Recent Developments of Advanced Ti 3+ -Self-Doped TiO 2 for Efficient Visible-Light-Driven Photocatalysis

: Research into the development of efficient semiconductor photocatalytic materials is a promising approach to solving environmental and energy problems worldwide. Among these materials, TiO 2 photocatalysts are one of the most commonly used due to their efficient photoactivity, high stability, low cost and environmental friendliness. However, since the UV content of sunlight is less than 5%, the development of visible light-activated TiO 2 -based photocatalysts is essential to increase the solar energy efficiency. Here, we review recent works on advanced visible light-activated Ti 3+ -self-doped TiO 2 (Ti 3+ –TiO 2 ) photocatalysts with improved electronic band structures for efficient charge separation. We analyze the different methods used to produce Ti 3+ –TiO 2 photocatalysts, where Ti 3+ with a high oxygen defect density can be used for energy production from visible light. We categorize advanced modifications in electronic states of Ti 3+ –TiO 2 by improving their photocatalytic activity. Ti 3+ –TiO 2 photocatalysts with large charge separation and low recombination of photogenerated electrons and holes can be practically applied for energy conversion and advanced oxidation processes in natural environments and deserve significant attention.


Introduction
Environmental pollution and sustainable energy development are controversial issues we all face today [1]. In keeping with the pace of technological development, the development of renewable energy technologies must be studied in depth. Solar energy is the most abundantly available energy source. An amount as large as almost 99.9% of the energy used on Earth is produced by the sun [2][3][4]. Harnessing this enormous amount of solar energy would provide sustainable and environmentally friendly energy. It is also a promising option for economically feasible technology development.
However, there are also disadvantages that limit the practical application of TiO2. First, the recombination of light-generated electrons and holes easily occurs, hindering the efficient use of absorbed energy [54][55][56]. Second, the TiO2 surface is generally hydrophilic, so its absorption capacity for aromatic organic pollutants is low, limiting its responsiveness [54,[57][58][59][60]. Though there are many other issues to overcome, the wide bandgap (~3.2 eV) of TiO2 requires ultraviolet light for photoactivity [24,54]. Only 5% or less of sunlight is UV light [24,53,59]. This means that TiO2 cannot use the visible-near IR energy that comprises most sunlight. Therefore, great efforts have focused on research to improve charge separation, surface reactivity and light absorption to overcome the disadvantages of TiO2 [12,[61][62][63][64]. Increasing solar energy efficiency and using more energy are major challenges for expanding the light-activated region into the visible region.
In this review, we focus on the development of advanced Ti 3+ -TiO2 for efficient solar energy harvesting of TiO2 photocatalysts. Ti 3+ -TiO2 is oxygen-deficient (TiO2−x) and was first reported in 2011 by Chem et al. They produced defects on the TiO2 surface and called the resulting material 'disorderengineered black TiO2.' This method reduced the light absorption band gap of to 1.5 eV, suggesting the possibility of using it for visible light catalysis by increasing solar absorption efficiency. [92] Since then, TiO2 has been reduced to produce blue TiO2 [87], and great efforts have been put into developing blue-black TiO2 to use visible light, and much progress has been made. Figure 1 shows the schematic of the nanoparticle structure of Ti 3+ -TiO2 and the band location for the insertion of each dopant. We provide the latest reports on synthesis methods of Ti 3+ -TiO2-based photocatalysts and their application efficiencies. First, we analyze the physical concept of Ti 3+ -TiO2 photocatalysts and their production and advancement. We discuss the shortcomings in the production of Ti 3+ -TiO2 and recent reports on the use of other materials to enhance the absorption of visible light. Moreover, many efforts have been discussed to overcome the disadvantages of photocatalytic efficiencies of Ti 3+ -TiO2 due to low charge separation and high recombination of photogenerated electrons and holes. Finally, we present some perspectives on the future of modified Ti 3+ -TiO2 photocatalysts.

Preparation of Ti 3+ -TiO2
Ti 3+ -TiO2 (TiO2−x) has been demonstrated to enhance visible light absorption [95]. Ti 3+ -self doping causes defect levels due to oxygen vacancies in the lattice structure of self-doped TiO2. The defect levels overlap the conduction band of TiO2 and can induce vacancies in the electronic state just below the conduction band [96]. As a result, the band gap is reduced, resulting in enhanced visible light absorption. Methods for producing Ti 3+ -TiO2 have been a focus of many studies. In general, high pressure and high temperature are used to reduce oxygen to form oxygen vacancies. Ti 3+ -TiO2 doped with high oxygen vacancies is super hydrophilic. Thus, Ti 3+ -TiO2 on hydrophilic substrates can result in strong bonding to the surface, thus increasing the coating ability.
According to recent trends, there are many reduction methods of TiO2, which we divide into five main branches: hydrogenation, hydrothermal reaction, alkaline metal reduction, sol-gelation and phase-selective reduction. As generations go through, the preparation method of Ti 3+ -TiO2 is not always superior, but it is considered to be a little more relaxed and more sophisticated.

