 materials can be prepared by hydrothermal and sol-gel methods [380,[384][385][386].
Xie et al. [387] prepared the monodisperse Ce-TiO 2 microspheres via a facile solvothermal process. Their results showed that the photocatalytic activity of TiO 2 was improved activity for MB under visible light irradiation upon introduction with Ce. Aman et al. [388] presented a 5% Ce-TiO 2 and their results show an improved activity for MB (50 ppm) with photodegradation of MB of~100% within 60 min in the presence of visible light. Table 21 presents the summary of recent progress on lanthanides modified TiO 2 photocatalysts for organic pollutants degradation.

Modification with Other Metal Ions
The other metal ions are also introduced in TiO 2 crystal lattices to replace Ti ions, thus improving the transferring rate of interfacial electrons, lowering the recombination between e − and h + , and adjusting the energy band structure [408]. Simultaneously, the energy level is introduced into the bandgap of TiO 2 to broaden the absorption band edge to the visible light range, and finally, improve the activity of TiO 2 under visible light [408,409].
Indium (In)-TiO 2 has been observed to increase the photoelectron chemical responses and photocatalytic activity of TiO 2 by enhancing the transfer of electrons and reduction in recombination rate of photoinduced charge carriers due to its CB/VB potential difference from TiO 2 [410,411]. Indium has lower toxicity, is relatively cheap, and has multiple oxidation states (In • , In +1 , In +3 ), which can help to improve charge mobility and electron trapping over the surface of TiO 2 [412]. The result by [413] shows an enhanced photocatalytic activity for 2,4-dichlorophenoxyacetic acid in the presence of UV light irradiation. Pozynak et al. [414] reported an efficient separation of photogenerated charges in nanocrystalline In 2 O 3 /TiO 2 photocatalyst during the degradation of 2-CP.
The presence of transition metal such as Gallium ion (Ga 3+ ) at the Ti 4+ sites can induce oxygen vacancies gaps [415,416] and build deficiencies near the CB in TiO 2 , which function as electron traps and increase the isolation of e-to h + pairs [417,418]. Simultaneously, modifying with Ga shift the absorption edge towards visible regime as well as improving the separation between photoexcited charge carriers [419]. The synthesis of Ga-TiO 2 can be achieved by laser pyrolysis [420], sol-gel method [417], traditional solid-state reaction [421,422], and hydrothermal method [419,[423][424][425]. However, these methods often require either a high experimental temperature to promote the reactions [420] or post heat treatment for crystallization [417,423]. Generally, a higher heating temperature always leads to grain growth and agglomeration, which decreases the specific surface area and is detrimental to photocatalytic activity. Table 22 presents the summary of recent progress on other metals modified TiO 2 photocatalysts for organic pollutants degradation.

Conclusions and Future Outlooks
TiO 2 is the most widely used photocatalyst for the photodegradation of organic pollutants. This paper presents the recent development of metal ion modified TiO 2 for the photodegradation of organic pollutants. Among the metal ions, metal such as Ni, Fe, Co, Cu, Au, Ag, Zr, W, and Mn have been widely explored and found to show beneficial influence on photocatalytic activity. However, the application of metal ion modified TiO 2 photocatalyst for organic pollutants continues to be limited due to several obstacles (preparation method, security, cost, commercial use, and efficiency). Some of the recommendations drawn from this review are listed as follows: The activity of TiO 2 photocatalyst has a lot to do with its preparation method. The preparation methods are different, and the shape and size, surface, and structural properties of the catalyst are different. The main methods for preparing metal ion modified TiO 2 include sol-gel, precipitation, immersion, and hydrothermal method. It was found that the samples prepared by sol-gel and hydrothermal methods produce better results. Its photocatalytic performance is much higher than that of photocatalysts prepared by other methods. This is mainly because the reaction process of these two methods is simple, the operation is controllable, and the prepared powder has a relatively small particle size, high purity, and good chemical uniformity. However, there is a limitation in the long preparation cycle. Therefore, future research should be devoted to new ways to find simple and effective ways to improve some defects of metal ions modified TiO 2 .
There is a need for continuous in-depth study on the use of sunlight irradiation, and the economical applicability of this approach for removing organic pollutants. To date, various metal ion modified TiO 2 photocatalysts have been developed and reported. However, their preparations consume considerable chemicals which are expensive, timeconsuming and complex. Traditional TiO 2 is generally a massive particle, and the control of the morphology of TiO 2 is an effective way to increase their contact with pollutants. For example, the preparation of hollow, porous, or larger specific surface area TiO 2 nanoscale particles can effectively solve the traditional small contact surface of TiO 2 and pollutants. In addition, TiO 2 itself can be modified to add its hydrophilic functional groups. It can increase the compatibility of TiO 2 with H 2 O, thereby promoting the working process of TiO 2 . Therefore, it is a necessity to develop a relatively scalable, inexpensive, environmentally friendly, and easy synthesis route.
There are also many defects in the application of TiO 2 in daily life. The limiting factor for its development is the photocatalytic performance problem. In addition, if the traditional photocatalyst is unmodified, it cannot adapt to the change to other light sources (e.g., sunlight). The water body purified by TiO 2 in daily life will take away part of the TiO 2 , and at the same time, the photocatalytic degradation of organic pollutants often produces some intermediate products. However, in most cases, the toxicity of the intermediate products tends to be stronger than the initial organic pollutants, and as a consequence poses greater harm to the environment and human beings. Therefore, there is a need for an extensive toxicological study of these intermediate products. Therefore, in terms of these issues, how to improve the flexibility of particles in practical applications and how to improve the safety of TiO 2 also needs to be considered in future research.
The applicability of this method requires additional engineering amplification testing. However, it is hoped that through rapid and continuous evaluation of the pilot plant configuration, a large-scale solar-driven photocatalytic activity treatment process with low site area requirements and high efficacy can be realized in near future. Furthermore, the majority of the studies use artificial organic pollutants and studying the performance of TiO 2 photocatalysts under real situations should be the focus of future work.
Finally, we hope that this paper can assist researchers to better understand the recent trend in the removal of organic pollutants using metal ion modified TiO 2 photocatalysts and also hope that this field can flourish in the future.