Hydrogenation
Hydrogenation using hydrogen gas to react with oxygen atoms in the lattice of TiO2 is the most common method for Ti +3 doping. Ti 3+ is self-doped in an abundance of TiO2 by high thermal or electromagnetic energy [97,98]. Hydrogenation requires harsh synthetic conditions and dangerous production processes (i.e., high heat and pressure, risk of explosion of hydrogen gas) [92,[97][98][99]. To overcome this, other reducing agents that can provide hydrogen such as NaBH4 and TiH2 have been introduced as alternatives to hydrogen gas [98,[100][101][102]. Additionally, increasing the rich Ti 3+ doping involves higher heat, higher pressure and longer reaction times. Well-crystallized samples can effectively enhance visible and infrared absorption and exhibit hydrophilicity by oxygen doping with H in the lattice. However, in hydrogenated TiO2, a high concentration of hydrogen atoms was required for efficient photocatalytic activity [103].
Additionally, if the temperature increases under firing conditions, phase conversion from anatase to rutile may occur. This can result in a band gap shift from anatase (~3.2 eV) to rutile (~3.0 eV). Phase transformation requires 600 to 700 degrees of energy in the absence of impurities, but the presence and amount of oxygen vacancies can lower the firing temperature. The increased oxygen deficiency causes relaxation of the large oxygen sublattice and facilitates the rearrangement and modification of atoms from anatase to rutile. In a high-pressure reducing atmosphere, heat transfer is also advantageous because of the high material density. Fast heat transfer will further promote anatase-to-rutile phase transformation [104]. However, the abundant doping of Ti 3+ results in an irregular crystalline phase, called a disordered phase. Disordered crystals may behave differently from the normal physicochemical properties of anatase and rutile crystals.

Hydrothermal Reaction
A method of manufacturing Ti 3+ -TiO2 with abundant defects has been reported using TiCl3 and TiF4 as precursors. Ti 3+ can easily be oxidized under high heat and pressure conditions. Introduction of Ti 4+ can inhibit oxidation of Ti 3+ in this reaction, leading to abundant doping [105]. In addition, Fang et al. synthesized various reduced TiO2 samples using Zn powder as a reducing agent and HF as a solvent in a simple one-pot hydrothermal process. However, Ti 3+ introduced by Zn reduction is not stable in air and can be easily oxidized [106].
This method, which has been slightly improved under hydrogenation conditions, still requires high heat and pressure. It has an advantage in that the reaction proceeds in a liquid state that is easier to handle than hydrogen gas. However, it is unstable in the air and hydrogen saturation is not effective. A new solvent is needed to overcome this stability issue.

Alkaline Metal Reduction
Researchers tired of using high heat and pressure gases and liquids. In general, attempts have been made to reduce TiO2 by using alkali metal, the most commonly used solid reducing agent. It is possible to obtain Ti 3+ -TiO2 by simply replacing the high heat and pressure and simply mixing the powder. In 2013, Yin et al. produced gray TiO2 rutile nanowires by reducing the TiO2 surface by directly introducing aluminum metal at a temperature of 700 °C [107]. There have also been attempts to generate Ti 3+ under milder conditions. In 2017, Zhang et al. sufficiently reduced TiO2 using Na/NaCl powder under room temperature and an argon atmosphere and treated it with Ru to synthesize nanoparticles of TiO2 with Ti 3+ and Ru. Crystalline TiO2 was milled with Na and NaCl powder at room temperature for 0.25 to 4 h under an argon atmosphere. It can be seen from Figure 2 that the color of P25 becomes darker as it undergoes harsh reaction conditions. Ti 3+ -rich TiO2 nanocrystals can be used as effective support for Ru particles and the Ru/Ti 3+ -TiO2 catalysts showed excellent performance in catalytic hydrogenation of N-methyl pyrrole [108].

Sol-Gelation
Since then, people have shown how to synthesize TiO2 doped with Ti 3+ abundantly, starting with the precursor building block, rather than doping Ti 3+ into the already prepared TiO2. Zhang et al. synthesized TiO2 single-crystal nanorods using a sol-gelation method and succeeded in obtaining Ti 3+ -self-doped blue TiO2 single-crystal nanorods by further annealing at 350 °C. Blue TiO2 showed a 97.01% higher decomposition rate of RhB than did white TiO2 under visible light, and the photocatalytic hydrogen generation rate increased. As a result of these investigations, the band gap was reduced to 2.61 eV due to the synergistic effect of the 1D-shaped single crystal structure and this increased the photocatalyst's visible light absorption ability and photocatalytic reactivity [109]. In 2018, Yao et al. synthesized N/Ti 3+ -doped TiO2 in one step using TiN. Additionally, using BiOBr, N/Ti 3+ -doped TiO2 and heterojunctions were formed to enhance sonocatalytic activity. Doping of BiOBr and N/Ti 3+ leads to effective charge separation of electron-electron pairs, increasing the methylene blue removal efficiency by 1.81 times [110].
In general, the grown TiO2 crystal phase is anatase. However, depending on the reaction conditions, rutile anatase may also be present. Among the synthetic conditions, there is a possibility that BiOBr promoted rutile growth [110]. Optimal performance is achieved by mixing anatase and rutile in appropriate proportions (85:15). However, it is generally found that the photocatalytic efficiency of anatase is higher. Due to the high density of localized states in anatase, it is used for surface adsorbed hydroxyl radicals and slow charge carrier recombination. Rutile has a high charge recombination rate and consequently has a reduced ability to adsorb functional species, which is detrimental to photocatalytic performance [104].

Phase-Selective Reduction
There are many ways to produce Ti 3+ -TiO2, but almost all of these require high temperature and high pressure. In addition, it is not yet possible to selectively reduce anatase and rutile phase on TiO2 crystals in harsh conditions of high temperature and high pressure. In 2016, Zhang et al. first developed a method for selectively reducing the crystal phase of Degussa P25 TiO2 nanoparticles at room temperature and atmospheric pressure using a simple Li-Ethylene diamine (EDA) solution. They melted Li metal in EDA solution at room temperature and pressure to produce a blue electride solution of a strong reducing agent to reduce TiO2 nanocrystals. The Li-EDA solution is unique in reducing only the rutile phase while the anatase phase remained the same as in the composition of Degussa P25. They confirmed that Ti 3+ was selectively introduced into TiO2 to significantly improve visible and near-infrared absorption and increase the charge-hole charge separation efficiency through type II band gap alignment ( Figure 3). These effects promoted a strong hydrogen-generating surface reaction. Therefore, when using phase-selective disorder engineering, 0.5 wt% Pt and 3.46mmol were used to show high stability and a high hydrogen generation rate of 13.89 mmol•h −1 •g −1 [87]. In addition, in 2019, Hwang et al. discovered that when P25 is treated with Na-EDA solution, only the anatase phase is reduced. Engineered Ad/Ro TiO2 selectively treated with Na-EDA to reduce the anatase phase contained only Ti 3+ defect sites in the anatase phase and the internal energy band gap was narrowed. This indicates enhanced visible light activity. Hwang et al. subsequently reduced CO2 using Ti 3+ -self-doped Ad / Ro TiO2 that selectively reduced only the anatase phase to produce CH4 using light in the visible region [111]. The phase-selective method of reducing TiO2 using a metal-EDA electride solution has attracted attention as a promising method of self-doping rich Ti 3+ on the surface of TiO2 at room temperature and atmospheric pressure to improve the visible light activity of TiO2. The developed Blue TiO2 (BTO) was used for organic arylation reactions, the yield of which was supported with untreated P25 (40%) and BTO (63.4%) [112]. In addition, the BTO had excellent reactive oxygen species (ROS) generation and showed strong efficacy in decomposing algae (Chlamydomonas) under sunlight [113].
The phase-selective reduction of P25 does not require heat or pressure. In addition, while selectively reducing rutile and anatase TiO2 crystal phases, it became possible to impart selectivity [114]. With the selective reduction of rutile and anatase, their physicochemical properties are clearly different. We expect to pioneer new applications in various fields by applying this method. Reproduced from [1]. Copyright (2016), The Royal Society of Chemistry.

Enhancement of Photogenerated Charge Separation and Photocatalytic Activity in Advanced Ti 3+ -TiO2
Photocatalysts exhibit catalytic activity when introduced to light. The photogenerated electrons and holes move to the photocatalytic surface. The moved electrons and holes react with adsorbed electron acceptors and donors, respectively, to complete the catalytic reaction. Abundant Ti 3+ doping can lead the excitation region from ultraviolet to visible light. Ti 3+ -doped TiO2 can narrow the wide band gap of TiO2 for harvesting visible light and can provide an increase in electronic conductivity due to doped Ti 3+ defects. However, after photoexcitation, the generated electrons and holes may undergo significant electron recombination in the depletion region of Ti 3+ -doped TiO2 [115][116][117].
To overcome this and ultimately enhance photocatalytic activity in all applications, deformation studies with several different materials have been conducted. Performance can be improved by using Ti 3+ -TiO2 in combination with other materials or elements. Studies using various other materials have been conducted to overcome the previously mentioned limitations of Ti 3+ -TiO2 deformation.
We focus on the synthetic methods of advanced Ti 3+ -TiO2 with high photogenerated charge separation and efficiency and classify them into four categories according to recent developments. Ti 3+ -TiO2 modification has been extensively studied, including metal and nonmetal doping, semiconducting coupling, and stoichiometry modification to prepare and stabilize composites with other materials.

Metal-Doping
It has long been a major concern to increase the efficiency of TiO2 photocatalysts and expand their use in the visible region using easily accessible metal dopants. Metal dopants can be mixed with Ti 3+ atoms or replaced by Ti atoms, and the band gap can be adjusted in such a state. Moreover, a new band was produced as a conduction band to help better separate photogenerated charges after photoexcitation. Metal dopants act as an electron acceptor as well as a hole acceptor in the valence band. The introduction of metal dopants is still a promising option for Ti 3+ -TiO2. We summarized these efforts in Table 1. LSPR-based photosensitization can promote the absorption of visible or near infrared rays [122,123]. If the frequency of the incident photon coincides with the natural frequency of the vibrating surface electrons, resonant group oscillations of the valence electrons can occur to promote absorption of Ti 3+ -self-doped TiO2 visible light [124]. Usually, LSPR excitation occurs in the visible region when Au [125], Ag [126], and Pt [127] are combined with TiO2 [128]. Attempts to combine Ti 3+ -self-doped TiO2 and LSPR metals have continued. However, in almost all cases, noble metals should be used for plasmon resonance [125][126][127]. The use of precious metals is still valid. In 2019, phaseselective room-temperature solution engineering was used to enhance HER performance by attaching Pt particles to TiO2 richly doped with Ti 3+ [114]. This indicates that the simple phaseselective room-temperature solution engineering method does not lag behind TiO2 manufactured in other harsh conditions of different reducing conditions.
Recently, He et al. used Ti 3+ and Ni for plasmon-mediated carrier transfer to enhance the degradation of methylene blue in visible light. They deposited Ni-TiO2 structures on SiO2 spheres based on the modified nanosphere lithography method. It was found that plasmon-generated hot electrons and Ni holes can be transferred to TiO2 in a heterogeneous structure (Figure 4). The transferred hot electrons and holes occupy oxygen vacancies or produce Ti 3+ . TiO2 self-doped with Ti 3+ showed enhanced removal performance of methylene blue of 0.11 ± 0.04 μmol•L −1 •min −1 in visible light [69].
Furthermore, it is valuable to be able to achieve the same effect as LSPR by using inexpensive non-precious metal transition metals other than Ni. We believe that developing a method of applying transition metals will become the main approach to metal-doped Tt 3+ TiO2 in the future. KGaA, Weinheim.

Single Atom Site Doping Effects on Ti 3+ -TiO2
Isolated single atom site (ISAS) catalysts are a new approach in the field of catalysis due to their increased catalytic activity and excellent selectivity. Hejazi et al. created a stable platinum single atom site by grafting Pt single atoms to the Ti 3+ -self-doped TiO2 anatase (001) nanosheet by hydrogenation [89]. After reducing the thin Ti 3+ layer in the surface Ar/H2 environment, it was used as a Pt single atom support and tightly adhered from the diluted aqueous Pt solution. The amount of Pt deposition can be adjusted by considering the heat treatment temperature in Ar/H2. This showed a 150-fold improvement in hydrogen generation rate over that observed using the same amount of platinum nanoparticles ( Figure 5) [89]. Despite many issues, including the stability of ISAS catalysts, this is a very attractive approach. With a small amount of metal, a photocatalytic performance of tens to hundreds of times improvement is observed. In addition, we believe that a single atom leads to a widely applicable approach for many. The development of the ISAS catalyst using Ti 3+ as support can provide a new approach for Ti 3+ -self-doped TiO2 for absorbing visible light. In general, the bonding of transition metals and TiO2 is a classic method. Similarly, much effort has been put into grafting transition metals into Ti 3+ -TiO2. Recently, research on photocatalysts for removing microorganisms and viruses was actively conducted in addition to the removal of existing classical organic or inorganic contaminants. Zhang et al. reported cell lysis of E. coli and agropathogenic fungal spores by preparing classical Cu and Ti 3+ -TiO2 in a new way. It was reported that anhydrous ethanol can be used as a reducing agent in the hydrothermal process leading to the formation of metallic copper and Ti 3+ . They confirmed the photocatalytic disinfection effect on five agricultural pathogenic fungal spores, including F. graminearum and B. dothidea spores, by fluorescence staining using a FungaLight CFDA-AM/propidium iodide yeast vitality kit. Almost all bacteria were killed upon irradiation for 3 h. In Figure 6, killed bacteria are identified by fluorescent staining. Both copper and other metals are expected to be effective in bacterial and viral cell lysis when combined with Ti 3+ -self-doped TiO2 [120]. Recently, grafting of rare earth metals such as Bi (rather than classic commercially available metals such as Cu, Zn and Co) into Ti 3+ -TiO2 has been reported. Gul et al. increased the solar reactivity by grafting Bi to Ti 3+ -TiO2. They performed extensive doping with Ti 3+ and Bi-by-Bi doping and solgel technology and showed better photocatalytic performance than undoped TiO2 for flumequine decomposition [118].
As shown in Figure 7, Zhou et al. introduced doping with six rare earth metals (i.e., La, Ce, Pr, Nd, Eu and Gd) and sepiolite with Ti 3+ -TiO2 to improve visible light activity and provide low band gap energy. In addition, this strategy resulted in a high adsorption capacity of contaminants through the sepiolite and improved the photocatalytic performance. Figure 7 shows the schematic structure for modification of the conduction band maximum and the photodegradation effect of Orange G for each metal. They found that, upon doping of rare earth metal ions, the structure and chemical properties of the nanocomposite of TiO2 depended heavily on the radius of the rare earth metal ion. Rare-earth ions with a valence of +3 (i.e., RE +3 ) can be abundantly doped with Ti 3+ because the rare earth metal ions are much larger than that of Ti 4+ . However, this can cause a charge imbalance that can contribute to the adsorption of OH − . It is also reported that the empty 4f level of rare earth metals can act as a scavenger for light-generating electrons. Of the six rare earth metals, the Eu complex showed the best photocatalytic performance [121]. The combination of common transition metals and rare earth metals with Ti 3+ -self-doping can be an excellent way to enhance photocatalytic performance, and promising materials for cell lysis of viruses and bacteria can be developed.

Nonmetal Doping
Much effort has been made to expand the applications of TiO2 photocatalysts with metal and nonmetal based dopants into the visible light regime and increase the photocatalytic efficiency. In particular, many common elements such as nitrogen, carbon, sulfur and phosphorus have been used for grafting. The grafting of nonmetallic elements such as these can lower the surface energy for adsorption of the photocatalytic surface [129]. Unlike metal dopants, nonmetal dopants can be directly involved in the band of TiO2. In addition, nonmetallic elements with a different electronic environment from oxygen can have positive and negative effects on the band gap energy. We believe that such modification through band engineering is a unique possibility for nonmetal dopants. We summarize the recent Ti 3+ and nonmetallic dopant grafting in Table 2. Nitrogen is one of the most frequently used nonmetal elements for adjusting the valence band of TiO2. Doping with nitrogen may cause the maximum upward movement of the valence band, and visible light absorption may also be improved by controlling the red-shift of the absorption edge. Nitrogen can improve both carrier density and electron conductivity in the neutral region [129]. Additionally, it can act as an absorption site for organic pollutants. Recently, efforts have been made to enhance visible light absorption and enhance photocatalytic performance using Ti 3+ and N dopants together. By doping N and Ti 3+ together, Ti 3+ in the band decreases the conduction band, and N increases the valence band to reduce the band gap energy and enhance the photocatalytic performance. Jia et al. improved the charge separation by introducing carbon after N doping, introducing carbon components, and using carbon as a site to accept the separated charge after photoexcitation. This showed a significantly faster Rhodamine B decomposition rate than conventional nitrogen-doped Ti 3+ -self-doped TiO2 (Figure 8) [70].

Sulfur-Doped Ti 3+ -TiO2
Among nonmetal dopants, sulfur has the same valence as oxygen, has adequate energy, and may not cause charge imbalance in TiO2. The energy band occupied by the charge imbalance can act as a carrier recombination site and degrades the carrier transport process [129,135]. 4 Based on these advantages, sulfur has been noted as another promising nonmetallic dopant to drive the performance of TiO2 photocatalysts. Active research with Ti 3+ is ongoing. Ji et al. introduced S 2− into anatase TiO2 together with Ti 3+ , which significantly lowered the band gap energy and increased photocatalytic hydrogen production. They confirmed that, by introducing sulfur and Ti 3+ , the band gap decreased from 3.2 eV for the existing anatase TiO2 to 2.0 eV after co-doping [132]. As shown in Figure 9, Meng et al. added plasmonic Ag particles to TiO2 nanorods introduced with sulfur and Ti 3+ to synergistically reduce the band gap through impurity dopants and expand LSPR into the visible light regime. These are photocatalytic reactions where phenol's visible-light-driven photocatalytic decomposition rate and hydrogen production rate are as high as 98.67% and 209.2 μmol•h −1 •g −1 , approximately 2 and 5 times higher than TiO2 alone [133].

Multi-Doped Ti 3+ -TiO2
Several elemental dopants can be introduced at the same time. Yan et al. reported black TiO2 nanosheets with Ti 3+ by doping nitrogen, carbon and sulfur. The doped C, N, S elements can result in additional impurity levels above the valence band. The mass concentrations of doped C, N and S are 14.20%, 4.80% and 3.30%, respectively, and these worked synergistically with Ti 3+ . They used visible light to photocatalytically decompose methyl orange up to 92.13%. The amount of hydrogen generated by the photocatalyst was 149 μmol•h −1 •g −1 [134]. An attempt to use multiple materials without being limited to a single material may provide a new way to improve photocatalytic performance.

Semiconducting Coupling
Semiconducting heterogeneous catalytic junction enhances various properties by adding a new band to Ti 3+ -TiO2. The newly added band may additionally absorb visible light, promote optical junction carrier separation and transfer and enhance interactions with the target material. Semiconducting heterojunctions are advantageous for absorbing visible light of photocatalysts and increase photocatalytic activity by promoting separation and transfer of photojunction carriers of semiconductors [136,137]. Previous bands of semiconductor materials such as CdS [138], Ag3PO4 [139], Cu2O [140] and WO3 [141], have been combined with TiO2. Recently, g-C3N4 has shown overall water splitting band potential and high visible light response, leading to the current trend [142,143]. Likewise, when Ti 3+ -TiO2 is combined with semiconducting materials, better synergy can be expected than before. We looked at the recent trends of Ti 3+ -TiO2 and organized them in Table 3. Transition metal dichalcogenide (TMD) materials based on quasi-dimensional (2D) structures have recently attracted attention because of their excellent electronic and optical properties. These materials have great potential for high-tech applications and have different properties compared to conventional bulk counterpart materials. The state density of the TMD material increases in a semicontinuous stepwise fashion. Therefore, TMD materials can exhibit new electronic and optical properties [25,26]. MoS2 is a representative TMD material, and it is a popular material in HER photocatalysts and electrocatalysts. This is because the sandwiched MoS2 atomic layer can provide various unsaturated bonds at the edge of the layer. There have been attempts to enhance the photocatalytic performance with recent Ti 3+ -TiO2 using these new electronic and optical properties. There have been attempts to dope the MoS2 layer and other semiconducting or metal materials over spear-shaped black Ti 3+ -TiO2. Attempts have been made to bond N/Ti 3+ -TiO2 with spear and MoS2 and to apply hollow Ti 3+ -TiO2 and MoS2 / CdS together [76,147]. All of the materials were able to clearly identify the vertically grown layer and showed excellent performance in removing HER and Methylene orange. Figure 10 shows the schematic structure and mechanism of action of hollow black TiO2/MoS2/CdS. In addition to TMD metal, there have been attempts to combine Ti 3+ -TiO2 with a layered material represented by bismuth oxyhalides. Yao et al. confirmed the well-grown TiO2 and BiOBr nanostructures by synthesizing N/Ti 3+ -TiO2 and BiOBr in one step. These materials showed high efficiency in removing methylene blue due to increased visible light absorption [110]. In addition to 2D-based layered materials, zero-dimensional quantum dots using TMD materials are also attracting attention because of their small size, large specific surface area and shortened charge travel distance. Many recent TiO2 quantum dot complexes have attracted considerable attention. Coupling of TiO2 and semiconducting quantum dots can improve the charge separation of photo-generated electron-electron pairs and smaller quantum dot materials are uniformly dispersed on the TiO2 surface, preventing aggregation of quantum dots and providing more active sites [153,154]. However, the synergy of the existing TiO2/QD materials was not satisfactory [155,156]. Therefore, a catalyst using visible solar energy was achieved with the introduction of Ti 3+ -TiO2. Likewise, there have been recent attempts to modify the MoS2 and CdS QDs through conjugation of Ti 3+ -TiO2 as shown in Figure 11 [146,150]. Abundant Ti 3+ self-doping sufficiently improves the visible light response, and QD MoS2 and CdS can promote photocatalytic carrier separation through a builtin electric field to provide a rich specific surface area to improve photocatalytic performance. Figure 11. Process diagram of the preparation and photocatalytic hydrogen evolution reaction (HER) mechanism of the MoS2 QDs modified black Ti 3+ -TiO2/g-C3N4 hollow nanosphere heterojunction.
In addition to TMD materials, there have also been attempts to integrate Ti 3+ -TiO2 with QD. Ag3PO4 attracted attention because of its high quantum yield and band gap energy (2.45 eV) suitable for absorbing visible light. However, Ag3PO4 was only used as a photocatalyst due to fast charge recombination and relatively large particle size. Overcoming these shortcomings with QD Ag3PO4 and bonding with Ti 3+ -TiO2 increased visible light responsiveness and improved photocatalytic performance ( Figure 12). [145] The introduction of quantum dots suggests a new paradigm to solve problems that could not be solved with the existing classical materials. Richly doped Ti 3+ can serve as an effective support for quantum dot materials and as an electron carrier delivery site. Thus, richly doped Ti 3+ has very promising and potential possibilities, requiring more in-depth research into this area.

Carbon-Based Nonmetallic Semiconductors/Ti 3+ -TiO2
Carbon-based nonmetallic semiconducting materials are attracting great attention in the energy industry because of their attractive properties such as high conductivity and easy accessibility. The two-dimensional electronic network of sp 2 carbon atoms exhibits high electron mobility and conduction [157]. In the past few years, carbon-based materials have been extensively explored as photocatalysts. Recently, graphitic-carbon nitride (g-C3N4) has attracted much attention as a visible light active catalyst due to its appropriate band gap (2.69 eV). In particular, TiO2 and Z-structure photocatalyst band alignments are similar to the plant photosynthesis process and can provide high photocatalytic activity through more powerful charge separation than type I and type II band alignment [158,159]. Ti 3+ -TiO2 can strengthen this Z-structure with abundant Ti 3+ doping in the conduction band. Figure 13 shows the Z structure diagram between TiO2 and g-C3N4. Recently, studies on enhancing the performance of visible light-activated photocatalysts have been actively conducted through the combination of Ti 3+ -TiO2 and g-C3N4. There have been attempts to improve charge separation by doping plasmon metal between Ti 3+ -TiO/g-C3N4. Cao et al. introduced Agdoped black TiO2 nanosphere structures and g-C3N4. Figure 14 shows a schematic of the Ti 3+ -TiO2/Meso-g-C3N4 manufacturing method. They showed that Ag could act as an electron-conducting bridge between TiO2 and g-C3N4, contributing to electron donors and acceptors in the Z-structure [151].   [94]. Copyright (2017), Elsevier.

Metal Oxide/Ti 3+ -TiO2
Heterojunctions of metal oxide and TiO2 are a classic (and the easiest) method to enhance TiO2 photocatalytic activity after metal doping. To date, new heterojunctions of various metal oxides and TiO2 have been studied. Heterojunctions of Ti 3+ -TiO2 and metal oxides are also of great interest. Although many metal oxides are candidates, heterojunctions of WO3 and Ti 3+ -TiO2 have recently been developed. WO3 can achieve Z-structure band alignment with Ti 3+ -TiO2. The resulting Zstructure has a band structure similar to natural photosynthesis [158,159]. This can strongly increase the charge separation efficiency. In addition, two different redox sites can suppress the reverse reaction of radical production. In 2020, Nguyen et al. developed a catalyst that introduced blue Ti 3+ -TiO2 and WO3 produced by lithium (Li-EDA) of ethylenediamine, a strong reducing agent of the previous superbase. They show 100% selectivity in reducing CO2 to CO, and that blue Ti 3+ -TiO2/WO3 formed a perfect Z-structure ( Figure 15) [91]. Consequently, the introduction of a complete Zstructure can be a powerful modification of Ti 3+ -TiO2. This is a direct way to overcome the weakness of charge separation and recombination and dramatically enhance the performance of photocatalysts. Figure 15. Band alignments of P25 and blue TiO2 NPs and the proposed electron transfer mechanism of the 7BT/W1-A1 HNPs photocatalyst. The photoelectrons generated and excited electrons from the CB of WO3 to migrate to trap holes at the VB of blue TiO2 sites through a Z-scheme system. After separation, the excited electrons rapidly jump from the CB of blue TiO2 onto Ag NPs acting as active sites for conversion of CO2 into CO, while all of the photogenerated holes directly oxidize H2O to form O2 from the VB of the WO3 site. Reproduced from [91]. Copyright (2019) Elsevier.
There have also been attempts to make heterojunctions with CeO2, which has a band gap energy similar to TiO2. CeO2 has recently attracted attention due to the special redox properties between Ce (III) and Ce (IV) oxidation states, which include high thermal stability, excellent oxygen storage capacity and easy conversion. Xiu et al. developed Ti 3+ /Ce 3+ self-doped TiO2/CeO2 nanosheets by reducing TiO2 and CeO2 together. They showed that Ti 3+ and Ce 3+ complement each other and enhance electron separation ( Figure 16) [148]. Figure 16. Visible-light-driven photocatalytic mechanism for Ti 3+ -TiO2/Ce 3+ -CeO2 nanosheet heterojunctions. Reproduced from [148]. Copyright (2017), Elsevier.

Stoichiometry Modification
There have been attempts to change the stoichiometry of materials beyond controlling Ti 3+ -selfdoped intrinsic defects to enhance the visible light absorption of TiO2 photocatalysts. Introduction of defect states such as Ti 3+ is sensitive to the chemical state and spatial distribution of electronic structure modifiers. Therefore, photocatalytic activity may be reduced through the formation of an undesirable band structure. It is important to create favorable defects at the atomic level to improve absorption in the visible light band. However, it is still difficult to correct structural defects at the atomic level and produce an optimal TiO2 material. A study on correcting atomic-level structural defects is ongoing beyond the recent Ti 3+ -TiO2 work. Efforts have been made to replace hydrogen vacancies in Ti 3+ -TiO2 with atomic levels of hydrogen [81], carbon [82] and nitrogen [82,83]. We summarized in table 4.
In general, reduced Ti 3+ -TiO2 has a blue-black color, whereas substituted TiO2 has a different color. Red TiO2 appears when the space for a single oxygen atom is filled with two hydrogen atoms, called Ti 3+ H TiO2 [81]. As shown in Figure 17, the color and atomic/band structure of TiO2 was changed according to the vacancies in the crystal structure. In addition, the introduction of heterogeneous elements results in a new band state. The band gap is reduced due to changes in the conduction band and valence band, and both show excellent activity in visible light. We believe that engineering the oxidation state of Ti atoms (beyond Ti 3+ , which is simply reduced) and the surrounding electronic environment through complete stoichiometric control is a new paradigm for developing effective TiO2 visible photocatalysts. In the future, we can expect to expand and improve the photocatalytic performance through visible light for TiO2 photocatalysts via uniformly controlled stoichiometric modifications.

Conclusion and Outlook
TiO2-based photocatalytic reactions were first reported in the early 1930 s. They were first used to bleach dye using reactive oxygen species generated under UV light. TiO2 photocatalyst (called a "photosensitizer") has been studied extensively, and it is expected to play a role in solving the limited energy problem [24]. In particular, efforts have been made to improve the absorption band of TiO2 photocatalyst into the visible light regime to efficiently use solar energy. These efforts continued to Ti 3+ -TiO2, and since the report of black Ti 3+ -TiO2 reduced by Ward et al. in 2011, Ti 3+ -TiO2 research has made great progress. [92] We first studied the development of TiO2 reduction to produce abundant Ti 3+ for overcoming the high heat and pressure of the hydrogenation reaction (as the first generation of Ti 3+ -TiO2-based photocatalysts). Then, due to their long reaction times and risks, increasing mild and easy to handle methods have been developed as the next generation. A lithiumethylenediamine solution (Li-EDA) has been introduced for self-doping Ti 3+ effectively at room temperature and atmospheric pressure, as well as controlling the crystalline phase of reduced TiO2. Selective reduced crystal phases with different physicochemical properties will pioneer photocatalytic utilization for several properties [87]. However, although Ti 3+ is abundantly doped such that the band region for visible light is expanded, there are still limitations. The interaction and reactivity between target materials and photocatalysts, as well as effective suppression of charge separation and recombination, are still challenges that need to be overcome. The best way to improve the limited performance of these single materials is with several different composite materials.
In this review, we focused on the types and methods of Ti 3+ -TiO2 and the materials introduced and categorized them into four categories according to recent developments. Ti 3+ doping can be used as sufficient support for an isolated single-atom site catalyst and can stabilize the catalyst atom. The isolated single-atom catalyst has excellent selectivity and reactivity with a small amount of catalyst [89]. Introducing metals other than plasmonic precious metals will make commercialization easier. The introduction of LSPR using isolated single atom site catalysts and non-precious metals in metal dopants is the most effective modification method. Going further, bimetallic sites, which have been extensively studied recently, are being proposed as a new way to solve the selectivity and stability issues that a single atom catalyst cannot. There are different advantages depending on the metal combination, and there are even advantages to not using precious metals. It has recently been shown that bimetallic site catalysts can also be applied with TiO2 [160]. However, this approach requires more research of bimetal sites.
The introduction of nonmetal dopants is a way to engineer the fundamental band structure of TiO2. Modification of the band structure can make it easier to expand this approach to visible photocatalysts, and most nonmetallic dopants can improve the decomposition of harmful substances through strong interactions with organic harmful substances. Another approach is doping two or three heterogeneous elements beyond single element nonmetallic doping. The introduction of a suitable semiconducting material can be used to overcome the shortcomings of the aforementioned TiO2 photocatalysts. Beyond the classic 3D dopant, the introduction of 2D TMD and 0D QD materials shortens both the large specific surface area and charge travel distance. Because of this, it is possible to improve the charge separation of electron-electron pairs, which could not be solved before. The improved charge separation has high expectations for HER and water splitting. The introduction of metal oxides can also be a great alternative. Due to their excellent accessibility and stability, carbonbased materials can be promisingly combined with Ti 3+ TiO2. The Z-structure has a structure similar to that of photosynthesis. The formation of Z-structures through the introduction of metal oxides and carbon-based materials is a powerful approach to improving photocatalytic performance. Similar to a plant's photosynthetic structure, the photocatalyst can replace various functions of plants in the future. This research field has potential in the energy industry.
Furthermore, advances in material engineering technology can dominate atomic defects. This presents a new base of TiO2 photocatalysts through heteroatom insertion into the TiO2 grid beyond Ti 3+ single atom doping [81][82][83]. It is expected that the next generation of Ti 3+ -TiO2 is in stoichiometric modification. If the electronic environment is completely controlled by controlling the oxidation state of the Ti atom and the surrounding atoms rather than simply doping the reduced Ti 3+ , there is a possibility to produce a TiO2 photocatalyst suitable for each application method. TiO2 has been studied as a practical photocatalytic material. However, in addition to applications that can be directly applied, in-depth studies on the electron environment, oxidation state of various dopants in the crystal structure, enhanced charge separation, and defect-level band energy are feasible. By investigating the atomic state and bonding level in the final crystal structure, it is possible to identify the principles and mechanisms of theoretical photocatalytic applications. Such an understanding is essential to improving photocatalytic performance.
Advanced Ti 3+ -TiO2 approaches have recently been extended to visible light in solar energy harvesting and air purification. In particular, we believe that by applying air purification [152], diseases caused by viruses and bacteria [120] in the air-that have emerged as among the biggest problems faced by humanity-can be prevented by using Ti 3+ -TiO2-based photocatalysts, a visible photocatalyst that is harmless to humans. For this, it is necessary to study the interactions of cells with photocatalysts in a view point of biochemistry, as well as conduct theoretical studies about classical interactions between organic and inorganic and their mechanisms. Nonetheless, the application of visible light photocatalysts in this industry is still insufficient. It is essential to explore sufficient functions required for commercialization, [91] further study of Ti 3+ -TiO2-based materials and the development of more effective heterogeneous materials. This makes Ti 3+ -TiO2 a viable option for visible photocatalysts in the future.